1. Methods of physical vapor deposition. Thermal evaporation. Adhesion. Role of vacuum.

Monitoring of the deposition process.

2. Magnetron and RF-diod sputtering.

3. Electron-beam evaporation.

4. Engineering aspects of physical vapor deposition: vacuum chambers, pumps, cooling systems.

5. Monitoring and control of the deposition processes.

6. MBE based technologies.

Andrey Shishkin

Physical vapor deposition, PVD

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.

• 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.

• Several important examples of different coatings.

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

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

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 m shifts the resonant frequency by

N0 1.661 MHz*mm for Here 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.

f0 nN0 / D

0 2.648 g/cm3 – density of the quartz , 0 2.947 *10 11 g/cm s2 - shear modulus for the AT cut

of a quartz crystal.

Quartz crystal microbalance

For Au ( =19.3 g/cm3) film, one monolayer (d~3A) corresponds to f=33 Hz. Temperature stabilization is important condition for precise measurements!

Equation (Q1) is valid for f / f0 0.03

For a wide frequency range ( f/f0

Estimate for the thermal evaporation rate

ev

ma ev

2kT dm / dt P(T )

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 .

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 Maxwell‘s distribution.

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

,

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

V i s the specific volume change of the phase transition (the difference of

specific volumes of a liquid and vapor).

dT T 2 R

dP

PQ For an ideal gas and dQ/dT=0

ln P Q

A A B / T

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

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

a small solid angle d in the direction determined by angle is given by equation dw=(w/) cos( )d . 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( )cos( /R2 If the source is spherical, dw=(w/4) d and d= m cos( /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 = , cos =H/(H2+X2)1/2 , R2=H2+X2.

source

substrate

(H 2 X 2 )2

m H

4 (H 2 X 2 )3 / 2

H 2 d ( X ) m m 1

H 2

m 1

4 H 2

max d

d ( X ) max d Spherical source

Flat source

0.0 0.2 0.4 0.7

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 1.4 0.0

0.2

0.4

0.6

0.8

1.0

d/d

max

Spherical sourc(ea)

Flat 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.

Advantages 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.

• Estimation of the film thickness uniformity.

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)

http://www.vem-co.com/guidehttp://www.vem-co.com/guidehttp://www.vem-co.com/guidehttp://www.vem-co.com/guidehttp://www.vem-co.com/guidehttp://www.vem-co.com/guidehttp://www.vem-co.com/guidehttp://www.vem-co.com/guidehttp://www.vem-co.com/guidehttp://www.vem-co.com/guidehttp://www.vem-co.com/guidehttp://www.vem-co.com/guide

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

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

2meU c

| e | H R R[cm] 1.8 U[kV ] / H [kG]

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).

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

s p u t 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 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

Ar -argon atoms

+ -argon ions

E

H

e -electrons

-sputtered atoms

e e

A+r +

Ar e

anode

+

substrate

film

+

e

H

E + plasma

+

lines 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.

cathode-source (-)

N

S

S

N

N

S

magnetic system

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.

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

RF-magnetron High-frequency magnetron sputtering is used

when it is necessary to apply dielectric films.

Previously it was assumed that the sprayed

material has good electrical conductivity. At

the same time, the working gas ion hitting

the cathode is neutralized on it and returns to

the vacuum volume of the working chamber. Schematic representation of bipolar pulsed power

If the sprayed material is a dielectric, then the positive ions are not neutralized and within a short

period of time after the supply of a negative potential, the target is coated with a layer, creating a

positive charge on its surface.

The field of this charge compensates for the initial field of the cathode, which is under negative

potential, and further sputtering becomes impossible, since the ions from the discharge are not

attracted to the target.

Therefore, dielectric targets cannot be sprayed in a constant electric field.

To ensure sputtering of the dielectric target, it is necessary to neutralize the positive charge on its

surface by applying a high-frequency alternating potential.

When replacing a DC voltage with an alternating dielectric target, it is bombarded with ions only in

the negative half-period of the supply voltage.

In other words, the sputtering of the target does not occur continuously, as in cathode sputtering,

but discretely with the frequency of the supply voltage (usually 13.56 MHz).

At high frequency and the distance from the target to the substrate matched with it, the electrons in

the middle part of the high-frequency discharge do not have time to reach the electrodes during the

half-period, they remain in the discharge, making oscillatory movements and intensively ionizing

the working gas.

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.

•(--) 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

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 ablation.

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

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

Role of vacuum

To study the surface at the atomic level, it is absolutely necessary that during the experiment this surface should remained almost unchanged. This means that the flow of molecules from the surrounding volume to the surface should be very small.

