Apparatus and method for monitoring and controlling droplet generator performance

Abstract

An apparatus and method for controlling the formation of droplets for generating EUV radiation comprises an arrangement for generating a laser beam directed to an irradiation area and a droplet source. The droplet source comprises a fluid exiting a nozzle and a subsystem having an electrically actuatable element that produces a disturbance in the fluid. The droplet source produces a beam that breaks up into droplets that coalesce into larger droplets as they travel toward the irradiation area. The process is controlled by observing the beam at points in the beam where droplets have not yet fully coalesced.

Description

[0001] CROSS-REFERENCE TO RELATED APPLICATIONS

[0002] This application claims priority to U.S. Application No. 62 / 648,969, filed on March 28, 2018, the entire contents of which are incorporated herein by reference. Technical Field

[0003] The present application relates to extreme ultraviolet ("EUV") light sources and methods of operating the same. These light sources provide EUV light by generating a plasma from a source or target material. In one application, EUV light can be collected and used in a lithography process to produce semiconductor integrated circuits. Background Art

[0004] A patterned EUV beam 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 also called soft X-rays) is generally defined as electromagnetic radiation with a wavelength in the range of about 5 nm to about 100 nm. One wavelength of particular interest for lithography occurs at 13.5 nm.

[0005] Methods of generating EUV light include, but are not limited to, converting a source material into a plasma state having chemical elements that emit lines in the EUV range. These elements may include, but are not limited to, xenon, lithium, and tin.

[0006] In one method, generally referred to as laser produced plasma ("LPP"), it is desirable to generate plasma by irradiating a source material in the form of droplets, streams or lines with a laser beam. In another method, generally referred to as discharge produced plasma ("DPP"), the desired plasma can be generated by placing a source material having an appropriate emission line between a pair of electrodes and inducing a discharge between the electrodes.

[0007] A technique for generating droplets includes melting a target material, sometimes also referred to as a source material, such as tin, and then driving it through a relatively small diameter orifice, for example, an orifice having a diameter of about 0.1 μm to about 30 μm, under high pressure to produce a laminar fluid jet with a velocity in the range of about 30 m / s to about 200 m / s. Under most conditions, the jet breaks into droplets due to a hydrodynamic instability generally referred to as the Rayleigh-Plateau instability. Naturally occurring instabilities (e.g., thermal noise or vortex shedding in the beam leaving the orifice) can cause the beam to break into droplets. The velocities of these droplets may vary and may combine with each other to coalesce into larger droplets.

[0008] In the EUV generation process under consideration herein, it is desirable to control the splitting process / coalescence process. For example, in order to synchronize the droplets with the optical pulses of the LPP driving laser, a repetitive perturbation with an amplitude exceeding the amplitude of the random noise can be applied to the continuous laminar fluid jet emitted from the orifice. By applying a perturbation at the same frequency (or its higher harmonic) as the repetition frequency of the pulsed laser, the droplets can be synchronized with the laser pulses. For example, the perturbation can be applied to the beam by coupling an electrically actuable element (such as a piezoelectric material) to the beam and driving the electrically actuable element with a periodic waveform. In one embodiment, the diameter of the electrically actuable element shrinks and expands (nanometer scale). This size change is mechanically coupled to the capillary, which undergoes corresponding diameter shrinkage and expansion. This volume displacement generates acoustic and elastic waves in the capillary that terminates at the orifice. Then, the target material in the orifice is periodically accelerated by the acoustic wave. The frequency range of the droplets that are far apart provided by the frequency of the driving laser is much lower than the natural Rayleigh splitting frequency of the fluid microjet. The natural breakup frequency of the fluid jet is in the range of about 3 MHz to about 15 MHz, while the expected drive laser operation is in the range of about 50 kHz to about 160 kHz. This means that in order to obtain the desired final droplet of up to 200 small microdroplets, it is necessary to merge into a periodic droplet stream composed of multiple droplets much larger than the orifice diameter.

[0009] As used herein, the term "electrically actuatable element" and its derivatives refer to a material or structure that undergoes a dimensional change when subjected to a voltage, an electric field, a magnetic field, or a combination thereof, including but not limited to piezoelectric materials, electrostrictive materials, and magnetostrictive materials. Apparatus and methods for using electrically actuatable elements to control a droplet stream 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 on August 20, 2013, and U.S. Patent No. 8,513,629, entitled "Droplet Generator with Actuator Induced Nozzle Cleaning," published on January 15, 2009, both of which are incorporated herein by reference in their entirety.

[0010] Therefore, the task of the droplet generator is to place the droplets at the main focus, where these droplets are used as the target material for forming EUV. The droplets must arrive at the main focus within certain spatial and temporal stability criteria, that is, the position and timing are repeatable within acceptable tolerances. They must also arrive at a given frequency and speed. Furthermore, the droplets must be completely coalesced, which means that the droplets must be monodisperse (uniform in size) and arrive under given driving frequency conditions. For example, the droplet beam should have no "companion" droplets, that is, no smaller droplets of target material that have failed to coalesce into main droplets. Due to the use of small orifices and large pressures, meeting these standards becomes very complicated, so it may be necessary to use an electrically actuable element to drive the form to merge about 200 microdroplets. The operating window is usually very small, making the system sensitive to performance changes (such as changes in performance over time). For example, when the performance of the droplet generator changes, the droplets generated by the droplet generator may not be completely coalesced when they arrive at the main focus. Eventually, the performance of the droplet generator degrades to a point that necessitates taking the droplet generator offline for maintenance or replacement.

[0011] Therefore, there is a need to be able to control droplet generation and coalescence in a way that allows these processes to be optimized. Summary of the invention

[0012] A simplified overview of one or more embodiments is given below to provide a basic understanding of the embodiments. This overview is not an exhaustive overview of all contemplated embodiments, and is not intended to identify key elements or important elements of all embodiments, nor is it intended 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 detailed implementation presented later.

[0013] Therefore, it is desirable to confirm the correct operation of the droplet generator by determining whether the droplets fully coalesce before they reach the primary focus. This can be accomplished by supplying an optical feedback system to the EUV source that can identify whether a particular electrical waveform supplied to the droplet generator produces droplets that coalesce at the primary focus.

[0014] According to one aspect of the embodiment, an apparatus is disclosed, comprising: a target material dispenser arranged to provide a target material beam to an irradiation site in a vacuum chamber; an electrically actuatable element mechanically coupled to the target material dispenser and arranged to induce a velocity disturbance in the beam based on a droplet control signal; a detector arranged to observe droplets in the beam at points where the droplets in the beam have not yet fully coalesced and 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 electrically actuatable element and the controller for supplying the droplet control signal based at least in part on the waveform generator control signal. The electrically actuatable element may be a piezoelectric element.

