In a Scanning Electron Microscope (SEM), a beam is scanned over the sample surface in a raster pattern while a signal from secondary electrons (SE) or Back-scattered electrons (BSE) is recorded by specific electron detectors. The electron beam, which typically has an energy ranging from a few hundred eV up to 40 keV, is focused to a spot of about 0.4 nm to 5 nm in diameter. Latest generation SEMs indeed can achieve a resolution of 0.4 nm at 30 kV and 0.9 nm at 1 kV.
Beyond the ability to image a comparatively large area of the specimen, SEM can be equipped with a variety of analytical techniques for measuring the composition, crystallographic phase distribution and local texture of the specimen. Chemical composition analysis can be performed by Energy Dispersive X-ray Spectroscopy (EDS) which relies on the generation of an X-ray spectrum from the entire scan area of the SEM. An EDS detector mounted in the SEM chamber collects and separates the characteristic X-rays of different elements into an energy spectrum and EDS system software is used to analyze the energy spectrum in order to determine the abundance of specific elements. A typical EDS spectrum is portrayed as a plot of X-ray counts vs. energy (in keV). Energy peaks correspond to the various elements in the sample. EDS can be used to find the chemical composition of materials down to a spot size of a few microns and to create element composition maps over a much broader raster area. Together, these capabilities provide fundamental compositional information for a wide variety of materials.
Within the NFFA’s project activities, a ZEISS Field Emission (FE) SEM Supra 40, located in a dedicated lab at the CNR-IOM Institute, is made available to access. The microscope is equipped with a conventional Everhart-Thornley secondary electrons detector and with a last generation Gemini column featuring the innovative so-called beam booster and an objective lens that consists of a combined electrostatic/electromagnetic lens doublet. A Schottky field emitter serves as gun. Electrons are emitted from the heated filament while an electrical field is excited by applying the extractor (Uext) voltage. To suppress unwanted thermoionic emission from the shank of the Schottky field emitter, a suppressor voltage (Usup) is applied as well. The emitted electrons are accelerated by the acceleration voltage (Ueht). The beam booster (Ub, booster voltage), which is always at a high potential when the acceleration voltage is at most 20kV, is integrated directly after the anode. This guarantees that the energy of the electrons in the entire beam path is always much higher than the set acceleration voltage. This considerably reduces the sensitivity of the electron beam to magnetic stray fields and minimizes the beam broadening. The great advantages of such a design are superb resolution even at ultra low voltages with an increased signal to noise ratio. Thanks to the special Gemini’s lens shape minimizing the magnetic field at the specimen, high resolution imaging of dia-, para, or ferromagnetic materials is possible with very short working distances.
A high efficiency In-lens detector for high resolution SE imaging is integrated in the GEMINI column above the objective lens. Low energy secondary electrons are intercepted at the point of impact by the weak magnetic field at the specimen surface, then accelerated in the booster column and focused on the In-lens above the objective lens thus yielding images with a signal to noise ratio increased by a factor of 2-3 compared to conventional Everhart-Thornley detector.
A second In-column detector will be also installed on the microscope (scheduled on July 2015) enabling energy- and angle-selective simultaneous detection of backscattered electrons (BSE) positioned directly above the In-lens SE detector allowing for channelling contrast (crystal orientation), as well as compositional contrast.
An EDAX X-ray detection system is currently annexed to the microscope allowing to perform chemical analysis of the investigated samples which will be soon (commissioning scheduled on July 2015) replaced by a latest generation Oxford X-ray detection system – Aztec EDS, providing a high-degree accuracy at a fast data acquisition speed. The Oxford detector is a LN2-free X-Act Silicon Drift Detector with 10 mm2 active area and a Super Atmosphere Thin Window (SATW) Light Element Window allowing for:
Software is provided for performing quantitative analysis, digital imaging, line-scan x-ray profiles, multiple element x-ray maps, image analysis with: