Semiconductor device manufacturing: process – Including control responsive to sensed condition – Optical characteristic sensed
Reexamination Certificate
2002-01-31
2004-10-19
Fourson, George (Department: 2823)
Semiconductor device manufacturing: process
Including control responsive to sensed condition
Optical characteristic sensed
C438S486000, C438S016000
Reexamination Certificate
active
06806099
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to a process for producing a poly-silicon (hereinafter abbreviated as poly-Si) film for liquid crystals and semiconductor devices, and a method for inspecting the poly-silicon film.
The reason why a poly-silicon film is superior to an amorphous silicon (a-Si) film as the active layer of a thin film transistor (TFT) used as a driver element in a liquid crystal display is as follows: in the case of the poly-silicon film, since the mobility of a carrier (electrons in n channel or holes in p channel) is high, the cell size can be reduced, so that the precision and minuteness of the liquid crystal display can be enhanced. In addition, the formation of a conventional poly-Si TFT requires a high-temperature process at 1,000° C. or higher. On the other hand, a TFT having a high carrier mobility can be formed in a low-temperature process permitting employment of an inexpensive glass substrate, when there is adopted a low-temperature poly-silicon formation technique in which annealing of only a silicon layer with a laser does not make the temperature of the substrate high.
In this laser annealing, as shown in
FIG. 13
, an a-Si film formed on a glass substrate is scanned while being irradiated with light absorbable thereby, to make the whole a-Si film into a polycrystal, whereby a poly-Si film is obtained. As shown in
FIG. 14
, the poly-Si grain size varies with the surface density of irradiation energy (fluence) of a laser, so that the stability of the laser reflects on the grain size distribution of the poly-Si. The carrier mobility of the poly-Si film increases with an increase of the grain size. In order to attain high TFT characteristics with in-plane uniformity, it is necessary to make the grain size distribution uniform and maintain a large grain size. To attain the large grain size, employment of a fluence in the D region shown in
FIG. 14
is sufficient. However, if the fluence shifts upward owing to the instability of the laser, or the like, the fluence enters a region shown as the E region in
FIG. 14
, i.e., a region where the poly-Si film contains micro crystals with a grain size of 200 nm or less. In this case, the carrier mobility is decreased, resulting in a faulty device. The grain size varies not only with the laser fluence but also with the nonuniformity of thickness of the a-Si film before the laser annealing. Therefore, in order to form the poly-Si film so that its grain size may always be in a definite range, the laser instability and the thickness change of the substrate have to be kept slight. For this purpose, control of the grain size is necessary. Accordingly, it becomes important to control the poly-Si grain size to keep it constant, by checking the poly-Si grain size and feeding back the check result to the laser annealing conditions.
As a method for the control, measuring the grain size itself of the poly-Si is the most reliable. The grain size has been measured by incorporating a sample for the check into an initial or intermediate production lot, or by randomly sampling a product and directly observing the grain size of a poly-Si film formed in a production process, by an electron microscope or a scanning tunnel microscope. As other prior arts, there are the following methods. Japanese Patent Kokai No. 10-214869 discloses a method in which a poly-silicon film is evaluated on the basis of its transmittance. According to this method, the grain size cannot be estimated, though insufficient crystallization due to the insufficient fluence of laser beams can be monitored on the basis of the ratio between a-Si and a poly-Si by utilizing the difference in absorption coefficient between a-Si and the poly-Si. Japanese Patent Kokai No. 11-274078 discloses a method in which a poly-silicon film is evaluated on the basis of its surface gloss (reflectance). In this method, the change of the gloss with the poly-Si grain size is utilized and the gloss is considered to be minimal at an optimum poly-Si grain size. This optimum poly-Si grain size corresponds to a grain size at which the reflectance becomes minimal, namely, the surface roughness becomes maximal.
SUMMARY OF THE INVENTION
The pressure resistance of the gate insulating film of a device becomes insufficient if the surface roughness of the film is high. Thus, a grain size detected by the utilization of conditions under which the surface roughness becomes maximal is used in a method in which there is detected a region where the risk of insufficient pressure resistance due to a remarkable surface unevenness is the highest. If this region is employed, a process for reducing the surface roughness is required, resulting in a complicated production process. Thus, a device production process dependent on the above-mentioned prior art substrate examination methods requires a special process for reducing the surface roughness, and its adoption is limited to that at a grain size (about 300 nm) in the B region shown in FIG.
14
. However, a poly-Si film having a higher carrier mobility has to be formed in order to produce a liquid crystal which consumes less electricity and has higher precision and minuteness. To form such a poli-Si film, it is sufficient that there is employed the D region shown in
FIG. 14
, i.e., a region in which the grain size becomes maximal. For this purpose, it is necessary to estimate the grain size, independent of the surface roughness. As a method for determining the D region, the above prior arts are not suitable and examination by electron-microscopic observation is not suitable for determination on the site of a mass production line because it requires human labor and a long time for obtaining a measurement result. Accordingly, it is difficult to produce stably a poly-Si substrate having a low surface roughness and a grain size of more than 300 nm. The present invention was made in view of the above problems, and makes it possible to determine a region where the surface roughness is low and the grain size of a poly-Si is maximal, by a simple method. Thus, the present invention is intended to provide a process for producing a poly-Si film having a low surface roughness and a high carrier mobility, without product nonuniformity or in high yield.
For the achievement of the above object, the present invention provides a process for producing a poly-Si film which comprises a step of forming a poly-Si film by annealing a silicon film set on a substrate, by light irradiation, a step of measuring a light diffraction pattern of the poly-Si film, and a step of selecting the poly-Si film on the basis of the light diffraction pattern.
The aforesaid silicon film is composed of an a-Si film and is converted to a poly-Si film by annealing by laser beam irradiation. The grain size of the poly-Si film is estimated by measuring the angular distribution of scattered light intensities, and the quality of the poly-Si film is judged by knowing whether its grain size is in the range of the upper limit of the average grain size to the lower limit which range is defined by the relationship between the field-effect mobility and the grain size.
As shown in
FIG. 1
, a light source
2
used for the above-mentioned poly-silicon size measurement with angle dependency of scattered light intensity is a laser having an output wavelength of 540 nm or less and emits laser beams perpendicularly to a substrate
1
having the above-mentioned poly-Si film formed thereon. A plurality of light detector units
7
are located at their respective angles in a range of about 5° to about 45° in order to measure the angular distribution of the intensities of scattered lights from the irradiation region. As shown in
FIG. 7
, the relationship between the poly-Si grain size and the breadth of angular distribution of scattered light intensities in the light diffraction pattern of the poly-Si film is explainable in terms of a relation based on Fourier transformation which is such that in general, the breadth of angular distribution of the intensities of scattered light from particles de
Abe Hironobu
Kimura Yoshinobu
Ohkura Makoto
Saito Masakazu
Shiba Takeo
Fourson George
Miles & Stockbridge P.C.
Toledo Fernando L.
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