Center for Adaptive Optics15 Nov 1999 Meeting

Major William D. Cowan, Ph.D.Air Force Research Laboratory

Materials and Manufacturing Directorate, AFRL/MLWright-Patterson AFB, Ohio 45433

Microfabricated Segmented Micromirror Arrays

Microfabricated Segmented Micromirror Arrays

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2

• Introduction• Foundry Processes• MUMPs 19 MEM-DM• Continuous Facesheet Designs• Micromirror Surface Figure• Proposed CfAO SUMMiT Design

Overview

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Problem: Make practical deformable mirrors (DMs) for adaptive optics (AO) in foundry microfabrication processes

DMs among the most expensive components in AO systems: $1000/channel

Microelectromechanical systems (MEMS) ideally suited for optical applications - deflections consistent with optical wavelengths - photolithographic (parallel) fabrication of parts with identical characteristics

Deflection uniformity critical for low cost AO (eliminate 100% testing)

Use foundry fabrication processes to reduce cost for low volume applications

Lessons learned applicable to specialized microfabrication processes

(Reduce cost, size, weight, power dissipation)

Introduction

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Foundry Process Descriptions

SUMMiT MMPOLY3(2 m) SACOX3(1.5-2 m, CMP) MMPOLY1+2(2.5 m) SACOX1(2 m) MMPOLY0(0.3 m) SiN(0.8 m) Oxide(0.6 m) Substrate

MUMPs Metal(0.5 m) Poly2(1.5 m) Oxide2(0.75 m) Poly1 (2.0 m) Oxide1(2 m) Poly0(0.5 m) SiN(0.6 m) Substrate

Trade fill factor, mirror size, array size(wiring depth)Self-planarization may help fill factor

Planarization decouples mirror and actuator design

etch access holes

$3k2 mos.

$10k? mos.

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partial Poly2 self-planarization

1.5m wide Poly1 anchors

Incomplete etch of 1.5m wide Poly1 gap

wiring(MMPOLY0)anchor

actuator upperelectrode(MMPOLY1+2)

flexure

mirror (MMPOLY3)

mirror to actuator vias

etch access holes3 m 3 m

MUMPs self-planarization SUMMiT with CMP Planarization

MUMPs vs. SUMMiT Planarizaton

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Deflection of electrostatic piston micromirror

dV A

k t d=

−ε 2

22 ( )

Electrostatic Piston Micromirror

d

gt

Movable top electrode

Fixed bottom electrode

k, spring constant

V

A

top electrode, mirror plate

flexure anchor

to bottom electrode

k is a function of flexure number, geometry, andmaterial stiffness (note how unidirectional layout mitigates the effect of residual stress)t is fixed by sacrificial layer thickness of processd is defined by optical modulation requirementsTrade k and A for desired V, uniformity, yield, etc.

, for d=0 to ~t/3

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Testing Piston MicromirrorsGood deflection uniformity on die (wafer) but not necessarily die to dieDynamic laser interferometer testing is expensive in time/complexity

Def

lect

ion

(n

m)

0 5 10 15 200

50

100

150

200

250

300

350

Control voltage (V)

Static fringemeasurementV316=18 V

dynamic laser interferometermeasured modeled

Static fringe technique developed for interferometric microscope is very fastSimple procedure: Toggle electrode voltage between 0 and V, fringe lines appear static for deflection=/2, ,…, where is test wavelength

Interferometric microscope video also provides rapid characterization of yield and deflection uniformity

With good fit to model, only needone data point for characterization

Only need one data point from onedevice in an array

But why not model this simplestructure and avoid characterizationtesting?? Material Properties??

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Segmented MEM-DM (M19)

1212 Array203 m center-to-center mirror spacingStroke ~0.6 mTrapped oxide platePoly0 wires under flexuresPost foundry metallization requiredFill Factor: ~77%

M19 Piston Micromirror Element

M19 MEM-DM Image

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HeNeM1

Iris

BS1

Ll

Ls

MEM-DM

MEMSControl

PC

BeamExpander

AberratingLens

La

Lt1LF

LMBS2

PSF

Image

ImageCamera

PC

Lt2

PSFCamera

PC

OpticalAttenuator

OpticalPower Meter

Lw1

Lw2

Adaptive Optics Test Bed

M19 Optical Measurements

Optical input power normalized using attenuator and power meterIncrease magnification of far field pattern on PSF cameraPSF camera frame rate used to scale measured intensities

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M19 MEM-DM Aberration Correction

Incident Optical Signal

Plan

eR

OC

=0.

