A MEMS-based tracking milli-mirror for high

Sensors and Actuators A 133 (2007) 368–374
A MEMS-based tracking milli-mirror for high-density optical disk drives
J.P. Yang a,∗ , L.N. Low b , D. Johnson b
a
Data Storage Institute, DSI Building, 5 Engineering Drive 1, Singapore 117608, Republic of Singapore
b Singapore Polytechnic, 500 Dover Road, Singapore 139651, Republic of Singapore
Received 8 July 2005; received in revised form 9 March 2006; accepted 11 May 2006
Available online 17 July 2006
Abstract
We present a new MEMS-based milli-mirror for precise tracking in high-density optical disk drives (ODDs). The device consists of a torsionally
suspended mirror plate, one pair of torsion springs, which support the mirror plate and offer a restoring torque, and two pairs of electrodes attached
to the mirror plate and glass substrate. The dimensions of mirror plate and torsion springs were determined so that a 5 V dc bias ±4.5 V ac
drive voltage would provide the mirror with ±0.02◦ rotation to transmit laser beam spot on spinning disk. The MEMS-based milli-mirror was
successfully fabricated using MEMS technology. Displacement–voltage linearization scheme was implemented by differential voltage driving. The
static and dynamic performances of mirror prototype, such as capacitance versus driving voltage, rotation angle versus driving voltage, and resonant
frequency were characterized and compared well with the simulation solutions. The mechanical resonant frequency of the mirror is expected to be
high enough to satisfy the requirement of the servo bandwidth of precise tracking-control in high-density blue-laser optical disk drive.
© 2006 Elsevier B.V. All rights reserved.
Keywords: MEMS mirror; Microactuator; Optical disk drive; Servo control; Microfabrication
1. Introduction
In a high-density optical disk drive (ODD) that uses bluelaser, precise track-following control of the laser beam spot on
media disk is required. The conventional ODD uses a dual-stage
servo system, in which a sledge motor is used for the coarse
positioning while a voice coil motor (VCM) is for the fine positioning. A conventional optical pickup (OPU) head shown in
Fig. 1(a) includes a 2D VCM for both tracking and focusing.
Due to its size and weight, the OPU’s resonant frequency is
low. It is difficult to use the conventional OPU to realize precise
track-following control for high-density ODD.
On the other hand, a MEMS-based actuator has advantages
such as precise positioning, lightweight and small power consumption [1,2]. Moreover, an electrostatically actuated MEMS
device has good linearity compared to a PZT-actuated device and
fast response compared to a thermally actuated device. Therefore, an electrostatically actuated micromirror is proposed to
replace the conventional VCM as a fine tracking mechanism
of OPU in the high-density ODD in Fig. 1(b). It will be more
∗
Corresponding author. Tel.: +65 6874 8409; fax: +65 67771349.
E-mail address: Yang Jiaping@dsi.a-star.edu.sg (J.P. Yang).
0924-4247/$ – see front matter © 2006 Elsevier B.V. All rights reserved.
doi:10.1016/j.sna.2006.06.019
effective to steer the laser beam trace itself rather than drive the
whole OPU module. Comparing with the conventional VCM
OPU module, the MEMS mirror device can be fabricated by
mass fabrication process at low production cost.
Over the past few years, several MEMS mirrors have been
suggested for ODD applications. Ueda et al. developed a MEMS
torsional mirror for an optical disk drive [3]. The mirror consists
of three pieces of silicon substrates but an angular degree of 0.05◦
would need a voltage of 32 V driving voltage. Yonezawa et al.
suggested an electrostatic torsion mirror [4], in which groove
patterns on the backside of the mirror plate are proposed to
reduce damping ratio. Yang et al. designed, fabricated and characterized an electrothermal actuator integrated with miromirror
for fine-tracking-control [5]. Its intrinsic thermal response is
limited to a hundreds of microseconds. Yee et al. proposed a
PZT-actuated mirror, which moves along the out-of-plane vertical direction for ODD tracking-control [6]. The complex PZT
process could not be compatible with standard IC process. Bu et
al. investigated an annular shutter mirror for use as an adjusting
means of the numerical aperture (NA) in a DVD pickup module
[7].
