WO2025140811A1

WO2025140811A1 - Extreme ultraviolet light generation sequence for an extreme ultraviolet light source

Info

Publication number: WO2025140811A1
Application number: PCT/EP2024/083488
Authority: WO
Inventors: Giulia PIOVAN; Steven Chang; Robert Jay RAFAC; Daniel Leslie HALL; Haining Wang; Damon Erfan GHETMIRI; Eric Nicholas HAHN; Michael Anthony PURVIS; Daniel John William BROWN; Yezheng Tao
Original assignee: ASML Netherlands BV
Current assignee: ASML Netherlands BV
Priority date: 2023-12-29
Filing date: 2024-11-25
Publication date: 2025-07-03
Anticipated expiration: 2026-06-29
Also published as: TW202534426A

Abstract

A method for selecting operating parameters of an EUV light source includes selecting a temporal or spatial relationship between a plurality of illuminations of a target by first, second, and third laser beams. The method further includes setting an initial value of a temporal delay between the second laser beam and the third laser beam. The method further includes setting an initial value of an energy of the second laser beam. The method further includes measuring an EUV energy generated by the illuminations of the target based on the initial value of the temporal delay between the second laser beam and the third laser beam and the initial value of the energy of the second laser beam. The method further includes adjusting the temporal delay between the second laser beam and the third laser beam and the energy of the second laser beam in response to the measuring.

Description

EXTREME ULTRAVIOLET LIGHT GENERATION SEQUENCE FOR AN EXTREME ULTRAVIOLET LIGHT SOURCE

CROSS-REFERENCE TO RELATED APPLICATIONS

[0001] This application claims priority of US Application No. 63/616,024, filed on December 29, 2023, titled LASER BEAM POSITIONING SEQUENCE FOR EXTREME ULTRAVIOLET LIGHT SOURCE WITH INDEPENDENT LASER BEAMS, and US Application No. 63/717,932, filed on November 8, 2024, titled EXTREME ULTRAVIOLET LIGHT GENERATION SEQUENCE FORAN EXTREME ULTRAVIOLET LIGHT SOURCE, which are incorporated herein by reference in their entirety.

FIELD

[0002] The present application relates to extreme ultraviolet (“EUV”) radiation sources and methods thereof. EUV radiation can be used as, for example, exposure radiation in a lithographic process to fabricate semiconductor devices.

BACKGROUND

[0003] A lithographic apparatus is a machine that applies a desired pattern onto a substrate, usually onto a target portion of the substrate. A lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In that instance, a patterning device, which can be a mask or a reticle, can be used to generate a circuit pattern to be formed on an individual layer of the IC. This pattern can be transferred onto a target portion (e.g., comprising part of, one, or several dies) on a substrate (e.g., a silicon wafer). Transfer of the pattern is typically via imaging onto a layer of radiationsensitive material (photoresist or simply “resist”) provided on the substrate. In general, a single substrate will contain a network of adjacent target portions that are successively patterned. Known lithographic apparatuses include so-called steppers, in which each target portion is irradiated by exposing an entire pattern onto the target portion at one time, and so-called scanners, in which each target portion is irradiated by scanning the pattern through a radiation beam in a given direction (the “scanning”- direction) while synchronously scanning the target portions parallel or anti-parallel to this scanning direction. It is also possible to transfer the pattern from the patterning device to the substrate by imprinting the pattern onto the substrate.

[0004] A lithographic apparatus typically includes an illumination system that conditions radiation generated by a radiation source before the radiation is incident upon a patterning device. A patterned beam of EUV light can be used to produce extremely small features on a substrate. EUV light (also sometimes referred to as soft x-rays) is generally defined as electromagnetic radiation having wavelengths in the range of about 5-100 nm. One particular wavelength of interest for photolithography occurs at 13.5 nm. [0005] Methods to produce EUV light include, but are not necessarily limited to, converting a source material into a plasma state that has a chemical element with an emission line in the EUV range. These elements can include, but are not necessarily limited to, xenon, lithium and tin. In various implementations, these methods are suitable for use in the production of microelectronic circuits and/or semiconductor devices.

[0006] In one such method, often termed laser-produced plasma (“LPP”), the desired plasma can be produced by irradiating a source material, for example, in the form of a droplet, stream or wire, with a laser beam.

[0007] One technique for producing a laser-produced plasma involves irradiating a source material, often a droplet, with a series of laser pulses. A first pulse from a first laser, often termed a pre-pulse, can expand the droplet into a disc-like shape to form a so called “tin target”, or simply “target”. A second pulse from a second laser, often termed a rarefication pulse or a rarefaction pulse, can change the density of the target, resulting in a “rarefied target”. Lastly, a third pulse from a third laser can irradiate the rarefied target to generate a plasma.

[0008] To maximize energy generated per pulse and stability of generated EUV light, each of the three laser beams must be properly aligned with the droplet and target. Additionally, various operating parameters of an EUV light source (e.g., beam characteristics, droplet characteristics, etc.) can be set with countless permutations for generating EUV light. If the three laser beams are misaligned and/or the EUV light source parameters are set improperly, an EUV light source can produce lower and more unstable power, resulting in dose errors at the wafer level.

SUMMARY

[0009] Accordingly, it is desirable to determine a sequence for aligning three laser beams in an EUV light source and setting operating parameters for the three laser beams such that EUV power and stability are maximized.

[0010] In some aspects, a method for selecting operating parameters of an EUV light source can include selecting a temporal or spatial relationship between a plurality of illuminations of a target by a first laser beam, a second laser beam, and a third laser beam. In some aspects, the method can further include setting an initial value of a temporal delay between the second laser beam and the third laser beam. In some aspects, the method can further include setting an initial value of an energy of the second laser beam. In some aspects, the method can further include measuring an EUV energy generated by the illuminations of the target based on the initial value of the temporal delay between the second laser beam and the third laser beam and the initial value of the energy of the second laser beam. In some aspects, the method can further include adjusting the temporal delay between the second laser beam and the third laser beam and the energy of the second laser beam in response to the measuring.

[0011] In some aspects, an EUV light source can include one or more laser sources, a measurement system, and a controller. In some aspects, the one or more laser sources can be configured to illuminate a target with a first laser beam, a second laser beam, and a third laser beam. In some aspects, the measurement system can be configured to measure an EUV energy generated by the illuminations of the target. In some aspects, the controller can be configured to select a temporal or spatial relationship between a plurality of illuminations of the target by the first laser beam, the second laser beam, and the third laser beam. In some aspects, the controller can be further configured to set an initial value of a temporal delay between the second laser beam and the third laser beam. In some aspects, the controller can be further configured to set an initial value of an energy of the second laser beam. In some aspects, the controller can be further configured to measure, with the measurement system, the EUV energy generated by the illuminations of the target based on the initial value of the temporal delay between the second laser beam and the third laser beam and the initial value of the energy of the second laser beam. In some aspects, the controller can be further configured to adj ust the temporal delay between the second laser beam and the third laser beam and the energy of the second laser beam in response to the measuring.

[0012] In some aspects, a lithographic apparatus can include an EUV light source. In some aspects, the EUV light source can include one or more laser sources, a measurement system, and a controller. In some aspects, the one or more laser sources can be configured to illuminate a target with a first laser beam, a second laser beam, and a third laser beam. In some aspects, the measurement system can be configured to measure an EUV energy generated by the illuminations of the target. In some aspects, the controller can be configured to select a temporal or spatial relationship between a plurality of illuminations of the target by the first laser beam, the second laser beam, and the third laser beam. In some aspects, the controller can be further configured to set an initial value of a temporal delay between the second laser beam and the third laser beam. In some aspects, the controller can be further configured to set an initial value of an energy of the second laser beam. In some aspects, the controller can be further configured to measure, with the measurement system, the EUV energy generated by the illuminations of the target based on the initial value of the temporal delay between the second laser beam and the third laser beam and the initial value of the energy of the second laser beam. In some aspects, the controller can be further configured to adjust the temporal delay between the second laser beam and the third laser beam and the energy of the second laser beam in response to the measuring.

[0013] Further features of various aspects of the present disclosure are described in detail below with reference to the accompanying drawings. It is noted that the present disclosure is not limited to the specific aspects described herein. Such aspects are presented herein for illustrative purposes only. Additional aspects will be apparent to those skilled in the relevant art(s) based on the teachings contained herein.

BRIEF DESCRIPTION OF THE DRAWINGS/FIGURES

[0014] The accompanying drawings, which are incorporated herein and form part of the specification, illustrate the present disclosure and, together with the description, further serve to explain the principles of the present disclosure and to enable those skilled in the relevant art(s) to make and use aspects described herein.

[0015] FIG. 1 shows a reflective lithographic apparatus, according to some aspects.

[0016] FIGS. 2A, 2B, and 3 show more details of a reflective lithographic apparatus, according to some aspects.

[0017] FIG. 4 shows a lithographic cell, according to some aspects.

[0018] FIG. 5 shows a source material delivery system, according to some aspects.

[0019] FIG. 6 shows a method for optimizing positions of laser beams in an EUV light source, according to some aspects.

[0020] FIG. 7 shows a first sequence for optimizing positions of laser beams in an EUV light source, according to some aspects.

[0021] FIG. 8 shows a second sequence for optimizing positions of laser beams in an EUV light source, according to some aspects.

[0022] FIG. 9 shows a third sequence for optimizing positions of laser beams in an EUV light source, according to some aspects.

[0023] FIG. 10 shows a method for selecting operating parameters of an EUV light source, according to some aspects.

[0024] The features of the present disclosure will become more apparent from the detailed description set forth below when taken in conjunction with the drawings, in which like reference characters identify corresponding elements throughout. In the drawings, like reference numbers generally indicate identical, functionally similar, and/or structurally similar elements. Additionally, generally, the leftmost digit(s) of a reference number identifies the drawing in which the reference number first appears. Unless otherwise indicated, the drawings provided throughout the disclosure should not be interpreted as to-scale drawings.

DETAILED DESCRIPTION

[0025] The aspects described herein, and references in the specification to “one aspect,” “an aspect,” “an exemplary aspect,” “an example aspect,” etc., indicate that the aspects described can include a particular feature, structure, or characteristic, but every aspect may not necessarily include the particular feature, structure, or characteristic. Moreover, such phrases are not necessarily referring to the same aspect. Further, when a particular feature, structure, or characteristic is described in connection with an aspect, it is understood that it is within the knowledge of those skilled in the art to effect such feature, structure, or characteristic in connection with other aspects whether or not explicitly described.

[0026] Spatially relative terms, such as “beneath,” “below,” “lower,” “above,” “on,” “upper” and the like, can be used herein for ease of description to describe one element or feature’s relationship to another element(s) or feature(s) as illustrated in the figures. The spatially relative terms are intended to encompass different orientations of the device in use or operation in addition to the orientation depicted in the figures. The apparatus can be otherwise oriented (rotated 90 degrees or at other orientations) and the spatially relative descriptors used herein can likewise be interpreted accordingly.

