Adam Peczalski

Piezoelectrically Transduced Temperature-Compensated Flexural-Mode Silicon Resonators

In this paper, we explore the piezoelectric transduction of in-plane flexural-mode silicon resonators with a center frequency in the range of 1.3-1.6 MHz. A novel technique utilizing oxide-refilled trenches is implemented to achieve efficient temperature compensation. These trenches are encapsulated within the silicon resonator body so as to protect them during the device release process. By using this method, we demonstrate a high-Q (> 19 000) resonator having a low temperature coefficient of frequency of <; 2 ppm/°C and a turnover temperature of around 90 °C, ideally suited for use in an ovenized platform. Using electrostatic tuning, the temperature sensitivity of the resonator is compensated across a temperature range of +50 °C to +85 °C, demonstrating a frequency instability of less than 1 ppm. Using proportional feedback control on the applied electrostatic potential, the resonator frequency drift is reduced to less than 110 ppb during 1 h of continuous operation, indicating the ultimate stability that can be achieved for the resonator as a timing reference. The resonators show no visible distortion up to -1 dBm of input power, indicating their power handling capability.

Piezoelectrically transduced high-Q silica micro resonators

In this paper, we report on high-performance piezoelectric-on-silica micromechanical resonators for integrated timing applications. Fused silica is used as the resonator structural material for its excellent material properties, and thin film aluminum nitride is used as the piezoelectric transduction layer. A silica resonator is demonstrated with a high quality factor (QU~25,841), low motional impedance (Rm ~350 Ω), and good power handling capability. The measured f×Q product of this resonator is the highest amongst reported micromachined silica/fused quartz resonators.

Low-power ovenization of fused silica resonators for temperature-stable oscillators

In this paper, we report on temperature-stable operation of silica MEMS oscillators on an ovenized fused silica platform. Temperature servo-control circuits are implemented using an on-chip RTD-based temperature sensor and a resistive heater. A wide-range linear analog controller has been implemented to reduce the effective TCF of the fused silica resonator by an order of magnitude. Digital calibration method is used to mitigate offset errors caused by non-ideal temperature sensing. By effectively removing the offset errors, the frequency drift of an oscillator using a silica micromechanical resonator is reduced to less than 11 ppm over 105 °C of external temperature change. The power consumption to ovenize the entire platform consisting of four resonators is lower than 15.8 mW.

Investigation Into the Quality Factor of Piezoelectric-on-Silica Micromachined Resonators

In this paper, we investigate loss mechanisms in piezoelectric-on-silica bulk acoustic wave resonators, including those resulting from thermoelastic damping (TED), surface roughness, and supporting tethers. Alternate resonator designs, piezoelectric materials, and fabrication processes are demonstrated to empirically test these loss mechanisms. Quality factors (Qs) in the order of ~16000 at a center frequency of 5 MHz have been consistently measured for aluminum nitride (AlN)-on-silica coupled-ring resonators. It is shown that neither TED nor surface losses are the dominant sources of loss for AlN-on-silica resonators in the megahertz regime. Instead, it is suggested that charge redistribution loss resulting from nonuniform strain across the piezoelectric layer is the dominant loss mechanism, with a charge redistribution Q of ~38000 at 5 MHz for AlN-on-silica devices. When all loss mechanisms are considered, the total Q is estimated to be 25000, a value comparable to the measured results of the piezoelectric-on-silica resonators of this paper.

Temperature compensated silicon resonators for space applications

This paper presents piezoelectric transduction and frequency trimming of silicon-based resonators with a center frequency in the low megahertz regime. The temperature coefficient of frequency (TCF) of the resonators is reduced using both passive and active compensation schemes. Specifically, a novel technique utilizing oxide-refilled trenches is implemented to achieve efficient temperature compensation while maintaining compatibility with wet release processes. Using this method, we demonstrate high-Q resonators having a first-order TCF as low as 3 ppm/°C and a turnover temperature of around 90 °C, ideally suited for use in ovenized platforms. Using active tuning, the temperature sensitivity of the resonator is further compensated around the turnover temperature, demonstrating frequency instability of less than 400 ppb. Such devices are ideally suited as timing units in space applications where size, power consumption, and temperature stability are of critical importance.

Temperature compensated fused silica resonators using embedded nickel-refilled trenches

This paper reports a new fabrication process that utilizes nickel-refilled trenches to achieve passive temperature compensation in fused silica. Using this scheme, piezoelectrically actuated fused silica resonators are demonstrated with a temperature coefficient of frequency (TCF) of +50.28 ppm/K (reduced from +77.65 ppm/K) and quality factors of over 5,000. Additionally, a higher frequency mode at 16 MHz shows a TCF of +21.84 ppm/K (reduced from +71.94 ppm/K). This compensation method can be extended to actuate a compensated and an uncompensated mode of the same device, allowing for a temperature-stable dual-mode frequency reference. This is the first time that passive temperature compensation has been shown for fused silica micro-mechanical resonators.