Cleanroom Technologies Continue to Keep Contamination at Bay
Garments, filters, ionization and stable materials for cleanroom furnishings are meeting today
Alexander E. Braun, Associate Editor -- Semiconductor International, 3/1/1998
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At a Glance | | |
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Garments -- use vs. particulates
According to Bryan Weber of Aramark Cleanroom Services (Austin, Texas), the cleanroom garment industry faces several challenges. "Part of those challenges we have set for ourselves, others come from our customers. We are driven to do an even better job of cleaning garments while reducing costs; otherwise, we won't be competitive in the future." Weber added that while quality and cost are important factors, service is an important part of the industry's relationship with its customers. "We constantly need to consider ways to optimize routing structures, to keep our inventories at the appropriate levels for customers. It is not enough that they have the appropriate number of garments that they need on an ongoing basis. If there is an emergency evacuation, you have to be there to support that customer's needs."
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| Cleanroom fittings must be more than just furnishings. A chair must not only be ergonomically designed, adjustable and easy to move, but be composed of materials that will not particulate easily and are nonconductive. (Source: Biofit Engineered Products) |
The garment industry finds itself playing a balancing act composed of structuring delivery systems, fast garment turnaround when they come back from the customer and ensuring that garments do not degrade prematurely. When a garment begins to degrade, the generation of particulates begins climbing to unacceptable levels (Fig. 1).
Garment life varies. Some last as little as three years or as many as eight years. Weber said although customers generally want to know how many washings a garment will withstand, it is not necessarily the washings that cause degradation, but the environment that the garment is worn in and how long it is worn between washings.
"Some of our customers will wear the same garment for three or four days at a stretch," explained Weber. "They take it off, hang it up in the cleanroom and when the person comes back, he puts it back on. Others change garments more frequently. More frequent changes mean a larger inventory, but because the time you are actually wearing that garment is reduced, it does not degrade as rapidly." According to industry sources, studies show that within the first four to eight hours of wearing a garment, its effectiveness begins to drop off considerably.
As an industry executive who asked to remain unnamed said, "We do our best to be service-oriented and keep abreast of the latest fabrics and garment construction and different ways of making sure that the garment processing system works optimally. Then the customer ruins all our good work because he thinks he can save a few bucks by having his people wear the garments longer." The executive added that users often fail to recognize that the additional small cost of carrying the extra inventory required to change more frequently is more than likely to be offset by improved yields and contamination control.
In principle, Weber said he agrees with this. "The customer must, on his own, evaluate the cost benefit of the frequency of changes on a garment vs. improvements in contamination control -- and that applies to all supplies, from tacky mats up. What kind of benefit can be accrued if, for example, by changing garments three instead of two times per week, the result is a 1% yield improvement in your chip production?"
Furniture and the cleanroom
During the dim beginnings of the semiconductor industry when what passed for a cleanroom then needed shelving, tables, chairs or any kind of furniture, it was ordered from the same company that furnished the offices or from a medical supplies store. That time is now long past. Stainless steel has become an important fixture in the cleanroom. Manufacturers of cleanroom furnishings like Lynndale Stainless (Phoenix, Ariz.) provide cleanrooms with customized and production furnishings. "Although we do not work with exotic metals, we will make anything out of stainless steel," Sales Manager Dale Schreiner said.
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| 1. According to industry studies, cleanroom garments can begin degrading and producing particulates after four to eight hours of use. The environment the garment is in can also affect its life and effectiveness. (Source: Aramark Cleanroom Services) |
Stainless steel did not become common in the fab environment until about eight years ago. As semiconductor manufacturing began becoming more sensitive to particulates and yields started to suffer, this became an issue. Properly processed, stainless steel is a relatively low particle emission metal. Electropolishing is a crucial finishing step, regardless of whether the cleanroom is Class 100, 10, 1 or 0.5, because it brings up a micron layer of chromium to the steel's surface, and chrome is a very low particulate.
This is not to say that stainless steel does not particulate. According to Schreiner, "Everything particulates. It does not matter what it is; given enough time, even glass will particulate. Our challenge is to reduce it by as much as possible."
One of the major producers of this type of furnishings, Lynndale products are found in all major fabs. Customers consult them about their production areas, and the company meets their needs for workstations, tables of every kind, ergonomics solutions, cabinets and the like. "We adapt to their needs," affirmed Schreiner. "Cleanrooms must have furnishings built to the highest tolerances, and with the best finishes possible. Bargain fittings may produce particulates that contaminate and bring down yields. Also, when the time comes for the fab to expand, the cheap fittings may not be flexible enough to grow with it, requiring replacement."
Second only to contamination, one of the main concerns of cleanroom furnishing manufacturers today is meeting the various local, state and federal ergonomics requirements and guidelines. "We must design our products to fulfill safety requirements established to avoid conditions like carpal tunnel syndrome and back problems." Schreiner added that when a fab requires a table or a chair, it no longer is "just" a table or a chair. "Tables and chairs need to be adjustable -- whether hydraulically, mechanically, electrically or pneumatically -- other items, such as keyboards, must also meet local, state and federal regulations."
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| 2. Shown is AMD Fab 25 in Austin, Texas. Architectural cleanroom products, such as ceiling and wall systems and raised access flooring, play important parts in keeping cleanrooms up to specs, while providing size and configuration flexibility. (Source: Daw Technologies) |
Filtration, static charges and EMI
As the need to filter out ever-smaller particulates evolves, hepa and ulpa filtration and jell seal filter technologies appear to be holding their own. Some filter manufacturers we spoke to were unwilling to go on record, but there is a consensus that there may be hard times coming for this industry in the semiconductor manufacturing environment as robotics become more prevalent.
