The etch characteristics of the low-k dielectrics DVS BCB (BCB hereafter) and SILK in oxygen/ fluorine based plasmas are studied. In an O-2/NF3 plasma afterglow, the etch rate of both polymers first increases linearly with increasing NF3 concentration, then decreases monotonously. A pure fluorine plasma afterglow does not practically etch either BCB nor: SILK but significantly changes their chemical and optical properties; The fluorinated polymers are more chemically stable and have lower refractive indices, Reactive ion etching (RIE) of the polymers shows a different behavior. The etch rate of SILK in a pure oxygen plasma is maximal, but BCB is etched slowly, Fluorine addition results in an increased etch rate of BCB. However,the etch rate Of SILK decreases with increasing fluorine concentration. RIE in a pure SF6 plasma shows good etch characteristics and can be used for some practical applications. Mechanisms of the reactions are discussed.