WO2003003107A1

WO2003003107A1 - Method of forming a transflective electrode and liquid crystal display device

Info

Publication number: WO2003003107A1
Application number: PCT/IB2002/002438
Authority: WO
Inventors: Teizo Yukawa; Tomihisa Sunata
Original assignee: Koninklijke Philips Electronics N.V.
Priority date: 2001-06-29
Filing date: 2002-06-20
Publication date: 2003-01-09
Also published as: JP2003015145A; TW567369B

Abstract

An object of the present invention is to provide a method of for ming an electrode with reduced costs and a liquid crystal display device to which such method is applied. After forming the transparent electrode film 10 and the reflective electrode film 11, the resist film 12 is formed. The resist film 12 is patterned in such a way that the trench 12a and the thin portion 12b are formed in the resist film12. The reflective electrode film 11 and the transparent electrode film 10 are etched using the resist film 12 in which the trench 12a and the thin portion 12b have been formed as an etching mask. The thin portion 12b is removed by ashing the resist film 12. Then, the reflective electrode 110 is etched using the resist film 12 from which the thin portion 12 has been removed as an etching mask. Each pixed electrode is formed by a transparent electrode 100 and a reflective electrode 110. The liquid crystal display is a transfelctive-type liquid crystal display device.

Description

Method of forming an electrode and Liquid crystal display device

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a method of forming a transparent electrode and a reflective electrode on each predetermined region of a substrate, and also to a liquid crystal display device to which this method is applied.

2. Description of Related Art

Recently, a liquid crystal display device of the so-called transflective having both a transmissive function and a reflective function has been widely used. In such a transflective-type liquid crystal display device, a transparent electrode and a reflective electrode are formed in each pixel in such a way that the liquid crystal display device can achieve both the transmissive function and the reflective function.

The manufacturing process of the transflective-type liquid crystal display device requires not only a lithography step for forming the transparent electrode but also another lithography step for forming the reflective electrode, so that it has problems that the number of manufacturing steps and the manufacturing cost increase.

SUMMARY OF THE INVENTION

It is an object of the present invention is to provide a method of forming an electrode with reduced costs and a liquid crystal display device to which such method is applied.

A method of forming an electrode according to the present invention for attaining the object is characterized in that a method of forming a transparent electrode and a reflective electrode on each predetermined region of a substrate, the method comprising: a first step of forming a first film for the transparent electrode on the substrate; a second step of forming a second film for the reflective electrode on the first film; a third step of forming a third film on the second film; a fourth step of processing the third film in such a way that a portion of the third film corresponding to the predetermined region comprises a thinner portion and a thicker portion and that a portion of the third film corresponding to a periphery of the predetermined region is removed; a fifth step of etching the first film and the second film by using the processed third film as a mask; a sixth step of removing the thinner portion of the third film; and a seventh step of etching the second film by using the third film, from which the thin portion has been removed, as a mask. In a method of forming an electrode according to the present invention, in the fourth step, the third film is processed in such a way that a portion of the third film corresponding to a periphery of the predetermined region is removed and that a portion of the third film corresponding to the predetermined region comprises a thinner portion and a thicker portion. In the fifth step, since the first film for the transparent electrode and the second film for the reflective electrode are etched using the third film processed as described above as an etching mask, the first film for the transparent electrode and the second film for the reflective electrode can remain only on the predetermined region. In the sixth step, the thinner portion of the third film is removed, so that, in the seventh step, the second film for the reflective electrode can be etched using the third film from which the thinner portion has been removed as an etching mask. As described above, in a method of forming an electrode according to the present invention, after etching both the first film for the transparent electrode and the second film for the reflective electrode in the fifth step, the thinner portion of the third film is removed in the sixth step and the second film is etched using the third film from which the thinner portion has been removed as an etching mask in the seventh step. That is to say, to remove the thinner portion of the third film in the sixth step makes it possible to etch the first film for the transparent electrode and the second film for the reflective in the different shape. Therefore, the first film for the transparent electrode and the second film for the reflective can be etched in the different shape without separately forming an etching mask for etching only the first film and another etching mask for etching only the second film, so that the number of manufacturing steps and the manufacturing cost will be reduced.

In a method of forming an electrode according to the present invention, it is preferred that the third film is a photosensitive film, the fourth step comprises an exposure step of exposing the photosensitive film so as to apply different levels of exposure energy to respective areas of the photosensitive film and a development step of developing the exposed photosensitive film, and the sixth step is a step in which ashing of the developed photosensitive film is carried out.

