Green and Clean

Reducing water use in exhaust management systems

Joseph Van Gompel and Vikrant Chidgopkar, BOC Edwards

The semiconductor industry has been growing rapidly for several years, and this trend gives every indication of continuing. As a result, the consumption of resources and the emission of both air- and waterborne pollutants are under close observation. In order to remain even close to current levels, the consumption of resources per wafer (more specifically, per square inch of silicon) must be reduced. To this end, the SIA's National Technology Roadmap for Semiconductors (NTRS) lists targets for water, energy, and perfluorocarbon (PFC) emissions through 2012.¹ The SIA's 1997 figures, listed in Table I, are identical to those in the 1998 update of the roadmap. These targets include the reduction of water use from 30 to 2 gal/sq in. of silicon by 2009. While much of the water consumption in the semiconductor industry comes from wet benches and, increasingly, from CMP processes, exhaust gas scrubbers are by no means insignificant contributors.

Table I: Targets for water and energy use listed in the National Technology Roadmap for Semiconductors (1997 edition).

Water Consumption in a Typical Fab

A typical 200-mm wafer fab processing 40,000 wafers per month uses between 2 and 3 million gallons of water per day.² Approximately 70% of this water is used to produce ultra-high-purity water (UHPW), which is part of the wafer-cleaning process. Much of this water is either recycled or reclaimed for use in other areas of the fab. Approximately 20% of the water is used for other, non-UHPW, processes, such as cooling towers and heat exchangers. The remaining 10% is used for non-process-related parts of the fab, such as lawns, bathrooms, and sprinkler systems. Water can be drawn from aquifers, city water supplies, recycle streams, or other processes within the fab.

Because of state regulations and economic pressures, there is a general push to reduce raw water consumption in fabs. Changes in equipment design, such as the introduction of wet benches and reverse osmosis (RO), and the recycling and reclaiming of spent water allow for maximum cost savings without sacrificing water quality (and thus wafer yield). Depending on water quality, the effluent water from wet benches is recycled,
reclaimed, or discharged as industrial wastewater (IWW). Water that is reused to produce UHPW is known as recycled, and water that is used in scrubbers or cooling towers is known as reclaimed. Water that cannot be recycled or reclaimed is discharged as IWW. Water used in processes such as CMP must undergo significant treatment before being discharged as IWW.^3,4 Water containing contaminants from wafer-cleaning processes is especially difficult to segregate into recycle, reclaim, and IWW. For every 100 gallons of raw water entering the fab, approximately 21 gallons are recycled.

In sum, spent or used water is divided into recycle, reclaim, hazardous IWW, and nonhazardous water streams. Recycled water is used in the UHPW system and reclaimed water is used in cooling towers and scrubbers. Hazardous wastewater is discharged as IWW and nonhazardous water is discharged into the city sewer system.

Point-of-use (POU) abatement devices often use water for the cooling or scrubbing of particulates and acid gases from vacuum pumps before the gases are exhausted to the fab's ductwork. POU scrubbing is often performed in addition to rooftop scrubbing to remove corrosives such as HCl and HF (substances whose emission is controlled by the Environmental Protection Agency). POU scrubbing also removes reactive compounds such as BCl^₃, WF^₆, and DCS, which react with water to cause the formation of solids and thus lead to blockages. The water used in POU scrubbers can be from a range of sources. Invariably it is sent to either the acid drain (IWW treatment) or to the fab's fluoride treatment facility. The acid drain typically has a large settling tank in which either sulfuric acid or sodium hydroxide is added to achieve a neutral pH. Fluoride treatment typically uses calcium hydroxide to neutralize HF and to precipitate out calcium fluoride. Fluoride treatment has become increasingly important as more municipal sewer systems have adopted stringent maximum fluoride levels that are frequently below 20 ppm. When municipal water codes are coupled with the NTRS guidelines, dilution to attain low fluoride levels is not an option. Reducing water use is the only way to address the issue.

