Methods and systems for removing tin contaminants

Abstract

A method is provided for cleaning a structure, such as an EUV lithographic apparatus, to which tin contaminants are adhered, and the method comprises inducing the formation of hydrogen bubbles in β -sn adhered to the structure to form hydrogen infused β-Sn, cooling the hydrogen infused β -sn to be below a threshold temperature and thereby converting the β -sn to α − Sn, and then removing the converted α − Sn from the structure.

Description

METHODS AND SYSTEMS FOR REMOVING TIN CONTAMINANTSCROSS-REFERENCE TO RELATED APPLICATIONS

[0001] This application claims priority of US application 63 / 598,483 which was filed on 13 November 2023 and which is incorporated herein in its entirety by reference.FIELD OF THE INVENTION

[0002] In several aspects, the present invention relates to methods of cleaning a structure to which Sn and / or [3-Sn contaminants are adhered. In another aspects, the present invention relates to methods and systems for cleaning an EUV source.BACKGROUND OF THE INVENTION

[0003] Extreme ultra violet (“EUV”) lithography is an optical lithography technology used in recent years in the semiconductor industry to fabricate integrated circuits with extremely small features. EUV comprises radiation with a wavelength of approximately 13.5 nm. In an EUV lithography system, an EUV radiation source is used. The EUV radiation is emitted by a plasma that is formed by an electric discharge or by a laser pulse ionizing a target. The target can comprise gas or vapor, for example Xe gas, Li vapor or Sn vapor. In operation Li or Sn can accumulate on the components of the EUV source causing loss of transmission or disturbing gas flows. In addition to being used in a lithographic process, an EUV source can also be used for inspection or metrology. While tin-based laser plasma produced EUV source has many benefits, it has to be cleaned on a periodic basis to remove Tin contaminants from various machine components. Such cleaning often results in undesirable and expensive system downtime. While tin mitigation may be important when the EUV source is used in a lithographic process, tin mitigation may be of even higher importance when the EUV source is used in an inspection or metrology process. Note that the terms “Tin” or “tin” are used herein synonymously with its chemical element “Sn.”

[0004] There are two known methods for mechanically removing Tin from contaminated parts. First, mechanical Tin cleaning tools can be used to clean Tin off parts in-situ (i.e., while remaining installed in its operational position). For example, a commercial rotation brush (aluminum oxide or brass) can be placed on a drill to clean various parts of an EUV source (e .g . , during replacement of the EUV source collector). However, the reach of the brush is typically limited, even if mounted on an extension, and Tin adhesion is significant; this renders the brush method inefficient. Second, CO2 snow (or CO2 pellet, a more aggressive method) cleaning can also be used. CO2 snow cleaning is a promising method to successfully remove Tin from multiple parts. CO2 snow cleaning is a dry, non-abrasive, solvent-free, residue-free cleaning method. In this process, a jet of CO2 “snow” is created during rapid evaporation, as pressurized liquid CO2 jet exits a nozzle and breaks into droplets. When the nozzle is pointed at a contaminated surface (e.g., such as an EUV source mirror), the CO2 snow jet causes rapid cooling andembrittlement of contamination layers. Contamination is detached and rinsed as a result of CO2 snow sublimation upon impact and local gas / liquid flows. CO2 snow cleaning can be a powerful technique to clean microscopic amounts of Tin (e.g., a very thin layer or individual droplets of Tin, typically less than 10 um thick) that are typically found on the collector of an EUV source. However, for thick layers of tin (1 mm or more) it is significantly less effective.

[0005] Another method of cleaning Tin sometimes involves the allotropic transformation of white [3- tin into grey a-tin through freezing of [3-tin at very cold temperatures (typically, below -20 °C). In this process Tin is transformed from its malleable white phase ([3-tin) into its brittle grey phase (a-tin). The grey tin is a powdery brittle substance. Volume increase of approximately 27% is associated with the transformation. The expansion can reduce adhesion, since expanded grey Tin layer does not conform to the surface to which he original white Tin layer adhered conformally. The transformation occurs below approximately 13°C at normal pressure and is often referred to as “Tin pest.”

[0006] With this said, these technologies can be time consuming, sometimes requiring removal of parts from their operating environment, causing significant machine downtime. As example, lengthy periods of time may be required to clean Tin contaminated parts of the EUV laser produced plasma (“LPP”) source, a discharge produced plasma source, or a laser assisted discharge produced plasma source or any variations thereof. In addition, Tin contamination is detrimental to the performance of an LPP EUV source. It can cause loss of EUV power output (loss of collector mirror reflectivity), disturb gas flow (loss of gas conductivity) or result in tin vapor and particles co -propagating with EUV from the source (categorized as tin through intermediate focus, “Sn2IF”). Sn2IF can be generated for example from Tin contamination present on the surfaces close to the EUV beam and intermediate focus (IF) in the presence of H2 plasma. The IF-cap is an EUV source component that is closest to IF. Cleaning of the IF -cap installed in the EUV source with mechanical methods is challenging due to geometrical constraints: the narrow end of the generally conical IF-cap does not allow access of the brush or CO2 snow jet. At the same time uninstallation and re-installation of the IF-cap can take much more than a day, and cause undesired and costly downtime of EUV lithographic, inspection or metrology tool Finally, it should also be noted that transformation of white Tin into grey Tin, which could facilitate the mechanical cleaning by weakening of the adhesion, can also be a time-consuming process (e.g., weeks), depending in part on the Tin amount, purity, material and shape of the substrate contaminated with Tin, as well as process pressure and temperature.

[0007] Yet other aspects of this disclosure relate to cleaning of Tin from two buckets that are used in the EUV source. One of the Sn buckets stores Sn exposed to plasma, and the other stores the Sn unexposed to plasma . Capacity of both buckets typically is sufficient to store Tin corresponding to EUV source continuous operation for approximately one year, as it is limited to the available volume of the EUV source. Eventually, these buckets must be replaced or cleaned before EUV source can be operated again. Existing technology of renewing the buckets involves: docking bucket onto a speciallydesigned fixture, rotating the fixture, transporting docked bucket into a vacuum chamber (oven), pumping down the vacuum chamber, heating up the oven to above Sn melting temperature, flowing in molten Sn out of the bucket, cooling the vacuum chamber, venting the chamber and transporting the bucket out, and cleaning the drained Sn.

[0008] There are several disadvantages in this methodology for cleaning Sn buckets, including the need to heat a vacuum chamber to above 230 °C, Sn flow management, prevention of oxidation, the Sn draining process, etc.

[0009] The present disclosure addresses the above issues.SUMMARY OF THE INVENTION

[0010] In a first aspect, a method is provided for cleaning a structure to which Sn contaminants are adhered. The method comprises inducing the formation of H2 bubbles in [3-Sn adhered to the structure to form H2 infused [3-Sn; cooling the H2 infused [3-Sn to be below a threshold temperature and thereby convert the [3-Sn to a-Sn; and removing the a-Sn from the structure.

[0011] The inducing of the formation of H2 bubbles may comprise directing a H2 plasma toward the [3- Sn during normal operation of the structure’s operating environment.

[0012] The inducing of the formation of [3-Sn bubbles may exposing the [3-Sn to hydrogen radicals. A hydrogen radical generator may be used for exposing the [3-Sn to hydrogen radicals.

[0013] In one embodiment, the inducing of the formation of H2 bubbles comprises exposing the [3-Sn to H+hydrated ions.

[0014] The exposing the [3-Sn to H+hydrated ions may comprise placing the structure with the adhered [3-Sn in an electrolyte filled bath, and applying a voltage across the structure and a counter-electrode disposed in the bath.

[0015] The cooling may be conducted by placing the structure with the adhered [3-Sn into a temperature -controlled environment.

[0016] The temperature-controlled environment may comprise a container, wherein a liquid coolant and / or a gaseous coolant is provided in the container.

[0017] The removing may comprise physically agitating the a-Sn so as to free it from the structure.

[0018] The physically agitating may comprise applying a stream of CO2 snow or CO2 pellets (typically, a stream comprising CO2 snow or CO2 pellets) to the a-Sn.

[0019] The physically agitating may comprise mechanical brushing.

[0020] The cooling suitably comprises applying a coolant onto the H2 infused [3-Sn or within the structure with H2 infused [3-Sn.

[0021] The cooling may comprise circulating a liquid coolant and / or a gas coolant. The liquid coolant and / or the gas coolant may be circulated through a coolant tool.

[0022] The cooling may comprise applying a liquid coolant or a gas coolant directly onto the H2 infused P-Sn.

[0023] The structure may comprise a component of an EUV source, further comprising removing the structure from a housing of the EUV source prior to inducing the formation of H2 bubbles in the Sn contaminates adhered to the structure.

[0024] The -Sn adhered to the removed structure may have some H2 bubbles formed therein as a result of some exposure to hydrogen plasma during operation of the EUV source.

[0025] The P-Sn adhered to the removed structure may have minimal or zero H2 bubbles formed therein as a result of the structure being largely shielded from hydrogen plasma during operation of the EUV source or due to operation of the structure at the point above tin melting temperature.

[0026] The inducing is preferably conducted at a rate faster than the rate at which H2 bubbles would be formed in an operating environment of the structure.

[0027] In another aspect of the invention, there is provided a method of cleaning a structure to which Sn contaminates are adhered. The method comprises seeding a mass of P-Sn adhered to the structure to be cleaned with aconversion seed, so as to promote conversion of P-Sn to a-Sn; cooling the seeded mass of P-Sn to be below a threshold temperature and thereby convert the P-Sn to a-Sn; and removing the converted a-Sn from the structure.

[0028] The threshold temperature typically is 13.2 °C at atmospheric pressure.

[0029] The seeding may be conducted by spraying an a-Sn infused liquid onto the P-Sn.

[0030] Such a a-Sn infused liquid may comprise a-Sn particles and ethanol.

[0031] The seeding may comprise forming H2 bubbles in the P-Sn adhered to the structure to be cleaned.

[0032] The seeded mass may preferably be cooled to a temperature between 0 °C and -60 °C.

[0033] More preferably, the seeded mass may be cooled to a temperature between -20 °C and -40 °C.

[0034] In another aspect, there is provided a method of cleaning a structure having P-Sn contaminants adhered thereon, the structure having cooling channels therein for receiving a coolant at an operating temperature during operation thereof. The method comprises providing the coolant to the cooling channels at a temperature lower than the operating temperature, so as to induce the P-Sn contaminants adhered to the structure to convert to a-Sn; and removing the a-Sn from the structure.

[0035] The removing may comprise mechanical brushing of the a-Sn to remove it from the structure.

[0036] The removing may comprise applying a stream of CO2 snow or pellets (typically, a stream comprising CO2 snow or CO2 pellets) to the a-Sn.

[0037] The structure may be an intermediate focus cap of an EUV source.

[0038] The method may comprise removing the coolant used during operation of the structure and replacing it with a coolant having a lower freezing point so that it can be brought to the temperaturelower than the operating temperature so as to induce the P-Sn contaminants adhered to the structure to convert to a-Sn .

[0039] In one embodiment, the method comprises cooling the same coolant used at the operating temperature to be at the lower temperature so as to induce the -Sn contaminants adhered to the structure to convert to a-Sn .

