Significant advances in microfabricated electromechanical resonators as frequency-selective elements in RF circuits have been realized in recent years. Devices relying on electrostatic transduction have been demonstrated with exceptionally high quality factor (Q) values and resonance frequencies. Still, electrostatic microresonators exhibit high motional resistance due to low electromechanical coupling strength, presenting challenges for integration into 50Ω RF circuits. Furthermore, the power handling capacity of electrostatic resonators is generally quite low, with typical values on the order of -20 dBm. High DC bias voltages required to minimize motional resistance can also place limits on compatibility with conventional IC circuits.
Resonators based on piezoelectric transduction offer a path towards integrated RF transceivers with low motional resistance multi-frequency on-chip filters. While discrete surface acoustic wave (SAW) filters remain the dominant frequency control technology for wireless applications, advances in piezoelectric thin film resonator technology such as film bulk acoustic-wave resonators (FBARs) are making these devices increasingly attractive for high frequency applications. However, such thin film resonators cannot support multiple frequencies on a single chip, since resonance is defined entirely by the film thickness.
MML researchers are developing piezoelectric resonator arrays based on thin film materials including PZT (in collaboration with the Army Research Laboratory) and single crystal AlGaAs (in collaboration with the National Security Agency Laboratory for Physical Sciences). The AlGaAs family of materials offers piezoelectric coupling coefficients, dielectric constants, and wavespeeds similar to that of ZnO. However, unlike ZnO, high quality single crystal AlGaAs may be readily grown by molecular beam epitaxy with an excellent lattice match to the substrate (GaAs). Additionally, Si-doped AlGaAs can provide low-resistivity single crystal electrodes for applying actuation voltages and measuring charge output, thereby replacing the high loss metal layers conventionally used as electrodes in piezoelectric resonators. Thus, the effects of damping mechanisms associated with amorphous metals, grain boundaries, and interfacial defects may be reduced in AlGaAs devices, potentially leading to enhanced quality factors and lower motional resistance values. The AlGaAs fabrication technology is also compatible with direct integration into high-speed electronics through a modified high electron mobility transistor (HEMT) process.
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