Choosing the right scanning electron microscope for your laboratory
Author: Noriyuki Inoue, JEOL USA
What is scanning electron microscopy?
Scanning electron microscopy (SEM) is an imaging technique that produces images of a sample by scanning the surface with a focused beam of electrons. SEM differs from optical microscopy, as it uses electrons instead of light to “see” into the material’s surface. When comparing SEM to optical microscopy, optical microscopy is limited by the wavelength of light, which is physically set in a defined range. SEM has the advantage of breaking this limit and allows for resolution that can reach the sub-nanometer level.
SEM has a large depth of field and higher magnifications than traditional optical microscopy. This, combined with its ability to conduct chemical analyses using spectroscopic methods, makes it a very powerful research tool. SEMs provide a high degree of analytical capability and reveal surface details at nanoscale resolution. A single image from SEM can often be enough to achieve critical objectives i.e., visualizing microstructures. There are many types of SEMs, ranging from the more common type, which use a tungsten filament as an electron source, to the more specialized type which, with a field emission (FE) electron gun mounted, attains higher resolution and magnification.
How does scanning electron microscopy work?
SEM uses deflector coils which alter the path of the electron beam, scanning it in a zig-zag type pattern (raster scanning). Typically, three detectors are positioned at angles in the sample chamber, these are an X-ray detector, a back-scattered electron detector, and a secondary electron detector. Sample thickness is not an issue as none of these elements is reliant on transmission.
When operating an SEM, a high-energy beam of electrons is scanned across the sample. Magnets focus the electron beam to a point several nanometers in diameter. As the electron beam interacts with the surface of the sample, signals are produced and compiled by various imaging and analytical detectors. Thus, high-resolution nanoscale images are achieved along with precise measurements. An SEM may detect backscattered electrons (to reveal morphology and topography and give insight as to composition), or secondary electrons (to reveal surface topography).
SEM is often considered a quick, versatile, and convenient option over other microscopy techniques. Recently, it has been shown to be useful in more and more applications and choosing the ideal SEM instrument is dependent on many factors. Below, we summarize some of the main considerations when selecting an SEM.
1. Microscope magnification
Since electron wavelengths are up to 100,000 times smaller than the wavelengths of visible light, SEMs resolve details hundreds of thousands of times smaller than optical microscopes.
The field of view (FOV) in a microscope defines the size of the feature to be imaged. This value can range between millimeters, microns and nanometers. To define the FOV required to image samples, first the end goal must be decided. If the number of particles in a sample is what is of interest, having multiple particles per image is not an issue, so an SEM that provides a FOV of 100 times greater is enough. However, if the structure of a particle is what is of interest, a closer FOV is needed to see the required detail. This is shown in Figures 1 and 2, which compare tabletop, tungsten, multipurpose FE and ultra-high resolution (UHR) FE SEM instruments.
To simplify choosing the right magnification for specific applications, a tabletop SEM can be very efficient. The relaxed vacuum requirements and small evacuated volume enable fast image production, without the extensive sample preparation. Additionally, tabletop SEM is normally carried out by the individual who requires the information, which thereby eliminates time required for a dedicated SEM operator to carry out analysis and produce a report. As well as obtaining answers quickly, it is also beneficial to be able to carry out analysis straight away and for the user to be able to manage it in real-time response to observations.
For instances where higher magnification is needed, but space is also a limiting factor, conventional tungsten SEMs are an option to simplify specimen navigation and advanced automation delivers crisp secondary and backscatter images in seconds. If a specimen is challenging to analyze, FE SEMs and UHR FE SEMs provide topographical and elemental information at magnifications of 10x up to 1,000,000x.
Figure 1: Materials SEM comparisons.
Figure 2: Biological SEM Comparisons.
Figure 3: SEM comparisons for imaging Titanium alloy cross section.
2. Microscope resolution
The word resolution indicates the smallest observable element in an image. For the human eye, that is about 0.2 mm. SEM resolution is usually between 0.5 and 4 nanometers, meaning it provides the opportunity for particle diameters and geometries to be studied in great detail. There are many contributing factors that can affect the maximum resolution obtained in an SEM, like the electron spot size and interaction volume of the electron beam with the sample.
