Abstract: Filled trenches useful in semiconductor devices for isolation or other purposes are provided by etching a trench in the semiconductor substrate, lining the trench with a first material (e.g., silicon dioxide), covering the first material with a seed layer (e.g., polysilicon) for nucleating a further material, removing a part of the seed layer on the first material in an upper part of the trench, and then filling the trench by selectively depositing the further material (e.g., polysilicon) on the remaining seed layer.Thermally grown silicon dioxide is convenient for the first material. The first material and seed layer may extend over the substrate surface. The upper portion of the seed layer is conveniently removed by first covering the seed layer with a mask (e.g., photoresist) and then etching back the mask to expose the seed layer above the substrate surface and in the upper portion of the trench, leaving part of the mask in the lower portion of the trench.