Laser beam scanning method

[0002] 1. Field of the Invention

[0003] The present invention relates to a laser beam scanning method for laser activation and crystallization for a thin film transistor substrate in a large area-liquid crystal display.

[0004] 2. Discussion of the Related Art

[0005] Laser beam scanning is used to recover or recrystallize a part of a polycrystalline silicon layer which becomes amorphous due to doping with ions, or to crystallize an amorphous silicon layer.

[0006] In general, these processes (referred to as activation of the silicon layer using laser) proceeds by supplying a surface of the layer with laser energy by scanning the layer repeatedly with a laser beam having predetermined spatial and energy profiles.

[0007] FIG. 1 shows schematically laser scanning according to a technique of the related art. FIG. 2 shows schematically the spatial profile 21 and the energy profile 22 of the laser beam in the length direction of the spatial profile. The laser beam 13 which has an elongated spatial profile 21 is generated by a laser equipment known in the art. The energy profile 22 of the laser beam is substantially flat along the entire length of the beam and has an elongated step shape with steep edges.

[0008] Referring to FIG. 1, laser scanning is carried out by irradiating a thin film 10, such as an amorphous silicon film, with the laser beam 13 at a predetermined repetition rate and translating the thin film 10, thereby scanning the whole surface of the thin film. The translating direction of the thin film is opposite to the scanning direction of the laser beam.

[0009] However, the length of the laser beam generated by known laser equipment has an upper limit. Typically, the length of the laser beam is about 200-300 mm with a width of about 0.3-2.0 mm. Therefore, when scanning a large area silicon film that is wider than the length of the laser beam, the entire surface of a large area film cannot be irradiated with a single scanning of the laser beam. An example of such a large area film is a wide vision LCD panel having a diagonal dimension of 12.1 inches or more.

[0010] Accordingly, the present invention is directed to a laser beam scanning method that substantially obviates one or more of the problems due to limitations and disadvantages of the related art.

[0011] The object of the present invention is to provide a laser beam scanning method which supplies a large area substrate with uniform laser energy.

[0012] Additional features and advantages of the invention will be set forth in the description which follows and in part will be apparent from the description, or may be learned by practice of the invention. The objectives and other advantages of the invention will be realized and attained by the structure particularly pointed out in the written description and claims hereof as well as the appended drawings.

[0013] To achieve these and other advantages and in accordance with the purpose of the present invention, as embodied and broadly described, the present invention provides a laser beam scanning method, wherein the laser beam has a spatial profile of an elongated rectangular shape and a substantially flat energy profile, the end portions of the energy profile having energy slope regions, the energy slope being defined by the ratio of the laser energy density to the width of the slope region.

[0014] Another aspect of the present invention provides a laser beam scanning method, in which a large-area film is irradiated by a laser beam scanning across the film two or more times, to provide laser energy to the whole large-area film, the laser beam being displaced relative to the film in a direction perpendicular to the scan direction before each subsequent scan. The laser beam has a spatial profile having an elongated rectangular shape along an longitudinal axis, and a substantially flat energy profile along the longitudinal axis, where the end portions of the energy profile have energy slope regions, the energy slope being defined by the ratio of the laser energy density and the width of the slope region. The energy slope regions of the successive laser beams are selectively overlapped to supply the substrate with uniform laser energy.

[0015] It is to be understood that both the foregoing general description and the following detailed description are exemplary and explanatory and are intended to provide further explanation of the invention as claimed.

[0016] The accompanying drawings, which are included to provide a further understanding of the invention and are incorporated in and constitute a part of this application, illustrate embodiments of the inventing and together with the description serve to explain the principle of the invention.

