WO2019185370A1

WO2019185370A1 - Apparatus for and method of monitoring and controlling droplet generator performance

Info

Publication number: WO2019185370A1
Application number: PCT/EP2019/056528
Authority: WO
Inventors: Theodorus Wilhelmus DRIESSEN; Alexander Igorevich ERSHOV; Bob Rollinger; Georgiy O. Vaschenko; Koen Gerhardus WINKELS; Dietmar Uwe Herbert TREES
Original assignee: Asml Netherlands B.V.
Priority date: 2018-03-28
Filing date: 2019-03-15
Publication date: 2019-10-03
Also published as: NL2022747A; JP2021516775A; JP2024045309A; EP3804474A1; JP7428654B2; KR20200135798A; CN111919516A

Abstract

Apparatus for and method of controlling formation of droplets used to generate EUV radiation that comprise an arrangement producing a laser beam directed to an irradiation region and a droplet source. The droplet source includes a fluid exiting a nozzle and a sub-system having an electro-actuatable element producing a disturbance in the fluid. The droplet source produces a stream that breaks down into droplets that in turn coalesce into larger droplets as they progress towards the irradiation region. The process is controlled by observing the stream at a point where droplets in the stream have not fully coalesced.

Description

APPARATUS FOR AND METHOD OF MONITORING AND

CONTROLLING DROPLET GENERATOR PERFORMANCE

CROSS-REFERENCE TO RELATED APPLICATIONS

[00001] This application claims priority of US application 62/648,969 which was filed on March 28, 2018 and which is incorporated herein in its entirety by reference.

FIELD

[00002] 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 source or target material. In one application, the EUV light may be collected and used in a photolithography process to produce semiconductor integrated circuits.

BACKGROUND

[00003] 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.

[00004] Methods to produce EUV light include, but are not necessarily limited to, converting a source material into a plasma state that has a chemical element with an emission line in the EUV range. These elements can include, but are not limited to, xenon, lithium and tin.

[00005] In one such method, often termed laser produced plasma (“LPP”), the desired plasma can be produced by irradiating a source material, for example, in the form of a droplet, stream, or wire, with a laser beam. In another method, often termed discharge produced plasma (“DPP”), the 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.

[00006] One technique for generating droplets involves melting a target material, also sometimes referred to as a source material, such as tin and then forcing it under high pressure through a relatively small diameter orifice, such as an orifice having a diameter of about 0.1 pm to about 30 mih, 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 commonly known as the Rayleigh-Plateau instability. Naturally occurring instabilities, e.g. thermal noise or vortex shedding, in the stream exiting the orifice, will cause the stream to break up into droplets. These droplets may have varying velocities and may combine with each other to coalesce into larger droplets.

[00007] In the EUV generation processes under consideration here, it is desirable to control the break up / coalescence process. For example, in order to synchronize the droplets with the optical pulses of an LPP drive laser, a repetitive disturbance with an amplitude exceeding that of random noise may be applied to the continuous laminar fluid jet that emanates from the orifice. By applying a disturbance at the same frequency (or its higher harmonics) as the repetition rate of the pulsed laser, the droplets are synchronized with the laser pulses. For example, the disturbance may be applied to the stream by coupling an electro-actuatable element (such as a piezoelectric material) to the stream and driving the electro-actuatable element with a periodic waveform. In one embodiment, the electro-actuatable element will contract and expand in diameter (on the order of nanometers). This change in dimension is mechanically coupled to a capillary that undergoes a corresponding contraction and expansion of diameter. This volume displacement causes acoustic and elastic waves in the capillary tube that ends in the orifice. The target material in the orifice is then accelerated periodically by the acoustic waves. Providing the widely spaced droplets at the frequency of the drive laser happens in a frequency range that is far below the natural Rayleigh breakup frequency of the fluid microjets. The natural breakup frequency of the fluid jet is in the range between about 3 and about 15 MHz, whereas the drive laser operation is expected in the range between about 50 and about 160 kHz. This means that to obtain the desired final droplets up to 200 small microdroplets have to be merged into the periodic droplet stream consisting of droplets much larger than the orifice diameter.

[00008] As used herein, the term“electro-actuatable element” and its derivatives, means a material or structure which undergoes a dimensional change when subjected to a voltage, electric field, magnetic field, or combinations thereof and includes, but is not limited to, piezoelectric materials, electro strictive materials, and magnetostrictive materials. Apparatus for and methods of using an electro-actuatable element to control a droplet stream are disclosed, for example, in U.S. Patent Application Publication No. 2009/0014668 Al, titled“Faser Produced Plasma EUV Light Source Having a Droplet Stream Produced Using a Modulated Disturbance Wave” and published January 15, 2009, and U.S. Patent No. 8,513,629, titled“Droplet Generator with Actuator Induced Nozzle Cleaning” and issued August 20, 2013, both of which are hereby incorporated by reference in their entirety.

