Papers by Keyword: Stress Migration

Abstract: The effect of surface film on the Al whisker fabrication by utilizing stress migration was investigated. The sample was a thin aluminum film deposited on an oxidized silicon substrate and covered with a surface film. Aluminum oxide layer, silicon oxide layer and Cu oxide layer were used as the surface films. Al whiskers were obtained only in the samples with aluminum oxide layer and silicon oxide layer after heat treatment. It was found that both the brittle surface films and compressive stress determined the Al whisker growth.

Abstract: By means of FLAC^3D software, firstly 3-demensional model was built, and stress migration and evolution law of coal seam in the process of mine roadway excavation are studied. Then according to the stress evolution law, the mechanism of acoustic emission (AE) generation in coal after excavation was analyzed. The results show that: after excavation, due to lateral displacement constraints in the coal body near the free face is removed, the coal in this zone was in 2-dementional stress condition. Because the vertical stress exceeded the uniaxial strength of coal body, the coal got into the post-peak strain-softening state. With the evolution of time, the stress in the zone gradually reduced until a state of equilibrium. In this zone the principle of AE generation is that lateral stress reduction result in macro crack propagation, coal mass failure, then elastic wave emitted. Signals come from the zone, i.e. stress relief zone, is the main part of AE signals, and should be the focus of study in AE technology of predicting coal or rock dynamic disasters. In stress-peak zone, due to the constraints of the lateral displacement, the lateral stress and strength of coal increased, the vertical stress of coal body gradually increased until the strength peak in the 3-dimensional stress state. After the excavation of roadway, the stress in stress-concentration and stress-peak zone gradually increased to equilibrium state over time. The principle of AE generation in this region is that due to the increase of the vertical stress, coal and rock gradually damaged that resulted in the release of the elastic wave.

Abstract: The problem of whisker formation in tin (Sn) wiring in small electronic devices has become an important issue with the requirement of lead-free wiring, because doping of Pb to reduce whisker formation cannot be applied. It is therefore urged to better understand stress migration in tin, which is suspected to play a key role in whisker growth. We aim to study grain boundary diffusion in tin by atomistic simulation. After constructing an efficient interatomic potential suitable for diffusion of atoms using the genetic algorithm (GA), we perform molecular dynamics (MD) simulation of grain boundary diffusion in Sn under stress. We find that the magnitude of stress effect on diffusion depends on the boundary structure. Moreover, we examine the effect of impurities on vacancy migration by ab initio calculation to find atom doping that has potential to suppress diffusion.

Abstract: Migration of atoms is presented to be utilized for fabrication of metallic micro/nanomaterials by controlling the phenomenon. Two kinds of migration phenomena are treated; one is electromigration and the other is stress migration. In addition to the formation of micro/nanomaterials, some achievements in enhancing their functions are demonstrated. One is a technique to fabricate nanocoils from the formed Cu nanowires. The others are techniques to weld or cut the micro/nanowires by using Joule heating. Finally, regarding evaluation of mechanical and electrical properties of the micro/nanomaterials, the concentrated-mass cantilever technique in atomic force acoustic microscopy and the four-point atomic force microscope technique are shown to be powerful tools, respectively.

Abstract: Stress-assisted atomic migration occurs in thin films due to thermal stress development, followed by hillock and void formation on a film surface. Relation between thermal stresses and hillock formation was investigated on copper films with and without passivation layer. Copper films with a thickness of 10, 50 and 100 nm on oxidized silicon wafer were prepared for investigating thermal stress and hillock formation. In-situ thermal stress observation by X-ray measurement revealed that compressive stresses develop in an early stage of heating followed by a sudden decrease in the temperature region between 100 and 200 deg. In a cooling stage, stresses in a film linearly changed with decreasing temperature to form a tensile residual stress state. Surface morphology is observed by optical microscope and SEM after the heat cycle as well as at elevated temperatures in a vacuum chamber. Dome-like swells were formed on an AlN passivation layer. Almost of all of the swells on 100 nm thick film collapsed after the heat treatment up to 350 deg whereas the swells on 10 nm thick film had no collapse excepting a few case. Comparing with the film without passivation, the swell is considered to be the result of atomic migration of copper film to form hillocks in the interface between copper film and AlN passivation film during heating. Atoms are considered to migrate reversibly into the copper film in the cooling stage, resulting to make vacant hall in the swell of AlN film and then collapse due to tensile stress development.