Anisotropic plasma etching of polysilicon

Abstract

Plasma etching of polycrystalline silicon films for fabrication of silicon gate MOS integrated circuits has been studied with emphasis on fine-line devices. CF4–O2 plasmas, commonly used for etching silicon, are unacceptable for very fine features because the etching is isotropic and load dependent. This results in substantial undercutting and insufficient dimensional control. Several alternative gases were investigated in a parallel–plate reactor. CF3Cl and a 70% CF3Br–30% He mixture were found to provide selectivities of 30:1 and 16:1, respectively, over thermal SiO2, freedom from loading effects and a large vertical to lateral etch rate anisotropy which minimizes undercutting. Extensive measurements of etch rate and edge profile as a function of gas composition were made for C2F6–Cl2 plasmas. Fully anisotropic etching (zero lateral etch rate) was observed at low Cl2 concentrations with a selectivity ≳6:1 over thermal SiO2 when using conventional photoresist masks. The vertical and lateral etch rates and the selectivity increase with increasing Cl2 concentration for n-type polysilicon. The dependence of etch rate on Cl2 concentration for p-type polysilicon is nearly identical to that for undoped polysilicon, whereas n-type material etches faster than undoped material with the etch rate ratio (n-type:undoped) increasing with Cl2 concentration. Values for this ratio as large as 15:1 were measured. ’’Pattern sensitivity’’ was observed with photoresist masks such that at low Cl2 concentrations etching was slower in the vicinity of resist features while at high Cl2 concentrations etching was faster near resist features as compared to open (unmasked) areas of the same film. Pattern sensitivity was negligible at an intermediate Cl2 concentration. Results of effluent sampling with IR absorption and mass spectrometry and spectral analysis of optical emission from the plasma suggest that Cl atoms are the likely active species. It is conjectured that the anisotropy in etch rate is a consequence of enhanced chemical reaction between Cl and Si on surfaces which are ion (or electron) bombarded while lateral etching is suppressed by the combination of CF3 radicals and Cl atoms on surfaces which are not ion (or electron) bombarded.