The information gained from scanning electron microscopy can give you more than just an image. Michael Dixon takes a look at the advances in field emission SEM
The information gained from scanning electron microscopy can give you more than just an image. Michael Dixon takes a look at the advances in field emission SEM
The field emission scanning electron microscope (FESEM) is a well-established tool for high resolution imaging in the fields of semiconductors, materials and life sciences. Nanotechnology has strongly driven the development of recent FE-SEMs, with demands not only for increasing resolution but also for more information from the sample. The result is that the FESEM is evolving into a tool for compositional and structural imaging as well as high resolution. With the development of different electron imaging techniques, information can be easily derived on surface morphology, compositional makeup and internal structure. Two of the key areas of development have been:
• Advances in secondary electron/backscattered electron (SE/BSE) detector technology
• SEM-based scanning transmission electron microscope (STEM) imaging
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| Figure 1: Secondary electron detection with EXB filter and upper detector |
Hitachi’s latest in-lens SE/BSE detectors are designed to offer maximum flexibility with energy filtering, and operate throughout the whole kV range. Utilising the latest version of the patented ExB detection system1 an electron detector is positioned above the objective lens and perpendicular electrostatic and magnetic fields are applied. By setting these fields appropriately, the beam experiences no deflection from the axis on its way through to the sample. However, for electrons passing back up through the lens, the electrostatic force and the magnetic force are additive with the result that the electrons are guided towards the detector. This configuration not only significantly improves collection efficiency and signal-to-noise ratio in the images, but allows different signal combinations to be collected, giving a wealth of different information from the sample. The basic configuration is shown in Figure 1, where imaging is due purely to secondary electrons. This gives topographic detail with high surface sensitivity and can be used with beam energies as low as 100 kV.
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| Figure 2: Pure secondary electron image of titanium, barium tri-oxide, 100KX original magnification. (Sample courtesy of Dr. Tamaki, Ritsumeikan University) |
Examples of imaging this way are shown in Figures 2 and 3. Titanium, barium tri-oxides (TiBaO3) are well known materials used in nonvolatile memory. The sample here is studied for the production of a ferro-electric film. At 3kV, 800,000x images reveal the size and distribution of fine particles.
The detector configuration can also be used for angle selective backscattered imaging, by allowing either low angle or high angle backscattered electrons to be collected. Figure 4 shows the introduction of low angle backscattered electrons. Adjusting the filter to allow a combination of SE and low angle
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| Figure 3: Pure secondary electron image of titanium, barium tri-oxide, 800KX original magnification. (Sample courtesy of Dr. Tamaki, Ritsumeikan University) |
BSE information to the detector can provide images with compositional contrast and surface detail. The ratio of topographical and compositional information can be adjusted. This mode enables BSE imaging even at very low accelerating voltages with excellent surface sensitivity – something which is not possible with conventional scintillator or solid state BSE detectors (e.g. BSE imaging at 500V).
Figures 5 and 6 show images from a highly insulating PTFE sample. The charging effect often associated with imaging such highly insulating materials (Figure 5) can be easily eliminated by adjustment of the detector (Figure 6). Here the contribution from the lowest energy secondary electrons is reduced (i.e. those most susceptible to the surface charge) enabling an image free of charge-artefact to be produced from electrons of slightly higher energy.
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| Figure 4: Secondary and low angle backscattered electron detection |
Immuno-gold labeling is a technique that allows us to understand the distribution of specific proteins on cell surfaces. In Figures 7 and 8, the beta 3 antigen has reacted with antibodies that are marked by 10 nm colloidal gold particles on an activated human blood platelet. Gold label imaging can now be performed at much lower accelerating voltages by collecting SE and low angle BSE. This gives surface sensitive imaging ensuring that location of the gold and surface morphology can be associated.
High angle BSEs give almost pure composition information and this can now be performed with beam energies down to 100V. Figure 9 shows compositional information derived from high angle BSE imaging in a MEMS application. The technique provides excellent delineation of the Si-based layer structure, permitting accurate assessment and measurement of the different components.
