# Patent

## Title
LCD pixel array structure

## Abstract

Only five photomasks are used to fabricate a LCD pixel array structure. A gate dielectric layer of the LCD pixel array structure is formed by two deposition steps to increase the storage capacity of the storage capacitor.

## Background
RELATED APPLICATIONS
This application is a division of U.S. patent application Ser. No. 11/947,118, filed on Nov. 29, 2007, which claimed priority to Taiwan Application Serial Number 95144783, filed Dec. 1, 2006, which is herein incorporated by reference.


BACKGROUND
1. Field of Invention
The present invention relates to a liquid crystal display (LCD) and a fabrication method thereof. More particularly, the present invention relates to a LCD pixel array structure and a fabrication method thereof.
2. Description of Related Art
The ongoing of the optoelectronic technology in addition to the coming of the digital times propel the vigorous developing of the LCD market. Since LCDs have advantages of high display quality, small volume, light weight, low driving voltage, and low consuming power, LCDs are widely applied in consumable electronic products such as personal digital assistant (PDA), mobile phones, video recorders, notebook computers, displays of personal computers, displays used in cars, and projection televisions. Hence, LCDs have gradually replaced the cathode ray tube (CRT) displays and become the mainstream of displays in the market
The fabrication method of the LCDs is mainly composed by deposition, photolithography, and etching processes. In these three processes, the cost of photolithography is the highest. Hence, how to reduce the number of photolithography processes, that is, how to reduce the number of photomasks, in the whole fabrication processes of a TFT array substrate is an important issue in LCD fabrication.
SUMMARY
According to an embodiment of this invention, a pixel array structure of a liquid crystal display is provided.
At least a silicon island is on an active device area of a substrate, and two ends of the silicon island respectively has a doped area to be a source and a drain of a thin film transistor. A first gate dielectric layer is on the silicon island and the substrate. A data line is on an external side of the active device area. At least a first electrode is on a first gate dielectric with a capacitor area of the substrate underneath. A second gate dielectric layer is on the first gate dielectric layer, the data line and the first electrode. At least a gate is on the second gate dielectric layer with the central of the silicon island underneath. At least a second electrode is on the second gate dielectric layer with the first electrode underneath, wherein the first electrode, the second gate dielectric layer and the second electrode constitute a first storage capacitor. A dielectric layer is on the second dielectric layer, the gate and the second electrode, and a first, a second, a third, and a fourth openings in the dielectric layer respectively exposing the data line, the source, the drain, and the first electrode. A pixel electrode is on the first dielectric layer with a pixel area underneath, wherein the second electrode, the dielectric layer and the pixel electrode constitute a second storage capacitor. A first conductive line is on the dielectric layer and in the first opening and the second opening to connect the data line and the source. A second conductive line is on the dielectric layer and the in the third opening and the fourth opening to connect the drain and the first electrode.
According to an embodiment of this invention, a liquid crystal display using the pixel array structure described above is also provided. For example, the pixel array structure described above is disposed on a first substrate. A color filter array structure is disposed on a second substrate. Then, a liquid crystal layer is sandwiched between the first and the second substrate to constitute the liquid crystal display.
It is to be understood that both the foregoing general description and the following detailed description are made by use of examples and are intended to provide further explanation of the invention as claimed.



BRIEF DESCRIPTION OF THE DRAWINGS
The invention can be more fully understood by reading the following detailed description of the embodiment, with reference made to the accompanying drawings as follows:
FIGS. 1-5 are cross-sectional diagrams showing various steps of fabricating a pixel array structure according to an embodiment of this invention; and
FIG. 6 is a cross-sectional diagram showing a general liquid crystal display.



