SEM-EDS X-Ray Spectroscopy (Elemental Analysis)Scanning Electron Microscopy with Energy Dispersive Spectroscopy (SEM-EDS) allows for approximate quantification of elemental composition for elements larger than boron.
How it works: An electron beam scans across the sample surface. This is similar to how old fashioned CRT televisions employ an electromagnetically guided electron beam to scan along the TV screen to illuminate pixels. The beam knocks electrons in the sample out of their orbits. When electrons in the sample move to fill those vacated electron orbitals they emit X-rays with energies that are characteristic to each element. (Table of X-Ray Emission Lines - Lawrence Berkeley National Laboratory) Strengths:
Limitations:
Specifications:
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Background information on SEM Detectors used at MAP Labs:
- Backscattered Electron Detector (BSED): The BSED imaging detector is shown in blue in the above image. This detector surrounds the electron beam aperture. The beam rasters along the sample surface, similar to the electron beam in an old fashioned cathode-ray-tube (CRT) television. Dense materials are more likely to bounce the beam electrons back elastically, hitting the BSED detector, this is why dense materials appear brighter in the BSED imaging (for example gold, lead, copper will appear brighter than aluminum, carbon, or silicon). The BSED detector is divided into 4 quadrants with independent gain settings allowing visualization of surface topology, turning down the gain on one quadrant will cause "shadows" in regions of the sample surface that are angled in that direction (QUAD-BSED imaging).
- The Secondary Electron Detector (SED, also known as an Everhart–Thornley detector) observes electrons that were ejected from their orbits in the specimen by the electron beam. This detector sits off to the side, within a positively charged cage that helps direct electrons towards a scintillator (a device that emits light when struck by electrons - again, similar to an old CRT television screen). A photomultiplier converts the scintillator light intensity to a voltage that can be recorded for each pixel in the image. The SED does not provide information on the composition of the sample, but does provide high detail imaging of surface structures. Due to increased ease of secondary electron escape from the specimen surface, edges and highly textured regions of the specimen surface appear brighter in SED imaging.
- Energy Dispersive Spectrometer (EDS or EDAX): The secondary electrons ejected from the specimen leave vacant orbitals within the atoms that make up the specimen surface. When electrons in the specimen fall down to fill these vacated orbits they emit X-rays with energies that are characteristic to each element. The EDS contains a silicon drift detector (SDD) which counts and measures the energies of thousands of x-rays emitted from the specimen surface each second. The collected spectrum of x-rays is then automatically compared with reference libraries to correct for varying detector sensitivity to each element and quantify the composition of the specimen.
X-Ray Fluorescence Spectroscopy (XRF Elemental Analysis)
When necessary we're also able to work with local Universities to offer X-Ray fluorescence spectroscopy (XRF). In XRF an X-Ray source is used to excite electrons in a specimen instead of the electron beam of an SEM. The sample fluoresces with its own X-Rays when electrons fall to lower energy orbits to fill the vacant orbitals. The X-Rays from the sample are detected and used to determine its elemental composition.
XRF analysis is non destructive and unlike SEM analysis the XRF can operate without a vacuum. The instrument has a spot size of ~3mm and can accommodate samples up to 2" in height (up to 10 inches in width/length).
The X-Rays can penetrate several millimeters into low density materials. Having a sufficiently thick material is key to achieving accurate quantitation.
XRF can also be used to observe X-Ray diffraction peaks, bulk grain orientation in metals (crystallographic texture), and plating thickness.
XRF analysis is non destructive and unlike SEM analysis the XRF can operate without a vacuum. The instrument has a spot size of ~3mm and can accommodate samples up to 2" in height (up to 10 inches in width/length).
The X-Rays can penetrate several millimeters into low density materials. Having a sufficiently thick material is key to achieving accurate quantitation.
XRF can also be used to observe X-Ray diffraction peaks, bulk grain orientation in metals (crystallographic texture), and plating thickness.
Relevant Methods:
- ASTM E1621 - Standard Guide for Elemental Analysis by Wavelength Dispersive X-Ray Fluorescence Spectrometry
- ASTM B568 - Standard Test Method for Measurement of Coating Thickness by X-Ray Spectrometry