1. Field of the Invention
The present invention relates to a semiconductor device having a circuit structured with a thin film transistor. For example, it relates to the structure of an electro-optical device, typically a liquid crystal display device, and of an electric equipment loaded with such an electro-optical device as a component. Note that throughout this specification, the semiconductor device indicates general devices that may function by use of semiconductor characteristics, and that the above stated electro-optical device and electric equipment are categorized as the semiconductor device.
2. Description of the Related Art
In recent years, the technique of crystallizing and improving the crystallinity of an amorphous semiconductor film or a crystalline semiconductor film (a semiconductor film having crystallinity which is polycrystalline or microcrystalline, but is not a single crystal), in other words a non-single crystal semiconductor film, formed on an insulating substrate such as a glass, has been widely researched. Silicon film is often used as the above semiconductor film.
Comparing a glass substrate with a quartz substrate, which is often used conventionally, the glass substrate has the advantages of low cost and good workability, and can be easily formed into a large surface area substrate. This is why the above research is performed. In addition, the reason for preferably using a laser for crystallization is that the melting point of a glass substrate is low. High energy can be imparted to a non-single crystal film be means of a laser without causing much change in the temperature of the substrate.
A crystalline silicon film formed by performing laser annealing has a high mobility. Accordingly, it is actively used in monolithic type liquid crystal electro-optical devices, where thin film transistors (TFTs) are formed using this crystalline silicon film, for example, TFTs for driving pixels and for driver circuits, are formed on one glass substrate. The crystalline silicon film is formed from many crystal grains. Therefore, it is called a polycrystal silicon film or a polycrystal semiconductor film.
Further, a method of performing laser annealing by processing a high output pulse laser beam, such as an excimer laser by means of optical system, into a square spot of several centimeters, or into a linear shape with a length of 10 cm or more, on the surface to be irradiated, and scanning the laser beam (the laser beam irradiation position is moved relatively to the surface to be irradiated), has been preferably used because it has good mass productivity and is superior industrially. In addition, continuous emission lasers with very high output, such as an Ar laser, have been recently developed. There are reports of good results obtained when using a continuous emission laser for annealing a semiconductor film.
In particular, if a linear shape laser beam is used, then a high degree of mass productivity can be obtained because unlike the case of using a spot shape laser beam with which it is necessary to scan forward, back, left, and right, laser irradiation can be performed over the entire surface to be irradiated by scanning only at a right angle to the longitudinal direction of the linear shape laser. This is because scanning at a right angle to the longitudinal direction is the most efficient scanning direction. Due to this high mass productivity, the present use in laser annealing of a linear shape laser beam in which a pulse emission excimer laser beam is processed into a suitable optical system, is becoming a mainstream.
For the case of processing the above pulse emission excimer laser beam into a linear shape and irradiating the linear shape laser beam while scanning, for example, with a non-single crystal silicon film, the phenomenon of stripes at a portion where the beams overlap is noticeable. (Refer to FIG. 22A.)
The semiconductor characteristics of the film differ remarkably for each of these stripes, so if this striped film is used when forming an integrated driver and pixel (system on panel) liquid crystal display device, a drawback develops where these stipes appear on the screen, as is. The stripes which appear on the screen are caused by the non-uniform crystallinity in both the driver section and the pixel portion. This problem is being remedied by improving the film quality of the non-single crystal silicon film, the laser irradiation object, but this is not yet enough.
An object of the present invention is to solve this problem. The cause of the striped pattern is the energy diffusion in the width direction near the edges of the linear shape laser beam. In general, when a linear shape laser beam is formed, an optical system called a beam homogenizer is used to make the beam homogenous. A beam so processed has a very high homogeneity.
However, with respect to the light quality, there is a region in which the energy is gradually attenuated on the linear shape laser beam edge. The crystallinity of a semiconductor film irradiated with this region is poor relative to a region exposed to the center of the beam. A method is then taken of increasing the crystallinity of the regions in which crystallinity is poor by overlapping irradiation while gradually displacing the linear shape laser beam in the width direction of the beam.
