Recycling Spent Ultrapure Rinse Water--A Case Study in the Use of a Financial Analysis Tool.
This paper profiles the cost and the profitability potential associated with recycling and reuse of ultrapure rinse waters left over from semiconductor wafer-rinsing processes. An environmental health and safety financial software tool was used to a) account for costs, b) estimate their financial impact, and c) compare the profitability potential of recycling and nonrecycling options. The financial analysis was designed to help environmental health and safety specialists enhance the ways they present the economic soundness of spent-rinse-water recycling to fabrication managers. It was found that the financial software tool provided a flexible, low-maintenance, decision-oriented approach to understanding the full cost of owning water-recycling systems. A financial-analysis case study looked at a typical fabrication facility that processed 480,000 200-millimeter wafers annually, used 500 gallons per minute of ultrapure water, and recycled at 50 percent. Both direct and indirect costs were calculated. The financ ial analysis showed that the annual incremental cost difference between recycling ($2.03 million) and not recycling ($2.85 million) is approximately $823,878. This annual cost reduction, taken over the 15-year estimated productive/economic life of the system and discounted at 15 percent, with a combined tax rate of 40 percent and a five-year depreciation, results in a net present value of $2,577,212.
In the field of semiconductor wafer manufacturing, rapid industry growth, increases in wafer size and process steps, and new technologies that require higher water purity indicate a trend toward higher water usage per wafer. It is estimated that future fabrication facilities will use one to three million gallons of ultrapure water per day (1). Because water is lost during the purification process, the actual demand on the municipal feed-water supply will be approximately 25 percent greater than the amount of ultrapure water required. As a result, the consumption of water and the emission of waterborne pollutants are under close observation (2). The National Technology Roadmap for Semiconductors has identified targets through 2012 for decreasing the net use of feed water and ultrapure water use, as well as for lowering water purification costs. The target reductions for 1997 are based on actual industry data. Reduction requirements beyond 1997 are best estimates derived by consensus of a technical working grou p (Table 1).
Currently, a combination of strategies is being pursued, including higher-efficiency rinse processes, recycling of higher-quality water from process applications, and reuse of lower-quality water for nonprocess applications. The strategies outlined below are expected to be the ones design/process engineers and environment, safety, and health specialists use to successfully design and operate future water systems with recycling capabilities.
The primary strategies for decreasing net feed-water use are to
* conserve cooling, scrubbing, and washing water;
* develop water-balance modeling techniques;
* develop impurity-mapping techniques; and
* develop cost-optimization modeling techniques.
New strategies for decreasing ultrapure water use are to
* use more efficient rinse processes,
* lower idle flow when tools are not in use, and
* use intelligent and reliable hardware and software for control.
New strategies for lowering water purification costs are to
* develop more efficient separation methods with lower energy and chemical use;
* provide for high-efficiency ionic impurity removal;
* reliably remove all organic substances;
* match purity level to purity need; and
* combine and optimize reuse strategies according to feed-water quality, site conditions, and process requirements.
Since most of the ultrapure water is used for wafer-rinsing purposes, a financial analysis would profile the life cycle cost and profitability potential associated with recycling spent waters from semiconductor-wafer-rinsing processes. Recycling involves segregating, collecting, and monitoring spent rinse waters; removing any organic compounds that have been introduced into the spent rinse waters by the wafer fabrication process; and returning this water to the inlet of the ultrapure water. Figure 1 shows the main components of a typical ultrapure water system that incorporates recycling.
Before fabrication managers will allocate the capital and noncapital resources necessary for implementing such a strategy, they must be convinced that exposures to impurity risks (i.e., manufacturing-interruption incidents) are controlled. They also must see a positive benefit-cost relationship (i.e., improved water quality and lower water costs).
The case study reported here was designed to help environmental health and safety specialists, as well as design and process engineers, enhance the way they present the economic soundness of spent-rinse-water recycling to fabrication managers. This paper does not characterize risk burdens associated with recycling spent rinse waters, nor does it model the contingent liability (i.e., probability estimate of a contamination incident and expected range of costs) associated with a manufacturing shutdown. Exposures to anticipated risks are addressed qualitatively. A quantitative analysis of risk burdens and contingent liability would be an excellent undertaking for a future study.
Review of Literature
Studies concerned with profiling the life cycle cost and profitability potential associated with water recovery, recycling, and reuse efforts were bound to be nonexistent. A number of articles addressed the financial benefit of water-recycling efforts (1,3-5). None of the studies, however, used financial modeling tools to reasonably estimate the full range of costs and profitability potential.
