Transparent electrode made from indium-zinc-oxide and etchant for etching the same

A pixel electrode employs a transparent electrode made from indium-zinc-oxide (IZO) that is capable of preventing damage and bending thereof. In a liquid crystal display device containing pixel electrodes, the transparent electrode is made from indium-zinc-oxide (IZO) having an amorphous structure so that it can be etched within a short period of time with a low concentration of etchant. Accordingly, it is possible to prevent damage and bending of the transparent electrode upon the patterning thereof.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a transparent electrode capable of improving etching characteristics. The present invention is also directed to an etchant suitable for etching a transparent electrode.

2. Description of the Related Art

Generally, a liquid crystal display (LCD) of an active matrix driving system uses thin film transistors (TFT's) as switching devices to display a natural moving picture. Since such a LCD can be made into a smaller device in size than the existent Brown tube, it has been widely used for a monitor for a personal computer or a notebook computer as well as for office automation equipment such as a copy machine, etc. and portable equipment such as a cellular phone, a pager, etc.

Recently, the active matrix LCD trends toward overlapping a pixel electrode108of a transparent electrode with a signal wire106such as a gate line or a data line as shown inFIG. 1so as to enhance the aperture ratio. In this case, an organic insulating film104having a low dielectric constant is entirely coated on a substrate102provided with the signal wire106in order to minimize insulation between the signal wire106and the pixel electrode and a parasitic capacitance. The LCD having the pixel electrode108overlapped with the signal wire106has an aperture ratio which is improved by the overlapping area between the signal wire106and the pixel electrode108in comparison to a LCD in which the pixel electrode108does not overlap with the signal wire106, but rather is spaced, by a desired distance (i.e., about 5 to 10 μm) from the signal wire106.

The pixel electrode108is usually made from a transparent conductive material such as indium-tin-oxide (ITO). This ITO film is entirely deposited on the organic insulating film104and thereafter patterned in such a manner to overlap with the signal wire106. Upon patterning of the ITO film, however, the edge portion of the ITO film may be often twisted or damaged after being etched with an etchant. If the edge portion of the ITO film formed on the organic insulating film104is twisted or damaged, then the width of the overlapping portion between the pixel electrode108and the signal wire106is narrowed which generates light leakage from the overlapping portion. Pattern badness of the ITO film is caused by a poor interface-bonding characteristic of the ITO film to the organic insulating film104. Particularly, pattern badness is caused by a fact that the ITO film is etched with an etchant which has a high concentration of strong acid and also the etching of the ITO film is made at a low speed.

In order to reduce the bad pattern of the ITO film, a surface treatment process of the organic insulating film104for strengthening the adhesive force between the organic insulating film104and the ITO film is performed. An example of such a surface treatment includes a method of forming an acid film on the organic insulating film104using an acid such as HNO3or H2SO4, etc. Other examples include a method of forming a hydrogen film on the surface of the organic insulating film104or ion-doping the surface of the organic insulating film104with oxygen. Even when a surface treatment of the organic insulating film104is made, if the ITO film is etched by a doping system in which the substrate102is precipitated within a chamber filled with an etchant, then the etchant permeates into the interface between the organic insulating film104and the ITO film to generate the pattern badness of the ITO film as mentioned above. In this case, the etchant for etching the ITO film includes a high concentration of strong acid, and the etching thereof is conducted for a long time. For instance, the etchant for etching the ITO film has a very high acid concentration such that the ratio of oxalic acid (C2H2O4) to de-ionized water is less than 1 to 10. The time required for etching the ITO film using such a high-concentration of etchant is more than 1000 seconds.

The ITO film has applications for a display device and an ink-jet head besides a LCD. Also, the ITO film is applicable to a pixel electrode for an X-ray detecting device having a structure similar to the active matrix LCD. However, in the device or equipment employing the ITO film, a transparent electrode or a pixel electrode, etc. applied with ITO is liable to generate a bad pattern because the etching characteristics of the ITO is poor.

SUMMARY OF THE INVENTION

Accordingly, it is an object of the present invention to provide a transparent electrode that has improved etching characteristics.

A further object of the present invention is to provide an etchant suitable for etching said transparent electrode.

In order to achieve these and other objects of the invention, a transparent electrode applied to a liquid crystal display device according to the present invention is made from indium-zinc-oxide (IZO) having an amorphous structure so that it can be etched in a short period of time with a low concentration of etchant.

