Plasma etching of polysilicon and Si₃N₄in SF₆with some impact on MOS device characteristics

Abstract

Accurate delineation of the circuit materials polycrystalline silicon ("poly"), and silicon nitride are important requirements of most SFC process sequences. We have investigated the use of SF <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">6</inf> as an active species in the parallel-plate plasma etching of these materials. For the etching of poly there is good selectivity (better the 50:1) with respect to the etch rates of SiO <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</inf> and positive photoresist. This process has been used in the fabrication of MOS transistor with 3-µm poly-gate lengths and threshold voltages vary by less than 0.05 V both across a wafer and from wafer to wafer. Etching of nitride is less selective and less isotropic than that of poly.