Method for fabricating liquid crystal display device

Disclosed is a method for fabricating a liquid crystal display device comprising: providing a first substrate having a pixel portion and a pad portion; sequentially laminating a gate insulating layer, a semiconductor layer and a first conductive layer on the first substrate where a gate electrode is formed; forming a first PR pattern, which is patterned relatively thin on a channel region of a transistor to be formed, on the first conductive layer with a half-tone mask; patterning the first conductive layer with the first PR pattern; forming a second PR pattern which is aligned with an outer periphery of the first conductive layer by performing a first ashing process on the first PR pattern; patterning the semiconductor layer using the second PR pattern; forming source/drain electrodes using the second PR pattern; forming a passivation layer and a pixel electrode on the first substrate; attaching a second substrate to the first substrate; and forming a liquid crystal layer between the first substrate and the second substrate.

This application claims the benefit of Korean Patent Application No. 2006-061475, filed on Jun. 30, 2006, which is hereby incorporated by reference for all purposes as if fully set forth herein.

BACKGROUND OF THE INVENTION

The present invention relates to a method for fabricating a liquid crystal display (LCD) device, and more particularly, to a method for fabricating a liquid crystal display device which can obtain uniformity of a channel by replacing a slit mask used in a photolithography process with a half tone mask, and which based on the uniformity thus obtained, can reduce active tail and wavy noise phenomena by applying a pre-ashing process when forming source/drain electrodes.

Description of the Related Art

Among display devices, in particular, in a flat panel display including a liquid crystal display device, a display device is driven by active devices such as thin film transistors in each of the display pixels.

Such a method for driving the display device is generally called “an Active Matrix driving method.”

In the active matrix driving method, the active devices are disposed in each of the pixels that are arranged in a matrix shape to drive a corresponding pixel.

From this point of view, a related art liquid crystal display device will now be explained with reference toFIG. 1.

FIG. 1is a diagram showing a pixel of a liquid crystal display device implementing an active matrix pixel driving method.

As shown inFIG. 1, the related art liquid crystal display device is a thin film transistor liquid crystal display device, which uses a thin film transistor10as the active pixel driving device.

Referring toFIG. 1, each of the pixels in the thin film transistor liquid crystal display device (where N×M pixels are arranged vertically and horizontally) includes a thin film transistor10formed at an area where a gate line13to which a scanning signal is applied from an external driving circuit and a data line19cto which a picture signal is applied intersect with each other.

Here, the thin film transistor10includes a gate electrode13aconnected to the gate line13, an active pattern17aformed above the gate electrode13aand being active when the scanning signal is applied to the gate electrode13a, and a source electrode19aand a drain electrode19bformed on the active pattern17a, respectively.

A display area of the pixel is provided with a pixel electrode25which is connected to the source/drain electrodes19aand19band receives the pixel signal through the source/drain electrodes19aand19bwhen the active pattern17ais activated thus to operate a liquid crystal material layer (not shown).

Detailed description of the structure of the related art liquid crystal display device will now be given with reference toFIG. 2which is a cross-sectional view taken along line II-II inFIG. 1and shows a cross-section of the related art liquid crystal display device.

Referring toFIG. 2, the thin film transistor is disposed on a first substrate11which is formed of a transparent material (e.g., glass) and forms an array substrate.

Here, the thin film transistor includes the gate electrode13aformed on the first substrate11, a gate insulating layer5laminated on the entire first substrate11having the gate electrode13a, the active pattern17aformed on the gate insulating layer5, the source electrode19aand the drain electrode19bformed on the active pattern17a, and a passivation layer23formed on the entire first substrate11.

In addition, the pixel electrode25that is connected to the drain electrode19bof the thin film transistor through a contact hole (not shown) formed in the passivation layer23is formed on the passivation layer23.

Meanwhile, a color filter substrate facing toward the array substrate11includes a second substrate31formed of a transparent material (e.g., glass), a black matrix33formed on the second substrate31and formed on an image non-display area (e.g., the area having the thin film transistor or the area between pixels) so as to prevent the penetration of light through the image non-display area, and a color filter layer35formed of red, green, and blue filters so as to implement real colors.

In this case, when the color filter substrate and the array substrate are attached to each other, a liquid crystal layer41is filled therebetween thus to complete the liquid crystal display device.

