Particle formation during low pressure SF6/argon etching of silicon in a single wafer parallel plate reactor is studied. Particles are extracted from the exhaust and collected on the wafer. Particle composition and morphology depend on plasma power, etch time, gas composition, and pressure. Primary particles of tens of nm in diameter and spherical and chain aggregates as large as 5 mu m are observed. Critical powers and etch times are required for the formation of these aggregates. The presence of major gas phase species is determined using mass spectrometry and optical emission spectroscopy. Mechanisms for describing the formation processes of particles are presented, including gas phase precursor formation, nucleation, and coagulation. Precursor formation is determined from plasma-dependent, homogeneous, gas-phase reactions and etch product distributions predicted from electrical and etch rate measurements of the plasma. Dissociation of SF6 into lower molecular weight SFx species and unsaturated SFex, species is primarily responsible for initial particle formation.