[0027] The terms “about,” “approximately,” or the like can be used herein indicates the value of a given quantity that can vary based on a particular technology. Based on the particular technology, the terms “about,” “approximately,” or the like can indicate a value of a given quantity that varies within, for example, 10-30% of the value (e.g., ±10%, ±20%, or ±30% of the value).

[0028] Aspects of the present disclosure can be implemented in hardware, firmware, software, or any combination thereof. Aspects of the disclosure can also be implemented as instructions stored on a computer-readable medium, which can be read and executed by one or more processors. A machine- readable medium can 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 can comprise read only memory (ROM); random access memory (RAM); magnetic disk storage media; optical storage media; flash memory devices; electrical, optical, acoustical or other forms of propagated signals (e.g., carrier waves, infrared signals, digital signals, etc.), and others. Furthermore, firmware, software, routines, and/or instructions can be described herein as performing certain actions. However, it should be appreciated that such descriptions are merely for convenience and that such actions result from computing devices, processors, controllers, or other devices executing the firmware, software, routines, instructions, etc. The term “machine-readable medium” can be interchangeable with similar terms, for example, “computer program product,” “computer-readable medium,” “non-transitory computer- readable medium,” or the like. The term “non-transitory” can be used herein to characterize one or more forms of computer readable media except for a transitory, propagating signal.

[0029] Before describing such aspects in more detail, however, it is instructive to present an example environment in which aspects of the present disclosure can be implemented.

[0030] Example Lithographic Systems

[0031] FIG. 1 shows a lithographic apparatus 100 in which aspects of the present disclosure can be implemented. In some aspects, lithographic apparatus 100 can comprise the following: an illumination system (illuminator) IL configured to condition a radiation beam B (for example, deep ultra violet or extreme ultra violet radiation); a support structure (for example, a mask table) MT configured to support a patterning device (for example, a mask, a reticle, or a dynamic patterning device) MA and connected to a first positioner PM configured to accurately position patterning device MA; and, a substrate table (for example, a wafer table) WT configured to hold a substrate (for example, a resist coated wafer) W and connected to a second positioner PW configured to accurately position substrate W. Lithographic apparatus 100 also comprises a projection system PS configured to project a pattern imparted to radiation beam B by patterning device MA onto a target portion (for example, comprising one or more dies) C of substrate W. In lithographic apparatus 100, patterning device MA and the projection system PS are reflective. [0032] Illumination system IL can comprise various types of optical components, such as refractive, reflective, catadioptric, magnetic, electromagnetic, electrostatic, or other types of optical components, or any combination thereof, for directing, shaping, or controlling the radiation beam B. Illumination system IL can also comprise a sensor ES that provides a measurement of, for example, one or more of energy per pulse, photon energy, intensity, average power, and the like. Illumination system IL can comprise a measurement sensor MS for measuring a movement of radiation beam B and a uniformity compensator UC that allow an illumination slit uniformity to be controlled. Measurement sensor MS can also be disposed at other locations. For example, measurement sensor MS can be on or near substrate table WT.

[0033] In some aspects, support structure MT can support patterning device MA in a manner that depends on the orientation of the patterning device MA with respect to a reference frame, the design of lithographic apparatus 100, and other conditions, such as whether or not the patterning device MA is held in a vacuum environment. Support structure MT can implement mechanical, vacuum, electrostatic, or other clamping techniques to hold the patterning device MA. Support structure MT can be a frame or a table. Support structure MT can be fixed or movable. By using sensors, support structure MT can ensure that patterning device MA is at a desired position (e.g., a given position with respect to the projection system PS).

[0034] The term “patterning device” can be used herein to refer to any device that can be used to impart a radiation beam B with a pattern in its cross-section, such as to create a pattern in target portion C of substrate W. The pattern imparted to radiation beam B can correspond to a particular functional layer in a device being created in target portion C to form an integrated circuit.

[0035] Patterning device MA can be reflective. Examples of patterning devices MA include reticles, masks, programmable mirror arrays, or programmable LCD panels. Masks can include different mask types, such as binary, alternating phase shift, or attenuated phase shift, as well as various hybrid mask types. An example of a programmable mirror array employs a matrix arrangement of small mirrors, each of which can be individually tilted so as to reflect an incoming radiation beam in different directions. The tilted mirrors can impart a pattern in radiation beam B, which is reflected by a matrix of small mirrors.

[0036] In some aspects, the term “projection system” can be used herein to refer to any type of projection system, including refractive, reflective, catadioptric, magnetic, electromagnetic and electrostatic optical systems, or any combination thereof, as appropriate for the exposure radiation being used, or for other factors, such as the use of an immersion liquid on the substrate W or the use of a vacuum. Atmospheric gas can absorb EUV or electrons used for exposing a substrate. Therefore, a vacuum environment can be used for EUV or electron beam radiation. A vacuum environment can be provided to the whole beam path with the aid of a vacuum wall and vacuum pumps.

[0037] Lithographic apparatus 100 can be of a type having two (dual stage) or more substrate tables WT (and/or two or more mask tables). In such “multiple stage” machines, additional substrate tables WT can be used in parallel, or preparatory steps can be carried out on one or more tables while one or more other substrate tables WT are being used for exposure. In some situations, the additional table may be different from substrate table WT.

[0038] In some aspects, lithographic apparatus 100 can be of a type in which at least a portion of the substrate can be covered by a liquid having a relatively high refractive index, e.g., water, so as to fdl a space between the projection system and the substrate. An immersion liquid can also be applied to other spaces in the lithographic apparatus, for example, between the mask and the projection system. Immersion techniques can increase the numerical aperture of projection systems. The term “immersion” as used herein does not mean that a structure, such as a substrate, must be submerged in liquid. For example, a liquid can be located between the projection system and the substrate during exposure.

[0039] Illuminator IL can receive a radiation beam from a radiation source SO. Source SO and lithographic apparatus 100 can be separate physical entities. In such cases, source SO is not considered to be part of lithographic apparatus 100 and radiation beam B can pass from source SO to illuminator IL with the aid of a beam delivery system (not shown), which can include, for example, suitable directing mirrors and/or a beam expander. In other cases, source SO can be an integral part of the lithographic apparatus 100. A radiation system can comprise source SO, illuminator IL, and/or beam delivery system BD.

[0040] In some aspects, illuminator IL can be used to condition radiation beam B to have a desired uniformity and intensity distribution in its cross section. The desired uniformity of radiation beam B can be maintained by using uniformity compensator UC. Uniformity compensator UC can comprise a plurality of protrusions (e.g., fingers) that can be adjusted in the path of radiation beam B to control the uniformity of radiation beam B. Measurement sensor MS can be used to monitor the uniformity of radiation beam B.

[0041] Radiation beam B can be incident on patterning device MA, which is held on the support structure MT, and in this manner, radiation beam B can be patterned by the patterning device MA. In lithographic apparatus 100, radiation beam B can be reflected from the patterning device (for example, mask) MA. After being reflected from the patterning device MA, radiation beam B can pass through projection system PS, which focuses the radiation beam B onto a target portion C of the substrate W. With the aid of the second positioner PW and position sensor IF2 (for example, an interferometric device, linear encoder, or capacitive sensor), substrate table WT can be moved accurately (for example, so as to position different target portions C in the path of the radiation beam B). Similarly, first positioner PM and another position sensor IF1 can be used to accurately position the patterning device (for example, mask) MA with respect to the path of the radiation beam B. Patterning device MA and substrate W can be aligned using mask alignment marks Ml, M2 and substrate alignment marks Pl, P2.

[0042] In some aspects, lithographic apparatus 100 can be used in at least one of the following modes: [0043] 1. In step mode, support structure MT and substrate table WT can be kept essentially stationary, while an entire pattern imparted to radiation beam B is projected onto a target portion C at one time (e.g., a single static exposure). Substrate table WT can then be shifted in the X and/or Y direction so that a different target portion C can be exposed.

[0044] 2. In scan mode, support structure MT and substrate table WT can be scanned synchronously while a pattern imparted to radiation beam B is projected onto a target portion C (e.g., a single dynamic exposure). The velocity and direction of substrate table WT relative to support structure MT can be determined by (de-)magnification and image reversal characteristics of projection system PS.

[0045] 3. In another mode, support structure MT can be kept substantially stationary holding a programmable patterning device, and substrate table WT can be moved or scanned while a pattern imparted to radiation beam B is projected onto a target portion C. A pulsed radiation source SO can be employed and the programmable patterning device is updated after each movement of substrate table WT or in between successive radiation pulses during a scan. This mode of operation can be readily applied to maskless lithography that utilizes a programmable patterning device, such as a programmable mirror array.

[0046] Combinations and/or variations on the described modes of use or entirely different modes of use can also be employed.

[0047] In some aspects, lithographic apparatus 100 can comprise an EUV radiation source configured to generate a beam of EUV radiation for EUV lithography. The EUV radiation source can be configured in a radiation system, and a corresponding illumination system is configured to condition the EUV radiation beam of the EUV source.

[0048] FIG. 2A shows different view of lithographic apparatus 100, including source SO (e.g., source collector apparatus), illumination system IL, and projection system PS, according to some aspects. Source SO is constructed and arranged such that a vacuum environment can be maintained in an enclosing structure 220 of source SO. An EUV radiation emitting plasma 210 can be formed by a discharge-generated plasma source. In some aspects, a plasma of excited tin (Sn) (e.g., excited via a laser) is used to produce EUV radiation.

[0049] The radiation emitted by the EUV radiation emitting plasma 210 can be passed from a source chamber 211 into a collector chamber 212 via an optional gas barrier or contaminant trap 230 (in some cases also referred to as contaminant barrier or foil trap), which is positioned in or behind an opening in source chamber 211. Contaminant trap 230 can comprise a channel structure. Contamination trap 230 can also comprise a gas barrier and/or a channel structure.

[0050] In some aspects, collector chamber 212 can comprise a radiation collector CO. Radiation collector CO can be a so-called grazing incidence collector. Radiation collector CO can comprise an upstream radiation collector side 251 and a downstream radiation collector side 252. Radiation that traverses radiation collector CO can be reflected off a grating spectral filter 240 to be focused in a virtual source point INTF. Virtual source point INTF can be referred to as the intermediate focus. Source collector apparatus can be arranged such that the intermediate focus INTF is located at or near an opening 219 of enclosing structure 220. The virtual source point INTF can be an image of the EUV radiation emitting plasma 210. Grating spectral fdter 240 can be used for suppressing infrared (IR) radiation.

