NL2010236A - Lithographic apparatus and method. - Google Patents

Description

LITHOGRAPHIC APPARATUS AND METHOD

HELD

[0001] The present invention relates to a lithographic apparatus and a method, and in particular to an apparatus and method for decontaminating a surface on which debris accumulates.

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-f 4 nm, for example within the range of 5-f0 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 issue with such methods of generating EUV radiation is that the source collector module can become contaminated with fuel droplets. When tin is used for the generation for EUV tin droplets are known to be deposited on surfaces within the source collector module including the surface of the collector CO itself and other surfaces including for example metrology ports. This is detrimental to the performance of the source collector module. It is therefore desirable to provide a means for cleaning deposited tin from the collector CO and other surfaces within the source collector module.

[0009] One known way of cleaning tin from the surfaces is by creating hydrogen radicals that can contact the surface and by reaction with the tin to form gaseous SnH* can decontaminate such surfaces. Hydrogen is present in the source collector vessel and hydrogen radicals are created by the EUV-producing plasma. However as the plasma is made with tin as the fuel because of the tin debris created with the plasma the processes of cleaning and contamination occur simultaneously and contamination remains a problem.

SUMMARY

[0010] According to a first an aspect of the invention there is provided a lithographic apparatus comprising a source for generating a radiation beam, said source including a first fuel supply adapted to supply in use a first fuel to a first ignition point where a first plasma is created by a laser pulse, wherein said first plasma generates said radiation beam, and a second fuel supply adapted to supply in use a second fuel to a second ignition point where a second plasma is created by a laser pulse, wherein said second plasma generates hydrogen radicals without creating debris material and wherein in use said first and second plasmas are created by laser pulses emitted by a single laser source.

[0011] The first ignition point may be at a focal point of the laser source to provide maximum power intensity for the creation of the radiation generating plasma and the second ignition point may be coincident with the first ignition point. However, since creation of the second plasma may not require the same power intensity it may be preferable to locate the second ignition point at a distance from the first ignition point such that it is not at the focal point of the laser pulse. This has the advantage of creating a larger plasma volume and consequently a greater number of hydrogen radicals.

[0012] The second fuel may be solid, liquid or a gas stream. Preferably when the second fuel is a gas it is an inert gas. Preferably when the second fuel is a gas it is supplied to the second ignition point at a density of from 1016 to 1019cm"3.

[0013] In some embodiments of the invention the first fuel supply and the second fuel supply may be provided interchangeably at the same location in the apparatus. Alternatively they first fuel supply and the second fuel supply may be formed as an integral unit that can be provided at a single location. Alternatively the first fuel supply and the second fuel supply may be provided at different locations.

[0014] In some embodiments the apparatus may comprise a protective member that may be moved into a position for protecting a component within the apparatus during start-up of the source. The protective member may be adapted for movement between the protecting position and a second position where the component may be revealed for use. This movement may be a reciprocal movement or the protective member may be mounted on a roller.

[0015] According to another aspect of the invention there is provided a method of decontaminating a surface within a radiation source of a lithographic apparatus, comprising supplying a first fuel to a first ignition point where a first plasma is created by a laser pulse, wherein said plasma generates radiation, and supplying a second fuel to a second ignition point where a second plasma is created by a laser pulse, wherein said second plasma generates hydrogen radicals without creating debris material, wherein a single laser source is used to create said first and second plasmas.

[0016] The first ignition point may be at a focal point of the laser source to provide maximum power intensity for the creation of the radiation generating plasma and the second ignition point may be coincident with the first ignition point. However, since creation of the second plasma may not require the same power intensity it may be preferable to locate the second ignition point at a distance from the first ignition point such that it is not at the focal point of the laser pulse. This has the advantage of creating a larger plasma volume and consequently a greater number of hydrogen radicals.

[0017] The second fuel may be solid, liquid or a gas stream. Preferably when the second fuel is a gas it is an inert gas. Preferably when the second fuel is a gas it is supplied to the second ignition point at a density of from 1016 to 1019cm"3.

[0018] In some embodiments of the invention the first fuel supply and the second fuel supply may be provided interchangeably at the same location in the apparatus. Alternatively they first fuel supply and the second fuel supply may be formed as an integral unit that can be provided at a single location. Alternatively the first fuel supply and the second fuel supply may be provided at different locations.

[0019] Preferably the method may comprise adjusting the supply of hydrogen to the radiation source to enhance the flow of hydrogen radicals to the surface.

[0020] Further features and advantages of the present invention, as well as the structure and operation of various embodiments of the present invention, are described in detail below with reference to the accompanying drawings. It is noted that the present invention is not limited to the specific embodiments described herein. Such embodiments are presented herein for illustrative purposes only. Additional embodiments will be apparent to persons skilled in the relevant art(s) based on the teachings contained herein.