[0015] According to another aspect of the embodiment, an apparatus is disclosed, comprising: a target material dispenser arranged to provide a target material beam to an irradiation site in a vacuum chamber; an electrically actuatable element mechanically coupled to the target material dispenser and arranged to induce a velocity disturbance in the beam based on a droplet control signal; a detector arranged to observe droplets in the beam at a point in the beam where the droplets have not yet fully coalesced and generate a droplet detection signal, the detector comprising an irradiation source and a photosensitive sensor, the photosensitive sensor comprising at least one optical element arranged in 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 electrically actuatable element and the controller for supplying the droplet control signal based at least in part on the waveform generator control signal. The electrically 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 be 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. The light-sensitive sensor may be 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. The light-sensitive sensor may include an illumination source. The light-sensitive sensor may include: an illumination source arranged outside 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.

[0016] According to another aspect of the embodiment, a method is disclosed, comprising the steps of providing a beam of target material to a radiation site in a vacuum chamber using a target material dispenser, the target material dispenser comprising an electrically actuatable element arranged to induce a velocity disturbance in the beam based on a droplet control signal, observing droplets in the beam at points where the droplets in the beam have not yet 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 a droplet control signal based at least in part on the waveform generator control signal. The electrically actuatable element may be a piezoelectric element.

[0017] According to another aspect of the embodiment, a method for determining and using a transfer function of a droplet generator is disclosed, the droplet generator being adapted to deliver a beam of liquid target material to a radiation region in a system for generating EUV radiation, the method comprising the steps of providing a beam of target material from the droplet generator to a plasma generation system; generating a control signal; applying the control signal to an electrically actuatable element mechanically coupled to the droplet generator to introduce a velocity disturbance into the beam; determining a velocity amplitude; determining a 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.

[0018] According to another aspect of the embodiment, a method of controlling a droplet generator adapted to deliver a beam of liquid target material to a radiation region in a system for generating EUV radiation is disclosed, the method comprising the steps of providing a beam of target material from the droplet generator to a plasma generation system; generating a control signal; introducing a velocity disturbance into the beam by applying the control signal to an electrically actuatable element mechanically coupled to the droplet generator; observing the beam at a point in the beam where droplets have not yet fully coalesced; and modifying the control signal based at least in part on a result of the observing step.

[0019] According to another aspect of the embodiment, a method for estimating a coalescence length of a beam of droplets of a liquid target material generated by a droplet generator in a system for generating EUV radiation is disclosed, the method comprising the steps of providing a beam of target material from the droplet generator to a plasma generation system; generating a control signal; introducing a velocity perturbation into the beam by applying the control signal to an electrically actuatable element mechanically coupled to the droplet generator; observing the beam at a point where droplets in the beam have not yet fully coalesced to generate a droplet signal; and estimating the coalescence length based at least in part on a distance between peaks in the droplet signal. The estimated coalescence length can then be used to control the operation of the droplet generator.

[0020] According to another aspect of the embodiment, a method for estimating a condition of a droplet generator suitable for delivering a beam 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 beam of target material from a droplet generator to a plasma generation system; generating a control signal; introducing a velocity disturbance into the beam by applying the control signal to an electrically actuatable element of the target material mechanically coupled to the droplet generator; observing the beam at a point where droplets in the beam have not yet fully coalesced to produce a droplet signal; and evaluating the condition of the droplet generator based on the droplet signal.

[0021] Further embodiments, features, and advantages of the present invention, as well as the structure and operation of various embodiments are described in detail below with reference to the accompanying drawings. BRIEF DESCRIPTION OF THE DRAWINGS

[0022] The accompanying drawings, which are incorporated herein and constitute a part of the specification, illustrate the methods and systems of embodiments of the present invention by way of example and not limitation. Together with the detailed description, the drawings are also used to explain the principles and enable those skilled in the relevant art to make and use the methods and systems proposed herein. In the accompanying drawings, the same reference numerals indicate the same or functionally similar elements.

[0023] Figure 1 A simplified schematic diagram of an EUV light source coupled to an exposure device.

[0024] Figure 1A is a simplified schematic diagram of an apparatus including an EUV light source with an LPP EUV light radiator.

[0025] Figure 2 Schematic diagram of the droplet generation subsystem for an EUV light source.

[0026] Figure 3 , FIG. 3A to FIG. 3C , Figure 4 and Figure 5 Several different techniques are illustrated for coupling one or more electrically actuatable elements to a fluid to produce disturbances in the beam exiting an aperture.

[0027] Figure 6 is a diagram illustrating a coalescence state in a droplet stream.

[0028] Figure 7 is a diagram of a synthetic waveform such as may be used according to one aspect of an embodiment.

[0029] Figure 8 is a diagram of a droplet generation system with feedback such as may be used according to one aspect of an embodiment.

[0030] Fig. 9is a diagram of a droplet generation system with feedback such as may be used according to one aspect of an embodiment.

[0031] Fig. 10A and Fig. 10B is a diagram illustrating possible droplet signals according to one aspect of the embodiments.

[0032] Fig.11 is a flow chart illustrating a method of estimating coalescence length according to an aspect of an embodiment.

[0033] Fig.12 is a flow chart illustrating a method for determining a transfer function of a droplet generator according to an aspect of an embodiment.

[0034] Other features and advantages of the present invention and 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 proposed herein for illustrative purposes only. Based on the teachings contained herein, other embodiments will be apparent to those skilled in the relevant art. DETAILED DESCRIPTION

[0035] Now, various embodiments are described with reference to the accompanying drawings, wherein throughout the text, similar reference numerals are used to refer to similar elements. In the following description, for the purpose of explanation, many specific details are elaborated to facilitate a thorough understanding of one or more embodiments. However, in some or all cases, it may be apparent that any embodiment described below can be put into practice without adopting the specific design details described below. In other examples, known structures and devices are shown in block diagram form to facilitate the description of one or more embodiments. A simplified overview of one or more embodiments is presented below to provide a basic understanding of the embodiments. This overview is not an exhaustive overview of all contemplated embodiments, and is not intended to identify the key elements or important elements of all embodiments, nor is it intended to describe the scope of any or all embodiments.

[0036] However, before describing such embodiments in more detail, it is beneficial to provide an example environment in which embodiments of the present invention may be implemented. In the following description and in the claims, the terms "upper", "lower", "top", "bottom", "vertical", "horizontal", etc. may be employed. These terms are intended to illustrate relative orientations only, not any orientation relative to gravity.

