Case Study 2: Digital Micromirror Devices (DMD) study-DMD.pdf · Case Study 2: Digital Micromirror...
Transcript of Case Study 2: Digital Micromirror Devices (DMD) study-DMD.pdf · Case Study 2: Digital Micromirror...
M. C. Wu 1
EE M250B/MAE M282/BME M250B
Case Study 2:Digital Micromirror Devices (DMD)
Chapter 20 of Senturia
“A MEMS-based projection display,” Van Kessel, P.F.; Hornbeck, L.J.; Meier, R.E.; Douglass, M.R., Proc. IEEE , Vol. 86 pp.1687 -1704 1998
http://www.dlp.com/
http://www.dlp.com/dlp_technology/dlp_technology_research.asp
Other Literatures on Course Website
M. C. Wu 2
EE M250B/MAE M282/BME M250B
Optical MEMS
• MEMS are well-suited for interaction with light– Structural dimensions ~ wavelength– Small displacement has large effect (e.g., ON-OFF switching)
• Interferometric devices : ∆∆∆∆d ~ 0.25 λλλλ• Scanning devices : ∆θ∆θ∆θ∆θ ~ a few degrees
– Photon has no mass• Does not need large-force actuators
– MEMS enables large-scale systems• E.g., 1000x1000 display or optical switches
M. C. Wu 3
EE M250B/MAE M282/BME M250B
Digital Micromirror Device (DMD) --Texas Instruments
~ 1 million DMD’s on a chip
http://www.dlp.com/dlp/resources/dmmd.asp
M. C. Wu 4
EE M250B/MAE M282/BME M250B
Schematic of TI’s DMD
M. C. Wu 5
EE M250B/MAE M282/BME M250B
Principle of Projection SystemUsing DMD
M. C. Wu 6
EE M250B/MAE M282/BME M250B
Digital Micromirror Device (DMD)Texas Instruments
Top View of DMD
L. Hornbeck, Electronic Imaging, 1997
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EE M250B/MAE M282/BME M250B
MEMS-Based Projection Display
• High brightness• High contrast• Grey scale achieved by digital modulation (DMD) or analog
control (SLM)• Compact, light weight, low power• Particularly attractive for portable system
M. C. Wu 8
EE M250B/MAE M282/BME M250B
Early Development of MEMS Spatial Light Modulator
for Display Applications
M. C. Wu 9
EE M250B/MAE M282/BME M250B
Westinghouse Mirror-Matrix DeviceThomas, Westinghouse, 1975
R. N. Thomas, “The mirror matrix tube: A novel light valve for projection display,” IEEE T-ED, Vol. ED-22, pp.765-775, 1975.
• Surface micromachined• Built on SOS (Sapphire)• Addressed by electron beam• Deflect up to 4°• Contrast ratio: 10 to 1• Difficulty in HNA release etch• Limited resolution in electron
beam
M. C. Wu 10
EE M250B/MAE M282/BME M250B
2D Display with Electrostatic Cantilever Light Modulator Array
Petersen, IBM, 1977
• 16-element cantilever light modulator is used in conjunction with a galvanometer to scan the modulated beam across the screen
Petersen, K. E., “Micromechanical light modulator array fabricated on Silicon,” Appl. Phys. Lett., vol. 31, pp.521-3, 1977.
M. C. Wu 11
EE M250B/MAE M282/BME M250B
Silicon Cantilever Light ModulatorPetersen, IBM, 1977
Petersen, K. E., “Micromechanical light modulator array fabricated on Silicon,” Appl. Phys. Lett., vol. 31, pp.521-3, 1977.
• SiO2 structural layerSi sacrificial layerP++ Doped stop etch layer
• Cantilever beams biased by voltage
• Electrically actuated• Individually addressable• Pull-in observed
M. C. Wu 12
EE M250B/MAE M282/BME M250B
Silicon Torsional Electrostatic Light ModulatorsPetersen, IBM, 1980
• Stress in torsion beams is one order of magnitude smaller than fracture stress of single crystal Si
• Over 1012 cycles were demonstrated with ± 1° deflection
Petersen, K. E., “Silicon torsional scanning mirror,” IBM J. R&D, vol. 24, pp.631-7, 1980.
