UC Berkeley

This dissertation describes a MEMS-based frequency-selective power amplifier that performs both signal filtering and power amplification, while consuming zero power when there is no input, i.e., zero-quiescent power consumption. The frequency-selective power amplifier employs a micromechanical resonant switch (resoswitch) as a key building block similar to those recently used for zero-quiescent power radio receivers, but capable of handling higher powers. This document details the design, fabrication, and characterization of these higher frequency and higher power micromechanical resoswitches, and employs them as power amplifiers. Here, the mechanical Q of the resoswitch largely governs the threshold input level that instigates power gain. Theoretical and experimental studies of Q, as well as Q enhancement techniques and high-Q structural design, are discussed. Further, post-fabrication laser trimming addresses the frequency accuracy of the vibrating devices. A model that replaces laser blasted holes with stiffness-modifying cracks captures well the frequency shift dependence on laser blast location. The accuracy of this theory further enables a deterministic trimming protocol that specifies the laser targeting sequence needed to achieve a required amount of frequency tuning with minimal Q reduction.

The resoswitch used to demonstrate the frequency-selective power amplifier employs slots to engineer stiffness along orthogonal axes of a wine-glass disk resonator structure. The slots realize displacement amplification—a larger displacement magnitude along the output than the input axis—allowing impact switching only to the output electrodes, not the input, all of which improves reliability. A finite element analysis (FEA)-based model predicts the displacement gain as a function of disk size and slot dimension/location.

To improve impact contact resistance, this work employs metal for the slotted-disk resoswitch, achieved via a CMOS-compatible surface micromachining process that essentially replaces normally-used polysilicon material with aluminum metal, which serves as both structural and interconnect material. This not only improves contact resistance, but also reduces parasitic resistance, which in turn reduces feedthrough currents. When embedded in a switched-mode power amplifier circuit, the Al displacement-amplifying resoswitch performs signal filtering and power amplification that first filters an incoming signal with channel-like selectivity and then amplifies the signal with a power gain of 13.8 dB. More importantly, unlike transistor-based circuits, the resoswitch-based frequency-selective power amplifier consumes zero power while in standby.

Measurements indicate that sputtered aluminum has high mechanical Q at low frequencies, as a folded-beam capacitive-comb-driven micromechanical resonator fabricated via the aforementioned process achieved a Q up to ∼20,000. Unfortunately, the higher frequency slotted-disk used in the displacement amplifier did not fare as well, as its Q was only on the order of 1,000. An effort to study the Q limits of different resoswitch designs included an experimental study of intrinsic loss mechanisms using cryogenic operation to enhance resonator Q and better elucidate important energy loss mechanisms. Here, operation of a 61-MHz wine-glass disk resonator at temperatures as low as 5K reduces temperature-dependent energy loss and raises Q to as high as 362,000, likely devoid of thermoelastic friction. On the other hand, introduction of slots into a wine-glass disk structure (to effect displacement gain) reduces Q by introducing thermoelasting damping, which this document models in detail. A displacement-amplifying elliptic disk solves this problem by deriving gain-induced stiffness differences from geometric ratioing rather than Q-degrading slots, which permits simultaneous displacement gain and high-Q.

Finally, this work includes a study of laser trimming to trim power amplifier frequency to a desired range. Here, laser trimming offers flexible bidirectional frequency tuning with minimal effect on Q. Unlike other frequency tuning methods, laser trimming does not require added process steps, e.g., sputtering of frequency-shifting metal materials on structures. Rather, laser trimming offers precise post-fabrication frequency adjustment via blasts of controlled size and location on a given resonator. A model that captures the frequency shift dependency on laser blast location shows good agreement with measured results on micromechanical clamped-clamped (CC)-beam resonators. The theory provides a modeling framework for laser trimming applicable to other types of beam resonators, e.g., free-free beams, cantilevers, and even disks or rings by altering the boundary condition matrices.