In this paper, an electrostatically actuated MEMS millimirror is presented as a precise tracking mechanism of OPU
in high-density ODD. The design and fabrication of the MEMS
J.P. Yang et al. / Sensors and Actuators A 133 (2007) 368–374
369
Fig. 1. Conventional VCM and MEMS-based mirror for OPU tracking mechanism.
mirror are discussed. Static and dynamic performances of the
mirror prototype, such as capacitance versus driving voltage,
rotation angle versus driving voltage, and resonant frequency
were characterized and compared well with the simulation
solutions.
2. MEMS-based milli-mirror design and simulation
2.1. MEMS-based milli-mirror design
In the optical recording technology using semiconductor
blue-laser, the wavelength of laser is 410 nm. The NA is up
to 0.85. Thus, the diameter of the laser spot size (d = 0.5λ/NA)
is as small as 0.24 ␮m. Assuming the track pitch of the optical
drive is the same as this value, a 12 cm-diamter disk would be
able to storage up to 20 GB of data on a single side.
The diameter of the laser volume for an optical disk drive is
about 2 mm. The proposed MEMS milli-mirror is located on a
45◦ mount substrate in Fig. 1(b). A pair of aluminum (Al) reflective mirrors, as shown in Fig. 2, is formed on a silicon substrate
plate of B × L × H (2.2 mm × 3 mm × 0.3 mm), which is good
for mirror flatness. The solid line shows the mirror substrate
plate and the torsion springs on a silicon wafer. The dashed line
shows the two Al mirrors (electrodes) and the two pads on an
opposite side glass substrate with a small gap from the mirror
plate. The mirror plate is supported by one pair of the torsion
springs, whose dimensions are Tl = 250 ␮m length, Tw = 15 ␮m
width and Th = 70 ␮m height, respectively. The springs are also
coated with Al as electrical paths from outer pads to either sides
of mirror plate. Another pair of electrodes (Al) is attached to
the glass substrate with respect to the two Al mirrors, respectively. The gap between the mirror plates and glass substrates
is designed to meet the parallel plate electrodes performance
and fabrication process capability. In this study, the gap of 7 ␮m
is achieved using one Al patterned glass wafer bonded to the
backside of mirror silicon wafer, on which the etched cavity is
formed. Since the gap between the electrodes is very small in
relative to the size of the mirror plate, very large air damping
may be expected. Thus, a row of grooves in Fig. 3 is made on the
Fig. 2. Schematic view of MEMS mirror design.
backside of the mirror plate to reduce the damping [4]. When
an electrical voltage difference between the two parallel plates
is applied to the pads, the attraction force of the electric fields
between the plate conductors would drive the mirror to rotate
via the torsion springs.
Fig. 3. Mirror plate design using comb structures (grooves) for air damping.
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Fig. 5. Simulated rotation angle vs. driving voltages.
Fig. 4. Simulated C–V curve.
2.2. Analytical and FEM modeling
To study the mirror design, analytical and finite element
methods (FEM) were used. When a driving voltage is applied
to the parallel plate electrodes, the capacitance–voltage (C–V)
and the relationship between the rotation angle θ and the
given bias voltage V are derived using the following equations
[8,9]:
εB
2g − a1 θ
C=
ln
(1)
θ
2g − a2 θ
2g
εBT1 V 2
2g
2g − a2 θ
θ=
−
+
ln
(2)
4GIp θ 2 2g − a2 θ
2g − a1 θ
2g − a1 θ
where a1 is the distance between the pivot point of torsion spring
and the nearer electrode edge, a2 the distance between the pivot
point and the further electrode edge, g the air gap between the
mirror plate and opposite side substrate below the mirror plate, ε
the permittivity of air, G the shear modulus of the torsion spring,
Ip is the polar moment of inertia of the torsion spring, which is
given by
4
1
T
T
w
1 − w4
; Tw ≤ Th
− 0.21
(3)
Ip = Th Tw3
3
Th
12Th
Fig. 6. Differential voltage driving method.
Apply V0 + u on the one side of mirror, and V0 − u on the other
side. When the rotation angle is small enough, the electrostatic
force can be simplified as the function of driving voltage u:
F=
εA (V0 − u)2
2εAV0 u
εA (V0 + u)2
−
≈
2
2
2 (g0 − x)
2 (g0 + x)
g02
where A is the area of half mirror, g0 the initial gap, x the gap
change, V0 the bias voltage, and u is the dynamic driving voltage.
