NL2011306A - Method and apparatus for generating radiation. - Google Patents

Description

METHOD AND APPARATUS FOR GENERATING RADIATION

FIELD

[0001] The present invention relates to a method and apparatus for generating radiation.

BACKGROUND

[0002] 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.

[0003] 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] A theoretical estimate of the limits of pattern printing can be given by the Rayleigh criterion for resolution as shown in equation (1):

where λ is the wavelength of the radiation used, NA is the numerical aperture of the projection system used to print the pattern, ki is a process dependent adjustment factor, also called the Rayleigh constant, and CD is the feature size (or critical dimension) of the printed feature. It follows from equation (1) that reduction of the minimum printable size of features can be obtained in three ways: by shortening the exposure wavelength λ, by increasing the numerical aperture NA or by decreasing the value of ki.

[0005] 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 5-20 nm, for example within the range of 13-14 nm. It has further been proposed that EUV radiation with a wavelength of less than 10 nm could be used, for example within the range of 5-10 nm such as 6.7 nm or 6.8 nm. Such radiation is termed extreme ultraviolet radiation or soft x-ray radiation. Possible sources include, for example, laser-produced plasma sources, discharge plasma sources, or sources based on synchrotron radiation provided by an electron storage ring.

[0006] EUV radiation may be produced using a plasma. A radiation system for producing EUV radiation may include a laser for exciting a fuel to provide the plasma, and a radiation source for containing the plasma. The plasma may be created, for example, by directing a laser beam (i.e., initiating radiation) at a fuel, such as particles (usually droplets) of a suitable fuel material (e.g., tin, which is currently thought to be the most promising and thus likely choice of fuel for EUV radiation sources), or a stream of a suitable gas or vapor, such as Xe gas or Li vapor. The resulting plasma emits output radiation, e.g., EUV radiation, which is collected using a radiation collector. The radiation collector may be a mirrored normal incidence radiation collector (sometimes referred to as a near normal incidence radiation collector), which receives the radiation and focuses the radiation into a beam. The radiation collector may have any other suitable form. The radiation source may include an enclosing structure or chamber arranged to provide a vacuum environment to support the plasma. Such a radiation system is typically termed a laser produced plasma (LPP) source. In an alternative system, which may also employ the use of a laser, radiation may be generated by a plasma formed by the use of an electrical discharge - a discharge produced plasma (DPP) source.

[0007] As discussed above, a laser beam is directed at a target of a suitable fuel material to generate a radiation generating plasma. It has been proposed to direct an initial (e.g., lower energy) laser pulse at the target, prior to the directing of the main beam at the target. The initial (or first, or pre-pulse) laser pulse may be configured to, for example, in some way change a property of the target to optimize the radiation generation process in some way. For example, the optimization may be such that a larger amount of EUV radiation is generated for a given target, and/or such that the radiation is preferentially directed in a certain direction, and/or such that debris resulting from the generation of the radiation generating plasma is directed in a certain direction. Typically, the initial laser pulse will heat the target and/or change the shape of the target, either or both of which may assist in the aforementioned optimization.

[0008] Although the use of a first, lower energy laser pulse may indeed optimize the generation of radiation, there are still drawbacks associated with the use of droplet targets and an initial laser pulse. For example, one drawback is that the initial laser pulse may, when incident on the target, cause particulate contamination to be generated in the form of a large number of nanometre-sized particles of fuel. These particles may be small and fast enough to overcome any buffer gas or other mitigation arrangement that is present, which might otherwise prevent the particles from coming into contact with and contaminating the collector of the radiation source, or the like. Alternatively or additionally, the generation of such particles results in there being less material for use in the generation of the radiation generating plasma, which may reduce the conversion efficiency of the radiation emitting process as a whole (even if only slightly). Furthermore, the use of an initial laser pulse will still require a significant amount of energy, and may add design complexity to the laser arrangement as a whole that is used in the generation of the radiation generating plasma.

SUMMARY

[0009] It is desirable to obviate or mitigate at least one problem of the prior art, whether identified herein or elsewhere, or to provide an alternative to existing apparatus or methods.

[0010] According to a first aspect of the present invention, there is provided a method of generating radiation, the method comprising: directing a first jet from a first location to a second location, the first jet being continuous between the first and second locations; directing a second jet from a third location to a fourth location, the second jet being continuous between the third and fourth locations; wherein at least one of the first and second jets comprises a fuel for use in generating a radiation generating plasma and the at least one of the jets at least partially intersects a plasma formation location; directing initiating radiation at the plasma formation location to generate a radiation generating plasma, wherein expansion of the plasma is at least partially constrained by each of the first and second jets.

[0011] The first and second jets may be directed such that the first and second jets together form part of a generally concave surface at the plasma formation location.

[0012] The first and second jets may at least partially intersect the plasma formation location such that radiation is incident on at least part of each of the first and second jets.

[0013] The first and second jets may be directed such that they form a gap of a predetermined width at the plasma formation location. The gap between the jets may be in a range of 100 to 2000 pm.

[0014] The initiating radiation may be directed such that a focal point of the initiating radiation is wider than the gap between the first and second jets.

[0015] The initiating radiation may be directed substantially perpendicular to a longitudinal axis of the gap between the first and second jets.

[0016] The initiating radiation may be directed such that a focal point of the initiating radiation is further from a source of the initiating radiation than the plasma formation location.

[0017] The at least one of the first and second jets may be formed so as to comprise a substantially flattened surface, and the initiating radiation may be directed such that the initiating radiation is incident on the substantially flattened surface.

[0018] The first and second jets may be formed with a generally non-square rectangular cross-section and each of the first and second jets may be angled such that respective longitudinal axes of the first and second jets form an internal angle of less than 180 degrees and an external angle of more than 180 degrees. The initiation radiation may be directed towards the internal angle or may be directed towards the external angle.

[0019] The initiating radiation may be linearly polarized in a direction substantially perpendicular to a longitudinal axis of a gap between the first and second jets at the plasma formation location.