M. C. Wu 13
EE M250B/MAE M282/BME M250B
MEMS Optical Display (1)
Digital Micromirror DeviceTM
Texas Instruments
Ref: Larry J. Hornbeck, “Digital Light Processing™: A New MEMS-Based Display Technology” (http://www.dlp.com/dlp_technology/images/dynamic/white_papers/117_Digital_Light_Processing_MEMS_display_technology.pdf )
M. C. Wu 14
EE M250B/MAE M282/BME M250B
Before DMD was born, there was DMD (Deformable Mirror Device)
• First reported in 1980• Addressed by underlying
array of MOS transistor• DRAM like architecture
– Mirror response ~ 25 µµµµs– Floating source hold time ~ 200 ms
• Array size: 128 x 128• Pixel size: 51 µµµµm x 51 µµµµm• Air Gap: 620 nm• Active area ratio: 32%
• Ref: Larry Hornbeck, “128x128 Deformable Mirror Device”, IEEE Trans. Electron Devices, p. 539, 1983
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EE M250B/MAE M282/BME M250B
Early DMD’s
Hornbeck, “Deformable Mirror Spatial Light Modulator” SPIE Vol. 1150, p. 86, 1989
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EE M250B/MAE M282/BME M250B
Cantilever-Beam DMD’s
Fabricated by anisotropic wet etching
Fabricated by spun-on spacer and plasma etch
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EE M250B/MAE M282/BME M250B
Cantilever vs Torsion Beam DMD’s
• Torsion-Beam DMD• Symmetric torsion beam device
has amplitude-dominant modulation
• Cantilever-Beam DMD• Balanced flexure device has
phase-dominant modulation
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EE M250B/MAE M282/BME M250B
DMD Fabrication
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EE M250B/MAE M282/BME M250B
DMD Features and Requirements
Number of moving parts 0.5 to 1.2 million Mechanical motion Makes discrete contacts or landings Lifetime requirement 450 billion contacts per moving part Address voltage Limited by 5 volt CMOS technology Mechanical elements Aluminum Process Low temp., sputter deposition, plasma etch Sacrificial layer Organic, dry-etched, wafer-level removal Die separation After removal of sacrificial spacer Package Optical, hermetic, thermal vias Testing High-speed electro-optical before die
separation
L. Hornbeck
M. C. Wu 20
EE M250B/MAE M282/BME M250B
Fabrication Process for Basic DMD
• Aluminum alloy-based surface-micromachining process
• 2~3 µµµµm organic sacrificial layer (it also planarize the surface)
• Hinge: Aluminum alloy, typically 500 to 1000 angstrom thick
• Mirror: Aluminum alloy, typically 3000 to 5000 angstrom thick
• Dry releasing in isotropic plasma etching
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EE M250B/MAE M282/BME M250B
Digital Micromirror Device (DMD)Texas Instruments
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EE M250B/MAE M282/BME M250B
Detailed Layer Structure of DMD
L. Hornbeck, Electronic Imaging, 1997
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EE M250B/MAE M282/BME M250B
Fabrication of DMD
• Use aluminum instead of polysilicon as structural material→ Completely compatible with CMOS
• Use DUV-hardened photoresist for sacrificial material→ Dry releasing in Plasma Etcher to reduce stiction
• Aluminum alloys are used to improve performance– Al mirror may contains a small fraction of Cu and Si– One report mentioned 0.2% Ti, 1% Si is added to Al
hinges. Al compounds for anti-creep were discussed.• DMD superstructure built on CMOS memory (SRAM) circuit• 6 photomask layers
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EE M250B/MAE M282/BME M250B
Fabrication Process Flow
M. C. Wu
EE M250B/MAE M282/BME M250B
TI’s Digital Micromirror Devices (DMD)
(Texas Instruments, Digital Micromirror DeviceTM)
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EE M250B/MAE M282/BME M250B
DMD Analysis
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EE M250B/MAE M282/BME M250B
(1) Energy Domain Model
20
*
21
)( CVW =θVoltage-controlled actuation Co-energy
Calculate capacitanceV
QC =
Electrostatic potential distribution:
−=
0
1)(θθθφ V
Electric field distribution: θθ
ˆ0r
VE =r
Total charge on electrode: ∫ ⋅=electrode
AdEQrr
0ε
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EE M250B/MAE M282/BME M250B
Calculation of Capacitance
+−
−=
+−
−=
=⋅= ∫∫−
+−
01
01
0
0
1
1
0
0
)(0
00
tan1
tan1
ln
)(ln
)(1
1
θ
θ
θε
θε
θεε
gLx
gx
Vw
LxP
xPVw
drr
rwidthVAdEQ
xP
LxPelectrode
rr
Assume constant width
0tan/ θgP =Using
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EE M250B/MAE M282/BME M250B
Approximate Solution:Stable Angle and Pull-In Voltage
3
1
2
23
23
0
02031
2
0
0*
30301
2
0*
03010
3)0(3)0(3
)3(2)0()(
)1(2)0(
)(
)1()0()(
a
a
VCa
k
VCa
k
kaaVCW
aaVC
W
aaCC
−
±−=
=+−=∂
∂−=
++⋅=
++⋅=
θθ
θ
θ
θθθ
θτ
θθθ
θθθ
Cubic polynomial fit of capacitor:
Real solution 3
