Previous efforts from our group have shown encouraging initial results in stabilizing etch rates versus time during a run by using real-time, multivariable feedback control (RTC) in an Applied 8300 reactive ion etcher. That work indicated the need for improvements in our sensor systems, both the sensors currently used in feedback control and those monitoring the effects of the control on the wafers being etched. In this article we report on our efforts in the development and improvement of two such sensor systems. The first is an optical emission spectroscopy system which simultaneously measures two emission line intensities for use in actinometry. The second sensor system uses spectral reflectometry data to determine the in situ film thickness, from which we calculate the etch rate. We show examples of RTC using the actinometry sensor system during fluorine-based polycrystalline silicon etching. The results of using these sensors for RTC are presented by comparing open loop signals with those from real-time closed loop etch runs. In situ etch rate accuracies, estimated using our reflectometry system, are discussed. Film thicknesses calculated from in situ measurements are compared with those calculated by ex situ spectroscopic ellipsometry.