COVERSTORY
types are a Czochralski (CZ) crystal puller, or a directional solidification of silicon (DSS) furnace.
The CZ puller produces boules of single-crystal silicon that, when sliced into wafers, provide for high efficiency (17 to 20 percent) solar cells. The process time for a boule used for the solar industry is typically two days. The mass of the boule starts out at around 120kg, and then is typically less than 100 kg after it has been shaped. In contrast, a DSS furnace can process a much larger amount of silicon in the same process time, typically 450kg with capabilities reaching up to 800 kg, but the resulting polycrystalline silicon ingot, when sliced into wafers, generally results in a lower efficiency (13 to 16 percent) solar cell.
Vacuum Pump Considerations Vacuum considerations for both processes are similar, but CZ pullers generally require more argon gas at a lower pressure than required by a DSS furnace. Considerations include not only the actual process conditions required inside the chamber, and the resultant effects on the end product, but also issues related to base pressure, leak-tightness, pump location and associated piping, powder management and pump type. Argon, chosen for its inert nature, presents particular challenges to the vacuum pumps because of its poor thermal conductivity.
The level of vacuum required and the speed of evacuation are primary factors in determining the optimal pump size and type. Fast evacuation is generally desirable since it reduces overall processing time and cost, however, excessively fast evacuation of chambers containing poly-silicon fines can cause loss of the product into the vacuum line. In general, faster is better since the quicker a chamber is brought to base pressure, the sooner the crystallization process can start. A low base pressure is generally required for no other reason than to conduct a rate-of-rise leak test. Oxygen or nitrogen leaking into the chamber can contaminate the silicon.
Pump location and the design of the piping system significantly impact evacuation speed. Larger diameter pipe increases the conductance of the
Figure 1: Effects of piping system design on pumping speed
system, but also increases the volume of gas to be evacuated. Shorter pipe runs, i.e. pump locations closer to the chamber, are desirable, however short pipe runs are only applicable when pumps with low vibration are used, or where additional vibration damping is provided to noisy pumps. Figure 1 shows the effects of various piping arrangements on pumping speed for Edwards’ GXS160/1750 pump. Piping size most significantly impacts the pumping speed at lower pressures. Poor design results in much longer evacuation time.
Powder Management
The molten silicon generates silicon vapor that is pumped away by the vacuum system. The vapor forms a powder when it condenses in the piping and pump. The powder is abrasive and causes
The level of vacuum required and the speed of evacuation are primary factors in determining the optimal pump size and type. Fast evacuation is generally desirable since it reduces overall processing time and cost, however, excessively fast evacuation of chambers containing poly-silicon fines can cause loss of the product into the vacuum line
15
www.solar-pv-management.com Issue IV 2010
Page 1 |
Page 2 |
Page 3 |
Page 4 |
Page 5 |
Page 6 |
Page 7 |
Page 8 |
Page 9 |
Page 10 |
Page 11 |
Page 12 |
Page 13 |
Page 14 |
Page 15 |
Page 16 |
Page 17 |
Page 18 |
Page 19 |
Page 20 |
Page 21 |
Page 22 |
Page 23 |
Page 24 |
Page 25 |
Page 26 |
Page 27 |
Page 28 |
Page 29 |
Page 30 |
Page 31 |
Page 32 |
Page 33 |
Page 34 |
Page 35 |
Page 36 |
Page 37 |
Page 38 |
Page 39 |
Page 40 |
Page 41 |
Page 42 |
Page 43 |
Page 44