mems electrostatic double t-shaped spring mechanism for...

JMEMS-2012-0188.Final

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Abstract—A novel microelectromechanical systems (MEMS)

based micro-scanner is developed with the ultimate goal of

integrating it into an endoscopic probe for use in clinical

investigations. Micro-assembly technology is utilized to construct

this device, which consists of an electrostatic based micro-

actuator and a pyramidal polygonal micro-reflector. A two-stage

double T-shaped spring beam system is introduced to the

actuator for displacement amplification, and as well for motion

transfer from the translational movement of the in-plane comb

drives to the rotation of the central ring-shaped holder.

Meanwhile, an eight-slanted-facet-highly-reflective pyramidal

polygonal micro-reflector is developed using high precision

diamond turning and soft lithography technologies. This reflector

design requires only a small mechanical rotational angle to

achieve full circumferential scanning. This MEMS device is

developed with the goal of integrating it into an OCT probe that

could provide an alternative for endoscopic optical coherence

tomography (EOCT) applications that would have advantages of

circumferential imaging capability, fast scanning speed, and low

operational power consumption.

Index Terms — Microelectromechanical systems, optical

coherence tomography, two-stage double T-shaped spring

system, pyramidal polygon micro-reflector, soft lithography, full

circumferential scanning

I. INTRODUCTION

PTICAL coherence tomography (OCT) is a promising

technique for its cellular or even sub-cellular resolution

(1-10 μm) [1] cross-sectional imaging of biological tissues,

which is considered suitable for early detection and diagnosis

of cancerous growth. For some clinic applications, for

example, intravascular or gastrointestinal investigation, organs

normally have tubular shapes. In order to reduce sampling

errors, the whole cross-sectional images of the tubular organ

need to be captured, thus full circumferential scanning (FCS)

is highly desired. Early research efforts on FCS such as

Manuscript received Jun 9, 2012. This work was supported by the

Singapore Ministry of Education (MOE) Academic Research Fund under

Grant R-265-000-416-112.

X. Mu is with the National University of Singapore, Singapore 117576,

and also with the Institute of Microelectronics, Agency of Science,

Technology, and Research (A*STAR), Singapore (e-

mail:[email protected]).

G. Zhou, Dennis W. K. Neo, A.S. Kumar and F. S. Chau are with the

National University of Singapore, Singapore 117576 (e-mail:

[email protected]).

H. Yu and M. Tsai are with the Institute of Microelectronics, Agency for

Science, Technology, and Research (A*STAR), Singapore 117685.

proximal catheter actuation [2] were capable of scanning at a

speed of around 4 revolutions per second albeit with nonlinear

motion due to physical inertia, friction and compliance of the

device. Commercially- available micro-motors have also been

employed to spin mirrors or prisms to achieve FCS [3-6].

In recent times, MEMS technology [6-22] has demonstrated

strong potential in biomedical imaging applications due to its

outstanding advantages of small size, fast scanning speed and

convenience of batch fabrication. The recent development of a

dual reflective MEMS micromirror based on bimorph

actuators has further enriched and extended the MEMS

technology for FCS in clinical applications [18, 19].

The MEMS integrated OCT probes normally utilize fibers

and GRIN lenses to transmit and focus, and MEMS micro-

mirrors to deflect the incident light beams, respectively. The

image of a certain small tissue area can be detected and

reconstructed three-dimensionally by mirror rotational

scanning about the x and y axes respectively. Although it is

convenient to realize the probe miniaturization by this

configuration, the restricted scanning area makes it unsuitable

for intravascular applications. In order to realize FCS scan, we

have earlier proposed a compact four-pieces-in-one fiber-

pigtail GRIN lens bundle EOCT probe [20]. This proposed

configuration utilized multiple parallel incident light beams to

drastically reduce the required mechanical rotation angle of

the scanning mirror to achieve 360-degree circumferential

scanning with only minimal increase in package size. The core

light-scanning component placed at the distal side of the

endoscopic probe is a pyramidal polygonal micro-reflector

with four-highly reflective- slanted facets actuated by MEMS

chevron-beam electrothermal micro-actuators. This MEMS

platform is orientated perpendicularly to the incident light

beams. Although a large scanning range can be realized by

chevron-beam micro-actuator, the high power consumption

makes its surface temperature too high to be compatible with

clinic medical bioimaging, a side-effect which had been

reported previously [23-34].