70 m

ROC=0.35 mR

OC

=1.

60 m

Plane ROC=0.80 m

ME

M-D

M F

igu

re 1.0 (208@500 Hz) 0.07 (174 @40 Hz) 0.04 (108@40 Hz)

0.09 (96@99 Hz) 0.76 (158@500 Hz) 0.05 (121@40 Hz)

0.18 (91@203 Hz) 0.04 (97 @40 Hz) 0.27 (115 @244 Hz)

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M19 MEM-DM Demo

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Status of MUMPs 19 Design

Still have the same device operating in the AFIT AO testbed• Approaching 2 years of intermittent operation exposed to laboratory air• Stan Rogers using to demonstrate phase retrieval

Delivered 2 packaged devices to Dr Wild and Dr Kibblewhite at University of Chicago, Yerkes Observatory

• Don’t know status of their work, but recently had inquiry from MEMS Optical who had seen MUMPs 19 devices while visiting U of C

May have a couple left - have been requested by USAF Academy

For quick (~4 months), moderate performance, low-cost devices this design can be shoehorned into a 0.5 cm square die with 4 copies per MUMPs die site

• Will yield >50 devices for $3k + packaging costs• Still need post foundry metallization

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0 203 m 406 m 0 203 m 406 m

Hei

ght (

nm)

Hei

ght (

nm)

378.2

1349.0

360.

7

300.

6

240.

5

180.

4

120.

1

60.1

1213

.5

1011

.3

202.

340

4.5

606.

880

9.0

Heights (nm)

18 V 21 V

MUMPs Continuous Facesheet DM Influence Function

Interferometric Microscope Image Observed actuator coupling ~40% in good agreement with predicted

Actuator Couplingk

kact

FS

= 1

4 1

100%⏐⏐⏐

⏐↵√+

↔

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Single element of MUMPs 21 CF DM 144 actuatorsWired as a defocus corrector - elements equidistant from center are connectedOnly 16 voltages requiredActuators can flatten residual stress induced deformation

0 V applieddeformation due to residual stress

21 V applied to center 4 elements

21 V applied to 8 elements

Interferometric microscope images of MUMPs 21 DM center

etch holes

print-through ofactuator structure

MUMPs 21 CF MEM-DM

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Potential applications - optical aberration correction - laser communication - direct write photolithography - laser machining - consumer electro-optics

Optical Efficiency/Imaging Performance

- fill factor (% reflective surface area) - mirror surface figure -- curvature -- print through -- reflectivity - array surface figure (uniformity)

IdealCurvatureFF<100%

Print-through

Micromirrorarray surface

Far field

Micromirror Surface Figure

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1212Trapped oxide platePoly0 wires under flexuresPost foundry metalFill Factor: 77%

M19 M19_A

M19_B M19_C

88 Trapped oxide plateMUMPs metalFill Factor: 67.4%

88 Trapped oxide platePost foundry metalFill Factor: 67%

88 Poly2 mirror plate attached to actuator by viasPost foundry metalFill Factor: 71.9%

MUMPs Mirror DesignsAll arrays employ 203 m center-to-center mirror spacing

MUMPs flexures 4 m wide for better yield and deflection uniformity

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SUMMiT Mirror Design

wiring(MMPOLY0)

anchor

actuator upperelectrode(MMPOLY1+2)

flexure

metallization stop &actuator interconnect

mirror (MMPOLY3)

mirror to actuator vias10 m 10 m

etch access holes3 m 3 m

As-drawn fill-factor: 95%Post foundry metallization required

203 m center-to-center mirror spacing

gap 3 m

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Micromirror Surface Characterization

Instrument:Zygo Maxim 3-DLaser interferometric microscopeAccuracy: 3 nm RMSManual scan of mirror middle to get Peak-to-Valley (PV)

MUMPs devices - only Poly0 electrode under mirror - curvature due to residual material stresses in plate structure - metal ~50 MPa tensile - polys ~10 MPa compressive - trapped oxide ?

