1. Field of the Invention
The present invention relates to a semiconductor device using a semiconductor thin film and its manufacturing method and, in particular, to a thin film transistor (hereinafter referred to as TFT) using a crystalline semiconductor thin film containing silicon.
2. Description of the Related Art
In recent years, a technology for forming a TFT on a glass substrate or the like to constitute a semiconductor circuit has been rapidly advanced. As a typical semiconductor device, there is an active matrix type liquid crystal display (hereinafter referred to as AMLCD) integrated with a driver. The AMLCD integrated with a driver is a monolithic display device having a pixel section and a driver circuit on the same substrate. Further, a system-on panel in which a memory circuit, a clock generating circuit, and the like are built has been developed.
While a TFT in which an amorphous silicon (hereinafter referred to as a-Si) is used in an active layer is employed as a switching device of a pixel in a conventional AMLCD, in the peripheral circuit of the AMLCD integrated with a driver is mainly employed a TFT in which a polycrystalline silicon (hereinafter referred to as poly-Si) having a higher field effect mobility is used as the active layer because the a-Si is not suitable for the peripheral circuit which is required to operate at high speeds.
A conventional poly-Si TFT has a higher field effect mobility than an a-Si TFT. When a variety of circuits are mounted on a system-on panel, however, because the circuits are required to operate at higher speeds, the TFTs employed by the circuits are required to have a field effect mobility higher than the AMLCD integrated with a driver.
Also, even in the AMLCD integrated with a driver, TFTs having a higher field effect mobility are required because it is required to operate at high speeds due to an increase in the number of pixels and to reduce the area of a driver circuit.
Chief among factors determining the field effect mobility of the TFT is a surface dispersion effect. The flatness of an interface between the active layer of the TFT and a gate insulating film produces a large effect on the field effect mobility of the TFT, and as the interface becomes flatter, the effect of the surface dispersion becomes less and thus the field effect mobility becomes larger.
One of currently available methods of producing a crystalline silicon film is a laser crystallization method, and a method of applying an excimer laser to an amorphous silicon film to crystallize it has been known. An amorphous silicon film having a thickness of 10 nm to 150 nm (typically 30 nm to 60 nm) is formed on an insulating substrate by sputtering, CVD, or the like and subsequently irradiated with excimer laser light, whereby the amorphous silicon film is melted, solidified, and crystallized. In the case where the amorphous silicon film contains about 5% or more hydrogen, it is previously dehydrogenated by a heat treatment performed at temperatures of about 400° C. to 500° C. for several hours because hydrogen is explosively removed when it is annealed by a laser.
While the conditions of the laser crystallization are selected by an operator, when the excimer laser is employed, for example, a laser pulse oscillation frequency is 30 Hz and a laser energy density is 100 mJ/cm3 to 500 mJ/cm3 (typically, 300 mJ/cm3 to 400 mJ/cm3). A linear laser beam is applied to the whole surface of the substrate, wherein the overlapping ratio of the linear beam is 80% to 98%.
Protrusions (or bumps) called ridges are formed at random on the surface of the film crystallized in this way by the laser beam. It is thought that the protrusions are produced by surface tension waves induced on the surface of Si annealed and melted by the laser. Typically, the protrusions have a thickness two times the thickness of the thin film. The thickness of the thin film is usually 30 nm to 60 nm and thus the protrusions have a height of 30 nm to 60 nm from the surface of the film. The protrusions formed in this way give dispersion to the movements of electrons or holes due to the above-mentioned surface dispersion effect to reduce the field effect mobility of the TFT. The larger the protrusions are, the larger the effects are.