EDS Mapping. Ian Harvey Fall Practical Electron Microscopy

Transcription

1 EDS Mapping Ian Harvey Fall From: Energy Dispersive X-ray Microanalysis, An Introduction Kevex Corp Characteristic X-ray generation p.2 1

2 X-ray generation source (sim) p.3 X-ray Resolution effects p.4 2

3 X-ray detection A Guide to Energy Dispersive X-Ray Analysis Li-drifted Si (or h.p. Ge) crystal: Photoelectric effect # of electron / hole pairs α X-ray energy Charge pulse converted to V at FET CHIP LED switched on to reset saturated FET (deadtime) p.5 A Guide to Energy Dispersive X-Ray Analysis Optical Feedback for FET reset Counting time = live time Reset time = dead time (all pulses rejected) For increasing beam current, count rate peaks and falls off as dead-time increases p.6 3

4 Resolution vs. count rate tradeoff EDS system sorts as well as counts X-ray events, hence total count rate produced is much lower than that for WDS system To increase the count rate capability, shorten the time constant of the main amplifier But at the sacrifice of resolution For many SEM and TEM application, count rate are low and a 10 µs time constant is used for highest resolution p.7 EDS: gaussian peak shapes E=hν-(KeV energy of ejected core electron) - (ev work function of thousands of ejected valence electrons) FWHM α (C 2 E + N 2 ) 1/2 C= cons f(work funct) E= photon energy N= amplification noise p.8 Goldstein et al. Scanning Electron Microscopy and X-Ray Microanalysis, 3rd ed. Klewer Academic Press 4

5 X-ray spectrum depends on Vacc A Guide to Energy Dispersive X-Ray Analysis p.9 Another reason to keep the vacuum clean p.10 Goldstein et al. Scanning Electron Microscopy and X-Ray Microanalysis, 3rd ed. Klewer Academic Press 5

6 Care for quantitative analysis Assumptions: Flat polished sample Homogeneous within sampling region Calculations not simply ratio of intensities: Peaks may overlap (reason for WDX) Elements backscatter differently due to different density (producing different x-ray yields) Energy losses of incoming beam produce different x-ray generation distributions for different elements Re-absorption of X-rays plays a role (Z-dependent) Some re-absorbed X-rays produce other X-rays (fluorescence) Φρz and ZAF corrections p.11 Goldstein et al. Scanning Electron Microscopy and X-Ray Microanalysis, 3rd ed. Klewer Academic Press WDS: sharp peak shapes E=hν=hc/λ λ = 2d sin θ Counts are digital according to angular position of crystal spectrophotometer Only collect x-rays of the one energy at a given position ==> SLOW Superior for quant- microprobe Figure 10.5er.3 p.12 Goldstein et al. Scanning Electron Microscopy and X-Ray Microanalysis, 3rd ed. Klewer Academic Press 6

7 EDS or WDS: count the events within a given peak window and map beam position when each event occurs A Guide to Energy Dispersive X-Ray Analysis p.13 Mapping Because most of your samples are NOT flat smooth / polished homogeneous p.14 7

8 Trading peak resolution for data collection rates A Guide to Energy Dispersive X-Ray Analysis p.15 p.16 8

9 Ti Sub ion pump filament p.17 Hoar frost : Take a nice picture first p.18 9

10 Use a background window p.19 after letting it run for ~10 minutes p.20 10

11 p.21 p.22 11

12 setup p.23 Collect Maps p.24 12

13 p.25 End collection, release ext p.26 13

14 p.27 p.28 14

15 p.29 p.30 15

16 p.31 p.32 16

17 p.33 Run at hi V acc to identify all peaks look for relevant L, M do you need better resolution? How to analyze thin organic film? Drop V acc Tilt the sample p.34 17

18 SEM (BSE) view of gross residual BCB (DOA) BSE (Z-sensitive) image shows dramatic contrast between organic (dark), primary Sn phase (grey), and primary Pb phase (white) p.35 Miscellaneous Failure Mechanisms: Assembly Solderability 5x5 750/500 TC test First failure at ~150 cycles (Ni/Au only) Catastrophic failure rate on affected boards Unknown source of process sensitivity Solder lift-off from pad Adjacent good pad p.36 18

19 EDS Maps of a Typical Pad Interface (Where-n-tar-blazes did the Cu come from??) BSE image Ni map Cu map p.37 Typical Ni/Au device (post-polish / 50% nitric; 3 min. decorative etch) Etched bulk eutectic solder Cu/Sn intermetallic Columnar Ni grains Etched Cu pad p.38 19

20 EDS Maps of a Failed Pad Interface (note lack of Cu/Sn intermetallic) p.39 XRF: X-Ray Fluorescence p.40 20

21 XRF: High Sensitivity vs. SEM-based Bulk analysis technique Lower spatial resolution (mm-mapping) non-vacuum or low vacuum quantitation; liquids possible Reduced background p.41 Microspot XRF (June) p.42 21

22 Spectral Mapping - Bone Fossilization Fe K P Si Na p.43 Map Image Overlays: Bone Fossilization Fe Red K Blue Si Yellow P Gray Na - Green p.44 22