The Role of Electrons

in Sputter Deposition

of Thin Films by Frank Arthur Green

A thesis submitted for the degree of Ph.D at the University of London, and also for the Diploma of Imperial College of Science and Technology.

Department of Electrical Engineering [Science of Materials]

Imperial College May 1979

ACKNOWLEDGEMENTS

My thanks are due to Professor J. C. Anderson for his

supervision of the latter part of the thesis and for the

facilities provided at the Materials Section, Imperial College.

Thanks are also due to Dr. B. N. Chapman for his supervision of

the experimental work and to the Science Research Council for

the provision of a grant.

I am also indebted to Judy, my wife, and to my parents for

the help and encouragement they have given in numerous ways.

Finally, I would like to thank Mrs. Alyson Phillips for

her careful typing of the manuscript and also my friends and

colleagues at the Materials Section, Imperial College, Dover

Grammar School for Boys and Folkestone Grammar School for Girls

for their helpful assistance.

ABSTRACT "The Role of Electrons in Sputter Deposition of Thin Films."

F. A. Green.

Energy and power measurements of the electrons bombarding

the anode in a D.C. planar sputtering system show that a small

but significant number travel in straight lines to the anode

without collision and are responsible for a large percentage

of the power input.

A magnetic field was used in a cylindrical geometry to

alter the electron trajectories and improve plasma efficiency,

measurements being taken to show the distribution of the input

power in this new system. The contribution to the substrate

power input from neutral species was seen to be more important

in this high deposition rate system. Radiation from an uncooled

target was also an important energy source. Modified magnetron

geometries were also investigated as was the anticipated film

thickness uniformity problem resulting from this cylindrical

arrangement.

Over a range of sputtering conditions in the cylindrical

magnetron system, the current-pressure characteristics show an

unexpected discontinuity and current-magnetic field curves ex-

hibit a maximum current. These phenomena seem to depend on the

ease with which the plasma can sustain itself and are related

to the relationship between the maximum distance of the secon-

dary electron initial trajectory from the target surface, and

the target dark space thickness.

The problem of plasma sustainment was investigated by making

theoretical calculations of the main region of electron impact

ionisation together with the predicted electron energy spectrum

in the negative glow. This spectrum was compared to the experi-

mental analysis results.

2 TABLE OF CONTENTS

Title Page .. .. .. .. .. .. .. ..

Acknowledgements .. .. .. .. .. ..

Abstract .. .. .. .. .. .. ..

Contents .. .. .. .. .. .. ..

Chapter One - Introduction to Electron Effects

..

..

..

1

2

7

1.1 Sputtering in General .. .. .. 7

1.2 The Basic Process .. .. .. Of 00 9

1.3 Sustaining the Discharge .. .. .. 12

1.4 Collisional Probability .. .. .. 14

1.5 Efficiency Aspects .. .. .. 15

1.6 Substrate Bombardment .. .. .. 15

1.7 Heating Effects .. .. .. 17

1.8 Nucleation Effects .. .. .. .. 20

1.9 Guidelines of the Work .. .. .. .. 20

Chapter Two - Energy Analysis .. .. .. .. .. 22

2.1 General Introduction .. .. .. .. .. 22

2.1.1 Analysis in the Plasma Region .. .. .. 22

2.1.2 Analysis at the Substrate .. .. .. .. 22

2.2 Energy Analysis .. .. .. .. .. .. 24

2.3 Theory of the Spherical Condenser .. .. 25 2.3.1 Focusing .. .. .. .. .. 00 .. 25 2.3.2 Resolution of the Analyser .. .. .. .. 29 2.4 Experimental System .. .. .. .. .. 29 2.4.1 Introduction .. .. .. .. .. .. .. 29 2.4.2 Vacuum Considerations .. .. .. .. .. 30

2.4.3 Sputtering Chamber .. .. .. .. .. 31

2.4.4 Target Assembly .. .. .. .. .. Of 31

2.4.5 Particle Extractor .. .. .. .. .. 33 2.4.6 Energy Analyser .. .. 00 .. 00 . . 33 2.5 Calibration of the Energy Analyser .. .. 34 2.6 Accuracy of Results .. .. .. .. .. 35 2.7 Experimental Results .. .. .. .. .. 37

3

2.7.1 General Curve Shape .. .. .. .. .. 37 2.7.2 Constant Voltage with Varying Pressure .. .. 37

2.7.3 Constant Pressure, Variable Voltage .. .. 39 2.7.4 Variation of Target/Substrate Separation and

Sampling Aperture .. .. •• •• .. .. 41

2.8 Analysis of Results .. .. .. .. .. 42

2.9 Analyser Acceptance Bandwidth .. .. .. 43 2.10 Heat Input to the Substrate .. .. .. .. 45 2.11 Conclusions .. .. .. .. .. .. .. 46 2.11.1 General Comments .. .. .. .. .. .. 46 2.11.2 Summary of Results .. .. .. .. .. .. 46

Chapter Three - Higher Rate Sputtering .. .. .. 49 3.1 Introduction .. .. .. .. .. .. .. 49 3.2 Factors Influencing Deposition Rate .. 51 3.2.1 Target/Substrate Separation .. .. .. .. 51 3.2.2 The Temperature of Substrate and Target .. 51

3.2.3 Pressure .. .. .. .. .. .. .. .. 52 3.2.4 Voltage Between Target and Substrate .. .. 55 3.2.5 The Current Drawn by the Discharge .. .. 57

3.3 The Concept of Efficient Sputtering .. .. 58

3.4 Plasma Efficiency .. .. .. .. .. .. 61

3.4.1 Triode Systems .. .. .. .. .. .. 62 3.4.2 The Use of an External Magnetic Field .. .. 63 3.4.3 The Penning Cell .. .. .. .. .. .. 65 3.4.4 The Cylindrical Magnetron .. .. .. .. 66 3.5 'Novel' Magnetron Systems .. .. .. .. 67

3.5.1 The Sputtergun .. .. .. .. .. .. 67

3.5.2 The Planar Magnetron .. .. .. .. .. 68 3.6 Experimental Systems .. .. .. .. .. 70 3.6.1 Conventional Cylindrical Magnetron .. .. 70 3.6.2 A 'Modified' Cylinder - Introduction .. .. 72 3.6.3 A 'Modified Cylinder' - Construction Details 74 3.7 Deposition Rates .. .. .. .. .. .. 76

4

Chapter Four - Film Uniformity .. .. .. .. .. 77 4.1 The Factors Affecting Thin Film Uniformity .. 77 4.1.1 Introduction .. .. .. .. .. .. .. 77 4.1.2 Solid Angle .. .. .. .. .. .. .. 77 4.1.3 The Mean Free Path of Sputtered Species .. 78 4.1.4 The Distribution of Sputtered Material .. .. 79 4.1.5 Condensation Coefficient .. .. .. .. 80 4.2 Experimental Investigation for a Cylinder .. 81 4.2.1 Measuring Technique .. .. .. .. .. 82 4.2.2 Experimental Results .. .. .. .. .. 84 4.3 Improving Film Uniformity .. .. .. .. 85 4.4 Split Target Experimental System .. .. .. 86 4.4.1 General System .. .. .. .. .. .. 86 4.4.2 Five Bar System .. .. .. .. .. .. 88 4.4.3 Experimental Results .. .. .. .. .. 90 4.5 Simplified Theory for the Thickness

Distribution .. .. .. .. .. .. .. 92 4.5.1 Extension to the Sectioned Bar .. .. .. 95 4.6 Conclusions .. .. .. .. .. .. .. 98

Chapter Five - Electrons in a Magnetically Supported Plasma .. .. .. .. .. .. .. .. .. .. 99 5.1 Introduction .. .. .. .. .. .. .. 99 5.2 The Role of the Magnetic Field .. .. .. 102 5.2.1 Electron Trajectory .. .. .. .. .. 103 5.2.2 Target/Substrate Potential Distribution .. 104 5.3 Region of Ion Production .. .. .. .. 108 5.3.1 Secondary Electron Coefficient .. .. .. 110 5.3.2 Electron Energy Distribution at the CDS/NG

Boundary .. .. .. .. .. .. .. .. 111

5.3.3 Analysis Conclusions .. .. .. .. .. 114 5.3.4 Energy Distribution Assuming Constancy of Qi 116

5.3.5 Analysis Conclusions .. .. .. .. .. 120 5.3.6 Flux Multiplication Model .. .. .. .. 121 5.3.7 Conclusions on Ion Production .. .. .. 123 5.4 The Pressure/Current Characteristic .. .. 123 5.5 Magnetic Field Variations .. .. .. .. 124

.. 129 5.6 General Conclusions .. .. .. ..

5

Chapter Six - System Performance .. .. .. .. 131

6.1 Optimum Operating Parameters .. .. .. .. 131

6.1.1 Operating Voltage .. .. .. .. .. .. 131

6.1.2 Operating Current .. .... .. .. .. 132

6.1.3 Optimum Condition Conclusions .. .. .. 136 6.2 General Characteristics .. .. .. .. .. 136 6.2.1 Current/Voltage Relationship .. .. .. .. 137 6.2.2 Magnetic Field Variation .. .. .. .. 138 6.3 Substrate Heating .. .. .. .. .. .. 140 6.3.1 Introduction .. .. .. .. .. .. .. 140 6.4 Distribution of the Input Power .. .. .. 141 6.4.1 Power to the Substrate .. .. .. .. .. 141 6.4.2 Distribution of Power to the Substrate .. .. 143 6.4.3 Power to the Target .. .. .. .. .. 145 6.5 Experimental Results .. .. .. .. .. 146 6.5.1 Deposition Rate .. .. .. .. .. .. 146 6.5.2 Variation of Substrate Input Power .. .. 147 6.5.3 Equilibrium Temperature Te .. .. .. .. 152

6.5.4 Magnetron Compared to Non Magnetron .. .. 153 6.6 Distribution of Power to the Substrate (con) 154 6.7 Conclusions .. .. .. .. .. .. .. 157

6.7.1 General Comments .. .. .. .. .. .. 157 6.7.2 Factors Necessary for an Efficient High Rate

System, .. .. .. .. .. .. .. .. 158

Chapter Seven - Conclusions .. .. .. .. .. 160

7.1 Introduction .. .. .. .. .. .. .. 160 7.2 Review of the Major Conclusions .. .. .. 161

7.2.1 Energy Analysis .. .. .. .. .. .. 161 7.2.2 Processes at the Substrate .. .. .. .. 164 7.2.3 Switching Phenomena .. .. .. .. .. 165 7.3 An H Configuration .. .. .. .. .. .. 168 7.3.1 Experimental Results .. •• .. .. .. 169 7.3.2 Switching With Smaller End Plates .. .. .. 171 7.3.3 Concluding Remarks on the H Configuration .. 174 7.4 Conclusions on Switching .. .. .. 174 7.5 The Role of Photons .. .. .. .. .. 177 7.6 Concluding Remarks .. .. .. .. .. .. 178

E

References for Chapter One .. .. .. .. .. .. 179 References for Chapter Two .. .. .. .. .. .. 181 References for Chapter Three .. .. .. .. .. 182 ,References for Chapter Four .. .. .. .. .. 184 References for Chapter Five .. .. .. .. .. 185 References for Chapter Six .. .. .. .. .. .. 186 References for Chapter Seven .. .. .. .. .. 187

Appendix I .. .. .. .. OS .. .. .. . • 188 Appendix II .. .. .. .. .. .. .. .. .. 189

CHAPTER ONE

INTRODUCTION TO ELECTRON EFFECTS 5

1.1 s U'''TERI::G IN GE'.'ERAL

The name sputtering is given to the process whereby atoms

or molecules of a material are ejected from its surface due to

bombardment of that surface by positive ions. Sputtering was

first observed in early incandescent laps which darkened as

the carbon filament material was deposited on the glass envelope.

In this case the necessary ions arose from the residual air left .

in the imperfect vacuum which was achieved in the lamps at that

time. The resultant sputtering was obviously not considered to

be an advantage.

Whilst the earliest reports on the process occur in the mid

nineteenth century(1), more detailed work was produced later by

Fruth(2) and Von Hippel(3)(4) who proposed that sputtering took

place by virtue of momentum transfer between the bombarding ion

and the atoms or molecules of the target material. The general

literature on sputtering has grown immensely in recent years and

has been reviewed in depth by many authors includin 7ehner(5),

Kay(6), Maissel(7), We P and Anderson(8)

, Jackson(9) and

Holland and Priestland ~I. Perhaps the main reason for the

growth in the interest in the process has been its exceptional

versatility as a• deposition technique. This can be illustrated

by looking at some of the advantages of the basic sputtering

technology.

(a) Versatility

It is possible to sputter virtually any material onto any

surface. Whilst sputtering may not be the most convenient sol-

ution to a particular problem, there are times' when it is the

only possibility. High melting point metals such as tungsten

are difficult to evaporate whilst using the sputtering process,

tungsten deposition presents no special problems.

(b) Adhesion

Good adhesion between the film and the substrate can usually

be achieved in sputtering. This is normally attributed to the

relatively high arrival energies of the sputtered species.

(c) Compound Films .

Stoichiometry is preserved with compound targets providing

sputtering takes place in molecular and not atomic form. When

8

ejection takes place in the form of atoms, stoichiometry can

still be achieved by the use of target cooling to inhibit solid

state diffusion in the target material.

Reactive sputtering can also be used for the production of

compound films. In this technique the inert gas commonly used

is either partially or wholly replaced by a gas such as 02, N2,

or H2S. Now in addition to momentum transfer sputtering from

the target, the possibility of chemical reaction exists either

at the target or in the gas phase or at the substrate.

(d) Thickness Uniformity

The angular distribution of the ejected atoms from each

point of the source follows the familiar cosine law for both

• thermal evaporation and sputtering sources. However, the larger

source area available with a sputtering target provides a much

larger area over which uniform distribution is possible.

(e) Deposition Rate

The relatively slow deposition rates obtained in sputtering

mean that film thickness control can be achieved to a high degree

of accuracy. In applications where the low rate is a disadvan-

tage, such as in the production of thick coatings, the rate can

be increased by using an improved configuration such as a

magnetron device which. is discussed in more'detail in chapter

three.

(f) Substrate Cleaning,

A-sputter etching stage can easily be incorporated into

production apparatus. This consists of an electrically insulated

substrate table to which the negative sputtering potential can be

applied thereby making the substrate the target for the ions.

This feature can be of great value for cleaning substrates,before

deposition starts,to improve adhesion.

Another application of this facility is in the etching of a

pattern on the substrate through a substrate mask. Sputter

etching does not produce the undercutting effects usually

exhibited by chemical etching processes.

(g) Co-Suutterino

Several targets of identical or different materials can be •

employed simultaneously to produce films of varying•composition.

(h) Sequential Snutterin

Targets can be utilized sequentially to produce a layered

9

coating such as in a thin film transistor. It should be admitted

here.that radiation damage prineipaily from x rays produced by

high energy elections impacting onto metal surfaces can limit the

value of the process as a means of producing semiconducting

devices.

(i) Emitaxv

There is often a critical temperature above which epitaxial

film growth is possible. This temperature is usually fairly low

for sputtered films. Unfortunately this potential advantage is

nullified by the fact that sputtered semiconductors have very low

mobilities because of damage to the crystal caused by the high

arrival energies of the sputtered species. Sputtered epitaxial

films are not usually thought to be advantageous.

Sputtering in general will not be reviewed in any detail in

this work which is directed at the properties of electrons as

related to the sputtering process and the effects which they can

cause. To this end, the basic sputtering process will be briefly

discussed in the simplest D.C. case with emphasis being placed on

the electron properties.

1.2 THE BASIC PROCESS

When a beam of energetic particles is incident on a target,

material is ejected or sputtered from the target surface in a

momentum transfer process. Since for a given energy E, particle

momentum p is given by p = (2mE) where m is the mass, it might

be thought that heavy ions are most suitable for the promotion of

the sputtering process, but closer inspection reveals that maxi-

mum momentum transfer occurs when the target atoms and sputtering

ions are of comparable mass. Electrons are usually thought to.be

totally inadequate for sputtering purposes because of low mass

although work by Townsend(11) suggests that high energy electrons

can cause sputtering. However, the process is very inefficient

and can normally be safely ignored.

The necessary high energy can easily be imparted to gaseous

ions by accelerating them by means of an electric field. The

sputtering ions can be produced externally using.an ion gun but

it is more convenient to produce them internally by creating a

glow discharge. The requirement for gaseous ions limits the

choice of sputtering environment and argon is often.chosen

10

because it is chemically inert, is a relatively massive ion and

the pure gas is readily obtainable.' Glow. discharge phenomena

have been reviewed by 'baissel(7) and Cobine(12) and will only be

discussed briefly here.

When a high electric potential is applied between two flat

parallel electrodes in a low pressure system (typically 1 to 100

millitorr) a current will flow and there will be a gaseous dis-

charge between the electrodes. Positive argon ions (Ar+) created

in the breakdown are attracted towards the negative electrode

(target) and strike it causing sputtering of the target material.

When breakdown first occurs, a normal glow discharge is initially

established in which the current density at the electrode remains

constant. For a small voltage increase, the current increases at

constant current density as the discharge spreads .to use the

whole of the electrode surface. Once all of the surface is in

use, larger voltages'are needed to increase the current flow and

the discharge is said to be in the abnormal glow condition. This

situation is normally applicable to sputtering applications since

it ensures that all of the target surface is being used. Also,

the abnormal glow is the only situation in which the current den-

sity is controllable and it is only in this region that current

densities are high enough for atoms to be sputtered out of the

target at useful rates.

11

Fig. 1.1 Soati•ai Distribution of Dark and luminous Zones, Electric

Field X, Potential V, and Scace Charge Der sitiec iO+/O in a Glow

Discharge.

1 ASTON D4RK SPACE

TARGET; 2 CATHODE LAYER

/4/ 3

4 3CROOKES DARK SPACE

4NEGATIVE GLOW

1 2

t

I iD DISTANCE FROM I TARGET

The transport of current through the discharge is due to the

axial motion of electrons and positive ions. An electron is

emitted from the cathode by virtue of a positive ion impinging on:

it and gains energy as it is accelerated away by the strong field

12

near the cathode. Since an electron will leave the cathode with

an energy of about one electron volt, initially no excitation of

gas molecules can occur and there is a'dark region (the Aston

dark space) next to the cathode. Further from the cathode,.

electron energies are as high as the excitation potential of the

gas (about 15ev) and there is a luminous layer. Beyond this

area, electron energies are far above the Maximum excitation

potential of the gas and a relatively dark region, the Crooke's

dark space (CDS) results. This region is of funda.r:ental impor-

tance to the sputtering process and elementary texts would

suggest that most of the ions striking the target come from the

CDS. A more detailed examination later in this work does,

however, cast doubt on the assumption.

The distribution of the electric field (Fig. 1.1) is such

that most of the inter-electrode potential is dropped across the

CDS. Beyond the CDS here is a negative glow (NG) region which

contains electrons from two groups:-

(1) Fast electrons produced at or near the cathode which

have not suffered any energy loss due to inelastic collision in

the CDS and therefore have the full fall of potential at the NG

region:

(2) A larger group of slow electrons created in the discharge

which have made many inelastic collisions. Since these slow elec-

trons cannot gain energy rapidly in the weak electric field, they

tend to have energies below the gas ionisation potential but above

the excitation potential and they, therefore, produce many

exciting collisions and a negative glow.

Whilst other light and dark regions can exist in a plasma, they are usually absent under sputtering conditions where the sub-

strates are normally positioned in the NG.

1.3 SUSTAINING THE DISCHARGE

There are several processes which can occur when an ion is

incident on a target in the glow discharge situation.

(a) The ion can implant itself into the target material.

(b) It can be reflected either as an ion or as a neutral

species.

(c) Ejection of the target material can occur either in

neutral or ionic form.

13

(d) Electrons are produced and being negatively charged they

tend to be reseller. from the target area which is also at a nega-

tive potential. It is these secondary electrons which are

responsible for sustaining the discharge since they are able to

create more ions by co?l_Lsion. Since this electron impact o -

ionisation creates another electron (Ar + e--- Ar+ + 2e-) we

have a breeding mechanism by which a small 'number of electrons

leaving the target can multiply by multiple collision to a large

number by the time the GDS/NG boundary is reached..

A criterion for the discharge to be sustained is that each

ion incident on the target must produce sufficient electrons so

that they in turn will produce another ion which is incident on

the target. It should be realised that because of collisions,

any ion created does not have unit probability of reaching the

target.

An electron/gas atom collision need not result in ionisation

and the formation of an ion/electron pair; other processes occur,

notably:-

(a) An elastic collision in which the electron 'bounces' off

• the gas atom with a direction change but no kinetic energy loss

• and no ion production.

• (b) An excitation type collision in which the gas atom is

raised to an excited state and then decays back to the ground

state emitting light at a frequency characteristic of the gas.

This process is responsible for the various glow regions of a gaseous discharge. The electron will typically change direction

and lose kinetic energy in a collision of this type and will

migrate to the walls of the containing vacuum vessel thus being

•lost to the plasma. This represents a serious loss mechanism

since an electron lost to the plasma cannot contribute to plasma

efficiency by causing ionising collisions.

A further important point is that there is a higher probab-

ility of electron/ion recombination on the walls of the containing

vessel. Whilst this process is unlikely to happen in the plasma

area due to the requirement for energy and momentum to be con-

served in the two body problem; the conservation is much easier

to satisfy in the corresponding three body problem which is

relevant when the wall enters into the calculations. This is

14

because the wall allows for the dissipation of kinetic energy in

the form of heat. -

Hence, the wall acts as a trap for the charge carriers'which

could participate in additional useful collisions. In general

the walls ought to be kept as far away as possible from the tar-

get area and attempts have been made to keep the electrons away

from the wall area by using magnetic fields. This will be looked

at in detail in a later chapter.

1.4 COLLISIO_: 4L PROIt3ILITY

So far, no consideration, has been given to the probability

that an electron emitted from the target will strike a gas atom

causing ionisation or excitation. The thickness of the CDS is

often thought of as being approximately the mean distance travel-

led by an electron before it makes an ionising collision. Whilst

this concept is often useful, much of the later work in this

thesis casts doubts of the validity of the assumption. Certainly

the CDS is a fundamental plasma parameter and defines a lower

limit on the pressure which can be used. If the anode (substrate

in the simple D.C. case) is placed inside the CDS, the discharge

will be extinguished because sufficient ionising collisions will

not take place to maintain the discharge. Whilst a discharge can

be maintained with the substrate just outside the CDS, it is

found that ion production is affected unless the target/substrate

separation is at least two CDS lengths suggesting that the NG has

some part to play in ion production and discharge sustairment,

The CDS length should not be confused with the electron mean

free path which is about ten times smaller since it also includes

the probability of excitation and elastic collision as well as

ionisation.

As the gas pressure is reduced the CDS lengthens since col-

lisions of any sort including ionisation become less likely. The

target/substrate separation has, therefore, to be increased to

prevent the discharge being extinguished. In a practical simple

D.C. sputtering system where the aim is an .acceptable deposition

rate, a low pressure limit is imposed by the fact that the elec-

trode separation becomes unrealistic. In the theoretical case

(i.e. a plasma but not suitable for useful sputtering) a low

pressure limit is set by the fact that'the discharge cannot be

15

sustained when the necessary electrode separation is so large

that the consequentially.weak electric field results in electron

loss being excessive. This lowest possible pressure is about 15

microns in the simple D.C. case depending on geometry.

Results presented by Chapman et al( 13) and Ball ( 1) indi-

cate that some electrons strike the substrate with an energy

corresponding to the full fall of cathode potential suggesting

that they have undergone no collision of any type in traversing

the plasma. This is, perhaps, surprising at first sight in view .

of the expected collision cross sections as presented by Massey

and Burhop(15).

Whilst these fast electrons are contributing nothing to the

creation of further ions by collisional processes, they are also

of interest in terms of the substrate heating and damage to the

deposited film which can be caused and this will be reviewed'

later.

1.5 EFFICIENCY ASPECTS

Previous sections have concentrated on how electrons are

produced in the sputtering process, how they interact with gas

atoms causing the discharge to be self sustaining and how they

are lost to the anode and containing walls. One of the major

' disadvantages of the sputtering process is its slow deposition

rate but this can be improved either by creating more electrons'

or by making those present more efficient at causing ionisation.

The two main methods are to support the discharge either mag-

netically by attempting to confine the electrons to the plasma region using a magnetic field or thermionically by injecting an extra supply of thermal electrons. Both of these methods will be

reviewed in chapter three but it is worth mentioning here that it

is the modification of the role of the electron which is respon-

sible for the high deposition rate devices which are now being

developed. •

1.6 SUBSTRATE BO:.:BARD' TENT

The role of the contairing.wall as a loss mechanism has

already been discussed and it is obvious that the substrate to be

coated must also be bombarded by charged species as well as

neutral material. Furthermore, since the substrate is typically

held at anode potential it is particularly susceptible to bom-

16 bardment by electrons. Since an ion current is striking the

cathode, it follows from charge continuity considerations that an

electron current must be drawn from the, anode.

The physical properties of sputtered films are dependent on

the conditions under which the film was prepared. Each particle

which is eventually incorporated into a film must have been

previously adsorbed at the surface and it is, therefore, advan-

tageous to know the charge and energy of the flux of particles

incident on the substrate during deposition since this might help

explain the film properties.

Not all of the material arriving at the substrate is incor-

porated into the film since there is a possibility of

re-sputtering leading to a sticking probability of less than

unity. Thermal desorption of material is also possible. In the

case of a compound target, these processes could lead to the

production of a non-stoichiometric film since the constituent

atoms might exhibit different sticking characteristics on

reaching the substrate. Without information about the incident

particle flux to the substrate, it is not possible to say which

process, if any, is significant.

The energy and mass spectra of positive ions incident on the

substrate in a sputtering system have been investigated by Coburn •

and Kay(16-18). They have been able to demonstrate that the •

Penning Ionisation mechanism is the most important method of ion

production near to the substrate as opposed to electron impact

ionisation which is most significant in the general plasma region.

It should be noted that these ions produced close to the substrate

are unlikely to reach the target because of the relatively large

distance involved. In the Penning mechanism, a metastable gas

(argon) atom produced by collisions itself collides with a neutral

atom of the target material. If the ionisation potential of the

atom is less than the energy of the metastable, then the latter

can de-excite to the ground state by ionising the neutral species

according to a general formula:-

Arm + + +Ar+ e

Work by Gilkinson et al (19) suggests that the following

mechanisms could also take place though the probability cross

sections are expected to be very low.

17

Arm + Ar Ar,+ f- e-

Ar+ + 2Ar ----~ Ar2+ + Ar

Ar+ + X --a Ar + X+

Ar2+ + X 2Ar + X+

Since the plasma potential can be a few volts positive with

respect to the most positive electrode (i..e. anode) the Penning

process provides a mechanism for bombarding the substrate with

positive ions. Coburn and Kay(20)

have pointed out that re-

sputtering would be negligible at the energies liely to be

involved though this might not be the case in bias sputtering

applications. It is difficult to say whether the Penning mech-

nism could represent a significant electron source but this seems

unlikely.

Problems in pumping a vacuum chamber for sputtering appli-

cations have been reviewed by Lamont(21)

and Ennos(22) has

indicated that contamination can be produced by the interaction

of an electron beam with organic molecules which are always

present(23). Whilst this problem will not be discussed in this

work it provides yet another example of the part played by

electrons in the sputtering process.

The effect of electron bombardment on the substrate can,

perhaps, best be considered in terms of the heating effects and.

possible changes in growth mechanism which can be caused.

1.7 HEATING EFFECTS

It is an observed experimental fact that the substrates in

a typical sputtering system tend to heat up unless special

arrangements are made to prevent this. The consequences are many

and varied and have been noted by many workers.; only a few exam-

ples will be quoted here.

Muth(24) has pointed out that the heat received by substrates

during sputtering of multilayer films is often enough to cause

inter diffusion between.film layers. This had the effect in his

experiments of altering film conductivity. Vincett(25) has sug-

gested that there exists an optimum substrate temperature allowing

the correct amount of movement by newly condensed• atoms to promote

good film growth. Riegert(26)

has concluded-that in a high rate

sputtering system, the deposition rate may be limited by the

amount of heat that can be tolerated•iri the substrate. This rate

18

is typically less than that v:hich can be achieved by evaporation,

making conventional sputtering unsuitable for many production

applications.

MarinKovic and .Roy(27) observed that in the sputtering of

tellurium, the film orientation changes from the 100 to 101 plane

as the film gets thicker. This has been explained by assuming

that recrystallisation occurs because of heating during film

growth.

Holland et al(28) in describing an R.F. device with non

grounded electrodes concluded that whilst surfaces at the boun-

dary of the plasma are bombarded by both low energy ions and

electrons, the observed heat levels must be due to energetic

electrons. Brodie et al(29) present figures to suggest that the

total energy dissipated at the substrate in their 'inverted V'

R.F. device was 0.55 watts cm-2. They conclude ( in-agreement

with Holland) that high energy electrons represent the major

source of energy to the substrate. It has been.suggested(30)

that in the D.C. mode with an uncooled cathode, 13% of the

applied power appears as heat at the anode. Whilst electron

bombardment is thought to be the main factor, the kinetic energy

of the sputtered atoms and their latent heat of condensation are

also thought to be important especially at higher deposition

rates.

By using a grid arrangement(30) it is possible to prevent

positive ions and high energy electrons from reaching the sub-

strate and thus demonstrate their affect. The main conclusion concerning film heating is that with electron bombardment during

the entire sputtering process, for a thick film (above fifty

.nanometres approximately) the film temperature is considerably

greater than that of the substrate. As a result, surface

reflectivity of the film can be reduced and, more importantly, a

large amount of stress can be induced into the surface of the

film and this can result in poor adhesion. In thin films, the

usual worry is the mechanical integrity of the, film but stress

can also cause changes in the magnetic and supercond cting

transition temperatures of materials. Stress in thin films has

been reviewed in some detail by Sun et al(31) and Lau(32).

Goldstein and Beliina(33) used a system of grids to prevent

19 electron bombardment of the substrate so that tantalum could be

sputtered onto teflon. In general, delicate substrates can only

be subjected to low input power or the heating effect due to

electron bombardment will cause degradation.

Similar problems have been observed in the coating of

P.T.F.E. with platinum. At 30 microns pressure and target sub-

strate separation of six centimetres, the maximum permissible

power input is 5 watts producing a deposition rate of a mere 50

nanometres per hour. At higher input levels serious blisters

develop on the P.T.F.E. surface. Chapman(34) has observed damage

on perspex substrates and attributed this .to electron heating

effect's.

Apart from causing degradation of delicate substrates, high

temperatures can also affect deposition rate. Davidse and

Maissel(35) quote figures on the variation of deposition rate

with substrate temperature. For example, at 800 watts R.F. power,

the deposition rate of quartz in their system was 120 n m•min 1

at 100°C and only 60 n m min-1

at 500°C. In general the depo-

sition rate decreases with increasing substrate temperature and

it can be concluded that it is imperative to have a uniform

substrate temperature if a uniform film thickness distribution is

to be maintained. Conversely, it might be possible to explain a

deposition profile in terms of a temperature profile along the

substrate though it must be remembered that target/substrate

geometry is also likely to contribute to any effect. Jones et t36i

al have suggested that re-sputtering is also a possibility.

ChapmanC37) has observed a focusing effect in which the pattern

of a composite target is reproduced on the substrate. This is

thought to be due to differing secondary electron coefficients

for the two parts of the target causing differing temperatures on

the corresponding parts of the substrate.

The effect of substrate temperature is reviewed at length by

Vossen(38) who points out that the heat generated by bombardment

has essentially the same effect.as when a substrate is deliber-

ately heated to a given temperature. It is concluded that a

major difficulty is that a hot substrate can cause sublimation

of volatile constituents. As substrate temperature is increased,

grain size can increase causing changes in fila properties such

20

as mechanical hardness. In some applicati.ens, electron bombard-

ment causing heating might be useful whilst in other cases it may

be a disadvantage. This would clearly depend on the individual

circumstances of the deposition in question.

Typical temperatures obtained at the substrate in R.F.

sputtering have been reviewed(30). The temperature obviously

depends on the deposition rate and the materials in question but

• figures of order hundreds of degrees Centigrade are not uncommon.

It has also been noted that cooling the substrate could be dif-

ficult because of problems associated. with obtaining a good

thermal contact between the substrate and the cooling system.

1.8 NUCLEATIO:, EFFECTS

Electron bombardment of substrates has been shown to have

other effects besides those associated with heating.

