Targeting water use for chemical mechanical polishing
06/01/2000
overview
Burgeoning water use and wastewater generation impose a chilling effect upon current and anticipated semiconductor industry growth. Traditional waste treatment is costly and cannot meet escalating demands. One solution is to target the point of use with reclaim technology geared to the chemical mechanical planarization process. This approach has achieved water savings that have ranged from 62-86%, paying for itself within months. In addition to effectively reducing CMP effluent, reclaim systems must separately target both components of the waste stream — water and slurry.
Gary Corlett, Lucid Treatment Systems, Hollister, California
There seems no end to the semiconductor industry's insatiable need for water and subsequent wastewater treatment. After consuming an estimated 108 billion gallons of water in 1999, semiconductor producers expect to use more than 138 billion gallons in 2000 [1].
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Behind these numbers is a heightened demand for water use at chemical mechanical planarization (CMP). In 1997, wastewater from the industry's use of CMP was 325 million gallons. By 1999, CMP wastewater topped 5 billion gallons. This year, CMP could account for 40% of water use in the semiconductor industry. As linewidths shrink to <100nm by 2005, these water-use numbers are destined to soar. The correlation between critical dimension and water consumption is not clear.
 Figure 1. Typical effluent turbidity during a CMP operation showing the predominance of "clear" water effluent. |
The sheer volume of water requirements for CMP places a double strain on resources, taxing supplies and overwhelming waste treatment facilities around the world. From the location of Intel's largest fab in Albuquerque, NM, to the location of Motorola's newest fab in Tianjin, China, water and waste resource issues constrain the expansion of fabs and the overall growth of the industry.
Noting that little progress has been made in the past two years, the 1999 International Technology Roadmap for Semiconductors (ITRS) specifies that net feed water use be reduced from 13gal/in2 to 5gal/in2 by 2005, with a 2008 and beyond long-term goal of 2gal/in2, and a proportional decrease in ultrapure water (UPW) use (Table 1).
These goals are complicated by the industry's emerging transition to 300mm wafers. The ITRS recognizes that part of the industry's water use must be controlled through better effluent management.
CMP's variable waste stream
CMP is an extremely precise and repetitive process using slurry as the chemical component. Separating each cycle of planarization are the much longer steps of pad conditioning, wafer buffing, rinse, wafer handling, and idle periods. Each of these steps consumes a constant flow of UPW, creating a unique high-volume waste stream that is intermittently concentrated, but extremely dilute overall. For example, assuming an average UPW use of 5.1gal/min on a continuous basis (7 days, 24hr), 10 commercially available CMP tools will consume approximately 86,400gal of UPW daily, generating an equal amount of wastewater. This translates into annual consumption of 31.5 million gallons of UPW requiring approximately 52.57 million gallons of feed water.
Despite its volume, the nonslurry process steps dilute the waste stream so significantly that approximately 70% of this effluent is actually "clear" water. (Here we define "clear" as having a turbidity level of 0.75 nephelometric turbidity units [NTU], the average standard for US drinking water. By comparison, DI water should be = 0.5NTU.) In practice, CMP slurry solids may be as high as 30% during the actual CMP step, but the high volumes of UPW water surrounding the process dilute the collected waste stream to an average of 0.02%.
Consider the process pattern on one commercially available tool (Fig. 1), monitoring the turbidity of the effluent over time. As wafer polishing commences, slurry feed begins and total solids concentrations on the platen typically range from 12-30/wt%. These slurry solids enter the waste stream and increase its turbidity. When planarization is complete, slurry feed stops and the series of steps that follow use UPW water only. The turbidity of the effluent drops quickly below 0.75NTU. Review of the entire cycle shows that the effluent turbidity is below 0.75NTU for 70% of the total time.
 Figure 2. CMP effluent reclaim technology — dubbed waste interface systems and remote diverter (WISARD) — based on point-of-use diversion. |
Turbidity is a measure of the suspended solids or material in water that cause a cloudy appearance. In the case of CMP waste, these solids are most commonly silica or alumina abrasives used in CMP slurries. Using a nephelometer or light-scattering device on CMP waste, it is very easy to characterize the waste stream and identify opportunities for reuse. Tracking turbidity throughout a typical 2:42 min cycle reveals that 70% of the wastewater stream is "clear" with a turbidity level of <0.075NTU. This means that 70% of the discharge does not need to enter the waste treatment system. This water, actually cleaner than city water supplied to the UPW facility, is ideal for many nonprocess applications such as equipment rinsers, fume scrubbers, cooling towers, and even source water for UPW systems. Separating it at the point of use and putting it to work within the fab provides a unique opportunity to reduce CMP water use and waste production, and a means to help meet ITRS goals.
