CROSS-REFERENCE TO RELATED APPLICATIONS

[0001] This application claims the benefit of European patent application 14160565.9, which was filed on 18 March 2014, and which is incorporated herein in its entirety by reference.

FIELD

[0002] The present invention relates to droplet stream generators, and particular, though not exclusively, fuel stream generators for use within a radiation source such as an EUV radiation source.

BACKGROUND

[0003] 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 from a patterning device (e.g. a mask) onto a layer of radiation- sensitive material (resist) provided on a substrate.

[0004] The wavelength of radiation used by a lithographic apparatus to project a pattern onto a substrate determines the minimum size of features which can be formed on that substrate. A lithographic apparatus which uses EUV radiation, being electromagnetic radiation having a wavelength within the range 4-20 nm, may be used to form smaller features on a substrate than a conventional lithographic apparatus (which may for example use electromagnetic radiation with a wavelength of 193 nm).

[0005] EUV radiation may be generated within a radiation source. In one type of radiation source, known as a Laser Produced Plasma (LPP) radiation source, EUV radiation is generated by igniting a fuel source to generate a radiation producing plasma. In an LPP, the fuel source may be provided as a stream of droplets. Plasma initiating radiation is directed at the droplets to form a plasma. In order to increase the amount of EUV radiation produced by a fuel source it is desirable to generate droplets at high frequencies but with a sufficient distance between each droplet in the fuel stream to prevent generated plasma from affecting subsequent droplets. SUMMARY

[0006] It is an object of at least one embodiment of the present invention to obviate or mitigate at least one of the problems set out above.

[0007] According to a first aspect of the present invention, there is provided a fuel emitter. The fuel emitter comprises a first droplet stream generator arranged to emit a fuel stream comprising droplets separated by a first distance; and a droplet removal apparatus arranged to remove a first subset of droplets from the fuel stream before the first subset of droplets reach a target region such that the droplets of the fuel stream provided to the target region are separated a second distance greater than the first distance.

[0008] According to a second aspect of the present invention, there is provided a fuel emitter. The fuel emitter comprises a first droplet stream generator arranged to emit a fuel stream comprising droplets, wherein two consecutive droplets of the fuel stream are separated by a first distance; and a droplet removal apparatus arranged to remove a first subset of droplets from the fuel stream before the first subset of droplets reach a target region such that, as a result of removing the first subset, the droplets of the fuel stream provided to the target region are separated a second distance greater than the first distance.

[0009] In this way, a desired inter-droplet spacing within the fuel stream can be achieved without the use of complicated systems for inducing coalescence between fuel droplets.

[00010] The droplet removal apparatus may comprise a second droplet stream generator arranged to emit a cross stream of droplets towards the fuel stream such that in use at least some droplets of the cross stream collide with droplets of the fuel stream to remove at least some of the first subset of droplets from the fuel stream.

[00011] The droplet removal apparatus may comprise a plurality of second droplet stream generators each arranged to direct a respective cross stream of droplets at the fuel stream, such that in use at least some droplets of each cross stream collide with respective ones of the first subset of droplets to remove the respective ones of the first subset of droplets from the fuel stream.

[00012] Each of the droplet stream generators may be arranged to direct its respective cross stream in a different direction from others of the droplet stream generators.

[00013] The second droplet stream generator may be arranged to generate a cross stream of the same material as the fuel stream. In this way, droplets removed from the fuel stream by the cross stream can be recycled, with the cross stream, to provide additional cross streams and/or fuel streams. [00014] The second droplet stream generator may comprise a nozzle through which the cross stream is emitted. The second droplet generator may further comprise a transducer arranged to vibrate the nozzle to control a frequency and droplet separation of the cross stream. Similarly, the first droplet stream generator may comprise a nozzle, and a transducer may be provided to vibrate the nozzle to control a frequency and droplet separation of the fuel stream, by, for example, inducing coalescence of a plurality of droplets. Alternative or additional methods of inducing coalescence may also be used with either or both of the fuel stream generator and the second droplet stream generator.

[00015] Each of the second droplet stream generators may be arranged to emit a cross stream with a different droplet spacing to the cross streams emitted by the other second droplet stream generators.

[00016] The second droplet stream generator may be arranged to emit a cross stream that in use periodically removes a predetermined proportion of droplets from the fuel stream.