Based on the configuration of the EOCT probe we

developed before, we propose in this paper a MEMS micro-

scanner that is actuated by electrostatic means instead. This

MEMS device has the advantages of lower power

consumption and higher scanning speed while maintaining the

same scanning range of the previous design. One drawback of

electrostatic actuation based MEMS devices is the relatively

small scanning range due to its comb structure and working

principle [35-45]. To avoid this limitation, a two-stage double

T-shaped spring beam mechanism is introduced to convert

MEMS Electrostatic Double T-shaped Spring

Mechanism for Circumferential Scanning

Xiaojing Mu, Guangya Zhou, Hongbin Yu, Julius Ming-Lin Tsai, Dennis Wee Keong Neo, A Senthil

Kumar, Fook Siong Chau

O

This is the author's version of an article that has been published in this journal. Changes were made to this version by the publisher prior to publication.The final version of record is available athttp://dx.doi.org/10.1109/JMEMS.2013.2255115

Copyright (c) 2013 IEEE. Personal use is permitted. For any other purposes, permission must be obtained from the IEEE by emailing [email protected].


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relatively small translational movements of the comb drives

into a large rotational motion of the central suspended ring-

shaped holder. The two-stage structure (as shown in Fig. 1(b))

provides displacement/angular amplification relative to a

single-stage one. A pyramidal polygonal micro-reflector with

eight highly-reflective slanted facets is developed to further

lower the mechanical rotation angle required to realize full

circumferential scanning. Once the micro-reflector is driven to

rotate, a circumferential light scan will be realized. A

circumferential tissue image may be reconstructed by

recording the data from eight fiber optic “channels”

sequentially or simultaneously. This newly- proposed

configuration of an electrostatic MEMS micro-scanner based

probe not only features the advantages of the previous one

(compactness, small size and alleviation of the mirror

curvature issue induced by residual stresses that might exist in

traditional thin MEMS micro-mirrors), but also has a surface

temperature that makes it suitable for clinic in vivo

investigation and fast scanning capability.

This paper presents the theoretical modeling, structural

design and FEM simulation of the MEMS mechanism in

section II. Section III describes the detailed processes used to

fabricate the microactuators and pyramidal polygon micro-

reflector followed by the process of assembling these two

parts together. Section IV demonstrates the characterization of

this newly-developed device. Finally, the conclusions are

provided in Section V. An optical scan angle up to near-360°

and a rapid response time of 14 ms have been experimental

measured.

II. DEVICE DESIGN

A. Structure Design of Rotational Mechanism

Figure 1(a) shows the schematic of the proposed MEMS-

based micro-scanner. An electrostatic type MEMS-driven in-

plane rotational platform is the actuation part, which has two

lines of bidirectional-movable comb drives on the two sides

(Fig. 1(b)). A ring-shaped holder is suspended in the center by

a two-stage double T-shaped spring beams mechanism. In

order to realize light scanning, an eight-slanted-facet

pyramidal polygon micro-reflector is mounted on the ring-

shaped holder. The translational movement of the two lines of

comb drives in opposite directions results in the rotation of the

central suspended ring-shaped holder and thus realizes the

rotation of the pyramidal polygon micro-reflector. During

operation, the micro-reflector can be driven to rotate

clockwise and anti-clockwise by the electrostatic actuators. As

schematically shown in Fig. 1(c), mechanical rotation angle of

22.5° both in clockwise and anti-clockwise direction could

realize circumferential scanning (22.5°×2×8=360°) when eight

external laser beams are shining on the eight slanted facets of

the micro-reflector respectively at the same time.

(a)

(b)

(c)

Fig. 1. (a) Schematic of the proposed electrostatic MEMS micro-scanner

integrated OCT probe, (b) electrostatic in-plane rotational MEMS micro-

scanner, (c) the schematic for realizing full circumferential scan

Figure 2 shows the working principle of the two-stage

double T-shaped spring-beam mechanism electrostatic based

MEMS microactuator. It can be seen that the ring-shaped

holder is connected to a pair of electrostatic comb-drive

actuators (the movable fingers of which is suspended by the

folded-beam supporting structure) using a double T-shaped

two-stage beam mechanism. With this particular design, the

translational movement provided by the micro-actuator can be

eventually translated into rotation of the ring-shaped holder

about an equivalent pivot point. At the same time, compared




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with direct connection, the first stage T-shaped beam

configuration (a and b), especially the vertically- arranged

beam (a) as shown in Fig. 2, can help to release the

deformation-induced stress at the anchor point of the

horizontal beam (b) as well as in the beam itself, thus

improving structure stability and reducing the undesired

nonlinear effect. The introduction of the second stage T-

shaped beam configuration (b and c) helps to translate and

amplify the deformation of the first stage T-shaped spring to

the spring c. When the driving voltage V1 is energized, whilst

keeping V2 zero, the ring-shaped holder will be actuated to

rotate counter-clockwise, and vice versa.