A transparent electrode applied to an X-ray detecting device according to the present invention and electrodes consisting of a capacitor are made from indium-zinc-oxide (IZO) so is to have a fast etching speed.

An etchant for etching the transparent electrode, according to the present invention, is a mixture to which a desired compositional ratio of oxalic acid is added.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

Referring toFIG. 2andFIG. 3, there is shown a transparent electrode according to a first embodiment of the present invention, that is, a pixel electrode applied to a liquid crystal display device. In the present liquid crystal display device, a thin film transistor (TFT)30is provided at an intersection between a data line22and a gate line24, and pixel electrodes20overlapping with the data line22are arranged in a matrix type. The data line22applies a video signal to each liquid crystal pixel cell. The gate line24applies a gate pulse synchronized with the video signal to a gate electrode4of the TFT30. Each liquid crystal pixel cell includes a liquid crystal layer injected between the pixel electrode20and a common electrode (not shown). The liquid crystal layer is driven with an electric field between the pixel electrode20and the common electrode to control the transmitted light amount of incident light received thereto via a substrate.

The TFT30has a gate electrode4connected to the gate line24, a source electrode12connected to the data line22and a drain electrode14connected to the pixel electrode20. In order to provide the gate electrode4and the gate line24, a metal layer is deposited on the substrate2by sputtering or a vacuum vapor deposition technique to a thickness of about 2500 Å. The metal layer is patterned by reaction ion etching after the formation of a photo mask. After the gate electrode4and the gate line24are formed on the substrate2, a gate insulating film6made from a dielectric material such as SiNx, etc. is deposited on the substrate2by a plasma enhanced chemical evaporation technique to cover the gate electrode4and the gate line24. The gate insulating film6has a thickness of about 2000 to 3000 Å. A semiconductor layer8made from an amorphous silicon (a-Si) is deposited on the gate insulating film6to have a thickness of about 2000 Å, and an ohmic contact layer10made from an a-Si doped with n+ ions is deposited thereon having a thickness of about 500 Å. The semiconductor layer8and the ohmic contact layer10cover the gate insulating film6on the gate electrode4. The source electrode12and the drain electrode14, made from a metal, are deposited on the ohmic contact layer10to have a thickness of about 500 to 2000 Å. The source electrode12and the drain electrode14are patterned in such a manner as to be spaced by a predetermined channel width from each other. Subsequently, the ohmic contact layer10is etched along a channel defined between the source electrode12and the drain electrode14to expose the semiconductor layer8. On the substrate2provided with the TFT30, the data line22and the gate line24, an organic insulating film16is entirely coated on an even basis by a spin coating technique. After the organic insulating film16is cured under a nitrogen atmosphere, a portion of the organic insulating film16covering each drain electrode14is etched. At this time, a contact hole18defined on the drain electrode14connects the drain electrode14to the pixel electrode20. The pixel electrode20made from indium-zinc-oxide (IZO) is entirely deposited on the organic insulating film16provided with the contact hole18. The pixel electrode20is also deposited within the contact hole18to be connected to the drain electrode14. Finally, the pixel electrode20deposited on the organic insulating film16is photo-masked in such a manner to overlap with the side surface of the data line22, and thereafter patterned by an etching.

The IZO selected as the material of the pixel electrode20is a transparent conductive compound in which In2O3is mixed with ZnO in a desired ratio. The IZO can be etched by a low concentration of etchant and has the characteristic of being capable of being etched within a short period of time in a low concentration of etching liquid. Also, since the IZO has an amorphous structure, it has an excellent interface-bonding characteristic with respect to the organic insulating film16. Thus, if the material of the pixel electrode20is made from IZO, then a surface treatment process for strengthening the adhesive force to the organic insulating film16can be omitted.

The IZO is deposited on the organic insulating film16by a sputtering technique, for example, a DC magnetron sputtering technique. Process conditions of the sputtering technique are indicated by the following table:

Since the IZO film deposited on the organic insulating film16in this manner has an excellent etching characteristic, the pixel electrode20, particularly, the end portion of the pixel electrode20is not damaged or bent after it is etched. Also, the IZO film is precipitated in a low concentration of etchant having a low acid ratio and etched rapidly, even when it is etched by a dipping system, making it easy to etch.

