Apparatus and method for combining multiple laser beams

Abstract

An apparatus and method for combining multiple (i.e., two or more) laser beams to even reduce the lateral gap between the two or beams to a point of elimination, such that lateral gaps caused by, for example, the space between the edge of a mirror and the coating on the surface of the mirror or by optical geometry are avoided.

Description

[0001] Cross Reference to Related Applications

[0002] This application claims priority to U.S. Application No. 62 / 941,971, filed November 29, 2019, entitled “APPARATUS FOR AND METHODS OF COMBINING MULTIPLE LASER BEAMS,” which is incorporated herein by reference in its entirety. TECHNICAL FIELD

[0003] The present disclosure relates to combining multiple laser beams for use in, for example, a lithographic apparatus. BACKGROUND

[0004] A lithographic apparatus is used to create integrated circuits, or chips, on substrates, such as wafers. The chips are used in a wide range of electronic devices such as personal computers, mobile phones and many more. A lithographic apparatus can be used to create various parts of the chip, such as the transistor, resistor and capacitor parts.

[0005] A lithographic apparatus can be used to create various parts of the chip, such as the transistor, resistor and capacitor parts. The lithographic apparatus can be used to create multiple layers of devices, such as transistors, resistors and capacitors, on a substrate, such as a wafer. The devices can be electrically connected to one another to form the circuitry of the chip. The devices can be electrically connected to one another to form the circuitry of the chip.

[0006] The light source used to illuminate the pattern and project it onto the substrate can be any of a variety of configurations. Deep ultraviolet excimer lasers commonly used in lithography systems include krypton fluoride (KrF) lasers having a wavelength of 248 nm and argon fluoride (ArF) lasers having a wavelength of 193 nm.

[0007] There can be circumstances in which it is desirable to use multiple (e.g., two) laser beams. For example, it can be desirable to have the ability to change the wavelength between pulses. In another case, it can be desirable to increase the power and / or repetition rate of a laser system by combining two laser beams so that they deliver power simultaneously or in alternating pulses. Here and elsewhere, “combining” is used to mean causing the beams to propagate in a common direction that is collinear or parallel to one another.

[0008] In other words, wafer throughput can be increased if the need for additional exposure passes can be avoided. It is also beneficial to be able to increase the amount of power (dose) delivered to the wafer beyond the limits that a single laser can provide. This can be achieved in theory by using two lasers with overlapping beams fed to a scanner, allowing multi-wavelength exposure in a single pass. With maximum power combination of each KrF laser, higher total power can be achieved. SUMMARY

[0009] The following presents a simplified summary of one or more embodiments in order to provide a basic understanding of such embodiments. This summary is not an extensive overview of all contemplated embodiments, and is intended to neither identify key or critical elements of all embodiments nor delineate the scope of any or all embodiments. Its sole purpose is to present some embodiments in a simplified form as a prelude to the more detailed description that is presented later.

[0010] According to an aspect of an embodiment, a gas discharge laser system is disclosed, comprising: a first laser cavity module adapted to generate a first beam of laser radiation; a second laser cavity module configured to generate a second beam of laser radiation; and a beam combiner arranged to receive the first beam and the second beam and adapted to cause the first beam and the second beam to propagate in a common direction, the beam combiner comprising a mirror having a reflective surface and a beveled edge forming an acute angle with the reflective surface. The mirror is arranged such that the first beam of laser radiation is reflected off the reflective surface in the common direction and the second beam of laser radiation propagates in the common direction next to the acute angle. The system can further comprise an aperture arranged such that the second beam passes through the aperture and the aperture limits a size range of the second beam of laser radiation such that the second beam of laser radiation does not impinge on the acute angle. The width of the aperture can be adjusted.

[0011] According to another aspect of an embodiment, a gas discharge laser system is disclosed, comprising: a first laser cavity module adapted to generate a first beam of laser radiation; a second laser cavity module adapted to generate a second beam of laser radiation; and a beam combiner arranged to receive the first beam and the second beam and adapted to cause the first beam and the second beam to propagate in a common direction, the beam combiner comprising an optical element having a face comprising a first portion and a second portion, the first portion having a reflective coating, the second portion not having a reflective coating. The optical element is arranged such that the first beam of laser radiation is reflected off the reflective surface in the common direction and the second beam of laser radiation propagates in the common direction through the optical element and the second portion of the face of the optical element.

