KR20010002574A - The structure of thin film transistor in reflection type LCD and method of forming it - Google Patents

Abstract

PURPOSE: A TFT for an LCD and a forming method thereof are to reduce a process number in manufacturing a bottom gate type amorphous TFT LCD thus to simplify a manufacturing process of the TFT LCD. CONSTITUTION: A TFT LCD comprises a gate electrode(13), a gate line and a gate pad which are formed on a glass substrate(11); a gate isolation layer(15) and an amorphous silicon layer(17) which are serially formed on a region including the gate electrode, the gate line and the gate pad excluding a contact thereof; a source electrode, a drain electrode, a data line, and a data pad which are formed by serially depositing and patterning an ohmic contact layer(19) and a metallic layer on the amorphous silicon layer; a barrier layer which is formed on a region that includes the source electrode, the drain electrode, the data line, and the data pad but excludes the gate electrode, the gate line and the gate pad contact; and an opaque pixel electrode which is formed on the barrier layer and electrically connected to the source electrode through the contact.

Description

The structure of thin film transistor in reflection type LCD and method of forming it}

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a thin film transistor of a reflective liquid crystal display and a method of forming the same, and more particularly, to a thin film transistor and a method of forming the same in a bottom gate amorphous TFT LCD.

The importance of information display devices is very important in the development of the information society, and among the information display devices, LCD is the most rapidly developing field at present. In particular, TFT LCD, which uses thin film transistors to control the pixels, has the advantages of LCD, which is light weight, thin, and low power consumption, and is positioned as a high-quality information display device that can meet the demands of high resolution, fast operation speed, and colorization. It's widening.

TFT LCD is a typical form of an active matrix method, and an active nonlinear element called a transistor is used to control each pixel. In the LCD, unlike the semiconductor device in which the transistor element is formed on the semiconductor substrate, the transistor is formed on the glass substrate, thereby exhibiting some characteristics. TFT LCD can be divided into top gate type and bottom gate type depending on whether the gate is formed on the top or the bottom of the channel, and whether the semiconductor forming the channel is amorphous or polysilicon. Therefore, it can be divided into amorphous silicon type and active silicon type.

In either case, it is a major challenge in the process to reduce the transistor formation cost, which greatly affects the overall LCD cost, while maintaining the reliability and operating characteristics of the transistors to be formed. The key to reducing transistor formation costs is to simplify the process and reduce the number of expensive steps.

Hereinafter, a process of forming a thin film transistor of a top gate amorphous silicon type reflective LCD in which a gate is first formed on a glass substrate and an active region of a transistor device is formed of amorphous silicon due to a relatively simple process will be described.

According to the prior art, first, a single film or multiple films of aluminum or chromium are laminated on a glass substrate, and a gate electrode and a gate line are formed using photolithography and an etching process (1st mask). A gate pad is formed at the end of the gate line. Next, an amorphous silicon film for forming a gate insulating film, a channel and a source drain region is stacked over the gate pattern. An ohmic contact layer is formed on the amorphous silicon to lower the resistance in contact with the source drain electrode. The layer is doped with impurities such as phosphorous in the amorphous silicon to electrically connect the semiconductor layer with the electrode metal layer. Will increase.

The pattern is continuously formed on the three-layer film thus formed by photolithography and etching using a photomask corresponding to the active region (2nd mask). Then, a metal layer for forming a source drain electrode is stacked thereon, and a source, a drain electrode, and a data line are formed through a mask technique (3rd mask).

The protective film is stacked on the transistor basic structure thus formed. The protective film is a kind of insulating film, which is generally made of silicon oxide, but may be made thick of an organic film. After the protective film is laminated, the insulating film is removed on the gate pad or the data line pad and the source electrode to form a contact to prepare a connection with an external electrode or a pixel electrode. The process of partially removing the insulating layer also uses photolithography and an etching process (4th mask).

On this passivation layer, pixel electrodes are also formed by masking. Since the pixel electrode is a reflective liquid crystal display device, aluminum is mainly formed by sputtering to form a large portion of the pixel through photolithography and etching processes. The pixel electrode is electrically connected to the source electrode of the transistor through a contact and serves as a reflector. In the case of a backlight TFT LCD, the pixel electrode is formed of indium tin oxide (ITO) or the like (5th mask).

