We carried out the model-based analysis of the Er-doped silica glass films ($62%SiO_2-30%B_2O_3-8%P_2O_5+0.2%wt;Er_2O_3$) etch mechanism. Etching mechanism in the $CF_4/O_2$ inductively coupled plasma was investigated by using the combination of simplified models for plasma chemistry and etch kinetics. As the $O_2$ mixing ratio in the $CF_4/O_2$ plasma increases from $0{sim}30%$, the etch rate decreases monotonically in the range of $385{sim}190nm/min$ that contradicts with the behavior of F atom density and flux. It was found that, at low ion bombardment energies, the etch process followed the formal kinetics of ion-assisted chemical reaction and was controlled by both neutral and ion fluxes.