1.  Field of the Invention
The present invention relates to a laser annealing method and a laser annealing apparatus, which perform an annealing process in such a manner that a laser beam is transmitted through a subject plural times by reversing a direction of the laser beam to irradiate the subject. 
2.  Description of the Related Art
System On Glass (SOG)-TFT generally receives attention, in which a driving circuit, a signal processing circuit, and an image processing circuit are directly formed on a glass substrate of LCD, as well as a thin film transistor (TFT) for pixel display gate.  That is noted from viewpoint of reduction in size and weight, and cost saving of flat-panel displays such as a liquid crystal display (LCD) and an organic electro-luminescence (EL) display. 
Although amorphous silicon (a-Si) is used for TFT for pixel display gate, polysilicon (poly-Si) having large carrier mobility is required for SOG-TFT.  However, since deformation temperature of the glass is as low as 600° C., in formation of a poly-Si film, it is impossible to use a crystal growth technology utilizing a high temperature more than 600° C. Therefore, excimer laser anneal (ELA) is used for the formation of the poly-Si film.  In the excimer laser anneal, after an a-Si film is formed at lower temperature (100° C. to 300° C.), pulse irradiation of a XeCl excimer laser having a wavelength of 308 nm is performed to thermally fuse the a-Si film, and the a-Si film is crystallized in a cooling process.  The poly-Si film can be formed without thermally damaging the glass substrate by the use of ELA. 
The conventional laser annealing process in which a-Si is changed into poly-Si is performed by irradiating the a-Si film only from one side with the XeCl excimer laser having a wavelength of 308 nm.  Since an absorption coefficient of the XeCl excimer laser having a wavelength of 308 nm to the a-Si is as large as 1×106 cm−1, input energy is absorbed in a region which is extremely close to a surface (<1 nm). 
Therefore, when the excimer laser anneal is used, a large temperature gradient is generated in a depth direction in the Si layer fused by the absorption of laser energy and heat transfer, and sometimes the Si layer becomes a partially-fused state shown in FIG.  29. 
In this case, heat is diffused mainly in a substrate direction and solid-state phase transition into a crystal phase occurs at 800° C. in the remaining a-Si which has not been fused, so that a crystal nucleus is generated in a boundary portion between the fused Si phase and the a-Si phase.  With the generated crystal nucleus being a starting point, crystal grows in an upward direction of FIG.  29 along the temperature gradient.  A crystal grain which has grown from the crystal nucleus collides with another crystal grain which has grown from an adjacent crystal nucleus, and the crystal growth is stopped at the state in which the crystal grain is small and there are many crystal grain boundaries. 
High electric charge mobility is required for high performance of TFT.  Since the crystal grain boundary becomes obstacle of the movement for electron, in order to increase electric charge mobility, it is important to generate a crystal grain having few crystal grain boundaries, that is, a large crystal grain. 
Therefore, as shown in FIG.  30, in the excimer laser anneal, when output of the excimer laser is increased and the remaining a-Si phases are formed in the shape of an island, the number of crystal nucleuses generated is decreased and each crystal grain is grown in a large size. 
As shown in FIG.  31, in the excimer laser anneal, when the output of the excimer laser is further increased and the a-Si phases are completely fused, the Si layer becomes a supercooling state in which the crystallization is not started even if the temperature is decreased below a melting point.  Then, when the temperature is further decreased, crystal nucleuses are generated all at once to fill the Si layer with minute crystal grains. 
FIG.  32 quantitatively shows a relationship between the laser intensity and the diameter of the crystal grain described above.  As the laser intensity is increased, the diameter of the crystal grain is increased such that the a-Si layer changes from a partially fused (a) state to the fused state (b) in which the remaining a-Si phases are formed in the island shape.  Once the laser intensity exceeds the intensity at which a-Si is completely fused, the Si layer becomes fully fused state (c), and the diameter of the crystal grain is remarkably decreased.  Output stability of the excimer laser is not good, and usually a fluctuation in intensity in a range of 10 to 15% is unavoidable (hatching in FIG.  32).  Therefore, an effective diameter of the crystal grain obtained by the excimer laser is currently about 0.3 μm at most.  This is also the limitation by setting a crystal growth direction to a vertical direction (vertical direction in FIGS.  29 and 30). 
In consideration of the above problem, there has been devised an annealing method which controls the crystal growth in lateral direction in such a manner that a laser beam is slowly scanned on a substrate so as not to completely fuse the a-Si and not to create the supercooling state. 
