Droplet generator

Abstract

A droplet generator assembly comprising a droplet generator, a modulation controller and a measurement controller. The droplet generator comprises a nozzle and a modulator. The droplet generator is configured to receive fuel, apply a modulation to the fuel using the modulator so as to break up the fuel into droplets, and emit said droplets from the nozzle. The modulation controller is operable to provide a driving signal to the modulator, the modulator configured to apply the modulation based on the driving signal. The measurement controller is operable to obtain data representative of a temperature of the modulator.

Description

DROPLET GENERATORCROSS-REFERENCE TO RELATED APPLICATIONS

[0001] This application claims priority of US application 63 / 735,312 which was filed on December 17, 2024 which is incorporated herein in its entirety by referenceFIELD

[0002] The present disclosure relates to a droplet generator and apparatus and methods for improving the performance of a droplet generator. In particular, but not exclusively, the droplet generator may be used for the generation of EUV light. The droplet generator can be used in connection with a lithographic apparatus, particularly, but not exclusively, an EUV lithographic apparatus.BACKGROUND

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

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

[0005] Methods for generating EUV light include, but are not limited to, altering the physical state of a source material, also known as a target material, to a plasma state. The source materials include a compound or an element, for example, xenon, lithium, or tin, with an emission line in the EUV range. In one such method, often termed laser produced plasma (“LPP”), the required plasma is produced by irradiating a source material, for example, in theform of a droplet, stream, or cluster of source material, with an amplified light beam that can be referred to as a drive laser. For this process, the plasma is typically produced in a vessel, for example, a vacuum chamber. Where droplets are used, they are provided via a droplet generator apparatus, which is supplied with high pressure liquid source material.

[0006] As lithography apparatuses advance, existing assemblies used in such apparatuses will not have the required performance, stability, or reliability required. Existing droplet generators will be unable to meet the requirements of next generation lithography apparatuses, which will likely require higher frequencies of droplet generation, and so it is desirable to provide apparatuses and assemblies which provide improved performance, stability, and / or reliability.SUMMARY

[0007] According to a first aspect there is provided a droplet generator assembly. The droplet generator assembly comprises a droplet generator, a modulation controller and a measurement controller. The droplet generator comprises a nozzle and a modulator, wherein the droplet generator is configured to receive fuel, apply a modulation to the fuel using the modulator so as to break up the fuel into droplets, and emit said droplets from the nozzle. The modulation controller is operable to provide a driving signal to the modulator, the modulator configured to apply the modulation based on the driving signal. The measurement controller is operable to obtain data representative of a temperature of the modulator.

[0008] Self-heating of modulators within a droplet generator can be problematic, for example causing premature aging of the modulators and / or reducing their efficacy. By having a droplet generator assembly with both modulation and measurement controllers the modulator can be used for modulation and also have its temperature monitored. In this way, any overheating can be detected and remedied with minimal adverse effects to the apparatus, for example downtime or other interruption of droplet generation.

[0009] The nozzle may comprise a conduit. The nozzle may comprise a first orifice configured to fluidly couple to a reservoir storing fuel (e.g. in the form of a liquid) and a second orifice configured to emit the droplets. The emitted droplets may include sub-droplets (e.g. small droplets which are controlled, by the applied modulation, to coalesce into larger droplets following emission from the nozzle). The modulator is configured to apply a modulation to the fuel, for example when the fuel is within the nozzle of the droplet generator. The fuel may break up at least partially within the droplet generator (or the nozzle thereof) and / or at least partially following emission from the droplet generator (or the nozzle thereof). As such, the droplet generator may be said to be configured to receive fuel, emit fuel from the nozzle, andapply a modulation to the fuel such that the emitted fuel forms droplets during and / or following emission

[0010] The droplet generator assembly may be referred to as an assembly or an apparatus (e.g. a droplet generator apparatus). The modulator may be located in or on the nozzle. The modulator may at least partially surround the nozzle, for example around its circumference.

[0011] The modulator may comprise a piezoelectric actuator. For example, the piezoelectric actuator may comprise lead zirconite titanate (PZT). Of course, other piezoelectric materials may be used. Piezoelectric actuator may include one or more piezoelectric elements. PZT is particularly sensitive to the shape and frequency of the driving signal. Heating in PZT can lead to premature degradation.

[0012] The modulation controller and measurement controller may be switchably connected to the droplet generator. Beneficially, such a switchable connection can allow one to alternate between using the droplet generator assembly for droplet generation and monitoring the temperature of the modulator.

[0013] The modulation controller and measurement controller may be connected to the droplet generator by way of a switch mechanism. The switch mechanism may have (e.g. comprise) a first switching arrangement in which the droplet generator is connected to the modulation controller. The switch mechanism may have (e.g. comprise) a second switching arrangement in which the droplet generator is connected to the measurement controller. The switch mechanism may comprise a transistor switch or a fast relay switch, for example.

[0014] The measurement controller may comprise a capacitance measurement system. The capacitance measurement system may be operable to measure a capacitance of the modulator and determine a temperature of the modulator based on the measured capacitance.

[0015] The capacitance measurement system may comprise a series resistor. That is, the capacitance measurement system may comprise circuitry in the form of an RC circuit. The applied modulation may comprise a plurality of frequencies comprising a first frequency and at least one additional frequency. The at least one additional frequency may comprise a second frequency and a third frequency.

[0016] The first frequency may correspond to a resonant frequency of the modulator and / or a desired droplet generation rate and / or the resonance of a nozzle of the droplet generator. That is, the first frequency may be substantially equal to a resonant frequency of the modulator and / or to a resonant frequency of the nozzle of the droplet generator and / or to a desired droplet generation rate. Substantially equal to, or corresponding to, may mean that the frequencies (e.g. the first frequency and the resonant frequency) are similar in frequency, for example within acertain bandwidth of each other. An acceptable bandwidth may depend on the sharpness (e.g. Q factor) of the resonance. It should be understood that, due to real-world implementation issues, for example fluctuations, losses, error margins when setting a frequency and / or transmitting a modulation signal etc., the frequencies may not be identical.

[0017] Each of the at least one additional frequencies may have a frequency that is an integer multiple of the first frequency. The at least one additional frequencies may therefore have a higher frequency than the first frequency. The at least one additional frequency may comprise a second frequency of, for example, twice the first frequency and a third frequency of, for example, ten times the first frequency. As described above, the frequencies may not be identical to the integer multiple of the first frequency due to real- world implementation issues, for example fluctuations, losses, error margins when setting a frequency and / or transmitting a modulation signal etc. but the frequencies should be substantially equal to or similar to the selected integer multiples.

[0018] According to a second aspect there is provided a radiation source including the assembly of the first aspect. The radiation source may also comprise at least one laser operable to illuminate a droplet of fuel following emission from the nozzle so as to convert the droplet into a plasma state. The radiation source may also comprise means of collecting radiation emitted from the droplet following recombination of the plasma. The means of collecting radiation may include one or more collector mirrors. The radiation source may be configured to generate EUV radiation. The fuel may comprise tin.

[0019] According to a third aspect there is provided an exposure apparatus comprising the radiation source of the second aspect. The exposure apparatus may comprise a lithographic apparatus.

[0020] According to a fourth aspect there is provided a method determining a temperature of a modulator of a droplet generator. The method comprises switchably operating the droplet generator between a droplet generation mode and a monitoring mode. Operating the droplet generator in the droplet generation mode comprises providing a driving signal to the modulator of the droplet generator so as to apply a modulation, based on the driving signal, to fuel within the droplet generator such that the fuel forms droplets following emission from the droplet generator. Operating the droplet generator in the monitoring mode comprises measuring a capacitance of the modulator using a measuring signal and generating, based on the measured capacitance, data indicative of the temperature of the modulator.

