5a-OpticalTechniques-OM,PEM.pdf

1EE5514 IC YIELD,RELIABILITY AND FAILURE ANALYSISOPTICAL TECHNIQUES:Light Microscopy-based ToolsDr.Wai-Kin WongDepartment of Electrical&Computer Engineering National University of SingaporeE-mail:elewwknus.edu.sgTel:6516-2086 Room:E4-08-04EE5514 2006-07 WK Wong2OPTICAL TECHNIQUESLearning Objectives:Understand photonic imaging physics in semiconductor applications Attributes&operating principles of various optical toolsRole and application of optical tools in global failure site isolationEE5514 2006-07 WK Wong3LIGHT MICROSCOPY REFERENCESReferences/Suggested ReadingWK Chim,Semiconductor Device and Failure Analysis Using Photon Emission Microscopy,John Wiley,2000 Florida State University-Molecular Expressions Virtual Microscopy web(http:/micro.magnet.fsu.edu)Supplementary Reading PapersEE5514 2006-07 WK Wong4MICROSCOPY TIMELINEAtomic Force MicroscopeGerd Binnig,Calvin Quate,Christoph Gerber1986NSOM/SNOMDieter Pohl,Winfried Denk,and M.Lanz1984Scanning Tunneling MicroscopeGerd Binnig and Heinrich Rohrer,Gerber1981SIMSRichard Honig/HW Werner1958Confocal MicroscopyMarvin Minsky1955SEMVladimir Zworykin1942Scanning TEMManfred Von Ardenne1938TEMMax Knoll/Ernst Ruska1931AES(Concept)Pierre Auger1923XPS(from Photoelectron effect)Albert Einstein1905First compound light microscopeZaccharias and Hans Janssen1590EyeglassesSalvino DArmate1284InventionInventorDateEE5514 2006-07 WK Wong5OPTICAL MICROSCOPYBasic ConceptsDetection of photons from UV to IR rangeEE5514 2006-07 WK Wong6OPTICAL MICROSCOPYResolution generally proportional to wavelength,whether diffraction or optical aberration limited(for far-field optics,i.e.working distance )Bandgap eV-max wavelength for photon generation(direct bandgap recombination)105 10910-14-10-101019-102410-11-10-14Gamma Rays102 10510-17-10-141017-101910-9-10-11X-Rays3-10210-19-10-171014-101710-6-10-9Ultra Violet1 310-19101410-6Visible10-4 110-23-10-191012-101410-4-10-6Infra Red10-11-10-410-30-10-231010-101210-1-10-4Micro Waves10-11-10-410-30-10-23103-1010104-10-1Radio WavesEnergy(eV)Energy(J)Frequency(Hz)Wavelength(m)NameEE5514 2006-07 WK Wong7OPTICAL MICROSCOPYDefinitions(Microscopy):Resolution:The smallest feature size that can be resolved(discerned)Rayleighs criterionFocus:Definition 1:Optical configuration where the smallest possible probe illuminates the object(scanned probe systems SEM,TEM,SOM,etc.)Definition 2:Optical configuration where the object lies in the focal point of the objective lens(projection imaging systems such as LM)Astigmatism:Deviation of imaging probe from ideal(usually circular)shape,described in 2-dimensionsDefocused(low-res),focused(high-res)&astigmatic imagingEE5514 2006-07 WK Wong8OPTICAL MICROSCOPYExample of spatial resolutions:Compound LM=0.61 /NA(Abbes Law)Confocal microscopy=0.4 /NARef:http:/R=angular separation between adjacent pointsd=aperture sizeEE5514 2006-07 WK Wong9OPTICAL MICROSCOPYFinal probe resolution dependent on i)Wavelength(fundamental diffraction limit);ii)Illumination source size;iii)Aperturing;iv)Optical demagnification.Desirable to have small source sizes further delimited by apertures and/or opticsSource Size and ApertureEE5514 2006-07 WK Wong10OPTICAL MICROSCOPYResolution cost ease of use trade-off:Select appropriate tool for expected feature sizeApplication scope of inspection tools by feature sizeIncreasing costMore difficult to useLower costEase of useLarger defectsSmaller defectsEE5514 2006-07 WK Wong11OPTICAL MICROSCOPYRelative transparency of photons from 1.1 2.5 microns for intrinsic SiPhoton absorption characteristics of SiEE5514 2006-07 WK Wong12OPTICAL MICROSCOPYLight transmission efficiency through 625 m of p-doped Si having doping concentrations of(a)1.5x1016;(b)3.3x1017;(c)1.2x1018;(d)7.3x1018 