Coating Materials News Vol 6 Issue 1

Introduction:
Problems with adhesion present a recurring concern in the vacuum coating industry. It is an issue that must be dealt with regardless of deposition technique, substrate species, or coating material, and often because of these parameters. Current technology provides the advantages of coating a variety of substrate materials for applications ranging from, but not limited to, polymers of various species, reactive and inert metals, multi-element compound semiconductors, surfaces subjected to high abrasive forces, and operation at temperatures between liquid helium and hundreds of degrees, etc. The coatings we apply are measured in thicknesses of micrometers and are expected to adhere with high force to those substrates when subjected to some of the conditions mentioned. As discussed in previous CMN issues, adhesion is achieved when the internal and external stresses that a coating experiences are balanced with the bond strength to the substrate.

That vaporized materials can be made to nucleate and grow to form solid films on surfaces of dissimilar materials is remarkable in itself and has been the subject of numerous studies since the first vacuum-deposited film layer. This CMN issue will summarize some of the techniques developed to produce adhesion on the more common substrates. Much of the work on surface preparation and adhesion promotion is historically attributed to D. Mattox¹. Surface preparation before coating varies with the surface and coating materials; oxide, non-oxide, metal, and polymer surfaces require separate considerations. First we discuss cleaning, then the final preparation that promotes adhesion.

Preparation of Surfaces:
The definition of "clean" is subject to question since "clean" might mean free of visible particulates, lack of discoloration, freedom from haze, or "clean" at the invisible level. Sources of contaminants that can render a surface unsuitable for good coating adhesion include: the polishing process for glass, polymers, or metals; the etching chemistry for semiconductors; precipitation of hydrocarbons, alkali halide contamination, organic sealants and lubricants, photoresist residual, etc.

Cleaning Techniques:
General Mechano-Chemical Cleaning
This applies to the cleaning of glass, metal, groups II-VI compounds and IVA semiconductors surfaces. The cleaning process often requires several stages, each specifically intended to remove a surface residual contaminant or to condition the surface in preparation for nucleation of the thin film layer. After a glass surface is polished, an organic solvent washing is required to dissolve polishing lap material (pitch, wax), followed by scrubbing with cotton in an aqueous detergent (for example Alconox) to remove the insoluble polishing compound. The ground edges as well as the polished surfaces must be mechanically scrubbed. Thorough rinsing in water to remove all traces of detergent follows. Finally, rising in alcohol to dissolve water from the surface and blowing dry leaves the surface ready for immediate introduction to the vacuum system. The parts are blown dry with air or nitrogen filtered to 0.2 µm. The surfaces might be wiped (in one direction) with a clean material moistened in alcohol or acetone, but wiping is problematical for fear of recontaminating the surface and leaving particulates. This is critical for high energy applications where resident particulates become sites for damage, and for multilayer coatings where a loose particle creates voids leading to spectral leaks or to environmental penetration and instability. Handling of the parts during and after cleaning must be done with latex gloves or metal tooling to prevent transfer of skin oils to the surfaces.

Variations on the above basic procedure include immersion in an ultrasonic bath of the diluted detergent containing a non-ionic surfactant followed by ultrasonic rinsing in circulating water. The detergent might also contain inorganic builders that convert insoluble materials to water-soluble salts. The baths are generally heated to at least 40° C. Some glasses have slight solubility or suffer ion leaching and therefore the time of immersion, especially in de-ionized water, should be limited. The water should be changed frequently because it will become acidic through absorption of atmospheric CO₂. Soft glasses are subject to micro-scratching from residual surface particles during the agitation. These scratches often are not visible until the surface has been coated. Then they might give the appearance of a surface haze. Surface modification through dissolution or cation exchange is felt to be responsible for the susceptibility of some glasses to microscratching. Therefore, the detergent and its additives are important for minimizing scatter and staining.

Vapor degreasing in isopropyl alcohol, ethyl alcohol, acetone, or other organic solvents is used in large volume production. Sometimes the edges are not left as clean as the centers of the parts because the flow is discontinuous there. Some substrate materials are detrimentally affected by ultrasonic agitation, and microscopic surface scoring is observed.

Cleaning of Polymer Surfaces
Polymer surfaces are cleaned in a procedure similar to that for glass with the exception that they require special handling because of their softness and solubility to some organic solvents. In addition, during aqueous cleaning they absorb water which must later be removed to achieve vacuum coating adhesion. Lighter mechanical contact is necessary to prevent sub-microscopic scratching that becomes a source of light scatter or of stress sites after coating. Surface or absorbed solvent or halide salt residues are a greater problem because it is not possible to evaporate the residue by high temperature vacuum baking as it is for glasses.

