The volume required can be huge. In the U.S., chip fabs use far less water than agriculture and the power generation industry, and semiconductors have not fueled political tensions over water resources nationally, says Chris Miller, a history professor at Tufts University in Massachusetts and author of the recent study book chip war. Still, there was concern about shortages in TSMC’s native Taiwan, where droughts have turned local farmers, whose irrigation systems have been shut down, against the chipmaker.
Not just any water will do. Just as the air in a chip factory needs to be so dust-free that people have to wear full hazmat suits, the semiconductor industry uses a special category of “ultra-pure” water to clean silicon wafers throughout the manufacturing process. While regular drinking water can have a purity of 100 to 800 microsiemens per centimeter — a measure of electrical conductivity that serves as an indicator of contamination — ultrapure water has less than 0.055 microsiemens, according to Gradiant, a Boston-based water recycling startup per centimeter This works with chip manufacturers. Ultrapure water must have an extremely low conductivity, which corresponds to only a small number of interfering ions or charged atoms.
“Very often, if you want to get the highest possible performance from the material, you have to go to extreme purity,” says Grace Xing, a Cornell University professor of electrical and computer engineering who also directs a new inter-university semiconductor research center HIGHEST. “That’s one of the reasons why the semiconductor industry uses a lot of water.”
Producing ultrapure water is a multi-step process that removes a wide variety of contaminants, including microbes and other microscopic creatures you might find in oceans and lakes, as well as smaller particles, including even salt ions. One technique used is reverse osmosis, which is also used in desalination plants. Water is pushed through a membrane whose pores are small enough to filter out salts. (Chip factories also use less pure water, similar to that flowing from household faucets, to cool production facilities.)
Given the critical role of water in chip manufacturing, the recovery and reuse of wastewater has become a priority for the industry. The more of it that can be reused in a factory, the less need there is to tap the local water supply. Currently, the percentage of wastewater that can be recycled varies by company and factory, depending on the manufacturing processes used and investments in water treatment. However, they all face the same basic problem: when cleaning the wafers, ultrapure water becomes contaminated and must be thoroughly cleaned before it can be reused in a factory or discharged into a public wastewater treatment system.
Cleaning the polluted water is a complicated process as there are countless pollutants in the factory effluent. Lithography and etching can produce acidic effluent that can be contaminated even with strong hydrofluoric acid. Airborne silicon particles can become visible when thinning the wafers, while the use of solvents such as isopropyl alcohol can leave organic carbon residues.
“Industry has developed methods to separate different components of this 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 progressive companies, the American multinationals that we work with – but also the Korean and Taiwanese companies that we work with – all separate their wastewater in at least more than 10 ways, some of them 15 or 16.”