Post on 22-May-2018
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Texas Christian University Department of Engineering Ed Kolesar
Introduction toMicroeletromechanical Systems
(MEMS)Lecture 9 Topics
• MicroOptoElectroMechanical Systems (MOEMS)Grating Light ValvesCorner Cube Reflector (CCR)MEMS Light ModulatorOptical SwitchMicromirrorsTunable IR FilterInterferometryField Emission Display
Texas Christian University Department of Engineering Ed Kolesar
MEMS Overview
Micromachining: lithography, deposition, etching, …
Processes & Foundries
Devices & Structures
Methodology
History & Market
Introduction &
Background
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Texas Christian University Department of Engineering Ed Kolesar
Grating Light Valve
MEMS display array fabricated in CMOS compatible process.
Each pixel is made up of multiple ribbon-like structures, which can be moved up or down over a very small distance (only a fraction of the wavelength of light) by controlling electrostatic forces.
The ribbons are arranged such that each pixel is capable of either reflecting or diffracting light.
Invented by D. Bloom (Stanford)Developed since 1994 by Silicon Light Machines,
Sunnyvale CA, www.siliconlight.comAcquired by Cypress Semiconductors 7/2000
Exclusive license with Sony
Texas Christian University Department of Engineering Ed Kolesar
Grating Light Valve
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Texas Christian University Department of Engineering Ed Kolesar
Grating Light Valve
• Ribbons at same height: reflection
• Ribbons at height difference d: diffraction; no reflection when d=λ/4 (maximum diffraction)
• Intensity of 1st order diffraction lobes:
2d
Reflection
Diffraction
d
Movable Ribbons
( )intensity diffractedorder 1st maximum
2sin
max
2max1
IdII λπ=
[D.T. Amm and R.W. Corrigan 1998]
Texas Christian University Department of Engineering Ed Kolesar
Grating Light Valve
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Texas Christian University Department of Engineering Ed Kolesar
Grating Light Valve
Texas Christian University Department of Engineering Ed Kolesar
Grating Light Valve
• Design features:Usually 6 ribbons per pixel: 150µm x 200µm Very high switching speedsSwitching: thresholds, hysteresisCMOS fabrication compatible (Cypress Semiconductor)
• Allows a variety of system architectures, including very simple designs:
2D pixel array (passive array)1D scan lineAnalog display (note: non-linear transfer function, snap-in point)
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Texas Christian University Department of Engineering Ed Kolesar
Grating Light Valve
Texas Christian University Department of Engineering Ed Kolesar
Grating Light ValveDifferent GLV Arrangements
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Texas Christian University Department of Engineering Ed Kolesar
Corner Cube Reflector (CCR)
• Corner cube retroreflectorreflects light directly back down its incident path.
• Common in bicycle and road reflectors, and on the moon.
• [Comtois and Bright 1996]
• [Chu, Lo, Berg and Pister 1997] 1 kbps over 100 meters
Parallel Beams
Texas Christian University Department of Engineering Ed Kolesar
CCR Design
Specifications:• 500 m distance• 56 kbps
Issues:• Switching speed• Accuracy• Diffraction• Size
Parallel Beams
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Texas Christian University Department of Engineering Ed Kolesar
CCR Design
Schematic Design:
• Reflective material: Au
• 2 hinged plates (locked in vertical position)
• 1 membrane (horizontal, deformable)
• Use MCNC MUMPs (3 layer polysilicon process)
silicon nitrate poly0 poly1 poly2
Membrane
Hinged Plate
Texas Christian University Department of Engineering Ed Kolesar
CCR Design
Determine minimum size of plates and membranes:
• Want ring diameter of 1st diffractive minimum < 1 mλ / d = sin θ ≈ tan θ = 0.5/500 = 10-3
d = λ 103 = 222 10-6 m = 222 µm(KrCl excimer laser)
CCR
Laser
Detector (Ø 1m)
Distance 500 m
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Texas Christian University Department of Engineering Ed Kolesar
CCR Design
Accuracy of mirror plates:
• Center of reflected beam should lie within 1 m radiusθ ≈ tan θ = 0.5/500 = 10-3 = 1 mrad ≈ 0.057°
CCR
Laser
Detector (Ø 1m)
Distance 500 m
Texas Christian University Department of Engineering Ed Kolesar
CCR Design
Mirror deflection:
• Gap: oxide1 (2 µm) + oxide2 (0.75 µm) = 2.75 µm
• Radius of curvature:
silicon nitrate poly0 poly1 poly2
Membrane Thickness, d
Gap, g
mm22482)2()(
2
222
=+=
+−=
gdgrdgrr
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Texas Christian University Department of Engineering Ed Kolesar
CCR Design
Membrane (bridge)• Deflection:
• Required force (conservative estimate):
ρ
ρ
ρρ
ρ
⋅⋅≈
⋅⋅⋅⋅⋅⋅⋅=
=
=
−=
−
−−−
Nm7
366946
3
4
22
2
106
)12)m105.1(m10222Pa10169384()m10222(inertia ofmoment bending 12
(pressure)density force modulus, sYoung' 384)2(
))(24/()(
y
ywtI
EEIddy
xdEIxxy
mN740get weµm2for 106/ N
m7 2
.FgydydF
===
⋅⋅=⋅= −ρ
Texas Christian University Department of Engineering Ed Kolesar
CCR Design
Electrostatic actuation• Capacitance:
• Required voltage:
• This voltage is too high!solutions:
More accurate analysis of required force (non-linear, pull-in point, …)Design modifications
2mF82
21
626122
1088.2
pF159.m1075.2)m10222(mF1085.8
VgVCFgdC
⋅⋅==
=⋅⋅⋅==−
−−−ε
V1601088.2
N1074.0mF8
3=
⋅⋅= −
−V
1010
Texas Christian University Department of Engineering Ed Kolesar
CCR Design
Switching speed• Estimate spring constant:
• Calculate mass:
• Resonance frequency:
• 56 kbps is feasible!
mN370µm2mN74.0/ === yFK
kHz2332/
2
21 ≈==
==
mKf
mKf
ππω
πω
µg17.0kg105.12221033.2 18232 =⋅⋅⋅⋅=== −tdVm SiSi ρρ
Texas Christian University Department of Engineering Ed Kolesar
CCR Design
Power• Estimate work for switching membrane:
• Power:
• Compare with energy stored in capacitor
nJ4.1Nm)1075.2(370 26212
021
0
≈⋅⋅==== −∫∫ KgKsdsFdsEgg
µW78kHz56nJ4.1 =⋅== EfP
mW154kHz56µJ75.2µJ752J186101590 29
212
21
=⋅=
≈⋅⋅== −
P..CVE
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Texas Christian University Department of Engineering Ed Kolesar
CCR Design
Other assumptions:
• Very smooth mirror surfaces• No obstacles, no fog• Is deflected membrane really curved, or mostly flat?
Other problems:
• If we reduce the required voltage by reducing the membrane stiffness, then we also reduce the resonance frequency and the bit rate
• What else?
Texas Christian University Department of Engineering Ed Kolesar
MEMS Light Modulators
Modulator Type Motion Side View
Cantilever Bending
Torsional Plate Rotation
Membrane Drumhead
Suspended Plate Vertical
[Kovacs, 1998, p.462]
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Texas Christian University Department of Engineering Ed Kolesar
Optical Switches
MEMS Optical Switches: modulation of light w/o conversion to/from electrical signal
• All optical network switch: 256 input and output channels
• Switch time ca. 10 ms• 16 x 16 micromirror array• MUMPs like polysilicon process• Electrostatic 2DOF actuation
with position feedback
• “MicroStar Technology” (Lucent Bell Labs 1999)
Texas Christian University Department of Engineering Ed Kolesar
Micromirror on Crystal Planes
• MOEMS reflectors and beam splitters [Rosengren et al., 1994]
• V-groove for fiber positioning
Beam Splitter
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Texas Christian University Department of Engineering Ed Kolesar
Tunable IR Filter
• Parallel plate array polarizes light of wavelength λ > 2d
• Two orthogonal arrays act as filter with cutoff frequency λ = 2d
• Pull on structure to increase cutoff wavelength
• [Ohnstein et al., 1995 and 1996]
Flexures
Plates
dF
Texas Christian University Department of Engineering Ed Kolesar
Interferometry
• Basic idea:two plates at distance d = n λwith n small (usually 1)
• Constructive interference for all wavelengths λ = d/n
• λ for visible light (or IR, UV) is within range of many micromachined thin film thicknesses
• Refractive indices can be varied from 1.38 (MgF2) to 2.4 (TiO2)
Semi-transparent Mirrorsd
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Texas Christian University Department of Engineering Ed Kolesar
Field Emission Display
Strong electrostatic field pulls electrons off the sharp tips and accelerate them towards display
[See for example, W. Hofmann, L.-Y. Chen, J. H. Das and N. C. MacDonald, 1996]