MARL has a history of scanning electron microscopy dating back to 1971. In the more than 40 years since, it has seen revolutionary developments in instrumentation and techniques leading to the present equipment.
The SEM is extremely useful for its range of magnification and depth of field compared to a light microscope. The Quanta is specified for 1 nm resolution allowing for magnifications of hundreds of thousands of times. Often just as important as the extremely high magnification is the large depth-of-field at low magnification that enables researchers to view their material in a seemingly three-dimensional fashion.
In addition to structural information, a rich variety of phenomena occur as the electrons of the primary beam of the SEM interact with the specimen surface. Secondary electrons (SE) are often used to produce high-resolution images which highlight topography. Backscattered electrons (BSE) also reveal topography, but are particularly sensitive to the effective atomic number of the phase under the beam thus revealing compositional differences. Characteristic x-rays from the elements in the sample allow investigators to determine the elemental composition of phases as small as a few hundred nm.
FEI Quanta 250 FE-SEM
This is a field-emission SEM offering a maximum resolution on the order of 1.0 nm. It is capable of operating in high vacuum for conductive samples. It is capable of variable pressure and extended pressure (i.e., environmental) modes (up to 20 Torr) for observation of non-conductive 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. In addition the following options have been installed on the SEM:
- beam deceleration option enables adjustment of the landing energy of the electron beam on the sample for conductive samples
- a concentric backscattered (CBS) detector is available to enhance the performance of the beam deceleration option and improve the imaging capabilities at low voltage (<5kV)
- peltier stage with moderate heating and cooling capabilities (-25C to 55C)
- heating stage capable of temperatures up to 1000C
- EDS system for analysis of elements as low as Boron (Oxford Aztec)
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, a FEI Quanta 250, with high vacuum, variable pressure, and extended pressure modes of operation.