1. Field of the Invention
The present invention relates to a semiconductor device and a method for manufacturing the semiconductor device.
In this specification, a semiconductor device means all types of devices which can function by utilizing semiconductor characteristics, and an electro-optical device, a semiconductor circuit, and an electronic appliance are all semiconductor devices.
2. Description of the Related Art
In recent years, transistors that are formed using a semiconductor thin film having a thickness of several nanometers to several hundreds of nanometers over a substrate having an insulating surface such as a glass substrate have been attracting attentions. Transistors are widely used for electronic devices such as integrated circuits (ICs) and electro-optical devices. In particular, transistors are urgently developed as switching elements of display devices typified by liquid crystal display devices, EL display devices, electronic paper, and the like.
In an active matrix liquid crystal display device, a voltage is applied between a pixel electrode connected to a selected switching element and a counter electrode corresponding to the pixel electrode, and thus, a liquid crystal layer disposed between the pixel electrode and the counter electrode is modulated optically. The optical modulation can be recognized as a display pattern by an observer. An active matrix liquid crystal display device here means a liquid crystal display device which employs a method in which a display pattern is formed on a screen by driving pixel electrodes arranged in matrix using switching elements.
In an active matrix EL display device, where a plurality of switching elements is disposed in a pixel, voltage is applied to a light-emitting element electrically connected to at least one of the switching elements, whereby electrons and holes are separately injected from a pair of electrodes into a layer containing a light-emitting organic compound, and current flows. The carriers (electrons and holes) are recombined, and thus, the light-emitting organic compound is excited. The light-emitting organic compound returns to a ground state from the excited state, thereby emitting light. Because of such a mechanism, the light-emitting element is called a current-excitation light-emitting element.
The range of uses of such an active matrix display device is expanding, and demands for larger screen size, higher definition, and higher aperture ratio are increasing. In addition, it is demanded that a production method of the active matrix display device offer high fabrication yield and reduce production cost. Simplification of a process is one way for increasing productivity and reducing manufacturing cost.
In active matrix display devices, transistors are mainly used as switching elements. In manufacturing transistors, reduction in the number of photolithography steps or simplification of the photolithography step is important for simplification of the whole process. For example, when one photolithography step is added, the following steps are further needed: resist application, prebaking, light exposure, development, post-baking, and the like and, moreover, steps before and after the aforementioned steps, such as film formation, etching, resist removal, and cleaning and drying a substrate. The number of steps is significantly increased only by adding one photolithography step in the manufacturing process. Therefore, many techniques for reducing the number of photolithography steps or simplifying the photolithography step in a manufacturing process have been developed.
Transistors are broadly classified into top-gate transistors, in which a channel formation region is provided below a gate electrode, and bottom-gate transistors, in which a channel formation region is provided above a gate electrode. These transistors are generally manufactured using at least five photomasks. Further, when a spacer for keeping a space between a pair of substrates is formed through a photolithography step in order to hold a liquid crystal layer between the pair of substrates, a liquid crystal display device is manufactured with six or more photomasks in total.
Many conventional techniques for simplifying the photolithography step use a complicated technique such as backside light exposure (e.g., Patent Document 1), resist reflow, or a lift-off method, which requires a special apparatus in many cases. Using such complicated techniques may cause various problems, thereby leading to reduction in yield.