Introduction to EDX. Energy Dispersive X-ray Microanalysis (EDS, Energy dispersive Spectroscopy) Basics of EDX

Transcription

1 Introduction to EDX Energy Dispersive X-ray Microanalysis (EDS, Energy dispersive Spectroscopy) EDX Marco Cantoni 1 Basics of EDX a) Generation of X-rays b) Detection Si(Li) Detector, SDD Detector, EDS (<-> WDS) c) Quantification EDX in SEM, Interaction volume Monte-Carlo-Simulations EDX in TEM [ d) EDX in SEM-STEM ] EDX Marco Cantoni 2

2 Inelastic scattering of electrons at atoms E electron_in > E electron_out Continuum X-ray X production (Bremsstrahlung, Synchrotron) Inner shell ionization SE SE, BSE, EELS EDX Marco Cantoni 3 Core shell ionisation: chemical microanalysis by X-ray, Auger electrons and Electron Energy Loss Spectrometries e- + L 1 L2 K L3 e- M 5 Kα2 RX + L 1 L2 K L3 Ionisation 1ps + e- KL 2L3 L 1 L2 K L3 Ka 1 Ka 2 Kb La 1 La 2 KL 1L2 KL 1L3 KL 2L3 L 1M1M2 M 4 M 3 M 2 M 1 L 3 L 2 L 1 K Rayons X Electrons Auger Emission X Emission Auger EDX Marco Cantoni 4

3 Emission of characteristic X-ray and Auger electron EDX Marco Cantoni 5 Designation of x-ray emission lines EDX Marco Cantoni 6

4 EDX Marco Cantoni 7 Efficiency of X-ray generation Relative efficiency of X-ray and Auger emission vs. atomic number for K lines Ionization cross-section vs. overvoltage U=Eo/Eedge (electron in -> X-ray out) Light elements Auger Spectroscopy Heavy elements EDS SEM TEM -> Cu-K 8.1kV, HT 15kV U = 15/8.1 = 1.85 Light element atoms return to fundamental state mainly by Auger emission. For that reason, their K-lines are weak. In addition their low energy makes them easily absorbed. To ionized the incident electron MUST have an energy larger than the core shell level U>1. To be efficient, it should have about twice the edge energy U>2. EDX Marco Cantoni 8

5 X-ray production vs. atomic number Z Low efficiency for light elements! EDX Marco Cantoni 9 Characteristic lines: Moseley's Law EDS range ~ kev! To assess an element all detectables lines MUST be present!!! known ambiguities: Al Kα = Br Ll S Kα = Mo Ll EDX Marco Cantoni 10

6 Moseley s law for K-series Frequency ν of X-rays emitted from K-level vs. atomic number ( Z ) ν = E= hν et λ=c/ν with the Planck constant:h= (52) J s and 1eV = J Energy of characteristic X-rayX -> > Element Qualitative EDX-Analysis So, lets measure the X-rays emitted from my sample and determine the composition! But how to detect it? EDX Marco Cantoni 11 b) Detection of X-rays (EDX) EDX Marco Cantoni 12

7 EDX Marco Cantoni 13 X-Ray energy conversion to electrical charges: 3.8eV / electron-hole pair in average electronic noise+ imperfect charge collection: 130 ev resolution / Mn Ka line Detector acts like a diode: at room temperature the leak current for 1000V would be too high! The FET produces less noise if cooled! Li migration at room temperature! ->Detector cooling by L-N EDX Marco Cantoni 14

8 Pulse detection and analysis Pulse detection: Charge ~ energy Shorter time constant = process time to analyze voltage -> peak broadening (lower energy resolution) Longer time constant (higher energy resolution) ->pulse rejection (dead time) EDX Marco Cantoni 15 Process time time constant resolution peak identification beam current count rate Long low count rate Higher dead time narrow peaks Time const. (μsec.) Count rate, cts/sec. (30% dead time) ~ ~ ~4 500 ~2 000 short high count rate lower dead time broad peaks EDX Marco Cantoni 16

9 Silicon Drift Detectors Extremely fast (up to counts/sec.) No L-N cooling required Similar priced as Si/Li detectors Peak tail at lower energies Lower resolution for light elements EDX Marco Cantoni 17 Detection and artifacts X-Ray energy conversion to electrical charges: 3.8eV / electron-hole pair in average electronic noise+ imperfect charge collection: 130 ev resolution / Mn Ka line Take care when looking for trace elements (low concentrations). Don t confuse small peaks with escape peaks! EDX Marco Cantoni 18

