Method and system for EUV energy adjustment via rarefaction pulse adjustment

Abstract

A method can adjust an extreme ultraviolet (EUV) energy output of an EUV light generation system. The method can include irradiating a droplet with a pre-pulse laser beam suitable to reshape the droplet into a target. The method can further include irradiating the target with a rarefaction pulse laser beam suitable to modify one or more of a density of the target or an absorption characteristic of the target. The method can further include irradiating the modified target with a main pulse laser beam suitable to generate EUV light. The method can further include measuring an EUV energy output of the EUV light with a sensor. The method can further include adjusting at least one of the rarefaction pulse laser beam or the main pulse laser beam in response to the measured EUV energy output.

Description

METHOD AND SYSTEM FOR EUV ENERGY ADJUSTMENT VIA RAREFACTION PULSE ADJUSTMENTCROSS-REFERENCE TO RELATED APPLICATIONS

[0001] This application claims priority to US Application No. 63 / 561,491, filed March 5, 2024, titled METHOD AND SYSTEM FOR EUV ENERGY ADJUSTMENT VIA RAREFACTION PULSE ADJUSTMENT, which is incorporated herein by reference in its 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 ofthe 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.

[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 pulsed laser beam. In another method, often termed discharge produced plasma (“DPP”), the plasma can be generated by positioning source material having an appropriate emission line between a pair of electrodes and causing an electrical discharge to occur between the electrodes.

[0007] In an EUV LPP source, the EUV energy and power output can exhibit instability as a result of naturally varying conditions affecting the formation of the plasma (e.g., undesirable distortion of the source material, velocity or position of the source material in relation to the laser pulses, and the timing of the laser pulses relative the optimum for plasma generation). Because the lithographic processes require stable application of EUV power onto a wafer, it is necessary to control the power delivery (or the energy of the individual pulses) of EUV light.

[0008] Current EUV sources currently employ various methods for adjusting (e.g., modulating) and controlling EUV power output.

[0009] For example, a controller may be configured to adjust the power output of the main pulse laser by adjusting a power level of radio frequency (RF) waves that are used to excite or pump a gain medium in the laser. This adjusting in turn adjusts the EUV power output. This technique can adjust EUV power smoothly over a certain power output range. However, due to practical limitations of the main pulse laser, the achievable range of EUV adjustment or modulation (or modulation depth) is somewhat limited — the actuation range can be typically 75-100% of the power capability of current systems. Therefore, limited modulation depth constrains this method of EUV modulation to high power use cases. Additionally, the speed of EUV power adjustment is limited by the dynamic response of the laser itself. Limitations to power adjustment speed can result in undesirable dose errors.

[0010] As a result, the limitations with these adjustment methods, also known as modulation methods, can make it challenging to operate an EUV source under low power use cases. Furthermore, limitations in EUV adjustment speed hinders the ability to scale the control bandwidth (e.g., range of frequencies over which variability can be attenuated) as power output scales in future generations of the EUV source. Accordingly, the current adjustment methods pose numerous problems for controlling EUV stability, and thus for EUV operational power and throughput.SUMMARY

[0011] Accordingly, it is desirable to improve control of EUV energy and power output to reduce instabilities in EUV generation and to improve accuracy in EUV lithographic apparatuses.

[0012] In some aspects, a method can adjust an extreme ultraviolet (EUV) energy output of an EUV light generation system. In some aspects, the method can include irradiating a droplet with a pre -pulse laser beam suitable to reshape the droplet into a target. In some aspects, the method can further include irradiating the target with a rarefaction pulse laser beam suitable to modify one or more of a density of the target or an absorption characteristic of the target. In some aspects, the method can further includeirradiating the modified target with a main pulse laser beam suitable to generate EUV light. In some aspects, the method can further include measuring an EUV energy output of the EUV light with a sensor. In some aspects, the method can further include adjusting at least one of the rarefaction pulse laser beam or the main pulse laser beam in response to the measured EUV energy output.

[0013] In some aspects, an EUV light generation system can include one or more laser sources, a droplet generator, a controller, a collecting optical device, and a sensor. In some aspects, the one or more laser sources can be configured to produce a pre-pulse laser beam, a rarefaction pulse laser beam, and a main pulse laser beam. In some aspects, the droplet generator can be configured to provide a droplet at a predetermined irradiation region inside a chamber of the system. In some aspects, the controller can be configured to: (1) irradiate the droplet with the pre-pulse laser beam at the predetermined irradiation region inside the chamber, the pre-pulse laser beam suitable to reshape the droplet into a target; (2) irradiate the target with the rarefaction pulse laser beam at the predetermined irradiation region inside the chamber, the rarefaction pulse laser beam suitable to modify one or more of a density of the target or an absorption characteristic of the target; (3) irradiate the modified target with the main pulse laser beam at the predetermined irradiation region inside the chamber, the main pulse laser beam suitable to generate EUV light; and (4) adjust at least one of the rarefaction pulse laser beam or the main pulse laser beam in response to an EUV energy output measured by a sensor. In some aspects, the collecting optical device can be positioned inside the chamber and can be configured to collect the EUV light.

[0014] In some aspects, a lithographic apparatus can include an EUV light generation system, an illumination system, and a projection system. In some aspects, an EUV light generation system can include one or more laser sources, a droplet generator, a controller, a collecting optical device, and a sensor. In some aspects, the one or more laser sources can be configured to produce a pre-pulse laser beam, a rarefaction pulse laser beam, and a main pulse laser beam. In some aspects, the droplet generator can be configured to provide a droplet at a predetermined irradiation region inside a chamber of the system. In some aspects, the controller can be configured to: (1) irradiate the droplet with the pre -pulse laser beam at the predetermined irradiation region inside the chamber, the pre -pulse laser beam suitable to reshape the droplet into a target; (2) irradiate the target with the rarefaction pulse laser beam at the predetermined irradiation region inside the chamber, the rarefaction pulse laser beam suitable to modify one or more of a density of the target or an absorption characteristic of the target; (3) irradiate the modified target with the main pulse laser beam at the predetermined irradiation region inside the chamber, the main pulse laser beam suitable to generate EUV light; and (4) adjust at least one of the rarefaction pulse laser beam or the main pulse laser beam in response to an EUV energy output of the EUV light measured by a sensor. In some aspects, the collecting optical device can be positioned inside the chamber and can be configured to collect the EUV light. In some aspects, the illumination system can be configured to illuminate a pattern of a patterning device. In some aspects, the patterning devicecan be configured to impart a pattern on the EUV light. In some aspects, the projection system can be configured to project an image of the pattern onto a substrate.

[0015] 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

[0016] 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.

[0017] FIG. 1 shows a reflective lithographic apparatus, according to some aspects.

[0018] FIGS. 2A and 2B show more details of a reflective lithographic apparatus, according to some aspects.

[0019] FIG. 3 shows a lithographic cell, according to some aspects.

[0020] FIG. 4 shows an EUV light generation system, according to some aspects.

[0021] FIG. 5 shows a schematic plot indicating EUV pulse energy as a function of a rarefaction pulse energy, according to some aspects.

[0022] FIG. 6 shows a schematic plot indicating EUV pulse energy as a function of a rarefaction pulse duration, according to some aspects.

[0023] FIG. 7 shows a schematic box plot indicating a relative EUV energy as a function of a relative timing between a rarefaction pulse laser beam and a main pulse laser beam, according to some aspects.