The use of recirculated water to remove particulates and acid gases in POU wet scrubber devices is acceptable if the scrubbing efficiency of acid gases is not unduly compromised and if dissolved solids do not impair the functioning of the scrubbers. For example, the removal of HF from an exhaust stream is more efficient at neutral pH than at a 5% HF solution. To achieve scrubbing efficiency and a high level of safety, a target of 0.5% HF (5000 ppm) has been set. The water used for recirculating scrubbers must be relatively free of fluoride from the outset, requiring either the use of fresh water or RO reject water. However, RO reject water may be unacceptable if it has a high level of calcium, which causes the formation of calcium fluoride scale. Alternatively, a wet scrubber may use a pump to recirculate water at a rate of up to 50 gal/min while only consuming 1—2 gal/min of fresh water.

TPU and TCS Combustors and Scrubbers

In combination combustor/wet scrubber devices, water is used to remove combustion products and to cool the combustor effluent. Since gases dissolve better in cold water than in warm water, recirculation in combustor/scrubber POU devices requires that water be specially cooled. Moreover, the combustor/scrubber combination using an uncooled recirculation loop could conceivably get hot enough to boil and potentially damage the POU device. The cooling process is easily achieved through a closed-loop process chilled water (PCW) system, which also cools vacuum pumps and chillers in the subfab. Because thermal processor units (TPUs) and thermal conditioning systems (TCSs) both use water for cooling and scrubbing, they consume water at the high rate of 6 gal/min. A solution was sought to reduce the level of water consumption.

Figure 1: Combined combustor/scrubber thermal processor unit originally designed in response to the demand to reduce PFC emissions.

The TPU shown in Figure 1 was originally designed in response to the demand to reduce PFC emissions. Since the destruction of PFCs generates HF, a high-efficiency scrubber was incorporated into the TPU. Similarly, since particulates are generated in many processes that use PFCs for chamber cleans, such as PECVD nitride, tungsten deposition, and silicon carbide deposition, the scrubber system was designed to also handle solids efficiently. The TCS was developed with the same robustness as the TPU but was not designed to destroy PFC gases. However, even without this capability, it effectively removes deposition gases and certain non-PFC clean gases such as F^₂. Thus, the combustor and wet scrubber ends of both the TPU and TCS use water and handle particles in a similar manner.

Figure 2: Ceramic combustor used in the TPU and TCS combustor/scrubber POU abatement device shown in Figure 1.

The combustion system of the TPU and TCS contains up to four isolated inlets that feed into a porous ceramic combustor. Manufactured by Alzeta (Santa Clara, CA), the combustor is a 6 x 12-in. cylinder, as shown in Figure 2. A mixture of natural gas and air is forced into the cylinder from a surrounding plenum and is then ignited on the inner surface of the liner, much like the flameless incandescence of a camping lantern. The resulting isothermal zone, which typically operates between 750° and 1000°C depending on the process, serves to destroy the process gases. The inward flow of the fuel-air mixture is adequate to minimize particulate buildup in the combustor so that no scraping or cleaning mechanism is required.

Figure 3: Diagram of a three-stage wet scrubber used in TPU and TCS point-of-use (POU) combustor/scrubber abatement devices.

The exhaust gases, once destroyed in the combustor, are hot and corrosive. As depicted in Figure 3, the three-stage wet scrubber of both the TPU and TCS uses an overflow weir to continuously wash the inner surface of the quench zone with water, thus preventing corrosion and the deposition of solids. After the gases are cooled to below 90°C by a flux-force condenser water spray, the effluent then enters the cyclone scrubber. Made of polypropylene for corrosion resistance, the cyclone scrubber section rapidly rotates the gases, flinging the particulates and water droplets against the inner wall where they are washed off with a water spray. After HF removal by the packed tower scrubber, the gases are exhausted to the acid duct, where HF concentrations are typically at sub-threshold-limit-value (TLV) levels.

The limitations of the wet scrubber section of the TPU and TCS determine how water is used. For example, temperatures must be kept below 90°C to prevent the softening of the scrubber's polypropylene material and to maintain scrubbing efficiency. Even at low temperatures, the scrubber runs the risk of becoming a stripper if the HF concentration is too high, thus placing an effective upper limit on the recirculation of the scrubber water. The use of heat exchangers enables the scrubber to maximize HF removal.

