Charge in composite micromechanical resonators [electronic resource]

Silicon micromechanical resonators have recently come into view as competitors to quartz technology for frequency reference applications. Several researchers have investigated techniques for improving the frequency stability of silicon resonators against temperature. In particular, a promising temperature compensation technique exploiting the positive temperature coefficient of elastic modulus of silicon dioxide (a dielectric material) has been demonstrated both for MEMS, as well as for surface acoustic wave devices. The coating of silicon resonators with silicon dioxide enables a significant reduction of the temperature sensitivity. Broadly, dielectrics such as silicon dioxide play an important role in many classes of MEMS. They are frequently used for electrical isolation, as structural elements, for enhancing the transduction within electrostatic actuators and more generally, as sacrificial layers. However, it is also well known that dielectrics are susceptible to various charging phenomena. Charge build-up and charge motion can screen electrode potentials, affecting the overall electromechanical properties of the device. Such adverse effects due to charging are also seen in oxide-coated silicon resonators. Since charging is not always avoidable, techniques that can circumvent the undesirable effects of charge can be of value. In this work, we investigate the effects of dielectric charge on resonant frequency in thermally oxidized silicon resonators in a hermetic wafer-level package, with a double-ended tuning fork (DETF) design. We present theoretical models for the electromechanical effects of charge trapped in the dielectrics within a resonator. In addition, observations of time-varying resonator frequency indicate the presence of mobile oxide charge in a series of voltage biasing and temperature experiments. This work also introduces the use of ac polarization for resonant electrostatic MEMS as a drift-circumvention technique, that eliminates the frequency drift caused by dielectric charging and charge screening. It is mathematically and experimentally shown that an ac-polarized resonator can sustain stable oscillations when used in a positive feedback oscillator circuit. We also demonstrate an oscillator topology that generates a low-drift reference frequency signal (0.15 ppm/week of drift over 21 days) with this technique in spite of using a resonator that exhibits very large frequency drifts under dc polarization (about -90 ppm in 42.5 hours). Long-term data is presented for these drift-susceptible devices, showing more than 2 orders-of-magnitude improvement in frequency stability. In addition to reducing the drift due to charge motion in oxidized resonators, ac polarization provides a path to the independent control of resonance frequency and response amplitude in two-port electrostatic resonators without the need for additional electrodes. We also note a reduced frequency sensitivity to dc offset voltages, and discuss oscillator noise characteristics that differ from the dc polarization case.

Keywords: Electrical Engineering
Advisor: Kenny Thomas William primary advisor. Thesis advisor
Howe Roger Thomas advisor. Thesis advisor
Murmann Boris advisor. Thesis advisor