System for testing a reflector such as a collector reflector and method for testing a reflector such as a collector reflector

Abstract

A system is disclosed that is configured to test a collector reflector, the collector reflector having a first focus and a second focus, the system comprising: a test radiation subsystem that is operable to project test radiation from the second focus onto the collector reflector; a sensor subsystem that is operable to receive test radiation reflected from the collector reflector toward the first focus; and a radiation limiter subsystem that is operable to limit the test radiation received by the sensor to test radiation reflected from a restricted portion of the collector reflector; and a control subsystem that is operable to control movement of the radiation limiter subsystem along a series of different positions, thereby limiting the test radiation received by the sensor to test radiation reflected from a corresponding series of different restricted portions of the collector reflector.

Description

[0001] CROSS-REFERENCE TO RELATED APPLICATIONS

[0002] This application claims priority to European application 18169481.1 filed on April 26, 2018, the entire contents of which are incorporated herein by reference. Technical Field

[0003] The invention relates to a system and a method for testing a mirror.As an example of such a mirror, a collector mirror, such as used in an EUV radiation source, can be mentioned. Background Art

[0004] A lithographic apparatus is a machine that applies a desired pattern to a substrate (usually to a target portion of a substrate). A lithographic apparatus can be used, for example, in the manufacture of an integrated circuit (IC). In that case, a pattern forming device, alternatively referred to as a mask or reticle, can be used to produce a circuit pattern to be formed on a single layer of the IC. This pattern can be transferred to a target portion (e.g., a portion including a die, a die, or several dies) on a substrate (e.g., a silicon wafer). The transfer of the pattern is typically performed via imaging onto a radiation-sensitive material (resist) layer disposed on the substrate. Typically, a single substrate will contain a network of adjacent target portions that are patterned continuously.

[0005] Photolithography is widely recognized as a key step in the manufacture of ICs and other devices and / or structures. However, as the size of features manufactured using photolithography becomes smaller and smaller, photolithography is becoming a more important factor for enabling the manufacture of small ICs or other devices and / or structures.

[0006] A theoretical estimate of the pattern printing limit can be given by the Rayleigh resolution criterion, as shown in equation (1):

[0007]

[0008] where λ is the wavelength of the radiation used, NA is the numerical aperture of the projection system used to print the pattern, k1 is a process-dependent adjustment factor, also known as the Rayleigh constant, and CD is the feature size (or critical dimension) of the printed features. From equation (1), it can be seen that a reduction in the minimum printable size of a feature can be obtained in three ways: by shortening the exposure wavelength λ, by increasing the numerical aperture NA, or by reducing the value of k1.

[0009] 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 in the range of 5nm to 20nm, for example in the range of 13nm to 14nm. It has further been proposed that EUV radiation having a wavelength less than 10nm, for example in the range of 5nm to 10nm, such as 6.7nm or 6.8nm, may be used. Possible sources include laser produced plasma (LPP) sources, but other types of sources are possible.

[0010] An example of current progress in the development of LPP sources for EUV lithography is described in the paper "High power LPP EUV source system development status" by Benjamin Szu-MinLin, David Brandt, Nigel Farrar, SPIE Conference Proceedings Vol. 7520, Lithography Asia 2009, December 2009 (SPIE Digital Library Reference DOI: 10.1117 / 12.839488). In a lithography system, the source apparatus will typically be contained within its own vacuum housing, with a small exit aperture provided to couple the EUV radiation beam to the optical system where the radiation will be used.

[0011] For high resolution patterning for lithography, the EUV radiation beam must be conditioned to obtain desired parameters, such as uniformity of intensity distribution and angular distribution, when it reaches the mask. Examples of irradiation systems are described in U.S. Patent Application Publication Nos. 2005 / 0274897A1 (Carl Zeiss / ASML) and 2011 / 0063598A (Carl Zeiss). Example systems include a "fly's eye" illuminator that transforms the highly non-uniform intensity profile of an EUV source into a more uniform and controllable source.

[0012] For good imaging performance, it is important that the collector mirror as applied in an EUV source has a sufficiently high and uniform reflectivity. Such a collector mirror may be contaminated due to debris generated by the EUV radiation generation process. Thus, the collector mirror may be regularly subjected to a cleaning process followed by an inspection or testing process in order to assess the reflectivity.

[0013] Known methods for determining reflectivity are considered to be quite time-consuming and expensive. Other mirrors applied in, for example, EUV radiation sources or lithographic equipment may also require the aforementioned inspection and testing. Similarly, known methods for determining the reflectivity of such mirrors may also be time-consuming and expensive. Summary of the invention

[0014] Various aspects of embodiments of the present invention are directed to providing an alternative system and method for testing a collector mirror of an EUV source.

[0015] According to one aspect of the present invention, there is provided a system configured to test a collector reflector having a first focus and a second focus, the system comprising: a test radiation subsystem operable to project test radiation from the second focus onto the collector reflector; a sensor subsystem operable to receive test radiation reflected from the collector reflector toward the first focus; and a radiation limiter subsystem operable to limit the test radiation received by the sensor to test radiation reflected from a restricted portion of the collector reflector; and a control subsystem operable to control movement of the radiation limiter subsystem along a series of different positions, thereby limiting the test radiation received by the sensor to test radiation reflected from a corresponding series of different restricted portions of the collector reflector.

[0016] According to another aspect of the present invention, a method for testing a collector reflector is provided, wherein the collector reflector has a first focus and a second focus, the method comprising: projecting test radiation from the second focus onto the collector reflector; receiving, by a sensor, the test radiation reflected from the collector reflector toward the first focus; and limiting the test radiation received by the sensor to test radiation reflected from a restricted portion of the collector reflector; and controlling the movement of a radiation limiting system along a series of different positions, thereby limiting the test radiation received by the sensor to test radiation reflected from a corresponding series of different restricted portions of the collector reflector.

[0017] According to yet another aspect of the present invention, there is provided a system configured to test a reflector, the system comprising:

[0018] a test radiation subsystem operable to project test radiation onto the reflector;

[0019] a sensor subsystem operable to receive test radiation reflected from the reflector; and

[0020] a radiation limiter subsystem operable to limit the test radiation as received by the sensor subsystem to test radiation reflected from a restricted portion of the reflector;

[0021] A control subsystem is operable to control movement of the radiation limiter subsystem along a series of different positions to thereby limit the test radiation received by the sensor subsystem to test radiation reflected from a corresponding series of different restricted portions of the reflector.

[0022] These aspects of the present invention as well as its various optional features and embodiments will be understood by those skilled in the art from the following description of examples. BRIEF DESCRIPTION OF THE DRAWINGS

[0023] Embodiments of the invention will now be described, by way of example only, with reference to the accompanying schematic drawings, in which corresponding reference numerals indicate corresponding parts, and in which:

[0024] Figure 1 schematically depicts a lithography system according to an embodiment of the present invention;

[0025] Figure 2 yes Figure 1 A more detailed view of the system and showing a new monitoring and control system for an EUV radiation source;

[0026] Figure 3 schematically shows a cross section of an embodiment of a system according to the invention;

[0027] Figures 4a to 4c schematically illustrates a cross-sectional view of a test radiation subsystem as can be applied in the present invention;

[0028] Figure 5a to Figure 5f schematically illustrates a cross-sectional view of a sensor subsystem as can be applied in the present invention;

[0029] Figure 6 schematically shows a cross section of another embodiment of a system according to the invention;

[0030] Figure 7 schematically illustrates a radiation limiter subsystem as can be applied in a system according to the invention;

[0031] Figure 8 Schematically shows a cross section of a system according to the invention in a calibration position;

[0032] Fig. 9 schematically shows a cross section of a further embodiment of a system according to the invention;

[0033] Fig.10 A portion of a collector mirror as can be inspected by the system according to the invention is schematically shown.

[0034] Fig.11The system according to the invention and the inventors are schematically shown on a whiteboard.

[0035] Fig.12 schematically shows a cross section of an embodiment of a system according to the invention for testing or inspecting a mirror;

[0036] Fig.13 A further exemplary embodiment of a system according to the invention for testing or inspecting a mirror is schematically shown in cross section. DETAILED DESCRIPTION

[0037] Figure 1 A lithography system 100 according to an embodiment of the present invention is schematically depicted, the lithography system comprising a lithography apparatus and an EUV radiation source, the EUV radiation source being configured to generate EUV radiation, such as an EUV radiation beam. In the embodiment as shown, the EUV radiation source comprises a source collector module SO. In the embodiment as shown, the lithography scanning apparatus comprises: an illumination system (illuminator) IL, the illumination system being configured to condition a radiation beam B (e.g., EUV radiation); a support structure (e.g., a mask stage) MT, the support structure being configured to support a pattern forming device (e.g., a mask or reticle) MA, and being connected to a first positioner PM configured to accurately position the pattern forming device; a substrate stage (e.g., a wafer stage) WT, the substrate stage being configured to hold a substrate (e.g., a wafer coated with a resist) W, and being connected to a second positioner PW configured to accurately position the substrate; and a projection system (e.g., a reflective projection system) PS, the projection system being configured to project a pattern imparted to the radiation beam B by the pattern forming device MA onto a target portion C (e.g., comprising one or more dies) of the substrate W.

[0038] The illumination system IL may include various types of optical components for directing, shaping or controlling the radiation, such as refractive, reflective, magnetic, electromagnetic, electrostatic or other types of optical elements, or any combination thereof.

[0039] The support structure MT holds the patterning device MA in a manner that depends on the orientation of the patterning device MA, the design of the lithographic apparatus, and other conditions such as, for example, whether 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 can be, for example, a frame or a table, which can be fixed or movable as required. The support structure can ensure that the patterning device is in a desired position, for example relative to the projection system.

[0040] 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 so as to create a pattern in a target portion of a substrate. The pattern imparted to the radiation beam may correspond to a specific functional layer in a device (such as an integrated circuit) being created in the target portion.

[0041] 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 photolithography 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 uses a matrix arrangement of small mirrors, each of which can be individually tilted so as to reflect an incident radiation beam in different directions. The tilted mirrors impart a pattern to the radiation beam reflected by the mirror matrix.

[0042] Similar to the illumination system, the projection 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, suitable for the exposure radiation used or for other factors such as the use of a vacuum. It may be desirable to use a vacuum for EUV radiation because other gases may absorb too much radiation. Therefore, a vacuum environment may be provided to the entire beam path with the aid of vacuum walls and a vacuum pump.

[0043] As depicted here, the device is of the reflective type (eg, using a reflective mask).

