(1) Field of the Invention
This invention relates to a thin film transistor (TFT) substrate and a method for fabricating such a TFT substrate and, more particularly, to a stagger type TFT substrate fabricated through a plurality of exposure processes using a plurality of kinds of masks and a method for fabricating such a stagger type TFT substrate.
(2) Description of the Related Art
In recent years improvement in the performance of liquid crystal display units has been required greatly to obtain high brilliance, high intensity and high definition. In particular, the study and development of TFT substrates included in liquid crystal panels used in them are proceeding. In addition, today liquid crystal display units must be manufactured by efficient, more simplified processes from the viewpoint of corporate profits. Furthermore, consideration must be given to the environment on the earth in utilizing electric power and raw materials in these manufacturing processes.
Currently, TFT substrates are manufactured mainly by a reverse stagger system. TFT substrates are formed through a plurality of exposure processes by the use of a plurality of masks. In this case, at least five masks are used for forming a gate bus-line layer, an operation island layer, a drain bus-line layer, a protection film layer, and a pixel electrode layer on TFT substrates. That is to say, to manufacture reverse-stagger type TFT substrates, as many as five exposure processes must be performed. Therefore, to improve efficiency in the manufacture of such reverse-stagger type TFT substrates, a method, for example, for reducing the number of the exposure processes from five to three by the use of a half tone mask is proposed (see, for example, Japanese Unexamined Patent Publication No. 2001-311965).
By the way, there is another system, which is called a stagger system, for manufacturing TFT substrates. By adopting this system, TFT substrates which differ from reverse-stagger type TFT substrates in structure can be formed. Compared with reverse-stagger type TFT substrates, stagger type TFT substrates have, for example, the following advantages. Layers included in stagger type TFT substrates can be formed continuously. In addition, an interface treatment process is not necessary for manufacturing stagger type TFT substrates.
Conventionally, however, a plurality of exposure processes must be performed with five to eight masks to manufacture stagger type TFT substrates. Accordingly, manufacturing efficiency is low and it takes many a day to complete TFT substrates. Moreover, many (kinds of) masks are used and a plurality of exposure processes are performed. This increases the probability that a display defect will occur due to, for example, dust. As stated above, problems, such as manufacturing efficiency, a yield, and manufacturing costs, which will arise at the time of manufacturing stagger type TFT substrates remain unresolved.