Naokazu Murata

Fatigue strength of electroplated copper thin films under uni-axial stress

Fatigue strength of electroplated copper thin films was measured under uni-axial stress. Two kinds of electroplated films were prepared for the fatigue test. One was a commercial film mainly used for interconnections in printed wiring boards. The other film was grown on a stainless steel substrate by using acid copper sulfate bath without any additive agent. The micro texture of each film was observed by using SEM (Scanning Electro Microscope) and SIM (Scanning Ion Microscope). It was found that the micro texture of each film was quite different with each other. The mechanical properties such as the yield stress, fracture elongation and Youngs modulus of each film changed significantly from those of bulk copper depending on their micro structure. The low-cycle fatigue strength also varied drastically with each other, while the high-cycle fatigue strength was almost same. The fracture surfaces were observed by SEM after the fatigue test. It was found that there were two fracture modes under the fatigue test. One was a typical ductile fracture, and another was brittle one even under the fatigue load higher than its yield stress. The crack seemed to propagate through the grains when the ductile fracture occurred. On the other, the crack seemed to propagate along grain boundaries of columnar grains when the brittle fracture occurred. These results clearly indicated that the fatigue strength of electroplated copper thin films varies depending on their micro structure.

Micro texture dependence of the mechanical and electrical reliability of electroplated copper thin film interconnections

Both mechanical and electronic properties of electroplated copper films used for interconnections were investigated experimentally considering the change of their micro texture caused by heat treatment. Since those properties varied drastically depending on the electroplating conditions and thermal history after the electroplating, a novel evaluation method of the crystallinity of grains and grain boundaries of the electroplated copper thin films has been proposed by applying the conventional electron back scatter diffraction method. It was found the porous grain boundaries in the films cause brittle mechanical and electrical fractures of the films. The proposed method was effective for evaluating the crystallinity of grain boundaries quantitatively, and thus, estimating both the mechanical and electrical properties of the films used for mass production.

Effect of Crystallographic Quality of Grain Boundaries on Both Mechanical and Electrical Properties of Electroplated Copper Thin Film Interconnections

Effects of crystallographic quality of grain boundaries on mechanical and electrical properties were investigated experimentally. A novel method using two parameters of image quality (IQ) and confidence index (CI) values based on electron back-scattering diffraction (EBSD) analysis was proposed in order to evaluate crystallographic quality of grain boundaries. IQ value was defined as an index to evaluate crystallinity in region irradiated with electron beam. CI value determined existence of grain boundaries in the region. It was found that brittle intergranular fatigue fracture occurred in the film without annealing and the film annealed at 200 °C because network of grain boundaries with low crystallinity remained in these films. On the other hand, the film annealed at 400 °C caused only ductile transgranular fatigue fracture because grain boundaries with low crystallinity almost disappeared. From results of measurement of electrical properties, electrical resistivity of copper interconnection annealed at 400 °C with high crystallinity (2.09 × 10−8 Ωm) was low and electron migration (EM) resistance was high compared with an copper interconnection without annealing with low crystallinity (3.33 × 10−8 Ωm). It was clarified that the interconnection with high crystallinity had superior electrical properties. Thus, it was clarified that the crystallographic quality of grain boundaries has a strong correlation of mechanical and electrical reliability.

Improvement of the reliability of thin-film interconnections based on the control of the crystallinity of the thin films

In this study, the degradation mechanism of electronic performance of electroplated copper interconnections was investigated by considering their crystallinity of grain boundaries. The crystallinity of the interconnections was evaluated quantitatively by applying an EBSD (Electron Back-scattered Diffraction) method. It was found that the crystallinity of the interconnections varied drastically depending on the seed layer material for electroplating. A new design guideline for highly reliable electroplated copper thin film interconnections was proposed based on the measured results.

Quantitative Evaluation of the Crystallinity of Grain Boundaries in Polycrystalline Materials

The effect of the crystllinity of electroplated copper thin films on their mechanical and electrical reliability was investigated experimentally. The crystallinity of grains and grain boundaries was evaluated by IQ (Image Quality) value of EBSD (Electron Back-Scattering Diffraction) analysis. Mechanical properties of thin electroplated films were measured by tensile test. The crystallinity of the thin film was improved by annealing. Therefore, yield stress decrease and fracture strain increase with increase of annealing temperature. Thin film interconnections for measurement of electrical properties were prepared based on damascene process. The crystallinity of the interconnection without annealing was low. The crystallinity of interconnection was improved by annealing at 400°C for 30minutes. Though the EM (Electro-Migration) resistance of the annealed film was improved drastically, SM (Stress-induced Migration) was activated even though interconnection was kept at room temperature without any application of electrical current after annealing. This is because high residual stress was caused by shrinkage of electroplated copper due to change of crystallinity. Thus the control of the crystallinity of the electroplated film was very important to improve the reliability of the interconnection.Copyright

Improvement of Crystallographic Quality of Electroplated Copper Thin-Film Interconnections for Through-Silicon Vias

The relationship between the electrical properties and crystallographic quality (crystallinity) of electroplated copper thin-film interconnections was investigated. The crystallinity of the grains and grain boundaries of the interconnections was evaluated on the basis of the image quality (IQ) value obtained by electron back-scatter diffraction (EBSD) analysis. The electrical properties of the interconnections vary markedly depending on their crystallinity. The crystallinity also changed markedly as functions of electroplating conditions and the annealing temperature after electroplating. Although the electro migration (EM) resistance of the annealed interconnection was improved, stress-induced migration (SM) was activated by a high residual stress after annealing. To improve electrical reliability without heat treatment after electroplating, the effects of the seed layer under the interconnections on the crystallinity were investigated. As a result, the crystallinity was improved by changing the seed layer from Cu to Ru. In addition, the decrease in current density during electroplating also improved the crystallinity. Therefore, both introducing the Ru seed layer and decreasing the current density during electroplating are effective for developing highly reliable copper interconnections.

