Semiconductor device manufacturing: process – Formation of semiconductive active region on any substrate – Amorphous semiconductor
Reexamination Certificate
2000-07-27
2002-08-27
Elms, Richard (Department: 2824)
Semiconductor device manufacturing: process
Formation of semiconductive active region on any substrate
Amorphous semiconductor
C438S166000
Reexamination Certificate
active
06440824
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a method of crystallizing a semiconductor thin film, and a laser irradiation apparatus which is used for the method. Also, it relates to a thin film transistor and a display device, for example, LCD or organic EL display as are fabricated by utilizing the method and the apparatus.
2. Description of the Related Art
Crystallizing annealing which employs laser light has been developed as a part of an expedient by which a process for manufacturing a thin film transistor is turned into a low temperature process. This consists in that a non-monocrystalline semiconductor thin film of amorphous silicon, polycrystal silicon of comparatively small grain diameters, or the like formed on an insulating substrate, is irradiated with laser light to be locally heated, whereupon the semiconductor thin film is converted (crystallized) into a polycrystal of comparatively large grain diameters in the cooling process thereof. The thin film transistor is integrally formed using the crystallized semiconductor thin film as an active layer (channel region). Owing to the adoption of such crystallizing annealing, it is permitted to establish a low temperature process for a thin-film semiconductor device, and to use an inexpensive glass substrate, unlike an expensive quartz substrate of excellent heat resistance.
In the crystallizing annealing, it is common practice that the pulses of laser light in the shape of a line are intermittently projected while being overlapped in a scanning direction. The semiconductor thin film can be crystallized comparatively uniformly by projecting the laser light overlappingly. The crystallizing annealing which employs the laser light in the linear shape (line beam) is schematically illustrated in FIG.
11
. The laser light
50
shaped into the line extending in the Y-direction of an insulating substrate
1
made of glass or the like is projected from the front surface side of the insulating substrate
1
formed with a semiconductor thin film beforehand. On this occasion, the insulating substrate
1
is moved in an X-direction (scanning direction) relatively to the irradiation region thereof. Here, the irradiation is done while the line beam
50
emitted from the light source of an excimer laser is being intermittently moved in overlapping fashion. More specifically, the insulating substrate
1
is scanned through a stage member in the X-direction relatively to the line beam
50
. The stage member is moved at a pitch smaller than the widthwise dimension of the line beam
50
by one shot, so that the whole substrate
1
can be irradiated with the line beam
50
, thereby to carry out the crystallizing annealing.
Laser light is sequentially outputted as pulses from a laser light source. The intensities (energy densities per unit area) of the individual pulses are not always constant, but they fluctuate in excess of ±15 [%] even with an up-to-date laser light source. Therefore, in a case where the laser light has been projected by overlapping the pulses repeatedly, local dispersion comes out in the diameters of the crystal grains of a crystallized semiconductor thin film in accordance with dispersion in the intensities of the individual pulses. This appears as dispersion in the characteristics of thin film transistors which are integrally formed on an insulating substrate. In a case where a display device, such as liquid crystal panel, has been fabricated using such an insulating substrate, the characteristics dispersion appears as non-uniformity in an image quality or as pixel defects.
FIG. 12
is a schematic plan view illustrating an example of a region of irradiation with a line beam. The irradiation region is in, for example, an elongate shape having longer latera of 200 [mm] and shorter latera of 400 [&mgr;m], and it scans in the direction of the shorter latera. Irradiation regions adjacent to each other overlap at their parts of, for example, 95 [%]. Accordingly, the line beam having the shown irradiation region is moved stepwise at intervals of 20 [&mgr;m]. When note is taken of one point on a substrate, the line beam passes 20 times at the steps of 20 [&mgr;m], and the point is irradiated with laser light 20 times in total.
FIG. 13
is a graph schematically showing the sectional intensity distribution of the line beam along line X—X indicated in FIG.
12
. In general, the sectional intensity distribution of a line beam in the shorter axial direction thereof is in the shape of a rectangle. When the line beam scans at the steps of 20 [&mgr;m], a certain point on an insulating substrate is intermittently irradiated with laser light 20 times. Thus, a semiconductor thin film corresponding to the point repeats melting based on the laser irradiation and solidification based on cooling, 20 times, and crystal grains enlarge meantime. In actuality, however, dispersion is involved in the intensities of individual laser beams as stated before. Accordingly, when one point is noted, it is not irradiated with energy being always at the same level, repeatedly 20 times, but it is struck by energy having a dispersion of about ±15 [%]. In general, the crystal grains enlarges more as the laser light intensity is higher, but they turn into microcrystals contrariwise when a critical intensity is exceeded. Accordingly, when an abrupt upward fluctuation in the energy exists during the repeated pulse irradiation, the crystal grains might turn into the microcrystals on the contrary. Especially in the case of noting one place, when the abrupt upward fluctuation of the energy occurs at the final step among the 20 times of repeated irradiating steps, the crystal state of the place ends in a microcrystalline one left intact. Conversely, when the line beam of high energy is abruptly projected at the initial step among the 20 times of repeated irradiating steps, hydrogen might ablate on the occasion of the melting of the semiconductor thin film of amorphous silicon which has contained the hydrogen in large amounts at the stage of forming the film. When the ablation occurs, the semiconductor thin film itself changes in quality, and no normal crystal grains can be obtained even by thereafter irradiating the thin film with the line beam repeatedly.
SUMMARY OF THE INVENTION
In order to solve the problems of the prior art as stated above, means to be explained below have been adopted. The present invention consists in a method of crystallizing a semiconductor thin film, having the shaping step of shaping laser light emitted from a laser light source, thereby to define a laser beam which has a predetermined intensity distribution in a predetermined irradiation region; and the irradiating step of repeatedly irradiating the semiconductor thin film formed over a substrate beforehand, with the laser beam while scanning the film so that irradiation regions may be overlapped; characterized in that said shaping step shapes said laser beam so that a sectional intensity distribution of said laser beam in the irradiation region as taken in a direction of the scanning may be convex, and that a peak of the intensity distribution may lie at a position which is between a front end and a rear end of said irradiation region in relation to the scanning direction and which is nearer to the front end with respect to the middle of said irradiation region. Preferably, said shaping step shapes said laser beam so that an intensity at said front end of said irradiation region may become lower in a range within 30 [%], as compared with an intensity of the peak. Also, said shaping step shapes said laser beam so that an intensity at the rear end of said irradiation region may become lower in a range exceeding 5 [%], as compared with the intensity of the peak. Besides, said shaping step shapes said laser beam so that the intensity of the peak may become lower in a range exceeding 10 [%], as compared with a critical intensity
Hayashi Hisao
Ikeda Hiroyuki
Takatoku Makoto
Elms Richard
Kananen, Esq. Ronald P.
Pyonin Adam
Rader & Fishman & Grauer, PLLC
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