NL2008962A - Radiation source, lithographic apparatus and method of producing radiation. - Google Patents

Description

RADIATION SOURCE, LITHOGRAPHIC APPARATUS AND METHOD OF

PRODUCING RADIATION

HELD

[0001] The present invention relates to a radiation source, a lithographic apparatus and a method of producing 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):

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

[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-produced plasma sources, or sources based on synchrotron radiation provided by an electron storage ring.

[0006] EUV radiation may be produced using a plasma. One type of radiation system for producing EUV radiation (also known as a radiation source) may include a laser for exciting a fuel to provide the plasma, and a source collector apparatus for containing the plasma. The plasma may be created, for example, by directing a laser beam at a fuel, such as particles of a suitable material (e.g., Sn), or a stream of a suitable gas or vapour, such as Xe gas or Li vapour. 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. Such a radiation system is typically termed a laser produced plasma (LPP) source.

[0007] The source collector apparatus may include an enclosing structure or chamber arranged to provide a vacuum environment to support the plasma.

[0008] An alternative radiation system for producing EUV radiation may use an electrical discharge to excite fuel. For example, a very hot plasma of Sn, Li or Xe may be generated using an electrical discharge. The plasma emits output radiation, e.g., EUV radiation, which is collected using a radiation collector. The radiation collector may comprise a series of grazing incidence reflective surfaces. Such a source collector apparatus is typically termed a discharge produced plasma (DPP) source.

[0009] A DPP radiation source generates plasma by a electrical discharge in a substance, for example a gas or vapour, between an anode and a cathode, and may subsequently create a high-temperature discharge plasma by Ohmic heating caused by a pulsed current flowing through the plasma. In this case, the desired radiation is emitted by the high-temperature discharge plasma. Such a device is described in European Patent Application No. 03255825.6, filed September 17, 2003 in the name of the applicant.

[0010] A further type of radiation source for producing EUV radiation is a modification of a DPP radiation source. In this type of radiation source, radiation (for example infrared (IR) radiation) is incident on the fuel (for example Sn, Li or Xe) prior to the electrical discharge. The radiation that is incident on the fuel may be provided by a laser. The radiation incident on the fuel causes the fuel to become a modified fuel distribution (for example a cloud of evaporated fuel). This modified fuel distribution is then subjected to an electrical discharge between an anode and a cathode. As with a DPP source, the electrical discharge excites the modified fuel distribution so as to produce a radiation producing plasma that emits radiation (for example, EUV radiation). This type of radiation source may be referred to as a Laser-assisted Discharge Plasma (LDP) source.

[0011] In some embodiments of DPP and LDP radiation sources the anode and cathode each comprise a generally disk shaped portion. The disk shaped portions are located such that each of them is partially submerged in a bath containing a liquid fuel. The disk shaped portions are rotated such that the circumference of each disk shaped portion passes through the bath containing the liquid fuel. As the circumference of each disk shaped portion passes through the bath, the fuel adheres to the circumference of each disk shaped portion. The fuel is then carried, by the circumference of the disk shaped portion as it rotates, to a location referred to as the pinch. The pinch is the location at which the separation between the anode and the cathode is a minimum. When the fuel is at the pinch an electrical discharge passes between the anode and the cathode through the fuel (in the case of DPP radiation sources) or through the modified fuel distribution (in the case of LDP radiation sources), causing the radiation producing plasma to be produced.

[0012] In order to enable the disk shaped portions of the anode and cathode to rotate, the disk shaped portions are supported by bearings. These bearings have to support large loads and often may be required to place the anode and cathode in a complex positional relationship with one another. The contact between the disk shaped portions of the anode/cathode and their respective bearings may be critical in determining the bearing pretension and stiffness. These factors may be critical in controlling the rotation of the disk shaped portions. Furthermore, the operation of the bearings may be critical to the mnning accuracy of the disk shaped portions as they rotate, and in particular to the running accuracy of the circumference of the disk shaped portions.

[0013] It will be appreciated that the separation between the disk shaped portions of the anode and the cathode at the pinch may be critical in determining the properties of the plasma that is produced by the electrical discharge, and hence properties of the radiation produced by the plasma. The separation between the disk shaped portions of the anode and the cathode at the pinch is affected by the running accuracy of the circumference of the disk shaped portions, and consequently the operation of the bearings. Consequently, if one of the bearings is operating in an undesirable manner, the radiation produced by the plasma may have undesirable properties. Relatively small deviations from the desired operating parameters of the bearings may lead to relatively large deviations from the desired running position and/or accuracy of the circumference of the disk shaped portions and hence deviations from the desired pinch geometry and desired properties of the radiation producing plasma.

[0014] The operating lifetime of the bearings is adversely affected by the large pretension within the bearing required to support the disk shaped portions with the desired accuracy. The operating lifetime of the bearing may also be adversely affected by the fact that the bearings are commonly located in a vacuum. Locating the bearings within a vacuum means that the potential for use of lubricants is minimised. In some cases it is not possible to use lubricant.

[0015] If a bearing has to be repaired or replaced, then this may require shutting down the radiation source (and any lithographic apparatus of which the radiation source forms part). This ‘down time’ of the radiation source and/or lithographic apparatus will reduce the potential operating time of the radiation source and/or lithographic apparatus.

[0016] An additional problem with known DPP or LDP-based radiation sources is the thermal load on the electrodes due to their close proximity to the plasma. This may become particularly relevant when scaling the EUV source to meet the specifications for a production exposure tool.

[0017] A further problem with known DPP or LDP-based radiation sources is that the conversion of fuel (e.g., Sn) to radiation emitting plasma may be incomplete, and consequently particles of fuel material are projected from a plasma formation location when the plasma is generated. Fuel material may accumulate on surfaces of the source collector apparatus, and may reduce the efficiency of the source collector apparatus or prevent it from operating.

SUMMARY

[0018] It is desirable to provide a radiation source, lithographic apparatus and device manufacturing method that overcomes or mitigates a problem associated with the prior art. It is further desirable to provide an alternative radiation source.

[0019] According to a first aspect of the invention, there is provided a radiation source comprising an anode configured to rotate about a first axis; a cathode configured to rotate about a second axis; the anode and/or the cathode being supported by a bearing arrangement, which includes a satellite bearing that is located at position that is spaced from the first axis or the second axis respectively.

[0020] The bearing arrangement may include a plurality of satellite bearings angularly spaced about the first axis or the second axis.

[0021] The satellite bearings may be substantially equi-angularly spaced about the first axis or the second axis.

[0022] The anode and/or cathode may include a first engagement feature, and at least one satellite bearing may include a second engagement feature, the first and second engagement features being configured such that the first and second engagement figures engage one another such that the at least one satellite bearing rotatably supports the anode and/or cathode.

[0023] The first engagement portion may comprise an annular channel that is generally co-axial with the first or second axis.

[0024] The annular channel may be defined in part by a wall having a generally v-shaped, generally U-shaped or generally arcuate profile.

[0025] The second engagement feature may include an engagement surface having a generally arcuate profile.

[0026] The satellite bearing may be arranged to support the anode or cathode at a location closer to a circumference of the anode or cathode than to the first axis or the second axis respectively.

