Target fluid supply system for an EUV radiation source, and servicing method

Abstract

A target fluid supply system for an EUV radiation source with a reservoir configured to store and selectively expel a target fluid, a sensor configured to generate a signal indicative of a volume of target fluid stored in the reservoir, and an ejection assembly in fluid communication with the reservoir configured to eject target fluid. The ejection assembly has a service freeze valve configured to connect to a source of purge gas. The service freeze valve is in fluid communication with a fillable volume of the ejection assembly configured to be filled with target fluid. The service freeze valve is configured to selectively permit purge gas into at least a portion of the fillable volume. A control unit configured to determine whether a threshold fill level of target fluid is within the service freeze valve at least partly based on the signal and the fillable volume of the ejection assembly.

Description

TARGET FLUID SUPPLY SYSTEM FOR AN EUV RADIATION SOURCE. AND SERVICINGMETHODCROSS REFERENCE TO EARLIER APPLICATION

[0001] The present application claims priority benefits of European application No. 24218052.9 filed 6 December 2024.FIELD

[0002] The present invention relates to a target fluid supply system for an EUV radiation source. In other aspects, the present invention relates to an EUV radiation source comprising such a target fluid supply system, and an EUV exposure apparatus. In still other aspects, the invention relates to a method of servicing a target fluid supply system, a computer program comprising instructions configured to cause a controller to carry out this method, a non-transitory computer readable medium carrying the instructions, and a computer apparatus.BACKGROUND

[0003] Light generated by means of a radiation source can be used by exposure apparatuses for semiconductor manufacturing processes. Examples of such exposure apparatuses are a lithographic apparatus, a metrology, or an inspection apparatus, more specifically a mask inspection apparatus and even more specifically an actinic mask inspection apparatus.

[0004] A lithographic apparatus is a machine constructed to apply a desired pattern onto a substrate. A lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs). A lithographic apparatus may, for example, project a pattern at a patterning device (e.g., a mask) onto a layer of radiation-sensitive material (e.g., a photoresist or resist) provided on a substrate. To project a pattern on a substrate a lithographic apparatus may use electromagnetic radiation. The wavelength of this radiation determines the minimum size of features which can be formed on the substrate. A lithographic apparatus, which uses EUV radiation, having a wavelength within the range 4-20 nm, for example 6.7 nm or 13.5 nm, may be used to form smaller features on a substrate than a lithographic apparatus which uses, for example, radiation with a wavelength of 193 nm.

[0005] An (actinic) mask inspection apparatus is an apparatus that is configured for measuring dimensions or detecting defects in masks or mask blanks. EUV lithography uses reflective surfaces instead of transmissive optics such as lenses. Mask blanks used in EUV lithography generally have a multilayer structure which functions as a Bragg reflector. The multilayers may be altematingly Molybdenum and Silicon. If a defect exists in this structure, the projected pattern will be deformed in the lithographic process. Therefore, mask inspection to check whether a defect is present is considered a requirement for a mass-production process. EUV mask inspection may be used for several purposes and in several different stages. Firstly, it can be used for the detection of phase defects that may occurin mask blanks. Such phase defects may occur during the manufacturing of the multilayer stack of the mask blank. If undetected, these phase defects are printed on all chips printed with the part of a mask containing the phase defects. Such phase defects may be correctly detected by using the same or similar (13.5nm) actinic EUV wavelength as the lithographic tool. Secondly, mask inspection can be used for patterned mask inspection and can be carried out for the quality control of EUV patterned masks. For example, the mask inspection can be used to measure critical dimensions on the mask blank. In addition to phase defects, absorber pattern defects on the surface can be detected. Thirdly, mask inspection can be used for simulating exposure and determining the deterioration of optical contrast of a defect detected in the actinic inspection. Forth, the mask inspection can be used for optical proximity correction (OPC) evaluation or during mask repair process so as to improve pattern transfer fidelity. Further, it can be used for inspecting optical contrast after fixing the defect. In addition to the above, mask inspection can also be used to measure small particle / amplitude effects.

[0006] Exposure apparatuses, such as the lithographic apparatus and mask inspection apparatus, typically comprise a radiation source. The source may be a laser produced plasma (LPP) source, for example an EUV LPP source. The LPP sources typically comprise a target fluid supply system which is configured to eject droplets of target fluid (e.g. molten tin) for interception by a laser, producing radiation. The target fluid supply system comprises components which may be damaged by internal pressure fluctuations due to suboptimal information regarding the configuration of the system.SUMMARY

[0007] In accordance with an aspect of the present disclosure, there is provided a target fluid supply system for an EUV radiation source. The target fluid supply system comprises a reservoir configured to store and selectively expel a target fluid. The target fluid supply system comprises a sensor configured to generate a signal indicative of a volume of target fluid stored in the reservoir. The target fluid supply system comprises an ejection assembly in fluid communication with the reservoir configured to eject the target fluid expelled from the reservoir. The ejection assembly comprises a service freeze valve configured to connect to a source of purge gas. The service freeze valve is in fluid communication with a fillable volume of the ejection assembly. The fillable volume is configured to be filled with an amount of the target fluid that is expelled from the reservoir. The service freeze valve is configured to selectively permit purge gas into at least a portion of the fillable volume . The target fluid supply system comprises a control unit configured to determine whether a threshold fill level of target fluid is within the service freeze valve at least partly based on the signal and the fillable volume of the ejection assembly.

