Understanding the properties of new materials prior to fabrication can be challenging. Often, the structure of a material at very small scales is critical to its properties and performance. Visualization technologies continue to improve, allowing scientists to investigate solid materials at scales closer and closer to the scale of individual atoms. One of the most exceptional aspects of scanning electron microscopy is the ability to investigate relatively large samples, such as those that can be held in the hand, with nanometer-scale resolution. Enhancements in electron optics over the past decade have produced phenomenal improvements in secondary electron imaging, which is vital for imaging of surface morphology. More recently, developments in backscattered electron detectors have produced similar improvements in qualitative compositional imaging.
Earlier-generation backscattered electron detectors typically had a resolution of about 200 nm. Thus, even though scanning electron microscopes were capable of much higher resolution using secondary electron imaging, backscattered electron detectors were not able to provide complementary compositional images. However, the latest-generation electron backscattered detectors, coupled with new electron optics, yield compositional images with much higher resolutions. These new detectors produce qualitative compositional maps with nearly the same spatial resolution as secondary electron images, and provide a means of targeting the electron beam for X-ray spectrometer measurements for quantitative analysis.
One such instrument is the low-angle backscattered electron (LABE) detector offered by JEOL USA, Inc. (Peabody, MA),1 which is an option on the company’s field emission scanning electron microscopes (FESEMs). An example of an interesting material that can be imaged with the LABE detector is a new type of wound care dressing composed of silver-coated fibers. Silver has gained renewed interest as an antimicrobial weapon, especially with the overuse of antibiotics. The dressings use silver nanoparticle coatings on fibers to provide antimicrobial activity.
All scanning electron microscopes (SEMs) have features in common, though they vary in their degree of complexity. The common parts include a source of electrons with a means to accelerate the electrons toward the sample, a lens system to focus the electrons into a beam, an aperture to refine the beam further, a lens to focus the beam to a fine probe, scanning coils to scan the beam over the sample, and detectors to detect the signals generated by the interaction between the electron beam and the sample. Images are formed by scanning the beam over the sample, like reading the lines of a book, and collecting the electron signals generated in synchronized fashion with the scanning. Two basic types of electron signals are commonly detected: secondary electrons and backscattered electrons. It is important to understand the difference between the two types because images produced by each one present different information about the sample. Ideally, the greatest information about a sample is available when images from both signal types are acquired with exactly the same conditions and compared with matched image sets acquired at different beam conditions. Following is a brief explanation of each type of image.
Secondary electron images
Secondary electrons are generated when the electron beam from the SEM strikes the sample and ionizes the atoms of the sample. This interaction depth is very shallow, with most of the detected secondary electrons originating from very near the surface of the sample. Thus, secondary electron images (SEIs) provide images of a sample’s surface morphology. The energy of secondary electrons is relatively low. As a result, if the sample is a poor electrical conductor, then electrons from the beam begin to accumulate around the location of the electron beam’s contact with the sample; this is commonly referred to as “charging.” Charging degrades the image significantly because it can deflect the beam and disturb collection of the secondary electrons by the secondary electron detector. Brightness and contrast of secondary images is primarily a function of the electron flux or dose of the beam, and the sample morphology relative to the secondary electron detector. The ultimate resolving power or the ability of the microscope to distinguish the distance between two very closely spaced objects is primarily a function of the diameter of the beam and the ability of the microscope to raster the beam very precisely at small distances. In both cases, smaller beam diameters and rasters result in higher magnification and image resolution.2
Backscattered electron images (BEI)
Backscattered electrons are electrons from the SEM beam that strike the atoms of the sample and are ejected back out of the sample. This interaction occurs at deeper depths in the sample than secondary electron generation. One of the most important aspects about the production of backscattered electrons is that the number of electrons backscattered is proportional to the atomic number of the atoms comprising the sample. Higher-atomic-number elements have larger effective diameters for backscattering and therefore bounce back a larger number of beam electrons than lower-atomic-number elements. Thus, typical compositional mode backscattered electron images are actually qualitative compositional maps. Portions of a backscattered electron image that are bright have a higher atomic number than darker portions of the image.2
Image resolution and quality
Although the quality of SEM images is dependent on the quality of the detectors used, there are aspects of SEMs that affect their overall spatial resolving capability and the quality of images they can produce. Several of these factors do not depend on the qualities of the detectors used. These include magnification and scanning stability, that is, the ability to focus an electron beam with a small diameter, and the ability to produce an electron beam with a high electron flux, or bright beam. SEM manufacturers have been improving these microscope functions for decades, and certainly these improvements have been especially visible in secondary electron imaging capabilities. However, improving the imaging of backscattered electrons has been a more challenging task primarily due to the physics involved.
