Thick glass film technology for polysilicon surface micromachining

This paper explores the use of thick glass films as suitable alternatives t o CVD oxide films for use as sacrificial, planarization, and passivation la yers in polysilicon surface micro machining processes. Such glasses can be spin-coated to produce films up to 20 mu m thick in one step and to globall y planarize the wafer surface, extending the overall mechanical design capa bility by enabling additional device structural complexity. Glass optical c onstants were determined, and the film quality was evaluated using SEM, EDS , XPS, and XRD, The films were found to have low intrinsic stresses and oth er characteristics desirable for sacrificial layer applications. A glass ch emical-mechanical polishing process with 5300-Angstrom/min removal rate and acceptable selectivity to polysilicon was developed, along with a wet etch chemistry that preferentially etches the film at 3.24 mu m/min without aff ecting the silicon substrate or the structural polysilicon, The film was us ed to planarize up to 10-mu m-tall topographies associated with surface mic romachined features through spin-on and polish-back steps, and was in addit ion demonstrated to be a viable protective layer for silicon wafers during extended KOH etching in silicon bulk micromachining processes. The glass ha s stable constituents that do not diffuse or contaminate either the substra te or the device features during the application and firing procedures.