A method of performing an etching process is provided. A substrate is provided, wherein a first region and a second region are defined on the substrate, and an overlapping region of the first region and the second region is defined as a third region. A tri-layer structure comprising an organic layer, a bottom anti-reflection coating (BARC), and a photoresist layer is formed on the substrate. The photoresist layer and the BARC in the second region are removed. An etching process is performed to remove the organic layer in the second region by using the BARC and/or the photoresist layer as a mask, wherein the etching process uses an etchant comprises CO2.