US20070176180A1

US20070176180A1 - Polysilicon structure, thin film transistor panel using the same, and manufacturing method of the same

Info

Publication number: US20070176180A1
Application number: US11/689,687
Authority: US
Inventors: Chien-Chou Hsu; Tsung-Yi Hsu
Original assignee: AU Optronics Corp
Current assignee: AU Optronics Corp
Priority date: 2004-07-09
Filing date: 2007-03-22
Publication date: 2007-08-02
Also published as: US20060006390A1; TWI285783B; US7303981B2; US20070166898A1; TW200602776A

Abstract

A method for forming a polysilicon structure is provided. An amorphous silicon structure with a first amorphous silicon region and a second amorphous silicon region is formed in a first region and a second region of a substrate, respectively. The first amorphous silicon region is thinner than the second amorphous silicon region. The amorphous silicon structure is crystallized to form the polysilicon structure with a first polysilicon region and a second polysilicon region corresponding to the first amorphous silicon region and the second amorphous silicon region.

Description

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a Continuation of pending U.S. patent application Ser. No. 11/000,837, filed Nov. 30, 2004 and entitled “POLYSILICON STRUCTURE, THIN FILM TRANSISTOR PANEL USING THE SAME, AND MANUFACTURING METHOD OF THE SAME,” the contents of which are incorporated herein by reference.

BACKGROUND

The present invention relates to a low temperature polysilicon film and in particular to a polysilicon structure formed by crystallization of an amorphous silicon structure, methods for forming the same, and devices utilizing the film.
Typically, thin film transistors (TFTs) are used as the active devices in active matrix flat panel displays. For example, TFTs are employed to drive the liquid crystal display (LCD) or the organic light emitting display (OLED).
Conventionally, hydrogenated amorphous silicon (α—Si: H) is used as the semiconductor film (active layer) of a TFT. Polysilicon, however, may provide higher electron transmission than amorphous silicon due to more regular crystal orientation. Thus a development trend is to utilize the polysilicon instead of amorphous silicon in TFT technology.
Typically, there are three methods for forming the thin polysilicon film. In a first method, the polysilicon film is formed by deposition. In this method, the polysilicon film requires a sufficient thickness, so as to grow large grains. Thus, the polysilicon film has a poor surface uniformity that adversely affects the formation of the subsequent gate insulating layer. Moreover, the deposition temperature is high (600° C.), which also adversely affects the fabrication of the device. In a second method, thermal treatment is performed on an amorphous silicon layer, so as to transfer to a polysilicon layer. In this method, the polysilicon layer may be less thick and have a better surface uniformity. However, a high deposition temperature (6002C) and long deposition duration are required and the thermal budget increases. As a result, throughput and device reliability suffer. In a third method, perform a laser treatment to transfer an amorphous silicon layer into a polysilicon layer. The method is the most commonly used.
The polysilicon TFT display comprises a display region and a driving circuit region, wherein the switching devices on the driving circuit region require higher switching rate and readability. That is, the switching devices on the driving circuit region preferably have higher electron transmission and better sub-threshold swing. A polysilicon layer with large grain size can provide such electronic characteristics. Additionally, current leakage in the display region must be low. If the surface roughness of the polysilicon layer is too high, poor coverage of the gate insulating layer thereon results, thus increasing current leakage. If the grain size of the polysilicon layer is small, low surface roughness results, improving the coverage of the gate insulating layer thereon and reducing current leakage. That is, in order to improve the electronic characteristics of the polysilicon TFT display, the polysilicon layer on the driving circuit region must have a relatively large grain size and that on the display region a relatively small grain size.
In order to form a polysilicon film with different grain sizes on the driving circuit region and the display region, respectively, for fabrication of the polysilicon TFT display, the driving circuit region and the display region must be respectively treated. For example, a laser treatment with a relatively low scanning rate is performed on the amorphous silicon layer on the driving circuit region, to form the polysilicon layer with a relatively large grain size. The laser treatment with a relatively high scanning rate is subsequently performed on the amorphous silicon layer on the display region, to form the polysilicon layer with relatively small grain size. The problems presented by laser alignment and mask changes may reduce yield and throughput. Thus, an improved method for forming polysilicon layers with different grain sizes is desirable.

