Ultimate guide: SEM technology explained
Scanning electron microscopes (SEMs) produce images of a sample by scanning the surface with a focused beam of electrons. As the electrons interact with the sample, they produce backscattered electrons, secondary electrons, and characteristic X-rays, all of which are captured by a series of detectors and used to build up a high-resolution image of the sample surface and analyze its chemical structure. Some SEMs are capable of resolution better than one nanometer. In this post, we investigate the different hardware components SEMs use to achieve this.
SEM technology: Main components
From a hardware perspective, the main components of a standard SEM are the electron source, a series of lenses in the column, apertures, scanning coils, detectors, and the SEM chamber where you can mount one or more samples onto a holder. Everything in an SEM system is in a vacuum, where the vacuum in the upper part/column may be higher than the chamber levels, depending on the type of SEM you use.
Click image to enlarge
Scanning electron microscope: How it works
Let’s explain how these components all come together and how the underlying SEM technology works.
Electron source
The electron source (often called the electron gun) provides a stable beam of electrons of adjustable energy, usually from between 20–30 eV to 30 keV. Three types of gun are available: thermionic, lanthanum hexaboride, and field emission.
A thermionic emission gun features a thin tungsten filament that is heated to a high temperature (about 2,800°K) to generate an electron beam. The tungsten filament is inserted head-down into a cone called the Wehnelt cup. The filament and Wehnelt cup form the negatively charged cathode, which effectively “forces” the electrons to flow towards an anode, which is a metal disk with a hole in the center. This focuses the beam, and the finest point of the beam is emitted (usually called the cross over), creating the emergent electron beam.
A lanthanum hexaboride (LaB6) gun uses a similar principle, but a lanthanum hexaboride crystal is used instead of the tungsten filament. Typically, a LaB6 source achieves better resolutions and higher brightnesses compared to a tungsten source, but it is also a more expensive option, requiring a specialist technician to maintain the system and a superior vacuum to operate effectively.
A typical thermionic emission (LaB6) gun on the left and a field emission gun on the right.
A field emission gun heats a sharp metal tip, usually made of tungsten and with a radius of less than 100 nm, and uses two anodes to accelerate the resulting electron beam. Field emission guns achieve superior resolution, compared to thermionic emission and LaB6 guns, but they are also the most expensive type and require an ultra-high vacuum, which increases the O&M costs.
Lenses
In an SEM, the lenses are magnetic. Each lens is made of current-carrying coils, where the current is adjusted to change the strength of the lens. There are two types of lens: condenser and objective. A condenser lens demagnifies the electron beam and is usually used in conjunction with apertures to collimate the beam. An objective lens focuses the beam on the sample, determining the final diameter of the electron probe. The objective lens is a key component of any SEM, affecting the final resolution and image quality.
SEM detectors
A field emission gun heats a sharp metal tip, usually made of tungsten and with a radius of less than 100 nm, and uses two anodes to accelerate the resulting electron beam. Field emission guns achieve superior resolution, compared to thermionic emission and LaB6 guns, but they are also the most expensive type and require an ultra-high vacuum, which increases the O&M costs.
Example illustrations of secondary and back-scattered electrons and characteristic X-rays.
When the electron beam reaches the sample surface, it “enters” the sample and interacts with it. The picture above shows the interaction volume, sometimes called interaction pear because of its distinctive shape.
The size and the shape of this interaction volume depend on the acceleration voltage and the density of the material. For example, if the voltage is high, the beam will penetrate more deeply into the sample. When it comes to the sample density, you have to understand how a beam will penetrate different material types where, for example, it is easier to penetrate a polymer material, compared to a stainless steel sample.
When the electrons enter the sample, one of three phenomena occurs. First, when the electrons hit the sample’s atoms, some are scattered off the sample’s surface. These are back-scattered electrons (BSE) and are high-energy electrons, which belong to the primary (incident electron) beam. They give compositional information about the sample and material contrast information. When interpreting BSEs, a higher grayscale level is usually synonymous with a higher atomic number. So, for example, gold will appear brighter than a polymer (which is mostly carbon and oxygen-based, representing low atomic number elements).
Second, secondary electrons (SE) are also used to generate the resultant image. Here, electrons from the primary beam hit the material’s atoms, and electrons from the material’s atoms are also kicked out of the sample’s surface. These are the secondary electrons, coming from the surface of the sample, and they provide information on the topography and morphology of the sample. The thickness of the region from where they are ejected is proportional to the accelerated voltage and density of the material.
The third, and final, type of signal commonly detected is characteristic X-rays. These are generated when a secondary electron is kicked out from a specific atom. This, in turn, generates a vacancy in a specific electron shell of that atom. Consequently, an electron from an outer shell (of the same atom) is forced to fill that vacancy, moving from an outer shell to an inner shell. This “movement” causes the ejection of a photon that has an energy that is characteristic of an element (hence, the name characteristic X-rays). These X-rays are detected by a specific detector called EDS (energy dispersive X-rays) that provides elemental information on the sample.
Scanning electron microscopes: What are you waiting for?
At Thermo Fisher Scientific, our innovative microscopy and application expertise helps you find meaningful answers to the questions that accelerate breakthrough discoveries, increase productivity, and ultimately change the world. To discover more about SEM technology and see if a scanning electron microscope fits your research requirements, contact one of our expert teams today.
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