NL2010950A - Lithographic apparatus. - Google Patents

Description

LITHOGRAPHIC APPARATUS

FIELD

[0001] The present invention relates to a lithographic apparatus and to components thereof, including a collector module, suitable for use in conjunction with, or forming part of, a radiation source, and an infra-red detector.

BACKGROUND

[0002] A lithographic apparatus is a machine that applies a desired pattern onto a substrate, usually onto a target portion of the substrate. A lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In that instance, a patterning device, which is alternatively referred to as a mask or a reticle, may be used to generate a circuit pattern to be formed on an individual layer of the IC. This pattern can be transferred onto a target portion (e.g., comprising part of, one, or several dies) on a substrate (e.g., a silicon wafer). Transfer of the pattern is typically via imaging onto a layer of radiation-sensitive material (resist) provided on the substrate. In general, a single substrate will contain a network of adjacent target portions that are successively patterned.

[0003] Lithography is widely recognized as one of the key steps in the manufacture of ICs and other devices and/or structures. However, as the dimensions of features made using lithography become smaller, lithography is becoming a more critical factor for enabling miniature IC or other devices and/or structures to be manufactured.

[0004] A theoretical estimate of the limits of pattern printing can be given by the Rayleigh criterion for resolution as shown in equation (1):

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

[0005] In order to shorten the exposure wavelength and, thus, reduce the minimum printable size, it has been proposed to use an extreme ultraviolet (EUV) radiation source. EUV radiation is electromagnetic radiation having a wavelength within the range of 5-20 nm, for example within the range of 13-14 nm. It has further been proposed that EUV radiation with a wavelength of less than 10 nm could be used, for example within the range of 5-10 nm such as 6.7 nm or 6.8 nm. Such radiation is termed extreme ultraviolet radiation or soft x-ray radiation. Possible sources include, for example, laser-produced plasma sources, discharge plasma sources, or sources based on synchrotron radiation provided by an electron storage ring.

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

[0007] As well as generating EUV radiation (or whichever radiation is intended to be generated), unwanted and potentially damaging radiation may also be generated at, or reflected from, the plasma formation location (e.g., off the plasma or the fuel target), for example in the form of infrared radiation. For example, and most importantly, infrared radiation (e.g., laser radiation) used to form the plasma may reflect from the fuel target or the plasma. Without any preventative action, the infrared radiation will have the same beam path as EUV radiation that is generated. The infrared radiation may damage sensitive optical components along the beam path, which components are used to condition (e.g., shape and/or direct) the EUV radiation. Alternatively or additionally, the infrared radiation may pass onto the substrate, and cause distortion of that substrate, which could have an adverse affect on patterns applied to that substrate.

[0008] One way of attempting to prevent infrared radiation from passing onto and through sensitive parts of the lithographic apparatus might be to provide a diffractive element in a beam path between the plasma formation location and, for example, a focal point of a collector used to collect and focus the radiation that is generated/reflected from the plasma formation location. As known in the art, the diffractive element may take the form of, or be part of, a zone plate or a spectral purity filter, or the diffractive element could be part of the collector itself (i.e., in the form of a grating collector). The diffractive element is configured to ensure that EUV radiation reflected off or passing through the diffractive element is substantially unaffected by the diffractive element, while at the same infrared radiation in substantially diffracted in order to prevent at least some (and ideally most of, or even all) infrared radiation from passing through the focal point, and onto and through sensitive parts of the lithographic apparatus (e.g., the illuminator, projection system or the like).

[0009] Ordinarily such a diffractive element would be configured to ensure that a majority of infrared radiation is diffracted into diffraction orders other than the m = 0 order (zeroth order). This is because the m = 0 diffracted order radiation will have the same beam path as EUV radiation that is generated, and will thus pass into and through the lithographic apparatus. However, even with this preventative measure infrared radiation will still be diffracted into the m = 0 order and pass onto and through the lithographic apparatus.

[0010] Under normal or ideal operating conditions it may well be that the intensity of infrared radiation in the m = 0 diffracted order is insufficient to cause damage or significant damage to the components of the lithographic apparatus, or to adversely affect the patterning of substrate. There will be circumstances, however, where this is not the case - in the generation of radiation, high power density infrared radiation spots may be produced in the radiation source (e.g., in the generation of the plasma and/or in the radiation reflected from the plasma/fuel target), which will be reflected by the collector onto and through the lithographic apparatus. These may be referred to as "hotspots." These hotspots might be sufficient in intensity to cause damage to sensitive optical components of the lithographic apparatus and/or to adversely affect the patterning of substrates. Potential damage can be severe, for example resulting in damage to multilayer coated mirrors of the lithographic apparatus. Repairing and/or replacing optical components such as multilayer mirrors may be extremely time consuming (e.g., at least in terms of apparatus downtime) and/or require significant financial investment.

SUMMARY

[0011] It is desirable to obviate or mitigate at least one problem of the prior art, whether identified herein or elsewhere, or to provide an alternative to existing apparatus or methods.

[0012] According to a first aspect of the present invention, there is provided a collector module for a radiation source, the collector module comprising: a collector for collecting radiation generated by a radiation generating plasma at a plasma formation location, and for directing at least a portion of the generated radiation to a focal point; a structure adjacent to and upstream of the focal point, the structure extending at least partially around an expected position of a beam comprising the at least a portion of the collected radiation; a diffractive element located in a beam path between the plasma formation location and the focal point, the diffractive element being arranged to diffract infrared radiation that is reflected from the plasma formation location, such that when the plasma formation location is at an intended location, m = +1 order diffracted infrared radiation is directed towards a first region of the structure and away from passing through the focal point, and m = -1 order diffracted infrared radiation is directed towards a second region of the structure and away from passing through the focal point; the first region and second region of the structure forming part of a system for determining a property of the infrared radiation.

[0013] The system may comprise a plurality of sensors. The first region and second region may each be or comprise at least one of the plurality of sensors.

[0014] The plurality of sensors may extend around the expected position of the beam. The plurality of sensors may form a substantially segmented ring of sensors.

[0015] The system may comprise one or more surfaces for receiving infrared radiation, and one or more infrared sensitive cameras for inspecting the one or more surfaces. The first region and second region may each be or comprise one of those surfaces.

[0016] The diffractive element may be a part of the collector, such that the collector is a grating collector.

[0017] The diffractive element may be, or may form part of, or be, a zone plate or a spectral purity filter (transmissive or reflective).

[0018] The system for determining a property of the infrared radiation may be in connection with a controller, the controller being arranged to provide a response to a certain property being detected, or a certain level of a property being detected. In one example, the response may be: an issuance of a warning; and/or to change the operating state of a radiation source of which the collector module forms a part; and/or prevent radiation being generated by a radiation source of which the collector module forms a part; and/or to control a position or orientation of one or more parts of the radiation source to ensure that m = 0 (and/or m = 1, and/or m = -1) diffracted order radiation is prevented from passing through the focal point.

[0019] The property that determined is one or more of: an intensity or (resulting) temperature ofm = +l,m=-l,orm = 0 order diffracted radiation (which includes an intensity distribution, or location, or position, or shape of the m = +1, m = -1, or m = 0 order diffracted radiation); and/or a change in intensity or (resulting) temperature ofm-+l,m--l, or m = 0 order diffracted radiation (which includes a change in intensity distribution, or location, or position, or shape of the m = +1, m = -1, or m = 0 order diffracted radiation); and/or a position of a point of origin of the reflected infrared radiation; and/or a change in position of a point of origin of the reflected infrared radiation.

[0020] The collector may be a normal incidence collector.

[0021] According to another aspect of the invention, there is provided a collector module for a radiation source, the collector module comprising: a collector for collecting radiation generated by a radiation generating plasma at a plasma formation location, and for directing at least a portion of the generated radiation to a focal point; and an infrared radiation sensitive camera for inspecting a collection surface of the collector.

