Advancing EDS Analysis in the SEM with in-situ Quantitative XRF Brian J. Cross (1) & Kenny C. Witherspoon (2) 1) CrossRoads Scientific, El Granada, CA 94018, USA 2) ixrf Systems, Inc., Houston, TX 77059, USA International Microscopy Congress, September 5 th, 2006 1 Outline Why use XRF in the SEM? Low-power x-ray x microtubes External vs. re-entrant entrant designs Optics, collimation and shielding Performance characteristics MDLs Applications Quantitative analysis Point analysis & mapping IMC-16, Sapporo, Japan 2 1
Why use XRF in the SEM? Better x-ray x sensitivity at energies >2 kev Improved peak-to to-background ratios EDS count rates are limited more counts in peaks with XRF vs. SEM (high backgrounds) Lower Minimum Detection Limits (MDLs) At higher energies (medium-high Z elements) Analysis in air possible Almost no specimen charging IMC-16, Sapporo, Japan 3 Theoretical Comparison of XRF & SEM Sensitivities SEM-EDS EDS better <2 kev; XRF 10-1000x 1000x sensitivity >5 kev IMC-16, Sapporo, Japan 4 2
Some disadvantages of XRF Spatial resolution usually much worse 50-5000 5000 microns vs. <2 microns for SEM-EDS EDS Sample imaging not possible X-ray mapping with stage usually slower than with electron beam Low energy sensitivities (<1-2 2 kev) are better for SEM-EDS EDS than XRF (w/eds) IMC-16, Sapporo, Japan 5 Advantages of Integrated XRF Only one instrument required Smaller footprint in the lab SEM chambers typically have spare port Less expensive than two separate instruments Use common detector, electronics, stage, chamber, computer and software Use both techniques on same sample In situ analysis, without sample transfer IMC-16, Sapporo, Japan 6 3
Modern Transmission-Target Target Tubes Thin film targets deposited on Be window Elements in the range Mo Ag, operating at ~30 kv, key parameters are: Mean electron energy range = ~1.5-2 2 microns Integrated path length = ~2-3 3 microns Optimum thickness anode is ~1.5-3 3 microns High-energy electrons are mostly stopped in target Self-absorption of low-energy X rays is limited Be window provides final electron barrier & heat conductor for electron energy transfer Typical Be window thickness is ~250 microns IMC-16, Sapporo, Japan 7 fx TM Integrated fx TM Microtube Assembly Assembly ready for mounting on SEM Transmission-target tube with integral high-voltage transformer. Re-entrant entrant design allows close coupling of tube to sample. IMC-16, Sapporo, Japan 8 4
fx Tube Mounted on Hitachi SEM Left picture shows fx mounted on Hitachi S3000 SEM Right diagram shows cross-section section diagram Note horizontal mounting in this example, requires vertical stage rotation fx design is re-entrant, entrant, with tube inside SEM IMC-16, Sapporo, Japan 9 X-Ray Optics Enabling technology for microbeam XRF Simple apertures for beam collimation wide bandpass Curved mirrors low bandpass Curved crystals narrow bandpass Formed and graded multilayers rel. narrow bandpass Mono and Polycapillary lenses rel. wide bandpass True microbeams require microspot x-ray x tubes Spot size on tube target must be <100 micron (ideally <10) Obtain analytical spot sizes in the range 10-100 100 micron Larger target spot sizes limited to use w/apertures for x-ray x beam sizes in the range ~0.1 5 5 mm IMC-16, Sapporo, Japan 10 5