[0020] The initiating radiation may be a radiation pulse. The pulse may have an energy in a range of 1 and 5 Joules. The pulse may have an energy such that debris generated as a result of plasma generation is anisotropic. For example, the pulse may have an energy of 0.5 Joules or less.

[0021] The first and second jets may be directed at a speed in a range of 2.5 m/s to 200 m/s.

[0022] The first and second jets may have a diameter or an equivalent diameter in a range of 100 pm to 10 mm.

[0023] The second and fourth locations may be the same location.

[0024] According to a second aspect of the present invention, there is provided a method of generating radiation, the method comprising: directing a jet from a first location to a second location, the jet being continuous between the first and second locations, the jet comprising a fuel for use in generating a radiation generating plasma, the jet at least partially intersecting a plasma formation location and the jet being formed to comprise a substantially concave surface at the plasma formation location; and directing initiating radiation so that the initiating radiation is incident on the substantially concave surface at the plasma formation location to generate a radiation generating plasma.

[0025] According to a third aspect of the present invention, there is provided a method of generating radiation, comprising directing a jet from a first location to a second location, the jet being continuous between the first and second locations and the jet comprising a fuel for use in generating a radiation generating plasma; directing initiating radiation at a plasma formation location through which the continuous jet passes to generate, in use, a radiation generating plasma.

[0026] According to a fourth aspect of the present invention, there is provided a lithographic method, comprising: generating radiation according to a method of the first, second or third aspects; and using the generated radiation to apply a pattern to a substrate.

[0027] According to a fifth aspect of the present invention, there is provided a radiation source comprising: a first nozzle for directing a first jet from a first location to a second location, the first jet being continuous between the first and second locations; a second nozzle for directing a second jet from a third location to a fourth location, the second jet being continuous between the third and fourth locations; wherein at least one of the first and second jets comprises a fuel for use in generating a radiation generating plasma and, in use, the at least one of the jets at least partially intersects a plasma formation location; a secondary radiation source configured to direct initiating radiation at the plasma formation location to generate, in use, a radiation generating plasma, wherein, in use, expansion of the plasma is at least partially constrained by each of the first and second jets.

[0028] According to a sixth aspect of the present invention, there is provided a radiation source comprising: a first nozzle for directing a jet from a first location to a second location, the jet being continuous between the first and second locations, the jet comprising a fuel for use in generating a radiation generating plasma, the jet at least partially intersecting a plasma formation location and the jet being formed to comprise a substantially concave surface at the plasma formation location; a secondary radiation source configured to direct initiating radiation such that the initiating radiation is incident on the concave surface at the plasma formation location to generate, in use, a radiation generating plasma.

[0029] The radiation source may further comprise a radiation collector for collecting radiation generated by the radiation generating plasma at the plasma formation location, and for directing at least a portion of the generated radiation to a focal point.

[0030] According to a seventh aspect of the present invention, there is provided a lithographic apparatus comprising, or in connection with (e.g., physically connected together, or able to receive radiation from), the radiation source of the fifth or sixth aspect.

[0031] According to an eighth aspect, there is provided an apparatus comprising, or in connection with (e.g., physically connected together, or able to receive radiation from), a radiation source of the present invention. The apparatus may be an inspection tool (e.g., a patterning device inspection tool), or any apparatus that uses radiation for reasons other than lithography (e.g., illumination, inspection, experimentation, probing, or the like).

[0032] One or more aspects of the invention may, where appropriate to one skilled in the art, be combined with any one or more other aspects described herein, and/or with any one or more features described herein.

BRIEF DESCRIPTION OF THE DRAWINGS

[0033] Embodiments of the invention will now be described, by way of example only, with reference to the accompanying schematic drawings in which corresponding reference symbols indicate corresponding parts, and in which:

[0034] Figure 1 schematically depicts a lithographic apparatus according to an embodiment of the invention;

[0035] Figure 2 is a more detailed view of the apparatus of Figure 1, including an LLP radiation source;

[0036] Figure 3 schematically depicts a radiation source according to an embodiment of the present invention;

[0037] Figures 4 and 5 schematically depict different shapes of jet that may be formed using the radiation source of Figure 3;

[0038] Figure 6 schematically depicts a radiation source according to an alternative embodiment of the present invention; and

[0039] Figures 7 to 11 schematically depict different configurations of jets and incident radiation that may be formed using the radiation source of Figure 6.

DETAILED DESCRIPTION

[0040] Figure 1 schematically depicts a lithographic apparatus LAP including radiation source SO according to an 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 stmcture (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.

[0041] 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.

[0042] 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 stmcture 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.

[0043] 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.

[0044] 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 that is reflected by the mirror matrix.

[0045] 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 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.

[0046] As here depicted, the apparatus is of a reflective type (e.g., employing a reflective mask).

[0047] 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.

[0048] Referring to Figure 1, the illuminator IL receives an extreme ultra violet radiation beam from the radiation source 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, cluster or jet of material having the required line-emitting element, with a laser beam. The radiation source SO may be part of an EUV radiation system including a fuel stream generator for generating a stream of fuel and/or a laser (neither of which are shown in Figure 1), for providing the laser beam for exciting the fuel. The resulting plasma emits output radiation, e.g., EUV radiation, which is collected using a radiation collector, disposed in the radiation source. The laser and/or fuel stream generator and the collector module (often referred to as radiation source), may be separate entities, for example when a CO2 laser is used to provide the laser beam for fuel excitation.

[0049] 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 radiation source 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 radiation source, for example when the source is a discharge produced plasma EUV generator, often termed as a DPP source.

[0050] 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.

[0051] 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.

[0052] The depicted apparatus could be used in at least one of the following modes: 1. In step mode, the support structure (e.g., mask table) MT and the substrate table WT are kept essentially stationary, while an entire pattern imparted to the radiation beam is projected onto a target portion C at one time (i.e., a single static exposure). The substrate table WT is then shifted in the X and/or Y direction so that a different target portion C can be exposed.

