WO2018108468A1 - Radiation source apparatus and method, lithographic apparatus and inspection apparatus - Google Patents

Abstract

An EUV radiation source (42, 300) is used for example in a lithographic apparatus or inspection apparatus. For generating EUV radiation (302), laser radiation (304) in a second waveband is directed to a target of liquid tin fuel (69) to cause generation of a plasma. Grating radiation (328) is delivered to the target prior to and/or during delivery of the laser radiation (304), producing an electromagnetic field having a spatial distribution that includes multiple peaks and troughs across the target (406). This causes a corresponding spatial variation in a property of the target, such as the refractive index. If high enough in energy, or it may cause ablation or plasma formation in a grating pattern. By one or more mechanisms, such as surface plasmon polaritons, this periodic variation in the optical properties of the target enhances absorption of the laser energy to improve conversion efficiency of the radiation source apparatus.

Description

RADIATION SOURCE APPARATUS AND METHOD, LITHOGRAPHIC APPARATUS

AND INSPECTION APPARATUS

[0001] CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims priority of EP application 16203624.8 which was filed on December 13, 2016 and which is incorporated herein in its entirety by reference.

FIELD

[0002] The present invention relates to a radiation source apparatus and to methods of generating radiation. The present invention further relates to EUV optical systems including such a radiation source. The invention further relates to a device manufacturing method and to an inspection method.

BACKGROUND

[0003] A lithographic apparatus is a machine that applies a desired pattern onto a substrate, usually onto a target portion of the substrate. A lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In that instance, a patterning device, which is alternatively referred to as a mask or a reticle, may be used to generate a circuit pattern to be formed on an individual layer of the IC. This pattern can be transferred onto a target portion (e.g., comprising part of, one, or several dies) on a substrate (e.g., a silicon wafer). Transfer of the pattern is typically via imaging onto a layer of radiation-sensitive material (resist) provided on the substrate. In general, a single substrate will contain a network of adjacent target portions that are successively patterned. Lithography is widely recognized as one of the key steps in the manufacture of ICs and other devices and/or structures. However, as the dimensions of features made using lithography become smaller, lithography is becoming a more critical factor for enabling miniature IC or other devices and/or structures to be manufactured.

[0004] In order to shorten the exposure wavelength and, thus, reduce the minimum printable size, it has been proposed to use an extreme ultraviolet (EUV) radiation source. EUV radiation is electromagnetic radiation having a wavelength within the range of approximately 1 to 100 nm. For the purposes of lithography, it has been proposed to use wavelengths within the range 5-20 nm, for example within the range of 13-14 nm, or for example within the range of 5-10 nm such as 6.7 nm or 6.8 nm.

[0005] In addition, such short wavelength radiation is also useful for inspection of small structures, to determine their properties for example by reflectometry and/or scatterometry. In US20160282282 it is proposed to measure properties such as CD and overlay of target structures using EUV radiation. Spectroscopic reflectometry is performed using radiation scattered at zero and/or higher diffraction orders. Diffraction signals are further strengthened by the use of a conical mount between an EUV optical system and the substrate. The contents of the prior applications are hereby incorporated by reference in the present disclosure.

[0006] Possible radiation sources for EUV radiation include, for example, laser-produced plasma sources, discharge plasma sources, or sources based on synchrotron radiation provided by an electron storage ring. Laser-produced plasma sources use high energy lasers to generate a plasma in a target made of a suitable fuel material. Examples of LPP sources are described for example in published patents US2014264087 A 1 (Rafac et al) and US2014368802A1 (Yakunin et al). The contents of these documents are incorporated herein by reference. In these examples, liquid tin droplets are used as the target, for generating an EUV-emitting plasma.

[0007] A major challenge in the development and application of EUV sources for commercial applications is to obtain a higher output of the desired radiation. In LPP sources, part of the light in each pulse of laser radiation is reflected by the fuel material and by the plasma. This brings two drawbacks. Firstly, any light that is not absorbed, does not contribute to the production of the desired EUV radiation and is in that respect wasted. Secondly, given the high power of the laser beam, the reflected light has the potential to damage surrounding optics as well as the seed laser and laser amplifiers. As a customary step to improve absorption, a pre- pulse of radiation is used to prepare the target, for example to make the liquid droplet flatter and larger, and/or to convert it into a mist. Both of the patent documents cited above aim to further improve conversion efficiency by preparing the target with further pulses of radiation. US2014264087A1, for example, applies a further pre-pulse of a different wavelength to the main pulse to change some property of the target related to absorption of the main pulse laser radiation. US2014368802A1 applies pre-pulse radiation in particular directions to give the target a desired 3 -dimensional shape or orientation, so that the main pulse radiation is incident on a surface of the fuel material in a direction oblique to the surface rather than at normal incidence to improve absorption.

[0008] Nevertheless, the problems of reflection and how to increase efficiency of the conversion to EUV radiation remain significant.

[0009] An EUV radiation source with a solid Sn target material is disclosed in S. S. Harilal et al, "Efficient laser-produced plasma extreme ultraviolet sources using grooved Sn targets", APPLIED PHYSICS LETTERS 96, 111503 (2010), doi:10.1063/1.3364141. The groove, which is larger than the focused beam of laser radiation, is designed to confine the plasma so as to intensify it within an EUV production zone.

SUMMARY OF THE INVENTION

[0010] It is an object of the present invention to address one or more of the above challenges in the design of a radiation source, including in particular a source of EUV radiation.

[0011] The invention provides an apparatus for providing first radiation in a first waveband, the apparatus comprising a system configured for:

- directing second radiation in a second waveband to a target to cause generation of said first radiation; and

- directing third radiation to the target prior to and/or during delivery of the second radiation, the third radiation being delivered in such a way as to produce an

electromagnetic field having a spatial distribution of energy that includes multiple peaks and troughs across the target so as to cause a corresponding spatial variation in a property of the target.

[0012] The radiation in the first waveband may be for example EUV radiation with a wavelength in the range 1 nm to 100 nm, for example in the in the range 5 nm to 20 nm.

[0013] The inventors have recognized that creating grating-like structures in a target of an LPP radiation source could increase absorption of the laser radiation and so enhance conversion efficiency of the radiation source. The inventors have further recognized that radiation with a suitable spatial variation can be used to achieve effects similar to that of a grating etched in solid metal. The target may be prepared, for example, from a drop of liquid fuel material, but the invention is not limited to liquid fuel material. The third radiation may be the same as, or additional, to pre-pulse radiation used for preparing the target for the main-pulse radiation.

