Etching 0.35 μm polysilicon gates on a high-density helicon etcher

Abstract

This article describes an evaluation of a helicon high-density plasma etch tool for etching 0.35 μm polysilicon gates. The objective of the evaluation was to establish whether it is possible to etch 0.35 μm lines of both doped and undoped polysilicon (simultaneously) while maintaining straight profiles across the wafer and a very high polysilicon-to-oxide selectivity. Several high-density plasma (HDP) polysilicon etch tools for 0.35 μm technology have been tested at Texas Instruments. It is known that HDP tools operating at low pressures can obtain high polysilicon etch rates and anisotropic etch profiles. Most of the HDP sources utilize two separate radio-frequency (rf) power supplies; one to generate the high-density plasma and a second to bias the chuck. This allows relatively independent control of the ion density and the ion energy. It is possible to attain high polysilicon-to-oxide selectivity in the overetch step by using a high flux (high rf power to the plasma) of low energy (low rf power to the chuck) ions. This high selectivity is particularly important when long overetches are required with thin gate oxides.