SUMMARY

Embodiments of the invention provide a polysilicon structure on a substrate. The polysilicon structure comprises a first polysilicon region and a second polysilicon region. A thickness of the first polysilicon region is less than that of the second a thickness and a grain size of the first polysilicon region is larger than that of the second polysilicon region.
Embodiments of the invention additionally provide a method for forming a polysilicon structure. A substrate having a first region and a second region is provided. An amorphous silicon structure with a first amorphous silicon region and a second amorphous silicon region is formed in the first region and the second region, respectively. The first amorphous silicon region is thinner than the second amorphous silicon region. The amorphous silicon structure is crystallized to form the polysilicon structure with a first polysilicon region and a second polysilicon region corresponding to the first and second amorphous silicon regions, respectively.
Embodiments of the invention further provide a method for forming a polysilicon structure. A substrate having a first region, a second region, and a third region is provided. An amorphous silicon layer with a first amorphous silicon region, a second amorphous silicon region, and a third amorphous silicon region, is formed on the substrate, in which the first, second and third amorphous silicon regions, are respectively in the first region, the second region and the third region. The first amorphous silicon region is thinner than the second amorphous silicon region, which is thinner than the third amorphous silicon region. The amorphous silicon structure is crystallized to form the polysilicon structure with a first polysilicon region, a second polysilicon region and a third polysilicon region corresponding to the first, second and third amorphous silicon regions, respectively.
Embodiments of the invention still further provide a flat panel display. The display comprises a first transistor and a second transistor. The first transistor is disposed on the substrate and comprises a first polysilicon region serving as a first active region. The second transistor is disposed on the substrate and comprises a second polysilicon region serving as a second active region. Moreover, the first polysilicon region is thinner than the second polysilicon region and a grain size of the first polysilicon region is larger than that of the second polysilicon region.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments of the invention will become more fully understood from the detailed description given hereinbelow and the accompanying drawings, given by way of illustration only and thus not intended to be limitative of the invention.
FIGS. 1A to 1C are cross-sections of a method for forming a polysilicon structure of the first embodiment of the present invention.
FIGS. 2A to 2D are cross-sections of a method for forming a polysilicon structure of the second embodiment of the present invention.
FIGS. 3A to 3B are cross-sections of a method for forming a polysilicon structure of the third embodiment of the present invention.
FIGS. 4A to 4C are cross-sections of a method for forming thin film transistors for a flat panel display of the fourth embodiment of the present invention.

