NL2011327A - Source collector apparatus, lithographic apparatus and method. - Google Patents

Description

SOURCE COLLECTOR APPARATUS. LITHOGRAPHIC APPARATUS AND METHOD HELD

[0001] The present invention relates to a source collector apparatus in particular for use in a lithographic apparatus, and to a method for protecting a fuel droplet stream from disturbance.

BACKGROUND

[0002] A lithographic apparatus is a machine that applies a desired pattern onto a target portion of a substrate. Lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In that circumstance, a patterning device, which is alternatively referred to as a mask or a reticle, may be used to generate a circuit pattern corresponding to an individual layer of the IC, and this pattern can be imaged onto a target portion (e.g., comprising part of, one or several dies) on a substrate (e.g., a silicon wafer) that has a layer of radiation-sensitive material (resist). In general, a single substrate will contain a network of adjacent target portions that are successively exposed. Known lithographic apparatus include so-called steppers, in which each target portion is irradiated by exposing an entire pattern onto the target portion in one go, and so-called scanners, in which each target portion is irradiated by scanning the pattern through the beam in a given direction (the “scanning”-direction) while synchronously scanning the substrate parallel or anti parallel to this direction. It is also possible to transfer the pattern from the patterning device to the substrate by imprinting the pattern onto the substrate.

[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):

(1) where λ is the wavelength of the radiation used, NA is the numerical aperture of the projection system used to print the pattern, k\ 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 k\.

[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, for example within the range of 5-10 nm such as 6.7 nm or 6.8 nm. 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 source collector module for containing the plasma. The plasma may be created, for example, by directing a laser beam at a fuel, such as droplets of a suitable material (e.g., tin), 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, which receives the radiation and focuses the radiation into a beam. The source collector module 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.

[0007] Another known method of producing EUV radiation is known as dual laser pulsing (DLP). In the DLP method droplets are pre-heated by a Nd:YAG laser to cause the droplet (e.g., a tin droplet) to decompose into vapour and small particles that are then heated to a very high temperature by a CO2 laser.

[0008] One known drawback with using fuels such as droplets of molten tin is that over time components within the radiation source may become contaminated by tin deposition. This problem applies in particular to the collector and if tin accumulates on the surface of the collector the performance of the radiation source will be degraded. One known technique to minimize this problem is to provide a supply of hydrogen gas that is directed so as to prevent tin from reaching the collector surface. A disadvantage with this, however, is that this gas flow may disturb the tin droplets that are issued from the droplet generator and this may interfere with the droplets reaching the plasma formation location point as intended. To solve this problem it is known to provide a shroud around the droplet stream so that the flow of hydrogen gas does not disturb the stream, but the shroud can still result in contamination of the collector with tin droplets and tin ions being scattered from the shroud to the collector. Furthermore tin can accumulate on the shroud itself and then contaminate the collector (and other components) by dripping and/or sputtering from the shroud. The shroud may also block some of the generated EUV radiation and this may reduce the performance of the radiation source.

SUMMARY

[0009] According to a first aspect of the invention there is provided a source collector apparatus for use in a lithographic apparatus comprising, a fuel droplet generator configured in use to generate a stream of fuel droplets directed from an outlet of the fuel droplet generator towards a plasma formation location, and a gas supply configured in use to provide a flow of gas extending adjacent to the stream of fuel droplets over at least a major portion of the stream of fuel droplets from the outlet to the plasma formation location.

[0010] Preferably the flow of gas extends generally parallel to the stream of fuel droplets. The flow of gas in cross-section may extend circumferentially about the fuel droplet stream, and preferably the flow of gas may be cylindrical or part-cylindrical. The flow of gas may be an annular or part annular gas jet. The flow of gas may be positioned between the fuel droplet stream and a collector. The flow of gas may be formed of a plurality of individual gas streams.

[0011] The gas supply may be provided as part of the fuel droplet generator or may be provided surrounding at least a part of the fuel droplet generator. The gas may be hydrogen.

[0012] According to another aspect of the invention there is provided a lithographic apparatus comprising the source collector apparatus as defined aforesaid.

[0013] According to another aspect of the invention there is provided a method of shielding a stream of fuel droplets between an outlet of a fuel droplet generator and a plasma formation location from disturbance in a source collector apparatus, the method comprising providing a flow of gas adjacent to the stream of fuel droplets over at least a major portion of the stream of fuel droplets.