[0037] First reference Figure 1, showing a simplified schematic cross-sectional view of selected portions of an example of an EUV lithography apparatus, generally designated 10". The apparatus 10" may, for example, be used to expose a substrate 11, such as a wafer coated with a resist, using patterned EUV light. For the apparatus 10", an exposure device 12" (e.g., an integrated circuit lithography tool such as a stepper, a scanner, a step and scan system, a direct write system, an apparatus using a contact and / or proximity mask, etc.) utilizing EUV light may be provided, the exposure device 12" having, for example, one or more optical elements 13a, 13b for irradiating a patterned optical element 13c, such as a mask, with an EUV light beam to produce a patterned beam; and one or more reduced projection optical elements 13d, 13e for projecting the patterned beam onto the substrate 11. A mechanical component (not shown) may be provided for generating a controlled relative movement between the substrate 11 and the patterning device 13c. As Figure 1 As further shown, the device 10" may include an EUV light source 20" in a chamber 26", which includes an EUV light radiator 22 that emits EUV light, which is reflected by an optical element 24 along a path into the exposure device 12" to illuminate the substrate 11. The illumination system may include various types of optical components for directing, shaping or controlling radiation, such as refractive optical components, reflective optical components, electromagnetic optical components, electrostatic optical components, or other types of optical components, or any combination thereof.

[0038] As used herein, the term "optical element" and its derivatives are to be interpreted broadly to include, but are not limited to, one or more components that reflect and / or transmit and / or manipulate incident light, and include, but are not limited to, one or more lenses, windows, filters, wedges, prisms, prism gratings, 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, diffuse reflectors, and combinations thereof. Furthermore, unless otherwise specified, the term "optical element" or its derivatives as used herein are not limited to components that operate or have advantages only within one or more specific wavelength ranges, such as at EUV output light wavelengths, illumination laser wavelengths, wavelengths suitable for metrology, or any other specific wavelengths.

[0039] Figure 1AA specific example of the apparatus 10' is illustrated, which includes 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 transmitting the light pulses to a light source chamber 26. For the apparatus 10, the light pulses may propagate from the system 21 along one or more beam paths into the chamber 26 to irradiate the source material at the irradiation region 48, generating an EUV light output for substrate exposure in the exposure device 12.

[0040] Used for Figure 1A Suitable lasers for the illustrated system 21 may include pulsed laser devices, such as pulsed gas discharge CO2 laser devices, which, for example, use DC or RF excitation to produce radiation at 9.3 μm or 10.6 μm, which operate at relatively high power (e.g., 10 kW or more) and have a high pulse repetition frequency, for example, 50 kHz or more. In a specific embodiment, the laser may be an axial flow RF pumped CO2 laser having an oscillator-amplifier configuration with multi-stage amplification (e.g., master oscillator / power amplifier (MOPA) or power oscillator / power amplifier (POPA)) and having a seed pulse initiated by a relatively low energy and high repetition rate Q-switched oscillator capable of, for example, 100 kHz operation. The laser pulses from the oscillator may then be amplified, shaped and / or focused before reaching the irradiation region 48. A continuously pumped CO2 amplifier may be used for the laser system 21. Alternatively, the laser may be configured as a so-called "self-aligning" laser system in which the droplet acts as a reflector of the optical cavity.

[0041] Depending on the application, other types of lasers may also be suitable, for example, excimer or molecular fluorine lasers operating at high power and high pulse repetition rate. Other examples include solid-state lasers, for example, with an active medium in the form of an optical fiber, rod, slab, or disk; other laser architectures with one or more chambers (e.g., oscillator chambers) and one or more amplifier chambers (where the amplifier chambers are connected 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 implanted with one or more excimer, molecular fluorine, or CO2 amplifier or oscillator chambers may be suitable. Other designs may be suitable.

[0042] In some examples, the source material may be irradiated first 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 two separate oscillators. In some arrangements, one or more common amplifiers may be used to amplify both the pre-pulse seed and the main pulse seed. For other arrangements, separate amplifiers may be used to amplify the pre-pulse seed and the main pulse seed.

[0043] Figure 1A It is also shown that the device 10 can include a beam conditioning unit 50, which has one or more optical elements for beam conditioning, such as expanding, steering and / or focusing the beam between the laser source system 21 and the irradiation site 48. For example, the steering system includes one or more mirrors, prisms, lenses, etc., which can be configured and arranged to steer the laser focal spot to different positions in the chamber 26. For example, the steering system can include a first plane mirror and a second plane mirror, the first plane mirror being mounted on a tip-title actuator that can independently move the first plane mirror in two dimensions; and the second plane mirror being mounted on a tip-title actuator that can independently move the second plane mirror in two dimensions. With this arrangement, the steering system can controllably move the focal spot in a direction substantially orthogonal to the beam propagation direction (beam axis).

[0044] The beam conditioning unit 50 may include a focusing assembly for focusing the beam to the radiation location 48 and adjusting the position of the focal spot along the beam axis. For the focusing assembly, an optical element such as a focusing lens or a mirror may be used, which is coupled to an actuator for moving in a direction along the beam axis to move the focal spot along the beam axis.

[0045] like Figure 1AAs further shown, the EUV light source 20 may also include a source material delivery system 90 to deliver, for example, source material such as tin droplets into the interior of the chamber 26 to an irradiation region or primary focus 48 where the droplets will interact with light pulses from the system 21 to ultimately create a plasma and generate EUV emissions 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, entitled “Systems and Methods for Target Material Delivery in a Laser Produced Plasma EUV Light Source,” issued on January 18, 2011, U.S. Patent No. 7,405,416, entitled “Method and Apparatus For EUV Plasma Source Target Delivery,” issued on July 29, 2008, and U.S. Patent No. 7,372,056, entitled “LPP EUV Plasma Source Material Target Delivery System,” issued on May 13, 2008, the entire contents of which are incorporated herein by reference in their entirety.

[0046] Source materials for generating EUV light output for substrate exposure may include, but are not limited to, materials including tin, lithium, xenon, or combinations thereof. EUV emitting elements (e.g., tin, lithium, xenon, etc.) may be in the form of liquid droplets and / or solid particles contained within liquid droplets. For example, elemental tin may be used as pure tin, tin compounds (e.g., SnBr4, SnBr2, SnH4), tin alloys (e.g., tin-gallium alloys, tin-indium alloys, tin-indium-gallium alloys, or combinations thereof). Depending on the materials used, the source materials may be present in the irradiation area under a variety of temperature conditions including room temperature or near room temperature (e.g., tin alloys, SnBr4), elevated temperatures (e.g., pure tin), or temperatures below room temperature (e.g., SnH4), and in some cases, may be relatively volatile, such as SnBr4.

