441 research outputs found

    Parameter estimation for a morphochemical reaction-diffusion model of electrochemical pattern formation

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    The process of electrodeposition can be described in terms of a reaction-diffusion PDE system that models the dynamics of the morphology profile and the chemical composition. Here we fit such a model to the different patterns present in a range of electrodeposited and electrochemically modified alloys using PDE constrained optimization. Experiments with simulated data show how the parameter space of the model can be divided into zones corresponding to the different physical patterns by examining the structure of an appropriate cost function. We then use real data to demonstrate how numerical optimization of the cost function can allow the model to fit the rich variety of patterns arising in experiments. The computational technique developed provides a potential tool for tuning experimental parameters to produce desired patterns

    Enhanced electrodeposition for the filling of micro-vias

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    This thesis investigated the introduction of megasound (MS) (1MHz) acoustic technology as an enhanced agitation method of an electrolyte solution for the electrochemical deposition of copper (Cu), used in electroplating processes. The thesis, carried out at Merlin Circuit Technology Ltd, studied the possibility of improving processing capabilities for use in Printed Circuit Board (PCB) industrial manufacture. Prior laboratory experiments demonstrated increased metallisation of vertical interconnect access (via) features in a Printed Circuit Board (PCB), which, if applied within manufacturing, would enable increased connectivity throughout a PCB and result in cost savings. PCB manufacturing quality after MS-assisted Cu electroplating was assessed by measurements of the topography of the electrodeposits, using scanning electron microscopy and white-light interferometry. Cu plating rate changes were also measured on the surface of the PCB and inside the vias. After plating Cu with MS-assistance, the macro and microscale surface composition was demonstrated to alter due to the direct influence of the acoustic waves. Systematic characteristic of the surface was conducted by varying the settings of the acoustic transducer device as well as the process parameters including electrical current distribution, bath additive chemistry and solution temperature. MS processing was shown to produce unique Cu artefacts. Their deleterious formation was demonstrated to be influenced by acoustic standing waves and microbubble formations at the electrolyte solution/PCB interface. Causes of these artefacts, microfluidic streaming and cavitation, were also observed and controlled to reduce the creation of these artefacts. MS plating Cu down through-hole via (THV) and blind-via (BV) interconnects was shown to produce measureable benefits. These include, for THVs, a 700 % increase of Cu plating deposit thickness within a 175 μm diameter, depth-to-width aspect ratio (ar) of 5.7:1, compared with processing under no-agitation conditions. For BVs, a 60 % average increase in Cu deposition in 150 μm and 200 μm, ar 1:1, was demonstrated against plating under standard manufacturing conditions - bubble agitation and panel movement.Engineering and Physical Research Council (EPSRC) grant number EP/G037523/

    Turing pattern formation in the Brusselator system with nonlinear diffusion

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    In this work we investigate the effect of density dependent nonlinear diffusion on pattern formation in the Brusselator system. Through linear stability analysis of the basic solution we determine the Turing and the oscillatory instability boundaries. A comparison with the classical linear diffusion shows how nonlinear diffusion favors the occurrence of Turing pattern formation. We study the process of pattern formation both in 1D and 2D spatial domains. Through a weakly nonlinear multiple scales analysis we derive the equations for the amplitude of the stationary patterns. The analysis of the amplitude equations shows the occurrence of a number of different phenomena, including stable supercritical and subcritical Turing patterns with multiple branches of stable solutions leading to hysteresis. Moreover we consider traveling patterning waves: when the domain size is large, the pattern forms sequentially and traveling wavefronts are the precursors to patterning. We derive the Ginzburg-Landau equation and describe the traveling front enveloping a pattern which invades the domain. We show the emergence of radially symmetric target patterns, and through a matching procedure we construct the outer amplitude equation and the inner core solution.Comment: Physical Review E, 201

