WO2014121873A1

WO2014121873A1 - Radiation source for an euv optical lithographic apparatus, and lithographic apparatus comprising such a power source

Info

Publication number: WO2014121873A1
Application number: PCT/EP2013/076983
Authority: WO
Inventors: Johannes Damen; Hermannus LANGELER
Original assignee: Asml Netherlands B.V.
Priority date: 2013-02-08
Filing date: 2013-12-17
Publication date: 2014-08-14

Abstract

A radiation source apparatus is configured to generate a beam of radiation by excitation of a fuel into a plasma. The apparatus comprises a receiving structure (504) for capturing fuel debris during operation and a heating arrangement (524) for heating one or more surfaces ((520), (522)) of said receiving structure to a temperature sufficient to liquefy said fuel debris such that it can be made to flow along the surface to another part of the apparatus. The heating arrangement comprises one or more conductors (526) lying adjacent to conductive material of the receiving structure in the vicinity of said surfaces.

Description

RADIATION SOURCE FOR AN EUV OPTICAL LITHOGRAPHIC APPARATUS, AND LITHOGRAPHIC APPARATUS COMPRISING SUCH A POWER SOURCE

CROSS-REFERENCE TO RELATED APPLICATIONS

[0001 J This application claims the benefit of US provisional application 61 /762,566 which was filed on February 8^th, 2013, and which is incorporated herein in its entirety by reference.

FIELD

[0002] The present invention relates to a power source for a lithographic apparatus, and associated lithographic apparatus.

BACKGROUND

[0003] A lithographic apparatus is a machine that applies a desired pattern onto a substrate, usually onto a target portion of the substrate. A lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In that instance, a patterning device, which is alternatively referred to as a mask or a reticle, may be used to generate a circuit pattern to be formed on an individual layer of the IC. This pattern can be transferred onto a target portion (e.g. comprising part of, one, or several dies) on a substrate (e.g. a silicon wafer). Transfer of the pattern is typically via imaging onto a layer of radiation- sensitive material (resist) provided on the substrate. In general, a single substrate will contain a network of adjacent target portions that are successively patterned.

[0004] 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.

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

CD = k_l *— (1)

¹ NA

where λ is the wavelength of the radiation used, NA is the numerical aperture of the projection system used to print the pattern, k l 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 .

10005] 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 (LPP) sources, discharge-produced plasma (DPP) sources, or sources based on synchrotron radiation provided by an electron storage ring.

[0006] EUV radiation may be produced using a plasma. A radiation source apparatus for producing EUV radiation 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. 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 apparatus may include an enclosing structure or chamber arranged to provide a vacuum environment to support the plasma. Such a radiation source apparatus is typically termed a laser produced plasma (LPP) source.

[0007] As a by-product of the plasma generation, debris of the fuel material in the form of vapor, dust or droplets is ejected in many directions, as well as the wanted EUV radiation. The radiation source apparatus typically includes many structures and subsystems to manage this material, which otherwise would quickly contaminate optical surfaces and degrade performance. One such measure is to provide a receiving structure that surrounds the beam to intercept and trap the fuel debris. The structure may be heated to a temperature at which the debris will melt and run into drainage channels so that it can be removed from the environment, whether occasionally or continuously during operation. An example of such an apparatus is disclosed in published patent application US2008179548. To liquefy the debris in the case of tin as a fuel, the temperature of the receiving structure surface may be approximately 300°. [0008] To maximize the intercepting surface and discourage rebounding of debris back into the environment, the receiving structure typically has a convoluted surface, for example covered in fins or vanes or other local structural elements. The heating of all these elements to the correct temperature brings many challenges. The patent application US2008179548 mentions resistive heating, or alternatively fluid heating using water or liquid gallium. Because of the large number local structural elements requiring their own heating, the heating system becomes very complex. The prevailing method example of resistive heating requires a thermally conductive connection from each heating element into the receiving structure. Materials used for this element and connection must be EUV- and vacuum-compatible, limiting the choice of materials. Inefficiency in the transfer of heat to the structure implies that the elements themselves reach a much higher temperature. This brings further challenges for the selection of materials and reliability.

SUMMARY

10009] It is desirable to provide an alternative to resistive heating for surfaces of a receiving element in a plasma-based radiation source.

