Abstract: A gate structure is described, including a dielectric layer, a gate conductive layer and a stacked cap structure. The dielectric layer is disposed between a substrate and the gate conductive layer. The gate conductive layer is disposed between the dielectric layer and the stacked cap structure. The stacked cap structure disposed on the gate conductive layer includes at least two insulating layers having different materials and contacting each other.

Abstract: A semiconductor device includes a substrate and an isolation structure, which includes a trench in the substrate, a lower filling layer at the bottom of the trench, and an upper filling layer on the lower filling layer, wherein the lower filling layer is denser than the upper filling layer, and the lower filling layer contains chlorine. The method for forming an isolation structure includes the steps of forming a trench in a substrate wherein the trench comprises side surfaces and a bottom surface, forming a nitride liner on the side surfaces of the trench, growing an epitaxial silicon layer from to the bottom surface of the trench, oxidizing the epitaxial silicon layer to form a lower filling layer in the lower portion of the trench, and filling a portion of the trench above the lower filling layer with dielectric material.