Artificial Intelligence in Materials Science: Applications of Machine Learning to Extraction of Physically Meaningful Information from Atomic Resolution Microscopy Imaging

Materials science is the cornerstone for technological development of the modern world that has been largely shaped by the advances in fabrication of semiconductor materials and devices. However, the Moore’s Law is expected to stop by 2025 due to reaching the limits of traditional transistor scaling. However, the classical approach has shown to be unable to keep up with the needs of materials manufacturing, requiring more than 20 years to move a material from discovery to market. To adapt materials fabrication to the needs of the 21st century, it is necessary to develop methods for much faster processing of experimental data and connecting the results to theory, with feedback flow in both directions. However, state-of-the-art analysis remains selective and manual, prone to human error and unable to handle large quantities of data generated by modern equipment. Recent advances in scanning transmission electron and scanning tunneling microscopies have allowed imaging and manipulation of materials on the atomic level, and these capabilities require development of automated, robust, reproducible methods.Artificial intelligence and machine learning have dealt with similar issues in applications to image and speech recognition, autonomous vehicles, and other projects that are beginning to change the world around us. However, materials science faces significant challenges preventing direct application of the such models without taking physical constraints and domain expertise into account.Atomic resolution imaging can generate data that can lead to better understanding of materials and their properties through using artificial intelligence methods. Machine learning, in particular combinations of deep learning and probabilistic modeling, can learn to recognize physical features in imaging, making this process automated and speeding up characterization. By incorporating the knowledge from theory and simulations with such frameworks, it is possible to create the foundation for the automated atomic scale manufacturing

Analysis of the structural and optoelectronic properties of semiconductor materials and devices using photoacoustic spectroscopy and synchrotron x-ray topography

This thesis deals with the characterisation of semiconductor materials and devices through two complimentary experimental modalities. Synchrotron X-ray topography and photoacoustic spectroscopy are rapid, non-destructive and non-invasive techniques. The former may be used to elucidate the strain within a crystalline material due to localised structural defects causing deviations in the recorded X-ray intensity; whilst the latter can indirectly probe the non-radiative de-excitation processes within the bandstructure by measuring pressure variations within the gas in contact with the sample. In the first half of this work, a review of the theoretical description of the photoacoustic effect in condensed matter samples is presented. This classical review is extended to encompass the photoacoustic effect in semiconductor materials. Criteria governing the design o f a spectrometer are then extracted. A photoacoustic spectrometer based on the gas-microphone technique, with a wide spectral range (0.5 eV to 6.2 eV) was designed and constructed. The spectrometer was characterised across its spectral range using common semiconductor materials. The latter half of the thesis commences with a review of the kinematical and dynamical theories of X-ray diffraction. The properties of synchrotron radiation are discussed, with particular focus on their applicability to X-ray topography. The large area, section and grazing incidence topography techniques are presented. Several topographic studies of semiconductor materials and devices were performed. These included an analysis of the evolution of strain in ultra-bright light emitting diodes under varying degrees of electrical stress, strain induced by the epitaxial lateral overgrowth of gallium nitride on sapphire, stress due to rapid thermal processing of silicon wafers, characterisation of diamond crystals for use in a high energy monochromator, misfit dislocation generation at a Si/SiGe heterointerface and dynamical imaging of microdefects in nearly perfect silicon

Structural characterisation of semi-insulating LEC gallium arsenide

Double crystal x-ray topography using a synchrotron radiation source has been used to measure the lattice distortions present in 50mm diameter samples of (001) semi-insulating LEG gallium arsenide. Lattice strains and tilts have been mapped in In-doped and undoped samples as well as annealed and unannealed samples taken from the seed and tail ends of boules. The properties of the x-ray source which are necessary for these measurements are discussed and it is concluded that a synchrotron source is the only practical choice. Lattice strains of 90ppm and tilts greater than 100 arc seconds were measured in In-doped material both of which appear to be due to a combination of In concentration variations and the inhomogeneous dislocation distribution. Undoped samples were found to be more uniform with lattice strains of typically +20ppm towards the samples edges where the dislocation density is largest. The lattice tilt distribution in seed and undoped samples invariably exhibited a four-fold symmetry which was enhanced by the presence of lineage features lying along the directions. Tail end samples were generally less uniform in lattice strain and showed a lower symmetry in their lattice tilts. These results are discussed in the light of current ideas concerning the origin of variations in lattice strain and EL2 concentration. An x-ray diffraction method involving integrated intensity measurements of the quasi-forbidden 200 reflection, which is highly stoichiometry sensitive, is investigated. The results, however, show no conclusive stoichiometry variations but do highlight important experimental conditions which must be satisfied if such measurements are to be meaningful. The images of dislocations in double crystal x-ray topographs are investigated and compared with theoretical simulations in order to assess the effects of point defect environment on the dislocation strain field. The results suggest that the EL2-dislocation interaction is not significantly strain driven

Modelling and simulation of paradigms for printed circuit board assembly to support the UK's competency in high reliability electronics

The fundamental requirement of the research reported within this thesis is the provision of physical models to enable model based simulation of mainstream printed circuit assembly (PCA) process discrete events for use within to-be-developed (or under development) software tools which codify cause & effects knowledge for use in product and process design optimisation. To support a national competitive advantage in high reliability electronics UK based producers of aircraft electronic subsystems require advanced simulation tools which offer model based guidance. In turn, maximization of manufacturability and minimization of uncontrolled rework must therefore enhance inservice sustainability for ‘power-by-the-hour’ commercial aircraft operation business models. [Continues.

