[en] The vapor cloud produced by a magnetron sputtering gun in the vacuum chamber is not uniform. In order to coat substrates within a desired tolerance of thickness uniformity, a 'flattening' of the vapor flux distribution is required. This is achieved by means of mechanical masks which intercept the vapor cloud, by a carefully designed motion of the substrate through the vapor cloud, or by a combination of both. The present work describes a design method for masks, to be used with planar substrates, and its application in the case of a Varian S-GUN magnetron sputtering gun. By using the mask the coating thickness uniformity on large ring shaped flat substrates, 200 mm wide, has been improved from ±35% to ±9% around the mean value of the thickness. (author)