fx Tube Optics Transmission-target tube Demountable collimator assembly Collimator assembly contains both front and back apertures Spectrum with and without an initial aperture in front of Be window 1st aperture restricts view to center of target, away from contamination regions 2nd aperture provides collimation of the beam at working distances of 10-20 mm from the front aperture Total Counts Total Counts Typical Ag Tube Output Spectrum 35 kv, No pinhole aperture 5000 Ni Kα 4500 4000 3500 Cu Kα 3000 2500 Fe Kα 2000 Ag Kα 1500 1000 Ag Kβ 500 0 0 200 400 600 800 1000 1200 1400 1600 1800 2000 Channel Number Typical Ag Tube Output Spectrum 35 kv, with 1.5 mm pinhole in front of window 2500 2000 Ag Kα 1500 1000 Ag Kβ 500 0 0 200 400 600 800 1000 1200 1400 1600 1800 2000 Channel Number IMC-16, Sapporo, Japan 11 fx Tube Optics - 2 Demountable collimator assembly Collimator assembly contains both front and back apertures 1st aperture (~1 mm) restricts view to target center, away from contamination regions 2nd aperture (0.1-5 5 mm) provides beam collimation At working distances of 10-20 mm from front aperture, beam size is ~2x aperture size High-purity materials provide contamination-free spectra Rear Aperture Sleeve Front Aperture Spring Retainer IMC-16, Sapporo, Japan 12 6
X-Beam Polycapillary Optics Multiple total external reflection of X rays Large solid angle capture small focal point Performance gain vs. aperture is typically a factor of >100x under identical conditions Polycapillary provides broadband excitation with falloff at higher energies Courtesy of X-ray Optical Systems, Inc. IMC-16, Sapporo, Japan 13 X-Beam for the SEM Tube is external to SEM chamber Distance offset by polycapillary optic Transports collimated beam of X rays to focal spot at sample. Beam at sample is typically 20-100 microns in diameter Spot size is energy dependent. Integrated automatic shutter mechanism X rays can remain on for stability. Z adjustment, with XY interface flange adjustment, allows alignment of X-Beam X with e-beam. e Slide adjustment changes spot size. Operates at moderate power (50W) with fan cooling. IMC-16, Sapporo, Japan 14 7
Picture shows X-BeamX and fx tubes mounted on Hitachi 3000 SEM with EDS detector. XRF Inside the SEM X-Beam on Hitachi 3000 IMC-16, Sapporo, Japan 15 Scatter Spectra from X-Beam X and fx Tubes fx transmission-target target has more high- & low-energy source radiation vs. X-Beam X w/side window + polycapillary optics IMC-16, Sapporo, Japan 16 8
Determination of X-Beam X Spot Size Knife-edge scan of Mo Kα x rays directly from the X-Beam 7000 6000 5000 Knife-edge scan Derivative of the knife-edge scan Counts 4000 3000 2000 1000 0 FWHM = 24.8 μ m -1.30-1.28-1.26-1.24-1.22-1.20-1.18 Knife edge position (mm) X-ray image of a Cu grid (125um pitch and 12 um bars) IMC-16, Sapporo, Japan 17 SEM, X-Beam X and fx Spectra for Glass Standard CH2 4% SiO 2 2% As 3 O 2 0.2% ZrO 2 IMC-16, Sapporo, Japan 18 9