cm-3Signal attenuation in heavily-doped Si substrates in NIR/IR range:EE5514 2006-07 WK Wong13OPTICAL MICROSCOPY1.OPTICAL/LIGHT MICROSCOPY1.1BackgroundLimited resolving power of the naked eye-need more resolution and depth of fieldOptical microscopes are useful for observing and analyzing features with sizes in the micron rangeAlso called compound light microscope,reflected light microscope,wide/far-field microscopeBasic operation example:Ref:http:/EE5514 2006-07 WK Wong14OPTICAL MICROSCOPYAdvantages of Optical Microscopy:Relatively low costNatural,intuitive images interpretationEase of use no vacuum,specimen chamber constraintsColor informationTransparency of semiconductor dielectric films subsurface infoNo charging issues(SEM,FIB)Biological e.g.DaphniaPhysical e.g.IC inspectionEE5514 2006-07 WK Wong15OPTICAL MICROSCOPYCompound microscope optical pathsOverall Magnification of Compound Microscope=Lateral Magnification of Objective(from 2X to 100X)x Angular Magnification of Eyepiece(10 x standard)EE5514 2006-07 WK Wong16OPTICAL MICROSCOPYPrimary parameter for OM/LMs:Numerical Aperture(NA):Numerical Aperture,NA=n sin where n=refractive index of medium=half-angle aperture of the LM objective lens at its focal pointNA-measure of the light-gathering and resolving power of an objective lensOM Spatial resolution=0.61 /NAExample of numerical aperture variationObjective lensDUTWorking distancenEE5514 2006-07 WK Wong17OPTICAL MICROSCOPYSince spatial resolution=0.61 /n sin,resolution limit determined by:Optical wavelength (diffraction limited)Objective lens NA example&relationship with Rayleighs criterionRefractive index,n of mediumCoherence of illumination(dispersion effects)Type of specimenAccuracy of optics(lens profile,alignment)-astigmatism,chromatic&spherical aberrationPractical limits of NA 1.4 for oil-immersion(n=1.5)optics-resolution 0.25 m for green light(=550 nm)EE5514 2006-07 WK Wong18OPTICAL MICROSCOPYRefractive Indices of Common Materials3.9120Lead Sulphide2.4170Diamond1.8 2.1Silicon nitride1.9200Zircon1.6600Flint1.6440Quartz1.5100Oil1.5000Glass1.4700Glycerin1.4550Silicon dioxide1.3330Water1.3290Alcohol1.3090Ice1.0002Air1.0000VacuumnMediumEE5514 2006-07 WK Wong19OPTICAL MICROSCOPYDepth of focus(i.e.the thickness of the image space that is simultaneously in focus)is given by:Depth-of-Focus,DOF=n/(2 x NA2)(Born-Wolf definition)Depth of field(i.e.the thickness of the object space that is simultaneously in focus)is:Depth-of-Field,D =n -(NA)(NA)field222EE5514 2006-07 WK Wong20OPTICAL MICROSCOPYResolution-Dfield-Working Distance tradeoff:High NA gives high image brightness(image brightness NA)high resolutionHowever,high NA also corresponds to:small Dfield small WDRefer to previous NA exampleEE5514 2006-07 WK Wong21OPTICAL MICROSCOPYExamples of Working Distance,Magnification&NA(Dry Objective)0.5120.4300.420.8050X0.51.520.9090.610.5550X82.311.300.730.4620X35.721.720.840.4020X11Depth of Field(m)Depth of Focus(m)Theoretical Resolution(m)NAMagnificationWD(mm)EE5514 2006-07 WK Wong22OPTICAL MICROSCOPYResolution improvementShorter wavelength such as blue(=470 nm).In practice,green(=550 nm)is commonly usedIncrease NAImmersion objective to increase nPractical resolution limit 0.20-0.25m(NA=1.3-1.4 and green light)Example of immersion opticsEE5514 2006-07 WK Wong23OPTICAL MICROSCOPY1.2Applications of Reflected Light Microscopy1.2.1Bright-field and dark-field imagingDark field vs.bright field illuminationEE5514 2006-07 WK Wong24OPTICAL MICROSCOPY2.Less detailed view,esp.for subsurface features.2.Image quality is degraded by the presence of glare.1.Poorer sharpness.1.Images lack contrast&depth.Disadvantages2.Defect location and identification more easily achieved.2.Thickness and integrity