A manufacturing method for a semiconductor-device isolation structure comprises providing a substrate with at least one shallow trench isolation structure, performing a salicide process that forms a recess on the surface of the shallow trench isolation structure, forming a cap film covering the substrate and filling the recess, performing an etching process to remove the cap film outside the recess, and forming a contact etch stop layer covering the substrate and filling the recess. Due to the filling recess with the cap film first, the contact etch stop layer covering the substrate and filling the recess does not have seams or voids.