Patent Application
20080099760
The present application claims priority from Japanese application JP 2006-295667 filed on Oct. 31, 2006, the content of which is hereby incorporated by reference into this application.
The present invention relates to a picture element driving circuit of a display panel and a display device using the same, and particularly to a picture element driving circuit and a display device using the same having a structure in which a driving circuit can be configured only by laminating layers including elements for configuring a circuit, such as wires and constituent elements of an FET such as source, drain and gate electrodes, an insulation film and a semiconductor, and in which through-holes for connecting the layers are not required, in a configuration of the picture element driving circuit in which a driving circuit for one picture element is configured by using two field-effect transistors (FETs).
A cathode-ray tube display device uses an electron gun for lighting of a picture element. On the other hand, a plasma display panel (PDP) and a liquid crystal display (LCD) that are typical thin-model display devices use thin film transistors (TFTs), in place of the electron gun, for lighting and turning off of a picture element. The TFTs are arranged in all the picture elements to be used as switching elements so as to perform lighting control of the picture elements. Therefore, each picture element is provided with a circuit (picture element driving circuit), including the TFT, for controlling lighting of the picture element. For the TFT, an element using amorphous silicon or polycrystalline silicon as a material of a channel is used, and the TFT is manufactured by a method similar to that of manufacturing a large-scale integrated circuit (LSI).
The LSI is configured by laminating a plurality of thin films. Each layer includes the FET and wires that are constituent elements of the LSI. The FET is configured in such a manner that source/drain electrodes and a gate electrode, a gate insulation film, and a channel semiconductor are formed on different layers, and the layers are laminated while being aligned. The layers are etched to form the structure by photolithography, and are sequentially laminated from the bottom. Electrical connection of the layers includes a case in which electrically-direct contact is realized between two contacted layers that are adjacent in the upper and lower sides to each other such as connection in the FET, and a case in which holes across the plurality of layers are provided by photolithography and metals are embedded therein. The latter case is called through-holes, and the holes are formed by photolithography. Etching is required for hole-drilling, and a dry etching process such as reactive ion plasma etching is usually used.
Such a manufacturing process of LSI requires an exposure device used for photolithography and a vacuum device used for thin-film manufacturing and dry etching, which results in high cost. Accordingly, if the picture element driving circuit of the thin-model display device can be manufactured at low cost by a method different from the manufacturing process of LSI, the cost of LSI and also the cost of a product using the LSI can be reduced due to reduction in manufacturing cost.
As a method of reducing the manufacturing cost, a manufacturing method of the TFT and an electric circuit by coating and printing is currently being actively studied. The manufacture by coating and printing does not require a vacuum device indispensable in a silicon device and a heat treatment at a high temperature, so that significant reduction in manufacturing cost can be realized. On the other hand, it is difficult to miniaturize a circuit by coating and printing as compared to photolithography, and thus the coating and printing cannot be used for the manufacturing of the LSI.
However, since demands for miniaturization of a circuit and performance of the TFT in the picture element driving circuit of the thin-model display device are low as compared to LSI, there is a possibility that a circuit forming method using the coating/printing method can be applied. Since organic materials are soluble with an organic solvent and can be handled at a low temperature around room temperature, they are suitable for the coating/printing method. If a channel semiconductor and an insulation film of the TFT can be configured by organic materials, all the forming processes of TFT can be realized by the coating/printing method.
For example, manufacturing of a single FET element by the coating/printing method is described in Nature Materials, Vol. 3, pp. 171-176 (2004). In addition, an approach to formation of an active matrix control picture element driving circuit by the coating/printing method is described in Nature Materials, Vol. 3, pp. 106-110 (2004), Society for Information Display 2005 International Symposium 2005, pp. 19-21 (2005), Applied Physics Letters, Vol. 85, No. 10, pp. 1849-1851 (2004), and Advanced Materials, Vol. 13, No. 21, pp. 1601-1605 (2001). All of the driving circuits described in these documents are driving circuits for voltage-driven picture elements such as LCD, and there are no descriptions that driving circuits for current-driven picture elements are formed by the coating/printing method. In the case of voltage-driven picture elements, the number of TFT necessary for the picture element driving circuit is one, so that there is no need of through-holes in the circuit formation and the circuit can be easily formed by the coating/printing method.
