Cleaning with Plasma
Ultra-fine contaminations not visible to the naked eye are foundon all surfaces. Almost always, these contaminants must be removed as aprerequisite for faultless further surface treatment such as:
Plasma technology offers solutions for any types of contamination, substrate and post-treatment. In the process, molecular contamination residues can be removed as well. Different cleaning processes are available for various requirements of the specific case. The major processes used include the following:
1.Removing hydrocarbons in the oxygen plasma
Micro-cleaning – Degreasing in oxygen plasma
On nearly all surfaces, there are hydrocarbons, which are residues of greases, oils, or release agents. These coatings severely reduce the adhesion of other materials during subsequent surface treatment. For this reason, chemical removal of hydrocarbons in the oxygen plasma has become a standard treatment before coating, printing or gluing.
The plasma reactions during this cleaning process are illustrated by the example in the section “Small plasma physics”.
Ions, radicals and UV radiation act together. High-energy UV radiation splits macromolecules. Oxygen radicals, ions and split-off hydrogen radicals occupy free chain ends of the polymer chains, thereby forming
Being gaseous in the low-pressure plasma, the degradation products of the hydrocarbons are siphoned off.
On polymeric surfaces, activation by oxygen radicals starts in parallel with the degradation of surface contamination. This activation is a prerequisite for proper adhesion of non-polar plastics. For details, see Activating Materials.
Oil, grease or release agents containing additives cannot always be completely removed in oxygen plasma (without any residue). Solid oxides which adhere to the substrate may form. If necessary, these can be purified in additional downstream purification processes.
Cleaning in oxygen plasma works on virtually all materials. Often, purified dry air can be used instead of oxygen. Accordingly, hydrocarbons can be removed both in low and atmospheric pressure plasma.
2.Mechanical cleaning by micro-sandblasting
Noble gas plasma is a particularly simple plasma. It consists only of ions, electrons, and noble gas atoms. As the gas is always atomic, there are no radicals and, since noble gases do not react chemically, there are also no reaction products. Because of the kinetic energy of the heavy ions, argon plasma is active nevertheless.
The kinetic energy of the impacting ions chips away at the atoms and molecules forming the coat, gradually decomposing them.
The treatment is effective on virtually all surfaces, and thus on any kind of contamination. Almost all contamination that resists chemical corrosion can be removed by micro-sandblasting.
As the positively charged ions are accelerated to a negatively charged electrode, plasma excitation occurs in a parallel plate reactor.
Structuring – physical etching
High-energy ions chip away at fragments not only off the surface coating but also off the substrate material itself. This results in increased structuring and roughening of the surface on a molecular scale. As with sand blasting or grinding, this leads to an increase in surface area and possibly also to undercuts, thereby improving the adhesion of subsequently applied coatings.
Contrary to chemical etching effects in low-pressure plasma, micro-sandblasting is not isotropic, i.e., it does not act evenly on all surfaces of a component, but mainly in the direction of the electric field because the ions are accelerated in this direction.
3.Reduction of oxide layers
Oxide layers are found on many surfaces. After prolonged storage, most metals tend to form oxides. On many metals, oxide layers formduring plasma cleaning in oxygen plasma. These oxide layers interfere with all further processing stages:
Often, solid oxidized deposits are also found on non-metals,which may form during cleaning in an oxygen plasma. Oxide layers can oftenwithstand any corrosion by conventional solvents. Because of their highhardness, it is even difficult to get rid of them mechanically. They areremoved by reduction in hydrogen plasma.
In oxygen or air plasma, it is possible to oxidize extremely thin metal coats as thick as only a few atomic layers. These invisible coats harden and protect the metal from chemical and mechanical damage and further oxidation. They ensure a permanent metallic shine.
Surface oxidation is often carried out in atmospheric pressure plasma.
Typically, various types of contaminants need to be removed from a surface, so different cleaning processes are applied in sequence, such as:
1. Removing release agents (hydrocarbons) in the oxygen plasma
2. Micro-mechanical fine cleaning by micro-sandblasting in argonplasma
1. Degreasing in oxygen plasma
2. Reduction of oxide layers in hydrogen plasma
On the other hand, oxygen purification is followed immediately by the activation of non-polar surfaces by means of oxygen radicals. To this end, the process is continued for some time after cleaning. For details, see Activating Materials and for even more prolonged reaction, Downstream Etching of Materials.
Plasma cleaning has unique advantages compared to other cleaning processes:
Cleans even in the smallest cracks and gaps
Cleans all component surfaces in one step, even the interior of hollow components
Residue-free removal of breakdown products by vacuum extraction
No damage to solvent-sensitive surfaces by chemical cleaning agents
Removal of molecularly fine residues
Fit for immediate further processing (which is highly desired). No venting or removal of solvents
No storage and disposal of hazardous, polluting and harmful cleaning agents
Very low process costs