W3P.099
A MAGNETOSTATIC 2-AXIS MEMS SCANNER WITH I-SECTION RIB-REINFORCEMENT
AND SLENDER PERMANENT MAGNET PATTERNS
Hsin-Yu Huang
1
, Tsung-Lin Tang
1
, Wei-Lun Sung
1
, Hung-Yi Lin
3
, and Weileun Fang
1,2
1
Dept. of Power Mechanical Engineering,
2
Inst. of NanoEngineering and MicroSystems,
National Tsing Hua University, Hsinchu, Taiwan
3
Touch Micro-system Technology Corporation, Taoyuan, Taiwan
ABSTRACT
This study demonstrates the 2-axis epitaxial silicon
scanner driven by the coil-less magnetostatic force using
electroplated permanent magnet (CoNiMnP) film. The
present approach has four merits: (1) the process employs
the cheap silicon wafer with epitaxial layer; and the
electrochemical etching stop technique is used to
precisely control the thickness of scanner; (2) the
I-section rib-reinforced structure is implemented to
provide high stiffness of the mirror plate; (3) the
magnetostatic driving force on scanner is increased by
electroplated permanent magnet film with slender
patterns; (4) the size of packaged scanner is reduced since
the assembled permanent magnets are not required.
This study exploits the simple ECES process to
implement the scanner with I-section rib-reinforced
structure on an epi-wafer. Hence, the stiffness of mirror
plate is increased. Moreover, this study also implements
the electroplated permanent magnet film with slender
patterns on the scanner, and the magnetization direction
of the hard magnet material (CoNiMnP) is fixed. Thus,
the assembled permanent magnet in [5] is no longer
required, and the package size of the scanner is reduced.
CONCEPT AND DESIGN
This study employs the epitaxial silicon (epi-Si) to
realize the MEMS scanner. The ECES technique is thus
used to release single crystal silicon scanner and precisely
control its thickness [6]. Fig. 1 illustrates the schematic
design concept of the scanner. Fig. 1(a) shows the front
side view of the 2-axis scanner on epi-wafer. The scanner
consists of the mirror plate connects to the supporting
frame through one pair of torsional springs (fast scan
spring), and the supporting frame connects to the anchor
through another pair of torsional springs (slow scan
spring). The permanent magnet (CoNiMnP) film is
electroplated on the mirror plate and supporting frame for
magnetic actuation. Fig. 1(b) illustrates the backside view
(a)
Fast scan spring
Permanent magnet film
Mirror plate
KEYWORDS
Optical scanner, Magnetostatic actuation, I-section
rib-reinforcement, permanent magnet film, Electroplating
INTRODUCTION
The 2-axis MEMS scanner is a key enabling
component for the compact display system [1], laser
printer [2], spectroscopy [3], and barcode scanning [4]. In
general, the 2-axis MEMS scanner employs
micro-actuators to manipulate incident light in two
orthogonal directions. Thus, the design considerations of
the scanner include high scanning speed, high resolution,
small feature size, and low power consumption
requirements in such demanding application.
The SOI substrate with deep reactive ion etching
(DRIE) process is extensively employed to implement
MEMS scanners [5]. Nevertheless, cost is a major
concern for such technique. In [6], the low cost p-type
silicon wafer with n-type epitaxial silicon layer (named
epi-wafer) has been employed to realize a large size flat
mirror plate using the electrochemical etching stop
(ECES) technique. However, the stiffness and moment of
inertia are trade-off for the design of mirror thickness.
The electromagnetic forces, such as Lorentz force and
magnetostatic force, have been exploited to drive MEMS
scanner [1, 4, 5]. However, the process issues for
electrical routing, and the problem of joule heat can not be
ignored for Lorentz force scanner [7]. On the other hand,
the magnetostatic force can be easily induced by coating a
ferromagnetic material on the scanner [5, 8]. Thus, the
electrical routing is not required. However, the permanent
magnet is required to ensure the magnetization direction
of the ferromagnetic material. Hence, it remains a
challenge to shrink the package size for scanners driven
by magnetostatic force.
