景深对HAADF-STEM原子分辨率三维重构的影响

Influence of depth of field on three-dimensional reconstruction structure of HAADF-STEM atomic-resolution tomography

扫描透射电子显微镜高角环形暗场像通常被认为是样品内部结构的线性投影,被广泛地应用于倾转系列三维重构之中。但是,在像差校正电镜中,当电子束的横向分辨率提升到亚埃尺度时,其纵向景深也将缩小至纳米尺度。当景深小于样品的厚度时,高角环形暗场像将从样品内部结构的线性投影转变为样品内某一深度的光学层析。本论文通过理论模拟,研究了纳米尺度景深下原子分辨率三维重构的可行性。研究发现,当景深小于样品厚度时,三维重构技术只能正确重构样品中的局部区域。该区域的大小与入射电子束景深呈正相关,位置与入射束的欠焦量有关。另外,研究还发现实际正确重构的区域相对于电子束名义聚焦位置偏上,即存在提前聚焦现象,其偏离程度与样品内原子的原子序数、会聚角以及加速电压有关。当原子序数越大或会聚角越大时,其与名义聚焦位置偏离越大。

High angle annular dark field scanning transmission electron microscopy images are generally considered as linear projections of sample’s internal structure,and are widely used in the tilt series tomography.The improved resolution of transmission electron microscopy makes the three-dimensional reconstruction at atomic resolution possible.Nevertheless,the high spatial resolution in aberration-corrected microscope also results in reducing the depth of field to nanometer scale.When the depth of field is smaller than the sample’s thickness,the high angle annular dark field scanning transmission electron microscopy images will transform from the linear projection of the sample’s internal structure to an optical sectioning at a certain depth within the sample.In the present study,the feasibility of the atomic resolution tomography is investigated using theoretical simulation referring to the depth of field.It is found that when the depth of field is smaller than the sample’s thickness,the tomography is still feasible,but only a local area in the sample can be correctly reconstructed.The size of this area is related to the depth of field of the electron probe,and its position is determined by the defocus of the convergent beam.In addition,it is found that the well reconstructed area is always above the nominal focused position,which can be influenced by the sample’s composition,the convergence angle and the accelerating voltage.This can be explained by the pre-focusing effect of the focused beam in sample.