Method and apparatus for laser annealing

Abstract

A layer (22) on a substrate (24) is laser annealed by pulses in a plurality of laser beams (14A-14D) formed into a uniform linear beam (20). The laser beams (14A-14D) are partitioned into a first set of beams (14A, 14D) and a second set of beams (14B, 14C). The second set of beams (14B, 14C) is incident on the layer (22) from a smaller angular range than all of the beams (14A-14D) combined. The pulses in the beams (14A-14D) are synchronized such that the pulses in the first set of beams (14A, 14D) are incident on the layer (22) before the pulses in the second set of beams (14B, 14C). The pulses in the first set of beams (14A, 14D) melt the layer and the pulses in the second set of beams (14B, 14C) sustain the melting.

Description

[Technical Field]

[0001] (Priority) This application claims priority to U.S. Provisional Application No. 63 / 185,273, filed May 6, 2021, and U.S. Provisional Application No. 63 / 251,174, filed October 1, 2021, the disclosures of which are incorporated herein by reference in their entireties.

[0002] (Technical field of the invention) The present invention relates generally to annealing of amorphous silicon layers using ultraviolet laser radiation projected in a linear beam, and more particularly to annealing in which the projected linear beam is partitioned into temporally separated pulses delivered to the amorphous silicon layer at different angles of incidence. [Background technology]

[0003] (Discussion of Background Art) Laser silicon annealing is a key processing step in the production of high-resolution active-matrix liquid crystal displays (AMLCDs) and active-matrix organic light-emitting diode (AMOLED) displays. A thin amorphous silicon layer on a glass substrate is melted by a pulsed ultraviolet laser beam and crystallizes as it cools. The polycrystalline silicon layer becomes the semiconductor base upon which thin-film transistors (TFTs) and other electronic circuits are formed by conventional lithographic processes. During the annealing step, the substrate and the silicon layer above it are in the form of a "panel," which can later be separated into smaller individual displays, such as those used in consumer electronic devices.

[0004] Laser silicon annealing is a delicate process, and maintaining optimal energy density across the width of the panel during laser processing is essential. The laser beam is shaped into an elongated "line beam" that is projected onto the panel. The line beam is characterized by a major axis spanning the entire width of the panel and an orthogonal minor axis. The line beam has a uniform or "flat-top" intensity distribution along the width of the panel. Each panel is scanned in the minor axis direction along its length. The scan speed is selected to repeatedly melt and anneal the silicon at each location on the panel until the desired polycrystalline microstructure is obtained. Irradiation with approximately 20 consecutive laser pulses is typical. Exposing each location to multiple laser pulses also provides pulse-to-pulse averaging.

[0005] In an ideal microstructure, the grain agglomerates have uniform size and orientation, with a minimal density of defects that could adversely affect usable panel yield. Interference effects during exposure to the linear beam preferentially orient these grain agglomerates, producing "ripples" on the surface of the annealed silicon. The quality of the annealed silicon layer can be assessed by illuminating the annealed silicon at an oblique angle to observe the diffraction caused by the ripples. Non-uniformity in intensity along the major axis of the linear beam produces aligned stripes along the minor axis, known as "scan mura." Temporal variations in intensity produce aligned stripes along the major axis, known as "shot mura." Methods for characterizing silicon recrystallization are described in U.S. Pat. No. 9,335,276 and U.S. Patent Publication No. 2013 / 0341310 (each assigned to the assignee of the present invention, the complete disclosures of each of which are incorporated herein by reference). These methods can be applied during laser annealing to adjust the process in real time.

[0006] Excimer lasers are advantageous for laser silicon annealing, which requires a stable laser beam with low pulse energy noise (less than approximately 1% RMS) and ultraviolet wavelengths. Examples include argon fluoride (ArF) excimer lasers, which generate a 193-nanometer (nm) laser beam, and xenon fluoride (XeF) excimer lasers, which generate a 351-nm laser beam. The most powerful, state-of-the-art industrial excimer lasers use xenon chloride (XeCl) to generate a 308-nm laser beam with pulse energies up to 1 joule at pulse repetition rates up to 600 Hz. For example, the Lambda SX excimer laser manufactured by Coherent Inc. (Santa Clara, California) can maintain very stable pulse energy and beam parameters while operating sustainably for over 100 million pulses. For applications requiring even greater pulse energies, the output of two or more excimer lasers can be combined using beam-mixing optics and synchronized pulse delivery. Such beam mixing and synchronization is described in U.S. Pat. Nos. 7,408,714 and 8,238,400, respectively (each assigned to the assignee of the present invention, the complete disclosures of each of which are incorporated herein by reference).

[0007] Methods and apparatus for shaping a laser beam into a uniform linear beam are described in U.S. Patent Nos. 7,265,908, 7,428,039, and 7,615,722 (each of which is commonly owned and the complete disclosures of each are incorporated herein by reference). These methods incorporate beam homogenization, whereby a laser beam from one or more sources is spatially partitioned, and the partitioned beams are overlaid and projected onto a silicon layer. For example, 10 to 30 partitioned beams are overlaid and projected, each measuring approximately 0.4 mm in the short axis and 750 mm to 1,500 mm in the long axis. The beam homogenization device includes one or two "microlens arrays," which contain multiple individual "microlenses" that are much smaller than the incident beam. Each microlens is an illumination source that contributes to the overall overlaid beam. Additional optics collect all the beams emerging from the microlenses and shape them into a uniform linear beam.

