Eye-type scanning mirror with dual vertical combs for laser display
Young-Chul Koa,d,*, Jin-Woo Chob, Yong-Kweun Muna, Hyun-Gu Jeonga, Won-Kyoung Choic,
Ju-Hyun Leea, Jeong-Woo Kima, Ji-Beom Yood, Jin-Ho Leea,**
a
Devices Lab., bCSE Center, cNano Fab. Center, Samsung Advanced Institute of Technology,
P. O. Box 111, Suwon 440-600, Korea
d
Center for Nanotubes and Nanostructured Composite, Sungkyunkwan University,
Suwon, 440-746, Korea
ABSTRACT
Since lasers have the most saturated colors, laser display can express the natural color excellently. Laser scanning
display has merits of simple structure and high optical efficiency. We designed a new scanning mirror which has a
circular mirror plate with an elliptical outer frame and is electrostatically driven by vertical combs arranged at the
outer frame. This eye-type mirror showed a larger deflection angle compared to the rectangular and the elliptical
mirrors. To increase the driving force twice, stationary comb electrodes are arranged at the upper and lower sides
of the moving comb fingers, together. The diameter of the mirror plate is 1.0 mm, and the lengths of the major
and minor axes of the outer frame are 2.5 mm and 1.0 mm, respectively. Using this scanning mirror, we obtained
an optical scanning angle of 32° when driven by the ac control voltage of the resonant frequency in the range of
22.1 ~ 24.5 kHz with the 100 V dc bias voltages. We demonstrated the full color XGA-resolution video image
with the size over 30 inches using an eye-type scanning mirror. The successful development of compact laser TV
will open a new area of home application of the laser light.
Keywords: laser TV, laser display, scanning mirror, scanner, vertical comb, eye-type
1. INTRODUCTION
As the multimedia society has come, the needs for large area display is increasing more rapidly. So many kinds of
projection displays have been developed. Although flat panel displays, like LCD and PDP, increase its size very
fast, projection displays still have merits of cost and simplicity of structure in large area displays. Laser scanning
display is being developed as one of the future projection displays. It is well known that conventional displays
using phosphors or a lamp as a light source can express only about 30 % of all visible colors. Recently various
efforts to expand color gamut of displays are being tried. The most efficient one is the laser display which is
realized with lasers as the light source. Since lasers have the most saturated colors, the laser display has wider
color gamut than that of the conventional displays using phosphors or a lamp. Its color gamut is almost three
times wider than that of the conventional displays1, 2. So the trials to use lasers as light sources have been
continued. Figure 1 and Figure 2 show the comparison of color gamut between the conventional displays (sRGB)
and laser display. With three lasers of RGB colors, up to 83 % of human visible color area can be expressed, while
color gamut is only 36 % in the conventional displays. In spite of this excellent characteristic, laser TV for the
commercial displays could not be realized yet, for the lack of laser-related technologies. One of the most obstacles
for home theater of the laser display is the delay of the development of compact, high power blue and green laser
sources. It is however clear that we can use compact RGB laser sources within several years, due to the rapid
progress of the semiconductor laser technologies.
*[email protected]; **[email protected]; Tel: 82-31-280-9328; Fax: 82-31-280-6879
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MOEMS Display and Imaging Systems III, edited by Hakan Ürey,
David L. Dickensheets, Proc. of SPIE Vol. 5721 (SPIE, Bellingham,WA, 2005)
0277-786X/05/$15 · doi: 10.1117/12.591276
Colors
[Million]
Distinguishable Colors
3.5
3.24
3
2.7
100%
83%
2.5
2
1.5
1.16
36%
1
0.5
0
Figure 1. Color coordinates and color gamut.
sRGB
Laser
Optimal Color
Figure 2. Comparison of color gamut between conventional
displays (sRGB) and laser display.
There are several kinds of methods to make a video image with laser sources. We have been studying a scanning
type laser display which has a relatively simple structure3-11. Figure 3 shows the schematic drawing of the laser
scanning display. Scanning type laser display is mainly composed of lasers, modulators and scanners. Laser
beams are modulated according to the video signals and then a combined beam is projected to the screen by
scanners. Compared to the light valve type projection displays such as LCD and DLP, it has advantages that it can
be reached extremely small system volumes and has the highest optical efficiency in the case of using directly
modulated lasers.
