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 forma 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 p