Scanning electron microscope

The Scanning Electron Microscope (SEM) is a microscope that uses electrons rather than light to form an image. There are many advantages in using the SEM instead of a light microscope. The SEM has a large depth of field, which allows a large amount of the sample to be in focus at the same time. Preparation of the samples is relatively easy since most SEMs only require the sample to be conductive. A sputter coater is necessary if the sample is not conductive: it is used gold to observe the morphology of the sample or carbon to investigate the composition and the present phases. The

combination of higher magnification, larger depth of focus, greater resolution, and ease of sample observation, makes the SEM one of the most heavily used instruments in materials science. The basic theory of SEM functioning is explained.²

A beam of electrons is generated in the electron gun, located at the top of the column, (in Fig. 2.3).

Fig. 2.3: Beam's path through the column.

The most common filament is the tungsten hairpin gun. This filament is a loop of tungsten which functions as the cathode. A voltage is applied to the loop, causing it to heat up. The anode, which is positive with respect to the filament, forms powerful attractive forces for electrons. This causes electrons to accelerate toward the anode.

Some accelerate right by the anode and on down the column, to the sample. Other types of filaments are lanthanum hexaboride (LaB6) filaments and field emission guns.

The electron beam hits the sample, provoking the expulsion of electrons from the sample. These electrons are collected by a secondary detector or a backscattered detector, converted to a voltage, and amplified. The amplified voltage is applied to the grid of the CRT and causes the intensity of the spot of light to change. The image consists of thousands of spots of varying intensity on the face of a CRT that correspond to the topography of the sample.

While all these signals are present in the SEM, not all of them are detected and used for information. The most commonly used signals are the secondary electrons, the backscattered electrons and X-rays.

A schematic of the interaction between sample and incident electron beam is shown in Fig. 2.4.

Fig. 2.4: Electron beam/specimen interaction.

Secondary electrons are specimen electrons that obtain energy by inelastic collisions with beam electrons. They are defined as electrons emitted from the specimen with energy less than 50 eV.

Secondary electrons are predominantly produced by the interactions between energetic beam electrons and weakly bonded conduction-band electrons in metals or the valence electrons of insulators and semiconductors. There is a great difference between the amount of energy contained by beam electrons compared to the specimen electrons and because of this, only a small amount of kinetic energy can be transferred to the secondary electrons.

Elastic scattering occurs between the negative electron and the positive nucleus. This is essentially Rutherford scattering. Sometimes the angle is such that the electron comes back out of the sample. These are backscattered electrons.

During inelastic scattering, energy is transferred to the electrons surrounding the atoms and the kinetic energy of the energetic electron involved decreases. A single inelastic event can transfer a various amount of energy from the beam electron ranging from a fraction to many keV. The main processes include phonon excitation, plasmon excitation, secondary electron excitation, continuum X-ray generation, and ionization of inner shells. In all processes of inelastic scattering, energy is lost, though different processes lose energy at varying rates.

During utilization, a good vacuum level inside the column must always be insured for several reasons. If the sample is in a gas filled environment, an electron beam cannot be generated or maintained because of a high instability in the beam. Gases could react with the electron source, causing it to burn out, or could produce random discharges and lead to instability in the beam. The transmission of the beam through the electron optic column would be also hindered by the presence of other molecules. These molecules, which could come from the sample or the microscope itself, could form compounds and condense on the sample. This would lower the contrast and obscure details in the image.

The spatial resolution of the SEM depends on the size of the electron spot, which in turn depends on both the wavelength of the electrons and the electron-optical system which produces the scanning beam. The resolution is also limited by the size of the interaction volume, or the extent to which the material interacts with the electron beam. The spot size and the interaction volume are both large compared to the distances between atoms, so the resolution of the SEM is not high enough to image individual atoms, as it is possible in the shorter wavelength (i.e. higher energy) transmission electron microscope. The SEM has compensating advantages, though, including the ability to image a comparatively large area of the specimen, the ability to image bulk materials (not just thin films or foils) and the variety of analytical modes available for measuring the composition and properties of the specimen. Depending on the instrument, the resolution can fall somewhere between less than 1 nm and 20 nm.

The world's highest SEM resolution is obtained by the Hitachi S-5500. Resolution is 0.4 nm at 30kV and 1.6 nm at 1kV. In general, SEM images are easier to interpret than TEM images.