1. Field of the Invention
The present invention relates to a semiconductor device including thin film transistors (TFTs), a method for manufacturing the same, and a semiconductor manufacturing apparatus. More particularly, the present invention relates to a semiconductor device in which the active region is formed from a crystalline semiconductor film obtained by crystallizing an amorphous semiconductor layer, and a method for manufacturing the same.
2. Description of the Related Art
In recent years, attempts have been made in the art to form high-performance semiconductor elements on an insulative substrate such as a glass substrate or an insulating film, aiming at realization of liquid crystal display devices and organic EL display devices having larger sizes and higher resolutions, contact image sensors operating at higher speeds with higher resolutions, three-dimensional ICs, etc. Particularly, a type of liquid crystal display device having the pixel section and the driving circuit on the same substrate is finding its use in various household appliances, in addition to a monitor of a personal computer (PC). For example, liquid crystal displays are used as television sets, replacing CRTs (Cathode-Ray Tubes), and front projectors are used for home entertainment applications such as for watching movies and for playing video games. Thus, the market for liquid crystal display devices has been growing at a remarkable rate. Moreover, system-on-panel devices have been developed actively, in which logic circuits such as a memory circuit and a clock generation circuit are formed on a glass substrate.
Displaying high-resolution images means an increase in the amount of data to be written to pixels, and the data needs to be written within a short time. Otherwise, it is not possible to display a moving picture that has a very large amount of data for high-definition display. Therefore, TFTs used in a driving circuit are required to operate at a high speed. In order to achieve high-speed operations, there is a demand for forming the TFTs using a crystalline semiconductor layer having a desirable crystallinity, with which it is possible to obtain a high field-effect mobility.
A commonly-known method for obtaining a desirable crystalline semiconductor film on a glass substrate involves irradiating an amorphous semiconductor film with laser light such as excimer laser light so as to instantaneously melt/solidify and thus crystallize the amorphous semiconductor film. Another method that has been developed in the art involves adding a metal element capable of promoting crystallization to an amorphous semiconductor film, which is then subjected to a heat treatment. With this method, a desirable semiconductor film having a uniform crystal orientation can be obtained through a heat treatment performed at a lower temperature and for a shorter time than other conventional methods. In this method, laser light irradiation is often used in combination in order to further improve the crystallinity, by irradiating a crystalline semiconductor film obtained through a heat treatment with laser light so as to partially melt/solidify and thus recrystallized the crystalline semiconductor film, thereby reducing the crystal defects therein and obtaining a crystalline semiconductor film of a higher quality.
However, it has been found that with such a method of irradiating an amorphous or crystalline semiconductor film with laser light so as to melt/solidify and thus crystallize or recrystallize the amorphous or crystalline semiconductor film, surface irregularities are formed on the surface of the semiconductor film. The surface irregularities are formed as follows. After a semiconductor film is once melted by laser light irradiation, crystal nuclei are formed, and the melted portions are gradually solidified starting from the crystal nuclei. Then, the crystal grain boundary portions, which are solidified lastly, are raised in a mountain-range-like shape (where two crystal grains meet) or in a mountain-like shape (at multiple points where three or more crystal grains meet), due to the difference in volumetric expansion coefficient between a melted portion and a solidified portion. Those portions on the surface of a semiconductor film that are raised in a mountain-range-like shape or in a mountain-like shape will be hereinafter referred to as “protruding portions” or “ridges”. With a top-gate thin film transistor, the ridge is present at the channel interface between the semiconductor film and the gate insulating film, thereby deteriorating the interface characteristics and the field-effect mobility. Furthermore, an electric field is localized at the tip of a ridge. Thus, it is believed that a ridge deteriorates the voltage endurance of the gate insulating film, and lowers the overall device reliability including the hot-carrier resistance.
