Takenao Nemoto

Numerical analysis on hydrogen diffusion and concentration in solid with emission around the crack tip

Abstract Construction of a physical model and numerical analysis were carried out concerning hydrogen diffusion in solid under elastic-plastic local stress field with hydrogen emission around the crack tip due to the dissolvent anodic reaction. These analyses showed that hydrogen diffuses and concentrates at the site of the elastic-plastic boundary in the direction of crack length when several mechanical conditions are satisfied. Hydrogen accumulation becomes much more remarkable with increase of yield stress. The effect of the diffusion constant is found to correspond to the rate of hydrogen diffusion. It accelerates the rate of increase of hydrogen accumulation at the elastic-plastic boundary, although the maximum saturated value of the concentration is not affected. This analysis shows that a high strength steel is liable to cause the microcrack initiation and coalescence with the main crack due to the hydrogen accumulation and embrittlement at the elastic-plastic boundary. This gives the theoretical foundation to the mechanism on the stress corrosion cracking accompanied with subcritical crack growth for high strength steels.

The characteristics of hydrogen diffusion and concentration around a crack tip concerned with hydrogen embrittlement

Abstract Previously, we proposed a physical model for hydrogen diffusion and accumulation around the crack tip and performed accurate numerical analysis which takes account of the effects of both hydrogen diffusion and accumulation due to the stress gradient. Based on this analysis, the characteristics of hydrogen accumulation around the crack tip were clarified. Since the characteristics of stress corrosion cracking and corrosion fatigue are dominated by chemical anodic reaction, hydrogen embrittlement and dislocation mechanism, to perform the analysis on the competitive phenomenon by these mechanism and to relate the sensitivity of hydrogen embrittlement to the characteristics of corrosion fatigue, it is necessary to construct a exact physical law on the characteristics of hydrogen diffusion and concentration and to formulate the characteristics as a simple function such as diffusion constant, D, yield stress σys, and stress intensity factor, K. The effect of stress field such as plane strain and plane stress on the hydrogen embrittlement is necessary to be clarified as the effect of specimen thickness on the hydrogen embrittlement. In this paper, based on this view point, the effect of D, σys, and K on hydrogen embrittlement were investigated and formulated. A quantitative parameter which characterize hydrogen embrittlement was proposed for both cases of plane strain and plane stress conditions as the effect of specimen thickness on the hydrogen embrittlement.

Effect of Additives in Organic Acid Solutions for Post-CMP Cleaning on Polymer Low-k Fluorocarbon

The appropriate post- chemical mechanical polishing (CMP) cleaning solution for an advanced nonporous polymer, ultralow-k fluorocarbon film is proposed. While decrease in fluorine content measured by high-resolution X-ray photoelectron spectroscopy and increase in dielectric constant were observed after additives cleaning with oxalic acid, no significant change of fluorine content and dielectric constant were found after citric acid with different additives and acids without additives cleanings. This reveals that additives play an important role in degradation of the fluorocarbon film. An appropriate selection of additives is important to clean the fluorocarbon film in post-CMP cleaning in advanced large-scale integrated generations. To complete successful integration of the post-CMP cleaning on new dielectric materials such as nonporous low-k fluorocarbon film, an appropriate solution and additives should be carefully selected to avoid electrical degradation in subsequent process.

End-point detection of Ta/TaN chemical mechanical planarization via forces analysis

This study explores the transition of shear force spectral fingerprints during tantalum (Ta) and/or tantalum nitride (TaN) chemical mechanical planarization on patterned wafers using a polisher and tribometer that has the unique ability to measure shear force and down force in real-time. Fast Fourier Transformation is performed to convert the raw force data from time domain to frequency domain and to illustrate the amplitude distribution of shear force and down force. Results show that coefficient of friction, variance of shear force and variance of down force increase during polishing when the Ta/TaN layer is removed thus exposing the inter-layer dielectric layer. Unique and consistent spectral fingerprints are generated from shear force data showing significant changes in several fundamental peaks before, during and after Ta/TaN clearing. Results show that a combination of unique spectral fingerprinting, coefficient of friction and analysis of force variance can be used to monitor in real-time the polishing progress during Ta/TaN chemical mechanical planarization for optimal polishing time.

