Apparatus and method for monitoring and controlling droplet generator performance

Abstract

To control droplet generation and coalescence in a manner that allows optimization of these processes.SOLUTION: An apparatus and a method for controlling formation of droplets used to generate EUV radiation comprise a droplet source and an arrangement that produces a laser beam directed to an irradiated region. 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 progressing toward the irradiated region. The process is controlled by observing the stream at a point where droplets in the stream have not fully coalesced.SELECTED DRAWING: Figure 9

Description

[Technical Field]

[0001] CROSS-REFERENCE TO RELATED APPLICATIONS

[0001] This application claims priority to U.S. Application No. 62 / 648,969, filed March 28, 2018, which is incorporated herein by reference in its entirety.

[0002] This application relates to extreme ultraviolet ("EUV") light sources and methods of operation. These light sources provide EUV light by creating a plasma from a source or target material. In some applications, the EUV light is collected and used in photolithography processes to fabricate semiconductor integrated circuits. [Background technology]

[0003]

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

[0004]

[0004] Methods for producing EUV light include, but are not necessarily limited to, converting a source material into a plasma state having chemical elements with emission lines in the EUV region, which may include, but are not limited to, xenon, lithium, and tin.

[0005] In one such method, often referred to as laser-produced plasma ("LPP"), the desired plasma can be generated by irradiating a source material, for example in the form of droplets, a stream, or a wire, with a laser beam. In another method, often referred to as discharge-produced plasma ("DPP"), the required plasma can be generated by positioning a source material having a suitable emission line between a pair of electrodes and generating a discharge between the electrodes.

[0006] One technique for generating droplets involves melting a target material, such as tin, sometimes referred to as the source material, and forcing it under high pressure through a relatively small-diameter orifice, e.g., an orifice having a diameter of about 0.1 μm to about 30 μm, to produce a laminar fluid jet with a velocity ranging from about 30 m / s to about 200 m / s. Under most conditions, the jet breaks up into droplets due to a hydrodynamic instability commonly known as the Rayleigh-Plateau instability. Naturally occurring instabilities in the flow exiting the orifice, such as thermal noise or vortex shedding, will cause the stream to break up into droplets. These droplets may have a variety of velocities and may combine with each other to coalesce into larger droplets.

[0007] In the EUV generation process discussed herein, it is desirable to control the breakup / coalescence process. For example, to synchronize droplets with the optical pulses of an LPP drive laser, repetitive disturbances of amplitudes exceeding the amplitude of random noise can be applied to a continuous laminar fluid jet emanating from an orifice. By applying disturbances at the same frequency (or a harmonic thereof) as the repetition rate of the pulsed laser, droplets are synchronized with the laser pulse. For example, disturbances can be applied to the stream by coupling an electronically actuatable element (such as a piezoelectric material) to the stream and driving the electronically actuatable element with a periodic waveform. In one embodiment, the electronically actuatable element expands or contracts in diameter (on the order of a few nanometers). This dimensional change is mechanically coupled to a capillary tube, which correspondingly expands or contracts in diameter. This volumetric displacement induces acoustic and elastic waves in the capillary tube terminating at the orifice. The target material within the orifice is then periodically accelerated by the acoustic waves. The provision of widely spaced droplets at the drive laser frequency occurs in a frequency range far below the natural Rayleigh breakup frequency of the fluid microjet. While the natural breakup frequency of fluid jets is in the range of about 3 to about 15 MHz, the drive laser operation is expected to occur in the range of about 50 to about 160 kHz. This means that to obtain the desired final droplet, up to 200 small microdroplets must be merged into a periodic droplet stream consisting of droplets much larger than the orifice diameter.

[0008]

[0008] As used herein, the term "electronically actuatable element" and its derivatives refer to a material or structure that undergoes a dimensional change when exposed to an electric voltage, an electric field, a magnetic field, or a combination thereof, and includes, but is not limited to, piezoelectric, electrostrictive, and magnetostrictive materials. Apparatus and methods using electronically actuatable elements to control droplet streams are disclosed, for example, in U.S. Patent Application Publication No. 2009 / 0014668A1, entitled "Laser Produced Plasma EUV Light Source Having a Droplet Stream Produced Using a Modulated Disturbance Wave," published January 15, 2009, and U.S. Patent No. 8,513,629, entitled "Droplet Generator with Actuator Induced Nozzle Cleaning," issued August 20, 2013. Both documents are incorporated herein by reference in their entireties.

[0009] Thus, the challenge of a droplet generator is to place droplets at the prime focus, where they are used as target material for EUV generation. The droplets must arrive at the prime focus within specific spatial and temporal stability criteria—that is, at positions and timings that are repeatable within acceptable margins. The droplets must also arrive at a given frequency and velocity. Furthermore, the droplets must be fully coalesced. That is, they must be monodisperse (uniformly sized) and arrive at a given drive frequency. For example, the droplet stream must not contain “satellite” droplets—smaller droplets of the target material that fail to coalesce into the main droplet. Meeting these criteria is complicated by the fact that for smaller orifices and higher pressures, it would be necessary to merge approximately 200 microdroplets using a drive scheme for electronically actuable elements. The operating window is typically very small, making the system sensitive to performance variations, such as performance changes over time. For example, as the performance of a droplet generator changes, the droplet generator may produce droplets that do not fully coalesce by the time they reach the primary focus. Eventually, the performance of the droplet generator will deteriorate to the point where the droplet generator must be taken off-line for maintenance or replacement.

[0010]

[0010] There is therefore a need to be able to control droplet generation and coalescence in a manner that allows for optimization of these processes. Summary of the Invention

[0011] The following presents a simplified summary of one or more embodiments to provide a basic understanding of those embodiments. This summary is not an extensive overview of all possible embodiments, and is not intended to identify key or critical elements of all embodiments or to 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.

[0012]

[0012] It is therefore desirable to verify proper operation of the droplet generator by determining whether the droplets are fully coalesced by the time they reach the primary focus. This can be achieved by providing the EUV source with an optical feedback system that can identify whether a particular electrical waveform supplied to the droplet generator results in coalesced droplets at the primary focus.

[0013] According to one aspect of an embodiment, an apparatus is disclosed that includes a target material dispenser arranged to provide a stream of target material to an irradiation site within a vacuum chamber, an electronically actuatable element mechanically coupled to the target material dispenser and arranged to induce a velocity perturbation 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 generate a waveform generator control signal based at least in part on the droplet detection signal, and a waveform generator electrically coupled to the electronically actuatable element and the controller, the waveform generator providing the droplet control signal based at least in part on the waveform generator control signal. The electronically actuatable element may be a piezoelectric element.

