Patent Description:
Laser annealing is used to produce large high-resolution LCD and OLED displays by melting a thin amorphous silicon layer on a substrate, onto which thin-film transistors (TFTs) are to be defined, followed by crystallization of the cooling silicon. The annealing process requires a stable pulsed ultraviolet laser, with low RMS pulse-energy noise of less than about <NUM>%, at a wavelength in a range between <NUM> nanometers (nm) and <NUM>. The laser-beam generated by the ultraviolet laser is formed into a line-beam that is scanned across the silicon layer. The laser-beam must have very-low beam-quality in order to form a speckle-free homogenous line-beam. Beam-quality is measured by the unit-less beam-quality factor M<NUM>. Very-low beam-quality corresponds to a very-high value of M<NUM>.

At present, such laser-beams are provided by a high-power excimer laser or a combination of such excimer lasers. For example, a xenon chloride (XeCl) excimer laser, which generates laser-radiation having a wavelength of <NUM>. Excimer lasers can be generally characterized as super-atmospheric gas-discharge lasers. The laser-beams have an elongated cross-section, characterized by a long axis and a short axis, corresponding respectively to length and width dimensions of a line-beam. The line-beam has a uniform or "flat-top" intensity distribution along both the length and width dimensions.

Typically, the M<NUM> values of a laser-beam from a XeCl excimer laser used for annealing are about <NUM> in the short axis and about <NUM> in the long axis, with <MAT> full-width beam dimensions of about <NUM> millimeters (mm) in the short axis and about <NUM> in the long axis. Laser-beams from several such excimer lasers are combined and projected into a line-beam having a width of about <NUM> and a length of between about <NUM> and about <NUM>. The required ultraviolet power-per-millimeter-of-length is about <NUM> watts (W), at a pulse-repetition frequency of about <NUM> hertz (Hz) and a pulse duration of about <NUM> nanoseconds (ns). For a line-beam length of <NUM>, about <NUM> kilowatts (kW) of ultraviolet power is required, which can be achieved by combining the outputs of six individual <NUM> W lasers, each providing <NUM> joule (J) of pulse-energy.

One disadvantage of excimer lasers is a high capital cost, which is due, inter-alia, to a requirement for a complex gas tube that includes discharge electrodes and power supplies capable of delivering electrical pulses having peak voltages of greater than <NUM> kilovolts (kV) to these electrodes. Another disadvantage of excimer lasers is a high cost-of-operation, due to a limited gas-tube lifetime of less than one year and frequent replacement of gas-tube windows during that lifetime.

There is a need for ultraviolet laser-annealing apparatus having lower capital cost and lower cost-of-operation than excimer laser-annealing apparatus. Preferably, the laser-annealing apparatus would be capable of providing pulse-energies and beam-parameters comparable to those provided by the excimer laser-annealing apparatus discussed above.

US patent application with publication number <CIT> discloses an amorphous silicon layer on a glass substrate crystallized by concentrating continuous wave radiation from a number of OPS-lasers into a line of light on the layer. The layer is moved with respect to the line of light to control the dwell time of the line on any location on the layer and to crystallize an extended area of the layer.

US patent application with publication number <CIT> discloses a laser imaging system with reduced speckle. The laser imaging system includes spatially superpositioned <NUM>-D arrays, or alternatively <NUM>-D arrays, of independent emitters of laser radiation, with each emitter having a spectral bandwidth Δλi centered at some arbitrary wavelength ΔλOi. The elements of the array are allowed, by design, to have a slightly different central wavelength, thereby creating an ensemble bandwidth ΔΛ which is much greater than the bandwidth Δλi of any individual emitter in the array.

US patent application with publication number <CIT> discloses a method of intracavity frequency conversion in a continuous wave laser, including causing fundamental radiation to circulate in a laser resonator. The fundamental radiation makes a first pass through an optically nonlinear crystal where a fraction of the fundamental radiation generates second-harmonic radiation in a forward pass through the crystal. The residual fundamental radiation and the second-harmonic radiation are then sum-frequency mixed in forward and reverse passes through an optically nonlinear crystal such that a fraction of each is converted to third-harmonic radiation. The residual second-harmonic radiation and fundamental radiation from the sum-frequency mixing then make a reverse pass through the second-harmonic generating crystal where the second-harmonic radiation is converted back to fundamental radiation. The third harmonic radiation can be delivered from the resonator as output radiation, or can be used to pump another optically nonlinear crystal in an optical parametric oscillator. Second-harmonic radiation can also be used to pump an optical parametric oscillator.

