WO2020141057A1

WO2020141057A1 - Apparatus for controlling introduction of euv target material into an euv chamber

Info

Publication number: WO2020141057A1
Application number: PCT/EP2019/085023
Authority: WO
Inventors: Benjamin Andrew SAMS; Dietmar Uwe Herbert TREES; Georgiy O. Vaschenko
Original assignee: Asml Netherlands B.V.
Priority date: 2018-12-31
Filing date: 2019-12-13
Publication date: 2020-07-09
Also published as: JP2022515973A; NL2024324A; CN113273314A

Abstract

Apparatus for controlling introduction of EUV target material into an EUV chamber in which a flow restrictor orifice is placed in a flow path of target material through the droplet generator, for example, at a position upstream of a filter within the droplet generator. Also, a liquid tin sensor, for example, a conduction probe or electrical load detector, which may be located in the volume of a droplet generator heater blocks. The flow restrictor can be configured to serve additionally as a seal, for example, for the filter. Thus, the flow restrictor can be configured as a metal disk that can also serve as a sealing gasket. For example, the metal may be unannealed tantalum, tantalum-tungsten alloy, annealed molybdenum, molybdenumrhenium alloy, or annealed rhenium.

Description

APPARATUS FOR CONTROLLING INTRODUCTION OF EUV TARGET MATERIAL INTO AN EUV CHAMBER

CROSS-REFERENCE TO RELATED APPLICATIONS

[0001] This application claims priority of US application 62/786,622 which was filed on December 31, 2018 and which is incorporated herein in its entirety by reference.

FIELD

[0002] The present application relates to extreme ultraviolet (“EUV”) light sources and their methods of operation. These light sources provide EUV light by creating plasma from a target material. In one application, the EUV light may be collected and used in a photolithography process to produce semiconductor integrated circuits.

BACKGROUND

[0003] A patterned beam of EUV light can be used to expose a resist coated substrate, such as a silicon wafer, to produce extremely small features in the substrate. EUV light (also sometimes referred to as soft x-rays) is generally defined as electromagnetic radiation having wavelengths in the range of about 5 nm to about 100 nm. One particular wavelength of interest for photolithography occurs at 13.5 nm.

[0004] Methods to produce EUV light include, but are not necessarily limited to, converting a target material (also referred to as a source material) into a plasma state that has a chemical element with an emission line in the EUV range. These elements can include, but are not limited to, xenon, lithium, and tin.

[0005] In one such method, often termed laser produced plasma (“LPP”), the desired plasma can be produced by irradiating a target material, for example, in the form of a droplet, stream, or wire, with a laser beam. In another method, often termed discharge produced plasma (“DPP”), the required plasma can be generated by positioning source material having an appropriate emission line between a pair of electrodes and causing an electrical discharge to occur between the electrodes.

[0006] One technique for generating droplets involves melting a target material such as tin and then forcing it under high pressure through a relatively small diameter orifice in a droplet generator, such as an orifice having a diameter of about 0.1 pm to about 30 pm, to produce a laminar fluid jet having velocities in the range of about 30 m/s to about 200 m/s. Under most conditions, the jet will break up into droplets due to a hydrodynamic instability known as the Rayleigh-Plateau instability. These droplets may have varying velocities and may combine with each other to coalesce into larger droplets.

[0007] The molten target material is maintained under high pressure by connecting a reservoir holding the target material to a source of high pressure gas, such as an inert gas, e.g., argon. In such an arrangement there is a risk that the droplet generator will fail in a manner that leaks and permits an uncontrolled flow of target material under pressure into the chamber containing the system optics such as a collector mirror, thus contaminating the optics with target material.

[0008] For the droplet generators operating at 3,000-4,000 PSI pressure in EUV sources, a failsafe system using a device called a Rapid Venting System (RVS) is used. The RVS uses a signal from a gas flow switch to detect significant tin leaks caused by a failure of one of the droplet generator components to prevent massive contamination of the source vacuum chamber by shutting off the supply of high pressure gas to the reservoir and rapidly venting the droplet generator into a large gas tank referred to as a Rapid Vent Tank(RVT). Without the RVS, a nozzle failure can introduce heavy tin contamination into the chamber in a fraction of a second.

[0009] There are several possible disadvantages and limitation of an RVS system such as that just described. For example, during pressurization of the droplet generator, the gas volumetric flow rate changes (mass flow is constant). Before the gas pressure has reached -400 PSI the gas flow rate exceeds the threshold of the RVS’s flow switch, which results in the RVS being activated. Thus, as a practical matter, the RVS system is typically disabled for the beginning of droplet generator pressurization, permitting pressurization to proceed but leaving the EUV source unprotected for conditions such as, for example, a broken nozzle.