[0024] FIG. 8 shows a schematic plot indicating a compound effect on a relative EUV energy by adjusting both a main pulse laser source and a rarefaction pulse laser source, according to some aspects.

[0025] FIG. 9 shows a method of adjusting an EUV energy output of an EUV light generation system, according to some aspects.

[0026] 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

[0027] 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.

[0028] 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.

[0029] 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).

[0030] 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.

[0031] 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.

[0032] Example Lithographic Systems

[0033] 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.

[0034] 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.

[0035] 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).

[0036] 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.

[0037] 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.

[0038] 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.

[0039] 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.

[0040] 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.

[0041] 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.

[0042] 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.

[0043] 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. Inlithographic 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.

[0044] In some aspects, lithographic apparatus 100 can be used in at least one of the following modes:

[0045] 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.

[0046] 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.

[0047] 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.

[0048] Combinations and / or variations on the described modes of use or entirely different modes of use can also be employed.

[0049] 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.

[0050] FIG. 2A shows a 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 202 of source SO. An EUV radiation emitting plasma 204 can be formed by adischarge-generated plasma source. In some aspects, a plasma of excited tin (Sn) (e.g., excited via a laser) is used to produce EUV radiation.

[0051] The radiation emitted by the EUV radiation emitting plasma 204 can be passed from a source chamber 206 into a collector chamber 208 via an optional gas barrier or contaminant trap 210 (in some cases also referred to as contaminant barrier or foil trap), which is positioned in or behind an opening in source chamber 206. Contaminant trap 210 can comprise a channel structure. Contaminant trap 210 can also comprise a gas barrier and / or a channel structure.

[0052] In some aspects, collector chamber 208 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 212 and a downstream radiation collector side 214. Radiation that traverses radiation collector CO can be reflected off a grating spectral fdter 216 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 218 of enclosing structure 202. The virtual source point INTF can be an image of the EUV radiation emitting plasma 204. Grating spectral fdter 216 can be used for suppressing infrared (IR) radiation.

[0053] Subsequently, the radiation traverses the illumination system IL. Illumination system IL can include a faceted field mirror device 220 and a faceted pupil mirror device 222 arranged to provide a desired angular distribution of radiation beam 224, at patterning device MA, as well as a desired uniformity of radiation intensity at patterning device MA. Upon reflection of radiation beam 224 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 224 to patterning device MA.

[0054] More elements than shown can generally be present in illumination system IL and projection system PS . Grating spectral filter 216 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.

[0055] 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.

[0056] Collector CO, as illustrated in FIG. 2A, is depicted as an example of a nested collector with grazing incidence reflectors 230, 231, and 232 (or collector mirror). Grazing incidence reflectors 230, 231, and 232 can be disposed axially symmetric around an optical axis O. A collector optic of this type can be used in combination with a discharge-generated plasma source, often called a DPP source.

[0057] 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 shownin 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 234 (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 collector 236 that reflects EUV light from the EUV radiation emitting plasma 204 along a path into the exposure device 234 to irradiate substrate W. Collector 236 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.

[0058] Example Lithographic Cell

[0059] FIG. 3 shows a lithographic cell 300, also sometimes referred to a lithocell or cluster, according to some aspects. Lithographic apparatus 100 (FIGS. 1, 2A, and 2B) can form part of lithographic cell 300. Lithographic cell 300 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.

[0060] Example EUV Light Generation System

[0061] FIG. 4 shows an EUV light generation system as a detailed view of a portion of lithographic apparatus 100 (e.g., FIGS. 1, 2A, 2B, and 3), according to one or more aspects. Elements in FIG. 4 having the same reference numbers as those in FIGS. 1, 2A, 2B, and 3 have the same or substantially similar structures and functions as described in reference to FIGS. 1, 2A, 2B, and 3. In some aspects, source SO can be a LPP EUV source. Source SO can comprise a laser system 438 for generating light pulses and delivering the light pulses into a collector chamber 208. For the lithographic apparatus 100, the light pulses can travel along one or more beam paths from the laser system 438 and into the chamber 208 to illuminate a source material at an irradiation region 446 to generate a plasma (e.g., plasma region located at EUV radiation emitting plasma 204 in FIG. 2B) that produces EUV light for substrate exposure in the exposure device 234.

[0062] In some aspects, laser system 438 can comprise one or more pulsed laser devices, 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, a laser source within laser system 438 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 446. Continuously pumped CO2 amplifiers can be used for the laser system 438. 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.

[0063] 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 amplifier (MOPA) 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.

[0064] In some aspects, laser system 438 can include one or more light sources, optical amplifiers, lasers, or lamps configured to produce a pre -pulse laser beam, a rarefaction pulse laser beam, and a main pulse laser beam. For example, laser system 438 can include any combination of CO2 laser device and solid state laser devices to produce the pre -pulse laser beam, the rarefaction pulse laser beam, and the main pulse laser beam. In some aspects, laser system 438 comprises a pre -pulse laser source 440, a rarefaction pulse laser source 442, and a main pulse laser source 444. In one example aspect, all three of pre-pulse laser source 440, rarefaction pulse laser source 442, and main pulse laser source 444 can be the same laser source. In one example aspect, two of pre -pulse laser source 440, rarefaction pulse laser source 442, and main pulse laser source 444 can be the same laser source. In one example aspect, each of pre-pulse laser source 440, rarefaction pulse laser source 442, and main pulse laser source 444 can be a separate laser source.

[0065] In some aspects, pre-pulse laser source 440 can irradiate a droplet of source material at the predetermined irradiation region 446 inside chamber 208 to reshape the droplet into a target. Pre-pulse laser source 440 can spread the droplet of source material spatially to create, for example, a disk of molten metal.

[0066] In some aspects, pre-pulse laser source 440 can produce pre-pulse laser beams with a wavelength range of about 250 nm to about 10 pm. For example, pre-pulse laser source 440 can produce a pre-pulse laser beam with a wavelength of about 1 pm. In some aspects, pre-pulse laser source 440can produce pre-pulse laser beams with an energy per pulse ranging from below 1 mJ to about 150 mJ. For example, pre-pulse laser source 440 can produce a pre-pulse laser beam with an energy per pulse of 4 mJ. In some aspects, pre-pulse laser source 440 can produce pre-pulse laser beams with a duration ranging from below 1 ns to about 120 ns. For example, pre-pulse laser source 440 can produce a prepulse laser beam with a duration of 10 ns. In some aspects, pre -pulse laser source 440 can produce a pre-pulse laser beam with a duration lasting less than 1 ns. For example, pre-pulse laser source 440 can produce a pre-pulse laser beam with a duration of about 10 ps to about 300 ps. The pre-pulse source 440 can be any radiation or light source that produces pre-pulse laser beams with any combination of wavelengths, energies, and durations within the ranges listed above.

[0067] In some aspects, rarefaction pulse laser source 442 can irradiate the target at the predetermined irradiation region 446 inside chamber 208 to modify the target to have a modified absorption characteristic based on a density of the target. In some aspects, a rarefaction pulse (alternatively termed as a rarefication pulse) provided by rarefaction pulse laser source 442 can reduce the density of the target by evaporating or ionizing a portion of the target (or all of the target) such that a modified target forms in the shape of, for example, a volumetric cloud, a mist of micro- or nano-particles, or a cloud of atomic vapor. In some example applications, the modified target can have a lower density than the unmodified target.