[0008] More recently, an ultraviolet diode-pumped solid-state laser capable of laser silicon annealing to produce panels with the desired quality has been disclosed. This solid-state laser is described in U.S. Patent Publication No. 2020 / 0235544 (co-owned, the complete disclosure of which is incorporated herein by reference). Solid-state lasers have the advantage of lower capital costs and lower operating costs than excimer lasers. [Prior art documents] [Patent documents]

[0009] [Patent Document 1] U.S. Patent No. 9,335,276 [Patent Document 2] U.S. Patent Publication No. 2013 / 0341310 [Patent Document 3] U.S. Patent No. 7,408,714 [Patent Document 4] U.S. Patent No. 8,238,400 [Patent Document 5] U.S. Patent No. 7,265,908 Summary of the Invention [Means for solving the problem]

[0010] (Summary of the Invention) The gain medium in an excimer laser is a mixture of flowing and pressurized gases, typically including rare gases, halide gases, and buffer gases. The gas mixture is excited by a short current pulse in a gas discharge, generating excited weakly bound dimers. For example, xenon chloride dimers have a laser line at 308 nm. The laser pulse delivered by an excimer laser typically comprises two temporal peaks with different amplitudes and an overall pulse duration of tens of nanoseconds (ns). Pulse durations of 50 ns to 70 ns are typical for excimer lasers used in silicon laser annealing. The pulse duration affects the rate of ripple formation. If the overall pulse duration is too short, the scan speed must be reduced and processing time will increase. The duration of the pulse delivered to the panel can be extended by adding an optical delay or by using multiple synchronized excimer laser cavities to deliver temporally separated pulses.

[0011] The gain medium in the aforementioned diode-pumped solid-state laser is neodymium (Nd 3+ ) or ytterbium (Yb 3+ ) doped oxide crystal. Laser pulses are produced by Q-switching the laser cavity and frequency converted to ultraviolet wavelengths in an optically nonlinear crystal. Laser pulses delivered by this solid-state laser have pulse durations of tens of nanoseconds. Nd 3+An exemplary laser with a :YAG gain crystal produced a laser beam at 355 nm by frequency tripling in two LBO crystals. The pulse duration was 25 ns when the laser cavity was optimized for maximum output power.

[0012] A need exists for laser annealing apparatus and methods that utilize solid-state lasers to form polycrystalline silicon with the high-quality microstructure currently offered by excimer lasers, along with their capital and operating cost advantages. Preferably, these laser annealing apparatus and methods would form microstructures that even closer to the ideal of uniform grain size and orientation, and with fewer defects.

