In 1935, Max Knoll created the first scanning electron microscope (SEM). His design was further worked upon by Manfred von Ardenne over the next few years. However, the advanced SEM design created by Professor Charles Oatley at Cambridge University during the 1950s and 1960s is widely recognized as a commercially viable instrument. He is credited as the father of scanning electron microscopy.
The SEM comprises an electron gun at the top of the column, which thermionically produces a beam of electrons. As the electrons move through the column, they are conditioned by coils. The coils act as lenses and enable the beam of electrons to raster across a sample in a vacuum chamber. The beam scans the sample, and secondary electrons are emitted from the sample. The secondary electrons are collected by detectors in the chamber to produce an image of the sample.
One major advantage of the SEM is that unlike the optical microscopy, SEM is not restricted by the diffraction limit of visible light, thereby allowing researchers to image at more than hundred thousand times magnification. It also helps to maintain very good depth of field, which enables users to precisely resolve surface characteristics of the sample.
The other advantages of the SEM are as follows:
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It is a versatile platform for additional capabilities that extend beyond basic imaging.
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The thermionic gun in the SEM can be easily substituted with or complemented by a field emission gun (FEG) so as to greatly enhance the spatial resolution of the SEM.
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Energy-dispersive x-ray spectroscopy (EDS or EDX) provides the user with an opportunity to examine the chemical composition of a sample using x-rays generated when the electron beam strikes the sample.
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It can be equipped with focused ion beam (FIB) capability to enable ion beam milling and deposition.
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The chamber is fitted with probes to enable the user to examine the conductivity of a sample.
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SEM platforms can also be applied for lithography and micromachining applications.
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Figure 1. Zeiss Sigma VP SEM installed on AVI-400 Active Vibration Control System
Environmental Challenges
The SEM is susceptible to environmental disturbances, as it is used to observe extremely small feature sizes at extremely high levels of magnification. Manufacturers of SEM take this into consideration and develop criteria for maximum acceptable electromagnetic interference, acoustic noise levels, and vibration, so that their instruments will function well.
The electron beam, sample, and detectors must have a high level of coherence in order to obtain precise images. The manufacturers provide the criteria as a part of the installation requirements document for each instrument.
The SEM manufacturers may conduct a site survey using vibration measurement equipment before installation. In case the site does not meet the specification, the buyer of the SEM will need to sign a waiver indicating that the instrument may not realize maximum performance.
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Figure 2. SEM image of Silicon Balls - EMI effects can be seen on feature edges
Massive instruments like the electron microscopes are already provided with built-in passive vibration isolation mechanisms; hence they are comparatively unaffected by higher frequency vibrations. In the case of SEMs, they are still sensitive to low frequency vibrations such as building vibrations, however.
They have to be provided with additional vibration isolation systems if they have to be installed on higher floors in buildings or in areas with considerable seismic vibration levels. The recommended vibration isolation system is the Active Vibration Control System.
SEMs are also sensitive to EMI; hence they should not be placed near any source of EMI, such as moving vehicles, elevators, or HVAC machinery; however if high levels of EMI are present, then a large EMI shield has to be constructed around the SEM. An EMI Cancellation System can also be installed.
Typical Applications
The following are some of the areas where SEM is used:
- Forensics
- Data storage
- Failure analysis
- Medical devices
- Energy research
- Materials science
- Mining and geology
- Pharmaceutical research
- Semiconductor research
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Figure 3. NO ISOLATION - Sample: Gold on Carbon imaged with LEO SEM
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Figure 4. PASSIVE ISOLATION-Sample: Gold on Carbon imaged with LEO SEM
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Figure 5. ACTIVE ISOLATION - Sample: Gold on Carbon imaged with LEO SEM
About Herzan
Herzan provides high performance environmental solutions for precision research instruments. They include acoustic enclosures, vibration isolation systems, Faraday cages, and site survey tools. Herzan specializes in supporting nanotechnology research, but also offers solutions for product testing, in-vitro fertilization, and many other applications.
Herzan understands that every application and environment is different, so it collaborates with customers to create comprehensive integrated solutions that satisfy their unique demands.
Herzan was founded in 1992 by Ann Scanlan in Orange County, California. Originally, Herzan was established as an American subsidiary of Herz Company Ltd., a Japanese company specializing in vibration control. The name Herzan comes from the amalgamation of 'Herz' and 'Ann'.
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This information has been sourced, reviewed and adapted from materials provided by Herzan.
For more information on this source, please visit Herzan.