A thin silicon dioxide layer is formed on a substrate to act as a pad oxide layer. Subsequently, a Si.sub.3 N.sub.4 or BN layer is deposited on the pad oxide layer. An in situ doped polysilicon layer is deposited on the Si.sub.3 N.sub.4 or BN layer. A trench is formed in the substrate. An oxide liner is formed along the walls of the trench and on the surface of the in situ doped polysilicon layer. A CVD oxide layer is formed on the oxide liner and refilled into the trench. A two-step chemical mechanical polishing (CMP) removes the layers to the surface of the Si.sub.3 N.sub.4 or BN layer. The first step of the two-step CMP is an oxide slurry CMP that is stopped at about 100 to 500 angstroms from the in situ doped polysilicon layer. The second step of the two-step CMP is a poly slurry CMP that is controlled to stop at the surface of the Si.sub.3 N.sub.4 or BN layer.