basically determined by the smallest trench dimension and by the requirement for some overfill the surface topography of filling film 15a can be non planar due to erosion of the etch mask 11 or microloading effects in the deposition of the film for example in a first step shown in fig2 the filling film 15a on top of the structure is isotropically etched with high selectivity to the underlying layer 11 or layer 14 if on top of layer 11 this isotropical etch can be performed as a wet etch i e in a tank tool or a spin etch tool or by cde etch rate and etch time have to be adjusted in such a way that the film 15a is removed from the layer 11 but that the trench fill 15 is not etched below the interface between layers 11 and 12 or layer 11 and wafer 10 respectively the minimal thickness of the layer 11 is determined by the requirements for its use as a trench etch mask in the first place however a larger thickness of layer 11 can be chosen in order to guarantee that the above criterion is satisfied as shown in fig3 the layer 11 is now removed by an isotropical etch selective to the trench fill 15 if a trench sidewall film 14 is present that.