Various types of flat panel display devices have been developed as replacement of cathode-ray tubes (CRT). Since among others liquid crystal display devices have such advantageous features as light weight, thin thickness, low electric power consumption, etc., the liquid crystal display devices have been widely noticed. In addition, an active matrix type liquid crystal display device with switches connected electrically to pixel electrodes, respectively, has become a main stream of the liquid crystal display device because no substantial cross-talk takes place between adjacent pixels so that it realizes an excellent display quality.
The following description is directed to an optically transparent, active matrix liquid type crystal display device, for example, which includes thin film transistors as switching elements.
Such an active matrix liquid crystal display device includes an electronic circuit array substrate and its counter substrate between which a liquid crystal layer is held. Alignment layers are provided on the inner walls of the substrates to align molecular orientation of the liquid crystal. The array substrate further includes an insulation layer made of glass or quartz, a plurality of signal and scanning lines disposed in matrix on the insulation layer, and a plurality of pixel electrodes made of a transparent material, such as indium-tin-oxide (ITO), and disposed on regions defined by the matrix. Thin film transistors (TFTs) are provided adjacent to the cross points in the matrix as switching elements to control the pixel electrodes. Gate and drain electrodes of the TFTs are electrically connected to scanning and signal lines, respectively, while source electrodes of the TFTs are electrically connected to the pixel electrodes.
The counter substrate is optically transparent so that it includes a transparent insulation layer made of glass, and an ITO counter electrode disposed on such transparent insulation layer. A color filter layer is further provided on the counter substrate for a color image display.
The array substrate extends outward, to define a plate-like outer peripheral portion of the liquid crystal display device, as an electrically connecting portion to receive connecting pads connected to terminals of flexible printed circuits, driver integrated circuits (IC), etc. through isotropically conductive layers. The inner peripheral edge of the counter substrate and the opposite inner edge of the array substrate are sealed with sealant to prevent the liquid crystal from leaking.
The connecting pads include electric power supply lines to supply electric power to the counter electrode. The electric power supply lines are connected to the counter electrode through electric power supply lines and terminals disposed on the array substrate, and conducting materials provided between the substrates (see, for instance, Japanese Patent Publication Tokkai Hei 10-8326).
In this type of the liquid crystal display device, aluminum (Al) or its alloy is used for signal lines of such display technologies as high definition display functions and large display sizes. In the case, however, where the connecting pads are made of such a metal material, the pads partially move by mechanical scratching or rubbing (scrubbing), because the hardness and melting point of aluminum or its alloy are relatively low. As a result, short circuits take place between the adjacent pads.
In order to overcome such short circuits, signal line pads can be made of the same materials (such as molybdenum (Mo) or molybdenum wolfram (MoW), which have much higher hardness and higher temperature melting points) and made at the same time as the scanning lines and scanning line pads. In this case, the signal line pads or their extending portions are connected to the signal lines through electrically conductive materials, which are provided in contact holes located at the peripheral edge and in either the sealing portion or the portion adjacent to it.
Further, the electric power supply line pads are also made of molybdenum system metals and connected to terminals of the electric power supply lines though the conductive materials in the contact holes.
Meanwhile, the manufacturing cost of the array substrate is quite high the cost as a part of the cost of the liquid crystal display device and, in particular, the manufacturing cost of the TFTs is significant as a part of the cost as a part of the cost of the array substrate.
Thus, attempts at cost reduction of the TFTs and array substrate have been made by means of having a lower number of patterning processes, i.e., a lesser number of photo-masks to shorten the manufacturing process as a whole.
Japanese Patent Application Tokugan Hei 8-260572 discloses that the pixel electrodes are disposed on the top layer thereby carry out the patterning process of the signal lines, source and drain electrodes, and semiconductor coating layer at the same time and with the same photo-mask pattern. After that patterning process, the contact holes connecting the source electrodes to the pixel electrodes are made at the same time as the peripheral contact holes exposing connecting terminals of the signal and scanning lines.
In detail, the peripheral contact holes have first and second contact holes exposing first and second conductive layers, respectively, and small size conductive connecting patterns bridging the first and second holes. The conductive connecting patterns are made at the same time as the pixel electrodes.
A conventional liquid crystal display device is shown in FIGS. 6 and 7. FIG. 6 is a schematic plan view of a part (corner portion) of the liquid crystal display device. FIG. 7 is a schematic and sectional view of an edge of the liquid crystal display device cut along the line VII—VII as shown in FIG. 6.
A first electric power supply terminal 34′ disposed on a circuit array substrate 10′ is provided opposite to a second electric power supply terminal 21 disposed on a counter substrate 20, in a predetermined inside region surrounded by sealant 62. The terminals 34′ and 21 are electrically connected by an electrically conductive material 61 made of an electrically conductive paste. The second electric power supply terminal 21 is an extending portion of a counter electrode. The first electric power supply terminal 34′ extends an electric power supply line 34′a, which also extends toward an electric power supply line pad 13 in a plate-like connecting portion. The end of the electric power supply line 34′a is overlapped with, or disposed close to, a connecting wide-width portion 13a, which is a portion extending from the electric power supply line pad 13 to an inside portion of the substrate. An ITO film 54 is connected to the wide-width portion 13a and the edge of the power supply line 34′a through contact holes 45 and 46, i.e., the power supply fine 34′a and the pad 13 are connected through the ITO film 54.
In such a conventional liquid crystal display device, since the ITO film 54 electrically connects the power supply line 34′a to the pad 13, if it is broken, electric power is not supplied to the counter electrode, thereby causing a poor image display. In the case where edge surfaces of insulation layers 15 and 4 (defining contact holes 44 and 45, respectively) become steep in angle (acute angles), breaking-down of the ITO film 54 may occur.
Even though the power supply line 34′a is directly connected to the pad 13 or the wide-width portion 13a, through electrically conductive materials of the one contact hole provided in the insulation layer 15, the breaking-down of the electric power supply line 34′a may occur due to dust, etc., during the patterning process.