In one aspect, optical apparatus for annealing a layer on a substrate in accordance with the present disclosure comprises a plurality of frequency-converted repetitively-pulsed solid-state lasers. Each laser delivers an output beam having a wavelength in the ultraviolet region of the electromagnetic spectrum, a cross-section characterized by mutually-orthogonal first and second transverse axes, a beam-quality factor M<NUM> in the first transverse axis of greater than about <NUM>, and a beam-quality factor M<NUM> in the second transverse axis of greater than about <NUM>. Laser pulses in each output beam have a pulse-energy greater than about <NUM> millijoules and a pulse-repetition frequency greater than about <NUM> hertz. The optical apparatus further includes a line-projector arranged to receive the output beams, form the output beams into a line-beam, and project the line-beam onto the layer. The line-beam has a length and a width on the layer.

Aspects of the present invention are defined by the independent claims below to which reference should now be made. Optional features are defined by the dependent claims.

The accompanying drawings, which are incorporated in and constitute a part of the specification, schematically illustrate a preferred embodiment of the present disclosure, and together with the general description given above and the detailed description of the preferred embodiment given below, serve to explain principles of the present disclosure.

Turning now to the drawings, wherein like features are designated by like reference numerals, <FIG> schematically illustrates a preferred embodiment <NUM> of an externally-frequency-tripled repetitively-pulsed solid-state laser in accordance with the present invention. Laser <NUM> has a linear resonator <NUM> formed between a high-reflection resonator mirror <NUM> (HR mirror) and a partially-transmitting output-coupling resonator mirror <NUM> (OC mirror). Cartesian x-, y-, and z-axes are indicated on the drawing for reference.

Resonator <NUM> includes a gain-element <NUM> in the form of a slab that is located between resonator mirrors <NUM> and <NUM>. Resonator <NUM> is depicted schematically in a perspective view in <FIG>. Other components within the resonator are omitted from <FIG> for simplicity of illustration. Gain-element <NUM> is energized by pump-radiation from two-dimensional diode-laser arrays 20A and 20B. The pump-radiation is indicated in <FIG> by arrowheads E. The pump-radiation is folded into the gain-element by dichroic mirrors 22A and 22B, respectively. By way of example, the gain-element can be a neodymium (Nd<NUM>+) doped or ytterbium (Yb<NUM>+) doped yttrium aluminum garnet (YAG) or yttrium orthovanadate (YVO<NUM>) crystal. These crystals provide optical gain at wavelengths between about <NUM> and about <NUM>, characteristic of the dopant ion and crystal material, in the near-infrared region of the electromagnetic spectrum.

Energized gain-element <NUM> produces a beam of fundamental radiation, having a near-infrared wavelength, which circulates in resonator <NUM>. This fundamental radiation is indicated by arrowheads F. The circulating fundamental-radiation is linearly polarized, having a polarization-orientation indicted by arrows PF. This polarization-orientation is established by a thin-film polarizer <NUM> located in the resonator. Q-switched pulsed operation of the resonator is effected cooperatively by a Pockels cell <NUM> and a quarter-waveplate <NUM> located in resonator <NUM>.

The preferred end-pumping of the gain-element and Q-switch pulsed-operation are exemplary and should not be considered as limiting the present disclosure. Those skilled in the art would recognize that gain-element <NUM> may be side-pumped and that "cavity dumped" pulsed-operation may be utilized, without departing from the scope of the present disclosure.

A beam of output fundamental-radiation from resonator <NUM> is transmitted through OC mirror <NUM> and focused by a lens <NUM> into an optically nonlinear crystal <NUM>, which is arranged for type-<NUM> frequency-doubling of the fundamental radiation. A half-waveplate <NUM> rotates the polarization-orientation of the fundamental radiation directed into nonlinear crystal <NUM> by <NUM>°. A portion of the fundamental radiation is converted by nonlinear crystal <NUM> into a beam of second-harmonic radiation, having a wavelength in the visible region of the electromagnetic spectrum, leaving a residual beam of fundamental radiation. For example, up to about <NUM>% of the fundamental radiation is converted. Second-harmonic radiation is indicated in the drawing by double arrowheads <NUM>. The second harmonic-radiation has a polarization-orientation orthogonal to that of the fundamental radiation, indicated by arrows P<NUM>.

The second-harmonic radiation and the residual fundamental-radiation are both focused by a lens <NUM>, through a selective waveplate <NUM>, into an optically nonlinear crystal <NUM>. Nonlinear crystal <NUM> is arranged for type-<NUM> sum-frequency mixing of the second-harmonic radiation with the residual fundamental-radiation to generate a beam of third-harmonic radiation. Third-harmonic radiation is indicated in the drawing by triple arrowheads <NUM>. Suitable crystals for the second-harmonic generation and sum-frequency mixing include lithium triborate (LBO), beta barium borate (BBO), cesium borate (CB), and cesium lithium borate (CLBO). Frequency-tripling of the above-discussed wavelengths between <NUM> and <NUM> provides output wavelengths between about <NUM> and about <NUM>.

