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Ch31-I044963.fm Page 149 Tuesday, August 1, 2006 3:06 PM
3:06 PM
1, 2006
Tuesday, August
Page 149
Ch31-I044963.fm
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MICROFABRICATION OF
A PARYLENE SUSPENDED STRUCTURE
AND INVESTIGATION OF ITS RESONANT FREQUENCY

1
1
D. Yoshikawa , S. Aoyagi and Y. C. Tai 2
'Systems Mangement Engineering, Kansai University
3-3-35, Yamate-cho, Suita, Osaka 564-8680, Japan
California Institute of Technology
136-93, Pasadena, CA9112, USA

ABSTRACT
Polymer material of Parylene has intrinsic tensile stress on account of mismatch of thermal coefficient
of expansion (TCE) between the substrate and the deposited film. Therefore, the stiffness k of the
Parylene suspended structure under tensile stress is much higher than that under no stress, which also
leads to its higher resonant frequency f r. These mechanical characteristics are investigated in this
study. First, FEM simulation is employed, and it is proved that k <x 1// holds true under tensile
stress, while kxl/l 3 holds true under no tensile stress according to the theory of strength of
materials, where / is beam length. This means a relatively long beam is necessary under tensile stress
for the purpose of lowering /,., which leads to obtaining high sensitivity in case that the suspended
structure is applied to a sensor such as an accelerometer. Considering this, a structure with spiral
beams is proposed. Second, Parylene suspended structures are practically fabricated. Their
experimental resonant frequencies are obtained by a LDV. They coincide well with simulated ones. As
the result, it is proved that the structure with spiral beams is effective for lowering f r.

KEYWORDS
Parylene, Resonant frequency, Stiffness, Tensile stress, Spiral beam, Accelerometer

INTRODUCTION
Parylene is polymer material expected to be applied in micromachine field and many sensors and
actuators using Parylene has been investigated and reported 1|. For example, Parylene accelerometer
|
as shown in Fig.l is being developed by authors [2]. Parylene has intrinsic tensile stress on account of
mismatch of thermal coefficient of expansion (TCE, a) between substrate and Parylene deposited on it
[3]. The stiffness k of this accelerometer structure changes according to the tensile stress of Parylene.
In this study, the mechanical characteristics of suspended microstructures are investigated by using
FEM (Finite Element Method) analysis. From the result of FEM simulation, it is proved that k °c III