The specification describes a dual patterned polish stop layer technique for shallow trench isolation. The shallow trenches are formed by etching trenches in a semiconductor substrate wafer, backfilling with oxide, and polishing by chemical-mechanical polishing (CMP) to produce a planar, trench isolated, wafer. To ensure planarity of the wafer after CMP, and avoid dishing of the field oxide, a dual silicon nitride polish stop layer is used. The first polish stop layer is applied selectively to protect the active device regions, and the second polish stop layer is applied selectively to protect the field oxide regions.