Ph.D. Dissertation Defense (May 21st, Monday) - Bongsang Kim
bongsang at stanford.edu
Mon May 14 18:12:46 PDT 2007
Ph.D. Oral Examination, Stanford University
Stability and Performance of Wafer-scale Thin-film Encapsulated MEMS
Advisor : Professor Thomas W. Kenny
Co-Advisor : Professor Roger T. Howe
Date : Monday, May 21st, 2007
Time : 9AM (Refreshments served at 8:45 AM)
Place : CISX 101 (Auditorium)
Silicon resonators are one of the most promising devices due to their
application as frequency references in electronic circuits. Reduced size and
fabrication will make silicon resonators cost effective compared to the
oscillators which are the most widely used as circuit frequency references.
there have been many breakthroughs in the field of MEMS resonators, the
of packaging is yet to be solved. The stability of the resonant frequency
is absolutely essential for use as a frequency reference, and the frequency
depends on the quality of the package environment.
This work presents the stability and performance of MEMS resonators packaged
wafer-scale thin-film encapsulation process, called 'epi-seal'. This
formed by depositing polycrystalline silicon at CMOS clean and extremely
temperature (~980°C) environment. Mechanical robustness of the encapsulation
provide MEMS resonators extremely high yield even after harsh post
such as wafer sawing and wire bonding.
During more than 1 year of operation, resonant frequencies of these
resonators were stable in ppm level drift. This commercial level stability
achieved with the help of the secured operating environment provided by the
cleanliness and hermeticity of the 'epi-seal' encapsulation. For further
of encapsulation design, diffusive gas species and diffusion paths were
by 400°C accelerated diffusion experiment.
In addition, the other efforts to develop commercial level high performance
resonators are presented.
Quality Factor, Q is a description of energy loss of resonators, which is
important for designing oscillator circuits with the resonators. The
dependence of various energy losses is investigated. The quality factor of
resonators can be engineered to be either a strong or a weak function of
temperature. Especially for 'oven-based' active temperature compensation,
temperature dependent Q can be used as an effective, direct, and delayless
measure of temperature of resonators.
To achieve temperature stability, silicon dioxide, which becomes stiffer as
temperature increases while silicon becomes softer, can be used as
material. Si-SiO2 composite resonators were successfully fabricated inside
modified 'epi-seal' encapsulation. These encapsulated Si-SiO2 composite
resonators showed more than 20x improvement in temperature stability.
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