1
2
23 3)0(3 a
a
VCa
k ≥
θ
Pull-in Voltage 41
231 )0(3
=
Caa
kVPI
θ
M. C. Wu 30
EE M250B/MAE M282/BME M250B
(2) Parallel Plate Model by Hornbeck
dxzz
Vx
La
a 20
'
0
2
)(
1
2 −⋅⋅⋅= ∫
ετ
−
+−⋅⋅=αβ
αβαβαθ
ετ1
)1ln(1
tan2 22
2
M
aa
WV
Mθθα
tantan=
L
L'=β
Normalized angle
Actuator loading factor
Bias Electrode-1
Bias Electrode-2
Landing Electrode
Landing Electrode
V2 V1
L’
L
xz
Attractive torque
M. C. Wu 31
EE M250B/MAE M282/BME M250B
0=+ Sa ττ
Restoring Force from Torsion Beam
• Restoring torque
• Equilibrium
• Solve α (α (α (α (normalized angle) for any given voltage
CS
θτ −= C: torsion compliance
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EE M250B/MAE M282/BME M250B
Graphic Solution of Torsion Mirrors
• The solution is the intersection point of the two curves corresponding to electrostatic torque and spring restoring torque
– Low voltage (Curve A): two intersection points but only the smaller angle solution is stable
– Critical voltage (Curve B): the two curves are tangential and there is only one intersection point called pull-in angle or snap-down angle
– High voltage (Curve C): there is no intersection no stable solution
IncreasingVoltage
Angle αααα
To
rqu
e
Restoring Torque
Electrostatic Torque
ABC
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EE M250B/MAE M282/BME M250B
Transfer Characteristics of Torsion Mirrors
Voltage
An
gle
BistableRegime
AnalogRegime
Pull-inVoltage
CWL
zVS 3
302
ε⋅=
Pull-in voltage(or Stiffness Voltage)
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EE M250B/MAE M282/BME M250B
Temporal Response of DMD
• Analog operation• Resonant frequency ~ 50 kHz
• Digital operation• Step response time ~ 12 µµµµs
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EE M250B/MAE M282/BME M250B
DMD Addressing
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EE M250B/MAE M282/BME M250B
Addressing Scheme for DMD
L. Hornbeck, Electronic Imaging, 1997
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EE M250B/MAE M282/BME M250B
DMD Address and Reset Sequence
All memory cells under DMD has been loaded
All DMD’s reset in parallel
Allow mirror to release and begin to rotate
Loading memory cells for new mirror positions
Bias turned on to latch the mirrors in 10° or -10°
Field applied to yoke and mirrors
M. C. Wu 38
EE M250B/MAE M282/BME M250B
DMD Bias Cycles
∆∆∆∆V=19
∆∆∆∆V=33.5
∆∆∆∆V=0
∆∆∆∆V=16.5
∆∆∆∆V=19
∆∆∆∆V=24
∆∆∆∆V=26
∆∆∆∆V=7.5
∆∆∆∆V=24
∆∆∆∆V=24 ∆∆∆∆V=24
∆∆∆∆V=24
∆∆∆∆V=24
∆∆∆∆V=26
∆∆∆∆V=7.5
∆∆∆∆V=19
∆∆∆∆V=33.5
∆∆∆∆V=0
∆∆∆∆V=16.5
∆∆∆∆V=19
Va=5
Va=5
Va=7.5
Va=7.5
Va=7.5
Va=0
Va=0
Va=0
Va=0
Va=0
Va=0
Va=0
Va=0
Va=0
Va=0 Va=5
Va=5
Va=7.5
Va=7.5
Va=7.5
Vb=24
Vb=24
Vb=24
Vb=7.5
Vb=- 26
Vb=24
Vb=24
Vb=24
Vb=7.5
Vb=- 26
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EE M250B/MAE M282/BME M250B
Bias and Switching Waveforms
M. C. Wu 40
EE M250B/MAE M282/BME M250B
Electrostatic Torques on DMD
M. C. Wu 41
EE M250B/MAE M282/BME M250B
SEM of DMD before Deposition of Mirror
L. Hornbeck, Electronic Imaging, 1997
Spring Tip to reduce stiction
• A reset pulse is applied to the mirror and yoke, causing the spring tip to flex.
• As the spring tips unflex, the reaction force producing a reliable release from the surface.
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EE M250B/MAE M282/BME M250B
Efficiency of DMD Projectors
Light ProjectorEfficiency
DMD PixelEfficiency
L. Hornbeck, Electronic Imaging, 1997
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EE M250B/MAE M282/BME M250B
Grey Scale of DMD Projector:Pulsewidth Modulation Technique
L. Hornbeck, Electronic Imaging, 1997
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EE M250B/MAE M282/BME M250B
DMD Systems
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EE M250B/MAE M282/BME M250B
Projection Display UsingDigital Micromirror Display (DMD)
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EE M250B/MAE M282/BME M250B
DMD Projection System with2 DMD Chips
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EE M250B/MAE M282/BME M250B
DMD Projection System with3 DMD Chips
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EE M250B/MAE M282/BME M250B
Schematic of Projection Display System with 3 DMD’s
L. Hornbeck, Electronic Imaging, 1997
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EE M250B/MAE M282/BME M250B
Micromirror Array with Vertical SpringSouel National University, Korea
• Spring hidden under mirror– Large reflecting area, high
fill factor (84% achieved, 91% theoretical)
• Simpler fabrication process– One mask– One shadow evaporation
• 50µµµµm x 50 µµµµm Al micromirror• Pull down voltage = 8 V• Response time = 20 µµµµsec
(with 29 V applied)• Resonant frequency = 11 kHz