[0044] 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 a "multi-stage" machine, the extra 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.

[0045] refer to Figure 1 , the illuminator IL receives an extreme ultraviolet radiation beam from a source collector module SO of an EUV radiation source. Methods for generating EUV light include, but are not necessarily limited to, utilizing one or more emission lines in the EUV range to convert a material into a plasma state having at least one element (e.g., xenon, lithium, or tin). In one such method, often referred to as laser produced plasma ("LPP"), the desired plasma may be generated by irradiating a fuel such as a droplet, stream, or cluster of material having the desired line-emitting element with a laser beam. The source collector module SO may be a device including Figure 1The fuel is excited by the plasma. The laser is used to provide a laser beam for the fuel. The resulting plasma emits output radiation, such as EUV radiation, which is collected using a radiation collector disposed in a source collector module. For example, when a CO2 laser is used to provide the laser beam for fuel excitation, the laser and EUV radiation source may be separate entities.

[0046] In such cases, the laser is not considered to form part of the lithography system, and the radiation beam is delivered from the laser to the source collector module by means of a beam delivery system including, for example, suitable directing mirrors and / or a beam expander. In other cases, for example, when the source is a discharge produced plasma EUV generator, which is often referred to as a DPP source, the source may be an integral part of the source collector module.

[0047] The illuminator IL may include an adjuster for adjusting the angular intensity distribution of the radiation beam. Typically, at least the outer and / or inner radial extent of the intensity distribution in a pupil plane of the illuminator (commonly referred to as σ-outer and σ-inner, respectively) may be adjusted. In addition, the illuminator IL may include various other components, such as a faceted field mirror arrangement and a faceted pupil mirror arrangement. The illuminator may be used to adjust the radiation beam to have a desired uniformity and intensity distribution in its cross-section.

[0048] The radiation beam B is incident on a patterning device (e.g. a mask) MA held on a support structure (e.g. a mask table) MT and is patterned by the patterning device. After reflection from the patterning device (e.g. a mask) MA, the radiation beam B passes through a projection system PS which focuses the beam onto a target portion C of the substrate W. With the aid of a second positioner PW and a position sensor PS2 (e.g. an interferometric device, a linear encoder or a capacitive sensor), the substrate table WT can be accurately moved, for example in order to position a different target portion C in the path of the radiation beam B. Similarly, a first positioner PM and a further position sensor PS1 can be used to accurately position the patterning device (e.g. a mask) MA relative to the path of the radiation beam B. The patterning device (e.g. a mask) MA and the substrate W can be aligned using mask alignment marks M1, M2 and substrate alignment marks P1, P2.

[0049] The depicted device may be used in at least one of the following modes:

[0050] 1. In step mode, the support structure (e.g. mask table) MT and substrate table WT are kept substantially stationary while the entire pattern imparted to the radiation beam is projected at one time onto a target portion C (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.

[0051] 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 speed 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.

[0052] 3. In another mode, the support structure (e.g. mask table) MT holding the programmable patterning device is kept substantially stationary 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, a pulsed radiation source is typically used and the programmable patterning device is updated as required after each movement of the substrate table WT or between successive radiation pulses during a scan. This mode of operation can be readily applied to maskless lithography using a programmable patterning device, such as a programmable mirror array of the type mentioned above.

[0053] Combinations and / or variations on the above described modes of use or entirely different modes of use may also be used.The embodiments to be illustrated involve scanning, such as in modes 2 and 3 just mentioned.

[0054] Although specific reference may be made herein to the use of lithographic apparatus in IC manufacturing, it should be understood that the lithographic apparatus described herein may have other applications, such as manufacturing integrated optical systems, guidance and detection patterns for magnetic domain memories, flat panel displays, liquid crystal displays (LCDs), thin film magnetic heads, and the like. It will be appreciated by those skilled in the art that in the context of such alternative applications, any term "wafer" or "die" used herein may be considered synonymous with the more general term "substrate" or "target portion", respectively. The substrates referred to herein may be processed in, for example, a coating and developing system (a tool that typically applies a resist layer to a substrate and develops the exposed resist), a metrology tool, and / or an inspection tool before or after exposure. Where applicable, the disclosure herein may be applied to such and other substrate processing tools. In addition, the substrate may be processed more than once, for example, to produce a multi-layer IC, so that the term substrate used herein may also refer to a substrate that already contains multiple processed layers.

[0055] Figure 2The system 100 is shown in more detail, and the system includes an EUV radiation source including a source collector module SO, a lithography scanning device including an illumination system IL, and a projection system PS. The source collector module SO of the EUV radiation source is constructed and arranged so that a vacuum environment can be maintained in the closed structure 220 of the source collector module SO. The systems IL and PS are likewise contained in their own vacuum environments. The plasma 210 emitting EUV radiation can be formed by a laser-generated LPP plasma source. The function of the source collector module SO is to transfer the EUV radiation beam 20 from the plasma 210 so that it is focused in a virtual source point. The virtual source point is usually referred to as an intermediate focus (IF), and the source collector module is arranged so that the intermediate focus IF is located at or near the hole 221 in the closed structure 220. The virtual source point IF is an image of the plasma 210 emitting radiation.

[0056] From the aperture 221 at the intermediate focus IF, the radiation traverses an illumination system IL, which in this example comprises a faceted field mirror arrangement 22 and a faceted pupil mirror arrangement 24. These arrangements form a so-called "fly's eye" illuminator arranged to provide a desired angular distribution of the radiation beam 21 at the patterning device MA, and a desired uniformity of radiation intensity at the patterning device MA. After reflection of the beam 21 at the patterning device MA held by the support structure (mask table) MT, a patterned beam 26 is formed, and is imaged by the projection system PS via reflective elements 28, 30 onto a substrate W held by a wafer stage or substrate table WT.

[0057] Each system IL and PS is arranged within its own vacuum or near-vacuum environment defined by an enclosure structure similar to enclosure structure 220. There may typically be more elements than shown in illumination system IL and projection system PS. Also, there may be more mirrors than shown in the figures. For example, in addition to Figure 2 In addition to the reflective elements shown in FIG. 1 , there may be one to six additional reflective elements in the illumination system IL and / or the projection system PS. For example, the above-mentioned US patent application publication shows three additional elements in the illumination system.

[0058] Considering the source collector module SO in more detail, a laser energy source including a laser 223 is arranged to deposit laser energy 224 into a fuel such as xenon (Xe), tin (Sn) or lithium (Li), thereby producing a highly ionized plasma 210 with an electron temperature of tens of eV. Other fuel materials such as Tb and Gd can be used to produce higher energy EUV radiation. The high energy radiation produced during the deexcitation and recombination or recombination of these ions is emitted from the plasma, collected by a near normal incidence collector mirror CO and focused on an aperture 221. The plasma 210 and the aperture 221 are located at the first and second focal points of the collector or collector mirror CO, respectively.

[0059] To deliver a fuel, such as liquid tin, a droplet generator 226 is arranged within the enclosure 220, the droplet generator 226 being arranged to emit a high frequency droplet stream 228 toward a desired location of the plasma 210. In operation, laser energy 224 is delivered synchronously with the operation of the droplet generator 226 to deliver a radiation pulse to cause each fuel droplet to become a plasma 210. The delivery frequency of the droplets may be several kilohertz, such as 50 kHz. In practice, the laser energy 224 is delivered in at least two pulses: a pre-pulse and a main pulse. The pre-pulse delivers limited energy to the droplet before it reaches the plasma location, so as to condition the droplet for receiving the main pulse, such as by shaping the droplet into a pancake or by vaporizing the fuel material into a small cloud. The main pulse is delivered to the conditioned droplet at the desired location to generate the plasma 210. A catcher 230 is provided on the opposite side of the enclosure 220 to catch fuel that, for whatever reason, does not become a plasma.

[0060] There are numerous additional components in the source collector module and lithographic equipment in a typical lithography system, but they are not illustrated here. These components include arrangements for reducing or mitigating the effects of contamination within a closed vacuum, for example to prevent deposits of fuel material from damaging or impairing the performance of the collector mirror CO and other optical devices. In addition, one or more spectral purity filters may be included in the source collector module SO and / or the illumination system IL. In addition to EUV radiation of the desired wavelength, these filters are also used to eliminate as much radiation of unwanted wavelengths as possible generated by the laser and / or plasma 210. The spectral purity filter can be positioned near the virtual source point or at any point between the collector mirror CO and the virtual source point. The filter can be placed at other locations in the radiation path, such as downstream of the virtual source point IF. Multiple filters can be deployed. Those skilled in the art are familiar with the need for these measures and the manner in which the measures can be implemented, and no other details are required for the purposes of this disclosure.

[0061] For more details, refer to Figure 2 The laser 223 of the embodiment presented is of the master oscillator power amplifier (MOPA) type. This type consists of a "master" laser or "seed" laser, which is labeled MO in the diagram, followed by a power amplifier (PA). A beam delivery system 240 is provided to deliver laser energy 224 to module SO. In practice, the pre-pulse element of the laser energy will be delivered by a separate laser that is not separately shown in the diagram. The laser 223, the fuel source (i.e., droplet generator) 226 and other components can be controlled, for example, by a source control module 242.

[0062] Those of ordinary skill in the art will appreciate that reference axes X, Y, and Z may be defined to measure and describe the geometry and behavior of a lithographic system, its various components, and the radiation beams 20, 21, 26. At each portion of the lithographic system, a local reference coordinate system of the X, Y, and Z axes may be defined. The Z axis roughly coincides with the direction of the optical axis O at a given point in the system, and is generally orthogonal to the plane of the patterning device (reticle) MA and to the plane of the substrate W. In the source collector module, the X axis roughly coincides with the direction of the fuel flow (228, described below), while the Y axis is orthogonal to that direction, pointing out of the page, as shown in FIG. Figure 2 On the other hand, near the support structure MT holding the mask MA, the X-axis is generally transverse to the scanning direction aligned with the Y-axis. Figure 2 In such a region, 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 coordinate system can be chosen to describe the lithography system and its behavior.

[0063] Referring to the illumination system in slightly more detail, the faceted field mirror arrangement 22 comprises an array of individual facets such that the EUV radiation beam 20 is divided into a plurality of sub-beams, one of which is labeled 260 in the figure. Each sub-beam is directed toward an individual facet on the faceted pupil mirror arrangement 24. The facets of the pupil mirror arrangement 24 are arranged to direct their individual sub-beams onto a target which is a slit-shaped region of the pattern forming device MA. The division into sub-beams 260 and the combination into a single beam 21 are designed to produce highly uniform illumination over the slit region when the illumination arriving from the source collector module is highly non-uniform in its angular distribution. It is also known that the facets of the arrangements 22 and / or 24 may be steerable and / or shieldable in order to implement different illumination modes.

