GREEN AND CLEAN

Treating wastewater from CMP using ultrafiltration

Steven Browne, Cirent Semiconductor; and Vivien Krygier, Joseph O'Sullivan, and Eric L. Sandström, Pall Corporation

Although it differs radically from the clean technologies that characterize semiconductor manufacturing, chemical-mechanical planarization (CMP) is fast becoming the established technique for polishing multilevel devices.¹ As wafer fabs add an increasing number of such polishing tools, new challenges arise, including in such nonprocess areas as facility capabilities and environmental requirements. For example, because the multipart abrasive slurry used in CMP to oxidize and abrade the substrate surface contains an enormous number of ultrafine particles, large quantities of ultrapure water must be used for rinsing these particles off the polished wafers. The resulting wastewater stream then also contains large quantities of particles and may need to be treated in order to maintain an acceptable level of total suspended solids (TSS) in the facility's industrial wastewater effluent. In some communities, the total amount of effluent discharge is also limited, which makes recycling of some of the wastewater an attractive goal as well.

Figure 1: Particle-size distribution for a typical slurry.

The high level of particulates in CMP wastewater dictates that some form of cross-flow filtration should be used for their removal, as opposed to dead-end or once-through cartridge filtration, which would be quite expensive in terms of consumables. There are also several reasons why ultrafiltration (UF) technology should be chosen over microfiltration. As seen in Figure 1, silica particles in an oxide slurry can range in size from 20 to 80 nm, far too small to be retained by microfilters alone. To be effective, cross-flow microfilters need to build up a dynamic layer, similar to a filter precoat. Achieving this buildup frequently requires the addition of chemicals to flocculate the colloidal CMP slurry particles. The very low and varying concentration of suspended solids—200—1000 ppm—can make this process difficult to control. In addition, if the recovered water is to be recycled, the presence of excess chemicals would make posttreatment more complex. Furthermore, there is always the danger that any loss of pressure in the treatment system could cause the dynamic layer on the microfilter to collapse, allowing passage of particles. In contrast, ultrafiltration membranes have pore sizes substantially smaller than the silica particles in slurries and thus can provide an absolute barrier to all suspended particles regardless of concentration and flow conditions.² No chemical additives are required, which simplifies the posttreatment processing of recovered water for recycling. This case study describes the installation and operation at Cirent Semiconductor (Orlando, FL) of a CMP wastewater treatment system that incorporates ultrafiltration modules.

System Design and Operation

The ultrafiltration system selected by Cirent was designed to concentrate a 0.05—0.5% mixed oxide and metal slurry waste to about 10—15% by weight at a continuous feed flow of 50 gal/min. The key feature of this system is the ultrafiltration membrane, which has a molecular weight cutoff rating of 13,000 daltons. A cross section of the module is shown in Figure 2. The durable, long-lasting membrane is configured as a smooth, double-skinned hollow fiber of exceptional strength. Solids separation is effected by small pores on the inner surfaces of the fibers. Both skinned surfaces have identical removal ratings, thus ensuring protection against the passage of solids should the inner skin be inadvertently damaged. Such damage is rare, however, since the membrane readily tolerates the abrasive slurry suspensions. As seen in Figure 3, the Cirent UF system contains 26 modules, which are arrayed in parallel on a manifold.

The entire ultrafiltration system installed at Cirent's wastewater treatment facility is shown schematically in Figure 4. The oxide and metal slurry waste streams are first collected separately into redundant holding tanks located in the fab's service level beneath the CMP area. From these holding tanks, the oxide and tungsten wastewater is pumped 1800 ft to the water treatment facility. There the waste streams are mixed in a preultrafiltration balance, or buffer, tank, which is equipped with a mixer to keep the slurry wastewater in suspension. Because the UF system is operated in a continuous-feed batch mode, the buffer tank has to be large enough to accommodate the differences in wastewater production and the UF system's capacity, which varies as the amount of suspended solids changes during the batch processing cycle. Buffer tanks used in this type of application are usually designed to hold 2—4 hours of wastewater volume. This size also provides time for short-term maintenance or repairs without having to bypass to drain. The UF system is designed to be able to catch up for this lost processing time. To achieve greater security, Cirent decided to install buffer tanks that can hold the slurry wastewater generated in the facility over 24 hours.