[0022] The infrared sensitive camera may be arranged (e.g., located and/or orientated) to receive infrared radiation emitted by the collector. Additionally, and optionally, the infrared sensitive camera may be arranged such that infrared radiation reflected by the collector is directed away from (i.e., not received by) the infrared sensitive camera.

[0023] The collector may be a normal incidence collector.

[0024] According to another aspect of the present invention, there is provided a radiation source comprising the collector module of the first or second aspect of the invention. The radiation source might further comprise: a fuel stream generator configured to generate a stream of fuel and direct that stream towards the plasma formation location; and/or wherein the radiation source further comprises, or is arranged to received laser radiation from, a laser configured to direct laser radiation at a fuel at the plasma formation location to generate, in use, the radiation generating plasma.

[0025] According to yet another aspect of the present invention, there is provided a lithographic apparatus comprising: an illumination system for providing a radiation beam; a patterning device for imparting the radiation beam with a pattern in its cross-section; a substrate holder for holding a substrate; a projection system for projecting the patterned radiation beam onto a target portion of the substrate, and wherein the lithographic apparatus further comprises, or is in connection with, the collector module or radiation source of any preceding clause.

[0026] According to yet another aspect of the present invention, there is provided a method of determining a property of infrared radiation reflected from a plasma formation location in a collector module of a radiation source, the method comprising: diffracting infrared radiation that is reflected, such that when the plasma formation location is at an intended location, m = +1 order diffracted infrared radiation is directed away from passing through a focal point of the collector, and m = -1 order diffracted infrared radiation is directed away from passing through the focal point; detecting an intensity or temperature, or change therein, of the m = +1 order diffracted infrared radiation and/or the m = -1 order diffracted infrared radiation, to determine a different property of the infrared radiation.

[0027] According to still another aspect of the present invention, there is provided a method of determining a property of infrared radiation reflected from a plasma formation location in a collector module of a radiation source, the method comprising: inspecting a collection surface of a collector of the collector module with an infrared sensitive camera to determine the property.

[0028] According to a still further aspect of the invention there is provided an infra-red detector comprising a screen and a camera, the camera in use being arranged to detect hotspots on said screen caused by infra-red radiation, further comprising a heating system whereby said screen may be heated.

[0029] The screen may take any suitable form which is substantially transmissive to a desired radiation band, e.g., EUV radiation, but which is responsive to infra-red radiation. For example the screen may comprise a wire mesh. When the screen comprises a wire mesh the heating system may comprise a current source adapted to supply electrical current to the mesh. The wire mesh may comprise two independent wire arrays each adapted to be supplied with an electrical current. Each array may comprise a plurality of parallel wires, and the arrays may disposed at an angle relative to each other. The arrays may be disposed at a relative angle of between 45° and 90°, e.g., they may be mutually perpendicular. Such embodiments assist in ensuring uniform heating of the screen.

[0030] According to a still further aspect of the invention there is provided an infra-red detector comprising a screen and a camera, the camera in use being arranged to detect hotspots on the screen caused by infra-red radiation, wherein at least one beam splitter is provided between the screen and the camera, the at least one beam splitter being adapted to create two or more optical paths creating two or more images of the screen at the camera, and wherein the beam splitter directs different amounts of light along different optical paths.

[0031] Preferably a first optical component is provided between the screen and the beam splitter(s) for forming an image of the screen at the beam splitter(s), and a second optical component is provided between the beam splitter(s) and the camera to project the images onto the camera.

[0032] In the aforesaid two aspects of the invention an infra-red detector may be provided that is capable in use of detecting infra-red hotspots having a power density in the range of lW/cm2 to 100W/cm2 (or 5W/cm2 to 80W/cm2, or 10W/cm2 to 60W/cm2) using only a single camera and without requiring any change in camera settings.

[0033] In another aspect the invention also extends to a lithographic apparatus comprising: an illumination system for providing a radiation beam, and an infra-red detector as described above in the aforesaid aspects of the invention wherein said detector is provided in said illumination system.

[0034] In another aspect of the invention there is provided a method of detecting an infra-red hotspot in an optical system, comprising placing a screen in an optical path and directing a camera at said screen to detect hotspots on the screen caused by infra-red radiation, comprising heating the screen whereby the power density of low intensity hotspots is raised above a camera threshold.

[0035] Preferably the screen comprises a wire mesh, and the heating comprises supplying an electrical current to said mesh. The wire mesh may comprise two independent wire arrays and the method may comprise supplying each mesh with an electrical current.

[0036] According to a still further aspect of the invention there is provided a method of detecting an infra-red hotspot in an optical system, comprising placing a screen in an optical path and directing a camera at the screen to detect hotspots on the screen caused by infra-red radiation, comprising creating multiple optical paths creating multiple images of the screen at the camera, and wherein different amounts of light are directed along different optical paths.

[0037] One or more aspects of the invention may, where appropriate to one skilled in the art, be combined with any one or more other aspects described herein, and/or with any one or more features described herein.

BRIEF DESCRIPTION OF THE DRAWINGS

[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 depicts a lithographic apparatus according to an embodiment of the invention;

[0040] Figure 2 is a more detailed view of the apparatus of Figure 1, including an LPP source collector module;

[0041] Figure 3 schematically depicts a collector module according to an embodiment of the present invention;

[0042] Figure 4 schematically depicts detail of a structure used in the collector module of Figure 3, in accordance with an embodiment of the present invention;

[0043] Figure 5 schematically depicts detail of a structure used in the collector module of Figure 3, in accordance with another embodiment of the present invention;

[0044] Figure 6 schematically depicts an infrared hotspot on a collection surface of a collector;

[0045] Figure 7 schematically depicts how the hotspot shown in Figure 6 is incident on a structure of the collector module according to an embodiment of the present invention;

[0046] Figure 8 schematically depicts how the hotspot shown in Figure 6 is incident on a structure of the collector module according to another embodiment of the present invention;

[0047] Figure 9 schematically depicts a system for detecting hotspots on a collection surface of a collector, in accordance with another embodiment of the present invention;

[0048] Figure 10 shows a plot of camera bit-reading against IR hotspot power density as determined for different exposure times;

[0049] Figures ll(a)-(d) illustrate schematically an embodiment of the invention with Figure 11(a) illustrating an embodiment of the invention and Figures ll(b)-(d) being exemplary image views as seen by a camera sensor;

[0050] Figure 12 illustrates schematically another embodiment of the invention; and

[0051] Figure 13 is a view similar to Figure 10 but showing the effect of preheating the mesh.

[0052] The features and advantages of the present invention will become more apparent from the detailed description set forth below when taken in conjunction with the drawings, in which like reference characters identify corresponding elements throughout. In the drawings, like reference numbers generally indicate identical, functionally similar, and/or structurally similar elements. The drawing in which an element first appears is indicated by the leftmost digit(s) in the corresponding reference number.

DETAILED DESCRIPTION

[0053] This specification discloses one or more embodiments that incorporate the features of this invention. The disclosed embodiment(s) merely exemplify the invention. The scope of the invention is not limited to the disclosed embodiment(s). The invention is defined by the clauses appended hereto.

[0054] The embodiment(s) described, and references in the specification to "one embodiment," "an embodiment," "an example embodiment," etc., indicate that the embodimenl(s) described may include a particular feature, structure, or characteristic, but every embodiment may not necessarily include the particular feature, structure, or characteristic. Moreover, such phrases are not necessarily referring to the same embodiment. Further, when a particular feature, structure, or characteristic is described in connection with an embodiment, it is understood that it is within the knowledge of one skilled in the art to effect such feature, structure, or characteristic in connection with other embodiments whether or not explicitly described.

