a novel cmos-mems scanning micro-mirror using vertical comb drives
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7/23/2019 A Novel CMOS-MEMS Scanning Micro-mirror Using Vertical Comb Drives
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A Novel CMOS-MEMS Scanning Micro-mirror Using Vertical Comb Drives
Peng Qu, Hongwei Qu
Oakland University, 2200 N. Squirrel Road, Rochester, Michigan 48309, USA
Introduction
This paper presents design and simulation of an
improved CMOS-MEMS electrostatic torsional micro-
mirror for rotational scanning. The inclusion of substratesingle crystal silicon (SCS) in vertical comb drives
(VCDs) allows for large electrostatic force and scanning
angle. The uniqueness of the VCDs also includes
elevation tenability of the electrodes. The device design
and simulation for improvements are based on AMI 0.5
µm CMOS technology with which the previous devices
were fabricated. With typical technological parameters
included in simulation, a maximum scanning angle of
±10° can be obtained at a 27 V driving voltage.
Micro-mirror Design
Fig. 1 (a) depicts a 3D model of the proposed device
in which two sets of stator electrostatic comb drives, oneon each side of the mirror, are elevated by two
complementary sets of bimorph beams. The inset as Fig.
1(b) shows the detail of vertical mismatch between the
elevated stator comb drives and rotor comb drives that
are connected to mirror plate. The 400µm x 400µm
mirror plate is anchored to substrate via two torsional
springs to allow for rotational motion upon the
electrostatic driving by the aforementioned comb drives.
The inclusion of ~ 60 µm-thick SCS underneath the
mirror plate ensures an optical flatness of the mirror
plate upon the release of the entire mirror structure. This
silicon layer also determines the thickness thus the area
of the comb fingers. Other dimensional parameters for
the micro-mirror are given in Table 1.
At each side, two sets of complementary bimorphs
are used to elevate stator comb fingers above the mirror
plate that is in the same plane as substrate as shown in
Fig. 1(c) [1]. The two sets of bimorph beams are oriented
oppositely so that the curling of each group compensatesthe other, resulting in the elevation of stator comb drives
that are parallel to rotor comb drives. Without specific
material properties and process parameters, our previous
design has resulted in a measured 110~120 µm of the
stator comb drives, which failed the device due the
consequent disengagement of the comb drives, as shown
in Fig. 2. With the approximate data extracted from
previous measurements, we have redesigned the mirror.
The bimorph beams have been shortened to 50 µm for an
achievable engagement of the comb drives.
One important consideration in the design of the
torsional springs for the mirror plate is that its resonant
frequency should be much smaller than the stators for areliable actuation. For our application, a torsional spring
constant of 1.2 x 10-8 Nm/rad for a desired resonance
frequency of 1 kHz is designed. In order to significantly
reduce the overall device size, various folded serpentine
configurations of the torsional spring have been
exploited as listed in Table 2. Optimal spring
configuration #2, which results in the highest vertical-to-
torsional spring constant ratio (SCR) and largest
resonant mode separation, has been selected for a
reliable actuation.
Micro-mirror Simulation
The elevations of both previous and new stator
VCDs have been simulated using CoventorWare. Fig. 3shows the z-displacement along the top surface of the
stator comb drives. Considering the designed finger
thickness of ~60 µm, a ~10 µm engagement between the
rotor and stator fingers can be expected, with a
considerable surface flatness of the stator beams.
The modal analysis shows that the torsional mode
exists at 1.03 kHz, while the vertical motion mode exists
at a much higher 4.39 kHz, suggesting a practically
Figure 2. SEM image of bimorph beam and comb fingers
from old device
BimorphSet 2
ElevatedVCD
Anchor
Figure 1. (a). 3D Model of the proposed micro-mirror device;(b). Close-ups of the vertically elevated comb fingers; (c).
LVD bimorph structure.