[0010] A radiation source apparatus has been provided according to a first aspect, which is configured to generate a beam of radiation by excitation of a fuel into a plasma, said apparatus comprising a receiving structure for capturing fuel debris during operation and a heating arrangement for heating one or more surfaces of said receiving structure to a temperature sufficient to liquefy said fuel debris such that it can be made to flow along the surface to another part of the apparatus, said heating arrangement comprising one or more conductors lying adjacent to conductive material of the receiving structure in the vicinity of said surfaces, and a source of alternating current to flow in said conductors, thereby to cause heating directly within said conductive material by induction.

[0011 ] The invention in a further aspect provides for a lithographic apparatus, comprising a radiation source of the first aspect, configured to generate a beam of EUV radiation.

[0012] Further features and advantages of the invention, as well as the structure and operation of various embodiments of the invention, 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 TH E DRAWINGS/FIGURES

[0013] Embodiments of the invention will now be described, by way of example only, with reference to the accompanying drawings, in which:

Figure 1 depicts schematically a lithographic apparatus having reflective projection optics;

Figure 2 is a more detailed view of the apparatus of Figure 1 including a plasma- based radiation source apparatus;

Figure 3 is an enlarged schematic view of the radiation source apparatus having a receiving structure including numerous local structural elements in the form of vanes;

Figure 4 schematically depicts the form of a single vane with a known form of resistive heating element;

Figure 5 illustrates the principle of inductive heating applied to the receiving structure of Figure 3 in embodiments of the present invention;

Figure 6 schematically depicts the form of a single vane with an inductive heating arrangement in accordance with a first embodiment of the present invention; and

Figure 7 schematically depicts the form of a single vane with an inductive heating arrangement in accordance with a second embodiment of the present invention.

[0014] 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.

DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS

[0015] Figure 1 schematically depicts a lithographic apparatus 100 including a source module SO according to one embodiment of the invention. The apparatus comprises:

an illumination system (illuminator) II. 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.

[0016] 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.

[0017] The support structure MT holds the patterning device MA in a manner that depends on the orientation of the patterning device, the design of the lithographic apparatus, and other conditions, such as for example whether or not the patterning device is held in a vacuum environment. The support structure can use mechanical, vacuum, electrostatic or other clamping techniques to hold the patterning device. The support structure may be a frame or a table, for example, which may be fixed or movable as required. The support structure may ensure that the patterning device is at a desired position, for example with respect to the projection system.

[0018] 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.

[0019] 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 matri 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.

[0020] 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.

[0021] As here depicted, the apparatus is of a reflective type (e.g. employing a reflective mask).

[0022] 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.

[0023J Referring to Figure 1 , the illuminator IL receives an extreme ultra violet radiation beam from the source module SO. Methods to produce EUV light include, but are not necessarily limited to, converting a material into a plasma state that has at least one element, e.g., xenon, lithium or tin, with one or more emission lines in the EUV range. In one such method, often termed laser produced plasma ("LPP") the required plasma can be produced by irradiating a fuel, such as a droplet, stream or cluster of material having the required line- emitting element, with a laser beam. The source module SO may be part of an EUV radiation source apparatus including a laser, not shown in Figure 1, for providing the laser beam exciting the fuel. The resulting plasma emits output radiation, e.g., EUV radiation, which is collected using a radiation collector, disposed in the source module. The laser and the source module may be separate entities, for example when a C02 laser is used to provide the laser beam for fuel excitation.

[0024] In such cases, the laser is not considered to form part of the lithographic apparatus and the radiation beam is passed from the laser to the source module with the aid of a beam delivery system comprising, for example, suitable directing mirrors and/or a beam expander. In other cases the source may be an integral part of the source module, for example when the source is a discharge produced plasma EUV generator, often termed as a DPP source.

[0025[ 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. [0026] 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 M l , M2 and substrate alignment marks P I , P2.

[0027] The depicted apparatus could be used in at least one of the following modes:

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

2. In scan mode, the support structure (e.g. mask table) MT and the substrate table WT are scanned synchronously while a pattern imparted to the radiation beam is projected onto a target portion C (i.e. a single dynamic exposure). The velocity and direction of the substrate table WT relative to the support structure (e.g. mask table) MT may be determined by the (de-)magnification and image reversal characteristics of the projection system PS.

