Kouji Mimura

Thickness dependence of resistivity for Cu films deposited by ion beam deposition

Abstract The thickness dependence of the resistivity for Cu films deposited by ion beam deposition (IBD) was evaluated using Fuchs–Sondheimer (F–S) model for electron surface scattering and Mayadas–Shatzkes (M–S) model for electron grain boundary scattering. For fitting the F–S and M–S models to the experimental data, the approximate equations proposed in both models were discussed and it was confirmed that the experimental resistivity of the Cu films could be described well by a simple form combined of the approximate equations for both models. By means of the simple form in this work, the most reasonable fit to the experimental data could be obtained under the conditions of the surface scattering coefficient p =0 and the reflection coefficient at grain boundary R =0.40.

Electrical and thermal conductivities in quenched and aged high-purity Cu-Ti alloys

Abstract Electrical resistivity and thermal diffusivity at room temperature have been measured in quenched and aged high-purity copper–titanium alloys. The results show that the electrical and thermal conductivities increase as aging time at 720 K increases, which indicates that precipitation takes place in these alloys.

Removal of alloying elements from zirconium alloys by hydrogen plasma-arc melting

Abstract The removal of alloying elements such as Sn and Fe from zircaloy scrap was investigated using H2Ar (0% up to 50% H2) plasma-arc melting under atmospheric pressure and reduced pressure. While the removal rates of Sn and Fe are very low when the plasma gas is only Ar, they become higher with increasing hydrogen content of the plasma gas and with decreasing pressure during the melting process. Zirconium metal with above 99.9% purity is recovered from zircaloy scrap by hydrogen plasma-arc melting for 3 h at a reduced pressure of 9.3 kPa. Furthermore, hydrogen plasma-arc melting is confirmed to be superior to electron beam melting for the removal of alloying elements from the scrap. To clarify the refining effect and its mechanism, the hydrogen plasma-arc melting of Zr-1mass%Sn and Zr-1mass%Fe binary alloys was examined. The reactions of Sn and Fe removal from the Zr alloy melts obey a first-order rate law and their removal rates are found to increase proportionally to PH2 and P H2 3 4 for Sn and Fe respectively, where PH2 is the partial pressure of hydrogen in the plasma gas. This unique and excellent refining effect is supposed to be caused by activated hydrogen atoms formed in the high temperature plasma-arc.

Effect of substrate bias voltage on the thermal stability of Cu/Ta/Si structures deposited by ion beam deposition

The interfacial reactions of the Cu (100 nm)/Ta (50 nm)/Si structures and their relationship with the microstructure of Ta diffusion barrier are investigated. Ta films were deposited on Si (100) substrates using a non-mass separated ion beam deposition system at various bias voltages ranging from 0 to -200 V. An optimum applied substrate bias voltage of -125 V was found to yield a dominant α-Ta film with a noncolumnar structure, low electrical resistivity (about 40 µΩcm) and smooth surface. A Ta diffusion barrier which was deposited at the optimum bias voltage prevented Cu–Si interaction up to 600°C for 60 min in flowing purified H2, whereas a Ta layer with a columnar structure, deposited at zero bias voltage, degraded at 300°C. Two different reactions of the Cu/Ta (0 V)/Si and the Cu/Ta (-125 V)/Si structures concerning the thermal stability were investigated and discussed on the basis of the experimental results.

Influence of small amounts of impurities on copper oxidation at 600-1050°C

In order to study the influence of small amount of impurities on the copper oxidation kinetics, the oxidation was examined at 600–1050°C in 0.1 MPa oxygen atmosphere using 99.5% (2N) and 99.9999% (6N) pure copper specimens. The influence of impurities has been discussed considering the roles of the nonprotective CuO layer, the impurity layer at the Cu2O–Cu interface, and the diffusion of copper in the Cu2O layer. The nonprotective CuO layer for 2N copper can greatly enhance copper oxidation. However, the impurities concentrated at the region near the Cu2O–Cu interface for 2N copper can slow oxidation. Contrary to the presence of metallic impurities, such as Ni, Sb, and Pb, the nonmetallic elements As and Se dissolved in Cu2O have a deleterious influence on the outward diffusion of copper. Grain-boundary diffusion in Cu2O can somewhat contribute to 2N copper oxidation at 850–1050°C, but its effect in enhancing oxidation at 600–800°C is weaker than the effect of the impurity layer at the Cu2O–Cu interface in impeding oxidation.

