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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

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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