Systems and methods of laser-to-droplet positioning with tilt range keep assist during extreme ultraviolet radiation generation

Abstract

An extreme ultraviolet light source includes a laser, a target and a controller. The target may be a fuel droplet. The controller may determine optimal laser-to-droplet alignment positions in two orthogonal directions and update the position of the laser to maintain optimal alignment between the laser and fuel droplet during radiation generation. A method of determining an optimal laser-to-droplet alignment position includes fitting a polynomial to a radiation generation metric that varies based on the laser-to- droplet alignment position, and determining an optimal alignment position for the laser relative to the fuel droplet based on an apex of the polynomial fit; or evaluating commanded laser energy versus laser- to-droplet alignment position across an available alignment space to determine where commanded laser energy is maximum, and determining an optimal alignment position for the laser relative to the fuel droplet so that the alignment space is symmetric between the maximum points.

Description

SYSTEMS AND METHODS OF LASER-TO-DROPLET POSITIONING WITH TILT RANGE KEEP ASSIST DURING EXTREME ULTRAVIOLET RADIATION GENERATIONCROSS-REFERENCE TO RELATED APPLICATIONS

[0001] This application claims priority of US Application No. 63 / 615,995, filed on December 29, 2023, titled SYSTEMS AND METHODS OF LASER-TO-DROPLET POSITIONING WITH TILT RANGE KEEP ASSIST DURING EXTREME ULTRAVIOLET RADIATION GENERATION, and US Application No. 63 / 650,086, filed on May 21, 2024, titled SYSTEMS AND METHODS OF LASER-TO-DROPLET POSITIONING WITH TILT RANGE KEEP ASSIST DURING EXTREME ULTRAVIOLET RADIATION GENERATION, which are incorporated herein by reference in their entirety.FIELD

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

[0003] A lithographic apparatus is a machine that applies a desired pattern onto a substrate, usually onto a target portion of the substrate. A lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In that instance, a patterning device, which can be a mask or a reticle, can be used to generate a circuit pattern to be formed on an individual layer of the IC. This pattern can be transferred onto a target portion (e.g., comprising part of, one, or several dies) on a substrate (e.g., a silicon wafer). Transfer 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 havingwavelengths 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 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] When a EUV source system is set up at commissioning or after system service actions, a calibration test can be performed, which determines the parameter settings to be used for system operation in order to achieve optimum EUV power and dose performance. This parameter optimization leads to parameter settings that affect a target (source material) tilt. Due to system -to-system variability and system drifts, the parameter optimization leads to different parameter settings and thus different target tilts for different systems, or for the same system over time.SUMMARY

[0008] Accordingly, it is desirable to improve positioning of a laser relative to a fuel droplet / target to correct system drift and optimize EUV radiation generation.

[0009] In some aspects, an extreme ultraviolet (EUV) light source may produce plasma by irradiating a fuel droplet / target with a laser beam. In some aspects, a controller may continuously track optimal laser-to-droplet alignment positions in two orthogonal directions and update the position of the laser to maintain optimal alignment during radiation generation.

[0010] In some aspects, a method of optimizing a laser-to-droplet alignment position can include fitting a polynomial to a radiation generation metric that varies based on the laser-to-droplet alignment position, and determining an optimal alignment position for the laser relative to the fuel droplet / target based on an apex of the polynomial fit. In some aspects, a method of optimizing a laser-to-droplet alignment position can include evaluating commanded drive laser power versus laser-to-droplet alignment position. This can be performed across an available alignment space to determine where commanded drive laser power is maximum, and determining an optimal alignment position for the laser relative to the fuel droplet / target so that the alignment space is symmetric between the maximum points. In some aspects, a laser-to-droplet alignment position may be modified to adhere to an optimal laser- to-droplet alignment position.

[0011] In some aspects, an acceptable range of X tilt thresholds and an acceptable range of Y tilt thresholds for a fuel droplet / target relative to a laser may be determined. In some aspects, a minimum and a maximum possible laser-to-droplet X alignment position are calculated so that a fuel droplet / targetsatisfies the range of Y tilt thresholds, and a minimum and maximum laser-to-droplet Y alignment position are calculated so that the fuel droplet / target satisfies the range of X tilt thresholds. In some aspects, an optimal laser-to-droplet alignment position may be calculated, and an alignment position of a laser may be modified. In some aspects, the optimal laser-to-droplet alignment position may be checked to ensure that the optimal alignment position falls within the minimum and maximum X and Y alignment positions. In some aspects, the alignment position of the laser may be adjusted to comply with X and Y tilt thresholds, even if the calculated optimal laser X and Y alignment positions fall outside of the X and Y tilt thresholds.

[0012] In some aspects, a computer-readable storage medium contains program instructions for a method being executed by an application. In some aspects, the application comprises code for one or more components that are called by the application during runtime. Execution of the program instructions by one or more processors of a computer system causes the one or more processors to perform one or more operations similar to the method described above.

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

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

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

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

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

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

[0019] FIG. 6 shows a method of optimizing and modifying a laser-to-droplet alignment position, according to some aspects.

[0020] FIGS. 7A and 7B show greyscale plots of EUV radiation counts as a function of laser-to-droplet alignment position and EUV energy, according to some aspects.

[0021] FIGS. 8 A and 8B show greyscale plots of commanded drive laser power as a function of laser- to-droplet X and Y alignment positions, according to some aspects.

[0022] FIG. 9 shows a coordinate system for aligning a laser to a target, according to some aspects.

[0023] FIG. 10 shows an example of laser back reflection mitigation, according to some aspects.

[0024] FIG. 11 shows an example of debris mitigation, according to some aspects.

[0025] FIG. 12 shows a process window for optimal X and Y tilts of a target, according to some aspects.

[0026] FIG. 13 shows example EUV loss event, according to some aspects.

[0027] FIG. 14 shows an implementation of a range keep assist method, according to some aspects.

[0028] FIG. 15 shows a range keep assist control loop, according to some aspects.

[0029] FIG. 16 shows a computer system, according to some aspects.

[0030] 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

[0031] 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.

[0032] 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.

[0033] 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).

[0034] 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 readableby a machine (e.g., a computing device). For example, a machine -readable medium can include 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.

[0035] 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.

[0036] Example Lithographic Systems

[0037] FIG. 1 shows a lithographic apparatus 100 in which aspects of the present disclosure can be implemented. Lithographic apparatus 100 includes 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 the 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 the substrate W. Lithographic apparatus 100 also has a projection system PS configured to project a pattern imparted to the radiation beam B by patterning device MA onto a target portion (for example, comprising one or more dies) C of the substrate W. In lithographic apparatus 100, the patterning device MA and the projection system PS are reflective.

[0038] The illumination system IL can include 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. The illumination system IL can also include 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. The illumination system IL can include a measurement sensor MS for measuring a movement of the radiation beam B and uniformity compensators UC that allow an illumination slit uniformity to be controlled. The measurement sensor MS can also be disposed at other locations. For example, the measurement sensor MS can be on or near the substrate table WT.

[0039] The support structure MT holds the 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. The support structure MT can use mechanical, vacuum, electrostatic, or other clamping techniques to hold the patterning device MA. The support structure MT can be a frame or a table, for example, which can be fixed or movable. By using sensors, the support structure MT can ensure that the patterning device MA is at a desired position, for example, with respect to the projection system PS.

[0040] The term “patterning device” MA should be broadly interpreted as referring 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 the target portion C of the substrate W. The pattern imparted to the radiation beam B can correspond to a particular functional layer in a device being created in the target portion C to form an integrated circuit.

[0041] The patterning device MA can be reflective. Examples of patterning devices MA include reticles, masks, programmable mirror arrays, or programmable LCD panels. Masks are well known in lithography, and include 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 impart a pattern in the radiation beam B, which is reflected by a matrix of small mirrors.

[0042] The term “projection system” PS can encompass 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. A vacuum environment can be used for EUV or electron beam radiation since other gases can absorb too much radiation or electrons. A vacuum environment can therefore be provided to the whole beam path with the aid of a vacuum wall and vacuum pumps.

[0043] 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, the 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 not be a substrate table WT.

[0044] The lithographic apparatus can also be of a type wherein 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 fill 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 are well known in the art for increasing 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.

[0045] The illuminator IL receives a radiation beam from a radiation source SO. The source SO and the lithographic apparatus 100 can be separate physical entities, for example, when the source SO is an excimer laser. In such cases, the source SO is not considered to form part of the lithographic apparatus 100 and the radiation beam B passes from the source SO to the illuminator IL with the aid of a beam delivery system BD (not shown) including, for example, suitable directing mirrors and / or a beam expander. In other cases, the source SO can be an integral part of the lithographic apparatus 100, for example, when the source SO is a mercury lamp. A radiation system can comprise the source SO, the illuminator IL, and / or the beam delivery system BD.

[0046] The illuminator IL can be used to condition the 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 a uniformity compensator. Uniformity compensator comprises 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. A sensor can be used to monitor the uniformity of radiation beam B.

[0047] The radiation beam B is incident on the patterning device (for example, mask) MA, which is held on the support structure (for example, mask table) MT, and is patterned by the patterning device MA. In lithographic apparatus 100, the radiation beam B is reflected from the patterning device (for example, mask) MA. After being reflected from the patterning device (for example, mask) MA, the radiation beam B passes through the 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), the 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, the 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 (for example, mask) MA and substrate W can be aligned using mask alignment marks Ml, M2 and substrate alignment marks PI, P2.

[0048] The lithographic apparatus 100 can be used in at least one of the following modes:

[0049] 1. In step mode, the support structure (for example, mask table) MT and the substrate table WT are kept essentially stationary, while an entire pattern imparted to the radiation beam B is projected onto a target portion C at one time (i.e., a single static exposure). The substrate table WT is then shifted in the X and / or Y direction so that a different target portion C can be exposed.

[0050] 2. In scan mode, the support structure (for example, mask table) MT and the substrate table WT are scanned synchronously while a pattern imparted to the radiation beam B is projected onto a target portion C (i.e., a single dynamic exposure). The velocity and direction of the substrate tableWT relative to the support structure (for example, mask table) MT can be determined by the (de- )magnification and image reversal characteristics of the projection system PS.

[0051] 3. In another mode, the support structure (for example, mask table) MT is kept substantially stationary holding a programmable patterning device, and the substrate table WT is moved or scanned while a pattern imparted to the 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 as needed after each movement of the 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.

[0052] Combinations and / or variations on the described modes of use or entirely different modes of use can also be employed.

