ABSTRACT In this paper, a new micromirror structure has been proposed and fabricated. The proposed micromirror is electromagnetically actuated along two-axis at low voltage using an external magnetic field. The mirror plates and torsion bars are made of bulk silicon and the actuation coils are made of electroplated copper. The maximum deflection angles have been measured as ±4.35° for x-axis actuation and ±15.7° for y-axis actuation. The actuation voltages are below 4.2V for xaxis actuation and 1.76V for y-axis actuation, respectively

Euisik Yoon

Hyung-Kew Lee

Il-Joo Cho

Jun-Bo Yoon

Kwang-Seok Yun

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A Low-Voltage Two-Axis Electromagnetically Actuated Micromirror
With Bulk Silicon Mirror Plates and Torsion Bars
Il-Joo Cho, Kwang-Seok Yun, Hyung-kew Lee, Jun-Bo Yoon and Euisik Yoon
Department of Electrical Engineering and Computer Science
Korea Advanced Institute of Science and Technology (KAIST)
373-1 Kusong-dong, Yusong-gu, Taejon 305-701, Korea
(TEL) +82-42-869-5462, (FAX) +82-42-869-8530, E-mail: ijcho@iml.kaist.ac.kr
ABSTRACT
In this paper, a new micromirror structure has been 
proposed and fabricated.  The proposed micromirror is
electromagnetically actuated along two-axis at low
voltage using an external magnetic field.  The mirror
plates and torsion bars are made of bulk silicon and the
actuation coils are made of electroplated copper.  The
maximum deflection angles have been measured as
±4.35° for x-axis actuation and ±15.7° for y-axis
actuation.  The actuation voltages are below 4.2V for x-
axis actuation and 1.76V for y-axis actuation, respectively.
INTRODUCTION
There are many applications of micromirrors that
require large deflection angles and mechanically robust
structures such as optical switch [1], optical scanner [2], 
projection display [3], etc.  Most of the micromirrors
reported to date have used electrostatic force actuation [4-
6].  In the case of electrostatic actuation, the actuation
voltage is high (>100V) to acquire the deflection of a
required angle and it is relatively difficult to have
mechanically robust mirror structures because the torsion 
bars have to be thin to have a low spring constant.  On
the contrary, electromagnetic actuation can induce a large 
force with a lower voltage but require large power
consumption [7]. 
In this work, we used an external magnetic field to 
acquire large force at low voltage and low power
consumption.  This large force actuation enables the
micromirrors to be fabricated in a stable microstructure
using bulk silicon.  To obtain a stable microstructure, the 
proposed structure uses bulk silicon for torsion bars and a 
mirror plate. This structure requires low operating
voltages by using an external magnetic field.  In
addition to that, electromagnetic actuation enables bi-
directional motion, so that torsion bars are subjected only 
to torsional motion but not to bending motion.  This can 
make a stable actuation possible.  Previously,
micromirrors using an external magnetic field have been 
reported; however, they only allow one-axis actuation [8-
9].  In this paper, a two-axis magnetically actuated
micromirror has been proposed, fabricated, and
demonstrated using bulk silicon.
DESIGN AND SIMULATION
The proposed micromirror actuates with Lorentz
force using an external electromagnetic field as shown in 
Figure 1.  The proposed micromirror structure consists
of a mirror plate, frames, cantilever type actuators and
coils.  The cantilever type actuator is connected to the
frames using lifting bars.  The mechanical structure is
made of silicon.  The coils are electroplated copper
integrated on the mirror plate and cantilever type
actuators.  There are two types of actuators, x-axis
actuator and y-axis actuator.  The x-axis actuator is
implemented in the cantilevers on the outside of the
mirror plate and the y-axis actuator is integrated on the
mirror plate.
The operating direction of the micromirror is
determined by the direction of the current flowing in the 
coils on the mirror plate and the cantilever actuator as
shown in Fiure 2.  Among this coils, only the coils
perpendicular to the magnetic field induce Lorentz force.
In the case of x-axis actuation, the coils on the mirror
plate induce Lorenz force.  The coils which flow a
