Liquid crystal cells – elements and systems – Particular structure – Having significant detail of cell structure only
Reexamination Certificate
2001-01-23
2003-01-21
Ton, Toan (Department: 2871)
Liquid crystal cells, elements and systems
Particular structure
Having significant detail of cell structure only
C349S143000, C349S147000
Reexamination Certificate
active
06509942
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to a liquid crystal display device, a wiring substrate, and methods for fabricating the liquid crystal display device and the wiring substrate. More particularly, the present invention relates to a liquid crystal display device capable of realizing reflection-mode display by use of ambient light, and a method for fabricating such a liquid crystal display device.
In recent years, rapid progress has been made in the field of application of liquid crystal display (LCD) devices to wordprocessors, laptop computers, pocket TV sets, and the like. Among the LCD devices, a reflection type LCD device capable of realizing display by reflecting ambient light, in particular, has attracted attention for its possibility of reducing the power consumption, the thickness, and the weight taking advantage of requiring no backlight. Nowadays, also, there has been developed a dual-mode LCD device that can realize display in both reflection and transmission modes.
Conventionally, in LCD devices, a reflector or a reflection layer for reflecting ambient light was placed on the outer surface of a substrate (a TFT substrate or a substrate located farther from the viewer). However, as the pixel size becomes smaller with increase in the capacity of the LCD devices, this construction arises the following problem. That is, parallax is generated due to the difference in distance from a color filter (pixel portion) and the reflector (reflection layer), and this deteriorates display quality.
In order to solve the above problem, the construction of placing a reflection layer on the surface of a substrate facing a liquid crystal layer has become widely used. In this construction, also, the reflection layer itself is used as an electrode for applying a voltage to the liquid crystal layer (for example, as a pixel electrode in an LCD device using TFTs). As the material for the reflection layer, aluminum (Al) (and an Al alloy) is often used since Al is high in reflectance, easily patterned, and low in electric resistance.
It is known that when an ITO layer and an Al layer are put in contact with each other and together exposed to an alkaline solution, galvanic corrosion occurs between ITO and Al, resulting in partial loss of the ITO layer and the Al layer. Note that in the following description, the expression “ITO layer” includes not only the ITO layer before patterning but also a pattern of ITO layer after the patterning, unless otherwise specified.
Japanese Laid-Open Patent Publication No. 3-246524 discloses a solution for the problem of partial loss. That is, conventionally, a double-layer structure of Al layer/ITO layer is adopted for display electrodes or interconnections, in place of the original single ITO layer, to reduce the electric resistance of the display electrodes or interconnections. With this double-layer structure, however, part of the ITO layer tends to be lost in a process of developing a resist layer formed on the Al layer with an alkaline solution. The publication discloses providing a protection layer made of molybdenum (Mo) or a Mo alloy (Mo—Ti, for example) between the ITO layer and the Al layer. By providing this protection layer, even if a pinhole exists in the Al layer, the above problem of partial loss of the ITO layer can be solved.
However, the inventors of the present invention have found that the method disclosed in Japanese Laid-Open Patent Publication No. 3-246524 described above has a problem as follows. Galvanic corrosion between ITO and Al is prevented by the formation of a Mo layer (including a Mo alloy layer) between the ITO layer and the Al layer. However, if a pinhole exists in the Al layer, the Al layer is corroded at a portion surrounding the pinhole when the Al layer is exposed to an alkaline developer, remover, or etchant, resulting in partial loss of the Al layer. Note that the method disclosed in the above publication is not directed to suppression/prevention of formation of a pinhole in the Al layer, but directed to prevention of galvanic corrosion between ITO and Al, and for this purpose, a Mo layer (or a Mo alloy layer) that can be etched together with the Al layer is formed as an intermediate layer blocking direct contact between the Al layer and the ITO layer.
If an Al layer is partially lost in a construction using the Al layer as part of a reflection electrode, the area of the resultant reflection electrode (reflection layer) decreases, resulting in deterioration of the function of reflecting ambient light. To state more specifically, in a method for fabricating a liquid crystal display device including an Al layer as a reflection layer, the Al layer is subjected to a process of developing a resist layer with an alkaline developer for formation of a resist pattern that is used as a mask in patterning of the Al layer, and a process of removing the resist pattern with an alkaline remover after the patterning of the Al layer. When the Al layer has a pinhole and is exposed to such an alkaline developer or remover in the above processes, a portion of the Al layer surrounding the pinhole (typically, a circle having a diameter of 2 to 5 &mgr;m) is lost. This partial loss of the Al layer (growth of the pinhole) is more eminent in the removing process than in the developing process. This is because a remover has higher ability of decomposing the resist layer than a developer, and also the surface area of the Al layer exposed to the remover is larger than that exposed to the developer.
In the patterning of the Al layer, the phenomenon of partial loss of the Al layer also causes the problem that peripheries of the patterned Al layer are jagged (like edges of a stamp). The patterning of the Al layer may be performed with a variety of materials (alkaline and acid solutions, and the like) by a variety of methods. When etching is adopted, occurrence of partial loss of the Al layer is more eminent in wet etching, which can advantageously etch the layered structure of Al layer/Mo layer with a same etchant, than in dry etching.
When an Al layer is temporarily deposited on an ITO layer as the top layer of a terminal electrode and then removed, part of the ITO layer may disadvantageously be lost due to galvanic corrosion between ITO and Al. For example, there is a case where an ITO layer is formed as the top layer of a terminal electrode such as a scanning line terminal electrode in a terminal area and the Al layer for formation of the reflection layer is temporarily deposited on the ITO layer. When the temporarily deposited Al layer is removed with an etchant, the ITO layer may be partially lost due to galvanic corrosion, and as a result the reliability of the terminal electrode may be impaired.
The problem of partial loss of an Al layer is disadvantageous, not only in the LCD device using the Al layer as the reflection layer, but also in LCD devices or other devices including interconnections, electrodes, or the like having the double-layer structure of Al layer/ITO layer, such as a display device using organic electroluminescence (EL) and a solar battery. For example, in an active matrix LCD device using TFTs that includes signal lines having the double-layer structure of Al layer/ITO layer, if the extent of partial loss of the Al layer is great, the Al layer may be narrowed excessively or may be broken. As a result, the Al layer may fail to sufficiently supplement the conductance of the ITO layer. If the conductance of a signal line decreases, normal display may not be obtained due to signal delay or the like. Note that in the following description the term “break” sometimes refers to the state where the conductance of an interconnection such as a signal line is reduced to such a degree that normal display is no more possible, in addition to the state where an interconnection is completely broken.
SUMMARY OF THE INVENTION
Objects of the present invention are providing a liquid crystal display device and a wiring substrate, which can suppress/prevent occurrence of partial loss of an Al layer formed on a Mo layer due to
Takayama Masaya
Tanaka Naoyuki
Nguyen Hoan
Nixon & Vanderhye P.C.
Sharp Kabushiki Kaisha
Ton Toan
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