Apodization techniques for spurious mode suppression in 900MHz 1GHz MEMS aluminum nitride contour mode resonators

This dissertation introduces for the first time the concepts of apodization, a method to reduce spurious modes of vibration, for Aluminum Nitride (AlN) contour-mode one port MEMS resonators. In this thesis the concept of apodization, until now applied only to the SAW devices, is theoretically studied and experimentally implemented in the AlN MEMS resonators. A new method to shape the geometry of the electrodes in order to obtain a consistent reduction of spurious modes is introduced. The presence of spurious modes hinders the performance of filters by generating ripples in the pass-band and unwanted responses out-of-band. Therefore, the reduction of spurious vibrations in MEMS resonators is of paramount importance. Apodization techniques were successfully applied to 900 MHz and 1 GHz MEMS AlN Contour-Mode Resonators (CMR) to efficiently suppress spurious modes in close proximity of the main mechanical resonance. Complete elimination of spurious responses in the admittance plot of these resonators is attained without impacting their Q or electromechanical coupling coefficient, kt2. Moreover, the synthesis of modern filters requires having two set of resonators with closely spaced (few percent, 1-5%) center frequencies. Although the AlN MEMS technology can provide for multiple frequencies on the same chip, it is hard to lithographically control such small shifts. This thesis presents new methods to shift the resonance frequency of the resonator by a small range. These techniques are based on : (i) varying the pitch of the resonator fingers (the frequency setting dimensions) in specific regions of the device surface, (ii) introducing materials with different mass density, (iii) modifying the overall thickness of the metal electrodes. These concepts have been applied to the same apodized resonators, so that devices simultaneously characterized by the absence of spurious modes and slightly different resonance frequency could be demonstrated.