Comparison of MDLs for E-beam, E X-Beam X and fx Selected MDLs for Na-Mg Mg- Al borosilicate glass std. E-beam spectrum @30 kv Specimen carbon coated X-Beam spectrum @30 kv Mo anode, polycap. optics fx tube spectrum @35 kv Ag transmission-target target Given E-beam X-beam fx Elt (Wt.%) MDL MDL MDL Mg 6.03 0.164 0.747 0.466 Al 10.06 0.157 0.390 0.274 Si 1.87 0.160 0.260 0.188 P 2.18 0.132 0.136 0.092 Ca 10.01 0.108 0.037 0.049 V 0.84 0.132 0.032 0.040 Fe 0.42 0.147 0.015 0.018 Ge 1.04 0.274 0.009 0.013 As 1.67 0.339 0.009 0.015 Sr 0.42 0.803 0.006 0.016 Zr 0.15 1.220 0.010 0.018 Mo 0.67 2.299 0.017 0.019 Sb 1.67 0.321 0.130 0.178 La 0.85 0.390 0.122 0.148 Ce 0.85 0.388 0.120 0.147 W 0.32 0.838 0.033 0.041 IMC-16, Sapporo, Japan 19 SEM vs. fx Spectra for Glass Standard CH4 Dots: SEM spectrum @30 kv Bars: fx spectrum @35kV 1% ZrO 2 0.1% PbO 1% CdO IMC-16, Sapporo, Japan 20 10
10.000 1.000 Minimum Detection Limits from Glass Standards E-beam X-Beam fx MDL (3-sig Wt.%) 0.100 0.010 0.001 0 10 20 30 40 50 60 70 80 90 100 Atomic No. IMC-16, Sapporo, Japan 21 SEM & X-Beam X Spectra for Glass Standard SRM 610 NIST SRM 610 Component Wt% SiO2 72.00 CaO 14.00 NaO3 14.00 Al2O3 2.00 ppm Co 390 Cu 444 Fe 458 Mn 485 Ni 458 Ag 254 Sr 515 Th 457 Ti 437 Pb 426 K 461 Ru 425 Th 61 U 461 Zn 433 Overlay e-beam spectrum on XRF (X-Beam) Peak fitting to X-beam SRM 610 spectrum IMC-16, Sapporo, Japan 22 11
SEM and fx Spectra from Al Alloy 2014 Dots: SEM spectrum @20 kv Bars: fx spectrum @35kV 0.02% V 0.03% Pb IMC-16, Sapporo, Japan 23 Strategies for Quantitative Analysis Analyze same sample by both SEM-EDS EDS and XRF Assume sample is uniform within XRF beam area How to combine results from 2 techniques? Analyze both and select concentrations from each? Many times can t t analyze all elements (not detected)! Fix elements from one analysis while doing other? Does not work - ratio of two element totals is unknown! Iteratively fix some concentrations while analyze others and do vice versa for the other method Difficult and painful, and may not work very well! Solution is do both analyses simultaneously IMC-16, Sapporo, Japan 24 12
Combined SEM-XRF Analysis Standardless ZAF for SEM-EDS EDS k = c Z A Fc where, k = I/I p = intensity ratio to pure elt. and, I p = ƒ(p, (p,ω,r,q, )) can be calculated for each element on a relative basis. c is the weight fraction for each element Because absolute count rates are not calculated, assume that c i = 1. Standardless FP for XRF Similar set of equations (but different physics) IMC-16, Sapporo, Japan 25 Combined SEM-XRF Analysis - Solution Use common equation c i = 1 Can t t normalize pure intensities Relationship between ZAF and FP not known. Solution is to normalize the k-ratiosk Done iteratively in the same loop as the concentration normalization. Partial sum done for each method (ZAF, FP). Total of two sums must be 1. Update concentration estimates for both ZAF and FP at the end of each iteration. IMC-16, Sapporo, Japan 26 13
Combined SEM-XRF Analysis of AA 2014 Alloy Set both spectra to analyze same elements Choose XRF (FP) or SEM-EDS (ZAF) method for each element, or set to Automatic. Choose appropriate x-ray lines for each method IMC-16, Sapporo, Japan 27 Compare ZAF, FP & Combined Analysis All AA 2014 analyses were done by a standardless method Quantitative method chosen automatically Based on Peak-to to- Background Error ratios Main source of quant error is the spectrum model for the fx tube Can reduce by using FP calibration for XRF Mg 0.45 Ka 1.02 Ka 1.45 Ka ZAF 0.48 Al 92.54 Ka 93.04 Ka 