[0053] In a further aspect, lithographic apparatus 100 includes EUV radiation source configured to generate a beam of EUV radiation for EUV lithography. In general, the EUV radiation source is configured in a radiation system, and a corresponding illumination system is configured to condition the EUV radiation beam of the EUV source.

[0054] FIG. 2A shows a lithographic apparatus in more detail, including the source collector apparatus SO, the illumination system IL, and the projection system PS, according to some aspects. The source collector apparatus SO is constructed and arranged such that a vacuum environment can be maintained in an enclosing structure 220 of the source collector apparatus SO. An EUV radiation emitting plasma 210 can be formed by a discharge produced plasma source. In some aspects, a plasma of excited tin (Sn) (e.g., excited via a laser) is provided to produce EUV radiation.

[0055] The radiation emitted by the EUV radiation emitting plasma 210 is passed from a source chamber 211 into a collector chamber 212 via an optional gas barrier or contaminant trap 230 (in some cases also referred to as contaminant barrier or foil trap), which is positioned in or behind an opening in source chamber 211. The contaminant trap 230 can include a channel structure. Contamination trap 230 can also include a gas barrier or a combination of a gas barrier and a channel structure. The contaminant trap or contaminant barrier 230 further indicated herein at least includes a channel structure.

[0056] The collector chamber 212 can include a radiation collector CO, which can be a so-called grazing incidence collector. Radiation collector CO has an upstream radiation collector side 251 and a downstream radiation collector side 252. Radiation that traverses collector CO can be reflected off a grating spectral filter 240 to be focused in a virtual source point INTF. The virtual source point INTF is commonly referred to as the intermediate focus, and the source collector apparatus is arranged such that the intermediate focus INTF is located at or near an opening 219 in the enclosing structure 220. The virtual source point INTF is an image of the EUV radiation emitting plasma 210. Grating spectral filter 240 is used in particular for suppressing infra-red (IR) radiation.

[0057] Subsequently the radiation traverses the illumination system IL, which can include a faceted field mirror device 222 and a faceted pupil mirror device 224 arranged to provide a desired angular distribution of the radiation beam 221, at the patterning device MA, as well as a desired uniformity of radiation intensity at the patterning device MA. Upon reflection of the beam of radiation 221 at the patterning device MA, held by the support structure MT, a patterned beam 226 is formed and the patterned beam 226 is imaged by the projection system PS via reflective elements 228, 229 onto a substrate W held by the wafer stage or substrate table WT. This is intended to be exemplary only, as other illumination systems utilize various other different mirrors and optical devices to direct radiation beam 221 to the patterning device MA.

[0058] More elements than shown can generally be present in illumination optics unit IL and projection system PS. The grating spectral filter 240 can optionally be present, depending upon the type of lithographic apparatus. Further, there can be more mirrors present than those shown in the FIG. 2A, for example there can be one to six additional reflective elements present in the projection system PS than shown in FIG. 2A.

[0059] In some aspects, illumination optics unit IL can include 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 optics unit IL can include a measurement sensor MS for measuring a movement of the radiation beam B and uniformity compensators UC that allow an illumination slit uniformity to be controlled. The measurement sensor MS can also be disposed at other locations. For example, the measurement sensor MS can be on or near the substrate table WT.

[0060] Collector optic CO, as illustrated in FIG. 2A, is depicted as a nested collector with grazing incidence reflectors 253, 254, and 255, just as an example of a collector (or collector mirror). The grazing incidence reflectors 253, 254, and 255 are disposed axially symmetric around an optical axis O and a collector optic CO of this type is preferably used in combination with a discharge produced plasma source, often called a DPP source.

[0061] FIG. 2B shows selected portions of lithographic apparatus 100 (e.g., FIG. 1), but with alternative collection optics in the source collector apparatus SO, according to some aspects. It should be appreciated that structures shown in FIG. 2A that do not appear in FIG. 2B (for drawing clarity) can still be included in aspects referring to FIG. 2B. Elements in FIG. 2B having the same reference numbers as those in FIG. 2A have the same or substantially similar structures and functions as described in reference to FIG. 2A. In some aspects, the lithographic apparatus 100 can be used, for example, to expose a substrate W such as a resist coated wafer with a patterned beam of EUV light. In FIG. 2B, the illumination system IL and the projection system PS are represented combined as an exposure device 256 (e.g., an integrated circuit lithography tool such as a stepper, scanner, step and scan system, direct write system, device using a contact and / or proximity mask, etc.) that uses EUV light from the source collector apparatus SO. The lithographic apparatus 100 can also include collector optic 258 that reflects EUV light from the EUV radiation emitting plasma 210 along a path into the exposure device 256 toirradiate the substrate W. Collector optic 258 can comprise a near-normal incidence collector mirror having a reflective surface in the form of a prolate spheroid (i.e., an ellipse rotated about its major axis) having, e.g., 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.

[0062] FIG. 3 shows a detailed view of a portion of lithographic apparatus 100 (e.g., FIGS. 1, 2A, and 2B), according to one or more aspects. Elements in FIG. 3 having the same reference numbers as those in FIGS. 1, 2A, and 2B have the same or substantially similar structures and functions as described in reference to FIGS. 1, 2A, and 2B. In some aspects, lithographic apparatus 100 can include a source collector apparatus SO having an LPP EUV light radiator. As shown, the source collector apparatus SO can include a laser system 302 for generating a train of light pulses and delivering the light pulses into a light source chamber 212. For the lithographic apparatus 100, the light pulses can travel along one or more beam paths from the laser system 302 and into the chamber 212 to illuminate a source material at an irradiation region 304 to generate a plasma (e.g., plasma region located at EUV radiation emitting plasma 210 in FIG. 2B) that produces EUV light for substrate exposure in the exposure device 256.

[0063] In some aspects, suitable lasers for use in the laser system 302 can include a pulsed laser device, e.g., a pulsed gas discharge CO2 laser device producing radiation at 9.3 pm or 10.6 pm, e.g., with DC or RF excitation, operating at relatively high power, e.g., 10 kW or higher and high pulse repetition rate, e.g., 50 kHz or more. In some aspects, the laser can be an axial -flow RF-pumped CO2 laser having an oscillator amplifier configuration (e.g., master oscillator / power amplifier (MOPA) or power oscillator / power amplifier (POPA)) with multiple stages of amplification and having a seed pulse that is initiated by a Q-switched oscillator with relatively low energy and high repetition rate, e.g., capable of 100 kHz operation. From the oscillator, the laser pulse can then be amplified, shaped and / or focused before reaching the irradiation region 304. Continuously pumped CO2 amplifiers can be used for the laser system 302. Alternatively, the laser can be configured as a so-called “self-targeting” laser system in which the droplet serves as one mirror of the optical cavity of the laser.

[0064] In some aspects, depending on the application, other types of lasers can also be suitable, e.g., an excimer or molecular fluorine laser operating at high power and high pulse repetition rate. Some examples include, a solid state laser, e.g., having a fiber, rod, slab, or disk-shaped active media, other laser architectures having one or more chambers, e.g., an oscillator chamber and one or more amplifying chambers (with the amplifying chambers in parallel or in series), a master oscillator / power oscillator (MOPO) arrangement, a master oscillator / power ring amplifier (MOPRA) arrangement, or a solid state laser that seeds one or more excimer, molecular fluorine or CO2 amplifier or oscillator chambers, can be suitable. Other suitable designs are envisaged.

[0065] In some aspects, a source material can first be irradiated by a pre -pulse and thereafter irradiated by a main pulse. Pre-pulse and main pulse seeds can be generated by a single oscillator or two separate oscillators. One or more common amplifiers can be used to amplify both the pre -pulse seed and mainpulse seed. In some aspects, separate amplifiers can be used to amplify the pre-pulse and main pulse seeds.

[0066] In some aspects, the lithographic apparatus 100 can include a beam conditioning unit 306 having one or more optics for beam conditioning such as expanding, steering, and / or focusing the beam between the laser system 302 and irradiation region 304. For example, a steering system, which can include one or more mirrors, prisms, lenses, etc., can be provided and arranged to steer the laser focal spot to different locations in the chamber 212. For example, the steering system can include a first flat mirror mounted on a tip-tilt actuator which can move the first mirror independently in two dimensions, and a second flat mirror mounted on a tip-tilt actuator which can move the second mirror independently in two dimensions. With the described arrangement(s), the steering system can controllably move the focal spot in directions substantially orthogonal to the direction of beam propagation (beam axis or optical axis).

[0067] The beam conditioning unit 306 can include a focusing assembly to focus the beam to the irradiation region 304 and adjust the position of the focal spot along the beam axis. For the focusing assembly, an optic, such as a focusing lens or mirror, can be used that is coupled to an actuator for movement in a direction along the beam axis to move the focal spot along the beam axis.

[0068] In some aspects, the source collector apparatus SO can also include a source material delivery system 308, e.g., delivering source material, such as tin droplets, into the interior of chamber 212 to an irradiation region 304, where the droplets will interact with light pulses from the laser system 302, to ultimately produce plasma and generate an EUV emission to expose a substrate such as a resist coated wafer in the exposure device 256. More details regarding various droplet dispenser configurations can be found in, e.g., U.S. Pat. No. 7,872,245, issued on January 18, 2011, titled “Systems and Methods for Target Material Delivery in a Laser Produced Plasma EUV Light Source”, U.S. Pat. No. 7,405,416, issued on July 29, 2008, titled “Method and Apparatus For EUV Plasma Source Target Delivery”, U.S. Pat. No. 7,372,056, issued on May 13, 2008, titled “LPP EUV Plasma Source Material Target Delivery System”, and International Appl. No. WO 2019 / 137846, titled “Apparatus for and Method of Controlling Coalescence of Droplets In a Droplet Stream”, published on July 18, 2019, the contents of each of which are incorporated by reference herein in their entirety.

[0069] 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 EUV emitting element, e.g., tin, lithium, xenon, etc., 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 atin compound, e.g., SnBr4. SnBr2, SnH4. as atin 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 can be presented to the irradiation region at various temperatures including room temperature or near room temperature (e.g., tin alloys, SnBr4), at an elevated temperature, (e.g., pure tin) or attemperatures below room temperature, (e.g., SnH ). and in some cases, can be relatively volatile, e.g., SnBr4.