[0014] According to another aspect of an embodiment, an apparatus is disclosed that includes: a target material dispenser arranged to provide a stream of target material to an irradiation site within a vacuum chamber; an electronically actuatable element mechanically coupled to the target material dispenser and arranged to induce a velocity perturbation 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 photosensitive sensor, the photosensitive sensor comprising at least one optical element disposed within the vacuum chamber; a controller arranged to receive the droplet detection signal and generate a waveform generator control signal based at least in part on the droplet detection signal; and a waveform generator electrically coupled to the electronically actuatable element and the controller, the waveform generator providing the droplet control signal based at least in part on the waveform generator control signal. The electronically actuatable element may be a piezoelectric element. The photosensitive sensor may be a camera. The photosensitive sensor may be a photodiode. The photosensitive sensor may include a camera located outside the vacuum chamber, an optical module located within the vacuum chamber, and an optical fiber relaying light from the optical module to the camera. The photosensitive sensor may include a photodiode located outside the vacuum chamber, an optical module located within the vacuum chamber, and an optical fiber relaying light from the optical module to the photodiode. The photosensitive sensor may include an illumination source. The photosensitive sensor may include an illumination source located outside the vacuum chamber, an optical module located within the vacuum chamber, and an optical fiber relaying light from the illumination source to the optical module.

[0015] According to another aspect of an embodiment, a method is disclosed that includes providing a stream of target material to an irradiation site in a vacuum chamber using a target material dispenser, the target material dispenser including an electronically actuatable element positioned to induce a velocity perturbation 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 providing the droplet control signal based at least in part on the waveform generator control signal. The electronically actuatable element may be a piezoelectric element.

[0016]

[0016] According to another aspect of an embodiment, a method for determining and using a transfer function of a droplet generator adapted to deliver a flow of liquid target material to an irradiation area in a system for generating EUV radiation is disclosed, the method comprising the steps of providing a flow of target material from the droplet generator for a plasma generation system, generating a control signal, applying the control signal to an electronically actuatable element mechanically coupled to the droplet generator to introduce a velocity perturbation in the flow, determining a velocity amplitude, determining the transfer function of the droplet generator based at least in part on the velocity amplitude and the control signal, and controlling the droplet generator using the determined transfer function.

[0017]

[0017] According to another aspect of an embodiment, a method for controlling a droplet generator adapted to deliver a flow of liquid target material to an irradiation region in a system for generating EUV radiation is disclosed, the method comprising the steps of providing a flow of target material from the droplet generator for a plasma generation system, generating a control signal, introducing a velocity perturbation in the flow by applying the control signal to an electronically actuable element mechanically coupled to the droplet generator, observing the flow at a point where droplets in the flow have not fully coalesced, and modifying the control signal based at least in part on the results of the observing step.

[0018] According to another aspect of an embodiment, a method is disclosed for estimating a coalescence length of a stream of droplets of liquid target material generated by a droplet generator in a system for generating EUV radiation, the method comprising: providing a stream of target material from the droplet generator for a plasma generation system; generating a control signal; introducing a velocity perturbation into the stream by applying the control signal to an electronically actuatable element mechanically coupled to the droplet generator; observing the stream at a point in the stream where droplets have not fully coalesced to generate a droplet signal; and estimating the coalescence length based at least in part on the distance between peaks of the droplet signal. The estimated coalescence length may then be used to control operation of the droplet generator.

[0019]

[0019] According to another aspect of an embodiment, a method for evaluating the status of a droplet generator adapted to deliver a flow of liquid target material to an irradiation region in a system for generating EUV radiation is disclosed, the method comprising the steps of providing a flow of target material from the droplet generator for a plasma generation system, generating a control signal, introducing a velocity perturbation in the flow by applying the control signal to an electronically actuable element mechanically coupled to the target material in the droplet generator, observing the flow at a point where droplets in the flow have not fully coalesced to generate a droplet signal, and evaluating the status of the droplet generator based on the droplet signal.

[0020] 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 the accompanying drawings. [Brief explanation of the drawings]

[0021]

[0021] The accompanying drawings, which are incorporated in and form a part of this specification, illustrate by way of example, and not by way of limitation, methods and systems of embodiments of the present invention. The drawings, together with the detailed description, further serve to explain the principles of the methods and systems presented herein and to enable one skilled in the art to make and use the methods and systems. In the drawings, like reference numbers represent identical or functionally similar elements.

[0022] [Figure 1]

[0022] FIG. 1 is a simplified schematic diagram of an EUV light source coupled with an exposure device. [Figure 1A]

[0023] FIG. 1 is a simplified schematic diagram of an apparatus including an EUV light source with an LPP EUV illuminator. [Figure 2]

[0024] FIG. 1 is a schematic diagram of the droplet generation subsystem of an EUV light source. [Figure 3]

[0025] 1 illustrates a technique for coupling one or more electronically actuatable elements with a fluid to create a disturbance in the flow exiting the orifice. [Figure 3A]

[0025] A technique is illustrated in which one or more electronically actuatable elements are coupled to a fluid to create a disturbance in the flow exiting the orifice. [Figure 3B]

[0025] A technique is illustrated in which one or more electronically actuatable elements are coupled to a fluid to create a disturbance in the flow exiting the orifice. [Figure 3C]

[0025] A technique is illustrated in which one or more electronically actuatable elements are coupled to a fluid to create a disturbance in the flow exiting the orifice. [Figure 4]

[0025] A technique is illustrated in which one or more electronically actuatable elements are coupled to a fluid to create a disturbance in the flow exiting the orifice. [Figure 5]

[0025] A technique is illustrated in which one or more electronically actuatable elements are coupled to a fluid to create a disturbance in the flow exiting the orifice. [Figure 6]

[0026] FIG. 10 is a diagram showing the state of coalescence in a droplet stream. [Figure 7]

[0027] 1 is a graph of a composite waveform as may be used in accordance with an aspect of an embodiment. [Figure 8]

[0028] FIG. 1 is a diagram of a droplet generation system with feedback as may be used in accordance with an aspect of an embodiment. [Figure 9]

[0029] FIG. 1 is a diagram of a droplet generation system with feedback as may be used in accordance with an aspect of an embodiment. [Figure 10A]

[0030] FIG. 10 illustrates a possible droplet signal according to an aspect of an embodiment. [Figure 10B]

[0030] FIG. 1 illustrates a possible droplet signal according to an aspect of an embodiment. [Figure 11]

[0031] 1 is a flowchart illustrating a method for estimating a coalescence length according to an aspect of an embodiment. [Figure 12]

[0032] 10 is a flowchart illustrating a method for determining a transfer function of a drop generator in accordance with another aspect of an embodiment. DETAILED DESCRIPTION OF THE INVENTION

[0023]

[0033] Further features and advantages of the present invention, as well as the structure and operation of various embodiments of the present invention, are described in detail below with reference to the accompanying drawings. It should be noted that the present 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 those skilled in the art based on the teachings contained herein.

