GRATING LIGHT VALVESFOR

HIGH RESOLUTION DISPLAYS

RAJ B. APTE

ReportNo. 5192

A DISSERTATION

SUBMI11TEDTOTHE DEPARTMENTOF

ELECTRICAL ENGINEERING

AND THE COMMITFEEON GRADUATE STUDIES

OF STANFORDUNIVERSITY

IN PARTIAL FULFILLMENT OFTHE REQUIREMENTSFOR THE DEGREE

OF

DOCTOROF PHILOSOPHY

June1994

© COPYRTGHTBY RAJB. APTE

All Rights Reserved. No part of this publication may be

reproduced,storedin a retrievalsystemortransmittedin anyform

or by any means: electronic, electrostatic,optic, magnetic,

mechanical,photocopying, recording or otherwise, without

permissionin writing from theauthor.

II

I certify that I havereadthis dissertationandthat in my opinion it is fuiiy adequate,

in scopeandquality, asadissertationfor thedegreeofDoctorof Philosophy.

David M. Bloom, PrincipalAdvisor

I certify thatI havereadthis dissertationandthat in my opinion it is fuiiy adequate,

in scopeandquality, asadissertationfor thedegreeof Doctorof Philosophy.

JamesS. Harris

I certify thatI havereadthis dissertationandthat in my opinion it is fuiiy adequate,

in scopeandquality, asadissertationfor thedegreeofDoctorof Philosophy.

ThomasKenny

Approvedfor theUniversityCommitteeon GraduateStudies.

111

Abstract

The GratingLight Valve (GLV) is a micromechanicalphasegratingthatcan be usedfor

color displayapplications. Operationis basedon electricallycontrollingthe mechanical

positionsof gratingelementsto modulatediffractionefficiency. By choosingdimensions

of thegratingstructurescarefully,it is possibleto produceadigital opticaldevice.

Since gratingsare inherentlydispersive,theGLV canbeusedfor color displays. Full

NTSC-qualitycolors areavailable. In addition,thedevicesarebistableandmaybeableto

operatewith apassivematrix of contactsandstill achievethe performanceof an active

matrix light valve. Eight bitsof grayscalearepossibleusingtime division multiplexing

and the fast (20ns) switchingspeedof the GLV. The contrastratio of the device is

sensitiveto processingerrors,andaratioof20:1 wasmeasured.With betterprocessing,a

color contrastof 200:1 shouldbe achieved. Theoperatingvoltageis 20 V, but thereis

goodevidencethat5 V operationis feasible.

Oneproblemin thedevelopmentoflargeone-dimensionalandtwo-dimensionalarraysof

deviceswassticking during thefinal wet processingstep. This is a commonproblemin

micromachines.Our solutionis to userough(150A-RMS) polysilicon films to reducethe

areaof contactbetweenthemovingpartsandthesubstrate.In thecaseoftwo-dimensional

arrays,this film couldbedopedandfunctionasthe seconddimensionof interconnects.

Although this structuresuffersfrom someof the difficulties of anon-planarprocess,it

wasusedto demonstratetwo-dimensionalarraysofdevices.

iv

Acknowledgments

This work was sponsoredby Andy Yang and Ken Gabriel of the AdvancedResearch

ProjectsAdministrationunderContractsDAALO3-92-G-0232andF49620-93-I-0609.

In additionto thesponsorsI would like to thankmy advisor,DaveBloom,whosetireless

energyand enthusiasmneverfailed to rouseme from my processing-labstupor,andmy

researchassociate,Bill Banyai,whodeconstructedmy experiments.I hopethatsomedayI

will beworthyoftheirefforts.

v

Contents

Abstract iv

Acknowledgments v

Contents vi

List of Figures ix

List ofTables xii

List of Photographs xiii

Introduction 1

1.1 BasicDeviceFabricationandOperation 1

1.2 Comparisonwith LCDs andOtherMicromechanicalValves 2

1.2.1 Limitationsof LCDs 2

1.2.2 OtherMicromechanicalDisplays 3

1.3 ThisWork 6

1.4 Outline 6

OpticsoftheGLV 7

2.1 Diffraction GratingAnalysis 7

2.1.1 Basic Operationof the GLV 9

2.1.2 Scalar Diffraction Theory 9

2.2 Basic Optical Systems 12

2.3 MonochromeContrastRatio 15

vi

2.3.1 BrightnessandContrastRatio. 15

2.3.2 Effectof EtchAnisotropyon Contrast 18

2.4 ColorOperation 18

2.4.1 CIE ColorCoordinates 20

2.4.2 ColorDesign 22

2.5 ColorContrastRatio 25

2.6 PixelSizeLimits 27

MechanicsoftheGLV 30

3.1 Basicsof hysteresis 30

3.2 MaterialsParameters 32

3.3 StringModel 33

3.4 BeamModel 36

3.4.1 SwitchingVoltage 38

3.4.2 PeakStress 39

3.4.3 Hysteresis 39

3.5 Row-addressingMethod 40

3.6 Speed 42

3.7 TemperatureLimits 42

Fabricationof theGLV 45

4.1 BasicProcess 45

4.2 Isolation 47

4.3 InterconnectConductivityandReflectivity 50

4.4 Sticking 51

4.4.1 Water 52

4.4.2 BeamPeelingTheory 53

4.4.3 Stress 53

4.4.4 SurfaceTreatments 54

4.4.5 Striations 55

4.4.6 VanderWaalsBonding 56

4.4.7 SurfaceRoughnessandContrast 60

4.4.8 Progress in Reducing Sticking 61

4.5 Two-DimensionalArrays 63

4.5.1 Isolation 63

4.5.2 Thermalbudget 63

vii

4.5.3 BasicRecipe . 64

4.5.4 NonplanarProcessing 64

4.6 Reliability 66

4.7 DeviceFailure 67

4.8 FutureProcessDesign 67

Conclusion 71

5.1 Device Summary 71

5.2 Future Work 72

Bibliography 74

Appendix1 82

A1.1 StandardProcessSteps 82

Al.2 CurrentGLV process 83

Appendix2 85

vm

List of Figures

Figure 1.1

A single GLV pixel 2

Figure2.1

Diffraction efficiencyfor severalordersof an aluminumreflectionphase

grating 8

Figure2.2

Two states of the GLV 9

Figure2.3

Diffraction efficiency(diffractivity) ofthefirst orderasafunctionof

wavelengthfor a “down” pixel, SdOWfl(A~) 11

Figure2.4

Diffraction efficiency(diffractivity) ofthefirst orderasafunctionof

wavelengthfor an“up” pixel, 11

Figure2.5a

Simplemonochromeopticalsystem(I) 13

Figure2.5b

Simpleopticalsystem(II) for electicalcharacterizationofdevices 14

Figure2.6

BW responsivity of the human eye 16

Figure2.7

BW contrastratio vs. wavelengthfor narrowbandsources 16

Figure2.8

BW contrastratio vs. film thicknesserror 17

Figure2.9

Effectofbadsidewallson contrastratio 19

ix

Figure2.10

Contrast ratio vs. sidewall angle 19

Figure 2.11

Color responsivity of the human eye 21

Figure 2.12

Basic optics of Schlieren system (III) 24

Figure 2.13

MeasuredcolorcoordinatesandtheoreticalvaluesoftheGLV with NTSC

phosphorstandardsand the visible gamut 26

Figure 2.14

Color coordinatesof greenpixel asa functionof diffraction angle 26

Figure 3.1

Basicmodelfor theGLV beammechanics 30

Figure 3.2

Origin ofhysteresis 31

Figure3.4

Results of beam model 38

Figure 3.5

Simulatedhysteresiscurve 40

Figure 3.6

Measuredhysteresiscurveofasinglepixel 41

Figure 3.7

Pixel switching in 20.5 ns 43

Figure4.1

Single pixel of the one mask GLV process 46

Figure4.2

Schematicof singlemaskprocess 46

Figure4.3

Device to device isolation 47

Figure4.4

Field concentrationcausedby poor sidewalls 50

Figure4.5

Controlofresidualstressin LPCVD nitride 54

Figure4.6

Beampeellengthasafunctionofresidualstressandstriations 58

x

Figure 4.7

Surface roughnessand beam sticking 59

Figure 4.8

Contrastratio versusfilm thickness 59

Figure4.9

Schematicoftwo dimensionalarray 65

xi

List of Tables

Table2.1

Basicvaluesofcolorparameters 22

Table2.2

Contrastratio forunoptimizedsystem 27

Table2.3

Contrastratio for optimizedsystem 27

Table3.1

BasicphysicalandgeometricfactorsoftheGLV 33

Table3.2

Threedifferenttypesofnitride in use 33

Table3.3

Experimentalsecondinstability voltages 38

Table3.4

Materialsparametersfor thermalexpansion 44

Table4.1

Progressin decreasingsticking 62

XII

List of Photographs

Photograph2.1

SEMof colorpixels 25

Photograph2.2

Sideviewofstuckbeams 29

Photograph4.1

Gratingdestroyedby shortingandfusing 48

Photograph4.2

Closeupof fusedbeam 49

Photograph4.3

Harpteststructure 52

Photograph4.4

Atomic forcemicroscopeimageofharpstructure 56

Photograph4.5

Striationsin onedimensionalarrays 57

Photograph4.6a

Rough polysilicon surface 60

Photograph4.6b

Nitride deformedby roughpolysilicon 61

Photograph4.7

Singlepixel in atwo-dimensionalarray 62

Photograph4.8

4x4 pixel array 66

Photograph4.9

Fourcornersof atwo dimensionalarray 68

Photograph4.10

Fourcornerspointof two-dimensionalarray,alternateview 69

xjn

Chapter 1

Introduction

1.1 BasicDeviceFabrication and OperationThe GratingLight Valve (hereafterGLV) is a micromechanicallight valve intendedfor

displayapplications.A singlepixel is shownin Figure 1.1. Thebody of the deviceis a

collectionof tenbeamsstretchedacrossa frame. This frameis attachedby a spacerto

thesubstrate,leavingthebeamssuspendedin air. By movingthebeamselectrostatically

it is possibleto modulatethediffraction efficiencyof light incidenton thestructure.This

device, whosefabricationusesonly standardSilicon processes,is the subjectof this

thesis.

As a light valve for display,the GLV hasa numberof interestingproperties. GLV

fabricationis fairly simple,requiringonly onemaskstep forbasicdevicesandonly three

or four for completearrayfabrication. This should translateinto low productioncost.

The GLV is capableof eitherblack-and-white(BW) orcolor operationwith whitelight

illumination. The pixels of the GLV areextremelyfast, switching in under25 ns.

Furthermore,thepixels arebistablewith appliedvoltage: it maybepossibleto operate

the GLV and achieveactive matrix performancewith only a passivematrix. The

combinationof speedandbistability maybe usedfor spatial light modulatorapplications

aswell asfor simplifying the designof drivers (fastpixelscanbeaddressedby apassive

matrix, while slowpixelsrequirethe increasedcomplexityof anactive matrix structure).

1

1.2 Comparison with LCDs and Other MicromechanicalValvesFrom the mid-seventiesmicromechanicaldisplay technologieshave been under

investigation. Commercializationhasfocusedon pivoting or moving mirrors to steer

light into or out of collection optics [Sampsell1992]. An alternativetechnologyuses

elastomersas the micromechanicallayer in a diffractive configuration [Gerhard-

Multhaupt 1990]; this technologyis themostsimilar to theGLV. A comparisonof GLV

technologywith the dominantnewdisplaytechnology,liquid crystaldisplays(LCDs),

andothermicromechanicaldisplaytechnologieswill serveto explainthemotivation for

this work.

Figure 1.1: A singleGLV pixel

1.2.1 Limitationsof LCDsThe attractionof micromechanicaldisplaysis that they do not sufferfrom the limited

speedand efficiency of LCDs. NematicLCDs switch in milliseconds,and while new,

fasterliquid crystaltechnologiesareunderinvestigation,commercialLCD pixelsoperate

2

at little more than the video framerefreshrate. This complicatesthe designof device

drivers, since simple row-by-row addressingrequiresdevicesto respondin a small

fraction of the framerate. LCD panelstypically include anactivematrix of perhapsa

million transistorswhichcanlatch quickly. A secondproblemwith LCDsis theirlimited

opticalefficiency. Typically around5%of thelight that entersacolor LC valvemakesit

to the screen. This problemis especiallyacutefor projectiondisplays,which require

maximumdeliveryof screenlumens. Micromechanicaldisplaysarepotentiallycapable

of a 500%improvementin optical throughputoverLCDs (i.e., 25% of theincidentlight

reachingthe screen). In applicationswhere lamp power is limited, this may be an

importantfactoragainstLCDs.

If lamp technologyis not a limiting factor,thenthebrightnessof a light valvedisplayis

governedby the generationof heatin the valve. All the light that is not transmitted

througha LC light valve is dissipatedin the valve itself as heat,so the low opticalthroughputtranslatesinto deviceheating. Theproblemis compoundedby thefact that

LCDs arevery temperaturesensitive,with only a 40° C operatingrange. The GLV is

constructedof hightemperatureceramicmaterialsandis very insensitiveto temperature

variations. In addition,micromechanicallight valvesmodulatelight by switchingit from

thecollectionopticsinto abeamdump: theenergynot transmittedfrom darkpixelsdoes

not heatthe device. Only about8% of the incidentlight is absorbedby the aluminum

reflectoron the surfaceof the chip Thesefacts combineto make it likely that much

largerlamps can be usedwith micromechanicaldisplayscomparedto LCDs. Larger

lampsandhigherefficienciesmeanmorescreenlumens.

Thepromisesolving the problemsof LCDs—speed,optical efficiency,andtemperature

sensitivity/deviceheating—makesmicromechanicaldisplaysinterestingto a numberof

companies[Sampsell 1992], including those with LCD manufacturingcapability

[Yoshida 1993].

1.2.2 OtherMicromechanicalDisplaysPioneersof micromachiningfirst proposedmicromechanicaldisplays in the mid-

seventies[Petersen1982]. Commercialdevelopmentcommencedat TexasInstrumentssoonafterand continuesto the present[Hornbeck 1991a; Hornbeck1991b; Sampsell

1990]. Theirwork is basedon electrostaticpivoting or movingmirrors. Sincemirrors

haveto be rigid while beamsin the GLV are flexible, mirrors arenearlyan order of

magnitudethickerthanbeams.This translatesinto a largermomentof inertiaandslower

3

accelerationsfor a given driving torque. Also, mirrors must be deflectedby several

microns while beamsrequireless than one seventhof a micron deflection. For these

reasonsGLV is more thantwo ordersof magnitudefaster than TexasInstruments’

DeformableMirror Device(DMD).

