To synthesis anything, from housing bricks to your comfortable clothes to rockets and satellites, you need materials. Right? Without proper materials at our service, it’s indeed difficult to imagine our life. But, how do we determine which material to use for which purpose? Well, this is what material scientists do! They study the properties of materials, analyze their composition and structure, and look for their possible applications. But how do they analyze and study the properties of different materials? Well, to do this, they make use of different techniques and instruments and one such important technology in this regime is that of “Scanning Electron Microscope”.
What is SEM?
An abbreviation for ” Scanning Electron Microscope”, SEM is an important electron microscope. Just like the light microscopes use visible light to produce the image of the sample under consideration, the electron microscopes use electrons for imaging.
Now, there are mainly two types of electron microscopes, TEM (Transmission electron microscope) and SEM (Scanning electron microscope). However, contrary to TEM which studies the electrons that pass through the specimen, the SEM technology uses the electrons that are reflected back or get knocked off the near-surface region of a sample to create its image.
The Scanning Electron Microscope Apparatus
In a Scanning Electron microscope, several components work together to form a perfect image of the specimen. Some of these components and their working is as follows:
At the top of the column, an electron source is placed that emits a beam of electrons. One common example of an electron source is a heated tungsten cathode. However, with the technology improving day by day, different electron sources are reaching the market. This electron beam then travels to a positive anode, from where it goes further on its journey to the sample.
Lens and coils :
We have generated an electron beam, but now, we need something to align and guide it properly towards the sample. How to do this? Well, this is where lenses and coils come to our rescue.
The electron beam is kept aligned using suitable combinations of lenses. The lenses used here are simple electromagnetic lenses that consist of coils of wires placed inside metal pole pieces. When a current passes through the coils, a magnetic field is generated. This magnetic field guides the path of electrons towards the sample.
The condenser lens converges the beam before the electron beam cone opens up again. And, the objective lens converges it again before it hits the sample. The condenser lens defines the size of the electron beam, which further defines the image resolution, while the objective lens focuses the beam onto the sample. As far as the scanning coils are concerned, these are used to scan the electron beam properly over the sample.
The detectors are the heart of every instrument and the same goes for SEM as well. When an electron beam interacts with the surface of a sample, it can result in the generation of different radiations and electrons from the sample.
However, in SEM, mainly two types of electrons, which are the Backscattered Electrons (BSEs) and the Secondary Electrons (SEs) are analyzed deeply.
The back scattered electrons are those electrons that belong to the primary electron beam and got reflected back after interacting elastically with the sample. Whereas, the secondary electrons originate from the atoms of the sample itself as a result of inelastic interactions between the electron beam and the sample.
The SEs originate from the deeper regions of the sample while BSEs originate from surface regions. So, both these electrons carry different types of information regarding the sample and hence, require different detectors for their detection.
Keeping this in mind and taking into consideration the energies of these two different categories of electrons, generally, solid-state detectors are used for the detection of BSEs, while the detection of SEs requires the use of Everhart-Thornley detector.
How are the SEM images formed?
The electron beam is here, the detectors are here. But how do they lead to image formation? Let’s see!
At the surface of the specimen, the electron beam scans it following a raster scan. At any given moment, the specimen is bombarded with electrons over a very small area.
As mentioned earlier also, several things may happen when these electrons interact with the sample. They may be elastically reflected from the specimen thus generating BSEs, may be absorbed by the specimen giving rise to secondary electrons of very low energy, together with X- rays or may also be absorbed giving rise to the emission of visible light. Not only this, but they may also even give rise to electric currents within the specimen.
Now, all these contributions can be used to produce an image. However, the most famous is the image formation by means of the low-energy secondary electrons. But how do these electrons form an image? Well, it involves a number of steps.
As already mentioned, an Everhart-Thornley detector is mainly used to detect SEs. This detector consists of a scintillator disk inside a Faraday cage, which is positively charged and attracts the SEs. The scintillator is then used to accelerate the electrons and converts them into light before reaching a photomultiplier for amplification. The photomultiplier tube then converts the photons of light into a voltage.
The strength of this voltage depends upon the number of secondary electrons that are striking the disc. Thus the secondary electrons produced from a small area of the specimen give rise to a voltage signal of a particular strength. This voltage is then fed to an electronic console, where it is then processed and amplified to generate a point of brightness on a cathode ray tube screen.
An SEM image is thus built up in this manner simply by scanning the electron beam across the specimen and can easily be viewed and analyzed on a computer. The 1D images can be further processed to have a 3D view as well.
Here are some of the most spectacular SEM images:
Working conditions and suitable samples:
SEM is a highly sensitive instrument, so the most important condition for its proper functioning is the presence of a high vacuum. The entire apparatus needs to be kept under vacuum. So all the components of SEM are sealed inside a special chamber in order to preserve vacuum and to protect it against contamination, vibrations, or noise. The vacuum not only protects the electron source from being contaminated but also allows the user to acquire a high-resolution image.
As far as the specifications of the sample are considered, the sample should be small enough to fit inside the apparatus properly. Moreover, the sample should be electrically conductive at least at the surface because non-conductive samples lead to the accumulation of charge over the surface which further leads to scanning faults and other image artifacts.
So, the non-conductive samples are coated suitably to make them conductive on the surface. The specimens should also be completely dried before scanning to preserve the vacuum.
Undoubtedly, SEM has become an important characterization technique these days. From research scholars to renowned scientists, everyone uses SEM. It has not only helped researchers to optimize their material characterization processes but has also saved a lot of their valuable time due to its instantaneous results.
Not only in material sciences, but it has also marked a niche for itself in biological sciences due to its ability to scan and study biological tissues. Although it is very delicate to operate and has some limitations related to the samples it can analyze, still, it remains one of the best characterization techniques that Max Knoll and Ernst Ruska have bestowed the scientific community with!