Fig. 2. Working principle of the MEMS micro-actuator

B. Theoretical Study of the Two-stage Double T-shaped

Spring Mechanism

(a)

(b)

Fig. 3. Simplified model for the two-stage double T-shaped beam spring

mechanism of (a) the Whole system and (b) the Individual components

Figure 3(a) shows the simplified model of the two-stage

double T-shaped beam mechanism together with the ring-

shaped holder, in which the holder is treated as a rigid body

with only rotational degree of freedom. When an external

force F is applied onto this T-shaped beam mechanism, the

free-body diagrams of each component can be schematically

drawn, as in Fig. 3(b).

Under the effect of this external force, all the beams will be

deformed (mainly bending deformation), therefore creating

bending energy in the beam bodies, which can be described by

[46]

Component 1:

0 ( )D

M F Ak

(1)

where

22 1 2

22 1 2 1 2

1 1 1

3 8 21 1 1 1

4 16 4 16

l l l E I

A= ,D

l l l l l

Considering the force and moment equilibrium of the

structure, we get




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1 0 1 2

1 2 6 7 1 2 6 7

1 2 6 7 1 2 6 7

3 4 3 4

0 3 3 3

3 3 4

2 2

( + )

M M F l

M M M M M ' M ' M ' M '

F F F F F ' F ' F ' F ' F

F F F ' F '

M M F l

M F l R

(2)

Substituting Eq. (2) into Eqs. (3), (4), (5), (6), (7), (8) and (9),

2 2

2

22

1 0 11 11

0

2 2 221 2 1 0 2 0

( )

2 2

2 1[ + ]

2 3 2

2l l

l

M M F xM F xU dx dx

EI EI

lF l F M l M

EI

(3)

Component 6:

2 2

2

22

6 0 66 66

0

2 2 226 2 6 0 2 0

( )

2 2

2 1[ + ]

2 3 2

2l l

l

M M F xM F xU dx dx

EI EI

lF l F M l M

EI

(4)

Component 2:

2

2 1

216

'M lU

EI

(5)

Component 3:

2

1 13

16

M ' lU

EI

(6)

Component 5:

2

6 1

516

'M lU

EI

(7)

Component 7:

2

7 1

716

'M lU

EI

(8)

Component 4:

42

3 34

0

2 220 4 4

424 3

( )2

2

1( + )

3( + + )

l F x MU dx

EI

M l ll R+ R

EI l l R

(9)

12

3whI

(10)

where E is the Young’s modulus of structural material, I is the

moment of inertia of the beam structure, which is dependent

on the beam width w and thickness h, R is the radius of the

central ring-shape holder.

Substituting Eq. (1) into Eqs. (3), (4), (5), (6), (7), (8) and (9),

2 2 221 6 2 2

2 1[ ( ) + ( ) ]

2 3 2

l D DU U F l A l A

EI k k

(11)

2 212 3 5 7 2[2 ( )]

64

l DU U U U F l A

EI k

(12)

22

2 24 44 42

4 3

( )

( + )3( )

DA

l lkU F l R+ REI l l R

(13)

As a result, the total bending energy in the two-stage double

T-shaped spring beam mechanism caused by the external force

can be calculated, ie

1 2 4

2 22 2 2

212

22

244 42

4 3

2 4

2 1[ ( ) + ( ) ]3 2

+ [2 ( )]16

( )

( + )3( )

total

2

U U U U

D Dl l A l A

k k

lF Dl A

EI k

DA

lkl l R+ Rl l R

(14)

The resultant deflection δ can be deduced by performing the

first order differentiation of Utotal with respect to the external

force F,

2 22 2 2

212

22

244 42

4 3

2 1[ ( ) + ( ) ]3 2

2+ [2 ( )]

16

( )

( + )3( )

totalU

F

D Dl l A l A

k k

lF Dl A

EI k

DA

lkl l R+ Rl l R

(15)

It can be seen that the equivalent spring constant of this

two-stage double T-shaped spring beam mechanism k can be

expressed by,




5

2 22 2 2

212

22

244 42

4 3

1

2 1[ ( ) + ( ) ]3 2

2+ [2 ( )]

16

( )