[0064] The conditioned EUV radiation beam 21 is delivered to the patterning device MA by means of a conditioning mask module 262. Such a module comprises a mask unit, also referred to as a mask mask (REMA), which may have movable blades defining the extent of an illumination slit in the X and Y directions. Typically, the illumination slit applied, as in an EUV type lithography apparatus, may be curved.

[0065] In front of the REMA there may also be an illumination uniformity correction module (UNICOM).

[0066] To expose a target portion C on substrate W, a pulse of radiation is generated on substrate table WT and the shielded table MT performs a synchronized movement 266, 268 to scan a pattern on patterning device MA through the illumination slit.

[0067] Examples of illumination systems that include REMA and UNICOM functionality are described in US Patent Application Publication Nos. 2005 / 0274897A1 and 2011 / 0063598A.

[0068] A number of measures are applied in the source controller 242. Such measures include monitoring to ensure that the virtual source point IF is aligned with the aperture 221 at the exit of the source collector module SO. In systems based on LPP sources, control of the alignment is usually achieved by controlling the position of the plasma 210 rather than by moving the collector mirror CO. The collector mirror, the exit aperture 221 and the illuminator IL are accurately aligned during the set-up process so that the aperture 221 is located at the second focus of the collector mirror. However, the exact position of the virtual source point IF formed by the EUV radiation at the exit of the source optics depends on the exact position of the plasma 210 relative to the first focus of the collector mirror. Fixing this position accurately enough to maintain adequate alignment usually requires active monitoring and control.

[0069] For this purpose, a source control module (controller) 242 controls the location of the plasma 210 (EUV radiation source) in this example by controlling the injection of fuel and also controlling, for example, the timing of the excitation pulses from the laser. In a typical example, the excitation pulses of laser radiation 224 are delivered at a rate of 50kHz (period 20μs) and in bursts lasting anywhere from, for example, 20ms to 20 seconds. The duration of each main laser pulse may be about 1μs, while the resulting EUV radiation pulse may last about 2μs. By appropriate control, the EUV radiation beam is maintained accurately focused on the aperture 221 by the collector mirror CO. If this is not achieved, all or part of the beam will impinge on the surrounding material of the enclosed structure.

[0070] The source control module 242 is supplied with monitoring data from one or more sensor arrays (not shown) that provide a first feedback path for information about the location of the plasma. The sensors may be of various types, such as described in the U.S. Patent Application Publication No. 2005 / 0274897A1 mentioned above. The sensors may be located at more than one position along the radiation beam path. For example only, the sensors may be located around and / or behind the field reflector device 22. The sensor signals just described may be used to control the optical systems of the illuminator IL and the projection system PS. The sensor signals may also be used via a feedback path to assist the control module 242 of the source collector module SO to adjust the intensity and position of the EUV plasma source 210. The sensor signals may be processed, for example, to determine the observed location of the virtual source IF, and such location may be extrapolated to indirectly determine the location of the EUV source. If the virtual source location drifts, as indicated by the sensor signal, a correction is applied by the control module 242 to re-center the beam in the aperture 221.

[0071] Additional sensors and feedback paths can typically be provided in the source collector module SO itself, rather than relying entirely on signals from the illuminator sensors, to achieve faster, direct and / or independent control of the radiation source. Such sensors may include, for example, one or more cameras that monitor the location of the plasma. In this way, the location beam 20 is maintained in the aperture 221, and damage to the equipment is avoided and efficient use of the radiation is maintained.

[0072] In order to ensure that the substrate W is provided with a suitable dose of radiation, it is important to ensure that the generated EUV radiation is sufficiently reflected by the collector or collector mirror CO. In particular, it is desirable for the collector mirror CO to have a sufficiently high reflectivity and for the reflectivity to be as uniform as possible.

[0073] In this regard, it may be noted that during EUV radiation generation as described above, debris may be generated due to the interaction of the fuel droplet stream 228 with the laser pulses. Such debris may cause contamination of the collector mirror CO, thus adversely affecting the reflectivity and the uniformity of the reflectivity. Furthermore, other sources of debris may adversely affect the reflectivity and the uniformity of the reflectivity of the collector mirror CO. Possible sources of such contamination include, for example, droplets or particles or contaminants falling from components of the EUV source arranged above the collector mirror CO, or contamination due to spraying from surfaces such as blade surfaces or tin catchers.

[0074] To reduce the adverse effects of pollution, it is possible to Figure 2 The collector reflector mirror CO shown in FIG. 1 is subjected to a cleaning process to remove contamination. Such a cleaning process may, for example, include a carbon dioxide snow cleaning process.

[0075] In order to check that the cleaning process has been effective, it is preferred to subject the collector mirror to an inspection after the cleaning process, for example evaluating the reflectivity of the collector mirror at a plurality of locations on the collector mirror.

[0076] The present invention provides a system for performing such inspection or testing of a collector mirror of an EUV lithography system, and a method of inspecting or testing a collector mirror of an EUV lithography system.

[0077] More generally, the present invention provides a system for performing inspection or testing of a mirror. Various types of mirrors may be inspected or tested using the present invention. Examples of such mirrors include mirrors applied in, for example, EUV radiation sources, lithographic projection systems, illumination systems such as used in lithographic apparatus, and the like. Figure 2 The invention can, for example, be applied to inspect or test a collector mirror CO, any mirror such as the facet field device 22 or the pupil mirror device 24 as applied in an illuminator IL, any mirror such as the reflective elements 28 or 30 as applied in a projection system PS.

[0078] Figure 3 An embodiment of a system according to the invention for testing a collector mirror is schematically shown. The collector mirror 300 has a first focus (focus point) 310 and a second focus 320. The focus 310 may correspond substantially to, for example, Figure 2 The radiation-emitting plasma 210 shown in FIG. 1 is formed at a location, and the focus 320 corresponds to, for example, Figure 2 The intermediate focus IF is shown in FIG.

[0079] The system according to the invention comprises a test radiation subsystem 330 operable to project test radiation 330.1 from the second focus 320 to the collector mirror 300. Test radiation within the meaning of the invention refers to radiation used during testing or inspection of a mirror, such as the illustrated collector mirror 300.

[0080] In an embodiment of the present invention, the test radiation as applied has a wavelength or wavelength spectrum selected based on the radiation used under normal operating conditions of the collector reflector. Specifically, if the collector reflector is to be used in an EUV lithography system, the wavelength or wavelength spectrum of the test radiation can be selected based on the EUV radiation applied in the lithography equipment. Thus, in an embodiment of the present invention, the test radiation may include radiation having a wavelength in the range of 10nm to 20nm. In an embodiment, the test radiation as applied may have a wavelength spectrum of 13.5nm+ / -1nm half-maximum width (FWHM). In an embodiment, the test radiation subsystem as applied in the system according to the present invention may be configured to apply radiation of different wavelengths or different wavelength ranges to inspect or test the reflector. In such an embodiment, the test radiation subsystem may include multiple radiation sources and / or multiple filters, which are used to filter the generated radiation so as to achieve the desired test radiation. As an example, it may be useful to evaluate the reflectivity of a reflector such as the collector reflector 300 at different wavelengths or wavelength ranges. It may be possible, for example, to evaluate the reflectivity of a collector mirror in the aforementioned spectrum of 13.5 nm + / - 1 nm, but also in a spectrum covering or including infrared (IR) radiation.

[0081] In an embodiment, the test radiation subsystem is configured to generate test radiation across an angle α0 such that in principle the entire collector mirror 300 can be irradiated. Note that Figure 3 The illustration of represents the collector mirror 300 in a two-dimensional cross section, and in practice, the test radiation is typically emitted in three dimensions. Figure 3 The angle α0 in α is the angle subtended by the three-dimensional test radiation pattern when projected onto a two-dimensional plane having a cross-section of the collector mirror 300. In an embodiment, the test radiation subsystem is configured to generate test radiation spanning an angle less than the angle α0. In such an embodiment, the entire collector mirror 300 may still be scanned or irradiated, for example, by rotating the test radiation subsystem 330 or a portion thereof.

[0082] The system according to the present invention also includes a sensor subsystem, schematically indicated by reference numeral 340, which is operable to receive test radiation reflected from the collector reflector 300 toward the first focus 310, such as the test radiation indicated by arrow 342. In embodiments, as will be discussed in more detail below, the sensor subsystem 340 may include one or more sensors or detectors for measuring the test radiation reflected from the reflector (i.e., the collector reflector 300).

[0083] According to the present invention, the system for testing the collector reflector further comprises a radiation limiter subsystem 360. The radiation limiter subsystem 360 is configured to limit the test radiation received by the sensor subsystem 340 to the test radiation reflected from the restricted portion of the collector reflector 300. Thereby, the radiation measured or detected by the sensor subsystem will only relate to the radiation reflected from the restricted portion of the collector reflector 300. In the embodiment as shown, the restricted portion of the collector reflector is indicated by the reflector segment 300.1. By limiting the test radiation received by the sensor subsystem to the test radiation reflected from the restricted portion of the collector reflector, the performance of said specific portion of the collector reflector can be evaluated, for example the reflectivity of said portion can be evaluated.

[0084] In the embodiment as shown, the radiation limiter subsystem 360 is configured to enable only a small portion of the generated radiation 330.1 to pass towards the collector reflector. The small portion of the radiation 330.1 subtends an angle that is much smaller than the angle α0. In order to achieve this, the radiation limiter subsystem 360 includes a shielding member 360.1 that blocks the test radiation, whereby the shielding member 360.1 includes holes, such as tubular holes 360.2, through which the small portion of the test radiation 330.1 can pass. The radiation limiter subsystem 360 thus enables only a limited portion, such as the portion 300.1 of the collector reflector 300, to be irradiated by the test radiation generated by the test radiation subsystem 330.

[0085] According to the invention, the system further comprises a control subsystem 370, which is configured to control the movement of the radiation limiter subsystem 360, for example by providing a control signal 370.1 to the radiation limiter subsystem 360. In particular, the control subsystem 370 is configured to control the position of the radiation limiter subsystem 360 such that the restricted portion of the collector reflector irradiated by the test radiation can be changed. Thus, the control subsystem can, for example, be configured to control the movement of the radiation limiter subsystem along a series of different positions, thereby limiting the test radiation received by the sensor subsystem 340 to the test radiation reflected from a corresponding series of different restricted portions of the collector reflector 300. Thereby, the performance of the different restricted portions of the collector reflector can be evaluated, for example the reflectivity of these portions can be evaluated.