Viewed over a longer time period, the variability of the waste stream is even more apparent. From the equipment maker's perspective, CMP tools must be able to achieve more than 90% uptime. But in the fab, preventive maintenance, buff pad conditioning, test wafer runs, and other routine process activities can drive down utilization to as low as 50%.
Tracking use of one CMP tool during a fab's single extended 9:06-hr shift (546 min), operators ran two test wafers during the first seven hours of the shift, then one 25-wafer lot at the end of the shift. The CMP tools were in operation for 81 min of the 546 min measured; for the remaining 465 min, the waste stream was clear. During this shift, 85% of the wastewater did not need to be treated. However, the next shift might produce a decidedly different wastewater pattern. The point is that with multiple CMP tools, each in a different phase of utilization or maintenance, the solids content of the waste stream is impossible to predict from one moment to the next.
Though CMP produces high volumes of extremely dilute waste, the suspended solids in the waste do pose serious challenges for disposal. Slurry solids readily clog municipal waste systems. Within many fab waste systems, mixed oxide and metal slurries agglomerate into a viscous mass, and slurry solidifying in discharge lines is a chronic problem that requires frequent attention. Increasingly, local jurisdictions are restricting the release of suspended solids into municipal waste systems. Tungsten, copper, and other trace metals are already restricted.
Point-of-use water reclamation
We believe that the ITRS claims tend to distort the industry's problems with water and wastewater because most efforts at CMP water reclamation and reuse have focused on end-of-process filtration. They have targeted the entire effluent stream emerging from the waste drain, from the data above clearly an inefficient and costly approach that
cannot keep pace with the explosive growth of CMP. A more effective strategy addresses the sources of water use within a CMP tool, using a system that monitors the waste stream from individual polishers, deciding in real time whether to reclaim the water or to divert it for treatment (Fig. 2).
This approach involves a stand-alone, self-contained unit that connects to the CMP tool's waste line. As Fig. 2 shows, all waste passes through the system and its installed sensors that measure the turbidity of the effluent stream in real time. If the turbidity content is sufficiently low, the water is diverted to "clean" or reclamation and used within the fab. Before the stream enters the clean or reclaim lines, a second process sensor confirms that turbidity falls within established clean water requirements. When slurry enters the waste line, the process sensor detects the change in turbidity and diverts this contaminated water to the effluent treatment facility. By modifying water use for CMP, as outlined above, this approach can help IC manufacturers reduce water and associated waste production at CMP operations by as much as 75%.
Fab trials
Planning a fab expansion, a major IC manufacturer considered additional water sources, including a costly new well. The company was also exploring options for controlling water use and decided to test the waste interface system described above. Its engineers conducted a six-week trial with the diversion system connected to one polisher to characterize its performance.
During tests of the single polisher, the daily ratio of effluent vs. reclaimed water varied widely, with reuse reaching a low of 40% and a high of 97%. This machine performed >4344 process cycles and generated >337,700gal of effluent at an average rate of 7342gal/day. A daily average of 2559gal, just 35% of the total volume, was sent to waste treatment. During the six-week trial, the polisher averaged 65% water reclaim.
 SpeedFam-IPEC's Auriga C tool equipped with Lucid's WISARD technology. |
Tests of random samples taken during the evaluation period compared the quality of UPW, source water from the municipal system, slurry-laden waste, and water reclaimed for nonprocess uses. The results of ion chromatography and atomic element spectrum analyses revealed that water reclaimed by the system was of substantially higher quality than source water (Table 2). In addition to serving nonprocess applications such as equipment rinse, this water offered an excellent feed source for the reverse osmosis (RO) function of the UPW plant with a cost savings.
It can take up to two gallons of city water to produce each gallon of UPW, but reclaimed water, such as that in column two of Table 2, provides a cleaner and more efficient source for UPW processing. Because of its quality, such reclaimed water used to generate UPW may translate into less frequent back flushing of purification sand filters and extended life of carbon beds. In addition, because the particle content of reclaimed water is much lower than that of city water, RO membranes should generate higher flux rates, producing UPW more efficiently.
Before the tests cited above, CMP waste had averaged 0.02wt% solids. After incorporating the waste interface system and reducing effluent by 65%, the waste diverted for treatment averaged 0.31wt%. Reduced in volume and more concentrated, this waste is easier and less expensive to treat.
Based on these results, the semiconductor manufacturer was able to increase CMP capacity without expanding its UPW plant or waste treatment facility. In addition, it has improved the efficiency and cost-effectiveness of the waste treatment process.
