High Resolution Imaging in the Field Emission Scanning Electron Microscope at Low Accelerating Voltage and with Energy-Filtration of the Electron Signals

For developing new technologies, it is important to characterize the microstructure of materials with high spatial resolution at the nanoscale. To achieve high resolution, field emission scanning electron microscopes (FE-SEM) were developed. These microscopes allow working at low accelerating voltage, below 5 kV, to take advantage of the reduction of the interaction volume with accelerating voltage (from 1 μm in Al at 10 kV to 10 nm at 1 kV). Furthermore, their higher gun brightness compared to conventional thermo-electronic emitters [1], allow a probe size at the nanoscale. However, technical problems arise when SEM operates at low kV, i.e., the source brightness decreases and the chromatic aberration increases, all SEM parameters being equal. Using deceleration mode minimizes these problems and further improvement is achieved by using a cold-field emitter, which has a smaller energy spread and providing the highest brightness and the smallest source size of a FE-SEM. O.C. Wells proposed another way to attain higher resolution by separating the high-resolution signals from the lowresolution signals during the detection [2-3]. At low accelerating voltage, the emission volume of backscatter (BSE) and secondary (SEII, emitted by BSEs) electrons signals approach that of SEI (emitted by the primary electrons) signals. However it is not enough to reach the highest resolution. The use of a magnetic field above the sample and probe deceleration allows the improvement of the spatial resolution by collecting mostly high-resolution signals. In addition, the energy-filtration of the electron signals allows selecting the type of contrast detected: surface topography, compositional, or crystallographic.