SEM’s high resolution is attributed to the fact that the wavelength of the electrons becomes shorter because the accelerating voltage of the electrons used in the SEM is as high as several kV to several tens kV, and to the characteristic difference of the electromagnetic lenses used to converge the electron beams. By utilizing several images together with software, the size distribution of particles may be determined and a concentration versus particle diameter may also be calculated.
SEM images are stored in an image file (e.g., JPEG, TIFF) with a user-defined number of pixels. An SEM will scan small areas with an electron beam, which means the portions of the surface will become a pixel of the final image. More pixels result in a longer processing time; however, a long analysis process can have a detrimental effect on the sample.
Tabletop SEMs can generate an electron beam at the specimen surface with spot size of several nanometers, and a price range similar to that of a high-end optical device, thus they are slowly revolutionizing the industry, realigning production standards to a new level of miniaturization. Tungsten SEM is suitable for analysis of sub-micron structures (hundreds of nm), the lower kV allows for a smaller X-ray signal depth within the sample and thus higher X-ray spatial resolution. If ultra-high X-ray spatial resolution is needed to resolve ~50nm layers, then an FE SEM is the best option, since FE emitters maintain a very small spot size even at low kV. Table 1 shows a comparison of some relevant parameters between thermionic tungsten emitters and Schottky FE emitters.
Table 1: A comparison of parameters between thermionic tungsten and Schottky field emission emitters.
3. Microscope applications
SEMs are used in a wide range of industries including electronics, chemicals, machinery and pharmaceuticals and are used in research, quality control and product inspection.
SEM is very popular with scientists in the materials and life science research areas as its resolution and depth of field capabilities are a significant improvement over those of traditional optical microscopy. In materials science, investigations into nanotubes and nanofibers, high temperature superconductors, mesoporous architectures and alloy strength, all rely heavily on the use of SEMs for research and investigation. Many material science industries, from aerospace and chemistry to electronics and energy usage, have been made possible with the help of SEMs.
For pharmaceutical applications, SEM can play a pivotal role in the rapid and efficient characterization of new drug treatments, providing insights into their interactions with human cells and their applications in complex therapies.
A tabletop SEM can produce the necessary results at a lower cost as long as the application is routine and well defined. A key consideration is any future laboratory requirements, which may need resolutions unachievable by tabletop SEM. To combat this, outsourcing to a laboratory with a larger model is a potential option or tabletop SEM can be utilized to aid a future floor model system. For example, tabletop SEM can be used to screen capacity and to carry out routine analyses, leaving the floor model system available for more challenging uses. Due to the nearly unrestricted FOV of FE and UHR FE SEMs, high resolution imaging and high current analyses can be achieved without sacrificing performance. These SEMs are particularly suited for imaging and analyzing magnetic light element materials and nanostructures.
4. Microscope users
As electron microscopy has become more available, the user experience has been redesigned to suit any operator. SEM has historically been restricted to substantial laboratories with the budget and space to justify the installation of full-scale, floor standing SEMs. Traditional SEM instruments have offered unrivalled details of every material, from insects to crystals and bacteria, but can be complex to use, requiring expert understanding, and also necessitate a large, dedicated space.
However, the arrival and advancement of compact and user-friendly tabletop SEMs is changing this picture. The invention of compact SEM instruments intended to fit on a tabletop and the onset of new technologies has made it feasible to incorporate further analysis equipment into SEMs, creating self-contained nano-laboratories through the addition of an assortment of electrical, mechanical, and chemical test equipment.
When considering what type of SEM to buy or use, it is worth thinking about the number of individuals who will be utilizing the system, how much training they have, and the amount of time it might take to train them. Whereas floor-based systems with their built-in automation are simple to use, training is recommended to optimize performance. Tabletop SEMs are uncomplicated, and, for the majority of applications, they require much less training.
SEM is an innovative tool with countless applications. However, it is important that the user has a defined idea of what type of analysis is required – and of how the different spot sizes, electron beams, and accelerating voltages will influence the SEM imaging quality. Selecting the best parameters for any given experiment is crucial in selecting the most appropriate SEM.
Overall, tabletop SEM meets the imaging requirements in most situations. The resolution limits in the several nanometer range is enough for 80-90% of all applications making them a smart choice for most laboratories.