The etching chemistry of benzocyclobutene (BCB) low-k dielectric films was studied in a high density plasma etcher using F-2+O-2 and Cl-2+O-2 plasmas. The etching rate in F-2+O-2 plasmas exceeded 1.2 mum/min with selectivity over oxide and nitride of 16 and 32, respectively. The etching rate in Cl-2+O-2 plasmas exceeded 0.6 mum/min with selectivity over oxide and nitride of 40 and 80, respectively. BCB films do not etch in pure Cl-2 or pure O-2 plasmas without ion bombardment, but etching rates of 1000 Angstrom /min were observed using F-2 plasmas without ion bombardment. The ion flux in F-2+O-2 plasmas is primarily O-2(+) and O+, whereas in Cl-2+O-2 the dominant ion is ClO+. BCB etching yields in F-2+O-2 plasmas were measured with a plasma beam/quartz crystal microbalance system. The etching yields suggest that the neutral fluxes and surface chemistry control the etching rates under these conditions, not the ion flux. Using x-ray photoelectron spectroscopy, it was determined that oxygen plasmas preferentially remove the carbon content of BCB, leaving behind a silicon oxide surface. Chlorine plasmas preferentially remove the silicon, leaving behind a carbon surface. F-2+O-2 plasmas etch BCB through a fluorocarbon film layer, the thickness of which increases with increasing fluorine concentration in the plasma.