An activity-based life cycle cost framework incorporating present-value financial analysis served as the architecture guiding the financial-analysis case study. A template listing the life cycle phases of spent-rinse-water recycling was used to categorize cost factors and related activities. Definitions of the phases appear in the sidebar on page 19.
There were three reasons for the emphasis on activity-based costing:
1. It is a useful and proven method for assigning costs (6-8).
2. Life cycle analysis is a useful and proven method for inventorying resource inputs and outputs (9,10).
3. Net-present-value analysis provides the most reliable method of comparing the financial performance of mutually exclusive alternatives (11).
To conduct the case study, the authors examined a typical facility manufacturing 200-millimeter (mm) wafers, processing 480,000 wafers annually, using 500 gallons per minute of ultrapure water for wafer-rinsing purposes, and recycling spent rinse water at a rate of 50 percent. This percentage gave the optimum ratio found in research conducted at the Texas Instruments DP1/DMOS5 Wafer Fabrication Facility located in Dallas, Texas (12). The authors then used an environmental health and safety financial-modeling software tool to compare the life cycle cost and profitability potential of ultrapure-water recycling and nonrecycling options (13).
The financial analysis was performed under the assumption that a rate of 500 gallons per minute of ultrapure water is needed throughout the year for wafer rinsing. The efficiency of a typical water purification system is approximately 75 percent, which implies that 666 gallons per minute of feed water must be purchased to supply 500 gallons per minute of ultrapure water. The cost of feed water is $1.00 per 1,000 gallons, and municipal sewage costs $1.50 per 1,000 gallons. The recycling equipment has an estimated-efficiency rating of 50 percent, which implies that out of the 500 gallons per minute of ultrapure water used in wafer rinsing, only 250 gallons can be recycled--and combined with 470 gallons of new feed water. Even after the switch from a nonrecycle strategy to a recycle strategy, therefore, there is still a requirement for purification of new feed water. The numbers in Figure 1 show the flow rates (in gallons per minute) of municipal feed water, ultrapure water, and spent rinse water through the ult rapure-water-processing, wafer-rinsing, and spent-rinse-water recycling processes. For this study, the recycling system includes monitoring of spent-rinse-water quality to determine whether the water is suitable to be used as feed water for the ultrapure water system, a diverter valve that sends unsuitable water to the industrial wastewater treatment facility, and a dedicated treatment system that removes organic compounds from the spent rinse water with ultraviolet light and hydrogen peroxide.
To complete the financial analysis, including the determination of life cycle cost and profitability potential associated with a spent-rinse-water recycling system, the study used the financial default values given in Table 2 and the cost categories given in Table 3. Default values and cost categories are reasonable estimates based on data derived from semiconductor fabrication facilities.
Table 4 compares the estimated life cycle costs and profitability potential associated with recycle and nonrecycle strategies. Of chief importance are the annual overall cost savings gained from a recycle strategy ($823,878). These savings are due primarily to significant reductions in costs associated with waste and utilities. Thus, the cost savings and profitability potential of implementing a recycle strategy can be substantial. In this study, both direct and indirect benefits were identified.
The annual incremental cost difference between the nonrecycle strategy ($2,857,512, or $5.95 per wafer) and the recycle strategy ($2,033,634, or $4.30 per wafer) is $823,878, or $1.65 per wafer. Taken over the 15-year estimated productive/economic life of the system, the annual cost reduction results in a net present value of $2,577,212. This amount is basically the difference between the discounted present value of benefits and the discounted present value of costs-or the value today of an amount to be received later, discounted at a rate of 15 percent. Thus, the recycle strategy should be considered highly acceptable from an investment standpoint.
Further review of the data shows an incremental annual cost savings of $197,100 in waste disposal alone. That figure, however, only tells part of the story For instance, if waste disposal costs were to increase at a rate of 10 percent over the life of the nonrecycle strategy, the after-tax present value of these costs would increase from $1,844,027 to $3,069,331. Under the recycle strategy, they would increase from $1,152,517 to $1,918,332. The most significant aspect of the escalation in waste disposal costs, how- ever, would be that the net present value of overall cost would increase by $3,036,701. Clearly the power in the cost-modeling tool lies in calculating the impact that escalations in certain cost factors would have on overall cost and profitability potential.
