Excimer laser with uniform beam

Abstract

By positioning multiple resonant cavity mirrors (32, 34) at each end of a linear excimer laser (30), fine structure in the transverse mode of the excimer laser beam is minimized. At one end, a highly reflective end mirror (32) and a partially reflective end mirror (34) are tilted at a small angle relative to each other. At the other end, two output coupling mirrors (36, 38) are tilted at a small angle relative to each other. This arrangement of resonant cavity mirrors (32, 34) produces a composite laser beam that blurs any fine structure.

Description

[0001] priority

[0002] This application claims priority to U.S. Provisional Application Serial No. 62 / 821,239, filed on March 20, 2019, the disclosure of which is incorporated herein in its entirety. Technical Field

[0003] The present invention relates generally to gas discharge lasers and more particularly to resonator mirror arrangements for excimer gas discharge lasers. Background Art

[0004] Excimer lasers produce high-power laser radiation in the ultraviolet region of the electromagnetic spectrum. The gain medium is a flowing pressurized gas mixture, usually including a noble gas, a halide gas, and a buffer gas. The gas mixture is energized by a short current pulse between two elongated electrodes to produce excited weakly bonded dimers in the gas discharge. For example, the gas mixture can include xenon (Xe) and hydrogen chloride (HCl) gases, and helium (He) or neon (Ne) as a neutral buffer gas. When energized, electronically excited XeCl dimers are produced, which have a laser line at 308nm. Other examples are argon fluoride (ArF) dimers with a 193nm laser line and xenon fluoride (XeF) dimers with a 351nm laser line.

[0005] Typically, the gas mixture can be energized at a repetition rate of up to several kilohertz, the gas discharge can last for a duration of a few nanoseconds to a few hundred nanoseconds, the duration of the output laser pulse can reach a few nanoseconds to tens of nanoseconds, and the average output laser power can reach hundreds of watts. The most powerful state-of-the-art industrial excimer lasers are based on XeCl and produce pulse energies up to 1 joule at pulse repetition rates of up to 600 Hz. For example, the Lambda SX excimer laser from Coherent Inc. of Santa Clara, California. This excimer laser can operate continuously for more than 100 million pulses while maintaining very high pulse energy stability and stable beam parameters. In applications requiring even greater pulse energies, the output of two or more excimer lasers can be combined by beam mixing optics and pulse transmission synchronization. Such beam mixing and synchronization are described in U.S. Patents 7,408,714 and 8,238,400, respectively, both of which are assigned to the assignee of the present invention, and the complete disclosure of each patent is incorporated herein by reference.

[0006] An important application is the laser recrystallization of silicon, a process used to manufacture flat panels for screens of consumer electronic devices and large-size display screens. Silicon is a semiconductor substrate on which the electronic circuits in the screen are formed by a photolithographic process. During the recrystallization process, a thin layer of amorphous silicon on a glass substrate is repeatedly melted by a pulsed laser radiation beam until the desired crystalline microstructure is obtained. The substrate and the silicon layer thereon are scanned relative to the laser radiation source and the optics that deliver the laser radiation. The laser radiation beam incident on the silicon layer is shaped into an elongated "beam" with a uniform intensity distribution in a direction orthogonal to the scanning direction. Excimer lasers are the light source of choice in this process, which is called "excimer laser annealing". This is a delicate process. It is crucial to maintain a stable and uniform intensity distribution with an optimal energy density.

[0007] Methods and apparatus for shaping an excimer laser beam into a beam are described in U.S. Patents 7,428,039 and 7,615,722, which are assigned to the assignee of the present invention and whose entire disclosures are incorporated herein by reference. These methods incorporate beam homogenization whereby a laser beam from one or more sources is spatially split and the split beams are projected onto and overlaid on a silicon layer. For example, ten to thirty projected and overlapping beams, each having a minor axis dimension of about 0.4 mm and a major axis dimension of 750 mm to 1500 mm. Beam homogenization requires expensive and complex optics, including lens arrays that can accommodate large beams from excimer lasers. For example, a beam having a lateral dimension of 15 mm x 35 mm.