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| Figure 5: SE/ LA BSE image of PTFE showing charging |
Although the examples shown here have been imaged at low beam energies, angle selective BSE imaging can be carried out at any accelerating voltage offering full flexibilty to have pure SE (topography), adjustable low angle SE/BSE (for topographical and compositional information or elimination of charge artefact) and high angle BSE (composition only) under all conditions.
Transmission electron imaging is conventionally performed in dedicated transmission electron microscopes and it has
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| Figure 6: Increasing proportion of LA BSE eliminates charging effect |
become a powerful tool for many biologists and material scientists. Transmission electron imaging, however, can also be performed in the SEM - providing the sample can be made sufficiently thin e.g. 200nm. Fig 10 shows a basic schematic of transmission imaging in the SEM – for brightfield imaging a simple electron detector is placed beneath the specimen. For darkfield imaging an annular electron detector is placed beneath the specimen to collect electrons scattered by larger angles, ignoring those which are not scattered or scattered by low angles.
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| Figure 7: Gold immuno labeling of human platelet ( 50 Kx original magnification) |
SEM-based STEM has become important for a variety of reasons. Firstly, other enabling technologies like FIB have made it quick and easy to prepare thin-sections of bulk materials. Secondly, research into new nano-materials has demanded high throughput but detailed structural investigation i.e. not just near-surface information from SE and BSE. In addition, the increased use of carbon-based nano-materials has led to an understanding that high beam energy is not always required, since light C-based material is easily penetrated by less energetic electrons.
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| Figure 8: Gold immuno labeling of human platelet (100 Kx original magnification) |
STEM imaging of thin specimens also has some potential resolution advantages over SE imaging of thicker specimens as usually performed in the SEM since the broadening effect of the interaction volume is reduced. With the latest ultra-high resolution SEMs able to provide resolution better than 4Å, the capability of SEM-based STEM has moved closer to that of TEM.
SEM-based STEM is particularly well suited to ultra-high-resolution work for fundamental materials research such as carbon nanotubes and mesoporous silica. For these
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| Figure 9: High angle BSE detection |
types of materials, the latest FE-SEMs can approach the resolution of TEM, and in many cases provide significantly better contrast. Mesoporous silica is used in a wide variety of chemical systems, including catalysis, gas adsorption, biological cell labelling and enzyme distribution. Observation of surface morphology and pore diameter are critical factors in tailoring the material to the required usage and highlight the high resolution capabilities in both imaging modes. The technique is also extremely useful for light / low density materials such as wax and resins where the high accelerating voltages of the TEM gives insufficient contrast but lower kV STEM imaging gives clear delineation of two materials with very similar density. In the life sciences, high resolution imaging of biological sections and viruses is also now possible, which was previously only carried out in the TEM.
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| Figure 10: Ultra high resolution SE image of mesoporous silica (500 KX original magnification). (Courtesy Professor John et.al, Tulane University, Louisiana, USA) |
The latest STEM detectors in Hitachi SEMs also allow the angle of collection to be varied and the image to be optimised for the required contrast. The ideal collection angle for dark field imaging depends upon the information required, sample material and sample thickness. Adjustable DF detectors provide flexibility for a variety of contrast mechanisms and specimens. Figure 13 shows the effect of collection angle on Z contrast on a semiconductor sample – in this case the collection angle can be small to give crystallographic diffraction imaging or larger to give atomic number contrast imaging. This type of adjustability in collection angle is something that, until now, was only seen in TEM.
In summary, SEM-based STEM imaging is developing an increasingly wide range of applications in material and biological sciences. While the technique should not be seen as a replacement for high energy TEM for all applications (due to its lower resolution and lower penetration capabilities) it nonetheless has its own strengths, such as imaging of low density materials and the ability to relate surface and bulk properties as well as other benefits such as cost and ease of use.
Reference:
1. Sato, M et al: Proc. SPIE, 2014,17, 1993
Michael Dixon. Michael has been working with Hitachi for 10 years on the application of electron microscope technology to a variety of disciplines.
