A study of reaction mechanisms in plasmas related to glass-fiber productionCitation for published version (APA):Saes, L. H. (1987). A study of reaction mechanisms in plasmas related to glass-fiber production. TechnischeUniversiteit Eindhoven. https://doi.org/10.6100/IR256689

DOI:10.6100/IR256689

Document status and date:Published: 01/01/1987

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a study of reaction mechanisms

_ in plasmas ..~...,. \~ related to

.••.. ·•.. r ~ glass-fiber production

l.h. saes

A STUDY OF REACTION MECHANISMS IN PLASMAS

RELATED TO GLASS-FIBER PRODUCTION

A Sl.UDY OF REACTION MECHANISMS IN PLASMAS

RELATED TO GLASS-FIBER PRODUCTION

PROEFSCHRIFT

ter verkrijging van de graad van doctor aan de T echnische Universiteit Eindhoven, op gezag van de rector magnificus, prof. dr. F.N. Hooge, voor een commissie aangewezen door het college van dekanen in het openbaar te verdedigen op

vrijdag 6 februari 1987 te 16.00 uur

door

Ludovicus Hen ricus Saes

geboren te Nederweert

Druk: Oissertatiedrukkerij Wibro, Hetmond.

Dit proefschrift is goedgekeurd

door de promotoren:

Prof.dr. H.H. Brongersma Prof.dr. F.J. de Hoog

aan mijn ouders

Wie aileen van zichzel£ hee£t geleerd.

hee£t een dwaas tot leermeester gehad.

Hen Jonson.

-v-

TABLE OF CONTENTS

1 . GENERAL INTRODUCTION. 1 1.1. The production of solar grade silicon. 1 1.2. The production of opticaL fibers. 2

1.2.1. Processes for the production of opticaL fibers. 3 1.2.2. Important properties of opticaL fibers. 6 1.2.3. A possible application of SiF4 in opticaL fiber

production. 9 1.3. Framework of the thesis. 11

2. FUNDAMENTAL PROCESSES INVOLVING IONS OF SiF4 AND SiCl4. 13 2.1. Introduction. 13 2.2. Energetics of ion production by electron impact. 14

2.2.1. Definitions. 14 2.2.2. Relation between AP, IP, EA and other

thermochemical quantities. 16 2.3. Preutousty reported data on thermochemistry and

reactions of ions of SiF4 and StCL4. 17 2.3.1. ThermochemicaL data. 17 2.3.2. Ion chemistry. 21

2.4. ExperimentaL methods used to study the ions of SiF4 and StCt4. 22 2.4.1. Introduction. 22 2.4.2. The trapped-electron apparatus. 22 2.4.3. The Ion Cyclotron Resonance (ICR) technique. 24

2.5. Ion production and ton chemistry. Results of measurements. 26 2.5.1. Trapped-electron measurements. 26 2.5.2. ICR measurements. 27

2.5.2.1. Reactions with parent gas. 27 2.5.2.2. Reactions in mixtures. 29 2.5.2.3. Influence of impurities. 30

2.6. Discussion. 31 2.6.1. Thermochemical data. 31 2.6.2. Ion-molecule reactions. 35

3. THE USE OF THERMODYNAMIC CALCULATIONS FOR THE STUDY OF THE CHEMISTRY IN SiF4-D2 AND SiCl4-D2 PLASMAS. 38 3.1. Introduction. 38 3.2. GeneraL strategy of the method. 41 3.3. The principles of chemicaL equilibrium caLcuLations. 44

3.3.1. Basic theory. 44 3.3.2. The computer program. 45

3.4. CaLcuLations of equiLibria in Si-F-0 and Si-CL-0 systems. 48 3.4.1. Introduction. 48 3.4.2. St-F-0 systems, the infLuence of eLementaL

composition. 49 3.4.3. St-F-0 systems, the influence of totaL pressure. 50 3.4.4. Comparison of the Si-CL-0 and Si-F-0 systems. 50

3.5. CaLcuLations for incompLete systems. 54 3.5.1. Introduction. 54 3.5.2. ChemicaL equiLibria in Si-F-0 systems when SiF4

and/or SiF3 are omit ted from the catcuLat tons. 55

-VI-

3.5.3. Chemical. equilibria in Si-F-0 systems when the 02 11tol.ecul.e ts excluded. 58

3.5.4. Equilibria in systems with excess SiF4. 59 3.6. Perturbation of equilibrium calculations For

investigation of equilibrium shifts. 62 3.6.1. Introduction. 62 3.6.2. Perturbation oF the equilibrium in Si-F-0 systems. 64 3.6.3. Perturbation oF the equilibrium in Si-Cl.-0 systems. 72

3.7. Discussion. 73

4. A MASS SPECTROMETER SYSTEM FOR MEASURING THE GAS PHASE COMPOSITION OF A PLASMA. 80 4.1. Introduction. 80 4.2. Description oF the apparatus For mass spectrometry. 81

4.2.1. The microwave system. 81 4.2.2. The expansion compartment and the analysis section. 84 4.2.3. Theoretical. background of the design of the

extraction system. 88 4.2.4. The quadrupole service electronics and computer

Facilities. 93 4.3. Experimental. results and discussion. 95 Appendix A. 97

5. LIGliT EMISSION SPECTROSCOPY OF SiF4, 02 AND SiF4-o2 MICROWAVE DISCHARGES. AN ORIENTATION. 102 5.1. Introduction. 102 5.2. Earlier studies on the emission spectra of

SiF4 fragments. 102 5.3. Experimental.. 103 5.4. Results. 104 5.5. Discussion. 107

6. THE UV SPECfRUM OF AN SiF4 MICROWAVE DISCHARGE IN THE REGION OF 220 - 250 nm. 110 6.1. Introduction. 110 6.2. Experimental.. 113 6.3. Results and discussion. 114

7. REACfiON MEOIANISMS IN A roNTAMINATED OXYGEN PLASMA AFTERGLOW. 124 7.1. Introduction. 124 7 .2. Experimental.. 128 7 .3. Theoretical description of the decay of the optical.

emission in the afterglow of discharges. 132 7.3.1. Introduction. 132 7.3.2. Exponential. decay. 132 7.3.3. Non-exponential. decay. 135 7.3.4. Some aspects oF the analysis of the data. 136

7.4. Measurements of the afterglow emission intensity oF the OH ~A2.r --+ X2 l1i) transition. 138

7.5. Measurements oF the aFterglow emission intensity of the NO + 0 chemiluminescence. 143

7.6. Discussion. 149

8. mNCLUDING REMARKS. 150

SUMMARY.

SAMENVATIING.

REFERENCES.

NAWOORD.

aJRRICULUM VITAE.

-VII-

154

156

158

169

173

-1-

Chapter 1. General Introduction.

The main subject of this thesis has to do with the possible use of

SiF4 for the manufacturing of optical fibers for telecommunication

purposes. SiF4 is a waste product of the fertilizer industries. For

the production of fertilizers calcium salts ( calciumfluoride,

calciumphosphates and calciumsilicates) are extracted from the

phosphate ore by washing the minerals that contain these salts with

concentrated sulphuric acid. The main byproduct of this process is

HzSiF&. In the Netherlands the annually produced quantity of this acid

amounts to several hundreds of tons (1986). At present H2SiFo is

treated as a waste product.

But from H2SiFo SiF4 can be produced. At room temperature this is a

gas. Its sublimation point is 178.35 K. A relatively simple

destillation procedure can be used to remove impurities such as HF

{GME59) and the (salts of the) transition metals. Once this

purification has been performed, SiF4 might be a proper raw material

for production of solar grade silicon {photovoltaic cells} or pure

quartz {optical fibers). Of course, the production of pure Si and pure

Si02 from the purified SiF4 should be performed in such a way that no

impurities are introduced by the production process. In section 1.1

and 1.2 a few of these processes are mentioned.

1.1. The production o£ solar grade silicon.

Hunt {HUN77) has studied the feasibility of several methods to

produce solar grade silicon. In his comparison he includes the

following source materials:

silica {Si02). which can be reduced by carbon either in an arc

furnace process or in a high temperature plasma;

- metalurgical grade silicon, which can be purified through vapour

phase transport (see below) or through production of SiH4;

- SiF4 , which can be reduced by reaction with sodium;

SiC1 4 for which he proposes a plasma reduction process with

sodium, a fluidized bed reduction with zink and a plasma

reduction with hydrogen.

-2-

He includes an economical analysis of these processes except for the

SiF4 source material. For this species he indicates that further

purification of the produced SiF4 may be necessary. This could be done

by vapour phase transport via SiF2 (HUN77). Then SiF4 is led over

silicon at a temperature of about 1300-1400 °C. The reaction

Si(s) + SiF4(g) ~ 2 SiF2(g) • l.I

produces SiF2(g) that is transported and converted back to SiF4 in

colder regions of the set-up.

Amouroux and coworkers (AM079, CXJU82, MEX83) have looked at the

production of solar grade Si from various starting materials with high

temperature plasmas. The objective of the use of high temperature

plasmas is to dissociate all gas constituents into atoms. In the

quenching zone downstream the plasma the silicon is deposited on a

cold wall. From thermodynamic equilibrium calculations at high

temperatures (>3000 K) and economical considerations they concluded

that SiF4 probably is not the most profitable starting material for

this process. The stability of SiF, even at very high temperatures

(>6000 K) will most likely introduce considerable amounts of fluorine

in the Si deposit (AM079, MEX83).

1.2. The production or optical Cibers.

The second option for a high-tech application of SiF4 is in the

manufacturing of optical fibers for telecommunication. For a better

understanding of the applicability of SiF4 in this field it seems

justified to give a short introduction to the production processes and

desired properties of optical fibers.

In the last decade significant progress has been made in the

manufacturing and application of optical fibers (see KA081. BEN81.

GEI86 for reviews). The application of fibers for telecommunucation

has several adva__:ages over the use of copper wires (GEI86):

- smaller cable size and weight,

- elimination of earth loops,

- no electromagnetic emission and few cross-talk problems,

-3-

- much greater bandwidth

- immunity to electromagnetic interference,

- handling of high voltages without isolation transformers,

usage under most circumstances is possible (high temperatures,

explosive atmosphere, humidity etc.),

potentially lower cost and energy saving.

As disadvantages one could name:

- the small dimensions of the fiber core demand high-precision

connectors and accurate manipulation to obtain good connections,

- the polarization state of the light is not constant,

- reflections can occur at connector interfaces.

We will now look at the production process for optical fibers and at

the desired properties before coming back to the applicability of SiF4

in this field.

1.2.1. Processes for the production of optical fibers.

Several methods for the manufacturing of optical fibers have been

proposed (see GEIS6). All of these methods consist of two steps. First

a preform is made of which subsequently the fiber is drawn.

At present, routinely used industrial processes for the

manufacturing of fibers start from the gases SiC14 and 02.

In essention these are heated and through the overall reaction 1. II

Si02 is produced and deposited.

l.II

The nature of the final product and the rheological properties of

Si02 require that it is deposited in an almost final form. It seems

not economical to produce bulk Si02 in first instance and use this for

high purity and high performance fiber production in a subsequent

step.

Three most commonly used methods to produce preforms will be

described here. Keck (KEC73) uses the Outside Vapour Phase Oxidation

(OVPO) process (fig. 1.1a). In this process he starts with a mandrel

on the outside of which Si02 is deposited. The SiCl4 and 02 are

supplied to a burner that is moved up and down the mandrel. In this

-4-

way several layers of SiOz soot are deposited. The mandrel is rotated

during the deposition process to obtain homogeneous layers of soot.

At the end of the deposition process the mandrel is removed and the

sooty preform is collapsed (at high temperature) to a solid glassy rod

from which the fiber is drawn.

soot deposit

\

(a)

(b)

Fig. 1.1. Principles of three processes for optical fiber preform

production.

a. Outside Vapour Phase Oxidation (OVPO) (KUPBO)

b. Modified Chemical. Vapour Deposition (MCVD) (KUPBO)

c. Plasma-activated Dlemical Vapour Deposition (PCVD)

(BAC85).

In the Modified Chemical Vapour Deposition (MCVD) process (NAG82)

the reactant gases are led through a rotating quartz tube (fig. l.lb).

An Oz-Hz burner is used to heat the reactant mixture to temperatures

where glassy particles are formed in the gas phase. These glassy

particles deposit downstream of the hot zone. By moving the torch up

and down the quartz tube previously deposited layers are sintered to

opaque layers and new layers are simultaneously deposited.

-5-

A third method that is used for the deposition of silica is the

Plasma-activated Chemical Vapour Deposition (PCVD) process (KUPSO,

BACS5). The gases SiCl4 and 02 again are fed into a quartz tube in

which they are made to react. This time, however, the reaction is

induced by a microwave plasma {fig l.lc} that is generated in a

cavity. The deposition of Si02 is instantly vitrified because a

furnace keeps the quartz tube constantly at 1400 K (KUP78, KOE75,

KUPSO}. By moving the cavity the reaction zone is moved and layers of

Si02 are deposited on top of each other.

Fig. 1.2. PrincipLe of the drawing of fibers from preforms (GEI86).

The preform is heated to its softening point (top section).

The fiber that ts drawn from it ts led through a bath with

coating material (lower part of the top section). Then the

coating is hardened and the fiber is wound on a collecting

drum.

-6-

Once the preforms have been made, they are collapsed to rods

(diameter in the order of a few em) and subsequently drawn to fibers

(overall diameter of about 150 Mffi).

In fig. 1.2 a schematic view is given of the drawing process. The

preform is placed in the top of the drawing apparatus and heated by

oxyhydrogen burners to its softening point. The blank fiber that is

drawn from the tip of the preform is very vulnerable and therefore it

is coated as soon as possible by leading it through a bath of coating

material. The coating is hardened subsequently and the fiber is wound

onto a collecting drum. In the drawing process the radial geometry of

the fiber stays the same as in the preform.

1.2.2. Important properties of optical fibers.

In order to be a useful means of telecommunication an optical fiber

should meet at least two essential demands: low loss and low pulse

broadening.

In order to achieve low loss in the fiber it should first of all be

free of cracks and gas bubbles. Also impurities that absorb the light

that is to be transmitted should not be present. Light sources that

are used in connection with fibers are LED's (0.8 0.9 Mffi and

1.1 - 1.7 Mro) and lasers (0.85, 1.3 and 1.55 Mro) (GEI86). The

obtainable attenuation for these wavelengths is in the order of a few

tenths of a dB per km (GEI86) for silica fibers. Impurities of the

transition elements (Mn, Ni. Cr. Fe, Cu etc.) have large disastrous

effects on the attenuation properties of the fiber (KUPSO). The

concentrations of these elements should be kept below 1 ppb to achieve

the demanded low loss.

Another important impurity that influences the absorption of the

transmitted light is formed by OH-groups that are incorporated in the

silica structure. In fig. 1.3 for instance the calculated attenuation

as a function of the wavelength is given for a fused quartz fiber with

0.2 ppm OH incorporation. The presence of OH groups thus forms a

serious problem in the production processes of the fibers. Much care

is taken to avoid their incorporation.

-7-

6 7 INTRPo!SIC ATTENUATION, ;tm

r---r~-~--~ 8 9 10 I I YT' 2C.JI'i'-4-'l' 1'-...-''ih

r

SCATTERING

lNFRAREO AEISORPTION

ZERO l O!SPERS!ON

\ l \]

Fig. 1.3. Theoretical spectral loss of a fused quartz wave-guide with

0.2 ppm OH. The peaks indicate absorptions of OH-groups

(Olshansky in BEN81).

A fiber can only guide light when there exists a difference in

refractive index between the core of the fiber and its surroundings.

Therefore, an optical fiber exists of a core and a cladding.

In principle three types of optical fibers are produced nowadays (see

fig. 1.4).

The propagation of light rays through the fiber can be explained in

terms of geometrical optics. A light ray is trapped in the core of the

fiber as long as it is incident on the interface core-cladding at an

angle larger than the angle for total reflection (see fig. 1.4).

If the arriving angle of the ray is steeper, the path is longer and

the time necessary for traversing the fiber is also longer. In this

way pulse broadening arises.

In a stepped index fiber this effect is largest.

The refractive index profile in a graded index fiber is chosen such

(nearly parabolic) that the various light rays need equally much time

to travel through the fiber. This enhances the bandwidth by a factor

of about ten (see table 1.1).

CROSS SECTIONAL VIEW

G Step index multirnode tl ..... 25-150 m1crons

Graded index multtmode "- 20-150

INDEX PFlOcllE

Rad1al distance

-8-

INPUT PULSE

8 .. ~U" Jl Single mode T steu mdtrx ,,, ~ 1$-8 mtcrons

LIGHT PATHS OUTPUT

PULSE

Fig. 1.4. Types of opticaL fibers. The muttimode step index fiber is

shown on top. Next the graded index fiber and finatty the

monomode fiber are shown. Geametricat cross sections,

refractive index profiles, input pulse, tight paths in the

fiber and output pulse are given. (Giattorenzi in KA081).

In a monomode stepped index fiber the core diameter is about equal

to the wavelength of the light that is used (1.5-8 IJ.m). In such a

fiber only one mode of the electromagnetic wave can propagate in the

core. This reduces the dispersion considerably compared to the graded

index fiber: the bandwidth is again enhanced by a factor of 10 to

about 1 GHz km.

A disadvantage of the single-mode fibers is the low diameter of the

core. This introduces d1fficul ties in connecting two fibers to each

other. because the alignment of the cores is a crucial parameter in a

telecommunication system.

T

-9-

TabLe 1.1. TypicaL vaLues for some characteristics of fibers

compared with conventionaL techniques (GEI86).

Fiber Indicative Single Multi Graded Electrical Radio

values mode mode index cable step index

Attenuation 10-3 in dB/km to 1 10 1 to 10 10 2

Dispersion in nslkm w-2 10 to 50 0.2 to 2 20 10-3

Bandwidth, 6 dB in 1000 10 100 50 15 MHz km

In the scope of this thesis it is not relevant to give more details

about these matters. For further information one can read the reviews

of Klippers {KUPBO) and Bachmann {BAC85) or the compilations of studies

edited by Kao {KAOBl), Bendow {BENB1) and Geisler {GEIB6).

1.2.3. A possible application of SiF4 in optical fiber production.

The variations in refractive index necessary to produce fibers are

introduced by adding dopants to the deposition of Si02. Graded index

profiles can be achieved by adding more dopant in subsequent layers of

the deposition. The effects of various dopants are shown in fig. 1.5.

In fig. 1.6 the refractive index profiles of several types of fibers

produced with the PCVD technique are shown. Commonly used dopants are

Ge02 , P20s, B203 and F (GEI76, ABE79, KUP7B, KUPBo). The first three

of these are mostly introduced in the form of the chlorides of Ge. P

and B respectively. Several sources of fluorine have been used.

Kuppers has used SiF4 {KUP78) while Bachmann reports the use of C2F6

{BACB5), both in PCVD processes.

-10-

refrQcltve llldex

Fig. 1.5. Variation of the refractive index of Si02 with the

concentration of several dopants (BAC85).

(a) (b) (C)

(d) (e) (f)

Fig. 1.6. Refractive index profiLes for fibers prepared with the

PCVD technique (BAC85),

a. graded index muLtimode, dopant Ge02 ;

b. graded index mul timode , dopant F;

c. step index muL timode, dopant Ge02;

d. depressed cladding singLe mode, dopants Ge02, F;

e. deeply depressed cLadding single mode, dopants Ge02, F;

f. dispersion flattened singLe mode, dopants Ge02, F.

-11-

Not only is fluorine a cheap dopant, it also positively influences

the long term stability of the fiber and it reduces the O» absorption

peak (BAC85). The combination of a decrease of the refractive index

and the other advantages make 1 t extremely sui table for doping the

cladding of the fiber. It is routinely used for this purpose nowadays

(see fig. 1.6).

As far as we know SiF4 has not routinely been used as a supply for

fluorine atoms.

1.3. Framework of this thesis.

In the investigations presented in this thesis it is tried to

better understand the reaction mechanisms in an SiF4-02 plasma and to

compare this plasma with an SiC14-Q2 plasma. A better understanding of

both systems could lead to the application of SiF4 in the fiber

manufacturing and help to solve the waste problem of the fertilizer

industries.

The search for reaction mechanisms in a chemically active plasma

can start at several points. One can look at elementary processes to

study the properties of the molecules and radicals the plasma consists

of. This gives information about microsopic processes that may occur

in the plasma. One can also look at macroscopic properties and overall

reactions in order to find clues for reaction mechanisms.

In this work several techniques have been used. Each technique that

has been used is dealt with in a separate chapter.

In chapter 2 elementary processes are described and studied.

Electron-molecule interactions that lead to the formation of ions are

dealt with. The trapped-electron technique is used to investigate

inelastic scattering and negative ion formation in electron-SiF4

interactions. Ion formation and ion chemistry of the positive and

negative ions of SiF4 and 02 is described. Finally, in this chapter an

analysis of thermochemical data that have been reported in literature

is presented.

Chapter 3 deals with the thermodynamic modelling of the chemistry

in the plasma. Although the plasma is not in thermodynamic equilibrium

it is shown that thermodynamic equilibrium calculations in a model

with perturbed parameters are a valuable tool to investigate the

-12-

reaction mechanisms that lead to the formation of Si02 in SiF 4-o2

plasmas. A description of the computer program that is used for these

calculations is also given. The method presented in this chapter may

turn out to be useful for the modelling of other plasma processes.

An experimental set-up for mass spectrometric analysis of the

discharge is discussed in chapter 4. The design of this apparatus has

been directed to obtain a good picture of the compost tion of the

plasma and to eliminate effects of background pressure. Because of the

high reactivity of water with SiF4 and SiCl" special care has been

taken to minimize the water background pressure. Cryoshields have been

applied to achieve this goal.

Chapter 5 is an introduction to chapters 6 and 7. All three

chapters deal with different aspects of emission spectroscopy.

In chapter 5 the spectra of plasmas in SiF4, 02 and SiF4-o2 mixtures

are presented and the main features are discussed. The differences

between the SiF4-o2 spectrum and the sum of the spectra of SiF4 and 02

plasmas are pointed out and analysed.

A controversy that exists in literature on part of the spectrum of

an SiF4 plasma is the subject of chapter 6. The band system in the UV

spectrum between 220 and 250 nm has been measured with a 2 m

spectrograph and analysed. Comparison with absorption experiments

performed on SiF2 shows that the emitter of this band system must be

SiF2.

Impurities in oxygen gas cause an afterglow emission of an OXYgen

discharge. Especially water impurities are important in the light of

the aforementioned incorporation of OH groups in deposited silica

layers. In chapter 7 a study of the afterglow emission of an OXYgen

discharge is presented. A method to purify oxygen from incorporated

H20 is also investigated.

In chapter 8 some general concluding remarks are given.

-13-

Chapter 2. Fundamental processes involving ions of SiF4 and SiC1 4 •

2.1. Introduction.

In plasmas ions are produced, among others, through electron impact

on molecules and atoms, through Penning ionization and through charge

transfer. They form an essential component in plasmas. In order to

obtain insight into the chemical processes that occur in plasmas, it

is, therefore, necessary to study the ion chemistry.

In our case oxygen and halogens are present in the discharges.

so not only positive ions, but also negative ions must be taken into

account. Verhaart (VER78) showed that some halocarbons (CFzClz and

CF3Cl) easily form stable negative ions on electron impact. Also, the

CF3Cl and CF4 molecules show energy resonances that indicate the

existence of temporary negative ion states of CF3Cl- and CF4 . These

processes all occur at electron energies below 2 eV. Since in plasmas

most of the electrons have low energies, negative ions could very well

play an important role.

The chemistry

paths. Radicals,

in plasmas can proceed through various reaction

for instance, can determine the way in which a

product is formed. But if ion chemistry of positive or negative ions

would show reaction paths along which the same product can be formed,

such a mechanism could be energetically advantageous. Especially when

negative ions, formed at low electron impact energies, play a role in

such a process, reaction thresholds could be lowered.

This chapter deals with the study of ion chemistry. First of all.

some important quanti ties related to the production of ions will be

defined in section 2.2. Several relations between these and other

thermochemical quantities are discussed. These quantities can be

helpful to decide whether certain processes are energetically possible

or not. Literature data on these quantities and on ion chemistry of

Sif4 and SiCl4 are presented and discussed in section 2.3. In section

2.4 the experimental techniques are described, which are used in the

present study. Section 2.5 gives the results of the measurements and

finally the conclusions are given in section 2.6.

-14-

2.2. Energetics of ion production by electron impact.

2.2.1. Definitions.

Appearance Potential (AP).

Electron impact on atoms or molecules may lead to the following

ionization processes:

e + M ~ M• + 2 e ionization 2.I

e + AB ~ A• + B + 2 e dissociative ionization 2.II

e + M ~ M-" electron attachment 2. III

e + AB ~ A- + B dissociative attachment 2.IV

Here is M a molecule or atom,

A and B are atoms or polyatomic groups.

Each of these processes can occur only if the energy of the electron

exceeds the threshold value for that process. The observed value of

the threshold is called the appearance potential (AP).

Ionization Potential.

The ionization potential for process 2. I can be defined as the

energy required to remove an electron completely from the neutral

particle in the ground state, to form the corresponding molecular ion

(or atomic ion) also in its ground state (see e.g. FI£57 p 26). The

ionization potential and the appearance potential are often unequal.

The product ion will not necessarily be formed in its ground state:

IP(M) + E"r , 2.1

in which E"r is the sum of kinetic and excitation energies of the

M+ ion and the electron.

Electron Affinity (EA).

The electron affinity is defined as the difference in energy

between the ground state neutral molecule plus an electron at rest at

-15-

infinity and the ground state negative (molecular) ion (see e.g. Clffi84

p 484}. The electron affinity is positive if the ground state D6gative

ion is more stable than the ground state neutral molecule plus an

electron at infinity.

The relation between the electron affinity and the appearance

potential is:

AP(W) EA(M) + El<r . 2.2

It follows that if EA > 0 also E~r ) 0 and the product ion will always

be formed in an excited state.

Excess energies.

In electron impact measurements of appearance potentials one should

always reckon with the possibility that product ions are formed in an

excited state or that the products have non-zero kinetic energies.

In other words, E~r # 0. When positive ions are produced (processes

2.1 and 2.11), the kinetic energy of the molecular or atomic species

in general is small. The electrons that are involved in the process

will, because of their low mass. carry the major part of the excess

energy. In cases of predissociation with an excited (AB+)"

intermediate this does not necessarily hold. When negative ions are

produced, there are no free electrons after the collision. Therefore.

in processes 2.111 and 2.IV all excess energy will be transfered to

the molecular products and ions.

The excess energy generally consists of three contributions:

61 + tv + te .

Here is t 1 the translational energy of the products,

tv the vibrational energy of the products;

te the electronic excitation energy of the products.

2.3

For the case of negative ion production Franklin and coworkers have

developed a way to determine E" r from measurement of tt .

A semi-empirical relationship on the basis of classical statistical

mechanics for the average kinetic energy of the products, et. and the

quantity (E"r - te) has been given as (HAN6S):

appearance:

ground state excess. energy potential+

of positive ion

i

initial ground ~ state

(a)

ionization ?Otentioi

-16-

appearance cmte:nt1al T

' ... r I e)(cess energy

tmtal grQUnd . .'tate ·--r-r electron

ground state _jJ_ affinity of negative ion

(b)

Fig. 2.1. ReLation between the appearance potentiaL, excess energy

and ionization potentiaL (a) or eLectron affinity (b) for

formation of positive and negative ions in eLectron impact

processes.

f:e ) I a N , 2.4

with aN the "effective" number of oscillators in the products.

Experiments showed that a has to be taken 0.44 in most of the cases

studied (HAN68).

In fig. 2.1 the relations between appearance potential, ionization

potential. electron affinity and excess energies are summarized for

processes 2.1 and 2.1II.

2.2.2. Relation between AP. IP. EA and other thermochemical

quanti ties.

For the processes 2.11 and 2.IV. dissociative ionization and

dissociative attachment, the appearance potential can be expressed as:

D(A-B) + 1P(A) + E~r , 2.5

D(A-B) EA(A) + E"r . 2.6

w1th D(A-B) the bond strength betwe~n A and B.

-17-

E E

a b

Fig. 2.2. Relation between appearance potential, excess energy and

ionization potential (a) or electron affinity (b) for

dissociative ionization and dissociative attachment

processes respectively.

In fig. 2.2 these relations are illustrated. The bond strength is

related to the standard heats of formation according to:

D{A-B}

Applying this to 2.5 and 2.6 gives:

2.3. Previously reported data on thermochemistry and reactions of

ions of SiF4 and SiCl4.

2.3.1. Thermochemical data.

2.7

2.9

In tables 2.1 to 2.3 literature data on heats of formation,

ionization potentials, appearance potentials and electron affinl ties

TabLe 2.1. ThermochemicaL data reported in Literature for the siLicon fLuorides. ALL vaLues are given in eV.

Heat of formation Ionization potential Appearance potential of SiFn• from SiF4

AHf 0 IP AP

SiFs

SiF4 -16.75 WIS63 ca 15.71 DON68 e -16.75 DON68 a 15.0 FAR77a t

15.81 ROS77 pe 16.1 JAD77 pe

SiF:J -10.19 DON68 c 8.5 HAS69 c 16.20 DON68 e -11.41 STU71 c 11.5 FRA77a t -11.24 FAR77a t -10.37 SCH78 c -10.62 WAL81 t

SiFz -5.90 DON68 e li.29 DON68 e 27.35 DON68 e -6.10 STU71 t 11.0 EHL64 e -6.13 FAR77a t 11.08 FEH74 pe -6.02 EHL64 t 11.5 FAR77a t

SiF -0.3 JOH58 s 7.26 JOH58 s 28.75 DON68 e -o.14 EHL64 e 7.3 DON68 e -o.09 DON68 e 7.5 EHL64 e -o.20 STU71 t 8.5 FAR77a t -o.25 FAR77a t 0.07 ROS77 a

Techniques used for the determination of the quantities: a value adopted from other authors c calculations ca calorimetry e electron impact m magnetron/surface ionization

Appearance potential Electron affinity of SiFn from SiF4

AP EA

6.3 CHR84 c

10.65 WAN73a e 3.35 PAG69 m 2.04 WAN73a e 2.04 FRA74

<2.95 RIC75

1.0 RCS77

p photo detachment pe photo-electron spectroscop:r s light emission spectroscop;t t thermodynamic equilibrium :malysis

e p

c

I ..... 00 I

TabLe 2.2. ThermochemicaL data reported in Literature for the siLicon chLorides. ALL vaLues are given in eV.

Heat of formation Ionization potential Appearance potential of SiCln+ from SiCl4

AHr 0 IP AP

SiCl4 -6.81 SfU71 ca 11.79 ROS77 pe -6.81 ROS77 11.6 VOU47 e -6.81 PAB77 a

SiCb -4.16 SfU71 c 12.9 VOU47 e -3.95 PAB77 a 12.48 ROS77 e

SiCh -1.70 SfU71 ca 11.8 VOU47 e 18.4 VOU47 e

-1.72 PAB77 a

SiCl 1.98 SfU71 ca 20.5 VOU47 e 1.97 PAB77 a

Techniques used for determination of the quantities a value adopted from other authors c calculation ca calorimetry e electron impact pe photo electron spectroscopy

Appearance potential Electron affinity of SiCln- from SiCl4

AP EA

6.5 PAB77 e 1.1 PAB77 e

~-7} JAG68 e 5.9

:.7}

e 0.64 PAB77 e JAG68

>2.5 JAG68 e 0.8 VOU47 e >2.6 VOU47 e

I ,_ '"' I

Table 2.3. Thermochemical properttes of seueral relevant spectes. The data were taken from seueral comptlattons (STU7l, ROS77, LEV82, CHR8~).

All ualues are gtuen tn eV.

Heat of formation Ionization potential Appearance potential of M• Electron affinity AH,o IP value parent byproduct EA

H 2.2591 13.598 17.3 H2 H- 0.7542 19.415 HF F

F 0.82 17.422 15.6 F2 F- 3.445 19.008 F2 F

Cl 1.25 12.967 11.86 Ch Cl- 3.613

0 2.5826 13.618 17.272 02 o- 1.461 18.99 02 0

I~ 4.67 8.151 1.385 I

OH 0.41 13.18 18.05 H20 H 1.8254 ~ I

HF -2.82 16.007

HCl -0.956 12.748

H20 -2.506 12.614 1.65

SiO -1.04 11.58

S102(g) -3.2

H2 0 15.4256

F2 0 15.686 2.9

Ch 0 11.48 2.46

02 0 12.059 o2•(a1 ITu) at 16.104 0.15

-21-

for the silicon fluorides, the silicon chlorides and other for our

studies relevant species have been collected. Most of the data

initially have been found in compilations as the JANAF tables (STU71),

the issues by the National Bureau of Standards on the energetics of

gaseous ions (ROS77. LEVB2) and reviews as given by Field (FIE57) and

Christophorou (aiRB4). From the tables 2.1 and 2.2 it becomes clear

that the studies on SiF4 have yielded more consistent results than

those on SiC1 4. We will return to this issue in section 2.6.

