1. Introduction

Electron microscopes are scientific devices that employ a beam of highly energetic electrons to study objects on a very fine scale. This inspection can yield information about the topography (surface features of an object), morphology (shape and size of the particles making up the object), composition (the elements and compounds that the object is composed of and the relative amounts of them) and crystallographic information (how the atoms are arranged in the object) of the sample. The wide spread use of electron microscopes are based on the fact that they allow the observation and characterization of materials on a nanometer (nm) to micrometer (μm) scale. Electron microscopes were made due to the limitations of bright-field light microscopes which are limited by the physics of light to 500x or 1000x magnification and a resolution of 0.2 micrometers.

The Transmission Electron Microscope (TEM) was the first category of electron microscope that was developed by Max Knoll and Ernst Ruska in Germany in 1931. The first Scanning Electron Microscope (SEM) debuted in 1942 with the first commercial instruments around 1965. Electron microscopes work exactly like their optical counterparts except that they use a focused beam of electrons instead of light to form image of the specimen and gain information as to its structure and composition.

The basic steps involved in all electron microscopes are:

• a.  A stream of electrons is formed in high vacuum by electron guns

b.  This stream is accelerated towards the specimen with a positive electrical potential while is confined and focused using metal apertures and magnetic lenses into a thin, focused, monochromatic beam.

c.  The sample is irradiated by the beam and interactions occur inside the irradiated sample, affecting the electron beam.

d.  These interactions and effects are detected and transformed into an image.

 

1.1. Electron gun

The first and basic part of the microscope is the source of electrons. It is usually a V-shaped filament made up of tungsten that is wreathed with Wehnelt electrode (Wehnelt Cap). Owing to negative potential of the electrode, the electrons are emitted from a small area of the filament called a point source. A point source is important because it emits electrons with similar energy. The two common types of electron guns (Figure 18.1) are the conventional electron guns and the field emission guns (FEG).

In a conventional electron gun the positive electrical potential is applied to the anode and the filament (cathode) is heated until a stream of electrons is generated. The electrons gather speed by the positive potential down the column, and due to the negative potential of cap, all electrons are repelled toward the optic axis. A group of electrons occur in the space between the filament tip and cap, which is called a space charge. Those electrons at the base of the space charge (nearest to the anode) can exit the gun area through the small (<1 mm) hole in the cap and then travel down the column to be used in imaging.

A field emission gun comprises of sharply pointed tungsten tip held at several kilovolts negative potential relative to a nearby electrode, so that there exist a very high potential gradient at the surface of the tungsten tip. This results in the potential energy of an electron as a function of distance from the metal surface to have a sharp peak, then drops off quickly (due to electron charge travelling through an electric field). Since electrons are quantum particles and have a probability distribution to their location, a certain number of electrons that are nominally at the metal surface will find themselves at some distance from the surface, such that they can reduce their energy by moving further away from the surface. This transport-via-delocalization is called 'tunnelling', and is the basis for the field emission effect. FEGs produce much higher source brightness than in conventional guns (electron current > 1000 times), better monochromaticity, but requires a very good vacuum (~10-7 Pa).