1. Field of the Invention
The present invention relates to a method of forming a buffer dielectric layer in a semiconductor device and a method of manufacturing a thin film transistor using the same, and, more particularly, to a method of forming a buffer dielectric layer in a semiconductor device and a method of manufacturing a thin film transistor using the same, in which grain size can be maximized, while suppressing the damage of a substrate, by minimizing heat transfer into the substrate, in a procedure of transforming an amorphous silicon layer into a polysilicon layer.
2. Description of the Prior Art
The polysilicon thin film transistor has been used as a device for driving a pixel of an active type liquid crystal display device or an active type organic EL and a device for driving a gate and a data driving circuit. In manufacturing of a SOD (System on a Display) having a driving circuit integrated on a panel, crystallization characteristics of the polysilicon is an important factor to determine a limit of the driving circuit block, capable of being integrated on the panel. For these reasons, the maximization of the grain size and the improvement of crystallization characteristics are required.
As a method for crystallizing the amorphous silicon using the laser, there are a Excimer Laser Annealing (ELA) method for obtaining polysilicon by scanning a pulse laser having a line beam shape and a Sequential Lateral Solidification (hereinafter, referred to as SLS) method for obtaining the polysilicon similar to the single crystaline silicon using the lateral crystal growth. Hereinafter, the method of forming a polysilicon layer using the ELA method and the SLS method will be explained with reference to FIGS. 1 and 2.
FIG. 1 is a perspective view illustrating a conventional method of forming the polysilicon active layer using the ELA method.
Referring to FIG. 1, an amorphous silicon layer 106a is formed on a buffer dielectric film 104 of SiO2, which is formed on a glass or silicon wafer (Si-wafer) substrate 101. Then, the polysilicon active layer 106b having a grain size of about 200 to 400 nm is formed through ELA method. The scanning laser has a shape of line beam. Through the ELA method, a charge mobility of approximately 100 cm2/v.sec can be obtained.
However, the grain size of the polysilicon should be larger in order to improve the mobility sufficiently. But, the heat is quickly transferred into the SiO2 of the buffer dielectric film 104 and the substrate 101 after the melting, before the crystal is sufficiently grown. Therefore, it is not easy to secure growth time enough and thus there is a limitation in enlarging the grain size.
FIG. 2 is a concept diagram illustrating a conventional method of forming the polysilicon active layer using the SLS method.
Referring to FIG. 2, unlike the ELA method, in the SLS method, a buffer dielectric film 104 of SiO2 is formed on a substrate 101. An amorphous silicon layer 106a is formed on the SiO2 film 104, and the laser beam is irradiated to the amorphous silicon layer 106a under a condition that a mask is patterned with constant intervals. Irradiation on the mask pattern 201 allows the portion shadowed by the mask pattern 201 to be grown in direction of the middle point. The amorphous silicon layer is transported with a pitch smaller than the interval of the mask pattern to be sequentially exposed to the laser, thereby the side of the side-grown crystal can be maximized. The SLS method was suggested by James S. Im et al., in U.S. Pat. No. 6,368,945B1, very improved charge mobility can be obtained, compared with the ELA method.
However, in the procedure of crystallizing the amorphous silicon layer by the SLS method, in case the size of the SLGs (Super Lateral Grain) is shorter than the half of the interval of the mask pattern, a new nucleation 202 is generated in the middle portion to which grain boundaries. For these reasons, the successive lateral-grown grain can not be obtained with ease. Accordingly, the interval of the mask pattern and the laser energy should be adjusted such that the nucleation 202 is not generated. But, a limitation in intervals of the mask pattern increases the number of the process mask to reduce the throughput thereof.
In addition, since the crystallization of the amorphous silicon layer uses laser energy as strong as melting the amorphous silicon to sufficiently grow the grain, in case of irradiation on the upper portion of the plastic substrate, the substrate may be damaged due to the heat. Accordingly, when the buffer dielectric film is formed with a single film of SiO2, the laser beams can not be irradiated with sufficient energy. Thus, the laser energy should be reduced or the pulse time should be decreased in order to increase the grain size. But, this process has a limitation in increasing grain size.
On the other hand, Korean Laid-Open Patent Publication No. 2000-3176 describes the technique that a porous silica film is formed on a glass substrate, a silicon nitride film is formed thereon, and then a silicon layer is formed thereon. In the publication, the silicon nitride film is deposited with a chemical vapor deposition method using SiH4 gas and NH3 gas, and thus plenty of hydrogen is contained in the silicon nitride film after the deposition. This hydrogen generate a problem in that, when the amorphous silicon is crystallized by the laser annealing, it can generate the defects in the polysilicon film. In case the silicon nitride film exists over the porous silica film, since the heat conductivity of the silicon nitride film is somewhat higher than that of the silicon oxide film, the heat loss of the silicon nitride film may be larger than that of the silicon oxide film. For these reasons, the crystallization characteristics can be deteriorated. Further, in case the nitrogen components in the silicon nitride film are diffused into the channel of the polysilicon device, the device characteristics can be deteriorated.