2.3.2. Ion chemistry.

Only a few earlier studies on the ion chemistry of ions derived ·

from SiF4 are known. Only ion-molecule reactions of the positive ions

have been studied. Reents (REEB4. REE85) has used Fourier Transform

Mass Spectrometry. Senzer {SENB3) has used an ion-beam/scattering gas

set-up in which the ions had kinetic energies in the range of 0.2 to

7.0 eV.

These studies have shown that the ions of SiF4 are not very

reactive towards their parent gas. Reents (REEB5) has indicated that

SiF4 normally is contaminated with substantial amounts of Si2F&O.

Both studies, however, did not consider reactions of SiFn+ (n = 1-4)

ions with oxygen.

The only results reported on negative ion chemistry come from mass

spectrometric studies. Harland {HAR71) and MacNeil {MAC70) have

reported the formation of the SiFs ion at higher pressures

(> 10-3 Pa) in their ionization chamber. They have not studied the

chemistry of this ion. The study of Jager (JAG6B) has shown that o­reacts with SiCl4 forming SiCl30-. Still the picture of the reactions

of positive and negative ions of both SiCl4 and SiF4 with 02 is far

from complete. This is one of the reasons that we have carried out a

study on these reactions.

-22-

2.4. Experimental methods used to study the ions of SiF4 and SiCl4.

2.4.1. Introduction.

Two experimental methods have been used to obtain more information

on the ions of interest. In section 2.4.2 the trapped-electron

apparatus for the study of the negative ion formation is discussed.

In section 2.4.3 the ICR technique is presented.

The SiF4 that has been used for our studies was obtained from

Matheson (purity 99.6%). It has been used without further

purification. The SiC14 has been obtained from Wacker and was

subjected to a few freeze-pump-thaw cycles before it was admitted to

the ICR-cell in order to remove incorporated gases.

2.4.2. The trapped-electron apparatus.

In 1958 Schultz {SCH58) introduced the trapped-electron method to

study interactions of low-energy {0 - 30 eV} electrons with atoms and

molecules. In this method electrons that underwent inelastic

collisions with gas molecules, having a final energy lower than a

threshold value, are collected and detected. It enables the study of

both optically allowed and optically forbidden excitation processes.

In addition ion formation can be investigated with this technique.

The essential part of the apparatus to obtain a trapped-electron

(TE) spectrum with sufficiently high resolution is the electron gun.

In the beginning {SCH58, BR068) the RPD method {Retarding Potential

Difference) was used to produce a monoenergetic electron beam with a

fwhm of 100 meV (SCH58). Lateron this was substantially improved with

the introduction of the TEM {Trochoidal Electron Monochromator)

{STA70, ROY72, VEE76). This feature improved the performance to a fwhm

of 40 meV at a beam current of 1 nA.- Verhaart finally introduced a

slit electrode that allows for higher beam currents (up to 10 nA} at

the same resolution.

For our experiments we have used the apparatus described by

Verhaart (VERSO). In fig. 2.3 a schematic drawing of the electrode

system and the potentials applied to the electrodes is given. The

-23-

collision chamber is differentially pumped by two oil diffusion pumps

(background pressure in the 10-6 Pa range). During operation the gas

pressure in the collision chamber is in the 0.01- 0.1 Pa range.

(a)

v

w 0~+~=====+====~----r--

(b)

Fig. 2.3. The trapped-electron apparatus. Electrode system (a) and

potentials (b).

The vacuum system is surrounded by a solenoid creating a magnetic

field (0.3 T) along the beam axis. This field confines the electrons

to a helical path near the axis of the apparatus. The magnetic field

plays an important role in the energy selection systems (STA70, VERSO

ch 2) and the collection of the trapped electrons (SCH58).

On the left-hand side in fig. 2.3 the electron gun with the slit

and the TEM is drawn. In our experiments with SiF4 a fwhm of 30 meV

has been achieved.

On the right-hand side the collector for the electron beam, a

Faraday cup, is located.

-24-

In between, the collision chamber with the collector for the

trapped electrons is visualized. The potential (W) of the collector

plates is chosen just above. zero. Electrons that suffered inelastic

collisions and have a final energy smaller than eW cannot escape the

well and are collected on C2 (fig. 2.3). If ions are formed they will

always reach C2 because of their large cyclotron radius. This is even

independent of the well depth W for a wide range of positive and

negative values of this parameter. Thus a signal on C2. independent of

W, indicates ion formation. A signal that depends on W points out

processes in which excitation of molecules takes place. From the shape

of a TE-peak (a peak in the trapped-electron spectrum) and its

behaviour as a function of W. one can derive information about the

kind of excitation it is linked with (VEE76, VERSO).

2.4.3. The Ion Cyclotron Resonance (ICR) technique.

The Ion Cyclotron Resonance (ICR) principle was first used by

Hipple (HIP49) for gas analysis. Since then it has been developed as

an important tool for studying many phenomena in which ions are

involved. A good survey of the applicability of ICR has been given by

Wanczek (WAN84) .

In fig. 2.4 the essential part of an ICR apparatus, the resonance

cell, is drawn. The walls of this cell carry a small positive

potential to prevent ions that are produced in the cell to drift to

the walls. This potential is called the trapping potential. A magnetic

field is applied to the cell to make the ions move in helical paths

around the field axis. The characteristic quantity of this movement is

the cyclotron frequency We given by:

eB/m. 2.10

To detect ions of mass m, an rf field of frequency We is applied to

two walls of the cell such that the direction of the rf field and the

magnetic field are perpendicular. The energy absorbed from the rf

field by the ions is measured. This energy is proportional to the

number of ions in the cell. By scanning either the magnetic field

strength or the rf frequency an energy absorption spectrum,

representing a mass spectrum of the ions in the cell, can be measured.

-1V

-1V

Ftg. 2.4. The ICR-cett.

-25-

,..'4 7 I

I I I I I

To study gas phase reactions of ions in an ICR-cell the

trapped-mode ICR technique is commonly used nowadays {a.o. LEH76,

VEL81, REE84). In this mode of operation the electron beam that

produces the ions is pulsed. After the electron beam pulse the ions

are trapped in the cell during time intervals up to a few seconds,

allowing for chemical reactions. At the end of this time interval a

mass spectrum is recorded during the detect pulse. Finally all ions

are ejected from the cell by a DC pulse on one of the trapping plates

and then the whole cycle starts all over again. This sequence is

further illustrated in fig. 2.5.

It is also possible to eject specific ions from the cell directly

after the electron beam pulse. This is done by applying an rf pulse of

sufficient power and of frequency w0 , characteristic for the ions to

be removed. These ions recombine at the walls of the cell and

reactions starting from these ions cannot take place anymore. With

this so called double-resonance technique one can identify reaction

paths in ion chemistry. A more detailed description of this technique

can be found in LEH76 or VEL81.

-26-

electron beam

n double resonance r -----~ .__ __ -----JI , reaction time

--------~1_·----------~·r--l~ __ d_e_~_c_t_io_n ________ __

ejection n ---------J '----

Fig. 2.5. The puLse cycLe used in trapped-mode ICR measurements.

For the measurements described in this chapter we have used a home

built ICR-spectrometer that was described earlier (HAR77 and VEL81}.

Some additional features were added since then, which, among others,

make it possible to perform rapid scan measurements.

The measurements have been carried out at electron energies of 27

and 51 eV for positive ions and at lower energies for the negative

ions. Pressures in the ICR-cell ranged from 10-6 to 10- 3 Pa. Trapping

times of the ions have been varied between a few ms and 10 s.

2.5. Ion production and ion chemistry.

Results of measurements.

2.5.1. Trapped-electron measurements.

The results of the yrapped-electron measurements for SiF4 are given

in fig. 2.6. The cu~nt on the trapped-electron collector is plotted

versus the electron energy at three values of the well depth. The

energy calibrati..m has been carried out by admixing helium and

d~termining the positions of the various excitation maxima relative to

the 23 S and 2 1S excitations at 19.82 eV and 20.96 eV respectively.

In the spectra at the two well depths unequal to zero the tail of

the elastic scattering peak at the low energy side of the spectrum can

be recognized. No inelastic scattering, and thus no excitation, occurs

-27-

' ~,_;,..! i ~W•0.06V' ~ i--·~ w=-o.,oov:

0 5 10 15 20 E-eW leV)

Fig. 2.6. Trapped-eLectron current for SiF4 as a function of the

eLectron energy at various weLL potentiaLs.

for energies below 10 eV. The first excitation above 10 eV is due to

the formation of negative ions. This can be concluded from the

constant intensity of this peak in the spectra for W = 0.00 eV and

W = 0.06 eV. The maximum of the negative ion peak lies at

11.12 ± 0.08 eV. The appearance potential can be determined at

10.2 ± 0.2 eV. The other excitation maxima are found at

11.85 ± 0.06 eV, 12.99 ± 0.14 eV, 13.79 ± 0.15 eV and 15.09 ± 0.12 eV.

There is not enough information available for their assignment.

With the Trapping Well Modulation technique (TWM) as described by

Verhaart (VERSO) it has been attempted to find resonances of temporary

negative SiF4 ions analogous to those in CF4 (VER78), but no evidence

has been found for their existence.

2.5.2. ICR measurements.

2.5.2.1. Reactions with parent gas.

SiF4.

As it could be expected from the appearance potentials in table

2.2. at 27 eV electron impact energy only SiF4+ and SiF3+ were formed

directly from SiF4. At 51 eV also SiF2+ and SiF+ were seen. The ion

intensities of these four ions were in good agreement with those

reported by Cornu (COR66).

-28-

In double resonance measurements the following reactions could be

detected:

2.V

2.VI

At low pressures {10-6 Pa). reaction 2.V was slow compared to reaction

2.VI At higher pressures (> 10- 5 Pa) the reaction rate of reaction

2.V increased rapidly as could be expected for a recombination

reaction.

Reaction 2.VI was so fast that at high pressures, the SiF4• ion is

not found at all. At low pressures a few tens of ms were sufficient to

let it completely disappear. In agreement with Reents (REE84) we found

that SiF+ and SiF2+ are unreactive towards SiF4.

The presence of F+ and st• ions was not observed.

Measurements of negative ions showed a maximum ion pr.oduction at an

electron energy of about 10.5 eV. Both the formation of F- and SiF3-

were at their maximum at this energy. The secondary ion SiFs- (earlier

reported in aqueous solutions (CLA67) ) was also observed, but the ion

intensities were too low to identify its precursor.

SiCl4.

In SiC1 4 at electron energies of both 27 eV and 51 eV the primary

ions SiCl•. SiCl2•. SiCl3+ and SiCl4+ were seen. SiCl3+ was the most

abundant ion followed by SiCl4•. SiCl+ and SiCl2•. Adduct formation

from SiC13+ (as in reaction 2.V) did not occur:

2.VII

The Si2Cl7+ ion was still formed however. but out of SiCl4•:

2. VIII

The ion intensity of Si2Cl7•. stayed low, also at higher pressures.

The other ions, SiCl+ and SiCl2+ showed no reaction with SiCl4.

-29-

Similar to the situation for SiF4, Cl+ and Si+ were not seen.

Measurements of negative ions showed the formation of Cl and

SiCb- at an electron energy of 8.8 eV. Again both ion intensities

were at their maximum at this energy. The SiCl5- ion was not formed.

2.5.2.2. Reactions in mixtures.

SiF4 + 02.

In mixtures of SiF4 and 02 (50% - 50%) charge transfer was observed

in addition to reactions 2.V and 2.VI:

2.IX

This reaction turned out to be very rapid. The formation of SiF3+ in

a reaction of 02+ was not observed. The negative ions of SiF4 did not

react with 02. The negative o- ion. however. formed at an electron

energy of 5.4 eV, shows the following reaction:

2.X

o2- formation was not observed.

SiCl4 + 02.

Charge fransfer from 02 + to SiCl4 was the most important feature of

the ion molecule reactions in an SiCl4-Q2 (50% - 50%) mixture. In

agreement with the thermochemical data the reaction

2.XI

was observed. This charge transfer reaction was reasonably fast. After

150 ms at a pressure of 2.10- 4 Pa no 02 + was found anymore. Charge

transfer from 02+ to SiCl4, forming SiCl3+, has not been observed.

Using double resonance ICR the formation of SiCl30- could be shown to

proceed according to:

2.XII

Again the ions Cl- and SiCl3- did not react with 02.

-30-

2.5.2.3. Influence of impurities.

Siloxanes.

In the mass spectra of SiF4 peaks were also detected at higher

masses. These had to be attributed to Si2FsO+ and Si2F60•. The latter

of these peaks was very small and not always present. The former ion

is formed in an ion-molecule reaction with SiF3•:

2.XIII

Another possibility for the formation of Si2FsO+ might be:

2.XIV

but the (minor) presence of Si2F60+ and the absence of SiF30H+ make

reaction 2.XIII more probable.

Analogous features occurred for SiCl4. The formation of Si2Cls0+

proceeds according to:

2.XV

In the negative ion mass spectrum also impurities in SiF4 could be

seen. The ions Si2FsO-, Si2F7o- and SiF30- were present. In the

spectrum of SiCl4 only SiCI30- could be detected. The intensities of

these ions all were too low, however, to carry out double-resonance

measurements to trace the precursors.

H20.

Much more important than the impurities in SiF4 and SiCl4 was the

water background pressure (about 10-7 Pa). Water reacted with the ions

in such a way that halogen atoms were exchanged by OH-groups:

+ HF ' 2.XVI

2.XVII

2.XVIII

-31-

The ions that showed this behaviour were SiF3+, Si2F7+, SiCl4+,

SiCl3 +. SiCl2 +. SiC!+ and the impurity ions SbF5o• and Si2Cl50+.

Of other ions (such as SbCh+) the concentration was too low to

observe this effect.

The exchange of halogen atoms did not stop after one step, but went

all the way until the Si-atom was completely surrounded by OH-groups.

Addition of 02 to the mixture enhanced the reactions considerably, but

this should be ascribed to the water content of the OxYgen gas from

the cylinder (see chapter 7).

2.6. Discussion.

2.6.1. Thermochemical data.

The data found in the literature on the thermochemical properties

of the silicon fluorides show, in several cases, major differences and

some inconsistencies.

One of the difficulties is the determination of the heat of

formation of SiF3. This quantity is needed to describe the following

processes with their respective thermochemical relations:

2.XIX

e 2.XX

2.XXI

D{SiF3-H} 2.14

-32-

The appearance potentials in 2.11, 2.12 and 2.13 have been measured

(DON68, WAN73a) just as the kinetic energies of the negative ions in

2.XX and 2.XXI. From the latter (according to 2.4) the non-electronic

part of E2o and E21 has been determined (WAN73a}. Using these values

and AH1°(H), AH1°(F) and AH1°(SiF4) from table 2.3 and 2.1 and

AH1 °(SiF3H) and D(SiF3-H) from WA.Lsl. DON78 and FAR77b one finds

(numerical values in eV):

2.15

10.70 = AH1°(SiF3} EA(SiF3) + 0.82 + 16.75 + 5.36 + E2o , 2.16

10.65 = AH1°(SiF3) - 3.45 + 0.82 + 16.75 + 1.39 + E21 . 2.17

4.34 = AH1°{SiF3) + 2.26 + 12.70 . 2.18

From equation 2.18 it follows that AH1°(SiF3) equals -10.62 eV.

With this value it follows from equation 2.15, 2.16 and 2.17 that

IP(SiF3} = 9.25 eV, EA(SiF3) 1.61 eV . E2o = 0 eV and E21 = 5.76 eV.

Of these values only the electron affinity of SiF3 seems to be too low

compared to other values (see table 2.1).

The other possibilities for solving equations 2.15 to 2.19 seem

even more unreliable.

The value for AH1°(SiF3) in the JANAF tables is based upon the

assumption that AHr for reaction 2.XXIII is 0.02 eV.

2.XXIII

With this equation AH1°(SiF3} is determined to be -11.42 eV.

Substitution of this value in 2.15 to 2.18 gives: IP(SiF3) = 10.04 eV,

EA(SiF3) = 0.87 eV, E2o = 0 eV, E21 = 6.56 eV. The electron affinity

of SiF3 becomes even smaller. It also gives a 0.87 eV discrepancy in

equation 2.18.

McDonald {DON68} has based his calculation of AH1°(SiF3) on

equation 2.15 and a value for IP(SiF3) calculated by extended Hlickel

-33-

theory, of 8.5 eV. Later on this value has been published by Hastie

(HAS69). With this as a starting point McDonald has calculated that

AH1°(SiF3) = -10.19 eV. Wang has derived EA(SiF3) = 2.04 eV and E21 = 5.33 eV from this value. The discrepancy in 2.18 now becomes 0.43 eV.

Farber {FAR77a) also gives a consistent set of values for the heats

of formation, close to those in the JANAF tables.

At this stage the value for AH1°{SiF3) derived from 2.XXII seems

most reliable, because this is an isodesmic reaction. This means that

the reactants and the products contain the same numbers of the same

bonds. Calculated and measured results for the change of enthalpy in

this reaction. therefore, easily harmonize {WAL81. SCH78. OON78). Also

this reaction is independent of the SiF4-SiF3-SiF2-SiF system.

A minor problem in the list of values in tables 2.1 to 2.3 is

AH1°(SiF). For this we adopt the spectroscopic value given by Johns

(JOH58a) for D(Si-F). In table 2.4 the adopted values and calculated

bond dissociation energies are listed.

Table 2.4. Thermochemical properties of the fluorosilanes.

All values in eV.

n AH1°(SiFn) IP(SiFn) EA(SiFn) D(SiFn-t-F) D{SiFn-1+-F)

1 - 0.02 7.26 5.42 5.58

2 - 5.90 11.29 6.42 2.67

3 -10.62 9.25 1.61 5.54 7.58

4 -16.75 15.71 6.95 0.49

The SiC14 data in literature are much less consistent than those

for SiF4. The processes in which negative ions are formed analogous to

SiF4 are:

+ Cl . 2.XXIV

AH1°(SiCl4) + Ez4 .

2.20

-34-

2.XXV

Besides these processes also the'following ones are reported:

2.XXVI

2.XXVII

2.XXVIII

Not all authors, however, agree on the mechanism along which these

fragments are formed and on the neutral products that are formed.

Also. there is some controversy on the appearance potentials of these

ions. A comparison of refs. PAB77, JAG68 and VOU47 gives an impression

of these differences (see also table 2.2). Our measurements will be

discussed in the next section.

At this point we only calculate AH, 0 (SiC13) and IP(SiCl3) from the

data on the following reactions:

2.XXIX

2.22

2.XXX

2.23

Using the values for D(SiCb-H), AH, 0 (SiCl3H). AH, 0 (SiCL!) and

AP(SiCl3+) from Steele (STE62) and Walsh (WALSl) we find (numerical

values in eV):

12.46 = AH, 0 (SiC13) + IP(SiCl3) + 1.25 + 6.81 . 2.24

3.96 = AH, 0 (SiC13) + 2.26 + 4.99 . 2.25

-35-

This gives AH1°(SiCb) = -3.29 eV and IP(SiCl3) = 7. 71 eV. In this

case we have the possibility to (more or less) check these values

against the data for the process {STE62):

e + SiHCl3 ·~ SiCb+ + H . 2.XXXI

AP(SiCl3+) 2.26

It follows that there is a discrepancy of 0.24 eV. This is well within

the error limits and neglectance of excess energies (see section 2.2).

As far as the data allow calculation of bond strengths, these are

presented in table 2.5.

Tab~e 2.5. Thermochemica~ properties of the ch~orosi~anes.

ALL ua~ues in eV.

n AH1°(SiCln) IP{SiCln) D(SiCln-1-Cl) D(SiCln-1+-Gl)

1 1.98 3.94

2 -1.70 11.8 4.93

3 -3.29 7.71 2.84 5.68

4 -6.81 11.79 4.77 0.69 .II

2.6.2. Ion-molecule reactions.

Ion chemistry in silicon tetrafluoride and silicon tetrachloride

shows no major differences. Positive and negative ions are not very

reactive towards their parent neutrals or towards oxygen. The most

important reactions with the parent neutrals lead to formation of

Si2F7+ and Si2Cl7+ respectively.

In mixtures with oxygen the charge transfer from 02+ to SiF4 and

SiCl4 is the most important reaction. The interesting feature of this

reaction is that charge transfer to SiF4 is very rapid, but it stops

after a few ms, leaving considerable amounts of 02+ untouched, while

the reaction with SiCl4 consumes all the 02 +.

-36-

This can be understood easily from the appearance potentials of the

various ions: SiF4+ at 15.71 eV, SiCl4+ at 11.79 eV and 02+ at 12.059

eV in its ground state (X2~9 +) and at 16.104 eV in its first excited

state (a4Uu). Apparently, in the case of SiF4 , the first excited state

of 02 + is responsible for this charge transfer; the ground state

carries not enough energy for this process to occur. In the case of

SiCl4 the ground state of 02+ is capable of ionizing the SiCl4+ and

takes part in the charge transfer.

From the fact that the charge transfer process of 02+ (a4Uu) to

SiF4 can be followed in time during a few ms, it follows that the

lifetime of this level under our experimental circumstances lies in

the order of at least a few ms.

Formation of SiF3+ in a charge transfer process

(AP(SiF3+) = 16.20 eV) is not possible. SiCl3+, however,

(AP(SiCl3+) = 12.48 eV) can be formed, but is not observed either.

Negative ions are formed at rather high electron energies. In the

case of SiCl4, in contrast to other authors (JAG6S and VOU47}, we find

only one negative ion resonance peak with a maximum at 8.8 eV. The

only ions detected are Cl- and SiCb-. The only reaction of the

negative oxygen ions with SiF4 and SiC1 4 is the formation of SiF3o­

and SiCbO- respectively. These ions are the only products of ion

chemistry of oxygen and the two silicon halides in which a

silicon-halide bond is replaced by a silicon-oxygen bond. These ions

also turn out to be hardly formed and they are unreactive to further

replacement of halogens by oxygen.

In fact it follows from these results that ion chemistry is

probably not responsible for the production of Si02 in the SiF4-D2 and

SiCl4-02 discharges. The ions (also the negative ones} are formed at

relatively high electron energies compared to those that are the most

common in plasmas as used for Si02 deposition (see chapter 5-7}.

Therefore, the production of ions will not be very high. These ions

are not very reactive to their parent molecules, so reaction paths

along such lines are very unlikely. The only possibility for the ions

to play a role in the chemistry is a reaction mechanism in which they

primarily react with radicals.

The main feature of their reactions with parent molecules is,

however, adduct formation (Si2F7+. SiFs-. Si2Cl7+). Such reactions can

-37-

also be expected for their interactions with radicals because of the

high bond strenghts in all molecules involved.

In our experiments the ions are produced having kinetic energies

that lie in the thermal range {Ek { 0.05 eV). Of course, at elevated

temperatures there can be reactions that become possible. Senzer

(SEN83) studied the SiF4-D2 system such that the reactant ions have

energies ranging from 0.2 {corresponding to T = 2300 K) to 7 eV. The

most striking difference. in ion chemistry between his experiments and

ours, is that he has not seen the formation of Si2F7+. This means that

the ion is only weakly bound. This is not surprising when the

structure of this ion is as suggested by Reents (REE84):

F F I I

F - Si F - Si - F I I

F F

An estimate of its binding energy, however, can be formulated as

D(SiF3+-SiF4) ) D(SiF3+-F). Otherwise reaction 2.VI would not occur.

This also implies that the energy stored in vibrations in the adduct

formation (2.V) is at least 0.5 eV.

In reactions with water the ions show strong reactivities. Also the

most stable ion (SiF3+) with a bond strength D(SiF2+-F) 7.58 eV is

attacked by water molecules. From this reaction (2.XXVI) we can give a

lower limit for the bond strength D(SiF2+-0H). In reaction 2.XVI the

SiF2•-F bond and the H-QH bond are broken. The H-F and SiF2•-oH bond

are formed. This leads to the inequality:

D{SiF2•-oH) + D{H-F) ~ D(SiF2+-F) + D(H-OH) . 2.27

Since D(H-OH) = 5.17 eV and D(H-F) = 5.90 eV (table 2.3), we find

D(SiF2+-oH) ~ 6.65eV. The reactions of the positive ions with water,

therefore, are. in general, highly exothermic and it is not surprising

that even small amounts of water (estimated partial pressure

10-7 Pa) give rise to detection of these-substitutions.

The same kind of estimation can be made for the SiCl4. There we

find that D(SiCl2+-QH) ~ 6.39 eV.

-38-

Chapter 3. The use of thermodynamic calculations for the study of

the chemistry in SiF4-02 and SiCl4-02 plasmas.

3.1. Introduction.

The modelling of the chemical processes in plasmas has been the

object of several st1:1dies. In general these studies can be divided

into two groups.

The first group consists of kinetic studies of plasmas such as

reported by Capitelli and coworkers (CAP80, and references therein).

Such studies include the modelling of dissociation of molecules,

recombination and excitation (electronic as well as vibrational)

processes. The kinetics of these interactions are used to calculate

the electron energy distribution function in the plasma. In these

calculations also the plasma composition is determined.

Among others Capi telli studied oxygen plasmas with this method

(CAPBO). In his studies he incorporated processes in which:

- electrons excite vibrations in the 02 molecule,

-collisions of the gas constituents excite vibrations,

- electrons dissociate 02 molecules,

- 0-atoms recombine in three-body collisions,

-vibrations are de-excited in collisions with other gas particles,

- dissociation of 02 is induced by collisions with other gas

particles,

-electronically excited particles transfer their energy,

- 03 is involved,

- recombination of two Q-atoms at the wall occurs,

- negative ions are produced in dissociative attachment,

- negative ions recombine to form 02.

In order to be able to give a description of the plasma in such a

way one must know all the rate constants. of the reactions and the

processes involved. The composition of the plasma can be calculated by

solving the set of differential equations that describe the production

and destruction of the various species. Such calculations require

large computer programs and lots of processing time.

-39-

So far, to our knowledge, this way of modelling has only been

applied to simple cases. Gas mixtures with many different species

require knowledge of the kinetics of the above mentioned processes for

each of the components of the mixture. If chemical changes occur, rate

constants for these processes also must be known.

So. if one wants a quick means to estimate what processes could

dominate in the plasma, this method does not seem attractive.

Especially if one looks at chemically active plasmas with many

chemically different species and chemical reactions, the set of

equations becomes large and calculations take much processing time.

Besides, the lack of kinetic data on processes involving chemical

intermediates also becomes limiting in such cases.

A second line of methods for the modelling of the chemical changes

in discharges and especially in the afterglow, is based upon the use

of thermodynamic (equilibrium) calculations. In their studies

Fauchais, Amouroux and coworkers (AM079, COUS2, MEXS3) consider the

plasma as a source of energy to heat the reactant gases. Their plasma

is an arc heater with a temperature of about 4250 K (axJS2). The

evolution of the chemical compost tion of the plasma afterglow is

described by Coudert on the basis of thermodynamically calculated rate

constants, the measured temperature profile and the flow

characteristics of the gas. Amouroux (AM079) uses thermodynamic

equilibrium calculations to estimate the composition of the gas flow

that leaves a plasma of high temperature (3000 - 7000 K). From his

calculations he predicts the possibility of making pure Si from

various compounds such as SiOz, SiCl4, SiF4 and SiH4. In these studies

the quenching of chemical equilibrium that occurs at the exit of the

plasma torch plays an essential role.

So far no studies are known that use thermodynamic equilibrium

calculations to model the chemistry in low density, low temperature

plasmas. This is not surprising, because such plasmas are not in

thermodynamic equilibrium. Even PLTE (Partial Local Thermal

Equilibrium) is not reached in such plasmas { cf. MULS6). In the

discharges that are used for deposition of {thin) layers of various

materials typical values of plasma parameters are:

-40-

- pressure 1 - 1000 Pa,

- power dissipation in the plasma 10 - 1000 W,

-plasma volume up to 500 cm3,

- gas flow in the order of tens of seem.

In such plasmas the excitational temperatures of the gas particles

are different. The electron temperature is also different from the gas

temperature. Still, .the kinetic temperatures of the molecules are

equal for all gas constituents. The number of collisions between gas

molecules is high enough to establish such an equilibrium. For

instance, at a pressure of 100 Pa (1 mbar) the mean free path for

molecules in air at 25 °C is about 6.8 10- 5 m (ROT82). A molecule with

thermal kinetic energy (0.025 eV) thus suffers about 5.5 106

collisions per second.

But although the kinetic energy of the gas particles is in thermal

equilibrium, the chemical composition of the gas may differ from

chemical equilibrium. This can be due to several processes, among

others:

- dissociation of molecules by electron impact,

- reactions induced by vibrationally excited particles,

- reactions induced by electronically excited particles,

- ion chemistry.

These phenomena make up the fundamental differences between a chemical

vapour deposition (CVD) process and a plasma chemical process.

A chemical equilibrium is thus not reached, because the principle of

detailed balancing is not fulfilled.

In this chapter an effort is made to explore possibilities of using

modified thermodynamical equilibrium calculations for prediction of

shifts in thermochemical equilibria in plasmas. Although it will not

be possible to calculate the plasma composition exactly, it is

suggested, that it is possible to predict shifts in thermochemical

equilibrium composition due to plasma interactions and to indicate

which plasma process (dissociation of a specific species, temperature

increase) is responsible for these shifts.

For obvious reasons we concentrate upon SiF4-D2 and SiCl4-D2

pl:'!smac~

-41-

The general strategy of the method will be discussed in section

3.2. Next, in section 3.3 the principles of chemical equilibrium

calculations will be discussed. A description of the computer program

that has been used will also be given. In section 3.4 the results of

the calculations on Si-F-0 and Si-Cl-0 systems in thermodynamic

equilibrium will be presented. Sections 3.5 and 3.6 will deal with two

ways of modifying the equilibrium calculations. Finally, in section

3.7 a discussion of the results will be presented and conclusions will

be drawn.

3.2. General strategy of the method.

The aim of this study is to compare the importance of radicals in

the plasma with respect to the chemical changes that take place in

chemically active plasmas. This is done with a method that is rather

simple compared to kinetic studies. The basis of this method is formed

by thermodynamic equilibrium calculations. If this method works for

the Si-F-0 and Si-Cl-0 systems that we study, it may be a useful tool

for the study of other plasma chemical reactions as well.

In comparing the fluoro- and chloro-silane it is, first of all,

illustrative to look at the enthalpy change of the overall reactions

3.1 and 3.II as a function of temperature.

+ Oz ~ SiOz + 2 Fz ,

+ Oz ~ SiOz + 2 Clz

The enthalpy changes for thes~ reactions are given by:

3.I

3. II

= AH1°(SiOz) + 2 AH1°(Fz)- AH1°(SiF4)- AH1°(0z} 3.1

AH1°(SiOz) + 2 AH1°(Clz) - AHt 0 (SiCl4} AH1°(0z). 3.2

When AHr ( 0 this means that the reaction is exothermic. When AHr ) 0

the reaction is endothermic and cannot proceed without an additional

supply of energy.

So, from fig. 3.1 it becomes clear that reaction 3.II becomes

possible at temperatures higher than 3000 K. while reaction 3.1 stays

-42-

endothermic up to high values of the temperature. In fact fig. 3.1

shows that the reverse (=etching) reactions of 3.I and 3.II are more

probable (in the low temperature region) than the ones proposed .

0 ...... E

" .... (0 388 0 ~

c: 2118 0 ...,

100 100 0 (II (II (..

••• • oa !,.. 0

::J) -II 110 a. .... tD

.I: -~ .. ..., c: w

Fig. 3.1. Heat of reaction as a function of temperature for reactions

(1) SiCL4 + 02 ~ Si02 + 2 CL2

(2} SiF4 + 02 ~ Si02 + 2 F2

Data from EKVISYSf (NOL85).

In the PCVD process (KtiPSO, KUP7S, BEES5) it was found that both

systems, the SiCl4-02 and the SiF4-02 discharges. give rise to Si02

production. Especially in the SiF4 case this means that energy

barriers in the various partial reactions, that ultimately result in

the overall reaction 3. I. can be conquered with the aid of plasma

specific properties.