[00009] The task of the droplet generator is thus to place droplets in the primary focus where they will be used as target material for the EUV production. The droplets must arrive at primary focus within certain spatial and temporal stability criteria, that is, with position and timing that is repeatable within acceptable margins. They must also arrive at a given frequency and velocity. Furthermore, the droplets must be fully coalesced, meaning that the droplets must be monodisperse (of uniform size) and arrive at the given drive frequency. For example, the droplet stream should be free of“satellite” droplets, that is, smaller droplets of target material that have failed to coalesce into a main droplet. Meeting these criteria is complicated by the fact that for small orifices and large pressures, it may be necessary to merge around 200

microdroplets using the electro-actuable element drive form. The operation window is usually very small, making the system sensitive to variations in performance such as performance changes over time. For example, when the performance of the droplet generator changes, it may produce droplets that are not fully coalesced by the time they reach the primary focus. Eventually the droplet generator performance will deteriorate to the point that the droplet generator must be taken offline for maintenance or replacement.

[00010] There is thus a need to be able to control droplet generation and coalescence in a manner that allows for optimization of these processes.

SUMMARY

[00011] 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.

[00012] It is thus desired to confirm proper operation of the droplet generator by determining whether droplets are fully coalesced by the time they reach the primary focus. This can be accomplished by supplying the EUV source with an optical feedback system that can identify whether or not a certain electrical waveform supplied to the droplet generator results in coalesced droplets at primary focus.

[00013] According to one aspect of an embodiment there is disclosed an apparatus comprising a target material dispenser arranged to provide a stream of target material to an irradiation site in a vacuum chamber, an electro-actuatable element mechanically coupled to the target material dispenser and arranged to induce velocity perturbations in the stream based on a droplet control signal, a detector arranged to observe droplets in the stream at a point in the stream where the droplets have not fully coalesced and to generate a droplet detection signal, a controller arranged to receive the droplet detection signal and generating a waveform generator control signal based at least in part on the droplet detection signal, and a waveform generator electrically coupled to the electro-actuatable element and to the controller for supplying the droplet control signal based at least in part on the waveform generator control signal. The electro-actuatable element may be a piezoelectric element.

[00014] According to another aspect of an embodiment there is disclosed an apparatus comprising a target material dispenser arranged to provide a stream of target material to an irradiation site in a vacuum chamber, an electro-actuatable element mechanically coupled to the target material dispenser and arranged to induce velocity perturbations in the stream based on a droplet control signal, a detector arranged to observe droplets in the stream at a point in the stream where the droplets have not fully coalesced and to generate a droplet detection signal, the detector comprising an illumination source and a light sensitive sensor, the light sensitive sensor comprising at least one optical element arranged in the vacuum chamber, a controller arranged to receive the droplet detection signal and generating a waveform generator control signal based at least in part on the droplet detection signal, and a waveform generator electrically coupled to the electro-actuatable element and to the controller for supplying the droplet control signal based at least in part on the waveform generator control signal. The electro-actuatable element may be a piezoelectric element. The light sensitive sensor may be a camera. The light sensitive sensor may be a photodiode. The light sensitive sensor may be a camera arranged outside of the vacuum chamber, an optical module arranged in the vacuum chamber, and an optical fiber for relaying light from the optical module to the camera. The light sensitive sensor may be a photodiode arranged outside of the vacuum chamber, an optical module arranged in the vacuum chamber, and an optical fiber for relaying light from the optical module to the photodiode. The light sensitive sensor may include an illumination source. The light sensitive sensor may include an illumination source arranged outside of the vacuum chamber, an optical module arranged in the vacuum chamber, and an optical fiber for relaying light from the illumination source to the optical module.

[00015] According to another aspect of an embodiment there is disclosed a method comprising the steps of providing a stream of target material to an irradiation site in a vacuum chamber using a target material dispenser, the target material dispenser comprising an electro- actuatable element arranged to induce velocity perturbations in the stream based on a droplet control signal observing droplets in the stream at a point in the stream where the droplets have not fully coalesced and generating a droplet detection signal, generating a waveform generator control signal based at least in part on the droplet detection signal, and supplying the droplet control signal based at least in part on the waveform generator control signal. The electro- actuatable element may be a piezoelectric element.

[00016] According to another aspect of an embodiment there is disclosed a method of determining and using a transfer function of a droplet generator adapted to deliver a stream of liquid target material to an irradiation region in a system for generating EUV radiation, the method comprising the steps of providing the stream of target material for a plasma generating system from the droplet generator, generating a control signal, applying the control signal an electro-actuatable element mechanically coupled to the droplet generator to introduce a velocity perturbation into the stream, determining a velocity amplitude, determining a transfer function for the droplet generator based at least in part on the velocity amplitude and the control signal, and using the determined transfer function to control the droplet generator.