Figure 3: Ultrafiltration modules arrayed in parallel on a manifold. Photo Courtesy of Cirent Semiconductor

From the buffer tanks the wastewater is pumped through a prefilter to the UF system feed/batch, or process, tank, and from there it is pumped to the ultrafiltration manifold. Each UF module is equipped with isolation valves for ease of changeout while the system is operating and has a valved sampling port that permits the inspection and analysis of its filtrate, or permeate. To minimize membrane fouling and to maintain optimal performance, the feed is recirculated through the UF modules at 2 m/sec, which maintains a high tangential velocity across the membrane surface. The permeate flows from the modules to the permeate, or reverse filtration, tank. A small portion of this clear liquid is used for reverse filtration (RF) and for seal water for the UF system's pumps. The spent seal water, in turn, is used to humidify the process tank to prevent scaling. Performed automatically every 20 minutes for 20 seconds, reverse filtration helps to keep the membrane surfaces clean. During this process permeate is pumped from the RF tank back through the UF module membranes to the process tank. By using reverse filtration, a stable flux can be maintained during the solids concentration process.

After a predetermined time interval, the flow from the buffer tank is stopped while the UF system continues to operate until the process tank liquid is drawn down. The final slurry concentration can range from 2 to 10% (wt/vol), depending on the initial suspended-solids level of the CMP wastewater. A forwarding pump transfers this concentrate to another tank, where it is treated together with the fab's hydrofluoric acid waste. The resultant sludge is then dewatered by a filter press to a dry cake, which is suitable for disposal in a landfill. Once the concentrate has been transferred from the process tank, the UF system is rinsed, drained, and flushed automatically prior to the next concentrating cycle, and this rinsewater is returned to the process tank.

In addition to the features described above, the UF system is fully automated and incorporates a programmable logic controller together with a PC and flat-panel man-machine interface (MMI) for data acquisition and a full graphic presentation of the process. The system's automation controller is fully integrated with the waste treatment plant's central control room, and remote monitoring via modem is being considered for off-site performance analysis and troubleshooting.

System Safety Features

Because hard shutdowns—that is, immediate suspensions of processing with no advance warning—are very costly, a number of countermeasures were taken during the design of the UF system to minimize the risks involved in such occurrences. The first line of defense against a hard shutdown is provided by the buffer tank upstream of the UF system. The tank's hydraulic capacity is described above. Within the UF system, alarm and self-diagnostic features are triggered automatically if a component fails. Because the system is supplied with redundant pumps, downtime for repair is not expected to exceed 2 hours.

Should there be an insufficient flow of wastewater, the UF system will not stop immediately but will go automatically into a no-permeate mode in which only enough permeate is forwarded from the UF modules to maintain the RF tank at the appropriate level. If sufficient wastewater again becomes available within a specified period, the system will automatically resume full operation. Should the loss of wastewater be protracted, however, the system will then automatically drain, go through a rinse/cleaning cycle, and stop.

If there is an electricity loss, the system and its pumps will stop. All automated (air-actuated) valves will go to their "fail" positions, which were carefully selected to provide the best possible protection for the system and the process. In addition, the control system is protected via an uninterruptible power supply and a redundant PLC. When electricity is restored, the system must be restarted by an operator to ensure that the outage has done no damage.

Permeate Quality

Following start-up and the commissioning of the UF system at Cirent, a number of feed and permeate samples were collected for analysis. The wastewater generated by oxide and metal CMP tools can vary significantly based on the throughput and volume of the dispensed slurry. During the weeklong sampling period, the total suspended solids in the feed material, as measured downstream of the buffer tanks with the aid of a 0.2-µm nylon-66 analysis disk, ranged from 0.02 to 0.10% (wt/vol). The permeate from the UF system was, as system specifications require, free of suspended solids. The samples were also analyzed in terms of ionic contamination using inductively coupled plasma atomic emission spectrometry (ICP-AES), as shown in Table I. All levels detected in the permeate were <10 ppb with the exception of boron, calcium, nickel, potassium, silicon, and sodium. These generally low ionic levels suggest that the water recovered from the UF system is a suitable candidate for recycling. The high potassium level detected in the permeate results from an additive in the CMP slurries, while the high silicon level is attributable to soluble silica. Use of a colorimetric technique (the heteropoly blue method) confirmed that soluble silica levels were <4 ppm.

