A method of forming a hard mask layer on a substrate includes forming an amorphous carbon layer using nitrous oxide (N2O). A source of carbon and the nitrous oxide (N2O) are introduced to the substrate under a plasma ambient of an inert gas. The amorphous carbon layer has a nitrogen content ranging from about 0.05 at % to about 30 at % and an oxygen content ranging from about 0.05 at % to about 10 at %. In forming a semiconductor device, the hard mask layer is patterned, and a target layer beneath the hard mask layer is etched using the patterned hard mask layer as an etch mask.