[0012] According to another aspect of embodiments, a gas discharge laser system is disclosed, comprising: a first laser cavity module adapted to generate a first beam of laser radiation; a second laser cavity module adapted to generate a second beam of laser radiation; and a beam combiner arranged to receive the first beam and the second beam and adapted to cause the first beam and the second beam to propagate in a common direction, the beam combiner comprising a mirror and an actuator mechanically coupled to the mirror. The first beam of laser radiation initially travels in a first direction and impinges the mirror at a first position, and the second beam of laser radiation initially travels in a second direction at an angle to the first direction and impinges the mirror at the first position, the actuator being arranged to cause the mirror to rotate between a first position and a second position, the first beam of laser radiation propagating in a third direction at the first position, the second beam of laser radiation propagating in the third direction at the second position. The first direction can be at an angle θ to the second direction, in which case the actuator is adapted to cause the mirror to rotate by an angle θ / 2. The first laser cavity module can be adapted to generate a first beam of pulses of laser radiation, and the second laser cavity module can be adapted to generate a second beam of pulses of laser radiation between the pulses of the first beam of pulses of laser radiation, and the actuator can be adapted to place the mirror in the first position during the pulses of the first beam of pulses of laser radiation, and to place the mirror in the second position during the pulses of the second beam of pulses of laser radiation.

[0013] According to another aspect of embodiments, a method of combining a first pulsed laser beam and a second pulsed laser beam in a lithographic apparatus is disclosed, the method comprising the steps of: placing a mirror in a first state in which a pulse of the first pulsed laser beam impinging the mirror is reflected in a direction in which the pulse is allowed to be used; generating one or more pulses of a first beam of laser radiation; placing the mirror in a second state in which a pulse of the second pulsed laser beam impinging the mirror is reflected in a direction in which the pulse is allowed to be used; and generating one or more pulses of a second beam of laser radiation. The step of placing the mirror in the first state can comprise placing the mirror in a first rotational position in which a pulse of the first pulsed laser beam would propagate in a direction in which the pulse is allowed to be used in the first state, and the step of placing the mirror in the second state can comprise placing the mirror in a second rotational position in which a pulse of the second pulsed laser beam would propagate in a direction in which the pulse is allowed to be used in the second state. The first pulsed laser beam and the second pulsed laser beam can impinge the mirror at an angle θ relative to each other, and the first rotational position and the second rotational position can be at an angle θ relative to each other.

[0014] Other features and advantages of the present application, as well as the structure and operation of the various embodiments of the present application, are described in detail below with reference to the accompanying drawings. It is noted that the application is not limited to the specific embodiments described herein. The embodiments presented herein are by way of illustration only. Other embodiments based on the teachings of this specification will be apparent to those skilled in the relevant arts. BRIEF DESCRIPTION OF DRAWINGS

[0015] The accompanying drawings, which are incorporated herein and form a part of the specification, illustrate the present application and, together with the description, further serve to explain the principles of the application and to enable a person skilled in the pertinent art to make and use the application.

[0016] Figure 1 is a functional block diagram of a conventional dual cavity laser system.

[0017] Figure 2 is a functional block diagram of a dual cavity laser system according to an aspect of an embodiment.

[0018] Figure 3A is a layout of a beam combiner using a cut-off mirror.

[0019] Figure 3B is an enlarged portion of Figure 3B

[0020] Figure 4 is a possible layout of a beam combiner according to an aspect of an embodiment.

[0021] Figure 5A is a plan view of optical elements to be used in a beam combiner according to an aspect of an embodiment, Figure 5B is a possible layout of a beam combiner comprising Figure 5A optical elements of

[0022] Figure 6A is a plan view of a beam combiner in a first position according to an aspect of an embodiment, and Figure 6B is a plan view of the beam combiner of Figure 6A in a second position.

