Periodic structures have been obtained in porous silicon by photodissolution of the material in hydrofluoric acid under interferometric illumination. Spatial modulation of the illumination will result in correlated spatial modulation of the porosity. This process can be performed after or during the electrochemical formation of the porous silicon layer and is generalised to different types of porous silicon. The period of the structure is easily modified by changing the wavelength or the incident angles of the illuminating laser beams. One-, two- or three-directional porosity submicron sized modulations have been obtained in porous silicon. We show several examples of hexagonal patterning. This technique is an easy and cheap type of in-depth lithography and we show how it can replace standard lithography for generating regular patterns of uniform macropores in silicon.