A method of forming a semiconductor device structure includes forming a resist structure over a substrate, the resist structure includes an anti-reflective coating (ARC) layer and a photoresist layer over the ARC layer. The method further includes patterning the photoresist layer to form a trench therein. The method further includes performing a hydrogen plasma treatment to the patterned photoresist layer, wherein the hydrogen plasma treatment is configured to smooth sidewalls of the trench, and the hydrogen plasma treatment is performed at a temperature ranging from about 200° C. to about 600° C. The method further includes patterning the ARC layer using the patterned photoresist layer as a etch mask.