[0008] The fillable volume of the ejection assembly may correspond to a predetermined volume of the ejection assembly which is to be filled by target fluid during operation. The Tillable volume of the ejection assembly may not be occupied by the target fluid, in some conditions, e.g. when it is occupied by purge gas.

[0009] In previous target fluid supply systems, the amount of target fluid within the service freeze valve is not well-determined, leading to an insufficient amount of target fluid within the service freeze valve for proper function. Insufficiently filled service freeze valves may result in formation of easily displaced plugs of solid target material within the service freeze valve. On sudden displacement of a plug (e.g. under pressure from adjacent target fluid), the ejection assembly may experience sudden target fluid pressure fluctuation. Certain components of the ejection assembly are susceptible to damage from sudden target fluid pressure fluctuation.

[0010] Advantageously, the provision of the control unit configured to determine the fill level, or the amount of the target fluid present within a fillable volume in the service freeze valve, allows robust determination of the amount of target fluid within the service freeze valve compared to the threshold fill level of the service freeze valve, ensuring sufficient filling of the service freeze valve, and avoiding target fluid pressure fluctuation.

[0011] The EUV radiation source may be a laser produced plasma (LPP) or a discharge produced plasma (DPP) source, which employ the target fluid as fuel to produce the plasma.

[0012] The fillable volume and geometry of the ejection assembly may be predetermined, so the relation between total target fluid volume and target fluid level in the service freeze valve is well-characterised.

[0013] The sensor may be a level sensor. The level measured may be indicative of a fill level of target fluid stored in the reservoir.

[0014] The sensor may be a contactless sensor.

[0015] The term contactless sensor refers in this specification to sensors in which the sensor does not contact the measured part (i.e. the target fluid). Advantageously, this allows the separation of the sensor from a potentially inhospitable environment.

[0016] Preferably the sensor is a Hall-effect sensor. Alternatively, the sensor may be an acoustic sensor.

[0017] The ejection assembly and reservoir may be in fluid communication through a target fluid supply conduit.

[0018] The control unit may be configured to control the reservoir to expel target fluid.

[0019] The control unit may be configured to control the reservoir such that the target fluid reaches the threshold fill level in the service freeze valve.

[0020] The threshold fill level may correspond with a cross-section change within a fillable volume in the service freeze valve. Preferably, the cross-section change is a reduction of cross-sectional area encountered by the target material as it is filling up the fillable volume.

[0021] The control unit may be configured to control the reservoir such that the target fluid level is at most at the connection to the purge gas source.

[0022] The reservoir may comprise a deformable separator defining a first chamber for containing the target fluid. The reservoir may comprise a pressurizing system configured to pressurize a hydraulic fluid. The deformable separator may be configured to separate the hydraulic fluid from the target fluid in the first chamber. The deformable separator may be configured to deform under pressure from thehydraulic fluid and thereby expel target fluid. The sensor may be configured to generate a signal indicative of a state of deformation of the deformable separator.

[0023] The state of deformation may be indicative of the volume of target fluid stored in the reservoir.

[0024] The deformable separator may comprise a bellows.

[0025] The ejection assembly may be detachable from the reservoir such that the ejection assembly and the reservoir are not in fluid communication.

[0026] The ejection assembly may comprise a further freeze valve configured to selectively permit passage of target fluid through the fillable volume of the ejection assembly.

[0027] The target fluid employed in the target fluid supply system may comprise tin. The target fluid employed in the target fluid supply system preferably essentially consists of tin (e.g. consist of tin to a purity level of at least 99.99% by weight, preferably of at least 99.999% by weight).

[0028] According to another aspect of the present disclosure, there is provided an EUV radiation source comprising a target fluid supply system according to the preceding aspect. The EUV radiation source comprises a plasma formation region configured to receive target fluid ejected from the ejection assembly.

[0029] According to another aspect of the present disclosure, there is provided an EUV exposure system comprising the EUV radiation source of the preceding aspect and an EUV exposure apparatus.

[0030] The EUV exposure system may comprise a lithographic apparatus or a lithographic tool, such as for example a metrology apparatus, or a mask inspection apparatus.

[0031] According to another aspect of the present disclosure, there is provided a method of servicing a target fluid supply system. The target fluid supply system comprises a reservoir configured to store and selectively expel a target fluid. The target fluid supply system comprises an ejection assembly in fluid communication with the reservoir configured to eject target fluid expelled from the reservoir. The ejection assembly comprises a service freeze valve configured to connect to a source of purge gas. The service freeze valve is in fluid communication with a fillable volume of the ejection assembly. The Tillable volume of the ejection assembly is configured to be filled with an amount of the target fluid that is expelled from the reservoir. The service freeze valve is configured to selectively permit purge gas into at least a portion of the fillable volume. The method comprises connecting the ejection assembly to the reservoir. The method comprises driving the reservoir to expel target fluid into the ejection assembly. The method comprises sensing a volume of target fluid stored in the reservoir to generate a signal. The method comprises determining whether a threshold fill level of target fluid is within the service freeze valve at least partly based on the signal and a volume of the ejection assembly.

[0032] The method may further comprise stopping driving the reservoir once the threshold fill level of target fluid within the service freeze valve is reached.