DETAILED DESCRIPTION
FIGS. 1-5 are cross-sectional diagrams showing various steps of fabricating a pixel array structure according to an embodiment of this invention
In FIG. 1, a semiconductor layer, such as a polysilicon layer or an amorphous layer, is formed on a substrate 100 having an active device area 102, a capacitor area 104, and a pixel area 106. The semiconductor layer, i.e. the polysilicon layer, is then patterned to form a patterned semiconductor layer, such as a silicon island 115, on the active device area 102. The forming method and the patterning method of the polysilicon layer are, for example, chemical vapor deposition and photolithography/etching, respectively.
In FIG. 2, a first gate dielectric layer 120 and a first metal layer are sequentially formed on the substrate 100 and the silicon island 115. The first metal layer is then patterned to form a data line 125 on the external side of the active device area 102 and a first electrode 130, respectively.
The material of the first gate dielectric layer 120 is, for example, silicon oxide, silicon nitride, or silicon oxynitride. The forming method of the first gate dielectric layer 120 is, for example, chemical vapor deposition. The material of the first metal layer is, for example, aluminum-copper alloy, copper, or other proper conductive materials. The forming method of the first metal layer can be physical or chemical vapor deposition, and the patterning method of the first metal layer can be photolithography and etching.
In FIG. 3, a second gate dielectric layer 135 and a second metal layer are sequentially formed on the first dielectric layer 120, the data line 125 and the first electrode 130. The second metal layer is then patterned to form a gate 140 above the silicon island 115 and a second electrode 145 above the first electrode 130. Next, dopants are implanted into the silicon island 115 by using the gate 140 as an implantation mask.
The overlapped parts of the first electrode 130, the second gate dielectric layer 135 and the second electrode 145 constitute a first storage capacitor. The first storage capacitor can therefore be formed simultaneously without any additional photomask during the formation of the data line 125 and the first electrode 130.
An additional wet etching process can be further performed to etch gate 140 for reducing the size of the gate 40. Then, an ion implantation process is performed to form lightly doped drain 160 on the internal sides of the source 150 and the drain 155 by using the reduced gate 140 as the implantation mask to improve the operation properties of the transistor composed of the gate 140, the source 150, and the drain 155.
The material of the second gate dielectric layer 135 can be, for example, silicon oxide, silicon nitride, or silicon oxynitride. The forming method of the second gate dielectric layer 135 can be, for example, chemical vapor deposition. The material of the second metal layer can be, for example, aluminum-copper alloy, copper, or other proper conductive layers. The forming method of the second metal layer can be physical or chemical vapor deposition, and the patterning method of the second metal layer can be photolithography and etching.
In FIG. 4, a dielectric layer 165 and a transparent conductive layer 170 are sequentially formed on the second gate dialectic layer 135, the gate 140 and the second electrode 145. The dielectric layer 165 and the transparent conductive layer 170 are then patterned to form openings 175, 180, 185, and 190 to respectively expose the data line 125, the source 150, the drain 155, and the first electrode 130.
The material and the forming method of the dielectric layer 165 can be, for example, silicon oxide and chemical vapor deposition, respectively. The material of the transparent conductive layer 170 can be, for example, indium tin oxide, aluminum zinc oxide, or indium zinc oxide. The forming method of the transparent conductive layer 170 can be, for example, chemical or physical vapor deposition. The patterning method of the dielectric layer 165 and the transparent conductive layer 170 can be, for example, photolithography and etching.
In FIG. 5, a third metal layer is formed on the transparent conductive layer 170 and in the openings 145, 180, 185, and 190. The third metal layer and the transparent conductive layer 170 are then patterned to form a pixel electrode 170a on the pixel area 105, a first conductive line 195 between the source 150 and the data line 125, and the second conductive line 200 between the drain 155 and the first electrode 130. The overlapped parts of the second electrode 145, the dielectric layer 165, and the pixel electrode 170a constitute a second storage capacitor.
The material of the third metal layer can be, for example, aluminum-copper alloy, copper, or other proper conductive material. The forming method of the third metal layer can be physical or chemical vapor deposition. The patterning method of the third metal layer and the transparent conductive layer 170 can be photolithography and etching. There are two types of removed depths in the step of pattering the third metal layer and the transparent conductive layer 170. One type of removed depth is to remove only the third metal layer on the exposed area of the pixel electrode 170a, the other type of removed depth is to remove both the third metal layer and the transparent conductive layer 170 outside the exposed pixel electrode 170a. Hence, the photomask used in the photolithography can be, for example, a half-tone photomask.
The pixel array structure can be applied on any suitable flat panel display, such as liquid crystal display. FIG. 6 is a cross-sectional diagram showing a general liquid crystal display. In FIG. 6, liquid crystal display 200 has a first substrate 205, a second substrate 210, and a liquid crystal layer 215 therebetween. If the pixel array structure is disposed on the first substrate 205, the color filter array can be disposed on the second substrate 210. Since various modifications for liquid crystal display are well known by persons skilled in the art and are thus omitted here.
From the embodiments described above, only five photomasks are needed to accomplish the pixel array structure. Moreover, the storage capacitor is made by series connecting the first and the second storage capacitor from FIG. 5. Further, only the first gate dielectric layer is the dielectric layer of the first storage capacitor, and the first electrode and the second electrode of the first storage capacitor are made by metal. These features can largely increase the storage capacitance of the storage capacitor.
It will be apparent to those skilled in the art that various modifications and variations can be made to the structure of the present invention without departing from the scope or spirit of the invention. In view of the foregoing, it is intended that the present invention cover modifications and variations of this invention provided they fall within the scope of the following claims and their equivalents.