[0068] In some aspects, rarefaction pulse laser source 442 can produce rarefaction pulse laser beams with a wavelength range of about 250 nm to about 10 pm. For example, rarefaction pulse laser source 442 can produce a rarefaction pulse laser beam with a wavelength of about 1 pm. In some aspects, rarefaction pulse laser source 442 can produce rarefaction pulse laser beams with an energy per pulse ranging from below 1 mJ to about 150 mJ. For example, rarefaction pulse laser source 442 can produce a rarefaction pulse laser beam with an energy per pulse of 4 mJ. In some aspects, rarefaction pulse laser source 442 can produce rarefaction pulse laser beams with a duration ranging from below 1 ns to about 120 ns. For example, rarefaction pulse laser source 442 can produce a rarefaction pulse laser beam with a duration of 10 ns. In some aspects, rarefaction pulse laser source 442 can produce a rarefaction pulse laser beam with a duration lasting less than 1 ns. For example, rarefaction pulse laser source 442 can produce a rarefaction pulse laser beam with a duration of about 10 ps to about 300 ps. In some aspects, rarefaction pulse laser beam can produce a rarefaction pulse laser beam with a relative timing of below 1 ps to about 5 ps after pre-pulse laser source 440 produces a pre-pulse laser beam. In some aspects, rarefaction pulse laser beam can produce a rarefaction pulse laser beam with a relative timing of below 1 ns to about 500 ns before main pulse laser source 444 produces a main pulse laser beam. The rarefaction pulse source 442 can be any radiation or light source that produces rarefaction pulse laser beams with any combination of wavelengths, energies, durations, and relative timings within the ranges listed above.

[0069] In some aspects, main pulse laser source 444 can irradiate the modified target at the predetermined irradiation region 446 inside chamber 208 to convert the modified target into EUV- emitting plasma (e.g., EUV radiation emitting plasma 204 in FIGS. 2A and 2B) to generate EUV light.

[0070] In some aspects, main pulse laser source 444 can produce main pulse laser beams with a wavelength range of about 250 nm to about 10 pm. For example, main pulse laser source 444 can produce a main pulse laser beam with a wavelength of about 10 pm. In some aspects, main pulse laser source 444 can produce main pulse laser beams with an energy per pulse at the EUV location ranging from about 100 mJ to about 800 mJ. For example, main pulse laser source 444 can produce a main pulse laser beam with an energy per pulse at the EUV location of 500 mJ. In some aspects, main pulse laser source 444 can produce main pulse laser beams with a duration ranging from below 1 ns to about 300 ns. For example, main pulse laser source 444 can produce a main pulse laser beam with a duration of 50 ns. In some aspects, main pulse laser source 444 can produce a main pulse laser beam with a duration lasting less than 1 ns. For example, main pulse laser source 444 can produce a main pulse laser beam with a duration of about 10 ps to about 300 ps. The main pulse source 444 can be any radiation or light source that produces main pulse laser beams with any combination of wavelengths, energies, and durations within the ranges listed above.

[0071] In some aspects, source SO can also comprise a beam conditioning unit 448 having one or more optics for beam conditioning, such as expanding, steering, and / or focusing the pre-pulse laser beam, the rarefaction pulse laser beam, and main pulse laser beam between the laser system 438 and irradiation region 446. 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 208. 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 tiptilt 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).

[0072] Beam conditioning unit 448 can comprise a focusing assembly to focus pre-pulse laser beam, the rarefaction pulse laser beam, and main pulse laser beam to irradiation region 446 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.

[0073] In some aspects, the source SO can also comprise a droplet generator (e.g., source material delivery system 450) for delivering source material, such as tin droplets, to a predetermined irradiation region 446 in chamber 208, where the droplets can interact with light pulses from the laser system 438 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 234. 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”, and U.S. Pat. No. 7,372,056, issued on May 13, 2008, titled “LPP EUV Plasma Source Material Target Delivery System”, the contents of each of which are incorporated by reference herein in their entirety.

[0074] 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, SnH4. 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 446, 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., SnH4).

[0075] In some aspects, the source SO can comprise a controller 452 for controlling devices in laser system 438 to generate light pulses for delivery into the chamber 208 and / or for controlling movement of optics in beam conditioning unit 448. In some aspects, controller 452 can manipulate an amount of excitation provided in radio frequency (RF) energy to the CO2 power amplifiers as a means for adjusting the power output of a CO2 laser (e.g., main pulse laser source 444). Given a linear or otherwise monotonic relationship between CO2 power and EUV energy / pulse, controller 452 can achieve a pulse - to-pulse adjustment of the CO2 energy level to maintain a desired EUV energy output.

[0076] In some aspects, source SO can comprise an EUV output sensor 454 configured to measure an EUV energy output of the EUV light and to provide a power amplitude value as a measurement output signal to feedback system 456. In some aspects, feedback system 456 can compute an EUV output error based on a comparison between a predetermined EUV set point and the measured power amplitude value of the EUV output. Feedback system 456 can transmit the EUV output error as an input to controller 452 for corrective action. In response to the EUV output error, controller 452 can, for example, provide a power, duration, timing, position, and / or direction correction signal to laser system 438 to control laser trigger timing and / or to control movement of optics in beam conditioning unit 448, e.g., to change the location and / or focal power of the light pulses being delivered to irradiation region 446 in chamber 208.

[0077] In some aspects, controller 452 can control at least one of rarefaction pulse laser source 440 or main pulse laser source 444. In some aspects, controller 452 can output a control signal to control and adjust a characteristic of rarefaction pulse laser source 442 alone to adjust EUV power output. For example, controller 452 can output a control signal to control and adjust at least one of a rarefaction pulse energy, a rarefaction pulse duration, and a rarefaction pulse relative timing delay with respect to a pre-pulse laser beam or a main pulse laser beam. In some aspects, controller 452 can output a controlsignal to control and adjust a characteristic of main pulse laser source 444 alone to adjust EUV power output. For example, controller 452 can output a control signal to control and adjust at least one of a main pulse energy or a main pulse duration. In some aspects, controller 452 can output a control signal to control and adjust a characteristic of rarefaction pulse laser source 442 and a characteristic of main pulse laser source 444, alone or in combination, to adjust EUV power output.

[0078] In some aspects, controller 452 can increase the adjustment range of EUV output by adjusting characteristics of rarefaction pulse laser source 442 and main pulse laser source 444, alone or in combination. For example, controller 452 can expand the adjustment range to include lower EUV levels compared to current systems. Therefore, the additional actuation depth of controller 452 can improve low-dose operations.

[0079] In some aspects, latency in adjusting rarefaction pulse energy and rarefaction pulse duration of rarefaction pulse laser source 442 (e.g., a latency about 3 pulses or less for adjusting rarefaction pulse laser source 442) can be lower than latency in adjusting RF level for main pulse laser source 444 (e.g., a latency about 8 pulses for main pulse laser source 444). In this configuration, corrective action can be nearly instantaneous, allowing for adjustments on a shot-by-shot basis. Accordingly, controller 452 can operate at a higher bandwidth compared to current systems, thereby improving EUV dose performance. In an exemplary practical benefit, the improved EUV dose performance can enable lithographic apparatus 100 to recover from EUV level drops and prevent errors on a die on wafer W.