2. In scan mode, the support structure (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 velocity and direction of the substrate table WT relative to the support structure (e.g., mask table) MT may be determined by the (de-)magnification and image reversal characteristics of the projection system PS.

3. In another mode, the support structure (e.g., mask table) MT is kept essentially stationary holding a programmable patterning device, and the substrate table WT is moved or scanned while a pattern imparted to the radiation beam is projected onto a target portion C. In this mode, generally a pulsed radiation source is employed and the programmable patterning device is updated as required after each movement of the substrate table WT or in between successive radiation pulses during a scan. This mode of operation can be readily applied to maskless lithography that utilizes a programmable patterning device, such as a programmable mirror array of a type as referred to above.

[0053] Combinations and/or variations on the above described modes of use or entirely different modes of use may also be employed.

[0054] Figure 2 shows the lithographic apparatus LAP in more detail, including the radiation source SO, the illumination system IL, and the projection system PS. The radiation source SO is constructed and arranged such that a vacuum environment can be maintained in an enclosing structure 2 of the radiation source.

[0055] A laser 4 is arranged to deposit laser energy via a laser beam 6 into a fuel, such as xenon (Xe), tin (Sn) or lithium (Li), which is provided from a fuel stream generator in the form of a fuel jet generator 8. Liquid (i.e., molten) tin, or another metal in liquid form, is currently thought to be the most promising and thus likely choice of fuel for EUV radiation sources. A fuel jet trap 9 is arranged to receive fuel not spent during plasma creation. The deposition of laser energy into the fuel creates a highly ionized plasma 10 at a plasma formation location 12, which has electron temperatures of several tens of electronvolts (eV). The energetic radiation generated during de-excitation and recombination of these ions is emitted from the plasma 10, collected and focused by a near normal incidence radiation collector 14 (sometimes referred to more generally as a normal incidence radiation collector). The collector 14 may have a multilayer structure, for example one tuned to reflect, more readily reflect, or preferentially reflect, radiation of a specific wavelength (e.g., radiation of a specific EUV wavelength). The collector 14 may have an elliptical configuration, having two natural ellipse focus points. One focus point will be at the plasma formation location 10, and the other focus point will be at the intermediate focus, discussed below.

[0056] A laser 4 and/or radiation source and/or a collector 14 may together be considered to comprise a radiation source, specifically an EUV radiation source. The EUV radiation source may be referred to as a laser produced plasma (LPP) radiation source. The collector 14 in the enclosing structure 2 may form a collector module, which forms a part of the radiation source (in this example).

[0057] A second laser (not shown) may be provided, the second laser being configured to preheat the fuel before the laser beam 6 is incident upon it. An LPP source that uses this approach may be referred to as a dual laser pulsing (DLP) source. Such a second laser may be described as providing a pre-pulse into a fuel target, for example to change a property of that target in order to provide a modified target. The change in property may be, for example, a change in temperature, size, shape or the like, and will generally be caused by heating of the target.

[0058] Although not shown, the fuel jet generator 8 will comprise, or be in connection with, a nozzle configured to direct fuel, along a trajectory towards the plasma formation location 12.

[0059] Radiation B that is reflected by the radiation collector 14 is focused at a virtual source point 16. The virtual source point 16 is commonly referred to as the intermediate focus, and the radiation source SO is arranged such that the intermediate focus 16 is located at or near to an opening 18 in the enclosing structure 2. The virtual source point 16 is an image of the radiation emitting plasma 10.

[0060] Subsequently, the radiation B traverses the illumination system IL, which may include a facetted field mirror device 20 and a facetted pupil mirror device 22 arranged to provide a desired angular distribution of the radiation beam B at the patterning device MA, as well as a desired uniformity of radiation intensity at the patterning device MA. Upon reflection of the beam of radiation at the patterning device MA, held by the support structure MT, a patterned beam 24 is formed and the patterned beam 24 is imaged by the projection system PS via reflective elements 26, 28 onto a substrate W held by the wafer stage or substrate table WT.

[0061] More elements than shown may generally be present in the illumination system IL and projection system PS. Furthermore, there may be more mirrors present than those shown in the figures, for example there may be 1-6 additional reflective elements present in the projection system PS than shown in Figure 2.

[0062] As described above, fuel can be provided in droplet form, each droplet forming a respective target for successive laser pulses. Problems may arise, however, from use of droplet targets. In particular, fuel droplets are particularly susceptible to interactions with plasma and debris created from prior fuel droplets. Such interactions may lead to variation in fuel droplet timing. Further, plasma generated by respective laser pulses and respective fuel droplets may vary in unpredictable ways, leading to unpredictable variation in fuel droplet timing. Variation in the timing of fuel droplets may lead to difficulty in accurately timing respective laser pulses, such that expensive and complicated droplet timing mechanisms may be required. Additionally, heat from earlier plasma may result in deformations in the shape of subsequent fuel droplets and such shape deformations may reduce conversion efficiency.

[0063] According to an embodiment of the present invention, the abovementioned problems may be obviated or mitigated. According to an embodiment of the present invention there is provided a method of generating radiation, and an apparatus (i.e., a radiation source) for use in implementing that method. The method comprises directing a continuous jet of liquid fuel from a first location to a second location. The method further comprises directing initiating (that is, radiation sufficient to generate a plasma from the target) radiation at a plasma formation location through which the continuous jet passes to generate, in use, a radiation generating plasma from the continuous jet.

[0064] The use of continuous jets mitigates at least some of the problems associated with use of fuel droplets. One advantage is that unlike fuel droplets which, as described above are highly susceptible to detrimental interactions with plasma and debris generated from prior fuel droplets, the inertia provided by continuous jets of liquid fuel mean that they are less susceptible to such interactions. As such, it is easier to ensure that fuel is present at the plasma formation location when the initiating radiation is released. Additionally, the shape of the continuous jet is less susceptible to shape deformation that may result from heat generated by earlier plasma. Furthermore, as described above but not depicted in Figure 2, the fuel jet generator 8 is provided with or is in connection with a nozzle through which the fuel jet is directed. Use of continuous jets allows for the use of relatively simple, passive fuel nozzles with wider diameters (compared with droplet forming nozzles), resulting in lower costs and longer nozzle lifetimes.