Ultra-high purification of iron by anion exchange in hydrochloric acid solutions

Promising applications in advanced electronics have created a demand for ultra-high purity iron. Anion exchange in chloride solutions may offer an efficient and practical way of purification. Examination of anion-exchange distribution functions revealed a variety of behavior patterns to be exploited. Elution tests in laboratory-scale columns suggested that virtually all the impurities can be separated from the iron chloride solution by anion exchange. The hydrochloric acid concentration and volume of the rinsing and eluent solutions determine the efficiency of the procedure. Separation of certain elements (e.g. copper and molybdenum) requires strict control of redox conditions. The designed procedure consists of two anion-exchange steps. In the first step and during solution preparation, those impurities are separated which can be precipitated by iron, or sorbed in the resin at low HCl solutions under reducing conditions determined by the addition of Fe powder. The remaining impurities are separated from iron, applying a second ion exchange step under oxidizing conditions. Performance characteristics (yield, purification ratio and volume efficiency) of the separation indicate the optimum parameters of the rinsing and eluent solutions and the suitable way of defining the collected volume fraction of the effluent.

Purification of tantalum by means of hydrogen plasma arc melting

Abstract Further purification of commercially pure Ta metal ( > 99.9%) by hydrogen plasma arc melting (HPAM) has been examined under a pressure of 101.3 kPa using Ar-30 vol% H 2 plasma gas. Most impurities, except for Nb, Mo and W, are removed down to a few mass ppb levels. Concentrations of the elements which deteriorate the large scale integrated circuit performance, such as Fe, Th and U, are also decreased by two or more orders of magnitude. The final concentrations of many impurities in Ta purified by HPAM are lower than those achieved by electron beam melting (EBM) of Ta. The use of Ta, instead of W, as a cathode inside the plasma torch, makes it possible to avoid contamination of the Ta melt with W and Th from the thoriated W cathode commonly used. Possibility for the removal of Th, U and W from the Ta melt during HPAM was also discussed on the basis of thermodynamic calculations.

Effect of Alloying Mg on Corrosion Resistance of Cu at High Temperature

Effect of Mg alloying in Cu with pretreatments on corrosion behavior of Cu was studied, which was compared with that of a CuAl alloy. It was found that, at 673 K under oxygen pressure of 0.1 MPa, although the corrosion resistance of CuMg alloy was evidently improved in comparison with that of pure Cu, it was worse than that of CuAl alloy. Three possible reasons were: (i) Cu atoms were incorporated into the MgO surface layer. (ii) Fissures were produced in MgO layer. (iii) The adherence of MgO surface layer on Cu was poor. The first one could be due to formation of a new intermetallic phase of MgCu 2 on the surface of Cu sample during the formation of MgO surface layers, while the latter two were attributed to low Pilling-Bedworth ratio of Mg-O system smaller than 1.

Purity Effect on Oxidation Kinetics of Copper at 800-1050°C

To clarify the purity effect on copper oxidation kinetics, oxidation was carried out at 800-1050°C using 99.99% (4 N), 99.9999% (6 N) and floating zone refined (FZR, >99.9999%) copper specimens. In the Arrhenius plots of the parabolic oxidation rate constants for the double-layer (dl) formation (Cu 2 O + CuO) at 800-1050°C in 0.1 MPa O 2 atmosphere, the points for FZR copper essentially followed a straight line. For 6 N and 4 N coppers, the oxidation kinetics at high temperatures (above 950°C) was almost the same as that for FZR copper, but the points at lower temperatures followed two lines with smaller slopes. This result suggests that the oxidation of FZR copper was governed by the lattice diffusion at 800-1050°C. Trace impurities had almost no influence on the oxidation kinetics of 6 N and 4 N coppers at temperatures above 950°C where the lattice diffusion predominated. However, at lower temperatures where the grain boundary diffusion also contributes to oxidation, trace impurities decreased the activation energy by impeding the growth of Cu 2 O grains to facilitate the grain boundary diffusion. The activation energy governed by the lattice diffusion was 173 kJ/mol for the dl formation, while it was 98 kJ/mol for the single-layer formation (Cu 2 O). This supports a (pO 2 ) 1/4 dependence of the parabolic rate constant associated with the diffusion of copper atoms via neutral copper vacancies in the Cu 2 O layer.

Fast, vacancy-free climb of prismatic dislocation loops in bcc metals

Vacancy-mediated climb models cannot account for the fast, direct coalescence of dislocation loops seen experimentally. An alternative mechanism, self climb, allows prismatic dislocation loops to move away from their glide surface via pipe diffusion around the loop perimeter, independent of any vacancy atmosphere. Despite the known importance of self climb, theoretical models require a typically unknown activation energy, hindering implementation in materials modeling. Here, extensive molecular statics calculations of pipe diffusion processes around irregular prismatic loops are used to map the energy landscape for self climb in iron and tungsten, finding a simple, material independent energy model after normalizing by the vacancy migration barrier. Kinetic Monte Carlo simulations yield a self climb activation energy of 2 (2.5) times the vacancy migration barrier for 1/2〈111〉 (〈100〉) dislocation loops. Dislocation dynamics simulations allowing self climb and glide show quantitative agreement with transmission electron microscopy observations of climbing prismatic loops in iron and tungsten, confirming that this novel form of vacancy-free climb is many orders of magnitude faster than what is predicted by traditional climb models. Self climb significantly influences the coarsening rate of defect networks, with important implications for post-irradiation annealing.