Two Variables Feedback Control of Plasma ETCH Processing

In this study, we have experimentally demonstrated two realtime feedback control systems in a chlorine inductively coupled plasma etcher. One is the closed-loop control of the electron density and the ion energy, the other one is the closed-loop control of the ion energy and the ion flux on the wafer surface. A novel transmission line microwave interferometer was used to measure plasma electron density´. The principle of this technique is the same as the conventional microwave interferometers except that the sensing microwave propagates along a transmission line structure, thus, it avoids the multi-path problem which often reduces the sensor´s dynamic range and accuracy. An impedance meter was adopted to measure the rf voltage, current and their relative phase angle. The electrical characterizations on the electrostatic chuck can be obtained by compensating the parasitic effect of the coaxial cable which located between the impedance meter and the electrostatic chuck via transmission line theory. The rf voltage is linearly dependent on the sheath voltage, or the ion energv and the ion flux can be calculated from sheath model method. A back-propagation artificial neural network was adopted to estimate the ion flux at low bias power level. In both of the control systems, the actuators are two 13.56 MHz rf power generator which drive the inductive coil and the electrostatic chuck to adjust the plasma density and ion energy, respectively. The information of the plasma electron density, ion flux and peak voltage step responses are used to set up two first order liner time-invariant systems. The closed-loop proportional-integral controllers are designed to compensate for process drift, process disturbance, and to minimize steady-state error of plasma parameters. The experimental results showed that the closed-loop control had a better repeatability of plasma parameters and a better reproducibility in etch rate compared with open-loop control.