[0086] In an embodiment of the present invention, a set of different restricted portions substantially covers the entire collector mirror 300 .

[0087] In an embodiment of the invention, the radiation limiter subsystem comprises one or more actuators to displace the aperture tube 360.2, for example together with the shielding member 360.1, so as to change the restricted portion of the collector reflector 300 irradiated by the test radiation. In such an embodiment, the aperture tube 360.2 can be rotated, for example, around an axis perpendicular to the plane of the drawing and, for example, passing through the second focal point 320. By doing so, the angle α defining the angle at which the test radiation impinges on the collector reflector can be modified. In a similar manner, the aperture tube 360.2 can be rotated, for example, around an axis parallel to the indicated Z axis and, for example, passing through the second focal point 320. By combining the two rotational movements, the entire surface of the collector reflector can be selectively scanned by the test radiation. Such an embodiment will be explained in more detail below.

[0088] Those skilled in the art will appreciate that other types of actuators, such as linear actuators or motors, may also be applied to displace the radiation limiter subsystem, thereby controlling which limited portion of the collector mirror is being irradiated.

[0089] In an embodiment of the present invention, the test radiation subsystem 330 or the radiation limiter subsystem 360 is configured to shape the generated test radiation into a beam. In such an embodiment, the test radiation subsystem 330 and / or the radiation limiter subsystem 360 is configured to project the test radiation as a beam onto the collector reflector 300. In such an embodiment, the control subsystem 370 can be configured to direct the beam or beam-shaped test radiation onto a plurality of different restricted portions of the collector reflector, one restricted portion after another. Such sequential projection of the test radiation beam onto different restricted portions of the collector reflector can also be referred to as scanning the collector reflector with the test radiation beam. During such a scanning process, whereby a portion or the entire reflector of the collector reflector 300 is scanned, the sensor subsystem 340 can generate a set of measurement data representing the test radiation reflected from the different restricted portions of the collector reflector 300. Such measurement data can be provided to the control subsystem 370 of the system according to the present invention, for example via a data channel, indicated by arrow 370.2. In an embodiment, the measurement data 370.2 as received may for example be processed by a processing unit 380 of the control subsystem 370. Such a processing unit may for example be implemented as a processor, a microprocessor, a computer etc. Such a processing unit 380 may for example comprise a memory unit for storing the measurement data.

[0090] In an embodiment, the processing unit 380 may be configured to process the measurement data in order to determine a spatial reflectivity distribution of at least a portion of the collector reflector. In such an embodiment, measurement data representing test radiation reflected from different confined portions of the collector reflector 300 may be compared with data representing generated or emitted test radiation to determine the reflectivity of the different confined portions of the collector reflector. Examples of measures of reflectivity are the ratio of the amount of test radiation received to the amount of test radiation generated or emitted, and the difference between (on the one hand) the amount of test radiation generated or emitted and (on the other hand) the amount of test radiation received. Thus, in an embodiment of the present invention, the measurement data obtained from the sensor subsystem 340 may be supplemented with source measurement data representing the amount of test radiation emitted or generated during the scanning process. Such source measurement data may be provided to the processing unit 380 of the control subsystem 370, indicated by arrow 370.3, for example via a data channel. In such an embodiment, test radiation subsystem 330 may also include a sensor, referred to as a source sensor, that is configured to generate a source measurement signal representative of the test radiation projected by test radiation subsystem 330. By combining the measurement data of the sensor subsystem with the source measurement data of the test radiation subsystem, the reflectivity across collector mirror 300 may be more accurately assessed because any fluctuations in the test radiation, as generated and emitted during the scanning process, may be accounted for based on the source measurement data.

[0091] As an alternative to applying a source sensor in the test radiation subsystem 330, it is worth mentioning that a sensor may also be applied in the aperture tube 360.2 in order to obtain a measure of the amount of test radiation emitted towards the collector reflector 300. In a similar manner, the measurement data of such an aperture sensor may be combined with the measurement data obtained from the sensors of the sensor subsystem 340 in order to determine a spatial reflectivity map of at least a portion of the collector reflector 300.

[0092] Preferably, as already indicated above, for example when the collector mirror is used as Figure 2 , the wavelength or wavelength spectrum of the applied test radiation preferably corresponds to the wavelength or wavelength spectrum as used during normal operation of the collector mirror. By doing so, the reflectivity of the collector mirror can be determined more accurately for the relevant wavelength or wavelength spectrum. To achieve this, there are different options.

[0093] In an embodiment of the present invention, the test radiation subsystem 330 includes a radiation source and a multilayer reflector assembly, which is used to spectrally filter the generated radiation to achieve the desired test radiation, that is, test radiation with an appropriate wavelength or wavelength spectrum. As is known, a multilayer reflector, also known as a Bragg reflector, is a type of reflective optical element composed of multiple layers with alternating thicknesses and refractive indices. The thickness is tuned to control the constructive interference of radiation of the desired wavelength reflected from subsequent layers. In an embodiment, the radiation source of the test radiation subsystem 330 includes a Xe EUV source. In such an embodiment, the multilayer reflector arrangement can be configured to filter the radiation into a wavelength spectrum of, for example, 13.5nm+ / -1nm half-width at half maximum (FWHM).

[0094] Fig. 4 schematically shows three possible multilayer mirror arrangements for generating suitable test radiation, which can be applied, for example, to a test radiation subsystem 330 of a system according to the invention.

[0095] Figure 4a Schematically shown is a radiation source 420, for example a radiation source for generating EUV radiation. In the embodiment as shown, an aperture 410 is provided to allow a portion of the generated radiation indicated by arrow 420.1 to impinge on a first reflector 430, for example a multilayer reflector. The first reflector 430 is configured to reflect the received radiation towards a second reflector 440 (for example a multilayer reflector), indicated by arrow 420.2. The second reflector 440 is configured to reflect the received radiation towards an aperture 450, indicated by arrow 420.3. The aperture may be, for example, an outlet of an aperture tube 460, which is configured to receive radiation 420.3 reflected from the reflector 440. In the embodiment as shown, the reflectors 430 and 440 may be used to filter undesired wavelengths or wavelength components from the radiation generated by the source 420. Alternatively or in addition, mirrors 430 and 440 may form a beam expander or beam reducer for controlling the width, and hence the intensity, of a beam formed by test radiation impinging on collector 300 .

[0096] In an embodiment of the present invention, the reflector assembly includes reflectors 430 and 440, and the aperture tube 460 can be used as a radiation limiter subsystem as discussed above. In such an embodiment, the reflector assembly and the aperture tube can, for example, be configured to be displaceable relative to the radiation source 420 so as to change the direction of the radiation 420.3 emitted from the aperture. By appropriately setting the size of the reflector or reflector assembly, the radiation beam 420.3 emitted from the aperture 450 can be a collimated beam 420.4, that is, a parallel radiation beam that can impinge on a restricted portion of the collector reflector. In order for the collimated beam 420.4 to be reflected from the collector reflector towards the first focus of the reflector, it is necessary to emit the collimated beam 420.4 from the second focus. This can be achieved, for example, by arranging the reflector assembly and the aperture tube so that the second focus is within the aperture tube, such as at location 470.

[0097] Figure 4b Another embodiment of a mirror assembly that may be applied to filter radiation from a radiation source is schematically shown. Figure 4b A radiation source 520 is schematically shown, for example a radiation source for generating EUV radiation. In the embodiment as shown, the radiation source 520 is configured to emit a portion of the generated radiation, indicated by arrow 520.1, to impinge on a first reflector 530, such as a multilayer reflector, shown here in cross-section. The first reflector 530 is configured to reflect the received radiation toward a second reflector 540 (e.g., a multilayer reflector), indicated by arrow 520.2. The second reflector 540 is configured to reflect the received radiation toward a focal point 550. As discussed above, the reflectors 530 and 540 can be used to filter the radiation generated by the radiation source 520 to obtain test radiation having an appropriate wavelength or spectrum. From the focal point 550 forward, the filtered radiation 520.3 can be used, for example, for scanning as described above with reference to FIG. Figure 3 300). In such an embodiment, the generated test radiation may be limited, for example, by a radiation limiter 360 configured to limit the generated test radiation, such as radiation 520.3, to impinge on a limited portion of the collector reflector. In order to ensure that the emitted test radiation 520.3, or the portion thereof impinging on the collector reflector, is reflected from the collector reflector (e.g., collector reflector 300) towards the first focus of the reflector, it is necessary to emit the test radiation 520.3 from the second focus of the collector reflector. This may be achieved, for example, by arranging the reflector assemblies 530, 540 so that the second focus of the collector reflector substantially coincides with the focus 550.

[0098] The mirror arrangement comprising mirrors 530 and 540 is known as a Schwarzschild objective.

[0099] Figure 4c A further embodiment of a mirror assembly which can be applied to filter radiation from a radiation source is schematically shown, said mirror assembly also comprising a Schwarzschild objective including mirrors 630 and 640 . Figure 4c A radiation source 620 is schematically shown, for example a radiation source for generating EUV radiation. In the embodiment as shown, the radiation source 620 is configured to emit a portion of the generated radiation indicated by arrow 620.1 to impinge on a first mirror 630, for example a multilayer mirror, shown here in cross-section. The first mirror 630 is configured to reflect the received radiation towards a second mirror 640 (for example a multilayer mirror), indicated by arrow 620.2. The second mirror 640 is configured to reflect the received radiation through an aperture 660 of the first mirror 630 as a collimated beam 620.3. Those skilled in the art will appreciate that by appropriately sizing and positioning the mirrors 630 and 640 of the Schwarzschild objective, it can be ensured that the emitted radiation is shaped into a collimated beam 620.3, rather than as a collimated beam. Figure 4b 6. The reflector arrangement 630, 640 can thus both act as filters for the generated test radiation having an appropriate wavelength or spectrum of wavelengths and shape the test radiation so as to obtain a test radiation beam 620.3 configured to impinge on a restricted portion of the inspected collector mirror. Thus, the reflector arrangement can be considered to act as a radiation limiter that limits the test radiation to impinge on a restricted portion of the collector mirror.

[0100] Again, to ensure that radiation is reflected from a collector mirror (e.g., collector mirror 300) toward the first focus of the mirror, it is necessary to emit test radiation 620.3 from the second focus of the collector mirror. This can be achieved, for example, by arranging a second mirror 640 at or near the second focus of the collector mirror under inspection.