Therefore, the transistor electrode structure of a conventional bottom gate amorphous silicon type reflective TFT LCD is completed through a five-sheet mask process, which is cumbersome due to the mask process such as film formation, photoresist deposition, exposure, development, and etching. There were a lot.

An object of the present invention is to provide a method for completing a transistor electrode of a TFT LCD through a four-sheet mask process by improving the need for a cumbersome mask process five times in the conventional five-sheet mask method. An object of the present invention is to provide a reflective liquid crystal display device thin film transistor having a configuration that can be formed through the method.

1 shows a state in which a gate electrode is formed on a lower glass substrate that will constitute an LCD.

FIG. 2 shows a gate insulating film made of a silicon oxide film or a silicon nitride film, an amorphous silicon film, and an ohmic contact layer of an amorphous silicon material doped with phosphorus with impurities in the state of FIG. 1, and then source and drain electrodes are formed thereon. Indicates a state.

3 is a view illustrating a step of etching and removing an ohmic contact layer among the lower three layer films using an electrode pattern formed in FIG. 2 as an etching mask.

FIG. 4 illustrates a state in which a protective film is stacked on the entire surface and etched according to a predetermined pattern in the state of FIG.

FIG. 5 illustrates a state in which the pixel electrode is formed by forming a pixel electrode layer on the front surface and patterning pixel electrodes in the state of FIG.

Fig. 6 is a graph showing the source drain current value characteristics of the gate low voltage in the transistor formed by the four masks according to the present invention.

Fig. 7 shows the relationship between the used voltage and the reflectivity when the LCD is completed through this electrode.

8 is a plan view of a pixel portion showing an embodiment of the present invention.

※ Explanation of symbols for main parts of drawing

11: lower glass substrate 13: gate electrode

15: gate insulating film 17: amorphous silicon film

19: ohmic contact 21: drain electrode

23: source electrode 25, 35: protective film

27: mask pattern 37: pixel electrode

In order to achieve the above object, a thin film transistor for a reflective liquid crystal display device according to an embodiment of the present invention has a gate electrode, a gate line, a gate pad, a gate electrode, a gate line, and a gate pad formed on a glass substrate except for a contact on the gate pad. A source electrode, a drain electrode, a data line and a data pad, and a source electrode, a drain electrode, a data line, and a data, which are formed by sequentially stacking and patterning a gate insulating film, an amorphous silicon film, and an ohmic contact layer and a metal layer over the amorphous silicon film. And a passivation layer formed over the pad except for the source electrode, the gate pad, and a data pad upper contact region, and an opaque pixel electrode formed over the passivation layer and electrically connected to the source electrode through a contact. Gong.

In the thin film transistor structure formed above, the reflector may be formed of a simple flat (FLAT) type, but may be curved to widen the LCD viewing angle or to increase luminance within a certain viewing angle.

In the present invention, the amorphous silicon film is generally formed thicker in the region where the electrode, such as the source drain, is formed, than the surroundings. This is because when the electrode pattern is etched and the ohmic contact layer of the lower layer is removed using the etching mask, the upper part of the amorphous silicon film is also removed. Since the degree of etching, that is, the thickness of the amorphous silicon film becomes the thickness of the channel layer, the thickness of the amorphous silicon film should be such that there is no problem in the function of the channel.

In order to achieve the above object, a method of forming a thin film transistor for a reflective liquid crystal display device according to the present invention includes forming a gate electrode, a gate line, and a gate pad on a glass substrate, and sequentially stacking a gate insulating film, an amorphous silicon film, and an ohmic contact layer. Stacking a source drain electrode layer and forming a source drain electrode, a data line, and a data pad pattern; etching and removing the ohmic contact layer with an etch mask of the source drain electrode, the data line, and the data pad pattern; And forming contact holes on the pad portion and the source electrode, and forming an opaque pixel electrode connected to the source electrode through the contact hole.

In the present invention, each step encompasses a series of processes, such as laminating a film for forming a pattern, applying a photoresist, exposing, developing, and etching, except for etching the ohmic contact layer by using a source drain electrode pattern or the like as an etch mask. It includes the mask process to make. In the present invention, the amorphous silicon film should be left to the extent that there is no problem in forming the channel although a certain thickness of the upper portion is removed by etching.

