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The formation of Cu thin film directly on Ti diffusion barrier by electrodeposition is important for next generation Cu interconnect in Si-based microelectronic devices. It depends on the surface morphology evolution of Cu deposit at the initial stage of nucleation and growth. In this study, we investigated the surface morphology evolution of Cu thin film on Ti in citric acid solution, introducing several simple ways to determine the stages of nucleation, Cu nuclei overlap, and Cu film formation with deposition time. During potentiostatic Cu deposition, studies on morphology change using the chronoamperometry, the change of surface roughness, and the change of sheet resistance and sheet resistivity are carried out along with microscopic observation. Resultant surface morphology of Cu deposit on Ti is greatly influenced by nucleation behavior, and its dependence on deposition potential is also studied in relation with the calculation of critical Cu nucleus size. According to the suggested ways, it is available to indirectly observe the transition of Cu nuclei to Cu thin film on Ti diffusion barrier during electrodeposition. [ABSTRACT FROM AUTHOR]

Cu-Sn alloy foams were produced by electrodeposition in a 0.5 M sulfuric acid solution using a porous polyurethane (PU) template. A thin seed layer of Ti/Ni was sputter-deposited on the PU template. The sulfuric acid solution contained 10 mM Cu and 1 mM Sn to electrodeposit Cu-Sn alloy under diffusion-controlled condition. The effect of the solution temperature on the chemical composition, microstructure, and electrical resistivity of Cu-Sn alloy foams was fundamentally investigated. Increasing the solution temperature has reduced cathodic overpotential for alloy reduction, resulting in improved reduction rate and deposition efficiency of Cu-Sn alloy. Thermal treatment for the removal of the PU template contributed to the microstructural recovery of as-electrodeposited Cu-Sn alloy. Electrical resistivity of Cu-Sn alloy foams was measured using a Ag-wired plastic holder. Their relatively low values, compared to theoretically predicted values, were resulted from the formation of intermetallic compounds within Cu-Sn alloy foam. [ABSTRACT FROM AUTHOR]

Electrochemical deposition of iron in HNO3 solution containing iron dissolved from copper slag was investigated with varying pH and chemical components of the solution in this study. The pH dependency of reduction potentials of ionized iron and copper species facilitated the elimination of copper inclusion in electrodeposited iron-based layers. Hydrogen evolution in competition with iron reduction from preferentially ferric ions was also diminished by increasing pH of the solution with NaOH addition. Adding malonic acid was effective to considerably diminish the oxidation of reduced iron due to the complexation between ferric ion and malonate (Mai) ligand. However, iron oxidation could not be completely inhibited by adding malonic acid because a partial transition of Fe(Mal)+ ions to ferrous ions, which can react readily with oxygen during electrodeposition, would occur at low cathodic potentials. Controlling malonic acid concentration could lead to the formation of a-Fe with very low oxygen content less than 0.15 wt%. [ABSTRACT FROM AUTHOR]

Cu-Co alloy substrate was formed on the Au-seeded p-GaN side of sapphire wafer specimens to function as a secondary substrate of vertical structure GaN-based light emitting diode (LED), using electrodeposition method at constant potentials in an acidic citrate electrolyte. Potentiostatic deposition process was available to change the Co content of Cu-Co alloy substrates with varying cathodic deposition potential. Rotation speed of wafer specimens during electrodeposition also affected seriously the microstructure as well as Co content of the substrates. Cu-Co alloy layers deposited on non-rotating specimens showed coarse, particulate microstructures, whereas Cu-Co alloy substrates formed with applying specimen rotation had dense, grained microstructures. The chemical composition and microstructure of electrodeposited Cu-Co alloy substrates were closely related with their electrical and mechanical properties, which should be essentially considered for a appropriate secondary substrate of vertical structure LEDs. Residual stress, electrical resistivity, and Vickers hardness of the substrates were measured, respectively, according to cathodic deposition potential and rotation speed. In conclusion, mechanically strengthened Cu-Co alloy substrate in comparison with ductile Cu substrate was fabricated having very low residual stress and acceptable electrical resistivity by alloying Cu with low Co content of less than 3.15 at% in potentiostatic deposition process. [ABSTRACT FROM AUTHOR]

