Results for the reactive ion etching and patterning of benzocyclobutene (BCB) in CF4/O-2 and SF6/O-2 plasmas in a parallel-plate reactor with Langmuir probe and optical emission diagnostics are reported. The behavior of the O atom concentration [O] is similar in both plasmas, showing a strong maximum at a concentration of about 70% O-2. The F atom concentration [F] has a maximum at 30% O-2 in CF4/O-2, but the etch rate has a maximum at 70% O-2. In SF6/O-2, by contrast, [F] and the etch rate increase continuously as the SF6 content is increased, and the highest etch rate is obtained with pure SF6. Also, BCB etched in pure SF6 gives a very smooth surface. Patterning is performed using two different masks: a conventional hard mask with Si oxide as the top layer and a different type of mask with BCB itself as the top layer. The anisotropy of the etching is evaluated in terms of the plasma conditions and type of masking. The aspect ratio dependence of the etch rate is investigated up to an aspect ratio of 2.5. The results are discussed in terms of possible etch mechanisms and their differences between SF6/O-2 and C-4/O-2 plasmas. A simple model is used to show how the latter mask reduces the dependence of the etch rate on the aspect ratio. (c) 2006 American Vacuum Society.