A method includes providing a substrate having insulating layers thereon; forming a first trench in a first region of the substrate and a second trench in a second region of the substrate; thermally growing layers of oxide along the sides of the trenches; filling the first trench and the second trench with a polysilicon material, planarizing the polysilicon material, and creating a shallow trench isolation between the first region and the second region, wherein the step f) of creating the shallow trench isolation is performed only after the steps of d) filling and e) planarizing.