An organic EL (Electro Luminescence) element has attracted attention as an element for the next-generation display panel which can improve faults of PDP and LCD, such as life duration, contrast, responsiveness, and power consumption, and studies toward the practical use have been advanced. The organic EL element is a thin film element of an organic material, and can be made much thinner than PDP and LCD. Further, the organic EL element uses an organic material as a raw material, so that a wall-hanging display and a flexible display can be realized by taking advantage of the characteristics of the organic material such as light weight and plastic properties. Further, as described above, since the organic material is a material suitable for the coating/printing method, there is an advantage that a novel display panel with high added-value can be realized at low cost if the organic EL element and the picture element driving circuit can be realized by the coating/printing method.
The organic EL element is a current-driven element.
In the case of using the organic EL element as a picture element, an FET for current control is required in addition to that for picture element switching as described in Japanese Patent No. 2784615, and it is conceived that at least two FETs are required in the picture element driving circuit.
An example of a basic picture element driving circuit of an organic EL element according to the present invention is shown in
Data of the data line 202 is taken in by the switch FET 211 located on a line selected by the scanning line 201 to be held into the holding capacity 212, a signal in accordance with the data of the data line 202 is held for a predetermined period of time, and the signal is applied to a gate of the driving FET,213. A current is supplied or not supplied from the current supplying line 203 to the organic EL element 214 in accordance with the data held by the holding capacity 212, and thus contrasting of each picture element is controlled.
As shown in
In the case of configuring a circuit by the coating/printing method, a technique of forming the circuit structure by laminating materials is basically employed, which does not include a technique of scraping off substances on the substrate. Therefore, it is difficult to form through-holes by the coating/printing method. The through-holes can be principally formed if the coating/printing method is repeated so as to leave holes at positions of the through-holes. However, this scheme is not practical in consideration of the required sizes and accuracy of alignment of the through-holes. In Advanced Materials, vol. 13, No. 21, pp. 1601-1605 (2001), there is disclosed a method in which after laminating necessary layers, a solvent for a material configuring layers in which the through-holes are provided is locally coated so as to form the through-holes by coating/printing method.
However, this method does not solve the problem of the accuracy of alignment. The width of a wire used for the picture element driving circuit of the display device is normally several tens of micrometers, and it is difficult, even for a device that is still being studied and capable of aligning with high accuracy, to form the through-holes with accuracy in accordance with the width. Further, since an organic material is used as a solvent, a material used for a foundation layer and a top layer is restricted so that the organic solvent does not affect layers other than the through-holes-formed layers. The layers in which the through-holes need to be formed require characteristics, such as electric insulation properties, chemical-resistance properties, water-resistance properties, and gas barrier properties, and a material insoluble with chemicals generally exhibits a higher performance than a material soluble with chemicals due to such characteristics, thus it is difficult to select a material from this aspect.
Accordingly, in the method according to Advanced Materials, vol. 13, No. 21, pp. 1601-1605 (2001), it is necessary to select a film made of a material with low performance, and as a result, there is a possibility that the performance of the FET remains low and the material cannot be used for the FET for the picture element driving circuit. In addition, it is conceivable that usage of a film without sufficient barrier properties results in deterioration of the element earlier than the required life duration of the element. Furthermore, the organic thin films are low in adhesion between the films and mechanical strength as compared to inorganic materials, and thus local peeling and reduction in mechanical strength are caused around the through-holes, resulting in acceleration of deterioration of the element.
As described above, in the case of manufacturing the picture element driving circuit for the organic EL element by the coating/printing method, the formation of through-holes involves serious problems in both aspects of manufacturing and performance. Accordingly, it is absolutely necessary to devise so as to avoid difficulty of the accuracy of alignment in the formation of through-holes, restriction of selecting a material accompanied by usage of a solvent, and acceleration of reduction and deterioration in element performance due to presence of through-holes. The object of the present invention is to solve such problems.
The present invention has been achieved by paying attention to the fact that a picture element is controlled by two FETs and that a channel of the FETs is controlled by a gate electrode that is arranged through an insulation layer. That is, attention has been paid to the fact that the channel of two FETs and the gate thereof are configured while being faced to each other through the insulation layer. Specifically, a gate and a drain of each FET are formed on a layer that is arranged through the insulation layer, and the gate of each FET is arranged on a layer different from the layer on which a source and a drain of the other FET are formed.
These and other objects and many of the attendant advantages of the invention will be readily appreciated as the same becomes better understood by reference to the following detailed description when considered in connection with the accompanying drawings.
