Free Radicals in an Inductively Coupled Etching Plasma

Abstract

A high-density (>10 11 cm -3 ) CF 4 /H 2 plasma was produced in an inductively coupled plasma (ICP) reactor where an external helical coil is wound around a quartz tube. Capacitive coupling from the coil to the plasma caused the release of a large number of impurities ( SiF 4 and CO) from the warm quartz wall close to the coil. These impurities significantly deteriorate the etch selectivity of SiO 2 to Si in the ICP reactor. Water cooling and a Faraday shield are effective to suppress the release of impurities. Neutral radicals CF 3 , CF 2 , CF and F were measured in addition to ionic species. The high-density high electron-temperature ICP causes the formation of a large number of F atoms and CF + ions with fewer CF x radicals, in comparison to a low-density capacitively coupled plasma (CCP). H 2 addition to the CF 4 discharge drastically modifies the CF 3 and CF 2 densities in the ICP as well as in the CCP. The high etch rates and the low selectivity of SiO 2 to Si obtained in the ICP were discussed taking account of the residence time and the dissociation time of reactive species in the etching reactor.