The differencein speedis sufficient to allow row-addressingof the GLVwhile limiting

the DMD to frameaddressing.In row-addressing,eachrow pixels is selected,one at a

time. Simulaneouslypixel datais put on thecolumndrivers. Following thewrite cycle,

the currentrow is de-selectedand the next row is selected. If eachpixel contains

memory,either by the integrationof atransistoror some inherentbistability, then this

methoddoesnot sufferfrom thelimited contrastofpassive-matrixaddressing[Alt 1973].

Clearly thepixelsmustrespondfasterthantheframe-ratetimes thenumberof rows. For

digital pixels (which bothDMD andGLV use), theresponseof thepixel mustbefaster

still by thenumberof distinctgraylevels. For a 1000row displaywith 8 bitsof grayscale

(percolor) addressedat 60 Hz, thepixelsmustbe capableof respondingat 15 MHz. The

GLV is capableof this speed.

For theslowerDMD pixels, morecumbersomeframe-addressingis needed.A matrix of

fast master-slaveflip-flops is locatedbeneaththe pixels, oneflip-flop per pixel, The

masteris connectedto the addressinglinesof that pixel, while the slaveis connectedto

the mirror immediatelyaboveit. The masterflip-flops are row addressed.After the

completeframeof masterflip-flops hasbeenprogrammedfor the nextframe,the data

from eachmasteris latchedto its slaveandthemirror that standsaboveit. Themasterto

slave latchingis donefor the entire framesimultaneously.Finally, row-addressingof

masterflip-flops continuesfor thenext frame.

Frame-addressingachievesthe sameperformanceasrow-addressing,but requireseight

transistorsperpixel (four for eachflip-flop). Eight million transistorsareneededin a

megapixeldisplay. Only two-thousanddriversareneededfor the GLV, eachof which

must switch amongthree logic states. Sincetn-statelogic requiresfewer than 10

transistorsperdriver, fewer thantwentythousandtransistorsareneededfor a megapixel

GLV. Thereducedcomplexityof theGLV shouldmakeit lessexpensiveto manufacture

thantheDMD.

A secondadvantageof theGLV over theDMD is that theGLV is capableof producing

color from awhite illumination sourcewithoutany additionalcomponents.Although the

4

DMD canbeusedfor color, theaddition of a large,rotating color wheel on othercolor

selectoris neededto illuminate theDMD with red, green,and bluelight sequentially.

Theframerateis tripled, andthe DMD imagesthe red, green,andblue componentin

succession.The eye integratesthe threeprimary imagesinto one color image. The

additionof a color selectoris not neededfor theGLV: it is convertedfrom BW to colon

operation simply by narrowing a slit in the projection optics. The intrinsic color

generationof theGLV will bevery usefulfor manufacturingcompactcolor displaysfor

pagerandhead-mountedapplications.

Other micromechanicaldisplay technologiesarebasedon electronbeam,active-matrix

Silicon, or CRT/photoconductoraddressingof viscoelasticand oil films [Gerhard-

Muithaupt 1991]. Electron beamsand CRT/photoconductoraddresseddisplays are

unlikely to haveamajorimpactbecauseof theirhighcostandthereforesmall penetration

into the low andmiddlepartsof themarket. However,thereis promisethat researchon

active-matrixviscoelasticsystemsmayleadto mainstreamproductsin thefuturebecause

of theirsimplicity offabricationandcompatibilitywith CMOS processintegration.

Viscoelasticspatial light modulators(VSLMs) [Gerhard-Multhaupt1990] usea thin

viscoelasticlayersandwichedbetweena flexible layerof metalandarigid substratewith

transistorsandmetal lines on the other. If a voltage is appliedto the lines, which are

shapedlike gratings,the top metal is attractedandthe viscoelasticlayerand top metal

deformtogether.This formsa sinusoidalgratingon thetop metal. Essentially,theGLV

andVSLM arebasedon thesameprinciplesof operation,with different implementations

of the spacerlayer: air vs. plastic. Sincethesurfacedeformationsaresimilar in shape,

the optical systemsare very similar. One advantageof the VSLM is that it doesnot

requireany high temperatureprocessing,so integrationwith driver circuits and active

matricesis greatly simplified. Nevertheless,a passivematrix GLV that only requiresa

few thousanddriver transistorsmay beeasierto manufacturethan the VSLM with an

activematrix of millions of transistors.

5

1.3 This WorkOriginal contributionsof this thesisare thediscussionsof color, devicemodelingand

thefabricationof two-dimensionalarraysof devices.Specifically:

• Generatingcolor using thedispersivepropertiesof gratingsandSchlierenoptics.

• Modelingthecontrastratiofor broadbandillumination.

• Modelingthemechanicalpropertiesof beams.

• Usingstriationsto reducestickingof thebeamsto thesubstrate.

• Usingsurfaceroughnessto reducestickingof thebeamsto thesubstrate.

• Fabricatingtwo-dimensionalarraysofdevices.

• Proposingatwo-dimensionaladdressingscheme.

Other students,OIavSolgaardandFranciscoSandejas,did the initial processdesignand

first masklayout. In addition,Franciscodevelopedotherprocessesto decreasesticking

andto obtaincritical sidewall(anddimensional)controlfor highercontrast.

1.4 OutlineThis chapterpresenteda sketchof micromechanicalentriesinto displaydevelopmentand

a comparisonwith LCDs. Chapter2 explains the optical propertiesof the device,

including theprincipleof operation,color generation,contrastratio,andscalability. Two

modelsfor the electromechanicaloperationof the deviceare presentedin Chapter3.

Thesemodelsareusedin Chapter4 to analyzetheproblemof stiction encounteredduring

fabrication. The processdesign for two-dimensionalarrays is reviewed. Chapter5

summarizesthe researchon this deviceand discussesfuture researchtopics. The two

appendicesgive specific processrecipe stepsfor the devicesand the details of the

numericalbeamcalculations.

6

Chapter 2

Optics of the GLV

2.1 Diffraction Grating AnalysisA diffraction gratingis a periodic structurethat affectseitherthe amplitudeor phaseof

incidentlight. Typically the periodis severaltimes thewavelengthof light. A detailed

analysisof diffraction gratings[Born 1980] showsthatincidentlight is diffractedby the

gratinginto severaldirectionswhichconformto theBraggcondition. Amplitudegratings

areformedby alternatingstripesof absorbingandtransmittingmaterial. Phasegratings

modulatethephaseratherthantheamplitudeoflight.

The GLV is amicroelectromechanicalphasediffraction grating. The amplitudesof the

diffractedmodesof a 2.00 ~.tmperiodphasegratingwith rectangulargroovesconstructed

from aluminum asa functionof groovedepthareshownin Figure 2.1. The specular

modehasapeakreflectivity of 92%whennogroovesarepresent(92%is thereflectivity

of aluminum). This value decreasesas the light is diffracted rather thanreflected.

However,whenthegroovesare?J2deep,thereflectivity is againmaximum. Shadowing

effects (causedby reflectionsfrom the sidewallsof the grating elements)limit this

maximumreflectivity to 82%.

The light that is not reflectedinto the specularmode is diffracted. For small grating

depthsthereis little diffraction. As theround-trip depthapproachesX/4 in phase,the

diffraction peaks,with 41%of the light in eachof thefirst orderdiffraction modes. At a

gratingdepthof X12 the diffractedlight is againnulled. In this casethe gratingfunctions

like a perfectlyflat mirror, for nearlyan octaveof wavelengthsof light. Sinceeachof the

7

Incidentand -1 diffracted-2 diffracted

0.8

0.6

0.4

0.2

0

0

(b)

Figure 2.1: Diffraction efficiency for severalordersof an aluminum

reflection phasegratingwith a 2.00 p.m period, illuminated at 13.5°

incidenceat 550 nm. (a) showsschematicallythe diffractedmodes.(b)

shows the diffractedintensitiesasa functionof gratingdepth. Note the

finite reflectivity of aluminumlimits thespecularreflectionwith nogroves

to 92%. [Gaither1988; Veldkamp1989]

-3diffracted

0-specular

+1 diffracted

+2 diffracted

(a)

1

~1)

C

0.1 0.2 0.3GratingDepth [p.m]

8

diffracted modeshasa different diffraction angle,the componentof the wavevector

normal to the gratingvarieswith ordernumber. This variationis what causesthe higher

ordermodesto null furtherout thanthe±1 order. Thus,not all thediffractedmodescan

be nulled at one gratingdepth. Fortunately,thesehigher ordersare fairly small in

magnitude,sothatthetotal powerlost if theyarespatiallyfilteredis negligible.

2.1.1 BasicOperationof the GLVThe switchingof thediffraction efficiency canbeusedto makedevicesin anumberof

ways. The two basic methodsdependon whetherthe reflectedor diffracted light is

collected by the optical system. Theseare demonstratedin Figure 2.2. In the

undeflected,or“up” case,which correspondsto A12 the GLV is purelyreflecting. In the

deflected,or “down” case,the phasedelay is X /4 and the diffraction into the ±1

diffraction ordersis maximized.Thekey to deviceoperationis that thespacerandbeam

thicknessesarechosento beX/4 deep.

incidentlightincidentlight

0-specular -1 -diffracted + 1-diffracted

X/2

(b) Down: Diffraction

Figure 2.2: Two statesof the GLV. In (a), the beamsare up and the

devicereflects the incidentlight. In (b), whenthe beamsaredown, the

GLV diffracts all the light. The top two illustrations show the cross

sectionthroughthe beams.Thebottomtwo show a crosssectionthat is

parallelto thebeams.

2.1.2 ScalarDiffraction Theory

Scalardiffraction theory for normal incidenceis largely in agreementwith Figure 2.1.

Thescalartheorygives for the±1 diffraction orders[Solgaard1992]

II

(a) Up: Reflection

9

I . 2”~’~ 2rdI=—~-sincI — I 1—cos — 1+

2 ~2)

where I~is the incidentlight intensity, d is thegratingdepth, A. is thewavelengthof the

incidentlight, and p is theperiodicityof thegrating. Forsmall diffraction angles(2 p.m

periodicity gives a diffraction angleof 15° at 550 nm) and a grating designedwith

d = for a designwavelengthof 2~,theintensity of the±1 diffraction ordersfor a

“down” pixel is

SdOWfl(A.)—0.412 [1_cos~~-]

Foran “up” pixel thecorrespondingexpressionis

S~~(A.)=O.4lso~)[i_cos2~~0]

10

0.4

0.1

380 480 580 680

Wavelength [nmj

Figure 2.3: Diffraction efficiency (diffractivity) of the first order asafunction of wavelengthfor a “down” pixel, SdOWfl(A.). This gratingis

designedfor 550nm.

0.1

0380 480 580 680 780

Wavelength[nm]

Figure 2.4: Diffraction efficiency (diffractivity) of the first order asafunctionof wavelengthfor an “up” pixel, S~~(A.).This gratingis designed

for 550 nm.

Theplotsof SdOWfl(A.) and Sn,,(A.), Figures2.3 and 2.4, respectively,illustratethebasisof

thebandwidthand thecontrastratioof thedevice. It is apparentthat theGLV modulates

0.3

0.2

0

780

0.4

0.3

0.2

11

light overa 200 nm bandwidth. Thecontrastat anyparticularwavelengthis simply theratioof SdOWfl(A.) to S~,(A.).BecauseS~~(A.)is anull atonly asinglewavelength(550nm

in this case),thecontrastratiopeaksat thatpoint anddeclinesto eitherside. In Section

2.3 theseparameterswill beusedalongwith a developmentof humanvisualperception

to calculatecontrastratios for thesedeviceswhenusedin optical systemsdescribedin

Section2.2.

2.2 BasicOptical SystemsOptical systemscanbe constructedto view eitherthe reflectedor diffractedlight. The

latterhastwo clearadvantages.Sincethe non-diffractiongratingportionsof the device,

includingbondpadsandotherlargeareas,remainequallyreflectingin both the“up” and

“down” positions,therewill be aproblemgeneratingadequatecontrastwithout theuseof

maskingfilms orspatialfilters to removetheunmodulatedlight. Thesecondadvantage

to viewing thediffracted light is that the spatialdispersionof the grating,discussedin

Section2.4 andFigure2.12,canbeusedto makecolor pixels.

The basisof BW operationwasshownin Figure 2.2. Whenthebeamsare “up,” the

deviceis reflective,andthenormally incidentlight is reflectedbackto thesource. If the

beamsarebroughtinto the“down” position,thenthepixel diffracts82% of theincident

light into the±1 diffraction modes. Additional light is diffractedinto higherordermodes

(about10%of theincident),but theoptics usedhad too small anapertureto collectthis

light.

The opticalsystemswere usedin devicetesting. The prototypicalBW optical system

(hereafter,systemI), is shownin Figure2.5. Theilluminationsourcewaseithera250 W

metalhalide arc lamp with an integratedreflectoror a40 W tungsten-halogenlamp with

dielectric reflector. The light was condensedwith ff2.4 optics and imagedwithout

magnificationat an intermediatepoint. At theintermediatepoint the imagewasspatially

filtered to insureadequatecollimation. Sincecollimationwithin theplaneof Figure2.5

is essentialfor goodcontrast,thearcor filamentof the lamp is shownperpendicularto

this plane. The sourceimagewas thencollimatedand directedby a turningmirror onto

the face of the device. The specularreflection was returnedto the lamp, while the

diffractedorderswerecollectedby aprojectionlensplacedjust overafocal length away.

The distancebetweenthedeviceandtheprojectionlens wasadjustedto focustheimage

on adistant screen. In this systemtheprojectionlens is usedboth for projectionto the

12

screenaswell asspatialfiltering of thediffractedlight. A telecentricstopwasplacedata

distanceof one focal length from theprojectionlens. At this plane,all raysfrom the

device plane with the sameangle all pass though the samepoint, i.e., all the +1

diffraction orderraysfocusatonepoint while all the-1 raysfocusatanother.By placing

a stop with slits in it at thosetwo points, all non-diffractedlight is blockedfrom the

screen.

TelecentricStop

f

SpatialFilter I_____

________ ~ ProjectionLens

~

Lamplr ~ TurningMirror

Illumination Optics

Device

Figure 2.5a: Simple monochromeopticalsystem(I). SystemI wasused

as a prototypefor a secondsystem, vide infra, to do electricaltesting

(systemII) anda third to do optical testing(systemIll).