(b) (c)
(a)
Bimorph
Beams
SpringElevatedStator
Fingers
Mirror
Plate
Anchor
Stator
Rotor
63
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7/23/2019 A Novel CMOS-MEMS Scanning Micro-mirror Using Vertical Comb Drives
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useful design. The simulated resonant frequency of the
LVD is 4.45 kHz, corresponding to a torsional stiffness
of 4.61 x 10-7 Nm/rad, which is 30 times greater than
torsional spring constant. This design can ensure stable
actuations of the mirror plate.
Fig. 4 shows the predicted rotational angles of the
mirror plate as responses to applied voltage and
electrostatic force, respectively. For a maximum
rotational angle of 10° that is determined by dimensionsof driving comb drives, a DC voltage of 27
V is expected,
corresponding to a 8 µ N electrostatic force generated by
the comb drives. The analytical results match the
Coventor simulations with an overall error within 5%.
Fabrication Process
AMI 0.5µm technology with three metal layers was
and will be used in the device fabrications. The CMOS
foundry fabricated chips have a profile as shown inFig. 5(a). The post-CMOS microfabrication process
starts with a backside etching of substrate SCS, to define
the mirror plate thickness of 60 µm. (5(b)). Then
anisotropic SiO2 etching is performed on the front side to
expose the regions of bimorph beams (5(c)). A unique
wet aluminum etching follows to remove the top Al
layer M3; and a silicon DRIE followed by an isotropic
silicon etch is performed to undercut the silicon
underneath the bimorph beams and mirror springs (5(d)).
This step also electrically isolates the comb fingers on
mirror plates and LVD plate from silicon substrate. Next,
the second anisotropic SiO2 etch defines VCD comb
fingers (5(e)). Finally, a silicon DRIE etches through the
comb fingers and completes the release (5(f)).
Table 1 Design parameters for the micro-mirror Parameter Definition Value
l m Mirror Length 0.4 mm
wm Mirror Width 0.4 mm
t SCS Thickness 60 µm g VCD Finger Gap 2 µmw f VCD Finger Width 6 µml f VCD Finger Length 100 µm
N Number of Fingers 25
l b Length of bimorph beam 50 µmwb Width of bimorph beam 9 µmt b Thickness of bimorph beam 1.8 µm
N Number of bimorph beams 24
Lb Length of amplifier beam 150 µmW b Width of connector beam 30 µm
Table 2 Design and optimization of torsional springs
a
w
a
wa=200 µm
w s=6 µm
t s=3.6 µm
f φ :f z=1:1.97
k z :k φ =1
a
bw
a
bw
a=62 µm
b=20 µm
w s=6 µm
t s=3.6 µm
f φ :f z=1:4.27
k z :k φ =4.67
a
b
w
a
b
w a=32 µm
b=360 µm
w s=8 µm
t s=3.6 µm
f φ :f z=1:1.4
k z :k φ =0.5
a
b
w
a
b
w a=150 µm
b=15 µm
w s=4 µm
t s=3.6 µm
f φ :f z=1:2.31
k z :k φ =1.37
References
[1] A. Jain, et al , 2004 Solid State Sensor, Actuator and
Microsystems Workshop, Hilton Head, SC, pp.228-231.
Figure 5. The fabrication process flow of the mirror
(c)
Spring Comb drive Bimorph Beams
(a) (d)
(e)(b)
(f)
0 5 10 15 20 250
2
4
6
8
10
12
Voltage (V)
R o t a t i o n A n g l e ( D e g r e
e s )
Analytical Calculation
Coventor SimulationCurve Fitting
(b)
0 1 2 3 4 5 6 7 8 90
2
4
6
8
10
12
Electrostatic Force in Z (uN)
R o t a t i o n A n g l e ( D e g r e e s )
Analytical CalculationCoventor Simulation
Curve Fitting
(a)
Figure 4. Predicted scanning responses of the mirror. (a)
rotation angel vs. applied dc voltage; and (b) scanning
angle vs. force generated by the comb drives.
#1
#2
#3
#4
Figure 3. Coventor simulation of elevation effects of
both old and new designs.
0 20 40 60 80 100 12040
60
80
100
120
Length of Stator (um)
S t a t o r D i s p l a c e m e n t ( u m )
Old design
New design
Figure 3. Coventor simulation of elevation effects of
both old and new designs.
64
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