Scanning Electron Microscopy: Theory, History and Development of the Field Emission Scanning Electron Microscope

Since its initial development (Everhart and Thornley, 1958) the scanning electron microscope (SEM) has earned a reputation for being the most widely used, high performance, imaging technology that is available for applications ranging from imaging, fabrication, patterning, and chemical analysis, and for materials of all types and applications. It is estimated that 150 000 or so such instruments are now currently in use worldwide, varying in performance and complexity from simple desk-top systems to state-of-the-art field emission gun systems that can now cost in excess of $5 million. The basic principle of the scanning electron microscope is simple. An incident electron beam is brought to a focus that typically varies in size from a fraction of a centimeter in diameter down to a spot that can be smaller by a factor of many thousands of times, and with an energy varying from 100 eV or less to a maximum of 30 keV or more. This beam spot is typically then scanned (Figure 1.1) in a linear “raster” pattern across the region of interest, although other patterns – such as a radial beam – are sometimes employed for special purposes. Typically the final deposited pattern will contain of the order of 1000 × 1000 or more individual imaging points. The incident beam electrons can interact with the sample atoms through either elastic or inelastic scattering. Elastic scattering is where the incident electrons are deflected with no loss of energy. Inelastic scattering involves a loss of energy, often by ionizing the sample atoms. The incident electrons will scatter (both elastically and inelastically) many times in