A process for forming shallow trench isolation (STI) structures. It includes the steps of: (a) depositing a composite silicon nitride on to the silicon substrate; (b) forming a shallow trench on the silicon substrate by etching, using the composite silicon nitride as the hard mask; (c) depositing a filler oxide layer inside the shallow trench as well as on top of the composite silicon nitride, using a chemical vapor deposition (CVD) method; and (d) using a chemical-mechanical polishing (CMP) process to planarize the filler oxide layer using the composite nitride as a CMP stop. The composite silicon nitride comprises a plurality of silicon nitride layers whose CMP removal rate increases with the distance from the silicon substrate. Additionally, a composite silicon oxide layer can be formed on top of the filler oxide layer which comprises a plurality of silicon oxide layers whose CMP removal rate increases with the distance from the silicon substrate.