[0051] Subsequently, the radiation traverses the illumination system IL. Illumination system IL can include a faceted field mirror device 222 and a faceted pupil mirror device 224 arranged to provide a desired angular distribution of radiation beam 221, at patterning device MA, as well as a desired uniformity of radiation intensity at patterning device MA. Upon reflection of beam of radiation 221 at patterning device MA, held by support structure MT, a patterned beam 226 is formed and the patterned beam 226 is imaged by projection system PS via reflective elements 228, 229 onto substrate W held by the wafer stage or substrate table WT. In some aspects, other configurations of mirrors and/or optical devices can be used to direct radiation beam 221 to patterning device MA.

[0052] More elements than shown can generally be present in illumination system IL and projection system PS. Grating spectral filter 240 can optionally be present, depending upon the type of lithographic apparatus. Further, there can be more mirrors present than those shown in the FIG. 2A, for example there can be one to six additional reflective elements present in the projection system PS than shown in FIG. 2A.

[0053] In some aspects, uniformity compensator UC, sensor ES, and/or measurement sensor MS shown in FIGS. 2A and 2B can be as described above in reference to FIG. 1.

[0054] Collector CO (also called a collector mirror or collector optic), as illustrated in FIG. 2A, is depicted as an example of a nested collector with grazing incidence reflectors 253, 254, and 255. Grazing incidence reflectors 253, 254, and 255 can be disposed axially symmetric around an optical axis O. A collector of this type can be used in combination with a discharge -generated plasma source, often called a DPP source.

[0055] FIG. 2B shows a portion of lithographic apparatus 100 (e.g., FIG. 1), but with alternative collection optics in source SO, according to some aspects. It should be appreciated that structures shown in FIG. 2A that do not appear in FIG. 2B (for drawing clarity) can still be included in aspects referring to FIG. 2B. Elements in FIG. 2B having the same reference numbers as those in FIG. 2A have the same or substantially similar structures and functions as described in reference to FIG. 2A. In some aspects, the lithographic apparatus 100 can be used, for example, to expose a substrate W such as a resist-coated wafer with a patterned beam of EUV illumination. In FIG. 2B, illumination system IL and projection system PS are represented combined as an exposure device 256 (e.g., an integrated circuit lithography tool such as a stepper, scanner, step and scan system, direct write system, device using a contact and/or proximity mask, etc.) that uses EUV light from source SO. Lithographic apparatus 100 can also comprise a collector 258 that reflects EUV light from the EUV radiation emitting plasma 210 along a path into the exposure device 256 to irradiate substrate W. Collector 258 can comprise a near-normal incidence collector mirror having a reflective surface in the form of a prolate spheroid (e.g., an ellipse rotated about its major axis). The prolate spheroid structure can have a graded multi-layer coating with alternating layers of molybdenum and silicon, and in some cases, one or more high temperature diffusion barrier layers, smoothing layers, capping layers and/or etch stop layers.

[0056] FIG. 3 shows a detailed view of a portion of lithographic apparatus 100 (e.g., FIGS. 1, 2A, and 2B), according to one or more aspects. Elements in FIG. 3 having the same reference numbers as those in FIGS. 1, 2A, and 2B have the same or substantially similar structures and functions as described in reference to FIGS. 1, 2A, and 2B. In some aspects, source SO can be a LPP EUV source. Source SO can comprise a laser system 302 for generating a train of light pulses and delivering the light pulses into a light source chamber 212. For the lithographic apparatus 100, the light pulses can travel along one or more beam paths from the laser system 302 and into the chamber 212 to illuminate a source material at an irradiation region 304 to generate a plasma (e.g., plasma region located at EUV radiation emitting plasma 210 in FIG. 2B) that produces EUV light for substrate exposure in the exposure device 256.

[0057] In some aspects, laser system 302 can comprise a pulsed laser device, e.g., a pulsed gas discharge CO2 laser device producing radiation at 9.3 pm or 10.6 pm, e.g., with DC or RF excitation, operating at relatively high power, e.g., 10 kW or higher and high pulse repetition rate, e.g., 50 kHz or more. In some aspects, the laser can be an axial-flow RF-pumped CO2 laser having an oscillator amplifier configuration (e.g., master oscillator/power amplifier (MOPA) or power oscillator/power amplifier (POPA)) with multiple stages of amplification and having a seed pulse that is initiated by a Q-switched oscillator with relatively low energy and high repetition rate, e.g., capable of 100 kHz operation. From the oscillator, the laser pulse can then be amplified, shaped and/or focused before reaching the irradiation region 304. Continuously pumped CO2 amplifiers can be used for the laser system 302. Alternatively, the laser can be configured as a so-called “self-targeting” laser system in which the droplet serves as one mirror of the optical cavity of the laser.

[0058] In some aspects, depending on the application, other types of lasers can also be suitable, e.g., an excimer or molecular fluorine laser operating at high power and high pulse repetition rate. Some examples include, a solid state laser, e.g., having a fiber, rod, slab, or disk-shaped active media, other laser architectures having one or more chambers, e.g., an oscillator chamber and one or more amplifying chambers (with the amplifying chambers in parallel or in series), a master oscillator/power oscillator (MOPO) arrangement, a master oscillator/power ring amplifier (MOPRA) arrangement, or a solid state laser that seeds one or more excimer, molecular fluorine or CO2 amplifier or oscillator chambers, can be suitable. Other suitable designs are envisaged.

[0059] In some aspects, a source material can first be irradiated by a pre -pulse and thereafter irradiated by a main pulse. Pre-pulse and main pulse seeds can be generated by a single oscillator or two separate oscillators. One or more common amplifiers can be used to amplify both the pre-pulse seed and main pulse seed. In some aspects, separate amplifiers can be used to amplify the pre-pulse and main pulse seeds. [0060] In some aspects, a source material can first be irradiated by a pre-pulse, thereafter irradiated by a rarefication pulse, and thereafter irradiated by a main pulse. Pre-pulse, rarefication pulse, and main pulse seeds can be generated by up to three separate oscillators. One or more common amplifiers can be used to amplify the pre-pulse, rarefication pulse, and main pulse seeds.

[0061] In some aspects, source SO can also comprise a beam conditioning unit 306 having one or more optics for beam conditioning, such as expanding, steering, and/or focusing the beam between the laser system 302 and irradiation region 304. For example, a steering system, which can comprise one or more mirrors, prisms, lenses, etc., can be provided and arranged to steer the laser focal spot to different locations in the chamber 212. For example, the steering system can comprise a first flat mirror mounted on a tip-tilt actuator, which can move the first mirror independently in two dimensions, and a second flat mirror mounted on a tip-tilt actuator which can move the second mirror independently in two dimensions. With the described arrangement(s), the steering system can controllably move the focal spot in directions substantially orthogonal to the direction of beam propagation (beam axis or optical axis).

[0062] Beam conditioning unit 306 can comprise a focusing assembly to focus the beam to irradiation region 304 and adjust the position of the focal spot along the beam axis. For the focusing assembly, an optic, such as a focusing lens or mirror, can be used that is coupled to an actuator for movement in a direction along the beam axis to move the focal spot along the beam axis.

[0063] In some aspects, the source SO can also comprise a source material delivery system 308 for delivering source material, such as tin droplets, to irradiation region 304, where the droplets can interact with light pulses from the laser system 302 to produce plasma and generate an EUV emission. The EUV emission is used to expose a substrate such as a resist-coated wafer at exposure device 256. More details regarding various droplet dispenser configurations can be found in, e.g., U.S. Pat. No. 7,872,245, issued on January 18, 2011, titled “Systems and Methods for Target Material Delivery in a Laser Produced Plasma EUV Light Source”, U.S. Pat. No. 7,405,416, issued on July 29, 2008, titled “Method and Apparatus For EUV Plasma Source Target Delivery”, U.S. Pat. No. 7,372,056, issued on May 13, 2008, titled “LPP EUV Plasma Source Material Target Delivery System”, and International Appl. No. WO 2019/137846, titled “Apparatus for and Method of Controlling Coalescence of Droplets In a Droplet Stream”, published on July 18, 2019, the contents of each of which are incorporated by reference herein in their entirety.

[0064] In some aspects, the source material for producing an EUV light output for substrate exposure can include, but is not necessarily limited to, a material that includes tin, lithium, xenon or combinations thereof. The source material can be in the form of liquid droplets and/or solid particles contained within liquid droplets. For example, the element tin can be used as pure tin, as a tin compound, e.g., SnBr4, SnBr2, SnFL, as a tin alloy, e.g., tin-gallium alloys, tin-indium alloys, tin-indium -gallium alloys, or a combination thereof. Depending on the material used, the source material, when sent to irradiation region 304, can be at various temperatures, for example, room temperature or near room temperature (e.g., tin alloys, SnBr4), at an elevated temperature (e.g., pure tin), or at temperatures below room temperature (e.g., SnH ).

[0065] In some aspects, the source SO can also comprise a controller 310 and/or a drive laser control system 312 for controlling devices in laser system 302 to generate light pulses for delivery into the chamber 212 and/or for controlling movement of optics in beam conditioning unit 306. Source SO can also comprise a droplet position detection system which can comprise one or more droplet imagers 314 that provide an output signal indicative of the position of one or more droplets (e.g., to ensure that droplets arrive on target at irradiation region 304). The droplet imager(s) 314 can provide measurement output to a droplet position detection feedback system 316. Droplet position detection feedback system 316 can compute a droplet position and trajectory, from which a droplet error can be computed (e.g., on a droplet-by-droplet basis, or on average). The droplet error can then be provided as an input to controller 310, which can, for example, provide a position, direction and/or timing correction signal to laser system 302 to control laser trigger timing and/or to control movement of optics in beam conditioning unit 306, e.g., to change the location and/or focal power of the light pulses being delivered to irradiation region 304 in chamber 212. Furthermore, source material delivery system 308 can comprise a control system operable in response to a signal from controller 310 (which in some implementations can include the droplet error described above, or some quantity derived therefrom) to modify the release point, initial droplet stream direction, droplet release timing and/or droplet modulation to correct for errors in the droplets arriving at irradiation region 304.

[0066] In some aspects, the lithographic apparatus 100 can also comprise collector 258 and a gas dispenser device 320. Gas dispenser device 320 can dispense gas in the path of the source material from source material delivery system 308 (e.g., irradiation region 304). Gas dispenser device 320 can comprise a nozzle through which dispensed gas can exit. Gas dispenser device 320 can be structured (e.g., having an aperture) such that, when placed near the optical path of laser system 302, light from laser system 302 is not blocked by gas dispenser device 320 and is allowed to reach irradiation region 304. A buffer gas such as hydrogen, helium, argon or combinations thereof, can be introduced into chamber 212. The buffer gas can be present in the chamber 212 during plasma discharge and can act to slow plasma-created ions, reduce degradation of optics, and/or increase plasma efficiency. Alternatively, a magnetic field and/or electric field (not shown) can be used alone, or in combination with a buffer gas, to reduce damage caused by fast-moving ions.