Stirland(39) working with e ✓aporated gold deposits on a rock salt

substrate has shown that electron bombardment during the depo-

sition increases the nucleation density and produces a single

(001) orientated deposit. He further concludes that electron bombardment results in a higher island density in thin deposits

and a more continuous film in thicker deposits. It is suggested

that these changes caused by electron bombardment are related to

changes at the substrate. Possibly the incident electron flux -

causes dissociation producing nucleation sites in the form of

vacancies or adsorbed substrate atoms which have moved from their

normal positions.

Chambers and Prutte n(40)

in agreeing with Stirland's obser-

vations point out that direct heating of the substrate by electron

bombardment is unlikey to• be the reason for the observed improve-

ment in the orientation of sputtered deposits.

Brodie et a1-(29) concluded that electron bombardment during

the initial stages of growth greatly improves the adhesion of the -

sputtered film. This is attributed to the creation of nucleation

sites on the substrate either by the formation'of surface defects

or by the removal of surface contaminants.

• 1.9 GUIDELI"ES OF THE WORK

As has already been indicated, electrons in sputtering play

an important part in determining film thickness and properties.

21

The starting point of t..i.s work is to look at the energy spectrum

of electrons incident on the substrate during the planar D.C.

sputtering of copper. This is accomplished using an electro-

static energy analyser which is described in chapter two.

Low deposition rate is perhAps the major disadvantage of the

sputtering process and figures already quoted suggest that 'cool'

substrates favour higher rates as well As less substrate thermal

damage.

It is well known that the use of a magnetic field in a

cylindrical configuration ( a cylindrical magnetron) will alter

the electron bombardment of the substrate and at the same time

increase ionising efficiency by confining the electrons to the

target region. This configuration is investigated further since

it is, perhaps, the best example of the modification of electron

motion to improve the sputtering parameters. Most of the work in

this section is directed at three main areas; substrate heating

effects, deposition rate achieved and film thickness uniformity.

If high deposition rate is the major consideration, then the

trend is towards larger power inputs and systems of larger physi-

cal dimensions. This introduces problems for the conventional

cylindrical magnetron since the construction of a large system is

not always convenient. Attempts are made to overcome this by

using a 'modified', cylinder which is essentially a flat rectan-

gular target with the ends rounded into a semicircular cross

section. The construction of this shape is possibly simpler and

it would appear to have advantages in 'on line' continuous feed

production applications. Some surprising results concerning the

role of the electrons were observed and this was investigated in

detail.

This 'modified system' was studied further; the main aims

being a clearer understanding of the part played by the electrons

and the production of a high rate system with reduced electron

loss and improved efficiency.

22

CHAPTER TWO

ENERGY ANALYSIS

2.1 GENERAL INTRODUCTION

Discussion in Chapter One has centred on the general role

which electrons play in a sputtering environment. Various inter-

actions which can occur involving electrons at the target,

substrate and in the plasma regions have been reviewed.

Without information concerning the electron flux density and

energy spectrum at various positions in the discharge, it is dif-

ficult to determine which of the possible mechanisms involving

electrons are significant and which are not Since discharge

characteristics can be modified by altering electron trajectories

and energies (for example by the use of an external magnetic

field) it is obviously necessary to have the basic data available

on electron energies and distributions in a normal D.C. sput-

tering system before modifications aimed at improvement of the

sputtering process can be devised.

2.11ANALYSIS IN THE PL.' S',IA REGION

Electrostatic probes represent a possible diagnostic tech-

nique which cas. be utilised to establish plasma parameters. A.

measurement is made by applying a voltage to the probe and

recording the current flowing from the plasma. Most of the

original wor• was performed by Langmo"ir(1 ) and is reviewed by

Cobine(2). Early results were obtained using a low pressure arc

discharge and are, therefore, not directly relevant but more

recently Ball(3) has performed some measurements for discharge .

parameters in the sputtering region. Summarising his conclusions,

the negative glow of the plasma was found to contain two electron

energy distributions, one of mean energy about 0.6ev and the other

at about 6ev. Both Of these energies are too ]ow to cause

appreciable excitation or ionisation of gas atoms though the

energetic tail of the 6ev distribution should be effective. This •

probe method failed to detect the presence of higher energy

electrons (as discussed by Ball).

- 2.1 .2 ANALYSIS AT S::. :'E

An analysis of the electrons striking the substrate is

advantageous for - two reasons:-

(a) It gives a measure of the electron bombardment of the .

23

growing film; an essential prerequisite if film characteristics,

as discussed in Chapter One, are to. be controlled.

(b) Since the secsndary electrons. from the target must pass

through the plasma region to reach the substrate, a knowledge of

the electron energy spectrum at the substrate should provide

information concerning the interaction of electrons with gas

atoms in the plasma region. This is clearly of interest since

electron impact ionisation - is mainly responsible for sustaining

the discharge and it is important to have an idea of the effi-

ciency of the process (i.e. the collision cross-section).

At the commencement of this project, relatively little work

had been done on the energy analysis of particles hitting the

substrate during D.C. sputtering. Coburn(4) has constructed a

system for determining the mass and energy of particles striking

the substrate in a planer diode system but has, so far, only

obtained results for positive ions.

Various workers have looked at the sputtered neutral species

present in a discharge, some of their conclusions being summa-

rised below. Honig(5) ionised neutral particles with an electron

beam prior to analysis and concluded that neutrals are about one

hundred times more abundant than positive ions on the assumption

that the efficiency of the ionisation process is very low at

about 10-4. Bradley(6) attempted a similar analysis but con-'eluded that his instrument was not sufficiently sensitive to

permit a qualitative study of neutrals,

A novel technique was employed by Wehner(7) in which he deposited sputtered atoms onto a balance pan. Arriving atoms

exert a force MV where M is the mass/second hitting the pan and

V is the average normal velocity component at' the surface. If this force is allowed to displace the pan upwards, the original

position will eventually be restored because the pan is gaining

weight as sputtered atoms condense on it.

Hence MV = ?tg and V can be measured by finding the time t

for the original position to be restored. This method gave an

average ejection energy for sputtered atoms of about 10ev, much

higher than typical values in thermal evaporation.

In another experiment, Stuart and Wehner(8) immersed a

target in a plasma which was pulsed for short periods (,:'i 1 ,t1 s)

24

so that atoms were sputtered from the target as a grotip. The

atoms are ejected as neutral, unexcited particles, but they

undergo excitational collisions with plasma electrons and emit

their characteristic spectra. The atoms in a particular group

become spatially dispersed as a consequence of their distribution

of velocities and this dispersion was observed as a time dis-

tribution of photons emitted by sputtered atoms as they pass an

observation volume a known distance away from the target. This

time distribution was converted to a velocity or energy distri-

bution. Results showed an energy distribution, with a mean energy

of about 10ev (in agreement with the previous ekperiment which

could only measure the mean value) but a most probable value of

about 5ev indicating that the distribution is skewed with a high

energy tail,

Previous work on electron energy spectra at the substrate

was carried out at Imperial College by Guimaraes(9) who obtained

some preliminary results using a 90° deflection electrostatic

analyser. The starting point of this project was, therefore, an

attempt at the repetition of the results of Guimaraes with sub-

sequent extension both in terms of the accuracy of the results

obtained and in their analysis.

More recently, some work has been published by Ball(3) who obtained results on the electron energy spectrum at the anode of a D.C. sputtering system using a 127° electrostatic analyser. His results will be discussed later for comparison with our work.

2.2 E':ERGY ANALYSERS

General principles of energy analysis for charged particle

beams have been reviewed by Steckelmacher(10) who points out that

there are three fundamental methods of analysis:- (1) By the use of a retarding field to allow only charged

particles of a certain minimum energy to reach a collector..

'(2) Deflection caused by electric or magnetic fields.

(3) Time of flight measurements between two fixed points (a method Also available for neutral species).,

As pointed out(10) a general comparison of the merits of

different analysers is difficult if not impossible in view of the

variety of possible, applications each requiring different opera-

ting conditions. As a general principle, retarding fields and

25

deflection types of analyser are simplest in the - case of measuring

charged parti..les. For our specific application, there are advan-

ta?es in filtering out neutral species suggesting that a deflection

type analyser would be most suitable. It should be pointed out

that deflection using a magnetic _field was not thought to be

advisable since a magnetic field significantly alters the plasma

characteristics as will be discussed in a later chapter.

In this work, it was decided to use a 900 deflection elec-

trostatic analyser (a spherical condenser analyser). An advantage

of this type is that there is a focusing, effect in two dimensions

though there are inherent constructional difficulties associated

with the accurate machining of the deflection plates. A major

reason for the choice of this particular analyser was the fact

that the equipment had already been built(9) though it was in need of re-assembly and calibration.

2.3 THEORY 0" THE SPHERICAL C07:DE7SER

2.3.1. FOCUSING

The focusing of charged particle beams by a spherical con-

denser was first studied in detail by Purcell(11). The basic

electrostatic theory required to solve the focusing problem can

be found in any standard text(12); here we develop only the

outline.

Consider (Fig. 2.1) two concentric spheres A and B of radius

R1, R2 (R2} R1) with A carrying a charge of +Q coulombs and B a charge of -Q (i.e. there is a potential difference (electric field) across the plates of the capacitor.

Fig. 2.1. The Spherical Condenser

Since the field strength due to a single conducting sphere

must be zero inside that sphere it follows that sphere B contri-

butes nothing to the electric field in the space between the two spheres. Hence, the field strength between the spheres depends

26

on A only and is given by Gauss's law as:-

£ _ rrE,

0 (r)R1 ) 0

where r is the distance from the centre of the sphere; E is the electric field strength and E is the permittivity of free

space.

Since E. -dV where V is the potential difference

dr between the spheres..

R1

V = Q

dr 4116 o R 2

V - Q 1 -1 4 Tr E0 R1 52

From (1) E r2 4irrE 0

V = E r? 1 - 1 R1

72

• V = E r2( R2 R1

R1 R2

or E = VR1 R2 (R2 - 1) r2

This is an expression for the radial electric field . (r) between the plates.

Now consider the situation of the spherical analyser in

Fig. 2.2 where a beam of charged particles is allowed to enter

through an aperture into the space between the two concentric

(1)

(3)

spheres.

27

Fig. 2.2 Charj e.i Particle Trajectory

Consider two spherical plates radii of curvature R1 and

R2 at potentials V1 and V2 respectively and let the centre line

radius be r1. The arrangement of Fig. 2.2 is essentially.a capacitor and hence the charges on the plates must be equal and

opposite since there is no net current flow. With the above

arrangement, negatively ch ;.rged particles will follow the curved

trajectory between the plates, a reversal of the charges would

favour positively charged species.

Prom (3)

E(r) = Vf R1 R2 ( 4) (R2 - R1) r2

where Vf = (V2 - V1) and is the focusing potential.

From force balance considerations in a circular orbit

radius r1; for a particle of charge e.

2 e C (r) = mv1 = 2Ee

r1 1

where v1 is the velocity in the circular orbit and E is the

energy of the charged particle in electron volts. The neces-

sary force towards the centre to produce circular motion is

provided by the electric field E(r)

28

E (r) = 2E and from (4) r1

Vf R1 R2 (R2 - R1)

1 = 2E

r2 r1 1

Vf R1R2 1 = 2E (R., - R1)

r1

and since r1 = R1 + R2 for the central trajectory 2

Vf R1R2 = E(R22 - R1 2)

or Vf = E R2 - R1

R1 R2

which is the usual way of expressing the relationship.

This can be written as:-

Vf = kE where k is a'constant for the analyser depending on

its geometry and k = R2 - R1

R1 R2

Hence the energy of charged particles passing round the

curved trajectory of the analyser is related to the potential

difference between the spheres and the energy in electron volts

(eV) can be evaluated once the analyser has been calibrated.

A theoretical value for the constant could be evaluated

from the expression for k but a practical calibration.is usually

required because of the constructional difficulties in producing

and assembling two spheres in an exactly concentric manner.

In practice we choose to apply voltages of ± Vf /2 to the

spheres so that the centre line will be close to earth potential.

In fact this arrangement will nOt provide a centre line at

precisely earth potential since the distribution of the potential

between the spheres does not fall off linearly but shows a 1/r

29

dependence:

The reasons for wanting a centre line at earth potential

are associated with the fact that the analyser should not change

the energy of the electrons under observation and the substrate is earthed in a typical D.C. planar system. Setting - 1dr /2 on

the spheres is convenient experimentally and does not rroduce a.

large departure from the requirement of having an earthed

central trajectory.

2.3.2 RESOLUTION OF THE A'"ALYSER

In any deflection type analyser the electric field con-figuration causes charged particles to follow trajectories which

are a function of their energy. It is important to realise that

a band or spread'of eneries will be transmitted and not a single

energy and that there will be an energy distribution about'a

central maximum. This transmission energy band is selected by

the size of the entrance and exit apertures of the analyser.

If the analyser is set to transmit particles of energy E,

then it will allow the passage of particles in the energy range

E - RoE where Ro is the resolution of the instrument defined by:-Ro p E/E where LE needs to be defined in relation to the peak shape of the transmitted beam. Q E is usually related to the width at half maximum intensity. The resolving power /4D of the

instrument is the inverse of the resolution and is, therefore,

defined from AD = E/Q E.

In a practical device, a compromise has to be achieved

between an acceptable resolution and a reasonable number of

charged particles being allowed into the analyser. Clearly a

minimum particle flux must be allowed into the analyser if a 'measurable output current is to result and the consequential

spread of transmission energies has to be tolerated. In the

limiting case with the apertures stopped dour, to zero width, the

resolution would be perfect but no'particles would enter the

analyser.

2. 4 EXPERII..E.:T Ab SYSTEM

2.4.1 I?:TRODU' TION

The system has already been described in detail(9) and it is not intended to 'duplicate that work. A schematic diagram of

GAS HANDLING SYSTEM

ELECTRICAL CONNEC TIONS

TARGET

ENERGY ANA YS£R

CURRENT COLffCTOR

VACUUM CHAMBER

the experimental arrangement is shown in Fig. 2.3

F_.; 2.3 Experimental system

30

1 TO OIL SUBSTRATE AND

DIFFUSION PARTICLE EXTRACTOR PUMP

2.4.2 VACUOL COSI DERATI O?T S

The vacuum chamber consisted of a 45cm diameter cylindrical

glass cylinder with target asse._.bly and energy analyser supported

from the top plate to facilitate easy removal. .The chamber is

evacuated by conventional means using an oil diffusion pump backed

by an oil rotary pump, liquid nitrogen trapping being used. The

best pressure obtainable was about 1 x 10 torr.

SPUT TERING CHAMBER

31 Whilst D.C. sputtering requires an operating pressure in

the range 20 to 100 mtorr, the energy analyser requires a much

lower pressure so that electrons extracted from the sputtering

environment will not have their energies altered by collision

with gas atoms in the analyser. The sputtering chamber is,

therefore, differentially pumped through the particle extractor

(sampling aperture) and, for example, with a 2 m.m. aperture

pressures of 90 mtorr and 4 x 10-1 mtorr can be achieved in the

sputtering chamber and analyser respectively.

The mean free path of an electron is a function of its

energy but at 4 x 10-1 mtorr it is of order 10 ems which is con-

sidered to be satisfactory for these purposes.

2.4.3 SPUTTERI.7G C A :DER

The chamber consisted of a cylindrical glass vessel 15 cm

long and 15 cm diameter with two aluminium end plates held

together by long bol ; and sealed onto the glass by two large

'0' rings. One end plate supports the target assembly and dark

space shield, the other lupi,orting the energy analyser system.

During the preliminary pump down prior to the commencement

of sputtering, this chamber can be pumped directly to the same

• pressure as the rest of the vacuum chamber. Closing a valve

introduces differential pumping so that a pressure gradient can

be set up as already discussed.

2.4.4 TARGET AS SE7BLY

This is shown In Fig. 2.4.

The target can be easily replaced and the complete assembly

including dark space shield is mounted onto a micalex shaft

which can be screwed through the aluminium end plate so that the

target position and hence target/substrate separation can be

varied. Since water cooling is not available, input power to

the target is 1_mited and the'system requires a long time period

to reach an equilibrium temperature. The implications of this

will be discussed later in the chapter.

2.4 Tan et Asse::5ly

MI CAL EX SHAFT

32

TARGET

ALUMINIUM SEPARATOR END PLATE

DARK SPACE SHIELD

))

2 • .1. 5PARTIG1JE EXTR.~C~OR

This is sh01nl in Fig. 2.5. Fi~. 2.5 Parti~le Exfractor

SUBSTRATE SAHPUNG APERTURE

ENERGY ANAlYSER

Insulated from the main portion of the substrate (anode)

end plate so that the a'1alyser has the capability for investi­

gating bias sputtering, the sampling aperture is easily

demountable so that the effect of different apertures can also

be studied. In our application, as already discussed, the anode

an~ particle extractor were kept at earth potential wi~h the

centre line of the energy analyser also at earth potential so

that unwanted particle acceleration would not be introduced into

the system."

2.4. 6 E~;ERGY AX ALYSER

Theoretical aspects h~ve already been considered. The

"actual device consisted of two sectors of alwninium hemispheres

10 cm and 11 cm diruneter respectively th~reby yielding a centre

line radius of 5.25 cm. The theoretical nllue of k (the cali-

bratio!1 constant ) -1 of the analyser is, therefore, 0.191 volts av

[k = I R2 - R1 I J

R1 R2

Some of the electrons which Vlould normally strike the anode

in a conventional sputtering system are ~ollected ~y the sampling"

aperture (particle extractor) and pass into the" analyser which is

34 set to transmi a ba=d of energies. The output current associated

with this Und. is measured using a Faraday cage collector.

When the voltage across the analyser plates iS changed, a new

output current results since a different energy band is now-being

transmitted. A spectrum of current collectel against energy

transmitted can, therefore, be built up.

2,5 L L _; ,_~ ; 0, 07 T__F. _... ER"Y A • :,Y` R ~, v. s r:

As already discussed, an experimental calibration is required

and this wee achieved by suspending the tip of a tungsten filament

in front of the analyser sampling aperture. The filament was held

at a negative potential of a few hundred volts with respect to the

earthed sampling aperture and then heated electrically to 'white

heat' (2000 °C) to produce thermionic electron emission from the

tip. Under these conditions, a near moroe.ergetic.electron beam

of energy corresponding to the potential of the filament can be

considered to strike he sampling aperture since at the low

pressure of 1 x 10-6

torr collisions can be neglected.

For these operating parameters kT Pe, 0.1 eV (where k is Boltzmann's constant) which is negligible compared to an accele-

rating potential between' filament and sampling aperture of order

hundreds of volts. Hernie the electron thermal energy can be ignored as can any energy distribution.

The electron beam was energy analysed by varying the poten-

tial across the analyser plates, the following results shown in

Fig. 2.6 being obtained.

Fip. 2.6 Current recorded (I) v Fo^usln? Volt ~..,fe (Vf fpr an

Acre!-erang,Potential of 400 volts.

44 t ' 160 V(Volfsl

35 Hence K = V.f/E = 90.2/400 = 0.226 volts eV 1.

The experiment was repeated with five different accelerating

voltages of 450, 5G0, 550, 600 and 700 volts respectively and an

average vaue of K eva . a :ted. The results were all in the range 1.223 to 0.227 volts eV-1 yielding a value for iC, ., r , of 0.225

-1 nV:~~n.,rF; ..1 volts eV compared to a theoretical value of 0.191 volts eV .

It is worth noting from Fig. 2.6 that the value of I when

Vf = 0 is surprisingly high. This should be borne in mind when

• viewing later energy curves since .it may be that the low energy •

values are unreliable. This possibility will be discussed later

with reference to the energy curves.

2.6 ACCURACY OF RESULTS

Before prese:.ting the observed energy spectra in detail, it

is necessary to discuss the precautions needed to achieve accurate

reproducible results since it soon became apparent that this was

a difficult problem.

Plotting the output current (I) from the Faraday Cage col-

lector against the analyser focusing voltage (Vf) is not satis-

factory since it gives no reliable indication of the input power

setting and would render meaningless any comparison of results

obtained on different days under different operating conditions.

The output current was, therefore, normalised to the total sputter

current (1T) and results were presented as I:/ against V.

Unfortunately the parameter 7, is not a constant and the results

tended to drift with time as discussed below.

(a) The current drawn by the discharge (IR,) is clearly a

function of the sputtering pressure and a small unwanted pressure

fluctuation can have a significant effect on the output current.

It has, in fact, been suggested by some worker s that an alter-

native to using a pirani gauge to monitor sputtering pressure

would be to measure the discharge current, though some of our

subsequent work with magnetic fields might cast doubts on this

suggestion.

Pressure stabilisation was found to take about two hours to

achieve, possibly because as the uncooled target warms up on the

. commencement of sputtering, pockets of occluded gas molecules are

constantly being released.

36

(b) Pre-svutteri:.S has long been recognised as an essential step to remove target surface contamination prior to film depo-

sition. As the surface layer is removed it was found that the

discharge current is reduced because of a difference in the

secondary electron coefficient of the copper target and its sur-

face coating. The following typical results were obtained over

a period of one hour.

Fi? 2.7 Total Shutter Current v Time

It was eventually found to be necessary to allow a warm up '

time of about two hours before steady conditions could be achieved.

Since as TT varies the current I collected by the analyser also

varies it was hoped that I/IT would remain approximately constant

rendering a long warm up time unnecessary but experiments on

these lines did not produce reliable results.

It would be interesting to see if a water'cooled target would

require the same time period for stabilisation but this was not

investigated.

Under the above conditions, reproducibility to within 5% was

obtainable on different days providing the Keithley electrometer

used to measure the analyser current was re-zeroed before every

reading. Pre-sputtering times of this long duration tended to

lead to excessive accumulations of deposited copper on the target

dark space shield and sampling aperture necessitating a frequent '

37 cleaning operation, An even longer pre-sputter period was then

required on the first experimen after cleaning. Whilst this

experimental technique is not convenient, it was found to be

necessary if reproducibility of 5% or better was to be achieved.

2.7 E `'RI=AL RESULTS

2.7.1 GE ``ERAL CURVE SHAPE

A series of experiments for a variety P of sputtering pressures,

voltages and target/substrate separations revealed that the general

shape of the energy 'spectra was of the form shown in Fig. 2.8 and

it is convenient to split the curve intd two separate parts con-

sisting of a low energy region I and a high energy region II.

Fig. 2'.8 General Shape of Energy Soec tra

r

too+

T ' 50-

L

> V (Volts)

The low energy peak (section I) would usually be poorly

defined with a shallow maximum at about 25 eV in contrast to the

high energy peak (section II) which had a much sharper outline

and a well defined cut off at high energy. The analyser voltage

Vf at the cut off was a function of the inter-electrode potential

between target and substrate.

2.7.2 C=TL7T `TOLT.^AGE V,ITH VARYI':G PRESSURE

A series of experiments was performed in which ' the discharge

90 180 Vf (Volts)

—1

IT T

V=1095 Volts P=43ji

-- -V=1095 Volts P= 68p

100

50

38

voltage was held constant and the pressure in the s;; te:: varied

so that a series of energy spectra, each at a different pressure,

could be obtained. Some typical results are shov.n in Fig. 2.9.

Fi , . 2.0 Energy Srestra Pt rsns t sar t V o l to _e Varl alp'+ e Pressure

Fig. 2.9 illustrates the difficulty of presenting information

concerning the - two peaks on the same scale and in future they will

be considered separately.

The high energ y part of the curve is perhaps more striking if

it is replotted (Fig. 2.10) as I/IT against E where E is the energy

in eV of electrons hitting the substrate.

i

1

_ 400

100

133

50

F';r. 2.10 I/IT -r E at Constant Voltage

39

x 10-1 T

v=1095 vat is p=43).1

_ -y_1095Vo(ts p=6624

I I I 800 1067 E. (e0

We can now see that the well defined pea?: of the curve cor-

responds to an energy in electron volts which is the sale as the

inter-electrode potential.

2.7.3 COx'STA:'!T PRESSURE, VARI!t LE VOLTAGE This second series of experiments involved keeping the pres-

sure constant and varying the sputtering voltage to produce a

series of spectra at differing voltages. On both the high and

low energy curves the values of Vf (volts) and E (eV) are shown

on the abscissa. The results are summarised in Figs. 2.11 and

2.12.

15--

10 --

40 Fig. 2.11 Energy Soectra at Constant ?re:; ure Variablē Volta e — Low Enf.,7 Rao,-=e

A / 0

_7 • P 30/u

I t I I I 1 I 2 4 6

• 1 I 1 I I I

I I 1

t 1

I I 1

. I 1

1300 Vol ts ,1

1000 "

I I> V( Volts 1 , 1 1 1 I I

8.8 17.6 26.4 E (e v)

X10-1

_ S

100 — T V4120 Volts

---V-815 Volts

P=40»

50 -- i

I I 30 90 400

1 I { ( 1 >

180 240 V l Vol ts)

800 1067 f E ley)

/

41

TF-14. 2.12 Energy Spectra at Constant Pressure Variab_e Voltage -

H .h_ __: r R to e

The above two graphs are representative of. many in the

pressure range 25 - 100 microns each showing the same general

trend. The low energy peak is not very well defined and does

not strongly depend on either pressure or voltage always occur-

ring at around 25 eV. There is a slight trend for the peak to

move to . higher energies for higher applied sputtering voltages.

The high energy peak does not seem to ne pressure sensitive

.but is strongly dependent on voltage; the energy at the peak

corresponding very closely to the inter-electrode potential.

2.7.4 VA.RI,TIO'. 07 T RGET/SU .STRATI: SEP ARATI .': 1::D SA'"PLING

APERTURE

. All the results presented so far were for a target/substrate

separation of 8 ems and a sa::!plin:; aperture of .1.E m.m. diameter.

Increasing; the target/substrate separation by adjusting the

target position caused a slight reduction in the number of high

energy electrons detected whilst increasing the sampling aperture

42

diameter causei a slight increase in the number of electrons

detected at all energies. ..either change ignificantly affected

the curve shape though the variations were, admittedly, not

studied in any great detail.

2.8 ANALYSIS OF RESULTS

Whilst these experimental curves are perhaps surprising at

first sight in that they seem to imply that a lot of energetic

electrons are striking the substrate, they are also misleading

and it has already been pointed out that care is needed in their .

interpretation.

The value of the parameter Io (the current with no voltage

across the analyser plates) is disappointingly high since with

no voltage across the plates it is not possible for a charged

particle created within the discharge to reach the Faraday Cage

collector.

This Io figure c_nnot be considered simply as a background

count to be subtracted from the detected current to give the net

increase current since once the low energy peak is passed cur-

rents are observed which are below the Io value. Clearly,

however, some sort of background pick up must be represented by

Io though stray signals being induced through the carefully

screened electrometer leads was thought to be unlikely. Obvious

possibilities are either charged particle creation within the

analyser itself, despite the lower pressure due to differential

pumping, or a significant contribution from secondary electrons

being ejected from the analyser walls by primary particles from

the discharge hitting them.

- Despite the reservations introduced by the uncertainty of

this Io factor, the low energy peak does appear to be detectable

under all conditions of pressure and voltage though it was not

observed by Ball(3). Thoughts on its significance, if any, will

be deferred until the acceptance bandwidth of the analyser is

included in the analysis later in the chapter.

The high energy peak which was observed by Ball(3) is,

perhaps, more interesting since it is better defined and shows

that some electrons are striking the substrate with the full

inter-electrode potential having traversed the.discharge without '

43

making an inelastic collision. These high energy electrons are

detrimental for two reasons: firstly because they are not con-

tributing to the enhancement of the spi tterin.g rate by creating

more ions and secondly because they are providing a significant

heat input to the substrate which may cause damage to certain

delicate substrates.

These. two points are crucial to the development of the rest

of the work but before they can be considered further an analysis

of the analyser acceptance bandwidth is required.

2.9 ANALYSER .ACCEPTf?_:c_? BANDWIDTH

As previously discussed, an analyser set to transmit an

energy E will in fact pass energies in the range E ± RoE where

Ro = E/E and is the analyser resolution. Since Ro is a con-

stant for our analyser it follows that as the energy under

analysis is increased in magnitude, the spread of energies which

will pass is also increased and this tends to bias the results

in favour of the higher energies.

If ō F(E) is the flux of particles bombarding the substrate

with energies between E and E + 6 E, then when set to E the

analyser transmits & F(E) particles per unit time.

S F(E) = Q E S F(E) = R I: ō P(E) d E ° S E

and in the limit as 6 F(E) --) 0 we can write:-

& F(E) = RE dF(E) o dE

Since Q = It (by defn) Q = charge

I = current

t = time.

I = Q/t = charge arriving/unit time.

I = numbers arriving/unit time x charge on a single particle.

dI = 8 F(E). e

where dI is the fraction of the total current'

I caused by the energy range dE.

44 dI = R E dF(E). e where e is the electronic charge.

o dE

= dI oe dI dE R.

0Ee E

So far we have presented curves of di (called I/IT in the

experimental results) against E whilst curves of dF(E)/dE against

E or dI/E against E would be more advantageous since they would

show the number of particles actually hitting the substrate from

the discharge.

Hence the curves of current aainst energy.previously pre-

sented need to be modified by dividing the ordinate by E to

produce the following general curve of flux against energy.

Fig. 2.13 ..'.odified Energy Spectra

_ X A.r E

25 1000 E (ev)

In interpreting this curve it should be realised that when E

is sall, I/I, x 1/E is tending towards infinity and any small

error in E will cause very large uncertainties in the value of

the ordinate.

45 When the results are v:.'csented in this way the so called

high e er y peak. ie greatly diminished and the low energy peak

from the previous curves is removed completely to be replaced by

a Slight deviation away from the smooth downward trend. The

previous results could, therefore, be easily interpreted wrongly

since the small low energy peak in fact corresponds to a large

signal whilst the large high energy pear, corresponds to a small

signal.

:,:ore importantly the results show that most electrons

reaching the substrate are of low energy though we must remember

that large uncertainties are to be expected. Bearing in mind

the fact that sputtering is a collision dominated low mean free

path process, this result is not too surprising. There is,

however, still a peak correspohding to the full inter-electrode

fall of potential showing that a small but significant number of

electrons are crossing the plasma without col.lisicn.

It should be noted that bandwidth considerations will shift

the analyser calibration curve (Fig. 2.6) to a slightly lower

voltage maximum producing a lower value for the calibration

constant (K) but the correction needed is very small.

One obvious question still needing to be discussed is

whether the original spectra of I/IT against E with their two

peaks have in themselves any significance and this will be con-

sidered in the next section.

2.10 HEAT INPUT TO THE SUBSTRATE

If SP(E) is the power carried to the substrate by electrons

with energies in the range E to E + S E, then since power is the

rate of dissipation of energy we can write:-

b P(E ) = S F(E).E.

dP(E) = dF(E).E ot dI.E oC dI dE dE L

Therefore, the curves of I/Il against E originally presented

are in fact curves of the 'power input to the substrate caused by

electrons against electron energy.

46 This means that the high energy electrons, although

relatively few in number are largely responsibl:. for the electron

power input to the substrate (the other contributions to sub-

strate power input will be discussed in detail in a later chapter).

Since this power input to the substrate can be large enough to

cause degradation of heat sensitive substrates(13) it is clear

that a knowledge of the electron energy distribution curves is

.useful in the deposition onto these substrates especially.

2.11 CONCLUSIONS

2.11.1 GE7ERAL CO'`"E',TS

At this stage of the work a fundamental decision was needed

on whether to try and reproduce the experimental data more

accurately by improved instrumentation or whether to accept the

results as being a reasonable approximation to the energy spectra

and investigate the implications of these results. The latter

course of action was hosen for a number of reasons but mainly

because it seemed to offer a more interesting line of research

with the possibility of using electron properties to achieve a

higher sputtering rate.

Apart from some work in a later chapter on justifying the

experimental curves by considering the theoretical electron

distribution on the cathode dar: space/negative glow boundary,.

no further effort was put into the energy analysis and a summary

of our conclusions should serve to indicate the dominant lines

of thought.

2.11 .2 SU." .'.ARY OF RESULTS

The electron energy spectra for electrons incident on the

substrate in a D.C. sputtering process show that most electrons

arrive with relatively low energy but some have almost the com-

plete inter-electrode energy (in equivalent eV) and are largely

responsible for the power input to the substrate. The other

departure from the ge::ral trend occurs at about 25eV where there

are more electrons present than might have been expected and a

consequential larger power input than might have been anticipated..

The reasons for this behaviour are not clear but the effect does

seem to be real rather than some function of the energy analyser

since it is reproducible under varied conditions of pressure and

47 voltage. As a first step, more detailed accurate spectra would

be _'eau re'i to dis , ver the precise energy y rage over which

increased signals ;';ere being detected. Since many of the

ionisation and excitation levels for argon are in the region of

20eV it could be that this is responsible for the observed results

but without more information it is difficult to comment con-

clusively.