[0010] Also, the activation threshold of the RVS flow switch must be selected to be sensitive to anticipated failures, but insensitive to conditions that would be encountered during normal operation. Trying to satisfy these criteria simultaneously forces a compromise resulting in a switch that does not always operate when it should or sometimes operates when it should not, for example, during initial pressurization as described above. For the flow switch to detect tin leaks, the tin flow must exceed a minimum value. There are cases where detection fails such as when the capillary diameter is too small, when the orifice is chipped, or when the capillary only partially breaks.

[0011] An example of a limitation is that the pressure rating of the current RVS components effectively limit droplet generator operation to pressures of about 4,000 PSI. A higher-pressure system can be developed, but increasing the pressure rating comes at the expense of reduced conductance of the components and therefore the rate of depressurization will be reduced, making the rapid depressurization scheme described above ineffective at protecting the source.

[0012] An example of a disadvantage is that the RVS is bulky, heavy, and requires connection to stiff hoses This adds an extra load on the droplet generator steering system and reduces the control performance. Also, the RVT is very large in size (about 300 L) and occupies a significant of space in the fabrication facility. For operation at greater droplet generator pressures, it would be necessary to make the hoses even stiffer and the RVT even larger. Also, rapidly depressurizing an operable droplet generator can impart a damaging shock to the droplet generator that may cause it to fail on the next start.

[0013] There thus remains a need address target material leakage from the droplet generator that avoids these limitations and disadvantages.

SUMMARY

[0014] The following presents a simplified summary of one or more embodiments in order to provide a basic understanding of the embodiments. This summary is not an extensive overview of all contemplated embodiments, and is not intended to identify key or critical elements of all embodiments nor delineate the scope of any or all embodiments. Its sole purpose is to present some concepts of one or more embodiments in a simplified form as a prelude to the more detailed description that is presented later.

[0015] According to one aspect of an embodiment, a flow restrictor orifice is placed in a flow path of target material through the droplet generator, for example, at a position upstream of a filter within the droplet generator. Also, a liquid target material level sensor, for example, a conduction probe or electrical load detector (ELD), may be used and may be located in the volume of a droplet generator heater block. The flow restrictor can be configured to serve additionally as a seal, for example, for the filter. Thus, the flow restrictor may be configured as a disk that can also serve as a sealing gasket. For example, the disk may be made of a metal such as unannealed tantalum, tantalum-tungsten alloy, annealed molybdenum, molybdenum- rhenium alloy, and annealed rhenium or of a refractory material.

[0016] Further embodiments, features, and advantages of the present invention, as well as the structure and operation of the various embodiments are described in detail below with reference to accompanying drawings. BRIEF DESCRIPTION OF THE DRAWINGS

[0017] The accompanying drawings, which are incorporated herein and form part of the specification, illustrate the methods and systems of embodiments of the invention by way of example, and not by way of limitation. Together with the detailed description, the drawings further serve to explain the principles of and to enable a person skilled in the relevant art(s) to make and use the methods and systems presented herein. In the drawings, like reference numbers indicate identical or functionally similar elements.

[0018] FIG. 1 is a simplified schematic view of an EUV light source coupled with an exposure device.

[0019] FIG. 1A is a simplified, schematic diagram of an apparatus including an EUV light source having an EPP EUV light radiator.

[0020] FIG. 2 is a schematic diagram of a droplet generation subsystem for an EUV light source.

[0021] FIG. 3 is a schematic diagram of a conventional system for limiting target material leakage in an EUV production system.

[0022] FIG. 4 is a not-to-scale diagram of an arrangement for limiting leakage of EUV target material into an EUV chamber according to one aspect of an embodiment.

[0023] FIGS. 5A - 5D are plan views of various exemplary implementations of a flow restrictor according to aspects of embodiments.

[0024] FIG. 6 is a partially cutaway view of a portion of a system for limiting target material leakage in an EUV production system according to one aspect of an embodiment.

[0025] Further features and advantages of the invention, as well as the structure and operation of various embodiments of the invention, are described in detail below with reference to the accompanying drawings. It is noted that the invention is not limited to the specific embodiments described herein. Such embodiments are presented herein for illustrative purposes only. Additional embodiments will be apparent to persons skilled in the relevant art based on the teachings contained herein.

DETAILED DESCRIPTION

[0026] Various embodiments are now described with reference to the drawings, wherein like reference numerals are used to refer to like elements throughout. In the following description, for purposes of explanation, numerous specific details are set forth in order to promote a thorough understanding of one or more embodiments. It may be evident in some or all instances, however, that any embodiment described below can be practiced without adopting the specific design details described below. In other instances, well-known structures and devices are shown in block diagram form in order to facilitate description of one or more embodiments. The following presents is not an extensive overview of all contemplated embodiments, and is not intended to identify key or critical elements of all embodiments nor delineate the scope of any or all embodiments.