[0070] In some aspects, the lithographic apparatus 100 can also include a controller 310, which can also include a drive laser control system 312 for controlling devices in the laser system 302 to thereby generate light pulses for delivery into the chamber 212, and / or for controlling movement of optics in the beam conditioning unit 306. The lithographic apparatus 100 can also include a droplet position detection system which can include one or more droplet imagers 314 that provide an output signal indicative of the position of one or more droplets, e.g., relative to the irradiation region 304. The droplet imager(s) 314 can provide this output to a droplet position detection feedback system 316, which can, e.g., compute a droplet position and trajectory, from which a droplet error can be computed, e.g., on a droplet-by-droplet basis, or on average. The droplet error can then be provided as an input to the controller 310, which can, for example, provide a position, direction and / or timing correction signal to the laser system 302 to control laser trigger timing and / or to control movement of optics in the beam conditioning unit 306, e.g., to change the location and / or focal power of the light pulses being delivered to the irradiation region 304 in the chamber 212. Also for the source collector apparatus SO, the source material delivery system 308 can have a control system operable in response to a signal (which in some implementations can include the droplet error described above, or some quantity derived therefrom) from the controller 310, to e.g., modify the release point, initial droplet stream direction, droplet release timing and / or droplet modulation to correct for errors in the droplets arriving at the irradiation region 304.

[0071] In some aspects, the lithographic apparatus 100 can also include a collector optic 258 and a gas dispenser device 320. Gas dispenser device 320 can dispense gas in the path of the source material from the source material delivery system 308 (e.g., irradiation region 304). Gas dispenser device 320 can comprise a nozzle through which dispensed gas can exit. Gas dispenser device 320 can be structured (e.g., having an aperture) such that, when placed near the optical path of laser system 302, light from laser system 302 is not blocked by gas dispenser device 320 and is allowed to reach the irradiation region 304. A buffer gas such as hydrogen, helium, argon or combinations thereof, can be introduced into, replenished and / or removed from the chamber 212. The buffer gas can be present in the chamber 212 during plasma discharge and can act to slow plasma created ions, to 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 fast ion damage.

[0072] In some aspects, the lithographic apparatus 100 can also include a collector optic 258 such as a near-normal incidence collector mirror having a reflective surface in the form of a prolate spheroid (i.e., an ellipse rotated about its major axis) having, e.g., 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. Collector optic 258 can be formed with an aperture to allow the light pulses generated by the laser system 302 to pass through and reach theirradiation region 304. The same, or another similar aperture, can be used to allow gas from the gas dispenser device 320 to flow into chamber 212. As shown, the collector optic 258 can be, e.g., a prolate spheroid mirror that has a first focus within or near the irradiation region 304 and a second focus at a so-called intermediate region 318, where the EUV light can be output from the source collector apparatus SO and input to an exposure device 256 utilizing EUV light, e.g., an integrated circuit lithography tool. 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. Aspects using the collector optic CO (FIG. 2A) with structures and functions described in reference to FIG. 3 are also envisaged.

[0073] Example Lithographic Cell

[0074] FIG. 4 shows a lithographic cell 400, also sometimes referred to a lithocell or cluster, according to some aspects. Lithographic apparatus 100 can form part of lithographic cell 400. Lithographic cell 400 can also include one or more apparatuses to perform pre- and post-exposure processes on a substrate. Conventionally these 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.

[0075] Example Plasma Material Droplet Source

[0076] FIG. 5 shows a source material delivery system 500, according to some aspects. In some aspects, source material delivery system 500 can be used in a lithographic apparatus 100 (e.g., source material delivery system 90 in FIG. 3). Source material delivery system 500 can comprise a nozzle 502, an electromechanical element 504, and a waveform generator 506. Nozzle 502 can comprise a capillary 508. Source material delivery system 500 can further comprise a shroud 510, a controller 512, a detector 514, and / or a detector 516. Controller 512 can comprise a processor.

[0077] As used herein, the terms “electromechanical,” “electro-actuatable,” or the like can refer to a material or structure which undergoes a dimensional change (e.g., movement, deflection, contraction, and the like) when subjected to a voltage, electric field, magnetic field, or combinations thereof and includes, but is not limited to, piezoelectric materials, electrostrictive materials, and magnetostrictive materials. Apparatuses and methods for using an electro -actuatable element to control a droplet stream are disclosed, for example, in U.S. Patent No. 7,897,947, titled “Laser Produced Plasma EUV Light Source Having a Droplet Stream Produced Using a Modulated Disturbance Wave” and issued March01, 2011, and U.S. Patent No. 8,513,629, titled “Droplet Generator with Actuator Induced Nozzle Cleaning” and issued August 20, 2013, both of which are incorporated by reference herein in their entireties.

[0078] In some aspects, electromechanical element 504 can be disposed on (e.g., surrounding) nozzle 502. It should be appreciated that interactions between nozzle 502 and electromechanical element 504 described herein can be directed to interactions between a pressure-sensitive element of nozzle 502 and electromechanical element 504 (e.g., electromechanical element 504 is disposed on capillary 508). Waveform generator 506 can be electrically coupled to electromechanical element 504. Controller 512 can be electrically coupled to waveform generator 506.

[0079] As explained earlier, in some aspects, an EUV -generating -plasma can be generated by irradiating target material (e.g., Sn) with a laser, which ionizes the target material (i.e., excitation). The target material can be provided as a stream of coalesced droplets that intersects the laser path. Small deviations in the positioning of a coalesced target material droplet in relation to the laser can affect efficiency and stability of EUV radiation, which in turn can impact lithographic processes that depend on the EUV radiation. Therefore, it is desirable to control the positioning between coalesced droplet and the laser such that EUV -generation is stable and efficient. One method to improve stability and efficiency is to ensure repeatable coalescence of target material droplets so that each coalesced droplet produces a repeatable interaction with the laser. Another method is to ensure accurate timing of the laser pulses as they target the passing target material droplets. Various structures and functions in aspects of the present disclosure can allow for repeatable laser-to-droplet alignment and / or can enable laser-to-droplet alignments that are optimized or partly-optimized for high EUV pulse energy or EUV conversion efficiency, for example.

[0080] In some aspects, nozzle 502 can eject initial droplets of target material, shown in FIG. 5 as a stream of target material 518. electromechanical element 504 can transduce electrical energy from the waveform generator 506 to apply a pressure on nozzle 502 (e.g., on capillary 508). This introduces a velocity perturbation in stream of target material 518 exiting nozzle 502. Stream of target material 518 ultimately coalesces into droplets which are detected by detector 514 and / or detector 516 to generate a signal (e.g., a detection signal). As used herein, the term “detect” or the like can be used to refer to capturing an image (e.g., using a camera) of the droplet and / or binary indication of the presence or absence of a droplet or when a droplet crosses a given location (e.g., using a laser curtain). Detectors 514 and 516 can be trigger detectors, gating detectors, gate detectors or other suitable detectors that generate a detection signal in response to a fulfillment of one or more conditions, for example the detected presence of a droplet. One of detectors 514 and 516 can be an image capture device and the other can be a gate detector. Controller 512 can determine properties of stream of target material 518 based on the signal from detector 514. Properties of the stream of target material 518 can comprise, for example, velocity profde of the droplet stream at the detection point, gap (time and / or distance) between droplets, presence of uncoalesced droplets (satellite droplets, or simply “satellites”), droplet size,coalescence length, droplet path (or aim), or the like. Controller 512 can use the information from detectors 514 and / or 516 to generate a feedback signal to control operation of the waveform generator 506.

[0081] In some aspects, controller 512 can adjust parameters of electrical signals (e.g., waveform, hybrid waveform) generated by waveform generator 506. Parameters of waveforms can comprise, for example, relative phase difference(s) between two or more waveforms in superposition, amplitude, wavelength, and the like. Controller 512 can also determine an adjustment of a waveform parameter based on an external input 520, which can originate from another controller or be based on a user input.

[0082] In some aspects, shroud 510 can be disposed on nozzle 502. Shroud 510 can be disposed so as to cover and protect stream of target material 518 from forces that can disrupt coalescence and droplet generation.

[0083] In some aspects, waveform generator 506 is configured to generate an electrical signal to control the applied pressure on nozzle 502. The electrical signal can comprise a superposition (e.g., hybrid waveform) of a first periodic waveform having a first frequency (e.g., a low frequency sine wave) and a second periodic waveform having a second frequency different from the first frequency (e.g., a high frequency square wave). The term “sine” can be used herein to refer to sinusoidal patterns. The second frequency can be an integer multiple of the first frequency. The resulting velocity perturbations in stream of target material 518 allow the initial droplets that are ejected from nozzle 502 to coalesce as they travel away from nozzle 502. A fully coalesced droplet 522 can form at a distance L (“coalescence length”) from the orifice of nozzle 502. In other words, a distance, measured from the nozzle, at which coalesced droplet 522 forms without remnant uncoalesced droplets (e.g., satellites) defines a coalescence length.

[0084] In some aspects, the coalescence length can be adjusted by adjusting parameters of the electrical signal from waveform generator 506 (e.g., relative phase of waveforms), which ultimately influences coalescence behavior via velocity perturbations of the initial droplets (additional details regarding the use of hybrid waveforms in coalescence-based droplet generation can be found in International Appl. No. WO 2019 / 137846). Initial droplets can be generated at a rate of, for example, between 3x l06and 10* 106initial droplets per second (e.g., frequency of 3, 4, 5, 8, 10 MHz). The frequency of initial droplets can be a function of, for example, the size of the orifice on nozzle 502 (or capillary 508) and a so-called Rayleigh breakup phenomenon. In some aspects, source material delivery system 500 generates fully coalesced droplets 522 having a lower frequency (e.g., 20, 30, 40, 50, 60, 75, 100 kHz) and without any satellites — from the initial droplets of a higher frequency (e.g., 5 MHz).

[0085] In some aspects, source material delivery system 500 is configured to control the breakup / coalescence process to reduce instabilities in the EUV -generating -plasma. It can be instructive to first describe some factors that can influence droplet coalescence. Referring back to FIG. 3, an EUV radiation source can employ gas dispenser device 320 to introduce a gas flow (e.g., hydrogen gas) into irradiation region 304. The gas flow from gas dispenser device 320 can introduce drag to the dropletsin stream of target material 518 (FIG. 5), thereby affecting the velocities of droplets. Therefore, the coalescence process — being significantly influenced by the velocity perturbation of droplets — can be substantially impacted by the presence of gas. A reason for using gas can be for allowing some useful features. For example, the gas can be used as a chemical radical for cleaning collector optic 258. More details regarding the use of hydrogen gas can be found in U.S. Pat. No. 10,359,710, issued on January 18, 2011, titled “Radiation System and Optical Device,” which is incorporated by reference herein in its entirety. For the use of at least these features, drag can be tolerated in some aspects.

