Plasma Cleaning

Plasma Cleaning and EtchingWhy Plasma Clean?
Plasma cleaning is primarily used in a diverse range of industries to modify materials surfaces prior to bonding (adhesion). Plasma cleaning can replace chemical treatment and etching processes (leaving no organic residue and requiring no subsequent solvent clean) and mechanical abrasion methodologies such as sand blasting. Surface modification uniformity is significantly improved with plasma cleaning with respect to other techniques; observed in the improved uniformity of adhesion results after cleaning.

Typical applications which routinely employ plasma cleaning include:

Surface modification of a wide range of materials for improved bonding

Photoresist stripping

Silicon etching

What is a Plasma?

The plasma state is generated when a gas is subjected to sufficient energy to brake down its molecular integrity and dissociate it into ions, electrons and other sub atomic species. During recombination, photons are released.

In a plasma cleaning system, the plasma is generated by a high energy discharge (from an RF source) between two (or three) electrodes. Positive ions are accelerated in one direction, while electrons (negative polarity) are accelerated in the opposite direction.

Ideally, the materials to be cleaned are positioned on a sample tray that is parallel to the electrode sets such that the plasma action is evenly distributed across the sample plane, ensuring uniformity and consistent cleaning results.   This is an improvement over traditional “Barrel” cleaning systems as the generated plasma has an even distribution over each of the parallel electrodes within the processing chamber – while cylindrical samples are well suited to the traditional barrel electrode setup; planar samples, such as chips or substrates in the semiconductor industry will achieve improved levels of cleaning uniformity.

Surface Modification

Appropriate gas(es) are selected accordingly to the specific contaminants to be removed, or the nature of the surface modification.

“Molecular Sandblasting” or surface ablation is achieved by bombarding the material surface with high energy plasma particles which dislodge molecules from the sample surface. The surface area of any given planar dimension is thus increased by the molecular peaks and troughs produced. Obviously to prevent any chemical reaction taking place, the gases employed for ablation alone must be inert such as Argon and Nitrogen.

A further mechanism by which surfaces can be modified occurs when the plasma generation chemically react with the surface of the sample being treated.  Gases such as Oxygen and Carbon Tetraflouride provide a chemical plasma clean.

For the removal of organic contaminants, an oxygen plasma in commonly employed. Oxygen plasma will crack the organic (hydrocarbon) contaminant molecules generating H2O, CO2 and CO.  An inert gas, such as Argon can then be employed to remove any latent oxide layers in a subsequent treatment.