Semiconductor device manufacturing: process – Making field effect device having pair of active regions... – On insulating substrate or layer
Reexamination Certificate
2000-01-12
2001-08-14
Nelms, David (Department: 2818)
Semiconductor device manufacturing: process
Making field effect device having pair of active regions...
On insulating substrate or layer
C438S158000, C438S265000
Reexamination Certificate
active
06274413
ABSTRACT:
FIELD OF THE INVENTION
The present invention is related to a method for fabricating a polysilicon thin film transistor.
BACKGROUND OF THE INVENTION
As the development of the integrated circuit is now mushrooming, the polysilicon thin film transistor grown by the low pressure chemical vapor deposition (LPCVD) is extensively employed in the application of liquid crystal display (LCD) and high-density static random access memory. As an example, the active-matrix LCD (AM-LCD) which is the most dominant LCD product adopts the thin film transistor (TFT) as the display unit. Accordingly, the AM-LCD is power-saving and is able to display both the grayscale and the color. However, because there are a great deal of traps at the grain boundary in the polysilicon thin film, the polysilicon thin film transistor has larger threshold voltage and worse subthreshold swing. It has been reported that the plasma hydrogenation process is an efficient method to passivate these trap states. Nevertheless, the UV emitted from the discharged plasma glow will damage the boundary of the polysilicon/silicon oxide. Moreover, the plasma hydrogenation process brings about a low efficiency in passivating the tail state traps thereby limiting its applications. In conclusion, the conventional fabrication process for a polysilicon thin film transistor has the following shortcomings:
1. It is inefficient to passivate the tail state traps if the plasma hydrogenation process is combined into the conventional fabrication process for fabricating the polysilicon thin film transistor.
2. It will somewhat damage the boundary of the polysilicon/silicon oxide if the plasma hydrogenation process is combined into the conventional fabrication process for fabricating the polysilicon thin film transistor.
3. There is another approach for improving the characteristics of the polysilicon thin film transistor, that is, the channel oxidation process. The channel oxidation process provides a better efficiency for passivating the tail state traps than the plasma hydrogenation process does, and can prevent the damage caused by the UV emitted from the discharged plasma glow during the plasma hydrogenation process. Nevertheless, the application of channel oxidation will be limited as the gate oxide is getting thick.
Therefore, it is tried by the applicant to minimize the damage caused by the UV emitted from the discharged plasma glow and increase the efficiency for passivating the tail state traps.
SUMMARY OF THE INVENTION
An object of the present invention is to provide a process for fabricating the polysilicon thin film transistor for minimizing the damage cause by UV emitted from the discharged plasma glow during plasma hydrogenation process and improving the efficiency for passivating the tail state traps.
Another object of the present invention is to provide a process for fabricating the polysilicon thin film transistor for shortening the fabrication time and reducing the leakage current.
Another further object of the present invention is to provide a process for fabricating the polysilicon thin film transistor for preventing the gate oxide from getting thick, increasing the on/off current ratio, and lowering the subthreshold swing.
According to the present invention, the channel oxidation process and the plasma hydrogenation process are combined into the fabrication process for a polysilicon thin film transistor. The polysilicon thin film transistor is fabricated by the following steps: (a) forming an oxide layer on a substrate, (b) forming a polysilicon layer on a portion of the oxide layer to serve as a gate, (c) forming a gate oxide on the polysilicon layer and another portion of the oxide layer, (d) forming a polysilicon channel on the gate oxide layer, (e) defining a source region and a drain region in a portion of the polysilicon channel, (f) oxidizing the polysilicon channel, (g) forming a dielectric layer on the polysilicon channel, and (h) hydrogenating the polysilicon thin film transistor by plasma.
In accordance with the present invention, the substrate is an N-type silicon substrate.
Preferably, the oxide layer is a field oxide with a thickness of 5000 Å.
In accordance with the present invention, the polysilicon layer is formed by low pressure chemical vapor deposition (LPCVD).
Preferably, the polysilicon layer has a thickness of 550 Å.
In accordance with the present invention, the gate is formed by implanting phosphine (PH
3
) ions with a dosage of 5×10
15
cm
−2
into the polysilicon layer.
In accordance with the present invention, the gate oxide is formed by the low pressure chemical vapor deposition (LPCVD). Preferably, the gate oxide has a thickness of 300 Å.
After the gate oxide is formed, the gate oxide is further treated by a subsequent heat treatment at an ambient temperature of 800° C.
In accordance with the present invention, the polysilicon channel is formed by the steps of: first, forming an amorphous silicon layer with a thickness of 300 Å on the gate oxide by low pressure chemical vapor deposition (LPCVD) at an ambient temperature of 525° C. Second, annealing the amorphous silicon layer to form a recrystallized polysilicon layer in the presence of a nitrogen gas at a temperature of 600° C. Third, implanting phosphorous ions with a dosage of 5×10
12
cm
−2
into the recrystallized polysilicon layer to form the polysilicon channel.
In accordance with the present invention, the drain region and the source region are formed by implanting boron ions with a dosage of 5×10
15
cm
−2
into the portion of the polysilicon channel.
In accordance with the present invention, after the source region and the drain region are formed, the polysilicon channel is oxidized by a thermal oxidation process at a preferably temperature of 800° C. for one hour.
In accordance with the present invention, the dielectric layer is a borophosphosilicate glass (BPSG) layer with a thickness of 6000 Å, and after the BPSG layer is formed, the BPSG layer is further treated by a high-temperature flow process at a temperature ranged from 850° C. to 950° C.
In accordance with the present invention, before the step of hydrogenating the polysilicon thin film transistor by plasma, the method further includes the steps of: (a) forming contact windows on the drain region and the source region respectively, (b) depositing a metal layer in the drain region and the source region to provide a drain contact and a source contact for said drain region and source region respectively, (c) forming a first passivation layer on the dielectric layer, and (d) forming a second passivation layer on the first passivation layer, thereby forming the polysilicon thin film transistor.
In accordance with the present invention, the step of hydrogenating the polysilicon thin film transistor by plasma is executed in a plasma enhanced chemical vapor deposition (PECVD) system for four hours.
Now the foregoing and other features and advantages of the present invention will be more clearly understood through the following descriptions with reference to the accompanying drawings, in which:
REFERENCES:
patent: 5661051 (1997-08-01), Yeh et al.
patent: 5693959 (1997-12-01), Inoue et al.
patent: 5796150 (1998-08-01), Wuu et al.
patent: 6117733 (2000-09-01), Sung et al.
Fang Yean-Kuen
Wang Yung-Chi
Yang Dun-Nien
Corless Peter F.
Dang Phuc T.
Edwards & Angell LLP
National Science Council
Nelms David
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