As the rotation angle is proportional to the driving torque generated by the force F, the angle is obviously linear to the driving
voltage u, from Eq. (5). Fig. 7 shows the simulated results when
V0 = 5 V. It is observed that the linear relationship between the
rotation angle and the driving voltage is implemented.
Moreover, dynamic analysis of the device is performed by
the analytical solution and FEM, respectively. In the modeling,
By neglecting high-order terms due to (a1 θ/2g) 1 and
(a2 θ/2g) 1, Eq. (2) is further simplified as the following relationship between the rotation angle and the driving voltage:
2 a2 2
a1
εBTl V 2
∼
(4)
−
θ=
8GIp
2g
2g
Figs. 4 and 5 show the C–V curve and the rotation angle versus
driving voltage V. The mirror is rotated with maximum angle of
0.27◦ , while pull-in voltage is 13.7 V at 0.12◦ .
Examination of Fig. 5 or Eq. (4) reveals that θ is proportional to V2 . The non-linearization may cause difficulty for the
servo controller design. Here, the differential voltage driving
method [10], as shown in Fig. 6, is used to realize linear actuation between rotation angle and driving voltage.
(5)
Fig. 7. Simulated rotation angle vs. differential driving voltages.
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371
Fig. 8. Process flow of MEMS mirror.
Table 1
Resonant frequencies by analytical and FEM methods
Mode no.
1
2
Description
Rotating around torsion springs
Mirror plate torsion
Frequency (kHz)
Analytical
FEM
0.525
201
0.566
239
the air damping is neglected due to small effect by the etched
grooves [4]. The results are shown in Table 1. It is observed that
except the 1st frequency of 566 Hz, the 2nd frequency is over
239 kHz. As a rule of thumb, the servo bandwidth can be as
high as 1/4 of the critical resonant frequency. For this case, 1/4
of 239 kHz would be 60 kHz, which is high enough to support
a very high bandwidth of tracking-follow control design in a
blue-laser optical disk drive [11].
3. Device fabrication and characterizations
The MEMS mirror device was successfully fabricated by
microfabrication technologies such as lithography, deep RIE and
anodic wafer bonding. The process flow is given in Fig. 8. Referring to Fig. 8, (A) a silicon dioxide layer is grown by the thermal
oxidation method in a diffusion furnace. The oxide is patterned
via standard photolithography techniques using the mask for
the gap etch. The oxide is then etched using a reactive ion etch
system. In (B) the mask for the deep etch (223 ␮m) is used to
pattern the photoresist layer that acts as the mask when the deep
etch is run. (C) using the previously patterned oxide, the etch for
the 7 ␮m gap is run. The oxide coating is then removed by HF
acid. (D) On the top side of the wafer the reflective aluminium
sufrace is coated using the DC sputtering method for metalization. The aluminium is patterned using the surface mask and
then etched in an acid based wet etch solution. (E) the top etch
(70 ␮m) mask is used to pattern the photoresist for the top etch.
(F) aluminium is sputtered onto the glass substrate and patterned
using the electrode mask. (G) both wafers are bonded together
using andoic bonding. Additional wafer cleaning processes are
needed thoughout the process for the removal of photoresists
and other materials.
Fig. 9 shows the SEM photos of the micromirror structure and
surrounding substrate prior to anodic bonding, and the close-up
view of the damping grooves and beam. Fig. 10 is a plan view
of the mirror structure after bonding with glass substrate.
4. Characterizations
The prototype performance was evaluated at the experimental set-up, as shown in Fig. 11. Firstly, a probe station and an
Fig. 9. SEM photos of the prototype before wafer bonding.
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Fig. 10. MEMS mirror prototype after bonding.
impedance analyzer Agilent 4294A are used to measure the
capacitance versus voltage (C–V) curve, as shown in Fig. 12.
By measuring the capacitance value C0 at driving voltage V = 0,
it is estimated that the gap is 7.05 ␮m, which is close to the
designed value of 7.0 ␮m.
Next, laser doppler vibrometer (LDV) is used to measure the
static rotation angle of micromirror under the applied voltages.