## Claims

1. A pixel array structure of a liquid crystal display, comprising:
at least a patterned semiconductor layer on an active device area of a substrate, two ends of the patterned semiconductor layer respectively having a doped area to be a source and a drain of a thin film transistor;
a first gate dielectric layer on the patterned semiconductor layer and the substrate;
a data line on an external side of the active device area;
at least a first electrode on a first gate dielectric with an capacitor area of the substrate underneath;
a second gate dielectric layer on the first gate dielectric layer, the data line and the first electrode;
at least a gate on the second gate dielectric layer with the central of the patterned semiconductor layer underneath;
at least a second electrode on the second gate dielectric layer with the first electrode underneath, wherein the first electrode, the second gate dielectric layer and the second electrode constitute a first storage capacitor;
a dielectric layer on the second gate dielectric layer, the gate, and the second electrode;
a first, a second, a third and a fourth openings in the dielectric layer, the second gate dielectric layer, and the first dielectric layer respectively exposing the data line, the source, the drain, and the first electrode;
a pixel electrode on the first dielectric layer on a pixel area, wherein the second electrode, the dielectric layer and the pixel electrode constitute a second storage capacitor;
a first conductive line on the dielectric layer and in the first opening and the second opening to connect the data line and the source; and
a second conductive line on the dielectric layer and the in the third opening and the fourth opening to connect the drain and the first electrode.

2. The pixel array structure of claim 1, wherein a material of the patterned semiconductor layer comprises polysilicon or amorphous silicon.

3. The pixel array structure of claim 1, wherein a material of the first gate dielectric layer comprises silicon oxide, silicon nitride, or silicon oxynitride.

4. The pixel array structure of claim 1, wherein a material of the second gate dielectric layer comprises silicon oxide, silicon nitride, or silicon oxynitride.

5. The pixel array structure of claim 1, wherein a material of the dielectric layer comprises silicon oxide.

6. The pixel array structure of claim 1, wherein a material of the transparent conductive layer is indium tin oxide, indium zinc oxide, or aluminum zinc oxide.

7. A liquid crystal display, comprises:
a pixel array substrate having a pixel array structure;
a color filter substrate; and
a liquid crystal layer between the pixel array substrate and the color filter substrate,
wherein the pixel array structure comprises:
at least a patterned semiconductor layer on an active device area of a substrate, two ends of the patterned semiconductor layer respectively having a doped area to be a source and a drain of a thin film transistor;
a first gate dielectric layer on the patterned semiconductor layer and the substrate;
a data line on an external side of the active device area;
at least a first electrode on a first gate dielectric above an capacitor area of the substrate;
a second gate dielectric layer on the first gate dielectric layer, the data line and the first electrode;
at least a gate on the second gate dielectric layer above the central of the patterned semiconductor layer;
at least a second electrode on the second gate dielectric layer above the first electrode, wherein the first electrode, the second gate dielectric layer and the second electrode constitute a first storage capacitor;
a dielectric layer on the second gate dielectric layer, the gate, and the second electrode;
a first, a second, a third and a fourth openings in the dielectric layer, the second gate dielectric layer, and the first dielectric layer respectively exposing the data line, the source, the drain, and the first electrode;
a pixel electrode on the first dielectric layer on a pixel area, wherein the second electrode, the dielectric layer and the pixel electrode constitute a second storage capacitor;
a first conductive line on the dielectric layer and in the first opening and the second opening to connect the data line and the source; and
a second conductive line on the dielectric layer and the in the third opening and the fourth opening to connect the drain and the first electrode.