Download A Large-Size MEMS Scanning Mirror for Speckle Reduction

micromachines
Article
A Large-Size MEMS Scanning Mirror for Speckle
Reduction Application †
Fanya Li 1,2 , Peng Zhou 2 , Tingting Wang 1,2 , Jiahui He 1,2 , Huijun Yu 2 and Wenjiang Shen 2, *
1
2
*
†
The School of Materials Science and Engineering, Xi’an Jiaotong University, Xi’an 710049, China;
[email protected] (F.L.); [email protected] (T.W.); [email protected] (J.H.)
Key Lab of Nanodevices and Applications, Suzhou Institute of Nano-tech and Nano-bionics,
Chinese Academy of Sciences, Suzhou 215123, China; [email protected] (P.Z.);
[email protected] (H.Y.)
Correspondence: [email protected]; Tel.: +86-512-6287-2688
This paper is an extended version of our paper published in the 12th IEEE International Conference on
Nano/Micro Engineered and Molecular Systems, 9–12 April 2017, Los Angeles, CA, USA.
Academic Editor: Huikai Xie
Received: 30 March 2017; Accepted: 24 April 2017; Published: 3 May 2017
Abstract: Based on microelectronic mechanical system (MEMS) processing, a large-size 2-D scanning
mirror (6.5 mm in diameter) driven by electromagnetic force was designed and implemented in this
paper. We fabricated the micromirror with a silicon wafer and selectively electroplated Ni film on
the back of the mirror. The nickel film was magnetized in the magnetic field produced by external
current coils, and created the force to drive the mirror’s angular deflection. This electromagnetically
actuated micromirror effectively eliminates the ohmic heat and power loss on the mirror plate, which
always occurs in the other types of electromagnetic micromirrors with the coil on the mirror plate.
The resonant frequency for the scanning mirror is 674 Hz along the slow axis, and 1870 Hz along
the fast axis. Furthermore, the scanning angles could achieve ±4.5◦ for the slow axis with 13.2 mW
power consumption, and ±7.6◦ for the fast axis with 43.3 mW power consumption. The application
of the MEMS mirror to a laser display system effectively reduces the laser speckle. With 2-D scanning
of the MEMS mirror, the speckle contrast can be reduced from 18.19% to 4.58%. We demonstrated
that the image quality of a laser display system could be greatly improved by the MEMS mirror.
Keywords: microelectronic mechanical system (MEMS); speckle reduction; electromagnetic force;
optical scanning
1. Introduction
Solid-state lasers can provide wider color gamut, longer lifetime, and higher brightness and
contrast of images compared to light emitting diodes (LEDs), a popular light source for projection
displays [1]. Laser display technology plays a significant role in our life, and can be applied in various
fields such as movie theatres, home televisions and conference rooms. However, the existence of
speckle degrades the images quality severely, which is an irregularly distributed pattern of light and
dark particles caused by the interference of the reflective coherent laser beam from the rough screen
comparable to optical wavelength [2–4]. One of the promising speckle reduction technologies in laser
projection [5,6] is to employ MEMS scanning mirrors. At present, research on MEMS scanning mirrors
are mostly focused on small diameter MEMS mirrors, while rarely on the larger size. Large-size mirrors
can not only tolerate high optical power, but also ensure maximum utilization of light energy [7].
Microvision Company in the United States developed an electromagnetic two-dimensional scanning
mirror, and successfully applied it to a laser Pico projection system, but the 1-mm diameter of the
mirror was unable to meet the requirements of the high lumen imaging display [8]. Oliveira et al. were
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the pioneers who applied a MEMS scanning mirror to eliminate laser speckle. Limited to a 0.8-mm
diameter, the MEMS mirror had issues when used in practical laser display systems [9]. Akram et al.
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in Vestfold University in Norway further improved the MEMS two-dimensional scanning mirror and
improved
the quality
of the
display
images,abut
its diameter
was only
2 mm, and
cannot be
[8]. Oliveira
et al. were
the laser
pioneers
who applied
MEMS
scanning mirror
to eliminate
laseralso
speckle.
usedLimited
in hightopower
laser
displays
[10].
The
large
size
and
mass
of
a
MEMS
two-dimensional
a 0.8-mm diameter, the MEMS mirror had issues when used in practical laser displaymirror
[9]. Akram
et al. in Vestfold
University
inthe
Norway
further
improved
MEMSAttwolimitsystems
the possibility
of achieving
a larger angle
unless
driving
moment
is highthe
enough.
the same
scanning
mirror andused
improved
the quality
of the
laser display
but its
diameter
time,dimensional
the oscillating
micromirror
to reduce
the laser
speckle
shouldimages,
also have
a high
operating
was only
2 mm, and also cannot be used in high power laser displays [10]. The large size and mass
frequency
[11].
of a MEMS two-dimensional mirror limit the possibility of achieving a larger angle unless the driving
Based on the above requirements, this paper proposes a 6.5-mm diameter, two-dimensional
moment is high enough. At the same time, the oscillating micromirror used to reduce the laser speckle
MEMS scanning mirror driven by the electromagnetic method. The efficient electromagnetic drive
should also have a high operating frequency [11].
mode not
onlyonoffered
the requirements,
driving moment
of theproposes
large angle
required,
but also
realized the high
Based
the above
this paper
a 6.5-mm
diameter,
two-dimensional
frequency.
The
mirror
is
used
in
a
laser
projection
system
to
suppress
laser
speckle.
Thisdrive
scanning
MEMS scanning mirror driven by the electromagnetic method. The efficient electromagnetic
mirror
with
a
large
diameter
could
be
used
in
high
power
laser
illumination
for
high
lumen
projection.
mode not only offered the driving moment of the large angle required, but also realized the high
Moreover,
the The
highmirror
frequency
of in
thea scanning
mirrorsystem
couldtoeffectively
reduce
the speckle
contrast and
frequency.
is used
laser projection
suppress laser
speckle.