[0023] The features and advantages of the present application will become more apparent from the detailed description set forth below, taken in conjunction with the accompanying drawings, in which like reference characters identify corresponding elements throughout. In the drawings, like reference numbers generally indicate like, functionally similar, and / or structurally similar elements. DETAILED DESCRIPTION

[0024] The present specification discloses one or more embodiments comprising features of the present application. The disclosed embodiment(s) are merely illustrative of the present application. The scope of the present application is not limited to the disclosed embodiment(s). The present application is defined by the claims appended hereto.​

[0025] The described embodiment(s) and the reference in the specification to "one embodiment", "an embodiment”, “example embodiment”, etc., indicate that the embodiment(s) described can include a particular feature, structure, or characteristic, but every embodiment can not necessarily include the particular feature, structure, or characteristic. Moreover, such phrases are not necessarily referring to the same embodiment. Further, where a particular feature, structure, or characteristic is described in connection with an embodiment, it is submitted that it is within the knowledge of those skilled in the art to effect such feature, structure, or characteristic in connection with other embodiments whether or not explicitly described.

[0026] Before describing the embodiments in further detail, it is beneficial to provide an example environment in which embodiments of the present application can be implemented. Figure 1 is a functional block diagram of a conventional dual cavity laser system 10. In this embodiment, a laser beam is provided at an input port of a lithography machine 12 such as a stepper or scanner. The main components of the laser system 10 can be mounted underneath a floor 14, as shown, with the scanner 12 mounted on the floor 14. The laser system 10 includes a beam delivery unit 16 that provides an enclosed beam path for delivering the laser beam to the input port of the scanner 12. The particular light source system illustrated includes a master oscillator 18 and a power amplifier 20, and is a laser system known as a master oscillator power amplifier or MOPA system. The laser system 10 also includes various components for controlling the spectral properties of the pulses, shaping the pulses, etc., generally indicated as optics 22 and optics 24. The master oscillator 18 produces a first laser beam that is amplified twice by the power amplifier 20 to produce an output laser beam as indicated by the arrow in Figure 1 The single MOPA configuration just described is merely an example. It will be apparent to those of ordinary skill in the art that the principles set forth herein are applicable to laser systems having other configurations such as MOMO and multiple MOPA configurations.

[0027] According to an aspect of the embodiments, the need for multiple passes of the wafer is avoided by exposing the wafer to two beams. In Figure 2In the illustrated arrangement, the first resonant charger 40 provides electrical energy to the first commutator 42 and the second commutator 44. The first commutator 42 provides pulses to the first compression head 46. The second commutator 44 provides pulses to the second compression head 48. The first compression head 46 causes a discharge in the first laser cavity module 50. The second compression head 48 causes a discharge in the second laser cavity module 52. Also shown in FIG. 3 are optics for conditioning the laser beams, such as first and second line narrowing modules 54 and 56, first and second optical couplers 58 and 60, and first and second stabilization modules 62 and 64. The beams generated by the laser cavity modules are line narrowed to produce a bandwidth much smaller than the natural bandwidth of the gas discharge system. A control circuit 70 can control the first and second line narrowing modules 54 and 56 so that the bandwidth and wavelength of the beams generated by the first and second laser cavity modules 50 and 52 can be different from each other.

[0028] Various triggering arrangements can be employed. For example, one trigger can be used to fire both cavities, with or without a delay between the time of the trigger and the discharge for one or both of the cavities. Alternatively, the triggers can be generated separately, i.e., by separate circuitry, so that the two cavities will have different voltage / energy commands.

[0029] Also indicated generally by 68 are various system components that can be used in common by the first and second laser cavity modules 50 and 52, such as a gas handling system, a control system, an interface, a power distribution system, a cooling water system, capabilities for cavity filters and blowers, and a beam path purge system. Thus, in the illustrated arrangement, the two laser cavities can share these components, and each of them need not have two.

[0030] Figure 2 The apparatus of FIG. 1 also includes a control circuit 70 capable of operating the two lasers with independent energy, controlling relative firing times, bandwidths, and wavelengths, and a scanner interface capable of dividing energy / pulse commands between the two lasers.

[0031] In one embodiment, the first laser cavity module 50 can generate laser radiation of a first wavelength while the second laser cavity module 52 generates laser radiation of a second wavelength different from the first wavelength. Thus, in this case, the two cavities cooperate to produce different wavelengths of radiation with different focal planes on the wafer and operate at different depths.

