Toshihiko Koseki

Cu Wettability and Diffusion Barrier Property of Ru Thin Film for Cu Metallization

The main issue of Cu metallization is the electromigration of Cu through the interface between Cu and the barrier or capping layer. To improve electromigration resistance at the Cu and barrier metal interface, insertion of a glue layer which enhances the adhesion of Cu onto the under layer may be effective. The wettability of Cu on Ru and Ta glue layers was evaluated as the index of Cu adhesion strength onto glue layers. The wetting angle of Cu (43°) on a Ru substrate was three times lower than that of Cu (123°) on a Ta substrate after annealing. Lower wetting angle of Cu on a Ru substrate indicates a good adhesion property between Cu and Ru and may imply a high electromigration resistance. The better Cu wettability of Ru compared to Ta can be explained by the concept of lattice misfit. A Ru(002) plane has lower lattice misfit, which suggests lower interface energy, and enhanced the adhesion of Cu onto Ru. However, the Ru film showed poor Cu diffusion barrier properties, which suggests Ru should be used as a glue layer in combination with another barrier layer.

Solidification of undercooled Fe-Cr-Ni alloys: Part I. Thermal behavior

Solidification of undercooled Fe-Cr-Ni alloys was studied by high-speed pyrometry during and after recalescence of levitated, gas-cooled droplets. Alloys were of 70 wt pct Fe, with Cr varying from 15 to 19.7 wt pct, balance was Ni. Undercoolings were up to about 300 K. Alloys of Cr content less than that of the eutectic (18.1 wt pct) have face-centered cubic (fee) (austenite) as their equilibrium primary phase, and alloys of higher Cr content have body-centered cubic (bcc) (ferrite) as their equilibrium primary phase. However, except at low undercoolings in the hypoeutectic alloys, all samples solidified with bcc as the primary phase; the bcc then transformed to fcc during initial recalescence for the lower Cr contents or during subsequent cooling for the higher Cr contents. The bcc-to-fcc transformation, whether in the semisolid or solid state, was detected by a second recalescence. In the hypoeutectic alloys, the growth of primary metastable bcc apparently results from preferred nucleation of bcc. The subsequent nucleation of fcc may occur at bcc/bcc grain boundaries.

Numerical simulation of equiaxed grain formation in weld solidification

Abstract Numerical simulation is developed on grain structure development during weld solidification of steel. Monte Carlo (MC) method is applied to the simulation of nucleation and growth of solid, being combined with finite difference calculation of heat conduction and solute diffusion. Based on the experimental result that titanium nitride (TiN) works as a nucleating agent of equiaxed grain formation, given number density of TiN is allocated to MC cells randomly and instantaneous nucleation is assumed to occur when the melt undercooling of the cell exceeds a given critical level. It is found that the simulation can reproduce the weld solidification by reflecting the effect of TiN as a nucleating agent and the effect of soluble titanium that increases melt undercooling. Those effects have been recognized only as combined effects in previous experimental studies, but through the present simulation, they are first investigated in a separate manner. The simulation results indicate that equiaxed grain formation is promoted not only by a nucleating agent, TiN, but also by melt undercooling increased by soluble titanium and that both are requisite to bring about the columnar-to-equiaxed transition (CET). The results are discussed in connection with theoretical models on CET and confirm the rationality of the models.

Numerical modeling of solidification and subsequent transformation of Fe-Cr-Ni alloys

A computational method for the analysis of phase transformation involving solidification was developed with the assumption of thermodynamic equilibria at interfaces. The region of interest was divided into finite segments, and solute diffusion across the segments was computed by the use of the direct finite difference method (FDM). Simultaneously, thermodynamic equilibrium at each interface was updated at every step of the diffusion analysis to determine the location of the interfaces. The temperature decrease and the increment of fraction solid were calculated based on thermal balance, including a heat extraction condition. Solid state transformation from δ to γ phase within each FDM segment was modeled by the use of a Clyne-Kurz (C-K) type analysis with assumptions of complete mixing of solutes in theδ phase and limited back diffusion in theγ phase. The calculation results were compared with welding solidification experiments in the iron-chromium-nickel ternary system. Good agreement was obtained with respect to solute distribution and residual fraction ofδ phase over different compositions and solidification modes of the alloys used.