[0017] FIG. 1 is a schematic drawing showing laser scanning according to related art;

[0018] FIG. 2 is a schematic drawing showing a spatial profile and an energy profile of a laser beam;

[0019] FIG. 3 is a drawing showing laser scanning method according to an embodiment of the present invention;

[0020] FIG. 4 is a schematic drawing showing a spatial profile and an energy profile of a first laser beam according to an embodiment of the present invention;

[0021] FIG. 5 is a schematic drawing showing the laser energy distribution of a laser beam overlapping region during laser beam scanning according to a first embodiment of the present invention;

[0022] FIG. 6 is a schematic drawing showing the laser energy distribution of a laser beam overlapping region during laser beam scanning according to a second embodiment of the present invention; and

[0023] FIGS. 7A to 7D are schematic drawings showing the laser energy distribution of a laser beam overlapping region during laser beam scanning according to a third, a fourth, a fifth and a sixth embodiments of the present invention;

[0024] Reference will now be made in detail to the preferred embodiment of the present invention, an example of which is illustrated in the accompanying drawings.

[0025] FIG. 3 is a drawing showing laser scanning according to a first embodiment of the present invention. FIG. 3 shows an overlapped portions of laser beams when a large area-film 30 is scanned by a laser beam 33. The large area film or substrate 30 is irradiated by the laser beam scanning across the film 30 two times (or more than two times if necessary to cover the entire film), including a first scan 31 and a second scan 32, to provide laser energy to the entire large-area film 30. For the second scan 32, the laser beam 33 is displaced with respect to the film 30 in the direction perpendicular to the scan direction (indicated by the arrow 35) by a distance less than the length of the laser beam. As a result, the laser beam for the first and second scans overlaps in an area 34. Such overlap of the laser beams can cause voids and other problems in the film 30.

[0026] When displacing the laser beam with respect to the film, either one of the laser beam and the film may be physically displaced while the other one stays stationary. When two laser beams are said to overlap in an area, such as the area 34, what is meant is that the area 34 of the film is irradiated by both laser beams. In particular, when a first portion of a first laser beam is said to overlap a second portion of a second laser beam in an area, what is meant is that that area of the film is irradiated by both the first portion of the first laser beam and the second portion of the second laser beam.

[0027] FIG. 4 shows schematically a spatial profile 41 and an energy profile 42 of the laser beam according to an embodiment of the present invention. The spatial profile 41 of the laser beam has an elongated rectangular shape defining an longitudinal axis perpendicular to the direction of the scan. The energy profile 42 of the laser beam is substantially flat in the region 44 over the entire length of the beam except at each end portion of the laser beam, where the energy profile define energy slope regions 43. The energy density slope in the regions 43 can be controlled by manipulating the laser equipment, such as filters and lenses, and is preferably linear. Wherein the end portions of the laser beam has slope angle below 90 degrees, preferably, about 45 degrees.

[0028] FIG. 5 shows schematically the laser energy distribution of a laser beam overlapping region during a laser beam scanning according to a first embodiment of the present invention. As described above, a large area film is scanned by the laser beam two times (or more than two times if necessary to cover the entire film), including a first scan 57 and a second scan 58 to provide laser energy to the entire large area film. The dotted lines show a spatial profile 51 and an energy profile 52 of the laser beam in the first scan, and the solid lines show a spatial profile 53 and energy profile 54 of the laser beam in the second scan.

[0029] The two laser scans 57 and 58 overlap in an area 55. The width of the overlapping region is determined by the displacement of the laser beam relative to the film before the second scan. In the embodiment of FIG. 5, the overlapping region 55 includes both the energy slope regions 56aand 56bof the first and second laser beams, as well as a portion 56cof the flat energy region of each beam adjacent the respective energy slope region.

[0030] FIG. 6 shows schematically the laser energy distribution of a laser beam overlapping region during a laser beam scanning according to a second embodiment of the present invention. Again, a large area film is scanned by the laser beam two times (or more than two times if necessary to cover the entire film), including a first scan 67 and a second scan 68, to provide laser energy to the entire large area film. The dotted line shows a spatial profile 61 and an energy profile 62 of the laser beam in the first scan, and the full line shows a spatial profile 63 and an energy profile 64 of the laser beam in the second scan.