[0067] In some aspects, collector 258 can be a near-normal incidence collector mirror having a reflective surface in the form of a prolate spheroid as described above. Collector 258 can be formed with an aperture to allow the light pulses generated by laser system 302 to pass through and reach irradiation region 304. The same, or another aperture, can be used to allow gas from the gas dispenser device 320 to flow into chamber 212. As shown, the collector 258 can be, e.g., aprolate spheroid mirror that has a first focus within or near the irradiation region 304 and a second focus at an intermediate region 318, where the EUV light can be transmitted to exposure device 256. It is to be appreciated that other optics can be used in place of the prolate spheroid mirror for collecting and directing light to an intermediate location for subsequent delivery to a device utilizing EUV light. It is also envisaged that structures and functions described in reference to FIG. 3 can be used with collectors other than collector 258 (e.g., collector CO (FIG. 2A)).

[0068] Example Lithographic Cell

[0069] FIG. 4 shows a lithographic cell 400, also sometimes referred to a lithocell or cluster, according to some aspects. Lithographic apparatus 100 (FIGS. 1, 2A, 2B, and 3) can form part of lithographic cell 400. Lithographic cell 400 can also comprise one or more apparatuses to perform pre -exposure and post-exposure processes on a substrate. These can include spin coaters SC to deposit resist layers, developers DE to develop exposed resist, chill plates CH, and bake plates BK. A substrate handler, or robot, RO picks up substrates from input/output ports I/Ol, I/O2, moves them between the different process apparatuses and delivers them to the loading bay LB of the lithographic apparatus 100. These devices, which are often collectively referred to as the track, are under the control of a track control unit TCU, which is itself controlled by a supervisory control system SCS, which also controls the lithographic apparatus via lithography control unit LACU. Thus, the different apparatuses can be operated to maximize throughput and processing efficiency.

[0070] Example Plasma Material Droplet Source

[0071] FIG. 5 shows a source material delivery system 500, according to some aspects. In some aspects, source material delivery system 500 can be used in a lithographic apparatus 100. For example, source material delivery system 500 can be used as source material delivery system 308 in FIG. 3. Source material delivery system 500 can comprise a nozzle 502, an electromechanical element 504, and a waveform generator 506. Nozzle 502 can comprise a capillary 508. Source material delivery system 500 can further comprise a shroud 510, a controller 512, a detector 514, and/or a detector 516. Controller 512 can comprise a processor.

[0072] In some aspects, terms such as “electromechanical,” “electro -actuated,” or the like can be used herein to refer to a material or structure which undergoes a dimensional change (e.g., movement, deflection, contraction, rotation, and the like) when subjected to a voltage, electric field, magnetic field, or combinations thereof. Some examples can include piezoelectric materials, electrostrictive materials, and magnetostrictive materials. Apparatuses and methods for using an electro-actuated element to control a droplet stream are disclosed, for example, in U.S. Pat. No. US 7,897,947, titled “Laser Produced Plasma EUV Light Source Having a Droplet Stream Produced Using a Modulated Disturbance Wave” and issued March 01, 2011, and U.S. Patent No. 8,513,629, titled “Droplet Generator with Actuator Induced Nozzle Cleaning” and issued August 20, 2013, both of which are incorporated by reference herein in their entireties. [0073] In some aspects, electromechanical element 504 can be disposed on (e.g., surrounding) nozzle 502. It should be appreciated that interactions between nozzle 502 and electromechanical element 504 described herein can be directed to interactions between a pressure-sensitive element of nozzle 502 and electromechanical element 504 (e.g., electromechanical element 504 is disposed on capillary 508). Waveform generator 506 can be electrically coupled to electromechanical element 504. Controller 512 can be electrically coupled to waveform generator 506.

[0074] In some aspects, an EUV-generating -plasma can be generated by irradiating target material (e.g., Sn) with a laser, which ionizes some or all of the target material (i.e., excitation). The target material can be provided as a stream of coalesced droplets that intersects the laser path. Microscopic interactions between a coalesced target material droplet and the laser can affect efficiency and stability of EUV radiation, which in turn can impact lithographic processes that depend on the EUV radiation. Therefore, it is desirable to control the interaction between coalesced droplet and the laser such that EUV-generation is stable and efficient. One method to improve stability and efficiency is to ensure repeatable coalescence of target material droplets so that each coalesced droplet produces a repeatable interaction with the laser. Structures and functions in aspects of the present disclosure allow for repeatable coalescence of target material droplets.

[0075] In some aspects, nozzle 502 can eject initial droplets of target material, shown in FIG. 5 as a stream of target material 518. electromechanical element 504 can transduce electrical energy from the waveform generator 506 to apply a pressure on nozzle 502 (e.g., on capillary 508). This introduces a velocity perturbation in stream of target material 518 exiting nozzle 502. Stream of target material 518 ultimately coalesces into droplets which are detected by detector 514 and/or detector 516 to generate a signal (e.g., a detection signal). As used herein, the term “detect” or the like can be used to refer to capturing an image (e.g., using a camera) of the droplet and/or binary indication of the presence or absence of a droplet or when a droplet crosses a given location (e.g., using a laser curtain). Detectors 514 and 516 can be trigger detectors, gating detectors, gate detectors or other suitable detectors that generate a detection signal in response to a fulfillment of one or more conditions, for example the detected presence of a droplet. One of detectors 514 and 516 can be an image capture device and the other can be a gate detector. Controller 512 can determine properties of stream of target material 518 based on the signal from detector 514. Properties of the stream of target material 518 can comprise, for example, velocity profde of the droplet stream at the detection point, gap (time and/or distance) between droplets, presence of uncoalesced droplets (satellite droplets, or simply “satellites”), droplet size, coalescence length, droplet path (or aim), or the like. Controller 512 can use the information from detectors 514 and/or 516 to generate a feedback signal to control operation of the waveform generator 506.

[0076] In some aspects, controller 512 can adjust parameters of electrical signals (e.g., waveform, hybrid waveform) generated by waveform generator 506. Parameters of waveforms can comprise, for example, relative phase difference(s) between two or more waveforms in superposition, amplitude, wavelength, and the like. Controller 512 can also determine an adjustment of a waveform parameter based on an external input 520, which can originate from another controller or be based on a user input. [0077] In some aspects, shroud 510 can be disposed on nozzle 502. Shroud 510 can be disposed so as to cover and protect stream of target material 518 from forces that can disrupt coalescence and droplet generation.

[0078] In some aspects, waveform generator 506 is configured to generate an electrical signal to control the applied pressure on nozzle 502. The electrical signal can comprise a superposition (e.g., hybrid waveform) of a first periodic waveform having a first frequency (e.g., a low frequency sine wave) and a second periodic waveform having a second frequency different from the first frequency (e.g., a high frequency square wave). The term “sine” can be used herein to refer to sinusoidal patterns. The second frequency can be an integer multiple of the first frequency. The resulting velocity perturbations in stream of target material 518 allow the initial droplets that are ejected from nozzle 502 to coalesce as they travel away from nozzle 502. A fully coalesced droplet 522 can form at a distance L (“coalescence length”) from the orifice of nozzle 502. In other words, a distance, measured from the nozzle, at which coalesced droplet 522 forms without remnant uncoalesced droplets (e.g., satellites) defines a coalescence length.

[0079] In some aspects, the coalescence length can be adjusted by adjusting parameters of the electrical signal from waveform generator 506 (e.g., relative phase of waveforms), which ultimately influences coalescence behavior via velocity perturbations of the initial droplets (additional details regarding the use of hybrid waveforms in coalescence-based droplet generation can be found in International Appl. No. WO 2019/137846). Initial droplets can be generated at a rate of, for example, between 3x l0⁶ and 10* 10⁶ initial droplets per second (e.g., frequency of 3, 4, 5, 8, 10 MHz). The frequency of initial droplets can be a function of, for example, the size of the orifice on nozzle 502 (or capillary 508) and a so-called Rayleigh breakup phenomenon. In some aspects, source material delivery system 500 generates fully coalesced droplets 522 having a lower frequency (e.g., 20, 30, 40, 50, 60, 75, 100 kHz) and without any satellites — from the initial droplets of a higher frequency (e.g., 5 MHz).

[0080] In some aspects, source material delivery system 500 is configured to control the breakup/coalescence process to reduce instabilities in the EUV -generating -plasma. It can be instructive to first describe some factors that can influence droplet coalescence. Referring back to FIG. 3, an EUV radiation source can employ gas dispenser device 320 to introduce a gas flow (e.g., hydrogen gas) into irradiation region 304. The gas flow from gas dispenser device 320 can introduce drag to the droplets in stream of target material 518 (FIG. 5), thereby affecting the velocities of droplets. Therefore, the coalescence process — being significantly influenced by the velocity perturbation of droplets — can be substantially impacted by the presence of gas. A reason for using gas can be for allowing some useful features. For example, the gas can be used as a chemical radical for cleaning collector 258. More details regarding the use of hydrogen gas can be found in U.S. Pat. No. 10,359,710, issued on January 18, 2011, titled “Radiation System and Optical Device,” which is incorporated by reference herein in its entirety. For the use of at least these features, drag can be tolerated in some aspects.

[0081] Plasma forces can also affect coalescence. The EUV -generating -plasma can be characterized as a complex flow of ionized matter. Therefore, droplets in the vicinity of the EUV -generating -plasma can be subject to electromagnetic and fluid-mechanical forces. Consequently, uncoalesced droplets may not be able to fully coalesce if they are still in fragmented form (e.g., satellites) by the time they enter the influence of the plasma forces. The presence of satellites at irradiation region 304 can impact stability of EUV-generation, which in turn can be undesirable for lithographic processes that depend on precise energy dosages from the EUV source.

[0082] In some aspects, it is desirable for fully coalesced droplets to form at a given distance from irradiation region 304. In some aspects, full coalescence of droplets at a given distance away from irradiation region 304 can be achieved by positioning source material delivery system 308 (or its nozzle, e.g., nozzle 502 of FIG. 5) further away from irradiation region 304. A nozzle has a range of possible coalescence lengths (e.g., having a minimum and/or maximum) based on, for example, parameters of the electrical signal from waveform generator 506 (FIG. 5). A maximum coalescence length of source material delivery system 308 can be, for example, approximately 700 mm. Therefore, the tip of such a nozzle would need to be placed at least 700 mm away from irradiation region 304 for full coalescence of droplets prior to arriving at irradiation region 304. However, there can exist reasons that caution against placing the nozzle at such distances from irradiation region 304. For example, aiming the droplets precisely and reproducibly for intersection with a laser is desirable for EUV-generation stability. However, as source material delivery system 308 is positioned further away, the droplets can be under the influence of drag for longer periods of time, leading to higher uncertainties in the aim of coalesced droplet and sub-optimal interaction between the droplets and the laser.

