A surface free from undesired extrinsic deposites is a fundamental condition for any subsequent process. Therefore a plasma cleaning process needs to be executed prior to any further plasma treatment.
Surface cleaning by Oxygene plasma:
Surface deposites, most particularly those which remain on surfaces after mechanical and chemical treatment with liquid cleaning agents often are organic. Usually remains of grease, silicones, release agents or biological material are not completely removed by these conventional cleaning methods. However even minimal remains may inhibit tremendously the quality of subsequent treatments like coating, bonding or painting or in case of medical implants may inhibit the biointegration or cause infects.
Plasma cleaning by an oxygene plasma, alternatively often by a dry air plasma usually can remove even smallest remains of organic substance.
Stimulated oxygene molecules and radicals are very reactive. The UV-radiation of the plasma is able to crack polymeric chains of hydrocarbons. Oxygene radicals rapidly link to unsaturated links of cracked hydrocarbons and so prevent those from recombination. This process causes the degradiation of macromolecules to smaller molecules which are gaseous and can be removed by the vacuum pump.
Removing surface oxidation
Almost any kind of metal carries a surface layer of oxidized material, if it has been exposed even short time to the open atmosphere. In case of non-noble metals this is a most desired effect, because this oxidized layer which usually is very thin but hard and stable prevents oxidation of the bulk material.
However oxidized layers strongly inhibit subsequent attachment of additional components, such as soldering, bonding and electrical conductivity.
In a hydrogene plasma stimulated hydrogene molecules, ions and radicals cause reactions with the oxygene of the oxidized layer to aqueous vapour which will be removed by the vacuum pump.
Micro sandblasting by an argon plasma
Neither oxygene nor hydrogene plasma can remove any kind of deposites, Molecules which remain are most essentially salt compounds and ceramics. Even those can be removed by the micro sandblasting effect caused by the mechanical impact of inert gas ions accelerated by an electric field which is not selective and effective on almost any kind of substrate.
The inert gas used for micro sandblasting usually is argon.
Because of the non selectivity also the substrate by itself will be treated by micro sandblasting. Very often this is a desired effect because it causes roughing of the surface and improves bonding. If this etching effect is not desired on the substrate the time of argon treatment must be optimized to minimise substrate treatment.
The treatment velocity of micro sandblasting is small. As far as hydrocarbons are involved therefore oxygene plasma cleaning usually is executed in advance. Additional remains are removed by subsequent argon plasma treatment. An argon/hydrogene-plasma can be used if oxid layers need to be removed in addition.
The video shows the principles of plasma cleaning.