BRIEF DESCRIPTION OF THE DRAWINGS

[0021] 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:

[0022] Figure 1 schematically depicts a lithographic apparatus according to an embodiment of the invention;

[0023] Figure 2 is a more detailed schematic view of the lithographic apparatus;

[0024] Figure 3 is a schematic view of a first embodiment of the invention;

[0025] Figures 4 to 7 are schematic views of possible hydrogen flows in embodiments of the invention,

[0026] Figure 8 is a schematic view of a further embodiment of the invention, and

[0027] Figure 9 is a schematic view of a further embodiment of the invention

[0028] 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

[0029] This specification discloses one or more embodiments that incorporate the features of this invention. The disclosed embodiment(s) merely exemplify the invention. The scope of the invention is not limited to the disclosed embodiment(s). The invention is defined by the claims appended hereto.

[0030] 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.

[0031] 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 structure (e.g., a mask table) MT constructed to support a patterning device (e.g., a mask or a reticle) MA and connected to a first positioner PM configured to accurately position the patterning device; a substrate table (e.g., a wafer table) WT constructed to hold a substrate (e.g., a resist-coated wafer) W and connected to a second positioner PW configured to accurately position the substrate; and a projection system (e.g., a reflective projection system) PS configured to project a pattern imparted to the radiation beam B by patterning device MA onto a target portion C (e.g., comprising one or more dies) of the substrate W.

[0032] 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.

[0033] 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.

[0034] 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.

[0035] The patterning device may be transmissive or reflective. Examples of patterning devices include masks, programmable mirror arrays, and programmable LCD panels. Masks are well known in lithography, and include mask types such as binary, alternating phase-shift, and attenuated phase-shift, as well as various hybrid mask types. An example of a programmable mirror array employs a matrix arrangement of small mirrors, each of which can be individually tilted so as to reflect an incoming radiation beam in different directions. The tilted mirrors impart a pattern in a radiation beam, which is reflected by the mirror matrix.

[0036] 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.

[0037] As here depicted, the apparatus is of a reflective type (e.g., employing a reflective mask).

[0038] 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.

[0039] 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 CO2 laser is used to provide the laser beam for fuel excitation.

[0040] 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.

[0041] 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.

[0042] 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.

[0043] The depicted apparatus could be used in at least one of the following modes:

[0044] 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.

[0045] 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.

[0046] 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.

[0047] Combinations and/or variations on the above described modes of use or entirely different modes of use may also be employed.

[0048] 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, e.g., 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.

[0049] 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.

[0050] 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.

[0051] Referring now to Figure 3 there is shown a first embodiment of the invention in which a neutral fuel is supplied for ignition by the laser beam in order to generate hydrogen radicals. By the term "neutral" in this context is intended a fuel for the creation of hydrogen radicals that does not cause any further particulate, molecular or atomic contamination. 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 as shown in Figure 2. 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 hydrogen serves a number of purposes including maximising 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. A further neutral fuel - such as He - can be supplied with the hydrogen so as to increase the gas density for better plasma conditioning. Also shown in Figure 3 is a second fuel supply unit 3that supplies a second fuel used to generate hydrogen radicals H*, which as will be described in more detail below serve to clean the collector of tin that has been deposited thereon. Second fuel supply 3 comprises a fuel reservoir 4 and a fuel delivery needle 5 for delivering fuel to an ignition point where the fuel is ignited by a laser pulse from laser source LA to create a plasma 6. The plasma 6 creates hydrogen radicals H* from the hydrogen gas already present in the source collector module SO. If the tin droplet generator is considered to be a first fuel supply in that it supplies fuel for the creation of the EUV generating plasma, then the fuel supply 3 may be considered to be a second fuel supply.

[0052] The fuel supply 3 can supply fuel as a stream of droplets, or as a liquid jet, or as a high density gas stream or in solid form. Examples of possible fuels for creating hydrogen radicals include inert gases such as Xe, Kr, Ar, Ne and He. A plasma created from such an inert gas generates hydrogen radicals from hydrogen gas by a number of mechanisms including dissociation through UV radiation, collisions between H2 molecules and fast inert gas ions, and secondary electron generation at the collector surface. Importantly the fuel does not generate debris material by which is understood to be particulate, molecular or atomic matter that can accumulate on the collector. It will also be understood that while the discussion here is primarily in terms of cleaning the collector of accumulated tin debris, tin also accumulates on other surfaces within the source collector module such as metrology ports and the creation of hydrogen radicals also serves to clean these surfaces.

