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MEMS 3D Optical Mirror/Scanner
Tiansheng Zhou∗, Pat Wright
∗,
Jared Crawford∗, Graham Mckinnon
∗
∗Micralyne Inc.
1911-94 Street, Edmonton, Alberta,
T6N 1E6, Canada
Email: tzhou@micralyne.com
Yunfa Zhang∗∗
∗∗Mechanical Engineering Department
University of Alberta
Edmonton, Alberta
T6G 2M7 Canada
Abstract
This paper presents a novel MEMS optical mirror
based on a proprietary fabrication process. The mirror is
fabricated with single crystal silicon and has hexagonal
reflective surface 600 µm across, with a measured surface
roughness is less than 20 angstroms RMS and a radius of
curvature of greater than 5 meters. The device has a full
360° of Z rotation at up to 3° (~1° controllable) of out of
X-Y plane tilt angle depending the design parameters.
This mirror has no perforation holes on the reflective
surface and no stiction problems during fabrication or
operation. The addition of lateral comb drive actuators
gives the mirror up to 4 um X and Y in-plane movement.
The control of X and Y translation is totally independent
and free of movement interference. Due to all electrostatic
actuation, the device has lower power consumption, with
a designed driving voltage of less than 120 volts.
Simulation results, including modal analysis, are included.
1. Introduction
DWDM technology in optical networks has increased
capacity in point to point connections, enhancing network
scalability. However, issues of effective bandwidth
management have yet to be addressed. Optical cross-
connect switches, allow for manipulation of connections
at the wavelength level by users at remote locations.
Presently, microelectromechanical systems (MEMS) are
positioned to become the leading technology in this
application. Transparent all optical (OOO) systems help
minimize degradation of optical channels enabling long-
range networks. It is seen that MEMS based OOO
networks offer several advantages, such as cost-
effectiveness inherent in batch fabrication, signal
immunity from electromagnetic interference, bit
rate/protocol transparency, and ease of DWDM
implementation.
Another very promising MEMS mirror application is
the implementation in miniaturized, cost effective optical
scanners. A miniaturized MEMS 3D scanner with low
power and low cost can be used in high throughput 3D
imaging, retinal displays and 3D MEMS memory [1-4].
In this paper, we present the design, fabrication,
simulation and characterization of a new MEMS 3D
optical mirror/scanner. Our device has both out-of-plane
X and Y rotations and in-plane X and Y translations. All
actuation is electrostatically driven. The comb drive
electrodes for translation are offset to achieve larger
electrostatic force and avoid X and Y movement inference
at the same time. For larger area fill factor, the
mirror/scanner has a hexagonal reflective surface 600 µm
across. Fabricated using Micralyne’s proprietary process,
our 3D mirror/scanner has no perforation holes on the
entire reflective surface and no stiction during fabrication
or operation. The mirror is formed using the device layer
of an SOI wafer, minimizing surface roughness and mirror
curvature.
2. Design
Our 3D optical mirror/scanner has two actuation
modules; a rotation module and a translation module.
They all are electrostatic driven. The rotation actuator
module consists of top electrode that is the mirror and
lower metal electrode. The lower electrodes, including
ground electrode, are divided into 14 electrically isolated
metal areas in order to achieve full 360° of Z rotation at
up to a 3° of out of X-Y plane tilt angle. The ground
electrode is to prevent operational stiction due to
accidental electrodes snapping.
The translation actuator (Figure 1 and 2) is a comb
drive with offset electrodes. This design has two
advantages over comb drives with evenly spaced
electrodes. One is that it has a larger capacitor area,
therefore a larger electrostatic force with the same applied
voltage. The other is that X and Y translation is totally
Proceedings of the International Conference on MEMS, NANO and Smart Systems (ICMENS’03)
0-7695-1947-4/03 $17.00 © 2003 IEEE
2
independent and free of movement interference. For
regular comb drive with even gap electrodes, interference
between X and Y translation could cause motion
instability, which positive charged electrode fingers will
move sideway to contact with negative charged ones.
Considering our mirror as ideal Torsion Beam
structure (Figure. 3), for rotational X or Y actuation, the
following formula can be used to calculate the address
potential, tV [5]:
−−
−+
−
−=
x
dxz
z
x
dWz
z
x
dxz
x
dWz
xW
dk
V
AxA
x
y
t
t
0
0
0
0
0
0
3
0
3
2
2ln
2
ε (1)
where tk is the spring constant for X or Y rotation, 0ε is
the free space dielectric constant, 0z is the resting
separation when no voltage tV is applied, xW and yW are
mirror widths along the X and Y axes respectively, Ax is
the lateral position at which the edge of the address
electrode is located, d is the desired deflection at some
position x away from mirror center, the approximation of
mirror rotation angle θ can be express as:
( )x
d=≈ θθ sin (2)
For the translation actuator (Figure 1&2), we neglect
the deformations of the electrodes during operation as
well as fringing effects of the electric field around the
edges of the electrodes. For one pair of electrodes, the
voltage, V for translating the mirror a desired distance,
f in X or Y direction can be obtained by using following
equation [5]:
( ) ( )+−
−
=
2
1
2
0
0
11
2
fzfzA
fkV
xy
ε (3)
where 0z and 1z are the resting separations between
electrodes when no electrode voltage is applied. Refer to
Figure 2 for the definition of 0z and 1z . xyk is the
spring constant of translation actuator in X or Y direction.
