Etching of high aspect ratio patterns induces reactive ion etching (RIE) lag. This effect is studied for oxide features etched in a high density plasma excited by electron cyclotron resonance using different fluorocarbon gases. A new RIE lag model is proposed which depends on the assumption that the oxide etch rate is, as on a blanket sample, strongly influenced by the deposition of fluorocarbon film on the oxide surface during the etching process.