[0021] The modulator may comprise a piezoelectric actuator, for example a PZT. In use, the nozzle may contain fuel and modulation of the fuel can cause the fuel to form droplets afteremission from the nozzle. Switchably operating may be interpreted to mean operating the droplet generator in a first mode (the droplet generation mode) during a first time period and operating the droplet generator in a second mode (the monitoring mode) during a second time period, where the first and second time periods do not overlap. Switchably operating may comprise alternating between the droplet generation mode and the monitoring mode multiple times. That is, the method may comprise, for multiple iterations, switchably operating the droplet generator between the droplet generation mode and the monitoring mode. The method may comprise, after operating the droplet generator in the droplet generation mode, switching the droplet generator to monitoring mode. The method may comprise, after operating the droplet generator in the monitoring mode, switching the droplet generator to droplet generation mode. Generating data indicative of the temperature of the modulator may be performed as part of operating the droplet generator in monitoring mode. Alternatively, the processes can be kept separate, for example the temperature may be determined in an offline process.

[0022] Operating the droplet generator in the droplet generation mode may comprise providing the driving signal via first circuity, for example circuitry connecting a signal generator and the modulator. Operating the droplet generator in the monitoring mode may comprise measuring the capacitance of the modulator using second circuity, for example circuitry connecting a measurement controller and the modulator. The measurement controller may comprise, for example, an RC circuit or a capacitance bridge. The measurement controller may provide the measuring signal to the modulator using the second circuitry. The second circuitry may comprise an RC circuit. The capacitance may be measured based on, for example, a voltage drop over a resistor of the RC circuit following provision of the measuring signal.

[0023] By operating the droplet generator in the droplet generation mode, the modulator of the droplet generator will tend to heat, for example because at least part of the driving signal may correspond to a resonant frequency of the modulator. By switching to the monitoring mode, the method allows for the temperature of the modulator to be determined in-situ and immediately following and / or during operation of the droplet generator for the generation of fuel droplets.

[0024] Generally it is difficult to monitor the temperature of a modulator in-situ, especially when the modulator is used in extreme environments such as in a droplet generator of a radiation source (for example a radiation source of an EUV lithographic apparatus). By providing the ability to switchably operate between a droplet generation mode and a monitoring mode, the methods described herein allow for the monitoring of temperature of the modulatorin-situ. As such, a real indication of the temperature of the modulator while in use can be obtained.

[0025] By obtaining temperature data for the modulator when driven by a particular driving signal, the negative effects of heating (e.g. premature aging of the modulator) can be mitigated or avoided, for example by selecting a different driving signal which provides less heating to the modulator.

[0026] The droplet generator may be operated in the droplet generation mode for a first time period. The droplet generator may be operated in the monitoring mode for a second time period. The second time period may be significantly shorter than the first time period.

[0027] For example, the first time period may comprise a second, multiple seconds, minutes, or even hours. The second time period may comprise, for example, less than a second, for example milliseconds or less than a millisecond. In this way, the temperature of the modulator may be monitored periodically while using the droplet generator to generate droplets, without causing a significant time interruption to the droplet generation process. As such, the output of the droplet generator is minimally affected by the monitoring of temperature, minimizing or avoiding downtime. A reduction in downtime is particularly important when the droplet generator is used in applications where high yield is desired, for example for lithography.

[0028] The measuring signal may comprise an alternating current having a measurement period. The second time period may comprise one or more measurement periods. In this example, the measuring signal will have a measurement frequency and an associated measurement period. The measurement period is the reciprocal of the measurement frequency. The measurement period may be referred to simply as the period of the measuring signal. As such, the second time period may comprise one or more periods of the measuring signal.

[0029] The second time period may comprise, for example, 3 measurement periods, 100 measurement periods or 1000 measurement periods. Beneficially multiple measurement periods are used because it may take a few measurement periods for the modulator response to stabilize. It is also desirable for the second time period to be as small as possible so as to reduce down-time of the modulator. In an example where the measuring signal comprises an AC signal with a measurement frequency of 50 kHz, a time period of 1000 measurement periods will correspond to approximately 20 ms.

[0030] The measuring signal may comprise one or more frequencies that do not correspond to a resonant frequency of the modulator and / or a nozzle of the droplet generator and / or a droplet generation rate.

[0031] For example, the measuring signal may be an electrical signal which has one or more frequencies (e.g. a voltage with one or more frequencies). By providing off-resonant frequencies, minimal heating is provided by the measuring signal to the nozzle of the droplet generator. The one or more frequencies may also be selected such that they do not correspond to an integer multiple of a resonant frequency of the modulator of the droplet generator. Not corresponding to a resonant frequency may comprise not having a frequency that is equal to a resonant frequency, and optionally not having a frequency that is within a certain bandwidth around the resonant frequency. For example, the bandwidth may be associated with (e.g. selected based upon) the sharpness or Q-value of the resonance, or any other metric which quantifies the response of a particular resonant frequency to different driving frequencies (e.g. the full width half max of its respective resonance curve).

[0032] Measuring a capacitance of the modulator may comprise providing a circuit comprising the modulator, a resistor, and a source providing the measuring signal, measuring a difference in voltage in the circuit before and after the modulator, and determining the capacitance based on the voltage drop and a resistance of the resistor. That is, an RC circuit can be used to measure the capacitance. In other implementations, measuring the capacitance may comprise using a capacitance bridge

[0033] The applied modulation may comprise a plurality of frequencies comprising a first frequency and at least one additional frequency. The at least one additional frequency may comprise a second frequency and a third frequency.

[0034] The first frequency may correspond to a resonant frequency of the modulator and / or nozzle and / or droplet generation rate. That is, the first frequency may be substantially equal to a resonant frequency of the modulator, nozzle and / or droplet generation rate. Substantially equal to, or corresponding to, may mean that the frequencies (e.g. the first frequency and the resonant frequency) are similar in frequency, for example within a certain bandwidth of each other. An acceptable bandwidth may depend on the sharpness (e.g. Q factor) of the resonance. It should be understood that, due to real- world implementation issues, for example fluctuations, losses, error margins when setting a frequency and / or transmitting a modulation signal etc., the frequencies may not be identical

[0035] Each of the at least one additional frequencies may have a frequency that is an integer multiple of the first frequency. The at least one additional frequencies may therefore have a higher frequency than the first frequency. The at least one additional frequency may comprise a second frequency of, for example, twice the first frequency and a third frequency of, for example, ten times the first frequency.

[0036] According to a fifth aspect, there is provided a method of selecting a driving signal for a modulator of a droplet generator. The method comprises providing a first driving signal to the modulator of the droplet generator so as to apply a modulation, based on the first driving signal, to fuel within the droplet generator such that the fuel forms droplets following emission from the droplet generator, measuring a first capacitance of the modulator and generating, at least partially based on the measured first capacitance, first temperature data indicative of the temperature of the modulator in response to the first driving signal. The method further comprises providing a second driving signal to the modulator of the droplet generator so as to apply a modulation, based on the second driving signal, to fuel within the droplet generator such that the fuel forms droplets following emission from the droplet generator, measuring a second capacitance of the modulator and generating, at least partially based on the measured second capacitance, second temperature data indicative of the temperature of the modulator in response to the second driving signal. The method further comprises selecting, at least partially based on the first temperature data and second temperature data, the first driving signal or second driving signal. Generating or selecting at least partially based on specific data means that the specific data is used (e.g. as an input) in the generation or selection process, but that other data may additionally be used (e.g. as an input) for the generation or selection process. The additional data may include, for example, calibration data, simulation data, preselected parameters, sensor data, metrology data etc.. In an example, the additional data includes droplet coalescence metrology, such that the selected driving signal(s) are selected based on both inline droplet tuning data and temperature data.