10 Wavelength Dispersive Spectroscopy (WDS) The specimen, the diffracting crystal and the detector stay on the Rowland circle. To scan the wavelength, this circle rotates around the specimen to satisfying the Bragg law Johansson focusing spectrometer: The diffracting crystal is bent with a curvature radius double of that of the Rowland circle The crystal surface is cylindrically ground to the radius of the Rowland circle EDX Marco Cantoni 19 Electron Microprobe EPMA (Electron Probe MicroAnalyser) with WDS EDX Marco Cantoni 20

11 EDS <-> WDS Energy Resolution Acquisition time Use Standardless Analysis Peak to background ratio EDS eV 1 min. Easy :1 WDS 5eV 5-30min. difficult difficult 1000:1 EDX Marco Cantoni 21 Continuum, Bremsstrahlung (K,Na)NbO 3 Overvoltage, 10keV Electron beam: 10keV Duane-Hunt limit EDX Marco Cantoni 22

12 Bremstrahlung (background) when a charged particle (des-) accelarates or changes direction, it emits an electromagnetic wave. This is widely used to produce synchrotron radiation On a bulk sample of atomic number Z: N(E) is the number of photons of energy E, E0 the energy of the incident electron and K the Kramers constant N ( E) ( E) KZ E0 = E EDX Marco Cantoni 23 EDS in SEM Acquisition under best conditions Flat surface without contamination (no Au coating, use C instead) Sample must be homogenous at the place of analysis (interaction volume!!) Horizontal orientation of the surface High count rate (but dead time below 30%) Overvoltage U=Eo/Ec >1.5-2 For acquisition times of 100sec. : detection of ~0.5at% for almost all elements EDX Marco Cantoni 24

13 Continuum, Bremsstrahlung (K,Na)NbO 3 Not ideal! Overvoltage, 10keV Duane-Hunt limit EDX Marco Cantoni 25 (K,Na)NbO3 Spectrum Na K Nb O Total Spectrum Spectrum Spectrum Spectrum Spectrum Spectrum Spectrum Spectrum Max Min EDX Marco Cantoni 26

14 c) Quantification Yes, but.. First approach: compare X-ray intensity with a standard (sample with known concentration, same beam current of the electron beam) c i : wt concentration of element i I i : X-ray intensity of char. Line k i : concentration ratio c c i std i = I I i std i = k i EDX Marco Cantoni 27 Intensity ~ Concentration? How many different samples? EDX Marco Cantoni 28

15 EDX Marco Cantoni 29 Electron Flight Simulator EDX Marco Cantoni 30

16 Casino EDX Marco Cantoni 31 EDX Marco Cantoni 32

17 Quantification When the going gets tough.. Correction matrix c c [ ] i i Z A F = = ki std i I I std i "Z" describe how the electron beam penetrates in the sample (Z-dependant and density dependant) and loose energy "A" takes in account the absorption of the X-rays photons along the path to sample surface "F" adds some photons when (secondary) fluorescence occurs EDX Marco Cantoni 33 Flow chart of quantification Measure the intensities and calculate the concentrations without ZAF corrections Calculate the ZAF corrections and the density of the sample Calculate the concentrations with the corrections Is the difference between the new and the old concentrations smaller than the calculation error? no Yes! stop EDX Marco Cantoni 34

18 Correction methods: ZAF (purely theoretical) PROZA Phi-Rho-Z PaP (Pouchou and Pichoir) XPP (extended Puchou/Pichoir) with standards (same HT, current, detector settings) Standardless: theoretical calculation of I std Standardless optimized: «hidden» standards, user defined peak profiles EDX Marco Cantoni 35 Quantitative EDX in SEM Acquisition under best conditions Flat surface without contamination, horizontal orientation of the surface (no Au coating, use C instead) Sample must be homogenous at the place of analysis (interaction volume!!) High count rate (but dead time below 30%) Overvoltage U=Eo/Ec >1.5-2 For acquisition times of 100sec. : detection of ~0.5at% possible for almost all elements Standardless acquisition acquisition possible with high accuracy (intensities of references under the given conditions can be calculated for a great range of elements), test with samples of known composition, light elements (like O) are critical Spatial resolution depends strongly on HT and the density of the sample EDX Marco Cantoni 36