Hereinafter, an electrode and a method for forming the same according to an embodiment of the present invention will be described in more detail with reference to the accompanying drawings.

FIG. 1 shows a state in which the gate electrode 13 is formed on the lower glass substrate 11 that will constitute the LCD. This is achieved by laminating the gate layers and patterning them with a mask technique. The gate layer is generally made of a metal material such as aluminum or chromium or a multilayer structure thereof. In this case, all of the gate electrode, the gate line, and the gate pad are formed. Usually the thickness of the gate pattern is not constant, but thousands of microseconds are common.

2, a gate insulating film 15 made of a silicon oxide film or a silicon nitride film, an amorphous silicon film 17, and an ohmic contact layer 19 made of an amorphous silicon material doped with phosphorus are sequentially stacked thereon in the state of FIG. Again, the source and drain electrodes 21 and 23 are formed thereon. The source drain electrode is formed of a conductive layer such as a metal, and data lines and pads for applying a signal to the drain electrode 21 are also formed. The source drain electrode is formed by laminating electrode layers and forming a pattern using a mask technique.

FIG. 3 is a view illustrating a step of etching and removing the ohmic contact layer 19 among the lower three layer films using the electrode pattern formed in FIG. 2 as an etching mask. The overetching of the ohmic contact layer was performed, and the upper portion of the amorphous silicon film 17, which is the same material, was also removed. Therefore, the thickness of the channel portion of the transistor region and the thickness of the amorphous silicon film in the region other than the active region are the same. Therefore, the overetching amount should be adjusted so that the amorphous silicon film has a thickness capable of appropriately acting as a channel. In general, the thickness of the amorphous silicon film under the source and drain electrodes remains slightly thicker than that of the adjacent amorphous silicon film due to overetching. Looking at the actual value, the thickness of the ohmic contact layer and the amorphous silicon film to be etched is about 1000 GPa to 1500 GPa, and the resulting amorphous silicon film is etched about 500 GPa to 1000 GPa, leaving about 1000 GPa to 15000 GPa.

In the related art, an amorphous silicon film remains as a channel in a channel part, and a silicon film is removed in all other parts except the active area. However, in the present invention, there is almost no risk of causing functional side effects of the transistor even if the amorphous silicon film remains in other parts, and since the opaque property of the amorphous silicon film does not degrade the quality of the LCD because it is a reflective LCD, the channel layer and the active layer are not required without a separate mask. The outer area can be etched simultaneously to reduce masking in one step.

6 is a graph showing the source drain current value characteristics with respect to the gate voltage in the transistor formed by the four masks in the present invention. The off current (Ioff) is 4.84E-1pA, and the on current (Ion) is 2.57uA, which shows no significant problem in the operating characteristics. FIG. 7 shows the relationship between the voltage used and the reflectivity when the LCD is completed through this electrode. . Through the above experimental values, it can be seen that the four mask process of the present invention has no problem in terms of transistor characteristics.

FIG. 4 shows a state in which the protective film 25 is laminated on the entire surface in the state of FIG. 3 and the exposure is performed according to the predetermined mask pattern 27. After exposure, development and etching are performed. In the case of using a thick photosensitive organic film, which is advantageous for flattening as a protective film, it is more convenient since the patterning can be completed by developing without additional etching. Contact holes are formed on the source region. In the patterning process of forming the contact hole, a protective film of a portion to be connected to an external electrode such as a gate pad or a data pad is generally opened together to form a contact hole. At this time, anisotropic dry etching is mainly used. However, by using an etchant having less etching property for metal and etching property for amorphous silicon film or amorphous silicon film, there is a big problem in opening the gate pad part by controlling some etching time. It is possible without. It is conceivable that the protective film is formed of an inorganic insulating layer such as a silicon oxide film or a case of being formed of an organic film having a considerable thickness having photosensitivity. In the present embodiment shown in the drawing, the protective film 25 is formed of a photosensitive organic film having a predetermined thickness.

