A method of forming a patterned mask layer includes the following steps. A plurality of support features is formed on a mask layer. A plurality of spacers is formed on side walls of the support features. A patterned protection layer is formed on the support features and top surfaces of the spacers. At least a part of side surfaces of the spacers are not covered by the patterned protection layer, and the patterned protection layer is formed in a process environment containing methane (CH4). A trimming process is then performed to remove a part of each of the spacers. Tapered parts of the spacers may be removed by the trimming process before the step of etching the mask layer with the spacers as a mask, and the critical dimension uniformity of the patterned mask layer may be improved accordingly.