When the etching characteristics and the resistivity of such an IZO material is compared with those of the ITO, the IZO can be etched within a shorter period of time at a lower (or normal) temperature using a lower concentration of etchant when compared with ITO as indicated by the following table. Also, the IZO has a resistivity slightly higher than the ITO.

As seen from the above Table 1, an etchant for etching IZO according to the present invention is a mixture having a ratio of oxalic acid (C2H2O4) to de-ionized water is 1:180. An interfacial active agent for improving the mixing uniformity of C2H2O4to de-ionized water may be added to the etchant. To set the ratio of C2H2O4to de-ionized water to be 1:180 aims at assuring a sufficient etching control margin, even when the acid concentration is low and the etching speed is fast, as seen from the following Table 3 andFIG. 4.

More specifically, as seen from Table 3 andFIG. 4, the concentration and etching speed is too high when the ratio of C2H2O4to de-ionized water is 1:30 or 1:128, thus, it is difficult to provide an etching control when etching IZO. When the concentration of C2H2O4is low such that the ratio of C2H2O4to de-ionized water is 1:350, the etching speed becomes slow. Thus, it is desirable that a composition of an etchant for etching the IZO film, that is, the ratio of C2H2O4to de-ionized water should be 1:180 in consideration providing an etching control margin, an etching speed and an acid concentration.

FIG. 5shows the structure of a transparent electrode according to a second embodiment of the present invention wherein a pixel electrode is applied to an X-ray detecting device. Referring toFIG. 5, a TFT array of the present X-ray detecting device includes a TFT containing a gate electrode34, a source electrode42, a drain electrode44, an organic insulating film46covering the TFT, a pixel electrode50provided on the organic insulating film46, a storage capacitor Cst provided between an upper transparent electrode56and a lower transparent electrode48, and a ground line52connected to the storage capacitor Cst. The TFT, the storage capacitor Cst and the ground line are covered with the organic insulating film46. The pixel electrode50formed on the organic insulating film46covers the TFT, the storage capacitor Cst and the ground line52. The storage capacitor Cst plays the role of charging an X-ray sensing signal applied from the pixel electrode50, and applies the X-ray sensing signal to a data line connected to the source electrode42in a time interval at which a channel is being formed between the source electrode42and the drain electrode44of the TFT. An inorganic insulating film54such as SiNx, etc. is formed between the upper and lower transparent electrodes56and48serving as an electrode for the storage capacitor Cst. A photo-detecting layer for detecting an X-ray is provided on the pixel electrode50. The photo-detecting layer is made from selenium.

In the present invention, the upper and lower transparent electrodes56and48are made from IZO. As mentioned above, the IZO has an excellent interface adhesion characteristic with respect to the organic insulating film46and can be etched within a short time with a low concentration of etchant. Accordingly, the edge of the pixel electrode50is not bent or damaged after etching thereof. In this case, since the thickness of the ITO film existing in the bottom surface of the contact hole60is approximately two times larger than that of the ITO film for the pixel electrode50upon patterning of the pixel electrode50, a residual film may be often left after etching of the ITO film. On the other hand, if the pixel electrode50and the upper and lower transparent electrodes56and48are IZO films, an etching of the ITO film is conducted rapidly even when the ITO film, having approximately twice the thickness at the bottom surface of the contact hole60, is simultaneously etched upon etching of the ITO film for the pixel electrode50. Thus, it becomes possible to minimize the defect of an IZO residual film left at the bottom surface of the contact hole60. More specifically, upon patterning of the pixel electrode50, the ITO film for the pixel electrode50and the ITO film having the twice thickness within the contact hole60are etched simultaneously. At this time, since the etching rate of the IZO film is approximately 60 times higher than that of the ITO film as seen from the above Table 2, the ITO film for the pixel electrode50having a thickness of about 500 Å and the IZO film having a thickness of about 1000 Å formed on the bottom surface of the contact hole60can be almost simultaneously etched. Accordingly, a patterning of the pixel electrode50can be easily made with no residual IZO film within the contact hole60.

As described above, according to the present invention, the IZO is able to be etched within a short time with a low concentration of etchant and is applied to the pixel electrode to improve the etching characteristic. An etchant for etching the transparent electrode according to the present invention is a mixture of oxalic acid C2H2O4and de-ionized water in ratio of 1:180, respectively. Thus, a low acid concentration sufficiently assures an etching control margin for the IZO, and has a fast etching speed with respect to the IZO, so that it is suitable for etching a transparent electrode employing the IZO.