Method for controlled selective polysilicon etching

Abstract

An ammonia-based etchant is employed, in dilute aqueous solution and preferably with a moderating agent, to etch polysilicon. Ammonium fluoride and ammonium hydroxide are the preferred etchants, with acetic acid and isopropyl alcohol the preferred moderating agents for use with the respective etchants. Dilute solutions of these etchants and their respective moderating agents provide a controllable, uniform polysilicon etch with reasonably good selectivity to undoped polysilicon over doped polysilicon. A dilute solution of ammonium fluoride and acetic acid provides particularly good selectivity. These etchants are applied to the etching of doped polysilicon upon which undoped hemispherical grain (HSG) polysilicon has been formed. The undoped HSG polysilicon is etched at a slower rate than the doped polysilicon which is etched at a greater but controllable and uniform rate. The result is a surface with greater total surface area contained within the same wafer area. This allows an increase in capacitance for capacitors formed with HSG polysilicon, without any increase in wafer area occupied thereby.