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|CMP Report||October 2000|
Assessing the Environmental Impact of Copper CMP
Benoît Maag, Duane Boning and Bettina Voelker
The required transition from aluminum to copper interconnects poses new challenges in wastewater treatment. Specifically, current microprocessors having six to seven layers of copper interconnect (Fig. 1) generate waste from both the copper electroplating solution and the copper CMP/post-CMP cleaning processes. Modeling of a typical 200 mm fab facility processing 25,000 wafers per month shows approximately 0.28 grams of copper removed during CMP per layer of interconnect, resulting in 0.28 ppm of copper in the facility's final effluent. This is well above the limit allowed for direct discharge into a water body (usually on the order of 10 ppb), but under typical local limits of around 1 ppm. In fact, up to 1.3 ppm of copper is allowed in drinking water. The U.S. regulatory guidelines address both solid and liquid copper-containing waste. Solid waste from copper CMP is not listed as hazardous unless it contacts rinse waters from copper electroplating. The effectiveness of the copper CMP waste treatment approach depends on the form the copper takes, also known as copper speciation.
Modeling the typical facility
Many factors can affect a real facility's departure from this model. First, a facility usually has several manufacturing lines; it is likely only some of them will implement copper interconnect technology at first, while others will use aluminum. Second, evaporation is not taken into account and may comprise up to 40% of water use. Third, when facilities begin processing 300 mm wafers, water consumption and waste volume are likely to change significantly. Finally, water use reduction through reuse and recycling is expected in the future.1
This calculation estimates copper CMP generates 0.28 ppm of copper in the final facility effluent, assuming no copper removal treatment is applied. In addition, the copper contained in city water, which may be as high as several ppm (0.05-3.2 ppm in Massachusetts2), can find its way to the final effluents through the UPW plant reclaimed water, further boosting copper concentration in the final effluent.
From an environmental protection standpoint, toxicity of copper or its treatment at a wastewater treatment plant also strongly depends on copper speciation.
U.S. regulatory framework
Figure 4 provides an overall map of applicable regulations. Copper-bearing effluents are controlled by two different regulatory mechanisms, depending whether the semiconductor manufacturing facility discharges (after on-site treatment) directly into a water body (direct discharge) or into the collection system of a wastewater treatment plant (indirect discharge). For direct discharge, the facility must be in compliance with its National Pollutant Discharge Elimination System (NPDES) permit. This state-issued permit specifies pollutant discharge limits and monitoring requirements. The state periodically updates the discharge limits to maintain a pollutant load below the total maximum daily load (TMDL) in ambient water.
Indirect dischargers are subject to compliance with "local limits" established by the receiving wastewater treatment plant in accordance with the National Pretreatment Program for Indirect Dischargers.4 Such limits are established so the pollutant does not interfere with the operation of the wastewater treatment plant or will pass through it. Copper local limits are on the order of 1 ppm (Table 1).
Both NPDES limits and local limits apply at the property line. To prevent dischargers from diluting pollutants to reach compliance, the EPA has established categorical limits that apply at the end of the manufacturing process rather than at the property line. Point source categories such as "metal finishing" or "electrical and electronic components" are listed with specific end-of-process pollutant content limitations. The debate over whether the CMP process should be listed under metal finishing (40 CFR 433 in the code of federal regulations) or electrical and electronic components (40 CFR 469) was clarified by an April 21, 1998, memo from the EPA, stating that "the Agency believes that the metal finishing requirements contained in part 433 only cover the process after wafer fabrication which is used to deposit a layer of metal onto the surface of the wafer to provide contact points for final assembly."5 The interpretation of the agency is that copper CMP, as a process which is performed in the fab cleanroom before final assembly, is to be classified under the electrical and electronics component point source category. However, that listing does not specify a copper concentration limit. This interpretation is nonetheless not as strong as a ruling, and could be re-evaluated when the EPA issues the new Metal Plating and Machining categorical standards currently under development.
aUnder revision, will probably be 1.0 ppm or lower; bExpected; cOne-day average; dMaximum concentration, "grab" sample; eMaximum concentration, composite sample
Solid waste generated by the CMP process is subject to regulation under the RCRA. The hazardous waste F-006 listing, which applies to semiconductor manufacturing sludge at most facilities, has become a disputed piece of legislation. Abrasive slurries used in CMP processes typically contain alumina or silica particles in large quantities (3-7% wt alumina in copper polishing slurries, 8-15% wt fumed silica or 25% wt colloidal silica in other CMP slurries). If these particles are totally removed on the facility site by gravity settling, filtration or coagulation-flocculation processes, they are processed into a sludge cake, which, after drying, can be disposed of in a landfill.
