The present invention relates to an active-matrix substrate and an inspecting method thereof, more specifically, relates to the active-matrix substrate which is used for a device such as an active-matrix liquid crystal display device in which driving voltage is applied to a pixel electrode via a switching element and liquid crystal is driven in accordance with a potential difference between the pixel electrode and an opposing electrode so as to provide a display, and concerns the inspecting method thereof.
Conventionally, an active-matrix liquid crystal display device has been arranged in a state in which individual pixel portions are independently disposed in a matrix form, and each pixel portion is provided with a pixel electrode and a switching element.
In the above-mentioned active-matrix liquid crystal display device, driving voltage is applied to the pixel electrode via the switching element, liquid crystal is driven in accordance with a potential difference between the pixel electrode and an opposing electrode which is disposed in a manner so as to oppose the pixel electrode via the liquid crystal, and transmitted light or reflected light is subjected to an optical modulation so as to display an image on the liquid crystal display panel.
In the above-mentioned liquid crystal display device, an MIM(Metal Insulator Metal) element or TFT(Thin Film Transistor) element has been used as the switching element. Especially, a liquid crystal panel using the TFT element has been most widely used as the active-matrix liquid crystal display device thanks to its quality and cost efficiency.
In the above-mentioned liquid crystal display device in which the TFT element is used, with regard to the pixel portions arranged in a matrix form, scanning lines for inputting a scanning signal which controls the switching element and signal lines for inputting a signal of an image which is to be displayed on the liquid crystal panel are vertically and horizontally disposed. Further, a between layer insulating film is formed between the scanning and signal lines and the pixel electrode.
In the liquid crystal display device having the above-mentioned construction, the between-layer insulating film allows the pixel electrode to be stacked on the scanning lines and the signal lines. Further, for example, Japanese Published Unexamined Patent Application No. 172685/1983 (Tokukaisho 58-172685, published on Oct. 11, 1983) discloses a liquid crystal display device which has the above-mentioned construction so as to improve an aperture rate of each pixel, and furthermore, so as to allow the insulating film to shield from electric field induced by a signal line, thereby reducing defects in alignment of liquid crystal.
Incidentally, a switching element such as the TFT element is generally weak against strong electric field. Therefore, static electricity appearing during a manufacturing process of the liquid crystal display device tends to destroy the TFT element. For example, the liquid crystal display device is provided with an alignment film made of polyimide that determines the alignment direction of the liquid crystal on the substrate. Further, the alignment film is rubbed with a cloth in one direction so as to determine the aligning direction of a liquid crystal molecule. At this time, static electricity appears by the rubbing operation.
When the scanning lines and signal lines on the substrate become charged by the static electricity, an influence is given on a crystal structure of a semiconductive layer within the TFT element. Therefore, the threshold of the TFT element is shifted by several volts. As a result, the switching operation of the switching element is not normally performed so that a portion which has received static electricity is recognized as a defected pixel.
In order to prevent the above-mentioned problem, in the manufacturing process of the substrate, each input terminal of the scanning lines and the signal lines is generally short-circuited by a metal pattern referred to as a short ring. However, after the active-matrix substrate and an opposing substrate have been bonded to each other so as to form the liquid crystal panel, the short ring is removed before peripheral circuits such as a driver have been installed on the input terminals. Therefore, the input terminal of the short ring is not appropriate for a measure against static electricity appearing during the installing process.
For this reason, in the vicinity of the input terminals of the scanning lines and/or the signal lines, protecting circuits are provided so as to connect the adjacent scanning lines and/or the adjacent signal lines.
FIG. 9 is a plan view schematically showing the construction of a conventional active-matrix substrate 51. The liquid crystal panel has a construction in which an active-matrix substrate 51 and an opposing substrate 52 are bonded to each other with a sealing member(not shown), and liquid crystal(not shown) is sealed into a gap between the substrates 51 and 52.
On the active-matrix substrate 51, a plurality of scanning lines 53 and a plurality of signal lines 54 are vertically and horizontally disposed. Each of the areas divided by the scanning lines 53 and the signal lines 54 is referred to as a pixel portion 55, and an effective display area 56 is constituted by the pixel portions 55 which are disposed in a matrix form.
