A method of forming openings is disclosed. A substrate is first provided, and the tri-layer structure is formed on the substrate. The tri-layer structure includes a bottom photoresist layer, a silicon-containing layer and a top photoresist layer form bottom to top. Subsequently, the top photoresist layer is patterned, and the silicon-containing layer is etched by utilizing the top photoresist layer as an etching mask to partially expose the bottom photoresist layer. Next, the partially exposed bottom photoresist layer is etched through two etching steps in turn by utilizing the patterned silicon-containing layer as an etching mask. The first etching step includes an oxygen gas and at least one non-carbon-containing halogen-containing gas, while the second etching step includes at least one halogen-containing gas. The substrate is thereafter etched by utilizing the patterned bottom photoresist layer as an etching mask to form at least an opening in the substrate.