Next generation direct electron detectors for diffraction measurements in the scanning electron microscope

本项目的目的是为电子背散射衍射（EBSD）的扫描电子显微镜（SEM）技术开发下一代探测器。EBSD利用背散射电子的衍射，以几十纳米的空间分辨率，从样品表面的大片区域快速、非破坏性地提供材料结构特性的信息。它是一种成熟的织构分析技术，用于量化晶界和晶体相，并且使用基于互相关的分析可以测量相对应变，几何上必要的位错，晶格倾斜和扭曲，反相域和晶体极性。在EBSD中，碰撞电子通过高角度非弹性散射，形成可衍射的背散射电子发散源。由此产生的电子背散射图（EBSP）由大量重叠带组成，称为菊池带，它们与晶体结构的二维投影密切相关。EBSP通常由电子敏感荧光粉或闪烁屏和电荷耦合器件（CCD）或互补金属氧化物半导体（CMOS）相机检测。在这个项目中，直接电子探测器正在开发，以取代传统的荧光粉屏幕和可见光相机组合。直接电子探测器的优点包括能够降低电子束能量；电子束电流;与传统系统相比，采集时间短。也没有扭曲的光学；采集可以是无噪声的；对于本项目将要开发的探测器，也可以对背散射电子进行能量判别。这将提高横向和空间分辨率，并允许在EBSP中获取小规模细节，这是现有商用EBSD系统在实践中无法获得的。这可能提供路线，例如，确定晶格常数，晶体相识别，并以更高的灵敏度绘制应变。降低电压和电流的能力也将允许对通常在光束下损坏的材料进行微观结构分析，例如下一代太阳能电池的卤化物钙钛矿。能量过滤直接电子探测EBSD的首次演示是在Strathclyde进行的，演示了Timepix的能力，这是一种数字CMOS混合像素探测器。该探测器是欧洲核子研究中心主持的国际合作项目（Medipix2）的成果，该项目旨在为x射线和伽马射线成像中的一系列问题提供解决方案。在与国家物理实验室（NPL）的合作中，Timepix用于3D EBSD，当样品在聚焦离子束扫描电镜中切片时，EBSD地图将被依次获取。EDAX最近还推出了基于Timepix探测器的商业直接电子探测器EBSD系统。这个博士项目将超越第一个商业系统。在与格拉斯哥大学和国家物理实验室的合作下，将开发用于EBSD的最新混合像素探测器。新的探测器将允许EBSD在更低的电压下执行（约1千伏，目前系统的工作电压降至约5千伏），并具有更高的采集速率（每秒10,000帧）。将与Aimo Winkelmann博士（波兰克拉科夫AGH科技大学，斯特拉斯克莱德大学客座教授）合作，使用他的世界领先的动态模拟分析软件，对固定的ebsp进行复杂的数据分析。

The aim of this project is to develop next generation detectors for the scanning electron microscopy (SEM) techniques of electron backscatter diffraction (EBSD).EBSD exploits diffraction of backscattered electrons to provide information on the structural properties of materials rapidly and non-destructively, with a spatial resolution of tens of nanometres, from large areas of the surface of a sample. It is a well-established technique for texture analysis, quantifying grain boundaries and crystal phases and the use of cross-correlation-based analysis of allows the measurements of relative strain, geometrically necessary dislocations, lattice tilt and twist, antiphase domains, and crystal polarity.In EBSD, the impinging electrons are scattered inelastically through high angles forming a diverging source of backscattered electrons which can be diffracted. The resultant electron backscatter pattern (EBSP) consists of a large number of overlapping bands, known as Kikuchi bands, which are closely related to a 2D projection of the crystal structure. EBSP are generally detected by an electron-sensitive phosphor or scintillator screen and a charge-coupled device (CCD) or complementary metal-oxide-semiconductor (CMOS) camera. In this project, direct electron detectors are being developed as a replacement for the conventional phosphor screen and visible camera combination. The advantages of direct electron detectors include the ability to reduce the electron beam energy; beam current; and acquisition time compared with conventional systems. There is also no distorting optics; acquisition can be noiseless; and for the detectors to be exploited in the present project, energy discrimination of the backscattered electrons is also possible. This will lead to improved lateral and spatial resolution and will allow the acquisition of small-scale details in the EBSP which are not in practice obtainable with existing commercial EBSD systems. This may provide routes to, for example, the determination of lattice constant, crystal phase identification, and the mapping of strain with greater sensitivity. The reduced voltage and current capabilities will also allow the microstructural analysis of materials which generally damage under the beam, such as halide perovskites for next generation solar cells. The first demonstrations of energy filtered direct electron detection EBSD were carried out here at Strathclyde, which demonstrated the capabilities of Timepix, a digital CMOS hybrid pixel detector. This detector was an outcome of an international collaboration (Medipix2) hosted at CERN, established to provide a solution for a range of problems in X-ray and gamma-ray imaging. In collaboration with National Physical Laboratory (NPL), the use of Timepix for 3D EBSD where EBSD maps are acquired sequentially as a sample is sliced in a focussed ion beam SEM. EDAX has also very recently launched a commercial direct electron detector EBSD system based on the Timepix detector.This PhD project will go beyond this first commercial system. In collaboration with the University of Glasgow and NPL, the newest hybrid pixel detectors for EBSD will be exploited. New detectors will allow EBSD to be performed at even lower voltages (of order 1 kV, present systems operate down to around 5 kV) and with higher acquisition rates (of order 10,000 frames per second). The complex data analysis of the squired EBSPs will be in collaboration with Dr Aimo Winkelmann (AGH University of Science and Technology, Krakow, Poland and visiting professor at Strathclyde) using his world-leading dynamical simulations analysis software.