[00017] Each of the second droplet stream generators may be arranged to emit a cross stream that in use periodically removes a different number of droplets from the fuel stream than the cross streams emitted by the other second droplet stream generators.

[00018] The fuel emitter may comprise n second droplet generators arranged to generate cross streams that together, in use, periodically remove (n ² -l) of every n ² droplets from the fuel stream.

[00019] The droplet removal apparatus may comprise a first laser arranged to direct a laser beam pulse at respective ones of the first plurality of droplets to remove the respective ones of the droplets from the fuel stream.

[00020] The first laser may be arranged to emit a beam pulse having an energy of approximately 0.1 mJ and lasting approximately 10 ns. It will be appreciated, however, that other the first laser may be arranged to emit beam pluses having different energies, pulse widths and may comprise laser radiation having any of a plurality of different wavelengths.

[00021] The first laser may be an Nd:Yag laser. The first laser be a laser of a different type.

[00022] The first laser may be arranged to periodically remove a predetermined portion of the droplets of the fuel stream.

[00023] The target region may be a plasma formation region within a radiation source for a lithographic apparatus. [00024] The plasma formation region may be within a first chamber of the radiation source, and the droplet removal apparatus may be external to the first chamber such that in use droplets in the first subset do not enter the first chamber.

[00025] The fuel emitter may comprise a droplet trap positioned to receive droplets removed from the fuel stream.

[00026] The fuel stream may comprises a molton metal, such as molton tin.

[00027] The second distance may be 1 mm or more.

[00028] According to a third aspect, there is provided a radiation source, comprising: a fuel emitter for providing fuel targets to a plasma formation region, the fuel emitter comprising: a first droplet stream generator arranged to emit a fuel stream towards the plasma formation region, the fuel stream comprising droplets separated by a first distance; a droplet removal apparatus arranged to remove a first subset of droplets from the fuel stream before the first subset of droplets reach the plasma formation region such that the droplets of the fuel stream provided to the plasma formation region are separated a second distance greater than the first distance; wherein the radiation source is arranged to receive an initiating radiation beam and to direct the initiating radiation beam at the plasma formation region to interact with the droplets of the fuel stream to generate a radiation emitting plasma.

[00029] The radiation source may further comprise a radiation collector for collecting radiation generated by a radiation generating plasma at the plasma formation region, and for directing at least a portion of the generated radiation to a focal point.

[00030] According to a fourth aspect, there is provided a radiation system comprising: a radiation source according to the second aspect; and a second laser arranged to provide the initiating radiation beam.

[00031] The radiation system may further comprise a third laser arranged to direct a fuel modifying radiation beam at a droplet of the fuel stream to alter a property of the droplet before the initiating radiation is incident on the droplet at the plasma formation region.

[00032] According to a fifth aspect, there is provided a lithographic system comprising: a radiation source according to the second aspect arranged to generate a radiation emitting plasma; and a lithographic tool arranged to receive radiation emitted by the radiation emitting plasma.

[00033] According to a sixth aspect, there is provided a method comprising emitting a fuel stream comprising droplets separated by a first distance; and removing a first subset of droplets from the fuel stream before the first subset of droplets reach a target region such that the droplets of the fuel stream provided to the target region are separated a second distance greater than the first distance.

[00034] According to a seventh aspect, there is provided a method comprising emitting a fuel stream comprising droplets, wherein two consecutive droplets of the fuel stream are separated by a first distance; and

removing a first subset of droplets from the fuel stream before the first subset of droplets reach a target region such that, as a result of removing the first subset, two consecutive droplets of the fuel stream provided to the target region are separated at a second distance greater than the first distance.

[00035] Removing a first subset of droplets may comprise emitting a cross stream of droplets towards the fuel stream such at least some droplets of the cross stream collide with droplets of the fuel stream to remove at least some of the first subset of droplets from the fuel stream.

[00036] Removing a first subset of droplets may comprise emitting a plurality of cross streams of droplets at the fuel stream, such that at least some droplets of each cross stream collide with respective ones of the first subset of droplets to remove the respective ones of the first subset of droplets from the fuel stream.

[00037] Emitting a cross stream of droplets may comprise emitting a cross stream comprising droplets having a same composition as droplets of the fuel stream.

[00038] Emitting a cross stream of droplets may comprise vibrating a nozzle through which the cross stream is emitted to control a frequency and droplet separation of the cross stream.

