CHEM-E5225 :Electron Microscopy X-Ray Spectrometry

Transcription

1 CHEM-E5225 :Electron Microscopy X-Ray Spectrometry Yanling Ge

2 Outline X-ray Spectrometry X-ray Spectra and Images Qualitative and Quantitative X-ray Analysis and Imaging Discussion of homework

3 X-ray Analysis: Why Bother? The elemental information is of great facilitation for subsequent image and/or diffraction analysis. Basic Operations Mode spot mode TEM focused-spot analysis mode: strong C1 lens (larger spot size), smaller C2 aperture, (alignment is a problem). It is recommend to do XEDS in STEM mode, which meet all the requirements for a focused-spot analysis. Always use a: Low-background (Be) holder Liquid-N2 cooled holder Double-tilt holder

4 The Energy-Dispersive Spectrometer The detector generates a charge pulse proportional to the X- ray energy. This pulse is first converted to a voltage. The voltage is amplified through a field-effect transistor (FET), isolated from other pulsed, further amplified, then identified electronically as resulting from an X-ray of specific energy. A digitized signal is stored in the channel assigned to that energy in the computer display.

5 Semiconductor Detectors Si(Li)

6 Semiconductor Detectors Silicon-Drift Detectors (SDD) High throughput. No cooling or minimal thermoelectric (Peltier) cooling. Energy resolution competitive with Si(Li) detectors. The small X-ray count rates negating the high throughput advantage of SDDs.

7 Comparison of detectors

8 Turning X-rays Into Spectra (Si(Li)/SDD) 1 X-ray electron-hole pairs a charge pulse a voltage pulse digitized signal assigned with energy of X-ray Display resolution: keep the resolution at 10 ev per channel or better. Time constant (τ) is the time allowed for the analog processor to evaluate the magnitude of the charge pulse. Shortest τ will allow more counts per second to be processed but with a greater error in the assignment of a specific energy to the pulse, so the energy resolution will be poorer. A longer τ will give you better resolution but the count rate will be lower. Dead time is the time period when the detector is effectively switched off for the pulse processor to analyze that pulse.

9 Energy Resolution P: FWHM of processing electronics; X: FWHM of detector leakage current; I: intrinsic line width of the detector Measure the resolution in column, NiO foil or Cr films may be used instead of Mn. Typically, Si(Li) detectors have a resolution of 140 ev at Mn Kα with the best being <130 ev. The best reported IG resolution is 114 ev. SDDs offer about 140 ev but can get down to 130 ev with Peltier cooling.

10 Detector Characteristics The detector resolution on column using the Mn or Ni Kα line (typically 150 ev for Si(Li) and 140 ev for an IG or SDD). The Incomplete charge collection ICC defined by the FWTH/FWHM ratio of the Ni Kα line (ideally 1.82). The ice/contamination build-up reduces the detection efficiency for low-energy X-rays which reflected in the Ni Kα/Lα ratio. DO NOT generate high fluxes of X-rays or Back-scattered electrons unless your detector is shuttered. DO NOT ever warm up the detector yourself; get help. But make sure that the bias is turned off and the manufacturer is consulted. DO NOT use unfiltered or re-cycled liquid N 2.

11 Processing Variables Check the energy calibration of the computer display. Check the dead-time circuitry by the linearity of the output count rate versus beam current. Check the counts in a fixed clock time as a function of beam current to determine the maximum output count rate. Fixed clock time

12 The XEDS-AEM Interface Collection Angle: is a solid angle subtended at the analysis point on the specimen by the active area of the front face of the detector. Take-off Angle α: the angle between the specimen surface (at 0 tilt) and a line to the center of the detector. High take-off angle to minimize the absorption effect, which is more important for SEM than STEM.

13 Orientation of the Detector to the Specimen (a) Is the detector pointing on the axis? The ideal position of detector is that it is looking at the region of your specimen that is on the optic axis when the specimen is encuentric and at zero tilt. (b) where is the detector with respect to the image? It is best if the detector is looking toward a thin region of the specimen, rather than toward a thicker region.

14 X-ray Spectra and Images

15 The Ideal Spectrum The Characteristic Peaks: the characteristic X-rays have a well-defined energy and a natural line width (the FWHM of the Gaussian districution of X-ray energies) of typically 1-5 ev. But the XEDS degrades this width to a Gaussian shaped peak with a FWHM of about 135 ± 10 ev. The Continuum Bremsstrahlung Background: it arise as beam electron are slowed down or stopped by electrostatic interactions with nuclei in the specimen. What make X-ray spectrometry challenging is that the spectrum generated within your thin specimen, the spectrum detected by your Si(Li) detector, and the spectrum displayed on your computer screen are all quite different.