[0086] Plasma forces can also affect coalescence. The EUV -generating -plasma can be characterized as a complex flow of ionized matter. Therefore, droplets in the vicinity of the EUV -generating -plasma can be subject to electromagnetic and fluid-mechanical forces. Consequently, uncoalesced droplets may not be able to fully coalesce if they are still in fragmented form (e.g., satellites) by the time they enter the influence of the plasma forces. The presence of satellites at irradiation region 304 can impact stability of EUV-generation, which in turn can be undesirable for lithographic processes that depend on precise energy dosages from the EUV source.

[0087] In some aspects, it is desirable for fully coalesced droplets to form at a given distance from irradiation region 304. In some aspects, full coalescence of droplets at a given distance away from irradiation region 304 can be achieved by positioning source material delivery system 308 (or its nozzle, e.g., nozzle 502 of FIG. 5) further away from irradiation region 304. A nozzle has a range of possible coalescence lengths (e.g., having a minimum and / or maximum) based on, for example, parameters of the electrical signal from waveform generator 506 (FIG. 5). A maximum coalescence length of source material delivery system 308 can be, for example, approximately 700 mm. Therefore, the tip of such a nozzle would need to be placed at least 700 mm away from irradiation region 304 for full coalescence of droplets prior to arriving at irradiation region 304. However, there can exist reasons that caution against placing the nozzle at such distances from irradiation region 304. For example, aiming the droplets precisely and reproducibly for intersection with a laser is desirable for EUV-generation stability. However, as source material delivery system 308 is positioned further away, the droplets can be under the influence of drag for longer periods of time, leading to higher uncertainties in the aim of coalesced droplet and sub-optimal interaction between the droplets and the laser. Therefore, the method to position source material delivery system 308 further away from irradiation region 304 can have limits.

[0088] Alternatively, or in addition to, the method of positioning a source material delivery system further away from an irradiation region, aspects of the present disclosure allow for manipulation of the maximum coalescence length of a source material delivery system. In some aspects, a maximum coalescence length is decreased as much as possible.

[0089] As used herein, the term “maximum coalescence length” can be used to describe a maximum distance, measured from a source material delivery system (e.g., from a nozzle thereof), at which fully coalesced droplets form without remnant uncoalesced droplets (satellites). Moreover, the maximumcoalescence length can refer to a maximum of a range of coalescence lengths (e.g., a range can be determined by adjusting a single parameter of the source material delivery system while keeping other parameters fixed, as described further below). The term “minimum coalescence length” follows a logic similar to the maximum coalescence length.

[0090] Earlier it was described that a coalescence length of a source material delivery system can be manipulated using a superposition of a first periodic waveform (e.g., a low frequency sine wave) and a second periodic waveform (a high frequency square wave) as an electrical signal to actuate an electromechanical element on the source material delivery system. For simplifying descriptions that follow, the first and second periodic waveform will be respectively referred to as sine and square waves. However, this should not be interpreted as limiting and it should be understood that other suitable waveforms for the first and second periodic waveforms are envisaged. For example, a triangle wave, a sawtooth wave, sharp periodic peaks (e.g., periodic delta-like), and / or variants thereof can be used.

[0091] In some aspects, the range of coalescence lengths of a source material delivery system can be a function of at least the amplitude of the sine wave, frequency of the square wave, and / or a relative phase difference between the sine and square waves. With the coalescence length being multi -parameter dependent, it should be appreciated that it can be simpler to consider, as a non-limiting example, adjusting one knob (e.g., an adjustable parameter), while leaving other knobs fixed, when examining the coalescence length and quantities derived therefrom. For example, the range of coalescence lengths with respect to the full range of relative phase difference of the sine and square waves (e.g., 0-2TI radians or 0-360 degrees of the square wave) can be examined for a given value of the amplitude of the sine wave. For this given value of the sine wave amplitude, it is possible to determine a minimum and maximum of the coalescence length over the full range of the relative phase between the sine wave and the square wave. If, for example, a different sine wave amplitude is under consideration, then examining the range of coalescence lengths, with respect to the full range of relative phase difference of the sine and square waves, can result in a new range of coalescence lengths, along with a new minimum and maximum. In this manner, the maximum coalescence length with respect to a given knob can be a variable (and adjustable) quantity when another knob is adjusted.

[0092] In some aspects, for frequencies of the square wave below, e.g., 1 MHz, the amplitude of the sine wave can appreciably influence the maximum coalescence length. However, at frequencies of the square wave above 1 MHz (e.g., 2 MHz), the dependence of the maximum coalescence length on the sine wave amplitude can be considerably negligible (e.g., flat line), for a given range of a sine wave amplitudes. Waveform amplitudes can be measured in, for example, voltages of the electrical signal from a waveform generator (e.g., waveform generator 506 of FIG. 5). Sine wave amplitudes that can be used in aspects herein can be, for example, values between approximately 0.1-10.0, 0. 1-6.0, 0.5-5.0, or 1.0-4.0 V.

[0093] In some aspects, having a flat line behavior with respect to changes of the sine wave amplitude can allow not having to tune or optimize the sine wave amplitude at all. The ability to not have to tuneknobs of an EUV source unit allows for confident deployment of the system without having to further tune them in the field. Having working configurations out of the factory can save on setup costs, field downtime, and further maintenance.

[0094] In some aspects, the presence of drag can also contribute to shortening the maximum coalescence length. Traditionally, the impact of gas flow at the plasma formation region in an EUV source can have been seen as a minor inconvenience of sorts, where the difficulties of accommodating the presence of gas are outweighed by the features it allows. However, some aspects of the present disclosure make novel use of the drag imparted on droplets in the stream of target material.

[0095] In some aspects, drag can be used to limit the maximum coalescence length. Nozzle 502 can eject initial droplets of target material through the gas (e.g., provided by gas dispenser device 320, FIG. 3) such that the initial droplets experience drag. During the coalescence process, a first set of intermediate droplets are formed by coalescing. The term “intermediate droplet” can be used herein to describe droplets that have coalesced from the initial droplets but have not yet achieved the final form for interacting with the laser for EUV generation. Fully coalesced droplet 522 (FIG. 5) is an example of the final form. As intermediate droplets merge to form larger intermediate coalesced droplets, there can be instances where some intermediate droplets are larger than others. Deceleration due to the drag force can be greater on smaller droplets. Therefore, the drag mechanism can be used to slow down smaller droplets such that they collide with larger droplets. The intermediate droplets can arise at a frequency that is based on the frequency of the square wave. By increasing the ratio of the frequency of the square wave to the frequency of the sine wave, smaller and more intermediate droplets can be generated. Consequently, the deceleration due to drag can be greater on the smaller droplets, leading to quicker coalescence.

[0096] In some aspects, gas parameters can be adjusted to adjust the maximum coalescence length. For example, an illumination system that employs source material delivery system 500 (FIG. 5) can adjust the maximum coalescence length by adjusting at least a density or temperature of the gas. Increasing the density of the gas (e.g., injecting more gas), the temperature of the gas, or both, can increase the effects of drag to shorten the maximum coalescence length.

[0097] Example Laser-to-Droplet Positioning System

[0098] During the production of EUV radiation, a source material, often a droplet, can be irradiated by a series of laser pulses. A first pulse from a first laser, often termed a pre-pulse, can expand the droplet into a disc -like shape, hereby referred to as a target. A second pulse from a second laser, often termed a rarefication pulse, can change the density of the target. Lastly, a third pulse from a third laser can irradiate the target to generate a plasma.

[0099] To maximize power and stability of generated EUV radiation, each laser pulse must be correctly positioned relative to droplet / target. The position of a laser beam relative to a droplet or target can be referred to as a laser-to-droplet position.

[0100] FIG. 6 shows a method 600 of optimizing and modifying laser-to-droplet position alignment during EUV radiation generation, according to some aspects. Method 600 can comprise steps 602, 604, and 606.

[0101] Step 602 can comprise calculating an optimal laser-to-droplet alignment position. In some aspects, step 602 can further comprise steps 602A and 602B.

[0102] In some aspects, step 602A may comprise fitting a polynomial to a radiation generation metric that varies based on a laser-to-droplet alignment position. An optimal alignment position for a laser relative to a fuel droplet or target may be based on an apex of a polynomial fit. In some aspects, a radiation generation metric may comprise the EUV energy generated by interaction of a laser pulse with a target, or the conversion efficiency of that interaction, for example. In some aspects, conversion efficiency can be defined as the energy of generated EUV radiation divided by the energy of the drive laser pulse used to produce the EUV radiation.

[0103] Step 602B may comprise evaluating commanded drive laser power versus laser-to-droplet alignment positions across an available alignment space to determine positions where commanded drive laser power reaches a maximum. In some aspects, an optimal alignment position for a laser relative to a fuel droplet or target ensures that the alignment space is symmetric between the maximum points.

[0104] Step 604 can comprise calculating minimum and maximum laser-to-droplet X and Y alignment positions that satisfy an acceptable range of target X and Y tilts. In some aspects, a range of target X and Y tilts can be chosen to minimize contamination (e.g. fuel debris on collector and vessel walls) and / or to minimize back reflections into a source laser.

[0105] Step 606 can comprise modifying a laser-to-droplet alignment position. In some aspects, a laser-to-droplet alignment position may be modified to align with the optimal laser-to-droplet position calculated in step 602. In some aspects, a range keep assist implementation may adjust the laser-to- droplet position if it falls outside of the range of minimum and maximum X and Y positions calculated in step 604.

[0106] The method steps of FIG. 6 can be performed in any conceivable order and it is not required that all steps be performed. Moreover, the method steps of FIG. 6 described above merely reflect an example of steps and are not limiting. That is, further method steps and functions are envisaged based aspects described in reference to FIGS. 1-5 and 7-14.

[0107] FIGS. 7A and 7B illustrate polynomial fit method 700, according to some aspects. In some aspects, polynomial fit method 700 determines an optimal laser-to-droplet alignment position for maximizing EUV radiation.

