The market for tabletop Scanning Electron Microscopes has increased considerably due to the general shift toward smaller, faster, and more economical instrumentation. For a prospective buyer, the landscape of current offerings can be difficult to ascertain what features are important. Although these instruments effectively bridge the gap between Optical Microscopes and conventional SEMs, some offerings still deprive users of some major elements in the SEM workflow.
We recently met with several microscopists having years of experience and asked them what they thought were important things to investigate when evaluating a new Tabletop SEM. Here is a summary of 10 key questions to consider or to ask prospective suppliers, and how the answers to those questions can help you make better buying decisions.
1) Who’s Going to Use it… and for What Type of Application?
Traditional floor model SEMs often need a separate electron microscopy suite, separated from vibration, thermal, electrical and acoustical disturbances. Additional cost can be incurred for highly trained staff to operate and, at a fundamental level, maintain those instruments. Although these rigorous parameters apply for highly complex research on large SEMs, tabletop or desktop SEMs provide the perfect solution if a facility is involved in more general research, QA/QC, process development, failure analysis and/or education. Tabletop SEMs can also reduce the work load of full-size SEMs having a much higher cost-of-ownership.
The second half of this question concentrates on the application. Standard desktop SEMs are suitable for single-user or routine applications, but if a laboratory has a larger user group with a different experience and needs, then one of the latest breed of more sophisticated table top systems might be a better fit.
Do users’ applications involve non-conductive or biological samples? While low vacuum mode for charge reduction is offered by most tabletop SEMs, magnification is rather limited on many sample types. Sputter coating, sample freezing or drying should still be considered as it often provides much better imaging.
If users require Low Vacuum mode, how well does the SEM handle the chamber pressure? Does it employ an actual pressure feedback control or cut corners with a simplified air bleed orifice that can have performance fluctuations over time? Does it require special expensive inserts which limit the type of sample holders that can be used?
2) How Intuitive and Complete Are the Operating System and Software?
Maybe the most fundamental question to ask about any microscopy instrument is which computer operating system does it use? For instance, one SEM brand uses Linux operating system for the SEM and then adds a Windows PC with for EDS. Likewise, Microsoft will not be supporting Windows 7 much longer – is Windows 10 an option? Having a common operating system for the microscope and EDS will shorten the learning curve and simplify the workflow making overall operation faster and easier for users.
“What image controls do I need at my fingertips?” is the next question. As shown in the video below, the graphical user interface or GUI needs to be intuitive and should allow easy adjustment of the electron optics and the stage. Direct dimensional measurements and image annotation are also invaluable. Being able to view the BSE and SE images side by side can also be helpful.
3) Which Imaging Modalities Do You Need?
There are many ways through which an electron beam interacts with the sample, as illustrated in Figure 1. Simpler tabletop SEMs usually have a single backscatter electron detector (BSE): one that is helpful for seeing elemental contrast or atomic number in the sample, but not surface topography (Figure 1a). Usually, these will be 4-quadrant BSE detectors that are capable of creating pseudo-topography images using just one or two of those quadrants. Yet, the latest entrants in the market now provide an extra Secondary Electron (SE) detector that creates excellent images for surface morphology and topography.
Users should look for SE detectors to be true Everhart-Thornley type – the same as used in full-size SEM. However, one drawback of SE detectors is that they cannot image in charge reduction or low vacuum modes in the case of non-conductive samples. Without a true Everhart-Thornley type SE detector your images will never give the best topographic information and will lack the contrast of an image done with an Everhart-Thornley style detector.
Figure 1. (a) Electron beam/sample interactions (b) Secondary electrons (“SE”) capture surface topography while (c) Backscattered Electrons (“BSE”) image contrast based on the atomic weight of individual elements.
4) What Accelerating Voltage Best Suits Your Application?
Are users’ samples beam-sensitive? Most Tabletop SEMs which are restricted to 5 kV on the low end, so if your samples are beam sensitive, consider a system with variable voltages down to 1kV which is more comparable to full-size SEM that some allow voltages down to 0.1kV. Equally important is to test how well the SE detector can image at those low voltages.
Another consideration is charging. As shown in Figure 2, a good tabletop SEM will be able to capture images in high vacuum mode of un-coated nonconductive samples by using beam control features like widely adjustable spot size and detector amplification.