[0013] In one aspect, a method for annealing a layer on a substrate is disclosed. A plurality of pulsed laser beams are provided and split into a first set of laser beams having a first set of laser pulses and a second set of laser beams having a second set of laser pulses. The laser beams are formed into a linear beam. The linear beam has a major dimension, an orthogonal minor dimension, and a uniform intensity distribution along the major dimension. Each location along the major dimension of the linear beam is illuminated by all of the laser beams. The layer is illuminated using the linear beams. The second set of laser beams are incident on the layer from a smaller angular range than all of the laser beams combined. The pulses in the laser beams are synchronized so that the first set of laser pulses are incident on the layer before the second set of laser pulses. The pulses in the first set of laser beams melt material in the layer illuminated by the linear beams, and the pulses in the second set of laser beams are incident on the layer before the melted material solidifies. The present invention provides, for example, the following. (Item 1) 1. A method for annealing a layer on a substrate, comprising: providing a plurality of pulsed laser beams, the laser beams being split into a first set of laser beams having a first set of laser pulses and a second set of laser beams having a second set of laser pulses; forming the laser beam into a linear beam, the linear beam having a major dimension and an orthogonal minor dimension, the linear beam having a uniform intensity distribution along the major dimension, and each location along the major dimension of the linear beam being illuminated by all of the laser beam; illuminating the layer with the linear beams, wherein the second set of laser beams are incident on the layer from a smaller angular range than all of the laser beams combined; synchronizing pulses in the laser beams such that a first set of laser pulses is incident on the layer before a second set of laser pulses, the pulses in the first set of laser beams melt material in the layer illuminated by the linear beam, and the pulses in the second set of laser beams are incident on the layer before the melted material solidifies; A method comprising: (Item 2) 2. The method for annealing according to item 1, wherein the layer is made from amorphous silicon, which is transformed by the annealing into polycrystalline silicon. (Item 3) 3. The method for annealing according to claim 1 or 2, wherein the sequential pulses in the first and second sets of laser beams partially overlap and the temporal separation between the sequential pulses is equal, thereby extending the time that the layer is continuously illuminated by the linear beam. (Item 4) 10. The method for annealing of claim 1, wherein the first set of laser pulses are incident on the layer simultaneously, the first set of pulses and the second set of pulses are separated by a delay time ΔT, and the second set of laser pulses are incident on the layer simultaneously. (Item 5) 10. The method for annealing of claim 1, wherein the first set of pulses are separated from one another by a time δt, the first set of pulses and the second set of pulses are separated by a delay time ΔT, and the second set of pulses are separated from one another by the time δt. (Item 6) 6. The method for annealing according to item 5, wherein ΔT≧2δt. (Item 7) 6. The method for annealing according to item 5, wherein the delay time ΔT is in the range of 20 to 120 nanoseconds, and the time Δt is in the range of 0 to 60 nanoseconds. (Item 8) 10. The method for annealing according to any of the preceding items, wherein the first set of pulses are incident on the layer simultaneously, the first set of pulses and the second set of pulses are separated by a delay time ΔT, and the second set of pulses are separated from each other by a time δt. (Item 9) Item 9. The method for annealing according to item 8, wherein ΔT=1.3δt. (Item 10) 9. The method for annealing according to item 8, wherein the delay time ΔT is in the range of 20 to 120 nanoseconds, and the time Δt is in the range of 0 to 60 nanoseconds. (Item 11) The first set of laser beams are oriented in a first angular range α with respect to the central axis of the linear beam. F and the second set of laser beams are incident on the layer within a second angular range α relative to the central axis. S is incident on the layer within the second angular range α S is the first angle range α F 10. The method for annealing according to any of the preceding items, wherein the central axis is closer to the center axis than the center axis. (Item 12) The first angular range α F is in the range of 1° to 9°, and the second angle range α S Item 12. The method for annealing according to item 11, wherein the angle is in the range of 0° to 3°. (Item 13) 13. The method for annealing according to claim 11 or 12, wherein the central axis is perpendicular to the layer in the major dimension. (Item 14) Item 13. The method for annealing according to item 11 or 12, wherein the second set of laser beams have a smaller angle of incidence on the layer than the first set of laser beams. (Item 15) Item 12. The method for annealing according to item 11, wherein the central axis is tilted in the short dimension by an angle θ in the range of 4° to 12° relative to the normal to the layer. (Item 16) 10. The method for annealing according to any of the preceding items, wherein the second set of laser beams is unpolarized. (Item 17) 10. The method for annealing of claim 1, wherein the first and second sets of laser beams have a wavelength of 355 nanometers and the first and second sets of laser pulses have a pulse duration in a range of 20 to 30 nanoseconds. (Item 18) 1. An optical apparatus for annealing a layer on a substrate, comprising: a plurality of pulsed laser sources, each of which provides a pulsed laser beam; a beam homogenizer having a major axis and an orthogonal minor axis, the laser beam being directed into and transmitted through the beam homogenizer, the directed laser beam being split into a first set of laser beams and a second set of laser beams arranged serially along the major axis of the beam homogenizer; a beam projector including a plurality of lenses arranged in series between the beam homogenizer and the layer, the beam projector arranged to intercept the laser beam transmitted through the beam homogenizer, the beam homogenizer and the beam projector cooperating to form the laser beam into a linear beam that illuminates the layer, the linear beam having a major dimension and an orthogonal minor dimension, each location along the major dimension of the linear beam being illuminated by all of the laser beams; and a controller for synchronizing pulses in said laser beam; Equipped with the second set of laser beams are incident on the layer from a smaller angular range than all of the laser beams combined; The controller synchronizes the pulses in the laser beams such that pulses in the first set of laser beams are incident on the layer before the pulses in the second set of laser beams, the pulses in the first set of laser beams melt material in the layer illuminated by the linear beams, and the pulses in the second set of laser beams are incident on the layer before the melted material solidifies. (Item 19) 1. An optical apparatus for annealing a layer on a substrate, comprising: a plurality of pulsed laser sources, each of which provides a pulsed laser beam; a beam homogenizer, into which the laser beam is directed and transmitted through the beam homogenizer; a beam projector including at least two positive lenses arranged in series between the beam homogenizer and the layer, the beam projector arranged to intercept the laser beam transmitted through the beam homogenizer, the beam homogenizer and the beam projector cooperating to form the laser beam into a linear beam that illuminates the layer, the linear beam having a major dimension and an orthogonal minor dimension, each location along the major dimension of the linear beam being illuminated by all of the laser beams, the laser beams collectively spanning a range of incidence angles α over locations within the linear beam; a controller for synchronizing pulses in said laser beam; Equipped with The optical device, wherein the controller synchronizes pulses in at least one laser beam such that pulses in the laser beams illuminate the layer after pulses in all other laser beams, and the at least one laser beam spans a range of incidence angles less than 0.5α. (Item 20) 20. The optical device of claim 19, wherein the at least one laser beam spans a range of incidence angles less than 0.25α. [Brief explanation of the drawings]

[0014] The accompanying drawings, which are incorporated in and constitute a part of this specification, illustrate generally preferred embodiments of the present invention and, together with the general description given above and the detailed description of preferred embodiments given below, serve to explain the principles of the invention.

[0015] [Figure 1] FIG. 1 illustrates a schematic diagram of one preferred embodiment of an optical apparatus according to the present invention for illuminating and annealing a layer on a substrate, comprising a controller, a plurality of laser sources, a beam homogenizer including two sets of linear microlens arrays, and a beam projector including four cylindrical lenses.

[0016] [Figure 2]FIG. 2 is an enlarged view of the optical apparatus of FIG. 1 illustrating multiple pulsed laser beams provided by a laser source focused onto one location in the long dimension of a linear beam for annealing a layer, with the linear beam being cooperatively formed by a beam homogenizer and a beam projector.

[0017] [Figure 3] FIG. 3 is a perspective view of a practical arrangement of the optical device of FIG.

[0018] [Figure 4] FIG. 4 is a graph of optical magnification versus time, illustrating schematically one preferred embodiment for synchronization of pulses in the laser beam of FIG. 2 incident on locations in a linear beam.

[0019] [Figure 5A] 5A, 5B, and 5C are graphs of optical magnification versus time, illustrating three preferred embodiments for synchronization of pulses in the laser beam of FIG. 2 incident on locations within a linear beam. [Figure 5B] 5A, 5B, and 5C are graphs of optical magnification versus time, illustrating three preferred embodiments for synchronization of pulses in the laser beam of FIG. 2 incident on locations within a linear beam. [Figure 5C] 5A, 5B, and 5C are graphs of optical magnification versus time, illustrating three preferred embodiments for synchronization of pulses in the laser beam of FIG. 2 incident on locations within a linear beam.