Meanwhile, a common electrode37may be further provided on the color filter layer35for supplying an electric field to the liquid crystal layer41in addition to the pixel electrode25.

Such liquid crystal display devices are generally fabricated by complicated processes, such as by photolithography using a mask.

Referring toFIGS. 3A through 3G, description of a method for fabricating a liquid crystal display device by a related art 4-mask process using a slit mask will now be given in detail.

FIGS. 3A through 3Gare cross-sectional views showing sequentially the process of the fabrication method for a liquid crystal display device by applying a 4-mask process using a slit mask.

First, referring toFIG. 3A, a metal layer to be used for forming a gate electrode is formed over an entire surface of a first substrate11, and then a photoresist film (not shown) is coated thereon. Through a photolithography process, a gate line (not shown) and a gate electrode13aconnected to the gate line are formed.

Referring toFIG. 3B, a gate insulating layer15, a semiconductor layer17, an ohmic contact layer (an n+ amorphous silicon thin film is generally used, not shown), and a conductive layer19are sequentially formed over the entire surface of the first substrate11having the gate electrode13a.

Herein, the conductive layer19is a layer to be patterned into the source electrode and the drain electrode through following procedures.

And, a photoresist film (not shown) is coated on the conductive layer19, and light is then irradiated onto the photoresist film (not shown) through a slit mask20having a light shielding portion20a, a semi-transmissive portion20b, and a transmissive portion20c. Then, a photoresist film pattern21is formed on the conductive layer19after exposing and developing procedures.

Here, since the photoresist film pattern21is formed by using the slit mask20, a photoresist film pattern21aformed on an upper portion of a channel area is thinner, compared to a photoresist film pattern21bformed on another area.

Then, referring toFIG. 3C, the photoresist film pattern21is utilized as an etching mask such that the conductive layer19, the ohmic contact layer (not shown), and the semiconductor layer17are sequentially etched thus to form an active pattern17a.

Next, referring toFIG. 3D, an ashing process is performed on the photoresist film pattern21. Herein, since the photoresist film pattern portion21aover the channel region (that is, the relatively thin area of the photoresist film pattern) is removed during the ashing process, the conductive layer19is exposed.

The ashing process is a process whereby the photoresist film as organic matter is oxidized for removal. Some portion21aof the photoresist film pattern21is removed by oxidization, thereby reducing its overall volume. Here, the photoresist film pattern21at the edges of the channel area and the active pattern is also removed.

Referring toFIG. 3E, the photoresist film pattern21after having been ashed is utilized as an etching mask such that the conductive layer on the channel area and the ohmic contact layer are removed thus to form the source electrode19aand the drain electrode19b.

Next, referring toFIG. 3F, after the photoresist film pattern21having been ashed is removed, the passivation layer23is formed on the substrate having the source and drain electrodes19aand19b.

Referring toFIG. 3G, a contact hole (not shown) for exposing the drain electrode19bis formed in the passivation layer23through a photolithography procedure.

Then, the pixel electrode25, which is connected to the drain electrode19band formed of a transparent electrode material, is formed.

The related art thin film transistor formed according to the sequence of the above-mentioned procedures is fabricated by the 4-mask process, in which a first mask is used for forming the gate electrode, a second mask is used for forming the active pattern and source/drain electrodes, a third mask is used for forming the contact hole to expose the drain electrode and a fourth mask is used for forming the pixel electrode.

According to the related art method for fabricating the liquid crystal display device by using the above procedures, as shown inFIG. 3E, since the photoresist film pattern21having been ashed also exposes the edge of the active pattern17a, the ohmic contact layer (not shown) and the conductive layer19formed on the edge of the active pattern17aare removed. Consequently, the active pattern17ais more protruded than the source/drain electrodes, thereby causing “an active tail defect.”

Detailed description of the active tail defect will be given in reference toFIGS. 4A and 4B, which are cross-sectional views showing the fabrication sequence for forming source/drain electrodes in the related art process using a slit mask, as viewed from the data line side.

As shown inFIG. 4A, the semiconductor layer17, the ohmic contact layer18, the patterned source/drain electrode forming conductive material19, and the patterned photoresist film pattern21are disposed on the substrate11. Detailed explanations thereof will be given in comparison withFIG. 3C.