[0101] In a similar manner as discussed above, the multilayer mirror assemblies 630, 640 may be configured to be displaceable so as to scan at least a portion of the surface of the collector mirror with test radiation 620.3 so as to determine the reflectivity of the mirror.

[0102] In an embodiment of the invention, the test radiation subsystem as applied further comprises a sensor for sensing the radiation emitted by the source. Such a sensor (also referred to as a source sensor) may, for example, be configured to directly receive the radiation emitted by the source. Figure 4c, such a sensor 650 is schematically shown, the sensor 650 being arranged to receive radiation 620.4 emitted by the source 620. Alternatively, the source sensor 650 may be arranged to receive a portion of the test radiation reflected from a reflector (e.g. the first reflector 630 or the second reflector 640) of a reflector arrangement as applied in embodiments of the present invention. In such an arrangement, the source sensor may thus measure radiation having the same wavelength or wavelength spectrum as the test radiation received by the collector reflector.

[0103] In embodiments of the present invention, a source sensor (i.e., a sensor configured to generate source measurement data representative of radiation emitted by a source of the test radiation subsystem or test radiation) may also be arranged in a location such as Figure 3 360 . In such an embodiment, the source sensor may be arranged at or near or in the orifice tube 360 ​​. 2 of the radiation limiter subsystem 360 so as to capture a portion of the radiation irradiated by the test radiation subsystem 330 .

[0104] As already indicated above, the system according to the invention comprises a sensor subsystem, e.g. Figure 3 5 . Various different embodiments of such a sensor subsystem as applied herein are discussed.

[0105] Figure 5a A first arrangement of a sensor subsystem 700 as may be applied in a system according to the invention is schematically shown, together with a collector mirror 300 whose reflectivity is to be checked. The sensor subsystem 700 comprises a sensor 700.1 having an active region 700.2 facing the collector mirror 300. In the embodiment as shown, the active region 700.2 is considered to be located at or near a first focus of the collector mirror 300 in order to receive test radiation emitted from a second focus and then reflected from the mirror 300.

[0106] In an embodiment, the sensor 700.1 may, for example, comprise an EUV photodiode.The sensor subsystem 700 further comprises an output 700.3 for outputting a measurement signal 700.4 representative of the received radiation.

[0107] In the arrangement as shown, the sensor subsystem is configured to remain stationary relative to the collector mirror 300. Thus, depending on the angle of the emitted radiation, the angle of incidence of the test radiation reflected from the collector mirror 300 may vary relative to the optical axis O, as illustrated by beams 710.1 and 710.2. By means of calibration, the effect of the angle of incidence on the received radiation may be taken into account.

[0108] As an alternative, Figure 5bAs schematically shown in , a sensor subsystem 750 may be applied comprising a plurality of sensors 750.1, 750.2, 750.3 having different orientations with respect to the optical axis O. By doing so, depending on the angle of incidence of the received radiation, the most sensitive sensor may be selected, i.e. the sensor whose active area best faces the received radiation.

[0109] Figure 5c A further alternative sensor subsystem 760 is schematically shown together with the collector reflector 300, the sensor subsystem 760 comprising a sensor 760.1 having an active region 760.2. The sensor subsystem further comprises an actuator or actuator arrangement 770 configured to displace the sensor 760.1 relative to the optical axis O. In particular, the actuator arrangement may be configured to rotate the sensor 760.1 about an axis 780 parallel to the indicated Z direction and passing through the first focus of the collector reflector, and about an axis perpendicular to the figure and passing through the first focus of the collector reflector. In such an arrangement, the sensor 760.1, in particular the active region 760.2 of the sensor 760.1, may be directed to face the test radiation reflected towards the first focus, for example towards the reflected beam of the radiation beam 710.2.

[0110] In an embodiment, Figure 5d As schematically shown in , a sensor 790.1 of a sensor subsystem 790 as applied in a system according to the invention may be mounted inside a tube, such as an orifice tube 795. In the embodiment as shown, the sensor 790.1 and the orifice tube 795 may be displaced by an actuator or actuator arrangement 770 as discussed above to face the direction of reflection from the collector reflector, such as to face the reflected beam of the radiation beam 710.2. By arranging the sensor 790.1 inside the orifice tube 795 or at the distal end of the orifice tube 795, the sensor 790.1 may sense only radiation received by the orifice tube 795. By doing so, measurement disturbances due to stray radiation may be mitigated or avoided.

[0111] like Figure 5a to Figure 5d The sensor subsystem schematically shown in FIG. 1 is configured to receive reflected radiation directly, i.e., without applying spectral filtering of the reflected radiation. As discussed above, it may be preferred to inspect or test the collector reflector using test radiation having a wavelength that is the same or similar to the wavelength applied during normal operation, because one is particularly interested in characterizing the collector reflector for that specific wavelength or spectrum of wavelengths. Figure 5a to Figure 5d A sensor subsystem for performing such an assessment may advantageously be combined with a test radiation subsystem configured to filter the test radiation (e.g. Figures 4a to 4cThese systems may be applied using the test radiation subsystem illustrated in ), where the first reflector and / or the second reflector may be implemented with a spectral purity filter.

[0112] Regardless of whether or not the test radiation subsystem applies filtering of the test radiation, filtering of the test radiation reflected from the collector mirror may be applied in a similar manner. Figure 5e to Figure 5f Such an embodiment is schematically shown in FIG.

[0113] Figure 5e A sensor subsystem 800 as may be applied in a system according to the invention, and a collector mirror 300 are schematically shown. The sensor subsystem 800 as schematically shown comprises a sensor 800.1, and the multilayer mirror assembly comprises a pair of mirrors 810, 820, which are configured to reflect test radiation 830.1 reflected from the collector mirror 300 towards the sensor 800.1 according to indicated arrows 830.2. In the embodiment as shown, the incident radiation 830.1 is received by an aperture tube 840, which is arranged to receive reflected radiation towards a first focus 850 of the collector mirror 300. In embodiments of the invention, the sensor subsystem 800 may also be provided with an actuator or an actuator arrangement for adjusting at least one of the aperture tube, the multilayer mirror assembly and the sensor in order to receive radiation reflected from different restricted portions of the collector mirror.

[0114] In an embodiment of the invention, a sensor subsystem as implemented in a system according to the invention may use a Schwarzschild objective to spectrally filter any incident radiation (ie, test radiation reflected from the collector mirror 300). Figure 5f Such an embodiment is schematically shown in FIG. Figure 5fSchematically illustrated is a sensor subsystem 900 as may be applied in a system according to the invention, and a collector reflector 300. The sensor subsystem 900 as schematically illustrated comprises a sensor 900.1, and the multilayer reflector assembly comprises a pair of reflectors 910, 920 shown in cross section, the reflectors 910, 920 being configured to reflect a test radiation 930.1 reflected from the collector reflector 300 towards the sensor 900.1 according to the indicated arrows 930.2. It can be seen that the reflectors 910 and 920 form a Schwarzschild objective, which is configured to receive a test radiation beam 930.1 reflected from the collector reflector 300 towards a first focus of the collector reflector. In the embodiment as shown, the first focus of the collector reflector may, for example, be arranged to substantially coincide with the first reflector 910. In the embodiment as shown, incident radiation 930.1 is received via an aperture tube 940, which is arranged to receive reflected radiation towards a first focus of the collector mirror 300, i.e., at the location where the mirror 910 is located. In embodiments of the present invention, the sensor subsystem 900 may also be equipped with an actuator or actuator arrangement for adjusting at least one of the aperture tube, the multilayer mirror assembly and the sensor to receive radiation reflected from different restricted portions of the collector mirror 300. It may be noted that the mirror arrangements 910, 920 may be used in conjunction with the embodiment as shown. Figure 4c The reflector arrangement shown in is configured in a similar manner, but is used in the opposite manner; that is, in Figure 4c In an embodiment of the invention, a Schwarzschild objective is used to generate a collimated beam from a radiation source, and Figure 5f In the embodiment of , the Schwarzschild objective is configured to receive the collimated beam 930.1 and project it onto the sensor.

[0115] exist Figure 5e or Figure 5f In the embodiment of the sensor subsystem shown in , a mirror arrangement is used to filter the test radiation received by the sensor. When such an embodiment is used, filtering of the radiation generated by the source can be omitted.

[0116] In such Figure 3In the arrangement shown in , the radiation limiter subsystem 360 is arranged in the beam path between the test radiation subsystem 330 and the collector reflector 300. Thus, only a restricted portion 300.1 of the collector reflector 300 is irradiated at a time. Alternatively, the radiation limiter subsystem may be implemented in the radiation path between the collector reflector 300 and the sensor subsystem 340. In such an embodiment, the test radiation emitted towards the collector reflector 300 does not need to be spatially limited to impinging on only the restricted portion of the collector reflector, because the limitation of the test radiation received by the sensor is limited by the radiation limiter subsystem arranged in the radiation path between the collector reflector 300 and the sensor subsystem 340. In an embodiment of the present invention, the radiation limiter subsystem comprises: a first radiation limiter subsystem arranged in the path between the test radiation subsystem and the collector reflector; and a second radiation limiter subsystem arranged in the path between the collector reflector and the sensor subsystem. In such radiation limiter subsystems, the first radiation limiter subsystem may for example comprise a first shielding member having a first hole therein, the first shielding member being arranged in the optical path of the test radiation between the second focus and the collector reflector, and the second radiation limiter subsystem may for example comprise a second shielding member having a second hole therein, the second shielding member being arranged in the optical path of the test radiation between the collector reflector and the first focus. It should be noted that within the meaning of the present invention, an optical path may refer to a trajectory followed by the test radiation, even though the applied radiation may not be visible.

[0117] Figure 6 Such an embodiment is schematically shown in FIG. Figure 6 Schematically showing the Figure 3 The same features and components. In addition, the embodiment includes another radiation limiter subsystem 390, which is configured to limit the test radiation received by the sensor subsystem 340 to radiation reflected from the restricted portion 300.1 of the collector reflector 300. In the embodiment as shown, the another radiation limiter subsystem 390 includes a shielding member 390.1 protruding from the aperture tube 390.2. Since the radiation limiter subsystem is included in both the optical path between the collector reflector and the first focus and the optical path between the second focus and the collector reflector, stray light effects can be mitigated or avoided. Those skilled in the art will appreciate that the two radiation limiter subsystems are shifted synchronously because the two radiation limiter subsystems need to be oriented toward the same restricted portion, such as portion 300.1 of the collector reflector 300.

