Method of fabricating CMOS thin film transistor
A method of fabricating a CMOS thin film transistor. First and second active layers are formed at first and second transistor areas of a transparent substrate. A gate insulating film is formed at middle portions of the first and second active layers while the sides of each are exposed. Each exposed side is ion-doped with a first conductive impurity at a first energy and at a first concentration using the first and second gate electrodes as mask, thus forming first and second high-concentrated areas. A photoresist pattern that covers the first transistor area is formed on the transparent substrate. The second high-concentrated impurity area is ion-doped with a second conductive impurity, at a second energy that is greater than the first energy and at a second concentration that is greater than the first concentration, to form a third high-concentrated impurity area that is counter-doped.
This application claims the benefit of Korean Patent Application No. 2000-08947, filed on Feb. 24, 2000, which is hereby incorporated by reference for all purposes as if fully set forth herein.
BACKGROUND OF THE INVENTION1. Field of the Invention
The present invention relates to a method of fabricating a CMOS thin film transistor, and more particularly to a method of fabricating a CMOS thin film transistor that reduces an off current of an n-type thin film transistor.
2. Discussion of the Related Art
Generally, in a CMOS thin film transistor (TFT), the n-type TFT has a high current driving ratio when in the “ON” state, but a large leakage current in the “OFF” state. When used in a liquid crystal display (LCD), the large leakage current of the n-type TFT changes the value of a signal stored in a pixel electrode, which leads to reduced picture quality. Accordingly, to reduce leakage, n-type TFTs having LDD (lightly doped drain) structures or offset structures have been suggested.
Referring to
Referring now to
The conductive metal film and the insulating material film are then patterned by photolithography so as remain only on the middle portions of the first and second active layers 15 and 16. Thus, both sides of the first and second active layers 15 and 16 are exposed. The insulating material film remaining on the first and second active layers 15 and 16 becomes a gate insulating film 18. The conductive metal film remaining over the first active layer 15 becomes a first electrode 20. The conductive metal film remaining over the second active layer 16 becomes a second gate electrode 21.
Subsequently, the first and second active layers 15 and 16 are doped with n-type impurity ions, such as P or As, at a low concentration while using the first and second gate electrodes 20 and 21 as masks. The result is first and second low-concentrated impurity areas 23 and 24.
Referring to
The exposed portion of the first low-concentrated impurity area 23 (see
Referring to
The exposed portion of the second low-concentrated impurity area 24 is then doped with n-type impurity ions, such as P or As, at a high concentration using the photoresist pattern 25 as a mask. The result is a second high-concentrated impurity area 31, which becomes source and drain areas of the n-type TFT. The second low-concentrated impurity area 24 that remains becomes an LDD (lightly doped drain) area of the n-type TFT. The portion of the second active layer 16 under the second gate electrode 21 becomes a channel area of the n-type TFT.
Thereafter, although not shown, the second photoresist pattern 29 and the photoresist pattern 25 are removed.
While useful, the conventional CMOS-TFT fabricating method described above has a problem in that its fabrication process is rather complicated. The impurity areas of the n-type TFT and the p-type TFT require two photo processes and three ion-doping processes. Another problem is that since the LDD areas of the n-type TFT are controlled by formation of the n-type high-concentrated impurity areas, the LDD areas can have non-uniform lengths. This deteriorates the device characteristics.
SUMMARY OF THE INVENTIONAccordingly, the present invention is directed to a method of fabricating CMOS thin film transistors that substantially obviates one or more of the problems due to limitations and disadvantages of the related art.
An object of the present invention is to provide a method of fabricating a CMOS thin film transistor wherein n-type and p-type impurity areas are formed using simplified photo processes and ion-doping processes.
A further object of the present invention is to provide a method of fabricating a CMOS thin film transistor wherein an LDD area of a n-type TFT is formed on a self-alignment basis to have a uniform length so as to improve device characteristics.
Additional features and advantages of the invention will be set forth in the description that follows, and in part will be apparent from the description, or may be learned by practice of the invention. The objectives and other advantages of the invention will be realized and attained by the structure particularly pointed out in the written description and claims hereof as well as the appended drawings.
To achieve these and other advantages and in accordance with the purpose of the present invention, as embodied and broadly described, a method of fabricating a CMOS thin film transistor according to the present invention includes the steps of forming first and second active layers on desired portions of first and second transistor areas of a transparent substrate, and then forming a gate insulating film on the middle portions of the first and second active layers. Then, forming first and second gate electrodes on the gate insulating film. The method continues by ion-doping the exposed sides of the first and second active layers with a first conductive impurity at a first energy and at a first concentration using the first and second gate electrodes as masks so as to form first and second high-concentrated doped areas. Then, a photoresist pattern covering the first transistor area is formed. The exposed second high-concentrated impurity area on the transparent substrate is then ion-doped with a second conductive impurity at a second energy that is higher than the first energy and at a second concentration that is greater than the first concentration. This forms a third high-concentrated impurity area that is counter-doped.
It is to be understood that both the foregoing general description and the following detailed description are exemplary and explanatory and are intended to provide further explanation of the invention as claimed.
BRIEF DESCRIPTION OF THE DRAWINGThe accompanying drawings, which are included to provide a further understanding of the invention and are incorporated in and constitute a part of this specification, illustrate embodiments of the invention and together with the description serve to explain the principles of the invention.
In the drawings:
Reference will now be made in detail to the preferred embodiment of the present invention, an example of which is illustrated in the accompanying drawings.
Referring to
While the first and second active layers 45 and 46 may be formed by depositing a polycrystalline silicon, they may also be formed by depositing an amorphous silicon by the CVD technique and then annealing it with a laser beam to become a polycrystalline silicon. The buffer layer 43 prevents an impurity on the substrate 41 from being diffused into the first and second active layers 45 and 46.