z
x
y
Figure 1. Schematic of X and Y translation actuation
Z1
Z0
f
V
Figure 2. Schematic of offset electrodes for X and Y translation actuation
yz
x
Figure 3. Schematic of both rotation and translation actuation
Figure 3 shows the schematic of 3D mirror/scanner
with both rotational and translation actuators.
3. Fabrication
There are two common techniques for the fabrication
of most MEMS devices; surface micromachining and bulk
micromachining. Bulk micromachining requires the
removal of material from bulk substrates to form three-
dimensional structures. This technique is very
straightforward and is a well-documented fabrication
process. However, it is suitable only for very simple
geometries. Surface micromachining involves multiple
deposition, lithography, and etch steps. By selectively
etching sacrificial films, freestanding structures can be
Proceedings of the International Conference on MEMS, NANO and Smart Systems (ICMENS’03)
0-7695-1947-4/03 $17.00 © 2003 IEEE
3
released as microstructures. This technique has the
capability to create more complex structures, and has a
larger toolbox of possible materials. However, it is a more
expensive process and suffers from problems with film
stresses and stiction, which hinder device performance.
An emerging technology in the fabrication of MEMS
devices is silicon-on-insulator (SOI) substrates due to the
robustness of the silicon device layer as a structural
material.
A. Pattern and etch Pyrex 15 µm. Pattern Metal 1 and perform lift-off
B. Anodically bond SOI wafer to Pyrex
C. Etch back silicon backside
D. Pattern and etch low stress Metal 2. Pattern and DRIE etch 15 µm to release
Figure 4. Process flow for mirror fabrication
A generalized MEMS process has been developed at
Micralyne. Our proprietary process, is a simple, versatile,
and robust process that allows for fast prototyping of
micromachined components in various sectors of the
MEMS field.
A schematic of the process flow can be found in Figure
4. The starting material is a 7740 Pyrex wafer. The first
mask is used to pattern the Pyrex for the first etch (Figure
4A). This etch can be used to define the gap between the
mirror and bottom electrodes and grooves for electrode
lines, etc. The Pyrex is etched isotropically in an HF
solution to a depth of up to 35µm. Here we are reporting
on a mirror design with a 15 µm gap. The substrate is
subsequently lithographically patterned with the second
mask. This mask is used to define actuation electrodes,
metal lines, and bonding pads. The deposited metal stack
can be any metal stack suitable for MEMS applications,
such as aluminum, chrome/gold, or
titanium/platinum/gold. Following that, an SOI wafer is
anodically bonded with device side down to the patterned
side of the Pyrex wafer (Figure 4B). No bond alignment is
required. The SOI wafer consists of a thick (typically 525
µm) handle wafer, a thin (here we have used 15 µm)
single crystal silicon device layer, and a buried oxide
layer. The handle wafer is subsequently etched away in a
wet heated KOH process, and the buried oxide is etched in
a buffered oxide etch. This leaves a single crystal silicon
membrane over patterned electrodes in cavities/gaps
etched in Pyrex (Figure 4C). A low stress and high
reflective gold metal stack is deposited on the silicon
surface, and lithographically patterned with the third
mask. The Au metal is then etched to form electrodes,
actuation lines, reflective surfaces, etc. on the silicon
surface. Finally, the silicon surface is patterned with a
fourth mask to expose the silicon using another wet gold
metal etch process. The final etch is in the DRIE, where
the structures are released in a dry plasma (Figure 4D).
This method of release is preferable since it eliminates
stiction problems. Finally, the wafer is diced, and we are
left with our final 3D optical mirror/scanner.
A SEM photo of the fabricated 3D optical
mirror/scanner with X and Y translation electrodes is
shown in Figure 5A. Figure 5B shows a close-up of the
translation combs and spring found at the corner of the
mirror platform.
A. Top view of mirror/scanner
B. Close-up of the translation combs and spring
Figure 5. SEM photo of 3D MEMS mirror/scanner with X and Y translation actuation
Proceedings of the International Conference on MEMS, NANO and Smart Systems (ICMENS’03)
0-7695-1947-4/03 $17.00 © 2003 IEEE
4
4. Simulation
FEM software ANSYS is used to simulate the 3D
MEMS mirror and scanner. Figure 6 shows the modal
analysis results. The graph on the left is the first modal
shape with rotation frequency of 2496 Hz around X, while
the graph on the right shows the second mode shape with
rotation frequency of 2621 Hz around Y. Small frequency
difference between X and Y rotation resulting from
careful hinge design implies that the diving voltage for X
and Y rotation shall be very close. Therefore the area
design of the driving electrodes on the Pyrex would be
simpler in terms of driving voltage requirement and
driving circuit design.
