Thin film technology and physics of thin-film based microstructures.

Sputtering of thin films

Part 1. Physical vapor deposition (PVD) of thin films. Methods and equipment.

Part 2. Molecular beam epitaxy (MBE) of semiconductor heterostructures.

Heterostructure lasers. High electron mobility transistors (HEMTs).

Properties of two-dimensional electron systems.

Part 3. Microfabrication: main steps. Optical and electron-beam lithography.

Coulomb blockade. Single-electron transistor.

Part 1. Physical vapor deposition, PVD

• Introduction. Several important examples of different coatings.

• Thermal evaporation

• In-situ control of the film thickness. Estimations of the film uniformity.

• Electron-beam physical vapor deposition

• Sputtering deposition, magnetron sputtering deposition

• Cathodic arc deposition.

• Reactive deposition

• Laser ablation

• Role of vacuum conditions

• Vacuum systems for the physical vapor deposition. Pumping systems.

• Adhesion of coatings.

• Types (modes) of thin film growth.

S.I.Dorozhkin

Physical vapor deposition (PVD) systems deposit thin films and coatings by a process in

which a target material is vaporized, transported in vacuum, and condensed on to a substrate.

PVD processes include Sputtering, Electron beam, and Thermal Evaporation.

Nanotechnology: metallization in the fabrication of integrated

circuits and photomasks

Metallization is usually the final step in the fabrication of integrated circuits.

(1) Electrical connections of all components of a circuit.

(2) Producing of bonding pads

Thin aluminum films are usually used for the metallization.

Simple photomasks are usually made of glass covered by chromium.

(1) (2)

Thin metal films in superconducting logic elements

Examples of large-area coatings. Thin-film solar cells

CdTe panels mounted on a supporting structure

belong to a second generation of photovoltaic solar sells.

This is one of examples of the largest area thin-film

coatings.

Thin-film silicon laminates being installed onto a roof.

CIGS solar cell on a flexible plastic substrate

Modern thin-film semiconductor materials for

solar cells:

Amorphous silicon (a-Si)

Cadmium telluride (CdTe)

Copper indium gallium diselenide (CIGS)

Produced mainly by methods based on the chemical vapor deposition.

p-type

n-type

Hard and decorative coatings (TiN,…) are mainly produced by the cathodic arc deposition and magnetron sputtering

Several additional examples

1. Molecular beam epitaxy of semiconductor structures for lasers, high electron

mobility transistors (HEMTs), etc.

To be considered in more detail in following lectures.

2. Optical coatings.

3. ….

Thermal evaporation

Thermal evaporation is realized by thermal heating (usually

resistive) of an evaporated material placed in an evaporation

source.

It is carried out in a vacuum at pressure P<10-5 torr.

The thermal evaporation is the most gentle PVD method with

evaporated particle energies ~1500 K (0.12 eV).

This method is the simplest PVD method and needs

comparatively low power consumption.

Main types of simple evaporation sources are metallic

boats (1,2) and heaters made of refractory wires (3) (Wo,

Mo,…) which can be covered by a passive material (2)

(Al2O3)

More complicated effusion cells with a crucible made

of passive material (e.g., boron nitride, BN) and an

external heater are used for precision evaporation

processes like the molecular beam epitaxy.

A simple system for thermal evaporation

Vacuum chamber 1 is evacuated to pressure of the order

of 10-6 Torr via the pumping line. The pressure is

measured by vacuum gauge 2. A material 3 is evaporated

from a metal boat 4 heated by an electric current. The

current flows through an isolated vacuum tight lead 5,

which is usually cooled by water (not shown in Fig.), and

grounded lead 6. A substrate 7 is mounted on a holder 8.

The holder may have options of rotating around vertical

axis and tilting relative to it. The latter option

corresponds to the so named oblique evaporation. These

options help to avoid undesirable shadows. The

temperature of the holder may be varied and stabilized in a wide range. The thickness of

evaporated film is controlled by a thickness monitor based on a quartz crystal microbalance 9

(QCM). QCM measures variation of a mass on a surface of a quartz crystal resonator. For a

high accuracy measurements the crystal should stay at a constant temperature. The evaporation

process is controlled by the current through the boat (i.e., the boat temperature) and the shutter

10 position. The shutter is closed till reaching the desired boat temperature and beginning of the

material vaporization. It closes at the end of evaporation process.

