Delft University of Technology, Lorentzweg 1, 2628 CJ Delft, The Netherlands.
Ultramicroscopy. 2011 Jul;111(8):982-94. doi: 10.1016/j.ultramic.2011.03.019. Epub 2011 Apr 6.
Scanning ion microscopy has received a boost in the last decade, thanks to the development of novel ion sources employing light ions, like He(+), or ions from inert gases, like Ne(+) and Ar(+). Scanning ion images, however, might not be as easy to interpret as SEM micrographs. The contrast mechanisms are different, and there is always a certain degree of sample sputtering. The latter effect, on the one hand, prevents assessing the resolution on the basis of a single image, and, on the other hand, limits the probing time and thus the signal-to-noise ratio that can be obtained. In order to fully simulate what happens when energetic ions impact on a sample, a Monte Carlo approach is often used. In this paper, a different approach is proposed. The contrast is simulated using curves of secondary electron yields versus the incidence angle of the beam, while the surface modification prediction is based on similar curves for the sputtering yield. Finally, Poisson noise from primary ions and secondary electrons is added to the image. It is shown that the evaluation of an ion imaging tool cannot be condensed in a single number, like the spot size or the edge steepness, but must be based on a more complex analysis taking into account at least three parameters: sputtering, contrast and signal-to-noise ratio. It is also pointed out that noise contributions from the detector cannot be neglected for they can actually be the limiting factor in imaging with focused ion beams. While providing already good agreement with experimental data in some imaging aspects, the proposed approach is highly modular. Further effects, like edge enhancement and detection, can be added separately.
扫描离子显微镜在过去十年中得到了发展，这要归功于采用氦(+)或氖(+)和氩(+)等惰性气体离子的新型离子源的发展。然而，扫描离子图像可能不像 SEM 显微照片那样易于解释。对比机制不同，而且总会有一定程度的样品溅射。后一种效应一方面阻止了根据单个图像评估分辨率，另一方面限制了探测时间和因此可以获得的信噪比。为了充分模拟高能离子撞击样品时的情况，通常采用蒙特卡罗方法。在本文中，提出了一种不同的方法。使用二次电子产额与束入射角的曲线来模拟对比度，而溅射产额的类似曲线则用于表面改性预测。最后，将初级离子和二次电子的泊松噪声添加到图像中。结果表明，不能像光斑大小或边缘陡度那样将离子成像工具的评估归结为一个单一的数字，而必须基于更复杂的分析，至少考虑三个参数：溅射、对比度和信噪比。还指出，由于探测器的噪声贡献不能被忽略，因此它们实际上可能是聚焦离子束成像的限制因素。虽然在某些成像方面已经与实验数据很好地吻合，但所提出的方法具有高度的模块化。可以单独添加其他效果，如边缘增强和检测。
