CROSS REFERENCE TO RELATED APPLICATIONS

[0001] This application claims priority to U.S. Application No. 63/294,273, filed December 28, 2021, titled STEERING FOR CONDITIONING LASERS; and U.S. Application No. 63/421,732, filed November 2, 2022, titled LASER BEAM STEERING SYSTEM AND METHOD, both of which are incorporated herein in their entireties by reference.

TECHNICAL FIELD

[0002] The present disclosure relates to extreme ultraviolet radiation generators which produce light by excitation of a source material, in particular to the steering of lasers that irradiate the source material in such generators.

BACKGROUND

[0003] Extreme ultraviolet (“EUV”) radiation, for example, electromagnetic radiation having a wavelength of around 50 nm or less (also sometimes referred to as soft x-rays), and including light at a wavelength of about 13 nm, is used in photolithography processes to produce extremely small features in substrates, for example, silicon wafers.

[0004] Methods for generating EUV radiation include, but are not limited to, altering the physical state of the source material to a plasma state. The source material includes an element, for example, xenon, lithium, or tin, with an emission line in the EUV range. In one such method, often termed laser produced plasma (“LPP”), the required plasma is produced by irradiating a source material in the form of, for example, a droplet, stream, or cluster of source material, with an amplified laser beam that can be referred to as a drive laser beam. For this process, the plasma is typically produced in a sealed vessel, for example, a vacuum chamber, and monitored using various types of metrology equipment.

[0005] CO2 amplifiers and lasers, which output an amplified light beam at a wavelength of about 10600 nm, can present certain advantages as drive lasers for irradiating the source material in an LPP process. This may be especially true for certain source materials, for example, for materials containing tin. For example, one advantage is the ability to produce a relatively high conversion efficiency between the drive laser input power and the output EUV power.

[0006] EUV radiation may be produced in a multi-step process in which a target, e.g., a droplet of source material, is hit before reaching an irradiation site by one or more pulses of conditioning radiation that condition or prepare the target for ultimate phase conversion at the irradiation site. Conditioning in this context may include altering the shape of the droplet, e.g., flattening the droplet, or the distribution of the droplet, e.g., at least partially dispersing some of the droplet as a mist, or even partial phase change. For the purposes of this disclosure, these pulses which are preliminary to the main heating pulse are referred to as conditioning pulses and include pre-pulses, rarefication pulses, and pedestal pulses, regardless of whether produced by a main drive laser or another laser. The term “pulses” refers to all manner of radiation pulses, regardless of their purpose and regardless of whether produced by a primary drive laser or another laser or some other device capable of producing radiation pulses.

[0007] Also, as implied above, as a result of conditioning, the droplet of source material will undergo physical changes preliminary to the phase change induced by the main heating pulse, including shape changes and mass distribution changes. Sometimes the mass of source material is referred to as a droplet before it is conditioned and as a target after it has been conditioned at least once. Herein, the term “droplet” will be used to refer to the mass of source material before any conditioning and the term “target” will be used to refer to the mass of source material both before and after conditioning so that a droplet is a type of target, unless the context indicates otherwise.

[0008] One objective in the efficient production of EUV light is attaining the proper relative positioning of the main pulse and, if used one or more conditioning pulses, and the target. In general this involves aligning the beam centroid with the center of mass of the target, although it can also involve deliberate “off-center” strikes. This is also referred to as aligning the pulse and the target. It is generally important to align the target and the pulse to within a few micrometers for efficient and debris-minimized operation of the EUV radiation generator. In general, the alignment state is determined by determining the position of the pulse, determining the position of the target, and finding a difference (i.e., distance) between those two positions. For example, U.S. Patent No. 7,372,056, issued May 13, 2008, and titled “LPP EUV Plasma Source Material Target Delivery System,” discloses the use of a droplet detection radiation source and a droplet radiation detector that detects droplet detection radiation reflected from a droplet of source material. See also U.S. Patent No. 8,158,960, issued April 17, 2012, and titled “Laser Produced Plasma EUV Light Source,” U.S. Patent No. 9,241,395, issued January 19, 2016, and titled “System and Method for Controlling Droplet Timing in an LPP EUV Light Source,” and U.S. Patent No. 9,497,840, issued November 15, 2016, and titled “System and Method for Creating and Utilizing Dual Laser Curtains from a Single Laser in an LPP EUV Light Source.”