[0008] Further averaging is achieved by exposing each location on the silicon layer to multiple laser pulses. For example, 10 to 20 consecutive laser pulses. Nevertheless, the performance of screens made using this process can still be improved by reducing the differences in the crystal microstructure, by forming a more stable and uniform line beam. One of the causes of instability and inhomogeneity is the fine structure of the lateral spatial mode of the excimer laser beam, which is caused by instabilities in the gas discharge, turbulence in the gas flow, and contamination of the gas containment window. This fine structure is unstable and varies significantly from pulse to pulse. The standard deviation of the energy density at any location in the output beam of an excimer laser can be greater than 10%, or even greater than 20%, while the standard deviation of the integrated energy of the entire beam is less than 0.3%. Fine structure is more difficult to homogenize than coarse structure, especially when it becomes smaller than a single lens element in a lens array.

[0009] There is a need for an excimer laser that produces a laser beam with minimal fine structure. Specifically, a laser beam with minimal brightness contrast in the fine structure. Preferably, such a reduction in brightness contrast would be achieved without compromising efficiency, pulse energy, average power, or any other critical beam parameter. Other applications of excimer lasers would also benefit from reducing brightness contrast in fine structures, such as microlithography and laser lift-off. SUMMARY OF THE INVENTION

[0011] In one aspect of the invention, an excimer laser that generates a laser beam includes a laser chamber containing an excited gas mixture, a first rear mirror having a high reflectivity at the wavelength of the laser beam, and a second rear mirror that is partially reflective at the wavelength of the laser beam. The first rear mirror and the second rear mirror are located at one end of the laser chamber. The second rear mirror is located between the first rear mirror and the laser chamber. The first rear mirror and the second rear mirror are tilted relative to each other at a first angle. A first output coupling mirror and a second output coupling mirror are provided. The first output coupling mirror and the second output coupling mirror are located at opposite ends of the laser chamber. The first output coupling mirror and the second output coupling mirror are tilted relative to each other at a second angle. BRIEF DESCRIPTION OF THE DRAWINGS

[0012] The accompanying drawings, which are incorporated in and constitute a part of the specification, illustrate schematically preferred embodiments of the invention and, together with the general description given above and the detailed description of the preferred embodiments given below, serve to explain the principles of the invention.

[0013] 1A is a side view schematically illustrating a prior art excimer laser that generates a pulsed laser beam, including a laser chamber, two windows, a high reflective mirror, and an output coupling mirror.

[0014] Figure 1B is a side view schematically illustrating a preferred embodiment of an excimer laser according to the present invention, which generates a composite pulsed laser beam and includes two rear mirrors and two output coupling mirrors, the two rear mirrors are tilted relative to each other and the two output coupling mirrors are tilted relative to each other.

[0015] 2A is an enlarged side view schematically illustrating further details of the prior art excimer laser of FIG. 1A .

[0016] Figure 2B It is schematically shown Figure 1B A magnified side view of the excimer laser with further details.

[0017] 3A is a near-field camera image of a lateral spatial mode of a pulsed laser beam from a prior art excimer laser of the type shown in FIGS. 1A and 2A .

[0018] Figure 3B Is from Figure 1B and2B Near-field camera image of the transverse spatial modes of a composite pulsed laser beam of an excimer laser of the type shown.

[0019] 4A is a graph schematically showing the relative energy deviation at the centroid of the pulsed laser beam from the prior art excimer laser of FIG. 3A versus the pulse number.

