IBM Corp.'s electronic chip-making plant in Burlington, Vt., used to destroy its leftover silicon wafers. Now the plant is turning them into clean solar-generated electricity. Engineers at the plant have devised a process that scrubs used wafers clean, without the use of corrosive chemicals. The cleaned wafers are reused multiple times at the Burlington plant, then resold to companies that use the same kind of silicon to make solar power cells.
"It's clean, it's easy, and it works great," said Eric White, an IBM engineer who helped develop the process. IBM plans to share its technology with other chip makers. The company estimates that up to 3 million wafers discarded worldwide each year could be recycled using the process.
"This is good news, if indeed this is proven and it's replicable," said Paula Mints, associate director at Navigant Consulting Inc. of Chicago, an energy consulting firm. Mints said prices are rising for silicon used in solar cells, and the industry will welcome a way to make supplies go further.
Electricity made with solar cells is used worldwide to run everything from street lamps to factories. According to research from Solarbuzz LLC, the solar cell industry produced enough cells last year to generate 2,200 megawatts of power - about as much as two large coal-fired power plants.
Electronic chips are carved from circular wafers of ultra-pure silicon. Each wafer produces hundreds of finished chips, each one etched with complex electronic circuits. But the manufacturing process is long and complex. Sometimes etching defects will ruin a wafer. In addition, chip makers use "monitor" wafers to test various manufacturing processes. These monitor wafers will never be used to make finished chips. Still, they're etched with circuit designs belonging to the chip company or to customers who've ordered the chips specially made. An engineer from a rival firm could learn valuable trade secrets by putting the etched wafers under a microscope. So the etchings on defective or monitor wafers must be destroyed.
Until recently, IBM simply ground up the silicon wafers and had them dumped in a landfill. Though the silicon is nontoxic, it's costly, due to soaring demand for solar cells. Other companies avoid discarding the valuable silicon by scrubbing off the circuit etchings. The wafers can then be reused at the chip factory, or sold to makers of solar power cells. But until recently, de-etching a wafer required toxic chemicals such as nitric acid and sulfuric acid.
IBM engineers came up with a cleaner way, using water and a specially designed abrasive pad that uses friction rather than harsh chemicals. White said many of his colleagues were skeptical at first. "They didn't think it would work," said White. "I didn't, until we tried it."
Wafers can be reused six or seven times, until the abrasive scrubbing process wears them too thin. With monitor wafers costing about $42 each, getting extra uses from each one will save IBM a good deal of money. White estimated the process will let IBM cut its purchases of new monitor wafers by 25 percent, saving about $1.5 million a year. And the abandoned wafers, instead of being discarded, can be sold.
ReneSola, a Chinese solar cell maker, has been using the recycled silicon from Burlington for about six months. "These pretreated scrap wafers must be further etched and cleaned to remove all possible contamination before they are remelted to form solar ingot," said ReneSola executive vice president Paul Li.
"They can buy the wafers that we're done with for a quarter of what new wafers cost," said White. Besides, making new silicon for solar cells requires huge amounts of energy. Creating the cells on recycled silicon makes solar power even more energy-efficient.
Hiawatha Bray can be reached at bray@globe.com.
