Ultrapure Materials—Fluids

Designing practical DI-water recycling systems for use in semiconductor fabs

Jack Martyak, Jacobs Engineering

Water conservation is a concern throughout the United States, Asia, and Europe, and in some locations it is mandatory. This situation is particularly important to the semiconductor industry because water use in wafer processing continues to increase. As wafer size has grown from 150 to 200 mm, there has been an approximately 250% increase in ultrapure water (UPW) consumption per wafer, the result of both the need for additional processing steps and the added topography on the wafer surface. The forthcoming increase from 200- to 300-mm wafers is expected to add another six- to sevenfold increase in water consumption. Recent developments in process technology such as chemical-mechanical planarization (CMP) also have increased the demand for UPW in the fab.

It is common industry practice to install a water reclamation system that processes wastewater for use in a facility's evaporative cooling towers. This practice not only reduces the amount of water drawn from municipal or private sources, it also lowers waste discharge levels and is a good engineering practice since the fab wastewater—deionized (DI) water with low levels of process wastes—is often of higher quality than the sourcewater.

The recycling of DI water to the manufacturing area has also been practiced since 1984, although the earliest treatment systems were inherently flawed. At that time, fab technicians mixed process chemicals manually, which introduced the potential for human error and unexpected contaminant loads, and the recycling systems could not be designed to remove all possible contaminants. Today, there is little operator intervention in wafer processing. The use of programmable logic—controlled (PLC) tools ensures that each wafer lot is processed in exactly the same manner, and chemicals are added automatically. This greater predictability has led to the creation of recycling equipment that is designed to treat DI water with known contaminants at specific concentrations. The new systems collect the wastewater from the third through eighth rinses in the wet tools in a dedicated drain system, which transports it to reclaim-water tanks and the DI-water recycling equipment (see Figure 1). The highly concentrated wastes from the first and second wafer rinses are discharged through a separate drainage system. The treated recycled water can either be blended with virgin UPW feedwater streams or used in process steps that do not require the highest purity levels, such as steps where dissolved oxygen concentration is not a concern. It has been projected that 30—50% of a fab's UPW can be reused. This article reviews the concepts and planning considerations behind the design of a successful DI-water recycling system.

Figure 1: Schematic of a typical system for treating DI water for reuse. In this system, some rinsewater is reclaimed for nonprocess use as detailed here, and some is recycled for use in process-related and other applications.

Basic Design Considerations

When designing a once-through DI-water system for a semiconductor fab, both the quality of the feedwater and the water requirements at the wafer must be known. Seasonal variations in water quality as well as sourcewater variations also have to be taken into account to ensure that the specified purity levels are maintained. The design of a water-recycling system also involves such issues, but it requires review of many additional items as well, such as tool layout in the fab, collection and redistribution systems, tool drain connections, and—most important of all—the process recipes used in the wet-process tools. The typical fab has hundreds of tools discharging water wastes to collection drains. Their different process recipes can lead to hundreds of potential variations in the quality of the wastewater going into the water-recycling equipment. The leading contaminants found in such water can be classified as bacteria (both viable and nonviable), colloidal material, dissolved gases, inorganic salts, organic carbon componds, particulate, pH (alkalinity or acidity), and silica (dissolved and reactive). The interaction between these various forms of contaminents and the level of detection required are also of concern to recycling system designers, as is the potential for untreatable contaminants entering the water treatment equipment. Even though wafer processes are automated, the possibility of human error still exists (an example being during tool maintenance, when PLC is overriden) and wastewater may be delivered to the recycling system in an out-of-spec condition.

At the outset of the design process, the designers must meet with the fab management responsible for the project to define specific project constraints, goals, and requirements. For example, one constraint may be that no site or facility shutdowns will be permitted to tie in new equipment. A goal may be to recycle 30% of the UPW being consumed, and a requirement may be to have the system deliver specific percentages of recycled water to various fab locations, or to blend a particular percentage with the incoming water supply. The programming session leads to the development of engineering concepts for the recycling system design. These concepts also reflect the cost-effectiveness of the various treatment equipment options in relation to the quality and quantity of the wastewater streams.

The design process is usually separated into two major areas: the collection and redistribution piping networks, and the equipment and process treatment flow required to ensure the contaminants in the waste streams are removed. The design of the piping is no trivial task and must be preceded by a complete facility and wet-tool survey. This survey must include such items as all drains and tool connections and types, separate drain systems for dilute acids, DI-water recycling, solvents, and hydrofluoric acids and heavy metals, as well as sanitary and storm drains. The survey must also include tool layout and the congregation of like tools; existing and potential pipe routings; and tool and process requirements involving water, such as wafer rinsing; chemical dilution; steam generation; and tool, wall, and floor cleaning. The distance between processing areas and mechanical rooms, and the existing wastewater collection and repumping systems must also be covered in the survey.