[0027] Alternatively, the satellite bearing may be arranged to support the anode or cathode at a location adjacent an inner or outer circumference of the anode or cathode.

[0028] The anode and/or cathode may be rotated by a motor via at least one satellite bearing.

[0029] The anode and/or cathode may be generally disc or annular shaped.

[0030] The anode and/or cathode may comprise at least one fuel channel that extends from a radially inner location with respect to the first or second axis, to a radially outer location with respect to said first or second axis.

[0031] The radiation source may comprise a fuel supply configured to supply fuel to said radially inner location.

[0032] The satellite bearing may be movable in a plane perpendicular to the first or second axis.

[0033] The satellite bearing may be movable in a direction that is substantially parallel to the first or second axis.

[0034] According to a second aspect of the invention there may be provided a lithographic apparatus comprising the radiation source of the first aspect of the invention, and further comprising an illumination system configured to condition radiation produced by the radiation source to form a conditioned radiation beam; a support constructed to support a patterning device, the patterning device being capable of imparting the conditioned 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.

[0035] According to a third aspect of the invention there may be provided a method of producing radiation, the method comprising providing a radiation source comprising an anode and a cathode; rotating the anode about a first axis; rotating the cathode about a second axis; and supporting the anode or the cathode by a bearing arrangement including a satellite bearing that is located at position that is spaced from the first axis or the second axis respectively.

[0036] The method further may further comprise controlling the position of the satellite bearing in a plane perpendicular to the first or second axis and/or in a direction that is substantially parallel to the first or second axis so as to control at least one of the following the relative positioning between the anode or cathode and a portion of the radiation source that is not the anode or the cathode; the relative positioning between the anode and the cathode; the geometry of the pinch defined between the anode and the cathode; the pre-tension of the anode or cathode; the stiffness of the anode or cathode with regards to rotation about the respective first or second axis; and the shape of the anode or cathode.

[0037] Further features and advantages as well as the structure and operation of various embodiments are described in detail below with reference to the accompanying drawings. It is noted that the 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

[0038] 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:

[0039] Figure 1 schematically depicts a lithographic apparatus according to an embodiment of the invention;

[0040] Figure 2 is a more detailed view of the lithographic apparatus shown in Figure l;

[0041] Figure 3 is a schematic side view of a portion of a radiation source according to an embodiment of the present invention;

[0042] Figure 4 is a schematic plan view of a portion of a known radiation source;

[0043] Figure 5 is a schematic perspective view of a portion of the radiation source shown in Figure 3;

[0044] Figure 6 is a cross section through the portion of the radiation source shown in

Figure 5;

[0045] Figure 7 is a further view of the cross section shown in Figure 6;

[0046] Figure 8 is an enlarged view of a portion of the radiation source shown in

Figure 7;

[0047] Figure 9 is a schematic representation of a portion of a radiation source according to an embodiment of the present invention;

[0048] Figure 10 is schematic representation of a portion of a radiation source according to a further embodiment of the present invention;

[0049] Figure 11 is a schematic representation of a portion of a radiation source according to a yet further embodiment of the present invention;

[0050] Figure 12 is schematic representation of a portion of a radiation source according to another embodiment of the present invention; and

[0051] Figure 13 schematic representation of a portion of a radiation source according to a yet further embodiment of the present invention.

[0052] Embodiments are described below with reference to the accompanying drawings. In the drawings, like reference numbers generally refer to identical or functionally similar elements. Additionally, the leftmost digit(s) of a reference number generally identifies the drawing in which the reference number first appears.

DETAILED DESCRIPTION

[0053] It is noted that reference in this specification to “one embodiment,” “an embodiment,” “an example embodiment,” etc., indicate that the embodiment described may include a particular feature, structure, or characteristic, but not every embodiment may necessarily include the particular feature, stmcture, 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 submitted 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.

[0054] 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.

[0055] 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.

[0056] The support structure MT holds the patterning device MA in a manner that depends on the orientation of the patterning device, the design of the lithographic apparatus, and other conditions, such as for example whether or not the patterning device is held in a vacuum environment. The support structure can use mechanical, vacuum, electrostatic or other clamping techniques to hold the patterning device. The support stmcture may be a frame or a table, for example, which may be fixed or movable as required. The support stmcture may ensure that the patterning device is at a desired position, for example with respect to the projection system.

[0057] 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.

[0058] 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.

[0059] 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.

[0060] As here depicted, the apparatus is of a reflective type (e.g., employing a reflective mask).

[0061] 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.

[0062] Referring to Figure 1, the illuminator IL receives an extreme ultra violet (EUV) 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 discharge produced plasma (“DPP”), the EUV emitting plasma is produced by using an electrical discharge to vaporize a fuel. The fuel may be an element such as xenon, lithium or tin that has one or more emission lines in the EUV range. The electrical discharge may be generated by a power supply that may form part of the source collector apparatus or may be a separate entity that is connected via an electrical connection to the source collector apparatus.

[0063] 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.

[0064] 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 PS 1 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.

[0065] The depicted apparatus could be used in at least one of the following modes:

[0066] In step mode, the support stmcture (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.

[0067] In scan mode, the support stmcture (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 stmcture (e.g., mask table) MT may be determined by the (de-)magnification and image reversal characteristics of the projection system PS.

[0068] In another mode, the support stmcture (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.

[0069] Combinations and/or variations on the above described modes of use or entirely different modes of use may also be employed.

[0070] Figure 2 shows the lithographic apparatus 100 in more detail, including the source collector apparatus SO, the illumination system IL, and the projection system PS. The source collector apparatus SO is constructed and arranged such that a vacuum environment can be maintained in an enclosing structure 220 of the source collector apparatus SO. An EUV radiation emitting plasma 210 may be formed by a discharge produced plasma source. EUV radiation may be produced by a gas or vapour, for example Xe gas, Li vapour or Sn vapour in which a very hot plasma is created to emit radiation in the EUV range of the electromagnetic spectrum. The very hot plasma 210 is created by, for example, an electrical discharge causing an at least partially ionized plasma. Partial pressures of, for example, 10 Pa of Xe, Li, Sn vapour or any other suitable gas or vapour may be required for efficient generation of the radiation. In an embodiment, a plasma of excited tin (Sn) is provided to produce EUV radiation.

[0071] The radiation emitted by the hot plasma 210 is passed from a source chamber 211 into a collector chamber 212 via a rotating foil trap 260 and a static foil trap 261 (described further below), either or both of which may be positioned in or behind an opening in source chamber 211.

[0072] The collector chamber 212 may include a radiation collector CO, which may be a so-called grazing incidence collector. Radiation collector CO has an upstream radiation collector side 251 and a downstream radiation collector side 252. Radiation that traverses collector CO can be reflected off a grating spectral filter 240 to be focused in a virtual source point IF. The virtual source point IF is commonly referred to as the intermediate focus, and the source collector apparatus SO 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.

[0073] 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.

[0074] More elements than shown may generally be present in illumination optics unit IL and projection system PS. The grating spectral filter 240 may optionally be present, depending upon the type of lithographic apparatus. Further, 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.