Ni substrates were electrochemically deposited on p-GaN side of vertical structure GaN-based light emitting diodes (LEDs). The Ni substrate functions as a part of p-electrode, a heat sink, and a mechanical supporter of vertical structure LED devices. Using a proper Ni electrodeposition bath, the residual stress of 50-μm thick Ni substrates was lower than 10 MPa, which is very low to be regarded as stress-free. Stress-free Ni substrate with better mechanical strength than Cu substrate was effective in minimizing mechanical damage possibly generated during LED chip fabrication. Vertical structure LED devices comprised in a 2-inch Ni-supported wafer had forward voltage on average as low as 3.35 V at 20 mA compared with 3.54 V of Cu-supported LED devices. Ni-supported LED devices showed comparable optical performance to Cu-supported LED devices in output power and operation reliability at constant forward currents. Electrical and optical performances of vertical structure LEDs with WO3-added Cu substrate, which was previously considered to enhance the mechanical property of pure Cu substrate, were also presented for reference. In conclusion, Ni substrate affects the efficiency of vertical structure LEDs positively more than Cu and WO3-added Cu substrates as long as those substrates are electrodeposited equally in stress-free conditions. [ABSTRACT FROM AUTHOR]

Pure Cu substrate has been previously electroplated on p-GaN side of vertical structure GaN-based light emitting diode (LED) to mechanically hold thin GaN-based multilayer after removal of a sapphire substrate by laser lift-off process. The ductile Cu substrate needs to be mechanically strengthened to prevent the GaN-based multilayer from being damaged during vertical LED chip fabrication. In this study, the formation of Cu substrate with W addition on Au-seeded LED wafer by electroplating is introduced. W presents as W03 in the Cu substrate. W content of W-added Cu deposits was dependent on constant reduction potential in an acidic citrate bath. The reduction potential also influenced directly the electrical resistivity, hardness, and residual stress of W-added Cu substrates. Applying moderate reduction potentials like -1.1 V resulted in 1.5 times larger hardness than that of pure Cu substrate by adding around 1.0 at % W to Cu. Nevertheless, the electrical resistivity of W-added Cu substrate was maintained as low as that of pure Cu substrate. At more positive reduction potentials than -1.1 V, very low residual Stress between 4 and 42 MPa was measured. Wafer bowing induced by large residual stress of W-added Cu substrate can cause serious mechanical damage to LED structure. [ABSTRACT FROM AUTHOR]

Cast Ni-Al bronze (NAB) alloy, which is a most common material of ship propeller operating in sea water, is coated with the same NAB alloy by high-velocity oxy-fuel (HVOF) thermal spray process. Application of a HVOF NAB coating to cast NAB can be considered to enhance the resistance to corrosion fatigue failure of a NAB-based ship propeller and repair its damaged location at low cost. Linear sweep voltammetries, potentiostatic corrosion tests, and 80-day immersion corrosion tests are conducted in a 3.5% NaCl solution to investigate the corroding behavior of HVOF-NAB-coated cast NAB. Corrosion pits and microcracks through which the NaCl solution can easily permeate are developed from many pores included in the HVOF NAB coating. More serious corrosion compared to uncoated cast NAB occurs with producing corrosion products of Cu20, Al2O3, and Fe2O3. Corrosion fatigue tests of both HVOF-NAB-coated Cast NAB and uncoated cast NAB for reference are also conducted in the NaCl solution. Corrosion fatigue strength of the HVOF-NAB-coated cast NAB is measured as 140 MPa, which is much higher than 90 MPa of the uncoated cast NAB. [ABSTRACT FROM AUTHOR]

