Electron Probe Microanalysis EPMAWhat’s the point?Key pointsGeneric EMP/SEMElectron GunsW filament:biased Wehnelt CapSX50 Gun and WehneltW filament:Some electron units/valuesSaturationProducing minimum beam diameterColumn: focusing the electronsCondenser & Objective Lenses: working distanceCondenser Lenses: adjusting beam currentProbe diameterProbe current: monitoring and stabilizationProbe current monitoringSlide 18Probe current regulationScanning CoilsSX51 specsOptical MicroscopeGo to Vacuum ModuleElectron Probe MicroanalysisEPMA Electron Optical ColumnUW- Madison Geology 777What’s the point?We need to create a focused column of electrons to impact our specimen, to create the signals we want to measure. This process is identical for both scanning electron microscope (SEM) and electron microprobe (EMP). We use conventional terminology, from light optics, to describe many similar features here.UW- Madison Geology 777Key points• Source of electrons: various electron guns; W in particular. We want high, stable current with small beam diameter.• Lenses are used to focus the beam and adjust the current• Current regulation and measurement essential• Beam can be either fixed (point for quant. analysis) or scanning (for images)• Optical microscope essential to position sample (stage) height, Z axis (= X-ray focus)• Vacuum system essentialUW- Madison Geology 777Generic EMP/SEMElectron gunColumn/ Electron opticsOptical microscopeWDS spectrometersScanning coilsEDS detectorVacuum pumpsSE,BSE detectorsFaraday current measurementUW- Madison Geology 777Electron GunsSeveral possible electron sources: most common is the W filament, thermoionic type. A W wire is heated by ~2 amps of current, emitting electrons at ~2700 K – the thermal energy permits electrons to overcome the work-function energy barrier of the material. Another thermoionic source is LaB6, which has added benefits (“brighter”, smaller beam) but it is more expensive and fragile. Both have very good (~1%) beam stability, compared to a different variety of sources, the field emission guns, which are “brighter” and have much smaller beams (great for high resolution SEM images) but lower beam stability and require ultra high vacuum.UW- Madison Geology 777W filament:biased Wehnelt CapCurrent (~2 A) flows thru the thin W filament, releasing electrons by thermoionic emission. There is an HV potential (E0) between the filament (cathode) and the anode below it, e.g. 15 keV. The electrons are focused by the Wehnelt or grid cap, which has a negative potential (~ -400 V), producing the first electron cross over.Goldstein Fig 2.4, p. 27First electron cross-overUW- Madison Geology 777SX50 Gun and WehneltWehnelt diameter (below) is ~20 mm UW- Madison Geology 777W filament:W filament is ~125 m diameter wire, bent into hairpin, spotwelded to posts. W has low work function (4.5 eV) and high melting T (3643 K), permitting high working temperature. Accidental overheating will cause quick failure (top right). Under normal usage, the filament will slowly ablate W, thinning down to ultimate failure (uncertain why offset). With care/luck, a filament may last 6-9 months, though 1-2 month life is not uncommon. Goldstein Fig 2.8, p. 33UW- Madison Geology 777Some electron units/valuesBrightness is a measure of the current emitted/unit area of source/unit solid area of beam (not used in daily activities)BaseballHV: speed of the ballcurr: size of the ballWater through hoseHV: water pressurecurr: size of the stream of waterHigh voltage and Current - AnalogiesUW- Madison Geology 777Saturation“Saturation” is the optimization of 1) current stability (on the plateau) and 2) filament life (minimal heating). The Operating or Saturation point is at the “knee” of the plot. On the SX51, “HEAT” is the variable, with saturation usually between 228 and 200, with new filaments at the upper value, and gradually declining as the filament ages (thins). These are unitless values (0-255 scale)Goldstein et al Fig 2.5, p. 278Producing minimum beam diameter(Goldstein et al, 1992, p. 49)Similar to light optics (though inverted: reducing image size):d0 is the demagnified gun (filament) crossover--typically 10-50 um, then after first condenser lens, it is further demagnified to crossover d1. After C2 and objective lens, the final spot is 1 nm-1um.Light Optics Electron Optics1/f = 1/p + 1/qUW- Madison Geology 777Column: focusing the electrons(Goldstein et al, 1992, p. 44)Simple iron electromagnet: a current through a coil induces a magnetic field, which causes a response in the direction of electrons passing through the field.Rotationally symmetric electron lens: beam electrons are focused, as they are imparted with radial forces by the magnetic field, causing them to curve toward the optic axis and cross it.Condenser & Objective Lenses:working distance(Goldstein et al, 1992, p. 51)Left: shorter working distance (~q2), greater convergence (2): smaller depth of field, smaller spot (d2), thus higher spatial resolution. Right: longer WD, smaller convergence: larger depth of field, larger spot, decreased resolution.UW- Madison Geology 777Condenser Lenses:adjusting beam current(Goldstein et al, 1992, p. 52)Probe current (Faraday cup current, e.g. 20 nA) is adjusted by increasing or decreasing the strength of the condenser lens(es): a) weaker condenser lens gives smaller convergence 1 so more electrons go thru aperture. Thus higher current with larger probe (d2) and decreased spatial resolution (2). b) is converse case, for low current situation.UW- Madison Geology 777Probe diameterReed 1993, Fig 4.11, p. 46“What is the beam/probe size”? I would suggest this is a philosophical question: what is the theoretical size of the beam--before it enters the specimen?-- a question of limited importance in EPMA For that hypothetical question, Reed provides a ballpark estimate (for ~5 nA of beam current, the minimum diameter is 0.2 m.(D is demagnification,  is beam semi-angle). However, in the real world, the actual “interaction volume” (due to electron scattering) and thus size of analysis volume is larger, as you can appreciate from your Monte Carlo simulations.Probe current: monitoring and stabilizationEPMA requires precise measurement of X-ray counts. X-ray count intensity is a function of many things, but here we focus on electron dosage. If we get 100 counts for 10 nA of probe (or beam or Farady)