[0075] Collector optic CO, as illustrated in Figure 2, is depicted as a nested collector with grazing incidence reflectors 253, 254 and 255, just as an example of a collector (or collector mirror). The grazing incidence reflectors 253, 254 and 255 are disposed axially symmetrically around an optical axis O. A collector optic CO of this type is preferably used in combination with a discharge produced plasma source (DPP source).

[0076] The radiation source collector SO shown in Figures 1 and 2 may include a laser-assisted discharge plasma (LDP) type of radiation source. An LDP radiation source is a modification of a DDP radiation source. Figure 3 shows an LDP radiation source. The radiation source 300 comprises two rotating wheels 302 and 304. The first rotating wheel 304 forms part of an anode, whereas the second rotating wheel 302 forms part of a cathode. The wheels rotate in the directions indicated by arrows 306 and 308. A liquid fuel is 310 is continuously applied to the wheels 302 and 304. This is achieved by partly immersing each of the wheels 302 and 304 in baths 312, 314 of liquid fuel 310. In this case, the liquid fuel is liquid tin, but any appropriate fuel may be used. The liquid fuel 310 within each of the baths 312, 314 is maintained in a liquid state by heaters 316, 318. Rotation of the wheels 302, 304 through the liquid fuel 310 in the fuel baths 312, 314 causes the liquid fuel to form a film 320, 322 on the outer circumference of each of the wheels 302, 304.

[0077] Any appropriate number of heaters may be used. In some embodiments one or more heaters is used to heat the liquid fuel in the fuel baths. In some embodiments one or more heaters may be used to heat the anode and/or cathode. Any suitable type of heater may be used. In some embodiments an induction heater may be used, for example, the anode and/or cathode may be heated by a heater via inductive heating.

[0078] The film 320, 322 of fuel is carried by the rotating wheels 302, 304 to a pinch location 324 (also referred to as the pinch). At the pinch 324, the separation between the wheels 302 and 304 is a minimum.

[0079] Radiation 326 produced by a secondary radiation source 328 (in this case a radiation pulse produced by a laser) is directed on to a portion of the fuel at the pinch 324. It will be appreciated that in other embodiments any appropriate secondary radiation source may be used. The radiation 326 is directed onto the film 322 of fuel on the wheel 304 at the pinch 324. The fuel absorbs at least part of the radiation 326 such that at least part of the fuel evaporates to form a modified fuel distribution 330 at the pinch 324. The modified fuel distribution may be described in some instances as a cloud of partially ionized fuel. The modified fuel distribution 330 expands towards the other wheel 302.

[0080] A capacitor bank 332 is connected across the wheels 302, 304 (in this case wheel 302 is a portion of the cathode and wheel 304 is a portion of the anode). In this case the fuel is conductive and hence the capacitor bank is connected across the wheels 302, 304 via the fuel 310 in the fuel baths 312, 314. When the fuel density of the modified fuel distribution 330 between the wheels 302, 304 at the pinch 324 is sufficiently high, a discharge is initiated between the wheels 302, 304 at the pinch. The high current flow through the modified fuel distribution 330 at the pinch 324 converts at least part of the modified fuel distribution 330 into a radiation emitting plasma (in this case, emitting EUV radiation).

[0081] The process whereby the fuel forms a modified fuel distribution and whereby at least part of the modified fuel distribution is converted into a radiation producing plasma may be repeated several thousand times per second.

[0082] The fuel within the LDP source has several functions other than being the fuel for creating the radiation producing plasma. The fuel on each wheel 302, 304 may be electrically conductive. If so, the fuel may form the at least part of the electrodes (anode, cathode) between which the electrical discharge occurs. The fuel may protect the wheels 302, 304 from being ablated by the radiation 326. The fuel may also act as a coolant, which helps to dissipate heat generated by the radiation 326 and/or by the electrical discharge.

[0083] Generating a radiation producing plasma in this way may result in the production of a significant amount of fuel debris. Such fuel debris may take the form of ions, droplets or clusters. If the fuel debris is allowed to pass to the illuminator IL of the lithographic apparatus then optics within the illuminator IL may become dirty or be damaged. This may lead to a reduction in the imaging performance of the lithographic apparatus, which is undesirable. Consequently, the radiation source may include debris collection apparatus, debris deflection apparatus or debris cleaning apparatus to prevent fuel debris from passing to the illuminator. Examples of debris collection apparatus, debris deflection apparatus or debris cleaning apparatus include rotating foil traps, reflective foil traps, directional gas flows and cleaning with hydrogen radicals.

[0084] Figure 4 shows a schematic representation of a portion of a known LDP radiation source. Compared to the embodiment of the invention shown in Figure 3, due to the fact that the portion of the LDP source shown in Figure 4 is shown from above, features such as the fuel baths are not visible. The radiation source shown in Figure 4 has two rotating wheels 302a and 304a. The wheel 302a forms part of a cathode and the wheel 304a forms part of an anode. Between the wheels 302a and 304a is a pinch 324a. A secondary radiation source 328a produces radiation 326a, which is incident on fuel (not shown) on wheel 304a at the pinch 324a. This causes the fuel on the wheel 304a at the pinch 324a to form a modified fuel distribution 330a at the pinch 324a. An electrical discharge between the cathode and anode (and hence between wheels 302a and 304a) through the modified fuel distribution 330a causes at least part of the modified fuel distribution 330a to be converted into a radiation producing plasma. The radiation producing plasma emits radiation 333a. In this case the emitted radiation 333a forms a beam of radiation that is generally conical in cross-section.

[0085] The wheels 302a and 304a rotate about respective axes 334a and 336a. In order to enable the wheels 302a and 304a to rotate about axes 334a and 336a, the wheels 302a, 304a are supported by respective bearings 338a and 340a. The wheels 302a and 304a are driven for rotation by respective motors 334a and 336a.

[0086] It can be seen that the axes 334a and 336a are orientated with respect to one another such that there is an angle a between the respective planes containing each of the wheels 302a and 304a. The reason that there is an angle a between the planes containing the wheels 302a and 304a is that to maximise the amount of usable radiation that is produced by the radiation producing plasma and that is emitted by the radiation source. This is achieved by minimising the amount of radiation produced by the radiation producing plasma that is absorbed by the anode/cathode.

[0087] Each of the wheels 302a and 304a is supported by its respective bearing 338a and 340a. Each of the wheels 302a, 304a is attached to a separate solid axle (not shown). The axle is attached to the centre of each wheel 302a and 304a and the axle passes through the respective bearing 338a, 340a. Because the axel and bearing of each wheel are located substantially at the centre of each wheel, the bearings 338a, 340a may be referred to as central bearings. Each of the axles is mechanically linked to the respective motors 334a, 336a. The motors 334a, 336a drive the wheel 302a, 304a to which they are attached via the respective axle such that the wheels 302a, 304a rotate about respective axes 334a, 336a, which pass through the centre of the respective wheels 302a, 304a.