( + )3( )

k F

k F

D Dl l A l A

k k

l Dl A

EI k

DA

lkl l R+ Rl l R

(16)

Equation (16) can be simplified as a function of k, ie

1 3 2II II II 02k k

(17)

We then get

23 1 2

31

II 4 II IIII

2 IIk

(18)

where 2

1 1 2 3

22 3

3 2 3

2 21 21 2 2

1 22 2 2

221 2 4 4

3 424 3

II = I - I + I

II I

II I 2 I 1

2 2I 4

8 3

2I 4

8

2 1I ( )

8 3( )

A A

D

D A D

l ll l

EI EI

l ll l

EI EI

l l l ll R R

EI EI EI l l R

At the same time, the resultant rotational angle of the

central ring-shape holder θ can be calculated using beam

deflection function [47], 2 34 4

4 4 04 3 4 3

2 34 4

4 44 3 4 3

2 34 4

4 44 3 4 3

( )1 1 1= [ ( + ) ]

( + + ) 2 3 ( + + )

( )( )1 1

[ ( + ) ]( + + ) 2 3 ( + + )

( )( )12 1

[ ( + ) ]( + + ) 2 3 ( + + )3

l l Rθ l R l M

EI l l R l l R

DA

l l RkF l R lEI l l R l l R

DA

l l RkF l R ll l R l l RE h w

(19)

After analyzing the two-stage double T-shaped beam

mechanism, the model for the whole system involving the

folded-beam suspension for the actuator is built as shown in

Fig. 3(a), in which the folded-beam suspensions are

represented by springs with equivalent spring constant of ks.

3

1 3

44

f

s

E hwk k

l

(20)

where l and wf are the length and width of the folded-beam,

respectively.

For further simplification, the whole system can be treated

as a spring system as given in Fig. 3(a). It is obvious that when

the electrostatic driven force Fa is applied at both sides, the

resultant force F exerted on the T-shaped beam (namely k) can

be deduced to be

2

2

a

s

F kF

k k

(21)

where 20 ra

hF N V

g

(N is the number of finger pairs of the

comb-drive micro-actuator, ε0 (=8.85×10-12 F/m) is the

vacuum permittivity, εr (=1) is the relatively permittivity of

air, V is the applied voltage, h and g are the finger thickness

and gap, respectively.

Combining Eqs. (18), (19), (20) and (21), the resultant

rotational angle of the central ring holder can be calculated.

In the current design, all the structures are fabricated using

an SOI wafer with a 25 µm-thick device layer, ie all the

structure thicknesses are equal to 25 µm. Consequently, the

structural dimensions of the folded-beam suspension are first

determined to be 800 µm-long (l) and 6 µm-wide (wf) to

achieve moderate in-plane spring constant along the

movement direction whilst providing sufficient support in the

out-of-plane direction, according to our previous experience.

At the same time, a comb-drive actuator with 372 pairs of

fingers and 2 µm finger gap is adopted. From Eq. (19), it can

be seen that a smaller beam width (w) of the T-shaped beam

mechanism will result in a larger rotational angle. However,

with the decrease of beam width, the structure strength in

vertical direction will be reduced, thus affecting the operation

stability of the ring-shaped holder. Moreover, a design with

too small a beam width will also present a challenge to

fabrication as well as the yield. As trade-off, the beam width

of the T-shaped structure is selected to be 3 µm. Calculations

of the resultant rotational angle with different lengths of

central bar (l3) and central spring (l4) show that most of the

corresponding lengths of T-spring vertical beam (l1) and the

horizontal beam (l2) to the peak value are located in the range

of 150-250 μm. In order to simplify the computation, l1 and l2

are selected to be 200 μm.

Figure 4(a) shows a sample contour of the function of

resultant rotational angle with the variables of central bar (l3)

and central spring (l4) when T-spring vertical beam length (l1)

and the horizontal beam length (l2) are selected as 200 μm and

200 μm respectively. The two-axis relationship curves of

resultant rotation angle of the ring-shaped holder under

different structure designs of the central bar spring mechanism

under an applied 100 V DC are demonstrated in Fig. 4(b),

from which 1000 μm and 800 μm are deduced to be the

optimized values for the central bar (l3) and central spring (l4).




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After reaching a maximum point (of around 20.5º), further

increasing the length of the central bar (l3) and central spring

(l4) will inversely reduce the resultant rotation angle at a

relatively rapid rate. It is because of the limitation of the

deformation compliance of the two-stage double T-shaped

spring beam system in a relative small and unchanged

designed chip area.