Pressure, p, Torr

Concentration of molecules n, cm-3

Flux of molecules to the surface, I,

cm-2 s-1

Mean free path,

λ

Monolayer

formation time, τ

760 2x1019 3x1023 700 A 3 ns

1 3x1016 4x1020 50 μm 2 μs

10-3 3х1013 4х1017 5 cm 2 ms

10-6 3х1010 4х1014 50 m 2 s

10-9 3х107 4х1011 50 km 1 h

Table of characteristic values for nitrogen molecules at room temperature. The sticking coefficient is assumed to be 1. The surface concentration of one monolayer is assumed to be equal (which is close to the actual magnitudes for solid surfaces).

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)

Adhesion

Adhesion is the tendency of dissimilar particles or surfaces to cling to one another (cohesion refers to the tendency of similar or identical particles/surfaces to cling to one another). The forces that cause adhesion and cohesion can be divided into several types. The intermolecular forces responsible for the function of various kinds of stickers and sticky tape fall into the categories of chemical adhesion, dispersive adhesion, and diffusive adhesion. In addition to the cumulative magnitudes of these intermolecular forces, there are also certain emergent mechanical effects.

Dew drops adhering to a spider web.

Dependence of friction coefficient on the environment

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.

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

Pumping speed *1

PGT-3F PGT-6F

24m3/(s·m2) / 64m3/(s·m2)

Applicable pressure

range

Applicable gases

Inapplicable gases

Leak volume

Baking temperature

Power consumption

Filament material

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

H2, N2, O2, H2O, CO, CO2

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

1.3 x 10-11Pa·m3/s max.

250ºC

270W

Titanium alloy

Filament life span

Rated continuous operation time *2 : About 75

hours/filament

Continuous ON time : About 25 hours/filament

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.

Number of filaments 3 6

Connection flange *3 UFC070-FH UFC114-FH

Weight 1.0kg 2.5kg

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

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.

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.

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

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.

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.

Modern thin-film semiconductor materials for

solar cells:

Amorphous silicon (a-Si)

Cadmium telluride (CdTe)

Thin-film silicon laminates being installed onto a roof.

CIGS solar cell on a flexible plastic substrate

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.

Optical coatings.

….

2.

3.

MBE is the layer by layer growth of a high quality single crystal by physical vapor

deposition in ultra- high vacuum. The method is the most widely used for growth of

layered semiconductor materials constituted by layers with different chemical

composition (heterostructures). (‘semiconductor engineering’) . The grown material is

used in production of semiconductor lasers, high-electron-mobility transistors (HEMTs),

… This method, in fact, is the technological culmination of the physical vapor

deposition technique. The most strict requirements are met in growing of the material

for HEMTs. Here we consider growth of the GaAs/AlGaAs heterostructures with a two-

dimensional electron channel.

The following main conditions should be met:

(i) Ultra clean materials (Ga, Al, As) with impurity concentrations significantly below

1x1014 atoms/cm3.

(ii) Ultra-high vacuum P

MBE of GaAs/AlGaAs heterostructures: vacuum

Standard ultra-high vacuum systems

manufactured by several companies are usually

adapted for a particular consumer. Most

frequently a set of closed cycle helium cryopumps

is added.

One example of pumping system of growth

chamber: (i) three helium cryopumps with

pumping rate 3000 l/s each (9000 l/s together)

(ii) a liquid nitrogen–cooled titanium sublimation

pump,

(iii) a liquid nitrogen–filled panel which

surrounds the sample area.

The pressure in the growth chamber is kept below 10-11 Torr several years after initial

charging of effusion cells with necessary materials, long-term backing and evacuation. The

system consists of three ultra-high-vacuum chambers: a growth chamber, a preparation

chamber and loading chamber (the latter two are not shown in the Fig.). Grown

heterostructures are transported to the preparation chamber and exchanged by new substrates

(GaAs wafers) by a manipulator.

MBE -machine

Peculiarities of the GaAs/AlGaAs growth

AlxGa1-xAs

Crystal structure - Zinc Blende

Lattice constant - 5.6533+0.0078x

A lattice mismatch between AlAs and the

GaAs is ~0.1%

To avoid the formation of the Ga droplets

there should be excess flux of As. Then the

growth rate is determined by the Ga flux.

Typical growth parameters

Cell temperatures:

As – 350o C

Ga – 850o C

Al – 980o C

Dots show the melting points

Substrate (wafer) temperature

can lie in a rather wide temperature range

~500o C – 650o C with typical value ~635o C

At 350o C As sublimes in a form of tetramer

As4. Sometimes an additional thermal cracker

cell is used to decompose As4 into dimmers

As2.

The substrate rotation is used to improve the

layer uniformity while it complicates the real-

time control of the thickness by RHEED.

The growth rate is determined by the Ga flux,

depends on a layer and is typically ~1 A/s

Growth interruptions are extensively used to

facilitate interface smoothness.

