A method is disclosed for the formation of a planarizing coating film on the surface of a substrate having a stepped level difference under processing for the manufacture of semiconductor devices. The coil has a first endwinding that is substantially straight to a longitudinal axis of the coil and a second endwinding that is bent relative to the longitudinal axis of the coil. The method comprises centering the optical fiber relative to the outside diameter of the proximal portion of the guidewire tubing.