[0033] The threshold fill level of target fluid may be such that the target fluid reaches a cross-section change within the service freeze valve.

[0034] The method may further comprise actuating the service freeze valve to solidify at least a portion of the target fluid within the service freeze valve.

[0035] The method may further comprise subsequently driving the reservoir to reach and maintain an operating pressure of the ejection assembly.

[0036] Sensing the volume of target fluid stored in the reservoir may comprise using a magnetic field.

[0037] According to another aspect of the present disclosure, there is provided a computer program comprising computer readable instructions configured to cause a controller to carry out the method of servicing a target fluid supply system.

[0038] According to another aspect of the present disclosure, there is provided a non-transitory computer readable medium carrying computer readable instructions configured to cause a controller to carry out the method of servicing a target fluid supply system.

[0039] According to another aspect of the present disclosure, there is provided a computer apparatus for a control unit. The computer apparatus comprises a memory storing processor readable instructions. The computer apparatus comprises a processor arranged to read and execute instructions stored in said memory. Said processor readable instructions comprise instructions arranged to control the computer to carry out the method of servicing a target fluid supply system.BRIEF DESCRIPTION OF THE DRAWINGS

[0040] Embodiments of the invention will now be described, by way of example only, with reference to the accompanying schematic drawings, in which:Figure 1 depicts a lithographic system comprising a lithographic apparatus and a radiation source;Figure 2 depicts a system for (actinic) mask inspection;Figure 3 schematically illustrates a target fluid supply system;Figure 4 schematically illustrates a cross section through a service freeze valve of the target fluid supply system shown in Figure 3;Figure 5A schematically illustrates a portion of the reservoir of the target fluid supply system shown in Figures 3 and 4;Figure 5B schematically illustrates the portion of the reservoir of the target fluid supply system shown in Figure 5A in an operational state; andFigure 6 schematically illustrates a method of servicing a target fluid supply system.DETAILED DESCRIPTION

[0041] Figure 1 schematically shows a lithographic system comprising a radiation source SO and a lithographic apparatus LA. 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 patterningdevice MA (e.g., a mask), a projection system PS and a substrate table WT configured to support a substrate W.

[0042] 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 faceted 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.

[0043] 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 projection system 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 1, the projection system PS may include a different number of mirrors (e.g., six or eight mirrors).

[0044] 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.

[0045] 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.

[0046] The lithographic apparatus LA and radiation source SO described herein can be used in method for performing a circuit layout patterning process. A circuit layout patterning method comprises receiving a substrate with a photoresist layer. The method further comprises directing EUV radiation from radiation source to the photoresist layer to form a patterned photoresist layer. The method further comprises developing and etching the patterned photoresist layer to form a circuit layout.

[0047] Figure 2 depicts a system for (actinic) mask inspection. A mask inspection system can be used to identify or inspect defects in a mask to be used in a lithographic process by means of a lithographic system such as for example the one described in Figure 1. The mask inspection system comprises a radiation source SO and an illumination system IL and a detection system DS. A patterning device (i.e. mask) MA is placed on a support structure MT, which may be a mask stage, and illuminated by the illumination system IL reflecting radiation incident from the radiation source SO. The radiation coming from the illuminated patterning device MA is reflected by the detection system. In this way an image isformed on a detector DE. The radiation source SO of Figure 2 may be modelled after the radiation source system of Figure 1.

[0048] The radiation source SO shown in Figure 1 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 laser, is arranged to deposit energy via a laser beam 2 into a fuel (i.e., a target fluid), such as tin (Sn) which is provided from, e.g., a target fluid supply system 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 target fluid supply system 3 is configured to eject droplets of target fluid in the form of droplets, along a trajectory towards a plasma formation region 4. The laser beam 2 is incident upon the fuel at the plasma formation region 4. The deposition of laser energy into the target material creates a plasma 7 at the plasma formation region 4. Radiation, including EUV radiation, is emitted from the plasma 7 during de-excitation and recombination of electrons with ions of the plasma 7.

[0049] The EUV radiation from the plasma 7 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.

[0050] 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.

[0051] 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.

[0052] Although shown contained within the enclosing structure 9 of the source SO, in practice target fluid supply system 3 may be disposed at least partially outside the enclosing structure.

[0053] Figure 3 schematically illustrates a target fluid supply system 3. The target fluid supply system 3 be employed for supplying the target fluid to a radiation source, in particular an EUV radiation source. The radiation source may be a plasma-based source wherein radiation is generated by a plasma producedwith the target fluid. The target fluid supply system 3 may be employed to provide the target fluid to a laser produced plasma source or a discharge produced plasma source.

[0054] The target fluid supply system 3 of Fig. 3 comprises: a reservoir 32, a target fluid inlet conduit 34, target fluid supply conduit 36 and a refill module 38. Target fluid supply system 3 may form a part of the radiation source SO of an EUV exposure system, such as the lithographic system of Figure 1 and / or the radiation source SO of the mask inspection system of Figure 2.

[0055] The reservoir 32 is configured to store and selectively expel target fluid (i.e. liquid target material). The target material may for example comprise of tin (Sn), lithium (Li), or xenon (Xe), preferably tin. Target fluid entering an inlet 24 of the reservoir 32 is expelled at an outlet 26 of the reservoir 32. In some operating modes, the reservoir 32 may be driven to expel target fluid at a selected pressure. Alternatively, or additionally, the reservoir 32 may be driven to expel a selected volume of target fluid.