[0055] Figure 1 schematically depicts a lithographic apparatus LAP including a source collector module SO according to an embodiment of the invention. The apparatus comprises: an illumination system (illuminator) IL configured to condition a radiation beam B (e.g., EUV radiation); a support 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.

[0056] 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.

[0057] 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.

[0058] 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.

[0059] 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.

[0060] The projection system, like the illumination system, may include various types of optical components, such as refractive, reflective, magnetic, electromagnetic, electrostatic or other types of optical components, or any combination thereof, as appropriate for the exposure radiation being used, or for other factors such as the use of a vacuum. It may be desired to use a vacuum for EUV radiation since gases may absorb too much radiation. A vacuum environment may therefore be provided to the whole beam path with the aid of a vacuum wall and vacuum pumps.

[0061] As here depicted, the apparatus is of a reflective type (e.g., employing a reflective mask).

[0062] 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.

[0063] Referring to Figure 1, the illuminator IL receives an extreme ultra violet radiation beam from the source collector 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 collector module SO may be part of an EUV radiation system including a fuel stream generating for generating a stream of fuel and/or a laser (neither of which are shown in Figure 1), for providing the laser beam for exciting the fuel. The resulting plasma emits output radiation, e.g., EUV radiation, which is collected using a radiation collector, disposed in the source collector module. The laser and/or fuel stream generator and the collector module (often referred to as a source collector module), may be separate entities, for example when a CO2 laser is used to provide the laser beam for fuel excitation.

[0064] 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 collector 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 collector module, for example when the source is a discharge produced plasma EUV generator, often termed as a DPP source.

[0065] 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.

[0066] The radiation beam B is incident on the patterning device (e.g., mask) MA, which is held on the support structure (e.g., mask table) MT, and is patterned by the patterning device. After being reflected from the patterning device (e.g., mask) MA, the radiation beam B passes through the projection system PS, which focuses the beam onto a target portion C of the substrate W. With the aid of the second positioner PW and position sensor PS2 (e.g., an interferometric device, linear encoder or capacitive sensor), the substrate table WT can be moved accurately, e.g., so as to position different target portions C in the path of the radiation beam B. Similarly, the first positioner PM and another position sensor PS1 can be used to accurately position the patterning device (e.g., mask) MA with respect to the path of the radiation beam B. Patterning device (e.g., mask) MA and substrate W may be aligned using mask alignment marks Ml, M2 and substrate alignment marks PI, P2.

[0067] The depicted apparatus could be used in at least one of the following modes: 1. In step mode, the support structure (e.g., mask table) MT and the substrate table WT are kept essentially stationary, while an entire pattern imparted to the radiation beam is projected onto a target portion C at one time (i.e., a single static exposure). The substrate table WT is then shifted in the X and/or Y direction so that a different target portion C can be exposed.

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

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

[0068] Combinations and/or variations on the above described modes of use or entirely different modes of use may also be employed.

[0069] Figure 2 shows the lithographic apparatus LAP in more detail, including the source collector module SO, the illumination system IL, and the projection system PS. The source collector module SO is constructed and arranged such that a vacuum environment can be maintained in an enclosing structure 2 of the source collector module.

[0070] A laser 4 is arranged to deposit laser energy via a laser beam 6 into a fuel, such as xenon (Xe), tin (Sn) or lithium (Li), which is provided from a fluid stream generator 8. Liquid (i.e., molten) tin (most likely in the form of droplets), or another metal in liquid form, is currently thought to be the most promising and thus likely choice of fuel for EUY radiation sources. The deposition of laser energy into the fuel creates a highly ionized plasma 10 at a plasma formation location 12, which has electron temperatures of several tens of electronvolts (eV). The energetic radiation generated during de-excitation and recombination of these ions is emitted from the plasma 10, collected and focused by a near normal incidence radiation collector 14 (sometimes referred to more generally as a normal incidence radiation collector). The collector 14 may have a multilayer structure, for example one tuned to reflect, more readily reflect, or preferentially reflect, radiation of a specific wavelength (e.g., radiation of a specific EUV wavelength). The collector 14 may have an elliptical configuration, having two natural ellipse focus points. One focus point will be at the plasma formation location 10, and the other focus point will be at the intermediate focus, discussed below.

[0071] A laser 4 and/or fluid stream generator 8 and/or a collector 14 may together be considered to comprise a radiation source, specifically an EUV radiation source. The EUV radiation source may be referred to as a laser produced plasma (LPP) radiation source. The collector 14 in the enclosing structure 2 may form a collector module, which forms a part of the radiation source (in this example).

[0072] A second laser (not shown) may be provided, the second laser being configured to preheat the fuel before the laser beam 6 is incident upon it. An LPP source that uses this approach may be referred to as a dual laser pulsing (DLP) source.

[0073] Although not shown, the fuel stream generator will comprise, or be in connection with, a nozzle configured to direct a stream of, for example, fuel droplets along a trajectory towards the plasma formation location 12.

[0074] Radiation B that is reflected by the radiation collector 14 is focused at a virtual source point 16. The virtual source point 16 is commonly referred to as the intermediate focus, and the source collector module SO is arranged such that the intermediate focus 16 is located at or near to an opening 18 in the enclosing structure 2. The virtual source point 16 is an image of the radiation emitting plasma 10.

[0075] Subsequently, the radiation B traverses the illumination system EL, which may include a facetted field mirror device 20 and a facetted pupil mirror device 22 arranged to provide a desired angular distribution of the radiation beam B at the patterning device MA, as well as a desired uniformity of radiation intensity at the patterning device MA. Upon reflection of the beam of radiation at the patterning device MA, held by the support structure MT, a patterned beam 24 is formed and the patterned beam 24 is imaged by the projection system PS via reflective elements 26, 28 onto a substrate W held by the wafer stage or substrate table WT.

[0076] More elements than shown may generally be present in the illumination system IL and projection system PS. Furthermore, there may be more mirrors present than those shown in the figures, for example there may be 1-6 additional reflective elements present in the projection system PS than shown in Figure 2.

[0077] As discussed above, unwanted (and potentially damaging) infrared radiation reflected from the plasma formation location (e.g., from the fuel target and/or the resulting plasma) may be substantially prevented from passing onto and though the lithographic apparatus as by appropriate diffraction of the reflected infrared radiation. However, even with such diffraction, m = 0 diffracted order infrared radiation will still propagate onto and through the lithographic apparatus since it will have the same beam path as undiffracted radiation (e.g., EUV radiation). The m = 0 diffracted order infrared radiation may cause damage to the lithographic apparatus if the m = 0 diffracted order radiation is of sufficient intensity, for example when a high power density infrared radiation spot is reflected from the plasma formation location.

[0078] Presently, the level of infrared radiation that is reflected is not monitored. Instead, the infrared radiation is simply diffracted to ensure that the amount of radiation that passes into and through the lithographic apparatus is reduced or eliminated. However, as discussed above, such diffraction does not in fact prevent m = 0 diffracted order radiation passing into and through the lithographic apparatus. It is therefore desirable to be able to be to determine what infrared radiation is passing into and through the lithographic apparatus (e.g., m = 0 diffracted order infrared radiation). A problem is that m = 0 diffracted order infrared radiation will have the same beam path as EUV radiation, and so detecting infrared radiation of that order might require a slight obscuration of the EUV (e.g., with an infrared detector), which is undesirable, since it might reduce the amount of EUV radiation available for patterning of substrates. It is also desirable to be able to react to levels of infrared radiation that might be damaging to the lithographic apparatus.