[0024]

[0034] Various embodiments will now be described with reference to the drawings. Like reference numerals will be used to refer to like elements throughout. In the following description, for purposes of explanation, numerous specific details are set forth in order to facilitate a thorough understanding of one or more embodiments. It will be apparent, however, that in some or all cases, any of the embodiments described below can be practiced without employing the specific design details described below. In other instances, well-known structures and devices are shown in block diagram form in order to facilitate describing one or more embodiments. The following presents a simplified summary of one or more embodiments to provide a basic understanding of those embodiments. This summary is not an extensive overview of all possible embodiments, and is not intended to identify key or critical elements of all embodiments or to delineate the scope of any or all embodiments.

[0025]

[0035] Before describing such embodiments in more detail, however, it is helpful to present an exemplary environment in which embodiments of the present invention may be practiced. In the following description and claims, terms such as "up," "down," "top," "bottom," "vertical," "horizontal," and similar terms may be used. These terms are intended to indicate relative orientation only, and are not intended to indicate orientation with respect to gravity.

[0026]

[0036] Referring first to FIG. 1 , there is shown a simplified schematic cross-sectional view of selected portions of an example of an EUV photolithography apparatus, generally designated 10″. The apparatus 10″ can be used to expose a substrate 11, such as a resist-coated wafer, with a patterned beam of EUV light. The apparatus 10″ may be provided with an exposure device 12″ (e.g., an integrated circuit lithography tool such as a stepper, scanner, step-and-scan system, direct-write system, device using contact and / or proximity masks, etc.) that utilizes EUV light, which may include, for example, one or more optical elements 13 a,b for illuminating a patterning optical element 13 c, such as a reticle, with the beam of EUV light to produce a patterned beam, and one or more reduction projection optical elements 13 d, 13 e for projecting the patterned beam onto the substrate 11. A mechanical assembly (not shown) may be provided to generate controlled relative movement between the substrate 11 and the patterning means 13 c. As further shown in FIG. 1 , the apparatus 10″ may include an EUV light source 20″ including an EUV light illuminator 22 that emits EUV light within a chamber 26″, where the EUV light is reflected by optical element 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, to direct, shape, or control the radiation.

[0027]

[0037] As used herein, the term "optical element" and its derivatives are intended to be broadly interpreted to include, but are not necessarily limited to, one or more components that reflect and / or transmit and / or act on incident light, including, but 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 multilayer mirrors, near-normal incidence mirrors, grazing incidence mirrors, specular reflectors, diffusive reflectors, and combinations thereof. Also, unless otherwise specified, the term "optical element" and its derivatives, as used herein, are intended to be limited to components that operate only or advantageously within one or more specific wavelength ranges, such as EUV output light wavelengths, illuminating laser wavelengths, wavelengths suitable for metrology, or some other specific wavelength.

[0028]

[0038] FIG. 1A illustrates one specific example of an apparatus 10″ that includes an EUV light source 20 with an LPP EUV light illuminator. As shown, the EUV light source 20 may include a system 21 that generates a train of light pulses and delivers the light pulses into a light source chamber 26. With respect to the apparatus 10, the light pulses may travel along one or more beam paths from the system 21 into the chamber 26 to illuminate a source material at an irradiation region 48 and generate an EUV light output for substrate exposure in the exposure device 12.

[0029]

[0039] Lasers suitable for use in the laser system 21 shown in FIG. 1A may include pulsed laser devices, such as pulsed gas discharge CO2 laser devices, that generate radiation at approximately 9.3 μm or 10.6 μm, e.g., via DC or RF excitation, and operate at relatively high powers, e.g., 10 kW or higher, and high pulse repetition rates, e.g., 50 kHz or higher. In one particular implementation, the laser may be an axially RF-pumped CO2 laser with a multi-stage oscillator-amplifier configuration (e.g., a master oscillator / power amplifier (MOPA) or a power oscillator / power amplifier (POPA)), with a seed pulse initiated by a Q-switched oscillator at relatively low energy and high repetition rates, 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 also be used in the laser system 21. Alternatively, the laser may be configured as a so-called "self-targeting" laser system, in which a droplet serves as one mirror of the optical cavity.

[0030]

[0040] Depending on the application, other types of lasers may also be suitable, such as excimer or molecular fluorine lasers operating at high power and high pulse repetition rates. Other examples include solid-state lasers with active media in, for example, fiber, rod, slab, or disk shape; other laser architectures with one or more chambers, such as one oscillator chamber and one or more amplifier chambers (the amplifier chambers may be in parallel or in series); master oscillator / power oscillator (MOPO) arrangements; master oscillator / power ring amplifier (MOPRA) arrangements; or a solid-state laser seeding one or more excimer, molecular fluorine, or CO amplifier or oscillator chambers. Other designs may also be suitable.

[0031]

[0041] In some cases, the source material may be first irradiated by a pre-pulse and then by a main pulse. The pre-pulse seed and the main pulse seed may be generated by a single oscillator or by two separate oscillators. In some configurations, one or more common amplifiers may be used to amplify both the pre-pulse seed and the main pulse seed. In other arrangements, separate amplifiers may be used to amplify the pre-pulse seed and the main pulse seed.

[0032]

[0042] 1A also shows that apparatus 10 can include a beam conditioning unit 50 having one or more optical elements for beam conditioning, such as expanding, steering, and / or focusing, between laser source system 21 and irradiation site 48. A steering system, which can include, for example, one or more mirrors, prisms, lenses, etc., can be provided and arranged to steer the laser focal point to different locations within chamber 26. For example, the steering system can include a first planar mirror mounted on a tip-tilt actuator that can independently move the first mirror in two dimensions and a second planar mirror mounted on a tip-tilt actuator that can independently move the second mirror in two dimensions. With this arrangement, the steering system can controllably move the focal point in a direction substantially perpendicular to the direction of beam propagation (beam axis).

[0033]

[0043] The beam adjustment unit 50 may include a focusing assembly for focusing the beam at the illumination site 48 and adjusting the position of the focal point along the beam axis. The focusing assembly may employ an optical element, such as a focusing lens or a focusing mirror, coupled to an actuator for movement along the beam axis to move the focal point along the beam axis.

