PROCESSCONTROL
t is impossible to attend any major gathering of the solar energy industry without noticing its fascinatingly ambivalent relationship with semiconductor manufacturers. This is particularly true when meetings of both groups coincide in space and time, such as at InterSolar and Semicon West every summer in San Francisco.
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As a provider of process analysis and control systems to the semiconductor industry, it is clear to us at Rudolph that solar device manufacturers can realize tremendous benefits from applying a similar, yet unique, approach to the control of their manufacturing process.
After the investment to develop a system for solar applications, it is equally clear to us that the differences between the manufacturing processes are just as important as the similarities, and it is not possible to simply apply a software tool developed for one to the other. It is deadly to assume that, just because the solar process involves a fraction of the number of steps, it is less challenging to control than a semiconductor process.
The similarities are found in the processes and materials used by both industries: silicon substrates, metal and insulator film deposition, diffusion, etc. The primary differences lie in the lower requirements in geometric precision of the much larger solar devices, fewer process steps (10-15 for solar vs. several hundred for semiconductor manufacturing), and the much higher volume of wafers that are produced during solar manufacturing. A typical solar fab may start the same number of wafers in one day that a large semiconductor fab starts in a month.
This consideration permeates through the entire process, and can profoundly affect processing decisions at every step. It is a much different decision to scrap a $5 solar wafer than a $5000 semiconductor wafer. Interestingly, it is also a much different decision to scrap one thousand $5 solar wafers than one $5000 semiconductor wafer.
Know your process
Yield, throughput and efficiency are the primary process performance criteria in PV manufacturing. Efficiency refers to the conversion of incident radiation to output power. Small manufacturing process variations can cause large differences in device efficiency. Devices are often binned by efficiency and sold at prices commensurate with performance. A decrease in efficiency, therefore, translates directly to lost revenue.
Given the greater wafer volume, one challenge facing PV manufacturers is deciding “when to jump”, that is, when does a detected excursion in process performance warrant corrective action. Statistical tools exist that can balance costs and benefits of such decisions and recommend action, but they are different from similar processes in lower volume semiconductor manufacturing.
Understanding data
Another challenge confronting solar manufacturers is the difficulty in distinguishing causes rooted in incoming material variations from those resulting from process tool variations. Variations in the quality of incoming silicon substrates used for solar devices have a large impact on overall performance of the cell or device. Considerations such as the position within the ingot from which the wafer originated are entirely foreign to semiconductor processes, but may be significant for solar manufacturing. This is one of the reasons we see more vertically integrated factories, which require analysis engines to be able to look across the entire manufacturing process (casting, wafering, cell, module) for issues and root cause.
Another challenge for solar manufacturers is the perceived cost of metrology. Semiconductor manufacturers are accustomed to metrology data available at virtually every step in the process. For solar manufacturers the perceived cost of metrology systems and concerns about reduced throughput can be daunting. Solar manufacturers are looking to technologies like fault detection classification (FDC) to provide early indications of process problems, predict tool failures, and extend the life of consumables. The perception of cost
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www.solar-pv-management.com Issue V 2010
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