Ph.D. Oral Examination - Shasha Wang
wangss at stanford.edu
Mon Mar 19 00:17:39 PDT 2012
Stanford University Ph.D. Oral Examination – Department of Electrical Engineering
Hermetically Encapsulated Fully Differential Breathe-Mode Ring Resonator
Speaker: Shasha Wang
Advisor: Professor Thomas W. Kenny
Date: Thursday, March 22, 2012
Time: 2:00 pm (refreshments at 1:45 pm)
Location: Allen-X Auditorium (formerly CIS-X Auditorium) - Room 101
As the modern electronic devices continue to miniaturize and integrate more functionalities, silicon based MEMS timing references attract more attentions and are replacing quartz crystal in the $5 billion market. They offer a lot of advantages such as small foot-print, low cost, low power consumption, etc. However, the performance of MEMS based timing reference still need improving to overperform the well-establised quartz crystal. The oscillators' phase noise performance is particularly important, since the frequency references in RF devices must satisfy stringent phase noise specifications. In this talk, I will talk about how to design a MEMS oscillator to achieve good phase noise performance.
First, I will talk about how to design MEMS resonators with high quality factor through minimizing air damping, anchor loss and thermoelastic dissipation. We have designed and fabricated a fully-differential breathe-mode ring resonators with a quality factor as high as 473,000 at 10MHz. This quality factor is approaching the theoretical maximum quality factor, set by the phonon to phonon scattering in the material. However, the motional impedance of our resonator is very high due to large transduction gap size limited by the fabrication process. We designed and analyzed an OP-AMP based three-stage trans-impedance amplifier to provide sufficient gain for close loop oscillation. Close-to-carrier (1kHz offset) phase noise performance of -120dBc/Hz is achieved. The relatively poor noise performance is due to high motional impedance of our resonator.
In the second part, I will talk about how to modify our fabrication process flow to reduce the resonator's motional impedance. We used surface mico-machining combined with bulk micro-machining to achieve a transduction gap size as small as 260nm, which reduces the motional impedance by 6x and also lowers DC bias voltage requirement. The resonator's quality factor and stability is well maintained too.
In the last part, I will briefly talk about using InvenSense Nasiri Fabrication platform to achieve integrated MEMS-CMOS structures. Open loop transmission response of the ring resonator with integrated CMOS Trans-impedance amplifier will be presented. Additionally, mechanical coupling method is used to form ultra-narrow bandwidth (<0.05% BW) breathe-mode ring filters.
Electrical Engineering Department
Stanford Micro Structures and Sensors Lab
Stanford University, California, USA
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