Metal Ion	Level
Aluminum	<10 ppb
Arsenic	<10 ppb
Barium	<10 ppb
Boron	72 ppb
Cadmium	<10 ppb
Calcium	61 ppb
Chromium	<10 ppb
Cobalt	<10 ppb
Copper	<10 ppb
Gallium	<10 ppb
Germanium	<10 ppb
Gold	<10 ppb
Iron	<10 ppb
Lead	<10 ppb
Lithium	<10 ppb
Magnesium	<10 ppb
Manganese	<10 ppb
Molybdenum	<10 ppb
Nickel	32 ppb
Phosphorus	<10 ppb
Potassium	8.719 ppm
Silicon	1.070 ppm
Silver	<10 ppb
Sodium	59 ppb
Strontium	<10 ppb
Tin	<10 ppb
Titanium	<10 ppb
Zinc	<10 ppb
Zirconium	<10 ppb

The use of ultrafiltration to treat CMP wastewater also results in low levels of total organic carbon because of the filter membrane's removal of long-chained organic molecules. As mentioned above, the UF modules have a molecular weight cutoff of 13,000 daltons.

In order to be recycled, water recovered from the CMP process must be subjected to further treatment downstream of the UF system using reverse osmosis (RO) or ion exchange systems. The degree of recovery achievable with an RO unit, however, is highly dependent on the soluble silica level in the water being treated. The typical soluble silica feed specification for an RO unit is <30 ppm. Fortunately, the soluble silica levels detected in the permeate at Cirent are well within this specification. It has been reported that the soluble silica level in CMP wastewater is strongly influenced by pH: At high pH levels the solubility of silica increases, whereas it is at a minimum at a pH between 6 and 8.³ The mixed wastewater in the Cirent buffer tanks typically has a pH ranging from 4.8 to 6.2.

Chemical Cleaning

Chemical cleaning of CMP UF treatment systems is performed only when the flux has decreased to an unacceptable level. This typically occurs about once a year. During chemical cleaning the permate and all DI water rinses are returned to the UF process tank. The cleaning sequence provides >85% recovery of the initial pure-water flux for the UF modules and includes draining of the UF system, rinsing with permeate, cleaning with a 0.4% (wt/vol) NaOH solution at a low recirculation rate for 1 hour, draining of the caustic solution, and a final DI water rinse. (A caustic solution is recommended by the manufacturer for removal of bacteria and organic materials, and the use of 1% oxalic or acetic acid is recommended for the removal of inorganic materials.) The UF system at Cirent has undergone one such chemical cleaning sequence since its installation in July 1997.

Conclusion

Using an ultrafiltration system to treat CMP wastewater has helped Cirent Semiconductor to meet local effluent discharge limits in terms of total suspended solids. The UF system has handled mixed oxide and metal CMP wastewater successfully, and an analysis of the system permeate has indicated that the recovered water is a suitable candidate for recycling. In addition, no significant fouling of the membrane has been observed, and only one chemical cleaning sequence has needed to be performed in more than 15 months of service.

References

1. AR Sethuraman, "CMP—Past, Present and Future," Future Fab International 5 (July 1998): 261—264.

2. V Krygier, "Membranes—Advanced Ultrafiltration Technology for High Purity Water," Ultrapure Water 11, no. 2 (1994): 56—59.

3. LF Comb, "Silica—Chemistry and Reverse Osmosis," Ultrapure Water 13, no. 13 (1996): 41—43.

Steven Browne is a member of the technical staff at Cirent Semiconductor (Orlando, FL), which is a joint venture between Lucent Technologies and Cirrus Logic. He is responsible for the design and operation of the fab's ultrapure water, industrial wastewater treatment, and recycle water systems. He also oversees regulatory issues surrounding industrial wastewater effluent discharge and the site's UPW well-water permits. (Browne can be reached at 407/371-6443.)

Vivien Krygier, PhD, has been with Pall Corporation (East Hills, NY) for more than 20 years and is involved with filtration systems for CMP processing. Previously, she spent several years in the scientific and laboratory services group, which is involved with system recommendations and problem solving. Krygier has also worked in various marketing positions, developing the company's filtration products and promoting them in the semiconductor industry. She received her PhD in biochemistry from McGill University, Montreal. (Krygier can be reached at 516/484-5400.)

Joseph O'Sullivan, PhD, is a senior staff scientist in the scientific and laboratory services group at Pall Corporation. In this post he has provided technical support to the semiconductor industry for more than seven years. The author of several articles on filtration and contamination control, O'Sullivan received his BSc in chemistry and his PhD in physical chemistry from University College, Cork, Ireland.

Eric L. Sandström is the director of cross-flow filtration systems engineering at Pall Trinity Micro in Cortland, NY. He has 30 years' experience in the development, design, and applications of industrial filtration systems with a specialization in ultrafiltration—the last 7 years with Pall. A professional engineer (PE), Sandström earned a BSME from Bradley University, Peoria, IL.

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