The mechanical stability and optical performance of vertical structure GaN-based light emitting diodes (LEDs) were investigated in company with the analysis of residual stresses of layer components constituting LED structure. Residual stress of a PR passivation structure, an Au seed layer, and an electrodeposited Cu substrate was measured according to the layer thickness that is the most important parameter to affect the residual stress. They were formed covering the whole LED wafer, which includes many isolated vertical LED chip structures. It was concluded that their residual stresses according to the layer thickness influenced critically the mechanical damage and light output power of vertical LED structures after the laser lift-off (LLO) of a sapphire substrate. [ABSTRACT FROM AUTHOR]

Ruthenium (Ru) thin films were grown on thermally-grown SiO2 substrates by thermal atomic layer deposition (ALD) using a .sequential supply of a zero metal valence precursor, isopropyl-methylbenzene-cyclohexadienc Ru(O) (IMBCHRu, C16,H22Ru) and molecular oxygen (O2) at substrate temperatures ranging from 185 to 310 °C. The growth rate at 185 C was approximately 0.059 nm/cycle but its resistivity was >3000 μΩ cm. When the deposition was done at 200° C, the resistivity was decreased drastically to ~100 μΩ cm, and the film growth rate increased to 0.075 nm/cycle. An ALD temperature window from 225 to 270 °C was observed. A high growth rate of 0.086-0.089 nm/cycle was obtained at this ALD temperature window. The film deposited at 270°C showed a minimum resistivity of ~30 μΩ cm and a high density of 11.7 g/cm3 and with no impurities in the film, such as oxygen and carbon. At 310°C, the growth rate increased to 0.136 nm/cycle due to a partial decomposition of the precursor. In addition, the film resistivity increased slightly to ~40 μΩ cm with the incorporation of carbon and the formation of a less-dense film. The step coverage of the ALD-Ru film was dependent on the dimensions of the contact and deposition temperature. At the contact with an aspect ratio of ~4.6 (top opening diameter: 80 nm), the step coverage was excellent irrespective of the deposition temperature. However, at the contact with an aspect ratio of ~25. the step coverage of the film deposited at 310° C (or above ALD temperature window) was degraded, even though those prepared within ALD temperature window were ~100%. Finally, ALD-Ru film was used successfully as a seed layer for Cu electroplating. [ABSTRACT FROM AUTHOR]

Copper was electrodeposited directly onto a sputter-deposited tungsten diffusion barrier layer in an acidic copper-ammonium-citrate (Cu-NH3-Cit) bath. The nucleation characteristic of copper deposits according to applied cathodic potential was investigated. The nucleation mode, nuclei density, and dominant copper species to be preferentially reduced were critically dependent on the applied cathodic potential. Pure instantaneous or progressive nucleation of copper was not observed, but instead, mixed nucleation mechanisms appeared. For low cathodic potentials, the copper deposits, which were reduced mainly from cupric ions, showed consequently a progressive nucleation characteristic despite initial instantaneous nucleation. For large cathodic potentials, the instantaneous nucleation of copper was achieved from the reduction of copper complexes following the early stage of progressive nucleation. The copper nuclei density of 2×1010 cm-2 calculated for filling the sub-70 nm damascene structures was approached by applying large cathodic potentials. [ABSTRACT FROM AUTHOR]

Thermal stability of indium tin oxide (ITO)/Ag-based reflective contacts to p-GaN was investigated for their application to vertical Structure GaN-based light-emitting diodes (LEDs). The ITO/Ag-based p-contacts showed good ohmic property with low specific contact resistance of 10-4-10-5 Ω cm². In order to study the thermal degradation of the ITO/Ag-based p-contacts, they were annealed at 400°C for 72 h in air. The reflectance of ITO/Ag and ITO/Ni/Ag was significantly reduced due to Ag degradation on annealing. However, the reflectance of ITO/Ni/Ag/Ni/Au and ITO/Ni/Ag/W/Cu was not degraded on annealing, but instead it was slightly increased to 81.4% and 83.7% at 460 nm, respectively, because the Ag reflector was protected with Ni/Au or W/Cu encapsulation layers. Moreover, on annealing of the ITO/Ag-based p-contacts, the vertical structure GaN-based LEDs with thermally stable ITO/Ni/Ag/Ni/Au and ITO/Ni/Ag/W/Cu p-contacts showed higher output power of 132 mW and 142 mW at 350 mA, respectively, compared to those with thermally unstable ITO/Ag and ITO/Ni/Ag p-contacts. [ABSTRACT FROM AUTHOR]