[0079] According to an embodiment of the present invention, there is provided a solution to the problems mentioned above. There is provided a collector module for a radiation source. The collector module comprises a collector for collecting radiation generated by a radiation generating plasma at a plasma formation location, and for directing at least a portion of the generated radiation to a focal point (e.g., an intermediate focal point or intermediate focus). A structure is provided that is adjacent to and upstream of the focal point. The structure extends at least partially around an expected position of a beam comprising the at least a portion of the collected radiation. For instance, the structure may have a shape like a frustoconical shell, which may surround an EUV radiation beam when that beam is aligned substantially as intended - i.e., an expected position. The collector module further comprises a diffractive element located in a beam path between the plasma formation location and the focal point. The diffractive element can be an independent component, or form a part of another component of the collector module. The diffractive element is arranged to diffract infrared radiation that is reflected from the plasma formation location, such that when the plasma formation location is at an intended location (e.g., an intended operating position, which includes an intended range of positions) m = +1 diffracted order infrared radiation (first order) is directed towards a first region of the aforementioned structure, and away from passing through the focal point. At the same time, m = -1 diffracted order infrared radiation is directed toward a second region of the structure and (again) away from passing through the focal point. The diffraction is such that, under normal operation conditions, a majority of the infrared radiation is diffracted in to orders other than the m = 0 order. The first region and second region of the structure form part of a system for detecting a property of the infrared radiation. The property may be, for example, one or more of an intensity or temperature ofm = +l,m = -l orm = 0 diffracted order radiation; and/or change in intensity or temperature ofm = +l,m = -lorm = 0 diffracted order radiation; and/or a position of a point of origin of the reflected infrared radiation; and/or a change in position of a point of origin of the reflected infrared radiation.

[0080] A key advantage of the present invention (although not the only advantage) is that m = +/-1 diffracted order radiation may be used to determine a property of m = 0 diffracted order radiation, which m = 0 diffracted order radiation will pass onto and through the lithographic apparatus. Thus, without needing the measure the m = 0 diffracted order radiation directly (which might be difficult or impractical), properties of the m - 0 diffracted order can be determined. Depending on the determined property (e.g., a certain intensity), or change therein, a reaction might be to change the state of operation of a radiation source connected to the collector module (or of which the collector module forms a part). For example, the radiation source might be shut down, to prevent damage to the lithographic apparatus. At the same time, because the level of infrared radiation passing into the lithographic apparatus can now be determined, the source may have its power increased to increase EUV generation, but only up to a level just below where reflected infrared radiation might cause damage. Other advantages are associated with the invention, as will be apparent from a description of embodiments of the invention that follows.

[0081] Figure 3 shows a collector module SO. The collector module may be viewed as a more detailed version of the collector module as shown in and described with reference to Figures 1 and 2, or as a collector module that can be used in place of that collector module shown in those Figures. Features appearing in Figure 3 that have already been shown in and described with reference to Figure 2 are given the same reference numerals for consistency and clarity.

[0082] Referring to Figure 3, the collector module SO shares has all of the features already shown in and described with reference to the collector module of Figure 2. In addition, however, there is now shown a structure 30 that is adjacent to and upstream of the intermediate focus 16, which structure 30 extends at least partially around an expected position of a beam B comprising radiation that is to be used to pattern a substrate (e.g., EUV radiation). ‘Expected position’ might be the position of the beam B under normal operating conditions (e.g., when the beam is well aligned). The structure 30 is located adjacent to and upstream of an exit opening 18 of the enclosing structure 2, for example an opening that leads to an illuminator. In another embodiment, the structure may for part of, or define, that opening 18. The structure 18 may be used to separate an environment within the enclosure 2 from an environment outside of the enclosure 2, for example an environment in a downstream illuminator. The structure 30 may comprise one or more gas inlets or outlets, and/or one or more sensors, to facilitate this separation. The shape of the structure may also facilitate such separation. For example, the stmcture 30 is shown as having a frustoconical shell-like stmcture, and the conical nature of the stmcture 30 may assist in preventing contamination from passing out of the enclosing stmcture 2, and/or may additionally be used as a guide for guiding or defining a cross sectional shape of the EUV radiation beam B.

[0083] Radiation coming from the plasma formation location 12 does not only comprise EUV radiation B (shown in solid lines in the Figure) but also comprises infrared radiation (shown in dotted lines in the Figure). In this embodiment the collector 14 is provided with a grating structure (i.e., is a grating collector), such that the collector 14 also serves as a diffractive element for appropriately diffracting infrared radiation that is reflected from the plasma formation location 10. The grating collector is configured such that m = +1 diffracted order radiation 32 is directed towards a first region of the structure 30 and away from passing through the intermediate focus 16. Although not shown, m = -1 diffracted order infrared radiation is directed towards a second region of the structure 30, and away from passing through the intermediate focus 16. At the same time, m = 0 diffracted order radiation passes through a conduit (or other passage way - e.g., a hollow center) provided through the structure, and through the intermediate focus 16. As mentioned above, and as discussed in more detail below, the first region and second region of the structure 30 form part of a system for detecting a property of the infrared radiation.

[0084] Figure 4 schematically depicts an end-on view of the structure 30, the view being taken in the direction in which radiation propagates onto and/or through the structure 30. In this embodiment, the system for detecting a property of the infrared radiation comprises a plurality of sensors 36, the first region and second region of the system each being or comprising at least one of the plurality of sensors 36. The plurality of sensors 36 extend around the expected position of the beam (and thus around the structure 30) to define a substantially segmented ring of sensors 36. By extending around the expected position of the beam, the m = +/-1 orders can be detected wherever they originate from (i.e., 360 degrees of detection coverage is provided).

[0085] The sensors 36 may be infrared sensors located on or form part of the structure 30. In another example the sensor 30 may comprise a surface to which is connected certain electronics that may be used in the determination of the temperature of that surface, for example one or more resistors, thermocouples, thermistors or the like. The choice of size of each sensor 36 may be a balance between measurement of power (more accurate with larger sensor) or power density (more accurate with smaller sensor). The number of sensors will depend on the required resolution.

[0086] In Figure 5, and in another embodiment, the system for detecting a property of the infrared radiation may comprise one or more surfaces 38 for receiving the m = +/-1 diffracted order infrared radiation, and one or more infrared radiation sensitive cameras 40 for inspecting the one or more surfaces 38. The first region and second region of the structure each constitute or comprise one of those surfaces 38.

[0087] In the embodiments shown in Figures 4 and 5, the first and second region of the stmcture wifi not, in normal use, receive any EUV radiation or m = 0 diffracted order radiation, but only specifically directed m = +/-1 diffracted order infrared radiation. It will be understood that the first and second regions are regions to which m = +/-1 diffracted order infrared radiation is directed, and can, depending on the direction of diffraction, be different parts of the stmcture at different times (e.g., for different hotspot locations).

[0088] Figure 6 shows an infrared hotspot 42 as it might appear on a collecting surface of the collector 14. Figure 7 shows how this hotspot is, by diffraction, incident on and passes through the stmcture 30: m = +1 diffracted order infrared radiation 44 is diffracted onto a first region of the stmcture 30 and onto one of a plurality of sensors 36, whereas m = -1 diffracted order infrared radiation 46 is diffracted onto and incident on a second region of the stmcture 30, which comprises another (substantially opposite) sensor of the plurality of sensors 36. Radiation diffracted into the m = 0 order radiation 48 is not incident on the structure, and passes through the stmcture 30 and onto and through the lithographic apparatus.

[0089] It is known that there is a well established mathematical relationship between the intensity/temperature of m = +/-1 diffracted order radiation (first order) and the associated m = 0 diffracted order radiation (zeroth order). Thus, the detecting of a property of m = +1 and/or m = -1 diffracted order radiation may be used to indirectly determine one or more properties of the m = 0 diffracted order radiation.

