Abstract: A method for forming a shallow trench isolation (STI) structure with a predetermined target height is provided. A substrate having a pad oxide layer formed on the substrate is provided. A nitride-containing layer with a thickness is formed on the pad oxide. A STI structure is formed and extends through the nitride-containing layer, the pad oxide layer, into the substrate. The thickness of the nitride-containing layer is measured to calculate the height of STI structure according to a correlation between the thickness of the nitride-containing layer and the height of STI structure. A thickness of the top portion STI structure to be removed is determined according to the difference between the height of the STI structure and the predetermined target height and is removed in a first etching process. The nitride-containing layer is removed without etching the STI structure or the pad oxide layer in a second etching process.