This scanning
mirror
with
a large
diameter
could be
used in high power laser illumination for high lumen projection.
bring
clearer
and
more
comfortable
images.
Moreover, the high frequency of the scanning mirror could effectively reduce the speckle contrast
and bring clearer and more comfortable images.
2. Design
study employed the electromagnetic scanner in Figure 1 to demonstrate the proposed design
2.This
Design
concept. As indicated in Figure 1a, four external coils (A, B, C, and D) are symmetrically placed in
This study employed the electromagnetic scanner in Figure 1 to demonstrate the proposed
corresponding positions and kept a certain distance for the mirror’s free deflection. Specifically, the coils
design concept. As indicated in Figure 1a, four external coils (A, B, C, and D) are symmetrically placed
A and
C are located beneath the ferromagnetic film on the outer frame, and coils B and D are placed
in corresponding positions and kept a certain distance for the mirror’s free deflection. Specifically,
beneath
the rectangular
on the back offilm
theon
mirror.
Coils
A and
C coils
are responsible
for
the coils
A and C are ferromagnetic
located beneathfilm
the ferromagnetic
the outer
frame,
and
B and D
the slow
axis, while
B and
D coils areferromagnetic
for the fast axis.
A of
and
coils are
driven
byCthe
are placed
beneath
the rectangular
filmWhen
on thethe
back
theCmirror.
Coils
A and
aresquare
◦ phase difference, the two coils, the bottom magnetic
wave
signals with
theslow
sameaxis,
frequency
responsible
for the
while B and
and 180
D coils
are for the fast axis. When the A and C coils are
driven
theNi
square
signals
the will
samecompose
frequencyaand
180°magnetic
phase difference,
two
bar and
thebysoft
filmwave
on the
outerwith
frame
closed
circuit, the
then
ancoils,
attractive
the
bottom
magnetic
bar
and
the
soft
Ni
film
on
the
outer
frame
will
compose
a
closed
magnetic
force will drive the mirror to deflect a certain angle around the slow axis. Similarly, coils B, D, the
circuit,
then anbar
attractive
drive
thecompose
mirror toanother
deflect amagnetic
certain angle
around
slow axis.signals
bottom
magnetic
and theforce
soft will
Ni film
will
circuit
whenthe
excitation
Similarly, coils B, D, the bottom magnetic bar and the soft Ni film will compose another magnetic
are applied to B and D coils, so as to achieve the purpose of the two-dimensional scanning with our
circuit when excitation signals are applied to B and D coils, so as to achieve the purpose of the twoproposed model. Figure 1b,c shows the backside of mirror and the external coils, respectively. The two
dimensional scanning with our proposed model. Figure 1b,c shows the backside of mirror and the
groups
of coils
the signals
aretwo
controlled
separately,
independence,
scanning
external
coils,and
respectively.
The
groups of
coils and therefore,
the signals good
are controlled
separately,
linearity
and accuracy
can be achieved
for
the biaxial
scanner.can
Detailed
dimensions
of thescanner.
scanner and
therefore,
good independence,
scanning
linearity
and accuracy
be achieved
for the biaxial
coilsDetailed
in our design
are
summarized
in
Table
1.
dimensions of the scanner and coils in our design are summarized in Table 1.
Figure 1. (a) Illustration of the microelectronic mechanical system (MEMS) mirror and the actuation
Figure
(a) Illustration
of the
microelectronic
system
(MEMS) mirror and the actuation
coils1.underneath;
(b) Back
profile
of mirror withmechanical
nickel; (c) External
coils.
coils underneath; (b) Back profile of mirror with nickel; (c) External coils.
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Table 1. The dimensions of designed scanner and coils.
Parameter
Value
Unites
Diameter of the mirror
Thickness of the mirror
6.5
200
mm
µm
Width of the axis
Slow axis
Fast axis
100
160
µm
Length of the axis
Slow axis
Fast axis
1500
1750
µm
200
20
3
2
10
900
30
µm
µm
mm
mm
mm
Ω
Thickness of the axes
Thickness of the nickel film
Outer diameter of the coil
Inner diameter of the coil
Height of the coil
Number of turns for the coil
Resistence of the coil
We introduced a magnetic circuit model to solve the theoretical value of force shown in Figure 2.
Based on the hypothesis [12] that all the magnetic fluxes pass through the core (no leakage except
for the air gap), and according to the Maxwell’s magnetic force formula, attractive force could be
expressed by:
B0 2 A0
F =
(1)
µ0
In the equation, B0 is defined as the magnetic flux density of the air gap, A0 is the cross-sectional
area of the gap, and µ0 is the magnetic permeability of air. According to the Ampere’s Law, we could
derive that:
I
N ·i =
Hds = H m lm + H c lc + H 0 ·2x ,
(2)
where N is the number of coil turns, and i is the current flowing through the coil. Hm , Hc , and H0
respectively represent the strength of the magnetic field for the magnetic core, the clapper
(ferromagnetic film) and the gap. lm , lc , and x represent the length of the magnetic core, the clapper
and the air gap.
Because:
B
Φ
H =
=
(3)
µ
µA
the permeability of air can be negligible compared with the ferromagnetic materials’ permeability, that
is to say:
µ0 µm , µ0 µc
By assuming:
Ac = Am = A0 = A
(4)
Then, substituting (2)–(4) into the Equation (1), the magnetic force can be expressed as follows:
F = k
2
i2
x2
(5)
where k = µ0 N4 A . Obviously, the value of the driving force is proportional to the square of the current
and inversely proportional to the square of the gap length.