Solid waste can be listed as hazardous if it falls within a defined category (40 CFR 261.30-33) or if it displays ignitability, corrosivity, reactivity or toxicity characteristics (40 CFR 261.21-24). Usually, solid waste from copper CMP is not listed as hazardous unless it comes into contact with the rinse waters from copper electroplating. Under the RCRA, such solid waste is listed as F-006 hazardous waste according to 40 CFR 261.31. The F-006 listing is one of the most contested of RCRA's provisions and has generated a large number of petitions for exemption. It is argued that the F-006 listing was established at a time when electroplating bath solutions contained cadmium or cyanides and should not apply to new processes, such as the one used in semiconductor copper interconnect technology, where these chemicals are not used.
For that matter, IBM's Burlington, Vt., plant entered a project with the EPA, which was in its final phase of approval at the time of this writing.6 The rationale for this project is that copper coming from the electroplating rinse process triggers the F-006 listing while representing only a minute fraction of the discharged metal (0.3% of total copper in sludge compared to 81% from the copper CMP process in 20016). The five-year project provides IBM with an exemption for its facility sludge, which does not otherwise display hazardous characteristics, in exchange for a reduction in greenhouse gases. After five years, if the project is deemed successful in reducing overall pollutant discharge, the EPA will exempt the IBM Burlington facility from the F-006 listing. The EPA could then grant overall exemption from the F-006 listing for other semiconductor manufacturing facilities using copper electroplating, but this would require additional rulemaking.
Although extremely unlikely, another piece of regulation that may apply to CMP waste is the Superfund Act (CERCLA: Comprehensive Environment Response, Compensation and Liability Act). Even though some landfill sites listed as Superfund sites have shown copper contamination, their classification was based on pollutants other than copper. Since copper content in CMP sludge is limited (less than 1.25 g of copper per kg of sludge, dry weight basis according to the facility model), it is unlikely that semiconductor manufacturers disposing of copper CMP solid waste would be held responsible in the future. Indeed, this is close to the accepted level for fertilizers made from sewage sludge (1.5 g of copper per kg of fertilizer, listed in 40 CFR 503.13) or found in U.S. domestic waste landfills (2.5 g per kg of waste of non-ferrous metals, including copper and zinc but not aluminum and lead7).
Copper's environmental impact
Copper is an abundant metal, naturally found in all bodies of water (Table 2) and is an essential nutrient for plant and animal life. An extensive survey conducted by the U.S. Geological Survey9 showed total copper concentrations as low as 5-10 ppb could cause toxic effects such as reduced growth or photosynthesis in algae or teratogenic effects in sensitive species of fish or amphibians.
aLevels higher than oceanic levels indicate copper input from rivers and anthropogenic sources. Levels one magnitude lower are common; bCopper piping leaches into drinking water and wastewater, levels are higher during rainfall; cUS EPA's maximum concentration level (MCL) for drinking water. If over 10% samples exceed MCL, treatment is necessary
In natural waters, copper is largely bound to dissolved organic compounds, and free cupric ion is usually found in concentrations several orders of magnitude lower than that of complexed copper. Free cupric ion is the most readily bioavailable form of the metal and, as a result, can be toxic if present in excessive quantities. Many tests to evaluate copper toxicity are conducted in the laboratory with water that is relatively free of complexing substances. In such tests, the toxic-free cupric ion represents a higher proportion of total copper present in the water than it would in a real natural water body. These tests are therefore conservative, as they should be, because they need to consider the worst-case scenario whereby free cupric ion is released under changing environmental conditions, such as a drop in pH or an influx of calcium ion that can replace copper in some complexed forms. To protect wildlife in natural waters, the EPA has recommended water quality criteria for copper (Table 3).
According to current knowledge, direct discharge of effluents containing copper at levels below the above guidelines can be considered safe for the environment. In some cases, direct dischargers may obtain a "dilution credit" as their effluents are diluted into a large river to a level below the safe ambient water quality level, in which case adverse effects close to the discharge point need to be taken into account.
The facility model described above shows that, without treatment, copper in the facility effluent is at levels too high (280 ppb) to be acceptable for direct discharge (Fig. 3). Therefore a treatment system capable of removing significant amounts of copper is indispensable before direct discharge.
Impact on a wastewater treatment plant
The increased influx of copper into the wastewater treatment collection system could potentially give rise to several problems. It could have an adverse effect on the biological stage of wastewater treatment (activated sludge, trickling filters or other), or it could increase copper content either in the effluents or in the sludge generated by the wastewater treatment plant, which in turn would have an environmental impact.
The effect of copper on the effectiveness of a wastewater treatment plant's biological treatment stage has been studied extensively at locations that receive effluents from large electroplating operations.10 Such studies reveal that inhibition of microorganisms in the activated sludge occurs at copper concentrations in excess of 1 ppm. Since local limits are in the range of 0.5-2.0 ppm (Table 1), it is unlikely that mixing semiconductor facility effluents with domestic wastewater and street run-off, whose copper content is usually in the range of 60-100 ppb, would cause any disruption of the treatment plant.