19 Demo NSS/INCA Peak finding, synthetic spectrum Spectrum imaging (extraction of elements) EDX Marco Cantoni 37 Modern EDX systems: User friendly interfaces New and more powerful electronics (stability of calibrations, higher count rate) Drift compensation for long acquisition times (element mapping on CM300 at high mag, sitelock ) Synthesized spectra (spectrum overlay) easier identification Advanced element mapping: Spectral imaging (data cube), selection of elements and regions post-acquisition Powerfull reporting and Export tools (Word, Powerpoint, html, tif etc.) EDX Marco Cantoni 38

20 Spectrum imaging Data cube Synthesized spectrum Extraction of element maps EDX Marco Cantoni 39 PZT bulk EDS in TEM High spatial resolution! 20nm thick PZT EDX Marco Cantoni 40

21 Thin samples -> correction factors weak (A and F can be neglected) Very weak beam broadening -> high spatial resolution ~ beam diameter (~nm) EDS in TEM High energy: artifacts! If only there wasn t this specimen preparation EDX Marco Cantoni 41 STEM point analysis PbMg 1/3 Nb 2/3 O 3 (bulk) Processing option : Oxygen by stoichiometry (Normalised) Spectrum Mg Si Nb Pb O Total Spectrum Spectrum Spectrum Spectrum Spectrum Spectrum Spectrum Spectrum Spectrum Max Min All results in Atomic Percent EDX Marco Cantoni 42

22 STEM linescan Pb(Zr,Ti)O 3 (thick film), slight Pb excess EDX Marco Cantoni 43 STEM Element Mapping STEM Element Mapping PMN/PT 90/10 (bulk) EDX Marco Cantoni 44

23 Artifacts how to recognize/minimize them EDX Marco Cantoni 45 X-Ray collection in TEM EDS upper obj. lens polepiece sample holder collection solid angle collimator EDS detector lower obj. lens polepiece EDX Marco Cantoni 46

24 X-Ray collection in TEM EDS upper obj. lens polepiece sample holder collection solid angle collimator EDS detector lower obj. lens polepiece EDX Marco Cantoni 47 Stray X-Rays in TEM EDS X-rays shower hole count upper obj. lens polepiece sample holder collection solid angle collimator EDS detector lower obj. lens polepiece EDX Marco Cantoni 48

25 Stray X-Rays in TEM EDS upper obj. lens polepiece EDS detector back-scattered e - to thick sample collimator lower obj. lens polepiece EDX Marco Cantoni 49 Stray X-Rays in TEM EDS upper obj. lens polepiece EDS detector collimator scattered e - to polepieces + thick sample lower obj. lens polepiece EDX Marco Cantoni 50

26 Stray X-Rays in TEM EDS: characteristic x-rays upper obj. lens polepiece EDS detector collimator lower obj. lens polepiece EDX Marco Cantoni 51 Stray X-Rays in TEM EDS: continuum (Bremsstrahlung) upper obj. lens polepiece EDS detector collimator lower obj. lens polepiece EDX Marco Cantoni 52

27 Ideal samples: FIB samples: almost uniform thickness, small sample size(less bulk material around) EDX Marco Cantoni 53 EDS in TEM Thin samples -> correction factors weak (A and F can be neglected), quantification easy Very weak beam broadening -> high spatial resolution ~ beam diameter (~nm) High energy -> artifacts Sample preparation, sample geometry EDX Marco Cantoni 54

28 Bonus EDX Marco Cantoni 55 EDX of Powders nano particles SEM or TEM??? TEM holey Carbon film KNbO 3 Nano-rods SEM Optical microscope reflected light EDX Marco Cantoni 56

29 Simulation Electron Flight Simulator EFS Sample holder x x C film EDX Marco Cantoni 57 SEM 10kV TEM 300kV x x EDX Marco Cantoni 58

30 SEM 10kV SEM-STEM 30kV x x EDX Marco Cantoni 59 SEM 10kV STEM 30kV Interaction volume >> particle size Sample holder analyzed No deconvolution possible Low HT = limited energy range for ionization energies Easy interpretation no contribution from substrate (C) MBTF corrections for quantification EDX Marco Cantoni 60

31 XL30-SFEG SEM/STEM TEM sample Annular dark field Bright field BSE-Detector (upside-down) Poor man s s TEM EDX Marco Cantoni 61 SEM-STEM STEM BF SEM-STEM STEM ADF KNbO3 EDX Marco Cantoni 62

32 SEM-STEM BF SE detector EDX Marco Cantoni 63 SEM-STEM STEM ADF 30kV, FIB lamella SEM-SE 30kV EDX Marco Cantoni 64

33 SEM SE of TEM sample FIB true STEM, CM300, 300kV. ADF SEM STEM ADF, 30kV EDX Marco Cantoni 65