Basic Processes

For our discussion we will treat the photoresist usually as a polymer consisting of carbon (C) and hydrogen (H), the other species present in resist are of so low concentration that they can be ignored (maybe not always for contamination issues, but for the topic here). In plasma assisted resist removal there are to basic reaction ways to form volatile reaction products:

Oxidation:

here the resist polymer is oxidized, the basic reaction is:

$$\rm C_xH_y + O_2 \; \rightarrow \; CO_2 + H_2O.$$

Reduction:

here the resist polymer is reduced by $\rm H_2$, the basic reaction is:

$$\rm C_xH_y + H_2 \; \rightarrow \; CH_4,$$

more commonly used is $\rm NH_3$:

$$\rm C_xH_y + NH_3 \; \rightarrow \; HCN + CH_4 + others.$$

Today in nearly all ashing processes oxygen in one or the other form ($\rm O_2$, Ozone $\rm O_3$, $\rm H_2O$) is used to oxidize the resist and transfere it to gasenous species which are pumped away. Reducing chemistries were proposed time by time, but they are not widespread in mass production because of safety concerns. Technically they would be interesting because some materials redeposited from the etching process like Ti form nonvolatile oxides that could be avoided by reducing chemistries. The rest of the section will deal, due to the importance of the oxidizing processes, primarily with those.