The increased signal at hig.n energies is of greater interest

• for our purposes since high energy-electrons are detrimental to

the efficiency of the sputtering process on the following grounds:-

(a) The sputtering process is in general slow in that

deposition rates are often unacceptably low under normal operating

conditions. These high energy electrons having traversed the

plasma region without making an ionising or 'exciting collision

have produced no electron/ion pairs either directly or indirectly

and have, therefore, contributed nothing to the achievement of a

higher deposition rate.

(b) The high energy electrons are primarily responsible in

many cases for substrate heating and whilst this can be beneficial

in certain circumstancesit is undoubtedly detrimental in others

•notably in the coating of delicate substrates such as P.T.F.E. or

perspex. Substrate heating, where required, is usually better

obtained under controlled conditions using an external source and

in some cases substrate cooling may be required because of the

excess heat being generated. Since substrate cooling can be

difficult because of the need for good contact between the sub-

strate and its backing plate, it is preferable to eliminate the

cooling requirement by controlling the incident electron flux.

(c) As higher anode/cathode voltages are tised in the search

for higher deposition rates, X-ray production can become sig-

nificant because of Eremsstrahlen caused by the high energy .

electrons which are produced. Whilst this radiation is unlikely

to constitute a significant health hazard it can cause damage in

certain substrates.

The rest of the wort, was, therefore, devoted to an investi-

gation of the high energy electron contribution to the sputtering

48 process from the point of view of substrate heating and the

production of higher deposition rates by the removal of the

energetic electrons from the substrate. If the electrons could

in some v:ay be trapped in the plasma region then they must even-

tually undergo a collision which has a probability of being

inelastic' thereby causing er.er7y loss to the electron. Since an

ionising collision would produce an extra ion which could eject

more target material there is a pos ibilit;,; of enhancin the

sputtering rate by electron trapping with a chance.t at substrate

heating might be reduced by the resulting modification of the

electron flux.

Whilst this concept was to prove slightly optimistic it

provides a guideline for the rest of the work and is investigated

in the following chapters.

49

CHAPTER THREE

RICHER RATE S U'T':'ERI: G

3.1 ^ROT'."CT -0.r

One of the major problems of the sputtering process is

that power consumption is high in relation to the amount of

material transported resulting in deposition rates comparing

unfavourably with those obtainable in other processes such as

thermal evaporation. Hecht and .ullaly(1) have noted that

because of its versatility, sputtering is in demand for the

fabrication of numerous unique components where thick films

(,'i 6000 microns) are needed but production problems exist because of the excessively long deposition times required to

produce the coatings. Examples include the production of

strong permanent alloy magnets and the production of rocket

thrust chambers.

Further thoughts• on industrial economic lines are pro-vided by Thornton

(2) who suggests the most significant parameter

to be deposition rate multiplied by substrate area (i.e.

coverage) rather than the more usual deposition rate. On this .

basis, sputtering becomes. competitive on a large scale (pro-

vided a reasonably high rate is achieved) principally because

scaling up to physically large production systems is not an

insurmountable problem. Since R.F. power supplies are more

expensive than D.C. and R.F. systems cannot be scaled up as

easily, Thornton further suggests that the most spectacular

improvements can be expected for D.C. systems operating with-

cylindrically symmetric. geometries where the sputtering process

is scale independent in the plane of the substrate.

Apart from the inconvenience of long deposition times due

to low sputtering rates, they can lead to poor quality films.

We have been able to demonstrate this by producing a series of

constant thickness/different deposition rate, copper films on

glass substrates. For example a 1000 oA copper film deposited

at 1000 oA hr-1

(deposition time one hour) appears black,

exhibits very poor adhesion and has a very high resistance

whilst a film of the same thickness deposited in three minutes o -1

(20,000o A hr) ~ h a.. a characteristic copper appearance,

50

exhibits such improved adhesion and has negligible resistance

suggesting a much improved copper. film. Obviously as the arrival rate of sputtered material is lowered, the ratio of impurity species (e.g. oxygen and water vapour) to sputtered

species is increased with an effect on file quality. However,

in the case of reactive sputtering of dielectrics (e.g. S102 in

Ar-02 mixtures) low deposition rates lead. to higher quality

films.

Whilst it might seem reasonable to assume that lower pres-

sures must lead to better film purity in terms of less sputtering

gas (e.g. argon) being embedded in the growing. film, this is not

necessarily (s as was demonstrated by Winters and Kay(3,4) and

Jones et al (5). Winters and Kay were able to show by analysing

the deposited film content of nickel samples that any gas species

can be embedded in a growing film providing the gas atoms' have

sufficient kinetic energy When they arrive at the film surface.

The degree to which inert gas trapping will take place was

thought to depend primarily on the kinetic energy of the incident

gas species and secondarily on the chemical and physical micro-.

structure of the film surface. Film temperature and whether

bias sputtering is employed are other important factors.

At high pressure all of the high energy argon reflected

from the target is likely to be reduced to thermal energy before

reaching the substrate. However, as the pressure decreases, the

average energy of the reflected argon which reaches the substrate

will tend to increase resulting in more argon embedding into the

film in the absence of bias. The main conclusion seems to be

that the purest films are obtained by sputtering at the highest

possible pressure provided that the sputtering gas is itself

pure.

Whilst high deposition rates would appear to be advantageous

on economic and film purity arguments, it should be remembered (1,6) that film structure is also a function of deposition rate '

and it may be that in some applications the structure produced

at high rate cannot be tolerated. This point is not usually of

importance in the deposition of metals and will not be considered

further in our work.

51 3.2 FACTORS I.,F LTJ E7CI..:n UE ^ITT„ . RATE

The choice of sputtering gas has already been considered

in Chapter One, if this is not now considered to .be a variable

parameter there are basically five factors which can influence

the deposition rate:-

(a) The target/substrate separation.

(b) The temperature of the substrate'and target.

(c) Pressure.

(d) Voltage difference between target and counter-electrode

(the inter-electrode potential).

(e) The current drawn by the discharge.

3.2.1. TARGET/SnSTPAT7. L77PARATION

An optimum separation was not evaluated experimentally in

our work but .the .following basic principles(?) were taken into

account in constructing the system.

The counter-electrode (often also acting as substrate

position) must be situated outside the cathode dark space (CDS)

or the discharge will not be self sustaining and as a general

rule it should be at least twice the CDS length away from the

target so that ion production is not significantly affected by

the close proximity of the substrate.

The less the electrode separation the greater the depo-

sition rate, since the substrate is subtending a larger angle

at the target, but the poorer the film uniformity and the higher

the substrate temperature. Since deposition rate is a function

of substrate position and for that matter target size, it is

more realistic in terms of the efficiency of the sputtering

process (i.e. momentum transfer) to consider the amount of tar-

get material removed per unit target area and no further detailed

thought was given to substrate positioning.

3.2.2 THE TEMPERATURE OF SU `;;TRATT A';n TXRGET

Whilst substrate temperature considerations will be dis-

cussed in detail in a later chapter, we should remember here

that since the condensation coefficient of the sputtered atoms

is a function of substrate temperature, this parameter will

affect the deposition rite.

52

The temperature of the target is not usually considered in

a saratus design other than from the point of view that water

cooling is preferable to prevent solid state diffusion in alloy

targets and the melt n:, of seine targets at high power input

levels. an alternative apr,:roach of liquid phase sputtering has (3) who allowed their bee::.investigated by :lr=.zterat and Gesic:: allowed

to melt in order to provide hi,gh.depositon rates in a

combined sputtering-evaporatio:. mode. Little support seems to

have been given to this idea as yet, presumably because of the

inconvenience-of handling a target in this farm.

3.2.3 i R1=RE As the pressure increases, the mean free path of sputtered

atoms will decrease causi_n , them to spread out more because of

enhanced scattering. For example, at 50/u pressure the mean

free heath of atoms is about one millimetre whilst at 5/u it is

one centimetre though this latter pressure is not usually

suitable for unassisted D.C. sputtering.

The disadvantage of increased spread tends to be offset by

the fact that the mean free path of electrons is also reduced

at higher pressure giving an increased probability for electron

impact ionisation and resulting in higher currents and enhanced

sputtering at high pressure.

A further advantage of higher pressures is that positive

ions travelling towards the target will be more susceptible to

collision and will be more likely to strike the target at angles

other than normally. The variation of sputtering yield with

angle of incidence has been investigated by Cheney and Pitkin(9)

who find for a Xe+ on Cu system that the maximum yield (atoms/

incident ion) is when the ions strike the target at an angle

of incidence of about 200 to the normal. This fact can be used

to advantage in the "Sputtergun" to be discussed later.

The reap limiting factor to ' sputtering at high pressure is

the probability of sputtered atoms returning to the target by

virtue of scattering in the gas and it has been estimated that

at 10 pressure approximately 90% of the sputtered material

never leaves the CDS region. Also at high pressure the CDS

53

length is reduced causing problems in the construction of

efficient cathode dark hi.eld. to sunpress sputtering at

unwanted parts of the target (e.g. water cooling connections)

and proper masking of selected areas of substrate becomes

difficult.

At low pressure, the CDS becomes long causing a large

target/substrate separation with a subsequent lowering of

deposition rate and in practice a compromise must be achieved

at an optimum pressure choice.

The distribution of ion energies striking the target of

a glow dischar.e has heel: studied by Davis and Vanderslice(IO)

who obtained an energy spectrum of the form shown in Fig. 3.1.

Fit-. 3.1 Distribution of Ion enemy at the Tar[-et

1.0 — A INTENSITY

0.8

06 -

04 -

02 —

Vo=farget potential V =energy of ion in equivalent volts

r

06 08 10 c

02 04

This distribution curve is thought to be caused principally

by charge exchange interactions of the form:- o +

Ar+ + Ar° '~ Ar + Ar (for argon ions).

The equation represents a moving ion hitting and charge

exchanging with a near stationary atom to produce a moving atom

and a stationary ion which immediately moves towards the target

provided it is in an electric field region of the discharge.

Sputtering at the target is, therefore, caused by a distribution

54

of energetic ions and also a distribution of energetic neutral

species, both di striL'utionr being a func ! cn of pressure since

pressure changes alter the ion mean free path and hence the

charge exchange col_isien_l probability. -

īf sputtering; is truly a momentum exchange phenomenon, we

might expect the yield under neutral atom bombardment to be the

same as that using the corresponding ion.- This was confirmed '

by Bader et al(11)

and ::enKni:..-,t and Wehner(12)

but not by

','ahadevan et al(13) Medved(14) and Hagstrua(1 5) perhaps

illustrating the fact that reliable data in this field is

difficult to obtain.

The work of :redved et al(1 rt ) and Hagstrum(1 5) suggests that neutral bombarding species show two important differences when

compared to ionic species.

(a) Reflection properties are different. There is a

finite but low probability that the bombarding species will be

reflected from the target surface rather than cause sputtering.

This probability would appear to be about twice as high for

neutrals as for ionic bombarding species.

(b) As a consequence of the bombardment of the target,

secondary electrons are ejected as well as sputtered species.

The number of electrons ejected per incident particle is

measured by the parameter Z;i . Since these secondary electrons

are responsible for creating further ions and hence producing a

self sustaining plasma, it follows that the X;, value is critical.

It is observed that ZSz (ions) is greater than Xi (neutrals)

partly because the neutrals are more likely to be reflected and

partly because ions exhibit two possible electron ejection

mechanisms (potential and kinetic ejection) whilst neutrals only

show kinetic ejection. This Xi concept will be discussed more

fully in a later chapter dealing with self sustaining plasmas.

A theoretical calculation of deposition rate as a function

of pressure would clearly be difficult because of the many fac-

tors involved but Laegreid and Wehner(16) have shown that the

sputtering yield (atoms per incident ion ) begins to fall off

above 30 microns pressure as shown in Fig. 3.2

55

?. 2 Spnt ter r:, Yield as a Function of Pressure

YIELD (atoms/ion) 0-5

0.4

03 --

02

D•1 —

10 100 PRESSURE (microns) ( log scale)

This fall off above 30 microns is initially compensated

by the fact that more ions are present at the higher pressure.

The turning point is at about 125 microns and above this value

the increased ion current can no longer offset the reduced

yield and the deposition rate falls. The best compromise is

often found to be around 50 microns since backstreaming from

the diffusion pump often occurs at pressures as high as 125 microns.

Since in practice, film quality may well determine pres-

sure choice, it is usually preferable to choose a suitable

working pressure and concentrate on the voltage and current

parameters for manipulation of the deposition. rate.

3.2.4 VOLTAGE B7-:7;;E:717 T PI T A: 1 SUBSTRATE

The variation of sputtering yield for copper with argon

ions has been studied in some detail by many workers(16-20)~

Since a glow discharge produces an ion energy distribution at

the tar,et(10), most of the above work has been carried out

using ion beam techniques and some care would be needed if

precise information was required for a glow discharge sputtering

application using these data.

YIELD (atoms/ion)

8 -

6—

56

Some of the main points to emerge are that the sputtering

yield is a function of the crystal plane exposed to the ion beam and that the state of the surface of the sample is an

important consideration. Not surprisingly slightly different

results are produced with different copper targets showing that

purity is important.

All samples seem to exhibit the sane' general trend (Fig.

3.3) for yield as a function of incident energy sho '. ing an

exponential rise above a threshold value then rising linearly,

less than linearly and approaching a flat maximum at about 25KeV. At higher incident energ ies the yield falls slightly as

the probability of ion implantation increases.

Fig. 3.3 Sputtering Yield as a Function of Incident fon Energy

10 20 30 ENERGY (KeV)

Summarising these results, it would seem that if high

deposition rate was the sole criterion of the deposition

process then there would be no point in having a target poten-

tial of greater than 20KV and little point in going past about

8KV since the yield does not increase much above this value.

The chosen experimental sputtering voltage is', however, usually

much lower than this for reasons which will be discussed in the

folloviing sections.

57

2,a,1272- : r q n. ; T .7 '.', ; : :'. `. ".. : S C H.: RG E

Since the _,ember of io.ne striking the target is proportional

to the current density, then for ions of the same energy the

amount of material reine ;'e'1 from the target should be proportional'

to the current being drawn by it. We might, therefore, expect

that as the current is increased. at constant voltage the depo-

sition rate will also increase. The conclusion is based on the

usual assumption that spattering yield is not itself a function

of current though work by Jackson(21)

on contamination and damage

effects has led to doubts on the validity of this.

Some interesting work was done by Guseva(22) who studied the

sputtering yield for copper/argon and copper/copper systems in

the energy range 3 25 KeV. The following results were obtained.

Flee 3.4 Sputtering Yields Against Current Density

A YIELD 10 __(atoms/ionl

8 --

100 200 300 CURRENT DENSITY p.AA cm")

For low ion currents the yield depends on target current

density but above 100/& A cm2 this is not true and the curve

is relatively flat. This effect was interpreted by assuming

that a surface layer (presumably oxide or nitride) tends to form

on the target material and that higher current densities are able

to demolish it easily and are successful in preventing reformation.

58

as sputtering proceeds. The build up of oxide layers causing

low sputtering rates in aluminium or some types of refractory

metal is a common problem but is not usually considered for

materials such as copper which sputter easily.

A further interesting re.:ul t from this work is that copper/

coper yields are about a factor of. two higher than copper/argon

yields supporting the theory that S oC :'.1d 2/i ~'1 + ."2)2

n where S is

the yield and Mi M2 are the masses of target and gas atoms

respectively. On this theory, maximum sputtering of copper will

be produced by copper ions.

A reasonable assumption would seem to be that as the dis-

charge current increases the sputtering rate also increases.

The consequence of this is that if high deposition rate was the

only objective then the hig:.est possible discharge current ought

to be drawn.

3. 3 TiiI; CO=PT OP ..-,,, r•i I :T SPUTTERING

Of all the factors influencing deposition rate, current and

voltage are perhaps the most important and most easily understood

and the initial part of the work was concentrated on these two

parameters.

The deposition rate of tantalum films as a function of vol-

tage and current has been studied by Schuetze et al(23) and is

summarised in Fig. 3.5.

1000 2000 3000 VOLTAGE

0.7 -

06 RATE (microns hrl

0.5

0.4 -

03 -

02 -

0.1

15mA

5mA

59

Pic'. 3.5 De^ositicn. Rite c` Tantalum Under Varyin7, Conditions

Consider the effect on deposition rate of doubling the

power input (volts x amps) to the target in various operating

regions. Starting at 5mA and 1000 volts we can see that doub-.

ling the current at constant voltage would approximately double the deposition rate whilst doubling the voltage at constant current would-produce a five fold increase. Whilst the change

in power input is the same in both cases, the resulting depo-

sition rate change is different and it would clearly be

preferable to produce a voltage increase under these conditions.

Repeating this exercise starting at 5mA and 1500 volts we can

see that doubling the voltage or doubling the current will

produce about the same change in deposition rate but at voltages

in excess of this 1500 volt figure it is change of current which

produces the greater change in deposition rate which has now

become fairly insensitive to voltage charge. These figures.

would, therefore, seen to suggest that the optimum sputtering.

voltage is about 1500 volts (considerably lower than the 8KV mentioned earlier) and that if high deposition rates are required.

they would be best obtained by choosing an operating voltage of

1500 volts and then increasing the current as. auch as possible.

60

In attempting to achieve high deposition rates, one pos-

sibility is to build a 1 rge power supply on the assumption

that as voltage and current keep increasing then the deposition

rate must also increase. Whilst this idea would seem to be

valid there are three important objections:

(a) Although the size of the power 2 r sut,ply could theoreti-.

tally be scaled up indefinitely, cost would eventually become

an important 'consideration.

(b) A certain percentage of the input power will be

converted to heat at the substrate. Eventually the substrate

temperature would become unreasonably high thus limiting the

allowed input power.

(c) Large power inputs to a target lead to large thermal

stresses and there is a limit to the heat input Which can be

sustained before damage results. This would suggest that the

power ought to be introduced as efficiently as possible,

presumably by choosing an optimum voltage and then producing

power increase by current increase at constant voltage.

The efficiency of a sputtering system will be measured in

terms of the amount of material deposited for a given input

power. Vie, therefore, use a 'reduced rate' defined by:-

Deposition Rate at the Substrate Reduced Rate = Power Input in Watts

This 'reduced rate' is a convenient parameter by which to

measure the performance of the sputtering system.

If the curves of Fig. 3.5 are replotted as reduced rate

against voltage, the results are as shown in Fig. 3.6.

61

fi x 5mA 15mA 10mA 16

Ve t?. 7 .6 Vc1uon:3. ~?t' Arai..=' Snutter!ng Voltage

2000 3000 VOLTAGE 1000

RA TE/POWER x10-3

24

Whilst these results would seem to support the idea: of an

optimum voltage, it does not follow that the same optimum vol-

tage could be expected for systems with a different geometry.

Since there are several factors which can affect the deposition

rate, a good theoretical model correlating the rate to the input

variables would be di_'ficult to produce and a practical cali-

bration would. be needed for each individual system. Sets of

data for reduced rate against voltage were obtained for a

cylindrical magnetron system which was constructed and the

results will be presented in a later chapter.

3.4 PL 3:A EFT'T (;T F'-r;Y

So far we have discussed the choice oT operating parameters

to obtain minimum wastage of the input power; we now turn to

improving the efficiency of the plasma itself. Since electron

impact ionisation is largely responsible for the creation'of

ions which ultimately cause sputtering, it follows that elect-

rons are the key factor in producing high deposition rates.

There are two obvious requirements, both of which we would like

to satisfy if the plasm?. efficiency is to be improved.-

62

(a) To increase the number of electrons present in the

plasma.

(b) To make better use of those 'electrons already present.

3.4.1 =ODE s' 'S

Electrons having . _:?ro ri_lte energy can be supplied ther-

mionically from an indepen'dent source so that ionisation does

not depend primarily on secondary electrons being emitted from

the cathode. A typical schematic arrangement is shown in

Fig. • .

~.... 3.7 Triode System

TARGET -2 KV

I ANODE

THERMIONIC

+f00V EMIT TER

SUBSTRATE

The anode draws electrons from the heated filament into

the main discharge region and is biased positively with respect

to the substrate which is, therefore, protected from .electron

bombardment. Applying a high voltage across the target/substrate

'electrodes ensures that the target is subjected to a flux of

energetic ions as usual. Deposition rates of 1C00 °A min-I

are feasible using this type of system.

63

~ae~ctic field I, it ex~erie~ces a force K x V so th~t if the

velo::;i ty is p:lral:l.el to t:le :::'!.3!'letic fiel:i it is una:fected

whilst i: it is pe'!"pe:::li<!t.:.l'J.r :~ t:le field it experiC!.8es a

fo!"ce at right angles to both it::elf and the :r;3.r?;netic field

directio:l re:.;ul tir.g in ci.rcul:1.r r:!otion a':Jbut the :.1ClC;1:etic field

line.

In general the path o~ an e~ectron in a m~gnetlc field is a helix but for the special c~se in which the direction of

motion is e~tirely at right a!:slcs to the ma.gn~tiG flelj direc-

tion, this path th~!l sir;-.plifies to an arc of a circle.

If the ~agnetic field is applied perpendicul2rly to the

electric field in a sputterir-g syste;n .'13 in a ma[,;lletron dC'fice,

then seconda'!"y elec trons 1·~8. ving th(~ tarGet surfac e nOrI:1ally t

will follo'lI a curved tr2.jectory (not cll'cular) as L:'licated in

3.8 which assumes the absence of collisions with gas

atoms.

Fi8_ 3.8 Electron Trajectory in n ~~Gnctron Confi~uration

ELECTRIC FIELD

;." /

I

S S \

'~

--'--7 DR! FT DIRECTION

1 8 HAGNETIC FIELD

----., ...... , .... , electron path '" ' '"" ~ " ~ \ / ;' , \, \ I \ I V ,

\ \ \ S S \ S:::s::J

TARGET

64

This curve shape will be analysed in detail later. Once

a collision occurs, the electron nay be knocked out of the

trajectory in the plane at right angles to the electric and

magnetic fields and the path taken would then become helical and would be much more difficult to analyse. 77e could,

however, still say that there would be a general drift in the

direction indicated by the • arrows on Fig. 3.9.

A longitudinal magnetic field (anode - cathode direction)

which is parallel to the electric field is often-used to advan-

tage in planar devices and is successful in reducing electron

wall losses by restraining the motion onto orbits round the

magnetic field lines and channeling the electrons into the

anode. This configuration, however, has no effect on the high

energy electrons discussed in chapter two which move parallel

to the magnetic field without collision and are, therefore,

unaffected by it. Whilst the arrangement is successful in increasing plasma efficiency and, therefore, deposition rate

by reducing electron wall loss, it contributes nothing to the

control of substrate temperature.

A magnetic field transverse to the electric field direction

(Fig. 3.8) would inhibit the high energy electron effect by

deflecting these electrons away from the substrate but unfor-

tunately the drift motion tends to concentrate the current onto

one side of the plane target. The resulting asymmetry is

detrimental to film thickness uniformity since most material

is ejected from one side of the target. A similar effect could

be produced using a longitudinal magnetic field with a target

offset from the centre line axis of the system.

It should be pointed out that the magnetic field strengths

generally used are not strong enough to significantly influence

the ions in the discharge. We assume that the only charged particles to be affected are electrons. It has been noted(24)

that at very high magnetic field strengths, (of order 1T) the potential distribution across the discharge is altered

significantly and the system behaves differently.

CATHODE

65

3.4.3 T: P '...T..0

Perh ns thr' e t known ar_:, u:L.cnt incor oratin'; electric

and magnetic fields is the 'Fermin; Cell which is shown in Fig.

3.9 Fi- 3.9 The

CATHODE ANODE

This device uses two cathodes, one at each end of a cylin-

drical anode. A linear magnetic field is used to trap the

electrons and cause them to gyrate about the magnetic field

lines and reflect successively at each cathode until a collision

is made. The concept is most useful as an ion pump or as a

pressure gauge but is of limited value as a sputtering device

because much of the ejected material is deposited on the inside

wall of the cylindrical anode and cannot be conveniently

deposited onto a substrate.

TOP VIEW

OB

E.= ELECTRIC FIELD J

An alternative anproach is to use a cylindrical post

cathode with an axial magnetic field.

Fi? 3.!0 Electn)n Drift )tion in a Cy1i :ri^a-; acrnetron

66

ANODE TARGET

With this configuration the electron drift motions can

close on themselves as shown in Fig. 3.10, permitting a

uniform current distribution over the target surface. An

electron is, therefore, trapped in close proximity to the tar-

get and keeps returning to its surface not being able to

escape until a collision is made. Furthermore, after the col-

lision the electron will again be confined to some new orbit

and will only make progress towards the anode when a further

collision is made. Ion production is increased close to the

• cathode surface causing a greater current to be drawn by the

device. Since symmetry is preserved, scaling up to large sizes

is relatively easy.

The attractiveness of this cylindrical magnetron device

has been recognised by many workers and in recent years many

'novel' magnetron systems extending the basic • idea have been

constructed. Two of these devices will now be hriefly reviewed

as a prelude to our later work.

- MAGNET

ANODE

TARGET

SUBS TRATES

- - -MAGNETIC FIELD LINES

67 Z r ••il~.N' t -~m:~y• cY TE.,S

The follJi g : evi e:. can be considered together because

they have the basic similarity of being2 devices with the

subsequent modification of electron trajectories and impact

ionisation characteristics.

c. 1 ,-:T S ,,, r, (•rr„

The Sputtergun (Fig. 3.11) uses a per...anent tsagnet bolted

onto the back of the anode to produce a non linear magnetic

field. Electrons are 'trapped' in the magnetron field and are

forced to travel in long helical paths less than 5 m.r;. from

the target surface.

3.11 The S utters u.n

TARGET

One major advantage of this system is that the substrate

is isolated both physically and electrically from the sputtering

source and .goes not at tract high velocity electrons. The 'prob-

lems of having a cool substrate and a high deposition rate are,

therefore, largely overcome whilst uniformity control can be

achieved by altering the angle of the target,' typically to a

cone shape as shownig. 3.12. .

Va : MAGNET

-l.-----ANODE

TARGET (-500 V)

68

VZZ?ZZZZ177ZZZZZ//zjg -SUBSTRATES

rrhis arrangerilcnt h~3 the added adVl1!ltagc tha. t ions tend

to strike the target obliquely incre~sinG the deposition rate

and 'sprayinG' m~terial preferentially in the direction of the

substrate.

3.5.2 THE PLA::AH ~-AG:~E'l'ROH

'rhe prQble:fls of .usinG a planar t~l.rget VIi th a. magnetic

field have already been di~cutsed and led to the idea that a

cylindrical target was p::--eferable. The planar :na~netron

(Fig. 3.13) represents illl attempt at solving the electron drift

problems whilst retaining ~i planar gemcetry.

3.13 T .e Fl:...__ '. ..: etron

69

MAGNETIC FIELD LINES

TARGET ,

The magnetic field which is created by pole pieces behind

the cathode enters and leaves the electrode surface perpen-dicularly but is nearly parallel to the surface at the furthest

distance away. The region on the cathode surface between the curved magnetic field lines is called the area of erosion and

an erosion channel is created by arranging for the field lines.

to form a closed path as shown in Fig. 3.13. Under these con-ditions the E x B electrol drift motions close on themselves and a cycloidal electron path results within the closed loop

created by the magnetic field arrangement. This is shown in

Fig. 3.14 viewing part of the erosion channel only.

-Pic,]. 3.14 Electr3n Path._ in the Planar ..'agnetron

-•,' V

------>-- MAGNETIC FIELD LINE

---)--ELECTRON PATH ALONG EROSION CHANNEL

70

Whilst high deooxitio.. rates are achievable, one

e disadvan-

tage

an-

t_re 1: thatthe target is c Ād prefere: :tiNl_ r in the reg_or

of the closed trac:: and. typically only 255J of it is usable.

There is no obvious way to change to another track once the

first is worn and wastage of target material is, therefore,

high.

E xpi ^T' •T • ,

SYSTE .S •

Some experimental devices were constructed to investigate

several of the ideas discussed in the review of high rate con-

cepts which has been presented. It was eventually decided to construct a co:l:ention:l cylindrical magnetron to test some

basic ideas concerning the removal of energetic electrons from the substrate and a modified cylinder was also devised to see

if the concepts were still valid with a changed geometry. The construction of these devices will now be discussed

briefly.

3.5.1 CONVE':TIC`: L CY=RTCA , 7 AG 'ETRON

The basic design is shown in Fig. 3.15, the final assembly -

having approximate dimensions of 30 cm length and 3 cm diameter.

The target consisted of two copper tubes, the outer one

having one copper end plate brazed in position and the inner

one having two end plates with water connections through one

of them.

11

11

P.T. FE. SPA[ER

TOP PLATE

o RING UPPER COS SHIELD

y-t-t---ALUHI NIUH POWDER

t-trl-+-_INNER COPPER CYLINDER OjTER COPPER > )

L YLINDER I~~~I ~/ / 1:'1 ~- LOWER COS SHIELD

____ TI.J..l.-..--L.L.I---~-e-- BAFFLE PLA TE

The outer cylinder is co~pletely isolated from the cooling

water so that a water leak would not result in water entering

the vacuum chamber but :l good thern~al cont:)ct betVlee~1 the two

cylinders VIas ensured by packing the gap Vii th fine aluminium

powder chosen for its non m~gnetic properties. The inner

cylindp.r is, therefore, .easil.y demountable from the outer but

it was in fact left perm8.::1ently in posit-Lon once the efficiency

of the water cooling h::td bcefi established •

. The co:nplete a~,;:;e:;.bly was ~3upported from the top plate of

the vacuwn systeIrl but :~?:lced fro'::1 it using P.T.F.E. insulation

and 0' ri~gs as shown i~ Fig. 3.15. An axi~l magnetic field

was provided by a c~lr of Helrnholtz coils mounted externally.

72

End effects were ,eliminated by constructing two 'CDS shields,

the upper one being upported by the top plate of the vacuum

chamber and the lower one from a baffle plate assembly on top of

the diffusion pump inlet.

Without the baffle plate assembly, the discharge from the

centrally mounted cylindrical target tended to be drawn down

into the throat of the diffusion pump. .Apart from this possibly

causing crac'_i ng of the diffusion pump oil it seemed likely that

unwanted pressure gradients were being established and it was

decided to remove the effect by using the baffle plate.

Various series of experiments concerning substrate tempe-

rature, efficiency of the target water cooling and deposition

rate as a func :.ion of the adjustable parameters were conducted.

The results

7~will i~be

`

discussed in later chapters.*

.3.6.2 A '::O i/IFIED' rYLI.:Df:R - T'1TRODYJCTIOi:

A method of increasin7, coverage which is likely to become

popular is the 'On Line' deposition system in which substrates

are fed past a stationary target. A continuous deposition

process can be Achieved either by using an air lock to load

and unload batches of substrates or by feeding substrates past

the target in a continuous stream.

Unfortunately a cylindrical target cannot be easily engi- _

neered for either of these two possibilities:-

(a) In the batch process it is not possible to include too

many substrates in the batch since those near to the centre of

the cylindrical target will receive a thicker coating than those

near the ends - this will be discussed in the next chapter.

(b) In a continuous feed process (Fig. 3.16) the shaded

areas of the target surface can contribute little to the coverage

of the substrates as they move along on the conveyor belt.

When a substrate is a long way from the target (having just

been fed into the system) the deposition rate would be low

resulting in poor film quality. Whilst this problems can be

overcome by using a mase to keep sputtered material off the

substrates until a position is reached where the deposition rate

73 Fir-. 3.16 'On Line' re.ociti cn

E--

ARROWS SHOW DIRECTION OF TRAVEL OF SUBSTRATES

would be high, a consequen.e is that about 50% of the target

area is wasted. A better arrangement is shown in Fig. 3.17

using a rectangular target.

Fig. 3.17 'On Line' Deno!:i Lion. Rectangular Taruet

••>. ->, -~- --;. ~-

-E'

- - - ELECTRON TRAJECTORY

74 The wasted area is now clearly reduced. Thilst.the

arreenr7emeht of Fig. 3.17 may appear to be simply two planar

tar:7ets bac: to beca, it does have further advantages. Assuming

that it behaves as a cylinder (a modified cylinder) then the

symmetry problems of the planar system have been overcome since

the electron trajectories will now close on themselves in the

mashetror field as shown by the dotted lines on the diagram. A target as described above was constructed on a small

scale to see -if the new geometry was a feasible alternative to

the conventional cylindrical arrangemeilt.