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and inversely proportional to the square of the gap length.
and inversely proportional to the square of the gap length.
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Figure
2. 2.
AA
magnetic
circuit
model
consists
ofof
a magnetic
core
with
copper
winding
and
a clapper
Figure
magnetic
circuit
model
consists
a magnetic
core
with
copper
winding
and
a clapper
Figure
2.
A
magnetic
circuit
model
consists
of
a
magnetic
core
with
copper
winding
and
a clapper
made
out
of
ferromagnetic
materials.
The
magnetic
field
created
by
the
copper
coil
is
concentrated
in in
made out of ferromagnetic materials. The magnetic field created by the copper coil is concentrated
made
out ofcore
ferromagnetic
materials.
The
magnetic
field created
by
the copper
coilpath
ispath
concentrated
in
the
magnetic
and
clapper
due
toto
their
high
permeability.
The
magnetic
circuit
is is
shown
byby
the
magnetic
core
and
clapper
due
their
high
permeability.
The
magnetic
circuit
shown
the
magnetic
core
and
clapper
due
to
their
high
permeability.
The
magnetic
circuit
path
is
shown
by
dashed
lines.
dashed lines.
dashed lines.
3. 3.
Fabrication
Fabrication
3. Fabrication
Figure
3 shows
the
detailed
fabrication
process.
AsAs
inin
Figure
3a,3a,
wewe
used
a double
sided
polished
Figure
3 shows
the
detailed
fabrication
process.
Figure
used
a double
sided
polished
Figure
3
shows
the
detailed
fabrication
process.
As
in
Figure
3a,
we
used
a
double
sided
n-type
(100)
200-µm
thick
Si wafer
as the
starting
substrate.
A 20-nm
Ti adhesion
layerlayer
and aand
100-nm
Au
n-type
(100)
200-μm
thick
Si wafer
as the
starting
substrate.
A 20-nm
Ti adhesion
apolished
100-nm
n-type
(100)sputtered
200-μm
thick
Sibackside
wafer
asof
the
A 20-nm
Tiseed
adhesion
and following
a 100-nm
coating
were
on the
Sistarting
substrate
the seed
layer
for
the
following
Au coating
were sputtered
on the
backside
of substrate.
Siassubstrate
as the
layerlayer
forelectroplating
the
Au
coating
sputtered
on thick
the backside
Si substrate
astothe
seed patterned
layer
for the
following
step.
A 20-µmwere
thick
film
of photoresist
(AZ4620)
was
patterned
selectively
electroplate
nickel.
electroplating
step.
A 20-μm
film
of of
photoresist
(AZ4620)
was
to selectively
electroplating
step.
A
20-μm
thick
film
of
photoresist
(AZ4620)
was
patterned
to
selectively
After
the soft-magnetic
Ni electroplating
step, the
andstep,
the Ti/Au
thin films and
werethe
removed
electroplate
nickel. After
the soft-magnetic
Ni photoresist
electroplating
the photoresist
Ti/Au by
thin
electroplate
nickel.
After
the
soft-magnetic
Ni
electroplating
step,
the
photoresist
and
the
Ti/Au
thin
acetone
solution
and
IBE
(ion
beam
etching)
technology,
respectively.
Then,
a
second
photolithography
films were removed by acetone solution and IBE (ion beam etching) technology, respectively. Then,
films
were
removed
bythe
acetone
and
IBE (ion
etching)
technology,
respectively.
Then,
step
was
used
to define
window
bulk
etching,
as shown
in silicon
Figure etching,
3d.
Afterasthat,
thein
a second
photolithography
step solution
wasfor
used
to silicon
define
thebeam
window
for bulk
shown
a
second
photolithography
step
was
used
to
define
the
window
for
bulk
silicon
etching,
as
shown
in
structure
of the
mirror
and axes of
were
by the
deep
reactive
ion etching
(DRIE)
Figure 3d.
After
that,plate
the structure
thereleased
mirror plate
and
axes
were released
by the
deepprocess.
reactive
Figure
3d.
After
that,
the silicon
structure
ofcoated
the
mirror
plate
axes
were
released
thethe
deep
reactive
Lastly,
the
front
side
of
the
was
withof
a 120-nm
aluminum
layer
to by
form
reflective
ion etching
(DRIE)
process.
Lastly,
the
front
side
the and
silicon
was
coated
with
a 120-nm
aluminum
ion
etching
(DRIE)
process.
Lastly,
the
front
side
of
the
silicon
was
coated
with
a
120-nm
aluminum
mirror
layer surface.
to form the reflective mirror surface.
layer to form the reflective mirror surface.
Figure
3. 3.
Fabrication
flow
forfor
thethe
MEMS
mirror.
(a)(a)
Ti/Au
seed
layer
sputtering;
(b)(b)
thick
photoresist
Figure
Fabrication
flow
MEMS
mirror.
Ti/Au
seed
layer
sputtering;
thick
photoresist
Figure
3.spinning
Fabrication
flow
for the MEMS
mirror.
(a) Ti/Au seed
layer
sputtering;
(b) thick
(AZ4620)
exposure;
(c)(c)nickel
electroplating
and
photoresist
removal;
(d)photoresist
(AZ4620)
spinningand
and
exposure;
nickel
electroplating
and
photoresist
removal;
(d)second
second
(AZ4620)
spinning
and
exposure;
(c) nickel
and
photoresist
removal; (d) second
photolithography
and
Ti/Au
films
removal;
(e)(e)
Sielectroplating
etching
(DRIE);
(f)(f)
Al
layer
coating.
photolithography
and
Ti/Au
films
removal;
Si
etching
(DRIE);
Al
layer
coating.
photolithography and Ti/Au films removal; (e) Si etching (DRIE); (f) Al layer coating.
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4.