Figure 2. Impact of changing the accelerating voltage – A beetle claw imaged at 1kV at high vacuum without coating using SE detector. (Image: Courtesy of NanoImages, LLC)
Will users be doing EDS? The majority of tabletop SEMs are restricted to 15 kV maximum accelerating voltage for the electron beam. This limited accelerating voltage does not promote collecting satisfactory spectra in the range above 8-10 keV for many critical elements like Molybdenum, Bromine, Zinc and several others. Further, 15 kV forces the use of spectral peaks at energies less than 5 keV where there are frequently many overlapping elements. As shown in Figure 2, an electron beam with accelerating voltage up to 30 kV can generate better imaging and better EDS results by matching the beam to the composition of the sample.
Figure 3. EDS Spectra of CIGS Solar cross section at 15 kV (navy) and 30 kV (red) zoomed to 7 keV to 22 keV range showing increased precision and peak detection at 30 kV. Inset – full spectra showing cluttered region below 5 keV.
5) What About Magnification and Resolution?
Magnifications of 100,000x or more are advertised by many companies. Users should consider what that number actually means in terms of their application. Most importantly, the actual Magnification is highly dependent on how the image is displayed whereas Resolution is a better measure of the microscope capabilities.
What may be 10,000x on a microscope display could well be 100,000x when projected in a presentation, but the important thing is whether that increase is really giving users some extra information. At some point, magnifying any digital image reaches a point known as “hollow or empty magnification” where no additional detail can be revealed.
However, Resolution refers to the ability to separate and image fine detail (Figure 4). Resolution is also a function of the electron optics. Consider the type of electron gun and factors that impact beam cohesion such as a fixed aperture or the ability to vary the aperture size allowing the optics to be adjusted to produce the best image detail. Similarly, having functional control over beam stigmation at high magnification will produce better images.
A users’ best guide is their own application. What do users actually need to “see” in their sample? While conventional desktop SEMs will be fine for samples with lower resolution needs, users should plan to invest more for a desktop SEM that will easily resolve finer detail.
Figure 4. Achieving higher resolution may require a larger investment but will be worth the money. The fine detail captured in this Lanthanum powder allows measuring features as small as 10-15 nm. (Image: Courtesy of SEC, Korea)
6) Planning on Publishing?
If users are planning to publish, then they should consider the pixel density used for capturing SEM images. Usually, the higher the pixel count, the better. If a SEM can only collect an image at 2048 x 2048 or 1280 x 960 pixels, it is likely to be soft or have the jagged edged artifact known as “pixilation” when projected or printed in journals. A higher pixel density, on the other hand, can be useful for producing a high-quality image with rich topography and clean, well-defined edges.
Users should look for an SEM that is capable of collecting images in various modes to suit their requirements such as fast scan mode (640 x480) up to a photo quality of 5000 pixels or greater.
When initially introduced, small stage size and chamber limited the adoption of table top technology. As technology and capabilities have progressed, so have the size of sample that can be imaged with some systems now allowing samples up to 80x80mm.
What about viewing angle? The ability to translate the sample in XYZ, tilt and rotate allows users to move features of interest to the middle of the field of view and view them from the most optimum viewing angle or to accentuate x-ray counts for EDS, as illustrated in the video below.
Tabletop SEMs are available with both manual and motorized stages. Entry level Tabletop SEMs have the ability to move the sample laterally side-to-side (X) or front-to-back (Y) in order to position the sample in the field of view. Motorized stages offer the convenience of software control of the positioning while a Manual stage is lower cost and allows easier modifications for special sample presentation requirements. A system with a rotate axis can make fast work of imaging multiple samples using a holder as pictured in Figure 5, even with a manual operated stage.
Figure 5. Flexible sample holders for multiple samples that take advantage of a stage’s rotary axis to move sample-to-sample quickly. (Image: Courtesy of NanoImages, LLC)
The method of Z or sample height adjustment should also be considered. Is it possible to adjust the stage height externally or will it need a chamber an open/close cycle that would interrupt and slow down the work flow? Having no external Z control can be an advantage in a situation where novice users access the SEM, making it less prone to user damage but the lack of real-time sample positioning can have costs in productivity.