[0014] 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

[0015] 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:

[0016] Figure 1 schematically depicts a lithographic apparatus according to an embodiment of the invention;

[0017] Figure 2 is a more detailed schematic view of the lithographic apparatus;

[0018] Figure 3 is a schematic view of the prior art;

[0019] Figure 4 is a schematic view of an embodiment of the invention;

[0020] Figure 5 illustrates operation of an embodiment of the invention; and

[0021] Figure 6 shows schematically a hydrogen jet as formed in an embodiment of the invention.

[0022] The features and advantages of the present invention will become more apparent from the detailed description set forth below when taken in conjunction with the drawings, in which like reference characters identify corresponding elements throughout. In the drawings, like reference numbers generally indicate identical, functionally similar, and/or structurally similar elements. The drawing in which an element first appears is indicated by the leftmost digit(s) in the corresponding reference number.

DETAILED DESCRIPTION

[0023] This specification discloses one or more embodiments that incorporate the features of this invention. The disclosed embodiment(s) merely exemplify the present invention. The scope of the present invention is not limited to the disclosed embodiment(s). The present invention is defined by the clauses appended hereto.

[0024] The embodiment(s) described, and references in the specification to “one embodiment,” “an embodiment,” “an example embodiment,” etc., indicate that the embodiment(s) described may include a particular feature, structure, or characteristic, but every embodiment may not necessarily include the particular feature, structure, or characteristic. Moreover, such phrases are not necessarily referring to the same embodiment. Further, when a particular feature, structure, or characteristic is described in connection with an embodiment, it is understood that it is within the knowledge of one skilled in the art to effect such feature, structure, or characteristic in connection with other embodiments whether or not explicitly described.

[0025] Figure 1 schematically depicts a lithographic apparatus 100 including a source collector apparatus 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 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.

[0026] 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.

[0027] 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.

[0028] 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.

[0029] 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.

[0030] 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.

[0031] As here depicted, the apparatus is of a reflective type (e.g., employing a reflective mask).

[0032] 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.

[0033] Referring to Figure 1, the illuminator IL receives an extreme ultra violet radiation beam from the source collector apparatus SO. Methods to produce EUV radiation 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 element, with a laser beam. The source collector apparatus 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 collector apparatus. The laser and the source collector apparatus may be separate entities, for example when a C02 laser is used to provide the laser beam for fuel excitation.

[0034] In such cases, the laser is not considered to form part of the lithographic apparatus and the laser beam is passed from the laser to the source collector apparatus with the aid of a beam delivery system comprising, for example, suitable directing mirrors and/or a beam expander.

[0035] 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.

[0036] 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.

[0037] The depicted apparatus could be used in at least one of the following modes:

[0038] 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.

[0039] 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.

[0040] 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 programmable patterning device, such as a programmable mirror array of a type as referred to above.

[0041] Combinations and/or variations on the above described modes of use or entirely different modes of use may also be employed.

[0042] Figure 2 shows the apparatus 100 in more detail, including the source collector module SO, the illumination system IL, and the projection system PS. The source collector module SO is constructed and arranged such that a vacuum environment can be maintained in an enclosing stmcture 220 of the source collector module SO. An EUV radiation emitting plasma 210 may be formed by a laser produced plasma (LPP) source. EUV radiation may be produced by a gas or vapor, for example Xe gas, Li vapor or Sn vapor in which the very hot plasma 210 is created to emit radiation in the EUV range of the electromagnetic spectrum. As will be discussed in more detail below in the case of a laser produced plasma (LPP) source the very hot plasma 210 is created by configuring laser LA to emit a beam of laser radiation 205 that is focused on target area 211 to which is supplied a first fuel, eg a droplet of tin (Sn), from a first fuel supply. The laser generates a plasma of Sn vapour, which emits EUV radiation as is known in the art.

[0043] The source module SO further includes a radiation collector CO that collects the generated EUV radiation and focuses the EUV radiation at a virtual source point IF. The virtual source point IF is commonly referred to as the intermediate focus, and the source collector module is arranged such that the intermediate focus IF is located at or near an opening 221 in the enclosing structure 220. The virtual source point IF is an image of the radiation emitting plasma 210.

[0(344] Subsequently the radiation traverses the illumination system IL, which may include a facetted field mirror device 22 and a facetted pupil mirror device 24 arranged to provide a desired angular distribution of the radiation beam 21, 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 21 at the patterning device MA, held by the support structure MT, a patterned beam 26 is formed and the patterned beam 26 is imaged by the projection system PS via reflective elements 28, 30 onto a substrate W held by the wafer stage or substrate table WT.