Selective waveplate <NUM> is configured to provide no polarization rotation of the fundamental radiation and <NUM>-degrees polarization rotation of the second-harmonic radiation, thereby aligning the polarization orientations for type-<NUM> sum-frequency mixing. The third-harmonic radiation has a polarization-orientation orthogonal to the orientation of the fundamental radiation and second-harmonic radiation, indicated by arrows P<NUM>. The polarization-orientation of the output third-harmonic radiation can be rotated by another half-waveplate (not shown), if required for an application. The output third-harmonic radiation is collimated by a lens <NUM>, forming a collimated ultraviolet output beam <NUM>. There is some remaining fundamental-radiation and remaining second-harmonic radiation (not shown) following the sum-frequency mixing. This remaining radiation can be separated from the output third-harmonic radiation by a filter (also not shown), such as a thin-film interference filter. Remaining radiation is preferably removed for most applications.

The beam of output fundamental-radiation from resonator <NUM> and ultraviolet output beam <NUM> have a first transverse axis parallel to the x-axis, a second transverse axis parallel to the y-axis, and propagate along the z-axis, as depicted in <FIG> and <FIG>. The transverse x-axis and transverse y-axis are mutually orthogonal. HR mirror <NUM> and OC mirror <NUM> may be flat or may be cylindrical having catoptric power only in the second transverse axis.

The first transverse axis is referred to hereinafter as the "horizontal axis" or "long axis". The second transverse axis is referred to hereinafter as the "vertical axis" or "short axis". The z-axis is referred to as the "propagation axis". Terms such as "horizontal", "vertical", "long", and "short" are used herein for convenience of description. "Horizontal" and "vertical" are not meant to limit the spatial orientation of the laser apparatus in use. Similarly, "long" and "short" are not meant to limit the aspect ratio of a beam, which is easily transformed by lenses and mirrors. Those skilled in the art would recognize that the short and long axes of the output beam could be interchanged using a periscope and that the beam-dimensions can be adjusted using one or more telescopes. These changes can be made without departing from the scope of the present disclosure.

<FIG> is a perspective view schematically illustrating the frequency-tripled pulsed solid-state laser of <FIG> in a housing <NUM> having a window <NUM>, through which ultraviolet output beam <NUM> is delivered. Exemplary M<NUM> values and exemplary cross-sectional dimensions for the short and long axes of the ultraviolet output beam are indicated on the drawing. The exemplary M<NUM> values are respectively greater than <NUM> and greater than <NUM>. The exemplary cross-sectional dimensions of ultraviolet output beam <NUM> are respectively about <NUM> and about <NUM>. Ultraviolet output beam <NUM> from inventive laser <NUM>, having slab-shaped gain-element <NUM>, may be rectangular or may be elliptical in cross-section. The cross-sectional shape depends, for example, on whether the gain-element is side-pumped or end-pumped.

Inventive laser <NUM> is capable of delivering ultraviolet output pulses having a full-width at half maximum (FWHM) pulse-duration greater than about <NUM> ns and a pulse-energy greater than about <NUM> mJ-per-pulse at a pulse-repetition frequency greater than about <NUM>.

It should be noted here that the design of inventive laser <NUM> represent a radical departure from conventional solid-state laser design, in which efforts are directed to maximizing beam quality (minimizing M<NUM>) for operations such as laser cutting and laser drilling. In these operations, precise focusing of laser radiation is required. Such conventional solid-state lasers typically deliver a laser beam with a nominally circular crosssection and an M<NUM> value less than about <NUM> in both transverse axes. Multimode solid-state lasers emitting ultraviolet radiation are also designed to have the lowest possible M<NUM> values for a given output power. Typically, these lasers produce ultraviolet output beams having M<NUM> values of less than <NUM> in both transverse axes.

An objective in designing inventive laser <NUM> is to maximize M<NUM> to reach values greater than about <NUM> in the short axis and greater than about <NUM> in the long axis, with the long-axis M<NUM> value greater than the short-axis M<NUM> value. These large beam-quality factors are characteristic of the above-discussed excimer lasers that the inventive laser is intended to replace. Key to maximizing M<NUM> values is slab-shaped gain-element <NUM>, cooperative with the above-discussed cylindrical resonator mirrors <NUM> and <NUM>. Gain-element <NUM> has a horizontal width W, a vertical thickness T, and a length L, as depicted in <FIG>.

The above-described end-pumping with elongated beams from two-dimensional diode-laser arrays 20A and 20B provides an elongated gain-volume, having a horizontal width w, a vertical height h, and a length L (the length of the gain-element). A cross-sectional gain-area <NUM> with dimensions width w by height h defines the cross-section of the beam of fundamental radiation within gain-element <NUM>. Gain-area <NUM> acts as a soft aperture within resonator <NUM>. Width w is significantly greater than height h, preferably at least three times greater. These representative dimensions are referred to below in describing calculated performance of examples of inventive laser <NUM>.

