PFC-Free Dry Etching Method for Si Using Narrow-Gap VHF Plasma at Subatmospheric Pressure

We propose a Si dry etching method that does not require any perfluorocarbon (PFC) source gases with a high global warming potential. This method uses a narrow-gap, very high frequency (VHF) glow plasma of an O(2)/He gas mixture at subatmospheric pressure to generate etchant species (such as fluorine atoms) from the stable solid poly(tetrafluoroethylene) (PTFE) resin. The gas component generated by the chemical reaction between the plasma and PTFE is studied by Fourier transform IR gas analysis. In using a pure He plasma, the generated gases almost consist of PFC molecules. On the contrary, in the O(2)/He mixture plasma, stable gaseous species generated from the PTFE source mainly consist of CO(2) and COF(2). The etching rate of Si depends on both oxygen partial pressure (P(O2)) and input electric power (W(VHF)). Especially, the Si etching rate is very sensitive to P(O2). These results combined with the detailed analysis of optical emission spectra suggest that the oxygen addition suppresses the generation of higher order C(x)F(y) (x>1) molecules and increases the etchant species, such as F(2) and COF(2). A very high Si etching rate of 27 mu m/min at P(O2)=40 Torr and W(VHF)=250 W has been attained.