[0080] In some aspects, controller 452 can comprise a multi-stage controller that employs at least one faster controller stage configured to adjust laser timing of a laser source in laser system 438 and at least one slower stage configured to respond to an EUV output error and an output of the at least one fast controller. In this configuration, the plurality of controller stages of controller 452 can coordinate to compensate for measured errors in EUV power and to keep the output of the at least one faster controller (e.g., laser timing correction signals) centered about a nominal value. For example, controller 452 can employ the at least one slower controller stage to adjust the gain of main pulse laser source 444 and can employ the at least one faster controller stage to adjust the timing of the rarefaction pulse laser source 440 affecting the density of the source material. In this configuration, controller 452 can increase a depth of modulation of EUV output by actuating two separate plasma parameters and can increase adjustment speed by manipulating a relative timing delay between laser pulses. The resulting improvement of controller performance can lead to better overall EUV stability, which translates to more EUV power allocated to wafer production rather than energy control overhead.

[0081] In some aspects, the lithographic apparatus 100 can also comprise a collector 236 and a gas dispenser device 458. Gas dispenser device 458 can dispense gas in the path of the source material from source material delivery system 450 (e.g., irradiation region 446). Gas dispenser device 458 can comprise a nozzle through which dispensed gas can exit. Gas dispenser device 458 can be structured (e.g., having an aperture) such that, when placed near the optical path of laser system 438, light from laser system 438 is not blocked by gas dispenser device 458 and is allowed to reach irradiation region446. A buffer gas such as hydrogen, helium, argon or combinations thereof, can be introduced into chamber 208. The buffer gas can be present in the chamber 208 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.

[0082] In some aspects, collector 236 can be a collecting optical device, positioned inside chamber 208, configured to collect the EUV light. In some aspects, collector 236 can be a near-normal incidence collector mirror having a reflective surface in the form of a prolate spheroid as described above. Collector 236 can be formed with an aperture to allow the light pulses generated by laser system 438 to pass through and reach irradiation region 446. The same, or another aperture, can be used to allow gas from the gas dispenser device 458 to flow into chamber 208. As shown, the collector 236 can be, e.g., a prolate spheroid mirror that has a first focus within or near the irradiation region 446 and a second focus at an intermediate region 460, where the EUV light can be transmitted to exposure device 234. 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. 4 can be used with collectors other than collector 236 (e.g., collector CO (FIG. 2A)).

[0083] Example EUV Energy Adjustment Responses

[0084] FIGS. 5, 6, 7, and 8 show schematic plots indicating EUV energy adjustment responses performed by the EUV light generation system shown in FIG. 4, according to some aspects. In some aspects, controller 452 shown in FIG. 4 can be configured to adjust EUV energy output according to the following EUV energy adjustment responses described below.

[0085] FIG. 5 shows a schematic plot 500 indicating EUV pulse energy 562 (arbitrary units) as a function of a rarefaction pulse energy 564 (arbitrary units), according to some aspects. In some aspects, EUV pulse energy 562 can be adjusted by adjusting rarefaction pulse energy 564 on a shot-to-shot basis. As a result, rarefaction pulse energy 564 can be leveraged as an actuator to control EUV pulse energy 562.

[0086] In some aspects, controller 452 shown in FIG. 4 can adjust EUV pulse energy 562 according to adjustment response 566, which is a mean line for data collected regarding the exemplary aspect shown in FIG. 5. Adjustment response 566 can show a sensitivity of EUV pulse energy 562 to rarefaction pulse energy 564. Adjustment response 566 can be representative of the relationship between EUV pulse energy 562 and rarefaction pulse energy 564, as illustrated with a scale of arbitrary units, for example as illustrated in the figure.

[0087] In some aspects, controller 452 can adjust rarefaction pulse energy 564 within a range of about 0 to about 100, as illustrated in arbitrary units, for example. In some aspects, the adjustment of rarefaction pulse energy 564 can produce an adjustment of EUV pulse energy 562 within a range of about 50 to about 100, which is illustrated in arbitrary units, for example. For this exemplary adjustmentrelationship between EUV pulse energy 562 and rarefaction pulse energy 564, an adjustment of rarefaction pulse energy 564 between about 35% and about 80% of a maximum value for rarefaction pulse energy 564 can reduce EUV pulse energy 562 by up to 40%-50% of its maximum value, thereby providing an additional 50% modulation depth to the output of EUV pulse energy 562 with controller 452.

[0088] In some aspects, at a first rarefaction pulse energy range 561, there is not enough rarefaction pulse energy 564 to change the density of a target, which results in no increase in EUV pulse energy 562 relative to the amount of EUV pulse energy 562 present in the absence of rarefaction pulse energy 564. In the exemplary aspect of adjustment response 566 shown in FIG. 5, in a circumstance where a low dose of EUV pulse energy 562 is desired (e.g., during die repair), rarefaction pulse energy 564 can be set to about 35, as illustrated in arbitrary units, for example, in the particular plasma recipe of this example. Using such rarefaction pulse energy 564 can enable lithographic apparatus 100 to operate with low-power EUV shots to compensate for dose error on a die on wafer W. The energy values for rarefaction pulse energy 564 that produce a low dose of EUV pulse energy 562 can vary depending on a variety of factors. For example, the size of the target may influence these values because larger targets may absorb more rarefaction pulse energy 564 than a smaller target absorbs to produce a certain amount of EUV pulse energy 562.

[0089] In some aspects, at a second rarefaction pulse energy range 563, a target can be properly rarefied by rarefaction pulse energy 564 such that EUV pulse energy 562 can increase to a maximum value from the amount of EUV pulse energy 562 present in first rarefaction pulse energy range 561. In the exemplary aspect of adjustment response 566 shown in FIG. 5, in a circumstance where a maximum EUV pulse energy 562 is desired, rarefaction pulse energy 564 can be operated with the EUV pulse energy 562 at a chosen setpoint value of about 80, as illustrated in arbitrary units, for example, in the particular plasma recipe of this example. A setpoint in the second rarefaction pulse energy range 563 may be chosen in view of a variety of considerations. For example, the setpoint may be selected so that it is not too high, thus allowing “overhead” capacity to enable temporary increases in pulse energy when needed. Alternatively, or in addition, the setpoint may be selected so that it is not too low, thus avoiding waste of capacity and avoiding inefficient overall operation. The energy values for rarefaction pulse energy 564 that produce a maximum EUV pulse energy 562 can vary depending on the size of the target and other factors.

[0090] In some aspects, at a third rarefaction pulse energy range 565, any additional increases in rarefaction pulse energy 564 can over-rarefy a target, thereby limiting the ability to produce any more EUV pulse energy 562. The energy values for rarefaction pulse energy 564 that over-rarefy the target can vary depending on a variety of factors, such as the size of the target.

[0091] In one exemplary aspect, controller 452 can adjust rarefaction pulse energy 564 within a range of about 0.0 mJ to about 3.5 mJ. In some aspects, the adjustment of rarefaction pulse energy 564 can produce an adjustment of EUV pulse energy 562 within a range of about 2 mJ , 4 mJ, or 5 mJ to about8 mJ, 8.5 mJ, 9 mJ, 10 mJ, or 11 mJ. In a circumstance where a low dose of EUV pulse energy 562 is desired (e.g., during die repair), rarefaction pulse energy 564 can be set to about 1 mJ, 2 mJ, or 3 mJ, for example, in the particular plasma recipe of this example. In a circumstance where a maximum EUV pulse energy 562 is desired, rarefaction pulse energy 564 can be set to about 2 mJ, 3 mJ, 4, mJ, or 5 mJ, for example, depending on factors such as the size of the target, the angle of incidence of the rarefaction pulse on the target, the spatial profde of the rarefaction pulse, the duration and temporal profile of the rarefaction pulse, the chemical composition of the target, the density of the target following the pre- pulse, and other factors.