[0065] Figure 3 schematically illustrates the radiation source (SO) of Figure 2 in further detail. As described above, rather than providing a fuel in droplet form, the fuel jet generator 8 is arranged to provide a jet 40 from a nozzle 41, the jet 40 being continuous (i.e., not formed from a plurality of droplets) between the nozzle 41 and the fuel jet trap 9. The nozzle 41 may be comprised of Tungsten, Molybdenum, or an alloy thereof. The nozzle may be resistant to damage caused by tin debris and may allow for high operational temperatures (for example, up to 1000 °C). Tolerance of high operational temperatures may be required to prevent degradation of the nozzle 41, which may result from high temperatures experienced in the vicinity of the plasma formation location. Additionally, or alternatively, the nozzle 41 may be coated with a disposable coating, such as a tin coating. Furthermore, the plasma formation location may be chosen to be a minimum distance from the nozzle 41. For example, the plasma formation location may be a minimum of 10mm from the nozzle 41 such that heat from the plasma generated at the plasma formation location and energy of tin debris has dissipated to some degree at the location of the nozzle 41.

[0066] The fuel jet trap 9 is configured to receive unspent fuel (i.c., fuel that was not converted into a plasma state by the laser beam 6). Unspent fuel arriving at the fuel jet trap 9 is recirculated to the fuel jet generator 8 via a fuel pump 43. The collector 14 is shown as a normal incidence collector, but in other embodiments could be a grazing incidence collector, or any other suitable form of collector.

[0067] The jet 40 is ejected from the nozzle 41 at a speed sufficient to supply new fuel at the plasma formation location for each pulse of the laser beam 6. Generally, jet speeds may typically be within the range of 25 m/s and 100 m/s. For example, for a laser beam pulse repetition rate of 80 kHZ and a diameter of interaction between the laser and the jet 40 of less than 500 pm, the jet 40 may be ejected from the nozzle 41 at a speed of 40 m/s. In order to maintain the continuous nature of the jet 40, the jet may be formed with dimensions that ensure that plasma generated does not bum through the jet 40. fuel jet trap 9

[0068] In some embodiments, the jet 40 may be formed by the nozzle 41 to present a particularly shaped surface to the incident laser beam 6. Figures 5 and 6 schematically depict different shapes that may be utilised in accordance with embodiments of the invention. Referring to Figure 4, a jet 40’ is shown in cross-section through a plane normal to the longitudinal axis of the jet 40’. The jet 40’ is formed by the nozzle 41 so as to present a flattened surface 50 to the laser beam 6. In particular, in the embodiment depicted in Figure 4, the jet 40’ is shown as having a non-square rectangular cross-section (although it will be appreciated that a suitable flattened surface could be provided with other shapes). The flattened surface 50 of the jet 40’ is substantially perpendicular to the optical axis 36 of the radiation collector 14 (not shown) of the radiation source (SO), and also substantially perpendicular to the trajectory (i.e., direction of propagation) of the laser beam 6. As described above with reference to droplet targets flattened by a pre-pulse laser, it has been found that this arrangement ensures that most radiation will be generated along the optical axis 36 and that debris produced might generally propagate in a direction that is in some way parallel to the optical axis 36, aiding control or accommodation of such debris (e.g., by the appropriate location of one or more debris or contamination traps, or some other form of mitigation arrangement - e.g., a buffer gas or the like).

[0069] While a single initiating laser is depicted in the preceding Figures, in alternative embodiments, additional initiating (that is, radiation sufficient to generate a plasma from the target) lasers may be provided. Additional initiating lasers may result in higher conversion efficiency.

[0070] Figure 6 schematically illustrates an embodiment of the present invention that increases an amount of plasma that is directed toward a beam waist of the laser beam 6. Generally, it is desirable to produce plasma that remains within the beam waist of the laser beam 6 as plasma that expands outside the beam waist does not contribute to the emission of EUV radiation (the energy that has been used to generate that plasma is therefore wasted). In more detail, it is desirable to produce a plasma having an electron density of 1019 electrons per cm3 at the beam waist of the laser beam 6, at which electron density the plasma is substantially opaque to the laser beam 6. At an electron density of 1019 electrons per cm3, therefore, a greater portion of electromagnetic radiation of the laser beam 6 is transferred to the plasma. In the embodiment depicted in Figure 6, a jet 40” (again shown in cross-section through a plane normal to its longitudinal axis) is formed by the nozzle 41 so as to present a generally concave surface substantially perpendicular to the trajectory of the laser beam 6. Compared to point targets (such as spherical droplets), or flattened surfaces such as that presented by the jet 40’, it has been found that concave surfaces act to direct more of the plasma ignited by the laser beam 6 into the beam waist of the laser beam 6 by preventing the plasma from expanding in directions bounded by the unspent fuel. By causing more plasma to be directed into the beam waist of the laser beam 6, the concave surface of the jet 40” may increase conversion efficiency. For example, concave surfaces such as that schematically depicted in Figure 6 may increase conversion efficiency by 20% to 30%.