[0088] As previously discussed, the axes 334a and 336a about which the wheels 302a and 304a rotate are at an angle to one another (in this case the axes 334a and 336a subtend an angle of 180 - a). In addition, the axes 334a and 336a may not be perpendicular to the direction of gravitational force that acts on the respective wheels 302a and 304a. Due to this complicated geometry, the bearings 338a and 340a must cope with a complicated array of forces acting upon them. In addition, for the complicate geometry of the wheels 302a and 304a to be maintained relative to one another, the bearings 338a and 340a must have very precise running accuracies. These issues are discussed in more detail below.

[0089] Due to the complicated array of the forces that act on each bearing (for example, forces due to the rotation of the respective wheel and forces that act on the bearing due to the non-perpendicular direction of gravity acting on the wheel relative to the axis of rotation of the wheel) the pre-tension and stiffness of the bearing are critical to its operation efficiency. For example, if the pre-tension between axial load and radial load within the bearing is not within predetermined limits or (which are determined having regard to the forces acting on the bearing during operation), then this may cause portions of the bearing to grind against one another. For this reason, the pre-tension between axial load and radial load within the bearing may be chosen such that the sum of forces acting on the bearing provides minimum friction to rotation of the anode/cathode, which is supported by the bearing.

[0090] For example, depending on the type of bearing, bearing surfaces and/or rolling elements within the bearing may grind against one another. Such grinding will result in the wear of the bearing and will eventually lead to a reduction in the operating performance of the bearing. In extreme cases, the bearing may seize. If portions of the bearing begin to grind, not only will the operating performance of the bearing be reduced (i.e., frictional loss of useful energy within the bearing will increase), but also the relative alignment between components of the bearing will also be degraded due to material within the bearing being grinded away. In this situation the alignment between any article being supported by the bearing and a portion of the bearing may also be degraded. This may lead to a reduction in the running accuracy of the wheel, which is supported by the respective bearing.

[0091] This problem is exacerbated by the fact that any misalignment of the centre of the wheel that is caused by an inaccuracy in the running accuracy of the bearing will be amplified so as to cause a much greater inaccuracy in the running accuracy of the outer circumference of the wheel (this effect may also be referred to as being due to the fact that the circumference of the wheel has a large Abbe arm with respect to the centre of the wheel (or the axel supporting the wheel that is supported by the bearing). The term Abbe arm refers to the distance of something from a pivot point. In this case, the pivot point is may be a portion of the bearing and the Abbe arm of the outer circumference of the wheel (i.e., its distance from the pivot point) is relatively large. This means that any pivotal movement of the wheel at a location closer to the pivot point than the outer circumference will be amplified. This may be of particular importance due to the fact that the pinch 324a of the radiation source (which in part defines properties of the radiation producing plasma produced by the radiation source) is defined between the circumference of the two wheels 302a and 304a.

[0092] These problems may be further enhanced by the high pre-tension that is required within the bearings and also by the fact that the bearings may be located in an environment, such as a vacuum, which prevents the use of a lubricant within the bearing.

[0093] Furthermore, the bearings 338a, 340a (and hence the relative alignment between the wheels 302a, 304a defining the pinch 324a) may be susceptible to expansion and/or contraction due to changes in temperature that may occur whilst heating the apparatus up so that it is at a temperature that is sufficient to maintain the fuel in a liquid state.

[0094] Figures 5 to 7 show three different views of a portion of a radiation source according to the embodiment of the present invention shown in Figure 3 and that may form part of a lithographic apparatus as shown in Figures 1 and 2. Figure 5 shows a perspective view, whereas Figures 6 and 7 show two separate cross-sectional views. The radiation source shown in these Figures comprises an anode 304 in the form of a rotating wheel or disk. The anode is configured to rotate about a first axis 336. The radiation source also comprises a cathode 302, which is also in the form of a rotating wheel or disk, which rotates about a second axis 334. The anode and cathode are each supported by a bearing arrangement 340, 338. The bearing arrangements 338, 340 are each supported by an interface plate 342, which may form part of the housing of the radiation source.

[0095] Each of the bearing arrangements 338, 340 shown in Figures 5 to 7 has three satellite bearings 344. Due to the perspective of each of the Figures 5 to 7, only one satellite bearing 344 of each of the bearing arrangements 338, 340 can be seen. The three satellite bearings 344, which form part of each bearing arrangement 338, 340 are substantially equi-angularly spaced about the respective axis 334 or 336. That is to say, there is an angular spacing of about 120° about the axis 334, 336 between adjacent satellite bearings 334, which form part of the bearing arrangement 338, 340.

[0096] Each of the satellite bearings is located at a position that is spaced from the first axis 336 or the second axis 334 respectively. That is to say, each of the satellite bearings is located such that it is off-axis with respect to the axis of rotation of the respective anode or cathode.

[0097] For the sake of completeness, Figures 5 to 7 also show the pinch 324 between the anode 304 and the cathode 302; and the cone of radiation 333 that is produced by the radiation producing plasma.

[0098] It can be seen that the anode and cathode 304 and 302 both have the same size and shape and also (as seen best in Figure 7) have the same cross-sectional profile.

[0099] The cross-sectional profile of the anode 304 and cathode 302 is such that the diameter of the anode/cathode 304, 302 is greater at a first side 346 of the anode/cathode 304, 302, which is furthest from the bearing arrangement 340, 338 than the diameter of the anode/cathode 304, 302 at a second side 348 of the anode/cathode 304, 302, which is closest to the bearing arrangement 340, 338. The diameter of the anode/cathode 304, 302 decreases linearly from the first side 346 to the second side 348 of the anode/cathode 304, 302. Consequently, the profile of the anode 304 and cathode 302 is such that the circumferential edge of the anode and cathode 304, 302 that extends between the first side 346 of the anode/cathode and the second side 348 of the anode/cathode is generally planar (i.e., has a generally straight line profile). The angle β that the circumferential edge of the anode/cathode 304, 302 subtends with respect to the direction of the axes of rotation 336, 334 of the anode 304 and cathode 302 is chosen such that the anode and cathode 304, 302 do not block the path of the radiation produced by the radiation producing plasma as it travels from the radiation producing plasma to an output of the radiation source (and hence the illuminator of a lithographic apparatus of which the radiation source may form part).

[00100] It will be appreciated that although the size, shape and profile of the anode and cathode in the present embodiment are the same, in other embodiments this may not be the case. Furthermore, any appropriate size, shape and/or cross-sectional profile of anode or cathode may be used. For example, a generally disc shaped anode may be used in combination with a generally annular cathode, or vice-versa.

[00101] The bearing arrangements 340 and 338 that support the anode 304 and cathode 302 respectively are such that, via the satellite bearings 344, they support the anode/cathode 304, 302 at a location on the anode/cathode that is spaced from the centre of the respective anode/cathode and consequently spaced from the axis of rotation (first axis 336 and second axis 334) of the anode/cathode 304, 302. In fact, the satellite bearings 334 shown in Figures 5 to 7 support the anode/cathode 304, 302 at locations that are adjacent to the circumference of the anode or cathode 304, 302.

