1. Field of the Invention
The present invention relates to a transistor array for use in a liquid crystal display, an organic EL display, an x-ray sensor, a memory and other electronic apparatuses, and also relates to an active-matrix substrate including such a transistor array and methods for fabricating the transistor array and the active-matrix substrate. The present invention further relates to a function line for use to fabricate the transistor array, a semiconductor device including the transistor array, and a display device including the active-matrix substrate.
2. Description of the Related Art
An active-matrix-addressed liquid crystal display (LCD), which uses thin-film transistors (TFTs) as its switching elements, achieves a display quality that is at least comparable to, and often superior to, that of the conventional cathode-ray tube (CRT) display. Also, an active-matrix-addressed LCD is a thin and lightweight display device that realizes a high resolution with significantly reduced power dissipation. Furthermore, an active-matrix-addressed LCD is also implementable as a display device with a huge screen. Having all of these advantageous features, the active-matrix-addressed LCD is currently being popularized rapidly in various fields of applications and is expected to be a next-generation image display device that replaces the conventional CRT in the very near future. Meanwhile, an organic EL display, which uses an organic electroluminescent material as a display medium, has also attracted much attention in the art recently and is also considered to be among potential next-generation image display devices.
Recently, these next-generation image display devices are not just used as alternatives to the conventional CRTs but are also the objects of vigorous research and development to realize so-called “electronic paper”. That is to say, these display devices are currently under modification so as to provide replacements for conventional paper-printed materials. For that purpose, those image display devices should be made as “flexible displays”, which never experience failure even when folded or rolled and which are readily portable at any time.
To make such a flexible display by conventional manufacturing technologies that have been applied to fabricate an LCD or an organic EL display, glass, which has been used extensively as a substrate material, needs to be replaced with some elastic substrate material that is deformable even at room temperature (e.g., plastic or stainless steel).
However, such an elastic material poorly resists the heat. For example, when exposed to intense heat, a plastic substrate may deteriorate, emit a toxic gas, or be deformed or warped excessively. Also, even if a heat-resistant plastic or stainless steel is used as a substrate material, the substrate made of such a material is also deformed or warped due to a non-negligible difference in thermal expansion coefficients between that substrate and a thin film to be deposited on the substrate. Accordingly, to avoid these unwanted situations, a substrate made of such a poorly heat resistant material should not be exposed to heat in excess of about 200° C.
However, some of the conventional manufacturing and processing steps that have been carried out to fabricate an image display device require a processing temperature that exceeds about 200° C. For example, in the manufacturing and processing step of making an amorphous silicon TFT, which is used as a switching element for an image display device, the gate insulating film and amorphous silicon film of the TFT are normally formed at temperatures exceeding about 300° C. Accordingly, if the heat resistant plastic or stainless steel is used as a substrate material, the conventional manufacturing process cannot be used as it is.
Stated otherwise, the plastic or stainless steel may be used as a substrate material for an image display device if the gate insulating film and amorphous silicon film of the amorphous silicon TFT can be formed at temperatures of less than about 200° C. In that case, however, a high quality gate insulating film or amorphous silicon film (e.g., a film exhibiting good dielectric strength, in particular) is very hard to obtain. Also, when a TFT is made of those poor quality films, the threshold value of the TFT will change significantly after long hours of operation.
Recently, an LCD that uses polysilicon TFTs as its switching elements has been researched and developed vigorously. An image display device of this type is partly characterized by providing a circuit for controlling a drive signal for the display device on a glass substrate. Thus, compared to the LCD that uses amorphous silicon TFTs as its switching elements, the LCD including the polysilicon TFTs achieves a higher resolution.
In an LCD including polysilicon TFTs, however, the driver thereof (e.g., a CMOS inverter) needs to be made of a polysilicon with an electron mobility typically exceeding about 100 cm2/V·s. It is not easy to deposit such a polysilicon on a glass substrate. For example, the silicon on the glass substrate should be fused with the temperature of the glass substrate maintained at about 600° C. or less by some special technique such as a laser annealing process. Accordingly, it is difficult to apply the technique requiring such a high-temperature process to an image display device that uses plastic or stainless steel as a substrate material.
To overcome these problems, Japanese Laid-Open Publication No. 10-91097 discloses a display device including a string of transistors. In the transistor string, a number of transistors are arranged along the length of a conductor core on which an insulating film, a silicon film and an n+-type ohmic contact layer are stacked in this order. In this technique, the gate insulating film and the silicon film that require high-temperature processing are included in the transistor string, which is bonded onto a substrate after those films have been stacked on the conductor core. Thus, the substrate is never exposed to heat. For that reason, a substrate having a low distortion point (e.g., a plastic substrate) may be used.