Before discussing backscattered electron imaging, a few definitions pertinent to imaging overall are necessary. Magnification is the ratio of the horizontal scan or raster distance of the SEM’s imaging window (cathode ray tube monitor on old SEMs, but simply an imaging window of the software interface of modern SEMs) divided by the horizontal distance of the beam scan or raster. With recent technology, the beam can be scanned to finer and finer distances. The latest-generation FESEMs routinely support magnifications of one million times with the ability to resolve the distance between two objects spaced at about 1 nm (1 × 10–9 m). The distance between individual atoms is only another factor of 10 smaller. Rastering a tiny diameter beam can be accomplished repeatedly and stably. This requires high precision of the scanning coils. The ability of the scan coils to precisely scan the beam very slowly while the detectors concurrently sample the generated electron signal a sufficient number of times per scan to produce an image with a large number of pixels results in a high-resolution image.
In order to take the greatest advantage of the extremely fine scanning capability, one requires an extremely fine probe. Therefore, the ability to focus a beam with a very fine diameter onto the sample will produce an image with greater spatial resolution than one with a much larger beam diameter. Also, accomplishing this at lower beam energies, or accelerating voltages, whereby the interaction volume is relatively closer to the surface, produces even more clarity because the secondary and backscattered electrons interact with less sample material. At lower beam energies, the smaller interaction volume means that the resolution of backscattered images is much closer to the resolution of secondary electrons.3 However, since the number of generated signal electrons is smaller at lower energies, actually visualizing both signals at nearly the same resolution required the invention of new electron optics. The new optics are essential not only for high-resolution secondary electron imaging, but also for high-resolution backscattered electron imaging.
Better imaging facilitated by key new technologies
Historically, secondary electron imaging has been capable of 10–100× better resolution than backscattered electron imaging. A major reason for this is that secondary electrons are lower-energy electrons and can be attracted to a conventional secondary electron detector. In contrast, the higher-energy backscattered electrons cannot be similarly attracted, so that once one produces a very fine beam and rasters it over smaller distances, the overall number of backscattered electrons captured is too low to be useful.
A recent advance in the latest FESEM electron optics is the semi-in-lens objective lens, which focuses the beam to a sharp spot. The magnetic field of the semi-in-lens actually extends below the pole piece onto the sample surface.2 The presence of the magnetic field around the area of observation assists with collection of the secondary and backscattered electrons. Electrons from the sample follow spiral paths in the magnetic field on their way to a detector. Secondary electrons, which are low energy, spiral with a small, tight radius, whereas higher-energy backscattered electrons spiral with a larger radius.4 This collection process provides enough signal boost so that the FESEM can be optimized for the finest resolution using low accelerating voltage, or beam energy, and low probe current to avoid charging and damaging the sample, which is most desirable for realworld nanometer-scale samples. The physics of the beam and sample interaction means that low-energy/low-current operating conditions produce smaller interaction volumes closer to the sample surface, and provide the best resolution, but reduce the number of signal electrons to the sample. To accommodate this, in-lens detectors utilize shorter working distances than conventional SEMs.