[0014] In photonics more generally, it is known that reflection of radiation by a metal target can be influenced by a grating structure etched into the target surface. For example, if and etched periodic grating arrangement is suitably tuned to properties of incident laser radiation, so-called surface plasmon polaritons can be excited in the surface of the fuel material. These amount to surface-bound oscillations of free charge carriers to the effect that more of the incident light is absorbed than in the case of a light wave incident on a flat interface. The physics of surface plasmon polaritons is described in S.A. Maier, "Plasmonics: Fundamentals and Applications", Springer Business & Science Media, LLC (2007). Surface plasmon polaritons are just one example of the type of phenomenon which might be exploited to increase conversion efficiency in an LPP radiation source. The physics of laser excitation of metals generally is described for example in E.G. Gamaly and A.V. Rode, "Physics of ultra-short laser interaction with matter: From phonon excitation to ultimate transformations", Prog. Quant. Electron. 37, 215-323 (2013).

[0015] The invention further provides an EUV optical apparatus comprising a radiation source and an EUV optical system, wherein the radiation source comprises an apparatus according to the invention as set forth above. The EUV optical apparatus may be, for example, a lithographic apparatus, said EUV optical system comprising a projection system for applying patterns to substrates using EUV radiation from the radiation source. The EUV optical apparatus may be, for example, an inspection apparatus, said EUV optical system an illumination system for directing radiation EUV radiation from the radiation source to a structure of interest and for collecting EUV radiation after interaction with the structure.

[0016] The invention further provides a method of generating first radiation in a first waveband, wherein second radiation in a second waveband is directed to a target to cause generation of said first radiation, the method further comprising delivering third radiation to the target prior to and/or during delivery of the second radiation, the third radiation producing an electromagnetic field having a spatial distribution of energy that includes multiple peaks and troughs across the target so as to cause a corresponding spatial variation in a property of the target.

[0017] These and other aspects of the invention will be understood by the skilled person from a consideration of the examples described below and illustrated in the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

[0018] Embodiments of the invention will now be described, by way of example only, with reference to the accompanying drawings, in which:

Figure 1 depicts schematically a lithographic apparatus having reflective projection optics, as an example of an optical apparatus using EUV radiation;

Figure 2 is a more detailed view of the apparatus of Figure 1 , in an embodiment having an LPP source apparatus, for generation of EUV radiation;

Figure 3 is a schematic block diagram of arrangements for delivering second radiation, third radiation and pre-pulse radiation in an LPP radiation source apparatus according to one embodiment of the present invention, viewed along the line of travel of the target; Figure 4 illustrates (a) the principle of generation of the third radiation and (b) the effect of the third radiation, in the radiation source apparatus of Figure 3 ; Figure 5 illustrates various time points in the operation of the radiation source apparatus in one embodiment of the invention, viewed in a direction transverse to the line of travel of the target;

Figure 6 illustrates one mechanism by which absorption of the second radiation can be increased by generation of surface plasmon polaritons in an embodiment of the present invention;

Figure 7 illustrates three variations of the mechanism of Figure 6 representing further embodiments of the present invention;

Figures 8, 9 and 10 illustrate timings of second radiation, third radiation and pre-pulse radiation in embodiments of the present invention;

Figure 11 illustrates (a) the principle of generation of the third radiation and (b, c) the effect of the third radiation, in alternative embodiments of a radiation source apparatus according to the present invention; and

Figure 12 is a schematic diagram of an inspection apparatus including a radiation source apparatus according to the present invention.

[0019] Throughout the Figures, the same reference numerals indicate similar or corresponding features.

DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS

[0020] Before describing radiation source apparatus in detail, a lithographic apparatus with reflective optics will be described, as an example of an EUV optical apparatus in which the invention may be applied.

[0021] Figure 1 schematically depicts a lithographic apparatus 100 including a source module SO according to one embodiment of the invention. The apparatus comprises:

- an illumination system (illuminator) IL configured to condition a radiation beam B (e.g. EUV radiation).

- a support structure (e.g. a mask table) MT constructed to support a patterning device (e.g. a mask or a reticle) MA and connected to a first positioner PM configured to accurately position the patterning device;

- a substrate table (e.g. a wafer table) WT constructed to hold a substrate (e.g. a resist-coated wafer) W and connected to a second positioner PW configured to accurately position the substrate; and

- a projection system (e.g. a reflective projection system) PS configured to project a pattern imparted to the radiation beam B by patterning device MA onto a target portion C

(e.g. comprising one or more dies) of the substrate W.

[0022] The illumination system may include various types of optical components, such as refractive, reflective, magnetic, electromagnetic, electrostatic or other types of optical components, or any combination thereof, for directing, shaping, or controlling radiation.

[0023] The support structure MT holds the patterning device MA in a manner that depends on the orientation of the patterning device, the design of the lithographic apparatus, and other conditions, such as for example whether or not the patterning device is held in a vacuum environment. The support structure can use mechanical, vacuum, electrostatic or other clamping techniques to hold the patterning device. The support structure may be a frame or a table, for example, which may be fixed or movable as required. The support structure may ensure that the patterning device is at a desired position, for example with respect to the projection system.

[0024] The term "patterning device" should be broadly interpreted as referring to any device that can be used to impart a radiation beam with a pattern in its cross-section such as to create a pattern in a target portion of the substrate. The pattern imparted to the radiation beam may correspond to a particular functional layer in a device being created in the target portion, such as an integrated circuit.

[0025] The patterning device may be transmissive or reflective. Examples of patterning devices include masks, programmable mirror arrays, and programmable LCD panels. Masks are well known in lithography, and include mask types such as binary, alternating phase-shift, and attenuated phase-shift, as well as various hybrid mask types. An example of a programmable mirror array employs a matrix arrangement of small mirrors, each of which can be individually tilted so as to reflect an incoming radiation beam in different directions. The tilted mirrors impart a pattern in a radiation beam which is reflected by the mirror matrix. [0026] The projection system, like the illumination system, may include various types of optical components, such as refractive, reflective, magnetic, electromagnetic, electrostatic or other types of optical components, or any combination thereof, as appropriate for the exposure radiation being used, or for other factors such as the use of a vacuum. It may be desired to use a vacuum for EUV radiation since other gases may absorb too much radiation. A vacuum environment may therefore be provided to the whole beam path with the aid of a vacuum wall and vacuum pumps.

[0027] As here depicted, the apparatus is of a reflective type (e.g. employing a reflective mask). [0028] The lithographic apparatus may be of a type having two (dual stage) or more substrate tables (and/or two or more mask tables). In such "multiple stage" machines the additional tables may be used in parallel, or preparatory steps may be carried out on one or more tables while one or more other tables are being used for exposure.

[0029] Referring to Figure 1 , the illuminator IL receives an extreme ultra violet radiation beam from the source module SO. Methods to produce EUV light include, but are not necessarily limited to, converting a material into a plasma state that has at least one element, e.g., xenon, lithium or tin, with one or more emission lines in the EUV range. In one such method, often termed laser produced plasma ("LPP") the required plasma can be produced by irradiating a fuel, such as a droplet, stream or cluster of material having the required line-emitting chemical element, with a laser beam. The source module SO may be part of an EUV radiation system including a laser, not shown in Figure 1 , for providing the laser beam exciting the fuel. The resulting plasma emits output radiation, e.g., EUV radiation, which is collected using a radiation collector, disposed in the source module. The laser and the source module may be separate entities, for example when a C02 laser is used to provide the laser beam for fuel excitation.