[0027] With initial reference to FIG. 1, there is shown a simplified, schematic, sectional view of selected portions of one example of an EUV photolithography apparatus, generally designated 10". The apparatus 10" may be used, for example, to expose a substrate 11 such as a resist coated wafer with a patterned beam of EUV light. For the apparatus 10", an exposure device 12" utilizing EUV light, (e.g., an integrated circuit lithography tool such as a stepper, scanner, step and scan system, direct write system, device using a contact and/or proximity mask, etc.), may be provided having one or more optics 13a, 13b, for example, to illuminate a patterning optic 13 c with a beam of EUV light, such as a reticle, to produce a patterned beam, and one or more reduction projection optic(s) 13 d, 13 e, for projecting the patterned beam onto the substrate 11. A mechanical assembly (not shown) may be provided for generating a controlled relative movement between the substrate 11 and patterning means 13 c. As further shown in FIG. 1, the apparatus 10" may include an EUV light source 20" including an EUV light radiator 22 emitting EUV light in a chamber 26" that is reflected by optic 24 along a path into the exposure device 12" to irradiate the substrate l l.The illumination system may include various types of optical components, such as refractive, reflective, electromagnetic, electrostatic or other types of optical components, or any combination thereof, for directing, shaping, or controlling radiation.

[0028] As used herein, the term“optic” and its derivatives is meant to be broadly construed to include, and not necessarily be limited to, one or more components which reflect and/or transmit and/or operate on incident light, and includes, but is not limited to, one or more lenses, windows, filters, prisms, grisms, gratings, transmission fibers, etalons, diffusers, homogenizers, detectors and other instrument components, apertures, axicons and mirrors including multi-layer mirrors, near-normal incidence mirrors, grazing incidence mirrors, specular reflectors, diffuse reflectors and combinations thereof. Moreover, unless otherwise specified, neither the term“optic” nor its derivatives, as used herein, are meant to be limited to components which operate solely or to advantage within one or more specific wavelength range(s) such as at the EUV output light wavelength, the irradiation laser wavelength, a wavelength suitable for metrology or any other specific wavelength. [0029] FIG. 1A illustrates a specific example of an apparatus 10 including an EUV light source 20 having an LPP EUV light radiator. As shown, the apparatus 10 may include a system 21 for generating a train of light pulses and delivering the light pulses into a light source chamber 26. For the apparatus 10, the light pulses may travel along one or more beam paths from the system 21 and into the chamber 26 to illuminate source material at an irradiation region 48 to produce an EUV light output for substrate exposure in the exposure device 12.

[0030] Suitable lasers for use in the system 21 shown in FIG. 1 A, may include a pulsed laser device, e.g., a pulsed gas discharge CO2 laser device producing radiation at 9.3 pm or 10.6 pm, e.g., with DC or RF excitation, operating at relatively high power, e.g., 10 kW or higher and high pulse repetition rate, e.g., 50 kHz or more. In one particular implementation, the laser may be an axial-flow RF-pumped CO2 laser having an oscillator-amplifier configuration (e.g., master oscillator/power amplifier (MOPA) or power oscillator/power amplifier (POPA)) with multiple stages of amplification and having a seed pulse that is initiated by a Q-switched oscillator with relatively low energy and high repetition rate, e.g., capable of 100 kHz operation. From the oscillator, the laser pulse may then be amplified, shaped and/or focused before reaching the irradiation region 48. Continuously pumped CO2 amplifiers may be used for the laser system 21. Alternatively, the laser may be configured as a so-called“self targeting” laser system in which the droplet serves as one mirror of the optical cavity.

[0031] Depending on the application, other types of lasers may also be suitable, e.g., an excimer or molecular fluorine laser operating at high power and high pulse repetition rate. Other examples include, a solid state laser, e.g., having a fiber, rod, slab, or disk-shaped active media, other laser architectures having one or more chambers, e.g., an oscillator chamber and one or more amplifying chambers (with the amplifying chambers in parallel or in series), a master oscillator/power oscillator (MOPO) arrangement, a master oscillator/power ring amplifier (MOPRA) arrangement, or a solid state laser that seeds one or more excimer, molecular fluorine or CO2 amplifier or oscillator chambers, may be suitable. Other designs may be suitable.

[0032] In some instances, a source material may first be irradiated by a pre-pulse and thereafter irradiated by a main pulse. Pre -pulse and main pulse seeds may be generated by a single oscillator or two separate oscillators. In some setups, one or more common amplifiers may be used to amplify both the pre-pulse seed and main pulse seed. For other arrangements, separate amplifiers may be used to amplify the pre-pulse and main pulse seeds.

[0033] FIG. 1 A also shows that the apparatus 10 may include a beam conditioning unit

50 having one or more optics for beam conditioning such as expanding, steering, and/or focusing the beam between the laser source system 21 and irradiation site 48. For example, a steering system, which may include one or more mirrors, prisms, lenses, etc., may be provided and arranged to steer the laser focal spot to different locations in the chamber 26. For example, the steering system may include a first flat mirror mounted on a tip-tilt actuator which may move the first mirror independently in two dimensions, and a second flat mirror mounted on a tip-tilt actuator which may move the second mirror independently in two dimensions. With this arrangement, the steering system may controllably move the focal spot in directions substantially orthogonal to the direction of beam propagation (beam axis).

