DUV laser system beam divergence control apparatus and process

Abstract

A deep ultraviolet laser system includes a pulse stretcher including multiple steerable mirrors; a sensor positioned to measure a divergence of beams passed through the pulse stretcher; and a controller connected to the sensor and to the mirrors, the controller configured to (1) using the sensor, measure and record an initial divergence of a beam passed through the pulse stretcher (2) separately dither each steerable mirror and measure and record corresponding divergences (3) calculate, based on the corresponding divergences, new angular positions for each steerable mirror to produce a divergence at, within, or closer to a target or target range, and (4) position each respective steerable mirror of the pulse stretcher at its new angular position and, (5) measure and record a new divergence. A process of controlling divergence is also disclosed.

Description

DUV LASER SYSTEM BEAM DIVERGENCE CONTROL APPARATUS AND PROCESSCROSS-REFERENCE TO RELATED APPLICATIONS

[0001] This application claims priority to US Application No. 63 / 738,888, filed December 26, 2024, titled DUV LASER SYSTEM BEAM DIVERGENCE CONTROL APPARATUS AND PROCESS, which is incorporated herein by reference in its entirety.FIELD

[0002] The disclosed subject matter relates to control of beam properties in deep ultraviolet (DUV) laser systems, particularly to control of beam divergence in DUV laser systems used for photolithographic integrated circuit manufacturing and similar processes.BACKGROUND

[0003] Photolithography is a process by which semiconductor circuitry is patterned on a substrate such as a silicon wafer. A photolithography optical source provides deep ultraviolet (DUV) light used to expose a photoresist on the wafer. Often, the optical source is an excimer laser source and the light is a pulsed laser beam. The light beam is passed through a beam delivery unit and a reticle or a mask, and then projected onto a prepared silicon wafer. In this way, a portion of a chip design is patterned into a photoresist that is then developed, etched, and cleaned, then used as a mask for implantation, deposition, etching, and / or other processes on or in the structure of the wafer, and then the process repeats.

[0004] For best performance when projecting the light beam onto the prepared wafer, it is often desirable to minimize the divergence of the beam or to keep the divergence of the beam within certain limits. Optical components and / or their mounting and positioning can change with time due to various thermal, mechanical, or chemical effects, which can alter the divergence of the light beam produced by a laser system. Improved beam divergence control processes and apparatuses are subjects of the present disclosure.SUMMARY

[0005] In some general aspects, a deep ultraviolet (DUV) laser system includes: a pulse stretcher, the pulse stretcher configured to receive and pass through DUV beams, the pulse stretcher including multiple steerable mirrors steerable by actuators, each steerable mirror being steerable in one or more angular degrees of freedom; a sensor positioned and configured to measure a divergence of beams passed through the pulse stretcher; and a controller connected to the sensor and to the actuators, the controller configured to (1) using the sensor, measure and record an initial divergence of a beam or beams passed through the pulse stretcher with the steerable mirrors in respective initial angular positions, (2) using the actuators, separately dither each degree of freedom of each steerable mirror ofthe pulse stretcher and, using the sensor, measure and record corresponding divergences of a beam or beams passed through the pulse stretcher, (3) calculate, based on the respective initial angular positions, on the initial divergence, based on the dithering, and based on the corresponding divergences, respective new angular positions for each steerable mirror to produce a divergence, in a beam or beams passed through the pulse stretcher, either (a) within a target divergence range or (b) closer to the target divergence range, or to a divergence target, than the initial divergence, or both, and (4) using the actuators, position each respective steerable mirror of the pulse stretcher at its respective new angular position and, (5) using the sensor, measure and record a new divergence of a beam or beams passed through the pulse stretcher.

[0006] Implementations can include one or more of the following.

[0007] The controller can be configured to, using the actuators, separately and bidirectionally dither each degree of freedom of each steerable mirror of the pulse stretcher. The controller can be configured to iterate actions (1) through (5), with the new divergence from the previous iteration used as the initial divergence of the current iteration, until the new divergence of the current iteration is within the target divergence range. The controller is further configured to iterate actions (1) through (5), with the new divergence from the previous iteration used as the initial divergence of the current iteration, until the new divergence of the current iteration is within 0.1 percent the new divergence of the previous iteration. The controller can be configured to separately and bidirectionally dither each degree of freedom of each steerable mirror by dithering each degree of freedom of each steerable mirror separately in time. The controller can be configured to separately and bidirectionally dither each degree of freedom of each steerable mirror by dithering each degree of freedom of each steerable mirror separately in frequency.

[0008] The pulse stretcher can include at least four steerable mirrors. Each steerable mirror can be steerable in two angular degrees of freedom.

[0009] The sensor can be positioned and configured to sense the properties, of a beam passed through the pulse stretcher, at a far-field of the beam. The DUV laser system can further include a second sensor connected to the controller and positioned and configured to measure one or more near-field properties of beams passed through the pulse stretcher, and the controller can be further configured to: measure and record an initial value or initial values of the one or more near-field properties of one or more beams passed through the pulse stretcher; during dithering measure and record corresponding values of the one or more near-field properties of one or more beams passed through the pulse stretcher; calculate the respective new angular positions for each steerable mirror to produce (A) a divergence either (1) within a target divergence range or (2) closer to the target divergence range, or to a target divergence value, than the initial divergence, and (B) one or more of the one or more near-field properties either (1) within a target range or (2) closer to a target range, or to a target value, than the respective initial corresponding value, in a beam or beams passed throughthe pulse stretcher. The one or more near-field properties include one or more of beam footprint, beam uniformity or spatial profile, and beam symmetry.

[0010] The controller is further configured to: measure and record an initial value or initial values of one or more far-field properties other than divergence of one or more beams passed through the pulse stretcher; measure and record corresponding values of the one or more far-field properties of one or more beams passed through the pulse stretcher; calculate the respective new angular positions for each steerable mirror to produce (A) a divergence either (1) within a target divergence range or (2) closer to the target divergence range, or to a target divergence value, than the initial divergence, and (B) a stasis of, or an improvement of, one or more of the one or more far-field properties, in a beam or beams passed through the pulse stretcher. The one or more far-field properties can include one or more of beam angular space profile and beam pointing.

[0011] In additional general aspects, a process of operating a deep ultraviolet (DUV) laser system is provided, the DUV laser having a pulse stretcher, the pulse stretcher configured to receive and pass through DUV beams and including multiple steerable mirrors steerable by actuators controlled by a controller, each steerable mirror being steerable in one or more angular degrees of freedom, the process including: measuring and recording an initial divergence of a beam or beams passed through the pulse stretcher with the steerable mirrors in respective initial angular positions; separately and dithering each degree of freedom of each steerable mirror of the pulse stretcher and measuring and recording corresponding divergences of a beam or beams passed through the pulse stretcher; determining, based on the respective initial angular positions, the initial divergence, the dithering, and the corresponding divergences, respective new angular positions, for each steerable mirror, calculated to result in a divergence, in a beam or beams passed through the pulse stretcher, either (1) within a target divergence range or (2) closer to the target divergence range or to a divergence target than the initial divergence; and controlling the actuators to position each respective steerable mirror of the pulse stretcher at its respective new angular position and measuring and recording a new divergence of a beam or beams passed through the pulse stretcher.

