Important aspects of cleaning are the conditions that exist in the processing area, the handling of the surfaces and storage of the cleaned part. Dust is a particular concern in many instances, because particulates on the substrate surface will result in pinholes in the deposited film, and there is no good way to remove the particulates by in situ cleaning. The figure shows the recontamination of a clean, gold surface in a very clean environment, atypical "cleanroom" environment, and in a "machine shop" environment, as determined by coefficient of adhesion measurements between gold surfaces.

Avoidable contaminants in the processing area include particulates; some vapors; and some reactive gases, such as chlorine. The first action that should be taken to reduce contaminants in the processing area is to remove as many sources of contamination as possible. This can mean good "housekeeping," separation of contaminant-producing processing from the cleaning area; elimination or minimization of particulate-producing materials, such as excessive amounts of common paper, books; and short-fiber fabrics, like rugs, cotton-fiber clothing and furniture coverings. Vapor producing materials, such as many molded plastics, many types of paint and vinyl floor, and furniture coverings should be eliminated or minimized. Personnel performing the cleaning should not use particulate-producing personal products, like mascara or body powders, nor vapor producing products, like hand creams.

The second action that can be taken is to contain contaminant-producing sources as much as it is feasible to do. Humans and their clothing shed large amounts of particulates that are "pumped out" through the loose weave of the fabric as the person moves about. The use of head coverings, facial hair coverings and coats of tightly woven cloth will contain the particulates somewhat. Mouth or nose covering should be used to contain aerosols produced when speaking or sneezing. Necessary vapor-producing processes should be performed in ventilated work areas, such as "chemical hoods."

The third action is to contain the substrates being processed, and expose them as little as possible to the ambient. During cleaning, substrates should be held in fixtures made of materials that can be easily cleaned and do not contain potentially contaminating materials such as the plasticizers in many molded polymers. Glass, ceramics, hard metals or unplasticized polymers are used as fixturing materials. It is best if the fixtures do not touch areas of concern, because "abrasive transfer" of materials in contact can result in contamination. The use of fixtures reduces the amount of touching of critical surfaces by the operator during the cleaning process. When substrates are handled, they should be held in non-critical areas, such as edges, with as little abrasion as possible. Fixtures and parts should be stored in clean containers when not being processed.

Handling by the operator should be done, either with tooling to contact the surfaces, or with "low-lint" and low-extractable" gloves, wiping material, toweling, etc. Low-extractable means that solutions used in the cleaning process will not extract materials from the item being used. For example, vinyl gloves should never be used with alcohol, a common "wipe-down" and drying solution, because alcohol will extract phthalates from most vinyl materials. Unplasticized polyethylene, fluoropolymer and most latex gloves are generally suitable for handling clean surfaces. Commonly obtained latex gloves are powdered to aid in putting on the glove, and gloves used in the cleaning area should be unpowdered. Gloves used with "glove boxes" often contain waxes and should be used with care.

The use of gloves must be monitored, because persons wearing gloves often handle contaminated surfaces, then touch the clean surfaces, thereby transferring contamination. When performing critical processing, operators should be trained to put on polyethylene gloves for the actual operation, then discard them when they are through, even if they are continuously wearing latex gloves.

The fourth action to reduce contamination is to use small processing areas, with filtered air sources. These can be "clean benches," which use a horizontal laminar flow of air, filtered through fiber filters to remove particulates; or areas that are separated from the processing area by plastic curtains and use of vertical laminar flow of filtered air, which exits beneath the curtain . When this type of equipment is used, the disruption of the laminar flow, which gives turbulence, should be minimized. Air flow rate through the filters should be monitored and checked for uniformity over the filter surface. In a few cases, activated carbon filters are used with the clean bench filters to remove vapors from the filtered air. The fiber filters can degrade from chemical attack, and should be monitored for particle generation.

Finally, the ultimate action in controlling the cleaning environment is the "cleanroom." The cleanroom uses filters to remove particulates from the air in the whole room. Often, the filters are staged with the prefilter capable of filtering 99.97 percent of particles larger than 0.3 microns, and the second stage filtering 99.999 percent of particles 0.12 microns or larger. Cleanroom standards are measured by the number of particles/cu ft of air, with particle size greater than 0.5 microns. Class 1 cleanrooms are attainable with very stringent design, fabrication and operating conditions. Class 100 cleanrooms are common in fabrication and assembly of particulate-sensitive components and devices. Class 10,000 rooms are usually attainable with modification of existing facilities and moderate control of conditions in the room. Uncontrolled areas with rugs and cloth furniture can be Class 100,000 or greater. Airflow in the cleanroom should be monitored, to prevent "dead spots" where there is no active flow of filtered air. Particle counts in the air are made using light scattering techniques.

The term "cleanroom" is a misnomer, in that in a cleanroom, no attempt is made to remove vapors from the air. Vapors are minimized in the cleanroom by eliminating the potential sources of vapors. Generally, the humidity in a cleanroom is controlled to about 45 percent, in order to minimize electrostatic-charge build-up and still have a comfortable working environment.

Efficient cleanrooms are very expensive to build, operate and maintain . Careful consideration should be given to the use of other techniques for contamination control before deciding on the need for a high-class (really a low-Class) cleanroom.

References

1. D.L. Tolliver, ed., Handbook of Contamination Control, Noyes, Publications (1988).

2. Institute of Environmental Sciences (IES) Compendium of Standards, Practices, Methods., etc., Relating to Contamination Control, I ES-C-CC009-84-T, Mount Prospect, IL

3. "Annual Buyers Guide: A Guide to Ultraclean Suppliers and Products," Microcontamination Magazine.