4. Characterization
Characterization and
and Results
Results
4. Characterization and Results
Figure
Figure 44 shows
shows the
the package
package method
method and
and fully
fully assembled
assembled prototype.
prototype. The
The whole
whole package
package is
is
Figure
shows the
packageThe
method
and fully
assembled
The
package
similar
the4sandwich
sandwich
structure:
The
uppermost
cover
isprototype.
to protect
the whole
mirror
from
theis
similar
to the
structure:
uppermost
glassglass
cover
is to protect
the mirror
from
the
external
similar to
the sandwich
Thetop
uppermost
glassthe
cover
isthe
to is
protect
the
mirror
from
theincident
external
external
environment;
1.2-mm
thick
plastic
topabove
spacer
above
mirror
isto
chosen
a large
environment;
a 1.2-mma structure:
thick
plastic
spacer
mirror
chosen
ensureto
aensure
large
environment;
athe
1.2-mm
thick
plastic
top Ni
spacer
above
the
mirror
is chosen
to ensure
ais large
incident
incident
angle;
distance
between
theand
film
and thecoils
bottom
coils0.45
is about
0.45
mm,
which
is the
angle;
the
distance
between
the
Ni film
the bottom
is about
mm,
which
the
thickness
angle;
theofdistance
between
theThe
Ni film
andbottom
the
bottom
coilsspacer
is about
0.45
mm,
which
is theallowable
thickness
thickness
thespacer.
bottomThe
spacer.
thickness
of thespacer
bottomdefines
defines
the maximum
of
the bottom
thickness
of the
the
maximum
allowable
rotation
of the coils
bottom
spacer.
The thickness
ofplaced
the coil
bottom
spacer
defines
the maximum
allowable
rotation
angle;
arecoils
symmetrically
placed in
the
the driving
are
applied
rotation
angle;
are symmetrically
in holder,
the coiland
holder,
and the currents
driving currents
are through
applied
angle; coils
are symmetrically
placed in theboard
coil holder,
and
the
driving
currents
are applied
through
electrical
connections
to the printed
(PCB)
pads.
The
whole
device
is only
through
electrical
connections
to the circuit
printed circuit
board
(PCB)
pads.
The
wholesize
device
size16ismm
only×
electrical
connections
to
the
printed
circuit
board
(PCB)
pads.
The
whole
device
size
is
only
16
mm ×
16mm
mm×
× 12
16
16 mm.
mm × 12 mm.
16 mm × 12 mm.
Figure 4.
4. (a)
(a) Package
Package method;
method; (b)
(b) Fully
Fully assembled
assembled prototype.
prototype.
Figure
Figure 4. (a) Package method; (b) Fully assembled prototype.
After packaging, the device was tested with the setup shown in Figure 5. In the measurement,
After
the
was tested
with the
in
5. In
Afterpackaging,
packaging,
thedevice
device
tested
thesetup
setupshown
shown
inFigure
Figure
Inthe
themeasurement,
measurement,
two Ampere
meters were
used
towas
record
the with
relation
between
the current
and5.the
mirror’s
scanning
two
Ampere
meters
were
used
to
record
the
relation
between
the
current
and
the
mirror’s
scanning
two Ampere
meters were
used was
to record
the relation
the current
the mirror’s
scanning
property.
A function
generator
employed
to sendbetween
square waves
with aand
certain
frequency
to the
property.
AAfunction
generator
was
employed
to
send
square
waves
with
aacertain
frequency
to
property.
function
generator
was
employed
to
send
square
waves
with
certain
frequency
tothe
the
coils as the excited signal, and to the oscilloscope for monitoring phase difference between input
coils
as
the
excited
signal,
and
to
the
oscilloscope
for
monitoring
phase
difference
between
input
coils
as
the
excited
signal,
and
to
the
oscilloscope
for
monitoring
phase
difference
between
input
channels. The 2-D scanning mirror’s vertical and horizontal axes were driven independently by
channels.
The
2-D scanning
mirror’s
vertical
and and
horizontal
axes were
independently
by signals
channels.
The
scanning
mirror’s
vertical
horizontal
axes driven
were driven
independently
by
signals
from
the2-D
function
generator.
from
thefrom
function
generator.
signals
the function
generator.
Function generator
Function generator
CH2 CH1
CH2 CH1
Oscilloscope
Oscilloscope
CH1CH2
CH1CH2
Ampere meter
Ampere meter
Ampere meter
Ampere meter
Figure 5.5. Schematic measurement setup: the 2-D scanning mirror was
was driven
driven independently
independently by
by
Figure
Figure 5. Schematic measurement setup: the 2-D scanning mirror was driven independently by
function generators.
generators.
function
function generators.
The results are plotted in Figure 6. The deflection angle can be calculated from the length of the
The results
in Figure
6. The
deflection
can be calculated
fromthe
themicro-mirror
length of the
scanning
line andare
theplotted
distance
between
the screen
and angle
the scanning
mirror. When
scanning line and the distance between the screen and the scanning mirror. When the micro-mirror
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66of
of10
10
worked
the
vibration
state,
torsion
angles
increased
with
the
AC
(alternating
Theat
results
are plotted
in Figure
6. mechanical
The
deflection
angle
can be
calculated
from
of the
worked
at
the resonant
resonant
vibration
state,
mechanical
torsion
angles
increased
with
thethe
AClength
(alternating
current)
driving
currents
(refer
to
root-mean-square
values
of
the
current
flowing
in
the
coils)
scanning
line
and
the
distance
between
the
screen
and
the
scanning
mirror.