By performing the steps described above, it is possible to provide a portion of the photosensitive film corresponding to the predetermined region with a thinner portion and a thicker portion and to remove a portion of the photosensitive film corresponding to a periphery of the predetermined region.

In a method of forming an electrode according to the present invention, it is preferred that the method comprises an eighth step of forming a fourth film having recesses or proj ections before the first step .

It is possible to provide the reflective electrode with recesses or projections by forming a fourth film having recesses or projections before the first step.

A liquid crystal display device according to the present invention is characterized in that the device comprises the reflective electrode and the transparent electrode formed by using the method as claimed in any one of claims 1 to 3.

A liquid crystal display device according to the present invention is characterised in that the reflective electrode is formed directly on the transparent electrode.

In a liquid crystal display device according to the present invention, the transparent electrode or the reflective electrode may comprise a multilayer construction.

BRIEF DESCRIPTION OF THE DRAWINGS

Fig. 1 is a cross-sectional view of a part of the transflective-type liquid crystal display device comprising a transparent electrode and a reflective electrode which are formed using an embodiment of the method of forming an electrode according to the present invention.

Fig. 2 is a plan view of a glass substrate 1 of the TFT substrate assembly 51 shown in Fig.l on which the TFTs 50 have been formed.

Fig. 3 is a cross-sectional view taken along a line I-I in Fig. 2.

Fig. 4 is a cross-sectional view of the glass substrate on which the insulating film and the planarizing film have been formed.

Fig. 5 is a cross-sectional view of the substrate on which the planarizing film 9 has been exposed to light and developed.

Fig. 6 is a cross-sectional view of the substrate on which the insulating film 8 has been etched. Fig. 7 is a cross-sectional view of the glass substrate on which a transparent electrode film 10, a reflective electrode film 11 and a resist film 12 have been formed.

Fig. 8 is a plan view of the substrate on which some parts of the resist film 12 have been removed. Fig. 9 is a cross-sectional view taken along a line LT-IT in Fig. 8.

Fig. 10 is a cross-sectional view of the substrate on which the reflective electrode film 11 and the transparent electrode film 10 have been etched.

Fig. 11 is a cross-sectional view of the substrate on which the separate resist films 12c have been ashed. Fig. 12 is a plan view of the substrate on which the reflective electrodes 110 have been etched.

Fig. 13 is a cross-sectional view taken along a line Hl-ffl in Fig. 12. Fig. 14 is a cross-sectional view of a substrate on which a transparent electrode and a reflective electrode have been formed in an example of the conventional method.

DESCRIPTION OF THE PREFERRED EMBODIMENT(S)

Hereinafter, an embodiment of the present invention will be described below.

The liquid crystal display device comprises a TFT substrate assembly 51 in which TFTs 50, transparent electrodes 100 and reflective electrodes 110 and others are formed and a color filter substrate assembly 52 in which a color filter and others are formed. In Fig. 1, the color filter substrate assembly 52 is illustrated in a simplified form since the structure of the color filter substrate assembly 52 is not relevant to the characteristic part of the present invention. A liquid crystal layer 53 is present between the TFT substrate assembly 51 and the color filter substrate assembly 52. A backlight 54 is provided on the back side of the TFT substrate assembly 51.

Hereinafter, a method of manufacturing the TFT substrate assembly 51 in which the transparent electrode 100 and the reflective electrode 110 corresponding to the characteristic part of the present invention are formed will be described. Fig. 2 is a plan view of a glass substrate 1 of the TFT substrate assembly 51 shown in Fig.l on which the TFTs 50 have been formed. Fig. 3 is a cross-sectional view taken along a line I-I in Fig. 2.

First, as shown in Fig. 3, gate electrodes 2 and gate insulating film 3 are formed on the glass substrate 1. Then, semiconductor layers 4 such as a-Si:H (hydro genated amorphous silicon), ohmic contact layers 5 such as n-l- a-Si:H (n+ hydrogenated amorphous silicon), source electrodes 6 and drain electrodes 7 are formed, so that the TFT 50 is formed in each pixel area. As shown in Fig. 2, the gate electrodes 2 arranged in a row direction are electrically connected via a gate bus 20 to each other and the source electrodes 6 arranged in a column direction are electrically connected via a source bus 60 to each other. After forming the TFTs 50, an insulating film and a planarizing film will be formed on the glass substrate 1 (see Fig. 4).

Fig. 4 is a cross-sectional view of the glass substrate on which the insulating film and the planarizing film have been formed. As shown in Fig. 4, the insulating film 8 is formed on the glass substrate 1 so as to cover the TFTs 50. The planarizing film 9 is then formed by applying a photosensitive material to the insulating film 8 over its entire surface. After forming the planarizing film 9, the planarizing film 9 will be exposed to light and developed (see Fig. 5).

