WO1998023997A1

WO1998023997A1 - Thin film transistor-liquid crystal display and method of fabricating the same

Info

Publication number: WO1998023997A1
Application number: PCT/KR1996/000279
Authority: WO
Inventors: Chul Ha Kim; Young Woo Seo
Original assignee: Orion Electric Co. Ltd.
Priority date: 1996-11-30
Filing date: 1996-12-31
Publication date: 1998-06-04
Also published as: TW348323B; KR19980041088A

Abstract

A TFT-LCD is disclosed and includes a gate line extended in one direction on an insulating substrate; a semiconductor layer pattern superposed on the gate line, one end of the semiconductor layer pattern being projected from both sides of the gate line, the other end of the semiconductor layer pattern being projected in one direction from the one side of the gate line; a data line intersected with the gate line, and superposed on a portion of the semiconductor layer pattern at the portion where the semiconductor layer pattern is projected from both sides of the gate line, the data line coming into contact with the semiconductor layer pattern at the superposed portion; and a pixel electrode connected to the other end of the semiconductor layer pattern, to thereby improve the aperture rate, picture quality and luminance.

Description

THIN FILM TRANSISTOR-LIQUID CRYSTAL DISPLAY AND METHOD OF FABRICATING THE SAME

TECHNICAL FIELD The present invention relates to a liquid crystal display (LCD) employing a thin film transistor (TFT) as its switching device and a method of fabricating the same.

More particularly, the present invention relates to a TFT-LCD employing a bottom-gate TFT whose gate electrode is formed under its channel and a method of fabricating the same, in which a semiconductor layer pattern is formed in such a manner that a portion of the semiconductor layer pattern to be a channel in the TFT is superposed on top of a gate line, and the other portion of the semiconductor layer pattern is superposed on a data line with the gate line in order to simplify its process, prevent the gate line and data line from short-circuiting, increase the pixel aperture ratio by reducing the area occupied in the TFT, and reduce the leakage current by forming an offset region or LDD region in the source and drain junction region, thereby increasing the aperture rate and improving the production yield and reliability of the device.

BACKGROUND ART An LCD, a kind of flat panel display, forms a picture using the brightness level obtained by applying an electric field to a liquid crystal having the fluidity of liquid and the optical characteristics of crystal, and changing the optical anisotropy of the liquid crystal. There are various kinds of LCDs, such as a twisted nematic (TN) LCD, super twisted nematic LCD or ferroelectric LCD. A TFT-LCD, which includes a TFT of a switching element of the pixel, is widely used.

The LCD is available in a wider range than the conventional CRT since it has an easily formed compact structure, a lower rate of power consumption, a higher definition in color and greater size. Recently, the TFT-LCD has drawn attention because it has a faster response time and greater picture quality than the conventional CRT.

A conventional LCD is constructed in a type of liquid crystal that is sealed between upper and bottom substrates on which each of the transparent electrode pattern are formed. The LCD includes a transparent pixel electrode pattern formed of indium tin oxide (ITO) or Sn0₂ formed on the upper and bottom substrates made of a transparent material such as quartz, glass or plastic film, an insulating protective film for insulating the transparent electrode pattern, and an alignment film for directing the liquid crystal in a predetermined direction. The alignment film has valleys in a predetermined direction thereon. Thus, the film has an orientation. The valleys are formed in such a manner that the orientation film is rubbed by a rubbing roll of cylindrical core surrounded by a texture. A color filter is formed on the bottom substrate.

The upper and bottom substrates are sealed at their marginal portion and have a predetermined cell gap, defining a predetermined clearance into which a liquid crystal is sealed. The liquid crystal is sealed between the upper and bottom substrates .

Since the LCD cannot display pictures for itself, it is used in the form of a module including an optical source such as a light -emitting device, for example, a electroluminescence device, a light emitting diode panel, or a cold cathode fluorescence lamp. The LCD forms pictures with ground colors which appear when the liquid crystal is driven.

In a TFT-LCD, a TFT for switching the pixel electrode is formed on one side of the pixel electrode formed on the bottom substrate. A bottom-gate TFT is formed in such a manner that its gate is formed under its channel of silicon layer. A top-gate TFT is formed in such a manner that its gate is formed on its channel. Fig. 1 is a layout of a conventional TFT-LCD employing a bottom-gate TFT. Referring to Fig. l,the bottom of the TFT is comprised of a gate line 12, gate electrode 13 and semiconductor layer pattern 14. The gate line 12 which is extended in a longitude direction thereof on the bottom of TFT is located on top of a transparent insulating substrate 10 . The gate electrode 13 is projected from one side of gate line 12. The semiconductor layer pattern 14 to be a channel in the form of a rectangle is superposed on the center of gate electrode 13. A data line 16 is formed perpendicular to gate line 12. A drain electrode 17 is projected from one side of data line 16 and is connected to semiconductor layer pattern 14. Here, a gate insulating layer (not shown) is formed on gate line 12 and gate electrode 13, and insulates them from semiconductor layer pattern 14 and data line 16.

