Dry Etching (Plasma Etching) removes materials from a substrate using plasma in a vacuum environment. In short, a reactive species within the plasma has to react with the material you want to remove from the surface of the substrate with the formation of forming volatile reaction byproducts. This process can be broken into four steps or processes as follows.
- Formation of Active Gas Species: Gas species such as ions, electrons and radicals are activated in the zone of plasma discharge.
- Transport Active Species: Neutral species are diffused to the substrate surface. The negative bias on the substrate cathode accelerates charged species to the surface.
- Surface Reaction: A number of mechanisms fire during three key sub-steps. While processes depending on the energy of the impinging ions have a significant physical component, during a chemical etch, activated neutrals react independently of their kinetic energy. Normally, most processes involve both physical and chemical reactions. The three key sub-steps of this surface reaction are:
- Absorption of precursors
- Surface reaction
- Desorption of products.
- Pump Down Effect: Following step “desorption of products”, volatile reaction products diffuse back to the bulk plasma and are removed by vacuum pump. Diffusion directions of etchants and reaction products are determined by the respective concentration gradients at the substrate surface and in the bulk plasma.