Materials Science: Essential Science for Contamination Control
HAROLD D. FITCH
While most contamination control professionals do not claim to be experts in materials science, many of the problems we encounter relate to surprises caused by materials reacting unexpectedly in special environments.
Being aware of this specialized physical science can often put us on the right track to finding solutions to unusual problems. It is challenging because our obvious assumptions about the nature of things can be dead wrong when just one variable changes.
Consider iron and its use in steel. We see steel all around us in building and construction projects. One of the usual things we associate with steel is its oxidized form–rust. It occurs when steel reacts with moisture to form iron oxide, creating a reddish brown appearance.
We expect to see rust wherever steel is used in an unprotected state and exposed to moisture. On the other hand, we are all acquainted with stainless steel. Its shiny decorative surface is resistant to oxidation or rust. Stainless steel is often the material of choice where contamination control and ease of cleaning or sterilizing is important. Steel and stainless steel have different reactions to moisture caused by adding chromium and sometimes another element such as nickel or molybdenum to the steel alloy.
Copper is an element that is well known for its ductility and malleability. If a small amount of beryllium is added to copper, we have a material that can be heat treated and that makes very good springs. This is not at all what we expect from copper.
When small changes are made in material composition, millions of fascinating changes occur. New discoveries are made every day. The contamination control engineer must be aware of the properties of the materials being used for contamination control applications. Last month, we talked about chemical compatibility or incompatibility in the case of a filter that was dissolved by the very material it was being used to filter. We learned that the filter must be compatible with what is being filtered. And, the chemical being filtered must be compatible with all the materials it contacts, such as support screens, housings, plumbing, and pumps.
For example, examine some situations in which the wrong choice of materials caused a serious problem in a contamination control application.
Choosing the Right Materials
The first case involves the choice of materials for air duct work in semiconductor fab facilities in Japan. It was decided that plastic duct work would be more corrosion resistant and easier to clean than the standard metal duct work currently used for the major air handling system in semiconductor fabrication plants.
Plastic ducts were installed in several fabricators. This was OK until a fire broke out in one of the plants. The plastic ducts burned vigorously and the fires spread throughout the entire plant in a matter of a few minutes. A costly lesson in materials selection!
In another case, plastic liners were fabricated onto line-heated metal tanks in chemical cleaning and etching operations to prevent corrosion of the base metal tanks in one of our plants. This worked well at first, but led to disaster when the solution temperature was raised and the liners ignited.
It is hard to achieve a pure environment not contaminated by some source. Fortunately, much of this contamination does not harm operations. Sometimes, we intentionally add materials that may contaminate when the value of adding the contaminating material outweighs the potential contamination problem.
One such case is the addition of carbon filler to reduce static charge in plastic product carriers. The carbon reduces harmful static charge, and although there is some contamination from the carbon itself, the net result is positive. The reduction in static charge and its associated contamination problems far outweighs the small contamination from carbon, which is easily removed.
Consider one more real life example. While purchasing an acid for one of our semiconductor processing lines, we found another potential source. The specifications were reviewed and the second source met all of the known requirements. The new source was brought on line.
Although the new material met all our specifications, devices made with this new material consistently failed, even though we could discern no difference in the materials.
We learned two important lessons from that experience. First, it demonstrated differences in materials that were beyond current measurement techniques to identify. However, these differences could still be significant to our processes. Second, it showed the importance of including a functionality test as part of the overall specification.
This last example is important because it shows that even in a careful evaluation, with well motivated people working hard to do the best job possible, there can still be unforeseen occurrences that can bring disaster.
The Role of a Contamination Control Engineer
The role of a contamination control engineer is never easy. However, the chances of success are much greater the more one knows about potential problems and the more careful one is in making the best evaluations initially.
The contamination control engineer may not know all of the properties of the materials being selected, but it is his or her responsibility to be aware of the potential dangers. The contamination control engineer should check out the potential problems and at least eliminate the obvious. An awareness of materials science and consideration of its ramifications must be a part of the contamination control engineer`s arsenal!
Harold Fitch is president of Future Resource Development, a consulting firm in Burlington, VT, specializing in cleanroom education and problem-solving. He conducts international training seminars for CleanRooms` shows and seminars. n