Spatial resolution of strain state analysis by convergent nano beam electron diffraction

发展亚纳米空间分辨率应变测量方法对材料科学和工业微电子生产具有重要意义。局部应变场影响半导体纳米结构的力学和电子性能，并控制外延生长的动力学过程。对于会聚纳米束电子衍射，用直径在亚纳米范围内的聚焦电子束扫描样品表面。这导致在物镜的后焦平面上形成衍射盘。衍射圆盘与未衍射圆盘的距离取决于局部晶格参数，因此由局部晶格应变给出。利用这一原理，本文作者开发并发表了SANBED （strain analysis by nano beam electron diffraction）应变状态分析方法，该方法使用宽度为0.5-0.7 nm的电子束。这种方法产生了非常高的精度，可与文献中发表的使用透射电子显微镜的最佳值相媲美。然而，分析靠近界面的半导体纳米结构，电子束尾部的一部分可能延伸到界面之外。虽然这部分电子束的局部强度很小，但这种影响以复杂的方式降低了应变测量的空间分辨率。本文的目的是利用SANBED方法测量和优化应变分析的空间分辨率。计划以三种已发表的知名成分谱的半导体纳米结构样品作为参考样品，采用已发表的标准- sanbed方法测量应变谱。然后，将实验结果与图像仿真结果进行比较。为此，我们将测试不同复杂度的各种近似，力求在足够小的计算时间内达到高精度。在项目的第二部分，我们将通过优化实验参数和评估方法来研究提高该方法的空间分辨率。实验参数包括不同反射的选择和激发以及聚光镜孔径的几何形状。关于评估程序，我们打算测试额外的滤波和替代方法来测量衍射盘的位置。为了有效地进行优化，将采用实验测量和图像模拟两种方法。在项目的第三部分，将增加光束的半会聚角，使衍射盘开始重叠，从而导致照明电子束的直径变小。我们期望这也能提高应变评估的空间分辨率。另一方面，必须调整圆盘位置的测量方法，使其能够适用于重叠的衍射盘。

Development of methods for measurement of strain with sub-nanometer spatial resolution is of high relevance for material science as well as industrial microelectronics production. Local strain fields influence mechanic and electronic properties of semiconductor nanostructures and govern kinetic processes of epitaxial growth. For convergent nano-beam electron diffraction the specimen surface is scanned with a focused electron beam whose diameter lies in the sub-nanometer range. This leads to formation of diffraction disks in the back focal plane of the objective lens. The distance of a diffracted disk from the undiffracted one depends on the local lattice parameter and thus is given by the local lattice strain. Using this principle, the authors of this proposal developed and published the SANBED (strain analysis by nano beam electron diffraction) method for strain state analysis, which uses an electron beam with 0.5-0.7 nm width. This method yields a very high precision which is comparable with the best values published in literature using transmission electron microscopy. However, analysing a semiconductor nanostructure close to an interface, it is possible that a part of the tail of the electron beam extends beyond the interface. Although this part of the electron beam has a small local intensity, this effect reduces the spatial resolution of strain measurement in a complex manner. Aim of this proposal is measurement and optimisation of the spatial resolution of the strain analysis with the SANBED method. It is planned to use three semiconductor nanostructure samples with published well-known composition profiles as reference samples to measure strain profiles with the published standard-SANBED method. Then, the experimental results will be compared with image simulation. To this end, various approximations with different complexity will be tested, aiming at high accuracy at sufficiently small computation time. In the second part of the project we will investigate improvement of the spatial resolution of the method by optimizing the experimental parameters as well as the evaluation method. Experimental parameters include selection and excitation of different reflections and the geometry of the condenser aperture. Concerning the evaluation procedure we intend to test additional filtering and alternative methods to measure positions of diffraction disks. To perform the optimizations in an efficient way, both experimental measurement and image simulation will be applied. In the third part of the project the beam semi convergence angle will be increased so that the diffraction disks start to overlap, which leads to a smaller diameter of the illuminating electron beam. We expect that this also improves the spatial resolution of the strain evaluation. On the other hand, the method for measurement of disk position must be adapted so that it can be applied to overlapping diffraction disks.