[0034]

[0044] As further shown in FIG. 1A, the EUV light source 20 may also include a source material delivery system 90 that delivers a source material, such as, for example, tin droplets, to an irradiation region or prime focus 48 within the chamber 26 where the droplets interact with light pulses from the system 21, ultimately generating a plasma and generating EUV radiation to expose a substrate, such as a resist-coated wafer, in the exposure device 12. Further details regarding various droplet dispenser configurations and their relative advantages can be found, for example, in U.S. Patent No. 7,872,245, issued January 18, 2011, entitled "Systems and Methods for Target Material Delivery in a Laser Produced Plasma EUV Light Source," U.S. Patent No. 7,405,416, issued July 29, 2008, entitled "Method and Apparatus For EUV Plasma Source Target Delivery," and U.S. Patent No. 7,372,056, issued May 13, 2008, entitled "LPP EUV Plasma Source Material Target Delivery System," the contents of each of which are incorporated herein by reference in their entirety.

[0035]

[0045] Source materials for generating EUV light output for substrate exposure can include, but are not necessarily limited to, materials containing tin, lithium, xenon, or combinations thereof. EUV-emitting elements, such as tin, lithium, xenon, etc., can be in the form of liquid droplets and / or solid particles contained in liquid droplets. For example, elemental tin can be used as pure tin, as a tin compound, such as SnBr4, SnBr2, or SnH4, or as a tin alloy, such as a tin-gallium alloy, a tin-indium alloy, a tin-indium-gallium alloy, or a combination thereof. Depending on the material used, the source material may be presented to the irradiation region at various temperatures, including at or near room temperature (e.g., tin alloy, SnBr4), at elevated temperatures (e.g., pure tin), or below room temperature (e.g., SnH4), and in some cases, may be relatively volatile, such as SnBr4.

[0036]

[0046] 1A , apparatus 10 may also include an EUV controller 60. The apparatus may also include a drive laser control system 65 for controlling devices in system 21 to generate light pulses for delivery into chamber 26 and / or for controlling movement of optical elements in beam conditioning unit 50. Apparatus 10 may also include a droplet position detection system, which may include, for example, one or more droplet imagers 70 that provide an output representative of the position of one or more droplets relative to illumination area 48. Imager 70 may provide this output to droplet position detection feedback system 62. This system may, for example, calculate droplet position and trajectory, from which droplet error may be calculated, for example, on a drop-by-drop or average basis. The droplet error may then be provided as an input to controller 60, which may, for example, provide position, direction, and / or timing correction signals to system 21 to control laser trigger timing and / or control movement of optical elements in beam conditioning unit 50, for example, to change the location and / or focusing power of the light pulses being delivered to irradiation region 48 in chamber 26. Also, with respect to EUV light source 20, source material delivery system 90 may have a control system operable in response to signals from controller 60 (which, depending on the implementation, may include the droplet error described above or some quantity derived therefrom) to, for example, modify the ejection point, initial droplet stream direction, droplet ejection timing, and / or droplet modulation to correct for errors in droplets reaching desired irradiation region 48.

[0037]

[0047] Continuing with FIG. 1A , apparatus 10 may also include optical element 24″, such as a near-normal incidence collector mirror having a reflective surface in the shape of a prolate spheroid (i.e., an ellipse rotated about its major axis) having a graded multilayer coating, e.g., with alternating layers of molybdenum and silicon, and optionally one or more high-temperature diffusion barrier layers, smoothing layers, capping layers, and / or etch stop layers. FIG. 1A shows that optical element 24″ may be formed with an aperture to allow light pulses generated by system 21 to pass through and reach illumination region 48. As shown, optical element 24″ may be, for example, a prolate spheroidal mirror having a first focal point in or near illumination region 48 and a second focal point in a so-called intermediate region 40, where EUV light may be output from EUV light source 20 and input to an exposure device that utilizes EUV light, e.g., an integrated circuit lithography tool. It should be understood that other optical elements may be used in place of a prolate spheroidal mirror to collect and direct the light to an intermediate location for subsequent delivery to a device that utilizes EUV light.

[0038]

[0048] A buffer gas, such as hydrogen, helium, argon, or combinations thereof, may be introduced, replenished, and / or removed from chamber 26. The buffer gas may be present in chamber 26 during plasma discharge and may act to slow ions created by the plasma to reduce optical element degradation and / or increase plasma efficiency. Alternatively, magnetic and / or electric fields (not shown) may be used alone or in combination with the buffer gas to reduce fast ion damage.

[0039]

[0049] FIG. 2 illustrates the droplet generation system in more detail. A source material delivery system 90 delivers droplets to the illumination site / prime focal point 48 within the chamber 26. A waveform generator 230 provides drive waveforms to electronically actuatable elements within the droplet generator 90, which induce velocity perturbations in the droplet stream. The waveform generator operates under the control of a controller 250, based at least in part on data from a data processing module 252. The data processing module receives data from one or more detectors. In the illustrated example, the detectors include a camera 254 and a photodiode 256. The droplets are illuminated by one or more lasers 258. In this exemplary arrangement, the detectors detect / image the droplets at points in the stream where coalescence is expected to occur. The detectors and lasers are also positioned outside the vacuum chamber 26, observing the stream through a window in the wall of the vacuum chamber 26.

[0040]

[0050] 3 illustrates, in schematic form, the components of a simplified droplet source 92. As shown, droplet source 92 may include a reservoir 94 that holds a fluid, such as molten tin, under pressure. As also shown, reservoir 94 may be formed with an orifice 98, allowing pressurized fluid 96 to flow through the orifice and establish a continuous stream 100 that subsequently breaks up into a plurality of droplets 102 a,b.

[0041]

[0051] The system uses one or more lasers to illuminate the droplet stream and one or more detectors (e.g., one or more cameras, photodiodes, or some combination thereof) to measure the optical response of the droplets as they pass through the laser beam. In such designs, the lasers and detectors are located outside the vacuum vessel; that is, the laser beam must be precisely projected onto the droplet stream, and the droplet optical response must be imaged by remote optics. This imposes limitations on the accuracy and resolution of the measurement and limits the manner in which this metrology can be used.

[0042]

[0052] As previously mentioned, this system can be used to establish whether droplets are coalescing (or not) near the primary focus. If the droplets are not coalescing, the droplet generator's operating parameters are adjusted to ensure coalescence. However, this process can generally only be performed when the drive laser is turned off, potentially resulting in significant machine downtime. It would be advantageous to be able to adjust the droplet generator signal while generating plasma. It would also be advantageous to obtain in-line measurements of droplet generator performance that could be used to plan droplet generator maintenance and predict droplet generator lifespan. Droplet generator performance has been shown to change over time. An ideal signal at start-up may result in satellite droplets after a few hours. In such a case, the drive laser is turned off for droplets, and the drive signal optimization is repeated. Typically, this is done daily, resulting in approximately one hour of downtime.

