Molecular Dynamic Simulation of Thin Film Growth Stress Evolution

With the increasing demand for thin films across a wide range of technology, especially in electronic and magnetic applications, controlling the stresses in deposited thin films has become one of the more important challenges in modern engineering. It is well known that large intrinsic stress—in the magnitude of several gigapascals—can result during the thin film preparation. The magnitude of stress depends on the deposition technique, film thickness, types and structures of materials used as films and substrates, as well as other factors. Such large intrinsic stress may lead to film cracking and peeling in case of tensile stress, and delamination and blistering in case of compression. However it may also have beneficial effects on optoelectronics and its applications. For example, intrinsic stresses can be used to change the electronic band gap of semiconducting materials. The far-reaching fields of microelectronics and optoelectronics depend critically on the properties, behavior, and reliable performance of deposited thin films. Thus, understanding and controlling the origins and behavior of such intrinsic stresses in deposited thin films is a highly active field of research.;In this study, on-going tensile stress evolution during Volmer-Weber growth mode was analyzed through numerical methods. A realistic model with semi-cylinder shape free surfaces was used and molecular dynamics simulations were conducted. Simulations were at room temperature (300 K), and 10 nanometer diameter of islands were used. A deposition rate that every 3 picoseconds deposit one atom was chosen for simulations. The deposition energy was and lattice orientation is [0 0 1]. Five different random seeds were used to ensure average behaviors.;In the first part of this study, initial coalescence stress was first calculated by comparing two similar models, which only differed in the distance between two neighboring islands. Three different substrate thickness systems were analyzed to ensure no simulation artifacts were introduced by this parameter. Results from the calculations showed that initial coalescence stress of 5 nanometer thickness substrate system is significantly lower than that of the other two systems. Then histogram analysis and stress coloring analysis were conducted to analyze the distribution of stress within thin films. It was concluded that substrates 10 nm thick were sufficient for subsequent stress evolution simulation studies.;In the second part of this study, on-going tensile stress evolution was examined by modeling atomic scale deposition (i.e. film growth) for at least 30 nanoseconds. Intrinsic stress as a function of effective island thickness, and force per unit width as a function of effective island thickness were obtained from simulations. Average stress behaviors and corresponding atomistic structure changes were analyzed.