[0047] Continue to refer Figure 1AThe 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 generate light pulses for delivery to the chamber 26, and / or for controlling the movement of optical elements 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, which provide an output indicating the position of one or more droplets, for example relative to the irradiation area 48. The one or more imagers 70 may provide the output to a droplet position detection feedback system 62, which may, for example, calculate the droplet position and trajectory, from which the droplet error may be calculated, for example, droplet by droplet or on an average. The droplet error may then be provided as an input to the controller 60, which may, for example, provide position, direction, and / or timing correction signals to the system 21 to control the laser trigger timing and / or control the movement of optical elements in the beam conditioning unit 50, for example, to change the position and / or focus power of the light pulses delivered to the irradiation area 48 in the chamber 26. In addition, for the EUV light source 20, the source material delivery system 90 can have a control system that can operate in response to signals from the controller 60 (which in some embodiments can include the droplet errors described above or a certain amount derived therefrom) to, for example, modify the release point, initial droplet beam direction, droplet release timing and / or droplet modulation to correct errors in droplets arriving at the desired irradiation area 48.

[0048] continue Figure 1A , the device 10 may also include an optical element 24", such as a near-normal incidence collector reflector having a reflective surface in the form of a prolate spheroid (i.e., an ellipse rotated about its major axis) having a graded multilayer coating (for example) 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. Figure 1A It is shown that the optical element 24" can be formed with an aperture to allow light pulses generated by the system 21 to pass through and reach the illumination area 48. As shown, the optical element 24" can be, for example, a prolate ellipsoidal reflector having a first focus within or near the illumination area 48 and a second focus in a so-called intermediate area 40, where EUV light can be output from the EUV light source 20 and input to the exposure equipment 12, such as an integrated circuit lithography tool, using the EUV light. It should be appreciated that other optical elements can be used in place of the prolate ellipsoidal reflector for collecting light and directing it to an intermediate location for subsequent delivery to an equipment utilizing EUV light.

[0049] A buffer gas such as hydrogen, helium, argon, or a combination thereof may be introduced, replenished, and / or removed from chamber 26. The buffer gas may be present in chamber 26 during the plasma discharge and may be used to slow down ions produced by the plasma to reduce optical component degradation and / or improve 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.

[0050] Figure 2 The droplet generation system is illustrated in more detail. The source material delivery system 90 delivers droplets to the radiation site / primary focus 48 within the chamber 26. The waveform generator 230 provides a drive waveform to the electrically actuatable element in the droplet generator 90, which introduces a velocity disturbance into the droplet beam. The waveform generator operates under the control of the controller 250 based at least in part on data from the data processing module 252. The data processing module receives data from one or more detectors. In the example shown, the detector includes a camera 254 and a photodiode 256. A droplet is irradiated by one or more lasers 258. In this typical arrangement, the detector detects / images the droplet at a point in the beam where coalescence is expected to occur. In addition, the detector and laser are arranged outside the vacuum chamber 26, and the beam is observed through a window in the wall of the vacuum chamber 26.

[0051] Figure 3 The components of a simplified droplet source 92 are shown in a schematic format. As shown, the droplet source 92 can include a reservoir 94 that holds a fluid (e.g., molten tin) under pressure. In addition, as shown, the reservoir 94 can be formed with an orifice 98, thereby allowing a pressurized fluid 96 to flow through the orifice, thereby establishing a continuous stream 100 that subsequently breaks into a plurality of droplets 102a, 102b.

[0052] 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 a design, the lasers and detectors are located outside the vacuum vessel, which means that the laser beam must be precisely projected onto the droplet stream and the droplet optical response must be imaged by remote optical elements. This provides certain limitations in terms of the accuracy and resolution of the measurement, and restricts the ways in which the measurement can be used.

[0053] As mentioned, the system can be used to establish whether droplets are coalescing (or not) near the primary focus. If the droplets are not coalescing, the operating parameters of the droplet generator can be adjusted to ensure coalescence. However, in general, this process can only be performed when the drive laser is turned off, which may result in a large amount of machine downtime. It may be advantageous to be able to adjust the droplet generator signal while the plasma is being generated. It may also be advantageous to be able to obtain online measurements of droplet generator performance, which can be used to plan maintenance of the droplet generator and predict the life of the droplet generator. It has been shown that the performance of the droplet generator changes over time. A signal that is ideal at the beginning may result in satellite droplets after a few hours. In this case, the drive laser droplets are turned off and the driver signal optimization is repeated. Typically, this operation is performed every day, which results in about 1 hour of downtime.

[0054] Figure 3 One possible configuration of a droplet source 92 is shown as part of a droplet generator 90. The droplet source 92 also includes a subsystem for generating disturbances in a fluid having an electrically actuatable element 104 operably coupled to the fluid 96 and a signal generator 106 that drives the electrically actuatable element 104. FIG. 3A to FIG. 3C , Figure 4 and Figure 5 Various ways in which one or more electrically actuatable elements can be operably coupled to a fluid to generate droplets are shown. Figure 3A Initially, an arrangement is shown in which a fluid is forced under pressure from a reservoir 108 through a tube 110, such as a capillary tube, having an inner diameter between about 0.2 mm and about 0.8 mm and a length between about 10 mm and about 50 mm, thereby producing a continuous stream 112 that exits an orifice 114 of the tube 110 and subsequently breaks into droplets 116a, 116b. As shown, an electrically actuatable element 118 may be coupled to the tube. For example, the electrically actuatable element may be coupled to the tube 110 to deflect the tube 110 and perturb the stream 112. Figure 3B A similar arrangement is shown having a reservoir 120, a tube 122, and a pair of electrically actuatable elements 124, 126, each coupled to the tube 122 to deflect the tube 122 at a corresponding frequency. Figure 3C Another variation is shown in which a plate 128 is located in a reservoir 130 that is movable to drive fluid through an orifice 132 to produce a beam 134 that breaks into droplets 136a, 136b. As shown, a force may be applied to the plate 128 and one or more electrically actuatable elements 138 may be coupled to the plate to perturb the beam 134. It should be appreciated that the capillary may be coupled to the Figure 3C The embodiments shown are used together.

[0055] Figure 4 Another variation is shown in which fluid is driven under pressure from a reservoir 140 through a tube 142, thereby producing a continuous stream 144 that exits an orifice 146 of the tube 142, which subsequently breaks up into droplets 148a, 148b. As shown, an electrically actuatable element 150 having, for example, an annular shape or a cylindrical tube shape may be positioned around the circumference of the tube 142. When actuated, the electrically actuatable element 150 may selectively squeeze and / or release the tube 142 to disturb the stream 144. It should be appreciated that two or more electrically actuatable elements may be employed to selectively squeeze the tube 142 at respective frequencies.