current, I1, induce a upward force, F1, while the coils
which flow a current, I2, induce a downward force, F2.
These forces enable the mirror plate to rotate along the y-
axis with inner torsion bars as a centeral axis.  In the
case of x-axis actuation, the coils on the cantilever
actuators induce Lorentz force. The coils which flow a 
current, I3, induce a upward force, F3, and this force lifts
up the pair of cantilevers.  The lifted cantilevers
accordingly lift the frame. 
Y-axis X-axis
B-field
X-axis actuator
Y-axis actuator
Figure 1. Shemetic diagram of the proposed micromirror.
0-7803-7185-2/02/$10.00 ©2002 IEEE 540
Figure 2. Operating principle of the proposed micromirror.
  (a) (b)
Figure 3. Simulation results using ANSYS: (a) x-axis actuation, 
(b) y-axis actuation.
The other pair of cantilevers move downward by the 
Lorentz force, F4, induced from a current, I4, which
flows in the opposite direction of I3, and this makes the
frame move downward the frame.  From this cantilever 
actuation, the frame rotates along x-axis with outer
torsion bars as a central axis. Figure 3 shows the
actuation modes of each direction simulated using
ANSYS 5.3.  The resonant frequency of x-axis actuation
is 5.369kHz and that of y-axs actuation is 3.391kHz,
respectively.  This resonant frequencies allows this
micromirror to be operated in sub-milliseconds.
FABRICATION
The fabrication processes of the proposed
micromirror are shown in Figure 4.  Starting material is 
an SOI wafer with top silicon of 5µm and buried oxide of 
1µm.  The thickness of the top silicon determines the
thickness of the proposed micromirror directly.  Mirror
frames and mirror plates are defined at the top silicon by 
RIE (Reactive Ion Etching) using thermal oxide as an
etch mask (Figure 4 (b)).  Then, Cr/Cu is deposited for a 
seed metal layer for electroplating.  Next, bottom copper 
coils are electroplated as shown in Figure 4 (c).  The
5µm-thick copper coils are electroplated using 10µm-
thick AZ9260 photoresist as a mold. After electroplating 
the bottom copper coils, photoresist is coated for
scrificial layer to make an air gap and patterned for
contact. Then, another copper layer is deposited as a seed 
layer.
Table I. Dimension of the proposed micromirror
X-axis actuation Y-axis actuation
Mirror plate 800µm × 800µm
Torsion-bar length 100 µm
Torsion-bar height 10µm(silicon 5µm +Cu coil 5µm)
Lifting-bar length 50 µm
Lifting-bar width 20 µm
Coil width 10µm
Coil height 5µm
Resonant frequency 
(simulated)
5.369kHz 3.391kHz
With photoresist as a mold, bridge metal is electroplated 
as shown in Figure 4 (d).  Using the buried oxide as an 
etch stop, silicon is etched from the backside in KOH
with silicon nitride as an etch mask (Fig. 4 (e)).  After
removing the buried oxide, the mirror and frame are
released as shown in Figure 3 (f).
Figure 5 shows the SEM pictures of the fabricated
micromirror.  Figure 5 (a) shows the side view of the
fabricated micromirror and Figure 5 (b) shows the top
view.  Air bridge metal lines are seen at the cross point 
of the coils and there are four metal pads used for x-axis
and y-axis actuation inputs.  Metal lead wires are
electroplated on the torsion bars for electrical connection.
The total thickness of the torsion bars will be the addition 
of top silicon thickness and lead wire thickness, which is 
about 10 µm. These thick torsion bars enable
mechanically-stable operation under the large
electromagnetic force induced from an external magnetic 
field. The detailed dimensions of the fabricated
structure are summarized in Table I.
Figure 4. Process flow of the proposed structure: (a) Starting 
wafer, (b) Mirror structure define by dry etch, (c) Bottom Cu
coil electroplating, (d) Air bridge coil electroplating, (e) Si
anisotropic etch in KOH, (f) Buried oxide removal.
F3
F4
F1
F2
I2
I1
I3
I4
B-field
Cu Coil
Buried Oxide
Silicon
(a)
(f)(e)
(d)(c)
(b)
0-7803-7185-2/02/$10.00 ©2002 IEEE 541
(a)
(b)
Figure 5. SEM pictures of the fabricated mirror structure: (a)
side view, (b) top view.
MEASUREMENT AND RESULTS
To measure deflection angles, current is applied to
the copper coil.  The diced micromirror is mounted on a 
PCB board and the metal pads on the mircromirror and
PCB board are connected with bonding wires.  The
actuation current is applied to the micromirror via the
PCB board.  External magnetic field is applied using a