FIG. 4Ashows a state that the source/drain electrode forming conductive material19shown inFIG. 3Cis patterned, as viewed from the data line side. That is, it is the state that the source/drain electrode forming conductive material19(e.g. a metallic material including molybdenum) is patterned by applying a wet etching using the photoresist film pattern21.

InFIG. 4A, the gate electrode and a gate insulating layer are not shown. Also, the photoresist film having a relatively thin channel area is not shown, since it has a different section fromFIG. 3C.

Undesirably, this may cause the active tail defect, which will be described now, in reference to the following procedures.

InFIG. 4A, the source/drain electrode forming conductive material19is patterned, and then a dry etching process is performed for patterning the ohmic contact layer18and the semiconductor layer17by using the photoresist film pattern21as an etching mask, thereby forming the active pattern.

Here, as shown inFIG. 4A, due to the shape of the photoresist film pattern21applied as the etching mask, an outer periphery of the etched active pattern17aand an outer periphery of the patterned source/drain electrode forming conductive material19are not aligned with each other.

That is, referring toFIG. 4B, the edge of the active pattern17ais not completely etched leaving some portion thereof remnant, resulting in undesirably having a shape of a tail.

This is called “an active tail phenomenon,” having a protrusion of almost 1.7 μm as shown inFIG. 4B. The active tail phenomenon causes a reduction in an area of the pixel electrode and generates about a 2% loss in aperture ratio as a result of the area reduction.

Further, as an amorphous silicon thin film having a very thin thickness, the ohmic contact layer18is fully etched so as to be aligned with the outer periphery of the patterned source/drain electrode forming conductive material19in the above-mentioned dry etching process.

FIG. 4Bis a cross-sectional view showing the state that the passivation layer23and the pixel electrode25are formed, as viewed from the data line side.

Because the conductive layer is always present under the source/drain electrode forming conductive material, light from a backlight projected from the data line side penetrates the gate insulating layer thus to directly impinge upon the semiconductor layer.

The backlight light penetrating the gate insulating layer and impinging the semiconductor layer may activate the semiconductor layer and cause defects, such as wavy noise.

At the time of forming the source/drain electrodes, the wavy noise occurs in a displayed image when the active pattern protruding more than the source/drain electrodes diffracts the backlight light or a channel signal is distracted by the backlight light.

SUMMARY OF THE INVENTION

Therefore, in order to overcome the above-mentioned problems, it is an object of the present invention to provide a method for fabricating a liquid crystal display device which can reduce active tail and wavy noise defects without requiring an additional masking process.

To achieve these and other advantages and in accordance with the purpose of the present invention, as embodied and broadly described herein, there is provided a method for fabricating a liquid crystal display device, comprising: providing a first substrate having a pixel portion and a pad portion; sequentially laminating a gate insulating layer, a semiconductor layer, a conductive layer, and a photoresist film on the first substrate where a gate electrode is patterned; forming a photoresist film pattern by patterning the photoresist film with a half-tone mask; patterning the conductive layer and the semiconductor layer with the photoresist film pattern; partially removing the photoresist film pattern through a first ashing process; forming source/drain electrodes by patterning the conductive layer with the remnant photoresist film pattern; forming a passivation layer and a pixel electrode on the first substrate; attaching a second substrate to the first substrate; and forming a liquid crystal layer between the first substrate and the second substrate.

According to another embodiment of the present invention, there is provided a method for fabricating a liquid crystal display device, comprising: providing a first substrate having a pixel portion and a pad portion; forming a gate electrode on the first substrate using a first mask; sequentially laminating a gate insulating layer, a semiconductor layer, a first conductive layer on the first substrate where the gate electrode is formed; forming a Photo Resist (PR) pattern, which is patterned relatively thin on a channel area of a transistor, on the first conductive layer with a half-tone mask; patterning the first conductive layer and the semiconductor layer using the PR pattern; partially removing the PR pattern by performing a first ashing process on the PR pattern; forming source/drain electrodes by patterning the first conductive layer using the remnant PR pattern; forming a passivation layer on the first substrate having the source/drain electrodes; partially exposing the drain electrode by patterning the passivation layer using a second mask on the first substrate; forming a second conductive layer on the first substrate; forming a pixel electrode by patterning the second conductive layer using a third mask on the first substrate; attaching a second substrate to the first substrate; and forming a liquid crystal layer between the first substrate and the second substrate.