[0020] [Figure 6A] FIG. 6A is an enlarged view of the optical arrangement of FIG. 1, similar to FIG. 2, and illustrates schematically a pulsed laser beam focused onto one location in the major dimension of the linear beam.

[0021] [Figure 6B]FIG. 6B is an enlarged view of another preferred embodiment of an optical device according to the present invention, similar to the embodiment of FIGS. 1 and 2, and schematically illustrates multiple pulsed laser beams focused onto one location in the short dimension of a linear beam. DETAILED DESCRIPTION OF THE INVENTION

[0022] (Detailed Description of the Invention) Referring now to the drawings, wherein like components are designated by like numerals, FIG. 1 schematically illustrates a preferred embodiment of an optical apparatus 10 according to the present invention. The optical apparatus 10 includes multiple pulsed laser sources 12A-12D, each of which provides a pulsed laser beam 14A-14D that is directed into a beam homogenizer 16. The laser beams 14A-14D are transmitted through the beam homogenizer 16 and intercepted by a beam projector 18. The beam homogenizer 16 and the beam projector 18 cooperate to shape the laser beams 14A-14D into a linear beam 20 that illuminates a layer 22 on a substrate 24. The layer 22 is made of a material to be annealed, e.g., amorphous silicon. The substrate 24 is made of a support material, such as glass.

[0023] Cartesian axes on the drawings are for reference purposes. Laser beams 14A-14D propagate along a "Z" axis between laser sources 12A-12D and layer 22. Laser beams 14A-14D have orthogonal lateral "X" and "Y" axes, respectively, that correspond to the major and minor dimensions of linear beam 20. Laser beams 14A-14D are depicted with different arrowheads to identify and distinguish them in the drawings. Each beam is depicted in FIG. 1 as a central ray and two peripheral rays.

[0024] Here, beam homogenizer 16 comprises two sets of linear microlens arrays, 26A-26D and 28A-28D. Each microlens array includes a plurality of cylindrical microlenses 30 or 32 linearly arranged along the X-axis. The individual microlens arrays within each set are also linearly arranged along the X-axis. Beam homogenizer 16 therefore has a long axis parallel to the X-axis and an orthogonal short axis parallel to the Y-axis. In an alternative arrangement, the multiple individual microlens arrays within each set of microlens arrays may be replaced with a single microlens array, which intercepts all laser beams. Laser beams 14A-14D propagate from laser sources 12A-12D through microlens arrays 26A-26D and then through microlens arrays 28A-28D.

[0025] The laser beams 14A-14D directed onto the beam homogenizer 16 are arranged serially along its long axis. Here, the laser beams 14A-14D illuminate the linear microlens arrays 26A-26D and then the microlens arrays 28A-28D, respectively. The laser beams 14A-14D emerging from the microlens arrays 28A-28D are divergent in the long axis (X-axis). When the laser beams 14A-14D directed onto the beam homogenizer 16 are well collimated, the microlens arrays 26A-26D may be omitted, since their practical purpose is to increase the angular acceptance of the beam homogenizer 16.

[0026] Here, beam projector 18 includes four cylindrical lenses 34, 36, 38, and 40, arranged serially in numerical order between beam homogenizer 16 and layer 22. Cylindrical lenses 34 and 38 have positive optical power in the long dimension (X-axis). Cylindrical lens 38 has an optical axis 42 that is approximately perpendicular to substrate 24 in at least the long dimension. Cylindrical lens 34 is arranged to intercept diverging laser radiation from all cylindrical microlenses 32 and, together with cylindrical lens 38, forms linear beam 20 in the long dimension. While cylindrical lens 34 alone would be sufficient to form a linear beam in the long dimension by converging and overlapping laser radiation from all microlenses, beam projectors typically have multiple optical elements. Here, one practical advantage of including cylindrical lens 38 in beam projector 18 is that the overall length of optical device 10 is reduced.

[0027] Each cylindrical microlens 32 in the microlens arrays 28A-28D is a source of divergent laser radiation. The laser radiation emerging from each cylindrical microlens 32 overlaps with the laser radiation from all of the other cylindrical microlenses 32 in the linear beam 20. Each location along the major dimension of the linear beam 20 is thereby illuminated and homogenized by all of the laser beams 14A-14D. The linear beam 20 is positioned above the layer 22 and has a uniform intensity distribution along its major dimension (X-axis). The linear beam 20 thereby uniformly illuminates the layer 22 during annealing.

[0028] Cylindrical lenses 36 and 40 have optical power in the short dimension (Y-axis). Cylindrical lens 36 is arranged to intercept all of the laser radiation and, together with cylindrical lens 40, forms linear beam 20 in the short dimension. Overall, all of the light rays within the laser radiation are focused in the long dimension (X-axis) by cylindrical lenses 34 and 38 and in the short dimension by cylindrical lenses 36 and 40. All of the light rays converge toward linear beam 20.

[0029] FIG. 2 is a schematic illustration of further details of laser beams 14A-14D formed into linear beam 20, enlarged compared to FIG. 1. The drawing depicts portions of laser beams 14A-14D focused in the major dimension (X-axis) onto one location 44 within linear beam 20. In FIG. 1, laser beams 14A-14D are each depicted as a single ray incident on each location within linear beam 20. In FIG. 2, laser beams 14A-14D are each depicted as two bounded rays, extending across a range of angles of incidence. Laser beam 14D is focused, for example, at angle α D Collectively, laser beams 14A-14D subtend a larger angle α. Central axis 46 is the center of gravity of the portions of laser beams 14A-14D that are focused onto location 44.

