Electron Probe Micro-Analysis (EPMA)

Transcription

1 Electron Probe Micro-Analysis (EPMA) Nilanjan Chatterjee, Ph.D. Principal Research Scientist 1 Electron Microprobe Facility Department of Earth, Atmospheric and Planetary Sciences Massachusetts Institute of Technology Massachusetts Institute of Technology

2 Uses of the EPMA 2 Quantitative micro-chemical analysis (Spot mode): Micron-scale, complete chemical analysis Be to U (10-50 ppm under favorable conditions) High resolution imaging (Raster mode): Back-scattered electron: composition Secondary electron: topography Cathodoluminescence (light): trace element and defects X-ray map: spatial distribution of elements

3 JEOL JXA-8200 Superprobe 3 Liquid nitrogen for EDS Electron gun Electron beam column Energy dispersive X-ray spectrometer (EDS) Optical microscope Sample exchanging chamber Wavelength dispersive X-ray spectrometer (WDS) Secondary electron detector High vacuum pump (inside)

4 Signal types 4 (W hairpin filament or LaB 6 crystal or field emission-type)

5 Sample preparation 5 Sample cut and mounted in epoxy, if necessary Coarse polished with SiC paper (53-16 mm) Qualitative analysis Quantitative analysis Fine polished with alumina slurry or diamond paste on cloth (grit size mm) Washed with water in ultrasonic cleaner after each polishing step, dried after final step, wiped with ethanol

6 Carbon coating 6 25 nm Thickness is monitored on a polished surface of brass, coated with the samples

7 d p (nm) 7 Different electron sources Vertical electron beam W hairpin Horizontal sample surface LaB 6 Field Emission Monte Carlo simulation of electron trajectories inside the sample Tungsten hairpin, 15 kv, 10 na, d p 100 nm Width of interaction volume >> probe diameter

8 Spatial resolution for different signals 8 20 kev, W filament diameter ~150 nm 150 nm 450 nm Fluorescence of CoKa by CuKa ~60 mm (not to scale) CuKa ~1 mm CuLa ~1.5 mm Production volume for different signals is different The onion shell model: Cu-10%Co alloy

9 Defocused beam operation 9 Necessary to use a defocused beam (large spot size) for some types of analysis: Fine-grained material groundmass of a porphyritic rock (spot > grain size) Na or F containing glass Na migrates away, F migrates toward the spot (~ 10 mm) Hydrous glass and minerals amphibole and mica (~ 10 mm)

10 Back-scattered electron (BSE) 10 Compositional contrast

11 Secondary electron (SE) 11 (SE) Produced by ionizations in the target atoms Topographic contrast

12 Intensity Cathodoluminescence (CL) 12 Light wavelength (nm) Semiconductors Trace impurities generate different light

13 Characteristic X-ray generation 13 Ti Ka Characteristic X- rays Fe Ka Ka x-ray Ti Kb Fe Kb Inner-shell ionization Overvoltage, U = E/E c > 1 E : electron beam energy E c : critical excitation energy of shell in target atom

14 Mean Atomic Number Qualitative analysis with BSE and EDS ilmenite 14 hornblende plagioclase

15 Wavelength Dispersive Spectrometer (WDS) 15 l q q for n=1, ABC = 1l A Bragg s Law: nl = 2d sin q B C d L-value : L = nl R d sin q = L 2R n: order of diffraction l: wavelength of x-ray d: lattice spacing in diffracting crystal q: angle of incidence or diffraction L: distance between sample and crystal R: radius of focusing (Rowland) circle

16 WDS operation 16 Radius of focusing circle (R) is fixed

17 WDS vs. EDS spectral resolution 17 Peak resolution with WDS (FWHM ~10 ev) with EDS (FWHM ~150 ev)

18 X-ray elemental mapping with WDS and EDS 18

19 Mapping modes and image quality 19 Beam-rastered image: electron beam rasters over the area to be imaged (defocusing problem) Stage-rastered image: electron beam is stationary, stage moves Image resolution: number of points measured within the imaged area X-ray Signal: beam current and counting (dwell) time per point

20 EPMA Quantitative analysis 20 X-ray intensity is proportional to the concentration, C I C i, I i : concentration and intensity in sample C (i), I (i) : concentration and intensity in standard ZAF Matrix corrections: Z: Atomic Number correction A: Absorption correction F: Fluorescence correction

21 Typical quantitative analysis output 21 k-ratio Element Wt% Oxide Wt% Atomic proportion Element k-ratio El-Wt% MDLel% Ox-Wt% MDLox% At-Prop Error% SiO2 Ka % TiO2 Ka % Al2O3 Ka % Cr2O3 Ka % FeO Ka % MnO Ka % MgO Ka % CaO Ka % Na2O Ka % O Total: Cation: Typical mineral analysis reports Oxide Minimum wt% and Atomic Minimum proportions detection detection The wt% total and cation sum give us limit an idea of the quality limit of the analysis Atomic proportions give us the formula: (El-Wt%) Mg (Ox-Wt%) 0.27 Fe 1.84 Mn 0.25 Ca 0.62 Al 2.04 Si 2.96 O 12 (garnet) Uncertainty Cation Sum based on a certain #O

22 Types of errors and counting statistics 22 Systematic Error Fixed (but unknown) offsets in all measurements Low systematic error: high accuracy Random Error Fluctuations in each measurement Low random error: high precision For N counts, N ± N N i.e., the uncertainty is X 100, or 1 X 100 % N N The percent uncertainty decreases as N increases (more precise) E.g., uncertainty for 25 counts is 20% for 100 counts is 10% for 10,000 counts is 1%

23 EPMA Analytical procedure 23 Sample preparation (mounting, polishing, carbon coating) Carbon coating non-conducting material Setting up the Electron Microprobe Accelerating voltage, beam current, probe diameter, WDS crystal/proportional counters, counting time for each element Qualitative analysis with EDS If nothing is known about the chemical composition Measurement of X-ray intensities in standards (calibration) by WDS Measurement of X-ray intensities in the specimen by WDS and data reduction Matrix corrections