[0045] Referring now to Figure 3 there is shown in more detail a part of the radiation source SO. Shown in Figure 3 is a collector CO that has a central opening 1 through which the laser beam 2 passes to strike the tin droplets at a plasma formation location 6. A supply of hydrogen is also provided through the opening 1 generally along the same axis as the laser beam. Hydrogen may also be supplied around the perimeter of the collector CO, and/or optionally through supply ports. The flow of hydrogen is indicated schematically by the arrows in Figure 3. The hydrogen serves a number of purposes including maximizing suppression of contamination of the collector CO (and also metrology modules not shown), acting as a source of hydrogen radicals for decontamination, and conditioning the plasma to keep hot ionized gas away from the collector CO and metrology modules.

[0046] A disadvantage of the provision of hydrogen into the source is that the flow of hydrogen has the potential to disrupt the stream of fuel droplets. This may, for example, cause a fuel droplet to deviate from its intended trajectory and since the timing of laser pulses generated by laser LA with the arrival of fuel droplets at the plasma formation location 6 must be very accurately synchronised, any deviations or perturbations in the stream of fuel droplets can be highly detrimental to system performance. For that reason, in the prior art a fuel delivery system 3 includes a fuel droplet generator 4 that issues a stream of fuel droplets towards the plasma formation location 6, which stream of droplets is protected by a shroud 5. While the shroud 5 serves well the intended purpose of protecting the droplet stream from undue interference from the hydrogen flows within the source collector module SO it comes with its own drawbacks as discussed in the introduction to this specification.

[0047] Figure 4 illustrates an embodiment of the invention. In this embodiment of the invention the shroud is replaced by a stream of hydrogen gas 7 that is provided adjacent to the fuel droplet stream 8. In particular the stream 7 of hydrogen gas is generally concentric to the fuel droplet stream 8. The stream of hydrogen gas 7 is directed such that it flows generally parallel to and in the same direction as the fuel droplet stream 8. While the hydrogen gas stream 7 may flow parallel to the fuel droplet stream 8, in practice the hydrogen stream may diverge away from the fuel droplet stream in the direction of the plasma formation location. The stream of hydrogen gas 7 may also be considered to be an annular or part-annular hydrogen gas jet. With a sufficiently large flow of hydrogen (e.g., 10 slm) the hydrogen stream 7 is sufficient to prevent the other hydrogen gas flows in the source collector module SO from disturbing the fuel droplet stream 7. In effect the hydrogen stream 7 forms a generally cylindrical tube surrounding the fuel droplet stream. The hydrogen stream 7 will extend for at least a major portion of the fuel droplet stream, ie it will extend from at least close to the outlet of the fuel droplet generator to at least as far as a conventional shroud. However, as a conventional shroud must stop at a certain distance from the plasma formation location, for a number of reasons including avoiding directly reflecting tin particles, one advantage of embodiments of the invention is that the hydrogen stream can extend closer to the plasma formation location than would a conventional shroud thus providing a greater degree of protection.

[0048] The hydrogen for the hydrogen gas stream 7 may be provided by one or more outlets that may be formed as part of the fuel droplet generator 4 but surrounding the fuel droplet outlet, or may be provided from a hydrogen supply formed with one or more outlets surrounding the fuel droplet generator 4. The number and form of the hydrogen outlets will depend on the cross-section shape required for the hydrogen gas stream. For example, an annular outlet may be provided if a continuous “cylinder” of hydrogen is required that completely surrounds the fuel droplet stream. Alternatively it may not be necessary to provide the hydrogen gas stream completely around the fuel droplet stream and in such cases an arcuate part-circular hydrogen gas outlet may be used to generate a part-cylindrical “shield” of hydrogen gas flow located where necessary, ie between the fuel droplet stream 7 and the collector CO. This may be less preferred however as an asymmetric gas flow may generate asymmetric pressures that might disturb the droplet stream. Another possibility is that two arcuate openings may be provided respectively on either side of the fuel droplet outlet providing part-cylindrical streams. It should be noted however that while a cylindrical or part-cylindrical stream or streams may be desirable, in practice the hydrogen gas flow may diverge away from the fuel droplet stream and the terms cylindrical and part-cylindrical should be understood accordingly.

[0049] Another possibility is that rather than providing a single hydrogen gas outlet, a plurality of individual small outlets may be provided close to each other either in a circle or a part-circle such that the hydrogen gas stream is in fact formed of multiple smaller streams.