SystemI wasusedto demonstratetheutility of the GLV for projectionandcontaineda

static GLV device. The static GLV devicewasdesignedwith a fixed, VGA bitmapped

image on it. At eachpixel of the bitmap, the values for red, green,and blue were

quantizedto six bits each. At thecorrespondinglocationon thestatic GLV, threepixels

13

werewritten, red,green,andblue, andthe beamlengthof eachwasvariedfrom 0.25 to

16 p.m dependingon the six bits for eachcolor. A dark pixel wassimply left blank.

Exceptfor contrastratio, this staticdisplaygives a faithful impressionof whata GLV

displaywill look like.

A secondsystem(systemII) wasconstructedfor devicetestingusingamicroscope/probe

stationto allow electricaloperationof thebeams. In this system,theillumination optics

were reducedto a tungsten-halogenlamp with integratedcollimating reflector. The

turning mirror was moved to the side so that the device was illuminated from the

diffraction angle. The diffraction angleof the -1 orderwas thennormalto the device.

This light wasimagedthroughamicroscopeobjectiveandeyepiece.Theotherdiffracted

modes(+1, ±2, ±3) were discarded.At thetelecentricpoint of the objectivelenstwo

strips of black tapewere usedto define a slit. This systemwasusedfor most of the

electricaldevicetesting (including hysteresismeasurements).A third optical system

(systemIII, seeSection2.4.3)wasusedto measurethecolorpropertiesof theGLV.

TelecentricStopLampwith IntegratedCollimating Reflector

MicroscopeObjective

ElectricalProbe

Figure 2.5b: Simple optical system(II) for electicalcharacterizationof

devices.This systemis constructedon aprobestationwith microscope.

Microscope —~...--

Eyepiece

f

14

2.3 MonochromeContrast RatioTo understandthe differencebetweendynamicrangeor extinction ratio, termsusedin

describing optical modulators, and contrast ratio, it is necessaryto discussthe

responsivityof the humaneye [Hunt 1991] and its effect on perceivedbrightness.

Theoreticaland experimentalvaluesof the BW contrastratio are discussed,and the

differenceis explainedasa resultof processparameters.

2.3.1 BrightnessandContrastRatio

Basedon subjectivedescriptionsof comparativebrightnessfor different colors, the

CommissionInternationalede 1’ Eclairage (hereafterC~)establishedin 1931 the Cifi

standardphotometricobserver. The basis of this observeris the photopic spectral

luminous efficiency function, V(A.) , plotted in Figure 2.6. This function gives the

relativebrightnessof narrowbandoptical sourcesof constantoptical powerover the

visible spectrumwhich peaksat 555 nm. Becauseof thelinearity of thehumaneye, theapparentbrightness,Y,~,ofa sourcewith somespectraldistribution,S~(A), is equalto

780 urn

Y~=Jv(A.)s~(A.)dA.38Orim

Using the valuesof S~(A.) for “up” and“down” pixelsin Section2.1.2, theapparent

brightnessof “up” and“down” pixelscanbecalculatedfor agivenillumination spectrum,S0(A.). Thecontrastratio is definedas ~down

To calculatethenarrowbandcontrastratioof theGLV, i.e. thecontrastratio for a light-emitting diode (LED) or othernarrowbandillumination, we take S0(A)= — A0),

where5(A. — A.0) is the deltadistribution at A.0. This contrastratio is plottedvs. A.0 in

Figure2.7. Thenarrowbandcontrastratio is extremelyhighat thedesignwavelengthand

is still betterthan 100:1 overnearly 100 nm of spectrum.In thecasewheretheGLV is

illuminatedwith LEDs, contrastratiosof betterthan i04 shouldbeexpected—perhapsas

high as108 if thedeviceis grownwith perfectdimensionsandvery smoothaluminum.

15

I~1.1.1.11

I

I

I

II

.11.I

I

I

III

•l._

480 580

Wavelength[nm]

BW responsivityofthehumaneye.[Hunt1991]

680 780

1.0.90.8

C

~ 0.4~ 0.3

0.20.1

0380

Figure2.6:

For white light illumination, which is flat over the entire visible spectrum,the contrast

ratio is calculatedto be 82:1. This valueof thecontrastratiois representativeof avariety

of high-temperatureblack-bodyandmultiline, white arcsources.It is alsorepresentative

of contrastratiosseenon existingLCD projectors[Yoshida1993].

Wavelength[nm]

Figure 2.7: BW contrastratio vs. wavelengthfor narrowbandsources.

For narrowbandoperation, the GLV is capableof operating as an

extremelyhigh contrastratiomodulator. For a deviceoptimizedfor the

greenthecontrastis betterthan100:1 from 520 to 580nm.

780

108

106

io4I102

1480 580 680

16

Becauseof thenarrownessof thepeakof thenarrowbandcontrastratio, it wasfearedthat

thecontrastratiowould be avery sensitivefunctionof processingvariations,particularly

thicknesserrorsthat causewavelengthshifts in the optical properties.This hypothesis

was testedby recalculatingthe contrastratio, as above,but for pixels with design

thicknessesother than 555 nm. Thesecontrastratiosare plotted in Figure 2.8 as a

function of the changein oxide plus spacerthickness (i.e., a device with design

wavelengthof 565 nm insteadof 555 nm has 10 nm ofwavelengtherror. Sincetheoxide

plus spacerthicknessis X/2, this correspondsto a 5 nmthicknesserror). This calculation

showsthat a few percenterrorin film growthwill not destroytheBW contrastratioof the

device.

Figure2.8: BW contrastratio vs. film thicknesserror. Thecontrastratio

is plottedasafunctionof total film (spacerandbeam)thicknesserrorfor a

pixel with 140 nmtotal nominalfilm thickness. Thus,at least5%control

of thicknessesis neededto constructsatisfactorycontrastratiodevices.

C

0 2 4 6 8 10 12 14

Film ThicknessError [nm]

90

80

70

60

50

40

30

20

17

2.3.2 Effect of EtchAnisotropyon ContrastThemeasuredvalueof the contrastratio was20:1[Apte 1993]. From Figure 2.8, it is

apparentthat thefourfold discrepancyfrom theorycannotbeexplainedby film thickness

errors(accuracyto within 5 nm was typical). In fact, theerror lies in the quality of the

sidewallsof the gratings,andthus in the maskingandetchingprocessused. Sincethe

diffraction efficiencyof the lowerpartof theslopedsidewallin the“up,” or dark,caseis

comparableto thatofthe“down,” or light, case,thecontrastis spoiledby thepoorquality

of the darkstate.

To maketheeffect of poorsidewallspreciseis difficult, sincea vectordiffraction theory

will be neededto handle500 to 1000A features[Gaylord 1982]. A simpleestimateis

possibleusing theexpressionfor diffraction givenin Section2.1.2. In this expressionthe

intensity of diffraction as a function of grating depthis given. If we averagethis

expressionover the range of heightsof the beam—includingthe downwardsloping

sidwalls—thisgives ourestimateof thediffraction efficiency of the “up” beams. The

substitutionof this value for the denominatorin the contrastratio underwhile light

illumination is plottedin Figure 2.10. A contrastratioof only 11:1 is expectedfor 45°

sidewalls.

Themeasuredvaluefor thesidewallslopefor thenitride etchrecipegiven in Appendix 2

is 25°,which is measuredfrom SEM photomicrographs.This slopegivesacontrastratio

of 21:1. Although themethodof this calculationneglectsthe very fine structuresof the

the sidewallsandtheirpreciseeffect upon thediffraction, it seemslikely that improved

sidewallslopeswill resultin improvedcontrastratio. Work on usingmetalmasksinstead

of photoresistduringtheetchsteparebeingexploredfor this reason.

2.4 Color OperationBy usingthe dispersivepropertiesof thegrating[Born 1980] theGLV canactbothasa

light valve anda colorfilter. For adiffractiongratingwith normally incidentillumination

andperiod, p, therelationshipbetweendiffraction angle,0 ,wavelength,A, andorder

number,m, is givenby

ASin 0 = m—

p

18

Beam

Substrate

Figure2.9: Effect of badsidewallson contrastratio. If thesidewallsof

the beamsare slopedby insufficient anisotropyof the beametch or

unsatisfactorymasking,thenthelower partsof theslopeareatthe height

for maximumdiffraction. Poorcontrastresults.

90

80

70

~ 60

~50~ 40Cc-)30

20

10

0

Sidewall Angle [degrees]

Figure 2.10: Contrastratiovs. sidewallangle. This figure demonstrates~

the importanceof goodanisotropyandmaskingfor thegratingbeametch

step.

SlopedSidewall

0 10 20 30 40

19

For the first order, in the caseof small angles,this reducesto 0 = A/p. Thus, if an

opticalsystemis constructedthat acceptsdiffractedraysfrom only anarrowsetof angles,6~± (90, then it will image theGLV only in thespectralrange

In order to image the GLV at A1 which is not in A0 ± (9A., wechooseanothervaluefor

theperiodicityof thegrating,p,, sothat

00± (90= A1 ± dApi

Thus, it is possibleto choosethreedifferent gratingperiodicitiessuchthat eachone

diffracts a different wavelength through the samediffraction angleandthus throughthe

sameslit in the telectric stop. This is a generalprocessthat could be usedfor more

sophisticatedadditivecolor systemsthantheusualred-green-blue(RGB) of theNational

TelevisionStandardsCommittee(hereafterNTSC). The opticsof sucha systemare

shownin Figure2.12,which showsthethirdopticalsystemusedwith theGLV [Hopkins

1992]. Thebasicinnovationof this systemis to place thecollimatinglens ontothe face

of the grating package. This puts the collimating lens into the optical path of the

diffractedlight. Beforea discussionof thedesignof color devices,a reviewof human

colorperceptionandcolorimetryis presented.

2.4.1 CIE Color Coordinates

Theresponsivitiesof thethreetypesof conesin thehumaneye[Hunt 19911areplottedin

Figure 2.11. If thesecouldbe measuredaccuratelyfor a largenumberof individuals,

then they couldform the basisof a color coordinatesystem. Sincethis study wasnot

possible, an alternativemethodologywas arrived at which used color matching

experiments. An observerwas presentedwith two illuminated boxes,one with an

unknownsource,andtheotherwith variableamountsof red,greenandblue (which are

definedin this caseat 700 nm, 546.1 nm, and435.8 nm andtermedR, G andB). The

observerthenchangedtheamountsof R, G andB until hematchedtheunknownsource.

Assumingthat thebrightnessof theunknownsourceis suchthat R+G+B is constant(ie,

brightnessis not a factor),then thecolor of theunknownsourcecouldbedescribedby

(R,G).

20

This explanationoversimplifies the basisof colorimetry, but gives a flavor of how to

interpretCIE color coordinates(x,y):x is theamountof red;y is theamountof green;and

whenboth decreasethecolor is blue(sinceaggregatebrightnessis constant).Becauseof

complicationsin the system,the rangeof visible colors is not describedby a simple

geometryin thex,yplanebut by a roundedtriangle. Theedgeof thetriangleconsistsof

highly saturated colors, like laser or LED illumination. In thecenter,with equalmixesof

red, blue, and green, are shades of white. Thus, moving from the centerto theedgeofthe

triangle increases saturation. Moving clockwiseincreaseswavelength. SeeFigure2.13.

100

.~50

£

0

Wavelength[nm]

Figure 2.11: Color responsivityof the humaneye.~3,y, and p are the

spectralresposivitiesof thethreetypesof conesin the humaneye. B, G,

and R are the spectal lines used to define the 1931 CIE Standard

ColorimetricObserver. [Hunt 1991]

If a display is constructedwith threecolor sources,suchas a color TV with three

phosphors,theneachcolor sourcemaybe plotted on thex,y plane. The set of all the

perceived colors madeby mixing thesethreesourcesin varying ratios is called the

“gamut” of the colors. Becauseof the linearity of the humaneyeandthe Cifi color

coordinatedefinition, the coordinatesof all of thecolorsin thegamutdefinedby three

21

primary sources form the triangle in the x,y plane definedby the coordinateof the

primaries.

2.4.2 ColorDesign

The procedurefor designinga color GLV thenreducesto thequestionof how to design

thegratingsandslits to achievea desiredcolor gamut. Forreasonsof compatibility with

existingtechnology,the targetgamutis theNTSCphosphorprimaries. Sincetheoptical

systemswe usedhave a common slit for all threecolors to passthrough, the key

parametersin color designarethechoiceof centerwavelengths,the slit position,andthe

slit width. Referring again to Figure 2.12, the slit position is given by f~?—L~Q~.The slitp0

width by f—. Thus, the designconsistsof choosingvaluesfor p0, A.0, A.1, A.2, andp0

(9A..

If p0 is large, thenthe diffraction angleis small and it may be difficult to spatially

separatethe diffracted light from the lamp mechanically. Also, the collimationrequirementswill be higher (videinfra). If p0 is too small,then thelithographybecomes

more difficult and the the diffraction anglesbecomevery large. In this case,scalar

diffraction theory breaks down, and the grating depth for a null in the specular reflection

no longerconincideswith the peakin first order diffraction. An intermediatevalueofp0 = 2.25 p.m was chosento yield high first order diffraction efficiency at a workable

angle.

Wavelength

[nm]

Periodicity

theory [pm]

Periodicity

exp. [p.m]

Diffraction

Angle [mrad]

x, y

theory

x, y

measured

x, y

NTSC

625 ± 30 2.65 2.75 236±13 0.66,

0.33

0.54,

0.41

0.67,

0.33

530±30 2.25 2.25 0.22,

0.71

0.31,

0.62

0.21,

0.71

465±30 1.97 2.00 0.14,

0.05

0.17,

0.04

0.14,

0.08

Table2.1: Basicvaluesofcolor parameters.

22

The choicesof A~,A.1, A.2, and ~9Aare governed by colorimetry. A white source

spectrum S(?~) was chosen for generality. It is also a good approximation for a blackbody

sourceat 3500°K,suchasatungstenhalogenlamp. After the illumination spectrumwas

chosen, it was chopped into three possibly overlapping segments,A0 ± dA, A.1 ± dA., and

A2 ± dA. The color coordinates of eachsegmentwerecalculatedwhile A0, A1, A.2, and

dA. were varied. Since making dA. aslargeaspossiblewould resultin aminimumof

light being wasted atthetelecentricstop, dA was increased from zero until the color

coordinatesshowedsigns of decreasingsaturationrelative to the NTSC standardphosphors; a valueof 30 nmmatchedthesaturationoftheNTSCphosphors. A.0, A1, and

A2 were selected by trying to matchthewavelengthsof green,red,andbluephosphors.

The resulting values are summarized in Table 2.1 and plotted in Figure 2.13. A

photomicrographof thepixels themselvesis shownin Photograph2.1.

