Arrays of high-performance ultra-high-frequency aluminum nitride trampoline resonators with gold-aluminum electrodes

In this work, a novel design for a micro-scale aluminum nitride (AlN) trampoline-shape piezoelectric resonator is assessed for ultra-high-frequency (UHF) control and filtering applications. The device is targeted to operate in frequencies in the 1.5GHz-6GHz range suitable for wireless communication systems. A finite element simulation package (ABAQUS) is used to solve for the three dimensional mechanical displacements and for the electric potential in the structure during forced oscillations covering the relevant range of frequencies. Material and air damping are accounted for in the model. The resonators are first arranged in singletons, then in pairs and fours to model an element array of a RF filter. As a first approximation, the electrodes thicknesses are neglected. We vary a wide range of design parameters and analyze the performance of the device by extracting the electromechanical coupling coefficient (K) and the quality factor (Q-factor) from the electrical impedance response. In the second part of the work, we consider the various parts of the trampoline resonator: a piezoelectric layer bearing two metal electrodes with two metallic sublayers inserted between them. The structure is meant to model a similar trampoline device that was fabricated and tested on site at UCSB. The effect of the surrounding substrate on the performance of the resonator is then analyzed when finite element simulations of the complete structure---aluminum nitride trampoline resonator with gold-aluminum electrodes surrounded by silicon---are performed. Analytical expressions obtained from an exact one-dimensional analysis of two structures subjected to the same mechanical and electrical loading as in the experiment are then derived. The results show good agreement with those of the simulations which indicate that for the array arrangement, the energy is split among the resonators and sum up to the value of the electromechanical coupling coefficient of a single resonator and that the value of the Q-factors is almost unchanged. When the electrodes are considered in the analysis, it is concluded that electrodes a quarter-wavelength thick maximize the Q-factor and that added thin metallic sublayers return better performance values. Finally, simulations contrasting air environment with vacuum indicate that damping due to air is insignificant.