The following indirect benefits are associated with recycling spent rinse waters:
* improved final ultrapure water quality (Table 5);
* improved reliability of the ultrapure water facility;
* less downtime and reductions in the frequency of reverse-osmosis membrane cleaning;
* less ion-exchange regeneration, fewer filter backwashes and rinses, less treatment processing; and
* less demand on the municipal water supply and wastewater treatment systems (1).
There are, however, risks associated with the recycling of spent rinse water back into the ultrapure water facility As with any feedwater source, a significant change in quality may significantly affect the quality of ultrapure water. Nevertheless, recycling of the spent rinse water, when it replaces incoming feed water from municipal systems, frequently improves the consistency of the overall feed water. Many municipalities provide water to manufacturing sites from various sources, such as surface water and well water. As communities strive to keep up with community growth, they often find it necessary to rely on a variety of reservoirs, replacing water from one source with water from another to keep up with demand. They may also alter chemical pretreatment steps. In some cases, the quality of water received from a municipality changes significantly with seasonal and temperature changes. A number of manufacturing interruption incidents have, even recently forced shutdowns of wafer fabrication facilities beca use of changes in the quality of the municipal water supplied to an ultrapure water facility Municipalities are not required to inform water users of these changes and, in fact, must comply with no quality specifications other than drinking-water standards. Incoming feed water is one of the few resources admitted to manufacturing sites without rigorous quality verification.
Through recycling of spent rinse water, much of this inconsistency can be minimized because the quality of ultrapure water is known and the chemistry used in wafer processing also is known. The data in Table 5 show that the quality of recycled spent rinse water is far superior to that of the typical municipal water supply Dilution of incoming municipal water with higher-quality recycled rinse water thus can significantly reduce many of the risks associated with feed-water incidents.
What is not well known, however, is how well processes typically used in ultrapure water facilities remove compounds that are generated in wafer processing and that are not normally monitored in incoming municipal feed water. Nor is it known what types of chemical reactions occur among these process-generated impurities. DeGenova and Shadman found that the risks associated with ultrapure-water recycling include
* the introduction of new impurity spikes into the system,
* the buildup of recalcitrant compounds,
* inadequacy of the present purification methods in removing process-generated contaminants,
* risk of new chemical interactions caused by recycling, and
* contamination from biofouling (1).
Nevertheless, DeGenova and Shadman report that in the six years that the Texas Instruments DP1/DMOS5 recycling system has been in operation, there have been very few instances in which the spent rinse water was diverted to industrial waste because of impurity spikes. (The facility reported eight minor diversions of approximately five minutes each.) Also, since the facility is continuing to recycle spent rinse water, one can assume that the recycling process has had no detrimental effect on product quality. In addition, it should be pointed out that facilities outside of the United States, particularly in Asia Pacific countries, have been recycling spent rinse water for over 20 years. In these countries, the cost of municipal water and waste effluent treatment is typically much higher than in North America, and regulations restrict the discharge of industrial waste water.
Typically, recycling strategies in the United States have been implemented only when additional resources are not readily available. Recycling has been perceived as a way of obtaining second-hand resources whose quality must be inferior to that of materials that come from the original source. Because the semiconductor industry has been associated with ultrapure, ultraclean recycling of a resource that has long been thought of as waste (spent rinse waters), it offers a paradigm shift, which may well be met with apprehension. The fear is that the recycled water will be of inferior quality When, however, the quality of ultrapurified water recycled from wafer rinsing is compared with the quality of typical municipal water supplies, it becomes apparent that the opposite is true. In fact, DeGenova and Shadman found that most of the spent rinse water is not equal to incoming municipal water but far superior in all respects (1). Recycling is not, therefore, a compromise. Instead, it can lead to significant benefits, including better, cleaner ultrapure water for wafer processes. The financial analysis model presented in this paper will, it is hoped, help semiconductor process engineers, financial experts, and environmental health and safety specialists demonstrate the soundness of environmental investments to organizational stakeholders.
Corresponding Author: Anthony Veltri, Associate Professor, Environmental Health and Safety Oregon State University, Waldo Hall, Room 308, Corvallis, OR 97331.
(1.) DeGenova, J., and F. Shadman (1997), "Recovery, Reuse, and Recycle in Semiconductor Wafer Fabrication Facilities," Environmental Progress, 16(4):263-267.
(2.) Van Gompel, J., and V. Chidgopkar (1999), "Reducing Water Use in Exhaust Management Systems," MICRO, 17(4):67-79.
(3.) Martyak, J. (1999), "Designing Practical DI-Water Recycling Systems for Use in Semiconductor Fabs," MICRO, 17(1):41-47.