[0037] The method of selecting a driving signal may comprise providing additional driving signals and generating additional temperature data indicative of the temperature of the modulator in response to (each of) the additional driving signals. In these examples, the driving signal may be selected from the first, second, or any of the additional driving signals, based on the first, second and additional temperature data.

[0038] Selecting the first driving signal or second driving signal may comprise, if the first temperature data indicates a lower temperature increase than the second temperature data, selecting the first driving signal. Selecting the first driving signal or second driving signal may comprise, if the second temperature data indicates a lower temperature increase than the first temperature data, selecting the second driving signal. That is, the driving signal that results in the lowest (smallest) temperature increase is selected. Similarly, if any of the additional driving signals result in temperature data that indicates a lower temperature increase than the first or second temperature data, the appropriate additional driving signal may be selected.

[0039] The first driving signal may be selected so as to apply an applied modulation comprising a first frequency and at least one additional frequency. The first frequency may correspond to a resonant frequency of the modulator, nozzle and / or droplet generation rate. Each of the at least one additional frequencies may have a frequency that is an integer multiple of the first frequency.

[0040] Measuring the first and second capacitance of the modulator may comprise providing a measuring signal to the modulator. The measuring signal may comprise one or more frequencies that do not correspond to a resonant frequency of the modulator, nozzle and / or droplet generation rate.

[0041] The first driving signal and second driving signal (and optionally one or more additional driving signals) may be selected from a set of driving signals. Each driving signal of the set of driving signals may be associated with one of a set of optimal frequency combinations. Each of the set of optimal frequency combinations may correspond to a combination of one or more frequencies that is predicted to induce effective break-up and coalescence of fuel into droplets (e.g. at a desired droplet rate) when applied as an applied modulation by the modulator. The set of optimal frequency combinations (and associated set of driving signals) may be selected based on an algorithmic process, by trial and error, empirically or otherwise.

[0042] The method may further comprise generating droplets, using a droplet generator, using the selected driving signal.

[0043] Any of the features of any of the above-described aspects may be combined. For example, the methods of the fourth and fifth aspect may be performed using the apparatus of any of the first, second or third aspects.BRIEF DESCRIPTION OF THE DRAWINGS

[0044] Embodiments of the invention will now be described, by way of example only, with reference to the accompanying schematic drawings, in which:- Figure 1 depicts a lithographic system comprising a lithographic apparatus and a radiation source;- Figure 2 depicts an example droplet generator assembly;- Figure 3 depicts another example droplet generator assembly;- Figure 4 is a flow diagram of an example method for determining a temperature of a modulator of a droplet generator; and- Figure 5 is a flow diagram of an example method for selecting a driving signal for a modulator of a droplet generator.DETAILED DESCRIPTION

[0045] Figure 1 shows a lithographic system comprising a radiation source SO and a lithographic apparatus LA. The radiation source SO is configured to generate an EUV radiation beam B and to supply the EUV radiation beam B to the lithographic apparatus LA. The lithographic apparatus LA comprises an illumination system IL, a support structure MT configured to support a patterning device MA (e.g., a mask), a projection system PS and a substrate table WT configured to support a substrate W.

[0046] The illumination system IL is configured to condition the EUV radiation beam B before the EUV radiation beam B is incident upon the patterning device MA. Thereto, the illumination system IL may include a facetted field mirror device 10 and a facetted pupil mirror device 11. The faceted field mirror device 10 and faceted pupil mirror device 11 together provide the EUV radiation beam B with a desired cross-sectional shape and a desired intensity distribution. The illumination system IL may include other mirrors or devices in addition to, or instead of, the faceted field mirror device 10 and faceted pupil mirror device 11.

[0047] After being thus conditioned, the EUV radiation beam B interacts with the patterning device MA. As a result of this interaction, a patterned EUV radiation beam B’ is generated. The projection system PS is configured to project the patterned EUV radiation beam B’ onto the substrate W. For that purpose, the projection system PS may comprise a plurality of mirrors 13,14 which are configured to project the patterned EUV radiation beam B’ onto the substrate W held by the substrate table WT. The projection system PS may apply a reduction factor to the patterned EUV radiation beam B’, thus forming an image with features that are smaller than corresponding features on the patterning device MA. For example, a reduction factor of 4 or 8 may be applied. Although the projection system PS is illustrated as having only two mirrors 13, 14 in Figure 1, the projection system PS may include a different number of mirrors (e.g., six or eight mirrors).

[0048] The substrate W may include previously formed patterns. Where this is the case, the lithographic apparatus LA aligns the image, formed by the patterned EUV radiation beam B’, with a pattern previously formed on the substrate W.

[0049] A relative vacuum, i.e. a small amount of gas (e.g. hydrogen) at a pressure well below atmospheric pressure, may be provided in the radiation source SO, in the illumination system IL, and / or in the projection system PS.

[0050] The lithographic apparatus LA and radiation source SO described herein can be used in method for performing a circuit layout patterning process. A circuit layout patterning method comprises receiving a substrate with a photoresist layer. The method further comprises directing EUV radiation from radiation source to the photoresist layer to form a patterned photoresist layer. The method further comprises developing and etching the patterned photoresist layer to form a circuit layout.

[0051] The radiation source SO shown in Figure 1 is, for example, of a type which may be referred to as a laser produced plasma (LPP) source. A laser system 1, which may, for example, include a CO2 laser, is arranged to deposit energy via a laser beam 2 into a fuel (i.e., a target material), such as tin (Sn) which is provided from, e.g., a droplet generator assembly 3. Although tin is referred to in the following description, any suitable fuel may be used. The fuel may, for example, be in liquid form, and may, for example, be a metal or alloy. The droplet generator assembly 3 may comprise a droplet generator with a nozzle configured to direct the fuel, e.g. in the form of droplets, along a trajectory towards a plasma formation region 4. The laser beam 2 is incident upon the fuel at the plasma formation region 4. The deposition of laser energy into the tin creates a plasma 7 at the plasma formation region 4. Radiation, including EUV radiation, is emitted from the plasma 7 during de-excitation and recombination of electrons with ions of the plasma 7.

[0052] The EUV radiation from the plasma 7 is collected and focused by a collector 5. Collector 5 comprises, for example, a near-normal incidence radiation collector 5 (sometimes referred to more generally as a normal-incidence radiation collector). The collector 5 may have a multilayer mirror structure which is arranged to reflect EUV radiation (e.g., EUV radiation having a desired wavelength such as 13.5 nm). The collector 5 may have an ellipsoidal configuration, having two focal points. A first one of the focal points may be at the plasma formation region 4, and a second one of the focal points may be at an intermediate focus 6, as discussed below.

[0053] The laser system 1 may be spatially separated from the radiation source SO. Where this is the case, the laser beam 2 may be passed from the laser system 1 to the radiation source SO with the aid of a beam delivery system (not shown) comprising, for example, suitable directing mirrors and / or a beam expander, and / or other optics. The laser system 1, the radiation source SO and the beam delivery system may together be considered to be a radiation system.

[0054] Radiation that is reflected by the collector 5 forms the EUV radiation beam B. The EUV radiation beam B is focused at intermediate focus 6 to form an image at the intermediate focus 6 of the plasma present at the plasma formation region 4. The image at the intermediatefocus 6 acts as a virtual radiation source for the illumination system IL. The radiation source SO is arranged such that the intermediate focus 6 is located at or near to an opening 8 in an enclosing structure 9 of the radiation source SO.

[0055] Figure 2 depicts a droplet generator assembly 200. The droplet generator assembly 200 may be implemented in the radiation source SO depicted in Figure 1, for example as at least part of the droplet generator assembly 3 of Figure 1. The droplet generator assembly 200 comprises a droplet generator 20. The droplet generator 20 includes a nozzle 22 which receives molten fuel from a fuel reservoir. The provision of molten fuel may be assisted (e.g. supplied) by an inline refill system. The fuel reservoir is not shown in Figure 2 but may, for example, be at least partially housed in a main body 21 of the droplet generator 20. An associated inline refill system may be at least partially housed in the main body 21 of the droplet generator 20 or may be implemented separately. The nozzle 22 is configured to receive molten fuel from the fuel reservoir, for example via a first aperture. The nozzle 22 has a second aperture 22a through which droplets can be emitted from the nozzle 22.