Pumping

line

Features of the thermal evaporation method

• (+) The most gentle PVD method, due to the lowest possible energy of

evaporated particles (~1500 K~0.12 eV).

• (+) Atomic or molecular deposition from the vapor.

• (+) Appropriate for deposition of a large number of materials.

• (+) Appropriate for ultra-high vacuum conditions.

• (+) In-situ control of the deposited layer thickness.

• (+) Wide range of the evaporation rate.

• (?) Nearly line-of-sight motion of evaporated atoms. Shadow.

• (+) Estimation of the film thickness uniformity.

• (-) Potential contamination of an evaporator material (in simple systems)

About thermal evaporation of particular materials see

1. Thin Film Evaporation Guide http://www.vem-co.com/guide

2. Напыление пленок в вакууме (Physical vapor deposition, PVD)

by С.И. Дорожкин, МФТИ (2012)

Thickness of deposited films: control and uniformity.

I. Thermal evaporation provides the best conditions for in-situ control of the average film

thickness.

A real – time film thickness monitor can use different sensors: (i) A quartz crystal microbalance,

(ii) Reflection high-energy electron diffraction (RHEED) (in the molecular beam epitaxy), (iii)

optical methods including ellipsometry.

A simplified geometry of the quartz resonator,

produced by a cylindrical quartz plate 1 and metallic

films 2,3 on opposite sides of the plate. To excite the

mechanical oscillations in the plate the ac voltage Vf is

applied between films 2 and 3. The resonator is a part

of an oscillator which frequency f is determined by the

resonant frequency of the mechanical resonator

(typical value ~5 MHz). The Q-factor of the

mechanical resonance can be as high as 106.

Evaporation of a thin film with mass Dm shifts the resonant frequency by mS

ff DD

00

2

02

661.10 NHere is the resonance frequency without evaporated film,

the quartz AT cut; n - is an integer numbering different resonant overtones, D-the plate thickness,

S- active area of the resonator.

DnNf /00 MHz*mm for

648.20 g/cm3 – density of the quartz , 11

0 10*947.2 g/cm s2 - shear modulus for the AT cut

of a quartz crystal.

(Q1)

Quartz crystal microbalance

For Au ( =19.3 g/cm3) film, one monolayer (d~3A) corresponds to Df=33 Hz.

Temperature stabilization is important condition for precise measurements !

.

Equation (Q1) is valid for 03.0/ 0 D ff

For a wide frequency range (Df/f0<~0.3) a more complicated equation is used:

D

0

000

f

ffZtgarctg

Zf

N

S

m

00ZHere , and are the density and shear modulus of evaporated film.

Factors limiting the accuracy of the thickness measurements:

1. Different positions of the substrate and the sensor needs calibration for particular geometry

and evaporator.

2. Heating of the quartz resonator can be eliminated by a cooling system.

3. Deposition of different materials on the same sensor van be eliminated by the use of several

sensors: one for each material.

Estimate for the thermal evaporation rate

ev

aev

kT

mTPdtdm

2)(/

This formula is equivalent to the well known equation of the molecular kinetic theory for the

particle flow Ns /t on a unit flat area: Ns /t=nV av /4 where n is the particle density and

Vav=(8kT/ma)1/2 is the average particle velocity determined from the Maxwellian distribution.

Here dm/dt – the mass rate of evaporation from a unit area, ma – mass of evaporated particle,

P(Tev) – the vapor pressure at evaporation temperature Tev .

VT

Q

dT

dP

DP(T) is usually described by the Clausius–Clapeyron equation

,

is the specific latent heat of the phase transition (vaporization) Q

DV is the specific volume change of the phase transition (the difference of specific

volumes of a liquid and vapor).

RT

PQ

dT

dP2

For an ideal gas and dQ/dT=0 TBAATR

QP /ln which gives (Q2)

Temperature dependence of the vapor pressure

Temperature dependences of the vapor pressure P on the

temperature T (horizontal scale is inversely proportional

to T) for Au and Ti. Solid lines were calculated in

accordance with Eq. (Q2) with the use of parameters

AAu=20.84 and BAu=4.26x104 K (liquid-vapor).