[0012] Implementations can include one or more of the following.

[0013] The dithering can include bidirectional dithering. The process can include iterating the process, with the new divergence from the previous iteration of the process used as the initial divergence of the current iteration of the process, until the new divergence of the current iteration of the process is within the target divergence range. The process can include iterating the process with the new divergence of the previous iteration of the process used as the initial divergence of the current iteration of the process until the new divergence of the current iteration of the process within 0.1 percent of the new divergence of the previous iteration.

[0014] Separately and dithering each degree of freedom can include dithering each degree of freedom separately in time. Separately and dithering each degree of freedom can include dithering each degree of freedom separately in frequency.

[0015] The pulse stretcher can have at least four steerable mirrors. Each steerable mirror can be steerable in two angular degrees of freedom. Measuring and recording an initial divergence and measuring and recording corresponding divergences can both include sensing the properties of a beam passed through the pulse stretcher at a far-field of the beam.

[0016] The process can further include: measuring and recording an initial value or initial values of one or more near-field properties of one or more beams passed through the pulse stretcher; and measuring and recording corresponding values of the one or more near-field properties of one or more beams passed through the pulse stretcher; and wherein the determined respective new angular positions for each steerable mirror are calculated to result in (A) a divergence either (1) within a target divergence range or (2) closer to the target divergence range, or to a target divergence value, than the initial divergence, and (B) one or more of the one or more near-field properties either (1) within a target range or (2) closer to a target range, or to a target value, than the respective initial corresponding value, in a beam or beams passed through the pulse stretcher. The one or more near-field properties can include one or more of beam footprint, beam uniformity or spatial profile, and beam symmetry.

[0017] The process can further include: measuring and recording an initial value or initial values of one or more far-field properties other than divergence of one or more beams passed through the pulse stretcher; and measuring and recording corresponding values of the one or more far-field properties of one or more beams passed through the pulse stretcher; and wherein the determined respective new angular positions for each steerable mirror are calculated to result in (A) a divergence either (1) within a target divergence range or (2) closer to the target divergence range, or to a target divergence value, than the initial divergence, and (B) a stasis of, or an improvement of, one or more of the one or more far-field properties, in a beam or beams passed through the pulse stretcher. The one or more far-field properties include one or more of beam angular space profile and beam pointing.

[0018] The details of one or more implementations are set forth in the accompanying drawings and the description below. Other features will be apparent from the description and drawings, and from the claims.DRAWING DESCRIPTION

[0019] FIG. 1 is a schematic diagram, not to scale, of an overall broad conception of a photolithography system.

[0020] FIG. 2 is a schematic diagram, not to scale, of an overall broad conception of an illumination system such as can be used in the photolithography system of FIG. 1.

[0021] FIG. 3 is a schematic diagram, not to scale, of aspects of a system for divergence control in a light source according to the present disclosure.

[0022] FIG. 4 is a cross-sectional schematic diagram of an optical pulse stretcher useful for beam divergence control according to an aspect of the present disclosure.

[0023] FIG. 5 is a flow diagram of a process of beam divergence control in a light source according to another aspect of the present disclosure.

[0024] FIG. 5A is a flow diagram showing an implementation of a portion of the process shown in the flow diagram of FIG. 5.DETAILED DESCRIPTION

[0025] Various aspects and implementations are now described with reference to the drawings, wherein like reference numerals are used to refer to like elements throughout. When a particular feature, structure, or characteristic is described in connection with an implementation, it is understood that it is within the knowledge of one skilled in the art to effect such feature, structure, or characteristic in connection with other implementations whether or not explicitly described.

[0026] Some features or aspects of the present disclosure may be implemented in hardware, firmware, software, or any combination thereof. For example, implementations of the present disclosure may also be implemented as instructions stored on a machine -readable medium, which may be read and executed by one or more processors. A machine -readable medium may include any mechanism for storing or transmitting information in a form readable by a machine (e.g., a computing device). For example, a machine-readable medium may include read only memory (ROM); random access memory (RAM); magnetic disk storage media; optical storage media; flash memory devices; cloud-implemented storage, electrical, optical, acoustic, or other forms of propagated signals (e.g., carrier waves, infrared signals, digital signals, etc.), and others. Further, firmware, software, routines, instructions may be described herein as performing certain actions. However, it should be appreciated that such descriptions are merely for convenience and that such actions in fact result from computing devices, processors, controllers, or other devices executing the firmware, software, routines, instructions, and so forth.

[0027] FIG. 1 shows a photolithography system 100 that includes a light source 102. As described more fully below, the light source 102 produces a pulsed light beam 104 and directs it to a photolithography exposure apparatus 106 that patterns microelectronic and other features on a wafer 110. The wafer 110 is placed on a wafer table 112 constructed to hold the wafer 110 and connected to a positioner 114 configured to accurately position the wafer 110 in accordance with certain parameters.

[0028] The pulsed light beam 104 has a wavelength in the DUV range, with a wavelength of 248 nanometers (nm) or 193 nm, for example. The photolithography exposure apparatus 106 includes an optical arrangement 108 having, for example, one or more condenser lenses, a mask, and an objective arrangement. The mask is movable along one or more directions, such as along an optical axis of the pulsed light beam 104 or in a plane that is perpendicular to the optical axis. The objective arrangement includes a projection lens and enables an image transfer to occur from the mask to photoresist on the wafer 110. The optical arrangement 108 adjusts the range of angles for the pulsedlight beam 104 impinging on the mask 110. The optical arrangement 108 also homogenizes (makes uniform) the intensity distribution of the pulsed light beam 104 across the mask 110.

[0029] The photolithography exposure apparatus 106 can include, among other features, a lithography controller 116 that controls how layers are printed on the wafer 110. The lithography controller 116 may include a memory that stores information such as process recipes that determine the parameters including a length of the exposure on the wafer 110 based on, for example, the mask used, as well as other factors that affect exposure. During lithography, a burst of pulses of the pulsed light beam 104 illuminates the same area of the wafer 110 to constitute an illumination dose.

[0030] The photolithography system 100 also preferably includes a control system 118. In general, the control system 118 includes one or more of digital electronic circuitry, computer hardware, firmware, and software. The control system 118 can be centralized or be partially or wholly distributed throughout the photolithography system 100, and / or may be at least in part distributed even beyond any of the physical structure of the photolithography system 100.