[0053] The fuel is supplied to the ignition point through a fuel delivery needle 5 to ensure that a high density of fuel is provided at the ignition point. For fuel in the form of a gas the local partial pressure at the ignition point should be such that the density of the gas is in the range of 1016 to 1019cm"3. The ignition point may be located at a distance from the focal point of the laser beam 6 (where the EUV generating plasma is created). This is because the power of the laser source necessary for creation of the EUV generating plasma may be higher than is necessary to create the hydrogen radical generating plasma from the fuel. It may therefore be possible to create the hydrogen radical generating plasma with a defocused beam and this has the advantage of providing a higher plasma volume and thus generating a greater number of hydrogen radicals. This can be achieved either by providing the fuel to an ignition point not at the beam focus, and/or by defocusing the laser beam itself. It is important to note however that the same laser that is used to generate the EUV radiation is also used to generate hydrogen radicals.

[0054] The second fuel supply 3 may be provided in the same location as the first fuel supply, e.g., the droplet generator that supplies tin droplets for the creation of an EUV generating plasma, or alternatively the second fuel supply may be provided at another location in the source collector module SO. In embodiments of the invention where the fuel supply 3 is provided at the same location as the droplet generator, collector decontamination is carried out in an in situ and offline manner. That is to say when contamination of the collector CO is detected, or alternatively at a scheduled time, the droplet generator may be removed and replaced with the fuel supply 3. A

decontamination process is then carried out by supplying neutral fuel to an ignition point to generate hydrogen radicals as discussed above. When this process is complete the fuel supply unit 3 is removed and replaced with the droplet generator. Contamination of the collector CO can be determined using known techniques either by measuring the reflectivity of the collector CO or by detecting a decrease in emitted EUV energy. Another option would be for the first and second fuel supplies to be formed as an integral unit. For example the droplet generator may be modified so that it had two outlets: one for supplying fuel droplets for EUV generation, and the other for supplying fuel for hydrogen radical creation. In such an embodiment offline decontamination could be performed without requiring removal of the droplet generator, or decontamination could be carried out at the same time as EUV generation with, for example, the laser pulse frequency being increased and with alternate laser pulses being timed and synchronised with the droplet generator and the second fuel supply so as to create EUV radiation or hydrogen radicals.

[0055] In embodiments where the second fuel supply 3 is provided at a different location from the droplet generator, the second fuel supply 3 could be operated to supply neutral fuel to generate hydrogen radicals either at scheduled times or when contamination of the collector is detected as in the above embodiment where the fuel supply unit is provided at the same location as the droplet generator. However, an alternative option is that the second fuel supply 3 may be operated continuously such that the hydrogen radicals are generated at the same time that EUV radiation is generated. This may be achieved by increasing the laser pulse rate for example to 80kHz with half of the pulses being used to produce EUV radiation and half the pulses being used to generate hydrogen radicals. As the fuel for the hydrogen radical generation does not need to be supplied to the primary focus position it will have minimal impact on the EUV generation process.

[0056] In embodiments of the invention it may be desirable to adjust the supply of hydrogen gas in order to maximise the efficiency of the decontamination process. In particular it is preferable to increase the flow of hydrogen across the collector surface both to enhance the delivery of atomic hydrogen to the collector surface and also to facilitate the removal of SnH4 away from the collector. This is illustrated in Figures 4 to 7, which show various options for the hydrogen flow. Figure 4 shows an embodiment of the invention where approximately 50% of the hydrogen is supplied through the collector opening 1, i.e., generally coaxially with the laser beam. The remaining 50% of the hydrogen is supplied around the perimeter of the collector CO. In this embodiment, however, there may be a zone between the center and the perimeter of the collector CO where the atomic hydrogen is only carried to the collector surface by diffusion and likewise the SnH4 is only carried away from the collector surface by diffusion. Figure 5 shows another embodiment in which the majority of the hydrogen is provided through the central opening 1 of the collector CO. In this embodiment with majority of the flow through the collector center hydrogen from the perimeter is drawn towards the collector center (along the collector surface) by the high hydrogen flow, and the atomic hydrogen formed at the plasma will diffuse to the collector surface and then be transported along the collector surface. SnH4 formed at the collector surface will be further transported by the hydrogen flow along the collector surface into the high central steam of hydrogen and moved away from the collector to an exhaust. Figure 6 shows an embodiment in which the majority of the hydrogen is supplied around the collector perimeter. This is similar to the embodiment of Figure 5 except that the flow of hydrogen is from the center of the collector to the perimeter but has the same effect that atomic hydrogen diffusing to the collector surface will be transported along the collector surface and SnH4 will be carried away by the hydrogen flow along the collector surface. Figure 7 shows another embodiment in which in addition to the main hydrogen supply (the majority of which is supplied through the collector aperture 1) there is pumping close of the collector perimeter, which increases further the hydrogen flow across the collector surface. It will be understood that by appropriately adjusting the hydrogen flow the transport of generated hydrogen radicals H* to the surface of the collector CO and the transport of resultant SnH4 away from the collector CO may be enhanced.

