Neutral species that arrive at the wafer surface can stick to the surface and react. Depending on the sticking coefficients and reaction probabilities, reactive and condensable species can be distinguished among the species in the feed gas and the reaction products (slide 1). The balance between reaction and condensation influences the etch profile.
Reactive species: React chemically with surfaces. Reactions are not very temperature sensitive because of low activation energies for the reactions. Surface coverage is typically saturated at a few monolayers.
Condensable species: Form liquid or solid films on surfaces. Surface coverage dependents strongly on the substrate temperature.
Slide 2 shows reactive and condensable species for the example of tungsten Etching with Cl2/O2.
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