[0030] In such cases, the laser is not considered to form part of the lithographic apparatus and the radiation beam is passed from the laser to the source module with the aid of a beam delivery system comprising, for example, suitable directing mirrors and/or a beam expander. In other cases the source may be an integral part of the source module, for example when the source is a discharge produced plasma EUV generator, often termed as a DPP source.

[0031] The illuminator IL may comprise an adjuster for adjusting the angular intensity distribution of the radiation beam. Generally, at least the outer and/or inner radial extent (commonly referred to as σ-outer and σ-inner, respectively) of the intensity distribution in a pupil plane of the illuminator can be adjusted. In addition, the illuminator IL may comprise various other components, such as facetted field and pupil mirror devices. The illuminator may be used to condition the radiation beam, to have a desired uniformity and intensity distribution in its cross-section.

[0032] The radiation beam B is incident on the patterning device (e.g., mask) MA, which is held on the support structure (e.g., mask table) MT, and is patterned by the patterning device. After being reflected from the patterning device (e.g. mask) MA, the radiation beam B passes through the projection system PS, which focuses the beam onto a target portion C of the substrate W. With the aid of the second positioner PW and position sensor PS2 (e.g. an interferometric device, linear encoder or capacitive sensor), the substrate table WT can be moved accurately, e.g. so as to position different target portions C in the path of the radiation beam B. Similarly, the first positioner PM and another position sensor PS1 can be used to accurately position the patterning device (e.g. mask) MA with respect to the path of the radiation beam B. Patterning device (e.g. mask) MA and substrate W may be aligned using mask alignment marks Ml, M2 and substrate alignment marks PI, P2.

[0033] An EUV membrane, for example a pellicle PE, is provided to prevent contamination of the patterning device from particles within the system. Such pellicles may be provided at the location shown and/or at other locations. A further EUV membrane SPF may be provided as a spectral purity filter, operable to filter out unwanted radiation wavelengths (for example DUV). Such unwanted wavelengths can affect the photoresist on wafer W in an undesirable manner. The SPF may also optionally help prevent contamination of the projection optics within projection system PS from particles released during outgassing (or alternatively a pellicle may be provided in place of the SPF to do this). Either of these EUV membranes may comprise any of the EUV membranes disclosed herein.

[0034] The depicted apparatus could be used in a variety of modes. In a scan mode, the patterning device support (e.g., mask table) MT and the substrate table WT are scanned synchronously while a pattern imparted to the radiation beam is projected onto a target portion C (i.e., a single dynamic exposure). The speed and direction of the substrate table WT relative to the patterning device support (e.g., mask table) MT may be determined by the (de- )magnification and image reversal characteristics of the projection system PS. In scan mode, the maximum size of the exposure field limits the width (in the non-scanning direction) of the target portion in a single dynamic exposure, whereas the length of the scanning motion determines the height (in the scanning direction) of the target portion. Other types of lithographic apparatus and modes of operation are possible, as is well-known in the art. For example, a step mode is known. In so-called "maskless" lithography, a programmable patterning device is held stationary but with a changing pattern, and the substrate table WT is moved or scanned.

[0035] Combinations and/or variations on the above described modes of use or entirely different modes of use may also be employed.

[0036] Figure 2 shows an embodiment of the lithographic apparatus in more detail, including radiation source apparatus in the form of a radiation system 42, the illumination system IL, and the projection system PS. The radiation system 42 as shown in Figure 2 is of the type that uses a laser-produced plasma as a radiation source. EUV radiation may be produced by a gas or vapor, for example Xe gas, Li vapor or Sn vapor in which a very hot plasma is created to emit radiation in the EUV range of the electromagnetic spectrum. The very hot plasma is created by causing an at least partially ionized plasma by, for example, optical excitation using CO2 laser light. In an embodiment, Sn is used to create the plasma in order to emit the radiation in the EUV range.

[0037] The radiation system 42 embodies the function of source SO in the apparatus of Figure 1. Radiation system 42 comprises a source chamber 47, in this embodiment not only substantially enclosing a source of EUV radiation, but also collector 50 which, in the example of Figure 2, is a normal-incidence collector, for instance a multi-layer mirror.

[0038] As part of an LPP radiation source, a laser system 61 is constructed and arranged to provide a laser beam 63 which is delivered by a beam delivering system 65 through an aperture 67 provided in the collector 50. Also, the radiation system includes a material 69, such as Sn or Xe, which is supplied by target material supply 71. The beam delivering system 65, in this embodiment, is arranged to establish a beam path focused substantially upon a desired plasma formation position 73.

[0039] In operation, the target material 69, which may also be referred to as fuel, is supplied by the target material supply 71 in the form of droplets. A trap 72 is provided on the opposite side of the source chamber 47, to capture fuel that is not, for whatever reason, turned into plasma. When such a droplet of the target material 69 reaches the plasma formation position 73, the laser beam 63 impinges on the droplet and an EUV radiation-emitting plasma forms inside the source chamber 47. In the case of a pulsed laser, this involves timing the pulse of laser radiation to coincide with the passage of the droplet through the position 73. These create a highly ionized plasma with electron temperatures of several 10⁵ K. The energetic radiation generated during de-excitation and recombination of these ions includes the wanted EUV which is emitted from the plasma at position 73. The plasma formation position 73 and the aperture 52 are located at first and second focal points of collector 50, respectively and the EUV radiation is focused by the normal-incidence collector mirror 50 onto the intermediate focus point IF.

[0040] The beam of radiation emanating from the source chamber 47 traverses the illumination system IL via so-called normal-incidence reflectors 53, 54, as indicated in Figure 2 by the radiation beam 56. The normal incidence reflectors direct the beam 56, via pellicle PE, onto a patterning device (e.g. reticle or mask) positioned on a support (e.g. reticle or mask table) MT. A patterned beam 57 is formed, which is imaged by projection system PS via reflective elements 58, 59 onto a substrate carried by wafer stage or substrate table WT. More elements than shown may generally be present in illumination system IL and projection system PS. For example there may be one, two, three, four or even more reflective elements present, rather than the two elements 58 and 59 shown in Figure 2.

[0041] As the skilled reader will know, reference axes X, Y and Z may be defined for measuring and describing the geometry and behavior of the apparatus, its various components, and the radiation beams 55, 56, 57. At each part of the apparatus, a local reference frame of X, Y and Z axes may be defined. The Z axis broadly coincides with the direction of optical axis O at a given point in the system, and is generally normal to the plane of a patterning device (reticle) MA and normal to the plane of substrate W. In the radiation source apparatus (radiation system) 42, the X axis coincides broadly with the direction of fuel stream (69, described below), while the Y axis is orthogonal to that, pointing out of the page as indicated. On the other hand, in the vicinity of the support structure MT that holds the reticle MA, the X axis is generally transverse to a scanning direction aligned with the Y axis. For convenience, in this area of the schematic diagram Figure 2, the X axis points out of the page, again as marked. These designations are conventional in the art and will be adopted herein for convenience. In principle, any reference frame can be chosen to describe the apparatus and its behavior.