[0071] While forming a continuous jet with a concave surface is advantageous with respect to conversion efficiency as described above, maintaining a suitable concave surface may be difficult, due to, for example, surface tension of the jet 40” and forces generated during plasma generation. In alternative embodiments of the present invention, a plurality of continuous jets of fuel are provided. Figure 6 schematically illustrates an alternative embodiment in which two jets 40a, 40b are directed from a fuel source generator 8’. In particular, the fuel source generator comprises two nozzles 41a, 41b, each configured to direct a respective jet 40a, 40b between the nozzles 41a, 41b and the fuel jet trap 9. While shown as two separate components in Figure 6, the nozzles 41a, 41b may be provided by a single component having, for example, a plurality of openings. The jets 40a, 40b arc parallel and separated by a gap in a plane normal to the optical axis 36. In addition to a separation in a plane normal to the optical axis 36, the jets 40a, 40b may be separated in other planes, or may not be separated. For example, the jets 40a, 40b may be positioned such that respective edges of the jets 40a, 40b touch at one or more points along their length, or along the whole of their lengths. For example, the jets 40a, 40b may graze along their length creating a generally concave surface at the point at which the jets 40a, 40b meet. Further, in alternative embodiments, the jets 40a, 40b may not extend in parallel, but may extend cross-directionally between their respective nozzles 41a, 41b and the fuel jet trap 9.

[0072] The dimensions of the jets and the gap may vary in dependence upon numerous factors such as power of the laser beam 6, capabilities of the fuel source generator 8, etc. In general, however, the jets 40a, 40b may have diameters in the range of 100 pm to 10 mm. It should be noted, however, that increases in the dimensions of the jets 40a, 40b may result in obstructions of EUV radiation. For example, increases in the dimensions of the jets 40a, 40b may prevent radiation reaching the collector 14. The gap between the jets 40a, 40b may be in the range of 100 to 2000 pm. As with the single jet 40 of Figure 3, each of the jets 40a, 40b is continuous (not comprised of a plurality of droplets) between the respective nozzles 41a, 41b and the fuel jet trap 9. As described in more detail below, by providing two (or more) jets, the shapes formed by the gap between the two jets can be exploited to provide similar benefits to those provided by the concave surface of the jet 40.”

[0073] Figure 7 schematically illustrates the incidence of the laser beam 6 on the jets 40a, 40b shown in cross-section through a plane normal to the longitudinal axis of the jets 40a, 40b. The jets 40a, 40b have generally circular cross-section. In the embodiment of Figure 7, the laser beam 6 is directed towards the gap between the jets 40a, 40b. The gap between the jets and the width of the laser beam 6 at the point of incidence with the jets 40a, 40b is such that the laser beam 6 is incident upon a portion of the surfaces of both of the jets 40a, 40b. As such, fuel from the surface of each of the jets 40a, 40b is converted into plasma by the laser beam 6. As with the concave surface of the jet 40” (see Figure 6), the shape of the jets 40a, 40b is such that the mass of fuel not spent in the plasma generation acts to prevent plasma expanding away from the beam waist of the laser beam 6. A greater portion of the generated plasma therefore remains within the beam waist of the laser beam 6 where it can contribute to the generation of EUV radiation. Indeed, it can be considered that the portions of the surfaces of each jet 40a, 40b, which generally face one another (highlighted in thicker line in Figure 7) form part of a generally concave surface. As such, while it will be appreciated that the gap between the jets 40a, 40b is such that they do not form a continuous surface, the term “generally concave surface” is used in the following description to refer to configurations such as that shown in Figure 7.

[0074] In addition to the advantages described above, providing two jets in place of a single jet may be further advantageous with respect to the direction of debris created during plasma formation. For example, for a pulse with a relatively low energy (for example in the region of 0.5 J to 2 J), debris is emitted substantially at an angle of approximately 135° to the left and right of the plasma formation location. As the direction of debris is known, static debris traps may be placed at likely debris formation positions within the housing 2 of the radiation source. It should be noted that, conversely, where a relatively high energy pulse is used, debris emission is largely isotropic. As such, where it is desired to reduce scattering of debris, relatively low energy laser pulses may be used.

[0075] As with the single jet 40, each of the jets 40a, 40b may be shaped to increase the efficiency of the conversion of laser radiation to EUV radiation. Figures 8 and 9 schematically illustrate alternative configurations, again utilizing two continuous jets. In Figures 9 and 10, two jets 40a’, 40b’ are formed with non-square rectangular cross section and angled to present a generally concave surface to the optical axis 36. In the configuration depicted in Figure 8, respective longitudinal faces 90a, 90b of the respective jets 40a’, 40b’ (i.e., faces extending parallel to the longitudinal axis of the cross-section of the respective jets 40a’, 40b’) are angled away from each other at acute angles to the optical axis 36. A generally concave surface is therefore provided by the two faces 90a, 90b, at the point where the laser beam 6 is incident on each of the faces 90a, 90b in the vicinity of the gap between the jets 40a’, 40b’. In the configuration depicted in Figure 9, the longitudinal faces 90a, 90b are angled away from each other at obtuse angles to the optical axis 36 such that a generally concave surface is formed by transverse faces 91a, 91b of the respective jets 40a’, 40b’ (i.e., faces extending perpendicular to the longitudinal axis of the cross-sections of the respective jets 40a’, 40b’). Put another way, in both of Figures 9 and 10, the jets 40a’, 40b’ arc angled such that their respective longitudinal axes form an internal angle of less than 180 degrees and an external angle of more than 180 degrees. In Figure 8, the laser beam 6 is directed towards the internal angle, while in Figure 9, the laser beam 6 is directed towards the external angle.

[0076] By forming jets 40a’, 40b’ with non-square rectangular cross-section as illustrated in Figures 8 and 9, the volume of fuel that the radiation source SO is required to pump may be advantageously reduced. Referring in particular to Figure 9, fuel is generally spent from the surface of the faces 91a, 91b during the creation of plasma. Forming the jets 40a’, 40b’ in non-square rectangular cross section provides a significant depth of unspent fuel in a direction substantially parallel to expansion of created plasma, with a smaller volume of fuel than is required to provide the same depth of unspent fuel using spherical jets 40a, 40b.