Material Consideration on Ta, Mo, Ru, and Os as Glue Layer for Ultra Large Scale Integration Cu Interconnects

The electromigration resistance of ultra-large scale integration (ULSI) Cu interconnects can be improved by inserting an adhesion promoter between Cu and the diffusion barrier. A metallurgical survey was accomplished to select the element having a good Cu adhesion property. For adoption as an interconnect material, it should have a low resistivity and should not react with Cu to avoid increasing the resistance of Cu interconnects. Ru, Os, Mo, W, and Ta satisfied the above conditions. The Cu adhesion property of these elements was estimated by the lattice misfit concept. The Cu adhesion property was experimentally examined and compared hcp elements (Ru and Os), which have a good matching interface with fcc Cu, with the bcc elements (Mo and Ta). Ru and Os, which had lower lattice misfit values, showed a better adhesion property than the bcc elements having higher lattice misfit values. Among these elements, Ru had the best Cu adhesion property and thus it can be an optimum glue layer element for Cu interconnects.

Formation mechanism of vermicular and lacy ferrite in austenitic stainless steel weld metals

Abstract Solidification and subsequent transformation of austenitic stainless steel weld metals that solidified in the ferritic–austenitic mode were investigated from the viewpoint of crystallography. The formation mechanisms for the vermicular and lacy ferrite observed in the weld metals were clarified. The ferrite morphology is determined by both the crystallographic orientation relationship between ferrite and austenite established at the stage of ferrite nucleation and the relationship between the welding heat source direction and the preferential growth directions of ferrite and austenite. In particular, for the formation of continuous lacy ferrite, it is necessary that the ferrite continues to grow with the Kurdjumov–Sachs orientation relationship with austenite that is established at the stage of ferrite nucleation.

Solidification of undercooled Fe-Cr-Ni alloys: Part II. Microstructural evolution

Results are reported on microstructures of Fe-Cr-Ni alloys, solidified over a range of undercoolings and quenched during or after recalescence. Alloys studied contained 70 wt pct Fe and with Cr varying from approximately 15 to 20 wt pct. The three lower Cr alloys were hypoeutectic (with fee as primary phase in equilibrium solidification); the two higher Cr alloys were hypereutectic (with bcc as primary phase in equilibrium solidification). Results obtained are in agreement with predictions based on thermal analyses previously presented; they confirm and extend the understanding gained in that work. The primary phase to solidify in the hypoeutectic alloys is bec when undercooling is greater than an amount which decreases with increasing Cr content. At the lower Cr contents, the stable fcc phase then forms by solid-state transformation of the metastable phase and its subsequent engulfment by additional fcc. At the higher Cr content, transformation is by a peritectic-like reaction in the semisolid state, except near the surface at higher undercoolings where the transformation is massive. In the hypereutectic alloys, primary solidification at all undercoolings is the stable bcc phase. Partial transformation to fcc occurs in the semisolid or solid state, depending on composition and undercooling.

Characterisation of interface of steel/magnesium FSW

Abstract Mild steel with/without zinc coating and magnesium alloy AZ31B were lap joined by friction stir welding during the tool plunging through the magnesium into the steel. With increasing welding speed, the fracture strength increases. At the interface of zinc coated steel and magnesium, a liquid eutectic layer was detected for higher but not for lower welding speeds. At the interface between the steel without zinc coating and magnesium, no melting could be detected.

Influence of impact parameters of zirconia droplets on splat formation and morphology in plasma spraying

In this study, the effects of the impact parameters, namely, the diameter d0, velocity V0, and temperature T0, of an impacting droplet of yttria-stabilized zirconia (YSZ) on splat morphology have been investigated systematically under plasma spraying conditions. In particular, fully molten droplets of 30–90μm in d0 that impact on a preheated quartz glass substrate at V0 of 10–70m∕s have been examined via hybrid plasma spraying. The degree of flattening of final splat morphology, ξ, was found to be predicted by the relationship ξ=0.43Re1∕3, where Re is the Reynolds number. The dimensionless spreading time of droplets, ts*=tsV0∕d0, was distributed around 2.7, where ts is the spreading time of the droplet. The ideal maximum spread factor derived from the splat height was approximately proportional to Re1∕4. The latter two findings suggest that the analytical model developed by Pasandideh-Fard et al. [Phys. Fluids 8, 650 (1996)] can be applied to the droplet impact in plasma spraying especially for the case o...

Effect of surfactant redistribution on weld pool shape during gas tungsten arc welding

Abstract In previous evaluations of GTA welding on stainless steel plates the surfactant content in the workpiece has been assumed to be in a state of equilibrium. However, surfactant concentration usually had to be adjusted in order to achieve agreement between the weld pool shapes in experiments and those in simulations. In this study a physicochemical approach to the redistribution of surfactant at the surface and in the bulk is presented for the first time. Sulphur was considered as a representative surfactant. The model allows surfactant molecular transport via convection, diffusion, and sorption. It is shown that sulphur atoms accumulate at the surface at stagnation points, affecting the coefficient of surface tension and thus the final weld pool shape. Thus, the sulphur content in the simulation approaches the nominal sulphur content in the steel plate. The theory is strengthened by both electron probe microanalysis and Auger electron spectroscopy.