The color coordinatesweremeasuredwith a spectra-colorimeter[PhotoResearch1992]

and arepresentedin 1931 CIE color coordinates[Hunt 1991]. Figure 2.13 showsthe

NTSC color gamut along with the theoreticaland experimentalgamuts. There is a

definite lossof saturationof thegreenandred,althoughtheblueis well-saturated.Figure

2.14 showsthecolorasa functionof diffraction angle. From this plot it is clearthat the

problemis primarily in the collimation of the incident light: in the absenseof good

collimation,it is possibleto saturatethered andblueby over-tuning,withoutevergetting

a saturatedgreen. For example,if thegratingis over-tunedto theblue,then its spectrum

will be mostly violet, regardlessof the grating pitch. Although there is a loss of

brightness(andcontrast),it is possibleto saturatethecolorfully. Thesameis alsotrueof

the red, sinceboth red and blue are at the endsof the visible spectrum. The green

primaryis impossibleto over-tune.Thus,thefactthatthe greencoordinateis unsaturated

in Figure 2.14 indicatesthat poor collimation is causingwavelengthsoutside the

530±30nm to passthroughtheslit. To correctthis, an angularsourcesizeof lessthan13

mradis needed,which correspondsto a linearsourcedimensionof 0.65 mm. A sourceof

approximately 1.0 mm wasused.

23

P4

4—p pProjectionLens

Collimating Lens

Figure 2.12: Basic opticsof Schlierensystem(Ill). This optical system

places the lamp collimating lens in the diffracted light path. This

innovationproducesa very compact,folded optical system. The lamp

illumination is collimatedat the Collimating Lens. This light strikesthe

GLV normally, anda diffraction spectumis producedafocal lengthaway

at the TelecentricStop. Only a portionof thespectrumpassesthrough,

dependingon theperiodicityof thegrating. Not shownis an eyepieceor

viewer.

Telecentric Stop

f

red

f~=1.6°=28 mradp

f~2~137°236 mradp

24

RED

BLUE

GREEN

c==::==:=~c====:== crcn:— ——w ~ 1~~ ~ ~ r ~ ~

~_~c__.%_*_•~~,•_ ~ ~ ~*—~ ‘~ ~* ~ ~ ~

p— ~ ra~ a—~-~ ~~- - —~ ~- — ~— - ~

•E•d~~~~•~ ~~ .~ ~ ~ ~ ~ ‘ ~~ ~-,_____s._4r_ ‘~4~M_~_ ~ ~ ~‘ ~ ~ ‘~ ~

~— —~ r— —~ r- -- - - —‘— - —, —— — L_ ~ ~ -

~ ~ p—~ ~ —~——~~ r—~,~— —_$ ~— — ~— — —d—_ ~ ~ — — —~ -~—— ~ --

~ ——-——-~ ...~~ : : ~ ~.~. ~ ~“~‘ ~

~____~_J ~ - — — -- —r—— ~ ~ ~a—~ ~ ~

~ ————

— — —~

~!EE:Tt±ee~= E2EEE==:±=—~ ~C4: ~ ~ ~ ~ . ~ . ~ ~~ r-

~tzt=t==—tz==: ~==c—t====~ ~ .

~t==r~ <2:n t~r~ ,~t~aacr ~ ~t ~~. ~;=n=rtt= ~2?==~=~ ~~t===:~~ ~ =z~—~~- ~

~ fstzzt==ntz~azz~ ~~ — r~r~g~ >v~tn~.

— ‘‘ -‘—‘—‘—~ ~—“—‘-—- ~~%~—-~+~%%-%-. ~ & ~—~-— ~ ~—

~ -

~ -~a~ ;az~r_ctn~.~tr~ç ~at_~~ztr-~’ ~nc ~an ~

. . ~— -~ — — r~- —

~——r<- — —

—a

~ .~—-- ~

r _ -— — _44_— _Y_~~ ~ —

~ ~ ~*~~:b~

\___ ———-—.——— —————a-— .‘——-~c fl— a— — ~— — ——

r’”’ ~ ‘‘~— — — \__

—< §___~_‘__ -

- ~ a~nrr —:==Z rt~zz~ —: —

ç~~: ç,_._,_~ ~

Ills

25 ~tm

Photograph2.1: SEM of color pixels. The first row correspondsto red,the second blue, and the third green. Each row consists of a series of

devices (three are shown) that are electrically connected, which is why

each pixel lacks a separateframe.

2.5 Color Contrast RatioThecontrastratioin thecolor caseis not asimple functionof devicegeometry. Rather,

it is a functionof the typeof systemusedto projectthecolor, thenumberof light valves,

thenumberof masklevels,andhowthecontrastis defined. We will examinetwo types

of systemsunder two definitions. The unoptimizedcasehasa single light valve with

only onebeam/spacerthickness. This will ordinarily be optimized for the green. The

secondcaseis for a projectorwith threelight valves,eachoptimized for asingle color, or

for aprojectorwith asinglelight valvethat hasthreedifferentbeam/spacerthicknesses.

25

y 0.5

0

x

Figure 2.13: Measuredcolor coordinatesand theoreticalvaluesof the

GLV with NTSC phosphorstandardsand the visible gamut. The outer

ring indicates the visible gamut.

1.0

y 0.5

Figure2.14: Color coordinatesof greenpixel asafunction of diffraction

angle.Theangleis variedfrom 160 to 320 mrad. Points outsidetheedge

of theroundedtriangleof visible light arenoisy.

0.4 0.8

0.4

x

0.8

26

The unoptimized case is typical of a low costandweightproduct,while theoptimized

case is for higherperformance.

Themonochromecontrastratio is the“on” brightness divided by the “off’ brightnessofa

singlepixel of a singlecolor. The pixel contrastratioassumesthat eachpixel consistsof

aRGB triadof devices.The pixel contrastratio is the“on” brightnessofa single device

dividedby the“off’ brightnessof thewhole triad. The valuesof monochromeandpixel

contrastratios for both unoptimizedand optimizedsystemsaregivenin Tables2.2 and

2.3. Thesecalculationsassumethat the devicesdo not scatterlight, have perfect

sidewalls andhavehighbeam/spacerthicknessuniformity.

Color MonochromeContrastRatio Pixel ContrastRatio

red 20 36

green 341 59

blue 22 6

Table2.2: Contrastratiofor unoptimizedsystem.

Color MonochromeContrastRatio PixelContrastRatio

red 434 78

green 341 257

blue 375 27

Table2.3: Contrastratio for optimizedsystem.

2.6 Pixel SizeLimitsThe GLV exhibitsvery highpixel densities.Earlydeviceswereconstructedwith 20 x 25

p.m frames. Themostrecentdevices,with shorterbeams,are20 x 15 p.m. This givesa

monochromepixel density of 0.33 megapixel/cm2,or a color density of 0.11

megapixeb/cm2.Thequestionarises,howmuchfurthercanthepixel sizebereduced?

Thebasicexpressionfor diffraction from a pixel is [Solgaard1992]

I = l~(Diff OrderIntensities)(sin Na)2

wherea = sin 0. Thewidth of thecentrallobe is givenby

27

A dAsin(90= —Np p

so the condition on N, the numberof gratinglines, for greenlight, A. = 530nm and

dA =60nm, is

N>~=8.8

Thesetsthe limit on color pixels as 8.8x2.25=l9.9p.macross. For BW pixels

dA = 200nm andthelimit is threetimes smaller: 7 p.m.

In the otherdirection,along the lengthof the beams,thelimit is given by the aperture

ratio. As the beamsget shorter both the switching voltage and the amount of

undeflectablebeamat the endsincrease. Sinceapproximately3 p.m of the beamis

wasted(4 p.mis moretypical, but this includesthe0.5 p.mof theframethatis undercut),

it is impracticalto makea pixel shorterthan 10 p.m. SeePhotograph2.2. The one

exceptionis if singly-supportedcantileversareusedinsteadof doubly-supportedbeams.

In this casepixelsmight beshortenedto 6 p.mwith a 50%apertureratio.

28

Photograph2.2: Sideviewof stuck beams. This micrographshows the

distanceoverwhichthebeamsbendto thesubstrate.

29

Chapter 3Mechanicsof the GLV

3.1 Basicsof hysteresisThemoststriking featureof themechanicaloperationof theGLV is thehysteresisof the

deflectionof the beams—and hencediffraction efficiency—asa function of applied

voltage. Mechanicalmodelsof thedeflectioncanprovidescalinglawsto helpdesignand

control thehysteresis.Two modelswill bepresented,oneanalyticaland onenumerical.

In both casesthe reasonfor the hysteresisis the same,that the electrostaticattraction

between the top and bottom electrodesis a nonlinearfunction of deflectionwhile the

mechanical restoring force caused by the beamstiffnessand tensionis linear. This is

shown in Figure 3.1.

~ing ~ Foc~

t Capacitor

Foc(1_~)2

Figure 3.1: Basic model for the GLV beammechanics. The spring

representsthe restoringforce causedby the beamstiffnessand tension.

Thecapacitorrepresentstheelectrostaticattractionbetweentheelectrodes.

30

Mechanical

(a)

V<V1

(b)

V1<V<V2

(c)

V>V2

Figure 3.2: Origin of hysteresis.Thesecurvesplot electrostaticand

mechanicalforcesasa functionof normalizeddisplacement(seeFigure

3.1 for equations).When the appliedvoltageV<V ~, the first instability

voltage, thereis onestablesolutionin whichtheforcesbalance(a). If V is

increasedpastVi, thenthereare two stablesolutions,one up and one

down (b). For V>V2, thesecondinstability voltage, thebeammustbe in

thedownposition,pinnedto thesubstrate.

Electrostatic

F

1

31

Thesourceof thefirst equationin Figure 3.1 is a linearapproximationof thedeflectionof

beams with force. Unless the beam material violates Hooke’ slaw, this approximationis

sound. The second equation is the nonlinear force between the platesof acapacitor.The

consequenceof this nonlinearityof theelectrostaticforceis shownmoreclearly in Figure

3.2. The two curves for spring and electrostatic force are plotted vs. normalized

displacementof thecenterofthebeamfor threevoltageranges. For small voltages,there

is only one “loadline” solution,in which thebeamis slightly deflected.For intermediate

voltagesthereare two solutions,onelightly deflectedandthe otherin full contactwith

the substrate. For large voltages, the only stablesolution is in full contactwith the

substrate.Thus,thedevicesshowsahysteresisreminiscentof amagneticcore.

The simplestmodel for themechanicaloperationof thebeamsof theDGLV neglectsthe

the momentof inertiaof thebeams. In this casethebeamis consideredasa stringunder

tension,andthe electrostaticforcethat drivesthebeamis lumpedinto thecenterof the

beam. While thefirst approximationwill beshownto berigorous,thesecondis amajor

sourceof error,sinceit tendsto dramaticallyunderestimatethevoltageneededto switch

the beams.This fact is mitigatedby theutility of thestring model in examiningscaling

laws in analyticform, which is not possiblewith the numericalsimulationsthat are in

agreementwith measuredswitchingvoltages.

3.2 Materials ParametersThematerialsparametersandnominalgeometryof thebeamsaregivenin Table3.2. In

this developmentwe treat the beamand spacerthicknessesasequal. The Young’s

Modulusof oursamplesis imperfectlyknown. Measurementsdonewith samplesfrom

the sameLPCVD furnacegive 200 ± 100 GPa [Hong 1990]. However,otherworkers

havereportedinconsistentvaluesfor similargrowthconditions[Kiesewetter1992]. The

averagetensionin the beamswas determinedusing a scanningHelium Neon laser

deflectionsystem[Flinn 1987] asafunctionof dichlorosilaneto ammoniaflows within

the furnace [Beck 1990].

32

Parameter Symbol Value

Young’sModulus E 200± 100 GPa

AverageIntrinsicTension T 100-800 MPa

OpticalIndexof Refraction n 2.0 - 2.39

RelativePermittivity 8 6.45

TopElectrodeThickness 400 ABot. Electrode Thickness 3000 - 6000AIsolationThickness 5000ASpacerThickness t 1325ABeamThickness t 1325ABeamWidth w 1.0 - 1.5 p.m

BeamLength L 6 -40p.m

AreaMomentof Inertia I = 2.4x 10~p.m4

Table3.1: BasicphysicalandgeometricfactorsoftheGLV.

Dichlorosilane/AmmoniaFlow ResidualStress Indexof Refraction

1.0 800MPa 2.04

3.0 420 MPa 2.19

5.2 8OMPa 2.38

Table3.2: Threedifferenttypesof nitride in use.

3.3 StringModel

The force on a string, for small deflections at a point at the center, is linear. (See Figure

3.3 and Table3.1 for definitions).x is thedeflectionof thebeamat thecenter, ~ =

and K is aparameterin unitsof force.

F = 4 T tw ~ = K ~L

For thecasethat L = 15 p.m and T = 800 MPawehaveK = 3.75 p.N, which is the scale

of therestoringforceon a singlebeamelement. Theelectrostaticforce[Solgaard1992]

is afunctionof theappliedvoltage,v. ThedimensionlessparameterV is defined below.

33

1CL 2 ij2F---° -___

2 (t-x)2 (1_~)2

In this case we have neglected the finite contribution to the capacitancefrom the

dielectric in the beam.Whenthebeamis up, this contribution increases the capacitance

by ,V~, 16%. At the inflection point the effect is 10%. The parameters are:

K= 4Tt2wL

and

V— /e0Lwv2‘1~ t2Thecondition for the secondinstability point is that the numberof crossingpoints in

Figure3.2 decreasesfrom threeto one. Theforceandspringconstantsof theelectrostatic

attractionand thebeamtensionmust be equal,which is equivalentto saying that the

secondinstability point occurs when the spring line is tangent to the nonlinear

electrostatic curve.

Tension = Ttw

Electrostatic (9Attraction = —

dx 2

Figure 3.3: Modelling beams as strings. The upward restoring force is

caused by tensile stress T in the beams, while the downward electrostatic

attraction is the derivative of the stored energy in the beam capacitance, C,

with deflection x.

Tension

34

The solution for the second instability point is

~ xl— t 3

Thus, the instability point, beyond which the beam collapses with increasing voltage to

the substrate, occurs at one-third deflection. As the voltage is retarded, the beam will

spring up at the first instability point. This is given by

K=e~V~

If we now proceed to solve for the second instability voltage, we have

v — /32 T21~27e Land

V1=~~J~V2=0.4V2

Numerically,theseexpressionsunderestimatethe instability voltagesby a factor of 2

comparedto experiment. This is becauseof the assumptionthat the entire distributed

electrostaticattraction is focusedat the centerof the beam. Nevertheless,the string

model accuratelypredictsthe scalingbehaviorof the switching voltage. Extreme

sensitivity to material thickness has been observed, as well as the inverse linear

dependence on beam length. Finally, a weak dependence on stress levels has been

observed as well. Thisdatawill bepresentedin Section3.4.1.

