Apodization technique for spurious mode suppression in AlN contour-mode resonators

• Use of apodization technique (i.e. electrode shaping) to suppress spurious vibrations.
• Explain theory of apodization as applied to MEMS resonators.
• Finite element analysis of apodization showing effective suppression of spurious vibrations.
• Experimental verification of apodization in resonators formed by different AlN film thicknesses (500 nm and 1 μm) and metal electrodes.

This paper reports on the application of apodization techniques to 900 MHz–1 GHz MEMS AlN Contour-Mode Resonators (CMRs [1], [2], [3], [4], [5] and [6]) to efficiently suppress spurious modes in close proximity of the main mechanical resonance. This concept has been applied with excellent results to a variety of one port resonators formed by patterned top electrodes made out of aluminum, and a floating bottom electrode made out of platinum sandwiching the AlN film. As also predicted by 3D COMSOL analysis, a complete elimination of the spurious responses (>90% suppression) is attained without significantly impacting the quality factor, Q  , and electromechanical coupling coefficient, kt2, of the device. On average, the Q improves except for resonators with thick (220 nm) top electrodes for which <11% degradation in Q   is recorded. The kt2 reduction is <20% and it has an absolute value > 1% for all designs.