Another concern is the increase of copper either in the treatment plant sludge, which often is processed to make fertilizer, or in the plant's effluents. Most of the copper in particulate form and some of the dissolved copper is removed by the wastewater treatment process.11 It is therefore expected that the copper content of wastewater treatment sludge would increase. The fertilizer made from this sludge has to satisfy quality criteria including a copper concentration limit — for example, 1.5 g of copper per kg of fertilizer (federal limit listed in 40 CFR 503.13). In cases where the copper content of fertilizer is already close to regulatory limits (e.g., the fertilizer made from sludge generated at the Boston Deer Island wastewater treatment facility contains 0.9 g of copper per kg, the Massachusetts limit being 1.0 g of copper per kg), further additions from semiconductor effluents may be problematic.
At other wastewater treatment plants, such as those discharging into the San Francisco Bay south of Dumbarton Bridge, an increased copper load may cause out-of-compliance levels. For example, the Sunnyvale treatment plant, which discharges into the south of San Francisco Bay — a relatively stagnant water body — has no dilution credit and has a copper discharge limit of 8.6 ppb and an annual discharge limit of 715 pounds (324 kg). These levels are barely being met despite high copper removal efficiency. For such sites, an influx of strongly complexed copper, which may pass through the treatment system, could force a facility to be out of compliance. Specifically, if untreated effluent were discharged from the copper CMP facility modeled above, with copper strongly bound to a non bio-degradable complexing agent (especially EDTA), it would add more than 450 kg of copper per year to the Sunnyvale treated effluent, well above its annual limit.
Such issues, highly dependent on the local environment, may drive some treatment plants to revise local limits.
Several options can be considered for copper removal. A gravity/settling system can remove copper by the production of a sludge that may or may not be considered hazardous, depending whether electroplating rinse water has been in contact with it. If the sludge is not considered hazardous, then this type of system can be both economical and safe. Another option is to install a complete copper removal system that involves activated carbon oxidant removal, filtration of slurry particles, and ion exchange to extract copper from the effluent. Such systems are being tested at various locations including Motorola and SEMATECH in Austin. In these two cases, copper speciation, which is a result of slurry and post-CMP clean solution chemical composition, is having a strong impact on the treatment system's performance. Specifically, strongly complexed copper is hard to precipitate or remove. A more detailed study of copper speciation and its impact on removal performance is available.3
is sales and marketing director of the Semiconductor and Optical Fiber Division of Saint-Gobain Quartz (Paris). Maag holds engineering degrees from Ecole Polytechnique and the Ecole Nationale Supérieure des Techniques Avancées (ENSTA) in France, a joint ENSTA/University of Paris VI Masters Degree in Fluid Mechanics, and an M.E. from MIT. His master's thesis pertained to the environmental impact of copper CMP. Duane Boning
is associate professor of electrical engineering and computer science at MIT, and associate director of the MIT Microsystems Technology Laboratories. His research focuses on variation modeling, interconnect technology and semiconductor process control. He formerly was a member of technical staff at Texas Instruments in Dallas. He is associate editor of the IEEE Transactions on Semiconductor Manufacturing, and holds M.S. and Ph.D. degrees from MIT in electrical engineering and computer science. Bettina Voelker
joined the faculty of MIT as assistant professor of civil and environmental engineering in 1996. She holds a B.S. in chemistry and an M.S. in civil engineering from MIT as well as a Ph.D. in environmental sciences from the Swiss Federal Institute of Technology (ETH) in Zurich.
Benoit Maag is sales and marketing director of the Semiconductor and Optical Fiber Division of Saint-Gobain Quartz (Paris). Maag holds engineering degrees from Ecole Polytechnique and the Ecole Nationale Supérieure des Techniques Avancées (ENSTA) in France, a joint ENSTA/University of Paris VI Masters Degree in Fluid Mechanics, and an M.E. from MIT. His master's thesis pertained to the environmental impact of copper CMP.
Duane Boning is associate professor of electrical engineering and computer science at MIT, and associate director of the MIT Microsystems Technology Laboratories. His research focuses on variation modeling, interconnect technology and semiconductor process control. He formerly was a member of technical staff at Texas Instruments in Dallas. He is associate editor of the IEEE Transactions on Semiconductor Manufacturing, and holds M.S. and Ph.D. degrees from MIT in electrical engineering and computer science.
Bettina Voelker joined the faculty of MIT as assistant professor of civil and environmental engineering in 1996. She holds a B.S. in chemistry and an M.S. in civil engineering from MIT as well as a Ph.D. in environmental sciences from the Swiss Federal Institute of Technology (ETH) in Zurich.
We would like to acknowledge university and industrial participants in the NSF/SRC Center for Environmentally Benign Semiconductor Manufacturing, SEMATECH, Massachusetts Water Resources Authority, Austin Water & Wastewater Utility, Sunnyvale Department of Public Works and EPA Region 1 for valuable discussions. •