[0056] The reservoir 32 comprises a sensor 33 configured to generate a signal indicative of a volume of target fluid stored in the reservoir. Monitoring or tracking the volume of target fluid stored in the reservoir 32 allows determination of the volume of target fluid that has been expelled from the reservoir or added to the reservoir.

[0057] The inlet conduit 34 and the target fluid supply conduit 36 are both configured to convey target fluid, and are fluidly connected to the reservoir 32. The inlet conduit 34 is connected to the inlet 24 of the reservoir 32. The target fluid supply conduit 36 is connected to the outlet 26 of the reservoir 32.

[0058] The target fluid supply conduit 36 fluidly connects an ejection assembly 40 to the reservoir 32. The ejection assembly is arranged to eject target fluid that has been expelled from the reservoir 32 into the plasma formation region of the radiation source. In case of a laser produced plasma source, the ejection assembly 40 may be configured to eject droplets of target fluid expelled from the reservoir 32. The ejection assembly 40 and the target fluid supply conduit 36 are configured to reversibly disconnect, for example during maintenance or replacement of the ejection assembly 40.

[0059] In the following description the terms ‘downstream’ and ‘upstream’ are used here with reference to the direction of target fluid flow in normal operation of target fluid supply system 3 downstream of the refill module 38 - that is, from the reservoir 32 to the ejection assembly 40.

[0060] The refill module 38 is configured to supply target fluid to reservoir 32 via the inlet conduit 34.

[0061] In some embodiments, target material may be fed into the target fluid supply system 3 in the form of solid ingots at the refill module 38. The refill module may be configured to melt the target material, forming target fluid and condition it for use downstream (e.g. in the reservoir 32 and the ejection assembly 40).

[0062] The ejection assembly 40 comprises a service freeze valve 42 and an ejection portion 44.

[0063] The service freeze valve 42 is configured to connect to a source of purge gas 48 via a gas line 49. Purge gas may selectively be permitted into at least a part of the ejection assembly 40 by opening the service freeze valve 42. During servicing actions of a component, the residual tin in portions of theejection assembly 40 adjacent the service freeze valve 42 is purged by means of the purge gas. Purging may involve displacing of a certain purge volume of the target fluid. Preferably, the purge gas is inert with respect to the target fluid. However, it may be slightly chemically reducing in order to remove oxides from the target fluid. The purge gas may be substantially composed of argon and / or hydrogen. The purge gas may be pressurized.

[0064] The ejection portion 44 is configured to receive a continuous supply of target fluid from upstream and eject an generally uninterrupted stream of target fluid into the plasma formation region. The ejection portion 44 may be provided in the form of a droplet generation portion which comprises a target fluid droplet generator configured generate an uninterrupted stream of target fluid droplets 50 (e.g. by jet breakup), ejected towards the plasma formation region 4 (see Figure 1).

[0065] The ejection portion 44 may eject the target fluid via a nozzle (not shown). The ejection portion 44 is configured to receive a substantially continuous supply of target fluid at a substantially constant operating pressure, and may be damaged by pressure fluctuations.

[0066] The ejection portion 44 and the service freeze valve 42 are in fluid communication via an intermediate junction 45 and a further freeze valve 46, configured to selectively permit passage of target fluid from upstream into the ejection portion 44.

[0067] The target fluid supply system 3 further comprises a control unit 52 configured to determine an amount of target fluid within the service freeze valve 42 at least partly based on the signal generated by sensor 33 and a tillable volume of the ejection assembly 40. The control unit 52 is additionally configured to control (or drive) the reservoir 32 to expel target fluid, or to stop expelling target fluid, based at least in part on the signal generated by the sensor 33. For example, control unit 52 may be configured to drive reservoir 32 to expel a selected volume of target fluid. The selected volume may be a predetermined volume based at least in part on a specification of volume to be filled with target fluid in the service freeze valve 42.

[0068] In an example, the tillable volume of the ejection assembly 40 may correspond to the predetermined volume of the ejection assembly which is filled by target fluid expelled from the reservoir 32 during operation. Typically, a newly-installed or reinstalled ejection assembly 40 is unoccupied by tin upstream of the further freeze valve 46 (i.e. intermediate junction 45 and service freeze valve 42). The further freeze valve 46 selectively blocks fluid flow to the ejection portion 44.

[0069] The tillable volume and geometry of the ejection assembly 40 is predetermined, so the relationship between total target fluid volume expelled (into the ejection assembly by reservoir 32) and the amount of target fluid in the service freeze valve 42 is well-characterised. Thus, the control unit 52 can robustly determine the amount of target fluid in the service freeze valve 42 based on the volume of target fluid expelled (as indicated by the change in volume of target fluid in the reservoir 32) and the fillable volume of the ejection assembly 40. The amount of target fluid within the service freeze valve 42 corresponds to a level or height (see Figure 4) within the service freeze valve.

[0070] Figure 4 schematically illustrates an example cross-section of the service freeze valve 42. The service freeze valve 42 comprises a valve conduit 62, configured to permit passage of fluids (e.g. target fluid material or purge gas). The valve conduit terminates at opposite ends with a target fluid port 63 on one end and a gas port 64 at an opposing end. Target fluid port 63 connects to the rest of the ejection assembly 40, for example via intermediate junction 45. Gas port 64 connects to the source of purge gas 48 via gas line 49.

