Gyroscopes are angular velocity sensors that are used for measuring rate or angle of rotation. The application domain of silicon microgyroscopes is quickly expanding from automotive to aerospace and consumer electronics industries. Examples include anti-skid and safety systems in cars, inertial measurement units (IMUs), image stabilization in digital cameras, and smart user interfaces in handheld devices. As potential high volume consumer applications for micromachined gyroscopes continue to emerge, design and manufacturing techniques that improve the performance, reliability and shock survivability of gyroscope while providing multi-axial functionality become increasingly important.
Today, state-of-the-art silicon micromachined vibratory gyroscopes can achieve high performance with low operational frequency (3–30kHz) at the cost of large form factor, high operating voltages and very low pressure package environment. Additionally, temperature compensation is required to guarantee stable performance over temperature. These all add up to make the finished product elaborate and costly. In this dissertation, capacitive bulk acoustic wave (BAW) silicon disk gyroscopes are introduced as a new class of micromachined vibratory gyroscope to investigate the operation of Coriolis-based gyroscopes at high frequency and further meet consumer electronics market demands. Capacitive BAW gyroscopes, operating at high frequency of 1–10MHz, are stationary devices with vibration amplitudes less than 20nm, resulting in high operational bandwidth and high shock tolerance, which are generally unavailable in low frequency gyroscopes. BAW gyroscopes require low operating voltages, which simplifies the interface circuit design and implementation in low-voltage CMOS technologies. They also demonstrate appropriate thermally-stable performance in air, which eliminates the need for vacuum packaging and temperature compensation, resulting in superior reliability and reduced cost.
This dissertation presents the design, implementation and characterization of z-axis capacitive BAW disk gyroscopes in (100) and (111) single crystal silicon. A revised high aspect-ratio poly- and single crystalline silicon (HARPSS) process was utilized to implement these devices in thick silicon-on-insulator (SOI) substrates (35–60µm) with very small capacitive gap sizes (∼200 nm). The prototype devices show ultra-high quality factors ( Q) in excess of 200,000 and large bandwidth of 15–30Hz under very high-Q mode-matched condition. The measured rate sensitivity for a 6MHz-disk gyroscope with Qmatched-mode of 235,000 was 270µV/°/sec in (100) silicon.
Another major contribution of this dissertation is to optimize the design and implementation of BAW disk gyroscopes for self-matched mode operation. Operating a vibratory gyroscope in matched mode is a straightforward way to improve performance parameters. But, it is very challenging to achieve without applying large voltages, which are difficult to generate with CMOS electronics. In this work, self-matched mode operation was provided by enhanced design of the perforations of the disk structure. In addition, the operating frequencies of the secondary elliptic modes were high enough to marginalize air damping losses. At the same time, the high operating frequency offers a very large device bandwidth of ∼ 400Hz when these devices are operated in air. The rate sensitivity of the optimized device in air was measured to be 65µV/°/sec for a 7.3MHz device with Q matched-mode of 15,000. In addition, these most advanced devices were characterized over a typical consumer electronics temperature range. It was observed that the modes remained matched and the measured Q and scale factor demonstrate the high performance stability of BAW gyroscopes even at elevated temperatures.
To complete this thesis, a gyroscope with planar-axis sensitivity (x-axis) is developed as an extension of the z-axis BAW gyroscope design. The x-axis gyroscope uses out-of-plane modes of a silicon disk structure. A rate sensitivity of 73µV/°/sec around the x-axis was measured for this device with a Qmatched-mode of 17,000 in (100) silicon. A multi-axis single-proof-mass gyroscope was introduced to measure the rotation rate around the x or y-axis and the z-axis by operating in in-plane and out-of plane modes. Like the single-axis devices, these gyroscopes were also optimized to achieve self-matched mode operation. The optimized multi-axis gyroscope exhibits matched in-plane mode and out-of-plane modes.
In conclusion, the experimental results establish the suitability of BAW gyroscopes for consumer electronic applications.