Real-time feedback control of plasma density in inductively coupled plasmas

Summary form only given. The real-time feedback control has the potential to enhance the process reliability of VLSI fabrication. Most of the studies on real-time feedback control of plasma processing tools were performed on RIE systems. Optical emission spectroscopy was usually employed to serve as the in situ measurements. For inductively coupled plasmas (ICP), a controller based on response surface methodology was designed to control plasma parameters such as species densities, fluxes and energies by employing numerical plasma simulations. The purpose of this study is to demonstrate experimentally the real-time feedback control of plasma density in an ICP. The sensor of the controller was a 36 GHz heterodyne interferometer which measures the line-averaged plasma density. The 13.56 MHz RF power driving the ICP antenna was used as the actuator while the gas pressure was varied to serve as a disturbance causing the plasma density to deviate from the set point. To compensate for the effect of the disturbance, the ICP RF power was varied accordingly. For this proof-of-principle experiment, the PID algorithm was chosen for the controller. A forward controller with a specific trajectory planning was also embedded into the system to smooth out the overshoot and oscillations of electron density when a sudden change of RF power occurs, such as power on or off. The experiment was conducted in an ICP reactor, which used a planar type coil and had a chamber diameter of 40 cm and a height of 20 cm. The feed gas was argon and the flow rate was kept at 15 sccm during the experiment. The heterodyne interferometer was temperature controlled to enhance its stability on phase measurement to within 1/spl deg/.