Directions in Contamination Control
Robert McIlvaine
Alpa Bagga
The McIlvaine Company, Northbrook, Ill.
-- Semiconductor International, 7/1/1999
As the dimensions on semiconductor wafers
decrease, the purity requirements for the air and water used in the fab
increase. To meet this challenge, the supplier industry is upgrading its
technology and is also modifying the ways in which it does
business.
Business changes
Substantial consolidation has taken place in the contamination control arena. Purchasers of systems to purify air and water are finding that their suppliers are now large companies with global reach. For example, Paris-based Vivendi has achieved a major consolidation with its acquisition of US Filter, headquartered in Palm Desert, Calif. US Filter, in turn, has purchased hundreds of companies in the water and ultrapure water business including Fullman International Inc. (Portland, Ore.), one of the nation's largest installers of process piping systems for carrying high purity water, chemicals and gasses through ultraclean manufacturing systems, with annual sales exceeding $130M.
Cleanroom service companies are also consolidating with five acquired by MPW Industrial Services Group Inc. (Hebron, Ohio) and operated under the brand name of The Pentagon Group. Suppliers have followed suit with, for example, Cintas Cleanroom Resources' (Cincinnati, Ohio) recent merger with Ameritech Resources (Austin, Texas). Increasingly, semiconductor device makers are able to buy all their disposables from a single source such as VWR (West Chester, Pa.), thus benefiting from cost efficiencies by eliminating large numbers of small purchases. The end result is that the semiconductor manufacturer will be dealing with fewer companies who will supply a greater variety of products and services.
Innovations in air purification
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| Fig 1 In the next two years, total cleanroom orders will approach $650M with HVAC (heating, ventilation and air conditioning) dominating at ~$400M. |
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| Fig 2 Over a five-year period, fluctuating expenditures reflect an increase in demand and a decrease in capital equipment costs. |
Semiconductor companies spend a great deal of money to achieve ultrapure air in their fabrication facilities. The semiconductor industry will spend $479M on cleanrooms this year and, by 2001, $646M worldwide -- $399M in heating, ventilation and air conditioning and $247M on other cleanroom hardware such as perforated floors and ceilings comprised of HEPA filters (Fig. 1). A significant portion of these purchases will be for technology that has only recently become available. Currently, the traditional ballroom cleanroom is being replaced by minienvironments, a concept based on the assumption that wafers need to be kept as far away from people as possible. Isolation of the wafers is achieved by enclosing each process in its own environment. Robots then move the wafers in sealed containers between processes. Taiwan Semiconductor, Zilog and other chipmakers have achieved high yields with this approach. Japanese as well as U.S. suppliers are committing to the minienvironment concept for 300 mm wafer plants.
Despite minienvironments, reduction of molecular contamination and submicron particles throughout the fab now has a higher priority than in the past. Conventional HEPA and ULPA filters that are used for air filtration are typically made of fiberglass. While these filters have been effectively used for the removal of particulates, there has been substantial evidence in recent years of boron contamination in the filtered air, seemingly from the fiberglass in the filters. Air samples that are exposed to the HEPA/ULPA air environment during digestion exhibit increased boron levels when compared to unexposed samples. One solution has been a new design of ULPA filter made of W.L. Gore's (Elkton, Md.) expanded PTFE membrane. In a side-by-side evaluation, PTFE membrane filters showed substantially less boron contamination than those exposed to fiberglass. While the PTFE medium is more expensive than micro-fiberglass, suppliers claim greater durability. Initially, Gore was the supplier of both the membranes and the completed filter; now, Sweden-based Camfil will use the Gore media in its ULPA filters. Suppliers of glass media used in filters are also making improvements. U.K.-based Hollingsworth and Vose has a new glass medium that has 100 times less boron than the traditional product and is also claimed to be substantially less costly than PTFE.
Other filter innovations on the horizon include electrostatically-charged depth filters. Developed at the University of Tennessee's Textiles and Nonwovens Development Center (TANDEC), USA, and the Technical University of Liberec, Czech Republic, these filters are especially suited for the increasingly stringent requirements placed on HVAC and HEPA filters. By combining the high filtration efficiency of nonwoven fabric with bulk and resiliency provided by staple fibers, these new composites have both bulk and low pressure-drop, thereby providing notably increased particle-holding capacity while maintaining high filtration efficiency. Several other companies in Japan and North America are also developing electrostatically-charged media.
