SEM - Electron Beam.pdf

EE5514 IC Yield,Reliability&Failure AnalysisElectron Beam TechniquesW.K.CHIMAssociate ProfessorDepartment of Electrical and Computer EngineeringNational University of SingaporeE-mail:elecwknus.edu.sgTel:(65)6516-6287Office Location:E2-03-31Electron Beam TechniquesReferences R.E.Lee,“Scanning Electron Microscopy and X-Ray Microanalysis”,Prentice Hall,1983.D.B.Holt and D.C.Joy(Eds.),“SEM Microcharacterization of Semiconductors”,Academic Press,1989.D.E.Newbury et al.,“Advanced Scanning Electron Microscopy and X-Ray Microanalysis”,Plenum Press,1986.D.Chescoe and P.J.Goodhew,“The Operation of Transmission and Scanning Electron Microscopes”,Oxford University Press,1990.Electron Beam TechniquesScanning Electron Microscopy(SEM)-Introduction-SEM Signals-Instrumentation and Resolution-Sampling Volume Limitation&Sample Charging SEM Based Techniques-X-Ray Microanalysis-SEM Voltage Contrast-Electron Beam Induced Current-Cathodoluminescence Transmission Electron Microscopy(TEM)Electron Beam-Introduction0.11101970197519801985199019952000YearChannel length(m)size of human blood cellsize of a virus2-orders of magnitude reduction in transistor size over 30 years!Photo from IBMtransistor acts as a switch0.18 mRabies virusTransistor channelTransistor Size ScalingElectron Beam-IntroductionSpatial resolution issue-Size of a disturbing or killer defect decreasing(Single missing atom,equivalent to Si-O Bond length 0.2 nm,in a 1 to 2 nm thick gate dielectric could result in a disturbing defect!)-Need for higher resolution microscopy techniques(e.g.,SEM,TEM,STM/AFM)(TEM micrograph of a high-K gate dielectric stack,taken from Chen et al.,J.Electrochem.Soc.,vol.149,no.6,pp.F49-F55,2002.)SEM-Introduction The SEM is a highly versatile instrument that is widely used for imaging,material characterization and failure analysis.SEM offers the advantages of higher magnification,greater depth of focus and various modes of imaging,such as:-Secondary electron(SE)imaging-Backscattered electron(BSE)imaging-Energy/Wavelength dispersive X-ray(EDX/WDX)analysis-Voltage contrast(VC)-Electron beam induced current(EBIC)-Cathodoluminescence(CL)-Magnetic contrast(MC)SEM-IntroductionDepth of Focus(Depth of Field)Out of Focus Region(Ref:C.E.Lyman et al.,Plenum Press,1990.)SEM-IntroductionDepth of Focus(Optical Microscope vs SEM)(Ref:O.C.Wells.,“Scanning Electron Microscopy”,McGraw-Hill,1974.)SEM-Introduction Primary electron beam is focused to a spot of 0.01 1 m diameter by 2 to 3 lenses,and then scanned over the sample surface,in synchronism to the scanning on the CRT display.Primary electrons striking the sample generates secondary electrons(SE),backscattered electrons(BSE)and other types of signals.SE and BSE are collected,amplified and detected with a scintillator-photomultiplier (Everhart-Thornley)detector.SEM-IntroductionSEM Signals Interaction of the primary beam with the sample creates an excitation volume,in which electrons are scattered(elastic and inelastic scattering).Electrons in elastic collisions lose only a small fraction of their original energy but undergo large-angle deflection and are known as backscattered electrons(BSE).Inelastically scattered electrons lose much of their original energy and those with energies ThermionicEmissionThermionic emission is governed by the Richardson-Dushmann Equation:Jo=A T2exp(-m/kT)SEM InstrumentationElectron Gun(Field Emission Gun,FEG)Field emission is the tunneling of an electron at an energy EF(Fermi energy)through the narrow potential energy(PE)barrier.The narrowing of the barrier is induced by a large applied electric field.JField emission=envxexp(-Ecrit/E)e=Electronic chargen=Electron concentrationvx=Mean speed of electron along x-direction(or the normal to surface from