[0034] The beam conditioning unit 50 may include a focusing assembly to focus the beam to the irradiation site 48 and adjust the position of the focal spot along the beam axis. For the focusing assembly, an optic, such as a focusing lens or mirror, may be used that is coupled to an actuator for movement in a direction along the beam axis to move the focal spot along the beam axis.

[0035] As further shown in FIG. 1 A, the EUV light source 20 may also include a source material delivery system 90, e.g., delivering target or source material, such as tin droplets, into the interior of chamber 26 to an irradiation region or primary focus 48, where the droplets will interact with light pulses from the system 21, to ultimately produce plasma and generate an EUV emission to expose a substrate such as a resist coated wafer in the exposure device 12. More details regarding various droplet dispenser configurations and their relative advantages may be found for example in U.S. Pat. No. 7,872,245, issued on January 18, 2011, titled “Systems and Methods for Target Material Delivery in a Laser Produced Plasma EUV Light Source”, U.S. Pat. No. 7,405,416, issued on July 29, 2008, titled“Method and Apparatus For EUV Plasma Source Target Delivery”, and U.S. Pat. No. 7,372,056, issued on May 13, 2008, titled“LPP EUV Plasma Source Material Target Delivery System”, the contents of each of which are hereby incorporated by reference in their entirety.

[0036] The source material for producing an EUV light output for substrate exposure may include, but is not necessarily limited to, a material that includes tin, lithium, xenon or combinations thereof. The EUV emitting element, e.g., tin, lithium, xenon, etc., may be in the form of liquid droplets and/or solid particles contained within liquid droplets. For example, the element tin may be used as pure tin, as a tin compound, e.g., SnBr₄, SnBr2, SnEL, as a tin alloy, e.g., tin-gallium alloys, tin-indium alloys, tin-indium-gallium alloys, or a combination thereof. Depending on the material used, the source material may be presented to the irradiation region at various temperatures including room temperature or near room temperature (e.g., tin alloys, SnBr₄), at an elevated temperature, (e.g., pure tin) or at temperatures below room temperature, (e.g., SnEB), and in some cases, can be relatively volatile, e.g., SnBr₄.

[0037] Continuing with reference to FIG. 1A, the apparatus 10 may also include an

EUV controller 60, which may also include a drive laser control system 65 for controlling devices in the system 21 to thereby generate light pulses for delivery into the chamber 26, and/or for controlling movement of optics in the beam conditioning unit 50. The apparatus 10 may also include a droplet position detection system which may include one or more droplet imagers 70 that provide an output indicative of the position of one or more droplets, e.g., relative to the irradiation region 48. The imager(s) 70 may provide this output to a droplet position detection feedback system 62, which can, e.g., compute a droplet position and trajectory, from which a droplet error can be computed, e.g., on a droplet-by-droplet basis, or on average. The droplet error may then be provided as an input to the controller 60, which can, for example, provide a position, direction and/or timing correction signal to the system 21 to control laser trigger timing and/or to control movement of optics in the beam conditioning unit 50, e.g., to change the location and/or focal power of the light pulses being delivered to the irradiation region 48 in the chamber 26. Also for the EUV light source 20, the source material delivery system 90 may have a control system operable in response to a signal (which in some implementations may include the droplet error described above, or some quantity derived therefrom) from the controller 60, to e.g., modify the release point, initial droplet stream direction, droplet release timing and/or droplet modulation to correct for errors in the droplets arriving at the desired irradiation region 48.

[0038] Continuing with FIG. 1A, the apparatus 10 may also include an optic 24" such as a near-normal incidence collector mirror having a reflective surface in the form of a prolate spheroid (i.e., an ellipse rotated about its major axis) having, e.g., a graded multi-layer coating with alternating layers of molybdenum and silicon, and in some cases, one or more high temperature diffusion barrier layers, smoothing layers, capping layers and/or etch stop layers. FIG. 1A shows that the optic 24" may be formed with an aperture to allow the light pulses generated by the system 21 to pass through and reach the irradiation region 48. As shown, the optic 24" may be, e.g., a prolate spheroid mirror that has a first or primary focus within or near the irradiation region 48 and a second focus at a so-called intermediate region 40, where the EUV light may be output from the EUV light source 20 and input to an exposure device 12 utilizing EUV light, e.g., an integrated circuit lithography tool. It is to be appreciated that other optics may be used in place of the prolate spheroid mirror for collecting and directing light to an intermediate location for subsequent delivery to a device utilizing EUV light. [0039] A buffer gas such as hydrogen, helium, argon or combinations thereof, may be introduced into, replenished and/or removed from the chamber 26. The buffer gas may be present in the chamber 26 during plasma discharge and may act to slow plasma created ions to reduce optic degradation and/or increase plasma efficiency. Alternatively, a magnetic field and/or electric field (not shown) may be used alone, or in combination with a buffer gas, to reduce fast ion damage.