    Scale-up of enface electrochemical reactor systems

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    Ph.DPhotolithography, the standard pattern transfer technique, has many sustainable issues due to the application of a mask to the substrate. A ‘maskless’ pattern transfer method, called the Enface technique, has recently been proposed for metal plating and etching. This method introduces the idea of bringing a patterned tool and a substrate together in close proximity and a current or voltage is passed between them enabling metal to be selectively deposited or removed from the substrate. The process requires sufficient electrolyte agitation within a narrow inter-electrode gap and has previously been shown to hold in a vertical flow channel reactor. However, the process has to be adapted for tank-type systems for industrial implementation. Mass transfer during electrodeposition can be enhanced by ultrasonic waves. It has therefore been investigated whether this would be an appropriate agitation method for Enface. In order to scale-up the process, 3 types of Enface reactors were investigated; a vertical flow cell, a 500 ml lab-scale tank-type cell and an 18 L ultrasound plating tank. The limiting current technique was used to study the mass transfer in these systems. Electrodeposition of copper pattern features in 0.1 M CuSO4 was achieved in each of these geometries. The scalability was quantified by measuring the uniformity of deposit roughness and deposit thickness of the features across the substrate using profilometry. The lab-scale tank-type cell with a 20 kHz ultrasound probe was used to investigate the effect of ultrasound agitation within narrow inter-electrode gaps. Mass transfer correlations showed that turbulent flow becomes fully developed when using ultrasound in this narrow geometry. Limiting current experiments showed that relatively low ultrasound powers of 9 – 18 W/cm2 should be used and current distribution modelling showed that the ultrasound source should be placed no less than 30 mm from the substrate. Copper pattern features were deposited onto 10 mm diameter substrates and using long current pulses with bursts of ultrasound during the off-time was the most suitable plating mode. Specially designed electrode holders in the large-scale 18 L ultrasound tank was used to deposit copper patterns onto larger substrates. Features of μm-scale were deposited onto A7 size substrates, but there was an unacceptable variation in deposit thickness of ±80% due to the non-uniformity of the electrode gap across the plate. However, mm-scale features were successfully deposited onto A7 size substrates with an acceptable deposit thickness uniformity and deposit roughness uniformity of ±18% and ±40% respectively across the plate. Enface is therefore currently scalable for mm-scale features on substrates of this size.EPSRC, E

    Micropattern transfer without photolithography of substrate :Ni electrodeposition using enface technology

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    PhD ThesisSince the standard photolithographic patterning technology possesses a number of sustainable issues, a “maskless” technology, Enface, has been proposed. Here, a patterned ‘tool’ placed opposite to the substrate within micrometre range is required. Etching or plating occurs by passing tailored current or voltage waveforms, provided that the electrolyte resistance is high. Enface is a resource efficient process as the use of chemicals is greatly reduced. This research project aimed to investigate the feasibility of Ni pattern transfer using Enface under stagnant conditions. It would be advantageous if Enface could be used for nickel deposition as it is a slow kinetic system and controlled by mixed mass transfer and kinetics which is a system where Enface has never been used before. An electrochemical cell has been specifically designed and an electrolyte was systemically developed as required by Enface. Polarisation experiments were carried out to determine applied current densities that would be used in galvanostatic plating experiments for pattern transfer of millimetre and micron scale features. Deposited features were comprehensively characterised to see the performance of the patterning process. Current distribution during the pattern transfer experiments was investigated by simulation and modelling using Elsy software. An electrolyte of 0.19 M nickel sulfamate was selected and shown to be capable of depositing nickel. Polarisation data from experiments in Enface system showed that each feature size requires a different applied current density. As expected, pattern transfers of metallic nickel were achieved for millimetre and micron scale features at a current efficiency of around 90 % with current spreading were observed. The deposited feature width broadens with increasing time and decreasing feature size. In addition, maximum thickness that could be achieved was around 0.5 μm due to entrapped gas bubbles leading to process termination. The gas bubbles were detrimental to the deposits resulting in a rough and inhomogeneous surface as well as photoresist degradation. Ultrasound agitation was shown to be capable of diminishing the effect of gas bubbles. However it requires an optimisation of applied power density to avoid negative effects of cavitation bubbles. The result of simulation showed a non-uniform current distribution across the feature width with the highest current density at the centre resulting in a bell-shaped surface profile which is in agreement with the experiments. However, the deposited shape evolution obtained from the experiments is consistently much better than those obtained from the simulation.Ministry of Education and Culture through Directorate General for Higher Education, Government of Republic of Indonesia for funding my PhD study, Poc-Enface for research studentshi

    Investigation of Electrodeposited Magnetite Films: Formation and Characterization