Mechanical and electrical reliability of copper interconnections for 3DIC

The mechanical and electrical properties of electroplated copper thin films were found to vary drastically depending on their electroplating conditions and thermal history after electroplating. The change of their microstructure was the main reason for the variation. The crystallinity of the micro texture was evaluated quantitatively by applying an electron back-scattering diffraction method.

HIGH TEMPERATURE DAMAGE OF NI-BASE SUPERALLOY CAUSED BY THE CHANGE OF MICROTEXTURE DUE TO THE STRAIN-INDUCED ANISOTROPIC DIFFUSION OF COMPONENT ELEMENTS

In order to assure the reliability of advanced gas turbine systems, it is very important to evaluate the damage of high temperature materials such as Ni-base superalloys under creep and fatigue conditions quantitatively. Since the micro texture of the gamma-prime (γ′ ) phase was found to vary during the creep damage process, it is possible, therefore, to evaluate the creep damage of this material quantitatively by measuring the change of the micro texture. The mechanism of the directional coarsening of γ′ phasesof Ni-base superalloy under uni-axial strain at high temperatures, which is called rafting, was analyzed by using molecular dynamics (MD) analysis. The stress-induced anisotropic diffusion of Al atoms perpendicular to the finely dispersed γ/γ′ interface in the superalloy was observed clearly in a Ni(001)/Ni3 Al(001) interface structure. The stress-induced anisotropic diffusion was validated by experiment using the stacked thin films structures which consisted of the (001) face-centered cubic (FCC) interface. The reduction of the diffusion of Al atoms perpendicular to the interface is thus, effective for improving the creep and fatigue resistance of the alloy. It was also found by MD analysis that the dopant elements in the superalloy also affected the strain-induced diffusion of Al atoms. Both palladium and tantalum were effective elements which restrain Al atoms from moving around the interface under the applied stress, while titanium and tungsten accelerated the strain-induced anisotropic diffusion, and thus, the rafting phenomenon.Copyright

Stress-Induced and Electro-Migration of Electroplated Copper Thin Film Interconnections Used for 3D Integration

Electroplated copper thin films have started to be applied to not only interconnections in printed wiring boards, but also thin film interconnections and TSV (Through Silicon Via) in semiconductor devices because of its low electric resistivity and high thermal conductivity. Thus, the electrical reliability of the electroplated copper interconnections was investigated experimentally. Self-made electroplated copper thin film interconnections were used for the evaluation. Electroplating conditions are as follows. The thin film interconnections were made by damascene process for electromigration tests. The applied current density during the test was varied from 1 MA/cm2 to 10 MA/cm2 . Abrupt fracture caused by the local fusion was often observed in the as-electroplated interconnections within a few hours during the test. Since there were a lot of porous grain boundaries in the as-electroplated thin films, the local high Joule heating should have caused the fusion at one of the porous grain boundaries. Actually, it was confirmed that the failure rate increased linearly with the square of the amplitude of the applied current density. However, the diffusion of copper atoms caused by electromigration was enhanced significantly when the film was annealed at 400°C. Many voids and hillocks were observed on their surfaces. This change of the fracture mode clearly indicates the improvement of the crystallographic quality of the annealed film. It was also observed that the stress-induced migration was activated substantially in the annealed film. Large voids and hillocks grew during the custody of the film even at room temperature without any application of current. This stress-induced migration was caused by the increase of residual tensile stress of about 200 MPa in the annealed film. It was also found that sulfur atoms segregated in the grown hillocks, though no sulfur atoms were found by EDX in the initial as-electroplated interconnections or other area in the annealed thin film interconnections. Thus, the hillock formation in the annealed interconnections was enhanced by the segregation of sulfur atoms. These sulfur atoms should have been introduced into the electroplated films during electroplating. Therefore, it is very important to control the micro texture, the residual stress and the concentration of sulfur in the electroplated copper thin film interconnections to assure the stable life, in other words, to eliminate their sudden brittle fracture and time-dependent degradation caused by the residual stress in the thin film interconnections.Copyright

Micro-texture dependence of stress-induced migration of electroplated copper thin film interconnections used for 3D integration

Effect of the micro texture of electroplated copper thin film interconnections on stress-induced migration was investigated experimentally and theoretically. The micro texture of electroplated copper thin films changed drastically as a function of the annealing temperature after the electroplating. However, stress-induced migration was activated even though the thin film interconnection was kept at room temperature after annealing. As a result, voids and hillocks appeared on the thin film interconnection. This is because high residual stress was caused by shrinkage of the thin film interconnection due to the densification caused by recrystallization. Molecular dynamics simulations showed that the diffusivity of copper atoms along grain boundaries with low crystallinity was enhanced significantly by high tensile residual stress. Therefore, the grain boundary diffusion accelerated by tensile residual stress is the main reason for the formation of hillocks and voids in the thin film interconnection after annealing.