[0031] The two laser scans 67 and 68 overlap in an area 65. In this embodiment, the energy slope regions 66aand 66bof the respective laser beams overlap completely. In addition, only the energy slope regions 66aand 66boverlap, and no portion of the flat regions overlap. As a result, the total laser energy density received in the overlapping region 65 is a constant and is the same as that received in other regions which are irradiated once by the flat energy portion of the laser beam. This allows spatially uniform laser energy to be supplied to the entire substrate.

[0032] Spatial uniformity in the irradiation energy is important for laser activation, as both insufficient and excess laser energy may cause defects in the silicon layer formed. When the method of the present invention is applied to laser annealing of an active layer after doping, the impurity regions of the active layer where activation occurs receive laser energy uniformly, and activation proceeds uniformly. This enhances uniformity of the thin film transistors formed in the substrate.

[0033] FIGS. 7A to 7D are schematic drawings showing the laser energy distribution of a laser beam overlapping region during laser beam scanning according to a third, a fourth, a fifth and a sixth embodiments of the present invention.

[0034] In these four embodiments of the present invention, a first laser beam having a first energy profile and a second laser beam having a second energy profile. The first energy profile is substantially flat over the entire length of the beam except at least one of the edge portions which is called energy slop region. And, the second energy profile is substantially flat along the entire length of the beam and has an elongated step shape with steep edges.

[0035] In these four embodiments of the present invention, the solid lines show an energy profile of the laser beam in the first scan and the dotted lines show an energy profile of the laser beam in the second scan.

[0036] Referring to FIG. 7A a large area film is scanned by the laser beam two times including a first scan 71-1 with the first laser beam having the first energy profile and a second scan 71-2 with the second laser beam. Having a second energy profile. Only the energy slop region of the first laser beam in the first scan 71-1 overlaps the second laser beam in the second scan 71-2 . Accordingly, the total laser energy density received in the overlapping region 71ais a constant and is the same as that received in other regions.

[0037] Referring to FIG. 7B a large area film is scanned by the laser beam two times including a first scan 72-1 with the second laser beam having the second energy profile and a second scan 71-2 with the first laser beam having the first energy profile. Only the energy slop region of the first laser beam in the second scan 71-2 overlaps the first laser beam in the first scan 72-1. Accordingly, the total laser energy density received in the overlapping region 72ais a constant and is the same as that received in other regions.

[0038] Referring to FIG. 7C, a large area film is scanned by the laser beam two times including a first scan 73-1 with the first laser beam having the first energy profile and a second scan 73-2 with the second laser beam having the second energy profile. The two laser scans 73-1 and 73-2 overlap in an area 73a. Both the energy slop region and a portion of the flat energy region adjacent the energy slop region of the first laser beam in the first laser scan 73-1 overlap the second laser beam in the second scan 73-2.

[0039] Referring to FIG. 7D, a large area film is scanned by the laser beam two times including a first scan 74-1 with the second laser beam having the second energy profile and a second scan 74-2 with the first laser beam having the first energy profile. The two laser scans 74-1 and 74-2 overlap in an area 74a. Both the energy slop region and a portion of the flat energy region adjacent the energy slop region of the first laser beam in the second laser scan 74-2 overlap the second laser beam in the first scan 74-1

[0040] When the method of the present invention is applied in crystallizing an amorphous silicon layer, regions of discontinuity in crystallization are reduced or eliminated because laser energy is distributed uniformly to crystallize the entire amorphous silicon layer. Uniform crystallization of the entire layer is therefore achieved.

[0041] It will be apparent to those skilled in the art that various modifications and variations can be made in a laser irradiation method of the present invention without departing from the spirit or scope of the inventions. Thus, it is intended that the present invention cover the modifications and variations of this invention provided they come within the scope of the appended claims and equivalents.