[0031] FIG. 2 shows a DUV laser system 202 in the form of a DUV gas-discharge pulsed laser system that can serve as the light source 102 of FIG. 1 and produces a pulsed beam 204 that can serve as the light beam 104 of FIG. 1. FIG. 2 shows a two-chamber laser system as a nonlimiting example. It will be understood that the principles explained herein are equally applicable to a single chamber laser system or a laser system having more than two chambers. The DUV laser system 202 may include, for instance, a solid state or gas discharge master oscillator (“MO”) seed laser 220, an amplification stage 240 such as a power ring amplifier (“PRA”) 240, relay optics 230, and an output subsystem 250. In other implementations of the DUV laser system 202, a power oscillator (“PO”) may be used instead of the PRA 240.

[0032] The MO seed laser 220 may include, e.g., an MO chamber 224 which includes a pair of electrodes 223 and 225. The MO seed laser 220 may also include a master oscillator output coupler (“MO OC”) 228, which may comprise a partially reflective mirror (not shown), that forms, together with a reflective grating (not shown) in a line narrowing module (“LNM”) 222, an oscillator (optical cavity) in which a beam oscillates to form a seed laser output pulse. The MO seed laser 220 may also include a line-center analysis module (“UAM”) 226. The relay optics 230 may include an MO wavefront engineering box (“WEB”) 232 that may serve to redirect the output of the MO seed laser 220 toward the PRA 240, and may include, a multi prism beam expander (not shown) and an optical delay path (not shown).

[0033] The PRA 240 may include, for example, a PRA discharge chamber 244. The PRA discharge chamber 244 may include a pair of electrodes 243 and 245. The PRA discharge chamber 244 may be part of an oscillator. The oscillator may be formed or defined by (1) seed-beam injection and output coupling optics (not shown) that may be incorporated into a PRA WEB 248 and (2) a beam reverser (“BR”) 242. The PRA WEB 248 may incorporate a partially reflective input / output coupler (notshown) and a maximally reflective mirror for the nominal operating wavelength (e.g., at around 193 nm for an ArF system) and one or more prisms.

[0034] A bandwidth analysis module (“BAM”) 246 may receive the light beam oscillating in the PRA 240 and pick off a portion of the light beam for metrology purposes, e.g., to measure the output bandwidth and pulse energy. The laser light beam 249 of pulses then passes through the PRA WEB 248 to an optical pulse stretcher (“OPuS”) 252, and then to an autoshutter, both within the output subsystem 250. In the implementation of the output subsystem 250 shown in FIG. 2, the autoshutter is in the form of, or included within, a combined autoshutter metrology module (“CASMM”) 254, which may also include a pulse energy meter.

[0035] A purpose of the OPuS 252 is to convert an individual initial pulse into a pulse train.Secondary pulses created from the initial pulse by the OPuS 252 are delayed with respect to each other, and can overlap each other. By distributing the energy of the initial pulse into a train of secondary pulses, the effective length of the pulse is expanded at the same time the peak intensity of the pulse is reduced.

[0036] The PRA discharge chamber 244 and the MO discharge chamber 224 are configured as chambers in which electrical discharges between the electrodes create an inverted population of high energy molecules, including, e.g., Ar, Kr, F2, and / or Xe to produce a relatively broad-band light amplification potential. The wavelength(s) that are permitted to oscillate, and accordingly receive significant amplification, can be line-narrowed to a relatively very narrow bandwidth around a center wavelength selected by adjustments made in the LNM 222.

[0037] FIG. 3 is a schematic diagram, not to scale, of aspects of a system for divergence control in a light source according to the present disclosure, including an OPuS 352 and a CASMM metrology module 354 that may be used, for example, as the OPuS 252 and CASMM 254 of FIG. 2. The OPuS 352 receives a laser light beam 349 of pulses and stretches the pulses as will be discussed below, then passes abeam of stretched pulses 353 to the CASMM 354. The CASMM 354 includes a far-field sensor 358 that senses far-field characteristics of the beam of stretched pulses 353. A beam splitter 356 at a location A splits off a small portion 353a of the light of the beam of stretched pulses 353 and directs it to the far-field sensor 358. The location A can correspond to a far-field location of the beam of stretched pulses 353 (a location not at or near a beam waist) and / or there can be optical elements (not shown) in the path of the small portion 353a that ensure that the far-field sensor is at a position corresponding to a far-field of the small portion 353a of the beam of stretched pulses 353. An image sensor sensitive to the DUV wavelength(s) of the beam of stretched pulses 353 can be used as the far-field sensor 358 to capture images of the beam in the far-field. The CASMM can also include a nearfield sensor such as an image capture module 362 that senses near-field characteristics of the beam of stretched pulses 353. A beam splitter 360 at a location B can split off a small portion 353b of the light of the beam of stretched pulses 353 and direct it to the near-field sensor 362. The location B can correspond to a near-field location of the beam of stretched pulses 353 (a location at or near a beamwaist) and / or there can be optical elements (not shown) in the path of the small portion 353b that ensure that the near-field sensor is at a position corresponding to a near-field of the small portion 353b of the beam of stretched pulses 353. Similarly to the far-field sensor, an image sensor sensitive to DUV wavelength(s) of the beam of stretched pulses 353 can be used as the near-field sensor 358 to capture images of the beam in the near-field. Sensors 358 and 362 may include elements such as photodiode arrays, phosphor screens, cameras, quad-cell detectors, or other elements suitable for gathering information on the spatial distribution of the intensity of a DUV beam.

[0038] In some implementations the CASMM 354 is not present as such, and a separate or individual far-field sensor 358 can be used. In some implementations an individual or separate near-field sensor 362 can also be used. This is reflected in FIG. 3 by the presence of a dashed line to represent the CASMM 354, to indicate that the CASMM 354 itself is not necessarily present in all implementations.

[0039] Data from the far-field sensor 358, representing characteristics of the beam 353 in the far-field, and data from the near-field sensor 362 (if present), representing characteristics of the beam 353 in the near-field, are transmitted to a control unit 364. It will be understood that the control unit 364 may be implemented in hardware or firmware or software or combinations of these, and may be implemented in a single physical location or may be distributed over multiple locations, including in no fixed physical location, such as in a cloud service or cloud services. The control unit 364, or the sensor 358, or both in combination, may perform image analysis of an image of the beam 353 in the far-field, or a combination of image analyses, such as, for example, one or more of edge detection, contour detection / labeling, aperture detection, and computer vision / pattem recognition-based identification techniques. The control unit 364 and the sensor 358, when present, can operate similarly to analyze a near-field image of the beam 353. The control unit 364 generates control signals, based on information from the sensor 358 and optionally from the sensor 362, such as signals Cl, C2, C3, C4, and AC, each of which may include one or more individual signals, to control the OPuS 352.

[0040] A user interface 366 is configured to exchange data with the control unit 364. The user interface 366 and the control unit 364 may be connected by a hard wire connection or wirelessly. The user interface 366 and the control unit 364 may be connected directly or be connected indirectly through intermediary components, ports, buses, and so forth. The user interface 366 permits a user such as a field service engineer to monitor the control of the OPuS and review any acquired images of the beam 353 obtained by the sensors 358 and 362. The user interface 366 also permits a user to override any automatic processes if desired, such as by entering into a manual control mode, or by stepping through an otherwise automatic process, allowing review and confirmation and / or adjustment of each step if desired.

