Active-matrix type liquid crystal devices have been well known which utilize thin film transistors (generally called as TFT). In a conventional active-matrix liquid crystal display device, a peripheral circuit member is constituted by IC and is externally connected to terminals of matrix wirings of the pixels. Further, it is also known to form TFTs for forming a peripheral circuit on the same substrate on which TFTs are arranged in a matrix as a switching element in a pixel region.
Since the TFTs formed on a peripheral circuit portion are to drive the TFTs formed in a matrix form on a pixel portion, they are required to have a capability of passing a large amount of electric current therethrough. Specifically, they are required to have a large ON current and a large mobility.
On the other hand, the TFTs formed on the pixel portion are not required to have a large mobility. Rather, they need to have a lower OFF current (leak current) in order to maintain electric charges on pixel electrodes. Accordingly, the required characteristics for the TFTs of a peripheral circuit are different from those for the TFTs of a pixel portion.
Further, an amorphous silicon film has been used to form a TFT but its characteristics are not satisfactory. Therefore, a TFT using a crystalline silicon film has been investigated. Generally, it is necessary to perform a thermal annealing at 600° C. or higher and for more than 24 hours for obtain a crystalline silicon film from an amorphous silicon film. However, a glass substrate which is usually used as a substrate of a liquid crystal device can not endure such a thermal annealing because for example, a Corning 7059 glass has a distortion point of 593° C. In particular, one problem is that the glass substrate tends to be distorted because of the high temperature so that it is difficult to increase the size of the substrate.
The inventors of the present application have confirmed through their experiments that by contacting a slight amount of catalyst metal such as nickel or platinum with an amorphous silicon film, it is possible to crystallize the silicon film at lower temperatures, for example, at 550° C. for about 4 hours. And the resultant crystallinity is comparable with that obtained the above conventional thermal treatment at 600° C. The inventors considered that these metal functions as a catalyst to promote the crystallization of an amorphous silicon film.
The inventors also confirmed that there are two types of crystallization in the case of using a catalyst as indicated below.
(1) A crystallization proceeds in a direction normal to a substrate in a region where a catalyst was introduced.
(2) A crystallization proceeds in a direction parallel with a substrate from a region to which a catalyst was introduced toward a region to which a catalyst was not introduced.
The crystal structure in the case of (2) was confirmed by using a TEM (transmission type electron microscope) that columnar crystals grow in a direction parallel with a substrate. Also, the amount of nickel necessary for causing the above first type of crystallization is different from the amount of nickel necessary for causing the above second type of crystallization. For example, when the (2) type of crystallization extends about 30 μm, the amount of the nickel necessary to be introduced thereto is 10 times as much as that required in the case of (1).
In the present specification, hereinbelow, the region where the above (1) type of crystallization occurs will be called as a vertical growth region and the region where the above (2) type of crystallization occurs will be called as a lateral growth region.