SurfaceAnalysis.pdf

W.K.CHIMAssociate ProfessorDepartment of Electrical and Computer EngineeringNational University of SingaporeE-mail:elecwknus.edu.sgTel:(65)6516-6287Office Location:E2-03-31EE5514 IC Yield,Reliability&Failure AnalysisSurface Analysis TechniquesSurface Analysis Techniques Auger Electron Spectroscopy(AES)Secondary Ion Mass Spectrometry(SIMS)Electron Spectroscopy for Chemical Analysis(ESCA)or X-ray Photoelectron Spectroscopy(XPS)Surface Analysis TechniquesBackgroundAuger Electron EmissionAES InstrumentationAuger SpectrumDepth ProfilingAuger Electron Spectroscopy(AES)Auger Electron Spectroscopy(AES)Background AES is a true surface analysis technique(Unlike EDS/EDX which is a bulk analysis technique).D.K.Schroder,Semiconductor Material and Device Characterization,Wiley,1990,Chapter 10.Escape zones AES is used to identify the elemental composition of surface films,or with ion sputtering,to measure the concentration of elements as a function of depth(i.e.,depth profiling).Auger Electron Spectroscopy(AES)Background(Contd)Auger electrons are low energy electrons(50 to 2500 eV)that are emitted from a sample when it is scanned by an electron beam of up to 5 keV.Secondary ElectronsEnergy DistributionAuger ElectronsBSEs Higher energy beams produce Auger electrons deeper within the sample that have little chance of escaping.The low energy of the Auger electrons limit their escape depth from the sample to only a few monolayers of the surface.AES sampling depths is typically in the region of between 5 to 50.All elements,with the exception of hydrogen and helium,can be detected using AES.Auger Electron Spectroscopy(AES)Background(Contd)AES is particularly sensitive to the light-element range.It therefore complements X-Ray Spectroscopy which has low sensitivity for the light elements.AES Detection Limit:0.1 to 1%but varies from element to element.Quantitative AES analysis is difficult.Reported accuracies are currently about 20-50%,with 5%precision for simple semiconductor samples.Auger Electron Spectroscopy(AES)Auger Electron Emission A primary electron from the electron gun ejects an electron from the K shell.D.K.Schroder,Semiconductor Material and Device Characterization,Wiley,1990,Chapter 10.X-ray and Auger.url The K-shell vacancy is filled by an outer-shell electron(L1in this case).The energy E=EL1-EKis then transferred to a third electron(the Auger electron),originating in this case at the L2,3level.The atom remains in the doubly ionized state and the entire process is labelled KL1L2,3 or simply as KLL.Auger Electron Spectroscopy(AES)Auger Electron Emission(Contd)Since the Auger process is a three-electron process,it is obvious why hydrogen and helium cannot be detected,as both elements have less than 3 electrons.The dominant Auger energy transitions are:For 3 Z 14:KLL transitionsFor 14 Z 40:LMM transitionsFor 40 Z 82:MNN transitionsAuger Electron Spectroscopy(AES)Auger Electron Emission(Contd)In general,when an electron is excited from level A,the vacancy filled by an electron from level B,and an Auger electron is ejected from level C,the kinetic energy of the Auger electron(EABC)is given by:EABC=EA(Z)-EB(Z)-EC(Z+)-q where =Sample work function Z =Atomic number of the sample element =Approximately equal to 1(To account forenergy of final doubly-ionized state beinglarger than the sum of energies forindividual ionization of the same levels)Auger Electron Spectroscopy(AES)AES InstrumentationD.K.Schroder,Semiconductor Material and Device Characterization,Wiley,1990,Chapter 10.The emitted Auger electrons are detected with a retarding potential analyzer,a hemispherical analyzer,or a cylindrical mirror analyzer(CMA).The CMA allows electrons with energies of around Vato pass through the exit slit.AES System with CMA Analyser An ultra-high vacuum of 10-9torr is required to protect the sample from carbon contamination which will interfere with