Online proceedings - EDA Publishing Association

11-13
May 2011, Aix-en-Provence, France
Parameter Design of Triaxial Microaccelerometers
with Piezoelectric Thin-Film
Jyh-Cheng YU
National Kaohsiung First University of Science and Technology
2 Jhuoyue Rd.,Nanzih , Kaohsiung City 811,Taiwan, R.O.C.
Chungda Lee
Department of Mechanical and Automation Engineering
I-Shou University, 1, Sec. 1, Syuecheng Rd., Dashu, Kaohsiung 840, Taiwan, R.O.C.
Abstract - This study proposes an analytical model for a
high sensitivity piezoelectric thin film triaxial
microaccelerometer, and investigates the influence of the
fabrication processes on the parameter design. The structure
design is consisted of four parallel suspension beams, a
central seismic mass, and eight piezoelectric thin film
transducers. The sensitivity consistence between out-of-plane
and in-plane accelerations is a key issue for the following
signal processing. A simplified system modeling scheme
based on anisotropic material properties using area moment
method and laminated beam theory is presented and applied
to the parameter design. An optimized thickness ratio
between piezoelectric thin film and the silicon substrate of
the laminated supporting beam is derived to maximize the
sensitivity. The study shows that the aspect ratio of the
seismic mass is the deterministic factor to the differences
among triaxial sensitivities. The triaxial sensitivity
performances for two structure designs with the seismic
masses fabricated using chemical wet etching and deep
reactive ion etching (DRIE) are compared. The design using
DRIE provides more even triaxial sensitivity, while the
design using wet etching shows cost advantage with
additional parameter constraints due to the required
compensation pattern for convex corner etching.
I. INTRODUCTION
For the last two decades, the design and research of
piezoelectric sensing accelerometers have drawn great
attention due to the advantages of low cost, energy saving,
simple structure, high sensitivity, and excellent dynamic
performance. Bulk micromachining based piezoelectric
accelerometers have a lower detection level that is suitable
for precision measurement. Piezoelectric accelerometers
consisted of a single seismic mass and cross supported beams
have been investigated for uniaxial measurement [1][2][3]
and triaxial measurement [4] applications. Most analytical
models assume piezoelectric thin film for simplification.
Hindrichsen [5] proposed an analytical model for a PZT thick
film triaxial accelerometer based on anisotropic material
tensors and Euler’s beam.
In general, accelerometers with PZT thick film
transducers have higher charge and voltage sensitivities in
comparison with those thin film devices in the same given
dimensions. Another configuration of piezoelectric thin-film
triaxial microaccelerometers consisted of parallel beam
suspensions and a central seismic mass has been studied
[6][9]. Parallel beam suspensions provide higher sensitivity
for in-plane accelerations comparing with those using a cross
beam structure.
Different fabrication processes of seismic mass, such as
chemical wet etching and deep reactive ion etching (DRIE),
affect the parameter design for device dimensions. Most of
the triaxial accelerometers do not have equal sensitivity for
all the three directions. Accelerometers fabricated using
DRIE of a thick SOI wafer could provide better consistence
of sensitivity in all the three orthogonal axes with a proper
parameter design. On the other hand, wet etching process has
a great cost advantage with dimensional limitations due to
sloping walls and a required compensation pattern for convex
corners.
This work presents the system modeling and parameter
design based on fabrication constraints of a triaxial
piezoelectric accelerometer. The proposed configuration
suspension of the device adopts parallel beams at both ends
of a seismic mass using etching of (100) SOI wafer. This
study adopts the area moment method to determine the
stiffness matrix of the supporting beams describing the
relationship between the end forces and moments and the
boundary conditions. The elastic property of the suspension
beams considers both the silicon beams and the piezoelectric
films using the laminated beam theory based on anisotropic
material properties. Analytical models of the resonant
frequency and the sensitivity are verified with the results
using finite element method to justify the model accuracy.
Three axial sensitivities for applicable conditions of design
geometry due to different fabrication processes are
compared.
II. DESIGN OF MICROACCELEROMETER
The proposed microaccelerometer consists of a
quadri-beam suspension, a seismic mass, and eight
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