[0030] In the exemplary apparatus depicted in Figures 1 and 2, laser beams 14A-14D pass through separate but identical pairs of linear microlens arrays, causing the laser beams to be incident on location 44 over a non-overlapping range of angles. Each laser beam illuminates location 44 while spanning approximately the same angle. In the exemplary apparatus, there are four laser sources, and therefore each laser beam incident on location 44 spans an angle approximately equal to or slightly less than 0.25α. For example, α is about 8°, and α is about 0.25α. D is about 2°.

[0031] The inventors have found that illumination of layer 22 from a smaller angular range is advantageous for ripple formation and high-quality laser annealing. For example, if all laser sources 12A-12D are utilized, a smaller angle α is advantageous. Alternatively, if only laser source 12D is utilized, a smaller angle α is advantageous. D However, it is advantageous to minimize these angles. To minimize these angles, the focal lengths of cylindrical lenses 32, 34, and 38 would be made as long as practical. The focal length of cylindrical microlens 30 and the widths of cylindrical microlenses 30 and 32 may need to be adjusted accordingly.

[0032] FIG. 3 is a perspective view of a practical arrangement of optical device 10, with laser sources 12A-12D omitted for clarity. In this arrangement, optical device 10 includes an additional steering mirror 50, which directs laser beams 14C and 14D into linear microlens arrays 26C and 26D. Steering optics such as steering mirror 50 enable a compact arrangement of optical elements inside an integrated tool for laser annealing. Mirror 52 and another mirror 54 both direct laser radiation onto substrate 24 and layer 22 thereon, and are preferably horizontal during annealing. One or both of mirrors 52 and 54 can be rapidly and repeatedly tilted through a small angular range relative to a “micro-smooth” linear beam 20 in its major dimension, as described in commonly owned U.S. Pat. No. 7,723,169 (the complete disclosure of which is incorporated herein by reference).

[0033] The Cartesian axes on the drawing indicate the propagation axis Z and the transverse axes X and Y as the laser radiation is directed onto layer 22 through beam homogenizer 16 and beam projector 18. During annealing, substrate 24 and layer 22 thereon are translated in the minor dimension (Y axis), deforming portion 56 of layer 22 illuminated by linear beam 20. In a working embodiment of optical device 10, layer 22 is made from amorphous silicon that is transformed at portion 56 into polycrystalline silicon by laser annealing.

[0034] 1, optical apparatus 10 further includes a controller 60 connected to each of laser sources 12A-12D. Controller 60 transmits a clock or trigger signal that governs when laser sources 12A-12D each produce a pulse of laser radiation. Controller 60 thereby synchronizes the pulses in laser beams 14A-14D relative to one another.

[0035] FIG. 4 is a graph of optical magnification versus time, schematically illustrating pulses in linear beam 20 and laser beams 14A-14D incident on location 44 in illumination layer 22. Pulses 72A-72D are provided by laser sources 12A-12D. FIG. 4 illustrates one embodiment of pulse synchronization 70 by controller 60, in which sequential pulses overlap and the temporal separation between sequential pulses is approximately equal. In the depicted pulse synchronization example, pulses 72A, 72D, 72B, and 72C are incident on location 44 in that order. Pulses 72A and 72D are separated by time τ, pulses 72D and 72B are separated by time τ, and pulses 72B and 72C are separated by time τ, where τ≒τ≒τ. Collectively, the four individual pulses 72A-72D form a longer pulse, extending the time that layer 22 is continuously illuminated. For example, a pulse having an effective duration of up to about 100 ns can be formed from four individual pulses generated by a Q-switched laser, each having a duration of 25 ns.

[0036] FIG. 5A is a graph of optical magnification versus time illustrating another embodiment of pulse synchronization 80 by controller 60. A first set 82 of pulses provided by two of the laser sources is incident on layer 22 prior to a second set 84 of pulses provided by the other two laser sources. The pulses in first set 82 are incident simultaneously or nearly simultaneously (as depicted). The pulses in first set 82 melt the material in layer 22 illuminated by linear beam 20. After a delay time ΔT, pulses in second set 84 are incident simultaneously or nearly simultaneously on layer 22 before the melted material solidifies. The nearly simultaneous illumination by pulses in each set maximizes the total optical magnification and initiates and sustains melting. The delay time ΔT extends the time the material is in the molten liquid phase, which is beneficial for material crystallization and the formation of high-quality microstructures. Pulse synchronization 80 effectively simulates longer pulses than could be provided by any of laser sources 12A-12D individually.

[0037] In one example of pulse synchronization 80, a first set of pulses 82 comprises pulses 72A and 72D, and a second set of pulses 84 comprises pulses 72B and 72C. In this example, with reference to FIG. 4, τ≈0, τ≈ΔT, and τ≈0. With reference to FIG. 2, a first set of laser beams 14A and 14D are incident on location 44. Then, after a delay time ΔT, a second set of laser beams 14B and 14C are incident on location 44. The second set of laser beams are incident at an angle approximately α, which is less than angle α. B +α C As discussed above, a smaller angular distribution produces higher quality microstructures in the laser-annealed polycrystalline silicon. Furthermore, the inventors have found that it is most advantageous to illuminate the molten silicon with a smaller angular distribution at the time that the microstructures are being precipitated from the molten material. Generally, precipitation will occur toward the end of each repeated cycle of melting and annealing. Applying this insight to pulse synchronization 80, the pulses can be delimited such that the second set of pulses can be either 72A and 72B, 72B and 72C, or 72C and 72D.