Figure 6. First and second mode shape of 3D mirror/scanner with rotation actuator
Figure 7 shows the titling of 3D mirror/scanner with
100 volts on one of bottom electrodes. This titling
involves with X and Y rotation. One of mirror corner has
15 micron of Z translation and touches the bottom ground
electrode.
Figure 7. Titling of 3D mirror/scanner with 100 volts on one of bottom electrodes
If the voltage is applied on the electrodes of Y
translation actuator, the mirror will move only in Y
direction without any X movement. Figure 8 shows the
analysis results of 120 volts applied on the electrodes of Y
translation actuator. The mirror moves about 2.7 microns
in Y direction while the electrode gap of X translation
actuator are marinating unchanged.
Figure 8. Translation of 3D mirror/scanner in Y direction only
5. Characterization
Characterization measurements, including surface
roughness, radius of curvature and preliminary voltage vs.
tilt angle, were performed on the mirror. All
measurements were performed using a Zygo New View
5000 optical profilometer. Surface scans were made
across ~600µm of the mirror surface, and indicated a
surface roughness of ~10 angstroms rms, and a radius of
curvature of greater than 5 meters. Note that
measurements were made on a number of the batch
fabricated devices, and ROC measurements were always
greater than 5 meters, and ranged up to ~ 40 meters on
some devices.
Figure 9 shows a Zygo plot of the mirror deflection
with 40 volts applied to one of the rotational actuation
electrodes. This corresponds to a tilt angle of 0.165°. This
is close to the simulation result. For the same amount of
tilt angle, our simulation shows the driving voltage on the
electrode is 50 volts. The deviation may be due to slight
undercutting of the hinge structures during the deep
Proceedings of the International Conference on MEMS, NANO and Smart Systems (ICMENS’03)
0-7695-1947-4/03 $17.00 © 2003 IEEE
5
silicon etch. Thinning of the hinges will reduce the
actuation/snap voltage of the device.
Figure 9. Rotation of 3D mirror/scanner with 40 volts applied to a single electrode
6. Summary
We have demonstrated the design, fabrication,
simulation and characterization of 3D mirror/scanner with
X-Y translation. The fabrication process is robust and
mature. The single crystal SOI device layer attributes the
excellent optical performances of the 3D mirror/scanner.
There is no movement interference between in-plane X
and Y translations. This device can be fabricated into the
array structure with its good area filling hexagonal shape.
It will be very attractive for the applications in optical
telecommunication, high throughput 3D imaging, retinal
displays and high-density memory.
7. References
[1] H. Xie, Y. Pan, G. K. Fedder, “Endoscopic Optical
Coherence Tomographic Imaging With a CMOS-MEMS
Micromirror”, Sensors and Actuators A: Physical, Jan.
2003
[2] J. M. Zara, S. W. Smith, "Optical Scanner Using a
MEMS Actuator", Sensors and Actuators A: Physical,
Dec. 2002
[3] R. R. A. Syms, D. F. Moore, "Optical MEMS for
Telecoms", Materials Today, Aug. 2002
[4] O. Packer etc., "Characterization and Use of a Digital
Light Projector for Vision Research", Vision Research,
Feb. 2001
[5] M. Adrian Michalicek, Daren E. Sene and Victor M.
Bright, “ Advanced Modeling of Micromirror Devices”,
International Conference on Integrated
Micro/Nanotechnology for Space Applications, 1995,
pp.214-229
[6] Sunghoon Kwon, Veljko Milanovic, and Luke P. Lee,
"Vertical Scanner for 3D Imaging”, Technical Digest of the 2002 Solid-State Sensor and Actuator Workshop,
Hilton Head Isl., SC, 1995
[7] V.A. Aksyuk et al., “Lucent Microstar Micromirror
Array Technology for Large Optical Crossconnects”,
Proc. SPIE, vol. 4178, 2000.
[8] Dooyoung Hah et al., “A Low Voltage, Large Scan
Angle MEMS Micromirror Array with Hidden Vertical
Comb-Drive Actuators for WDM Routers”, OFC
Technical Digest, March 2002.
[9] Ferdinand Beer et al., “Mechanics of Materials”
MacGraw Hill, London, 1992.
[10] William Thomson, “Theory of Vibrations with
Applications”, Prentice-Hall, New Jersey, 1981.
Proceedings of the International Conference on MEMS, NANO and Smart Systems (ICMENS’03)
0-7695-1947-4/03 $17.00 © 2003 IEEE
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