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How to Correlate Micro-XRF and SEM-EDS for Optimal X-ray Characterization of Materials

Introduction

Micro X-ray Fluorescence (Micro-XRF) and Energy Dispersive Spectroscopy (EDS) are similar techniques in that they both detect generated X-rays after interaction with the sample. For EDS, X-rays are generated by electrons bombarding the sample, while in a Micro-XRF unit, fluorescent X-rays are excited by high-energy X-rays emitted from the X-ray tube. Silicon Drift Detectors (SDDs) are used for X-ray detection in modern EDS and Micro-XRF systems. Data collection is also similar between these two techniques because it is possible to utilize either one to do qualitative and quantitative analysis, mapping, and linescan. This application note discusses the relative advantages of SEM-EDS and benchtop Micro-XRF analysis and suggests how the two techniques can be used together for the optimal X-ray characterization of materials.

Spectra overlay for an SRM 610 glass standard with different elements doped at 300 – 500 ppm concentrations. SEMEDS spectrum in blue outline and XRF in red.
Figure 1. Spectra overlay for an SRM 610 glass standard with different elements doped at 300 – 500 ppm concentrations. SEM-EDS spectrum in blue outline and XRF in red.

 

Micro-XRF

A benchtop Micro-XRF unit utilizes the benefits of conventional XRF while implementing micro-spot X-rays with a moveable stage. For higher Z elements, micro-XRF improves the detection limits ten times or more than SEM-EDS. Figure 1 shows the EDS and Micro-XRF spectra overlay of a glass standard with different elements doped at 300 – 500 ppm concentrations. It shows the significant difference in trace element sensitivity between the two techniques. Micro-XRF uses higher-energy X-rays to generate lines that are not detectable with EDS, such as Sr K, Zr K, and Ag K, which is useful when lower energy lines have overlaps in the EDS spectrum.

Micro-XRF analysis is non-destructive with no beam damage to the sample and minimal sample preparation. Grinding and polishing of the sample are not generally required, and conductivity is not an issue. Sample loading is flexible in that thicker samples can be loaded directly on the stage, and thinner samples, particulates, and fibers can be mounted. The sample shape and height can be irregular, and the large sample chamber in a benchtop Micro-XRF unit can accommodate a wide range of sample sizes. The penetration depth of X-rays is microns to millimeters to give better detection of sub-surface composition than an electron beam. The smallest spot size in a Micro-XRF system is approximately 20 – 30 μm by employing a poly-capillary technique. These features mean Micro-XRF is more appropriate than SEM-EDS for analyzing larger-scale features. For example, Figure 2 reveals the Micro-XRF mapping of a relatively large concrete pavement sample from a roadway to determine the depth and paths of ion infiltration. Micro-XRF mapping on this type of sample is much faster and has fewer limitations on sample preparation than using a SEM-EDS. Samples can be run either in low-vacuum mode or air mode, allowing the analysis of liquids or samples that will dehydrate in a vacuum.

a) Montage image of a concrete pavement sample from a roadway routinely exposed to acetate and formate salt-based de-icing solutions during winter months. The top surface of the concrete is on the left side, while the bottom surface is on the right. b) Overlay of K, S, and Ca elemental maps. Blue = K, green = S, and red = Ca.
Figure 2. a) Montage image of a concrete pavement sample from a roadway routinely exposed to acetate and formate salt-based de-icing solutions during winter months. The top surface of the concrete is on the left side, while the bottom surface is on the right. b) Overlay of K, S, and Ca elemental maps. Blue = K, green = S, and red = Ca.

 

Air atmosphere will scatter or absorb low-energy X-rays; however, some Micro-XRF units equipped with the helium flush option allow vacuum performance at atmospheric pressure because attenuation of X-rays in helium is much lower than in the air.

EDS elemental maps of an Al tape on SiC were collected at 2 kV using a Si3N4 window SDD detector. Al L (73 eV) and Si L (92 eV) are separated in the maps.
Figure 3. EDS elemental maps of an Al tape on SiC were collected at 2 kV using a Si3N4 window SDD detector. Al L (73 eV) and Si L (92 eV) are separated in the maps.

