Title:  Advanced HARPSS Processes for High Q and High Frequency Inertial Sensors

Committee: 

Dr. Ayazi, Advisor       

Dr. Ansari, Chair

Dr. Brand

Abstract:

The objective of this research is to introduce several advances into the HARPSS fabrication platform to improve the performance metrics of resonant MEMS devices and enable new functionalities. Many resonators including bulk acoustic wave (BAW) gyrsocopes require ultra-high quality factor (Q) to improve signal to noise ratio. The use of isotropic device layers such as thick epitaxially grown polysilicon can enable Akhiezer-limited Qs due to suppressed thermoelastic damping (TED) and anchor loss in centrally-supported gyroscopic disk resonators. However, thick epi-poly is prone to large stress at high processing temperatures (>900C). Hence, a reduced-temperautre HARPSS process has been devised to fabricate and test protoype thick solid disk epi-poly BAW gyroscopes. Another performance metric of a silicon resonator is the temperature coefficient of frequency (TCF). In this work, a novel Distributed Lame mode BAW resonator has been designed and fabricated on highly-doped single crystalline silicon substrates, utilizing a high frequency resonance mode that can achieve a frequency turnover point at a high temperature (>90C), making this device suitable for low-jitter oven-controlled crystal oscillators (OCXO). Lastly, motional impedance of capacitive resonators can be reduced by scaling the capacitive gaps to sub-100nm, requiring hydrogen annealing of device sidewalls to eliminate scalloping and roughness induced by Bosch process. In this work, a frequency output piezoelectrically-transduced resonant BAW accelerometer is introduced, combining 100nm gap HARPSS process with a metal-less AlN-on-Si resonator, which operates based on electrostatic softening effect and utilizes a novel moving electrode.