A method for controlling critical dimension (CD) of etch features in an etch layer disposed below a functionalized organic mask layer disposed below an intermediate mask layer, disposed below a patterned photoresist mask, which forms a stack is provided. The intermediate mask layer is opened by selectively etching the intermediate mask layer with respect to the patterned photoresist mask. The functionalized organic mask layer is opened. The functionalized organic mask layer opening comprises flowing an open gas comprising COS, forming a plasma, and stopping the flowing of the open gas. The etch layer is etched.