1. Field of the Invention
The present invention relates to a semiconductor device having a circuit which includes a thin film transistor (hereinafter referred to as a TFT), and also relates to a manufacturing method thereof. For example, the present invention relates to an electronic appliance in which an electro-optic device typified by a liquid crystal display panel or a light-emitting display device including an organic light-emitting element is provided as a component.
It is to be noted that the semiconductor device in this specification refers to all devices that can function by utilizing semiconductor characteristics, and electro-optic devices, semiconductor circuits, and electronic appliances are all semiconductor devices.
2. Description of the Related Art
In recent years, a technique for manufacturing a thin film transistor (TFT) by using a semiconductor thin film (with a thickness of from about several to several hundred nanometers) formed over a substrate having an insulating surface has attracted attention. Thin film transistors are widely applied to electronic devices such as ICs and electro-optic devices, and urgent development is expected on thin film transistors as switching elements of image display devices in particular.
Among a variety of applications of such image display devices, which have been devised, application to portable appliances has particularly attracted attention. Glass substrates and quartz substrates are often used; however, they have disadvantages in that they are easily broken and they are heavy. Therefore, it has been tried to form TFT elements over a substrate which has flexibility, typically a plastic film which is flexible.
Consequently, a technique has been suggested in which an element formed over a glass substrate is separated from the substrate and transferred to another base material such as a plastic film.
The present applicant has suggested a separation and transfer technique in Patent Document 1. Patent Document 1 describes a technique in which a metal layer (Ti, Al, Ta, W, Mo, Cu, Cr, Nd, Fe, Ni, Co, Ru, Rh, Pd, Os, or Ir) is formed over a substrate and an oxide layer is stacked thereover. In this technique, when the oxide layer is formed, a metal oxide layer of the metal layer is formed at an interface between the metal layer and the oxide layer, and separation is carried out in a later step by utilizing this metal oxide layer.    Patent Document 1: Japanese Published Patent Application No. 2003-174153