Efficient Physical Embedding of Topologically Complex Information Processing Networks in Brains and Computer Circuits

Nervous systems are information processing networks that evolved by natural selection, whereas very large scale integrated (VLSI) computer circuits have evolved by commercially driven technology development. Here we follow historic intuition that all physical information processing systems will share key organizational properties, such as modularity, that generally confer adaptivity of function. It has long been observed that modular VLSI circuits demonstrate an isometric scaling relationship between the number of processing elements and the number of connections, known as Rent's rule, which is related to the dimensionality of the circuit's interconnect topology and its logical capacity. We show that human brain structural networks, and the nervous system of the nematode C. elegans, also obey Rent's rule, and exhibit some degree of hierarchical modularity. We further show that the estimated Rent exponent of human brain networks, derived from MRI data, can explain the allometric scaling relations between gray and white matter volumes across a wide range of mammalian species, again suggesting that these principles of nervous system design are highly conserved. For each of these fractal modular networks, the dimensionality of the interconnect topology was greater than the 2 or 3 Euclidean dimensions of the space in which it was embedded. This relatively high complexity entailed extra cost in physical wiring: although all networks were economically or cost-efficiently wired they did not strictly minimize wiring costs. Artificial and biological information processing systems both may evolve to optimize a trade-off between physical cost and topological complexity, resulting in the emergence of homologous principles of economical, fractal and modular design across many different kinds of nervous and computational networks

Geometric conductive filament confinement by nanotips for resistive switching of HfO2-RRAM devices with high performance

Filament-type HfO2-based RRAM has been considered as one of the most promising candidates for future non-volatile memories. Further improvement of the stability, particularly at the "OFF" state, of such devices is mainly hindered by resistance variation induced by the uncontrolled oxygen vacancies distribution and filament growth in HfO2 films. We report highly stable endurance of TiN/Ti/HfO2/Si-tip RRAM devices using a CMOS compatible nanotip method. Simulations indicate that the nanotip bottom electrode provides a local confinement for the electrical field and ionic current density; thus a nano-confinement for the oxygen vacancy distribution and nano-filament location is created by this approach. Conductive atomic force microscopy measurements confirm that the filaments form only on the nanotip region. Resistance switching by using pulses shows highly stable endurance for both ON and OFF modes, thanks to the geometric confinement of the conductive path and filament only above the nanotip. This nano-engineering approach opens a new pathway to realize forming-free RRAM devices with improved stability and reliability

Cost modelling and concurrent engineering for testable design

This thesis was submitted for the degree of Doctor of Philosophy and awarded by Brunel University.As integrated circuits and printed circuit boards increase in complexity, testing becomes a major cost factor of the design and production of the complex devices. Testability has to be considered during the design of complex electronic systems, and automatic test systems have to be used in order to facilitate the test. This fact is now widely accepted in industry. Both design for testability and the usage of automatic test systems aim at reducing the cost of production testing or, sometimes, making it possible at all. Many design for testability methods and test systems are available which can be configured into a production test strategy, in order to achieve high quality of the final product. The designer has to select from the various options for creating a test strategy, by maximising the quality and minimising the total cost for the electronic system. This thesis presents a methodology for test strategy generation which is based on consideration of the economics during the life cycle of the electronic system. This methodology is a concurrent engineering approach which takes into account all effects of a test strategy on the electronic system during its life cycle by evaluating its related cost. This objective methodology is used in an original test strategy planning advisory system, which allows for test strategy planning for VLSI circuits as well as for digital electronic systems. The cost models which are used for evaluating the economics of test strategies are described in detail and the test strategy planning system is presented. A methodology for making decisions which are based on estimated costing data is presented. Results of using the cost models and the test strategy planning system for evaluating the economics of test strategies for selected industrial designs are presented

Laser-assisted scanning probe alloying nanolithography (LASPAN) and its application in gold-silicon system

Nanoscale science and technology demand novel approaches and new knowledge to further advance. Nanoscale fabrication has been widely employed in both modern science and engineering. Micro/nano lithography is the most common technique to deposit nanostructures. Fundamental research is also being conducted to investigate structural, physical and chemical properties of the nanostructures. This research contributes fundamental understanding in surface science through development of a new methodology. Doing so, experimental approaches combined with energy analysis were carried out. A delicate hardware system was designed and constructed to realize the nanometer scale lithography. We developed a complete process, namely laser-assisted scanning probe alloying nanolithography (LASPAN), to fabricate well-defined nanostructures in gold-silicon (Au-Si) system. As a result, four aspects of nanostructures were made through different experimental trials. A non-equilibrium phase (AuSi3) was discovered, along with a non-equilibrium phase diagram. Energy dissipation and mechanism of nanocrystalization in the process have been extensively discussed. The mechanical energy input and laser radiation induced thermal energy input were estimated. An energy model was derived to represent the whole process of LASPAN