The reflecting plate and pixel electrode to be formed in the following process may be formed as a simple flat surface, but in the present embodiment, the curved plate is formed to increase the luminance in the front direction. This curvature determines its distribution by certain calculations, which acts as a kind of microlens. In order to form such curvature, a thin slit pattern may be formed in the pixel electrode region in the step of patterning the protective layer to perform a kind of two-step tone exposure. In the two-stage tone exposure, a small amount of light is applied to a portion of the photoresist, that is, the photosensitive organic protective film, so that the amount of light is reduced by using a thin slit or the like so that only an upper layer is removed and a certain thickness is left when developing after exposure. In addition, the two-step exposure may be used while appropriately adjusting the opening of the pad part.

FIG. 5 illustrates a state in which the pixel electrode is formed by forming a pixel electrode layer on the front surface and patterning pixel electrodes in the state of FIG. The contact window of FIG. 4 is covered with a pixel electrode material. For the material of the electrode, aluminum metal is commonly used for the reflector, and sputtering method is mainly used. After forming the pixel electrode layer, a mask technique is used to form the pixel electrode pattern. The pixel electrode 37 is connected to the source electrode through a contact, and a bend is formed on the surface of the pixel electrode 37 according to the bend of the surface of the lower passivation layer 35. To this end, the thickness of the reflective film should not be significantly thicker than the bending on the protective film. Figure 5 is one embodiment showing the electrode of the present invention at the same time.

8 is a plan view of a pixel portion showing an embodiment of the present invention. A pixel electrode serving as a reflective film is covered over the entire pixel portion. Most of the opaque regions of the signal lines such as the gate lines and the data lines are formed to overlap the pixel regions. In the case of the transmissive TFT LCD, the signal lines overlap the area where the black matrix is formed between the pixel parts to increase the aperture ratio. However, in the case of the reflective type, when the signal line is present in the area between the pixels, it may act as a reflective film. In order to improve the contrast ratio, it is preferable to make it overlap with the reflective film in this way.

According to the method of the present invention, one mask process can be reduced by using the reflection type property that the LCD thin film transistor is not required to be transparent in the film below the reflector, thereby reducing the process cost.

Claims (8)

A gate electrode, a gate line, a gate pad formed on a glass substrate, A gate insulating film and an amorphous silicon film sequentially stacked on the gate electrode, the gate line, and the gate pad except for a contact on the gate pad; A source electrode, a drain electrode, a data line, and a data pad formed by sequentially stacking and patterning an ohmic contact layer and a metal layer on the amorphous silicon film; A passivation layer formed on the source electrode, the drain electrode, the data line, and the data pad except for an upper contact region of the source electrode, the gate pad, and the data pad; And an opaque pixel electrode formed over the passivation layer and electrically connected to the source electrode through contact with the source electrode. The method of claim 1, The protective film is a thin film transistor for a reflective liquid crystal display, characterized in that formed by a photosensitive organic insulating film. The method of claim 2, The pixel electrode is a thin film transistor for a reflective liquid crystal display device, characterized in that to form a bend to have a predetermined distribution in the region to act as a micro lens. The method according to any one of claims 1, 2 or 3, A thin film transistor for a reflective liquid crystal display device, wherein an amorphous silicon film in a region where the electrode is formed such that the source drain is thicker than a surrounding amorphous silicon film. The method according to any one of claims 1, 2 or 3, And the data line and the gate line overlap the pixel electrode. Forming a gate electrode, a gate pad, and a gate line on the lower glass substrate, Stacking a gate insulating film, an amorphous silicon film, and an ohmic contact layer, stacking a source drain electrode layer, and then forming a pattern of a source drain electrode, a data line, and a data pad, Etching the ohmic contact layer by using the source drain electrode, the data line, and the data pad pattern as an etch mask; Stacking and patterning a protective film to form a contact on the top of the source electrode and the pad part; and And forming an opaque pixel electrode electrically connected to the source electrode. The method of claim 6, And the exposure is performed in two steps tones in the patterning of the passivation layer so that a bend is formed on an upper surface of the passivation layer in the pixel electrode region. The method according to claim 6 or 7, The protective film is a method of forming a thin film transistor for a reflective liquid crystal display device, characterized in that using a photosensitive organic insulating film.