DETAILED DESCRIPTION OF THE INVENTION

Reference will now be made in detail to an embodiment of the present invention, example of which is illustrated in the accompanying drawings.

Description will now be given in detail of the method for fabricating a liquid crystal display device according to the present invention, examples of which are illustrated in the accompanying drawings.

FIG. 5is a diagram showing the intensity of light irradiated to a channel region of a thin film transistor during a photolithography process using a slit mask. InFIG. 5, reference numeral121denotes a transparent substrate and reference numeral123denotes a shielding material (e.g., chrome) formed on a shielding area in the slit mask.

Referring toFIG. 5, the slit mask120includes a transmissive region through which light is transmitted at 100%, a slit region through which light is transmitted at more than 0% and less than 100%, and a shielding region where light transmission is blocked.

The slit area has a slit structure, and the intensity of light irradiated through the slit structure is less than that through the transmissive region where light is fully transmitted. Accordingly, after the photoresist film113is coated and if the slit mask120, which partially has the slit region and the transmissive region disposed over the photoresist film113, is used for exposure, a thickness of the photoresist film113aremaining under the slit region and that of the photoresist film113B remaining under the transmissive region are formed to be different.

That is, for the case of a positive photoresist film, the thickness of the photoresist film113airradiated through the slit region is formed to be thicker than that under the transmissive region. However, for the case of a negative photoresist film, a thickness of the photoresist film remaining under the transmissive region is formed to be thicker than that under the slit region.

Referring toFIG. 5, when the slit mask120is used, the light intensity irradiated onto the channel region during the exposure process is not uniform. Accordingly, a resulting surface of the channel region thus formed is non-uniform and uneven, thereby reducing its uniformity.

Due to these problems, when the slit mask is used, it was difficult to apply a pre-ashing process before an active pattern was patterned.

FIG. 6is a diagram showing the intensity of light irradiated onto a channel region of a thin film transistor during a photolithography process using a half-tone mask, instead of using a slit mask, in fabricating a liquid crystal display device according to an embodiment of the present invention.

Similarly to using the slit mask, the half-tone mask220used in the present invention includes a transmissive region, a half-tone region (i.e., a semi-transmissive region), and a shielding region.

The half-tone region is formed of a metallic material that can control an amount of light transmitted according to its thickness (e.g., molybdenum silicide, MoSi). And, the intensity of light irradiated through the half-tone region is less than that through the transmissive region where light is fully transmitted. Accordingly, after the photoresist film213is coated and if the half-tone mask220over the photoresist film213is used for exposure, a thickness of the photoresist film213aremaining under the half-tone region and that of the photoresist film213B remaining under the transmissive region are formed to be different.

That is, for the case of a positive photoresist film, the thickness of the photoresist film irradiated through the half-tone region is formed to be thicker than that under the transmissive region. However, for the case of a negative photoresist film, a thickness of the photoresist film remaining under the transmissive region is formed to be thicker than that under the half-tone region.

InFIG. 6, reference numeral221denotes a transparent substrate, reference numeral223denotes a chrome layer for shielding light, and reference numeral225denotes a molybdenum silicide (MoSi) layer formed on a half-tone region. Here, if the thickness of the molybdenum silicide (MoSi) layer225is adjusted, the amount of transmitted light irradiated onto the photoresist film213may be controlled.

Referring toFIG. 6, since the intensity of light irradiated onto the channel region during the exposure process is uniform, the surface of the channel region is formed to be smooth, thereby enhancing its uniformity.

Accordingly, when the half-tone mask220is used, a pre-ashing process can be applied before an active pattern is patterned.

Description will now be given in detail of the method for fabricating a liquid crystal display device according to the present invention with reference toFIGS. 7A through 7C.

FIGS. 7A through 7Care sequential cross-sectional views showing the fabrication process utilizing a half-tone mask and applying a pre-ashing process before an active pattern is patterned, as viewed from a data line side, according to one embodiment of the present invention.

FIG. 7Ashows the state that a source/drain electrode forming conductive material307is patterned by a wet etching process.

