Making solar cells: This *is* your father’s fab

by Phil LoPiccolo, Editor-in-Chief, Solid State Technology

What’s the difference between semiconductor equipment and solar equipment? “A factor of ten, squared,” said T.J. Rodgers, chairman of SunPower Corp. and president and CEO of Cypress Semiconductor, during his Wednesday (July 18) keynote address at SEMICON West. “If you want to sell a solar fab some equipment, it has to be 10x cheaper — for example, $400,000 not $4 million,” he explained, “and it has to be 10x faster — say, 800 wafers per hour, not 80 wafers per hour.”

To illustrate how silicon plants contrast with typical semiconductor fabs, Rodgers offered a glimpse inside SunPower’s solar cell facility in Manila, Philippines, a “mega-monster fab” compared to the scale of semiconductor plants, he said. (SunPower, a former Cypress business, was spun out in 2002.) For example, the facility produces wafers with a peak power capability of 100 million watts/year, which translates to 32 million wafers per year. In contrast, a large semiconductor plant produces about 1 million wafers per year, he explained. Moreover, given that solar cells require about 8 grams of silicon per watt, at 100MW, the plant consumes 800 tons of silicon, which is nearly 15 tons per week or some 2 tons per day.

To accommodate the high-volume throughput, the solar facility is a continuous-flow operation that runs “more like a razor-blade plant than a semiconductor fab,” Rodgers said. The silicon flows in a continuous stream (24 hours a day/seven days a week), with several wafers moving along side-by-side on tracks at the rate of 3600 wafers/hour, the equivalent of one wafer being produced every second.

Given the rapid flow, if any machine goes down the line will typically shut down within an hour, Rodgers noted. Therefore, another prerequisite for solar equipment is a minimum uptime requirement of ~99.8%. “If your machine were capable of being up and running only 90% of the time, I couldn’t afford to put it on my line, even if you gave to me for free,” he said, “because I’d lose the entire value of your machine within a few weeks.”

In terms of the individual processes used to fabricate the solar cells — e.g., lithography, diffusion, etching, metallization, etc. — the requirements are far less stringent in solar cell plants than in present-day semi fabs. “Our plant looks a semi fab from 1985, because it has the sophistication of a fab from 1985,” Rodgers said. For example, in lithography, the design rules are currently 100 microns. “We’d like to get down to about 30 microns,” he said, “but we have no use for going below that.”

The diffusion process is also old school. “We have giant racks of tubes loaded with thousand-wafer lots,” Rodgers said. “If anyone wanted to make a continuous machine that dopes wafers, our industry would really appreciate it,” he said. As long as it costs <$400,000, runs >800 wafers/hour, and has virtually no downtime. — P.L.

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