DigitalMicromirrorArrayforProjectionTV
MichaelA.Mignardi
TexasInstruments
Alightmodulatorforhigh-definitionprojectiontelevisionsystemshasbeen
constructedusinganarrayofmicro-mirrors.Thin-filmandsurface
micromachiningtechnologyprovidetheprimarymeansoffabricationforthearray,
whichisbuiltupoverconventionalCMOSSRAMaddresscircuitry.Thisarticle
providesdetailsofthemanufacturingprocess.
Areflective,spatial,lightmodulatorcomposedofanarrayofrotatablealuminummirrors
hasshownpromiseasanalternativetoLCDtechnologyforstandardandhigh-definition
projectiontelevision(PTV)systems.Resultsdemonstratethatthemodulator,calleda
digitalmicromirrordevice(DMD),offersperformanceequaltoorbetterthanacathode
raytube.Fundamentally,theDMDisanadvancedmicro-electromechanicalsystem
(MEMS)thatutilizesamatureaddresscircuittechnology.Itofferspotentialasadisplay
device(Fig.1)thatcanbemanufacturedwithhighyieldatmarketablecost[1][2].
DigitalMicromirrorReflectingArray
MicromirrorandDMDarray
Figure2aillustratesthemicromirrorelement,analuminummirrorsuspendedoveranair
gapbytwothin,post-supported,mechanicallycomplianttorsionhingesthatpermita
mirrorrotationof±10°.
DigitalMicromirrorElement
Figure2a&2b
Thepostsareelectricallyconnectedtoanunderlyingbias/resetbus(Fig.2b)that
connectsallthemirrorsofthearraydirectlytoabondpad.Fabricatedoveraddress
circuitryconsistingofconventionalCMOSSRAMcells,thereflectingelements
individuallyconsistofmirrors16μmwideonapitchof17μm(Fig.3).Inthismanner,a
bias/resetvoltagewaveformcanbeappliedtothemirrorsbyanoff-chipcircuit.Below
eachmirrorareapairofaddresselectrodes,whichareconnectedtothecomplementary
sidesoftheunderlyingSRAM.
Figure4showsthelayoutofthe
integratedDMDstructure.The
SRAMunderlyingeachmirrorisa
conventionalsix-transistorcircuitor
storagecellfabricatedusingatwin-
wellCMOS,0.8μm,double-level
metallizationprocess.Dependingon
theSRAMstate,acombinationof
thebiasandaddressvoltages
electrostaticallyattractseachmirror
elementtooneoftheaddress
electrodes.Themirrorrotatesuntil
itstiptouchesalandingelectrode
thatisheldatthesamepotentialas
themirror.A"1"inthememorycell
rotatesthemirror+10°,whilea"0"
causesarotationof-10°.Although
theDMDcanbeoperatedinananalogmode,themirrorsarebiasedsothatonlythe
digitallandingstatesof±10°arepossible.Thisdigitaloperatingmodepermitsuseof
low-voltageCMOScircuitsandensureslarge,uniformdeflectionangles.
DMDMemoryArrayModuleandPackage
ProjectionTVsystem
Figure5illustratestheoperationofasingle-chip,DMD-basedPTVsystem.Acondenser
lenscollectslightfromawhitelightsource,e.g.,ametalhalideorxenonarclamp.The
illuminationisdirectedata+20°anglefromthenormaltotheDMDsurfaceand
orthogonaltotherotationalaxesoftheindividualmirrorsofthearray.Alenslocated
abovethechipprojectsanenlargedimageoftheDMDonascreen.Whenrotatedat+l0°,
theindividualmirrorelementsreflectincominglightintotheprojectionlenspupilto
produceabrightimageonthescreen.Whenrotated-10°,themirrorelementsappear
dark.Lightreflected,scattered,ordiffractedintotheopticalpathbythehingeormirror
supportpostsincreasestheilluminationlevelduringaDMDpixel-offconditionand
reducestheprojectionsystem''scontrastratio.