There are two major deficiencies with thestring model. Thefirst, which is minor, is that

the model assumes the beams are long and floppy. An analysisthat includesthe finite

stiffness of the beams gives [Cho 1992]

V22=~~~~ 127 e0L (*—Tanh*)

where k = ~ Forourgeometries,k = 4p.m~,kL>> 1, and this expression reduces

to thepreviousone.Thus, this error seemsminor. However,whenpredictingtheprofile

35

of the beamsnear the spacer, the beam stiffnessbecomesimportant. The second

shortcomingis that thedeflectingforceis lumped into thecenterof thebeam,ratherthan

distributedalong the length. To overcomebothof theseproblems,a numericalmodel

wasconstructedto predictswitchingvoltagesmoreaccurately.

3.4 BeamModelThe beammodelovercomesboth failuresof the stringmodelandgivesaccurateresults

for switchingvoltages.It is basedon solving the4thorderbeamequation[Hartog 1961]:

~(4) T~(2) W.ØEl El

for the local deflection y(l), a function of theposition 1 along the beam. W(y) is the

one-dimensionalelectrostaticpressure(N/rn) forcingfunction

W(y)=-~’4’ 1

This inhomogeneous,nonlinearequationis best solved by the methodof Green’s

functionswith self-consistency.First, theequationis solvedassumingthat the forcing

functionis thedeltafunctionat position a. For 1 < a thesolutionis 5’(l; a) = y1 (1), and

for 1 > a the solution is 5~(l;a)= y2(l). Then, using an assumedbeamdisplacement

function °y(1) we calculatea trial forcing function °W(°y(l)). A new displacement

functionis generatedby convolvingGreen’sfunctionwith thetrial forcing function:

= ~(1;a)*~W(~y(a))

Theiterationsareperformeduntil I’~’y(l)—~y(l)~is small. The algorithmcanbe made

efficient by samplingthebeampositionat n grid points. In this case,5~(l;a)is an n x n

matrix, andtheconvolutionis amatrix multiplication.

To calculatey1(l) and y2(l) thehomogeneousequationmustbe solved:

WtT(2) = oEl

36

andtheboundaryconditionsare,for aunitymagnitudedeltafunctionalatposition a are:

y2(L)=0

y~(O)=O

y~(L)= 0

y1(a) = y2(a)y~’~(a)= y~(a)

y~2~(a)= y~2~(a)

and

y~3~(a)— y~3~(a)+ 1=0

Theseboundaryconditions arederivedby integratingthe delta function-forcedbeam

equationfrom a— S to a+ S. Thehomogeneoussolutionsaresimpleexponentials

= a1 + b1x+ c1 e”~+d1 e~’~

y2(l)=a2+b2x+c2e~+d2e’~

Solution of theseeight equationsin 8 unknownsuseda commercialmath package

[Wolfram 1991].

While manyof the detailsof the solutionarestraightforward,onesubtletyaroseduring

the analysis: how to model the hard contactbetweenthe collapsedbeamand the

substrate.This is not a trivial problem. In this casewe assumedthat the substratewas

springy, i.e. that it respondedwith a forceproportionalto theamountit wascompressed.

Sometuningof this parameterwasneededto help keepthe deflectioninterationsfrom

oscillating. While for the presentanalysisthis provedsufficient, any furthermodeling

must include a viscous dampingtermto keepthe solution from vibrating. It mustbe

notedthat oscillationsin thesolutionarephysical,in thesensethat in vacuumthebeams

do vibrate with high Q [Solgaard1992]. The viscousterm must then representthe

dampingofair on themotion of thebeam.

37

3.4.1 SwitchingVoltageThe first questionwe addressedwith the beammodel was the predictionof the second

instability voltage(switchingvoltage). Theexperimentaldataaregiven in Table 3.3.

The simulationresultsarein Figure 3.4.

Stress[MPa] BeamLength[j.tm] Voltage,V2

800 20 18

800 16 26

400 16 18

100 16 11

Table3.3: Experimentalsecondinstability voltages.

25

I I

600

TensileStress [MPa]

Figure3.4: Resultsof beammodel. Secondinstability voltageis plotted

as afunction of nitride tensilestressfor threedifferent lengthbeams,15

Jim,20 Jim, and25 jim.

The simulatedvaluesareapproximately25% lower than the measuredvalues,with no

fitting parameters.As canbe seen,theresultsare also consistentwith the scalinglaws

pertainingto length andintrinsic stress. Theunderestimationofboth thestringandbeam

.~

CC.)

20

15

10

5,

0

0 200

—

400 800 1000

38

modelsmaybe dueto acommonerror, theuseof thesemi-infiniteparallelplatecapacitor

model for thebeamattraction. Sincethegapto width ratio is about1:10,stray field lines

maydecreasethecapacitanceenoughto accountfor partof the 25%error. Twomaterial

parametersthatarepoorlyknown,Young’smodulusandthedielectricconstantof nitride,

may also contribute. The dominantcontributionis probablydueto the addition of top

electrodealuminumto thebeams.

3.4.2 PeakStressThe yield stressof our LPCVD nitride is not well known, so it is not possible to

definitively determinehow dangerousstressconcentrationsin the beamswill be.

Accordingto thebendingseenin SEM micrographsandthebeammodel, it takesa beam

from 2 to 4 micronsto deformdown to the substrate.This meansthat the increaseof

peakstressover theaveragestress[DenHartog 1949] is lessthan400 MPa.

das = —4Et—

dz

where s is peakstressminustheaveragestress,z is thedirectionalongthelength of the

beam,and a is theanglebetweenthe tangentto thebeamat z andthehorizontal. The

expressioncanbe evaluatedin the string model to give an estimateof the peakstress.

Thusthe maximumstressseenin high-stressdevicesis 1.2 GPa(400MPaplus 800 MPa

averagestress),while the yield stressis nominally 14 GPa [Petersen1982] (this value

wasmeasuredfor muchthickerfilms and for only one reagentgasratio). It is likely that

stressconcentrationsdueto surfaceroughnessorcrackscouldproducemuchlargestress

concentrations.

3.4.3 HysteresisA simulationof deflectionof the centerof a beamin the beammodel asa functionof

voltage is presentedin Figure 3.5. Thehysteresiswidth is representedquite well by

V1 0.4V2, which wasderivedfor thestring model. Theopennessof the curvemaybe

useful in passive-matrixaddressing. Also, as per the string model, the normalized

deflectionat the secondinstability point is less than the 0.33 predictedby the string

model.

In Figure 3.6 is ameasuredhysteresiscurvefor a singlepixel on a striatedsubstrate(as

describedin Section4.4.5). Optical systemII wasusedto makethemeasurement,with a

39

CCD camerain themicroscopeframeasthedetector. Thereis a significantdarkcurrent

in theCCD, sono contrastdatacanbe takenfrom this curve.

b

Voltage[arb. units]

Figure 3.5: Simulatedhysteresiscurve.Note that V1 0.4172, andthatthe

beamis deflectedby lessthat 1/3 at V2.

3.5 Row-adressingMethodThe useof inherentdevicebistability for a passivelydrivenarrayof devicesis aunique

featureof micromechanicaldisplays. Passivematrix row-addressingusesthreestate

drivers. Rows arebiasedat ground,and the columnsarebiasedat (V1 + 172)/ 2. See

Figure3.5. Theframeis addressedtwice,onceto turn on pixelsthat areoff, andtheother

to turn off pixels that are on. In the first case,the row is selectedby applying

—(V2 — V1) / 3. Individualcolumnsareturnedon by applying (V1 + 172)!2 +(V2 — V~)/ 3.

In this casethetotal voltageacrossthedesiredpixel is greaterthan V2. sothepixel turns

4(V1+V2)

40

on. The voltagesonpixels in otherrows and columnsareall betweenl7~and V2, sono

pixels switch. Similarly, to turn on pixels off, therows aregroundedexceptfor one,to

which +(V2 — V1)/ 3 is applied. The columns that are to be unchangedremain at

(V1 + V2) / 2, but to the columnsto be switchedis applied (V1 + V2) / 2— (V2 — 17k) / 3.

The desiredpixel thenhas (V1 + V2) / 2— 2(V2 — V1) / 3, which is less than V~,so the

pixel switchesoff. Otherpixels areleft betweenV1 and V~anddo not switch. In this

case,at the cost of addressingthe frame twice asoften, activematrix performanceis

achievedat passivematrix complexity. Severaldevicesexhibitedthis behaviour,though

theyweredestroyedin testing.

AppliedVoltage

Figure 3.6: Measuredhysteresiscurveof asinglepixel. Thefinite slope

at the instability voltagesaredueto variationacrossthepixel of individual

beams.

Contrastwill be degradedfrom 80:1 to 40:1 becausethe beamsarepartially deflected

undera (V1 + 172) / 2bias. The way to fix this is to makethe spacerthicker, to shift the

hysteresiscurvedown. In this way thethicknessescanbe adjustedsothat (V1 + V2) / 2

producesminimal diffraction.

0 4 8 12 16 20 24

41

3.6 SpeedTheresonantfrequencyof thesedevicesis closeto 10 MHz [Solgaard1992]. Theyare

faster thanearlierdevicesbecauseof smallerdimensionsand higherresidualstresses.

The lO-to-90switchingspeedis 20.5 ns(seeFigure3.7). This speedof switchingmakes

the GLV the fastestlight valve of which the authoris aware,roughly 500 times faster

than TI’ s valveand500,000times fasterthanLCDs on themarket. This speedis useful

becauseit allows thedeviceto operatein arow by row fashion. This eliminatesthe need

for a full set of datalatchesfor the entire frame—twoordersof magnitudesavingsin

transistorcount.

Rate EventsperFrame Frequency

FrameRate 1 60 Hz

FrameAddress 2 120 Hz

5 bitsgray scale 32 3.8 KHz

Line Rate (VGA) 480 1.8 MHz

noninterlaced

Table3.4: Timebudgetfor row by row addressing.A 1.8 MHz line rate

is usedto addressa non-interlacedVGA displaywith 15 bits (i.e., 5 bits

percolor).

3.7 Temperature LimitsIf the GLV is usedin a projectionsystem,it is likely that anextremelybright andhigh

power sourcewill be used. About 5% of the incident light will be absorbedby the

aluminumtop reflector,andthis light will heatthedevice. Sincethematerialsusedare

fairly stablewith respectto temperatureup to 400°C,themeltingpoint of aluminum,it is

expectedthat theGLV will berobustwith temperature.However,it is importantfor the

driver designthatthe instability voltagesnotchangetoo muchwith temperature.

Usingthevaluesin Table3.4, theaveragestressin beamscomposedof 1325A of nitride

and400 A of aluminumdecreasesby 0.25 MPaJ°C. Therefore,at 400°Cthe average

tensilestressin thebeamsdecreasesby only 100 MPa, which resultsin approximatelya

25% shift in instability voltagesfor low residual stressdevices. The changeis

correspondinglysmallerfor higherstressdevices.

42

Another problemis the fact that the aluminum,which becomescompressedby the

smallerthermalexpansioncoefficientof silicon, may forcethe beamto deflectup past

the planeof the spacer. Becauseof the dependencyof the materialsparameterson

depositiontechnique,this possibility shouldbeexploredexperimentally.Sinceonly 5%

of theincidentillumination is absorbed,it is extremelyunlikely that any lamp couldheat

aGLV past150 or 200°C.

000

Figure3.7: Pixel switchingin 20.5 ns. Thephotocurrentfrom a single25

x 25 p.m pixel is shownasmeasuredon a silicon photodetectoron optical

systemI. Thepixel switchesfrom theup to thedown position. Therise

beforethetransitionis thoughtto be an artifactof themeasurement.

20 nsec/divhorizontalsweepspeed

43

Material YoungsModulus[GPa] ThermalExpansion[106/°C]

Al 70 25.0

Si 73 2.33

Si3N4 200 0.8

Table3.4: Materialsparametersfor thermalexpansion.[Petersen1982]

44

Chapter 4Fabrication of the GLV

4.1 Basic ProcessThe GLV can be fabricatedin its simplest form with only a single mask. The

morphologyof a singlepixel is shownin Figure4.1. The pixel is definedby a frame

which extendsalongthe front andbackedges. Connectingthetwo piecesof the frame

areseveralbeams,which are the movingparts of the device. Beneaththe frameis a

spacerlayer,which supportstheframeawayfrom thesubstrate.An air gapseparatesthe

beamsfrom thesubstrate,which is conducting. On top of thebeams,theframe,andthe

exposedareasof thesubstrateis a thin layerof metal,whichenhancesthereflectivity of

thestructureandservesasthetopelectrode.

Fabricationoflinear arraysof thesedevicesis diagrammedin Figure4.2. Thefirst stepis

to depositon anprime silicon wafer a 1325 A thick layerof silicon dioxide (hereafter

abbreviated“oxide”) followed by 1325 A of silicon nitride (“nitride”). The nitride is

patternedto form theframeandbeamsof thedevice. Then an isotropic, selectiveetchis

usedto undercutthe oxide from beneaththebeams. In orderto freethebeams,at least

0.75 p.m of undercutis needed.However,this is not enoughtocompletelyundercutthe

oxidefrom beneaththeframe. In this way theframeremainssupportedby theoxide,and

the beamsare freebut supportedat theirends. Since a silicon rich LPCVD nitride is

used,thebeamsareundertension. Finally, 400A of aluminumareevaporatedonto the

top of thestructureto form thetopelectrodeandreflector. Thewafersarethendiced.

45

silicon nitride

silicon dioxidespacer

siliconsubstrate

1300A

aluminumtopelectrodeandreflector

beamsheldup

by tensilestress

(b)

beampulled down

electrostatically

(c)

Figure4.1: Singlepixel of theonemaskGLV process(a). Beamsin the

undeflectedposition(b). Beamspulled againstthe substrate(c). Not to

scale.

1: Depositionof sacrificial layerandbridgematerial

3: Etchingof sacrificial layer

2: Patterningof bridges

nitride siliconoxide aluminum

Figure4.2: Schematicof singlemaskprocess.

(a)

4: Metallization

46

4.2 IsolationDeviceto device isolation is essentialfor deviceoperation. Althoughelectricaldevices

typically rely on reverse-biasedjunctions or mesas,only mesasare suited to

micromechanicaldevices.