One may, as a first try, think of reaction 3.1 to be composed of

SiF4 + 02 ~ SiOF2 + 0 + F2 •

SiOF2 + 0 ~ Si02 + F2

3. III

3.IV

Reaction 3. III probably is the rate limiting step in this set of

reactions, because of the br~?aking up of the oxygen molecule. This

costs about 5.16 eV (table 2.3). In a plasma, where 02 is partially

-43-

dissociated by the electrons. this extra energy is no longer a bottle

neck and the formation of SiOF2 may become easier.

In a real chemical plasma, however, reactions 3.III and 3.IV are

not the only ones that take place. All radicals that can be formed

from Si, F and 0 are present in larger or smaller amounts. Each of

these has its own reaction possibilities. Still there are rate

limiting steps in the production of Si02. Also. by the dissociative

action of the plasma, equilibrium rate limitations may be removed.

In order to understand this better, we have carried out

thermochemical equilibrium calculations for the normal systems and for

modified systems in which the dissociation by the plasma is simulated.

The differences in composition at equilibrium of these systems give

information about the rate limiting steps.

The simulation of dissociation by the plasma is performed in two

ways. First the 02 {respectively SiF4 or SiCl4) molecule is omitted

from the list of species that can be present. Oxygen is supposed to be

present in atomic form as far as it is not incorporated in

heteronuclear molecules. Of course, such an equilibrium calculation

does not give a description of a real situation, but one can trace

reaction tendencies as will be shown in section 3.5. Omission of 02

from the chemical equilibrium could in practice be achieved, if the

cross section for electron induced dissociation were very high and

also the possibility of recombination were very low.

A second way of simulating dissociations by plasma specific

interactions is by changing thermochemical data of a molecule. In this

case we enhance the dissociation of 02 by using an "effective" value

for AH1°{02)eff which is larger than AHt 0 (02)· Such a modification

enhances the dissociation of 02. This is more realistic than the

former method, but differences in the equilibrium composition are much

smaller. In section 3.6 this method will be discussed.

Both methods are essentially the same. When AHt 0 {02)et 1 in the

second case is taken infinitely large, the first method results. So,

with increasing AH1°{02)eff the solution of the thermochemical

equilibrium calculations in the second case must change in a

continuous manner from the normal equilibrium composition to that of

the first method.

-44-

3.3. The principles of chemical equilibrium calculations.

3.3.1. Basic theory.

The calculations of chemical equilibrium compositions can be

performed in various different ways. The basic strategy for

calculating chemical equilibria is the minimization of the Gibbs free

energy, given by:

N G = 2 n; 1-lt •

i::::1

in which n; is the molar amount of substance i,

1-lt is the chemical potential of substance i.

3.3

N is the number of species that are taken into account.

The chemical potential of a (perfect) gas species is given by:

with R

T

Pt

0 = 1-l I + R T ln(p; I Po) •

the universal gas constant (8.3143 103 Jlkmol).

the (absolute) temperature,

the partial pressure of substance i,

3.4

Po

!-lot

a reference pressure (in most definitions taken 105 Pa),

the reference chemical potential of substance i at pressure

p •. This quantity is a function of temperature only.

The partial pressure of substance i is given by:

in which Pt is the total pressure in the system, which is kept

n

constant,

n is the total molar amount of gas:

N 2 n; .

i=l

3.5

3.6

-45-

So, the minimization of the Gibbs free energy for a mixture of gases

comes down to the minimization of the sum

N N

~ n 1 ~0 1 + ~ n; R T ln(ni PI / n Po) . 3.7

i=l i=l

Extra conditions for finding the equilibrium composition (n 1 ) for a

real system are of course the restriction that n 1 L 0 and the

conservation of the elemental abundances. If we fix the stoichiometry

of the species that can occur in the mixture in a matrix A and the

total amounts of the elements in a vector ~. then this condition can

be expressed as:

in which n is the vector with the amounts of all species (n 1 } as

components.

3.8

The actual problem thus comes down to the minimization of 3.7 with the

restrictions that 3.8 holds and that n; L 0.

3.3.2. The computer program.

The minimization problem, sketched in the previous section,

resembles standard problems that can be solved with computer

techniques. The procedure usually applied to such problems is an

iterative algorithm. First an estimate of the solution is made. This

is subsequently used to get a better estimate, for instance by

applying a Newton-Raphson algorithm. In this case, however,

straightforward application of such techniques leads to difficulties.

In the course of the calculations it is possible that some values of

n 1 become very small compared to others. In itself this is no problem,

but the calculation of the logarithm in 3.7 leads to numbers that are

out of range for representation in the available word length of the

computer (values ( 10-35 in typical cases).

Transformation of the mathematical problem so that ln(n;) becomes

-46-

the variable also leads to a transformation of the numerical problem.

Then the subtract ion of almost equal numbers (to obtain the next

estimate of a small ln(n1) ) leads to accuracy problems {loss of

significant digits).

In both cases a calculation with double precision numbers does not

solve the problems, because the extension of the range of

possibilities is by far not sufficient for the purpose. This has been

shown in our own trials and also by other authors (cited in NOLS3).

A more sophisticated algorithm is needed to solve this problem.

In this study ,the EKVICALC program has been used (NOL85). This program

is an updated version of the FREEMIN program described by Nolang

(NOLS3).

The EKVICALC program is based on the Brinkley-NASA-Rand (BNR)

method for the calculation of chemical equilibria. Some improvements,

which will be indicated below, have been implemented. The BNR method

uses Lagrangian multiplicators to solve 3.7 and 3.8. Minimization of

3.7 can now be written as the set of equations (for systems with only

gaseous species):

0

~ + ln(n1 Pt I n Po)

where Y is the vector of which the elements are the Lagrangian

mul tipl icators,

M is the number of elements.

3.9

the problem is thus transformed into solving the set of equations

3.6, 3.8 and 3.9. This set is a non-linear set of equations in the

components of Y and !!· It can be transformed into a linear set for

iterative use in two ways,

The first method to do thfs is by chosing the n1 as independent

variables and using a Taylor series expansion for the logarithm of n 1

that is truncated after ti1e first order term. The second method choses

ln(n 1) as the independent variables. The Taylor series for

exp {ln{n 1 )) is then tr:mcated after the first order term. Both

me';hods lead to the same coefficient matrix ·hat has to be solved each

iteration (NOLS3). This matrix is given by:

-47-

F 3.10

with

p A • nd • A1 , 3.11

where nd is the diago~l matrix containing the elements of D·

So far nothing has been done about the numerical problems described

before. These problems turn up as singularities in the matrix F. The

solution suggested by Nolang for these problems is based on a careful

analysis of the matrix P. The elements of this matrix are calculated

according to:

3.12

So n1 values that are too small will not significantly change the

value of the matrix element. This allows a division of the species

into two groups, major and minor. The matrix P can then be written as:

p Pmaj + Pmin , 3.13

where PmaJ contains the contributions of the significant species,

Pmtn contains the contributions of the non-significant

species.

If PmaJ is of full rank then P is non-singular and no difficulties

occur. The equilibrium composition can be computed by taking into

account only the major species. The values of the minor species can be

derived from these. In such a way the calculations can be sped up

considerably.

If PmaJ is singular, one can use Pmln to make it non-singular. This

is done by substituting linear dependent rows of PmaJ by the same rows

of Pmin· This technique is illustrated by Nolang in a few case studies

(NOLS3).

-48-

Another difficulty that arises in using an iterative algori thrn is

the finding of a starting estimate close enough to the real solution.

This should guarantee convergence of the calculation within reasonable

time. Based on equations 3.3. 3.4. 3.6 and 3.8 Nolang introduces a new

method to obtain this starting estimate (NOLS3).

By setting PtiPo = PtiPo in equation 3.4, the non-linearity is

removed from the set of equations and the solution of the remaining

set can easily be obtained. In some cases this leads to a degenerate

system of equations which needs a special treatment. A two-step

approach is used in such cases. In the first step the set of equations

is reduced to as many independent equations as possible. From this the

estimated values for corresponding nt are determined by using a

modified matrix A. Nolang (NOLS3) has shown that this method generates

considerably better starting estimates than other methods (e.g. van

Zeggeren (ZEG70), Clasen (CLA}, Smith {SMI6S}. cited in NOL83).

Special problems arise in the iterative procedures when, besides

gases, also condensed phases are introduced. Special care has to be

taken chosing the condensed phase assemblage in order to keep F (eq.

3.10} non-singular. It is beyond the scope of this chapter to fully

explain these problems and their solutions. Nolang (NOL83) discusses

them in more detail. He incorporated these solutions in a computer

program for calculating chemical equilibria called FREEMIN. An updated

version of this program and the thermochemical database belonging to

it are used in this investigation to calculate the thermochemical

equilibria. This program runs on an IBM compatible microcomputer

equipped with 640 kB of memory and two diskdrives. A typical

calculation of an equilibrium composition for one given temperature

and pressure condition with 16 gaseous species takes only a few tens

of seconds.

3.4. Calculations of equilibria in Si-F-o and Si-Cl-0 systems.

3.4.1. Introduction.

One should first analyse the results of the calculations of

-49-

equilibrium compositions of the normal systems to be able to estimate

the significance of calculations on incomplete or perturbed systems.

In figures 3.2 to 3.6 calculations for normal systems are summarized.

Firstly, the influence of the composition and the pressure on the

equilibrium composition of the Si-F-o system will be discussed.

Secondly, the differences between the Si-Cl-D and Si-F-D systems and

their similarities will be pointed out.

3.4.2. Si-F-o systems, the influence of elemental composition.

In fig. 3.2 to 3.4 equilibrium compositions are plotted as a

function of temperature for three different Si:F:O ratios. The species

that were included in the calculations are: SiF-4. SiF3 • SiF2 • SiF.

Si02 (gas), SiO, SiOF2 , OF, OF2 , 02 F. 02 , F2 , 0. F and Si{gas).

Calculations have been performed at a pressure of 100 Pa (1 mbar).

This is about the pressure at which our discharges are operated. The

thermochemical data for the different species were taken from the

EKVIBASE database, which is part of the Ekvi System thermochemical

calculation programs {NOL85).

From these calculations several chemical tendencies become clear:

First of all it can be stated that exotic molecules such as

OF. 02 F and OF2 play a minor role as could be expected.

- The temperature at which dissociation of 0 2 or SiF4 occurs does

not vary much when another composition is chosen.

- Formation of Si02 has its maximum around 2500 K. The ratio

[02 ]/[Si02 ] is independent of the Si:F:O ratios and equals about

103.

- Also the formation of SiO in the temperature interval 3000

4000 K seems to be fully determined by the ~ount of oxygen atoms

in the system. The ratio [02]2ooK/[Si02]3500K is roughly unity

when there are enough Si atoms to fulfil this. In the case of

excess 0 atoms (fig. 3.3) all Si atoms are transformed to SiO

molecules in this temperature range.

From all three figures anri the indicated features it becomes clear,

that at low temperatures {up to 1700 K roughly} the 02 and the SiF-4

-50-

molecules barely react. At higher temperatures the fluorine atoms are

replaced by oxygen although this is in competition with the overall

dissociation of molecules at these high temepratures. Of course. this

replacement is easier when excess oxygen is present.

3.4.3. Si-F-o systems, the influence of total pressure.

In figs. 3.5 and 3.6 the equilibrium composition of Si-F-o {1:4:2)

mixtures is given for pressures of 1 Pa and 104 Pa respectively.

It can be seen that for lower pressures the temperature at which

radicals are formed in substantial quantities decreases. At higher

pressures this temperature increases. This is a normal feature. For

the Si-F-Q system it can now be quantified. For instance, the

temperature at which the [SiF4] is 1 % of the concentration at low

temperatures (500 K) can be taken for this quantification. We find the

results in table 3.1.

Table 3.1. The temperature at which [SiF4] is 1% of the [SiF4] at

low temperatures (500 K) in a 1:1 mixture of

SiF4 and 02 at three different total pressures.

total pressure T1% inK in Pa

1 2350

102 2850 104 3550

3.4.4. Comparison of the Si-ci-o and Si-F-Q systems.

A difficu1ty arises when one tries to compare the St-cl-o and the

Si-F-0 systems. Thermodynamic data for SiOCh are not available in

EKVIBASE. so this particle cannot be taken into account. The data for

SiOF:a in EKVIBASE have been estimated from those for SiF4 and Si02

(taken from the JANAF tables (811n1)). Now there are two ways to

compare Si-ci-o and Si-F-Q systems. The first way is to estimate

0 E

.,., c :J 0 E (1)

-51-

10~---------------------------------------------------------------

F

10

10-<

10

10

10"'"::---:-r:::-7r::---:-r:::-::<-=-:--r:-"-'l:'::-:-r:-cc-r::---:--l-!il-:.-:-..,.-,--;:-l-L+---:--.---.---::-r--;:::..o'--r--,-,-"......-1-.----i>---T uo o.2o o.4V o.61l :o.ao t.oo 1. Temperature

Fig. 3.2. Thermodynamic equHibrtum composition of an

SiF4-02 (1:1) mixture at 100 Pa.

thermochemical data for Si0Cl2 and apply these to calculate the

equilibrium composition. The second way is to omit SiOF2 from the

calculations in the Si-F-D system and compare the result of such

calculations with that of the Si-Cl-0 system. This second way can be

chosen in this case, because SiOF2 in figs. 3.2 to 3.6 behaves

essentially the same as SiF3. No big influence has to be expected from

the omission. It can be seen in fig. 3.7 that this expectation does

not lead to difficulties.

In fig. 3.8 the calculation corresponding with that in fig. 3.7 has

been carried out for the Si-Cl-D system. Some main features of this

figure are summarized here:

Intermediate radicals such as SiCl3 and SiCl2 are less abundant

than in the Si-F-D system.

-52-

10

10 I

SiF4 10 • "

tO"'

a E

..., 10"2

c :I a E CD

10"3

Fig. 3.3. Thermodynamic equU.ibrium composition of an

SiF4-02 (1:10) mixture at 100 Pa.

- The presence of the chlorine molecule is much more pronounced

than that of the fluorine (at all temperatures). This is not

surprising in view of the big difference in bond strengths

(2.9 eV) of these molecules.

-The presence of Si02 is at its maximum at a temperature of about

1600 K instead of 2500 K in the Si-F-0 system. Moreover the

maximum concentration of this product is about 103 times higher

than in the Si-F-Q system:

In figs. 3.9 and 3.10 the results for the same calculations have

been presented for other compost tions and mixtures. From these it

becomes clear that at about 1400 - 1500 K the equilibrium for the

reaction to form Si02 is completely on the side of its production. The

-53-

!0',_-----------------------------------------------------------~·······--,

10 •

w 0 E

...., w' c ::J 0 E (1J

to-<

te·'L·:c::---r-,-.::::-:-o-::c:c-:--rc--IJl~~--:r:-...,-J~r'-!r::--::-f:--:~~~~:;:!r.~'F."'!;-,.;~..--:-:r.~~'---,l 0.00 0.20 0.40 0.60 O.Sllt.OO !.

Temperature

Fig. 3.4. Thermodynamic equilibrium composition of an

SiF4-02 (40:1) mixture at 100 Pa.

Si02 amount at these temperatures is limited by the least abundant of

the two 02 and SiCl4.

This is a big difference between the Si-F-Q systems and the Si-Gl-Q

system. In the Si-F-0 system the Si02 concentration was fully

determined by the 02 amount introduced (section 3.4.2). In that case

the equilibrium between Si02 and SiO seems to be.important instead of

a reaction as in

3. II

-54-

10~---------------------------------------------------------------,

0 E

Fig. 3.5. Thermodynamic equilibrium composition of an

SiF4-02 (1:1) mixture at 1 Pa.

3.5. Calculations for incomplete $YStems.

3.5.1. Introduction.

As it has been indicated in section 3.2, the calculation of

chemical equilibria of incomplete systems can give insight into

reaction tendencies in a chemically active plasma. In this section the

results of ~uch calculations will be discussed. First it will be

assumed that the dissociation of SiF4 in the discharge is so efficient

that the SiF4 molecule is no longer present in the equilibrium

compost tion. The result of this calculation is presented in section

3.5.2. In section 3.5.3 the calculations with exclusion of the 02

-55-

10

11 I

te·•

w• 0 E

...> w' c :::J 0 E ro

to"'

19 ...

10 ..

Temperature

Fig. 3.6. Thermodynamic equilibrium composttton of an

SiF4-02 {1:1) mixture nt 104 Pn.

molecule are discussed. All these calculations are carried out at a

pressure of 100 Pa and an excess amount of oxygen (Si:F:O 1:4:20) to

simulate deposition plasma conditions. In section 3.5.4 the system

with excess SiF4 is analysed.

3.5.2. Chemical equilibria in Si-F-Q systems when SiF4 and/or SiF3

are omitted from the calculations.

In fig. 3.11 the resalt of calculations for a system without the

SiF4 molecule is presented. Again, such incomplete systems could exist

if the dissociation of SiF4 by the plasma would be the only

equilibrium disturbance caused by the plasma and if this dissociation

-56-

10

10°

to-'

10-2

0 E

_,..) 10-3 c :J 0 E (1l

1o-•

10-s

10-<

10_, o.oo

Fig. 3.7. Thermodynamic equilibrium composition of an

SiF4-02 (1:1) mixture at 100 Pa without SiOF2.

has a very high cross section.

It can be seen that for high temperatures there is no difference

between this system and the one with SiF4 included (fig. 3.3). This is

not surprising, because SiF4 does not play a role in the system at

temperatures above 2500 K, because it has dissociated on thermal

grounds.

In the low temperature range instead of SiF4 the SiF3 radical turns

out to be the stable species. Also some exotic species such as 02F and

OF2 are formed in minor quantities besides F2 and F atoms.

One thing that should be noted is the following. In the complete

systems (fig. 3.2 to 3.4) the [SiOF2]/[SiF3] ratio is close to unity

(as long as there is enough oxygen in the system). In the system

without SiF4 this is not true anymore. The reason for this will become

-57-

10q----------------------------------------------------------------,

10 •

w•

18 ..

0 E

.,.., w' c ::J 0 E (1l

10 ..

10 ..

10 ..

Temperature

Fig. 3.8. Thermodynamic equilibrium composition of an

SiCi4-02 (1:1) mixture at 100 Pa.

clear later' on.

Cl

In the emission spectrum of SiF4 discharges several bands of the

SiF2 radical can be identified (chapter 5). Therefore, also

calculations on a system without SiF4 and without SiF3 have been

performed (fig. 3. 12). Except for one characteristic the outcome of

these calculations does not differ from the previous case: the SiOF2

turns out to be the replacing species for SiF3 at low temperatures

( < 2000 K).

-58-

10

10 ' 02

10. SiC1 4

to·• 0 E

.,..> to·• c ::J 0 E (0

to·•

to'"'

10-s

Temperature

Fig. 3.9. Thermodynamic equiLibrium compostion of an

StCl4-02 (1:10) mixture at 100 Pa.

3.5.3. Chemical equilibria in Si-F-o systems when

the 02 molecule is excluded.

·In the previous section equilibrium calculations on systems with

total dissociation of Sif4 have been presented. We will now look at a

system in which the 02 molecule does not exist.

The equilibrium composition for this system is given in fig. 3.13.

In this case for low temperatures the differences with the original

system (fig. 3.3) turn out to be much greater than in the case of

dissociation of Sif4. The formation of Si02 and 02F below 1000 K

suggests that the 0 radical is capable of attacking Sif4 molecules.

However. this conclusion can not be drawn just like that. In reality a

reaction barrier may exist that hinders this transformation. A higher

0 E

.,.., c :J 0 E (l)

-59-

10~---------------------------------------------------------

10.

Temperature

Fig. 3.10. Thermodynamic equilibrium composition of an

SiCt4-D2 (40:1} mixture at 100 Pa.

temperature or other means of excitation would then be necessary for

the reaction between 0 and SiF4. Nevertheless. on thermodynamical

grounds it is justified to assume that in a system with dissociated 02

molecules and SiF4. Si02 is formed (in the gas phase) at low

temperatures. Around 1500 K the equilibrium composition resembles more

or less the one without the extra dissociation of 02 although the

SiOF2 concentration is somewhat higher and the SiF4 seems to have

reacted a bit more than in the original equilibrium.

3.5.4. Equilibria in systems with excess SiF4.

It may be challenging to draw conclusions about reaction tendenciP!'.

on the basls of the calcmiRtitms in the previous sections. but thi&

-60-

IOq----------------------------------------------------------------,

0 E

-.-> c :J 0 E (U

10 I

10°

10 ...

ID-s

Temperature

Fig. 3.11. Thermodynamic equilibrium composition of an

SiF4-02 (1:10) mixture at 100 Pa without SiF4 .

would be a bit premature. For instance, the conclusion that oxygen

atoms react with SiF4 molecules is not valid yet. The presence of Si02

in the equilibrium composition could also be due to the excess amount

of oxygen introduced in the system. This pushes the equilibrium to the

side of Si02 and explains the presence of Si02. The same set of

calculations has been performed for systems with excess SiF4 to verify

such mechanisms.

Fig. 3.14 shows the system from which SiF4 has been excluded. There

is no visible tendency to form SiOF2 in this case. The general picture

of the equilibrium composition is equal to that in fig. 3.11. SiF3

shows no affinity to react with the 02 molecule.

The same can be said for the case where SiF2 is the most fluor­

enriched particle (fig. 3.15}. The excess of SiF2 again does not show

-61-

!Oq----------------------------------------------------------------,

!0 I

!0 I

0 E

.,.., c :::J 0 E (ll

w'

II""

to"'

1'o.oo ;

Temperature

Fig. 3.12. Thermodynamic equiLibrium composition of an

SiF4-02 (1:10) mixture at 100 Pa without SiF4 and SiF3 .

a tendency to react to Si02 (for instance).

For the SiF4+0 mixture {fig. 3.16) the excess concentration of SiF4

has a major influence. Si02 is not formed in large quantities anymore

and SiOF2 is present instead. This indicates that the equilibrium in

this case depends strongly on the composition of the gas phase. But it

also indicates that SiF4 is attacked by 0 radicals, even if the SiF4

concentration is much higher than the 0 concentration.

-62-

10~---------------------------------------------------------------.

10'

to' Si

W'

0 E

+' w• c :J 0 E (1l

w'

10 ...

10-s

10;~~~~~~~~~~~'"~~~~~~~~~~77.~~~~~~~~~_. 0.00 0.20 0.40 0.60 0.80 I. 0 I.

Temperature

Fig. 3.13. Thermodynamic equiLibrium composition of an

SiF4-02 (1:10) mixture at 100 Pa without Oz.

3.6. Perturbation of equilibrium calculations for investigation

of equilibrium shifts.

3.6.1. Introduction.

The previous section dealt with equilibrium calculations on

incomplete systems. This is a rather drastic way to simulate extra

dissociation of the gas molecules by a plasma. Therefore, in this

section another approach will be used to refine the method. Extra

dissociation will now be simulated by introducing "effective"

thermodynamic data for the relevant substances.

If we study the influence of extra dissociation of the 02 molecule,

-63-

10

10'

10"'

02

w• 0 E

_,j 10"3 c :::J 0 E (0

10"' OzF \ 10 ..

10 _,,j_.....,.,.---,r,-:-,-r:-...,-,..L,,...-+-~---cr:--4::--::-r:-c!-~+-:-t--X.r:-:lt=~"":=F."':--:r::-l'::-r.c-:-:r::-:-':r:--:-r.~ 0.00 00.2000.4000.6000.8111.0 l.

Temperature

Fig. 3.14. Thermodynamic equiLibrium composition of an

SiF4-02 (40:1) mixture at 100 Pa without SiF4.

we change the enthalpy of formation of this molecule so that it

becomes harder to form it (AH1°(02)eff > AHf 0 (02)). In this way the

degree of dissociation can be adjusted and varied smoothly. A more

realistic model of a palsma in which electrons dissociate 02 (for

instance} than the model in which 02 is excluded, is thus achieved.

Unfortunately the results of the calculations are more difficult to

interprete.

In the calculations we change the enthalpy of formation for 02 or

SiF4 (c.q. SiCl4) such that the degree of dissociation (the ratios

[0]/[02]. [SiF3]/[SiF4]. [SiCb]/[SiC14]) increases by 1 %. So for

each temperature the degree of dissociation due to the thermodynamic

equilibrium under normal conditions is calculated; for instance for

oxygen. Then the ratio [0]/[02] is increased by 1 % by changing the

-64-

10 •

Si to'

!0"'

to·• 0 E

.,.., !O"' c

:J 0 E (0

10 ..

to""

10~

to·' 0.00

Fig. 3.15. Thermodynamic equilibrium composition of an

SiF4-02 (40:1) mixture at 100 Pa without SiF4 and Si.

AHf 0 {02). The elemental composition of the system is, of course. kept

constant. The value of 1 % for the increase in dissociation degree is

rather arbitrary and has no further meaning. The adjustment of the

AHi 0 {02) is performed for each temperature seperately. Calculations

are carried out for temperatures in the range 1000 - 3000 K and for

the elemental compositions Si:F{Cl}:O = 1:4:0.05, 1:4:0.5, 1:4:2,

1:4:20. The influence cf the perturbation of the thermodynamic values

on the equilibrium com~osition is measured.

3.6.2. Perturbation or the equilibriLm in Si-F-o s: stems.

The calculations in which extra dis~ociation of SiF4 is simulated

will be discussed first. The shifts in concentrations of all gas

-65-

!0

SiF4

!0'

to-t

0/

to-•

0 E

...., 10-3 c ::J 0 E (li

to"'

10""'

to-<

w' 0,00

Fig. 3.16. Thermodynamic equitibrium composition of an

SiF4-02 (40:1) mixture at 100 Pa without 02 .

Si

'\ SiF3

constituents due to the extra dissociation of SiF4 are given in table

3. 2. The changes in the amounts of the various species are given

relative to their abundance in the original equilibrium composition.

For all four elemental compositions it can be seen that the changes

at 3000 K are very small (< 0.05 %). The [SiF4] changes by 1% at this

temperature, because we have imposed an increase of 1 % for the

[SiF::~]/[SiF4] ratio. The [SiF4] is, at 3000 K. several orders of

magnitude smaller than the [SiF::t] (see fig. 3.2). Therefore, a 1 %

change in the [SiF4] causes a 1 % change in the [SiF::~]/[SiF4] ratio

without any influence on the concentrations of the other species.

In fact we can state that an extra dissociation of 1 % has no

influence at temperatures where the parent molecules are dissociated

T/species

!si:F:O=

1000

1500

! 2000

2500

3000

I St:F:O =

1000

1500

2000

2500

3000

TabLe 3.2a. Changes in the abundances of the species in the equiLibrium composition of Si-F-0 systems due to 1% extra dissociation of SiF4.

Changes are expressed in %. Temperature is expressed in KeLvin.

SiF4 SiF3 SiF2 SiF Si02 SiO SiOF2 02F OF2 OF 02 Fz Si

1:4:0.05

0.0 1.0 0.4 -0.2 -0.7 -0.7 0.4 0.6 1.2 0.6 0.0 1.2 0.0

0.0 1.0 0.2 -0.5 -1.3 -1.3 0.2 0.8 1.5 0.7 0.0 1.5 -1.3

-0.0 0.9 0.1 -0.7 -25.1 -1.6 0.1 0.8 1.8 0.9 0.0 1.7 -1.6

-0.4 0.5 0.2 -0.2 -0.7 -o.6 0.1 -5.5 0.7 0.4 -0.1 0.8 -0.5

-1.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0

1:4:0.5

0.0 1.0 0.5 -0.1 -0.7 -0.6 0.4 0.5 1.1 0.5 0.0 1.1 0.0

0.0 1.0 0.3 -0.3 -0.9 -o.9 0.3 0.6 1.3 1.6 0.0 1.3 -0.9

-o.o 1.0 0.3 -0.5 0.9 -1.3 0.3 o.s 1.5 0.8 0.0 1.5 -1.3

-0.4 0.5 0.3 0.1 -0.5 -0.3 1.2 0.2 0.6 0.3 0.0 0.7 -0.2

-1.0 0.0 0.0 0.0 0.0 0.0 0.0 -0.0 0.0 0.0 0.1 0.0 0.0

0 F

I 0.0 0.6

0.0 0.8

0.0 0.9

0.0 0.5 I

~ I

0.0 0.0

I 0.0 0.5

0.0 0.6

0.0 0.8

0.0 0.4

0.0 0.0

TabLe 3.2b. Dw.nges in the abundances of the species in the equiLibrium composition of Si-F-0 systems due to 1% extra dissociation of SiF4.

Dw.nges are expressed in %. Temperature is expressed in KeLvin.

0.5 -0.0 -0.6 -0.6 0.5 0.5 -2•105 0.0 1.0 0.0

-0.2 -1.1 -0.8 0.4 0.6 -3•103 0.0 1.2 -0.8

0.3 -0.3 6.9 -1.0 0.3 0.6 -103 0.7 0.0 1.3 -1.0

0.3 0.1 -0.3 -0.1 0.3 0.2 -7•102 0.2 -0.0 0.4 -0.1

0.0 0.0 0.0 0.0 0.0 0.0 0.0 -6•102 0.0 0.0 0.0 0.0

0.0 1.0 0.5 0.0 -0.5 -0.5 0.5 0.6 -2•105 0.5 0.0 1.0 0.0

-0.0 0.9 0.4 -0.2 -1.2 -0.7 0.4 0.5 -103 0.5 0.0 1.2 -0.7

-0.7 0.3 0.1 0.1 0.0 0.0 0.1 0.1 -7·102 0.1 0.0 -.1 0.0

-1.0 0.0 -o.o -0.0 0.0 0.0 -0.0 0.0 -6·102 0.0

-0.9 0.0 0.0 0.0 0.0 0.0 0.0 0.0 -5•102 0.0

F

0.0 0.5

0.0 0.6 I

(])

0.0 0.7 -..J I

0.0 0.3

0.0 0.0

0.0 0.5

0.0 0.6

0.0

0.0 0.0

0.0 0.0

-68-

on thermal grounds. So, in a plasma with very high gas temperatures,

in the situation we studied here above 3000 K, the extra dissociation

caused by the plasma specific interactions is a minor contribution to

the total degree of dissociation.

The changes in [OF2] become very large for systems in which the

[02] is equal to or larger than the [SiF4]. This can, in principle,

have two causes. If the absolute value of [OF2] is very small in the

original equilibrium, a change by a few orders of magnitude can be

easily induced when the concentration of a more abundant species is

changed by 1 %. For example we consider the situation at elemental

composition Si:F:O = 1:4:2 at 2000 K. The value for [OF2] in the

original equilibrium is 9.93 10- 15 mol. In the perturbed equilibrium

we find [OF2] = 1.29 10-13 mol, an increase by a factor of 10 (103 %). Under the same conditions the value of [OJ changes from 2.99 10-2 to

3.02 10-2 (1 %). This change of 3 10-4 mol can very easily cause a

change by 10- 13 mol in the [OF2]. But then one would expect changes of

this order of magnitude for other minor species as well.

We can now compare this to the situation with Si:F:O = 1:4:0.5 at

2000 K. There [OF2] changes from 4.94 10- 15 mol to 4.99 10- 15 mol

(1.2 %) when [OJ changes by 10- 4 mol from 1.17 10-2 mol to 1.18 10-2

mol (0.9 %). The absolute changes for [OJ are in both cases comparable

in magnitude, but those in [OF2] differ largely. Therefore, a second

explanation for these large changes is more likely.

Numerical instability for the calculation of [OF2] could in these

circumstances occur. The values for the various heats of formation in

Si-'F-0 systems are such that the number of iterations necessary to

find the equi 1 ibrium solution in normal systems is quite high for

certain temperatures. This indicates that the numerical stability of

the iteration procedure described in section 3.2 can be poor in these

cases. A small change in the starting data can cause large effects in

the solution. The large numbers we find for the changes in [OF2] are

probably due to this. The numerical stability is, however, not so bad

that calculations of this kind should not be performed with the

EKVICALC program. The changes in the equilibrium composition as a

whole for small changes in the input data are very small in absolute

-69-

sense and of the same order of magnitude as the changes in the input

data. The most abundant species behave as in a numerical stable

situation, so there is no reason to doubt the general trends in the

outcome of the calculations. Only the [OFz] data should be excluded

from the analysis.

At lower temperatures (1000, 1500.K) the 1% increase of the degree

of dissociation of SiF4 .is achieved by a 1 % increase of [SiF3]. For

higher temperatures the [SiF4] changes by 1 %. This can be understood.

from fig. 3.2. In this figure it is shown that the [SiF3] is very

small compared to the [SiF4] for low temperatures. A 1 % change in

[SiF3] thus causes a 1 % change in [SiF3]/[SiF4] for equal [SiF4].