[0077] It will be appreciated that jets 40a’, 40b’ will be subject to forces that may result in deformation of the jets 40a’, 40b’. For example, surface tension may cause the jets 40a’, 40b’ to be vortical, forming a helix-like shape, as a result of surface tension. As such, the gap between the jets 40a’, 40b’ may be smaller at some points along their length (between the nozzles 41a, 41b, and the fuel jet trap 9) than at other points. The plasma formation location may therefore be chosen to be a point at which the jets 40a’, 40b’ are a predetermined desired distance from each other, which distance may depend upon a number of factors including the composition of the fuel and the power and dimensions of the laser beam 6. As described above, however, it is generally desirable for the plasma formation location to be a minimum distance from the nozzles 41a, 41b, in order to prevent excessive wear of the nozzles 41a, 41b. For example, while dependent upon the composition of the nozzles 41a, 41b, it is generally preferable for the plasma formation location to be at least 10 mm from the nozzles 41a, 41b. Where natural deformation of the jets 40a’, 40b’ does not allow for plasma formation at a point sufficiently distant the nozzles 41a, 41b, (because, for example, the jets 40a’, 40b’ do not pass sufficiently close or sufficiently distant to one another), dimensions and deformations of the jets 40a’, 40b’ may be purposefully modulated to ensure that the jets 40a’, 40b’ pass within a suitable distance from one another at a suitable distance from the nozzles 41a, 41b. For example, the nozzles 41a, 41b may be configured to spin the jets 40a, 40b, 40a’, 40b’.

[0078] In each of the examples schematically depicted in Figures 8 to 10, the laser beam 6 is depicted as striking each of the jets 40a, 40b (or 40a’, 40b’) symmetrically. In alternative embodiments, the laser beam 6 may be directed to strike only one of the jets 40a, 40b, 40a’, 40b’ (schematically depicted in Figure 10), or such that a greater portion of laser beam 6 strikes one of the jets than strikes the other of the jets. For example, where the laser beam 6 is comprised of a relatively short laser pulse (for example below 10 ns), the plasma that is generated upon the laser beam 6 striking the jets may not close the gap between the jets. As such, if the laser beam 6 were to be directed perpendicular to the gap between the jets 40a, 40b (as depicted in Figure 7), a portion of the laser radiation would pass through the gap between the jets 40a, 40b without contributing to plasma generation or EUV radiation. In Figure 10, the laser beam 6 is directed towards only the jet 40b. In this configuration, the presence of the jet 40a still acts to prevent plasma expanding outside the beam waist of the laser beam 6, while the direction of the laser beam 6 is such that less laser radiation is wasted as a result of the plasma not fully closing the gap between the jets 40a, 40b.

[0079] Referring to Figure 12, the laser beam 6 may be focussed such that the beam waist 6a of the laser beam 6 is beyond the jets 40a, 40b (i.e., such that the jets 40a, 40b are between the beam waist 6a and the collector 14). The collector 14 is also depicted in Figure 12 for reference. It has been found that by focussing the laser beam 6 such that the beam waist 6a is beyond the jets 40a, 40b, more of the plasma stays within the laser beam 6, and can therefore contribute to EUV radiation generation. This is because EUV emitting plasma generally travels towards the collector 14. If the beam waist 6a is beyond the jets 40a, 40b, the laser beam 6 is wider before the jets 40a, 40b such that more of the EUV emitting plasma will be within the laser beam 6. In other embodiments, the laser beam 6 may be focussed such that the beam waist 6a is directly between the jets 40a, 40b or in front of the jets 40a, 40b (i.e., between the collector 14 and the jets 40a, 40b).

[0080] In some embodiments, the laser beam 6 has a linear polarization that is substantially perpendicular to the longitudinal axis of the gap between the jets at the point at which the laser beam 6 is incident on the jets. For example, in the example that is schematically depicted in Figure 6, in which the jets 40a, 40b are parallel, the linear polarization of the laser beam 6 may be perpendicular to the longitudinal axis of the jets. As a consequence of this polarization, the laser radiation may cause electrons in the generated plasma to move with a generally circular motion. The electrons may collide with one another and with tin ions, leading to Brcmsstrahlung emission of radiation. This may increase the efficiency of the generation of radiation by the plasma (for example by up to around 30%).

[0081] In some embodiments the polarization (i.e., E-field orientation) of the laser beam may be non-perpendicular to the longitudinal axis of one or both of the jets at the plasma formation location. Where this is the case, the polarization may include a component that is perpendicular to the longitudinal axis of one or both of the jets at the plasma formation location. The size of this component may determine the efficiency of laser radiation coupling into plasma through plasma resonance at critical electron density surface.

[0082] In the above described embodiments comprising two jets, the jets are oriented in parallel. In alternative embodiments, however, the jets may be cross-directional. Further, while the foregoing has described particular configurations of two continuous jets, a greater number of jets may be used. Additionally, while the foregoing has assumed that all jets comprise fuel to be converted into plasma, in alternative embodiments only one of the jets may comprise fuel, while the others of the jets constrain expansion of plasma. Where only a subset of the jets comprises fuel, it may be necessary to provide separate re-circulation means for fuel containing and non-fuel containing jets.

[0083] It will also be appreciated that while the Figures described above have depicted continuous jets having generally circular, rectangular and concave cross-sections, jets may be formed into other shapes (for example, triangular, elliptical, or convex). Additionally, where two or more jets are provided, two or more of the jets may be formed into differing shapes. The choice of shapes with which to form the jets may be application dependent. For example, while some shapes may be more easily formed and maintained during plasma generation, other shapes may provide better conversion efficiency.

[0084] It was described above, with reference to the use of droplet targets, that an initial (e.g., lower energy) pre-pulse of laser radiation may be directed onto a droplet target in order to modify the shape and/or temperature of the target. Similarly, in some embodiments of the present invention, a pre pulse of laser radiation may be directed onto one or more of the continuous jets. For example, an yttrium aluminium garnet (YAG) or a CO2 laser may provide the pre pulse.

[0085] In comparison with radiation sources comprising fuel stream generators configured to provide droplet targets, use of jet targets requires the pumping of a significantly greater volume of liquid fuel over a given period. As such, it may be beneficial to the operation and longevity of components such as the fuel jet generator 8, the fuel jet trap 9 and the fuel pump 43 if operating temperatures of these components can be reduced. The composition of the fuel used to form the jets may therefore be selected for lower temperature operation. For example, the fuel may be an eutectic alloy. For example, a fuel comprising an alloy of tin and gallium, or an alloy of tin and indium may allow room-temperature operation.