[0118] In an embodiment of the present invention, the radiation limiter subsystem includes a hemispherical shield member and an orifice tube extending through the apex of the hemisphere. Figure 7Such an embodiment is schematically shown in FIG. Figure 7 Schematically illustrated is a radiation limiter subsystem comprising a hemispherical shield member 950 and an orifice tube 960 extending through the apex of the hemisphere. The radiation limiter subsystem also comprises an actuator arrangement 970 comprising a first actuator 970.1 configured to rotate the shield member 950 and the orifice tube 960 about an axis parallel to the indicated X-axis and a second actuator 970.2 configured to rotate the shield member 950 and the orifice tube 960 about an axis parallel to the indicated Z-axis. Figure 7 The radiation limiter subsystem schematically shown in FIG. 1 can be applied, for example, as Figure 6 The radiation limiter subsystem 360 shown in Figure 6 The radiation limiter subsystem 390 shown in FIG. 1 may also be used to cause the actuator assembly 970 to Figures 4a to 4c Any of the test radiation subsystems shown in the figure may be displaced, and / or Figure 5c to Figure 5f Any of the sensor subsystems shown in is displaced.

[0119] In an embodiment of the present invention, applying Figure 7 The radiation limiter subsystem schematically shown in FIG. 9 is such that a center 975 of a sphere formed by a portion of the hemisphere corresponds approximately to the location of the first focus or the second focus.

[0120] Before checking or testing a mirror such as collector mirror 300 by a system according to the invention, it may be advantageous to calibrate the system. In an embodiment of the invention, a calibration method is proposed whereby test radiation emitted by a test radiation subsystem is emitted directly towards the sensor subsystem. Figure 8 The system according to the invention is schematically shown arranged in a calibration position. The system as schematically shown corresponds approximately to Figure 6 The system shown in FIG. 1 includes a test radiation subsystem 330, a sensor subsystem 340, a control subsystem 370, and a radiation limiter subsystem, wherein the radiation limiter subsystem includes a first radiation limiter subsystem 360 and a second or another radiation limiter subsystem 390. Figure 6 Compared with the situation described in Figure 8The radiation limiter subsystems 360 and 390 shown in are arranged (i.e., rotated) so that the apertures 360.2 and 390.2 point towards each other. In such a position, which is also referred to as the calibration position, the test radiation beam 1000 can be emitted directly into the aperture 390.2 via the aperture 360.2 so as to be received by the sensor subsystem 340, in particular by the sensor of the sensor subsystem 340. The system can be calibrated by comparing the source measurement data (i.e., data indicating the amount of radiation emitted by the test radiation subsystem 330) with the measurement data obtained from the sensor subsystem 340 in the calibration position, i.e., determining the relationship between the emitted test radiation and the received test radiation in the absence of a collector reflector. Figure 8 3, reference numerals 370.2 and 370.3 may indicate obtained measurement data and source measurement data, respectively, as provided to the control unit 370. In an embodiment of the invention, prior to determining a measurement or test sequence of the reflectivity of the collector mirror, the ratio of the amount of received radiation to the amount of emitted radiation, or the difference between the amount of received radiation and the amount of emitted radiation may be determined.

[0121] In an embodiment of the invention, after a measurement or test sequence to determine the reflectivity of the collector mirror, the ratio of received radiation to emitted radiation, or the difference, can be determined again. By determining the mentioned ratio or difference before and after the measurement sequence, any drift or degradation of the test radiation subsystem 330 or the sensor subsystem 340 can be detected and taken into account.

[0122] With respect to the calibration method as discussed, it may be noted that the same method may also be applied when the first radiation limiter subsystem 360 or the second radiation limiter subsystem 390 is omitted.

[0123] In an embodiment of the present invention, a mirror to be inspected or tested (eg, collector mirror 300) is mounted on a frame that is substantially stationary during inspection or testing.

[0124] In an embodiment of the invention, the reflector to be inspected or tested is mounted at such a frame in an orientation with respect to gravity that substantially corresponds to the orientation of said reflector during normal operation. Fig. 9 The collector mirror 1100 in illustratively illustrates such an embodiment.

[0125] Fig. 9A collector mirror 1100 is schematically shown mounted inside a system 1200 according to an embodiment of the invention. In the embodiment as shown, the system comprises a first vacuum chamber or container 1210 in which the collector mirror 1100 may be arranged, for example, on a frame 1220. In the embodiment as shown, the container 1210 is mounted to an external frame 1230 at an angle, the angle being selected, for example, so that the collector mirror 1100, when mounted to the frame 1220, is arranged at approximately the same angle as would be applied during normal operation. By doing so, it can be assumed that any deformation of the collector mirror 1100, for example due to gravity, will correspond approximately to the deformation of the mirror 1100 during normal operation (i.e. when mounted, for example, in a housing such as Figure 2 The system 1200 further includes: a test radiation subsystem 1240 arranged to emit test radiation 1240.1 from a second focus 1250 of the collector mirror; and a sensor subsystem 1260 arranged to receive test radiation 1260.1 reflected from the collector mirror 1100 toward the first focus 1255 of the collector mirror 1100. In the embodiment shown, as shown, Figure 6, the system 1200 further includes a radiation limiter subsystem including a first radiation limiter subsystem 1270 and a second or further radiation limiter subsystem 1280, the radiation limiter subsystem being configured to limit the test radiation received by the sensor to the test radiation reflected from the restricted portion of the collector reflector 1100. Arrows 1285 schematically illustrate possible displacements of the radiation limiter subsystems 1270 and 1280 during testing of the collector reflector. In the embodiment as shown, the first radiation limiter subsystem 1270 includes a shielding member 1270.1 and an orifice tube 1270.2. In the embodiment as shown, the test radiation subsystem 1240 is arranged in a second vacuum chamber or container 1290, the first vacuum chamber 1210 and the second vacuum chamber 1290 are separated by a wall 1300, the wall comprising a pair of generally parallel wall portions 1300.1 and 1300.2, with holes in the wall portions 1300.1 and 1300.2 to allow the test radiation to pass toward the collector reflector. In the embodiment as shown, the wall portions 1300.1 and 1300.2 are separated by a gap 1310, the gap 1310 is configured to receive the shielding member 1270.1 of the first radiation limiter subsystem 1270. In the embodiment as shown, the holes in the wall 1300 are configured so that the test radiation as emitted can reach all portions of the collector reflector 1100. By arranging the shielding member 1270.1 of the first radiation limiter subsystem between the two wall portions 1300.1 and 1300.2 of the wall separating the two vacuum chambers 1210 and 1290, a labyrinth seal is created which blocks or hinders any debris generated by the test radiation subsystem 1240 from propagating into the vacuum chamber 1210 containing the collector mirror. In order to further hinder or hinder contamination of the vacuum chamber 1210, the system according to the present invention may also include a purge gas subsystem configured to introduce a purge gas flow towards the second vacuum chamber 1290 in order to hinder the propagation of debris into the first vacuum chamber 1210. Reference numeral 1320 refers to such a purge gas subsystem. It may also be noted that the purge gas subsystem may be used to purge any of the orifice tubes applied in embodiments of the present invention in order to prevent the propagation of debris towards the collector mirror.

[0126] As already discussed above, in an embodiment of the present invention, a system for inspecting or testing a collector mirror, such as a collector mirror of an EUV radiation source, is configured to irradiate a restricted portion of the collector mirror by a test radiation, such as a test radiation beam. In other words, the test radiation subsystem as applied in the system according to the present invention may be configured to irradiate a spot on the collector mirror, the size of the irradiated spot corresponding to the size of the radiation beam (i.e. the test radiation beam as applied). In principle, the size of the spot as applied may be chosen randomly. A person skilled in the art will appreciate that the smaller the spot size (i.e. the restricted portion of the collector mirror irradiated corresponding to each measurement), the higher the resolution of the reflectivity map as obtained. Furthermore, the smaller the spot size, the longer the measurement that can be taken for the entire collector mirror and the smaller the signal-to-noise ratio of the measurement may be.

[0127] Typically, the collector mirror as applied in an EUV radiation source is provided with a grating arranged to reduce the second focus (e.g. Figure 2 IR radiation reflected by the intermediate focus IF) shown in FIG. Fig.10 Such a grating is schematically shown in . Fig.10 A portion 1400 of a collector mirror is schematically shown, comprising a grating 1410 having a period P. Fig.10 Further shown is a test radiation beam 1420 configured to impinge on a confined portion of the collector mirror portion 1400. The radiation beam 1420 has a cross-section Pb corresponding to the size of the spot or confined portion of the collector mirror being irradiated.

[0128] The inventors have considered that the edges 1410.1 of the grating may cause perturbations in the measurement as performed. To avoid such perturbations, it has been found that it may be advantageous to choose the size of the spot irradiated by the radiation beam 1420 to be an integer multiple of the period of the grating, i.e. to choose Pb=n×P, n being an integer.

[0129] Fig.11 Schematically shown is a cross-sectional view of a system according to the invention, drawn on a whiteboard 1500 by the inventor HF.

[0130] In an embodiment of the system according to the invention as described above, the system according to the invention is described as an inspection or test system for a collector reflector, in particular a collector reflector having a first focus and a second focus. When the reflector to be tested has a first focus and a second focus, as described above, it is advantageous to arrange the test radiation subsystem at one of the first focus and the second focus and to arrange the sensor subsystem at the other of the first focus and the second focus. By doing so, the reflector property that the radiation emitted from the first focus will reach the second focus (or vice versa) can be used, regardless of the angle at which the radiation is emitted. However, the present invention can also be implemented to inspect or test other reflectors, i.e. reflectors that do not have a first focus and / or a second focus.

[0131] In the absence of the first focus and / or the second focus, appropriate measures may need to be taken to ensure that the test radiation emitted by the test radiation subsystem and reflected by the reflector is captured by the sensor subsystem of the system according to the invention. Such measures may, for example, include applying one or more actuators or motors that can displace the sensor subsystem and / or the test radiation subsystem and / or the radiation limiter subsystem and / or the reflector being tested or inspected.

[0132] Fig.12 A system according to the invention comprising such measures is schematically illustrated in FIG.