M19_A

False color image of surface height

Mesh of surface figure

Scan line

PV=303 nm

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Micromirror Surface Characterization

False color image of surface height

Mesh of surface figure

Scan line

PV=291 nm

SUMMiTSUMMiT - design employs actuator and wiring under mirror plate - planarization incomplete - print-through of underlying structures - some residual stress curvature

Zygo results confirmed by checking an unreleased die on stylus surface profilometer

Note!:Devices fabricated on early SUMMiT runsPlanarization targeted at mechanical vice optical flatnessSandia has now fixed problem (new SUMMiT Optical process)

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Image

PSF

Optical Perf vs. Micromirror FigureM19_A MUMPs Metal303.4 nm PV concave

M19_B AFIT Metal55.6 nm (convex)

SUMMiT AFIT Metal 291.1 nm PV

(print-through + concave)<

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Reflected Optical PSFMirror Description Optical Efficiency Peak Intensity Effective FWHM Power Normalized Normalized Fill Factor Normalized

% % % % % MUMPs Plane Mirror 76.3 100 100 100 100M19 No Metal 29.2 38.3 5.2 22.8 104M19_A MUMPs Metal 56.9 74.5 0.6 10.0 221M19 AFIT Metal 1 62.6 82.0 24.9 49.9 98M19 AFIT Metal 2 60.8 79.7 25.8 50.8 99M19_B AFIT Metal 53.6 70.2 35.7 59.8 105M19_C AFIT Metal 30.0 39.3 7.8 28.0 117SUMMiT No Metal 44.0 57.7 7.8 28.0 116SUMMiT AFIT Metal 1 66.0 86.5 7.2 26.7 109SUMMiT AFIT Metal 2 67.5 88.4 6.9 26.2 111

Optical Measurement Summary

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Surface Figure Study Results

Fill factor and optical efficiency (power) not good metrics- don’t measure imaging performance

Surface figure is most important factor for imaging performance

“Good” polysilicon piston micromirror arrays require- planarization- residual stress control/characterization

Sputtered chromium/gold metallization promising

Proposed fabrication approach- design in an initial convex curvature using residual stresses- sample lot (release and measure curvature)- design metallization to yield flat mirror surfaces- metallize lot

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Latest SUMMiT Optical Design

32 32 Array of segmented micromirrors (1024 total)100 m pitch (center-to-center), Nominal fill-factor ~95% Employs unproven Row-Column address scheme

• Only 2N wires for N2 array• Wiring limits maximum array size in foundry processes • Row-Column (line) addressing demonstrated for bistable mirror arrays• Pulse width & pulse amplitude modulation also demonstrated

(Rounsaval AFIT thesis)

StatusOnly a few samples tested - 15 min partial, 30, 45 min release etches (1:1, HF:HCl)Mirror element flatness <30 nm peak to valleyUnreleased array(s) shows global convex curvature

• May be artifact of CMP process, or residual stress in oxide• Probably can minimize by design “tricks”• Can also correct out or “flatten” array in use

Discuss findings with Sandia to determine cause/fix

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SUMMiT 32 x 32 Row/Col Array

One array so far had problems with MMPOLY3 attachment to underlying actuators

•May suggest non-uniformity of CMP oxide thickness across wafers•Have heard CMP “wedge” problem anecdotes

Actuator-only global curvature is convex(~120nm peak to valley)

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Interferometer Images

MUMPs 19

MUMPs PlaneMirror (Gold) SUMMiT Optical

= 632 nm

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Testbed Images & PSFs (Preliminary data)

MUMPs 19MUMPs PlaneMirror (Gold)

SUMMiT OpticalPartial Release(?)

SUMMiT OpticalFull Release (30/45?)

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Proposed CfAO SUMMiT Design

128 to 256 element array of segmented micromirrorsSingle wire per element address scheme (die size/wire bond limited design)

• Wire-bonded electrical connectionsMinimum 100 m pitch (center-to-center)

• Larger element size for increased fill & lower operating voltage• Have 128 element 203 m designs on 0.5 cm square die• Trade of bond pad space & mirror size required to optimize

Minimum fill-factor ~95%Minimum stroke: 0.5 m Mirror element flatness <30 nm peak to valleyOptimize global flatness by design and study of process using current arrays

StatusHave had initial discussions with Sandia about approach

• Want design that they will agree to release/package/bond• Standard module run should yield 50-75 finished parts (untested)

Will explore progress of metallization - use if available• Otherwise design for ease of post-foundry (user) metallization