[0043]

[0053] FIG. 3 illustrates a possible configuration of a droplet source 92 as part of a droplet generator 90. The droplet source 92 further includes a fluid disturbance generation subsystem having an electronically actuatable element 104 operably coupled to the fluid 96 and a signal generator 106 that drives the electronically actuatable element 104. FIGS. 3A-3C, 4, and 5 illustrate various ways in which one or more electronically actuatable elements may be operably coupled to the fluid to create droplets. Beginning with FIG. 3A, an arrangement is shown in which a fluid is flowed from a reservoir 108 under pressure through a tube 110, e.g., a capillary tube, having an inner diameter of about 0.2 mm to about 0.8 mm and a length of about 10 mm to 50 mm, exiting an orifice 114 of the tube 110 to create a continuous stream 112 that subsequently breaks up into droplets 116a,b. An electronically actuatable element 118 may be coupled to the tube as shown. For example, an electronically actuatable element may be coupled to the tube 110 to deflect the tube 110 and impede the flow 112. FIG. 3B shows a similar arrangement having a reservoir 120, a tube 122, and a pair of electronically actuatable elements 124, 126 coupled to the tube 122, each of which deflects the tube 122 at a respective frequency. FIG. 3C shows another variation, in which a movable plate 128 is positioned within the reservoir 130 to force fluid through an orifice 132 to create a stream 134 that breaks into droplets 136 a, b. As shown, a force may be applied to the plate 128, and one or more electronically actuatable elements 138 may be coupled to the plate to impede the flow 134. It should be understood that a capillary tube may be used with the embodiment shown in FIG. 3C.

[0044]

[0054] 4 shows another variation in which fluid is flowed from a reservoir 140 under pressure through a tube 142, exiting an orifice 146 in the tube 142 to create a continuous stream 144 that then breaks up into droplets 148a,b. As shown, an electronically actuatable element 150, having, for example, a ring or cylindrical tube shape, may be positioned to surround the periphery of the tube 142. When actuated, the electronically actuatable element 150 may selectively squeeze and / or unsqueez- le the tube 142 to impede the flow 144. It should be understood that more than one electronically actuatable element may be employed to selectively squeeze the tube 142 at respective frequencies.

[0045]

[0055] FIG. 5 shows another variation in which fluid is flowed from a reservoir 140′ under pressure through a tube 142′, exiting an orifice 146′ of the tube 142′ to create a continuous stream 144′ that then breaks into droplets 148a′, b′. As shown, an electronically actuatable element 150a, e.g., having a ring shape, can be positioned to surround the periphery of the tube 142′. When activated, the electronically actuatable element 150a can selectively squeeze the tube 142′ to impede the flow 144′ and generate droplets. FIG. 5 also shows that a second electronically actuatable element 150b, e.g., having a ring shape, can be positioned to surround the periphery of the tube 142′. When activated, the electronically actuatable element 150b can selectively squeeze the tube 142′ to impede the flow 144′ and remove contaminants from the orifice 152. For the illustrated embodiment, the electronically actuatable elements 150a and 150b may be driven by the same signal generator, or different signal generators may be used. As described below, waveforms having different waveform amplitudes, periodic frequencies, and / or waveform shapes may be used to drive the electronically actuatable element 150a to generate droplets for EUV output. The electronically actuatable element generates disturbances in the fluid that generate droplets with different initial velocities, causing at least some adjacent droplet pairs to coalesce before reaching the irradiation region. The ratio of initial microdroplets to coalesced droplets may be any number, for example, ranging from about 10 droplets to about 500 droplets.

[0046]

[0056] Thus, controlling the breakup / coalescence process involves controlling the droplets to coalesce sufficiently before they reach the irradiated area and to have a frequency corresponding to the pulse rate of the laser used to irradiate the coalesced droplets. To control the coalescence process of Rayleigh breakup microdroplets into fully coalesced droplets at a frequency corresponding to the laser pulse rate, a designer synthesized waveform consisting of a linear superposition of multiple voltage and multiple frequency sinusoidal waveforms may be supplied to an electronically actuatable element. The control system may allow the phase of each spectral component to be individually adjusted using only separate sinusoids. The waveform may be defined as a voltage or current signal.

[0047]

[0057] On-axis droplet velocity profiles are obtained by imaging the droplet stream at a fixed location downstream of the coalescence and are used as feedback to control the droplet generation / coalescence process. As a form of imaging, a light barrier can be used to temporally resolve the droplet paths, and from this information the droplet coalescence pattern can be reconstructed.

[0048]

[0058] Coalescence of microdroplets and subcoalesced droplets is controlled by a periodic electrical drive signal on the droplet generator's electronically actuable actuator. This signal is automatically monitored during source operation. Based on the crossover interval and DFC data, the best operating point can be selected. The selected signal is applied to the droplet generator, and the pre-pulse and drive laser are optimized for optimal plasma conditions.

[0049]

[0059] The use of designer waveforms allows users to target specific droplet coalescence lengths at user-specified frequencies using feedback from imaging metrology at fixed points downstream of fully coalesced droplets. One form of designer waveform may consist of (1) a sinusoidal waveform with a fundamental frequency approximately equal to the laser pulse rate and (2) a set of higher frequency sinusoidal waveforms. The higher frequency waveforms are all harmonics of the fundamental frequency, i.e., multiples of the fundamental frequency. The use of designer waveforms also allows for the determination of the nozzle's transfer function of the on-axis target material flow velocity perturbation / profile, which in turn can be used to optimize the parameters of the designer waveform that drives the electronically actuable elements.

[0050]

[0060] The entire droplet coalescence process may be viewed as a succession of subcoalescence steps or regimes that evolve as a function of distance from the nozzle. This is illustrated in FIG. 6. For example, in a first regime 161, i.e., when the target material initially exits the orifice or nozzle, it 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 with varying velocities. In a third regime 163, measured by time of flight or distance from the nozzle, the microdroplets coalesce into intermediate-sized droplets with varying velocities relative to each other, referred to as subcoalescence droplets 165. In a fourth regime 166, the subcoalescence droplets coalesce into droplets of the desired final size, i.e., fully coalesced droplets 168. The number of subcoalescence steps is variable. The distance from the nozzle exit 169 to the point where the droplets reach their final coalesced state is the coalescence distance or coalescence length 170. Ideally, the droplet coalescence distance 170 is as short as possible. Once droplets coalesce into larger droplets, they are less susceptible to source conditions such as hydrogen flow and ion bombardment.

