Application Example 4: Nano Inclusions in Co-Hardened Gold Plating for Electronic Applications – Further Evidence for High Lateral Resolution in Low-Voltage, High-Angle BSE Imaging

Abstract

Through Application Example 3, it has been demonstrated clearly that high-angle BSEs detected for imaging are generated only from the outermost surface regions, less than 2 nm, even for alumina film with a relatively low mean Z and density at the accelerating voltage of 1.50 kV. For other metals of industrial importance, such as Fe, Co, Ni, Cu, Zn, Au, Pd, etc., where the atomic numbers are significantly higher than that for aluminum oxide, it is expected that, at similar accelerating voltages, high-angle BSEs are generated from even smaller depths of ~1 nm or less. In such situations, the interaction volumes for high-angle BSEs become comparable to, or even smaller than, the diameter of the primary electron beam, ~1.2 nm at 1 kV for the present instrument. Thus, the lateral resolution comparable to, or even higher than, that of in-lens SE imaging is expected for high-angle BSE imaging, although the information included in respective images is of different nature. An excellent resolution in low-voltage, high-angle BSE imaging has already been demonstrated in Application Example 1, where Pb particles of sizes ~5 nm have been revealed successfully at an accelerating voltage of 1.64 kV. Here, a further example is presented to show that the lateral resolution in low-voltage, high-angle BSE imaging at similar accelerating voltages could be even higher than that of in-lens SE imaging; this is quite contrary to what most of the SEM users generally believe. However, it is readily understood that the situation here is entirely different from the cases where samples are examined at high accelerating voltages of over 10 kV.