The Gas Plasma Alternative to Wet Cleaning

by: Don Paquin
Pages: 45 - 49; March, 1994

Ever-tightening restrictions on chemical use have intensified the search for "drop-in" substitute liquid cleaners. Some manufacturers have discovered, however, that the best approach is to minimize or eliminate gross cleaning altogether and do a final dry clean with gas plasma.

Plasma is not a laboratory curiosity. This process has been an essential production tool for more than 20 years on the fabrication of microelectronic devices. ^1,2,3 Over this period, plasma has gradually been accepted by a much broader range of industries: automotive, medical, textiles, and plastics to name a few. Today it is routinely used to clean and surface treat plastic automotive bumpers, stainless steel syringe needles, angioplasty balloon catheters, plastic lenses, golf balls, lawnmower distributor covers, and many other diverse products.

This non-toxic gaseous (usually oxygen or air) discharge process will remove even the toughest organic residues from parts within minutes and, best of all, chemical safety, storage, disposal, and cost are virtually non-issues.

Chemical safety is assured since all cleaning takes place in a closed vacuum chamber and the reaction by-products are evacuated through a vacuum pump as soon as they are formed. These vapors do not require scrubbing and are simply vented to the atmosphere through a standard house exhaust.

Plasma Chemistry Breakdown

Given enough electrical excitation, almost any gas will break apart to form a glowing mixture of electrons, charged ions, and neutral molecular fragments (free radicals). Although some chemical reactions are not well understood, most of the products formed are easily predicted from more conventional chemical reactions.

To utilize plasma it is only necessary to understand that these reactive molecular fragments will combine with thin organic surface films to form volatile gaseous by-products which are pumped away. Since the reaction occurs on the surface of a part, the bulk of the material is unaffected. Most metals, ceramics, and glass materials show no visible sign of cleaning. Plastics are only superficially etched by oxygen after extended cleaning.

For most cleaning applications, industrial-grade oxygen gas (O₂) is used at a pressure of about .1 to 1 Torr. Under these conditions, a pale blue gas discharge will occur when a radio-frequency source is connected across a set of electrodes placed within the vacuum chamber. Parts to be cleaned are simply placed on these inner electrodes which may be in the form of a cage or removable shelves.

Cleaning takes place by the combined action of ultraviolet light (generated in the discharge) and atomic oxygen fragments reacting with organic residues on the part. Since most residues are hydrocarbon-based molecules, a generalized formula for the two stages of reactions may be written as follows:

1. O₂ + RF energy - 2 O atoms, ions, and electrons plus UV and visible light.

2. CxHy (residue) + Oxygen plasma - CO₂ and H₂O vapor with trace amounts of carbon monoxide and smaller hydrocarbons.

Overall, the reaction is similar to normal combustion of hydrocarbons; but in this case it takes place at low temperature (usually 25 to 50° C), and extra energy is supplied by highly energized ions and ultraviolet light. The latter helps break apart hydrocarbons and provides the activation energy to initiate the chemical reaction.

Small Doses

Only a small fraction of the gas is actually broken down to reactive fragments, and most of the by-product stream is unreacted oxygen. A typical cleaning process uses only about 30 grams of oxygen and produces only a few grams of by-products per hour.

Most of these by-products are small quantities of harmless gasses such as carbon dioxide, and water vapor with trace amounts of carbon monoxide and other hydrocarbons. To put this in perspective, 10 minutes of automobile exhaust is approximately equivalent to one year of plasma cleaning exhaust.

The removal rate of organic films from silicon wafers has been studied extensively using plasma. ^4,5 Sophisticated plasma systems used in integrated circuit plasma systems used in integrated circuit manufacturing are capable of strip rates exceeding 1 micron (40 micro-inches) per minute at temperatures near 200° C. Lower-cost industrial systems are easily capable of rates up to .2 microns per minute under more modest conditions of 100° C or less.

In applications where heat is not a problem, thicker organic residues up to .001 inch may be removed in about an hour at 150° C. Most lighter cleaning and surface treatment is carried out in 15 minutes or less including pump downtime. This assumes that most of the gross cleaning already has been done by hot water rinsing or wiping.

Dry vs. Wet Processing

Many manufacturers already have discovered that it makes no sense to use gallons of solvents and acids just to remove thin surface films. This is especially important when the application requires a hyper-clean surface, such as in printing, potting an adhesive bonding applications where as little as one atomic layer of organic contamination may be detrimental.