[00039] Emitting a plurality of cross streams may comprise emitting a plurality of cross streams each having a different droplet spacing.

[00040] Emitting a cross stream may comprise emitting a cross stream to periodically remove a predetermined proportion of droplets from the fuel stream.

[00041] Each of the plurality of cross streams may remove a different number of droplets from the fuel stream than the others of the plurality of cross streams.

[00042] The method may comprise generating n cross streams, the n cross streams together periodically removing (n ² -l) of every n ² droplets from the fuel stream.

[00043] Removing a first subset of droplets from the fuel stream may comprise directing a laser beam pulse at respective ones of the first plurality of droplets to remove the respective ones of the droplets from the fuel stream. [00044] Emitting a laser beam pulse may comprises emitting a laser beam pulse having an energy of approximately 0.1 mJ and lasting approximately 10 ns.

[00045] The laser beam pules may be emitted by an Nd: Yag laser.

[00046] The method may further comprise directing laser beam pulses at the fuel stream to periodically remove a predetermined portion of the droplets from the fuel stream.

[00047] The target region may be a plasma formation region within a radiation source for a lithographic apparatus.

[00048] The plasma formation region may be within a first chamber of the radiation source. The droplet removal apparatus may be external to the first chamber such that in use droplets in the first subset do not enter the first chamber.

[00049] Removing the first subset of droplets from the fuel stream may comprise directing the first subset of droplets towards a droplet trap positioned to receive droplets removed from the fuel stream.

[00050] The fuel stream may comprises molton metal, such as molton tin.

[00051] The second distance may be 1 mm or more.

[00052] It is to be understood that features described in relation to one aspect above may be combined with features of other aspects.

BRIEF DESCRIPTION OF THE DRAWINGS

[00053] Embodiments of the invention will now be described, by way of example only, with reference to the accompanying schematic drawings, in which:

Figure 1 schematically depicts a lithographic system comprising a lithographic apparatus and a radiation source according to an embodiment of the invention;

Figure 2 schematically depicts a radiation source according to an embodiment of the invention;

Figure 3 schematically depicts a fuel emitter of the fuel source shown in Figures 1 and 2 according to an embodiment;

Figure 4 schematically depicts an alternative fuel emitter according to an embodiment;

- Figure 5 schematically depicts another alternative fuel emitter according to an embodiment; and

Figure 6 schematically depicts another alternative fuel emitter according to an embodiment. DETAILED DESCRIPTION

[00054] Figure 1 shows a lithographic system including a fuel emitter according to one embodiment of the invention. The lithographic system comprises a radiation source SO and a lithographic apparatus LA. The radiation source SO is configured to generate an extreme ultraviolet (EUV) radiation beam B. 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. The illumination system IL is configured to condition the radiation beam B before it is incident upon the patterning device MA. The projection system is configured to project the radiation beam B (now patterned by the mask MA) onto the substrate W. The substrate W may include previously formed patterns. Where this is the case, the lithographic apparatus aligns the patterned radiation beam B with a pattern previously formed on the substrate W.

[00055] The radiation source SO, illumination system IL, and projection system PS may all be constructed and arranged such that they can be isolated from the external environment. A gas at a pressure below atmospheric pressure (e.g. hydrogen) may be provided in the radiation source SO. A vacuum may be provided in illumination system IL and/or the projection system PS. A small amount of gas (e.g. hydrogen) at a pressure well below atmospheric pressure may be provided in the illumination system IL and/or the projection system PS.

[00056] The radiation source SO shown in Figure 1 is of a type which may be referred to as a laser produced plasma (LPP) source. A laser 1, which may for example be a C0 ₂ laser, is arranged to deposit energy via a laser beam 2 into a fuel, such as tin (Sn) which is provided from a fuel emitter 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 fuel emitter 3 comprises a nozzle configured to direct tin, in the form of droplets, along a trajectory towards a plasma formation region 4. The laser beam 2 is incident upon the tin 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 ions of the plasma.

[00057] The EUV radiation is collected and focused by 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 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 elliptical configuration, having two ellipse focal points. A first focal point may be at the plasma formation region 4, and a second focal point may be at an intermediate focus 6, as discussed below.

[00058] The laser 1 may be separated from the radiation source SO. Where this is the case, the laser beam 2 may be passed from the laser 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 1 and the radiation source SO may together be considered to be a radiation system.

