Method for reducing line width roughness with plasma pre-etch treatment on photoresist

2 A method for reducing line width roughness (LWR) of a feature in an etch layer below a patterned photoresist mask having mask features is provided. The method includes (a) non-etching plasma pre-etch treatment of the photoresist mask, and (b) etching of a feature in the etch layer through the pre-treated photoresist mask using an etching gas. The non-etching plasma pre-etch treatment includes (a1) providing a treatment gas containing Hand COS, (a2) forming a plasma from the treatment gas, and (a3) stopping the treatment gas.