[0009] All patent applications, patents, and printed publications cited herein are incorporated herein by reference in their entireties, except for any definitions, subject matter disclaimers or disavowals, and except to the extent that the incorporated material is inconsistent with the express disclosure herein, in which case the language in this disclosure controls.

[0010] Thus, in some systems, the pulse as reflected from the target is used to locate the target in space by collecting the reflected light and imaging it on a sensor. In other systems a secondary light source in addition to the pulse laser is used to illuminate the target, and a camera is positioned to image the illuminated target. [0011] The laser typically includes one or more elements intended to shape and control the amplitude of the output conditioning laser pulses. One such element is an acousto-optical modulator (“AOM”) that provides the amplitude control.

[0012] Once the targets have been located, it is necessary to strike them with the laser in a manner that is optimal for the operation (e.g., conditioning or conversion) being performed, i.e., align the laser pulse and the target. It will be understood that information from a given target is used to correct alignment for subsequent targets. One way to perform this is by steering the pulses from the laser. This steering can be accomplished combined action of a “slow steering” system for larger corrections and a more responsive “fast steering” system for smaller but quicker corrections. The use of such systems can, however, introduce insertion losses which decrease the available laser output energy. [0013] There is therefore a need for a target pulse alignment system which avoids this drawback.

SUMMARY

[0014] The following presents a concise summary of one or more embodiments in order to provide a basic understanding of the embodiments. This summary is not a comprehensive overview of all contemplated embodiments and is not intended to identify key or critical elements of all embodiments nor set limits on the scope of any or all embodiments. Its sole purpose is to present some concepts of one or more embodiments in a simplified form as a prelude to the more detailed description that is presented later.

[0015] According to one aspect of an embodiment there is disclosed an apparatus for steering a pulse from a laser to align the pulse with a target of source material. An acousto-optical device such as an acousto-optical deflector (AOD) is provided within the laser to perform a pulse steering function in addition to the performing the functions more typically performed by the AOM. This avoids the need for a separate, external AOD in the beam path between the laser and the target.

[0016] According to another aspect of an embodiment there is disclosed a laser for producing a beam of radiation, the laser comprising at least one laser gain medium for generating the beam of radiation and an acousto-optical device arranged to receive the beam of radiation and adapted to both control an output amplitude of the beam of radiation and to steer the beam of radiation. The acousto-optical device may be an acousto-optical deflector. The laser may include an enclosure with the at least one gain medium and the acousto-optical device being arranged inside the enclosure. The acousto-optical device may be an acousto-optical deflector. The laser may include a beam shaping module which may include the acousto-optical device.

[0017] The beam of radiation generated by the laser may comprise at least one prepulse. The beam of radiation generated by the laser may comprise at least one pedestal pulse. The beam of radiation generated by the laser may comprise at least one rarefication pulse. The laser may be a solid state laser. The laser may be a CO2 laser.

[0018] The acousto-optical device may be arranged to steer the beam of radiation along a first axis and the apparatus may further comprise a beam rotator positioned to receive the beam of radiation after the acousto-optical device and arranged to rotate the beam of radiation such that the beam of radiation is steered along a second axis different from the first axis.

[0019] According to another aspect of an embodiment there is disclosed a system for a target of source material with a beam of radiation, the system comprising a seed laser, a gain medium arranged to amplify a seed laser beam from the seed laser to generate the beam of radiation, and an acousto- optical device arranged to receive the beam of radiation and adapted to control an output amplitude of and to steer the beam of radiation. The acousto-optical device may be an acousto-optical deflector. The system may further comprise an enclosure with the seed laser, the gain medium, and the acousto- optical device being arranged within the enclosure. The acousto-optical device may be an acousto- optical deflector.

[0020] According to another aspect of an embodiment there is disclosed an apparatus for steering a beam of radiation along a first dimension transverse to a streamwise direction of a stream of targets of source material, the apparatus comprising a source material dispenser arranged to dispense the stream of targets in the streamwise direction, a laser configured to generate the beam of radiation, and a detector arranged to detect an alignment state between one of the targets and the beam of radiation and adapted to generate a steering control signal, the laser including an opto-acoustical device arranged to control an amplitude of the beam of radiation, wherein the acousto-optical device located within the laser is additionally arranged to receive the steering control signal and adapted to steer the beam of radiation in response to the steering control signal.

[0021] The acousto-optical device located within the laser may be adapted to steer the beam of radiation along the first dimension. The acousto-optical device located within the laser may be adapted to steer the beam of radiation along a second dimension different from the first dimension and the apparatus may further comprise a beam rotator arranged to receive the beam of radiation from the acousto-optical device for rotating the beam of radiation so that steering after the beam rotator is along the first dimension. The acousto-optical device may be an acousto-optical deflector.

