Abstract: A method, system, and medium of spectroscopically modeling and/or controlling a semiconductor manufacturing process. The modeling/controlling includes the steps of conducting a plurality of semiconductor manufacturing process runs by changing at least one of process parameters from its target value, and collecting spectral data indicative of the light emitted by plasma during each of said semiconductor manufacturing process runs. The modeling/controlling also includes the step of formulating a ratio based on a relationship between the collected spectral data and the changes in the at least one of the plurality of process parameters.