Types of cleanroom contamination
There are six major cleanroom contaminants. All of these are treated in a clean environment to reduce the level of contamination that they cause. These major contaminants are:
- The production facility
- The production personnel
- Process water and chemicals
- Process gases
- Static electric charge
The following table lists the major sources of particulate cleanroom contamination and some common types of these particulates.
|Source of particulate contamination||Examples|
|The room itself||dust and aerosols in the air|
|The operator||hair, skin flakes, bacteria, clothing fibers, finger prints|
|The equipment||flecks of dried processing chemicals, dust, paint flakes, fiber dust, wiper dust|
|Glass or plastic dust||fragments of glass or plastic from when they are cut|
|Dirty solvents||particles in water, cleaning solvents, and the like|
Chemical Cleanroom Contamination
A remarkable number of chemicals are used in many cleanroom manufacturing environments. Chemical contamination generally occurs when unwanted chemicals get accidentally added to the desired processing materials. The amount that gets added is often a very small or “trace” amount. Under normal circumstances this would seem to be of minimal important. However, due to the sensitive nature of many industries that manufacture in cleanroom environments such as semiconductor, LCD and micr0-electronics even trace amounts can have significant effect on yield and performance. A particularly insidious source of contamination, both particulate and chemical may be the processing water that is used in manufacturing.
Bacteria Cleanroom Contamination
Another major source of contamination in cleanroom manufacturing is bacteria. Bacteria are a natural part of the environment and may act as either a chemical or a particulate contaminant, or in the worst case both. While a clean environment minimizes the number of bacteria that are initially in the air, this ignores potential operator contributions of bacteria to the clean environment. For example, a sneeze or a cough will generally put both bacteria and aerosol particles into the air. Bacteria are on everyone’s skin and scratching exposed skin will place both skin flakes and bacteria into the air. Another problem is the ability of bacteria to mutate; thus it is not uncommon for bacteria to grow in water systems and the like. In light of their size, it is not surprising that bacteria are particulate contaminants. But, how are they chemical contaminants? Recall that they are alive and contain a vast array of chemicals. Included among these chemicals are electrically charged molecules called ions.
Sources of Cleanroom Contamination
The discussion will now focus on the sources of contamination encountered in cleanroom manufacturing, and briefly discuss some of ways that these contaminants are minimised and why this minisization is important. The major sources of contamination in cleanroom manufacturing are:
- The air in the facility
- The personnel in the facility (including things brought into the facility with them).
- The water used within the manufacturing process
- The chemicals and gases used in the process
- Static electric charge
- The production facility and equipment in the facility
Each of these contaminants requires special control techniques.
The major approach to minimising contamination in the work environment is to have the manufacturing done in a cleanroom. This is a special room with treated air, personnel containment clothing, filtered water, high purity chemicals, and specially designed equipment.
The contaminants in the air are a major problem. The particulates and aerosols in the air do not behave in the same way as large particles. This is critical because the particles are very small, and thus have a large surface area. This area is available for chemical reactions with other substances as well as physical effects such as electrostatic attraction. The particles that are critical contaminants in air do not behave similarly to large particles. For example, a large particle, say a paper clip, when dropped from ones hand falls to the ground. The situation with very small particles is different, they randomly move around and execute “Brownian motion”—they float in the air for a long time. Fortunately, a large fraction of the particles in the air in a cleanroom are removed via HEPA (high-efficiency particle attenuation) filters. These are typically 99.99+% effective. Thus, at least in the absence of new sources of contamination, only the smaller of the small particles remain in the air. These typically are removed by colliding with other particles until they are so large that they are removed by other means or they stick to surfaces and are removed in this way. This is a bit like dust in a room; large aggregates of dust form “dust bunnies” that “hide” under furniture and the like, while other dust just sticks to the tops of the furniture.
It would seem that the filtered air in a lower class number cleanroom is rather free of particulates, so where does particulate contamination come from? It turns out that nearly everything that comes into the cleanroom brings in contamination. Moreover, while they are essential to the program and process, people are a large source of cleanroom contamination due to the shedding of skin particles.
This shedding does not disappear in a cleanroom. For example, while sitting in a cleanroom an operator can give off between 100,000 and 1,000,000 particles per minute. This is made worse by movement. Walking at 2 mph generates 5,000,000 particles per minute. Furthermore, faster motion makes the situation worse. At 5 mph (a fairly good pace) 10,000,000 particles per minute are shed. Everyday activities also increase the number of particles shed. A sneeze increases the number of particles shed by a factor of twenty.
Of course we also wear clothes so this is not the whole story. Normal clothes add millions of particles to the air even when they are under a cleanroom garment. This is a sufficiently serious problem that wool, cotton or high collars may be prohibited in certain areas. This is not a control issue but a yield issue. Human breath is also a problem. In climates where the temperature falls to near freezing, it is common to see a white cloud of condensing water vapor when people breathe and it is cold. This water vapor is always there, however, we rarely see it. This water vapor from everyone’s breath may also be a problem. Furthermore, smokers’ breath contains five times more particulates than that of non-smokers. In summary, every person entering a cleanroom is a source of contamination.
You are probably aware of how the personnel contribution to contamination is minimised. First, you cover up as much as possible with low contamination clothing. Since our heads and feet are among the worst offenders it is standard to have shoe coverings and head coverings of some sort. Furthermore, our faces are sources of contamination; thus, masks and safety glasses with side shields are required for more than the very important safety reasons. Lastly, gloves are generally required because our hands are a great source of oils, and other contaminants. It is good practice to dress from the top down.
Water Contamination Issues
Water is used both for cleaning (both with added surfactants and in the absence of added surfactants) and for rinsing before or after cleaning with other solvents in many cleanroom production facilities. For this reason, water purity is critical. It is not uncommon to hear of pure spring water, or pure tap water. From the view point of cleanroom manufacturing these are extremely impure. The water used in many cleanroom manufacturing applications is treated to remove the following contaminants: dissolved minerals and salts, particulates, bacteria and organics. Organics are best described as substances that contain carbon, but this is not really sufficient. For now think of them as greasy or oily substances and most common solvents.
Process Chemical Contamination
In some cleanroom manufacturing environments a fairly large number of chemicals are used. Each individual chemical could be a source of contamination. To keep chemicals clean and particulate free highly purified variants are required. These will need to be delivered in clean, non-corrosive containers, transported ‘cleanly’ and not cross contaminated.
In many manufacturing environments a range of process gases can be used for a number of different purposes. These gases need to be correct for the application, be of the highest quality and often filtered to reduce the level of particulates. Consideration also has to be given to any by products or vapors from the gases as these could react to produce an additional unwanted contaminant.
Equipment and Consumables used within the Cleanroom
Any equipment, consumables and furniture brought into the cleanroom could be a source of contamination. Therefore any of these items brought in must be compatible for used in a cleanroom. For example equipment and furniture would be manufactured from the highest grade stainless steel, cleanroom seating from high grade solid resin and consumables such as stationery would be made from special cleanroom bonded paper. Cleaning of a cleanroom can be highly problematic, therefore specialised cleanroom wipes, cleaning equipment and disinfectants will always be used.
Sourced from a number of cleanroom articles