Numerical Analysis of Vacancy Transport by Residual Stress in Electromigration on LSI Interconnects

The Li multiplication method for the driving force induced particle migration equation was proposed to solve numerically the stress and electric fields induced vacancy migration equation. On the basis of this method, vacancy migration behaviors were found to be predicted under the competitive relationship between stress and electric field. When a residual stress is dominant, vacancies concentrate around the maximum hydrostatic stress region, such as elastic–plastic boundary. On the other hand, when the electric field is dominant, vacancies do not concentrate around the maximum hydrostatic stress region, but move from the cathode end to the anode end. An in situ observation of electromigration on AlCuSi interconnect was conducted to verify the simulation results. A void nucleated and grew at the tip of a notch on an AlCuSi line without passivation film, while many voids appeared and grew in cathode side in a line with passivation film. Greater hydrostatic stress gradient is considered to occur in the line without passivation film, owing to a small scale of yielding, and this leads to concentrate void formation at the tip of the notch. These results indicate that numerical analysis of electromigration is valid to represent the experimental result. Based on results mentioned above, various failure modes in interconnects cased by electromigration are predictable by the proposed Li multiplication method.

In situ Observation of Grain Growth on Electroplated Cu Film by Electron Backscatter Diffraction

In situ observations using an automated electron backscatter diffraction (EBSD) system with a heating stage were carried out to clarify the mechanism of grain growth on electroplated Cu and Cu deposited by physical vapor deposition (PVD). Hardness measurements and EBSD observations revealed that the strain in electroplated Cu and Cu deposited by PVD was released by annealing. Little preferential orientation of the texture was observed in electroplated Cu, while Cu deposited by PVD showed strong (111) texture. A greater number of twin boundaries were observed in electroplated Cu than in Cu deposited by PVD. Grains surrounded by random grain boundaries were more enlarged by annealing in electroplated Cu than those in Cu deposited by PVD. Since segregation of impurities with a low melting point, such as sulfur, to grain boundaries could promote grain growth, which is often accompanied by twin boundary formation, the reason that many twin boundaries are found in electroplated Cu is considered to be the existence of impurities such as sulfur at grain boundaries. The effect of a twin grain boundary on resistivity was calculated and the contribution of a twin grain boundary to resistivity was found to be much less than that of a random grain boundary.

Tribological Effects of Brush Scrubbing in Post Chemical Mechanical Planarization Cleaning on Electrical Characteristics in Novel Non-porous Low-k Dielectric Fluorocarbon on Cu Interconnects

Damage reduction during planarization is strongly required to avoid scratch generation and the variation in the electrical properties of low-k dielectrics leading to yield loss in an integrated circuit after the implementation of an ultralow-k dielectric in Cu damascene interconnects. An optimum process condition to reduce damage on brush scrubbing in post-chemical–mechanical-planarization (post-CMP) cleaning was proposed for advanced nonporous organic ultralow-k dielectric fluorocarbon/Cu interconnects. Increasing brush rotation rate by decreasing down pressures results in the improvement in both electric properties and particle removal efficiency. The tribological effects of brush scrubbing in post-CMP cleaning on the electrical characteristics were explored. The brush scrubbing condition of a high brush rotation rate at low down pressures contributes to the suppression of damage generation.

Damage-Free Post-CMP Cleaning Solution for Low-k Fluorocarbon on Advanced Interconnects

As technology node progressing, ultra low-k film has been implemented to reduce RC delay in LSI circuit. A fluorocarbon (CFx) film is proposed as foreground ultra low-k film because of non-porous structure [1]. Although CFx film is expected to be stable for its structure advantage, damage-less process is anticipated to avoid dielectric constants change in subsequence process steps [2]. CMP and post CMP process are concerned to bring damage on devices, so the effect of post CMP cleaning solutions on CFx structure and electric property is evaluated.

Electrical Characteristics of Novel Non-porous Low-

A novel non-porous low-k dielectric, fluorocarbon, deposited by new microwave excited plasma enhanced chemical vapor deposition was successfully integrated into Cu damascene interconnects for the first time. Electrical characteristics of fluorocarbon/Cu damascene lines are investigated. A compatible line to line leakage current to the one with porous low-k carbon doped silicon oxide and a low effective dielectric constant as a value of 2.5 are achieved. The novel non-porous ultralow-k dielectric, fluorocarbon, is considered as a promising candidate to extendible for 22 nm generation and beyond.

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Electromigration was analyzed mathematically by solving Huntingtons equation. A general solution assuming steady-state conditions was derived by Fourier transformation and complex integration. The analysis revealed that a significant accumulation of atoms at the anode induced hillock formation, and that atomic depletion at the cathode induced void formation. An electromigration acceleration test was performed to observe void formation using Al–0.5%Cu–1.0%Si on a SiO2/Si substrate. Experimental results of in situ observation were in good agreement with the theoretical result. The effect of a local stress field on void formation is discussed.