[0092] The method of adjusting rarefaction pulse energy 564 can be applied alone or in combination with the other methods of adjusting EUV energy described in FIGS. 6 and 7 below. For example, controller 452 can increase both the range of adjustment in EUV pulse energy 562 and the speed of adjustment in EUV pulse energy 562 by combining multiple adjustment methods.

[0093] FIG. 6 shows a schematic plot 600 indicating EUV pulse energy 668 (arbitrary units) as a function of a rarefaction pulse duration 670 (arbitrary units), according to some aspects. In some aspects, EUV pulse energy 562 can be adjusted by adjusting rarefaction pulse duration 670 on a shot- to-shot basis. As a result, rarefaction pulse duration 670 can be leveraged as an actuator to control EUV pulse energy 668.

[0094] In some aspects, controller 452 shown in FIG. 4 can adjust EUV pulse energy 668 according to adjustment response 672, which indicates a parabolic relationship between EUV pulse energy 668 and rarefaction pulse duration 670. Adjustment response 672 can show a sensitivity of EUV pulse energy 668 to rarefaction pulse duration 670. Adjustment response 672 can be representative of the relationship between EUV pulse energy 668 and rarefaction pulse duration 670, as illustrated with a scale of arbitrary units, for example as illustrated in the figure.

[0095] In some aspects, controller 452 can adjust rarefaction pulse duration 670 within a range of about 0 to about 100, as illustrated in arbitrary units, for example. In some aspects, the adjustment of rarefaction pulse duration 670 can produce an adjustment of EUV pulse energy 668 within a range of about 90 to about 100, which is illustrated in arbitrary units, for example. For this exemplary adjustment relationship between EUV pulse energy 668 and rarefaction pulse duration 670, an adjustment of rarefaction pulse duration 670 can reduce EUV pulse energy 668 by about 10% of its maximum value, thereby providing an additional 10% modulation depth to the output of EUV pulse energy 668 with controller 452. In some aspects, sensitivity of EUV pulse energy 668 to adjustment of rarefaction pulse duration 670 can be higher than this exemplary aspect. For example, a plasma recipe can be selected such that adjusting rarefaction pulse duration 670 offers a comparatively larger modulation depth for the output of EUV pulse energy 668 with controller 452, such as a modulation depth of 50% in one example.

[0096] In the exemplary aspect of adjustment response 672 shown in FIG. 6, in a circumstance where a maximum EUV pulse energy 668 is desired, rarefaction pulse duration 670 can be set to about 50, asillustrated in arbitrary units in the figure. In some aspects, controller 452 can command rarefaction pulse duration 670 to operate slightly offset from 50, as illustrated in arbitrary units in the figure, to help compensate with a corrective command if EUV pulse energy 668 drops below a desired level. In the exemplary aspect of adjustment response 672 shown in FIG. 6, in a circumstance where a low dose of EUV pulse energy 668 is desired (e.g., during die repair), rarefaction pulse duration 670 can be set to a duration shorter than 25, as illustrated in arbitrary units in the figure.

[0097] In one exemplary aspect, controller 452 can adjust rarefaction pulse duration 670 within a range of about 5 ns to about 25 ns. In some aspects, the adjustment of rarefaction pulse duration 670 can produce an adjustment of EUV pulse energy 668 within a range of about ±1%, ±2%, ±3%, ±5%, or ±10% of a center value of about 6 mJ, 7 mJ, 8 mJ, 9 mJ, 10 mJ. For example, adjustment of rarefaction pulse duration 670 may reduce EUV pulse energy 668 by about 3%-20% of its maximum value, thereby providing additional modulation depth to the output of EUV pulse energy 668 with controller 452.

[0098] The method of adjusting rarefaction pulse duration 670 can be applied alone or in combination with the other methods of adjusting EUV energy described in FIGS. 5 and 7. For example, controller 452 can increase both the range of adjustment in EUV pulse energy 668 and the speed of adjustment in EUV pulse energy 668 by combining multiple adjustment methods.

[0099] FIG. 7 shows a schematic box plot 700 indicating a relative EUV energy 774 (% of maximum EUV energy) as a function of a relative timing delay 776 (arbitrary units) between a rarefaction pulse and a main pulse, according to some aspects. This plot includes boxes 778a-778e, medians 780a-780e, maximums 782a-782e, minimums 784a-784e, distributions 786a-786e, and a representative adjustment response 788.

[0100] In some aspects, each box 778a-778e can indicate an interquartile range of a spread of data regarding relative EUV energy 774 for predetermined values of relative timing delay 776. Each of boxes 778a-778e can be representative of the relationship between relative EUV energy 774 and relative timing delay 776, as illustrated with a scale of arbitrary units for example as illustrated in the figure. For example, box 778a can correspond to a spread of relative EUV energy 774 for a relative timing delay 776 of about 20, as illustrated in arbitrary units in the figure; box 778b can correspond to a spread of relative EUV energy 774 for a relative timing delay 776 of about 40, as illustrated in arbitrary units in the figure; box 778c can correspond to a spread of relative EUV energy 774 for a relative timing delay 776 of about 60, as illustrated in arbitrary units in the figure; box 778d can correspond to a spread of relative EUV energy 774 for a relative timing delay 776 of about 80, as illustrated in arbitrary units in the figure; and box 778e can correspond to a spread of relative EUV energy 774 for a relative timing delay 776 of about 100, as illustrated in arbitrary units in the figure. In some aspects, each of boxes 778a-778e can include a median 780a-780e, respectively, that indicates the middle of each data set of relative EUV energy 774.

[0101] In some aspects, each of boxes 778a-778e can include a first whisker extending to a maximum 782a-782e, respectively, and can include a second whisker extending to a minimum 784a-784e,respectively. In some aspects, maximum 782a can indicate the largest number in each data set of relative EUV energy 774. In some aspects, minimum 784a-784e can indicate the smallest number in each data set of relative EUV energy 774.

[0102] In some aspects, each of boxes 778a-778e can include a distribution 786a-786e, respectively, that indicates a distribution of each data set of relative EUV energy 774 (e.g., most values of each data set of relative EUV energy 774 can be found at the median 780a-780e of the data set).

[0103] In some aspects, relative EUV energy 774 can be adjusted by adjusting relative timing delay 776 on a shot-to-shot basis. As a result, relative timing delay 776 can be leveraged as an actuator to control relative EUV energy 774. In some aspects, controller 452 shown in FIG. 4 can adjust relative EUV energy 774 according to adjustment response 788. In some aspects, controller 452 can adjust relative timing delay 776 (e.g., between a rarefaction pulse laser beam and a main pulse laser beam) within a range of about 0 to about 100, as illustrated in arbitrary units, for example. In some aspects, controller 452 can delay an electrical trigger for rarefaction pulse laser source 442 to adjust the duration of relative timing delay 776. In some aspects, the adjustment of relative timing delay 776 can produce an adjustment of relative EUV energy 774 within a range of about 65% to about 100% of maximum EUV power level. For this exemplary adjustment range of relative timing delay 776, adjustment of relative timing delay 776 can reduce a EUV pulse energy by about 35% of its maximum value, thereby providing an additional 35% modulation depth to the output of EUV pulse energy with controller 452.

