The Top Five Technical Challenges in Poly-crystalline Silicon

1. The Top Five Technical Challenges in Poly-crystalline Silicon Canisius College April 5, 2011 Larry Coleman, Engineering Consultant

2. What is Polysilicon ? • Industry jargon for polycrystalline silicon • Key raw material in manufacture of photovoltaic arrays = solar cells • Only abundant element to exhibit photovoltaic response ~ 20% efficiency • Other photovoltaic materials with better efficiency are either rare or toxic • Can be amorphous, poly- or single crystal

3. Photovoltaic Array Economics • Average US residential use is 20- 30 KW-hr/day • Locally we pay $0.18/KWH, some places double • Array cost = $2500/KW, get about 8 KW-hr/day from a 1 KW array, averaged over the year • Payouts are about 5 years locally, no subsidy • No disconnect from the grid, need battery backup • Adding in batteries & inverters doubles the payout time • Europe leads the way with subsidies

4. Photovoltaic Basics • Solar radiation is function of latitude and cloud cover • Typical silicon efficiency is up to 17% of solar spectrum. Room for improvement. • Uses p-n junction to generate DC power • Wafer is about 1/100 inch thick

5. What an array looks like

6. Silicon the Element

7. Where do you find silicon • Silicon is ~25% of the earth’s crust, as silica and silicates • You make MG silicon by reducing quartz silica with carbon (coke) at 2600°F = 1430°C in a submerged electric arc furnace SiO2 + 2 C → Si + 2CO • Cool it down to solidify. Break it up with automatic jack-hammers to 4” chunks • Grind it to suit in a ball-mill or hammer-mill • Metallurgical Grade Silicon is about 98.5% pure • Used historically in steel-making and aluminum alloys

8. MGS toSolar Silicon in 10 steps • React quartz with coke → MGS • Fluidize and react with HCl → HSiCl3 (TCS) • Purify the TCS with distillation & adsorbents • Decompose in CVD reactor → poly rods • Break the rods & CZ pull single crystal boules • Slice the boules into wafers • Epitaxially react with doped gas to make p-n junction solar wafers • Photomask on the collection grid • Mount and make electrical connections • Encapsulate with glass as an array

9. Five biggest technology challenges • Challenges are where technology is lacking and improvements will make a big difference in production costs • Does not / can not include new advances and developments • European $$ support is now, especially in Germany. US support is lagging behind. • Where science needs more development, development needs more scientists • US technology is very prized globally

10. Tech Challenge #1 - Status • Metallurgical silicon grinding and fluidization • MGS is used primarily in steel- and aluminum making for alloying strength, where particle size doesn’t matter • Solids fluidization is mature for FCC catalyst and coal combustion – MGS is hardly known to the industry • Fluidization is a new and niche science

11. Tech Challenge #1 - Needs • How to make a tighter grind distribution • How to characterize pneumatic conveying • How to model reaction shrinking • How to minimize losses, while maximizing reactivity • How to track electronic impurities

12. Tech Challenge #1 – Science Tie-ins • Fracture Physics & Materials Science • Interphase Physics ( solids + gas = liquid) • Engineering process modeling • Statistical analysis

13. Tech Challenge #2 - Status • Disproportionation Reactions • Used to change trichlorosilane to silane • Solid catalyst reactors re-arrange Si-H and Si-Cl bonds • Adds purification to the process ( removes Boron and Phosphorus) • Mechanisms and kinetics are unknown • Diffusion suspected in playing a role

14. Tech Challenge #2 - Needs • How to model the reactions • How to minimize the reactor sizes • How to enhance the kinetics • How to maximize impurity retention • How to construct processes to recycle waste streams

15. Tech Challenge #2 – Science Tie-ins • Chemistry • Engineering process modeling • Better thermodynamic data analysis • Chemical kinetics

16. Tech Challenge #3 - Status • Molecular Sieve and Membrane Purification • Modified zeolites and other structures have potential for removing contaminants from electronic gases - minimum R&D has been done • Metallic membranes can remove hydrogen and other light gases, but fouling is still a problem • Most purification by distillation is expensive and limited in application by diffusion

17. Molecular sieves ZSM-5 5-8 A° 3-5 A° 8-13 A°

18. Tech Challenge #3 - Needs • Use molecular size and polarity differences to drive separations to higher degree @ low price • Follow the example of air separation, but to greater purification on smaller quantities • Develop new types of sieves, rather than just “A” and “X” types. Example = ZSM-5 for gasoline • Better membrane coatings that can stand up to corrosive environments. Ties to a hydrogen economy.

19. Tech Challenge #3 – Science Tie-ins • Nano-technology / Nano-engineering • Molecular structure of zeolites and molecules - physics • Chemical modification of zeolites and other mole sieve structures • Physics of membrane processes – molecular diffusion

20. Tech Challenge #4 - Status • Chemical Vapor Deposition (CVD) • Some crude models available for CVD onto silicon rods and silicon wafers • Anecdotal evidence is that deposition rates can be boosted by 50% = large power reduction • Good thermophysical data is scarce • On-going work with CVD deposit of carbon onto graphite for better high temperature reactors

21. CVD Reactor

22. Tech Challenge #4 - Needs • Accurate thermophysical values of components • Decomposition model driven by hard production data, including diffusion effects near the rod • Development of SiC CVD technology to retain purity and lower energy costs • Development of decomposition models with higher purity silane • Develop CVD model for carbon-graphite

23. Tech Challenge #4 – Science Tie-ins • Physics and material science • Thermodynamics • Chemistry at high temperatures • Computerized Flow Dynamic (CFD) modeling

24. Tech Challenge #5 - Status • Crystal and Ribbon Pulling of molten silicon at 1410°C = 2570°F • Continuous melt replentishment promises lower costs but hampered by crucible materials • Existing quartzware only lasts 100 hours • New ceramics and coatings are only partial solution • Ribbon pulling has the best long term potential • 30% of the silicon is lost in wafering

25. Crystal Pulling Silicon

26. More about CZ pullers

27. Parts of a silicon boule

28. Tech Challenge #5 - Needs • Improved materials of construction for crucibles that will last more than 2 runs. Quartz dissolves • Some work has started on silicon nitride coatings to prevent wetting. • Best option is improved pyroltyic graphite made by CVD deposit of carbon vapor @ 1800°C • Melt replentishment is a target technology to make crystal pulling more continuous. Problems with hydrogen content being too high. • Ultimate goal is being able to pull single crystal ribbon

29. Tech Challenge #5 – Science Tie-ins • Materials Science • Physics of sub-cooled liquids • High temperature chemistry (reactions) with quartzware and ceramics • Measurement of impurities at electronic levels: FTIR, mass-spec GC

30. Education levels needed • Physics – likely PhD, with specialization in high temperature silicon processing, crystallography, ceramics, and zeolites • Chemistry – likely MS with specialization in analytical sciences ( FTIR, GC-MS, epitaxy) • Chem. Engineering – BS or MS, with concentration in thermodynamics, silicon chemistry, fluidization, and chemical kinetics • Engineering Simulation Science - MS • Nano-engineering – likely PhD, specialized in membranes and zeolite modification

31. CZ Puller Videos Kayex Video: http://www.kayex.com/page.asp?tid=129&name=Kayex-Silicon-Crystal-Growing-Process-Demo From inside the pull chamber, looking into the crucible: http://www.youtube.com/watch?v=cYj_vqcyI78