Metal-Mold Reactions in CMSX-4 Single Crystal Superalloy Castings

Metal-mold reaction (MMR) layers are often found on the surface of as-cast CMSX-4 single-crystal alloy parts. These layers cannot be removed prior to solution heat treatment of cast parts because of the sensitivity of the single-crystal castings to recrystallization (RX) defect formation. Removal of the reaction layer after solution heat treatments is very costly, as the layer is very hard, and requires abrasive water jet and pressure (grit) blasting processes. To address this manufacturing concern, it was desirable to understand the mechanisms of reaction layer formation and hardening after solution heat treatment. With this understanding, we developed methods for minimizing the reaction layer formation, which will potentially bring a big cost saving for the CMSX-4 casting processes.In this work, we confirmed experimentally that silica (SiO2) reacts with Al, Hf, and Ti, facilitating surface oxidation and formation of a tenacious surface eutectic phase. To avoid this, elimination of the Si is desired. However, Si is present in many of the refractory and pattern materials used in the casting system, which can transfer into the alloy during the solidification process in both liquid and solid state. A two-path solution was investigated: 1) eliminate all Si sources, and 2) create surface diffusion barriers to prevent reaction of the SiO2 with the metal. Potential sources of silica/silicon in the casting system include: the CMSX-4 charge material (nominal Si content less than 400 ppm), thermocouple protection quartz tube (100% silica), crucible (4% silica), bushing (80% silica), funnel (if used-60% silica), ash in pattern waxes, mold release (silicone-based), binder for facecoating (includes nanoscale silica-4% in facecoat), and cores (when used-80% silica).For a diffusion barrier, an yttria slurry (short lifetime and high cost of fine yttria flour), was replaced with an yttria aerosol spray coating, applied directly to the wax pattern before normal zircon primary facecoating. This was followed by an yttria binder washing and soaking, applied on top of the spray coating shell after dewaxing for strengthening. This process showed good bonding in casting trials in an argon atmosphere Bridgman casting furnace. Optical microscopy, SEM/EDS, AES and XPS techniques were employed for characterization of MMR interfaces of both the CMSX-4 casting and the shell mold. These characterization methods revealed MMR layers with oxidation, Si and Hf rich features.In this study, the yttria spray (alone) slightly reduces the amount of MMR of CMSX-4, but when the yttria spray was combined with the binder wash, the reaction was further reduced

In-situ analysis of polymer film coats using AFM and thermal probe methods

The in-situ analysis of pharmaceutical solid dosage forms films is of great interest to pharmaceutical research, especially with the drive towards continuous processing compared to batch processing requiring either in-line or on-line analysis of pharmaceuticals. The size and shape of many solid dosage forms impedes their in-situ analysis, particularly when using thermal analysis techniques such as differential scanning calorimetry (DSC). Thermal probe methods, which combine atomic force microscopy (AFM) with thermal analysis, permits imaging and characterisation of solid dosage forms in-situ. The goal of this thesis is to perform the in-situ analysis of film coated minitablets through the use of thermal probe methods. Minitablets, which are small diameter tablets, were coated with Opadry I, Opadry II or Surelease, and blends of Surelease and Opadry I and Surelease and Opadry II. In addition the same systems were prepared as cast free films to ascertain differences resulting from the two methods of producing the films. Distinct morphologies were observed between the film coated minitablets and cast free films, particularly for the polymer blends. Thermal analysis by modulated temperature DSC (MTDSC) and localised thermomechanical analysis (L-TMA) of film coated minitablets coated with polymer blends indicating miscibility. However the novel technique of heated tip tapping mode was employed, which was able to resolve separate domains of the polymer blends, which appeared as nanophases. Heated tip tapping mode was further employed to determine changes in the surface morphology of film coated minitablets which had undergone curing. It was successfully used to observe the further coalescence of Surelease and the alteration in the phase distribution of the polymer blends after film coated minitablets had been cured. Polymer films with incorporated pigments were also analysed by thermal probe methods with differences in pigment distribution being observed between film coated minitablets and cast free films. Tapping mode AFM was able to determine the location of pigment particles and the novel use of L-TMA performed in tapping mode allowed thermal analysis without changing AFM mode. Overall the use of AFM combined with thermal probes achieved the aim of performing in-situ analysis of polymer film coats; however further research to correlate in-vivo behaviour of film coats would be of value