<FIG> graphically illustrate calculated beam-quality factor M<NUM> for the output fundamental radiation from resonator <NUM> in the vertical axis versus resonator length, in an example of inventive laser <NUM> of <FIG>. The resonator length is the distance between resonator mirrors <NUM> and <NUM>. Resonator mirrors <NUM> and <NUM> are flat in the calculation of <FIG>. Resonator mirrors <NUM> and <NUM> are concave in the calculation of <FIG>, having a radius-of-curvature of <NUM>. Gain-element <NUM> is located in the center of the resonator. M<NUM> values are depicted for different thermal lenses in the gain--element, from <NUM> diopter (D) to <NUM> D, in the vertical axis. In this example, fundamental radiation is generated having a wavelength of about <NUM>. The dimensions width W, length L, and thickness T of gain-element <NUM> are assumed to be <NUM>, <NUM>, and <NUM>, respectively. Gain-area <NUM> has a width w of <NUM> and a height h of <NUM>.

<FIG> graphically illustrates calculated beam-quality factor M<NUM> for the output fundamental radiation in the horizontal axis versus resonator length, in the resonator of <FIG>. , which has flat resonator mirrors. The thermal lens in gain-element <NUM> is approximately <NUM> D in the horizontal axis. However, intra-cavity focusing or defocusing can be introduced by adding cylindrical lenses having dioptric power in the horizontal axis. M<NUM> values are depicted for different pairs of cylindrical lenses, from <NUM> D to <NUM> D total dioptric power, that are symmetrically located adjacent to each end of the gain-element. Adding positive dioptric power to the resonator increases the M<NUM> values in the horizontal axis.

<FIG> graphically illustrates calculated beam-quality factor M<NUM> for the output fundamental radiation in the vertical axis versus resonator length, in the resonator of <FIG>. Here, the thermal lens in gain-element <NUM> is <NUM> D in the vertical axis. M<NUM> values are depicted for different radii-of-curvature of the resonator mirrors, from <NUM> to <NUM>, in the vertical axis. <FIG> shows that adding positive dioptric power to the resonator increases the M<NUM> values in the vertical axis. Similarly, <FIG> shows that adding positive catoptric power to the resonator considerably increase the accessible M<NUM> values in the vertical axis.

The examples demonstrate that a lasing resonator-mode is established having beam-quality factors that are determined by the resonator length, gain-area, intra-cavity lensing, and curvature of the resonator mirrors. Thermal-lensing is strongest in the vertical axis and negligible in the horizontal axis. The resulting M<NUM> values are still smaller in the vertical axis than in the horizontal axis due to the slab-shape of gain-element <NUM> and the elongation of gain-area <NUM>. Dioptric or catoptric power in each of the transverse axes can be modified independently using intra-cavity lenses or curved resonator mirrors to achieve desired M<NUM> values for the output fundamental radiation. Again, in the exemplary laser, the vertical axis corresponds to the short axis and the horizontal axis to the long axis of the fundamental radiation.

It is further assumed that gain-element <NUM> is a Nd<NUM>+ doped YAG crystal. If the gain-element is energized by <NUM> J-per-pulse of absorbed pump-radiation, the dioptric power of the slab--shaped gain-element is about <NUM> D in the short axis and about <NUM> D in the long axis. Under these conditions, resonator <NUM> can be configured to reliably produce M<NUM> values for the output fundamental radiation greater than <NUM> in the short axis and greater than <NUM> in the long axis. Preferably, resonator <NUM> would be configured to produce M<NUM> values greater than <NUM> in the short axis and greater than <NUM> in the long axis.

<FIG> is a graph of calculated output fundamental-radiation power as a function of OC-mirror reflectivity, with the exemplary Nd<NUM>+: YAG resonator operated at different pulse-repetition frequencies, from <NUM> to <NUM>. Here, the resonator length is <NUM> and the resonator mirrors are flat. The other resonator parameters are the same as for <FIG>. It can be seen that at a pulse-repetition frequency of <NUM> and for an OC-mirror reflectivity of about <NUM>%, the output fundamental-radiation power peaks at about <NUM>,<NUM> W, which corresponds to a pulse-energy of about <NUM> Joules. In examples presented herein below it is assumed that the resonator is pumped at and operates at a pulse-repetition frequency of <NUM>. <FIG> is a graph of FWHM pulse-duration as a function of OC-mirror reflectivity in the example of <FIG>. It can be seen that the optimal <NUM>% OC-mirror reflectivity results in a pulse-duration of about <NUM> ns.

Model calculations indicate that the output third-harmonic radiation power from inventive laser <NUM> can be expected to be about <NUM>% of the output fundamental-radiation power from resonator <NUM>, when the harmonic-conversion efficiencies are optimized. Accordingly, the inventive frequency-tripled solid-state laser can be expected to deliver an ultraviolet output power of about <NUM> W at a wavelength of <NUM>. Higher powers may be achieved through further refinement of the inventive near-infrared resonator. At this ultraviolet output power, the combined outputs of about twelve of the inventive lasers would be required to provide the same power produced by six excimer lasers.