[00059] Radiation that is reflected by the collector 5 forms a radiation beam B. The radiation beam B is focused at point 6 to form an image of the plasma formation region 4, which acts as a virtual radiation source for the illumination system IL. The point 6 at which the radiation beam B is focused may be referred to as the intermediate focus. 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.

[00060] The fuel emitter 3 may be separated from other components of the radiation source SO. In the example of Figure 1, the fuel emitter is within a first chamber 15 of the radiation source SO, while the collector 5 and plasma formation region 4 are within a second chamber 16. By separating the fuel emitter 3 from the chamber 16, tin debris resulting from the operation of the fuel emitter 3 can be prevented from entering the chamber 16.

[00061] The radiation beam B passes from the radiation source SO into the illumination system IL, which is configured to condition the radiation beam. 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 radiation beam B with a desired cross-sectional shape and a desired angular distribution. The radiation beam B passes from the illumination system IL and is incident upon the patterning device MA held by the support structure MT. The patterning device MA reflects and patterns the radiation beam B. 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.

[00062] Following reflection from the patterning device MA the patterned radiation beam B enters the projection system PS. The projection system comprises a plurality of mirrors 13, 14 which are configured to project the radiation beam B onto a substrate W held by the substrate table WT. The projection system PS may apply a reduction factor to the radiation beam, forming an image with features that are smaller than corresponding features on the patterning device MA. A reduction factor of 4 may for example be applied. Although the projection system PS has two mirrors in Figure 1, the projection system may include any number of mirrors (e.g. six mirrors).

[00063] Figure 2 shows a laser produced plasma (LPP) radiation source SO which has an alternative configuration to the radiation source shown in Figure 1. The radiation source SO includes a fuel emitter 3 which is configured to deliver fuel to a plasma formation region 4. The fuel may for example be tin, although any suitable fuel may be used. A pre-pulse laser

17 emits a pre-pulse laser beam 18 which is incident upon the fuel. The pre-pulse laser beam

18 acts to preheat the fuel, thereby changing a property of the fuel such as its size and/or shape. A main laser 19 emits a main laser beam 20 which is incident upon the fuel after the pre-pulse laser beam 18. The main laser beam 20 delivers energy to the fuel and thereby converts the fuel into an EUV radiation emitting plasma 7.

[00064] A radiation collector 21, which may be a so-called grazing incidence collector, is configured to collect the EUV radiation and focus the EUV radiation at a point 6 which may be referred to as the intermediate focus. Thus, an image of the radiation emitting plasma 7 is formed at the intermediate focus 6. An enclosure structure 22 of the radiation source SO includes an opening 23 which is at or near to the intermediate focus 6. The EUV radiation passes through the opening 23 to an illumination system of a lithographic apparatus (e.g. of the form shown schematically in Figure 1).

[00065] The radiation collector 21 may be a nested collector, with a plurality of grazing incidence reflectors 24, 25 and 26 (e.g. as schematically depicted). The grazing incidence reflectors 24, 25 and 26 may be disposed axially symmetrically around an optical axis O. The illustrated radiation collector 21 is shown merely as an example, and other radiation collectors may be used.

[00066] A contamination trap 27 is located between the plasma formation region 4 and the radiation collector 21. The contamination trap 27 may for example be a rotating foil trap, or may be any other suitable form of contamination trap. In some embodiments the contamination trap 27 may be omitted.

[00067] The enclosure 22 of the radiation source SO includes a window 28 through which the pre-pulse laser beam 18 can pass to the plasma formation region 4, and a window 29 through which the main laser beam 20 can pass to the plasma formation region 4. A mirror 30 is used to direct the main laser beam 20 through an opening in the contamination trap 27 to the plasma formation region 4. [00068] The radiation sources SO shown in Figures 1 and 2 may include components which are not illustrated. For example, a spectral filter may be provided in the radiation source. The spectral filter may be substantially transmissive for EUV radiation but substantially blocking for other wavelengths of radiation such as infrared radiation.

[00069] It is known for a fuel emitter to emit fuel droplets with a frequency of approximately 40 kHz, a speed of approximately 40 m/s, and inter-droplet spacing (i.e. the distance between each droplet at the plasma formation region) of approximately 1 mm.