[0083] Example Beam Positioning Method

[0084] In some aspects, a plasma is generated through irradiation by three successive light pulses. A first pulse, often termed a pre-pulse, can irradiate a source material (e.g., a droplet) to generate an expanded disc (e.g., a target). The target can then be irradiated by a second pulse, often termed a rarefication pulse or a rarefaction pulse. In some aspects, the rarefication pulse can change the density of the target to ensure optimal absorption. Subsequently, a third pulse, often termed a main pulse, irradiates the rarefied target to generate a plasma that produces EUV light.

[0085] Light pulses from a laser system are aligned with the droplet/target to generate a plasma. In some aspects, the first pulse is generated by a first laser beam, a second pulse is generated by second laser beam, and a third pulse is generated by a third laser beam. To ensure each pulse irradiates the desired droplet/target, positions of the laser beams are determined independently. In some aspects the first and second laser beams comprise a 1 pm laser and the third laser beam comprises a CO2 laser. [0086] In some aspects, laser beams can be positioned in two orthogonal directions. Moreover, in some aspects, positions of the laser beams can be adjusted in three orthogonal directions (e.g., x, y, and z). These positions can be defined in terms of, for example, relative or absolute positions of intersection among the laser beams and/or relative or absolute positions of intersection of the laser beams with the path of travel of material such as droplets or targets used to generate EUV light. FIG. 3 defines x, y, and z axes relative to elements in chamber 212. In some aspects, the center of the coordinate system can be defined as irradiation region 304. In some aspects, “fuel” droplets such as liquid tin droplets travel from source material delivery system 308 to irradiation region 304 along the x-axis. In some aspects, the y-axis is perpendicular to the x-z plane (e.g., points out of the plane of the page). In some aspects, the z-axis can point from a center of the collector 258 to irradiation region 304.

[0087] In some aspects, a light source can comprise sensors that measure aspects of EUV radiation generation, for example, target formation and EUV energy generated per pulse.

[0088] In some aspects, atarget formation sensor, for example a camera, can measure the size, position, and/or shape of the droplet a few microseconds after the droplet is hit by the first pulse. Size, position, and/or shape measurements can be used as feedback to correctly position the first laser beam.

[0089] In some aspects, a camera can capture dispersion of tin after a tin target is hit by the second pulse. In some aspects, dispersion measurements can determine an alignment offset of the second laser beam. In some aspects, the alignment offset can be used to position the second laser beam.

[0090] In some aspects, an EUV sensor can measure the amount and power of EUV light generated by the plasma. In some aspects, the EUV sensor can be located behind an EUV collector or in a scanner of an EUV light source. In some aspects, EUV energy per pulse can be measured for a set of x and y positions of the second and third laser beams.

[0091] In some aspects, stability key performance indicators (KPIs) can be extracted from EUV sensor measurements. Stability KPIs can include EUV standard deviation or moving standard deviation (e.g. how standard deviation changes overtime). In some aspects, a stability KPI can include any metric that can estimate dose margin from one or two minutes or more of data at the same frequency as a droplet repetition rate. In some aspects, stability KPI’s and EUV energy per pulse can be used to position the second and third laser beams.

[0092] FIG. 6 shows a method 600 of determining positions of a set of three laser beams in an EUV light source, according to some aspects. Method 600 can comprise a first sequence 602, a second sequence, 604, and a third sequence 608.

[0093] In some aspects, first sequence 602 comprises calibrating positions of the first, second, and third laser beams to hit the droplet/target.

[0094] In some aspects, first sequence 602 can be performed when an EUV light source is operating at a low duty cycle. A low duty cycle can generally be defined as a cycle where few droplets are hit and many are missed. In some aspects, 0.4% of droplets are hit by a laser pulse during a low duty cycle. [0095] In some aspects, second sequence 604 comprises further adjusting positions of the first, second, and third laser beams calibrated in first sequence 602. In some aspects, the beam positions are adjusted to maximize EUV energy generated per pulse.

[0096] In some aspects, second sequence 604 can be performed when an EUV light source is operating at a low duty cycle. In some aspects, performing second sequence 604 at a low duty cycle allows a user to control positions of the laser beams before plasma dynamics are introduced into the EUV light source. In some aspects, second sequence 604 can be initiated after successful completion of first sequence 602. [0097] In some aspects, third sequence 606 comprises further adjusting positions of the first, second, and third laser beams. In some aspects, positions of the laser beams are adjusted to concurrently maximize EUV energy per pulse and EUV stability. In some aspects, third sequence 606 can be initiated after successful completion of second sequence 604.

[0098] In some aspects, third sequence 606 is performed while an EUV light source is operating at a high duty cycle. In some aspects, a high duty cycle comprises an operation of the EUV light source wherein the laser pulses hit 80% or more of the droplets (e.g., roughly 85%, 90%, 95%, 99%, 99.5%, 99.7%, 99.9% of droplets are hit during a burst). In some aspects, a high duty cycle can be a baseline operation for the EUV light source.

[0099] In some aspects, the number of droplets irradiated during a high duty cycle can be determined by a plasma recipe. In some aspects, a plasma recipe can outline relevant parameters, such as time delays between the pre-pulse, rarefication pulse, and main pulse and energy ranges of the pre-pulse, rarefication pulse, and main pulse.

[0100] FIG. 7 shows first sequence 702, according to some aspects. In some aspects, the steps in first sequence 702 can represent a more detailed description of first sequence 602 in method 600 (FIG. 6). In some aspects, first sequence 702 comprises steps 708, 710, 712, and 714.

[0101] Step 708 comprises calibration of x, y and z positions of the third beam. In some aspects, the first beam and the second beam are turned off during calibration of the third beam. In some aspects, x and y positions of the third beam are scanned until a droplet is hit. Scanning the x position of a beam may involve steering the direction (angle) of the beam, or adjusting a timing of pulses of the beam relative to the timing of passing droplets, or a combination thereof. Scanning the y position of a beam may involve steering the direction (angle) of the beam, or steering the direction (angle) of the path of travel of the droplets, or a combination thereof. In some aspects, the z position of the third beam can be calibrated by scanning z positions until target expansion reaches a maximum. Scanning the z position of a beam may involve adjusting a focus point of the beam, or steering the direction (angle) of the path of travel of the droplets, or a combination thereof. The maximum target expansion can be measured by a target formation sensor. In some aspects, the target formation sensor is a camera.

[0102] Step 710 comprises calibration of x, y, and z positions of the first beam. In some aspects, x and y positions of the first beam are scanned until the droplet is hit. In some aspects, z-positions of the beam can be scanned until the target reaches a maximum expansion. In some aspects, a sensor, such as a camera, can detect the size and position of the droplet/target. In some aspects, the second beam is prevented from hitting the target during calibration of the first beam. The second beam can be prevented from hitting the target by turning off the beam, moving the beam in space, or changing the timing of the beam.

[0103] Step 712 comprises calibration of the second beam. In some aspects, x and y positions of the second beam are scanned until the target is hit.

[0104] Step 714 comprises setting the positions of the first, second, and third beams using the calibrations determined in steps 708, 710 and 712.

[0105] FIG. 8 illustrates second sequence 804, according to some aspects. In some aspects, the steps in second sequence 804 can represent a more detailed description of second sequence 604 in method 600 (FIG. 6). In some aspects, second sequence 804 comprises steps 816, 818, 820, 822, 824 and 826. [0106] Step 816 comprises adjusting the positions of the second beam so that the second beam does not hit the target. In some aspects, the second beam can be turned off, moved in space, or moved in time such that a pulse from the second beam misses the target.

[0107] Step 818 comprises adjusting x and y positions of the first beam to a desired target rotation. In some aspects, a sensor (e.g., a target formation camera) can measure the rotation of the target relative to the beam. In some aspects, a controller can adjust the x and y positions of the first beam based on measurements from the sensor. In some aspects, a desired target rotation can be specified by a plasma recipe (e.g., radiation dose, radiation timing) or production factors (e.g., number of wafers per production lot, etc.).

[0108] Step 820 comprises adjusting x and y positions of the third beam to maximize EUV energy generated per pulse. In some aspects, a series of x and y positions are scanned to determine the positions where EUV energy per pulse is maximized. In some aspects, a feedback algorithm can be implemented to determine the desired x and y positions of the third beam.

[0109] Step 822 comprises readjusting the second beam so that the second beam hits the target. In some aspects, the second beam is set to the x and y positions found in step 712.

[0110] Step 824 comprises adjusting x and y positions of the second beam to maximize EUV energy generated per pulse. In some aspects, a series of x and y positions are scanned to determine the positions where EUV energy per pulse is maximized. In some aspects, a feedback algorithm can be implemented to determine the desired x and y positions of the second beam.

[0111] Step 826 comprises adjusting the positions of the third beam to account for cross-talk between steps 822 and 824. In some aspects, a series of x and y positions are scanned to determine the positions where EUV energy per pulse is maximized. In some aspects, a feedback algorithm can be implemented to determine positions of the third beam where EUV energy per pulse is maximized.

[0112] FIG. 9 shows third sequence 906, according to some aspects. In some aspects, the steps in third sequence 906 can represent a more detailed description of third sequence 606 in method 600 (FIG. 6). In some aspects, third sequence 906 comprises steps 928, 930, 932, 934, and 936. Third sequence 906 can be executed at a high duty cycle where the laser pulses hit 80% or more of the droplets.

[0113] Step 928 comprises adjusting x and y positions of the first beam to achieve a desired target rotation. In some aspects, a sensor (e.g., a target formation camera) can measure the rotation of the target. In some aspects, a controller can adjust the x and y positions of the first beam based on measurements from the sensor.

[0114] Step 930 comprises adjusting the x and y positions of the third beam to maximize the amount of EUV energy generated per pulse. In some aspects, a series of x and y positions are scanned to determine the positions where EUV energy per pulse is maximized. In some aspects, a feedback algorithm can be implemented to determine desired x and y positions of the third beam.

[0115] Step 932 comprises further adjusting the x and y positions of the second beam. In some aspects, optimal x and y positions for the second beam concurrently maximize EUV energy generated per pulse and EUV stability. In some aspects, EUV energy generated per pulse and EUV stability KPI’s are measured for a series of x and y positions to determine desired x and y positions of the second beam.

[0116] Step 934 comprises readjusting x and y positions of the third beam to account for cross-talk between steps 930 and 932. In some aspects, a series of x and y positions are scarmed to determine the positions where EUV energy per pulse reaches a maximum. In some aspects, a feedback algorithm can be implemented to determine the positions where EUV energy per pulse is maximized.

[0117] Step 936 comprises closing an energy control loop of the laser system on a desired set point.