At higher temperatures than 3000 K this is the other way around.

The most important feature in table 3.2, finally, is the decrease

of the [SiOz] and [SiO] when SiF4 is (extra) dissociated. From this we

can conclude that the SiF3 radical is stable against attack by 0 2

molecules. The reaction of F atoms (also formed in dissociation of

SiF4) with SiOz and SiO to form SiOFz and other fluorine containing

molecules turns out to be favoured. The SiOz is attacked (etched) by

the fluorine atoms as has. been shown in many etch processes (e.g.

DONSO) for the solid phase.

When we look at the Si-F-0 system in which the dissociation of the

Oz molecule is increased by 1 %, the picture is very different from

the previous one. In table 3.3 the results of this calculation are

given. Two features are the same for this situation and the extra

dissociation of SiF4: at 3000 K no remarkable influence on the

equilibrium composition can be noticed and again the [OFz] shows a

more or less unstable behaviour for the systems with higher oxygen

concentrations.

The extra dissociation of 02 molecules causes extra formation of

Si02 and SiO by breaking up Si-F bonds in the radicals SiF3, SiFz and

SiF. This can be derived from the changes in abundances of these

species.

In both cases, increase of dissociation of Oz and increase of

dissociation of SiF4, the concentrations of exotic molecules like OzF.

T\species

Si:F:O = 1000

Table 3.3a. Changes in the abundances of the species in the equilibrium composition of Si~F-0 systems due to 1% extra dissociation of 02.

Changes are expressed in %. Temperature is expressed in Kelvin.

SiF4 SiF3 SiF2 SiF Si02 SiO SiOF2~02F OF2 OF 02 F2 Si

1:4:0.05

0.0 -0.3 -0.6 -o.8 0.9 -o.1 0.5 2.2 1.5 1.3 0.0 0.5

0 F

1.0 0.3

1500 JLo.o -o.1 -o.3 -0.4 1.4 0.4 0.7 2.1 1.3 1.1 0.0 0.3 -0.5 1.0 0.1

2000 0.0 -o.1 -0.1 -o.2 -21.2 0.7 0.8 I 1.9 1.0 1.0 -0.1 0.1 -0.3 0.9 0.1

2500 0.0 0.0 0.0 0.0 0.2 0.1 0.1 -5.5 0.1 0.1 -o.9 0.0 0.~~ 0.0

3000 0. 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 -1.0 0.0 0.0 0.0 Sl Si:F:O = 1:4:0.5

1000 0.0 -o.3 -o.6 -0.9 0.8 -0.2 0.4 2.3 1.6 1.3 0.0 0.6 0.0 1.0 0.3

1500 0.0 -o.2 -0.4 -0.6 1.1 0.2 0.6 2.2 1.4 1.2 0.0 0.4 ! -0.8 1.0 0.2

2000 i 0.0 -o.1 -o.2 -0.4 3.5 0.5 0.7 2.0. 1.2 1.0 -o.o 0.2 -0.5 0.9 0.1

2500 -o.o -0.0 -0.1 -0.1 0.7 0.3 0.4 0.9 0.5 I o.5 -o.5 0.1 -0.1 0.5 0.1

3000 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 I o.o -1.0 0.0 0.0 0.0 0.0

T'"species

Si :F:O =

1000

1500

2000

2500

3000

ist=F=O=

1000

1500

2000

2500

3000

TabLe 3.3b. Changes in the abundances of the species in the equiLibrium composition of Si~F-0 systems due to 1% extra dissociation of 02.

Changes are expressed in %. Temperature is expressed in KeLvin.

SiF4 SiF3 SiF2 SiF Si02 SiO SiOF2 02F OF2 OF 02 F2 Si

1:4:2

0.0 -0.3 -0.6 -0.9 0.8 -0.2 0.4 2.3 -2•105 1.3 0.0 0.6 0.0

0.0 -0.2 -4.4 -0.7 0.7 0.0 0.5 2.2 -2•103 1.2 0.0 0.5 -1.0

0.0 -0.2 -0.4 -0.5 9.0 0.3 0.6 2.1 -103 1.1 -0.0 0.4 -0.7

-0.1 -0.1 -0.1 -0.2 0.8 0.3 0.5 1..2 -7·102 0.7 -0.4 0.2 -0.2

-o.o -0.0 -0.0 -0.0 0.0 0.0 -0.0 0.1 -6•102 0.0 -1.0 0.0 0.0

1:4:20

0.0 -0.3 -0.6 -0.9 0.7 -0.3 0.3 2.3 -2•105 1.3 0.0 0.6 0.0

0.0 -0.6 -1.1 -0.9 -6.7 -0.1 -0.3 2.2 -3·103 0.5 0.0 0.6 -1.2

-0.0 -0.2 -0.5 -0.7 0.6 0.0 0.5 2.1 -103 1.2 -0.0 0.5 -1.0

-0.3 -0.3 -0.4 -0.4 0.8 0.2 0.2 1.4 -7•102 0.8 -0.3 0.2 -0.5

-0.2 -0.1 -0.1 -0.1 0.1 0.0 -0.1 0.1 -6•102 0.1 -0.9 -0.0 -0.1

0

1.0

1.0

1.0

0.6

0.0

1.0

1.0

1.0

0.8

0.1

F

0.3

0.3

0.2

0.1

0.0

0.3

-0.4

0.2

0.1

0.0

I

I

I -J .... I

-72-

OF2 and OF tend to increase. In a real system these molecules may not

be found at the outlet, because they will react to 02 and F2 before

they leave the vacuum pump. The extra production or breaking up of

Si02 and SiO, however. may be detected because the Si02 is either

deposited (extra dissociation of 02) or attacked (extra dissociation

of SiF4). It is not necessary that, in a real plasma system, these

reactions take place in the plasma region. This can happen downstream

the discharge in a region where the gas is cooled and the equilibrium

of the higher temperature may be frozen.

Anyway, the most important conclusion from these calculations is

that the production of Si02 in an SiF4-02 plasma is most likely caused

by the extra dissociation of the 02 molecule due to plasma

interactions rather than extra dissociation of SiF4. This is in

contrast to the fact that breaking up the first Si-F bond would

facilitate the overall reaction 3.I by decreasing the endothermicity

by about 6.95 eV compared to the 5.12 eV that can be won by

dissociation of 02 (see tables 2.3 and 2.4).

One should be able to confirm this conclusion in experiments with

separate discharges for 02 and SiF4. Such an experiment is in

preparation at our laboratory at this moment.

3.6.3. Perturbation of the equilibirium in Si-Cl-o systems.

In tables 3.4 and 3.5 the results of the perturbation calculations

for Si-cl-0 systems are presented. For these calculations the

thermodynamic properties for the species 0Cl2. OCl and Si0Cl2 had to

be estimated. For the former two this was done on the basis of the

bond energies in 02Cl. For the latter the value given by Dittmer

(DIT82) was adopted.

In table 3.4 it can be seen that extra dissociation of SiCl4

compared to the unperturbed equilibrium calculation is only effective

at low temperatures (1000 K). Then a decrease of [Si02] and the exotic

molecule concentrations is related to an increase of the [SiCln]

{n ( 4). At higher temperatures than 1000 K extra dissociation of

SiC14 by 1 % has no effect on the equilibrium composition since the

[SiC1 4] is exceeded {in equilibrium) by the [Si02] and [02]

respectively from 1400 K upwards {see figs. 3.8 and 3.9).

-73-

Table 3.5 shows that extra dissociation of 02 molecules has very

little effect on the equilibrium composition. The [Si02] is increased

at the cost of the [SiCln]· The abundance of the exotic species is

also somewhat increased.

3.7. Discussion.

In this chapter an attempt has been made to use thermodynamic

equilibrium calculations for the search for reaction mechanisms in

chemically active plasmas. These calculations have shown to be helpful

in finding the effects of plasma induced dissociation on the chemical

equilibrium composition of the gas.

We have looked at two methods to simulate plasma induced

dissociation.

On the basis of calculations for incomplete systems we have been

able to deduce that the 0 radical probably is an important species in

the reaction mechanism for the formation of Si02 out of 02 and SiF4.

Dissociated 02 can attack the SiF4 molecule and form Si-0 bonds.

Dissociated SiF4 is not capable of reacting with 02.

Perturbation of the normal thermodynamic equilibrium calculations

leads to the same conclusions. In this case the thermodynamic data of

certain substances are changed such that dissociation of these

substances is enhanced. The effects on the concentrations of the other

substances present in the equilibrium are then studied.

In both ways the consequences of the dissociation of one species

for the equilibrium composition can be seen. In our case: when SiF4 is

dissociated, Si02 and SiO are transformed into SiOF2 and exotic

species such as 02F. OF2 and OF are produced; when 02 is dissociated

Si02, SiO and SiOFz are present in larger quanti ties than in the

original equilibrium and al~o the concentrations of the exotic species

are increased.

In the real plasma a mixture of SiF4 and 02 is exposed to the

plasma interactions. The extra dissociation due to this plasma

specific interactions will not be equal for all species. The overall

result will depend upon ls.i..:.<:tic parameters. Besides, not only SiF4 and

T\species

I Si:Cl :o =

1000

1500

2000

2500

3000

Si:Cl:O =

1000

1500

2000

2500

3000

TabLe 3.4a. Changes in. the abundances of the species in the equiLibrium composition of Si-CL-0 systems due to 1% extra dissociation of SiCL4.

Changes are expressed in %. Temperature is expressed in Ketvin.

SiCl4 SiCl:J SiClz SiCl SiOz SiO SiOClz OzCl OClz OCl Oz Clz Si

1:4:0.05

-0.0 1.0 1.0 0.9 -0.9 0.0 0.1 -1.9 -0.9 -o.9 -1.9 0.0 0.9

-0.0 0.9 0.7 0.5 -1.1 -o.5 0.0 -1.2 -0.3 -0.5 -1.4 0.5 0.3

-1.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.1

-1.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0

1:4:0.5

-0.1 0.9 0.7 0.4 -0.6 0.0 0.5 -o.5 0.3 0.1 -o.3 0.5 0.2

-o.o 1.0 1.0 1.0 -1.0 0.0 0.0 -2.0 -1.0 -1.0 -2.0 0.0 1.0

-1.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0

-1.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0

-1.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0

0 Cl

I -0.9 0.0

-0.7 0.3

0.0 0.0

0.0 0.0

-0.1 0.3

-1.0 0.0

0.0 0.0

0.0 0.0

0.0 0.0

T\species

Si:Cl:O =

1000

1500

2000

2500

3000

TabLe 3.4b. Changes in the abundances of the species in the equiLibrium composition of St-CL-0 systems due to 1% extra dissociation of SiCt4.

Changes are expressed in %. Temperature is expressed in KeLvin.

SiCl4 SiCb SiCh SiC I Si02 SiO Si0Cl2 02Cl ~12 OCl 02 Cl2 Si

1:4:2

-Q.2 0.8 0.6 0.3 -0.1 0.0 0.5 0.2 0.4 0.3 -0.1 0.5 0.1

-0.9 -0.9 -1.0 -1.0 0.9 0.0 0.0 1.9 1.0 1.0 -0.0 0.1 -1.0

-1.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0

-1.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 O.Q 0.0

-1.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0

Si:Cl:O = 1:4:20

1000 8 0.5 0.3 0.2 0.0 0.0 0.3 0.1 0.3 0.2 -0.0 0.3 0.0

1500 0.0 Q.O 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0

2000 -1.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0

2500 -1.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0

1000 -1.0 0 0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0

0 Cl

-0.0 0.2

1.0 0.1 I

I

0.0 0.0 ~ 0.0 0.0.

0.0 0.0

0.0 ~~ 0.0 0.1

0.0 0.0

0.0 0.0

0.0 0.0

T\species

Si:Cl:O =

1000

1500

2000

2500

3000

Si:Cl:O =

1000

1500

2000

2500

3000

TabLe 3.5a. Changes in the abundances of the species in the equiLibrium composition of Si-CL-0 systems due to 1% extra dissociation of 02.

Changes are expressed in %. Temperature is expressed in KeLvin.

SiCl4 SiCb SiCl2 SiC I Si02 SiO SiOCb 02Cl 0Cl2 OCI 02 Cl2 J 1:4:0.05

0.0 0.0 0.0 -0.1 0.0 0.0 0.0 0.1 0.1 0.1 -0.9 0.0

0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 -1.0 0.0

0.0 0.0 0.0 0.0 1.8 0.9 0.9 1.7 0.9 0.9 -0.1 0.0

0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 -1.0 0.0

0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 -1.0 0.0

1:4:0.5

-0.1 -Q.3 -0.4 -0.6 0.5 0.0 0.3 1.8 1.2 1.0 -0.2 0.4

0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 -1.0 0.0

0.0 0.0 0.0 0.6 1.8 0.9 0.9 .1.8 0.9 0.9 -0.0 0.0

0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 -1.0 0.0

0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 -1.0 0.0

Si 0 Cl

-0.1 0.1 0.0

0.0 0.0 0.0

0.0 0.9 0.0

0.0 0.0 0.0

0.0 0.0 0.0

-0.8 0.8 0.2

0.0 0.0 0.0

0.0 0.9 0.0

0.0 0.0 0.0

0.0 0.0 0.0

T\.species

I Si:Cl:O =

' 1000

1500

2000

2500

3000

lsi:Cl:O=

1000

1500

2000

2500

3000

Table 3.5b. Changes in het abundances of the species in the equilibrium composition of Si-cl-D systems due to l% extra dissociation of 02 .

Changes are expressed in %. Temperature is expressed in Kelvin.

SiCl4 SiCb SiCl2 SiC! Si02 SiO SiOCb 02Cl OCb OCl 02 Cl2

1:4:2 ~ . ..

-0.2 -0.3 -0.5 -D.7 0.9 0.0 -0.4 2.0 1.3 1.1 -D.1 0.4

-o.9 -0.9 -1.0 -1.0 0.9 0.0 0.0 1.9 1.0 1.0 -0.0 0.1

0.0 -o.o 0.0 0.0 1.9 1.0 1.0 1.9 1.0 1.0 -0.0 0.0

-0.5 -0.5 -0.5 -0.5 0.4 0.0 -0.1 0.9 0.4 0.4 -0.5 0.0

0.0 -0.0 -0.0 -0.0 0.0 0.0 -0.0 0.0 0.0 0.0 -1.0 0.0

1:4:20

-0.5 -0.6 -0.7 -0.8 1.0 0.0 0.3 2.1 1.2 1.1 -0.0 0.2

-1.0 -1.0 -1.0 -1.0 0.8 -0.1 -0.0 1.9 1.0 1.0 0.0 0.1

-0.0 0.0 0.0 0.0 2.0 1.0 1.0 2.0 1.0 1.0 -0.0 -0.0

-1.1 -1.0 -0.9 -0.8 0.6 -0.0 -0.3 1.2 0.5 0.6 -0.3 -0.1

-0.2 -0.2 -0.1 -0.1 0.1 0.0 -0.1 0.1 0.0 0.1 -0.9 0.0

Si 0 Cl

I -0.9 0.9 0.2

1.0 1.0 0.1

0.0 1.0 0.0 ~ I

-0.5 0.5 0.0

0.0 0.0 0.0

I -1.0 1.0 0.1

-1.1 1.0 0.1

0.0 1.0 0.0

0.7 0.7 0.0

0.1 0.1 0.0

-78-

02 will suffer extra dissociation. Other molecules will also

experience this phenomenon. The effect of all extra dissociation on

the equilibrium composition is, therefore, quite uncertain. In a

subsequent study one could look at these effects in more detail.

Apart from this the most important conclusion of this chapter

stands: the dissociation of the 02 molecule is more advantageous for

production of Si02 from SiF4 than the dissociation of SiF4.

In experiments we found that in an SiF4-o2 microwave discharge, about

5% of the SiF4 that was supplied was transformed into Si02 (BEE85).

This suggests that in an SiF4-o2 plasma of this kind the degree of

dissociation of the 02 molecules must be considerably higher than that

of SiF4 molecules (see also fig. 3.3 to see that the [Si02] is always

below 0.5 %) .

So it seems promising to try another set-up for production of Si02

from SiF4 and 02. In this set-up the discharge should be generated in

02 and the SiF4 should be admixed downstream the discharge region.

A convenient property of 0 atoms is that they can be transported

through quartz tubes over fairly long distances without considerable

loss by recombination. At the point were the SiF4 is admixed the gas

should be heated to approximately 2500 K. This is the temperature

where, according to the equilibrium calculations the [Si02] is at its

maximum (e.g. fig. 3.2).

So far we have only discussed the results for the Si-F-Q system.

Si -Cl-0 sys terns behave roughly the same although there is one big

difference. In Si-cl-0 systems the [Si02] at its maximum becomes equal

to the [SiC1 4] or [02]. whichever is the lowest, at their maxima. So

in such systems, on thermal grounds, a 100 % transformation of SiCl4

into Si02 is possible (cf. figs. 3.9 and 3.10). The formation of Si02

takes place at lower temperatures than in the case of SiF4. Extra

dissociation due to plasma specific reactions may thus enhance the

reaction rate, but it seems not essential.

This can also be seen in the perturbation studies. The effects of

extra dissociation of 02 and SiCl4 are not as large as in comparable

situations for SiF4 Still, also in this case it could be ar!vantageous

to use a separate discharge for 02 and perform the admixing of SiCl4

in a heated region downstream. The temperature in this region (of the

gases) should be about 1500 K for optimal Si02 production.

-79-

The method we have presented here to study reaction tendencies in

plasmas is promising. Further research is, however, necessary before

it can be applied as a standard teclmique. Experimental evidence is

lacking at the moment, but experiments are in preparation.

We used the EKVICALC thermochemical computer programs for this

study. They provide a user-friendly environment for interactive

thermodynamic equilibrium calculations. However, these programs are

not ideal for the studies we undertook, because they lack options for

fixing concentration ratios (a very unuseful feature in normal

equilibrium calculations). Therefore, time consuming work like the

perturbation studies can only be done, if the programs are made

suitable for this.

During the preparation of this chapter for printing, a new version

of the EKVICALC program became available. In this version an error in

the iterative calculation procedure has been fixed. The iteration was

stopped too early in specific cases in the old program. This could be

the cause of the large changes in [OFz] mentioned in section 3.6.2.

Since this only affects the results for the minor species in the

system, the overall results that we obtained are still valid.

Unfortunately a check on the consequences of this error for the

results of our calculations could not be performed in time.

-80-

Chapter 4. A mass spectrometer system for measuring the gas phase

composition of a plasma.

4.1. Introduction.

The previous chapter deals with calculations on equilibria in gas

phase systems. It has been shown that the method that was used to

predict a shift in equilibria in plasmas can be a useful tool in the

design of plasma processing. It is desirable to try and verify these

predictions by experiments.

Mass spectrometry has been adopted to analyse gas samples from a

discharge in SiF4-G2 mixtures, thus opening a way to verify the

conclusions of chapter 3.

In the design of an apparatus that is used to measure the gas

composition several demands should be met and several problems should

be solved:

- care should be taken to the design of the extraction system such

that the particles arriving in the ionization chamber are

originating from the plasma. This question will be dealt with in

section 4.2.3,

- a differential pumping system must be used to bring down the

pressure from the 102 Pa range in the plasma zone to about

10-4 Pa or lower in the analyser compartment,

- contamination of the quadrupole by Si02 should be avoided.

Deposition on the quadrupole rods could lead to serious

malfunctioning of this instrument. Therefore, we chose to mount

the quadropole in a crossbeam position (figs. 4.1 and 4.3).

In this way the particle beam only traverses the ionization

chamber,

- the reactivity of water with SiF4 is very high. Therefore, the

water partial pressure in the analysis section should be as low

as possible otherwise the SiFn particles coming from the

discharge zone have a fair chance to react before they are

analysed. In order to achieve this the main pump of the system is

a cryopump,

-81-

- the use of cryoshields at 20 K enforces a set of 80 K shields

to block a direct view from the 300 K surroundings to the 20 K

shields,

- the fact that a plasma must be generated (with microwave power)

at the same spot where the extraction of particles must be

carried out and that the plasma must preferably be generated in

a quartz tube causes construction problems that will be

discussed in section 4.2.1.

i ~~~~~~~ p plasmaregion

n: n nozzle

][ ds s skimmer q quadrupole

q I source compartment II expansion compartment III analysis compartment

Fig. 4.1. Principle of the apparatus for analysis of the gas phase composition of a plasma. The discharge is generated in a quartz tube (I) with a small hole in its wall. The particle beam that emerges fram this hole is expanded {II) and a skimmer selects a small part of this beam that can enter the analysis campartment (III) with the quadrupole mass analyser.

The principle of the appartus that has been designed and built

according to these starting-points is given in fig. 4.1.

A small hole in the wall of a quartz tube permits plasma particles

to enter an expansion compartment. A small section of this particle

beam is allowed to pass through z,, ~:kimmer to the analysis chamber with

a quadrupole mass analyser. Both the expansion compartment and the

analysis chamber are mainly cryopumped.

4.2. Description of the apparatus for mass spectrometry.

4.2.1. The microwave system.

The microwave system that we use for generation of discharges is a

travel! ing wave system. It consists essentially of a doubly bent

-82-

rectangular wave-guide. A cross sectional view is given in fig. 4.2.

The microwaves are produced by a magnetron (Toshiba 2Ml72). The quartz

tube (inner diameter 2.3 em) in which the plasma is generated is

positioned along one wall of the wave-guide. A 10 ~m hole in the wall

of this tube is situated above a hole (diameter 1 em) in the wall of

the wave-guide and serves as the opening for plasma particles to enter

the analysis chamber underneath (see section 4.2.2).

If the quartz tube were placed in an atmospheric section of the

waveguide, a good seal between the atmospheric surroundings and the

analysis section would be necessary. Such a removable seal between a

curved surface (of the tube) and a flat one (the wave-guide wall) that

can stand the elevated temperatures expected to arise at this spot (up

A, B quartz windows c. D bellows E grid in waveguide F nozzle

~quartz

~aluminium

D brass

• opening

F

H water load I magnetron

wall J power supply

Fig. 4.2. The microwave system. The parts of the wave-guide with the

magnetron and the water!oad are at atmospheric pressure.

The part with the quartz tube is at 10- 3 Pa.

-83-

to 1000 °C), would be very cumbersome. We have chosen for evacuation

(below 10- 3 Pa) of part of the wave-guide to avoid this sealing

problem.

The problem has been transferred to the windows to be used in the

wave-guide. The material of which these windows are made must fulfil

several demands:

- microwave power absorption should be low,

- microwave reflection should be low.

mechanical strength should be high enough to stand the

atmospheric pressure drop,

- its expansion coefficient should be low

- it should not be porous or have high outgassing,

it should be possible to fix it to aluminium with a vacuum

adhesive.

We chose quartz (4 mm thickness) of optical quality for this

purpose. Provided that the adhesive is spread uniformly along the

interface and no grains arise during the hardening of the adhesive

microwave powers up to 1 kW can be used without problems. When the

windows suffer point loads a crack is almost inevitable. The adhesive

we have used is a polyamide based vacuum adhesive {Scotch weld 2216

B/A).

The wave-guide section that is to be pumped should be connected to

a seperate pump because the aperture of 1 em diameter to the analysis

chamber is too small. It is not allowed to make large openings in the

walls of the wave-guide if one wants to prevent leakage of radiation,

bad guidance of the waves or dissipation of energy at the edges of

such openings. Therefore an opening with a grid has been used (see

fig. 4.2). An oil diffusion pump equipped with a Peltier baffle is

used to pump this section. The ultimate pressure in this part is about

3.10- 4 Pa, measured with a Penning gauge (Leybold PR30).

The quartz tube is pumped with a rotary pump (Leybold Dl6BC) filled

with Fomblin oil because of the corrosive gases that are used. The

ultimate pressure reached in this tube is in the 0. 1 Pa range as

measured with a Pirani gauge (Leybold TR201).

In a previous set-up, as discussed in section 5.3, we also made use

of a wave-guide with a quartz tube to generate discharges. In that

case some problems arose with the leakage of microwave power along the

-84-

plasma column. In some experiments with that set-up the power that

could be applied was limited by the safety requirements allowing

10 mW/cm2 as the maximum radiation level at the surface of an

apparatus (IRPSl). In the present set-up the bellows C and D in

fig. 4.2 cause substantial reduction of the radiation level along the

quartz tube. At the outer edge of these bellows the microwave

radiation level measured with a Narda 8211 leakage detector is below

0.1 mW/cm2 for all plasmas we have studied.

The magnetron is used in the continuous Wa.ve mode. Its power supply

has been built by the electronics workshop at our institute.

In essention it is an adjustable 12-phase rectified AC voltage that is

further flattened with a few capacitors and choke-coils. The maximum

power that can be generated is about 1 kW.

The plasmas that are generated with this set-up are (in a wide

range indepent of pressure, microwave power and kind of gas) all

axially symmetric and stable (i.e. without flickering).

4.2.2. The expansion compartment and the analysis section.

Below the microwave system, that has been described in the previous

section, the expansion and analysis compartments of the apparatus are

situated. In fig. 4.1 the principle of this part is illustrated. The

body of this part has been constructed out of one piece of aluminium

that was X-rayed beforehand to ensure that there are no cracks in it.

The expansion compartment is pumped with a turbo pump (Leybold

Turbovac 360C) and cryoshields. Under operating conditions the main

pump is the cryoshield at 20 K. This shield also forms the wall

between the expansion and the analysis compartments.

The outside of this wall is used to pump the major part of the gas

flow from the tube. Where other types of pumps always require special

design to bring the pump as close to the orifice as possible, the

cryoshields are easier in this respect. The disadvange of this system

is that 20 K shields should not confront surfaces at 300 K. A direct

view from a cryoshield to surroundings at 300 K leads to considerable

heat loads on the 20 K shield due to radiation. The 20 K shield in our

set-up is attached to a Leybold Heraeus R210 cryogenerator giving 2 W

cooling power to the 20 K shield.

-85-

a

a. wave-guide e. skillllller b. quartz tube f. 80 K cryoshield c. plasma region and nozzle g. 20 K cryoshield d. displacement for drawing h. quadrupole head

Fig. 4.3. Artists impression of the construction of the cryoshields and the design of the exp.m.sion comp:trtment. The microwave system has been displaced over a distance d to permit a view of the skimmer. In the innermost comp:trtment the quadrupole head with the ionization chamber is situated. An 80 K shieLd blocks the direct view from the quadrupole to the 20 K cryopump. The skimmer has a netting downstream. to prouide a field-free sp:tce for the exp.m.sion beam..

-86-

The direct view from 300 K parts of the set-up to the 20 K shield

is blocked by a system of 80 K shields. In fig. 4.3 an artists

impression of the inner of the expansion compartment is given. It can

be seen that most of this compartment is filled with cryoshields.

In order to keep the 80 K shields at the desired temperature a

liquid nitrogen reservoir on which these shields are mounted is built

in the vacuum vessel. It is important to keep the level of the liquid

nitrogen in this reservoir as constant as possible to prevent pr~ssure

variations caused by subsequent cooling and warming up of the shields.

This problem was solved with a liquid nitrogen supply system as is

schematically given in fig. 4.4.

f

c

~--- -, I a I

I :k I

F===;;:::tl I I I I

b a 80 K shield b upper 1 N2 container c lower 1 N2 container d level indicator e siphon f from 1 N2 supply tank g to gas ballast of rotary

pump h heater

Fig 4.4. The liquid nitrogen supply system. The upper container is

placed in the main uacuum chamber. The lower container is

placed outside the uacuum system. The level of the liquid

in the upper container is kept constant while in the lower

container this level fluctuates. The level indicator

regulates the supply from the supply tank.

The liquid N2 container in the vacuum system is filled through

a siphon from a container placed underneath. this second container is

filled from a large liquid nitrogen supply tank. Filling of the top

-fSl-

container is done by evacuation. The nitrogen is pumped upwards until

the level of the liquid reaches the edge of the pumping tube. The

liquid level stabilizes at this point. When the level sinks below the

edge of the pumping tube, the pump creates a vacuum above the liquid

that causes refilling from the bottom container. When the pump

capacity at the pumping tube grows too high and liquid nitrogen flows

through this tube, the evaporation of the liquid on its way to the

pump saturates the pumps demand for gas completely. The pressure above

the liquid can then increase again and no liquid flows through the

tube any longer. Of course, this only works if the pumping capacity at

the pumping tube entrance is higher than the amount of nitrogen that

evaporizes due to the heatload of the shields. In our experiments we

use 02 and SiF4, both gases that can react with the oil of the rotary

pumps. The pump that is used to pump the quartz tube (having the

highest load of these gases) is, therefore, operated with fomblin oil.

This pump also needs continuous gas ballast with a dry gas. The gas

ballast of this pump is, therefore, connected to the pumping tube of

the upper liquid nitrogen container. With a gas flow of 1600 1/h this

would require evaporation of about 1,6 1 liquid nitrogen per hour. The

heat load of the shields was estimated to be about 70 W. This is

equivalent to the evaporation of 1 1 liquid nitrogen per hour.

The gas supply for the gas ballast of the rotary pump and the

regulation of the liquid nitrogen level in the upper container can be

combined. In practice the heat load of the shields turned out to be a

bit lower and the gas ballast supply was reduced a bit. Before the gas

enters the pump it is heated to about room temperature.

So the expansion chamber is pumped by a turbopump, a cryoshield at

20 K and several shields at BO K. Together these pumps can reach an

ultimate pressure of about 10- 6 Pa (10- 8 mbar) measured at the bottom

of the compartment with a Penning gauge (Leybold PM310).

The analysis compartment with the quadrupole mass spectrometer is

pumped with an oil diffusion pump {Edwards E02) and the inner wall of

the 20 K shield. This shield encloses the head of the mass

spectrometer as much as possible. Consequently, the pumping efficiency

for gases in this compartment is maximized. The diffusion pump mainly

pumps the region at the end of the quadrupole rods where the

multiplier is situated. The only connection between the expansion

-88-

compartment and the analysis chamber is the hole in the skimmer. The

openings between the cryoshields have been filled with teflon to

prevent gas particles to pass through.

4.2.3. Theoretical background of the design of the extraction system.

The extraction of particles from the discharge must be carried out

with care if one wants to measure the composition of the plasma. Once

the particles have left the plasma, they should not react any further.

Therefore. the chance that these particles collide with background gas

in the analyser section of the apparatus should be minimized. Also

collisions of the particles with each other should be prevented.

A supersonic expansion of the gas from the plasma to the ionization

chamber of the quadrupole can meet these demands. A general review of

supersonic expansions has been given by Anderson (AND74}. In this

chapter we will follow the approach of Beijerinck {BEIS1} which is

more suited for practical calculations.

In our experimental set-up the supersonic expansion is achieved

according to the schematic views in figs. 4.1 and 4.3. Both the

expansion compartment and the analysis chamber are differentially

pumped. The pumping speed in the second compartment is too low to

allow for a skimmerless set-up.

If we use an orifice our first concern is that the flow through

this orifice does not cause changes in the chemical composition of the

effluent gas. A boundary layer in the orifice is to be prevented. If

the orifice has a geometrical radius RN.o. the effective radius RN is

smaller due to the boundary layer that is formed due to the viscosity

of the gas. The thickness of the boundary layer is closely related to

the Reynolds number of the flow. Values for RN/RN. o for several

conditions have been calculated and are presented in table 4.1. As it

can be seen from these data, especially at pressures of the order of

100 Pa the boundary layer becomes a problem. Our experiments will

generally be performed at pressures of 400-1000 Pa.

-89-

Table 4.1. Some characteristics of the source regio;t that are

relevant for the calculations related to the

supersonic expansion. VaLues of a1, u 100 and 11 are

caLcuLated for 02.

P1 n1 T1 a1 111(1) 11 RN/RN,O

Pa 1022m-3 K m s- 1 m s- 1 10-5kg m- 1 s - 1

100 2.44 300 390 730 2.0 0.53

300 7.32 0.82

1000 24.2 0.85

100 2.44 600 550 1000 3.4 0.49

300 7.32 0.71

1000 24.2 0.84

100 2.44 900 680 1270 4.5 0.47

300 7.32 0.69

1000 24.4 0.83

TabLe 4.2. The reLation between Zref/RN and 0 •

0 5/3 7/5 9n

Zr e f/RN 0.806 0.591 0.490

-90-

A second important point to consider is the number of collisions

that the gas particles suffer after they have left the orifice. In the

"sudden freeze model" of the supersonic expansion (BEISl) one can

calculate the posi t1on z,, relative to the nozzle, beyond which the

mean number of collisions is equal to some small fixed number N,. This

position is given by (BEISl):

(1 875 N -1 -~)3/(1+2) . . F 4.1

Here is 1 the ratio cp/cv for the expanding gas,

Zref a scaling length, related toRN and 1 (table 4.2).

