1. Field of the Invention
The present invention relates to a semiconductor device which has circuits structured by thin film transistors (hereafter referred to as TFTs) and to a method for fabricating the same. For example, the present invention relates to an electro-optical device, typically represented by a liquid crystal display panel, and to an electronic apparatus in which a light emitting display device having light emitting elements is installed as a part.
Note that, in the specification, the term, semiconductor devices, indicates a category of general devices which are capable of functioning by utilizing semiconductor characteristics, and electro-optical devices, semiconductor circuits, and electronic apparatuses are all included in the category of semiconductor devices.
2. Description of the Related Art
Thin film transistors (hereinafter referred to as TFTs) are known as a typical semiconductor element that use a semiconductor film having a crystal structure. The TFTs are attracting attention as a technique of forming an integrated circuit on a glass or other insulating substrates, and devices utilizing the TFTs, such as a liquid crystal display device with a built-in driving circuit, are beginning to appear on the market. In prior art, a semiconductor film with a crystal structure is formed by using a heat treatment or a laser annealing to crystallize an amorphous semiconductor film that is obtained by deposition through a plasma CVD method or a reduced pressure CVD method. (Laser annealing is a technique of crystallizing a semiconductor film through irradiation of laser light.)
The thus formed semiconductor film with a crystal structure is a mass of crystal grains. Since the crystal grains are randomly oriented and the orientation thereof cannot be controlled, the semiconductor film affects TFT characteristics. JP 07-183540 A discloses a technique to tackle this problem. The technique involves doping with a metallic element such as nickel that helps crystallization of a semiconductor film, to form a semiconductor film having a crystal structure. The technique can cause a large proportion of crystal grains to orient in the same direction, and can lower the heating temperature required for crystallization as well. When such a semiconductor film having a crystal structure is used to form a TFT, not only the field effect mobility will be improved, but also the sub-threshold coefficient (S value) will be reduced to improve the electric characteristics of the TFT greatly.
By using a metallic element for promoting crystallization, generation of nuclei in crystallization can be controlled. Therefore, film quality thus obtained is uniform in comparison with another crystallization method in which nuclei are generated at random, and ideally, it is desirable that metallic elements are completely removed or reduced to an allowable range. However, the metallic element added for helping crystallization remains in the semiconductor film having a crystal structure, or on the surface thereof, causing problems such as fluctuation in characteristic of semiconductor elements obtained. For example, the remained metallic element increases OFF current in the TFTs to cause fluctuation between the semiconductor elements. In short, the metallic element for helping crystallization becomes an unwanted presence once the semiconductor film having a crystal structure is formed.
Gettering using phosphorus is actively employed as an effective method of removing a metallic element that helps crystallization from a specific region of a semiconductor film having a crystal structure. For instance, the metallic element can readily be removed from a channel forming region by doping a source-drain region of a TFT with phosphorus and subjecting the film to heat treatment at 450 to 700° C.
Phosphorus is injected to the semiconductor film having a crystal structure by ion doping (ion doping is a method of dissociating PH3 or the like by plasma and helping the obtained ions in the electric field to inject the ions into a semiconductor, and basically does not include ion mass separation). For gettering, the concentration of phosphorus in the semiconductor film has to be 1×1020/cm3 or higher. Phosphorus doping by ion doping makes the semiconductor film having a crystal structure amorphous, and an increased phosphorus concentration inhibits recrystallization during the subsequent annealing. In addition to this problem, high concentration phosphorus doping prolongs the treatment time required for doping and lowers the throughput in the doping step.
Furthermore, a source-drain region of a p-channel TFT which is doped with phosphorus needs boron in a concentration 1.5 to 3 times higher than the phosphorus concentration in order to reverse the conductivity type of the region. This makes the recrystallization difficult and raises the resistance of the source drain region undesirably.
Also, when sufficient gettering is not conducted in the substrate and a variation in gettering is caused, a slight difference, that is, a variation among respective TFT characteristics is caused. In the case of a transmission type liquid crystal display device, when there is a variation in the electrical characteristics of the TFTs located in a pixel portion, a variation in a voltage applied to each pixel electrode is caused. Thus, a variation in the amount of light to be transmitted is caused. This results in uneven display which is reflected in an eye of an observer.
Also, for a light emitting device using an OLED, a TFT becomes an essential element to realize an active matrix drive method. Thus, at least, a TFT which serves as a switching element and a TFT for supplying a current to the OLED are provided to each pixel of the light emitting device using the OLED. The luminance of the pixel is determined based on an on current (Ion) of the TFT which is electrically connected with the OLED and supplies a current to the OLED regardless of the circuit configuration of the pixel, and drive method. Accordingly, for example, when an entire white display is conducted, there is a problem in that a variation in luminance is caused if the on current is not kept constant.