Etching plasmas based on CF4have been studied in a magnetron ion etching (MIE) system. Optical emission and mass spectra were measured as a function of applied power, pressure, and gas composition. Electrical probes were used to measure the plasma floating potential and the dc self‐bias of the rf electrode. The magnetic field has an effect not only on the physics of the plasma (ion density, plasma impedance, etc.) but also on the chemistry. Two regimes of operation are found to exist. Under typical high‐power MIE operating conditions, the species present in the plasma tend to be more highly dissociated than in a conventional reactive ion etching (RIE) plasma. As a consequence, the tendency for C–F polymer to form on etched Si surfaces is suppressed and selective etching of SiO2over Si does not occur. At lower powers, the plasma characteristics and etching response revert back to RIE‐like behavior as the degree of dissociation decreases. The implications for etching processes are discussed.