DETAILED DESCRIPTION

In the first embodiment, as shown in FIG. 1A, a substrate 100 comprising a driving circuit region 1 and a display region 2 is provided. A first amorphous silicon layer 110 is formed on the substrate 100 corresponding to the display region 2 of the substrate 100 by deposition, lithography, and etching. For example, an amorphous silicon layer (not shown) is formed by chemical vapor deposition (CVD). Thereafter, lithography and etching are performed on the amorphous silicon layer to form the first amorphous silicon layer 110 in the display region 2 of the substrate 100, wherein the etching may comprise wet or dry etching.
A second amorphous silicon layer 120 is subsequently formed on the substrate 100 corresponding to the driving circuit region 1 of the substrate 100 and covers the first amorphous silicon layer 110 in the display region 2, as shown in FIG. 1B. As a result, an amorphous silicon structure with a first amorphous silicon region 121 a in the driving circuit region 1 and a second amorphous silicon region 121 b in the display region 2 is formed on the substrate 100. The difference in thickness between the first amorphous silicon region 121 a in the driving circuit region 1 and the second amorphous silicon region 121 b in the display region 2 must be maintained within a specific range. If the difference in thickness of the first amorphous silicon region 121 a and the second amorphous silicon region 121 b is too low, the difference in grain size therebetween is small, thus, the electronic characteristics of the polysilicon TFT are reduced. Conversely, if the difference in thickness of the first amorphous silicon region 121 a and the second amorphous silicon region 121 b is too high, further adjustment of process conditions is required, such as the ion implanting energy and dosage, complicating the process. Therefore, the difference in thickness must be maintained within a range of about 100 to 1000 Å, and preferably 200 to 400 Å.
The amorphous silicon structure is crystallized. That is, crystallization is simultaneously performed on the first and second amorphous silicon regions 121 a and 121 b, such that the amorphous silicon structure is transferred into a polysilicon structure 130 with a first polysilicon region 130 a and a second polysilicon region 130 b corresponding to the first amorphous silicon region 121 a and a second amorphous silicon region 121 b, respectively, as shown in FIG. 1C. In some embodiments, the crystallization may comprise a laser treatment that employs, for example, an excimer laser, continuous wave laser (CW laser), or laser beam pulse. Moreover, the laser treatment may comprise lateral solidification (LS), sequential lateral solidification (SLS), continuous grain silicon (CGS), or metal induced lateral crystallization (MILC).
Since the thickness of the second amorphous silicon layer 120 in the driving circuit region 1 is less than the total thickness of the first and second amorphous silicon layers 110 and 120 in the display region 2, the meltability of the second amorphous silicon layer 120 in the driving circuit region 1 after the laser treatment, is higher than that of the first and second amorphous silicon layers 110 and 120 in the display region 2. Accordingly, the first polysilicon region 130 a in the driving circuit region 1 can be formed with larger grain size, thereby effectively improving electron mobility and sub-threshold swing. Conversely, since the total thickness of the first and second amorphous silicon layers 110 and 120 in the display region 2 is thicker than that of the second amorphous silicon layer 120 in the driving circuit region 1, the meltability of the first and second amorphous silicon layers 110 and 120 in the display region 2 after the laser treatment, is lower than that of the second amorphous silicon layer 120 in the driving circuit region 1. Accordingly, the second polysilicon region 130 b in the display region 2 can be formed with smaller grain size, thereby lowering the surface roughness of the polysilicon layer 130 in the display region 2 to reduce current leakage. In the first embodiment, the mentioned potential advantages can be obtained with one laser treatment procedure, eliminating problems presented by laser alignment and mask changes, thus reducing process time.
In the second embodiment, as shown in FIG. 2A, a substrate 200 comprising a driving circuit region 3 and a display region 4 is provided. A first amorphous silicon layer 210 is formed on the substrate 200. For example, the amorphous silicon layer 210 is formed by chemical vapor deposition (CVD).
Thereafter, the first amorphous silicon layer 210 in the driving circuit region 3 is partially removed by lithography and etching to form the first amorphous silicon layer 210 with differing thicknesses respectively in the driving circuit region 3 and display region 4, as shown in FIG. 2B. Here, the etching may comprise wet or dry etching. Moreover, the difference in thickness between the first amorphous silicon layer 210 in the driving circuit region 3 and that in the display region 4 must be maintained within a specific range.