Even the purest solvents leave behind some residue, resulting in a weak boundary layer between part and adhesive. Plasma is usually considered a "final clean" for achieving a surface totally free of organic contamination.

When cleaning plastics, plasma offers a tremendous advantage over solvent cleaning since it both cleans and surface-treats at the same time. Atomic oxygen removes organics and also chemically combines with the material surface to enhance chemical properties for adhesive bonding. After plasma treatment most surfaces are considerably more polar and wettable, allowing adhesives to fill surface micro-pores and form stronger covalent bonds (see Table 1).

Unlike liquid chemicals, plasma affects only the surface of the material being treated. All chemical, physical, mechanical, electrical, and optical properties of he bulk material are left intact. Solvents and acids may cause serious damage to surfaces of plastics or affect mechanical properties if absorbed into the bulk. Gases can penetrate into hidden recesses and microcracks that would otherwise collect and concentrate solvent residues.

Regular monitoring, replenishing, and disposal of chemicals are unnecessary with plasma. Fresh gas flows through the chamber throughout the process, and the by-products are continually evacuated.

The strongest argument for dry processing can be expressed in one work: consistency. Several parameters define the process, and all are microprocessor controlled. The cleaning strength of a gaseous discharge will depend mainly upon power, time, pressure, gas flow rate, and gas type. All of these variables can be precisely repeated during each cleaning cycle. The net result is a high degree of day-to-day repeatability and higher yields.

Economics and Efficiency

Compared to liquid cleaning systems, plasma is considerably less expensive when all costs of chemical storage, disposal, and liability are factored in. After an initial capital purchase of $60K for a medium-sized plasma system, operating costs are minimal.

For example, one tank of industrial-grade oxygen gas (about $20) will often last a year or more for light cleaning. Comparable liquid cleaning processes typically require gallons of solvents or aqueous cleaners costing at least hundreds of dollars a month plus disposal expense up to $500 per drum, not including permits. Additional savings will be realized as operators spend less time on chemical monitoring and safety systems.

Training time is also minimized with dry processing since plasma is an easy to use as a microwave oven. After loading parts into the reactor, one button activates the system t carry out each step of a pre-programmed process. Fail safe shut-off controls automatically stop the process if some part of the system falls outside of preset operating specifications. Operators usually are able to carry out side tasks while parts are being cleaned.

System Setup

Many types of gas plasma systems are currently available for cleaning applications, ranging in size from small, modified microwave ovens to large chambers designed to hold several car bumpers. Most applications utilize closed-vacuum chambers where cleaning occurs in a batch mode, but continuous treatment also is possible for polymer sheets or fibers on spools.

A typical batch system consists of a square or cylindrical vacuum chamber made of aluminum with a hinged door for loading parts. In addition, all systems have several key features in common: a vacuum pump, a radio-frequency generator, a gas flow module, and a microprocessor-based controller.

After parts are loaded into the reaction chamber, the process begins by first pumping down to a vacuum of about .05 Torr. The process gas then may flow through the system at a regulated pressure of between .1 to 1 Torr while pumping continues. The radio frequency generator, operating at 13.56 MHz, supplies excitation power.

Most systems allow for automatic control of many process variables, including pressure, power, gas flow, temperature, and time. Multi-step processes may be stored in the controller memory which insures a high degree of process repeatability.

Plasma may not fit every cleaning application, but it eliminates most problems associated with wet chemicals without the undesirable health, safety, or environmental risks.

References

1. J.R. Hollahan and A.T. Bell, Techniques and Applications of Plasma Chemistry. John Wiley & Sons, New York, NY 1974

2. B. Chapman, Glow Discharge Processes. John Wiley & Sons, New York, NY 1980

3. R. d'Agostino, Editor, Plasma Deposition, Treatment and Etching of Polymers. Academic Press, Inc., San Diego, CA 1990

4. R.F. Reichelderfer, M. Welty, and J.F. Battey, Journal of Electrochemical Society. 124, (12) 1926, (1977)

5. R.L. Bersin, Solid-State Technology. 124, 147 (1977)

About the Author

Don Paquin is a senior applications engineer for GaSonics International (San Jose, Calif.), responsible for chemical process support for the company's industrial plasma-cleaning division, and management of its Industrial Applications Laboratory.