Yasushi Sasajima

Molecular dynamics study of cluster deposition in thermal plasma flash evaporation

Abstract We simulated the rearrangement process of a high-temperature cluster put on a low-temperature substrate using molecular dynamics to verify the rearrangement behavior of the hot clusters. In the first step, a monoatomic homogeneous system was simulated. A spherical fcc stacked cluster consisting of 418 atoms corresponding to a 2 nm cluster, was placed on the 6 planes of a fcc (111) substrate. The interaction potential was assumed to be the Morse function corresponding to a material with a melting point of 1680 K. The substrate temperature Tsub and the initial cluster temperature Tinitcluster were varied from 300 to 1000 K and from 1450 to 3000 K, respectively. In the case of Tinitcluster 2400 K rearranged markedly into almost two-dimensional clusters even at Tsub=300 K and the degree of rearrangement seemed to be independent of the substrate temperature. It was revealed that high temperature nanoscale clusters could deform easily even on a room-temperature substrate owing to their high internal energy. This was confirmed by scanning tunneling microscopy.

Monte Carlo simulations of film growth

The difference and similarity between Monte Carlo simulations and molecular dynamics simulations are discussed, and the combined method between the two methods is also examined. These methods are used for the study of elemental processes of thin film formation. First, the computer simulations of film growth are carried out using general expressions of interatomic potentials, i.e. the Morse function and the Lennard-Jones function. The growth mode of thin films and the structure of the interface between film and substrate are studied as functions of the energy parameters and the lattice mismatch between film and substrate materials. Next, realistic simulations of film growth are made using the embedded atom method potential, and the structure of the interface is studied for some particular materials. It has been shown that the general expressions of the interatomic potential are useful for general discussions of the growth mode of thin films, while the realistic potential is suitable for obtaining information of film growth of particular materials. Assumptions and simplifications introduced in the simulations are discussed in detail. The role of the model in the computer simulations is particularly emphasised.

Monte Carlo simulation of grain boundary cross effect on hydrogen diffusivity in aluminum

Assuming the jumping of interstitial impurities to neighboring sites, the diffusivity is simulated by the Monte Carlo method based on the random walk theory. The jumping probability along a grain boundary is greater than that in the lattice of an inside grain. The crossing point of grain boundaries has a smaller jummping probability to neighboring sites on the grain boundary than that in the lattice. The results of simulation support the grain boundary cross effect, proposed experimentally by us, on the grain size dependence of hydrogen diffusivity in an aluminum polycrystal. The diffusivity shows a peak at some critical grain size. The peak includes two conflicting effects of grain boundaries: enhancement of diffusion along grain boundaries and suppression of diffusion by trapping at grain boundary junctions.

Reduction in resistivity of 50 nm wide Cu wire by high heating rate and short time annealing utilizing misorientation energy

The resistivities and microstructures for 50 nm Cu wires fabricated by high heating rate (3 K/s) and short time (1 min) annealing using infrared rapid thermal annealing equipment have been investigated as a function of annealing temperature and compared to those properties for wires fabricated by a slow heating rate (0.08 K/s), long time (30 min) conventional H2 annealing process. The resistivity of wires annealed by the new process decreased substantially with increasing annealing temperature from 573 to 773 K. The resistivity had its lowest value between 773 and 873 K, and it increased rapidly with annealing temperature above 923 K. The average ρ value was 2.98 μΩ cm for 773 K new process wires, whereas average ρ values were about 3.55 μΩ cm for 573 K and 3.42 μΩ cm for 673 K conventionally H2 annealed wires. This resistivity value for the new process wires was about 16% lower than the value for wires annealed at 573 K and 13% lower than the value for the wires annealed at 673 K by the conventional H2 a...

Molecular Dynamics Study of the Structural Changes of Ultra-fine Particles

The dynamics of ultra-fine particles (UFPs) have been studied by the molecular dynamics method. The Morse-type function has been assumed as the interaction potential between the atoms in UFPs. The numbers of atoms of the calculated systems have been 13 to 459. The particles with magic numbers, 13, 55 and 147, have shown the structural change from fcc to icosahedron, while the system of 459 atoms has not.

Monte Carlo simulation of diffusion process on quasi-lattice

The characteristics of the diffusion process of quasicrystals are studied for one-dimensional and two-dimensional models by the conventional Monte Carlo simulation. For the one-dimensional model, the tracer diffusion coefficients of an isolated particle are calculated for the three types of lattice models, i.e., single crystal, binary solid solution and quasicrystal. The characteristics of diffusivity of the one-dimensional quasicrystal model are derived from comparisons among the three types of models. For the two-dimensional model, the characteristics of diffusivity of a quasicrystal are discussed in comparison with those of the honeycomb lattice. In particular, the concentration dependence of diffusivity on the two-dimensional Penrose lattice is calculated and the effects of interatomic potential are discussed.

Molecular Dynamics Study of the Thin Film Formation Process

Abstract The initial stage of thin film formation process was studied by the molecular dynamics method. The Morse function was assumed as the interaction potential between the substrate-substrate, substrate-deposited and deposited-deposited atoms. The interfacial structure of the calculated system was restricted to be coherent, i.e. the structures of the substrate and the deposited atoms were both fcc and had the same lattice constants. The depth of the potentials for the three types of interaction was changed to study its effects on the film structure. The substrate temperature was also varied to see the temperature dependence of the relaxation process of the film structure. Two kinds of structure of the deposited films were considered; three atomic layers of fcc(111) and a monolayer of fcc(111). Both consisted of 19 atoms. They were considered as the nucleus of the film at the initial stage of the deposition process. The deposited nuclei were relaxed into equilibrium shapes such as the monolayer, the 3 ...

Void generation during the annealing process of very narrow copper wires

We carried out experiments on stress-induced void formation in ultrathin Cu wires while varying heat-treatment temperature, wire dimensions, and overlayer thickness. We also did molecular dynamics simulations of void formation in a buried wire of nanometer scale and compared these results with experimental results to clarify details of the void formation mechanism. The experimental and simulation results showed good accordance in explaining the effects of wire width, overlayer thickness, and cooling rate on void formation. (1) The narrower the wire width, the easier the void formation. (2) The thicker the overlayer, the easier the void formation. (3) The larger the cooling rate, the greater the suppression of void formation. From the obtained results, we constructed a void formation model for a buried wire. The basic concept of the model describes how local strain at four trench corners is relaxed in the buried wire in the annealing process. There are two ways to relax the local strain: (1) structural rel...

Computer Simulation of the Film Growth Process on the Two-Dimensional Penrose Pattern

Abstract We performed computer simulation of the film growth process on the two-dimensional (2D) Penrose pattern, which is considered a typical structural model of quasicrystal. The atomistic structure of the deposited atoms was obtained as a function of time under various conditions of atomic binding energy, temperature and deposition rate. The Monte Carlo method based upon the solid-on-solid model was utilized for the present calculation. We found a geometrical restriction on the growth front of grains in the 2D Penrose pattern, which is also expected for the growth of an actual quasicrystal.

Computer Simulation of Film Growth Process on the Two-Dimensional Penrose Pattern

We performed computer simulation of the film growth process on the two-dimensional (2D) Penrose pattern, which is considered a typical structural model of quasicrystal. The atomistic structure of the deposited atoms was calculated as a function of time under various conditions of atomic binding energy, temperature and deposition rate. The Monte Carlo method based upon the solid-on-solid model was utilized for the present calculation. We found a geometrical restriction on the growth front of grains in the 2D Penrose pattern, which is also expected for the growth of an actual quasicrystal.