ATi=23.81 and Bti=5.67x104 K (solid-vapor).

The solid signs are taken from handbook “Physical

quantitites” edited by Grigor’eva and Meilikhov (1991).

The melting points Tm are marked by vertical by arrows.

Equilibrium vapor pressures of selected

materials. The slashes indicate the

melting points (MPs).

Typical for the thermal deposition value

of the vapor pressure 10-2 Torr is marked

by the horizontal arrow. Some of the

materials are evaporated and some are

sublimated.

Sublimation of metals is possible by

their direct heating by passed current

(e.g., Ti).

Angular distribution of atomic flow

A general geometry of evaporation. q is the angle between the

normal to the surface element dS1 of the evaporator and a direction

R to the surface element dS2 of the substrate. R is a distance between

these elements. The angle between R and a normal to dS2 is b.

Projection of the dS2 on the plane normal to R is dS0= dS2cos(b) .

A part dw of a total flux w from a flat source into a small

solid angle dW in the direction determined by angle q is given by

equation dw=(w/) cos(q )dW . This relation is the result of the

maxwellian distribution and an analog of the Lambert law in the

optics.

For evaporated mass m, the film thickness is d= m cos(q )cos(b) /R2

If the source is spherical, dw=(w/4) dW and d= m cos(b) /4R2.

Coordinate dependence of the thickness of the deposited films

Consider thermal vapor deposition from a small flat source on parallel

substrate . This corresponds to q =b , cos q =H/(H2+X2)1/2 , R2=H2+X2.

source

substrate

222

2

)()(

XH

HmXd

2max

1

H

md

2/322 )(4)(

XH

HmXd

2max

1

4 H

md

Spherical source

Flat source

0.0 0.2 0.40.7

0.8

0.9

1.0

1.1

(b)

d/d

max

X/H

0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.40.0

0.2

0.4

0.6

0.8

1.0

d/d

max

(a)

Flat source

Spherical source Thickness distribution along the substrate for a small flat (black

lines) and spherical (red lines) sources. Panel b shows initial part of

panel a.

These data show that condition of thickness uniformity leads to

requirement of a large distance between the source and the substrate,

i.e., to a large size of a vacuum system.

Theoretical calculations may be inaccurate due to a finite size of the

source and redistribution of the temperature along it. However some

of these factors may even lead to a better film uniformity. In fact, in

practice the optimal conditions are chosen empirically and

reproduced from one evaporation cycle to another.

Electron-beam physical vapor deposition (EBPVD)

A focused electron beam is emitted from an electron

gun and deflected to the evaporated material by

magnetic field. Energy of electrons is determined by the

acceleration voltage of 1-10 kV. The electron beam

diameter is usually about several mm and the total

power can reach 10 kW. At such conditions, local

temperature of the evaporated material can be in excess

of 3500 K, which allows for evaporation even very

refractory materials. The crucible is kept rather cold due

to the cooling by a water flow.

High vacuum is necessary not only for evaporated atom

transport but also for long life of the cathode of the

electron gun. Typical pressure P<10-5 Torr.

Magnetic field helps to avoid the direct flow of desorbed atoms from the electron gun area to

the substrate and the appearance of the evaporated material and ions in the gun area.

A local pressure at the heated surface may be rather high, so that the evaporated atoms first will

move diffusively. Ballistic motion begins at some distance from the source surface.

Electrons may also ionize the atoms of residual gas and of evaporated material.

Electron-beam physical vapor deposition (EBPVD)

Electron beam evaporators can be rather compact due to

the use of the magnetic field H

The electron beam radius is given by formula

H

c

e

UmR e

||

2 ][/][8.1][ kGHkVUcmR

Features of the electron beam evaporation method.

• (+) A very wide range of evaporation rates (0.01 nm/s – 0.1 m/s)

• (+) A large number of metals including refractory ones.

• (+) Atomic deposition from the vapor.

• (+) Evaporation of source material only.

• (+) Appropriate for high vacuum conditions.

• (+-) In-situ control of the evaporated layer thickness (small thicknesses).