Preparation for Coating:
Surfaces will remain clean for only minutes unless placed in a special environment such as a dry nitrogen-purged container or in a UV/ozone chamber. Recontamination occurs from air exposure and takes the form of water vapor, hydrocarbons, and other atmospheric pollutants precipitating onto the surface. These may act to prevent or reduce the adhesive bond to the deposited coating. Contamination by hydrocarbons is nearly inevitable. Heating a surface to high temperatures in an attempt to effect cleaning instead causes stubborn carbides to form. Fortunately hydrocarbons can be removed by oxidation through UV-ozone exposure, and this step is sometimes required for critical surface cleaning.

Surface Conditioning and Final Cleaning:
It is often necessary to provide a final cleaning or surface conditioning step immediately before deposition begins. The final cleaning is intended to remove barrier layers that prevent intimate bonds from forming. Surface conditioning forms nucleation interfaces that promote strong bonds. When oxide compound layers are deposited on dissimilar polymer surfaces, an example of the former case is observed. The latter case is exemplified by the growth of metal oxide compound layers on glass and metal substrates, where an oxide interface grows.

Conditioning to Strengthen Adhesion
Some cleaning chemistries leave broken bonds on surfaces, where the contaminants mentioned previously can attach. If the new bond is strong, it will be difficult to achieve a clean surface. If a favorable interface can be created, as in the case of metal oxides on silicates such as glass, then adhesion can be strengthened. These "adhesor" layers are 25Å to 200Å deposits of a metal which readily forms an oxide compound. Examples are chromium, nickel and aluminum, which oxidize in the residual oxygen or water vapor present in moderate vacuum pressures.

Adhesion Requirements
Adhesion of some coating materials often requires a thin deposited layer of the same composition as the substrate surface to provide nucleation sites. Examples are on substrates of glass, fused silica, silicon, and zinc sulfide. Adhesion of gold to fused silica and surfaces has been the subject of much work. Thin coatings of materials such as bismuth oxide, silica, etc. are often required.

In the case of carbon-based coatings on materials like zinc sulfide, where surface and coating chemistries are very different, it is necessary that a carbide compound interface be created.

Glow Discharge
In-vacuum cleaning of polymers and glass is often done with a high pressure (~200 mTorr) oxygen/nitrogen plasma known as glow discharge. The high energy ions in the plasma might operate by oxidizing residual hydrocarbons, might drive water off by heating, might deposit a monolayer of metal, or might disrupt the surface. Some or all of these mechanisms operate to different degrees not fully understood. Generally the glow discharge cleaning appreciably improves adhesion. In some cases adhesion is significantly improved by sputtered metal from the electrode or by surface damage because the presence of foreign material acts as sites for nucleation. In other cases, increased absorption can result.

Ion Gun Irradiation
Another in-vacuum cleaning procedure is ion gun irradiation. Here a stream of high energy argon (sometimes with oxygen added) ions is directed toward the surfaces as they rotate into and out of the beam. The bombarding ions discharge or erode surface contaminants enhancing adhesion. As above, excessive energies can cause surface damage, particularly on semiconductor surfaces. An extension of this process is the mechanism of very high adatom energies achieved in ion beam sputtering where the surfaces are etched or species are implanted or caused to diffuse into the surface to achieve a strong bond.

Deposition Considerations:
The deposition technique and its parameters influence the ability of a coating to adhere to the substrate of another film layer. Partial pressure level and composition, deposition rates, material composition, and substrate temperature affect the stress nature of the deposited layer. For situations where the adhesive bond is weak, it is essential to minimize (or balance) the accumulated stress of the coating. Amorphous layers generally contain less stress than microcrystalline structures. We have treated this subject in earlier issues of CMN.

Environmental Effects:
For many coating-substrate combinations, there is often a delayed reaction that causes a coating that failed adhesion testing immediately after deposition to pass a number of hours later. The reasons for this time delay are variously given as interface diffusion, chemical reaction, and others even less well understood. The phenomenon is frequently seen with metal-silicate interfaces. Another effect is the diffusion of moisture or other gasses that result in weakening of the adhesive bond by changing the interface chemistry or by increasing the stress of the layer. High temperature can cause chemical reaction that results in increased stress that subdues the adhesive forces. These are the reasons that durability testing in the form of humidity and temperature cycles are established means for qualifying a coating.

Conclusion:
The science of the adhesion of coatings to surfaces is a complex one that contains a degree of mystery in the sense that we cannot always explain why it works or does not work. Various process techniques available to improve adhesion were discussed. When the coater is satisfied that the surface is as clean as can be achieved, it sometimes requires deliberate "contamination" with a foreign material, the adhesor layer, to make it work. Generally using materials with low inherent stress is a step in the right direction. Thorough testing under various environmental conditions is necessary to verify and establish the process developed for achieving adherent coatings.

Reference:
D. M. Mattox, Thin Solid Films 53 (1978) 81.

Samuel Pellicori, Principal Contributor
Pellicori Optical Consulting
P.O. Box 60723 | Santa Barbara, CA 93160
Phone/FAX: 805-682-1922

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