The chemical processes that underlie metal etching in high-density CO/NH3 plasmas have been investigated using supersonic pulse, plasma sampling mass spectrometry. The chemical processes have been found to be consistent with Fischer-Tropsch chemistry resulting in the generation of formamide in the plasma environment. Formamide, closely related to acetic acid in both chemical structure and ligand binding capabilities, is suggested as being responsible for the two and one-half-fold increase in etch rate previously observed [K.B. Jung, J. Hong, H. Cho, S. Onishi, D. Johnson, YD. Park, J.R. Childress, and S.J. Pearton, J. Vac. Sci. Technol. A 17(2), 535 (1999)] when small amounts of CO are added to NH3 plasmas. 0 2005 American Vacuum Society. Carbon MonoxideAmmoniaNickelFormamide