[0090] In one example, the intensity/temperature of the m = +/-1 diffracted order radiation may be determined to be of a certain level. This level may indicate that m = 0 diffracted order radiation is at or above a threshold level that might cause damage to the lithographic apparatus. In this scenario, a controller forming part of the detection system may be arranged to provide a response. For instance, the response may be an issuance of a warning (e.g., a visual or audio warning); and/or to change the operating state of a radiation source of which the collector module forms a part (e.g., to close a shutter or the like for the radiation source to prevent radiation being passed onto and through the lithographic apparatus, or to change one or more operational settings such as laser firing rate or intensity, or the like); and/or to prevent radiation being generated by a radiation source of which the collector module forms a part (e.g., by preventing the laser of the radiation source from firing, and/or from preventing the fuel stream generator from generating a fuel stream); and/or to control a position or orientation of one or more parts of the radiation source to ensure that m = 0 (and/or m = 1, and/or m = -1) diffracted order radiation is prevented from passing through the focal point (e.g., to ensure that such diffracted order(s) is (are) directed at an obscuration or the like located in-between the plasma formation location and the focal point. Such a response can prevent damage to the lithographic apparatus, or at least warn that such damage is taking place. This could save downtown required to repair any damage, or costs associated with such repair. Alternatively or additionally, the response may allow for maintenance or checks to be rapidly undertaken.

[0091] Alternatively and/or additionally, instead of or as well as detecting the intensity of m = 0 diffracted order radiation, the position of the m = 1 and/or m = -1 orders (e.g., around the structure in a circumferential direction) will allow information to be obtained as to the angular intensity distribution of radiation reflected from the plasma formation location.

[0092] One example has been provided as to how the detection of m = +/-1 diffracted order radiation may be used to determine properties of m = 0 diffracted order radiation, and to act in response, accordingly. However, there are also other applications of the detection of the detection of m = +/-1 diffracted order infrared radiation, in response to which the controller may again provided a response.

[0093] In one example, if the intensity/temperature of m = +1 diffracted order radiation is determined to be higher than m = -1 diffracted order radiation, then this may indicate that a spatial location of the origin of the infrared radiation (e.g., a plasma formation location) has changed from an intended position. For example, an intended (centralised or well aligned) position of origin would usually result in the intensity/temperatures being determined as being equal. Thus, the detection of m=+/-l diffracted order infrared radiation may be used to, alternatively or additionally, determine the position or change of position of the plasma formation location. If a positional change is detected (e.g., outside of a certain range), the controller may provide an appropriate response.

[0094] In general, it will be apparent that the present invention can be used to determine (at least) a property that is one or more of: an intensity or (resulting) temperature of m = +1, m = -1, or m = 0 diffracted radiation (which includes an intensity distribution, or location, or position, or shape of the m = +l,m = -l,orm = 0 diffracted radiation); and/or a change in intensity or (resulting) temperature of m = +1, m = -1, or m = 0 diffracted radiation (which includes a change in intensity distribution, or location, or position, or shape of the m = +1, m = -1, or m = 0 diffracted radiation);; and/or a position of a point of origin of the reflected infrared radiation; and/or a change in position of a point of origin of the reflected infrared radiation. For instance, with the above described embodiments, it is possible to determine the angular intensity distribution of infrared radiation reflected from the plasma formation location. This may be achieved by determination of the intensity and/or position (which includes a shape and/or size, and a location around the structure) of m = 1 and/or m = -1 diffracted orders.

[0095] In yet another example, detecting the temperature/intensity of m = +/-1 diffracted order infrared radiation may be used to determine how effectively diffraction of such radiation is being implemented by the diffractive element. For instance, if the m = +/-1 diffracted order radiation intensity level is seen to drop or drift downwards over time, then this could be an indication that the diffractive element is no longer diffracting infrared radiation as strongly as intended, for example due to one or more contaminants beginning to coat the grating collector, or, in an alternative embodiment, a zone plate or spectral purity filter that serves as the diffractive element. If the diffraction is not as strong as intended, more radiation may be diffracted into the m = 0 order. Alternatively or additionally, the m = +/-1 diffracted orders may begin to have an angular separation from the m = 0 diffracted order, which results in at least a portion of the m = +/-1 diffracted order radiation passing towards and through the aforementioned focal point and onto and through the lithographic apparatus. This is of course undesirable. However, in accordance with this example, it should be kept in mind that drops in infrared radiation intensity levels may occur due to general drops in levels of generated radiation at the plasma formation location. For instance, even though the m = +/-1 diffracted orders of infrared radiation have decreasing intensity, this does not necessarily mean that that radiation is now being directed into the m = 0 diffracted order or the like - this could be attributable to a drop in generated radiation as a whole, or some other factor (e.g., change in conversion efficiency, change in shape or location of plasma, or the like). Thus, in at least one example, it may also be useful to determine the generated level of EUV radiation as a baseline or reference point, and to see how this correlates with detected drops or changes in m = +/-1 diffracted order infrared radiation. If the detected m = +/-1 diffracted order infrared radiation drops in intensity when the EUV radiation intensity does not, this may be indicative of weaker diffraction as mentioned previously, and the controller may provide an appropriate response.

[0096] Figure 8 shows an alternative way of achieving the determination or detection referred to above, but now in conjunction with the system as shown in relation to Figure 5. In this embodiment, one or more infrared radiation sensitive cameras 40 may be used to inspect one or more surfaces 38 on which the m = +1 diffracted order radiation 44 and/or m = -1 diffracted order radiation 46 is incident, for example to determine one or more properties of the m = 0 diffracted order radiation 48, all as described above.

[0097] Preferably the infrared radiation sensitive camera 40 may be arranged to receive infrared radiation emitted by the surfaces 38 due to appropriate heating thereof. That is, the radiation sensitive camera 40 may be arranged such that any infrared reflected by the surfaces 38 is directed away from (i.e., not received by) the infrared radiation sensitive camera 40. This may be advantageous, since this may prevent the infrared sensitive camera being blinded (e.g., saturation levels being exceeded, or the like) by the reflected infrared radiation.

[0098] In the embodiments described so far, intensity/temperature of diffracted m = +/-1 radiation has been used to determine a different property of the infrared radiation (i.e., a property other than the intensity/temperature of diffracted m - +/-1 radiation, for example a property of m - 0 diffracted order radiation, or an origin of the infrared radiation, or the like). Figure 9 shows an alternative approach. Figure 9 shows that an infrared radiation sensitive camera 50 may be used for directly inspecting a collection surface of a collector 14 for infrared radiation hotspots 42. If such a hotspot is detected, or a hotspot with a certain temperature, location or intensity, the aforementioned controller may be used to respond as previously described.

[0099] Preferably the infrared radiation sensitive camera 50 may be arranged to receive infrared radiation emitted by the collector 14 due to appropriate heating thereof. That is, the radiation sensitive camera 50 is arranged such that infrared radiation reflected by the collector 14 is directed away from (i.e., not received by) the infrared radiation sensitive camera 50. This may be advantageous, since this may prevent the infrared sensitive camera 50 being blinded (e.g., saturation levels being exceeded, or the like) by the reflected infrared radiation.

[00100] In embodiments where a camera has been described, a gas flow or other form of contamination barrier may be provided to prevent problematic build up of contamination on a window/lens of the camera.

[00101] The structure referred to above is sometimes referred to in the art as an intermediate focus cap. The structure usually has a frustoconical shell-like shape. In another embodiment (not shown) the structure for receiving m = +/-1 diffracted order infrared radiation might be a different shape, for example an annulus or part annulus or the like.

[00102] In the above embodiments, the diffractive element has been described as taking the form of, or being part of, a zone plate or a spectral purity filter. However, the diffractive element forming part of the collector is preferable, since this may reduce any losses EUV losses that might be associated with transmissive components such as, at least in some examples, a zone plate or spectral purity filter, and/or reduced the number of components off that the EUV radiation must reflect.

