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. In addition to the ability to image a comparatively large area of the specimen, the SEM can be equipped with a variety of analytical techniques to measure 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 scanning area of the SEM. AnEDS detector mounted in the SEM chamber collects and separates characteristic X-rays of different elements in 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 represented as a plot of X-ray counts versus energy (in keV). The 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 composition maps of elements over a much broader raster area. Together, these capabilities provide critical compositional information for a wide variety of materials.
The ZEISS Field Emission (FE) SEM SUPRA™ 40 is located in a dedicated lab at the CNR-IOM Institute. The microscope is equipped with a conventional Everhart-Thornley secondary electrons detector and with thelast 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 thermionic 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 into 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 is also installed on the microscope, 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.
AZtecOne Energy Dispersive X-ray Spectroscopy (EDS) system with a 10 mm2 x-act Silicon Drift Detector (SDD) is attached to the microscope enabling chemical analysis of the examined samples to be performed. Aztec EDS provides high accuracy at a high data acquisition rate. It allows:
Software is available to perform quantitative analysis, digital imaging, line-scan x-ray profiles, multiple element x-ray maps, image analysis with:
A ZEISS 4-Channel annular Scanning Transmission Electron Microscopy (aSTEM) detector, used in high angle annular dark-field (HAADF) mode, enables the detection of transmitted electrons with atomic number contrast, in this method the regions with a higher atomic number appear brighter due to their higher angle scattering of the electron beam.