Thermally-Assisted Pattern Formation on Heteroepitaxial Thin Films. (Paperback)

The controlled growth of self-assembled patterns on initially flat and strained thin film heterostructures by photothermal assistance is developed in the present study. The material systems under consideration are lattice-mismatched layers, such as silicon-germanium (SiGe) films, for the numerical study, and coefficient of thermal expansion (CTE)-mismatched layers, such as metal/polymer films, for the experimental study. In the first part of the study, a governing equation is formed to simulate the growth behavior of SiGe films for the case of constant surface diffusivity, and then the investigation is further extended to the case of spatially-varying diffusivity by photothermal modulation. Results show that a stable film can be perturbed to self-assemble patterns of which surface profile wavelength follows that of modulation. In the second part, it is experimentally shown that planar aluminum/polymethyl methacrylate films also form ordered patterns on the surface by spatially non-uniform laser heating that locally induces anisotropic compressive stress in the metal layer as well as partial relaxation in the underlying polymer layer. More complicated structures, such as line-gratings and concentric rings, are realized with an interferometric laser heating system by using the thermomechanical growth behavior. Regardless of the source of strain, whether it is coming from the lattice-mismatch or the CTE-mismatch, it is shown that the morphology instability can be photothermally perturbed, leading to the achievement of spatial selectivity and controllability of growth that allows regularization of patterns. This eliminates the need for any prior photolithographic processes, as typically seen in other previous studies, required to order the orientation of self-assembly. Thus, the present investigation provides a path for a novel fabrication method in which a simple laser-treatment produces well-arranged periodic structures such as quantum-dots or nanogratings for device applications in microelectromechanical systems and nanotechnology.