Accurate and stable frequency references are essential for most electrical systems. While quartz oscillators are precise, they cannot be miniaturized for higher frequencies. This makes MEMS resonators an attractive alternative due to their small size and low power consumption despite their relatively large frequency drift over temperature. This research proposes a novel solution for silicon-based MEMS devices that offer low-frequency variations across a wide temperature range, making them suitable for integration into modern communication and digital systems.
Two main research avenues are explored. First, the thermal behavior of S0 Lamb wave X-cut Lithium Tantalate (LT) resonators is investigated, demonstrating a zero-temperature coefficient of frequency (TCF). Through numerical methods and simulations, the resonators are optimized for high electromechanical coupling and high turnover temperature, which is crucial for designing stable micro-oven-controlled oscillators. Experimental fabrication confirms their high-quality factor (~2000 at 200 MHz), high turnover temperature (>80°C), and moderately large effective electromechanical coupling (~5% at 200 MHz). Then, the unique characteristic of S0 Lamb wave X-cut LT resonators is used to propose a novel passive temperature compensation method, potentially canceling both first and second-order TCFs in silicon-based resonators. The combined heterostructure of highly doped silicon and rotated X-cut LT resonators, with opposing quadratic TCF curves, provides near-perfect compensation for temperature-induced frequency fluctuations. Implementing this design with a thin LT film (1.5 µm) on an SOI wafer resulted in a frequency drift as low as 70 ppm over a temperature range of 20°C to 100°C at a 313 MHz operation frequency, significantly improving over conventional silicon-based resonators and approaching quartz performance.
Secondly, the relationship between thermally-induced frequency fluctuations and phase noise of thin-film Piezoelectric-on-silicon (TPOS) resonators is examined. The results provide experimental validation suppression of overall oscillator circuit noise through the operation of the resonator at turnover temperature.
Reza Abdolvand, Committee Chair.
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