Reminder : PhD Oral defense (Bongsang Kim) , 5/21, Monday 9am CISX-auditorium

Bongsang Kim bongsang at stanford.edu
Sun May 20 01:55:38 PDT 2007


Hi all,

This is the reminder for my PhD Oral defense, this Monday 9am at CISX
auditorium.
You can download the summary slides under following link.
http://www.stanford.edu/~bongsang/defense_slide/BK_PhD_Defense_Slides_Summary.pdf

Best regards,

Bongsang



Ph.D. Oral Examination, Stanford University
Title : Stability and Performance of Wafer-scale Thin-film Encapsulated MEMS
Resonators

Ph.D. Candidate : Bongsang Kim
 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)

Abstract

Silicon resonators are one of the most promising devices due to their
potential
application as frequency references in electronic circuits. Reduced size and
batch
fabrication will make silicon resonators cost effective compared to the
quartz
oscillators which are the most widely used as circuit frequency references.
While
there have been many breakthroughs in the field of MEMS resonators, the
problem
of packaging is yet to be solved. The stability of the resonant frequency
over time
is absolutely essential for use as a frequency reference, and the frequency
stability
depends on the quality of the package environment.

This work presents the stability and performance of MEMS resonators packaged
in
wafer-scale thin-film encapsulation process, called 'epi-seal'. This
encapsulation is
formed by depositing polycrystalline silicon at CMOS clean and extremely
high
temperature (~980°C) environment. Mechanical robustness of the encapsulation

provide MEMS resonators extremely high yield even after harsh post
processing,
such as wafer sawing and wire bonding.

During more than 1 year of operation, resonant frequencies of these
encapsulated
resonators were stable in ppm level drift. This commercial level stability
was
achieved with the help of the secured operating environment provided by the
cleanliness and hermeticity of the 'epi-seal' encapsulation. For further
optimization
of encapsulation design, diffusive gas species and diffusion paths were
investigated
by 400°C accelerated diffusion experiment.

In addition, the other efforts to develop commercial level high performance
MEMS
resonators are presented.

Quality Factor, Q is a description of energy loss of resonators, which is
very
important for designing oscillator circuits with the resonators. The
temperature
dependence of various energy losses is investigated. The quality factor of
MEMS
resonators can be engineered to be either a strong or a weak function of
temperature. Especially for 'oven-based' active temperature compensation,
strong
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
compensating
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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