Structural analysing Essay Example | Topics and Well Written Essays - 1750 words

Structural analysing - Essay Example Today, an SEM has become an indispensible instrument for any R&D or production set up dealing with advanced materials science. In the subsequent sections interaction of electrons with matter, basic principles of electron microscopy, architecture and working of scanning electron microscope, different attachments to an SEM and applications of SEM in materials science will be discussed in brief with special emphasis on specifications of a modern SEM. Interaction of Electrons with Matter Electrons as a probe are extremely versatile as they generate a wide range of signals which can be detected and processed to get useful and meaningful insight about surface topography, microstructure, microchemistry and micro-texture of the material being probed. Different kinds of signal generated as a result of interaction of electron probe are shown in Fig. 1 [1]. Fig. 1: Schematic Drawing Showing Electron – Matter Interaction When thickness of the specimen is less than ~ 100 nm only then the i ncident electron beam is able be pass through it and generate different kind of transmitted signals. However, the transmitted signals are relevant for Transmission Electron Microscope (TEM) and not for Scanning Electron Microscope (SEM); therefore, we will not discuss about transmitted signals here. Among the reflected signals secondary electrons (SE) and Back Scattered Electrons (BSE) are relevant for SEM for imaging and characteristic X-rays are useful for chemical analysis in SEM. Besides, BSE is also useful in micro-texture analysis using Electron Back Scattered Diffraction (EBSD) attachment. Therefore, we will limit our focus to these signals only. Secondary Electrons (SE) These are low energy electrons (less than 50 eV) generated after the incident beam hits the substrate. Back Scattered Electrons (BSE) These are high energy electrons (more than 50 eV up to beam energy) generated after the incident beam hits the substrate. Characteristic X-Rays When electron beam strikes the s pecimen, it knocks out the inner shell electrons and the vacancy thus created is immediately filled by an electron from higher shells. This electronic transition leads to generation of X-rays which are characteristics of the element. Thus these X-rays can be used for micro-chemical analysis using Energy Dispersive Spectroscopy (EDS) and / or Wavelength Dispersive Spectroscopy (WDS). Basic Architecture of SEM Basic architecture of an SEM is presented in Fig. 2 [2]. It is a column always under vacuum consisting of different subcomponents. It consists of an electron gun or electron source. This can be different types, which will be discussed afterwards. This is followed by condenser lenses to focus the beam. There are apertures in the path to allow only and an aperture to allow only the useful (central) portion of the beam to the subsequent stages. The focused beam is double scanned and made to pass through an aperture to fall onto the specimen. The beam is scanned over the specimen in a raster and the generated signals – secondary or back scattered electrons are collected, amplified and again scanned in a raster onto a CRT screen in synchronized manner. Thus image is formed pixel by pixel. The magnification is ratio of