The etching of Fe, Ni and Co foils in CO/NH@sub 3@ plasmas have been investigated using supersonic pulse plasma sampling mass spectrometry. It has been previously reported@footnote 1@ that plasmas based on high mole fractions of CO are observed to result in the deposition of carbide films, while pure NH@sub 3@ plasmas and high NH@sub 3@ content mixtures are reported to etch the metals, with a maximum etch rate of 500A/min at ~ 13% CO in NH@sub 3@. Mass spectra were collected over the entire range of composition from 100% CO to 100% NH@sub 3@ plasmas. In pure CO plasmas, the major plasma products that were observed are CO@sub 2@, C@sub 2@O@sub 2@, C@sub 2@O@sub 3@ and C@sub 3@O@sub 2@. The addition of NH@sub 3@ to the feed gas results in a complete elimination of the C@sub 3@O@sub 2@ species at ~60% CO, while C@sub 2@O@sub 3@ persists to ~40% CO. As the C@sub 3@O@sub 2@ and C@sub 2@O@sub 3@ disappear, a series of peaks around N@sub 4@H@sub 6@ appears. The CO@sub 2@ signal initially decreases rapidly as the CO composition is lowered to ~60%, and then remains relatively constant until disappearing completely in the 100% NH@sub 3@ plasma. The C@sub 2@O@sub 2@ species, believed to be a weakly bound dimer, is the only product that shows a smooth transition to nitrogen containing analogs as the NH@sub 3@ percentage increases. C@sub 3@O@sub 2@ is well-know to act as a carbon atom donor, through successive losses of CO, and consequently, it is concluded that this species is responsible for the deposition of the carbide layer that inhibits the etching of metals in pure CO plasmas. The rapid suppression of the chemical sequence responsible for forming this species, as NH@sub 3@ is added, makes etching possible for higher fractions of NH@sub 3@ in the plasma.
@FootnoteText@
@footnote 1@ K.B. Jung, et al. J. Vac. Sci. Technol. A 17(2), 535 (1999)