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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