which field emission occurs)Ecrit=Critical field for tunneling which depends on the effective barrier height effSEM InstrumentationElectron Gun(FEG)The high electric field at the tip surface causes narrowing of the tunneling barrier and some electrons can tunnel through and escape from the tip.The improvement in brightness(A cm-2Sr-1)over the conventional tungsten filament and LaB6gun is a factor of 103to 104.A field emission gun(FEG)consists of a metal surface(typically single crystal tungsten tip)with a radius of curvature of 50 nm separated from a plane metal surface(extraction electrode)by a millimeter or so.A potential difference of several thousand volts is applied between the tip and extraction electrode.Example shown for FEG in a STEM.For the SEM,the acceleration voltage is lower(30 kV instead of 100kV)SEM InstrumentationElectron Gun(FEG)Field emission tip can be operated at room temperature(Cold FEG)or heated to 1500-2000 K(Thermal FEG).Cold FEG produces electrons with a smaller energy spread(0.3 eV)than thermal FEG(0.5 eV).The virtual source size is very small,typically of the order of 5 nm(much smaller than the 50 m crossover point for a tungsten filament gun).Thus,the demagnification required to produce a probe size of 0.5 nm is about 10 x for a FEG system(much smaller than that required for a tungsten filament gun system,which is about 104 x,as explained later).SEM InstrumentationElectron Lenses(Electromagnetic Lenses)An electromagnetic lens is essentially a length of wire coiled around a metal cylinder containing a soft iron pole piece.Electron lenses are used to demagnify the crossover at the electron gun from a size of 50 m to a final probe diameter of 5 nm.2 to 3 electromagnetic lenses are usually used.SEM InstrumentationLorentz Force:F=-e(v x B)Also:mv2/2=eVoWave-Particle Duality(De Broglies Law):=h/(mv)Electromagnetic Lenses.urlSEM ResolutionConcept of Resolution Resolution is defined as the smallest distance between 2 separate features of the specimen which permits these features to be reliably distinguished in the image.Improved Resolution=Decreased Probe Size=Increased Noise or Decreased Signal-to-Noise Ratio(SNR).dminResolution of a light microscope(Abbes Law-Far-field criterion)dmin=(0.612 )/(n sin )where =wavelength of light,n=refractive index of medium,and =aperture angle.Resolution.urlSEM Resolution Resolution in the SEM depends on the smallest electron probe spot achievable.Electron optics in the SEM are designed to achieve the smallest electron spot with maximum current.Signal-to-noise ratio is determined by the electron probe current,which decreases with decrease in probe spot size.SEM ResolutionSEM ResolutionSpherical Aberration Electrons moving in trajectories further away from the optic axis are focused more strongly.dS=0.5 CS 3CS=Spherical aberration coefficient(Typical CS 2 cm)=Semi-angle subtended by focused electron probe For a point object,the image in the focal plane is a disk of least confusion of diameter dS,whereSEM ResolutionExample:For a final aperture size=100 mand working distance=10 mm,=50 m/10 mm=5 x 10-3radiandS=0.5 x 2 x 10-2x(5 x 10-3)3=12.5 To minimise dS,should be kept small.However,if is too small,probe current is also small=Decreased SNR.SEM ResolutionChromatic AberrationChromatic aberration is due to the energy spread in the primary electron beam,resulting from:(1)Stability of HV supply(2)Energy spread from electron gun(Tungsten Gun:3 4 eV;LaB6Gun:2 3 eV;Field Emission Gun:0.2 0.5 eV)dC=(E/Eo)CC CC=Chromatic aberration of lens(typically 1 cm)Focusing action of the lens is stronger for electrons with lower energy.SEM ResolutionChromatic AberrationExample:For EO=30 keV,E=3 eV,and=5 x 10-3radian,dC=(3/30 x 103)x 1 x 10-2x 5 x 10-3=50 To minimise dC:(1)Minimise E(run gun at saturation).