As a result of exposing the planarizing film 9 to light and developing it, the planarizing film 9 is provided with a plurality of recesses 9a and through holes 9b passing therethrough. The through holes 9b are formed just above the drain electrode 7 of each of the TFTs 50. In order to form the recesses 9a and the through holes 9b as shown in Fig. 5, the planarizing film 9 is exposed to light in such a way that an amount of exposure energy absorbed in portions in which the recesses 9a are to be formed is different from an amount of exposure energy absorbed in portions in which the through holes 9b are to be formed. As a result of such an exposure of the planarizing film 9, it is possible to adjust a depth of each portion of the exposed planarizing film 9 to be removed when the exposed planarizing film 9 is developed, depending on the amount of the absorbed exposure energy. Therefore, the recesses 9a and the through holes 9b can be formed. After forming the recesses 9a and the through holes 9b in the planarizing film 9, the insulating film 8 is etched using the planarizing film 9 as an etching mask (see Fig. 6). Fig. 6 is a cross-sectional view of the substrate on which the insulating film 8 has been etched.

Since the through hole 9b formed in the planarizing film 9 is formed above the drain electrode 7, each through hole 8 a for exposing each drain electrode 7 is formed in the insulating film 8 when the insulating film 8 is etched using the planarizing film 9 as an etching mask. After etching the insulating film 8, the transparent electrode and the reflective electrode are formed in a method shown in Figs. 7 to 13. Next, description will be made with reference to Figs. 7 to 13.

Fig. 7 a cross-sectional view of the glass substrate on which a transparent electrode film 10, a reflective electrode film 11 and a resist film 12 have been formed.

After etching the insulating film 8 (see Fig. 6), a material for the transparent electrode and a material for the reflective electrode are sequentially deposited, so that the transparent electrode film 10 and the reflective electrode film 11 are formed as shown in Fig. 7. A material such as an ITO can be used as the material for the transparent electrode film 10, while a material such as an Al alloy and an Ag alloy can be used as the material for the reflective electrode film 11. Since the recesses 9a are formed in the planarizing film 9 which is formed below the transparent electrode film 10 (see Fig. 6), recesses 10a and recesses 11a are formed in the transparent electrode film 10 and the reflective electrode film 11, respectively, following the shape of the recesses 9a of the planarizing film 9. After forming the transparent electrode film 10 and the reflective electrode film 11, a resist material is applied to the entire surface of the reflective electrode film 11, so that a resist film 12 is formed. After forming the resist film 12, a part of the resist film 12 is removed as follows.

Fig. 8 is a plan view of the substrate on which some parts of the resist film 12 have been removed. Fig. 9 is a cross-sectional view taken along a line 11-11 in Fig. 8. After forming the resist film 12 (see Fig. 7), the resist film 12 is exposed to light and developed, so that trenches 12a for dividing the resist film 12 into pixel regions and thin portions 12b each having a reduced thickness are formed as shown in Fig. 9. By the trenches 12a, resist films 12c separated into for each pixel region (hereinafter, referred to as 'separate resist film' ) are formed from the resist film 12. In Fig. 8, each separate resist film 12c is shown by hatching and the thin portion 12b formed in each separate resist film 12c is shown by cross-hatching. The trenches 12a are formed along the gate buses 20 and the source buses 60 in such a way that a part of reflective electrode film 11 is exposed by the trenches 12a, so that the material of the resist film 12 has been removed in each area surrounding each separate resist film 12c. Since the thin portion 12b is formed so as to have a reduced thickness, each separate resist film 12c is provided with both the thicker portion having a greater thickness and the thinner portion having a less thickness as shown in Fig. 9. Therefore, the reflective electrode film 11 is exposed at the trenches 12a, but the reflective electrode film 11 is not exposed at the thin portion 12b. In order to form the trenches 12a and the thin portions 12b, the resist film 12 is exposed to light in such a way that an amount of exposure energy absorbed in portions in which the trenches 12a are to be formed is different from an amount of exposure energy absorbed in portions in which the thin portions 12b are to be formed. As a result of such an exposure of the resist film 12, it is possible to adjust a depth of each portion of the exposed resist film 12 to be removed when the exposed resist film 12 is developed, depending on the amount of the absorbed exposure energy. In this manner, the trenches 12a and the thin portions 12b can be formed. After forming the trenches 12a and the thin portions 12b in the resist film 12, the reflective electrode film 11 and the transparent electrode film 10 are etched using the separate resist films 12c as etching masks (see Fig. 10). Fig. 10 is a cross-sectional view of the substrate on which the reflective electrode film 11 and the transparent electrode film 10 have been etched.