[0040] FIG. 2 illustrates the droplet generation system in more detail. The source material delivery system 90 delivers droplets to an irradiation site / primary focus 48 within chamber 26. A waveform generator 230 provides a drive waveform to an electro-actuatable element in the droplet generator 90 which induces a velocity perturbation into the droplet stream. The waveform generator operates under the control of a controller 250 least partially on the basis of data from a data processing module 252. The data processing module receives data from one or more detectors. In the example shown, the detectors include a camera 254 and a photodiode 256. The droplets are illuminated by one or more lasers 258. In this typical arrangement, the detectors detect / image droplets at a point in the stream where coalescence is expected to have occurred. Also, the detectors and lasers are arranged outside of the vacuum chamber 26 and view the stream through windows in the walls of vacuum chamber 26.

[0041] FIG. 3 illustrates the components of a simplified droplet source 92 in schematic format. As shown there, the droplet source may include a reservoir 94 holding a fluid 96, e.g. molten tin, under pressure. Also shown, the reservoir 94 may be formed with an orifice allowing the pressurized fluid 96 to flow through to a nozzle 95 establishing a stream which subsequently breaks into a plurality of droplets. Droplet source 92 further includes a sub system 98 producing a disturbance in the fluid having an electro-actuatable element that is operably coupled with the fluid 96. The droplet source 92 also includes a filter 100 for preventing contaminants in the fluid 96 from reaching downstream components of the droplet source 92 and contaminating them. The droplet source 92 also includes a heater block 104 for maintaining the components of the droplet generator positioned in a cavity defined by the heater block 104 at a temperature sufficiently high to keep the fluid 96 in its molten state.

[0042] As described above, the fluid 96 in the reservoir 94 is pressurized by connection to a gas source 106. The gas is supplied to the reservoir 94 through a valve 108. As described above, the valve 108 is a three part valve which is capable of venting the reservoir 94 to a holding tank 110 if it is determined that the fluid 96 is leaking. The various disadvantages and limitations of a leakage control system such as that described are set forth above. [0043] FIG. 4 shows improved system for preventing leakage of droplet source 92 from excessively contaminating an EUV chamber. As can be seen, the arrangement of FIG. 4 includes a flow restrictor 200 positioned in the conduit that carries the fluid 96 from the reservoir 94 through the droplet source 92. In the arrangement shown, the flow restrictor 200 is positioned upstream of the filter 100. The function of the flow restrictor 200 can be combined with that of the seal for the filter 100. In other words, the flow restrictor 200 can be in the form of a metal disk that can also serve as a sealing gasket.

[0044] FIGS. 5 A - 5D are plan views of various exemplary implementations of the flow restrictor 200 according to aspects of embodiments. FIG. 5A shows a flow restrictor 200 configured as a disk 210 with a single orifice 220. FIG. 5B shows a flow restrictor 200 configured as a disk 210 with a group 230 of orifices. FIG. 5C shows a flow restrictor 200 configured as an annular disk 240 holding a porous membrane 250. FIG. 5D shows a flow restrictor 200 configured as disk 210 with an orifice 220 partially obstructed by a pin 260. The pin 260 may be a tapered pin and the orifice 220 may be surrounded by a cylindrical element such as a short tube which may be tapered so as to be conical so that the tapered pin and conical tube together define a throttle. It will be clear to one having ordinary skill in the art that other configurations could work as well.

[0045] The flow restrictor 200 is designed to allow adequate tin flow to fill the droplet generator nozzle volume without impacting jetting performance, but it must be restrictive enough to limit fluid flow when one of the elements of the droplet generator downstream fails. In addition, the flow restrictor 200 is configured to enable sufficient gas flow to evacuate the volume of a new droplet generator nozzle and filter when the droplet generator is heated up for the first time. If the conductance is too small, then contaminants, such as water vapor, may not be removed and may react with tin and form particles that can clog the nozzle. For example, in the case of a flow restrictor configured as in FIG. 5A and a droplet generator operating at 8,000 PSI, the orifice diameter may be in the range of about 65pm to about 75 pm. In the case of molten tin as the fluid 26, its flow through a broken capillary (having internal diameter of 500 pm) is slow enough to decrease without escaping the droplet generator heater block cavity. This flow rate also offers a relatively long time, on the order of 100-180 seconds, to depressurize without the tin level in the cavity reaching a hole from which it can spill.

[0046] In various implementations the flow restrictor 200 is able to survive a high differential pressure that would be applied across the flow restrictor when one of the elements of the droplet generator downstream fails. This may be achieved by the proper design and material selection of the flow restrictor 200. The material must be soft enough to be deformed by the fittings to form a seal, yet it must have enough strength to handle the maximum applied pressure. The material may be a metal such as unannealed tantalum, tantalum tungsten alloy, annealed molybdenum, molybdenum-rhenium alloy, and annealed rhenium or a refractory material.