Furthermore, on the active-matrix substrate 51, a plurality of spare wires 57 are disposed on the input side and the non-input side of the signal lines 54.
Additionally, on the active-matrix substrate 51, a scanning-line input terminal 58 and a signal-line input terminal 59 are respectively formed on each end of the scanning lines 53 and each end of the signal lines 54. And protecting circuits 60 are formed so as to connect the adjacent scanning lines 53 and to connect the adjacent signal lines 54 on the active-matrix substrate 51.
The protecting circuit 60 is, for example, formed by using a switching element adopting a diode. Namely, as shown in FIG. 10, the two switching elements are arranged in parallel in the opposite direction from each other so as to form a diode ring structure, which serves as the protecting circuit 60. The protecting circuits 60 are arranged in a manner so as to connect the adjacent scanning lines 53 and to connect the adjacent signal lines 54.
For example, Japanese Published Unexamined Patent Application No. 106788/1988 (Tokukaisho 63-106788, published on May 11, 1988) discloses the above-mentioned construction. With this arrangement, even in the case when an electric field exceeding a certain value is concentrated on a certain portion, the electric charge can be released to the adjacent scanning line 53 and the adjacent signal line 54; thus, it is possible to prevent defects caused by the aforementioned electrostatic destruction.
Moreover, as shown in FIG. 9, the active-matrix substrate 51 is provided with a plurality of terminals 61 for a lighting inspection. The active-matrix substrate 51 is cut along a division line 62 upon completion of the lighting inspection so that the terminals 61 are removed.
As shown in FIG. 9, the spare wires 57 are normally disposed outside the effective display area 56 so as to intersect the end portions of the signal lines 54 and/or the scanning lines 53. Therefore, in many cases, terminal portions 63 of the spare wires 57 are placed closer to the corners of the liquid crystal panel as compared with the terminals 58, 59, and 60 of the scanning lines 53 and the signal lines 54. Hence, upon handling the panel, the terminal portions 63 of the spare wires 57 tend to come into contact with static electricity prior to other portions. Thus, in the case when the panel comes into contact with an object which is electrostatically charged, or in the case when the panel has been electrostatically charged and the terminal portions 63 of the spare wires 57 come into contact with the object so that the object is subjected to an electric discharge, high voltage tends to be applied to the spare wires 57.
However, the conventional protecting circuits 60 are provided in order to prevent an electrostatic destruction of the TFT elements so that the spare wires 57, which have no semiconductor element thereon, are not provided with the protecting circuits 60.
Incidentally, the spare wires 57 are connected with the signal lines 54 and the scanning lines 53 by using laser, only after defects have been found on the signal lines 54 and/or the scanning lines 53. Thus, the spare wires 57 are normally arranged so as not to be electrically connected with the signal lines 54 and the scanning lines 53. Therefore, the impedance is extremely high. In the case when high voltage is applied from terminal portions 63, etc. due to static electricity, puncture caused by the high voltage appears at intersecting portions 64, which are the intersections of the spare wires 57 and the signal lines 54 and/or the scanning lines 53, and which is provided for a connection with laser. As a result, electric problems appear in some cases.
Further, even in the case when puncture does not occur at the intersecting portions 64, the potential of the signal lines 54 and/or the scanning lines 53 partially increases up to around the potential of the spare wires 57. Consequently, a problem occurs in the operation of the TFT element disposed on each of the pixel portions 55.
The objective of the present invention is to provide an active-matrix substrate which prevents puncture from occurring at the intersections of spare wires and signal lines or scanning lines and prevents degradation in a property of TFT elements that is caused by a surge of potential, and to provide an inspecting method thereof.
In order to achieve the above-mentioned objective, the active-matrix substrate of the present invention is characterized by: a substrate, a plurality of scanning lines which are disposed in parallel on the substrate, signal lines which are arranged in parallel so as to intersect the scanning lines on the substrate, a plurality of spare wires which are arranged on the input side and non-input side of the signal lines and/or the scanning lines so as to intersect the signal lines and/or the scanning lines, and first protecting circuits which connect the adjacent spare wires so as to prevent an undesired high voltage from being applied to the spare wire.