[0022] According to another aspect of an embodiment there is disclosed a method of delivering a beam of radiation to a target of source material, the method comprising generating a beam of radiation, using an acousto-optical deflector both to control the amplitude of and steer the beam of radiation, and using the beam of radiation to irradiate the target of source material. The method may further comprise a step after using the acousto-optical deflector of rotating the beam of radiation.

[0023] Further embodiments, features, and advantages of the subject matter of the present disclosure, as well as the structure and operation of the various embodiments, are described in detail below with reference to accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWING

[0024] FIG. 1 is a partially schematic functional block diagram of an overall broad conception for a laser-produced plasma EUV radiation source system according to an aspect of the present invention. [0025] FIG. 2 is a partially schematic functional block diagram of a conditioning pulse delivery system such as might be used in the arrangement of FIG. 1.

[0026] FIGS. 3 A and 3B are diagrams illustrating certain targeting principles in a system such as that shown in FIG. 2.

[0027] FIG. 4A is a schematic, not-to-scale view of a laser such as might be used in the arrangement of FIG. 2.

[0028] FIG. 4B is a schematic, not-to-scale view of another laser such as might be used in the arrangement of FIG. 2.

[0029] FIG. 5A is a not-to-scale schematic diagram of a laser for producing a conditioning pulse according to one aspect of an embodiment.

[0030] FIG. 5B is a not-to-scale schematic diagram of a laser for producing a conditioning pulse according to another aspect of an embodiment.

[0031] FIG. 6A is a partially schematic functional block diagram of a conditioning pulse delivery system according to one aspect of an embodiment.

[0032] FIG. 6B is a partially schematic functional block diagram of a conditioning pulse delivery system according to one aspect of an embodiment.

[0033] FIG. 7 is a partially schematic functional block diagram of a conditioning pulse delivery system according to one aspect of an embodiment.

[0034] FIG. 8A is a partially schematic functional block diagram of a conditioning pulse delivery system according to one aspect of an embodiment.

[0035] FIG. 8B is a partially schematic functional block diagram of a conditioning pulse delivery system according to one aspect of an embodiment.

[0036] FIG. 9A is a flowchart showing steps of a conditioning pulse delivery method according to one aspect of an embodiment.

[0037] FIG. 9B is a flowchart showing steps of a conditioning pulse delivery method according to another aspect of an embodiment.

[0038] Further features and advantages of the invention, as well as the structure and operation of various embodiments of the invention, are described in detail below with reference to the accompanying drawings. It is noted that the invention is not limited to the specific embodiments described herein. Such embodiments are presented herein for illustrative purposes only. Additional embodiments will be apparent to persons skilled in the relevant art based on the teachings contained herein.

DETAIEED DESCRIPTION

[0039] Various embodiments are now described with reference to the drawings, wherein like reference numerals are used to refer to like elements throughout. In the following description, for purposes of explanation, numerous specific details are set forth in order to promote a thorough understanding of multiple embodiments. It may be evident in some or all instances, however, that any embodiment described below can be practiced without adopting the specific design details described below.

[0040] FIG. 1 is a schematic view of an example of an EUV radiation source, e.g., a laser produced plasma EUV radiation source 10, according to one aspect of an embodiment. As shown, the EUV radiation source 10 may include a pulsed or continuous laser source 22, which may, for example, be a pulsed gas discharge CO2 laser source producing a beam 22b of pulses of radiation at a wavelength generally below 20 pm, for example, in the range of about 11 pm to about 9 pm or less. The pulsed gas discharge CO2 laser source may have DC or RF excitation operating at high power and at a high pulse repetition rate.

[0041] The EUV radiation source 10 also includes a source material delivery system 24 for delivering source material in the form of liquid droplets or a continuous liquid stream. In this example, the source material is a liquid, but it could also be a solid. The source material may be made up of tin or a tin compound, although other materials could be used. In the system depicted the source material delivery system 24 introduces droplets 14 of the source material into the interior of a vacuum chamber 26 to an irradiation region 28 where the droplets 14 may be irradiated to produce plasma. It should be noted that as used herein an irradiation region is a region where source material irradiation is to occur and is an irradiation region even at times when no irradiation is actually occurring. The EUV light source 10 also includes a beam focusing and steering system 32. Beam steering may be accomplished combined action of a “slow steering” system for larger corrections and a more responsive “fast steering” system for smaller but quicker corrections. The slow steering module may include, for example, mechanically actuated or electro-mechanically actuated steering elements such as mirrors or prisms that can be adjusted with motors, servo-motors, stepper motors, piezo-electric actuators or similar devices. The fast steering system may be implemented using an acousto-optical deflector (“AOD”) inserted into the pulse path. Doing so, however, incurs an insertion loss. In other words, adding the AOD to the path between the conditioning laser and the target reduces the amount of power the conditioning laser can deliver and hence, the amount of power available for conditioning the target.

