The decisive criterion associated with the species emission intensity ratio (H alpha/SiH*) which characterizes the crystallinity of microcrystalline silicon (mu c-Si) film was found to display an unstable behavior resulting from species concentration variation during mu c-Si film growth with optical emission spectroscopy (OES) tool. In this study, a real-time process control system i.e. closed-loop system was developed. It aims to control the species intensity ratio with OES device in a very high frequency (VHF) plasma enhanced chemical vapor deposition reactor, via modulating the VHF power and silane dilution to improve mu c-Si film growth for high efficiency a-Si/mu c-Si tandem solar cell. The experiment results show that the closed-loop system stabilized the H alpha/SiH* intensity ratio within a variation of 5% during the mu c-Si film deposition process. Higher growth rate of mu c-Si film with the same crystallinity was obtained in the closed loop system which consumed less power and SiH4 gas than in the open loop system, i.e. without process control. (c) 2011 Elsevier B.V. All rights reserved.