[0038] FIG. 5B is a graph of optical magnification versus time illustrating yet another embodiment of pulse synchronization 90 by controller 60. Individual pulses within each of first set 82 and second set 84 are separated in time. In the depicted example, pulses within first set 82 are separated from one another by a time δt, and pulses within second set 84 are separated from one another by a substantially identical time δt, which is approximately half the delay time ΔT. Overall, first set 82 melts material within layer 22 illuminated by linear beam 20, while second set 84 extends the time the material is in the molten liquid phase. More generally, ΔT≧2×δt, meaning pulse synchronization 90 sustains melting for a longer overall time than pulse synchronizations 70 or 80.

[0039] FIG. 5C is a graph of optical magnification versus time illustrating yet another embodiment of pulse synchronization 100 by controller 60. The integrated optical magnification 102 provided by all individual pulses is depicted by the thicker line. The pulses in the first set 82 are incident simultaneously or nearly simultaneously. The pulses in the second set 84 are separated in time by a time δt. In the depicted example, the delay time ΔT is equal to approximately 1.3 × δt. Overall, the first set 82 of pulses provides sufficient energy to melt the material in layer 22 illuminated by linear beam 20, and the second set 84 of pulses provides sufficient optical magnification to extend the time the material remains in the molten liquid phase. The integrated optical magnification 102 of pulse synchronization 100 more closely replicates the two temporal amplitude peaks typical of excimer lasers used for laser annealing. However, pulse synchronization 100 using optical device 10 may be superior because the integrated optical magnification 102 is more constant over the duration of the melt.

[0040] Again, in pulse synchronization 90 or 100, the second set of pulses can be either 72A and 72B, 72B and 72C, or 72C and 72D. Generally, the last pulse will have the greatest impact on the quality of the microstructure produced by laser annealing. In pulse synchronization 90 or 100, that last pulse occurs when the laser beams 14A-14D, each incident on location 44, are angled at approximately the same angle (α A ≒α B ≒α C ≒α D ), it can be any pulse in the second set 84.

[0041] Referring again to FIG. 2, laser beams 14B and 14C are closer to central axis 46 and are both at an angle less than angle α, approximately α B +α CLaser beams 14A and 14D are farther from central axis 46 and span an angle α. FIG. 6A is a schematic illustration of laser beams 14A-14D formed into a linear beam 20, similar to FIG. 2. A first set of laser beams 14A and 14D span a first angular range α relative to central axis 46. F The second set of laser beams 14B and 14C are incident on location 44 within a second angular range α S The second angular range α S is the first angle range α F For example, the angle α F is in the range of about 2° to about 4°, and the angle α S is in the range of 0° to approximately 2°.

[0042] When pulse synchronization 80, 90, or 100 is applied to this arrangement, high quality microstructures are produced when a first set of pulses 82 is in the first set of laser beams and a second set of pulses 84 is in the second set of laser beams. That is, pulses 72B and 72C in laser beams 14B and 14C are selected to be in second set of pulses 84.

[0043] In the exemplary arrangement depicted in FIG. 6A , the central axis 46 of the linear beam 20 is approximately perpendicular to the layer 22 at location 44. If the beam projector 18 were telecentric, the central axis 46 would be approximately perpendicular to the layer 22 at each location within the linear beam 20, and the central axis 46 would be approximately parallel to the optical axis 42 of the cylindrical lens 38. In the depicted arrangement, the laser beams 14B and 14C have smaller angles of incidence on the layer 22 than the laser beams 14A and 14D. In some annealing applications, higher quality microstructures are produced by selecting a second set of pulses within the laser beams that has both a smaller angular distribution and a smaller angle of incidence. Here, the laser beams 14B and 14C are incident on the layer 22 within a second angular range α , which is smaller than the angle α. S Laser beams 14B and 14C are also incident on location 44 within a maximum of about αB ≒α C Laser beams 14A and 14D have a smaller angle of incidence up to about α A +α B ≒α C +α D It has a larger angle of incidence up to

[0044] FIG. 6B schematically illustrates details of another preferred embodiment of an optical device 110 according to the present invention. Similar to the optical device 10 of FIG. 1, the optical device 110 includes a beam homogenizer 16 and a beam projector 18 that cooperate to form a linear beam 20. The optical device 110 focuses the laser beams 14A-14D onto a location 44 in the short dimension (Y-axis). In the short dimension, the central axis 46 may be tilted at a small angle θ relative to the normal 112 of the layer 22, as depicted here. In practice, this is achieved by tilting the optical axes of the cylindrical lenses 38 and 40. Tilting helps reduce back reflections from the layer 22. The angle θ is typically in the range of 4° to 12°. As an example, an angle of 8° is depicted in the drawings. Note that the linear beam 20 is depicted in FIGS. 6A and 6B as a wavefront of laser radiation.

[0045] The combined optical device would have elements of optical devices 10 and 110. Optical device 110 may include an additional beam homogenizer (not shown) located between laser sources 12A-12D and beam projector 18. The additional beam homogenizer would have cylindrical microlenses linearly arranged along its short dimension (Y-axis), and these microlenses would have positive optical power in the short dimension. Laser beams 14A-14D would be directed onto the additional beam homogenizer at different angles in the short dimension. Laser beams 14A and 14D would be directed closer to the periphery of the additional beam homogenizer, while laser beams 14B and 14C would be directed closer to the center of the additional beam homogenizer.

[0046] Alternatively, optical device 110 can omit the additional beam homogenizer while still properly accounting for the imaging properties of cylindrical lenses 36 and 40 in the short dimension by simply directing laser beams 14A-14D through beam homogenizer 16 and beam projector 18. At the image plane of layer 22, laser beams 14A and 14D will be directed at a larger angle relative to laser beams 14B and 14C. At the Fourier plane of layer 22, laser beams 14A and 14D will have a larger displacement relative to laser beams 14B and 14C.