(4.) Peters, L. (1998), "Ultrapure Water: Rewards of Recycling," Semiconductor International, 21(2):71-76.
(5.) Roche, T., and T. Peterson (1996), "Reducing DI Water Use," Solid State Technology, 39(12):78-82.
(6.) Cooper, R., and R. Kaplan (1988), "Measure Costs Right: Make the Right Decisions," Harvard Business Review, 66(5):99-103.
(7.) Compton, T. (1994), "Using Activity Based-Costing in Your Organization--Part 1," Journal of Systems Engineering, 45(3):32-40.
(8.) Compton, T. (1994), "Using Activity Based-Costing in Your Organization--Part 2," Journal of Systems Engineering, 45(4):36-42.
(9.) A Technical Framework for Life Cycle Assessments (1991), Washington D.C.: Society of Environmental Toxicology and Chemistry.
(10.) Graedel, T., B. Allenby, and R. Comrie (1995), "Matrix Approaches to Abridged Life-Cycle Assessments," Environmental Science and Technology, 29(3): 134-140.
(11.) Newman, D. (1983), Engineering Economic Analysis, San Jose, Calif.: Engineering Press.
(12.) Weber, T., J. DeGenova, and J. Joiner (1999), "Deionized Water Recycling Results and Benefits from a Case Study at Texas Instruments' DMOS 5 Facility," Future Fab, Vol., 8(5):1-10.
(13.) Veltri, A. (1997), Environment, Safety and Health Cost Modeling, Technology Transfer Report #97093350A-ENG, Austin, Tex.: SEMATECH, Inc.
Life Cycle Phases of Spent Rinse Water Recycling
Phase I--Up front. This phase profiles the risk (e.g., business interruption resulting from impurities) and cost burdens associated with a spent-rinse-water recycling system over its productive/economic life. A balance is maintained between water usage priorities and competing business performance factors. Costs associated with up-front activities are considered one-time costs.
Phase II--Acquisition. This phase involves obtaining wastewater or chemical-use permits and procuring capital equipment for the water-recycling system that will reduce water usage and control exposure to impurity spikes. Costs associated with acquisition activities are considered one-time costs.
Phase III--Use disposal. This phase is concerned with protecting and productively using and disposing of spent rinse waters and ancillary water purification chemicals in a manner that prevents injury, illness, and environmental incidents and that reduces pollution and waste. Costs associated with use and disposal activities are considered annual costs.
Phase IV--Postdisposal. This phase monitors waterborne pollutants and other waste after the waste is no longer under the control of the manufacturing process, has left the internal factory site, and has been transferred to another company for management. Costs associated with postdisposal activities are considered annual costs.
Phase V--Closure. This phase retires the water-recycling system at the end of its useful/economic life and prepares the area for other productive uses. Costs incurred during the closure process are considered one-time costs.
A final cost category, Incident Area, comprises costs incurred as a result of an impurity spike incident that adversely affects the manufacturing process, internal factory, or external environment. Included are costs resulting from incidents of contamination, pollution, alteration, occupational injury or illness, and noncompliance fines associated with use of ancillary water purification chemicals.
* In the field of semiconductor wafer manufacturing, rapid industry growth, increases in wafer size, and new technologies will increase water usage.
* It is estimated that future fabrication facilities will use one to three million gallons of ultrapure water per day.
* Because water is lost during the purification process, the actual demand on the municipal feed-water supply will be approximately 25 percent greater.
* As a result, the consumption of water and the emission of waterborne pollutants are under close observation.
* The National Technology Road Map for Semiconductors has identified targets through 2012 for decreasing the net use of feed water, as well as for lowering water purification costs.
* Recycling of water used to rinse wafers will have a prominent place in these reductions.
* Typically recycling strategies in the United States have been implemented only when additional resources are not readily available.
* Recycling has been perceived as a way of obtaining second-hand resources whose quality must be inferior.
* The semiconductor industry offers a paradigm shift.
* When the quality of ultrapurified water recycled from wafer rinsing is compared with the quality of typical municipal water supplies, it becomes apparent that most of the spent rinse water is superior.
* Recycling is not, therefore, a compromise. Because recycled water is purer," manufacturers save on purification costs.
* Recycling also cuts disposal costs.
* With the use of a financial analysis tool, environmental health and safety specialists can demonstrate the potential savings to factory managers and encourage recycling.
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|Author:||Airth, Gerald L.|
|Publication:||Journal of Environmental Health|
|Date:||Nov 1, 2000|
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