Laser Diode
MEMS Scanner
Modulator
Figure 3. Schematic drawing of the laser scanning display.
Our ultimate goal is a palm-top-sized laser TV for targeting consumer electronics. As a recent results, we
demonstrated the full color video image using a high speed MEMS scanning mirror. The successful development
of compact laser TV will open a new area of home application of the laser light.
Proc. of SPIE Vol. 5721
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2. EYE-TYPE SCANNING MIRROR
Scanning type laser display has merits of simple structure and high optical efficiency. So it has a big advantage of
a compact-sized system. For a palm-top-sized laser TV, we need small components of lasers and scanners.
Typical mechanical scanners are not proper for such compact consumer applications, because of the large size and
high cost. MEMS scanners have a very high potential of acquiring the small size and low cost. And it doesn’t
make any sound noise unlike mechanical rotating mirror.
For increasing the performance of the scanning mirror such as a high driving frequency and a large scanning angle
by reducing the moment of inertia and increasing the rotation moment, we designed a scanning mirror which has a
circular mirror plate with an elliptical outer frame and is electrostatically driven by vertical combs arranged at the
outer frame, as shown in Figure 4. This new scanning mirror looks like a human eye, so we named it an eye-type
scanning mirror. And we compared the dynamic actuation angle of the eye-type scanning mirror with the various
shapes of the mirror plate like rectangular and elliptical mirror. For this purpose, we integrated the simulation
procedure and performed the optimization process using ANSYS and iSIGHT software, as shown in Figure 5.
After the structure dimensions were determined, parametric modeling and modal analysis were performed by
ANSYS. From the acquired static angle, we estimated the dynamic actuation angle by analytic formula. The
iSIGHT program iterated the whole process cycle in a given variable range. Using this procedure, we examined
the dynamic actuation angle with the various resonant frequency range about three types of the scanning mirror, as
shown in Figure 4.
Figure 4. Three types of the scanning mirror.
Determine mirror dimensions
3D Parametric Modeling (ANSYS)
iSIGHT
Optimization modules
(MMFD, Genetic)
Modal analysis (ANSYS)
Calculation static actuation angle (ANSYS)
Estimate dynamic actuation angle
Using analytic formula
Figure 5. Optimization process.
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Proc. of SPIE Vol. 5721
Figure 6 shows the dynamic actuation angle of three types of the mirror plate. As a result, it can be shown that the
eye-type mirror has a larger deflection angle compared to the rectangular and the elliptical mirrors in a given
resonant frequency.
Figure 6. Optimization simulation results of mirror shapes by iSIGHT software.
To increase the driving force twice, stationary comb electrodes are arranged at the upper and lower sides of the
moving comb fingers, together. Figure 7 shows the schematic drawing of vertical comb structures. This dual
vertical comb structure also has an advantage of decreasing the vertical movement of the mirror induced by the
unbalanced forces in the vertical direction.
Vertical oscillation
Comb
Upper comb
Comb
(a) Single vertical comb structure
Lower comb
(b) Dual vertical comb structure
Figure 7. Schematic drawing of vertical comb structures.
Figure 8 shows the schematic drawing of the eye-type scanning mirror with dual vertical comb structures. It is
mainly composed of two parts. The upper structure is composed of vertical comb fingers (stationary electrodes), a
supporting frame, gold signal lines and pads on a trenched Pyrex glass substrate. The lower structure is composed
of a scanning mirror plate, two torsion bars, a supporting frame, vertical comb fingers (moving electrodes and
stationary electrodes), gold signal lines and pads on a Pyrex glass substrate. The diameter of the scanning mirror
plate is 1.0 mm, and the lengths of the major and minor axes of the outer frame are 2.5 mm and 1.0 mm,
respectively. The rotation hinges are square torsion bars with a thickness of 70 µm, which is the same as that of
the mirror plate. Considering the variance of the CMP process, we designed three kinds of spring length. Table 1
shows the design parameters and simulation results of the eye-type scanning mirror for HD-TV (1280 × 720P)
video image resolution. We assumed that the quality factor would be 20.
