Method of etching oxide layer and nitride layer

The present invention generally relates to a method of etching oxide layer and nitride layer. In particular, a substrate is provided. A surface of the substrate has an isolating structure projecting therefrom. A first oxide layer, a nitride layer and a second oxide layer are sequentially provided on the surface of the substrate, wherein the first oxide layer is uncovered on the isolating structure, the nitride layer is formed on the first oxide layer, and the second oxide layer is formed on the nitride layer. An isotropic etching process is performed by using an etching mask unmasking the isolating structure, to remove the unmasked portion of the second oxide layer and the unmasked portion of the nitride layer and expose sidewalls of the isolating structure.