[0042] In addition to the wanted EUV radiation, the plasma produces other wavelengths of radiation, for example in the visible, UV and DUV range. There is also IR (infrared) radiation present from the laser beam 63. The non-EUV wavelengths are not wanted in the illumination system IL and projection system PS and various measures may be deployed to block the non- EUV radiation. As schematically depicted in Figure 2, an EUV membrane filter in the form of a spectral purity filter SPF may be applied upstream of the virtual source point IF, for IR, DUV and/or other unwanted wavelengths. In the specific example shown in Figure 2, two spectral purity filters are depicted, one within the source chamber 47 and one at the output of the projection system PS. In a practical embodiment, only one spectral purity filter SPF may be provided, which may be in either of these locations or elsewhere between the plasma formation position 73 and wafer W.

[0043] As mentioned above, lithographic apparatus is not the only type of EUV optical system in which the radiation source apparatus and methods of the present disclosure may be applied. In particular, inspection apparatus and methods may be envisaged using any improved EUV radiation source. For examples of EUV inspection apparatus, we refer to the publication US20160282282, mentioned above, the contents of which are incorporated herein by reference. An example inspection apparatus from US20160282282 is illustrated in Figure 12, described below.

[0044] Figure 3 illustrates schematically the main components of a radiation source apparatus 300, which may be used as the radiation system 42 in the lithographic apparatus of Figure 2, or in another lithographic apparatus, or in another EUV optical system, such as an inspection apparatus. At the right hand side, the source chamber 47 is shown schematically, along with the fuel stream comprising droplets of target material 69 and the plasma formation location 73. The view in Figure 3 is along the X axis, that is, along the direction of the fuel stream. The view transverse to the direction of the fuel stream can be seen in Figure 5, described below.

[0045] Radiation including first radiation 302 having a desired EUV wavelength is emitted by a plasma formed at the plasma formation location 73. To generate the plasma, second radiation 304 comprising a main pulse of laser radiation is provided by a first laser source 306, which may be part of the laser system 61 in the example of Figure 2. To prepare the target material 69 for interaction with the main pulse of laser radiation, pre -pulse radiation 308 is provided by a second laser source 310, which may also be part of the same laser system as the laser source 306. Detail of the generation of the main pulse and pre-pulse radiation is not provided in the present disclosure, but may be found for example in the publications explained in the publications US2014264087A1 and US2014368802A1, mentioned above.

[0046] In the illustrated example, the second radiation 304 and pre-pulse radiation 308 are directed using a beam combining optical element 312 and focusing arrangement 314. Not seen in the orientation shown in Figure 3, the beam of pre-pulse radiation 308 typically impinges on the droplets of target material 69 at a time earlier in their flight than the main pulse comprising second radiation 304. The beam of pre-pulse radiation may therefore be focused on a location displaced on the X axis, relative to the main pulse. A controller 316 synchronizes operation of the first laser source 306 and second laser source 310, to achieve the appropriate timing for each droplet of fuel material 69. The delivery of droplets, as well as the pulsed operation of the laser sources 306, 310 is repeated with a high frequency, for example several tens of kilohertz, for example 50 kHz. Consequently, the desired EUV radiation 302 is emitted in pulses with the same frequency.

[0047] As in the prior publication US2014264087A1, the present disclosure envisages the delivery of third radiation to condition the target material 69, to improve absorption of the second radiation 304, to improve the efficiency of generation of the EUV first radiation 302. In accordance with the principles of the present disclosure, however, the third radiation is provided in such a way as to produce at the target an electromagnetic field having a distribution of energy that includes multiple peaks and troughs across the target so. This spatial variation in the electromagnetic field is such as to cause a corresponding spatial variation in a property of the target. An energy distribution having multiple peaks and troughs, which may also be referred to as a repetitive spatial variation, can be distinguished from the routine energy distribution in which a beam of radiation has a central peak in energy that decreases towards the periphery of the beam. As will be described further below, the spatial variation may be for example a periodic variation in one or two directions, which applies a grating pattern to the target, in one or more properties. This grating pattern can be tuned to improve absorption of the second radiation by the fuel material, thereby increasing efficiency of the EUV radiation source apparatus.

[0048] In the example of Figure 3, two beams 328a and 328b of radiation are illustrated which together form third radiation 328 impinging on the target material. Radiation sources 330a and 330b generate these beams. As will be described further below, the third radiation may impinge on the target material at a point in time prior to and or during application of the main pulse of laser radiation (second radiation 304). In the following discussion, the third radiation will be described as "grating radiation", in reference to its function. The timing of delivery of the grating radiation is synchronized with the delivery of the main pulse and pre-pulse radiation by the controller 316.

[0049] In the illustrated example, each beam 328a/328b of grating radiation has its own focusing arrangement 332a and 332b. In other embodiments, the grating radiation may be passed through the same focusing arrangement 314 as the main pulse radiation and/or pre-pulse radiation. In the illustrated example, the beams of grating radiation are positioned either side of the beam of main pulse radiation. In other embodiments, the grating radiation may be delivered from one side or another of the main pulse radiation. Any of the second radiation (main pulse), the pre-pulse radiation and the third radiation (grating radiation) may be delivered broadly along the z-axis as illustrated, or at an angle oblique to the z-axis. [0050] Figure 4 (a) illustrates the principle of application of the grating radiation 328 to the target in the example of Figure 3. The beams 328a and 328b of grating radiation are shown meeting at an angle Θ to one another in the vicinity of a target 402. In the present example, it is assumed that target 402 is a droplet of fuel material 69 that has been prepared already by application of pre-pulse radiation 308. In some embodiments, for example, pre-pulse radiation 308 is used to transform a substantially spherical droplet into a flatter, "pancake" shape, which is shown schematically in the drawing. In other embodiments, the pre-pulse radiation may be sufficient to disperse the liquid fuel material into a mist of smaller droplets or vapor.

[0051] For simplicity, it is assumed that each of the beams 328a and 328b comprises coherent radiation with a planar wavefront, illustrated schematically at 404a and 404b. Interference between these beams at the face of the target 402 produces an interference pattern which has a periodic spatial variation of intensity across the face of target 402. In physical terms, the two beams of radiation interfere to produce an electromagnetic field having spatial distribution of energy which is illustrated schematically as the spatial variation 406. Figure 4 (b) shows the face of the target 402, with the lines of the grating pattern formed by the spatial variation 406. Each shaded line in Figure 4 (b) represents, for example, a band of high energy in the electromagnetic field, with lower-intensity regions in between. While simple bands of high energy are shown, it will be understood that the electromagnetic field may have, for example, a sinusoidal variation in energy.

[0052] Interfering two beams of grating radiation with an angle between them is a simple way of producing an electromagnetic field with the desired spatial variation of energy. In principle, any method can be used, and an alternative method will be described below, with reference to Figure 11.

