1. Field of the Invention
The present invention relates to a semiconductor device having a thin film transistor formed using a crystalline semiconductor film of high crystallinity. The present invention also relates to a crystallizing method for manufacturing thereof.
2. Related Art
Recently, a research has been advanced concerning a sophisticated semiconductor device using a thin film transistor. Particularly in the semiconductor device requiring rapidity and high functionality, it is necessary to obtain the thin film transistor (hereinafter also referred to as TFT) having high mobility.
In order to enhance the crystallinity of the semiconductor film, the crystallization is performed to form a crystalline semiconductor film in such a way that a metal element for promoting the crystallization typified by nickel element (Ni) is added, formed, or coated to the semiconductor film, and then a heat treatment is performed thereto (for example, refer to patent document 1).
When the metal element for promoting the crystallization typified by Ni is used in a such crystallizing step, it is possible to obtain the crystalline semiconductor film having a large grain size and to obtain the crystalline semiconductor film in which the grain boundaries are likely to unite, thereby having fewer defects in the grain.
In addition, as the different crystallizing method from the heat treatment, a crystallizing method by a laser irradiation has been studied. The crystallization by the laser irradiation is conventionally performed in such a way that an ultraviolet beam is irradiated in a pulse oscillation to an amorphous or poly-crystalline silicon layer to form a silicon thin film including a group of silicon single crystal grains (refer to patent document 2, for example). In the laser crystallization according to the patent document 2, the travel distance of a rectangular ultraviolet beam between the position where it ends to be irradiated and the position where it begins to be irradiated next is set to 40 μm or less and the proportion of the travel distance with respect to the width of the ultraviolet beam measured along the moving direction is set in the range of 0.1 to 5%. The patent document 2 discloses that the silicon single crystal grain to be obtained has the preferred orientation approximately <100> to the surface of the base substance.
It is reported that when the polarized laser light is used in the crystallization, a ridge is formed in the direction perpendicular to the direction of polarization by optimizing the condition at the laser irradiation (refer to non-patent document 1). The non-patent document 1 describes that the interval between the ridges depends on the wavelength and the irradiation angle of the laser light, which is expressed with λ/(1±sin θ) in p-polarized laser light where λ is the wavelength of the laser light and θ is the irradiation angle of the laser light.
The non-patent document 1 discloses in FIG. 2A that the first irradiation of the linear pulsed laser forms a linear ridge. Moreover, the non-patent document 1 discloses in FIG. 2B that the second irradiation of the linear pulsed laser light at an angle 90° with respect to the direction of the first laser irradiation forms a grid pattern of the ridge.
The non-patent document 1 reports an experiment where an amorphous silicon film formed over a glass substrate is irradiated with a pulsed Nd: YAG laser in a film-forming chamber of ultra high vacuum in which the temperature is set to be the same as that of the substrate, which is 350° C.
[Patent Document 1]
Unexamined Patent Publication H7-161634 bulletin
[Patent Document 2]
Unexamined Patent Publication H10-41234 bulletin
[Non-Patent Literature 1]
“AM-LCD 2000”, p. 265-268 written by Y. Nakata, A. Shimoyama, and S. Horita,
In the crystallizing method according to patent document 1, many aggregations of pillar crystals (each aggregation is also referred to as a domain) having a large size from 200 to 300 μm are formed. The crystals in one domain have the same crystal orientation. However, the orientation is different in the adjacent domains, and they have a boundary therebetween. When a TFT is manufactured by forming a channel-forming region within one domain, high electrical characteristic can be obtained.
However, the domain is formed at random and it is difficult to form the channel-forming region within one domain. Therefore, it becomes difficult to form all the channel-forming regions which are arranged in a pixel portion and a driver circuit portion respectively within one domain.
As a result, although there is an advantage of high mobility when such a crystalline semiconductor film is used as an active layer of TFT (an island-shaped semiconductor film including a channel-forming region and an impurity region), there is a risk of a small difference, which is the variation, in the characteristic between respective TFTs due to the absence or presence of the boundary between the adjacent domains (the domains having the different orientation) in the channel-forming region or due to the difference of the size of the domain to be formed.
The variation in the electrical characteristic of TFT arranged in the pixel portion or the driver circuit portion causes the variation in the current and the voltage applied to each pixel electrode, which becomes the display unevenness that is visible for the observer.
At present, this variation is acceptable and it does not lead to any problems. In the future, however, when the pixel size is miniaturized further and the more precise image is demanded, this variation is considered to become a serious problem. In the future, as the width of the gate wiring becomes narrower, the size of the channel-forming region (channel width) becomes smaller. Therefore, there is a risk that a TFT having a boundary between the domains in the channel-forming region is formed. The characteristic of such a TFT (mobility, S-value, on-current, off-current, and the like) varies compared with that of TFT having a channel-forming region with no boundaries, and therefore this variation is considered to cause the display unevenness.
According to crystallizing method disclosed in the patent document 2, the travel distance of a rectangular ultraviolet beam between the position where it ends to be irradiated and the position where it begins to be irradiated next is set to 40 μm or less, and the proportion of the travel distance with respect to the width of the ultraviolet beam measured along the moving direction is set in the range of 0.1 to 5%. In the embodiment 1 of the patent document 2, any point in the amorphous silicon layer is irradiated with the pulsed ultraviolet laser 100 times.
Since the laser is irradiated to the silicon layer many times, for example 100 times, in such a crystallizing method, the processing takes much time.
In particular, when the crystalline semiconductor film formed by controlling the orientation with the use of a metal such as Ni is irradiated with the laser many times, it is impossible to maintain its orientation. In other words, when the laser light is irradiated many times as shown in the conditions described in the patent document 2 after forming the crystalline semiconductor film with the orientation controlled using the metal element as shown in the patent document 1, it is impossible to maintain the crystalline semiconductor film with the orientation controlled.
In addition, as shown in the non-patent document 1, the ridge forms a grid pattern only when the second laser irradiation is performed at an angle 90° with respect to the direction of the first laser irradiation under the conditions where the Nd: YAG laser is irradiated in a vacuum film-forming chamber with the substrate temperature set to 350° C. Therefore, in such a method, the laser irradiation takes much time. This means it takes much time to manufacture a semiconductor device or a thin film transistor, and therefore this method is unsuitable for mass production.