3.6.3 A '=DTT,IED CYII77EF:' - CT:=CTTO7 DETAILS

The basic design is shown in Fig. 3.18. The rectangular

target assembly (8 x 0 x 1 cm) was supported by metal studding

which was screwed onto a vacuum feedthrough mounted through but

insulated from the top plate of the system. Sputtering of the

studding is prevented by a glass CDS shield whilst the top of

the target is protected by a metal CDS shield supported on two

ceramic spacers set into the top of the target.

Fig. 3.18 A 1 7.odified* Cylinder

NON MAGNETIC STUDDING

GLASS CDS SHIELD

DURAL CDS SHIELD

CERAMIC SPACER

8 cm

RECTANGULAR '1AC117 COPPER TARGET

75

Cleaning the glasss after about five lours ru iinig time was

found to be beneficial for r prolonging the CDS shield life to

about thirty `lours. A ceramic shield would probably last longer. Attempts at shielding the studding by plasma arc spraying a

layer of alumina ont it were not successful but were not

extensively investigated since the glass shield could easily be

replaced.

Power was fed into the target along the studding; water

cooling could also be incorporated by the same method in a

larger design.

The first model incorporated a CDS shield supported from

the baseplate for the bottom of the target but this was sub-

sequently removed since satisfactory deposition could be •

achieved without it.

One possible problem was thought to be the sharp edges of

the target shown in Fig. 3.18 and the geometry shown in Fig.

3.19 was used in the initial trials.

Fig. 3.19 Initial r'odiried Target - Too new

8cm

8cm

..j ,''0•5 cm

Experiments showed that the target would sputter satisfac-

: torily whether the edges were rounded or not and subsequent

models used a simple rectangular geometry without the edges

rounded Off; this shape being much easier to construct.

Apart from the possible 'on line' advantages, this rectan-

gular shape may also be easier to fabricate than the conventional

cylinder in some cases.

76 . 3.7 T)EP0SI7T0:: _V'S

Exnerimental results for the two targets will be presented in detail in later chpters, here we should siciply mention that deDosition rates of about 1000

oA tnin

-1 were easily achievable,

though the jstems were usually run at lower levels than this

to avoid the risk of overloading the power supply.

As previously discussed, our intention wra.::: never to aim

purely for high deposition rate at the e.xclusion of other coil-

siderations and no - ei::phasis on the actual rate achieved will be made in the results presented later.

77

! =.._PT F. 'POUR

4.1 T ` __ _S °._ F OT:.71 EI~...

4.1.1 I :TRT 1'; `:'": 0',

One of the usual prerequisites of a successful film is

that it should exhibit good uniformity throughout the coverage

area. ':Whilst this quality may be of little value in reflection

coatings for example it is clearly of vital importance in any

film to be used for electrical conduction purposes since resis-

tance is a strong function of thickness. Although large area

uniformity is unlikely to be of importance in the construction

of a single small device, it is of interest if a batch of

devices is to be fabricated with comparable characteristics.

In device fabrication, it is usual to deposit a uniform film

and then use photolithography and etching to define the required

pattern since this provides more precise patterns than can be

obtained by mechanical masking during deposition. This is

especially true for sputtering applications where tine short

mean free path of sputtered atoms means that sharp line edges

are unobtainable using mechanical masking.

In this section we consider briefly some of the factors

which can influence film uniformity and then discuss hog°, it

could be improved when appropriate.

4.1 .2 SOLI?) A_:GLE

The solid angle subtended by the target at the substrate

is significant since it gives a measure of the percentage of

ejected target material which can be expected to strike the

substrate. Whilst thi amounts to saying that the smaller the

target/substrate separation the greater the solid angle and the

greater the deposition rate, a long cylindrical target (as dis-

cussed in chapter three) is likely to produce additional

problems as shown in Fig. 4.1. •

4.1 IIon Tarr7et

78

TARGET

c<= ANGLE SUBTENDED AT SUBSTRATE EDGE )3 =ANGLE SUBTENDED AT CENTRE

It is clear that p > a. and we might, therefore, expect to

observe more sputtered material at the centre of the substrate

plane than at the end. Whilst this effect is likely to occur

with any target geometry, the variation will be greater using

a long cylinder.

4.1.3 THE :7A7 FREE PATH OP SPUTT=D SPECIES

The conclusion of he previous section is not totally

valid since it assumes that sputtering is a line of sight pro-

cess with atoms following linear trajectories from target to

substrate. Scattering by ar;,on as atoms will in fact modify

the observed dist-f.ibution and is likely to completely dominate

the deposition if the target/substrate separation or the pres-

sure were very large. It is usually assumed that these latter

cases are unlikely to be found in most sputtering systems aiming

at a reasonably high deosition rate and scattering is ignored,

principally because the analysis required becomes complicated.

79

Recently. Westwood(1) has pointed out th7:t this line of

sight `Oie. is _a ):r O 'l at e to tt usual diod e sputtering-con-

ditions where scattering is important and has suggested an

alternative idea which he uses to deal with sputtering through

an aperture onto a substrate. He suggests that the tra.:sport

of atoms to the substrate is controlled by diffusion through

the gas once the sputtered atoms have travelled a Se':! Ci"e.an

free paths from the target and have been scattered by gas atoms. We should, therefore, consider an almost plane front of atoms,

whose dimensions are approximately the same as those of the

target, wite a uniform density at all points in the front. A film thickness uniformity problem would, therefore, not exist

provided the substrate was far enough away so that sufficient

collisions could take place to set up the uniform density atomic

wavefront. Thearetl s'a1 calculations suggest that this minimum

substrate distance is about five centimetres for diode sputtering at 30)U pressure but the number can be varied by a factor of

about two depending on the assumptions made.

Since a typical target/substrate separation for diode

sputtering is about five centimetres, we are just in the region

where this effect is beginning to become important though it was ' thought to be less significant for magnetrons where the pressure

is typically lower and scattering therefore less pronounced.

Westwood also points out that there will be distortion of

the plane front near to the edges of the target because of

diffusion parallel to the front. He therefore, also predicts

ā thinner film at the target extremities but for different reasons.

The expected film thickness distribution profile will be

considered in more detail later in this chapter. 4.1.4 THE fISTRI';UTIC. OF SPIJTTRRET) ?.1TERIAL

The angular distribution of mterial sputtered from tar-

gets has been studied by various workers( 2- 4)

, the results being

summarised in Fig. 4.2.

OVER COSINE

COSINE

UNDER COSINE

N N

80

4. ? ...1R An ;.'.."' Di t.'l ,. ,;tio.'- o: S'_ wt tered :ate-in?

TARGET SURFACE

In general the distribution is found to be approximately

cosine for target potentials in the range 1 - 3 ?V tending to

over cosine at higher voltages and under cosine at low voltages.

In the case of oblique incidence, ejection is favoured in the

direction of specular reflection for the incident Lori. Target

temperature or current density apparently have little affect

on any of the distributions.

4.1 . 5 CO777:S?TT ;_. COE .A'CIP.'T

As discussed in Chapter One, the condensation coefficient

for sputtered species is a function of substrate temperature

and is reduced at increased temperature. Since efficient sub-

strate cooling is usually difficult to achieve, the substrate

usually heats up during deposition and we might expect a thermal

gradient to be established along it under certain circumstances.

Whilst a metal film would not sustain a temperature gradient

(except perhaps when it was very thin) it should be possible for

this sroperty to be exdiibited by insulating films (for which

D. C. s:;uttering would not be successful) and for films deposited.

onto tie .:rate s.., _! 1 substrates which were physically isolated from each other.

81

In this :itu,t 7 or the substrates in the centre May be

hotter than those at the extremities. This will tend to reduce

condensation and hence lber deposition rate in the centre and

therefore oppose the solii anle effet which indicated increased

deposition in the centre. The temperature gradient which the

substrates would typically experience is, however, quite small.

and is unlikely to croduce a significant change in deposition

profile. It is usual to ic;nore this effect and consider a

sticking factor of unity irrespective of temperature.

4.2 EX7EaI=TA". 1.777:7-1A71-7= T-TOr: A CYLT7D7R

In view of the conflicting factors affecting uniformity it

was decided to test the cylindrical target experimentally to see

if a distribution problem existed, no thought being given at

this stage to the extra possible complication of using a magnet-

ron system. . The target geometry used is shown schematically in

Pig. 4.3, the microscooe cLide glass substrates being held in

a gantry which was supported from the top plate of the vacuum

system.

111-. 4.3 The 7.7.2.r.netron. System

E.

SUBSTRATES

TA RGET

82

Early problems were encountered in the achievement of

accirstable repro:luoibility and befbre pxese tirig experimental

results it is necessary to discuss the tech:AqueS required for

the production of satisfaotory readings.

z1.2.1 777,A=7"

Four possible thiekness measuring techniques were Given

cenc,ideration.

(a) A mechanioal measurement of step heiGht using a

Talysurf apparatus: In this techfAque a stylus is mechanically .

trac1-zed over a step ma7le by depositing the film onto part of

the substrate only. The film thickness is LLeasured from a tape chart output. This method is most suitable for hard films which

will not be easily damged by the stylus. A sharp step is

preferable though not essential.

(b) An interferometric technique using a Varian interfero-

meter, the theory of which can be found in any standard text(5)

A step film is produced as with the Talysurf method and the

whole substrate is they: metallised, usually by evaporation; with

a reflecting layer of silver or more commonly aluminium. Since

the step is faithfully reproduced by the aluminium coating,

reflection interfererce patterns can be obtained from light

reflecting at the tor. and bottom of the step. The spacing of

the i_terference fri:fgas is related to the step height which •

can, therefore, be measured.

A well defined step was found to be essential if satisfac-

tory fringes were to be produced. Also the aluminium coating

needed to exhibit good reflectivity for well defined fringes.

This reflectivity could only be achieved if the evaporation system base pressure was better than 5 x 10 torr prior to the com..4=ncement of evaporat4 on.

(c) A resistivity technique in which the resistance of the

film is measured by depositing electrodes onto it. This idea

was not investigated experimentally since it was felt that

electrode contact resistance problems might he difficult to

overcome ani in any case non uniformity of the film would defeat

it.

83

(d) A 1Tethod of direct weight gain in which strate is

wei7hed both 'zefore after depooition. One d'ifficulty here

is that the :7.ht not exhit bulk density though it

should be r: Lf olese Us: C5 tne film “..,s very thin.

Choice of substr-Ae wou1.1 .sesubly be ir:.port'ant though the

idea vis not researche:: in any depth since the interfercetric

technique was found to be If:iving aocel:table results.

The first two fnethis were e:ktessively investigated, both being tried on the sa!:.e fili by making a mecnanic;n1Aep measure-ment and then ::Letallising for interferometry. It soon became clear that the interferoetric Z:echcique was more reproduCible; practice in using the instrument leading to repeatable measuring

to 40 GA 11 5000 eA (an error of 0.9).

This method is most suitable for films in the thickness

rn-nge 2000 °A. 10,000 °A because of the way in which the

interference fringe patern (Pig. 4.4) needs to be analysed.

4 t Inte-Per1::leter Fr!h7e Pat terns

( (41) .

A good quality fringe sycte::: is shown in (i), the deviation of the fringes being proportional to the step heiht of the

film. It is necessary to evaluate the fringe displacement a/b,

diagra,m (ii) showing a systez: where a b and. the step

height is in fact 29470A (hIlf the wavelength of yellow sodium

light).

84

A dislace::.ent of exactly three fringes corresp:onds to a

hei7ht of 2'=41 A hut once a larger step is. used a co!:-Tli-

c ,, tel =- inge t.i ,' ture such as is. (iii) often e:erges and it can

be d'ficu7 t to decdo treisely how larEe the displaceent is.

Thr, calculatel c3ull in fact easily be in error by units

of 2947 °A if the nut1er of fringes displaced was estimated we scull use the Talysurf to give

an approximate thickness and then use the interferolneter, but in practice it is easier to restrict filt, thickness to the

preferrel region Around 5000 °A by adjustis.g'deposition times

accordingly. The following procedures were found to be advantageous:- (a) Using - photo-resist to mask a sharp edge to the film

on the substrate with subse'iuent reoval of the photo-ref3ist layer. :.:echenil makiln;: by putting a layer of metal foil in

close proximity to the cutstrate was found to he unsatisfactory

in agreement with Westv.00d(1)

and !,.leny other authors.

(b) Depositing the film at a suicieutly high rate such

that a shiny 'copper li!te.' aeosit was produced. Liquid nitrogen trapping was also beneficial by preventing backstream-

ing diffusion pump oil fro:n coAaminating the film. Copper

films which appeared .dull did not generally metallise success-

fully for interferoetrte work becnuse the reflectivity was

'too low.

(c) Pre-sputtering usig a large shutter to prevent depo-sition on the substrates us til such time as the plasma operating

variables (voltage, curreht and pressure) had stabilised. This

problem has already been discussed in Chapter Two; presputtering

for at least thirty minutes being required. This plasma eqibriunl was found to be esnential if film thickness .as to

be -n.-produced on successive experi.:tental. runs.

2.2 F=.171=2

The initial experiments were .arried out using the system

shon in Fig. 4.3 but with no magnetic field. Sputtering at

30)u pressure and target potential of 27/N the thickness unifor-

mity curve of Fig. 4.5 was observed with a target/substrate

separation of five centietres.

85

7x7=tal 7-117.,,et

THICKNESS

5000 --

4000 —

3000 —

2000

1000

5 10 15 20 25 30 LENGTH ALONG CYLINDER (cm)

As expected, the film is approximately twice as thick in

the centre (15cia mirk) than at the extremities. When the

experiment was rt7Teted using the magnetron configuration

(magnetic field present) at 500 volts, the thickness distri-

bution at the substrate was not significantly different. This

is not surprising since 955 of ejected sputtered material is neutral and is, therefore, unaffected by the magnetic field

which increases the deposition rate but does not alter the

relative spatil distribution of material.

4.3 I7T7,07171 77,7 1.1';TY3R7.7ITY

Having illustrated in Fig. 4.5 that the anticipated

distribution problems does exist for the qTliildrical arrange-

ment, we now need to look at waysof improlAng the distribution

profile. The following possibilities merit consideration:—

86

(a) :i:ov1 1:g the substrate further away from the target.

This is not a very elegant solution since a reduction in

deposition rate wou?_d h:/e to be tolerated.

(b) ovin the target tst the deposition is in orogress

so that the substrates are in effect 'sprayed' with sputtered

material. ProbleJ,s would .:learly h.- ve . to be anticipated

because of the need to move target and water cooling connections whilst they are electrically live. Since the target would

require movement in a vertical plane, vacuum feedthrough diffi-

culties might also be anticipated further suggesting that this is not a very attractive cozsibility.

(c) :,7oving the substrates whilst the deposition is in pro— .

gress. Whilst this would be preferable to target movement, it

would still lead to wasted space since vertical movement would

again be required. This would still present difficulties for •

bias sputtering applications because of the need to move

electrically live parts.

(d) The use of a split target with differing power inputs

to each section as discussed briefly by Hajzak(6). The thick-

ness distribution of Fig. 4.5 would be reproduced approximately

with equal power inputs to each section but the distribution could, presumably, be made more uniform by supplying more power

to the outer sections of the split target. This idea was investigated in some detail using a thin straight bar rather

than a cylindrical target since a sectioned cylinder seemed to

provide unnecessary constructional problems.

4.4 SPLIT TARGET IVI.P7PT.77=1. SYSTFA

4.4.1 G=HAL SYSTFd

The first experiments were carried out using a single bar

(one piece) to ensure tht a similar distribution was produced

to that using the cylindrical target. The most convenient method was to support the target from the vacuum system base-plate using cefamic spacers (Fig. 4.6) and to sputter upwards

rather than horizontally as with the cylir.drical target.

CERAMIC SPACER

SUPPORTING PLATE

TARGET BAR (later split into

sections )

BASEPLATE

TO DIF FUSION PUMP

87 Pig. A.6 Ex7-my..imnt:,.1 Target

SLIDE SUBSTRATES -GANTRY TO SUPPORT MICROSCOPE

(adjus table height) 1

The thickness of thc.ceramics was chosen to be less than the CDS length under the chosen sputtering conditions so that the supporting plate could act as a CDS shield for the under-

side of the bar. This arrangement of coutterisg ucwards has the advantage

that the target geometry can be changed quickly since a per-

manent supporting gantry is not required. Whilst a 30 cm bar would have been preferable for a

direct comearison with .the cylinder, a bar of this length when

arranged horizontally would only just have fitted into the vacuum chamber and it was felt that the side walls might intro-duce spurious edge effects. A 20 cm bar was used to eliminate this possil:Hity.

Using 30/A pressure, 2KV target potential and a target/

substrate separaton.of five centimetres, the result given in Pig. 4.7 was obtained.

68

.71-7. 4.7 Sir 1e :a-- - 7z7e-imental Results

THICKNESS ;4) 5000

4 000 -

3000 7

2000-

1000-

I I i I

5 10 15 20 LENGTH ALONG BAR

This result shows the bar system to be a reasonable analogue

of the cylinder (Fig. 4.5). The end thickness is 42 of the

central thickness for the bar and 46% for the cylinder.

4.4.2 FIVE TIR SYSTE77,

Preliminary experients in building up a split bar of total

length 20 cm showed that five sections was the optimum choice.

Whilst it would be desirable to have as many small bars as

possible to maximise the variability of the system, it was found

that the individual sections became too difficult to handle if

they were much smaller than 4 cm in length.

The bands system c3ncisted of five bars each of s length

3.9 cm laid close to bat insulated from each other. Five poten-

tlometePs were used to vary independently the potential on each

bar and, therefore, the current drawn by it. A schematic

diagram of the electriCal arrangements is shown in Fig. 4.8.

89

PLASMA

ANODE AND SUBSTRATES

_ indicates limits of vacuum chamb©r

POWER SUPPLY

The followig point were fJund to te iu..portant:-

(a) The of !putteri ,at thr vacuu feed-

throuchs and alon th,) cor.necti.v the feedthrouhr3 to the

cthcr.rice th, curre:1t

reaiins be tc,c jh I the pote;:tLreter setcs incor-

rect. The c;onneti.i:j wire oLto them

to act a a QDS the fee,Ithrohs ';re fitted with

T7;C sh!.]ds.

90

(b) The target/substrate separation must be the sae for

all f'7e bars or %r. acyetry de-velcs in the unif:Jrmity

It was, threfore, ncessary to ensure that the tet

all. the :=1 ht off sup:)rti. plate and tne substrate

bar was narliel to the surfs..::e of the tar:ets. This was

checked with a travelling microscope.

(c) Exneriments on altering the potential and hence cur-

rent on one bar o.:.e1 tnat this did not sg -ifi(:antly afect

the sae parameters . oc the other four bars. The targets,

there.'ore, work :7 :1decenently simplifying any adjustment.

Since ',111 five bars are linked to one plasma it mi;7nt be expec-

ted that change:! in ccc part of the cyte:: would affect the

behavioar in another but this plasma coupling affect does not

see;:. to be strong.

(d) As the supply vo'ltage is increased, the targets all

draw more power but in the sa:::e proportion. The thickness

distribution does not appear to be altered significantly by the

choice of supply voltaze wnch is net a critical parameter.

(e) Pressure chnii:e did not significantly affect the uni-

formity results though the range studied (30/u - '72.1) was

admittedly small.

4.4.3 7177":77=T A7. pn!--,ulTs

Consider a series of experimental runs, all at 39/4 pres-

sure, 5 centimetres target/substrate separation and a power

supply settinz of 2.5K7 (though not all of this will appear

across the discharge 25...Ice some will 'se dropped across the

potentiometers). The table on the next page shows the currents

drawn by the five bars for five different experiments. The

unifority results are shown in Fig. 4.9, the positions of the

bars also being indicated. Deposi tior. times were adjusted to

orAuce films ;:f about the preferred thickness (5000 o

5000 —

4000 —

3000

2000 --

1000 —

I

8 12 16 4 20

91

RUT:: ER

7AP 1 (NIA)

.7.,..F: (,:::. A,)

2 '.. AR 3 (::, A)

T.':1/ (--n A)

4 2AR 5 (rr, ;,)

TOT A.:, 01.7.RENT (7': A)

a 3 3 3 3 • 3 15

1-, 3.1 3 . 2 .9 3 3.1 15.1

, 3.2 2.9 2 . 8 2.9 3.2 1 5

d 4.2 2.9 2.8 2.9 4 . 2 17

e 3.5 2.35 2.75 2.35 • ..) , •) r 15.45

^ THICKNESS A

BAR 1 BAR 2 BAR 3 BAR 4 BAR 5 . 3

LENGTH ALONG BAR (cm)

92

(a) ',7-1.07 the e'fect of runz.inr-- all tho bars at equal

ou.:rent and iz; thercfre, siu larto the earlier rezolts pre- 2:e:Ited for a s. -inF,le ('c) ani (c) sh,; the result of

radually th., current to thc cuii bars. (d)

shov:s better unifor:Lity touj-h over-correction 7,1th the ext 7-elr.ities noN cotributiliT, too mush. Thee c::.rves :,hov, that .

in -..)rincinle it should be possible to achieve improved unifor-mity with a sectioned target, the be::+. attet (e) ;:howi5L2; a uniform film to within 4% over 75% of the substrate and 10% over all the substrate.

.Unfortunately, the improvements madefl.om (a) to (e) were

achieved totally by trial and error and we hn.ve no way of knowing

boa to use the inFormation ;ailed on a bar of differe -ct length.

It would obviously be an advantage to possess a theoretical

model for the observed distributions.

4. ST=LTFIED TT.:07.Y 1-20 TT.E. MTTK7= TTITPI7T5TION

Consider a bar L cm long situated a distaLce T cm from the

substrate (Fig. 4.10). F17. 4.10 TheTlry of ti1,7, Distribution

17ti TARGET

I I

dx I -x)

SUBSTRATE

93

If we assume:-

(a) A unifDrm c'_12rent dictritution over the tar,.;et area.

(b) A cosine dietr:bution of ::.aterial sputterel from each

point on the target surface.

(c) ro csilie:o:is of <ecte'l material with gas soiccuies. (This is not a very assumptiin since the mean frt:e path

at even a low pressure such as 5 micr7)nsis still only of

order 1 cm.)

We can show (Acoendix 1) that the ncnt of material

deposited (D) onto a small element of substrate of area dx is

given by:-

D 2T [ Kdx tan-1 (1-xl + (1-x) T

where K.is a constant ani I can take values from 0 to L.

Choosing some suitable values of L 20 cm, T 5 cm and evaluating the part of the expression in square brac'nets, the

following results arc obtained where x is the distance from

one end of the bar in centimetres and the thickness is in

arbitrary units of Kdx/10.

POSIT= EALONG

SI_TrTRAT. :,:. (cm) 0 2 5 7 10 13 15 18 .20'

xKqx 1.57 2.28 2.84 2.96 3.02 2.96 2.34 2.28 1.57 THICKNESS --- 10

If we normalise these results to fit the experimental

results of Pig. 4.7 at the mid point of the bar then the resul-

ting theoretical curve calculated from the table of values above

is shown by the dotted line in Fig. 4,11.

4.1

0

94

Ti7.. 4.11 Comrariscn 7etw,aen F.,::-:ri7ental Result and Theoretical

Prediation

0 THICKNESS A EXPERIMENTAL

-- -THEORETICAL 5000

4000

3000

2000

1000

4

16 20 DISTANCE ALONG BAR (clli

It ls seen that the two curves correspond very well though the experimental uniformity is slightly worse than the predicted theoretical result.

This comparison is surpri2ingly good bearing in mind the

simplifying no collision assumption which was made. Another

factor which has been overloked(7) is th.:tt the cathode dark

space 11-,s a focusing effect on ions ncar the edge of the tar-

get casing an increase in current deity in this region and consequential enhanced suttering.

Film temperature nas measured at different points in the substrate plane ucing therocouples. The method of measurement

is discussed in detail in Chapter six. Any temperature

varlatioa was found to be negligible and Could not, therefore, be reson,:ible for modifzTing the tir•-ess distribution.

0.6

0'2

BAR 12 16 20

DISTANCE (cm)

1.4 THICKNESS

(arbitrary units K (ix )

10 10

95

1 ''-lrr".7",- =T 77T-7 7-.-7,

Sinee the. tical correlation to the experimental

res1;_its ap::ears to 'oe quite 4 -ood, it seems wcrthwhile trying

to tend the tIlecry to ftvebnrs des:.ite reservations about

the validity of the basic a=”rptions.

Given that we can calculate the thickness distribution from

P sin1e bar, it should be ^cosible to ue the p:sinciele of

superposition to add the five individual contributions and pro-

duce a resultant curve. Since the bars are drawing different

currents, they will be eaiittin different amoants CI material;

this can be allowed for by using the current an a weighting

factor in the theory.

Tne fact that the Lors are at different volta,;es and will

produce slightly differing sputtering yields can be ignored

since the distribution of ion energies caused by Th,lr;e exchange

collisions (as discussed in Chpter three) will be fairly

similar.

If we use equation 4.1 to calculate the distribution pro-

duced along the whole substrate by one of the five bars only,

the shape given in Fig. 4.12 is produced:—

Pig. 4.12 Unifurmity Distribution From! A Single Bar

96

of 7alues

S-_,atrate

(cm)

Thic',:ness x7f7dx 10

0 1.16

2 1.45

4 1,16

o 0.643

8 0.3

10 0.139

12 0.07

14 0.037

16 0.021

13 0.013

20 0.007

If all of the five bars vere drawing the sa;:le current, they

would all produce the came distribution (symmetrical about the

maximum) only shifted qlong for each bar such that the maximum

of the distribution alv;ayc corresponded to the centre

bar v:hich prodsced it. We can, therefore, produce

table of values to show the relative contribution of

on the suttrate from each of the five barn separately.

Substrate Position flAR 1 3AR 2 BAR 3 1-',?.R 4 :BAR 5

(cm)

of the

the following

any point

Total xKdx 10

0 1.16 0.3 0.07 0.021 0.007 1.56

2 1.45 0.643 0.139 0.037 0.013 2.28

4 1.16 1.16 0.3 0.07 0.021 2.71

6 0.643 1.45 0.643 0.139 0.037 2.91

8 0.3 1.16 1.16 0.3 0.07 2.99

10 0.139 0.643 1.45 0.643 0.139 3.02

12 0.07 0.3 1.16 1.16 0.3 2.99

14 0.037 0.139 0.643 1.45 0.643 2.91

16 0.021 0.07 0 1.16 1.16 2.71

18 0.013 0.037 0.139 0.643 .1.45 2.28

20 0.007 0.021 0.07 0.3 1.16 1.56

97

b%sio LLe no.:: only eoh to be modifie,d by scaline,

u- '0 'Is the b -,r is

drain/. Con:..ider thi eu fexerturt e on

4.9 where birc one T:_a draW 3.5 mA, two and four

draw 2.C5 :nA rindtr Jr...,.ws 2.75 71A.

Havi:u fol:nd the r.,:odified co...trbution from eacl', hr to

each point on the :::ubntrate plane, the t'ive contri-

' butionL: are then adled to Jive the r.;itant i:..-:trution and

the re cult tJ the exi_eri:ente.1 curve at

the centre.

The re2u1',11.. copari::on betweerl the e.7r:,ntal read.;nzs

and the theoretical ;:ode1 :thown in Pig. 4.13.

Prc:diotion

EXPERIMENTAL - THEORY

I 4 8 12 76 20 DISTANCE

(cm)

5000

4000

3000

2000

1000

98

.::h1lct there ar,e i,oints of :7imilarity t the t'::o curves

it is ovus thtf'it is no: :7articul-?.rly 27cod. This is

not sur-:riin oicc, the thc-retial mei i: 1-7.:ic7.n to be

oversim?lifiel.

}-.e a::::arel-,t reaLonL7e fit between theoretical and

excerimental re ,ults fc.. acin.j:le bar doen not, in fast, T':.ean

that th theoreticc:. m.)ael Ic f:tfactol-y. The re it v:hen

the theory is extended to a multi2le bar system are not co ecouraing. A better ::.odei i_ccrporatin :c tein ideas

would be reqred before the cu-rehts nee..:ed to produce unifor-

mity.could bc calculated theoretically. 'hilot it is c.fortunate tht thf,oreticl 1-,redictions were

of little value in solving the. Ilhiformity roblem, it was

nevel-th,-le:: cs:-Jible to achive imnroved uniformity ging ,3

Solit taret cystem in a planer co_figuratin. Since results with a planar target and a cylindrical taret show a si:cilar

trend and intreducinG a ::i.Agnetic field did not appear to sig-nificantly alter the thickness distribution, it seen reasonable

to conclude tl'at a :-:plit target cylinder could be used to improve

uniformity in a manetron configuration.

The main purpose. of these experiments vas to demonstrate

that the unifo=ity prol;lem could in principle he overcome. It

wai' not inteded that this shoul be a major part of the work. Altelmative aoi.roacher: for future work are discussed in the final chapter.

99

7:71T

7.7 :POT' ) ?':A

A series o° tests wa:- :initiated ts.compare the conventional

r tr try to the f3J.) celled 'modified

cylinder' az discunsei in Ch.:,.pter three. This latter configur-

ation was thou=7ht tc have possible alv”.ntages over the

con_ventional arraemest in ter; of its m!pre convenient con-

stv'uction 3,nd 'on line' 1,1re scale benefits where the uniformity

problems as discussed in Chapter four would not he relevailt.

Initial results were encourac;ing in that the system

exhibited similar results to those observed usin a conventional

ma6netron. However, in an experiment to monitor the effect of

pressure chn,,e o: dischae current, the- characteristie.given

n Fig. 5.1 war oh rvo. 'ins a chart r3corder.

I Nal —3

B= 8x10 T

50-r

I I I 5 15 25 P(microns)

Fin . 5.1 Curreni: 1 v. Prersure n Fiela B S x 10-3T

100

I (ma)

100

/ /

100

Start._: at high pressure, as the pressure was reduced the

discharge current initially fell teadily as might be expected

:;til at some p Yes ?~:( t re was r sharp decrease w2 current re_t

for a neraigible re: sare change s'J.g4estintg that the plasma had .

in scne way s itc_._3 fr...:,m cre stable state to another. Further

investigation showed that this transition was both reversible

and repro...uci :le end a series of curves (Fig. 5.2) was t-l::en

for differingma.e ie field strengths. s.

Fil:. 5.2 Current T . v Pres,,ure s at m_f rerc" :t xraq, ..t_ic Field

StLp: Lh

-3 B2.$,10 T

, B = 7x10-3T _3 f3=6x10 T

' B =4x10- 4x 10-3T / / _3

/ B=240 T /

50

15 25 P (microns)

The magnitude of.the current change on switching with con-

stant field B increased as B ;r.:s increased but the pressure at which switChng occurred was not a strong function_ of B. :The

range of ca gnet.ic field variation was admittedly small being

.limited by the bzax:i sum permitted current to the Helmholtz coil configuration which was providing the field.

101

The results at 8 7_ I0 3T show the current approximately

halvin for negl;Ible :-,ressure change and it was observed

this the rlasma colour switched fro::: 7reen (cop; er dominated)

to blue (argon doriinatel), drawing less current and appearing

less intenf-e. This in the ourrent/prossure

characteristic is accompanied by funda,„..ental change in CD3

len;:th at the switching pressure (Fig. 5.3); these results

havin been Obtained by observing the discharge with a kathe-

tometer.. This is essentially a travelling :ilicroscope with a

long focal length objective lens so that the target edge and

CDS/ITG boundary can be observed from outside the discharge;

the CDS length being the diffese-ce of the two readings which

were noed on the vernier scale of the irstrument. •

Fir-r.. 5.3 crY3 Distance d v Presure n with 7 = 8 x 10 3T

d (cms)

2-0

15

1•0

0.5

1 1 5 15

4

25 P (microns )

102

Larger errors have to be tolerated at low pressures because

the definition of the ODS/::G ede'is not as clear. The error

of about 1mm at higher pressures is (aused by the fact that the

CDS/1.71 edge is not quite parallel to the target edge resulting

in variable readings depending on which part of the target the

observation is made. •

The variatien of d with pressure has been studied by

.kston(1) Who derived the empirical relaticnship.

A C • d = 4. (j)i

(5.1)

where A and C are constants and j is the current density

at the cathode. Sino C is usually small, the second term can

be neglected leaving a relationship suggesting the increase of

d as p is reduced but predicting a smooth transition rather

than -a discoetine;j • A relationship of this form was observed

experimentally for •a planar diode non magnetron arrangement and

at first sight there is no obvious reason why the introduction

of a magnetic field should produce a discontinuity in the

relation between d and p. •

Similar results Were observed using a conventional cylin-

drical magnetron indicating that the effect W9.2 not solely a

function of the unusual target geemetry. Extensive tests over.

a variety of current/pressure regions revealed that the switching

effect was totally absent in the absence of on external magnetic

field confirming that en investigation of the role of the mag-

netic field was necessary.

