Recent Advances in Monolithic Film Resonator Technology

Abstract

During recent years, work has been progressing toward the development of stable, high-Q, acoustic resonators capable of operation in the UHF band. The aim of these efforts is twofold: (1) circuit simplification and performance improvement associated with stable signal generation and narrow-band signal sorting directly at UHF, and (2) resonator volume reduction and potential use as an MMIC circuit element. In the latter case, the resonator can be formed by sputter deposition of a piezoelectric film directly onto supporting membranes formed in semiconductor substrates. The resonator equivalent circuit consists of a clamped capacitance in parallel with a series combination of motional inductance, capacitance, and resistance. The motional branch-Q and clampedto-motional capacitance ratio are two important performance parameters whose ratio constitutes a figure of merit (FOM) that defines device usefulness in oscillator and filter circuitry. One-dimensional resonator analysis provides valuable insight regarding the effects of film-to-membrane layer thickness ratio, electrode placement within the layers, and vibrational mode on the resulting resonator FOM. In connection with extending device operating frequencies, analysis results indicating high obtainable FOM for resonator harmonic vibrational modes are of particular interest. In this paper, we discuss relationships between resonator FOM and obtainable filter and oscillator electrical performance, including practical methods of achieving desired circuit performance using low FOM devices. Results of resonator one-dimensional analysis are presented and compared to measured characteristics. Included is a description mode of ZnO on SiO, resonators exhibiting exceptionally good performance in the second harmonic mode. Several of these devices have been used to construct hybrid two- and three-pole ladder filters having 1% to 3% bandwidth at 700 MHz.