of oxide can be assessed.1.Better contrast&depth perception for non-planar surface features e.g.step edges,surface roughness.1.Internal(subsurface)structures of device can be examined in better detail e.g.metallization,contact window,step coverage.AdvantagesExample(Indentation on Metal)Dark fieldBright fieldEE5514 2006-07 WK Wong25OPTICAL MICROSCOPY1.2.2 Dielectric thickness estimation?Approximate measure of oxide/nitride thickness?Color due to constructive and destructive interference of light?Wavelengths of light in SiO2which undergo constructive interference given by:where:=wavelengtht=oxide thicknessn,m=integer series 0,1,2,3,etc.mnt2=EE5514 2006-07 WK Wong26OPTICAL MICROSCOPYSilicon Dioxide/Nitride Color vs.Film Thickness and Viewing Angle Calculator:http:/ee.byu.edu/cleanroom/color_chart.phtmlEE5514 2006-07 WK Wong27PHOTON EMISSION MICROSCOPY2.PHOTON EMISSION MICROSCOPY2.1BackgroundDeveloped in the 1990s for global failure site isolationPrimary advantages:Large field of viewDeeper signal collection zone due to Si/Si dielectric transparency to photons in the NIR/IR rangeImaging from frontside(active topside of die)or backside(via die substrate)Unique spectral signature of failure mechanisms in pn junctions and MOS devicesBest case resolution=half of photon wavelength(around 300 nm)Applicable devices:p-n junctions(Si,III-V),Si MOSFETsEE5514 2006-07 WK Wong28PHOTON EMISSION MICROSCOPY2.3Typical PEM InstrumentationCooled CCDObjective lens arrayLightproof boxPC-based control,acquisition and storageMotorized stageMonochromator,Stage and Microscope controllersEE5514 2006-07 WK Wong29PHOTON EMISSION MICROSCOPY2.2 Physics of OperationDetection of visible(390-770 nm)and NIR(770-1500 nm)photonsPhotons emitted from saturated transistors,pn junctions,latchup,gate oxide breakdown and other photon-generating structures under electrical biasBias(Energisation)-EHP Generation-EHP recombination/Trapping-Photon EmissionLink between emission spectra and device parameters(pn junction/MOS,e-fields,current,etc.)EE5514 2006-07 WK Wong30PHOTON EMISSION MICROSCOPYRadiative interband transition in a direct bandgap semiconductorEE5514 2006-07 WK Wong31PHOTON EMISSION MICROSCOPYRadiative interband transition in an indirect bandgap semiconductorEE5514 2006-07 WK Wong32PHOTON EMISSION MICROSCOPYEmission spectrum of pn junction showing emission peak at 1107 nm corresponding to Si bandgapEE5514 2006-07 WK Wong33PHOTON EMISSION MICROSCOPYEmission spectrum of forward and reverse-biased pn junctionEE5514 2006-07 WK Wong34PHOTON EMISSION MICROSCOPYBand diagram of reverse biased pn junction+E-field-EE5514 2006-07 WK Wong35PHOTON EMISSION MICROSCOPY2.4Application2.4.1 Panchromatic Mode Qualitative imaging technique Resolution 1 m Detection sensitivity at least an order of magnitude better than liquid crystal hot spot technique Extensively used for leakage current localization Minimal sample preparation High sensitivity CCD camera to detect very low levels of light emission Good light isolation and long integration periods required EE5514 2006-07 WK Wong36PHOTON EMISSION MICROSCOPYApplication Example(a)PEM-reflected light overlay image showing light emission of faulty n-MOSFET;(b)Higher mag SEM image of source-drain punchthrough defect after metal-polySi deprocessing(a)(b)EE5514 2006-07 WK Wong37PHOTON EMISSION MICROSCOPYApplication ExamplePEM micrograph showing current leakage in MOSFET due to hot electronsPEM micrograph showing photo-emission due to oxide leakage in VLSI chipEE5514 2006-07 WK Wong38PHOTON EMISSION MICROSCOPYApplication ExamplePhoton emission-reflected light images of hot carrier effects in(a)Double-diffused drain(DDD)n-MOSFET;(b)Large-angle-tilt implanted drain(LATID)n-MOSFET(a)(b)EE5514 2006-07 WK