[0032] Figure 2 Also shown in FIG. 1 is a first metrology unit 72 arranged to measure parameters of the beam generated by the first laser cavity module 50, including wavelength. Figure 2The illustrated apparatus also includes a second metrology unit 74 arranged to measure parameters of the beam generated by the second laser cavity module 52, including wavelength. A third metrology unit 76 is arranged to measure parameters, including wavelength, of the combined beam, i.e., the parameters of the combination of the beam from the first laser cavity module and the beam from the second cavity module. It will be appreciated that at times the combined beam is only the beam from one of the laser cavities if the other cavity is not fired. The metrology units provide their measurements to the control circuit 70. The metrology units as illustrated can measure wavelength independently of the light from both laser cavity modules and the combined beam. The control unit 70 can use the measurements to control the wavelength of the beam produced by each laser cavity module.

[0033] Firing of the two cavities can be alternated to achieve an effective double repetition rate. The beams from the two lasers can be combined to achieve an effective repetition rate that is twice the repetition rate of either of the two lasers as illustrated in the bottom timing diagram. As described above, any of a variety of arrangements can be used to combine the beam paths of the multiple lasers.

[0034] The firing sequence of the two (or more) laser cavity modules can be arranged in any of a variety of patterns. For example, the sequence can be arranged so that the cavities fire alternately in a sequential shot. Alternatively, the sequence can be arranged so that the first laser cavity module fires a first number of shots and then the second cavity fires a second number of shots, where the first and second numbers can be equal or unequal. The sequences can be employed with laser cavity modules that generate light of the same wavelength or different wavelengths. Also, laser cavity modules that generate light of two different wavelengths can be fired at substantially different repetition rates to produce a spectrum with different energy content in each of the two wavelength beams. The second firing rate can be, for example, an integer multiple of the first firing rate, so that the firing rate can be, for example, in a ratio of 2: 1. The relationship of the first firing rate to the second firing rate can also be in a ratio of two integers, for example, 3:2.

[0035] For the foregoing embodiment, the timing difference At between firing of the two lasers can be arranged substantially to any value, including zero (assuming that the optics can withstand a doubled instantaneous power level) or small enough so that the two pulses do not occur at the same time, but one after the other (i.e., during the same exposure), thereby doubling the effective dose. Thus, the beams from the two lasers can be generated one after the other as illustrated to be combined to achieve an effective dose that is twice the dose of either of the two lasers, as illustrated in the bottom timing diagram.

[0036] The beams from the first laser cavity module 50 and the beams from the second laser cavity module 52 are combined by the beam combiner 66. If the two beams have different wavelengths, another way to combine them is by using a dichroic mirror. A dichroic mirror operates in such a way that one wavelength (short pass) is transmitted while the other wavelength is reflected. However, this technique is only used to combine beams with different wavelengths.

[0037] Techniques for combining two laser beams with the same or different wavelengths include the use of intercepting mirrors. For example, in Figure 3A In the process, beam 100 is reflected away by intercepting mirror 110 to travel in a first direction A, and a second beam 120 also propagates in direction A near the edge of intercepting mirror 110. Direction A is propagated as a direction for further use as a beam(s). Due to manufacturing limitations in placing the reflective coating 130 on the surface of mirror 110, a space B exists between the edge 112 of mirror 110 and the edge 132 of coating 130. Figure 3B The middle is magnified. Figure 3B yes Figure 3A (Enlarged version shown in the dashed box). A gap C also exists between the coating 130 on the top surface 114 of the mirror and the back surface 116 of the mirror. The coating 130 is typically applied within 1 mm of the edge of the mirror 110, i.e., at distance B, and has a thickness of approximately 10 mm. The space B and the thickness of the mirror 110 (represented by the gap C) contribute to the thickness of the gap G between the two beams 110 and 120, which is approximately 10 mm wide. This gap G can cause problems in downstream optics, and from an engineering point of view, it may be desirable to minimize the width of the gap G for some applications.

[0038] To combine the two bundles 100 and 120 to minimize the gap between them, one can use, for example... Figure 4 The arrangement shown. In Figure 4 In the figure, a beveled reflector 150 is cut to allow beam 120 to propagate without striking any reflector surface. An adjustable aperture can be positioned and sized to help ensure beam 120 propagates without striking any reflector surface. Beam 100 is reflected away by a beveled reflector 150 that is close to the edge of a beveled reflector 160. The resulting combined beam profile has a reduced gap, for example, on the order of approximately 1 mm, compared to gap G. Beams 100 and 120 are propagated parallel to each other in direction A, adjacent to each other, and one is translated relative to the other in a direction transverse to direction A.