[0042] In the system shown, the components are arranged so that the droplets 14 travel substantially horizontally. The direction from the laser source 22 towards the irradiation region 28, that is, the nominal direction of propagation of the pulse 22b, may be taken as the Z axis. The path the droplets 14 take from the source material delivery system 24 to the irradiation region 28 may be taken as the X axis. The view of FIG. 1 is thus normal to the XZ plane. While a system in which the droplets 14 travel substantially horizontally is depicted, it will be understood by one having ordinary skill in the art that other arrangements can be used in which the droplets 14 travel vertically or at some angle with respect to gravity between and including 90 degrees (horizontal) and 0 degrees (vertical). [0043] The EUV radiation source 10 may also include an EUV light source controller system 60 and a laser firing control system 65 along with the beam steering system 32. The EUV radiation source 10 may also include a detector such as a target position detection system which may include one or more droplet imagers 70 that generate an output indicative of the absolute or relative position of a target droplet, e.g., relative to the irradiation region 28, and provide this output to a target position detection feedback system 62.

[0044] The target position detection feedback system 62 may use the output of the droplet imager 70 to compute a target position and trajectory, from which a target error can be computed. The target error can be computed on a target-by-target basis, or on the basis of an average, or on some other basis. The target error may then be provided as an input to the light source controller 60. In response, the light source controller 60 can generate control signals such as a drive laser position, direction, or timing correction signals and provide these control signals to the laser beam steering system 32. The laser beam steering system 32 can use the control signal to change the location and/or focal power of the drive laser beam focal spot within the chamber 26. The laser beam steering system 32 can also use the control signal to change the geometry of the interaction of the drive pulse 22b and the target 14. For example, the drive pulse 22b can be made to strike the target 14 off-center or at an angle of incidence other than directly head-on.

[0045] The system shown in FIG. 1 also includes a conditioning laser 23 for generating a conditioning beam 23b. This conditioning beam 23b is made up of pulses that prepare the target for subsequent heating by the main drive pulse. The conditioning pulse can change the shape or distribution of the target. It includes pulses variously referred to as pre-pulses, pedestal pulses, and rarefication pulses. It is also necessary that the conditioning pulse from the laser 23 strike the target in an optimal manner. Towards this end, the laser beam steering system 32 has the capability of steering the pulse 23b generated by the conditioning laser 23 as described below.

[0046] It will be understood that the use of a separate laser, which may be a solid state laser producing radiation having a wavelength of about 1 pm, to produce these conditioning pulses is only one of multiple ways of producing these pulses. For example, the laser source 22 itself could be used to produce conditioning pulses. Also, while in the description which follows a conditioning pulse generated by a conditioning laser is used for the sake of having a concrete example to facilitate understanding, one of ordinary skill in the art will understand that the principles elucidated apply as well to other pulses including main or heating pulses produced by the main or a single laser source. [0047] As shown in FIG. 1, the source material delivery system 24 may include a target delivery control system 90. The target delivery control system 90 is operable in response to a signal, for example, the target error described above, or some quantity derived from the target error provided by the system controller 60, to adjust paths of the targets 14 through the irradiation region 28. This may be accomplished, for example, by repositioning the point at which a target delivery mechanism 92 releases the droplets 14. The droplet release point may be repositioned, for example, by tilting or shifting the target delivery mechanism 92. The target delivery mechanism 92 extends into the chamber 26 and is preferably externally supplied with source material and a gas source to place the source material in the target delivery mechanism 92 under pressure. The system also includes a source material catch 80 that catches and retains source material which has not been vaporized to limit contamination from such source material.

