Process for forming re-entrant geometry for gate electrode of integrated

Abstract

A crystalline semiconductor gate electrode having a re-entrant geometry and a process for making same are disclosed. The novel gate electrode may be formed from a polysilicon layer on a substrate by first implanting a masked polysilicon layer with a neutral species, i.e., a species which will not introduce a dopant into the polysilicon, such as a Group IV element, e.g., silicon, or a Group VIII element, e.g., argon. The neutral species is implanted into the masked polysilicon layer at an angle to provide a tapered implanted region which undercuts one side of the length (long dimension) of the mask. The substrate may then be rotated 180.degree. and then again implanted to provide a tapered implanted region which undercuts the opposite side of the length of the mask. When gate electrodes with such re-entrant geometry are to be formed on a substrate with their long axes at right angles to one another, i.e., some lying along an X axis in the plane of the masked polysilicon layer on the substrate and others lying along a Y axis in the plane of the masked polysilicon layer on the substrate, the substrate may be rotated 90.degree., rather than 180.degree., between each implantation, and four implantations, rather than two, are performed. After the implantations, the implanted masked polysilicon layer is then subject to an etch, preferably an anisotropic etch, which will remove the unmasked implanted portions of the polysilicon layer, as well as the implanted regions beneath the mask, resulting in a gate electrode with re-entrant or tapered sidewalls, i.e., resembling an inverted trapezoid in cross-section.