DMDsingle-chipcolorprotectiondisplay
Sincemirrorresponsetimeisfast(~10microseconds),grayscaleoutputcanberealized
withtheDMDbyemployingpulsewidthmodulationofthemirroroutput.Inthiscase,
eachvideofieldissubdividedintotimeintervalsorbittimes.Forexample,an8-bit
modulationschemeenables2tothe8thpower,or256graylevels[3].
ThePTVsystemcanutilizeeitherasingle-orathree-chipDMDarrangement.Inathree-
chipsystem,eachchipisusedforoneoftheprimarycolors—red,green,orblue.Thisis
analogoustosystemsemployedinmostLCDprojectiondisplays.However,asingle-chip
systemisalsopracticalbecauseoftheDMD''shighlightefficiency.Inthiscase,acolor
wheelmaybeusedtosequentiallyilluminatetheDMDwiththethreeprimarycolors.
Thesingle-chipapproachislessexpensiveandprovidespixelself-convergence.The
mostrecentDMDdevice,a768x576pixelarraywith442,368mirrors,hasbeen
demonstratedinbothone-andthree-chipPTVsystemswithprojectionscreendiagonals
rangingfrom42in.to13ft.
Surfacemicromachining
Fourphotolithographicallydefinedlayersaresurfacemicro-machinedtoformthe
electrode,sacrificiallayer,hinge,andmirror.Thesacrificiallayerisanorganicmaterial
thatisplasma-ashedtoformtheairgapbetweentheaddresselectrodesandmirror.The
otherthreelayersareformedfromdryplasmaetched,sputter-depositedaluminum.
StructuraldetailsandthefabricationsequenceareshowninFigure6.
MirrorElementFabricationSequence
DMDsurfacemicromachiningbeginsaftercontactopeningsfortheaddresscircuit
electrodehavebeenformedinthecircuit''sprotectiveoxide.Theaddresselectrodefilmis
thensputter-depositedontotheoxide,lithographicallypatterned,andplasmaetched.An
organicsacrificialplanarizinglayeristhenspunovertheelectrodeandpatternedwith
vias,whichwillbeusedsubsequentlytoformthesupportposts.StandardICtechnology
tightlycontrolsthecriticaldimensionsinthexandydirections.However,thez-axis
dimension(i.e.,thickness)mustbeequallytightlycontrolledtoachievereproducible,
uniformmirrortiltangles.
Twometallayersareemployedfortheburiedhingeprocess.Thehingealuminumalloy
sputteredontopofthesacrificiallayerisfollowedbyaplasma-depositedmaskingoxide
thatispatternedtodefinethehinges.Depositionofasecondaluminumalloylayerthen
buriesthehingeoxideandformsthebeam.This,inturn,isfollowedbystillanother
plasma-depositedmaskingoxide,whichispatternedtoformthemirrors.Thesupport
postsconsistofboththehingeandmirrormetals.Finally,asinglealuminumplasma
etchingsteppatternsthehingesandmirrors.Themirrormetaloverlyingthehingeregion
isetchedawaytoexposetheburied-hingeoxide,whichfunctionsasanetchstop.When
thealuminumplasmaetchinghasbeencompleted,whatremainsareregionsofhingeand
mirrormetalthathavebeensimultaneouslypatterned[4].Recentlya"hiddenhinge"
structurehasbeenemployed(Fig.7).Thisenablesfabricationofapixelarraywitha
compact,closelyspacedsetofsquaremirrorsthatprovidehighopticalcontrastratioand
efficiency;thehingesandtheirassociatedsupportpostsarehiddenunderthemirror.A
mirrorsupportpostconnectsthemirrortoanunderlyingyoke,andtorsionhingesconnect
theyoketohingesupportposts.Withthisparticulararchitecture,thehingeandthe
addressandlandingelectrodesarecoplanar.
Thehiddenhingearchitecturerequirestwosacrificiallayers—onebetweenthemirror
andtheunderlyinghingesandaddresselectrodes,andasecondbetweenthecoplanar
addresselectrodesandhingeandtheunderlyingthirdlevelofmetalthatisincontactwith
theCMOSSRAMstructure.