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

[0028] Combinations and/or variations on the above described modes of use or entirely different modes of use may also be employed.

10029] Figure 2 shows an embodiment of the lithographic apparatus in more detail, including a source module SO, the illumination system II., and the projection system PS. The source module in this example comprises a radiation source apparatus 42 of the type that uses a laser-produced plasma as a radiation source. EUV radiation may be produced by a gas or vapor, for example Xe gas, Li vapor or Sn vapor in which a very hot plasma is created to emit radiation in the EUV range of the electromagnetic spectrum. The very hot plasma is created by causing an at least partially ionized plasma by, for example, optical excitation using C0₂ laser light. Partial pressures of, for example, 10 Pa of Xe, Li, Sn vapor or any other suitable gas or vapor may be required for efficient generation of the radiation. In an embodiment, Sn is used to create the plasma in order to emit the radiation in the EUV range.

[0030] The radiation source apparatus 42 embodies the function of source SO in the apparatus of Figure 1 . Apparatus 42 comprises a source chamber 47, in this embodiment not only substantially enclosing a source of EUV radiation, but also collector 50 which, in the example of Figure 2, is a normal-incidence collector, for instance a multi -layer mirror.

[0031 ] As part of an LPP radiation source, a laser system 61 (described in more detail below) is constructed and arranged to provide a laser beam 63 which is delivered by a beam delivering system 65 through an aperture 67 provided in the collector 50. Also, the apparatus includes a target material 69, such as Sn or Xe, which is supplied by target material supply 71. The beam delivering system 65, in this embodiment, is arranged to establish a beam path focused substantially upon a desired plasma formation position 73.

[0032] In operation, the target material 69, which may also be referred to as fuel, is supplied by the target material supply 71 in the form of droplets. When such a droplet of the target material 69 reaches the plasma formation position 73, the laser beam 63 impinges on the droplet and an EUV radiation-emitting plasma forms inside the source chamber 47. In the case of a pulsed laser, this involves timing the pulse of laser radiation to coincide with the passage of the droplet through the position 73. As mentioned, the fuel may be for example xenon (Xe), tin (Sn) or lithium (Li). These create a highly ionized plasma with electron temperatures of several 10's of eV. Higher energy EUV radiation may be generated with other fuel materials, for example Tb and Gd. The energetic radiation generated during de- excitation and recombination of these ions includes the wanted EUV radiation which is emitted from the plasma at position 73. The plasma formation position 73 and the aperture 52 are located at first and second focal points of collector 50, respectively and the EUV radiation is focused by the normal -incidence collector 50 onto the intermediate focus point IF.

[0033] The beam of radiation emanating from the source chamber 47 traverses the illumination system IL via so-called normal incidence reflectors 53, 54, as indicated in Figure 2 by the radiation beam 56. The normal incidence reflectors direct the beam 56 onto a patterning device (e.g. reticle or mask) positioned on a support (e.g. reticle or mask table) MT. A patterned beam 57 is formed, which is imaged by projection system PS via reflective elements 58, 59 onto a substrate carried by wafer stage or substrate table WT. More elements than shown may generally be present in illumination system IL and projection system PS. For example there may be one, two, three, four or even more reflective elements present than the two elements 58 and 59 shown in Figure 2. Radiation collectors similar to radiation collector 50 are known from the prior art.

[0034] As the skilled reader will know, reference axes X, Y and Z may be defined for measuring and describing the geometry and behavior of the apparatus, its various components, and the radiation beams 55, 56, 57. At each part of the apparatus, a local reference frame of X, Y and Z axes may be defined. The Z axis broadly coincides with the direction of optical axis O at a given point in the system, and is generally normal to the plane of a patterning device (reticle) MA and normal to the plane of substrate W. In the source collector module 42, the X axis coincides broadly with the direction of fuel stream (69, described below), while the Y axis is orthogonal to that, pointing out of the page as indicated in Figure 2. On the other hand, in the vicinity of the support structure MT that holds the reticle MA, the X axis is generally transverse to a scanning direction aligned with the Y axis. Arrows 80 and 82 indicate the scanning directions of the reticle MA and the substrate W, respectively. For convenience, in this area of the schematic diagram Figure 2, the X axis points out of the page, again as marked. These designations are conventional in the art and will be adopted herein for convenience. In principle, any reference frame can be chosen to describe the apparatus and its behavior.