[0048] Continuing with FIG. 1, the radiation source 10 may also include one or more optical elements. In the following discussion, a collector 30 is used as an example of such an optical element, but the discussion applies to other optical elements as well. The collector 30 may be a normal incidence reflector, for example, implemented as a multilayer mirror (“MLM”) with additional thin barrier layers, for example B4C, ZrC, S i 3N1 or C, deposited at each layer interface to effectively block thermally-induced interlayer diffusion. The collector 30 may be in the form of a prolate ellipsoid, with a central aperture to allow the laser radiation 22b and 23b to pass through and reach the irradiation region 28. The collector 30 may have a first focus at the irradiation region 28 and a second focus at a so-called intermediate point 40 (also called the intermediate focus 40) where the EUV radiation may be output from the EUV radiation source 10 and input to, e.g., an integrated circuit lithography scanner or stepper 50. The integrated circuit lithography scanner or stepper 50 uses the radiation, for example, to process a silicon wafer workpiece 52 in a known manner using a reticle or mask 54. The silicon wafer workpiece 52 is then additionally processed in a known manner to obtain an integrated circuit device.

[0049] FIG. 2 is a diagram illustrating certain principles of operation of a conditioning pulse delivery system such as might be used in the arrangement of FIG. 1. As can be seen, a droplet generator 92 generates a stream of targets 14 in the chamber 26. At a time Ti one of the targets is struck by a conditioning pulse 23b generated by conditioning laser 23. The conditioning pulse 23b is steered by an acousto-optical deflector (AOD) 200 included in the laser beam steering system 32. This prepares the target 14 at time T2 as indicated by the lightened sphere. It will be understood, however, that target preparation and conditioning may include changes in the target’s shape (e.g., flattening), mass distribution, and so on. Then, at time T3, the target reaches the point in the irradiation region 28 where it is struck and converted to a plasma state by the main heating pulse 22b.

[0050] The AOD 200 is useful in systems requiring fast beam steering, that is, steering fast enough to suppress AY excursions as shown in FIG. 3B, also referred to a laser-to-droplet excursions in the Y dimension or L2DY excursions, that increase the dose margin. The fast steering enabled by the AOD 200 thus improves EUV performance. The presence of the AOD 200, however, reduces the conditioning laser power by about 15% due to the insertion loss occasioned by use of the AOD 200. [0051] With reference to L2DY excursions, in general, the metrology directed to target detection and measurement determines a degree of alignment of the targets with the laser beams including the conditioning beams. As mentioned, for a reference coordinate system, as shown in FIG. 3A, Z is the direction along which the laser beam/pulse 23b propagates and is also the direction from the collector 30 to the irradiation site 28 and the EUV intermediate focus. X is in the droplet propagation plane. Y is orthogonal to the XZ plane. To make this a right-handed coordinate system, the trajectory of the targets 14 is taken to be in the -X direction.

[0052] The primary components of targeting alignment error are AX and AY as shown in FIG. 3B. These errors typically need to be kept to less than about 5 pm. Errors in the Z-direction are less critical because the Rayleigh length of the laser focus is relatively long, so errors on the order of 100 pm or so are tolerable. The X position error AX is mostly a consequence of droplet coalescence variation. Any timing error correction can be accomplished by detecting the time at which the droplet crosses a laser curtain 115 within the irradiation region near the irradiation site 28. This measurement can be made even when the laser is operational because the laser curtain 115 is provided by a separate laser source and so can always be on. Also, the measurement performed using the laser curtain 115 is relatively tolerant to misalignment in Y, Z because the curtain is made to be wide in the YZ plane. The Y error AY can be determined using the reflection of the conditioning beam 23b from the target 14. In principle, to measure the droplet Y position it is possible use a separate illuminator like the laser curtain 115. In general, beam steering is used to reduce the Y error, AY.

[0053] FIG. 4A is a simplified schematic of an embodiment of a conditioning laser 400 such as might be used in the above-described systems as conditioning laser 23. As shown, the conditioning laser 400 may include a laser diode 410 acting as a seed laser, a focusing lens 415, a laser rod 420 acting as a gain medium, and an output coupler 425. The laser rod 420 is provided with a highly reflective coating 422 and an antireflective coating 424. The laser 400 also includes a beam shaping module 430 which includes optical elements for beam shaping and an AOM 435 that controls the amplitude of the output beam. These elements are all arranged within an enclosure 440.