Supporting frame
(b)
Slow scan spring
Epi-Si diaphragm
I-section rib
Figure 1: Schematic diagram of (a) front side, and (b)
backside views of the scanner design on the epi-wafer.
978-1-4577-0156-6/11/$26.00 ©2011 IEEE
2366
Transducers’11, Beijing, China, June 5-9, 2011
Fast scan axis
Vertical pattern
(a)
y
x z
(b)
y
x z
Slow scan axis
Figure 4: Modal analysis results by Finite-element
(ANSYS), (a) 1
st
mode for slow scan: 1,078 Hz, and (b)
6
th
mode for fast scan: 21,119 Hz.
Fig. 4 shows the modal analysis result of the designed
scanner. The first mode is the slow scan mode at 1,078 Hz,
where the mirror plate rotates about the slow scan springs
(Y-axis scanning). The sixth mode of the scanner with a
frequency of 21,119 Hz is employed for the fast scan
mode. The mirror plate rotates about the fast scan springs
(X-axis scanning).
Horizontal pattern
Figure 2: Orthogonal electroplated slender permanent
magnet patterns for the fast and slow scan axes.
of the scanner. It shows the scanner with I-section ribs
suspended on a cavity etched by anisotropic ECES
process. The I-section rib structures made of the Si
substrate are employed to increase the stiffness of mirror
plate. The ECES process is also employed to precisely
control the epi-Si diaphragm.
As indicated in Fig. 1(a), the permanent magnet film is
patterned to slender rectangles to increase the shape
anisotropy so as to further enhance magnetization [5].
This study designs permanent magnet patterns to induce
the magnetostatic force for the actuation of the 2-axis
scanner. As indicated in Fig. 2, the mirror plate is covered
with horizontal permanent magnet patterns for the
actuation of fast scan axis; and the supporting frame is
covered with vertical permanent magnet patterns for the
actuation of slow scan axis. Fig.3 shows the driving
scheme for the present 2-axis scanner. A time varying
magnetic field was produced when an ac current with
driving frequency is introduced into the solenoid.
Moreover, the solenoid can be driven by two function
generator, so that it can provide the ac magnetic field with
multiple excitation frequencies. The 2-axis scanner is
actuated by the solenoid with input ac signals consisting
of two frequencies respectively for slow scan and fast
scan.
Scanning pattern
Laser
FABRICATION AND RESULTS
The process steps shown in Fig. 5 have been
established to realize the presented scanner. The process
started with a (100) epi-wafer which had a 50µm thick
n-type silicon epitaxial layer on top of a 400µm thick
p-type silicon substrate. As shown in Fig. 5(a), a Si
x
N
y
layer (150nm) was deposited on the wafer by low pressure
chemical vapor deposition (LPCVD). After that, a 200nm
thick Cr layer was deposited onto the backside of the
epi-wafer by electron beam evaporation. As illustrated in
Fig. 5(b), the backside Si
x
N
y
and Cr layers were
simultaneously patterned to define the etching window
for the subsequent wet etching and DRIE processes. As
depicted in Fig. 5(c), a lift-off process was employed to
pattern a 100nm thick evaporated Al layer to increase the
reflectivity of the mirror plate. Fig. 5(d) shows the 20nm
thick Ti adhesion layer and the 200nm thick Au seed layer
were deposited onto the top side of an epi-wafer for the
following electroplating process. These Ti/Au layers also
acted as the electric conduction layers for the subsequent
ECES process. A 12m thick photoresist was patterned to
act as a mold, and then a 10µm thick CoNiMnP film was
electroplated and molded using the photoresist [9], as
illustrated in Fig. 5(e). As depicted in Fig. 5(f), a
time-controlled DRIE process was employed to define the
thickness of the p-type Si layer (30m in this study) for
the I-section rib structure. As shown in Fig. 5(g), the
anisotropic ECES process was employed to precisely
control the thickness of epi-Si diaphragm [6] and to form
the I-section rib structure as well. Finally, as indicated in
Fig. 5(h), the epi-Si diaphragm was patterned by a DRIE
process to release the scanner.