• (?) Nearly line-of-sight motion of evaporated atoms. Shadow.

• (-) Poor estimation of the film thickness uniformity.

• (-) Charging of the substrate. Possible damage of the evaporated film or a substrate

material by high-energy electrons.

Deposition of a material sputtered by ions

Sputtering by ions is a very effective method due

to high possible ion energy and a large mass. The

sputtering normally leads to extraction of atoms

from a source material. The extracted atoms may

have energy of several eV.

While the high energy ions can be produced by

separate ion sources, usually a scheme with

ionization of the incoming sputtering gas by high

energy electrons directly in the coating chamber

is used. Inert argon is frequently used as a

sputtering gas.

The method is suitable for a reactive deposition of different compounds of the sputtered

material with the use of an appropriate sputtering gas

Sputtering process

One ion usually transfer its energy

to many atoms of a solid target and

can sputter many atoms if its energy Ei greatly exceeds the

binding energy Et. It also leads to

the fact that the threshold ion energy Ethr >> Et (Ethr~20-50 eV

while Et~3-6 eV). The sputtering

coefficient

gsput depends on the ion energy and

mass.

for Ar+ at Ei=600 eV

Theoretical energy distribution of sputtered atoms has maximum at E=Et/2.

Here c is the angle to the target normal.

Magnetron evaporation

Schematic of the magnetron operation. The cathode is made

of evaporated material. It is bombarded by ions produced

due to collisions of a gas (usually Ar atoms) with electrons.

Both ions and electrons are accelerated by electric field

applied between cathode and anode (typical electrical

voltage is several hundred volts). The gas is supplied

through the fine control needle valve to a typical pressure

~10-3 Torr. To enhance the probability of ionization of the

gas atoms by electrons, the magnetic field (0.2 – 2 kG) is

applied which causes electrons to move along spiral orbits

centered on the magnetic field lines instead of along short -argon ions H

EAre

ee

Ar+

ArAr e

+

S

SN

N

cathode-source (-)

substrate

magnetic system

N

S

film

anode

+

e

H

EAr

plasma+

+

+

+

-electrons

-sputtered atoms

-argon atomslines of

electric and

magnetic fields

straight lines parallel to the electric field. The magnetic field only slightly effects motion of ions

due to their large mass. The plasma arises in the region with the largest magnetic field. The

cathode material is sputtered by the ions. The atoms of the sputtered material move in different

directions and, in particular, cover the substrate.

Typical parameters of the process.

•The gas pressure 10-3-10-1 Torr,

•Electrical voltage applied between cathode and anode 300 – 1000 V;

•Magnetic field at the cathode 0.2-2 kG;

•Current density on the cathode up to 10 А/cm2;

•The supplied electric power up to 10 kW.

Uniformity of the film thickness produced by the magnetron

deposition method

Coordinate dependence of the relative copper film thickness produced by the magnetron

deposition method. Diameter of the source material on cathode is 76 mm. The data are

obtained for two different depositions (different symbols) for two different distances (102

mm, red symbols) and 152 mm (blue symbols) between the cathode and the substrate.

(Data from PVD Products Company)

Features of the magnetron deposition method.

• (+) A high evaporation rate.

• (+) Rather good adhesion of the coating due to high energy of sputtered atoms.

• (+) A large number of materials including refractory ones. Dielectrics are deposited with the

use of the RF voltage.

• (+) Atomic deposition from the vapor.

• (+) Possibility of a reactive deposition with the use of an appropriate sputtering gas.

• (-) Inappropriate for high vacuum conditions.

• (-) Not very clean coatings.

• (-) Difficulty to control uniformity of the film thickness.

• (+-) In-situ control of the evaporated layer thickness is problematic.

• (?) Essential deviations from line-of-sight motion of sputtered atoms. Poor shadow.

• (-) Possible damage of the evaporated film or a substrate material by high-energy electrons.

Cathodic arc deposition

Electrical arc in a vacuum is a region of plasma produced by

positive ions of a cathode material and electrons. The arc current

is concentrated in small cathode spots about several microns in

diameter . The current density in the spots is of order of 106

A/cm2. It produces local temperature up to 15000 K, which is

accompanied by intensive material evaporation, emission of

electrons and vapor ionization. The spots disappear in a one place

of a cathode and appear in another shifting along the cathode.

