Abstract: A spectroscopic method is disclosed to provide a non-intrusive and in-situ monitoring of plasma etching conditions during the fabrication of semiconductor devices using RF power. It includes the steps of: (a) selecting a single plasma gas as a probe, in a cleaned plasma etch chamber; (b) measuring the spectral intensities of the plasma gas; and (c) plotting the measured spectral intensities either directly or indirectly against the RF time. A single plasma gas is selected which exhibits opposite relationships with RF time at two respective wavelengths.