pair of permanent maganets as shown in Figure 6.  The
micromirror mounted on the PCB board is inserted
between the permanent magnets in a way to apply a
magnetic field parallel to the x-axis of the micromirror as 
shown Figure 1.  The measured external magnetic field
intensity is 0.16T(=1600Gauss) at the actuation point. 
Figure 6. Measurement setup: (a) Displacement measured
using 3D profiler (Keyence VF-7500), (b) Permanent magnet
setup.
Figure 7. Picture of the rotating micro mirror using a 3D 
profiler
The 3D laser profiler is used to measure rotational
angles of the actuated micromirror.  The used laser
profiler is a Keyence VF-7500. This laser profiler scans
sample surface with a focused laser beam and displays a 
3D profile as shown in Figure 7.  To measure the
actuation angle, the mirror plate is scanned with a
focused laser line from the 3D profiler and the actuation 
angle is obtained by measuring the difference in height
between the left side and the right side of the mirror plate.
As a result of rotation, the laser line on the left of the
mirror is in focus and that on the other side is out of focus 
and seen blurred as shown in Figure 7.  Rotation angle
can be also confirmed from the dimensional change in the 
sidewall of the mirror plate. 
(a)
(b)
Figure 8. Measured deflection angle as a function of applied
current: (a) x-axis actuation, (b) y-axis actuation.
Permanent magnet
Diced mirror
Laser line
0-7803-7185-2/02/$10.00 ©2002 IEEE 542
Table II. Experimental results of the fabricated micromirror
X-axis actuation Y-axis actuation
Magnetic Field 0.16Tesla
Actuation Angle 0 ~ ± 4.35° 0 ~ ± 15.7°
Actuation Current 0 ~ 120mA 0 ~ 80mA
Actuation Voltage 0 ~ 4.2V 0 ~ 1.76V
Maximum Power 50.4mW 14.1mW
Angular deflection is measured by applying a
constant current.  Figure 8 shows the measured angular
deflection of the micromirror as a function of applied
current for x-axis and y-axis actuations, respectively,
compared to the simulation results.  The simulation
results of angular deflection are obtained using ANSYS
5.3.
In the case of x-axis actuation, the maximum
deflection angle has been measured as 4.35° at the
applied current of 120mA and the actuation voltage of
4.2V.  The total resistance of the colis on the cantilever 
type actuator is 35Ω and power consumption is 50.2mW. 
In the case of y-axis actuation, the maximum deflection 
angle has been measured as 15.7° at the applied current
of 80mA and the power consumption of 14.1mW.
Therefore, the total maximum power consumption is
about 64.5mW for two-axis simultaneous actuation.
Table II summarizes the characteristics of the fabricated
micromirror.
CONCLUSTIONS
We have proposed and fabricated a new two-axis
actuated micromirror.  This fabricated micromirror
operates electromagnetically using an external magnetic
field.  The maximum actuation angle has been measured 
as 4.35° at 120mA for x-axis actuation and 15.7° at
80mA for y-axis actuation, respectively.  The actuation 
voltages are below 4.2V for x-axis actuation and 1.76V
for y-axis actuation, respectively.  Total maximum
power consumption is 64.5mW and simulated resonant
frequencies are 5.369kHz for x-axis actuation and
3.391kHz for y-axis actuation, respectively.  The
fabricated micromirror has demonstrated large deflection 
angles at low voltage and low power consumption.  The
robust torsion bar structure formed by bulk silicon allows 
the proposed micromirror to stably operate at high
frequencies.  The actuation angle of the y-axis actuation 
can be increased by optimizing the dimension of lifting
bars through the further work. 
ACKNOWLEDGEMENTS
This work has been supported in part by the Korea
Science and Engineering Foundation through the
MICROS center at KAIST and National Research
Laboratory program from Ministry of Science and
Technology of Korea. The authors would like to thank
Eun-Chul Park and Yun-Seok Choi for their help on
technical support.
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0-7803-7185-2/02/$10.00 ©2002 IEEE 543


A low-voltage Two-axis Electromagnetically Actuated Micromirror with Bulk Silicon Mirror

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A low-voltage Two-axis Electromagnetically Actuated Micromirror with Bulk Silicon Mirror

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