On Low Voltage Scanning Electron Microscopy and Chemical Microanalysis.

The current status and general applicability of scanning electron microscopy (SEM) at low voltages is reviewed for both imaging (low voltage scanning electron microscopy, LVSEM) and chemical microanalysis (low voltage energy-dispersive X-ray spectrometry, LVEDX). With improved instrument performance low beam energies continue to have the expected advantages for the secondary electron imaging of low atomic number (Z) and electrically non-conducting samples. They also provide general improvements in the veracity of surface topographic analysis with conducting samples of all Z and at both low and high magnifications. In new experiments the backscattered electron (BSE) signal retains monotonic Z dependence to low voltages (<1 kV). This is contrary to long standing results in the prior literature and opens up fast chemical mapping with low dose and very high (nm-scale) spatial resolution. Similarly, energy-dispersive X-ray chemical microanalysis of bulk samples is extended to submicron, and in some cases to <0.1 µm, spatial resolution in three dimensions at voltages <5 kV. In favorable cases, such as the analysis of carbon overlayers at 1.5 kV, the thickness sensitivity for surface layers is extended to <2 nm, but the integrity of the sample surface is then of concern. At low beam energies (E(0)) the penetration range into the sample, and hence the X-ray escape path length out of it, is systematically restricted (R = F(E(0)(5/3))), with advantages for the accuracy or elimination of complex analysis-by-analysis matrix corrections for absorption (A) and fluorescence (F). The Z terms become more sensitive to E(0) but they require only one-time calibrations for each element. The new approach is to make the physics of the beam-specimen interactions the primary factor and to design enabling instrumentation accordingly.