Referring toFIG. 7A, there is provided a substrate300, a semiconductor layer303formed on the substrate300and to be patterned into an active pattern through following procedures, an n+ silicon thin film305formed on the semiconductor layer303for making an ohmic contact with source/drain electrodes to be formed later, a source/drain electrode forming conductive material307formed on the semiconductor layer having the n+ silicon thin film and having been wet-etched, and a photoresist film309formed above the source/drain electrode forming conductive material307and patterned by using a half-tone mask (not shown).

FIG. 7Adoes not show much difference fromFIG. 4A. That is, as shown inFIG. 7A, a source/drain electrode forming conductive material307, having been patterned more than the photoresist film309, is more etched inwardly by distance d2.

FIG. 7Bshows the state that a pre-ashing process is applied before an active pattern is formed by patterning of the semiconductor layer303.

The photoresist film309aremaining after the pre-ashing process has a reduced lateral width. In addition, the outer periphery of the photoresist film309aand that of the source/drain electrode forming conductive material307are aligned with each other, thereby remarkably reducing the possibility of generating an active tail phenomenon in following processes.

After the pre-ashing process shown inFIG. 7B, the related art fabrication process steps for a liquid crystal display device are performed. That is, the photoresist film309aremnant after the pre-ashing process is used as an etching mask to pattern the n+ silicon thin film305and the semiconductor layer303, thereby forming the active pattern, and then proceeding to an ashing process on the channel region of the thin film transistor.

FIG. 3Dshows the result of the ashing process performed on the channel region. The half-tone exposed photoresist film that is partially remaining over the channel area is completely removed, thereby exposing the source/drain electrode forming conductive material (reference numeral19inFIG. 3D).

Next, a dry etching process is performed to remove the source/drain electrode forming conductive material (19, reference numeral307inFIG. 7B) over the channel region.

Then, a dry etching process is performed to remove the n+ silicon thin film over the channel region (not shown inFIG. 3D, reference numeral305inFIG. 7B).

Preferably, the above-mentioned pre-ashing process and the dry etching process for removing the n+ silicon thin film over the channel region (not shown inFIG. 3D, reference numeral305inFIG. 7B) are integrally performed in one chamber.

Thereafter, a PR stripping process is performed for removing the remaining photoresist film, to complete the source/drain electrode formation (not shown).

Lastly, processes including a passivation layer formation, a pixel electrode formation, a liquid crystal layer formation, etc. are sequentially performed to fabricate the liquid crystal display device.

FIG. 7Cis a cross-sectional view showing the state that a pixel electrode is formed, as viewed from a data line side.

InFIG. 7C, there is provided a substrate300, a semiconductor layer303formed on the substrate300, an n+ silicon thin film305, a source/drain electrode forming conductive material307, a passivation layer311, and a pixel electrode313.

Referring toFIG. 7C, an active tail in the range of only 0.3-0.5 μm is formed in the liquid crystal display device according to one embodiment of the present invention, resulting in remarkable enhancement compared to the related art active tail phenomenon occurrence.

As described above, the uniformity in the channel region of the thin film transistor may be obtained by using the half-tone mask, instead of using the related art slit-mask, according to one embodiment of the present invention. Further, based on the obtained uniformity in the channel region, the pre-ashing process may be applied before the active pattern is patterned, thereby preventing or reducing the generation of the active tail phenomenon.

Hereinafter, detailed description of a method for fabricating a liquid crystal display device according to another embodiment of the present invention will now be given with reference toFIGS. 8A through 8E, which are sequential cross-sectional diagrams showing the process of the fabrication method for a liquid crystal display device according to another exemplary embodiment of the present invention.

InFIG. 8A, after an initial washing process is performed, a first conductive layer (not shown) to be used for a gate electrode is formed on a transparent substrate300(e.g., glass). Then, a patterning process (e.g., a wet etching) is performed using a first mask (not shown) to form a gate electrode301a, a gate line301, and a capacitor lower electrode301b.

Here, the first mask (not shown) may be a generally used mask, not necessarily an expensive slit mask or an expensive half-tone mask.

Further, the first conductive layer (not shown) may be formed as a thin film of an opaque conductive material with a low resistance, such as aluminum (Al), aluminum alloy, tungsten (W), copper (Cu), chromium (Cr), molybdenum (Mo), etc.

The first conductive film (not shown) may be formed in a multilayer structure laminated with two or more low-resistance conductive materials.

