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Vacuum Science and Technology for Thin Film Device Processing


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

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Vacuum Science and Technology for Thin Film Device Processing

  2. 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. 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. 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. 5. Thin film PV R&D PETEC
  6. 6. CIGS R&D NREL
  7. 7. OLED lighting IPMS
  8. 8. CMOS foundry
  9. 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. 10. Introduction to vacuum  Pressure, mean free path, residual gas  Pumps and system design  Pressure measurement
  11. 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. 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. 13. Flow in vacuum  Viscous, turbulent  Molecular, laminar
  14. 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. 15. Vacuum pumps pment/vacuum_pumps/vacuum_pumps_all_types
  16. 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. 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
  18. 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. 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. 20. Turbo  Light gases pump more slowly  Full pump rate only below 10-3 mbar Pfeiffer
  21. 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. 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. 23. Chamber design  Short path to pump  Simple shapes  Pressure gauge close to chamber but not looking directly at the pump (ie. Opposite)
  24. 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. 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. 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. 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. 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. 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. 30. Physical vapour deposition  Layer by layer deposition of materials  Roughness, adhesion  Thickness control – intra and inter substrate  Defectivity – pinholes, particles
  31. 31. Physical vapour deposition Hauzer
  32. 32. PVD – what can be deposited?
  33. 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. 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
  35. 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. 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. 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. ChooseIndPwrSup-270-01.pdf
  38. 38. Ion assisted deposition  Can be used with ebeam, thermal or sputter deposition  Increases the adhesion and density of the film
  39. 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. 40. Chemical vapour deposition  Precursor heated (in a furnace) with a substrate and converted to inorganic layer  Different precursors give different films
  41. 41. Plasma enhanced CVD (PECVD) Applied Materials – TCO deposition for gen8 display glass
  42. 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. 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)
  44. 44. Ion beam etching  A beam of energetic ions bombard a substrate sputtering away the surface  High vacuum  Etches anything
  45. 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.