High resolution dopant profiling in the SEM, image widths and surface band-bending

To study the mechanisms of dopant contrast in secondary electron (SE) imaging in the SEM, we have measured the image widths of a series of thin p-doped layers in Si, from 1 nm upwards. We have used computer modelling to simulate the effects of surface band-bending due to a realistic density of surface states on the Si, and we have also calculated the magnitude of the external patch fields. We have found a good correlation between the intensity widths and slopes of experimentally measured SE images of thin p-doped layers and the calculated widths and slopes of the energy distributions across these layers at a depth of 5-10 nm below the surface. This is consistent with the mean escape depth of SEs in Si being about 7 nm. We conclude that doping contrast in the SEM is mainly a function of bulk built-in voltages modified by surface band-bending effects within about 5-10 nm of the surface.