.2 TF 0L5 OF THF. !.7AG7777TC FTrLD

Consider an electron leaving the target in a magnetron

arrangement and being influenced by the perpendicular electric

and magnetic fields (Fig. 5.4). •

103

717..1_5.4 Elt'ctrJ .. "ect2rv

SUBSTRATE

NG

T •1 d WS

I .~ Yin , tg i .

i TARGET 1

E. = electric field strength B =magnetic field strength

The electron (charge e) will experience a force e. in the

direction perpendicular to the targ et/substrate plane and

e(v x B) from the magi:otic field ;there v is the velocity at

any particular in:Aant along the trajectory. A curved path

will, therefore, result and in the absence of collisions with

gas atoms the electron will return to the target surface pro-

vided the magnetic field is strong enough. In the case of a

weak magnetic field, the substrate or vacuum vessel containing

walls will intervene before the orbit can close on itself.

This situation is fundamentally different from the non magnetron

case where in the absence of collisions the electron is confined

to a line between target and substrate and has no chance of

returning to the target.

5.2.1 E=»_„m7.37 TDAT^-':'RY

Sup_ oce Y:7:0 is the maximum distance of the curved trajec-

tory from the target assu! ir.g no collisions and that an electron

has fallen through V volts to reach yMAX'

x>

104

Potential energy lost = eV joules ac.~d this will be true

irrespective of how the voltage drop i distributed between

target and substrate. This loss in potential energy will

appear as kinetic encrg7 of the movin electron.

2 L^ =

.2 -.eV

V _ _~t since at y ,, the electron of mass m MAX

is moving in the x direction only.

Applying Newton's second law in the x direction we can

write:-

mx = eyB.

Integrating with respect to time produces:-

mx = eyfl + K where K is a constant.

If the electron leaves the cathode normally then x = 0 at

y = 0 and we can set K = 0 simplifying the expression to

mk = eyB (5.2)

m e.2 ( Y, .A )2B

2 •

. . eV = mj

iii

Pr:V] 1 and Y"TAX =

If Vt is the target potential and VxM is the potential at

)( MAX, then V = Vt - Vyt and we can write:-

y MAX _ 1 ?2 (Vt - VrT) 1.

Before any further progress can be made it is essential

to know the potential distribution across the discharge so that

a value forVrm can be estimated.

~ S f.~ .T T<<. TTS TE r T TT Is T .T 5.2.2 ,1F:i S 7 :~:. 0 .. ~h D ~T T":U_IO.,

Aston() has le7onstr tei that the electric field strength

across a glow discharge falls linearly with distance from the

target and that the negative glow region is essentially field

free (i.e. nearly all of the voltage dro_ ned 'between_ target and

substrate is in fact dropped across the ODS with no drop across

the rest of the discharge to a first approximation).

(5.3)

105

The, linear reduction in field strength E. with distance y can be expressed as:-

E = - y) _ v dy

where C is a constant and d is the CDS length. This

expression correcp -Jnds to the field being strongest in close

proximity to the target and filling to zero at the CDS/`:G edge.

Integrating with respect to y yields the expression.

(vt - v ) _ t (yd - y2/2) (5.4)

where Vt is the target potential (y = 0) and (Vt - V,) J

represents the potential through v.hich the electrons ons have fallen

to reach the po.i,lt y. Putting iii typical spattering values of say V1 . -1KV and '

d = 2cm allows us to plot the electron energy (e [17 t - Vy] ) as a function of distance from the target assuming no collisions

to change the energy.

D1.-tnce across TABLE OF Electron Energy discharge e (y)(cm) VALUES (ev) (iĪt - V )

0 0

0.2 190

0.4 360

0.6 510

0.8 640 1.0 750 1.2 840

1.4 910 1.6 960 1.8 990

2.0 1000

000

Soo

600

4co

Zoo

EL ECTRON NERG Y (ev)

1 COS NG

2.0 DISTANCE ACROSS DISCHARGE (cm)

10 05 1.5

106

Fig.. 5.5 Electron Ener:*'r ncreass Across the Discharpe

Fig. 5.5 shows the electron at first gaining energy rapidly as it is accelerated in the strong field close to the

target but this acceleration, tending; to zero by the time the

relatively field free NG is reached.

The fore. oing work on the significance of the para..leters

y',iAX and d has been published (4) and the papers e.re enclosed

in Appendix %i. The problem will now be considered in more

detail.

Since the NG is essenti=J1y ficid free, then provided

_.~ d we will be justified in setting Vni = 0 and writing

equation (5.3) in the fōrm:-

1 201 B e .. (5.5) .

Since y is pressure independent and the CDS length

dcc1/p, then provided the pressure is high enough, we should

always he „ale to achieve the condition y > d when the

107

trapped electron trajectories are outside the CDS.

In the limit ..he : )I ., ~Y - d We will obtain the limiting A

mag:etic field Blim such that for B $h T we will not be

justified in us_ equation (5.5) since 'with the trajectory

inside the CDS we could no longer set V. = 0. Putting in the values of B = 8 x 10~ 3T and V, = .1KV e

.

predict y-- 1.3cm using equation (5.5) . Referring to ? :AY.

Fig. (5.3) this suggest; that at pressures above the 'switch'

whilst at lower pressures .`/ ..,AX Z d and that in moving through the switch region they ,AX = d relationship is satisfied at some time.

The significance of the y ,'Ax

:d ratio is not only of

importance theoretically, giving, as it does, information on

the region of ion production in the discharge, but also

experimentally since the y d resion should be avoided

in the interests of plasia stability and the consequential

non controllability of deposition .rates.

5.3 REGIO".' OF TO: PfOD:3CTTON

Before any further progress can be made it is essential

to know the main region of ion production in the discharge

or more precisely the origin of the ions which strike the

target since it is these ions which are ultimately responsible

for sustaining the .risen, .rge. Various diverse ideas appear to

exist notably Druyvesteyn and Penning(5) and Brewer and

We s th_aver(6) who suggest th _t most ions come from the negative

glow and Little and Von Engel(7) and Holmes and Cozens(8) who

conclude that ion production is mainly in the cathode dark

space with a significant contribution to target secondary

electron emission from photons. in the negative glow. An

important point is that both ideas agree on the need for an

NG to exist, agreeing with the erperi~..er:ta_i observations that

ion production ie affecte:1 if the, target/.substrate separation

is less than two CDS lengths.

Cor_. ides 3. secondary electron bei.:; libe_rwted 'from the

target acel beln j accelerated acroe.s the di :rhar'ge by the

103

electric field. If Q. is the effi~iency of the ionisation.

. e'=.:''.1.:':3'.. 't s r u:JJers of i7ositive charges Jr'aluced per

electron rer cen i etr e path r_ re:s rr, then in passing

through an e'_ aLLe ,a. .. i;_;1h S s, the nu..ber :f ionisati..n.s c,

occurri is.

Sn =Q_ p Ss (5.6)

rovide d S s < <X where X is the me1n free path for

ionisation.

The para:_:eter Qi is a function of e.:.ectr:;n energy and has

"been measured by Smith(9). The basic .results are shown. in

Fig. 5.6.

5.6 Efficiency of Ionisation sation for Electrons in Irgon .

90 ENERGY (e v) The main point to emerge is that 90eV i7 the most efficient •

ionising energy for electron imp=-cting into argon when

0. = 13 per cm per m.m. pressure. It should be realised that

this figure of 90eV is rel::.tively low when compared to the

energies obtained in the high field regions close to the target

of a glow dischrge and the optimum ionisation energy is soon

exceeded as the electrons accelerate away from the target:

Hence, a.t a typical sputtering : ressu_,e of 30 microns

.(3 -Z ) we can expect 0.39 collisions 1~J ia..~,. we ,9 ia..isii_g col.~isia..s per

centimetre implying a mean free path for io.:isation of about

2.5cm for 90eV electrons. Since the mean free path at other

109

energies is longer, thc, neglecting of collisions in Calculating

the electron trajectory in a magnetic field would appear to be

reasonable, since excitation collisions in the CDS are relatively

rare and elastic collisions with neutral atoms would not produce

a oh.-_nge in energy.

Usin: the data of Fig. (5.6) it is possible to calculate . .

the nu_Lber of ions produced by one secondary electron during

transit from the taret to the edge of the CDS. The cal-

culation is complicated Ìy the fact -Ulat each ionising collision

produces an extra electron which can itself cause further

ionisation (a breeding effect).

Examinationoftheparal%eter Q, (Fig. 5.6) reveals that 1

it is constant to within a factor of about three once an energy

of approximately 20eV has bechl exceeded. If we make the.simp-

lifying assumption that Q. is cnztant for all electron energy

values, it is possible to use integration to calculate the

number of electrons reaching the CDS/NG edge for every electron

leaving the target.

Electron Proiucton Across a CDS of Lerith 2cm

te-- CDS

NG —

TARGE T

I I I I I I

n 1 1

I I I

X l")

X=0 6X

Every electron leaving the target will have multiplied to

n electrons ly the time .ah ele:.:ental length Sx is reached a

X=2

110

distance .x from the target. Since each electron entering S x

will producce Q r, C x electron in the element then the total - i.

number of electrons. produced in the element (8n) oan be

expressed as:-

Sn - nQp Sx i. w dr = Q pd.x

n ~ a

loge,i 2Qip

N = e2Qip

(5.7)

If we put Q. equal to its maximum value of 13 and, there-

fore, deliberately over-enti:date the number of electrons '_`1

reaching the CDS/`IG boundary we predict N = 2.18 at 3~

pressure using equation (5.7). This would .:u Best that at

most, every electron leaving the target creates 1.18 new elec-

trons and, therefore, 1.18 ions whLch will be attracted towards

the target to cause further sputtering. A more realistic

average value for Qi is, perhaps, 6.5 (Fig. 5.6) when N = 1.48

suggesting the creation of 0.88 electrons and ions.

5.3.1 S C _P Tv r. CT._ )' COEFFICIENT

Since we are interested in whether the CDS alone can

produce sufficient ions to sustain the discharge, a knowledge

of the secondary electron coefficient of the target (ō )

(the number of electrons emitted/incident ion striking the

target) is clearly essential.

Following the early work of Oliphant(10) most of the recent

work has been by Hagstrum(11-15)

Some of the major conclusions are:-

(a) Dirty surfaces give much higher 'ā values than clean

ones.

(b) iii depends on the actual sample condition and past history.

(c) ii i increases vjth_ increasing ion energy but this is not a strong function for clean surfaces. No value of ō is i

111

greater than unity within- the energy range 0 -- 1000eV

irrespective of sputtering gas or target condition. A figure

of less than 0.3 electrons/incident ion would seem most likely

forO_r copee_' target.

(d) Two distinct electron ejection mechanisms are possible:

(i) Potential ejection requiring a charged particle,

the ō values for which have already been i di:cussed.

(ii) Kinetic ejection in v.hich the energy source is

the kinetic enerEy energy of the bombard' r particles.

As expected, kinetic ejection by neutral species tends to zero with d creasing energy and is too small to measure accurately below 300eV. Since

energetic neutrals with energy in excess of this figure could only be produced by charge exchange

collisions rrit'h ions in the CDS, it seems

reasonable to ignore this mechanism and rely on

111 = 0.3 electrons/ion as the figure for cal-

culating whether the plasma is self sustaining.

If the plasm:. is to sustain n itself, each electron leaving

the target must create sufficient ions striking the target so

that they in turn will cause a further electron to be ejected.

If '. = 0.3 electrons/ion we require 3.3 ion:, to produee one electron and since there is a maximum of 1.18 ions produced in

the CDS with 0.48 ions thought to be a better estimate, it is

reasonable to conclude that ionisaticn in the CDS cannot alone

produce sufficient ions to sustain the plasma. This conclusion

eme (5-8)

is in agreement with earlier v:or;:

5.3.2 ... rp''TaD 77177- Y DIST2 7=1?? AT Ti.. r?12 ,.0 r0J"DARY

Sir.:L, one electro:: leaving the target only multiplies to about 1.48 electrons at the CDS/':;G boundary it follows that at least 50(1., o t the electrons leaving the target undergo no ionising collision in the CDS an1 their detection as high energy electrons in energy analysis is eot so surprising as was at first thought.'

We can obtain furthe ^ information on how many electrons undergo

no iori.. e J -I -ion by allowing Q1 to vary (Pig. 5.6) and using an iterative technique to chart the progress of the electrons

across the ';DS,

112

C=nsider our operating parameters 3' pressure and

targetpotential of -1kV when ~i' r~ external i•~ ~-gnet is field

the CDS length is 2cm. if the CCD S region is split into ten

equal segments each of length ā s = 0.2cm then the necessary C

c ndi. Bio-_ O ei > .rill be satisfied and we can obtain an

estimate of the electron multiplication by assuming that all

of the ionisation within a segment occurs at its .,:id point

which can be givee a characteristic energy using Fig. 5.5.

Using the CDS potential distribution of Pig. (5.5) we can

assign an average electron en ergy of 97.5eV to the first seg-

ment and obtain a value ofQ. = 13 from Fig. (5.6). Then this

value is inserted into equation (5.6) we predict 0.039 ionising .

collisions in the first segment. Therefore, 0.039 ions and new

electrons are created in this ee;mont by one electron leaving

the target surface and 1.039 electrons enter the :second segment

of the CDS.

Since electrons will, in fact, cause.ionising collisions

anywhere in the first segment with energies in the range from

the ionisation threshold to 190eV we have over estimated the

number of icnisations since the representative energy chosen

(97.5eV) happens to co •respond. to the mo:,t efficient energy

for electrons ionising argon atoms. The prediction of only

3.9% of the electrons causing ionisation in this first segment"

is, therefore, high but illustrates the point that this type

of collision is a relatively rare event close to the target.

Leaving the first segment we hive 0.961 electrons of

energy 190eV and 0.078 electrons whose energy is uncertain

since we would require precise information concerning the col-

lision. If we now repeat the calculation for the second segment

there are 0.961 electrons of representative energy 278eV giving

Qi =9 and producing 0.026 new electrons together with 0.078

electrons of unknown energy. Even if we deliberately over-

.estimate the contribution from these. 0.078 electrons by assign-

ing the;:: the maximum possible Qi value of 13 _they will still

on'y produce 0.003 new electrons which is about 105 of the '

electrons formed in this segment. Even in the ioost optimistic

113

caee the breeding effect 'is, therefore, relatively Unimportant in this region of the disellarge a:.1 the choLce of energy for

electrons affected by a collision is not critical, especially

as Q. is not a rapidly varyieg funtion of energy.

To simplify the caecdiation, let us aeoeme that any col-

lision affected electron has zero energy after the collision and then gains energy ae it accelerates in the electric field of the CDS. • These 0.076 electron:, therefore, started with

zero energy at the mid point of 'the first segment corres;onding.

to an energy of 97.5eV and should be assigned an energy of

160.5eV(278-97.5)and Q.=11 at the mid point in the second a

segment. They, therefore, create 0.0026 extra electrons which is 10% of the electrons created in the cegalent.

Three electron energy groups need to be carried into the third segment:- (a) those.electrons ehich have not yet collided. (b) Those formed in the first segelent and have not collided

since. (c) Those formed in the second segment. The calculation

becomes progressively more involved until by the end of the

tenth segment we have eleven groups of electron energies, the

relative abundance of which is shows in the table below.

Electrons Created Pr One Electron Leavine The Target

Energy Number of (eV) Electrons

1000 0.835

903 0.066

723 0.049

563 0.040

423 0.038

303 ' 0.032

203 0.032

123 0.031

63 0.031

23 0.034

3 0.036

114

The following points should be noted:-

(a) The main conclusion from these results is that about

80% of the electrons le•ivi the target cross the CDS without _.c;iY g an ionising collision and t1a; the number of electrons 2

with the maximum possible energy is far greater than that at

any other energy.

The excitational collision probability has been ex.4.mined

by Zapesochnyi and Feltsan(16) who found an energy/collision

probability relationship of similar shape to the ionisation

results of Smith(9) but with a maximum at about 15eV. This

explains why the CDS region is relatively dark since in a high

electric field area, 15ev and :maximum excitation is soon

achieved and exceeded by the accelerating electrons. Since it

is usually assumed(S) that the cross section for excitation by

electron collision . s significant only for e.iergie s below 30eV

it follows that excitation in the CDS will be even less sig-

nificant than ionisation.

Given that elastic collision will not change electron

energy, it follows th.t a large percentage of electrons released from the target will reach the C';S edge with the full

.inter-electrode drop of potential.

The ener:;y distribution at the CDS/NG edge therefore shows a large number of high energy electrons and a much smaller

almost constant number of electrons at all the other lower

energies. Since the relative nu:_.hers of these lower energy

electron_, are so small it is doubtful whether the variation in

their number is significant and the only firm conclusion we should draw is that the highest possible electron energy value

is by far the most heavily populated. .

(b) Allowing Qi to vary predicts 1.22 electrons at the

CDS/NG edge for every electron leaving the target once again

suggesting that ion production in the CDS cannot alone sustain

the plasma.

(c) The poor comparison between this predicted energy

distribution and the energy analysis curves of Chapter Two

which shoved most electrons with low energy and a few at the

115

maximum possible energy is not surprising since they refer to

different regions of the discharge. The above analysis refers

to the cps/rG bound'ary whilst the experimental analyser results are taken well into the G.

In the nearly field free rG region, any new electrons

produced by ionising collisions will heve no opportunity to

increase their energy significantly since the electric field

streegth is low. These electrons will, therefore, retain low

energy which faveUrs further ionising aad excitational col-

lisions and the breeding effect will be lore significant. This NG region should be an area of colour, as is observed by looking at tiie dischare, and ion production. We would, therefore, expect an increase in the number of low energy electrons and a reduction in the percentage of high energy electrons detected as we move through the NG region away from the target. The experimental rerealts of Chapter Two chow the extent of this restructuring of the distribution. The change is admittedly large and no close correlation could be claimed between the

theoretical CDS/NG boundary distribution and earlier experi-mental results though some broad agreement is evident.

(d) The number of segments chosen for the prediction of

the number of ions created in the CD3 is not an important

factor. A simpler model using only r'our segments predicts 1.4 electrons arri7ieg at the COSPIG boundery for every one leaving the target as compared with 1.22 using ten segments. This is a

result of the breeding effect not being very important in the nsa-Aalsolererelat-ivecelleteeicyof Q. over the energy 1 range 20-1000eV. This seems to suggest that we might obtain a

satisfactorVelectrollelleriD'clistributiollbykeeping Q.con-stant; the great advantage of this being that the analysis can now be extended into the rG so that the theoretical distri-bution produced could be compared directly with the experimental

-results of Chapter Two.

116

Qi

S}inoe, the nw-.ber of el€trons n at a position in the dis-

oha_ ;e a distance :c from the target is given by:-

n _ `ioix x

We can use this e1:pre:_ si,_. to see ho.: the number of elec-

trons increases a :ross the CDS. Assu:::ing Qi = 6.5 and p = 3` 1

the results are .,hov.n in Table 5.1 where S n x refers to the

number of electrons formed in the segen t of the ddisch tr;e and E shows the enervy which the newly for.::ed electrons would

possess assuming they 7ere13rmed in the centre of the segment

and reached the CDS/::G boundary witho'wt further collision.

Table 5.1 .'ultinli c:.ti.en of Electrons Across the CDS

X(cm) nx Snx Ex(eV)

0 1 1000 0.2 1.035 0.035 903

0.4 1.081 0.046 723

0.6 1.124 0.043 563

0.8 1.170 0.046 423

1.0 1.216 0.046 303

1.2 1.264 0.048 203

1.4 1.314 0.05 123

1.6 1.365 0.051 63

1.8 1.420 0.055 23

2.0 1.477 0.057 3

This analysis can be extended well into the VG (to

x = 10cm say) but unfortunately we cannot assign a value to Ey

since at the moment we have a r;ode1 where all the inter-

electrode potential is dropped across the CDS which would

require E = 0 in the G. S:..ce the .:G is in fact only relatively field free( 2) we should assign a ::.call potential to

each segnent in the ."G. Table .2 shows the result where we assume Q. = 6.5 and the proule:.. of the value of Ex has been i ignored for the r.•,oment.

117

T -11 "';.2 l atir. Electrens Acro: the 7G

7(=-.)

3 1.795 0.07

4 2.192 0.134

5 2.651 0.103

6 3.222 0.126

7 3.916 0.153

8 4.759 0.186

9 5.784 0.225

10 7.029 0.274

In table (5.2) Sn refers to the number of electrons

created in a segL.lent 0.2cm long centred on x. Table (5.2)

shows the number of electrons formed in each seg.:.ent of the

discharge; we require the number which would actually reach

an energy anabs(ar situated in the G. If L iz the mean free

path for an ionising colltsionand Xi is the distace which

an electron has to travel to reach an energy analyser in the

NG, then the probability o arrival without further collision

is :71.ven by exp(-2:1/1), and the bn values in tables (5.1) and

(5.2) should be multiplied by this factor. This correction

will tend to discriminate against the high energy electrons

since they have the furthest distance to travel. From table

(5.1) we expect to create 0.035 electrons in the first 0.2cm

so for every electron leavig the target we expect 0.965 to

trave:L 0.2cm without colliflion. The probability of arrival

(exo -X1/L) is therefore equal to0.965 and putting X1 = 0.2cm

allows us to calculate the mean free path L as being aoprox-

imately 5 cehtimetres. Table (5.3) shows the reconstructed

energy distribution in the case when the analyser is 10cm

from the target (8cm into the NG).

118

T7Ible r.3 oner?, the : rrr,-Y Ar ,: °er

Z(co) X (c.-) .Snx

exp(- il )

' ~~ (' v ) x

0.0 10.0 0.135 1000

0.2 9.8 0.005 903

0.4 9.6 0.007 . 723

0.6 9.4 0.007 563

0.8 9.2 0.0073 423

1.0 9.0 0.0076 •303

1.2 8.8 0.0083 203

1.4 8.6 0.0089 123

1.6 8.4 0.0095 63

1.8 8.2 0.011 23

2.0 8.0 0.012 3

3.0 7.0 0.017

4.0 6.0 0.025

5.0 5.0 0.038

6.0 4.0 0.057

7.0 3.0 0.084

8.0 2.0 0.125

9.0 1.0 0.184

10.0 0.0 0.274

In order to plot this distribution we must m: Ike an

assum?tion concerning the value of Ex in the NG when

2 < x < 10. Let us assume a constant pote.,tial gradient in

the NG of 10 volt cm-1 which would give 1000 volt across 2cm

of CDS and 80 volts across 8cm of NG. This is in line with

the idea thi.t the Z'G is essentially field free but it must be

stressed that the estimate is purely convenient, though hope-

fully realistic, to enable the energy characteristic to be

plotted. The theoretical distribution is shown in Fig. 5.8.

C: r Distribtion

—~--;— - -

the G

119

27

21

15

9

T — — — — - a > 2 0 50 00 1080

ENERGY (ev)

Two important points should be noted concerning Fig. 5.8.

(i) The rate at which the predicted number falls off from

its low energy value is determil:Ol i ari:eiy by the assumption of

a potential gradient of 10 volt cm-1 in the ';G. An assusi:ption

of say 5 volt cm-1

would produce a sharer decline and the only

firm conclusion we should draw is that there would be a large

low energy signal from the analyser.

(ii) Fig. 5.3 is a. plot of the energy distribution of the

electrons created in the discharge together with the predicted

signal for the electrons which were not involved in an ionising

collision. The electrons which collided with gas atoms to

cause the ionisation are nos, at the i:.omen`:, included. Consider

again Table 5.1 where for x = 0.2cm, .. nx = 0.035 electrons

created. We should also i:.c1u.e a further 0.035 electrons which

must have collided and lost energy in the ionisation. If we

120

made thesimplifyinp. assumption that these electrons had their

energy reduced to zero in collidiag wi'Gh heavy gas atoms we

should then double all of the S.nx values producing a theor-

etical distribution e.ith an even larger lo'.': energy contribution.

Alternatively, we ceuld assue.e thet the colliding electron loses only the energy equivient to the ionisation poteatiul of

argon (approx 15eV). In this case we would expect a slightly greater high energy sijaal but most of the additional electrons

would still be low energy having been created in the NG and not ,

increased their energy significantly before themselves colliding

to create further ionisation (nx=1.48 at x=2cm rising to nx=7.03 at x=10cm).

221t5 ) The following points should be noted:- (a) Most electrons hitting an analyser situated in the NG

themselves ccice from the 7'7G and are, therefore, of low energy.

Since the excitation anl ionisation collisional probabilities are similar functions of electron energy(9)(16), we might expect that the NG would be the main region for the production of ions and electron:, since observing the discharge shows that the NG

is the main colour area where most excitation is occurring. (b) Comparison between Fig. 5.8 and the experimental

results of Chapter Two shows some agreeieent in th - t both curves

exhibit the maximum signal at low energy with a euhsidâå'y high energy peak corresponding to electrons v.hich heve traversed the discharge without collision. The main differences are that the theoretical curve (Fig. 5.8) falls off more steeply at low

'energies and shows a larger, better defined, signal at the high

ene= peak. ' Discrepancies are to be expected in view of the

simplifying assumptions made in the theory and our uncertainty about the experimental results at low energy when the energy analeeser acceptance bandwidth (SE) becomes very small.

(c) The fact that the theoretical energy distribution is

similar to our experimental results 'from Chapter Two suggests •

that the assuleptions msde in the derivation are reasonably well justified. Attempts at an exact carve fit were not thought

121

to be worthwhile, however, since a very much simrilified model

has been used. ePore •su,.'ther pro6-ess could be made, a

vriable colliLion cross section Q. would be required and a

means found or ihcludih:2; excitation of. atoms in, or due to,

the nezative This Intecfacto-?- would -L-_erease the num-

bur of low eery electro,..s r.h1h ;oul'I be consitc.:t with the

experimental results which. showa large low encry

Furthermore, ae would require infor:Lation oonerning the

snali pote:.tial.op in the :1% (d)- Whilst eomprison with experimental energy analysis

res-.11ts has been considered at some length, the main conclusion

as fir as the rest of the work in this chapter is concerned is

that i'nisatio I. the CDS anot alone sustain the discherge.

This conclusion largely depends on the accuracy of the O. '1 values as oresent0 by Smith(9); fortunately we can obtain an

check by considerin a 21:htly different collision

model.

7=T7IC;Tin 700EL

Consider now a similar altern2tive approach by looking at

the increase in the iux of electrons (number/area sec) as they

move :,eross the CD.

0 TTIN o lectron ovin Across thr, CPS

I I I I I I I I I

11,2Ve I I I I

I I

X X+SX

• X =0 TARGET

x=cl CDS/NG EDGE

122

If ne any Vo are the e'_cctr. r. density oili velocity at

_` amiel er _ni t :c a the flux o.. electrons moving across the

di.._'iir„e at that point (the nue.ber crossing unit area per

second) will be "z.en r ii V • As the electrons accelerate e c

aero—, the di,..her,ee, then .in. the absence of colli:..:._.- ,flux.

(n rt)remaine uY:cr'iar'e ed. Collision e, however, can lead to

flux generation as discussed below.

1-len the following simplifying assumptions are made:

(a) No density gradient in the g-s of density n0.

(b) The electrons are available to produce Ionisation

whatever their energy, i.e. ignore the small threshold energy.

(c) The cross-section for ionisation (Q ) is not a func-

tion of electron energy. In fact the cross-cect'.on ices vary

with energy but in cor:'tnna t to within a factor oftiYo(17) for

our experimental rare of .0-1kV.

Consider the electron flux api:rc :cii'.. a section of the

CDS of thickness S x.

Flux out = Flux in + Flux generated in S x.

(neVe)x + c x = (neVe)x + nOQ.Sx(neVe)

i.e. n0Qi S x(nee) represents the extra flux created

since , by definition, n0gi S x represents the fraction of the flux colliding in the element.

(neVe)x + dd(neVe)

S x = (neVe )x + n0QiS x (neVe).

and since we have set ai constant it will be possible to form

and solve an integral:-

d/dx(neVe)

(neVe)._

ilog (neVe I P 7

n00.

d

0 n0Q.dx

where d represents the CDS length and is the limiting value •

of x.

(ne'Te)d = (neVe)0 exp(n0Qid) (5.8)

This expression for the flux multiplication across the dis-

charge has a tiimi.ar form as equation (5.7) for the increase in

the actual number of electrons across the discharge.

Put tin- in soa' typical figures of n0 = 1.06 x 1021

m_323

(%d ;'re.".....re) d = 2x10-2m 3n•j ~• = 3x10

-20m2

as given by

I.7aseey and _urho:~( 17) which shows Q 'to be with_., a~ factor of Q.

two of 3x10 2' m2 for electron energies between 40 and 800e7;

we then find the flux nulti:e i c''t icn to be 1.89.

Whilst this figure is high enough to ;'un gest that a

significant number of electrons did actually undergo an iori ting

collision, it in not high enous-h to provide a self sustaining

plasma bearing in mind a X value of about 0.3 electrons/

incident ion.

5.3.7 ?'C:.0"IO'"`. 0' I0': P':0T`;7CTIOT

The rain conclusion to be drawn is that the two models

presented both lead to the name result th::t the CDS does not

seem to be able to produce sufficient ions on its own to main-

tain the ditch_.rge and the ;;G is needed as an extra source of

ions whether they be created by electrons or photons.

Whilst conditions for producing ionisation in the NG are

favourable since any new electron formed is not immediately

accelerated to energies in excess of the most favourable energy,

it should be remembered that any ion formed has a reduced chance

of ever reaching the t«r?.et since it will 'drift' randomly in

the relatively field free ':'t region. In the CDS, conditions are'

contrasting in thet whilst ion production is not favoured, the

electric field ensures that any ion produced has a high probab-

ility of reaching the target. (Since charge exchange collisions

would still produce an ion reaching the target).

5.4 '1 - °.7:)r/CU: E..,

Since it ht's been shown that the 'switch' in the pressure/

current ch"racteris tic of Fig. 5.1 is associated with the ratio

of y nx :d and that the CDS is apparently incapable of sus-

tair!in . the discharge alone, we can speculate further on the

reasons for this switching behaviour.

Consider the plasma in a state where the pressure is suf-

ficiently hieh that )1 :14:X> d (say 30/1 ) and ion production is

predominantly in the NG.. As the pressure. i s reduced, d increases

whilst V ,,AX remains unch'.nged provided y ; AX > d. The equality

124

= d which eerresponds to the 'switch' in eurreet (Fig. 5.1)

veuld, therefore, be achieved evehtually. We now consider the

mee-Aaniszis operatin'e ih the :.itching re( ion.

When the i:reseure is reduced, fewer ieniein.1 and exciting

collisiens wi21 be nede and the dih-charge might be expeoted to

extinguish. However, with the electron trajectory wholly in

the CDS region moct ions created will reach the tar:et, in con-

trast to the situatien in the ITG region where m -eny ions will

be lost. Thus we nove into a 'different regime in which fewer

ions are created but the majoritj will be able to produce

further electron ciuiscion from the target. Clearly the most

effeCtive situation will occur when maximum height of the

trajectory (1/.. ,) is just inside the CDS since an ion formed /

in this regien is subject to the full accelerating potential

before hitting the cathode and therefore, produce the

maximum possible number of new electrons. In addition, any

new electrons generated near the CDS/NG boundary are in .a

relatively weak field and, therefore, remain at relatively low

energies, increasing their ionising efficiency in collisions.

with gas atoms.

In the magnetron configuration, the magnetic field acts

to confine these electrons to the CDS further increasing the

overall ionisin:7 efficiency. Thus we argue that, when y AX passes from being greater than to less than the CDS length the

plasma changes from being etaihtaihed mainly by copious ion

pro.iuction in the 71G region to a relatively osriller ion pro-duction almost entirely in the CDS region -which is nevertheless

capable of maintaining the discharge. in view of this change

in mechanism, it is not surprising that there is an abrupt

change in the plasna ourrent.

5.5 7.7A777Tir.! 7=7) VARTVPI07S

As the magnetic field strength is increased it is observed

eveat.bothy.arid d decrease and we might expect greater

difficulty in observing a characteristic switch aeseei.,,.ted with

d moving through y 77 since hoth parmeters are changing. A series of experieLentAl curves of target current against

nar;netic field streegth at constant voltage of 1000 volts are

given in Figs. 5.10 and 5,11.

125

r. .10 Ivt 71.-Te'd 7 f-.1r D4fferent'Precs.Ares

I (ma) A

100 -

p.2u

P=201A

50 -

P-18/u

40 65 90 B (Tx10-"I)

1 26

Pi. 5.11 a=ent T v ielY! 7 DiffPrent Prer:cures

10

P 19/A

P

P =19.1

P =13,A P=10ju. ).