[00102] Supporting each of the anode 304 and cathode 302 using satellite bearings 334 at locations that are closer to the circumference of the respective anode 304 or cathode 302 than the central bearing known in the prior art has several advantages. First, due to the fact that the anode/cathode 304, 302 is supported by a plurality of (three in the bearing arrangements of Figures 5 to 7) separate satellite bearings 344 as opposed to a single central bearing, each of the satellite bearings experiences less load than the single central bearing in the prior art. Consequently, the satellite bearings may be less susceptible to wear than a single central bearing. Secondly, the satellite bearings each support the anode/cathode 304, 302 at a location that is closer to the circumference of the anode/cathode compared to a central bearing. As previously discussed, the circumferential edges of the anode 304 and cathode 302 define the pinch. Due to the fact that the electrical discharge and hence plasma production occur at the pinch, the geometry of the pinch defined between the circumferential edges of the anode and cathode 304 and 302 affects the characteristics of the radiation produced by the radiation producing plasma. Consequently, the running accuracy of the circumferential edges of the anode and cathode is important to producing radiation via the radiation producing plasma with desired properties. It will be appreciated that, by using satellite bearings to support the anode and cathode at a plurality of locations that are closer to the circumferential edge of the anode/cathode 304, 302 than a central bearing, the running accuracy of the circumferential edges of the anode and cathode will be improved (compared to a central bearing). Consequently the operating performance of the radiation source is improved (compared to that having central bearings) by virtue of the characteristics of the radiation producing plasma being substantially maintained as the desired characteristics of the radiation producing plasma because of a greater running accuracy of the anode and cathode.

[0100] Figure 8 shows an enlarged portion of Figure 7. In particular, Figure 8 shows one of the satellite bearings 344, which supports the anode 304. It can be seen that the satellite bearing 344 is supported by the bearing base 343, which is in turn supported by the interface plate 342.

[0101] The anode 304 includes a first engagement feature 350. The first engagement feature 350 of the anode 304 takes the form of an annular channel 352. The annular channel 352 is a recess in the second side 348 of the anode and is coaxial with the axis of rotation 336 (and hence the centre) of the anode 304. The channel 352 has a radially outer circumferential wall 354, which has a generally v-shaped profile. It may be said that the annular channel 352 is defined in part by the wall 354 having the generally v-shaped profile. The satellite bearing 344 includes a second engagement feature 356 in the form of an enlarged head portion 358 of the satellite bearing 344, which is located at one end of a shank portion 360 of the satellite bearing 344.

[0102] The first and second engagement features 350, 356 are configured such that the first and second engagement features 350, 356 engage one another in order that the satellite bearing 344 supports the anode 304. The head portion 358 of the satellite bearing 344 (the head portion 358 forming part of the second engagement feature 356) includes an engagement surface 362, which has a generally arcuate profile. The engagement surface 362 (which has a generally arcuate profile) engages the generally v-shaped radially outer wall 354 of the annular channel 352, which forms part of the first engagement portion 350. Due to the fact that the generally arcuate profiled engagement surface is received by the generally v-shaped wall 354 of the annular channel 352 helps to correctly locate the channel relative to the engagement surface of the satellite bearing 344, and consequently to locate the anode 304 relative to the satellite bearing 344.

[0103] It will be appreciated that although Figure 8 only shows one of the satellite bearings 344, which supports the anode 304, all of the satellite bearings 344, which support the anode and cathode are substantially similar.

[0104] In the embodiment shown in Figures 5 to 8 the anode and cathode are each connected to a motor (not shown), which drives the anode and cathode such that they rotate as previously discussed.

[0105] The embodiment shown in Figures 5 to 8 is also configured such that the shank 360 of the satellite bearings 344 is rotatably mounted to the bearing base 343. Consequently, as the anode and cathode rotate, the contact between the first engagement portion 350 of the anode/cathode and the second engagement portion 356 of the satellite bearings 344 cause the satellite bearings 344 (including their shanks 360) to rotate within the bearing base 343.

[0106] In other embodiments, the shank 360 of the satellite bearing 344 may be fixed to the bearing base 343 and the second engagement portion 356 of the satellite bearing 344 (which includes the head 358 of the satellite bearing 344) may be rotatably mounted to the shank 360 of the satellite bearing 344. In this case, as the anode/cathode rotates, the head 358 of the satellite bearing 344 rotates relative to the shank 360 of the satellite bearing 344.

[0107] It will be appreciated that it is within the scope of the invention for the first engagement portion 350 of the anode/cathode and the second engagement portion 356 of the satellite bearing 344 may have any appropriate configuration. For example, within the presently described embodiment, the first engagement portion 350, which forms part of the anode/cathode, includes a wall that has a v-shaped profile; and the second engagement portion 356, which forms part of the satellite bearing 344, includes an engagement surface that has a generally arcuate profile. In other embodiments, the first engagement portion of the anode/cathode may include an engagement surface having a generally arcuate profile and the second engagement portion of the satellite bearing may include a wall having a generally v-shaped profile. Furthermore, it will be appreciated that the first engagement portion and second engagement portion may have any appropriate complementary shape/surface. For example, the first engagement portion and second engagement portions may comprise wall having a generally v-shaped, generally U-shaped or generally arcuate profile. In addition, it will be appreciated that in the present embodiment each satellite bearing 344 engages with a radially outer wall of the channel 352. In other embodiments each satellite bearing may engage with a radially inner wall of the channel having any appropriate complementary shape/surface.

[0108] Figure 9 shows a schematic representation of a portion of a radiation source according to a further embodiment of the present invention. In this embodiment the anode 400 and cathode 402 are each supported by three satellite bearings 404 such that the anode 400 and cathode 402 can rotate about a first axis 406 and a second axis 408 respectively.

[0109] This embodiment differs from the previously described embodiment in that the satellite bearings 404 engage a circumferential radially outer edge 410 (also referred to as the outer circumference) of the anode 400 and cathode 402 respectively so as to support the anode 400 and cathode 402. It will be appreciated that the surface of the outer circumference 410 of the anode and cathode and the surface of the satellite bearings 404 may be any appropriate corresponding surfaces provided that they allow the satellite bearings 404 and anode/cathode to rotatable engage one another.

[0110] However, it will be appreciated that, as discussed in relation to the previous embodiment, the surface of the outer circumferences 410 of the anode and cathode 400, 402 are used to transport the film of liquid fuel (not shown) from the fuel bath (not shown) to the pinch 412. Because of this, the anode, cathode and satellite bearings are configured such that the engagement between the anode/cathode and the satellite bearings does not prevent the surface of the outer circumference 410 of the anode/cathode from transporting the film of liquid fuel to the pinch 412. An example of a way in which this may be achieved is that the satellite bearings 404 may only contact a portion of the surface of the outer circumferences of the anode/cathode 400, 402. Consequently, a portion of the surface of the outer circumferences 410, which is not contacted by the satellite bearings 404, may be used to transport the film of liquid fuel to the pinch.

[0111] In some embodiments the anode and/or cathode and at least one satellite bearing may be arranged such that the liquid fuel lubricates the at least one satellite bearing. In some embodiments, the anode and/or cathode and at least one satellite bearing may be configured such that rotation of the anode and/or cathode causes liquid fuel to be transported to the at least one satellite bearing such that the liquid fuel lubricates the at least one satellite bearing.

