Plasma General Information The type of plasma used in cleaning results when a gas is subjected to an electric field. The excited gas forms ions, free radicals, or other reactive species. Under vacuum this can occur near room temperature, so that heat damage is not a problem. In the plasma cleaning of metals, ceramics, and glass, contaminants are desorbed from a surface as a result of electron and ion bombardment and the surface heating associated with these impacts. Plasma cleaning requires no solvents, eliminating waste and residue. Plasma cleaning produces small quantities of CO2, water vapor, and hydrocarbons that may not require scrubbing and are simply vented to the atmosphere. Plasma cleaning processes have been used for at least 20 years in the electronics industry to clean contacts requiring high conductivity. Plasma cleaning has been used for semiconductor wafer, automotive bumpers, stainless steel syringe needles, angioplasty balloon catheters, plastic lenses, golf balls, lawnmower distributor covers. Plasma cleaning is not limited to readily accessible sections of a part. Blind holes, and any other sections that are gas accessible, are readily cleaned with plasma. One advantage of plasma is its ability to clean assemblies containing different materials. The four major surface effects of plasma are removal of organic contamination, removal of material by ablation (micro-etching) to increase surface area or to remove a weak boundary layer, cross-linking or branching to strengthen the surface cohesively, and surface chemistry modification to improve chemical and physical interactions at the bonding interphase. Oxygen and air are commonly used as the gases producing the plasma. Other cited gases include argon, nitrogen, and an oxygen/CF4 mixture. Oxygen plasmas will react with organic impurities on the surface to form volatile oxides, which can be pumped out of the system. A nitrogen plasma produced less cleaning of stainless steel than an argon/oxygen mix but did not harm the chromium dioxide corrosion protection layer. Most metals, ceramics, and glass materials show no visible sign of cleaning. Plastics are only superficially etched by oxygen after extended cleaning. Only the surface of the material is affected. Plasma can be used to remove oxide, polysilicon, and nitride from quartz materials. Plasma may be used for a final cleaning to totally remove organics after initial cleaning to remove heavy contamination. For example, a hot water rinse could be used before the plasma cleaning to reduce the required cleaning time. Plasma both cleans and surface treats plastics. Atomic oxygen removes organics and chemically combines with the material surfaces to enhance chemical properties for adhesive bonding. Plastic surfaces usually become more polar and wetable, allowing stronger adhesive bonds. Lap shear bond strengths of some polymers are increased following oxygen plasma cleaning. Plasma cleaning of polymers such as polypropylene, polyethylene, polycarbonate, polystyrene, and nylon may eliminate the need for additional surface preparation. Systems range in size from small modified microwave ovens to large chambers designed to hold several car bumpers. Batch cleaning is more common than continuous cleaning, but continuous treatment may be used for polymer sheets or fibers on spools. Plasma cleaning processes have been used for many years in the electronics industry to clean contacts requiring high conductivity. Plasma cleaning has been used for semiconductor wafer cleaning. Oxygen plasmas will react with organic impurities on the surface to form volatile oxides, which can be pumped out of the system. A typical plasma system consists of a vacuum system, a radio frequency (RF) generator, an impedance matching network, a gas controller, a reaction chamber, and a microprocessor-based controller. The process may be developed for batch or continuous processes. Typical prices for batch systems range from $70,000 to 200,000. For continuous systems, prices range upward from $130,000. The usual temperature for this process is 25-50 °C. The removal rate of sophisticated plasma systems used in integrated circuit manufacturing are capable of strip rates exceeding one micron per minute at temperatures near 200 °C. Lower-cost industrial systems are easily capable of rates up to 0.2 microns at up to 100 °C. Pressures in the plasma units usually vary from 0.1 to 1 torr. Plasma may be used to dry parts or materials. Lap shear bond strengths of some polymers are increased following oxygen plasma cleaning. Plasma cleaning of polymers such as polypropylene, polyethylene, polycarbonate, polystyrene, and nylon may eliminate the need for additional surface preparation. State Information | Glossary | Conversion Checklist | Comments Home | Process Advisor | Alternatives | Links | Search All SAGE material, Copyright© 1992, Research Triangle Institute Last Update: 18 March 1995 sage@rti.org http://clean.rti.org/alt.cfm?id=pl&cat=gi