[0118] In some aspects, one or more controllers can maintain the positions of the first, second, and third lasers during EUV light generation.

[0119] In some aspects, the method steps of FIGS. 6-9 can be automated.

[0120] The method steps of FIGS. 6-9 can be performed in any conceivable order selected as appropriate for a particular implementation of an EUV source and it is not required that all steps be performed. Moreover, the method steps of FIGS. 6-9 described above merely reflect an example of steps and are not limiting. That is, further method steps and functions are envisaged based aspects described in reference to FIGS. 1-5.

[0121] Example Operating Parameter Selection Method

[0122] In some aspects, a plasma is generated by an EUV light source (e.g., source SO as shown in and described with regard to FIGS. 1, 2A, and 2B) by irradiating a source material with a plurality of successive pulses of laser beams. In some aspects, the EUV light source can include one or more light sources configured to illuminate a source material (e.g., a droplet, a target, etc.) with a first laser beam, a second laser beam, and a third laser beam. The first laser beam, often termed a pre-pulse, can irradiate a source material (e.g., a droplet) to generate an expanded disc (e.g., a target). The target can then be irradiated by the second laser beam, often termed a rarefication pulse or a rarefaction pulse. In some aspects, the rarefaction pulse can change the density of the target to ensure optimal absorption of the third laser beam, often termed a main pulse. Subsequently, the third laser beam (e.g., the main pulse) irradiates the rarefied target to generate a plasma that produces EUV light. In some aspects, the EUV light source can include a measurement system configured to measure an EUV energy generated by the illuminations of the target. For example, the measurement system can include an EUV sensor located behind an EUV collector or in an EUV light source chamber or in a scanner coupled to an EUV light source. In some aspects, the EUV light source can include a controller configured to set and adjust operating parameters for the first, second, and third laser beams according to a plasma recipe.

[0123] In some aspects, a plasma recipe can be a combination of operating parameters that produce a corresponding output of EUV light. In some aspects, an EUV light source can determine a plasma recipe that maximizes the output of EUV power based on the setup parameters of the rarefaction pulse. An optimized plasma recipe can maximize the amount of usable target material at the right density within a light source chamber (e.g. chamber 212 as shown in and described with regard to FIGS. 2A and 2B), thereby maximizing the amount of EUV power generated when the main pulse irradiates the rarefied target.

[0124] In some aspects, a plasma recipe can include one or more operating parameters. For example, the one or more operating parameters can include at least one of: a pre -pulse energy, a temporal delay between the pre-pulse and the main pulse, a relative x, y, and z alignment positioning between the prepulse and the droplet, a pre-pulse duration, a pre-pulse beam profile, a pre-pulse beam size, a rarefaction pulse energy, a temporal delay between the rarefaction pulse and the main pulse, a relative x, y, and z alignment positioning between the rarefaction pulse and the target, a rarefaction pulse duration, a rarefaction pulse beam profile, a rarefaction pulse beam size, a main pulse energy, a relative x, y, and z alignment positioning between the main pulse and the target, a main pulse duration, a main pulse beam profile, and a main pulse beam size. In some aspects, a parameter space can be locally convex for various combinations of values for operating parameters, which means that a set of optimal parameter values can be found that lead to a peak EUV output within a limited (local) parameter space used for EUV production.

[0125] In some aspects, the method described below can apply to a second laser beam (e.g., the rarefaction pulse) having any type of beam profile and beam size. In some aspects, the second laser beam can have a Gaussian beam profile with a beam size of about 400 microns to about 2000 microns D4-sigma. For example, the Gaussian beam profile can have a beam size of about 500, 550, 600, 650, 700, 750, or 800 microns D4-sigma. In some aspects, the second laser beam can have a flat top beam profile with a beam size of about 500 microns to about 1000 microns at about 90% peak amplitude. For example, the flat top beam profile can have a beam size of about 500, 550, 600, 650, 700, 750, or 800 microns at about 90% peak amplitude.

[0126] In some aspects, the method described below can adjust energies and timings of the three laser beams, either independently of or together with adjustments of relative alignment positions between the three laser beams with respect to the source material. [0127] FIG. 10 shows a method 1050 for selecting operating parameters of an EUV light source, according to some aspects. In some aspects, method 1050 can include steps 1052, 1054, 1056, 1058, and 1060.

[0128] In some aspects, at step 1052, a temporal or spatial relationship between a plurality of illuminations of a target by a first laser beam, a second laser beam, and a third laser beam can be selected. In some aspects, throughout method 1050, the first laser beam can be interchangeably called a pre-pulse, the second laser beam can be interchangeably called a rarefaction pulse, and the third laser beam can be interchangeably called a main pulse. In some aspects, step 1052 can include aligning each of the three laser beams with the source material, which is called geometric overlap. For example, step 1052 can include method 600 and its corresponding steps first sequence 602, second sequence, 604, and third sequence 608, as described above with regard to FIG. 6. Additionally, for example, step 1052 can include first sequence 702 as described above with regard to FIG. 7, second sequence 804 as described above with regard to FIG. 8, and third sequence 906 as described with regard to FIG. 9. In some aspects, step 1052 can include initializing the temporal relationship (e.g., a time delay) between the pre-pulse and the rarefaction pulse. In some aspects, step 1052 can include initializing the energies of the prepulse and the main pulse using an EUV plasma excitation configuration. In some aspects, step 1052 can include setting initial values for plasma recipe parameters such as, for example, energy, duration, beam profile, and/or beam size for the pre-pulse and the main pulse. In some aspects, step 1052 can include setting initial values for plasma recipe parameters such as, for example, beam profile and/or beam size for the rarefaction pulse.

[0129] In some aspects, at step 1054, an initial value of a temporal delay between the second laser beam and the third laser beam can be set. The initial value of the temporal delay between the second laser beam and the third laser beam can be set according to a type of beam profile of the second laser beam such as, for example, a Gaussian beam profile or a flat top beam profile. In some aspects, the initial value of the temporal delay between the second laser beam and the third laser beam can be set between about 10 ns and about 200 ns from a first rising edge to a second rising edge of the waveform. For example, for a Gaussian beam profile, the initial value of the temporal delay between the second laser beam and the third laser beam can be set to about 50, 60, 70, 80, 90, 100, 110, or 120 ns. In another example, for a flat top beam profile, the initial value of the temporal delay between the second laser beam and the third laser beam can be set to about 50, 60, 70, 80, 90, 100, 110, 120, 130, 140, or 150 ns. [0130] In some aspects, at step 1056, an initial value of an energy (which may correspond to a value for a power or a fluence) of the second laser beam can be set. The initial value of the energy of the second laser beam can be set according to a type of beam profile of the second laser beam such as, for example, a Gaussian beam profile or a flat top beam profile. In some aspects, the initial value of the energy of the second laser beam can be set between about 1 J/cm² and about 10 J/cm². For example, for a Gaussian beam profile, the initial value of the energy of the second laser beam can be set to about 1, 1.5, 2, 2.5, 3, 3.5, or 4 J/cm² at a peak of the beam profile. In another example, for a flat top beam profile, the initial value of the energy of the second laser beam can be set to about 1, 1.5, 2, 2.5, 3, 3.5, or 4 J/cm² for a region of 90% peak amplitude.

[0131] In some aspects, before step 1058, method 1050 can further include a step in which an initial value of a duration of the second laser beam can be set. The initial value of the duration of the second laser beam can be set according to a type of beam profile of the second laser beam such as, for example, a Gaussian beam profile or a flat top beam profile. In some aspects, the duration of the second laser beam can be a length of time for a full-width half-maximum (FWHM) Gaussian pulse for each type of beam profile. In some aspects, the initial value of the duration of the second laser beam can be set between about 1 ns to about 20 ns. For example, for each type of beam profile such as, for example, a Gaussian beam profile or a flat top beam profile, the duration of the second laser beam can be set to about 5, 6, 7, 8, 9, 10, 11, or 12 ns for the FWHM Gaussian pulse.

[0132] In some aspects, at step 1058, an EUV energy generated by the illuminations of the target based on the initial value of the temporal delay between the second laser beam and the third laser beam and the initial value of the energy of the second laser beam can be measured. For example, a measurement system coupled to the EUV light source can include an EUV sensor located behind an EUV collector or in a scanner of an EUV light source. In some aspects, wherein the initial value of the duration of the second laser beam has been set prior to step 1058, an EUV energy generated by the illuminations of the target based on at least one of the initial value of the temporal delay between the second laser beam and the third laser beam, the initial value of the energy of the second laser beam, and the initial value of the duration of the second laser beam can be measured.

[0133] In some aspects, at step 1060, the temporal delay between the second laser beam and the third laser beam and the energy of the second laser beam can be adjusted in response to the measuring. In some aspects, the temporal delay between the second laser beam and the third laser beam and the energy of the second laser beam can be adjusted to updated corresponding values separately as one -dimensional scans or simultaneously as a two-dimensional scan. For example, a controller coupled to the EUV light source can be configured to adjust the temporal delay between the second laser beam and the third laser beam independently of an adjustment of the energy of the second laser beam. In another example, the controller can be configured to co-optimize the temporal delay between the second laser beam and the third laser beam and the energy of the second laser beam based on a two-dimensional scan. In some aspects, wherein the initial value of the duration of the second laser beam has been set prior to step 1058, at least one of the temporal delay between the second laser beam and the third laser beam, the energy of the second laser beam, and the duration of the second laser beam can be adjusted in response to the measuring.

[0134] In some aspects, a scan or scanning in this context can be defined as stepwise testing of a sequence of values for one or more operating parameters at various set points to collect data on an EUV power output by each of a plurality of combinations of the one or more parameters. In some aspects, the controller can automatically scan values around an initial combination of one or more operating parameters (e.g., temporal delay between the second laser beam and the third laser beam, energy of the second laser beam, duration of the second laser beam, etc.) to determine which updated combination of the operating parameters can generate a maximum EUV power output for the parameter space. In some aspects, the controller can perform a raster scan or a spiral scan to map the parameter space with a sufficient resolution of the separation of set points.

[0135] In some aspects, in step 1060, the controller can automatically scan values around an initial combination of one or more operating parameters. Such scanning can determine which updated combination of the operating parameters generates a maximum EUV power output for the parameter space. In some aspects, the controller can scan a two-dimensional parameter space based on the initial value of the temporal delay between the second laser beam and the third laser beam and the initial value of the energy of the second laser beam. In some aspects, in step 1060, the controller can perform two separate one -dimensional scans for each of the two operating parameters or a two-dimensional scan for both operating parameters at once. For example, a one-dimensional scan around the initial value of the energy of the second laser beam can include testing values for the energy of the second laser beam around the predetermined initial value to determine which updated value of the energy of the second laser beam produces the maximum EUV power output. For example, a one -dimensional scan around the initial value of the temporal delay between the second laser beam and the third laser beam can include testing values for the temporal delay between the second laser beam and the third laser beam around the predetermined initial value to determine which updated value of the temporal delay between the second laser beam and the third laser beam produces the maximum EUV power output. In another example, a two-dimensional scan around the initial value of the temporal delay between the second laser beam and the third laser beam and the initial value of the energy of the second laser beam can include simultaneously testing values of both the temporal delay between the second laser beam and the third laser beam and the energy of the second laser beam to determine which updated values of both operating parameters produce the maximum EUV power output.

