The objective of this study is to apply a physically based impedance model of a Cl-2/He plasma discharge to a commercially available etcher, operating under typical production conditions. This model is to be used in conjunction with a commercial rf monitor for the purpose of process monitoring and diagnosis. A designed experiment was run to extract the impedance model for the Lam Rainbow 4400 plasma etcher. Additional test runs were performed over a range of input rf power and pressure values to determine the accuracy of the extracted impedance model. The results indicate that a nonlinear impedance model can be used to characterize the electrical behavior of a Cl-2/Me discharge during normal production plasma etch processes. Therefore, once drifts in the electrical signals are detected by the rf sensor, subsequent analysis of the impedance can help determine the cause of the shift and eliminate any lengthy downtime.