When
the
micro-mirror
current) driving currents (refer to root-mean-square values of the current flowing in the coils) for
for
both
axis
fast
scanning
angle
±4.5°
at
the
applied
current
worked
the resonant
state,
mechanical
torsion
angles
increased
with
(alternating
both the
theatslow
slow
axis and
and vibration
fast axis.
axis. The
The slow
slow
scanning
angle could
could achieve
achieve
±4.5°
atthe
theAC
applied
current
of
and
consumption
of
the
scanning
could
reach
±7.6°
the
current)
driving
currents
(refer to root-mean-square
values
of the
currentangle
flowing
in the
coils)
forat
both
of 21
21 mA
mA
and power
power
consumption
of 13.2
13.2 mw,
mw, and
and
the fast
fast
scanning
angle
could
reach
±7.6°
at
the
◦We
applied
current
38
mA
and
maximum
power
consumption
of
43.3
mw.
used
a
0.45-mm
thick
the
slow
axis
and
fast
axis.
The
slow
scanning
angle
could
achieve
±
4.5
at
the
applied
current
of
applied current 38 mA and maximum power consumption of 43.3 mw. We used a 0.45-mm thick
◦
bottom
spacer
here,
so
the
maximum
allowable
rotation
angle
for
the
slow
axis
and
fast
axis
were
21
mA and
power
consumption
of 13.2allowable
mW, androtation
the fastangle
scanning
angle
could
±7.6
the
bottom
spacer
here,
so the maximum
for the
slow
axisreach
and fast
axisatwere
±4.5°
and
±8°
theoretically,
matching
well
with
the
experimental
data.
By
tuning
the
driving
applied
current
38
mA
and
maximum
power
consumption
of
43.3
mW.
We
used
a
0.45-mm
thick
±4.5° and ±8° theoretically, matching well with the experimental data. By tuning the driving
◦
frequency,
the
frequency
response
of
recorded.
shows
the
bottom
spacer
so the maximum
rotationangle
angle could
for
thebe
slow
axis andFigure
fast axis77were
±4.5
frequency,
thehere,
frequency
response allowable
of the
the scanning
scanning
angle
could
be
recorded.
Figure
shows
the
◦
frequency
responses
of
the
slow
and
fast
axes
when
20
mA
was
applied
to
a
coil.
According
to
the
results,
and
±8 theoretically,
matching
the experimental
data.
By tuning
the According
driving frequency,
the
frequency
responses of
the slowwell
and with
fast axes
when 20 mA was
applied
to a coil.
to the results,
the
resonant
frequencies
were
674
Hz
and
1870
Hz
for
the
slow
and
fast
axes,
respectively.
The
quality
frequency
response
of
the
scanning
angle
could
be
recorded.
Figure
7
shows
the
frequency
responses
the resonant frequencies were 674 Hz and 1870 Hz for the slow and fast axes, respectively. The quality
factor
Q
calculated
the
equation
[13]:
of
the slow
and
fast axesby
when
20 mA was
applied
to a coil. According to the results, the resonant
factor
Q was
was
calculated
by
the following
following
equation
[13]:
frequencies were 674 Hz and 1870 Hz for the slow andf fast axes, respectively. The quality factor Q was
f0
(6)
Q
calculated by the following equation [13]:
(6)
Q == 0
Δf
Δf
f0
where
is
bandwidth.
= half-power
(6)
where is
isthe
theresonant
resonantfrequency,
frequency,and
and ∆∆ Q
isthe
the
half-power
bandwidth.From
Fromthe
themeasured
measuredcurve,
curve,
∆
f
we
can
derive
the
Q
value;
122
for
the
slow
axis
and
623
for
the
fast
axis.
The
large
difference
of
we can derive the Q value; 122 for the slow axis and 623 for the fast axis. The large difference of Q
Q
where
f
is
the
resonant
frequency,
and
∆
f
is
the
half-power
bandwidth.
From
the
measured
curve,
we
0
values
values between
between the
the slow
slow and
and fast
fast axes
axes was
was due
due to
to the
the dependence
dependence of
of damping
damping on
on the
the resonant
resonant
can
derive
the
Q
value;
122
for
the
slow
axis
and
623
for
the
fast
axis.
The
large
difference
of
Q
values
0.5
frequency.
frequency. The
The Q
Q value
value increases
increases with
with the
the resonant
resonant mode
modeand
andis
is proportional
proportional to
toff0.5 according
accordingto
to Chu
Chu
between the slow and fast axes was due to the dependence of damping on the resonant frequency.
et
[14]. As
shown
Figure
1,
scanning
of the
by
C coils,
and the
et al.
al.
Asincreases
shown in
in
Figure
1, the
the slow
slow
scanning
the mirror
mirror is
istodriven
driven
by A
A and
and
coils,
the
The
Q[14].
value
with
the resonant
mode
and isofproportional
f0.5 according
toCChu
et and
al. [14].
outer
frame
is
scanning
together
with
the
mirror
plate.
So,
the
air
damping
is
severer
and
the
Q
value
As
shown
in is
Figure
1, thetogether
slow scanning
the mirror
driven
bydamping
A and C coils,
and the
outer
frame
scanning
with theofmirror
plate.isSo,
the air
is severer
andouter
the Qframe
value
is
smaller
the
axis
is
together
with
thescanning.
mirror plate. So, the air damping is severer and the Q value is smaller for
isscanning
smaller for
for
the slow
slow
axis
scanning.
the slow axis scanning.
slow
slowaxis
axis
Mechanicalangle(deg)
angle(deg)
Mechanical
Mechanicalangle(deg)
angle(deg)
Mechanical
5
5
4
4
3
3
2
2
1
1
0
0
4
4
6
6
8
8
10 12 14 16 18 20 22
10 12 14 16 18 20 22
Current(mA)
Current(mA)
8
8
7
7
6
6
5
5
4
4
3
3
2
2
1
1
0
0
0
0
fast
fastaxis
axis
5
5
10
10
(a)
(a)
15
15
20
20
25
25
30
30
Current(mA)
Current(mA)
35
35
40
40
(b)
(b)
Resonant frequency = 674Hz
4 Resonant frequency = 674Hz
4
Q = 122
Q = 122
slow
slowaxis
axis
3
3
2
2
1
1
0
0
400
400
500
500
600
600
700
700
Frequency(Hz)
Frequency(Hz)
(a)
(a)
800
800
900
900
Mechanicalangle(deg)
angle(deg)
Mechanical
Mechanicalangle(deg)
angle(deg)
Mechanical
Figure
6.