Therefore, various methods have been developed in the art for reducing the surface irregularities/ridges of a semiconductor film. Japanese Laid-Open Patent Publication No. 8-213637 discloses a method in which a semiconductor film has an island-like shape with an inclined edge, after which the semiconductor film is irradiated with laser light, in an attempt to prevent the formation of protruding portions on the surface of the semiconductor film. Japanese Laid-Open Patent Publication No. 10-92745 discloses a method in which a natural oxide film on the surface of an amorphous silicon film is removed by dry etching, after which the amorphous silicon film is irradiated with laser light in a vacuum process. Japanese Laid-Open Patent Publication No. 10-106951 discloses a method in which the surface of a silicon film on which protruding portions have been formed through laser light irradiation is oxidized to reduce the surface irregularities utilizing the fact that the oxidization rate is higher at ridges. Japanese Laid-Open Patent Publication No. 10-200120 discloses a method in which surface irregularities formed through laser light irradiation are reduced by surface polishing. Japanese Laid-Open Patent Publication No. 11-186552 discloses a method in which surface irregularities formed through laser light irradiation are reduced by etching the surface of a semiconductor film.
All of these publications disclose methods for reducing ridges on the surface of a semiconductor film. As described above, one factor for the formation of ridges is the difference in volumetric expansion coefficient between a melted portion and a solidified portion in the process of melting/solidifying a semiconductor film. Therefore, physically speaking, it is difficult to improve the surface configuration by making changes in the process conditions, or the like. Thus, various methods have been proposed in the art as disclosed in the publications mentioned above.
However, the methods disclosed in the publications all increase the number of process steps and complicate the manufacturing apparatus, and the increase in the number of process steps inevitably increases the cost and decreases the production yield. Moreover, these publications aim at preventing the decrease in the channel interface characteristics, the field-effect mobility, the voltage endurance of the gate insulating film and the reliability of a top-gate thin film transistor due to the presence of ridges on the surface of the semiconductor film.
The present inventors quantitatively examined the influence of the presence of ridges on a thin film transistor, finding that the decrease in the channel interface characteristics, the field-effect mobility, the voltage endurance of the gate insulating film and the device reliability was not so significant. Needless to say, the surface irregularities on the semiconductor film should preferably be as small as possible. However, it has been found that the channel interface characteristics and the field-effect mobility, for example, are influenced significantly by the condition of the semiconductor film itself, i.e., the crystalline condition thereof, whereas the influence of ridges is negligible, comparatively speaking. It has been found that the voltage endurance of the gate insulating film and the device reliability are also influenced primarily by the bulk properties and the thickness of the gate insulating film itself, and the influence of ridges is not so significant when the thickness of the gate insulating film is 50 nm or more.
However, a new problem has been discovered by the inventors of the present application. The problem is related to the production yield, and is one that cannot be found except by a quantitative experiment. A top-gate thin film transistor (TFT) using a semiconductor layer having surface irregularities that is formed by laser irradiation normally exhibits Vg-Id characteristics as shown in FIG. 10B. In FIG. 10B, curves 10a and 10b show the Vg-Id characteristics for drain-source voltages of 8 V and 1 V, respectively. However, while normal TFTs exhibit characteristics as shown in FIG. 10B, there were some TFTs with Vg-Id (gate voltage-drain current) characteristics as shown in FIG. 10A at an occurrence rate on the order of 0.01% to 0.1%. In FIG. 10A, curves 10a and 10b show the Vg-Id characteristics for drain-source voltages of 8 V and 1 V, respectively. When a gate voltage is applied in the off-state direction in such TFTs, abnormal humped curves of leak current occur as indicated by an arrow X for lower voltage values. The leak current values in the humped portion are greater than those of normal TFTs for the same gate voltages by one or two orders of magnitude. As the gate voltage is further increased in the off-state direction, these TFTs exhibit leak current curves similar to those of normal TFTs. The occurrence of an off-state current in a TFT using a non-monocrystalline crystalline semiconductor film is commonly modeled with thermal excitation and tunneling via a trap level occurring due to crystal defects, etc., present near the center of the bandgap. However, the abnormal humped curves of leak current as described above cannot be explained with this model because the leak current increases as the gate voltage increases in the off-state direction.
In a liquid crystal display device, for example, such an abnormal off-state leak current may prevent a TFT for switching a pixel electrode from sufficiently holding a charge written to the pixel electrode, resulting in a point defect. In a driver circuit for driving a display section, such an abnormal off-state leak current may prevent a charge written to a bus line from being held in a sampling TFT such as an analog switch, resulting in a line defect. As a result, the production yield is lowered substantially.