[0056] The droplet generator 20 also includes a modulator 23, which may be in the form of a piezoelectric actuator. The modulator may comprise one or more piezoelectric elements which deform in response to an electrical driving signal. The piezoelectric actuator may comprise lead zirconite titanate (PZT), although one skilled in the art will understand that many different piezoelectric materials could be used. When driven by a driving signal, the modulator 23 deforms the nozzle 22 and thereby perturbs the velocity of the fuel within the nozzle 22. This perturbation of the velocity of the fuel may be referred to as applying a modulation to the fuel. By applying a modulation to the fuel, the modulator 23 encourages the breakup and subsequent coalescence of fuel, following ejection from the nozzle 22, such that it forms droplets. Droplet formation may be said to be an ongoing process as the fuel travels through the nozzle, out of the nozzle, and towards a desired position (e.g. the plasma formation region 4 illustrated in Figure 1). Typically, the fuel is emitted as a solid jet that breaks apart into droplets, for example within 1 to 2 mm from the nozzle. The fuel emitted from the nozzle 22 may include droplets of any size (e.g. larger than and / or smaller than the eventual desired droplet size) which subsequently break up and / or coalesce during their travel towards the desired position. For example, the fuel emitted from the nozzle 22 may include sub-droplets (smaller than the desired droplet size) and / or super-droplets (larger than the desired droplet size), which subsequently coalesce or break up, respectively. The sub-droplets can include very small droplets, commonly referred to as satellites. It is desirable for all of these satellites to eventually coalesce into a main droplet at or before the desired position, although satellites can be difficult to obviatealtogether. Typically, droplets of the desired droplet size form within approximately 200 - 300 mm from the nozzle. The modulation applied by the modulator 23 can affect various parameters associated with droplet generation, for example the droplet rate, droplet size, droplet shape, droplet quality, number of satellites, droplet trajectory, droplet stability (e.g. stability of size, shape, speed or trajectory) etc.

[0057] The driving signal and the applied modulation (applied by the modulator 23) may be considered relatively coterminous. The driving signal refers to an electrical signal (e.g. an AC signal) provided, from a source (e.g. associated with the control assembly 24 described below), to the modulator 23. Upon receipt of the driving signal, the modulator 23 applies a modulation, based on the driving signal. As such, the frequency(ies) of the driving signal generally correspond to the frequency(ies) of the applied modulation.

[0058] The droplet generator 20 is also connected to a control assembly 24. The control assembly 24 has a modulation controller 25 which is operable to provide a driving signal S to the modulator 23. For example, the driving signal S can be provided in the form of an electrical signal. The driving signal contains multiple different frequency components. A specific example is described below, which is particularly applicable for generating droplets for an EUV radiation source for a lithographic apparatus or other EUV exposure apparatus applications. However, this is not to be construed as limiting and other driving signals (e.g. with more or fewer frequency components and / or different frequencies) may be used in combination with the apparatuses and methods described herein.

[0059] For generating droplets for an EUV radiation source, it is typically desirable to generate droplets at a high frequency (for example, tens of thousands of drops per second). In this example, the frequency components of the driving signal should also be high frequency, so as to encourage coalescence of droplets at the desired frequency. In an example, the driving signal S has three frequency components. A first frequency component has a first frequency fl at or around the frequency at which droplet generation is desired. For example, to achieve a droplet rate (e.g. a rate of droplets arriving at a desired location such as the plasma formation region 4) of 62,000 drops per second, a first frequency fl of approximately 62 kHz can be selected. The first frequency fl can be referred to as a resonant frequency, for example a resonant frequency of the droplet generation process and / or a resonant frequency of the droplet generator 20 itself (or a component thereof). Typically, the resonant frequency corresponds to the droplet generation rate (e.g. the desired droplet generation rate), which may be referred to as a droplet frequency. This generally corresponds to a resonant frequency of at least part of the droplet generator 20. The resonant frequency can correspond to a resonant frequency of the nozzle 22of the droplet generator 20, for example an acoustic resonance (relating to an acoustic standing wave) in the nozzle capillary. The nozzle 22 may be designed or selected so as to have a comparable (e.g. same or similar) resonant frequency to the droplet generation rate. The resonant frequency may also correspond to a resonant frequency of the modulator 23 of the droplet generator 20. Due to electro-mechanical coupling of the modulator 23 to the nozzle 22, the acoustic resonance of the nozzle 22 leads to electric resonance in the modulator 23. In this way, the resonant frequency of the droplet generation process (e.g. the droplet generation rate) and the resonant frequency of the droplet generator 20, nozzle 22 and modulator 23 may be considered coterminous. Herein, reference is made to the resonant frequency, which may be interpreted as any of the above described resonances. Components such as the nozzle and modulator may have more than one resonant frequency. For example, there may be more than one acoustic resonance in the nozzle. In such instances, reference to corresponding to the resonant frequency should be taken to mean corresponding to a resonant frequency of the multiple resonant frequencies. Similarly, in such instances, off-resonance refers to not corresponding to any of the multiple resonant frequencies.

[0060] To further encourage breakup and droplet coalescence, a second frequency component with a second frequency f2 and third frequency f3 component with a third frequency are also used. The second f2 has a higher, and usually a significantly higher, frequency compared to the first frequency fl. The third frequency f3 has a higher, and usually a significantly higher, frequency compared to the second frequency fl. The highest frequency component, i.e. the third frequency f3, encourages the jet of molten fuel in the nozzle to break up into droplets and sub-droplets. The intermediate frequency component, i.e. the second frequency f2, encourages a change in velocity of the droplets and sub-droplets. The second frequency f2 can be selected so as to encourage subcoalescence of the droplets and sub-droplets. The lowest frequency component, i.e. the first frequency fl corresponding to the resonant frequency, further changes the velocity of the droplets and sub-droplets so as to encourage final droplet coalescence. The droplets are encouraged to coalesce at the same frequency as the first frequency fl .

[0061] The effectiveness of break-up, sub-coalescence and coalescence can be improved by selection and tuning of the first, second and third frequencies fl, f2, f3. Typically, effective break-up and coalescence is achieved when the second and third frequencies f2, f3 are integer multiples of the first frequency fl. That is, f2 = Axfl and f3 = Bxfl, wherein A and B are integers. There are numerous combinations of A and B which lead to effective break-up and coalescence. In one example, the following frequency values are used: fl = 62 kHz, f2 = 620 kHz (i.e. A = 10) and f3 = 3.1 MHz (i.e. B = 50). However, other combinations are possible,for example A can be any number greater than 2 and B can be any number greater than 3 and greater than the selected value of A.

[0062] Typically, these combinations (e.g. the values of A and B) are selected based on an algorithmic process which assesses the effectiveness of a plurality of combinations in achieving effective break-up and coalescence at the desired droplet rate. The algorithmic process can output a number of combinations as optimal frequency combinations which are anticipated to provide the most effective break-up and coalescence.

[0063] While it would be desirable for all of the applied modulation to be applied to the fuel, parasitic modulation of mechanical parts of the droplet generator 20 also occur. For example, the nozzle 22 and modulator 23 experience significant modulation due to the driving signal S. Some of this modulation leads to heating (self-heating) of the modulator 23 and subsequent damage to the modulator 23. Heating of the modulator 23 is particularly dependent on the amplitude and frequency of modulation. Certain combinations of frequencies in the driving signal S can lead to increased parasitic modulation, increased heating, and therefore increased damage, compared to other combinations.