Current-angle
relationship:
(a)
slow
axis;
(b)
fast
axis.
Figure6.
6.Current-angle
Current-anglerelationship:
relationship:(a)
(a)slow
slowaxis;
axis;(b)
(b)fast
fastaxis.
axis.
Figure
3.5
3.5 Resonant frequency = 1870Hz
Resonant frequency = 1870Hz
Q = 623
3.0
Q = 623
3.0
fast
fastaxis
axis
2.5
2.5
2.0
2.0
1.5
1.5
1.0
1.0
0.5
0.5
0.0
0.0
1700
1700
1750
1750
1800
1800
1850
1850
1900
1900
Frequency(Hz)
Frequency(Hz)
(b)
(b)
Figure
7.
Frequency
response:
(a)
slow
axis;
(b)
fast
axis.
Figure
Figure7.
7.Frequency
Frequencyresponse:
response:(a)
(a)slow
slowaxis;
axis;(b)
(b)fast
fastaxis.
axis.
1950
1950
2000
2000
Micromachines 2017, 8, 140
7 of 10
Reliability is crucial to the successful application of MEMS devices when they reach
commercialization [15,16]. To study the shock resistance of our fabricated devices, a shock test was
performed in air at room temperature as follows [17]. A scanner prototype was fixed to a shock table
by 3 M Epoxy Adhesive. Acceleration corresponded to the height of the table from which it was
dropped. A piezoelectric transducer (PZT) sensor was mounted to the shock table to record actual
acceleration. In the test, we changed the fixed direction so that the shock was applied in three (X, Y, Z)
orientations, where the X and Y directions were along the fast axis and the slow axis respectively,
and the Z direction was perpendicular to the mirror plane. The table was dropped to the floor three
times in a row with three orthorhombic orientations, and half sine shock pulses with certain widths
were produced. No significant fracture or electrical failures were observed until the prototype was
tested at 900 g for X direction and 1500 g for Y, Z directions, where g is the gravity acceleration
(g ≈ 9.8 m/s2 ), demonstrating that the micro-mirror and applied package structure have good shock
resistance. In addition, a vibration test was carried out on Electro Dynamic Shakers with varying
frequencies (from 20 Hz to 2000 Hz) at a constant acceleration of 20 g [17]. After three periods of
shaking, the device could still operate as before. These results show that the device is reliable and
durable for practical applications.
5. Application to Speckle Reduction
In the application to laser projectors, the speckle phenomenon emerges by reflecting highly
coherent laser beams with single wavelengths on random rough surfaces, resulting in a random spatial
intensity distribution [18]. One of the criterion to describe speckle is speckle contrast ratio, which is
defined as [19]:
q
C =
h I 2 i−h I i2
hIi
× 100%
(7)
q
where h I 2 i and h I i represent the square mean value and the mean light intensity, h I 2 i − h I i2 denotes
the standard deviation. The lower the C value, the clearer images could be derived, which is now of
great concern.
As presented in Figure 8, the simplified speckle reduction system consists of a laser diode,
a fabricated scanning mirror, a light pipe, optics elements such as a focusing lens and a diffuser with
high transmittance, and a charge-coupled device (CCD) camera for acquiring pattern information.
With the two-dimensional scanning of the mirror, the laser beam was reflected onto the diffuser placed
at the entrance of the light pipe with angle diversity at different times. After multiple reflections inside
the light pipe, the uniform illumination will be formed at the exit surface, which, with the existence of
subsequent imaging optics, form the picture on the screen. The scanning area can be changed with
different driving currents, however, we need to control the total reflected light entering into the light
pipe. According to speckle suppression theory [20,21], once the speckle images at different times and
different positions are uncorrelated, then these irrelevant speckle figures are finally superimposed
on each other during a frame image formed on the screen. If N independent speckle patterns are
overlapped on an intensity basis, and we assume that each pattern has an equal mean intensity, the
speckle contrast C in the integrated image is reduced to [4]:
1
C= √
N
(8)
where N is the number of independent speckle patterns. We could derive that the higher the value of
N, the lower the value of speckle contrast results.
In our measurement, the focus length and aperture f-number of the CCD imaging lens are 25 mm
and 8, respectively. The CCD has a pixel size of 3.75 µm × 3.75 µm and is located at 2 m away from the
screen. We explored the mirror’s stationary and vibrating conditions and their influence on the speckle
contrast ratio. The calculated data by Matlab based on Equation (7) is presented in Table 2. When the
Micromachines 2017, 8, 140
Micromachines 2017,
Micromachines
2017, 8,
8, 140
140
8 of 10
of 10
10
88 of
2. When the mirror was working, the contrast was 4.58% during 50 ms integration time of the CCD
2. When the mirror was working, the contrast was 4.58% during 50 ms integration time of the CCD
camera,
whenthe
thecontrast
mirror was
off, the 50
contrast
was 18.19%
atof
the
same
time.
mirror
was and
working,
wasturned
4.58% during
ms integration
time
the
CCDintegration
camera, and
camera, and when the mirror was turned off, the contrast was 18.19% at the same integration time.
Figure
9 shows
speckle
images
and
without
workingtime.
mirror.