Next,FIGS. 8B and 8Cshows that source/drain electrodes307aand307bare formed. In the process, a half-tone mask is used and a pre-ashing process is applied.

Detailed description of the process to which the pre-ashing process is applied will now be given with reference toFIGS. 8B and 8C.

Referring toFIG. 8B, a gate insulating layer302is formed on the substrate300on which the gate electrode301is patterned. Here, the gate insulating layer302may be formed of a silicon nitride (SiNx) layer, a silicon oxide layer or of other inorganic insulating materials.

A hydrogenated amorphous silicon layer303, an n+ amorphous silicon thin film (not shown), and a second conductive layer307for forming source/drain electrodes are sequentially laminated on the gate insulating layer302.

Here, the hydrogenated amorphous silicon layer303serves as an active area of the thin film transistor, and is a layer on which the active pattern is patterned and a transistor channel is formed through following procedures.

Further, the hydrogenated amorphous silicon layer303is used as a semiconductor layer for forming the active pattern, which allows to perform a low temperature process and to use an inexpensive insulating substrate.

And, the n+ amorphous silicon thin film (not shown) is an ohmic contact layer. The source electrode and drain electrode make an ohmic contact with a certain area of the active pattern through the ohmic contact layer that is formed of the n+ amorphous silicon thin film.

Herein, the second conductive layer307for forming source/drain electrodes may be formed of an opaque conductive material with a low resistance, such as aluminum (Al), aluminum alloy, tungsten (W), copper (Cu), chromium (Cr), molybdenum (Mo), etc.

As shown inFIG. 8C, the gate insulating layer302, the hydrogenated amorphous silicon layer303, n+ amorphous silicon thin film (not shown), second conductive layer307for forming source/drain electrodes and a photoresist film (not shown) are sequentially laminated on the substrate300. Then, the source/drain electrodes307aand307bare formed by using a half-tone mask320.

First, the photoresist film (not shown) is patterned by using a half-tone mask320, and the second conductive layer307is then wet-etched by using the patterned photoresist film309as a mask to thus form the source/drain electrodes307aand307b.

In this case, the source/drain electrodes307aand307bmay be formed in a “U-shape” so as to increase a switching speed as the channel becomes wider.

As shown inFIG. 8B, the photoresist film formed on the channel region has a thickness less than that on another area, but has a uniformity over the channel region.

After the patterning process on the second conductive layer307for forming source/drain electrodes, a pre-ashing process is performed according to one embodiment of the present invention.

As described above, occurrence of the active tail phenomenon can also be prevented through the pre-ashing process.

Next, as shown inFIG. 8C, the hydrogenated amorphous silicon layer303is dry-etched and patterned, and thereafter an ashing process is performed to remove all of photoresist film309left on the channel region.

Then, the second conductive layer307for forming source/drain electrodes which is formed on the channel region is removed by a dry etching. The n+ amorphous silicon thin film (not shown) which is formed on the channel area is removed, thereby exposing the hydrogenated amorphous silicon layer303on the channel area.

Further, when a PR Stripping process is performed for removing the remaining photoresist film309, the source/drain electrodes307aand307bare formed.

Referring toFIG. 8D, a passivation layer311is formed over the entire resulting structure so as to protect the device from moisture and scratches.

Then, a photolithography process is performed using a third mask (not shown) to form a contact hole (312) that exposes the source electrode307by penetrating a certain area of the passivation layer311.

As shown inFIG. 8E, after a transparent conductive material (not shown) is deposited over an entire surface of the substrate and then is patterned through a photolithography process using a fourth mask, a pixel electrode313is formed to be electrically connected to the source electrode307bthrough the contact hole312.

Herein, material for forming the pixel electrode may be a transparent film of a conductive material having an excellent light transmissivity, such as Indium Tin Oxide (ITO) or Indium Zinc Oxide (IZO).

After this, general processes such as a process for filling a liquid crystal material layer in the liquid crystal display device, etc. are performed to complete the fabrication of the liquid crystal display device.

As described so far, the method for fabricating a liquid crystal display device according to the present invention can obtain uniformity of a channel region without requiring an additional masking process, can reduce occurrence of an active tail phenomenon, and improve upon wavy noise occurrence compared to the conventional art based on the uniformity obtained by additionally applying a pre-ashing process step when forming source/drain electrodes.