CMOS image sensor fabrication technologies Pixel design ...

1 LectureNotes5 CMOSI mageSensor DeviceandFabrication CMOS imagesensor fabricationtechnologies Pixeldesignandlayout Imagingperformanceenhancementtechniques Technologyscaling,industrytrends Microlens Color lterarrayEE392B:DeviceandFabrication5-1 ModernCMOSD eviceStructureEE392B:DeviceandFabricatio n5-2 ImagingIs Di erentfromDigitalLogicFeaturesDigitalLogi cImagingSilicideImprovescontactresistanc eAbsorbslight{ low photosensitivityIncreasedjunctionleakage STIE nablestighterdesignrulesLeadsto largerdark currentduetodefectsfromstressShallow junctionReducesshort-channele ectReducesquantume ciencyformediumto longwavelengthlightLower power supplyvoltageReducespower consumption,enablesdevicescalingReducesh eadroomfor signalswingLower thresholdvoltageImprovesdrivecurrentIncr easessubthresholdleakageThingateoxideEna blesdevicescalingtoshorterchannellengthI ncreasesgateleakageMultiplelevelsof interconnectImproveswire-abilityIncrease sdistancefrommicrolensorcolor lterto photodetectorEE392B.}

2 DeviceandFabrication5-3 BaselineModi cationsof CMOSfor ImagingModi cationsare generallyneededonlyin thepixelarea Modi cationsto improveopticalperformance: Non-silicidedsource/drainfor photodiode andPolySigateforphotogate Deeper n-well to p-substratediode for improvedquantume ciency Episubstratethicknessoptimizationfor quantume ciency, spectraltailoringandcrosstalkoptimizatio n Customizeddielectriclayersto reducere ectionfrommaterialwithmis-matchedrefract iveindex Reducedmetallightshieldheight,tight(vert icalandhorizontal)lightshieldEE392B:Devi ceandFabrication5-4 Modi cationsto reducedark current: Avoidlandedcontacts,minimizegateedge,iso lationedge GentleSTIprocessanddefectrepair/avoidanc earoundSTI Modi cationsto in-pixeltransistors: Thicker gateoxideto handlehigherpixel(analog)power supply AdjustvTto maximizesignalswingandminimizeleakage Longerthanminimumgatelengthto reducehot-carrierinducedphotonemissionan dimpactionizationEE392B:DeviceandFabrica tion5-5 ExampleCMOSI mageSensor Cross-SectionH.

3 Rhodeset al.,\CMOS imagertechnologyshrinksandimageperforman ce,"IEEEW orkshoponMicroelectronicsandElectronDevi ces, (2004)EE392 al.,\ m CMOS color imagertechnologywithnon-silicidesource/d rainpixel," , (2000) Wong,\TechnologyandDeviceScalingConsider ationsfor CMOSI magers," (1996)EE392B:DeviceandFabrication5-7 Non-SilicidedSource/Drain Silicideconsumessilicon,causesstressandl argerleakage(cornerleakage) N-well to p-substratediode al.,\Nonsilicidesource/drainpixelfor m CMOS imagesensor,"IEEEE lectronDeviceLetters., (2001)EE392B:DeviceandFabrication5-8N-We ll to P-SubstratePhotodiode Higherquantume ciencydueto deeper al.,\ m CMOS color imagertechnologywithnon-silicidesource/d rainpixel," , (2000)EE392B:DeviceandFabrication5-9N-We ll to P-SubstratePhotodiode UnderSTI Deep( m)n-well (MeV)implant,lightdose al.,\Ahighperformanceactivepixelsensor m CMOS color imagertechnology," , (2001)EE392B:DeviceandFabrication5-10 EpiSubstrateThicknessTailoring P+-substrate(more costly)cutsdownoncarrierdi usiondueto redandinfra-redlightbecausedi usionlengthin heavilydoped semiconductor isshort Typicalp-epionp+-substrateis<2 m,notdeepenoughfor goodgreen/redlightabsorption Epi-layer too et al.

