Abstract: A method for pulling back SiN to increase rounding effect in a shallow trench isolation process, includes the steps of preparing a substrate of Si and forming a SiO2 layer on the substrate, forming a Si3N4 layer on the SiO2 layer, defining Si3N4 trenches by plasma etching, etching the remaining Si3N4 with SF6/HBr gas, etching SiO2 layer to form a platform and enhance the rounding of the platform, etching the substrate to have a third shallow trench and a reinforced platform, filling the third shallow trench with oxide, planarizing the filled oxide using chemical mechanical polishing, and removing the Si3N4 layer, wherein after the removal of the Si3N4 layer, multiple cleaning processes are performed.