16 Artifacts Common to Si(Li) XEDS Systems Escape peak: Due to the imperfection of detector a small fraction of the energy is lost, for example excited Si Kα. You can only see escape peaks at (E-1.74) kev if there are major characteristic peaks! Internal fluorescence peak: this is a characteristic peak from the Si (or Ge) in the detector dead layer. Depending on the dead layer thickness, the Si K peak intensity corresponds to 0.1 1% of the specimen composition. Sum peak: arises when the count rate exceeds the electronics ability to discriminate all the individual pulse and so-called pulse pile-up occurs. The input count rate is high. The dead times are > 60%. There are major characteristic peaks in the spectrum.

17 Pre-specimen Effects The TEM illumination system produces high-energy bremsstrahlung X-rays and electrons scattered outside the main beam, both of which may stride the specimen, producing spurious X-rays. Stray X-rays will give a high Ag (or Mo) K/L ratio. Stray electrons will give a low Ag (or Mo) K/L ratio. Always operate with clean, thick, top-hat C2 diaphragms. Use very thin flake specimens or uniform thin films, if possible. Always image the electron beam on the TEM screen prior to analysis, to insure that it is well collimated by the C2 aperture. Use a Cs-corrected AEM if you can find one.

18 Post-specimen Scatter System X-rays come from parts of the AEM other than the specimen. Electrons scattered by the specimen may create X- rays characteristic of the materials in the holder (brass), the polepiece (Fe and Cu), and the collimator (e.g., Al, W) and any of these X-rays could be picked up by the XEDS detector. Always remove the objective diaphragm. Operate as close to zero tilt as possible. Use a Be specimen holder and Be Grids. Use thin foils, flakes, or films rather than self-supporting disks.

19 Coherent Bremsstrahlung In thin single-crystal specimens illuminated by high-energy electrons, it is possible to generate a bremsstrahlung X-ray spectrum that contains small, Gaussian-shaped peaks known as coherent bremsstrahlung (CB). They arise by the nature of the coulomb interaction of the beam electrons with the regularly spaced nuclei in the crystal specimen. Where β is the electron velocity (v) divided by the velocity of light (c), L is the real lattice spacing in the beam direction and α is detector take-off angle. You can t remove the CB effects entirely but they will move depending on both the accelerating voltage and the specimen orientation which will change the value of L.

20 Acquiring X-ray Spectra Spot mode: time consuming, statistical confidence appalling, operator bias. Spectrum-Line Profiles, no operator bias, information limited. X-ray images: the best way to gather X-ray information with some semiblance of statistical significance and without operator bias. Be carful with specimen drift, contamination and damage during acquiring. A. Analog Dot Mapping: using a specific energy to build a dot image. B. Digital Mapping: collect X-rays from multiple channels or windows and thus acquire several maps simultaneously. C. Spectrum Imaging: collects a full spectrum at every pixel in the digital image. D. Position-Tagged Spectrometry (PTS): sophisticated processing software in the analysis computer can be used to interrogate the data during acquisition.

21 Qualitative and Quantitative X-ray Analysis and Imaging

22 Microscopy and Specimen Variables Combine the specimen and microscopy variable the maximum kv is used and only choose a lower voltage if knock-on damage is a problem. Pick a portion of your specimen that is single phase in the area of interest and make sure it is tilted well away from strong diffraction condition to minimize crystallographic effects and coherent bremsstrahlung. Spatial resolution: distance measured in nm. Chemical resolution: analytical sensitivity/detection limits depending on P/B. Energy resolution: identifying elements by distinguishing their spectra peaks at different energies.

23 Basic Acquisition Requirements: Counts and Counts and more Caffeine. The first and most important step in qualitative analysis is to acquire a spectrum across the complete X-ray energy range. Collect a spectrum over 0-40 kev. Re-gather spectrum over the reduced range. Final spectrum with no more than 10 ev per channel. Try to increase counts Watch dead-time below about 50-60%. The total counts in this qualitative spectrum should exceed 1,000,000 over the full energy range.

