1. Technical Field of the Invention
The present invention relates to a laser annealing method and a laser annealing apparatus that reform a semiconductor film by irradiating the semiconductor film with pulse laser beam shaped into a rectangular beam.
2. Description of the Related Art
Laser annealing is a process of forming a polycrystalline silicon film by melting and solidifying an amorphous silicon film (hereinafter referred to as “a-Si film”) deposited on a substrate made of low-melting point glass (typically alkali-free glass) by irradiating to the a-Si film with a laser beam (For example, see Patent Document 1). Because of their electric properties superior to those of a-Si films, crystallized silicon films are being used for transistors to drive liquid crystal displays, which require high definition display, of portable telephones, digital cameras and so on.
The laser annealing is performed by scanning a semiconductor film (for example, an a-Si film), relatively in a minor (shorter) axial direction of the beam, on a substrate with a rectangular beam having its elongated section. The rectangular beam is shaped from a pulse laser beam from a laser source, using an optical system. In general, the scanning of the rectangular beam is done by moving the substrate. This scanning is carried out in such a manner that laser irradiated regions are partially overlapped with each other.
The following Patent Documents 2 and 3 disclose an entire configuration of an optical system of a laser annealing apparatus using an excimer laser as a laser source (hereinafter referred to as “excimer laser annealing apparatus”). The shown optical system includes a cylindrical lens array that splits a laser beam into a plurality of beams in major (longer) and minor (shorter) axial directions, and a condense lens that condenses the beams split by the cylindrical lens array. In the minor axial direction, reduction-projection is performed by a projection lens after once making an energy profile of the beams uniform.
In the above-described excimer laser annealing apparatus, the dimension of the beam is a degree of 365 mm in major axis and 0.4 mm in minor axis. An excimer laser has large width of minor axis and hence large depth of focus because of its poor quality of laser beam. On this account, positional variation of a laser irradiated surface, which is caused by a mechanical error of a substrate transferring apparatus and a machining error of a substrate surface, has little effect on annealing performance. Here, the positional variation refers to positional variation in a vertical direction of a semiconductor film.
On the other hand, the excimer laser annealing has a problem in that carrier mobility as an annealing property is greatly changed depending on laser irradiation energy. As one of measures against this problem, a great attention is paid to a laser annealing apparatus (hereinafter referred to as “solid green laser annealing apparatus”) using pulsed green laser beam, which is obtained from a second harmonic wave of an Nd:YAG laser, as a light source (for example, see Patent Documents 4 and 5). The use of this pulsed green laser allows a process margin for certain irradiation energy which is wider than that of an excimer laser.
However, since the solid green laser has power (below 0.1 J/pulse) significantly lower than that (maximum 1 J/pulse) of an commercialized excimer laser, there is a need to narrow a beam size in minor axis up to 100 μm or below. As a result, the depth of focus in minor axis is decreased and thus positional variation of a semiconductor film may have some effect on annealing performance.
The following Patent Documents 4 to 6 disclose an auto-focus mechanism applied to laser machining such as perforation and the like. This auto-focus mechanism monitors variation of a machining surface and keeps a light focusing point constant on the machining surface by moving an objective lens, which focuses laser beam on the machining surface, in an optical axis direction.    Patent Document 1: Japanese Patent No. 3204307    Patent Document 2: Japanese Unexamined Patent Application Publication No. 2000-338447    Patent Document 3: Japanese Patent No. 3191702    Patent Document 4: Japanese Unexamined Patent Application Publication No. 11-58053    Patent Document 5: Japanese Unexamined Patent Application Publication No. 11-23952    Patent Document 6: Japanese Patent No. 2835924    Non-patent Document 1: K. Nishida et. al., “Performance of Polycrystallization with High Power Solid Green Laser”, AM-FPD 2006.    Non-patent Document 2: OKAMOTO Tatsuki et. al., “Development of Green Laser Annealing Optical System for Low-Temperature Polysilicon”, RTM-05-28.
Non-Patent Document 2 shows an entire configuration of an optical system of a solid green laser annealing apparatus. For a major axial direction, this optical system splits laser beam into a plurality of laser beams in a rectangular waveguide made of quartz glass and superposes the plurality of laser beams on a semiconductor film on a glass substrate by means of an image lens. For a minor axial direction, the optical system turns the laser beam into parallel light of φ80 mm by means of an expander lens and focuses the parallel light on the glass substrate by means of a focusing lens as an objective lens.
In the solid green laser annealing apparatus shown in Non-Patent Document 2, when a machining surface is changed in a direction perpendicular to the machining surface, since the substrate is deviated from a focus position for the minor axial direction, energy density of the laser beam on a silicon film of the machining surface is changed. In a case of the optical system shown in Non-Patent Document 2, it is possible to avoid variation of the energy density by correcting a position of the condensing lens as the objective lens, like the auto-focus mechanism shown in Patent Documents 4 to 6.
However, a condensing lens used for a laser machining apparatus for perforation is relatively small while an objective lens (condensing lens or projection lens) used for a laser annealing apparatus is typically large, for example, has a dimension of 100 mm or above in minor axial direction×about 150 mm in major axial direction. On this account, a holder to hold such a lens group is large and very heavy. Accordingly, it is very difficult to move and vibrate the objective lens of the laser annealing apparatus with a precision of several microns in an optical axis direction in real time. In addition, although it may be considered to move and vibrate a substrate in a direction perpendicular to the substrate, since a substrate size for laser annealing is typically large (for example, more than 700 mm×900 mm), it is also difficult to vibrate a stage to support the substrate with a high precision. In addition, as to the major axial direction, since a depth of focus for the major axial direction is even larger than that for the minor axial direction, variation of a focus position has little effect on the annealing performance.