The arc starts by touching a cathode by an ignitor electrically

connected to anode.

A low pressure gas, if present in the chamber, is also ionized and

produces a compound with cathode material (Superhard coatings

by nitrides like TiN, etc).

Steered cathodic arc source

Random cathodic arc source

(photo)

Features of the cathodic arc deposition

• (+) Very high evaporation rate.

• (+) Ability to produce extremely hard coatings in the reactive process.

• (+) A large number of materials including refractory ones.

• (-) Difficulty to control uniformity of the film thickness.

• (-) In-situ control of the evaporated layer thickness is problematic.

• (+-) Essential deviations from line-of-sight motion of vaporized atoms. Poor shadow but

incomplete.

• (--) Presence of small drops in the ionized vapor of the cathode material. Needs special

arrangement to be avoided.

Laser ablation

Under high power laser irradiation the target injects a

vapor formed by individual atoms, ions, atomic

clusters and even small drops. A pulsed regime is

usually used.

For example: Nd:YAG laser (neodymium-doped

yttrium aluminum garnet; Nd:Y3Al5O12) , ~10 nc pulse,

~1J/pulse, 10 Hz repetition.

Particular composition depends on the evaporated

material and laser power.

The method is compatible with ultra-high vacuum

conditions and allow to keep a complicated

composition of a compound in the course of deposition

Role of vacuum

Three main factors:

I. Purity of a deposited film.

II. Ballistic (good shadow) or diffusive (poor shadow) transport of atoms from a source to a

substrate.

III. Ability of producing ions (in sputtering processes).

Several estimates.

I. A flow of residual gas molecules incident on the surface (dN/dt)/S=nVav/4=P/(2makT)1/2

gives for P=10-6 Torr of O2, T=300 K, (dN/dt)/S=3.4x1014 cm-2s-1 or about half of a

monolayer per second. This estimates shows that one needs extremely high vacuum

conditions to deposit very clean films.

II. For an ideal gas of particles with cross section s and the Maxwellian distribution of

velocities, the free path reads as l=(2)-1/2 /ns(2)-1/2kT/Ps , which gives for s = 16x10-16

cm2, T=300 K and P=10-4 Torr l~0.5 m.

III. Typical pressure in vaporization by ion sputtering (e.g., magnetron sputtering) is of the order

of 10-3 Torr.

Pressure dependences of characteristic parameters (an estimate)

A unique line correspond to either P (left scales) or P-1 (right scales) dependences. Some absolute

values depend on the molecular mass and cross-section.

Measurements of the pressure

Three main physical properties:

1. Mechanical deformation allows measurements of a differential pressure. If the reference

pressure is known constant an absolute pressure is measured.

1000 bar – 0.1 bar – usual dial bourdon gauges with atmospheric reference pressure (1).

100 Torr – 10-1 Torr – precision mechanical manometers with evacuated reference volume.

1000 Torr – 10-3 Torr – precision capacitance manometers (2).

2. Measurements of the gas thermoconductivity allows for measurements in the range 1000

Torr -10-5 Torr. (Modern Pirani vacuum transducers (3))

3. Measurements of ion current with the ion ionization being produced by a high-energy

electrons allows measurements from 10-2 Torr down to 10-12 Torr. (4)

(1) (2) (3) (4)

Comparison of different PVD methods.

Thermal evaporation.

Low energy atomic beams, compatibility with ultra-high vacuum conditions, stability and precision control of deposition rate, possibility of simultaneous deposition of several materials from different sources.

Inappropriate for some refractory materials, special cautions to improve the adhesion to some substrates.

Electron beam evaporation

Appropriate for many materials including refractory ones, compatible with ultra-high vacuum conditions, wide range of deposition rate, heating of the evaporated material only.

Inappropriate for films and substrates sensitive to the electric charge, problems with stabilization and control of the evaporation rate, limited target utilization.

Ion sputtering.

Appropriate for many materials including refractory ones, possibility of reactive deposition.

Incompatible with high-vacuum conditions, possibility of undesirable contamination of the sputtering gas atoms in the deposited material, limited target utilization (~30% for magnetrons)

Laser oblation.