[0133] Fig.12 A system 1100 according to the present invention is schematically illustrated, the system being configured to inspect a reflector 1110 having a reflective surface 1110.1. The reflective surface 1110.1 can generally have an arbitrary shape. The reflector 1110 can be, for example, a substantially flat reflector, a parabolic reflector, or a free-form reflector. A system according to an embodiment of the present invention comprises a test radiation subsystem 1120, a sensor subsystem 1130, and a radiation limiter subsystem, the radiation limiter subsystem comprising a first radiation limiter subsystem 1140.1 and a second radiation limiter subsystem 1140.2. The subsystems have substantially the same functions as the subsystems described above. In the embodiment as shown, the system further comprises an actuator subsystem 1150 configured to displace the sensor subsystem 1130 and the radiation limiter subsystem 1140.2. Specifically, the actuator subsystem 1150 is configured to displace the sensor subsystem 1130 and the radiation limiter subsystem 1140.2 along the X-axis.

[0134] The system 1100 according to the invention comprises a test radiation subsystem 1120 operable to project test radiation 1120.1 onto the reflector 1100. With regard to the type of test radiation, in particular the wavelength or wavelength range of the radiation, similar considerations as described above apply. In an embodiment, the test radiation subsystem 1120 is configured to generate test radiation spanning an angle α0, so that in principle the entire reflector 1100 can be irradiated. Note that Fig.12 The illustration of represents collector mirror 300 in a two-dimensional cross section, and in practice, test radiation is typically emitted in three dimensions. In an embodiment, test radiation subsystem 1120 is configured to generate test radiation spanning an angle less than angle α0. In such an embodiment, the entire mirror 1100 may still be scanned or irradiated, for example by rotating test radiation subsystem 1120 or portions thereof.

[0135] System 1100 according to the present invention also includes a sensor subsystem 1130 that is operable to receive test radiation reflected from reflector 1100 toward sensor subsystem 1130, such as test radiation indicated by arrow 1142. In an embodiment, sensor subsystem 1130 may include one or more sensors or detectors for measuring test radiation reflected from reflector 1110, such as described above.

[0136] According to the present invention, the system 1100 for testing the reflector 1110 also includes a radiation limiter subsystem, which includes a first radiation limiter subsystem 1140.1 and a second radiation limiter subsystem 1140.2. The radiation limiter subsystems 1140.1, 1140.2 are configured to limit the test radiation received by the sensor subsystem 1130 to the test radiation reflected from the restricted portion of the reflector 1110. By doing so, the radiation measured or detected by the sensor subsystem 1130 will only involve radiation reflected from the restricted portion of the reflector 1110. In the embodiment as shown, the restricted portion of the reflector is indicated by the reflector segment 1110.2. By limiting the radiation received by the sensor subsystem 1130 to the radiation reflected from the restricted portion 1110.2 of the reflector, the performance of the specific portion of the reflector can be evaluated, for example, the reflectivity of the portion can be evaluated.

[0137] In the embodiment as shown, the radiation limiter subsystem 1140.1 is configured to enable only a small portion 1141 of the generated radiation 1120.1 to pass towards the reflector 1110. The small portion 1141 of the radiation 1120.1 subtends an angle that is much smaller than the angle α0. In order to achieve the described situation, the radiation limiter subsystem 1140.1 includes a shielding member having a hole, such as a tubular hole through which a small portion of the test radiation 1120.1 can pass. The radiation limiter subsystem 1140.1 thus enables only a restricted portion (e.g., portion 1110.2 of the reflector) to be irradiated by the test radiation generated by the test radiation subsystem 330. In a similar manner, the radiation limiter subsystem 1140.2 is configured to allow the sensor subsystem to receive test radiation only from the restricted portion of the reflector 1110. It should be noted that the first radiation limiter subsystem 1140.1 and the second radiation limiter subsystem 1140.2 can therefore have the same configuration as shown. Figure 3 and Figure 6 The radiation limiter subsystem described in . Figure 3 In a similar manner as described in , any one of the first radiation limiter subsystem 1140.1 and the second radiation limiter subsystem 1140.2 may be omitted.

[0138] According to the present invention, the system further comprises a control subsystem 1170, which is configured to control the movement of the radiation limiter subsystem, for example by providing control signals 1170.1 and 1170.2 to the first radiation limiter subsystem 1140.1 and the second radiation limiter subsystem 1140.2. Specifically, the control subsystem 1170 is configured to control the position of the radiation limiter subsystem so that the restricted portion of the reflector irradiated by the test radiation can be changed. Thus, the control subsystem 1170 can, for example, be configured to control the movement of the radiation limiter subsystem along a series of different positions, thereby limiting the test radiation received by the sensor subsystem 1130 to the test radiation reflected from the corresponding series of different restricted portions of the reflector 1110. By doing so, the performance of the different restricted portions of the reflector 1110 can be evaluated, for example the reflectivity of these portions can be evaluated.

[0139] In an embodiment of the present invention, a set of different restricted portions substantially covers the entire reflector 1110 .

[0140] In an embodiment of the present invention, the radiation limiter subsystem may include one or more actuators to change the direction of the test radiation 1141 projected onto the reflector 1110 and to change the direction of the test radiation 1142 received by the sensor subsystem 1130 .

[0141] In an embodiment, the control subsystem 1170 may be configured to direct a beam or beam-shaped test radiation onto a plurality of different restricted portions of the reflector 1110, one restricted portion after another. Such sequential projection of the test radiation beam onto different restricted portions of the reflector 1110 may also be referred to as scanning the reflector with the test radiation beam. During such a scanning process, whereby a portion or the entire reflector of the reflector 1110 is scanned, the sensor subsystem 1130 may generate a set of measurement data representing the test radiation reflected from the different restricted portions of the collector reflector 1110. Such measurement data may be provided to the control subsystem 1170 of the system according to the invention, for example via a data channel, indicated by arrow 1170.3. In an embodiment, the measurement data 1170.3 as received may be processed, for example, by a processing unit 1180 of the control subsystem 1170. Such a processing unit may, for example, be implemented as a processor, a microprocessor, a computer, etc. Such a processing unit 1180 may, for example, include a memory unit for storing the measurement data.

[0142] In an embodiment, the processing unit 1180 may be configured to process the measurement data in order to determine a spatial reflectivity distribution of at least a portion of the collector reflector. In such an embodiment, measurement data representing test radiation reflected from different confined portions of the reflector 1110 may be compared with data representing generated or emitted test radiation to determine the reflectivity of the different confined portions of the reflector. Examples of measures of reflectivity are the ratio of the amount of test radiation received to the amount of test radiation generated or emitted, the difference between (on the one hand) the amount of test radiation generated or emitted and (on the other hand) the amount of test radiation received. Thus, in an embodiment of the present invention, the measurement data obtained from the sensor subsystem 1130 may be supplemented with source measurement data representing the amount of test radiation emitted or generated during the scanning process. Such source measurement data may be provided to the processing unit 1180 of the control subsystem 1170, for example, via a data channel.

[0143] like Fig.12The embodiment of the system according to the invention schematically shown in also includes an actuator subsystem 1150 configured to shift the sensor subsystem 1130 and the radiation limiter subsystem 1140.2. The goal of such an actuator subsystem 1150 is to ensure that test radiation reflected from the reflector, such as the radiation indicated by reference numeral 1142, is captured by the sensor subsystem 1130. In the absence of a first focus and a second focus, it should be clear to a person skilled in the art that test radiation directed toward different locations on the reflector will also be reflected to different locations. Therefore, in an embodiment of the present invention, the fact that test radiation directed toward different locations on the reflector will be reflected to different locations is anticipated by providing a shift of the sensor subsystem 1130 and the radiation limiter subsystem 1140.2.

[0144] Fig.12 Such a required shift is schematically illustrated in FIG. Fig.12 110.2, the dashed arrow 1143 represents test radiation directed toward the restricted portion 1110.3 of the reflector 1110, which is at a different location on the reflector than the restricted portion 1110.2. The dashed arrow 1144 represents test radiation reflected from the restricted portion 1110.3. It can be seen that the reflected test radiation 1144 is at a different location and at a different angle than the reflected test radiation 1142. In order to capture the reflected test radiation 1144, the sensor subsystem 1130 and the radiation limiter subsystem 1140.2 can be shifted to a location schematically indicated by the dashed line 1190. The dashed line 1190 thus schematically represents the location of the sensor subsystem 1130, and the direction of the radiation limiter subsystem 1140.2 that enables the capture of the test radiation 1144. In an embodiment of the present invention, the control unit 1170 may be configured to determine where the reflected test radiation will be directed for a specific limited portion of the reflector targeted with the test radiation based on the known shape or predetermined shape information of the reflector to be inspected, and the known relative position of the test radiation subsystem and the reflector. Based on this determined direction, the control unit 1170 may then be configured to control the actuator subsystem 1150, for example by providing an appropriate control signal 1170.4 to the actuator subsystem 1150, so as to shift the sensor subsystem 1130 and the radiation limiter subsystem 1140.2 to the appropriate position to capture the reflected radiation.

[0145] In an embodiment, the actuator subsystem 1150 may, for example, include one or more actuators (such as electromagnetic actuators or piezoelectric actuators) and / or one or more linear or planar motors for displacing the sensor subsystem 1130 and the radiation limiter subsystem 1140.2. In an embodiment, the actuator subsystem 1150 may be configured to displace the sensor subsystem 1130 and the radiation limiter subsystem 1140.2 with multiple degrees of freedom. In the embodiment as shown, the actuator subsystem 1150 may, for example, be configured to displace the sensor subsystem 1130 and the radiation limiter subsystem 1140.2 in the X direction and the Y direction, the Y direction being perpendicular to the XZ plane as indicated.

[0146] In the embodiment as shown, the test radiation subsystem 1120 is mounted to the frame 1200, and the sensor subsystem 1130 and the radiation limiter subsystem 1140.2 can be displaced relative to the frame 1200 by the actuator subsystem 1150. In such an embodiment, the relative position of the test radiation subsystem and the reflector to be inspected can remain fixed during the inspection.

[0147] As an alternative to the displacement of the sensor subsystem 1130 and the radiation limiter subsystem 1140 . 2 by the actuator subsystem 1150 , the system according to the invention may also comprise an actuator subsystem configured to displace the mirror 1110 relative to the test radiation subsystem 1120 .