Dedicated Fab-Scale Recirculation

Water consumption can be reduced by developing closed-loop, dedicated water recirculation systems for specific classes of POU devices, as illustrated in Figure 4. Several companies design and install water systems with skid-mounted platforms for this type of application. A 15-TPU installation such as that installed and operated successfully at a fab in the UK circulates about 90 gal/min of water. An appropriately sized holding tank in the subfab serves as a reservoir for the recirculation system. All drain water from the TPU installation is piped into the reservoir, which is equipped with pH control, an overflow drain, and makeup-water inlet. The recirculation system requires filtration and a pump to return the water to the TPUs. A heat exchanger is also necessary to handle the heat load generated by the combustor.

Figure 4: Diagram of a closed-loop, dedicated water recirculation system.

The amount of makeup water used directly determines the cost of operating a dedicated recirculation system. While solids can be removed by filtration, unacceptable levels of total dissolved solids (TDS), such as the sodium fluoride generated by HF neutralization (from PFC destruction), can accumulate. In a worst-case scenario, a TPU based on a water flow of 23.5 L/min that abates 1.6 std L/min of C^₂F^₆ in each of four chambers simultaneously would yield an instantaneous HF concentration of about 1450 ppm. Under heavy production schedules, a process tool performing accounting or deposition steps and wafer loading and unloading could be in chamber clean about 20% of the time. This would reduce time-weighted HF loading to about 290 ppm. If the makeup water for the 15-TPU recirculation system is set at 10%, or 9 gal/min, the HF concentration would eventually build to a maximum of 2900 ppm, or 0.29%, as depicted in Figure 5. This waste stream could then be diverted to a fluoride treatment facility, since total volume and fluoride concentrations would be suitable for processing. The actual percentage of operating makeup water could be determined experimentally by comparing fresh water costs with the maintenance costs that might be incurred as a result of component deposition or corrosion at high TDS levels. Under less aggressive production schedules, the final fluoride concentration would be reduced accordingly.

Figure 5: Fluoride buildup in a dedicated recirculation system under heavy production conditions. When the makeup water is set at 10%, or 9 gal/min, the HF concentration builds to a maximum of 2900 ppm, or 0.29%.

Industrial Wastewater Reclaim

The direct reclaim of wastewater from an IWW facility, a generalized scheme of which is shown in Figure 6, is a very cost-effective option for POU scrubber systems if fluoride emissions to the municipal sewage system is not an issue. In the direct reclaim procedure, a portion of the wastewater from the fab's wastewater treatment plant is reclaimed for use in a one-pass manner. The facilities required to accomplish this process include pumps, filters, perhaps a heat exchanger, and adequate pipelines from the treatment plant to the POU installation. However, since reclaimed water from IWW systems likely contains significant quantities of sodium, fluoride, sulfate, and a variety of other dissolved solids, it is unsuitable for use with some POU abatement tools, such as wet scrubbers, which already internally recirculate water.

Figure 6: Diagram of a POU system for the reclamation of water, in which a portion of the wastewater from the fab's treatment plant is reclaimed for use in a one-pass manner.

Because water directly reclaimed from an IWW facility would otherwise be destined for the sewer, it is effectively free. Thus, several U.S. fabs have begun to reclaim water in this manner, enjoying a return on investment in as little as six months. The cost of ownership for water and wastewater treatment can be about $20,000 per year per scrubber. The use of reclaimed water in a 15-TPU combination combustor/scrubber could save a fab up to $300,000 in facilities costs in addition to about 50 million gallons of fresh water annually.

The high concentrations of the various dissolved solids and fine particulates that get through the filtration system can make reclaimed water too corrosive for some materials. For example, under the influence of fine particles, brass fittings and nozzles may degrade rapidly. Some wetted surfaces, such as those in valves, may stick and fail. Because of this, POU scrubbers must be robust enough to endure high levels of dissolved solids, often with very high ionic strength. Polymer construction is noncorrosive and often best suited for this application. For a combined combustor/scrubber, the quench zone also must be adequately robust to prevent blockages and corrosion. Often, quench components must be made of 316 stainless steel or be nickel plated to avoid pitting in the harsh and hot environment. Plastic quench zones have short life spans. Also, adequate filtration is required to minimize abrasion.

Individual POU Recirculation

The TPU and TCS described earlier have an optional individual water recirculation system that is incorporated into the associated service module. This recirculation system uses a 6-gal reservoir and a pump to move the water through the scrubber, filter, and heat exchanger. It is also equipped with valves at the freshwater inlet and at the drain line. The valves are controlled by the programmable logic controller of the POU device. A schematic of this POU recirculation system is shown in Figure 7.