Japanese Laid-Open Publication No. 9-203910 also discloses a similar technique of making a display device by using a string of transistors that are arranged along the length of a metal line.
In this conventional technique, first, an insulating film and a semiconductor film are deposited in this order all over a metal core and then an n+ film is further deposited thereon so as to cover the semiconductor film. Next, the n+ film is patterned by a normal photolithographic process, thereby defining source/drain regions of the n+ layer and arranging a number of transistors along the metal line. Thereafter, the metal line is bonded onto a substrate such that the source/drain regions are connected to the source and pixel electrodes, respectively.
In this case, the patterned n+ layer on the metal line all needs to be connected with metal interconnects. To connect the patterned n+ layer on the metal line in its entirety just as intended, at least approximately 1,000 metal interconnects need to be arranged on the substrate with no misalignment at all, which is rather difficult to realize in an actual manufacturing process.
For example, suppose a metal line 504, in which a transistor 503 is defined so as to have source/drain regions (n+ layer 501) and a channel region 502, needs to be arranged with respect to a source line 505 and a drain electrode 506 as shown in FIG. 73. Also, suppose the n+ layer 501 has a length of 20 μm, the distance from the end of the source line 505 or the end of the drain electrode 506 to the channel region 502 is 5 μm each, and the channel length is 5 μm. These dimensions are just as defined by the design rules of a typical conventional TFT and are approximately equal to the exemplary design values disclosed in Japanese Laid-Open Publication No. 9-203910.
If the metal line 504 could be arranged exactly at the intended location with no shifting at all, then the source line 505 and drain electrode 506 would be located over the n+ layer 501 of the metal line 504 with a margin of 5 μm provided on each side of the channel region 502 as shown in FIG. 73. However, if just one of approximately 1,000 metal lines 504 shifted from its intended location by 5 μm or more, then all of the transistors on the metal line 504 would have their effective channel length changed and exhibit a different characteristic from that of the transistors on the other metal lines 504. Also, if the metal line 504 shifted from its intended location by 10 μm or more as shown in FIG. 74, then leakage failure would occur between the source/drain regions to make all of the transistors on the metal line 504 defective (i.e., to cause a line defect). Such a problem may occur even when the technique disclosed in Japanese Laid-Open Publication No. 10-91097 is adopted.
In this manner, according to the conventional technique described above, even the shift of just one metal line should affect all of the transistors on that metal line to cause a line defect. Then, the display device itself could become a non-repairable defective product.
Furthermore, Japanese Laid-Open Publication No. 9-203910 discloses that the entire surface be covered with an organic insulating film except portions of the source lines and drain electrodes for the purpose of planarization, for example. More specifically, as shown in FIG. 75, an organic insulating film 508 is provided so as to cover the source lines 505 and drain electrodes 506 and have contact holes 507 that expose portions of the source lines 505 and portions of the drain electrodes 506. The metal line 504 is arranged such that the n+ layer 501 of the transistor 503 is connected to the source line 505 and drain electrode 506 by way of those contact holes 507. In that case, once the contact holes 507 have shifted from their intended locations, even if the source line 505 and drain electrode 506 are arranged appropriately with respect to the metal line 504, the metal line 504 cannot be electrically connected to the source line 505 or drain electrode 506 anymore as shown in FIG. 76. In other words, alignment must be carried out in this case among the contact holes 507 of the metal line 504, source line 505 and drain electrode 506.
As for a liquid crystal display device including conventional TFTs to be provided on a glass substrate by a thin film technology, such alignment is sometimes not so difficult. However, if a thick organic insulating film 508 is provided for the purpose of planarization or if a plastic substrate to be deformed significantly when absorbing water or heat is used, it is very difficult to carry out the alignment highly accurately.
In order to overcome the problems described above, an object of the present invention is to provide an active-matrix substrate, in which a plurality of transistors are arranged with no misalignment at all on a base substrate that is deformable elastically at an ordinary temperature, and a display device including such an active-matrix substrate. Another object of the present invention is to provide a transistor array, in which a plurality of transistors are arranged with no misalignment at all, and a semiconductor device including such a transistor array. Still another object of the present invention is to provide a method for fabricating such a transistor array, a method for fabricating such an active-matrix substrate, and a function line for use in such a transistor array.