surface a mask layer 6 which exposes a well defined first region 8 of the substrate 39 s top surface the first region will have a specified width and will be located over a region where a trench is to be subsequently formed the substrate typically is a single crystal silicon wafer or other semiconductor material in which devices are to be formed the mask layer may be made of photoresist silicon oxide silicon nitride glass etc or alternating layers of any these materials chosen as needed to protect the unexposed substrate from subsequent implantation and etching processes impurity species are introduced into the substrate by any suitable technique through the exposed first region 8 of the substrate to make the undulating structure exemplified in this embodiment ion implantation generally will be employed as shown in fig1 b the impurity species are implanted so as to create one or more layers 10 that alternate with unimplanted substrate material 4 the impurity or impurities are chosen to create regions that are more or less susceptible to chemical etching in subsequent steps the exact impurity used will be chosen for use in combination with a specific etchant as explained in more detail below possible impurities for implantation include germanium oxygen o 2 and argon for example in order to provide the structure shown in fig1 b at least three separate implant steps must be performed each.