4 ,\Highsensitivity andno-crosstalkpixeltechnologyfor embeddedCMOS imagesensor," , (2001)EE392B:DeviceandFabrication5-11 CustomizedBack-EndDielectrics Gradetherefractiveindex,matchrefractivei ndexat boundariesas far aspossible Dielectrics:Si3N4, PECVD oxide,silicon-richoxide,SiO2, PECVD nitride Make suredielectricsare et al.,\Highsensitivity andno-crosstalkpixeltechnologyfor embeddedCMOS imagesensor," , (2001)EE392B:DeviceandFabrication5-12 DielectricThicknessOptimization Wavelengthdependentdueto multiplere et al.,\Highsensitivity andno-crosstalkpixeltechnologyfor embeddedCMOS imagesensor," , (2001)EE392 et al.,\Highsensitivity andno-crosstalkpixeltechnologyfor embeddedCMOS imagesensor," , (2001)EE392B:DeviceandFabrication5-14 Lower et al.,\Highsensitivity andno-crosstalkpixeltechnologyfor embeddedCMOS imagesensor," , (2001)EE392B:DeviceandFabrication5-15 OpticalPathOptimizationat theBackend Utilizedi erentdielectricrefractiveindexto achievetotalinternalre ection Snell'slaw:n1sin 1=n2sin al.

5 ,\Lightguidefor pixelcrosstalkimprovementin deepsubmicronCMOS imagesensor,"IEEEE lectronDeviceLetters, (2004)EE392B:DeviceandFabrication5-16 OpticalPathOptimizationat al.,\Lightguidefor pixelcrosstalkimprovementin deepsubmicronCMOS imagesensor,"IEEEE lectronDeviceLetters, (2004)EE392B:DeviceandFabrication5-17 AirGapGuard Ring Anextensionof thetotalinternalre al.,\Dramaticreductionof opticalcrosstalkin deep-submicrometerCMOS imagerwithairgapguardring,"IEEEE lectronDeviceLetters, (2004)EE392B:DeviceandFabrication5-18 LeakageCurrent Chargeleakagefromhighimpedancenode duringsignalintegrationorreadout Sources: Di usioncurrent(proportionalton2i) Generationcurrentin spacechargeregion(proportionaltoni) PNjunctiontunnelingcurrent(bandto bandtunneling) O -current{ subthresholdconductiondueto lowvT Gatecurrent{ importantat<130nmnode Hot-carriere ects{ presentfor transistors operatedin thesaturationregion Defectgeneratedleakage{ processstress(strainedsilicon,STI,silici de,contactetch)EE392 Wong,\TechnologyandDeviceScalingConsider ationsfor CMOSI magers," (1996)EE392B:DeviceandFabrication5-20 HotCarriers Carriersgainenergyas theytravelalongthechannel Whyare carrierscalled\hot"carriers?}}}}

6 Theenergyof thecarrierscanbe described by a carrierdistributioncharacterizedby a \temperature"thatis higherthanthelatticetemperature,hencethe term\hot"carriers Two maine ectscausedby hot-carriers Impactionization,generateselectron-holep airs PhotonemissionEE392B:DeviceandFabricatio n5-21 DeviceIn SaturationRegionWillEmitLightJ. C. Tsang,J. A. Kash, Vallett,IBMJ. ResearchandDevelopment, ,p. 583(2000)EE392B:DeviceandFabrication5-22 Hot-CarrierInducedPhotonEmission Intra-band(conductionband)transition{ onlyfor nFETs Photonsgeneratedin theinfra-redwavelengths Photonstravelquitefar in thesiliconsubstrate PNjunctionguard ringis note ectivein isolatingpixelfromphotons PNjunctionguard ringis usefulto block electronsfromimpactionizationEE392B:Devi ceandFabrication5-23 OptimizedSTIP rocess GentleSTIetch ReduceSTIdielectricstress(engineerthelin er)induceddefects(stackingfaults) Implantp+doped regionaroundSTIto pushelectronsaway fromSTIsurfaceEE392B.}