[00070] A stream of droplets may be provided by the decomposition of an initially continuous jet or stream of fuel into droplets to form the fuel stream. The fuel emitter may comprise a nozzle (not shown) through which molten/liquid fuel is driven under pressure to be ejected from the nozzle. The natural break-up of a stream of liquid issuing from a nozzle is known as Rayleigh break-up. The Rayleigh frequency, which corresponds to the rate of droplet production from the break-up of a jet ejected from the nozzle is related to the mean velocity of the fuel at the nozzle and the diameter of the jet : f average velocity .. .

1 Rayleigh ~ ₄ . _{5 jet} diameter ^{( 1}

[00071] Although Rayleigh break-up of a stream of fuel may occur without excitation, other methods to induce controlled jet perturbation, such as vibrations induced by, for example, a piezoelectric actuator, may be used to control the Rayleigh break-up by modulating or oscillating the pressure of molten fuel in the nozzle. Modulating the pressure inside the nozzle may modulate the exit velocity of the liquid fuel from the nozzle, and cause the stream of liquid fuel to break-up into droplets in a controlled manner directly after leaving the nozzle.

[00072] If the frequency of oscillation applied by a vibrator is sufficiently close to the Rayleigh frequency of the nozzle, droplets of fuel are formed, the droplets being separated by a distance which is determined by the mean exit velocity from the fuel nozzle and by the oscillation frequency applied by the vibrator. If the oscillation frequency applied by the vibrator is substantially lower than the Rayleigh frequency, then instead of a periodic stream of small fuel droplets being formed, aligned groups of fuel small droplets may be generated. A given aligned group of fuel may include a group of small droplets travelling at a relatively high speed and a group of small droplets travelling at a relatively low speed (the speeds being relative to the average speed of the stream of fuel exiting the nozzle). These aligned groups may coalesce together to form single larger fuel droplets. In this way a periodic stream of fuel droplets may be generated by applying an oscillation frequency (or multiple driving frequencies) to the vibrator which is significantly lower than the Rayleigh frequency. The spacing between the droplets is still governed by the mean exit velocity and the oscillation frequency: the spacing between the droplets increases with decreasing oscillation frequency.

[00073] Typically, a piezoelectric transducer may be used as a vibrator to apply oscillation to a nozzle such as a glass capillary. The piezoelectric transducer may be driven by a waveform generator with a signal that may contain a high frequency to break up the jet and a low frequency to control the coalescence behavior. Molten fuel may be stored in a heated reservoir vessel and forced to flow towards the nozzle through a filter. The flow rate may be maintained by a gas pressure over the molten fluid fuel in the reservoir vessel.

[00074] It is generally desired to increase the power of EUV radiation output by radiation sources SO. One way to achieve an increase in power is to increase the frequency of droplets emitted from a fuel emitter by emitting the droplets at a higher speed while retaining a desired inter-droplet spacing in order to obtain a desired intermediate droplet distance. A number of obstacles prevent an increase of the droplet speed with known methods. For example, as described above, an inter-droplet spacing of 1 mm may be provided for a fuel stream having a speed of 40 m/s. A droplet spacing of 1 mm is almost two orders of magnitude larger than the natural spacing caused by Rayleigh break-up. In order to achieve a larger spacing, oscillations are provided to the nozzle as described above, to cause coalescence of emitted droplets. Causing coalescence to achieve a desired spacing becomes increasingly difficult as the speed of the fuel stream increases.

[00075] Further, forcing fuel though a nozzle at higher speeds requires the fuel to be emitted at a higher pressure to obtain sufficient kinetic energy, causing additional wear on components of a fuel emitter and increasing a likelihood and frequency of failure, such as nozzle clogging. Additionally, to achieve desired pressure, the nozzle size of the fuel emitter may need to be reduced. This increases the likelihood that the nozzle will become clogged with fuel.

[00076] Figure 3 schematically depicts an embodiment of the fuel emitter 3 according to an embodiment in more detail. The fuel emitter 3 comprises fuel stream generator 40 in the form of a droplet generator. The fuel stream generator 40 is arranged to generate a stream of fuel droplets 41 (fuel stream 41). The droplets in the fuel stream 41 are, initially, each separated by a distance d _initia i and each travel at a speed Uf _Uei . d _initia i may be determined by the Rayleigh frequency of the fuel stream 41 as described above. It will be appreciated that while not depicted in figure 3, the fuel stream 41 may be a continuous stream for a period before separating into droplets separated by the distance d _initia i .