[0035] In addition to the wanted EUV radiation, the plasma produces other wavelengths of radiation, for example in the visible, UV and DUV range. There is also IR radiation present from the laser beam 63. The non-EUV wavelengths are not wanted in the illumination system II. and projection system PS and various measures may be deployed to block the non- EUV radiation. As schematically depicted in Figure 2, a transmissive SPF may be applied upstream of virtual source point IF. Alternatively or in addition to such a filter, filtering functions can be integrated into other optics. For example a diffractive filter can be integrated in collector 50 and/or mirrors 53, 54 etc., by provision of a grating structure tuned to divert the longer, IR radiation away from the virtual source point IF. Filters for IR, DUV and other unwanted wavelengths may thus be provided at one or more locations along the paths of beams 55, 56, 57, within source module ( apparatus 42), the illumination system II. and/or projection system PS. [0036] Alternative plasma-based sources may be used in place of that illustrated in Figures 2 and 3. In one alternative LPP source, the main pulse laser beam is directed onto the fuel droplet from the direction of the intermediate focus point IF, such that the collected EUV radiation is that which is emitted generally in the direction from which the main laser pulse was received. The main pulse beam may in that case be delivered to a plasma generation site via at least one optical element (such as a lens or folding mirror). The EUV radiation may be collected by a grazing incidence collector such as those used in discharge produced plasma (DPP) sources. Various ancillary components may be included in practice. For example a debris trap may comprise one or more stationary foil traps and/or a rotating foil trap.

[0037] To deliver the fuel, which for example is liquid tin, a droplet generator or target material supply 71 is arranged within the source chamber 47, to fire a stream of droplets towards the plasma formation position 73. In operation, laser beam 63 may be delivered in synchronization with the operation of target material supply 71 , to deliver impulses of radiation to turn each fuel droplet into a plasma. The frequency of delivery of droplets may be several kilohertz, or even several tens or hundreds of kilohertz. Typically, in present arrangements, a laser beam 63 may be delivered by a laser system 61 in at least two pulses: a pre pulse PP with limited energy is delivered to the droplet before it reaches the plasma location, in order to deform (change geometry) or vaporize the fuel material into a preferably pancake or cigar like shape or a small cloud (preconditioning the droplet), and then a main pulse MP of laser energy is delivered to the cloud at the desired location, to generate the plasma. In a typical example, the diameter of the plasma is about 2-3 mm. A trap 72 is provided on the opposite side of the enclosing structure 47, to capture fuel that is not, for whatever reason, turned into plasma.

[0038J Laser system 61 may be for example the MO PA (Master Oscillator Power Amplifier) type. Such a laser system 61 includes a "master" laser or "seed" laser, followed by a power amplifier system PA, for firing a main pulse of laser energy towards an expanded droplet cloud. A beam delivery system 24 is provided to deliver the laser energy 63 into the source chamber 47. Laser system 61 , target material supply 71 and other components can be controlled by a controller (not shown separately). The controller performs many control functions, and has sensor inputs and control outputs for various elements of the system. Sensors may be located in and around the elements of apparatus 42, and optionally elsewhere in the lithographic apparatus.

[0039] Numerous additional components may be present in a typical apparatus, though not illustrated here. These include arrangements for reducing or mitigating the effects of contamination within the enclosed vacuum, for example to prevent deposits of fuel material damaging or impairing the performance of collector 50 and other optics. Various systems for the delivery and extraction of gases to control the pressure and composition of the environment will also be included, as well as to create gas flows to steer contaminants away from the most critical and/or vulnerable locations. For example, hydrogen gas in molecular and/or atomic form may be used, due to its cleaning properties. Other features present but not described in detail are all the sensors, controllers and actuators involved in controlling of the various components and sub-systems of the lithographic apparatus.

