Structural analysis Essay Example | Topics and Well Written Essays - 1750 words - 1

Structural analysis - Essay Example This usually depends on wavelength of the radiation. The beams that enter the lens form an image by overlapping on each diffraction pattern. As per the below diagram, Rayleigh found out that distinction between two points would be possible if maximum of the first diffraction pattern matched with the minimum or the beginning of the second diffraction pattern. Therefore the distance d1 indicated is inversely proportional to diameter of the lens opening. In brief the gap r1 is dependent on wavelength ?, refractive index of the medium  µ and the angle formed by the beam ?. r1=d1/2=0.61 ?/ ( µ sin?). Therefore a high resolution or a lower value of r1 can be obtained by a shorter wavelength, a higher refractive index of the traversing medium and a smaller distance to the specimen causing a larger value of ( µsin?). When ordinary light-optical microscopes are used in air with refractive index=1, wavelengths of light being 400-700nm the maximum resolution that can be achieved is up to 200nm. Hence a magnification ratio above 1000 would be difficult to achieve. It is in these areas where a high amount of magnification is required that Electron microscopes prove the most beneficial. The Scanning Electron Microscope De Broglie’s relation describes the basic working principle of an electron microscope. The equation derived is ?= [1.5/ (V+ 10-6 V2)] 1/2 nm. Hence the wavelength can be adjusted by controlling the voltage of the electron beam. Electrons tend to get highly scattered in air and therefore a vacuum atmosphere needs to be maintained. Specimens also need to be made electrically conductive to avoid getting overcharged with electrons during testing. The diagram shows the main components of a Scanning Electron Microscope (SEM). These function in close cohesion in the running of seven prominent systems which are notably 1. Vacuum system- To prevent the scattering of electron beams a vacuum atmosphere is maintained to prevent dispersion. To achieve this two classes of pump are used. A low vacuum pump brings down the air pressure from atmosphere to 10-3 Torr and a high vacuum pump bring it further down from 10-3 Torr to 10-6 Torr. 2. Electron beam Generation system- This system produces the ‘illuminating’ or the primary electron beam for impingement on the sample. An electron gun generates the beam in a SEM. It is composed of a filament made of tungsten wire, Cerium Hexaboride or Lanthanum Hexaboride. A grid cap that directs the flow of electrons and a positively charged anode that accelerates the electrons onto the surface of the specimen. 3. Electron beam manipulation system- a system of lenses and coils control the shape, size and position of the electron beam to be directed on the sample surface. Electrostatic and magnetic fields control electron motion Electrostatic fields are found in the electron gun while magnetic field is present in the rest of the SEM. By passing electric current through a copper wire a magnetic field is made to form an electron microscope lens. A series of these lenses also known as condenser lens removes any kind of spherical aberration or astigmatism in the image. When the beam traverses the final condenser lens two sets of magnetic scanning coils move the beam thereby scanning in the X and Y direction in a raster pattern i.e the specimen is scanned from the upper left hand corner to the right corner after which it drops