OPTICAL AND ELECTRICAL CHARACTERIZATION OF IN-PLANE MODES
OF ALN-BASED PIEZOELECTRIC VIBRATING MEMS
V. Ruiz1, T. Manzaneque1, J. Hernando-García1, A, Ababneh2, H. Seidel3, E. Vikhagen4, J.L. Sánchez-Rojas1
1
Área de Tecnología Electrónica, Universidad de Castilla-La Mancha, 13071 Ciudad Real, Spain
2
Hijjawi Faculty for Engineering Technology, Yarmouk University, 21163 Irbid, Jordan
3
Chair of Micromechanics, Microuidics/Microactuators, Faculty of Natural Sciences and Technology II,
Saarland University, 66123 Saarbrücken, Germany
4
Optonor, 7047 Trondheim, Norway
Abstract — In this paper we study the modes of
vibration for three different aluminum nitrideactuated piezoelectric microstructures: a rectangular
cantilever, a crab leg anchored square paddle and a
four crab legs supported square plate. We combine
electrical and optical techniques to fully characterize
the vibrating modes of these types of MEMS structures, with special attention to the in-plane modes.
An electronic speckle pattern interferometry technique is used for a full 3D detection of the movement
of the structures. Quality factors as high as 2700
were obtained for in-plane modes in air. This work
shows the great flexibility in the selection of resonant
modes in piezoelectric resonators and actuators,
implemented by a proper design of the electrode
layout geometry.
Keywords: MEMS, piezoelectric, in-plane motion,
AlN.
I - Introduction
Nowadays there is an increasing interest in the use
of the in-plane modes of vibrating MEMS structures [1].
Under the proper design, such modes are characterized
by higher quality factors than out-of-plane modes, what
results in a better mass resolution for mass sensing
applications, as well as the possibility of detection in
high viscous media. For example, Brand et al. [3]
reported on the design of microcantilevers for mass
sensing with the first laterally vibrating mode, while
Dufour et al. [4] reported the electrical detection of the
first longitudinal mode of a millimeter-sized cantilever
in liquids of different viscosities.
The identification of the vibrating modes is a fundamental point to take into account. When working with
piezoelectric devices, the induced piezoelectric current
can be registered using different types of electronic
circuits [5]. However, this electrical approach does not
provide information about the shape of the mode, and
only a comparison of the experimental electrical measurement with theoretical analytical calculations or
simulations allows for an indirect determination of the
mode under study.
Here we combine electrical impedance measurements with an electronic speckle pattern interferometry,
that detects both out-of-plane and in-plane motion, to
characterize the modes of vibration of different types of
structures. The recognition of the shape of the mode is
done with the help of the optical data, and the impedance data is used to calculate the quality factor of the
modes.
II - Description of the structures
Three different structures were studied: a microcantilever (figure 1) with a top area of 1000x250 µm2, a
crab leg anchored square paddle of 500x500 µm2 (figure
2) and a four crab legs anchored square plate of
500x500 µm2 (figure 3). For all designs, the device
structure is as follows: a 20 µm thick, p-doped (100)
silicon plate serves as bottom electrode, which is covered with a 1 µm thick AlN piezoelectric film synthesized in a reactive sputter process from an aluminum
target in pure nitrogen atmosphere. As top electrode, a
10 nm Ti / 500 nm thick Au electrode is deposited. The
top electrode was designed to maximize the displacement of specific in-plane modes. A complete description
of the fabrication process can be found in [6].
Figure 1: Optical micrograph of the 1000x250 µm2 microcantilever. Brighter zones represent the Au electrodes.
ment system is controlling the excitation and a kind of
lock-in
in technique is used for the recording, the phase
relations between different directions are found. Both
in-plane vibrations and out-of-plane
plane vibrations can be
shown by animated displays,
plays, and frequencies up to 240
MHz can be measured.
IV - Results and Discussions
A. Cantilever structure
Figure 2: Optical micrograph of the 500 x500 µm2 micropaddle and its crab leg anchor. Brighter zones represent the Au
electrodes.
The first structure under study is a 1000x250 µm2
cantilver. The top electrode has been split into two strips
to improve excitation of lateral modes [8].
[
Several out of plane modes were found as it can be
seen in figure 4 but also some purely in-plane
in
modes
like the ones whose modal forms are represented in
figures 5 to 8. It is worth
th noting that the first lateral inplane mode has the highest quality
qu
factor in air but
another in-plane mode, the longitudinal one,
one has the
lowest quality factor,, about 25. Such a low value could
be related to the importance of the damping at the
anchor of the cantilever.
3025
Bending (n0)
Quality Factor
2525
500 µm microplate
Figure 3: Optical micrograph of the 500 x500
showing the four crab legs. Brighter zones represent the Au
electrodes.