[0054] It will be apparent to one of ordinary skill in the art that any suitable type of laser may be used, such as Diode Pumped Solid State (DPSS) lasers including q-switched InnoSlab DPSS lasers with internal amplifiers and CW seed lasers with an AOM pulse slicer, and internal amplifiers. FIG. 4B is a schematic diagram of a DPSS laser 450 as a non-limiting example. In FIG. 4B one pump diode laser in a stack of pump diode lasers 455 pumps a slab crystal 460 in a q-switch seed 465. The remaining pump diode lasers pump respective ones of amplifiers 470. The q-switch seed 465 has coupler mirrors 467 to circulate the seed light. The q-switch seed 465 includes a q-switch AOM 469. [0055] The beam 472 coming out of the final amplifier 470 is not symmetric, being elongated in one axis due to the InnoSlab laser and amplifier design. A beam shaping section 475 is used to convert the asymmetric output beam to a circular beam The beam shaping section 475 includes a beam shaper 480 and an AOM 485. The AOM 485 controls the amplitude of an output beam 487. These components may be housed within an enclosure 490.

[0056] While an AOM is used at the output of the laser for amplitude control in these examples, an electro optic module (“EOM”) can be used instead for the same purpose. As is true of an AOM, however, an EOM is used only for amplitude control. Neither the AOM nor EOM normally provides steering capability. Steering capability is more typically provided by an AOD, but the AOD can also perform the amplitude control function. As discussed below, it is possible to take advantage of this dual functionality, that is, amplitude control and steering, by using an AOD in place of either an AOM or EOM.

[0057] According to an aspect of an embodiment, the insertion loss incurred by the insertion of a separate acousto-optical device in the beam path between the conditioning laser and the target to perform steering is avoided by having this steering function performed as an additional function by an acousto-optical device inside the conditioning laser. Using a single acousto-optical device to perform dual functions instead of one for each function avoids the insertion loss that would otherwise be incurred by the use of a second acousto-optical device. This increases the pulse energy available for the conditioning beam.

[0058] Thus, as shown in FIG. 5A, according to an aspect of an embodiment, a conditioning laser 500 includes an AOD 510 which performs both the amplitude control function of the AOM 435 in the arrangement of FIG. 4A and additionally performs a beam steering function. The AOD 510 operates under the control of a control signal 520 generated based on a detected alignment state of a conditioning beam pulse and a target. As shown, according to an aspect of an embodiment, the AOD 510 is placed in the enclosure of laser 500 at a position optically after (i.e., down beam of) the output coupler 425. While the AOD 510 is shown as being part of the beam shaping module 430 it will be understood that the AOD 510 is not necessarily part of the beam shaping module 430.

[0059] Similarly, FIG. 5B shows this improvement implemented in a laser 550 (similar to the laser shown in FIG. 4B). The laser 550 includes an AOD 555 which replaces the AOM 485 of FIG. 4B and performs both the amplitude control function of the AOM 485 in the arrangement of FIG. 4B and additionally performs a beam deflection/ steering function. The AOD 555 operates under the control of a control signal 560 generated based on a detected state of alignment of a conditioning beam pulse and a target. While the AOD 555 is shown as being part of the beam shaping section 475 it will be understood that the AOD 555 is not necessarily part of the beam shaping section 475.

[0060] Placing the steering functionality within the laser 500 or 550 makes it possible to make the beam path between the laser 500 or 550 and the target 14 free of a separate, external steering AOD, and its consequent insertion loss. Thus, as shown in FIG. 6A, a slow steering module 600 in a pulse generation system 650 does not include an AOD. The AOD 510 in the laser 500 provides a fast steering capability rapid enough to be able to carry out steering pulse-to-pulse if desired. Because the AOD 510 also performs a beam amplitude modulation function there is no need for a separate operating AOM in the laser 500. “Operating” in this context means in the internal beam path and operational. The same is true of the laser 550 and the AOD 555 of FIG. 5B.

[0061] In the above embodiments, combining the steering functionality and the amplitude control functions within the laser 500 or 550 eliminates the need for a separate, external steering AOD. According to another aspect of an embodiment, however, and as shown in FIG. 6B, an AOD 620 in a pulse generation system 660 which both steers and controls the amplitude of the beam 23b can be placed in the output beam path of a laser 610. The laser 610 does not have an operating AOM or AOD. That is, such elements may be present in the laser 610 but are not being used in a manner that causes any appreciable insertion loss. The AOD 620 operates under the control of a signal 625.

