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

该项目的目的是开发用于电子背散射衍射(EBSD)的扫描电子显微镜(SEM)技术的新一代探测器。EBSD利用背散射电子的衍射从样品表面的大片区域快速且非破坏性地提供关于材料结构性质的信息，空间分辨率为数十纳米。它是一种成熟的织构分析技术，可以量化晶界和晶相，使用基于互相关的分析方法可以测量相对应变、几何上必要的位错、晶格倾斜和扭曲、反相区和晶体极化。在EBSD中，入射电子通过高角度非弹性散射，形成发散的背散射电子源，可以进行衍射。由此产生的电子背向散射图案(EBSP)由大量重叠的带组成，称为菊池带，这些带与晶体结构的2D投影密切相关。EBSP通常由电子敏感的荧光粉或闪烁体屏幕和电荷耦合器件(CCD)或互补金属氧化物半导体(CMOS)相机检测。在这个项目中，正在开发直接电子探测器，以取代传统的荧光屏和可见光相机组合。与传统系统相比，直接电子探测器的优点包括能够减少电子束能量、束电流和采集时间。也没有扭曲的光学；采集可以是无噪音的；对于本项目中要开发的探测器，背向散射电子的能量辨别也是可能的。这将导致横向和空间分辨率的提高，并使人们能够获得EBSP中的小范围细节，而这些细节实际上是现有的商业EBSD系统无法获得的。这可以提供例如测定晶格常数、晶相鉴定和更灵敏地绘制应变图的途径。降低的电压和电流能力还将允许对束下通常损坏的材料进行微观结构分析，例如用于下一代太阳能电池的卤化物钙钛矿。在Strathclyde进行了首次能量过滤直接电子检测EBSD演示，展示了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办的一项国际合作(MediPix2)的成果，该合作旨在为X射线和伽马射线成像中的一系列问题提供解决方案。与国家物理实验室(NPL)合作，使用Timepix进行3D EBSD，在聚焦离子束扫描电子显微镜中连续获取EBSD图。EDAX最近还推出了一个基于Timepix探测器的商用直接电子探测器EBSD系统。这个博士项目将超越这个第一个商用系统。与格拉斯哥大学和NPL合作，将开发用于EBSD的最新混合像素探测器。新的探测器将允许在更低的电压(大约1千伏，目前的系统运行到大约5千伏)和更高的采集速率(大约每秒10,000帧)下进行EBSD。对Squired EBSP的复杂数据分析将与Aimo Winkelmann博士(波兰克拉科夫的AGH科技大学、Strathclyde客座教授)合作，使用他世界领先的动态模拟分析软件。

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.