Scanning Electron Microscopy (SEM) technique is utilized to analyze the surface morphology of a specimen sample at higher magnification, higher resolution and depth of focus 13 compared to an optical microscope. In this technique, a very high energetic electron beam is focused onto the surface of the sample and it is scanned over a small area. Different signals are produced and suitable ones are selected depending on the way of its operation. The obtained signal is magnified and made to form synchronous image on a cathode ray tube. An optical camera is used to take the photograph of image or it may be processed on the computer. The amplification of the displayed image is states as the ratio of the size of the image on the cathode ray tube (CRT) to the size of the electron beam scanning on the sample surface. The type of information achieved can be changed by switching the signal. In this way, the definite desired features of the specimen surface can be seen on the CRT on a magnified scale. The first Scanning electron microscope (SEM) was discovered in 1938 (Von Ardenne) with the first commercial instruments around 1965. Its late development was due to the electronics involved in “scanning” the beam of electrons across the sample
The resolution of object image using SEM can approach a few nanometers and the magnifications of SEM can be easily adjusted from about 10 times to 300,000 times. In SEM, electron beam, accelerated by a relatively low voltage of 1-20 kV, is scanned on the specimen surface. As the electron beam strikes the surface, a large number of signals are produced from the specimen surface in the form of electrons or photons. These signals emitted from the specimen are collected by detectors to form images and the images are displayed on a cathode ray tube screen. Three types of images formed in SEM: backscattered electron images, secondary electron images, and elemental X-ray maping. Secondary electrons (SE) are produced due to inelastic scattering with atomic electrons and with the energy less than 50 eV. The secondary electrons which are emitted from sample surface are commonly confined to an area near the beam impact zone that allows images to be obtained at very high magnification or resolution. Backscattered electrons (BSE) are considered to be the electrons resulted from elastic scattering with the atomic nucleus and with the energy greater than 50 eV 24. The backscattering will likely occur in a material of higher atomic 50 numbers, so the contrast caused by elemental differences can be built up. After the primary electron beam collides with an atom in the specimen and ejects a core electron from the atom, the excited atom then decays to its ground state and emit either a characteristic X-ray photon or an Auger electron 25. The energy dispersive X-ray detector (EDX) can sort the X-ray signal by energy and produce elemental images, so the spatial distribution of particular elements can be detected by SEM. Mostly SEM has resolution of 1 nm for 1 KV, even resolution of 0.6 nm is possible for 5 KV.