[0064] The control assembly 24 also has a measurement controller 26 which is operable to measure a capacitance of the modulator 23. The capacitance of a modulator 23, for example a PZT piezoelectric actuator, is dependent on its temperature. Therefore, by measuring the capacitance of the modulator 23 data indicative of its temperature can be obtained. Each type of modulator will exhibit a different relationship between capacitance and temperature, for example based on its size and material. Calibration curves are available, or can be empirically determined, for the modulator in use, thereby enabling a conversion between the measured capacitance and the temperature of the modulator 23.

[0065] By providing a droplet generator assembly 200 with both a modulation controller 25 and a measurement controller 26, the droplet generator assembly 200 can alternate between applying a driving signal S using the modulation controller 25 and measuring the self-heating experienced by the modulator 23 using the measurement controller 26. As such, the heating effect of a particular driving signal S can be determined.

[0066] In an example implementation, the algorithmic process for generating optimal frequency combinations can be used and outputs a set of M driving signals, each associated with one of a set of optimal frequency combinations. The modulation controller 25 of the droplet generator assembly can be configured to sequentially apply each of M driving signals, each driving signal corresponding to one of the set of optimal frequency combinations. By alternately applying each of the M driving signals using the modulation controller 25 andmeasuring the capacitance of the modulator 23 using the measurement controller 26, the heating caused by each of the M driving signals can be determined. In this way, a driving signal which is optimized for both improved break-up and coalescence frequency and reduced heating can be identified.

[0067] The measurement controller 26 can be implemented in multiple ways, for example any means for measuring capacitance can be used. In one example, the measurement controller 26 comprises an RC circuit comprising a resistor and a capacitor, wherein the modulator 23 comprises the capacitor. A measuring signal comprising an alternating voltage is applied to the modulator 23 and a voltage drop is experienced due to the impedance of the modulator 23. The subsequent voltage drop can therefore be measured, for example based on the applied voltage and the resistance of the resistor. An RC circuit such as this is particularly useful as it can measure the capacitance of the modulator 23 relatively quickly, for example in less than a millisecond (ms). In order to reduce self-heating of the modulator 23 during measurement, the applied alternating voltage is selected so as to cause minimal self-heating of the modulator 23. The measuring signal may be selected to have a measurement frequency that is non-resonant (i.e. is substantially different in frequency compared to the first frequency fl). In an example where the first frequency is 62 kHz, a measurement frequency of 50 kHz may be selected such that it is far from the resonant frequency, not a multiple of the resonant frequency, and also not higher in frequency and so less likely to cause significant heating. Selection of the measurement frequency of the measuring signal can be guided based on theoretical or empirical knowledge of the modulator 23 or characteristics thereof (e.g. the material from which it is made). Any measurement frequency has an associated measurement period (i.e. the reciprocal of the frequency). Non-resonant may also be referred to as off-resonant (or off-resonance).

[0068] To measure capacitance with a higher accuracy, the measuring signal can be applied for a number of measurement periods (i.e. time periods of the measurement frequency). This is because it may take a few periods for the measuring signal and modulator response to stabilize. Any number of periods may be used, for example three periods or 1000 periods. At a measurement frequency of 50 kHz, measuring for 1000 periods will take approximately 20 ms.

[0069] Alternatively, the measurement controller 26 can comprise a capacitance bridge, or any other means of measuring capacitance. The measurement time of a capacitance bridge may be significantly longer than that of an RC circuit, for example on the order of hundreds of ms. In circumstances where down-time of the modulator 23 is undesirable, for example in lithographicprocesses where a high throughput is desired, use of a faster capacitance measuring means, for example the RC circuit, is preferred.

[0070] In Figure 2, the measurement controller 26 and modulation controller 25 are depicted as part of a control assembly 24. However, this the control assembly 24 is illustrative in nature, intended to illustrate that both the measurement controller 26 and modulation controller 25 are connected to the droplet generator 20. However, the control assembly 24 itself need not be implemented in any material form. That is, the droplet generator assembly may comprise the measurement controller 26 and modulation controller 25 separate to a control assembly 24.

[0071] Figure 3 depicts another droplet generator assembly 300. The droplet generator assembly 300 depicts a specific implementation of the droplet generator assembly 200 of Figure 2.

[0072] The droplet generator assembly 300 comprises a droplet generator 30, which may be of the same form as the droplet generator 20 described with reference to Figure 2. The droplet generator 30 has a modulator (not shown) which is operable to apply a driving signal S to a fuel contained within the droplet generator 30 so as to encourage break-up and coalescence of the fuel into droplets.

[0073] The droplet generator assembly 300 comprises a modulation controller 32, which may be comparable to the modulation controller 25 of Figure 2, and a measurement controller 34, which may be comparable to the measurement controller 26 of Figure 2.

[0074] The droplet generator 30 is connected to each of the measurement controller 34 and the modulation controller 32 by way of a switch 38. That is, the droplet generator 30 is switchably connected to the measurement controller 34 and the modulation controller 32. The switch 38 can be implemented by way of any switching means. For example, the switch 38 may be electronic or electromechanical, for example a transistor switch or fast relay switch. By providing a switch 38 between the droplet generator 30 and each of the measurement controller 34 and the modulation controller 32, the droplet generator 30 can be alternatively provided with a driving signal S from the modulation controller 32 and provided with a measuring signal from the measurement controller 34.

[0075] By providing switching means 38, the droplet generator 30 can be operated in a droplet generating process, whilst also periodically being monitored for temperature (through a capacitance measurement) in situ. The switch allows for in-situ measurement, with minimal down-time before switching back to the droplet generating process.

[0076] The droplet generator 300 in Figure 3 also comprises an oscilloscope 36. The oscilloscope 36 is provided between the switch 38 and the droplet generator 30 such thatwhichever signal is being provided to the droplet generator 30 (e.g. the driving signal or the measuring signal) can be monitored while it is being applied to the droplet generator 30 (and the modulator thereof). In alternative arrangements, other visualization and / or data recordal means (e.g. apparatus for recording data) may be used in place of, or in addition to, the oscilloscope 36. For example, a computer may be connected which is configured to record data (e.g. frequency data, capacitance data) in place of, or in addition to, the oscilloscope 36. The oscilloscope 36 or other visualization and / or data recordal means may also be positioned differently to the oscilloscope 36 depicted in Figure 3. In an example, each of the measurement controller 34 and modulation controller 32 have their own visualization and / or data recordal means. In these instances, the means may be connected between the switch 38 and each of the respective measurement controller 34 and modulation controller 32. In yet other arrangements, the droplet generator 300 may comprise no oscilloscope or other visualization means. Rather, data may be transferred to an external system for visualization and / or recordal and / or analysis.

[0077] Figure 4 is a flow diagram of an example method 400 for determining a temperature of a modulator of a droplet generator. For example, the method 400 may be used with the droplet generator assembly 200 of Figure 2 or droplet generator assembly 300 of Figure 3.

[0078] The method 400 comprises providing 402 a driving signal to the modulator. The driving signal can be applied to the modulator using a modulation controller, for example the modulation controller 25 of Figure 2, or the modulation controller 32 of Figure 3. Providing the driving signal to the modulator causes the modulator to apply a modulation, based on the driving signal, to fuel within the droplet generation (e.g. within a nozzle therein), thereby causing the fuel to break up and / or coalesce into droplets upon emission from the droplet generator. As such, the step of providing 402 a driving signal to the modulator may be considered as part of a process of operating the droplet generator in a droplet generation mode. That is, when the driving signal is provided to the modulator and fuel is provided to the droplet generator, droplet generation can take place. The droplet generator may be considered to be ‘in use’ (e.g. generating droplets) when in droplet generation mode.