Proc. of SPIE Vol. 5721
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Gold signal line & pad
Supporting frame
Torsion bar
Moving comb electrodes
Stationary comb electrodes
Scanning mirror plate
Figure 8. Schematic drawing of the eye-type scanning mirror with dual vertical comb structures.
Table 1. Design parameters and simulation results of the eye-type scanning mirror.
Mirror
Type
Model
1
2
3
1
2
3
22.50kHz
22.50kHz
22.50kHz
22.50kHz
22.50kHz
22.50kHz
Spring
Comb
Width Height Thick Width Length Length Width
2500
2500
2500
2500
2500
2500
1000
1000
1000
1000
1000
1000
70.0
70.0
70.0
67.5
70.0
72.5
100
100
100
100
100
100
1400
1600
1800
1400
1600
1800
100
100
100
100
100
100
4
4
4
4
4
4
Beam
Frequency
Gap Width Bending Tilting Torsion
4
4
4
4
4
4
60
60
60
60
60
60
10,804
9,388
8,236
10,427
9,388
8,525
23,169
19,096
16,063
23,169
19,096
16,063
24,721
23,338
22,165
24,101
23,338
22,699
Actuation Angle
Static Dynamic(Resonance)
(Q=20)
0.44
9.63
0.49
10.21
0.55
10.76
0.48
10.24
0.49
10.21
0.50
10.14
Figure 9 shows the novel fabrication processes of the scanning mirror. The fabrication processes are divided into
two parts, the upper and lower structure process. The lower structure fabrication processes are as follows. A Pyrex
7740 glass is trenched in the shape of signal lines and pads by RIE (a). Cr/Au signal lines and pads are formed on
a Pyrex glass substrate along the etched grooves (b). A SOI wafer, which has a 2 µm-thick buried oxide layer
under 80 µm in depth, is etched to the oxide layer by ICPRIE (c). The SOI wafer is anodically bonded to the Pyrex
glass substrate. The bonded wafer is polished to leave a 152 µm-thick of silicon on a glass substrate (d). A
deposited Cr/Au layer is remained at the supporting frame for bonding with an electroplated AuSn solder layer of
the upper structure (e). After patterning of combs, the silicon is etched to 70 µm in depth by ICPRIE (f). Unit
devices of the lower structure are separated by means of dicing and then cleaned. The upper structure fabrication
processes are as follows. A Pyrex 7740 glass is patterned using a dry film resist and through-holes are fabricated
by the sand blasting method (g). A silicon wafer is etched to 25 µm in depth by ICPRIE (h). The silicon and glass
wafers are anodically bonded together. The bonded wafer is polished to leave a 95 µm-thick of silicon on a glass
substrate (i). Cr/Au signal lines and pads are formed on the trenched glass substrate (j). A Cr/Au seed layer is
deposited on the polished silicon surface for an electroplating of the AuSn solder layer which has a 3 µm in
thickness. The removal of the Cr/Au seed layer at the outside of the frame is followed (k). After patterning of
combs, silicon is etched to 80 µm in depth by ICPRIE (l). Unit devices of the upper structure are separated by
means of dicing and then cleaned. Finally, each unit device of the upper and the lower structures is aligned and
bonded by the flip chip bonding (m).
Figure 10 shows the upper and lower structures of the scanning mirror and Figure 11 shows the eye-type scanning
mirror prototype.
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Proc. of SPIE Vol. 5721
Lower structure
Upper structure
(a) Line groove etching
(g) Through-hole formation
(b) Cr/Au signal line formation
(h) Backside etching
(c) Stationary comb etching
(i) Anodic bonding & Polishing
(d) Anodic bonding & Polishing
(j) Cr/Au signal line formation
(e) Cr/Au layer formation
(k) AuSn electroplating
(f) Comb & Torsion bar etching
(l) Comb etching
(m) Assembly (Flip chip bonding)
Pyrex glass
Cr/Au
Si
SiO2
AuSn
Figure 9. Fabrication processes of the scanning mirror.
