COVERSTORY


increased loads and wear in the pump. It also has the potential to generate hazardous levels of heat by reacting with oxygen or nitrogen when the system is vented. In particular, where a quartz crucible is used, silicon monoxide may also be generated. Depending on process conditions, there may be as much as a kilogram of powder produced over a two day process cycle. Powder may be managed by trapping before the pump or oxidizing it and collecting it after the pump. Either option requires periodic maintenance. In the case of pre-pump trapping, the efficiency of the trap must be balanced against its effects on pumping efficiency.


Powder handling is a fundamental consideration in the choice between conventional oil-sealed rotary or piston pumps and high performance dry pumps. Oil-sealed pumps use oil to seal the reciprocal or rotary pumping mechanism. The initial cost of an oil-sealed pump is low; however, maintenance costs can mount quickly to generate a high total cost of ownership. Because the powder becomes trapped in the oil, the oil must be changed frequently, as often as every two process cycles, and the pumps must be completely overhauled as frequently as every month. Maintenance downtime reduces productivity, and disposal of the contaminated oil contributes additional cost. An oil cleaner unit is an option; however, it too adds additional cost as a result of the added electrical consumption and maintenance requirements. The relatively low frequency mechanical vibrations generated by these pumps are readily transmitted


to the process chamber where they can impact crystal formation. Rotary pumps also generate considerable noise.


Dry Pumps For these reasons, the industry is moving toward the adoption of dry pumping technologies. The move follows a similar transition made by the semiconductor industry a decade ago. Although their initial cost is higher, dry pumps often demonstrate a lower total cost of ownership, due largely to lower maintenance costs. There is no oil to change or dispose of and periodic maintenance is required infrequently or not at all depending on the pump design. Powder handling remains important. If there is a powder trap mounted before the pump, then powder handling requirements depend on the trap’s efficiency.


Dry pump compressor design is very important. Dry pumps require tight mechanical clearances, making thermal management a primary design consideration. Unless managed effectively, thermal expansion can reduce the clearance between the


Because the powder becomes trapped in the oil, the oil must be changed frequently, as often as every two process cycles, and the pumps must be completely overhauled as frequently as every month


Figure 2: Vacuum side of seal (left) shows significant powder accumulation. Clean surface downstream of the seal (right) demonstrates seal efficiency


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www.solar-pv-management.com Issue IV 2010


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