Floats like a butterfly, Stings like a bee...What to do about AMC?
04/01/2001
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Many are fighting the good fight—and winning
by Chris Anderson
It's never really been enough just to get a handle on all the particles floating around in cleanrooms. Today's manufacturing engineers, who have a pretty good handle on taking particles out of the air, are now turning their attention to a new potential yield-limiting culprit: airborne molecular contamination (AMC). And unlike particle contamination, airborne molecular contamination is sneaky.
It's in the air. It's part of the air. And those whose job it is to measure and detect the gases and acids and bases that mingle with the air in a cleanroom, say that concentrations even on the miniscule magnitude of parts per billion can be enough to seriously limit production yields.
So where does this contamination come from? Lots of places. It comes from the off-gassing of the finish on that new piece of equipment. It comes from the chemical spill that was cleaned off the floor, but not cleaned out of the air or from the paper plant five miles away. It can even come from the chips themselves during manufacturing. Traces of chemicals used in one part of your production may get a free ride into the next room, then wreak havoc on that process.
You wouldn't be alone if you thought getting a handle on molecular contamination is nearly impossible. But those in the business of keeping the air in cleanrooms clean say they have many of the answers. From a new generation of activated carbon filters treated with a variety of chemicals designed to pull particular elements out of the air, to highly sensitive air-quality measuring devices that help determine optimal manufacturing conditions, there are many who are fighting the good fight against airborne molecular contamination—and winning.
Matt Middlebrooks, senior research engineer at AQF Technologies LLC, a filtration media supplier in Charlotte, NC, says advances in the control of airborne contaminants have ridden on the wings of cooperation among the three major parties involved: cleanroom manufacturers, environmental monitoring companies and filtration providers. "The three industries are jointly progressing and developing in tandem," he notes. "The cleanroom industry is trying to figure out what they have and how to control it. The monitoring industry is trying to help them find what levels and types of chemicals they can measure. And, finally, the mediation or the filtration industry is also going through rapid development in terms of what kinds of products are best suited to take out these chemicals that are usually found at very low concentrations."
Competition slowing knowledge flow?
While various organizations work to develop parameters for defining and classifying clean environments with respect to acceptable levels of molecular contamination, many companies hold their methods for reducing the levels of certain elements as closely guarded secrets.
"We are under a lot of non-disclosure agreements with our customers," says Chris Muller technical services manager for Atlanta-based filtration company Purafil Inc. "Many of the companies we visit consider their particular process and what they are looking to filter out as proprietary. They see it as something that potentially can give them an advantage over their competitors."
That can make it hard for someone like Muller to share what he has learned of what works and what doesn't work in particular situations, especially in a field that is still developing baselines of acceptable substance levels in the manufacturing environment of substances such as ammonia, acids or boron. But Allyson Hartzell, a senior staff scientist, micro mechanical products with Analog Devices (Cambridge, MA), contends there is plenty of literature available to help plant managers determine when, or if, they have a problem with certain contaminants. "There is plenty of literature available in this field that has information that includes thresholds for things like boron auto doping that can guide people and help provide information to conduct an accurate cost-benefit analysis to solve the problem," she says, "That kind of analysis is happening all the time."
Still, certain industries are holding their AMC practices close to the vest. "In the optics industry they are concerned with anything that may cloud or obscure the passage of light through the products," says Muller. "Anything they find that can reduce this clouding can provide a huge advantage, so that information is kept secret."
Starting inside
It's no secret that many of the chemicals used in semiconductor manufacturing can hurt the end product if introduced during the wrong process. Control of the environment inside a plant is often the first place to look if a company has noticed a decrease in its yield.
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"Acids are a major problem within wafer fabs," says Mark Camenzind, senior research chemist at Balazs Analytical Laboratory (Sunnyvale, CA). "Spills within a plant are fairly common and these can affect both the manufacturing and the performance of HEPA filters in some cases."
Dopants such as boron, phosphorous and arsenic circulating in parts per billion quantities can be enough to materially affect the resistivity or conductivity of circuits and result in lower yields. And bases like ammonia can have an equally pernicious affect.
The problem, of course, is that these are the very same materials used throughout the manufacturing process, so limiting the flow of these "fugitive emissions" within the plant is a paramount concern. To help tackle the problems, filter manufacturers offer a range of filters, usually activated carbon that is impregnated with materials designed to create a chemical reaction that in turn removes the trace elements from the environment.
