With many copper backend integration schemes, silicon nitride (SiN) etch stop layers are etched over copper. Because of misalignment, the SiN etch may also occur over oxide, where high selectivity to oxide would be desired. Typical plasma conditions use fluorocarbons with O@sub 2@ or Ar at low plasma power, which avoids Cu sputtering; however, the selectivity to oxide is typically low (~2:1). Etch conditions in a remote plasma reactor have been reported which give higher nitride:oxide selectivity based on the role of NO to enhance etching of SiN.@footnote 1@ We report the use of optical emission spectroscopy to optimize NO concentration in N@sub 2@/O@sub 2@/Ar plasmas, with or without a fluorocarbon source, for use in SiN etching. Signature spectra of excited molecular (N@sub 2@*, NO*) and atomic (O*, Ar*) species were identified and tracked over various plasma conditions in a commercial etch reactor. Small corrections were made using the Ar* intensities. While the relative concentrations of N@sub 2@* and O* species are linear with N@sub 2@ and O@sub 2@ flow, the NO concentration shows a maximum which occurs at a flow ratio of ~90/10 N@sub 2@/O@sub 2@. Survey etch experiments were run with blanket oxide and nitride wafers, using conditions of maximum NO concentration. Variation of the other process conditions showed that decreased bias power, along with increased NO and F conditions, lead to increased nitride/oxide selectivity, including values higher than the baseline process. The extension of this work to include profile and etch rate results from patterned nitride and nitride/oxide wafers will also be reported. @FootnoteText@ @footnote 1@ B. E. E. Kastenmeier, P. J. Matsuo and G. S. Oehrlein, J. Vac. Sci. Technol., A17 (1999) 3179.