An open form is produced with a plurality of in each case two-dimensionally structured layers. The form is made of silicon which is etchable in dependence on its doping. A first silicon layer is first produced, and a portion of the first layer which belongs to the form to be produced, is marked by doping at least one zone of the first layer. Subsequently, at least one further silicon layer is applied, and a portion belonging to the form is also marked therein. Finally, every unmarked portion of the layers is removed by etching depending on the respective doping of each layer. The open form is, in particular, a photonic crystal.