the Auger signal.VaAuger Electron Spectroscopy(AES)Auger SpectrumD.K.Schroder,Semiconductor Material and Device Characterization,Wiley,1990,Chapter 10.A typical Auger Spectrum for a silver sample is shown.The Auger electron peaks appear as small perturbations on a high background consisting of backscattered electrons that have lost varying amounts of energy.In order to enhance the Auger peaks,it is a common practice to differentiate the Auger signal and present it as dN(E)/dE versus E.The Auger energy position is indicated by the peak in the N(E)spectrum or by the maximum negative excursion of the dN(E)/dE peak in the differentiated spectrum.Auger Electron Spectroscopy(AES)Depth ProfilingDepth profiles are generated by alternately sputtering with an inert Ar ion beam and acquiring the Auger signal versus sputtering time.Tantalum Silicide film on Polysilicon on Oxidized Si WaferSurface Analysis TechniquesBackgroundPrinciple of SIMSSIMS InstrumentationSecondary Ion YieldModes of OperationAdvantages&Disadvantages of SIMS for Depth ProfilingSecondary Ion Mass Spectrometry(SIMS)Secondary Ion Mass Spectrometry(SIMS)Background SIMS,also known as Ion Microprobe and Ion Microscope,is one of the most sensitive and versatile surface analytical technique for semiconductor characterization.The technique is element specific and is capable of detecting all elements as well as isotopes and molecular species.Of all the microbeam analytical techniques,SIMS is the most sensitive,with detection limits for some elements in the 1014to 1015atoms/cm3range(1 ppm atomic=5 x 1016atoms/cm3).Lateral resolution is typically 100 m but can be as small as 1 m and depth resolution is 50 to 100.Secondary Ion Mass Spectrometry(SIMS)Background(Contd)Secondary Ion Mass Spectrometry(SIMS)Principle of SIMS(Contd)Destructive removal of material from the sample by sputtering with a primary ion beam and the analysis of the sputtered species by a mass spectrometer.Secondary Ion Mass Spectrometry(SIMS)Principle of SIMS(Contd)Sputtering is a process in which incident ions lose their energy mainly by momentum transfer as they come to rest within the solid.In the process,they displace atoms within the sample.The escape depth of the sputtered species is generally a few monolayers for primary energies of 10 to 20 keV,typically used in SIMS.Only about 1%of the sputtered species are ejected as positive and negative ions;the rest are neutral atoms and cannot be detected by conventional SIMS.Secondary Ion Mass Spectrometry(SIMS)SIMS InstrumentationD.K.Schroder,Semiconductor Material and Device Characterization,Wiley,1990,Chapter 10.Mass/Charge ratios of the secondary ions are analyzed,detected as a mass spectrum,as a count,or displayed on a CRT screen.Secondary Ion Mass Spectrometry(SIMS)SIMS Instrumentation(Contd)Secondary Ion Mass Spectrometry(SIMS)Secondary Ion Yield The secondary ion yield from sputtering is dependent on:-Type of sample material-Crystallographic orientation of sample-Nature,energy and incident angle of the primary ions Argon ions is commonly used as the primary ion source.To enhance the secondary ion yield for electropositive elements(e.g.,B and Al in Si),electronegative oxygen(O2-)ion source is used.Secondary Ion Mass Spectrometry(SIMS)Secondary Ion Yield(Contd)Electropositive Cesium(Cs+)ion source is used to enhance the yield of electronegative elements(e.g.,P,As and Sb in Si).Liquid metal ion sources,with a small beam size of about 500,is also used for high resolution work.The secondary ion yield is also dependent on the sample matrix material(i.e.,matrix effect).For example,the secondary ion yield for oxidized surfaces can be higher than that for bare surfaces by as much as 100 x.The yield of Si in SiO2is about 100 times higher than the yield of Si from the Si substrate.This effect has to be taken into account in quantitative analysis.Secondary Ion Mass Spectrometry(SIMS)Modes