Henry Barrow
Department of Electrical Engineering and Computer Sciences, UC Berkeley
BSAC Graduate Researcher
Dissertation Presentation
December 15, 2015 | 12:00 to 01:00 | 540 Cory Hall
Host: Clark Nguyen
In modern communication systems, conventional transistor technologies cannot sufficiently meet the demand for high performance filters in terms of insertion loss, percent bandwidth, and dynamic range. Instead these filters are typically implemented using large, off-chip components with orders of magnitude higher quality factor (Q), such as crystal and SAW resonators. These mechanical components constitute a bottleneck in system miniaturization because they interface with integrated electronics at the board level. Polycrystalline silicon micromechanical resonators with high Qs can potentially serve well as miniaturized substitutes for crystals in these filter applications, enabling on-chip high-performance filters. While micromechanical filters comprised of up to 3 mechanically coupled resonators have been demonstrated in the past, there exists a demand for bandpass filters with even sharper roll-offs and larger stopband rejections, and this requires higher order filters utilizing more than 3 coupled resonators.Filters comprised of 4 coupled resonators have been designed and fabricated. The measurement of a tunable 4-resonator filter yielded an impressively small 20-dB shape factor of only 1.59, representing the sharpest roll-off of any micromechanical filter measured to date. A protocol for automating the tuning process was developed and presented at the 2014 International Frequency Control Symposium in Taipei, Taiwan.
32-kHz real-time clock oscillators encompass a significant share of the multi-billion dollar oscillator market. Currently, quartz crystal-based oscillators at this frequency dominate the market because they offer the best combination of cost and performance. However, the physical dimensions of these oscillators are presently too large for future small form-factor electronic applications, such as ones that fit within credit cards. While attempts have been made to shrink quartz resonating elements, the increasingly difficult fabrication steps required to produce such devices raises manufacturing costs, thereby preventing widespread adoption (so far). Also, quartz crystal motional resistance values typically increase as resonator dimensions shrink, which in many oscillator configurations raises power consumption. Unlike common quartz resonators, properly designed MEMS resonators benefit greatly from scaling in that reductions in lateral dimensions lead to a rapid decrease in motional resistance by a square law. A capacitive-comb transduced micromechanical resonator using aggressive lithography to occupy only 0.0154-mm² of die area has been combined via bond-wiring with a custom ASIC sustaining amplifier and a supply voltage of only 1.65V to realize a 32.768-kHz real-time clock oscillator more than 100× smaller by area than miniaturized quartz crystal implementations and at least 4× smaller than other MEMS-based approaches, including those using piezoelectric material. This work was presented at the 2012 International Frequency Control Symposium in Baltimore, MD and was selected as a best student paper finalist.
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