Adding filters in the re-circulation process can be an especially tricky task in older plants. "Often there is just not the space necessary to put the filters in if it was not considered in the original design of the plant," says Muller.
That problem doesn't exist on some of the newer plants built today. "There is no question that as the geometries in the industry get smaller and smaller, contamination of this kind is more of a concern," says James Empson, worldwide market manager for semiconductors at Donaldson Co. Inc. (Minneapolis). That means if the plant is not already equipped with filters for AMC at the outset, the designers at least included space where the filtration systems can be added at a later date.
But new plants have their own problems, Camenzind notes. "The new equipment, the sealants and the paint and everything in a new plant is new and that means there can be significant off-gassing problems," he says. Suppliers of equipment to the semiconductor industry have heard these complaints loud and clear, say AMC experts, and are already making strides to use materials that have a significantly lessened impact on the plant's environment.
"The thing to keep in mind is that, aside from optics, molecular contamination problems can occur everywhere in the food chain of a product," says Camenzind. From the parts supplier to the device maker to the one who integrates it in a product to the user, these are all areas where it can be a problem."
Camenzind and his company are often called in to monitor the atmosphere and measure the elements in the air inside a manufacturing plant in an attempt to detect problems. Elevated levels of particular elements are red flags. But merely limiting or targeting specific contaminants when looking for a solution can be short-sighted, according to Empson.
"What you have inside a fab is really a very dynamic chemical soup," he says. "You never know, but removing or targeting one thing can have other consequences. It may be that what you are targeting kept something else in check. So once you remove it, another problem crops up."
For this reason, filtering system providers tend to encourage plant managers to take a wider approach to reducing the contamination levels within a plant. They may encourage using specific activated carbon filters designed to remove a range of acids or bases, thus lowering the overall contamination across a wider spectrum.
"Precise monitoring can be expensive," notes Muller. "If you can come in and do some reactivity monitoring, it won't give a precise read but it can help us suggest a solution."
Still, Camenzind stands behind the precise monitoring his company conducts as a major tool in helping plant operators manage AMC. "It is important to have this information not just from when there is a particular problem at a plant, but also readings from when the plant is running as it should be," he says.
These base lines can make it easier to diagnose AMC problems if the plant experiences a significant drop in yield and more precisely determine where, if at all, in the manufacturing process a problem exists.
"In the case of a catastrophic spill at a plant, having these numbers can also help return the plant to normal," he says. "It also helps with insurance claims for a spill to be able to show the proper levels you should be at after clean up."
How's the air outside?
While fugitive emissions inside a plant can wreak havoc on yield, the same can be true of the make-up air drawn from outside a plant. Sulfur compounds, heavy metals and other pollutants can also affect operations. But there is another culprit from this source: odor.
Muller notes an instance where one client's plant was close to a Burger King, and when the wind was just right you could smell the food inside the plant. While he admitted it might not be an unpleasant smell, his company nonetheless was charged with eliminating the odor from the makeup air. Why?
"Because the semiconductor industry works with such extreme and toxic chemicals in their process, the operators are trained that if they smell something out of the ordinary they evacuate," says Middlebrooks. "So that is a situation where they are just trying to control an odor to prevent an unnecessary shut-down, which can be very expensive."
The type of industry nearby also will have an affect on what kind of filtration is used for the make up air. Paper plants, coal-fired plants, agricultural operations and even other wafer fabs nearby all have their own pollution signatures and ones that should be addressed on a plant-by-plant basis.
Which is really the only way to treat AMC issues, say those interviewed for this report. "Everyone has their own processes and comfort levels with how they do things," says Empson. "In some older plants, the process may not be perfect, but the people have worked hard to get where they are and they don't want to tinker with it."
Muller concurs, noting that installing one type of makeup air system for one plant, doesn't necessarily mean that another nearby will automatically follow suit. "It just doesn't work in this industry to go in and tell people at one plant what you just put in across the street and they need it too," he says. "They may not need it or may not even care. There is no one solution, it all needs to be geared toward each individual operation."