[0047] FIG. 6 is a partially cut away view showing details of the positioning of a flow restrictor 200 also serving as a seal between a portion of a conduit portion 600 connected to the reservoir 94 (not shown) and a conduit portion 610 connected to the filter 100 (not shown). As can be seen, the flow restrictor 200 restricts the conduit at the interface and also serves as a sealing member between the two conduit portions.

[0048] Referring again to FIG.4, also shown therein is a fluid level sensor 300 designed to detect whether fluid is leaking within the droplet source 92 . The fluid level sensor 300 may be any appropriate sensor for detecting the fluid level in the base of the droplet generator. In the case where the fluid 96 is electrically conductive, as it would be for molten tin, the fluid level sensor 300 may be, for example, a conductivity type sensor or an electrical load detector. The fluid level sensor 300 develops an output signal indicated by the arrow A which can be supplied to, for example, data processing module 252 ((FIG. 2) to depressurize the droplet generator and to shut down operation of the source when the fluid level sensor 300 determines that fluid has accumulated in the droplet generator.

[0049] As shown, the fluid level sensor 300 may be configured as a tin leak sensor in the droplet generator heater block cavity. It may be configured as a contact supplied with a voltage and supported in a position spaced away from the conductive metal chassis of the droplet source 92 by, for example, an insulator block 310. If the tin level rises, the liquid tin will complete a circuit between the contact 300 and the chassis, thus shorting the circuit and showing 0V on the return signal.

[0050] The present invention has been described above with the aid of functional building blocks illustrating the implementation of specified functions and relationships thereof. The boundaries of these functional building blocks have been arbitrarily defined herein for the convenience of the description. Alternate boundaries can be defined so long as the specified functions and relationships thereof are appropriately performed.

[0051] The foregoing description of the specific embodiments will so fully reveal the general nature of the invention that others can, by applying knowledge within the skill of the art, readily modify and/or adapt for various applications such specific embodiments, without undue experimentation, without departing from the general concept of the present invention. Therefore, such adaptations and modifications are intended to be within the meaning and range of equivalents of the disclosed embodiments, based on the teaching and guidance presented herein. It is to be understood that the phraseology or terminology herein is for the purpose of description and not of limitation, such that the terminology or phraseology of the present specification is to be interpreted by the skilled artisan in light of the teachings and guidance. The breadth and scope of the present invention should not be limited by any of the above- described exemplary embodiments, but should be defined only in accordance with the following claims and their equivalents. Other aspects of the invention are set out in the following numbered clauses.

1. Apparatus comprising:

a vacuum chamber;

an optical element positioned within the vacuum chamber, the optical element having a primary focus within the vacuum chamber; and

a target material dispenser arranged to dispense target material to an irradiation site at the primary focus in the vacuum chamber, the target material dispenser comprising a target material reservoir, a nozzle, a conduit for conveying target material from the target material reservoir to the nozzle, and a flow restrictor positioned in the conduit for restricting a flow of target material through the conduit.

2. Apparatus as in clause 1 wherein the flow restrictor is configured as a seal.

3. Apparatus as in clause 1 wherein the flow restrictor is configured as a disk.

4. Apparatus as in clause 3 wherein the disk is configured as a seal.

5. Apparatus as in clause 3 or 4 wherein the disk comprises a metal.

6. Apparatus as in clause 5 wherein the disk comprises unannealed tantalum.

7. Apparatus as in clause 5 wherein the metal disk comprises a tantalum-tungsten alloy.

8. Apparatus as in clause 5 wherein the disk comprises annealed molybdenum.

9. Apparatus as in clause 5 wherein the disk comprises a molybdenum-rhenium alloy.

10. Apparatus as in clause 5 wherein the disk comprises annealed rhenium.

11. Apparatus as in clause 1 wherein the flow restrictor is configured as a disk with a single orifice.

12. Apparatus as in clause 1 wherein the flow restrictor is configured as a disk with a plurality of orifices.

13. Apparatus as in clause 1 wherein the flow restrictor is configured as an annular disk holding a porous membrane.

14. Apparatus as in clause 1 wherein the flow restrictor is configured as a disk with an orifice partially obstructed by a pin. 15. Apparatus as in clause 14 wherein the pin is tapered.

16. Apparatus as in clause 15 wherein the flow restrictor comprises an orifice surrounded by a cylindrical element.

17. Apparatus as in clause 16 wherein the cylindrical element comprises a tube.

18. Apparatus as in clause 17 wherein the tube is tapered so as to be conical.

19. Apparatus as in clause 1 further comprising a liquid level sensor arranged to sense an accumulation of target material in the target material dispenser.

20. Apparatus as in clause 19 further wherein the liquid level sensor comprises a conduction probe.

21. Apparatus as in clause 19 wherein the liquid level sensor comprises an electrical load detector.

22. Apparatus as in clause 19 wherein the target material dispenser further comprises a heater block and the liquid level sensor is located in the heater block.

