Piezoelectric Fused Silica Resonators for Timing References.

Abstract

MEMS resonators have the capability to match or exceed the performance of state-of-the-art quartz timing references at a fraction of the size, power, and cost. To enable this capability, this work investigates the use of fused silica as a substrate for piezoelectrically actuated resonators for applications in timing references.

This thesis presents the design and fabrication of a piezoelectrically actuated fused silica resonator. The fabricated resonators show a quality factor (Q) of 19,671 at 4.96 MHz with an insertion loss of 16.9 dB, and is the first reported example of a piezoelectrically actuated fused silica resonator in literature. An in-depth investigation into loss mechanisms in fused silica is performed in order to identify and address the major losses in the device and maximize potential performance. Multiple experimental and analytical investigations are presented, with a new form of loss, known as charge redistribution, presented as a possible dominant loss in these piezoelectric resonators. This loss mechanism is analytically modeled and simulated to have a Q of 25,100, within 20% of the experimentally measured devices.

The temperature sensitivity of fused silica is also addressed; as fused silica shows a temperature coefficient of elasticity almost three times higher than that shown in uncompensated silicon. Both active and passive methods of temperature compensation are implemented, including a fused silica ovenized platform and nickel-refilled trenches for temperature compensation. The nickel-refilled trenches are shown to reduce temperature sensitivity in piezoelectrically actuated fused silica resonators from 78 ppm/K to 50 ppm/K, with larger compensation possible but complete compensation infeasible from passive techniques alone. From this, a dual-mode system is proposed for use in ovenized systems where two modes are simultaneously activated in a single device volume. In this system, one mode acts as a stable reference frequency and the second mode acts as a temperature sensor, allowing for extremely accurate ovenization. A silicon-based prototype is developed, showing a +14 ppm/K temperature difference between the two modes, with additional temperature differential possible through the addition of material-based passive compensation.