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  • Environmental Electron Scanning Microscope

    Abstract

    This is the term paper for the course AEP 6610, reviewing the techniques and development of environmental scanning electron microscope (ESEM). The ESEM provides the ability to image samples down to nanometer scale without the necessity of high vacuum in the chamber and sample processing procedures. Therefore, samples can be imaged in its original hydrated state, preserving its dynamics, interior structures and morphology. Secondary electrons are collected to reveal the topology of the sample while backscattered electrons are collected to distinguish the element composition. In this paper, the resolution and limitations for ESEM are presented. The applications of ESEM on both organic and inorganic materials are discussed. Finally, future prospects an comparison with competing imaging technologies conclude the paper.

    Introduction

    The ESEM(Stokes 2008) was developed from conventional scanning electron microscopy (SEM)(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 handled in ESEM.

    ESEM consists of similar components as SEM. The ESEM components are shown in Fig.1(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 gas 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.

    A schematic components representation for ESEM. The chambers are kept at different pressures by differential pumping.

    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 enter the column lining chamber as divergent and broadened beams; therefore, condenser lenses are required to converge and focus the beam on a small crossover and toward the two environmental chambers.

    Environmental Chambers and PLAs

    One of the key development in ESEM is the chamber design that keeps high vacuum at the electron gun and relatively low vacuum in t