Disclosed is a method of forming a semiconductor device structure in a semiconductor layer. The method includes forming a first trench of a first width and a second trench of a second width in the semiconductor layer; depositing a layer of first material which conforms to a wall of the first trench but does not fill it and which fills the second trench; removing the first material from the first trench, the first material remaining in the second trench; depositing a second material into and filling the first trench and over a top of the first material in the second trench; and uniformly removing the second material from the top of the first material in the second trench, wherein the first trench is filled with the second material and the second trench is filled with the first material and wherein the first material is different from the second material.