Also, at the other portion of substrate 10 in the TFT, a pixel electrode 18 having a transparent electrode pattern is formed. A source electrode 19 connects pixel electrode 18 to semiconductor layer pattern 14.

The aforementioned conventional TFT-LCD is fabricated through the following procedure. That is, patterning a pixel electrode on an insulating substrate, patterning a gate line and gate electrode, depositing a gate insulating layer, patterning a semiconductor layer, depositing an insulating layer on the overall surface of the substrate structure, forming source and drain contact holes, and patterning a data line and, a source and drain electrode. These processes require many fabrication steps using masks. Accordingly, as the fabrication process becomes complicated, the production yield is decreased but the fabrication cost is increased.

Furthermore, when contacts for the source and drain electrodes are formed on both sides of the gate electrode through photolithography, misalignment may occur, resulting in short-circuiting between electrodes. This reduces the production yield. Moreover, in the conventional TFT-LCD, the TFT is separately formed having a predetermined distance from the gate line and data line. Also, the TFT portion is covered by a block matrix. Accordingly, the aperture rate is decreased, thereby deteriorating the picture quality and luminance . For the purpose of preventing the aperture rate from decreasing, a TFT-LCD structure has been proposed. This structure, as shown in Fig. 2, is constructed in the following manner. That is, a portion of gate line 12 is used as a gate electrode, without employing a separate gate electrode, and semiconductor layer pattern 14 is formed on gate line 12. One side of semiconductor layer pattern 14 is connected to pixel electrode 18 through source electrode 19, a portion of data line 16 is projected and this projected portion is used as drain electrode 17 which is connected to the other side of semiconductor layer pattern 14. However, in this structure, the projected drain electrode decreases the aperture rate, and the source electrode 19 and the source and drain contacts are formed through separate processes, making the process complicated and reducing the production yield.

DISCLOSURE OF INVENTION

An object of the present invention is to provide a TFT- LCD in which a semiconductor layer pattern is formed on a gate line, a data line is superposed on the semiconductor layer pattern, and a TFT is placed on conducting lines, to thereby improve its aperture rate and reliability.

Another object of the present invention is to provide a method of fabricating a TFT-LCD in which source and drain electrodes are not formed through separate processes so as to simplify its fabrication process and increase its aperture rate, resulting in an improvement in its production yield and reliability. To accomplish the object of the present invention, there is provided, a TFT-LCD including a gate line extended in one direction on an insulating substrate; a semiconductor layer pattern superposed on the gate line, one end of the semiconductor layer pattern being projected from both sides of the gate line, the other end of the semiconductor layer pattern being projected in one direction from one side of the gate line; a data line formed so that the data line is intersected perpendicularly with the gate line and superposed on a portion projected from both sides of the gate line in the semiconductor layer pattern; and, a pixel electrode contacted with the other portion projected from the end of the semiconductor layer pattern. To accomplish the object of the present invention, there is further provided a method of fabricating a TFT-LCD comprising the steps of : forming a gate line on an insulating layer; forming a gate insulating layer on the overall surface of the substrate; forming a semiconductor layer pattern superposed on the gate line, one end of the semiconductor layer pattern being projected from top to bottom on both sides of the gate line, the other end of the semiconductor layer pattern being projected in one direction from the top of one side of the gate line; forming a heavily doped impurity region in a portion of the semiconductor layer pattern where the semiconductor layer pattern is not superposed on the gate line; forming an insulating layer on the semiconductor layer pattern; removing a portion of the insulating layer placed on the heavily doped impurity region, to form source and drain contact holes; and forming a data line coming into contact with the heavily doped impurity region, the data line being intersected with the gate line.

BRIEF DESCRIPTION OF DRAWINGS

Fig. 1 is a layout of a conventional TFT-LCD;

Fig. 2 is a layout of another conventional TFT-LCD;

Fig. 3 is a layout of a TFT-LCD according to a first embodiment of the present invention; Fig. 4 is a cross-sectional view taken along line I-I of Fig. 3;

Figs. 5A to 5D are cross-sectional views showing a process of fabricating the TFT-LCD according to the first embodiment of the present invention; Figs. 6A, 6B and 6C are plane views showing a process of fabricating a TFT-LCD according to a second embodiment of the present invention; and

Figs . 7A and 7B are plane views showing a process of fabricating a TFT-LCD according to a third embodiment of the present invention.

BEST MODE FOR CARRYING OUT THE INVENTION Reference will now be made in detail to the preferred embodiment of the present invention, examples of which are illustrated in the accompanying drawings.

