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

点击翻译按钮获取中文摘要

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.