With the above-mentioned arrangement, in the case when an object which has been electrostatically charged comes into contact with one of the spare wires, or in the case when one of the spare wires comes into contact with an object which has been grounded or an object having a large capacity in a state in which the panel bears electric charge, the first protecting circuit allows the electric charge to be released to the adjacent spare wire so that it is possible to prevent an electric field from being concentrated on a certain spare wire. This effect is realized by the following function: relative to a certain amount of electric charge, the capacitance increases in accordance with the number of spare wires connected with the first protecting circuits so that the voltage caused by the electric charge is effectively reduced.
Therefore, it is possible to prevent puncture from occurring at the intersections of the spare wires and the signal lines or the scanning lines, and to prevent degradation in properties of the TFT elements that is caused by a surge of potential.
Further, with the above-mentioned arrangement, the active-matrix substrate of the present invention is also allowed to be provided with terminals connected with the scanning lines for the lighting inspection, terminals connected with the signal lines for the lighting inspection, terminals connected with the spare wires for the lighting inspection, and fourth protecting circuits for connecting the adjacent spare wires, in an area which is to be cut off upon completion of the lighting inspection.
The above-mentioned arrangement is provided with the fourth protecting circuit; therefore, it is possible to prevent puncture from occurring at the intersections of the spare wires and the signal lines or the scanning lines, and to prevent degradation in property of the TFT elements that is caused by a surge of the potential.
In the case when defects are found on lines in the lighting inspection, it is necessary to electrically inspect whether the spare wires are connected with the signal lines or the scanning lines with sufficiently low resistance by using laser. Thus, it is not possible to leave a short circuit among a plurality of the spare wires and between the spare wires and the signal lines or the scanning lines. Therefore, the conventional arrangement has the spare wires which are electrically independent from each other, thereby offering less protection against static electricity than the signal lines and the scanning lines.
The above-mentioned arrangement is provided with the fourth protecting circuits so that it is possible to effectively protect the spare wires from static electricity in a more positive manner. Further, the fourth protecting circuits are disposed in an area which is to be cut off upon completion of the lighting inspection so that the fourth protecting circuits are cut off before a displaying operation; thus, the fourth protecting circuits are free from a restraint in which the resistance value needs to be not less than 2 Mxcexa9. Namely, the fourth protecting circuit merely needs to have a resistance which is sufficiently high for determining whether the resistance at the connected portion is sufficiently low or not upon correcting the above-mentioned defects on lines by using laser. High reliability is not required. Thus, any problems are not caused even when an extremely large channel width is arranged for releasing a large amount of static electricity.
Furthermore, as described above, in the liquid crystal panel, a lighting inspection is performed before the external circuits have been installed. At this time, generally, some or all the signal lines are supplied with the same signal, or a plurality of the scanning lines are electrically bundled in a simple manner so that the inspection is carried out in a simpler manner with more efficiency. Therefore, an area, which is to be cut off upon completion of the lighting inspection, is provided with terminals which are connected with some or all the signal lines for the lighting inspection, and terminals which are connected with a plurality of the scanning lines for the lighting inspection. Upon completion of the lighting inspection, these terminals are cut off from the signal lines and the scanning lines so as to allow each of the signal lines and the scanning lines to become electrically independent.
Furthermore, an inspecting method of the active-matrix substrate is characterized by including a step of determining voltage applied to a resistance measuring device, which measures loop resistance, so as to allow the resistance value of the fourth protecting circuit to be more than 20 times the resistance value of one spare wire.
The above-mentioned method makes it possible to readily determine whether the resistance is sufficiently low at the connecting portion upon correcting defects on lines by using laser.
In the case when defects are found on lines, it is necessary to electrically inspect whether the spare wires are connected with sufficiently low resistance after the spare wires have been connected by using laser. In this case, this inspection is performed as follows: after a correction has been made by using the adjacent spare wires, loop resistance is measured between the adjacent spare wires via a portion connected by using laser.
For this reason, the resistance value via the fourth protecting circuit needs to be sufficiently larger than the loop resistance in order to find a correct resistance of the connection. Therefore, voltage applied to the resistance measuring device is determined so as to allow the resistance value of the fourth protecting circuit disposed between the spare wires to exceed 20 times the resistance value of one spare wire; thus, it is possible to smoothly inspect the resistance.
For a fuller understanding of the nature and advantages of the invention, reference should be made to the ensuing detailed description taken in conjunction with the accompanying drawings.
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