The parameter - is a non-dimensional source parameter given by:

4.2

Here is N1 the particle density in the source region

(compartment I),

T1 the gas temperature in compartment I.

k Boltzmanns constant (1.3805 10-23 JK- 1),

C6 the gas specific constant in the long range attractive

branch of the Van der Waals intermolecular potential

(V(r) = ~6 r- 6).

Values for the parameter~ and for ZriZref have been calculated for

some specific cases and for NF=2. From table 4.2 and the relation

between Zref and the nozzle radius RN it is clear that RN should be

made as small as possible if one wants to minimize the mutual

collisions between the particles.

However, RN should not be too small, because the number of

particles reaching the analysis compartment is proportional to RN 2.

Three different contributions to the particle density in the

ionization cbamber of the detector can be distinguished:

the particle density due to the flow of particles from the

plasma, na.

the particle density due to the flow of the background gas in the

second compartment, nb.

the background particle density in the analysis cbamber, nc.

-91-

The ratio na/(nb+nc) is the signal to noise ratio of the set-up.

In appendix A it is shown that for the set-up tl~t we use, this ratio

is given by:

9.03 105 (m/s) 4.3 U1oo

Here is u1 the velocity of the molecules after the expansion from

the plasma· region.

This quantity is given by:

( ....1.._)1/2 U1oo = a1 1 . 1'-

4.4

With a 1 the characteristic velocity of the gas particles in

compartment I.

The value of the constant in 4.3 (9.03 105 m/s) is independent of the

conditions in the plasma region. Only geometrical parameters {but not

the nozzle radius} and the pumping speed in the expansion compartment

determine its value {see appendix A). From 4.3 it can be derived that,

if the temperature in the plasma is 600 K (u1 03 1000 m/s, table 4.1},

the ratio na/(nb+nc) is about 900. So only one out of 900 particles

counted by the detector does not originate from the plasma.

The main reason why this value can be reached is the use of the

cryoshield to seperate compartments II and III. The reflection

coefficient of such a shield is very low ( (5 10-3, HAB75). Typical

pumping speeds per unit area for such shields can be calculated to be

10 1/s cm2 {ROT82}. In our set-up the pumping speeds in the expansion

and analysis compartments amount to about 800 1/s.

In practice the value for na/(nb+nc) is not as high as calculated

because of two effects. The skimmerhole is not the only connection

between the expansion compartment and the analysis section.

In fig. 4.2 it can be seen that the netting attached to the skimmer

pierces through the 80 K shield surrounded by a tube at 20 K.

A second leak in the shielding is the quadrupole. It is surrounded

by a teflon belt at the place where it pierces the 80 K shields, but

its inner part cannot be partitioned off. Thus there is a connection,

although its conductance is low because of many obstacles, between the

-92-

Table 4.3. Parameters of the supersonic expansion of 02 in dependence

of p1, T1 and the nozzle radius RN.O·

I RN.O = 5 J.Lffi ]II

P1 T1 - ZFIZref Zref ZF

Pa K NF = 2 J.Lffi J.Lffi

~

100 300 0.018 0.027 1.6 0.04

300 0.083 0.11 2.4 0.26

1000 0.28 0.31 2.5 0.77

100 600 0.013 0.020 1.4 0.03

300 0.056 0.075 2.1 0.16

1000 0.22 0.25 2.5 0.63

100 900 0.011 0.017 1.4 0.02

300 0.048 0.064 2.0 0.13

1000 0.19 0.22 2.5 0.55

RN.O = 50 J.Lffi

P1 T1 - ZF/Zref Zref ZF

Pa K NF = 2 J.Lffi J.Lffi

100 300 0.18 0.16 16 2.6

300 0.83 0.80 24 19

1000 2.8 2.34 25 59

100 600 0.13 0.11 14 1.5

300 0.56 0.56 21 12

1000 2.2 1.89 25 47

100 900 0.11 0.13 14 1.8

300 0.48 0.49 20 9.8

1000 1.9 1.66 25 42

-93-

ionization chamber and the pumping section at the end of the

quadrupole. The pressure in the pumping section should, therefore, be

brought down to about 10-& Pa if the theoretical value for na/(nb+nc)

should be obtained.

The values for RN.o and Rs that were used in the calculations of

na/(nb+nc). can now be used to determine Zref and zr for different

conditions. These values are given in table 4.3. From the values of zr

it can be seen that in most cases the number of collisions for a

particle is only 2 from 0.5 Jllll downstream of the orifice to the ·

detector. So, the values chosen for RN,o and Rs do not interfere with

the minimization of mutual collisions.

So far we only considered the expansion of oxygen. The other main

constituent of our gas mixtures is SiF4 or SiCl4. So actually we have

a seeded beam in our apparatus (BEISl). The lighter component of the

gas mixture (02) drags along the heavier component. The expansion of

SiF4 (SiCl4), therefore, cannot be treated separately. Normally an

enrichment of the centre-line beam for. the heavy component occurs. The

skimmer then will select only part of the beam and the detector will

see relatively more heavy particles than light ones. In our case,

however, because of the extremely low values for 2. we do not have to

reckon with this. The influence of the skimmer on the centre-line

intensity is too low. Another effect, that occurs in beams seeded with

noble gas metastable atoms, is that energy transfer occurs between the

two components of the gas mixture (BEISl}. In our case this could mean

that the particles flowing through the nozzle react chemically. It is

very difficult to estimate the importance of this effect. The fact

that zr is relatively small may be reassuring.

4.2.4. The quadrupole service electronics and computer facilities.

The quadrupole that is used in this set-up is a Balzers QMG511 with

cross beam ionization chamber and a 90° of-axis multiplier. The

service electronics (apart from the HF generator) have been developed

at our laboratory. In fig. 4.5 a block diagram of the electronics is

given. An IBM PC is used for controlling the mass scans and the data

acquisition. It is equipped with 4 extension boards:

-94-

a RAM memory extension to 640 kB,

- a Graphics Master high resolution (640*400) graphics card

(Tecmar),

-a Lab Tender data-acquisition board (S bits) (Tecmar),

-a Lab Master data-acquisition board (12 bits) (Tecmar).

So far only the quadrupole is controlled with the computer, but

additional facilities for control and measurement of other variables

in the experiments such as pressures, microwave power and gas flow

rates are available.

power supplies

for ion source

interface boards

IBM PC

preamplifier

discriminator pulsshaper

Fig. 4.5. Btock diagram of the quadrupote controt system.

The 12 bits DAC driving the HF generator allows a resolution of S

datapoints per amu. The pulses coming from the multiplier of the

quadrupole are first amplified in a pre-amplifier. These pulses have a

rise-time of about 6 ns and a decay time of about 25 ns. The overall

gain of the anode current coming from the multiplier to the load of

the preamplifier {50 Q) is about 15 mY/~. These pulses are sent to a

discriminator/pulse shaper to make them sui table for counting. The

pulses leaving this device are TTL compatible and 100 ns wide. They

are counted with the Am 9513 timer-counter on the Lab Master board of

the computer. A second output gives pulses of only 22 ns width, but

these turned out to be too fast for the counter. A binary counter

chip, used as a divider, can be inserted in the output line when high

count rates are expected.

-95-

Software has been written in Marco Assembler and MS-Pascal.

Assembler has been used for real time parts such as the counting.

Several software modules were developed to serve easy and user

friendly programming, for control of the interfaces to the experiment,

for graphics and plotter facilities and for handling and storing mass

spectra.

4.3. Experi~~ental results and discussion.

So far, a systematic study of mass spectra with the set-up that has

been built could not be performed. The delayed availability and some

technical problems have caused this. Some preliminary results can,

however. been given.

Although the testing and calibration of the apparatus is not

finished yet, we can state that the data-collection facilities operate

satisfactory. Also the microwave system for generation of plasmas

gives no problems. Discharges in 02. SiF4 and SiF4-o2 mixtures can

readily be generated in the pressure range from about 102 Pa to

2 103 Pa. The power dissipation in these plasmas has been measured

with a directional coupler and microwave power meter. It lies between

roughly 200 and 300 W. The general tendency is that in a plasma at

lower pressure the dissipation is lower than in a plasma at higher

pressure. In the waterload typically 50 W is absorbed.

The first aim of the mass spectrometric measurements has been to

determine the degree of dissociation in the plasma. So far, however,

no decisive experiments could be performed in this direction.

The differences between mass spectra of 02 gas and 02 plasmas at the

same pressure were too small to indicate dissociation. Variation of

the electron energy in the ionization chamber of the quadrupole will

be used to solve this problem.

In discharges in mixtures of SiF4 and 02 two products of chemical

changes have been identified: SiO and SiOF2. The fact that both peaks

are seen in the mass spectrum indicates that at least part of the

conversion of SiF4 into Si02 is a gas phase process. No data on the

molecular ion of Si02 have been found. We can, therefore, not decide

whether the SiO peak comes from SiO or from Si02. It is important to

-~-

know this because it is not known yet whether Si02 is formed in the

gas phase or by recombination of SiO and 0 at the wall, simultaneously

depositing Si02.

-97-

Appendix A. Calculations in relation to the vacuum system oF the

quadrupole mass spectrometer system.

In fig. A.l the configuration as used in our set-up is drawn

schematically. Several quantities are defined in this figure and their

actual values are given. To find an expression for the

signal/background ratio na/(nb+nc), we will first give expressions for

the three particle densities na. nb and nc.

The contribution to the count rate in the detector that is due to .

the gas flow from the plasma region is given by:

A.l

Here is I13(0) the centre-line intensity of the expansion from the

plasma (see BEISl for a definition),

01 the solid angle of this expansion seen by the

detector,

U1ro the final velocity of the particles after the

expansion,

Ro the radius of the detection volume {ionization

chamber).

Analogously the contribution nb to the particle density in the

ionization chamber due to the expansion of the background gas in

compartment II can be written as:

Here is I23(0) the centre-line intensity of the expansion of

background gas in compartment II to the analyser

compartment,

02 the solid angle that the detector sees of I23(0),

A.2

U2ro the final velocity of particles from compartment II.

The third contribution to the particle density in the ionization

chamber comes from the background gas in compartment III. This

contribution, denoted by nc_, is given by the ratio of the amount of

particles entering this compartment per unit of time and the pumping

speed in compartment III:

Fig. A.l.

nc

-98-

n m

I source compartment

II expansion compartment

III analysis compartment -t-- -t-P R I I s 1 R0 1

I I

RN .0 = 5 J.Llll

Rs = 0.1 mm

Ro 2 mm

dNs = 20 mm

dso 80 mm

dNo = 100 mm

Schematic view of the principle of the extraction system.

The numbres I, II and III indicate the different

compartments: I, the plasma region, II, the expansion

region and III the analysts compartment.

A.3

Here is ~ the peaking factor of the expansion (definition in

BEIBl),

no the solid angle seen by the skimmer of the expansion from

the plasma region,

83 the pumping speed in compartment III.

If we want to express I23(0) in 113(0) and the characteristics of the

apparatus we must introduce the equations for n2 and 123(0):

A.4

A.5

-99-

Here is s2 the pumping speed in the second compartment,

f(-)') a 1-dependent correction factor (BEI81},

a2 the characteristic velocity of the molecules in

compartment II,

Rs the radius of the skimmer hole.

A.4 and A.5 can be solved for n2 and I23(0) expressed in I 13 (0):

A.6

= 1 A.7

1 +

The centre-line intensity I13(0) is given by the conditions in the

plasma region:

With a 1 the characteristic velocity of the gas particles in the

plasma region.

A.S

The characteristic velocity a can be related to the gas temperature:

a = (2vkT/m) 112 . A.9

The final velocity Ueo of the gas particles after the expansion is

given by:

A.lO

So by exprensing all particle densities in It3(0} they are indirectly

expressed in n1 and Tt. the conditions in the plasma region. The solid

angles Oo through 02 can also be expressed in the geometrical

quantities of the set-up:

-100-

A.ll

A.l2

A.13

Combination of A.l, A.2, A.3 and A.7 and some rearrangement yields:

with

n a

f(l)

A.14

A.l5

The relevant quantities that were not given in fig. A.l are collected

in table A.l.

Table A.l. Relevant quantities for the calculation of nal(nb+nc)·

quantity value

T2 SO K

a2 200rnls

U2 375 m/s

82 0.8 m3/s

With these values and the different relations given above we can now

write the following:

-101-

n /(n. + n ) a o c = A.16

When the temperature in the plasma region is 300 K, Uim is 730 m/s

(table 4.1) and this means a signal to background ratio of about 900.

This value is independent of l13(0) and RN.O· Only the geometry of the

system determines its performance.

-102-

Chapter 5. Light emission spectroscopy of SiF4 , 02 and SiF4-02

microwave discharges.

An orientation.

5.1. Introduction.

In the previous chapter an apparatus has been described to measure

the chemical composition of the plasma by mass spectrometry. This

technique gives important information about the types of radicals that

are present in the plasma. It cannot. however, give information about

the electronic and vibrational states these particles are in. These

characteristics can be deduced from light emission spectroscopy.

In this chapter a study is presented on the application of emission

spectroscopy to discharges in SiF4, 02 and SiF4-D2 mixtures. First

some results of spectroscopy on SiF4 discharges as reported in

literature are reviewed (section 5. 2). After this the experimental

set-up for our measurements is discussed in section 5.3. In section

5.4 the results of these measurements are presented and finally in

section 5.5 a discussion of these results with respect to the

chemistry in SiF4-02 plasmas is given.

5.2. Earlier studies on the emission spectra of SiF4 fragments.

The emission spectrum of SiF4 discharges has been investigated by

various authors (e.g. PORll. EYS37. JOH58a, RA061, GOL72. WAN73b).

Most features in the spectrum have been ascribed to SiF and SiF2

transitions. The band systems of SiF2 have been identified by Johns

(JOH58b). This analysis has been extended considerably by Rao (RA061

and RA070). Gole (GOL72). Cal dow (CAL81) and Khanna {I<HA67a, b) used

absorption measurements to verify the analysis by Rao, confirming most

of his conclusions.

Apart from these spectroscopic studies two papers report

measurements of radiative lifetimes of excited levels in SiF4

fragments after electron or ion impact. Hesser (HES67) tentatively

ascribed a continumn emission near 300 nm to the SiF:1 radical, but

-103-

Aarts (AAR86} showed that this emission originates from an excited

level of SiF4 •. The spectra reported by these authors differ

considerably from those obtained in a discharge.

The emission spectra of 02 and 0 have been extensively described by

Krupenie (KRU72} and Striganov (STR68} respectively. In the rest of

this chapter we will focus on the influences on the spectrum when 02

is added to SiF4 or vice versa.

5.3. Experimental.

A schematic drawing of the experimental set-up is given in

fig. 5.1. The plasma is generated in a quartz tube that diagonally

crosses a waveguide at an angle of 30 degrees. A magnetron (Toshiba

2M172} is used to produce the microwave power. A waterload is

positioned at the end of the wave-guide to absorb the transmitted

power. In this configuration operating powers of the plasma up to 300

W can be achieved. This power is measured by measuring the increase of

the temperature of the water flowing through the waterload.

from gas .ntet system photomul hpl ier

nochro mat or

thermocOI..Jlle ~~----~------~~~~------------~~----.

thermocouple

t to pump system

Fig. 5.1. ExperimentaL set-up for the measurements of the light

emission of 02, SiF4 and SiF4-02 discharges.

-104-

The quartz tube can be evacuated down to 10-4 Pa (10- 6 mbar) with

an oil diffusion pump. The operating pressure of the plasma

(102 - 104 Pa) is maintained with a throttle valve in conjunction with

a rotary pump (Leybold D16BC) operated with Fomblin oil. Two mass flow

controllers are used to regulate the gas flows

(SiF4: 0- 0.32 Pa.m3 s- 1 (= 200 mbar.l.min- 1 ), 02: 0- 0.83 Pa.m3 s- 1

(= 500 mbar.l.min- 1)). SiF4 and 02 were purchased from Matheson and

Hoekloos respectively and used without further purification.

The emission spectra were measured end on. In the range of 200 -

500 nm a 25 em Carl-Leiss monochromator was used with a holographic

grating (1200 lines/mm, blaze 250 nm) with an EMI 9789 QB

photomultiplier. In the range of 360 850 nm a 50 em Jena

monochromator with flintglas prism was used in connection with an EMI

9558 photomultiplier.

5.4. Results.

Emission spectra of pure 02 and SiF4 discharges were studied as

well as those of several mixtures of SiF4 and 02. Differences between

the sum of the pure spectra and spectra of the mixtures give an

indication of the processes occurring in the plasma.

Spectra in the wavelength range 200 - 450 nm are shown in fig. 5.2.

In the 02 spectrum (fig. 5.2a) only atomic 0 lines can be identified

to originate from OXYgen. The two band structures with band heads at

284 and 306 nm are transitions in the OH radical (A2~+ ~ X2IT 1 ) (TAB70,

MOH77). These bands turn out to be the most intensive features in this

regiv-. of the spectrum although no hydrogen containing species was

administered deliberately. The water contamination of the OXYgen (ppm

range) in the cylinder must be responsible for this phenomenon.

In chapter 7 this is investigated in more detail.

In fig. 5.2b the spectrum of the pure SiF4 discharge is shown.

In this spectrum most structures can be recognized and attributed to

the SlF and SiF2 radicals. The a, (3, 1 and T/ bands of SiF and the

a 3 B1 ~ X1A1 and A1B1 ~ X1 A1 , bands of SiF2 are clearly visible. Also a

few Si lines can be seen.

0 0

x100

x1

Si

4SO 400

x1

-105-

x10

3SO

a o2 I I X10 x100 I

I I I I

x1

Si

300 2SO cSiF4+02

I I

: x1/3 I

i ,s; ,, J ~~JJ ~ ~1

}.(nm)

Ftg. 5.2. Light emtsston spectra tn the region between 225 and 475 nm

of SiF4, 02 and StF4-02 (1:1} discharges at 100 Pa, 300 W

absorbed power.

800

500 600 550 Alnml

soo

Fig. 5.3. Light emission spectra (4so~aso nm) frnm SiF4 and SiF4-o2

discharges at 100 Pa, 300 W absorbed power. Lines marked

with one dot(.) are F~atnm Lines; tines marked with two

dots(:) are O-atom lines and three dots (!) indicate

st~atom lines.

-107-

The spectrum of the SiF4-Q2 (50% - 50%) plasma as shown in

fig. 5.2c is not a simple superposition of the pure SiF4 and 02

spectra. The most remarkable features are:

1. Upon the~ and 1 bands of SiF and the a 3 Bt ~x1At band of

SiFa a new and intense structure arises. This can be ascribed to

the 1H ~X1 L+ transition in SiO (TAB70).

2. The intensity of the a band of SiF decreases by a factor of 5.

3. Adjacent to the a band of SiF a structure that most likely can

be attributed to SiO ( 1]- ~ 3 H (TAB70) or b3 L ~ a 3 Hr (SUC75))

increases in intensity by a factor of 10. The appearance of this

hand in the pure SiF4 spectrum may be due to evaporation

(etching) of SiOa or SiO from the tube-wall.

4. The intensity of the a 3 Bt ~ X1At band of SiFa is increased by

a factor of 7.

5. The OH hand systems have completely disappeared in the spectrum

of the mixture.

The spectra at longer wavelengths (fig. 5.3a. b) show a strong

presence of atomic lines and remarkably low emission from ionic

species. This occurs as well in pure SiF4 (fig. 5.3a} and Oa as in the

mixtures (fig. 5.3b}. In the SiF4 spectrum also a few molecular hands

in the 500 - 575 nm region are visible.These have not been identified

yet. A weak continuum from 450 to 725 nm also could not be ascribed to

a known transition. The differences between the spectra of the pure

compounds and the mixtures of SiF4 and 02 in the longer wavelength

region ·are:

1. The F lines increase in intensity when Oa is added.

2. The 0 lines decrease in intensity when SiF4. is added.

3. The bands in the SiF4 spectrum between 500 and 575 nm have

completely disappeared in the mixtures.

5.5. Discussion.

In the course of the experiments it has become clear that addition

of small quanti ties of SiF4 to Oa discharges and of Oa to SiF4

-108-

discharges has a great influence on the intensities of several

features in the emission spectrum of the plasma. The most remarkable

changes occur when SiF4 is added to an 0 2 plasma. The OH band

structure disappears completely when only 1% {the smallest quantity

that can be admixed reproducibly with this set-up} of the mixture is

SiF4. This is such a strong effect that its understanding seems very

important for the understanding of the reaction mechanisms in the

plasma that lead to the formation of SiOz.

Of course, addition of only small quantities of SiF4 to 02 could

change the discharge completely. Formation of negative ions {F-,

SiF3 } and also vibronic excitation of the various fragments of SiF4

could influence the electron energy distribution considerably. The

result of this could be that the OH bands do not occur any longer,

simply because OH is not excited in this new situation.

Another explanation that is more likely is that the HzO in the

oxygen gas doesn't enter the discharge region any longer when SiF4 is

added. The reaction between SiF4 and HzO is known to be extremely

fast. Both reactions 5.1 and 5.II are possible.

~ 2 HF + SiOFz .

5.1

5.II

Also multiple exchange of fluorine atoms by OH groups (as in the

ion-molecule reactions of chapter 2) is feasible:

5.III

The mixing of the gases in the inlet system would in this way prevent

water molecules to enter the discharge region. Thus. whatever the

mechanism for excitation of the OH bands may be. the disappearance of

H20 will also induce the disappearance of these bands.

The production of HF in these processes could lead to the

assumption tr2': HF bands must be seen in the spectrum of a mixture.

This, however, is very unlikely because the HF bands are known to be

very weak and they are in the same wavelength region as the

SiF2 (A1B1 ~ X1A1 ) and SiO (H ~ X1I+) transitions (LON73). Since the

HF concentration in the mixture will be very low (ppm region) it is

not surprising that its emission bands cannot be recognized.

-109-

In the spectrum of the mixture (fig. 5.2c) SiO bands in the

200 - 300 nm range and possibly near 425 nm are strongly present. This

indicates that in the gas phase of the plasma Si-D bonds are formed.

This means that at least part of the reaction mechanism leading to the

formation of Si02 takes place in the gas phase and not at the wall.

Measurements by van Morgen (MOR85) showed that the intensity of these

bands increases when the 02 concentration in the mixture increases.

The spectra at longer wavelengths show an increasing intensity for

F lines when 02 is added and a decreasing intensity of the 0-lines

when SiF4 is added. This means that the model that we suggested in

chapter 3 for the reaction of 02 and SiF4 is supported. When 02 is

added to an SiF4 plasma it is partially dissociated and reacts with

SiF4 to form F atoms as a byproduct. These are then excited in the

plasma and the intensity of the F emission increases. Also when SiF4

is added to an 02 discharge the 0-atoms in the discharge will react

with the SiF4 and are not available for excitation anymore. Thus the

Q-line emission is decreased. This explantation is not the only

possible one. A plasma as we have here is a too complicated system to

base conclusions on such relatively simple evidence. This study was

undertaken however, as a pilot study to find entries for further and

more detailed investigations. In this respect one should certainly

consider the aforementioned possibilities. In the next two chapters we

will investigate two subjects in more detail. Chapter 6 deals with the

spectrum of SiF4 discharges in the region between 220 nm and 250 nm.

A controver-sy in literature exists on the true emitter of the band

system that is seen there. In chapter 7 we will discuss the influence

of water contamination in 02 discharges.

-no-

Chapter 6. The UV spectrum of an SiF4 microwave discharp in the

region of 220 - 250 nm.

6.1. Introduction.

The UV emission spectrum of an SiF4 discharge has been studied by

various authors as stated in the previous chapter. In these studies a

controversy shows up regarding the origin of the emission spectrum in

the 220 - 250 nm range. Some authors have attributed the band spectrum

in this region to SiF2 while others have pointed out that SiF3 is the

emitter.

AI though Sa let reported on the use of SiF4 discharges (SAL73),

Porlezza (POR11) was the first to study the spectrum of such a plasma.

Johns (JOH58b) was the first to present an analysis of the band

spectrum in the aformentioned region, which he ascribed to the

non-linear SiF2 molecule on the basis of similarity of the spectrum

with the earlier identified band spectrum of CF2. He found two

vibrational frequencies (v' 1 = 937 cm- 1 and v'2 = 427 cm- 1) which he

ascribed to the ground state vibrational modes as indicated in table

6.1.

This study was extended by D. Rao (RA061). He recorded the spectrum

over a larger wavelength range: (217.9 - 275.5 nm). He included 120

bands in his analysis compared to 50 reported by Johns. In the long

wavelength part of the spectrum Rao could match his data with the

vibrational frequencies found by Johns (937 cm- 1 and 427 cm- 1). In the

shorter wavelength range he found vibrational progressions with

frequencies 778 cm- 1, 597.5 cm- 1 and 342 cm- 1

• The assignments to the

vibrational modes. however, differ from those of Johns (see table

6.1).

In another band system of SiF2 (between 365 nm and 420 nm) D. Rao later on found vibrational frequencies 277 cm- 1 and 343 cm- 1 (RA070).

Sankaranarayanan (SAN62) studied the wavelength range from 222.9 -

216.8 nm. He also based his analysis on that of Johns (JOH5Sb) and

fQund two additional vibrational frequencies (table 6.1). In contrast

to Rao he ascribed both vibrations to the excited state.

After the work of Timms (TIM65), who described a method for

synthesis of SiF2. other authors used absorption techniques to study

Table 6.1. Vibrational frequencies ascribed to the SiF2 radical.

· Data from uctrious authors and their assignments.

All unlues in cm- 1•

ground state (X1At} first singlet electronic method \ excited level (A 1Bd

etric stretch bending antisymm. stretch symm. stretch bending Dt" V2,. u3" Vt

. V2 .

937 427 UV emission

778 427 937 598 342 UV emission

937 427 766 338 UV emission

345 microwave abs.

345 252 UV absorption

855 872 IR absorption

343 277 UV emission

860 790 ~320 Vac. UV em. & abs.

855.01 870.40 IR emission

Values found by Milligan (MIL68} for SiF3:

953 cm- 1 832 cm- 1 807 cm- 1 406 cm- 1•

reference

JOH58b

RA061

SAN62

RA065

KHA67a

KHA67b

RA070

GOL72

CAL81

I ..... ..... ..... I

·-H?-

the properties of the SiFz molecule. V.Rao reported on a microwave

absorption study (RA065, RA066) in which he found the vibrational

frequency of 345 cm- 1 which makes that this frequency should be

assigned to the electronic ground state of the radical. Khanna

(KHA67a, KHA67b) performed a study of IR and of UV absorption by SiFz.

In the UV region the absorption spectrum revealed vibrational

frequencies of 345 cm- 1 and 252 cm- 1• TheIR study gave 855 cm- 1 and

872 cm- 1 as the frequencies for the symmetric and the antisymmetric

stretching mode respectively (table 6.1). Later on these measurements

were confirmed by Shoji (SH073) and Caldow (CAL81) who used microwave

absorption and IR emission spectroscopy respectively.

Gole (GOL72) extended the analysis to the vacuum UV region of the

spectrum and measured absorption as well as emission spectra

(wavelengths down to 60 nm). Two sets of absorptions bands (near

160.6 nm and 158.6 nm) were found and three emission bands were

assigned to SiF2 (160.5, 162.8 and 165.0 nm). He could deduce the

vibrational frequencies 320 cm- 1 for the upper state, 86,1 cm- 1 for the

ground state and 790 cm- 1 that he tentatively ascribed to the upper

state.

The discrepancies that follow from table 6.1 seemed to be solved

after the analysis of Wang (WAN73b). He remeasured the emission

spectrum in the 210 - 260 nm range. From his analysis vibrational

progressions follow with frequencies of 937 cm- 1, 427 cm- 1

, 345 cm- 1

and 252 cm- 1. He compared these values with the matrix IR studies

performed by Milligan of the SiF3 radical (MIL68). The latter found

absorptions that indicate vibrations with frequencies 954 cm- 1,

832 cm- 1, 807 cm- 1

• 406 cm- 1 and 290 cm- 1• Wang assigned the emissions

in the 210 260 nm range to the decay of the first electronic excited

level of the SiF3 radical on the basis of the similarity in

vibrational frequencies. His main argument for this assignment is a

study of negative ion formation from SiF4 (WAN73a) (chapter 2). The

excited state of SiF3 is produced in the reaction

6.I

According to Wangs analysis tha energy of SiF3~< is 5.47 eV above the

-113-

ground state. This energy corresponds to a wavelength of 227 nm, well

in the range of the observed UV emission.

In order to discuss the chemistry in SiF4-o2 plasmas it is

important to solve the controversies indicated above. The true emitter

of the UV band system should be found. Therefore, we studied the UV

emission spectrum once more and we re-analysed the data of Wang

carefully.

6.2. Experimental.

The plasma in our experiments is generated within a quartz tube.

having a flat window at one end. The spectrometer is placed at this

end viewing the quartz tube end on. A schematically drawn picture of

the set-up is given in fig. 6.1. The spectrometer is a modified JENA

Plangitterspectrograph (PGS2) with a focal length of 2 m. The grating

has 651 1/mm and is blazed for 300 nm. We have modified this

instrument such that a wavelength scan can be performed, by installing

an exit slit (RUY} and a photomultiplier (EMI 9558 QC) for recording

the spectra.

2.4SGHz power 1--------.. supply

cavity

t gas

2m monochromator

lN2 trap

Ftg. 6.1. Expertmentat set-up for measurements of the 200 - 500 nm

spectrum of StF4 dtscharges.

-114-

The microwave power for the generation of the discharge is supplied

~ Electro Medical Supplies Ltd Mark 3. The power delivered to the

ca.vi ty is in the range 50

2.45 GHz.

100 Watt at a microwave frequency of

The quartz tube is pumped by a two-stage rotary pump {background

pressure 0.5 Pa {5.10- 3 mbar)). The discharges are operated in the

200 Pa range. A cooled trap (lN2) is included in the vacuum system to

protect the pump against the reactive gases coming from the discharge.

6.3. Results and discussion.

Besides the emission bands in the 220 nm to 250 nm region the

measured spectra also include the a-SiF band system between 422 and

454 nm, the ~-SiF system (280 - 310 nm) and the 0-SiF system between

250 nm and 260 nm. Here only the analysis of the 220 - 250 nm band

system will be discussed.

The measured spectrum is shown in fig. 6.2. There is a very close

resemblance to that published by Wang {WAN73b).

Fig. 6.2. The emisston spectrum of an SiF4 microwave discharge

between 220 nm and 250 nm.

-115-

The search for vibrational frequencies is performed by placing the

wavenumbers of the peaks in the spectrum in a Deslandres table. This

can be done in several ways as shown in tables 6.2 to 6.7, resulting

in different vibrational frequencies. An assignment to the ground

electronic state (") and upper electronic state {') of these

frequencies can also be made. The various frequencies we have found

are:

v .. , = 853 ± 5 cm- 1

} Vutl 1 = 860 ± 5 cm- 1

} table 6.2 table 6.3 V"t i =343 ± 3 cm- 1

Vutv = 943 ± 6 cm- 1

v"v = 427 ± 2 cm- 1

} " 942 ± 3 cm- 1

} V vII = table 6.4 table 6.5

v" = 324 ± 1 cm- 1 V .. vll i 345 ± 2 cm- 1

vI

v' 1 = 255 ± 1 cm- 1

} table 6.6 v"x = 341 ± 8 cm-

1}

· table 6.7 v"tx 342 ± 1 cm- 1 v" = 771 ± 13 cm- 1

X I

We will now argue why SiF2 is the emitter of this band system.

In total 7 different frequencies are found from these tables, but not

all 7 can be true vibrational frequencies if SiF2 is the emitting

species. The IR absorption and emission data given by Khanna {KHA67b)

and Caldow {CAL81) agree fairly well with our values for v" 1 and

v"111· The microwave absorption and UV absorption data given by D. Rao

(RA066) and Khanna {KHA67a) respectively are also in good agreement

with our values for v" 1 1 , v" v 1 , v" v 1 1 1, v" 1 x, v" x and v' 1 • Since in

their experiments these authors had no doubt on the presence of SiF2

in their gas samples these values can be adopted as true vibrational

frequencies in SiF2. All four frequencies can be deduced from the

distances of the emission lines, so SiF2 is the emitter of at least

part of this band system. In table 6.6, however, all spectral lines

that we have measured are listed. This indicates that SiF2 is the

emitter of all these lines.