In Japan, at companies such as Toyoka Riken and Fujitsu and at the Tokyo Institute of Technology, researchers are working on UV irradiation devices to remove gaseous contaminants in semiconductor cleanrooms. Studies confirm that substances such as organic compounds are substantially reduced when the air is irradiated with UV light. UV irradiation combined with air washing and condensation provides effective removal of gaseous substances such as organic compounds and ammonia.
Water purification
As line width is reduced in silicon wafer fabrication, water purity requirements go up. This means a substantial increase in water consumption. A new 200 mm wafer fabrication facility with 5000 wafer starts per week will require at least 3M gallons of water a day. Increases in wafer size increase water demand. For example, the increase from 150 to 200 mm wafers created a nearly 250% increase in demand for ultrapure water. The forthcoming increase from 200 to 300 mm wafers is expected to add another six- to seven-fold increase in wafer fab water consumption.
Purchases of ultrapure water systems for the semiconductor industry were just over $1B in 1997. These systems include engineering and construction, reverse osmosis (RO) systems, instrumentation and controls, pumps and piping. In 1998, capital requirements for ultrapure water systems came down resulting in fewer dollars being spent on equipment. However, the demand for ultrapure water continues to rise. Figure 2 shows ultrapure water system expenditures through 2001.
During the course of fabrication processing, a wafer will be chemically etched and cleaned many times; each of the etching or cleaning steps is followed by a water rinse. Water from a city system contains unacceptable amounts of dissolved minerals from salts in the water, particulates and bacteria. The salts separate into ions creating contaminants in semiconductor devices and circuits and are removed from the water by RO and ion exchange systems.
A typical water treatment system at a fab uses all available options in varying combinations to attain water of the highest purity. Raw feedwater, generally a combination of city water and recycled wastewater, initially undergoes multimedia filtration for removal of particles greater than 5 µm. By adding HCl and H2SO4 to alkaline water and NaOH to acidic water, pH adjustments are made that can reduce CO2 and, therefore, scale formation, thus preventing the pollution of downstream RO membranes. Next, removal of ionic, organic, particle, bacteria and silica contamination is performed with RO. Generally, two reverse osmosis units are used in series, separated by a forced draft degasifier that removes any remaining CO. A 185 nm UV sterilization and/or anionic exchange resin usually follow to oxidize and remove organic compounds and silica, which may have passed through the two RO systems.
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| Fig 3 Schematic of the electrodeionization process illustrates the use of direct current to remove impurities from water. (Source: E-Cell Corp.) |
Demineralization, most commonly mixed bed, is the next step to achieve 18 M+ resistivity. Water exiting the mixed bed resin tank enters a PVDF-lined storage tank, which has been sparged with ozone to oxidize organics and prevent bacterial growth. It then passes through a 254 nm UV unit to convert the ozone to oxygen and subsequently into a vacuum degasifier to reduce dissolved oxygen, CO2 and volatile organics. A mixed bed demineralization follows as well as additional UV treatments. The last step is final filtration or point-of-use treatment. Microfiltration, ultrafiltration and reverse osmosis are used in various combinations as final barriers to ions, particles, bacteria and silica. Often a UV sterilizer and membrane filter are added to prevent bacteria, which is sloughed off in the preceding UF/RO step from entering the system. At this point, the water is ready for use at ambient temperature.
Electrodeionization (EDI)
New concepts in ion exchangers have been triggered by a growing need for higher purity water coupled with a trend away from processes using chemicals. Electrodeionization (EDI) is being used in the semiconductor industry as an alternative to traditional mixed-bed ion exchange systems.
EDI is a combination of electrodialysis and ion exchange in one apparatus. The technology uses electricity and a combination of membranes and ion-exchange resins to force contaminants from the feed stream and into a waste stream while continuously regenerating the resin bed. This electrochemical regeneration replaces the chemical regeneration of conventional ion exchange systems, eliminating the need for hazardous chemicals.
