Low Voltage Scanning Electron Microscopy
Write 7 pages with APA style on Low Voltage Scanning Electron Microscopy. Working Principles of Scanning Electron Microscopy (SEM) Scanning electron microscopes (SEMs) are microscopes, which produce sample images by scanning the samples using a beam of focused electrons. The focused beam of electrons interacts with electrons within the sample to produce differing signal strengths that can be detected by a secondary electron detector. The focused electrons hit the sample surface, where they either get reflected as back-scattered electrons (BSE) due to inelastic scattering or cause an emission of electrons from the sample surface as secondary electrons (SE) due to elastic scattering (Lyman, 1990). The SE are emitted from the k-orbitals of the sample once the electrons in the beam hit them in a strong enough way that displaces them from the orbital. Both BSE and SE show different properties of the sample material, which commonly includes composition and topography (Boyes, 2000). The focused beam of electrons is scanned by raster scanning and position of the beam is combined with the signal from the BSE and SE to develop an image. This type of microscopic observation can attain resolutions that are better than a nanometer. The focused beam used in SEM is produced by thermionic means from an electron gun consisting of a cathode made of tungsten. Tungsten is preferable because of its relatively lower vapor pressure and high melting point. Additionally, it is cost-effective because it is relatively cheap. The produced beam passes through two lenses that focus it onto the specimen. The energy of the beam ranges from 0.2 keV to 40 keV depending on the design of the microscope. The focus offers a spot diameter of approximately 0.4 nanometers to 5 nanometers (Joy & Carolyn, 1996). On reaching the final lenses in the column the beam is deflected by a pair of deflecting plates, which deflect it in two planes (X, Y planes) so as to create a raster scan area that is rectangular over the samples surface (Asmar et al., 2004). The interaction of beam electrons and the sample make the electrons lose energy through repeated random scattering, which occurs near the surface or much deeper depending on the projection energy in keV. This interaction volume from which energy is released in form of electrons to create the image is known as the interaction volume. The interaction volume is determined by the landing energy of the electrons from the projected beam as well as density and atomic number of the material under observation as the sample. Technically, the differences between HVEM and LVEM are a result of differences in landing energy determined by the electron accelerating voltage. The acquired signals are detected and amplified to develop images. The differences of detected signals are represented by variations in brightness on the visual display. The resultant image is thus a representative of electron energy emitted from the samples surface through inelastic and elastic scattering. The Unique Nature of Low voltage scanning electron microscopy (LVSEM) According to Joy and Carolyn (1996), even prior to practical use of scanning electron microscopes, it had been recognized that scanning at low electron accelerating voltage (<.5 keV) would improve the process.