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Airborne Molecular Contaminant filters and particle sloughing characteristics
by Scott Moore
 Figure 1. Particle shedding characteristics - textile vs. carbon tray system. |
The microelectronics industry has found a new enemy to contend with in the form of chemical contamination. Where high efficiency particle filters such as HEPAs and ULPAs used to be sufficient for cleanroom fabrication area protection, this is now not the case. Airborne Molecular Contaminants (AMCs) can now create wafer and hard drive failures reducing product yield and costing thousands, if not millions, of dollars worth of revenue loss.
Microelectronics firms are just starting to get a better understanding of the identity of these contaminants, their sources and at what levels (concentrations) they become detrimental to the process. For example, many wafer fabs are currently using microfine glass (borosilicate glass) in the HEPA and ULPA filters they have employed for particle control. Unfortunately, these filters emit Boron, which is a dopant and adversely effects the final product and yield thereof. As a result, many fab owners have all the components used in the manufacture of a HEPA or ULPA tested for out-gassing characteristics before they can be accepted for use. Many fab managers are now starting to use PTFE media in their ULPA filters as this media does not emit Boron. These managers are also starting to realize the importance of using chemical filtration can be both an added safety factor for fab personnel as well as increase product yield and overall fab output (chemical filters can be used to remove Boron).
Chemical filters themselves should not be specified without the utmost scrutiny as they are the only line of defense against unwanted AMCs whether generated within the fab (recirculation) or originating from outside (make-up air). When choosing a chemical filter, most fab owners concern themselves with capacity, weight, mass transfer zones, pressure drop, out-gassing and cost. Particle sloughing is a characteristic that is frequently overlooked or entirely dismissed.
The various styles of chemical filters currently used by fab managers include textile-based filters (see Photo 1), honeycomb-filled panels, carbon tray systems and carbon bonded panels (see Photo 2). Carbon tray systems and honeycomb-filled panel filters have been used for many years. However, they are best used on the make-up air-handling units rather than at the fab level due to their extensive particle sloughing characteristics.
 Photo 1: Close-up of carbon bonded panel. Latex binder prevents out-gassing and particle sloughing. |
Figure 1 compares a textile-based product to a carbon tray system with regard to particle sloughing. It is apparent that the textile-based filter produces very little sloughing in a very short time compared with the tray system. Honeycomb-filled products exhibit sloughing characteristics similar to those of a full retention tray system but to a lesser degree due to less fill of sorbent media. However, both honeycomb and full retention carbon tray systems can be used quite effectively in the make-up air handlers for the fab. High efficiency ASHRAE filters utilized downstream capture any sloughed carbon particles, protecting the expensive final filters and the fab area.
Carbon-bonded panels can be used on both the recirculation side of the fab or on the make-up air handlers, depending on fab owner needs. The reason for this is that the bonded panels exhibit very low sloughing characteristics and they have nearly the capacity of a full retention granular carbon tray system. Another nice feature is that unlike granular carbon tray systems, which have to be refilled and require spare trays available for replacement during the regeneration process, carbon-bonded panels are completely disposable.
 Photo 2: Carbon textile bonding technique reduces particle sloughing. |
There are downsides to every product and carbon-bonded panels are no exception. They are very heavy and require a tray type housing to be effective, that is 10 trays per housing with many housings needed can get very expensive for the fab owner. The type of metal frame used is critical in some fabrication areas as ions emitted from these frames can be deleterious to the process. For example some chip manufacturers cannot accept galvanized frames on their bonded panels. Stainless steel grades must be used as they will not emit these harmful ions. In addition, these panels are very fragile and must be packaged, shipped and handled with utmost care as breakage of panels is quite costly.
Chemical filters are becoming more and more prominent in cleanroom fabrication areas as the fight against dopants, acids, bases and condensables (AMC) rages on. As fab managers and owners continue to search for ways to provide the best atmosphere possible in their cleanrooms, chemical filtration has been recognized as a leading technology for AMC control. Although this leading technology is needed and in many cases required, it is not without scrutiny itself. This scrutiny should revolve around which style of chemical filter should be used in which application. Contaminant removal efficiencies, out-gassing and especially sloughing characteristics of chemical filters should be taken seriously by fab owners as these qualities are very important when selecting the correct AMC control filter for the correct application.
Scott Moore is product manager of Air Quality Division at Airguard, Louisville, KY.