[0040] In yet another aspect of the invention, a method may be provided for cleaning an EUV source, the EUV source including a source housing and a component within the housing, the component having P-Sn contaminants adhered thereto. The method comprises cooling the P-Sn contaminants on the component to a temperature that induces conversion of P-Sn to a-Sn while the component remains positioned within the housing; and agitating the a-Sn to remove it from the component while the component remains positioned within the housing.

[0041] The component may comprise an intermediate focus cap (IF Cap).

[0042] The EUV source may include additional components in the housing, wherein the additional components are removed from the EUV housing and then cooled to induce P-Sn adhered thereto to convert to a-Sn, and then removing the a-Sn from the additional components.

[0043] Such additional components may comprise one or more of the following: a scrubber, a liner, a near normal incidence collector, a gas distribution system, a tin catcher, or an obscuration bar. The method may further inducing the formation of EE bubbles in the P-Sn adhered to the component and or the additional component(s), such as the exhaust, prior to cooling of the P-Sn thereon.

[0044] In still another aspect of the invention, there is provided an EUV source cleaning system for cleaning a structure of an EUV source having P-Sn contaminants adhered thereon. The system comprises a refrigeration pump configured to cool and circulate a coolant, wherein the refrigeration pump is constructed and arranged to cool the coolant to a temperature below the freezing point of water; a coolant applicator constructed and arranged to provide a thermal conduit between the coolant and the P-Sn contaminants adhered to the structure to facilitate conversion of the P-Sn to a-Sn ; inlet and outlet lines connected between the coolant applicator and the refrigeration pump to enable the refrigeration pump to provide the coolant to the applicator; and an agitator configured to remove the a-Sn from the structure. The structure of the EUV source may be a component of the EUV source.

[0045] The coolant applicator may comprise cooling channels within the structure to be cleaned.

[0046] The coolant applicator may comprise a coolant housing configured to be moved into direct contact with the P-Sn contaminants adhered to the structure to facilitate conversion of the P-Sn to a-Sn.

[0047] The agitator may comprise a nozzle for ejecting a substance that impacts and removes the converted a-Sn from the structure.

[0048] The agitator may comprise a brush.

[0049] The coolant may comprise ethanol or glycol.

[0050] The structure may comprise an intermediate focus cap (IF-Cap) that remains within a housing of the EUV source during cleaning thereof.

[0051] In still another aspect of the invention, there is provided an EUV source cleaning system for cleaning a structure of an EUV source having [3-Sn contaminants adhered thereon. The system comprises a source of hydrogen configured to induce the formation of H2 bubbles in [3-Sn adhered to the structure to form H2 infused [3-Sn; a refrigeration system configured to cool the H2 infused [3-Sn to be below a threshold temperature and thereby convert the [3-Sn to a-Sn; and an agitator configured to remove the a-Sn from the structure. The structure of the EUV source may be a component of the EUV source.

[0052] The source of hydrogen preferably is an additional source of hydrogen configured to generate additional hydrogen above any H2 plasma that has been directed at the [3-Sn during normal operation of the structure’s operating environment.

[0053] The source of hydrogen may comprise a hydrogen radical generator configured to expose the [3-Sn to hydrogen radicals.

[0054] The source of hydrogen may comprise a source of H+hydrated ions. The source of H+hydrated ions may comprise an electrolyte filled bath configured to receive the structure; a counter-electrode disposed in the bath; and a power supply connectable to the structure and the counter-electrode, to apply a voltage across the structure and the counter-electrode.BRIEF DESCRIPTION OF THE DRAWINGS

[0055] Fig. 1 schematically depicts a first example of a lithographic system.

[0056] Fig. 2 is a schematic diagram of a second example of a lithographic system.

[0057] Fig. 3 depicts another lithographic system comprising a lithographic apparatus and an EUV radiation source .

[0058] Fig. 4 is a detailed schematic diagram of an embodiment of the source of Fig. 3.

[0059] Figs. 5A, 5B, 5C, and 5D are schematic diagram illustrating a cleaning process for parts which were exposed to H2 plasma during operation.

[0060] Fig. 6 is a schematic diagram of an alternative method and apparatus.

[0061] Fig. 7 is a schematic diagram of another alternative method and apparatus.

[0062] Fig. 8 is a schematic diagram of an exhaust in a temperature controlled environment.

[0063] Fig. 9 is a schematic diagram of applying a mechanical cleaning tool to the exhaust.

[0064] Fig. 10 is a schematic diagram of an exhaust that has been cleaned in accordance with any of the apparatus and / or methods disclosed herein.

[0065] Fig. 11 is a schematic diagram illustrating a method and apparatus for in-situ cleaning of an IF -cap.

[0066] Fig. 12 is a schematic diagram of an alternative or additional method and apparatus for in-situ cleaning of an IF -cap.

[0067] Fig. 13 is a schematic diagram of a further alternative or additional method and apparatus for in-situ cleaning of an IF -cap.

[0068] Fig. 14 is a schematic diagramillustrating removal of grey tin from the IF-cap of any of the Figs. 11 to 13.

[0069] Fig. 15 is a schematic diagram of a source segment that has been cleaned in accordance with one or more of the methods or apparatus disclosed herein.DETAILED DESCRIPTION

[0070] By way of a brief introduction, the description herein relates generally to semiconductor device manufacturing and patterning processes. More particularly, the following paragraphs describe several components of a system and / or related systems, as well as methods for determining relative positions of alignment marks in layers of a structure. As described above these systems and methods may be used for measuring alignment in a semiconductor device manufacturing process, for example, or during other operations.

[0071] Although specific reference may be made in this text to the manufacture of integrated circuits (ICs) for semiconductor devices, it should be understood that the description herein has many other possible applications. For example, it may be employed in the manufacture of integrated optical systems, guidance and detection patterns for magnetic domain memories, liquid crystal display panels, thin film magnetic heads, etc. The skilled artisan will appreciate that, in the context of such alternative applications, any use of the terms “reticle”, “wafer” or “die” in this text should be considered as interchangeable with the more general terms “mask”, “substrate” and “target portion”, respectively.

[0072] In the context of the present specification, the term “lithographic apparatus” is used to describe a lithographic projection apparatus. The term “projection optics” as used herein should be broadly interpreted as encompassing various types of optical systems, including refractive optics, reflective optics, apertures and catadioptric optics, for example. The term “projection optics” may also include components operating according to any of these design types for directing, shaping or controlling the projection beam of radiation, collectively or singularly. The term “projection optics” may include any optical component in the lithographic apparatus, no matter where the optical component is located on an optical path of the lithographic apparatus. Projection optics may include optical components for shaping, adjusting and / or projecting radiation from the source before the radiation passes the patterning device, and / or optical components for shaping, adjusting and / or projecting the radiation after the radiation passes the patterning device. The projection optics generally exclude the source and the patterning device.

[0073] Fig. 1 schematically depicts a lithographic system comprising an embodiment of a lithographic apparatus LA. The apparatus comprises an illumination system (illuminator) IL configured to condition a radiation beam B (e.g. UV radiation, DUV radiation, or EUV radiation); a support structure (e.g. a mask table) MT constructed to support a patterning device (e.g. a mask) MA and connected to a first positioner PM configured to accurately position the patterning device in accordance with certain parameters; a substrate table (e.g. a wafer table) WT (e.g., WTa, WTb or both) configured to hold a substrate (e.g. a resist coated wafer) W and coupled to a second positioner PW configured to accurately position the substrate in accordance with certain parameters; and a projection system (e.g. a refractive projection lens system) PS configured to project a pattern imparted to the radiation beam B by patterning device MA onto a target portion C (e.g. comprising one or more dies and often referred to as fields) of the substrate W. The projection system is supported on a reference frame RF.

[0074] As depicted, the apparatus is of a transmissive type (e.g., employing a transmissive mask). Alternatively, the apparatus may be of a reflective type (e.g., employing a programmable mirror array of a type as referred to above, or employing a reflective mask).

[0075] The illuminator IL receives a beam of radiation from a radiation source SO. The source and the lithographic apparatus may be separate entities, for example when the source is an excimer laser. In such cases, the source is not considered to form part of the lithographic apparatus and the radiation beam is passed from the source SO to the illuminator IL with the aid of a beam delivery system BD comprising for example suitable directing mirrors and / or a beam expander. In other cases, the source may be an integral part of the apparatus, or at least integrated into the apparatus, for example when the source is a mercury lamp or an EUV source, particularly EUV laser produced plasma source . The source SO and the illuminator IL, together with the beam delivery system BD if required, may be referred to as a radiation system.

[0076] The illuminator IL may alter the intensity distribution of the beam. The illuminator may be arranged to limit the radial extent of the radiation beam such that the intensity distribution is non -zero within an annular region in a pupil plane of the illuminator IL. Additionally, or alternatively, the illuminator IL may be operable to limit the distribution of the beam in the pupil plane such that the intensity distribution is non-zero in a plurality of equally spaced sectors in the pupil plane. The intensity distribution of the radiation beam in a pupil plane of the illuminator IL may be referred to as an illumination mode.

[0077] The illuminator IL may comprise adjuster AD configured to adjust the (angular / spatial) intensity distribution of the beam. Generally, at least the outer and / or inner radial extent (commonly referred to as o-outer and o-inner, respectively) of the intensity distribution in a pupil plane of the illuminator can be adjusted. The illuminator IL may be operable to vary the angular distribution of the beam. For example, the illuminator may be operable to alter the number, and angular extent, of sectors in the pupil plane wherein the intensity distribution is non-zero. By adjusting the intensity distributionof the beam in the pupil plane of the illuminator, different illumination modes may be achieved. For example, by limiting the radial and angular extent of the intensity distribution in the pupil plane of the illuminator IL, the intensity distribution may have a multi-pole distribution such as, for example, a dipole, quadrupole or hexapole distribution. A desired illumination mode may be obtained, e.g., by inserting an optic which provides that illumination mode into the illuminator IL or using a spatial light modulator.

[0078] The illuminator IL may be operable to alter the polarization of the beam and may be operable to adjust the polarization using adjuster AD. The polarization state of the radiation beam across a pupil plane of the illuminator IL may be referred to as a polarization mode. The use of different polarization modes may allow greater contrast to be achieved in the image formed on the substrate W. The radiation beam may be unpolarized. Alternatively, the illuminator may be arranged to linearly polarize the radiation beam. The polarization direction of the radiation beam may vary across a pupil plane of the illuminator IL. The polarization direction of radiation may be different in different regions in the pupil plane of the illuminator IL. The polarization state of the radiation may be chosen in dependence on the illumination mode. For multi -pole illumination modes, the polarization of each pole of the radiation beam may be generally perpendicular to the position vector of that pole in the pupil plane of the illuminator IL. For example, for a dipole illumination mode, the radiation may be linearly polarized in a direction that is substantially perpendicular to a line that bisects the two opposing sectors of the dipole . The radiation beam may be polarized in one of two different orthogonal directions, which may be referred to as X-polarized and Y-polarized states. For a quadrupole illumination mode, the radiation in the sector of each pole may be linearly polarized in a direction that is substantially perpendicular to a line that bisects that sector. This polarization mode may be referred to as XY polarization. Similarly, for a hexapole illumination mode the radiation in the sector of each pole may be linearly polarized in a direction that is substantially perpendicular to a line that bisects that sector. This polarization mode may be referred to as TE polarization.