[0086] 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, such as 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, LEDs, solar cells, etc. The skilled artisan will appreciate that, in the context of such alternative applications, any use of the terms “wafer” or “die” herein may be considered as synonymous with the more general terms “substrate” or “target portion,” respectively. The substrate referred to herein may be processed, before or after exposure, in for example a track (a tool that typically applies a layer of resist to a substrate and develops the exposed resist), a metrology tool and/or an inspection tool. Where applicable, the disclosure herein may be applied to such and other substrate processing tools. Further, the substrate may be processed more than once, for example in order to create a multi-layer IC, so that the term substrate used herein may also refer to a substrate that already contains multiple processed layers.

[0087] When describing the lithographic apparatus, the term “lens,” where the context allow, may refer to any one or combination of various types of optical components, including refractive, reflective, magnetic, electromagnetic and electrostatic optical components.

[0088] 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 clauses that follow. Other aspects of the invention are set out as in the following numbered clauses: 1. A method of generating radiation, the method comprising: directing a first jet from a first location to a second location, the first jet being continuous between the first and second locations; directing a second jet from a third location to a fourth location, the second jet being continuous between the third and fourth locations; wherein at least one of the first and second jets comprises a fuel for use in generating a radiation generating plasma and the at least one of the jets at least partially intersects a plasma formation location; directing initiating radiation at the plasma formation location to generate a radiation generating plasma, wherein expansion of the plasma is at least partially constrained by each of the first and second jets.

2. The method of clause 1, wherein first and second jets are directed such that the first and second jets together form part of a generally concave surface at the plasma formation location.

3. The method of clause 1 or 2, wherein the first and second jets at least partially intersect the plasma formation location such that radiation is incident on at least part of each of the first and second jets.

4. The method of clause 1, 2 or 3, wherein the first and second jets are directed such that they form a gap of a predetermined width at the plasma formation location.

5. The method of clause 4, wherein the gap is in a range of 1ÜÜ to 2ÜÜÜ pm.

6. The method of clause 4 or 5, wherein the initiating radiation is directed such that a focal point of the initiating radiation is wider than the gap between the first and second jets.

7. The method of one of clauses 4 to 6, wherein the initiating radiation is directed substantially perpendicular to a longitudinal axis of the gap between the first and second jets.

8. The method of any preceding clause, wherein the initiating radiation is directed such that a focal point of the initiating radiation is further from a source of the initiating radiation than the plasma formation location.

9. The method of any preceding clause, wherein the at least one of the first and second jets is formed so as to comprise a substantially flattened surface, and the initiating radiation is directed such that the initiating radiation is incident on the substantially flattened surface.

10. The method of any preceding clause, wherein the first and second jets are formed with a generally non-square rectangular cross-section; each of the first and second jets is angled such that respective longitudinal axes of the first and second jets form an internal angle of less than 180 degrees and an external angle of more than 180 degrees; and wherein the initiation radiation is directed towards the internal angle; or wherein the initiation radiation is directed towards the external angle.

11. The method of any preceding clause, wherein the initiating radiation is linearly polarized in a direction substantially perpendicular to a longitudinal axis of a gap between the first and second jets at the plasma formation location.

12. A method according to any preceding clause, wherein the initiating radiation is a radiation pulse, the pulse having an energy in the range of 0.1 and 5 Joules.

13. A method according to clause 12, wherein the pulse has an energy of 0.5 Joules or less.

14. A method according to any preceding clause, wherein the first and second jets arc directed at a speed in a range of 2.5 m/s to 200 m/s.

15. A method according to any preceding clause, wherein the first and second jets have a diameter or an equivalent diameter in a range of 100 pm to 10 mm.

16. A method of generating radiation, the method comprising: directing a jet from a first location to a second location, the jet being continuous between the first and second locations, the jet comprising a fuel for use in generating a radiation generating plasma, the jet at least partially intersecting a plasma formation location and the jet being formed to comprise a substantially concave surface at the plasma formation location; and directing initiating radiation so that the initiating radiation is incident on the substantially concave surface at the plasma formation location to generate a radiation generating plasma.

17. A lithographic method, comprising: generating radiation according to the method of any preceding clauses; and using the generated radiation to apply a pattern to a substrate.

18. A radiation source comprising: a first nozzle for directing a first jet from a first location to a second location, the first jet being continuous between the first and second locations; a second nozzle for directing a second jet from a third location to a fourth location, the second jet being continuous between the third and fourth locations; wherein at least one of the first and second jets comprises a fuel for use in generating a radiation generating plasma and, in use, the at least one of the jets at least partially intersects a plasma formation location; a secondary radiation source configured to direct initiating radiation at the plasma formation location to generate, in use, a radiation generating plasma, wherein, in use, expansion of the plasma is at least partially constrained by each of the first and second jets.

19. A radiation source comprising: a first nozzle for directing a jet from a first location to a second location, the first jet being continuous between the first and second locations, the jet comprising a fuel for use in generating a radiation generating plasma, the jet at least partially intersecting a plasma formation location and the jet being formed to comprise a substantially concave surface at the plasma formation location; a secondary radiation source configured to direct initiating radiation so that the initiating radiation is incident on the concave surface at the plasma formation location to generate, in use, a radiation generating plasma.

20. The radiation source of clause 18 or 19, further comprising a radiation collector for collecting radiation generated by the radiation generating plasma at the plasma formation location, and for directing at least a portion of the generated radiation to a focal point.

