introduction to microeletromechanical systems (mems)horizontal, deformable) c mnec•us mumps ......

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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


MEMS Overview

Micromachining: lithography, deposition, etching, …

Processes & Foundries

Devices & Structures

Methodology

History & Market

Introduction &

Background

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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


Grating Light Valve

33


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]


Grating Light Valve

44


Grating Light Valve


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)

55


Grating Light Valve


Grating Light ValveDifferent GLV Arrangements

66


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


CCR Design

Specifications:• 500 m distance• 56 kbps

Issues:• Switching speed• Accuracy• Diffraction• Size

Parallel Beams

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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


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

88


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


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

99


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


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


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


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

1111


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?


MEMS Light Modulators

Modulator Type Motion Side View

Cantilever Bending

Torsional Plate Rotation

Membrane Drumhead

Suspended Plate Vertical

[Kovacs, 1998, p.462]

1212


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)


Micromirror on Crystal Planes

• MOEMS reflectors and beam splitters [Rosengren et al., 1994]

• V-groove for fiber positioning

Beam Splitter

1313


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


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

1414


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]

introduction to microeletromechanical systems (mems)horizontal, deformable) c mnec•us mumps ......

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