[0079] In addition, the illuminator IL generally comprises various other components, such as an integrator IN and a condenser CO. The illumination system may include various types of optical components, such as refractive, reflective, magnetic, electromagnetic, electrostatic or other types of optical components, or any combination thereof, for directing, shaping, or controlling radiation. Thus, the illuminator provides a conditioned beam of radiation B, having a desired uniformity and intensity distribution in its cross section.

[0080] The support structure MT supports the patterning device in a manner that depends on the orientation of the patterning device, the design of the lithographic apparatus, and other conditions, such as for example whether or not the patterning device is held in a vacuum environment. The support structure may use mechanical, vacuum, electrostatic or other clamping techniques to hold the patterning device. The support structure may be a frame or a table, for example, which may be fixed or movable as required. The support structure may ensure that the patterning device is at a desired position, forexample with respect to the projection system. Any use of the terms “reticle” or “mask” herein may be considered synonymous with the more general term “patterning device.”

[0081] The term “patterning device” used herein should be broadly interpreted as referring to any device that can be used to impart a pattern in a target portion of the substrate. In an embodiment, a patterning device is any device that can be used to impart a radiation beam with a pattern in its crosssection to create a pattern in a target portion of the substrate . It should be noted that the pattern imparted to the radiation beam may not exactly correspond to the desired pattern in the target portion of the substrate, for example if the pattern includes phase -shifting features or so called assist features. Generally, the pattern imparted to the radiation beam will correspond to a particular functional layer in a device being created in a target portion of the device, such as an integrated circuit.

[0082] A patterning device may be transmissive or reflective. Examples of patterning devices include masks, programmable mirror arrays, and programmable LCD panels. Masks are well known in lithography, and include mask types such as binary, alternating phase-shift, and 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 to reflect an incoming radiation beam in different directions. The tilted mirrors impart a pattern in a radiation beam, which is reflected by the mirror matrix.

[0083] The term “projection system” used herein should be broadly interpreted as encompassing 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 or the use of a vacuum. Any use of the term “projection lens” herein may be considered as synonymous with the more general term “projection system”.

[0084] The projection system PS has an optical transfer function which may be non-uniform, which can affect the pattern imaged on the substrate W. For unpolarized radiation such effects can be fairly well described by two scalar maps, which describe the transmission (apodization) and relative phase (aberration) of radiation exiting the projection system PS as a function of position in a pupil plane thereof. These scalar maps, which may be referred to as the transmission map and the relative phase map, may be expressed as a linear combination of a complete set of basis functions. A convenient set is the Zemike polynomials, which form a set of orthogonal polynomials defined on a unit circle. A determination of each scalar map may involve determining the coefficients in such an expansion. Since the Zemike polynomials are orthogonal on the unit circle, the Zemike coefficients may be determined by calculating the inner product of a measured scalar map with each Zemike polynomial in turn and dividing this by the square of the norm of that Zemike polynomial.

[0085] The transmission map and the relative phase map are field and system dependent. That is, in general, each projection system PS will have a different Zemike expansion for each field point (i.e. foreach spatial location in its image plane). The relative phase of the projection system PS in its pupil plane may be determined by projecting radiation, for example from a point -like source in an object plane of the projection system PS (i.e. the plane of the patterning device MA), through the projection system PS and using a shearing interferometer to measure a wavefront (i.e. a locus of points with the same phase). A shearing interferometer is a common path interferometer and therefore, advantageously, no secondary reference beam is required to measure the wavefront. The shearing interferometer may comprise a geometric feature such as a diffraction grating, for example a two-dimensional grid, in an image plane of the projection system (i.e. the substrate table WT) and a detector arranged to detect an interference pattern in a plane that is conjugate to a pupil plane of the projection system PS. The interference pattern is related to the derivative of the phase of the radiation with respect to a coordinate in the pupil plane in the shearing direction. The detector may comprise an array of sensing elements such as, for example, charge coupled devices (CCDs).

[0086] The diffraction grating may be sequentially scanned in two perpendicular directions, which may coincide with axes of a co-ordinate system of the projection system PS (x and y) or may be at an angle such as 45 degrees to these axes. Scanning may be performed over an integer number of grating periods, for example one grating period. The scanning averages out phase variation in one direction, allowing phase variation in the other direction to be reconstructed. This allows the wavefront to be determined as a function of both directions.

[0087] The transmission (apodization) of the projection system PS in its pupil plane may be determined by projecting radiation, for example from a point-like source in an object plane of the projection system PS (i.e. the plane of the patterning device MA), through the projection system PS and measuring the intensity of radiation in a plane that is conjugate to a pupil plane of the projection system PS, using a detector. The same detector as is used to measure the wavefront to determine aberrations may be used.

[0088] The projection system PS may comprise a plurality of optical (e.g., lens) elements and may further comprise an adjustment mechanism configured to adjust one or more of the optical elements to correct for aberrations (phase variations across the pupil plane throughout the field). To achieve this, the adjustment mechanism may be operable to manipulate one or more optical (e.g., lens) elements within the projection system PS in one or more different ways. The projection system may have a coordinate system wherein its optical axis extends in the z direction. The adjustment mechanism may be operable to do any combination of the following: displace one or more optical elements; tilt one or more optical elements; and / or deform one or more optical elements. Displacement of an optical element may be in any direction (x, y, z or a combination thereof). Tilting of an optical element is typically out of a plane perpendicular to the optical axis, by rotating about an axis in the x and / or y directions although a rotation about the z axis may be used for a non-rotationally symmetric aspherical optical element. Deformation of an optical element may include a low frequency shape (e.g. astigmatic) and / or a high frequency shape (e.g. free form aspheres). Deformation of an optical element may be performed for example by using one or more actuators to exert force on one or more sides of the optical element and / orby using one or more heating elements to heat one or more selected regions of the optical element. In general, it may not be possible to adjust the projection system PS to correct for apodization (transmission variation across the pupil plane). The transmission map of a projection system PS may be used when designing a patterning device (e.g., mask) MA for the lithographic apparatus LA. Using a computational lithography technique, the patterning device MA may be designed to at least partially correct for apodization.

[0089] The lithographic apparatus may be of a type having two (dual stage) or more tables (e.g., two or more substrate tables WTa, WTb, two or more patterning device tables, a substrate table WTa and a table WTb below the projection system without a substrate that is dedicated to, for example, facilitating measurement, and / or cleaning, etc.). In such “multiple stage” machines, the additional tables may be used in parallel, or preparatory steps may be carried out on one or more tables while one or more other tables are being used for exposure. For example, alignment measurements using an alignment sensor AS and / or level (height, tilt, etc.) measurements using a level sensor LS may be made.

[0090] The lithographic apparatus may also be of a type wherein at least a portion of the substrate may be covered by a liquid having a relatively high refractive index, e.g. water, to fdl a space between the projection system and the substrate. An immersion liquid may also be applied to other spaces in the lithographic apparatus, for example, between the patterning device 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, but rather only means that liquid is located between the projection system and the substrate during exposure.

[0091] In operation of the lithographic apparatus, a radiation beam is conditioned and provided by the illumination system IL. The radiation beam B is incident on the patterning device (e.g., mask) MA, which is held on the support structure (e.g., mask table) MT, and is patterned by the patterning device. Having traversed the patterning device MA, the radiation beam B passes through the projection system PS, which focuses the beam onto a target portion C of the substrate W. With the aid of the second positioner PW and position sensor ID (e.g. an interferometric device, linear encoder, 2-D encoder or capacitive sensor), the substrate table WT can be moved accurately, e.g. to position different target portions C in the path of the radiation beam B. Similarly, the first positioner PM and another position sensor (which is not explicitly depicted in Fig. 1) can be used to accurately position the patterning device MA with respect to the path of the radiation beam B, e.g. after mechanical retrieval from a mask library, or during a scan. In general, movement of the support structure MT may be realized with the aid of a long-stroke module (coarse positioning) and a short-stroke module (fine positioning), which form part of the first positioner PM. Similarly, movement of the substrate table WT may be realized using a long-stroke module and a short-stroke module, which form part of the second positioner PW. In the case of a stepper (as opposed to a scanner), the support structure MT may be connected to a shortstroke actuator only, or may be fixed. Patterning device MA and substrate W may be aligned usingpaterning device alignment marks Ml, M2 and substrate alignment marks Pl, P2. Although the substrate alignment marks as illustrated occupy dedicated target portions, they may be located in spaces between target portions (these are known as scribe -lane alignment marks). Similarly, in situations in which more than one die is provided on the paterning device MA, the paterning device alignment marks may be located between the dies.

[0092] The depicted apparatus may be used in at least one of the following modes. In step mode, the support structure MT and the substrate table WT are kept essentially stationary, while a patern imparted to the radiation beam 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. In step mode, the maximum size of the exposure field limits the size of the target portion C imaged in a single static exposure. In scan mode, the support structure MT and the substrate table WT are scanned synchronously while a patern imparted to the radiation beam is projected onto a target portion C (i.e. a single dynamic exposure). The velocity and direction of the substrate table WT relative to the support structure MT may be determined by the (de-) magnification and image reversal characteristics of the projection system PS. In scan mode, the maximum size of the exposure field limits the width (in the non-scanning direction) of the target portion in a single dynamic exposure, whereas the length of the scanning motion determines the height (in the scanning direction) of the target portion. In another mode, the support structure MT is kept essentially stationary holding a programmable paterning device, and the substrate table WT is moved or scanned while a patern imparted to the radiation beam is projected onto a target portion C. In this mode, generally a pulsed radiation source is employed, and the programmable paterning device is updated as required 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 programmable paterning device, such as a programmable mirror array of a type as referred to above. Combinations and / or variations on the above - described modes of use or entirely different modes of use may also be employed.

[0093] The substrate referred to herein may be processed, before or after exposure, in for example a track (a tool that typically applies a layer of resist to a substrate and develops the exposed resist) or a metrology or inspection tool. Where applicable, the disclosure herein may be applied to such and other substrate processing tools. Further, the substrate may be processed more than once, for example in order to create a multi-layer IC, so that the term substrate used herein may also refer to a substrate that already includes multiple processed layers.

[0094] The inspection system referred to herein may be configured to inspect a mask for potential defects. This could be achieved by illuminating the mask by EUV light generated by means of the EUV source. The inspection apparatus may include an illumination system and an optical detection system. The EUV light may be reflected by means of the optical detection system to the mask to be inspected. In this way an image may be formed on a detector. This detector could for example be a time delay integration camera.

[0095] The terms “radiation” and “beam” used herein with respect to lithography encompass all types of electromagnetic radiation, including ultraviolet (UV) or deep ultraviolet (DUV) radiation (e.g. having a wavelength of 365, 248, 193, 157 or 126 nm) and extreme ultra-violet (EUV) radiation (e.g. having a wavelength in the range of 5-20 nm, which includes, for example, 13.5 nm), as well as particle beams, such as ion beams or electron beams. Some aspects of the present disclosure pertain more particularly to use of EUV radiation.

