WDS identifies the elemental composition of materials imaged in the SEM with an order of magnitude better spectral resolution and sensitivity than is achievable with EDS. Also, with the use of standards, WDS has the ability to determine the concentrations of light elements (B, C, O, and N). Most elements are detected below 0.1% and some as low as a few ppm.

Applications: EDS applications include:

• Identification of spectrally overlapped elements, such as
• S in the presence of Pb or Mo
• W or Ta in Si, or N in Ti
• Br in Al, common in semiconductor device failure
• Detection of low concentration species (down to 100 or even 10 ppm)
• P or S in metals
• Trace heavy metal contamination
• Performance-degrading impurities in high temperature solder alloys
• Analysis of low atomic number elements
• Composition of advanced ceramics and composites
• B in BPSG films (sensitivity to 2000 ppm)

Principle of Operation

The characteristic X-ray photons excited by the electron beam are sorted using a diffracting crystal, whose angular placement relative to the sample and photodetector is a unique measure of their wavelengths. As with EDS, the resulting spectral distribution is automatically compared with those from actual standards or synthetic X-ray fluorescence spectra of material formulations.

WDS vs. EDS

X-ray microanalysis in the scanning electron microscope is accomplished using EDS and/or WDS. EDS is more commonly applied due to its simplicity and speed, while WDS offers an important and often critical refinement of EDS data by providing

• Analysis for light elements with at least an order of magnitude higher sensitivity than available (ultrathin X-ray window) EDS instruments
• Resolution of severely overlapped spectrum peaks for improved element specificity
• Lowered detection limits over the entire periodic table
• More accurate quantitative analyses.