In the annealing method, as shown in FIG.  33, although the crystal nucleus is generated in the a-Si layer which is not irradiated with the laser beam, the crystal growth proceeds from the crystal nucleus in a bottom portion of the boundary between the a-Si and the fused layer to an obliquely upward direction due to a temperature gradient.  It is considered that, since the temperature gradient is present in the depth direction, a solid-liquid interface is inclined and the crystal growth proceeds perpendicular to the oblique solid-liquid interface. 
The size of the crystal grain is restricted by a film thickness and the collision with other crystal grains from other directions.  The essential cause thereof is the large temperature gradient in the depth direction of the fused layer. 
In consideration of the problem of the crystal growth in the lateral direction in the excimer laser, there has been devised a laser anneal which uses a light beam having a wavelength of 532 nm of a high-output Nd:YVO4 laser in which a laser output stability is high (as 1%). 
Since the absorption coefficient of the light beam having a wavelength of 532 nm of the Nd:YVO4 laser with respect to a-Si is 5×104 cm−1, a film thickness of 460 nm is required to absorb 90% of the input energy.  The absorption coefficient of the light beam having a wavelength of 532 nm of the Nd:YVO4 laser is smaller than that of the light beam having a wavelength of 308 nm of the excimer laser by 1.5 digits.  As shown if FIG.  34, when the laser beams are compared to each other with the same film thickness, the temperature gradient in the depth direction becomes more flat and the solid-liquid interface is likely to be vertical in the case of the light beam having a wavelength of 532 nm.  Therefore, a growth distance in the lateral direction can be longer, and a large crystal grain is generated. 
In consideration of the problem of the crystal growth in the lateral direction in the excimer laser, there is disclosed a laser annealing method in which a sample having a four-layer structure of a-Si/SiO2 insulating thin film/Cr light absorber thin film/substrate is irradiated from both sides with the laser beam (308 nm) of the excimer laser.  In this method, a heat bath under the SiO2 layer is generated by absorbing the laser energy from a backside in the Cr light absorber thin film.  As a result, the heat of the Si layer generated by the laser energy from the front surface side is hardly transferred to the substrate direction.  As transfer velocity of thermal energy accumulated in the Si layer is decreased, the heat is transferred in the direction of the Si film surface, and the crystal growth in the lateral direction is controlled (for example, see Surface Science vol.  21, No.  5, pp.  278 to 287 (2000)). 
Further, there is disclosed a laser annealing apparatus of the both-side irradiation by a solid-state laser, in which a second harmonic wave (532 nm), a third harmonic wave (355 nm), and a fourth harmonic wave (266 nm) of a Nd:YAG laser are utilized. 
In the above both-side irradiation laser annealing apparatus, the individual laser beam passes through the Si layer one time from the front surface and the backside surface.  That is to say, the annealing process is performed in such a manner that one position of Si film is irradiated with the laser beam from the backside surface, while the same position of Si film is irradiated with the laser beam from the front surface side (see, for example, Japanese Patent Application Laid-Open (JP-A) No.  2001-144027). 
In the above laser annealing method, which utilizes the XeCl excimer laser, the output of the light beam is not stable, and output intensity fluctuates in the range within ±10%.  In ELA, the diameter sizes of the crystal grains are varied in the poly-Si film and reproducibility is poor.  In the XeCl excimer laser, a repeat frequency of pulse drive is as low as 300 Hz.  In ELA, therefore, it is difficult to form a continuous crystal grain boundary, high-carrier mobility is not obtained, and annealing a large area of the Si film at high speed can not be realized.  Further, in the XeCl excimer laser, there is intrinsic problems that maintenance cost is high due to short lives of a laser tube and laser gas as low as about 1×107 shots, the apparatus is enlarged, and energy efficiency is as low as 3%. 
In order to improve performance of TFT, it is also important to thin the crystal film (not more than 50 nm) in addition to the increase in the diameter of the crystal grain. 
In the laser annealing method which utilizes the light beam having a wavelength of 532 nm of the Nd:YVO4 laser, a solid-liquid interface can become vertical, which is effective to the formation of a large crystal grain.  However, since the absorption coefficient of the light beam having a wavelength of 532 nm of the Nd:YVO4 laser with respect to a-Si is small, although the solid-liquid interface is vertical, the film thickness not lower than 150 nm is required in order to secure the energy absorption necessary to fuse the a-Si film. 
Therefore, in the laser annealing method, the vertical formation of the solid-liquid interface effective to the formation of a large crystal grain is contradictory to the thinning of the crystal film.  Optical properties of a-Si cause the contradiction, and it is difficult to balance these contradictory demands with each other. 
Further, in the laser annealing method which utilizes the light beam having a wavelength of 532 nm of the Nd:YVO4 laser, since the film thickness of 460 nm is required to absorb 90% of the input energy, waste of the input energy tends to increase when the a-Si film is thinned.