Semiconductor devices and methods of manufacture thereof are disclosed. In a preferred embodiment, a semiconductor device includes a workpiece and a trench formed within the workpiece. The trench has an upper portion and a lower portion, the upper portion having a first width and the lower portion having a second width, the second width being greater than the first width. A first material is disposed in the lower portion of the trench at least partially in regions where the second width of the lower portion is greater than the first width of the upper portion. A second material is disposed in the upper portion of the trench and at least in the lower portion of the trench beneath the upper portion.