[0047] In these arrangements, laser beams 14A and 14D focused on layer 22 are farther from central axis 46, and laser beams 14B and 14C are closer in both the major and minor dimensions to central axis 46. This focusing arrangement, when combined with the inventive synchronization of pulses in laser beams 14A-14D by controller 60, also provides a higher quality microstructure in layer 22 after annealing.

[0048] The optical device of the present invention splits the pulses into a first set 82 within a first set of laser beams and then a second set 84 within a second set of laser beams, with the second set of laser beams incident from a smaller angular range than all of the laser beams that form a combined linear beam. The linear beam forms an annealed microstructure with approximately the same quality as would be formed by a linear beam when all of the laser beams are incident from a smaller angular range. However, the optical device of the present invention is more compact along the beam propagation axis (Z-axis) than a comparable optical device that delivers all of the laser beams within the second, smaller angular range. In the major dimension (X-axis), the propagation length from cylindrical lens 34 to layer 22 determines the angular distribution for laser radiation focused onto one location on layer 22. As an example, forming a 2,300 millimeter (mm) long linear beam with an angular distribution of 1.7° in one embodiment of the optical device requires a propagation length of approximately 50 m. In comparison, forming the same straight beam in a comparable device with a smaller maximum angular distribution of 0.85° would require a longer propagation length of approximately 90 m. By delivering only the second set of pulses within a smaller angular range, the optical device of the present invention can provide the desired laser annealing quality in a much more compact tool.

[0049] The optimal parameters for laser annealing vary depending on factors such as the thickness of layer 22, the thickness of substrate 24, the heat capacity and thermal conductivity of substrate 24, the wavelength of the laser radiation, and the duration of individual pulses. For a silicon layer, a glass substrate, and laser radiation having a wavelength of about 355 nm and a pulse duration of 20-30 ns, the energy deposited on layer 22 by each individual pulse is between 0.1 and 0.5 millijoules per square centimeter (mJ / cm). 2 ), the delay time ΔT can be in the range of 20 to 120 ns, the time δt can be in the range of 0 to 60 ns, and the individual laser beams can be separated by an angle (α A , α B , α C , or α D), and all laser beams may span an angle α of up to 18°. Similarly, the first angular range α F can be in the range of 1° to 9°, and the second angle range α S can be in the range of 0° to 3°.

[0050] The periodic microstructures in the annealed polycrystalline silicon are formed by interference within the focused laser radiation, with the periodicity being primarily in the direction of the electric field. It has been discovered that an unpolarized linear beam can produce microstructures that are aligned along both the long and short dimensions of the linear beam. Either optical device 10 or optical device 110 can use an unpolarized laser beam to produce annealed silicon layers of sufficient quality for making commercial displays. Alternatively, a polarized or partially polarized laser beam may be selectively directed into optical device 10 or optical device 110 to achieve an optimal mix of polarizations incident on the silicon layer.

[0051] Although the optical device of the present invention is depicted herein as having four laser sources, it may have any number of laser sources, which may be divided into at least a first set of pulses and a second set of pulses. The laser source may be of any type suitable for melting the material to be annealed. In the case of amorphous silicon, excimer lasers and diode-pumped solid-state lasers, as described in the aforementioned U.S. Patent Publication No. 2020 / 0235544, have proven suitable for laser annealing.

[0052] All pulses in the first set of pulses together have sufficient energy to melt the material in the layer. Application of the first or initial pulse in the first set preheats the material, and melting can occur following application of a later or even final pulse in the first set. Alternatively, the first set of pulses can be only one pulse with sufficient energy to melt the material in the layer.

[0053] The second set of pulses may be only one pulse, incident on the layer within the second angular range, with a delay sufficient to extend the time during which the material is melted. For example, referring to FIG. 4, the first set 82 may be pulses 72A, 72D, and 72B, while the second set 84 is pulse 72C. The last pulse (here, pulse 72C within laser beam 14C) will have the greatest impact on the quality of the microstructure produced by laser annealing, assuming that the microstructure is precipitated from the melted material during this last pulse. Optical interference of the laser radiation within the last pulse will produce the microstructure, e.g., the grain structure in the laser-annealed silicon.

[0054] The laser beams collectively span a range of incidence angles α, with at least one pulse of the laser beams illuminating the layer after the pulses in all other laser beams but before the material melted by the first set of pulses solidifies. The at least one laser beam preferably spans a range of incidence angles less than 0.5α, and most preferably less than 0.25α.

[0055] In summary, multiple laser beams are split into a first set of laser beams having a first set of laser pulses and a second set of laser beams having a second set of pulses. The beam homogenizer and beam projector cooperate to form the laser beams into a linear beam having a long dimension and a short dimension. The linear beam has a uniform intensity distribution along the long dimension to anneal the layer on the substrate. The second set of laser beams are incident on the layer from a smaller angular range than all of the laser beams combined. A controller synchronizes the pulses so that the first set of laser pulses is incident on the layer before the second set of pulses.

[0056] The present invention is described above in terms of preferred and alternative embodiments. However, the present invention is not limited to the embodiments described and depicted herein. Rather, the present invention is limited only by the claims appended hereto.

