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.

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