SEM-EDXS (Scanning Electron Microscopy and Energy Dispersive X-ray Spectroscopy)

Introduction of SEM-EDXS

SEM-EDS combines two powerful analytical techniques: Scanning Electron Microscopy (SEM), which provides images, and Energy Dispersive X-ray Spectroscopy (EDXS or EDS), which enables the analysis of elements. SEM involves the use of a focused electron beam to scan the surface of a sample, generating high-resolution images through the use of secondary or backscattered emissions. At the same time, the interaction of the electron beam with atoms of the sample creates the characteristic X-rays. The EDS detectors can detect and analyze such X-rays to identify which elements are present and, in some cases, their relative abundance. This dual functionality enables users to associate surface characteristics and material composition, which is useful in research, development, and quality testing.

Methodology of SEM-EDXS

It is placed in the SEM chamber under a vacuum after sticking the sample on a conductive stub. The surface is scanned by a focused electron beam that results in several signals, primarily secondary electrons, used to provide topography (and, to a smaller extent, backscattered electrons used to provide compositional contrast). In the case of EDS analysis, the detector collects the X-rays released by the atoms within the sample as they decay to a lower energy state, having been stimulated by the beam. The energy of these X-rays is characteristic of each element, permitting the software to produce an elemental spectrum and even compositional maps. SEM-EDXS is particularly applicable in the detection of elements heavier than boron, and in most cases, it is used for point analysis, line scans, or area mapping.

Instrumentation of SEM-EDXS

The typical SEM-EDXS instrumentation consists of an electron column comprising an electron beam column and electromagnetic lenses, as well as secondary electron and backscattered electron detectors and an energy-dispersive X-ray detector placed close to the sample. The system is run in a high vacuum, but environmental SEMs do run non-conductive or stay-hydrated samples. Real-time elemental quantification and imaging of element distributions are possible using high-resolution monitors and analytical software. The more sophisticated systems could incorporate more than one detector or a couple of EDSS with alternate methods such as WDS or EBSD.

Strengths of SEM-EDXS

The combination of visual and chemical information provided by SEM-EDXS is very strong, with only one instrument to use. It can study tiny volumes (microns to sub-microns), detect most elements in the periodic table, and map elements, giving an elemental map. Many samples are not destroyed by the technique, which offers immediate results and requires minimal sample preparation in the case of conductive materials. It finds extensive applications in quality control, forensic analysis, corrosion characterization, and microstructural characterization.

Limitations of SEM-EDXS

SEM-EDXS is less sensitive to light elements (less than atomic number 5), and quantification must take into consideration the effect of roughness, sample geometry, and the detector angle. Surface analysis may be interfered with by coating non-conductive samples, e.g., with gold or carbon. It is also surface-biased, and the signals provided by X-ray normally start with the upper 1-2 microns. To detect even at a trace level or to provide a more in-depth composition, complementary methods may be necessary, such as ICP-MS or SIMS.