[0108] FIG. 7A presents data observed during a test period of operation of an EUV source. FIG. 7A illustrates a greyscale plot of a number of measured radiation shots observed as a function of (a) laser- to-droplet X position and (b) EUV energy. The x-axis in 702a indicates laser-to-droplet X position in arbitrary units. The y-axis in 702a indicates EUV energy 704a in arbitrary units. The laser-to-droplet X position can indicate a spatial distance between a droplet and the laser beam. For example, the laser-to-droplet X position may represent a distance between a centroid of a droplet along the x-axis and a centroid of an intensity profde of a laser beam along the x-axis. (Other metrics are also contemplated. For example, in other approaches to indicating the X position, this numerical value may represent, for example, distances between edges of the full-width-half-max envelopes of the droplet mass distribution and the laser intensity distribution.) The laser-to-droplet X position may typically be in the range of microns or tens of microns. The EUV energy may be in the range of milliJoules or tens of milliJoules. FIG. 7A includes a two-dimensional histogram of EUV energy data 706a collected during an experimental test. In this example, EUV energy data 706a indicates the number of measured radiation shots that were observed during the test at each value of laser-to-droplet X position and at each value of EUV energy. Darker shading in EUV energy data 706a indicates a greater number of observed radiation shots with the corresponding values of laser-to-droplet X position and EUV energy. Value bar 708a indicates a numerical count of the observed instances corresponding to the shading of EUV energy data 706a.

[0109] FIG. 7A includes a roughly oval dark region that evidences a cluster of observed shots with EUV energy between 55 and 85 units, and with laser-to-droplet X positions between 0 and 5 units. FIG. 7A includes a mean value curve 712a. This curve 712a indicates the mean EUV energy of the two- dimensional histogram as a function of the laser-to-droplet X positions 702a. FIG. 7A also has standard deviation curves 714a and 716a. These curves indicate the envelope values of EUV energy that are two standard deviations below (714a) and above (716a) the mean value curve 712a, as a function of laser- to-droplet X positions 702a. In addition to these statistical markers of mean and standard deviation, FIG. 7 also includes a polynomial fit curve 710a. Polynomial fit curve 710a models a polynomial dependence of EUV energy 704a as a function of laser-to-droplet X positions 702a, based on energy data 706a. As can be seen from FIG. 7A, polynomial fit curve 710a provides a close estimate of mean value curve 712a for EUV energy data 706a.

[0110] In some aspects, polynomial fit curve 710a can be used to guide or update the operation of a system that produces the droplet and the laser pulses represented by energy data 706a. For example, an apex of polynomial fit curve 710a may be used as a new set point 722a for laser-to-droplet X position. In some aspects, set point 722a can update a prior laser-to-droplet set point 720a or a nominal set point 718a. As can be seen from FIG. 7A, resetting operation around laser-to-droplet set point 720a can cause the system to operate with a laser-to-droplet X position 702a that corresponds to a maximum value of EUV energy 704a. Moreover, in various situations, selecting an apex of polynomial fit curve 710a may improve the stability of system: as can be seen from FIG. 7A, in the vicinity of this set point, EUV energy 704a has a reduced sensitivity to noise or disturbances or other variations in laser-to-droplet X position 702a.[oni] FIG. 7B presents data observed during a test period of operation of an EUV source. FIG. 7B illustrates a greyscale plot of a number of measured radiation shots as a function of (a) laser-to-droplet Y position and (b) EUV energy . The x-axis in 702b indicates laser-to-droplet Y position in arbitraryunits. The y-axis in 702a indicates EUV energy 704b in arbitrary units. The laser-to-droplet Y position can indicate a spatial distance between a droplet and the laser beam. For example, the laser-to-droplet Y position may represent a distance between a centroid of a droplet along the y-axis and a centroid of an intensity profde of a laser beam along the y-axis. (Other metrics are also contemplated. For example, in other approaches for indicating the Y position, this numerical value may represent, for example, distances between edges of the full-width-half-max envelopes of the droplet mass distribution and the laser intensity distribution.) The laser-to-droplet Y position may typically be in the range of microns or tens of microns. The EUV energy may be in the range of milliJoules or tens of milliJoules. FIG. 7B illustrates a two-dimensional histogram of EUV energy data 706b collected during an experimental test. In this example, EUV energy data 706b indicates the number of measured radiation shots that were observed during the test at each value of laser-to-droplet X position and at each value of EUV energy. Darker shading in EUV energy data 706b indicates a greater number of observed radiation shots with the corresponding values of laser-to-droplet X position and EUV energy. Value bar 708b indicates a numerical count of the observed instances corresponding to the shading of EUV energy data 706b.

[0112] FIG. 7B illustrated a roughly oval dark region that evidences a cluster of observed shots with EUV energy between 50 and 90 units, and with laser-to-droplet Y positions between -7.5 and 2.5 units. FIG. 7B includes a mean value curve 712b. This curve 712b indicates the mean EUV energy of the two- dimensional histogram as a function of the laser-to-droplet Y positions 702b. FIG. 7B also has standard deviation curves 714b and 716b. These curves indicate envelope values of EUV energy that are two standard deviations below (714b) and above (716b) the mean value curve 712b, as a function of laser- to-droplet Y positions 702b. In addition to these statistical markers of mean and standard deviation, FIG. 7B also includes a polynomial fit curve 710b. Polynomial fit curve 710b models a polynomial dependence of EUV energy 704b as a function of laser-to-droplet Y positions 702b, based on energy data 706b. As can be seen from FIG. 7B, polynomial fit curve 710b provides a close estimate of mean value curve 712b for EUV energy data 706b.

[0113] In some aspects, polynomial fit curve 710b can be used to guide or update the operation of a system that produces the droplet and the laser pulses represented by energy data 706b. For example, an apex of polynomial fit curve 710b may be used as a new set point 722b for laser-to-droplet Y position. In some aspects, set point 722b can update a prior laser-to-droplet set point 720b or a nominal set point 718b. As can be seen from FIG. 7B, resetting operation around laser-to-droplet set point 720b can cause the system to operate with a laser-to-droplet Y position 702b that corresponds to a maximum value of EUV energy 704b. Moreover, in various situations, selecting an apex of polynomial fit curve 710b may improve the stability of system. As can be seen from FIG. 7B, in the vicinity of this set point, EUV energy 704b has a reduced sensitivity to noise or disturbances or other variations in laser-to-droplet Y position 702b.

[0114] FIGS. 8 A and 8B illustrate laser energy command values as a function of laser-to-droplet X and Y alignment positions, according to some aspects. As described in reference to FIG. 3 and FIG. 5,a laser irradiates a droplet / target with one or more pulses to create an EUV emitting plasma. In some aspects, the power of the laser or the energy of the laser pulses is determined by an energy control loop. An output of the energy control loop may be referred to as a laser energy command. In various implementations of a laser system, a laser energy command may be a control signal that controls a timing of an output shutter such as an electro-optic modulator, or controls an amplitude of an output shutter such as an acousto-optic modulator, or controls a charging energy applied to a laser gain medium, or a combination thereof. An energy control loop may be configured to adjustment the laser power based on how much EUV energy has been generated by recent pulses. For example, the laser energy command can adjust the power of the laser to keep the amount of EUV energy generated approximately constant, or to increase / decrease the amount of EUV energy generated. In some aspects, a laser energy command value can indicate how much the power is adjusted to reach the desired EUV energy. A laser energy command value may be between zero and one, for example. A higher laser energy command value can indicate larger laser adjustments, and the like.

[0115] In some aspects, a laser is controlled to hit a droplet / target at a preferred laser-to-droplet X andY position. However, due to system variations, such as laser pulse timing, the laser may not always hit the droplet / target at the preferred position. This leads to a statistical variation of laser-to-droplet X andY positions. The laser-to-droplet X and Y positions can also naturally drift away from the preferred position over time.

[0116] FIGS. 8 A and 8B show greyscale plots of laser energy command values for a statistical variation of laser-to-droplet X and Y positions collected during operation of an EUV light source. The x-axes represent laser-to-droplet X positions in arbitrary units. The y-axes represent laser-to-droplet Y positions in arbitrary units. Laser to droplet X and Y positions may be measured in microns. Laser energy command values 806a and 806b indicate the magnitude of the laser energy command signal sent to the laser for each pulse. Value bars 808a and 808b can depict numerical values of laser energy command signals, with brighter values indicating higher laser energy command signals and darker values indicating lower laser energy command signals. Regions with lower values of laser energy command indicate that less laser power is needed to generate the desired amount of EUV in those regions. Thus, the laser-to-droplet alignment position is better for EUV generation in regions with darker shading.

[0117] FIG. 8A illustrates asymmetric laser energy command values across an alignment space. In some aspects, the center of laser energy command plot 800 is the laser-to-droplet X and Y set point. For example, in FIG. 8A, laser-to-droplet set point is (0, 3). In some aspects, an alignment space may the set of laser-to-droplet positions around the set laser-to-droplet X and Y set point that a laser is likely to hit during operation of a light source. In FIG. 8 A, maximum values 810a of commanded laser energy 806a are concentrated in the bottom (lighter) region of the alignment space (i.e., laser-to-droplet Y positions below -5). Lower values of laser energy command 812a are concentrated in the top region of the alignment space. When a laser-to-droplet Y position drifts below -5, the generated EUV radiationis likely to have less energy (e.g., a control loop is required to provide a higher laser energy command signal to achieve a desired energy). . Lower EUV energy can lead to lower die yield in a lithography system. As can be seen from FIG. 8A, the required laser energy command values are lowest in a dark region centered around (2, 3).

[0118] FIG. 8B illustrates symmetric laser energy command values across an alignment space. In some aspects, the center of laser energy command plot 800 is the laser-to-droplet X and Y set point. For example, in FIG. 8B, the data represent observations in which a set point is around (2, 3). In some aspects, an alignment space may the set of laser-to-droplet positions around the laser-to-droplet X andY set point that a laser is likely to hit during operation of a light source. In FIG. 8B, higher values 810b of laser energy command 806b are more symmetrically distributed around the edges of the alignment space. Lower values 812b of laser energy commanded 806b are approximately symmetric about the laser-to-droplet set point (2, 3). As laser-to-droplet X and Y positions drift away from the set point, the stability and energy of generated EUV radiation are likely to stay constant or not experience large variations. Stable and / or constant EUV radiation can improve die yield in a lithography system. As will be appreciated from FIGs. 8A and 8B, in an environment subject to variations in laser-to-droplet X andY positions, centering of a laser-to-droplet set point may be helpful to increase EUV energy delivery and / or to improve stability of the delivered EUV energy.

[0119] FIG. 9 shows a coordinate system 900, according to some aspects. Coordinate system 900 can be used to describe the orientation of a target 902 (i.e., a fuel droplet that has been hit by a pre-pulse). As discussed above, a target can be a an expanded droplet, expanded with a disc -like shape such as illustrated by the grey shape in FIG. 9. In the example of coordinate system 900, a target travels in the negative x direction along a path between a droplet generator 904 and a tin catch 906. A laser pulse, incident on the target, travels in the z-direction from a laser output 908 in a direction towards an intermediate focus 910 of a collector mirror, such as collector mirror 258. In this coordinate system, the y-axis represents the axis perpendicular to the x and z axes, and extends out of the page (as illustrated by the encircled dot at the intersection of the x and z axes).