 

Note that an SEM-based Micro-XRF system does not have many of the benefits mentioned above. Once the sample is loaded in an SEM chamber, all the requirements of SEM samples apply. The chamber size and stage of an SEM largely limit the sample dimensions, and non-conductive samples must be coated. The ability to analyze samples that cannot tolerate a vacuum atmosphere is also lost.

SEM-EDS

Compared to Micro-XRF, SEM-EDS has much better light-element/low-energy line sensitivity. SDD detectors equipped with a silicon nitride window and fast and low-noise pulse processors can routinely detect Si L. The same is true for detecting Al L, which is even lower at 73 eV. Figure 3 shows that Al L and Si L can even be separated in EDS maps. Using this type of EDS detector, quantification of trace carbon can be done accurately at high output count rates (Figure 4). Due to the efficiency of X-ray excitation at low energies and the window materials of the X-ray tube and SDD detector in a benchtop Micro-XRF system, detecting energy lines below one keV is challenging.

SEM-EDS requires more involved sample preparation. To get rid of the surface topography, cross-sectioning, grinding, or polishing of the sample is needed. Non-conductive samples need to be coated with conductive layers to eliminate charging. Samples must be small enough to fit on the sample stage and are generally mounted on a holder or stub. Due to the weaker penetration of the electron beam, the analysis depth of SEM-EDS is not as great as Micro-XRF. The spot size of the electron beam is much smaller than the X-ray beam size in Micro-XRF to provide nanoscale spatial resolution. Therefore, SEM-EDS is more appropriate for analyzing more precise locations and smaller-scale features. Typically, EDS analysis is conducted in vacuum conditions only. Low-vacuum mode is possible on some SEMs.

EDS quantitative results for trace carbon in a steel standard at 20 kV and 30% dead time. a) Carbon peak acquired at 15,000 input cps with 7.86 μs amp time (red) and 200,000 input cps with 0.96 μs amp time (blue outline). b) With increasing input count rate, measured carbon concentration is stable, ranging from 0.46 wt% to 0.52 wt% compared with the given known value of 0.50 wt%.
Figure 4. EDS quantitative results for trace carbon in a steel standard at 20 kV and 30% dead time. a) Carbon peak acquired at 15,000 input cps with 7.86 μs amp time (red) and 200,000 input cps with 0.96 μs amp time (blue outline). b) With increasing input count rate, measured carbon concentration is stable, ranging from 0.46 wt% to 0.52 wt% compared with the given known value of 0.50 wt%.

 

Correlative Micro-XRF and SEM-EDS

EDS and Micro-XRF analysis of the same spot on a solder. a) Trace Au and Bi are suspected in the EDS spectrum. b) Micro-XRF analysis with the Rh filter shows Au L and Bi L lines.
Figure 5. EDS and Micro-XRF analysis of the same spot on a solder. a) Trace Au and Bi are suspected in the EDS spectrum. b) Micro-XRF analysis with the Rh filter shows Au L and Bi L lines.

 

A correlative Micro-XRF and SEM-EDS approach provides the benefits of both techniques. As shown in Figure 5, a spot on the solder was analyzed by both techniques, and the trace Au and Bi suspected in the EDS spectrum can be confirmed by Micro-XRF using higher energy lines and the Rh beam filter. The higher magnification and more precise beam location make SEM-EDS shine while analyzing the fine features that cannot be resolved by the smallest spot size of a Micro-XRF system (Figure 6).

EDS analysis of fine features in the solder sample at 1,000X magnification.
Figure 6. EDS analysis of fine features in the solder sample at 1,000X magnification.

 

Conclusion

Micro-XRF has improved detection limits for higher Z elements, even more with beam filters, allowing the operator to return to EDS and find trace elements missed the first time. The minimal sample preparation required by this technique guarantees “as delivered” samples quickly and easily in a low-vacuum and even ambient conditions. It is also more suitable than SEM-EDS for analyzing larger-scale samples with rough surface topography. SEM-EDS is more appropriate for smaller-scale features and more precise locations. The detection limits for light elements or low-energy X-rays are vastly improved, and Al L can be regularly detected using a silicon nitride window SDD detector. In an ideal world, every lab would have both tools to accomplish the complete needs of spectral analysis.