Applications of silicon micromachining to resonator fabrication

This paper surveys recent developments in using silicon micromachining processes to fabricate micromechanical resonant structures for sensors, filters, and oscillators. Single-crystal silicon and polycrystalline silicon are excellent mechanical materials with quality factors approaching those of quartz. By etching sacrificial layers from a sandwich of thin films, microstructures can be surface micromachined from a deposited polycrystalline silicon (polysilicon) him. This technology can be applied to fabricate micromachined vacuum encapsulation for the resonant structure, as well. Surface micromachining has been merged with CMOS processes to enable fabrication of integrated micro electromechanical systems (MEMS) on a single silicon chip. An alternative process fabricates crystalline silicon resonators by means of wafer bonding and etch-back. These microstructures can be over 10 μm thick-much thicker than surface-micromachined polysilicon structures. Integration with this technology is feasible by processing CMOS electronics on wafers containing sealed cavity microstructures, with the microresonators defined at the end by reactive-ion etching. Trimming silicon microresonators is possible using micromechanical fusing and welding structures, or by electrical tuning