Microelectromechanical Filters for Signal Processing

Microelectromechanical filters based on coupled lateral microresonators are demonstrated. This new class of microelectromechanical systems (MEMS) has potential signalprocessing applications for filters which require narrow bandwidth (high Q), good signal-to-noise ratio, and stable temperature and aging characteristics. Microfilters presented in this paper are made by surface-micromachining technologies and tested by using an off-chip modulation technique. The frequency range of these filters is from approximately 5 kHz to on the order of 1 MHz for polysilicon microstructures with suspension beams having a 2m-square cross section. A series-coupled resonator pair, designed for operation at atmospheric pressure, has a measured center frequency of 18.7 kHz and a pass bandwidth of 1.2 kHz. A planar hermetic sealing process has been developed to enable high-quality factors for these mechanical filters and make possible wafer-level vacuum encapsulations. This process uses a low-stress silicon nitride shell for vacuum sealing, and experimental results show that a measured quality factor of 2200 for comb-shape microresonators can be achieved. [265]