Delivered by Dr. Alastair Buckley, University of Sheffield as part of CDT-PV core level training, Jan 2016

1. VACUUM SCIENCE AND
TECHNOLOGY FOR THIN
FILM DEVICE
PROCESSING
Alastair Buckley
University of Sheffield

2. 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 /kT
R
J A T e 


3. 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

4. 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

5. Thin film PV R&D
PETEC

6. CIGS R&D
NREL

7. OLED lighting
IPMS

8. CMOS foundry

9. 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..

10. Introduction to vacuum
 Pressure, mean free path, residual gas
 Pumps and system design
 Pressure measurement

11. Pressure
Vacuum 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

12. Mean free path
How far does a molecule travel before it collides
If you want a stable plasma then you will need collisions
If you want to thermally evaporate material then you want no collisions
The threshold for chambers that are about 1 m wide is around 10-3 to 10-4 mbar

13. Flow in vacuum
 Viscous, turbulent
 Molecular, laminar

14. 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?

15. Vacuum pumps
http://www.globalspec.com/learnmore/manufacturing_process_equipment/vacuum_equi
pment/vacuum_pumps/vacuum_pumps_all_types

16. Vacuum pumps
 Backing pumps
 Rotary
 High vac pumps
 Cryo
 Turbo
 Diffusion
 UHV pumps
 Sublimation
chamber
High vac pump Backing pump
exhaust
foreline
UHV pump (if needed)

17. 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

18. 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

19. 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

20. Turbo
 Light gases pump more slowly
 Full pump rate only below 10-3 mbar
Pfeiffer

21. 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

22. Sublimation pump
 Metals like chromium and titanium sublime
and as they do so they condense on the
chamber wall trapping residual gas with them.

23. Chamber design
 Short path to pump
 Simple shapes
 Pressure gauge close to chamber but not
looking directly at the pump (ie. Opposite)

24. Pressure measurement
Type 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 filament
Ion guage
10-10-10-4 Ionisation current using thermionic
electrons
Cold cathode
Penning
10-6-10-2 Ionisation current using high EM field

25. 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
grid
collector
emitter

26. 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

27. 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

28. 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

29. 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.

30. Physical vapour deposition
 Layer by layer deposition of materials
 Roughness, adhesion
 Thickness control – intra and inter substrate
 Defectivity – pinholes, particles

31. Physical vapour deposition
Hauzer

32. PVD – what can be deposited?
http://www.lesker.com/newweb/deposition_materials/materialdeposition.cfm?pg

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

34. 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

35. 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.

36. 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

37. 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

38. Ion assisted deposition
 Can be used with ebeam, thermal or sputter
deposition
 Increases the adhesion and density of the film

39. 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

40. 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

41. Plasma enhanced CVD (PECVD)
Applied Materials – TCO deposition for gen8 display glass
http://www.appliedmaterials.com/display

42. 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

43. 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/

44. 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/

45. 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.