[00077] A droplet removal apparatus in the form of a second droplet generator 43 is arranged to generate a further droplet stream (referred to herein as a cross stream) directed so as to cross the fuel stream. The cross stream 44 is not a fuel stream. That is, the droplets of the cross stream 44 are not used to generate a radiation emitting plasma at the plasma formation region 4. While the cross stream 44 is not a fuel stream, the droplets of the cross stream 44 may be made from the same material as the droplets of the fuel stream 41. For example, both the fuel stream 41 and the cross stream 44 may comprise droplets of molten tin. Indeed, each of the droplet generators 40, 43 may be served from a common reservoir of material from which droplets are to be formed. Alternatively, each droplet generator may be served from a different reservoir, and the material used to form the cross stream may be different from the material used to form the fuel stream.

[00078] The second droplet generator 43 is arranged to emit the cross stream 44 to cross the fuel stream 41 so as to periodically remove droplets from the fuel stream 41. In Figure 3 there is illustrated a collision 45 between a droplet of the fuel stream 41 and a droplet of the cross scream 44. In the example depicted in Figure 3, the droplet generator 43 is arranged to emit the cross stream 44 so as to collide with, and therefore remove, every other droplet (i.e. one droplet out of every two) of the fuel stream 41, thereby increasing the final distance dfinai between the droplets of the fuel stream 41 to 2 * d _initia i .

[00079] Without altering the properties of the fuel stream 41, the number of droplets that are periodically removed from the fuel stream 41 may be varied by adjustments to the frequency f _c i and/or phase of the cross stream 44 in order to adjust the timing of collisions between the droplets of the cross stream 44 and the fuel stream 41. It will be appreciated that the frequency f _c i may be adjusted by adjusting the speed U _c i of the cross stream 44 and/or the distance d _c i between the droplets of the cross stream 44. Further control as to the droplets removed from the fuel stream 41 may be achieved by adjusting the frequency of both the fuel stream 41 and the cross stream 44.

[00080] Additionally, while the droplets of both the fuel stream 41 and the cross stream 44 are depicted as being of generally the same size in Figure 3, by varying the size of the droplets of the cross stream 44 an additional parameter by which to modify the interaction between the fuel stream 41 and the cross stream 44 is provided. For example, by configuring the second droplet generator 43 to emit sufficiently large droplets, each droplet of the cross stream 44 may remove two or more droplets from the fuel stream 41.

[00081] The cross stream 44 may be directed at a fuel trap 46. Where the cross stream 44 is made up of the same material as the fuel stream 41, the fuel trap 46 may be connected to one or more reservoirs (not shown) supplying one or both of the droplet generators 40, 43. In this way, fuel that is removed from the fuel stream 41 may be recycled.

[00082] Where the fuel stream becomes unstable (for example the streams exhibit velocity, position and size fluctuations between droplets of the fuel stream) coalescence techniques, such as those described above (and also described in US 2011/0233429) may be used to coalesce two or more droplets in order to provide a stabilised fuel stream. It will be appreciated, however, that even where instability arises as a result of the methods described herein, an amount and complexity of coalescence operations required is considerably less than in prior art methods.

[00083] Figure 4 schematically illustrates an alternative embodiment of the fuel emitter 3 in which three cross streams 44, 47, 48 are provided. The three cross streams 44, 47, 48 may be provided by respective droplet generators (not shown). In the example of Figure 4, it is to be assumed that each of the cross streams 44, 47, 48 propagates at the same speed u _cn in the direction of the fuel trap 46 but each have a different inter-droplet distance. In particular, droplets of the first cross stream 44 are separated by a distance d _c i, droplets of the second cross stream 47 are separated by a distance da and droplets of the third cross stream 48 are separated by a distance d _C 3. It will be appreciated that figure 4 is merely schematic. For example, while in Figure 4 each of the cross streams stream are illustrated as lying in the same plane, such an arrangement is not necessary. For example, one or more (or each) of the cross streams may be directed along any trajectory that intersects the fuel stream 41.