[0071] The service freeze valve 42 is configured to selectively solidify at least a portion of the target fluid disposed in the valve conduit 62 on actuation, so as to stop flow through the valve conduit. Put alternatively, service freeze valve 42 selectively cools a portion of the target fluid to form a solid plug 66 of target material which resists displacement and obstructs flow of fluids (i.e. purge gas or target fluid) through the valve conduit 62.

[0072] The transverse cross-section of valve conduit 62 changes at section X-X. The portion of the valve conduit between section X-X and the gas port 64 is smaller in cross-section than the portion between section X-X and the target fluid port 63, defining a shoulder interface 69. The solid plug 66 formed resists displacement towards the gas port 64 when exposed to pressure applied at the target fluid port 63, due to abutment against shoulder interface 69. Thus, for optimal function of the service freeze valve 42, the fluid level (indicated by line Y-Y) of the target fluid must reach the cross-section change at section X-X before selective cooling or solidification takes place. Alternative and / or additional features may be provided to hold the solid plug 66 in place, such as an area of increased inward surface roughness or texture, or inwardly protruding pins of a target fluid resistant material.

[0073] In cases where the target fluid level does not reach the cross-section change at section X-X before selective cooling or solidification takes place, the solid plug 66 does not abut shoulder interface 69 and can be easily displaced under pressure from upstream target fluid. The displacement of solid plug 66 causes undesirable damaging pressure fluctuations in the ejection assembly 40.

[0074] The control unit 52 is configured to control the reservoir such that the target fluid reaches a cross-section change within the service freeze valve 42 before selective cooling or solidification takes place. The control unit 52 may drive the reservoir 32 to expel a total volume target fluid volume corresponding to a target fluid level within service freeze valve 42 at or above the cross-section change.

[0075] In general, the control unit 52 may be configured to drive the reservoir 32 such that the target fluid level is at most at the gas port 64. This avoids ingress of target fluid into the gas line 49, fouling the gas line and / or the purge gas source 48.

[0076] The reservoir may comprise a hydraulic assembly 80 for pressurizing the target fluid. An example is shown in Figures 5A. In this example, the hydraulic assembly 80 comprises a deformable separator 82, a pressurizing system 84 and a containment vessel 86. The deformable separator 82 is contained within the containment vessel 86. The deformable separator 82 defines a first chamber 88 for containing target fluid 83 which is separated by the deformable separator 82 from a complementary space 85 within the containment vessel 86. The pressurizing system 84 is configured to pressurize ahydraulic fluid 87 within the complementary space 85 where the hydraulic fluid 87 immerses the deformable separator 82. The deformable separator 82 physically separates the target fluid 83 in the first chamber 86 from the hydraulic fluid 87 while being capable of transferring pressure from the hydraulic fluid 87 to the target fluid 83. Under pressure from the hydraulic fluid 87, the deformable separator 82 deforms, e.g. by collapsing, thereby expelling target fluid via an outlet port 90. Outlet port 90 is in fluid communication with the outlet 26 of reservoir 32.

[0077] The deformable separator 82 may comprise a bellows having a corrugated or concertina-like sidewall 92 and a rigid endcap 94. The concertina-like sidewall 92 deforms by collapsing along a principal axis, as shown in Figure 5B.

[0078] The pressurizing system 84 pressurizes the hydraulic fluid 87, for example by heating the hydraulic fluid, thereby inducing thermal expansion of the hydraulic fluid. The pressurizing system 84 is controlled by control unit 52, so that controlled volumes of target fluid can be expelled from hydraulic assembly 80 (and by extension, reservoir 32). At lower pressures (e.g. between about 1 bara and about 8 bara), a gas pressure within the reservoir may be utilized to pressurize the hydraulic fluid 87 and deform the deformable separator 82.

[0079] The displacement A of the deformable separator 82 (in particular, the endcap 94) along the principal axis directly relates to the volume of target fluid 83 within first chamber 88.

[0080] Sensor 33 is preferably a contactless sensor that can be disposed outside the containment vessel 86 and is capable of generating a signal indicative of a state of deformation of the deformable separator 82 without being in physical contact with the deformable separator and the hydraulic fluid 87. This is advantageous because the hydraulic fluid 87 may be at high pressure and temperature, providing an unsuitable environment for sensor operation. For example, sensor 33 may comprise a pair of Hall-effect sensor elements 33a, 33b configured to detect a magnetic field generated by a magnet 96 disposed within the endcap 94. Sensor 33 is configured to determine the displacement A of the deformable separator 82 from magnetic flux densities detected by the Hall-effect sensor elements 33a, 33b. The sensor 33 generates a signal indicative of a volume of target fluid stored in the reservoir 32 corresponding to the detected displacement A.

[0081] Whilst two Hall-effect sensor elements are shown in Figures 5A and 5B, it will be appreciated that in other embodiments, the sensor 33 may comprise as few as a single sensor element. Alternatively, the sensor 33 may comprise three or more Hall -effect sensor elements. The provision of multiple Halleffect sensor elements provides redundancy in the event of a sensor failure and may provide further information regarding the status of the deformable separator 82 (e.g. tilt).