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    Magnetite (Fe3O4) is of both scientific and technological interest because of its fascinating magnetic properties. It has a high Curie temperature of 860 K and a theoretical 100% spin polarization at the Fermi level. There are a variety of deposition techniques to form thin films of magnetite, such as molecular beam epitaxy (MBE), pulsed laser deposition (PLD), iron oxidation, sputtering and so on. In comparison with other deposition methods mentioned above, electrodeposition has a key advantage of relatively low processing temperature. The intention of this work was to investigate magnetite (Fe3O4) thin films grown via an electrochemical route by using various kinds of characterization techniques, especially on morphology, chemical composition, structure and magnetic properties. Fe3O4 thin films were obtained by using a galvanostatic or potentiostatic deposition from simple aqueous solutions of ferrous salts. Iron oxide thin films have been grown at different current densities and temperatures onto polycrystalline copper substrates. XRD results indicate that Fe3O4 is formed at 90 oC at an applied current density of 0.05 mA·cm-2. Lower growth temperatures can cause the formation of another phase, α-FeOOH at a certain concentration of Fe2+ and pH buffer. Time-dependent growth of the iron oxides exhibits nucleation and coalescence. In order to obtain uniform Fe3O4 film surface, longer deposition times are needed. The influence of applied potential on the characteristics of the deposited iron oxide was examined. The formation of Fe3O4 in a low potential regime (< 100 mV) vs. gold reference electrode while iron oxyhydroxides such as goethite (α-FeOOH) and lepidocrocite (γ-FeOOH) are favoured for E > 100 mV. The magnetic properties of the films were found to be strongly dependent on the deposition potential. The multi-layer structure of Fe3O4/α-FeOOH/Fe3O4 onto NiO/Ni substrates has been demonstrated via successive deposition. A TEM cross-section image shows α-FeOOH is coherently formed between two ferromagnetic layers. ADF-STEM micrographs show that Fe3O4 has a columnar structure and has less composition variation compared to that grown onto a polycrystalline copper substrate. Synchrotron techniques, i.e. x-ray absorption near edge structure (XANES) and x-ray magnetic circular dichroism (XMCD), were performed to examine the iron oxide film. Fe K-edge x-ray absorption spectra demonstrate that the films grown at low potential regime (< 100 mV) have a comparable valency state with the standard Fe3O4 sample. The identification of the iron oxide was further confirmed by using XMCD technique. The calculation of the asymmetry ratio suggests that the total magnetic moment increased with decreasing applied potential. In addition, vibrating sample magnetometer (VSM) data show that the magnetic response is somewhat slower for the iron oxide grown at higher potential regime. A change of pH in the electrolyte does not change the lattice constant and film morphology or texture but does affect particle sizes in Fe3O4 thin films. This decrease with the pH is due to the reaction of FeOH+ ions with molecular oxygen in electrolyte

    Electrodeposition of zinc oxide nanostructured films

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    ZnO nanostructures have great promise in a wide range of applications such as sensors, optoelectronics, piezoelectronics, healthcare. Preparation of oxide films by electrodeposition from aqueous solution presents several advantages over other techniques such as controlling the rate and morphology through several well-defined parameters (electrode potential, current, temperature, pH, etc.), the fact that electrolytic processing is a well-established technology and readily scalable for production, and the non-equilibrium nature of the electrochemical interface often gives rise to morphologies and compositions not attainable through other, usually high-temperature, routes. Despite a large amount of research in this area the detailed mechanism of nucleation and growth is still controversial. Only a good understanding of it will allow the expected industrial applications to be achieved. One of the main difficulties to overcome is that tiny amounts of material are involved and the required in-situ measurements are thus very delicate. The ability of synchrotron radiation to probe material structure during deposition makes it the ideal tool for the study of nucleation and growth of these materials as a function of the processing parameters. Here we will present two synchrotron-based approaches involving both X-ray absorption and scattering. The first method, together with ex-situ characterisation, provides detailed information about how the kinetics of the growth and/or dissolution is influenced by the electrochemical parameters. The effect of time, potential, zinc ions concentration, oxygen precursor, temperature and electrolyte composition have been studied. Following this understanding of the influence of the parameters, films of desired structure can be synthesised and new structures have been made. Beside the electrochemical parameters, the growth of the film is influenced by the interaction with substrate in the early stage of nucleation. The second synchrotron technique allows the direct observation of the development of the crystal orientation of the films during the deposition. It gives promising results to study how the substrate influences the growth and thus the properties of the films
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