Thick glass film technology for polysilicon surface micromachining

Abstract

This paper explores the use of thick glass films as suitable alternatives to CVD oxide films for use as sacrificial, planarization, and passivation layers in polysilicon surface micro-machining processes. Such glasses can be spin-coated to produce films up to 20 /spl mu/m thick in one step and to globally planarize the wafer surface, extending the overall mechanical design capability by enabling additional device structural complexity. Glass optical constants were determined, and the film quality was evaluated using SEM, EDS, XPS, and XRD. The films were found to have low intrinsic stresses and other characteristics desirable for sacrificial layer applications. A glass chemical-mechanical polishing process with 5300/spl sim//spl Aring//min removal rate and acceptable selectivity to polysilicon was developed, along with a wet etch chemistry that preferentially etches the film at 3.24 /spl mu/m/min without affecting the silicon substrate or the structural polysilicon. The film was used to planarize up to 10-/spl mu/m-tall topographies associated with surface micromachined features through spin-on and polish-back steps, and was in addition demonstrated to be a viable protective layer for silicon wafers during extended KOH etching in silicon bulk micro-machining processes. The glass has stable constituents that do not diffuse or contaminate either the substrate or the device features during the application and firing procedures.