in fig2 the measured depths of the porous strips according to the invention are shown after the removal of the porous layer by means of caustic soda it can be appreciated that in the case of a strip width of 100 u03bcm the interface of an individual strip is no longer curved but of straight configuration one should also note the underetching below the mask which is only 100 u03bcm in width in fig3 an etching mask for producing a continuous variation of the etching rate according to the invention is shown this mask is positioned on the surface to be etched the surface region below the dark regions of the etching mask are covered the clear regions of the surface are etched in this manner the etching rate can be continuously adjustable or variable in the strips between both wedge structures along the indicated line a b c d the depth profile of the porous layer according to the invention produced in this manner is measured in fig4 this surface profile measurement measured after the etching away of the porous region according to the invention has been shown to be noted is the interface between porous silicon and crystalline silicon the region in which the etching rate is configured to be continuously variable by means of the lateral structuring is given between the points b and c with the process according to the invention it is possible to achieve discrete as well as continuously variable changes in.