[0136] In some aspects, steps 1058 and 1060 can be performed iteratively according to a feedback algorithm to determine a combination of values for one or more operating parameters within a convex parameter space that produces the maximum EUV power output.

[0137] The method steps of FIG. 10 can be performed in any conceivable order selected as appropriate for a particular implementation of an EUV source and it is not required that all steps be performed. For example, steps 1054 and/or 1056 (and/or the step in which the initial value of the duration of the second laser beam is set) can be performed first to set up initial values for one or more operating parameters for a plasma recipe, then step 1052 can be performed to determine geometric overlap between each of the three laser beams and the source material, and then steps 1058 and 1060 can be performed to adjust values of the one or more operating parameters to maximize the EUV power output. In another example , in some aspects, the EUV light source can include a metrology system such as, for example, sensors or cameras, configured to determine the relative positioning of the three laser beams. In this example, the metrology system can determine a geometric overlap between the second laser beam and target independent of any determination of the values of temporal delay between the second laser beam and the third laser beam, energy of the second laser beam, duration of the second laser beam that produce the maximum EUV power output. Accordingly, steps 1058 and 1060 can be performed at any time independent of the relative alignment positioning between each of the three laser beams and the source material.

[0138] Moreover, the method steps of FIG. 10 described above merely reflect an example of steps and are not limiting. That is, further method steps and functions are envisaged based aspects described in reference to FIGS. 1-5.

[0139] The terms “radiation,” “beam,” “light,” “illumination,” or the like can be used herein to refer to one or more types of electromagnetic radiation, for example, ultraviolet (UV) radiation (for example, having a wavelength X of 365, 248, 193, 157 or 126 nm), extreme ultraviolet (EUV or soft X-ray) radiation (for example, having a wavelength in the range of 5-100 nm such as, for example, 13.5 nm), or hard X-ray working at less than 5 nm, as well as particle beams, such as ion beams or electron beams. Generally, radiation having wavelengths between about 400 to about 700 nm is considered visible radiation; radiation having wavelengths between about 780-3000 nm (or larger) is considered IR radiation. UV refers to radiation with wavelengths of approximately 100-400 nm. Within lithography, the term “UV” also applies to the wavelengths that can be produced by a mercury discharge lamp: G- line 436 nm; H-line 405 nm; and/or, I-line 365 nm. Vacuum UV, or VUV (i.e., UV absorbed by gas), refers to radiation having a wavelength of approximately 100-200 nm. Deep UV (DUV) generally refers to radiation having wavelengths ranging from 126 nm to 428 nm, and in some aspects, an excimer laser can generate DUV radiation used within a lithographic apparatus. It should be appreciated that radiation having a wavelength in the range of, for example, 5-20 nm relates to radiation with a certain wavelength band, of which at least part is in the range of 5-20 nm.

[0140] Although some aspects of the present disclosure are described in the context of lithographic apparatuses in the manufacture of ICs, it should be understood that lithographic apparatuses described herein can be used in other applications, for example, in the manufacture of integrated optical systems, guidance and detection patterns for magnetic domain memories, flat-panel displays, LCDs, thin-film magnetic heads, etc. Those skilled in the art will appreciate that, in the context of such alternative applications, any use of the terms “wafer” or “die” herein can be considered as specific examples of the more general terms “substrate” or “target portion”, respectively. A substrate can be processed before or after exposure in, for example, a track unit (a tool that typically applies a layer of resist to a substrate and develops the exposed resist) and/or a metrology unit. Where applicable, aspects disclosed herein can be applied to such and other substrate processing tools. Furthermore, a substrate can be processed more than once, for example in order to create a multi-layer IC, so that the term substrate used herein can also refer to a substrate that already contains multiple processed layers. [0141] Furthermore, although some aspects of the present disclosure are described in the context of optical lithography, it should be understood that aspects of the present disclosure are not limited to optical lithography. For example, in imprint lithography, a topography in a patterning device defines the pattern created on a substrate. The topography of the patterning device can be pressed into a layer of resist supplied to the substrate whereupon the resist is cured by applying electromagnetic radiation, heat, pressure or a combination thereof. The patterning device is moved out of the resist leaving a pattern in it after the resist is cured.

[0142] It is to be understood that the phraseology or terminology herein is for the purpose of description and not of limitation, such that the terminology or phraseology of the present specification is to be interpreted by those skilled in relevant art(s) in light of the teachings herein.

[0143] The present disclosure has been described above 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. The foregoing description of specific aspects will so fully reveal the general nature of the present disclosure that others can, by applying knowledge within the skill of the art, readily modify and/or adapt for various applications such specific aspects, without undue experimentation and without departing from the general concept of the present disclosure. Therefore, such adaptations and modifications are intended to be within the meaning and range of equivalents of the disclosed aspects, based on the teaching and guidance presented herein.

[0144] It is to be understood that the Detailed Description section, and not the Summary and Abstract sections, is intended to be used to interpret the claims. The Summary and Abstract sections can set forth one or more, but not necessarily all, aspects of the present disclosure as contemplated by the inventor(s), and thus, are not intended to limit the present disclosure and the appended claims in any way. The breadth and scope of the protected subject matter should not be limited by any of the above -described aspects, but should be defined in accordance with the following claims and their equivalents.

Claims

1. A lithography system comprising: an extreme ultraviolet (EUV) light source comprising: a target; a first laser beam configured to supply a first pulse to the target; a second laser beam configured to supply a second pulse to the target; a third laser beam configured to supply a third pulse to the target; wherein the first, second and third laser beams are positioned independently.

2. The lithography system of claim 1, wherein a first sequence, a second sequence, and a third sequence are initiated sequentially to position the first, second, and third laser beams.

3. The lithography system of claim 2, wherein the first sequence comprises initializing positions of the first, second, and third laser beams relative to the target.

4. The lithography system of claim 2, wherein the second sequence comprises further adjusting positions of the first, second and third laser beams to maximize energy of EUV radiation generated on a per pulse basis when the third laser beam interacts with the target.

5. The lithography system of claim 2, wherein the third sequence still further adjusts positions of the first, second, and third laser beams are adjusted to maximize stability and/or power of EUV radiation generated on a per pulse basis when the third laser beam interacts with the target.

6. A method of producing a semiconductor device, the method comprising positioning laser beams in an extreme ultraviolet (EUV) light source, the positioning comprising: initiating a first sequence, wherein positions of a first beam, a second beam, and a third beam are adjusted relative to a target inside a vessel; initiating a second sequence, wherein positions of the first beam, the second beam and the third beam are further adjusted to maximize energy of EUV radiation generated on a per pulse basis when the third beam interacts with the target; and initiating a third sequence, wherein positions of the first beam, the second beam, and the third beam are still further adjusted to maximize stability and/or power of the EUV radiation; wherein the first sequence, the second sequence, and the third sequence can be executed independently.

7. The method of claim 6, wherein the first sequence comprises: calibrating the third beam while the first beam and the second beam are prevented from hitting the target; calibrating the first beam while the third beam is prevented from hitting the target; calibrating the second beam; and setting initial positions of the first beam, the second beam, and the third beam based on the calibrating the first beam and the calibrating the second beam; wherein the calibrating of the first beam, the second beam, and the third beam comprises scanning positions of the beams in two orthogonal directions until the corresponding beam hits the target; and wherein the calibrating of the first beam and the third beam further comprises scanning positions of the beams in a third orthogonal direction to maximize an expansion of the target.

8. The method of claim 7, wherein the first sequence occurs during a low EUV duty cycle, wherein about 0.4% of laser pulses hit the target.

9. The method of claim 7, wherein the expansion of the target is measured by a target formation sensor.

10. The method of claim 6, wherein the second sequence comprises: adjusting the second beam so that it does not hit the target; adjusting positions of the first beam in two orthogonal directions to achieve a desired target rotation; adjusting positions of the third beam in two orthogonal directions to maximize EUV energy generated per pulse; adjusting the second beam to hit the target; and adjusting the second beam in two orthogonal directions to maximize EUV energy generated per pulse.

11. The method of claim 10, wherein the second sequence occurs during a low duty cycle, wherein about 0.4% of laser pulses hit the target.

12. The method of claim 6, wherein the third sequence comprises: adjusting the first beam in two orthogonal directions to achieve desired target rotation; adjusting the third beam in two orthogonal directions to achieve maximum EUV energy generated per pulse; adjusting the second beam in two orthogonal directions to concurrently maximize power and stability of generated EUV radiation; and closing an energy control loop.

13. The method of claim 12, wherein EUV energy generated per pulse and stability are concurrently maximized based on average EUV energy measurements, EUV standard deviation, and/or moving standard deviation.

14. The method of claim 12, wherein the third sequence occurs during a high duty cycle, wherein at least 80% of laser pulses hit the target.

15. The method of claim 6, wherein maximum EUV power generated per pulse is determined using a liner scan across positions in two orthogonal directions or an automated feedback algorithm.

16. The method of claim 10 or 12, wherein the positions of the third beam in two orthogonal directions is adjusted a second time after the position of the second beam is adjusted, to account for cross talk between steps.

17. The method of claim 6, wherein the first pulse is configured to hit the target and expand the target into a disc shape.

18. The method of claim 6, wherein the second pulse is configured to change a density of the target.

19. The method of claim 6, wherein the third pulse is configured to interact with the target to generate EUV radiation.

20. An extreme ultraviolet (EUV) light source comprising: at least two laser sources, the laser sources configured to supply a first pulse, a second and a third pulse to a moving target; and a controller configured to control a positioning of one or more of the laser sources and configured to control a timing of one or more of the laser sources, wherein the controller is configured to perform an adjustment of a position of the first pulse in relation to the target, the controller is configured to perform an adjustment of a position of the second pulse in relation to the target, independently of the adjustment of the first pulse, and the controller is configured to perform an adjustment of a position of the third pulse in relation to the target, independently of the adjustments of the first and second pulses.

21. The EUV light source of claim 20, wherein: the controller is further configured to initiate a first sequence, wherein positions of the first pulse, the second pulse, and the third pulse are adjusted relative to a target inside a vessel; the controller is further configured to initiate a second sequence, wherein positions of the first pulse, the second pulse and the third pulse are further adjusted to maximize energy of EUV radiation generated when the third pulse interacts with the target; the controller is further configured to initiate a third sequence, wherein positions of the first pulse, the second pulse, and the third pulse are further adjusted to maximize stability of the EUV radiation.