[00017] According to another aspect of an embodiment there is disclosed a method of controlling a droplet generator adapted to deliver a stream of liquid target material to an irradiation region in a system for generating EUV radiation, the method comprising the steps of providing the stream of target material for a plasma generating system from the droplet generator, generating a control signal, introducing a velocity perturbation into the stream by applying the control signal to an electro-actuatable element mechanically coupled to the droplet generator, observing the stream at a point where droplets in the stream have not fully coalesced, and modifying the control signal based at least in part on results of the observing step. [00018] According to another aspect of an embodiment there is disclosed a method of estimating a coalescence length of a stream of droplets of liquid target material produced by a droplet generator in a system for generating EUV radiation, the method comprising the steps of providing the stream of target material for a plasma generating system from the droplet generator, generating a control signal, introducing a velocity perturbation into the stream by applying the control signal to an electro-actuatable element mechanically coupled to the droplet generator, observing the stream at a point where droplets in the stream have not fully coalesced to produce a droplet signal, and estimating the coalescence length based at least in part by a distance between peaks in the droplet signal. The estimated coalescence length may then be used to control operation of the droplet generator.

[00019] According to another aspect of an embodiment there is disclosed a method of assessing a condition of a droplet generator adapted to deliver a stream of liquid target material to an irradiation region in a system for generating EUV radiation, the method comprising the steps of providing the stream of target material for a plasma generating system from the droplet generator, generating a control signal, introducing a velocity perturbation into the stream by applying the control signal to an electro-actuatable element mechanically coupled to target material in the droplet generator, observing the stream at a point where droplets in the stream have not fully coalesced to produce a droplet signal, and assessing the condition of the droplet generator based on the droplet signal.

[00020] 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 DRAWING

[00021] 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. [00022] FIG. 1 is a simplified schematic view of an EUV light source coupled with an exposure device.

[00023] FIG. 1A is a simplified, schematic diagram of an apparatus including an EUV light source having an LPP EUV light radiator.

[00024] FIG. 2 is a schematic diagram of a droplet generation subsystem for an EUV light source.

[00025] FIGS. 3, 3A-3C, 4, and 5 illustrate several different techniques for coupling one or more electro-actuatable element(s) with a fluid to create a disturbance in a stream exiting an orifice;

[00026] FIG. 6 is a diagram illustrating states of coalescence in a droplet stream.

[00027] FIG. 7 is a graph of a composite waveform such as may be used according to one aspect of an embodiment.

[00028] FIG. 8 is a diagram of a droplet generation system with feedback such as may be used according to one aspect of an embodiment.

[00029] FIG. 9 is a diagram of a droplet generation system with feedback such as may be used according to one aspect of an embodiment.

[00030] FIGS 10A and 10B are diagram illustrating a possible droplet signals according to one aspect of an embodiment.

[00031] FIG. 11 is a flowchart showing a method of estimating coalescence length according to one aspect of an embodiment.

[00032] FIG. 12 is a flowchart showing a method of determining a transfer function for a droplet generator according to one aspect of an embodiment.

[00033] 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

[00034] 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 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.

[00035] Before describing such embodiments in more detail, however, it is instructive to present an example environment in which embodiments of the present invention may be implemented. In the description that follows and in the claims the terms“up,”“down,”“top,” “bottom,”“vertical,”“horizontal,” and like terms may be employed. These terms are intended to show relative orientation only and not any orientation with respect to gravity.

[00036] 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 l3a,b, for example, to illuminate a patterning optic l3c with a beam of EUV light, such as a reticle, to produce a patterned beam, and one or more reduction projection optic(s) l3d, l3e, 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 l3c. 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 11. 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.

[00037] 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, wedges, 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.

[00038] 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 EUV light source 20 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.

[00039] Suitable lasers for use in the system 21 shown in FIG. 1A, may include a pulsed laser device, e.g., a pulsed gas discharge C0₂ 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 C0₂ 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 C0₂ 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.

[00040] 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 C0₂ amplifier or oscillator chambers, may be suitable. Other designs may be suitable.

[00041] 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.

[00042] FIG. 1A 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).

[00043] 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. [00044] As further shown in FIG. 1A, the EUV light source 20 may also include a source material delivery system 90, e.g., delivering 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.

[00045] 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, SnH₄, as a tin alloy, e.g., tin-gallium alloys, tin-indium alloys, tin-indium- gallium alloys, or a combination thereof. Depending on the material used, the source material 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., SnH₄), and in some cases, can be relatively volatile, e.g., SnBr₄.

[00046] 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.

[00047] 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 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.

[00048] 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.

[00049] 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.

[00050] FIG. 3 illustrates the components of a simplified droplet source 92 in schematic format. As shown there, the droplet source 92 may include a reservoir 94 holding a fluid, e.g. molten tin, under pressure. Also shown, the reservoir 94 may be formed with an orifice 98 allowing the pressurized fluid 96 to flow through the orifice establishing a continuous stream 100 which subsequently breaks into a plurality of droplets 102 a, b.

[00051] The system uses one or more lasers to illuminate the droplet stream and one or more detectors (for example, one or more cameras, photodiodes, or some combination of same) to measure the light response of the droplets as they pass the laser beam. In such a design the lasers and detectors are located outside of the vacuum vessel meaning that the laser beam has to be precisely projected onto the droplet stream and the droplet light response has to be imaged by a remote optic. This provides certain limitations in the accuracy and resolution of the

measurements and limits the ways this metrology can be used.