7 DeviceandFabrication5-24 Transistor design Pixeltransistors HighvDDto providesignalswingheadroom Thickoxideto handlehighervDDandreducegateleakage BoostedResetGatevoltagefor hard reset Avoidhot-carriergenerationusinglongertha nminimumdevices vTadjustments HighervTfor resettransistor LowervTfor sourcefollower transistor Peripheraltransistors Standard CMOS logictransistors to reducepower consumptionandattainhighcircuitspeed Similar strategyas DRAM Separate\array" transistors,and\support circuit"transistorsEE392B:DeviceandFabri cation5-25 PixelLayoutandPixelSize Pixelsizemostlydeterminedby Contactsize Poly-gateto contactspacing Metalto metalspacing 20 Ffor 4 Tcell 13-16 Ffor 3 TcellEE392B:DeviceandFabrication5-26 PixelLayoutExamples{ 3 TPhotodiode Maximumphotodiode areamay notgivethebestimagingperformance Leakage,conversiongainA. I.}

8 Krymski,N. E. Bock,N. Tu, Blerkom,andE. R. Fossum,\Ahigh-speed,240-frames/s, ," , , ,January 2003I. Shcherback, ,\Photoresponseanalysisandpixelshape optimizationfor CMOS activepixelsensors," , (2003)EE392B:DeviceandFabrication5-27 PixelLayoutExamples{ 4T-PhotogateS. K. Mendis,S. E. Kemeny, R. C. Gee,B. Pain,C. , ,andE. R. Fossum,\CMOS activepixelimagesensors for highlyintegratedimagingsystems,"IEEEJ ournalof Solid-StateCircuits, , ,February 1997EE392B:DeviceandFabrication5-28 TechnologyScaling Today'sadvancedCMOS imagesensors are fabricatedin m CMOS Mostadvancedlogictechnologyis 90 nm(willbe 65 nmin 2006) CanCMOS imagesensor usenanometerscaleCMOS technologies ?EE392B:DeviceandFabrication 5-29 ,Intel(2003)Source:G. Moore,Intel(2003)Source:P. Gelsinger,Intel(2003)EE392B:DeviceandFab rication5-30 State-of-the-ArtTechnology90 nm65 ,Intel(2004)EE392B:DeviceandFabrication5 -31 Bene tsof Scaling More devicesper unitarea Highergateleakage Highersubthresholdleakage Lower power supplyvoltageEE392B:DeviceandFabrication 5-32 ScalingExamplesB.}

9 Doyleet al.,\Transistor elementsfor 30 nmphysicalgatelengthandbeyond,"IntelTech nologyJournal, (2002)EE392B:DeviceandFabrication5-33 Scalingfor CMOSI mageSensor Straight-forward scalingdoes notwork for CMOS imagesensors Photogateandphotodiode collectionregiontoo shallow Leakagetoo high Gatedielectric,subthresholdcurrent,pnjun ctionbandto bandtunneling, Power supplytoo lowEE392B:DeviceandFabrication5-34 IndustryTrends MostCMOS imagesensors m CMOS ,thickoxidetransistors for thepixel Pinnedphotodiode for CMOS imagesensors (atlow voltage) Cubackendto reducedielectricstackheight Migrationto m CMOSmay needsubstantialprocesschanges Pixelsizereductionto 2 m drivenmostlyby costEE392B:DeviceandFabrication5-35 Microlens Focuslightontophoto-sensitiveregion{ increasese ective ll factor from25-40%to 60-80%(andsensitivity by 2X) Lesse ectiveif photosensitiveareais irregularlyshapedA.}

10 Theuwissen,\SolidStateImagingwithCharge- CoupledDevices,"Kluwer (1995) al.,\ m CMOS color imagertechnologywithnon-silicidesource/d rainpixel," , (2000)EE392B:DeviceandFabrication5-36 MicrolensTypes (a)Hemisphericallens (b)Semi-cylindricallens (c)Rectangular domelensA. Theuwissen,\SolidStateImagingwithCharge- CoupledDevices,"Kluwer (1995)EE392B:DeviceandFabrication5-37 MicrolensFabrication Lensmaterialrequirements: Highlytransparentin thevisiblelightregion Indexof refraction> Canbe appliedbelow 500C Nodegradationor aging Semiconductor processingcom-patible Canbe patternedwithfeaturesizecommensuratewith thepixelsize Lensmaterialsaretypicallyi-lineorDUVresi sts Basematerialsare acrylic-basedresists,polyimideresists,ep oxyresists,poly-organosiloxane,polyorgan osilicateEE392B:DeviceandFabrication5-38 ExampleCMOSI mageSensor ChipH.