The scanning electron microscope (SEM)
micrographs in Fig. 6 show a typical fabricated scanner.
Fig. 6(a) shows the front side view of the whole device
consisting of mirror plate, supporting frame and torsional
springs. The zoom-in micrograph in Fig. 6(b) shows the
distribution of slender CoNiMnP patterns. The vertical
Magnetic field
Solenoid
Function
generator
Adder
Function
generator
Figure 3: Schematic illustration of the experiment setup
for scanner driving test.
2367
(a)
(e)
(b)
(f)
(c)
(g)
(d)
(h)
p-type Si n-type Si
Si
x
N
y
Cr
Al
Ti/Au
magnet
Figure 5: Fabrication process steps of the 2-axis scanner
with electroplated permanent magnet film and I-section
rib-reinforced structure.
(a)
(b)
shown in Fig. 7(a), the material composition of
electroplated permanent magnet film was analyzed by
Energy Dispersive Spectrometer (EDS). The magnet film
is composed of Co: 74.5 %, Ni: 19.4 %, P: 3.9 % and Mn:
2.2 % in weight percentage scale. It indicates the major
composition of this material is cobalt. The magnetic
properties of the permanent magnet film are characterized
with a vibrating sample magnetometer (VSM). The
typical demagnetization hysteresis loops for the lateral
direction (in-plane) is shown in Fig. 7(b). During
electroplating, external magnetic field was applied
parallel to the substrate plane, and the alloy with a
magnetic coercivity of 550 Oe, a retentivity of 0.58 Teslas,
and a maximum energy density of 8.8 kJ/m
3
.
Fig. 8 shows the driving test of the scanner to
demonstrate the 2-axis scanning. Fig. 8(a) shows the
driving stage established in this study for scanning test.
The scanner fixed to the stage is observed. The solenoid
embedded in the stage provides the magnetic field to drive
the scanner. The magnetostatic force is induced by
electroplated hard magnet (CoNiMnP) film on the
scanner. Since permanent magnets on the stage are not
required, the size of packaged scanner is relatively small,
as compared with the scanner in [5]. Fig. 8(b) shows the
typical measured frequency response of the 2-axis
scanner. The two orthogonal scanning resonant modes
were 1.1 kHz (slow scan) and 21.4 kHz (fast scan),
respectively. It indicates the orthogonal permanent
magnet patterns respectively induce magnetostatic force
on the mirror plate and the supporting frame. This study
also demonstrated the scanning pattern using the 2-axis
(a)
(c)
(d)
(b)
Figure 6: SEM image of the fabricated scanner, (a) front
side view, (b) vertical and horizontal electroplated
permanent magnet patterns, and (c-d) I-section rib
structure.
and horizontal patterns are electroplated on the supporting
frame and the mirror plate respectively. The SEM
micrographs in Fig. 6(c)-(d) show the I-section
rib-reinforced structure at the backside of mirror plate.
1.5
1.0
4
M (Teslas)
0.5
0.0
-0.5
-1.0
-1.5
-4000
-2000
0
2000
4000
H (Oersted)
EXPERIMENTS AND DISCUSSIONS
Fig. 7 shows the characterization of the electroplated
permanent magnet (CoNiMnP) films in this study. As
Figure 7: Characterization of the electroplated magnet,
(a) Energy dispersive spectrum (EDS), and (b) 4πM-H
hysteresis loop.
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(a)
(c)
-120
-130
1
st
mode:
1.1 kHz
6
th
mode:
21.4 kHz
Magnitude (dB)
5mm
-140
-150
-160
-170
-180
0
5
10
the Center for Nanotechnology, Materials Science and
Microsystems of National Tsing Hua University, the
Nano Facility Center of National Chiao Tung University,
the National Nano Device Laboratories, and the NEMS
Research Center of National Taiwan University for
providing the fabrication facilities.