[0120] In some aspects, an incident light pulse hits the disc of target 902 at an angle. That is, the light pulse (directed along the z axis) may not be normally incident onto the disc of target 902. The angle can be described as a rotation of a central axis 912 of target 902 about the x and / or y-axis. A Y-tilt (e.g., rotation about the y-axis: Ry) of target 902 can be measured as the angle between central axis 912 of target 902 and the x-axis. Similarly, an X-tilt (e.g., rotation about the x-axis: Rx) of target 902 can be measured as the angle between central axis 912 and the y-axis. (Other measurement approaches are also envisioned. For example, an X-tilt may represent an angle between the y-axis and a median plane of a target disc while a Y -tilt may represent an angle between the x-axis and a median plane of a target disc.)

[0121] Ranges of X and / or Y-tilts for target 902 may be chosen to improve performance of an EUV source. The targets may be produced with tilts that are selected or commanded in order to, for example, reduce tin deposition on chamber walls and / or to reduce back -reflections into a laser. An X and / or Y -tilt can be changed during EUV light source operation by altering the laser-to-droplet X and / or Y positions, as described in reference to FIGS. 7A-B and 8A-B.

[0122] FIG. 10 shows an illustrative drawing of reverse power reflection at varying target tilts inside a chamber according to some aspects. When a light pulse hits a target, part of the light can be reflected back towards the laser and captured by one or more sensors (not illustrated). The one or more sensors can measure properties of the interaction between the drive laser and the target, such as target tilt. While back-reflections can be useful for measurements, strong back -reflections can damage the laser. The amount of back-reflected light can be reduced by altering the Y tilt of the target, as described in reference to FIG. 9.

[0123] In FIG. 10, a target 1004a or 1004b is irradiated by a pulse of light 1003 from laser 1002 inside a chamber 1000. Chamber 1000 can be an embodiment of chamber 212 described in FIG. 3. Light 1003 can travel along the z-axis. A portion of laser light 1003 incident on target 1004a or 1004b can be reflected back towards laser 1002 and / or a collector mirror 1005 to form reflected light 1006a or 1006b, respectively.

[0124] Target 1004a illustrates a target with a Y tilt of zero, as described in reference to FIG. 9. Laser light 1003 incident on target 1004a may experience a back -reflection 1006a. In some aspects, the majority of back-reflection 1006a is directed towards laser 1002.

[0125] Target 1004b illustrates a target with a non-zero Y tilt. Laser light 1003 incident on target 1004b may experience back-reflection 1006b. In some aspects, back -reflection 1006b directs less light towards laser 1002. In some aspects, back -reflection 1006b may minimize damage to a laser 1002.

[0126] FIG. 11 illustrates paths of tin debris resulting from various target tilts, according to some aspects. After a target is irradiated by a light pulse, residual material (e.g., tin ions, tin vapor, tin droplets) from an EUV generating plasma can deposit on vessel walls 1102 and / or on radiation collector 1104. Tin deposition can reduce the lifetime of vessel walls, the collector, and other components in a radiation generation chamber. Tin debris can be directed out of the chamber by gas flows. In some aspects, the X tilt of a target can determine how tin debris interacts with gas flows in a chamber. In some aspects, a range of X tilts, as described in reference to FIG. 9, of a target can be selected such that residual material is directed towards an exhaust 1110.

[0127] In some aspects, target 1106a illustrates a target with an X tilt equal to zero. Debris path 1108a can show a possible debris path when target 1106a interacts with a light pulse. In this example, debris path 1108a directs debris towards exhaust 1110. In other examples, targets with other tilts may direct debris along a desired path to exhaust 1110.

[0128] In some aspects, target 1106b illustrates a target with a positive X tilt. Debris path 1108b shows a possible debris path when target 1106b interacts with a light pulse. In this example, debris path 1108b directs debris towards vessel wall 1102 below exhaust 1110.

[0129] In some aspects, target 1106c illustrates a target with a negative X tilt. Debris path 1108c shows a possible debris path when target 1106c interacts with a light pulse. In this example, debris path 1108c directs residual debris towards vessel wall 1102 above exhaust 1110.

[0130] In some aspects, targets with an X tilt near zero reduce vessel wall contamination by directing residual material towards an exhaust. In some aspects targets with an X tilt near zero (e.g., more than about -15, -10, -5 degrees and less than about +5, +10, or +15 degrees) may reduce vessel wall contamination by directing residual tin towards an exhaust.

[0131] In some aspects, the paths 1108a-c are simplified examples, and do not represent the full ion deposition dynamics of a radiation generation chamber.

[0132] FIG. 12 presents a scatterplot 1200 of X and Y tilts of targets measured during operation of one or more EUV radiation sources. The x-axis 1202 indicates X tilts in arbitrary units. The y-axis 1204 indicates Y tilts in arbitrary units. An X tilt can be measured as the angle between a central axis of a target and the y-axis, and Y tilt can be measured as the angle between the central axis of the target and the x-axis, as described in reference to FIG. 9. X and Y tilts may have units of degrees. Data points 1206 may indicate the X and Y tilts of each of a plurality of targets represented scatterplot 1200.

[0133] In some aspects, an EUV source may have various ranges of X and Y tilts that are expected (from modelling and / or form empirical observation) to minimize tin deposition on a collector and / or vessel walls, and / orto reduce harmful back -reflections into alaser. For example, window 1208 indicates an example of an acceptable range of X and Y tilts that minimize undesired tin deposition on vessel walls or a collector. Similarly, window 1210 indicates an example of an acceptable range of X and Y tilts that minimize undesired back-reflection into a laser. Moreover, window 1214 indicates an example of an acceptable range of X and Y tilts that minimize measurement error due to low signal -to-noise ratio of a sensor, as described below in reference to FIG. 13. In some aspects, window 1212 shows the overlap of window 1208, window 1210 and window 1214. Window 1212 can be interpreted as an ideal range of X and Y tilts that simultaneously reduce tin deposition and back -reflections. In some aspects, an EUV source may include a controller that ensures that X and Y tilts of a target stay within the bounds of window 1212. For example, a controller may limit, restrict, or otherwise adjust a laser-to-droplet X or Y position if the otherwise-desired X or Y tilt falls outside of window 1212.

[0134] FIG. 13 shows a plot 1300 illustrating an EUV loss event due to erroneous laser-to-droplet measurements, according to some aspects. Plot 1300 can be divided into sub-plots 1310, 1320, 1330 and 1340, which share a common time axis 1302. Axis 1302 may show time in arbitrary units.

[0135] Subplot 1310 shows target Y tilts 1312 as a function of time. Axis 1314 shows Y tilt values in arbitrary units. The moving average ofY tilts 1312 is plotted by line 1316.

[0136] Subplot 1320 shows laser-to-droplet X positions 1322 as a function of time. Axis 1324 shows laser-to-droplet X position values in arbitrary units. Laser-to-droplet X positions 1322 may be optimal laser-to-droplet X positions as determined via method 600 described in FIG. 6.

[0137] Subplot 1330 shows laser t-fire 1332, which represents the amount of time between detection of a droplet and a firing of a pre-pulse laser, as a function of time. Axis 1334 shows laser t-fire timing intervals in arbitrary units. Laser t-fire can be affected by laser-to-droplet X positions 1322. For example, an erroneous laser-to-droplet X position measurement may cause a laser pulse to miss a target, which causes a drop in t-fire.

[0138] Subplot 1340 shows EUV energy 1342 as a function of time. EUV energy 1342. Axis 1344 shows EUV energy values in arbitrary units. In some aspects, EUV energy 1342 is controlled by a laser energy command, as described in reference to FIGS. 8A and 8B.

[0139] In a light source, a retum-beam-diagnostic quad cell can detect light that is reflected from a droplet when the droplet is hit by a pre-pulse. When the pre-pulse hits an edge of the droplet, less light is reflected, and the quad cell intensity measurement decreases. If the quad cell intensity measurement gets too low, the measurement is dominated by noise. This leads to inaccuracies in quad cell measurements which can result in large errors in laser-to-droplet X positions and, in some cases, loss of generated EUV energy.

[0140] In some aspects, errors in quad cell measurements can be more likely to occur at large Y tilt angles. For example, in FIG. 13, Y tilts 1312 drift upwards to a value of above 30 around time 15 in subplot 1310. At time 15, laser-to-droplet X positions 1322 spike from a value of around -3 to a value of around 7. This spike may be due to erroneous quad cell measurements obtained due to the simultaneous large Y tilt angles. A laser-to-droplet X / timing optimization controller can react to the erroneous measurements by reducing laser firing time interval 1332, as evidenced by the drop in laser firing time interval 1322 at time 15 in subplot 1330. A reduction is laser firing time interval 1322 can result in a reduction of EUV energy 1342, as evidenced in subplot 1340.

[0141] Limiting Y tilt angles to a threshold value (e.g., to below an upper bound value or an upper range of values) can reduce the likelihood of erroneous quad cell measurements, and therefore sudden drops in generated EUV energy. The threshold Y tilt value can be determined by considering Y tilt angles / laser-to- droplet X positions where the signal-to-noise ratio of the quad cell measurement is high enough to consistently distinguish between the signal and noise.

[0142] FIG. 14 shows a range keep assist implementation 1400, according to some aspects. During operation of an EUV light source, a calculated optimal laser-to-droplet X or Y position (e.g., an optimal position calculated in step 602 of method 600) can fall outside of an ideal range of X and / or Y tilts (calculated to reduce ion deposition and back -reflections). Range keep assist implementation 1400 can adjust, via a controller or the like, an initially calculated laser-to-droplet X and / or Y position during operation of an EUV light source to ensure that X and / or Y tilts of a target stay within ideal range. In some aspects, X tilt angles are a function of laser-to-droplet Y positions, and Y tilt angles are a function of laser-to-droplet X positions. An initially calculated laser-to-droplet Y position can be changed to alter an X tilt of a target. Similarly, an initially calculated laser-to-droplet X position can be changed to alter a Y tilt of a target

[0143] In some aspects, range keep assist implementation 1400 continuously monitors X and / or Y tilts. Range keep assist implementation 1400 can activate when a measured X tilt angle or Y tilt angle falls above or below a calculated minimum or maximum value. Alternatively, or in addition, keep assist implementation 1400 can activate when an initially calculated laser-to -droplet X or Y position would lead to a X tilt angle or Y tilt angle that would fall above or below a calculated minimum or maximum value. Range keep assist implementation 1400 is shown through three corresponding plots. Plots 1410, 1430, and 1450 share a common time axis 1402.