[00052] As mentioned, this system may be used to establish whether droplets are coalescent (or not) near primary focus. If the droplets are not coalescent, then operational parameters of the droplet generator can be adjusted to ensure coalescence. In general, however, this process can be carried out only when the drive laser is turned off, potentially resulting in significant machine downtime. It would be advantageous to be able to tune the droplet generator signal while producing plasma. It would also be advantageous to be able to obtain in line measurements of droplet generator performance which may be used to plan droplet generator maintenance and predict the droplet generator lifetime. It has been shown that the performance of a droplet generator changes over time. A signal that is ideal at the start may result in satellite droplets a few hours later. In such case the drive laser is turned off droplet, and the drive signal optimization is repeated. Typically this is done on a daily basis, causing approximately 1 hour of downtime.

[00053] FIG. 3 shows a possible configuration for a droplet source 92 as part of the droplet generator 90. Droplet source 92 further includes a sub-system producing a disturbance in the fluid having an electro-actuatable element 104 that is operably coupled with the fluid 96 and a signal generator 106 driving the electro-actuatable element 104. FIGS. 3A-3C, 4 and 5 show various ways in which one or more electro-actuatable element(s) may be operably coupled with the fluid to create droplets. Beginning with FIG. 3 A, an arrangement is shown in which the fluid is forced to flow from a reservoir 108 under pressure through a tube 110, e.g., capillary tube, having an inside diameter between about 0.2 mm to about 0.8 mm, and a length of about 10 mm to about 50 mm, creating a continuous stream 112 exiting an orifice 114 of the tube 110 which subsequently breaks up into droplets H6a,b. As shown, an electro-actuatable element 118 may be coupled to the tube. For example, an electro-actuatable element may be coupled to the tube 110 to deflect the tube 110 and disturb the stream 112. FIG. 3B shows a similar arrangement having a reservoir 120, tube 122 and a pair of electro-actuatable elements 124, 126, each coupled to the tube 122 to deflect the tube 122 at a respective frequency. FIG. 3C shows another variation in which a plate 128 is positioned in a reservoir 130 moveable to force fluid through an orifice 132 to create a stream 134 which breaks into droplets l36a, b. As shown, a force may be applied to the plate 128 and one or more electro-actuatable elements 138 may be coupled to the plate to disturb the stream 134. It is to be appreciated that a capillary tube may be used with the embodiment shown in FIG. 3C.

[00054] FIG. 4 shows another variation, in which a fluid is forced to flow from a reservoir 140 under pressure through a tube 142 creating a continuous stream 144, exiting an orifice 146 of the tube 142, which subsequently breaks-up into droplets l48a, b. As shown, an electro- actuatable element 150, e.g., having a ring-shape or cylindrical tube shape, may be positioned to surround a circumference of the tube 142. When driven, the electro-actuatable element 150 may selectively squeeze and/or un-squeeze the tube 142 to disturb the stream 144. It is to be appreciated that two or more electro-actuatable elements may be employed to selectively squeeze the tube 142 at respective frequencies.

[00055] FIG. 5 shows another variation, in which a fluid is forced to flow from a reservoir 140' under pressure through a tube 142' creating a continuous stream 144', exiting an orifice 146' of the tube 142', which subsequently breaks-up into droplets l48a',b'. As shown, an electro- actuatable element l50a, e.g., having a ring-shape, may be positioned to surround a

circumference of the tube 142'. When driven, the electro-actuatable element l50a may selectively squeeze the tube 142' to disturb the stream 144' and produce droplets. FIG. 5 also shows that a second electro-actuatable element l50b, e.g. having a ring-shape, may be positioned to surround a circumference of the tube 142'. When driven, the electro-actuatable element l50b may selectively squeeze the tube 142' to disturb the stream 144' and dislodge contaminants from the orifice 152. For the embodiment shown, electro-actuatable elements l50a and l50b may be driven by the same signal generator or different signal generators may be used. As described further below, waveforms having different waveform amplitude, periodic frequency and/or waveform shape may be used to drive electro-actuatable element l50a to produce droplets for EUV output. The electro-actuatable element produces a disturbance in the fluid which generates droplets having differing initial velocities causing at least some adjacent droplet pairs to coalesce together prior to reaching the irradiation region. The ratio of initial microdroplets to coalesced droplets may be any number, for example, in the range of about 10 droplets to about 500 droplets.

[00056] Control of the breakup / coalescence process thus involves controlling the droplets such that they coalesce sufficiently before reaching the irradiation region and have a frequency corresponding to the pulse rate of the laser being used to irradiate the coalesced droplets. A designer composite waveform made up of the linear superposition of multiple voltage and multiple frequency sinusoidal waveforms may be supplied to electro-actuatable element to control the coalescence process of Rayleigh breakup microdroplets into fully coalesced droplets of a frequency corresponding to the laser pulse rate. The control system may use only separate sine waves, permitting the phase of every spectral component to be individually tuned. The waveform may be defined as a voltage or current signal.

[00057] The on-axis droplet velocity profile is obtained by imaging the droplet stream at fixed location downstream of coalescence and used as feedback to control the droplet generation / coalescence process. As a form of imaging, it is possible to use a light barrier to resolve droplet passage in time and reconstruct the droplet coalescence pattern from this information.