[0079] The method further comprises measuring 404 a capacitance of the modulator. Measuring 404 a capacitance of the modulator may comprise providing a measuring signal to the modulator. The measuring signal may be provided using a measurement controller, for example the measurement controller 26 of Figure 2 or the measurement controller 34 of Figure 3. As described above, there are multiple ways of measuring capacitance. One such method is using an RC circuit. When using an RC circuit, a circuit is provided which connects the measurement controller (or other driving signal source), modulator, and a resistor. Bymeasuring a difference in the voltage before and after the modulator, and using the known resistance of the resistor, the capacitance of the modulator may be determined. Of course, other means of determining capacitance may be used, and appropriate circuity may be provided.

[0080] The method further comprises determining 406 a temperature of the modulator. The temperature may be determined based on the measured capacitance, that is the capacitance measured in step 404. As described above, calibration curves for each modulator may be available or empirically determined. The calibration curves can be used to convert the measured capacitance into a temperature. As part of the determination step 406, the data output may be data indicative of a temperature. For example, the output data may represent an amount of heating, a percentage increase in temperature, or any other metric which provides information regarding the temperature of the modulator.

[0081] Measuring 404 the capacitance of the modulator and determining 406 the temperature may be considered part of a process of operating the droplet generator in a monitoring mode. That is, when the measuring signal is being provided to the droplet generator, monitoring of the temperature of the modulator can take place. It is possible, in other example implementations, to take the determination step 406 may be taken offline and perform the step of determining 406 temperature separately.

[0082] Between operating in droplet generation mode (e.g. step 402) and operating in monitoring mode (e.g. steps 404 and 406), a switching operation may take place. That is, the method may include switching between the droplet generation mode and the monitoring mode. Means for switching are described in more detail above.

[0083] The method 400 may be repeated multiple times, to achieve multiple temperature measurements and / or to monitor the temperature associated with different driving signals. As such, following the step of determining 406 temperature, the method 400 may further comprise operating again in droplet generation mode (e.g. step 402). Between operation in the monitoring mode and operating in the droplet generation mode, a switching operation may take place. That is, the method may include switching between the monitoring mode and the droplet generation mode. Means for switching are described in more detail above.

[0084] In practice, this method allows for the monitoring of modulator temperature while the droplet generator is in use, by switching between droplet generation mode and monitoring mode. In some example implementations, the amount of time spent in the droplet generation mode is significantly longer than the amount of time spent in the monitoring mode. In this way, the droplet generator can spend most of its time ‘in use’ and generating droplets, but briefly beswitching into a monitoring mode in which its temperature can be checked. This allows the temperature to be monitored in use, causing minimal down-time in droplet generation.

[0085] The droplet generator may operate in the droplet generation mode for a first time period and operate in the monitoring mode for a second time period. In general, the second time period is significantly shorter than the first time period. As described above, the second time period may be selected based on a frequency (and associated period) of the measuring signal. For example, the measuring signal may comprise an alternating current with a measuring frequency f and a measuring period P. The second time period may be selected to be a multiple of the measuring period P. Generally, it is useful to use a second time period of at least 2P or 3P, because it can take multiple oscillations for the measured capacitance signal to settle (e.g. stop fluctuating). However, it is also useful to minimize the second time period so as to reduce the down-time of the droplet generator, that is the time the droplet generator spends not in droplet generation mode. A second time period of approximately 100P or 1000P may be used, for example. A second time period approximately equal to milliseconds, or less than a millisecond, may be used. On the other hand, it is desirable to maximize the first time period, and so the first time period may comprise, for example, seconds, minutes or hours.

[0086] As described above, the driving signal may be selected so as to optimize droplet generation of the droplet generator. The driving signal may comprise multiple frequencies, one or more of which may be a resonant frequency of the modulator. The multiple frequencies can comprise a first frequency (which may be a resonant frequency) and one or more additional frequencies. The one or more additional frequencies can be integer multiples of the first frequency. The driving signal and its component frequencies is described in more detail above.

[0087] Figure 5 is a flow diagram of an example method 500 for selecting a driving signal for a modulator of a droplet generator. That is, the method 500 is for selecting an optimal driving signal for droplet generation. The optimization is in view of both efficient droplet generation and reduced temperature increase, thereby increasing the lifetime of the droplet generator.

[0088] The method 500 can be performed on a droplet generator with a modulator, for example it may be performed using the droplet generator assembly 200 of Figure 2 or the droplet generator assembly 300 of Figure 3.

[0089] Many of the steps of the method 500 for selecting a driving signal are comparable to the steps of the method 400 for determining a temperature of a droplet generator. For example, the method 500 for selecting a driving signal may be considered to comprise performing the method 400 for determining a temperature multiple times, each time for a different driving signal, and then selecting a driving signal at least in part based on the determined temperatures.That is, the determined temperatures may be used as an input to the selection process (e.g. a selection calculation which may be performed by a computer) but also additional data may be used as an input to the selection process. The additional data may include further sensor data, for example droplet metrology data. In this way, the selection of the driving signal can be based upon both the modulator temperature and the quality of the generated droplets.

[0090] The method 500 includes providing 501 a first driving signal to the modulator of the droplet generator. The first driving signal may be selected from a set of M driving signals, each associated with one of a set of optimal frequency combinations, as described in more detail above.

[0091] The method 500 further includes measuring 502 a first capacitance of the modulator. The first capacitance is associated with the capacitance of the modulator following (e.g. in response to, after being driven by) the first driving signal. The capacitance is therefore measured following operation of the droplet generator in droplet generation mode using the first driving signal.

[0092] The method also includes determining 503 first temperature data. The first temperature data is data indicative of the temperature of the modulator, following (e.g. in response to, after being driven by) the first driving signal. The first temperature data can be stored for use later in the method 500, for example on a computer storage medium.

[0093] The method 500 further comprises providing 504 a second driving signal to the modulator of the droplet generator. The second driving signal is provided instead of the first driving signal. As such, the method enables the response of the modulator to be probed in response to different driving signals. The second driving signal may be selected from a set of M driving signals, each associated with one of a set of optimal frequency combinations, as described in more detail above.

[0094] The method further includes measuring 506 a second capacitance of the modulator. The second capacitance is associated with the capacitance of the modulator following (e.g. in response to, after being driven by) the second driving signal. The capacitance is therefore measured following operation of the droplet generator in droplet generation mode using the second driving signal.

[0095] The method further includes determining 507 second temperature data. The second temperature data is data indicative of the temperature of the modulator, following (e.g. in response to, after being driven by) the second driving signal. The second temperature data can be stored for use later in the method 500, for example on a computer storage medium.

[0096] The method further comprises selecting 508 a driving signal. The driving signal is selected, from the first and second driving signals. The driving signal is selected based on the first and second temperature data. For example, selecting 508 a driving signal may comprise comparing the first and second temperature data. The driving signal associated with the lowest (smallest) change in temperature may be selected. For example, if the first temperature data indicates a lower temperature increase than the second temperature data, the first driving signal may be selected. If, on the other hand, the first temperature data indicates a higher temperature increase than the second temperature data (i.e. the second temperature data indicates a lower temperature increase than the first temperature data), the second driving signal is selected.

[0097] The method 500 may be performed for more than two driving signals. For example, between the step of determining 507 second temperature data and selecting 508 a driving signal, further iterations of the process of providing a driving signal, measuring a capacitance, and determining temperature data may be performed, for further driving signals. For example, the process may be performed for each of M driving signals from a set of optimal driving signals. Following the determination of temperature data associated with each of the applied driving signals, a driving signal can be selected, for example based on the temperature data associated with the lowest increase in temperature.

[0098] Any of the above methods can be incorporated as part of a method of generating droplets using a droplet generator. Beneficially, by generating droplets and incorporating the methods described herein, a driving signal can be selected which is optimized for both droplet generation efficiency and efficient lifetime management of the droplet generator (due to reduced heating). Furthermore, by generating droplets and incorporating the methods described herein, the temperature (and therefore maintenance state) of the droplet generator can be monitored in real time, in situ, and with minimal down-time to the droplet generation process.