Figure 7: POU individual recirculation system with a 6-gal reservoir and a pump to move the water through the scrubber, filter, and heat exchanger.

Individual recirculation capability is common in abatement systems, especially dedicated wet scrubbers. In this procedure, a quantity of water, most commonly 1—2 gal/min, is continuously fed into a reservoir in the scrubber. This water is then recirculated internally at rates of up to 50 gal/min while an equivalent 1—2 gal/min is continuously drained off. Whereas absolute water consumption is relatively low, the concentration of pollutants in the recirculated water can become significant. To combat this when scrubbing large quantities of the acid gases HCl and HBr, the reservoir can be equipped with pH monitoring and be continuously treated with NaOH or KOH. This treatment may be required to keep the pH in check and to prevent the scrubber from becoming a stripper at high loading. This occurs when more of the pollutant is removed from the dirty water and sent up the exhaust duct than is removed from the exhaust gas stream. Increasing the pH to shift the equilibrium from HCl to NaCl is a common way to sustain efficiency and maintain low water consumption.

Dosing requires careful thought. Chlorine, for example, is sparingly soluble in neutral water but goes into solution as the hypochlorite ion when scrubbed at a higher pH. In fact, bleach is a 5.25% solution of sodium hypochlorite in water and typically has a pH of 11 to 12. In contrast, fluorine (F^₂), while also a halogen, is incompatible with water. When F^₂ comes into contact with water, a spontaneous reaction results in the formation of various new compounds, not hypofluorite. These compounds always include HF and different levels of oxygen difluoride (OF^₂). With a TLV of 0.05 ppm, OF^₂ is very toxic, and its formation is enhanced at a higher pH.⁵ However, dosing with sodium thiosulfate has been shown to work in reducing OF^₂ generation to nearly undetectable levels.⁶ Thiosulfate dosing also results in the reduction of F^₂ breakthrough at high loadings. Conversely, a properly designed combustor/scrubber system converts F^₂ quantitatively into HF, which is then scrubbed to TLV levels and removed for fluoride treatment.

Typical individual recirculation systems use a constant makeup flow and consequently dump a continuous stream to the waste drain. With TPU and TCS abatement devices that can communicate with the process tool, the operating parameters change when different process gases are flowing. CF^₄, C^₂F^₆, C^₃F^₈, SF^₆, and ClF^₃ require aggressive combustion and scrubbing in order to be quantitatively converted into safe by-products. Since these compounds also increase the concentration of the acid gases HF and HCl in the waste stream, the recirculation system changes its makeup water rate accordingly to keep the HF below 0.5%.

This "intelligent" recirculation system has an internal tank that is initially filled with about 24 L of water. After filling, the freshwater input valve closes and the system circulates the water in the tank for a predetermined period of time. The default time, such as during the deposition step or between batches when the tool is idle, is 4.5 minutes. After this delay, 12 L of water are pumped from the tank to the drain. Then the drain valve closes and the tank is filled again to the 24-L mark. The span of time between refills is about 5.5 minutes, so that the average consumption of water is about 2 L/min, or 0.6 gal/min. Since HF is not generated during the deposition step unless WF^₆ is in use, the system could theoretically recirculate indefinitely. However, several deposition gases, such as DCS, generate HCl when abated and must be flushed through. In addition, some makeup water is required to offset minor evaporative losses from the wet scrubber.

Figure 8: Water refill logic for a POU recirculation system.

During the clean step, the destruction of a fluorine-containing gas causes the HF level to rise. Since the POU combustor/scrubber can abate up to four process chambers, the recirculation time varies with the number of chambers undergoing clean, as shown in Figure 8. Maximum water consumption in this situation is 3 gal/min, which occurs when the tank drains and refills without delay. Thus, time-weighted average water consumption is process dependent. Since a tool typically spends about 15% of its process time in chamber clean, the average water consumption for the intelligent recirculation system is < 1 gal/min, as illustrated in Figure 9.

Figure 9: Water consumption TPU/TCS water recirculation system with four process chambers in which average water consumption is 1 gal/min.