[0082] In some embodiments, the sensor 33 may not be a Hall-effect sensor, but another type of position sensor. In general, any sensor capable of indicating the volume target fluid stored in the reservoir may be employed, such as a level switch or acoustic sensor.

[0083] In some embodiments, the reservoir 32 may comprise two or more hydraulic assemblies 80.

[0084] Figure 6 is a flowchart of a method 100 of servicing the tin fluid supply system 3.

[0085] At SI, the ejection assembly 40 is connected to the reservoir 32, via the target fluid supply conduit.

[0086] At S2, the reservoir 32 is driven by control unit 52 to expel a volume of target fluid into the ejection assembly, progressively filling a volume within the ejection assembly. Control unit 52 causes pressurizing system 84 to pressurize the hydraulic fluid and deform the deformable separator 82, thereby expelling target fluid. For example, the control unit 52 may cause the pressurizing system 84 to heat the hydraulic fluid 87, inducing thermal expansion of the hydraulic fluid and deforming the separator 82 under pressure from the hydraulic fluid, expelling target fluid.

[0087] At S3, the controller 52 monitors the signal generated from sensor 33 indicative of a volume of target fluid stored in the reservoir 32, and determines the volume of target fluid expelled. Sensor 33 uses the magnetic field generated by the magnet 96 to determine the state (i.e. the deformable separator 82 displacement A and the corresponding target fluid volume) of the reservoir 32. The amount of target fluid within the service freeze valve 42 is determined (e.g. by controller 52) at based on the signal and the unoccupied volume of the ejection assembly.

[0088] At S4, the controller 52 stops driving reservoir 32 to expel target fluid, once a threshold amount of target fluid within the service freeze valve 42 is achieved. For example, the threshold amount of target fluid may correspond to a target fluid level reaching the cross-section change within the service freeze valve 42.

[0089] At S5, the service freeze valve 42 is actuated to solidify at least a portion of the target fluid within the freeze valve conduit 62.

[0090] At S6, the reservoir 32 is driven to reach and maintain an operating pressure of the ejection assembly 40. The operating pressure is the pressure of target fluid which the ejection portion 44 is configured to operate at. The operating pressure may be about 300 bara or more, preferably about 600 bara or more, more preferably about 800 bara or more, most preferably about 1200 bara or more. The operating pressure may be about 3000 bara or less, preferably about 2200 bara or less, more preferably about 1800 bara or less.

[0091] In general, method 100 is performed with a further freeze valve 46 actuated to block fuel flow out of the ejection portion 44 whilst the method takes place, thereby avoiding a continuous negative term (i.e. a leak) over time. The method may comprise a preliminary step of closing the service freeze valve to avoid tin contamination of gas supply lines.

[0092] Optionally, the service freeze valve 42 can be filled with target fluid that is expelled from a reservoir in the refill module 38, instead of from the reservoir 32 that is between the refill module 38 and the ejection assembly 40. Suitably, the reservoir 32 is bypassed when that mode of operation is used. The ejection assembly 40 may be in fluid communication with the reservoir in the refill module 38 via the target fluid inlet conduit 34 and the the target fluid supply conduit 36. Suitably, the reservoir 32 is isolated from these conduits by freezing the conduit that corresponds to the outlet port 90 that is in fluid communication with the outlet 26 of reservoir 32. Alternatively, the reservoir in the refillmodule 38 may be in fluid communication with the ejection assembly 40 via a separate bypass line (not shown). In these options, the sensor configured to generate a signal indicative of a volume of the target fluid stored in the reservoir may be a fill level sensor within the reservoir in the refill module 38. Examples of such sensors have been disclosed in publications, including: WO 2021 / 239382 Al (optical triangulation); US 8,154,000 B5 (resistance wire); Research Disclosure No. 652053 published in Vol. 652, No. 53 (optical time-of-flight and electrical resistance). The pressurizing system configured to pressurize the reservoir in the refill module 38 may be pneumatic. Alternatively, the service freeze valve is filled with the target fluid being expelled from the reservoir in the refill module 38 by gravity. In the latter case, a controlled backpressure may be applied in gas line 49 to control the filling. If reservoir 32 is not bypassed, then any sensor data from this reservoir would also need to be taken into account.

[0093] Although specific reference may be made in this text to the use of lithographic apparatus in the manufacture of ICs, it should be understood that the lithographic apparatus described herein may have other applications. Possible other applications include the manufacture of integrated optical systems, guidance and detection patterns for magnetic domain memories, flat-panel displays, liquid-crystal displays (LCDs), thin-film magnetic heads, etc.

[0094] Although specific reference may be made in this text to embodiments of the invention in the context of a lithographic apparatus, embodiments of the invention may be used in other apparatus. Embodiments of the invention may form part of a mask inspection apparatus, a metrology apparatus, or any apparatus that measures or processes an object such as a wafer (or other substrates) or mask (or other patterning devices). These apparatus may be generally referred to as lithographic tools. Such a lithographic tool may use vacuum conditions or ambient (non-vacuum) conditions.

