Plasma enhanced chemical vapor deposition: Modeling and control

This paper focuses on modeling and control of a single-wafer parallel electrode plasma-enhanced chemical vapor deposition process with showerhead arrangement used to deposit a 500 Angstrom amorphous silicon thin film on an 8 cm wafer. Initially, a two-dimensional unsteady-state model is developed for the process that accounts for diffusive and convective mass transfer, bulk and deposition reactions, and nonuniform fluid flow and plasma electron density profiles. The model is solved using finite-difference techniques and the radial nonuniformity of the final film thickness is computed to be almost 19%. Then, a feedback control system is designed and implemented on the process to reduce the film thickness nonuniformity. The control system consists of three spatially distributed proportional integral controllers that use measurements of the deposition rate at several locations across the wafer, to manipulate the inlet concentration of silane in the showerhead and achieve a uniform deposition rate across the wafer. The implementation of the proposed control system is shown to reduce the film thickness radial nonuniformity to 3.8 %. (C) 1999 Elsevier Science Ltd. All rights reserved.