Three-dimensional structures for photonic crystal applications have been fabricated up to now either by pure bottom-up approaches like colloidal self-assembly, by pure top-down approaches using VLSI technology or by interference lithography. Here we demonstrate the fabrication of three-dimensional silicon networks by a self-stabilized electrochemical etching technique on lithographically prestructured single crystalline substrates. Using a square two-dimensional lattice and a modulated current-voltage-profile, a columnar porous structure with strong diameter modulations is obtained. Subsequent homogenous and isotropic (i) or even anisotropic (ii) etching converts the columnar structure to a cubic geometry of high porosity and an air filling fraction of 80%. In the case of an isotropic etching the porous material is build up of intersecting air spheres in silicon. The optical characterizations along various high symmetry directions of the crystal confirm the achieved periodicity and shape and suggest this material for photonic crystal applications. According to theory this arrangement of air spheres in silicon opens a complete three-dimensional photonic bandgap of about 4.9% centered at 3 µm. Moreover, the subsequent anisotropic etching of the initial porous structure, which exploits the crystallographic nature of the substrate used, converts the former circular cross-section of the pores into an almost squared one. We theoretically study the dispersion behavior of PCs being fabricated by this developed technique. In addition, we present experimentally realized structures and characterize the photonic crystal optically. The reflectance measurements are in good agreement with corresponding bandstructure calculations. A major part of this work deals with the investigation and characterization of fabrication-related disorder and its prevention. This process allows the introduction of defect layers laterally during the etching process as well as vertically - by lithography - achieving a three-dimensional nanopositioning control. Moreover, the introduced process extends the variety of designing and sculpturing three-dimensional microstructures to meet the requirements of a multitude of micro- and nano-technological applications.