Semiconductor device manufacturing: process – Making field effect device having pair of active regions... – On insulating substrate or layer
Reexamination Certificate
1997-04-09
2001-08-21
Chaudhuri, Olik (Department: 2814)
Semiconductor device manufacturing: process
Making field effect device having pair of active regions...
On insulating substrate or layer
C438S152000, C438S783000, C438S786000, C438S162000
Reexamination Certificate
active
06277678
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to methods for manufacturing liquid crystal displays, and more particularly to methods for manufacturing thin film transistor-liquid crystal displays in which thin film transistors (hereinafter, TFT) are used as active elements.
BACKGROUND OF THE INVENTION
Liquid crystal displays (hereinafter, LCD) are flat panel display devices utilizing liquid crystal technology and semiconductor technology. LCD technology is based on the optical characteristics of the liquid crystal whose molecular arrangement is changed by an electric field.
A thin film transistor-LCD (hereinafter TFT-LCD) can consume low power, use low driving voltage, and be thin and light. The TFTs may be divided into polysilicon TFTs and amorphous silicon TFTs, based upon the material of the semiconductor layer used as a channel.
Processes for manufacturing the polysilicon TFT-LCD may be divided into low temperature processing (for example performed at a temperature of not more than 400° C.) and high temperature processing (for example performed at a temperature of more than 900° C.). In particular, low temperature processing performed at a temperature of not more than 350° may be most appropriate for manufacturing an LCD having a large area since the processing can be performed on a glass substrate. This may make the polysilicon TFT-LCD more competitive than the amorphous silicon TFT-LCD.
However, trapping centers which may hinder the movement of electrons or holes may exist in both the low temperature processing and the high temperature processing due to grain boundaries in the polysilicon. The trapping centers may lower the mobility of the carriers and may increase the threshold voltage of the TFT.
Various techniques are generally used in order to manufacture a high performance TFT using low temperature processing.
First, a technique for forming a polysilicon film of good quality on a substrate without thermal transformation of the substrate is often used. Rapid crystallization by laser induction can be used to form a polysilicon film of good quality. Since crystallization of the silicon thin film using a laser can be performed without heating the substrate at a high temperature, the substrate may be damaged less. Thus, the mobility of the carriers in polysilicon which is formed by laser anneal may be larger than about 100 cm
2
/Vs.
Second, a technique for forming a surface of good quality between the polysilicon film and an oxide film at a low temperature is used. When the trap density is high on the surface between the polysilicon film and the oxide film, the threshold voltage of the TFT may become higher. Although a plasma enhanced CVD (hereinafter, PECVD) may be appropriate for forming the oxide film at a low temperature, for example, 200° C. to 400° C., the PECVD may severely deteriorate the surface between the polysilicon film and the oxide film. There are various methods for reducing the deterioration of the surface, such as electron cyclotron resonance plasma CVD (ECR CVD) and radio frequency (RF) parallel-plate remote plasma CVD. In RF parallel-plate remote plasma CVD, it is possible to form a uniform thin film over a wide area. The polysilicon TFT having an oxide film formed by the remote plasma CVD may have a high mobility of about 400 cm
2
/Vs and a low threshold voltage of about 1.5V.
Third, an annealing technique after metallization is often used. The characteristic of the TFT may be improved when the TFT is annealed at a temperature of 270° C. in air. However, such an annealing method may impact the reproduction of the oxide film and the surface between the oxide film and the silicon film. In such an annealing method, it may be difficult to control humidity of air. Therefore, an annealing in which vapor is controlled at a low temperature is generally provided. The vapor may be most effective for annealing the TFT having an oxide film formed by the remote plasma CVD in the air at a low temperature.
The above-mentioned basic technologies are described in a publication entitled “
High Quality SiO
2
/Si Interfaces of Poly
-
Crystalline Silicon Thin Film Transistors by Annealing in Wet Atmosphere
”, Vol. 16, No. 5, May 1995, pp. 157-160 by Naoki Sano, et al.
In the above-mentioned conventional technology, the substrate may be annealed in a vapor atmosphere in order to remove trapping centers on the surface between the polysilicon film and the oxide film during the manufacture of the TFT. The oxide film is generally not formed by a general CVD method, but generally may be forced by the remote plasma CVD method in order to more effectively remove the trapping centers.
SUMMARY OF THE INVENTION
It is therefore an object of the present invention to provide improved thin film transistors and liquid crystal displays using thin film transistors.
It is another object of the present invention to provide thin film transistors and liquid crystal displays which include reduced numbers of trapping centers in the polysilicon films thereof.
These and other objects are provided, according to the present invention, by forming an insulating film containing water (H
2
O) on a thin film transistor and diffusing the water contained in the insulating film into the polysilicon film of the thin film transistor. Diffused water can thereby reduce trapping sites in the polysilicon film. An insulating film of spin-on glass (SOG), containing water is preferably used.
In particular, thin film transistors are manufactured according to the present invention by forming a thin film transistor comprising a gate, a source and a drain on a substrate. An insulating film containing H
2
O is formed on the thin film transistor. H
2
O contained in the insulating film is diffused into the thin film transistor.
The diffusing step preferably comprises the step of thermally processing the insulating film containing H
2
O to diffuse H
2
O contained therein into the thin film transistor. After the diffusing step, the insulating film containing H
2
O may be planarized.
The insulating film containing H
2
O preferably comprises a spin-on glass (SOG) film having a thickness of between 1000 Å and 3000 Å. The diffusing step is preferably performed at temperatures between 300° C. and 40° C.
After the insulating film containing H
2
O is formed, another insulating film may be formed, selected from the group consisting of nitride or oxide nitride films. Moreover, a second thermal processing step may be used to rearrange the hydrogen in the water diffused into the thin film transistor and to also relax the stress in the insulating film containing H
2
O. The second thermal processing step may be performed at temperatures between 150° C. and 200° C. A contact hole may be formed in the insulating films to expose the drain, and a pixel electrode be formed connected to the drain electrode through the contact hole.
The thin film transistor itself may be formed by depositing an amorphous film on a substrate and crystallizing the amorphous film. A semiconductor film pattern is then formed by patterning the semiconductor film. A gate insulating film and the gate electrode are formed on the semiconductor film pattern and a source and a drain are formed by doping the semiconductor film pattern. An interlayer dielectric film including contact holes for exposing the source and the drain is formed on the semiconductor film pattern, and a source electrode and a drain electrode is connected to the source and drain through the contact holes. The amorphous silicon may be crystallized using a laser, and the source and drain may be formed by ion showered doping the semiconductor film pattern. The steps of forming an insulating film containing H
2
O on the thin film transistor and then diffusing the H
2
O in the insulating film containing H
2
O into the thin film transistor may be repeatedly performed.
According to the present invention, an insulating film containing much H
2
O such as SOG is used. Therefore, it is possible to lower the threshold voltage of the transistor by reducing the traps formed on the surface betwe
Chaudhuri Olik
Myers Bigel & Sibley & Sajovec
Rao Shrinivas H.
Samsung Electronics Co,. Ltd.
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