Fig. 3 is a layout of one pixel of a TFT-LCD according to a first embodiment of the present invention. Referring to Fig. 3, TFT- LCD includes a gate line 12 extended in one direction on a transparent insulating substrate 10, a semiconductor layer pattern 14 to be a channel superposed on a portion of gate line 12, a pixel electrode 18 formed on a center portion of transparent insulating substrate 10 which is defined by gate line 12 and data line 16

In accordance with the TFT-LCD of the present invention, a transparent insulating substrate 10 is made of glass, quartz or plastic material, and a gate line 12 is formed with conductive material like Al, W, Ti or Cr. A semiconductor layer pattern 14 is formed of polysilicon, amorphous silicon or micro-crystal silicon. Also, a semiconductor layer pattern 14 is formed of in a predetermined form such as a J-shape. One side of the J- shape semiconductor layer pat tern 14 superposed on the data line 16 covers the gate line 12 as this shape is extended in two direction, and the other side of the semiconductor pattern is projected in one direction. Here, a heavily doped impurity region 24 to be the source and drain regions is formed in the projected portion of semiconductor layer pattern 14.

A predetermined end portion which is projected from both sides of the semiconductor layer pattern 14 comes into contact with the portion of the data line 16 from top to bottom, while the other portion projected from one side of the semiconductor layer pattern 14 is connected to a pixel electrode. A pixel electrode 18 is formed on a portion of transparent insulating substrate 10, defined by gate line 12 and data line 16. In the above-described layout, the TFT hardly occupies the pixel region so that the aperture rate is increased. Furthermore, the semiconductor layer pattern is formed on the gate line, and thus good topology can be obtained, resulting in accurate patterning during the formation of layers through photolithography.

Figs. 5A to 5D are cross-sectional views showing a process of fabricating the TFT-LCD according to the first embodiment of the present invention. Referring to Fig. 5A, gate line 12 is formed on transparent insulating substrate 10, and a gate insulating layer 22 is formed of a silicon oxide or silicon nitride on the overall surface of the substrate . Semiconductor layer pattern 14 is formed on gate insulating layer 22 in a J-shape as shown in Fig. 3. Referring to Fig. 5B, an impurity like As, P or B is heavily ion- implanted into a portion of the semiconductor layer pattern, where semiconductor layer pattern 14 is not superposed on gate line 12, to thereby form heavily doped impurity region 24 for ohmic contact. Then, an insulating layer 28 is formed of a silicon oxide or silicon nitride on semiconductor layer pattern 14. Here, insulating layer 28 is formed only on semiconductor layer pattern 14. However, it can also be formed on the overall surface of the substrate.

Referring to Fig. 5C, a predetermined portion of insulating layer 28 is selectively removed to form a contact hole 26, thereby exposing heavily doped impurity region 24.

Referring to Fig. 5D, contacts for connecting semiconductor layer pattern 14 and data line 16 may be formed on both projected portions of the semiconductor layer pattern for the purpose of reducing the contact resistance. Data line 16 is formed to come into contact with semiconductor layer pattern 14 and to intersect gate line 12. Pixel electrode 18 is formed of a transparent material such as ITO, Sn0₂ or In₂0₃-ZnO on a predetermined portion of the substrate, which is defined by gate line 12 and data line 16. Here, pixel electrode 18 comes into contact with heavily doped impurity region 24 through contact hole 26. Pixel electrode 18 may be formed before the formation of the gate line. In this case, a source electrode to which the pixel electrode is connected must be formed separately.

Figs. 6A to 6C are plane views showing a method of fabricating a TFT-LCD having an offset region in its source and drain regions according to a second embodiment of the present invention. Referring to Fig. 6A, gate line 12 is formed on transparent insulating substrate 10, extending in one direction. A gate insulating layer (not shown) is formed of a silicon oxide or silicon nitride on the overall surface of the substrate. Then, semiconductor layer pattern 14 is formed in J-shape on the gate line 12 in such a manner that a bidirectional protrude portion 14A and a monodirectional protrude portion 14B are extended from gate line 12.

Referring to Fig. 6B, a photoresist pattern 30 is formed on the portion where a data line is not superposed on the semiconductor layer pattern. Here, photoresist pattern 30 covers a portion of projected portion 14B of the semiconductor layer pattern 14.

Referring to Fig. 6C, an n-type or p-type impurity such as As, P or B is heavily ion-implanted into a portion of semiconductor layer pattern 14 where photoresist pattern 30 is not formed, to thereby form heavily doped impurity region 24 for ohmic contact. Here, an offset region 32 having a predetermined distance from gate line 12 is formed in heavily doped impurity region 24. Thereafter, the photoresist pattern is removed, an insulating layer (not shown) having source and drain contact holes, a data line and a pixel electrode (not shown) are formed on semiconductor layer pattern 14, to thereby accomplish the TFT-LCD according to the second embodiment of the present invention. In this case, offset region 32 reduces the leakage current .