With
the 2-D
when
the mirror
wasthe
turned
off, contrast
the contrast
was with
18.19%
at the
same the
integration
Figure
9 shows
Figure
9 shows
theMEMS
speckle
contrast
images contrast
with andforwithout
the
working display
mirror.could
With be
thereduced
2-D
of the
mirror,
the laser
projection
thescanning
speckle contrast
images
withthe
andspeckle
without the working
mirror.
With the 2-D scanning
of the
scanning of the MEMS mirror, the speckle contrast for the laser projection display could be reduced
frommirror,
18.19%the
to 4.58%.
This
resultfor
demonstrated
that the scanning
of thebe
mirror
can from
disturb
the spatial
MEMS
speckle
contrast
the laser projection
display could
reduced
18.19%
to
from 18.19% to 4.58%. This result demonstrated that the scanning of the mirror can disturb the spatial
and This
temporal
of the
source and
suppress
forand
lasertemporal
projection
4.58%.
resultcoherences
demonstrated
thatlaser
the scanning
of the
mirror the
canspeckle
disturb pattern
the spatial
and temporal coherences of the laser source and suppress the speckle pattern for laser projection
images. of the laser source and suppress the speckle pattern for laser projection images.
coherences
images.
MEMS mirror
MEMS mirror t1
Diffuser
Light pipe
Diffuser
Light pipe
Lens
Lens
t1 t2
t2 t3
t3
Laser beam
Laser beam
Computer
Computer
CCD
CCD
Screen
Screen
LD
LD
Figure
8. Speckle
reduction
system.
Figure
8. Speckle
reduction
system.
Figure 8. Speckle reduction system.
Table
2. The
speckle
contrast
by different
measurement
systems.
Table
2. The
speckle
contrast
by different
measurement
systems.
Table 2. The speckle contrast by different measurement systems.
Measurement Integration
IntegrationMaxmium
MaxmiumMinimum
Minimum MeanMean Contrast
Contrast
Measurement
Measurement
Integration
Maxmium
Minimum
Mean
Contrast
System
Time/ms
Intensity
Intensity
Intensity
Value/%
System
Time/ms
Intensity
Intensity
Intensity
Value/%
System
Time/ms
Intensity
Intensity
Intensity
Value/%
With
mirror
50
157
108
126
4.58
With
mirror
50
157
108
126
4.58
With
mirror
50
157
108
126
4.58
Without
mirror
50
255
94
179
18.19
Without mirror
50
255
94
179
18.19
Without mirror
50
255
94
179
18.19
Figure
9. Speckle
contrast
images
when
CCD
integration
time
to ms:
50 ms:
(a) Speckle
pattern
Figure
9. Speckle
contrast
images
when
CCD
integration
time
waswas
set set
to 50
(a) Speckle
pattern
Figure 9. Speckle contrast images when CCD integration time was set to 50 ms: (a) Speckle pattern
with
inactive
scanning
mirror,
C
=
18.19%;
(b)
Speckle
reduction
pattern
with
active
mirror,
C
=
4.58%.
with inactive scanning mirror, C = 18.19%; (b) Speckle reduction pattern with active mirror, C = 4.58%.
with inactive scanning mirror, C = 18.19%; (b) Speckle reduction pattern with active mirror, C = 4.58%.
6. Conclusions
6. Conclusions
6. Conclusions
WeWe
have
proposed
and
fabricated
a alarge-size
mirror can
canmeet
meetthe
have
proposed
and
fabricated
large-sizeMEMS
MEMSscanning
scanningmirror.
mirror. Our mirror
We have proposed
and
fabricated
a large-size
MEMS
scanningangle
mirror.
Our
mirror
canreduction
meet the
therequirements
requirements
of
high
resonant
frequency
and
large
deflection
used
for
speckle
of high
frequency and large deflection angle used for speckle reduction
requirements
of successfully
high resonant
frequency
and
large contrast
deflection
angle
used
for speckle
reduction
application,
and
reduces
thethe
laser
speckle
from
18.19%
to 4.58%.
In addition,
thethe
application,
and successfully
reduces
laser
speckle contrast
from
18.19%
to 4.58%.
In addition,
application,
and
successfully
reduces
the
laser
speckle
contrast
from
18.19%
to
4.58%.
In
addition,
the
fabricated
devices
have
a shock
resistance
of more
than
900900
g and
good
vibration
resistance,
which
is is
fabricated
devices
have
a shock
resistance
of more
than
g and
good
vibration
resistance,
which
fabricated
devices
havein
a shock
resistance
of more than 900 g and good vibration resistance, which is
alsoalso
crucial
when
used
commercial
applications.
crucial when used in commercial applications.
also crucial when used in commercial applications.
Micromachines 2017, 8, 140
9 of 10
Acknowledgments: This research was supported by the National Key Research and Development Program of
China (Grant No. 2016YFB0402003). The authors are grateful to SINANO’s Nano Fabrication Facility (NFF) for
their support in the fabrication process of micromirrors and related reliability tests.
Author Contributions: W.S. offered conceptual advices for finishing this article; J.H. and H.Y. gave technical
support and offered assistance in the whole experiment process; F.L. mainly collected data about the performance
for the mirror and wrote the manuscript; P.Z. and T.W. were responsible for the data of speckle reduction.
All authors discussed the results and commented on the manuscript at all stages.
Conflicts of Interest: The authors declare no conflict of interest.
References
1.
2.
3.
4.
5.
6.
7.
8.
9.
10.
11.
12.
13.
14.
15.
16.