[00084] The first cross stream 44 is arranged to remove a first droplet and then remove every other droplet (i.e. one droplet out of every two) after the first droplet from the fuel stream 41. The second cross stream 47 is arranged to remove a second droplet (the second droplet immediately following the first droplet) and then every fourth droplet from the fuel stream 41. The third cross stream 48 is arranged to remove a third droplet (the third droplet being three droplets after the first droplet) and to remove every eighth subsequent droplet of the fuel stream 41. Together, therefore, the cross streams 44, 47, 48 are arranged to remove seven of every eight droplets from the fuel stream 41. In the example of Figure 4, therefore, dfinal = 8 * d _initia i. [00085] It will be appreciated from the foregoing that there are a number of variables (which may be adjusted in order to control collisions) between the cross streams 44, 47, 48 and the fuel stream 41. In particular, for each cross stream n, the distance d _cn, the speed u _cn and the position at which the cross stream intercepts the fuel stream, may each be controlled, together with the speed of the fuel stream Uf _Uel , to remove a desired number of droplets from the fuel stream 41 to obtain a desired droplet separation at the plasma formation region d final - Generally, while u _cn may be varied, if only a single droplet is to be effected by a single collision, u _cn is maintained sufficiently fast to prevent a collision from effecting a subsequent adjacent droplet in the fuel stream 41.

[00086] In a particularly efficient arrangement, to obtain a df _inai that is x times greater than d _initia i , x— 1 droplets are periodically removed from every x droplets of the fuel stream 41. To remove x— 1 droplets, n cross streams may be used, where n = -Jx and where -Jx is a positive integer. In this case, each respective cross stream is arranged to periodically remove 2 ¹ droplets from the fuel stream 41, where i indexes each cross stream (that is i=l for the first cross stream, i=2 for the second cross stream, and i=n for the final cross stream). For example, to increase the separation of droplets in the fuel stream 41 by a factor of sixteen, four cross streams may be used to periodically remove fifteen droplets from the fuel stream 41 , the first cross stream removing every other droplet (2 ¹ ), the second cross stream removing every fourth droplet (2 ² ), the third cross stream removing every eighth droplet (2 ³ ) and the fourth cross stream removing every sixteenth droplet (2 ⁴ ). More generally, this arrangement may be expressed by equation (2). d ' ^■ final (2) [00087] As an example, if the fuel stream generator 40 comprises a nozzle arranged to eject a jet of approximately 16 μιη in diameter, and the fuel stream 41 is ejected at a speed of 500 m/s, fRayieigh ^¾ 7MHz causing droplets of approximately 30.4 μιη to form at a distance, dinitiai °f 72 μιη. To increase the distance between droplets in the fuel stream 41 to a distance of approximately 1.15 mm, therefore, df _inai = 2 ⁴ * d _initiai . This requires fifteen droplets to be periodically removed from the fuel stream 41, which as described above may be accomplished with four cross streams.

[00088] It is to be appreciated that while the above examples utilise cross streams each having a different inter-droplet spacing, other arrangements are possible. For example, to periodically remove two out of every three droplets from a fuel stream, two cross streams may be used, each cross stream periodically removing one out of every three droplets and the cross streams arranged so that each cross stream removes adjacent droplets. This is illustrated in Figure 5 in which two cross streams 44' 47' are directed at the fuel stream 41. Each of the cross streams 44', 47' is arranged to remove one out of three droplets, and the cross streams are exactly one droplet out of phase so that the cross streams 44', 47' always remove adjacent droplets from the fuel stream 41. In this case, therefore, df _inai = d _initia i * 3. As in Figure 4, it is to be appreciated that while the cross streams 44', 47' of Figure 5 are depicted propagating in the same plane, this is not required.

[00089] More generally, it is to be understood that any cross stream arrangement which periodically removes a desired number of droplets from the fuel stream 41 may be utilised.

[00090] The apparatus and method described above with reference to Figures 3 and 4 provides for accurate control of the inter-droplet separation of the fuel stream 41. Further, while the total amount of tin pumped by the fuel emitter 3 may be increased by the provision of one or more cross streams, because the periodic removal of droplets may be performed outside the chamber 16, the additional tin need not necessarily enter the chamber 16. As such, while the presence of additional tin within a fuel source may generally result in additional detrital debris, the presence of such additional detrimental may be mitigated by embodiments of the present invention.