Since the LDV can only measure displacement (not absolute
position), we measure the out-of-plane displacement of the mirror edge under a 1 Hz sinusoidal driving voltage as the static
displacement. From the distance between the mirror edge and
the center of torsion beam, we can approximately calculate the
mirror static rotation angle values versus the varying driving
voltages as shown in Fig. 13.
Then, the rotation angles by differential driving voltage where
V0 = 5 V are characterized as given in Fig. 14. It is seen that
the relationship between the rotation angle and the driving
voltage is close to be linear. Compared the designed results
(Figs. 4, 5 and 7) with the measured results (Figs. 12–14), we
found that there are a few variations. This is mainly due to the differences between the designed and fabricated parameters. One
parameter is the gap between electrodes, where the fabricated
gap is 7.05 ␮m compared to the designed value of 7 ␮m. The
other parameters include the dimension of the torsion beam. For
Fig. 12. C–V curve measured (circle) and designed (solid) for MEMS mirror
prototyped.
Fig. 13. Averaged (solid) and calibrated (circle) curves of rotation angle vs.
driving voltage.
example, the designed width 15 ␮m of torsion beam is fabricated
with 16.9 ␮m, while the beam height of 70 ␮m is fabricated with
75 ␮m.
Lastly, the dynamics performance of the MEMS mirror is
measured by LDV. Fig. 15 shows the frequency response of
Fig. 11. Experimental set-up of MEMS device prototype.
J.P. Yang et al. / Sensors and Actuators A 133 (2007) 368–374
373
enough to satisfy the requirement of the servo bandwidth of
precise tracking-control in high-density blue-laser optical disk
drive.
Acknowledgements
This work is funded by the Agency for Science, Technology
and Research (A*STAR), Singapore. The authors would like to
express their thanks to Dr. Li Qinghua, the former DSI staff, for
his help in the device testing and data analysis.
References
Fig. 14. Averaged (solid) and calibrated (circle) curves of rotation angle vs.
differential driving voltage.
Fig. 15. Frequency response measured for the MEMS mirror.
the MEMS mirror prototype, whose 1st resonant frequency
is at 599 Hz and well close to the designed 566 Hz with 5.8%
difference.
5. Conclusions
We designed and fabricated a MEMS-based mirror used
for a high-density optical disk. The device consists of a mirror plate, one pair of torsion springs and two pairs of electrodes on the mirror plate and glass substrate. The characterization of a MEMS mirror prototype is evaluated. Differential voltage driving is used for displacement–voltage linearization. The static and dynamics performances of mirror,
such as capacitance versus driving voltage, rotation angle versus driving voltage, and resonant frequency are measured and
compared well with the designed results. The mirror prototype can rotate up to ±0.02◦ at 5 V dc bias ± 4.5 V ac drive
voltage to transmit blue-laser beam spot on spinning disk. Its
very high resonant frequency of 239 kHz is expected to be high
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Biographies
J.P. Yang is Research Scientist in the Mechatronics and Recording Channel
Division at the Data Storage Institute (DSI). He received his PhD degree in
1996 from the Nanyang Technological University, Singapore. He was Lecturer
at the Shanghai Jiaotong University (1988–1993). He is co-recipient of the 1996
Best Journal Paper Award of the ASCE Technical Council on Computing Practices & Information Technology. His research interests include computational
mechanics, dynamics of disk spindle and microsystem, MEMS and probe storage devices, and multi-scale simulation for nanodevices. He has published over
80 international journal and conference papers in these areas.
L.N. Low received her MSc and BE (first class Honours) degrees in Electrical Engineering from the National University of Singapore in 1991 and 1987,
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respectively. She is currently working as a Senior Lecturer in the School of
Electrical and Electronics Engineering in the Singapore Polytechnic (SP) and
manages the Technology Centre for Nanofabrication and Materials in SP. Her
research interests in the MEMS area are microfluidic devices, biochips, microgripper, micromirror, etc.
D. Johnson is Research Engineer with the Technology Centre for Nanofabrication and Materials at the Singapore Polytechnic (SP). He received his ME
and BE degrees from the Messy University, New Zealand, in 2005 and 2002,
respectively. His current research activities are microfabrication processing
development for MEMS and BioMEMS.