How to Select the Right Desktop SEM?

When making the decision to purchase a scanning electron microscope (SEM) it’s important to choose an electron source appropriate for your application because the source has a significant impact on image quality. This article will compare a CaB₆ electron source with a tungsten electron source to determine which is most appropriate for a desktop SEM.

The electron source, which can be in the form of an electron gun, a cathode or a filament, is a key component of a desktop SEM and is used to form a stable electron beam. There are two main different types of SEM electron sources, which use different amounts of current to produce different beam sizes (spots) of differing stabilities and with different operational lifetimes.

Thermionic electron sources are used in desktop SEMs. This article will explore two thermionic electron sources - Tungsten and Cerium Hexaboride (CeB₆)

Thermionic Electron Sources

The heating of any solid material results in the thermionic emission of electrons. This emission occurs at a significant level when the electrons are given sufficient energy as to overcome the work function of the material. Cathodes in thermionic electron sources are produced from high melting point materials that have a low work function – facilitating the large-scale emission of electrons.

Figure 1. Cross-section view of an electron column with a schematic view of the source assembly.

The electron beam that is scanned over samples in an SEM is the result of the acceleration of electrons from the high negative potential of the source towards the anode (inside the electron columns) ground potential. The beam can form due to the vacuum environment of the column and is focused using lenses.

When comparing electron sources the important parameters to focus on are:

Source Temperature

The source temperature is the temperature at which electrons have enough kinetic energy to overcome the work function and are therefore omitted. The source temperature for thermionic electron emission is between 1,800 and 2,800 K.

Source Brightness

The brightness of a source is measured as the current density of the beam in terms of the solid angle. At higher the current densities (i.e. electron densities) more electrons are present in a small spot size, meaning that higher current densities facilitate high-resolution images to be taken at high magnifications.

Electron beam brightness has a positive linear relationship with the acceleration voltage. This means that any electron source will be ten times brighter at a voltage of 10 kV, than a voltage of 1 kV.

At high currents the beam spot size can be made finer to increase the resolution however eventually the signal-to-noise ratio will prevent resolution improvements from being made.

Source Size

As mentioned previously a small spot size corresponds to a higher resolution. The column lenses (in particular the condenser lens) demagnify the spot size with less demagnification required for sources, which produce a smaller spot.

• Electron Beam Energy Spread

The energy spread of the electron beam describes the different energies of individual electrons in the beam. Energy spread can result in chromatic aberration, where the beam is slightly less focused due to these energetic differences. When a low acceleration voltage is used the energy spread is large and chromatic aberration is a significant reason for lower resolution.

• Lifetime

An electron source’s lifetime is the time the system needs before it fails or requires replacements. A durable electron source that indicates when it will fail (to provide time for replacement) is best.

Figure 2. Comparison between a Tungsten and CeB₆ electron source.

Comparing Tungsten and CeB₆ Electron Sources

Tungsten filaments are a popular choice in SEMs. Tungsten has the lowest thermal expansion, highest melting point and lowest vapor pressure of all of the metals. Taking this into consideration alongside it high tensile strength it is obvious why tungsten is a popular choice.

However, it is not perfect, and by comparing tungsten with cerium hexaboride (CeB₆) this becomes clear:

Brightness

Tungsten has a brightness of 106 A/cm² sr, which is smaller than that of CeB₆.

CeB₆ has a lower work function than tungsten, which allows higher beam currents to be generated at lower temperatures, resulting in a higher beam brightness regardless of the voltage used. This means that a CeB₆ cathode is ten times brighter than tungsten. This higher brightness is advantageous because:

• For the same signal-to-noise ratio the spot from a CeB₆ source can be made smaller to give a better resolution.
• A greater current density can be achieved which means a better signal : noise ratio in a CeB₆ spot, than a tungsten spot of the same size

Figure 3. Image from TiO₂ powder made with CeB₆ system | Image from TiO₂ powder made with Tungsten system.

Electron Source Temperature

CaB₆ has an operational temperature of 1,800 K, whereas tungsten filaments require 2,800 K. This increased temperature has a detrimental effect on the source.

Source Size

The source size is of Tungsten is elliptically shaped and has a dimension ranging from 50µm to 100µm, depending on the source configurations and operating conditions.

Compared to a CeB₆ source, which has a dimension of <25µm, it means that considerable electron optic demagnification is required for a Tungsten source to achieve a small electron probe needed for good resolution in SEM.

Electron Beam Energy Spread

The higher temperature requirements of a tungsten source means that it has a greater energy spread than a CeB₆ source, meaning it produces images of lower quality - this effect is particularly pronounced at lower voltages. The energy spread in a typical tungsten beam is 2.5 eV, compared to 1 eV for a CeB₆ beam.

Electron Source Lifetime

The high operating temperature of tungsten filaments means they are white-hot, which results in them slowly evaporating whilst in use. This ultimately results in them becoming thinner and breaking during use.

The breaking of the filament can result in the contamination of the upper section of the column. For this reason when a tungsten filament is replaced the column components should either be cleaned or replaced.

CeB₆ sources degrade slowly over time with no sudden failure meaning its lifetime is more easily predicted. This allows researchers to determine the optimum time to replace their source, allowing replacement to occur at non-critical times and preventing analyses from being stopped by a failing electron source.

More importantly, column contamination is not an issue with CeB₆ sources meaning components do not need to be cleaned or replaced following failure.

Comparing the Lifetime of Tungsten and CeB₆ Sources

CeB₆ sources typically have a service life of more than 1,500 hours, whereas tungsten sources have an average service life that is 10 times less (vacuum dependent) at around 100 hours.

Phenom-World’s Recommended Electron Source

Phenom-World recommends that researchers use a CeB₆ source because of the many advantages it provides in terms of reduced maintenance, efficient operation and (most importantly) high-resolution imaging.

The only disadvantage of CeB₆ (against a long list of advantages which include a better signal-to-noise ratio, clearer images, a brighter beam and a predictable point of failure) is that it is more expensive than tungsten.

However, when you consider the longer lifetime of CeB₆ and the reduced risk of contamination (which is expensive to fix) tungsten is the more expensive option.

For these reasons the decision between choosing CeB₆ sources (and a desktop SEM) over tungsten an easy one.

This information has been sourced, reviewed and adapted from materials provided by Phenom-World BV.

For more information on this source, please visit Phenom-World BV.