1.  Field of the Invention
The present invention relates to a semiconductor device having a circuit including a thin film transistor (also referred to as a TFT) and a manufacturing method thereof.  For example, the present invention relates to electronic devices having, as a part, an electro-optic device typified by a liquid crystal display panel or a light-emitting display device having an organic light-emitting element or an inorganic light-emitting element. 
Note that a semiconductor device in this specification means a generic device which can operate with use of semi-conductive characteristics and includes all types of devices such as an electro-optic device, a semiconductor circuit and an electronic device. 
2.  Description of the Related Art
In recent years, a technology for forming a thin film transistor (TFT) using a semiconductor thin film (a thickness of around several nanometers to several hundred nanometers) formed over a substrate having an insulating surface has been attracting attention.  Thin film transistors are broadly applied to electronic devices such as an IC or an electro-optic device, and are particularly developed as switching elements for image display devices at a rapid rate. 
Light emitting elements, which use organic compounds as a light emitter and are characterized by their thinness, lightweight, fast response, and direct current low voltage drive, are expected to be applied to next-generation flat panel displays.  Among display devices, ones having light emitting elements arranged in matrix have a wide viewing angle and excellent visibility. 
In a light-emitting display device in which thin film transistors are arranged in matrix over a substrate having an insulating surface, convex or concave portions are formed due to a wiring or an electrode in the surface of the substrate.  When an insulating film is formed thereover by a coating method, it is difficult to have an even thickness.  In addition, a film thickness over the convex portion becomes thin, by a centrifugal force in forming a film by a coating method.  There is a problem in that resistance to pressure is reduced in the convex portion whose film thickness of the insulating film is thin. 
When the film thickness is made thicker to increase the resistance to pressure of the insulating film simply, the stress of the whole insulating film is increased.  Thus, there is a risk of peeling of the insulating film or a warpage of a substrate. 
In addition, a light-emitting display device in which thin film transistors are arranged in matrix has a structure described in Reference 1, specifically a structure in which an insulating film is provided between adjacent pixels (Reference 1: Japanese Patent Application Laid-Open No.  2002-164181). 
The insulating film provided between adjacent pixels is referred to as a bank, a partition, a barrier, an embankment or the like, and is provided over a source wiring or a drain wiring for a thin film transistor, or a power supply line.  Specifically, at an intersection portion of these wirings provided in different layers, a larger step is formed there than in other portions. 
Even in the case that the insulating film provided between adjacent pixels is formed by a coating method, there is a problem in that thin portions are partially formed due to this step and the resistance to pressure is reduced.