In a previous paper we showed that selective etching of porous SiOCH with respect to SiO2 and SiCH is clearly enhanced when using a pulsed bias in inductively coupled fluorocarbon plasma. To understand this pulsed process, a model for etch rate calculation is developed in the present paper. This model explains the etching/deposition threshold shift toward higher bias voltage in pulsed conditions. Rather good confidence is obtained with experimental SiO2, Si, SiOCH and SiCH etch rates. Porous SiOCH etching is found to behave slightly differently compared to SiO2 or SiCH; its chemical etching is assumed to occur even during the beginning of off period. This point could explain why good selectivities between porous SiOCH and SiO2 and SiCH are obtained.