[0096] Fig. 2 schematically depicts another example of a lithographic system 1000 that can be used in conjunction with the techniques described herein. The system 1000 comprises a radiation source SO, and a lithographic apparatus LA which comprises an illumination system IL, to condition a beam of radiation, and a projection system (“lens”) PS (e.g., a refractive, catoptric or catadioptric optical system). The lithographic apparatus LA also comprises a support structure (e.g., patterning device table) MT provided with a patterning device holder to hold a patterning device MA (e.g., a reticle); and a substrate table WT provided with a substrate holder to hold a substrate W The projection system PS is configured to image an irradiated portion of the patterning device MA onto a target portion (e.g., comprising one or more dies) (not shown) of the substrate W.

[0097] The patterning device MA of the apparatus in Figure 2 is of a reflective type. The apparatus may employ a different kind of patterning device to classic mask; examples include a programmable mirror array or LCD matrix.

[0098] The radiation source SO in this case comprises a discharge produced plasma source (DPP) or an LPP (laser produced plasma) EUV source that produces a beam of EUV radiation. This beam is fed into the illumination system (illuminator) IL, either directly or after having traversed conditioning means, such as a beam expander, for example. The illuminator IL may comprise adjusting means for setting the outer and / or inner radial extent (commonly referred to as <j-outcr and o-inncr. respectively) of the intensity distribution in the beam. In addition, it will generally comprise various other components, such as an integrator and a condenser. In this way, the beam impinging on the patterning device MA has a desired uniformity and intensity distribution in its cross-section.

[0099] The beam subsequently intercepts the patterning device MA, which is held on patterning device table MT. Having traversed the patterning device MA, the beam passes through the projection system PS, which focuses the beam onto a target portion of the substrate W. With the aid of WT positioning means (and interferometric measuring means) (not shown), the substrate table WT can be moved accurately, e.g. to position different target portions in the path of the beam. Similarly, MT positioning means (not shown) can be used to accurately position the patterning device MA with respect to the path of the beam, e.g., after mechanical retrieval of the patterning device MA from a patterning device library, or during a scan. In general, movement of the patterning device table MT and substrate table WT will be realized with the aid of a long-stroke module (coarse positioning) and a short-stroke module (fine positioning), which are not explicitly depicted. However, in the case of a stepper (asopposed to a step-and-scan tool) the patterning device table MT may just be connected to a short stroke actuator, or may be fixed.

[0100] The depicted tool can be used in different modes. In step mode, the patterning device table MT is kept essentially stationary, and an entire patterning device image is projected in one operation (i.e., a single “flash”) onto a target portion. The substrate table WT is then shifted in the x and / or y directions so that a different target portion can be irradiated by the beam. In scan mode, essentially the same scenario applies, except that a given target portion is not exposed in a single “flash”. Instead, the patterning device table MT is movable in a given direction (the so-called “scan direction”, e.g., the y direction) with a speed v, so that the projection beam is caused to scan over a patterning device image; concurrently, the substrate table WT is simultaneously moved in the same or opposite direction at a speed V = Mv, in which M is the magnification of the lens (typically, M = 1 / 4 or 1 / 5). In this manner, a relatively large target portion can be exposed, without having to compromise on resolution.

[0101] The radiation source SO is constructed and arranged such that a vacuum environment can be maintained in an enclosing structure 220 of the radiation source SO. Because EUV radiation is readily absorbed, in order to propagate it with minimal losses, pressures below atmospheric are used in the EUV beam path within the EUV source and within the EUV scanner. An EUV radiation emitting plasma 210 may be formed by a discharge produced plasma source or laser produced plasma source. EUV radiation may be produced by a gas or vapor, for example Xe gas, Li vapor or Sn vapor in which the hot plasma 210 is created to emit radiation in the EUV range of the electromagnetic spectrum. The plasma 210 is created by, for example, an electrical discharge causing at least partially ionized gas. In an embodiment, a plasma of excited tin (Sn) is provided to produce EUV radiation.

[0102] The radiation emitted by 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 may include a channel structure. Contamination trap 230 may also include a gas barrier or a combination of a gas barrier and a channel structure. The contaminant trap 230 further indicated herein at least includes a channel structure.

[0103] The source chamber 211 may include a radiation collector RC which may be a so-called grazing incidence collector. Radiation collector RC has an upstream radiation collector side 251 and a downstream radiation collector side 252. Radiation that traverses collector RC can be reflected off a grating spectral filter 240 to be focused in an intermediate focus IFalong the optical axis indicated by the line ‘O’. The intermediate focus IF is equally referred to as virtual source point, the radiation source is arranged such that the intermediate focus IF is located at or near an opening 221 in the enclosing structure 220. The virtual source point is an image of the radiation emitted by the plasma 210. Other optical elements can be used to condense the EUV radiation emitted by a discharge or laser plasma source to an intermediate focus, such a normal incidence collector. Normal incidence collector is more often used in conjunction with laser produced plasma sources.

[0104] Subsequently the radiation traverses the illumination system IL, which may include a facetted field mirror device 10 and a facetted pupil mirror device 11 arranged to provide a desired angular distribution of the radiation beam 21, 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 21 at the patterning device MA, held by the support structure MT, a patterned beam 26 is formed and the patterned beam 26 is imaged by the projection system PS via reflective elements, such as mirrors 13,14, onto the substrate W held by the substrate table WT.

[0105] More elements than shown may generally be present in illumination system IL and projection system PS. The grating spectral filter 240 may optionally be present, depending upon the type of lithographic system. In case of normal incidence collector, spectral filtering may be realized by the grating integrated into the collector itself. Further, there may be more mirrors present than those shown in the figures, for example there may be 1- 6 additional reflective elements present in the projection system PS than shown in Fig. 2.

[0106] Radiation collector RC, as illustrated in Fig. 2, 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 the optical axis O and a collector of this type may be used in combination with a discharge produced plasma source, often called a DPP source.

[0107] Alternatively, as described in more detail in connection with other figures herein, the collector optics may include a near-normal incidence radiation collector. Figure 3 illustrates an embodiment wherein the radiation source SO is based on an LPP radiation system as . A laser (not shown) is arranged to deposit laser energy into a fuel, such as xenon (Xe), tin (Sn) or lithium (Li), creating the highly ionized plasma with electron temperatures of several 10's of eV. The radiation source SO is configured to generate an EUV radiation beam B and to supply the EUV radiation beam B to the lithographic apparatus LA. The lithographic apparatus LA comprises an illumination system IL, a support structure MT configured to support a patterning device MA (e.g., a mask), a projection system PS and a substrate table WT configured to support a substrate W.

[0108] The illumination system IL is configured to condition the EUV radiation beam B before the EUV radiation beam B is incident upon the patterning device MA. Thereto, the illumination system IL may include a facetted field mirror device 10 and a facetted pupil mirror device 11. The faceted field mirror device 10 and facetted pupil mirror device 11 together provide the EUV radiation beam B with a desired cross-sectional shape and a desired intensity distribution. The illumination system IL may include other mirrors or devices in addition to, or instead of, the faceted field mirror device 10 and faceted pupil mirror device 11.

[0109] After being thus conditioned, the EUV radiation beam B interacts with the patterning device MA. As a result of this interaction, a patterned EUV radiation beam B’ is generated. The projectionsystem PS is configured to project the patterned EUV radiation beam B’ onto the substrate W. For that purpose, the projection system PS may comprise a plurality of mirrors 13,14 which are configured to project the patterned EUV radiation beam B’ onto the substrate W held by the substrate table WT. The projection system PS may apply a reduction factor to the patterned EUV radiation beam B’, thus forming an image with features that are smaller than corresponding features on the patterning device MA. For example, a reduction factor of 4 or 8 may be applied. Although the projection system PS is illustrated as having only two mirrors 13,14 in Figure 3, the projection system PS may include a different number of mirrors (e.g., six or eight mirrors).

[0110] The substrate W may include previously formed patterns. Where this is the case, the lithographic apparatus LA aligns the image, formed by the patterned EUV radiation beam B’, with a pattern previously formed on the substrate W.

[0111] A relative vacuum, i.e. a small amount of gas (e.g. hydrogen) at a pressure well below atmospheric pressure, may be provided in the radiation source SO, in the illumination system IL, and / or in the projection system PS.

[0112] The radiation source SO shown in Figure 3 is, for example, of a type which may be referred to as a laser produced plasma (LPP) source. A laser system 1, which may, for example, include a CO2-1laser, is arranged to deposit energy via a laser beam 2 into a fuel, such as tin (Sn) which is provided from, e.g., a fuel emitter 3. Although tin is referred to in the following description, any suitable fuel may be used. The fuel may, for example, be in liquid form, and may, for example, be a metal or alloy. The fuel emitter 3 may comprise a nozzle configured to direct tin, e.g. in the form of droplets, along a trajectory towards a plasma formation region 4. The laser beam 2 is incident upon the tin at the plasma formation region 4. The deposition of laser energy into the tin creates a tin plasma 7 at the plasma formation region 4. Radiation, including EUV radiation, is emitted from the plasma 7 during deexcitation and recombination of electrons with ions of the plasma.

[0113] The EUV radiation from the plasma is collected and focused by a collector 5. Collector 5 comprises, for example, a near-normal incidence radiation collector 5 (sometimes referred to more generally as a normal -incidence radiation collector). The collector 5 may have a multilayer mirror structure which is arranged to reflect EUV radiation (e.g., EUV radiation having a desired wavelength such as 13.5 nm). The collector 5 may have an ellipsoidal configuration, having two focal points. A first one of the focal points may be at the plasma formation region 4, and a second one of the focal points may be at an intermediate focus 6, as discussed below.

[0114] The laser system 1 may be spatially separated from the radiation source SO. Where this is the case, the laser beam 2 may be passed from the laser system 1 to the radiation source SO with the aid of a beam delivery system (not shown) comprising, for example, suitable directing mirrors and / or a beam expander, and / or other optics. The laser system 1, the radiation source SO and the beam delivery system may together be considered to be a radiation system.

[0115] Radiation that is reflected by the collector 5 forms the EUV radiation beam B. The EUV radiation beam B is focused at intermediate focus 6 to form an image at the intermediate focus 6 of the plasma present at the plasma formation region 4. The image at the intermediate focus 6 acts as a virtual radiation source for the illumination system IL. The radiation source SO is arranged such that the intermediate focus 6 is located at or near to an opening 8 in an enclosing structure 9 of the radiation source SO.

[0116] The concepts disclosed herein may be especially useful with emerging imaging technologies capable of producing increasingly shorter wavelengths. Emerging technologies already in use include EUV (extreme ultra violet), DUV lithography that is capable of producing a 193nm wavelength with the use of an ArF laser, and even a 157nm wavelength with the use of a Fluorine laser. Moreover, EUV lithography is capable of producing wavelengths within a range of 20-5nm by using a synchrotron or by hitting a material (either solid or a plasma) with high energy electrons in order to produce photons within this range.

[0117] Figure 4 shows a more detailed schematic overview of an LPP EUV source SO including a source housing 220. The source housing 220 has a liner 301, with three segments 301a, 301b, and 301c. Two of the segments 301b, 301c can easily be reached and hence cleaned, either in-situ, or by being removed from the housing 220 and cleaned ex-situ, as these segments can be shifted in and out of the machine within a reasonable amount of time. When removed, these segments can by cleaned by various methods (such as cryo-treatment to cause tin phase transformation, mechanical cleaning with a brush or CO2 snow / pellets etc.) as will be appreciated from various aspects of this disclosure. The present disclosure also provides a method to efficiently clean segment 301a (known as the intermediate focus cap or “IF-cap,” which is arguably one of the most significant parts for Sn2IF performance) in-situ (i.e., without removal from the housing 220) to provide significant time savings. This part of the source has portions thereof that are remote from the areas of easiest access, and also are of a reduced diameter as it approaches toward the interior point of a cone (as can be appreciated from the figures), making it difficult to access and clean in conventional methodologies.