[0053] Figure 5 is a schematic view of the interaction between the different beams of radiation and the fuel material, at five time points in the operation of the radiation source apparatus 300. These time points are labeled tl to t5. In general, it will be understood that Figure 5 presents for each respective one of the time points a respective view of a specific target at a respective location along the X direction which is the direction of the stream of fuel droplets. At time tl, a representative droplet 502 has been emitted by fuel material supply 71. The direction of travel of the droplet 502 is indicated by a downward arrow. Pre-pulse radiation 308 impinges on the droplet 502. At time t2, the droplet 502 continues its travel, and begins to adopt a flatter shape caused by the pre-pulse radiation 308. The next droplet 502' is shown leaving the fuel material source 71, to illustrate the repetitive nature of operation of the radiation source apparatus 300 in a practical embodiment. [0054] At time t3, grating radiation 328 having the spatial variation 406 impinges on the surface of the fuel material in the droplet 502. In a radiation source apparatus with the layout shown in Figures 3 and 4, the orientation of the interfering beams 328a and 328b of the grating radiation is such that the spatial variation 406 in the electromagnetic field will be periodic in a direction into the plane of the drawing in Figure 5 as a whole. The axes have been rotated locally, as shown, purely so that the spatial variation 406 can be seen in the drawing. There is no fundamental requirement for the spatial variation to be in a particular direction. However, if the droplet 502 travels an appreciable distance during the application of the grating radiation, it may be advantageous for the bands of high intensity to be aligned with the direction of travel, so that the grating pattern is not blurred on the target.

[0055] At time t4 the main pulse of laser radiation (second radiation 304) is applied to the target 502. Under the influence of the electromagnetic field having the spatial variation 406, one or more properties of the material of the target 502 are modified at locations 504 corresponding to the high-intensity portions in the grating radiation. The modification can take different forms, as will be discussed below. The spatially varying modification in one example includes a spatially varying complex dielectric function (complex permittivity) resulting from density changes in an electron gas within the surface 506 of the target material. This spatially varying dielectric function determines the (complex) refractive index of the material, and so influences the interaction of the target material with the second radiation 304 of the main laser pulse. The effect of this can be similar to the effect of a grating structure etched into a target made of solid material. By a range of different effects, including but not limited to the generation of surface plasmon polaritons, described below, it is known that reflection properties of a surface can be changed. In other words, although there is no possibility to etch a grating structure onto the target in the case of a liquid fuel droplet, the provision of an electromagnetic field having a suitable spatial distribution of energy across the target allows optical properties of the target to be modified.

[0056] At time t5 the second radiation 304 has caused target 502 to form completely or partially a plasma 508 which emits the desired first radiation 302. For use in the lithographic apparatus of Figure 2, the target material may be chosen so that the plasma emits EUV radiation, for example in a waveband between 5 and 20 nm. Tuning the spatial variation of the optical properties of the target to achieve a reduction in reflection and an increase in absorption of the second radiation can allow an increased conversion efficiency of the LPP radiation source. There is no fundamental requirement for the spatial variation to be periodic, or to have a fixed spatial frequency or to have only a single spatial frequency. [0057] For clarity of illustration and explanation, different events in the operation of the radiation source apparatus 300 have been illustrated as occurring at discrete time points tl to t5. Any of these time points can in practice be made simultaneous or overlapping with one or more of the others. For example, it would be possible in principle to apply the spatial variation in the pre-pulse radiation 308, or even prior to the pre-pulse radiation. However, in practice it may be difficult to ensure that the applied modification would survive in the material until the point of application of the main pulse radiation 304, and/or to maintain or achieve the desired spacing. More likely, the time points t3, t4 and/or t5 may be combined or overlapping. For example, the grating radiation 328 may be applied simultaneously with the main pulse radiation 304, or may be applied slightly before the main pulse radiation 304, and maintained throughout the main pulse radiation 304 overlapping in time. In principle, the pre-pulse radiation 308 may be optional. The application of third radiation 328 in accordance with the present disclosure can be used in combination with a variety of techniques known in the field of LPP sources, and the use of a pre-pulse is a customary measure to enhance plasma generation and efficiency.

[0058] Summarizing the above discussion, a spatially varying intensity in the field generated by the third radiation will, if the achieved electric field strength is large enough, lead to a spatial modulation of the complex dielectric function, for example the refractive index, of the fuel material. This modulation can arise through various mechanisms. One such mechanism is to excite the surface plasmon polariton mode, which requires a specific geometry, because in theory there is only a single suitable wave vector value that succeeds. A mathematical derivation of the surface plasmon polariton wave vector can be found for example in S.A. Maier, "Plasmonics: Fundamentals and Applications", Springer Business & Science Media, LLC (2007), section 2.2. Chapters 1 to 3 of the Maier book are incorporated herein by reference. . In practice, this means that the spatial periodicity of the modulation, the wavelength of the main pulse and its angle of incidence must be coordinated with the dielectric properties of the target material to excite precisely this wavelength. Tuning of the angle Θ as illustrated in Figure 4 would be a practical way to realize this. Tuning of the grating radiation wavelength and angle of incidence is another mechanism. Some options and considerations for achieving surface plasmon polariton excitation are illustrated in Figures 6 and 7.

[0059] Referring firstly to Figure 6, embodiments may be considered in which the grating radiation 328 and the main pulse radiation 304 have the same wavelength. This may be a result of conveniently deriving them from the same source, for example, in the same way that the pre-pulse radiation is often derived from the same source as the main pulse radiation. A grating pattern of modifications 504 spaced periodically along the surface 506 of the fuel material is established, for example by modifying the density of an electron gas within a metallic fuel material. The periodicity of the grating pattern is represented by a wave number (spatial frequency value) which is effectively the reciprocal of the period p shown in the drawing. The main pulse radiation 304 impinges upon the surface 506 at an angle so that the sum of the grating wave number and the wave vector component of the main pulse radiation along the surface match the surface plasmon polariton excitation condition. The plasmons propagate to the right, represented by arrow 602.

[0060] Using grating radiation of the same wavelength as the main pulse radiation, the shortest grating period that one can make will be half a wavelength (and that is in the impractical case of having the grating pulses impinge on the target surface at shearing incidence). In that case, for the grating spatial period p to match the surface plasmon polariton excitation condition, the main pulse should be incident at an oblique angle a, in which case the main pulse itself contributes a sufficiently large wave vector component along the target surface to excite the plasmons. In other words, the incidence angle of the main radiation becomes constrained by the desire to excite surface plasmons.

[0061] Figure 7 illustrates some variations that are enabled if the grating radiation has a shorter wavelength than the main pulse radiation. Referring to Figure 7 (a), if the wavelength of the grating radiation is sufficiently short, the grating pattern can be made with a spatial period p that exactly matches the surface plasmon polariton excitation condition, given a normally incident main pulse (a = 0). In such an arrangement, it may be possible to excite plasmons in both directions along the surface, indicated by arrows 604, 606.

