Aaron Ribner, Ph.D.
Plasmatic Systems, Inc.
1327 Aaron Road
North Brunswick, NJ 08902
Extended Abstract of Talk to the Society of Vacuum Coaters
For presentation at the Contamination
Control Session Thursday March 26, 1992
OBTAINING MOLECULARLY CLEAN SURFACES BY PLASMA PROCESSING
Plasma processing as a method of preparing substrate surfaces for vacuum coating and improve the adhesion of deposited films is described. Surfaces that have been plasma cleaned are suitable for further processes such as film deposition, bonding, gluing etc.
The process of vacuum coating requires the substrate surface be free of material that acts as a barrier to the proper adhesion of the deposited material. Also required for proper adhesion is a substrate capable of sustaining a strong physical, and or chemical bond to the deposited coating. Of the may cleaning methods available, plasma cleaning is one of the more versatile and cost effective.
Plasma cleaning removes thin layers of contamination without having to use great quantities of material. In solvent cleaning one uses 1 liter of solvent to remove 100 Angstroms of contamination. The solvent must be disposed of in an acceptable manner. In plasma cleaning a few cubic centimeters of process gas are used and the residue of carbon dioxide and water can be easily vented to the atmosphere. The parts come out dry after a short treatment. This clearly represents a savings of time and money. Plasma processing alters the surface properties materials by converting hydrophobic surfaces to hydrophilic surfaces. This substantially increases the adhesion of many types of coatings.
Cleaning of Organic Contamination
Thin layers (Up to 25 microns) of organic (or other low molecular weight material) can be removed from surfaces by plasma processing. Parts are placed in vacuum chamber, the chamber is backfilled with a process gas such as argon or oxygen to a pressure of 1-2 Torr and subjected to microwave or radio frequency radiation. See Figure 1 Below:
Schematic of the Plasma-Preen System
The radiation interacts with the process gas and forms ions, electrons and energetic neutral atoms. These species bombard the surface of the sample and transfer their kinetic energy to the low molecular weight atoms present such as carbon, oxygen and hydrogen. The low molecular weight atoms have enough energy to escape from the surface leaving behind a dangling surface bond. These surface bonds recombine to form new substrate devoid of low molecular weight contaminants.
Metal Oxide Reduction
Plasma processing with argon will reduce some metal oxides such as copper and gold. This is accomplished by removing the weakly bonded oxygen atoms. See Figure 2. Argon ions transfer their energy to the oxygen atoms by a collision process. This supplies enough energy for the oxygen atoms to escape from the surface. Argon, being inert, also escapes. The remaining metallic surface bonds recombine forming a free metal surface.
High Energy Argon Ions (MW 40) strike Low Mass Oxygen atoms (MW 16) and Transfer Energy.
Ar+ Ar+ Ar+ Ar+
\ \ \ \
\ \ \ \
O O O O
| | | |
Enough energy is added to the Oxygen atom to Break the Copper Oxygen Bond.
/ / / /
O O O O
| | | |
Pure Copper metal remains behind.
Sputtering of low Molecular weight atoms
In the case of a reactive process gas such as oxygen a chemical transformation occurs in addition to the sputtering process described above. This transformation transpires at significantly reduced temperatures. The reactive species has its chemical activation energy supplied by the ionizing radiation, without having to be heated. This low temperature processing does not damage heat sensitive plastics and enables them to act as suitable substrates.
Surface activation of materials such as polyethylene and polysulphone require the surface chemistries to be altered. Specifically a hydrophobic surface must be changed to a hydrophilic surface. Hydrophobic surfaces are characterized by covalent bonds such as carbon hydrogen bonds (--C--H ) that are present in oils, waxes, paraffin's and polyethylene. These surface bonds must be converted to polar bonds inorder for adhesives to bond and oxide forming metals to adhere.
Typical polar bonds are alcohol O-H and
epoxy bonds ----C---C----
The plasma processing parameters are adjusted to enabling oxygen to react with the surface at a temperature below the melting or softening point of the plastic. The surface is oxygenated forming polar alcohol and epoxy bonds. See Figure 3.
A good method of evaluating if a surface is hydrophobic or hydrophilic is to place a drop of distilled water on it. If the surface is hydrophobic the water will "Ball Up" and not wet the surface. If the surface is hydrophilic the water will wet out the surface in one continuous sheet. Plasma processed surfaces tend to be very hydrophilic with low contact angles between the water and the surface.
Virgin Polyethylene Surface
H H H H H H
| | | | | |
High Energy Oxygen Ion Strike the surface.
+ n ---O--O---
Formation of Alcohol and Epoxy groups on the surface.
H H H
O O O O O
| | | / \ / \
Surface Oxidation of Polyethylene
Some type of contaminants such as fingerprints are compound in nature. Fingerprints are composed of inorganic sodium chloride, and other salts, held in place by an organic binder of finger oils. Plasma processing will remove the organic binder. Once the binder is removed water can be used to dissolve the remaining salts. This technique is useful for cleaning optical components before anti-reflective or other coatings are applied.
The Plasma-Preen System
Plasmatic Systems, Inc. has developed a method of using microwave energy generated from a microwave oven to excite a plasma1. In the Plasma-Preen system microwave energy is directed into a vacuum bell jar, containing the work to be processed. The walls of the bell jar are transparent to microwave energy. Controls are added to vary the analog power of the microwave radiation. The base plate is water cooled to improve substrate temperature control at high plasma densities. Microwave ovens are supplied with duty cycle and microprocessor controls all ready built in, which are incorporated into the Plasma-Preen system. Using a microwave oven allows for a significant reduction in the cost of the equipment to make high power plasma cleaning equipment.
Plasma processing is capable of producing surfaces that are free of oils and other hydrophobic contamination. These surfaces facilitate the adhesion of thin films. The use of a microwave oven in the manufacture of plasma processing equipment makes the equipment affordable to all.
1.A. Ribner, US. Pat. #4,804,431 (February 14, 1989)