[0056] Figure 5 Another variation is shown in which fluid is forced under pressure from a reservoir 140' through a tube 142', producing a continuous stream 144' that exits an orifice 146' of the tube 142', which subsequently breaks up into droplets 148a', 148b'. As shown, an electrically actuatable element 150a, for example having an annular shape, may be positioned to surround the circumference of the tube 142'. When actuated, the electrically actuatable element 150a may selectively squeeze the tube 142' to disturb the stream 144' and produce droplets. Figure 5 It is also shown that a second electrically actuatable element 150b, for example having an annular shape, can be positioned around the circumference of tube 142'. When driven, electrically actuatable element 150b can selectively squeeze tube 142' to disturb beam 144' and remove contaminants from orifice 152. For the illustrated embodiment, electrically actuatable elements 150a and 150b can be driven by the same signal generator or different signal generators. As further described below, waveforms having different waveform amplitudes, cycle frequencies and / or waveform shapes can be used to drive electrically actuatable element 150a to generate droplets for EUV output. The electrically actuatable element generates a disturbance in the fluid that generates droplets with different initial velocities, thereby causing at least some adjacent droplet pairs to coalesce before reaching the irradiation area. The ratio of initial micro-droplets to coalesced droplets can be any number, for example, in the range of about 10 droplets to about 500 droplets.

[0057] Thus, controlling the breakup / coalescing process includes controlling the droplets so that they are fully coalesced before reaching the irradiation area and have a frequency corresponding to the pulse frequency of the laser used to irradiate the coalesced droplets. A designer synthesized waveform consisting of a linear superposition of multiple voltage and multiple frequency sinusoidal waveforms can be supplied to the electrically actuatable element to control the coalescence process of the Rayleigh-broken microdroplets to fully coalesced droplets with a frequency corresponding to the laser pulse frequency. The control system can use only a single sinusoidal wave, thereby allowing the phase of each spectral component to be fine-tuned separately. The waveform can be defined as a voltage signal or a current signal.

[0058] The droplet velocity profile on the axis is obtained by imaging the droplet stream at a fixed position downstream of the coalescence and used as feedback to control the droplet generation / coalescence process. As a form of imaging, an optical barrier can be used to resolve the droplet passage in time and the droplet coalescence pattern reconstructed from this information.

[0059] The coalescence of microdroplets and sub-coalesced droplets is controlled by a periodic electrical drive signal on the electrically actuatable actuator of the droplet generator. This signal is automatically monitored during source operation. Based on the crossing interval and DFC data, the optimal operating point can be selected. The selected signal is applied to the droplet generator, and the pre-pulse and drive lasers are optimized for optimal plasma conditions.

[0060] The use of designer waveforms enables the user to target a specific droplet coalescence length at a user-specified frequency using feedback from imaging measurements at a fixed point downstream of a fully coalesced droplet. One form of designer waveform may include: (1) a sine wave having a fundamental frequency approximately equal to the laser pulse frequency; and (2) a collection of higher frequency sine waveforms. All higher frequency waveforms are harmonics of the fundamental frequency, i.e., they are multiples of the fundamental frequency. The use of designer waveforms also permits the determination of the nozzle transfer function of the on-axis target material beam velocity perturbation / profile, which in turn can be used to optimize the parameters of the designer waveform that drives the electrically actuatable element.

[0061] The overall droplet coalescence process can be viewed as a series of multiple sub-coalescence steps or states that vary according to the distance from the nozzle. Figure 6. For example, in a first state 161, that is, when the target material first leaves the orifice or nozzle, the target material is in the form of a velocity-perturbed laminar fluid jet. In a second state 162, the fluid jet breaks up into a series of micro-droplets 164 having different velocities. In a third state 163, the micro-droplets coalesce into droplets of intermediate sizes, referred to as sub-coalesced droplets 165, which have varying velocities relative to each other, as measured by time of flight or by distance from the nozzle. In a fourth state 166, the sub-coalesced droplets coalesce into droplets of the desired final size, i.e., fully coalesced droplets 168. The number of sub-coalescing steps may vary. The distance from the nozzle outlet 169 to the point where the droplet reaches its 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 larger droplets, they are less sensitive to source conditions such as hydrogen gas flow and ion impact.

[0062] As mentioned, if complete coalescence is not achieved, the droplet beam will include smaller droplets known as satellite droplets or micro-satellites. The presence of satellite droplets can be detected by any one or a combination of several methods, for example, using a droplet detection module (DDM), cross spacing, DFC, or even by monitoring changes in the EUV signal. Systems and methods for monitoring droplet beams are disclosed, for example, in U.S. Patent No. 9,241,395, entitled “System and Method for Controlling Droplet Timing in an LPP EUV Light Source,” issued on January 19, 2016, the entire contents of which are incorporated herein by reference. Such measurements are typically used to determine where and when droplets are at the primary focus and the quality of the plasma, so the detection location is at or very close to the plasma location, typically between about 30 cm and 40 cm from the nozzle outlet. Detecting satellites at this distance is challenging because the measurement to detect these uncoalesced droplets may be, for example, half a meter from the wall of the source container bottle, and the satellites may be blown out of the field of view by the container flow.

[0063] Using sensors arranged at a distance (e.g., outside the chamber), this translates into attempting to observe microdroplets on the order of 4 microns in size at a distance of about 40 cm from the nozzle using an optical detector focused on a small area of ​​interest and fine-tuned to detect fully coalesced droplets on the order of 27 microns in size. In addition, the dominant gas flows in the chamber, for example, lateral "side winds" from a cone flow can cause these micro-associates to be spread over a large volume. Placing the measurement closer to the nozzle where the liquid has not yet coalesced can permit observation and detection of micro-associates at points where they are less spread out as they evolve, making it easier to detect all of them. Measuring at this location enables control and optimization of the associative process.

[0064] Now, combined with Figure 7 Some characteristics of the designer waveform examples are described. Figure 7 The upper waveform in is a basic waveform, the frequency of which is generally the same as or related to the pulse rate of the laser used to vaporize the droplets. In this example, the basic waveform is a sine wave. Figure 7 The lower waveform in is a higher frequency waveform, the frequency of which is generally an integer multiple of the frequency of the fundamental waveform. Any arbitrary periodic wave can be used; in this example, the higher frequency waveform is a series of triangular spikes. The two waveforms are superimposed to obtain a composite waveform. The waveform used for sub-coalescing is a superposition of harmonics of the sub-coalescing frequency, which may also include the sub-coalescing frequency.