94.65 Ka ZAF 91.69 Si 0.89 Ka 0.46 Ka 1.21 Ka FP 0.85 Ti 0.051 Ka 0.07 Ka 0.031 Ka FP 0.085 V 0.024 Ka 0.01 Ka 0.051 Ka ZAF 0.034 Cr 0.043 Ka 0.07 Ka 0.026 Ka FP 0.076 Mn 0.77 Ka 0.62 Ka 0.48 Ka ZAF 0.63 Fe 0.48 Ka 0.35 Ka 0.27 Ka ZAF 0.35 Ni 0.04 Ka 0.09 Ka 0.03 Ka FP 0.10 Cu 4.51 Ka 3.74 Ka 1.71 Ka FP 5.34 Zn 0.053 Ka 0.05 Ka 0.022 Ka FP 0.071 Ga 0.028 Ka 0.07 Ka 0.019 Ka FP 0.076 Zr 0.011 La 0.04 Ka 0.005 Ka FP 0.034 Sn 0.036 La 0.10 Ka 0.012 Ka FP 0.089 Pb 0.034 Ma 0.04 La 0.024 La FP 0.060 Bi 0.027 Ma 0.22 La 0.01 La FP 0.033 Used automatically selected methods, except V, Mn and Fe changed to SEM spectrum method to avoid diffraction peaks in XRF spectrum IMC-16, Sapporo, Japan 28 14
XRF Multilayer Thin-Film Applications Analyze up to 6 layers for thickness & composition Calibrate with single or multi-element thin-film standards Example shows 6-layer stack with 1 component per layer IMC-16, Sapporo, Japan 29 X-Beam Map of Standards Block IMC-16, Sapporo, Japan 30 15
X-Beam XRF Mapping of Thin Rock Section Thin-rock section Full map spectra from X-beam X (bars) and e-beam e (dots) XY scans. IMC-16, Sapporo, Japan 31 X-Beam XRF Mapping of Mining Sample Composite of Si, Cl, K & Fe IMC-16, Sapporo, Japan 32 16
XRF Elemental Maps of Printed Circuit Board 350x120 Maps Composite XRF Spectrum IMC-16, Sapporo, Japan 33 XRF Composite Maps of Printed Circuit Board Cu Al Pb Pd Ca Zn Ag Sb Mo Incoherent Photo PCB on C Stub Mo Coherent IMC-16, Sapporo, Japan 34 17
Chip Area X-Beam X Map and Integral Spectra Bars: XRF Composite Spectrum Overlay: SEM Spectrum @20 kv IMC-16, Sapporo, Japan 35 Chip Area SEM-EDS EDS X-Map X and Integral Spectrum IMC-16, Sapporo, Japan 36 18
Composite X-ray X Maps of Chip Area Composite XRF Map Ca Zn Zr Si Composite SEM-EDS Map Ca Zn O Bi Pd Ag-L Pb Pd Ag Pb Cu Fe Br Si Si Pb Al Pb Si C IMC-16, Sapporo, Japan 37 Summary 1 fx Re-entrant entrant Microtube Integrated re-entrant entrant transmission-target target micro x-ray x tube inside several SEMs. Thin target (1-2 2 microns) important for wide energy range of excitation. Long & small diameter of tube important for getting close coupling with sample. Low power (<5W) allows passive heat conduction no active cooling required. Sample analysis sizes from ~0.5 5 mm. IMC-16, Sapporo, Japan 38 19
Summary 2 External X-Beam X w/optics X-Beam has side-window microbeam x-ray x tube and capillary optics Moderate-power (50W) use requires fan cooling. Polycapillary optic has wide energy range, but less efficient at higher & lower energies. Interlocked integrated shutter mechanism allows tube to remain on for stability. Use stage Z-axis Z and tube XY interface flange to align X-Beam to E-beam. E Typical performance of better than 50 x 90 micron (FWHM) beam at the sample, with 25x45 possible. XY stage scanning allows the acquisition of x-ray x maps and line scans. IMC-16, Sapporo, Japan 39 Conclusions Demonstrated integration of several x-ray x tubes with various SEMs. Advantage of XRF for trace analysis is complementary to SEM-EDS EDS analysis. XRF software added to typical EDS analysis package to allow separate or combined qualitative and quant analysis. Stage automation allows x-ray x chemical mapping using electron or x-ray x beams. IMC-16, Sapporo, Japan 40 20