(a)

(b)

Fig. 4. (a) Sample contour of the resultant rotation angle with the variable of

l3 and l4, (b) Curves of resultant rotation angle of the ring-shaped holder

under different structure designs of the central bar spring mechanism

C. Simulation

Fig. 5. Simulation results for the proposed two-stage double T-shaped spring

beam mechanism

Using the designed structure parameters above, the

equivalent model of the device is also built and analyzed using

ABAQUS software. For simplification, an electrostatic driven

force equivalent to 100 V actuation voltage is directly applied

to the model during the analysis. Figure 5 shows the

simulation results for the case of clockwise actuation, from

which the in-plane rotation of the ring-shaped holder about the

virtual center pivot caused by the translation movements of the

side supports is clearly observed. The rotation angle is

calculated to be 18.3º under the comb drive stroke of around

38 μm, which agrees well with the theoretical analysis result

of 20.5º in Fig. 4.

TABLE I

FINAL STRUCTURE DIMENSIONS ADOPTED IN CURRENT DEVICE DESIGN

Item Dimension

Comb drive actuator

Finger pairs:372

Finger gap: 2 µm

Finger width: 5 µm

Folder beam suspension Length: 800 µm

Width: 6 µm

T-shaped beam

suspension

Length of vertical beam (l1): 200 µm

Length of horizontal beam (l2): 200 µm

Beam width: 3 µm

Central Spring

mechanism

Length of central bar (l3): 1000 µm

Length of central spring (l4): 800 µm

Beam width: 3 µm

Ring-shaped holder Inner diameter: 140 µm

Outer diameter: 240 µm

III. DEVICE FABRICATION

A. MEMS Actuator Fabrication

Fig. 6. Fabrication process flow for the actuator (a) Phosphosilicate glass layer

(PSG) deposition and bottom side oxide layer deposition, (b) Front side




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photoresist coating and lithographically pattern, and 20 nm chrome/500 nm

gold deposition by e-beam evaporation followed by lift-off to form bond pads,

(c) Front side structure patterning by deep reactive ion etching (DRIE), (d)

Front side protection material coating on the top surface, (e) Backside oxide

layer is removed by reactive ion etching (RIE) and trench is formed by DRIE,

(f) Wet oxide etch process to remove the backside oxide, (g) Front side

protection material is stripped using a dry etch process

The proposed MEMS electrostatic microactuator is

fabricated using a commercial foundry process called

SOIMUMPs (MEMSCAP, USA). It begins with a blank

double-sided polished SOI wafer; the top surface of the silicon

layer is doped by depositing a phosphosilicate glass (PSG)

layer and annealing at 1050°C for 1 hour in Argon (Fig. 6(a)).

The thicknesses of device layer, buried oxide and substrate are

25 µm, 1 µm and 400 µm, respectively. After that, a metal

stack of 20 nm of chrome and 500 nm of gold is patterned

through a liftoff process to form electrical connection pads

(Fig. 6(b)). Subsequently, using standard photolithography

process, the desired structure patterns, including the actuator,

the folded- beam and the T-shaped suspensions and the ring-

shaped holder, are fabricated into a photo resist layer, acting

as the hard mask. Then, all of these patterns are

simultaneously transferred into the silicon device layer with

one-step DRIE etching (Fig. 6(c)). Next, a frontside protection

material is applied to the top surface of the silicon layer (Fig.

6(d)). The wafers are then reversed, and the substrate layer is

lithographically patterned and this pattern is then etched into

the bottom side oxide layer using Reactive Ion Etching (RIE).

A DRIE silicon etches is subsequently used to etch these

features completely through the substrate layer that is under

the movable structure region as shown in Fig. 6(e). A wet

oxide etch process is then used to remove the exposed buried

oxide layer (Fig. 6(f)). Finally, the frontside protection

material is stripped in a dry etch process to totally release the

whole device (Fig. 6(g)).

Fig. 7. Scanning electron microscope (SEM) image of the as-fabricated

electrostatic based MEMS micro-actuator

Figure 7 shows the fully released MEMS micro-actuator.

The inset on the top left corner is the zoom-in image of the

ring-shaped holder, two-side flexure beam deformation of

which will drive it rotate accordingly. The image of comb-

drive actuator and the folded-beam suspension are shown in

the bottom right corner inset.