[0104] In the exemplary aspect of adjustment response 788 shown in FIG. 7, when a maximum relative EUV energy 774 of about 100% EUV power level is desired, relative timing delay 776 can be set to about 60, as illustrated in arbitrary units in the figure. In some aspects, significant adjustment of relative EUV energy 774 can be achieved by adjusting relative timing delay 776 below 60, as illustrated in arbitrary units in the figure. For example, controller 452 can command relative timing delay 776 to operate within a range of about 0 to about 60, as illustrated in arbitrary units, for example, to help compensate with a corrective command if relative EUV energy 774 drops below a desired level.

[0105] In one exemplary aspect, controller 452 can adjust relative timing delay 776 (e.g., between a rarefaction pulse laser beam and a main pulse laser beam) within a range of about 30 ns to about 150 ns. In a circumstance when a maximum relative EUV energy 774 of about 100% EUV power level is desired, relative timing delay 776 can be set to about 80 ns, 100 ns, 120 ns, or 140 ns, depending on factors such as the size of the target, the angle of incidence of the rarefaction pulse on the target, the spatial profile of the rarefaction pulse, the duration and temporal profile of the rarefaction pulse, the chemical composition of the target, the density of the target following the pre-pulse, and other factors. In some aspects, significant adjustment of relative EUV energy 774 can be achieved by controlling the relative timing delay 776 around a setpoint that corresponds to a lower than maximum value of the relative EUV energy.

[0106] In some aspects, adjusting relative timing delay 776 can allow for relatively uninterrupted adjustment of relative EUV energy 774. In some aspects, there can be minimal latency between theapplied adjustment of relative timing delay 776 and the resulting adjustment in relative EUV energy 774. The method of adjusting relative timing delay 776 can be applied alone or in combination with the other methods of adjusting EUV described in FIGS. 5 and 6 above. For example, controller 452 can increase both the range of adjustment in relative EUV energy 774 and the speed of adjustment in relative EUV energy 774 by combining multiple adjustment methods.

[0107] FIG. 8 shows a schematic plot 800 indicating a compound effect on a relative EUV energy 890 (% of maximum EUV energy) as a function of main pulse laser gain command 892 (arbitrary units), as shown by main pulse adjustment response 894 (e.g., adjusting main pulse laser source 444) and rarefaction pulse adjustment response 896 (e.g., adjusting rarefaction pulse laser 442), according to some aspects. In some aspects, main pulse laser gain command 892 can be the control signal applied to the RF level of the main pulse laser source 444. Specifically, main pulse laser gain command 892 with a magnitude of 1.0 can produce the maximum allowed RF amplification of main pulse laser source 444, while main pulse laser gain command 892 with a magnitude of 0.0 can produce the minimum allowed RF amplification of main pulse laser source 444. In some aspects, relative EUV energy 890 can be adjusted by adjusting characteristics of both main pulse laser source 444 and rarefaction pulse laser 442 on a shot-to-shot basis. In some aspects, the adjusted characteristics of each laser source can be pulse energy, pulse duration, and relative timing delay between pulses. As a result, these characteristics of both main pulse laser source 444 and rarefaction pulse laser 442 can be used separately or together as an actuator to control relative EUV energy 890.

[0108] In some aspects, main pulse adjustment response 894 can be obtained by adjusting the RF level of main pulse laser source 444 for a fixed rarefaction pulse energy, rarefaction pulse duration, and rarefaction pulse relative timing delay to achieve a maximum EUV energy. In some aspects, the adjustment of main pulse laser source 444 alone can produce an adjustment of relative EUV energy 890 within a range of about 40% to about 100% of maximum EUV power level. In the exemplary aspect shown in FIG. 8, the adjustment of main pulse laser source 444 alone can produce an adjustment of relative EUV energy 890 within a range of about 70% to about 100% of maximum EUV power level. Accordingly, in this example, adjustment of main pulse laser source 444 alone can yield about 30% modulation depth for the output of EUV pulse energy with controller 452.

[0109] In some aspects, rarefaction pulse adjustment response 896 can be obtained by adjusting the RF level of main pulse laser source 444 but with either rarefaction pulse energy, rarefaction pulse duration, or rarefaction pulse relative timing delay detuned so that the EUV energy achieved by a main pulse laser gain command 892 of 1.0 is a smaller percentage of the maximum EUV energy. In some aspects, the adjustment of rarefaction pulse laser source 442 alone can produce an adjustment of relative EUV energy 890 within a range of about 50% to about 70% of maximum EUV power level. Accordingly, adjustment of rarefaction pulse laser source 442 alone can yield about 20% modulation depth for the output of EUV pulse energy with controller 452. In the exemplary aspect shown in FIG.8, rarefaction pulse adjustment response 896 can produce 70% of the maximum EUV energy at a main pulse laser gain command 892 of 1.0.

[0110] In some aspects, the adjustment of main pulse laser source 444 and rarefaction pulse laser source 442 in combination can produce an adjustment of relative EUV energy 890 within a range of about 50% to about 100% of maximum EUV power level. Accordingly, an adjustment of main pulse laser source 444 and rarefaction pulse laser source 442 in combination can yield about 50% modulation depth for the output of EUV pulse energy with controller 452.

[0111] Example EUV Energy Output Adjustment Method

[0112] FIG. 9 shows a method 900 of adjusting an EUV energy output of an EUV light generation system, according to some aspects. In some aspects, at step S902, a droplet can be irradiated with a prepulse laser beam suitable to reshape the droplet into a target.

[0113] In some aspects, at step S904, the target can be irradiated with a rarefaction pulse laser beam suitable to modify one or more of a density of the target or an absorption characteristic of the target.

[0114] In some aspects, at step S906, the modified target can be irradiated with a main pulse laser beam suitable to generate EUV light.

[0115] In some aspects, at step S908, an EUV energy output of the EUV light can be measured with a sensor.

[0116] In some aspects, at step S910, a controller can adjust at least one of the rarefaction pulse laser beam or the main pulse laser beam in response to the measured EUV energy output. For example, a controller can adjust at least one of the rarefaction pulse laser beam and the main pulse laser beam, either alone, in alternation, or in combination, to control the EUV energy output.

[0117] In some aspects, adjusting the rarefaction pulse laser beam can comprise adjusting a rarefaction pulse energy. In some aspects, adjusting the rarefaction pulse energy can comprise providing up to about 50% of a modulation depth to the controller in addition to any modulation depth provided by adjusting the main pulse laser beam.

[0118] In some aspects, adjusting the rarefaction pulse laser beam can comprise adjusting a rarefaction pulse duration. In some aspects, adjusting the rarefaction pulse duration can comprise providing up to about 10% of a modulation depth to the controller in addition to any modulation depth provided by adjusting the main pulse laser beam.

[0119] In some aspects, adjusting the rarefaction pulse laser beam can comprise adjusting a relative timing between the rarefaction pulse laser beam and at least one of the pre -pulse laser beam or the main pulse laser beam. In some aspects, adjusting the relative timing between the rarefaction pulse laser beam and at least one of the pre-pulse laser beam or the main pulse laser beam can comprise providing up to about 35% of a modulation depth to the controller in addition to any modulation depth provided by adjusting the main pulse laser beam.

[0120] The method steps of FIG. 9 can be performed in any conceivable order and it is not required that all steps be performed. Moreover, the method steps of FIG. 9 described above merely reflect anexample of steps and are not limiting. That is, further method steps and functions are envisaged based aspects described in reference to FIGS. 1-8.

[0121] 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. 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.

[0122] 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, UCDs, 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.

[0123] 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.

[0124] 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 andmodifications are intended to be within the meaning and range of equivalents of the disclosed aspects, based on the teaching and guidance presented herein.