[0065] One way to enhance the measurement of droplet generator performance is to provide a method for observing droplet generation more directly, which provides detectors that are arranged in the vacuum chamber to observe droplet generation more closely. These detectors can be used to supplement or replace detectors located outside the chamber. These in-situ detectors provide real-time high-resolution feedback and control the operation of the droplet generator. They provide the ability to reduce machine downtime, which would otherwise be used to identify parameters that produce fully coalesced droplets without companion droplets. Minimizing companion droplets reduces dose stability errors due to the presence of on-axis companion droplets, as well as collector life issues due to the presence of on-axis companion droplets. In-situ detectors can also help reduce unplanned downtime caused by the sudden and unexpected need to replace the droplet generator.

[0066] To provide the possibility of more accurate measurements, the measurement can be placed relatively close to the nozzle outlet. The measurement is arranged to sense the droplet pattern at a position between about 0.5 cm and about 5 cm after the nozzle outlet. At this position, 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 position. The droplet passes through the detector. For example, the detector can be a focused laser curtain, in which case the amount of light reflected by the droplet is proportional to the cross-sectional area of ​​the droplet. A portion of the reflected light is collected by optical elements and converted into a time signal, for example, by a high sampling rate photodiode. In the case of a narrow focus, another example is the use of extinction of a laser curtain when a droplet passes. If the focus position can be controlled in a small 3D volume, the measurement can also be used to sense the position of the droplet beam, thereby providing detailed information for the droplet generator manipulation system. The amplitude and relative position of the reflection peak give quantitative information about the current coalescence process and about the current performance level of the droplet generator.

[0067] Reference now Figure 8 , showing an electrically actuatable element 200 located around a capillary 210 of a nozzle 220. The electrically actuatable element 200 converts electrical energy from a waveform generator 230 to apply different pressures to the capillary 210. This introduces velocity disturbances in the beam 240 of the molten target material 240 leaving the nozzle 220. The droplets are imaged by a camera 250 at an imaging point in a state where the droplets have formed but have not yet fully coalesced. If a camera is used, the system will also have a light source that is arranged to illuminate the droplets or is arranged to be blocked by a single droplet as the droplet passes through the beam produced by the light source. Imaging herein encompasses images of the formed droplets and only binary indications of the presence or absence of droplets. Imaging forms a velocity profile of the droplet beam at an imaging point in a state where the droplets have formed but have not yet fully coalesced. The control unit 260 uses the imaging data from the camera 250 to generate a feedback signal to control the operation of the arbitrary wave generator 230. The control unit 260 may control the relative phases of the low frequency periodic waves and the higher order arbitrary periodic waveforms and the amplitudes of the low frequency periodic waves and the higher order arbitrary periodic waveforms based on control input 265 which may originate from another controller or based on user input.

[0068] Fig. 9 Another conceptualization of a system according to one aspect of the embodiment is shown. Fig. 9In the system of FIG. 3 , droplet detection is performed by a laser curtain consisting of a laser 300 and a photodiode 310. Light from the laser 300 is transmitted through an optical fiber 302 to the interior of the chamber 26, where it illuminates a stream of droplets at a point near the droplet generator 90 using an optical element 305. Light reflected from the stream of droplets is transmitted back to the camera 310 by an optical fiber 312 through an optical element 315.

[0069] This optical feedback can be used to optimize the voltage drive signal to be applied to the electrically actuatable element while continuing to generate plasma at the primary focus. As an example, Fig. 10A The expected time signature of a droplet with a frequency of 500kHz and two companion drops is shown. A fully coalesced droplet pattern may consist of one Gaussian peak 400 per cycle. The companions result in a smaller Gaussian peak 410 being present adjacent to the 500kHz main Gaussian peak. The spectral content of the 500kHz drive signal (which may be, for example, a square wave signal) may be fine-tuned to achieve coalescence.

[0070] The sensor can also be used to determine the transfer function for frequencies below the sub-coalescing frequency. Fig. 10B An example is shown in which the droplet velocity of a sub-coalescing droplet stream is modulated using a 50kHz sine wave. Curve 420 shows a completely undisturbed droplet distribution; curve 430 shows a distribution of droplets traveling at 0.2 m / s, which is perturbed using a 50kHz sine perturbation; and, curve 440 shows a distribution of droplets traveling at 0.4 m / s, which is perturbed using a 50kHz sine perturbation. By measuring the relative delay time of the droplets at the sensor location, the velocity can be determined, and from this, the transfer function for 50kHz can be calculated. The units of the transfer function are m / s per volt. By also performing this transfer function measurement for other harmonics besides 50kHz, such as 100kHz and 150kHz, an optimal waveform can be constructed to achieve the shortest coalescence distance for a given voltage budget of the signal generator.

[0071] Once sub-coalescence is ensured, a lower frequency can be superimposed to drive the 500kHz micro-droplets to move closer together. In this example, a 50kHz signal is used to control the coalescence of the 500kHz droplets. At the measured position, coalescence has not yet occurred. (The coalescence process starts at the nozzle, and the various parts of the fluid flow from the orifice outlet have already moved relative to each other.) However, based on the relative spacing of the peaks, the coalescence length can be estimated. Fig.11One method of doing so is shown. In step S50, a beam is started. In step S52, a drive signal is applied to a droplet generator. In step S56, the beam is observed upstream of coalescence, that is, upstream of complete coalescence. In step S58, the spacing between peaks of a signal generated based on observing the beam is determined. In step S60, a coalescence length is estimated based on the determined spacing. The determined coalescence length can then be used, for example, to characterize, control and / or optimize the operation of the droplet generator.

[0072] The velocity amplitude at 50 kHz can also be determined. Combined with the applied voltage, this yields the transfer function of the droplet generator at 50 kHz (m / s per V). Fig.12 A method for implementing the process is shown. Steps S50, S52 and S56 are as described above. In step S60, the velocity amplitude of the droplet is determined. In step S64, the applied voltage is determined. These determined values ​​are used in step S66 to determine a transfer function. The determined transfer function can then be used, for example, to characterize, control and / or optimize the operation of the droplet generator.

[0073] The transfer function can be defined as the velocity perturbation obtained at the nozzle outlet per unit applied voltage at a specific frequency. For the considered nozzle transfer function, the signal applied to the electrically actuatable element (characterized by frequency, amplitude and phase) is the input and the velocity perturbation imposed on the liquid jet is the output. The coalescence length varies with the velocity amplitude of the sinusoidal component, where the frequency is below the sub-coalescing frequency. A larger sinusoidal amplitude implies an increase in the velocity perturbation and therefore a decrease in the coalescence length.

