An interlayer comprised of a thermoplastic resin, at least one high refractive index plasticizer and, optionally, a conventional plasticizer. The downhole tool may include a pair of directional locking members, at least one of which is operable to transfer a loading experienced by the downhole tool through rigid components of the downhole tool, thereby bypassing the sealing member. The stator unit includes an exposure region where the substrate is to be subjected to a process of exposing the substrate, and a measurement region where the substrate is to be subjected to a process of measuring a position of the substrate. It is preferable that there is used the gas flow-guide cylinder that Fe concentration is 0.