Department of Applied Science, Delft University of Technology, Lorentzweg 1, Delft, Zuid-holland 2628CJ, The Netherlands.
Microsc Microanal. 2011 Aug;17(4):624-36. doi: 10.1017/S1431927611000225. Epub 2011 Jun 16.
In recent years, novel ion sources have been designed and developed that have enabled focused ion beam machines to go beyond their use as nano-fabrication tools. Secondary electrons are usually taken to form images, for their yield is high and strongly dependent on the surface characteristics, in terms of chemical composition and topography. In particular, the secondary electron yield varies characteristically with the angle formed by the beam and the direction normal to the sample surface in the point of impact. Knowledge of this dependence, for different ion/atom pairs, is thus the first step toward a complete understanding of the contrast mechanism in scanning ion microscopy. In this article, experimentally obtained ion-induced secondary electron yields as a function of the incidence angle of the beam on flat surfaces of Al and Cr are reported, for usual conditions in Ga+ and He+ microscopes. The curves have been compared with models and simulations, showing a good agreement for most of the angle range; deviations from the expected behavior are addressed and explanations are suggested. It appears that the maximum value of the ion-induced secondary electron yield is very similar in all the studied cases; the yield range, however, is consistently larger for helium than for gallium, which partially explains the enhanced topographical contrast of helium microscopes over the gallium focused ion beams.
近年来，设计和开发了新型离子源，使聚焦离子束机超越了其作为纳米制造工具的用途。二次电子通常被用来形成图像，因为它们的产额很高，并且强烈依赖于表面特性，包括化学成分和形貌。特别是，二次电子产额随入射束与样品表面法线之间形成的角度而特征性地变化。对于不同的离子/原子对，了解这种依赖性是全面理解扫描离子显微镜对比度机制的第一步。在本文中，报道了在 Ga+和 He+显微镜的常用条件下，作为入射束对 Al 和 Cr 平面表面的入射角的函数，实验获得的离子诱导二次电子产额。将曲线与模型和模拟进行了比较，在大部分角度范围内都表现出很好的一致性；对偏离预期行为进行了讨论并提出了解释。似乎在所有研究的情况下，离子诱导的二次电子产额的最大值都非常相似；然而，氦的产额范围比镓大，这部分解释了氦显微镜相对于镓聚焦离子束的增强形貌对比度。