[0099] While droplet generation for use in lithographic applications has been described, the methods described herein may be applied to droplet generation in any field. The techniques described herein are particularly well suited to extreme environments where it is otherwise difficult to monitor the performance of components therein without causing significant downtime.

[0100] Temperature is discussed herein, and specifically the change in temperature of the modulator of the droplet generator. It should be understood that temperature change generally affects the performance of a modulator, rather than absolute temperature. In many environments, for example within an EUV radiation source, the ambient temperature may be particularly high, for example between 230 to 320 degrees Celsius. However, a smallertemperature increase due modulation self-heating and higher operational temperature will generally cause less damage to a modulator than a smaller operational temperature and high temperature increase due to modulation self-heating. For example, an ambient temperature of 250 degrees, a temperature increase of 10 degrees, leading to a modulator operational temperature of 260 degrees, may be preferable compared to an ambient temperature of 230 degrees, a temperature increase of 20 degrees, leading to a modulator operational temperature of 250 degrees. It may be preferable to find a driving signal which leads to a temperature increase of less than 10 degrees Celsius per volt of input amplitude. It may be preferable to find a driving signal which leads to a temperature increase of less than 7, less than 5, or less than 3 degrees Celsius per volt of input amplitude.

[0101] While specific examples of frequencies (e.g. of driving signals and measuring signals) are described herein, they are illustrative in nature and non-limiting. Any range of frequencies may be used, for example for resonant frequencies up to 200 kHz. In such an example, the first frequency may comprise up to 200 kHz and the second and third frequencies may comprise frequencies in excess of 200 kHz, e.g. integer multiples of 200 kHz. A limit of 200 kHz is not enforced on the methods described herein, but is more associated with the maximum possible or desirable droplet generation rate. For higher droplet generation rates, the first frequency and further frequencies may be similarly increased. In some examples, the first frequency is in the range 50 - 100 kHz, the second frequency is in the range 500 kHz - 1 MHz, and the third frequency is in the range 2 - 4 MHz. In other examples, the first frequency (e.g. the resonant frequency) may be in the range 50 kHz to 200 kHz. The second frequency may be in the range of 3x to 20x the first frequency (i.e. three times to twenty times the first frequency). The second frequency may, for example, be between 150 kHz to 4 MHz. The third frequency may be in the range 5x to 50x the first frequency (i.e. five times to fifty times the first frequency). The third frequency may be, for example, between 250 kHz to 10 MHz. These values are particularly pertinent to the frequencies of the driving signals. The measuring signals may also comprise any frequency, for example in the range 1 kHz to 200 kHz. Beneficially, the measuring signal should comprise frequencies that do not correspond to a resonant frequency of the system and / or any frequency components of the driving signal (e.g. the first, second or third frequency).

[0102] Reference is made herein to fuel. Fuel in this instance should not be narrowly interpreted e.g. as combustible fuel. Rather, the fuel described with reference to the LPP source herein acts as a target material. The target material is provided, in droplet form, so that laser radiation can deposit energy onto the target, causing it to form a plasma. In other exampleimplementations, for example where droplet generation is used in non-LPP source applications, the term fuel may refer to the material which the droplet generator causes to form into droplets.

[0103] Although specific reference may be made in this text to the use of lithographic apparatus in the manufacture of ICs, it should be understood that the lithographic apparatus described herein may have other applications. Possible other applications include the manufacture of integrated optical systems, guidance and detection patterns for magnetic domain memories, flat-panel displays, liquid-crystal displays (LCDs), thin-film magnetic heads, etc.

[0104] Although specific reference may be made in this text to embodiments of the invention in the context of a lithographic apparatus, embodiments of the invention may be used in other apparatus. Embodiments of the invention may form part of a mask inspection apparatus, a metrology apparatus, or any apparatus that measures or processes an object such as a wafer (or other substrates) or mask (or other patterning devices). These apparatus may be generally referred to as lithographic tools. Such a lithographic tool may use vacuum conditions or ambient (non-vacuum) conditions.

[0105] Although specific reference may have been made above to the use of embodiments of the invention in the context of lithography, it will be appreciated that the invention, where the context allows, is not limited to lithography and may be used in other applications, for example in any apparatus which uses EUV radiation provided by an LPP source, or any apparatus which uses a droplet generator.

[0106] Where the context allows, embodiments of the invention may be implemented in hardware, firmware, software, or any combination thereof. Embodiments of the invention may also be implemented as instructions stored on a machine-readable medium, which may be read and executed by one or more processors. A machine-readable medium may include any mechanism for storing or transmitting information in a form readable by a machine (e.g., a computing device). For example, a machine-readable medium may include read only memory (ROM); random access memory (RAM); magnetic storage media; optical storage media; flash memory devices; electrical, optical, acoustical, and others. Further, firmware, software, routines, instructions may be described herein as performing certain actions. However, it should be appreciated that such descriptions are merely for convenience and that such actions in fact result from computing devices, processors, controllers, or other devices executing the firmware, software, routines, instructions, etc. and in doing that may cause actuators or other devices to interact with the physical world.

[0107] Other aspects of the invention are set out in the following numbered clauses:1. A droplet generator assembly comprising: a droplet generator comprising a nozzle and a modulator, wherein the droplet generator is configured to receive fuel, apply a modulation to the fuel using the modulator so as to break up the fuel into droplets, and emit said droplets from the nozzle; a modulation controller operable to provide a driving signal to the modulator, the modulator configured to apply the modulation based on the driving signal; and a measurement controller operable to obtain data representative of a temperature of the modulator.2. The assembly of clause 1, wherein the modulator comprises a piezoelectric actuator.3. The assembly of clause 1 or 2, wherein the modulation controller and measurement controller are switchably connected to the droplet generator.4. The assembly of clause 3, wherein the modulation controller and measurement controller are connected to the droplet generator by way of a switch mechanism, the switch mechanism having a first switching arrangement in which the droplet generator is connected to the modulation controller and a second switching arrangement in which the droplet generator is connected to the measurement controller.5. The assembly of any preceding clause, wherein the measurement controller comprises a capacitance measurement system operable to measure a capacitance of the modulator and determine a temperature of the modulator based on the measured capacitance.6. The assembly of clause 5, wherein the capacitance measurement system comprises a series resistor.7. The assembly of any preceding clause, wherein the applied modulation comprises a plurality of frequencies comprising a first frequency and at least one additional frequency.8. The assembly of clause 7, wherein the first frequency corresponds to a resonant frequency of the modulator.9. The assembly of clause 7 or 8, wherein each of the at least one additional frequencies has a frequency that is an integer multiple of the first frequency.10. A radiation source including the assembly of any preceding clause.11. An exposure apparatus comprising the radiation source of clause 10.12. A method for determining a temperature of a modulator of a droplet generator, the method comprising switchably operating the droplet generator between a droplet generation mode and a monitoring mode, wherein: operating the droplet generator in the droplet generation mode comprises providing a driving signal to the modulator of the droplet generator so as to apply a modulation, based onthe driving signal, to fuel within the droplet generator such that the fuel forms droplets following emission from the droplet generator; and operating the droplet generator in the monitoring mode comprises measuring a capacitance of the modulator using a measuring signal and generating, based on the measured capacitance, data indicative of the temperature of the modulator.13. The method of clause 12, wherein: the droplet generator is operated in the droplet generation mode for a first time period; the droplet generator is operated in the monitoring mode for a second time period; and the second time period is significantly shorter than the first time period.14. The method of clause 13, wherein the measuring signal comprises an alternating current having a measurement period, and the second time period comprises one or more measurement periods.15. The method of any of clauses 12 to 14, wherein the measuring signal comprises one or more frequencies that do not correspond to a resonant frequency of the modulator.16. The method of any of clauses 12 to 15, wherein the measuring a capacitance of the modulator comprises: providing a circuit comprising the modulator, a resistor, and a source providing the measuring signal; measuring a difference in voltage in the circuit before and after the modulator; and determining the capacitance based on the voltage drop and a resistance of the resistor.17. The method of any of clauses 12 to 16, wherein the applied modulation comprises a plurality of frequencies comprising a first frequency and at least one additional frequency.18. The method of clause 17, wherein first frequency corresponds to a resonant frequency of the modulator.19. The method of clause 17 or 18, wherein each of the at least one additional frequencies has a frequency that is an integer multiple of the first frequency.20. A method of selecting a driving signal for a modulator of a droplet generator, the method comprising: providing a first driving signal to the modulator of the droplet generator so as to apply a modulation, based on the first driving signal, to fuel within the droplet generator such that the fuel forms droplets following emission from the droplet generator; measuring a first capacitance of the modulator; generating, based on the measured first capacitance, first temperature data indicative of the temperature of the modulator in response to the first driving signal;providing a second driving signal to the modulator of the droplet generator so as to apply a modulation, based on the second driving signal, to fuel within the droplet generator such that the fuel forms droplets following emission from the droplet generator; measuring a second capacitance of the modulator; generating, based on the measured second capacitance, second temperature data indicative of the temperature of the modulator in response to the second driving signal; selecting, based at least in part on the first temperature data and second temperature data, the first driving signal or second driving signal.21. The method of clause 20, wherein selecting the first driving signal or second driving signal comprises: if the first temperature data indicates a lower temperature increase than the second temperature data, selecting the first driving signal; and if the second temperature data indicates a lower temperature increase than the first temperature data, selecting the second driving signal.22. The method of clause 20 or 21, wherein the first driving signal is selected so as to apply an applied modulation comprising a first frequency and at least one additional frequency.23. The method of clause 22, wherein the first frequency corresponds to a resonant frequency of the modulator.24. The method of clauses 22 or 23, wherein each of the at least one additional frequencies has a frequency that is an integer multiple of the first frequency.25. The method of any of clauses 20 to 24, wherein measuring the first and second capacitance of the modulator comprise providing a measuring signal to the modulator.26. The method of clause 25, wherein the measuring signal comprises one or more frequencies that do not correspond to a resonant frequency of the modulator.27. The method of any of clauses 20 to 26, wherein the first driving signal and second driving signal are selected from a set of driving signals, each driving signal associated with one of a set of optimal frequency combinations.

