Coherent light generators – Particular beam control device – Modulation
Reexamination Certificate
2002-09-12
2004-05-25
Ip, Paul (Department: 2828)
Coherent light generators
Particular beam control device
Modulation
C372S027000
Reexamination Certificate
active
06741621
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to a laser irradiation apparatus and a method of treating a semiconductor thin film, and particularly to a laser irradiation apparatus used preferably for a treatment for crystallizing a semiconductor thin film, and a method of treating a semiconductor thin film conducted by use of the laser irradiation apparatus.
Thin film transistors (abbreviated to TFT) widely used as switching devices for flat-type display systems such as liquid crystal display systems include the TFT using polycrystalline silicon as an active layer (polycrystalline silicon TFT) and the TFT using amorphous silicon as an active layer (amorphous silicon TFT). Among these, the polycrystalline silicon TFT is higher in driving current as compared with the amorphous silicon TFT, so that it has the merit that it is possible to miniaturize the switching devices and enlarge the numerical aperture of pixel. In addition, the polycrystalline silicon TFT can be used not only as the switching device but also as peripheral driving circuits, for example, a shift register or a level converter, and these peripheral circuits can be formed on a display substrate in the same step as formation of the switching device. For these reasons, the polycrystalline silicon TFT is used as a device for high-precision display systems.
In recent years, a technology for fabricating the polycrystalline silicon TFT by a low-temperature process at or below 600° C. (the so-called low temperature polysilicon process) has been developed and put to practical use. By applying the low temperature polysilicon process to the production of a flat-type display system, it becomes unnecessary to use a high heat-resistant but expensive substrate such as quartz and single-crystalline silicon as the display substrate, so that it is possible to achieve reductions in cost and size of the high-precision flat-type display system.
Here, the low temperature polysilicon process is a method of obtaining a polycrystalline silicon layer by irradiating a silicon layer (amorphous silicon layer) formed on a substrate with laser light or electron beam to rapidly heat and melt the silicon without damaging the substrate, and crystallizing the silicon through the subsequent cooling process to obtain the polycrystalline silicon layer.
In order to obtain a polycrystalline silicon layer with a greater grain diameter in such a low temperature polysilicon process, the method of irradiating the silicon layer with the laser light or electron beam is important. In the low temperature polysilicon process at present, a multi-shot irradiation method is widely used. In the multi-shot irradiation method, the laser beam is scanningly radiated onto the silicon layer, and the same portion of the silicon layer is irradiated with laser at least two times, typically, 10 to 20 times. By this, it is known that, for example, in the case of a silicon layer having a thickness of 50 nm, a polycrystalline silicon layer with a grain diameter of 0.1 to 5 &mgr;m, typically, about 0.3 to 1 &mgr;m is obtained.
Other than the low temperature polysilicon process applying the multi-shot irradiation method mentioned above, there is known a sequential lateral solidification method (hereinafter referred to as SLS method) as reported, for example, in Applied Physics Letters, vol. 69, pp. 2864 to 2866 (1996).
FIG. 13
shows an outline of a treatment of a semiconductor thin film by the SLS method. In the method shown in the figure, first, a laser beam H oscillated from a laser light generating means
1
is made to be incident on a mask
6
having a periodic light-dark pattern, by use of optical means
2
to
5
such as lenses and reflectors. The laser beam H transmitted through the mask
6
is radiated onto a silicon layer on the surface of a substrate W mounted on a stage
9
through a focusing lens
7
and a reflector
8
, whereby the silicon layer is perfectly melted in a width of several &mgr;m. At the time of cooling, crystals are grown laterally from peripheral portions toward the inside of the melting regions, and stripe form lateral growth regions are obtained. Next, the mask
6
or the stage
9
with the substrate W mounted thereon is mechanically moved by a distance not more than the width of the melting regions, typically, about 0.75 &mgr;m, and then lateral growth is caused similarly to the above. It is reported that by such a method, it is possible to obtain an elongate polycrystalline silicon thin film which is uniform over a wide area and has grain boundaries parallel to the scanning direction.
Further, as an example of application of the SLS method, Japanese Patent Laid-open No. 2000-150412 discloses a method in which the above-mentioned periodic light-dark pattern is formed as an interference fringe by interference of laser light. The publication also discloses the technique of changing the positions of the interference fringe and, hence, the melting positions of the silicon layer, by moving mirrors and a stepped transmissive plate disposed on the optical path of the laser light by a mechanical means.
However, in the process applying the multi-shot irradiation method, of the above-mentioned low temperature polysilicon processes, the crystal size of the polycrystalline silicon obtained (grain diameter: 0.1 to 5 &mgr;m) is extremely small as compared with the size of the thin film transistors at present (about 5 to 50 &mgr;m square). Therefore, the characteristic of the thin film transistor fabricated by use of the polycrystalline silicon is, for example, such that the electron mobility is as low as 100 cm
2
/Vs due to carrier trap at grain boundaries of the polycrystalline silicon; thus, the thin film transistor obtained is inferior to the transistor fabricated in single-crystalline silicon.
Here, in a display system using a thin film transistor, if the characteristics of the thin film transistor in a display area are dispersed, it is recognized as dispersion of display characteristics, resulting in low display quality. The dispersion of the thin film transistor characteristics is due primarily to the dispersion of polycrystalline grain diameter, which arises from dispersion of laser energy in the polycrystallization process, specifically, dispersion on an irradiation shot basis and distribution of light intensity in the irradiation plane.
FIG. 14
shows the variation of mean grain diameter of polycrystalline silicon against laser energy in the case where the same portion is irradiated with laser light 20 times. From the figure, it is seen that where the laser energy may possibly vary by ±8%, if the laser energy exceeds the irradiation energy of 380 mJ/cm
2
corresponding to the maximum grain diameter even once in the 20 times of irradiation, the grain diameter is rapidly lowered, and fine crystallization occurs partly, so that it is necessary to perform irradiation at 350 mJ/cm
2
. It is also seen that the dependency of grain diameter on laser energy is heavy and, for example, a dispersion of energy of only ±1% leads to a dispersion of grain diameter of no less than about ±10%. However, suppressing the dispersion of laser energy to, for example, within ±0.5% is difficult to achieve at present, because the pulse oscillation is performed in a short time (for example, in the case of excimer laser, the pulse width is 20 to 200 ns); accordingly, the crystal grain diameter is dispersed.
On the other hand, in the SLS method, it is possible to obtain large-grain-diameter crystals uniform over a wide area. However, since the irradiation of a semiconductor thin film with laser light is conducted through the mask
6
and the lenses
5
,
7
of the focusing optical system, utilization efficiency of light energy is lowered, resulting in increases in treatment time and cost per substrate. In addition, there is need for a mechanism for correction of focusing errors due to waviness of the substrate or the like, which also leads to increase in treatment time and cost per substrate. Further, the stage on which to mount the substr
Depke Robert J.
Holland & Knight LLP
Ip Paul
Nguyen Tuan
Sony Corporation
LandOfFree
Laser irradiation apparatus and method of treating... does not yet have a rating. At this time, there are no reviews or comments for this patent.
If you have personal experience with Laser irradiation apparatus and method of treating..., we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Laser irradiation apparatus and method of treating... will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-3228662