[0118] As also shown in Fig. 4, the source SO further contains a collector 302, one or more H2 supplies (331, 332, 333), exhaust 310, turbopump 320, pipe 330, rough pump 340, and exhaust pipe 350. Arrow 380 in Fig. 4 represents H2 flow that contains tin particles, tin vapor and SnH4.A scrubber 345 largely removes tin particles, tin vapor and SnH4 from the flow passing therethrough, so that only H2 can pass to the turbo-pump 320 and the rough pump 340. The arrows 381 and 382 in Fig. 4 correspond to a clean H2 flow, largely free of tin particles and tin vapor and SnH4.

[0119] Within the source SO, Tin vapor and particles are produced mainly in the area of primary focus “PF” (305) near the collector 302, as this is where the majority of the laser-plasma interaction takes place. The tin vapor and tin particles 360 partially deposit on the liner 301 and will then fully deposit 370 on exhaust 310 or scrubber 345 and deplete tin-loaded flow of H2 380 so as to preventdamage of the turbopump 320 and rough pump 340. In operation, EUV and out of band radiation, as well as energetic tin atoms and ions, are believed to cause ionization of hydrogen gas, protecting collector from tin contamination. Liner 301 (sections 301a, 301b, and 301c) faces or (directly receives) the EUV H2 plasma continuously and remains below melting of tin. On the other hand, the exhaust 310 is remote from the EUV H2 plasma and does not face the EUV H2 plasma, and occasionally or continuously is heated up above tin melting temperature. This makes tin contaminant properties different (indicated by the different hatching) in the liner 301 as compared to the exhaust 310 (and other parts that are not in the line-of-sight of the plasma), and translates into different responses of Tin to freezing. EUV hydrogen plasma loads solid Tin with atomic hydrogen that ends up in trapped bubbles of molecular hydrogen. Such bubbles generate tension up to or exceeding ultimate tensile strength (several kBar equivalent of tension), alternatively or additionally such bubbles account for surfaces of tin free of native oxide, which possess lower threshold for lattice reconfiguration, compared to bulk tin atoms, or tin atoms trapped under the native oxide layer.

[0120] Specifically, H2 bubbles in solid (white) Sn have been found to promote the conversion of white Sn into grey Sn during cooling of Sn, which will be discussed later in this disclosure. In any case, these bubbles are normally only present in solid Sn which was exposed to the H2 plasma during normal operation of the source SO (i.e., during normal EUV source operation), particularly Sn deposited on surfaces near the primary focus, for example with line-of-sight to the primary focus.

[0121] To accelerate the conversion from white Sn to grey Sn for the parts which are not directly exposed to the H2 plasma during the source operation or experience periodic or continuous heating above Sn melting temperature (that promotes Hydrogen outgassing and removes hydrogen bubbles and seeds of grey Sn), the present disclosure includes exposing white Sn to H2 plasma and / or a Hydrogen radical generator prior to cleaning (i.e., mechanical cleaning with or without prior cryo-treating). Alternatively, or additionally, the present disclosure includes loading the Sn layer with Hydrogen electrochemically.

[0122] Figures 5A-5D show the cleaning process for parts which were exposed to H2 plasma during operation. Basically, after operation of the source SO, one or more of the parts can be cooled down to convert white Sn 360 to grey Sn 361. Next, the weakened grey Sn 361 is removed mechanically, for example, by brushing, CO2 snow cleaning, or CO2 pellet cleaning.

[0123] As shown in Fig. 5 A, the liner 301 contaminated with Sn 360 is placed into a controlled environment or chamber 400, and a gas or liquid coolant 401 is applied through input line or port 402 and exhausted through exhaust line or port 403. The coolant 401 can be applied externally to the liner 301, as shown, or internally to at least some portions of the liner 301 using the water-cooling channels 410 available in the liner (see Fig. 4). Typically, the cooling of liner 301 to a temperature of between about 0 °C to -60 °C for at least 1 hour, in one embodiment between 5 to 10 hours, and in anotherembodiment at least 10 hours. In one embodiment the temperature can be held for as long as 40 hours or longer if desired for optimal cleaning.

[0124] As shown in Fig. 5B, the Sn layer on the liner 301 is completely (or almost completely) transformed from white Sn 360 to grey Sn 361 (as indicated by the difference in hatching of Figs. 5A and 5B) and is now easier to remove.

[0125] It should be noted that components of the EUV source SO with s line of sight to the primary focus, can be cooled to below 0 °C, in one embodiment between -20 °C to -40 °C, and in another embodiment between -40 °C to -60 °C. The cooling may be applied via integrated cooling channel, for at least 1 hour, possibly longer than 10 hours, possibly between 10 to 20 hours, and induce white to grey Tin transformation in Tin saturated with hydrogen. Such cold treatment can be executed in situ, for components installed in the EUV source housing, or ex-situ in an environment chamber. Tin contamination, weakened by the phase transformation can be removed using a brush or CO2 snow / pellets.

[0126] As shown in Fig. 5C, the liner 301 with grey Sn 361 is optionally placed in controlled environment (450) and a mechanical cleaning is applied. This can include CO2 pellet pressure nozzle, CO2 snow jet nozzle, or by water pressure ejected from a water nozzle (such nozzles being schematically indicated by reference numeral 460, which ejects the associated material 461 to agitate and thereby loosen and remove the grey Sn from the liner surface. Alternatively, mechanical brushing can be used. For example, reference numeral 460 can also be considered to schematically represent a physical brush that can rotate or otherwise move to contact and agitate and thereby remove the grey Sn from the liner surface, liner surface.

[0127] Fig. 5D illustrates the cleaned liner 301 after cleaning.

[0128] Parts having Sn contaminants, but which were distant or out of the line of sight from EUV plasma during operation of the EUV source, may be subject to different or additional treatment prior to removal. Specifically, Figures 6 and 7 illustrate two alternative processes (or optionally that can be used in combination) in accordance with the present disclosure. Figs. 6 and 7 primarily deal with components, such as the exhaust 310, that are not exposed to the H2 plasma during source operation or have been heated above the Sn melting point during such operation. Other components can also be cleaned, using some of the principles set forth herein. For example, as illustrated in Fig. 4, a Sn catch and bucket for one-year Sn collection and a big bucket for one-year Sn collection are some other nonlimiting examples of such parts that are not directly exposed to H2 plasma during source operation, or contain molten tin that can outgas hydrogen even when exposed to hydrogen plasma. Specifically, the tin bucket or container 392 contains tin from the droplet generator 394, wherein such tin droplets were not exposed to the main laser pulse and laser pre-pulse, and so tin droplets 396 are collected by tin catcher 393 and eventually end up in the container 392.

[0129] Another tin bucket or container 391 collects tin dripping from scrubber 345 - a structure to separate tin vapor and SnH4 from the H2 (such that clean H2 is directed to the turbo pump and rough pump). Such parts, like these tin buckets 391, 392 and the exhaust 310 do not remain exposed to H2 plasma during ongoing operation of the source SO and thus may benefit from the methods illustrated in Figs. 6 and / or 7. - On the other hand, various other components with solid tin contamination accumulating during source operation that are directly exposed to H2 plasma during source operation and thus may not benefit (or benefit as much) from the methodologies explained in conjunction with Figs. 6 and / or 7.

[0130] With respect to the embodiment of Fig. 6, prior to cooling, the part to be cleaned is exposed to H2 plasma for a sufficient period of time (e.g., 1-10 hours, but longer if desired). Alternatively (or in addition), as shown in Fig. 7, Sn can be exposed to H+hydrated ions by means of electrochemistry prior to cooling. Both methods of Figs. 6 and 7 will introduce H2 bubbles into the Sn, resulting in accelerated Sn conversion from white to grey. When this method is applied, the cleaning time of Sn can be reduced significantly, since transformation of Sn is no longer a rate limiting step. The introduction of H2 bubbles into Sn essentially provides tension to the tin around bubbles and provides oxide-free tin surface facing the bubbles, unlike the surface of tin developing native oxide, once exposed to air, and functions as a catalyst to promote rearrangement of the crystal lattice at sufficiently low temperature. Hence, the introduction of H2 bubbles within the tin accelerates conversion of white tin to grey tin and should be considered a form of “seeding” to promote such conversion.

[0131] Referring more specifically to the embodiment of Fig. 6, an exemplary part or structure, such as exhaust 310, with a Sn contamination layer 370 is placed into a vacuum chamber 500 and H2 510 is supplied through input port 501 and exhausted through exhaust port 502. A plasma source or hydrogen radical generator 520 with electrical, gas and / or cooling supplies 521 generates hydrogen ions and / or radicals 522) in vicinity of the exhaust 310 within the chamber 500.

[0132] The embodiment of Figure 7 shows an example of parts or structures that may undergo an alternative process to that shown and described with respect to Fig. 6, or optionally in addition to the process of Fig. 6. As an example, a part or structure (such as exhaust 310) with Sn layer 370 is place into a bath container 600, fdled with electrolyte 630 to expose the Sn to H+hydrated ions. The electrolyte 630 is optionally refreshed through supply port 641 and removed through exhaust port 651. Exhaust 310 is negatively biased with respect to a counter-electrode 620 with a voltage supply 610 and wires 611, 621. As a result H2 bubbles are produced in the Sn adhered to the exhaust 301, which approximates effect of H2 plasma. In one optional embodiment, the electrolyte is water and organic acid with a pH of between about 2 to 4. In one non-limiting example, the counter electrode can be graphite. The Voltage applied may be between 10 V to 100 V. The current applied may be between 0. 1 A to 10 A.

[0133] After the treatment through the methods described with respect to Figs. 6 and / or 7, the Sn layer 371 on the exhaust 310 resembles Sn on the liner 301.

[0134] From the above, it should be appreciated that the Sn layer which is exposed to the EUV plasma directly during normal source operation is already saturated with hydrogen bubbles and thus will convert to grey Sn at cryo-cooling quickly. Should further exposure to hydrogen plasma or hydrogen radicals be deemed useful for cleaning the liner 301, for example to accelerate conversion of Sn even further, in yet another embodiment this can be done by placing another plasma source or hydrogen radical source into the source itself within line-of-sight to the Sn contaminated surface (e.g„ where the primary focus 305 of the collector 302 is typically located). This other plasma source or hydrogen radical source is specifically dedicated for in-situ cleaning during a cleaning operation. Similarly, a plasma generator or hydrogen radical generator can be placed at a position on axis of the cone, closer to the narrow end of the cone section 30 la of the IF -cap, where the effect of EUV plasma during normal operation of the EUV source (e.g., during EUV lithography operation) is typically lower, and where additional treatment for cleaning purposes is more beneficial. Such treatment can be done in situ, or in a separate environment (e.g., in another chamber) with hydrogen supplied at low pressure, e.g. P<100 Pa, and a plasma source, or electrolytic cell, similar to devices in Figs. 6 and 7.