Growth behavior of copper on TiN and TaN diffusion barriers according to the compositional change of copper-citrate (Cu-Cit) complex baths was studied in seedless copper electrochemical deposition. The peak potentials of cyclic voltammograms in the forward potential sweep were dependent on the concentration ratio of Cu2+ and Cit3- in the Cu-Cit baths. For the bath of [Cu2+]/[Cit3-] = 1 when one-step deposition of copper was performed at a peak potential for competitive reduction of both Cu2+ and CuCit- ions, the 3D island growth of copper including lamella-like structure was shown. A peak for Cu2+ reduction and a peak for CuCit- reduction existed separately for the bath of [Cu2+]/[Cit3-] = 2. Two-step deposition of copper in this bath gave birth to the 3D island plus layer-by-layer growth of copper. Interfacial adhesion between copper and TaN was measured as 12 and 34 J m-2, respectively, for both deposition conditions. The electrical resistivity of copper films grown by two-step deposition was lower than that by one-step deposition for the same thickness. [ABSTRACT FROM AUTHOR]

The preacceleration of electrons through reflection and shock drift acceleration (SDA) is essential for the diffusive shock acceleration of nonthermal electrons in collisionless shocks. Previous studies suggested that, in weak quasi-perpendicular (Q⊥) shocks in the high-β () intracluster medium (ICM), the temperature anisotropy due to SDA-reflected electrons can drive the electron firehose instability, which excites oblique nonpropagating waves in the shock foot. In this paper, we investigate, through a linear analysis and particle-in-cell simulations, the firehose instabilities driven by an electron temperature anisotropy (ETAFI) and also by a drifting electron beam (EBFI) in β ∼ 100 ICM plasmas. The EBFI should be more relevant to describing the self-excitation of upstream waves in Q⊥-shocks, since backstreaming electrons in the shock foot behave more like an electron beam rather than an anisotropic bi-Maxwellian population. We find that the basic properties of the two instabilities, such as the growth rate, γ, and the wavenumber of fast-growing oblique modes, are similar in the ICM environment, with one exception; while the waves excited by the ETAFI are nonpropagating (ωr = 0), those excited by the EBFI have a nonzero frequency (). However, the frequency is small with ωr < γ. Thus, we conclude that the interpretation of previous studies for the nature of upstream waves based on the ETAFI remains valid in Q⊥-shocks in the ICM. [ABSTRACT FROM AUTHOR]

The present paper provides a theoretical approach for the evaluation of the low frequency spontaneously emitted electromagnetic (EM) fluctuations in Kappa magnetized plasmas, which include the kinetic Alfvén, fast magnetosonic/whistler, kinetic slow mode, ion Bernstein cyclotron modes, and higher-order modes. The model predictions are consistent with particle-in-cell simulations. Effects of suprathermal particles on low frequency fluctuations are studied by varying the power index, either for ions (κi) or for electrons (κe). Computations for an arbitrary wave vector orientation and wave polarization provide the intensity of spontaneous emissions to be enhanced in the presence of suprathermal populations. These results strongly suggest that spontaneous fluctuations may significantly contribute to the EM fluctuations observed in space plasmas, where suprathermal Kappa distributed particles are ubiquitous. [ABSTRACT FROM AUTHOR]