Control of layer by layer growth by reflection high-energy electron

diffraction (RHEED)

nd )cos(cosSimilar to a diffraction grating o

AkeVEmEp ][][/39.02/2/2

RHEED pattern

Example of the MBE-grown structure

Transmission electron microscope image of a

7-nm AlAs, 5-nm GaAs 50-period superlattice.

Main growth steps:

1. Preparation of a GaAs substrate wafer

(i) Removal of the water by heating in the

introduction chamber

(ii) outgassing in the buffer chamber at a

temperature about 400o C

(iii) oxide removal in the growth chamber at a

temperature about 600o C under As flux

2. Growth of a short period GaAs/AlGaAs

superlattice to minimize effects of the GaAs

wafer surface imperfections.

3. Growth of the main layers, including doped

ones.

The best growth conditions are usually reached by trial and error.

Modification of electron energy spectrum

Eg=(1.424+1.155x+0.37x2) eV

(static) =12.90-2.84x

(high frequency)=10.89-2.73x

AlxGa1-xAs GaAs

Double heterostructure laser

There no strong requirements to the quality of layers in

such lasers. Such heterostructures can be even grown by

liquid-phase epitaxy.

Lasers utilizing transitions between subbands

In a quantum well laser, motion of electrons and holes in the growth

direction is quantized.

In a quantum cascade laser electron transitions occur between

different subbands

AlInAs/GaInAs

multiple quantum well

he

yx

heyx

he

nm

pp

am

nppE

,

22

2,

222,

22),(

])1[(*2

22

2

22

nnam

h

Selective doping as a method to produce high-electron mobility

systems (single heterojunction)

Profile of the conduction band minimum for

selectively doped AlGaAs-GaAs heterojunction

High electron mobility transistor (HEMT)

)(4/4/ 00

0

sdcdC

VCne gs

At small voltages between source and

drain, HEMT is similar to a plane capacitor

2)(/*)( eVnmVR gsg

Electron motion in

Z direction is quantized

High electron mobility transistor (HEMT) =

heterostructure FET (HFET) = modulation-doped FET (MODFET)

2)(/*)( eVnmVR gsg

Example of a static I-V charachteristic.

At small voltages between drain and source

I-V nonlinearity is the result of the

potential drop along the current carrying channel,

which leads to coordinate dependence of the

electron density ns.

Such transistors can be produced on other semiconductior

materials: InGaAs/AlGaAs, AlGaN/InGaN, etc. However the

modulation doping is the common method to produce high

electron mobility.

High frequency HEMTs

HEMTs are used in high-frequency circuits and can operate at frequencies ~ 1 THz.

In such transistors, the channel length Lsd should be well below 1mm.

Two- dimensional electron systems (2DESs)

Two-dimensional electron systems correspond to situation when all electrons reside in the

lowest subband

*2*2),(

22

2

222

m

pp

am

nppE

yx

yxn

kTm

p

amEE F

*2*2

3 2

2

22

12

degenerate 2DES

Two-dimensional electron systems arise in

many selectively doped semiconductor

heterostructures as a result of size quantization

of electron motion in the growth direction

(normal to the heterostructure layers) and free

electron motion along the layer.

Energy spectrum for electrons in an infinite

quantum well of a width a

(electrons with isotropic effective mass m*)

Classical electron dynamics in a magnetic field (2D)

Electron orbit

drift

HE

HE

Vd =cE/H

Less probable scattering

More probable scattering

Jx=jH=nseVd=nsecEy/H Hall resistance RH=H/nsec

Current along the electric field is the result of

electron scattering Jy=nse2 /(1+wc

22)Ey

yxxxxyy

yxyxxxx

EEj

EEj

0

2

2

0

2

~

)(

)(

)(1

1

w

w

w

cs

cs

c

H

ecnH

ecn

*

2

0m

ens

yxxxxyy

yxyxxxx

jjE

jjE

1

0

1

0~

ecn

H

ecn

H

s

s

In the absence of scattering

electrons drift along equipotential

lines. conductivity at H=0

cmeHc */w cyclotron frequency

Two- dimensional electron systems in a quantizing magnetic field

(wc>>1)

Unique property of 2DES is a discrete energy spectrum

in quantizing magnetic field H normal to 2DES

ckn kam

nE w

)2/1(

*2 2

222

,

cm

eHc

*wwhere

is the cyclotron frequency of electrons

with effective mass m*

Remarkably, degeneracy of a spin-split Landau level N0=eH/hc

is independent of the electron energy spectrum (effectve mass

m*, spin-orbit splitting, etc).

e

)(eD

)(eD

e

cw

Scattering frequency 1~D(eF) has minima in the minima of D(eF).

