The volume required can be huge. In the U.S., chip factories use far less water than agriculture and power generation industries, and semiconductors have not sparked political tension over water resources on a national scale, said Chris Miller, a history professor at Tufts University in Massachusetts and author of the recent book Chip war. Still, the issues have been a concern in Taiwan, home of TSMC, where droughts have turned local farmers, who have seen their irrigation systems shut down, against the chip maker.
Not just water is enough. Just as the air in a chip factory must be so dust-free that people must wear full-length overalls, the semiconductor industry uses a special category of “ultra-pure” water to clean silicon wafers during the manufacturing process. While standard drinking water can have a purity of 100 to 800 microsiemens per centimeter — a measure of electrical conductivity used as an indicator of contamination — ultrapure water has less than 0.055 microsiemens per centimeter, according to Gradiant, a water recycling start-up in Boston. that works with chipmakers. Ultrapure water should have extremely low conductivity, equivalent to only a small number of troublesome ions or charged atoms.
“If you want to get the highest possible performance from the material, you have to go to extreme purity very often,” said Grace Xing, Cornell electrical and computer engineering professor, who also directs a new inter-university semiconductor research center called SUPREME. “That’s one of the reasons why the semiconductor industry needs a lot of water.”
Producing ultrapure water is a multi-step process that removes a variety of contaminants, including microbes and other microscopic creatures found in oceans and lakes, as well as smaller particles, including even salt ions. One technique used is reverse osmosis, also used in desalination plants, where water is forced through a membrane with pores small enough to filter out salts. (Chip factories also use less pure water, similar to the water that flows from household faucets, to cool production equipment.)
Given the crucial role of water in chip production, recovering and reusing wastewater has become a priority for the industry. The more that can be reused within a fab, the less it is necessary to tap into the local water supply. Currently, the share of wastewater that can be recycled varies between companies and factories, depending on the production processes used and the investment in water treatment. Yet they all face the same fundamental problem: When wafers are cleaned, ultrapure water becomes contaminated and must be thoroughly cleaned before it can be reused by a factory or discharged into a public wastewater treatment system.
Cleaning up the polluted water is a complicated process as countless contaminants can be found in fantastic wastewater. Lithography and etching can produce acidic wastewater and can even contaminate it with powerful hydrofluoric acid. Suspended silicon particles can appear when wafers are thinned, while the use of solvents, including isopropyl alcohol, can leave organic carbon residue.
The industry has developed ways to separate different components of that wastewater, similar to how the general population sorts recycling, says Prakash Govindan, co-founder and COO of Gradiant. “The semiconductor industry is actually very advanced when it comes to dealing with wastewater,” he says. “The advanced companies, the US multinationals we work with, as well as the Korean and Taiwanese companies we work with, all separate their wastewater into at least more than 10 types, and some into 15 or 16.”
