This paper reports a new method for making multi-layer substrates (MLS) for high aspect ratio single crystalline movable microstructures using a group of technologies, such as direct wafer bonding (DWB), chemical mechanical polishing (CMP), and reactive ion etching (RIE). As a first example, Si-SiO2-polySi-SiO2-Si sandwich wafers were fabricated using CMP and DWB. Subsequently, free-standing micro cantilever beams and double side clamped bridges were fabricated on these sandwich wafers using a one-run self-aligned RIE process, where polysilicon was used as the sacrificial layer. Polishing and bonding of low pressure chemical vapour deposition (LPCVD) polysilicon were studied. An LPCVD Si3+xN4 polishing stop layer technique was presented to accurately control the final thickness of the device layer. The uniformity of the device layer was improved as well. © 1998 Elsevier Science S.A. All rights reserved.