[0125] 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.

[0126] Aspects and implementations of the present disclosure can be further described using the following numbered clauses:1. A method of adjusting an extreme ultraviolet (EUV) energy output of an EUV light generation system, the method comprising: irradiating a droplet with a pre-pulse laser beam suitable to reshape the droplet into a target; irradiating the target with a rarefaction pulse laser beam suitable to modify one or more of a density of the target or an absorption characteristic of the target; irradiating the modified target with a main pulse laser beam suitable to generate EUV light; measuring an EUV energy output of the EUV light with a sensor; and adjusting at least one of the rarefaction pulse laser beam or the main pulse laser beam in response to the measured EUV energy output.2. The method of clause 1, wherein adjusting the rarefaction pulse laser beam comprises adjusting a rarefaction pulse energy.3. The method of clause 2, wherein adjusting the rarefaction pulse energy comprises providing up to about 50% of a modulation depth of the EUV energy output in addition to any modulation depth of the EUV energy output provided by adjusting the main pulse laser beam.4. The method of clause 2, wherein adjusting the rarefaction pulse energy comprises adjusting the rarefaction pulse energy between about 35% and about 80% of a maximum rarefaction pulse energy value.5. The method of clause 1 , wherein adjusting the rarefaction pulse laser beam comprises adjusting a rarefaction pulse duration.6. The method of clause 5, wherein adjusting the rarefaction pulse duration comprises providing up to about 10% of a modulation depth of the EUV energy output in addition to any modulation depth of the EUV energy output provided by adjusting the main pulse laser beam.7. The method of clause 5, wherein adjusting the rarefaction pulse duration comprises adjusting the rarefaction pulse duration between about 5 ns and about 25 ns.8. The method of clause 1, wherein adjusting the rarefaction pulse laser beam comprises adjusting a relative timing between the rarefaction pulse laser beam and at least one of the pre -pulse laser beam or the main pulse laser beam.9. The method of clause 8, wherein adjusting the relative timing between the rarefaction pulse laser beam and at least one of the pre-pulse laser beam or the main pulse laser beam comprises providing up to about 35% of a modulation depth of the EUV energy output in addition to any modulation depth of the EUV energy output provided by adjusting the main pulse laser beam.10. The method of clause 8, wherein adjusting the relative timing between the rarefaction pulse laser beam and at least one of the pre-pulse laser beam or the main pulse laser beam comprises adjusting the relative timing between about 30 ns to about 150 ns.11. An extreme ultraviolet (EUV) light generation system comprising: one or more laser sources configured to produce a pre-pulse laser beam, a rarefaction pulse laser beam, and a main pulse laser beam; a droplet generator configured to provide a droplet at a predetermined irradiation region inside a chamber of the system; a controller configured to: irradiate the droplet with the pre-pulse laser beam at the predetermined irradiation region inside the chamber, the pre-pulse laser beam suitable to reshape the droplet into a target, irradiate the target with the rarefaction pulse laser beam at the predetermined irradiation region inside the chamber, the rarefaction pulse laser beam suitable to modify one or more of a density of the target or an absorption characteristic of the target, irradiate the modified target with the main pulse laser beam at the predetermined irradiation region inside the chamber, the main pulse laser beam suitable to generate EUV light, and adjust at least one of the rarefaction pulse laser beam or the main pulse laser beam in response to an EUV energy output of the EUV light measured by a sensor; and a collecting optical device, positioned inside the chamber, configured to collect the EUV light.12. The EUV light generation system of clause 11, wherein the controller is configured to adjust a rarefaction pulse energy.13. The EUV light generation system of clause 12, wherein adjusting the rarefaction pulse energy comprises providing up to about 50% of a modulation depth of the EUV energy output in addition to any modulation depth of the EUV energy output provided by adjusting the main pulse laser beam.14. The EUV light generation system of clause 12, wherein adjusting the rarefaction pulse energy comprises adjusting the rarefaction pulse energy between about 35% and about 80% of a maximum rarefaction pulse energy value.15. The EUV light generation system of clause 11, wherein the controller is configured to adjust a rarefaction pulse duration.16. The EUV light generation system of clause 15, wherein adjusting the rarefaction pulse duration comprises providing up to about 10% of a modulation depth of the EUV energy output in addition to any modulation depth of the EUV energy output provided by adjusting the main pulse laser beam.17. The EUV light generation system of clause 15, wherein adjusting the rarefaction pulse duration comprises adjusting the rarefaction pulse duration between about 5 ns and about 25 ns.18. The EUV light generation system of clause 11, wherein the controller is configured to adjust a relative timing between the rarefaction pulse laser beam and at least one of the pre -pulse laser beam or the main pulse laser beam.19. The EUV light generation system of clause 18, wherein adjusting the relative timing between the rarefaction pulse laser beam and at least one of the pre -pulse laser beam or the main pulse laser beam comprises providing up to about 35% of a modulation depth of the EUV energy output in addition to any modulation depth of the EUV energy output provided by adjusting the main pulse laser beam.20. The EUV light generation system of clause 18, wherein adjusting the relative timing between the rarefaction pulse laser beam and at least one of the pre -pulse laser beam or the main pulse laser beam comprises adjusting the relative timing between about 30 ns to about 150 ns.21. A lithographic apparatus comprising: an extreme ultraviolet (EUV) light generation system comprising: one or more laser sources configured to produce a pre-pulse laser beam, a rarefaction pulse laser beam, and a main pulse laser beam; a droplet generator configured to provide a droplet at a predetermined irradiation region inside a chamber of the system; a controller configured to: irradiate the droplet with the pre-pulse laser beam at the predetermined irradiation region inside the chamber, the pre-pulse laser beam suitable to reshape the droplet into a target, irradiate the target with the rarefaction pulse laser beam at the predetermined irradiation region inside the chamber, the rarefaction pulse laser beam suitable to modify one or more of a density of the target or an absorption characteristic of the target, irradiate the modified target with the main pulse laser beam at the predetermined irradiation region inside the chamber, the main pulse laser beam suitable to generate EUV light, and adjust at least one of the rarefaction pulse laser beam or the main pulse laser beam in response to an EUV energy output of the EUV light measured by a sensor; and a collecting optical device, positioned inside the chamber, configured to collect the EUV light; an illumination system configured to illuminate a pattern of a patterning device, the patterning device configured to impart a pattern on the EUV light; and a projection system configured to project an image of the pattern onto a substrate.22. The lithographic apparatus of clause 21, wherein the controller is configured to adjust a rarefaction pulse energy.23. The lithographic apparatus of clause 22, wherein adjusting the rarefaction pulse energy comprises providing up to about 50% of a modulation depth of the EUV energy output in addition to any modulation depth of the EUV energy output provided by adjusting the main pulse laser beam.24. The lithographic apparatus of clause 22, wherein adjusting the rarefaction pulse energy comprises adjusting the rarefaction pulse energy between about 35% and about 80% of a maximum rarefaction pulse energy value.25. The lithographic apparatus of clause 21, wherein the controller is configured to adjust a rarefaction pulse duration.26. The lithographic apparatus of clause 25, wherein adjusting the rarefaction pulse duration comprises providing up to about 10% of a modulation depth of the EUV energy output in addition to any modulation depth of the EUV energy output provided by adjusting the main pulse laser beam.27. The lithographic apparatus of clause 25, wherein adjusting the rarefaction pulse duration comprises adjusting the rarefaction pulse duration between about 5 ns and about 25 ns.28. The lithographic apparatus of clause 21 , wherein the controller is configured to adjust a relative timing between the rarefaction pulse laser beam and at least one of the pre -pulse laser beam or the main pulse laser beam.29. The lithographic apparatus of clause 28, wherein adjusting the relative timing between the rarefaction pulse laser beam and at least one of the pre -pulse laser beam or the main pulse laser beam comprises providing up to about 35% of a modulation depth of the EUV energy output in addition to any modulation depth of the EUV energy output provided by adjusting the main pulse laser beam.30. The lithographic apparatus of clause 28, wherein adjusting the relative timing between the rarefaction pulse laser beam and at least one of the pre -pulse laser beam or the main pulse laser beam comprises adjusting the relative timing between about 30 ns to about 150 ns.

