High-Q Low-Impedance MEMS Resonators
- Author(s): Hung, Li-Wen
- Advisor(s): Nguyen, Clark T.-C.
- et al.
The ever increasing need for regional and global roaming together with continuous advances in wireless communication standards continue to push future transceivers towards an ability to support multi-mode operation with minimal increases in cost, hardware complexity, and power consumption. RF channel-select filter banks pose a particularly attractive method for achieving multiband reconfigurability, since they not only provide the needed front-end reconfigurability, but also allow for power efficient and versatile transceiver designs, e.g., software-defined radio. Such channel-select filters, however, impose requirements on their constituent resonators that are not yet achievable on the micro-scale. Specifically, capacitively-transduced micromechanical resonators achieve high Q, but suffer from high impedance; while piezoelectric micromechanical resonators offer low impedance, but with insufficient Q. This dissertation demonstrates four new techniques to address the issues in both technologies.
Two of the methods recognize that sub-30 nm gap spacing enables electrostatic resonators to achieve acceptably low impedance. Unfortunately, however, such small gaps with the needed high aspect ratios are difficult to achieve via wafer-level batch processing. Two new methods are proposed and experimentally verified for forming sub-30 nm gaps: 1) partial-filling of electrode-to-resonator gaps with atomic layer deposition (ALD) of high-k dielectric; and 2) generating gaps via the volume reduction associated with a silicidation reaction. Among the many benefits provided by a silicide-based approach to gap formation is speed of release, where sub-30 nm gaps can be formed and high-aspect-ratio microstructures can be released via anneals lasting from seconds to a few minutes, regardless the lateral dimensions of the devices. Silicide-induced gap formation further does not require any etching and is applicable to a wide range of applications, from electronics to vacuum packaging.
The next two methods seek to circumvent the fact that AlN thin-film resonators have historically been measured with much lower Q than capacitive ones at similar frequencies. As a result, it was commonly accepted that the AlN thin films sputtered at low temperatures are to blame for the lower Q. This dissertation provides experimental evidence that it is not AlN material loss that restricts the Q of conventional AlN resonators, but rather the losses associated with their contacting electrodes. Specifically, a new transducer dubbed the "capacitive-piezoelectric" transducer is introduced that lifts the electrodes away from a piezoelectric resonator by tiny nanometer scale gaps that retain strong electric fields for good electromechanical coupling, while eliminating electrode-derived losses. After removing the electrode losses, the Q of piezoelectric AlN resonators rise by up to 9 times. A new surface-micromachining fabrication process has been developed for the capacitive-piezoelectric resonators, where the metal electrodes are separated from the AlN resonators by small air (or vacuum) gaps. The second approach for tapping the material Q of AlN uses Q-boosting mechanical circuits, where the electrode-equipped AlN resonators are mechanically coupled to electrode-less ones to form a composite-array. In this structure, the energy shared among all of the resonators in the composite-array effectively boost the Q of the electrode-equipped resonators. The Q of electrode-less resonators are extrapolated from the measurement data to be from 14,040 to 15,795. Both methods achieve measured Q exceeding 10,000, posting the highest reported Q for resonators constructed of sputtered AlN and confirming that AlN is indeed a high-Q material.