[0062] In general, AODs are able to provide beam deflection / steering along a single axis or dimension. According to an aspect of an embodiment, the laser 500 is positioned so that an acousto- optical device, in the example of FIG. 7 the AOD 510, is oriented to steer the beam in either direction along the Y axis as shown in FIGS. 3A and 3B. Alternatively, the AOD 510 can be oriented as desired and the beam could be rotated after the AOD 510 to properly orient the steering axis. A system to accomplish this is shown in FIG. 7 in which a beam rotator 710 which may be, for example, a dove prism or K-rotator, is arranged to rotate the beam 23b. This provides flexibility in rotating the steering axis without having to specifically orient the AOD inside the laser. This may be helpful in applications in which source beam delivery layouts vary between fabs. While FIG. 7 shows an arrangement in which the beam rotator 710 is external to both the laser 500 and the slow steering module 600, it will be appreciated by one of ordinary skill in the art that the beam rotator 710 could be implemented as being part of either of these components.

[0063] Also, an AOM can be driven to perform both the beam power modulation function and the beam deflection function by appropriate application of the driver/control signal to the AOM. Such an arrangement of a pulse generator 650 is shown in FIG. 8A in which a driver/control signal is applied to an AOM 810 by a controller 800. The steering function is performed by modulating the frequency of the driver/control signal and the amplitude is controlled by modulating the amplitude of the driver/control signal. In the arrangement of FIG. 8 A the AOM 810 is located within the laser 500. The arrangement of FIG. 8B is similar except that the AOM 810 is located on the beam path external to the laser 610.

[0064] In all of these arrangements in general the amplitude of the light being deflected varies as a function of the angle of the deflection. For some implementations in which there is a requirement to maintain the same laser pulse energy for all different angles it will prove advantageous to control the amplitude so that the amplitude of the light remains the same at all angles of defection. In other words, to maintain the same amplitude of the deflected light a corresponding compensation of the driver signal amplitude is performed.

[0065] FIG. 9A is a flowchart showing steps of a method of controlling a beam according to another aspect of an embodiment. A pulse of the laser beam, which may be a conditioning pulse or a main pulse, is generated in a step S10. This can be accomplished, for example, by using the lasers with acousto-optical devices as described above. Then, in a step S20, the acousto-optical device is used both to control the amplitude of and steer the beam pulse. In this context, “steer” is used to refer to the fast steering provided by the acousto-optical device as opposed to such slower steering as may be provided by a slow steering module. It will be understood that the slow steering module may additionally be present and used to perform a slow steering function. Then, in a step S30, it is determined whether the beam pulse and target are properly aligned. This can occur while the beam pulse is being used to irradiate the target. If the beam and target are properly aligned then the process reverts to step S10 and another pulse of the beam is generated and aligned. If the beam and target are not properly aligned then the process progresses to a step S40 in which the steering is corrected by, for example, applying a control signal to the acousto-optical device. Then the process reverts to step S10 and another pulse of the conditioning beam is generated.

[0066] FIG. 9B is a flowchart showing steps of a method of controlling a beam according to another aspect of an embodiment. The method of FIG. 9B is the same as the method of FIG. 9A except that an additional step S50 of rotating the beam is included.

[0067] The ability to use one component to perform the dual functions of beam amplitude control and beam steering provides an opportunity to retrofit systems already deployed in the field to enhance their functionality. For example, control over steering pulses from existing CO2 main lasers may be enhanced by controlling acousto-optical devices in the beam path present for the purposes of controlling pulse amplitude so that these same acousto-optical devices also steer the pulses. This may be effected by changing the drive or control signal applied to the acousto-optical device. More specifically, for an AOM that is already present in the beam path, and is configured to be able to serve also as an AOD, then the AOM could be driven to perform a deflection function in addition to an amplitude control function. Or the AOM could be replaced by an AOD which is used for both functions. As another alternative an AOD could be inserted into the existing beam path and used to provide an additional steering or used for steering and in lieu of an AOM in the beam path for amplitude control.

[0068] The present disclosure is made with the aid of functional building blocks illustrating the implementation of specified functions and relationships thereof. The boundaries of these functional building blocks have been arbitrarily defined herein for the convenience of the description. Alternate boundaries can be defined so long as the specified functions and relationships thereof are appropriately performed. For example, control module functions can be divided among several systems or performed at least in part by an overall control system.

[0069] The above description includes examples of one or more embodiments. It is, of course, not possible to describe every conceivable combination of components or methodologies for purposes of describing the aforementioned embodiments, but one of ordinary skill in the art will recognize that many further combinations and permutations of various embodiments are possible. Accordingly, the described embodiments are intended to embrace all such alterations, modifications, and variations that fall within the spirit and scope of the appended claims. Furthermore, to the extent that the term “includes” is used in either the detailed description or the claims, such term is intended to be inclusive in a manner similar to the term “comprising” as “comprising” is construed when employed as a transitional word in a claim. Furthermore, although elements of the described aspects and/or embodiments may be described or claimed in the singular, the plural is contemplated unless limitation to the singular is explicitly stated. Additionally, all or a portion of any aspect and/or embodiment may be utilized with all or a portion of any other aspect and/or embodiment, unless stated otherwise.