The steady-state model recently put forth for the solar wind electron velocity distribution function during quiet time conditions, was originally composed of three population electrons (core, halo, and superhalo) with the core remaining nonresonant with any plasma waves while the halo and superhalo separately maintained steady-state resonance with whistler- and Langmuir-frequency range fluctuations, respectively. However, a recent paper demonstrates that whistler-range fluctuations in fact have no significant contribution. The present paper represents a consummation of the model in that a self-consistent model of the suprathermal electron population, which encompasses both the halo and the superhalo, is constructed solely on the basis of the Langmuir fluctuation spectrum. Numerical solutions to steady-state particle and wave kinetic equations are obtained on the basis of an initial trial electron distribution and Langmuir wave spectrum. Such a finding offers a self-consistent explanation for the observed steady-state electron distribution in the solar wind. [ABSTRACT FROM AUTHOR]

A recent study discussed the steady-state model for solar wind electrons during quiet time conditions. The electrons emanating from the Sun are treated in a composite three-population model—the low-energy Maxwellian core with an energy range of tens of eV, the intermediate ∼102–103 eV energy-range (“halo”) electrons, and the high ∼103–105 eV energy-range (“super-halo”) electrons. In the model, the intermediate energy halo electrons are assumed to be in resonance with transverse EM fluctuations in the whistler frequency range (∼102 Hz), while the high-energy super-halo electrons are presumed to be in steady-state wave–particle resonance with higher-frequency electrostatic fluctuations in the Langmuir frequency range (∼105 Hz). A comparison with STEREO and WIND spacecraft data was also made. However, ignoring the influence of Langmuir fluctuations on the halo population turns out to be an unjustifiable assumption. The present paper rectifies the previous approach by including both Langmuir and whistler fluctuations in the construction of the steady-state velocity distribution function for the halo population, and demonstrates that the role of whistler-range fluctuation is minimal unless the fluctuation intensity is arbitrarily raised. This implies that the Langmuir-range fluctuations, known as the quasi thermal noise, are important for both halo and super-halo electron velocity distribution. [ABSTRACT FROM AUTHOR]

In a recent paper, Kim et al. put forth a steady-state model for the solar wind electrons. The model assumed local equilibrium between the halo electrons, characterized by an intermediate energy range, and the whistler-range fluctuations. The basic wave–particle interaction is assumed to be the cyclotron resonance. Similarly, it was assumed that a dynamical steady state is established between the highly energetic superhalo electrons and high-frequency Langmuir fluctuations. Comparisons with the measured solar wind electron velocity distribution function (VDF) during quiet times were also made, and reasonable agreements were obtained. In such a model, however, only the steady-state solution for the Fokker–Planck type of electron particle kinetic equation was considered. The present paper complements the previous analysis by considering both the steady-state particle and wave kinetic equations. It is shown that the model halo and superhalo electron VDFs, as well as the assumed wave intensity spectra for the whistler and Langmuir fluctuations, approximately satisfy the quasi-linear wave kinetic equations in an approximate sense, thus further validating the local equilibrium model constructed in the paper by Kim et al. [ABSTRACT FROM AUTHOR]

This paper presents a theory for the asymptotically steady-state solar wind electron velocity distribution function (VDF) in a local equilibrium with plasma wave turbulence. By treating the local solar wind electron VDF as a superposition of three populations—the low-energy Maxwellian core electrons with an energy range of tens of eV, the intermediate –103 eV energy-range halo electrons, and the high –105 eV energy-range superhalo electrons—the present paper puts forth a model in which the halo electrons are in dynamical steady state with the pervasive whistler fluctuations, while the superhalo electrons maintain dynamical steady-state equilibrium with the Langmuir fluctuations, known as the quasi-thermal noise. Customary models of the solar wind electrons include only the Maxwellian core and the halo (plus highly field-aligned strahl). While the present paper does not consider the strahl population in the discussion, the highly energetic superhalo component, which is observed to be present in all solar conditions, is explicitly taken into account as part of the total solar wind electron model. Comparisons with STEREO and WIND spacecraft observations are also made. [ABSTRACT FROM AUTHOR]