B. Micro-pyramidal Polygon Reflector Fabrication

Fig. 8. The fabrication process flow of eight-slanted-facet pyramidal polygon

micro-reflector

The proposed general fabrication flow for the eight-slanted-

facet pyramidal polygonal micro-reflector, as summarized in

Fig.8, combines high-precision single-point diamond

machining on a copper (Cu) substrate and the soft lithographic

replication process with an elastomeric material known as

PDMS. Firstly, the required mold is produced by diamond

machining methods (inset in Fig. 8). The reverse of the mold

will be replicated through a cycle of soft lithography and that

serves as a PDMS mold for the next cycle of soft lithography.

Particularly, liquid PDMS prepolymer (Sylgard 184 silicone

elastomer - a base and curing agent of Dow Corning Corp -

mixed in a 10:1 weight ratio) is poured onto the surface of the

copper mold. After complete curing in an oven at 60°C for 2

hours, this thick PDMS substrate with the pyramidal polygon

cavity pattern is peeled off. Meanwhile, the positioning pillar

of the polygon is patterned on the SU8 (SU-8 2025,

MicroChem Corp.) layer, which has been pre-spun onto the

surface of a 4-inch polished silicon wafer. Then, the inverse of

the pillar pattern (cylindrical pillar) with 80 μm is transferred

from the SU-8 mold to a second PDMS mold. This PDMS

mold is also carefully peeled from the mold substrate with the

assistance of isopropyl alcohol (IPA) solution whereupon it is

fully cured in the oven. Subsequently, a droplet of optical

adhesive (Norland optical adhesive 83H, EDMUND) is

applied on the surface of the cylindrical cavity that is pre-

defined in the secondary PDMS mold. This cylindrical cavity

defines the shape of the positioning pillar of the polygon

reflector. There are also two other punched holes nearby to

serve as alignment marks to precisely fix the relative positions

of the polygon block and the pillar beneath. After that, the first

layer of the PDMS mold of the polygon cavity is bonded

together with the one of cylindrical cavities. Thus, the shape




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of the whole micro-pyramidal polygon is defined by the

optical adhesive that is trapped in the polygon-cylindrical

cavity. The optical adhesive droplet fully cures in two minutes

after it is exposed to ultraviolet light, thereby forming the

pyramidal polygonal micro-reflector. Following this, the tough

micro-pyramidal polygon replica is peeled off from the PDMS

substrate. Finally, a 2000 Å-thick Au layer is deposited onto

the pyramid polygon surface, by means of E-beam

evaporation, to enhance its reflectivity.

Fig. 9. (a) Measured surface roughness of a slanted facet of the copper master

mold (average roughness Ra: 7.02 nm, root-mean-squared roughness Rq: 9.02

nm, the peak-to-valley difference Rt: 77.58 nm), (b) Measured surface

roughness of the slanted facet of the Au-coated polymer polygonal micro-

reflector (average roughness Ra: 48.95 nm, root-mean-squared roughness Rq:

61.90 nm, the peak-to-valley difference Rt: 950.67 nm)

The surface roughness measurement is performed by an

optical profiler Veeco. The measurement results of the

diamond turned copper master mold and the Au-coated

polymer polygonal micro-reflector are shown in Figs .9 (a)

and (b), respectively. It indicates that the measured average

surface roughness and root-mean-square roughness of the

diamond turned master mold are 7.02 nm and 9.02 nm,

respectively. Meanwhile, those roughnesses of the Au-coated

polygonal micro-reflector are 48.95 nm and 61.90 nm,

respectively. Such roughness difference between the diamond

turned mold surface and the polygonal micro-reflector surface

might be attributed to the thermal cycle and UV irradiation

involved in the fabrication process. In OCT imaging

applications, flat micro-reflector with surface roughness <

λ/10 are normally desired, with the wavelengths of the near-

infrared light source ranging from 930 nm to 1550 nm. As a

result, our currently proposed micro-reflector shows promise

to meet the requirements for the OCT imaging applications.

Fig. 10. The set-up for assembly and schematic of device assembly

Fig. 11. The scanning electron microscope (SEM) image of the assembled

MEMS electrostatic micro-scanner

After the eight-slanted-facet pyramidal polygon micro-

reflector and the electrostatic micro-actuator have been

fabricated, they are assembled manually (as shown in Fig. 10).

Initially, the microactuator chip is manually placed on top of

the polygonal reflector and held with a vernier caliper with the

actuator facing down. The position of the vernier caliper can

be adjusted in both transverse and longitudinal in-plane

directions as well as vertical out-of-plane direction by a three-

axis precision positioning stage. Next, the circular ring-shaped

holder of the actuation mechanism is aligned under a

microscope to confirm that the mounting hole and the

connection pillar are concentric before the actuator chip is

lowered and the connection pillar inserted into the circular




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ring-shaped holder. Finally, the pillar and the circular ring-

shaped holder are bonded together by Araldite 2012 epoxy

adhesive. Figure 11 shows a scanning electron microscope

(SEM) image of the device after assembly.