[0039] exist Figure 5A and Figure 5B Another arrangement is shown for combining the two bundles 100 and 120 to minimize the gap between them. (See diagram for example.)Figure 5A As shown, a reflective coating 130 is applied to a portion, e.g., a portion of the mirror 180 (irrespective of the mirror shape). As shown, the laser beam 100 is reflected off the reflective coating 130 while the laser beam 120 propagates through the portion of the mirror 180 that is not coated. The two beams can be combined with a gap whose width can be substantially reduced to zero. Figure 5B As shown, a reflective coating 130 is applied to a portion, e.g., a portion of the mirror 180 (irrespective of the mirror shape). As shown, the laser beam 100 is reflected off the reflective coating 130 while the laser beam 120 propagates through the portion of the mirror 180 that is not coated. The two beams can be combined with a gap whose width can be substantially reduced to zero.

[0040] In Figure 6A and Figure 6B another arrangement for combining the two beams 100 and 120 to minimize the gap between them is shown, which is particularly useful in systems where the beams 100 and 120 are not emitted simultaneously but in alternating pulses. The solid lines are the active beams and the dashed lines show the positions of the mirror and beams when in the other position. The beams propagate with an angle θ between them. The beams 100 and 120 hit the mirror 200 in a first position as shown in Figure 6A where the beam 100 propagates in direction A (i.e., the use direction) while the second beam 120 propagates in direction B. Then, as shown in Figure 6B the mirror 200 is rotated by an amount θ / 2 to a second position so that the beam 120 propagates in direction A while the beam 100 propagates in direction C. Depending on the control signals applied by the controller 220, the mirror 200 is rotated between the first and second positions, i.e., dithered, by the actuator 210. If the beams 100 and 120 are emitted as alternating pulses with a given repetition rate and the mirror is dithered with the same repetition rate, the beams can be co-linearly propagated in direction A, substantially combining the two beams. The same effect can be achieved by keeping the mirror 200 stationary and dithering the propagation direction of one or both of the beams 100 and 120.

[0041] It should be understood that the detailed description and not the summary and abstract are intended to explain the claimed invention. The summary and abstract can set forth one or more, but not all exemplary embodiments of the present invention as contemplated by the inventor(s), and thus are not intended to limit the present invention and the appended claims in any way.

[0042] The present invention is described above by means of functional building blocks illustrating the implementation of certain functions and relationships of the present invention. The boundaries of these functional building blocks have been arbitrarily defined herein for the convenience of the description. Alternative boundaries can be defined so long as the specified functions and relationships of the

[0043] The above description of the specific embodiments will so fully reveal the general nature of the application that others can easily modify and / or adapt for various applications such specific embodiments, without the exercise of inventive faculty, and without the necessity of doing further experiments. Therefore, it is intended that the disclosure and examples presented herein be considered as illustrative only and not as limiting the scope of the application as construed according to the appended claims. Furthermore, it is intended that wherever aspects can be found that offer "preferred" or "advantageous" embodiments, those aspects are equally "preferred" or "advantageous" embodiments. As such, there is no intention of placing limitations on the disclosure.

[0044] Implementations of the present disclosure can be further described using the following clauses:

[0045] 1. A gas discharge laser system comprising:

[0046] a beam combiner arranged to receive a first laser radiation beam and a second laser radiation beam and adapted to cause the first beam and the second beam to propagate in a common direction, the beam combiner comprising a mirror having a reflective surface and a beveled edge forming an acute angle with the reflective surface, the mirror being arranged such that the first laser radiation beam reflects off the reflective surface in the common direction and the second laser radiation beam propagates proximate to the acute angle in the common direction.

[0047] 2. The gas discharge laser system according to clause 1, further comprising a first laser cavity module and a second laser cavity module, the first laser cavity module being adapted to generate the first laser radiation beam and the second laser cavity module being adapted to generate the second laser radiation beam.

[0048] 3. The gas discharge laser system according to clause 1, further comprising an aperture arranged such that the second beam passes through the aperture and the aperture limits a size range of the second laser radiation beam such that the second laser radiation beam does not impinge on the acute angle.

[0049] 4. The gas discharge laser system according to clause 3, wherein a width of the aperture is adjustable.

[0050] 5. The gas discharge laser system according to clause 1, wherein the beam combiner is adapted to cause the first beam and the second beam to propagate parallel to each other.

