Abstract: A method of forming shallow trench isolation (STI) in a substrate. A shield layer is formed on part of the substrate. Using the shield layer as a mask, part of the substrate is removed to form a trench in the substrate. A first insulation layer is formed in part of the trench, where the trench remains an opening. The first insulation layer is partially etched back to leave a remaining first insulation layer at the bottom of the trench and to expose the sidewall of the trench above the remaining first insulation layer. The trench is filled up with a second insulation layer extending onto the shield layer. A planarization is performed on the second insulation layer, where the shield layer serves as a stop layer for the planarization. Thus, a void-free trench isolation area is formed in a substrate.