Surface Cleaning using RF Plasma

Plasma cleaning is an environmentally safe and effective method for removing contamination from a wide range of materials and products. Plasma has major advantages over normal "wet" cleaning methods.

• No handling costs associated with the purchase and disposal of hazardous chemicals
• No residues that are normally left behind with "wet" processes
• Minimal safety risk, no expensive safety or contingency training is required
• Environmentally friendly, the reaction takes place in an interlocked vacuum chamber
• Low temperature process, a chemical reaction is generated by electrical energy.
• Fast simple and easily controllable process with minimal operator input

Aqueous cleaning processes are used to remove residues which are dissolved into the liquid. These residues will almost always re-deposit as a very thin film, (mostly as an atomic layer,) over the "cleaned" surface when a part is dried.

Due to surface energies these, thin films can then be extremely difficult to remove by any means other than by vacuum plasma. These very thin films or residues are frequently the cause of poor and inconsistent adhesion.
In a vacuum plasma, surface cleaning is accomplished at the molecular level. Contaminant molecules are volatalized by either the energy, or the chemical effects of the plasma. Depending on the gas mixture, heavy ions can also be used to remove contaminants by physical bombardment.

After RF plasma processing, components are truly "atomically clean"

Plasma Principles

RF plasma processing is conducted inside a sealed chamber which is reduced to a medium vacuum before selected gases are introduced. The gas or mixture of gases is then energized into a plasma by an AC electrical field which is generated between an array of electrodes. The plasma then reacts at the surface of any material placed inside the chamber, modifying the surface and removing contamination which is turned into volatile molecules. These volatiles are then evacuated from the chamber by the vacuum pump.

The two types of plasma that can be generated is a primary series plasma that is controllable in terms of direction of its etch, (this is beneficial for selective surface treatment,) or a secondary species plasma that is homogeneous, (and will treat all surface areas of any geometry at the same rate).

Plasma cleaning and surface modification is largely dictated by the selection of source gas. The most common plasmas and their reactions are listed below:
 

Oxygen. Oxygen plasma generates atomic oxygen which will react with carbon material to form CO2 and water.

Argon. Argon plasma which has heavy atoms and gives off ultra-violet energy, these combined with heat will cause carbon compounds to form into methane and other smaller volatiles.

Hydrogen. Hydrogen plasma generates volatile molecules when used with metal and metal oxides. This can reduce oxides back to the pure metal state.

Bonding Enhancement

Cleaning is necessary to facilitate the removal of an unwanted material, typically a material that will inhibit or be to the detriment of a subsequent process. An example of this is the removal of grease or oil from a surface that would otherwise prevent a paint or coating or ink from bonding.
A clean surface therefore enhances adhesion when bonding plastics, rubbers and metals. Organic contamination and metal oxide is usually hydrophobic, (bad wettability,) once removed and a surface is enhanced by plasma it can be made to be hydrophilic, (good wettability,) and superior bonding is achieved. Obviously there are numerous applications for RF Plasma cleaning which can be used to directly replace costly ozone depleting CFC solvents, or water wasteful Aqueous based cleaners.

Surface Engineering

Surface Engineering can be  defined as:

• The modification of a material's surface, without affecting its bulk properties, to improve its performance.
• The change of a material's performance making it suitable for new applications.

Surface engineering can change the mechanical , electrical, and chemical properties of the materials surface. By selecting appropriate gases for the RF plasma process, specific changes or new characteristics can be introduced into the the materials surface. The two main applications of Surface Engineering by RF plasma processing are:

• Surface Cleaning
• Adhesion Promotion.

 Adhesion Promotion

Adhesion Promotion can be achieved using an RF plasma for it can improve mechanical, electrical and chemical adhesion mechanisms. This can be done by selecting a process which either:

• Roughens the surface at the molecular level to improve the mechanical bonding by the physical impact of highly energized ionized particles
• Etches the surface by the chemical action of the free radicals.
• Radicalizes the surface, ensuring that it is chemically reactive with the adherent material.
• Changes the chemistry of the surface so that it has an affinity with the adherent material

 Typical Industrial Applications

Polymers. A plasma technique that increases adhesive bond strengths for polymers, is to briefly exposure the polymer to an oxygen or argon plasma. This will increase adhesive bond strengths for coatings or inks for marking, (bar-codes ID tags etc.).

The cleaning mechanism of this type of plasma includes, ionic bombardment. The high kinetic energy of these plasmas will bombard the surface of the polymer releasing trapped moisture and generating a molecularly rougher surface exposing more surface area for bonding.

Also The polymer becomes cross linked and the plasma process puts hydrophilic or active groups onto the surface. A combination of these factors improves adhesion.
Optical surfaces can be treated with plasma to improve surface adhesion. Plasma removes contamination from the surface, then roughens the surface as with polymers, and rearranges electrons to enhance electrical adhesion. The advantage of plasma treating as opposed to other standard cleaning methods is that there is no detrimental effect to the materials composition.
Medical uses for plasma treatments are vast, from sterilization and the removal of organic compounds to the surface modifications and bonding enhancements of a variety of different materials. Some of the typical uses in the medical industry are.
 
 

• Increasing the bond strength's between angioplasty balloons and catheters
• Improving lubricity and biocompatibility of catheters, including lumenal surfaces
• Depositing coatings on guide wires for better lubricity
• Increasing plastic to metal adhesion for syringe hubs
• Ultra cleaning and surface modification of intraocular lenses for better wetting characteristics
• Activating surfaces of biomedical sensors for enhanced performance

Plastics and Rubbers. Good surface wettability and activation is necessary to make good adhesion of inks, paints, lacquers, adhesives and metallizations to plastic and rubber surfaces. Adhesion is related to:

• The cleanliness of the substrate
• Its radicalization
• Its surface topography

RF plasma surface treatments can improve the wettability of plastics and rubbers by raising the surface energy of the material. This is achieved by the action of highly reactive free radical molecules created in the plasma which form functional groups on the substrate surface.

These molecules then have a much better affinity with the coating material than the unmodified surface and a superior adhesion is achieved. Plasma  can also improve surface adhesion by removing organic contamination, by radicalizing and by roughening the surface.

Printed Circuit Board manufacture. Plasma processing is currently being used in  many different processes in the manufacture of this technology:
 
 

• Desmearing and etching of a wide variety of printed circuit materials prior to metalization
• Treating PTFE panels to improve plating adhesion
• Descumming and removing photo resist residue from the fine line panels for better dry film solder mask adhesion
• Removing residual conformal coating from lands for better bonding and solder ability
• Cleaning the surface of polyimide inner layers in flex/rigid flex circuits prior to lamination to promote adhesion
• Cleaning gold contacts for better wire bonding strength
• Eliminating pink ring formation

General Engineering. There ia a vast potential for plasma to provide solutions for General Engineering in order to improve products, and processes. A variety of manufacturing companies have successfully identified Plasma as a means to provide fast, reliable processing, which eliminates hazardous risks and reduces costs. Some typical applications:

• Reel to reel processing of Webs, Filter material, and fabrics to produce hydrophobic surfaces
• Metal cleaning of wire / thin metal substrates
• Bonding of PVC
• Enhanced surfaces for extruded tubing
• Fiber treatment