23. Apparatus comprising:

a vacuum chamber;

a target material dispenser arranged to dispense target material to an irradiation site at the primary focus in the vacuum chamber, the target material dispenser comprising a target material reservoir, a filter, a fluid conduit between the target material reservoir and the filter and establishing fluid communication between the target material reservoir and the filter, and a flow restrictor positioned in the conduit for restricting a flow of target material through the conduit.

24. Apparatus as in clause 23 wherein the flow restrictor is configured as a seal.

25. Apparatus as in clause 23 wherein the flow restrictor is configured as a disk.

26. Apparatus as in clause 25 wherein the disk is configured as a seal.

27. Apparatus as in clause 25 or 26 wherein the disk comprises a metal.

28. Apparatus as in clause 27 wherein the disk comprises unannealed tantalum.

29. Apparatus as in clause 27 wherein the metal disk comprises a tantalum-tungsten alloy.

30. Apparatus as in clause 27 wherein the disk comprises annealed molybdenum.

31. Apparatus as in clause 27 wherein the disk comprises a molybdenum-rhenium alloy.

32. Apparatus as in clause 27 wherein the disk comprises annealed rhenium.

33. Apparatus as in clause 23 wherein the flow restrictor is configured as a disk with a single orifice. 34. Apparatus as in clause 23 wherein the flow restrictor is configured as a disk with a plurality of orifices.

35. Apparatus as in clause 23 wherein the flow restrictor is configured as an annular disk holding a porous membrane.

36. Apparatus as in clause 23 wherein the flow restrictor is configured as a disk with an orifice partially obstructed by a pin.

37. Apparatus as in clause 36 wherein the pin is tapered.

38. Apparatus as in clause 23 wherein the flow restrictor comprises an orifice surrounded by a cylindrical element.

39. Apparatus as in clause 38 wherein the cylindrical element comprises a tube.

40. Apparatus as in clause 39 wherein the tube is tapered so as to be conical.

41. Apparatus as in clause 23 further comprising a liquid level sensor arranged to sense a level of target material in the target material reservoir.

42. Apparatus as in clause 41 further wherein the liquid level sensor comprises a conduction probe.

43. Apparatus as in clause 41 wherein the liquid level sensor comprises an electrical load detector.

44. Apparatus as in clause 41 wherein the target material dispenser further comprises a heater block and the liquid level sensor is located in the heater block.

45. A target material dispenser for dispensing EUV target material, the target material dispenser comprising:

a target material reservoir;

a nozzle;

a conduit for conveying target material from the target material reservoir to the nozzle; and a flow restrictor positioned in the conduit for restricting a flow of target material through the conduit

46. A target material dispenser as in clause 45 wherein the flow restrictor is configured as a seal.

47. A target material dispenser as in clause 45 wherein the flow restrictor is configured as a metal disk.

48. Apparatus as in clause 45 wherein the disk is configured as a seal.

49. Apparatus as in clause 47 or 48 wherein the disk comprises a metal.

50. Apparatus as in clause 49 wherein the disk comprises unannealed tantalum.

51. Apparatus as in clause 49 wherein the metal disk comprises a tantalum-tungsten alloy. 52. Apparatus as in clause 49 wherein the disk comprises annealed molybdenum.

53. Apparatus as in clause 49 wherein the disk comprises a molybdenum-rhenium alloy.

54. Apparatus as in clause 49 wherein the disk comprises annealed rhenium.

55. Apparatus as in clause 45 wherein the flow restrictor is configured as a disk with a single orifice.

56. Apparatus as in clause 45 wherein the flow restrictor is configured as a disk with a plurality of orifices.

57. Apparatus as in clause 45 wherein the flow restrictor is configured as an annular disk holding a porous membrane.

58. Apparatus as in clause 45 wherein the flow restrictor is configured as a disk with an orifice partially obstructed by a pin.

59. Apparatus as in clause 58 wherein the pin is tapered.

60. Apparatus as in clause 45 wherein the flow restrictor comprises an orifice surrounded by a cylindrical element.

61. Apparatus as in clause 45 wherein the cylindrical element comprises a tube.

62. Apparatus as in clause 61 wherein the tube is tapered so as to be conical.

63. Apparatus as in clause 45 wherein the disk is configured as a seal.

64. Apparatus as in clause 45 wherein the disk comprises a metal.

65. Apparatus as in clause 64 wherein the disk comprises unannealed tantalum.

66. Apparatus as in clause 64 wherein the metal disk comprises a tantalum-tungsten alloy.

67. Apparatus as in clause 64 wherein the disk comprises annealed molybdenum.

68. Apparatus as in clause 64 wherein the disk comprises a molybdenum-rhenium alloy.

69. Apparatus as in clause 64 wherein the disk comprises annealed rhenium.

70. A target material dispenser as in clause 45 further comprising a liquid level sensor arranged to sense a level of target material in the target material reservoir.

