A device for fabricating a mask by plasma etching a semiconductor substrate comprises a semiconductor substrate part of the area whereof is partially covered by a mask for protecting at least one area that must not be etched and for exposing at least one area including a pattern to be etched, a support for the substrate and means for generating a plasma in the form of a flow of ions toward the substrate. According to the invention the device further comprises means for confining the ions, including a conductive material screen disposed over the substrate and along the limit between the pattern area to be etched and the area not to be etched.