IV. EXPERIMENT AND RESULTS

Fig.12. Measured rotation angle-applied voltage and comb drive stroke-

applied voltage curves

The comb drive strokes and rotation angles of the central

ring-shaped holder under different driving voltages are

characterized. During the test, a maximum voltage of 100 V

DC is applied to ensure a high safety factor of stable and safe

device operation. From the experimental results shown in Fig.

12, it can be observed that at a voltage of 100 V, the comb

drive can achieve a maximum translational displacement of 38

μm whilst the central ring-shaped holder can be gradually

driven to rotate nearly 20º in both clockwise and counter-

clockwise directions.

(a)

(b)

Fig. 13. (a) Setup of transient response experiment, (b) Measured transient

response of the MEMS device

Besides the static characterization of the MEMS

microactuator, the transient response of the assembled MEMS

device is also measured by connecting an AC

Function/Arbitrary waveform generator (Agilent 33522A), a

voltage Amplifier (A400D) and a digital oscilloscope

(DL1520) to it. A position sensitive detector (PSD) (as shown

in Fig. 13(a)) is utilized to detect the light trajectory scanned

by the MEMS micro-reflector. A square wave with 22Hz

frequency, 120Vp-p voltage, and 50% duty cycle is applied

onto the sets of comb drive actuators that can drive the

polygon micro-reflector to rotate clockwise.

The output of the detector together with the driving signal is

directly captured by an oscilloscope in real time as shown in

Fig. 13(b). The times taken for the detector to attain 90%

values from 10% are measured to be around 82ms and 96ms.

Compared with the electrothermal actuation based MEMS

micro-scanner we developed and presented before, this

relatively fast response (14 ms) undoubtedly makes the current

device more competitive.

Fig. 14. The applied voltage-optical scanning angle curve, laser scan trace

lines and zoom in optical image of the MEMS micro-scanner




10

Figure 14 is the optical scanning angle vs. applied voltage

curve. To demonstrate the circumferential scanning capability,

the MEMS device is driven into an oscillatory motion using an

AC Function/Arbitrary waveform generator (Agilent 33522A)

and a voltage Amplifier (A400D). A laser beam illuminates

the micro-pyramidal reflector from above. The reflected

beams from the slanted facets of the reflector project eight

laser spots onto a cylindrical screen when the device is

stationary. The device is then driven to oscillate by applying a

sinusoidal input voltage with an amplitude of 80 Vpp and a

frequency of 40 Hz. The captured scanning trace lines are

shown in the inset of Fig. 14. Due to the limitation of the

camera shooting angle, only half of the scan lines are visible

in the figure. It is observed that under such a driving

condition, a near-360° circumferential scanning range can be

obtained. As the envisioned whole structure of the OCT

system is shown in Fig.15, the developed double T-shape

spring mechanism based MEMS micro-scanner could be the

basis for FS-EOCT probes, which may help to make the probe

miniaturized and compact.

Fig. 15. The schematic of the use envisioned for double T-shaped spring

mechanism MEMS micro-scanner on OCT bioimaging

V. CONCLUSION

A novel MEMS micro-reflector based on electrostatic

actuation that is aimed at achieving near-360° light beam

scanning for OCT purposes has been successfully developed.

It consists of a pair of electrostatically-driven comb-drives to

actuate a two-stage double T-shaped spring beam mechanism

and an eight-slanted-facet pyramidal polygonal micro-reflector

with highly reflective surfaces. The polygon micro-reflector is

developed using high precision diamond turning and soft

lithography technologies. The two-stage double T-shaped

beam spring mechanism translates the translational movement

of the electrostatic actuator into in-plane rotation of the central

suspended ring-shaped holder which holds the micro-reflector.

This two-stage structure also provides displacement/angular

amplification. During operation, a 38 μm comb drive stroke is

achieved at 100 V, resulting in a rotation angle of around 20°

in clockwise or counter-clockwise direction. Tests using light

illuminated on the rotational polygon micro-reflector, verified

that a near-360° circumferential scanning can be realized. At

the same time, the response time of current device is measured

to be 14 millisecond representing greatly improved

performance over previous designs and good potential for the

applicability of the device. In summary, this MEMS-driven

micro-scanner could be the basis for FS-EOCT probes, which

may help to make the probe miniaturized and compact. At the

same time, the high surface temperature problem that existed

in the previous design is probably alleviated by the current

device.