[0074] The designer waveform can be characterized by several parameters. The exact number of parameters depends on the choice of a higher frequency arbitrary periodic waveform that may have several fine-tuning parameters. Generally speaking, the sinusoidal voltage, the voltage of the higher frequency waveform, and the relative phase will be included in the characterization parameters. Although 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 will cause the droplet timing to vary. Generally, the droplet timing variation must be limited so that the droplets can be synchronized with the laser pulses.

[0075] In-situ droplet generator metrology allows for high-resolution droplet detection at the droplet generator outlet. To provide high resolution, the metrology can use, for example, optical fibers to deliver the test laser beam and the detection signal. Advantages of this measurement include: online control of the droplet generator so that droplet generator fine-tuning can be performed during droplet generator operation without downtime. Real-time quantitative feedback on droplet generator performance parameters can be used to predict the time for droplet generator replacement. In addition, preventing on-axis companion droplets near the plasma can extend collector life and improve dose stability. Since the coalescence process is affected by the plasma, it is beneficial to be able to adjust the signal during operation as the plasma is generated. In addition, direct control of the coalescence length avoids the need to turn off the laser beam away from the droplet and perform a new signal search, effectively reducing planned maintenance downtime and unplanned maintenance downtime.

[0076] While the process described above is used in connection with in-situ metrology, it will be appreciated that the process may be used with data collected by remote metrology, ie, metrology with the light source and / or detector located outside the chamber.

[0077] 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 captured by a camera may also be used. This may provide more data on the location and characteristics of the droplet.

[0078] The present invention has been described above by means of functional building blocks illustrating the embodiments of specific functions and their relationships. For ease of description, the boundaries of these functional building blocks are arbitrarily defined in this article. Alternative boundaries can be defined as long as the specific functions and their relationships are properly performed.

[0079] The above description of specific embodiments discloses the general properties of the present invention so comprehensively that others can easily modify and / or adapt to various applications for such specific embodiments without excessive experiments by applying knowledge in the art, without departing from the overall concept of the present invention. Therefore, based on the teachings and guidance presented herein, such adaptations and modifications are intended to fall within the meaning and scope of the equivalents of the disclosed embodiments. It should be understood that the words or terms in this article are used for descriptive purposes rather than for limiting purposes, so that the terms or words in this specification are to be interpreted by the technician according to teachings and guidance. The breadth and scope of the present invention should not be limited by any of the exemplary embodiments described above, but should only be limited according to the attached claims and their equivalents.

[0080] Further aspects of the invention are set out in the following numbered clauses.

[0081] 1. A device comprising:

[0082] a target material dispenser arranged to provide a beam of target material to an irradiation site in the vacuum chamber;

[0083] an electrically actuatable element mechanically coupled to the target material dispenser and arranged to induce a velocity perturbation in the beam based on the droplet control signal;

[0084] a detector arranged to observe the droplets in the beam at points where the droplets have not fully coalesced and to generate a droplet detection signal;

[0085] a controller arranged to receive the drop detect signal and to generate a waveform generator control signal based at least in part on the drop detect signal; and

[0086] A waveform generator is electrically coupled to the electrically actuatable element and the controller for supplying the drop control signal based at least in part on the waveform generator control signal.

[0087] 2. A device according to clause 1, wherein the electrically actuatable element is a piezoelectric element.

[0088] 3. A device comprising:

[0089] a target material dispenser arranged to provide a beam of target material to an irradiation site in the vacuum chamber;

[0090] an electrically actuatable element mechanically coupled to the target material dispenser and arranged to induce a velocity perturbation in the beam based on a droplet control signal;

[0091] a detector arranged to observe the droplets in the beam at points where the droplets in the beam have not yet fully coalesced and to generate a droplet detection signal, the detector comprising an illumination source and a light-sensitive sensor comprising at least one optical element arranged in the vacuum chamber;

[0092] a controller arranged to receive the drop detect signal and to generate a waveform generator control signal based at least in part on the drop detect signal; and

[0093] A waveform generator is electrically coupled to the electrically actuatable element and the controller for supplying the drop control signal based at least in part on the waveform generator control signal.

[0094] 4. A device according to clause 3, wherein the electrically actuatable element is a piezoelectric element.

[0095] 5. The device of clause 3, wherein the light-sensitive sensor comprises a camera.

[0096] 6. The device of clause 3, wherein the light-sensitive sensor comprises a photodiode.

[0097] 7. The apparatus of clause 3, wherein the light-sensitive sensor comprises: a camera disposed outside the vacuum chamber; an optical module disposed in the vacuum chamber; and an optical fiber for relaying light from the optical module to the camera.

[0098] 8. The device of clause 3, wherein the photosensitive sensor comprises: a photodiode disposed outside the vacuum chamber; an optical module disposed in the vacuum chamber; and an optical fiber for relaying light from the optical module to the photodiode.

[0099] 9. The device of clause 3, wherein the light-sensitive sensor comprises an illumination source.

[0100] 10. The apparatus of clause 3, wherein the light-sensitive sensor comprises an illumination source disposed outside the vacuum chamber; an optical module disposed within the vacuum chamber; and an optical fiber for relaying light from the illumination source to the optical module.

[0101] 11. A method comprising the steps of:

[0102] providing a beam of target material to an irradiation site in the vacuum chamber using a target material dispenser, the target material dispenser comprising an electrically actuatable element arranged to induce a velocity disturbance in the beam based on a droplet control signal;

[0103] observing droplets in the beam at points where the droplets in the beam have not yet fully coalesced and generating a droplet detection signal;

[0104] generating a waveform generator control signal based at least in part on the drop detection signal; and

[0105] A drop control signal is supplied based at least in part on the waveform generator control signal.

[0106] 12. The method according to clause 11, wherein the electrically actuatable element is a piezoelectric element.

[0107] 13. A method of determining and using a transfer function of a droplet generator adapted to deliver a beam of liquid target material to a radiation region in a system for generating EUV radiation, the method comprising the steps of:

[0108] providing a target material beam from a droplet generator to a plasma generation system;

[0109] generating a control signal;

[0110] applying a control signal to an electrically actuatable element mechanically coupled to a droplet generator to introduce a velocity perturbation into the beam;

[0111] Determine velocity amplitude;

[0112] determining a transfer function for the droplet generator based at least in part on the velocity amplitude and the control signal; and

[0113] The droplet generator is controlled using the determined transfer function.