[0057] The problem of potential contamination is particularly acute at start-up before a steady stream of droplets is generated. In another embodiment of the invention therefore there may be provided a protective member which may be placed so as to protect a surface from potential contamination during start-up and which may then be removed or withdrawn when steady-state droplet generation is achieved.

[0058] Figure 8 shows an example of such an embodiment where the surface to be protected is the collector CO. A protective member in the form of a shutter 10 is provided that may be moved into a location between the collector CO and the droplet stream 11. The shutter 10 is adapted to be reciprocated in the direction X-X and once a steady state of droplet generation is achieved the shutter 10 may be moved away.

[0059] Figure 9 shows an alternative embodiment in which the shutter 10’ may be adapted to be rolled around a roller 12 when not required. Such an embodiment may have an additional advantage when the source is opened for servicing as the shutter may also be used during such a service operation to protect the collector.

[0060] It will be understood that the terms "first fuel", "first plasma", "second fuel" and "second plasma" are used for convenience of description. The first fuel is considered to be tin or other known material, which when struck by the laser pulse will form a plasma ("the first plasma") generates EUV radiation. This plasma may also generate hydrogen radicals that will contribute to reducing the deposition of tin on surfaces within the source collector module, but the creation of hydrogen radicals is not the primary function of the first plasma that is to generate EUV radiation. The second fuel - e.g., an inert gas - is a fuel that when ignited by a laser pulse creates a second plasma that generates hydrogen radicals. The second plasma may also generate radiation - EUV or otherwise - but this is not the main purpose of the second plasma which is the generation of hydrogen radicals. The second fuel for the creation of this secondary plasma is considered to be "neutral" in the sense that upon ignition of this fuel no debris (or at least minimal debris) is created that may accumulate on surfaces within the source collector module.

[0061] 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.

[0062] 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.

[0063] 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.

[0064] 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, f93, f57 or f26 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.

[0065] 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.

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 lithographic apparatus comprising a source for generating a radiation beam, said source including a first fuel supply adapted to supply in use a first fuel to a first ignition point where a first plasma is created by a laser pulse, wherein said first plasma generates said radiation beam, and a second fuel supply adapted to supply in use a second fuel to a second ignition point where a second plasma is created by a laser pulse, wherein said second plasma generates hydrogen radicals without creating debris material and wherein in use said first and second plasmas are created by laser pulses emitted by a single laser source.

2. Apparatus as claimed in clause 1 wherein said first ignition point is at a focal point of said laser pulse and wherein said second ignition point is coincident with said first ignition point.

3. Apparatus as claimed in clause 1 wherein said first ignition point is at a focal point of said laser pulse and wherein said second ignition point is at a distance from said first ignition point.

4. Apparatus as claimed in any of clauses 1, 2 or 3 wherein said second fuel is a gas.

5. Apparatus as claimed in clause 4 wherein said second fuel is an inert gas.

6. Apparatus as claimed in clause 4 or 5 wherein said second fuel supply supplies the gas to said second ignition point at a density of from f 016 to 1019cm"3.

7. Apparatus as claimed in any preceding clause wherein said first fuel supply and said second fuel supply are interchangeably provided at a single location in said apparatus.

8. Apparatus as claimed in any preceding of clauses 1 to 6 wherein said first fuel supply and said second fuel supply are formed as an integral unit.

9. Apparatus as claimed in any of clauses 1 to 6 wherein said first fuel supply and said second fuel supply are provided at different locations.

10. Apparatus as claimed in any preceding clause further comprising a protective member that may be moved into a position for protecting a component within the apparatus during start-up of said source.

11. Apparatus as claimed in clause 10 wherein said protective member is adapted for movement between said protecting position and a second position where said component is revealed for use.

12. Apparatus as claimed in clause 11 wherein said movement is reciprocal movement.

13. Apparatus as claimed in clause 11 wherein said protective member is mounted on a roller.

14. A method of decontaminating a surface within a radiation source of a lithographic apparatus, comprising supplying hydrogen to said radiation source, supplying a first fuel to a first ignition point where a first plasma is created by a laser pulse, wherein said plasma generates radiation, and supplying a second fuel to a second ignition point where a second plasma is created by a laser pulse, wherein said second plasma generates hydrogen radicals without creating debris material, wherein a single laser source is used to create said first and second plasmas.