21. A lithographic apparatus comprising, or in connection with, the radiation source of any of clauses 18 to 20.

22. An apparatus comprising, or in connection with, the radiation source of any of clauses 18 to 20.

23. A method of generating radiation, the method comprising: directing a first jet from a first location to a second location, the first jet being continuous between the first and second locations; directing a second jet from a third location to a fourth location, the second jet being continuous between the third and fourth locations; wherein at least one of the first and second jets comprises a fuel for use in generating a radiation generating plasma and the at least one of the jets at least partially intersects a plasma formation location; and directing initiating radiation at the plasma formation location to generate a radiation generating plasma, wherein expansion of the plasma is at least partially constrained by each of the first and second jets.

24. The method of clause 23, wherein first and second jets are directed such that the first and second jets together form part of a generally concave surface at the plasma formation location.

25. The method of clause 23, wherein the first and second jets at least partially intersect the plasma formation location such that radiation is incident on at least part of each of the first and second jets.

26. The method of clause 23, wherein the first and second jets are directed such that they form a gap of a predetermined width at the plasma formation location.

27. The method of clause 26, wherein the gap is in a range of 100 to 2000 pm.

28. The method of clause 26, wherein the initiating radiation is directed such that a focal point of the initiating radiation is wider than the gap between the first and second jets.

29. The method of clause 26, wherein the initiating radiation is directed substantially perpendicular to a longitudinal axis of the gap between the first and second jets.

30. The method of clause 23, wherein the initiating radiation is directed such that a focal point of the initiating radiation is further from a source of the initiating radiation than the plasma formation location.

31. The method of clause 23, wherein the at least one of the first and second jets is formed so as to comprise a substantially flattened surface, and the initiating radiation is directed such that the initiating radiation is incident on the substantially flattened surface.

32. The method of clause 23, wherein the first and second jets are formed with a generally non-square rectangular cross-section; each of the first and second jets is angled such that respective longitudinal axes of the first and second jets form an internal angle of less than 180 degrees and an external angle of more than 180 degrees; and wherein the initiation radiation is directed towards the internal angle; or wherein the initiation radiation is directed towards the external angle.

33. The method of clause 23, wherein the initiating radiation is linearly polarized in a direction substantially perpendicular to a longitudinal axis of a gap between the first and second jets at the plasma formation location.

34. A method of clause 23, wherein the initiating radiation is a radiation pulse, the pulse having an energy in the range of 0.1 and 5 Joules.

35. A method of clause 34, wherein the pulse has an energy of 0.5 Joules or less.

36. A method of clause 23, wherein the first and second jets are directed at a speed in a range of 2.5 m/s to 200 m/s.

37. A method of clause 23, wherein the first and second jets have a diameter or an equivalent diameter in a range of 100 pm to 10 mm.

38. A method of generating radiation, the method comprising: directing a jet from a first location to a second location, the jet being continuous between the first and second locations, the jet comprising a fuel for use in generating a radiation generating plasma, the jet at least partially intersecting a plasma formation location and the jet being formed to comprise a substantially concave surface at the plasma formation location; and directing initiating radiation so that the initiating radiation is incident on the substantially concave surface at the plasma formation location to generate a radiation generating plasma.

39. A lithographic method, comprising: generating radiation by: directing a first jet from a first location to a second location, the first jet being continuous between the first and second locations, directing a second jet from a third location to a fourth location, the second jet being continuous between the third and fourth locations, wherein at least one of the first and second jets comprises a fuel for use in generating a radiation generating plasma and the at least one of the jets at least partially intersects a plasma formation location, and directing initiating radiation at the plasma formation location to generate a radiation generating plasma, wherein expansion of the plasma is at least partially constrained by each of the first and second jets; and using the generated radiation to apply a pattern to a substrate.

40. A radiation source, comprising: a first nozzle configured to direct a first jet from a first location to a second location, the first jet being continuous between the first and second locations; a second nozzle configured to direct a second jet from a third location to a fourth location, the second jet being continuous between the third and fourth locations; wherein at least one of the first and second jets comprises a fuel for use in generating a radiation generating plasma and, in use, the at least one of the jets at least partially intersects a plasma formation location; and a secondary radiation source configured to direct initiating radiation at the plasma formation location to generate a radiation generating plasma, wherein expansion of the plasma is at least partially constrained by each of the first and second jets.

41. A radiation source, comprising: a first nozzle configured to direct a jet from a first location to a second location, the first jet being continuous between the first and second locations, the jet comprising a fuel for use in generating a radiation generating plasma, the jet at least partially intersecting a plasma formation location and the jet being formed to comprise a substantially concave surface at the plasma formation location; and a secondary radiation source configured to direct initiating radiation so that the initiating radiation is incident on the concave surface at the plasma formation location to generate, in use, a radiation generating plasma.

42. The radiation source of clause 41, further comprising a radiation collector configured to collect radiation generated by the radiation generating plasma at the plasma formation location, and to direct at least a portion of the generated radiation to a focal point.

43. A lithographic apparatus, comprising: a radiation source having a first nozzle configured to direct a first jet from a first location to a second location, the first jet being continuous between the first and second locations, a second nozzle configured to direct a second jet from a third location to a fourth location, the second jet being continuous between the third and fourth locations, wherein at least one of the first and second jets comprises a fuel for use in generating a radiation generating plasma and, in use, the at least one of the jets at least partially intersects a plasma formation location, and a secondary radiation source configured to direct initiating radiation at the plasma formation location to generate a radiation generating plasma, wherein expansion of the plasma is at least partially constrained by each of the first and second jets; an illumination system configured to provide the radiation; a patterning device configured to impart the radiation with a pattern in its cross-section; a substrate holder configured to hold a substrate; a projection system configured to project the patterned radiation onto a target portion of the substrate.

Claims (1)

A lithography device comprising: an exposure device adapted to provide a radiation beam; a carrier constructed to support a patterning device, the patterning device being capable of applying a pattern in a section of the radiation beam to form a patterned radiation beam; a substrate table constructed to support a substrate; and a projection device adapted to project the patterned radiation beam onto a target area of the substrate, characterized in that the substrate table is adapted to position the target area of the substrate in a focal plane of the projection device.