Measurement of the extinction coefficient k is employed for effective and prompt in-line monitoring and/or controlling of the metal film composition. The dependency of the extinction coefficient on the composition of a metal compound is characterized by measuring the extinction coefficients of a series of the metal compound with different compositions. A monitor metal film is then deposited on a wafer. The extinction coefficient k of the film on the wafer is measured and a film compositional parameter is extracted. The wafer processing may continue if k is in specification or the needed compositional change in the film may be extracted from the measured value of the k and the established dependence of k on the composition of the film for out-of-spec k values.