Scanning Electron Microscopy

REL is pleased to announce the addition of scanning electron microscopy to its service offerings. Scanning electron microscope (SEM) images have a large depth of field and are high resolution, which allows for greater clarity and magnification of closely-spaced, complex features.

The scanning electron microscope (SEM) is a type of electron microscope capable of producing high-resolution images of a sample surface. In SEM, the image of the surface morphology in a specimen as well as the information of the surface composition can be identified by reading the scattered electrons and the electromagnetic radiation, which is produced during the interaction between the projected electron beam and the specimens. Due to the manner in which the image is created, SEM images have a characteristic 3-dimensional appearance and are useful for judging the surface structure of the sample. Normally, the optimization among the resolution, the image quality and the depth of field in a SEM has to be considered because of the interrelationship among them. They rely on the interaction volume in the specimen, which is functional to the beam accelerating voltage, the spot size of the electron beam and the working distance as well as the properties of the specimen such as atomic number and density.

The JEOL JSM-820 is a research grade tungsten source scanning electron microscope that is coupled with a Beryllium window energy dispersive x-ray spectrometer (EDS). It provides some convenient and unique ways to achieve the desired images with the optimized combination of the resolution, the image quality and the depth of field. The 4Pi microanalysis package associated with the system can be easily used to perform surface morphology and composition studies on a wide variety of organic and inorganic materials. Images can be recorded with digital imaging system with subsequent output to a high resolution photo printer or various picture formats.

Detection of Secondary Electrons

The most common imaging mode monitors low energy secondary electrons (SE), which are generated during the interaction between the projected electron beam and the specimen. Due to their low energy, these electrons originate within a few nanometers from the surface. The electrons are detected by a scintillator-photomultiplier device and the resulting signal is rendered into a two-dimensional intensity distribution that can be viewed and saved as a digital image. The brightness of the signal depends on the number of the escaped secondary electrons from the specimen. Thus a sharp edge or boundary can always be easily detected due to the 'escape' effect, which results in images with a well-defined, three-dimensional appearance. Using this technique, resolutions less than 1 nm are possible. Example images are shown in Fig.2.

Fig. 1. SE image of alumina spheres (left) and the surface structure of an alumina sphere(right)

Detection of Backscattered Electrons

Backscattered electrons (BSE) consist of high-energy electrons reflected or back-scattered out of the specimen interaction volume. Backscattered electrons can be used to detect contrast between areas with different chemical compositions since the brightness of the BSE image tends to increase with the atomic number.

Fig. 2. Comparison of SE Image (left) and BSE Image (right) of Alumina fiber balls infultrated with AA5083.Alumina fiber is the dark phase in the right image. Bright phases in the right image are the precipitates containing elements of larger atomic number. Boundary between the alumina fibers and the AA5083 is clearly identified.

Fig. 3. Comparison of SE Image (left) and BSE Image (right) from a composite beneath the machined surface. Broken SiC particles under machine are detected.

X-ray Microanalysis

X-rays, which are also produced by the interaction of electrons with the sample, may be detected in an SEM equipped for energy-dispersive X-ray spectroscopy (EDS) or wavelength dispersive X-ray spectroscopy (WDS). Characteristic X-rays can be collected from as small as a cubic micrometer volume of the specimen. Elements from atomic number 5(B) to 92(U) can be detected in concentrations above 0.001wt%. Analysis modes include qualitative, standardless, and fully rigorous standards-based quantitative analysis. Additional capabilities of 4Pi software include analog and digital x-ray mapping and report generation. Both analog and digital x-ray/compositional mapping are available.

Fig. 4. X-ray EDS compositional map on a composite. Different colors indicate different compositions: Red phase--- Matrix alloy; Green phase--- Ceramic particles. Additionally, the composition from selected area can be identified easily from the map, shown in the bottom right image. (EDS image mode)

Fig. 5. Compositional identification on a selected area by EDS analysis. (EDS image mode).

Fig. 6. Crack analysis of a tested composite material. The material inside the crack and on the banks of the crack have been identified with EDS analysis.

Others

(i) Area Fraction

(ii) Particle Size

Summaries

Imaging modes of JEOL JSM-820 include secondary electrons, backscattered electrons and X-rays. Under ideal conditions, the resolution in secondary electron imaging mode is less than 5nm. Surface structures of a wide variety of materials include both morphology and composition. Also the corresponding analysis can be identified with this SEM.

Contact REL, inc. for all of your SEM and EDS needs. Their knowledgeable metallurgists and staff will help you understand the microstructural features of your materials. REL’s cost effective, fast turnaround service will allow you to quickly make decisions on process changes to speed up the product development cycle.