[0148] Fig.13 Such an embodiment is schematically depicted in FIG. Fig.13 The system 1300 schematically shown in FIG. 1 corresponds generally to Fig.12 The system 1100 shown in FIG. Fig.13In the embodiment shown in , the test radiation subsystem 1120 and the sensor subsystem 1130 are mounted to a common frame 1200 together with the radiation limiter subsystem. In the embodiment as shown, the test radiation subsystem 1120 and the sensor subsystem 1130 are therefore regarded as having a fixed relative position. In order to ensure that the test radiation reflected from the reflector 1110 is reflected towards the sensor subsystem 1130, the system is provided with an actuator subsystem 1155 configured to displace the reflector 1110. In particular, the actuator subsystem 1155 is configured to displace the reflector so that the reflected radiation 1145 reflected from the restricted portion 1110.3 is directed towards the same location as the reflected radiation 1142. In particular, in the embodiment as shown, the actuator subsystem 1155 is configured to tilt or rotate the reflector 1110 so that the test radiation 1143 is reflected as radiation 1145 towards the location of the sensor subsystem 1130. It should be noted that in order for the sensor subsystem 1130 to capture the reflected radiation 1145, it may be necessary to rotate the radiation limiter subsystem 1140.2. Fig.12 In a similar manner as described, control unit 1170 may be configured to determine where the reflected test radiation will be directed for a particular limited portion of the reflector at which the test radiation is aimed based on a known shape or predetermined shape information of the reflector to be inspected and a known relative position of the test radiation subsystem and the reflector. Based on this determined direction, control unit 1170 may then be configured to control actuator subsystem 1155, for example by providing appropriate control signals to actuator subsystem 1155, to displace the reflector to an appropriate location or position to reflect the test radiation received by the reflector to the sensor subsystem.

[0149] An alternative to shifting the sensor subsystem 1130 and the radiation limiter subsystem 1140.2 by the actuator subsystem 1150 or shifting the reflector 1110 relative to the test radiation subsystem by the actuator subsystem 1155 is to apply an actuator subsystem for shifting the test radiation subsystem 1120 relative to the reflector 1110, thereby arranging the reflected radiation to reach the sensor subsystem 1130 regardless of which restricted portion of the reflector is irradiated.

[0150] In reference Fig.12 and Fig.13In the described embodiments, an actuator subsystem is used to ensure that an optical path for the test radiation is established between the test radiation subsystem and the sensor subsystem. In such embodiments, the actuator subsystem is therefore used to ensure that the test radiation subsystem, the reflector and the sensor subsystem are in the appropriate relative position and orientation so that a specific part of the reflector being inspected or tested is illuminated and the test radiation reflected from the reflector is captured by the sensor subsystem. According to the present invention, the optical path of the test radiation will also pass through a radiation limiter subsystem or several radiation limiter subsystems. As described above, the system can also be positioned or displaced by the actuator subsystem.

[0151] In an embodiment of the present invention, an actuator subsystem is also implemented to control the angle at which the test radiation is incident on the reflector. Fig.12 , the angle of incidence of test radiation 1141 is different from the angle of incidence of test radiation 1143. This may be undesirable. In particular, it may be desirable to test the reflector, for example to determine the reflectivity of the reflector, so that the test radiation as applied illuminates the reflector in substantially the same manner (i.e., at the same angle of incidence) as occurs during normal use of the reflector. For particular applications, the radiation beam may illuminate the reflector at substantially the same angle across the entire reflector surface. In order to test such a reflector, it may therefore be advantageous to ensure that the test radiation illuminates the reflector at an appropriate angle, regardless of which part of the reflector is irradiated. Such an arrangement may be ensured, for example, by controlling the relative position and / or orientation of the test radiation subsystem and the reflector. Such control may be established, for example, by applying an actuator subsystem configured to control the relative position and / or orientation of the test radiation subsystem and the reflector. Such an actuator subsystem may, for example, be controlled by a control subsystem as described above, whereby the control subsystem is configured to generate control signals for controlling the actuator subsystem based on desired incident angle information of the mirror being tested or inspected.

[0152] Thus, in embodiments of the present invention, the actuator subsystem or actuator subsystems as applied may serve a dual purpose:

[0153] - providing an optical path for the test radiation between the test radiation subsystem and the sensor subsystem, and

[0154] - Ensure that the test radiation impinges on the reflector being tested at the appropriate or desired angle of incidence.

[0155] To meet both requirements, additional degrees of freedom or multiple degrees of freedom of movement or displacement may be required.

[0156] In an embodiment of the invention, the actuator subassembly may, for example, include a 5 or 6 degree of freedom robotic arm to displace the test radiation subsystem and / or the sensor subsystem.

[0157] It may be noted that the above-mentioned embodiments of the system according to the invention may also be combined. In such a combined embodiment, an actuator subsystem may be applied, for example, which is configured to displace both the reflector and the test radiation subsystem, or both the reflector and the sensor subsystem, or any other combination.

[0158] In such an embodiment it may for example be advantageous to distribute the required degrees of freedom that need to be actuated on different elements.

[0159] refer to Fig.12 In an embodiment of the present invention, it may be advantageous, for example, to arrange the actuator subsystem to scan the mirror 1110, the actuator subsystem being configured to displace the sensor subsystem along the X axis and the mirror along the Y axis. Thus, by using two linear motors or actuators, a two-dimensional scan of the mirror surface may be obtained.

[0160] As mentioned, as referenced above Fig.12 and Fig.13 The described system enables inspection or testing of mirrors that do not have a first focus and a second focus. The system can therefore be applied to evaluate the reflectivity of mirrors of arbitrary shape or mirrors with a single focus.

[0161] Typically, as will be apparent from the examples given above, it will be necessary to apply an actuator subsystem, then enable displacement or rotation in at least two degrees of freedom (applied to the same element or to different elements of the system), in order to scan or inspect the surface of the reflector. However, in the case of a reflector with an optical axis (i.e. an axis along which there is rotational symmetry), it may be sufficient to displace any one element of the system (i.e. the reflector or the test radiation subsystem or the sensor subsystem) with only one degree of freedom (e.g. a translational degree of freedom along the optical axis). In order to achieve this, both the test radiation subsystem and the sensor subsystem should be arranged on the optical axis, such as e.g. Figure 3 and Figure 6 In such an arrangement, for example, Figure 3 and / or Figure 6 The system shown in FIG. 1 is used to test a parabolic reflector, with the addition of an actuator subsystem configured to translate the sensor subsystem along the optical axis to ensure that reflected radiation is captured.

[0162] Although specific embodiments of the present invention have been described above, it will be appreciated that the present invention may be practiced in other ways than those described. The behavior of the system may be largely defined by a computer program containing one or more sequences of machine-readable instructions for implementing certain steps of the method disclosed above, or by a data storage medium (e.g., a semiconductor memory, a magnetic disk, or an optical disk) storing such a computer program. The above description is intended to be illustrative rather than restrictive. Therefore, it will be appreciated by those skilled in the art that the invention described may be modified without departing from the scope of the claims set forth below.

Claims

1. A system configured to test a collector mirror having a first focus and a second focus, the system comprising: a test radiation subsystem operable to project test radiation from the second focus onto the collector mirror; a sensor subsystem operable to receive test radiation reflected from the collector mirror toward the first focal point; and a radiation limiter subsystem operable to limit the test radiation received by the sensor subsystem to test radiation reflected from a restricted portion of the collector reflector; a control subsystem operable to control movement of the radiation limiter subsystem along a series of different positions to thereby limit the test radiation received by the sensor subsystem to test radiation reflected from a corresponding series of different restricted portions of the collector mirror; Wherein the radiation limiter subsystem comprises a shielding member having an aperture therein, the shielding member being arranged in an optical path of the test radiation.

2. The system of claim 1, wherein the control subsystem comprises a processing unit configured to: - receiving measurement data from the sensor subsystem, the measurement data representing test radiation reflected from the corresponding series of different confined portions of the collector reflector; - processing the measurement data to determine a spatial reflectivity distribution of at least a portion of the collector mirror.

3. The system of claim 1 or 2, wherein the test radiation subsystem comprises an EUV source for generating EUV radiation.

4. The system of claim 3, wherein the test radiation subsystem comprises a multilayer mirror assembly for filtering the EUV radiation.

5. The system of claim 4, wherein the multilayer mirror assembly comprises a pair of multilayer mirrors for spectrally shaping the spectrum of the EUV radiation projected from the second focus by the test radiation subsystem.

6. A system according to any of the preceding claims, wherein the sensor subsystem includes a sensor operable to receive the test radiation reflected from the collector mirror towards the first focus and to generate a measurement signal representative of the received test radiation.

7. The system of claim 6, wherein the sensor subsystem includes a multilayer mirror assembly for filtering the test radiation received by the sensor.

8. The system of any preceding claim, wherein the test radiation subsystem comprises a source sensor configured to generate a source measurement signal representative of the test radiation projected by the test radiation subsystem.

9. A system configured to test a reflector, the system comprising: a test radiation subsystem operable to project test radiation onto the reflector; a sensor subsystem operable to receive test radiation reflected from the reflector; and a radiation limiter subsystem operable to limit the test radiation received by the sensor to test radiation reflected from a restricted portion of the reflector, wherein the radiation limiter subsystem comprises a shield member having an aperture therein, the shield member being disposed in an optical path of the test radiation; A control subsystem is operable to control movement of the radiation limiter subsystem along a series of different positions to thereby limit the test radiation received by the sensor to test radiation reflected from a corresponding series of different restricted portions of the reflector.

10. The system according to claim 9, further comprising: An actuator subsystem is configured to displace at least one of the test radiation subsystem, the sensor subsystem, the radiation limiter subsystem, and the reflector.

11. The system of claim 10, wherein the control subsystem is configured to control the actuator subsystem to establish an optical path for the test radiation between the test radiation subsystem and the sensor subsystem.

12. A method of testing a collector reflector having a first focus and a second focus, the method comprising: projecting test radiation from the second focus onto the collector mirror; receiving, by a sensor, test radiation reflected from the collector mirror toward the first focal point; limiting the test radiation received by the sensor to that reflected from a restricted portion of the collector mirror by a shielding member having an aperture therein, the shielding member being arranged in an optical path of the test radiation; and Movement of the radiation limiting subsystem along a series of different positions is controlled to thereby limit the test radiation received by the sensor to test radiation reflected from a corresponding series of different limited portions of the collector mirror.

13. The method of claim 12, wherein the step of limiting the test radiation comprises: arranging a first shielding member in an optical path of the test radiation between the second focus and the collector mirror, the first shielding member having a first aperture therein; and A second shielding member having a second aperture therein is arranged in the optical path of the test radiation between the collector mirror and the first focus.

14. The method of claim 13, wherein the step of controlling the movement comprises: Movement of the first and second shield members is controlled to establish an optical path for the test radiation between the second focal point and the first focal point via both the first aperture and the second aperture.

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