[00058] The coalescence of the microdroplets and subcoalesced droplets is controlled by a periodical electrical drive signal on an electro-actuatable actuator of the droplet generator. This signal is monitored automatically during source operation. Based on the crossing interval and DFC data, the best operation point may be selected. The selected signal is applied to the droplet generator, and the pre-pulse and drive laser are optimized for optimal plasma conditions.

[00059] The use of the designer waveform enables a user to target a specific droplet coalescence length at a user specified frequency using feedback from imaging metrology at a fixed point downstream of the fully coalesced droplet. One form of designer waveform may be comprised of (1) a sine wave at a fundamental frequency that is substantially equal to the laser pulse rate and (2) a set of higher frequency sinusoidal waveforms. All higher frequency waveforms are harmonics of the fundamental frequency, i.e. they are multiples of the fundamental frequency. Use of the designer waveform also permits nozzle transfer function determinations of the on-axis target material stream velocity perturbations/profile which in turn can be used to optimize the parameters of the designer waveform driving the electro-actuatable element.

[00060] The overall droplet coalescence process may be regarded as a succession of multiple subcoalescence steps or regimes evolving as a function of distance from the nozzle.

This is shown in FIG. 6. For example, in a first regime 161, that is, when the target material first exits the orifice or nozzle, the target material is in the form of a velocity-perturbed laminar fluid jet. In a second regime 162, the fluid jet breaks up into a series of microdroplets 164 having varying velocities. In the third regime 163, measured either in time of flight or by distance from the nozzle, the microdroplets coalesce into droplets of an intermediate size, referred to as subcoalesced droplets 165, having varying velocities with respect to one another. In the fourth regime 166 the subcoalesced droplets coalesce into droplets having the desired final size, i.e. fully coalesced droplets 168. The number of subcoalescence steps can vary. The distance from the nozzle exit 169 to the point at which the droplets reach their final coalesced state is the coalescence distance or coalescence length 170. Ideally, the coalescence distance 170 of the droplets is as short as possible. When the droplets have coalesced into bigger droplets, they are less sensitive for source conditions such as hydrogen flow and ion impact.

[00061] As mentioned, if full coalescence is not achieved, then the droplet stream will include smaller droplets referred to as satellite droplets or microsatellites. The presence of satellite droplets can be detected by any one or combination of several methods, for example, the use of a droplet detection module (DDM), crossing interval, DFC, or even through monitoring changes in the EUV signals. Systems and methods for monitoring the droplet stream are disclosed, for example, in U.S. Patent No. 9,241,395, issued January 19, 2016, titled“System and Method for Controlling Droplet Timing in an LPP EUV Light Source”, the entire contents of which are hereby incorporated by reference. Such metrologies are typically used to determine where and when the droplets are in the primary focus and the quality of the plasma, so the probing location is at, or very close to the plasma location, typically between about 30 cm and about 40 cm away from the nozzle exit. It is challenging to detect satellites at this distance, because the metrology to detect these uncoalesced droplets may be, for example, half a meter away at the walls of the source vessel and satellites may be blown outside of the field of view by the vessel flows.

[00062] With sensors that are arranged at a distance, for example, outside the chamber, this translates to trying to observe microdroplets having a size on the order of 4 microns at a distance of about 40 cm away from the nozzle using optical detectors focused on small regions of interest and also tuned to detect the fully coalesced droplets having a size on the order of 27 microns. Also, the prevailing gas flows in the chamber, for example, the transverse“side wind” coming from the cone flow can spread these microsatellites over a large volume. Placing the metrology much closer to the nozzle where droplets have not coalesced permits observation and detection of microsatellites at a point in their evolution where the spread is smaller, making it easier to detect all of them. Measurements at this location enable control and optimization of the coalescence process.

[00063] Some characteristics of an example of a designer waveform will now be explained in conjunction with FIG. 7. The upper waveform in FIG. 7 is the fundamental waveform that will in general have a frequency the same as or otherwise related to the pulse rate of the laser used to vaporize the droplets. In the example the fundamental waveform is a sine wave. The lower waveform in FIG. 7 is the higher frequency waveform that will in general have a frequency that is an integral multiple of the frequency of the fundamental waveform. Any arbitrary periodic wave can be used; in the example the higher frequency waveform is a series of triangular spikes. These two waveforms are superposed to obtain the composite waveform. The waveform for the sub coalescence will be a superposition of harmonics of the sub-coalescence frequency, which may also include the sub coalescence frequency. [00064] One way to enhance measurement of droplet generator performance is to provide a way to observe droplet generation more directly providing detectors arranged in the vacuum chamber to observe droplet generation more closely. These detectors may be used in addition to or replace detectors positioned outside the chamber. These in situ detectors provide real-time high-resolution feedback and control over the operation of the droplet generator. They provide the ability to reduce machine downtime that would otherwise be used to identify parameters resulting in fully coalesced droplets without satellite droplets. The minimization of satellite droplets in turn reduces dose stability errors due to the presence of on-axis satellite droplets and collector lifetime issues due to the presence of on-axis satellite droplets. The in situ detectors can also help reduce unscheduled downtime due to a sudden unexpected need to replace the droplet generator.