[0114] 14. A method of controlling a droplet generator adapted to deliver a beam of liquid target material to an illumination area in a system for generating EUV radiation, the method comprising the steps of:

[0115] providing a target material beam from a droplet generator to a plasma generation system;

[0116] generating a control signal;

[0117] introducing a velocity perturbation into the beam by applying a control signal to an electrically actuatable element mechanically coupled to the droplet generator;

[0118] observing the beam at a point where droplets in the beam have not yet fully coalesced; and

[0119] The control signal is modified based at least in part on the results of the observing step.

[0120] 15. A method for estimating the coalescence length of a beam of droplets of a liquid target material generated by a droplet generator in a system for generating EUV radiation, the method comprising the steps of:

[0121] providing a target material beam from a droplet generator to a plasma generation system;

[0122] generating a control signal;

[0123] introducing a velocity perturbation into the beam by applying a control signal to an electrically actuatable element mechanically coupled to the droplet generator;

[0124] observing the beam at a point where droplets in the beam have not fully coalesced to produce a droplet signal; and

[0125] The coalescence length is estimated based at least in part on the distance between peaks in the droplet signal.

[0126] 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 the operation of the droplet generator.

[0127] 17. A method of evaluating the condition of a droplet generator suitable for delivering a beam of liquid target material to an irradiation area in a system for generating EUV radiation, the method comprising the steps of:

[0128] providing a target material beam from a droplet generator to a plasma generation system;

[0129] generating a control signal;

[0130] introducing a velocity perturbation into the beam by applying a control signal to an electrically actuatable element mechanically coupled to a target material in a droplet generator;

[0131] observing the beam at a point in the beam where droplets have not yet fully coalesced to produce a droplet signal; and

[0132] A condition of the droplet generator is evaluated based on the droplet signal.

Claims

1. A device comprising: a target material dispenser arranged to provide a beam of target material to an irradiation site in the vacuum chamber through a nozzle outlet; an electrically actuatable element mechanically coupled to the target material dispenser and arranged to induce a velocity disturbance in the beam based on a droplet control signal; a detector arranged to observe the droplets in the beam at a point where the droplets in the beam have not yet completely coalesced and at a position between 0.5 cm and 5 cm from the nozzle outlet and to generate a droplet detection signal; a controller arranged to receive the drop detection signal and to generate a waveform generator control signal based at least in part on the drop detection signal; as well as A waveform generator is electrically coupled to the electrically actuatable element and the controller for supplying the drop control signal based at least in part on the waveform generator control signal.

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

3. The device according to claim 1, wherein: The detector is arranged to observe droplets during operation of the target material dispenser and to generate a droplet detection signal; The controller is arranged to receive the drop detection signal and generate a waveform generator control signal during operation of the target material dispenser; as well as The waveform generator is adapted to provide the drop control signal during operation of the targeted material dispenser.

4. The device according to claim 1, wherein The detector includes an illumination source and a light-sensitive sensor including at least one optical element disposed in the vacuum chamber.

5. The device of claim 4, wherein the electrically actuatable element is a piezoelectric element. The device of claim 4 , wherein the light-sensitive sensor comprises a camera or a photodiode.

7. The device according to claim 4, wherein the light-sensitive sensor comprises: a camera disposed outside the vacuum chamber; an optical module, wherein the optical module is arranged in the vacuum chamber; and an optical fiber for relaying light from the optical module to the light-sensitive sensor.

8. The device of claim 4, wherein the light-sensitive sensor further comprises an illumination source.

9. The device according to claim 4, wherein the light-sensitive sensor comprises: an illumination source, the illumination source being arranged outside the vacuum chamber; an optical module, wherein the optical module is arranged in the vacuum chamber; and an optical fiber for relaying light from the illumination source to the optical module.

10. An apparatus according to claim 1, wherein the controller is arranged to receive the droplet detection signal and generate a waveform generator control signal during operation of the target material dispenser, and the electrically actuatable element is suitable for causing a velocity disturbance in the beam during operation of the target material dispenser.

11. A method comprising the steps of: providing a beam of target material through a nozzle outlet to an irradiation site in a vacuum chamber using a target material dispenser, the target material dispenser comprising an electrically actuatable element arranged to induce a velocity disturbance in the beam based on a droplet control signal; observing the droplets in the beam at a point where the droplets in the beam have not yet completely coalesced and at a position between 0.5 cm and 5 cm from the nozzle outlet and generating a droplet detection signal; generating a waveform generator control signal based at least in part on the drop detection signal; as well as The drop control signal is supplied based at least in part on the waveform generator control signal.

12. The method of claim 11, wherein the steps of observing a droplet, generating a droplet detection signal, and providing the droplet control signal occur in situ during operation of the target material dispenser.

13. The method of claim 11, wherein the step of providing the droplet control signal occurs in-situ during operation of the target material dispenser and the velocity perturbation is induced in the beam occurs in-situ during operation of the target material dispenser.

14. A method of determining and using a transfer function of a droplet generator adapted to deliver a beam of liquid target material to an irradiation region in a system for generating EUV radiation, the method comprising the steps of: The droplet generator provides the target material beam to a plasma generation system; generating a control signal; applying the control signal to an electrically actuatable element mechanically coupled to the droplet generator to introduce a velocity perturbation into the beam; Determine velocity amplitude; determining a transfer function for the droplet generator based at least in part on the velocity magnitude and the control signal; and The droplet generator is controlled using the determined transfer function.

15. The method of claim 14, wherein the method is performed during operation of the droplet generator.

16. A method of controlling a droplet generator adapted to deliver a beam of liquid target material to an illumination area in a system for generating EUV radiation, the method occurring during operation of the droplet generator and comprising the steps of: providing a beam of target material from the droplet generator to a plasma generating system; generating a control signal; introducing a velocity perturbation into the beam by applying the control signal to an electrically actuatable element mechanically coupled to the droplet generator; observing the stream at a point where droplets in the stream have not yet completely coalesced and at a location between 0.5 cm and 5 cm from a nozzle outlet of the droplet generator; as well as The control signal is modified based at least in part on a result of the observing step.

17. A method for estimating the coalescence length of a beam of droplets of a liquid target material generated by a droplet generator in a system for generating EUV radiation, the method comprising the steps of: providing a beam of the target material from the droplet generator to a plasma generating system; generating a control signal; introducing a velocity perturbation into the beam by applying the control signal to an electrically actuatable element mechanically coupled to the droplet generator; observing the beam at a point where droplets in the beam have not fully coalesced to produce a droplet signal; as well as The coalescence length is estimated based at least in part on a distance between peaks in the droplet signal.

18. The method according to claim 17, further comprising: Following the step of estimating the coalescing length, there is a step of using the estimated coalescing length to control the operation of the droplet generator during operation of the droplet generator.

Images