Multivariable model predictive control of thin film surface roughness and slope: Application to a 2D kinetic Monte-Carlo model

This work focuses on the application of a multivariable model predictive controller that regulates thin film surface roughness and mean slope to a two-dimensional kinetic Monte-Carlo thin film growth model using both substrate temperature and deposition rate as manipulated inputs. The description of the thin film growth involving both adsorption and surface migration is first given. Surface roughness and surface slope are defined as the root-mean-squares of the surface height profile and the surface slope profile, respectively. The dynamics of the evolution of the thin film surface height profile are assumed to be described by an Edwards-Wilkinson-type equation (a second-order stochastic partial differential equation) in two spatial dimensions. Analytical solutions of the expected surface roughness and surface slope are obtained on the basis of the Edwards-Wilkinson equation and are used in the controller design. The model parameters of the Edwards-Wilkinson equation depend on the substrate temperature and deposition rate. This dependence is used in the formulation of the predictive controller to predict the influence of the control action on the surface roughness and slope at the end of the growth process. The model predictive controller involves constraints on the magnitude and rate of change of the control action and optimizes a cost that involves penalty on both surface roughness and mean slope from the set-point values. The controller is applied to the two-dimensional kinetic Monte-Carlo thin film growth model and is shown to successfully regulate surface roughness and mean slope to set-point values at the end of the deposition.