[0112] Figures 10 and 11 show two further embodiments of the present invention that are modifications of the embodiments shown in Figures 5 to 8 and Figure 9 respectively. Within the embodiments shown in Figures 10 and 11, the generally disk-shaped anode and cathode of the embodiments shown in Figures 5 to 8 and Figure 9 have been replaced with anodes and cathodes that are generally annular.

[0113] Figure 10 shows a generally annular anode 304b and a generally annular cathode 302b, both of which are supported for rotation by satellite bearings 344b. As described in relation to the embodiment shown in Figures 5 to 8, the head 358b of each of the satellite bearings 344b is received within an annular channel 352b in each of the anode 304b and cathode 302b respectively. As before, a generally arcuate surface 362b of each of the satellite bearings 344b engages a wall 354b of the respective anode/cathode 304b, 302b which has a generally v-shaped profile.

[0114] Figure 11 shows a generally annular anode 400b and a generally annular cathode 402b that are each supported for rotation by three satellite bearings 404b that are substantially equi-angularly spaced about the axis of rotation of the respective anode 400b or cathode 402b. As discussed in relation to the embodiment of the invention shown in Figure 9, the satellite bearings 404b each engage an outer circumferential surface 410b of the respective anode/cathode 400b, 402b.

[0115] In certain applications of the present invention, the use of a generally annular anode and cathode may be advantageous as compared to the use of a generally diskshaped anode and cathode. For example, due to the fact that less material is used to manufacture an annular anode or cathode compared to manufacturing a generally disk-shaped anode or cathode, an annular anode/cathode may be cheaper and/or easier to manufacture, and may have less mass. Due to the fact that a generally annular anode or generally annular cathode has less mass than an anode/cathode that is generally disk-shaped, a generally annular anode/cathode will place less load on the bearing, which is supporting it. This may lead to an increase in bearing lifetime. Furthermore, due to the fact that generally annular anode/cathode will weigh less than a generally disk-shaped anode/cathode, a generally annular anode/cathode will require less energy than a generally disk-shaped anode/cathode in order to heat it to a given temperature. This is important because within the radiation source the anode and cathode must be heated to a temperature such that the film of liquid fuel does not solidify on the anode/cathode. In addition, due to the fact that generally annular anode/cathode will weigh less than a generally disk-shaped anode/cathode, a generally annular anode/cathode will require less energy than a generally disk-shaped anode/cathode in order to rotate the anode/cathode. Consequently, due to the fact that a generally annular anode/cathode will require less energy to rotate and to heat to the required temperature than a generally disk-shaped anode/cathode, the power requirements of a radiation source having a generally annular anode and a generally annular cathode may be less than that of a corresponding radiation source having a generally disk-shaped anode and a generally disk-shaped cathode.

[0116] Figure 12 shows a cross-section through an anode/cathode that may form part of a radiation source according to a further embodiment of the present invention. The anode/cathode shown in Figure 12 is a generally annular anode/cathode. The anode/cathode 500 may be supported by satellite bearings such that it may rotate about an axis of rotation according to any of the previously described embodiments. However, the satellite bearings that support anode/cathode 500 are not shown within Figure 12 so as to aid the simplicity of the Figure.

[0117] The generally annular anode/cathode 500 shown in Figure 12 differs from the annular cathode/electrodes shown in Figures 10 and 11 in that the anode/cathode 500 includes at least one fuel channel 502 that extends from a radially inner location with respect to the axis of rotation of the anode/cathode to a radially outer location with respect of the axis of rotation of the anode/cathode 500. In this case, the radially inner location is the radially inner circumferential surface 504 of the generally annular anode/cathode 500 and the radially outer location is the radially outer circumferential surface 506 of the generally annular anode/cathode 500.

[0118] In the embodiment shown in Figure 12 there are a plurality of fuel channels 502. It will be appreciated that only a few of the fuel channels 502 are shown in Figure 12. In practice, there may be any appropriate number of fuel channels and the fuel channels may be angularly spaced all the way around the anode/cathode 500. In the embodiment shown in Figure 12 the fuel channels 502 extend generally radially. In other embodiments, the at least one fuel passage 502 may take any appropriate path from the radially inner location to the radially outer location. For example, the at least one fuel passage may be at an angle to the radius or may be generally spiral shaped.

[0119] In this embodiment it can be seen that the radiation source comprises a fuel supply 508 configured to supply fuel (in this case liquid fuel) to the radially inner location (in this case the radially inner circumferential surface 504) of the anode/cathode 500.

[0120] Within Figure 12 the fuel supply 508 is shown schematically. The fuel supply 508 may take any appropriate form. For example, the fuel supply may be a bath as currently known provided that the level of the liquid fuel within the bath is sufficient so that it can reach the radially inner location (radially inner circumferential surface 504). In other embodiments, the fuel supply may take the form of a conduit that allows fuel from a location remote to the anode/cathode 500 to be conveyed to the radially inner location of the anode/cathode 500. For example, the fuel supply may be a pipe that is linked to a fuel reservoir, the pipe having an outlet that supplies fuel to the radially inner location of the anode/cathode. In other embodiments, the fuel supply 508 may comprise a fuel wire feed or a fuel pellet feed, in which a wire of the fuel material or pellets of the fuel material respectively are fed to a heater that melts the fuel material so that it can be supplied to the anode/cathode.

[0121] The use of a conduit to provide fuel to the anode/cathode as opposed to the use of a fuel bath may, in some applications of a radiation sources according to the present invention, provide an advantage. Fuel baths are such that a relatively large surface area of liquid fuel within the fuel bath is exposed to the surrounding atmosphere. Consequently, there may be a relatively large amount of fuel vapour that escapes from the surface of the liquid fuel. Fuel vapour escaping from the surface of the liquid fuel in the fuel bath may contaminate the surrounding atmosphere. This may lead to a reduction in the imaging performance of a lithographic apparatus to which the radiation source is attached and/or may result in the radiation source having to be cleaned so as to remove fuel deposits caused by the fuel vapour. Such cleaning may result in the radiation source having to be shut down and hence may reduce the potential amount of time for which the radiation source (and hence any device of which the radiation source forms part - for example a lithographic apparatus) to be reduced.

[0122] As the anode/cathode 500 rotates about the axis, a centrifugal force acts on the liquid fuel. This centrifugal force causes the liquid fuel to move through the fuel channels 502 from the radially inner location (radially inner circumferential surface 504) to the radially outer location (radially outer circumferential surface 506).

[0123] Once the liquid fuel reaches the radially outer location, the fuel may form beads 510 of fuel at the point where the fuel channels 502 reach the radially outer location.

[0124] In some applications, the beads of liquid fuel 510 may be advantageous as compared to a thin film of liquid fuel (as is known in the prior art), due to the fact that the beads 510 of fuel provide discrete amounts of fuel, properties (for example shape, volume and surface area) of which can be controlled. By controlling properties of discrete amounts of fuel it may be possible to precisely control the interaction between the fuel and the radiation provided to the fuel as part of an LDP process, and/or the interaction between the fuel (or modified fuel distribution) and the electrical discharge as part of an DPP (or LDP process). This may enable characteristics of the radiation producing plasma, and hence the radiation produced by the radiation producing plasma, to be controlled.

