Piezoelectric Microbeam Resonators Based on Epitaxial Al0.3Ga0.7As Films

Abstract

In this work, piezoelectric resonators based on single crystal Al0.3Ga0.7As films are implemented. The combination of Si doped Al0.3Ga0.7As as electrode layers and moderate piezoelectric properties of updoped Al0.3Ga0.7As film leads to lattice matched single crystal resonators with high attainable quality factors and capability of integration with high speed circuits. To validate the fabrication process, simple cantilever beam structures are developed and characterized by laser Doppler vibrometry. In order to achieve higher center frequencies, a clamped-clamped (c-c) beam design is explored. Important resonator parameters including resonance frequency, quality factor, and power handling ability are investigated. Measured quality factors of c-c beams were found to be limited by anchor losses to the substrate. A free-free (f-f) beam design is proposed in order to alleviate the energy dissipation due to anchor losses. Fabricated f-f beam devices show increased quality factors compared to the c-c beam design. Another improvement is the adoption of bimorph configuration instead of unimorph configuration. Compared to unimorph cantilever beam design, bimorph cantilevers showed 80% to 120% of increase in displacement with the same driving voltage without significant change in quality factors. The quality factors of flexural mode resonators in atmospheric pressure are low due to the effect of air damping. For this reason, proper working of flexural mode resonators requires a vacuum package which imposes unwanted complexity in packaging. To solve this problem, length-extensional mode resonators (bar resonators) are proposed to take advantage of low air shear damping. Bar resonators with lengths ranging from 1000 micro-m to 100 mico-m have been fabricated and tested. Measured resonant frequencies range from 2.5 MHz to 72 MHz with good matching to theoretical predictions. The quality factors of bar resonators at their first resonant frequency are measured in air and in high vacuum, showing values between 4,300 - 8,900 and 8,000 - 17,000, respectively, with corresponding measured motional resistances of 7.3 kohm - 10.5 kohm and 4.0 kohm - 7.8 kohm, respectively. The developed bar resonators showed excellent power handling ability up to -10 dBm which is much higher than equivalent electrostatic resonators.