Benchtop scanning electron microscopes (SEMs) have come a long way since they were introduced. With capabilities now including magnifications up to 60,000X, multiple accelerating voltages, and both secondary electrons (SE) and backscattered electrons (BSE) detectors, these smaller versions of full-size SEMs are increasingly finding their way into industrial and educational laboratories. Whether used by trained electron microscopists as a simple screening instrument, or by lab technicians as a higher-resolution alternative to the light microscope, these powerful instruments accelerate the pace of research in the life sciences, forensics, and pharmaceutical fields.
As we highlight the numerous advancements in benchtop SEMs, we would be remiss if we failed to mention the overall size of the instrument. In the past, entire laboratory rooms were necessary to accommodate the large footprint of a full-scale SEM. Other factors that needed consideration included the thickness of the floor (an SEM is a very sensitive instrument, and even the slightest vibration can have an impact on imaging), and ensuring that electrical fields around the instrument were minimized to ensure the overall functionality of the instrument. Current benchtop SEMs are as small as 20” x 18” x 17” and weigh as little as 100 pounds.
Brent Platt operates the NeoScope (Credit: McCrone Group)
Combined Detectors Offer Imaging Choices
Overall, today’s benchtop SEMs contain many of the same features as their larger, full-size cousins. With multiple accelerating voltages, and both secondary electron (SE) and backscattered electron (BSE) detectors, these compact instruments provide the user with more flexibility at a lower cost. The types of signals produced by today’s benchtop SEMS, such as the JEOL JCM6000+, for example, includes both SE and BSE. Secondary electrons are inherently low in energy and come from the top few nanometers of the sample, so SE imaging can produce high-resolution images of the sample surface. Topography dominates here. Since the intensity of the BSE signal is closely related to the atomic number of the sample, BSE imaging is particularly useful in providing information about the distribution of different elements within the sample. Brighter areas are indicative of denser, higher atomic number materials. The NeoScope now offers both an Everhart-Thornley type SE detector as well as a high-sensitivity solid-state BSE detector. This solid-state multi segment backscatter electron detector features three imaging modes: composition, topographic, and shadow (a combination of composition and topographic information).
Advances Improve Analysis
When first introduced, the majority of benchtop SEMs were low vacuum instruments. Low vacuum SEMs are ideal for imaging and X-ray analysis of wet, nonconductive, unprepared samples, whereas high vacuum is ideal for failure analysis, inspection, and characterization. In addition, earlier benchtop units were restricted to one accelerating voltage, thereby limiting resolution capability. Current benchtop designs have as many as three accelerating voltages. Coupling this powerful feature with both high and low vacuum modes, and the capability of detecting secondary and backscattered electrons, allows scanning electron microscopists to analyze a wider range of samples, and achieve higher resolution images.
A common question is: What sort of resolution can I achieve with a benchtop SEM? Resolution—the ability to distinguish the shortest distance between two points on a specimen—is a critical parameter on all microscopes. Full-size SEMs can produce very high resolution images of a sample surface, typically revealing details less than 1 nm in size. The smaller benchtop instruments are able to resolve around 15 nm—an incredible achievement!
Maximum specimen size and total sample travel range are also important. When benchtop SEMs were first introduced to the market, most models were only able to accommodate samples approximately 25 mm in diameter. These powerful instruments can now accept samples as large as 75 mm in diameter and 50 mm high. These “less strict” size limitations allow for a greater array of samples to be analyzed, and decrease sample preparation time, one of the key components of scanning electron microscopy. Not only has the maximum allowable sample size improved over the years, but so has the stage on which microscopists place their samples. Older benchtop instruments only allowed for manual x-y sample movement, which often proved challenging to locate and re-locate areas of interest, particularly at higher magnifications. Many current model benchtop SEMs allow for a fully automated stage, making navigation a breeze. The ability to tilt and rotate a sample is also of great importance and many instruments come with the ability to rotate a full 360° and tilt from -15° to 45°.
Perhaps the most powerful advancement in current benchtop SEMs are the powerful, full-featured energy-dispersive X-ray spectroscopy (EDS) detectors. The EDS process detects X-rays emitted from the sample during bombardment by an electron beam to determine the elemental composition or chemical characterization of the sample. Features as small as 1 µm or less can be analyzed. Modern benchtop SEM EDS detectors are typically the silicon drift detector type, possess ultra-thin windows for detection of elements from B to U, and offer full area, partial area, multiple point and line scan EDS analysis capabilities. Active spectral mapping (up to 4096 x 3072 pixels), quantitation maps, and time resolved maps are built in, all producing a more accurate representation of the elements present. Equipping benchtop SEMs with this powerful analytical technique enables scientists to fully characterize their samples and identify unknowns, at a fraction of the cost of a full size SEM.
Scanning electron microscopy has come a long way since its development nearly 80 years ago. Modern benchtop SEMs possess the same features and functionalities as larger, full-size instruments, and are as simple to operate as a digital camera. A wide range of users are taking advantage of the capabilities of these exciting microscopes. We are sure to see even more exciting features in the future as magnification and resolution increases. The smaller these microscopes get, the bigger they become.
Jeff McGinn is President of the instrument sales division of The McCrone Group, McCrone Microscopes & Accessories. He is also President of the company’s educational arm, Hooke College of Applied Sciences, where he has been an instructor, teaching industry professionals about microchemical techniques. Jeff has a passion for microscopy and for serving customers’ needs.