[0070] The embodiments can be further described using the following clauses:

1. A laser for producing a beam of radiation, the laser comprising: at least one laser gain medium for generating the beam of radiation; and an acousto-optical device arranged to receive the beam of radiation and adapted to both control an output amplitude of the beam of radiation and to steer the beam of radiation.

2. The laser of clause 1 wherein the acousto-optical device is an acousto-optical deflector.

3. The laser of clause 1 wherein the laser includes an enclosure and the at least one gain medium and the acousto-optical device are arranged inside the enclosure.

4. The laser of clause 3 wherein the acousto-optical device is an acousto-optical deflector.

5. The laser of clause 1 wherein the laser includes a beam shaping module which includes the acousto-optical device.

6. The laser of clause 1 wherein the beam of radiation generated by the laser comprises at least one prepulse.

7. The laser of clause 1 wherein the beam of radiation generated by the laser comprises at least one pedestal pulse.

8. The laser of clause 1 wherein the beam of radiation generated by the laser comprises at least one rarefication pulse.

9. The laser of clause 1 wherein the beam of radiation generated by the laser comprises at least one main heating pulse.

10. The laser of clause 1 wherein the laser is a solid state laser.

11. The laser of clause 1 wherein the laser is a CO2 laser.

12. The apparatus of clause 1 wherein the acousto-optical device is arranged to steer the beam of radiation along a first axis and further comprising a beam rotator positioned to receive the beam of radiation after the acousto-optical device and arranged to rotate the beam of radiation such that the beam of radiation is steered along a second axis different from the first axis.

13. A system for a target of source material with a beam of radiation, the system comprising: a seed laser; a gain medium arranged to amplify a seed laser beam from the seed laser to generate the beam of radiation; and an acousto-optical device arranged to receive the beam of radiation and adapted to control an output amplitude of and to steer the beam of radiation.

14. The system of clause 13 wherein the acousto-optical device is an acousto-optical deflector.

15. The system of clause 13 further comprising an enclosure wherein the seed laser, the gain medium, and the acousto-optical device are arranged within the enclosure. 16. The system of clause 15 wherein the acousto-optical device is an acousto-optical deflector.

17. The system of clause 13 wherein the beam of radiation generated by the gain medium comprises at least one prepulse.

18. The system of clause 13 wherein the beam of radiation generated by the gain medium comprises at least one pedestal pulse.

19. The system of clause 13 wherein the beam of radiation generated by the gain medium comprises at least one rarefication pulse.

20. The system of clause 13 wherein the beam of radiation generated by the laser comprises at least one main heating pulse.

21. The system of clause 13 wherein the laser is a solid state laser.

22. The system of clause 13 wherein the laser is a CO2 laser.

23. Apparatus for steering a beam of radiation along a first dimension transverse to a streamwise direction of a stream of targets of source material, the apparatus comprising: a source material dispenser arranged to dispense the stream of targets in the streamwise direction; a laser configured to generate the beam of radiation, the laser including an opto-acoustical device arranged to control an amplitude of the beam of radiation; and a detector arranged to detect an alignment state between one of the targets and the beam of radiation and adapted to generate a steering control signal, wherein the acousto-optical device located within the laser is additionally arranged to receive the steering control signal and adapted to steer the beam of radiation in response to the steering control signal.

24. The apparatus of clause 23 wherein the acousto-optical device located within the laser is adapted to steer the beam of radiation along the first dimension.

25. The apparatus of clause 23 wherein the acousto-optical device located within the laser is adapted to steer the beam of radiation along a second dimension different from the first dimension, and further comprising a beam rotator arranged to receive the beam of radiation from the acousto-optical device for rotating the beam of radiation so that steering after the beam rotator is along the first dimension.

26. The apparatus of clause 23 wherein the acousto-optical device is an acousto-optical deflector.

27. A method of delivering a beam of radiation to a target of source material, the method comprising: generating a beam of radiation; using an acousto-optical deflector both to control the amplitude of and steer the beam of radiation; and using the beam of radiation to irradiate the target of source material.

28. The method of clause 27 further comprising a step after using the acousto-optical deflector of rotating the beam of radiation.

[0071] Other implementations are within the scope of the claims.