Materials Analysis Research Laboratory - Scanning Electron Microscopy
MARL became involved in scanning electron microscopy in 1971 with the acquisition of a JEOL JSM-U3 SEM through a National Science Foundation grant. In March 1987, a second SEM, a JEOL JSM-840A, was acquired. In 1993, a Hitachi S-2460N "variable-pressure" SEM was installed for analysis of very large samples and those samples not lending themselves to the high vacuum of a conventional SEM. In 2010, MARL acquired its first field-emission SEM, an FEI Quanta 250, with high vacuum, variable pressure, and extended pressure modes of operation.
The SEM is extremely useful in the characterization of materials for its range of magnification and depth of field. Our microscopes are both specified for 4 nm resolution allowing for magnifications of tens of thousands of times. Often just as important as the extremely high magnification is the large depth-of-field that enables researchers to view their material in a seemingly three-dimensional fashion. This feature allows imaging of surface morphology, inclusions, boundary interfaces, and general characterization of a specimen surface .
Another advantage of the SEM is the ability to examine specimens at magnifications as low as 10x. The accompanying micrographs show that SEM imaging provides extremely revealing information not attainable by conventional light microscopy.
In addition to structural information, a rich variety of phenomena occur as the primary electron beam of the SEM interacts with the specimen surface. Secondary electrons (SE) are often used to produce high-resolution images which highlight the three-dimensional nature of most samples. Backscattered electrons (BSE) also reveal topography, but are also particularly sensitive to the effective atomic number of the phase under the beam. The BSE signal thus shows up compositional differences. Characteristic x-rays from the elements in the sample allow investigators to determine the elemental composition of phases as small as 1 um.
This unit is a field emission SEM offering a maximum resolution on the order of 1.2 nm. It is capable of operating in high vacuum, variable pressure, and extended pressure modes for observation of nonconducting or moist samples. It is equipped with conventional secondary and backscattered electron detectors but also has two detectors for collecting secondary electrons in variable and extended pressure modes. It has a motorized stage with moderate heating and cooling capabilities, and it is equipped with a light-element x-ray detector and an IXRF Systems energy-dispersive x-ray system.
This is a "variable pressure" microscope capable of operating at pressures up to 270 Pa (2 torr) for the observation of moist, oily, or non-conductive samples. Most samples can be examined at 40 Pa (0.3 torr) in a helium atmosphere with good resolution. It can also be operated in high-vacuum mode for high resolution work.
It is equipped with a standard secondary electron detector (for use at high vacuum), both Robinson and Link Tetra backscattered electron detectors, and a Deben motorized stage. It has an Oxford Instruments "Isis" energy-dispersive x-ray system with a light-element detector. In addition, image collection and archiving is provided through a Quartz PCI passive imaging system.
Both microscope systems have software for qualitative and quantitative x-ray analysis, digital imaging, x-ray maps, and image analysis.