The design of the recycling system can occur concurrently with the fab survey. The first step in this design process is to consider the hierarchy of potential uses for the treated water. As much as possible of the wastewater collected should be utilized in reclaim applications, such as in cooling towers, boilers, and scrubbers. Another high-priority use of the water, which should be quantified, is in non-wafer-contact processing, such as tool cleaning, quartz sputtering, and glass cleaning. Use in fab humidification systems is also a possibility. These applications usually do not require that DI water be as high in purity as that used at the wafer-processing wet benches. Those wet-process areas that do not require the removal of dissolved oxygen gas from the water should also be identified. Examples include open tanks and brush cleaners used in CMP. The last potential use for the recycled water is in the UPW system. The recycled water is often of higher quality than raw water from a municipal source, and its use will reduce the demand for such water. However, recycling to the UPW system is the riskiest option, because a major upset could shut down the entire fab.

Reclaim System Design

As seen in Figure 1, the wastewater that is to be reclaimed for use in such nonprocess applications as cooling towers and scrubbers is separated from the water to be recycled and undergoes minimal treatment. Before designing the reclaim system, the project team needs to develop a water management flow diagram for the site that takes seasonal variations in water consumption into account. The composition of the wastewater to be reclaimed (which can be from the third through eighth wafer-rinse steps) is then determined, and equipment is selected that can accommodate the expected flow volume and will ensure that the objectionable contaminants are neutralized or removed. In general, this wastewater is of a dilute acid nature, and therefore needs to be filtered and pH adjusted. Once this has been done, the reclaimed water can replace municipal water in various applications, lowering both the fab's water intake and its liquid discharge. Because the composition of the reclaimed water will vary from that of the sourcewater, the fab's chemical treatment programs will need to be adjusted. Depending on the use of the reclaimed water, changes may be needed in the following areas: oxidizing/nonoxidizing biocide addition, sequestrant addition, antifoam addition, antiscalant/anticorrosion addition, and water temperature. In addition, corrosion coupon racks should be installed to monitor the condition of equipment that contacts the reclaimed water, such as condenser tubes and tower decks.

Recycling System Design

The treatment equipment and processes used for DI-water recycling must be designed to remove the contaminants present in wastewater, which are very different from those in feedwater from a municipal source. Thus, the first steps in a practical recycling system are the collection and analysis of the wastewater (see Figure 2). Go/no-go decisions regarding recycling are then based on the water quality and contaminant concentration data determined by the analytical monitors. On-line, real-time analysis is used for such parameters as conductivity, pH, and temperature, but analyses for particles and total organic carbon (TOC) may take as long as 8 minutes. Because of this lag time, multiple tanks are needed to allow continuous water collection while the monitors are cycling. Tank sizes should be chosen based on the incoming flow volume and analysis time, and the capacity of the treatment equipment should be based on return-on-investment factors and on the quantity of wastewater the fab hopes to recycle. The usual goal is to recycle as much water as possible, which may be as much as 50% of the DI water used.

Figure 2: Schematic of the recycling process flow in a typical system for treating DI water for reuse.

The go/no-go upper contamination limits are set as follows:

• Conductivity: Capacity of treatment equipment, such as ion exchangers.

• Organics: Capacity of treatment equipment, such as chemical oxidation units.

• pH: Local codes governing dual-containment versus single-wall piping.

• Temperature: Capacity of treatment equipment, such as heat exchangers.

If the incoming wastewater is out of spec, the first option is to divert it to the reclaim system. If it cannot be accommodated there, it is drained to the industrial waste neutralization and treatment (IWNT) plant. If the latter action occurs, flows need to be adjusted so as not to overtax the IWNT plant's capacity. In addition, if the recycling system is going to be used to supplement UPW production capacity, a municipal water backup system should be installed to prevent loss of capacity during long-term diversions of recycled wastewater, such as during investigations into unclassified contaminants in the waste stream.

After the acceptance of a batch of water into the recycling system, the water is pumped from the analysis tanks to a succession of treatment systems for the removal of the wafer-processing wastes entrained in the once-ultrapure DI water. The first treatment step is the neutralization of the water's pH, which is usually done with the addition of a caustic (e.g., sodium hydroxide). The design of the succeeding process flow, such as the one shown in Figure 2, must take into account several engineering goals. The first is to remove the contaminants; the second is to configure the treatment process so that no downstream tanks or vessels will be fouled. Measures also must be taken to maximize water flow while minimizing the need for operator attention and equipment maintenance.

The most worrisome, and most difficult to remove, processing contaminants are low-molecular-weight organics, usually organic solvents, which can foul downstream equipment, such as the reverse osmosis (RO) skids and ion exchangers, and provide a nutrient source for microorganisms. The conventional RO treatment for the removal of TOC is not completely effective for semiconductor process rinsewater. High-molecular-weight organics, such as those found in raw water supplies, are removed easily by RO membranes, but low-molecular-weight organics pass through. Therefore, chemical oxidation processes are used in recycling systems to ensure that low-molecular-weight organics are removed.