[0125] In some embodiments of the present invention the anode/cathode may include at least one fuel channel that extends from a radially inner location with respect to the axis of rotation of the anode/cathode to a radially outer location with respect of the axis of rotation of the anode/cathode; and the anode/cathode may not be supported by satellite bearings. These embodiments may also have a fuel supply configured to supply fuel to the radially inner location of the anode/cathode.

[0126] It will be appreciated that within the previously described embodiments the satellite bearings engage a circumferential radially outer edge (also referred to as the outer circumference) of the anode and cathode respectively so as to support the anode and cathode. In some embodiments of the invention, at least one satellite bearing may engage a circumferential radially inner edge (also referred to as the inner circumference) of the anode and/or cathode so as to support the anode and/or cathode. It will be appreciated that the surface of the inner circumference of the anode and/or cathode and the surface of the satellite bearings may be any appropriate corresponding surfaces provided that they allow the satellite bearings and anode and/or cathode to rotatable engage one another.

[0127] Figure 13 shows a portion of a radiation source according to a further embodiment of the present invention. The Figure shows a modification to a portion of the embodiment shown in Figures 5 to 8, and in Figure 10. Figure 13 shows six satellite bearings 344c that are mounted as two groups of three to separate bearing bases 343c. The bearing bases 343c are mounted to an interface plate 342c. Within the Figure, the three satellite bearings 344c that are mounted to the bearing base 343c on the right of the Figure support the anode (not shown). Similarly, the three satellite bearings 344c mounted to the bearing base 343c on the left of the Figure support the cathode (not shown). The anode and cathode are not shown within Figure 13 so that the satellite bearings 344c can be seen more clearly.

[0128] As previously discussed, the anode and cathode are supported by the satellite bearings 344c such that the anode can rotate about a first axis 336c and the cathode can rotate about an axis 334c. Within Figure 13 the axes 334c and 336c are indicated as crosses due to the fact that both axes are perpendicular to the plane of the Figure.

[0129] It can be seen that each of the satellite bearings 344c is mounted on its respective bearing base 343c via a pair of parallel rails 380c. The rails 380c enable each satellite bearing 344c to be moved within a plane perpendicular to the first axis 336c or second axis 334c. The satellite bearings 344c are also mounted to their respective bearing base 343c such that the satellite bearings are movable in a direction that is substantially parallel to the first axis 336c or second axis 334c.

[0130] It will be appreciated that it is within the scope of the invention for any appropriate arrangement may be used to enable a satellite bearing to be moved within a plane perpendicular to the first axis or second axis; and/or moved in a direction that is substantially parallel to the first axis or second axis.

[0131] Due to the fact that the satellite bearings are mounted such that they can be moved independently in a plane perpendicular to the first or second axis and/or in a direction that is substantially parallel to the first or second axis, this enables various characteristics of the anode and/cathode to be controlled. For example, the position of at least one of the satellite bearings 344c may be controlled by a controller (not shown) so as to control at least one of the following: 1. The position of the anode/cathode relative to the bearing base, interface plate or any other appropriate portion of the radiation source; 2. The position of the anode relative to the cathode (and vice versa) and consequently the geometry of the pinch defined between the anode and the cathode; 3. The pre-tension of the anode/cathode (or the stiffness of the anode/cathode with regards to rotation); and 4. The shape (and in particular the profile of the outer circumference) of the anode/cathode.

[0132] In some embodiments of a radiation source according to the present invention, the use of a generally annular anode/cathode in combination with satellite bearings that are movable in the manner discussed in relation to Figure 13 may be advantageous when compared to the use of a generally disk-shaped anode/cathode. This is because the generally annular anode/cathode may be more flexible than a generally disk-shaped anode/cathode. Consequently, it may be possible to control the position of at least one of the satellite bearings so as to give the anode/cathode a desired shape, and/or so as to impart a desired pre-tension into the anode/cathode. This may result in the ability to more completely control the characteristics of the anode/cathode and hence the characteristics of the pinch defined between the anode and cathode- thereby resulting in greater control over characteristics of the radiation produced by the radiation producing plasma at the pinch.

[0133] Within all of the previously discussed embodiments, instead of a motor mounted to the centre of the anode/cathode driving the anode/cathode for rotation about the relevant axis, the anode/cathode may be driven by a motor that is mechanically linked to at least one of the satellite bearings supporting the anode/cathode. That is to say, at least one satellite bearing supporting the anode/cathode may comprise a motor configured to drive the anode/cathode, whereby the satellite bearing is rotated by the motor and the satellite bearing imparts rotation to the anode/cathode.

[0134] By using one of the satellite bearings to drive the rotation of the anode/cathode, it is possible to move the drive unit (in this case a motor) further away from the pinch. This has several possible advantages. First, the drive unit, being located further from the pinch and a conventional central drive unit will enable a better view of the pinch. This may be advantageous when remote viewing apparatus (for example a video camera) is used to view the operation of the pinch. Secondly, because the drive unit is located further away from the pinch, the potential for the drive unit to block useful radiation that is produced by the radiation producing plasma is minimised. Thirdly, due to the fact that the drive unit is located further away from the pinch, it may be possible to encapsulate the drive unit so that it does not have to be in the same environment as the pinch. The environment of the pinch may be adverse to the operation of the drive unit, and hence the ability to encapsulate the drive unit from this environment may improve the operating performance of the drive unit.

[0135] The anode/cathode may be driven for rotation in any appropriate manner. For example, a belt drive or gear drive system may be used.

[0136] It will also be appreciated that although in the embodiments shown each have an anode/cathode that is supported by three satellite bearings that are equi-angularly spaced around the axis of rotation of the anode/cathode, any appropriate number of satellite bearings (such as 1, 2, 3 4, 5, 6 or more satellite bearings) may be used and any appropriate angular spacing between adjacent satellite bearings may be employed.

[0137] It will also be appreciated that the anode/cathode may be driven in any appropriate rotation direction.

[0138] Furthermore, the anode and/or cathode of any of the previously described embodiments may include a skimmer (which is well known to a person skilled in the art), which controls characteristics of the film of fluid fuel that is carried by the anode/cathode. For example, the skimmer may control the thickness of the fluid fuel film.

[0139] It will be appreciated that the embodiments of the invention described are configured such that the anode and cathode are located in the same plane and such that the axes of rotation of the anode and cathode are substantially parallel to one another. This may be advantageous in certain applications of a radiation source according to the present invention due to the fact that the geometry of the radiation source (and in particular the relative alignment between components of the radiation source) is less complex. However, in some embodiments of a radiation source according to the present invention, the anode/cathode may not lie in the same plane and/or the axes of rotation of the anode and cathode may be non-parallel to one another.

[0140] Although some embodiments of the invention have been described with tin being used as a fuel material, embodiments of the invention may use any suitable fuel material (e.g., Li or Xe).

[0141] 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.

[0142] 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.

[0143] While specific embodiments of the invention have been described above, it will be appreciated that the invention may be practiced otherwise than as described. For example, the invention may take the form of a computer program containing one or more sequences of machine-readable instructions describing a method as disclosed above, or a data storage medium (e.g., semiconductor memory, magnetic or optical disk) having such a computer program stored therein. 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 set out below. Other aspects of the invention are set out as in the following numbered clauses: 1. A radiation source comprising: an anode configured to rotate about a first axis; a cathode configured to rotate about a second axis; the anode and/or the cathode being supported by a bearing arrangement which includes a satellite bearing which is located at position which is spaced from the first axis or the second axis respectively.

