A geometric etch-stop technology for bulk micromachining

Abstract

This paper describes a new fabrication method for thesimultaneous creation of multi-level single-crystalline siliconstructures, each with a different thickness. The method combines deep dryetching and wet anisotropic etching of silicon in order to avoid multipleback-side alignment steps and timed etches. The levels are defined in asingle lithographic step from the front side. The fabrication involvesetching of deep trenches from the front side of the wafer followed by arefill and etch back process. The final structure is defined by masklesswet etching of the bulk silicon. The progress of the anisotropic wet etchis impeded by the geometric pattern at the bottom of the trenches, andthus structures with various thickness ranging from ten to a few hundredmicrometres can be implemented. The effect of various design parameters,such as trench geometry, refill material and reactive ion etching lag, arediscussed and design rules are established. The capabilities of the methodare demonstrated by the fabrication of a number of devices, such as1200×1200×3.5 µm diaphragms supported by a 40 µmthick rim and (1800×10×3 µm) embedded hot-wire anemometerssuspended by a 0.2 µm thick dielectric bridge.