[0127] The above-described aspects and implementations and other implementations are within the scope of the following claims.

Claims

CLAIMS1. A method of adjusting an extreme ultraviolet (EUV) energy output of an EUV light generation system, the method comprising: irradiating a droplet with a pre-pulse laser beam suitable to reshape the droplet into a target; irradiating the target with a rarefaction pulse laser beam suitable to modify one or more of a density of the target or an absorption characteristic of the target; irradiating the modified target with a main pulse laser beam suitable to generate EUV light; measuring an EUV energy output of the EUV light with a sensor; and adjusting at least one of the rarefaction pulse laser beam or the main pulse laser beam in response to the measured EUV energy output.

2. The method of claim 1, wherein adjusting the rarefaction pulse laser beam comprises adjusting a rarefaction pulse energy.

3. The method of claim 2, wherein adjusting the rarefaction pulse energy comprises providing up to about 50% of a modulation depth of the EUV energy output in addition to any modulation depth of the EUV energy output provided by adjusting the main pulse laser beam.

4. The method of claim 2, wherein adjusting the rarefaction pulse energy comprises adjusting the rarefaction pulse energy between about 35% and about 80% of a maximum rarefaction pulse energy value.

5. The method of claim 1, wherein adjusting the rarefaction pulse laser beam comprises adjusting a rarefaction pulse duration, wherein adjusting the rarefaction pulse duration comprises providing up to about 10% of a modulation depth of the EUV energy output in addition to any modulation depth of the EUV energy output provided by adjusting the main pulse laser beam.

6. The method of claim 5, wherein adjusting the rarefaction pulse duration comprises adjusting the rarefaction pulse duration between about 5 ns and about 25 ns.

7. The method of claim 1, wherein adjusting the rarefaction pulse laser beam comprises adjusting a relative timing between the rarefaction pulse laser beam and at least one of the pre -pulse laser beam or the main pulse laser beam, wherein adjusting the relative timing between the rarefaction pulse laser beam and at least one of the pre-pulse laser beam or the main pulse laser beam comprises providing up to about 35% of a modulation depth of the EUV energy output in addition to any modulation depth of the EUV energy output provided by adjusting the main pulse laser beam.

8. The method of claim 7, wherein adjusting the relative timing between the rarefaction pulse laser beam and at least one of the pre-pulse laser beam or the main pulse laser beam comprises adjusting the relative timing between about 30 ns to about 150 ns.

9. An extreme ultraviolet (EUV) light generation system comprising: one or more laser sources configured to produce a pre-pulse laser beam, a rarefaction pulse laser beam, and a main pulse laser beam; a droplet generator configured to provide a droplet at a predetermined irradiation region inside a chamber of the system; a controller configured to: irradiate the droplet with the pre-pulse laser beam at the predetermined irradiation region inside the chamber, the pre-pulse laser beam suitable to reshape the droplet into a target, irradiate the target with the rarefaction pulse laser beam at the predetermined irradiation region inside the chamber, the rarefaction pulse laser beam suitable to modify one or more of a density of the target or an absorption characteristic of the target, irradiate the modified target with the main pulse laser beam at the predetermined irradiation region inside the chamber, the main pulse laser beam suitable to generate EUV light, and adjust at least one of the rarefaction pulse laser beam or the main pulse laser beam in response to an EUV energy output of the EUV light measured by a sensor; and a collecting optical device, positioned inside the chamber, configured to collect the EUV light.

10. The EUV light generation system of claim 9, wherein the controller is configured to adjust a rarefaction pulse energy.

11. The EUV light generation system of claim 10, wherein adjusting the rarefaction pulse energy comprises providing up to about 50% of a modulation depth of the EUV energy output in addition to any modulation depth of the EUV energy output provided by adjusting the main pulse laser beam.

12. The EUV light generation system of claim 10, wherein adjusting the rarefaction pulse energy comprises adjusting the rarefaction pulse energy between about 35% and about 80% of a maximum rarefaction pulse energy value.

13. The EUV light generation system of claim 9, wherein the controller is configured to adjust a rarefaction pulse duration.

14. The EUV light generation system of claim 13, wherein adjusting the rarefaction pulse duration comprises providing up to about 10% of a modulation depth of the EUV energy output in addition to any modulation depth of the EUV energy output provided by adjusting the main pulse laser beam.

15. The EUV light generation system of claim 13, wherein adjusting the rarefaction pulse duration comprises adjusting the rarefaction pulse duration between about 5 ns and about 25 ns.

16. The EUV light generation system of claim 9, wherein the controller is configured to adjust a relative timing between the rarefaction pulse laser beam and at least one of the pre-pulse laser beam or the main pulse laser beam.

17. The EUV light generation system of claim 16, wherein adjusting the relative timing between the rarefaction pulse laser beam and at least one of the pre-pulse laser beam or the main pulse laser beam comprises providing up to about 35% of a modulation depth of the EUV energy output in addition to any modulation depth of the EUV energy output provided by adjusting the main pulse laser beam.

18. The EUV light generation system of claim 16, wherein adjusting the relative timing between the rarefaction pulse laser beam and at least one of the pre-pulse laser beam or the main pulse laser beam comprises adjusting the relative timing between about 30 ns to about 150 ns.

19. A lithographic apparatus comprising: an extreme ultraviolet (EUV) light generation system comprising: one or more laser sources configured to produce a pre-pulse laser beam, a rarefaction pulse laser beam, and a main pulse laser beam; a droplet generator configured to provide a droplet at a predetermined irradiation region inside a chamber of the system; a controller configured to: irradiate the droplet with the pre-pulse laser beam at the predetermined irradiation region inside the chamber, the pre-pulse laser beam suitable to reshape the droplet into a target, irradiate the target with the rarefaction pulse laser beam at the predetermined irradiation region inside the chamber, the rarefaction pulse laser beam suitable to modify one or more of a density of the target or an absorption characteristic of the target, irradiate the modified target with the main pulse laser beam at the predetermined irradiation region inside the chamber, the main pulse laser beam suitable to generate EUV light, andadjust at least one of the rarefaction pulse laser beam or the main pulse laser beam in response to an EUV energy output of the EUV light measured by a sensor; and a collecting optical device, positioned inside the chamber, configured to collect the EUV light; an illumination system configured to illuminate a pattern of a patterning device, the patterning device configured to impart a pattern on the EUV light; and a projection system configured to project an image of the pattern onto a substrate.

20. The lithographic apparatus of claim 19, wherein the controller is configured to adjust a rarefaction pulse energy, wherein adjusting the rarefaction pulse energy comprises providing up to about 50% of a modulation depth of the EUV energy output in addition to any modulation depth of the EUV energy output provided by adjusting the main pulse laser beam.

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