Appropriate for many materials including refractory and compound ones, compatible with ultra-high vacuum conditions.

Complicated composition of the vapor: atoms, molecules, ions, droplets.

Pumping systems

Dry pumps

Wet pumps

Two main mechanisms of the gas evacuation:

(I) Compression and pumping out.

(II) Adsorption . Usually applicable under high vacuum conditions in the absence of a gas

flow.

Rotary vane vacuum pump Leibold Trivac B

100m3/h= 27.7 l/s; 1 mbar=0.75 Torr

The Principle of Operation is transfer of a mechanical pulse from oil vapor molecules to gas

molecules resulting in their average motion from a pumed area to forevacuum area

Oil diffusion pump

The principle of operation is transfer of a mechanical pulse from oil vapor molecules to gas

molecules resulting in their average motion from a pumped area to forevacuum area

Dry preliminary pumps: diaphragm pump.

Scroll pump

Screw pump

Pump Model SP250 SP630

Disp. CFM @ 60 Hz

(m3/hr @ 50 Hz) 177 (250) 371 (630)

Ult. Pressure Torr

(mBar) 7.5 x 10-3(1 x 10-2) 7.5 x 10-3(1 x 10-2)

Motor HP (kW) 7.9 (5.9) 20 (15)

Dimensions

LxWxH - in. (mm)

5.1 x 20.9 x 34.6

(1,350 x 530 x 880)

64.2 x 26 x 34.6

(1,630 x 660 x 880)

Weight lbs (kg) 992 (450) 1,166 (530)

Roots pumps

Roots pumps are usually used at the pressure range between 10-1 Torr and 10 Torr, where

they are very effective

Turbomolecular pumps

Turbomolecular pumps have very different rotor diameters covering pumping rate range

from tens to several thousands l/s.

Turbomolecular pums may have either mechanical or magnetic rotor suspension

The heart of a turbomolecular pump is a turbine rotating with a speed of the order of 50,000 rpm

(turns per minute).

The principle of operation is transfer of a mechanical pulse from turbine blades to gas molecules

resulting in their average motion from a pumped area to the forevacuum area.

High vacuum turbomolecular pumps

The typical ultimate pressure of the turbomolecular

pumps is 10-8 Torr but may be as low as 10-10 Torr.

Sputter ion pumps

The basic principle of operation is adsorption of residual gases by Ti film sputtered from Ti

cathode due to the ion bombardment. Other mechanisms are also involved. Magnetic field

increases probability of ionization by high-energy electrons.

Pressure-current dependence

Dependence of the current through the ion pump is a measure of the pressure .

However this dependence should be calibrated for a particular pump and is different for

different residual gases.

Titanium sublimation pump

Model PGT-3F PGT-6F

Pumping speed *1 24m3/(s·m2) <20ºC> / 64m3/(s·m2) <-196ºC>

Applicable pressure

range Ultra-high vacuum of between 10-1 to 10-9Pa

Applicable gases H2, N2, O2, H2O, CO, CO2

Inapplicable gases He, Ne, Ar, CH4, C2H6 , other organic gases

Leak volume 1.3 x 10-11Pa·m3/s max.

Baking temperature 250ºC

Power consumption 270W

Filament material Titanium alloy

Filament life span

Rated continuous operation time *2 : About 75

hours/filament

Continuous ON time : About 25 hours/filament

Number of filaments 3 6

Connection flange *3 UFC070-FH UFC114-FH

Weight 1.0kg 2.5kg

The principle of operation is adsorption of residual gases by thermally evaporated Ti film.

The pumping becomes very effective when Ti is evaporated onto a cooled (liquid nitrogen or

helium) panel.

Cryo-pumps

The principle of operation is condensation and physisorption of residual gases on a surface

cooled by liquid 4He (T>~4K). Porous materials (charcoal, zeolite, molecular sieves, etc) highly

enhance the sorption capacity and allow effective sorption of light gases (H2, He, etc) at

temperature ~4K.

Zeolites cooled with liquid nitrogen are frequently used as dry preliminary pumps (P=1000-

10-4 Torr). They should be backed up after each pumping circle.