[00103] While in the above embodiments information regarding the zero order IR radiation may be obtained from detecting first order radiation, additionally or alternatively it may be possible to directly detect zero order diffracted IR radiation that passes through opening 18 along the path of the radiation beam B through the illuminator IL and potentially the projection system PS.

[00104] This may be done by placing an infra-red detector including a hotspot detection screen at a suitable location in the path of the radiation beam. While the detection screen could be placed at any location it is preferable to locate the screen in the illuminator IL at a location just downstream of the intermediate focus 16 for ease of access whereby the screen can be reached for maintenance and may be swapped and replaced if necessary. The screen may take any suitable form provided that it is substantially transmissive to radiation of interest, in this example EUV radiation, but is responsive to IR radiation. The hotspot detection screen may for example comprise a wire mesh that is sensitive to incident IR radiation but which is highly transmissive to EUV radiation. An infra-red camera (eg a camera sensitive to near infra-red radiation with a wavelength of between lOOOnm to f2,000nm) is directed at the wire mesh and an output from the camera is sent to image processing means which may include real time video processing means and data storage. The camera may be located in the illuminator IL and may be calibrated by illuminating the wire mesh with laser pulses of varying power densities but of the same wavelength as the LPP drive laser. Since the wire mesh is located in the vacuum of the illuminator IL the wire mesh will glow and the number of bits from the pixels is linked to the IR power density.

[00105] The camera will include a sensor chip but a known problem with prior art hotspot detection systems is that if the camera is sufficiently sensitive to detect low power density hotspots the camera will quickly saturate as the power density increases. This is illustrated in Figure 10 which plots the camera bit-reading (number of bits, Y-axis) against IR power density (W/cm , X-axis) for four different exposure times (which includes pixel integration times): respectively from left to right (i) 10ms, (ii) 0.2ms, (iii) 30ps and (iv) 6ps. As can be seen from Figure 10 with an exposure time of 10ms low power densities down to around 10W/cm can be detected but the camera quickly saturates before the power density reaches 20W/cm . Decreasing the exposure time means that a higher power density can be detected before saturation but the price for this is that the lower limit for the power density that can be detected increases. In practice a camera setting that can detect hotspots with power densities over a range of l-100W/cm2 cannot be obtained with current technologies. One option to overcome this problem is to change the camera settings between frames, e.g., by using a high sensitivity setting for odd frames and a low sensitivity setting for even frames, but this reduces the effective number of frames per second by two, and may reduce the frame rate even further if time is required to change the camera setting. Another option may be to use two cameras with different sensitivities but this requires the use of extra space, extra cost, and the use of two processing means to monitor and analyze the images in real time.

[00106] Figure 11(a) shows schematically an infra-red hotspot detection system according to an embodiment of the invention. Figure 11(a) shows a detection screen 100 in the form of a wire mesh that may be placed in the radiation beam B. Figure 11(a) also shows a camera sensor 101. The detection screen may also comprise or be associated with a spectral purity filter if desired. Between the detection screen 100 and the camera sensor 101 is provided a beam splitter 102 and mirror 103 which will be described in more detail below. Also provided are an optical component 104 which focuses the image of the mesh on the beam splitter 102, and optical component 105 which projects the multiple images created by the beam splitter 102 onto the camera sensor 101 at a desired location. The optical components 104, 105 may also be designed to ensure that radiation passing along both paths as described below have equal path length before reaching the detection screen. It will be understood by those skilled in the art that optical components 104,105 may each comprise a number of optical elements. It will also be understood that the beam splitter(s) may be any optical component(s) that is/are capable of dividing a beam of radiation into two or more parallel beams with differing amounts of radiation in each beam.

[00107] Beam splitter 102 functions to split incoming light into a first beam that comprises X% of the original light and which passes along a first optical path directly through the beam splitter 102, optical component 105 and then forms a first image on camera sensor 101. The remaining light, i.e., 100-X% of the original light received by the beam splitter 102 is directed at right angles to the incoming light to mirror 103 which reflects that light so that it passes along a second optical path that is parallel to the first optical path and forms a second image on camera sensor 101. The value of X is not critical but may for example be 5, 10, 15, 20, 35, 45 or indeed any value less than 50, but what is important is that more light passes along one of the two optical paths than the other, preferably substantially more. The advantage of providing differential amounts of light in the two beams is that if the beam with the greater amount of light saturates the sensor then the image formed by the beam with less light may still be imaged by the sensor. Typically X may be 10 such that 90% of the received light passes straight through the beam splitter 102 and 10% is diverted by the beam splitter 102 to form the second image. If a hotspot 106 exists on the detection screen 100 Figure 11(b) shows the single image 106a that would be formed on the camera sensor 101 without the beam splitter 102 being present. With the beam splitter 102 a double image 106a, 106b is formed on the camera sensor 101 as shown in Figure 11(c).

[00108] The advantage provided by this arrangement will now be explained with reference to a hotspot 106 present on the detection screen. The camera sensor 101 can be set to a high sensitivity, i.e., a long exposure time, such that if the hotspot 106 has a low power density (e.g., lW/cm2) the camera sensor 101 will detect the hotspot as the first image 106a which receives 90% of the incoming light. The second image 106b formed from the remaining 10% will be too weak to be detected. If on the other hand the hotspot 106 is of a higher power density (e.g., 100W/cm2) the first image 106a may saturate the sensor 101, but the second image 106b formed by only 10% of the incoming light will not saturate and may be imaged and processed. This principle may be extended further by using multiple beam splitters to form more than two images. For example Figure 11(d) shows the formation of four images 106a, 106b, 106c, 106d using three beam splitters. The division of the camera sensor 101 into four such regions is possible because a camera may conventionally have a resolution of 640x480 pixels and this can be divided into four regions each of 320x240 pixels which is sufficient for imaging of a hotspot.

[00109] The embodiment of the invention shown in Figure 11 therefore increases the effective dynamic range of the hotspot detection system such that without requiring any changes in the camera settings the system can detect hotspots over a power density range of lW/cm2 to 100W/cm2, or 5W/cm2 to 80W/cm2, or 10W/cm2 to 60W/cm2.

[00110] A further advantage of this embodiment of the invention is that the two (or more) optical paths created by the beam splitter(s) may be provided with filters such that each image is sensitive to a particular bandwidth. For example, in Figure 11(c) one of the two images may be a visible image, the other may be an infra-red image. Such filters may for example be provided as part of optical component 105.

[00111] Figure 12 shows a further embodiment of the invention. In this embodiment of the invention the hotspot detection screen comprises a wire mesh 200 that is substantially transparent to EUV radiation. The detection screen may also function as a spectral purity filter or there may also be provided a spectral purity filter if desired. The wire mesh 200 is connected to a source 201 of alternating electric current I that is applied to the wire mesh 200. Preferably the wire mesh 200 comprises two separate arrays 200a, 200b of wires that are disposed at an angle to each other. Preferably the two arrays extend in mutually perpendicular directions but other angles between 45° and 90° are also possible. The two arrays 200a, 200b are connected in parallel to the current source 200.

[00112] In this embodiment a current I is applied to the wire mesh 200 which serves to heat the wire mesh. The advantage of providing the wire mesh as two arrays 200a, 200b each provided separately with a current is that the current distribution over the wire mesh 200 is more uniform than it would be if the current was applied to one side of a wire mesh formed as a single piece.