A second amorphous silicon layer 220 is subsequently formed on the first amorphous silicon layer 210 in the driving circuit region 3 and the display region 4, as shown in FIG. 2C. As a result, an amorphous silicon structure with a first amorphous silicon region 221 a in the driving circuit region 3 and a second amorphous silicon region 221 b in the display region 4 is formed on the substrate 100. Since the difference in thickness between the first amorphous silicon layer 210 in the driving circuit region 3 and that in the display region 4 is maintained within a specific range, the difference in thickness can be maintained within a specific range after forming the second amorphous silicon layer 220 on the first amorphous silicon layer 210. As mentioned, if the difference in thickness is too low, the difference in grain size between the subsequently formed polysilicon silicon layer in the driving circuit region 3 and that in the display region 4 is small, reducing the electronic characteristics of the polysilicon TFT. Conversely, if the difference in thickness is too high, further adjustment of process conditions is required, such as the ion implanting energy and dosage, complicating the process. Thus, the difference in thickness must be maintained within a range of about 100 to 1000Å, and preferably 200 to 400 Å.
The amorphous silicon structure is crystallized. That is, crystallization is simultaneously performed on the first and second amorphous silicon regions 221 a and 221 b, such that the amorphous silicon structure is transferred into a polysilicon structure 230 with a first polysilicon region 230 a and a second polysilicon region 230 b corresponding to the first amorphous silicon region 221 a and a second amorphous silicon region 221 b, respectively, as shown in FIG. 2D. In the second embodiment, the crystallization may comprise a laser treatment that employs, for example, an excimer laser, continuous wave laser (CW laser), or laser beam pulse. Moreover, the laser treatment may comprise lateral solidification (LS), sequential lateral solidification (SLS), continuous grain silicon (CGS), or metal induced lateral crystallization (MILC).
Since the thickness of the second amorphous silicon layer 220 in the driving circuit region 3 is less than the total thickness of the first and second amorphous silicon layers 210 and 220 in the display region 4, the meltability of the second amorphous silicon layer 220 in the driving circuit region 3 after the laser treatment, is higher than that of the first and second amorphous silicon layers 210 and 220 in the display region 4. Accordingly, the first polysilicon region 230 a in the driving circuit region 3 can be formed with larger grain size, thereby effectively improving electron mobility and sub-threshold swing. Conversely, since the total thickness of the first and second amorphous silicon layers 210 and 220 in the display region 4 is greater than that of the second amorphous silicon layer 220 in the driving circuit region 3, the meltability of the first and second amorphous silicon layers 210 and 220 in the display region 4 after the laser treatment, is lower than that of the amorphous silicon layer 220 in the driving circuit region 3. Accordingly, the second polysilicon region 230 b on the display region 4 can be formed with smaller grain size, thereby lowering the surface roughness of the polysilicon layer 230 in the display region 4 to reduce current leakage. In the second embodiment, the mentioned potential advantages can be obtained with one laser treatment procedure, eliminating problems presented by laser alignment and mask changes, thus reducing process time.
In the third embodiment, as shown in FIG. 3A, a substrate 300 comprising a first region 5, a second region 6 and a third region 7 is provided. An amorphous silicon structure 310 with a first amorphous silicon region 310 a in the first region 5, a second amorphous silicon region 310 b in the second region 6 and a third amorphous silicon region 310 c in the third region 7 is formed on the substrate 300, wherein the thicknesses of the amorphous silicon structure 310 in the first, second, and third regions 5, 6, and 7 are different. For example, the first amorphous silicon region 310 a in the first region 5 has a thickness less than that of the third amorphous silicon region 310 c in the third region 7, and the second amorphous silicon region 310 b in the second region 6 has a thickness between that of the first and third amorphous silicon regions 310 a and 310 c in the first and third regions 5 and 7.
The difference in thickness between the amorphous silicon structure 310 in the first region 5 and the third region 7 must be maintained within a specific range. If the difference in thickness is too low, the difference in grain size between the subsequently formed polysilicon silicon structure in different regions is small, thus the electronic characteristics of the polysilicon TFT are reduced. Conversely, if the difference in thickness is too high, further adjustment of process conditions is required, such as the ion implanting energy and dosage, complicating the process. In the third embodiment, the difference in thickness may be maintained within a range of about 100 to 1000 Å, and preferably 200 to 400 Å.
The amorphous silicon structure 310 is crystallized. That is, crystallization is simultaneously performed on the first, second, and third amorphous silicon regions 310 a, 310 b, and 310 c, such that the amorphous silicon structure 310 is transferred into a polysilicon structure 330 with a first polysilicon region 330 a, a second polysilicon region 330 b, and a third polysilicon region 330 c corresponding to the first, second, and third amorphous silicon regions 310 a, 310 b, and 310 c, respectively, as shown in FIG. 3B. In the third embodiments, the crystallization may comprise a laser treatment that employs, for example, an excimer laser, continuous wave laser (CW laser), or laser beam pulse. Moreover, the laser treatment may comprise lateral solidification (LS), sequential lateral solidification (SLS), continuous grain silicon (CGS), or metal induced lateral crystallization (MILC).