[00065] To provide for the possibility of more precise measurements, the metrology may be placed relatively closer to the nozzle exit. The metrology is arranged to sense the droplet pattern at a location between about 0.5cm and about 5 cm after the nozzle exit. At this location the coalescence process is still ongoing, and useful information about the current droplet performance can be extracted from the droplet arrival times at the sensor location. The droplets pass a detector. For example, the detector may be a focused laser curtain, in which case the droplets reflect an amount of light proportional to the droplet cross sectional area. Part of the reflected light is collected by optics and transformed into a time signal, for example, by a high sampling rate photodiode. Another example in cases where the focus is narrow is to use the extinction of the laser curtain when the droplet passes. If this focus location can be controlled in a small 3D volume, the metrology can also be used to sense the position of the droplet stream, providing detailed information for the droplet generator steering system. The amplitude and relative position of the reflection peaks gives quantitative information about the current coalescence process, and about the current performance level of the droplet generator.

[00066] Referring now to FIG. 8, there is shown an electro-actuatable element 200 positioned around a capillary 210 of a nozzle 220. The electro-actuatable element 200 transduces electrical energy from the waveform generator 230 to apply varying pressure to a capillary 210. This introduces a velocity perturbation in the stream 240 of molten target material 240 exiting the nozzle 220. The droplets which are imaged by a camera 250 at an imaging point in a regime where droplets have formed but are not fully coalesced. If a camera is used then the system will also have a light source arranged to illuminate the droplets or to be blocked by individual droplets when they travel across a beam produced by the light source. Imaged herein

encompasses both forming an image of the droplet as well as a mere binary indication of the presence or absence of a droplet. The imaging develops a velocity profile of the droplet stream at an imaging point in a regime where droplets have formed but are not fully coalesced. A control unit 260 uses the imaging data from the camera 250 to generate a feedback signal to control operation of the arbitrary wave generator 230. The control unit 260 can control the relative phase of the low frequency periodic wave and the higher order arbitrary periodic waveform as well as the amplitude of the low frequency periodic wave and the amplitude of the higher order arbitrary periodic waveform based on a control input 265 which may originate from another controller or be based on a user input.

[00067] Another conceptualization of a system in accordance with an aspect of an embodiment is shown in FIG. 9. In the system of FIG. 9 droplet detection is carried out by a laser curtain made up of a laser 300 and a photodiode 310. The light from the laser 300 is conveyed by an optical fiber 302 to inside the chamber 26 where it illuminates the droplet stream at a point near the droplet generator 90 using optics 305. Light reflected from the droplet stream is conveyed by an optical fiber 312 back to the camera 310 by optics 315.

[00068] This optical feedback can be used in a procedure that optimizes the voltage drive signal to be applied to the electro-actuatable element while the plasma production at primary focus continues. As an example, FIG. 10A shows an expected time signal for droplets with a frequency of 500 kHz droplet and two satellite droplets. A fully coalesced droplet pattern would consist of one Gaussian peak 400 per period. The satellites cause smaller Gaussian peaks 410 to be present adjacent to the main 500 kHz Gaussian peaks. The spectral content of the 500 kHz drive signal, which can be, for example, a square wave signal, can be tuned to achieve coalescence.

[00069] The sensor can also be used to determine the transfer function for frequencies below the sub-coalescence frequency. FIG. 10B shows an example in which the droplet velocity of a stream of subcoalesced droplets is modulated with a 50 kHz sinusoid. Curve 420 shows a droplet distribution that is not perturbed at all, curve 430 shows a distribution of droplets travelling at 0.2 m/s that is perturbed with a sinusoidal perturbation at 50 kHz, and curve 440 shows the distribution of droplets traveling at 0.4 m/s perturbed with a sinusoidal perturbation at 50 kHz. By measuring the relative delay times of the droplets at the sensor location, this velocity can be determined, and from there one can calculate the transfer function at 50 kHz. The transfer function is given in m/s per Volt. By also performing this transfer function measurement for other harmonics of 50 kHz, such as 100 kHz and 150 kHz, an optimal waveform can be composed to achieve minimum coalescence distance with the given voltage budget of the signal generator.

[00070] Once sub-coalescence is secured, lower frequencies can be superimposed to force the 500 kHz micro-droplets to move closer together. In this example a 50 kHz signal is used to control the coalescence of the 500 kHz droplets. At the location of the metrology, the

coalescence has not yet happened. (The coalescence process starts at the nozzle, from the orifice exit portions of the fluid stream are already relatively moving towards each other.) However, based on the relative spacing of the peaks one can estimate the coalescence length. A method for doing so is shown in FIG. 11. In step S50 the stream is started. In step S52 the drive signal is applied to the droplet generator. In step S56 the stream is observed upstream of coalescence, that is, upstream of full coalescence. In step S58 the spacing between peaks of the signal generated from observing the stream is determined. In step S60, the coalescence length is estimated based on the determined spacing. This determined coalescence length can then be used, for example, to characterize, control, and/or optimize the operation of the droplet generator.

