Semiconductor device manufacturing: process – Making field effect device having pair of active regions... – On insulating substrate or layer
Reexamination Certificate
1995-12-14
2002-01-08
Nguyen, Tuan H. (Department: 2813)
Semiconductor device manufacturing: process
Making field effect device having pair of active regions...
On insulating substrate or layer
C438S166000, C438S487000
Reexamination Certificate
active
06337229
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to a method of making a semiconductor whose major component is silicon having crystalline property, more particularly, to a method of making a crystalline silicon semiconductor used in a semiconductor element such as a thin film transistor.
BACKGROUND OF THE INVENTION
Conventionally, a thin film transistor (hereinafter, TFT) using a thin film semiconductor has been known. TFT is constituted by using a thin film semiconductor formed on a substrate. TFT is used in various integrated circuits, especially in an electrooptical device such as a liquid crystal display, particularly as a switching element at a respective pixel of a liquid crystal display device of an active matrix type or a driver element formed in a peripheral circuit portion.
Although it is simple and convenient that an amorphous silicon film is used as a thin film semiconductor of TFT, its electric properties are poor. It is preferable to use a silicon thin film having crystalline property for promoting the properties of TFT. To provide a silicon film having crystalline property, an amorphous silicon film is firstly formed and crystallized thereafter by heating it or by irradiating thereon an electromagnetic wave having high energy such as a laser beam.
However, the crystallization by heating needs to take a time period of 10 hours or more at a heating temperature of 600° C. or more and accordingly, it is difficult to use a glass substrate. For example, the glass strain point of Corning 7059 generally used in an active type liquid crystal display device is 593° C. and therefore, the heating at 600° C. or more is problematic when area expansion of a substrate is considered. Further, the properties of the obtained crystalline silicon film are poorer than those of a film provided by a laser beam irradiation as follows.
It has been revealed that an element having a catalytic action accelerating crystallization of amorphous silicon is used to solve the problem as is disclosed in Japanese Unexamined Patent Publication Nos. 244103/1994, 244104/1994, 244105/1994, 244205/1994 and 296023/1994. That is, it has been revealed that the crystallization can be performed in a processing time of approximately 4 hours at 600° C. or less, typically, 550° C. by making an element of nickel, palladium, lead or the like adhere to an amorphous silicon film in a very small amount and heating it thereafter.
However, the catalyst element remains in such a process in the silicon film obtained in such a short period of time at such a low temperature and the properties of TFT using the silicon film are not preferable. Especially, it is the most serious problem in TFT that when a backwardly biased voltage (negative voltage in N-channel TFT, positive voltage in P-channel TFT) is applied on the gate, the absolute value of a drain current (off current or leakage current) is large and the value is considerably dispersed among respective elements.
Especially, the large off current causes a serious problem when the silicon film is used in a switching transistor of a pixel electrode in an active matrix type liquid crystal display device. When the off current of a thin film transistor arranged at a pixel electrode is large, the pixel electrode cannot hold electric charge during a predetermined time period which causes flickering of a screen and an obscure display.
SUMMARY OF THE INVENTION
In view of the current status it is an object of the present invention to provide a method of making a crystal silicon semiconductor film capable of reducing the off current of TFT and reducing the off current value of each element and its dispersion by adopting a step of crystallizing a silicon film by using a catalyst element promoting crystallization of silicon, especially, a method of making a crystal silicon semiconductor capable of being treated at a low temperature and preferable to mass production.
To achieve the above-mentioned object the present invention provides a silicon film having crystalline property by using the following steps.
Firstly, a silicon oxide film is deposited on an insulating surface by various chemical vapor deposition (CVD) processes, for example, plasma CVD process or thermal CVD process. The film forming temperature in this step is 450° C. or less, preferably, 300 through 350° C. The deposition may be performed by the plasma CVD process using, for example, tetraethoxysilane (TEOS) and oxygen or mono-silane (SiH
4
) and dinitrogen monoxide (N
2
O) or the thermal CVD process using mono-silane and oxygen.
An amorphous silicon film is deposited by various CVD processes on the silicon oxide film which has been deposited as mentioned above. For example, the film forming temperature of 295 through 305° C. is preferable in obtaining an amorphous silicon film by the plasma CVD process with mono-silane as raw material. However, it is necessary to form the amorphous silicon film without bringing the silicon oxide film into contact with the atmosphere. That is, the formation of the silicon oxide film and the amorphous silicon film needs to be continuously performed. It is preferable for this purpose to use a film forming device (cluster tool) of a publicly-known multichamber system.
Thereafter, a single substance of a catalyst element or a compound containing the catalyst element promoting crystallization of the amorphous silicon film is formed on the amorphous silicon film in a layer, a film or a cluster. Hereinafter, a layer of a single substance of a catalyst element or a compound containing the catalyst element is called a catalyst layer. The method of forming a catalyst layer will be mentioned later.
Further, the inventors found that the most significant effect is achieved when nickel is used as a catalyst element. The other usable catalyst elements are Pt,Cu, Ag, Au, In, Sn, Pd, P, As and Sb.
Thereafter, a heating process is performed on the amorphous silicon film by which a portion or a total of the amorphous silicon film is crystallized. In the process of crystallization, when the catalyst layer does not cover the total face of the amorphous silicon film, not only a region which the catalyst layer covers is crystallized but the crystallization is progressed from the region to peripheral portions.
In the crystallizing step the amorphous silicon film is heated at a temperature of 400° C. or more such that crystallization of the amorphous silicon film in which the catalyst element has been introduced is progressed. In a general glass substrate the heating temperature is 400° C. to 750° C. However, the heat resistant temperature differs by the kind of the glass substrate and accordingly, the upper limit of the heating temperature may be the strain point of glass. For example, the glass strain point is 593° C. for Corning 7059 glass and 667° C. for Corning 1737 glass.
Specifically, it is appropriate to determine the heating temperature as approximately 550° C. in view of heat resistance and productivity of a glass substrate.
It has been clarified that the higher the heating temperature, the more improved is the crystalline property of the silicon film. Therefore, the silicon film is heated at a temperature as high as possible so far as the substrate can stand the temperature in case where the crystalline property of the silicon film is mostly preferred. In this case it is preferable to use a quartz substrate which can stand a temperature of approximately 1000° C. For example, a quartz substrate can be heated at a temperature of approximately 800° C. through 1000° C.
The crystallization may more be promoted by irradiating a laser beam or an equivalent strong beam after the heating step. By adding this step portions which could not be crystallized in the previous step can be crystallized in which portions that have been crystallized in the previous step are used as nuclei.
The basic difference between the crystallization by the present invention and the conventional crystallization performed by irradiating a laser beam lies in that conditions determining the crystalline property are very severe
Sakama Mitsunori
Takemura Yasuhiko
Yamazaki Shunpei
Nguyen Tuan H.
Nixon & Peabody LLP
Robinson Eric J.
Semiconductor Energy Laboratory Co,. Ltd.
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