[0135] It should be appreciated that each of the described embodiments, some of the structures or components that are not in the direct line-of-sight of the H2 plasma may nevertheless receive some plasma exposure indirectly and thus may over time have commenced conversion to grey Sn and thus not need additional treatment. On the other hand, even for those parts, treatment as described above may be beneficial.

[0136] As shown in FIG. 8, the conditioned Sn-contaminated exhaust 371 is placed into temperature controlled, optionally insulated, environment or chamber 700, and a gas or liquid coolant 701 is applied through an input port 702 and exhausted through an output port 703. After cooling treatment, the Sn layer on the exhaust 310 to a significant extent converts to grey Sn 372.

[0137] As shown in FIG. 9, the exhaust 310 with grey Sn layer is placed in a limited environment or chamber 800 (e.g., to prevent escape of tin dust during clearing) and a mechanical cleaning tool 810 is applied. The mechanical cleaner 810, may be one or more of the following cleaning tools 810: a CO2 pellet pressure nozzle or CO2 jet / snow pressure nozzle (each omitting frozen CO2 820 in different sizes), a water pressure nozzle (emitting high pressure water 820) or by mechanical brush. Grey Sn is easily removed.

[0138] Fig. 10 illustrates the clean exhaust 310 after cleaning.

[0139] From the above disclosure, it can thus be appreciated that in accordance with various aspects of the present disclosure a method of cleaning any structure to which Sn contaminants are adhered can be cleaned (e.g., the exhaust 310, as one non-limiting example). The method entails inducing the formation of H2 bubbles in [3-Sn adhered to the structure (e.g., as a result of such bubbles being formedduring operation of the source or by such bubbles being induced subsequently by means described in connection with Figs. 6 or 7. The method further entails cooling the H2 infused [3-Sn to be below the above-mentioned threshold temperature such that the -Sn is converted to a-Sn. It should be noted that the rate of conversion is temperature dependent and, for example, can have a maximum rate of conversion in the range of approximately -18 °C to -40 °C. After such conversion, the a-Sn can be easily removed from the structure. Such removal may be accomplished by mechanically agitating the a-Sn as described above to remove it from the surface of the structure. The agitating may simply be brushing a -Sn off the structure, and in another method where perhaps more vigorous agitation is needed, CO2 snow or pellet cleaning, or vacuum cleaning can be used as non-limiting examples.

[0140] As mentioned above, the inducing of the formation of H2 bubbles may comprise directing H2 plasma toward the P-Sn, or by using a hydrogen radical generator 520 to expose the P-Sn to hydrogen radicals. In terms of directing H2 plasma towards the P-Sn, such directing can be in a general sense so that the H2 plasma impacts the P-Sn, although the H2 plasma may be placed in immediate proximity to the part to be cleaned (for example, the H2 plasma source can be placed 1 meter or less from the part to be cleaned. In some embodiments it may be less than 0.1 meters. A combination of both of these methods may be used. The inducing of the formation of H2 bubbles may comprise exposing the P-Sn to H+ hydrated ions. As described above, the exposing of the P-Sn to H+ hydrated ions may comprise placing the structure with the adhered P-Sn in the electrolyte fdled bath 600, and applying a voltage to the submersed electrodes, one of which is Sn contaminated part. These methods of inducing the formation of bubbles outside of the operating embodiment of the SO will typically generate H2 bubbles at a rate faster than the rate at which H2 bubbles would be formed in the structure’s operating environment (e.g., during normal operation of the EUV source) to form H2 infused P-Sn. Specifically, although such structures may potentially have some H2 infused therein as a result of minor or modest exposure to plasma from the plasma source during normal EUV source operation, such structures may not be within line of sight of the plasma source and hence H2 formation would take place at a relatively slower rate or not at all.. To be clear, when referring to the “rate faster than the rate at which H2 bubbles would be formed in the structure’s operating environment” that includes components or structures that are contaminated with tin having absolutely zero H2 bubbles therein because the structure is entirely shielded from plasma in its operating environment, or such structure operate at the point above tin melting point which prevents trapping of hydrogen bubbles even in the presence of hydrogen plasma. Thus, one aspect of the present method places such structures in an environment where H2 bubbles would be formed much more rapidly than the structure’s normal operating environment.

[0141] IThe cooling may be conducted by placing the structure with the adhered [3-Sn into the temperature-controlled environment 700. The temperature-controlled environment may comprise a container, wherein a liquid or gaseous coolant 701 is provided in the container.

[0142] Optionally, reference numeral 460 can be schematically considered to be a coolant tool 460 through which liquid or gas coolant is circulated. This can be more easily appreciated by reference to the embodiments of Figs. 11 and 12, illustrating a cooling circuit 40, including a coolant pump / refrigerator 400 that circulates coolant 401 through inlet port 402 and exhaust line or port 403. Such a cooling circuit can be connected to the coolant tool 460.

[0143] While the above disclosure pertains to structures that form a component of an EUV source, it can be appreciated that this method can be applied to any parts that are contaminated with tin, whether it be part of an EUV lithography machine or not. In the case where the structure is a component of the EUV source, such component or structure may optionally be removed from the aforementioned housing 220 of the EUV source prior to inducing the formation of H2 bubbles (or additional H2 bubbles) in Sn adhered to the structure to be cleaned. The [3-Sn adhered to the removed structure may already have some H2 bubbles formed therein as a result of some exposure to hydrogen plasma during operation of the EUV source, as mentioned previously. With that said, the [3-Sn adhered to the removed structure typically has minimal or zero H2 bubbles formed therein as a result of the structure being largely shielded from hydrogen plasma during operation of the EUV source (i.e., no direct line of sight).

[0144] Alternatively, white Sn on the various parts of the source SO may be converted to grey Sn by being contacted by a seed, for example, one or more grey Sn particles. Such contacting of the seed with the Sn to be removed can be done with the part remaining in the EUV housing 220 (in-situ) or after being removed from the machine. Grey Sn growth will start from contacted spots on the white Sn during subsequent cooling of the tin. The seed material / structure should preferably be close in lattice parameter and chemical bonding to grey Sn (again, the seed may be a grey Sn particle). The seed may be Germanium, or another isomorphic crystal. The grey particle seed may be pierced within the native oxide film (SnO?) (which is formed as a result of tin being exposed to air), for example at a depth of between about 10-100 nm) with any convenient mechanical tool, so as to ensure molecular-scale contact of the seed material and white tin, buried under the native oxide. This methodology will accelerate the conversion of white Sn into grey tin. After conversion to grey tin, the Sn can be removed by snow removal, brushing, vacuuming, etc., as described above. Also, to reiterate from previous discussions, another approach for seeding of a catalyst to accelerate conversion can be the use of H2 bubbles.

[0145] Sn transformation starts in points of nucleation and propagates from such points, for pure and dense (bubble free) white Sn with native oxide, the barrier for nucleation is much higher than for propagation.

[0146] The incorporation of H2 bubbles into Sn promote the conversion of white to grey tin. This is due to the fact that the Sn layer surrounding the H2 bubbles is oxide free . The transformation and lattice restructuring is therefore not obstructed by the oxide layer. Additionally, the tension in Sn surrounding the incorporated bubbles promotes phase transformation, as becomes clear from P-T phase diagram of different Sn allotrope phases, especially when extrapolated to tension. It is well known in literature thatnucleation is the rate limiting step in this process. When the seeds are formed, surrounding the H2 bubbles, fast growth of grey Sn will follow. These H2 bubbles will be incorporated by H2 plasma exposure and / or hydrogen radical generation process.

[0147] It should be appreciated that the seeding methodology described above can be performed on various different structures, and can be done as a precursor to any of the other cleaning processes described above.

[0148] In another aspect of the present disclosure, various parts of an EUV source are cleaned in- si tu (without removal from the source housing), to reduce downtime.

[0149] Specifically, as illustrated in Figs. 4 and 11-13, segment 301a of the EUV source has water cooling channels 410, which function to facilitate cooling the parts during EUV source operation, as they are exposed to high thermal load (by EUV plasma and scattered radiation), to prevent molten Sn spitting.

[0150] In one aspect of the present disclosure, these cooling channels 410 are also used to induce facilitate white to grey Sn conversion (aka “Tin pest”) by running a coolant at sufficiently cold temperatures to accelerate this conversion during source downtime. For example, an ethanol / water or glycol / water mix may be used. These coolants may have a temperature of between 0 °C to -60 °C, or between -20 °C to -40 °C, or between -30 °C to -55 °C . In the broadest sense a coolant having a temperature of below 13.2 °C may be used, as this is generally the temperature at which conversion starts (at atmospheric pressure).

[0151] Alternatively, a gas or other liquid with low freezing point can be used. Note that any liquid or mixture that stays liquid at the desired temperature and not harmful to the module or environment can be used. The medium is cooled externally in a special chiller 400 or can be brought chilled in a thermally insulated vessel and supplied to the module by pumping (e.g. with a mechanical pump 400 that does not have cooling capability) or by pressure or gravity assisted way. The same can be done with gaseous medium (but less efficiently). A chiller cabinet or refrigerator, with coolant temperature in the range of 0 °C to - 60 °C, can be used and (periodically) connected to the cooling lines integrated into components of the source SO. Thus, the cooling channels 410 can be used to facilitate in-situ cleaning of tin -contaminated structures that have such cooling channels. Ordinary water may be removed from the cooling channels before a cooling operation at temperatures for cleaning, as freezing water expansion could potentially be damaging for the channels 410.

[0152] After the Sn conversion as a result of cooling (which may take in the range of 10-40 hours, but can potentially be reduced), the weak grey Sn is removed in several ways, e.g. CO2 snow or pellet cleaning, vacuum cleaning, or mechanical brushing. In this way, Sn contaminated parts can be efficiently cleaned during already scheduled downtime. As an example, an IF -cap can be cleaned during collector swap, improving Sn2IF of the machine.

[0153] It should be noted that the cryo-treatment and mechanical removal (i.e., snow, pellet, brush and or vacuum processes) can be iterative, e.g., freezing => brushing => freezing => brushing.

[0154] Although segments 301b, 301c may not need cooling channels during source operation, they may nevertheless be provided with cooling channels for the purpose of in-situ cleaning only and cleaned in the same manner as segment 301a described above. However, segments 301b and 301c may be uninstalled and cleaned outside the source via freezing to facilitate grey Sn formation, followed by brushing, snow cleaning, vacuuming, etc., as described above.

[0155] Fig. 11 shows a schematic representation of the use of the cooling channels 410 in the IF -cap for Tin pest conversion while the IF -cap remains in-situ and connected to the housing. The cooling channels 410 will be fdled with a cooling liquid (or gas) for cryo-conversion, such coolant having a lower freezing point than water (or whichever coolant is used during normal EUV source operation) . The channels 410 are fdled with coolant via the cooling circuit 40, including a coolant pump / refrigerator 400 that circulates coolant 401 through inlet port 402 and exhaust port 403. As a result, the channels 410 of segment 301a are fdled with low temperature liquid (e.g. ethanol) to replace the normally used water. The cooling circuit 40 is then used to bring the segment 301a to temperature below -20 °C (for example, in the range of between -30 °C to -60 °C) for a time of between about 1 hour to about 10 hours. After the cooling is complete and the Sn is grey, the coolant is replaced back to water.