[00113] In this embodiment of the invention the current I serves to heat the wire mesh to an extent depending on the applied current and the resistivity of the wires forming the mesh. Preferably the wire mesh may be heated to a temperature of about 700°C. This heating of the mesh will create a temperature offset to the detection screen. A low energy hotspot will therefore seem brighter to the camera and can be detected by a camera with a lower sensitivity setting. In effect, referring back to Figure 10 and as shown in Figure 13, the pre-heating of the wire mesh shifts the zero point of the graph to the right to a degree dependent on the amount of pre-heating and as illustrated by the different X axes (a)-(c) in Figure 13. Axis (a) is the conventional arrangement with no pre-heating of the mesh. Axes (b) and (c) show the effect of preheating with a greater level of pre-heating being applied in the case of axis (c). Looking, for example, at axis (b) the pre-heating of the mesh is equivalent to an incident IR power density of about 30W/cm2 and thus a low power density hotspot of, say, 10W/cm2 would be seen by the camera as if it were a 40W/cm2 hotspot and can thus easily be detected with an exposure time of less than 3Ops. With a sufficiently large pre-heating and thus a sufficiently large offset, the dynamic range of the camera can easily be adjusted to accommodate hotspots with a range of from 10W/cm2 to 100W/cm2 as can be seen for example with the axis offset marked (c).

[00114] It will also be understood that the embodiments of Figures 11 and 12 could be combined. That is to say, in the embodiment of Figure 11 the screen 100 may comprise a screen of the type described with reference to Figure 12 (e.g., a wire mesh) which may be heated to increase the dynamic level of low intensity hotspots.

[00115] The embodiments of Figures 11 to 13 increase the dynamic range of the detector to 10W/cm2 to 100W/cm2 and allow a single camera to simultaneously detect multiple hotspots within that range.

[00116] As with the embodiments of the inventions in which first order diffracted IR radiation is detected, in these embodiments in which zero order IR radiation is detected directly the processing means may be configured to issue a trigger once an IR hotspot is detected with a power density above a predefined critical level (e.g., 100W/cm2). This trigger may for example cause the lithographic apparatus - and in particular the radiation source - to be switched off while the cause of the hotspot is investigated. Depending on the intensity of the hotspot and/or its precise location and the potential for damage, the lithographic apparatus may be allowed to complete the processing of a current wafer before being switched off - a "soft landing" - or may be switched off immediately if the risk of damage is high even if a wafer is being processed - a "hard landing."

[00117] Although specific reference may be made in this text to the use of lithographic apparatus in the manufacture of ICs, it should be understood that the lithographic apparatus described herein may have other applications, such as the manufacture of integrated optical systems, guidance and detection patterns for magnetic domain memories, flat-panel displays, liquid-crystal displays (LCDs), thin-film magnetic heads, LEDs, solar cells, etc. The skilled artisan will appreciate that, in the context of such alternative applications, any use of the terms "wafer" or "die" herein may be considered as synonymous with the more general terms "substrate" or "target portion," respectively. The substrate referred to herein may be processed, before or after exposure, in for example a track (a tool that typically applies a layer of resist to a substrate and develops the exposed resist), a metrology tool and/or an inspection tool. Where applicable, the disclosure herein may be applied to such and other substrate processing tools. Further, the substrate may be processed more than once, for example in order to create a multi-layer IC, so that the term substrate used herein may also refer to a substrate that already contains multiple processed layers.

[00118] When describing the lithographic apparatus, the term "lens," where the context allow, may refer to any one or combination of various types of optical components, including refractive, reflective, magnetic, electromagnetic and electrostatic optical components.

[00119] While specific embodiments of the invention have been described above, it will be appreciated that the invention may be practiced otherwise than as described. The descriptions above are intended to be illustrative, not limiting. Thus it will be apparent to one skilled in the art that modifications may be made to the invention as described without departing from the scope of the clauses that follow. Other aspects of the invention are set out as in the following numbered clauses: 1. A collector module, the collector module comprising: a collector for collecting radiation generated by a radiation generating plasma at a plasma formation location, and for directing at least a portion of the generated radiation to a focal point; a structure adjacent to and upstream of the focal point, the structure extending at least partially around an expected position of a beam comprising the at least a portion of the collected radiation; a diffractive element located in a beam path between the plasma formation location and the focal point, the diffractive element being arranged to diffract infrared radiation that is reflected from the plasma formation location, such that when the plasma formation location is at an intended location, m = +1 order diffracted infrared radiation is directed towards a first region of the structure and away from passing through the focal point, and m = -1 order diffracted infrared radiation is directed towards a second region of the structure and away from passing through the focal point; the first region and second region of the structure forming part of a system for determining a property of the infrared radiation.

2. The collector module of clause 1, wherein the system comprises a plurality of sensors, the first region and second region each being or comprising at least one of the plurality of sensors.

3. The collector modules of clause 2, wherein the plurality of sensors extend around the expected position of the beam to form a substantially segmented ring of sensors.

4. The collector module of clause 1, wherein the system comprises one or more surfaces for receiving infrared radiation, and one or more infrared sensitive cameras for inspecting the one or more surfaces, the first region and second region each being or comprising one of those surfaces.

5. The collector module of any preceding clause, wherein the diffractive element is a part of the collector, such that the collector is a grating collector.

6. The collector module of any of clauses 1 to 4, wherein the diffractive element is, or forms part of, a zone plate or a spectral purity filter.

7. The collector module of any preceding clause, wherein the system for determining a property of the infrared radiation is in connection with a controller, the controller being arranged to provide a response to a certain property being detected, or a certain level of a property being detected, the response being one or more of: an issuance of a warning; and/or to change the operating state of a radiation source of which the collector module forms a part; and/or to prevent radiation being generated by a radiation source of which the collector module forms a part.

8. The collector module of any preceding clause, wherein the property is one or more of: an intensity or temperature of m = +1, m = -1, or m = 0 order diffracted radiation; and/or a change in intensity or temperature of m = +l,m = -1, orm = 0 order diffracted radiation; and/or a position of a point of origin of the reflected infrared radiation; and/or a change in position of a point of origin of the reflected infrared radiation.

9. A collector module, the collector module comprising: a collector for collecting radiation generated by a radiation generating plasma at a plasma formation location, and for directing at least a portion of the generated radiation to a focal point; and an infrared radiation sensitive camera for inspecting a collection surface of the collector.

10. The collector module of clause 9, wherein the infrared sensitive camera is arranged to receive infrared radiation emitted by the collector, and is arranged such that infrared radiation reflected by the collector is directed away from the infrared sensitive camera.

11. The collector module of any preceding clause, wherein the collector is a normal incidence collector.

12. A radiation source comprising the collector module of any preceding clause, wherein the radiation source further comprises: a fuel stream generator configured to generate a stream of fuel and direct that stream towards the plasma formation location; and/or wherein the radiation source further comprises, or is arranged to received laser radiation from, a laser configured to direct laser radiation at a fuel at the plasma formation location to generate, in use, the radiation generating plasma.

13. A lithographic apparatus comprising: an illumination system for providing a radiation beam; a patterning device for imparting the radiation beam with a pattern in its cross-section; a substrate holder for holding a substrate; a projection system for projecting the patterned radiation beam onto a target portion of the substrate, and wherein the lithographic apparatus further comprises, or is in connection with, the collector module or radiation source of any preceding clause.

14. A method of determining a property of infrared radiation reflected from a plasma formation location in a collector module of a radiation source, the method comprising: diffracting infrared radiation that is reflected, such that when the plasma formation location is at an intended location, m = +1 order diffracted infrared radiation is directed away from passing through a focal point of the collector, and m = -1 order diffracted infrared radiation is directed away from passing through the focal point; detecting an intensity or temperature, or change therein, of the m = +1 order diffracted infrared radiation and/or the m = -1 order diffracted infrared radiation, to determine a different property of the infrared radiation.

15. A method of determining a property of infrared radiation reflected from a plasma formation location in a collector module of a radiation source, the method comprising: inspecting a collection surface of a collector of the collector module with an infrared sensitive camera to determine the property.