[0051] 6. The gas discharge laser system according to clause 1, wherein the first laser radiation beam has a first wavelength and the second laser radiation beam has a second wavelength different from the first wavelength.

[0052] 7. The gas discharge laser system according to clause 1, wherein the first laser radiation beam is triggered to fire at a first time and the laser radiation beam is triggered to fire at a second time, wherein a difference between the first time and the second time is At.

[0053] 8. The gas discharge laser system according to clause 7, wherein At is substantially equal to zero.

[0054] 9. The gas discharge laser system according to clause 7, wherein At is chosen such that the second laser radiation beam is triggered immediately after the first laser radiation beam stops emitting.

[0055] 10. A gas discharge laser system, comprising:

[0056] a first laser cavity module adapted to generate a first laser radiation beam;

[0057] a second laser cavity module adapted to generate a second laser radiation beam; and

[0058] a beam combiner arranged to receive the first beam and the second beam and adapted to cause the first beam and the second beam to propagate in a common direction, the beam combiner comprising an optical element having a face comprising a first portion and a second portion, the first portion having a reflective coating, the second portion having no reflective coating, the optical element being arranged such that the first laser radiation beam is reflected off the reflective surface in the common direction and the second laser radiation beam passes through the optical element and the second portion of the face of the optical element in the common direction.

[0059] 11. The gas discharge laser system according to clause 10, wherein the first laser radiation beam has a first wavelength and the second laser radiation beam has a second wavelength different from the first wavelength.

[0060] 12. The gas discharge laser system according to clause 10, wherein the first laser radiation beam is pulsed and triggered to emit at a first time and the laser radiation beam is pulsed and triggered to emit at a second time, wherein the difference between the first time and the second time is At.

[0061] 13. The gas discharge laser system according to clause 12, wherein At is substantially equal to zero.

[0062] 14. The gas discharge laser system according to clause 12, wherein At is chosen such that the second laser radiation beam is triggered immediately after the first laser radiation beam stops emitting.

[0063] 15. A gas discharge laser system, comprising:

[0064] a first laser cavity module adapted to generate a first laser radiation beam;

[0065] a second laser cavity module adapted to generate a second laser radiation beam; and

[0066] a beam combiner arranged to receive the first beam and the second beam and adapted to cause the first beam and the second beam to propagate in a common direction, the beam combiner comprising a mirror and an actuator mechanically coupled to the mirror,

[0067] the first beam of laser radiation initially travels in a first direction and impinges the mirror at a first position, and the second beam of laser radiation initially travels in a second direction at an angle to the first direction and impinges the mirror at the first position,

[0068] the actuator is arranged to rotate the mirror between a first position and a second position, the first beam of laser radiation propagates in a third direction at the first position, the second beam of laser radiation propagates in the third direction at the second position.

[0069] 16. A gas discharge laser system according to clause 15, wherein the first direction is at an angle θ to the second direction, and wherein the actuator is adapted to rotate the mirror through an angle θ / 2.

[0070] 17. A gas discharge laser system according to clause 15, wherein the first laser cavity module is adapted to generate a first beam of pulses of laser radiation, and the second laser cavity module is adapted to generate a second beam of pulses of laser radiation between the pulses of the first beam of pulses of laser radiation, and wherein the actuator is adapted to place the mirror in the first position during the pulses of the first beam of pulses of laser radiation, and to place the mirror in the second position during the pulses of the second beam of pulses of laser radiation.

[0071] 18. A method of combining a first pulsed laser beam and a second pulsed laser beam in a lithographic apparatus, the method comprising the steps of:

[0072] placing the mirror in a first state in which pulses of the first pulsed laser beam impinging the mirror are reflected in a direction in which the pulses are allowed to be used;

[0073] generating one or more pulses of a first beam of laser radiation;

[0074] placing the mirror in a second state in which pulses of the second pulsed laser beam impinging the mirror are reflected in a direction in which the pulses are allowed to be used; and

[0075] generating one or more pulses of a second beam of laser radiation.

[0076] 19. The method according to clause 18, wherein the step of placing the mirror in the first state comprises placing the mirror in a first rotational position, the pulses of the first pulsed laser beam in the first state will propagate in a direction in which the pulses are allowed to be used, and wherein the step of placing the mirror in the second state comprises placing the mirror in a second rotational position, the pulses of the second pulsed laser beam in the second state will propagate in a direction in which the pulses are allowed to be used.