(a) Upper structure
(b) Lower structure
Figure 10. Photographs of the upper and lower structures.
Figure 11. Prototype of the eye-type scanning mirror.
Proc. of SPIE Vol. 5721
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Figure 12 shows resonant frequency of three type of the scanning mirror. The resonant frequency was measured in
the range of 22.1 ~ 24.5 kHz according to the spring length. We designed the scanning mirror with the quality
factor of 20, but we acquired around 60. Figure 13 shows the deflection angle of the scanning mirror with respect
to the applied voltage. The dc bias voltages were applied to the stationary comb electrodes of the upper and lower
structures, and the ac control voltage of a resonant frequency was applied to the moving comb electrodes of the
lower structure. As the driving voltage was increased, the scanning angle was also increased linearly. The
characteristic of linear control can be explained by the linear control scheme12. When the dc bias voltages are
applied to both comb electrodes of the lower structure with the opposite signs and the driving voltage is applied to
the moving comb electrode of the upper structure, the net moment of the scanning mirror can be expressed by
Resonant frequency(kHz)
τ = τ1 −τ 2 = α(Vcontrol +Vbias)2 −α(Vcontrol −Vbias)2 = 4αVcontrolVbias
26
24
22
20
1
2
3
Type
Figure 12. Resonant frequency of three types of the scanning mirror.
Optical scanning anlge (deg.)
35
30
25
20
15
10
5
0
0
10
20
30
40
50
60
70
80
Driving voltage (V)
Figure 13. Deflection angle of the scanning mirror.
where, α, Vcontrol and Vbias are force constant, control (driving) voltage and bias voltage, respectively. When the
scanning mirror is driven according to the control voltage, the bias voltages are maintained constant. Thus the net
moment, which is directly related to the scanning angle, is only controlled by the control voltage. linearly. Using
this scanning mirror, we acquired the optical scanning angle of 32° when driven by the 65 ~ 75 V ac control
voltage of the resonant frequency in the range of 22.1 ~ 24.5 kHz with the 100 V dc bias voltages.
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Proc. of SPIE Vol. 5721
3. LASER SCANNING DISPLAY
Scanning type laser display is mainly composed of lasers, modulators and scanners. Figure 14 shows the
demonstration setup for laser scanning display using an eye-type scanning mirror. In this demonstration setup, we
replaced the polygon scan mirror with an eye-type scanning mirror. Eye-type scanning mirror was used as a
horizontal scanner and a galvanometer was used as a vertical scanner. In case of lasers and modulators, installed
in the last laser TV prototype5 were used. Modulated RGB laser beams were inserted to the scanners and projected
to the screen.
Laser
Screen
Figure 14. Demonstration setup for laser scanning display.
Figure 15 shows the full color laser video image. We demonstrated XGA-resolution video image with the size over
30 inches. The image size can be increased more according to the projection distance and proper projection optics.
The brightness depends on the laser powers. So the larger image can be realized with high power lasers.
Figure 15. Full color laser video image using an eye-type scanning mirror.
Proc. of SPIE Vol. 5721
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4. CONCLUSION
Eye-type scanning mirror was designed and fabricated using a MEMS technology. And we obtained an optical
scanning angle of 32° with the resonant frequency of 22.1 ~ 24.5 kHz and the mirror size of 1mm. Full color laser
scanning display with XGA-resolution was demonstrated successfully using the eye-type scanning mirror.
To actualize the laser TV for home theater, laser TV must have a compact size, low cost and low power
consumption. Our ultimate goal is a palm-top-sized laser TV, and it can be accomplished by making small
components of lasers and scanners. In the near future, if the compact blue and green lasers are successfully
developed and become commercially available, laser TV can occupy home as the main media.
Future work is improving the performance of the scanning mirror on the resonant frequency up to 33.75 kHz and
mechanical scanning angle over 15° with a mirror size of 1.5 mm, to achieve the HD-TV (1920 × 1080P) video
image resolution.
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