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Title: Coherent X-ray diffraction imaging and ptychography on silicon-on-insulator nanostructures
Author: Shi, X.
Awarding Body: University College London (University of London)
Current Institution: University College London (University of London)
Date of Award: 2013
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My PhD project is on the use of coherent X-ray diffractive imaging (CDI) techniques to study strained Silicon-On-Insulator (SOI) and Strained-Silicon-On-Insulator (SSOI) nanostructures, which include nanowires and microsquares. To understand and distinguish SOI wafer properties and compare and analyze different wafer bonding techniques, various SOI wafers with different bonding techniques were measured using CDI, and analysis shows inhomogeneity across all categories of SOI wafers. Furthermore, I discovered a radiation-induced bending effect causing SOI nanowires to exhibit highly strained characteristics in diffraction patterns in reciprocal-space. There is a trend of peak splitting of the central maximum of the diffraction patterns when increasing X-ray illumination dose, i.e. time of X-ray exposure. We are able to propose a model of radiation-induced bending of SOI model and our model is confirmed by simulations of calculated diffraction patterns and by a Finite-Element-Analysis tool (COMSOL Multiphysics). The results are in agreement with experimental measurements, which indicates that our model is a generally correct description of the X-ray radiation effect. We developed a Guided-Phase-Error-Reduction (GPER) algorithm to do diffraction data inversion, obtaining direct-space amplitude and phase of strong-bent SOI nanowires. Most importantly, we observe mechanical breakdown that causes various kinds of dislocations within the nanowire structure when the X-ray irradiation dose has reached a certain level. Beyond this dose, the nanowire begins to undergo plastic deformation. The second part of my PhD project involves design and performance of experimental measurements of reflection Bragg-geometry ptychography at beamline 34-ID-C at the Advanced Photon Source. We have used both KB mirrors and zone-plate focused methods. Zone-plate based focusing method produces highly asymmetric curved-wave front X-ray illumination for studies on both Siemens-star test patterns and SOI micro- and nano squares. Our preliminary analyses have shown some promising results. We have performed series of measurements on Siemens-star test sample by using transmission-geometry Ptychography with various coherence properties of Xrays and oversampling ratios of Fourier-space diffraction intensities. In the long term, Bragg-geometry ptychography has great potential for imaging condensed matter structures and surface effects, aiming at better understanding of structural Physics, Materials sciences and Nanotechnology in general. Strains by various industrial processing of SOI based electronic devices, such as silicon-based MOSFETs, can be probed by either conventional Bragg CDI or Bragg-geometry ptychography. MOSFETs may behave differently under X-ray irradiation, because of this radiation effect, resulting to various strain states which could modify carriers mobility in semiconductor devices performance in general applications.
Supervisor: Not available Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available