[0108] While specific embodiments of the invention have been described above, it will be appreciated that the invention may be practiced otherwise than as described. The descriptions above are intended to be illustrative, not limiting. Thus it will be apparent to one skilled in the art that modifications may be made to the invention as described without departing from the scope of the claims set out below.

Claims

CLAIMS1. A droplet generator assembly comprising: a droplet generator comprising a nozzle and a modulator, wherein the droplet generator is configured to receive fuel, apply a modulation to the fuel using the modulator so as to break up the fuel into droplets, and emit said droplets from the nozzle; a modulation controller operable to provide a driving signal to the modulator, the modulator configured to apply the modulation based on the driving signal; and a measurement controller operable to obtain data representative of a temperature of the modulator.

2. The assembly of claim 1, wherein the modulator comprises a piezoelectric actuator.

3. The assembly of claim 1, wherein the modulation controller and measurement controller are switchably connected to the droplet generator by way of a switch mechanism, the switch mechanism having a first switching arrangement in which the droplet generator is connected to the modulation controller and a second switching arrangement in which the droplet generator is connected to the measurement controller.

4. The assembly of claim 1, wherein the measurement controller comprises a capacitance measurement system operable to measure a capacitance of the modulator and determine a temperature of the modulator based on the measured capacitance.

5. The assembly of claim 1, wherein the applied modulation comprises a plurality of frequencies comprising a first frequency and at least one additional frequency, and wherein the first frequency corresponds to a resonant frequency of the modulator.

6. The assembly of claim 1, wherein the applied modulation comprises a plurality of frequencies comprising a first frequency and at least one additional frequency, and wherein each of the at least one additional frequencies has a frequency that is an integer multiple of the first frequency.

7. A radiation source including the assembly of claim 1.

8. A method for determining a temperature of a modulator of a droplet generator, the method comprising switchably operating the droplet generator between a droplet generation mode and a monitoring mode, wherein: operating the droplet generator in the droplet generation mode comprises providing a driving signal to the modulator of the droplet generator so as to apply a modulation, based on the driving signal, to fuel within the droplet generator such that the fuel forms droplets following emission from the droplet generator; and operating the droplet generator in the monitoring mode comprises measuring a capacitance of the modulator using a measuring signal and generating, based on the measured capacitance, data indicative of the temperature of the modulator.

9. The method of claim 8, wherein: the droplet generator is operated in the droplet generation mode for a first time period; the droplet generator is operated in the monitoring mode for a second time period; and the second time period is significantly shorter than the first time period.

10. The method of claim 9, wherein the measuring signal comprises an alternating current having a measurement period, and the second time period comprises one or more measurement periods.

11. The method of claim 8, wherein the measuring signal comprises one or more frequencies that do not correspond to a resonant frequency of the modulator.

12. The method of claim 8, wherein the measuring a capacitance of the modulator comprises: providing a circuit comprising the modulator, a resistor, and a source providing the measuring signal; measuring a difference in voltage in the circuit before and after the modulator; and determining the capacitance based on the voltage drop and a resistance of the resistor.

13. The method of claim 8, wherein the applied modulation comprises a plurality of frequencies comprising a first frequency and at least one additional frequency, and wherein the first frequency corresponds to a resonant frequency of the modulator.

14. The method of claim 8, wherein the applied modulation comprises a plurality of frequencies comprising a first frequency and at least one additional frequency, wherein each of the at least one additional frequencies has a frequency that is an integer multiple of the first frequency.

15. A method of selecting a driving signal for a modulator of a droplet generator, the method comprising: providing a first driving signal to the modulator of the droplet generator so as to apply a modulation, based on the first driving signal, to fuel within the droplet generator such that the fuel forms droplets following emission from the droplet generator; measuring a first capacitance of the modulator; generating, based on the measured first capacitance, first temperature data indicative of the temperature of the modulator in response to the first driving signal; providing a second driving signal to the modulator of the droplet generator so as to apply a modulation, based on the second driving signal, to fuel within the droplet generator such that the fuel forms droplets following emission from the droplet generator; measuring a second capacitance of the modulator; generating, based on the measured second capacitance, second temperature data indicative of the temperature of the modulator in response to the second driving signal; selecting, based at least in part on the first temperature data and second temperature data, the first driving signal or second driving signal.

16. The method of claim 15, wherein selecting the first driving signal or second driving signal comprises: if the first temperature data indicates a lower temperature increase than the second temperature data, selecting the first driving signal; and if the second temperature data indicates a lower temperature increase than the first temperature data, selecting the second driving signal.

17. The method of claim 15, wherein the first driving signal is selected so as to apply an applied modulation comprising a first frequency and at least one additional frequency.

18. The method of claim 17, wherein the first frequency corresponds to a resonant frequency of the modulator.

19. The method of claim 17, wherein each of the at least one additional frequencies has a frequency that is an integer multiple of the first frequency.

20. The method of claim 15, wherein measuring the first and second capacitance of the modulator comprise providing a measuring signal to the modulator.

21. The method of claim 20, wherein the measuring signal comprises one or more frequencies that do not correspond to a resonant frequency of the modulator.

22. The method of claim 15, wherein the first driving signal and second driving signal are selected from a set of driving signals, each driving signal associated with one of a set of optimal frequency combinations.

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