I

-116-

Table 6.2. Deslandres table for peaks in the spectrum of fig 6.2

(vibrational frequencies: 853 cm- 1 and 3~2.5 cm- 1).

v" v" m m+1 m+2 m+3

---·

44892 844 44048 863 43185 845 42340

347 334 337 348

n+1 44545 831 43714 866 42848 856 41992

341 346 348 342

n+2 43867 826 43042 884 42158 874 41284

340 359 338

n+3 43528 846 42682 40946

340

n+4 40606

334

n+5 40271

336

n+6 39936

v"1 = 853 ± 5 cm- 1 v" t 1 = 342.5 ± 2.5 cm- 1

Table 6.3. Deslandres table for peaks in the spectrum of fig. 6.2

(vibrational frequencies: 860 cm- 1 and 9~3 cm- 1).

v" m m+l m+2 m+3 m+4 l)

n 44892 930 43962 920 . 43042 954 42087

844 843 883 887

n+l 44048 929 43119 961 42158 958 41200 929 40271

863 867 874 849

n+2 43185 933 42252 968 41284 933 40351

845 846

n+3 42340 40438

v" 111 = 860 ± 5 cm- 1 Vulv = 943 ± 6 cm- 1

I

-117-

Table 6.4 Deslandres table for peaks of the spectrum in fig.6.2

(vibrational frequencies: 427.3 cm- 1 and 342.0 cm- 1).

v" v" m m+1 m+2 m+3 m+4

n 44634 339 44295 332 43962

430 427 434

n+1 44892 347 44546 341 44204 336 43867 340 43528

444 428 419 415 409

n+2 44448 331 44117 333 43784 332 43453 333 43120

400 403 416 411 437

n+3 44048 334 43715 346 43369 327 43042 359 42682

427 437 430 451 430

n+4 43621 344 43277 339 42938 348 42590 338 42252

435 429 438 432

n+5 43185 337 42848 348 42500 342 42158

411 421 413 422

n+6 42775 347 42428 341 42087 351 41736 353 41382

435 435 436 452 437

n+7 42341 348 41993 342 41651 366 41284 338 40946

426 434 450 420 423

n+8 41915 356 41559 35S 41201 336 40864 341 40523

440 433 418 426 423

n+9. 41474 349 41125 342 40783 345 40438 339 40100

439 432 424

n+10 40687 336 40351 337 40014

415 415

n+ll 40606 334 40271 336 39936

420 427

n+l2 40185 342 39844

.. 427.3 ± 1. 7 cm- 1 v" vi = 342.0 ± 1.3 em

I

-118-

Table 6.5. Deslandres table for peaks in the spectrum in fig. 6.5

(vibrationaL frequencies: 941.6 cm- 1 and 344.8 cm- 1).

v" v" m m+1 m+2 m+3

n 44892 930 43962 920 43042 955 42087

347 341 359 351

n+1 44545 924 43621 939 42682 956 41736

341 344 342 353

n+2 44204 927 43277 937 42340 957 41383

336 339 348

n+3 43867 929 42938 946 41992

340 348 342

n+4 43528 938 42590 940 41650 964 40687

343 338 367 336

n+5 43185 933 42252 968 41284 933 40351

337 338 367 337

n+6 42848 934 41914 969 40946 932 40014

348 356 340

n+7 42500 942 41558 952 40606

342 358 334

n+8 42158 958 41200 929 40271

336 336

n+9 40864 928 39936

-1

VII vii = 941.6 ± 3 cm- 1 v" v 1 1 1 = 344.8 ± 1.7 em I

-119-

TabLe 6.6. DesLandres tabLe for peaks in the spectrum of fig. 6.2

(vibrationaL frequencies: 255 cm- 1 and 342 cm- 1).

v" v' m m+1 m+2 m+3 m+4

n 44117 44633 258 44892

333 339 347

n+1 43785 264 44048 246 44294 251 44545

332 334 332 341

n+2 43452 262 43714 248 43962 242 44204 244 44448

333 346 341 336

n+3 43119 249 43368 253 43621 247 43867

344 328 344 340

n+4 42775 267 43042 235 43277 251 43528

348 359 339 343

n+5 42428 255 42682 256 42938 247 43185

341 342 348 337

n+6 42087 253 42340 250 42590 258 42848

351 348 338 342

n+7 41736 256 41992 260 42252 248 42500

353 342 338 342

n+8 41383 267 41650 264 41914 244 42158

367 356

n+9 41284 275 41558

338 358

n+10 40687 259 40946 255 41200 274 41474

336 340 336 349

n+11 40351 255 40606 258 40864 261 41126

337 334 341 343

n+12 40014 258 40271 251 40522 260 40782

336 337 345

n+13 39936 250 40185 253 40438

342 339

n+l4 39844 255 40099

v' 1 = 255 ± 1 cm- 1 u",x = 342 ± 1 cm- 1

Tabte 6.7. Destandres tabLe for peaks in the spectrum of fig. 6.2.

(vibrationaL frequencies: 342 cm- 1 and 771 cm-1).

m+1 m+2 m+3 m+4 m+5 m+6 m+7 m+8

n 44448 331

n+1 44892 775 44117 749 43368 778 42590 40271 347 333 326 338 335

n+2 44545 761 43784 742 43042 790 42252 778 41474 787 40687 751 39936 341 332 360 338 349 336

n+3 44204 752 43452 770 42682 768 41914 789 41125 774 40351 337 333 342 356 343 337

I .... n+4 44633 766 43867 748 43119 779 42340 782 41558 776 40782 768 40014 t:S

339 339 344 348 358 344 I

:•" . .) 44294 766 43528 753 42775 783 41992 792 41200 762 40438 1!: 332 343 347 338 336 339 ~~ n+6 43962 777 43185 757 42428 778 41650 786 40864 765 40099 .I 341 337 341 366 342

n+7 43621 773 42848 761 42087 803 41284 762 40522 344 348 351 336 337

n+8 44048 771 43277 777 42500 764 41736 790 40946 761 40185 334 339 342 354 340 341

n+9 43714 776 42938 780 42158 776 41382 776 40606 762 39844 346 348

n+10 43368 778 42590

v'tx =342 ± 8 cm- 1 v" X 1 771 ± 13 cm- 1

TabLe 6.8. Re-anaLysis of the data presented by Wang (WAN73b).

v"v'l m m+1 m+2 m+3 m+4 m+5 m+6 m+7

--:--[ 44120.5 254.9 44375.4 249.7 44625.~ 250.5 44875.6 236.8 45112.4 259.6 45364.0 257.2 45621.2 252.5 45873.7 347.7 349.7 344.6 367.4 334.8

n+1 43772.8 252.9 44025.7 254.8 44280.5 248.7 44529.2 248.4 44777.6 354.2 340.4 344.3 347.2 339.6

n+2 43418.6 266.7 43685.3 250.9 43936.2 245.8 44182.0 256.6 44438.6 251.4 44690.0 327.6 344.1 342.8 340.9 342.8

n+3 43091.0 251.2 43341.2 252.2 43593.8 248.7 43841.1 254.7 44095.8 340.3 341.4 345.4 341.1 355.3

n+4 42750.7 249.1 42999.8 249.2 43248.0 252.0 43500.0 240.5 43740.5 348.7 358.8 342.8 324.1

n+5 42402.0 239.0 42641.0 264.2 42905.2 270.7 43175.9 342.0 319.1 353.0 355.5

n+6 42060.0 261.9 42321.9 230.3 42552.2 268.2 42820.4 343.7 348.9 318.3 345.3

n+7 41716.3 256.7 41973.0 260.9 42233.9 241.2 42475.1 359.8 345.7 342.7 346.0

n+S 41356.5 270.8 41627.3 263.9 41891.2 237.9 42129.1 314.3 347.9 349.9 342.9

n+9 41042.2 237.2 41279.4 261.9 41541.3 244.9 41786.2 350.9 328.0 345.6 320.1

n+10 40691.3 260.1 40951.4 244.3 41195.7 270.4 41466.1 335.3 337.7 331.4 352.1

n+11 40355.5 258.2 40613.7 250.6 40864.3 249.7 41114.0 337.2 338.6 340.3 338.0

n+12 40018.3 256.8 40275.1 248.9 40524.0 252.0 40776.0 337.5 335.6 336.0

n+13 39937.6 250.8 40188.4 252.2 40440.6 336.8 336.2

n+14 39851.6 252.2 40103.8

I ..... 1\:1 ..... I

-122-

The other frequencies that can be deduced from the Deslandres

schemes are 943 cm- 1, 427 cm- 1 and 771 cm- 1

• In table 6.1 also a value

of 598 cm- 1 (RA061) that we didn't find is listed. These four

frequencies are all linear combinations of 342 cm- 1 and 253 cm- 1 :

943::::: 253 + 2*342

771 ::::: -253 + 3 * 342

598::::: 253 + 1*342

427 ::::: -253 + 2 * 342

Therefore, these frequencies are much less probable fundamental

vibrational frequencies. All lines in the spectrum we study here can

be described with the four vibrational frequencies given in table 6.9.

SiF3 is very likely not the emitter of the studied band system. The

data given by Milligan (MIL68) (table 6.1) show frequencies that don't

agree with the ones found here within the error limits. He included

the 225 - 400 cm- 1 spectral region in his absorption measurements. So,

if SiF3 should have vibrational energy spacings as 253 cm- 1 or

342 cm- 1, a signal should have shown up in his experiments. These were

not seen.

Table 6.9. VibrationaL frequencies found for SiF2.

ground state (X' At) first singlet excited state

symmetric bend antisyrnrn. bend

stretch stretch

V1 " v2" U3" vz"

855 343 870 252

Still there is the analysis of the UV emission spectrum performed

by Wang. For process 6.1 we can calculate the excess energy. available

for kinetic energy of the products and for electronic excitation, with

the data from table 2.4 and the thermochemical relation for trc~

process (2.13). A value of 6.75 eV follows from the calculation.

-123-

From Wangs measurements it follows that 5.36 eV is available for

electronic excitation at the appearance potential of process 6. I

(WAN73b). This value corresponds to a transition that would emit in

the spectral region that we study here. However. a re-analysis of

Wangs data shows that his results can also be arranged in a Deslandres

table with vibrational frequencies 253 ± 1 cm- 1 and 342 ± 1 cm- 1

(table 6.9). While the tables given by Wang (WAN73b) are fragmentary

and do not contain all the measured peaks, in the new table all peaks

can be inserted.

So, the conclusion must be that the emitter of the band system in

the 220 - 250 nm region of the UV spectrum of an SiF4 discharge is

more likely to be SiF2.

This does not exclude the excited SiF3 particle to be formed in

such a discharge according to, for instance, process 6.!. A four­

atomic species, however, in general has several other ways of loosing

its excitational energy apart from photon emission. Such emissions are

mostly very weak. To investigate whether the excited SiF3 is formed in

process 6.1 and emits light, the light emission from electron impact

on SiF4 at about 11.0 eV (maximum cross section for process 6.1)

should be investigated. Then a more reliable conclusion on the

occurrence of process 6.1 and on the emission spectrum of SiF3M can be

drawn.

-124-

Chapter 7. Reaction mechanisms in a contaminated oxygen plasma

afterglow.

7.1. Introduction.

In chapter 5 we have discussed the most prominent features in the

light emission spect·ra of 02 and SiF4 discharges. In the wavelength

region from 200 to 800 nm the band system due to the OH (A2~+ ~ X2U1 )

transition is the most intense band system in this oxygen discharge.

The origin of the hydrogen atoms in this discharge is the water

contamination in the 0 2 cylinder and in the vacuum system.

The study of the OH contamination is important for two major

reasons. OH-groups are a very unwanted contaminant of fibers

(chapter 1) so one must be able to notice their presence. Also the

reaction between SiF4 and water is very fast and lmowledge of the

influence and behaviour of H20 in a discharge with SiF4 is, therefore,

useful. Since H20 is a common contaminant of oxygen in cylinders we

started the investigation of its behaviour in 0 2 plasmas.

In the course of the experiments it has turned out that the

aforementioned band emission around 306 nm is not only present in the

spectrum of the discharge, but also in that of the afterglow. In

fig. 7.1 the emission spectrum of an oxygen plasma with only a few ppm

water contamination is shown. The atomic oxygen line in this spectrum

(at 777 nm, 58~ 5 P transition) originates from the discharge region.

It is by far the most intense line in the spectrum of the plasma and

it is detected at some distance of the discharge because it is guided

to the afterglow detector through multiple reflections in the quartz

tube (wall). The other features are not coming from the plasma region

but are true afterglow emissions. This has been checked as described

in section 7.2.

The emission band near 635 nm is the bimolecular emission reported

by Awschalom (AWS84). It is due to the luminescence from 2 02{a1 A9 )

molecules that emit one photon and de-excite to the ground level

02(X3~ 9 -) according to:

7.I

-125-

800 700 600 500 400 300 >.!nm)

Fig. 7.1. Spectrum of the afterglow of an oxygen plasma with water

tmpurtty below 10 ppm.

The emission at 762 nm originates from the decay of the 0 2(b1};9 +)

state (BEC71, HER50). This state is formed in the process:

7.II

Both for the 02(a1A9 ) and the 02(b1.!9 +) states Becker (BEC71) has

given wall deactivation coefficients for a stainless steel surface of

about 10-6 and 10-2 respectively.

In fig. 7.2 the spectrum of the OH(A2.!+ ~ X2U1 ) as it has been

measured in the afterglow is given. This has even been measured at

distances beyond 75 em downstream the discharge. The plasma has been

operated at a pressure of about 400 Pa and an oxygen flow rate of

about

1.6 10- 2 Pa m3 s- 1 •

Also in the course of the experiments a green-yellowish emh;sion

has been found in the afterglow. This turned out to be due to the

presence of small quantities of N2 in the discharge. The spectrum of

this emission is shown in fig. 7.3.

This emission is a continuum that spreads from 400 to roughly

800 nm and originates from the recombination reaction of NO and 0

(KENS4). The origi~ ~f the 306 nm band emission and of the continuum

-126-

320 310 300

il{nml

Fig. 7.2. The OH (A2};+ -+ X2 l1 1 ) band emission measured t'n the

afterglow of an oxygen discharge with water impurity.

0

L---~.-----------r----------,-----------r---------~ 800 700 600 500 400

il(nmJ

Fig. 7.3. The spectrum of the green-yellowish emission in the

afterglow of an 02 microwave discharge due to small N2

contamination. This emission is due to the NO + 0

chemiluminescent recombination.

-127-

emission, the processes that lead to these emissions, have been

investigated by studying the decay of their intensity as a function of

the distance to the plasma zone.

In the following section the experimental set-up will be discussed.

Special attention will be paid to the inlet system for small

quantities of H20. Section 7.3 deals with the theoretical background

of the experiments. The results of the measurements on H2 0

contamination of an oxygen plasma are given in 7 .4. The reaction

mechanisms that lead to the observed phenomena are discussed. In

section 7.5 the same is done for N2 contamination. A general

discussion of the results and of the applied method is given in 7.6.

quartz tube guide

monoctromator 550nm

Fig. 7.4. The experimental set-up for measurement of the afterglow of 02 discharges. U.H.P. (Ultra High Purity) oxygen is first lead through a t-N2 bath for further purification. A flow controller regulates the gas flow. The needle valve at the end of the quartz tube is used as an independent controle for adjustment of the pressure. A fixed quantity of H20 vapour is finaLLy admixed to the gas stream before the oxygen is admitted to the discharge tube. The cavity in uhich the plasma is generated can move up ,and down the tube thus varying the distance between the discharge and the photomultiplier for detection of the afterglow emission. The cycle time of this movement is 1800 s, the span 0.75 m. Typical values for the throughput are between 0.015 and 0.25 Pa m3/s and for the gas flow velocity a few m/s. The pressure drop over- the tube is of the order of tens of Pa.

-128-

7.2. L•p~rimental.

The study of the afterglow of the 02 discharges was undertaken with

the experimental set-up given in fig. 7.4.

The discharge tube used in the experiments is a quartz tube with an

inner diameter of 6 mm. It is placed axially in a cylindrical cavity.

Microwave power generated with a magnetron (Toshiba 2M172) is fed into

this cavity through a waveguide. The total power consumption is 300 W

maximum. The cavity is mounted on a chassis that can move up and down

the discharge tube to vary the distance between the discharge and the

detectors for the afterglow emission.

These detectors are mounted at the end of the discharge tube at a

fixed position. For the detection of the 306 nm band system (fig. 7.2)

a photomultiplier (EMI 9789 QB) is used which is equipped with two

diaphragms and an interference filter. This filter transmits only in

the 295-320 nm region (fwhm) (Balzers 466/1445). The NO+ 0 continuum

emission (fig. 7.3) is detected with a monochromator/photomultiplier

combination. The monochromator is a Jarrel Ash 0.25 m type equipped

with two 1180 1/mm gratings blazed for 300 and 600 nm. The

photomultiplier is an EMI 9558 QB. cooled to -20°C for noise

reduction.

Both emission detectors are mounted at an angle of 90° with the

tube. The small aperture of the monochromator and the two diaphragms

in front of the 306 nm detector determine the magnitude of the

detection volume. One should, however, be on the alert for radiation

that comes from the discharge region and is transported through the

tube by multiple reflections. The influence of this effect has been

investigated by using a quartz tube with an opaque alumina section and

by using a quartz tube with a 90° bend. In both experiments is has

been found that only the 777 nm line that is seen in the afterglow

spectra originates from the discharge. In both experiments is has been

found that the 306 nm band system and the NO + 0 continuum are

afterglow emissions.

The pressure in the tube is in the 102-103 Pa range and is measured

with a Datametrics Barocel membrane manometer. The pump that is used

is a D16BC rotary pump (Leybold). A throttle valve (indicated by V in

-129-

fig. 7.4) and a flow controller (Q in fig. 7.4) are used to adjust the

pressure and the gas flow velocity independently (see section 7.3.3).

The oxygen used for these experiments is UHP grade (99.998 %) purchased from Matheson.

The gas inlet system has been specially designed to free the oxygen

gas from H20 contamination and subsequently add a fixed amount of H20

to the oxygen that flows through the discharge tube.

This inlet system consist of two parts that are shown in figs. 7.5

and 7.6.

to llowcontroller and cavity

Fig. 7.5. Schematic drawing of that part of the inLet system that is

used to remove H20 from the 02 that comes from the

cyLinder.

