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\section{Introduction}   The ESEM\cite{Stokes_2008} was developed from conventional scanning electron microscopy (SEM)\cite{Reimer_1978}, preserving the principles of imaging and the resolution. The advancement and modification of SEM simplified the preparation of samples and allowed more complex imaging environment. The conventional SEM is a surface analytical technique; it has to be operated in high vacuum ($\approx 10^{-5} \sim 10^{-7} \space Torr$) chambers to prevent surface contamination. Therefore the samples must be clean, dehydrated, fixed and also conductive to avoid the charging effects. Toward these purposes, the samples are mostly pre-processed such as cooled, dehydrated and distorted, and usually coated with conducting materials, compromising the topographical and morphological resolution. The lifetime of samples for SEM is also short. The ideal samples for SEM are metal surfaces. However, in ESEM, the samples can be both organic or inorganic and conductors or insulators which are imaged under a range of pressures, temperatures and gases. The problematic issues in SEM such as contamination, short sample lifetime and charging effects can be resolved in ESEM.  ESEM consists of similar components as SEM. The ESEM components are shown in Fig.1\cite{Donald_2003}.  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 divided into two parts; one is the electron gun and the other is the column lining chamber consisting 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 lining chamber. The accelerating voltage is typically in the range of hundreds to tens of thousands of volts. The electron gun must be operated in very high vacuum ($10^{-7} \space Torr$) conditions to minimize scattering. The electron beams come in the column lining chamber as divergent and broadened beams; therefore, condenser lenses are required to converge and focus the beam into a small crossover and toward the two environmental chambers.