15 A method as claimed in clause 14 wherein said first ignition point is at a focal point of said laser pulse and wherein said second ignition point is coincident with said first ignition point.

16. A method as claimed in clause 14 wherein said first ignition point is at a focal point of said laser pulse and wherein said second ignition point is at a distance from said first ignition point.

17. A method as claimed in any of clauses 14, 15 or 16 wherein said second fuel is a gas.

18. A method as claimed in clause 17 wherein said second fuel is an inert gas.

19. A method as claimed in clause 17 or 18 wherein said second fuel supply supplies the gas to said second ignition point at a density of from 1016 to 1019cm"3.

20. A method as claimed in any of clauses 14 to 19 wherein said first fuel supply and said second fuel supply are interchangeably provided at a single location in said apparatus.

21. A method as claimed in any of clauses 14 to 20 wherein said first fuel supply and said second fuel supply are formed as an integral unit.

22. A method as claimed in any of clauses 14 to 20 wherein said first fuel supply and said second fuel supply are provided at different locations.

23. A method as claimed in any of clauses 14 to 21 comprising adjusting the supply of hydrogen to the radiation source to enhance the flow of hydrogen radicals to said surface.

24. A lithographic apparatus, comprising: a source for generating a radiation beam, the source including a first fuel supply configured to supply a first fuel to a first ignition point where a first plasma is created by a laser pulse, wherein the first plasma generates the radiation beam, and a second fuel supply configured to supply in use a second fuel to a second ignition point where a second plasma is created by a further laser pulse, wherein the second plasma generates hydrogen radicals without creating debris material, and wherein in use the first and second plasmas are created by laser pulses emitted by a single laser source.

25. The apparatus of clause 24, wherein the first ignition point is at a focal point of the laser pulse, and wherein the second ignition point is coincident with the first ignition point.

26. The apparatus of clause 24, wherein the first ignition point is at a focal point of the laser pulse and wherein the second ignition point is at a distance from the first ignition point.

27. The apparatus of clause 24, wherein the second fuel is a gas.

28. The apparatus of clause 27, wherein the second fuel is an inert gas.

29. The apparatus of clause 27, wherein the second fuel supply supplies the gas to the second ignition point at a density of from 1016 to 1019cm"3.

30. The apparatus of clause 24, wherein the first fuel supply and the second fuel supply are interchangeably provided at a single location in the apparatus.

31. The apparatus of clause 24, wherein the first fuel supply and the second fuel supply are formed as an integral unit.

32. The apparatus of clause 24, wherein the first fuel supply and the second fuel supply are provided at different locations.33. Apparatus as claimed in any of clauses 24 to 32 clause further comprising a protective member that may be moved into a position for protecting a component within the apparatus during start-up of said source.

34. Apparatus as claimed in clause 33 wherein said protective member is adapted for movement between said protecting position and a second position where said component is revealed for use.

35. Apparatus as claimed in clause 34 wherein said movement is reciprocal movement.

36. Apparatus as claimed in clause 34 wherein said protective member is mounted on a roller.

37. A method of decontaminating a surface within a radiation source of a lithographic apparatus, comprising supplying hydrogen to the radiation source; supplying a first fuel to a first ignition point where a first plasma is created by a laser pulse, wherein the plasma generates radiation; and supplying a second fuel to a second ignition point where a second plasma is created by a further laser pulse, wherein the second plasma generates hydrogen radicals without creating debris material, and wherein a single laser source is used to create the first and second plasmas.

38. The method of claim 37, wherein the first ignition point is at a focal point of the laser pulse and wherein the second ignition point is coincident with the first ignition point.

39. The method of clause 37, wherein the first ignition point is at a focal point of the laser pulse and wherein the second ignition point is at a distance from the first ignition point.

40. The method of clause 37, wherein the second fuel is a gas.

41. The method of clause 40, wherein the second fuel is an inert gas.

42. The method of clause 40, wherein the second fuel supply supplies the gas to the second ignition point at a density of from 1016 to 1019cm"3.

43. The method of any of clause 40, wherein the first fuel supply and the second fuel supply are interchangeably provided at a single location in the apparatus.

44. The method of any of clause 40, wherein the first fuel supply and the second fuel supply are formed as an integral unit.

45. The method of any of clause 40, wherein the first fuel supply and the second fuel supply are provided at different locations.

46. The method of any of clause 40, comprising adjusting the supply of hydrogen to the radiation source to enhance the flow of hydrogen radicals to the surface.

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.