Tong, Z.; Shen, W.; Song, S.; Cheng, W.; Cai, Z.; Ma, Y.; Wei, L.; Ma, W.; Xiao, L.; Jia, S.; et al. Combination
of micro-scanning mirrors and multi-mode fibers for speckle reduction in high lumen laser projector
applications. Opt. Express 2017, 25, 3795–3804. [CrossRef] [PubMed]
Subramaniam, S.; Le, C.P.; Kaur, S.; Kalicinski, S.; Ekwinska, M.; Halvorsen, E.; Akram, M.N. Design,
modeling, and characterization of a microelectromechanical diffuser device for laser speckle reduction.
J. Microelectromech. Syst. 2014, 23, 117–127.
Young, E.J.; Kasdin, N.J.; Carlotti, A. Image Analysis with Speckles Altered by a Deformable Mirror.
Available online: http://proceedings.spiedigitallibrary.org/proceeding.aspx?articleid=1744172 (accessed on
25 April 2017).
Tong, Z.; Chen, X.; Akram, M.N.; Aksnes, A. Compound speckle characterization method and reduction by
optical design. J. Disp. Technol. 2012, 8, 132–137. [CrossRef]
Heger, A.; Schreiber, P.; Höfer, B. Development and Characterisation of a Miniaturized Laser Projection
Display Based on MEMS-Scanning-Mirrors. Available online: http://proceedings.spiedigitallibrary.org/
proceeding.aspx?articleid=1345482 (accessed on 25 April 2017).
Kim, S.; Han, Y.G. Suppression of speckle patterns based on temporal angular decorrelation induced by
multiple beamlets with diverse optical paths. J. Korean Phys. Soc. 2014, 64, 527–531. [CrossRef]
Chen, X.; Svensen, Ø.; Akram, M.N. Speckle reduction in laser projection using a dynamic deformable mirror.
Opt. Express 2014, 22, 11152–11166.
Sprague, R.B.; Montague, T.; Brown, D. Bi-Axial Magnetic Drive for Scanned Beam Display Mirrors.
Available online: http://proceedings.spiedigitallibrary.org/proceeding.aspx?articleid=859733 (accessed on
25 April 2017).
Oliveira, L.C.M.; Barbaroto, P.R.; Ferreira, L.O.S.; Doi, I. A novel Si micromachined moving-coil induction
actuated mm-sized resonant scanner. J. Micromech. Microeng. 2005, 16, 165. [CrossRef]
Akram, M.N.; Tong, Z.; Ouyang, G.; Chen, X.; Kartashov, V. Laser speckle reduction due to spatial and
angular diversity introduced by fast scanning micromirror. Appl. Opt. 2010, 49, 3297–3304. [CrossRef]
[PubMed]
Bayat, D. Large Hybrid High Precision MEMS Mirrors. Ph.D. Dissertation, École Polytechnique Fédérale de
Lausanne, Lausanne, Switzerland, 2011.
Bleuler, H.; Cole, M.; Keogh, P.; Larsonneur, R.; Larsonneur, R.; Maslen, E.; Okada, Y.; Traxler, A. Magnetic
Bearings: Theory, Design, and Application to Rotating Machinery; Springer Science & Business Media: Berlin,
Germany, 2009.
Chen, M.; Yu, H.; Guo, S.; Xu, R.; Shen, W. An electromagnetically-driven MEMS micromirror for laser
projection. In Proceedings of the 10th IEEE International Conference on Nano/Micro Engineered and
Molecular Systems (NEMS), Xi’an, China, 7–11 April 2015; pp. 605–607.
Chu, H.M.; Hane, K. Design, fabrication and vacuum operation characteristics of two-dimensional
comb-drive micro-scanner. Sens. Actuators A Phys. 2011, 165, 422–430. [CrossRef]
Tanner, D.M.; Walraven, J.A.; Helgesen, K.; Helgesen, K.; Irwin, L.W.; Brown, F.; Smith, N.F.; Masters, N.
MEMS reliability in shock environments. In Proceedings of the 38th Annual 2000 IEEE International
Reliability Physics Symposium, San Jose, CA, USA, 10–13 April 2000; pp. 129–138.
Naumann, M.; Dietze, O.; McNeil, A.; Mehner, J.; Daniel, S. Reliability of anchors at surface micromachined
devices in shock environments. In Proceedings of the 2013 Transducers & Eurosensors XXVII: The
17th International Conference on Solid-State Sensors, Actuators and Microsystems (TRANSDUCERS &
EUROSENSORS XXVII), Barcelona, Spain, 16–20 June 2013; pp. 570–573.
Micromachines 2017, 8, 140
17.
18.
19.
20.
21.
10 of 10
GB4590-84, Mechanical and Climatic Test Methods for Semiconductor Integrated Circuits. Available online:
http://down.bzwxw.com/12/GB%204590-1984.pdf (accessed on 25 April 2017). (In Chinese)
Lee, J.Y.; Kim, T.H.; Yim, B.B.; Bu, J.U.; Kim, Y.J. Speckle reduction in laser picoprojector by combining
optical phase matrix with twin green lasers and oscillating MEMS mirror for coherence suppression. Jpn. J.
Appl. Phys. 2016, 55, 08RF03. [CrossRef]
Pan, J.W.; Shih, C.H. Speckle reduction and maintaining contrast in a LASER pico-projector using a vibrating
symmetric diffuser. Opt. Express 2014, 22, 6464–6477. [CrossRef] [PubMed]
Pei, T.H.; Yeh, F.C.; Tsai, K.Y.; Li, J.H.; Liu, Z.R.; Hung, C.L. Simulation and experiment of speckle reduction
by the beam splitting method on a pico-projection system. Adv. Mater. Res. 2014, 933, 572–577. [CrossRef]
Goodman, J.W. Speckle Phenomena in Optics: Theory and Applications; Roberts and Company Publishers:
Greenwood Village, CO, USA, 2007.
© 2017 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access
article distributed under the terms and conditions of the Creative Commons Attribution
(CC BY) license (http://creativecommons.org/licenses/by/4.0/).