Semiconductor device manufacturing: process – Formation of semiconductive active region on any substrate – Amorphous semiconductor
Reexamination Certificate
2002-07-29
2004-08-03
Niebling, John F. (Department: 2812)
Semiconductor device manufacturing: process
Formation of semiconductive active region on any substrate
Amorphous semiconductor
C438S378000, C438S478000, C438S622000, C438S795000, C438S166000
Reexamination Certificate
active
06770546
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a laser treatment apparatus for crystallization or activation after ion implantation, of a semiconductor substrate, a semiconductor film or the like using a laser beam and to a method of manufacturing a semiconductor device using such a laser treatment apparatus.
2. Description of the Related Art
Laser annealing is performed to crystallize a semiconductor wafer or a non-single crystalline semiconductor film in a semiconductor manufacturing process or to recover the crystallinity after ion implantation. A conventional laser annealing method involves a method of uniformly irradiating the entire surface of an object with a laser beam as is disclosed in JP 2-181419 A, a method of scanning a spot-shaped beam as is disclosed in JP 62-104117 A, or a method of irradiating a linearly processed beam obtained by an optical system as in a laser treatment apparatus disclosed in JP 8-195357 A.
The term “laser annealing” herein indicates a technique for recrystallizing a damaged layer or an amorphous layer formed in a semiconductor substrate or a semiconductor film or a technique for crystallizing an amorphous semiconductor film formed on a substrate. The “laser annealing” also includes a technique that is applied to leveling or improvement of a surface quality of the semiconductor substrate or the semiconductor film. Applicable laser oscillation devices are: gas laser oscillation devices represented by an excimer laser; and solid laser oscillation devices represented by a YAG laser. Such laser oscillation devices are known to heat a surface layer of a semiconductor by laser beam irradiation for an extremely short period of time, i.e., about several tens to several hundreds of nanoseconds so as to crystallize the surface layer.
For example, the above-cited JP 62-104117 A discloses a technique of polycrystallizing an amorphous semiconductor film with a laser beam scanning speed set so as to be beam spot diameter×5000/seconds or higher, without bringing the amorphous semiconductor film into a completely melted state. U.S. Pat. No. 4,330,363 B discloses a technique of irradiating a semiconductor region formed in an island-like shape with an elongated laser beam to substantially form a single-crystalline region.
The laser annealing is characteristic in that a treatment time period can be remarkably reduced as compared with annealing utilizing radiant heating or thermal conductive heating, that a semiconductor or a semiconductor film is selectively and locally heated so that a substrate is scarcely thermally damaged, and the like.
Recently, the laser annealing has been positively utilized for forming a polycrystalline silicon film on a glass substrate. This formation process is applied to the manufacture of a thin film transistor (TFT) which is used as a switching element of a liquid crystal display device. Since only a region where a semiconductor film is formed is thermally affected with the use of an excimer laser, the laser annealing with the excimer laser allows the use of a cheap glass substrate, thereby realizing the application of a glass substrate to a large display.
Moreover, since a TFT, which is manufactured of a polycrystalline silicon film crystallized by the laser annealing, can be driven at a relatively high frequency, not only a switching element provided at a pixel but also a driver circuit can be formed on a glass substrate. A pattern design rule is about 5 to 20 &mgr;m. Thus, about 10
6
to 10
7
TFTs are formed in a driver circuit and a pixel portion on the glass substrate, respectively.
The crystallization of amorphous silicon by laser annealing is achieved through a process of melt-solidification. More specifically, this process is regarded as a two-stage process consisting of: the generation of crystal nuclei; and the crystal growth from the crystal nuclei. However, the laser annealing using a pulsed laser beam is not capable of controlling the positions where crystal nuclei are formed and the density of the formed crystal nuclei, leaving the crystal nuclei spontaneously generated. Therefore, crystal grains are formed at arbitrary positions in a plane of a glass substrate. Moreover, the size of the obtained crystal grain is small, i.e., about 0.2 to 0.5 &mgr;m. A number of defects are generated at crystal grain boundaries, which is regarded as a primary factor of limiting a field effect mobility of the TFT.
In a technique related to the above-mentioned JP 62-104117 A, however, the crystal growth due to crystal nuclei which are believed to be formed in an unmelted region is dominant because a semiconductor film is not completely melted. As a result, the increase in size of crystal grains cannot be realized. More specifically, substantially single-crystalline crystals cannot be formed over the entire surface of a semiconductor film on which a channel region of a TFT is to be formed.
In the first place, a crystallization method with continuous wave laser scanning while effecting melt-solidification is considered to be close to a zone melting method. Although a high energy density is required to melt a semiconductor, this crystallization method has drawbacks in that it is difficult to realize a high output with the continuous wave laser, resulting in increase in apparatus size. Ultimately, the beam size is reduced through an optical system to be radiated to a semiconductor. With such beam size, however, a considerably long time period for the treatment is required to achieve the crystallization over the entire surface of a large substrate.
A laser beam capable of heating a semiconductor film is present in a wide range over an ultraviolet range to an infrared range. It is preferred to use a laser beam having a wavelength in an ultraviolet to visible range for selectively heating a semiconductor film or a semiconductor region formed on a substrate in view of the relationship with an absorption coefficient of a semiconductor. However, since a laser beam emitted from a solid laser oscillation device exhibits a strong coherence, the interference is caused on the irradiated surface. As a result, a uniform laser beam cannot be radiated.
SUMMARY OF THE INVENTION
The present invention is devised in view of the above problems, and has an object to provide a laser treatment apparatus for irradiating the position where a TFT is to be formed with a laser beam over the entire surface of a large substrate to achieve the crystallization, thereby being capable of forming a crystalline semiconductor film having a large grain diameter with high throughput.
In order to achieve the above-mentioned object, according to the present invention, there is provided a laser irradiation apparatus including: a first movable mirror for deflecting a laser beam in a main scanning direction; and an elongated second movable mirror for receiving the laser beam deflected in the main scanning direction and scanning the laser beam in a sub scanning direction, in which the second movable mirror includes means for scanning the laser beam in the sub scanning direction at a rotation angle around an axis in a longitudinal direction of the second movable mirror and irradiating an object to be treated which is placed on a stage with the laser beam.
Also, according to another structure of the present invention, there is provided a laser irradiation apparatus including: a first laser beam scanning system including a first movable mirror for deflecting a laser beam in a first main scanning direction, and an elongated second movable mirror for receiving the laser beam deflected in the first main scanning direction and scanning the laser beam in a first sub scanning direction; and a second laser beam scanning system including a third movable mirror for deflecting a laser beam in a second main scanning direction, and a fourth movable mirror for receiving the laser beam deflected in the second main scanning direction and scanning the laser beam in a second sub scanning direction, in which: the second movable mirror includes means for sca
Costellia Jeffrey L.
Isaac Stanetta
Niebling John F.
Nixon & Peabody LLP
Semiconductor Energy Laboratory Co,. Ltd.
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