On the other end of the equation are higher-end systems having a 5-axis stage to navigate and optimally position your samples. With the advancements of these microscopes some offer a navigational camera with motorized 5-axis stage for point and click control, really simplifying the task of optimal sample positioning. Intelligent software controls prevent samples from contacting sensitive SEM hardware as well as storage of coordinates for repetitive sample imaging.
EDS manufacturers also offer analytical tools that require motorized stage control via the EDS software to perform tasks like Automated Particle Analysis on filters, metal inclusions, mineral mapping and much more. This capability turns the SEM/EDS into a very intelligent analytical tool.
A second component of this stage equation is beam shift, which enables a feature to be centered in the field of view without physically moving it. The ability to shift the beam and quickly center features up to 150 microns laterally will be very helpful to productivity and user satisfaction.
Finally, what about temperature? A system that enables adding cooling stages can considerably enhance results, particularly for polymer or delicate biological samples, or to image moist sample without pre-drying or to run transient imaging studies that require variation of sample temperature.
8) Do You Need Elemental Analysis?
Another major stumbling block with many of the earlier systems was combining a tabletop SEM with microanalysis or elemental analysis capabilities. A range of solutions are now available on the market, with some SEM companies providing their own, proprietary EDS hardware and software whilst others, readily interface with commercially available systems from major third-party manufacturers such as Bruker, Oxford and EDAX offering much greater expandability to meet users’ specific application requirements (Figure 6). The software from these third-party suppliers is much more developed and highly dependable given it is their specialty. Simply put, SEM manufacturers make great microscopes and EDS manufacturers make great elemental micro-analysis tools. Be cautious of simplified EDS software capabilities to make sure it does what you need.
Figure 6. Some Tabletop SEMs integrate smoothly with commercially available EDS systems. Left: Pseudo-colored SEM mineral map with EDS spectrum Right: Backscatter image and pseudo-colored hypermap showing elemental variation of particles.
9) What About Service?
Service is a major cost-of-ownership factor that can tilt the buying decision in favor of tabletop SEMs over conventional floor models routinely needing coverage under costly maintenance contracts. As the actual user, is it possible to perform trouble shooting on-site or through remote access?
How easy is it to service the apertures, column and filaments? Can all this service be done yourself or in-house, or will you have to pack up the system and ship it to a factory service center? This might mean being without the SEM system for 2 weeks a year simply for a filament exchange. For filaments that can be exchanged in your lab, do they include a pre-centered Wehnelt so that exchange can be done easily and quickly unders 10 minutes?
What about the electronics? Can circuit boards be effortlessly diagnosed and exchanged should something fail? Although failures are infrequent in contemporary electronics, circuit boards sporting LED status indicators that are rack mounted facilitate easy exchange even by the user keeping your service cost low. Although taking more volume to house, single function circuit boards are low cost compared to a large complex motherboard that is very expensive to replace for a simple failure of one function.
10) What’s the Projected Growth Path for Your Lab?
Finally, users should consider the future. Is this SEM going to be the only step in your analytical electron microscopy arsenal or does their laboratory need to shift to a full system or work along-side other available SEMs? If the later, then how far will an entry-level system without full-featured software take users? How easy it will be for users to make that change or move from platform to platform?
On the other hand, if an existing full-sized SEM is utilized near capacity, a Tabletop SEM system could reduce the workload and operation cost on those systems. Or is electron microscopy becoming so extensively used by the institution or facility that it needs to add a powerful, user-friendly system for more routine analysis work that is accessible directly in Undergraduate labs or by Engineers and technical staff closer to the factory floor?
Finally, does the organization presently send out much of its SEM work but requires something in-house for those rapid, routine analyzes? If so, then a tabletop SEM can provide an excellent solution.
As with all microscopy, the key to success is to match the instruments to the application. The question is invariably, “How good is good enough?” The next generation of advanced tabletop SEMs now provide much of the same functionality that was seen only in expensive floor model SEMs a few short years ago, but now in robust, easy-to-train and easy-to-use formats, and, at great prices that can really make the boss happy.
This information has been sourced, reviewed and adapted from materials provided by NanoImages, LLC.
For more information on this source, please visit NanoImages, LLC.