[0040] Figure 3 shows an enlarged schematic view of the radiation source apparatus 42, and in particular shows a receiving structure 300 which surrounds the EUV beam 55 within source chamber 47. Receiving structure 300 can take many forms but typically comprises a frusto-conical wall 302 lined with local structural elements presenting local receiving surfaces to entrap fuel material ejected from the plasma. The local structural elements in this example comprise vanes 304 running obliquely around the inside of the conical wall. There may be for example 50, 60 or more. As explained in the introduction, the vanes will be heated by means to be described, so that fuel debris does remain solidly attached to the surfaces, but melts and forms droplets that will run along and between the vanes to be collected and removed outside the chamber 47. The fuel will be assumed to be tin, for the sake of this example. The vanes and other channels are shaped and oriented to guide the falling tin to a drain 306 and so into a tin collector 308. The drains and optionally the collector may be heated also, to maintain liquid flow in the tin material. In an embodiment based on tin as fuel, the vanes (and other heated surfaces such as drains) are to be maintained at around 300°C. The same tin collector 308 may for example be shared with the trap 72 connected by another drain 310. The tin collector may be inside or outside the chamber 47. Collected tin may be purified and recycled to the droplet generator 71 , or discarded for other uses.

[00 1 ] Figure 4 illustrates an individual vane 304 in a current design of receiving structure 300. Figure 4(a) is a side view, while Figure (b) is an end view. The vane is formed effectively from a piece of sheet metal that is folded in a V-shape to form walls 320, 322 as shown. In the space between the walls there is a heater 324, which is a rod with a coil of resistance wire inside. Through external connections 326 a DC-current 328 is injected into the resistance wire that causes the wire and rod 324 to heat up. The heat is radiated to the vane. Heaters for multiple vanes (not shown in Figure 4) could be connected in series, or in parallel, or they may be supplied individually. This cause the heater rod to warm up. Heaters for multiple vanes (not shown in Figure 4) could be connected in series, or in parallel, or they may be supplied individually.

[0042] Since all this is located in a near-vacuum environment, heat transfer by conduction or convection is not available (at least, it is extremely inefficient). In order to heat the vane up to a desired temperature such as 300°C, it may be required to have the heater rod at a much higher temperature, for example 600°C. This is because heat transfer by radiation only becomes efficient if the temperature differences are high. Thermal conductive filler materials used in other applications to assist heat transfer will generally be unsuitable for use in the vacuum environment, for example due to outgassing.

[0043] The Figure 4 arrangement is used currently in EUV radiation source apparatuses for commercial lithographic apparatus. The indirect mode of heating that it uses suffers from some limitations, however. Heat transfer by radiation alone is difficult, as explained. The efficiency of heating is relatively low, and power consumption higher than would be liked. Use of standard PTFE cable insulation is impossible because of the high temperature on wiring. This patent application discloses a direct heating concept by which electrical power is dissipated as heat in the vane itself rather than a separate element. Thereby the vane is the heat source as well and problems of heat transfer and high temperatures can be avoided. A principle of this direct heating is that in some way a current is injected in the vane material. Due to the ohmic resistance of the vane material, heat is generated within the walls 320, 322. One option considered for direct heating is to pass a current directly through the vane by connecting it to a current source. This option presents difficulties in that each vane would need to be is not electrically isolated from the wall 302 of the receiving structure generally. The inventors propose to cause direct heating of the vane material by inducing a current in the vane using a magnetic field.

[0044] Figure 4 illustrates the principle of inductive heating applied in embodiments of the present invention. Metal part 402 represents material of the vane 304 or other structural element to be heated. An induction coil comprising conductive turns 404 is connected via lines 406 to a source 408 of high frequency alternating current. The coil is placed close to the metal part 402, and the oscillating magnetic field B induces eddy currents 410 in the metal. These eddy currents cause ohmic heating through the resistance of the metal of part 402. Because the heat is generated in the material of the part, no conduction, radiation or the like is needed to achieve the heating to 300°C.

[0045] Figure 6 shows the concept of inductive heating applied to a vane 504 which is the same in form as vane 304 in the known apparatus of Figure 4. Walls 520, 522 correspond to walls 320 and 322, and the vane may indeed be identical in all respects to the known vane. As a point of difference, however, and inductive heating coil 524 replaces resistive element 324. Conductors 526 lead AC current from alternating current source 528 to the coil. Coil 524 comprises turns similar to those illustrated in Figure 5, lying close behind the walls 520, 522. As seen in Figure 6 (b), the coil may be formed by turns of foil or wire conductor on an insulating substrate 530, like a printed circuit board. The conductors of the coils may be copper for example. In other embodiments the coil may be self-supporting.

|0046] Figure 7 illustrates another embodiment which is similar to that of Figure 6, except that a few smaller coils 524 are be used instead of one large coil. The multiple coils are connected in parallel to reduce the working voltage. Other variations can be envisaged.