[0156] After conversion, the lower temperature coolant is again replaced with water or solution that is used for cooling during normal EUV operation. Any remaining coolant (e.g, typically water) will have no effect on the cooling channels after it is put back into the channels, as a water-soluble liquid may be used, e.g. ethanol or glycol in normal cooling during regular EUV source operation. Specifically, instead of water, the coolant with the lower freezing point than water (e.g., ethanol or glycol) may be used as the normal cooling fluid for the cooling channels 410 during normal operation of the IF-cap or any other component OO. In such cases, the rate of circulation of the coolant refrigeration pump 400 may be increased for any such cooling fluids when operating at higher temperatures (e.g., if the cooling function of the pump 400 is turned off, and only the circulation function of pump 400 is used). The temperature of the ethanol or glycol may be regulated accordance with the cooling function needed. For example, the refrigerator / pump can be set to slightly chill the coolant (e.g., by decreasing the starting temperature of the coolant by an amount between 5 °C to 10 °C, for example) during normal of the source SO, and then increase the cooling power so that the fluid is at the aforementioned temperatures used for cleaning (e.g,, between -20 °C to -40 °C).

[0157] From the above, it should be appreciated that the present disclosure provides method of cleaning various structures having [3-Sn contaminants adhered thereon, and the structures that have cooling channels therein can be cleaned by taking advantage of those channels by using low temperature coolants during structure downtime. Fig. 12 shows an alternative or an additional approach to Fig. 11. Specifically, instead of (or in addition to) using of the cooling circuit 40, a cooling tool 420, such as acontainer 420 with coolant 422 therein can be used. The cooling can be a very cold substance, such as a evaporating or sublimating material (liquid or solid, for example dry ice) for the purpose of Sn conversion. As can be seen, this can be done in-situ by bringing the surface of the tool or container into direct contact with the Sn contaminant material. This tool 420 can be used instead of the cooling circuit 40, or in conjunction therewith. It can also be used with segments 301b and 301c (not shown) for cleaning those parts in-situ.

[0158] Fig. 13 shows a further alternative where a second cooling circuit 426 is used in addition to cooling circuit 40 for cooling the segment 301a. The second cooling circuit 426 includes a pump / refrigerator 433 that circulates a coolant 401 through an input port 431 into a chamber / tool 430 and exhausted through port 432 for recirculation through the pump / refrigerator 433. The surface of the tool 430 can be brought into contact with Sn contaminants on the segment 301a to bring the segment 30 la to temperature below -20 C. The temperature of the coolant in the circuit may be brought between -30 °C to -60 °C and the tool 430 is held in contact with the Sn contaminant for about between 1 hour to about 10 hours, although it can be up to 40 hours if desired.

[0159] After the cooling is complete and Sn is converted to grey, the second cooling circuit 433 is removed, and the coolant 401 is replaced by water or other normal operating coolant for the channels 410 for EUV source operation.

[0160] After conversion, Sn is very brittle and its adhesion to parts is weak. The grey Sn is then removed from the parts, which is schematically presented in Fig. 14. Several cleaning techniques can be used as previously described, e.g., by a mechanical agitator, schematically represented at 901. The mechanical agitator 901 can be a reciprocating, rotating, or mechanical brush, it can be a nozzle for ejecting CO2 snow, pellets, or water, or it can be a vacuum device. Any of the agitation described in this disclosure can be accomplished with such an agitator.

[0161] Fig. 15 illustrates that segment 301a has been cleaned. Other structures, such as segments 301b and 301c, and exhaust 310, if cooled and cleaned off-line rather than in-situ, can then be reinstalled and source operation can resume.

[0162] From the above description, it can be appreciated that another aspect of the disclosure pertains to an EUV source cleaning system (see embodiments of FIGS. 11-15) for cleaning a structure of an EUV source, the source having [3-Sn contaminants adhered thereon. The system comprises a refrigeration pump 400 configured to cool and circulate a coolant, wherein the refrigeration pump is constructed and arranged to cool the coolant to a temperature below the freezing point of water, and possibly as low as -60 °C (e.g., between -20 °C and -40 °C). A coolant applicator (such as cooling channels 410 the cooling tool 420, or the cooling tool 430) is constructed and arranged to provide a thermal conduit between the coolant and the [3-Sn contaminants adhered to the component (e.g., the IF- Cap or other structure) to facilitate conversion of the [3-Sn to a-Sn. In the case of cooling channels for example, the portions of the IF-Cap between the channels and the tin contaminants provide the thermalconductivity. For the cooling tools 420 or 430 for example, these may act as a heat exchanger made from a thermally conductive material, such as stainless steel, that is brought into surface contact with the tin contaminant to facilitate conversion.

[0163] In the examples of FIG. 11-13, inlet and outlet lines may be connected between the coolant applicator and the refrigeration pump to enable the refrigeration pump to provide the coolant to the applicator. The system further comprises an agitator (such as the nozzles for ejecting CO2, water, or other substance, or the brushes as described above) are configured to remove the a-Sn from the structure.

[0164] It can also be appreciated from the above that, the EUV source may include additional components (i.e., other than the IF -cap 301a) that can optionally be removed from the EUV housing 220, then cooled ex-situ to induced [3-Sn adhered thereto to convert to grey a-Sn, then removing the grey a-Sn therefrom before being replaced back into the housing 220. For example, such additional components can be segments 301b, or 301c, and / or may comprise the exhaust 310 or other components mentioned previously. The removed additional components may be subjected to the formation of H2 bubbles in the [3-Sn adhered thereto as described above, prior to cooling for conversion and subsequent removal. The other non-limiting example of a component that can be more effectively cleaned through the practice of the invention is a near normal incidence collector. Although a typical contamination of the tin on the collector is local and does not exceed 10-100 um and CO2 snow is sufficient to remove »90% of the tin, the cleaning effect of CO2 snow can be facilitated by preliminary cooling of the collector, since the tin present is saturated with H2 bubbles after direct exposure to EUV plasma. The collector has its own system of cooling channels and can be cooled in-situ and ex-situ, using environment chamber or using the coolant circulating within the cooling channels. Since the amount of tin is lower than other source parts the cooling time may be lower, for example less than 40 hours or less than 4 hours.

[0165] In yet another aspect of the present disclosure, buckets of Sn are cleaned. As noted previously, two buckets of Sn are used in the EUV source. One of the Sn buckets 391 stores Sn exposed to plasma, and the other bucket 392 stores the Sn largely unexposed to plasma. Generally, in operation both buckets contain the molten tin, so it is free of any H2 bubbles. Thus to accelerate the conversion of white tin to grey, mechanical seeding is proposed (it is easier than plasma exposure or electrochemical treatment, since the geometry is simple because the tin surface can be accessed at the top of the bucket). The present disclosure comprises a method in which one or more seeds of a-Sn are applied to the [3- form of Sn for initiation of the conversion of the [3-form of Sn to its alpha-form allotrope. The seeding can be done, for example, by spraying a-Sn onto the [3-Sn to commence the conversion.

[0166] The seeded Sn bucket is exposed to a temperature lower than 13.2 °C (e.g, in a freezer container) and held there for a sufficient period of time (e.g, 12-36 hours), allowing Sn to break from the buckets and turn into pieces of alpha Sn.

[0167] The Tin, if sufficiently loosened and broken free from the surface of the bucket (typically formed of stainless steel), can then simply be dumped out of the bucket without agitation, and the bucket can then be reused in the EUV source. If agitation of the Sn is required, then any of the methods described above for agitation can be used.

[0168] In practice, this process can be done by the following method steps: First, a suspension or a colloid solution of grey Sn particles is prepared. For example, the suspension may be a-Sn particles and ethanol. Next, a bucket having a mass of Sn therein is placed in a freezer container. Next, the operator sprays the suspension onto the Sn in the bucket to promote the conversion to alpha Sn, and then wait for the beta to alpha Sn conversion. It should be noted that the bucket can be cooled (e.g., by being placed in the freezer) after being seeded, until it reaches the desired threshold temperature or lower. The threshold temperature may be 13.2 °C or lower. The converted alpha Sn can then be dumped out of the bucket, using agitation if necessary. Then the bucket us replaced in its operating position.