Imaging of magnetization dynamics in temperature gradients

This work investigates the manipulation of magnetization dynamics in magnetic vortex oscillators by local temperature profiles. The movement of the vortex cores is investigated experimentally by high resolution magnetic imaging. The dynamics of this systems is modelled by a generalized Thiele equation approach combined with micro magnetic simulations. The first part demonstrates a new method of fine-grain phase control in a pair of magnetic vortex oscillators coupled via their stray fields. A pair of two disk shaped permalloy disks placed close to each other is investigated. A gyrotropic precession of the cores is excited via Spin Transfer Torque (STT) by application of a spin-polarized electric current density. The phase relation between the two oscillators is programmed by local manipulation of the saturation magnetization. The temperature dependent saturation magnetization of one oscillator is manipulated by Joule heating. The resonance frequency spectrum of the coupled system is investigated by Lorentz-TEM measurements. Phase control is demonstrated by high resolution, time resolved Scanning transmission X-ray microscopy (STXM) measurements performed at the MAXYMUS beamline at Bessy II (Berlin, Germany). The dynamics of this system is modelled using a coupled extended Thiele equations approach. In the second part the manipulation of the vortex core position by pure thermomagnonic torques is investigated. Traditionally the manipulation of topological solitons such as magnetic vortices and skyrmions is achieved by spin polarized electric charge currents due to the application of an electrical potential. As recently proposed by theory, thermally excited magnons created by temperature gradients can also be used to manipulate magnetic structures. Here the theoretically well understood dynamics of magnetic vortex cores, when subject to thermal gradients, is investigated. Transmission Electron Microscopy measurements on magnetic vortices in temperature gradients created by local static Joule heating were performed. A large deflection compared to electric spin transfer torque excitation of the magnetic vortex core is observed. It is shown that the magnitude of the deflection depends on the applied heating power and the polarization of the core. To analyze the experimental results, a generalized Thiele equation model, including the different forces acting on the vortex core due to the applied temperature gradient, was developed. This analysis allows the estimation of the magnitude of the core motion for the involved force terms and therefore the differentiation of the origin of the experimental observed vortex movement

Laser-assisted scanning probe alloying nanolithography (LASPAN) and its application in gold-silicon system

Nanoscale science and technology demand novel approaches and new knowledge to further advance. Nanoscale fabrication has been widely employed in both modern science and engineering. Micro/nano lithography is the most common technique to deposit nanostructures. Fundamental research is also being conducted to investigate structural, physical and chemical properties of the nanostructures. This research contributes fundamental understanding in surface science through development of a new methodology. Doing so, experimental approaches combined with energy analysis were carried out. A delicate hardware system was designed and constructed to realize the nanometer scale lithography. We developed a complete process, namely laser-assisted scanning probe alloying nanolithography (LASPAN), to fabricate well-defined nanostructures in gold-silicon (Au-Si) system. As a result, four aspects of nanostructures were made through different experimental trials. A non-equilibrium phase (AuSi3) was discovered, along with a non-equilibrium phase diagram. Energy dissipation and mechanism of nanocrystalization in the process have been extensively discussed. The mechanical energy input and laser radiation induced thermal energy input were estimated. An energy model was derived to represent the whole process of LASPAN

Factors Affecting EMCCD Technology for Use in Space

This thesis is an assessment of factors of the EMCCD that may prevent this technology from being used in the space environment. The factors of interest here are EMCCD ageing, radiation effects from gamma rays and protons, and finally, single event susceptibility to heavy ions. The theory and architecture of the CCD and EMCCD are described with the aim of providing a technical basis in which to explore the results. The practical methods and novel experimental techniques carried out in this thesis are also described. However, EMCCD ageing is the primarily focus of this work concentrating on characterisation, understanding and then an experimental investigation into uncovering the cause of the phenomenon. Many EMCCDs (CCD97s) were tested with a cause of ageing being attributed to hot hole trapping in the oxide layer. Radiation effects regarding the effects that gamma-rays and protons have on EMCCDs, specifically the ageing, is also investigated. This analysis has shown that gamma-ray radiation in particular, has a large effect on the ageing and gain of these devices. In addition to this work is an experimental campaign to investigate the susceptibility to heavy ions primarily focussing on Single event Gate Rupture; showing EMCCDs to be a resilient against this type of error

Development of High-Speed Silicon Devices and Their Design with Advanced Physical Models

In the field of high-speed silicon devices, silicon bipolar junction transistors (BJTs) had played a major role from the 1970s to the end of the 1980s. However, in the 1990s complementary metal-oxide-semiconductor (CMOS) .field effect transistors (FETs) have been replacing their position. This dissertation explains the reasons why BJTs were suitable for high-speed operation. This is concluded from the development of technologies for BJTs and the analyses of devices fabricated with these technologies. At the same time it clarifies why they were replaced by CMOS transistors. The BJT's high driving capability and large power dissipation were the both sides of a sword. In the case of high-speed CMOS devices, the driving current of MOSFET should be large enough, and device design must be based on precise comprehension of carrier transport in MOSFETs. Therefore, we need accurate device model as well as rigid device-structure information obtained by experiments. This dissertation describes the device design methodology not only based on inverse modeling to extract device structures consistent with all kinds of experimental results but also based on simulations by generalized hydrodynamic model and full-band Monte Carlo model. The background and concept of the methodology is also discussed, and its necessity in future development is clarified. Moreover, hot carrier modeling is discussed by employing full-band Monte Carlo device simulation. Also, this dissertation clarifies the fact there is no experimental evidence for the difference between the surface and bulk impact ionization mechanism in silicon. The reported difference in the literature was only caused by an unsound application of the local field model and was just an artifact. Finally, by using these sophisticated models, the saturation drain current as well as hot carrier effects of subquarter micron MOSFETs are analyzed. MOSFET design strategy for the 0.1 μ　m regime is discussed and the importance of shallow junction for source/drain extension is also clarified.広島大学(Hiroshima University)博士(工学)doctora