<FIG> graphically illustrates calculated second-harmonic generation efficiency as a function of propagation distance a type-<NUM> non-critically phase-matched LBO crystal. The conversion efficiency is expressed as a percentage. The beam waist of the fundamental radiation is located in the center of the LBO crystal. The cross-sectional dimensions of the beam waist are assumed to be <NUM> in the short axis and <NUM> in the long axis. Incident pulse energy is assumed to be <NUM> J and pulse duration is assumed to be <NUM> ns. TNCPM is the non-critical phase-matching temperature for a plane wave, which is typically calculated to be in a range between about <NUM> and about <NUM>. The precise temperature depends on the Sellmeier coefficients used in the calculation. Calculations are shown for two crystal temperatures, TNCPM + <NUM> and TNCPM + <NUM>. The optimum crystal temperature for the focused beams in the example is about TNCPM + <NUM>. This slight detuning from TNCPM partially compensates wave-front curvature of the fundamental radiation on both sides of the beam waist.

It was determined that the already high M<NUM> value of the output fundamental-radiation from resonator <NUM> can be significantly increased by nonlinear crystal <NUM>. Second-harmonic radiation is generated from the output fundamental-radiation by second-harmonic generation in the nonlinear crystal. <FIG> also illustrates calculated second-harmonic radiation beam-quality factor M<NUM> in the long axis as a function of propagation distance within the crystal. It is assumed that the incident beam of multimode fundamental-radiation has a wavelength of <NUM>, an M<NUM> value of <NUM> in the short axis, and an M<NUM> value of <NUM> in the long axis. The calculations show that the M<NUM> value in the long axis more than doubles, from <NUM> for the incident beam of fundamental radiation to <NUM> for the beam of second-harmonic radiation emerging from the LBO crystal, at the optimum crystal temperature. Even for short propagation distances of a few millimeters within the LBO crystal, the M<NUM> value increases to about <NUM> for the beam of second-harmonic radiation.

<FIG> is a graph of calculated beam-quality factor M<NUM> in the long axis for unconverted fundamental-radiation as a function of propagation distance in the non-critically phase-matched LBO crystal of <FIG>. <FIG> also illustrates calculated unconverted fundamental-radiation power as a function of propagation-distance within the LBO crystal. The unconverted power is expressed as a percentage of the incident fundamental-radiation power on the LBO crystal. Unconverted fundamental-radiation exiting the LBO crystal becomes the residual fundamental-radiation directed into nonlinear crystal <NUM>. Calculations are shown for the same crystal temperatures as <FIG>. The M<NUM> value of the unconverted fundamental-radiation in the long axis also increases at the optimum crystal temperature, more than doubling to a maximum of about <NUM> as the beam propagates through nonlinear crystal <NUM>. Accordingly, nonlinear crystal <NUM> generates thirdharmonic radiation by sum-frequency mixing the beam of residual fundamental-radiation and the beam of second-harmonic radiation, each having a M<NUM> value of more than <NUM> in the long axis.

<FIG> and <FIG> illustrate increases in the beam-quality factors M<NUM> of the generated second-harmonic beam and the residual fundamental beam that are beneficial for laser annealing. These increases arise from changes in wave-front curvature of the propagating beams coupled with the conversion of fundamental radiation to second harmonic radiation and back-conversion of second-harmonic radiation to fundamental radiation. In general, frequency doubling is most efficient when the fundamental beam and the second-harmonic beam have the same wave-front curvature. For multimode beams with a flat-top intensity distribution, precise matching of the wave-front curvatures occurs when the M<NUM> value of the second-harmonic beam is twice the M<NUM> value of the fundamental beam. Variations in the M<NUM> values as the beams propagate through the nonlinear crystal are produced by continuous conversion and back conversion between the various transverse modes.

<FIG> and <FIG> graphically illustrate calculated beam quality factor M<NUM> for third-harmonic radiation as a function of propagation distance through nonlinear crystal <NUM>, which has the form of two separate LBO crystals, each having a length of <NUM>. The two LBO crystals are configured and arranged for walk-off compensation. <FIG> has a temperature in each crystal of <NUM> and <FIG> has a temperature in each crystal of <NUM>. <FIG> and <FIG> also illustrate calculated third-harmonic conversion efficiency as a function of propagation distance within the two LBO crystals. Again, the overall conversion of fundamental radiation to third-harmonic radiation is most efficient when the wave-front curvatures of all the beams are matched. Precise matching occurs when the M<NUM> value of the third-harmonic beam is three times that of the fundamental beam and the M<NUM> value of the second-harmonic beam is two times that of the fundamental beam.