OEM development
Our success with effluent and wastewater diversion has been part of SpeedFam-IPEC's efforts with its Auriga and Auriga C CMP platform, and its efforts to improve water efficiency. Prior to learning about the Lucid system, project lead engineer Mary Reker was investigating an approach for reducing overall water consumption by recycling water from "cleanest" tool functions to "less clean" activities. For example, water captured during the wafer load and unload cycle can be reused in tub rinses or the conditioner ring "birdbath." In work at SpeedFam-IPEC, effluents from polisher-processing and rinse steps were analyzed for their level of contamination in order to determine the optimum reuse applications. Other water optimization approaches for the polisher included using UPW flow regulators, reduced UPW delivery line sizes, and other techniques. However, for a CMP tool OEM, implementation of these changes would require equipment redesign and process requalification by the users.
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Reker pursued the water reuse scenario through point-of-use water segregation techniques. She found that the most viable option was to recycle water from the wafer unload station, index table, and polish table rinse, back to the tub rinses (a nonprocess contact step). This method replaces 2.6gpm water consumption by the tub rinses.
SpeedFam-IPEC secured a Lucid system — an early version of the tool shown in Fig. 2 — installed it on an Auriga C, and quickly confirmed Reker's earlier findings: A large percentage of the tool's wastewater could be effectively segregated and reclaimed (Fig. 3). Since this system has the ability to deliver the reclaim water directly to the tub rinse inlet on the polisher, extensive plumbing expenses and system interruptions were avoided. However, only a percentage of the total water reclaimed can be used by the tub rinses, so the remaining reclaimed water can be sent back to the fab for other reclaim and reuse systems.
The nominal water consumption of the Auriga C is 8gpm of UPW water for wafer processing and 2.6gpm of city water for tub rinses. However, many fabs use UPW for the tub rinses for ease of plumbing during installation. Assuming 86% recovery of the total UPW usage of 8gpm and that 2.6gpm of this recovered water is used for tub rinses, the following potential annual cost savings can be realized:
- tub rinses if UPW is used, 2.6gpm at $16/1000gal, saving $16,860;
- reclaimed water, 4.3gpm at $2/1000gal, saving $4520; and
- treatment and disposal savings, 6.9gpm at $8/1000gal, saving $29,013.
While cost will vary by location, this simple example yields a total potential annual savings of $50,393.
SpeedFam-IPEC now supports use of this effluent-monitoring system on all rotational polishers as an efficient way to reduce water consumption without affecting the polisher's process, performance, programming, or systems.
Reducing slurry use
A comprehensive effort to reduce CMP effluent must specifically target slurry use, as well as water. Typically, process engineers are forced to specify too much slurry, indicating amounts that range from 250-1000ml on the platen. Too little slurry can result in poor planarization or damage to the wafer, so it makes sense to err on the side of caution.
 Figure 3. SpeedFam-IPEC Auriga polisher effluent profile. |
Process engineers have to be cautious, because peristaltic pumps supplying each polisher on a shared distribution loop may not perform consistently; though cost-effective, peristaltic pumps offer limited repeatability. Normal wear and tear within a pump's tubing can affect slurry flow. In addition, supply pressure on a shared slurry distribution loop fluctuates as polishers are brought online. As more tools go into simultaneous operation, line pressure drops, reducing flow to the polishers already online. These variations in flow and pressure translate into fluctuations in the amount of slurry provided to the platen. This is why, to be safe, engineers call for large applications of slurry.
The answer is to control pressure dynamically on the slurry distribution loop, so polishers can go in and out of service with little impact on line pressure. We have done this with a loop pressure control system incorporating a sensor that detects changes in line pressure, and, with a pneumatic valve, adjusts slurry flow accordingly. In addition, a flow meter tracks the rate of slurry moving through the line. With pressure steady and the resultant flow constant, peristaltic pumps perform more consistently. They deliver slurry more reliably, allowing process engineers to control slurry flow to the platen more tightly. Improved process control makes it possible to dramatically reduce slurry consumption.
Conclusion
A twin approach to waste reduction — independently targeting water and slurry use at each polisher — lowers the volume of the waste stream while increasing its concentration. A more consistent effluent makes flocculation and other treatments more predictable and less demanding to manage. And by separating oxide slurries from tungsten and copper slurries, an IC manufacturer can recycle the characteristically high-quality CMP waste for a variety of industrial applications. Reclaimed slurries find use in such areas as textile processing, optical grinding, and precision casting.
Reference
- Calculated from data in E. Schumann, Info Express: What's Up From SEMI? "Silicon wafer shipments rise 25% in 1999, but concerns remain regarding future investment and R&D," February 2000.
Gary Corlett received his BS in industrial engineering and his MS in engineering (IE/operations research) from California State Polytechnic University, Pomona. He is a registered industrial engineer. Prior to founding Lucid Treatment Systems (formerly CMP Technologies), he was manager of plant services at Rockwell International, and is the former director of marketing operations for FSI's Applied Chemical Solutions. Corlett is VP of technology at Lucid Treatment Systems, 2339 Technology Parkway, Suite A, Hollister, CA 95023; ph 831/634-4800, fax 831/634-4805, e-mail [email protected].