[0050] A gas other than hydrogen may be used, though hydrogen is particularly preferred as it is generally transparent to EUV radiation and therefore does not cause significant loss in the EUV output.

[0051] Figure 5 schematically shows the operation of an embodiment of the invention obtained by modelling of gas flows within the source collector apparatus. In Figure 5 the fuel droplet stream 8 is shown horizontally with the plasma formation location at the left-hand end. For clarity the gas flow 7 is not shown completely but is shown broken away. It will be understood that gas flow 7 extends circumferentially fully around the fuel droplet stream and extends from adjacent the droplet outlet to adjacent the plasma formation location. The gas flow 7 is thus from right to left in the figure. Gas flow 7 is generally parallel to the fuel droplet stream 8 but in practice may diverge slightly therefrom. Reference numeral 9 indicates flows of hydrogen gas coming from the collector or near to the collector and figure 5 shows how these gas flows 9 begin being substantially perpendicular to the fuel droplet stream 8 but by the action of the gas flow 7 they are directed away from the fuel droplet stream 8 and indeed take up a generally parallel path to the fuel droplet stream. The gas flow 7 thus prevents the gas flows 9 from the collector from interfering with or otherwise disturbing the fuel droplet stream 8.

[0052] Figure 6 shows by way of example a flow of hydrogen gas 7 that may be created and used in an embodiment of this invention. In this example for clarity a generally half-cylindrical gas jet 7 is shown that flows from right to left in the figure. It will be seen that the flow diverges slightly as shown by downstream flow portion 7’ but the flow remains adjacent to and generally parallel to a fuel droplet stream that would extend with the gas flow 7.

[0053] It is possible that under the influence of the hydrogen flows from the collector, particularly near to the plasma location formation, the direction of the hydrogen flow 7 may be disturbed. It is possible that this might be compensated for by suitably adjusting the direction of the hydrogen gas flow 7 in advance such that the effect of the other hydrogen flows is to make the gas flow 7 generally parallel to the fuel droplet stream.

[0054] In addition to avoiding the need for a conventional shroud embodiments of the invention have other advantages over the prior art. For example, the hydrogen stream may provide an improved droplet stability and an increased droplet speed resulting in increased droplet spacing. The increased droplet speed will also reduce droplet trajectory deviation during start-up such that devices provided for collecting misdirected tin droplets can be reduced in size and/or number.

[0055] 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, 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.

[0056] Although specific reference may have been made above to the use of embodiments of the invention in the context of optical lithography, it will be appreciated that the invention may be used in other applications, for example imprint lithography, and where the context allows, is not limited to optical lithography. In imprint lithography a topography in a patterning device defines the pattern created on a substrate. The topography of the patterning device may be pressed into a layer of resist supplied to the substrate whereupon the resist is cured by applying electromagnetic radiation, heat, pressure or a combination thereof. The patterning device is moved out of the resist leaving a pattern in it after the resist is cured.

[0057] Although specific reference may be made in this text to the use an electrostatic clamp in lithographic apparatus, it should be understood that the electrostatic clamp described herein may have other applications, such as for use in mask inspection apparatus, wafer inspection apparatus, aerial image metrology apparatus and more generally in any apparatus that measure or process an object such as a wafer (or other substrate) or mask (or other patterning device) either in vacuum or in ambient (nonvacuum) conditions, such as, for example in plasma etching apparatus or deposition apparatus.

[0058] The terms “radiation” and “beam” as used herein encompass all types of electromagnetic radiation, including ultraviolet (UV) radiation (e.g., having a wavelength of or about 365, 355, 248, 193, 157 or 126 nm) and extreme ultraviolet (EUV) radiation (e.g., having a wavelength in the range of 5-20 nm), as well as beams of charged particles, such as ion beams or electron beams.

[0059] 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.

[0060] 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 description is not intended to limit the invention. Other aspects of the invention are set out as in the following numbered clauses: 1. A source collector apparatus for use in a lithographic apparatus comprising, a fuel droplet generator configured in use to generate a stream of fuel droplets directed from an outlet of said fuel droplet generator towards a plasma formation location, and a gas supply configured in use to provide a flow of gas extending adjacent to said stream of fuel droplets over at least a major portion of the said stream of fuel droplets from said outlet to said plasma formation location.

2. A source collector apparatus as claimed in clause 1 wherein said flow of gas extends generally parallel to said stream of fuel droplets.

