Study of Secondary Electron Image Contrast of Sample Surfaces Scanned by Focused Ion Beams

点击翻译按钮获取中文摘要

A scanning ion microscope (SIM) has been employed both by scanning a gallium (Ga) focused ion beam (FIB) on a sample surface and by detecting secondary electrons (SEs) emitted from the sample. Although the image formation mechanism present in the SIM is similar to that in a scanning electron microscope (SEM) there are some differences in image properties for materials, surface topography, grain contrast, and so on. In order to study the difference between STM and SEM images, a Monte Carlo simulation of ion-induced and electron-induced secondary electron emission was performed for 17 metals with atomic numbers, Z_2, of 4 -79 under Ga ion and electron bombardment with energies of several tens of and several keV. The SEs are produced in the SIM through collisions with three different collision partners : projectile ions, recoiled target atoms, and target electrons. The basic concept of our Monte Carlo model is to simulate all the trajectories of collision partners.The SE yield for Ga ion bombardment generally decreases with increasing Z_2, whereas the SE yield for electron bombardment increases. This explains the origin of the opposite trend in the material contrast between SIM and SEM. The lateral distribution of SEs escaped from the surface, which was much narrower for Ga ions than for electrons, indicates that the spatial resolution of SE images is better for SIM than SEM. For comparison between topographic contrasts in SIM and SEM, pseudo-images of semicircular rods with different radii were reconstructed from a line profile of the SE intensity obtained by the beam scan on the pattern. They revealed the topographic contrast for heavy metals is clearer in SIM than in SEM, whereas for light metals both contrasts are similar to each other. When scanning a step with large height and small wall angle, the bright area on the step caused by an edge effect was less for Ga ions than for electrons, so that the inclined wall is clearly observed in the SIM.