An-Najah Staff

Why do we need a low voltage scanning electron microscope? The simple answer is that “higher voltage” scanning electron microscopes cannot answer adequately some practical issues associated with imaging of certain types of materials. The latter include non-conducting samples whether organic (such as polymers, enzymes, cells, membranes, etc.) or inorganic (such as ceramics, pigments, minerals, composite materials, etc.). These materials usually manifest one or both of these particular problems: (i) charging effect due to accumulation of electrons on the scanned area of sample; (ii) local radiation damage of the sample, induced by energetic electrons through different mechanisms such as decomposition, sputtering, sublimation, ionization, diffusion, or transformation. These effects lie beyond the scope of this note. Charging effect leads to a degraded image, poor resolution and render EDX analysis worthless. It causes the incident beam to be repelled from the investigated region and gives rise to unstable or distorted SEM images [1]. In conventional high voltage SEM (up to 35kV), charging effects are avoided or minimized for non-conducting materials by coating the sample with a thin conductive layer of gold, carbon, platinum, or gold-palladium. A relatively thick layer of gold may hide some nano- scale features of the sample surface. Some samples, where specimens cannot be cut or broken for SEM observation, cannot be coated. As this coating can alter the appearance of the sample or hinder its reuse or analysis by other techniques (e.g. atomic force microscopy or Raman). Another option that avoids sample coating in high voltage SEM is using environmental cell conditions which consist in maintaining a low vacuum environment in the sample chamber by allowing water vapor (0–20 torr). E-SEM is particularly helpful when studying biological, wet, or degassing samples. Nevertheless, E-SEM has its own limitations and difficulties. On one hand, there are the technical issues of conditioning a large chamber and the reduced contrast due to the low vacuum the electron beam has to travel through. However, the presence of water in the E-SEM causes more radiation damage to organic and biological samples due to ionization of water molecules and generation of free radicals [2]. The other problem is the radiation effects or damage caused by a high voltage electron beam [2–4].  Conventional SEMs usually operate at an accelerating voltage of 15 kV and FE-SEMs at a voltage that can go up to 35kV. The high energetic and focused electron beam can cause serious local radiation damage of certain samples. The latter include organic and biological samples and certain inorganic materials such metal sulfides [5].