Abstract: The present invention discloses a technology for forming a thin film transistor. There is provided an insulating substrate having a gate electrode and a gate insulating layer for protecting the gate electrode thereon. A first semiconductor layer is then formed on the substrate. An insulating layer for etch stopper is formed on the first semiconductor layer and the gate insulating layer. A photoresist film is coated on the whole surface of the resultant structure. A selected portion of the photoreist film is exposed to light by projecting a linear light to a section starting from the backside of the substrate to the photoresist film, the substrate being moved horizontally. Etch stopper layer is formed by developing the exposed photoresist film and then removing the remaining photoresist film.