of OperationStatic SIMSA low sputtering rate of approx.1 per hour.A complete mass spectrum can be recorded for surface analysis of the top 5 or so of the sample.Dynamic SIMSThe intensity of one peak for a particular mass/charge ratio can be recorded as the sample is sputtered at a higher sputtered rate(approx.10 m/hr)yielding a depth profile.Secondary Ion Mass Spectrometry(SIMS)Modes of Operation(Contd)SIMS Depth Profile of a BPSG/SiO2/Si structure showing gettering of the sodium contaminant in the BPSG layer.Secondary Ion Mass Spectrometry(SIMS)Advantages of SIMS for Depth Profiling Excellent sensitivity(Generally 1%)Semi-destructive techniqueSurface Analysis TechniquesBackgroundPhotoelectron EmissionXPS InstrumentationAdvantages and Disadvantages of XPSX-ray Photoelectron Spectroscopy(XPS)X-ray Photoelectron Spectroscopy(XPS)Background XPS is also known as Electron Spectroscopy for Chemical Analysis(ESCA).Used in identification of chemical compounds at sample surfaces,by noting energy shifts due to changes in the chemical structure of the sample atoms(e.g.,an element in the form of an oxide exhibits a different XPS spectrum from that of a pure element).XPS is particularly useful for analysis of organics,polymers and oxides.For example,it has been used to follow the oxidation kinetics of elements.X-ray Photoelectron Spectroscopy(XPS)Photoelectron Emission Primary X-rays of 1 to 2 keV energy(typically Mg K or Al K)eject photoelectrons from the sample via the photoelectric effect.The spectrometer and sample are connected forcing their Fermi levels to line up.D.K.Schroder,Semiconductor Material and Device Characterization,Wiley,1990,Chapter 10.X-ray Photoelectron Spectroscopy(XPS)Photoelectron Emission(Contd)The measured energy of the ejected electron at the spectrometer(Esp)is related to the binding energy(Eb),referenced to the Fermi energy(EF),by the following equation:Eb=h -Esp-q spwhere h =Energy of primary X-rays sp=Work function of spectrometer(3 to 4 eV)Ebdepends on:-Atomic Composition-Chemical Environment(i.e.,Bonding)X-ray Photoelectron Spectroscopy(XPS)XPS InstrumentationD.K.Schroder,Semiconductor Material and Device Characterization,Wiley,1990,Chapter 10.The 3 basic components of an XPS system are the X-ray source,the spectrometer and a high vacuum.The emitted photoelectrons are commonly detected by the cylindrical mirror analyzer(CMA)detector(similar to that used in AES).As the X-ray line width in XPS should be as narrow as possible(X-ray line widths are proportional to atomic number of the source material),light elements like Al(EK=1.4866 keV)or Mg(EK=1.2536 keV)are commonly used as X-ray sources.X-ray generation from low Z materials also has reduced background radiation.X-ray Photoelectron Spectroscopy(XPS)XPS Instrumentation(Contd)Chemical compounds or elements are identified by the location of energy peaks in the undifferentiated XPS spectrum.D.K.Schroder,Semiconductor Material and Device Characterization,Wiley,1990,Chapter 10.Quantitative analysis is usually difficult although peak heights and areas can be used with appropriate correction factors to obtain concentrations.X-ray Photoelectron Spectroscopy(XPS)XPS Instrumentation(Contd)XPS detection sensitivity is around 0.1%(similar to AES)X-ray Photoelectron Spectroscopy(XPS)Advantages of XPS XPS allows not only elemental detection but also chemical bonding identification.The electron binding energy(Eb)is influenced by its chemical surrounding,making it suitable for determining chemical states.Because X-rays are used,sample charging is practically absent unlike electron and ion beam techniques.X-rays are also less destructive.X-ray Photoelectron Spectroscopy(XPS)Disadvantages of XPS Spatial resolution is generally poor as X-ray techniques are generally large-area methods.However,the analyzed sampled area has been reduced over the years and spot sizes of 100 m are possible.