It is assumed that the incident beam of residual fundamental radiation directed into the first LBO crystal has an M<NUM> value of <NUM> in the short axis and <NUM> in the long axis. The incident beam of second harmonic-radiation directed into the first LBO crystal has an M<NUM> value of <NUM> in the short axis and <NUM> in the long axis. The cross-sectional dimensions of the beam waist in the center of each LBO crystal are assumed to be <NUM> in the short axis and <NUM> in the long axis. The incident pulse energies are assumed to be <NUM> J of residual fundamental-radiation and <NUM> J of second-harmonic radiation. That is, a ratio of <NUM>% to <NUM>% for pulse energy in the fundamental radiation to pulse energy in the second-harmonic radiation. The pulse duration is still assumed to be <NUM> ns.

It was determined that the M<NUM> value of the output third-harmonic radiation further increased to <NUM> in the short axis and <NUM> in the long axis after propagation through both LBO crystals at <NUM>. That is, the M<NUM> value in the long axis is greater than <NUM>-times the M<NUM> values of the incident second-harmonic radiation and residual fundamental radiation. The overall conversion efficiency is <NUM>%. To summarize results of the model calculations, output fundamental-radiation from the resonator having M<NUM> values of <NUM> and <NUM> is converted into output third-harmonic radiation having M<NUM> values of about <NUM> and <NUM>, respectively. <NUM> J-per-pulse of fundamental radiation is converted into about <NUM> J-per-pulse of third-harmonic radiation.

The model calculations indicate that, following second-harmonic generation in nonlinear crystal <NUM>, fundamental radiation M<NUM> values of between about <NUM> and <NUM> in the short axis and between about <NUM> and <NUM> in the long axis can increase to between about <NUM> and <NUM> in the in the short axis and between about <NUM> and <NUM> in the long axis. That is, the beam-quality factor about doubles in value in each axis. Following sum-frequency mixing in nonlinear crystal <NUM>, the third-harmonic radiation M<NUM> values can further increase to between about <NUM> and <NUM> in the short axis and between about <NUM> and <NUM> in the long axis. Ultimately, the M<NUM> value in the long axis following sum-frequency mixing may be constrained by angular-acceptance limitations of the nonlinear crystals. Nevertheless, the M<NUM> values of the output third-harmonic beam can be greater than about <NUM> in the short axis and greater than about <NUM> in the long axis. This compares with M<NUM> values of <NUM> in the short axis and <NUM> in the long axis for ultraviolet beams produced by excimer lasers currently used for laser annealing, which are discussed above.

<FIG> and <FIG> demonstrate that having multiple nonlinear crystals arranged in series to generate third-harmonic radiation can provide sufficient propagation distance to achieve a desired beam-quality factor. <FIG> schematically illustrates another preferred embodiment <NUM> of an externally-frequency-tripled repetitively-pulsed solid-state laser in accordance with the present disclosure. Laser <NUM> is similar to laser <NUM> of <FIG>, but has two nonlinear crystals <NUM> and <NUM> to generate third-harmonic radiation by sum-frequency mixing. Nonlinear crystals <NUM> and <NUM> may have the same length or may have different lengths, which are selected to optimize overall conversion of fundamental radiation to third-harmonic radiation and to provide a desired beam-quality factor. The optical elements of embodiment <NUM> to the left of waveplate <NUM> are the same as in the <FIG> embodiment, and are omitted for convenience of illustration.

A portion of the output fundamental radiation is converted by nonlinear crystal <NUM> into a beam of second-harmonic radiation. The second-harmonic radiation and the residual fundamental-radiation are both focused by a lens <NUM> into nonlinear crystal <NUM> to generate one beam of third-harmonic radiation by sum-frequency mixing. This third-harmonic beam is separated from the residual fundamental beam and residual second-harmonic beam by a mirror <NUM> and is directed by a mirror <NUM> through a half-waveplate <NUM> and onto a cube-prism polarizer <NUM>. Mirror <NUM> is transmissive for fundamental radiation and second-harmonic radiation. Mirrors <NUM> and <NUM> are reflective for third-harmonic radiation.

The copropagating residual fundamental beam and residual second-harmonic beam transmitted through mirror <NUM> are focused into nonlinear crystal <NUM> to generate another beam of third-harmonic radiation. This third-harmonic beam is separated from the remaining fundamental beam and the remaining second-harmonic beam by another mirror <NUM> and directed thereby onto polarizer <NUM>. The two beams of third-harmonic radiation incident on polarizer <NUM> have orthogonal linear polarizations and are combined thereby to form output beam of ultraviolet radiation <NUM>. Polarizer <NUM> has a polarization selective surface that is transmissive for one polarization and reflective for the orthogonal polarization. Ultraviolet output beam <NUM> therefore includes both linear polarizations. The various lenses focus the beams into the nonlinear crystals and collimate the beams as depicted. For example, lens <NUM>.