[0144] In some aspects, plot 1410 shows example X orY tilt values as a function of time during EUV radiation generation. Axis 1414 can depict X or Y tilts in arbitrary units. In some aspects, X or Y tilts are measured in unit degrees. Line 1422 illustrates a minimum X or Y tilt. In some aspects, a minimum X or Y tilt may be defined as described in step 604 of method 600. Line 1420 illustrates a threshold X or Y tilt. The threshold X or Y tilt can be defined as the minimum tilt plus a buffer. Adding a buffer to the minimum tilt additionally ensures that the X or Y tilt does not fall below the minimum tilt value during light source operation. In some aspects, range keep assist 1400 is activated when a measured X or Y tilt (or an anticipated upcoming X or Y tilt) of a target falls below the threshold.

[0145] In some aspects, X or Y tilt values 1418 show example X or Y tilts when range keep assist implementation 1400 is not activated. The measured X or Y tilts fall below the threshold line 1420. Alternatively, X or Y tilt values 1416 show example X or Y tilts when range keep assist implementation 1400 is activated. Once X or Y tilts fall below the threshold value (or are anticipated to fall below the threshold values), range keep assist 1400 activates to ensure that X or Y tilts stay above minimum tilt line 1422.

[0146] In some aspects, plot 1430 shows example laser-to-droplet X or Y positions as a function of time during EUV radiation generation. Axis 1434 can depict laser-to-droplet X or Y positions in arbitrary units. In some aspects, laser-to-droplet X or Y positions are measured in units of microns.

[0147] Line 1440 illustrates a maximum allowed laser-to-droplet X or Y position. When range keep assist 1400 is activated, the maximum allowed laser-to-droplet X or Y position drops to a value that corresponds to the minimum X or Y tilt threshold 1420 (e.g., laser-to-droplet X and Y positions are restricted by the X or Y tilt threshold). The maximum allowed laser-to-droplet X or Y position that corresponds to tilt threshold 1420 is indicated by dashed line 1442.

[0148] Laser-to-droplet X or Y position data 1436 can show measured laser-to-droplet X or Y positions when range keep assist 1400 is implemented. Values of laser-to-droplet X or Y position data 1436 are restricted by the maximum laser-to-droplet X or Y position.

[0149] In some aspects, laser-to-droplet X or Y position data 1438 shows measured laser-to-droplet X or Y positions when range keep assist 1400 is not implemented. Values of laser-to-droplet X or Y position data 1438 exceed the maximum laser-to-droplet X or Y position indicated by line 1442. Without range keep assist implementation 1400, laser-to-droplet X or Y positions would fall outside of an ideal range of X and Y tilts.

[0150] In some aspects, plot 1450 shows an activation of range keep assist implementation 1400 as a function of time. Y -axis 1454 indicates a binary on / off nature of range keep assist implementation 1400. In some aspects, line 1456 indicates the status of the range keep assist mechanism. For example, if a Y tilt falls below a threshold value, range keep assist implementation 1400 is activated to adjust the laser- to-droplet X position and bring the Y tilt back to the threshold value.

[0151] FIG. 15 shows a schematic drawing of a range keep assist control loop 1500, according to some aspects. Range keep assist control loop 1500 can ensure that a laser-to-droplet X or Y position stays within bounds set by minimum and maximum X and Y tilts. In some aspects, range keep assist control loop 1500 can adjust a laser-to-droplet X position to satisfy a minimum or maximum Y tilt, or adjust a laser-to-droplet Y position to satisfy a minimum or maximum X tilt. Range keep assist control loop 1500 can be used by a controller, or the like, in a light source to continually monitor and adjust laser- to-droplet X and / or Y positions. In some aspects, four range keep control loops 1500 run simultaneously to ensure adherence to a minimum Y tilt, a maximum Y tilt, a minimum X tilt, and a maximum X tilt.

[0152] In some aspects, range keep assist control loop 1500 can comprise a feedback loop A, a feedforward loop B, and a main loop C. In some aspects, feed-forward loop B calculates a laser-to-droplet X set point needed to maintain a minimum Y tilt 1502. In some aspects, feedback loop A can measure and compensate for disturbances and drifts in the system. In some aspects, main loop C can measure an adjusted Y tilt and return the measured value to feedback loop A.

[0153] In some aspects, a measured and filtered Y tilt angle 1538 may be subtracted from minimum Y tilt 1502 at summing point 1504 to generate error signal 1506. In some aspects, error signal 1506 can be used in feedback loop A and feed forward loop B.

[0154] In some aspects, error signal 1506 can be used by integral controller 1508 in feedback loop A to obtain a command signal. The command signal can determine a Y tilt needed to bring the Y tilt of a target above the minimum Y tilt 1502. In some aspects, the output signal can be converted to a maximum laser-to-droplet X position 1510 starting from input 1546 through an existing relationship between Y tilt and laser-to-droplet X position and corrective term using input 1506 to account for disturbances and drifts entering the system over time. In some aspects, saturation function 1516 can convert laser-to-droplet X position 1510 into laser-to-droplet X set point 1518. In some aspects, a saturation function 1516 bounds laser-to-droplet X position 1512 if it falls below a maximum X position 1510 or above a minimum X position 1514. In some aspects, the minimum X position 1514 is obtained by similar range assist control loop as 1500 to satisfy a maximum Y tilt angle. For example, if laser-to- droplet X position 1512 is greater than minimum X position 1514, the output 1518 of saturation function 1516 can be minimum X position 1514. In some aspects, maximum and minimum X position bounds 1510 / 1514 can be calculated as described in step 604 of FIG. 6.

[0155] In some aspects, laser-to-droplet X set point 1518 may be used in feed-forward loop B. In feedforward loop B, feed-forward control 1540 may utilize error signal 1506 to calculate predictive term 1542. In some aspects, feed-forward control 1540 can calculate the predictive laser-to-droplet Xposition from a set point when Y tilt angle is below a minimum threshold. In some aspects, laser-to- droplet X set point 1518 and predictive term 1542 can be added at summing point 1544 to generate integral controller input 1546. In some aspects, integral controller input 1546 can be used by integral controller 1506 as a starting position command to a laser-to-droplet X position 1510, as described above.

[0156] In some aspects, laser-to-droplet X set point 1518 may be used in main loop C. In some aspects, an unknown actuator offset 1520 may be added to laser-to-droplet X set point 1518 at summing point 1522 to generate a laser-to-droplet X position 1524. In some aspects, offset 1520 can be defined as the y-intercept in a plot of Y tilt (degrees, x-axis) vs. laser-to-droplet X position (microns, y-axis). In some aspects, offset 1520 may be an unknown system -to-system variable actuator offset.

[0157] In some aspects, laser-to-droplet X position 1524 may be used by target formation 1526 to update a laser-to-droplet X position. In some aspects, target formation 1526 may also measure multiple tilts 1528 of the droplet after the position is modified.

[0158] In some aspects, measured Y tilts 1528 and unknown process and measurement noise 1530 combine at summing point 1532 to generate measured Y tilts 1534. In some aspects, measured Y tilts can be filtered by moving average filter 1536 to generate measured and filtered Y tilts 1538. In some aspects, moving average filter 1536 can average forty Y tilt measurements.

[0159] The example described herein illustrates adjusting a laser-to-droplet X position to maintain a minimum Y tilt threshold. It will be understood to a person of ordinary skill in the art that range keep assist control loop 1500 can be used to adjust a laser-to-droplet X or Y position to maintain a minimum Y tilt, a maximum Y tilt, a minimum X tilt, or a maximum X tilt.

[0160] FIG. 1600 illustrates an example computer system useful for implementing various embodiments in Figures 1-15.

[0161] Various embodiments may be implemented, for example, using one or more well-known computer systems, such as computer system 1600 shown in FIG. 16. One or more computer systems 1600 may be used, for example, to implement any of the embodiments discussed herein, as well as combinations and sub -combinations thereof.

[0162] Computer system 1600 may include one or more processors (also called central processing units, or CPUs), such as a processor 1604. Processor 1604 may be connected to a communication infrastructure or bus 1606.

[0163] Computer system 1600 may also include user input / output device(s) 1603, such as monitors, keyboards, pointing devices, cameras, other imaging devices etc., which may communicate with communication infrastructure 1606 through user input / output interface(s) 1602.

[0164] One or more of processors 1604 may be a graphics processing unit (GPU). In an embodiment, a GPU may be a processor that is a specialized electronic circuit designed to process mathematically intensive applications. The GPU may have a parallel structure that is efficient for parallel processing of large blocks of data, such as mathematically intensive data common to computer graphics applications, images, videos, etc.

[0165] Computer system 1600 may also include a main or primary memory 1608, such as random access memory (RAM). Main memory 1608 may include one or more levels of cache. Main memory 1608 may have stored therein control logic (i.e., computer software) and / or data.

[0166] Computer system 1600 may also include one or more secondary storage devices or memory 1610. Secondary memory 1610 may include, for example, a hard disk drive 1612 and / or a removable storage device or drive 1614. Removable storage drive 1614 may be a floppy disk drive, a magnetic tape drive, a compact disk drive, an optical storage device, tape backup device, and / or any other storage device / drive.

[0167] Removable storage drive 1614 may interact with a removable storage unit 1618. Removable storage unit 1618 may include a computer usable or readable storage device having stored thereon computer software (control logic) and / or data. Removable storage unit 1618 may be a floppy disk, magnetic tape, compact disk, DVD, optical storage disk, and / any other computer data storage device. Removable storage drive 1614 may read from and / or write to removable storage unit 1618.

[0168] Secondary memory 1610 may include other means, devices, components, instrumentalities or other approaches for allowing computer programs and / or other instructions and / or data to be accessed by computer system 1600. Such means, devices, components, instrumentalities or other approaches may include, for example, a removable storage unit 1622 and an interface 1620. Examples of the removable storage unit 1622 and the interface 1620 may include a program cartridge and cartridge interface (such as that found in video game devices), a removable memory chip (such as an EPROM or PROM) and associated socket, a memory stick and USB port, a memory card and associated memory card slot, and / or any other removable storage unit and associated interface.

[0169] Computer system 1600 may further include a communication or network interface 1624. Communication interface 1624 may enable computer system 1600 to communicate and interact with any combination of external devices, external networks, external entities, etc. (individually and collectively referenced by reference number 1628). For example, communication interface 1624 may allow computer system 1600 to communicate with external or remote devices 1628 over communications path 1626, which may be wired and / or wireless (or a combination thereof), and which may include any combination of LANs, WANs, the Internet, etc. Control logic and / or data may be transmitted to and from computer system 1600 via communication path 1626.