[0020] Figure 4B is a schematic diagram showing the Figure 3B A graph showing the relationship between the relative energy deviation at the center of mass of a composite pulsed laser beam of an excimer laser and the pulse number. DETAILED DESCRIPTION OF THE INVENTION

[0022] Reference is now made to the accompanying drawings, in which like parts are represented by like numbers. FIG. 1A is a side view schematically showing a prior art excimer laser 10, which includes a laser chamber 12, two windows 14, a high reflector 16 and an output coupling mirror 18. The laser chamber 12 contains a gas mixture, which is flowing and pressurized. The gas mixture is energized by a short pulse current flowing between two elongated electrodes, thereby generating a pulsed gas discharge containing excited dimer molecules. For clarity, the elongated electrodes, the power supply for injecting the pulsed current, the gas storage tank, the pump for pressurizing the gas mixture and the blower for flowing the gas mixture are omitted in the accompanying drawings. The high reflector 16 and the output coupling mirror 18 form a linear laser resonator around the gas discharge, which generates a pulsed laser beam 20 propagating along the optical axis 22.

[0023] The window 14 has high transmittance at the wavelength of the laser beam 20. In the accompanying drawings, the window is depicted with the laser beam 20 at a normal angle of incidence. In this arrangement, the window will be coated with an anti-reflection coating at the wavelength of the laser beam. In another arrangement, the uncoated window will be oriented at the Brewster angle. The high reflective mirror 16 has a reflectivity of approximately 100%. The reflectivity of the output coupling mirror 18 is typically between 4% and 10%, most typically between 5% and 8%. Both mirrors typically have planar surfaces. The laser beam 20 is depicted by boundary rays, with the solid line representing the intracavity laser beam and the dashed line representing the output laser beam.

[0024] FIG. 2A is an enlarged view schematically showing further details of the excimer laser 10 at each end of the laser resonator. The high reflector 16 and the output coupling mirror 18 are both oriented at a normal angle of incidence relative to the laser beam 20. Due to the high gain of the gas discharge, a relatively high output coupler transmission is required, so a large portion of the intracavity beam incident on the output coupling mirror becomes the output beam. The output beam has a full-angle beam divergence in the far field φ, which is typically on the order of 1 milliradian (mrad), and the long dimension is slightly larger than the short dimension. The beam divergence in the short dimension is more typically in the range of 0.8 mrad to 1.5 mrad.

[0025] FIG3A is a near-field camera image of the lateral spatial mode of a laser beam from a prior art excimer laser of the type shown in FIGS. 1A and 2A. The dimensions of the laser beam are approximately 15 mm x 35 mm, here horizontal x vertical. The long vertical dimension of the laser beam is determined by the physical spacing between the electrodes. The current through the gas mixture is vertical. The short horizontal dimension of the laser beam is determined by the width and configuration of the electrodes.

[0026] The fine structure is dominated by vertical "streamers", some of which extend the full height of the beam, which are imprinted on the spatial pattern as the beam is amplified in the gas discharge. Spatial modulation in the gain (which varies from pulse to pulse) is due to instabilities in the gas discharge and turbulence in the gas flow, as described above. The static component of the fine structure, which is constant from pulse to pulse, results from spatial modulation of the losses due to window contamination and window damage caused by the intense UV laser radiation. Contamination and damage accumulate during laser operation, degrading performance until the window is replaced. Window replacement is a long-interval maintenance procedure.

[0027] Figure 1B 1 is a side view schematically illustrating a preferred embodiment of an excimer laser 30 according to the present invention. The excimer laser 30 has two mirrors located at each end of the laser chamber. At one end, there is a first back mirror 32 and a second back mirror 34, the second back mirror 34 being located between the first back mirror 32 and the laser chamber 12. The first back mirror 32 is similar to the high reflective mirror 16 and has a high reflectivity of about 100% at the wavelength of the laser beam 20. The second back mirror 34 is partially reflective and has a reflectivity in the range of between 20% and 80%, most preferably in the range of between 35% and 65%. At the opposite end, there are a first output coupling mirror 36 and a second output coupling mirror 38. Similar to the output coupling mirror 18 in the excimer laser 10, the first output coupling mirror 36 and the second output coupling mirror 38 both have relatively low reflectivity, so a large portion of the intracavity beam incident on the two output coupling mirrors becomes the output beam. For example, the reflectivity of each mirror is less than 5%.