For example, if an Al alloy or an Ag alloy is used as a material for the reflective electrode film 11, a mixture of liquid of a phosphorus acid, a nitric acid, an acetic acid and water can be used as an etchant. If an ITO is used as a material for the transparent electrode film 10, a mixture of liquid of a hydrochloric acid and water can be used as an etchant. By etching the reflective electrode film 11 and the transparent electrode film 10 with the separate resist films 12c used as etching masks, the reflective electrode film 11 and the transparent electrode film 10 are divided into pixel regions, so that a reflective electrode 100 and a transparent electrode 110 are formed in the each pixel region. As a result of the above- described steps, the reflective electrode 100 and the transparent electrode 110 are formed in the each pixel region. However, in Fig. 10, each reflective electrode 110 is formed on the corresponding transparent electrode 100 so as to cover the entire surface of the corresponding transparent electrode 100. Therefore, in the above-described case where each reflective electrode 110 covers the corresponding transparent electrode 100 entirely, the light from the backlight 54 (see Fig. 1) is blocked by the reflective electrode 110 when the liquid crystal display device is used in the transparent mode, so that the light from the backlight 54 can not enter the liquid crystal layer 53. Then, a passing window for passing the light from the backlight 54 is formed in the reflective electrode 110 in the method shown in Figs. 11 to 13.

After etching the reflective electrode film 11 and the transparent electrode film 10 (see Fig. 10), the separate resist films 12c are ashed as shown in Fig. 11. Fig. 11 is a cross-sectional view of the substrate on which the separate resist films 12c have been ashed.

As the separate resist films 12c are ashed, the thickness of the separate resist film 12c becomes thinner evenly. Since the thin portion 12b is formed in the separate resist film 12c, a region A corresponding to the thin portion 12b of the reflective electrode 110 is exposed first as shown in Fig. 11. The ashing is finished at the time when the region A is exposed. After finishing the ashing, each reflective electrode 110 is etched using each ashed separate resist film 12c as an etching mask (see Figs. 12 and 13).

Fig. 12 is a plan view of the substrate on which the reflective electrodes 110 have been etched. Fig. 13 is a cross-sectional view taken along a line 111-111 in Fig. 12. The region A of the reflective electrode 110 (see Fig. 11) is removed by etching the reflective electrode 110, so that a passing window 110a for passing the light from the backlight 54 is formed in the reflective electrode 110. The transparent electrode 100 is thus exposed by forming the passing window 110a. The region for entering the light from the backlight 54 into the liquid crystal layer 53 when the LCD is used in the transparent mode can be formed by exposing the transparent electrode 100 as described above.

After etching the reflective electrode, the separate resist films 12c are removed. Then, an alignment film 13 (see Fig. 1) is formed by printing method and the rubbing process is performed. In this way, the TFT substrate assembly 51 is manufactured. In this embodiment, the resist film 12 is formed on the transparent electrode film 10 and the reflective electrode film 11 and processed into the form shown in Fig. 9 by exposing the resist film 12 to light and developing it. Since the reflective electrode film 11 and the transparent electrode film 10 are etched using the separate resist films 12c in the form shown in Fig. 9 as etching masks, the reflective electrode film 11 and the transparent electrode film 10 are divided into pixel regions, so that the reflective electrode 110 and the transparent electrode 100 are formed in each pixel region as shown in Fig. 10. Thereafter, the separate resist films 12c are processed into the form shown in Fig. 11 by performing the ashing on the separate resist films 12c, and then the reflective electrode 110 is etched using the separate resist film 12c in the form shown in Fig. 11 as etching masks, so that the passing window 110a (see Figs. 12 and 13) is formed in the reflective electrode 110. As described above, in this embodiment, both the reflective electrode film 11 and the transparent electrode film 10 are patterned into the desired form by changing the shape of the resist film 12 in two steps as shown in Figs. 9 and 11. Therefore, when the reflective electrode film 11 and the transparent electrode film 10 are patterned into the desired form, there is no need to separately form a resist film for patterning only the reflective electrode film 11 and another resist film for patterning only the transparent electrode film 10, so that the number of manufacturing steps and the manufacturing cost will be reduced.