[0047] With the above embodiments and concepts in mind, direct inductive heating of the vane or other element or the receiving structure can be used to obtain one or more of the following benefits compared with indirect heating. Temperatures can be lower, given that the highest temperature is that of the vane 504 itself, which is 300°C. These allow the use of more common conductor and insulation materials like copper, PTFE and FEP. All heat in the vane is generated in the vane and not in another part. As a result, less power may be needed (in the indirect concept there is a separate heater which is heated up as well).

|0048] Various design considerations are identified that can be used to achieve desired performance characteristics. For example, to achieve efficient magnetic coupling, the vane 504 or other part, the metal to be heated can be made of a conductor that has a high magnetic permeability μ. The coil can placed close to the vane in order to create an efficient magnetic coupling. Due to the skin-effect, the induced eddy-current is confined to the surface of the vane that is facing the coil. The skin-effect increases the resistance so that more heat is generated. The skin-effect is related to the frequency of the current: the higher the frequency, the higher the skin-effect resulting in a higher resistance of the part of the vane in which the eddy currents flow.

|0049] The skin-effect is also related to the magnetic permeability μ of the vane: this should can be made high to benefit from the skin-effect at lower frequencies. If desired, a so- called mu-metal can be used for the metal part. Typical frequencies are between 10 kHz and 100 kHz.

[0050] A higher current in the coil results in a higher eddy current and, hence, more heating. More windings result also in a higher eddy current since the B-field is made larger while allowing lower currents in the coil. However, more windings also result in a higher voltage. In the near-vacuum environments within EUV apparatus a phenomenon known as Townsend breakdown or Paschen breakdown arises when quite modest voltages are present. Consequently the voltages used to drive the coils should be kept low (for example below 100 V, or below 1 10 V or 90 V). Other things being equal, voltage in the coil is related to the following:

• The frequency: the higher the frequency the higher the voltage. This is due to the impedance of the coil. This impedance is related to the coil conductor cross section (resistance) as well as the number of windings (inductance).

• The amount of heat generated in the vane: more heat yields more voltage due to the conservation law of power.

[0051 ] The parallel connection of several smaller coils as illustrated in Figure 7 can be useful in achieving the desired heating power without exceeding the breakdown voltage levels.

[0052] Another benefit of the skin effect and the construction shown in Figures 6 and 7 is that the current in the vane metal flows in the inside surface of the walls 520, 522, and not on the outside. Thus, the vane acts as a shield so that no magnetic fields arise in the environment outside the vane.

[0053] An additional source of heat is hysteresis loss in the metal part. This can be exploited if one keeps the temperature below the Curie temperature of the metal. Above the Curie temperature the material becomes paramagnetic so that hysteresis loss does not occur. Curie temperatures for some metals are as follows: Iron: 770°C, Nickel: 358°C and Cobalt: 1 130°C (from Wikipedia).

[0054] In a good design, one will also control the amount of heat generated in the conductive material of coils 524. The dissipation in the coil should be kept low so that 'ordinary' insulation materials can be used. Heating of the coil is caused by the following:

• Ohmic resistance of the coil. The coil should be made of a good conductor, like copper or aluminum.

• The skin-effect in the coil itself, which cause the current in the coil to become confined too. This effect is reduced by using a flat conductor in the coil.

• The heat that is transported from the vane to the coil, either by conduction or radiation (convection is not available since there is no gas). Thermal insulation can be applied between vane and coil.

[0055] If the coil can be kept at a temperature below 150°C then copper wire insulated with FEP or PTFE can be used for the coil. The temperature can be increased to 200°C if silver-plated copper wires are used. [0056] These and many other engineering details can be worked out by the skilled person when putting the invention into practice in a given application. The designer may consider factors such as:

• The static and dynamic relationship between voltages, currents and frequencies in the coil and the amount of heat generated in the vane, which is also related to the vane material. This also should be of use in defining a suitable control algorithm.

|0057] · The amount of parasitic heat generated in the coil and possible materials of the coil.

