However, forming the cover layer by laying down a resin film is advantageous, not only because the step of forming the cover layer is easier, but also because it is possible to form a cover layer of uniform thickness with ease. [0052] The cover layer can be made up of a highly insulating resin based on a resin such as polymethyl methacrylate, polyacrylate, or polymethyl isopropenyl ketone, and is