Further, in this embodiment, the planarizing film 9 formed below the reflective electrode 110 is provided with the plurality of recesses 9a, so that the reflective electrode 110 is also provided with the plurality of recesses 11a. To provide the reflective electrode 110 with the recesses 11a makes it possible to provide the reflective electrode 110 with the desired reflective characteristics. In this embodiment, the reflective electrode 110 has the recesses 1 la in order to provide this reflective electrode with desired reflective characteristics but it is noted that, if the reflective electrode 110 has projections instead of the recesses 11a, the reflective electrode 110 still can have desired reflective characteristics. The reflective electrode 110 can be provided with projections by changing an exposure pattern for the planarizing film 9. The change of the exposure pattern for the planarizing film 9 makes it possible to provide the planarizing film 9 with the projections instead of the recesses, so that the reflective electrode 110 can be provided with projections. Further, it is not always necessary to form the recesses in or projections on the reflective electrode 110 but it is preferable to form the recesses in or projections on the reflective electrode 110 in order to provide the reflective electrode 110 with the desired reflective characteristics.

In this embodiment, the insulating film 8 is formed below the planarizing film 9 but the insulating film 8 may be omitted. Further, in this embodiment, the planarizing film 9 having a plurality of recesses 9a is formed in order to form the recesses 1 la in the reflective electrode 110 but it is possible to form the recesses in the reflective electrode 110 without forming the planarizing film 9, for example, by forming the recesses in the insulating film 8. Although the transparent electrode 100 and the reflective electrode 110 are in a single layer structure in this embodiment, the transparent electrode 100 and the reflective electrode 110 may be in a multi layer structure having two or more layers. If it is desired that, for example, the reflective electrode 110 has a multi layer structure, not only the reflective electrode film 11 but also another reflective electrode film may be formed before forming the resist film 12. After forming such a plurality of reflective electrode films, these reflective electrode films are etched using the separate resist film 12 as etching masks, so that the reflective electrode 110 can have a multi layer structure. In a similar way, the transparent electrode 100 can also have a multi layer structure. Therefore, if it is desired that, for example, both the transparent electrode and the reflective electrode have each double layer structure, two transparent electrode films may be formed and two reflective electrode films may be formed. In this case, four layers are formed in total as the transparent electrode films and the reflective electrode films. It is possible to pattern these four layers in a desired form by changing the shape of the resist film 12.

Finally, the method of forming an electrode according to this embodiment is described comparing with the conventional method of forming an electrode.

Fig. 14 is a cross-sectional view of a substrate on which a transparent electrode and a reflective electrode have been formed in an example of the conventional method.

Conventionally, after forming the transparent electrode 30, for example, an insulating film 31 , a first planarizing film 32 and a second planarizing film 33 are formed and then a reflective electrode 34 is formed. Therefore, in order to form the transparent electrode 30 and the reflective electrode 34, it is needed to form a resist film for patterning a transparent electrode film and a resist film for patterning a reflective electrode film. In contrast, in the method of forming an electrode according to the present invention, both the transparent electrode film 10 and the reflective electrode film 11 can be patterned by changing the shape of the resist film in two steps. Therefore, the number of the manufacturing steps and the manufacturing cost will be reduced as compared to the conventional way. [Effect of the Invention] As described above, according to the present invention, there are provided a method of forming an electrode with reduced cost and a liquid crystal display device to which the method is applied.

Claims

CLAIMS:

1. A method of forming a transparent electrode and a reflective electrode on each predetermined region of a substrate, said method comprising: a first step of forming a first film for said transparent electrode on said substrate; a second step of forming a second film for said reflective electrode on said first film; a third step of forming a third film on said second film; a fourth step of processing said third film in such a way that a portion of said third film corresponding to said predetermined region comprises a thinner portion and a thicker portion and that a portion of said third film corresponding to a periphery of said predetermined region is removed; a fifth step of etching said first film and said second film by using said processed third film as a mask; a sixth step of removing said thinner portion of said third film; and a seventh step of etching said second film by using said third film, from which said thin portion has been removed, as a mask.

2. A method as claimed in claim 1, wherein said third film is a photosensitive film, said fourth step comprises an exposure step of exposing said photosensitive film so as to apply different levels of exposure energy to respective areas of said photosensitive film and a development step of developing said exposed photosensitive film, and said sixth step is a step in which ashing of said developed photosensitive film is carried out.

3. A method as claimed in claim 1 or 2, wherein said method comprises an eighth step of forming a fourth film having recesses or projections before said first step.

4. A liquid crystal display device comprising said reflective electrode and said transparent electrode formed by using said method as claimed in any one of claims 1 to 3.

5. A liquid crystal display device as claimed in claim 4, wherein said reflective electrode is formed directly on said transparent electrode.

6. A liquid crystal display device as claimed in claim 4 or 5, wherein said transparent electrode or said reflective electrode comprises a multilayer construction.