B Tx10-4

30

60

127

The most striking portion of these .=agnetic field charac-

teristics, which are different eren t shape at different pressures,

is that there is, in Pie. 5.11, a region ,.here increasing the

magn.etic field ; tre' t lowers the discharge current. This

rest is at varinnce with the u._u al ideas on th• .-agnetic

{ield role which state that as the field strength _s increased, f

electron trapping is __. . .roved and increased ionisation results

in a larger Cur e t 112s can ~e explained ed 1% the following

r Since y :„Al. = ~'' for y .., d we c3:. cal-

culate ~/ Y. for different values of B and compare the results• with the e_Yneri cn aI values of d. If thi is done for 35

pressure (well above the observed switching pressure) and

1000 volts across the dischrge, the following results e.orge.

B(Tx10-4) d(om)

41 0.80 2.32 3.53 68 0.57 1.7 2.98 89 0.48 1.3 2.7 110 0.40 1:05 2.6

Unfortunately higher magnetic field strengths were not

attainable with the Helmholtz coils available, although in any

case the s:i.all d values which would then be observed would be

difficult to measure accurately. It would seem that y /d

is tending to unity only very slowly implying that yL. TAX < d might not be achievable at 35/ul. The electron trajectories

are, therefore, into the NG which is the main region of ion

production. This is in agreement with the experimental obser-

vation that at 35/43. and 1000 volts, a typical plasma

in our system is easily self sustaining and strengthens as the

magnetic field strength increases.

way.

126

A similar a :ly.:i:- at 18,U pre"'su:e, 1000 volts, in the

unstable region of t::e plasm?. reveals the_ following result s .

B(Tx10-4) d(cn) Y,.,,x(cra) YL: Axid 41 2.51 2.82 1.12

68 2.35 1.7 0.72

89 2.10 1.3 0.62

Whilst it should be reuemt e.!'ed th t the latter tw ' values

of y MAX are not valid since once d > y'~nX we are no longer able to use y _ 1

2mV/e , these figures show that at

MAX 13 18" pressure and 1000 volts it is possible for y to

approach d. We can mase an improved attempt at calculating

y "AX by using the more general expression:-

y MAX

= [2m(Vt_V.0!)]

Since V 7 0, the value f or y MAX when ~/ MAX d will be smaller than the above results suggest and can be found

using:-

V -V = 2Vt (d

y:. x y:, AX 2/ ) t Yr d2 2

( 5.4)

Rearranging equation (5.3) then gives:-

4mVd MAX• , r. B2ed2+2rV

The recalculated table of values then becomes for n=18,M:-

B(Tx10-4) d(cm) y;;,AX( cm) slfLIAX/d

41 2.51 2.82 1.12

68 2.35 1.45 0.62

89 2.1 1.03 0.49

Thus at 18AA pressure, the magnetic field change can pull

y ,,AX inside d. yL?AY then becomes inversely proportional to B so that further increase of.B moves the electron trajectory

rapidly closer to the target within the CDS.

(5.3)

129

If we supose thet = d represents the optimum con-

4 tion for sustaining the discharge when the trajectory does

not go through the negative glow region, the reduction of

current as B is increased, beyond the value for which

y = d, can be understood.

At pressures less than 1,4, the discharge is weak ren- .

dering accurate estimten of d difficult because of poor

definition of' the CDS/N1 edge. The results, though admittedly

inaccurate, are consistent with the model described above and • correspond to the situation for which y_ . d for all

(:,AL pressures. The maxima in the curves for 13, 14 and 1tVA

correspond to the y,„ = d condition in each case. As the

pressure is reduced, d increases and the maximum will move to

a lower value of 13 as shown in Pig. 5.11.

5.6 GEAT, CT7CLUS707S

It is seen that our Liagc.etren assisted discharrre is

characterised by three regions.

(a) A high pressure region where ion pro- duction

M ) d ape: - , AX duction is predominantly in the NG. As the magnetic field

strength is increased )'X and d both decrease but the

situation y

= d is not easily attainable. Furthermore, mAX

increased magnetic fields introduce stronger electron trapping

in the relatively electric field free NG region and the disL

charge current increases. Since the discharge is continually

strengthening as the magnetic field increases, pressure

reductions ought to be possible at high field strengths if

this was thought to be advantageous.

(b) A transition region where y mAx z d and the discharge is unstable and likely to switch from y ) d toy Lux d

as a way of improving ion production. (c) A low pressure region where the discharge appears to

consist primarily of a CDS and the rG is not very pronounced, This would net normally represent a significant sputtering

ree.ion since the discharge is very weak and is characterised

by an optimum magnetic field such that ion production is maxi-mised by having the electron trajectories just inside the

CDS/NG boundary.

130

cc our e this 'transition' ln• pressure is in the

l~::il.. _.. r, r ~,~. te... isis tr .. iti~

region of 1 ~

, :a„:etIe^ systems Cry.. be operated successfully Llf

at low pressures as demonstrated by commercially available

P? r-er Yagetrcn an_ etter in systems. Both of these devices

use much hig er non u_ if - magnetic fields created by permanent

magnets. It seems likely that, in a higer mai•=e'tic -field,

electron trapping is s efficient that the plasma can adopt a

mode in which ion production is satisfactory even at low pres-

sure. This is supportei by the work. of Thornton~ 18) who used •

much higher :, ag _etic field s tre:.g ths of order several hundred

x 1 - ~0 'T.

In the next chapter we consider some experimental results

aimed at 1rO'iuoing an efficient s uttering system. Bearing in

mind the different regions of the discharge which appear to

exist, pressure values must be cro::e.. ..uch thatoperatir_g para-

meters are in the high re ere region and the discharge is

drawing large currents. Under these conditions, a high magnetic

Field strength will produce the intense plasma required.

131

SIX - •• •

.1 C-7.=7.77 7.-2A77e7.7aS

As diseesed 5n Chapter Three, it is useful to consider the

scutteeing e-ecess in terms of a reiusel rate defined as Depo-

sition Rate at the Substrate/Power Input to the System. since

high power inputs will dueage a heat senSitive substrate, it

is important to apply the power as efficiently as possible by

maximising this so-called reduced rate. A series of experiments was, therefore, initiated to try to

find the optimum operating parameters for the system, the 'main

aim being to check that the anticipated voltage optimum choice does exist whilst there is nc equivalent conditicn Par the current parameter.

G.1.1 OT- VG VO -YE

A series of exeerients was carried out at constant pressure to determine the variation of deposition rate with applied vol-tage, the current being kept constant by varying the applied

external magnetic field. Here we present some results taken

with the water cooled cylinder; a similar analysis was also

underta!:enn with the so called 'modified' cylinder.

The deposition rate was measured by interferometric tech-

niques using the Varian interferometer. Since this method is -

time consuming because the vacuum system needs to be opened

up after every experimental run to facilitate the removal of the test slide s some measurements were taken using a quartz

crystal rate monitor. Crystal oscillators are not usually used in sputtering because the plasma interferes with the oscillating crystal and the results cannot be trusted. This problem was found to be predomieent, results over different trials proving totally unreliable, an -I causing the technique to be rejected in

favour of the much slower interferometer method which gave reprodueiLility to better than 2%.

The graph of deposition rate against power input at con-

stant current and pressure is shown in Fig. 6.1.

9000 0 _I _RATE A HR

6000- I= 20mA

P=30)14

3000-

132

(") 1 De7)csition Rate 1 Power Suv::7 ier! at Constant Current

10 20 30 40 POWER (WATTS) - — I— — — I— — - -- --I —

500 1000 1500 2000 VOLTS.

This il,formation car. be replotted to show reduced rate

(deposition rate watts-1

) against voltage.

. 6.2 Reduced Rate v Volt9:e at Coitant Current

REDUCED RATE 1=20 mA

P = 30)A

340

220

60 500 1000 1500 2000 VOLTS

133

As exne.-..ted, folloveing the discussions on sputtering yield

and appliel voltage in.Chapter Three, an opti:nue, voltage does

exist and whilst deposition rate could be increased further by

raising the applied voltage, the reduced rate falls es the per-

centa.7e wa-.1, -te of input power increases. The voltage ohoice is

not, however, too critical providing it in below say .1400 volts,

the optifue volt approxiffitely 8C0 volts.

A eiIL,tiar curve shape was obtained uf:ing different constant

currents e.fel pressures and was also noted for both tynes of

copper target.

6.1.2 (:177'17:7(1 CTJR17.71'

A sifIviLr series of results was taken et constant pressure

and voltae to find the:relationship between (19.o:A.tio:i rate and

::Lpplied current, the current being changed at eonstant voltage

by regulating the magnetic field strength. The results are

surcnaried in Figs.' 6.3 and 6.4.

Fir. 5.3 lieeesition Rate v PowerSunnlied at Constan/ Volta=

A DEPOSITION RATE 6000 - 0 -1 A HR

3000 P =30jA -1200 VOLTS

12 24 36 • 48 60 - - -1 - - -I - - 1- POWER (WATTS) - - 1- ›- - f --

10 20 30 40 50 CUR R ENT (mA)

REDUCED RATE A HR-I W-

134 ir. 6.A .e . C...2rent at Constant ';cL-,a e

10 20 30 • 40

CURRENT (mA l

As the current is increased, more ions per seconJ are

striking the target and we should, therefore, expect the

deposition rate to rise. Since the voltage distribution of

the increased current flux is un1hsnr-eol, the sputtering yield

(atoms/incident ion) should remain constant and doubling the

current should double the amount of material ejected per unit

time from the target and, therefore, double the deposition

rate. However, at constant voltage, doubling the current also

doubles the power input and the reduced rate should, therefore,

remain unchanged as shown in Fig. 6.4.

Two further points merit cnsideration -.

(a) The initial rise in reiu.ed rate (Pig. 6.4) is attributed mainly to the fact th .t as the deposition rate

increases, the ratio of copper atoms to argon atoms increases

ire the space between the electrodes (i.e. the partial pressure

of copper rises from its initial value of zero). The incident

ion flux therefore, contains an increasing number of copper

ions.

100

60

135

This is easily observei visibly as the plasma colour becomes

an intense green shade iniioatiue of copper at higher power input ]evels sho:.ing improved copper excitation. and also ioni-

sation since tne two prcce:ses retuire similar conditions.

Since utteri;; is a momntam exchnge process, the ion with

the highest sputtering yield is that of the sa::.e species as the

target material. An increased copper ionic flux to the target

should, therefore, improve not only the deposition rate but

also the reduced rate.

(b) The above discussions assume that the sticizing coef-

ficient for sputtered atoms is not a function of substrate

temperature. In fy,ct as the power input is increased, substrate

temperature will also iserease and the reduced rate would

decrease if all the other operating parameters were kept.con-

stant. As discussed briefly in Chapter One, this effect should

not be too serious provided the substrate temperature is not

allowed to rice too high.

Fortunately from our point of view these two fe.ctors of

substrate temperature and a copper ion contribution are either

small and/or cancelling each other out and Fiats. 6.3 and 6.4

sugest that we are justified in assuming that deposition rate

is directly proportional to current at constant voltage and

reduced rate is constant.

Since Fig. 6.3 shows discharge current directly proportional

to deposition rate, we could, perhaps, sensibly take discharge

current at constant voltage as a r_Aigh measure of deposition rate and eliminate the need to open up the vacuum system for

interferometric measurements. ',lie should not, however, expect too great an accuracy from this assumntion sin.;e it has been

pointed out by Jackson ' that stulies of contamination and

damage effects cast doubts on the validity of our assumption

that stutterins yield is not a function of bombarding ion cur-

rent density. In a later section on substrate heating it was

found to be possible to use Fig. 6.1 and 6.2 as calibration

curves to calculate the .t7pected deposition rate under different

operating conditions but the results are only accurate to within

about 15%.

136

6.1.' 07)7T77- 7 '7:: -=s70rs

The experimeetal fe.:ol.,;s show an eeti= eperating voltage

of areend .T.00 volt bet no oetimum cureent eli can be explained by assuming the: seuttereng yield -1 a funetion of voltage but not current. Hie rote efetent seutterhg iu, therefere, most likely to be achieved with a high current low voltage device though voltage choice is not too critical providing 1400 volts is not exceeded. This eonolueion is similar to those reached with the Sputtergun and. Planar Mageetron whioh

operate in the 600-700 volt region.

Similar conclusions were reached with the modified cylinder

though the reduced rate figures are slightly different because

the sputtered material is ejected in a different distribution Profile from the new target.

The limiting fee.tor on deposition rate in our systems seems

to be the magnitude of the magnetie field achievable with the

Helmholtz coil arrangement. A rough comparisch at 700 volts

across the discharge between the planar magnetron and our system shows the planar magnetron using a magnetic field

strength of 3 x 10-2T aria a resulting power density of 6 Wera-2

to produce a deposition rate for copier of 10,000A min-1 -2 whilst our system draws 2 Wcm-2 at 1 x 10 -T to produce

1500A min . The production of higher deposition rates at

the optimum voltage would require either 'heavier' Helmholtz

coils tapable of carrying a larger current or the use of per-

manent magnets fixed in close proximity to the targot asseMbly.

6.2 rr7E2AT. r:H=TEi:T.S7TC3

Having eetablished that high rate deposition is possible,

it is necessary to look at the consequential substrate heating

to ensure that unacceptably high temperatures are not being

produced by virtue of the power input becoming unreasonably large. The general characteristics of the water cooled cylin-

drical target Je.ere, therefore,. investigated as an introduction

to the heating studies.

137

6.2.1 C'77.272:T/70ITAGF 7=707S1=

The relatihip beteel. current an l volte at constant

!:Lan:netic field str.,L7th and pressure is shown in Fig. 6.5; the

pressure of "vAA tc ';:ep well clear of the dis-

continuiV licc-,.;:ss.1 in Chater Pi7e.

at Const-,.t :arnaetic P4 c11 and Pressure

300

200

CURRENT (mA) B=9x10

3 T

ß=4x10-1T

30),A

100

B=0

1000 2000 3000 VOLTAGE(VOLTS)

Comparison between the magnetron case (B = 9 x 10-'T) and

the non magnetron (B = 0) is difficult since they operate in

totally different ways. When the power input is increased the

non magnetron voltage rises stead ly as the impedance changes

from 51JV to 33 Awhilst the magnetron tal:es in more current

at almost constant voltage and its impedance falls from 7J\. to

2.A. The impedances are, therefore, an order of magnitude

different as indicatel by the different gradients of the curves

of Fig. 6.5.

A.rNA) V(VOLTS)

2401- 2400

120 -- 1200

CURRENT

P=.3p1

VOLTAGE

139

Current and vo2t-1.,-,0 were investigated as a function of

aplted T'a:7netic fie2. 1, the power sunly output setting remaining ur.chen]:ed throut the ex7-:criment. The resalts are shown in 6.6.

7i2-. 6.6 Currcnt anl Volta::e Af.ainst 7ep::letic Field

20 40 60 80 MAGNETIC FIELD •

(Tx10-4.)

The fact that the output voltage reduces sharply as the current increases at higher ma'fnetic field strengths indicates

that the power su:2p1y regulation is not very effective. Since

the plasma impedace chan7es for differeat operating conditions,

we should expect the 'maxi:num power theorem' to apply and the

discharge to accept most power when its iLlpedance is matched to

the internal resistance of the supply.

Replotting the inforl.:ation on Fig. 6.6 shcws no maximum in

the power against magnetic fieli curve but most power being

drawn at .zero magnetic fl:ld. This suggests that the supply

resistance is never matched to the plasma and that the impedance

mism-tch gets worse as the magnetic field strength is illolneased resulting in less power being drawn. ,

139 The nower aga__n'et !naC: eti. field curve is, therefore,

largely governed :y the i._ pedaece os the power supply end care

is needed in interpreting t, e re lts. For exa• ,l o some .0 L_ or 1. „~_ ,: in orte

regimes of the discharge it is possible to increase the meee-

netic field streegth at cen , ta_nt power supply setting yet

reduce the deposition rate since the ch ,Ing plasma impedance

significantly v.oreens the mie match causing machh less power to be drawn by the discharge. •

For a fixed power supply setting, the following result is

obtained at 30/1 pressure.

?'agnetic Field Current Voltage Power Input Deposition Rate (T x 104) (mA) (volts) (watts) °A hr-1

0 60 2600 156 7,500

85 200 510 102 12,500

As exeec ted, the magnetron is showing the greater depo-

sition rate for lower power input. The power is being used

less efficiently in the non magnetron case and more has to be

dissipated as heat in some part of the system. It would be

more relevant to compare the two cases at the same power input

i.e. with the magnetron drawing 156 watts. This would lead to

it taking in more current at almost constant Vol ag e and the

deposition rate would rise even further.

As the systems are pushed to higher power levels, the

magnetron will draw a continually increasing current at almost

constant vo' tage and the deposition rate will rise; the non

magnetron will require an increasing voltage and the deposition

rite will increase only slightly since the sputtering yield

increases less the e-1 linearly with voltage increase. The mag-

netron eyste ;l is, therefore, iecreasir.giy advanta' 'eous as the

power in; ut is increased.

It is now neses5ary to examine the substrate temperatures

to determine where the power input is being distributed.

140

.3 s!..77!:7a,,,7,,: H7:-1

3.1 1.7=777.7;7. Seutterine. traditienally been vewed as a hot process

and many v.ores have studied the extent of the rero-e1F-- --.. The

heating revew in Chaeee- One dieeu-sed briefly -ueh ideas as

heat input limiting the depozition rate, stress being induced

into a film, delicate sebetrateS beine dameged and film growth

being affected by temperature; we nasi lose',: at the distribution

of the input power in sreater detail.

The sources of :ower input to the substrate in an R.F.

sputtering system (7:.aterials Receoreb Corporation multi-target

sputterins module) h:ve been studied by Lau(2). By using

thin film thermocouples at the substrate an: measurine .water

flow rates and inlet/outlet temperatures at the wetrr-cooled

target it was possible to anlyee where the input over was

being dissipated. The mein eonolusion was that approximately 55% of the input power ie dissipated at the target, 5% at the

substrate and 405 by other sources such as ejection of sput-

tered atoms and secondary electrons, ionisation end excitation

of the gas and he it loss in Lne R.F. matcning network. Using

a planar D.C. target which wee not water cooled, 135 of the aeplied power appeared at the substrate suggesting that

radiation from the uncooled target might be en important con-

tribution to 'substrate heating in the particular geometry used.

Considering solely the power at the substrate, 805 of this was

thought to be caused by electrons with the kinetic energy of

the Sputtered atoms contributing 15% and their latent heat of

condersatIon a further 55. Radiation either from the plasma

itself or from the water ee)oled target was not thought to be

significant. The power input to the system was surfi3iently

high to cause the relatively high substr.:Ite temperatures of

550 00; the greater the power input the greater the temperature

produced.

A similar type of anal,sie of the distribution of the

input power was attemeted by :Ill for a D.C. plasar system.

Perhaps not surprieingly since thL confl :uratioh is ii:ferent,

his results do not igree closely with the results of Leu(2).

141

Approately r•J:5 of theapplied power was attr5buted to the

target with the ee-cetr%te ta::in: Up the•ree,ainieg 4C5.

all of this elbetrate inpet is eroviied by electrons with the

kinetie energy of the eputterel species and their coedensation

energy gieing only 1'% depoeitien rate wae, however, very ei

low at a'eeut 70 oA zin and it was thou-let like' that this

centribution frore neutral eplc e would rise at higher depo-

sition rates. A further important at iL that the 7eltages

used were in the reeion 3-5:(71 where .the Chapter r'wo energy,

analysis curves of current (i.e. power) against electron energy

are suite sharp. It should be reeeembered that these curves

showed a relatively s,nali high energy signal being largely

recoonsible for the electron power input to the eutetrate

especially high voltages wheee the sharpness of the curves

increases. (7 Ileither Lau -' nor Ball(3) have attempt e. an analysis of

the newer distriHtion in a cylindrical arrangee,ont or, per-

haps more importantly, s a maenetron arrangeeent. Since both

seem to believe that electrene are largely responsible for the

power input to the su'eetrate and the electron traj cc tories are

significantly affected by the magactie field; an analysis in

the eageetrer eystem.seeme te be required and will be discussed

in the next section.

■.! A

' • r .!' r ," '11' T•Tp `ln 1,0.

)pp 0 a .

Thie cae be conveeieely subdivided into two sections,

namely the power to the substrete and the power to the target

and these will no be eoneiderel seperately.

6.L.1 '777 ;1,, TRT!

The eystee. ueed con5.ietei of the water cooled cylindrical

target wi-n a cepi,ee diee of 1.5em diameter suspended in

fro L ths eebetrefc to aet as a seeeer for the incoming

eeergy flux.. The use h-A a ehromel/alteeel thermocouple

attashci with ds; ti rain: to the side ehielc!edfrora the

inc4 1e: t flux eo Ul lt the eeeeoe temperature 'could be monitored,

the .er.ocourle wires nroviYi.eg the neceese-ry support to hold

the zeneor in position. Since the sensor has no cooling

142

facility, radiation iii be the principal heat loss mebhanism

end the thermocouple . 11 measure -the tenperature

T

”,,ich will be e.^_':a'„l b_ed after %a suitab_e time period. P

The main problem in readingd_r Te stray electrical pick-

up from the discharge. Whilst this was reduced by plactn% , a

capacitor across the dig tai vol t.._eter which monitored the thermocouple output, the effect could not beeliminated sh o:ing

that low frequency radiation was still being picked up. The

only satisfactory me thod was to allow the discharge to run for '

long enough to establish equilibrium (approx. 20 minutes) and

then to switch off the discharge and read the thermocouple

im5..ediate2y before cooling of the disc sensor could commence.

Freli:.;inary ex e: i_nents suggested, as expected, that the substrate heating (equilibriu::s te:_.perature) is largely con-

trolled by the applied power density and is directly propor-

tional to it.

I,et us assume that the power input to the target is Vt

watts and that a fraction K of this power arrives at the sub-

strate as incident energy flux. Whilst X will be constant

for a particular set of operating conditions, we might expect

it to change once those parameters are altered since the effic-iency of the plasma only have been affec ted.

For a target of length 1 and a target/sensor separation r, the total energy K?! will be distributed into a cylinder of area

2lTrl at the sensor ignoring the e':fect of the end of the

cylinder which is not covered with a cathode dark space shield.

The power density at the substrate, is, therefore, K',7/2'rTrl

watts m-2 and a copper disc sensor of radius X metres will receive (K';/2 Trrl)(1TX2) watts provided that X «2'R'r.

By Stefan's radiation law, the disc v.i.il lose energy according to its emi cci'. ity . e, the fourth power of its abso-

lute temperature Te4 and its surface area 21TX2 (since it

radiates from both faces). If we assume that the conducting paint does not alter the radiating characteristics in the small

region where it is in contact with the disc sensor and that conductive heat loss along. the thermocouple wires can also

that

neglected, we can express re. s the power loss P as:-

143 Y P = e . 2 ITĪ.2d Te

'r1a r t."i

6 r _ '2:1-4

When e y:..._ iorlu.. is a..:_eve't, power losses and gains must

balance for the disc and we can write as a first approximation:-

KW. 2 = 2 'NX2 ā eT 4 2 lT rl e

Prevost's theory of heat exchanges states that when a

body is at a constant temperature, it is losing heat by

radiation and gaining it by absorption from the surroundings

at equal rates. Therefore, when the system is at a temperature

T1, before sputtering commences, the disc will radiate

e. 2 TrX26 T14 Watts and receive the same amount from its sur-

roundings. Once sputtering is in progress, we are measuring

the energy radiated correctly by noting Te but ought to include

in the power equati^n the 'background' input and write:-

K = 4lTrl 6 e(Te4 - T14) (6.1)

If we put in some typical values of 1 = 3 x 10-1m,

r = 2.5 x 10-2m (target/sensor separation),

6 = 5.67 x 10-8Wm-2K-4, Te = 41.5K, T1 = 298K, e ='0.1(4)

'yI = 121 watts we then calculate K = 0.097 implying that 9.7% of the input power is arriving at_ the substrate. This result

is broadly in line with earlier conc.Lusions(2)(3) for a

planar disc target configuration suggesting that the analysis used for the cylindrical target is valid.

The variation of K with change of operating conditions will be discused later.

6.4.2 DISTRThU O'r OP PO'..17.R T3 THE S fl3 TRATE

Having estbli.hed t:c_t about 105 of the input power is

being dissipate:. at the substrate, we net attempt to estimate the fraction lue to electron bombardment and that due to the

kinetic energy of the sputtered species and their condensation.,

Since the expression depends on the fourth power of the

absolute temperature, the correction for the input from the surroundings becomes insignificant at higher Te values when

T4» T 4. e 1

144 Consider thLe energy flux (energy/area sec) caused by

sputtered atoms impacting onto the substrate. .Sup: ose each

atom occupies a vorwse V when it is incorporated into the

growing film and that N atoms arrive per second. The increase

in atomic volume per second io , therefore, NV. If these atoms

were deposited onto an area A, the thickness increase would ce

(NV/Alms -I and this is the deposition rate D which can be

measured experimentally.

Since the energy delivered to the substrate. per second is

equal to the number of atoms arriving 'per second multiplied by

the energy of each arrival, we can write:-

Energy/sec = N(E + All) (6.2)

where E is the average energy of the impacting atoms and

DH is the latent heat of condensation. This expression

assumes a sticking coefficient of unity and neglects surface

mobility of adatoms. Since I) - N /A we can write:-

Energy/sec = DA (E + H)

and the energy flux (J) due to the atoms is given by:-

D(E + QH) (6.3) V

where J is the energy arriving/area sec.

The emission energies for atoms sputtered from a copper

target have been measured by Stuart and Wehner(5) who observed

that the energy distribution peaks at 2.9eV though some atoms

have energies in excess of 50eV yieldi n'; an average value of

9.25eV. The results are slightly different for different

sputtering gases and different crystallographic faces of the

target. As discu._sed in Chapter Four, however, Westwood(6)

has pointed out that arrival energies at the substrate will be

less than this quoted figure of 9.25eV and that in certain

circumstances we should put E 0 since the majority of atoms

will arrive with thermal eher ies due to collisions.

The heat of condensation .AH can be found in standard

tables(7), a typical value for copper being 3.2eV.

145

The atomic velume of an element is defined as being the

volume occupied by one mole of that ele-ent and is obtained by

divng the atomi& weight by the density of the solid. A rp)

typical figure for ceeeer' is 7.1em3

mole-1 though this is

based on the assumption that bulk density is being exhibited,

an approximation which is probably satisfactory providing the

film is a certain mini:em -thickness, say 200°A. The volume V

occupied by a sinele atom is then calculated to be 1.2 x 10-29

m3

Since the deposition rate D can be measure& experimentally, the energy flu:: J due to neutrals can be calculated using

equation (6.3) which shows, as might be .expected, that the

enerTef flux is directly proportional to the deposition rate.

If the power input to the target is known, we can use the

parameter K to evaluate the power reaching the substrate and

compare this to J to find how much of the power is- due to

neutrals and hence what fraction is caused by electrons. This evaluation will, however, need to be postponed until the

variation of K with the operating parameters has been considered.

6.4.3 P0WE7 TO T= TAT

Consider a water cooled target where the inlet and outlet temperatures T1 and T2 can be measured by inserting a thermometer

into both flow lines. If m is the mass of water flowing per

second measured by noting the time to collect a litre and using

the fact that the density of water is one gm cm-3 we can write:- Heat energy removed/sec = mc(T2 - T1) (6.4)

where c is the specific heat capacity of water. We can,

therefore, calculate the power removed as heat by the water

using equation (6.4) and compare it to the input power to the

system.

This experiment was carried out at 3C,)a pressure for a variety of magnetic field strengths including the B = 0 non magnetron configuration. In every case the result was that

approximately 80% of the input power was being dissipated as

heat at the target, no significant change becoming apparent

with change of operatin.para;:.eters. ';;hilst this figure is

higher than earlier results(2)(3) it is comparable and illus-

trates the point that sputtering is a 'het' inefficient process.'

146

Sinee so such power is appearing at the target, this would

seen to su;gest that 7,ater cooling is an important feature of a

system minimising substrate teee2erature since .raliation from an

uncooled target ni;;ht be e::petted to contribute significantly.

.5EX-72T-':-',J. R:.S=

A progra^me of experiments was carried out to see how the

sensor equilibrium te'noera tu: e Te and the narameter K (the

fraction of the input pou.er reachir: the substrate) varied with

operating co n itions. The effects of varying target/substrate

separation, magnetic field strength at :onst=ant voltage and,

voltage at constant magnetic field were studied. The pressure •

was kept constant at 30,1 in each case to eliminate one of the

possible variables.

Before each reading the magnetron system was run for

thirty minutes to allow the plasma to stabilise and to give the

sensor time to reach its Equilibrium temperature T e which was

then recorded. K was then calculated using the theory given

earlier.

In all, two hundred and forty experimental runs were con-

ducted split into forty series of six, each series being aimed

at varying one of the parameters whilst keeping the others

constant as far as possible. In all cases `f (the electron MAX trajectory maximum assuming no collisions) was calculated _for

comparison with the target/substrate separation.

6.5.1 DrpJSTfI07 PATE

Since we require the relative contributions to substrate

heating from electrons and neutrals, the deposition rate is

required so that the neutral energy flux can be calculated

using equation (6.3). is previously discussed, the accurate

measurement of film thicinness requires an interferometric

technique but the need to o, ee up the vacuum system considerably

lengthens the time for each experimental run and renders imprac-

tical a deposition rate measurement for two hundred and forty

specimens. A simpler method was required to give a quick,

reliable, estimate of the deposition rate.

147

Consider again Pigs. (6.1) and (6.3) of deposition rate

against voltage and eureeet. If we take as e reference from

Fig. (6.1) 500 volt equivalent to 3400 OAhr

we can une the

graph to calculate a ':zrk up' factor for any other voltage.

For example 1200 volt oro laces 7800 oAhr

1and a scale un of

2.28.

Using as an exaeple the experimental results, quoted

earlier, from a magnetron drawing 200 mA at 500 volt where the

deposition rate, measured ueing the interferometer, was 12,500

0Ahr-1 , - the following result emerges. Fig. (6.3) shows that

say 4OmA at 1200 volt gives 5600Ahr-1 so 200 mA at 1200 volt

will give 28,000 oAhr

-1 if we assume deposition rate is pro-

portional to current. This now needs scaling do..n to 500 volts

(factor of 2.28) to give 12,300 oAhr_l .

The agreement between the observed experimental result and

the very :Ample roo. 1 predietion is about 2 in this case. The

deposition rate was actually measured on ten of the experi-

mental runs using the interferometric method so that the .

accuracy of the model could be checked. The example quoted

did, in fact, exhibit the beet fit; all the others being in

agreement to within 15% (most better than 10;) which was con-

sidered satisfactory for our purposes. It was, therefore,

assumed that deposition rate couli be evalueted to within 15% •

on any of the two hundred and forty experimental runs.

6.5.2 VARITIT: OF SUPT2AT7 777PUT PUNER

The paraTeter K was evaluated from each experiment and

the results analysed to see if any trend emerged. The fol-

lowing general observations were noted:-

(a) The parameter rices slowly as the input power rises

in the non megeetron care. This risireg trend is also noted

for the megeetron case (Pig 6.7) with the K value being

slightly lower for a:et eiven poner input.

K Innut Po7:er .7 Tart tior. Cf

148

100 200 300 4-00 POWER (WATTS)

This reduction in the K value suggests that magnetrons

are more efficient since leLs of the input energy is now

appearing at the sustrate. There are, also, other important

differences which will be dicussed later. The rise in K

flattens out at higher power inputs and at no time exceeds

0.16 with a wter cooled system. This non constancy of K, even in the non magnetron configuration, is inconvenient

thouFh not unexpected since not only can the plasma efficiency

vary as already discussed but also simplifying assumptions

have been made in calculating K. The sticki:ng coefficient for

sputtered atoms is probably not unity but perhaps more impor-

tant, radiation is not the only cooling mechanism for the

temperature sensor since there will be a small contribution

from ooeductioh via the as and the thermocouple wires. •

The fact that he K valuil flattens off at higher power • inputs and, therefore, higher Te values may be due to the

background inieut correction which is significiant only at lower

temceratures especially since the fourth power law is operative.

149

This oorre,ton was used in all the calcIllations for Fig. 6.7

anI may e respc:-.2ile for the K variation at low Te value

where =an error in the background input would besii7nificant.

