Etching of high aspect ratio patterns induces a phenomenon known as reactive ion etching (RIE) lag, i.e., a large feature etches faster than a smaller one. This effect is studied for oxide etched in a high density plasma excited by electron cyclotron resonance using different fluorocarbon gases. The magnitude of the RIE lag is correlated with the deposition rate of fluorocarbon film on an unbiased sample, showing that chemical effects are important to understand the mechanisms of RIE lag in high density plasmas.