[0062] In Figure 7 (b) a larger grating spatial period (similar to the one in Figure 6) is obtained by varying the angle Θ to a smaller value. The non-normal angle of incidence a would again be required for the main pulse radiation 304, in order to excite plasmon 608 in the same direction as in Figure 6. Using the shorter wavelength of grating radiation, allows the angle Θ between the grating pulses to be made smaller. This may allow a more compact arrangement of the components of the radiation source apparatus.

[0063] In Figure 7 (c) the grating radiation has a shorter wavelength and a wider angle Θ. In this case, the grating spatial period is very short, and using a tilted-incidence main pulse 304, the surface plasmon polaritons can be excited in the opposite direction 610 to the direction shown in Figure 6 and 7(b). This configuration might be desirable, because a short grating spatial period p in itself eliminates diffraction. If the grating spatial period p is shorter than the main pulse wavelength divided by N (with N a positive integer), then the Nth and -Nth diffracted orders cannot exist. Radiation that is diffracted represents a loss available for absorption and generation of EUV radiation, similar to radiation that is reflected. Eliminating diffraction orders, therefore, is another way in which to increase conversion efficiency.

[0064] The examples illustrated in Figures 6 and 7 are not the only ones possible, and the techniques disclosed herein can be applied in a variety or layouts, which gives the system designer extra flexibility in determining the grating pitch and other parameters of the spatial variation. We can mention embodiments in which the grating pulses are not symmetrically spaced either side of the normal to the target surface. In the case of a target comprising a mist or vapor of fuel material, where the light propagates deeper, one can also exploit 3-D relationships beneath the target surface. In the case of two beams 328a and 328b with mist as a target, the grating "lines" may now extend deeper into the mist forming a set of planes of excited regions. This may have the advantage of being able to further guide the main pulse radiation 304 into the mist if the planes are parallel to the main pulse propagation direction. It is also possible to make a 3-D optical equivalent of a lattice, a so-called optical lattice, by having for each coordinate x, y, and z, two beams propagating in opposite directions. An example of this has been described by in "Optical Lattices and the Mott insulator" by Austrian Academy of Sciences, Innsbruck, Austria (see https://www.uibk.ac.at/exphys/ultracold/projects/rubidium/mott_insulator/). Such a 3-D optical lattice, used to create a corresponding 3-D lattice made of excited/unexcited fuel material (excited where all beams constructively interfere, not excited where they destructively interfere), may act like a photonic crystal and may even trap light. While six laser beams are required to form a 3-D lattice can be formed, with four laser beams a 2-D lattice can be formed. When we use two beams, and supposing that these two beams penetrate sufficiently deep into the mist, this would be equivalent to the ID lattice, being a set of planes as explained above.

[0065] Figures 8, 9 and 10 are example timing diagrams indicating, in a highly schematic form, some different options for the relative timing (and hence the X position) of the different events illustrated in Figure 5. In each of these figures, three graphs illustrate the relative timing of the pre-pulse radiation PP, the grating radiation GP and the main pulse MP. A common time dimension is indicated along the horizontal axis of each graph, not to any particular scale. The vertical axes are not to scale. In each graph, time points tl, t3 and t4 are labeled, corresponding to the time points illustrated in Figure 5 during the processing of fuel droplet 502. In Figure 8, a time point tl ' is indicated, representing the beginning of the next pulse sequence, processing the next fuel droplet 502'.

[0066] In Figure 8, the timing implemented by controller 316 is effectively the same as illustrated in Figure 5. That is to say, each of the pre-pulse radiation, the grating radiation and the main pulse radiation arrives in a discrete time interval, labeled tl, t3 and t4, respectively. In this case, it is assumed that the influence of the grating radiation lasts longer than the duration of the grating radiation pulse, so that the spatial modulation of the target properties remains when the main pulse arrives at time t4.

[0067] Figure 9 illustrates an alternative timing which is similar to that shown in Figure 8, except that the grating radiation has a duration extending from a time t3 before the main pulse commences, and through the duration of the main pulse. This is to ensure that the spatial modulation of the target properties endures throughout the main pulse period.

[0068] Finally, Figure 10 illustrates an embodiment in which the grating radiation is present effectively continuously, again ensuring that the spatial modulation of target properties endures throughout the impulse period. It will be understood that many variations on these timing schemes are possible, and detailed design will be required to achieve the optimum performance. Multiple beams of grating radiation may be applied, with different spatial variations. For example, while the target is expanding in response to the application of the pre-pulse radiation, a continuous or stepwise expanding grating pattern may be applied to the target by suitable configuration of the grating radiation.

[0069] Referring to Figure 11 , this illustrates how a spatially varying intensity distribution can be made without necessarily using two or more interfering beams of grating radiation. As seen in Figure 11 (a), grating radiation 928 is generated from a single beam of source radiation 928' provided by a single laser 930, using a spatial light modulator (SLM) 934. Following the numbering of Figure 4, spatial light modulator transforms the planar wavefront 904 of the source radiation 928' into an electromagnetic field having an arbitrarily chosen energy distribution having multiple peaks and troughs across the target. The drawing illustrates schematically corresponding spatial variation 906 where it impinges target material droplet 902. Other optical elements such as focusing arrangements may of course be provided.

[0070] The spatial light modulator 934 may be of a fixed type (that is, a reflective or transmissive device having a pattern of opaque portions, or phase shift portions). Alternatively, SLM 934 may be programmable, for example being a transmissive liquid crystal array, or a reflective micro-mirror array. Both types of SLM are well known in practice. Apart from the possibility to vary the distribution 906 of the third radiation from time to time using a programmable SLM, SLM's also allow more arbitrary patterns to be generated.

[0071] In Figure 11 (b) a simple one-dimensional grating pattern is formed, similar to the one shown in Figure 4 (b). In Figure 11 (c) a two-dimensional grating pattern is shown as an alternative example. A two-dimensional grating pattern may not be attractive in practice, if the main pulse radiation is linearly polarized, as is often the case. In that case, a one-dimensional grating, properly oriented with respect to the polarization direction of the main pulse radiation, can be expected to optimize coupling of the main pulse radiation into the target material. Incidentally, such a two-dimensional grating pattern could be generated using multiple interfering beams, similar to the pair of beams 328a and 328b. Patterns with a spatially varying pitch, and three-dimensional patterns can also be defined, as already mentioned above. Some types of pattern can be defined more easily using an SLM than a pair or multiplicity of beams.

[0072] In the case of a programmable SLM 934, further options become available. The SLM 934 may for example be under the control of the same controller 916 that controls the third radiation laser 930, and the main pulse and pre-pulse lasers. In a first embodiment, fine control of the spatial intensity distribution 906 can be implemented in real time, for example using feedback of conversion efficiency, reflected light and so forth. As an example, the grating pitch may be varied, while of course more elaborate adjustments can also be made. The plane of focus of the grating pattern can also be adjusted using an SLM, potentially eliminating an adjustable focusing arrangement. In such an example, the variation of the spatial intensity distribution may be modulated over time during interaction with a succession of targets, to optimize performance of the apparatus in one or more parameters.

