A process of forming a slit in a substrate is provided. A mask layer is formed on a substrate, wherein the mask layer does not include carbon. An etching process is performed to be substrate by using the mask layer as a mask, so as to form a slit in the substrate. The etching gas includes Cl2, CF4 and CHF3, a molar ratio of CF4 to CHF3 is about 0.5-0.8, and a molar ratio of F to Cl is about 0.4-0.8, for example. Further, the step of performing the etching process simultaneously removes the mask layer.