[0051]

[0061] As previously mentioned, if complete coalescence is not achieved, the droplet stream will contain smaller droplets, referred to as satellite droplets or microsatellites. The presence of satellite droplets can be detected by any one or a combination of several methods, such as the use of a droplet detection module (DDM), crossover spacing, DFC, or even by monitoring changes in the EUV signal. Systems and methods for monitoring droplet streams are disclosed, for example, in U.S. Pat. No. 9,241,395, issued January 19, 2016, entitled "System and Method for Controlling Droplet Timing in an LPP EUV Light Source," the entire contents of which are incorporated herein by reference. Such metrology is typically used to determine where and when droplets are at the prime focus and the quality of the plasma; therefore, the probing location is at or very close to the plasma, typically about 30 cm to about 40 cm from the nozzle exit. Detecting satellites at this distance is difficult. This is because the metrology for detecting these uncoalesced droplets may be, for example, half a meter away at the wall of the source vessel, and the satellites may be blown out of the field of view by the vessel flow.

[0052]

[0062] With a sensor placed at a distance, e.g., outside the chamber, this would involve observing microdroplets with sizes on the order of 4 microns with a light detector focused on a small region of interest at a distance of approximately 40 cm from the nozzle and tuned to detect fully coalesced droplets with sizes on the order of 27 microns. Furthermore, the transverse "crosswind" resulting from the prevailing gas flow in the chamber, e.g., cone flow, can spread these microsatellites throughout a large volume. Placing metrology closer to the nozzle, where the droplets have not yet coalesced, allows for the observation and detection of microsatellites at a point in their development where they are less spread out, making it easier to detect all of them. Measurements at this location allow for control and optimization of the coalescence process.

[0053]

[0063] Some features of an example designer waveform will now be described with reference to FIG. 7. The upper waveform in FIG. 7 is a base waveform, generally having a frequency identical to or otherwise related to the pulse rate of the laser used to vaporize the droplets. In this example, the base waveform is a sine wave. The lower waveform in FIG. 7 is a higher frequency waveform, generally having a frequency that is an integer multiple of the frequency of the base waveform. Any periodic wave may be used. In this example, the higher frequency waveform is a series of triangular spikes. These two waveforms are superimposed to obtain a composite waveform. The sub-coalesced waveform is a superposition of harmonics of the sub-coalesced frequency, which may also include the sub-coalesced frequency.

[0054]

[0064] One approach to enhancing droplet generator performance measurement is to provide a more direct observation of droplet generation by providing detectors positioned within the vacuum chamber to more closely observe droplet generation. These detectors may be used in addition to or in place of detectors positioned outside the chamber. These in situ detectors provide real-time, high-resolution feedback and control over the operation of the droplet generator. These detectors offer the ability to reduce machine downtime that would otherwise be used to identify parameters that result in fully coalesced droplets without satellites. Minimizing satellite droplets, in turn, reduces dose stability errors and collector life issues due to the presence of on-axis satellite droplets. In situ detectors can also help reduce unscheduled downtime due to the need for sudden, unexpected replacement of the droplet generator.

[0055]

[0065] To provide the possibility of more accurate measurements, the metrology device may be placed relatively close to the nozzle exit. It is positioned to detect the droplet pattern between approximately 0.5 cm and approximately 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 time at the sensor location. The droplets pass through 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. A portion of the reflected light is collected by an optical element and converted into a time signal, for example, by a photodiode with a high sampling rate. Another example of a narrow focus is to use extinction of the laser curtain as the droplets pass through. If the location of this focus can be controlled within a small 3D volume, the metrology device can also be used to detect the position of the droplet stream, providing detailed information for the droplet generator steering system. The amplitude and relative position of the reflected peaks give quantitative information about the current coalescence process and about the current drop generator performance level.

[0056]

[0066] Referring now to FIG. 8 , an electronically actuatable element 200 is shown positioned around the capillary tube 210 of the nozzle 220. The electronically actuatable element 200 converts electrical energy from a waveform generator 230 to apply a fluctuating pressure to the capillary tube 210, which introduces velocity perturbations to the stream of molten target material 240 exiting the nozzle 220. The droplets are imaged by a camera 250 at imaging points in the regime where the droplets are forming but not fully coalescing. If a camera is used, the system will also have a light source. The light source is positioned to illuminate the droplets or to be blocked by individual droplets as they travel across the beam generated by the light source. As used herein, imaging encompasses both forming an image of the droplets and simply a binary indication of the presence or absence of droplets. The imaging develops a velocity profile of the droplet stream at imaging points in the regime where the droplets are forming but not fully coalescing. The control unit 260 uses the imaging data from the camera 250 to generate feedback signals to control the 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 control input 265, which may come from another controller or may be based on user input.

[0057]

[0067] Another conceptualization of a system according to an aspect of an embodiment is shown in Figure 9. In the system of Figure 9, droplet detection is performed by a laser curtain consisting of a laser 300 and a photodiode 310. Light from laser 300 is carried by optical fiber 302 into the interior of chamber 26, where it illuminates the droplet stream at a point near droplet generator 90 using optics 305. Light reflected from the droplet stream is carried by optical fiber 312 and returned to camera 310 by optics 315.

[0058]

[0068] This optical feedback can be used in a procedure to optimize the voltage drive signal applied to the electronically actuatable element while plasma generation at the primary focus continues. As an example, FIG. 10A shows the expected time signal for a droplet pattern with one droplet and two satellite droplets at a frequency of 500 kHz. A fully coalesced droplet pattern would consist of one Gaussian peak 400 per period. The satellites have smaller Gaussian peaks 410 adjacent to the main 500 kHz Gaussian peak. The spectral content of the 500 kHz drive signal, which may be a square wave signal, can be adjusted to achieve coalescence.

[0059]

[0069] The sensor can also be used to determine the transfer function for frequencies below the sub-coalescence frequency. Figure 10B shows an example in which the droplet velocity of a stream of sub-coalesced droplets is modulated with a 50 kHz sinusoid. Curve 420 shows the completely unperturbed droplet distribution, curve 430 shows the distribution of droplets traveling at 0.2 m / s perturbed with a 50 kHz sinusoidal perturbation, and curve 440 shows the distribution of droplets traveling at 0.4 m / s perturbed with a 50 kHz sinusoidal perturbation. By measuring the relative delay time of the droplets at the sensor location, this velocity can be determined, from which the 50 kHz transfer function can be calculated. The transfer function is given in m / s per volt. By performing this transfer function measurement for other harmonics of 50 kHz, such as 100 kHz and 150 kHz, an optimal waveform can be constructed to achieve the minimum coalescence distance for a given voltage budget of the signal generator.

[0060]

[0070] Once sub-coalescence is established, a lower frequency can be superimposed to move the 500 kHz droplets closer together. In this example, a 50 kHz signal is used to control the coalescence of the 500 kHz droplets. At the metrology site, coalescence has not yet occurred. (The coalescence process begins at the nozzle, with portions of the fluid streams already moving relatively toward each other from the orifice outlet.) However, the coalescence length can be estimated based on the relative spacing of the peaks. A method for doing so is shown in FIG. 11. In step S50, the flow is initiated. In step S52, a drive signal is applied to the droplet generator. In step S56, the flow is observed upstream of coalescence, i.e., upstream of full coalescence. In step S58, the spacing between the peaks in the signal generated from the flow observation 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.

