Particle formation during reactive ion etching of silicon with SF(6)

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 are tens of nanometers in diameter and spherical and chain aggregates as large as 5 mum 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 spectroscopy and optical emission spectroscopy. A three stage mechanism for describing the particle formation (nucleation, heterogeneous growth, and coagulation) is presented. Particle precursor and heterogeneous sources are determined from plasma-dependent, homogeneous, gas-phase reactions and etch product distributions predicted from electrical and etch rate measurements. Dissociation of SF6 into lower molecular weight SFx species and unsaturated SiFx species are primarily responsible for nuclei formation and subsequent, rapid heterogeneous growth by attachment of positive ions.