From the cleanroom perspective, the ongoing challenge to the filter industry is the further reduction of airborne contaminants. According to John Gregor, manager at Contamination Control Products (Marlboro, N.J.), "End-users' requirements pose an interesting challenge: the need to improve yields by reducing, among others, airborne contaminants. The problem is, of course, that although the cleansing and purging of the cleanroom environment are key, and the industry is trying to supply products that will support this need, airborne contaminants are always around -- people must breathe and do things in that air" (Fig. 2).
Filtration technology does not lag behind. Even when dealing with small particle sizes, filters are extremely efficient. Currently, filters can easily remove particles the size of a large bacterium. This is one of the rare instances in which the move to 300 mm will not cause additional problems. The future issue with filtration systems will probably be gaseous contaminants, which are expected to be more of a problem for 300 mm than for the smaller wafer and larger feature sizes. Filters will be gauged not by how well they remove particles, but how well they remove chemicals.
Arnold Steinman, CTO for Ion Systems (Berkeley, Calif.), has a simple and wide-ranging definition of contamination: "It is something that is present in the cleanroom, which you do not want there because it adversely affects your process."
This open-ended view of what is contamination is important because people tend to think of contamination in terms of particulates in the air. However, static charge is as much a cleanroom contaminant as any particle and has an effect on how particulates contaminate. Steinman said, "Static charge's direct impact on contamination is one of particle attraction: whenever anything is statically charged, it will attract particles out of the air, in much the way a TV screen attracts particles. Regardless of how much is spent to keep particles out of the incoming air, there is still some unavoidable particle generation in the cleanroom. It most often occurs very close to the wafer. Garments, furniture, equipment and the actual process itself are all particle-generating activities."
One of the problems the cleanroom manager faces is that the smaller the particle size, the more effect electrostatics has on particle attraction. As the move progresses to 300 mm and as feature sizes are reduced, smaller particles become more significant, acquiring the capability to inflict "killer" defects.
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| 3. This system pipes compressed ionized gas for balance charge neutralization, while keeping clear of moving products and robotics. (Source: Ion Systems) |
A decided advantage of ionization systems is that even though a particle may be undetectable, it still can be neutralized by the technology (Fig. 3). "As particles grow smaller they begin eluding filters, and ionization becomes more of a factor," explained Steinman. "There really is no metrology available for small-particle detection. The limit of wafer scanning equipment is generally 0.1 µm. There is already talk about Class 0.1 cleanrooms, and particle sizes down to 0.02 or 0.03 µm are becoming significant. The problem is that other than measuring particles in the air with a condensation nucleus counter, there is really no surface metrology able to detect the really small ones." Steinman added that fabs could use a 0.05 µm particle scanner.
While contamination is a big issue, the problem of static discharge cannot be ignored. Static discharge, the transfer of charge between one or more objects, is accompanied by electromagnetic interference (EMI). EMI control has become vital in the semiconductor industry because cleanrooms are becoming increasingly crowded. If equipment EMI is not controlled, equipment interactions can affect not only yields, but equipment itself, sometimes creating equipment lock-ups.
Electrostatic discharge (ESD) is a similar problem. It generates radio waves that are picked up by the production equipment and interferes with the operation and the equipment -- and in the cleanroom, everything is an antenna. "Eliminate the static charge, and you eliminate the problem," Steinman said.
As semiconductor technology has evolved, human interaction with the product has been steadily dropping, to the point where today people-created particles are not viewed as serious a problem as those generated by equipment processing, such as photoresist stripping and plasma processes.
The arrival of 300 mm will not make things easier. A larger wafer will require larger equipment, for example, cassettes. In fact, many of the processing and finishing steps will require somewhat larger equipment, and when objects get larger, so does the potential for a static charge; it is an area-related phenomenon. Because the wafers will be too big for people to deal with them using vacuum arms, more automation will be needed. This in turn means more chances for equipment problems, with consequences that are two-and-a-quarter times more serious.
According to Steinman, "The laws of physics are the same everywhere. Static charge generation cannot be prevented, and static charge will interact with your production processes. The only thing that will be different between one fab and the next will be how much tolerance they build up for the problems caused by static charge. The problems will get significantly worse when we go to 300 mm. It is better to solve these static problems during the construction process and in the equipment you buy, than to try to solve them later in the finished fab."
Improving particle detection
As IC geometries and linewidths shrink to 0.18 µm (180 nm) and below, new categories of material defects that were previously unimportant are becoming yield-critical. Also, new processes such as copper metalization will create new categories of difficult-to-detect defects. Traditional optical inspection techniques are rapidly becoming incapable of detecting small-particle defects at 0.15 µm (150 nm) and below on patterned wafers and cannot detect critical electrical ones. However, just as lithography has migrated to shorter wavelengths in the UV and DUV range, optical inspection tools can also migrate to these shorter wavelengths and provide increased defect detection sensitivities needed for subquarter-micron device inspection.
A number of semiconductor equipment manufacturers have numerous UV and DUV development programs under way and expect these technologies will have significant applications below 0.18 µm. Meanwhile, e-beam-based inspection technologies, such as KLA-Tencor's SEMSpec inspection system -- which combines high-speed electron beam imaging with digital imaging processing to provide SEM-quality inspection results in hours -- are playing an important role in detecting defects that cannot be seen optically, such as small defects hidden in dense features, unopened contacts or via holes, defect detection in grainy layers or those with rough surfaces and also electrical defects that exhibit voltage contrast effects.
It is certain that contamination, whether caused by particles or electric charges, will be a fab problem for a long time to come. However, the technology needed to combat or neutralize these yield killers is keeping up with the critical requirements demanded by larger wafers and smaller CDs and will continue doing so in the future.