[0095] Where the context allows, embodiments of the invention may be implemented in hardware, firmware, software, or any combination thereof. Embodiments of the invention may also be implemented as instructions stored on a machine -readable medium, which may be read and executed by one or more processors. A machine -readable medium may include any mechanism for storing or transmitting information in a form readable by a machine (e.g., a computing device). For example, a machine-readable medium may include read only memory (ROM); random access memory (RAM); magnetic storage media; optical storage media; flash memory devices; electrical, optical, acoustical, and others. Further, firmware, software, routines, instructions may be described herein as performing certain actions. However, it should be appreciated that such descriptions are merely for convenience and that such actions in fact result from computing devices, processors, controllers, or other devices executing the firmware, software, routines, instructions, etc. and in doing that may cause actuators or other devices to interact with the physical world. For example, a computer program comprising computer readable instructions may be configured to cause a controller to carry out the method of servicing the tin fluid supply system of Fig. 6. Alternatively, a non-transitory computer readable medium may carry computer readable instructions configured to cause a controller to carry out themethod of servicing the tin fluid supply system of Fig. 6. As another alternative, a computer apparatus for a control unit may comprise a memory storing processor readable instructions and a processor arranged to read and execute instructions stored in said memory. Said processor readable instructions may comprise instructions arranged to control the computer to carry out the method of servicing the tin fluid supply system of Fig. 6. For example, any of the computer implemented forms of performing the method of servicing the tin fluid supply system of Fig. 6 may act as ‘retrofitting’ software upgrade to enhance functionality on systems that already include a sensor (such as a Hall sensor) and a service freeze valve but do not include a control unit in accordance with the present disclosure.

[0096] The specification will be summarized below by the following clauses.1. A target fluid supply system for an EUV radiation source comprising: a reservoir configured to store and selectively expel a target fluid; a sensor configured to generate a signal indicative of a volume of the target fluid stored in the reservoir; an ejection assembly in fluid communication with the reservoir configured to eject target fluid expelled from the reservoir, the ejection assembly comprising a service freeze valve configured to connect to a source of purge gas, the service freeze valve being in fluid communication with a tillable volume of the ejection assembly, the tillable volume being configured to be filled with an amount of the target fluid that is expelled from the reservoir, the service freeze valve being configured to selectively permit purge gas into at least a portion of the fillable volume; and a control unit configured to determine whether a threshold fill level of target fluid is within the service freeze valve at least partly based on the signal and the fillable volume of the ejection assembly.2. The target fluid supply system of clause 1, wherein the sensor is a level sensor, and wherein the level measured is indicative of a fill level of target fluid stored in the reservoir.3. The target fluid supply system of any preceding clause, wherein the sensor is a contactless sensor.4. The target fluid supply system of any preceding clause, wherein the ejection assembly and reservoir are in fluid communication through a target fluid supply conduit.5. The target fluid supply system of any preceding clause, wherein the control unit is configured to control the reservoir to expel target fluid.6. The target fluid supply system of clause 5, wherein the control unit is configured to control the reservoir such that the target fluid reaches the threshold fill level.7. The target fluid supply system of clause 6, wherein the threshold fill level corresponds with a cross-section change within a fillable volume in the service freeze valve.8. The target fluid supply system of any of clauses 5 to 7, wherein the control unit is configured to control the reservoir such that the target fluid level is at most at the connection to the purge gas source.9. The target fluid supply system of any preceding clause, wherein the reservoir comprises:a deformable separator defining a first chamber for containing the target fluid; and, a pressurizing system configured to pressurize a hydraulic fluid, wherein the deformable separator is configured to: separate the hydraulic fluid from the target fluid in the first chamber; and, deform under pressure from the hydraulic fluid and thereby expel target fluid; wherein the sensor is configured to generate a signal indicative of a state of deformation of the deformable separator.10. The target fluid supply system of clause 9 wherein the deformable separator comprises a bellows.11. The target fluid supply system of any preceding clause, wherein the ejection assembly is detachable from the reservoir such that the ejection assembly and the reservoir are not in fluid communication.12. The target fluid supply system of any preceding clause, wherein the ejection assembly comprises a further freeze valve, configured to selectively permit passage of target fluid through the fillable volume of the ejection assembly.13. The target fluid supply system of any preceding clause, wherein the target fluid comprises tin.14. An EUV radiation source comprising a target fluid supply system according to any preceding clause and a plasma formation region configured to receive target fluid ejected from the ejection assembly.15. An EUV exposure system comprising the EUV radiation source of clause 14 and an EUV exposure apparatus.16. A method of servicing a target fluid supply system, the target fluid supply system comprising: a reservoir configured to store and selectively expel a target fluid; and an ejection assembly in fluid communication with the reservoir configured to eject target fluid expelled from the reservoir, the ejection assembly comprising a service freeze valve configured to connect to a source of purge gas, the service freeze valve being in fluid communication with a fillable volume of the ejection assembly, the fillable volume being configured to be filled with an amount of the target fluid that is expelled from the reservoir, the service freeze valve being configured to selectively permit purge gas into at least a portion of the fillable volume; wherein the method comprises: connecting the ejection assembly to the reservoir; driving the reservoir to expel target fluid into the ejection assembly; sensing a volume of target fluid stored in the reservoir to generate a signal; and, determining whether a threshold fill level of target fluid is within the service freeze valve at least partly based on the signal and a volume of the ejection assembly.17. The method of clause 16, further comprising stopping driving the reservoir once the threshold fill level of target fluid within the service freeze valve is reached.18. The method of clause 16 or 17, further comprising actuating the service freeze valve to solidify at least a portion of the target fluid within the service freeze valve.19. The method of clause 18, further comprising subsequently driving the reservoir to reach and maintain an operating pressure of the ejection assembly. 20. The method of any of clauses 16 to 19, wherein sensing the volume of target fluid stored in the reservoir comprises using a magnetic field.21. A computer program comprising computer readable instructions configured to cause a controller to carry out a method according to any of clauses 16 to 20.22. A non-transitory computer readable medium carrying computer readable instructions configured to cause a controller to carry out a method according to any of clauses 16 to 20.23. A computer apparatus for a control unit comprising: a memory storing processor readable instructions; and a processor arranged to read and execute instructions stored in said memory; wherein said processor readable instructions comprise instructions arranged to control the computer to carry out a method according to any of clauses 16 to 20.