(2)Use high EO.(3)Use low.SEM ResolutionDiffractionDue to wave nature of electrons,diffraction effects give rise to an intensity variation.dd=1.22 /is the de Broglieselectron wavelength:=h/(mv)v=2 e EO/m1/2Example:For EO=30 keV and =5 x 10-3radian,=7.04 x 10-12mdd=17.2 SEM ResolutionAstigmatism Astigmatism results in an elliptical(instead of spherical)electron probe shape because of asymmetry in the magnetic lens field.This is due to machining tolerances of the lens pole pieces.Significant at high magnifications.Asymmetric lens fieldStigmatorfieldCorrected lens field Can be corrected by a stigmator,which is a set of coils that applies a field to squeeze the electron beam back to its symmetrical configuration.Two controls Amplitude A and angle.Astigmatism can also result when electron column/aperture is dirty;but this cannot be adequately corrected by the stigmator.SEM ResolutionSEM ResolutionSEM ResolutionSEM ResolutionGold on carbon specimen at 50,000 x magnification(Ref:C.E.Lyman et al.,“SEM,X-Ray Microanalysis and Analytical Electron Microscopy A Laboratory Workbook”,Plenum Press,1990.)Beam Energy=30 keVBeam Energy=1 keVAt high beam energy(or acceleration voltage)of 30 keV,topographic details of individual gold crystals(edge brightness and relief contrast)can be observed,which are not visible at a low beam energy of 1 keV.SEM Sampling Volume Limitation Although the boundary of a specimen may be atomically sharp,there is a gradual transition of the detected signal due to the finite size of the interaction volume of the electron probe beam.This effect is more severe at high magnifications,resulting inloss of detail at the boundary,except at the edges of structures.SEM Sample(Insulator)Charging(Ref:C.E.Lyman et al.,“SEM,X-Ray Microanalysis and Analytical Electron Microscopy A Laboratory Workbook”,Plenum Press,1990.)Beam Energy=15 keV5 keV2 keV1.5 keVNote severe charging effects at high beam energy(15 keV and 5 keV).SEM Sample ChargingNegative Charging at High Primary Electron Beam Energies(EO)=SE yield=BSE yield=Total electron yield(=+)SEM Sample ChargingNegative Charging at High Primary Electron Beam Energies(EO)For best possible resolution,the SEM should be operated at the highest available beam energy(typically Eo=30 keV).At 30 keV,an insulator specimen will experience negative charging since the total electron yield()is less than unity.As the sample,charges negatively to a surface potential of US,the primary electron beam will be gradually retarded and the operating beam energy will decrease to eUS(if the sample has a finite resistance R)or to the 2ndcross-over energy EPEII(if the sample has infinite resistance a perfect insulator).For a perfect insulator,the sample can charge up to a maximum negative surface potential US(max)=(EPEII Eo)/e,where Uo=Eo/e,which is very large(close to 30 keV for Eoof 30 keV).This is why negative charging is very serious.SEM Sample ChargingPositive Charging at Low Primary Electron Beam Energies(EO)SEM Sample ChargingPositive Charging at Low Primary Electron Beam Energies(EO)For an operating beam energy between the two cross-over energies(EPEIand EPEII),the sample will charge positively since the total electron yield()is greater than unity.As the sample,charges positively to a surface potential of US,the primary electron beam will be gradually accelerated and the operating beam energy will increase to EF(beam energy at point F).The operating beam energy will generally not increase to EPEIIbecause the positive surface potential will reduce SE emission and hence the total electron yield.The insulator sample can charge up to a positive surface potential US=(EF Eo)/e,which is typically much smaller in magnitude than that under negative charging.