[0028] Figure 2B is an enlarged view schematically showing further details of the excimer laser 30 at each end of the laser resonator. The first rear mirror 32 and the second rear mirror 34 are both tilted. The normal of the first rear mirror 32 has an angle θ1 relative to the optical axis 22, while the normal of the second rear mirror 34 has an angle θ2 relative to the optical axis 22. The first output coupling mirror 36 and the second output coupling mirror 38 are also tilted. The normal of the first output coupling mirror 36 has an angle θ3 relative to the optical axis 22, while the normal of the second output coupling mirror 38 has an angle θ4 relative to the optical axis 22.

[0029] As shown, the first back mirror 32 is approximately parallel to the first output coupling mirror 36 (θ1≈θ3) and the second back mirror 34 is approximately parallel to the second output coupling mirror 38 (θ2≈θ4), although this is not a necessary condition for the excimer laser of the present invention to function. The first back mirror and the second back mirror are depicted as separate optical components, each optical component having parallel planar surfaces, one of which is a reflector. Another convenient arrangement is that the back mirrors are opposing surfaces of a wedge-shaped optical component. Similarly, the first output coupling mirror and the second output coupling mirror can be opposing surfaces of another wedge-shaped optical element.

[0030] exist Figure 1B The effect of mirror tilting is depicted in . Since the intracavity beam is separated by the partially reflecting second rear mirror 34 and the weakly reflecting output coupling mirrors 36 and 38, the laser beam 20 essentially becomes a composite beam. This separation produces a composite output beam, indicated by the different dashed boundary rays in the figure. The component output beams do not necessarily have the same power, although in some embodiments the major components may have approximately the same power. For illustrative purposes, the propagation angles of the component output beams relative to each other are exaggerated. In the excimer laser 30, the optical axis 22 can be defined as a line connecting the beam centroids at consecutive positions along the composite output beam, determined by the first-order moment of its transverse spatial mode.

[0031] Back to Figure 2B , the tilt angles θ1 to θ4 of the mirrors 32 to 38 are also exaggerated. The inventors have found that a significant reduction in brightness contrast in fine structures can be obtained with tilt angles in the range of 5% to 20% of the full-angle divergence φ of an equivalent excimer laser without resonator mirror tilting, such as the excimer laser 10. By way of example, for an excimer laser with a full-angle beam divergence of 1 mrad, with the resonator mirrors at normal incidence, tilting the mirrors 32 to 38 by angles θ1 to θ4 between 0.05 mrad and 0.2 mrad from normal incidence results in significant blurring of fine structures. For this exemplary excimer laser, in order to achieve useful blurring of fine structures, the first back mirror 32 will be tilted at least 0.05 mrad relative to the second back mirror 34 and the first output coupling mirror 36 will be tilted at least 0.05 mrad relative to the second output coupling mirror 38.

[0032] In selecting the tilt angles, there is a tradeoff between enhancing mixing of the component beams to blur fine structure and increasing the full-angle divergence φ′ of the composite output beam, which may not be desirable. Selecting small tilt angles relative to the full-angle divergence φ means that the orientation of each tilt about the optical axis 22 is not too important. Certain orientations were found to be more suitable for blurring fine structure; most likely because the fine structure is otherwise dominated by the vertical streamers evident in Figure 3A. Similarly, the relative tilt angles θ1+θ2 and θ3+θ4 are more critical to blurring fine structure than the individual tilt angles θ1 to θ4. However, choosing symmetrical tilts about the optical axis by selecting θ1≈θ2 and θ3≈θ4 helps to minimize the overall full-angle divergence φ′.