Torsional (n1)
Lateral
2025
Longitudinal
1525
1025
2
III – Characterization Systems
A. Impedance Analyzer
The advantage of integrating a piezoelectric layer is
the possibility of achieving an all--electrical actuation/detection scheme. To make use of this advantage,
we studied the electrical performance of the AlN film
when exciting the structures in their different modes,
recording the change in impedance in air. A 4294A
Agilent impedance analyzer was used. This impedance
spectrum is then fitted into a Butterworth-Van-Dyke
Butterworth
equivalent circuit model [7] in order to obtain the
quality factor and resonant frequency of each mode.
mode
525
25
0
1
2
3
4
5
6
7
8
9
10
Mode order (n)
Figure 4: Quality factors for the electrically detected modes in
the cantilever sample. For the out-ofof- plane modes, the
classification was made using Leissa’s nomenclature [9].
[
B. Electronic speckle pattern interferometer
A full field speckle pattern-based
based interferometric
system from Optonor (MEMSMap 510) was used to
measure vibrations in all 3 directions. The object is
illuminated by several laser beams from different
directions in turn, and the object excitation was conco
trolled by the MEMSMap system. Since
Sinc the measure-
odal shape of the cantilever’s first
Figure 5: Measured modal
lateral mode at 255 KHz. The colored scale represents the yy
axis displacement amplitude of the structure.
has been designed to optimize the excitation of the
lateral in-plane mode.
The out-of-plane
plane modes with higher quality factor as
well as the two firsts in-plane
plane modes are shown in
figures 9 to 12.
Figure 6: Measured modal
odal shape of the cantilever’s second
lateral mode at 1.3 MHz. The colored scale represents the yy
axis displacement amplitude of the structure.
Figure 9: Measured modal shape of the paddle’s first torsional
mode at 57 KHz with a quality factor of 1500. The colored
scale represents the z-axis
axis displacement amplitude of the
structure.
Figure 7: Measured modal
odal shape of the cantilever’s first
longitudinal mode at 1.6 MHz. The colored scale represents
the x-axis
axis displacement amplitude of the structure.
Figure 10: Measured modal
odal shape of the paddle’s
paddle first lateral
mode at 71 KHz with a quality factor of 700. The colored
scale represents the y-axis
axis displacement amplitude of the
structure.
Figure 8: Measured modal shape of a high order mode at 41
MHz with a quality factor of 3700. The colored scale
represents the z-axis
axis displacement amplitude of the structure.
B. Crab leg anchored paddle
The second structure under study is a 500x500
500
µm
square paddle with a crab leg anchor. Each part of the
leg has a top area of 200x100 µm2. The top electrode
2
Figure 11: Measured modal
odal shape of the paddle’s
paddle first longitudinal mode at 337 KHz with a quality factor of 700. The
colored scale represents the x-axis
axis displacement amplitude of
the structure.
Apart from the usual in-plane
plane modes like the lateral
one and the longitudinal, a rotational mode was found in
the four crab legs-anchored
anchored paddle, showing the richness of modes of these complex structures.
structures
Acknowledgments
This work was supported by Spanish
Spanis Ministerio de
Ciencia e Innovación project DPI2009DPI2009 07497. Víctor
Ruiz acknowledges financial support from FPU grant
with reference AP2010-6059 and Tomás Manzaneque
acknowledges financial support from FPI grant with
reference BES-2010-030770.
Figure 12: Measured modal
odal shape of the paddle’s
paddle second
torsional mode at 499 KHz with a quality factor of 950. The
colored scale represents the z-axis
axis displacement amplitude of
the structure.
C. Four crab legs anchored plate
The last structure is a 500x500 µm
µ 2 square plate
anchored in each corner using four crab legs. Each part
of the crab leg has a top area of 400x100 µm2. The top
electrode has been designed to optimize the excitation
of a rotational mode that can be seen in figure 13.
Figure 13: Measured modal
odal shape of the plate’s
plate rotational
mode at 504 KHz with a quality factor of 640. The colored
scale represents the total displacement amplitude of the
structure.
V – Conclusion
We report the optical and electrical detection of both
out-of-plane and in-plane
plane modes in different types of
structures. The modal shape of the modes was deterdete
mined by means of a laser-based interferometer,
interferomete and the
corresponding quality factor was determined by the
fitting of the electrical impedance to a BVD model.
model
Among the different structures under investigation, the
first laterally vibrating mode of a cantilever is the mode
with the highest quality factor. It is also remarkable that
the longitudinal mode in this structure showed
show a very
low quality factor in air, what can be attributed to
anchor losses.
For the one crab leg-anchored paddle structure, the
quality factors of the in-plane
plane modes were found to be
comparable against the best out-of-plane
plane modes.
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