<FIG> schematically illustrates yet another preferred embodiment <NUM> of an externally-frequency-tripled repetitively-pulsed solid-state laser in accordance with the present disclosure. Laser <NUM> is similar to laser <NUM>, but uses predominantly mirrors instead of lenses to focus and collimate the beams. For example, a mirror <NUM> in laser <NUM> instead of lens <NUM> in laser <NUM>. Again, two third-harmonic beams are generated in separate nonlinear crystals. In laser <NUM>, these third-harmonic beams are spatially combined by a mirror <NUM> that is reflective for third-harmonic radiation to form ultraviolet output beam <NUM>. One advantage of laser <NUM> is that ultraviolet output beam <NUM> is linearly polarized.

To demonstrate principles of the present disclosure described herein above, two externally-frequency-tripled repetitively-pulsed solid-state lasers similar to embodiment <NUM> of <FIG> were modeled. Each laser has a resonator that produces a beam of output fundamental-radiation at <NUM> having dimensions <NUM> x <NUM> and beam quality factors <NUM> x <NUM> in the short x long axes. One non-linear crystal made of LBO generates a beam of second-harmonic radiation at <NUM> by type-I frequency doubling. Two serially-arranged non-linear crystals made of LBO generate two beams of third-harmonic radiation by type-I sum-frequency mixing. These two beams of third-harmonic radiation are collimated and polarization combined to form an output beam of unpolarized ultraviolet radiation, as depicted in <FIG>. The polarization-combined output beam of each laser has dimensions of <NUM> x <NUM> and beam quality factors of <NUM> x <NUM> in the short x long axes.

<FIG> schematically illustrates a preferred embodiment <NUM> in accordance with the present disclosure to spatially combine the output beams of the two externally-frequency-tripled repetitively-pulsed solid-state lasers. Each laser has housing <NUM> and window <NUM>, through which output beam <NUM> is delivered. One of the output beams is directed by a mirror <NUM> onto a mirror <NUM> that is precisely located to spatially combine the two output beams into one combined beam of unpolarized ultraviolet radiation <NUM>. Combined beam <NUM> has dimensions of about <NUM> x <NUM> and beam quality factors of about <NUM> x <NUM> in the short x long axes. Combined beam <NUM> also has twice the power of each output beam <NUM>.

<FIG> schematically illustrates a preferred embodiment <NUM> of an internally-frequency-tripled repetitively-pulsed solid-state laser in accordance with the present disclosure. Laser <NUM> is similar to laser <NUM>, having resonator <NUM> formed between HR mirror <NUM> and another high-reflection resonator mirror <NUM>, which is highly reflective for both the fundamental radiation and the second-harmonic radiation. In laser <NUM>, nonlinear crystals <NUM> and <NUM> are located within resonator <NUM>. The beam of fundamental radiation circulates in resonator <NUM>. Output coupling is through partial conversion of fundamental radiation to harmonic radiation, rather than partial transmission of fundamental radiation through an output-coupling mirror.

While propagating from left to right in the drawing, the beam of intracavity fundamental radiation passes through half-waveplate <NUM>, nonlinear crystal <NUM>, nonlinear crystal <NUM>, and is reflected by an HR mirror <NUM>. Half-waveplate <NUM> rotates the polarization-orientation of the fundamental radiation, then nonlinear crystal <NUM> partially converts the fundamental radiation to a beam of second-harmonic radiation, which is also reflected by HR mirror <NUM>. The reflected fundamental radiation and second-harmonic radiation co-propagate from right to left through nonlinear crystal <NUM>, selective waveplate <NUM>, nonlinear crystal <NUM>, and onto an output mirror <NUM>. The reflected fundamental radiation is further converted into second-harmonic radiation by nonlinear crystal <NUM>, increasing the power of the beam of second-harmonic radiation. Selective waveplate <NUM> rotates the polarization of the second-harmonic radiation only. Nonlinear crystal <NUM> partially converts the fundamental radiation and the second-harmonic radiation to a beam of third-harmonic radiation. Output mirror <NUM> is highly transmissive for fundamental radiation and reflective for second-harmonic and third-harmonic radiation. Collimated ultraviolet output beam <NUM> is directed out of resonator <NUM> by output mirror <NUM>. The polarization of the output third-harmonic radiation is indicated by arrow P<NUM>. The three lenses focus the beams into the nonlinear crystals and collimate the beams as depicted.

<FIG> schematically illustrate a preferred embodiment <NUM> of a line-projector for combining and homogenizing the output of six frequency-tripled solid-state lasers of <FIG>. Line-projector <NUM> forms the combined output into a line-beam that is projected onto a silicon layer <NUM> to be annealed, which is supported on a substrate <NUM>. The distribution of polarization orientations in the projected line-beam is selectable.