Wong39PHOTON EMISSION MICROSCOPYLimitations:Cannot detect frontside emissions covered by metal layersCCD detection not effective for long wavelength(1000 nm)emissions such as those from ohmic shortsCCD spectral quantum efficiencyEE5514 2006-07 WK Wong40PHOTON EMISSION MICROSCOPY2.4.2 Spectroscopic ModeAddition of off-axis light collection element,light guide and spectral analyzer/spectrograph to panchromatic setupPhoton collection using either concave mirrors or prisms.Highest quantum efficiency(80%)with semi-ellipsoidal mirror designsSpectroscopic Photoemission system schematic EE5514 2006-07 WK Wong41PHOTON EMISSION MICROSCOPYImplementation of Spectral Analyzer:Discrete Bandpass filtersMonochromators2.4.2.1 Discrete Bandpass Filtered PEMConventional PEMs can be retrofitted for spectroscopy using bandpass filters.Simple,low-cost modificationLimitations:Poor collection efficiency-less than 5%of emission collected.Poor spectral resolution-limited by filters(typically 50nm resolution,even poorer at NIR)Low-throughput,requires significant data post-processingLow number of spectrum data points(typically 5-10 filters)EE5514 2006-07 WK Wong42PHOTON EMISSION MICROSCOPY2.4.2.2Monochromator Spectroscopic PEMIncreased spectral resolution down to 1 nm resolutionUses prisms or diffraction gratings as wavelength-dispersive elementDiffraction gratings preferred due to ease of use,wider wavelength range,more constant dispersion with wavelengthCCD/photodiode line array for high throughput parallel spectrum detectionEE5514 2006-07 WK Wong43PHOTON EMISSION MICROSCOPYSpectroscopic PEM using PMTd(sin sin)=mwhere =angle of incidence=angle of diffraction d=distance between adjacent grooves m=diffraction order(integers 1,2,3 etc.)=wavelength of the incident beam Grating surface normalLonger Shorter EE5514 2006-07 WK Wong44PHOTON EMISSION MICROSCOPYSpectroscopic PEM using photodiode arrayIncreasing EE5514 2006-07 WK Wong45PHOTON EMISSION MICROSCOPYSpectroscopic PEM Application:MOSFET defectsFor MOSFET,presence of pn-junction-type spectra indicative of defectSaturated/hot electron-related mechanisms IsubdependenceEE5514 2006-07 WK Wong46PHOTON EMISSION MICROSCOPYEmission mechanism in saturated n-MOSFETsIsubEE5514 2006-07 WK Wong47PHOTON EMISSION MICROSCOPYExtensive library of spectral signatures of failure mechanisms as well as that from known good devices(KGD)defect“finger-printing”approach to failure analysis PEM Defect Spectra ExamplesEE5514 2006-07 WK Wong48PHOTON EMISSION MICROSCOPY2.5 Application IssuesCalibration of system response required,e.g.using standard tungsten lamp(Tc=3200K):Example of(a)Calibration source spectrum;(b)Measured spectrum;(c)Correction curve)()()(measuredncalibratiocorrectionfff=EE5514 2006-07 WK Wong49PHOTON EMISSION MICROSCOPYMainstream CCD detectors limited to detection of photons below 900 nm;spectral artifacts due to limited detector bandwidthSpectral artifact in forward-biased pn junctionEE5514 2006-07 WK Wong50PHOTON EMISSION MICROSCOPYOther Application Considerations:Photon emission not necessarily due to defect,e.g.light emission from saturated transistors.Comparison with KGD&design specs neededSignificant thinning(200 m)and polishing to reduce artifacts and increase sensitivityScaling down of device dimensions and voltages lower emissions levels,longer wavelengths+insufficient resolution for deep submicron devicesDifficult to fabricate uniform large area array detectorsLiquid N cooling(77K)necessary to reduce noiseEE5514 2006-07 WK Wong51Small bandgap(0.1-0.25 eV)semiconductor detectors(InGaAs,HgCdTe,etc.)for low-level,long-wavelength(long wavelength cutoffs of 2500 nm and beyond)detection Volts/WattPhotoconductive MCT detector Spectral Data Relative ResponsePHOTON EMISSION MICRO