One advantage of POU recirculation is that a whole line of POU devices is not idled by a failure in a fabwide recirculation system. Also, as with dedicated fabwide recirculation systems, individual POU systems generate low volumes of effluent with a high HF concentration, which is desirable when draining wastewater to the fluoride waste treatment system. Individual recirculation devices do not take up additional floor space; in fact, they have the same footprint as standard TPUs and TCSs with an accompanying service module. This can be highly beneficial in fabs in which there is no available space for a multiscrubber recirculation system. However, use of individual recirculation systems mandates PCW connections.

Conclusion

The future of the semiconductor industry is guided not only by smaller geometries and faster clock speeds but also by environmental requirements and emissions limits. Because the SIA National Technology Roadmap for Semiconductors describes an industry that in the future will use dramatically less water per wafer than at present, the reduction of water use will make itself felt in all parts of the fab. A POU abatement scrubber using water is a high-profile target.

Three water recycling approaches indicate how water reduction goals can be achieved. In a fabwide water-recycling system, water is taken from the wastewater treatment facility and sent back to the fab for reuse. This type of system has no impact on overall water consumption in the fab and is effectively free. The water used in this system must meet certain pH and clarity specifications before being discharged to the municipal sewer system and is often clean enough for single-pass use in scrubber systems. A potential drawback of this type of system is its large footprint and its relatively corrosive water supply, which requires robust scrubber components.

Dedicated recirculation systems can be used on either a fabwide or an individual POU device basis. These systems are desirable in areas in which fluoride emissions to the municipal wastewater system are regulated, since acid drain water that has been reduced to a volume and concentration acceptable to the fab's fluoride treatment system can be handled appropriately. A fabwide system, while requiring more fab space, has the potential advantage of not needing a cooling mechanism, since heat is passed through the pipework to the room. While individual POU recirculation systems that combine a combustor and a wet scrubber have small footprints, they require PCW systems to remove the heat generated by combustion. By using either a fabwide system or individual systems, water consumption can be reduced significantly, helping fabs to meet the roadmap plans of the future.

References

1. National Technology Roadmap for Semiconductors, (San Jose: Semiconductor Industry Association, 1997), 153—162.

2. L Peters, "Ultrapure Water: Rewards of Recycling," Semiconductor International 21, no. 2 (1998): 71—76.

3. S Browne, V Krygier, J O'Sullivan, and EL Sandström, "Treating Wastewater from CMP Using Ultrafiltration," MICRO 17, no. 3 (1999): 77—82.

4. J Martyak, "Designing Practical DI-Water Recycling Systems for Use in Semiconductor Fabs," MICRO 17, no. 1 (1999): 41—47.

5. GH Cady, "Reactions of Fluorine with Water and with Hydroxides," Journal of the American Chemical Society 57, no. 1 (1935): 246—249.

6. J Arnó and JD Sweeney, "Facing the Challenges of Reducing PFC Emissions in Plasma Chamber Cleans," MICRO 16, no. 7 (1998): 87—98.

Joseph Van Gompel, PhD, is a product specialist for exhaust management systems at BOC Edwards (Wilmington, MA). Based in Austin, TX, he is responsible for customer and sales support for the Southwest region. He has taken an active interest in global warming issues as they relate to PFCs and the semiconductor industry. Also, he has worked as a bench chemist and a customer support/applications specialist and was involved in the testing of the TPU 4214 CF^₄ abatement device at Motorola (Irvine, CA) in 1996. Van Gompel is a member of the American Chemical Society and the Society of Applied Spectroscopy. He has written numerous articles and given presentations on semiconductor manufacturing. He has a PhD in physical organic chemistry from the University of Illinois (Urbana-Champaign). (Van Gompel can be reached at 800/848-9800, ext. 6111, or joe.vangompel@edwards.boc.com.)

Vikrant (Vic) Chidgopkar is a product specialist for exhaust management systems at BOC Edwards. Based in Wilmington, MA, he joined the company in 1996 after spending five years in the environmental and chemical industry. He is a professional engineer and has published and presented several technical papers. He received his BS in chemical engineering from Osmania University (Hyderabad, India) and his MS in chemical engineering from the University of New Hampshire (Durham) in 1992. (Chidgopkar can be reached at 800/848-9800, ext. 3574, or vic.chidgopkar@edwards.boc.com.)

MicroHome | Search | Current Issue | MicroArchives
Buyers Guide | Media Kit