At H=0 constmD 2/*)( e

ww

2

2

22

2

*)1(* c

s

c

sxx

m

en

m

en

222

xy

xx

xyxx

xxxx

This leads to minima of the dissipative conductivity

and resistivity since xy/xx=wc>>1

Condition of the minima is occupation of integer number n of Landau levels: ns=nN0=neHn/hc,

i.e. positions of the minima are periodical in the inverse magnetic field (Shubnikov- de Haas

oscillations : sn hcnneH /

1 where is an additional degeneracy of the Landau level

Landau levels

Integer quantum Hall effect

Hall resistance in the minima of the Shubnikov – de Haas oscillations has a universal quantized

value 2/)/(// nehhcneHecHecnHR sxyH

When xx~xx tends to zero, the Hall resistance stays quantized in a range of parameters ns or H

K. von Klitzing (1980), Nobel prize of 1985 2DES in Si MOSFET: ns~(Vg-Vg0)

Resistance standard h/e2 = 25812.807557(18) W

Explanation in terms of localized and extended states

Fractional quantum Hall effect (1982)

Nobel winners 1998 R.B. Laughlin, H.L. Stoermer, D.C. Tsui

2DES in GaAs/AlGaAs

n

m

e

hRQuH 2 Main fractions: )12(2

n

n

e

hRQuH

Many-body effect,

quasiparticles with a fractional charge

e/(2n+1)

Graphene

|| iF ppV

e222/||)( Fvs VD ee

VF=108 cm/s

v=2 – valley degeneracy

s=2 – psevdospin degeneracy

Integer quantum Hall effect in graphene

cneHVFn /||2e

)12(2 2

ne

hR Quxy

Qu

H h

neQuxy

)12(2 2

Nobel winners 2010 A. Geim and K. Novoselov

N0=eH/hc

The oscillation period is determined by relation w=nwc

Microwave induced resistance oscillations (MIRO)

and Zero-resistance states (ZRS)

MIRO (Zudov et.al., 2001) ZRS (Mani et.al., 2002)

MIRO show extremely large amplitude

with xx tending to zero in the minima

Oscillations of the magnetoresistance and conductivity are connected by the normal

relations, so that simultaneously xx 0 and xx 0

C.L. Yang, M.A.Zudov, T.A. Knuuttila, R.R. Du, L.N.Pfeiffer, and K.W. West, Phys. Rev.Lett. 91, 096803 (2003).

Two mechanisms: indirect inter-Landau-level transitions and non-equlibrium electron

energy distribution function f(e)

Ryzhii, 1969

Durst et.al, 2003

Dorozhkin, 2003

Dmitriev, et al 2003, 2005

w/ wc

Instability of a homogeneous state with xx

Fragment of a sample with 2DES at a GaAs/AlGaAs heterojunction

0.6 мм

Fragment of a Hall bar sample (a strip of 0.6 mm width) with internal (60x60 mm2) and

external alloyed contacts (Au/Ge/Ni), bonding pads (Cr/Au) and

wiring (gold wires with 25 mm diameter).

Irregular correlated switching of the MW photo-voltages

B = +97 mT, f = 50.0 GHz, P = -1 dBm B = -95 mT, f = 48.1 GHz, P = -2 dBm

S.I. Dorozhkin, L.N. Pfeiffer, K. W. West, K. von Klitzing, and J.H. Smet, Nature Physics 7, 336 (2011)

Quasi-periodical switching Irregular switching Wafer 2

fsw~102 s-1 fsw~10 s

-1

The essence of the switching effect are flips of spontaneous electric field in domains

Wafer 1

1.66 K

1.50 K

1.15 K

Temperature dependence of the average switching frequency

Signals of the microwave photo-voltage

measured at different temperatures

Microwave induced resistance oscillations

ZRS

Wafer 1

)(/ 122

2 CCLWfC

)(/ 1221 CCLWCfC

Temperature dependence of conductivity of the doped layer

The sample layout and a measurement circuit.

(c) Schematic of the HEMT layers.

(d) lateral geometry of the transistor.

D1=162 nm, d2=79 nm

L=2.8 mm, W=0.5 mm

Experimental dependencies of the measured

capacitance on the modulation frequency for a set

of temperatures (symbols). Results of calculations

in accordance with Equations (1) are shown as solid

lines for different values of fitting parameter

Similar temperature dependences of the conductivity and the

switching frequency.

)/exp(~

)/exp(~

Tf

T

fsw

Thermally activated temperature dependences

Close values of activation energies and f1 imply proportionality between conductivity of

the doped layer and the switching frequency, ~fsw, which is the basis of an idea of the

spontaneous electric field screening by charges in the doped layer as a physical origin

of the dynamical domain structure.