VACUUM SCIENCE AND TECHNOLOGY FOR THIN FILM DEVICE PROCESSINGAlastair BuckleyUniversity of Sheffield

The three things you can do in vacuum

Evaporate materials as a coating method

Thermionic emission from a hot metal surface Richardson-Shottky equation

Create a plasma Electrons, ions, neutrals Electron energy distribution function

Extract an ion beam Etching, sputtering

2 /kTRJ A T e

Outline Introduction to vacuum

Pressure, mean free path, residual gas Pumps and system design Pressure measurement

Physical Vapour Deposition Thermal evaporation Electron beam evaporation Sputtering Thickness monitoring

Chemical Vapour Deposition Reactive ion etching

Introduction Why vacuum processing?

Solar PV c-Si cell fabrication – doping, etching, electrode deposition a-si cell fabrication – deposition, etch, electrode deposition CdTe – deposition, etc.. OPV – electrode deposition Perovskite – electrode deposition

Microelectronics Plastic electronics Structural coatings, discharge lamps, CRTSs

Vacuum is how all “high tech” is done at the moment

Thin film PV R&D

PETEC

CIGS R&D

NREL

OLED lighting

IPMS

CMOS foundry

What can you do in vacuum?

Deposition Metals Dielectrics Organics

Etch Chemical Ion beam

Implant / doping Not going to cover this – this is how silicon transistors are made

Surface science (Not going to cover this either) SEM XPS Auger Etc..

Introduction to vacuum

Pressure, mean free path, residual gas Pumps and system design Pressure measurement

PressureVacuum quality Torr Pa mbar Gas

Atmospheric pressure

760 1.013×10+5 1013

Low vacuum 760 to 25 1×10+5 to 3×10+3 1000 to 30

Medium vacuum 25 to 1×10−3 3×10+3 to 1×10−1 30 to 1×10−3

High vacuum 1×10−3 to 1×10−9

1×10−1 to 1×10−7

1×10−3 to 1×10−9

Ultra high vacuum

1×10−9 to 1×10−12

1×10−7 to 1×10−10

1×10−9 to 1×10−12

Extremely high vacuum <1×10−12 <1×10−10 <1×10−12

Outer space 1×10−6 to <3×10−17

1×10−4 to < 3×10−15

1×10−6 to <3×10−17

Perfect vacuum 0 0 0

Mean free pathHow far does a molecule travel before it collides

If you want a stable plasma then you will need collisionsIf you want to thermally evaporate material then you want no collisionsThe threshold for chambers that are about 1 m wide is around 10-3 to 10-4 mbar

Flow in vacuum Viscous, turbulent Molecular, laminar

Residual gases I wanted to show a chart of the different pump speeds and different

residual gases in a vacuum chamber at different pressures.

I couldn’t find one though – so we will measure that in the practical! What do you think it will look like?

Which gases do you think dominate at (mbar) 10-2

10-4

10-6

10-8

What do you think the different pump rates of these gases are?

Vacuum pumps

http://www.globalspec.com/learnmore/manufacturing_process_equipment/vacuum_equipment/vacuum_pumps/vacuum_pumps_all_types

Vacuum pumps Backing pumps

Rotary High vac pumps

Cryo Turbo Diffusion

UHV pumps Sublimation

chamber

High vac pump Backing pump

exhaustforeline

UHV pump (if needed)

Rotary pump Compression by a mechanical motion P > 10-3 mbar “Roughing” to pump air out of chamber “Backing” to maintain foreline pressure

for high vacuum pump

https://www.youtube.com/watch?v=AFHogF-9eGA

Cryo pump “Freezes” residual gas to

internal surface – basically a very big fridge inside the vacuum chamber

Operates at ~10-20 K Need regenerating every

so often and routine maintenance in filters

Very effective for pumping water, N2 and O2

Turbopump Momentum transfer pump Gas molecules diffuse into

pump and are “hit” by rotor blades, changing the molecules direction into the body of the pump.

Expensive and bearings go eventually but otherwise maintenance free

Turbo Light gases pump more slowly Full pump rate only below 10-3 mbar

Pfeiffer

Diffusion pump

Like the turbo pump operates by momentum transfer

An oil spray generates a net momentum and gas compression towards the foreline

Oil contamination makes unsuitable for most processes in semicon

Used widely in old vacuum TV tube industry due to low cost

Sublimation pump Metals like chromium and titanium

sublime and as they do so they condense on the chamber wall trapping residual gas with them.

