Introduction to Scanning Electron Microscopy

Introduction To Scanning Electron Microscopy

At the completion of the prac, the practical experience of operating a scanning electron microscope is sufficient to operate the particular machine in the future. During the experiment, two different gold plated samples are analysed under the SEM and compositional and topographic information is identified and analysed. Both the information is derived by changing the working distance, accelerating voltage, aperture size, probe current, resolution and magnification.

AIM

To analyse samples and obtain micrographs of the surface, using a scanning electron microscope. The experimental images are compared against the different material structures and defects between the various samples at different magnifications and resolutions.

INTRODUCTION

A schematic of a scanning electron microscope is shown below. The instrument can be separated into three major sections including electron-optical 'column', vacuum system and electronics and display system.

Figure 1: Layout off scanning Electron Microscope.

In the optical part of instrument, the first thing we notice is the need for a source of illumination. No visible photons of light are produced within the SEM but an invisible beam of electrons. The beam is produced from an electron 'gun' and a cross-section through a simplified electron gun is shown below.

Figure 2: Cross-section through an electron gun

A V-shaped tungsten filament (I) is heated electrically to about 2700Kand the high temperature causes many of the electrons in the tungsten to become sufficiently excited for them to escape. This process is called "thermionic emission".

Once freed the electrons would be quickly recaptured by the filament, because in losing them, it will have become positively charged. Applying a high negative voltage (typically 2-25 kV) between the filament and a nearby earthed anode disc, accelerates the electrons away from the filament and their velocity depends on the accelerating voltage and is only a fraction of the speed of light. Because of the high voltages applied to the gun, good electrical insulation (3) is required.

Enclosing the filament in a metal cylinder (2), usually called the Wenhelt cylinder or cathode, shapes the beam electrostatically, so that it emerges 10 - 50 мm in diameter. Unfortunately in air, or any other atmosphere, the electrons would be scattered by collision with gas molecules and they could travel only a few millimetres. A vacuum system is connected (figure 1, 14) to the column, so as to remove most of the gas molecules from the beam's path.

The ultimate performance of the SEM is mainly limited by the diameter of the beam. In order to improve performance we must be able to control it. In figure 1 the upper two lenses, the condensers (3), control the beam's diameter and demagnification occurs, when the diameter is reduced from 50 мm to around 5 nm. Electron lenses are very different from their optical counterparts.

Figure 3: Simplified electron lens (cross-section)

A coil of wire, with its axis aligned along the beam's path, is partially enclosed in a cylindrical iron case. There is a small gap in the inner bore of the case. When a direct current is passed through the coil an electromagnet is produced, with magnetic poles (N and S), created at the gap in the iron case. This is the 'polepiece gap' and it is really the magnetic lines of force, bridging this space that forms the lens.

Due to their charge, electrons are deflected as they intersect the lines of force. All electrons, entering the bore at the top of the lens, converge at a focal point (f), below the lens. A single point of focus is only produced if all the electrons have the same energy. This means the gun's accelerating voltage must be kept very stable, any variation will result in electrons of different wavelength or chromatic aberration.

Changing the current through the coil changes the magnetic field strength. This in turn changes the angle through which the electrons are deflected, resulting in a change in the focal length of the lens.

Figure 1 shows a typical lens arrangement of two condenser lenses (3) control the beam diameter and a third lens, the objective (5), ensures that the beam has its smallest diameter when it strikes the specimen surface (6) and helping to will focus the image.

SEM can have a beam diameter of 5 nm, the diameter of the volume sampled, the so-called 'interaction volume', may be up to five hundred times larger. The effect is most apparent with 'bulk' samples (i.e. > 1 мm3). It is caused by electrons, and other resulting radiations scattering and diffusing through the sample, before emerging and being detected.

When bombarded, many different interactions occur between the specimen and the electrons. Electrons are emitted from the specimen and these are collected by a detector which converts them into a small electrical signal.

Figure 3: Important specimen/electron interactions

If the electrons have sufficient energy, they may pass right through the specimen suffering no effects at all. This is really a non-interaction; consequently the emerging electron beam contains no information about the specimen at all. For this to occur in the SEM, the beam energy is typically 20 - 30 keV and the specimens must be extremely thin (< 1 мm). A beam electron (typically 20 keV), which passes close to a positively charged atomic nucleus, may be attracted by its opposite charge. As a result the electron changes its direction, with hardly any (< 1 eV) loss of energy. The angle through which it is deflected (scatter angle), depends