Scanning Electron Microscope with Field Emission Gun (FE-SEM)

This instrument is used for studying sample surfaces at high magnifications (> 200000x) due to the presence of a hot Schottky field emission (FE) gun. Potential (called the extraction voltage) is applied to draw out the electrons. Due to this combination of high temperature and extraction voltage, the brilliance of the source beam, as also its stability, is high. This system is attached with an EDS unit and an electron back scatter diffraction (EBSD) unit.

A. Energy Dispersive Spectroscope

One of the interactions that occur when an incident electron beam falls on a sample is the production of x-rays from the sample. The x-ray frequency is characteristic of the element in the sample that produces it. Thus, several elements can be analysed simultaneously. The detector analyses the energies of the generated x-rays, leading to speedy detection of the elements.

B. Electron Back Scatter Diffraction unit

In the EBSD unit, the sample is kept inclined at an angle of 70 to the incident beam and the emergent backscattered electrons undergo diffraction. The diffracted beams are collected on a phosphor screen where they form bands that are indexed based on known crystallographic inputs. The electron beam moves on the sample surface in a regular manner based on a step size determined by the user. Electron diffraction occurs at each point and the entire area of interest on the sample surface is mapped. EBSD is a technique where the input is crystallographic information and the output is microstructural information. The orientation of individual grains can be measured using EBSD, as also the grain shape, size and boundary statistics. EBSD is also a powerful tool for the determination of crystallographic texture. A special feature of the unit at ARCI is that the EDS and EBSD units work in synchronisation and hence elemental and microstructual information can be collected from each point from the area of interest. A recent area of application is phase analysis using the EDS/EBSD combination. Since spatial information from the different grains is preserved in EBSD, the location of small amounts of secondary phases (at triple points or within grains) can be determined accurately.