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

利用扫描离子显微镜（SIM）对样品表面的镓（Ga）聚焦离子束（FIB）进行扫描，并对样品发射的二次电子（SEs）进行检测。虽然SIM中的成像机制与扫描电子显微镜（SEM）中的成像机制相似，但在材料的图像特性、表面形貌、晶粒对比度等方面存在一些差异。为了研究STM和SEM图像的差异，对17种原子序数Z_2为4 -79的金属在能量为几十keV和几keV的Ga离子和电子轰击下的离子诱导和电子诱导二次电子发射进行了蒙特卡罗模拟。在SIM中，SEs通过与三种不同的碰撞伙伴：抛射离子、反冲目标原子和目标电子碰撞而产生。我们的蒙特卡罗模型的基本概念是模拟所有碰撞伙伴的轨迹。随着Z_2的增加，镓离子轰击的SE产率普遍降低，而电子轰击的SE产率则增加。这就解释了SIM和SEM材料对比相反趋势的起源。从表面逸出的SE的横向分布（Ga离子比电子窄得多）表明，SEM图像的空间分辨率优于SIM图像。为了比较SIM和SEM的地形对比，根据波束扫描获得的SE强度线轮廓重建了具有不同半径的半圆形棒的伪图像。他们发现重金属的地形对比在SIM中比在SEM中更清晰，而对于轻金属，两种对比彼此相似。当扫描高度大、壁角小的台阶时，由于边缘效应导致的台阶上的亮区对镓离子的影响小于对电子的影响，因此在SIM中可以清晰地观察到倾斜壁。

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.