In the first embodiment, a configuration and a manufacturing procedure of a picture element driving circuit shown in
As shown in
Next, as shown in
After forming the current supplying lines 203 and the scanning lines 201, counter electrodes 313 of organic EL elements are formed in parallel to the branch wires 201a of the scanning lines 201 and the branch wires 203a of the current supplying lines 203. In this case, the counter electrodes are preferably formed by, in place of the discharger, a method capable of high-speed and large-area printing, such as relief printing and surface printing due to large areas for electrodes. However, since the organic EL elements are current-driven elements, the counter electrodes 313 need not spread throughout the picture elements, and thus the size of the electrode 313 may be arbitrary as long as current at a degree where the organic EL elements can be driven can be applied. In this case, the printing methods such as the above are not necessary. In this embodiment, from the view point of simplicity that an apparatus for forming the electrodes is standardized, an area of 100 μm times 300 μm is coated with a silver ink by using the discharger so as to form the counter electrode 313.
Finally, the organic semiconductors 314 used as channels of the driving FETs 213 are dropped between each wire 203a added to the current supplying lines 203 and each counter electrode 313 of the organic EL elements by the discharger so as to obtain an active matrix picture element driving circuit. The interval between each branch wire 203a added to the current supplying lines 203 and each counter electrode 313 of the organic EL elements is set to 20 μm, as similar to the case of the switch FET 211.
Accordingly, it can be found that the data lines 202, the wires 311, and the wires 201a, together with the organic semiconductors 312 shown in
In this way, the areas configured by the data lines 202, the current supplying lines 203, the scanning lines 201, and the counter electrodes 313 configure the picture elements 10.
In the first embodiment, the discharger is mainly used as a coating and printing method, but the present invention is not apparently limited to this. An ink jet printer or a surface printer such as screen printing can be used in place of the discharger. The former has a characteristic that print heads can be easily integrated and large-area printing can be easily handled. Further, since the former is of a non-contact printing method, it can easily cope with irregularities of a substrate. Although high-accurate position adjustment is difficult in the surface printing, the surface printing has an advantage that large-area printing can be performed in a short period of time, as compared to the local coating technique used in the discharger and the ink jet printer. Further, the surface printing requires no precision devices such as a print head, and is high in failure resistance.
In the first embodiment, the wiring pattern shown in
After the driving FETs 213 are formed by the current supplying lines 203, the scanning lines 201, and the organic semiconductors 314 and then the counter electrodes 313 are formed, the insulation film 315 is formed oh the whole surface. Next, as described with reference to
Accordingly, since the organic EL layer is formed first in the second embodiment, the counter electrodes 313 connected to the driving FETs 213 are formed first, and then the switch FETs are formed.
In the third embodiment, there will be explained an example in which the driving FETs are formed right above the substrate 11 and the switch FETs are formed thereon through the insulation film 315. Specifically, the wiring pattern shown in
In the above described embodiments, the coatable organic semiconductors are used as the semiconductors, however, the configuration of the picture element driving circuit without through-holes is not limited thereto. As described in, for example, Nature, vol. 440, pp. 783-786 (2006), channels can be formed with polysilicon by heating and burning with the use of a silane-based coating solution. In this case, a heat treatment requires a relatively high temperature of 550° C. On the contrary, there is an advantage that a circuit can be designed without difficulty due to the high degree of carrier movement of the semiconductor and high deterioration-resistance. In addition, since the performance of the semiconductor is high, the picture quality of the display device can be easily enhanced.
Other than the coatable semiconductor, an amorphous silicon semiconductor can be used by using, for example, a silicon lamination layer formed by Chemical Vapor Deposition (CVD) as a semiconductor layer, which is disadvantageous in cost due to usage of a CVD vacuum process in the formation of the semiconductor and the necessity of photolithography. However, a fine circuit can be advantageously formed as compared to a coating/printing method.
According to the present invention, a circuit composed of two FETs can be formed without through-holes, and, as a result, a drilling process is not required. Thus, it is advantageous that the manufacturing process is shortened, the degree of freedom of selecting a material therefor can be enhanced, and the reliability of the product can be improved.
It is further understood by those skilled in the art that the foregoing description is a preferred embodiment of the disclosed device and that various changes and modifications may be made in the invention without departing from the spirit and scope thereof.
| Number | Date | Country | Kind |
|---|---|---|---|
| 2006-295667 | Oct 2006 | JP | national |