[00071] One can also determine the velocity amplitude at 50 kHz. Combined with the applied voltage this results in the Transfer Function (m/s/ per V) for the droplet generator at 50 kHz. A method for carrying out this process is shown in FIG. 12. Steps S50, S52, and S56 are as described above. In step S60 to the velocity amplitude of the droplets is determined. In step S64, the applied voltage is determined. These determined values are used in step S66 to determine the transfer function. This determined transfer function can then be used, for example, to

characterize, control, and/or optimize the operation of the droplet generator.

[00072] The transfer function may be defined as the velocity perturbation that is obtained at the nozzle exit per unit applied voltage at a specific frequency. For the considered nozzle transfer function, the signal applied to the electro-actuatable element (characterized by frequency, magnitude, and phase) is the input, while the velocity perturbation as imposed on the liquid jet is the output. Coalescence length varies with the velocity amplitude of the sine components with a frequency below the sub-coalescence frequency. Larger sine amplitude implies an increased velocity perturbation, hence coalescence length decreases.

[00073] The designer waveform may be characterized by several parameters. The exact number of parameters depends on the choice of the higher frequency arbitrary periodic waveform that could have several tuning parameters. Sine voltage, voltage of the higher frequency waveform and relative phase would in general be included among the characterizing parameters. While sine voltage and phase determine coalescence length, as presented above, the voltage of the higher frequency arbitrary periodic waveform controls the velocity jitter of the low frequency droplets. Velocity jitter of droplets results in variations of droplet timing. Typically, droplet timing variations must be limited in order to enable synchronization of the droplets with the laser pulse.

[00074] The in-situ droplet generator metrology allows high-resolution droplet detection at droplet generator exit. To provide high-resolution, the metrology may use, for example, a fiber-optic to deliver the test laser beam and detection signal. The advantages of such metrology include: inline control of the droplet generator so that droplet generator tuning can be carried out during droplet generator operation without downtime. Real time quantitative feedback on droplet generator performance parameters can be used to predict timing of droplet generator

replacement. Also, prevention of on-axis satellite droplets near the plasma improves collector lifetime and dose stability. Since the coalescence process is influenced by the plasma, it is beneficial to be able to adjust the signal with the plasma production in operation. Also, direct control on the coalescence length avoids the need for turning the laser off droplet and to perform new searches for signals, effectively reducing planned and unplanned maintenance downtime.

[00075] While the use of the procedure for as described above was in connection with in- situ metrology, it will be appreciated that the procedure can be used with data gathered by remote metrology, i.e., metrology with light sources and or detectors are located outside of the chamber.

[00076] Instead of a detector such as a dark field light bridge, it will be appreciated that a system that forms an image of the droplet which is captured by a camera could also be used. This would provide more data about the location and characteristics of the droplets.

[00077] 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.

[00078] 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.

[00079] Other aspects of the invention are set out in the following numbered clauses.

1. Apparatus comprising:

a target material dispenser arranged to provide a stream of target material to an irradiation site in a vacuum chamber;

an electro-actuatable element mechanically coupled to the target material dispenser and arranged to induce velocity perturbations in the stream based on a droplet control signal;

a detector arranged to observe droplets in the stream at a point in the stream where the droplets have not fully coalesced and to generate a droplet detection signal;

a controller arranged to receive the droplet detection signal and generating a waveform generator control signal based at least in part on the droplet detection signal; and

a waveform generator electrically coupled to the electro-actuatable element and to the controller for supplying the droplet control signal based at least in part on the waveform generator control signal.

2. Apparatus as in clause 1 wherein the electro-actuatable element is a piezoelectric element.

3. Apparatus comprising: a target material dispenser arranged to provide a stream of target material to an irradiation site in a vacuum chamber;

a detector arranged to observe droplets in the stream at a point in the stream where the droplets have not fully coalesced and to generate a droplet detection signal, the detector comprising an illumination source and a light sensitive sensor, the light sensitive sensor comprising at least one optical element arranged in the vacuum chamber;

4. Apparatus as in clause 3 wherein the electro-actuatable element is a piezoelectric element.

5. Apparatus as in clause 3 wherein the light sensitive sensor comprises a camera.

6. Apparatus as in clause 3 wherein the light sensitive sensor comprises a photodiode.

7. Apparatus as in clause 3 wherein the light sensitive sensor comprises a camera arranged outside the vacuum chamber, an optical module arranged in the vacuum chamber, and an optical fiber for relaying light from the optical module to the camera.

8. Apparatus as in clause 3 wherein the light sensitive sensor comprises a photodiode arranged outside the vacuum chamber, an optical module arranged in the vacuum chamber, and an optical fiber for relaying light from the optical module to the photodiode.

9. Apparatus as in clause 3 wherein the light sensitive sensor comprises an illumination source.

10. Apparatus as in clause 3 wherein the light sensitive sensor comprises an illumination source arranged outside of the vacuum chamber, an optical module arranged in the vacuum chamber, and an optical fiber for relaying light from the illumination source to the optical module.

