The mechanism of selective SiO2 etching over Si3N4 has been studied with the simulation of a ladder-type surface profile and specially designed experiments in a submicron self-aligned contact (SAC) etching process. The ladder-type profile, which is the boundary of broken shoulder Si3N4, is normally observed during a highly selective SAC etching. The structure of two adjacent SiO2 and Si3N4 lines is prepared and etched in a surface wave plasma (SWP) etching system in order to investigate the effect of fluorocarbon polymer deposition on the selectivity and the ladder profile. This designed experiment shows that the removal (adding) of fluorocarbon polymer in a SAC etching directly decreases, (increases) the selectivity and changes the ladder-type profile. A new surface reaction model for SiO2 etching has been developed to simulate the behavior of polymer and the ion enhanced dry etching. The simulation reasonably recovers the ladder-type profiles with the effects of polymer deposition and SiO2 etch byproducts, which are thought to be the main control factors of surface reactions in a submicron SAC etching. From the results of simulation and experiment, it is suggested that the structure for SAC process has to be optimized to maximize the selectivity of shoulder Si3N4 and photoresist at the same time.