Oxidation processes based on hydrogen peroxide injection (H^₂O^₂) have been developed by several UPW equipment suppliers. The injection of H^₂O^₂, coupled with the transmission of ultraviolet light into the water, destroys any organic molecules, regardless of species, by oxidizing them to carbon dioxide. Various systems also include additional features to optimize organic destruction and removal, such as activated carbon adsorption vessels. The drawback of the chemical oxidation process is that a high dose of H^₂O^₂ is required, so further processing is needed downstream to remove any residual chemical.

The next concern is particulate loading on downstream RO or ion-exchange equipment. In most cases, the water will already have been deionized (probably to >18 m‡-cm at 25°C), so the total dissolved solids (TDS) that must be removed will consist of organic salts from various wet processes. The particulates in the wastewater stream can include precipitated chemical salts, metallic rinses, plastic shed from wafer carriers, processing reaction solids, silica/silicon from the wafer, tool-related erosion by-products, and even large pieces of broken wafers. Other particles can come from the airborne reaction of chemical vapors in a minienvironment, or chemical leaks in an equipment core area, and the resulting precipitation of particles into the DI-water rinse tanks if the cleanroom airflow is not adequate.

Large particles can be removed from the water by 1.0-µm cartridge filters. Once the water has been filtered, it needs to undergo treatment to remove bacteria, small particles, any remaining organics, and salt. The most cost-effective equipment for this purpose is an RO system. However, thin-film composite RO membranes are made of a polyamide material that can be damaged easily by contact with an oxidant, so the water must be reneutralized before it enters the RO unit. This can be done with an injection of sodium metabisulfite. The sulfite reacts with any residual H₂O₂ and the resulting product is completely rejected by the RO membranes. Because the TDS levels in fab wastewater are low compared with levels in water from a municipal source, a single-pass RO system can be utilized. The concentrated waste materials from the RO skid should be directed to the IWNT plant.

At this point in the recycling process, the water's pH has been neutralized and contaminants such as bacteria, low- and high-molecular-weight organics, particles, and salts have been removed. Water temperature remains a concern, however, and heat exchangers are used to cool the purified wastewater prior to its reuse. Some semiconductor fabs use "hot" (60°—80°C) DI water. In those cases, the wastewater will also require cooling before the RO separation step.

Once its temperature has been adjusted, the recycled water should be ready for reuse. In reality, however, because any excursion in water quality can upset fab production, the recycling system design must include another analysis step. As with the initial analyses, dual tanks are required. If monitors reveal that the water is out of spec, it can be returned to the first set of analysis tanks. Valuable processing has been done on this water, and a trim or dilution step should be all that is required.

If the recycled water meets specifications, a preprogrammed decision about the process flow direction occurs. (The potential applications for the water were discussed earlier.) For process-related applications not requiring resistivity levels of 18 m(omega)-cm at 25°C and low dissolved gas concentrations, the water may flow directly from the final analysis tanks to the point of use. If the water will come in direct contact with wafers, very low levels of contaminants and a high level of resistivity are required, so additional treatment steps are necessary. These final processes include ultraviolet radiation (to ensure TOC removal), electrodeionization, and 0.1-µm cartridge filtration. Recycled water also can be directed into the UPW system, where any dissolved gases can be easily removed using existing equipment. Because the UPW feedwater will become a blend of municipal and recycled water, the overall net effects will be reductions in contaminant loading on the UPW treatment equipment and in the use of municipal water.

Conclusion

The notions of abundant water resources and unlimited disposal capabilities have been replaced in the semiconductor industry with an acknowledgment of the need to limit both sourcewater use and wastewater discharges. Indeed, DI-water recycling is a necessity in areas where water from the municipal source cannot meet fab demand and in locales where large volumes of liquid discharge are prohibited.

A DI-water recycling system is a specialized succession of wastewater treatment processes that remove manufacturing contaminants from wet-process rinsewater. A practical, well-designed system can reclaim water for nonprocess uses, such as in cooling towers and scrubbers, and recycle purified water back to the fab for support and process-related applications, and for blending into the UPW system. As UPW usage continues to increase with increases in wafer size, such recycling may be required no matter where a fab is located.

Acknowledgment

This article is a revised version of a paper originally presented at the 44th annual technical meeting of the Institute of Environmental Sciences and Technology, which was held in Phoenix in April 1998. Used with permission.

Jack Martyak is manager of the chemical and ultrapure water technology group of Jacobs Engineering (Phoenix). He has 16 years of experience in the field of chemical process and piping design, having worked for IBM before joining Jacobs. As part of his involvement in developing and implementing new concepts for ultrapure-DI-water systems in support of advanced manufacturing technologies, Martyak has written site system reports, conducted feasibility studies, and implemented conceptual and final designs. He also has published and presented several papers on UPW systems and on semiconductor contamination modeling and yield calculations, chaired several technical conferences, and served on conference and magazine advisory boards, including MICRO's editorial advisory board. (Martyak can be reached at 914/677-5250.)

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