[0033] Figure 3B Is from Figure 1B and 2B Near-field camera image of the lateral spatial modes of the laser beam of an inventive excimer laser of the type shown. The rear mirrors 32 and 34 are coatings deposited on opposite sides of a transparent wedge-shaped substrate. The output coupling mirrors 36 and 38 are also coatings deposited on opposite sides of the same substrate. The thickness of the wedge substrate is about 5 mm. The wedge angle is about 0.12 mrad. The two wedges are at about 90° to each other and about 45° to the horizontal and vertical axes. These orientations are determined empirically to optimize the overall laser performance, including pulse energy, pulse energy stability, beam divergence, and beam symmetry. As shown Figure 3B As shown, the thin edge of each wedge is closest to the bottom corner. The reflectivity of the second rear mirror 34 is about 50%, and the reflectivity of the output coupling mirrors 36 and 38 is about 2.75%.

[0034] Compared to the beam divergence angle φ of FIG. 3A produced by an equivalent prior art excimer laser of approximately 1.0 mrad, Figure 3B The divergence angle φ′ of the composite output beam is about 1.3 mrad. Compared with the fine structure of the beam in Figure 3A, Figure 3B The fine structure in the composite output beam of FIG3A is substantially blurred. The vertical streamers evident in FIG3A are Figure 3B Almost invisible. Figure 3B The excimer laser of the present invention has about 99% of the efficiency of the prior art excimer laser of Figure 3 A. This relatively small 1% decrease in efficiency is comparable to the change in efficiency caused by replacing a window.

[0035] FIG4A is a diagram schematically showing the relative energy deviation versus pulse number measured from 1000 camera images of the laser beam from the prior art excimer laser of FIG3A. Each camera image captures one laser pulse. The energy deviation is the energy deviation of the pixel closest to the centroid of each image. The standard deviation is 6.46%. Figure 4B It is a schematic illustration of Figure 3BA graph showing the relationship between relative energy deviation and pulse number measured from 1000 camera images of the composite output beam of the excimer laser of the present invention. The standard deviation is 4.42%. Figure 3B and 4B The output beam of the excimer laser of the present invention is shown to be significantly more uniform and more stable than the output beam of an equivalent prior art excimer laser.

[0036] An advantage of the excimer laser of the present invention is that a weaker and therefore less expensive beam homogenizer can be used to achieve the same processing quality as the prior art excimer laser. Another advantage is that the time interval between gas changes and window changes can be extended, because the instabilities and inhomogeneities of the output beam require longer to reach the maximum tolerance of the process. Extending the average or scheduled time interval between these maintenance procedures reduces laser downtime and cost of ownership.

[0037] Although one embodiment of the present invention is described above as having two back mirrors and two output coupling mirrors, additional partially reflecting back mirrors and / or additional output coupling mirrors may be included to further mitigate fine structures without departing from the spirit of the present invention and the scope of the present invention. In another embodiment of the present invention, the excimer laser has a highly reflective back mirror and multiple output coupling mirrors. In yet another embodiment of the present invention, the excimer laser has a highly reflective back mirror, multiple partially reflecting back mirrors, and an output coupling mirror.

[0038] Although the resonator mirrors described above have flat surfaces, excimer laser mirrors sometimes have a slight curvature. Those skilled in the art will recognize that the principles of the present invention can be applied to resonator mirrors having a slight curvature.

[0039] The present invention has been described above according to preferred embodiments and other embodiments. However, the present invention is not limited to the embodiments described and depicted herein. Instead, the present invention is limited only by the appended claims.

Claims

1. Excimer lasers that produce laser beams with minimal fine structure, including: a laser chamber containing a gas mixture that is energized; a first rear mirror having high reflectivity at the wavelength of the laser beam; a second rear mirror partially reflecting at the wavelength of the laser beam, the first rear mirror and the second rear mirror being located at one end of the laser chamber, the second rear mirror being located between the first rear mirror and the laser chamber, the first rear mirror and the second rear mirror being inclined relative to each other at a first angle; a first output coupling mirror; as well as A second output coupling mirror, wherein the first output coupling mirror and the second output coupling mirror are partially reflective at the wavelength of the laser beam, the first output coupling mirror and the second output coupling mirror are located at opposite ends of the laser chamber, and the first output coupling mirror and the second output coupling mirror are tilted relative to each other at a second angle.

