The sheet metal or substrate material 22 is constructed by a photo-chemical machining or milling method. In this exemplary process, an etching mask 24 is applied to both sides of the substrate 22 and covers the structure of the flat-shaped member 12, which is to be uncovered in a subsequent etching step. The mask 24 is formed by coating the substrate 22 with a photoresist and is exposed through a photomask having the desired pattern that is arranged in front of the mask, whereby the photoresist is polymerized or hardened in the covered areas while the other areas are rinsed away after development.