AFM Studies On The Growth Mechanism And Surface Roughening Of Metallic Films On Liquid Substrates

It is well known that the microstructures and physical properties of thin films depend closely on the nature of the substrates. Since the fact that the liquid surfaces can be used as thin film substrates was discovered in 1990's, the formation mechanism, microstructures and physical properties of metallic thin films on liquid substrates have been studied systematically.Ultra-thin Al and Ni films, deposited on liquid substrates (i.e. silicone oil surfaces) by vapor phase deposition method, have been fabricated successfully and their growth processes, microstructures, dynamic scaling behavios in nanoscale as well as surface roughening mechanisms have been studied systematically in this dissertation.It is found firstly that the growth processes of the Al and Ni films approximately obey the two-stage growth model, which is similar to that of the other metallic thin film systems on liquid substrates. The first stage involves nucleation and growth of compact atomic clusters. In the subsequent stage, the clusters diffuse and rotate randomly on the liquid surface and then ramified aggregates are shaped. Continuous films finally form on the liquid substrates due to the growth and connection of these ramified aggregates.Further studies on the Al and Ni systems by atomic force microscopy (AFM) are presented. It is found that both the Ni and Al atomic clusters and ramified aggregates exhibit granular microstructures and the mean size of the grains is of the order of 101 nm. The diameter size of the granularities obeys a Gaussian distribution. If the deposition rate remain unchanged and the nominal film thickness increases, the mean size of the Ni atomic grains decreases quickly while that of the Al atomic grains does not change obviously. During the deposition process, the growth of the atomic grain mainly depends on the density of nuclei and deposition rate. As the nominal film thickness increases, the density of nuclei goes up rapidly and therefore the average number of the ad-atoms that each grain can meet decreases, which finally results in the decrease of the mean diameter. If increasing of the density of nuclei and keeping pace with the deposition rate, the increase of the diameter of the atomic grain owing to its capturing some new ad-atoms will keep a balance with the increase of the nuclei, which results in the saturation of the mean diameter of the atomic grains.Both the ultra-thin Al and Ni films exhibit a self-affine surface morphology and therefore their dynamic scaling behavior analysis in nanoscale and surface roughening mechanism analysis are performed. The root-mean-square roughness (wrms) and the frequency spectrum of the (1DPSD) were measured by the AFM software. It is found that:in the Al system, the growth exponentβ=0.23±0.05, and in the range d=0.1-1.0 nm, the roughness exponent a varies fromα≥1 to<1; In the Ni system,β=0.32±0.05 andα=0.7±0.1. Compared with the traditional statistical growth models, we found that the dynamic scaling behavior of the Al films can be well explained by the VLS (Villain and Lai-Das-Sarma) equation. While the dynamic scaling behavior of the Ni films maybe a multi-scaling result of the WV (Wolf-Villain) and VLS equation. Based to the exponental values of the Al and Ni systems, the surface roughening mechanisms were analysed.In the Al system:in the early growth stage, the smoothing term of surface diffusion would dominate at small time scales and the nonlinear roughening term does not play a role; as the deposition progress, the nonlinear roughening term begins to play an important role and finally the thin film surface approaches the situation which results in a balance between roughening mechanism and smoothing processes. In the Ni system, the experimental result indicates that the nonlinear roughening term of shadowing always plays an important role in the deposition process. The thin film surface maybe in the situation that there is a competition between the roughening mechanism caused by shadowing and white noise and smoothing process caused by the surface diffusion.This dissertation is organized as following:In chapterⅠ, a brief review of the growth mechanism of thin film systems on solid substrates and the research progress on dynamic scaling behavior and surface roughening is given. Then the research on the formation process, microstructures and physical properties of the metallic thin films deposited on liquid substrates is introduced.In chapterⅡ, the formation processs of the Al film system deposited on liquid substrates is studied systematically. The microstructure of the Al atomic ramified aggregates is studied by AFM.In chapterⅢ, the formation processs of the Ni film system deposited on liquid substrates is studied systematically. The microstructure of the Ni atomic ramified aggregates is studied by AFM. The dependence between the mean diameters of the Al and Ni atomic grains and the nominal film thickness are revealed.In chapterⅣ, the dynamic scaling behaviors and surface roughening mechanisms of the Al and Ni film systems are studied systematically.