Lightdiffractionissignificantlyreducedandtheprojectionsystem''scontrastratiois
improvedwhenthehingesandsupportpostsarehiddenunderthemirror.Themirror
edgesatthelandingtipareorthogonalandthelandingtipdiffractslesslight,sothat
degradationinthecontrastratioisreduced.Increasedmirrorareapromotesgreater
opticalefficiency[5].
Back-endassembly
Itisduringback-endoperationsthattheDMDdevicefinallybecomesfunctional.This
occursuponremovalofthesacrificiallayer.Duringthepreliminarywafersawingstep,it
isimportantthatindividualchipsnotbecontaminatedwithparticles,whichcaninterfere
withtheDMD''smechanicaloperationandcanproducedefectsassociatedwithlight
scattering.Whenwafersawingiscomplete,thechipsareplacedinaplasmaetching
chamber,whereisotropicetchingcreatesanairgapbyremovingthesacrificiallayerfrom
underthemirrorandhinges.
OncetheDMDmirrorsaremechanicallyreleased,theiroperationcanbetestedandDMD
assemblycanbegin.Theentireassemblyprocessrequiresacleanroomenvironmentand
particlemonitoring.Packagingisanimportantconsideration,sincethepackagenotonly
providessignalandpowerleadconnectionstothechip,butalsoprovidesprotectionfrom
theoutsideenvironment.
AlthoughafamiliarsequenceofassemblyoperationsisemployedfortheDMD—die
attach,wirebond,windowseal,andfinaltest—thestructureneverthelessrequiresspecial
handling.Front-side,pick-and-placecontactcolletswilldamagetheDMD''smirrorarray,
andparticulatecontaminationcancausenumerousproblems.Specialattentionmustbe
giventoparticlecontrolanddevicehandlingduringeachassemblyoperation.Thedie
attachandwindowsealingprocessesmustutilizeadhesiveswithlowoutgassing
properties,sinceadhesiveoutgassingcanpromotemirrorstiction.Arobustpackageis
alsorequiredtomaintainaproperambientenvironmentforthechip.Insomecases,chip
coolingmaybenecessary.
Summary
Amicromirrorarray—thelight-modulatingelementofaPTsystem—hasbeensurface
micromachinedontopofconventionalCMOSSRAMaddresscircuitrytoformaDMD.
Theelectrodes,hinges,posts,andmirrorscomprisingtheDMD''ssuperstructureare
formedinamultilayerthin-filmstack,whichincludesasacrificiallayerthatisremoved
byplasmaetchingduringback-endassembly.Specialattentionisrequiredtoavoid
particlecontaminationduringfabricationanddeviceassembly.
References
1.JackYounse,IEEESpectrum,"MirrorsonaChip",Nov.1993.
2.LJHornbeck,"Deformable-MirrorSpatialLightModulators,"SpatialLight
ModulatorsandApplicationsIII,SPIECriticalReviews,vol.1150,p.86,Aug.
1989.
3.J.B.Sampsell,"AnOverviewofTexasInstruments''DigitalMicromirrorDevice
(DMD)andItsApplicationtoProjectionDisplays",SocietyforInformation
DisplayInternationalSymposiumDigestofTechnicalPapers,vol.XXIV,p.
1012,May1993.
4.J.M.Younse,D.W.Monk,"TheDigitalMicromirrorDevice(DMD)andIts
TransitiontoHDTV",Procof13thInternationalDisplayResearchConference
(LateNewsPapers),p.613,Aug.31-Sept.3,1993.
5.C.E.Tewetal.,"ElectronicControlofDigitalMicromirrorDeviceforProjection
Displays",Int''l.Solid-StateCircuitsConference(ISSCC),SanFrancisco,1994.
MICHAELA.MIGNARDIisaseniormemberoftechnicalstaffatTexasInstruments,
responsibleforDMDprocessandarchitechuredevelopmentandproductengineering.
BeforejoiningTl,Dr.MignardiworkedintheDefenseAnalyticalLabsatSouthern
ResearchInstitute.HeholdsaB.S.andaPh.D.inChemistryfromtheUniversityof
Florida,andholdstwopatentsrelatedtoDMDprocessing.
ReprintedfromtheJuly1994editionofSOLIDSTATETECHNOLOGY
?1994byPennWellPublishingCompany
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