Semiconductor device manufacturing methods and methods of forming insulating material layers are disclosed. In one embodiment, a method of forming a composite insulating material layer of a semiconductor device includes providing a workpiece and forming a first sub-layer of the insulating material layer over the workpiece using a first plasma power level. A second sub-layer of the insulating material layer is formed over the first sub-layer of the insulating material layer using a second plasma power level, and the workpiece is annealed.