[0041] FIG. 4 is a cross-sectional schematic diagram of an optical pulse stretcher (OPuS) 452 useful for beam divergence control according to an aspect of the present disclosure. The OPuS 452 of FIG. 4 can be used as the OPuS 352 of FIG. 3, for example. In OPuS 452, a housing or wall 452w enclosesan inner volume 452i typically containing a low-pressure gas to minimize absorption of DUV wavelengths. The OPuS 452 receives a laser light beam 449 of pulses along an input beam path IBP through an entrance optical window BW1, and stretches the received pulses to produce a beam of stretched pulses 453 passed on from the OPuS 452 through an exit optical window BW2 along an output beam path OPB.

[0042] Pulse stretching is achieved by splitting off one or more portions of an incoming pulse by means of a beam splitter BS, then passing the one or more portions through optical delay path(s), and then merging the one or more portions with an original portion of the laser light beam 449 on the output beam path OBP to form the beam of stretched pulses 453. In the implementation shown, the beamsplitter BS is in the form of a beamsplitting cube having a beamsplitting interface IF, but other beamsplitters, such as a beamsplitting mirror, can be used. In the implementation shown, the optical delay path is formed by mirrors 471, 472, 473, and 474, which together receive a split-off portion of an input pulse of the laser light beam 449 (a portion reflected by the beam splitter BS) and return the portion back to the beamsplitter BS, at which at least a first part of the returning portion is reflected onto the output beam path OBP. A second part of the returning portion may pass through the beamsplitter BS, becoming a second split-off portion of the initial pulse, which then goes along the optical delay path again as above. More than two split-off portions with significant pulse energy can be generated from each input pulse, depending on factors such as the reflectivity of the beam splitter BS and the mirrors 471, 472, 473, and 474. In various implementations, a beam-blocking shutter (not shown), positioned within or downstream of the OPuS 452, can cut off the split-off portions at a desired number.

[0043] Two or more, or each, of the mirrors 471, 472, 473, 474, are adjustable (“steerable”). In the implementation shown, the mirrors 471, 472, 473, 474 are steerable in two degrees of freedom. Specifically, mirror 471 is pivotable within the plane of the figure as indicated by the arrow Al, as well as around the axis AA. Mirror 472 is pivotable within the plane of the figure, as indicated by the arrow A4, as well as around the axis AB. Mirror 473 is pivotable within the plane of the figure, as indicated by the arrow A2, as well as around the axis AA. And mirror 474 is pivotable within the plane of the figure, as indicated by the arrow A3, as well as around the axis AB. This permits the angular steering of a portion of a pulse at each of the mirrors 471, 472, 473, 474. The beamsplitter BS can also be steerable in two degrees of freedom, such as one within the plane of the figure as indicated by the arow A5, and one around the axis of the initial beam path IBP, for example.

[0044] In the implementation shown in FIG. 4, each of the mirrors 471, 472, 473, 474 is coupled to a pair of controllable actuators, with mirror 471 coupled to actuators TWA1 and TWA2 receiving respective control signals Clx and Cly, mirror 472 coupled to actuators TWA7 and TWA8 receiving respective control signals C4x and C4y, mirror 473 coupled to actuators TWA3 and TWA4 receiving respective control signals C2x and C2y, and mirror 474 coupled to actuators TWA5 and TWA6 receiving respective control signals C3x and C3y. Actuators (not shown) receiving respective controlsignals (not shown) can similarly be coupled to the beamsplitter BS. The actuators can be in the form of be through-the-wall actuators (“TWAs”) that extend through the wall 452w of the OPuS 452. Alternatively, some or all of the actuators may be deployed within inner volume 452i, enclosed by wall 452w. The TWAs may each include, for example, an electrically controlled motor which causes an end of the TWA to translate along its axis according to the direction of rotation of a shaft to alter the alignment of the mirror to which the TWA is coupled. Other actuator types may of course be used, such as linear electric motors, for one example. Use of automatically controllable TWAs enables automation of an adjustment process of the OPuS 452. In some implementations, for example in implementations in which small adjustments of position at high rates are desired, a TWA also includes a piezoelectric actuator at the tip nearest the respective mirror, as shown in FIG. 4 in the case of actuator TWA7, with piezoelectric actuator PAI at its tip, and actuator TWA8, with piezoelectric actuator PA2 at its tip. One or more of the actuators may have ranges of adjustment that enable adjustments over a range of or several degrees of angle for a corresponding optical element (e.g., 2 degrees, 5 degrees, 10 degrees) with an adjustment speed between 0.1 and 20 degrees per second. One or more of the actuators may have ranges of adjustment that enable adjustments over a range of fractions of milliradians (e.g., 0,02 milliradians, 0.04 milliradians, 0.10 milliradians, 0.50 milliradians, 0.8 milliradians) or milliradians (e.g., 1 milliradian, 2 milliradians, 3 milliradians, 5 milliradians, 10 milliradians, 20 milliradians, 30 milliradians) with an adjustment speed of between 5 and 100 milliradians per second. One or more of the actuators may include a combination of two or more short-throw and long-throw and / or slow-response and fast-response components, enabling both fine and coarse adjustments (e.g., slower adjustments in the range of degrees augmented by more rapid adjustments in the range of milliradians or sub-milliradians).

[0045] According to aspects of the present disclosure, dithering-based automatic adjustments of mirror elements of an OPuS such as OPuS 452 of FIG. 4 are used to minimize or otherwise control divergence of an output beam such as the beam of stretched pulses 453 of FIG. 4.

[0046] In deep ultraviolet (DUV) laser systems, beam divergence may have specified limits, typically a specified maximum, for certain uses. Further, a specified target or range of beam divergence (above a minimum achievable level) can be desirable as one tool to reduce the effective spatial coherence and resulting speckle contrast of the beam. Reductions in speckle contrast are desirable for improved performance in semiconductor lithography. For example, reduced speckle contrast can improve (decrease) line edge roughness of imaging at the wafer, enabling smaller features to be more reliably produced. Dithering of the mirrors, or of each degree of freedom of the mirrors, is used in conjunction with measurement of divergence of the output beam to detect the effects of mirror positioning changes on divergence of the beam, and then to make adjustments to mirror positions to optimize or bring to within a desired range the divergence of the output beam. Use of dithering for this purpose has the advantage of effectively detecting the as-is, where-is properties of a laser system in the field, and of laser system components such as the OPuS. Even unforeseen and / ornot fully understood changes or variations within the system can be detected, by dithering paired with measurement of changes in divergence due to the dithering, and then compensated for.

