EDS Energy Dispersive Spectroscopy

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EDS Energy Dispersive Spectroscopy. Background Theory. Introduction to the EDS System Hardware & Software X-Ray Signal Generation Signal Origin, Spatial Resolution, Direction of Signal, Sample Surface EDS Instrumentation & Signal Generation - PowerPoint PPT Presentation


<ul><li><p>EDS</p><p>Energy Dispersive Spectroscopy</p></li><li><p>Background TheoryIntroduction to the EDS SystemHardware &amp; SoftwareX-Ray Signal GenerationSignal Origin, Spatial Resolution, Direction of Signal, Sample SurfaceEDS Instrumentation &amp; Signal GenerationDetector and geometry efficiency, Signal processing, Energy Resolution, Collimation</p></li><li><p>Introduction to the EDS SystemHardwareSoftware</p></li><li><p>Hardware SchematicMonitor (MCA Display)HPComputerEDAMIIIPCIDewarPreampSEMColumnPole PieceSampleStageChamberDetectorWindowCollimatorFET</p></li><li><p>Processing SchematicSpectrumInterpretationSignalProcessingSignalDetectionX-RaySignalBeam-SpecimenInteractionElectronBeam</p></li><li><p>X-Ray Signal GenerationSignal OriginSpatial ResolutionDirectionality of SignalsAnalysis of Rough Surfaces or Particles</p></li><li><p>Bohr Model of the Atom (a simplified view) ---where X rays come fromMaLbLaKaKbReal life spectra are more complex because there are multiple orbitals (esp. for the L, M and N orbitals). L-series spectra in EDS can have 6 or 7 peaks.Nucleus</p></li><li><p>Atomic Number Order for the K Series Peaks</p></li><li><p>Chart of Lines visible 0-10 kVK Lines - Be (Z=4) to Ga (Z=31)L Lines - S (Z=16) to Au (Z=79)M Lines - Zr (Z=40) to the highest occurring atomic numbers.Every element (Z&gt;3) will have at least one line viewable between0.1 and 10 keV. In some overlap conditions it might be necessary to examine the area between 10 and 20 keV.</p></li><li><p>Interaction Volume Regionssebsex-rayssamplesurfaceprimarybeamThis diagram is somewhat misleading. High-energy and low-energy x rays behave very differently (just like e-).High energy x rays can not be excited at great depths. Low energy x rays can be excited at great depths, but will most likely be absorbed and will not escape.</p></li><li><p>SE vs BSE ImagesSE -- Edge effect, charge sensitive, very little Z contrast.BSE --Z contrast dominates, no edge effect, no charging seen.</p></li><li><p>X-Ray Spatial ResolutionLow ZHigh ZHigh kVLow kVSpot size does not determine the reso-lution but kV and Z are more significant.</p></li><li><p>Signal ResolutionSignal resolution (se) is determined by the width of the electron beam (spot size) and is proportional to the signal depth.sebsex-rayssamplesurfacex-raybsese</p></li><li><p>Directionality of SignalsSE Signal - attracted to positive voltage on wire mesh network in front of detector.BSE Signal - Detector is arranged to collect signals from a large, symmetrical area.X-ray Signal - most directional of all signals, only one detector with no way to influence the trajectory of x-rays</p></li><li><p>Spectrum AnomaliesAbsorption of x-raysDetectorElectron BeamFluorescenceX-rays InteractionvolumeSpecimen MatrixBackscatterelectrons</p></li><li><p>Directionality of X-ray Signal A B CDetectorDirectionsamplestage/mountTopography has a significant effect on spectrum count rate and on composition (take-off angle and absorption effects)</p></li><li><p>BA CA= Lower low end peaksB= NormalC= Higher low end peaksTake-off angle is highest at C and lowest at A. 3 different spectra at 3 locations on the same particle with a uniform composition.</p></li><li><p>Effects of Tilt (FeCO3)Peaks are autoscaled to the O K peak. Q: What if they were scaled to the background area? A: FeK same height, C K, O K and FeL would be higher at +30 degrees.</p></li><li><p>EDS Instrumentation &amp; Signal DetectionX-Ray DetectorsThe Detector EfficiencyGeometrical EfficiencySignal ProcessingThe Signal ProcessorEnergy ResolutionCollimation</p></li><li><p>X-section of window &amp; crystal (sapphire)x-ray(photon)</p><p>microscopevacuumDetector VacuumDetectorWindow8u Be or 0.3u Polymer+,-chargesDetector</p><p>SiLito preamplifier (FET)Metallization Layer,(85 angstroms) plusthe Si dead layer-500 to 1000 volts</p></li><li><p>Detector EfficiencyWindow Transmission CapabilitiesI / Io = e -(mr t)Where :I = Final IntensityIo = Initial Intensitym = mass absorption coefficientr = densityt = thickness</p></li><li><p>Transmission of K x-rays through various windows</p></li><li><p>Mass Absorption Coefficient0.284Absorption edgeor critical excita-tion energy(Kab)CAbsorptionX-ray Energy (keV)C Ka Energy N Ka Energy</p></li><li><p>Absorption evidence in SpectraThe background is lower on the high-energy side due toabsorption in the sample.</p></li><li><p>Solid AngleW = A/d 2</p><p>Where:A= detector area, mm 2d = the sample to detector distance</p><p>The solid angle (omega) is in steradians. Count rate at 70 mm scale setting = 1/4 that at 50 mm.</p></li><li><p>The PreamplifierDetectorResetFETCOutput50 ns/x-ray eventUltimate peak measurement time will be about 50 us (1000x 50 ns)</p></li><li><p>Output signal of an X-Ray Event (or 3 events)vVoltage(mv)TimeMultiple x-ray events too close to each other will be rejected.Higher dead time (all rejected)Lower dead time</p></li><li><p>Throughput CurvesLesson: High count rates and high dead times actually give fewer counts and poorer spectra. You might consider a faster time constant.</p></li><li><p>Multichannel Analyzer</p></li><li><p>Resolution EquationFWHM= SQRT[(FWHM)noise2 + (2.35 FEe)2] Where:F = fano factor= 0.11E = energy of the x-ray, ev e = 3.8 ev/charge pair (Si), 2.96 ev/charge pair (Ge)</p></li><li><p>Resolution vs Energy for 70ev noiseMn</p></li><li><p>Collimators Be Window with no magnets (BSE do not penetrate)SUTW or UTW Windowwith magnets (shown in yellow) to deflect BSEIf BSE reach the detector they will producebackground anomalies --a hump in thebackground at high energies.</p></li></ul>


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