3. A source collector apparatus as claimed in clause 1 or 2 wherein said flow of gas extends circumferentially about said fuel droplet stream.

4. A source collector apparatus as claimed in clause 3 wherein said flow of gas has a cylindrical form.

5. A source collector apparatus as claimed in clause 3 comprising a collector and wherein said flow of gas is provided between said fuel droplet stream and said collector.

6. A source collector apparatus as claimed in any preceding clause wherein said flow of gas is formed of a plurality of individual gas streams.

7. A source collector apparatus as claimed in any preceding clause wherein said gas supply is provided as part of the fuel droplet generator.

8. A source collector apparatus as claimed in any of clauses 1 to 6 wherein the gas supply is provided surrounding at least a part of the fuel droplet generator.

9. A source collector apparatus as claimed in any preceding clause wherein the gas is hydrogen.

10. A lithographic apparatus comprising the source collector apparatus of any preceding clauses, and further comprising; an illumination system configured to condition a radiation beam; a support constructed to support a patterning device, the patterning device being capable of imparting the radiation beam with a pattern in its cross-section to form a patterned radiation beam; a substrate table constructed to hold a substrate; and a projection system configured to project the patterned radiation beam onto a target portion of the substrate.

11. A method of shielding a stream of fuel droplets between an outlet of a fuel droplet generator and a plasma formation location from disturbance in a source collector apparatus, the method comprising providing a flow of gas adjacent to said stream of fuel droplets over at least a major portion of the said stream of fuel droplets.

12. A method as claimed in clause 11 wherein said flow of gas extends circumferentially about said fuel droplet stream.

13. A method as claimed in clause 12 wherein said How of gas has a cylindrical form.

14. A method as claimed in clause 11 wherein said flow of gas is provided between said fuel droplet stream and a collector.

15. A method as claimed in wherein said flow of gas is formed of a plurality of individual gas streams.

16. The source collector apparatus for use in a lithographic apparatus, comprising: a fuel droplet generator configured to generate a stream of fuel droplets directed from an outlet of said fuel droplet generator towards a plasma formation location; and a gas supply configured to provide a flow of gas extending adjacent to said stream of fuel droplets over at least a major portion of the said stream of fuel droplets from said outlet to said plasma formation location.

17. The source collector apparatus of clause 16, wherein said flow of gas extends generally parallel to said stream of fuel droplets.

18. The source collector apparatus of clause 16, wherein said flow of gas extends circumferentially about said fuel droplet stream.

19. The source collector apparatus of clause 18, wherein said flow of gas has a cylindrical form.

20. The source collector apparatus of clause 18, further comprising a collector, and wherein said flow of gas is provided between said fuel droplet stream and said collector.

21. The source collector apparatus of clause 16, wherein said flow of gas is formed of a plurality of individual gas streams.

22. A source collector apparatus as claimed in any preceding clause wherein said gas supply is provided as part of the fuel droplet generator.

23. The source collector apparatus of clause 16, wherein the gas supply is provided surrounding at least a part of the fuel droplet generator.

24. The source collector apparatus of clause 16, wherein the gas is hydrogen.

25. A lithographic apparatus, comprising: a source collector comprising a fuel droplet generator configured to generate a stream of fuel droplets directed from an outlet of said fuel droplet generator towards a plasma formation location, and a gas supply configured to provide a flow of gas extending adjacent to said stream of fuel droplets over at least a major portion of the said stream of fuel droplets from said outlet to said plasma formation location; an illumination system configured to condition a radiation beam formed from said plasma; a support constructed to support a patterning device, the patterning device being capable of imparting the radiation beam with a pattern in its cross-section to form a patterned radiation beam; a substrate table constructed to hold a substrate; and a projection system configured to project the patterned radiation beam onto a target portion of the substrate.

26. A method of shielding a stream of fuel droplets between an outlet of a fuel droplet generator and a plasma formation location from disturbance in a source collector apparatus, the method comprising: outputting a stream of fuel droplets and outputting a flow of gas adjacent to said stream of fuel droplets over at least a major portion of the said stream of fuel droplets.

27. The method of clause 26, wherein said flow of gas extends circumferentially about said fuel droplet stream.

28. The method of clause 27, wherein said flow of gas has a cylindrical form.

29. The method of clause 26, wherein said flow of gas is provided between said fuel droplet stream and a collector.

30. The method of clause 26, wherein said flow of gas is formed of a plurality of individual gas streams.

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.