2. A radiation source according to either clause 1, wherein the or each bearing arrangement includes a plurality of satellite bearings angularly spaced about the first axis or the second axis.

3. A radiation source according to clause 2, wherein the satellite bearings are substantially equi-angularly spaced about the first axis or the second axis.

4. A radiation source according to any preceding clause, wherein the anode and/or cathode includes a first engagement feature, and at least one satellite bearing includes a second engagement feature, the first and second engagement features being configured such that the first and second engagement figures engage one another such that the at least one satellite bearing rotatably supports the anode and/or cathode.

5. A radiation source according to clause 4, wherein the first engagement portion comprises an annular channel which is generally co-axial with the first or second axis.

6. A radiation source according to clause 5, wherein the annular channel is defined in part by a wall having a generally v-shaped, generally U-shaped or generally arcuate profile.

7. A radiation source according to either clause 5 or clause 6, wherein the second engagement feature includes an engagement surface having a generally arcuate profile.

8. A radiation source according to any preceding clause, wherein at least one satellite bearing of the or each bearing arrangement is arranged such that the at least one satellite bearing supports the anode or cathode at a location closer to a circumference of the anode or cathode than to the first axis or the second axis respectively.

9. A radiation source according to clause 8, wherein at least one satellite bearing of the or each bearing arrangement is arranged such that the at least one satellite bearing supports the anode or cathode at a location adjacent an inner or outer circumference of the anode or cathode.

fO. A radiation source according to any of the preceding clauses, wherein the anode and/or cathode is rotated by a motor via at least one satellite bearing.

11. A radiation source according to any previous clause, wherein the anode and/or cathode are/is generally disc or annular shaped.

12. A radiation source according to any proceeding clause, wherein the anode and/or cathode comprises at least one fuel channel which extends from a radially inner location with respect to the first or second axis, to a radially outer location with respect to said first or second axis.

13. A radiation source according to clause 12, the radiation source comprising a fuel supply configured to supply fuel to said radially inner location.

14. A radiation source according to any preceding clause, wherein the at least one satellite bearing of the or each bearing arrangement is movable in a plane perpendicular to the first or second axis.

15. A radiation source according to any preceding clause, wherein the at least one satellite bearing of the or each bearing arrangement is movable in a direction which is substantially parallel to the first or second axis.

16. A lithographic apparatus comprising the radiation source of any preceding clause, and further comprising: an illumination system configured to condition radiation produced by the radiation source to form a conditioned radiation beam; a support constructed to support a patterning device, the patterning device being capable of imparting the conditioned 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.

17. A method of producing radiation, the method comprising: providing a radiation source comprising an anode and a cathode; rotating the anode about a first axis; rotating the cathode about a second axis; and supporting the anode or the cathode by a bearing arrangement including a satellite bearing which is located at position which is spaced from the first axis or the second axis respectively.

18. A method according to clause 16, the method further comprising: controlling the position of the satellite bearing in a plane perpendicular to the first or second axis and/or in a direction which is substantially parallel to the first or second axis so as to control at least one of the following: the relative positioning between the anode or cathode and a portion of the radiation source which is not the anode or the catode; the relative positioning between the anode and the cathode; the geometry of the pinch defined between the anode and the cathode; the pre-tension of the anode or cathode; the stiffness of the anode or cathode with regards to rotation about the respective first or second axis; and the shape of the anode or cathode.

19. A radiation source comprising: an anode configured to rotate about a first axis; and a cathode configured to rotate about a second axis, wherein at least one of the anode and the cathode are supported by a bearing arrangement that includes a satellite bearing located at position spaced from the first axis or the second axis, respectively.

20. The radiation source of clause 19, wherein the bearing arrangement comprises a plurality of satellite bearings angularly spaced about the first axis or the second axis.

21. The radiation source of clause 20, wherein the satellite bearings are substantially equi-angularly spaced about the first axis or the second axis.

22. The radiation source of clause 20, wherein at least one of the anode and the cathode comprises a first engagement feature, and the at least one satellite bearing includes a second engagement feature, the first and second engagement features being configured such that the first and second engagement figures engage one another such that the at least one satellite bearing rotatably supports the anode and/or cathode.

23. The radiation source of clause 22, wherein the first engagement portion comprises an annular channel which is generally co-axial with the first or second axis.

24. The radiation source of clause 23, wherein the annular channel is defined in part by a wall having a generally v-shaped, generally U-shaped or generally arcuate profile.

25. The radiation source of clause 24, wherein the second engagement feature includes an engagement surface having a generally arcuate profile.

26. The radiation source of clause 20, wherein at least one satellite bearing is arranged to support the anode or cathode at a location closer to a circumference of the anode or cathode than to the first axis or the second axis respectively.

27. The radiation source of clause 26, wherein at least one satellite bearing is arranged to support the anode or cathode at a location adjacent an inner or outer circumference of the anode or cathode.

28. The radiation source of clause 19, wherein at least one of the anode and the cathode is rotated by a motor via at least one satellite bearing.

29. The radiation source of clause 19, wherein at least one of the anode and the cathode is generally disc or annular shaped.

30. The radiation source of clause 19, wherein at least one of the anode and the cathode comprises at least one fuel channel that extends from a radially inner location with respect to the first or second axis, to a radially outer location with respect to said first or second axis.

31. The radiation source of clause 30, the radiation source comprising a fuel supply configured to supply fuel to said radially inner location.

32. The radiation source of clause 20, wherein the at least one satellite bearing is movable in a plane perpendicular to the first or second axis.

33. The radiation source of clause 20, wherein the at least one satellite bearing is movable in a direction which is substantially parallel to the first or second axis.

34. A lithographic apparatus comprising: a radiation source having an anode configured to rotate about a first axis and a cathode configured to rotate about a second axis, wherein at least one of the anode and the cathode are supported by a bearing arrangement that includes a satellite bearing located at position spaced from the first axis or the second axis, respectively; an illumination system configured to condition radiation produced by the radiation source to form a conditioned radiation beam; a support constructed to support a patterning device, the patterning device being capable of imparting the conditioned 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.

35. A method of producing radiation, the method comprising: providing a radiation source comprising an anode and a cathode; rotating the anode about a first axis; rotating the cathode about a second axis; and supporting the anode or the cathode by a bearing arrangement including a satellite bearing which is located at position which is spaced from the first axis or the second axis respectively.

36. A method according to clause 34, the method further comprising: controlling the position of the satellite bearing in a plane perpendicular to the first or second axis and/or in a direction which is substantially parallel to the first or second axis so as to control at least one of the following: the relative positioning between the anode or cathode and a portion of the radiation source which is not the anode or the cathode; the relative positioning between the anode and the cathode; the geometry of the pinch defined between the anode and the cathode; the pre-tension of the anode or cathode; the stiffness of the anode or cathode with regards to rotation about the respective first or second axis; and the shape of the anode or cathode.

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.