Desktop scanning electron microscopy (SEM) fills a critical gap in imaging capability between convenient but relatively low-power light microscopy with useful magnification up to about 1000×, and high power but relatively difficult and expensive technologies such as transmission electron microscopy and atomic force or other scanning probe microscopies, with useful magnifications of 100,000× and above. Although conventional SEM also spans this gap in regard to useful magnification, its expense and operational requirements push it into the same category as the other high-power techniques in terms of practical utility. The latest generation of desktop SEMs (Phenom, FEI Co., Hillsboro, OR) offer magnification up to 20,000× with cost and convenience better than many research-grade light microscopes.
Desktop SEM offers a compelling value proposition to microscope users in at least three categories:
- SEM data users: Although they use SEM data regularly in their work, SEM data users do not own or have immediate access to an SEM in their laboratory or department. They may send samples to a centralized SEM laboratory or to an outside service laboratory. They cannot justify the expense—the capital expenditure for the instrument and the ongoing salary for specialized personnel—of conventional SEM.
- SEM owners: SEM owners own a conventional SEM but find that the desktop SEM meets the imaging requirements of many of their applications, providing faster results at lower cost. By redirecting those tasks to the desktop system, they can optimize the economic efficiency of their operation and provide faster response to their customers, internal or external.
- Light microscope users: Light microscope users need higher-power imaging to improve performance on existing tasks or to satisfy requirements of new applications.
At 20,000×, the Phenom can resolve features as small as 30 nm, which is an order of magnitude better than light microscopes. Its intuitive control interface is easier to use than many light microscopes, and operators with no experience can be productive with only a few minutes of training. The system’s vacuum technology allows rapid pump-down of the sample chamber, generating an image in as little as 30 sec. The entire system is not much larger than a light microscope and can be located on a benchtop in almost any laboratory environment. A rough calculation based on utilization of 40 hours per week yields a total cost of ownership of approximately $400 per week—not much more than the typical cost for one sample at an outside service laboratory. In addition to the cost benefit, users give equal importance to the immediacy and interactivity provided by a desktop SEM. Rather than wait days for a result, they see results in minutes and can use the information to guide the course of their work.
In addition to higher resolution than light microscopes, SEM imaging delivers much greater depth of field, allowing clearly focused images of three-dimensional structures and extreme surface topography. The intensity of the backscattered electron signal used to form images is strongly dependent on the average atomic number of the sample, providing strong material contrast that can be used to distinguish among features with different composition. The backscattered electron signal is also relatively insensitive to the effects of charge accumulation, permitting imaging of most insulating materials without having to first apply a conductive coating. Finally, the use of backscattered imaging also relaxes sample chamber vacuum requirements, increasing the system’s tolerance for wet, dirty, or outgassing samples.
Several elements of the system’s design are critical to its combination of high performance and small size:
- Electron column: The miniature electron column is only a few inches long and uses permanent magnets rather than the electromagnets found in a conventional SEM. Its light weight and short length increase its resonant frequency and thus reduce its sensitivity to interference from acoustic and mechanical vibrations found in a typical laboratory environment.
- Electron source: A thermionic electron source (LaB6) provides very good brightness at 5 kV, permitting high-contrast, low-noise imaging without excessive beam penetration that can degrade resolution and surface sensitivity. At the same time, it avoids the high vacuum needed for field emission sources, keeping system costs low and relaxing vacuum requirements in the sample chamber.
- Vacuum: A differentially pumped vacuum system maintains high, stable vacuum conditions in the source chamber while tolerating low vacuum levels in the sample chamber. Lower vacuum levels permit imaging of almost any sample without the elaborate drying, fixing, and coating preparations required for conventional SEM. A patented sample exchange mechanism enables pump-down times, from sample introduction to first image, in less than 30 sec.
- User interface: A highly simplified user interface, based on a touchscreen display and mouse-like controller, allows operators to become proficient with minimal training. The motorized stage and “never-lost” navigation provide fast, intuitive exploration of the sample.
Applications
Figure 1 - Comparison of light microscope image (a) at approximately 1000× and tabletop SEM image (b) at 14,000× showing dramatic improvement in resolution, contrast, and depth of field. (Note: All magnifications refer to original image.)
Figure 2 - Pill cross-section showing excipient core, active layer, and polymer coating.
Figure 3 - Commercial drug powder.
Figure 4 - Drug-coated stent.
Figure 5 - Bacteria on filter.
Figure 6 - Allergens.
Figures 1–6 illustrate several applications of desktop microscopy. Figure 1 compares light microscope and SEM images of diatoms often used as indicators of specific environmental conditions or activity. Figures 2–6 show applications from pharmaceutical and life sciences. SEM provides quick evaluation of structural characteristics in compounded drugs (Figures 2 and 3), such as pills and powders, accelerating product and process development and enabling tight control of manufacturing processes and product quality. In Figure 4, SEM is used to ensure the continuity of a drug-containing coating on an implantable stent. Figure 5 shows contaminating bacteria on a filter substrate. In Figure 6, SEM is used to detect the presence and type of allergens in the environment. Figures 7–9 are from materials science applications. Backscattered imaging of metals (Figure 7) readily distinguishes among the different phases of the material. Figure 8 shows silicon nanowires less than 60 nm in diam, and Figure 9 shows carbon nanotubes also less than 60 nm in diam.
Figure 7 - Backscattered electron images showing strong material contrast, allowing metallurgists to distinguish among phases with different compositions based on average atomic number.
Figure 8 - Silicon nanowires with diameters less than 60 nm.
Figure 9 - Carbon nanotubes with diameters less than 60 nm.
Dr. Berger is Senior Vice President, FEI Co., 5350 NE Dawson Creek Dr., Hillsboro, OR 97124, U.S.A.; tel.: 503-726-7500; fax: 503-726-2615; e-mail: [email protected].