Electric heating – Metal heating – By arc
Reexamination Certificate
2002-10-24
2004-06-15
Stoner, Kiley (Department: 1725)
Electric heating
Metal heating
By arc
Reexamination Certificate
active
06750423
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to a laser beam irradiation method and a laser irradiation apparatus for using the method (apparatus including a laser and an optical system for guiding laser beam emitted from the laser to an object to be irradiated). In addition, the present invention relates to a method of manufacturing a semiconductor device, which includes a laser beam irradiation step. Note that a semiconductor device described here includes an electro-optical device such as a liquid crystal display device or a light-emitting device and an electronic device that includes the electro-optical device as a part.
DESCRIPTION OF THE RELATED ART
In recent years, a wide study has been made on a technique in which laser annealing is performed for a semiconductor film formed on an insulating substrate made of glass or the like, to crystallize the film, to improve its crystallinity so that a crystalline semiconductor film is obtained, or to activate an impurity element. Note that a crystalline semiconductor film in this specification indicates a semiconductor film in which a crystallized region is present, and also includes a semiconductor film that is crystallized as a whole.
A method of forming pulse laser beam from an excimer laser or the like by an optical system such that it becomes a square shape or a linear shape on an irradiation surface, and scanning the laser beam (or relatively shifting an irradiation position of the laser beam with respect to the irradiation surface) to conduct annealing is superior in mass productivity and is excellent in technology. The “linear shape” described here means not a “line” in the strict sense but a rectangle (or a prolate ellipsoid shape) having a high aspect ratio. For example, it indicates a shape having an aspect ratio of 10 or more (preferably, 100 to 10000). Note that the linear shape is used to obtain an energy density required for sufficiently annealing an object to be irradiated. Thus, if sufficient annealing is conducted for the object to be irradiated, it may be either a rectangular shape or a planar. Presently, excimer lasers with 15 J/pulse come onto the market and there is a possibility to perform a laser anneal by a planar beam. Further, the spot of the laser light is made laser light's energy distribution on an irradiation surface of the laser light when there is not a special definition.
FIGS. 10A and 10B
show an example of a configuration of an optical system for forming laser beam in a linear shape on an irradiation surface. This configuration is extremely general. All optical systems described above are based on the configuration shown in
FIGS. 10A and 10B
. According to the configuration, a cross sectional shape of laser beam is converted into a linear shape, and simultaneously an energy density distribution of laser beam on the irradiation surface is homogenized. In general, an optical system for homogenizing the energy density distribution of laser beam is called a beam homogenizer.
The spot of the laser beam emitted from a laser
71
is divided by a cylindrical lens array
73
. The direction is called a first direction in this specification. It is assumed that, when a mirror is inserted in a course of an optical system, the above-mentioned first direction is changed in accordance with a direction of light bent by the above-mentioned mirror. In this configuration, the cylindrical lens array is divided into seven parts. Then, the laser beams are superposed on an irradiation surface
79
by a cylindrical lens
74
, thereby homogenizing an energy density distribution of the linear laser beam in the longitudinal direction, and the length of the longitudinal direction is determined.
Next, the configuration shown in the side view of
FIG. 10B
will be described. The spot of the laser beam emitted from a laser
71
is divided by cylindrical lens arrays
72
a
and
72
b
. The direction is called a second direction in this specification. It is assumed that, when a mirror is inserted in a course of an optical system, the second direction is changed in accordance with a direction of light bent by the mirror. In this configuration, the cylindrical lens arrays
72
a
and
72
b
each are divided into four parts. The divided laser beams are temporarily synthesized by a cylindrical lens
74
. After that, the laser beams are reflected by a mirror
77
and then condensed by a doublet cylindrical lens
78
so that they become again single laser beam on the irradiation surface
79
. The doublet cylindrical lens
78
is a lens composed of two cylindrical lenses. Thus, an energy density distribution of the linear laser beam in a width direction is homogenized, thereby homogenizing an energy density distribution of the linear laser beam in the longitudinal direction, and the length of the width direction is determined.
For example, an excimer laser in which a size in a laser window is 10 mm×30 mm (which each are a half-width in beam profile) is used as the laser
71
and laser beam is produced by the optical system having the configuration shown in
FIGS. 10A and 10B
. Then, linear laser beam which has a uniform energy density distribution and a size of 125 mm×0.4 mm can be obtained on the irradiation surface
79
.
At this time, when, for example, quartz is used for all base materials of the optical system, high transmittance is obtained. Note that coating is preferably conducted for the optical system such that transmittance of 99% or more is obtained at a frequency of the used excimer laser.
Then, the linear laser beam formed by the above configuration is irradiated with an overlap state while being gradually shifted in a width direction thereof. Thus, when laser annealing is performed for the entire surface of an amorphous semiconductor film, the amorphous semiconductor film can be crystallized, crystallinity can be improved to obtain a crystalline semiconductor film, or an impurity element can be activated.
At an edge of linear, rectangular, or sheet-like laser light formed on an irradiation surface or in the vicinity thereof by an optical system, the energy density is attenuated gradually due to aberration of a lens or the like (FIG.
11
A). In this specification, a region at a laser light edge where the energy density is gradually attenuated is called an attenuation region.
As the substrate area is increased and the laser power is raised, it is now possible to form a longer linear beam or rectangular beam and a larger sheet-like beam. Annealing with such laser light is more efficient. However, the energy density of laser light emitted from a laser is smaller at its edge than around the center. Therefore, if laser light is expanded by an optical system more than prior art, attenuation in the attenuation region is intensified.
In the attenuation region, the energy density is lower than a region having a high uniformity in energy density and the low energy density is attenuated gradually. For that reason, an irradiation object cannot be annealed uniformly by laser light that has the attenuation region (FIG.
11
B). Even when the laser light scans an irradiation object for annealing in a manner that makes the attenuation regions overlap each other, it still is impossible to anneal the irradiation object uniformly because annealing conditions of the attenuation region are entirely different from annealing conditions of the highly uniform region. Accordingly, a region annealed by the attenuation region of laser light and a region annealed by the highly uniform region of the laser light cannot be treated equally.
For example, when a semiconductor film is an irradiation object, a region of the semiconductor film that is annealed by the attenuation region and a region of the semiconductor film that is annealed by the highly uniform region have different crystallinity. Therefore, if this semiconductor film varied in crystallinity from one region to another is used to manufacture TFTs, the electric characteristic of a TFT formed from the region that is annealed by the attenuation region is inferior to other
Moriwaka Tomoaki
Tanaka Koichiro
Johnson Jonathan
Robinson Eric J.
Robinson Intellectual Property Law Office P.C.
Semiconductor Energy Laboratory Co,. Ltd.
Stoner Kiley
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