Third in a series: How semiconductor manufacturing impacts water supplies and quality.
After energy (discussed in Part 2 of this series), water is the largest fab input and the largest contributor to fab waste. Yet tools for analyzing a fab’s water footprint are generally less mature than tools for analyzing emissions of CO2 and other greenhouse gases.
In part, this may be because water consumption is primarily a local issue, while greenhouse gas emissions are a global concern. The cleanliness of fab wastewater may be subject to national-level regulations, but different communities have different climates and different water resources. Regions with arid climates will necessarily manage water resources differently from regions with abundant rainfall.
Water is also relatively easy to recycle. After treatment, process water is often reused in fab chillers, and then used again for landscape irrigation. TSMC claims that every drop of water drawn from the municipal supply is re-used 3.5 times. Net water consumption at most fabs is far less than the sum of all in-fab water uses.
Helping fab neighbors with water recycling
As a step toward comprehensive analysis, Tom Cooper and Joyann Pafumi at Intel noted that water consumption, like energy consumption, can be attributed to three primary causes. Scope 1 consumption, direct use by the fab, includes all water drawn from outside sources — whether wells or a municipal supply — for any purpose, from process water to employee hygiene. Water used in chillers, irrigation, and so on can be conserved by applying the same principles used in other industries. Landscaping with plants native to the local climate reduces the need for irrigation. Systems that circulate water for heating and cooling should match their capacity to the volume being heated or cooled.
Process water consumption, on the other hand, is largely “built-in” to the fab and process design. The dimensions of wet bench tanks are defined by wafer and batch size and by the need for uniform flow. Industry-wide water conservation efforts have driven the adoption of more water-efficient wet benches, reducing water consumption per wafer. On a larger scale, the same process trends that tend to increase energy use also tend to reduce water consumption. Over the last few process nodes, large batch wet processing steps have given way to plasma-based cleaning and etch steps with single-wafer spray rinsing and residue removal.
On the other hand, increasing use of CMP and multi-step lithography processes tends to increase the water used per device layer. One ISMI study estimated that CMP may account for half of process water consumption. Thus, measures such as reducing water flows during tool rinsing and reusing water lost during tool idle periods can bring significant savings. As a group, wet process steps may produce more or less toxic wastes, depending on the process chemistry. Toxicity, in turn, determines the extent to which the water can be re-used and how much treatment is required.
Here, too, local conditions can have a significant impact on the water treatment required and the best way to handle waste water. For example, the municipal water supplied to Intel’s Chandler, AZ, plant has high levels of dissolved minerals, requiring additional purification before being used in the fab. Purification generates large volumes of toxic brine that cannot be released to either the sanitary sewer or directly into the groundwater. To manage this brine more effectively, Intel worked with the City of Chandler to upgrade the City’s reverse osmosis water purification facility. Intel’s existing brine concentrator was reinstalled at the City facility and helped to improve water reclamation there. This facility, according to Todd Brady, Intel’s director of global sustainability, cleans waste water from the Chandler plant to drinking water standards, allowing it to be injected back into the local aquifer. Intel also uses a portion of Chandler’s gray water for scrubbers, cooling towers, and landscape irrigation.
Energy use drives water use
Scope 2, water use associated with purchased energy, can account for a surprisingly large share of the fab’s water footprint, depending on the energy source. Any form of steam turbine electricity generation consumes water, though much of it is re-used. Coal-fired generation, however, also uses water to rinse away coal ash and other residues. Oil and natural gas extraction can consume substantial water, as well. In both cases, toxic contaminants can prevent any further use of the water. But even “green” fuels are not entirely benign. For example, bio-fuels derived from purpose-grown crops — such as corn grown for ethanol production or soybeans grown for oil — may require substantial irrigation water. Indeed, Intel, found that Scope 2 consumption accounted for about 28% of its overall water footprint. By using renewable energy and auditing suppliers to ensure that only agricultural wastes and other by-products were used for fuel production, the company reduced Scope 2 water use by 60% relative to the U.S. standard energy distribution. Without those efforts, it estimates that Scope 2 water use would have accounted for more than half of the total, exceeding even direct use by the company’s fabs.
Enabling water savings
Scope 3, water consumption associated with the supply chain, is another potentially large contributor to a fab’s overall water footprint. Here, analysts face a severe lack of accurate data. How much water does production of fab chemicals use? How does that number change as purity increases? Are electronics grade chemicals responsible for a disproportionate share of chemical industry water consumption?
Just as integrated circuit technology can enable energy conservation, it can help optimize water use, both in the fab and outside of it. Better analysis of fab wastewater can help facilitate water reuse and identify opportunities for conservation. Better water purification technologies can both reduce waste from the fab’s ultra-pure water plant and allow treated fab wastewater to return to the public water supply. Better short and long-term forecasting can help fab managers and municipal authorities balance fab water use against other water users, and can inform location planning. As with energy, though, the sheer scale of the semiconductor industry demands proportionate attention to the needs of other water users.
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