This two-step set-up is necessary to obtain reproducible

circumstances. The system sketched in fig. 7.5 is used to remove as

much water as possible from 02 that comes from the gas cylinder. This

is done by leading the gas through a liquid nitrogen bath, condensing

the 0 2 and trapping the H20. The gas pressure is kept at 1.5 105 Pa

which is needed for the flow controller. Variations in the evaporation

-130-

from flowco~troller to cav•ty

~~~-,~~'

WATER 100'c

Fig. 7.6. Schematic drawing of that part of the inlet system that is

used to add a fixed amount of H20 to the 02 flow.

rate of the 02 are smoothed out by the capillary in combination with

the buffer volume. The system sketched in fig. 7.6 is used to add a

small out fixed amount of H20 vapour to the oxygen that has been

cleaned in the liquid nitrogen bath. The oxygen that comes from the

set-up of fig. 7.5 passes through a flow controller for regulation of

the flow and subsequently enters the device for addition of a fixed

amount of H20. This device consists of several parts. A large

contair:er that is filled with water serves to regulate the

temperature. The tube through which the oxygen flows is led through

this water at the point where the H20 is added. A small container is

immersed in the water bath and connected to the gas flow tube via a

capilla;:v. This container is partially filled with water. It can be

connected to a vacuum pump for removal of incorporated gases in the

water and for evacuation down to the water vapour pressure. By heating

tne water in the large container to 100°C the vapour pressure in the

small container will become 105 Pa. The pressure difference over the

<;"!pi llary will then be about 105 Pa (pi) the 02 gas flow is typically

-131-

at 5 102 Pa {p2) and the H20 vapour will flow through the capillary to

the 02 main stream.

We will now look at the flow through the capillary in more detail.

In the first part of the capillary certainly a viscous flow regime

will exist. The Knudsen number at the exit is given by:

Kn=D/X, 7.1

In which D is the diameter of the capillary {25 nm)

A is the mean free path of water molecules in oxygen.

This amounts to about 1 and is just around the limit for molecular

flow {D/A = 1) as given by Roth {ROT82). So near the end of the

capillary a transition from viscous to molecular flow may occur. In

order to obtain an estimate for the throughput of the capillary we

assume viscous flow throughout the whole capillary. The throughput is

given by:

Q ~ D4 { 2 2) = 25fu]L P1 - P2 ,

with Pl the pressure of the H20 vapour at the entrance of the

capillary,

P2 the pressure of the 02 gas in the discharge tube,

L the length of the capillary,

7.2

~ the viscosity of water vapour {11.98 10-6 N s m- 2, TOU75).

For a capillary with L = 25m we thus find:

7.2

This will be a slight over-estimation because of· deviations from

viscous flow that may arise. If we keep Pl· constant, which is done by

heating the water in the water supply system of fig. 7.6 to 100 °C,

the flow through the capillary will also be constant. Thus a fixed H20

amount can be added to the 02 gas stream in this way. Since the Qo2 is

in the order of 0.015 to 0.25 Pa m3 s-1, the H20 concentration can be

varied between 0.6 and 10 ppm.

-132-

1.3. Theoretical description of the decay of the light emission in

the afterglow of discharges.

7.3.1. Introduction.

The light emission intensity of the afterglow of the discharges we

study, is a function of the distance to the plasma region. Since this

intensity is proportional to the number of excited particles that can

(spontaneously) emit the radiation under consideration, the decay of

the intensity is a reflection of the decay of the concentration of the

excited particles as a function of the distance to the plasma zone.

In our experiments a quartz tube has been used with an inner

diameter of 6 mm (fig. 7.4). The gas flow velocity in this tube is of

the order of a few m/s. Both the OH(A2~· ~ X2lli) band emission and

the NO + 0 chemiluminescence are optically allowed transitions. The

lifetime of the OH(A2~·) state is about 7 10-7 s. This means that the

decay takes place almost at the same position where the excited state

is formed (under our experimental conditions). Since the emissions can

be detected at distances even beyond 75 em from the discharge, their

decay must be due to a decay in their production. So, in fact, the

decay that is measured is the decay of their precursor(s).

In order to find out these precursors and the reaction mechanisms

through which the afterglow emission is produced we will now look in

more detail to a description of the decay of these emissions.

7.3.2. Exponential decay.

Suppose that particle a whose density is denoted by na is the

precursor of one of the light emissions. During the journey of the gas

from the discharge region to the pump, particles a can be formed and

destroyed in various processes. In this section we will assume n to

be so small that interactions between two particles of this type can

be neglected. Section 7.3.3 will deal with the situation that this is

net valid.

The particle density na as a function of time (t) and position (x)

along the tube obeys the relation:

an a at=-

-133-

This kind of formulation has also been used by Phelps in his study of

the decay of metastable noble gas atoms (PHE59). In this formula the

different terms have the following meanings. a n a ~ is the change of na at a certain position {downstream

anv -~ ax

the plasma zone). per unit of time;

is the decrease of the particle density na at position

x due to the gas flow in the tube per unit of time, v9

is the mean gas flow velocity at position x;

stands for the amount of particles a that react at the

wall of the tube per unit of time. Da/nt is the

diffusion coefficient for a. The factor f is the wall

deactivation coefficient. the chance that a particle a

is deactivated in a collision with the wall so that it

is not the precursor of an emission anymore. A is the

diffusion length that is given by 7.4;

is the decrease of the particle density naper unit of

time due to two-particle interactions. If the nature of

the second particle is irrelevant, na is replaced by

the total particle density nt. ka is the rate

coefficient for this process. This term can contain

more than one contribution if particles a can react

with different other particles with different rate

coefficients;

- k3n~nana describes the decrease of na by three-particle

1 - -n te a

interactions. Again na and n~ can be replaced by nt if

the nature of these particles is irrelevant. This can also be composed of more than one contribution with

different rate coefficients k3;

is the decrease of naper unit of time due to

spontaneous deexcitation of-the particles. The

parameter te is the life time of the excited state.

This term is only relevant for excited particles;

-134-

finally stands for the production of particles a. It

can be written as the sum of several kinds of

production mechanisms as is done with the decay

mechanisms. The structure of ~a strongly depends upon

the particle under consideration.

The diffusion length A is given by:

A

With R the radius of the tube and

L the length of the tube.

7.4

Since we only look at stationary systems, the explicit time dependence

is 7.3 disappears:

an a --at 0 . 7.5

In most situations the convective term in 7.3 can be simplified

because the gas flow velocity v9 is nearly constant along the

discharge tube:

an v a g ax

an a v --gax 7.6

With these simplifications 7.3 can be w1·itten:

d n a

n ;:r-;( a

::. [ -f Vg

1l. nR - k3nRn + .!._ ~ ] , 7 . 7 ·"2 tJ tJ 'l na a

From this expression it can be seen that, if ~alna is independent of

x, na is an exponential function of x and the right-hand part of 7.7

is a constant, the inverse decay length. It is more illustrative,

however, to tranform the decay length into a decay frequency. This can

be done by dividing the gas flow velocity, v9 , by the decay length, ~:

7.8

-135-

In fact this embodies a transformation from the laboratory

coordinate system to the coordinate system that moves with the gas

flow. The decay frequency is thus related to the mentioned processes

according to:

1 T

L + t e

+ L~ n a a

7.9

In the case that the nature of the collision partners in two- and

three-body collisions is irrelevant, or the particle densities n~ and

n~ are fixed fractions of the total particle density n 1 , n~ and n~ can

be replaced by nt.

The production of new particles a, ~a. will, in general, be

negligible. Only in special cases this should be taken into account.

With these simplifications 7.9 becomes finally:

1 T

1 - + t

e + 7.10

Measurement of liT as a function of total density {nt) gives the

opportunity to determine the mechanism that is responsible for the

decrease of lla· If liT depends linearly on the density a two-body

process is most important, if liT is a quadratic function of the

density a three-body process is most important, and so on.

Summarizing we can say that, if the decay of a specific emission is

exponential, this tells that only one excited or active particle of

type a is involved in the reaction mechanism that is responsible for

the decay. Secondly, from the dependence of the decay frequency on the

particle density, the kind of reaction mechanism and the rate

coefficient can be deduced.

7.3.3. Non-exponential decay.

If the density of particles of type a becomes so high that

interactions between two such particles .become important, an extra

term is introduced in equation 7. 3 that is quadratic in na. An

analytical solution of this equation is not possible then. Only if

-136-

this process becomes the most important one and the linear terms can

be neglected, a solution can be given. Equation 7.3 then has the form:

8 n a ~

8 n v a g

8 X

Here is 1/J a constant.

,Pn2 a 7 1.1

Again, for stationary situations the explicit time dependance of r~

is zero and the gas flow velocity can be taken constant.

This results in:

,Pn2 a

The solution of this equation is an inverse proportionality:

= n a

Solving for

d

X

+ Lx na,O v g

d n a gives: dX

,p/v =

( 1/na.O + x • 2 .

,p/v > g

7.12

7.13

7.14

The decay of the emission intensity must follow this curve to

satisfy the conditions for a process in which two excited particles

are involved. Distinction from the case in which an exponential decay

is found may be possible if the decay of the light emission can be

measu .. ·ed over sufficiently long distances from the discharge region.

7.3.4. Some aspects of the analysis of the data.

In the previous sections it has been indicated in which way the

meu3urement of the decay of afterglow emission intensities can yield

iH.; .rmation on the processes in the afterglow. Measurements, however,

-137-

all have their specific uncertainties that lead to errors in the

determinations of the relevant parameters of a least squares fit.

Systematic errors, such as the background level of the signal, also

influence the results. Therefore, it would be helpful if an

experimental aid could be found to check the results. Such an aid is

presented in this section.

In equation 7.10 it can be seen that the decay frequency 1/T that

follows from an exponential fit to the data is a function of the total

particle density and of a few fixed parameters. It is no explicit

function of the throughput, Q, or the gas flow velocity v9 • So, if v9

or Q and the density in the discharge tube are varied independently,

the variation of v 9 and Q may have no effect on the value of T.

If, however, a variation of v 9 or Q at a constant density

influences T, this means that at least one of the premisses for

equation 7.10 is not fulfilled.

Several explanations are possible in such cases:

the initial data set may not represent an exponential curve,

the replacement of na and n, in 7.9 by nt is not allowed.

the reaction mechanism is such that explicit dependence on v 9

or Q is introduced. This can occur when the wall of the tube

plays an essential role,

the production of particles a plays a role and introduces this

dependence.

From the foregoing analysis it follows that it is desirable to have a

facility to independently vary two out of the three variables v9 , Q

and nt. These are related according to:

R T 2- 0 Q = 11' R v nt -N-- 7.15

g A

Here is R the radius of the tube,

Ro the univeral gas constant.

T the absolute temperature,

NA Avogadros number.

By adjusting the flow controller and the throttle valve at the end of

the tube independently (fig. 7.4) the extra information that is needed

can be obtained.

-138-

7.4. Measurements of the afterglow emission intensity of the

OH (A2~+ ~ X2 IT 1 ) transition.

It has been argued in section 7.3 that the OH emission originates

from a process in which OH" OH (A2}:+)) is formed from another

species. Production of OH" particles cannot be the result of electron

molecule interactions at such large distances from the plasma zone and

in such high quantities as the emission intensity indicates. Electrons

are not available for this process because of the fast recombination

downstream of the plasma zone. A chemical reaction must, therefore, be

the production channel for OH".

to' :J •' (1)

:::» ...> (0 to' c Q)

...> c I 0) to• to• 0

ta1""'.'-:!;81;--;r-;;-.;:-.....-;::;r.-r-::;-.:-r::::'l:"-r:=r.--r-:::""r::-T-::'""'"'"'-ko""' tU 28.0 :!o.t 48.1 Sl.l liU 78.1

dtstsnce. lcml

Fig. 7.7. Typicat decay curve for the intensity of the 306 nm

emission as a function of the distance to the discharge

region.

In fig. 7.7 a typical decay curve for the 306 nm bandsystem of an

afterglow of an oxygen plasma with a small admixture of H20 is shown.

This emission decreases exponentially with the distance to the

discharge region over nearly three decades. Thus (see section 7.3) the

decay can be characterized by a constant decay frequency determined by

-139-

the decay of the particle density of one of the precursors of OH". The

decrease of this particles density cannot be due . to a gas phase

process that involves more than one of these particles. Also the

production of precursor particles in the afterglow is limited to very

special processes (talna· constant). Processes that are energetically

possible for the formation of OH" are:

0 + H --+ OH" • 7. III

0 + H + M --+ OH" + M, 7. IV

0 + H + w --+ OH" + w 7.V

H + HOz --+ OH" + OH 7.VI

H + 03 --+ OH" + Oz 7.VII

Here is M a third body

w the wall of the tube.

In these reactions only products of the discharge in 02 with a small

(ppm range) HzO contamination occur.

After deexcitation of OH" in 7.III. 7.IV and 7.V a subsequent

reaction with atomic oxygen releases the H-atom again:

OH+O--+Oz+H, 7 .VIII

This reaction is rather fast (rate coefficient ka = 4 10- 11 cm3 s- 1

DEM79). In this way the H-atoms can serve as a catalyst for the

recombination of o-atoms.

Both processes 7.VI and 7.VII are improbable because they involve

two decaying particles that are only present in low quantities. Also

it has been established that the excitation of OH" in 7. VII is not

electronic, but only vibrational up to v = 9 (TOBB4).

The decay of the OH" emission intensity must be the result of the

decay of the particle densities of its pre~ursors. Because this decay

is exponential over such a wide range it is only possible that it is

governed by simple processes. In order to find out the nature of these

processes the decay frequency 1/T has been measured as a function of

the total particle density. The result of these measurements is shown

in fig. 7 .8. In this figure two sets of measurements are given, one

-140-

performed at a constant throughput of gas and the other at a constant

setting of the throttle valve. The results of these measurements lie

in each others confidence interval. This means that there is no

influence of the gas flow velocity on 1/T. The regression line

indicates a linear dependence of 1/T on nt.

ID

Total denstt~ [m-~

Fig. 7.8. The relation between the decay frequency (1/T) and the

total particle density in the discharge tube for the

OH(A2~+ ~X2IT 1 ) transition. The triangles tndtcate

measurements performed at constant throughput Q. The

crosses indicate measurements performed at a constant

setting of the throttle valve V tn fig. 7.4.

The slope of the regression ltne is 1.0.

There are three possible particles whose decay in density can cause

the decay of the OHM band emission intensity: 0-atoms, H-atoms and OH

radicals. The D-atom density decreases due to processes 7.III, 7.IV.

7 .V, 7. VIII and:

0+0 +M--+02+M,

O+~+M--+03+M,

7.IX

7.X

-141-

Its decay can, therefore, be described by a decay frequency given by:

This equation can now be analysed for its validity.

The particle densities of Hand OH atoms are a function of the gas

flow velocity. In the experiments a fixed amount of H20 is added to

the main stream of oxygen (section 7.2). Since the gas flow velocity

of the oxygen is variable, the H20 particle density varies according

to:

7.17

Here is NH20 the number of molecules that leaves the H2D-capilary

per unit of time,

v 9 the gas flow velocity in the discharge tube,

A the cross section of the discharge tube.

All particle densities related to that of H20 will, therefore, vary

in the same manner with the gas flow velocity.

In fig. 7.8 no explicit influence of v 9 is noticable and,

therefore, the terms in 7.16 containing nH and noH cannot be

responsible for the decay process.

The diffusion term would result in an inverse proportionality of

1/To with nt which is also not seen in the measurements. If we assume

that the particle density of 02 is almost equal to the total particle

density, a rather straightforward assumption, reaction 7.X can also be

excluded, because this would have resulted in a quadratic relation

between 1/To and nt. Reaction 7. IX, finally, also can be skipped,

because Kaufmann has shown {KAU58) that its rate coefficient is much

lower than that for reaction 7.X. So, if reaction 7.IX were important,

reaction 7.X should certainly have been seen. On the basis of these

arguments the decay of the 0-atom density can be excluded as the cause

of the decay of the OH (A2I+) emission intensity.

For the decay of the H-atom density reactions 7.III, 7.1V, 7.V,

7.VI, 7.VII and:

-142-

H + H + M -4 H2 + M

H + OH + M -4 H20 + M ,

are responsible.

7.XI

7.XII

The H-atom density is increased by process 7. VIII. For the decay

frequency of nH we can thus write

Dnl + k5no 2 - + k6nuo2

A nt +

+ kl2n0Hnt - ksnonOH 1 nH · 7.18

The diffusion term due to process 7.V can be excluded because liT is

found to be linearly proportional to nt. On the same ground process

7.IV can be excluded. Processes 7.VI, 7.XI and 7.XII can be excluded,

because no influence of the gas flow velocity is measured. Process

7.VII is neglected because two second order particle densities are

involved. Processes 7.III and 7.VIII are left:

1 - keno 7.19

Process 7.8 obviously is so fast that OH radicals immediately react to

02 and H. The ratio noHinH will, therefore. be very low. In equation

7.19 the second term cannot dominate because we see no increase of the

emission, but a decrease. With liTH in the order of 101 to 102

fig. 7.8 and ks equal to 4 10- 11 cm3 s- 1 and an estimated degree of

dissociation of 5% (GOL73) we find that noHinH must be smaller than

roughly 10- 4 to give a negligible contribution in equation 7 .19.

Process 7.III thus determines the decay of the emission intensity. The

rate constant for this process can be calculated from our

measurements. It follows from fig. 7.8 and the degree of dissociation

(5%. GOL73) that

7.20

The regeneration of H atoms (process 7.VIII) is 4 orders of magnitude

-143-

faster than this process which is in agreement with the assumption

that noH/nH is smaller that roughly 10- 4 .

In this model there is, so far, no physical reason for the decay of

the emission intensity. The no must be constant along the tube as we

have show above. The decay of the H-atom density cannot be due to a

process in which H-atoms are involved directly as we have shown just

now. A decay due to process 7. II I is immediately compensated by

process 7.VIII. Therefore, there must be another process that is

responsible for the decay of nH. A possible process is

OH + OH{w) -+ H2 0 + 0 . 7.XIII

The H-atoms that are lost in this reaction cannot be regenerated.

A gas phase reaction between two OH radicals would be very improbable

because their density is too low for this. In contrast to that, the

concentration of hydroxyl groups on the surface of quartz can be very

high (LAN86). In this way they can wait for another OH radical to

react with. So reaction 7 .XIII can explain the decay of the H-atom

density. It does not show up in the expression for 1/TH (7.18) because

H-atoms are not directly involved.

7.5. Measurements of the afterglow emission intensity of the NO+ 0

chemiluminescence.

The spectrum of this chemiluminescence is shown in fig. 7.3. This

emission is lmown as the "air afterglow" {KAU58) and originates from

the recombination reaction of NO and 0:

7.XIV

Unlike the 306 run band system it is only present in the spectrum of

the afterglow and not in that of the discharge itself.

It starts a few nun {dependent on pressure and microwave power)

downstream of the discharge, leaving a dark gap between the pink color

of the oxygen plasma and the green-yellowish afterglow. Also the

intensity of the NO + 0 chemiluminiscence depends on the quartz wall

-144-

temperature. If this is increased locally, the intensity decreases at

that spot. Downstream of the heated section the emission is visible

again. When the wall cools down. the emission reappears after some

time.

In tubes with an alumina section and in the stainless steel tubes

of the vacuum system we have found that these wall materials do not

quench the emission although the molecules and radicals suffer many

collisions with the wall. In the 6 rnm tube that we use, at a gas flow

velocity of 2 m/s and a pressure of a few 100 Pa, particles will hit

the wall almost every mm they travel in the flow direction.

30

1.0

.s 0.

0.4

2.0

Fig. 7.9. Relation between the intensity of the NO + 0

chemiLuminescence and the wall temperature of the quartz

tube.

In fig. 7. 9 the relation between the wall temperature and the

emission intensity is given. The regression line is given by

I

Here is I the intensity of the NO+ 0 chemiluminescence,

Io a reference intensity,

k Bol tzmanns constant 1.3805 10-23 JIC 1,

T the absolute wall temperature,

Ea a characteristic energy.

7.21

-145-

Ea is 0.092 eV for the live drawn in fig. 7.9. We will return to this

phenomenon later on.

As in this case of the 306 nm band system the intensity of the

chemiluminescence depends exponentially on the distance to the

discharge region {fig. 7.10). Thus one of the precursor particles of

the luminescence is withdrawn from the production of N02* in an

exponential decay.

180 SOD 600

tD 2 t02 :J

(11

:J) ..., (I) to' c CD ~ c I IJ) to• 0

w•-=--r-::"r:-.....-:::-o::-~-=-r:--r-~--.-r--.---r--..--+to-• D.OI tO.O 20.D 30.0 18.0 50.0 60.0 70.0

dlsi..anc-e lcml

Fig. 7.10. Typical decay curve for the emission intensity of the

NO + 0 chemiluminescence as a function of the distance to

the discharge region.

In fig. 7.11 the measurements of 1/T as a function of the total

particle density nt are shown.

At low particle densities 1/T is inversely proportional to the

particle density, indicating that a diffusion process is responsible

for the decay. The uncertainties in the measurements do not admit a

definite choice between a linear or a quadratic dependence of 1/T on

n 1 for higher particle densities. Unfortunately the used equipment did

not allow for a decisive check by measurement of 1/T at higher

pressures than 2•103 Pa.

-146-

I 12 I 8 1014 to2+----'---'--'-.1-J.....L.l...l.f----'---'-----'---'-.L..l...U., tot

.. " (/)

:::J) + • 0 + c Q) to' to' :::l t:T Q)

'-4-

:::J) (ll 0 Q)

1=1

t8• to• 1012 to8 1014

Total density [ m-3 J

Fig. 7.11. Measured relation between the decay frequency 1/T and the

totaL particle density nt in the discharge tube for the

NO + 0 chemiluminescence. Crosses indicate two series of

measurements at two different settings of the throttle

valve (V). The triangles represent a series of

measurements at constant throughput Q.

Let us now consider the possibilities that may lead to the

behaviour as found in our experiments.

The NO radicals that are consumed in reaction 7.XIV return through

7.XV

So, same as the H-atoms. NO radicals are a catalyst for the

recombination of o-atoms.

Some energetically possible reactions that NO can be involved in

are

N + NO ........ N2 + o. 7.XVI

NO + 03 - N02 + 02 7.XVII

N + 03 - NO + 02 7 .XVIII

NO + NO(w) ........ N2 + 02 . 7.XIX

NO + 0 + M - N02 + M. 7.XX

-147-

Reactions 7.XVI and 7.XVIII can be neglected because they involve two

particles with low densities and because the densities of N and l\0

depend on v9 (see section 7.4). Reaction 7.XVII is also negligible for

these reasons, but moreover the N02 produced in this reaction is

subjected to 7.XV and is thus returned to the reaction cycle. Reaction

7.XIX can be important in view of the fact that wall contaminations !Jy

NO are well known to be persistent (BEC71). This means that NO

radicals can stick to a wall for a long time and thereby have a fair

chance to react according to this reaction. Reaction 7.XX is known in

literature (KAU58) as a channel for reaction without light emission.

The overall decay frequency of NO can thus be written

+

From the measurements {fig. 7.10) we can conclude that for low values

of nt the diffusion term prevails. The No-radicals are removed from

the catalysis by wall recombination. The rate of removal is determined

by f19 which can be calculated to be (at nt = 8.6 101& crn-3 ,

1/T : 25 -1 s •

ft9 = 0.004 . 7.23

This means that only 4 out of 1000 collisions of NO with the wall lead

to reaction 7 .XIX. In view of the low concentration of NO (of the

order of 10-3 nt) this is not surprising.

For high particle densities (nt = 8.6 1017 cm-3 , p 20 mbar) the

effects of reaction 7.XIV and 7.XX can be compared on the basis of

rate coefficients that are known from literature. The rate constant

for reaction 7.XIV is reported to be about 7 10- 17 cm3/s (FON64,

GOL73). For k1s a value of 9.3 10- 12 cm3/s was found (DEM79) and k2o

was given by Kaufman {KAU58) to be 6.8 10-32 cm&/s.

For this situation 7.22 can be written

--1- = 7 10- 17 - 9.3 10- 12 ~2 n + 5.9 10- 14 r 7 NO ~ ~0 O

7.24

-148-

From the constants in this equation it follows that reaction 7.XX

prevails over 7.XIV. The light emission is quenched due to three-body

interactions. Reaction 7.XV is much faster than 7.XX. This makes that

the NOz particle density must be very small (< 10- 3 nNo) to retain the

independence of 1/TNO on the gas flow velocity. Thus, for high n 1

reaction 7.XX is the dominating process that determines 1/TNO· If we

write no in terms of the degree of dissociation according to

= a nt ,

we can calculate the degree of dissociation from

5.9 10- 14 a nt .

When nt = 8.6 1017 cm- 3 , -1 s . which gives:

7.25

7.26

7.27

This value is rather low compared to the value of 5% suggested by

Golde (GOL73). It could mean that the rate coefficients that we used

are inaccurate and that the assumption that ~02 << ~O does not hold.

Our measurements, however, are not decisive in this respect because

the pressure range we could use is too small.

When the temperature of the quartz wall is increased, the

chemiluminosity decreases as we have shown in the beginning of this

section. The mechanism that is responsible for this is still unknown,

but a recombination of NO radicals to N02 or even N03 that is

stimulated by a hot wall can be responsible for this phenomenon.

Downstream of the heated section these products can react with 0-atoms

to form NO and N02 respectively (DEM79 for rate constants) returning

the nitrogen atoms to the catalytic cycle and enabling the

green-yellowish glow to reappear.

Another explanation, heating of the gas and subsequently cooling

down again, seems less probable because the heat transfer from the

tube to the gas probably is too small to generate the effects we have

found in our experiments.

-149-

7.6. Discussion.

It has been shown in this chapter that the afterglow emission

a;:;ec~rum of a."l cxygen discharge exhibits several features that are

related to molecular metastables. The influences of H20 and N2

impurities on the afterglow emission have been studied. Both

impurities introduce an extra band system in the emission spectrum of

the afterglow. The reaction mechanisms that cause these emissions have

been studied.

For the H20 impurity is was found that the OH(A22:+ -+ X211i)

transition is added to the afterglow spectrum. The production of the

excited OH radical is a two particle recombination process of H- and

0-atoms. The rate coefficient of this process was found to be 2.3

10- 15 cm3 /s. In a subsequent reaction of OH with D-atoms the H-atoms

are released. In this way they serve as a catalyst for the

recombination of D-atoms to 02. The decrease of the emission intensity

along the tube was found to be due to a wall recombination reaction of

two OH radicals.

N2 impurities introduce the so-called air afterglow. a

green-yellowish emission that is due to the chemiluminescent

recombination of NO and 0 to N02. In this case also a subsequent

reaction of N02 and 0 returns the NO radicals to serve as a catalyst

for the recombination of D-atoms. NO radical recombination at the wall

causes the decrease of the intensity of the afterglow. The

chemiluminescent recombination has, however, a rival in the three-body

recombination process. For low pressures this process is not important

and the decay of the afterglow intensity is fully determined by the

recombination of NO radicals at the wall. At higher pressures this

recombination is overruled by the three-body process, but it sdll i«

responsible for the decay.

The method that has been used for the search for the reaction

mechanisms was derived from Phelps' way of determining the rate

coefficient for the decay of metastable densities. We introduced an

independent variation of two out of the three variables: throughput,

gas flow velocity and total particle density. This enables a better

decision on the kind of reaction mechanism that tak•~s pla;;: in the

afterglow as discussed in section 7.3.

-150-

Chapter 8. Concluding remarks.

The goal of the investigations that are described in this thesis

was to find out if SiF4 could have an application in the manufacturing

of optical fibers. In this final chapter some reflections on this

subject will be given. Also some more general ideas that follow from

our studies will be presented.

Optical fiber production processes nowadays use SiC14 as the

starting material for the deposition of Si02 in glassy layers. This is

done in discharges. in flame burners or through CVD-processes.

Application of SiF4 in a flame burner or a CVD-process is not feasible

on thermodynamic grounds as we have shown in chapter 3. In plasma

processes with SiF4 as the main component a sooty deposit is obtained.

The efficiency of this process stays behind by a factor of 20 when

compared to that with SiC14 (BEE85, BACS5).

Our study focussed on the reaction mechanisms in SiF4-o2 and

SiCl4-o2 discharges in order to find the fundamental reasons for this

difference.

An important reaction channel in plasmas can be the ion chemistry.

When comparing the ion chemistry in SiCl4-Q2 and SiF4-02 systems we

have found no major differences.

In the first place the (chemical) reactions between the ions are

not numerous, secondly the differences that occur between SiC1 4 and

SiF4 are only of minor importance. These differences are mainly in

reaction rates and cannot account for the difference in efficiency

between the deposition of Si02 from SiF4-Q2 and SiCl4-02 plasmas. Ion

chemistry is probably not responsible for Si02 deposition at all.

There are no ion-molecule reaction paths that appear to lead to oxygen

enrichment in the molecules. The only reactions that are reasonably

fast are a few charge transfer processes. The ions studied were formed

in electron-molecule interactions. The negative ions were formed at

relatively high electron energies (8.8 eV for SiCl4 and 11.0 eV for

SiF4). This is analogous to the situations for CF4 and 0Cl4. The high

cross section for negative ion production at very low electron

energies that was found for CF2Cl2 (VER7S) may be found for SiF2Cl2 as

well. but the chances that this will introduce a considerable

-151-

improvement in reaction rates to form Si-Q bonds are very low. The

results that we found give no indication that other ions than those

studied (e.g. SiF2-) would behave differently. In recent studies (a.o.

by Christophorou, (CHR86) and references therein) it has become clear

that the formation of negative ions at higher temperatures may be much

more efficient. Vibrational excitation of the ground state of a

molecule may enhance negative ion formation by a factor of 106 in

typical cases. These effects, however, are not likely to occur for

SiC1 4 and SiF4, because the formation of their negative ions occurs at

high electron energies.

When a background pressure of water is present the reactions of the

ions in which halogen atoms are replaced by OH-groups, without

exception. are very fast. This could be the reason why incorporation

of these groups in the deposit is a bigger problem in PCVD processes

than in other processes {BACS5).

In the emission spectra of SiF4 and SiCl4 discharges only bands of

the diatomic and tri-atomic species SiF, SiF2, SiCl and SiCh have

been recognized (see chapter 5 and 6 for SiF4 and SAM86, ROWS5, ASU3S,

HUB79 for SiCl4). Emissions from SiF3, SiF4, SiCl3 and SiCl4 have not

been positively identified. Donnelly {DONBO) has reported a continuum

emission with a maximum at 632 nm that tentatively was ascribed to

recombination of SiF2 and F to form SiF3~. but this emission was not

seen in our experiments. So it .seems that SiF4 and SiCl4 behave

analogously in a discharge. Their dissociation patterns appear similar

and the fragments are excited in a comparable way.

A big difference between the two molecules lies in thermodynamic

properties. The relevant bond strenghts are: SiF3-F = 6.95 eV,

SiCb-Gl = 4.77 eV. F-F = 1.62 eV, Cl-Gl = 2.51 eV. This makes that

for SiF4 not only the breaking of the bond Si-F costs more energy than

in the case of chlorine, but also the formation of an F2 molecule

yields less in return. The combination of these two facts is probably

responsible for the big difference in behaviour of SiF4 and SiCl4. The

dissociation due to plasma specific interactions, separates F and Cl

atoms from the molecules leaving one or more free bonds. In this way

the radicals capable of reacting with 02 are formed. But, in the case

of SiF4, the F atoms have no attractive alternative for chemical

bonding. The bond strength in other F-containing molecules like F2 is

-152-

too low to make the competition between Si-0 and Si-F bonds tend to

Si-0 bond formation. In other words: production or formation of Si-D

bonds may be a fast process, but the reaction with F (or F2) is also

fast and favoured by the relatively high F atom concentration. The net

production of Si-D bonds will thus be limited. It is, therefore, not

surprising that considerable amounts of fluorine are being

incorporated in the deposited layers (KUP78). In the case of chlorine

the thermodynamic values are much more favourable for the formation of

Si02. The chlorine molecule offers a stable way out for separated Cl­

atoms.

So it seems desirable to add a fluorine bonding species to the

discharge gas mixture if one wants to use SiF4 for deposition of

glassy Si02 layers. One could think of several possibilities in this

field: H2. CX>2 or Na, but H2 and Na are unwanted because they may

introduce very undesirable impurities into the deposit. Admixing of

CX>2, however, may work. Although CF4 is used as an etchant for Si and

Si02, the excess of oxygen in the deposition plasma may passify this

action. In production processes (BACS5) nowadays C2F& is used as a

supply of fluorine for doping of the fibers. Carbon atoms thus appear

not likely to be incorporated. A study in this direction should be

undertaken if SiF4 is to be used for Si02 deposition.

The use of modified thermodynamic equilibrium calculations to

investigate reaction tendencies in plasmas as it has been described in

chapter 3 is a new method to approach the problem of revealing

reaction mechanisms in plasmas. In our case it has shown that

predissociation of oxygen in a separate discharge probably increases

the reaction efficiency in the Si02 deposition process.

An experimental check of the predictions is very much wanted. The

method that has been described may be used in more general sense than

just in plasma chemistry. Other non-equilibrium plasmas and also laser

induced processes could also be studied with this technique.

The charm of the method is that no kinetic data are used. Starting

from the thermodynamic quanti ties of the involved species reaction

tendencies in non-equilibrium situations can be predicted.

At this point a remark with a much deeper meaning should be made.

The perturbation method that has been used in chapter 3 gives

information on three different subjects in one (numerical) experiment.

-153-

As we have shown reaction tendencies may be derived from the

perturbation of thermodynamic input variables. Secondly the fact that

a small variation in the input parameters gives variations of the

output parameters that are of similar magnitude or smaller tells that

the numerical procedure used for the solution of the set of equations

is numerically stable.

Thirdly the deviations of the solution of the calculation due to

variation of the input thermodynamic values gives information on the

desired degree of accuracy to which the thermodynamic values should be

known. This feature may be used in the testing of the reliability of

specific estimates of thermodynamic properties.

So, three very different conclusions can be drawn from the same

experiment. It may by worth while to investigate the implications of

this fact with respect to the relation between the numerical model and

reality.

In chapter 7 the studies of the afterglow light emission of an

oxygen discharge with small impurities are described. We found that

both H20 and N2 impurities are detectable in the afterglow emission

spectra even at considerable distances from the plasma. Knowing that

the incorpation of OH-groups in the deposit is very unwanted and that

N2 is an indication for leakage of the vacuum system, the emissions

due to these impurities can be used to monitor the production process.

In the studies of the reaction mechanisms that lead to these

emissions (the OH (A2~+ ~ X2Hi) transition and the NO + 0

chemiluminescence) we have used the method described by Phelps (PHE53,

PHE59). In order to obtain more information on the reaction mechanism

than this standard method gives, we have investigated the influence of