The six lasers are designated as lasers <NUM>A, <NUM>B, <NUM>C, <NUM>D, <NUM>E, and <NUM>F. The six lasers are assumed to have nominally the same output characteristics. The ultraviolet radiation emitted by each of the lasers is linearly-polarized, here, S-polarized. Each laser directs a beam of radiation through one of six corresponding polarization rotators <NUM>. In the example depicted, the polarization rotators are adjusted such that radiation from lasers <NUM>A and <NUM>F is not polarization rotated and remains S-polarized. Radiation from lasers 10c and <NUM>D is polarization rotated by <NUM>° and becomes P-polarized. Radiation from lasers <NUM>B and <NUM>E is polarization rotated by some angle between <NUM>° and <NUM>°, to an intermediate orientation (I-polarized) between the P and S polarizations. For example, rotated by an angle of <NUM>°.

The beams of radiation having various polarization orientations then traverse corresponding ones of six long-axis beam-homogenizers, each thereof including two cylindrical lens-arrays 68A and 68B. Output from all of the beam-homogenizers is collected by a spherical lens <NUM>. Spherical lens <NUM> cooperative with a cylindrical lens <NUM>, a spherical lens <NUM> and a cylindrical lens <NUM> combines the outputs of lasers <NUM>A-F into a line-beam on substrate <NUM>. Spherical lenses <NUM> and <NUM> determine the length LL of the line-beam. Lenses <NUM>, <NUM>, <NUM> and <NUM> define the width LW of the line-beam, which is depicted in <FIG>. Each of the lasers delivers radiation to the entire length of the homogenized line-beam illuminating silicon layer <NUM>. The line-projector of <FIG> is capable of combining and homogenizing beams from a plurality of lasers <NUM>, here six, and forming a line-beam having a width LW of less than about <NUM>.

<FIG> schematically illustrates how the polarization of radiation at any point on the length of the line-beam is angularly distributed. In the example depicted, the polarization-orientation transforms from S-polarized at highest angles-of-incidence, through I polarized, to P-polarized at near normal angles-of-incidence.

It should be noted that only sufficient detail of line-projector <NUM> is presented here for understanding principles of the present disclosure. A detailed description of a particular apparatus for polarizing, combining, homogenizing, and projecting outputs from a plurality of ultraviolet excimer lasers is provided in <CIT>, assigned to the assignee of the present invention. Those skilled in the art may use other beam combining and projecting apparatus without departing from the scope of the present disclosure.

In summary, described above is an inventive frequency-tripled solid-state laser producing an ultraviolet output beam having an average power, pulse energy, and beam-parameters comparable to those of excimer lasers. The frequency tripling can be internal or external. The design approach to the resonator of the inventive laser is radically different from that of conventional prior-art solid-state lasers, enabling production of a near-infrared beam having an elongated cross-section and M<NUM> values that are significantly greater than <NUM> in one transverse axis and <NUM> in an orthogonal transverse axis. This design approach provides that the M<NUM> values of the near-infrared beam are multiplied in the frequency-tripling process to values as high as <NUM> in the one transverse axis and <NUM> in the orthogonal transverse axis. Ultraviolet output beams of several of the inventive lasers can be combined in the manner used to combine the output beams of prior-art ultraviolet excimer lasers.

Claim 1:
Optical apparatus for annealing a layer (<NUM>) on a substrate, comprising:
a plurality of frequency-converted repetitively-pulsed solid-state lasers (<NUM>), each thereof including a laser resonator (<NUM>) formed between first (<NUM>) and second (<NUM>) resonator mirrors, the laser resonator (<NUM>) including a gain-element (<NUM>) in the form of a slab that is energized by optical pumping to provide a gain volume in the gain-element (<NUM>), the energized resonator (<NUM>) producing a fundamental radiation beam (F), each solid-state laser (<NUM>) further including at least one nonlinear crystal (<NUM>, <NUM>) for converting a portion of the fundamental radiation beam (F) into an output beam (<NUM>) having a wavelength in the ultraviolet region of the electromagnetic spectrum, each output beam (<NUM>) having a cross-section characterized by mutually-orthogonal first and second transverse axes;
a line-projector (<NUM>) arranged to receive the output beams (<NUM>), form the output beams (<NUM>) into a line-beam, and project the line-beam onto the layer (<NUM>) on the substrate (<NUM>), the line-beam having a length and a width on the layer (<NUM>);
wherein cross-sectional dimensions of the gain volume, a resonator length between the first (<NUM>) and second (<NUM>) resonator mirrors, optical powers of the first (<NUM>) and second (<NUM>) resonator mirrors, and a propagation distance in the at least one nonlinear crystal (<NUM>, <NUM>) are selected to achieve a beam-quality factor M<NUM> of the output beam (<NUM>) in the first transverse axis greater than <NUM> and a beam-quality factor M<NUM> of the output beam (<NUM>) in the second transverse axis greater than <NUM>; and
wherein output coupling from the laser resonator (<NUM>) and a pulse-repetition frequency are selected to produce laser pulses of the output beam (<NUM>) having a pulse energy greater than <NUM> millijoules, the pulse-repetition frequency being greater than <NUM> hertz.