[0169] CLAUSES1. A method of cleaning a structure to which Sn contaminants are adhered, comprising: inducing the formation of EE bubbles in P-Sn adhered to the structure to form EE infused P-Sn; cooling the EE infused -Sn to be below a threshold temperature and thereby convert the P-Sn to a-Sn; and removing the a-Sn from the structure.2. The method of clause 1, wherein the inducing is conducted at a rate faster than the rate at which EE bubbles would be formed in the structure’s operating environment.3. The method of clause 1 or 2, wherein the inducing of the formation of EE bubbles comprises using a hydrogen radical generator to expose the P-Sn to hydrogen radicals.4. The method of clause 1 or 2, wherein the inducing of the formation of EE bubbles comprises exposing the P-Sn to H+hydrated ions.5. The method of clause 4, wherein the exposing the P-Sn to H+hydrated ions comprises placing the structure with the adhered P-Sn in an electrolyte filled bath, and applying a voltage across the structure and a counter-electrode disposed in the bath.6. The method of clause 1 or 2, wherein the inducing of the formation of EE bubbles comprises directing an EE plasma toward the P-Sn during normal operation of the structure’s operating environment.7. The method of any one of the preceding clauses, wherein the cooling is conducted by placing the structure with the adhered P-Sn into a temperature-controlled environment.8. The method of clause 7, wherein the temperature-controlled environment comprises a container, and wherein a liquid or gaseous coolant is provided in the container.9. The method of any one of the preceding clauses, wherein the removing comprises physically agitating the a-Sn so as to free it from the structure.10. The method of clause 9, wherein the physically agitating comprises applying a stream of CO2 snow or CO2 pellets to the a-Sn.11. The method of clause 9 or 10, wherein the physically agitating comprises mechanical brushing.12. The method of any one of the preceding clauses, wherein the cooling comprises applying a coolant onto the adhered P-Sn and / or within the structure with adhered -Sn.13. The method of clause 12, wherein the cooling comprises a coolant tool through which a liquid coolant and / or a gas coolant is circulated.14. The method of clause 12, wherein the cooling comprises applying a liquid coolant and / or a gas coolant directly onto the adhered P-Sn.15. The method of any one of clauses 1 to 11, wherein the cooling comprises applying a coolant onto the H2 infused P-Sn and / or within the structure with the H2 infused P-Sn.16. The method of clause 15, wherein the cooling comprises a coolant tool through which a liquid coolant and / or a gas coolant is circulated.17. The method of clause 15, wherein the cooling comprises applying a liquid coolant and / or a gas coolant directly onto the H2 infused P-Sn.18. The method of any of the preceding clauses, wherein the structure comprises a component of an EUV source, further comprising removing the structure from a housing of the EUV source prior to inducing the formation of H2 bubbles in the Sn contaminates adhered to the structure.19. The method of clause 18, wherein the P-Sn adhered to the removed structure has some H2 bubbles formed therein as a result of some exposure to hydrogen plasma during operation of the EUV source.20. The method of clause 18 or 19, wherein the P-Sn adhered to the removed structure has minimal or zero H2 bubbles formed therein as a result of the structure being largely shielded from hydrogen plasma during operation of the EUV source or due to operation of the structure at the point above tin melting temperature.21. A method of cleaning a structure to which Sn contaminates are adhered, comprising: seeding a mass of P-Sn adhered to the structure to be cleaned with a conversion seed, so as to promote conversion of P-Sn to a-Sn; cooling the seeded mass of P-Sn to be below a threshold temperature and thereby convert the P-Sn to a-Sn; and removing the a-Sn from the structure.22. The method of clause 21, wherein the threshold temperature is 13.2 °C at atmospheric pressure.23. The method of clause 21 or 22, wherein the seeding is conducted by spraying an to a-Sn infused liquid onto the mass of P-Sn.24. The method of clause 23, wherein the a-Sn infused liquid comprises a-Sn particles and ethanol.25. The method of clause 21 or 22, wherein the seeding comprises forming H2 bubbles in the mass of P-Sn adhered to the structure to be cleaned.26. The method of any one of clauses 21 to 25, wherein the seeded mass is cooled to a temperature between 0 °C and -60 °C.27. The method of c any one of clauses 21 to 25, wherein the seeded mass is cooled to a temperature between -20 °C and -40 °C.28. A method of cleaning a structure having -Sn contaminants adhered thereon, the structure having cooling channels therein for receiving coolant at an operating temperature during operation thereof, the method comprising: providing a coolant to the cooling channels at a temperature lower than the operating temperature, so as to induce the P-Sn contaminants adhered to the structure to convert to a-Sn; and removing the a-Sn from the structure.29. The method of clause 28, wherein the removing comprises mechanical brushing of the a-Sn to remove it from the structure.30. The method of clause 28 or 29, wherein the removing comprises applying a stream of CO2 snow or CO2 pellets to the a-Sn.31. The method of clause 28 or 29, wherein the removing comprises applying a stream comprising CO2 snow or CO2 pellets to the a-Sn.32. The method of any one of clauses 28 to 31, wherein the structure is an intermediate focus cap of an EUV source.33. The method of any one of clauses 28 to 32, wherein providing the coolant comprises removing a first coolant used during operation of the structure and replacing it with a second coolant having a lower freezing point than the first coolant, so that it can be brought to the temperature lower than the operating temperature so as to induce the P-Sn contaminants adhered to the structure to convert to a-Sn .34. The method of any one of clauses 28 to 32, wherein providing the coolant comprises cooling the same coolant used at the operating temperature to be at the lower temperature so as to induce the P-Sn contaminants adhered to the structure to convert to a-Sn.35. A method of cleaning an EUV source, the EUV source including a source housing and a component within the housing, the component having P-Sn contaminants adhered thereto, the method comprising: cooling the P-Sn contaminants on the component to a temperature that induces conversion of P-Sn to a-Sn while the component remains positioned within the housing; andagitating the a-Sn to remove it from the component while the component remains positioned within the housing. The method of clause 35, wherein the component comprises an intermediate focus cap (IF Cap). The method of clause 35 or 36, wherein the EUV source includes additional components in the housing, wherein the additional components are removed from the EUV housing and then cooled to induce P-Sn adhered thereto to convert to a-Sn, and then removing the a-Sn from the additional components. The method of clause 37, wherein the additional components comprise one or more of an exhaust, a scrubber, a liner, a near normal incidence collector, a gas distribution system, a tin catcher, and / or an obscuration bar. The method of any of clauses 35 to 38, further comprising inducing the formation of EE bubbles in the -Sn contaminants on the component prior to cooling of the P-Sn thereon. An EUV source cleaning system for cleaning a structure of an EUV source having P-Sn contaminants adhered thereon, the system comprising: a refrigeration pump configured to cool and circulate a coolant, wherein the refrigeration pump is constructed and arranged to cool the coolant to a temperature below the freezing point of water; a coolant applicator constructed and arranged to provide a thermal conduit between the coolant and the P-Sn contaminants adhered to the structure to facilitate conversion of the P-Sn to a-Sn; inlet and outlet lines connected between the coolant applicator and the refrigeration pump to enable the refrigeration pump to provide the coolant to the applicator; and an agitator configured to remove the a-Sn from the structure. The cleaning system according to clause 40, wherein the coolant applicator comprises cooling channels within the structure to be cleaned. The cleaning system according to clause 40 or 41, wherein the coolant applicator comprises a coolant housing configured to be moved into direct contact with the P-Sn contaminants adhered to the structure to facilitate conversion of the P-Sn to a-Sn. The cleaning system according to any of clauses 40 to 42, wherein the agitator comprises a nozzle for ejecting a substance that impacts and removes the a-Sn from the structure. The cleaning system according to any of clauses 40 to 43, wherein the agitator comprises a brush. The cleaning system according to any of clauses 40 to 44, wherein the coolant comprises ethanol or glycol. The cleaning system according to any of clauses 40 to 45, wherein the structure comprises an intermediate focus cap (IF-Cap) that remains within a housing of the EUV source during cleaning thereof.7. The cleaning system according to any one of clauses 40 to 46, further comprising a source of hydrogen configured to induce the formation of H2 bubbles in P-Sn adhered to the structure to form H2 infused -Sn. 8. A EUV source cleaning system for cleaning a structure of an EUV source having P-Sn contaminants adhered thereon, the system comprising: a source of hydrogen configured to induce the formation of H2 bubbles in P-Sn adhered to the structure to form H2 infused P-Sn; a refrigeration system configured to cool the H2 infused P-Sn to be below a threshold temperature and thereby convert the P-Sn to a-Sn; and an agitator configured to remove the a-Sn from the structure. 9. The cleaning system according to clause 48, wherein the source of hydrogen preferably is an additional source of hydrogen configured to generate additional hydrogen over any H2 plasma that has been directed at the P-Sn during normal operation of the structure’s operating environment. 0. The cleaning system according to clause 48 or 49, wherein the source of hydrogen comprises a hydrogen radical generator configured to expose the P-Sn to hydrogen radicals. 1. The cleaning system according to clause 48 or 49, wherein the source of hydrogen comprises a source of H+hydrated ions. 2. The cleaning system according to clause 51, wherein the source of H+hydrated ions comprises an electrolyte filled bath configured to receive the structure; a counter-electrode disposed in the bath; and a power supply connectable to the structure and the counter-electrode, to apply a voltage across the structure and the counter-electrode.

[0170] While the concepts disclosed herein may be used for cleaning specific parts of an EUV tool (and the source in particular), it shall be understood that some of the disclosed concepts may be used with other parts that may have Sn contaminants thereon. In addition, the combination and sub- combinations of disclosed elements may comprise separate embodiments.

[0171] The descriptions above are intended to be illustrative, not limiting. Thus, it will be apparent to one skilled in the art that modifications may be made as described without departing from the scope of the claims set out below.

Claims

CLAIMS1. A method of cleaning a structure to which Sn contaminants are adhered, comprising: inducing the formation of H2 bubbles in P-Sn adhered to the structure to formH2 infused -Sn; cooling the H2 infused P-Sn to be below a threshold temperature and thereby convert the P-Sn to a-Sn; and removing the a-Sn from the structure.

2. The method of claim 1, wherein the inducing is conducted at a rate faster than the rate at which H2 bubbles would be formed in an operating environment of the structure.

3. The method of claim 1 or 2, wherein the inducing of the formation of H2 bubbles comprises exposing the P-Sn to hydrogen radicals.

4. The method of claim 1 or 2, wherein the inducing of the formation of H2 bubbles comprises exposing the P-Sn to H+hydrated ions.

5. The method of claim 4, wherein the exposing the P-Sn to H+hydrated ions comprises placing the structure with the adhered P-Sn in an electrolyte fdled bath, and applying a voltage across the structure and a counter-electrode disposed in the bath.

6. The method of claim 1 or 2, wherein the inducing of the formation of H2 bubbles comprises directing a H2 plasma toward the P-Sn during normal operation of the structure’s operating environment.

7. The method of any one of the preceding claims, wherein the cooling is conducted by placing the structure with the adhered P-Sn into a temperature-controlled environment.

8. The method of and one of the preceding claims, wherein the removing comprises physically agitating the a-Sn so as to free it from the structure.

9. The method of claim 8, wherein the physically agitating comprises mechanical brusing or applying a stream comprising CO2 snow or CO2 pellets to the a-Sn.

10. The method of any one of the preceding claims , wherein the cooling comprises applying a coolant onto the H2 infused P-Sn or within the structure with H2 infused -Sn.

11. The method of claim 10, wherein the cooling comprises circulating a liquid coolant and / or a gas coolant.

12. The method of claim 10, wherein the cooling comprises applying a liquid coolant and / or a gas coolant directly onto the H2 infused P-Sn.

13. The method of any of the above claims, wherein the structure comprises a component of an EUV source, further comprising removing the structure from a housing of the EUV source prior to inducing the formation of H2 bubbles in the Sn contaminates adhered to the structure.

14. The method of claim 13, wherein the P-Sn adhered to the removed structure has some H2 bubbles formed therein as a result of some exposure to hydrogen plasma during operation of the EUV source.

15. The method of claim 13, wherein the P-Sn adhered to the removed structure has minimal or zero H2 bubbles formed therein as a result of the structure being largely shielded from hydrogen plasma during operation of the EUV source or due to operation of the structure at the point above tin melting temperature.

16. A method of cleaning a structure to which Sn contaminates are adhered, comprising: seeding a mass of P-Sn adhered to the structure to be cleaned with a conversion seed, so as to promote conversion of P-Sn to a-Sn; cooling the seeded mass of P-Sn to be below a threshold temperature and thereby convert the P-Sn to a-Sn; and removing the converted a-Sn from the structure.

17. A method of cleaning a structure having P-Sn contaminants adhered thereon, the structure having cooling channels therein for receiving a coolant at an operating temperature during operation thereof, the method comprising: providing the coolant to the cooling channels at a temperature lower than the operating temperature, so as to induce the P-Sn contaminants adhered to the structure to convert to a-Sn; andremoving the a-Sn from the structure.

18. A method of cleaning an EUV source, the EUV source including a source housing and a component within the housing, the component having P-Sn contaminants adhered thereto, the method comprising: cooling the -Sn contaminants on the component to a temperature that induces conversion of P-Sn to a-Sn while the component remains positioned within the housing; and agitating the a-Sn to remove it from the component while the component remains positioned within the housing.

19. An EUV source cleaning system for cleaning a structure of an EUV source having P-Sn contaminants adhered thereon, the system comprising: a refrigeration pump configured to cool and circulate a coolant, wherein the refrigeration pump is constructed and arranged to cool the coolant to a temperature below the freezing point of water; a coolant applicator constructed and arranged to provide a thermal conduit between the coolant and the P-Sn contaminants adhered to the structure to facilitate conversion of the P-Sn to a-Sn ; inlet and outlet lines connected between the coolant applicator and the refrigeration pump to enable the refrigeration pump to provide the coolant to the applicator; and an agitator configured to remove the a-Sn from the structure.

20. An EUV source cleaning system for cleaning a structure of an EUV source having P-Sn contaminants adhered thereon, the system comprising: a source of hydrogen configured to induce the formation of H2 bubbles in P-Sn adhered to the structure to form H2 infused P-Sn; a refrigeration system configured to cool the H2 infused P-Sn to be below a threshold temperature and thereby convert the P-Sn to a-Sn; and an agitator configured to remove the a-Sn from the structure.

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