Reactive and Condensable Species

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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Knudsen Transport of Neutral Species

RIE lag or Aspect Ratio Dependent Etching (ARDE) is particularly important for high aspect ratio etching. One of the elementary mechanisms that can contribute to ARDE is the transport mechanism of the neutrals. Plasma etching or reactive ion etching relies on the presence of reactive neutrals and ions at the etch front. For very high aspect ratio features, the neutral flux gets attenuated. Coburn and Winters showed that this reduction of the neutral flux towards the bottom of the feature can be described by the Knudsen transport model used to describe vacuum systems. In this model, neutrals are lost either on the feature sidewall as described by the transmission probability or get reflected from the etching surface as described by the reaction probability. The remaining species contribute to the etching process (slide 1).

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