[0170] Computer system 1600 may also be any of a personal digital assistant (PDA), desktop workstation, laptop or notebook computer, netbook, tablet, smart phone, smart watch or other wearable, appliance, part of the Intemet-of-Things, and / or embedded system, to name a few nonlimiting examples, or any combination thereof.

[0171] Computer system 1600 may be a client or server, accessing or hosting any applications and / or data through any delivery paradigm, including but not limited to remote or distributed cloud computing solutions; local or on-premises software (“on-premise” cloud-based solutions); “as a service” models (e.g., content as a service (CaaS), digital content as a service (DCaaS), software as aservice (SaaS), managed software as a service (MSaaS), platform as a service (PaaS), desktop as a service (DaaS), framework as a service (FaaS), backend as a service (BaaS), mobile backend as a service (MBaaS), infrastructure as a service (laaS), etc.); and / or a hybrid model including any combination of the foregoing examples or other services or delivery paradigms.

[0172] Any applicable data structures, file formats, and schemas in computer system 1600 may be derived from standards including but not limited to JavaScript Object Notation (JSON), Extensible Markup Language (XML), Yet Another Markup Language (Y AML), Extensible Hypertext Markup Language (XHTML), Wireless Markup Language (WML), MessagePack, XML User Interface Language (XUL), or any other functionally similar representations alone or in combination.Alternatively, proprietary data structures, formats or schemas may be used, either exclusively or in combination with known or open standards.

[0173] In some embodiments, a tangible, non-transitory apparatus or article of manufacture comprising a tangible, non-transitory computer useable or readable medium having control logic (software) stored thereon may also be referred to herein as a computer program product or program storage device. This includes, but is not limited to, computer system 1600, main memory 1608, secondary memory 1610, and removable storage units 1618 and 1622, as well as tangible articles of manufacture embodying any combination of the foregoing. Such control logic, when executed by one or more data processing devices (such as computer system 1600), may cause such data processing devices to operate as described herein.

[0174] Based on the teachings contained in this disclosure, it will be apparent to persons skilled in the relevant art(s) how to make and use embodiments of this disclosure using data processing devices, computer systems and / or computer architectures other than that shown in FIG. 16. In particular, embodiments can operate with software, hardware, and / or operating system implementations other than those described herein.

[0175] The terms “radiation,” “beam,” “light,” “illumination,” or the like can be used herein to refer to one or more types of electromagnetic radiation, for example, ultraviolet (UV) radiation (for example, having a wavelength X of 365, 248, 193, 157 or 126 nm), extreme ultraviolet (EUV or soft X-ray) radiation (for example, having a wavelength in the range of 5-100 nm such as, for example, 13.5 nm), or hard X-ray working at less than 5 nm, as well as particle beams, such as ion beams or electron beams. Generally, radiation having wavelengths between about 400 to about 700 nm is considered visible radiation; radiation having wavelengths between about 780-3000 nm (or larger) is considered IR radiation. UV refers to radiation with wavelengths of approximately 100-400 nm. Within lithography, the term “UV” also applies to the wavelengths that can be produced by a mercury discharge lamp: G- line 436 nm; H-line 405 nm; and / or, I-line 365 nm. Vacuum UV, or VUV (i.e., UV absorbed by gas), refers to radiation having a wavelength of approximately 100-200 nm. Deep UV (DUV) generally refers to radiation having wavelengths ranging from 126 nm to 428 nm, and in some aspects, an excimer laser can generate DUV radiation used within a lithographic apparatus. It should be appreciated that radiationhaving 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.

[0176] Although some aspects of the present disclosure are described in the context of lithographic apparatuses in the manufacture of ICs, it should be understood that lithographic apparatuses described herein can be used in other applications, for example, in the manufacture of integrated optical systems, guidance and detection patterns for magnetic domain memories, flat -panel displays, LCDs, thin-fdm 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.

[0177] Furthermore, although some aspects of the present disclosure are described in the context of optical lithography, it should be understood that aspects of the present disclosure are not limited to optical lithography. For example, in imprint lithography, a topography in a patterning device defines the pattern created on a substrate. The topography of the patterning device can be pressed into a layer of resist supplied to the substrate whereupon the resist is cured by applying electromagnetic radiation, heat, pressure or a combination thereof. The patterning device is moved out of the resist leaving a pattern in it after the resist is cured.

[0178] 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.

[0179] The present disclosure has been described above with the aid of functional building blocks illustrating the implementation of specified functions and relationships thereof. The boundaries of these functional building blocks have been arbitrarily defined herein for the convenience of the description. Alternate boundaries can be defined so long as the specified functions and relationships thereof are appropriately performed. The foregoing description of specific aspects will so fully reveal the general nature of the present disclosure that others can, by applying knowledge within the skill of the art, readily modify and / or adapt for various applications such specific aspects, without undue experimentation and without departing from the general concept of the present disclosure. Therefore, such adaptations and modifications are intended to be within the meaning and range of equivalents of the disclosed aspects, based on the teaching and guidance presented herein.

[0180] It is to be understood that the Detailed Description section, and not the Summary and Abstract sections, is intended to be used to interpret the claims. The Summary and Abstract sections can set forth one or more, but not necessarily all, aspects of the present disclosure as contemplated by the inventor(s),and thus, are not intended to limit the present disclosure and the appended claims in any way. The breadth and scope of the protected subject matter should not be limited by any of the above -described aspects, but should be defined in accordance with the following claims and their equivalents.

Claims

CLAIMS1. A method for optimizing a laser-to-droplet alignment position during extreme ultraviolet (EUV) radiation generation by a light source, the method comprising: fitting a polynomial to a radiation generation metric that varies based on the laser-to-droplet alignment position, and determining an optimal alignment position for a laser relative to a target based on an apex of the polynomial fit; or evaluating commanded laser energy versus laser-to-droplet alignment position across an available alignment space to determine where commanded laser energy is maximum, and determining an optimal alignment position for the laser relative to the target so that the alignment space is symmetric between the maximum points; and modifying the alignment position based on the optimal alignment position.

2. The method of claim 1, further comprising limiting the optimal alignment position to a range of minimum and maximum possible positions to prevent over or under tilting of a target relative to an axis of the laser.

3. The method of claim 2, wherein the limiting comprises calculating a minimum and maximum possible laser-to-droplet X alignment position so that a target satisfies a range of Y tilt thresholds, and a minimum and maximum laser-to-droplet Y alignment position so that the target satisfies a range of X tilt thresholds, and ensuring conformance to the minimum and maximum X and Y alignment positions even if the optimal alignment position breaches the maximum X or minimum Y alignment positions.

4. The method of claim 1, wherein the laser-to-droplet alignment position is recalculated at a predetermined interval during the radiation generation.

5. The method of claim 2, wherein the maximum possible range of optimal alignment positions is recalculated at a predetermined interval during the radiation generation.

6. The method of claim 1, wherein the radiation generation metric is EUV intensity.

7. The method of claim 1, wherein the radiation generation metric is conversion efficiency.

8. The method of claim 3, wherein the minimum Y tilt threshold is chosen to mitigate light reflection back into a laser.

9. The method of claim 3, wherein a minimum X and a maximum X tilt threshold are chosen to minimize tin contamination on vessel walls or a collector of the light source.

10. The method of claim 3, wherein a maximum Y tilt threshold is chosen to reduce measurement error due to a low signal-to-noise ratio of a sensor.

11. The method of claim 1, wherein the polynomial fit is used to determine laser-to-droplet X andY alignment positions when an energy controller is open.

12. The method of claim 1, wherein the commanded laser energy is used to determine laser-to- droplet X and Y alignment positions when an energy controller is closed.

13. An extreme ultraviolet (EUV) light source comprising: a laser; a target, wherein the target comprises a droplet that has been irradiated by the laser; a controller, wherein the controller continuously tracks optimal laser-to-droplet alignment positions in two orthogonal directions and updates the position of the laser to maintain optimal alignment during radiation generation.

14. A EUV light source of claim 13, wherein the two orthogonal directions are X and Y, and the controller determines the optimal laser-to-droplet X and Y alignment positions by fitting a polynomial to a radiation generation metric that varies based on the laser-to-droplet alignment position, and adjusting an optimal alignment position for the laser relative to the target based on an apex of the polynomial fit.

15. A EUV light source of claim 14, wherein the radiation generation metric is EUV intensity or conversion efficiency.

16. A EUV light source of claim 13, wherein the two orthogonal directions are X and Y, and the controller determines the optimal laser-to-droplet X and Y alignment positions by evaluating commanded laser energy versus laser alignment position across an available alignment space to determine where the commanded laser energy is maximum, and adapting the alignment space to be symmetric around between the maximum points.

17. A EUV light source of claim 13, wherein the controller calculates a minimum and maximumY tilt threshold and a minimum and maximum X tilt threshold for the target relative to the laser.

18. A EUV light source of claim 17, wherein the controller calculates a minimum and maximum acceptable laser-to-droplet X alignment position based on the Y tilt thresholds and a minimum and maximum laser-to-droplet Y alignment position based on the X tilt thresholds.

19. A EUV light source of claim 17, wherein the controller adjusts the laser-to-droplet X alignment position to satisfy the Y tilt thresholds and the laser-to-droplet Y alignment position to satisfy the X tilt thresholds, even if the optimal laser-to-droplet X and Y alignment positions fall outside the X tilt thresholds and Y tilt thresholds.

20. A EUV light source of claim 13, wherein the EUV light source is used in an optical lithography system.

21. A non-transitory computer-readable medium having instructions embodied thereon, the instructions executable by one or more processors to perform operations comprising: determining a tilt of a target relative to a laser; determining an acceptable range of X tilt thresholds and an acceptable range of Y tilt thresholds for the target relative to the laser; calculating a minimum and a maximum possible laser-to-droplet X alignment position so that the target satisfies the range of Y tilt thresholds, and a minimum and maximum laser-to-fuel droplet Y alignment position so that the target satisfies the range of X tilt thresholds; calculating an optimal laser-to-droplet X and Y alignment position based on EUV radiation generation parameters and adjusting an alignment position of the laser; determining whether the calculated optimal laser-to-droplet X and Y alignment positions satisfy the range of X and Y tilt thresholds; adjusting the alignment position of the laser to comply with X and Y tilt thresholds, even if the calculated optimal laser X and Y alignment positions fall outside of the X and Y tilt thresholds.

Images