The problemwith reverse-biasedjuncitons arisesfrom the fact that the 10:1 width-to-

thicknessratiomakesit difficult to cleancontaminantsfrom underthe beams. For this

reason,it is undesirableto usephotoresiston the GLV afterthe top electrode/reflector

layer hasbeenevaporated.Without apatterning,theelectrodematerialwill short the

isolationjunctions. Anotherproblemis that thelargeoperatingvoltagesof theGLV, up

to 30V, cancauseavalanchebreakdownofthe isolationjunctions.

In orderto designamesaisolationprocessthat requiresno maskingstepsafterthemetal

deposition,overhangingfeaturesareneeded.Fortuitously,thenitride layerof theGLV

overhangsthe oxide spacerby overhalf a micronin all directions,providing maskiess

deviceisolation. Thisoverhangis an artifactfrom therelease-etchprocess,which hasto

undercutby at leastonehalfof thewidth ofthewidestbeam. In colordevicesthis means

at least0.75 p.mof nitride overhangtheoxide.

Pixel 1 Pixel 2

In the caseof an idealdevicewith a perfectlyevaporatedtop-electrode,thethicknessof

themetalcouldapproachthatof the oxidespacer,over 1000A. However,if thequality

of the sidewalls is poor, then it becomesapparentthat thick metalcan increasethe

likelihood of shortingwhenthebeamsarebrought into contactwith the substrateasin

Figure4.4. Peakfields in air andnitride canapproach200 V/p.m for an ideal device. If

the sidewallsare imperfect,the fields can increaseseveral-fold,to perhaps 1 KV/p.m.

Thesefields aredangerouslycloseto the dielectricbreakdownfields. For this reason

thinneraluminumwasused,with somelossin conductivityandreflectivity. Photographs

4.1 and4.2 showdevicesthatmayhavebeendestroyedby shorting(seeSection4.7).

IsolationRegion

Figure4.3: Deviceto deviceisolation.

47

Photograph4.1: Gratingdestroyedby shortingandfusing. The failure of

thesebeamsis attributedto poorly insulating nitride and electrostatic

breakdown. This hypothesisis supportedby the fact that singlebeams

havenot beenobservedto fail in this way: this processeitheraffectsan

entirewaferor is absent.Thedestructionof gratingsis far lesscommon

thansticking asa failure mode. Fusingoccursat low voltages,typically

lessthat 20 V.

48

Photograph4.2: Closeupof fusedbeam.

49

4.3 Interconnect Conductivity and ReflectivityTo enablesimple line-by-lineaddressingof the GLV in a megapixeldisplayrequiresan

RC chargingtime for arow (or column)of 100 nsor better. This in turn requiresa sheet

conductivity of 0.5 ~ (per square). This is achievablewith a variety of metal

interconnectsincluding aluminum,silver, and commongroup VIII metals. Of these,

aluminumand silver are thetwo with good reflectivity. Silver suffersfrom corrosion

problemscomparedto aluminum,soaluminumwaschosenfor thetop electrode(andthe

top surfaceof the bottom electrode,sincethe depositionisn’t maskedor etched). The

thicknessof aluminumneededfor 0.5 ~, which is 750A,is too thick giventhe slopein

thenitride sidewallsasdiscussedabove. Therefore,a thinneraluminumlayerwasused.

Thesheetconductivityof400A of aluminumis 1 ~. Thedecreasein conductivitywill be

problemfor largearraysbut is notan issuefor thesmallerarraystestedhere.

Thereflectivity ofa thin layerof aluminumon topof nitride is givenas[Ramo 1984]:

R= z-z0 2z+zo

where

= ~ Z~~ycosf3L+ jZA1 sin/3LAl ZA! cos/3L— JZSINsin/3L

and Zs~N=154Q, ZA! =5.3+53J~ [Palik 1985] Z0 =377~,and /3=2,r/A,. The

reflectivity R is 0.33 for L=200A of aluminum,and0.75 for 400A. The sameresult

Figure4.4: Field concentrationcausedby poorsidewalls.

50

holds for silicon, with Z~,= 108~andreflectivities of .38 and .84 for 200 and 400A,

respectively. Although 400 A is not desirablefrom either reflectivity or conductivity

considerations,it is a fair compromisewith minimizing the thickness. With improved

sidewall formation, 750A of aluminum will satisfy reflectivity and conductivity

requirementsfor thetop electrode.

Mechanically, the main effect of the aluminumis to add massand stiffnessto the

cantilevers.Young’smodulusof aluminumis 70 GPa,but the increasein beamstiffness

is primarily dueto theincreasein beamthickness.For normaloperationof the devices,

the elasticlimit of aluminumis exceeded,so Hooke’s law is no longervalid. For this

reasonthe aluminumwas neglectedin the modelling in Chapter3, with a resulting

underestimateof switchingvoltages.

Otherpossibilitiesfor thebottOmelectrodeincludemetalsuicidesand polysilicon. The

bottom electrodewill generallybe coveredwith a layer of materialsfrom the top

electrodedeposition: theonly opticalrequirementof thebottomelectrodeis smoothness.

But silicides andpolysiliconsuffer from unsatisfactoryconductivities. The advantage,

however,is thatbotharestableatmuchhighertemperaturesthansimplemetals[Murarka

1993]. In particular,polysilicon is stableat 785 °C, thetemperaturefor LPCVD nitride

deposition. In this work polysiliconis usedfor thebottomelectrode.Ultimately a more

conductivebottomelectrodewill berequired.

4.4 StickingWheneverthe beamsof the GLV are brought into contact with the substrate(or

underlying interconnect)thereis a possibility of sticking. Sticking generallyoccurs

eitherduring thedrying stepaftertheoxidereleaseetchor whenthebeamsareswitched

down into substratecontactby a voltage exceedingV2. In the absenceof a third

electrode,which could supply an upwardelectrostaticattractionto raise stuckbeams,

sticking mustbe regardedasa devicefailure. As afailure mode,it is not catastrophic,

since sticking usually in not accompaniedby short- or open-circuiting(which then

destroysa roworcolumn).

Thebasicmechanismof sticking dependson whenit occurs. During fabrication,asthe

water that fills the volume beneaththebeamsevaporates,surfacetensionof the fluid

pulls the beamsinto contactwith the substrate. It waspostulated[Alley 1992b] that

51

solutesin theevaporatingfluid that remainbehindcancovalentlybondthebeamsto the

substrate. Sincebeamsthat havebeenstuckin this way takeseveralhoursto become

unstuckwhenre-immersedin water,it is assumedthatthebindingenergyis large.

4.4.1 Water

If the beamsbecomestuck during operation,the likely culprit is hydrogenbonding

mediatedby moisturebetweenhydrogenatedandhydroxylatedsurfaces[Scheeper1992].

In the GLV the beamsaremadeof nitride, which canhydroxylate,and the substrateof

silicon, which canoxidize. Thetestof theapplicability of this theoryto theGLV is quite

simple: muchhigherdeviceyields andlongerdeviceoperationareseenfor light valves

operatedin a flowing dry nitrogenambient. Also, devicefailure from sticking during

operationis reducedfor wafersthat arestoredin a dry vacuumchamber. In order to

quantifytheseresults,beampeelingtheorywill be reviewedin thenextsection.

Photograph4.3: Harp structure.

52

Sensitivity to watervapouris not uniqueto GLVs, and whenmanufacturedtheycanbe

bakedandhermeticallysealedin packages.

4.4.2 BeamPeelingTheoryAn excellent theory that analyzesthe dynamicsof beamsticking hasrecentlybeen

publishedby Mastrangelo[Mastrangelo1992; Mastrangelo1 993a; Mastrangelo1993b].

Theapplicableformulafor Lh, theminimumbeamlengththat will be stuck, is givenasa

function of the averagestressin thebeam, ~R, the specificbindingenergy,Y~,andthe

ratioof surfacecontactareato totalareaunderthebeam,5:

Lh4 =1128Et5ii+4o~RLh2

L~r~5)(~~ 21Et2

The equationcanbe solvednumerically;theelasticmodulustermthat comesfrom the

stiffnessof the beamand the residualstressterm areboth significant—neitherbeamstiffnessnorbeamstressdominates.Mastrangelofinds y, = 270mJ/m2 for hydrophilic

(waterattracting)and y~= 100mJ/m2 for hydrophobic(waterrepelling) surfaces;our

data(seeFigure 4.6) agreeswith the hydrophilic finding. The reasonhydrophilic,

surfaceshavea higherbindingenergythanhydrophobicis that hydrophilic surfaceshave

hydroxl groupsthat canhydrogenbond,while hydrophobicsurfacesusetheweakerVan

derWaal’s bond.

Thebasicmeansof testingthespecificbindingenergyis thoughtheuseof ateststructure

thathasa numberof different lengthbeams,from 10 to 40 p.m. This “harp” structureisshownin Photograph4.3. Generally,all thebeamslongerthanacertainlength, Lh, will

be stuck, while all the shorterbeamswill be up. This is shownon an atomic force

micrographin Photograph4.4. To havehigh yield, a GLV device shouldbedesignedwith L < Lh. As discussedin Section2.6, it is desirableto make L between10 and20

p.m. This requiresthat Lh beat least20 p.mif not30. Initial valueswere9 ±2 p.m. The

resulting value of Lh for eachprocessvariation will be given, and the resultsare

summarizedin Section4.4.8.

4.4.3 StressThesimplestway to decreasesticking is to increasetheresidualstressin the film. This

canbe accomplishedby varying the ratio of dichiorosilaneto ammoniain the nitride

LPCVD reactor[Beck 1990]. Stressesfrom 100 MPato 800 MPa arepossiblewithout

53

compromisingthestability andbreakdowncharacteristicsof the film. The first step in

decreasingsticking was to move from using 100 MPa films to 400 and 800 MPa. Thisresultedin Lh increasingfrom less than 10 p.m to 12 or 13 jim, in agreementwith

Mastrangelo’s equation.

voltage.

The tradeoff for decreasedsticking is increasedoperating

1400

1200

1000

800

600

400

200

01:6 1:3 1:1 3:1

Dichlorosilaneto AmmoniaRatio

Figure 4.5: Control

1990].

Substrate

Si

Oxide

of residualstressin LPCVD nitride. From [Beck

4.4.4 SurfaceTreatmentsAfter increasingtheresidualstress,thenextapproachto increasingLh wasto decreasey~.

thespecific bindingenergy. Mastrangeloreporteda decreaseof y~tolOOmJ/m2 when

thebinding surfaceswerechangedfrom hydrophilic to hydrophobic. This is consistent

with [Scheeper1992]. Baresilicon is hydrophobic,but it’s oxide is hydrophilic. So

while afreshly releasedwafermayhaveahydrophobicsurface,within hoursthatsurface

becomeshydrophilic..

One strategyfor solving this problemis to bond a hydrophobicmonolayerover the

oxidized surface,since preventingthe formation of a surfaceoxide is difficult in a

packageddevice [Alley 1992a; Alley 1 992b]. Alley’s octadecyltrichlorosilanemethod

wastriedon theGLV with no success.Thebeamstickingbecameworsethanbefore;all

ResidualStress[MPa]

6:1

54

beamsin the harp structurestuck to the substrate.Since thechemistryof the surface

treatmentrequiredanhydrousconditions,waferswere transferredfrom the release-etch

bath to dry, organic solvents. If the largeaspectratio of the areasunderthe beams

impededdiffusion of waterfrom theseregions,then thesilatingagentwould bondto the

water ratherthan the oxidized surface. This produceda hydratedpolymericmaterial

beneaththebeamsthat ruinedthedevices. It is thoughtthat this processshouldbeableto

be developedfor the GLV to avoid this problem,but it promisesonly a two foldimprovementin ~ Since Lh dependson theproductof 5, and y,, anotherapproachis

to reducethe effectiveareaof contactbetweenthebottomof thebeamandthesubstrate,

which is discussedin the nextsection. Otherwork hasbeendoneto reducethe contact

areawhich is notdiscussedhere[Sandejas1993].

4.4.5 Striations

If the substratebeneaththebeamsis corrugated,then S is reducedby the ratio of the

changein surfacecontactarea. For example,if the substrateis etchedto produce100 Atall lines, lp.mwide, spacedevery5 jim, thenthetotal contactareais reducedby afactor

of five. Thelines, hereafterreferredto as“striations,” arein thedirectionperpendicular

to the beams. This assumesthat when the beamsarebrought into contactwith the

substrate,theyonly hit thetopsof the lines,not thespacesin between.In orderto insure

this, theswitchingvoltagefor 5 p.mlongbeamswith iooA gapswascalculatedusingthe

beammodel andfound to be largerthanthat neededto switch 15 p.m beamswith .1325

p.m gaps. SeePhotographs4.4 and4.5.

This methodof reducing S hasthreevariables,the heightof thestriations,thewidth of

the striations and the line-to-line spacing. Increasingthe height of the striations is

desirablebecauseit allows the line-to-line spacingto increase. The height of the

striations is limited by it’s effect on contrastratio. Heightsof 100 A do not reduce

contrastappreciably(seeFigure4.8 andthe fact that the striationonly occupies0.2 or

lessof the surface). With 100 A high striations,the line-to-line spacingcanbe several

microns,at least5 if not 10. Thestriationwidth is limited by lithographyor processing

techniques.Sincewe usedlithographicallydefinedstriations with no etch-stop(which

would uncouplethestriationwidth andheight),wewere limited to 1 p.m wide striations.This gives a total S= 5. This geometryincreasedLh from 13 to 22 jim, which again

agreeswith Mastrangelo’s equation.

55

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Photograph4.4: Atomic force microscopeimage of harp structure.

Striationsareclearlyevidenton bothsubstrateandnitride. In this casethe

reflow smoothingprocessfailed to provide a flat surfacefor nitride

deposition.

4.4.6 Van derWaalsBonding

Although water undoubtedlyplays an important role in the sticking forces, in a dry

ambient sticking still remainsa problem. In this casethe culprit is not a surface

chemistryproblembut a fundamentallimitation, theVander Waal’s attractionbetween

two insulators[Scheeper1992]. Thepressureoftheattractionis givenby

P=6irD3

where A is theHamakerconstantand D is theseparationbetweenthetwo materials. For

D> 30 nm thelong-rangeattractionis givenby

56

Photograph4.5: Striationsin one dimensionalarrays.Note the slight

striationin thenitrtidebeamsandthe 1 p.mundercutof theframe.

57

1:40

35

30

~25

~

5

0

100 1600

Stress {MPa]

Figure 4.6: Beam peel length as a function of residual stressand

striations. The three curves are for S = 1:1 (i.e., no striations), 1:4, and

1:16. The two data points are experimental for S = 1:1 and 1:5 with 800

MParesidual stress.

