COVERSTORY
not yet common in the silicon crystal manufacturing industry, performance monitoring has the potential to provide similar benefits.
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Figure 3: GXS160/1750 rotors with the exhaust end on the left hand side demonstrates successful result of running the pump on a CZ puller application with no inlet filter
stator and the rotor causing seizure. This consideration becomes even more significant with high powder loads or when pumping argon because of its poor heat transfer characteristics.
Environmental Concerns
Modern dry pumps are designed for optimal energy efficiency, reducing operational costs and environmental impact (carbon footprint). They typically use high speed screws or other types of pumping mechanisms that are quieter acoustically and have less vibration. Advanced thermal design reduces the volume of water used, when and if water cooling is required. Dry pumps have smaller footprints, optimizing the utilization of expensive fab floor space.
Rotating seals are used within the dry pump to prevent contamination of the vacuum space with lubricating oil from the gearbox and drive components. If oil enters the vacuum space, it can mix with powder in the pump and make a paste that can cause subsequent pump failure. Likewise, the seals prevent powder from entering the gearbox. Powder that enters the gearbox can then contaminate the bearings and cause premature failure, especially since the powder in this application is silicon, a very hard material.
In the semiconductor and flat panel manufacturing industry, where dry pumps are used almost exclusively, it is very common to remotely monitor pump operating parameters, such as temperature, input power, input current, and rotational frequency, in order to watch for trends that may suggest pump maintenance may be required. Automated monitoring and data logging has been used to help plan pump maintenance. Although
Although many pumps may address the concerns raised in this paper, an example dry pump from Edwards is used for illustrative purposes. The pump incorporates the features required for low cost of wafer manufacturing, high reliability, and has excellent environmental features: noise less than 64 dB (A) without a silencer and only a 0.43 m2 footprint. Elimination of the silencer improves reliability since the silencer can act as an unwanted trap of power. Shaft seal efficacy was confirmed on this pump that was used on a CZ puller without an inlet dust filter. Figure 2 shows how the shaft seals have prevented dust entering the gearbox, and also prevented oil from entering the vacuum space. Such high quality seals lead to long service life and, consequently, lower running costs. The rotors of the pump shown in Figure 3 demonstrate how powder was transported from the inlet to the outlet effectively. The GXS screw dry pump has an integral booster pump that can handle 120 slm of argon at 10 Torr, uses only 4 slm of seal purge, and has an ultimate power of 4.3 kW. With a repeatable powder handling capability of 750 g, many of the vacuum considerations discussed in this article are favorably addressed. In addition, an on-board web server and supported serial communication protocol provides the facility with pump operating parameter measurement in real time.
Conclusion
Photovoltaic device manufacturers continue their quest to achieve cost parity with other energy production technologies. With the likely prospect that crystalline silicon technologies will continue to dominate for some time, the industry must look closely at the cost of the wafer manufacturing processes.
Choices made in the design of the vacuum system and choice of pumps used in these processes can have a significant effect on profit. It is important that the industry consider the total cost of ownership as the primary criterion for these choices, including environmental costs. On this basis, dry pump technologies are likely to replace oil-sealed pumps in most applications.
www.solar-pv-management.com Issue IV 2010
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