2. The excimer laser according to claim 1, wherein The first rear mirror and the second rear mirror are both tilted relative to the optical axis of the laser beam.

3. The excimer laser according to claim 1, wherein The first output coupling mirror and the second output coupling mirror are both tilted relative to the optical axis of the laser beam.

4. The excimer laser according to claim 1, wherein The first angle is equal to the second angle.

5. The excimer laser according to claim 1, wherein The first rear mirror and the second rear mirror are located on opposite surfaces of the transparent wedge-shaped substrate.

6. The excimer laser according to claim 1, wherein The first output coupling mirror and the second output coupling mirror are located on opposite surfaces of the transparent wedge-shaped substrate.

7. The excimer laser according to claim 1, wherein The first angle and the second angle are both greater than 0.05 milliradians.

8. The excimer laser according to claim 1, wherein The second rear mirror has a reflectivity ranging between 20% and 80% at a wavelength of the laser beam.

9. The excimer laser according to claim 8, wherein: The second rear mirror has a reflectivity in a range between 35% and 65% at a wavelength of the laser beam.

10. The excimer laser according to claim 1, wherein Each of the first and second output coupling mirrors has a reflectivity of less than 5% at the wavelength of the laser beam.

11. The excimer laser according to claim 1, wherein The first output coupling mirror and the second output coupling mirror have the same reflectivity at the wavelength of the laser beam.

12. The excimer laser according to claim 1, wherein The first angle is in a range between 5% and 20% of a full-angle divergence of a laser beam from an equivalent excimer laser, with all resonant cavity mirrors of the equivalent excimer laser being perpendicularly incident to the laser beam.

13. The excimer laser according to claim 1, wherein The second angle is in a range between 5% and 20% of a full-angle divergence of a laser beam from an equivalent excimer laser, with all resonant cavity mirrors of the equivalent excimer laser being perpendicularly incident to the laser beam.

14. An excimer laser for generating a laser beam, comprising: a laser chamber containing an energized gas mixture; a rear mirror located at one end of the laser chamber, having high reflectivity at the wavelength of the laser beam; a first output coupling mirror; as well as a second output coupling mirror, the first output coupling mirror and the second output coupling mirror being partially reflective at the wavelength of the laser beam, the first output coupling mirror and the second output coupling mirror being located at opposite ends of the laser chamber, the first output coupling mirror and the second output coupling mirror being tilted relative to each other at a relative tilt angle, the normal of each of the first output coupling mirror and the second output coupling mirror being tilted at an angle between 0.05 and 0.2 milliradians relative to the optical axis of the laser beam, and the first output coupling mirror and the second output coupling mirror being tilted relative to the rear mirror.

15. The excimer laser according to claim 14, wherein The first output coupling mirror and the second output coupling mirror are located on opposite surfaces of the transparent wedge-shaped substrate.

16. An excimer laser for generating a laser beam, comprising: a laser chamber containing an energized gas mixture; a first rear mirror having high reflectivity at the wavelength of the laser beam; a second back mirror partially reflecting at the wavelength of the laser beam, the first back mirror and the second back mirror being located at one end of the laser chamber, the second back mirror being located between the first back mirror and the laser chamber, the first back mirror and the second back mirror being tilted relative to each other at a relative tilt angle; a normal of each of the first back mirror and the second back mirror being tilted relative to the optical axis of the laser beam at an angle between 0.05 and 0.2 milliradians, and An output coupling mirror partially reflecting at the wavelength of the laser beam, the output coupling mirror being located at an opposite end of the laser chamber, the output coupling mirror being tilted relative to the first and second rear mirrors.

17. The excimer laser according to claim 16, wherein The first rear mirror and the second rear mirror are located on opposite surfaces of the transparent wedge-shaped substrate.

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