Techniques
Energy Dispersive X-Ray Spectroscopy (EDS)

Scanning electron microscopy (SEM) gives high resolution images of a wide range of samples. Combined with an appropriate sample preparation SEM gives a detailed view into the morphology of surfaces, the structure of materials as well as biological specimens. The combination with EDS makes SEM able to determine also the chemical composition of the whole sample or just of very small sample volumes such as defects or particles.

Principle of function

The abbreviation EDS or sometimes EDX says spectroscopy of X-rays using an energy dispersive spectrometer usually fitted to a scanning electron microscope.

Primary electrons by the SEM gun generate X-rays within the interaction volume inside the sample. Since this volume is in the range of cubic micrometre the method is called microanalysis. Two different kinds of X-rays are produced and detected: Continuous Bremsstrahlung and characteristic X-rays. Especially the latter give information about the chemical elements present in the interaction volume and their quantitative amount.

The detectors are built as SDD, Si(Li) or Ge type whereas the SDD are nowadays the most common ones.

Interpretation of X-ray spectra

The spectra show the intensity of detected X-rays within a small energy range (multi-channel analyser). They consist of a continuous Bremstrahlung and the characteristic X-rays of the chemical elements present inside the interaction volume.

1.) Identification of elements

Nearly all chemical elements give more than one characteristic X-ray signal in an EDS spectrum. Their energy position, their shape, and the existence of all of them indicate the presence of a specific element. Inhomogeneities, the structure of the sample, and the exciting parameters influence these features partly and have to be considered but can even be used to determine these properties. Attention has to be payed to overlapping signals of different elements or detector artefacts. The correct identification is essential for all following interpretations.

2.) Quantitative analysis

The software takes several parameters such as accelerating voltage, tilt, absorption of X-rays, and fluorescence excitation into account to calculate the quantitative concentration of the identified elements in an iterative optimization process. The detection limit for most of the elements is around 0.1 wt%. Especially the elements with atomic numbers lower than 10 show worse detection limits. Hydrogen and helium are not detectable at all.

Spatial resolution

The lateral resolution as well as the depth of information of EDS spectra is worse than these of SEM images. The reason is that X-rays have a good transmission potential and are thus able to leave from all the interaction volume of primary electrons with the sample. Depending on the accelerating voltage of the primary electrons, the mean atomic number, and the density of the sample the size of the interaction volume typically ranges from 0.1 to 2 micrometres.

Although lower accelerating voltages lead to smaller interaction volumes it cannot be reduced too far because the electrons still need to be able to ionize the sample atoms which need be detected.