[0097] While specific embodiments of the invention have been described above, it will be appreciated that the invention may be practiced otherwise than as described. 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 to the invention as described without departing from the scope of the claims set out below.

Claims

CLAIMS1. A target fluid supply system for an EUV radiation source comprising: a reservoir configured to store and selectively expel a target fluid; a sensor configured to generate a signal indicative of a volume of the target fluid stored in the reservoir; an ejection assembly in fluid communication with the reservoir configured to eject target fluid expelled from the reservoir, the ejection assembly comprising a service freeze valve configured to connect to a source of purge gas, the service freeze valve being in fluid communication with a tillable volume of the ejection assembly, the tillable volume being configured to be filled with an amount of the target fluid that is expelled from the reservoir, the service freeze valve being configured to selectively permit purge gas into at least a portion of the tillable volume; and a control unit configured to determine whether a threshold fill level of target fluid is within the service freeze valve at least partly based on the signal and the tillable volume of the ejection assembly.

2. The target fluid supply system of claim 1, wherein the sensor is a level sensor, and wherein the level measured is indicative of a fill level of target fluid stored in the reservoir.

3. The target fluid supply system of any preceding claim, wherein the sensor is a contactless sensor.

4. The target fluid supply system of any preceding claim, wherein the ejection assembly and reservoir are in fluid communication through a target fluid supply conduit.

5. The target fluid supply system of any preceding claim, wherein the control unit is configured to control the reservoir to expel target fluid6. The target fluid supply system of claim 5, wherein the control unit is configured to control the reservoir such that the target fluid reaches the threshold fill level.

7. The target fluid supply system of claim 6, wherein the threshold fill level corresponds with a cross-section change within a fillable volume in the service freeze valve.

8. The target fluid supply system of any of claims 5 to 7, wherein the control unit is configured to control the reservoir such that the target fluid level is at most at the connection to the purge gas source.

9. The target fluid supply system of any preceding claim, wherein the reservoir comprises:a deformable separator defining a first chamber for containing the target fluid; and, a pressurizing system configured to pressurize a hydraulic fluid, wherein the deformable separator is configured to: separate the hydraulic fluid from the target fluid in the first chamber; and, deform under pressure from the hydraulic fluid and thereby expel target fluid; wherein the sensor is configured to generate a signal indicative of a state of deformation of the deformable separator.

10. The target fluid supply system of any preceding claim, wherein the ejection assembly is detachable from the reservoir such that the ejection assembly and the reservoir are not in fluid communication.

11. The target fluid supply system of any preceding claim, wherein the ej ection assembly comprises a further freeze valve, configured to selectively permit passage of target fluid through the fillable volume of the ejection assembly.

12. A method of servicing a target fluid supply system, the target fluid supply system comprising: a reservoir configured to store and selectively expel a target fluid; and an ejection assembly in fluid communication with the reservoir configured to eject target fluid expelled from the reservoir, the ejection assembly comprising a service freeze valve configured to connect to a source of purge gas, the service freeze valve being in fluid communication with a fillable volume of the ejection assembly, the fillable volume being configured to be filled with an amount of the target fluid that is expelled from the reservoir, the service freeze valve being configured to selectively permit purge gas into at least a portion of the fillable volume; wherein the method comprises: connecting the ejection assembly to the reservoir; driving the reservoir to expel target fluid into the ejection assembly; sensing a volume of target fluid stored in the reservoir to generate a signal; and, determining whether a threshold fill level of target fluid is within the service freeze valve at least partly based on the signal and a volume of the ejection assembly.

13. The method of claim 12, further comprising stopping driving the reservoir once the threshold fill level of target fluid within the service freeze valve is reached.

14. The method of claim 12 or 13, further comprising actuating the service freeze valve to solidify at least a portion of the target fluid within the service freeze valve.

15. The method of claim 14, further comprising subsequently driving the reservoir to reach and maintain an operating pressure of the ejection assembly.

16. A computer program comprising computer readable instructions configured to cause a controller to carry out a method according to any of claims 12 to 15.

17. A non-transitory computer readable medium carrying computer readable instructions configured to cause a controller to carry out a method according to any of claims 12 to 15.

18. A computer apparatus for a control unit comprising : a memory storing processor readable instructions; and a processor arranged to read and execute instructions stored in said memory; wherein said processor readable instructions comprise instructions arranged to control the computer to carry out a method according to any of claims 12 to 15.

19. An EUV radiation source comprising a target fluid supply system according to any one of claims 1 to 11.

20. An EUV exposure system comprising the EUV radiation source of claim 19.

Images