Free Radicals in an Inductively Coupled Etching Plasma

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Abstract
A high-density (>1011 cm-3) CF4/H2 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 ( SiF4 and CO) from the warm quartz wall close to the coil. These impurities significantly deteriorate the etch selectivity of SiO2 to Si in the ICP reactor. Water cooling and a Faraday shield are effective to suppress the release of impurities. Neutral radicals CF3, CF2, 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). H2 addition to the CF4 discharge drastically modifies the CF3 and CF2 densities in the ICP as well as in the CCP. The high etch rates and the low selectivity of SiO2 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.

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