(c)
REFERENCES
[1] A. D. Yalcinkaya, H. Urey, D. Brown, T. Montague,
and R. Sprague, “Two-axis Electromagnetic
Microscanner for High Resolution Displays,”
IEEE
Journal of Microelectromechanical Systems,
vol. 15,
pp. 786-794, 2006.
[2] W. O. Davis, D. Brown, M. Helsel, R. Sprague, G.
Gibson,
A.
Yalcinkaya,
and
H.
Urey,
“High-performance silicon scanning mirror for laser
printing,”
Proc. of SPIE,
vol. 6466, 64660D, 2007.
[3] C. Ataman, H. Urey, and A. Wolter, “A Fourier
transform spectrometer using resonant vertical comb
actuators,”
Journal of Micromechanics and
Microengineering,
vol. 16, pp. 2517-2523, 2006.
[4] A. D. Yalcinkaya, O. Ergeneman, and H. Urey,
“Polymer magnetic scanners for bar code
applications,”
Sensors and Actuators A,
vol. 135, pp.
236-243, 2007.
[5] T.-L. Tang, C.-P. Hsu, W.-C. Chen, and W. Fang,
“Design
and
implementation
of
a
torque-enhancement 2-axis magnetostatic SOI optical
scanner,”
Journal of Micromechanics and
Microengineering,
vol. 20, 025020, 2010.
[6] C. Lee, “Design and fabrication of Epitaxial Silicon
Micromirror Devices,”
Sensors and Actuators A,
vol.
115, pp. 581-590, 2004.
[7] T. Mitsui, Y. Takahashi, and Y. Watanabe, “A 2-axis
optical
scanner
driven
nonresonantly
by
electromagnetic force for OCT imaging,”
Journal of
Micromechanics and Microengineering,
vol. 16, pp.
2482-2487, 2006.
[8] A. D. Yalcinkaya, H. Urey, and S. Holmstrom, “NiFe
Plated Biaxial MEMS Scanner for 2-D Imaging,”
IEEE Photonics Technology Letters,
vol. 19, pp.
330-332, 2007.
[9] H. J. Cho, J. Yan, S. T. Kowel, F. R. Beyette, Jr., and
C. H. Ahn, “A scanning silicon micromirror using a
bi-directionally movable magnetic microactuator,”
Proc. of SPIE,
vol. 4178, pp. 106-115, 2000.
15
20
25
10mm
Frequency (kHz)
Figure 8: Driving test for the magnetostatic 2-axis
scanner with electroplated permanent magnet film, (a)
the scanner packaged in the driving stage with embedded
magnetic coils, (b) the frequency response, and (c) typical
2D scanning pattern.
scanner, as shown in Fig. 8(c). Such typical 2D scan
pattern was produced after applying sinusoidal ac currents
(root-mean-square value of
I
rms
= 400 mA) with
frequencies of 60 Hz and 21.4 kHz to the solenoid. The
optical scan angles are respectively 7.8 in horizontal
direction and 6.9 in vertical direction.
CONCLUSIONS
This study presents the design and implementation of
the magnetostatic 2-axis epi-Si scanner driven by the
electroplated permanent magnet (CoNiMnP) film. The
CoNiMnP film is electroplated on the top side of the
2-axis scanner with orthogonal slender patterns
respectively for the fast scan and slow scan axes. Since
the assembled permanent magnets are not required to
drive the presented scanner, the package size is
significantly reduced. Moreover, the thickness of the
epi-Si diaphragm and I-section rib-reinforced structure at
the backside of the mirror plate are implemented by ECES
process. In application, this study demonstrates the 2D
scan pattern of 7.8 in horizontal direction and 6.9 in
vertical direction after applying harmonic excitations of
I
rms
= 400 mA (root-mean-square value) and frequencies
of 60 Hz and 21.4 kHz to the solenoid.
ACKNOWLEDGMENTS
This study was partially supported by the National
Science Council of Taiwan under grant of NSC-96-2628-
E-007-008-MY3. The authors would like to appreciate
CONTACT
* W. Fang, Tel: +886-3-5742923; fang@pme.nthu.edu.tw
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