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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 and the development 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 ideal sample for SEM is metal surface. 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.   ESEM consists of similar components like 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 pressure from $0.1 \sim 30 \space Torr$. 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}