Liquid crystal cells – elements and systems – Particular excitation of liquid crystal – Electrical excitation of liquid crystal
Reexamination Certificate
2000-03-29
2002-08-13
Ton, Toan (Department: 2871)
Liquid crystal cells, elements and systems
Particular excitation of liquid crystal
Electrical excitation of liquid crystal
C349S044000, C349S046000, C349S047000
Reexamination Certificate
active
06433842
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to an active matrix liquid crystal display device driven by thin film transistors (TFTs), and an interconnection formation method for such a liquid crystal display device.
In recent years, the market for TFT-driven liquid crystal display devices (TFT-LCDs) has expanded, compared with conventional CRTs, as they have recognized as image display devices capable of realizing further reduction in size and weight and improvement in definition. Such a TFT-LCD mainly includes: a glass substrate on which gate lines, data lines, TFTs formed at the respective intersections between the gate lines and the data lines, pixel electrodes connected to the TFTs, a gate insulating film, and a protection film are formed; a counter substrate; and a liquid crystal layer sandwiched by the glass substrate and the counter substrate.
With the recent progress in the achievement of a larger screen size and higher definition of TFT-LCDs, the requirements for lower resistance of interconnections and lower production yield have become stricter. For lowering the resistance of interconnections, aluminum or an aluminum alloy has been conventionally used. However, when a single layer of aluminum or an aluminum alloy is used for interconnections, hillocks tend to be generated on the surface of the interconnections, resulting in that an insulating film formed over the interconnections fails to cover them sufficiently (coverage failure). Moreover, aluminum or an aluminum alloy exhibits high contact resistance against an indium-containing oxide used as the material of pixel electrodes, such as indium tin oxide (ITO) and indium zinc oxide (IZO). It is therefore impractical to attempt direct electrical connection therebetween. In order to overcome these problems, in an inverted-stagger type TFT-LCD, gate lines are formed of a cladding structure in which a metal having a high melting point covers a wiring pattern of aluminum or an aluminum alloy. In this cladding structure, the second conductive layer (metal) covering the wiring pattern of aluminum or an aluminum alloy provides the contact characteristics with the pixel electrode material, while the aluminum or an aluminum alloy provides the conductivity as the interconnections. Such a structure is disclosed in, for example, Japanese Patent Laid-Open Nos. 341299/1993, 64109/1995, 26602/1997, 127555/1997, and 213809/1998.
The formation of the above cladding structure requires a complicate process including two photolithographic steps, one for the aluminum or aluminum alloy and the other for the second conductive layer. In order to make the process simpler, the layer of aluminum or an aluminum alloy and the second conductive layer are sequentially formed and etched at one time in one photolithographic step to form a wiring pattern. For this procedure, molybdenum or a molybdenum alloy is used for the second conductive layer since molybdenum is a metal having a high melting point allowing for one-time etching with aluminum or an aluminum alloy. Japanese Patent Laid-Open No. 20930/1992, particular, discloses an interconnection structure using a molybdenum alloy containing 0.5 to 10 wt % of chromium as the second conductive layer. In this disclosure, the layered structure is wet-etched at one time with a mixed solution of phosphoric acid, nitric acid, and acetic acid. The section of the resultant interconnection has a taper angle of 50°. Such a molybdenum-chromium alloy has a resistance against dry etching with a fluoric gas such as SF6. This provides an advantage as follows. When a contact hole or the like is formed through an overlying SiN insulating film by dry etching with SF6 gas, the second conductive layer made of the molybdenum-chromium alloy will not be etched away at the bottom inside the contact hole, ensuring connection with the pixel electrode and thus providing good contact characteristics with the pixel electrode.
There is a report on examinations on the sectional shapes of an interconnection of a molybdenum/aluminum layered structure when etched by a dip method and a shower method (Digest of Technical Papers of 1994 International Workshop on Active-Matrix Liquid-Crystal Displays, Nov. 30-Dec. 1, 1994, Kogakuin University, Shinjuku, Tokyo, Japan, p.188). More specifically, the molybdenum/aluminum layered structure was subjected to one-time etching with a mixed solution of phosphoric acid and nitric acid by the dip method and the shower method, and the sectional shapes of the resultant interconnection obtained by the two etching methods were examined. According to this report, in the dip method, the section of the interconnection was tapered, while in the shower method, the molybdenum layer protrudes like a brim relative to the aluminum layer.
Japanese Laid-Open Patent Publication No. 331066 of 1997 discloses the use of titanium, molybdenum, tantalum, tungsten, zirconium, or a composite material thereof as the second conductive layer. According to this disclosure, a light-shading film and interconnections are simultaneously formed in the same process, with the second conductive layer playing a role of minimizing the reflectance of the light-shading film. No specific combination of elements for the composite material when selected as the second conductive material is mentioned in this disclosure.
Japanese Patent Laid-Open No. 258633/1999 discloses a fabrication method of an array substrate of a display device in which the second conductive layer is formed of a metal selected from chromium, molybdenum, tungsten, titanium, zirconium, hafnium, vanadium, niobium, and tantalum or an alloy thereof. According to this disclosure, the formation of such a second conductive layer prevents generation of a hillock on an aluminum alloy film and also prevents corrosion of the aluminum alloy during dry etching of pixel electrodes. No specific combination of elements for an alloy or the composition of such an alloy when selected as the second conductive layer is mentioned in this disclosure.
SUMMARY OF THE INVENTION
For enhancing the productivity of the array substrate of a liquid crystal display device, the size of the mother glass substrate has become larger. For example, according to Flat Panel Display 2000, Nikkei BP, P.56 (1999), the production lines for substrates of 590×670 mm
2
, 600×720 mm
2
, and 650×830 mm
2
were in actual operation in 1998, and in the year of 2000, production lines for 680×880 mm
2
and 730×920 mm
2
are expected to be in operation. To correspond to such size increase of the mother glass substrates, the size of the fabrication equipment has become larger. In the case of a wet etching apparatus, as the size of the apparatus increases, sufficient stirring of an etchant becomes difficult in the dip method, and thus it is almost impossible to realize uniform etching over a large-area substrate. In order to obtain highly uniform wet etching over a large-area substrate, therefore, the shower method must be employed. However, the shower method has the problem described above. That is, in the shower method, when a layered structure of a molybdenum layer and an aluminum layer is etched at one time with a mixed solution of phosphoric acid and nitric acid, the molybdenum layer protrudes like a brim relative to the aluminum layer. This sectional shape of the interconnection was also confirmed by the present inventors in an experiment where a layered structure composed of a molybdenum alloy layer and an aluminum alloy layer was etched at one time with a mixed solution of phosphoric acid, nitric acid, and acetic acid by the shower method. If an insulating film is formed over the resultant interconnection having such a sectional shape, the insulating film may generate coverage failure, resulting in lowering the production yield.
In view of the above, the first challenge of the present invention is to obtain a tapered sectional shape of the interconnection having a layered structure of a molybdenum alloy and an aluminum alloy by the shower wet etching method thereby to ensure g
Cyahara Kenichi
Harano Yuichi
Kaneko Toshiki
Ochiai Takahiro
Onisawa Ken-ichi
Antonelli Terry Stout & Kraus LLP
Hitachi , Ltd.
Nguyen Hoan
Ton Toan
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