16. A collector module, comprising: a collector configured to collect radiation generated by a radiation generating plasma at a plasma formation location, and to direct at least a portion of the generated radiation to a focal point; a structure, located adjacent to and upstream of the focal point, the structure extending at least partially around an expected position of a beam comprising the at least a portion of the collected radiation; a diffractive element, located in a beam path between the plasma formation location and the focal point, the diffractive element being arranged to diffract infrared radiation that is reflected from the plasma formation location, such that when the plasma formation location is at an intended location, m = +1 order diffracted infrared radiation is directed towards a first region of the structure and away from passing through the focal point, and m = -1 order diffracted infrared radiation is directed towards a second region of the structure and away from passing through the focal point, wherein the first region and second region of the structure form part of a system for determining a property of the infrared radiation.

17. The collector module of clause 16, wherein the system comprises a plurality of sensors, the first region and second region each being or comprising at least one of the plurality of sensors.

18. The collector modules of clause 17, wherein the plurality of sensors extend around the expected position of the beam to form a substantially segmented ring of sensors.

19. The collector module of clause 16, wherein the system comprises one or more surfaces for receiving infrared radiation, and one or more infrared sensitive cameras configured to inspect the one or more surfaces, the first region and second region each being or comprising one of those surfaces.

20. The collector module of clause 16, wherein the diffractive element is a part of the collector, such that the collector is a grating collector.

21. The collector module of clause 16, wherein the diffractive element is, or forms part of, a zone plate or a spectral purity filter.

22. The collector module of clause 16, wherein the system for determining a property of the infrared radiation is in connection with a controller, the controller being arranged to provide a response to a certain property being detected, or a certain level of a property being detected, the response being at least one of: an issuance of a warning; to change the operating state of a radiation source of which the collector module forms a part; and to prevent radiation being generated by a radiation source of which the collector module forms a part.

23. The collector module of clause 16, wherein the property is at least one of: an intensity or temperature of m = +1, m = -1, or m = 0 order diffracted radiation; a change in intensity or temperature of m = +1, m = -1, or m = 0 order diffracted radiation; a position of a point of origin of the reflected infrared radiation; and a change in position of a point of origin of the reflected infrared radiation.

24. A collector module, comprising: a collector configured to collect radiation generated by a radiation generating plasma at a plasma formation location, and to direct at least a portion of the generated radiation to a focal point; and an infrared radiation sensitive camera configured to inspect a collection surface of the collector.

25. The collector module of clause 24, wherein the infrared sensitive camera is arranged to receive infrared radiation emitted by the collector, and is arranged such that infrared radiation reflected by the collector is directed away from the infrared sensitive camera.

26. The collector module of clause 24, wherein the collector is a normal incidence collector.

27. A radiation source comprising the collector module of clause 24, wherein the radiation source further comprises: a fuel stream generator configured to generate a stream of fuel and direct that stream towards the plasma formation location, wherein the radiation source is arranged to receive laser radiation from a laser configured to direct laser radiation at a fuel at the plasma formation location.

28 A lithographic apparatus, comprising: an illumination system configured to provide a radiation beam; a patterning device configured to impart the radiation beam with a pattern in its cross- section; a substrate holder configured to hold a substrate; a projection system configured to project the patterned radiation beam onto a target portion of the substrate; and a collector module having a collector configured to collect radiation of the radiation beam at a plasma formation location, and to direct at least a portion of the generated radiation to a focal point and an infrared radiation sensitive camera configured to inspect a collection surface of the collector.

29. A method of determining a property of infrared radiation reflected from a plasma formation location in a collector module of a radiation source, the method comprising: diffracting infrared radiation that is reflected, such that when the plasma formation location is at an intended location, m = +1 order diffracted infrared radiation is directed away from passing through a focal point of the collector, and m = -1 order diffracted infrared radiation is directed away from passing through the focal point; and detecting an intensity or temperature, or change therein, of the m = +1 order diffracted infrared radiation and/or the m - -1 order diffracted infrared radiation, to determine a different property of the infrared radiation.

30. A method of determining a property of infrared radiation reflected from a plasma fonnation location in a collector module of a radiation source, the method comprising: inspecting a collection surface of a collector of the collector module with an infrared sensitive camera to determine the property.

31. An infra-red detector comprising a screen and a camera, the camera in use being arranged to detect hotspots on said screen caused by infra-red radiation, further comprising a heating system whereby said screen may be heated.

32. A detector as clauseed in clause 31 wherein said screen comprises a wire mesh.

33. A detector as clauseed in clause 32 wherein said heating system comprises a current source adapted to supply electrical current to said mesh.

34. A detector as clauseed in clause 33 wherein said wire mesh comprises two independent wire arrays each adapted to be supplied with an electrical current.

35. A detector as clauseed in clause 34 wherein each said array comprises a plurality of parallel wires, and wherein said arrays are disposed at an angle relative to each other.

36. A detector as clauseed in clause 35 wherein said arrays are disposed at a relative angle of between 45° and 90°.

37. A detector as clauseed in clause 36 wherein said arrays are mutually perpendicular.

38. A detector as clauseed in any of clauses 31 to 37 wherein at least one beam splitter is provided between said screen and said camera, said at least one beam splitter being adapted to create two or more optical paths creating two or more images of said screen at said camera, and wherein said beam splitter directs different amounts of radiation along different optical paths.

39. A detector as clauseed in clause 38 wherein a first optical component is provided between said screen and said beam splitter(s) for forming an image of said screen at said beam splitter(s), and a second optical component is provided between said beam splitter(s) and said camera to project said images onto said camera.

40. An infra-red detector comprising a screen and a camera, the camera in use being arranged to detect hotspots on said screen caused by infra-red radiation, wherein at least one beam splitter is provided between said screen and said camera, said at least one beam splitter being adapted to create two or more optical paths creating two or more images of said screen at said camera, and wherein said beam splitter directs different amounts of radiation along different optical paths.

41. A detector as clauseed in clause 40 wherein a first optical component is provided between said screen and said beam splitter(s) for forming an image of said screen at said beam splitter(s), and a second optical component is provided between said beam splitter(s) and said camera to project said images onto said camera.

42. A detector as clauseed in any of clauses 31 to 41 wherein said detector is capable in use of detecting hotspots having a power density within the range of from lW/cm2 to 100W/cm2.

43. A lithographic apparatus comprising: an illumination system for providing a radiation beam, and an infra-red detector as clauseed in any of clauses 31 to 42 wherein said detector is provided in said illumination system.

44. A method of detecting an infra-red hotspot in an optical system, comprising placing a screen in an optical path and directing a camera at said screen to detect hotspots on said screen caused by infra-red radiation, comprising heating the screen whereby the power density of low intensity hotspots is raised above a camera threshold.

45. A method as clauseed in clause 44 wherein said screen comprises a wire mesh, and wherein said heating comprises supplying an electrical current to said mesh.

46. A method as clauseed in clause 45 wherein said wire mesh comprises two independent wire arrays and said method comprises supplying each said mesh with an electrical current.

47. A method as clauseed in any of clauses 44 to 46 further comprising placing a screen in an optical path and directing a single camera at said screen to detect hotspots on said screen caused by infra-red radiation, comprising creating multiple optical paths creating multiple images of said screen at said camera, and wherein different amounts of radiation are directed along different optical paths.

48. A method as clauseed in clause 47 further comprising creating more than two optical paths creating more than two images of said screen at said camera, a different amount of radiation being directed along each said optical path.

49. A method of detecting an infra-red hotspot in an optical system, comprising placing a screen in an optical path and directing a single camera at said screen to detect hotspots on said screen caused by infra-red radiation, comprising creating multiple optical paths creating multiple images of said screen at said camera, and wherein different amounts of radiation are directed along different optical paths.

50. A method as clauseed in clause 49 comprising creating more than two optical paths creating more than two images of said screen at said camera, a different amount of radiation being directed along each said optical path.

Claims (1)

A lithography device comprising: an illumination 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.