[0047] In the case that high-speed dithering is desired, piezoelectric actuators can be included in the actuators as in actuators TWA7 and TWA8 described above. In at least some implementations, properties of the output beam in addition to divergence, for example properties such as beam size, beam uniformity, beam pointing stability, and others can be measured and adjusted or optimized, or at least preserved, while adjusting or optimizing beam divergence.

[0048] FIG. 5 is a flow diagram of a process P510 of beam divergence control using a pulse stretcher in a DUV laser system according to an aspect of the present disclosure. Process 510 may be executed during a designated calibration period or a down-time period, when the laser system is not in active use for lithography operations. Alternatively, or in addition, process 510 may be executed occasionally, periodically, or continuously while the laser system is in active use for lithography operations. First, a beam divergence of a beam or beams passing through the pulse stretcher is measured and recorded (S510). In implementations, the measured divergence can be compared to a desired range or target (S511), and if it is within the desired range or sufficiently close to the desired target (S511, “Y” branch), the process can return, at a selected periodic rate for example, to again measure and record the beam divergence of a beam or beams passing through the pulse stretcher. When conditions arise in which the measured divergence is not within the desired range or not sufficiently close to the desired target (S511, “N” branch), a current pulse stretcher (PS) element is perturbed about a current axis (such as axis AA of mirror 471 of FIG. 4, for example) in a first direction (S512). Alternatively, in an implementation in which item S511 is not present (such as a process P510 in which the initial determination of a need or desire for divergence adjustment has been determined outside of the process P510, for example), directly after the divergence has been measured and recorded (S510), a pulse stretcher (PS) element is perturbed about an axis (such as axis AA of mirror 471 of FIG. 4, for example) in a first direction (S512 of FIG. 5) (such as by the extension of actuator TWA2 of FIG. 4 beyond an initial position).

[0049] With the current axis of the current element perturbed in the first direction, a corresponding beam divergence of a beam or beams passing through the pulse stretcher is measured and recorded (S514). Next the current axis of the current element (such rotation around the axis AA of mirror 471 of FIG. 4, for example) is perturbed in a second direction (opposite the first) (S516) (such as by the retraction of actuator TWA2 of FIG. 4 beyond an initial position). With the current axis of the current element thus perturbed in the second direction, a corresponding beam divergence of a beam or beams passing through the pulse stretcher is measured and recorded (S518 of FIG. 5). If there is another element and axis to test (S520, “yes” branch) (such as rotation of mirror 471 of FIG. 4 around an axis perpendicular to the figure in the direction of arrow Al, for example), the current PS element and axis are updated to the next element and axis to test (S521), and the process returns to S512 of FIG. 5. Ifthere is no further element and axis to test (S520, “no” branch), the subprocess SP570 of dithering and measuring is complete.

[0050] The acts S520 and S521 indicate a temporal relationship in which the various pulse stretcher elements are addressed individually and sequentially within the subprocess S570. In other implementations of a subprocess, multiple optical elements may be adjusted simultaneously, with each iteration of the subprocess S570 resulting in measurements from a combination of dithering adjustments to multiple pulse stretcher elements. For example, instead of dithering with one-by-one adjustments of the optical elements, several optical elements may be simultaneously perturbed. Each successive iteration may apply different amounts of perturbation to the elements. For example, the amounts of adjustments to a set of optical elements may have sinusoidal temporal variations having different temporal frequencies for each of the elements (e.g., 10 Hz for the variations applied to actuator TWA1 from FIG. 4, 15 Hz for the variations applied to actuator TWA2, 20 Hz for the variations applied to actuator TWA5, 30 Hz for the variations applied to actuator TWA6, and 45 Hz for the variations applied to applied to actuator TWA8). The resulting images or other measurements can be captured and Fourier analysis can be used to analyze the responses to the dithering. (For example, if the resulting images indicate a large sinusoidal amplitude of divergence that varies with a frequency of 10 Hz and a lesser sinusoidal amplitude of divergence that varies with frequency 30 Hz, then an inference may be drawn that the adjustments of the corresponding actuators, e.g., TWA1 and TWA6, have corresponding impacts on the beam divergence.)

[0051] Once the subprocess SP570 of dithering and measuring is complete, and the data collected is used to calculate estimated (new) actuator positions for achieving (or moving toward) a minimum divergence, or a target divergence (S524). The actuators are then used to move the mirrors to these updated positions (S526). With the actuators in their respective updated positions, a new or updated baseline divergence is measured and stored (S528) and the (new) divergence is checked against the desired range or distance from a target (S530). If the divergence is within a desired range or within a desired distance from a target (S530, “yes” branch), the process can end. Optionally, the process can return to the start and continue to monitor divergence (S510 and S511). If the divergence is not within a desired range or not within a desired distance from a target (S530, “no” branch), the process returns to repeat the dithering and measuring of subprocess SP570. This repetition can continue until the divergence is within a desired range or within a desired distance from a target (S530, “yes” branch). In various implementations, the repetition can be repeated at relatively slow rates such as 2 Hz, 5 Hz, 10 Hz, 20 Hz, 50 Hz, 100 Hz, for example, or at relatively high rates such as 50 Hz, 100 Hz, 200 Hz, 500 Hz, 1 kHz, for example. Slower operations may, for example, use long-throw actuators, such as stepper motors, for example, suitable for general alignment activities. Faster operations may, for example, use short-throw actuators, such as piezoelectric actuators, for example, suitable for divergence control. In various applications, slower operations may be used during setup and calibration operations. In various applications, faster operations with smaller dithering amplitudesmay be used concurrently with lithographic production without affecting the performance of the lithographic process.

[0052] FIG. 5A is a flow diagram showing an implementation of a sub-process or portion of the process shown in the flow diagram of FIG. 5. Specifically, FIG. 5 A shows a flow diagram of a subprocess S524SP, which can be considered an alternate or more detailed version of act S524 of FIG. 5A. In the subprocess S524SP, a coefficient of divergence variation is calculated for each axis of each steerable element of the pulse stretcher (S524a). Then, using the calculated coefficients, actuator positions (or element positions) for moving divergence to or toward a desired range or target ae estimated and stored.

[0053] A process of the general type of process P510 of FIG. 5, expressed in other terms, can include (1) measuring and recording an initial divergence of a beam or beams passed through a pulse stretcher that has pulse stretcher elements in the form of steerable mirrors, while the steerable mirrors are in respective initial angular positions; (2) separately and bidirectionally dithering each degree of freedom of each steerable mirror of the pulse stretcher and measuring and recording corresponding divergences of a beam or beams passed through the pulse stretcher; (3) determining, for each steerable mirror, respective new angular positions calculated to result in a divergence, in a beam or beams passed through the pulse stretcher, either (a) within a target divergence range or (a) closer than the initial divergence to the target divergence range or to a divergence target; and (4) controlling the actuators to position each respective steerable mirror of the pulse stretcher at its respective new angular position and measuring and recording a new (new baseline) divergence of a beam or beams passed through the pulse stretcher. The pulse stretcher is configured to receive and to pass on in stretched form pulses of a DUV laser. Each steerable mirror of the pulse stretcher can be steerable in one or two degrees of (angular or pointing) freedom.