Figs. 7A and 7B are plane views showing a method of fabricating a TFT-LCD having an LDD structure in its source and drain regions according to a third embodiment of the present invention. Referring to Fig. 7A, the formation of gate line 12, gate insulating layer (not shown), semiconductor layer pattern 14 and heavily doped impurity region 24 including offset region 32 are formed on insulating substrate 10 through processes of Figs. 6A, 6B and 6C. Then, a photoresist pattern 34 is formed on gate line 12 in self-alignment through a backside exposure using gate line 12, to thereby expose offset region 32.

Referring to Fig. 7B, an impurity having the same conductivity as that of heavily doped impurity region 24 is lightly ion- implanted into the exposed portion of semiconductor layer pattern 14, to form a lightly doped impurity region 36, thereby forming an LDD structure. Thereafter, photoresist pattern 34 is removed, and the data line and pixel electrode (not shown) are formed, accomplishing the TFT having the LDD structure.

INDUSTRIAL APPLICABILITY

As described above, according to the TFT -LCD of the present invention, the benefit of the TFT-LCD is that the semiconductor layer pattern is projected from one side of the gate line at a portion where the semiconductor layer pattern is superposed on the gate line, and projected from both sides of the data line at a portion where the data line is superposed on the semiconductor layer pattern. One end of the semiconductor layer pattern comes into contact with the data line, and the other end is connected to the pixel electrode. Furthermore, the offset region or LDD region may be formed in the semiconductor layer pattern. By doing so, the fabrication process is simplified and the production yield and the aperture rate are increased, while the leakage current is reduced, thereby improving the picture quality and luminance of the TFT-LCD.

Claims

WHAT IS CLAIMED IS:

1. A TFT-LCD comprising: a gate line formed in one direction on an insulating substrate; a semiconductor layer pattern superposed on the gate line, one end of the semiconductor layer pattern being projected from top to bottom on both sides of the gate line, the other end of the semiconductor layer pattern being projected in one direction from the top of one side of the gate line; a data line formed so that the data line is intersected perpendicularly with the gate line and superposed on a portion projected from both sides of the gate line in the semiconductor layer pattern; and, a pixel electrode contacted with the other portion projected from the end of the semiconductor layer pattern.

2. The TFT-LCD as claimed in claim 1, wherein the pixel electrode is formed of ITO, Sn0₂ or In₂0₃-ZnO.

3. The TFT-LCD as claimed in claim 1, further comprising an offset region placed at a projected portion where the other end of the semiconductor layer pattern comes into contact with the gate line.

4. The TFT-LCD as claimed in claim 1, further comprising an LDD region placed at the projected portion where the other end of the semiconductor layer pattern comes into contact with the gate line.

5. A method of fabricating a TFT-LCD comprising the steps of: forming a gate line on an insulating layer; forming a gate insulating layer on the overall surface of the substrate; forming a semiconductor layer pattern superposed on the gate line, one end of the semiconductor layer pattern being projected from top to bottom on both sides of the gate line, the other end of the semiconductor layer pattern being projected in one direction from the top of one side of the gate line; forming a heavily doped impurity region in a portion of the semiconductor layer pattern where the semiconductor layer pattern is not superposed on the gate line; forming an insulating layer on the semiconductor layer pattern; removing a portion of the insulating layer placed on the heavily doped impurity region, to form source and drain contact holes; and forming a data line coming into contact with the heavily doped impurity region, the data line being intersected with the gate line.

6. The method of fabricating a TFT-LCD as claimed in claim 5, wherein a portion of the heavily doped impurity region, which comes into contact with a pixel electrode, includes an offset region having a predetermined distance from the gate line.

7. The method of fabricating a TFT-LCD as claimed in claim 5, further comprising the step of forming a lightly doped impurity region at the projected portion where the other end of the semiconductor comes into contact with the gate line.

8. The method of fabricating a TFT-LCD as claimed in claim 7, wherein the lightly doped impurity region is formed in such a manner that a photoresist pattern is formed through a backside exposure using the gate line, and then an impurity is ion-implanted using the photoresist pattern as an ion implantation mask.

9. The method of fabricating a TFT-LCD as claimed in claim 5, further comprising the steps of forming a pixel electrode coming into contact with the semiconductor layer pattern before the step of forming the gate line, and forming a source electrode through a separate process.

10. The method of fabricating a TFT-LCD as claimed in claim 5, further comprising the step of forming a pixel electrode and which comes into contact with the semiconductor layer pattern after the step of forming the source and drain contact holes, and then forming the source electrode.