Several embodiments of a method for manufacturing an optical assembly for use in an optical flying head are provided. The face has a central zone and a intermediate zone adjacent and surrounding the central zone. One of the walls of the first conductive region forms a first thin portion and has a first dimension in a first direction. A transistor including an oxide semiconductor layer on which dehydration or dehydrogenation treatment is performed through such heat treatment has improved stability, so that variation in electrical characteristics of the transistor due to light irradiation or a bias-temperature stress test is suppressed.