[0054] The process can be repeated whenever the new divergence is not within a target range or not sufficiently close to a target value. The process can be intentionally iterative (designed to be iterative in most every application), for example, the new angular positions can be chosen to approach calculated ideal positions gradually, such by repeatedly selecting a new angular position that is a given percentage of the angular distance to a calculated ideal position until a minimum step-size of angular change is reached, which is then used until target(s) are reached. Or a selected small step-size of angular change may be used for every new angular position, such that iteration is necessary for any sufficiently large change in position. In implementations, such as implementations in which the process is intended to seek minimum divergence, the process can iterate until the current (or “new”) divergence is sufficiently close to the previous divergence, such as within 1, 0.5, 0.1 or 0.05 percent, for example, of the previous divergence, such that further improvement may be unlikely.

[0055] In implementations, the task of separately and bidirectionally dithering each degree of freedom of each steerable mirror of the pulse stretcher can be accomplished by dithering separately in time (such as stepping through each degree of freedom of each steerable mirror, in order across time),or by dithering separately in frequency. In other words, each degree of freedom of each steerable mirror can be dithered at a different frequency, and video and / or images and / or other sensor signals indicative of the resulting divergence changes can be captured overtime. By frequency-fdtering the changes in the image data, the effects of individual degrees of freedom can be detected and approximate ideal mirror positions can be calculated accordingly. If steering angular displacements (perturbances) are sufficiently small (and lithographic exposure system sensitivity sufficiently low to the resulting variation), this process can even allow essentially real-time optimization of divergence and other beam properties during operation of the DUV laser system. Actuators such as actuators TWA7 and TWA8 with respective piezoelectric actuators PAI and PA2 at their tips can be used for this type of dithering, for example.

[0056] Some implementations can further include measuring and recording an initial value or initial values of one or more near-field properties of one or more beams passed through the pulse stretcher together with measuring and recording corresponding values of the one or more near-field properties with each mirror and axis perturbed in two opposing directions. A near-field sensor such as near-field sensor 362 of FIG. 3 can be used to detect or measure the one or more near-field properties, for example. The determined respective new angular positions for each steerable mirror can then be calculated to result in (A) a divergence either (1) within a target divergence range or (2) closer to the target divergence range, or to a target divergence value, than the initial divergence, and (B) one or more of the one or more near-field properties either (1) within a target range or (2) closer to a target range, or to a target value, than the respective initial corresponding value, in a beam or beams passed through the pulse stretcher. In this way divergence can be adjusted or optimized simultaneously with optimizing or at least holding within desired limits one or more near-field properties. Such near-field properties can include one or more of beam footprint, beam uniformity or spatial profile, and beam symmetry, for example.

[0057] Alternatively or additionally, some implementations can further include measuring and recording an initial value or initial values of one or more far-field properties other than divergence, and measuring and recording corresponding values of the one or more far-field properties with each mirror and axis perturbed in two opposing directions. The respective new angular positions for each steerable mirror can then be calculated to result in (A) a divergence either (1) within a target divergence range or (2) closer to the target divergence range, or to a target divergence value, than the initial divergence, and (B) a stasis of, or an improvement of, one or more of the one or more far-field properties, in a beam or beams passed through the pulse stretcher. The one or more far-field properties can include one or more of beam angular space profile and beam pointing, for example.

[0058] In other aspects, the dithering and measurement process can include additional features. For example, in a process in which divergence is to be minimized, if divergence measurements corresponding to the positive and negative perturbations of a given mirror and axis both indicate more divergence than an initially measured divergence value, the initial mirror position forthat axis may beideal or at or near a minimum in its contribution to divergence. In this situation, additional measurements can optionally be taken at one or more greater values of angular perturbations than for the initial perturbed measurements. In this way it can be possible to detect whether the initial or current position is merely a local minimum. If such additional measurements detect an even lower minimum to one side of the initial position of the axis, a new angular position at the corresponding to the position producing the lower minimum can be selected when new positions for the mirrors are calculated.

[0059] Various techniques may be used to allocate computational resources to the tasks involved in a beam divergence control procedure, such as process P510. For example, a control unit (such as control unit 364 from FIG. 3) may be configured with a microcontroller and / or a microprocessor and / or a field-programmable gate array (FPGA) and / or an application-specific integrated circuit (ASIC) with sufficient memory and processing / clock speed to execute one iteration of the procedure in in a suitably short time, such as 2 milliseconds, 5 milliseconds, 10 milliseconds, or 20 milliseconds, for example. Alternatively, or in addition, the control unit may include a multi -core microcontroller or microprocessor computing system with sufficient memory and processing capability and clock speed to distribute the various tasks of the procedure among various processing cores. For example, a 4-core processor may be used in various implementations with each core assigned to implement a different function in parallel. For example, one core may be assigned with programming instructions to process camera images and calculate the location (e.g., beam centroids) and dimensions of beam profiles in support of acts S514 and S518 (FIG. 5). Such operations may include background subtraction, background gradient removal using best-fit plane levelling, removal of DC offsets, removal of recurrent artifacts, and / or adjustments of sensor gain values, for example. A second core may be assigned with programming instructions to calculate and store desired target settings for actuators, in support of act S524. A third core may support communication among the cores and other components of the control unit and / or other components of a laser system. A fourth core may maintain and monitor input-output operations, one or more user interfaces, and one or more command and reporting interfaces (such as a communications interface with a lithographic system (such as photolithography exposure apparatus 106 from FIG. 1).