22. The EUV light source of claim 21, wherein the first sequence comprises: calibrating the third pulse while the first pulse and the second pulse are prevented from hitting the target; calibrating the first pulse while the third pulse is prevented from hitting the target; calibrating the second pulse; and setting initial positions of the first pulse, the second pulse, and the third pulse based on the calibrating the first pulse and the calibrating the second pulse; wherein the calibrating of the first pulse, the second pulse, and the third pulse comprises scanning positions of the pulses in two orthogonal directions until the corresponding pulse hits a target; and wherein the calibrating of the first pulse and the third pulse further comprises scanning positions of the pulses in a third orthogonal direction to maximize an expansion of the target.

23. The EUV light source of claim 22, wherein the controller is further configured to perform the first sequence during a low EUV duty cycle, wherein fewer than 2% of laser pulses hit a target.

24. The EUV light source of claim 22, further comprising: a target formation sensor configured to measure expansion of the target.

25. The EUV light source of claim 21, wherein the controller is further configured to perform, during the second sequence: an adjustment of the second pulse so that it does not hit the target; adjustments of positions of the first pulse in two orthogonal directions to achieve a desired target rotation; adjustments of positions of the third pulse in two orthogonal directions to maximize EUV energy generated per pulse; an adjustment of the second pulse to hit the target; and an adjustment of the second pulse in two orthogonal directions to maximize EUV energy generated per pulse.

26. The EUV light source of claim 25, wherein the controller is further configured to perform the second sequence during a low duty cycle, wherein less than about 0.5% of laser pulses hit the target.

27. The EUV light source of claim 21, wherein the controller is further configured to perform, during the third sequence: an adjustment of the first pulse in two orthogonal directions to achieve desired target rotation; an adjustment of the third pulse in two orthogonal directions to achieve maximum EUV energy generated per pulse; an adjustment of the second pulse in two orthogonal directions to concurrently maximize power and stability of generated EUV radiation.

28. The EUV light source of claim 27, wherein the controller is further configured to concurrently optimize (a) EUV energy generated per pulse and (b) EUV energy stability.

29. The EUV light source of claim 27, wherein the controller is further configured to perform the third sequence occurs during a high duty cycle, wherein at least 90% of laser pulses hit the target.

30. The EUV light source of claim 25 or 27, wherein the controller is further configured to adjust the position of the third pulse in two orthogonal directions after the position of the second pulse are adjusted.

31. The EUV light source of claim 20, wherein the laser sources and the controller are configured so that the first pulse hits the target and expands the target into a disc shape.

32. The EUV light source of claim 20, wherein the laser sources and the controller are configured so that the second pulse changes a density of the target.

33. The EUV light source of claim 20, wherein the laser sources and the controller are configured so that the third pulse interacts with the target to generate EUV radiation.

34. A method for selecting operating parameters of an extreme ultraviolet (EUV) light source, comprising: selecting a temporal or spatial relationship between a plurality of illuminations of a target by a first laser beam, a second laser beam, and a third laser beam; setting an initial value of a temporal delay between the second laser beam and the third laser beam; setting an initial value of an energy of the second laser beam; measuring an EUV energy generated by the illuminations of the target based on the initial value of the temporal delay between the second laser beam and the third laser beam and the initial value of the energy of the second laser beam; and adjusting the temporal delay between the second laser beam and the third laser beam and the energy of the second laser beam in response to the measuring.

35. The method of claim 34, wherein the first, second, and third laser beams are generated by a single laser source.

36. The method of claim 34, wherein the first laser beam comprises a pre-pulse beam, the second laser beam comprises a rarefaction pulse beam, and the third laser beam comprises a main pulse beam.

37. The method of claim 34, wherein the setting the initial value of the temporal delay between the second laser beam and the third laser beam comprises setting the initial value of the temporal delay between the second laser beam and the third laser beam between about 10 ns and about 200 ns.

38. The method of claim 34, wherein the setting the initial value of the temporal delay between the second laser beam and the third laser beam comprises setting the initial value of the temporal delay between the second laser beam and the third laser beam between about 80 ns and about 120 ns.

39. The method of claim 34, wherein the setting the initial value of the energy of the second laser beam comprises setting the initial value of the energy of the second laser beam between about 1 J/cm² and about 10 J/cm².

40. The method of claim 34, wherein the setting the initial value of the energy of the second laser beam comprises setting the initial value of the energy of the second laser beam between about 1.5 J/cm² and about 2.5 J/cm².

41. The method of claim 34, further comprising setting an initial value of a duration of the second laser beam.

42. The method of claim 41 , wherein the setting the initial value of the duration of the second laser beam comprises setting the initial value of the duration of the second laser beam between about 1 ns to about 20 ns.

43. The method of claim 34, wherein the second laser beam comprises a Gaussian beam profile with a beam size of about 400 microns to about 2000 microns D4-sigma.

44. The method of claim 34, wherein the second laser beam comprises a flat top beam profile with a beam size of about 500 microns to about 1000 microns at about 90% peak amplitude.

45. The method of claim 34, wherein the adjusting the temporal delay between the second laser beam and the third laser beam and the energy of the second laser beam comprises adjusting the temporal delay between the second laser beam and the third laser beam and the energy of the second laser beam to updated corresponding values separately as one -dimensional scans or simultaneously as a two- dimensional scan.

46. An EUV light source, comprising: one or more laser sources configured to illuminate a target with a first laser beam, a second laser beam, and a third laser beam; a measurement system configured to measure an EUV energy generated by the illuminations of the target; and a controller configured to: select a temporal or spatial relationship between a plurality of illuminations of the target by the first laser beam, the second laser beam, and the third laser beam; set an initial value of a temporal delay between the second laser beam and the third laser beam; set an initial value of an energy of the second laser beam; measure, with the measurement system, the EUV energy generated by the illuminations of the target based on the initial value of the temporal delay between the second laser beam and the third laser beam and the initial value of the energy of the second laser beam; and adjust the temporal delay between the second laser beam and the third laser beam and the energy of the second laser beam in response to the measuring.

47. The EUV light source of claim 46, wherein the first, second, and third laser beams are generated by a single laser source.

48. The EUV light source of claim 46, wherein the first laser beam comprises a pre-pulse beam, the second laser beam comprises a rarefaction pulse beam, and the third laser beam comprises a main pulse beam.

49. The EUV light source of claim 46, wherein the controller is configured to set the initial value of the temporal delay between the second laser beam and the third laser beam between about 10 ns and about 200 ns.

50. The EUV light source of claim 46, wherein the controller is configured to set the initial value of the temporal delay between the second laser beam and the third laser beam between about 80 ns and about 120 ns.

51. The EUV light source of claim 46, wherein the controller is configured to set the initial value of the energy of the second laser beam between about 1 J/cm² and about 10 J/cm².

52. The EUV light source of claim 46, wherein the controller is configured to set the initial value of the energy of the second laser beam between about 1.5 J/cm² and about 2.5 J/cm².

53. The EUV light source of claim 46, wherein the controller is further configured to set an initial value of a duration of the second laser beam.

54. The EUV light source of claim 53, wherein the controller is configured to set the initial value of the duration of the second laser beam between about 1 ns to about 20 ns.

55. The EUV light source of claim 46, wherein the second laser beam comprises a Gaussian beam profile with a beam size of about 400 microns to about 2000 microns D4-sigma.

56. The EUV light source of claim 46, wherein the second laser beam comprises a flat top beam profile with a beam size of about 500 microns to about 1000 microns at about 90% peak amplitude.

57. The EUV light source of claim 46, wherein the controller is configured to adjust the temporal delay between the second laser beam and the third laser beam independently of an adjustment of the energy of the second laser beam to updated corresponding values separately as one -dimensional scans or simultaneously as a two-dimensional scan.

58. The EUV light source of claim 46, wherein the controller is configured to co -optimize the temporal delay between the second laser beam and the third laser beam and the energy of the second laser beam based on a two-dimensional scan.

59. A lithographic apparatus, comprising: an EUV light source, comprising: one or more laser sources configured to illuminate a target with a first laser beam, a second laser beam, and a third laser beam; a measurement system configured to measure an EUV energy generated by the illuminations of the target; and a controller configured to: select a temporal or spatial relationship between a plurality of illuminations of the target by the first laser beam, the second laser beam, and the third laser beam; set an initial value of a temporal delay between the second laser beam and the third laser beam; set an initial value of an energy of the second laser beam; measure, with the measurement system, the EUV energy generated by the illuminations of the target based on the initial value of the temporal delay between the second laser beam and the third laser beam and the initial value of the energy of the second laser beam; and adjust the temporal delay between the second laser beam and the third laser beam and the energy of the second laser beam in response to the measuring.

60. The lithographic apparatus of claim 59, wherein the first, second, and third laser beams are generated by a single laser source.

61. The lithographic apparatus of claim 59, wherein the first laser beam comprises a pre-pulse beam, the second laser beam comprises a rarefaction pulse beam, and the third laser beam comprises a main pulse beam.

62. The lithographic apparatus of claim 59, wherein the controller is configured to set the initial value of the temporal delay between the second laser beam and the third laser beam between about 10 ns and about 200 ns.

63. The lithographic apparatus of claim 59, wherein the controller is configured to set the initial value of the temporal delay between the second laser beam and the third laser beam between about 80 ns and about 120 ns.

64. The lithographic apparatus of claim 59, wherein the controller is configured to set the initial value of the energy of the second laser beam between about 1 J/cm² and about 10 J/cm².

65. The lithographic apparatus of claim 59, wherein the controller is configured to set the initial value of the energy of the second laser beam between about 1.5 J/cm² and about 2.5 J/cm².

66. The lithographic apparatus of claim 59, wherein the controller is further configured to set an initial value of a duration of the second laser beam.

67. The lithographic apparatus of claim 66, wherein the controller is configured to set the initial value of the duration of the second laser beam between about 1 ns to about 20 ns.

68. The lithographic apparatus of claim 59, wherein the second laser beam comprises a Gaussian beam profile with a beam size of about 400 microns to about 2000 microns D4-sigma.

69. The lithographic apparatus of claim 59, wherein the second laser beam comprises a flat top beam profile with a beam size of about 500 microns to about 1000 microns at about 90% peak amplitude.

70. The lithographic apparatus of claim 59, wherein the controller is configured to adjust the temporal delay between the second laser beam and the third laser beam independently of an adjustment of the energy of the second laser beam to updated corresponding values separately as one -dimensional scans or simultaneously as a two-dimensional scan.

71. The lithographic apparatus of claim 59, wherein the controller is configured to co-optimize the temporal delay between the second laser beam and the third laser beam and the energy of the second laser beam based on a two-dimensional scan.