Fortunately, the variation of is both slow and regular

allowin..; comparisons to be made between the different experi-

mental series,

(b) An average value for K over all the experiments is

about 0.1. The experimental results suest that K is higher when the target/sensor separatioh <y so that energetic

AX electrons stri--le the sensor, but the improvement made is not as significant as the energy analysis results in Chapter Two

might have suggested. These results are shown in Fig. 6.8

where the so called N series has a relatively high magnetic field strength of 7 x 10-3T corresponding to an electron.

trajectory which does not -reach the substrate in the absence

of collisions whilst the C series (2.5 x 10-3T) has an electron trajectory which intersects the substrate.

- F17. 6.8 K v Ainlied Power For A ',:agnetron

012

0.06

0 SERIES B=2.5x10- T

N SERIES (3-740-3T

50

100

150

200 POWER(WATTS)

150

This seems to sugest that keeping the ellergetfc electrons off the sensor produccee apercximately a 20;-:, reduction in the

value cf X for a rieen power inut though we must be careful in

interpreting this result sinus the energy contribution from the

neutral flux has yet to be ceeside-ed. This result correlates fairly well with the energy analy-

sis of Chapter Two which snows most of. the power input to the

substrate coming from tne relatively few high energy electrons

in the incident flux. We should not, however, expect the sort

of improvement which the Chapter Two results might have pre-dicted since most of the curves then taken were at voltages in

excess of 1KV and had sharp, well defined maxima. (Correspon-

ding to fem high energy electrons producing most of the power

input.) As the vol tage was reduced the maxima became shallower

(corresponding to the power input being more evenly distributed

throughout the electron a:erival energies) aed it is, therefore,

less important to keep the energetic electrons off the substrate

in a magnetron operating at 500 volt than in a conventional

syste,A at say 2KV. Also, keeping the energetic electrons off

the substrate will mean that they will produce, by ionising

collisions, further electrons which may reach the substrate.

The energy input from an increased number of low energy elec-

trons is likely to be comparable to the input from fewer high

energy electrons and increase of magnetic field strength to

control the electron trajectories may not, therefore, control

a substrate heating problem.

(c)Sofarwehavecomparedtheconditions LAX Ygreater

than and less than the target/sensor separation in different

experimental series; consider now a set of results (n series) in which y is changed by altering B at constant voltage

7..AX such the.t we treverse the y mxx = target/sensor separation at

some point in the series.

151

75;. 6. . v Atrelled. Power

K

042 - 11 SERIES

V-500 VOLTS

ao6

1 50 - 100 150 200 POWER (WATTS)

Now the K v power curves show a region where K can be

reduced as the power input is increased. This behaviour is in

the part of the characteristic where V. , target/senser f

Separation once again sug;;esting an advantage in keeping the

energetic electrons off the se:.sor. It should be ad:ilitted,

however, that the effect, whilst always detectable, was not as

significant as had been expected and the posible reasons for

this will be discussed later.

(d) Providing water cooling for the target has a signifi-

cant effect on K s7Iggestinc: that radiation from an uncooled

target would be an important source of power input to the sub-

strate. Under identical operating conditions, the provision

of water cooling can reduce K by about 33., (roughly 0.15 to 0.1). • ( This result is in agree:nent wUth those of Lau who concludes

that in his planar D.C. non :r.agnetron syE.te::J, target radiation

can account for 6C-/ of the enerzy flux to the sUbstrate when

the target is uncooled.

152

6.5.3 FUT=RIT.J7 T=E72=77.] T

Since fror.., (6.1), Te4 is proportional to the •

pro,luct of K a:1d the rower in. t (W), we exp:t that Te

will rise as the power inp,-,zt ees of the trends

of the K

.' power viiritio curves will be exh4 bitPd in the Te

curves. This expected trend is observed and noted in Figs.

6.10 and .6.11.

Fig. 6.10 T4 v Applied Power

1

0 SERIES

N SERIES

100 200 POWER (WATTS)

As anticipated, the sensor te!;:perature rises with applied

power inr,ut and there is scfne alvastage in keeping the ener-

getic electrons off the sensor as f:Aicated by the frlot that

the N series temperatures are lower. The fact that the fourth

power of the sensor te;:.perature is proportional to the input

power sug:ests that assuming radiation to be the principal heat

loss mech'anism is reasoaille.

153

6.114 v Applied Power

4

3

1

. Te

* (K -x10

IV SERIES

100 200 POWER (WAT TS) •

It will be seen that the maximum in K of Fig. 6.9 corre-

sponds in Fig. 6.11 to an inflexion in the Te4 characteristic .

end at no time does it seem to be possible to Llorease applied

power yet reduce sensor temperature, a result which the energy

analysis in Chapter Two had su:gested might be posible.

6.5.4 7ATT77F07 CO7PAP,7D TO :m n.(37:ETRO7

Whilst Fig. 6.7 shows K being redued in the magnetron

compared to the non magnetron, both cohfigurations have roughly

10% of the input power appearing at the sensor.

The obvious question is therefore, whOsher the ma,-netron

is a significant improvement on the non magnetron if a com-

parable amount of the incut power is arriving at the substrate in each case. The ansv.er lie partly in the deposition rates being achinved and these will now be discussed further.'

154

6.6 Iff777.777:0- 12, 'CWER TO T7.7 f7:7'..:277'...TE (Con)

Ap:1;finL; the eciutiol. j = DIV (E + AH) to the cal-

oulatich:, of the enercy flux due to the sieuttered at=s in the

two previo=ly quoted exa_ples we use the values as follows:-

(a) ':'a.,7::etrer:

Power input 102 Watts, Deposition rate D = 12,500 °A ho--1

-10 -1 (3.47 x 10 ms ), average arrival eaerify "E .0 since

Westwood(6) has indicated that the eery of the sputtered

atoms will h,,ve been almost reduaal to thermal levels under

these operating coaditions; condensation energy All 3.18eV

(5.09 x 10-19Joules) and I = 1.2 x 10-29m3 giVing the energy

flux due to - the neutrals as 14.7 Wm-2. This value will be

slightly low si cc E will, in fact, be slightly greater than

zero but serves as a reason:..ble first approximation.

If we assume thi t 10% of the power input is dissipated at

the substrate then the total energy flux is calculated at

108 Wm-2 at the substrate 5cms away. The energy breakdown is,

therefore, that the sputtered neutral atoms contribute approx-

imately 14% of the substrate input energy with electrons

providing the remaining 86% on the assumption that the water

cooled target is not radiating significantly.

(b) Non Ma7,netron

If this calculation in repeated when P = 0 using

D = 7500 elA hr

and an input power of 160 watts, the corre-

sponding energy input brea.alown is:- sputtered neutral atoms 5% with the e]ectroas contributing the remaiaing 95%.

Whilst this non roagaotro analyais is in agreenent with (2)(3). previous results ' in that the electron contribution is by

far the most important, vie can see that the magnetron results

are differeat with the 1.1 -atra3. contribution having been

increased by a factor of about three. This trend of the .

neutrair contributing 5 in the non manetran and 15% in the

magnetron case is repeated over a variety of operating con-

ditions though it is not Irossible to be too specific about the

actual values slace there is a possible uncertainty of 15% in

the deposition rate measureaent and, therefore, in the calcu-

lated neutral eacz-vgy flux at the substrate.

155

These results est that as the magnetic field strength

is increased to eee; tnc eher eti.. electrons of the subs urate,

these electrons eroduce more _onisation collisions because of

improved tr _ cinq and __here are the deposition rate thus causing

more energy to rech the substrate when the increased atomic

ffluxC;.rt~.:~ n:'eS onto 1 ~a .

As the magnetic f ld strength_ is increased, the energy

distribution to the : lb ~trĀte , therefore, c.aYgas from eiectran

predoeinance at zero .a xle tic field 'strength to a distribution in which e:'.ectro .s are still the main energy source but the

neutrals are increasingly important. In the above example, the

neutral contribution at 15% in the magnetron case is probably

an under estimate, partly because E is slightly higher than

zero and also because both calculations use K = 0.1 when, in

fact, the meL4netroa. K value will be less than the non :magnetron

value.

It is interesting to repeat the calculations assuming

K = 0415 with no targ:t water cooling, the energy breakdowns

are then:-

Tegnetren: Neutrals 9%, electrons 58%, radiation from an

uncooled target 33%.

„on ae; ecron. neutral s 3%, electrons 64%, radiation from

an uncooled target 33%.

This provides a striking illustration of the need for

efficient target cooling so that the radiation contribution

can be eliminated.

Consider now the energy breakdown for the K v applied power

curves of Figs. (6.8) and (6.9) for the 0, N and M series. Com-

Daring the ': and 0 series at 100 watts power, both give

aoproxim:etely the sane deposition rate, though we should re-

me:eber that a large error is possible since we are in a region

where deposition rate is a function of volt' ā (Figs. (5 1) and ti. ~ o „~. -e .

(6.2)) and the v it age choice .is not too i,.Dortaat as far as

efficient sputtering is concerned. (N sup _lies 100W at 530V, 189mA and 0 supplies 100"; at 900V-and 111mA. )

156

The neutral energy flux -,';hich is proportional to deposition

rate is, therefele, the sa.;e f:r 'esoth examples and calculated

to be ap.oroximely.13 L-e Since the values of K are dif-

fseeent, di"'erie; aunts of the total input power are reaching

the substrate and the equilibrium sensor temperatures are

differert. Tne ca'oulate-1 total energy fluxes are 95 1cm

-2 (N series) anl 120 Wm (0 series) and the electron fluxes are,

-2 therefore, 82 Wm " (N series) aad 107 Wm

-2 (0 series), giving

a roueh idea o the effect of keeping the energetic electrons

off the sensor. Both systems are producing the same deposition

rate from the same nower intut but the N series has a cooler

sensor temperature (Fig. (6.10)) because the energetic elec-

trons do not strike the sensor. The electron flux is reduced

by approximately 23% wl,ich is a sLeeificant improvement hut not

as much as had been orieinally hoped for.

If this analysis is repeated on the 7 series (Fig. 6.9)

for two readings, 1.11 (energetic electrons hitting the sensor)

and. M2 (energetic electrons kept off the sensor) the following

results emerge.

Run. r1 M2

Power (r:atts) 57 82.5

Current (IA) 114 165

T (°c) 83 103

K e

0.095 0.078

Total flux

(Wm-2) 57.1 68

Neutrel flux 7.5

(Wm-2)

11

Electron flux _n

(W2) 49.6 57 m

q of inrut from fieutruas 13 16

The contrIbution from the neutrals rises steadily as the

power input is increased and this teL;ether with an increasing

number of lower energy electrons is sufficient to keep the

sensor temperature rising.

157 ; . 7 7,0777

1 GF-E7A: C3777:73 The nest important coeclusion to emerge from this part of

the war.: is that in mageetrcn sputtering, the contribution to nu'estrate heating Cr.:a the neutrae sputtered atoms in much more significant than was at first thought. ':;hilst in our system the pressure choice o± :10/A effectively reduces the atoms to

thermal enersies by the time they reach the sensor, this will

not be so in magnetrons operating at lcwer pressures when the .

parameter E could not be set to zero. Since the average atom

ejection energy is 9.28 eV, tnsn in the case at low pressure of an atom reaching the substrate without collision we should

set E = 9.28 eV and since A H 3.18 eV the neutral contri-

bution would need to be increared by a factor of three and

would be approaching 50?; of the total energy flux to the sub-strate in some cases. Whilst this is an over estimate since

some collisions will occur, the heating contribution from

.neutrals will obviously be even more important in lower pres-

sure syste.es.

In our magnetron system, approximately 80% of the input

power is removed by the target cooling water, 10% appears as heat at the substrate and the remaining 10% is presumably used

in a variety of ways including the ejection of target atoms

and secondary electrons. There are two main contributions to the input rower to the substrate, the neutral sputtered flux

contributing approximately 15% with electrons the remaining -

85% on the assumption that target cooling is effective.

Comparing magnetrons and non magnetrons at constant power is difficult because their operating characteristics are not similar but significant differences emerge at higher power

levels when the non masnetron voltage rises quickly to figures

in excess of 1400 volts. Under these conditions, as the power

input increases, the Deposition rate does not rise as quickly

and the neutrals play a less Significant part in substrate

heatng. In our system at 2.6KV and no magnetic field, the

electrens contribute aroroximately 95% of the substrate input power, a figure whioh might be expected to rise further at

higher inputs and, therefore, higher voltages.

158

The ma,e,netron behaves rather differently at higher power

levels dra•oing in more. current at edmost cer.stant volta7e,

The deposition rate, therefore, rises steaJily and the neutrals

play a more important part in substrate heating.

At constant sower, the substrate is cooler in the mar;rnetron

case with the energetic electrons removed compared to the non

magnetron but the effect was not as significant as had been

expected. The important difference is that at high constant

power, the marnetron deposition rate is muoii greater than that .

achieved in the non magnetron case. As the ma-netic field is

applied it provides no power itself but shifts the energy in-put distribution to the substrate nuob that the neutral

contribution rises. A high magnetic field strel:gth will keep energetic electrons off the substrate but replaces them with . a greater number of lower eherPy electrons and also an inc-

reased neutral flue. Therefore, the deposition rate rises at

constant power and the substrate temperature also increases. It would only be true to say that magnetrons provide '

'cool' sputtering if we looked at constant high deposition rate in an area where Ane non magnetron was having to operate

at an inefficient high voltage. The magnetron would then

achieve the required. rate with less power input than the non

magnetron and the substrate would, therefore, be cooler.

To summarise these comments; it would seem that the magnetron can provide increased deposition rate at constant .

substrate temperature or reduced temperature at constant rate

but it does not seem to be possible to increase the rate and

lower 't he temperature partly because of the increased neutral

flux contributing more to substrate heating.

6.7.2 FAT7eRC 77r7ESS'tv FCR A7 17'7I'3'177'7T HIGH R'‘TE =IPM

Depositioe rate can always be increased if the power input

level is allowed to rise indefinitely by using a mare capacity power supply. Problems will, however, arise when the substrate .

can no longer tolerate the heat input. The following factors appear to be desirable if the system is to be classed as

efficient.

159

(a) Coolinj the target is essential, water cooling being

the obvious choice.

(b) The cyste: should te run at the optimum voitage, the

power increase t:) impebve deposition rate being achieved by

using ::.axi:io.s:, current at this chosen voltoge. In 3:10 case, the

preferred voltage is about 800 volts but any choice in the

range 500-1000 volts 7,oul1- .:e. acceptable. Efficiency reduces

significantly above approximately 1400 volts explaining why

conventional D.C. 'sputtering cannot achieve high. deposition

rates successfully.

(c) Keeping energetic electrons off. the substrate isuse-

ful bat by no means eliminates the electron contribution to substrate heating. Ideally electrons shoull be kept off the

substrate altogether meaning that the substrate should not be

the counter-electrode of the sputtering system;

One possibility for reducing the substrate electron flux

is to introduce a biased grid between the target and substrate. Unfortunately, this is likely to cast a shadow giving non-

uniform deposits. Another approach(9) to control the secondary

electrons has been to incorporate a series of biased grids.

Whilst substrate heating is reduced, shielding is, unfortunately,

required to prevent the electric field associated with the grids from interfering with the plasma ani the resultant complex

geometry produces not only a non-uniform deposition but also a

lower deposition rate.

Perhaps the best solution is incorporated into the

Sputtergun which utilises the fact that most sputtered material

is uncharged to 'spray it' in a partioalar direction whilst

the electrons are drawn away from the substrates to the counter-

electrodes.

160 •

7.1 1-=D77.7707

In this, the final nheeter of the work, it is approrriate

to look back on and to so:;le extent restate, the major con-

clusions of the preceeding chapters and to consider problems

which have emerged. nilst some problems have been elucidated,

others have been identified as a consequence of the work and

this should be seen not as a disappointment but as the expected.

result since scientific progress throughout the centuries has

led to the commencement of further work.

The title of "The Role of Eleetrons in Sputter Deposition

of Thin Films" is a broad one and many of the basic facts have

been long established. The role of the electron in its simplest

senee, namely electrons causing impact ionisation to produce

ions which bombard the target, thereby releasing sputtered atoms

and further secon1ar electrons, can therefore he assumed. The

main function of this work should be seen as a more detailed

comment on the precise mechanism by which the discharge is

achieved and maintained. At the time of the eommenceent of this work, magnetron

sputtering was not the well established teehnoloPy ehich' it now

undoubtedly is, but the firdiegs of this work have not, to far

as the author is aware, been published elsewhere. Lluch of the published work on magnetrons has, so far,

tended to concentrate on the technology of producing thicker,

improved quality films, at a greater rate, the role of the electron often being summarised only as a brief statement to

the effect that ionieatien is enhanced by the trappirg of elec-

trons within the plasma region by the magnetic field.

One of the aims of this thesis has been to attempt to

maintain a balance between the theoretical approach based on electron trajectories pn the One hand and the practical aspects . of the technoloy of -„er.odu:...ing useful thin films on the other.

161

The-e is, however, little chance of producing satisfactory

thin films without se.::,c knowleds-e of the theor:: involved in

the deposition process and it was with this in Mini that the

work wa:: undertaken.

7777-77: 07 =7 -=

7.2.1 :7E7.7! A-A7.YEI3

The nair point fron Chapter Two was that a relatively

-mall .ruber of high energy electrons cause a large part of

the 'Cower inrut to the substrate in D.r.;. sputtering with a

planer target in the absence of a mag-etic fiel I, the analyser

curves of current against electron energy having been shown to

be equlvalent to power input against energy.

This enery ana'iyis was carried out relatively early in

the project and the point was made in Charter Two that a

decision was required on whether to si.)end more time accurately

producing more UUrVeS or whether to accept the broad trends of

the results as outlined, above and to observe the characteristics

of the discharge when aHri17,netic is introduced to modify

the elcct,.on trajectoris. In future work, further energy

analysis would be of value, in particular to extend our know-

ledge of the following ncints:-

(a) The energyanalyis curves of current against energy

were shown to be equivalent to plots of power input against

electron energy and po!,:sensed the general chase shown in Fig.

'1.1.

P1::. 7.1 Ene:-r--, Srectrum

PROPORTION OF TOTAL CURRENT

IT

ENERGY (eV)

162

The lo: ,,nerny -J.--xinum was found to occur at about 25e7

irrespeetive on' onerat.':ng condition- and was thought to be

relatively uni:nnoriant so far as substrate heating was concerned

when compared to the larn.er peak at an energy correspohnling to

the inter-eleotrode poteettal.

lost of these curves were obtained for inter-electrode

notentials 4 11 excess Of 1Y:V,whilst a typical magnetron operates

at lower voltages. Since the high energy cc-al: tends to be

better defined as the inter-electrode potential -rises, it is

necessary to study the distributions more carefully for tyrical

magnetron voltages in the region 500-1000 volts. A better

!tnoledge of the peal: definition would provide more information

on the advantage of kecping the energetic electrons off the

substrate and this could be compared with the substrate heating

results of Chapter Six. The energy analysis results which are

available at lower voltage would seem to suggest that the

eliminaticn of energetic electrons at thee low voltage's would.

be less advantageous than at a hieher operating voltage from

the point of view of substrate heating.

(b) One of the more interesting features of Chapter Five

was the approximate correlation between the theoretical pre-

dictions of the electron energy distribution at the substrate,

assuming a knowledge of collision cross sections, and the

experimental energy analysis of Chapter Two. Two points

emerge from the theoretical predictions which could now be

checked by further enerny analysis.

(i) The detected energy distribution should be altered by

placing the analyser at different positions in the NG and

should be changed slightly by pressure change. Neither of

these changes were extensively investigated. It would be of

particular value to position the analysar as close as possible

to the CDS/.70 elge to see if a greater number of high energy

.electrons could be detected; theory having predicted a rela-

tively large high energy signal in this region of the plasma.

Unfortunately this boundary cannot be appro,.ched too closely

or ion oroductlon is affected.

163

(ii),Hays estalichel that there is a peak in the enerz,y

distribution curve cr.:Tres-e'en:ling to electrons reech'n: the

ana7yser without co1741-ion, we might expect that there will

also be a pea':: corrennonding to electrons which ma:e a ningle

ionising collision in passing from target to ru.':estrate. These

electrons would lose nem? energy Curing the collision and sncuri,

therefore, give a pea'..: in-the energy distribution at an energy

below the maximum possible. It would be necessary to scan the

enerev distribution curve more closely looking, for fine

structure and subsidiary peak::, in order to confirm the nresence

of such a oeak. The present analyser would not be suitable for

this work since its acceptance bandwidth is too large but

imeroved instrumentation miht be succes-zful.

(c) So far no eaercy analysis experief:te have been per-

foried on a magnetron configuration and vie, therefore, have no

information on the r, recice energy distribution of the electrons

which strike the substrate in this type of system. At the

moment it in only possible te nay whether the co called 'ener-

getic electrons' are reaching he substrate or not and in the

cases where the magretic field is sufficiently strong, we cannot assign an energy value to the most energetic electrons which arrive at the substrate since the outcome of the necessary

collision is uncertain.

In addition to studying the "eicetron distribution at the

substrate, we also need the spectrum of electrons appearing at all the earth ;lanes within the system geometr;e to see if these

surfaces receive any of the higher energy electrons. Geometry

variations directed at keeping electr)ns of the substrate could then be investigated, as well as the results of ihtroducing.sub-strate bias.

Ne conclude tnat t'ne c:Ier,:y analy,As, whilst contributing much te our present understan:ing of the basic.seuttering pro-cess, could provide further inform-ition if a detailed programme

was Initiated aimed at different geometries and the effect of

varying them. This might '._e especially true in low pressure

magnetrons where repeated collisions with gas ate= would be

less likely to modify the observed electron energies.

164 1.2 .2 .7,2-.

This Vik VC split into teo. eeetiene concerning substrate

he-ting film unferity. Ele-tve:-.o were shown, as expected,

to be t principal energy input to substre but the energy

flux from c:.uttered neutral atoms was found to be more signifi-

cant in high depoelion rate eystem. This neutral contribution

is likely to be even mere important in low pressure systems

where the eollie'on probability in reduced.

Speelfic conclusions on ...ueh cf the film uniformity section

of the Work are difficult to fore:ale:to ei_ce the importance of

ac}eievieg a given uniformity tolerance clearly depends on the

use to be made of the fabricated film. All we should say is

that cylindrical magnetrons h.ve an unfavourable terget geometry

as far as film uniformity is concerned and this aspect of the

deposition process could be improved by using a split target

system with differ power input densities to each eection.

Whilst this concept is not cohvenient experimentally, the

alternatives are not convenient either and it does mean that

uniformity problems are not insurmountable.

The alternative uniformity solution, discussed in this

work, of using a continuous production system with the no

called 'modified cylindrical target' has demonstrated, on a

small scale, that such a target can operate suocessrully as a'

magnetron.

The most important aspect which merits farther investi

gation concerns the arrival energies of sputtered neutral atoms

and the idea, as ihtroluced by ';:estwood(1) that neutral ener-

gies will be thermaliced after the atoms have -progressed a

certain number of mean free paths from the target. 'A knowledge

of the energy distribution of the sputtered neutral atomic flux woula be alveetaeous for two separate reasons:-

(a) It would pro; ide information on the accuracy of the

predictions(1)

concerning the ther:Ialisation of sputtered

neutral atom energies. At the moment it is clear from experi-

mental results that a uniformity problem exists yet there is no totally satisfactory theoretical model, since the affect of scatter!_ng is not certain, especially in magnetrons winich can

operate at I ressure. stuly of the at.1 c ar2iv1. en6rT:42r;

,f cress7-:..e a better r1.C. de to the

theoretical s.hould be

(b) In substrate '- c''4"ir"stnh.cs e re:aire E (the a-rer-,-e

arrival ener.7y of spattered at;:.:.$) so th,t

signli:".icart) fre, atc,z striking

ths, bstrte cah be ccalatel: As disCu.:sed in Chapter Six,

the est inf7.-)rr_ation awallabl,:: favours :iattin E= 0 though

this ass_::ption is -less hiy"Lj ;Je true for z.agnetre:_s ol-,er-

atinz at low pressure. Whilst it is c.crtin thlt thc neutral

shery L:1at has under-esti:aatel by setting E =0, an

ex7eri.-..ents.1 value of E as a fu-:stior_ of pressure would indicate.

the significance cf the error. Einoe the neutral ato:.: energy

input contribution to the substrate heatin is more i.::.nortant

at ihrates, It Liay he that low pressure netrs)li.: have a

:ore si6nificant nutral eontri'l,ution th n that estii%ated in

Chapter Six.

The proble:a in measuring the energj of neutral species(2)

is th.tt only syste.:,: b..:xed on ;:eaguring the ti!: of flight

between two points an; avail,ible. Coburn(3) has discussed the

idea of ionistng t neutrals and then using deflection methods

but the obvious diffi(taity here is that the energy of the sput-

tered atom aunt not be altered in the ionisation T)roce-e.

S7:77t1T;

The discovery f.!..4.planation of the sharp switch of plas:na

characteristis v‘ith 2resure cln,r1:.:e perhaps represents the

major feature of +,he beil,-orLour does not seem to

have been hotel in othcr publi:-:hel work. The moot striking

oh-,rcteristio is the sharpness of the switch, the change in

oasa n;---1-.20ristic:7 being so simificant tht, when the effect

sac 7J1):- rved, the i=ediate diagnosis wan to suspect a

fault in the eTectri.o'l system. Passing references to plasma

.tran=it;ons bee:. male by ?-ra3is(4) ani Wasa and Hayakawa(5)

and will he di!:cur=srl later. 'They have ot,however, mentioned

changes in plasna nmJeters which are as large as our observed

transitions or under the same operating conditions.

166

Wh'ist we hve been 'able to :7 -low th: the switch is close-

ly lihf:ei to the y „,,:d ratio v:here ›/„., is the e.axi:..um dista .la from the target of the eleotrpn tractories before

collision anl d is the S len:;th, it SCEs unlikely that this

the sole criterion determining the state. Further

ex7e-riental :r;1 theo,-etical ::or !.: is re,luired if the m:lerctan-

:n of this switcl.n; behavicur is to be improved and two main

asoects of the proble.:1 seem, at this st-19e, to merit investi-

gation:-

(a) The so c,!3led switch in plasm% ch.:.racteritics has

been explained in tern,; of a pos.-:ile m,,ichanis:ii by which ion

production can be proved to prevent extinction of the dis-

charge. The traition oressure hrbe,h shown to correspond

to the electron trajectories, in the lbsence of collisions,

just touch-!.nL; th.7. CflS/TT1 edc. Further wor should establish

whether this transition preesure is a fuction of ;;.anetic

field stren3th,.in particular at .r.gher Field stren-. In

the present work, the viailable raLge of variatio of field

strensth was such that ohiindes in transition pressure were not

detectable.

It seei.ls surprisin that the trai.sition pressure is as

high as a:Troximately 1!>il since Many cylindrical masnetrons

operate successfully at lower Tressures. Commercial equipment,

however, tends to use much hiner mIE;netic ficl ri strenL;ths than (6)(7), for example, those available in this work. Thornton

has used ;:iagnetic field strengths about five times hij;her to

produce thick s:.:utterud high rate copper coatihEs for operating

pressures in the ral.ge. 1-39,LA.

The switching phenOmonc,:l, as discussed in Chapter Five,

occurs presure is alters.1 (thus chang.n6 d but not y lax) but not whe,.. st.rengtn in altered ch-i.n;in,E.both

and d. StartL.L: at say 39AA pressure and y d, then

as the L:2.-netic field strn.:eth (2) is increased, 'oeth y and

d reduce u:.1 the ratio y ,» fi tends to unity only very slowly.

Since the plasma b.-es 1..-2orensively str.)nger as P is ihoreased,

as indicated by increased current and colour at constant power

input, ne ought to be reduce the pressure to below 1y1.

Shaded areas indicate

colour regions

167

2nd still retain a -,trosg nlasma provded the magnetic: field

stregth has inorec..seacufficient:y. &n important poiht tyo

note 12 that v.:,e:-_ the plasma is :e- :n, the CDS length

shrdn'.;.2 to a very o:-l1 value wn:::h ws,.11 not be lable

by the .,:theto::-.eter teohni;ue wn!_h :3-s used an'l the plasma

ap:.eare_sce is totally dominated by tl-,e intense oolouration of

the 771. This wo.lii seem to indidate that the !77/ is the Lost

important region of the discharge under these oirou::.stances'

and the role of photons :Light be cloghi;:ica.nt anl nil be

briefly diricussed lter.

(b) Wtilz;t the switch in plasma -para:leters is detectable

for both the oonehtional and 'modifiedi . cylindrioI geometries,

the i_fluence of target ge3L:etry re,:uires further in7estigtion.

In particular there is a significaht power loss to the earthed

base plate of the vacuu;ii yte..,1 from the bottom of the targets

when COS shields are- not fitted. Confirmation of this loss is

pro7ided by tne existence of a glow region in the plasma from

the bottom of the target to the earth plane (Fig. 7.2), the

spreading of the plasma below the target being controlled by

the magnetic field.

Pie-. 7.2 C-)loar Renin- 11 the Cy'l!:!al_ 7,,;:net -oon - Sche:ratic

18,9

c'..: eh 4 7 related to the efficiency of icn pre-

tke chara-..terstics migt 'oe altered by

provi a ADSchiell elimir.ate this 52ov,er loss an, improve

efficiency. point was

ea'nstructci and will ed in the section.

7.3 A- H C.:-.777TFAT-TT

One target geon.e:ry which is ',,:nown to be 13::rticl_ziarly

effective in pro -lcin ir:nisation is the "Hollow Cathode" dis- (3) cused by Little and Von Engel . This f.uste.::: involves the

use of two parallel plates both acting as cathodes placed op-

posite each other a'::1 keft at the slme iotentil. Electrons

leaving one target and failing to collide with a gas ato:r1 are

therefore, reflected back into the p1,4sma by the other target

and losses are significantly reduced. In this geometry The

cathode separation ec.'tro Ic the width of the d-r% space:; and

'when this seE;aratlonis :Alfficiently reduced the two negative

glows coalesce and the lip-ht e:aitted as well cc the cathode

current density rises . greatly. Under these con,ftiticno with

the olas:.ias ccupled together, the device is extremely efficiAsa

et sustaining ionisation and the glow can be male sufficiently

intense for use as a spectroscor,ic cource.

We can, perhaps, coLthine the efficient ionin::ation of the

hollow cathode with the nigh rate depoeition of the magnetrOn

by constructing a tarF,et to the desigh of Pig. 7.3. In this

system the end :plates constitute a hollow cathode and will tend

to reflect electrons back into the main plasma region.

F1F. 7.3 An H CohfiUration

7cm

CERAMIC SPACER CDS SHIELD

END PLATE

ORIGINAL TARGET

Elcd 8cm '

::'i::-. 7. I.

ATTACHING

SCREW

169

-1------ MAIN TARGET

COS SHIELD FOR SCREW HEAD

END PLATE

ellS SHIELD

INSULATOR

SCREW HEAD

Si~~e ~~e scrc~ is ut tar~et pote~t:al it requires insu-

The p\ost s~ri~-:i;};; i~;.itial obscrv~ttio~: ·j:~:'t: that for a fixed

pressur~, voltu~e t~e introduction

Q:J~o·.~e 2SfA fer o[Je:r:-'.t:~~:~ volt:..'..;;C]s iYl €:':G(-,;:;~ 0: 500 '!(ll ts, the

·~::yste.:l dr3:;;S ~,o l;~'lCL ~~')'.·;(~r t!'":'11: th~? gln.::::::· eDS s;lielj, sur-

too A£ter

1 ,--:„.............----

BLUE ..------------, - ' COLOURATION

_-- -END PLATE

MAIN TARGET

. .-----". \

----°".

I

CDS/NG BOUNDARY

170

extin -Inin.: th:: !.1.= the t- rzet wIlic 1. not w.,,to,- cooled

.'.-:a.: a d',:17. r,---]. :, r:c a:_o :=.1:sti, excesive heat- L.:.

The :?la . ly ve-:: :tro.-1,- even at rresure:. ag low az: ;e4.J. and

as a rou:h .-u-YIc, the -::::-..te.:. '::' -th e.:d plater.: exli.1)its approxi-

r:lately the sa.:.e tar„,,e orent de:ILIty at 7/A _12 the syste:.%

without ei plates drfx at 33,A for a fixed value of voltae.

and -anetio flold strencth.

To s7,itohin wa ol-...T,erved In the 7recf.,3ure race ;AL-3;AL

which was

'..ted and it i'.. cear that the hollow cathode

e:feOt proll:,oel by tu e.11 plte:: d... do:-.:in-ltinE; the mode of

disce c-;:cration. The ai2:7:aarar.oe of th,_; plasr.a (7ig. 7.5)

is also oh ,z.ged With the CDS len,th hocor:.in,,L; 1-,21 uniform and

the intenLe plasL:a col,Dur 1-:1:1,1; 1)1 u,, rather th:u croon. A

green cop:;er .;low wa:: ab:7er.t In the central region of the tar-

got an,..: .w:e only observed in the shaded vo;;;ons of FiL:. 7.5.

