Finding the right type of SEM for your research can be difficult. There are many characteristics to which you need to pay attention. The majority of the factors depend on your specific research and the samples you want to image and analyze. In this context, several components, parameters and settings of the SEM influence the results of the analysis. In order to guide you in the process of choosing a SEM model, we highlight the important aspects to be considered:
Resolution
In physics, resolution is defined as the minimum distance between two objects that still allows the observer to distinguish them as separate entities. Therefore, this parameter is one of the main characteristics of microscopes in general. Users should remember the following rule of thumb: The resolution of the microscope should be 5 to 10 times lower than the size of the feature you want to resolve. That means for example: if you want to image structures with an average size of 50-100 nanometers a SEM with a resolution of 10 nanometers provides good results. Smaller features will look blurry and would require a more sophisticated device in order to be resolved. The resolution of modern SEMs can fall somewhere between less than 1 and 20 nanometers.
Magnification
The microscopic magnification describes the measure of the apparent increase in size of a viewed image or object. Even if this parameter seems kind of similar to the resolution, it is not the same. The statement "the microscope has a magnification of 1000 times" does not specify how sharp the image eventually is. Therefore, it is important to distinguish between magnification and resolution. Under ideal conditions high-end SEMs can magnify up to one million times.
When operating a SEM, the user increases the magnification by decreasing the scanned area. The required field of view for your sample imaging depends on the purpose of your analysis. For example: you image closely packed particles with an average size of one micrometer and you simply want to count them. Then you magnify only until that point at which you can distinguish single particles from each other. This means you can have several hundreds or even more particles in one image. On the other hand, if the purpose of your research lies in the analysis of the structure of a particle, you must, of course, zoom in further.
The electron source
The purpose of the electron source is to provide a stable beam of electrons. There are two groups of electron sources used in SEM, varying in the amount of current they produce into a small beam size, the stability of the beam and the lifetime of the source.
The acceleration voltage
The voltage is an indication of the electron’s energy content; the value of this parameter therefore determines what kind of interaction the beam has with the sample. As a general guideline, a high voltage corresponds with a higher penetration into the surface of the sample known as bigger interaction volume. Theoretically, an increase in accelerating voltage results in a stronger signal. However, there are also disadvantages such as charging effects and damage of the specimen that comes along with high voltages.
The current intensity
In modern SEMs, the user has the ability to control the size of the electron probe. This is mainly achieved by adjusting the condenser and the objective lenses of the system and by selecting different apertures. In general, the spot size should be kept as small as possible since this way the resolution is enhanced.
Customizability
In many cases SEMs can be equipped with several different detectors or accessories. Such additional components allow users to perform a variety of different analyses or to use samples, which can usually not be imaged with a SEM.
User experience and time to image
A SEM, which can be quickly loaded with the sample saves time. For this, an efficient evacuation of the chamber is crucial. To some extent also the user-friendliness defines how much time is needed to collect the results. A system that is easy to handle and has a short loading time can provide the user with results within 1-2 minutes.
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What are the costs of a SEM?
When choosing a SEM, besides its imaging and analysis capabilities, the initial and operational costs play an important role in the purchase decision. Three different cost items make up the total SEM costs:
Initial investment
The initial investment is the largest cost item and depends heavily on the type of SEM you choose. A floor model SEM, for example, requires a significantly higher investment than a desktop SEM and provides you with extra research possibilities.
The table below offers a cost comparison between a desktop and floor model SEM plus the facilities required by each type.
Maintenance costs
Besides the initial investment, maintenance is necessary to keep the SEM in good condition. The costs for this can be split as follows:
All desktop SEMs use a pumping system consisting of a pre-vacuum and turbo molecular pump. These components require a regular maintenance performed by a service engineer. Such service costs money.
From time to time the electron source has to be replaced. During a SEMs, lifetime this is the most expensive part of maintenance. Electron sources with longer lifetimes are usually more expensive, but in turn, save costs in the long run. The Phenom desktop SEM is the only desktop scanning electron microscope with a long lifetime CeB6 source with an average of 1500 operating hours.
Operational costs
Power consumption
A SEM runs on electricity. Clearly, the amount of energy needed strongly depends on how long the instrument is in use. A SEM with low power consumption can help to reduce costs.
Robustness
Sometimes, users by themselves cause problems to the system. This especially happens, when there are multiple operators working with the SEM. Internal damages can for example emerge in case the sample is not loaded correctly into the chamber. Choosing a SEM with a secure and user-friendly loading system avoids repair costs.
Time-to-image
A high sample throughput accelerates the analysis, and saves valuable working time. In contrast, a relatively slow loading and unloading time, however, will slow down your research process and increase your operational costs.
Types of SEMs