Figure 3 shows the EDI process. Anion- and cation-selective membranes are arranged perpendicularly to an electric field, which causes ion migration through the membranes. The anion exchange membrane is permeable to anions only, the cation exchange to cations only. Negative elements migrate towards the positive electrode (anode) and positive towards the negative (cathode). These ions, however, are unable to travel all the way to their respective electrodes as they approach the adjacent ion-selective membrane, which has the opposite charge. This prevents further migration of ions, which are then forced to collect in the concentrate channel. Here the ions are flushed from the system in a high ionic stream called the concentrate; the channel running through the resin bed produces a stream with low ionic content called the dilute. As water passes down this channel, it is progressively deionized. The application of an electric field will lead to water splitting that produces hydrogen and hydroxide ions to regenerate the ion exchange resins. The resulting water is typically 99.9% free of ions and dissolved inorganics. EDI also provides a high removal efficiency of components like B, SiO2 and TOC, which normally are extremely difficult to eliminate. It is a safe, non-polluting alternative to traditional ion exchange systems and is inexpensive to operate. Pre-treatment can be accomplished with an RO system.
Throughout the 1990s, the EDI market has been dominated by US Filter and Ionics. Both companies target niche industries like the semiconductor industry that are concerned about conventional water purification chemicals and use relatively modest amounts of ultrapure water. These systems generally have limited flow rates and can be costly.
E-Cell (a subsidiary of Glegg Water Conditioning in Canada) claims its EDI technology incorporates a fundamentally different modular approach that reduces costs and broadens their application to a wider range of customers. The company uses an approach that simplifies membrane composition and can handle flow rates up to 2000 gallons per minute.
Christ Ltd. of Sweden has taken yet another approach and patented its SEPTRON process, which uses a spiral wound technique of the module. Christ claims the results are higher degrees of deionization with a small membrane surface and regeneration of the mixed-bed resins, along with superior removal of CO2 and SiO2.
Recovery and recycling
Recycling is projected to be both a political and economical necessity. Tighter effluent standards are being implemented in many countries throughout the world. There are a number of examples of semiconductor manufacturers who have already implemented recycling programs. IBM (East Fishkill, N.Y.) has saved $1.75M over the last seven years through recycling. Intel (Rio Rancho, Ariz.) is recovering 65% of its ultrapure water. The site was saving up to 300,000 gallons per day of municipal water in 1996 and increased that figure to a million gallons per day the following year.
The design of a water-recycling system includes the tool layout in the fab, collection and redistribution systems, tool drain connections and the process parameters used in the wet-process tools. The typical fab has hundreds of tools discharging water wastes to collection drains. Their different process parameters can lead to hundreds of potential variations in the quality of the wastewater going into the recycling equipment.
Wastewater from the semiconductor industry usually contains elements such as silicon and fluoride, miscellaneous acids and organics. Design of a recycling system should include separation of rinsewaters from concentrates. Rinsewaters can be further separated to produce highly organic streams, fluoride streams and other acids. An efficient option commonly used in the semiconductor industry is the reclamation of secondary ultrapure water rinses from production tools. The ultrapure water production unit uses secondary rinse water as source water.
In an ideal reclamation system, incoming reclaim water drains into a set of collection and buffer tanks. The reclaim water quality parameters such as pH, TOC and conductivity are monitored continuously. When the predetermined set points are exceeded, the reclaim water is fed directly to the wastewater treatment. Reclaim water meeting the requirements is fed to the reclaim system where it is treated as required. The treated reclaim water is monitored once more to confirm it meets the necessary predetermined parameters. If it meets the parameters, it is then used as feedwater in the ultrapure water treatment system.
What's next?
In next-generation fabs, more sophisticated and accurate particle and gas
monitors will be required. The concern about molecular contamination means that
more contaminants need to be continually measured. As the line size is reduced,
the diameter of 'killer particles' is also reduced. This means the monitor has
to be more accurate in counting submicron particles. Furthermore, the advent of
the minienvironment technologies raises the need to measure contamination in
many different enclosures not in just one big ballroom style cleanroom. The
growing emphasis on ultrapure water in industrial applications that use fewer
chemicals and less water will produce improvements in filtration and membrane
technologies, spurring new innovations in contamination control. 
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