[0061]

[0071] A 50 kHz velocity amplitude can also be determined, which, in combination with the applied voltage, results in a transfer function (m / s / per V) for the 50 kHz droplet generator. A method for performing this process is shown in FIG. 12. Steps S50, S52, and S56 are described above. In step S60, the droplet velocity amplitude is determined. In step S64, the applied voltage is determined. These determined values ​​are used to determine a transfer function in step S66. This determined transfer function can then be used, for example, to characterize, control, and / or optimize the operation of the droplet generator.

[0062]

[0072] The transfer function may be defined as the velocity perturbation obtained at the nozzle exit per unit applied voltage of a particular frequency. For the nozzle transfer function considered, the signal (characterized by frequency, magnitude, and phase) applied to the electronically actuatable element is the input, while the velocity perturbation imposed on the liquid jet is the output. The coalescence length varies with the velocity amplitude of the sinusoidal component at frequencies below the sub-frequency. A larger sinusoidal amplitude means an increased velocity perturbation, and therefore a shorter coalescence length.

[0063]

[0073] The designer waveform can be characterized by several parameters. The exact number of parameters depends on the selection of a higher frequency arbitrary periodic waveform, which may have several tuning parameters. The characterizing parameters will generally include the sinusoidal voltage, the voltage of the higher frequency waveform, and the relative phase. While the sinusoidal voltage and phase determine the coalescence length, as presented above, the voltage of the higher frequency arbitrary periodic waveform controls the velocity jitter of the low frequency droplets. The velocity jitter of the droplets results in droplet timing variations. Typically, the droplet timing variations must be limited to allow for synchronization of the droplets with the laser pulse.

[0064]

[0074] In-situ droplet generator metrology enables high-resolution droplet detection at the droplet generator exit. To provide high resolution, metrology may use, for example, optical fibers to deliver a test laser beam and detection signal. Advantages of such metrology include in-line control of the droplet generator, so that droplet generator adjustments can be performed during droplet generator operation without downtime. Real-time quantitative feedback on droplet generator performance parameters can be used to predict the timing of droplet generator replacement. Prevention of on-axis satellite droplets near the plasma also improves collector life and dose stability. Because the coalescence process is influenced by the plasma, the signal can advantageously be adjusted by plasma generation during operation. Direct control over the coalescence length also avoids the need to turn off the laser and perform a new search for the signal, effectively reducing planned and unplanned maintenance downtime.

[0065]

[0075] While the use of the procedure described above has been in connection with in situ metrology, it will be appreciated that the procedure may also be used with data collected by remote metrology, i.e., metrology with a light source and / or detector located outside the chamber.

[0066]

[0076] It will be appreciated that instead of a detector such as a dark field light bridge, a system that forms an image of the droplet captured by a camera could also be used, which would provide more data about the location and characteristics of the droplet.

[0067]

[0077] The present invention has been described above with the aid of functional building blocks that illustrate implementations of certain functions and relationships thereof. The boundaries of these functional building blocks have been arbitrarily defined herein for the convenience of description. Alternative boundaries may be defined so long as the certain functions and relationships thereof are appropriately performed.

[0068]

[0078] The foregoing description of specific embodiments fully reveals the overall nature of the present invention, such that those skilled in the art can readily modify such specific embodiments and / or adapt them for various uses without undue experimentation and without departing from the overall concept of the present invention. Therefore, such adaptations and modifications are intended to fall within the meaning and range of equivalents of the disclosed embodiments, based on the teaching and guidance presented herein. It should be understood that the terms and phrases herein are for the purpose of description and not limitation, and should be interpreted in light of the teachings and guidance by those skilled in the art. The breadth and scope of the present invention should not be limited by any of the exemplary embodiments described above, but should be defined only by the following claims and their equivalents.

[0069]

[0079] Other aspects of the invention are described in the following numbered clauses: 1. a target material dispenser positioned to provide a flow of target material to an irradiation site within a vacuum chamber; an electronically actuatable element mechanically coupled to the target material dispenser and positioned to induce a velocity perturbation in the stream based on the droplet control signal; a detector positioned 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 disposed to receive the drop detect signal and configured to generate a waveform generator control signal based at least in part on the drop detect signal; a waveform generator electrically coupled to the electronically actuatable element and the controller for providing a droplet control signal based at least in part on the waveform generator control signal; An apparatus comprising: 2. The device of clause 1, wherein the electronically actuatable element is a piezoelectric element. 3. A target material dispenser positioned to provide a flow of target material to an irradiation site within the vacuum chamber; an electronically actuatable element mechanically coupled to the target material dispenser and positioned to induce a velocity perturbation in the stream based on the droplet control signal; a detector positioned 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 photosensitive sensor, the photosensitive sensor comprising at least one optical element disposed within the vacuum chamber; a controller disposed to receive the drop detect signal and configured to generate a waveform generator control signal based at least in part on the drop detect signal; a waveform generator electrically coupled to the electronically actuatable element and the controller for providing a droplet control signal based at least in part on the waveform generator control signal; An apparatus comprising: 4. The device of clause 3, wherein the electronically actuatable element is a piezoelectric element. 5. The apparatus of clause 3, wherein the photosensitive sensor comprises a camera. 6. The apparatus of clause 3, wherein the photosensitive sensor comprises a photodiode. 7. The apparatus of clause 3, wherein the photosensitive sensor comprises a camera positioned outside the vacuum chamber, an optical module positioned within the vacuum chamber, and an optical fiber that relays light from the optical module to the camera. 8. The apparatus of clause 3, wherein the photosensitive sensor comprises a photodiode positioned outside the vacuum chamber, an optical module positioned within the vacuum chamber, and an optical fiber that relays light from the optical module to the photodiode. 9. The device of clause 3, wherein the photosensitive sensor comprises an illumination source. 10. The apparatus described in clause 3, wherein the photosensitive sensor comprises an illumination source located outside the vacuum chamber, an optical module located within the vacuum chamber, and an optical fiber that relays light from the illumination source to the optical module. 11. Providing a stream of target material to an irradiation site within the vacuum chamber using a target material dispenser, the target material dispenser comprising an electronically actuatable element arranged to induce a velocity perturbation 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 drop detect signal; providing a droplet control signal based at least in part on the waveform generator control signal; A method comprising: 12. The method of clause 11, wherein the electronically actuatable element is a piezoelectric element. 13. A method of determining and using a transfer function of a droplet generator adapted to deliver a flow of liquid target material to an irradiation region in a system for generating EUV radiation, comprising: providing a flow of target material from a droplet generator to a plasma generation system; generating a control signal; applying a control signal to an electronically actuatable element mechanically coupled to the droplet generator to introduce a velocity perturbation into the stream; determining a velocity amplitude; determining a transfer function of the drop generator based at least in part on the velocity amplitude and the control signal; controlling a droplet generator using the determined transfer function; A method comprising: 14. A method of controlling a droplet generator adapted to deliver a flow of liquid target material to an irradiation region in a system for generating EUV radiation, comprising: providing a flow of target material from a droplet generator to a plasma generation system; generating a control signal; introducing a velocity perturbation into the stream by applying a control signal to an electronically actuatable element mechanically coupled to the droplet generator; observing the stream at a point in the stream where the droplets have not fully coalesced; modifying the control signal based at least in part on the results of the observing step; A method comprising: 15. A method for estimating the coalescence length of a stream of droplets of a liquid target material produced by a droplet generator in a system for generating EUV radiation, comprising: providing a flow of target material from a droplet generator to a plasma generation system; generating a control signal; introducing a velocity perturbation into the stream by applying a control signal to an electronically actuatable element mechanically coupled to the droplet generator; observing the stream at a point in the stream where droplets have not fully coalesced to generate a droplet signal; estimating a coalescence length based at least in part on the distance between peaks of the droplet signal; A method comprising: 16. The method of clause 15, further comprising, after the step of estimating the coalescence length, the step of using the estimated coalescence length to control operation of the droplet generator. 17. A method for evaluating a condition of a droplet generator adapted to deliver a flow of liquid target material to an irradiation region in a system for generating EUV radiation, comprising: providing a flow of target material from a droplet generator to a plasma generation system; generating a control signal; introducing a velocity perturbation into the stream by applying a control signal to an electronically actuatable element mechanically coupled to the target material within the droplet generator; observing the stream at a point in the stream where droplets have not fully coalesced to generate a droplet signal; assessing a condition of the droplet generator based on the droplet signal; A method comprising:

Claims

1. a target material dispenser positioned to provide a flow of target material to an irradiation site within the vacuum chamber; an electronically actuatable element mechanically coupled to the target material dispenser and positioned to induce a velocity perturbation in the stream based on a droplet control signal comprising an applied voltage; a detector positioned to observe droplets in the stream and to generate a droplet detection signal; a controller configured to receive the drop detect signal and to generate a waveform generator control signal based at least in part on the drop detect signal; a waveform generator electrically coupled to the electronically actuatable element and the controller, the waveform generator providing the droplet control signal based at least in part on the waveform generator control signal; the controller is configured to determine a transfer function based on the velocity perturbation and the applied voltage. Device.

2. The device of claim 1 , wherein the electrically actuatable element is a piezoelectric element.

3. the detector is positioned to observe droplets and generate a droplet detection signal during operation of the target material dispenser; the controller is configured to receive the droplet detect signal and generate a waveform generator control signal during operation of the target material dispenser; The apparatus of claim 1 , wherein the waveform generator is adapted to provide the droplet control signal during operation of the target material dispenser.

4. the detector comprises an illumination source and a photosensitive sensor; The apparatus of claim 1 , wherein the photosensitive sensor comprises at least one optical element disposed within the vacuum chamber.

5. The device of claim 4 , wherein the electrically actuatable element is a piezoelectric element.

6. The apparatus of claim 4 , wherein the photosensitive sensor comprises a camera or a photodiode.

7. 5. The apparatus of claim 4, wherein the photosensitive sensor comprises a camera disposed outside the vacuum chamber, an optical module disposed within the vacuum chamber, and an optical fiber relaying light from the optical module to the photosensitive sensor.

8. The apparatus of claim 4 , wherein the photosensitive sensor further comprises an illumination source.

9. 5. The apparatus of claim 4, wherein the photosensitive sensor comprises an illumination source located outside the vacuum chamber, an optical module located within the vacuum chamber, and an optical fiber relaying light from the illumination source to the optical module.

10. the controller is configured to receive the droplet detect signal and generate a waveform generator control signal during operation of the target material dispenser; The apparatus of claim 1 , wherein the electronically actuatable element is adapted to induce a velocity perturbation in the flow during operation of the target material dispenser.

11. 1. A method for estimating a coalescence length of a stream of droplets of a liquid target material produced by a droplet generator in a system for generating EUV radiation, comprising: providing a flow of the liquid target material from the droplet generator to a plasma generation system; generating a control signal; introducing a velocity perturbation into the stream by applying the control signal to an electronically actuatable element mechanically coupled to the droplet generator; observing droplets in said stream attempting to coalesce; generating a droplet signal in response to said observing; and estimating the coalescence length based at least in part on a distance between peaks of the droplet signal; The method, wherein the coalescence length is a metric that indicates the distance from a source orifice of the liquid target material flow to where the droplets reach their final coalesced state.

12. 12. The method of claim 11, further comprising, after estimating the coalescence length, controlling operation of the droplet generator using the estimated coalescence length during operation of the droplet generator.

13. a target material dispenser positioned to provide a flow of target material to an irradiation site within the vacuum chamber; an electronically actuatable element mechanically coupled to the target material dispenser and arranged to induce a velocity perturbation in the stream based on a droplet control signal comprising an applied voltage, wherein a coalescence length of the stream depends on the induced velocity perturbation; a detector positioned to optically detect uncoalesced droplets in the stream and to generate a droplet detection signal based on said detection of said uncoalesced droplets; a controller arranged to generate a waveform generator control signal; a waveform generator electrically coupled to the electronically actuatable element and the controller and configured to generate the droplet control signal based at least in part on the waveform generator control signal; the controller is coupled to the detector and configured to vary the coalescence length by modifying the waveform generator control signal based at least in part on the drop detection signal; the controller is configured to determine a transfer function based on the velocity perturbation and the applied voltage. Device.

14. The apparatus of claim 13 , wherein the coalescence length is the distance from a source orifice of the flow of the liquid target material to the point where the droplets reach their final coalesced state.

15. The apparatus of claim 14 , wherein the controller varies the merged length such that the merged length is less than a distance between the source orifice and the irradiation site.

16. the detector is positioned to observe droplets and generate a droplet detection signal during operation of the target material dispenser; the controller is configured to receive the droplet detect signal and generate a waveform generator control signal during operation of the target material dispenser; The apparatus of claim 13 , wherein the waveform generator is adapted to provide the droplet control signal during operation of the target material dispenser.

17. the detector comprises an illumination source and a photosensitive sensor; The apparatus of claim 13 , wherein the photosensitive sensor comprises at least one optical element disposed within the vacuum chamber.

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