,.,.g. 7.5 Pla:Ana Appearance '1 '''" 1 Pate: - at 7bt, p7:-(=ure

GREEN COLOURATION

The re:aoval of the switchin,! effect in an iw,proved

ionisation environ7est sui:;,:.,e2ts that sItching and efficiency

of ionisation are rell-..ted and we can clearly conclude that

tr7et Ewo:-.etry ifJ a[. -1:2.ortant factor. U.ifortunately, e

cannot now c.::::zare y,..., to J. since neither y 7ax nor d are -AA

constant'_.:: this gee:ietry. The :)ara::Icter d varies as shown in

17 . (7.5), no obvio....s CTS rc,:::i3n 1)eing n.:.Fo'.:iated with the

,,L,..! eleotrons coming off en 1.Dlates. r11/"(' ''O'''''-'1 -(' for

CURRENT (mA)

171

the el-:"1. pl:Ite7 i2 not e.:-.,roprate rice they w4 71 be channelled

aloc.: .'1. ,.:H 1:ie.: in.tne :,_1...tence of col1i.7.1oqz.

It ie n)t i -)e :-_.-1r. that th-; :-.1 ::.1 .1te lel:linate the

:laf:.:.a 1-)eh7.vi f..- i_er :he: '.ontril;z:.t.a'-)eut tw, t':- ir -1:: of the

f:,: .7tteLTI1.

he 1r(), :):_ j. :;1-at!:.: .;.ere rl eel . 1.th r.11er plater of

dieil:on e=xie:' so that the e%dr-: no co.f.tri.c.l.ite aroxi::lately

4: , Jr the tLtr,7t .area avail7,be for c-_utte--1:::,. The p1ast-2a is

not nov as intenee and coc:pari2on of operatini: currents' at 13U

(Fl;. 7.6) shows the :Laz:_Ltuic of the Cfect w...i.c11 the different

end ':,lateL prol- zee on the curront/mnetic field chracteri!;tic.

The a-eas give:. ,,-) the fl-::re refer to the :i.re"..,, o tarc:et cur-

face boil.:: :.puttered, thes'e f'..uree hein„: lower than a calci- on bd cir:.ply on th:. target din'n:ionc :11i.ht ::. ua;est since

the (nS slliel]. -1AibLt 2:2utte2.ing on eertaLl parts ();": the

target surface.

F.L. 7.G eI'Ispn '-".TLI=IlEilLL144-11 P,C0 volts

200

100

LARGE END PLATES ( 235 cm'. )

SMALL END PLATES ( g6cml )

1

NO END PLATES ( 81cmi)

2 3 4 5 MAGNETIC FIELD ( Tx101

CDS

NG

172

7de: thece cc::11t1:72.. the c):inal wth end

v.Lt71 an1 -2::_ectrDn

tr:.jectorie:: tlIc; a -,, cc: G7 collfL.Lo: the

This T:_ew wt Ea::cess-

fu.ily witht ".7DS s'L-11-: at 39A1 pressre

the 1-.er draw is n..;t but at Lower pl.e.;I:urec the plasi;la

intel-:sity is nDticeahly the is

aE:ain in evllee at :,-.1-1d Pl,;. (7.7) sho.:. the CDS

a:Tearance i. tho . '07'ht c:rre:It state the 'OFF' low

curre;lt state where the 8reel: ce or rolourati alet.

7 7 """4'''' 4'

MAIN TARGET

CDS

NG

ON OFF (high pressure) END PLATE (low pressure)

The follovAnz noiLts 7.noul.i. be noted:-

(a) The switch is now pre:;eLit but at a lower precure than

is ol.,served without ea- ] 2:lates. This that since

io'isation is improve by the end ;dates, the rlo.c.ma has less 1:;self until the pressure is further

decreased whe:-. fewer col:Li:I-Lens nul.iify the v1ae )f

improved ionisation.

(b) The 'switch C! and 'cwitch off' doot exhibit exactly

the (FiG. 7.8). The reaspns for this

a:parent hysteresis are not clear.

173

50

I (mA)

SWITCH OFF (pressure decreasing)

--- SWITCH ON (pressure increasing )

I III I I —›—

4 6 8 10 PRESSURE (microns)

The switch off teildz to he relatively olow takin;J: ap-

prox'7:1'2.tely one E,";'30:1 to co:nDlete drin. which time an

teri:lediats CDS shpe (Y3... 7.9) is trinie as the plazma

rove s •fro:: '.he O :;t::Lte toth 070 state.

7.9 T:Itelyr.E3' ,te Sta:-e of fli ir Sv.ttr!h Off

CDS

NG

MAIN TARGET

END PLATE The :.itch o lE ter..I t occur at hilicr

m1792surcs :J.:: chow_ (7.E); no interediate sae (Fig.

(7. (1)) iL

174

(c) In V..e 'CF:' state, the CL:S length lc larly' control-

by thc :imencina 3f ,:nd plates an1 CnS hncead

(7.7)) so ht hire.oentral t - - - et

d.:rk. s:.acc electrchs coin; off the end

late:' are pro.jec:.,_sj t - .-i ,- CDS. It shou"_: be %otel that

the CD3 has spread out;.ard:,. from the crigi:al central tar,:).et

ani not oatwards from thi enl plate anitIlere is no sign of

a separate ODS as.po.:ited with the e:Id. p17,.te.

on therercre, the system adopted a mode of

oPeration where icnisaticn in ar, en?argei CT)S re Lon is

an2-irentTy fave,.:red in a weak nl%Pma.

C -77.7!ri7-7 7=

If tni. ::y:.tem was to be I ves',IL:ated in donth,

we woul'i req-Are a better nyste design incorporating tar;et

water oin and impruvel. CDS shields so that ope2ation would

be possible at h1i.r power levels. These preliminary re-ults h.,.ve, therefore, been presented in this ch pter rather than Ch'ti:te.c Five since no firm conclusio.,,s c'.!.11 be drawr„ without

"urther work. The ,-eslts 20 far support the idea that switchin6

Ic anocia,.te'J. with plana sustai.:.ment thoi„h the idea of oquatin:,

)(..„7. and d early too simple ;t: the case 0-7 this rorc com-

plicated geometry.

7.4 P077.1.77.= It is n..t the intention here to state again the main con-

clusions of the worn oh switchi,,g but to T.:.ake a few concluding

remar.K-s. (A) who who usel an R.F. soter:-., briefly di!:cusses

"The rather a'orupt ch.:-.gcs occurring in electron imninge-

ient L:rofileat the :-vicstrate wi.th sm-1- --_ec in the magnetic

field ctroH,Ilth" and p:JJ,gEsts- the are in(lict -;.ve of the plass,a nw. toi:in t.j sligtly di.fferent oeratin modes. He further suagests that the energy distributins of the charged particles are beil:L; severely affe;ted at times by the strength of the lonitudinll magnetic field. Since thin is a small s.::itch

from one strong state tf another jn R:F. lonL;i-

tudihal ficl], it is not the trancition to which we

refer but 1...,..'ieates th'2.t the switc.:1LI_L o,r:ce:;t to an alter-

native

o-i_e17..-Itin2 mode is not new.

, '

(V. ) 1.

:J·1~~~· .. eti(; : ':'cl.l.

175

clo:Je

of

Tr:e o.;,ti::lI.lf:1

Vi,' l'ec.'"lced 8

The

176

The only tra,F_It!:::. -tween the p;:sltive .J.cl

-..riti.o.:1 val.:.e of magneti:: fiell w?.s re and -,,hc ,lischarge

be^ame u-i..t.,le a:-.d t':_ei. clr.g,..-:: a:.pear%::-.:e wiLth a sl'.- i-, chanre

in nt. Th:_, culo-..1.r w-.:..: 01;270:: tu ch-,.:.re be-

twe,:r. blue -.1..1 red in ..:-: arcon ,>.7.1copper tar:et syste:— Zinoe

the 1-,ress1;re. was 5 x 10-'' torr (:earlytL) order:: of :_ ..,:. -..1.tude

lower th-In ours) is .2...: ivertel r:..:.netron reome ry v;ith a

cr...tic,, mag..eti_ f-ii-:'d stmhgth of 3 x 10-2T (fivo til:.es hirher)

aLd 200 volts letwe:.n the eleotrodes (three. times hil7her) with

a switch fro:.. blue 1,o rel (rather th...n green) it sees unlil:ely

that ti.:- n;itch i, the s,,....e as our el7served transition..

. olr,f- i.-, -h( a'.,ocLe of a'. etorn, Li-,rnet! field

e7,-..hibits te u.:ui.1 CM a:.d. 7..1 regions of 1:-1. discharre as

described a previos :.hters a.:.3. is in the 'i-ositive. Hpao.e

crre mot. Sir,oe our p:..as,..as ol.erating at relatively high

pressure arld low m-.:..rT.etio fieil trenrtir also show there

characteristic rerions on both si,ies of the transition, it

seems rousonabls to as..:r::e that they are al!,o in the positive

space charre mode -.-:.(1 the switch 1:: from o!.e positive sp-ice

cho mode to another. Farther evidc,;,oe that our niass are

LI the positive ::::-!.ce cargo mode comes fro::. Gill and Kay(9)

themselves who are 1.51s to operate their Inverted li:ar..etron. at

1Ik -presslIre,650 volts in either the !:o:sitive or negative space

charge modes by ch2ncre of 1:-.:;:,eti-:; field strerth. These

operating par...cters 'ire similar t our o:in, though the pressure

is lower, and the positi -:e spaee cl.rre mode is prolu,:ed at

9.5 ..x. 10-3T (our :Larnit;,:.de of ma..etio field =.trength) whilst

5.7 :: 10-2T (a 512. foll'Llcrease) is required to produce the

ner,Itivc sac -=:har!._:e ::sa.-_,. An inverted mag_etron is, there-

fore, in tIv. !?:.;sltive -,:aco ch-re mole anlor our type of

co::.ditions, but it should be noted that the operating conditions

of the nor ni and invef.ted ma.-letro: are different.

iasa an! Eaya.ca72(5) operated :,. cor.vconal m:-Lgr.ctron at

hi,:,h ::1 -irnetio fi,,11 .stre.rths of order 1T and 1/(1 pressure to

produce a so railed -o-21.7ied ne:,7:it -ire space charge mode in

177

which there -Is an are.!' ,ble :oie a:-,: :tathode fai -. both of '::hich are cacle of prol..-.7.1 electron 7Imi-a-tt ienizal_on.

Th,:y f=ther calc_,I:.to that a ch:c L their :-:;-:.- te:” fro:.: the :.,oeitive tc :ti.-e s . ce ch..re mode (,-fc,aracteri: V he ) :::,..,1a, toretically, 1- e at a Llaneti-; ld fie streth as

e

2 low as 1.4 x 10- T. Thi: trarslt , n.1 ficli is about twice as hi L.:h as our :aximm f' ,:,1,1 .7-12.:-:--t':. ohe aiin tht our 6ystem is 1.-. tIle .:.cLiti-le s: cc chir;:e mode. Furthermore, si_ce we

havr_. h -7,her 2rescures .;:hieh r?.vou increased electroL radial velocity, a ,:,.;:-,itIT.ve :,7-,:ace charge node seeL.s even ::ore likely.

All the evidene -av:il.t.le ;_oin:ts to a positive r.itace charce mo.lo r1.1_ i thca_na.lynio in Ch.:..:to.r. Fivc.is, th:Irerore; 1,--.1id. The e;:=.1.r, cocc:111- lon from this di:-.cuscio:. i:. tt the

with ,; -1:-J::or:leno.1.: i? too coftl21icateq to -,c., c=1)letely

exp1ai7_ed ',y e,.3uatin.:. y to d. .. ion,: term aim should,

threfore, he t:, estilDn:1-1 accul-Ltely the potentia] distri-

but'on 2r)::!-: the di:,che for a varet:T of ,nac!:etror.-

operatin,:; oonditione ani ,:::eoc:trie:7 and to ::ursue e-leri;y

analyis in manctror:; to sec if less si.:_ific:Int re . ]ictri-butins of electron eheri.iec (i.e. tranitions) are, in fact, rez7:ular evets. , 7."1, TH7 r= "Y: DTC'7S.

One acne _:t of this won: which has bee:: almost completely ignored is the role whioh 1:hot_;:- from the 7G play in the ow;- t:linLnt of the plas. Little :2.n:1 Von EnL.el(8) in discussini; the hollow cathoe e'ffect c'Jg::cot th::t .:to-electrio e;Lission at the tat causel by ultra violet ql:lat from the :TG is an

i_oortal: fiotor in_ ni., 51:ooL,etry. Thej ''u.rther s.:Lest that

!,.:e ion cdrrL..t fro:.: the 73 is small compred with the current

from the (2D'.7, ani se:. thr O as a :.rovider of photons rather

the:. JL L ..on:-. The 1.ol,-. of the 73 in a normal lischarge

s, however, leas o1.,-r since the !TG il.tohL:Ity is very ::.uch

reduce,I co-,:ared wich the holl.o...- Cathode G.

Am2.1L.....;i: is Cha-2ter Five s....owed that the CDS could not

1:1-o7-1 :1e ,.;:-., -c..1. he

sropniary el;:ctron coeffd._:i.:nt fc--; ion imponot) ia too small.

17, the 7-..! a.: to ef-

fc.;cti7oly a; adlitional

wo;.1d Le . . he d.Lff.:.)1t, to

accenz I ;.;.

intc::::c ultra vi-)1,-_.t .t-A.1 as a 1.:rce

of ailitional • a.-.1 t cervc the :)ffert, " any, of

j thlo

7ro:. the poi.lt of view of the role of eloctr3)::,:, ionisation

and e::citation th and it is

not too i.:.,2ortIl.at .'h oh of the t'..o pos:dble 73 role:-, is do:.irart

sinoc they both prol- oe ea.2. :.ro.3uct, enterin

the CDS fro::: the 70 will h..:11 to sasti:: the piaL:::.a ,::111.1st enterin::, the CDAwill ctf.ie Lii. trot ro)_eacj.::.L: elec-

tron:: azId caaLe thor iJ aticn Ii the; CDS which

help to sustair. the --itcl- 'nj • hov:ever, benefit a clearer ,Inderc.:. tanding

of the 1:recicc 70 role.

7.6 Cfl7=T"r1 777 777S SihoP the ,:h:),ters 'nave all colot:entrated on

the which le. in: , it wa._ folt to Le al)proprLate

that this fir.al chiu Çj sho :eoulate on future- work, bath'

expe-iz:et:-.t1 a-2i theoretical , and on the proble which are

as yet, unrecolvc:d. Ulldoabtedly :;:ajor understan a plasma

discharc oompl.:tely is thc fact tat thel-e are :::any co:ncting,

inter-related 1.for:)e sec, 11 of which ',lave so:he probability of

occur-in,:; 'Ind it is diff'.oalt to an.?.14:e which are

and which are Lt. The r.)le of re'atc:1 to sputter Cieposition is

clearly a ,:!iar, one are all areas of the

sputtc2in„; iLeo:Let ,-j it i2 hopet th7).t this work has :Jade

scar contri:,:_ti_on to '.:r.clod,L;c of tlie sabject.

179

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187

SUBSTRATE

dt TARGET

L

T

E.74E14E- -

X dx

Substituting for e gives:- D = KT2dx

dl

[(T2 + (l-x)21 2 0

188 Appendix I

Assume:- (1) Uniform current distribution over the target area.

(2) Cosine distribution of material sputtered from each point on the cathode surface.

Total deposit (D) from the target strip onto the substrate

element dx is given by:-

L dldxcos2 6 D = K R2 R

0

where K is a constant

(The solid angle subtended by the substrate element at dl is dxcos 6/R2 , the cosine distribution introduces an extra factor

of cos e. )

and this integral solves to give the solution quoted in the

text.

50

10 11 a II

5 15 25 35 P(microns)

FIG. 1. Current (I) vs pressure (p) with magnetic field of 80 G.

189

Current-pressure transitions in a magnetically supported dc sputtering system

F. A. Green and B. N. Chapman Materials Section, Department of Electrical Engineering, Imperial College, London, SW7 2BT, England (Received 28 April 1975)

Sputtering in a cylindrical magnetron configuration, current-pressure characteristics show an unexpected discontinuity and current-magnetic held curves exhibit a tnaximum. These characteristics seem io be closely related to the ratio of the height from the target of the secondary electron initial trajectory compared to the cathode dark space distance.

PACS numbers: 79.20., 81.35.11

Secondary electrons emitted from the target during sputtering interact with the sputtering gas in three main ways causing ionization, excitation, or elastic impact, and are vital for maintaining the glow discharge. It has been indicated" that the major source of power input to the substrate is from secondary electrons, and en-ergy analysis''{ has shown that some electrons hit the growing film with the full interelectrode potential dif-ference and are primarily responsible for the often undesirable heating effects which occur.

It has long been recognized that the use of a magnetic

189 Applied Physics Loners, Vol. 27, No. d, 15 August 1975 Copyright © 1975 American Institute of Physics 189

FIG. 2. Cathode dark space B=60G iss distance (:(1 vs pressure (p)

with magnetic field of 80 G.

5 10 15 20 25 30 35 P (m<roM )

field enables gas discharges to operate at lower pres-sure and/or increases the deposition rate. Under the influence of perpendicular electric and magnetic fields (a magnetron configuration) the electron path to the earth planes is increased and the probability of a colli-sion causing ionization also increases. It is therefore expected that increasing the magnetic field would in-crease the current drawn by the target, and traditional arguments follow.along this line. In aiming to produce higher sputtering rates, various magnetic field confi-gurations have been investigated including quadrupole fields, S "planar magnetron", 6 "sputter ;run", ° and the cylindrical magnetron.8 Perhaps the main advantage of the magnetron concept is that it can both increase the ionizing efficiency of the sputtering process and also keep high-energy electrons away from the substrate (compare parallel electric and magnetic fields).

In this letter, some preliminary results are presented which are at first sight surprising. Their explanation may lead to a better understanding of ion production in a_dc sputtering discharge.

Using a conventional cylindrical magnetron arrange-ment with magnetic field parallel to the cylinder axis, it was noted that at current input levels of 2 mA cm" 2 the plasma was a typical bluish argon color next to the copper target but an intense green color beyond. This coloration was also present using nitrogen as the sput-tering gas but absent at comparable inputs using an aluminum alloy target. A flame test on copper gives a similar green coloration though it should be remem-bered that different types of emission spectra are now being compared. This green coloration has also been observed with the "sputtergun". g

As the pressure is varied, there is a sharp transition in the current-pressure curve as shown in Fig. 1. Our results were obtained with a modified "cylindrical" magnetron arrangement designed for high-rate sputter-ing production applications though a similar effect is

d(cros) 20

15

10 -

05,

p:75µ

p= 3µ 1- -p=1ov

10 50 90 B (Gauss I

FTG. 3. Current Cl) vs magnetic field (B) for different pressures.

I(mA) 10 9 a 7

6 5 4 3 2

p.1SJµ

• CATHODE DARK - ----1 SPACE

TARGET

190

SUBSTRATE

NEGATIVE GLOW

FIG. 4. Electron trajectory in perpendicular electric-magnetic fields.

seen with a conventional cylinder. The transition mani-fests itself as a sharp switch, possibly from one stable state to another, and therefore it is possible to sharply increase the current drawn by the target, and hence the deposition rate, by only slightly varying the pressure.

As the "switch" occurs, the cathode dark space length , also changes sharply as indicated in Fig. 2. •

If the current drawn by the target is plotted against the magnetic field as shown in Fig. 3, it can be seen that again the pressure choice has an important bearing on the current characteristic. In some regions, the curve exhibits a maximum and lowering the magnetic field can increase the current drawn; this fact appears to contradict, usual thoughts on the magnetic field role.

Calculations suggest that at the pressure correspond-ing to the transition, the maximum height from the tar-get, ymu (see Fig. 4), of the electron orbit (assuming no collisions) in the perpendicular electric-magnetic fields is equal to the cathode dark space distance d. With reference to Fig. 2, at 15-Am pressure ymu<d, while at 30 pm ymu> d. This seems to be connected with the important question of whether ion production is predominantly in the cathode dark space or negative glow region of the plasma.

As the magnetic field is reduced, the switching phe-nomenon becomes less sharp and is totally absent with no magnetic field. This would seem to suggest that the effect is concerned Ivith electron motion in the plasma and not with target geometry, especially since similar results have been seen on modified cylindrical configurations.

Further work is in progress in an attempt to fully explain the effect.

11. Brodie, L.T. Lamont, and D.O. Myers, J. Vac. Set. Technol. 6, 124 (1969).

2 L. Holland, T. Putner, and G.N. Jackson, J. Set. Instrum. 1, 32 (1968).

313.J. Ball, J. Apps. Phys. 43, 3047 (1972). 1B.N. Chapman, D. Downer, and L.J.M. Guimaraes, J. Appl. Phys. 45, 2115 (1974).

5E. Kay, J. Apps. Phys. 34, 760 (1963). 6J.S. Chapin, Iles. Dev. No. 1, 37 (1974). 'R.P. Riegert, Res. Des. No. 2, 64 (1973). B.T.A. Thornton, Trans. S.A.E. Detroit 1973, Paper No. 730544. '

3P.J.. Clarke (unpublished).

190 Appl. Phys. Lett, Vol. 27, No. 4, 15 August 1975 F.A. Green and B.N. Chapman 190

Iectron effects in magnetron sputtering F. A. Green and B. N. Chapman Department of Electrical Engineering. Imperial College. London SA7 2BT. England

(Received 18 August 1975; in final form 9 October 1975)

Over a range of sputtering conditions in a cylindrical magnetron system, current-pressure. characteristics show an unexpected discontinuity and current-magnetic-field curves show a maximum current. These phenomena seem to depend on the relationship between the maximum distance of the secondary electron initial trajectory from the target surface, and the target dark space thickness.

PACS numbers: 79.20.M, 81.20., 52.80.H

191

° INTRODUCTION

Considerable interest has been shown in recent years in the use of sputtering devices which can achieve a high deposition rate. Since sputtering is due to target iirabardment by ions and high-energy neutrals, higher rates are usually achieved by increasing the electron ar:d hence ion populations in the plasn' Two obvious ; cthods of achieving this are to inject extra electrons il:to the plasma region (triode system) or to use a mag-rctic field to increase the number of ionizing collisions et the existing electrons.

In this paper we report and discuss some results which have been obtained with a magnetically sup-ported dc sputtering system. Some preliminary results 'Inc already been published.'

Ft SULTS

i sing a conventional cylindrical magnetron arrange-` T-':nt (Fig. 1) with a magnetic field parallel to the axis

cf, a copper target, a sharp and reversible increase in `•i:rrent was observed with increasing argon pressure (I'd. 2). The current increase is accompanied by a '".responding decrease in cathode dark space (C DS) tl•ct'kness (Fig. 3), and the appearance of an intense

green glow in the discharge. Further measurements were made of target current against magnetic field (Figs. 4 and 5). Contrary to expectations, in some pressure regions these results showed the current reaching a maximum and then decreasing for further increases in magnetic field strength.

DISCUSSION

We are unsure about the mechanisms involved in the circumstances described above, and the main purpose of the following is to promote discussion.

To understand the effects of a magnetic field on a glow discharge, we first need to understand the.mecha-nism of the basic glow discharge. However, this mecha-nism is still unclear, particularly under sputtering con-• di tions, even though research on the subject dates from the last century.

In qualitative outline, the basic mechanism of the sputtering glow discharge is understood and is described in most introductory texts. However, as soon as we examine the theory in more detail, problems arise such as knowing the region of origin in the plasma of the ions which strike the target. Druyvesteyn and Penning' and Brewer and Westhaver3 suggest that most of the ions come from the negative glow, and Davis and Vanderslice' using energy analysis at the cathode,

B

I

Axial Magnetic Field

Chamber and Substrate•

90

j 80 (MA)70

60

50

. 40

30

20

it5

9.80 Gauss Fin. 1. Cylindrical magnetron splatter-ing arrangement.

10

5 10 20 30 p (microns)

FIG. 2. Current 1 vs pressure p with magnetic field of 80 G.

J. Vac. sei. Technol., Vol. 13, No. 1, Jan./Feb. 1976 Copyright © 1976 by the American Vacuum Society 165

;a.

192 left

(mA)t .0o - 90 -80 -

70 -60-50 -40 -30 20 10

40 65 90 B (Gauss)

p•18p

1000 Energy(&) I

FIG. 6. Efficiency of ionization for electrons in argon.

9

7 6

5

4

3 2

1

0

p.190

p•15p

166 F. A. Green and B. N. Chapman: Electron effects in magnetron sputtering

.d (cms)

2.0 5.50 Gauss

1.5

1.0

0.5

1 1 t I I t t —s 0 5 15 25 35

p(microns)

FtG. 3. Cathode dark-space distance d vs pressure p with magnetic field of 80 G.

conclude that ions originate in or near the negative glow. However, it has alternatively been concluded by Little and Von Engel' and by Holmes and Cozens' that ion production is mainly in the cathode fall, and further that there is a significant contribution to second-ary electron emission from the target due to photons from the negative glow. It should be pointed out, however, that both models have a point of similarity in that they both require the negative glow to sustain a plasma, in one case to provide additional ions and in the other to provide photons for increased secondary electron production. • Holmes and Cozens' have further suggested that a pressure gradient can exist in the negative glow with gas pressure increasing towards the cathode due to the ion flux. Their figures suggest that this would be very significant in high-rate sputtering where current densities may exceed 10 mA cm-2. They have, however, not considered the flux of sputtered atoms and reflected high-energy neutrals which would oppose or may even reverse the pressure gradient due to the ion flux.

In spite of these reservations about our understanding of the glow discharge, we are still able to suggest some qualitative interpretation of our results. Sputtering

(mA) 10

p•13y p•1Ou

1 1 I 1 1 t 1 1 1 10 20 30 40 50 60 70 eo 90

B (Gauss)

FIG. 4. Current 1 vs magnetic field B for different pressures.

J. Vac. Sol. • Technol., Vol. 13, No. 1, Jan./Feb. 1976

FIG. 5. Current I vs magnetic field B for different pressures.

conditions produce very high ratios of electric field tn, pressure so that electrons rapidly reach the ionization' threshold for argon with little chance of collision.;: The efficiency of the ionization process (defined numbers of positive charges produced per electron per cm path per mm l Ig pressure at 0°C) has been 1ne )surLj: by Smith' and is shown in Fig. 6. Data in Massey an; Hurhop'shows that the ionization cross section remains• within a factor of two of 3 X 10-2° m2 for electron energic . between 40 and 800 eV, and we may thus take it aL•• sensibly constant in this range. A simple collision mood then predicts

Fd/ Fo = exp (noq,d) ,

where Fo is the electron flux at the target, Fd is th electron flux at the end of the CDS, no is the ga molecular density, g; is the ionization cross section, an.i. d is the CDS thickness. For typical values of 30 pm gas pressure and d=2 cm, then Fd /Fo= 1.89. Then figures are in agreement with data from Von Engel9 ar": an analysis using the results of Smith.' Since secondar. electron coefficients due to ion impact are generally lc ,; than 0.3,2•'°.'I this would suggest either that more originate in the negative glow (charge exchange in tl: CDS does not produce an additional ion) or that a ic'• ., of secondary electrons are due to photon bombardreep• or from high energy neutrals.

Efficiency

193 F. A. Green and B. N. Chapman: Electron effects in magnetron sputtering 167

Since the electric field becomes small close to and in •=,y negative glow, electrons tend not to pick up energy

rapidly after collision, and hence their mean energy :ie,:reases; this will at first increase both, the ionization "r,j excitation cross sections, causing the negative glow.

ionization cross section decreases below 100 eV ; king excitation increasingly important until this also

ch a ppears below about 11 eV (the threshold for excita- • tion in argon) marking the end of the negative glow

al .ough the anode may have intervened first.

ELECTRON TRAJECTORY

The action of a magnetic field B (along a c axis) ;perpendicular to the electric field (along the y axis) is ro produce electron motion along an x axis (Fig 1).

The equations of motion are

9_ (e/m)CE(y) — Bx], x= (e/m)B y ,

where B is the magnetic field strength and e (y) is the electric field.

By considering the gains and losses of potential and khietic energy, we can show that the maximum height y._.„ of the electron trajectory (assuming no collisions)

- is given by 127r:

ymax= B[— e

(v,—v)] ,

where v, is the (negative) target voltage and v is the - Potential at y,„ax. This expression holds both inside and eitside the CDS and is a better approximation at lower pressures when there are fewer collisions to alter the trajectory.

In the negative glow, the electric field, and hence v, ns a weak function of y and is almost constant as, therefore, is ymax ; on the other hand, because of the M'rong field in the CDS, v, and hence ym „a, is a strong function of y.

Consider a situation where, for a given constant clagnetic field, the pressure is high enough that ymax

in the negative glow, i.e., ymax>d. As we now reduce the pressure, ymax remains sensibly constant until

d being a strong function of pressure. A further r'.x:uction of pressure will now cause a rapid reduction

ymax as ymax and v enter the strong-field CDS region '-i the discharge.

BY comparing the value of ym„x with the data of 3, it can be seen that the sharp change in d at •

20 µm corresponds closely to the ymax =d situation. 20 µm, ymax> d while below 20 µm, ymax<d.

.Phe CDS thickness is a fundamental parameter of discharge. By comparing the magnetic/nonmagnetic

•'"'d situations we see that to a first approximation, the CDS distance will be the corresponding y'displace-

°nL when the path length is equal to the previous `-i'S thickness, i.e., the addition of a magnetic field "creases the CDS thickness, as observed.

Vac. Set. Technol., Vol. 13, No. 1, J.an./Fab. 1976

Since ionization is more efficient in the negative glow than in the CDS, we might expect a significant change in

glow characteristics when we alter the electron popula-tions by changing from a ymax>d to a ymax<d situation.

INTERPRETATION OF RESULTS

A quantitative solution does not look hopeful. But consider qualitatively the situation at say 30 Am Hg pressure and y,aax>d with a suitable magnetic field. As the pressure p is reduced, d increases towards yn„x, and fewer collisions occur. The negative glow is now producing fewer ions and photons and hence the current falls. A situation eventually arises where the negative glow fails to provide sufficient ions or photons: and the glow would be extinguished in the absence of a magnetic field. However, with a magnetic field, the glow now adopts a different form as d goes to ymax and the CDS lengthens to increase ion production within the CDS, an important point being that for each ion created in the CDS, there is unit probability of an ion striking the target, while ions or photons formed in the negative glow are in a very low field region, are further from the target, and are more likely to be lost before reaching it.

Let us now turn to the y,,,ax <d situation (i.e., 10 to 19 µm) in Fig. 4; under these cōnditions the discharge is more dependent on the CDS than with ymax >d. Now, as B is reduced, ymax increases towards d and ionization will in general take place nearer to the CDS/negative-glow boundary. This will produce a higher current because the new electrons formed will be in the weaker electric-field region, will accelerate more slowly, and will therefore be more likely to cause further ionization within the CDS. This gives a possible mechanism for current increase with magnetic field reduction, and at lower pressure (larger d), the maximum current would occur at a lower magnetic field as observed in Fig. 4. As B is further reduced, electrons escape more easily from the glow causing the current to fall again (as it eventually must do since B= 0 at, say, 10 Am Hg pressure, is not normally a selfsustaining situation.)

The results in Fig. 5 also show that pressure choice is critical to the current characteristic, the switch from one state to the other being induced by the magnetic field rather than pressure in this case.

CONCLUSION

The current drawn by the discharge is a strong function of magnetic field and perhaps more surprisingly shows a sharp change in a specific pressure range. Whilst high pressures and magnetic fields are compatible with high deposition rates, a low operating pressure may be required on grounds of film purity and accurate masking of specific substrate areas. In the light of the above results, the parameter choices should be arranged such that the 'ym„x

=d. situation is avoided in the

interests of plasma stability. An important aspect of electron behavior in a

sputtering discharge is the important role electrons play

168 F. A. Green and B. N. Chapman: Electron effects in magnetron sputtering 194

in substrate bombardment." We expect this behavior to be modified in magnetron situations. Preliminary observations confirm this and will be investigated in more detail.

ACKNOWLEDGMENTS

Thanks are due to Dr. J. Cozens for his interest and advice, and to the Science Research Council for provid-ing a grant to one of us (F.A.G.).

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'A. Von Engel, Ionized Gases (Oxford University, New York, 1965), 2nd ed., p. 63.

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