[0073] If the SLM allows the grating pattern to be varied on a short enough time scale, one might even configure the apparatus to modulate the spatial variation of the third radiation over time during interaction with a single target. By varying the projected pattern during passage of the grating-producing pulse, one might, for example be able to adapt the intensity distribution to the evolving dielectric function of the target material. This dielectric function changes over time because of heating, evaporation and plasma formation. Consequently, the effect of the applied intensity distribution will vary, and a fixed intensity distribution will not be optimum at all times. If desired, an additional probe laser could be provided to measure the induced change in the target material in real-time, and using these measurements to give feedback to the SLM.

[0074] Summarizing the above examples, the spatially varying intensity distribution in the electromagnetic field produced by the grating radiation is designed to spatially modulate one or more properties of the material of the target. If the excitation is mild, the modulation may be caused by intensity-dependent changes in the occupancy spectra of the conduction band electrons. If the excitation is more energetic, there might be local plasma formation or ablation of the material. The expected end-result is some modulation of the surface, either physically (bands of plasma forming or ablation causing surface relief) or electrically (non-thermal electron distributions localized in space). In all cases, it results effectively in a spatial variation of the complex dielectric function across the target. Typically, though not necessarily, this may be a variation of the effective refractive index of the target, as the refractive index is simply the real component of the complex refractive index, determined by the dielectric function (complex permittivity).

[0075] Given the periodically varying refractive index, the surface of the target can act like a grating. If the periodicity of this grating (which is determined by the angles of incidence of the grating radiation beams as well as their wavelengths), the angle of incidence of the main pulse, its wavelength and the dielectric properties of the target are properly adjusted to each other, it could be possible to excite surface plasmon polaritons at the surface. These are waves of charge density with a wave vector pointed along the surface that take their energy from the main pulse. The excitation of these surface plasmon polaritons dissipates their energy in the material, with the net result that the overall absorption of the main pulse could be enhanced with respect to the situation without the grating pulses present.

[0076] The excitation of surface plasmon polaritons is not the only absorption-enhancing phenomenon that can occur by modification of the dielectric function within a metallic target. A number of different phenomena may be exploited, either by themselves or in combination with the others. The spatially periodic dielectric function contrast that is formed by the grating radiation may also allow waveguide modes to be established, to which the main pulse could, under the right circumstances, couple. The transfer of energy from the main pulse to these waveguide modes traveling along the surface may also lead to enhanced absorption because the fields will always penetrate a finite distance into the metal, thereby transferring energy to the electrons in the metal or plasma. This energy is then ultimately dissipated as heat, contributing to the desired enhanced plasma generation.

[0077] Figure 12 shows schematically the form of an inspection apparatus usable for metrology of very small features. An EUV inspection apparatus 1200 is provided for measuring properties of a metrology target T formed on substrate W. The target may be a structure formed by lithography, for example using the lithographic apparatus of Figure 2. Various hardware components are represented schematically, and described in more detail and in more variety in the US2016282282A, mentioned above and incorporated herein by reference. Briefly, a radiation source 1230 provides radiation to an illumination system 1232. In accordance with the principles of the present disclosure, radiation source 1230 is an LPP source of the type described above with reference to any of Figures 3 to 11. [0078] Illumination system 1232 provides a beam of EUV radiation represented by ray 1204 which forms a focused irradiation spot on target T. Radiation reflected by target T and substrate W is split into a spectrum 1210 of rays of different wavelengths, before it impinges on detector 1213. Detector 1213 may be for example a CCD (charge coupled device) image sensor. Illumination system 1232 also provides a reference spectrum 1220 to detector 1214. Components 1212, 1213 etc. may be conveniently considered as a detection system 1233.

[0079] Substrate W in this example is mounted on a movable support having a positioning system 1234 such that an angle of incidence a of ray 1204 can be adjusted. In order to catch the reflected ray 1208, detection system 1233 is provided with a further movable support 1236, so that it moves through an angle 2a relative to the stationary illumination system, or through an angle a relative to the substrate. Additional actuators, not shown, are provided for bringing each target T into a position where the focused spot S of radiation is located.

[0080] A processor 1240 receives signals from the detectors 1213 and 1214. In particular, signal ST from detector 1213 represents the target spectrum and signal SR from detector 1214 represents the reference spectrum. Processor 1240 can subtract the reference spectrum from the target spectrum to contain a reflection spectrum of the target, normalized against variation in the source spectrum. The resulting reflection spectra for one or more angles of incidence are used in the processor to calculate a measurement of property of the target, for example critical dimension (CD) or overlay.

CONCLUSION

[0081] In conclusion, the present disclosure provides radiation source apparatuses and methods in which EUV radiation, or other radiation having a desired wavelength can be generated from a plasma with improved control of reflection and absorption characteristics. Efficiency may be improved, and/or problems of reflection of laser radiation back into the laser apparatus can be avoided or reduced. The improved radiation source apparatus can be included in a lithographic apparatus, or an inspection apparatus, or any optical apparatus using radiation of the first waveband. With regard to particular commercial applications, EUV radiation may be generated with improved efficiency, for example in the range 5-20 nm.

[0082] The terms "light", "radiation" and "beam" used herein encompass all types of electromagnetic radiation, including ultraviolet (UV) radiation (e.g., having a wavelength of or about 365 to 126 nm) and extreme ultra-violet (EUV) radiation (e.g., having a wavelength in the range of 1-100 nm or 5-20 nm). Lithographic apparatus and inspection apparatus can use any of these wavelengths, as well as particle beams, such as ion beams or electron beams. [0083] The term "lens", where the context allows, may refer to any one or combination of various types of optical components, including refractive, reflective, magnetic, electromagnetic and electrostatic optical components.

[0084] The breadth and scope of the present invention should not be limited by any of the above-described exemplary embodiments, but should be defined only in accordance with the following claims and their equivalents.

Claims

1. An apparatus for providing first radiation in a first waveband, the apparatus comprising:

a system configured for directing second radiation in a second waveband to a target to cause generation of said first radiation; and configured for directing third radiation to the target prior to and/or during delivery of the second radiation, the third radiation being delivered in such a way as to produce an electromagnetic field having a spatial distribution of energy that includes multiple peaks and troughs across the target so as to cause a

corresponding spatial variation in a property of the target.

2. An apparatus as claimed in claim 1 wherein said spatial distribution of energy is periodic in one or more directions.

3. An apparatus as claimed in claim 1 or 2 wherein said spatial variation in the property of the target includes a variation in an effective refractive index of the target.

4. An apparatus as claimed in any preceding claim wherein the variation in the property of the target includes a spatial variation in a dielectric function of an electron gas within a fuel material in the target.

5. An apparatus as claimed in any preceding claim wherein the variation in the property of the target includes a variation in a density of a fuel material at different points in the target.