[0058] · The way the coil is fixated in the vane to avoid parasitic forces and movements.

[0059] · The way the connection is made from the coil to the electronics outside the vacuum

[0060] · Overheating protection of the coil so that it does not burn out due to a calamity.

Conclusion

Taking the above considerations into account, the disclosure enables the provision of a heating mechanism for receiving structures for fuel debris that solves the problems of obtaining heat transfer in a vacuum. Embodiments of the invention can have high efficiency and hence low power consumption, compared to conventional indirect heating. High internal temperatures are avoided on internal components and interconnects, allowing for example use of standard PTFE cable insulation.

[00611 While the concepts disclosed herein have been described specifically in combination with LPP sources, they are also applicable to other types of sources, such as DPP sources. 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] 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.

[0063] Further, the radiation source described may be applied in radiation source arrangements for EUV radiation in other optical systems besides lithographic apparatus. An example would be a metrology apparatus using EUV radiation to benefit from the short wavelengths.

[0064] While specific embodiments of the invention have been described above, it will be appreciated that the invention may be practiced otherwise than as described. The descriptions above are intended to be illustrative, not limiting. Thus it will be apparent to one skilled in the art that modifications may be made to the invention as described without departing from the scope of the claims set out below.

Claims

CLAIMS:

1 . A radiation source apparatus configured to generate a beam of radiation by excitation of a fuel into a plasma, said apparatus comprising a receiving structure for capturing fuel debris during operation and a heating arrangement for heating one or more surfaces of said receiving structure to a temperature sufficient to liquefy said fuel debris such that it can be made to flow along the surface to another part of the apparatus, said heating arrangement comprising one or more conductors lying adjacent to conductive material of the receiving structure in the vicinity of said surfaces, and a source of alternating current to flow in said conductors, thereby to cause heating directly within said conductive material by induction.

2. An apparatus as claimed in claim 1 wherein said conductor comprises a coil having a plurality of turns.

3. An apparatus as claimed in claim 2 wherein said coil is substantially flat and positioned at the back side of a conductive part carrying said surface.

4. An apparatus as claimed in any preceding claim wherein said conductor is one of a plurality of coils arrayed across the back side of said conductive part.

5. An apparatus as claimed in any preceding claim wherein said conductor is flat in cross-section and supported by an insulating substrate.

6. An apparatus as claimed in any preceding claim where said receiving structure comprises a plurality of local structural elements and each element is provided with one or more conductors for causing heating of the element by induction.

7. An apparatus as claimed in claim 6 wherein said at least some of said elements comprise elongate vanes running substantially parallel to one another and at angles oblique to a direction of said radiation beam.

8. An apparatus as claimed in claim 7 wherein each vane has two metal sides and said conductor or conductors is or are provided in a space between said sides.

9. An apparatus as claimed in any preceding claim wherein said conductive material of the structure has a magnetic permeability greater than, at least on a side facing said conductor or conductors.

10. An apparatus as claimed in any preceding claim wherein said heating arrangement is configured to heat said surface to a temperature in the range 250° to 500°C, for example a temperature above 280°C and below 350°C.

1 1. An apparatus as claimed in any preceding claim wherein said source of alternating current is configured to deliver said current using a voltage less than 1 10 volts, optionally less than 100 volts or les than 90 volts.

1 2. An apparatus as claimed in any preceding claim wherein said receiving structure comprises a substantially cylindrical or frusto-conical structure surrounding a path of said radiation beam, and lined with a plurality of local structural elements, each element being provided with one or more conductors for causing heating of the vane by induction.

13. A radiation source apparatus as claimed in any preceding claim wherein the fuel is excited by laser radiation to generate said beam of EUV radiation

14. A lithographic apparatus, comprising a radiation source as claimed in any preceding claim, configured to generate a beam of EUV radiation, and EUV optical systems configured to receive said beam and to use it to transfer a pattern from a patterning device to a substrate.

15. An optical apparatus, comprising a radiation source as claimed in any preceding claim, configured to generate a beam of EUV radiation, and EUV optical systems configured to receive and condition the beam and to deliver the beam to a target location.