Modeling of laser chemical vapor deposition of thin films

Laser Chemical Vapor Deposition (LCVD) provides an excellent way of modifying surfaces. The deposition rate is found to be very high in the LCVD technique compared to the conventional Chemical Vapor Deposition (CVD) technique. The usefulness of the deposited film depends on its shape and morphology which are affected by the laser process parameters. This paper presents a mathematical model to study the effects of the laser parameters and processing conditions on the shape and size of the film. In many cases, the film is found to have volcano-like morphology which is also explained by this model. The LCVD process involves several physical phenomena such as the laser-matter interactions, transport processes, chemical reaction kinetics, and adhesion of the film material to the substrate surface. Depending on the laser-matter interactions, the LCVD technique is divided into two types that are referred to as the photolytic and pyrolytic LCVD techniques. In the photolytic process, the reactant molecules absorb the laser to undergo dissociation, whereas the substrate absorbs the laser energy in the pyrolytic process to create a hot spot at the substrate surface where the reactant molecules undergo thermal decomposition. The model of this paper is concerned with the pyrolytic LCVD technique. To model the LCVD process, the diffusion of various species is considered to be the dominant transport mechanism inside the LCVD chamber, and the threedimensional transient mass diffusion equation is solved. The thermal decomposition of the reactant molecules at the hot substrate surface is considered to follow Arrhenius' law of the chemical reactions. This model allows to select the appropriate process parameters to obtain a good quality film, and provides a means of determining the activation energy and pre-exponential factor from the film thickness and film deposition temperature data. The Damkohler number is shown to affect the morphology of the film. Also, an optimum condition is found to exist for depositing thin films by using the LCVD technique.