71. A target material dispenser as in clause 70 further wherein the liquid level sensor comprises a conduction probe.

72. A target material dispenser as in clause 70 wherein the liquid level sensor comprises an electrical load detector.

73. A target material dispenser as in clause 70 wherein the target material dispenser further comprises a heater block and the liquid level sensor is located in the heater block.

74. A target material dispenser as in clause 45 wherein the target material dispenser further comprises a filter between the target material reservoir and the nozzle and wherein the flow restrictor is positioned in the conduit between the target material reservoir and the filter

Claims

CLAIMS What is claimed is:

1. Apparatus comprising:

a vacuum chamber;

a target material dispenser arranged to dispense target material to an irradiation site at the primary focus in the vacuum chamber, the target material dispenser comprising a target material reservoir, a dispenser component, a conduit for conveying target material from the target material reservoir to the dispenser component, and a flow restrictor positioned in the conduit for restricting a flow of target material through the conduit, the dispenser component being at least one of a nozzle and a filter.

2. Apparatus as claimed in claim 1 wherein the flow restrictor is configured as a seal.

3. Apparatus as claimed in claim 1 wherein the flow restrictor is configured as a disk.

4. Apparatus as claimed in claim 3 wherein the disk is configured as a seal.

5. Apparatus as claimed in claim 3 wherein the disk comprises a metal.

6. Apparatus as claimed in claim 5 wherein the disk comprises unannealed tantalum or a tantalum-tungsten alloy.

7. Apparatus as claimed in claim 5 wherein the disk comprises annealed molybdenum, annealed rhenium or a molybdenum-rhenium alloy.

8. Apparatus as claimed in claim 1 wherein the flow restrictor is configured as a disk with a single orifice.

9. Apparatus as claimed in claim 1 wherein the flow restrictor is configured as a disk with a plurality of orifices.

10. Apparatus as claimed in claim 1 wherein the flow restrictor is configured as an annular disk holding a porous membrane.

11. Apparatus as claimed in claim 1 wherein the flow restrictor is configured as a disk with an orifice partially obstructed by a pin.

12. Apparatus as claimed in claim 11 wherein the pin is tapered.

13. Apparatus as claimed in claim 11 wherein the flow restrictor comprises an orifice surrounded by a tube.

14. Apparatus as claimed in claim 13 wherein the tube is tapered so as to be conical.

15. Apparatus as claimed in claim 1 further comprising a liquid level sensor arranged to sense an accumulation of target material in the target material dispenser.

16. Apparatus as claimed in claim 15 wherein the liquid level sensor comprises a conduction probe or an electrical load detector.

17. Apparatus as claimed in claim 15 wherein the target material dispenser further comprises a heater block and the liquid level sensor is located in the heater block.

18. Apparatus as claimed in claim 1 wherein the dispenser component comprises the filter disposed between the flow restrictor and the nozzle.

19. Apparatus as claimed in claim 1 wherein the dispenser component comprises the nozzle.

20. A target material dispenser for dispensing EUV target material, the target material dispenser comprising:

a target material reservoir;

a nozzle;

a conduit for conveying target material from the target material reservoir to the nozzle; and

a flow restrictor positioned in the conduit for restricting a flow of target material through the conduit

21. A target material dispenser as claimed in claim 20 wherein the flow restrictor is configured as a seal.

22. A target material dispenser as claimed in claim 20 wherein the flow restrictor is configured as a metal disk.

23. Apparatus as claimed in claim 20 wherein the disk is configured as a seal.

24. Apparatus as claimed in claim 22 wherein the disk comprises a metal.

25. Apparatus as claimed in claim 22 wherein the disk comprises tantalum or a tantalum-tungsten alloy.

26. Apparatus as claimed in claim 22 wherein the disk comprises annealed molybdenum, annealed rhenium or a molybdenum-rhenium alloy.

27. Apparatus as claimed in claim 20 wherein the flow restrictor is configured as a disk with a plurality of orifices.

28. Apparatus as claimed in claim 20 wherein the flow restrictor is configured as an annular disk holding a porous membrane.

29. Apparatus as claimed in claim 20 wherein the flow restrictor is configured as a disk with an orifice partially obstructed by a tapered pin.

30. Apparatus as claimed in claim 20 wherein the flow restrictor comprises an orifice surrounded by a tapered tube.

31. A target material dispenser as claimed in claim 20 further comprising a liquid level sensor arranged to sense a level of target material in the target material reservoir.

32. A target material dispenser as claimed in claim 31 further wherein the liquid level sensor comprises a conduction probe or an electrical load detector.

33. A target material dispenser as claimed in claim 31 wherein the target material dispenser further comprises a heater block and the liquid level sensor is located in the heater block.

34. A target material dispenser as claimed in claim 20 wherein the target material dispenser further comprises a filter between the target material reservoir and the nozzle and wherein the flow restrictor is positioned in the conduit between the target material reservoir and the filter.