The most suitable overlap pitch has been found by experiment of the inventors of the present invention to be approximately one tenth of the beam breadth (half width). Thus the crystallinity of the above region with poor crystallinity is improved. In the above example, the half line width is 0.6 mm, so laser irradiation is performed with an excimer laser pulse frequency of 30 Hz at a scanning velocity of 1.8 mm/s. The energy density of the laser at this time is 380 mJ/cm2. The methods stated to this point are very general methods of using a linear shape laser to crystallize a semiconductor film.
Continuous light excimer emission laser devices have been developed recently. In order to promote the excitation of an emission gas, microwaves are used in this laser. By irradiating the emission gas with gigahertz order microwaves, the rate determining reaction of the emission is promoted. Thus the development of the continuous emission excimer laser, which has been not available, becomes possible.
The advantage of using an excimer laser for crystallization of a silicon film is the high absorption coefficient of an excimer laser for a silicon film. The absorption coefficient for a silicon film of a continuous emission argon laser having a wavelength of approximately 500 nm, the wavelength often used in crystallization of a silicon film, is on the order of 105/cm. The intensity of an argon laser is attenuated to 1/e (where e is the natural logarithm) at the point where it has transmitted 100 nm of the silicon film. However, an excimer laser has an absorption coefficient on the order of 106/cm, one order of magnitude higher, so its intensity is attenuated to 1/e at the point where it has transmitted 10 nm of the silicon film.
In general, it is suitable for the thickness of a silicon film, which becomes a semiconductor element material, formed on a glass substrate to be approximately 50 nm. If the silicon film is thicker than 50 nm, there is a tendency for the off characteristics to become poor, while a thinner film influences the reliability.
However, when a 50 nm silicon film is irradiated with an argon laser, over half of the argon laser light goes through the silicon film and is irradiated on the glass substrate. The glass substrate, which one does not want to be heated due to its melting point, is thus heated more than necessary. In practice, when attempting crystallization by argon laser of a 200 nm silicon oxide film and a 50 nm silicon film formed in order on a Corning 1737 substrate, the glass changes shape before there is sufficient crystallization.
On the other hand, in the case of irradiation by an excimer laser, almost all of the light energy is absorbed in the 50 nm silicon film. Therefore nearly all of the excimer laser light can be used in crystallizing the silicon film.
Considering the above, use of an excimer laser for crystallization of a silicon film is good. An excimer laser, with a high absorption coefficient in a silicon film, is becoming more and more important for crystallizing a semiconductor film because continuous emission types have become available.
Provided that a continuous emission excimer laser is used, the pulse laser irradiation marks do not form, which is the subject of the present invention. Therefore a film with very high homogeneity can be obtained.
The undulations of a silicon film formed by pulse laser irradiation are shown in FIGS. 22A to 22C, while the undulations of a silicon film formed by continuous emission laser irradiation are shown in FIGS. 1A to 1C.
A diagram, as seen from above, of a silicon film irradiated by scanning a conventional pulse emission excimer laser is shown in FIG. 22A. FIG. 22B is a cross sectional diagram of a cross section cut parallel to the scanning direction of the pulse emission excimer laser (in the vertical face of the silicon film which includes the line segment EF). In addition, FIG. 22C is a cross sectional diagram of a cross section of a vertical face of the silicon film face at a right angle to the above cross section (in the vertical face in the silicon film which includes the line segment GH).
As can be understood from FIG. 22B, undulations of the same order as the silicon film thickness have developed in the pulse laser irradiation marks. On the other hand, the undulations shown in FIG. 22C are occurred due to the energy non-uniformity in the longitudinal direction of the linear shape laser beam, and compared to the undulations of FIG. 22B, are very small.
The diagram shown in FIG. 1A is a view seen from above of a silicon film irradiated while scanning a continuous emission excimer laser. FIG. 1B is a cross sectional diagram of a cross section cut parallel to the scanning direction of the continuous emission excimer laser (in the vertical face of the silicon film which includes the line segment AB). In addition, FIG. 1C is a cross sectional diagram of a cross section of a vertical face of the silicon film face at a right angle to the above cross section (in the vertical face in the silicon film which includes the line segment CD).
As can be understood from at FIG. 1B, the irradiation marks of the continuous emission excimer laser can be nearly disregarded when compared with the irradiation marks of the pulse laser. On the other hand, the undulations shown in FIG. 1C are occurred due to the energy non-uniformity in the longitudinal direction of the linear shape laser beam.