Ultra-low voltage scanning electron microscopy

Abstract

Although the benefits of operating the scanning electron microscope at low beam energies have been evident since the earliest days of the instrument, the successful utilization of the SEM under these conditions has required the development of high brightness field emission electron source, advanced lenses, and clean vacuums. As these technologies became available the level at which imaging became regarded as “low energy” has fallen from 10keV, first to 5keV, and more recently to 1keV. At this energy state of the art instruments can now provide an excellent balance between resolution – which becomes worse with decreasing energy – and desirable goals such as the minimization of sample charging and the reduction of macroscopic radiation damage – which tend to become more challenging as the energy is increased.An interesting new opportunity is to perform imaging in the ultra-low energy region between leV and 500eV. Over this energy range significant changes in the details of electron-solid interactions take place offering the chance of novel contrast modes, and the rapid fall in the electron beam range leads to the condition where the penetration of the incident beam into the sample is effectively limited to 1 or 2 nanometers. The practical problem is that of achieving useful levels of resolution and acceptable signal to noise ratios in the image. At energies below IkeV chromatic aberration dominates the probe formation in conventional instruments even when using an FEG source. However, the use of optimized retarding field optics essentially maintains chromatic aberration independent of landing energy down to very low values. Figure (1) shows an example of the performance that can be achieved on a commercial instrument – an Hitachi S-4500 – modified to operate in this mode, in this case at 50eV landing energy. The resolution of the image is judged from edge sharpness and detail to be significantly better than 0.1µm and, from experimental observation, this performance is apparently limited by residual astigmatism caused by uncorrected sample charging rather than by fundamental aberrations in the probe forming optics. Comparable, if somewhat lower resolution, images have been achieved on this, and other FEG SEMs, at energies as low as leV.