24 Peak Identification First: aware of artifacts, spurious and system peaks, CB. Second: understand the complexities of X-ray families. Third: understand things like critical ionization energy, X-ray line energy, K,L,M families of X-rays lines, relative weights of lines, fluorescence yield, etc.. The principles: 1. If a Kα line matches the peak, look for Kβ line which has about 10 of the Kα intensity and must be present above 1.74 kev. 2. If a Kα and Kβ pair fits the peaks and the Kα is > 8keV (NiKα), look for the L lines at 0.9keV for a Be-window detector. 3. If a Kα line does not fit, check for an Lα or Mα line fit. 4. If an Lα fits, there must be accompanying lines in the L family. 5. If the L lines fit, there must be a higher energy Kα/Kβ pair. 6. The M lines are usually visible for elements above La for Be-window, above Nb for UTW detector. 7. The Mα/Mβ line overlap is difficult to resolve because all the M lines are < 4keV. If an Mα/Mβ line fits, look for three very small M lines. 8. If the Mα line fits there must be higher energy L line family and possibly the very high energy K lines may be detectable.

25 Peak Visibility How to determine a small intensity is statistically significant: Integrate the peak (I A ) and background (I Ab ) counts over the same number of channels; use FWHM if it can be discerned with any confidence; if not, then the whole peak integral will do. I > 3 b A I A The peak is statistically significant at the 99% confidence limit and must be identified. I < 3 b A I A The peak is not significant and should be ignored. The side effect of long time spectrum.

26 Castaing proposed in 1951: Historical Perspective C i /C (i) = [K] I i /I (i) Wher K ia a sensitivity factor (not a constant) that takes into account the difference between the generated and measured X-ray intensities for both the standard and the unknown specimen. The contributions to K come from three effects: Z The atomic number A The absorption of X-rays within the specimen F The fluorescence of X-rays within the specimen It is assumed that the TEM specimen is thin enough so that any absorption or fluorescence can be ignored. Cliff-Lorimer ratio technique

27 Practical Steps for Quantification Use Kα line wherever possible. Keep your specimen as close to 0 tilt as possible to minimize spurious effects. If you have a wedge specimen, orient it so the thin portion of the wedge faces the detector, to minimize X-ray absorption. If the area of interest in your specimen is close to a strong two-beam dynamical diffraction condition, tilt it slightly to kinematical conditions. Accumulate enough counts in the characteristic peaks, I A, I B, etc. for acceptable errors, there should ideally be at least 10 4 counts above background in each peak. Background Subtraction and Peak Integration 1. Window methods 2. Modeling the background - Kramers s law 3. Filtering out the background

28 Errors in Quantification: The Statistics Standard deviation σ for a single measurement Standard deviation for n measurement: If we take the average value N of many composition determinations, and all the data points fall within ±3(N) 1/2 of N then the specimen is homogeneous. It will takes a real effort to get quantitative data with errors < ± 5-10% relative.

29 Calculating K AB Where A is atomic weight, Z means the atomic correction, Q is ionization cross sections, ω is fluorescence yield, and a is relative transition probability, ε is detector efficiency. The calculated value should be accurate to within ±20% relative. Kα Lα

30 Quantitative X-ray mapping counts with errors ±10% relative, so counts, counts, more counts!

31 Spatial Resolution and Minimum Detection

32 Definition and Measurement of Spatial Resolution b: Beam spreading, either calculate or by Monte Carlo simulation Measurement of Spatial Resolution R = 1.414L (from 2% to 98%) R max = (b 2 + d 2 ) 1/2 R = 1.8L (from 10% to 90%)

33 Thickness measurement - TEM methods: using thickness fringes in BF and DF for a wedged crystalline specimen, or using an inclined planar defect adjacent to the analysis region.

34 Thickness measurement Contamination-Spot Separation Method

35 Thickness measurement Convergent-Beam Diffraction Method K-M fringe is concentric in a ZAP and parallel in two-beam condition.

36 Thickness measurement Electron Energy-Loss Spectrometry Method Best method! Thickness measurement X-ray Spectrometry method The essential point here is that in the ζ-factor method, the absorption-corrected compositions can be determined simultaneously with the specimen mass-thickness by only using X-ray intensity data.

37 Minimum Detection MMF: the minimum mass fraction; MDM: the minimum detectable mass The typical values of MMF are in the range 0.1-1wt%. 2-5 Mn atoms were dectected in a 10-nm Cu-Mn film with Cs-corrected 300-kV FEG TEM by ζ-factor quantification plus MSA data manipulation.

38 Question: T 35.1, T35.3, T35.12 homework