Optical: systems and elements – Single channel simultaneously to or from plural channels
Reexamination Certificate
2001-08-31
2003-11-18
Mack, Ricky (Department: 2872)
Optical: systems and elements
Single channel simultaneously to or from plural channels
C359S619000
Reexamination Certificate
active
06650480
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a method of making a distribution of energy of a laser beam uniform in a particular region, and to a laser irradiation apparatus (including a laser device and an optical system for guiding a laser beam output from the laser device to a target) for annealing semiconductor film by using a laser beam (hereinafter referred to as laser annealing). And also the present invention relates to a method of manufacturing a semiconductor device manufactured by a method including a process of the laser annealing. In this specification, “semiconductor device” denotes the category of any device capable of functioning by utilizing a semiconductor characteristic, covering electro-optic devices, such as liquid crystal display device and electroluminescent (EL) display devices, and electronic devices including such a kind of electro-optic device as a component.
2. Description of the Related Art
In recent years, studies have been widely made of techniques for performing laser annealing on an amorphous semiconductor film or a crystalline semiconductor film (a semiconductor film having a crystalline property but not single crystalline, i.e., a polycrystalline or microcrystalline semiconductor film), i.e., a non-single crystalline semiconductor film, formed on an insulating substrate such as a class substrate to crystallize the film or improve the crystallographic characteristics of the film. As the above-described kind of semiconductor film, silicon film is ordinarily used.
Glass substrates are low-priced and having high workability in contrast with quartz substrates conventionally used widely. Because of these characteristics, glass is frequently used as the material of a large-area substrate. This is the reason for making the above-mentioned studies. Laser is favorably used for crystallization because the melting point of a glass substrate is low. Laser enables supply of a large amount of energy only to a non-single crystalline film over a substrate without a considerable increase in the temperature of the substrate.
Conventionally, for crystallization of an amorphous semiconductor film by heat, heating at a temperature of 600° C. or higher for ten hours or longer and, preferably, twenty hours or longer is required. An example of a substrate capable of enduring under this crystallization condition is a quartz substrate. A quartz substrate, however, is high-priced and not sufficiently workable. In particular, it is extremely difficult to work quartz into a large-area substrate. Increasing the area of a substrate is an essential factor in increasing the efficiency with which a semiconductor device using the substrate. In recent years, schemes to increase the substrate area for the purpose of improving the production efficiency have been markedly advanced. A substrate size of 600×720 mm is now becoming a standard with respect to factory lines newly constructed.
It is difficult to work quartz into such a large-area substrate as long as a presently available technique is used. A large-area quartz substrate, if any, must be high-priced and is not industrially usable. On the other hand, glass is an example of a material from which a large-area substrate can be easily made. As a glass substrate, a glass called Corning 7059 may be mentioned. Corning 7059 is markedly low-priced and sufficiently workable and can be easily formed into a large-area substrate. Corning 7059, however, has a strain point of 593° C. and cannot be heated at a temperature of 600° C. or higher without a problem.
A Corning 1737 substrate having a comparatively high strain point, 667° C. is known as one of the existing glass substrates. The result of an experiment made by forming an amorphous semiconductor film on Corning 1737 and maintaining the amorphous semiconductor film at a temperature of 600° C. for 20 hours was that there was no such deformation of the substrate as to influence the fabrication process, and the amorphous semiconductor film was crystallized. However, the heating time 20 hours is excessively long if considered as a heating time in a practical production process, and it is desirable to reduce the heating temperature below 600° C. from the viewpoint of production cost.
To solve this problem, a new crystallization method has been devised, details of which are as described in Japanese Patent Application Laid-open Hei No. 7-183540. This method will be described briefly below. First, a small amount of an element, e.g., nickel, palladium, or lead is added to an amorphous semiconductor film. For this addition, a plasma processing or deposition method, an ion implantation method, a sputtering method, a solution application method, or the like may be used. After the addition, the amorphous semiconductor film is placed, for example, in a nitrogen atmosphere at 550° C. for 4 hours to obtain a polycrystalline semiconductor film having good characteristics. The heating temperature and heating time and other hearing conditions most suitable for crystallization depend on the amount of the added element and the state of the amorphous semiconductor film.
An example of crystallization of an amorphous semiconductor film by heating has been described. On the other hand, crystallization of a semiconductor film by laser annealing can be performed even on a plastic substrate or the like as well as on a glass substrate having a low strain point because laser annealing enables supply of a large amount of energy only to the semiconductor film without a considerable increase in a substrate temperature.
Examples of a laser used for laser annealing are an excimer laser and an Ar laser. As a laser annealing method having the advantage of improving the productivity and mass-producibility, a method is favorably used in which a high-power laser beam obtained by pulse oscillation is processed by an optical system so as to form a spot having the shape of a several centimeters square, or a stripe having, for example, a length of 10 cm or longer along an irradiation plane, and the laser irradiation position is moved relative to the irradiation plane in a scanning manner to perform laser annealing. In particular, a use of a laser beam forming a linear on the irradiation plane (hereinafter referred to as “linear beam”) is effective in improving the productivity in contrast with a use of a spot laser beam, because scanning with the linear beam only along the direction perpendicular to the lengthwise direction of the stripe formed by the linear beam may suffice for irradiation of the entire target surface while scanning with the spot laser beam must be performed along each of two directions perpendicular to each other. Scanning along the direction perpendicular to the lengthwise direction of the stripe formed by the linear beam has maximum scanning efficiency. Because of this advantage in terms of productivity, a use of a linear beam obtained by processing high-power laser light with a suitable optical system is now becoming mainstream in laser annealing.
FIG. 2
shows an example of an optical system for processing a laser beam so that the beam forms a stripe on a surface to be irradiated. The optical system also has the function of making the distribution of laser beam energy along the irradiation plane uniform as well as processing the laser beam in the form of stripe. In general, an optical system for making the distribution of beam energy uniform is called a beam homogenizer.
The optical system will be described first with reference to the side view in
FIG. 2. A
laser beam emitted from a laser oscillator
201
is divided by a cylindrical lens array
202
in a direction perpendicular to the direction in which the laser beam travels. This direction perpendicular to the laser beam traveling direction will be referred to as “short-dimension direction” in this specification. In the example shown in
FIG. 2
, the laser beam is divided into four. The divided laser beams are converged by a cylindrical lens
204
so as be temporarily combined into one. The beams are
Fish & Richardson P.C.
Mack Ricky
Semiconductor Energy Laboratory Co,. Ltd.
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