The need to print smaller feature sizes has led to the shift from 248 nm to 193 nm lithography. The disadvantages of 193 nm ArF single-layer resist materials, such as lower depth of focus and lower etch resistance, have made thin-film imaging (TFI) techniques an attractive alternative. In the bilayer (comprised of an image layer and an underlayer) approach, a form of TFI, O2-based plasma chemistries are used for the transfer etches whereby oxidation of silicon in the image layer provides the required etch resistance. However, use of oxygen results in profile and critical dimension (CD) control issues. While gas additives have helped minimize these problems, there are other accompanying disadvantages. In this work, the feasibility of a non-O2-containing NH3-based plasma etch chemistry was evaluated. Effects of additive gases, such as N2, H2, and C2H4, were investigated. Surface analysis of the resist showed that nitrogen from the gas phase was incorporated in the surface of the image layer during the etch, and the etch resistance increased with this nitrogen content. Gas phase analysis showed that the relative N2H species density in the system was correlated with the nitrogen incorporated in the image layer, and is thus believed to govern the etch resistance of the image layer. The underlayer etch behavior was correlated with that of the atomic hydrogen species density. The effect of the additives on the cross section profiles and line edge roughness were studied. The- - highest CD biases were obtained with H2 as the additive, while near-zero CD biases were attained with C2H4 as the additive. Also, with the C2H4 additive, minimum line edge roughness and smoothest profiles were obtained.