1. Field of the Invention
The present invention relates to a method for manufacturing an EL display device which includes thin film transistors.
2. Description of the Related Art
In recent years, thin film transistors each of which are formed using a semiconductor thin film with a thickness of several nanometers to several hundreds of nanometers over a substrate having an insulating surface such as a glass substrate have attracted attention. Such thin film transistors are widely used for ICs (integrated circuits) and electronic devices such as electrooptic devices. In particular, rapid development of thin film transistors as switching elements of image display devices typified by liquid crystal display devices, EL (electro luminescence) display devices, and the like has been demanded.
In an active matrix EL display device, voltage is applied between a pair of electrodes of a light-emitting element provided in a selected pixel, which sandwiches an EL layer, whereby current flows through the EL layer and then a light-emitting layer emits light. This light emission is viewed as a display pattern by a viewer. Note that the active matrix EL display device here indicates an EL display device which forms a display pattern on a screen by driving pixels arranged in matrix by switching elements.
The range of uses for such an active matrix EL display device has been increasing, and demanded levels of a size of a screen, definition, and an aperture ratio are growing higher. In addition, high reliability is demanded in the active matrix EL display device, and high productivity and low production cost are demanded in a production method thereof. As for increasing productivity and reducing production cost, simplification of a process is given as one of measures.
In the active matrix EL display device, thin film transistors are mainly used as switching elements. In manufacture of a thin film transistor, it is important for simplification of a process to reduce the number of photomasks used for photolithography. For example, if one photomask is added, the following steps are further needed: steps such as resist application, prebaking, light exposure, development, postbaking, and the like; steps such as film formation and etching before and after the aforementioned steps; and then steps such as resist removal, cleaning, drying, and the like. That is, the number of steps is significantly increased only by addition of one photomask used in the manufacture process. Therefore, many techniques for reducing photomasks in the manufacture process have been developed.
Thin film transistors are roughly classified into a top-gate type thin film transistor in which a channel formation region is provided below a gate electrode and a bottom-gate type thin film transistor in which a channel formation region is provided above a gate electrode. It is known that the number of photomasks used in the manufacture process of the bottom-gate type thin film transistor is less than the number of photomasks used in the manufacture process of the top-gate type thin film transistors. Three photomasks are generally used for manufacture of the bottom-gate type thin film transistor.
Many conventional techniques for reducing the number of photomasks use a complicated technique such as backside light exposure, resist reflow, or a lift-off method, which requires a special apparatus. There has been a concern that yield is reduced due to various problems caused by usage of such a complicated technique. The number of photomasks has managed to be reduced at the cost of electric characteristics of thin film transistors.
As typical means for reducing the number of photomasks in a manufacture process of a thin film transistor, a technique using a multi-tone mask (called a half-tone mask or a gray-tone mask) is widely known. For example, Reference 1 (Japanese Published Patent Application No. 2003-179069) discloses a technique for reducing the number of manufacturing steps by using a multi-tone mask.