Chamber design Short path to pump Simple shapes Pressure gauge close to chamber but not

looking directly at the pump (ie. Opposite)

Pressure measurementType Pressure

range/ mbar

Mechanism

Pirani 10-4 -1 Temperature/pressure relationship of hot filament

Baratron 10-3-102 Capacitance of plates deflected by pressure change

Hot filamentIon guage

10-10-10-4 Ionisation current using thermionic electrons

Cold cathodePenning

10-6-10-2 Ionisation current using high EM field

Hot filament ion guage Electrons are emitted thermally from the filament The electrons accelerate towards the grid (+ve) They ionise gas atoms/molecules The +ve ions accelerate to the collector The collector current is proportional to the ion density

and therefore the pressure

gridcollector

emitter

Baratron (capacitance) gauge

Capacitance of parallel pair of electrodes is measured

One electrode is displaced by the pressure in the vacuum chamber

Pressure is calibrated to capacitance change

Pirani guage Resistance of a hot wire depends on its

temperature and therefore its conductive heat loss

Conductive heat loss depends on gas pressure

Edwards

Cold cathode (Penning) guage

A kV bias is applied to the anode. This ionises gas in the gauge. A magnet confines the ions in a circular path within the gauge resulting in further collisions and ionisation generating a measurable current at the cathode.

Cheap

Residual gas analysis (RGA) Quadrupole RGA Gas is ionised by electron collision near the cathode. Ions are accelerated into a quadrupole that has an oscillating field

applied. Only certain masses make it through to the detector at certain

frequencies of oscillation.

Physical vapour deposition Layer by layer deposition of materials

Roughness, adhesion Thickness control – intra and inter substrate Defectivity – pinholes, particles

Physical vapour deposition

Hauzer

PVD – what can be deposited?

http://www.lesker.com/newweb/deposition_materials/materialdeposition.cfm?pgid=0

Resistive evaporation Resistive heating Boats, crucibles and furnaces Metals (Ca, Al, Ag, Ni, Cr…) Some salts (LiF, BaF2, etc..) Some oxides (MoO3, V2O5)

Electron beam evaporation A hot filament emits electrons into vacuum These electrons are accelerated towards a target

material and collide with the material having kinetic energy that heats the target to an evaporation temperature

The evaporant has a line of path to the substrate to be coated

https://www.youtube.com/watch?v=ZN7NZYXGSbk

Sputtering

A glow discharge plasma is formed in vacuum at around 10-2 to10-3 mbar

A target of a material to be deposited is biased negative with respect to the plasma and ions from the plasma accelerate into the target ejecting target material towards the substrate.

Magnetron sputtering If the plasma is confined close to the target then

the sputtering rate can be enhanced significantly. A magnetic field can be used and in this case the

technique is know as magnetron sputtering.

Electron race track

RF, DC and pulsed sputtering To maintain a stable plasma and a stable

sputter rate a stable bias between the plasma and the target is needed.

For conducting targets this is possible with a DC field

For insulating targets RF fields can be used but the sputter rate is often lower than for DC.

In industrial coating applications often pulsed DC is used as a compromise.http://www.advanced-energy.com/upload/File/White_Papers/ENG-

ChooseIndPwrSup-270-01.pdf

Ion assisted deposition Can be used with ebeam, thermal or

sputter deposition Increases the adhesion and density of

the film

Quartz crystal microbalance The resonant frequency

of a piezo electric crystal depends on its mass.

An oscillating field is applied across a quartz crystal and its resonance monitored

The rate of change of mass addition can be measured

Chemical vapour deposition Precursor heated (in a furnace) with a

substrate and converted to inorganic layer

Different precursors give different films

http://www1.phc.uni-kiel.de/cms/index.php/en/research-m-gfr/140-cvd.html

Plasma enhanced CVD (PECVD)

Applied Materials – TCO deposition for gen8 display glasshttp://www.appliedmaterials.com/display

Atomic layer deposition Layer by layer chemical deposition

technique that can make atomically perfect films

Great for hermetic encapsulation Great for conformal film forming Really slow

Reactive ion etching Reactive ions are generated in a plasma The ions react with the substrate creatin

volatile products that are pumps away. Fluorine ions react with most oxides

(SiO2) Chlorine ions react with most metals (Al)

http://www.sentech.com/en/Plasma-Process-Technology__2288/

Ion beam etching A beam of energetic ions bombard a

substrate sputtering away the surface High vacuum Etches anything

http://www.sentech.com/en/Plasma-Process-Technology__2288/

Final slide.. What I hope you have learned.

Vacuum processing is ubiquitous in high tech

There are loads of different process techniques Many are not available in the science lab – but they are

available in industry

When you do vacuum fabrication think: Pressure is mean free path – 10-4 mbar is transition to collision free Pressure is residual gas – 10-4 to10-6 mbar is mostly water

In the vacuum practical you will measure P vs t for different residual gases. You will measure the pump rates of the different gases.