The expressions for Van der Waals bonding and the string model of chapter 3 can be

combined to plot Lh as a function of the striation ratio, 5, and residual stress, ~. See

Figure 4.6. The results show an encouragingtrend. For lithographicallydefined

striations, with a maximum S of 1:10, there are modest gains possible. But through the

use of an etch stop layer,which would allow timed etching to narrowthe striations,S = 1:100 is possible. This shouldexhibitan extremelylong Lh. Althoughthis approach

is appealing, using surface roughness is simpler.

When D is approximately2 nm, which may not be a bad guess for the surface roughness

on thebottomsideofthenitridebeams,thenthis expressiongivesa goodestimatefor the

observedvalueof y~for asubstrateexposedto moisture. This suggeststhat increasing

1:1

200 400 800

58

70

800 MPa

MPa

60

~: ::~

10

Figure 4.7: Surfaceroughnessand beamsticking. The threedots show

datafor 800 MPaandRMS surfaceroughnessmeasuredby AFM.

0

90

80

70

60

50

40

30

20

Film ThicknessError [nm]

Figure4.8: Contrastratioversusfilm thickness.Although thicknesserror

and roughnessare not the same,this figure, repeatedfrom Chapter2,

showsthatonly afew nanometersof roughnessareacceptablebeforethe

contrastratio decreasesprecipitously.

1 2 3 4 5 6Roughness[nm]

0 2 4 6 8 10 12 14

59

Photograph4.6a: Roughpolysilicon surface.

the roughnessof the substrateor the bottom side of the beamswill be useful in

controllingstiction.

4.4.7 SurfaceRoughnessandContrastOne semiconductormaterial that hascontrollableroughnessis LPCVD silicon, which

maybe amorphousor polycrystalline[Bawolek 1993; Dana1993; Voutsas1993]. By

controlling growthtemperature,film thickness,andsubstratesurfacequality avery large

rangeof surfaceroughnessis possible[ Ibok, 1993#64J. If wemaketheidentity that D,

the distancebetweenthe beamand the substratein the down position, is equalto the

surfaceroughness,thenusing thestringmodel andtheVanDerWaalsforcegivesFigure4.7, in which Lh is plottedasa functionof residualstressand surfaceroughness.Three

data points are added for 800 MPa deviceswith various polysilicon roughnesses

(measuredby atomic force microscopy). Photographs4.6aand 4.6b show the rough

polysiliconsurfaceandthenitride that is grownoverit.

60

Photograph4.6b: Nitride deformedby roughpolysilicon. The reflow

processsmoothstheoxidesurface,but someroughnessremains.

It is clearfrom Figure4.7 that increasedroughness,althoughnot aseffectiveasstriationsatincreasingLh, is still very effective. Thetradeoffis with contrastratio. Althoughwe

do not have atheory for contrastthat includessub-wavelengthscaleroughness,Figure

4.8 givessomeideaofthe decreasein contrastratio in this circumstance.

4.4.8 Progressin ReducingSticking

The progressin solving the sticking problemis summarizedin Table4.1, in which Lh ~s

given for eachprocessvarient. Short beamshavevery high switching voltages. An

observedpracticallimit wasthat above40V electromigrationof thealuminumresultedin

thedevicesshortingto ground. If thetestcircuit had significantsourceimpedance,then

thebeamssimply do not move(althoughtheyareup). If a low-impedancesourceis used,

61

the deviceexplodes,scatteringdebrisfrom thebeams,frame,andbondpads.In orderto

keepoperatingvoltagesto 35 V or less,theminimumusablebeamlength is 15 p.m. Toinsure>99% yield of usabledevices,an Lh of 25 p.mis desired:

Process Lh [p.m]

Desired >25

Initial (low stress) 10

High stress 13

Surface treated 8

Striated 22

p-Si roughened >40

Table4.1: Progressin decreasingsticking.

Photograph4.7: Singlepixel in atwo-dimensionalarray.Roughnessin the

nitride resultsin lessthanoptimal lithography.

62

4.5 Two-Dimensional ArraysThe problemswith building two-dimensionalarraysof pixels are importantbecause

solving them is the first stepin integratingtheprocessesof light valveconstructionand

driver circuit integrationand fabrication. Regardlessof whetheran active or passive

matrix is used, the interconnectionof the drivers and the pixels will require an

interconnectstep,and the developmentof two-dimensionalarraysexhibits significant

difficulties.

Theprimarydifficulties with addinganinterconnectlayeraretwofold: the interconnects

mustremainisolatedfrom one another,the substrate,and the top electrodes;and the

interconnectlayermustbe thermallycompatiblewith subsequentprocessing.Thefirst of

theseproblemswehavesolved,thesecondis a sourceof continuingconcern.

4.5.1 IsolationThe interconnectlayer presentsspecial problemsbecauseof the fact that the present

processdoesnot havea maskingstepafterthetop electrodedeposition. Becauseof the

large aspectratio of thebeamsand the small thicknessof the spacer,it is difficult to

removephotoresistresiduefrom beneaththe releasedbeamsfollowing lithography.

Therefore,thebasicprocessincludesno wet processingafterthereleaseetch. Sincethe

top electrodedepositionfollows the release,thereis no patterningof the top electrode

metal. In thebasicprocess,isolationbetweentop electrodesdependson theoverhangof

the nitride over thespacerto preventshorting. The samemesaisolationstrategyis used

to isolatethebottomelectrodes.Theuseof mesaisolationrequirestheuseof avery non-

planarstructure,with theconcomitantproblems.(SeePhotographs4.8 and4.9).

4.5.2 ThermalbudgetThereasonsgivenfor choosingaluminumfor thetop electrodeareequallyvalid for the

bottomelectrode.Thedifficulty is that subsequentlayers,especiallytheLPCVD nitride

depositionat 785 °C, exceedthe thermalbudgetof the aluminum(<400 °C). For this

reasonpolysilicon interconnectsare used insteadof aluminum. Although the

conductivityof polysilicon will makearrayslargerthan200x200 be RC limited rather

thandevicelimited, polysiliconhastheadvantagesthat its roughnesscanbecontrolledby

processingandit is stableat thenitride depositiontemperature(providedit is cappedwith

oxide).

63

4.5.3 BasicRecipe

Processingbeginswith a 5000 A oxide isolation layer beinggrown on a baresilicon

substrate.This isolation layercanbegrownby any availabletechnique.On top of this

layeris growna 3000-6000A undopedpolysilicon layer. Thetemperatureof this growth

may bevariedto control surfaceroughness.Thelayeris probablyamorphousinitially,

but it crystallizesduringsubsequenthigh temperatureprocessing(thereflow step). The

polysilicon is pre-depdiffusion dopedwith phosphorous,cleaned,andpatternedinto the

bottominterconnectlayer. 1325 A of low temperatureLPCVD oxide aredepositedon

top of thenonplanarbottomelectrodetraces.This oxide is dopedwith 8% Phosphorous

to reducethereflow temperature.Theoxideis steamreflowedat 1000°C for 20 minutes.

1325A ofLPCVD nitride aredepositednext,thenpatternedanddry etched. Thewaferis

then cleanedto removeall tracesof photoresistand released. The oxide spaceris

removedfrom under thebeams. In addition,oxide is removedfrom under theedgesof

the framesand the bottom electrodetraces. The overhangingpolysilicon and nitride

providethe isolation. This processis depictedin Figure4.9, and completedetailsare

givenin Appendix1.

4.5.4 NonpianarProcessing

Thedifficulties in thedevelopmentof this processarebestshownin Photographs4.9 and

4.10, which are two views of the cornerregionsof the 4x4 array in Photograph4.8.

Photograph4.9 is in the sameorientationasPhotograph4.8, with the top electrode

makingconnectionsvertically andthepolysiliconlines underneathrunninghorizontally.

Photograph4.10 is from thetheotherorientation.Thenonplanarstructureusedto isolate

thebottomelectrodescreatesthepossibility of nitridestringersshortingbetweenadjacent

topelectrodes.Thesestringersareevidentat thebrighthorizontalline in Photograph4.9

andon theright sideof Photograph4.10. Second,stepcoveragealongtheedgesof the

polysiliconmakesit possiblefor thetop electrodeto be open-circuitedif thereflow is not

sufficient. This canbe seenin thetop part of Photographs4.10, wherethe nitride/top

electroderunshorizontallyover thepolysilicon,downto theisolation oxidebetweenthe

polysiliconlines,andthenbackup thenextpolysiliconline. Carefultuningof thereflow

processwasusedto solve both problemsby smoothingout the edgesof the nonplanar

structures.

64

004

I

Figure4.9: Schematicof two dimensionalarray.This figureshowspixels

with only threebeamsratherthantheusualten for clarity.

‘fi

z

65

4.6 ReliabilityAdequatedatafor reliability doesnotexist for theGLV. An initial experimentwasdone

that cycled pixels over300 billion cyclesat an acceleratedrate(1 MHz for 100 hours),

which correspondsto tenyearsof televisionusefor a color GLV with eight bits of gray

scale. Thedeviceswere operatedwith a 25 V squarewavein ambientconditions. No

pixel damage(in the form of sticking or fusing) was observed. However,recentwork

[Pryputniewicz 1994] suggeststhat acceleratedlifetime testing is not valid, since it

doesn’tgive the material time to deformplastically or for cracksto grow. A second

limitation is that this testingwasdoneon striateddevices. It is not known whetherthe

useof surfaceroughnessratherthanstriationswill increasebeamcrackingornot.

Photograph4.8: 4x4 pixel array.

66

4.7 DeviceFailureThreetypesof devicefailure havebeenobserved.Low voltagefusing is aprocessthatis

relatedto a failure in wafer processing. It first appearedto be correlatedto particular

LPCVD nitride depositions,althoughmeasurementof thicknessandrefractiveindexdid

not reveal anything unusual. A laterhypothesiswas that the useof thicker (1 p.m)

aluminumbondpadmetalizationscaused leachingof silicon from the nitride into the

aluminum. This processdegradesthenitride andis knownastheKirkendall Effect [Wolf

1990]. The solution is to this problemis to usean aluminumsputtertargetwith 4%

silicon. Subsequentwafershaveconfirmed this phenomenologically,although no

measurementshave been performedto establishcausality. Wafers coated with

aluminum/silicondid notexhibit fusing.

The secondtype of devicefailure is high voltageshorting. When > 40 V is appliedto

devicesarcingcauseslargecurrentsto flow and deviceheating. Usually thedevice is

destroyedcatastrophically.It is thoughtthat the largefield concentrationsdescribedin

Section4.2 areresponsible.

Sticking of the beamsto the substrateis the third andmost commonform of device

failure. In inital experiments,thebeamsstuckto thesubstrateduring thedrying process

following the release-etch.Previouswork establishedthat freeze-dryingthe devices

resultedin un-stuckdevices[Solgaard1992]. However, whenoperatedinto contactwith

the substratethesedeviceswould stick. After the developmentof thehigh stressnitride

process,thefreeze-dryingtechniquewasabandonedin favorof standardspin-drying. For

all subsequentwork, if acertainbeamdid not stickduringspin-drying,it generallywouldnot stickduring subsequentoperation. For this reasonthefigure of merit for yield, Lh,

was recordedfor devicesthat hadbeenrecentlyreleased.Somedegradationof Lh over

time wasobservedin devicesthat hadbeenleft in air for months,so laterdeviceswerestoredin vacuumand testedin dry nitrogen. The standarddeviationfor Lh on a wafer

wasbetween2 and3 p.m, so making Lh 10 p.m largerthanthedesignlengthresultedin

yieldsof greaterthan90%.

4.8 Future ProcessDesignThepresentwork lessenedthesticking problemandtheinterconnectproblemthoughthe

useof the surfaceroughnessand doping of polysilicon. It is very likely that stress

concentrationwhenthebeamsstrike aroughenedsubstratewill increasedamagerates.

67

Photograph4.9: Fourcornersof a two dimensionalarray. The nitride

runs vertically, and the middle shows the isolation region. A nitride

stringeris apparentin thebottomhalfof thepicture.

Also, it is possibleto redesignthe striationssothat theyarenot visible from thetop

surface. Thismodificationwouldpreventthecontrastratio from beingdegraded.Thus,

it is likely that futureprocesseswill makeuseof smoothmaterialsandetchedstriations.

68

Photograph4.10: Fourcornerspoint of two-dimensionalarray,alternate

view. Nitride/topelectrodesrunhorizontallyandmustbe continuousover

the stepformedby thepolysilicon lines,whichrun vertically. In theright

halfof the picturethenitride, spacer,andpolysiliconcan be clearlyseen.

A nitride stringershortstwo adjacenttop electrodestogether.

69

Anotherweaknessof thepresentprocessis thevery high thermalbudget,which is 785 °C

for thenitride and 1000°C for thereflow. While cleveretchingmight eliminatetheneed

for reflow, thenitride is aseriousproblem. Onealternativeis to usealowertemperature

nitride, sayPECVD. PECVD nitride is depositedat 200 °C andthroughtheuseof a two-

frequencysystemhascontrollableresidualstress. Sincesuchsystemscanalso deposit

oxide, PECVD appearsto solve thethermalbudgetproblem. Aluminum couldbeused

for the bottomelectrodeaswell asthetop. Theproblemis that thereleaseetchshould

havea good differential ratebetweenoxide and nitride. For LPCVD it is well over

100:1, which is necessaryfor etching 10:1 aspectratios. For PECVD, the best

differentialetchratewe foundwas 10:1. If anotheretchcouldbefound,thenthePECVD

optionbecomesan excellentsolution.

70

Chapter 5Conclusion

5.1 DeviceSummaryThegratinglight valveis arelativelynewdisplaytechnology. It is basedon reflectionphase

gratingsof electricallycontrollabledepth. Whenthebeamsaresuspended“up” from the

substratethedevicehasaminimumof diffraction, andnormally incidentlight is reflected.

If apotentialis appliedto bring thebeamsinto contactwith the substrate,thenthedevice

diffracts80%ofthe light into thefirst orderdiffractionmodes,which arethencollectedby a

Schlierenopticalsystem. Thecontrastratio wasmeasuredto be 20:1 for black-and-white

displays. A contrastof 80:1 shouldbe achievablewith improvedprocessingtechniques.A

color gamuta little smallerthanthatof televisionphosphorswasmeasured.Improvedlamp

collimationshouldimprovethesaturationof thecolors. Contrastratiosfor optimizedcolor

devicesshouldexceed200. Pixelsassmallas6x20 ~Lmarepossible.