11. A method comprising the steps of: providing a stream of target material to an irradiation site in a vacuum chamber using a target material dispenser, the target material dispenser comprising an electro-actuatable element arranged to induce velocity perturbations in the stream based on a droplet control signal;

observing droplets in the stream at a point in the stream where the droplets have not fully coalesced and generating a droplet detection signal;

generating a waveform generator control signal based at least in part on the droplet detection signal; and

supplying the droplet control signal based at least in part on the waveform generator control signal.

12. A method as in clause 11 wherein the electro-actuatable element is a piezoelectric element.

13. A method of determining and using a transfer function of a droplet generator adapted to deliver a stream of liquid target material to an irradiation region in a system for generating EUV radiation, the method comprising the steps of:

providing the stream of target material for a plasma generating system from the droplet generator;

generating a control signal;

applying the control signal an electro-actuatable element mechanically coupled to the droplet generator to introduce a velocity perturbation into the stream;

determining a velocity amplitude;

determining a transfer function for the droplet generator based at least in part on the velocity amplitude and the control signal; and

using the determined transfer function to control the droplet generator

14. A method of controlling a droplet generator adapted to deliver a stream of liquid target material to an irradiation region in a system for generating EUV radiation, the method comprising the steps of:

generating a control signal;

introducing a velocity perturbation into the stream by applying the control signal to an electro- actuatable element mechanically coupled to the droplet generator; observing the stream at a point where droplets in the stream have not fully coalesced; and modifying the control signal based at least in part on results of the observing step.

15. A method of estimating a coalescence length of a stream of droplets of liquid target material produced by a droplet generator in a system for generating EUV radiation, the method comprising the steps of:

generating a control signal;

introducing a velocity perturbation into the stream by applying the control signal to an electro- actuatable element mechanically coupled to the droplet generator;

observing the stream at a point where droplets in the stream have not fully coalesced to produce a droplet signal; and

estimating the coalescence length based at least in part by a distance between peaks in the droplet signal.

16. A method as in clause 15 additionally comprising a step after the step of estimating the coalescence length of using the estimated coalescence length to control operation of the droplet generator.

17. A method of assessing a condition of a droplet generator adapted to deliver a stream of liquid target material to an irradiation region in a system for generating EUV radiation, the method comprising the steps of:

generating a control signal;

introducing a velocity perturbation into the stream by applying the control signal to an electro- actuatable element mechanically coupled to target material in the droplet generator;

assessing the condition of the droplet generator based on the droplet signal.

Claims

1. Apparatus comprising:

2. Apparatus as claimed in claim 1 wherein the electro-actuatable element is a piezoelectric element.

3. Apparatus comprising:

a controller arranged to receive the droplet detection signal and generating a waveform generator control signal based at least in part on the droplet detection signal; and a waveform generator electrically coupled to the electro-actuatable element and to the controller for supplying the droplet control signal based at least in part on the waveform generator control signal.

4. Apparatus as claimed in claim 3 wherein the electro-actuatable element is a piezoelectric element.

5. Apparatus as claimed in claim 3 wherein the light sensitive sensor comprises a camera.

6. Apparatus as claimed in claim 3 wherein the light sensitive sensor comprises a photodiode.

7. Apparatus as claimed in claim 3 wherein the light sensitive sensor comprises a camera arranged outside the vacuum chamber, an optical module arranged in the vacuum chamber, and an optical fiber for relaying light from the optical module to the camera.

8. Apparatus as claimed in claim 3 wherein the light sensitive sensor comprises a photodiode arranged outside the vacuum chamber, an optical module arranged in the vacuum chamber, and an optical fiber for relaying light from the optical module to the photodiode.

9. Apparatus as claimed in claim 3 wherein the light sensitive sensor comprises an illumination source.

10. Apparatus as claimed in claim 3 wherein the light sensitive sensor comprises an illumination source arranged outside of the vacuum chamber, an optical module arranged in the vacuum chamber, and an optical fiber for relaying light from the illumination source to the optical module.

11. A method comprising the steps of:

providing a stream of target material to an irradiation site in a vacuum chamber using a target material dispenser, the target material dispenser comprising an electro-actuatable element arranged to induce velocity perturbations in the stream based on a droplet control signal;

12. A method as claimed in claim 11 wherein the electro-actuatable element is a piezoelectric element.

generating a control signal;

determining a velocity amplitude;

using the determined transfer function to control the droplet generator

14. A method of controlling a droplet generator adapted to deliver a stream of liquid target material to an irradiation region in a system for generating EUV radiation, the method comprising the steps of: providing the stream of target material for a plasma generating system from the droplet generator;

generating a control signal;

introducing a velocity perturbation into the stream by applying the control signal to an electro-actuatable element mechanically coupled to the droplet generator;

observing the stream at a point where droplets in the stream have not fully coalesced; and modifying the control signal based at least in part on results of the observing step.

generating a control signal;

16. A method as claimed in claim 15 additionally comprising a step after the step of estimating the coalescence length of using the estimated coalescence length to control operation of the droplet generator.

generating a control signal; introducing a velocity perturbation into the stream by applying the control signal to an electro-actuatable element mechanically coupled to target material in the droplet generator; observing the stream at a point where droplets in the stream have not fully coalesced to produce a droplet signal; and

assessing the condition of the droplet generator based on the droplet signal.