The Role of Ion Bombardment in Plasma Etching
When a surface is exposed to ion bombardment, atoms and molecules can be ejected. This is called Sputtering (slide 1).
Physical sputtering is a non selective phenomenon (materials of different nature can be sputtered at similar rates). It is also a directional phenomenon which helps in obtaining anisotropic etching profiles.
Ion induced damage and mixing: Ion bombardment may favor neutral dissociation at the surface and increase the number of adsorption sites by generating some surface roughness and generating dangling bonds.
Ion enhanced chemical reaction: Ion bombardment may favor the formation of etch products.
Chemical sputtering: Ion bombardment may favor the desorption of etch by products.
Ions as a source of reactants: Ions get neutralized when they reach the surface, they become an additional source of reactive species.
The synergy between ion bombardment and chemical etching was first shown by Coburn and Winters in the classical experiment shown on slide 2.
Besides enhancing the chemical etch, ions also play a major role in removing non-volatile by-products or etch products that require an activation energy to desorp from the surface. The removal of by-products and their redeposition onto the feature sidewall is the fundamental reason why plasma etching can obtain anisotropic profiles (slide 3)
Factors that influence the anisotropy are (slide 4):
Ion energy flux (ion density and ion energy): Primarily responsible for the etch anisotropy in plasma etching. In general an increase in ion energy flux leads to a better anisotropy.
Neutral to ion flux ratio: The lower the neutral to ion flux ration the better the anisotropy.
Natural species reactivity: Probability for spontaneous reaction of neutral species to react with the surface.
Deposition rate for non-volatile etch products forming a passivation layers at the feature sidewalls.
Substrate temperature: Influences neutral species reactivity and reaction product deposition.