Claims

1. 1. A method for annealing a layer on a substrate, comprising: providing a plurality of pulsed laser beams, the laser beams being split into a first set of laser beams having a first set of laser pulses and a second set of laser beams having a second set of laser pulses; forming the laser beam into a linear beam, the linear beam having a major dimension and an orthogonal minor dimension, the linear beam having a uniform intensity distribution along the major dimension, and each location along the major dimension of the linear beam being illuminated by all of the laser beam; illuminating the layer with the linear beams, wherein the second set of laser beams are incident on the layer from a smaller angular range than all of the laser beams combined; synchronizing pulses in the laser beams such that a first set of laser pulses is incident on the layer before a second set of laser pulses, the pulses in the first set of laser beams melt material in the layer illuminated by the linear beam, and the pulses in the second set of laser beams are incident on the layer before the melted material solidifies; A method comprising:

2. 10. The method for annealing of claim 1, wherein the layer is made of amorphous silicon that is transformed by the annealing into polycrystalline silicon.

3. 3. The method for annealing according to claim 1 or claim 2, wherein sequential pulses in the first and second sets of laser beams partially overlap and the temporal separation between the sequential pulses is equal, thereby extending the time that the layer is continuously illuminated by the linear beam.

4. 3. The method for annealing of claim 1 or 2, wherein the first set of laser pulses are incident on the layer simultaneously, the first set of pulses and the second set of pulses are separated by a delay time ΔT, and the second set of laser pulses are incident on the layer simultaneously.

5. 3. The method for annealing of claim 1 or 2, wherein the first set of pulses are separated from one another by a time δt, the first set of pulses and the second set of pulses are separated by a delay time ΔT, and the second set of pulses are separated from one another by the time δt.

6. 6. The method for annealing of claim 5, wherein ΔT≧2δt.

7. 6. The method for annealing of claim 5, wherein the delay time ΔT is in the range of 20 to 120 nanoseconds and the time δt is in the range of 0 to 60 nanoseconds.

8. 3. The method for annealing of claim 1 or 2, wherein the first set of pulses are incident on the layer simultaneously, the first set of pulses and the second set of pulses are separated by a delay time ΔT, and the second set of pulses are separated from each other by a time δt.

9. 9. The method for annealing of claim 8, wherein ΔT=1.3δt.

10. 9. The method for annealing of claim 8, wherein the delay time ΔT is in the range of 20 to 120 nanoseconds and the time δt is in the range of 0 to 60 nanoseconds.

11. The first set of laser beams are oriented in a first angular range α with respect to the central axis of the linear beam. Ｆ and the second set of laser beams are incident on the layer within a second angular range α relative to the central axis. Ｓ is incident on the layer within the second angular range α Ｓ is the first angle range α Ｆ The method for annealing according to claim 1 or 2, wherein the annealing is performed in such a manner that the annealing is performed in a manner closer to the central axis than the annealing.

12. The first angular range α Ｆ is in the range of 1° to 9°, and the second angle range α Ｓ 12. The method for annealing according to claim 11, wherein is in the range of 0° to 3°.

13. 12. The method for annealing of claim 11, wherein the central axis is perpendicular to the layer in the major dimension.

14. 12. The method for annealing of claim 11, wherein the second set of laser beams have a smaller angle of incidence on the layer than the first set of laser beams.

15. 12. The method for annealing of claim 11, wherein the central axis is tilted in the minor dimension relative to the layer normal by an angle θ in the range of 4° to 12°.

16. 3. The method for annealing of claim 1 or 2, wherein the second set of laser beams is unpolarized.

17. 3. The method for annealing of claim 1 or 2, wherein the first and second sets of laser beams have a wavelength of 355 nanometers and the first and second sets of laser pulses have a pulse duration in a range of 20 to 30 nanoseconds.

18. 1. An optical apparatus for annealing a layer on a substrate, comprising: a plurality of pulsed laser sources, each of which provides a pulsed laser beam; a beam homogenizer having a major axis and an orthogonal minor axis, the laser beam being directed into and transmitted through the beam homogenizer, the directed laser beam being split into a first set of laser beams and a second set of laser beams arranged serially along the major axis of the beam homogenizer; a beam projector including a plurality of lenses arranged in series between the beam homogenizer and the layer, the beam projector arranged to intercept the laser beam transmitted through the beam homogenizer, the beam homogenizer and the beam projector cooperating to form the laser beam into a linear beam that illuminates the layer, the linear beam having a major dimension and an orthogonal minor dimension, each location along the major dimension of the linear beam being illuminated by all of the laser beams; and a controller for synchronizing pulses in said laser beam; Equipped with the second set of laser beams are incident on the layer from a smaller angular range than all of the laser beams combined; The controller synchronizes the pulses in the laser beams such that pulses in a first set of laser beams are incident on the layer before the pulses in a second set of laser beams, the pulses in the first set of laser beams melt material in the layer illuminated by the linear beam, and the pulses in the second set of laser beams are incident on the layer before the melted material solidifies.

19. 1. An optical apparatus for annealing a layer on a substrate, comprising: a plurality of pulsed laser sources, each of which provides a pulsed laser beam; a beam homogenizer, into which the laser beam is directed and transmitted through the beam homogenizer; a beam projector including at least two positive lenses arranged in series between the beam homogenizer and the layer, the beam projector arranged to intercept the laser beam transmitted through the beam homogenizer, the beam homogenizer and the beam projector cooperating to form the laser beam into a linear beam that illuminates the layer, the linear beam having a major dimension and an orthogonal minor dimension, each location along the major dimension of the linear beam being illuminated by all of the laser beams, the laser beams collectively spanning a range of angles of incidence α over locations within the linear beam; a controller for synchronizing pulses in said laser beam; Equipped with the controller synchronizes pulses in at least one laser beam such that pulses in the laser beams illuminate the layer after pulses in all other laser beams, and the at least one laser beam spans a range of incidence angles less than 0.5α.

20. 20. The optical device of claim 19, wherein the at least one laser beam spans a range of angles of incidence that is less than 0.25α.

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