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ESEM consists of similar components as SEM. An electron chamber that sits on the top of the sample chamber contains a heated filament, accelerating anode, condenser lenses and objective lenses. Between the electron and sample chamber, there are two stages of environmental chambers. The sample chamber is usually contains different gases molecules and is kept at certain higher pressure from $0.1 \sim 30 \space Torr$ compared with the high vacuum required in SEM. Each chambers are kept at different pressures by vacuum pumps and separated by multiple pressure limiting apertures (PLA's). The functions of PLA's will be discussed later in this section.   \subsection{Electron Chamber}  The electron chamber is separated into to parts; one is the electron gun and the other is the column chamber of many magnetic lenses. Electron beams are generated in the electron chamber by a filament heated to high temperature (thermionic emission) or kept at strong electric field (field emission). The electrons emerging from the filament are then directed through a small spot before being accelerated in the electron gun toward the column chamber. The accelerating energy is typically in the range of hundreds to tens of thousands of electron volts. The electron gun must work in high vacuum ($10^(-7) Torr$)  to minimize scattering. The electron beams come in the column chamber as divergent and broadened beams; therefore, condenser lenses are required to converge and focus the beam into a small crossover and toward the sample. The   \subsection{}