[00091] Figure 6 illustrates a fuel emitter 3 according to an alternative embodiment in which droplets are removed from the fuel stream 41 by a laser. In the embodiment of Figure 5, the fuel emitter 3 comprises a fuel stream generator 40 arranged to generate a fuel stream 41. The fuel emitter 3 further comprises a laser 50. The laser 50 is arranged to emit a pulsed laser beam 51 in a direction substantially perpendicular to the direction of propagation of the fuel stream 41. Each pulse of the laser 50 is timed so as to coincide with a respective droplet that it is desired to remove from the fuel stream 41. The energy of the laser beam 51 is selected to impart sufficient energy to propel the droplets out of the fuel stream 41 towards the trap 46. A path of the droplets removed from the fuel stream 41 is schematically illustrated by the hollow droplets 52.

[00092] The laser beam 51 may be of a relatively low power. In one example, the laser beam 50 has an energy of the order of 0.1 mJ and each pulse may last for approximately 10 ns. The relatively low energy of the laser beam pulses 50 is such that droplets of the fuel stream 41 are not affected by shock and/or ablation that results from the removal of an adjacent droplet. The laser 50 may be, for example, an Nd:Yag laser. It will be appreciated, however, that other types of laser, energies and pulse widths may be used.

[00093] Further, while in the example embodiment of Figure 5, a single laser 50 is used to remove all of the desired number of droplets, it will be appreciated that in other embodiments, additional lasers may be provided.

[00094] Advantageously, each of the fuel emitters 3 of Figures 3 to 5 provide a method in which a spatial distance over which the fuel stream 41 forms droplets having a desired separation is smaller than prior art methods which use only natural Rayleigh break-up or induced coalescence principles. Further, as coalescence is not the primary method of generating the desired inter-droplet spacing of the fuel stream 41, the nozzle of the fuel stream generator 41 may be larger than nozzles used in prior art fuel stream generators.

[00095] For example, nozzles of 4 micrometres are used in known fuel emitters, where induced coalescence is used to cause small droplets (formed as a result of Rayleigh breakup) to coalesce into large droplets with a desired inter droplet spacing. Using embodiments described herein, larger nozzles may be used such that less, and less complicated, coalescence is required as the droplet diameter (provided by Rayleigh breakup) is at, or closer to, a desired droplet diameter. In some examples, nozzles having diameters of between 4 and 30 μιη may be used.

[00096] The use of a large nozzle results in less clogging and therefore increases the lifetime of the fuel emitter 3 and reduces the frequency of maintenance. Further, as complicated coalescence conditions are not required, where the nozzle of the fuel stream generator 41 is to be vibrated, the vibration frequencies applied may be much lower, while the signal provided to the vibration means (such as a piezoelectric transducer) may be simplified.

[00097] In an embodiment, the invention may form part of a mask inspection apparatus. The mask inspection apparatus may use EUV radiation to illuminate a mask and use an imaging sensor to monitor radiation reflected from the mask. Images received by the imaging sensor are used to determine whether or not defects are present in the mask. The mask inspection apparatus may include optics (e.g. mirrors) configured to receive EUV radiation from an EUV radiation source and form it into a radiation beam to be directed at a mask. The mask inspection apparatus may further include optics (e.g. mirrors) configured to collect EUV radiation reflected from the mask and form an image of the mask at the imaging sensor. The mask inspection apparatus may include a processor configured to analyse the image of the mask at the imaging sensor, and to determine from that analysis whether any defects are present on the mask. The processor may further be configured to determine whether a detected mask defect will cause an unacceptable defect in images projected onto a substrate when the mask is used by a lithographic apparatus.

[00098] In an embodiment, the invention may form part of a metrology apparatus. The metrology apparatus may be used to measure alignment of a projected pattern formed in resist on a substrate relative to a pattern already present on the substrate. This measurement of relative alignment may be referred to as overlay. The metrology apparatus may for example be located immediately adjacent to a lithographic apparatus and may be used to measure the overlay before the substrate (and the resist) has been processed.

[00099] 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 substrate) or mask (or other patterning device). These apparatus may be generally referred to as lithographic tools. Such a lithographic tool may use vacuum conditions or ambient (non- vacuum) conditions.

[000100] The term "EUV radiation" may be considered to encompass electromagnetic radiation having a wavelength within the range of 5-20 nm, for example within the range of 13-14 nm. EUV radiation may have a wavelength of less than 10 nm, for example within the range of 4-10 nm such as 6.7 nm or 6.8 nm.

[000101] 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.

[000102] 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 disk storage media; optical storage media; flash memory devices; electrical, optical, acoustical or other forms of propagated signals (e.g. carrier waves, infrared signals, digital signals, etc.), 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.

[000103] 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.