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Etch

Plasma etching ("dry" etching) is a plasma-based process that facilitates the removal of material from the surface of a substrate. Plasma etching is performed in a vacuum environment and involve four mechanisms, which must be understood for effective etch system design:

Formation of active gas species.
Gas species are activated within the plasma discharge area. These species include ions, electrons and radicals. 

Transport of active species to the surface.
Neutral species are transported to the substrate surface mainly by diffusion, while negative bias on the cathode supporting the substrate accelerates charged species to the material surface.

Reaction at the surface.
Three actions occur at the material surface: the adsorption of precursors, reaction with the surface material, and desorption of the reaction products. There are variations according to substrate composition and process parameters. For example, processes mainly relying on the energy of impinging ions are considered "physical. This is in contrast with "chemical" processes in which activated neutrals react with the substrate, independent of their kinetic energy. In practice, most processes have both physical and chemical aspects.

Removal of reaction products.
After desorption, the volatile reaction products diffuse back to the plasma and are exhausted by a vacuum pump. The diffusion directions of etchants and reaction products are a result of concentration gradients of both species, in the bulk plasma and at the substrate surface.

 
 
 
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