발간논문

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In ULSI fabrication community, copper is now widely accepted as a new interconnect material to replace aluminum and its alloys due to its lower resistivity and higher electromigration resistance. In this study Cu seed layers deposited by magnetron sputtering onto a tantalum nitride barrier layer were given plasma H₂ and rapid thermal annealing(RTA) treatment for Cu nucleation enhancement prior to Cu electroplating. The combined effects of plasma H₂ pretreatment and rapid thermal annealing pretreatment of the Cu seed layer have been investigated on the resistivity, grain size and surface roughness of the electroplated copper films. The optimum pretreatment conditions for the Cu seed layer to obtain desirable surface roughness, grain size and resistivity of electroplated copper films were found to be the rf-power of 100W and the exposure time of 10min followed by rapid thermal annealing at 350℃. The mechanism through which Cu nucleation in electroplating is enhanced by plasma H₂/RTA has also been discussed.

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Both Al-21Ti-23Cr(L1₂+Cr₂Al) and Al-37Ti-12Cr(γ+TiAlCr) two-phase alloys have been accepted to be the most pertinent coating materials in Al-Ti-Cr alloy system to improve the high-temperature oxidation resistance of TiAl alloy taking into consideration of the oxidation properties, the resistance to thermal stress and the chemical compatibility. In this study, the analysis on thermal stress through a thermal expansion coefficient, a tensile and compressive test and reaction properties with TiAl alloy by means of bonding coating materials and TiAl substrate was performed. This elucidates the long-term oxidation properties in L1₂-based and γ-based alloys. The difference from a short-term oxidation resistance between L1₂-based alloy and the γ-based alloy was not observed because both alloys could form a stable Al₂O₃ layer. However, although both alloys show similar resistance to thermal stress, it was confirmed that the L1₂-based two-phase alloy has much better oxidation resistance than γ-based two-phase alloy in a long-term oxidation behavior considering the interface reaction properties with TiAl alloy.

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A fully coupled numerical model was developed to predict the formation of macrosegregation with consideration of deformation in continuous casting of steel slabs. A continuum formulation was adopted to investigate the macroscopic transport of momentum, heat and species in the continuous casting process. Darcian damping and thermosolutal connections were considered in the simulation to evaluate solute redistribution in the mushy zone. The BFC(Boundary-Fitted Coordinate) method was adopted to simulate the deformed grid and the solid phase velocity in the mushy zone due to the bulging that occurred between rolls. The velocity of the solid phase due to the deformation of a slab was calculated to apply the continuum model. The effects of bulging and casting speed were investigated on the solidification characteristics of cast steel slabs, such as the formation of solidification shell and the temperature distribution. It was found that the bulging caused by ferro-static pressure had a great effect on the formation of both centerline and inverse segregation. In addition, segregation simulations were performed for various chemical components, such as C Mn, P and S. It was also found that the present model can be successfully applied to predict the formation of centerline segregation in continuous casting of steel slabs.

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A numerical simulation was carried out to predict the compressible fluid flow of oxygen jet from the top blown-lance in RH vessel under reduced pressure. The shock and expansion waves of a jet coming from a convergent-divergent nozzle were predicted with the change of pressure conditions in RH vessel using single-hole nozzle. The effects of tilt angle at a given PCD were examined in RH vessel using tri-hole nozzle. Finite volume method was used as space difference scheme, Rho`s Approximate Riemann scheme to calculate flux at the cell face and LU-SGS as time integral scheme. The results provide oxygen jet flow characteristics due to the change of pressure condition in RH vessel. At lowered pressure conditions of 100Pa a supersonic flow under expanded jet was Found and that of 5,000Pa oblique shock waves with over expanded jet was found. On the contrary pressure condition of 50,000Pa, subsonic flow was developed.

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Electronic pads and interconnects consisting of a passivation layer (hard layer) and a metal layer (ductile layer), e.g. SiN/Al/SiO₂/Si, are very important in many electronic devices. However, they are likely to fail due to cracking of the passivation layers during operation of the devices. This study investigates the failure mechanism through finite-element analysis(FEA). FEA calculations reveal that residual stress is produced in the passivation layer as a result of thermal cycling and increases as cycling continues. During thermal cycling, the multilayer films are deformed by various stresses due to differential thermal expansion of adjacent materials, particularly shear stresses applied to the chip (or die) surface by the large thermal mismatch between substrate and chip (or die). The cyclic shear stress leads to accumulation of asymmetric plastic deformation (called the ratcheting effect) in the metal layers. The residual strain or stress in the passivation layer may be induced by the ratcheting deformation of metal layer. The effects of the thickness and the yield stress of the passivation and metal layers were analyzed through FEA and discussed. The residual stress in the passivation layer decreases with passivation layer thickness and metal layer yield strength, which is closely related to the likelihood of plastic yielding in the metal layer.

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Magnetoresistance effects have been investigated for Fe₃O₄ samples sintered with three type Fe₃O₄ powders; commercial Fe₃O₄ powders, Fe₃O₄ powders milled in Ar and ethyl alcohol. The largest MR ratio, 8% at 10kOe, was obtained in Fe₃O₄ sample sintered with powders milled in ethyl alcohol. The enhanced MR ratio appears together with the excess resistance, which regarded as tunneling barriers, at the interface between Fe₃O₄ particles. It indicates that the enhanced MR ratio arises from the spin dependent tunneling effect and can be explained with increased interparticle contacts. On the other hand, high sensitivities of Fe₃O₄ samples at low magnetic fields were achieved by fabricating the rod and ring type hybrid structure, Fe_(19)Ni_(81)/Fe₃O₄/ Fe_(19)Ni_(81). Compared with the MR values, 4%, at 2kOe for single Fe₃O₄ sintered body, same MR values for rod and ring type hybrid structures were obtained at 120Oe and 58Oe, respectively. These high sensitivities at low magnetic fields were attributed to magnetic fields increased by high permeability of Fe_(19)Ni_(81), at the space between the Fe_(19)Ni_(81) bulks.

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