Optical: systems and elements – Single channel simultaneously to or from plural channels
Reexamination Certificate
2001-12-21
2003-09-16
Dang, Hung Xuan (Department: 2873)
Optical: systems and elements
Single channel simultaneously to or from plural channels
Reexamination Certificate
active
06621636
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a method of irradiating semiconductor films using laser light, and to a laser irradiation apparatus (apparatus containing a laser and an optical system for introducing laser light output from the laser to an irradiation subject) for performing irradiation of semiconductor films.
2. Description of the Related Art
Techniques for increasing crystallinity or performing crystallization by irradiating laser light to a semiconductor film formed on an insulating substrate such as glass have been widely researched in recent years. Silicon is often used in the semiconductor film. Means of crystallizing a semiconductor film by using laser light and obtaining a crystalline semiconductor film, is referred to as laser crystallization throughout this specification.
Compared to conventional synthetic quartz glass substrates that are in widespread use, glass substrates possess the advantages of having abundant workability at low cost, and of easily manufacturing a large surface area substrate. These are the reasons the aforementioned research is being carried out. Further, the use of lasers, preferably for crystallization, is due to the low melting point of glass substrates. Lasers are able to impart a high amount of energy to the semiconductor film only, without increasing the temperature of the substrate. Further, throughput is remarkably high in comparison with means of heat treatment using an electric furnace.
Crystalline semiconductors are made up of many crystal grains, and therefore are also referred to as polycrystalline semiconductor films. Crystalline semiconductor films formed by irradiating laser light, have high mobility, and therefore thin film transistors (TFTs) are formed using crystalline semiconductor films. For example, crystalline semiconductor films are utilized much in devices such as monolithic liquid crystal electro-optical devices in which pixel driver TFTs and driver circuit TFTs are manufactured on one glass substrate.
Further, a method in which pulse laser light such as an excimer laser having a high output is optically processed into a square spot of several centimeters per side, or into a linear shape having a length equal to or greater than 10 cm, and the laser light is then scanned (alternatively, the position of laser light irradiation is moved relative to the irradiation surface) and irradiated onto the surface, is good for mass production and is industrially superior. This method is therefore preferably used.
In particular, if a linear shape beam is used, laser irradiation can be performed on the entire irradiation subject by only scanning in a direction perpendicular to the longitudinal direction of the linear shape beam, differing from the case of using spot shape laser light in which forward and backward, and left and right scanning is necessary. Mass production is therefore good. The reason for scanning in a direction perpendicular to the longitudinal direction is because the scanning direction has the highest efficiency. In present methods of laser irradiation, the use of linear shape beams, in which pulse emission excimer laser light is processed by a suitable optical system, is gaining ground as a technique for manufacturing liquid crystal display devices using TFTs, due to its good mass production characteristics.
Semiconductor film crystallization after irradiating laser light to a semiconductor film is explained here. If laser light is irradiated to a semiconductor film, the semiconductor film will melt. However, the temperature of the semiconductor film drops as time passes, and crystal nuclei form. An almost countless number of uniform (or non-uniform) crystal nuclei are generated in the semiconductor film, and crystallization is complete after they nuclei grow. The position and size of the crystal grains obtained in this case, are random. Further, the crystal grain growth distance is known to be proportional to the product of the crystallization time and the growth speed. Here, the term crystallization time is the amount of time from when the crystal nuclei develop within the semiconductor film until crystallization of the semiconductor film is complete. If the amount of time from the melting of the semiconductor film until crystallization is complete is taken as melting time, the melting time increased, and the cooling speed of the semiconductor film is taken as being leisurely, then the crystallization time becomes long, and crystal grains having a large grain size can be formed.
There are several different types of laser light, but in general, laser crystallization utilizing laser light having a pulse emission excimer laser (hereafter referred to as excimer laser light) is used. Excimer lasers have the advantages of high output, and the capability of repeated irradiation at high frequency, and in addition, excimer laser light has the advantage of a high absorption coefficient with respect to silicon films.
KrF (wavelength 248 nm) and XeCl (wavelength 308 nm) are used as excitation gases in excimer lasers. However, Kr (krypton) and Xe (xenon) gasses are extremely high cost, and if the frequency of gas replacement becomes high, this invites an increase in manufacturing costs.
Further, it is necessary to replace parts such as a laser tube for performing laser emission, and a gas purification apparatus for removing unnecessary compounds generated in the process of emission, on a 2 to 3 year basis. These attached parts are often expensive, and this also invites a problem of increased manufacturing costs.
Laser irradiation devices using excimer laser light possess high performance, as stated above, but require an extreme amount of efforts for maintenance. In addition, they also possess the disadvantage of high running cost when used as mass production laser irradiation devices (the term running cost meaning costs that develop along with operation).
In order to realize a laser irradiation apparatus having a low running cost compared with an excimer laser, and to realize a laser irradiation method using the laser irradiation apparatus, a method of using a solid state laser (a laser which outputs laser light with crystal rods as resonance cavities) can be used.
However, the grain size of crystal grains formed in accordance with laser crystallization using a YAG laser, which is one typical solid state laser, is extremely small compared to crystal grains formed by laser crystallization using an excimer laser.
It is thought that one reason is that although solid state lasers have high output at present, the output time is extremely short. Methods such as LD (laser diode) excitation and flash lamp excitation exist as methods of solid state laser excitation. In order to obtain high output by LD excitation, it is necessary to have a large electric current flow in LD. The LD lifetime is therefore short and the cost is increased compared with flash lamp excitation. For this reason, almost all LD excitation solid state lasers are small output. High output lasers for use in mass production are still in a development state at present. On the other hand, flash lamp can output an extremely strong light, and therefore lasers excited by flash lamps have high power. However, atoms excited by energy introduced instantaneously are emitted all at once with emission by flash lamp excitation, and therefore the laser output time is extremely short. Thus, solid state lasers at present have high output, but their output time is extremely short. Consequently, it is difficult to form crystal grains by laser crystallization using a solid state laser that have a grain size which is in the same order as, or greater than, the grain size formed by performing laser crystallization using an excimer laser. Note that the term output time refers to the half width of one pulse within this specification.
SUMMARY OF THE INVENTION
An object of the present invention is to provide a laser irradiation apparatus having low running cost in comparison with conventional laser irradiation apparatuses. In addition
Ishii Mikito
Kawaguchi Norihito
Masaki Miyuki
Nakajima Setsuo
Nishida Kenichiro
Cook Alex McFarron Manzo Cummings & Mehler, Ltd.
Dang Hung Xuan
Semiconductor Energy Laboratory Co,. Ltd.
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