Since the thicknesses (surface levels) of the amorphous silicon structure 310 in the first, second, and third regions 5, 6, and 7 are successively increased, the meltability of the amorphous silicon structure 310 in the first, second, and third regions 5, 6, and 7 are successively decreased after the laser treatment. Accordingly, the grain sizes and surface roughness of the subsequently formed polysilicon layer 330 in the first, second, and third regions 5, 6, and 7 are successively decreased. That is, the polysilicon structure 330 with different grain sizes in different regions 5, 6, and 7 can be accomplished by performing one laser treatment procedure on the amorphous silicon structure 310 having different thicknesses in different regions 5, 6, and 7, thereby eliminating problems presented by laser alignment and mask changes, thus reducing process time.
In the fourth embodiment, as FIG. 4C illustrates, thin film transistors for a flat panel display are provided. The thin film transistors are disposed on a substrate 400 comprising a driving circuit region 8 and a display region 9. A polysilicon structure 430 with a first polysilicon region 410 a and a second polysilicon region 410 b is disposed on the substrate 400. The first and second polysilicon regions 410 a and 410 b have different thickness and grain sizes. For example, the first polysilicon region 430 a in the driving circuit region 8 is thinner, has a larger grain size, and a higher surface roughness than that of the second polysilicon region 430 b in the display region 9, thereby improving electron mobility and sub-threshold swing of the thin film transistor in the driving circuit region 8 and reducing current leakage of the thin film transistor in the display region 9.
FIGS. 4A to 4C illustrate a method for forming thin film transistors for a flat panel display of the fourth embodiment of the present invention. As shown in FIG. 4A, a substrate 400 comprising a driving circuit region 8 and a display region 9 is provided. A buffer layer 405 is subsequently formed on the substrate 400. An amorphous silicon structure 410 with a first amorphous silicon region 410 a and a second amorphous silicon region 410 b is formed on the buffer layer 405. The first amorphous silicon layer 410 a in the driving circuit region 8 has a thickness less than that of the second amorphous silicon region 410 b in the display region 9. The difference in thickness between the amorphous silicon structure 410 in the driving circuit region 8 and that in the display region 9 may be maintained within a specific range. For example, the difference in thickness may be maintained within a range of about 100 to 1000 Å, and preferably 200 to 400 Å.
Thereafter, crystallization is simultaneously performed on the amorphous silicon structure 410 in the driving circuit region 8 and the display region 9, such that the amorphous silicon structure 410 is transferred into a polysilicon structure 430, as shown in FIG. 4B. In this embodiment, the crystallization may comprise a laser treatment which employs, for example, an excimer laser, continuous wave laser (CW laser), or laser beam pulse. Moreover, the laser treatment may comprise lateral solidification (LS), sequential lateral solidification (SLS), continuous grain silicon (CGS), or metal induced lateral crystallization (MILC).
Gate insulating layer 440 is formed on the polysilicon structure 430. Gates 450 is subsequently formed on the gate insulating layer 440, as shown in FIG. 4B. Thereafter, ion implantation is performed on the polysilicon layer 430 using the gates 450 as masks to form sources S, drains D, and channels C.
An interlayer dielectric (ILD) layer 460 is formed on the gate insulating layer 440 and covers the gates 450. Next, interconnects 470 are formed in the ILD layer 460, as shown in FIG. 4C. Finally, processes for fabricating the flat panel display are then successively proceeded and the flat panel display (not shown) is complete.
While the invention has been described by way of example and in terms of preferred embodiments, it is to be understood that the invention is not limited thereto. To the contrary, it is intended to cover various modifications and similar arrangements (as would be apparent to those skilled in the art). Therefore, the scope of the appended claims should be accorded the broadest interpretation to encompass all such modifications and similar arrangements.

Claims

1. A thin film transistor panel, comprising:

a substrate;

a first transistor disposed on the substrate and comprising a first polysilicon region serving as a driving circuit region; and

a second transistor disposed on the substrate and comprising a second polysilicon region serving as a display region, wherein the second polysilicon region is formed by a single layer;

wherein the entire first polysilicon region is thinner than the entire second polysilicon region and a grain size of the entire first polysilicon region is larger than that of the entire second polysilicon region.

2. The panel as claimed in claim 1, wherein the difference in thickness between the first polysilicon region and the second polysilicon region is about 100 to 1000 Å.

3. The panel as claimed in claim 1, wherein the difference in thickness between the first polysilicon region and the second polysilicon region is about 200 to 400 Å.

4. The panel as claimed in claim 1, wherein the first polysilicon region has a surface roughness higher than that of the second polysilicon region.