[0060] Aspects and implementations of the present disclosure can be further described using the following numbered clauses:1. A deep ultraviolet (DUV) laser system including: a pulse stretcher, the pulse stretcher configured to receive and pass through DUV beams, the pulse stretcher including multiple steerable mirrors steerable by actuators, each steerable mirror being steerable in one or more angular degrees of freedom; a sensor positioned and configured to measure a divergence of beams passed through the pulse stretcher; and a controller connected to the sensor and to the actuators, the controller configured to (1) using the sensor, measure and record an initial divergence of a beam or beams passed through the pulse stretcher with the steerable mirrors in respective initial angular positions, (2) using theactuators, separately dither each degree of freedom of each steerable mirror of the pulse stretcher and, using the sensor, measure and record corresponding divergences of a beam or beams passed through the pulse stretcher, (3) calculate, based on the respective initial angular positions, on the initial divergence, based on the dithering, and based on the corresponding divergences, respective new angular positions for each steerable mirror to produce a divergence, in a beam or beams passed through the pulse stretcher, either (a) within a target divergence range or (b) closer to the target divergence range, or to a divergence target, than the initial divergence, or both, and (4) using the actuators, position each respective steerable mirror of the pulse stretcher at its respective new angular position and, (5) using the sensor, measure and record a new divergence of a beam or beams passed through the pulse stretcher.2. The DUV laser system of clause 1, wherein the controller is configured to, using the actuators, separately and bidirectionally dither each degree of freedom of each steerable mirror of the pulse stretcher.3. The DUV laser system of clause 1, wherein the controller is further configured to iterate actions (1) through (5), with the new divergence from the previous iteration used as the initial divergence of the current iteration, until the new divergence of the current iteration is within the target divergence range.4. The DUV laser system of clause 1, wherein the controller is further configured to iterate actions (1) through (5), with the new divergence from the previous iteration used as the initial divergence of the current iteration, until the new divergence of the current iteration is within 0.1 percent the new divergence of the previous iteration.5. The DUV laser system of clause 1, wherein the controller is configured to separately and bidirectionally dither each degree of freedom of each steerable mirror by dithering each degree of freedom of each steerable mirror separately in time.6. The DUV laser system of clause 1, wherein the controller is configured to separately and bidirectionally dither each degree of freedom of each steerable mirror by dithering each degree of freedom of each steerable mirror separately in frequency.7. The DUV laser system of clause 1, wherein the pulse stretcher includes at least four steerable mirrors.8. The DUV laser system of clause 1, wherein each steerable mirror is steerable in two angular degrees of freedom.9. The DUV laser system of clause 1, wherein the sensor is positioned and configured to sense the properties, of a beam passed through the pulse stretcher, at a far-field of the beam.10. The DUV laser system of clause 1, further including a second sensor connected to the controller and positioned and configured to measure one or more near-field properties of beams passed through the pulse stretcher; and wherein the controller is further configured to: measure and record an initial value or initial values of the one or more near-field properties of one or more beams passed throughthe pulse stretcher; during dithering measure and record corresponding values of the one or more near-field properties of one or more beams passed through the pulse stretcher; calculate the respective new angular positions for each steerable mirror to produce (A) a divergence either (1) within a target divergence range or (2) closer to the target divergence range, or to a target divergence value, than the initial divergence, and (B) one or more of the one or more near-field properties either (1) within a target range or (2) closer to a target range, or to a target value, than the respective initial corresponding value, in a beam or beams passed through the pulse stretcher.11. The DUV laser system of clause 10, wherein the one or more near-field properties include one or more of beam footprint, beam uniformity or spatial profile, and beam symmetry.12. The DUV laser system of clause 1, wherein the controller is further configured to: measure and record an initial value or initial values of one or more far-field properties other than divergence of one or more beams passed through the pulse stretcher; measure and record corresponding values of the one or more far-field properties of one or more beams passed through the pulse stretcher; calculate the respective new angular positions for each steerable mirror to produce (A) a divergence either (1) within a target divergence range or (2) closer to the target divergence range, or to a target divergence value, than the initial divergence, and (B) a stasis of, or an improvement of, one or more of the one or more far-field properties, in a beam or beams passed through the pulse stretcher.13. The DUV laser system of clause 12, wherein the one or more far-field properties include one or more of beam angular space profile and beam pointing.14. A process of operating a deep ultraviolet (DUV) laser system having a pulse stretcher, the pulse stretcher configured to receive and pass through DUV beams and including multiple steerable mirrors steerable by actuators controlled by a controller, each steerable mirror being steerable in one or more angular degrees of freedom, the process including: measuring and recording an initial divergence of a beam or beams passed through the pulse stretcher with the steerable mirrors in respective initial angular positions; separately and dithering each degree of freedom of each steerable mirror of the pulse stretcher and measuring and recording corresponding divergences of a beam or beams passed through the pulse stretcher; determining, based on the respective initial angular positions, the initial divergence, the dithering, and the corresponding divergences, respective new angular positions, for each steerable mirror, calculated to result in a divergence, in a beam or beams passed through the pulse stretcher, either (1) within a target divergence range or (2) closer to the target divergence range or to a divergence target than the initial divergence; and controlling the actuators to position each respective steerable mirror of the pulse stretcher at its respective new angular position and measuring and recording a new divergence of a beam or beams passed through the pulse stretcher.15. The process of clause 14, wherein the dithering includes bidirectional dithering.16. The process of clause 14, further including iterating the process, with the new divergence from the previous iteration of the process used as the initial divergence of the current iteration of theprocess, until the new divergence of the current iteration of the process is within the target divergence range.17. The process of clause 14, further including iterating the process with the new divergence of the previous iteration of the process used as the initial divergence of the current iteration of the process until the new divergence of the current iteration of the process within 0.1 percent of the new divergence of the previous iteration.18. The process of clause 14, wherein separately and dithering each degree of freedom includes dithering each degree of freedom separately in time.19. The process of clause 14, wherein separately and dithering each degree of freedom includes dithering each degree of freedom separately in frequency.20. The process of clause 14, wherein the pulse stretcher has at least four steerable mirrors.21. The process of clause 14, wherein each steerable mirror is steerable in two angular degrees of freedom.22. The process of clause 14, wherein measuring and recording an initial divergence and measuring and recording corresponding divergences both include sensing the properties of a beam passed through the pulse stretcher at a far-field of the beam.23. The process of clause 14, further including: measuring and recording an initial value or initial values of one or more near-field properties of one or more beams passed through the pulse stretcher; and measuring and recording corresponding values of the one or more near-field properties of one or more beams passed through the pulse stretcher; and wherein the determined respective new angular positions for each steerable mirror are calculated to result in (A) a divergence either (1) within a target divergence range or (2) closer to the target divergence range, or to a target divergence value, than the initial divergence, and (B) one or more of the one or more near-field properties either (1) within a target range or (2) closer to a target range, or to a target value, than the respective initial corresponding value, in a beam or beams passed through the pulse stretcher.24. The process of clause 23, wherein the one or more near-field properties include one or more of beam footprint, beam uniformity or spatial profile, and beam symmetry.25. The process of clause 14, further including: measuring and recording an initial value or initial values of one or more far-field properties other than divergence of one or more beams passed through the pulse stretcher; and measuring and recording corresponding values of the one or more far-field properties of one or more beams passed through the pulse stretcher; and wherein the determined respective new angular positions for each steerable mirror are calculated to result in (A) a divergence either (1) within a target divergence range or (2) closer to the target divergence range, or to a target divergence value, than the initial divergence, and (B) a stasis of, or an improvement of, one or more of the one or more far-field properties, in a beam or beams passed through the pulse stretcher.26. The process of clause 25, wherein the one or more far-field properties include one or more of beam angular space profile and beam pointing.

[0061] The above-described aspects and implementations and other implementations are within the scope of the following claims.