Referring now to
The conductive metal film and the insulating material film are then patterned by photolithography so as to remain only at the middle portions on the first and second active layers 45 and 46. Thus, both sides of the first and second active layers 45 and 46 are exposed. At this time, the insulating material film remaining on the first and second active layers 45 and 46 becomes a gate insulating film 48. Further, the conductive metal film remaining over the first active layer 45 becomes a first gate electrode 50. The conductive metal film remaining over the second active layer 46 becomes a second gate electrode 51.
Subsequently, the exposed portions of the first and second active layers 45 and 46 are doped with p-type impurity ions, such as B or BF2, at an energy of about 10 to 20 KeV and at a dose of about 5×1014 to 5×1015/cm2. During doping, the first and second gate electrodes 50 and 51 act as masks. The result is first and second high-concentrated impurity areas 53 and 54.
Referring to
The exposed portion of the second high-concentrated impurity area 54 is then ion-doped with an n-type impurity, such as P or As, at an energy of about 30 to 60 KeV and at a dose of about 1×1015 to 1×1016/cm2. The second gate electrode 51 acts as a mask and the photoresist pattern 55 protects the p-type transistor area P2. The result is a third high-concentrated impurity area 57. In this case, the third high-concentrated impurity area 57 is formed by doping an n-type impurity at one to three times greater concentration such that the second high-concentration impurity area 54 doped with a p-type impurity is counter-doped with an n-type impurity, thereby forming an area doped with an n-type impurity at a high concentration.
Since the doping energy of the n-type impurity that formed the third high-concentrated impurity area 57 is much greater than the p-type impurity that formed the second high-concentrated area 54, the n-type impurity is distributed more widely than the p-type impurity. Accordingly, a high resistance area in which only the n-type impurity is distributed is formed within the second active layer 46 at a region adjacent the second electrode 51. This high resistance area alleviates an electric field between the gate and the drain of the n-type TFT to prevent a flow of an off current Ioff.
The third high-concentrated impurity area 57, which is used as the source and drain areas of the n-type TFT, is formed in a self-alignment manner. The overall fabrication process is not only simplified, but also a revival property and device reliability can be improved.
The p-type TFT uses the first high-concentrated impurity area 53, which is p-type doped, as source and drain areas. The n-type TFT uses the third high-concentrated impurity area 57, which is n-type doped, as the source and drain areas. In the p-type and n-type TFTs, portions adjacent the first and second gate electrodes 50 and 51 of the first and second active layers 45 and 46, respectively, become channel areas.
Thereafter, although being not shown, the photoresist pattern 55 is removed.
The second high-concentrated impurity area 54 (see
If phosphorus (P) is ion-doped at 10 KeV, then, the drain-source current Ids, that is, the off current Ioff, becomes approximately 10−6 A under a gate voltage Vg of −15V because the concentration of phosphorus (P) is high at the surface.
However, if phosphorus (P) is ion-doped at 30 KKeV or 50 KeV, the drain-source current Ids, that is, the off current Ioff, becomes approximately 10−9 A or 10−10 A, respectively, under a gate voltage Vg of −15V because the concentration of phosphorus (P) at the surface is lowered.
Furthermore, if phosphorus (P) is ion-doped at 60 KeV, the drain-source current Ids, that is, the off current Ioff, increase to approximately 10−8 A, under a gate voltage Vg of −15V as the concentration of the phosphorus (P) becomes lower-at the surface than that of the boron (B).
As described above, according to the present invention, each exposed side of the first and second active layers is ion-doped with a P-type impurity at a high concentration and at a high energy using the first and second gates as a mask to form first and second high-concentrated impurity areas. This forms the p-type TFT at the p-type transistor area. Sequentially, the photoresist pattern covering the p-type transistor area is formed, and the second high-concentrated impurity area is doped with a n-type impurity, in an ion self-alignment manner, to form a third high-concentrated impurity area, thereby forming the n-type TFT at the n-type transistor area. Thus, the impurity area of the CMOS-TFT is formed using one photographic process and two ion dopings.
Accordingly, the photo processes and the ion-doping processes can be reduced, thus simplifying the overall fabrication process. Also, the LDD area of the n-type TFT is self-aligned to have a uniform length, thus the device characteristics can be improved.
It will be apparent to those skilled in the art that various modifications and variation can be made in the present invention without departing from the spirit or scope of the invention. Thus, it is intended that the present invention cover the modifications and variations of this invention provided they come within the scope of the appended claims and their equivalents.
Claims
1-16. (canceled)
17. A CMOS thin film transistor, comprising:
- a first active layer on a substrate, said first active layer having end portions that are ion-doped with a first conductive impurity of a first polarity;
- a first gate insulating film on a middle portion of said first active layer;
- a second active layer on said substrate, said second active layer having end portions that are ion-doped with said first conductive impurity and with a second conductive impurity of an opposite polarity, wherein said second conductive impurity changes the doping of said end portions from said first polarity to said second polarity;
- a second gate insulating film on a middle portion of said second active layer;
- a first gate over said first gate insulating film; and
- a second gate over said second gate insulation film.
18. The CMOS thin film transistor according to claim 17, wherein said first conductive impurity is a p-type impurity.
19. The CMOS thin film transistor according to claim 17, wherein said substrate is transparent.
20. The CMOS thin film transistor according to claim 17, further including an insulating layer between said first active layer and said substrate.
Type: Application
Filed: Dec 6, 2004
Publication Date: May 12, 2005
Inventor: Joon Yang (Kyounggi-do)
Application Number: 11/004,064