Liquid crystal cells – elements and systems – Particular excitation of liquid crystal – Electrical excitation of liquid crystal
Reexamination Certificate
2003-02-11
2004-03-16
Chowdhury, Tarifur R. (Department: 2871)
Liquid crystal cells, elements and systems
Particular excitation of liquid crystal
Electrical excitation of liquid crystal
C349S042000, C349S139000, C349S147000, C257S059000, C257S072000
Reexamination Certificate
active
06707512
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to active matrix type liquid crystal display units (AM-LCD units) driven by thin film transistors (TFTs), and to processes concerning the manufacture of these AM-LCD units.
The market for thin film transistor-driven liquid crystal display units (TFT-LCD units) is expanding as these imaging display units are further improved in thickness, weight, and resolution. In recent years, along with the tendency of TFT-LCD units to increase in screen size and in resolution, the need for reduced resistance values relating to signal lines and their terminals and improved production yields has been increasingly stringent. Reduction in production costs is also being demanded.
For LCD units of the active matrix type, scanning lines extending in a horizontal (x-axial) direction and arranged in parallel in a vertical (y-axial) direction, and data lines extending in a vertical (y-axial) direction and arranged in parallel in a horizontal (x-axial) direction, are formed on the liquid crystal side of one of a pair of transparent substrates opposed to each other via liquid crystals, and the rectangular areas surrounded by these signal lines are used as pixel areas. Each pixel area is provided with both thin film transistors driven by scanning signals supplied from the scanning signal lines on one side, and pixel electrodes driven by data signals supplied from the data lines on another side via the foregoing thin film transistors. Each pixel is driven by scanning signals or data signals, and these scanning signals or data signals are supplied through the terminals of the scanning lines or data lines extending to the outer area of the display section that is formed as a set of pixel areas.
Each scanning line and each data line are sheathed with either an insulating film that also functions as the gate insulator of a thin film transistor, or a passivation film that also functions to avoid direct contact with the liquid crystals of said thin film transistor. The terminals of these signal lines are exposed by provision of holes in the insulating film or passivation film. It is known that the occurrence of so-called “electrocorrosion” can be prevented by sheathing said exposed surfaces with poly-crystalline indium tin oxide (p-ITO), one type of indium tin oxide consisting mainly of a poly-crystalline phase.
Each scanning line is required to be low in electrical resistance, to have sufficient dry-etching resistance so as not to be lost by dry etching intended to provide holes in its insulating film or passivation film, and to come into proper contact with p-ITO at its terminals. For these reasons, scanning signal lines are made of materials such as a chromium-molybdenum (CrMo) alloy/chromium (Cr) film, where the slash “/” denotes lamination and the left and right sides of the slash denote the upper and lower layers, respectively, of the lamination. (Hereinafter, the slash and both sides thereof mean the same.) In addition to satisfying the above-mentioned requirements, each data line is further required to come into proper contact with amorphous silicon since the source electrode and drain electrode of the thin film transistor are to be formed at the same time. Also, it is desirable that each data line be capable of being provided with wet-etching to ensure the selectability between the gate insulator and amorphous silicon during etching. For these reasons, data lines are made of materials such as a CrMo alloy/Cr film. The use of these wiring materials enables the amount of side etching to be reduced below 1 &mgr;m and the cross section of the wiring to be processed into a tapered shape. These processing characteristics, in turn, enable the obtainment of thin film transistors provided with excellent characteristics, including a channel length of 7 &mgr;m or less, and can be applied to pixel electrode processing for an In-Plane Switching(IPS) type of LCD unit.
Signal wiring in an LCD unit is formed so that a voltage is applied by the driver chips mounted in the LCD unit, and the electrical connection resistances at the connections (contacts or terminals) of these driver chips are also required to be reduced. In recent years, significant decreases in the areas of the contacts, coupled with further improvements in display resolution, have resulted in a tendency towards increased connection resistance. For example, the method of mounting drivers in an LCD unit is shifting from the conventional TCP (Tape Carrier Package) method to the COG (Chip-on-Glass) method, and decreases in the areas of the contacts in the case of this method are significant. The terminals of the signal lines sheathed with the above-mentioned p-ITO satisfy the need for reduced electrical connection resistance. Also, p-ITO satisfies the functional requirements of transparent pixel electrodes, and the transparent pixel electrodes and the terminal sheathing mentioned above are formed at the same time during the manufacturing processes.
Further progress of LCD units in terms of dimensions and resolution results in a more stringent need for reduction in the electrical resistance of scanning lines, in particular. Accordingly, materials that include aluminum (Al) or Al alloy will be used, instead of the CrMo alloy/Cr film mentioned above. (Hereinafter, such signal lines are also referred to as aluminum wiring). Also, wiring structure in which Mo or the like is included in the upper layer of Al or Al alloy is usually employed so that proper contact with p-ITO can be obtained at the terminals of the scanning lines. For example, molybdenum-zirconium (MoZr)/Al-alloyed films whose upper layer is made of an MoZr alloy that includes Zr at a rate from 2.6 to 23 weight % enables favorable etching into the forward tapered cross section of the wiring, and gives sufficient dry-etching resistance to the MoZr layer required when the insulating film on the MoZr/Al-alloyed films is provided with through-hole processing.
It was verified, however, that the use of scanning signal lines made of aluminum wiring and the use of transparent pixel electrodes and terminal sheathing made of p-ITO or the like induce the following inconvenience: to etch p-ITO, it is absolutely necessary to use a strong halogen acid solution, such as a hydrobromic acid (HBr) solution, and these solutions permeate defects in the insulating film sheathing the signal line and also functioning as the gate insulator of the thin film transistor, and defects in the passive film functioning as an insulating film to avoid direct contact with the crystals of that thin film transistor. Thus, the signal line is disconnected, and this, in turn, reduces production yields and increases production costs.
One method that can be used to prevent the signal line from being disconnected is by using either amorphous ITO (a-ITO) or indium-zinc oxide (IZO), instead of p-ITO, as transparent pixel electrodes and terminals, and further using oxalic acid ((COOH)
2
) as the etchant for processing these electrodes and the terminal sheathing, since the rate of dissolution of aluminum and aluminum alloy in oxalic acid is sufficiently low that signal line disconnections are not caused.
The selection of a-ITO or IZO, however, will give significantly high connection resistance to the driving circuits and turn out to be a long way from achieving the intended purpose of reducing the electrical contact resistance.
FIG. 14
is a graph showing the contact resistance values of p-ITO, a-ITO, and IZO materials. In
FIG. 14
, the horizontal axis denotes changes in the needle pressure applied to each material, and the vertical axis denotes changes in the contact resistance between each material and the needle at that time. For LCD units, the bump pressures driving chips applied to the signal lines of the terminals are considered to almost stay within range A of the graph, and, in this case, it can be seen that, in terms of reduction in contact resistance, using p-ITO is much more effective than using a-ITO or IZO.
Such increases in connection resistance due to the selection of
Ishii Masahiro
Kaneko Toshiki
Kasai Tsutomu
Kawamura Tetsuya
Onisawa Ken-ichi
Antonelli Terry Stout & Kraus LLP
Chowdhury Tarifur R.
Hitachi , Ltd.
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