Claims

CLAIMS1. A deep ultraviolet (DUV) laser system comprising:a pulse stretcher, the pulse stretcher configured to receive and pass through DUV beams, the pulse stretcher including multiple steerable mirrors steerable by actuators, each steerable mirror being steerable in one or more angular degrees of freedom;a sensor positioned and configured to measure a divergence of beams passed through the pulse stretcher; anda controller connected to the sensor and to the actuators, the controller configured to (1) using the sensor, measure and record an initial divergence of a beam or beams passed through the pulse stretcher with the steerable mirrors in respective initial angular positions, (2) using the actuators, separately dither each degree of freedom of each steerable mirror of the pulse stretcher and, using the sensor, measure and record corresponding divergences of a beam or beams passed through the pulse stretcher, (3) calculate, based on the respective initial angular positions, on the initial divergence, based on the dithering, and based on the corresponding divergences, respective new angular positions for each steerable mirror to produce a divergence, in a beam or beams passed through the pulse stretcher, either (a) within a target divergence range or (b) closer to the target divergence range, or to a divergence target, than the initial divergence, or both, and (4) using the actuators, position each respective steerable mirror of the pulse stretcher at its respective new angular position and, (5) using the sensor, measure and record a new divergence of a beam or beams passed through the pulse stretcher.

2. The DUV laser system of claim 1, wherein the controller is configured to, using the actuators, separately and bidirectionally dither each degree of freedom of each steerable mirror of the pulse stretcher.

3. The DUV laser system of claim 1, wherein the controller is further configured to iterate actions (1) through (5), with the new divergence from the previous iteration used as the initial divergence of the current iteration, until the new divergence of the current iteration is within the target divergence range.

4. The DUV laser system of claim 1, wherein the controller is further configured to iterate actions (1) through (5), with the new divergence from the previous iteration used as the initial divergence of the current iteration, until the new divergence of the current iteration is within 0.1 percent the new divergence of the previous iteration.

5. The DUV laser system of claim 1, wherein the controller is configured to separately and bidirectionally dither each degree of freedom of each steerable mirror by dithering each degree of freedom of each steerable mirror separately in time.

6. The DUV laser system of claim 1, wherein the controller is configured to separately and bidirectionally dither each degree of freedom of each steerable mirror by dithering each degree of freedom of each steerable mirror separately in frequency.

7. The DUV laser system of claim 1, wherein the pulse stretcher includes at least four steerable mirrors.

8. The DUV laser system of claim 1, wherein each steerable mirror is steerable in two angular degrees of freedom.

9. The DUV laser system of claim 1, wherein the sensor is positioned and configured to sense the properties, of a beam passed through the pulse stretcher, at a far-field of the beam.

10. The DUV laser system of claim 1, further comprising a second sensor connected to the controller and positioned and configured to measure one or more near-field properties of beams passed through the pulse stretcher; and wherein the controller is further configured to: measure and record an initial value or initial values of the one or more near-field properties of one or more beams passed through the pulse stretcher; during dithering measure and record corresponding values of the one or more near-field properties of one or more beams passed through the pulse stretcher; calculate the respective new angular positions for each steerable mirror to produce (A) a divergence either (1) within a target divergence range or (2) closer to the target divergence range, or to a target divergence value, than the initial divergence, and (B) one or more of the one or more near-field properties either (1) within a target range or (2) closer to a target range, or to a target value, than the respective initial corresponding value, in a beam or beams passed through the pulse stretcher.

11. The DUV laser system of claim 10, wherein the one or more near-field properties comprise one or more of beam footprint, beam uniformity or spatial profile, and beam symmetry.

12. The DUV laser system of claim 1, wherein the controller is further configured to: measure and record an initial value or initial values of one or more far-field properties other than divergence of one or more beams passed through the pulse stretcher; measure and record corresponding values of the one or more far-field properties of one or more beams passed through the pulse stretcher; calculate the respective new angular positions for each steerable mirror to produce (A) a divergence either (1)within a target divergence range or (2) closer to the target divergence range, or to a target divergence value, than the initial divergence, and (B) a stasis of, or an improvement of, one or more of the one or more far-field properties, in a beam or beams passed through the pulse stretcher.

13. The DUV laser system of claim 12, wherein the one or more far-field properties comprise one or more of beam angular space profile and beam pointing.

14. A process of operating a deep ultraviolet (DUV) laser system having a pulse stretcher, the pulse stretcher configured to receive and pass through DUV beams and including multiple steerable mirrors steerable by actuators controlled by a controller, each steerable mirror being steerable in one or more angular degrees of freedom, the process comprising:measuring and recording an initial divergence of a beam or beams passed through the pulse stretcher with the steerable mirrors in respective initial angular positions;separately and dithering each degree of freedom of each steerable mirror of the pulse stretcher and measuring and recording corresponding divergences of a beam or beams passed through the pulse stretcher;determining, based on the respective initial angular positions, the initial divergence, the dithering, and the corresponding divergences, respective new angular positions, for each steerable mirror, calculated to result in a divergence, in a beam or beams passed through the pulse stretcher, either (1) within a target divergence range or (2) closer to the target divergence range or to a divergence target than the initial divergence; andcontrolling the actuators to position each respective steerable mirror of the pulse stretcher at its respective new angular position and measuring and recording a new divergence of a beam or beams passed through the pulse stretcher.

15. The process of claim 14, further comprising iterating the process, with the new divergence from the previous iteration of the process used as the initial divergence of the current iteration of the process, until the new divergence of the current iteration of the process is within the target divergence range.

16. The process of claim 14, further comprising iterating the process with the new divergence of the previous iteration of the process used as the initial divergence of the current iteration of the process until the new divergence of the current iteration of the process within 0.1 percent of the new divergence of the previous iteration.

17. The process of claim 14, wherein separately and dithering each degree of freedom comprises dithering each degree of freedom separately in time.

18. The process of claim 14, wherein separately and dithering each degree of freedom comprises dithering each degree of freedom separately in frequency.

19. The process of claim 14, further comprising: measuring and recording an initial value or initial values of one or more near-field properties of one or more beams passed through the pulse stretcher; and measuring and recording corresponding values of the one or more near-field properties of one or more beams passed through the pulse stretcher; and wherein the determined respective new angular positions for each steerable mirror are calculated to result in (A) a divergence either (1) within a target divergence range or (2) closer to the target divergence range, or to a target divergence value, than the initial divergence, and (B) one or more of the one or more near-field properties either (1) within a target range or (2) closer to a target range, or to a target value, than the respective initial corresponding value, in a beam or beams passed through the pulse stretcher.

20. The process of claim 14, further comprising: measuring and recording an initial value or initial values of one or more far-field properties other than divergence of one or more beams passed through the pulse stretcher; and measuring and recording corresponding values of the one or more far-field properties of one or more beams passed through the pulse stretcher; and wherein the determined respective new angular positions for each steerable mirror are calculated to result in (A) a divergence either (1) within a target divergence range or (2) closer to the target divergence range, or to a target divergence value, than the initial divergence, and (B) a stasis of, or an improvement of, one or more of the one or more far-field properties, in a beam or beams passed through the pulse stretcher.

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