ACKNOWLEDGMENT

The authors would like to thank Dr. Du Yu, Dr. Tian Feng,

and Mr. Kelvin Cheo Koon Lin for their generous assistance.

Financial support by the Singapore Ministry of Education

(MOE) Academic Research Fund under grant R-265-000-416-

112 is gratefully acknowledged.

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12

Xiaojing Mu received the B.S. and M.S.

degrees in the Department of Mechanical

Engineering from Chongqing University

(CQU), Chongqing, P.R China, in 2006 and

2008, respectively. He is previous a Ph.D

student in Micro/ Nano System Initiative

(MNSI) lab at National University of

Singapore (NUS), Singapore and also an

attached full-time research student with

miniature medical device (MMD) group

from the Institute of Microelectronics,

Agency for Science, Technology and

Research (A*STAR). Currently, he is a

research scientist with Sensors & Actuators

Microsystems Program (SAM) group from

the Institute of Microelectronics (IME), Agency for Science, Technology and

Research (A*STAR), Singapore. His interest lies in optical MEMS and

MEMS pressure sensors. His Ph.D research work is design and fabrication of

optical micro devices for medical applications and current work in IME is

focusing on AIN based MEMS microphone, micro-speaker and SAW sensors.

Guangya Zhou received the BEng and PhD

degrees in optical engineering from Zhejiang

Univeristy, Hangzhou, China, in 1992 and

1997, respectively. He joined the National

University of Singapore (NUS) in 2001 as a

research fellow. Currently, he is an associate

professor with the Department of Mechanical

Engineering, NUS. His main research

interests include microoptics, diffractive

optics, MEMS devices for optical

applications, and nanophotonics.

Hongbin Yu received a BS in mechanical

engineering, MS in electrical engineering and

PhD in optical engineering from Huazhong

University of Science and Technology, China,

in 1999, 2002 and 2005, respectively. He was

a senior research fellow at Micro/Nano

Systems Initiative Technology with the

Department of Mechanical Engineering,

National University of Singapore. Currently,

he is a research scientist with Miniature

Medical Device (MMD) group from the

Institute of Microelectronics, Agency for

Science, Technology and Research

(A*STAR). His research interests involve the

design, simulation and fabrication technology

of MEMS/NEMS devices and optofluidics.

Julius Ming-Lin Tsai was born in Taipei,

Taiwan, R.O.C., in June 1976. He received

the B.S. and Ph.D. degrees in power

mechanical engineering from National

Tsing Hua University, Hsin-Chu, Taiwan, in

1995 and 2004, respectively. From June

2003 to April 2004, he was a visiting

scholar with Carnegie–Mellon University,

Pittsburgh, PA, where he was involved with

low-noise CMOS accelerometers. From

2004 to 2009 he joined VIA Technologies,

where he was involved with high-frequency

small-signal modeling. He is now a

principle investigator in Institute of Microelectronics, Singapore and an

adjunct assistant professor with Department of Electrical and Computer

Engineering, National University of Singapore. His main research interests are

in RFMEMS, NEMS switch and MEMS ultrasound transducers. Dr. Tsai was

the recipient of a scholarship presented by the National Science Council of

Taiwan.

Dennis Wee Keong Neo received the BS.

(BTECH Honors) in the Department of

Mechanical Engineering from National

University of Singapore. Currently, he is

pursuing the Ph.D. degree in the Department

of Mechanical Engineering in National

University of Singapore. His main research

interests are in the Micro/Nano machining

and manufacturing of freeform surfaces.

A.Senthil Kumar received the BS.

(Mechanical) and MS (Eng. Design) in

College of Engineering, Guindy, Anna

Univ. and Ph.D. degrees in National

University of Singapore. He is currently an

associate professor in the Department of

Mechanical Engineering, National

University of Singapore. His main research

interests are in the development of

Miniature Machine Tool for Micro/Nano

Machining, fixture design and Micro/Nano

machining.

Fook Siong Chau received the B.Sc. (Eng.)

and Ph.D. degrees from the University of

Nottingham, Nottingham, U.K.. He is

currently an associate professor in the

Department of Mechanical Engineering,

National University of Singapore. His main

research interests are in the development and

applications of optical techniques for

nondestructive evaluation of components

and the modeling, simulation, and

characterization of microsystems,

particularly bio-MEMS and MOEMS.