[0077] 20. The method according to clause 18, wherein the first pulsed laser beam and the second pulsed laser beam impinge on the mirror at an angle Θ relative to each other, and wherein the first rotational position and the second rotational position are at an angle Θ relative to each other.

[0078] 21. A beam combiner arranged to receive a first beam and a second beam and adapted to cause the first beam and the second beam to propagate in a common direction, the beam combiner comprising a mirror having a reflective surface and a bevelled edge forming an acute angle with the reflective surface, the mirror being arranged such that the first beam of laser radiation is reflected off the reflective surface in the common direction and the second beam of laser radiation propagates in the common direction immediately adjacent to the acute angle.

[0079] 22. The beam combiner according to clause 21, wherein the first beam is generated by a first laser cavity module and the second beam is generated by a second laser cavity module.

Claims

1. A gas discharge laser system, comprising: The first laser cavity module is adapted to generate the first laser radiation beam; The second laser cavity module is adapted to generate a second laser radiation beam; as well as A beam combiner is configured to receive a first laser beam and a second laser beam, and adapted to propagate the first and second laser beams in a common direction. The beam combiner includes an optical element having a surface comprising a first portion and a second portion, the first portion having a reflective coating and the second portion not having a reflective coating. The optical element is configured such that the first laser beam is reflected away from the first portion of the surface of the optical element in the common direction, and the second laser beam passes through the optical element and the second portion of the surface of the optical element in the common direction. The first laser beam is pulsed and triggered to emit at a first time, and the second laser beam is pulsed and triggered to emit at a second time, wherein the difference between the first time and the second time is Δt. Δt is chosen to be substantially equal to zero or such that the second laser beam is triggered immediately after the first laser beam stops emitting.

2. The gas discharge laser system according to claim 1, wherein the first laser radiation beam has a first wavelength, and the second laser radiation beam has a second wavelength different from the first wavelength.

3. A gas discharge laser system, comprising: The first laser cavity module is adapted to generate the first laser radiation beam; The second laser cavity module is adapted to generate a second laser radiation beam; as well as A beam combiner is configured to receive a first laser radiation beam and a second laser radiation beam, and is adapted to cause the first laser radiation beam and the second laser radiation beam to propagate in a common direction. The beam combiner includes a mirror and an actuator mechanically coupled to the mirror. The first laser beam initially travels in a first direction and strikes the reflector at a first position, and the second laser beam initially travels in a second direction at an angle to the first direction and strikes the reflector at the first position. The actuator is configured to rotate the reflector between a first position and a second position, wherein the first laser beam propagates upward in a third direction at the first position, and the second laser beam propagates upward in a third direction at the second position. The first direction forms an angle θ with the second direction, and the actuator is adapted to rotate the mirror by an angle θ / 2.

4. The gas discharge laser system of claim 3, wherein the first laser cavity module is adapted to generate a first laser radiation pulse beam, and the second laser cavity module is adapted to generate a second laser radiation pulse beam between pulses of the first laser radiation pulse beam, and wherein the actuator is adapted to: place the mirror in the first position during the pulse of the first laser radiation pulse beam, and place the mirror in the second position during the pulse of the second laser radiation pulse beam.

5. A method for combining a first pulsed laser beam and a second pulsed laser beam in a photolithography apparatus, the method comprising the following steps: The reflector is placed in a first state, in which the pulse of the first pulsed laser beam striking the reflector is reflected in the direction in which the pulse is permitted to be used; Generate one or more pulses of the first pulsed laser beam; The reflector is placed in a second state, in which the pulse of the second pulsed laser beam striking the reflector is reflected in the direction in which the pulse is permitted to be used; as well as One or more pulses are generated to produce the second pulsed laser beam. The step of placing the reflector in a first state includes: placing the reflector in a first rotational position, wherein the pulse of the first pulsed laser beam will propagate in a direction in which the pulse is permitted to be used in the first state; and the step of placing the reflector in a second state includes: placing the reflector in a second rotational position, wherein the pulse of the second pulsed laser beam will propagate in a direction in which the pulse is permitted to be used in the second state. The first pulsed laser beam and the second pulsed laser beam strike the reflector at an angle θ relative to each other, and the first rotation position and the second rotation position are at an angle θ / 2 relative to each other.

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