6. An apparatus as claimed in claim 5 wherein the variation in the density of the fuel material arises at least in part from ablation of material at points in the target.

7. An apparatus as claimed in claim 4, 5 or 6 further including a target material supply for providing said fuel material in the form of a liquid metal droplet.

8. An apparatus as claimed in claim 7 wherein in operation said target comprises a liquid metal droplet when the third radiation is delivered.

9. An apparatus as claimed in claim 7 or 8 wherein in operation the target comprises at least partially a mist of liquid metal when the third radiation is delivered.

10. An apparatus as claimed in any of claims 4 to 9 wherein in operation the target comprises at least partially a plasma after interaction with the third radiation.

11. An apparatus as claimed in any preceding claim wherein the system is further configured for irradiating a quantity of fuel material irradiated with pre-pulse radiation to form said target prior to delivery of said third radiation.

12. An apparatus as claimed in claim 11 further including a target material supply for providing said fuel material in the form of a liquid metal droplet when the pre-pulse radiation is delivered.

13. An apparatus as claimed in claim 11 or 12 wherein the target comprises a flattened droplet or cloud of fuel material when the third radiation is delivered.

14. An apparatus as claimed in any preceding claim wherein the system is configured to deliver two or more beams of the third radiation which interfere to produce said

electromagnetic field.

15. An apparatus as claimed in any preceding claim wherein the system is configured to direct one or more beams of the third radiation via a spatial light modulator to produce said electromagnetic field.

16. An apparatus as claimed in any preceding claim the system is operable to modulate the spatial distribution of energy of the electromagnetic field over time during interaction with one target.

17. An apparatus as claimed in any preceding claim wherein the system is operable to modulate the spatial distribution of energy of the electromagnetic field over time during interaction with a succession of targets, to optimize performance of the apparatus in one or more parameters.

18. An apparatus as claimed in any preceding claim wherein the system is operable to tune said spatial variation in the property of the target to a surface plasmon polariton excitation condition of a fuel material surface of the target, for a given wavelength, polarization and incidence angle of the second radiation.

19. An apparatus as claimed in claim 18 wherein the incidence angle of the second radiation is normal to the fuel material surface.

20. An apparatus as claimed in any preceding claim wherein the third radiation has a wavelength shorter than a wavelength of the second radiation.

21. An apparatus as claimed in claim 20 wherein the second radiation has a wavelength on the order of 10 micron and the third radiation has a wavelength less than 2 micron.

22. An apparatus as claimed in any preceding claim wherein said first radiation has a wavelength in the range 1 nm to 100 nm.

23. An apparatus as claimed in claim 22 wherein said first radiation has a wavelength in the range 5 nm to 20 nm.

24. An EUV optical apparatus comprising a radiation source and an EUV optical system , wherein the radiation source comprises an apparatus as claimed in claim 22 or 23 and the EUV optical system is arranged to receive said first radiation from the radiation source.

25. A lithographic apparatus comprising an EUV optical apparatus as claimed in claim 24, said EUV optical system comprising a projection system for applying a pattern to a substrate using said first radiation from the radiation source.

26. An inspection apparatus comprising an EUV optical apparatus as claimed in claim 24, said EUV optical system comprising an illumination system for directing said first radiation from the radiation source to a structure of interest and for collecting said first radiation after interaction with the structure.

27. A method of generating first radiation in a first waveband, wherein second radiation in a second waveband is directed to a target to cause generation of said first radiation, the method further comprising delivering third radiation to the target prior to and/or during delivery of the second radiation, the third radiation producing an electromagnetic field having a spatial distribution of energy that includes multiple peaks and troughs across the target so as to cause a corresponding spatial variation in a property of the target.

28. A method as claimed in claim 27 wherein said spatial distribution of energy is periodic in one or more directions.

29. A method as claimed in claim 27 or 28 wherein said spatial variation in the property of the target includes a variation in an effective refractive index across the target.

30. A method as claimed in any of claims 27 to 29 wherein the variation in the property of the target includes a variation in a dielectric function of an electron gas within a fuel material of the target across the target.

31. A method as claimed in any of claims 27 to 30 wherein the variation in the property of the target includes a variation in a density of a fuel material of the target across the target.

32. A method as claimed in claim 31 wherein the variation in the density of the fuel material arises at least in part from ablation of the fuel material at points across the target.

33. A method as claimed in any of claims 27 to 32 wherein the target comprises a liquid metal droplet when the third radiation is delivered.

34. A method as claimed in any of claims 27 to 33 wherein the target comprises at least partially a mist of liquid metal when the third radiation is delivered.

35. A method as claimed in any of claims 27 to 34 wherein the target comprises at least partially a plasma after interaction with the third radiation.

36. A method as claimed in any of claims 27 to 35 wherein a quantity of fuel material is irradiated with pre -pulse radiation to form said target prior to delivery of said third radiation.

37. A method as claimed in claim 36 wherein the fuel material is in the form of a liquid metal droplet when the pre -pulse radiation is delivered.

38. A method as claimed in claim 37 wherein the target comprises a flattened droplet or cloud of fuel material when the third radiation is delivered.

39. A method as claimed in any of claims 27 to 38 wherein the electromagnetic field having said spatial distribution of energy across the target is produced by interfering two or more beams of said third radiation.

40. A method as claimed in any of claims 27 to 39 wherein the electromagnetic field having said spatial distribution of energy across the target is generated by interaction of one or more beams of said third radiation with a spatial light modulator.

41. A method as claimed in any of claims 27 to 40 wherein said spatial distribution of energy of the electromagnetic field is modulated over time during interaction with one target.

42. A method as claimed in any of claims 27 to 41 wherein said spatial distribution of energy of the electromagnetic field is modulated over time during interaction with a succession of targets, to optimize performance of the radiation source in one or more parameters.

43. A method as claimed in any of claims 27 to 42 wherein said spatial variation in the property of the target is tuned to a surface plasmon polariton excitation condition of a fuel material surface within the target, for a given wavelength, polarization and incidence angle of the second radiation.

44. A method as claimed in claim 43 wherein the incidence angle of the second radiation normal to the surface of fuel material surface.

45. A method as claimed in any of claims 27 to 44 wherein the third radiation has a wavelength shorter than a wavelength of the second radiation.

46. A method as claimed in claim 45 wherein the second radiation has a wavelength on the order of 10 micron and the third radiation has a wavelength less than 2 micron.

47. A method as claimed in any of claims 27 to 46 wherein said first radiation has a wavelength in the range 1 nm to 100 nm.

48. A method as claimed in claim 47 wherein said first radiation has a wavelength in the range 5 nm to 20 nm.

49. A method of manufacturing a device wherein a pattern is applied to a substrate using radiation of the first wavelength generated by a method as claimed in any of claims 27 to 48.

50. A method of determining a property of a structure wherein the structure is illuminated using radiation of the first wavelength generated by a method as claimed in any of claims 27 to 48, and collecting said radiation after interaction with the structure.