The position of the beamsis bistable for intermediatevoltages. For a qualitative

understanding,the beamscan be modelledas strings undertension. To get better

quantitativeresults,a full integrationof thefourth orderbeamequationwasused. The

validity ofthemodelwaslimited becausethecontributionof top electrodealuminumto the

beamstiffnesswas neglected. Switching voltagesbetween5 and 10 V should be

obtainable.Thelowestmeasuredin this work is 11 V. Thecombinationof bistability and

speed—thedevicesswitch in 20.5 ns—mightbeusedfor passivematrix addressingin a

row-by-row fashion. The device operation should not be significantly affectedby

temperaturesin excessof 200 °C.

Simple fabricationrequiresonly a singlemask. To makedeviceswith two-dimensional

arraysof contactstwo masksareneeded.Throughthe useof theoverhangof the frame

71

material the devicesare isolatedfrom eachotherwithout an additionalmaskingstep.

Aluminum is usedfor thetop layerofinterconnectsandthereflectivelayeron thebeamsand

spaces.Sticking of thebeamsto thesubstrateis causedby hydrationof thesurfaceand/or

Van der Waalsbonding. The two methodsof reducingthis problem,corrugatingthe

substratewith striationsandusing anaturallyroughsubstratemateriallike polysilicon,both

areextremelysuccessfulat reducingsticking. However,bothhavedeleteriouseffectson

the optical performanceof the device if overdone. Two-dimensionalarrays were

constructedwith a highly non-planarprocess.Despiteproblemswith shortsandopensin

thetop conductor,this methodwasusedto demonstrateworkingtwo-dimensionalarrays.

5.2 Future WorkTherearefourmajorareasof developmentneededfor thesedevices,presentedin orderof

increasingimportance.

In orderto usethebistabilityof thedevicesfor switching,it is necessaryto insurethatthe

hysteresisloopis madeasopenaspossible. This mightbeachievedby usingconducting

beamsor beamswith higherpermittivities. Conductingbeamswould havethe effectof

decreasingthegapbetweenthetop andbottomelectrodeswhenthebeamsaredown. This

increasein capacitancewouldcauseasubstantialdecreasein thefirst instability voltage.

A secondareaof researchis to determinetheeffectof surfaceroughnesson contrastratio.

This will helpdeterminewhetherroughnessor striationsshouldbeusedto reducesticking.

A newprocessfor striationsshould bedesignedthatis self-alignedandproducesquarter

micronstriations. This might haveaminimal effect oncontrastwhile decreasingsticking.

Ultimately, aself-alignedstriationthatwould not affectcontrastwould beideal.

Third, thedevicesof this thesiswereall producedwith LPCVD. Thisprocessrequirestoo

high temperaturesto be compatiblewith drivercircuit fabrication. Eitheramoveto PECVD

or to alternatematerialsmustbeconsidered.Possibilitiesfor alternatematerialsinclude

spin-onglasses,polymers,metals,andothertypesof oxynitrides. Thisprocessredesignis

absolutelynecessaryfor reliable operation,since the high temperaturesof the current

processmakeit necessaryto bond the drivers to the display ratherthan integratethem

monolithically.

Finally, the aging characteristicsof micromechanicaldisplays arenot at presentwell

understood.TexasInstruments’deformablemirror deviceusesmetal flexures,while the

72

gratinglight valveusessilicon nitride. Thelatter is expectedto havemuchbetterageing

characteristicsthanthemetalflexures. Although thisproblemis speculative,it mayproveto

bea critical decidingfactor. In addition,it is not clearwhat sort of packagingis necessary

to insurelong life of thesedevices,i.e. whetherfull hermeticityis needed.

Thesefour areasofresearchhighiight the limitations ofthis work andthepresentstateof the

gratinglight valve. With additional work in theseareasit is possiblethat thegratinglight

valvewill somedaybe commerciallyproduced.

73

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81

Appendix 1ProcessRecipe

Therecipefor two-dimensionalarraysof GLVs is givenin this appendix. Therecipeis

specific to the processesand equipmentof the Centerfor IntegratedSystems(CIS),

StanfordUniversity,duringthelatterpartof 1993.

Al.1 Standard ProcessStepsTherearetwo standardcleaningstepsthatarepartof any CIS process.

OrganicClean.

a. H2SO4:H2O2,9:1, 120C,for 00:20:00.

b. Dump rinseandspin dry (hereafter,DRSD).

Diffusion Clean

a. OrganicClean.

b. H2SO4:H202,3:1, 90C, for 00:10:00.

c. DR.

d. HF:H20, 1:50, for 00:00:15.

e. DR.

f. H202:H20:HC1,1:5:1,7OCforOO:10:00.

g. DRSD.

Thestandardlithographyprocessincludes:

a. Singe,150C,00:30:00.

82

b. SVGCoatrecipe 1 (includesadhesionpromoterandpre-bake).

c. Expose1 lOmJ/cm2,Ultratechstepper.

d. SVGDevrecipe 1 (includesdevelopmentandpost-bake).

Al.2 Current GLV process

1. SCRIBE

Buy L-Primewafers.

Scribe.

DRSD.

2. WETTHERMAL OXIDATION

Diffusion Clean.

FurnaceTylan 1,3, or 4. ProgramWET1000, processtime 02:15:00.

3a. POLYS1LICONDEPOSITION(option1)

FurnaceTylanpoly. ProgramAMOR4006, processtime 01:41:00.

target0.32 jim.T=560°C. SiH4=136 sccm. H2~110 sccm.

3b. POLYSILICON DEPOSITION(option2)

FurnaceTylanpoly. ProgramAMOR55O,processtime 01:41:00.

target0.26 Jim.

T=550°C. SiH4=136 sccm. H2=110 sccm.

4. POLYSILICON DOPING

FurnaceTylan 6. ProgramPOCL3900,processtime 00:40:00.

Predepdiffusionin POCI3 ambient. T=900°C.

HF:H2O, 1:50,for 00:00:30.

DRSD.

5. POLYSILICON LITHOGRAPHY

Standardlithography, field “POLY,” reticleTWOLEVELS—clearfield.

EtcherDrytek2, StandardPoly Etch(SF6:C2C1F5),00:01:15perwafer.

83

6. LOW TEMPERATUREOXIDE DEPOSITION

Diffusion Clean.

FurnaceTylanbpsg. ProgramPSG400,T = 400°C,SiH4=14sccm,PH3=86sccm,

processtime 00:05:10. target 1225 A—oxide will densify AND thicken during

thereflow stepby 100 A.

7. OXIDE REFLOW

FurnaceTylan 1,3, or4. ProgramREFLOW.

I = 950°C. Steamambient. 00:10:00.

8a. NITRIDE DEPOSITION(option 1)

FurnaceTylannitride. ProgramSIN5.2,processtime 00:37:45.

target1325A.T = 785°C. NH3=32sccm. DCS=l65sccm.

8b. NITRIDE DEPOSITION(option2)

FurnaceTylannitride. ProgramSIN3.0,processtime00:33:45.

target1325A.T = 785°C. NH3=50 sccm. DCS=150 sccm.

8c. NITRTDE DEPOSITION(option3)

FurnaceTylannitride. ProgramSIN1.0,processtime00:28:00.

target1325A.T = 785°C. NH3=100 sccm. DCS=100 sccm.

9. NITRIDE LITHOGRAPHY

Standardlithography,field “NITRIDE,” reticleTWOLEVELS—clearfield.

EtcherDrytek2,StandardNitride Etch(SF6:CF3Br),00:04:30perwafer.

NB: this is tooshort to clearup stringers;shouldbe00:08:00.

10. RELEASEETCH

Organicclean.

EtchBOB 6:1,00:02:50.

DRDRSD.

84

Appendix 2Beam Model

The following Mathematicascript was typical of thoseusedin modelling the GLV

mechanics.The first step is to assumeGreen’sfunction for the beamequationis of a

particularform. Then this solution with undeterminedconstantsis constrainedby the

homogeneousbeamequation,theboundaryconditionsfor rigid supports,andtheintegral

of thebeamequationacrossan arbitraryimpulse forcingfunction. Numericalvaluesare

substitutedandGreen’sfunction g [p. q] is evaluatedasa50x50 matrix. Thefunction

deform[P. v, z] returnstheconvolutionofGreen’sfunctionandthe nonlinearcapacitor

forcing functionasevaluatedfor deflectionsp, with voltagev. z is aviscosityor step-size

parameterto damposcillationsof thesolution. iter [v, k] is a routinethatcalculatesthe

self consistentbeamdeflectionfor voltagev, with k asalimit on thenumberof iterations.

Theremainderof theappendixshowsthedetailsof acalculationof ahysteresisloop.

85

yl= al + bi x + ci EA(T x) + di E#~(_ T x);y2=a2+b2x+c2E’(Tx) +d2E’~’(-Tx);

boundaryValues = Solve [ {yl==0 I. x->O,y2==O I. x->l,D(yi,{x,1}]==0 I. x—>O,D(y2,{x,1}]==O I. x->l,yl==y2 I. x -> a,D[yi,{x,1)]==D(y2,(x,1}] I. x -> a,D[yl,(x,2)]==D(y2,{x,2}] I. x -> a,D(yl,(x,3}] - D(y2,{x,3}] + W ==0 I. x -> a),{ai,bl,cl,dl,a2,b2,c2,d2}];

ee = 1.2 l0”il;epO = 8.85 l0”-12;tt = .13 10”-6;dd = 1 l0”-6;11 = 25 10~’-6;ii = dd ttA~3 / 12;to = .13 1OA_6;ten = 400 1O’~6;ww = 1O’~’~i2 epo 11

Normalizedtt = 1O”-.6 (ten tt

Normalized

Young’s Modulus (Pa]Permittivity of Free Space (F/rn]Beam Thickness (in]

Beam Width Em]Beam Length (in]Beam Moment of InertiaSpacer Thickness (in]

Residual Beam Stress [Pa]dd / (2 tO”2 ee ii)electrostatic attraction (equal to W)dd/ (ee ii))’°’.5restoring force (equal to T)

p1 = First(Simplify( si I.{T —> .070 1.414, W —> 8.2 1O”(—6), 1 —> 50)]];

p2 = First [Sirnplify( s2 I.{T -> .070 1.414, W -> 8.2 i0~(—6), 1 -> 50)]];

g(p_,q..] := If[p>q, p2 I. {x -> P~a ->q}, p1/.{x -> p, a ->q}] Green’s Function at q

bounds(i_] := {i,i,49,i)

deflectionTable = Table (g (x, a] ,Evaluate(bounds [a]], Evaluate (bounds (x]]];

Numerical evaluation of g

forcingFunction[d_] := If(d<.lO,((1 -

58 - 6 1O”3 (d—.2)] +

If(d>.135,5 10A3 (d - .135),0]

limitFunction[c_] := Max(Min[ .135, ci, 0]

sl= yl I. boundaryValues;s2= y2 I. boundaryValues;

86

middleMask = Table(If((x>15 && x<35),l,O],Evaluate (bounds (xl]]

(0, 0, 0, 0, 0, 0,0, 0, 0, 0, 0, 0,0, 0, 0, 1, 1, 1, 1, 1

1, 1, 1, 1, 1, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0,

errorFunc(a_,b_] := Apply(Plus,Abs((a-b)middleMask]]/49

yFull = clearY(.2];

yZero = clearY(O];

viter (start_, stop_, increment_, iterations_I : =

Module [{i,out,v},out = 0;

Doiter (v, iterations];out = Append(out,{v,y~25]]}];), {v, start, stop, increment)];

ListPlot(out,PlotJoined -> True];out

] This module calculates thethe deflection for voltages

clearY[s_] := Evaluate(Table(s,Evaluate(bounds(x]]]]

showY := Show(Table(ListPlot(m([i]], Plotjoined -> True,DisplayFunction -> Identity], U, Length(m])],DisplayFunctioxi -> $DisplayFunction,PlotRange -> All]

showF = ListPlot [forceVector, PlotJoined->True]

Plot (forcingFunction(w], {w, .11.25)]

60

40

20

—20

—40

0.12 0.14 0.16 0.18 0.2

87

-Graphics--

deform(p_,v_,z_] := Module (U),forceVector = Map[forcingFunction,p];qC = (vA2)/49 deflectionTable . forceVector;qC = Map [limitFunction, qC];p + z (qC-p)

] This module performs one iteration of the self-consistent algorithm

iter(v_,k_] := Module ({i},For (m= { } ; error= U;oldY=y;el=1;e2=errorFunc(yFull,y] ;step=

Min[1O e2,.9];i=l,((i<=k) && (el > i0”—5) && (e2 > i0’~-4)), i++,

newY = deform (y,v,step];m = Append (in, newy];el = errorFunc(y,newY];e2 = errorFunc(yFull,newY];error = Append(error, {el,e2,step)];step = If[ 200 e]. < step,step/2,step];step = If[ i0’~2 e2 < step,step/3,step];oldY = y;y = newY;

];showY

] This module calculates the deformation at voltage v_

y = yZero

(0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 00, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0,

viter(0,5, .5,50]

88

0.0

0.0

0.0

0

0

0.0

((0, 0), (0.5, 0.000377021), (1., 0.00157196), (1.5, 0(2.5, 0.0107871), (3., 0.0161082), (3.5, 0.0229396),(4.5, 0.0431862), (5., 0.0641659))

viter(5,6, .1,100]

0.2

0.18

0.16

0 .14

0 .12

0.08 ~ 5.4

((5, 0.0703352), (5.1,(5.5, 0.211044), (5.(6., 0.210744))

Join(9675,9676]

5.6 5.8 6

0.0775739), (5.2, 0.0866328),6, 0.211131), (5.7, 0.210948),

((0, 0), (0.5, 0.000377021), (1., 0. 00157196), (1.5,(2.5, 0.0107871), (3., 0.0161082), (3.5, 0.0229396)(4.5, 0.0431862), (5., 0.0641659), (5, 0.0703352),(5.3, 0.210394), (5.4, 0.210773), (5.5, 0.211044),(5.8, 0.210906), (5.9, 0.210802), (6., 0.210744))

viter(6,2,-.5,30]

(5.3,(5.8,

0.0036(4.,

(5.1,(5.6,

1 2 3 4 5

.0036(4.,

89

0.2

0.212

0.21

0.208

0.206

0.204

0.202

0 .198

((6, 0.210758), (5.5, 0.210734), (5., 0.210985), (4.5, 0.21

(3.5, 0.211063), (3., 0.210419), (2.5, 0.208765), (2., 0.

Show( ListPlot[9679,PlotJoined -> True],ListPlot(9683,Plotjoined -> True]]

0 .15

0.1

0.05

0.2

0 .15

0.1

0.05

1 2 3 4 5 6

90

0.2

0.15

0.1

0.05

-Graphics -

t=2000 A, T = 400 MPa, v = voltage * 2, L = 50jim

1 3 4 6

91