Case Study 2: Digital Micromirror Devices (DMD) study-DMD.pdf · Case Study 2: Digital Micromirror Devices (DMD) ... voltage • Electrically ... (

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


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


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


Schematic of TI’s DMD

M. C. Wu 5


Principle of Projection SystemUsing DMD

M. C. Wu 6


Digital Micromirror Device (DMD)Texas Instruments

Top View of DMD

L. Hornbeck, Electronic Imaging, 1997

M. C. Wu 7


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


Early Development of MEMS Spatial Light Modulator

for Display Applications

M. C. Wu 9


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


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


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


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


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


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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Early DMD’s

Hornbeck, “Deformable Mirror Spatial Light Modulator” SPIE Vol. 1150, p. 86, 1989

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Cantilever-Beam DMD’s

Fabricated by anisotropic wet etching

Fabricated by spun-on spacer and plasma etch

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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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DMD Fabrication

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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


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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Digital Micromirror Device (DMD)Texas Instruments

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Detailed Layer Structure of DMD


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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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Fabrication Process Flow

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TI’s Digital Micromirror Devices (DMD)

(Texas Instruments, Digital Micromirror DeviceTM)

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DMD Analysis

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(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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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

M. C. Wu 29


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


(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


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

M. C. Wu 32


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

M. C. Wu 33


Transfer Characteristics of Torsion Mirrors

Voltage

An

gle

BistableRegime

AnalogRegime

Pull-inVoltage

CWL

zVS 3

302

ε⋅=

Pull-in voltage(or Stiffness Voltage)

M. C. Wu 34


Temporal Response of DMD

• Analog operation• Resonant frequency ~ 50 kHz

• Digital operation• Step response time ~ 12 µµµµs

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DMD Addressing

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Addressing Scheme for DMD


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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


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

M. C. Wu 39


Bias and Switching Waveforms

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Electrostatic Torques on DMD

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SEM of DMD before Deposition of Mirror


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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Efficiency of DMD Projectors

Light ProjectorEfficiency

DMD PixelEfficiency


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Grey Scale of DMD Projector:Pulsewidth Modulation Technique


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DMD Systems

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Projection Display UsingDigital Micromirror Display (DMD)

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DMD Projection System with2 DMD Chips

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DMD Projection System with3 DMD Chips

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Schematic of Projection Display System with 3 DMD’s


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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

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Case Study 2: Digital Micromirror Devices (DMD) study-DMD.pdf · Case Study 2: Digital Micromirror Devices (DMD) ... voltage • Electrically ... (