Electric heating – Metal heating – By arc
Reexamination Certificate
2002-02-04
2004-02-17
Dunn, Tom (Department: 1725)
Electric heating
Metal heating
By arc
Reexamination Certificate
active
06693258
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a process for producing a thin film semiconductor device and a laser irradiation apparatus. The laser irradiation apparatus is used for crystallizing a semiconductor thin film by using excimer laser light in the process for producing a thin film semiconductor device.
2. Description of the Related Art
A thin film transistor used for switching pixels of an active matrix type liquid crystal display device, a thin film transistor formed in a peripheral circuit driving a switching transistor, and a thin film transistor used in a load device type static RAM employ an active layer comprising amorphous silicon or polycrystalline silicon. Since polycrystalline silicon has higher mobility than amorphous silicon, a thin film transistor of high performance can be obtained. However, because polycrystalline silicon contains non-bonding pairs of silicon atoms in a high density in comparison to single crystal silicon, the non-bonding pairs cause a leakage electric current on channel off. As a result, it becomes a cause of lowering the operation speed on switch on. Therefore, in order to improve the characteristics of the thin film transistor, it is demanded to form a semiconductor thin film of polycrystalline silicon having less crystal defects and excellent uniformity. As a process for forming such polycrystalline silicon thin film, a chemical gas phase growing method and a solid phase growing method are proposed. As means for decreasing the non-bonding pairs, which causes the leakage electric current, a hydrogenation technique is employed, in which the non-bonding pairs are terminated by doping hydrogen in the polycrystalline silicon thin film. However, when crystals having a large particle size are grown by the chemical gas phase growing method to form a polycrystalline silicon thin film, the film thickness thereof becomes uneven. Accordingly, it is difficult to form a transistor having uniform device characteristics by using the polycrystalline silicon thin film.
As a process for forming a polycrystalline semiconductor thin film considering the problems described above, an annealing treatment using excimer laser light is proposed. In this process, a non-single crystal semiconductor thin film, such as amorphous silicon and polycrystalline silicon having a relatively small particle diameter, formed on an insulating substrate is irradiated with laser light to locally heated, and the semiconductor thin film is converted into polycrystals having a relatively large particle diameter (crystallization) during the cooling process. A thin film transistor is formed by integrating using the semiconductor thin film thus crystallized as an active layer (channel region). By using the laser annealing, a thin film semiconductor device can be produced by a low temperature process, and an inexpensive glass substrate can be used instead of an expensive quartz substrate excellent in heat resistance. Furthermore, because the excimer laser light is in the ultraviolet region, which results in large absorption coefficient for silicon, it has an advantage in that only the surface of the silicon can be locally heated but the insulating substrate is not thermally damaged. A method of the laser annealing includes a first method, in which an amorphous silicon thin film is directly irradiated with excimer laser light to convert into polycrystals, and a second method, in which a polycrystalline silicon thin film formed by solid phase growth is irradiated with excimer laser light at such an energy level that the whole film is not melted to conduct annealing.
The first method, the direct annealing of the amorphous silicon thin film, is advantageous for mass production of an LSI in future since it is simple in process in comparison to the second method. Furthermore, when a large area can be subjected to the bulk annealing treatment by irradiation of excimer laser light at a time, it is further advantageous for mass production. However, in the case where a conventional laser irradiation apparatus is used for the direct annealing of an amorphous silicon thin film, it has been difficult to obtain excimer laser light that has a large area with uniform cross sectional distribution of energy at a single shot sufficient to obtain a polycrystalline silicon thin film excellent in crystallinity and having small density of grain boundary trap. In order to solve the problem, an excimer laser irradiation apparatus is being developed that has such high output energy that a large area can be subjected to the bulk annealing treatment with a single shot at a time. Furthermore, in order to improve the effect of annealing using excimer laser light, a method is proposed, in which a substrate is previously heated to several hundreds degrees centigrade, and then the direct annealing of the amorphous silicon is conducted. However, even by using the excimer laser irradiation apparatus of high output energy, process conditions for obtaining a polycrystalline silicon thin film excellent in crystallinity and having small density of grain boundary trap have not yet been established. Furthermore, in the conventional direct annealing method of amorphous silicon, the crystalline particle diameter of polycrystalline silicon obtained is 50 nm or less in average, and thus further increase in crystalline particles size is demanded. In the laser annealing process using such a laser irradiation apparatus of high output of an emission duration of 50 ns or more, the crystallization process is conventionally conducted in the air. In this case, however, crystal defects are formed by combining oxygen in the air and silicon, and therefore there is a problem in that the mobility of the thin film transistor is not so improved that expected from the particle diameter (grain size) of the polycrystalline silicon.
As for the laser irradiation apparatus emitting excimer laser light, because output energy of laser light of the conventional apparatus is small (about 0.5 J), a method has been generally employed in that a linear beam having an irradiation area of 200 mm×0.6 to 0.7 mm is irradiated with an overlap of about from 90 to 95%. In this method, however, the output stability of laser is poor (about ±10% in the current situation), and non-uniformity of crystals is caused at a part where the output energy becomes unexpectedly large or small. When a circuit is integrated and formed on such a part, it becomes a cause of operation failure. It is also considered to employ an overlap of the laser beam of about 99% to disperse the dispersion of output as possible. However, such a method involves a problem in that the throughput becomes extremely poor to bring about increase in production cost. When the crystallization is conducted by using the conventional linear laser beam with an overlap, for example, of 95%, it requires about 6 minutes for treating a substrate of 400 mm×500 mm. When the same substrate is treated with an overlap of the linear laser beam of 99%, it requires 30 minutes. Furthermore, since the laser annealing is generally conducted in vacuum, it requires about 5 minutes for loading and unloading of the substrate.
In recent years, an excimer laser irradiation apparatus having high output energy that can conduct the annealing treatment of a large area with a single shot at a time has been developed as described in the foregoing. For example, an area of about 27 mm×67 mm can be irradiated at a time by using, for example, an excimer laser light source having output of 10 J. However, in order to produce a large area LCD panel having a dimension across corners of about 20 inches (about 120 mm×160 mm) required for a large display, a “boundary part” of laser irradiation is necessarily formed in either method. There is a problem in that when the other part than the “boundary part” is irradiated at optimum energy, the “boundary part” is over-irradiated, in which the semiconductor thin film is microcrystallized to deteriorate the performance of the thin
Asano Akihiko
Fujino Masahiro
Ino Masumitsu
Mano Michio
Sugano Yukiyasu
Dunn Tom
Johnson Jonathan
Kananen Ronald P.
Rader & Fishman & Grauer, PLLC
Sony Corporation
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