the gas flow velocity on the decay frequency that follows from the

experiments. With this technique we have been able to exclude several

reaction paths from the possibilities.

The application of this method in our case has revealed that wall

recombination reactions of intermediate radicals are the cause of the

decay of the light emission due to H20 and N2 impurities. Both H20 and

N2 are dissociated in the discharge and the H and N atoms serve as a

catalyst in the recombination process of o-atoms. Chemiluminescent

reactions are· part of these catalytic reaction chains. OH and NO

radicals recombine at the wall in such a way that H and N atoms are

withdrawn from the catalytic process.

-154-

Sll!llllary.

The search for an application for SiF4 , a waste product of the

fertilizer industries, has been the motive for the study of reaction

mechanisms in SiF4-02 and SiCl4-Q2 plasmas in which Si02 is deposited.

In several ways it has been tried to gain insight in these reaction

mechanisms. Each of these ways is discussed in a separate chapter.

In chapter 2 fundamental processes are described. Especially the

ion chemistry of both SiCl 4 and SiF4 is dealt with. In a plasma this

can be an important reaction channel. It is shown, however, that in

SiF4-o2 and in SiCl4-Q2 plasmas this is not the case. Reactions in

which radicals play a major role dominate the chemistry that leads to

the formation of SiOz.

Both the positive and the negative ions of SiF4, SiCl4 and Oz do

not react in such a way with the neutral molecules that silicon-oxygen

bonds are formed.

The exchange of halogen atoms by OH-groups in reactions of the ions

with water, perhaps leading to OH incorporation in the deposit, may be

more important in PCVD processes than in other CVD processes, becau.se

these reactions are very fast.

Inconsistencies in thermochemical data from literature induced a

study of these data leading to a consistent set that could be used as

a basis for further studies. This set of data is also given in

chapter 2.

In chapter 3 a new method, based on modified thermodynamic

equilibrium calculations, in search for dominant reaction paths in the

plasma is presented. The influence of various radicals on the overall

reactions in the plasma is studied by the modification of

thermochemical data of species. With this method the plasma-specific

influences on the chemistry can be simulated. The charm of the method

is that no kinetic data are needed to obtain information on the

chemical reaction mechanisms.

From the calculations it follows that the dissociation of Oz is

more important than the dissociation of SiF4 if one wants to form Si02

in SiF4-02 discharges. At first sight this may seem a bit surprising

because the bond strengths in Oz and SiF4 are 5.16 eV and 6. 95 eV

-155-

respectively. The low value of the bond strength in F2 is responsible

for this phenomenon.

In chapter 4 a mass spectrometer set-up, that has been built for

the analysis of the plasma composition, is described. The results of

preliminary experiments with this apparatus are given.

Light emission spectroscopy has also been used to study the

reaction mechanisms in our plasmas. Chapter 5 is an introduction in

which the general characteristics of the emission spectra of SiF4, 02

and SiF4-D2 are given.

A study of the band system between 220 and 260 nm in the spectrum

of an SiF4 discharge is presented in chapter 6. Conflicting reports in

literature on the origin of this band system are investigated and it

is shown that SiF2 is the emitting species and not SiF3 as suggested

by some authors.

In chapter 7 our studies of the influence of small impurities on

the afterglow emission of an oxygen discharge and the reaction

mechanisms in this afterglow are described. Water impurities introduce

OH(A2~+ ~ X2U1 ) band emission in the afterglow. It is shown that this

is part of a process in which H-atoms serve as a catalyst for the

recombination of o-atoms. The presence of hydrogen in SiCl4-02 and

SiF4-D2 plasmas not only leads to incorporation of OR-groups in the

deposit, but it also quenches reactive particles. Nitrogen impurities

also act as a catalyst in the recombination of o-atoms. The

chemiluminescent reaction between NO and 0 is used to study this

phenomenon.

The method used by Phelps for his studies of the quenching of

metastables is extended for our investigations. Independent variation

of two parameters out of the three {throughput, gas flow velocity,

pressure) is used to distinguish among several reaction mechanisms. It

turns out that this extension is valuable for the study of reaction

mechanisms that lead to a decay of the emission intensity of plasma

afterglow.

In chapter 8 some general conclusions are given.

=156--

Samenvatting.

Op instigatie van de Nederlandse kunstmestindustrie,

hoogwaardige toepassing voor hun afvalprodukt SiF4 zocht,

die een

zijn de

reactiemechanismen in SiF4-o2 plasma's die leiden tot de vorming van

Si02 neerslag onderzocht. Er zijn zowel experimentele als oak

theoretische technieken gebruikt om inzicht te verkrijgen in deze

processen. Iedere techniek wordt behandeld in een apart hoofdstuk.

Na het inleidende hoofdstuk 1. waarin nader wordt aangegeven waarom

naar SiF4-02 en SiCI4-o2 ontladingen gekeken is, volgt hoofdstuk 2,

dat handelt over fundamentele processen. Vooral ionenchemie wordt hier

besproken omdat dat in een plasma een belangrijk reactiemechanisme kan

zijn. In het geval van SiF4-02 en SiCI 4-o2 plasma's wordt echter

duidelijk gemaakt dat de ionenchemie geen bijdrage levert aan de

vorming van Si02 maar dat deze reacties via radicalen lopen. Zowel de

positieve als oak de negatieve ionen van SiF4, SiCl4 ~n 02 reageren

niet zodanig dat er silicium-zuurstof bindingen gevormd worden.

Ion-molecuul reacties die wel belangrijk zijn, zijn de reacties met

H20 verontreinigingen, waarin halogeen atomen vervangen worden door

OH-groepen. De inbouw van OH-groepen in het neerslag, die in PCVD

processen relatief haag kan zijn in vergelijking met andere CVD

processen, zou hierdoor verklaard kunnen worden. Bij deze studie bleek

het nodig te zijn dat er een consistente set thermochemische data over

de bestudeerde moleculen voorhanden was. Deze data zijn uit de

literatuur verzameld en, ontdaan van inconsistenties, gegeven in

hoofdstuk 2.

Een nieuwe methode, waarin aangepaste thermodynamische

evenwichtsberekeningen toegepast worden voor het zoeken naar dominante

reactiepaden in plasma's, wordt behandeld in hoofdstuk 3. Door

thermochemische data van deeltjes te veranderen wordt de invloed van

radicalen op de overall reacties bestudeerd. Dit geeft ook een

mogelijkheid om de plasma specifieke invloed op het chemische

evenwicht te leren kennen. Doordat er voor deze methode geen

kinetische data nodig zijn, is zij eenvoudig bruikbaar.

De berekeningen laten zien dat de dissociatie van 02 belangrijker

is dan die van SiF4 als het gaat om de vorming van Si02 in een SiF4-o2

-157-

plasma. Dit lijkt in eerste instantie vreemd, omdat de bindingsenergie

in het 02 molecuul 5.16 eV bedraagt en die in SiF4 6.95 eV. De lage

bindingsenergie in F2 (1.62 eV) blijkt de oorzaak te zijn van deze

paradox.

Hoofdstuk 4 gaat over een massaspectrometeropstelling die ontworpen

en gebouwd is om de samenstelling van bet plasma te meten. Enige

resultaten van inleidende experimenten met deze opstelling worden daar

gegeven.

In hoofdstuk 5, 6 en 7 wordt ingegaan op de licht emissie

spectroscopie als middel voor de bestudering van reactiemechanismen.

Hoofdstuk 5 vormt de inleiding waarin de algemene karakteristieken van

de spectra van SiF4, 02 en SiF4-D2 ontladingen behandeld worden.

In hoofdstuk 6 wordt extra aandacht geschonken aan een

bandenspectrum in bet gebied van 220 - 260 nm van een SiF4 ontlading.

De herkomst van deze band werd in de literatuur zowel aan SiF2 als aan

SiF3 toegeschreven. Nieuwe metingen en een hernieuwde analyse van de

data uit de literatuur zijn uitgevoerd om te Iaten zien dat SiF2 de

emitter is van dit bandensysteem.

Hoofdstuk 7 gaat over de invloed van kleine verontreinigingen op

het spectrum van de afterglow van een zuurstofontlading en over de

reactiemechanismen die daarbij een rol spelen. Water als

verontreiniging leidt tot de emissie van de OH(A2!+ ~ X2U1 ) band. Deze

emissie blijkt een onderdeel te zijn van een katalytisch proces waarin

H atomen de recombinatie van o-atomen bevorderen. Daardoor heeft de

aanwezigheid van waterstof in SiCl4-02 en SiF4-o2 ontladingen niet

aileen tot gevolg dat OH-groepen in het neerslag worden ingebouwd,

maar ook dat reactieve deel tjes gedesactiveerd worden. Een analoge

werking heeft een N2 verontreiniging. De chemiluminescentie van de

reactie NO + 0 ~ N02 is gebruikt om de katalytische werking van bet NO

radicaal voor de recombinatie van ~tomen te bestuderen.

De methode, die gebruikt is voor bet bestuderen van de

reactiemechanismeri in de afterglow, is in essentie gelijk aan die van

Phelps voor de studie van quenching van metastabiele edelgasatomen.

Voor ons onderzoek is deze methode uitgebreid met de onafhankelijke

variatie van twee van de drie parameters (druk, stroomsnelheid, gas

volume debiet) om de verschi llende mogelijke reactiemechanismen te

onderscheiden. Deze uitbreiding heeft haar nut bewezen.

Algemene conclusies worden gegeven in hoofdstuk S.

-158-

REFERENCES

AAR86 Aarts, J.F.M. Ion and electron impact ionization of SiF4 studied via UV emission Chern. Phys.

ABE79 Abe, K.

( 1986) 105-115

Fluorine doped silica for optical waveguides Proc. 2"d European Conf. Optical Fiber Comm. IEE Paris (1976) 59-61

AM079 Amouroux, J., Fauchais, P., Morvan, D., Rocher, D. Thermodynamic study of the ways of preparing silicon. Its application to the preparation of photovoltaique silicon by the plasma technique Ann. Chim. Fr. 1 (1979) 231-255

AND74 Anderson, J.B. Molecular beams and low density gas dynamics Dekker, New York, 1974

ASU38 Asundi, R.K., Karim, M., Samuel, R. Emission bands of SiCl2 and SnCl2 Proc. Phys. Soc. London 50 {1938) 581-598

AWS84 Awschalom, D.D., Shivashankar, S.A .. Robinson, B. Bimolecular processes in a low-density oxygen discharge Phys. Rev. A 29 (1984) 2578-2580

BAC85 Bachmann, P. Review of plasma deposition applications: preparation of optical waveguides. Pure & Appl. Chern. 57 (1985} 1299-1310

BAU73 Baulch, D.L. Drysdale, D.D., Horne, D.G. Evaluated data for high temperature reactions, VOL II Butterworths, London 1973

BEC71 Becker, K.H., Groth, W., Schurath, U. The quenching of metastable 02( 1 A9 ) and 02( 1l+ 9 ) molecules Chern. Phys. Lett. ~ (1971) 259-262

BEE85 Beek, F.M. van, Saes, L.H., Holscher. j.G.A .. Kipperman, A.H.M., Brongersma, H.H. Silica deposition from SiF4-o2 microwave discharges Proceedings of the 5 1

h European Conference on chemical vapour deposition, Uppsala, Sweden 1985

BEI81 Beijerinck, H.C.W., Verster, N.F. Absolute intensities and perpendicular temperatures of supersonic beams of polyatomic gases Physica ( 1981) 327-352

-159-

BENSl Bendow, B., Shasha.nka, S.M., ed. Physics of fiber optics Advances in ceramics g American Ceramic Society, Columbus, Ohio 1981

BOW6S Bowen, E. J. , ed. Luminescence in chemistry Van Nostrand 1 td. London 1968

BRI70 O'Brien, R.F., Myers, G.H. Direct flow measurement of 02(b1 ! 9 +) quenching rates J. Chem. Phys. 53 (1970} 3832-3835

BR06S Brongersma, H.H. The interaction of molecules with low energy (0-30 eV) electrons Thesis, RU Leiden, The Netherlands 1968

CAL81 Caldow, G.L., Deeley C.M .• Turner, P.H .. Mills, I.M. The infrared spectrum of silicon difluoride Chem. Phys. Lett. 82 (1981} 434-438

CAPSO Capitelli, M., Molinari, E. Kinetics of dissociation processes in plasmas in the low and intermediate pressure range Topics in current Chemistry 90 (1980) 60-109

CHR84 Christophorou, L.G .• ed. Electron-molecule interactions and their applications Volumes I and II Academic Press Inc .. New York 1984

CHR86 Christophorou. L.G. Electron attachment and detachment processes in electronegative gases Proceedings of the sth ESCAMPIG conference, Greifswald GDR, (1986) 3-6

CLA Clasen, R.j. Memo 4345 - PR AD 609 904 Rand Corp.

CLA67 Clark, H.C., Dixon, K.R. The SiFs- ion and evidence for the existence of GeFs­Chem. Comm. (1967) 717

OOR66 Cornu, A.,· Massot, R. Compilation of mass spectral data BF Goodrich Chemical Co., Calvert City, Ky. USA 1966

OOU82 Coudert, j.F., Bourdin, E .• Fauchais, P. Kinetic modelling of the reduction of SiCl4 in an arc heater for the production of silicon Plasma Chem. (1982) 399-419

-160-

DEM79 DeMore, W.B., et al. Chemical kinetic and photochemical data for use in stratospheric modelling Jet Propulsion Lab., JPL publication 79-27 NASA CR 158514

DIT82 Dittmer, G., Niemann, U. Thermodynamic properties of gaseous non-metallic main group element halides, oxyhalides and oxides Philips J. Res. 37 (1982) 1-30

DON68 McDonald, J.D .. Williams, C.H .. Thompson, J.C., Margrave, J.L. Appearance potentials, ionization potentials and heats of formation for perfluorosilanes and perfluoroborosilanes Adv. Chern. Ser. 72 (1968) 261-266

DON78 Doncaster, A.M .. Walsh, R. Kinetics of the gas phase reaction between iodine and trifluorosilane and the bond dissociation energy D(SiF3-H) Int. J. Chern. Kin. ~ (1978) 101-110

DON80 Donnelly, V.M., Flamm, D.L. Studies of chemiluminescence accompanying fluorine atom etching of silicon J. Appl. Phys. 51 (1980) 5273-5276

EHL64 Ehlert. T.C., Margrave, J.L. Mass spectrometric studies at high temperatures. II. The dissociation energies of the monofluorides and

difluorides of silicon and germanium J. Chern. Phys. 41 (1964) 1066-1072

EYS37 Eyster, E.H. The structure of the a and ~ band systems of SiF Phys. Rev. 51 (1937) 1078-1086

FAR77a Farber, M .. Srivastava, R.D. Mass spectrometric determination of the heats of formation of the silicon fluorides SiF(g), SiFz(g) and SiF3(g) J. Chern. Soc. Faraday Trans 1 74 (1977) 1089-1095

FAR77b Farber, M., Srivastava, R.D. Mass spectrometric determination of the heats of formation of the silane fluorides Chern. Phys. Lett. 51 (1977) 307-310

FEH74 Fehlner, T.P .. Turner, D.W. The photoelectron spectrum of SiFz Inorg. Chern. 13 (1974) 754-755

FIE57 Field, F.H., Franklin, J.L. Electron impact phenomena and the properties of gaseous ions Academic Press Inc., New York 1957

-161-

FON64 Fontijn, A., Meyer, C. B., Schiff, H. I. Absolute quantum yield measurements of the No-0 reaction and its use as a standard for chemiluminescent reactions j. Chern. Phys. 40 {1964} 64-70

FRA74 Franklin, J.L., Wang, J. Ling-Fai, Bennett, S.L., Harland, P.W., Margrave, J.L. Studies of the energies of negative ions at high temperatures Adv. Mass. Spectrom. 2 {1974) 319-325

GEI86 Geisler, j., Beaven, G .. Boutruche, J.P. Optical fibres Pergamon Press, Oxford 1986

GEI76 Geittner, P., ~tippers, D., Lydtin, H. Low-loss optical fibers prepared by plasma-activated chemical vapor deposition (CVD) Appl. Phys. Lett. 28 (1976) 645--646

GME59 Gmelins Handbuch der anorganische Chemie (15) Achte vollig neu bearbeitete Auflage Silicium Teil B. Verlag Chemie GmbH, Weinheim, West-Germany 1959

GOL72 Gale, J.L. The vacuum ultraviolet spectra of SiF2 and GeF2 J. Mol. Spectrosc. 43 (1972) 441-451

GOL73 Golde, M.F .• Roche, A.E., Kaufman, F. Absolute rate constant for the 0 + NO chemiluminescence in the near infrared j. Chern. Phys. 59 (1973) 3953-3959

GOL75 Golomb, D., Brown, J.H. The temperature dependence of the No-O chemiluminescent recombination. The RMC mechanism j. Chern. Phys. 63 (1975) 5246-5251

HAB75 Habets. A.H.M., Verster, N.F .• Vlimmeren. Q.A.G. van Simple device for measurements of the sticking coefficient of cryopumps Rev. Sci. Instr. 46 (1975) 613-616

HAN68 Haney, M.A .• Franklin. J.L. Correlation of excess energies of electron-impact dissociations with the translational energies of the products J. Chern. Phys. 48 ( 1968) 4093-4097

HAR71 Harland. P.W .. Thynne, J.C.J. Ionization by electron impact of some inorganic fluorides Adv. Mass Spectrom. § (1971) 493-496

-162-

HAR77 Hart, W.J. van der, Sprang, H.A. van Ion-molecule reactions of simple aliphatic ketones by continuous and trapped ICR J. Am. Chern. Soc. 99 (1977) 32-35

HAS69 Hastie, J.W., Margrave, J.L. Ionization potentials, electronic and molecular structures of metal halides from extended ~uckel theory J. Phys. Chern. 73 (1969) 1105-1116

HER50 Herzberg, G. Molecular spectra and molecular structure, I-III Van Nostrand, Princeton, New Jersey 1966

HES67 Hesser, J.E., Dressler, K. Radiative lifetimes of ultraviolet emission systems excited in BF3, CF4 and SiF4 J. Chern. Phys. 47 (1967) 3443-3450

HIP49 Hipple, J.A., Sommer, H., Thomas, H.A. A precise method of determining the Faraday by magnetic resonance Phys. Rev. 76 (1949) 1877-1878

HUB79 Huber, K.P., Herzberg, G. Molecular spectra and molecular structure. IV Constants of diatomic molecules Van Nostrand Reinhold Company 1979

HUN77 Hunt, L.P. Low-cost, low-energy processes for producing silicon The electrochemical society Luc. Princeton N.J. 08540 Semiconductor Silicon 1977

IRPSl International Radiation Protection Association (IRPA) 1981 Guidelines on limits of exposure to electromagnetic fields in the frequency range from 100 kHz to 300 GHz

JAD77 Jadrny, R., Karlsson, L., Mattson, L., Siegbahn, K. The valence electron spectrum of SiF4 Chern. Phys. Lett. 49 (1977) 203-206

JAG68 Jager, K., Henglein, A. Die Bildung negativer Ionen aus SiCl4 und organischen Siliciumchloriden durch Elektronenstoss Z. Naturforsch. (1968) 1122-1127

JOH58a Johns, J.W.C., Barrow, R.F. The band spectrum of silicon monof1uoride, SiF Proc. Phys. Soc. 71A (1958) 476-484

JOH5Sb Johns, J.W.C., Chantry, G.W., Barrow, R.F. The ultraviolet spectrum of silicon difluoride Trans. Faraday Soc. 54 (1958) 1589-1591

-163-

KAOEH Kao, C.K., ed. Optical fiber technology, II IEEE Press, New York 1981

KAU58 Kaufman, F. The air afterglow and its use in the study of some reactions of atomic oxygen Proc. Roy. Soc. London A 247 {1958) 123-138

I<EC73 Keck, D.B., Schultz, P.C., Zimar, F. Method of forming optical waveguide fibers United States Patent Office, 1973 No. 3, 737,292

KEN84 Kenner, R.D., ()gryzlo, E.A. Orange chemiluminescence from N02 j. Chern. Phys. 80 {1984) 1-6

KHA67a Khanna, V.M., Besenbruch, G., Margrave, J.L. Ultraviolet absorption spectrum of SiF2 J. Chern. Phys. 46 {1967) 2310-2314

KHA67b Khanna, V.M., Hauge, R., Curl jr, R.E., Margrave, J.L. Infrared spectrum, force constants, and thermodynamics functions of SiF2 J, Chern. Phys. 47 {1967) 5031-5037

KRU72 Krupen1e, P.H. The spectrum of molecular oxygen J. Phys. Chern. Ref. Data 1 {1972) 423-534

KUP78 ~uppers, D., Koenings, J .• Wilson, H. Deposition of fluorine-doped silica layers from a SiCl~/SiF~/02 gas mixture by the plasma-cvD method J. Electrochem. Soc. 125 {1978) 1298-1302

KUPso ~uppers, D., Lydtin, H. Preparation of optical waveguides with the aid of plasma­activated chemical vapour deposition at low pressures Topics in Current Chemistry 89, Plasma Chemistry 1 (1980) 107-131

LAN86 Langeveld, A.D. van Private communication

LEH76 Lehman, T.A., Bursey, M.M. Ion Cyclotron Resonance spectrometry John Wiley & Sons, New York 1976

LEV82 Levin, R.D., Lias, S.G. Ionization potential and appearance potential measurements 1971-1981 National Bureau of Standards NSRDS-NBS 71, Washington 1982

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LON73 Lonardo, G. di, Douglas, A.E. The electronic spectrum of HF. I. The B 1 ~+ ~ X 1 ~+ band system Can. J. Phys. 51 (1973) 434-445

MAC70 MacNeil, K.A.G., Thynne, J.C.j. The formation of negative ions by electron impact on silicon tetrafluoride and carbon tetrafluoride Int. j. Mass Spectrorn. Ion Phys. ~ (1970) 455-464

MEX83 Mexmain, j.M., Morvan, D., Bourdin, E .. Amouroux. J .. Fauchais. P. Thermodynamic study of the ways of preparing silicon. and its application to the preparation of photovoltaic silicon by the plasma technique Plasma Chern. Plasma Proc. ~ (1983) 393-421

MIL68 Milligan, D.E .. Jacox. M.E., Guillory, W.A. Matrix isolation study of the vacuum-ultraviolet photolysis of trifluorosilane. The infrared spectrum of the free radical SiF3

J. Chern. Phys 49 (1968) 5330-5335

MOH77 Mi:ihlrnann, G.R. Radiation produced by electrons incident on molecules Thesis, University of Leiden, The Netherlands 1977

MORSS Morgen. V.j.j. von Internal report TUE (dutch) N/VVS 1985

MUL86 Mullen, J. van der Excitation equilibria in plasmas. A classification Thesis, Technical University Eindhoven, The Netherlands 1968

NAG82 Nagel, S.R .. Mac Chesney, j.B .. Walker, L. An overview of the Modified Chemical Vapor Deposition (MCVD) process and performance IEEE Transactions on Microwave Theory and Techniques MTT-30 (1982) 305-322

NOL83 Nolang, B. Application of equilibrium computations to chemical vapour transport and related systems Thesis, University of Uppsala, Sweden 1983

NOL85 Nolang, B. The EKVISYST computer programs and manual Svensk Energidata, Balinge, Sweden 1985

PAB77 Pabst, R.E., Margrave, J.L., Franklin, J.L. Electron impact studies of the tetrachlorides and tetrabromides of silicon and germanium Int. J. Mass Spectrom. Ion Phys. 25 (1977) 361-374

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PAG69 Page, F.M., Goode, G.C. Negative ions and the magnetron John Wiley & Sons. London, 1969

PEA65 Pearse, R.W.B., Gaydon, A.G. The identification of molecular spectra Chapmann & Hall Ltd., London 1965

PHE53 Phelps, A.Y., Molnar, J.P. Lifetimes of metastable states of noble gases Phys. Rev. 89 (1953) 1202-1208

PHE59 Phelps, A.Y. Diffusion, de-excitation, and three-body collision coefficients for excited neon atoms Phys. Rev. 114 (1959) 1011-1025

POR11 Porlezza. C. Contributo alia conoscenza dello spettro a bande del tetrafluorruro di silico Rend. Accad. Naz. 20 (1911) 486-490

RA061 Rao, D.R., Yenkateswarlu. P. Emission spectrum of SiF2 Part I. The band system in the region 2755-2179 A J. Mol. Spectrose. I (1961) 287-303

RA065 Rao, Y.M .• Curl jr, R.F .. Timms. P.L., Margrave. J.L. Microwave spectrum of SiF2 J. Chern. Phys. 43 (1965) 2557-2558

RA066 Rao, Y.M .• Curl jr, R.F. Microwave spectrum and force constants of SiF2: centrifgal distortion J. Chern. Phys. 45 {1966) 2032-2036

RA070 Rao, D.R. New Electronic Emission from SiF2 J. Mol. Spectrosc. 34 (1970) 284-287

REE84 Reents jr .• W.D., Mujsce, A.M. Ion/molecule reactions of silicon tetrafluoride Int. J. Mass Spectrom. Ion Phys. 59 (1984) 65-75

REE85 Reents jr .• W.D., Wood, D.L., Mujsce, A.M. Impurities in silicon tetrafluoride determined by infrared spectrometry and fourier transform mass spectrometry Anal. Chern. 57 {1985) 104-109

RIC75 Richardson, J.H .. Stephenson, L.M .• Brauman, J.I. Photodetachment of electrons from trifluoromethyl and trifluorosilyl ions. The electron affinity of CF3 • and SiF3• Chern. Phys. Lett. 30 (1975} 17-20

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ROS77 Rosenstock, H.M .. Draxl, K .. Steiner. B.W., Herron, J.T. Energetics of gaseous ions j. Phys. Chern. Ref. Data suppl. 1 (1977)

ROT82 Roth, A. Vacuum technology North Holland Publishing Co .. Amsterdam 2"d revised edition 1982

ROW85 Rowe, M.D. Realtime multichannel emission spectroscopy of silicon tetrachloride plasmas involved in optical waveguide depositions Conference proceedings IPAT, Mlinchen (1985) 87-91

ROY72 Roy, D. Characteristics of the trochoidal electron monochromator by calculations of electron energy distribution Rev. Sci. Instr. 43 (1972) 535-541

RUY Ruyten, H. Verstelbare spleet (drawing) TUE afd W, section WP

SAL73 Salet, G. Sur les spectres des metallo1des Annales de chimie et de physique, serie IV 28 (1873) 1-71

SAM86 Sameith, D., Mi:inch, J.P., Tiller, H.j., Schade, K. Vibrational analysis of the UV emission spectrum of dichlorosilyl, SiCl2 Chern. Phys. Lett. 128 (1986) 483-488

SAN62 Sankaranarayanan, S. The ultraviolet band spectrum of silicon difluoride Proc. Nat. Inst. Sci. India, Part A 28 (1962) 311-316

SCH58 Schulz, G.j. Measurement of atomic and molecular excitation by a trapped­electron method. Phys. Rev. 112 (1958). 150-154

SCH67 Schafer, H., Brudereck, H., Marcher, B. Die Thermochemie der Silicium (II)-halogenide Z. Anorg. Allg. Chern. 352 (1967) 122-137

SCH78 Schlegel, H.B. Heats of formation of fluorine-substituted silylenes, silyl radicals and silanes j. Phys. Chern. 88 (1984) 6254-:-5258

SEN83 Senzer, S.N .• Lampe, F.W. Gaseous ion reactions in SiF4 and SiF4-D2 mixtures J. Appl. Phys. 54 (1983) 3524-3527

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SH073 Shoji, H., Tanaka, T., Hirota, E. Microwave spectrum of silicon difluoride in the excited vibrational sates, equilibrium structure, anharmonic potential function and Vt-V3 coriolis resonance J. Mol. Spectrosc. 47 (1973) 268-274

SMI68 Smith, W.R., Missen, R.W. Calculating complex chemical equilibria by ar1 improved reaction-adjustment method Can. J. Chern. Eng. 45 (1968) 269-272

STA70 Stamotovic, A .• Schulz. G.J. Characteristics of the trochoidal electron monochromator Rev. Sci. Instr. 41 (1970) 423-427

STE62 Steele, W.C .• Nichols, L.D., Stone, F.G.A. The determination of silicon-carbon and silicon-hydrogen bond dissociation energies by electron impact J. Am. Chern. Soc. 84 (1962) 4441-4445

STR68 Striganov, A.R .• Sventitskii, N.S. Tables of spectral lines of neutral and ionized atoms IFI Plenum, New York 1968

STU71 Stull, D.R., Prophet, H. JANAF Thermochemical tables NSRDS-NBS 37 National Bureau of Standards 1971

SUC75 Suchard, S. N. Spectroscopic data heteronuclear diatomic molecules IFI/Plenum 1975

TAB70 Tables of constant and numerical data, Selected constants. Spectroscopic data relative to diatomic molecules Pergamon Press, Paris 1979

TIM65 Timms, P.L .• Kent, R.A., Ehlert, T.C .• Margrave, J.L. Silicon-Fluorine chemistry I. Silicon difluoride and the perfluorosilanes J. Am. Chern. Soc. 87 (1965) 2824-2828

TOB84 Toby, S. Chemiluminescence in reactions of ozone Chern. Rev. 84 (1984) 277-285

UNG76 Ung. A.Y.M. Observations of the high vibrational levels of N2(B3 IT 9 )

in the Lewis-Rayleigh afterlow of nitrogen j. Chern. Phys. 65 (1976) 2987-2990

VEE76 Veen, E.H. van Direct and resonance excitation of molecules by low-energy (0-30 eV) electron-impact spectroscopy Thesis, RU Leiden, The Netherlands 1976

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VELS1 Velzen, P.N.T. van Photodissociation of ions trapped in an ion cyclotron resonance spectrometer. Thesis, RU Leiden, The Netherlands 1981

VER78 Verhaart, G.J .. van der Hart, W.J., Brongersma, H.H. Low energy electron impact on chlorofluoromethanes and CF4: resonances, dissociative attachment and excitation Chern. Phys. 34 {1978) 161-167

VERSO Verhaart, G.J. Excitation and dissociation of molecules by low energy (0-15 eV) electrons Thesis, RU Leiden, The Netherlands 1980

VOU47 Vought, R.H. Molecular dissociation by electron bombardment: a study of SiCl4 Phys. Rev. 71 {1947) 93-101

WALS1 Walsh, R. Bond dissociation energy values in silicon-containing compounds and some of their implications Ace. Chern. Res. 14 {1981) 246-252

WAN73a Wang, J. Ling-Fai, Margrave, J.L., Franklin, J.L. Interpretation of dissociative-electron attachment processes for carbon and silicon tetrafluorides J. Chern. Phys. 58 {1973) 5417-5421

WAN73b Wang, J. Ling-Fai, Krishnan, C.N., Margrave, J.L. Emission Spectrum of SiF3 J. Mol. Spectrosc. 48 {1973) 346-353

WANS4 Wanczek, K.P. Ion cyclotron resonance spectrometry - a review Int. J. Mass. Spectrom. Ion Proc. 60 {1984) 11-60

WES74 Westwood, N.P.C. The photoelectron spectrum of silicondifluoride Chern. Phys. Lett. 25 {1974) 558-561

WIS63 Wise, S.S., Margrave, J.L., Feder, H.M., Hubbard, W.N. Fluorine bomb calorimetry. I. The heats of formation of silicon tetrafluoride and silica J. Phys. Chern. 67 {1963) 815-821

W0077 Woolsey, G.A., Lee, P.H., Slafer, W.D. Measurement of the rate constant for N0-0 chemiluminescence using a calibrated piston source of light J. Chern. Phys. 67 (1977) 1220-1224

ZEG70 van Zeggeren, F., Storey, S.H. The computation of chemical equilibria Cambridge University Press 1970

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ffawoord.

In het promotieregelement van de TUE staat dat de promovendus eerst

tot de verdediging van het proefschrift toegelaten kan worden, wanneer

de leden van de prornotiecomrnissie in meerderheid vinden dat hij of zij

"blijk gegeven heeft zelfstandig onderzoek te kunnen verrichten". In

de vier en een half jaar die verstreken zijn tussen het begin van dit

project en het moment waarop dit proefschrift verdedigd wordt. heb ik

een ding zeker geleerd: zelfstandig onderzoek verrichten is niet

hetzelfde als alleen, in je eentje, onderzoek verrichten. Voor een

promotieonderzoek dat een wetenschappelijke toets van kritiek kan

doorstaan. is het nodig dat niet aileen de prornovendus werkt aan dat

onderzoek. Voor een promotieonderzoek waarin hoogstaand werk gedaan

kan worden. is het zelfs nodig dat aile teamleden van de projectgroep

met hart en ziel verbonden zijn aan dat ene doel: samen zo goed, maar

ook zo snel rnogelijk, verder komen in de wirwar van steeds nieuwe

keuzemogelijkheden voor het volgende experiment. Dit geldt in

verhoogde mate voor een onderzoeksproject, waarin met geavanceerde

apparatuur gewerkt wordt. Alle schakels in het team dat aan zo'n

onderzoek werkt zijn belangrijk.

Ik wil met deze inleiding duidelijk maken, dat iedereen in de

SiF4 groep bard nodig is geweest om uiteindelijk te kunnen komen tot

het boekje dat bier nu ligt. Daarom wil ik iedereen, Eddy, Freek,

Hans, Henk, Joop, Ronald en Ton van harte bedanken voor jullie inzet,

belangstelling en bijdragen.

Freek, doordat jij in de beginperiode betrokken was bij dit

onderzoek, hebben we een goede basis kunnen leggen voor de laatste

twee jaren. jouw onstuitbare dadendrang heeft er veel toe bijgedragen

dat we het onderzoek richting hebben kunnen geven.

Ton, ook jij hebt niet het gehele project meegernaakt, maar hebt

mede aan de basis gestaan. Je hebt mij, op de voor jou kenmerkende

manier. met beide ogen gericht op het bedrijfsleven, wegwijs gernaakt

in de ambtelljke molen van deze instelling en je hebt mijn

organisatorische interesses gewekt.

-170-

Eddy, jij stond steeds bereid en stak overal een helpende hand toe,

bij de vele verhuizingen, bij de opbouw van steeds weer nieuwe

electronica of als er weer eens een klusje was waarvan ik graag wilde

dat het gisteren of eergisteren klaar zou zijn.

Joop, je hebt je best gedaan om de mechanische constructies, die we

in ons enthousiasme bedachten maar niet precies op papier zetten, toch

volgens onze wensen te maken.

Henk, jij bent ergens halverwege in ons groepje binnengekomen, maar

wist je vrij snel tot een onmisbare praatpaal te rnaken. Ik waardeer

jouw manter van discussieren zeer. De rust die jij uitstraalt is een

welkome afwisseling geweest in de gejaagdheid van vooral de laatste

paar maanden. Evenals trouwens het dagelijkse half uurtje ontspanning,

dat je mijn hersens aanbood. Ik wil je tenslotte bedanken voor het

lezen van de laatste versie van het manuscript, waaruit je nog vele

typefouten hebt verwijderd.

Als laatste van ons vaste SiF4 groepje wil ik jou, Hans, heel

hartelijk bedanken voor alles wat jij hebt gedaan. Het aantal ideeen

dat jij hebt gelanceerd is schier oneindig. Je hebt je steeds met hart

en ziel ingezet voor het project. Altijd als er iets was waar ik over

wilde praten, kon ik bij jou terecht. En steeds wist je dan weer een

richting aan te geven waarin waarschijnlijk de oplossing zou liggen.

Als jij er niet geweest was ...... Maar ook buiten de werksfeer heb ik

veel van jou mogen leren op het gebied van de "levenswijsheid".

Naast de vaste kern zijn er een aantal studenten betrokken geweest

bij het onderzoek, die door hun stages of afstudeerwerk hebben

bijdgedragen aan de totstandkoming van di t boekje: Marijn van den

Bogaard, Erik van den Bosch, Steven Drenth, Adrie Ermes, Erik Giessen,

Jack Grupa, Pascal Hendriks, Jan Jacobs, Paul Janssen, Rob Janssen,

Maarten Luykx, Edward Maesen, Andries de Man, Peter van der Meulen,

Pim van Meurs, Gilles Moerdijk, Valentijn van Morgen, Leo Pel, Joris

Sparla, Marti Timmers, Geert Verhoofstad, Wim Veugelers, Johan Vlagsma

en Rita Zonneveld.

Niet aileen binnen de SiF4 groep, maar ook daarbuiten heb ik veel

steun gehad. D!t.i leden van de FOG groep ben ik dankbaar voor qe

prettige werksfeer en de bereidheid om in te springen op momenten, dat

dat nodig was. Speciaal wil ik daarbij noemen Giel Hoddenbagh, die met

-171-

zijn gouden handen de moeilijkste klusjes tot een goed einde wist te

brengen: Dick van Langeveld voor zijn bijdrage aan de discussies over

thermodynamica; Hans Ottevanger voor zijn adviezen op digitaal gebied.

De mechanische afdeling van de CTD heeft een belangrijk deel van de

massaspectrometeropstelling vervaardigd, nadat die door Piet Magendans

getekend was. De mensen van de afdelingswerkplaats. Jan van Asten,

Marius Bogers, Henk Heller, Henk van Helvoirt, Frank van Hoof, Rien de

Koning en Gerard Wijers hebben mij steeds met raad en daad bijgestaan

wanneer ik weer eens iets onmogelijks bedacht had.

De elektronische werkplaats onder leiding van de heer Huisman heeft

een groot deel van de electronica gebouwd. De grootste hulp op di t

gebied is echter gekomen van Rein Rumphorst, die de muziek ui t die

radio liet komen die daarvoor bedoeld was. Hij speelt nog steeds.

Grote bewondering heb ik voor de mensen van de glasblazerij (in het

bijzonder Toon van der Schoot), die de raarste kronkels in een mum van

tijd weten te maken. De mensen van de glastechnische dienst,

Versteegh. Kik en de Rijke. hebben steeds zonder morren hun uiterste

best gedaan om nieuwe ontladingsbuizen of nieuwe kwartsvensters te

maken als ik er weer eens een in de vernieling geholpen had.

In de periode dat er metingen in Leiden gedaan moesten worden, heb

ik veel steun ondervonden van Jacques Aarts, Wim van der Hart en Johan

Bakker. Ook de andere !eden van de theoretisch chemische club in

Leiden wil ik bedanken voor de prettige werksfeer.

Herman Beijerinck heeft mij leren denken met en over supersone

expansies.

De adviezen van ing. P. de Jong van Philips (CFT) zijn van grote

waarde geweest om een verantwoorde opzet van onze mikrogolfopstel­

lingen te bereiken.

De mensen van het kryogeen bedrijf (Jos van Amelsfoort en Wil

Delissen) ben ik dankbaar voor de nauwgezette levering van heel veel

vloeibare stikstof.

Ik wil Marga bedanken voor de snelheid waarmee ze niet-aanwezige

artikelen en boeken ui t andere bibliotheken kon laten komen en voor

haar niet aflatende vriendelijkheid.

Karin heeft ervoor gezorgd dat ik geen dag mijn melk heb hoeven

missen. Annie heeft mijn bureau en kamer van alle rotzooi ontdaan.

-172-

Ad, Bert, Helga, Marion, Rianne en Wouter hebben als goede buren de

sfeer op de gang opperbest gehouden.

Het grootste deel van de tekeningen in dit boekje werd vervaardigd

door Ruth Gruijters.

Het typewerk werd door Helga, Rianne en vooral ook door Ans

verzorgd. De omslag en figuur 4.3 werden door mijn broer Henk

getekend.

Aan bet eind van deze lange lijst moet ik van degenen die ik

vergeten ben te noemen begrip vragen. Bedenk echter dat er twee mensen

zijn. die niet in de lijst staan vernoemd, die mij tot nu toe steeds

met begrip en lief de begeleid hebben en die mij de mogel ijkheid

gegeven hebben dit alles tot een goed einde te brengen. Zij blijven

liever op de achtergrond.

Van harte bedankt.

Curriculum vitae.

2 8 - 1957

20 - 6 - 1975

18 - 12 - 1981

23 - 6 - 1982

-173-

geboren te Nederweert.

eindexamen gymnasium ~ aan bet Bisschoppelijk

College te Weert.

examen C-cursus stralingshygiene IRI, Delft.

doctoraal examen in de Technische Natuurkunde

aan de TH Eindhoven.

1 8 - 1982 tot 1 - 1 1987

wetenschappelijk onderzoekmedewerker hij SON/STW

in bet kader van bet SiF4 project.

1

Voor de bestudering van reacties van radicalen aan of met een wand

kan de methode van Phelps ui tgebreid worden tot een zeer bruikbaar

middel bij het zoeken naar reactiemechanismen.

A.V. PheLps, Phys. Rev. 114 (1959) 1011-1025,

Dit proefschrift, hoofdstuk 7.

2

Doordat Diana et al. slechts een fractie van bet aantal mogelijke

radicalen in hun thermodynamische evenwichtsberekeningen meenemen,

zijn hun conclusies ten aanzien van de depositie van Si uit Si-cl-H­

systemen aanvechtbaar.

lf. Diana, L. de lfa.rlno, L. lastrantuono, R. Rossi Rev. Int. Hautes.Temper. Refract. Fr. 18 (1981) 203-213,

Dit proefschrtft, hoofdstuk 3.

3

Reents en Mujsce suggereren ten onrechte dat Senzer de reactie

SiF4+ + SiF4 -+ ShF7 + + F over bet hoofd gezien heeft. Zij gaan

daarbij voorbij aan bet feit dat Senzer onder volledig andere

experimentele condities werkte.

W.D. Reents, A.lf. lfujsce, Int. ]. lfass Spectr. Ion Proc. 59 (1984) 65-75,

S.N. Senzer, F.W. Lampe, ]. AppL. Phys. 54 (1983) 3524-3527.

4

Het is onjuist dat Amouroux et al. SiF4 afwijzen als grondstof voor

de produktie van si licium voor fotovol ta'ische toepassingen op grond

van de stabiliteit van het SiF-radicaal bij hoge temperaturen.

]. Amouroux, P. Fauchais, D. Morvan, D. Rocher, Ann. Chim. Fr. !! (1979) 231-255,

J .M. Mexmain, D. Morvan, E. Bourdin, J. Amouroux, P. Fauchais, PLasma Chem. PLasma Proc. ~ (1983) 393-421.

5

De onverwacht grote aktiveringsenergie voor elektromigratie, die

Reddy et al. vinden in indium,

bandstruktuureffecten.

K.V. Reddy, ].].B. Prasad ]. AppL. Phys. 55 (1984) 1546-1550,

is toe te schrijven aan

K.V. Reddy, ].].B. Prasad, F. Beniere Revue Phys. AppL. 18 (1983) 613-617.

6

De methode die Proctor gebruikt voor het bepalen van de relatieve

frequentie responsfunctie uit de pulsresponsie van een

piezo-elektrische akoestische-emissie transducer, is aanvechtbaar.

T.H. Proctor, ]. Acoust. Soc. Am. 71 (1982) 1163-1168.

7

In de beschrijving van de energiespreiding van elektronen die een

trocho1dale elektronmonochromator verlaten, introduceren Stamatovic en

Schulz een term die evenredig is met het potentiaalverschil over de

uittree-opening. Gezien de grootte van deze term verdient het

aanbeveling de precieze vorm ervan nader te onderzoeken.

A. Stamatovic, G.]. SchuLz, Rev. Sci. Instr. 41 (1970) 423-427.

8

Het verdient aanbeveling om een systematische studie te verrichten

naar het effect van collimatorvormen en -materialen om de specifieke

voor- of nadelen van neutronentherapie beter te kunnen beoordelen.

Proc. 5th Symposium on Neutron Dosimetry, Mlinchen 1984 chapters XV, XVI, XVII, XVIII, XIX.

9

In sommige EG-landen maken subsidieregel ingen voor sociaal­

cul turele ui twisselingen bet voor verenigingen gemakkelijk om

gastvrijheid aan te bieden. Voor de gasten scbept dat soms financieel

moeilijke morele verplicbtingen. Een eenvormige EG-regeling zou dit

probleem opbeffen en zo de eenwordingsgedacbte bevorderen.

10

De uitgifte door posterijen van steeds nieuwe "filatelistiscbe"

stukken als bloks, eerste-dag enveloppen, jaarcollecties, maxikaarten

en onlangs weer mapjes. bevordert misscbien wel de omzet van de

posterijen, maar kan ertoe leiden. dat bet postzegelverzamelen

degradeert tot bet nemen van een abonnement op een tijdscbrift.

11

llet 'verschi 1 in kwal i tei t tussen vooral amateur-volksdansgroepen,

die optredens verzorgen, dient niet gemeten te worden in

danstechniscbe termen en grootbede~. Het enthousiasme van een publiek

van niet-kenners over een optreden geeft beter aan of er gevolksdanst

werd.