Temperature (K)

Elements of PVD systems

1. Vacuum chamber of a large volume to guarantee a uniform coating

2. Pumping systems.

3. Systems for evaporation (boats, electron beam guns, lasers, etc) and sputtering (regulated

needle valves, ion sources, magnetrons, etc.)

The PRO Line PVD 75 is compatible with the following

techniques:

Thermal Evaporation (up to four 4" individual boats, or

six 2" boat assemblies)

Torus® Magnetron Sputtering sources (up to six 2" or 3"

sources)

Electron Beam Evaporation Source (4 pocket 8cc, 8

pocket 12cc, 6 pocket 20cc)

LTE10 Organic deposition sources (up to two)

Combinations of the above techniques are also available.

Pumping system

Pfeiffer 790 l/s turbomolecular pump with KJLC RV212 oil sealed roughing pump. Base pressure

for a properly conditioned chamber is 5 x 10-7 torr (6.7 × 10-7 mbar).

Wide range gauge reads from atmosphere to 10-9 Torr.

Magnetron system

0 to 800V @ 0 to 2.5A

0 to 400V @ 0 to 5A Output Assembly of three magnetrons

Evaporation Sources

Tungsten tapered helix coil. Wire diam.=0.76 mm; Max diam 20 mm;

2.5 V, 23 A , 1800oC

Tungsten flat trough 0.25mm thick; 19 mm wide; 3 mm deep

1.47 V, 158 A, 1800oC

Boron Nitride Crucible inner Diam.=11.7 mm

Molibdenium heat shielded crucible heater

1.24 V, 273 A, 1600oC

Quartz sensors

Gold recommended for low film stress deposition,

such as Aluminum, Gold, Silver, etc. Silver or Alloy

recommended for high film stress deposition, such as

Chromium, Nickel, Inconel, etc. Alloy recommended

for dielectric material deposition, such as Magnesium

Fluoride, Silicone Monoxide, etc.

Control quartz crystal, plano-convex, 6 MHz, 0.55

inches (1.4 cm) diameter. Compatible with all 6MHz

crystal sensors. Choice of coating, silver, gold, or alloy

determined by the application.

Twelve crystals contained in one sensor

Thickness controller

A monitor measures only deposition rate and thickness.

A controller measures deposition rate and

thickness like a monitor, but also provides an

output signal to control source power supply

and deposition rate.

Radio-frequency oscillator

Adhesion of coatings.

Adhesion determines the quality of a mechanical contact between the substrate and the coating.

1. Inter-diffusion of materials in contact as well as chemical reactions between them provide

the strongest adhesion. Corresponding energy usually exceeds 1 eV per particle.

2. Van der Waals forces lead to so-named physisorption (10-100 meV).

Vacuum cleaning and ion (electron) bombardment can remove adsorbed atoms/molecules from

the substrate surface and highly improve the adhesion.

The best adhesion to a metallic substrate have metals producing alloys with the substrate

material.

A good adhesion to the oxide surfaces (e.g., glass) have an oxygen-active film materials such as

Ti, Cr, Mo, or Zr. E.g., the Cr films are usually used in photo-masks for optical lithography.

These materials are frequently used as an intermediate thin layer (~10 nm) between a substrate

and a main film.

In general, the higher energy of evaporated (sputtered) particles leads to their better adhesion.

As a result, one gets better adhesion in the cases of cathode arc deposition, magnetron

sputtering and electron-beam evaporation. It may be related to desorption of extrinsical

adsorbates from the substrate surface.

Some materials (e.g., Zn, Cd) may have very poor sticking coefficients to many substrates.

Film growth modes

There are three primary modes of thin-film growth for mutually insoluble materials:

(a) Volmer–Weber (VW: island formation), (b) Frank–van der Merwe (FM: layer-by-

layer), and (c) Stranski–Krastanov (SK: layer-plus-island). For each mode, the layers are

shown for three values of surface coverage characterized by number of monolayers (ML)

Θ with the same mass.

In the most common VW mode, individual islands coalescence into a continuous

polycrystalline film above some threshold thickness which depends on the substrate

material.

E.g., a gold film on a glass substrate gets continuous at average thickness about 7 nm.