EP3785078A1

EP3785078A1 - System for testing a mirror such as a collector mirror and method of testing a mirror such as a collector mirror

Info

Publication number: EP3785078A1
Application number: EP19717494.9A
Authority: EP
Inventors: Johannes Christiaan Leonardus Franken
Original assignee: ASML Netherlands BV
Current assignee: ASML Netherlands BV
Priority date: 2018-04-26
Filing date: 2019-04-17
Publication date: 2021-03-03
Also published as: KR102775472B1; KR20210003113A; NL2022961A; WO2019206767A1; TW201946499A; CN112041752A; CN112041752B; TWI863910B

Abstract

A system configured for testing a collector mirror having a first focus and a second focus is disclosed, the system comprises: a test radiation sub-system operative to project test radiation from the second focus onto the collector mirror; a sensor sub-system operative to receive test radiation reflected off the collector mirror towards the first focus; and a radiation limiter sub-system operative to limit the test radiation as received by the sensor to test radiation reflected off a limited portion of the collector mirror; a control sub-system operative to control a movement of the radiation limiter sub-system along a sequence of different positions, thereby limiting the test radiation as received by the sensor to test radiation reflected off a respective sequence of different limited portions of the collector mirror.

Description

SYSTEM FOR TESTING A MIRROR SUCH AS A COLLECTOR MIRROR AND METHOD OF TESTING A MIRROR SUCH AS A COLLECTOR MIRROR

CROSS-REFERENCE TO RELATED APPLICATIONS

[001] This application claims priority of EP application 18169481.1 which was filed on 26 April, 2018 and which is incorporated herein in its entirety by reference.

FIELD

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

BACKGROUND

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

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

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

CD = k *— (1)

¹ NA

where l is the wavelength of the radiation used, NA is the numerical aperture of the projection system used to print the pattern, ki is a process dependent adjustment factor, also called the Rayleigh constant, and CD is the feature size (or critical dimension) of the printed feature. It follows from equation (1) that reduction of the minimum printable size of features can be obtained in three ways: by shortening the exposure wavelength l, by increasing the numerical aperture NA or by decreasing the value of ki. [006] In order to shorten the exposure wavelength and, thus, reduce the minimum printable size, it has been proposed to use an extreme ultraviolet (EUV) radiation source. EUV radiation is electromagnetic radiation having a wavelength within the range of 5-20 nm, for example within the range of 13-14 nm. It has further been proposed that EUV radiation with a wavelength of less than 10 nm could be used, for example within the range of 5-10 nm such as 6.7 nm or 6.8 nm. Possible sources include laser-produced plasma (LPP) sources, although other types of source are possible.

[007] 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-Min Lin, David Brandt, Nigel Farrar, SPIE Proceedings Vol. 7520, Lithography Asia 2009, December 2009 (SPIE Digital Library reference DOI: 10.1117/12.839488). In a lithographic system, the source apparatus will typically be contained within its own vacuum housing, while a small exit aperture is provided to couple the EUV radiation beam into an optical system where the radiation is to be used.

[008] In order to be useful in high-resolution patterning for lithography, the EUV radiation beam must be conditioned to obtain desired parameters such as uniformity of intensity and angular distribution, when it reaches the reticle. Examples of an illumination system are described in United States Patent Application Publication Nos. US 2005/0274897A1 (Carl Zeiss/ASML) and

US 2011/0063598A (Carl Zeiss). The example systems include a‘fly’s eye’ illuminator which transforms the highly non-uniform intensity profile of the EUV source into a more uniform and controllable source.

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

[0010] Known methods for determining the reflectivity are found to be rather time-consuming and costly. Other mirrors as applied in an EUV radiation source or a lithographic apparatus may also require the aforementioned inspection and testing. Similarly, the known methods for determining the reflectivity of such mirrors may be time-consuming and costly as well.

SUMMARY

[0011] Aspects of embodiments of the present invention aim to provide an alternative system and method of testing a collector mirror of an EUV source.

[0012] According to an aspect of the present invention, there is provided a system configured for testing a collector mirror having a first focus and a second focus, the system comprises: a test radiation sub-system operative to project test radiation from the second focus onto the collector mirror; a sensor sub-system operative to receive test radiation reflected off the collector mirror towards the first focus; and a radiation limiter sub-system operative to limit the test radiation as received by the sensor to test radiation reflected off a limited portion of the collector mirror; a control sub-system operative to control a movement of the radiation limiter sub-system along a sequence of different positions, thereby limiting the test radiation as received by the sensor to test radiation reflected off a respective sequence of different limited portions of the collector mirror.

[0013] According to another aspect of the invention, there is provided a method of testing a collector mirror having a first focus and a second focus, the method comprises: projecting test radiation from the second focus onto the collector mirror; receiving test radiation reflected off the collector mirror towards the first focus by a sensor; and limiting the test radiation as received by the sensor to test radiation reflected off a limited portion of the collector mirror; controlling a movement of the radiation limiting system along a sequence of different positions, thereby limiting the test radiation as received by the sensor to test radiation reflected off a respective sequence of different limited portions of the collector mirror.

[0014] According to yet another aspect of the present invention, there is provide a system configured for testing a mirror, the system comprises:

a test radiation sub-system operative to project test radiation onto the mirror;

a sensor sub-system operative to receive test radiation reflected off the mirror; and a radiation limiter sub-system operative to limit the test radiation as received by the sensor sub- system to test radiation reflected off a limited portion of the mirror;

a control sub-system operative to control a movement of the radiation limiter sub system along a sequence of different positions, thereby limiting the test radiation as received by the sensor sub-system to test radiation reflected off a respective sequence of different limited portions of the mirror.

[0015] These aspects of the invention and various optional features and implementations thereof will be understood by the skilled reader from the description of examples which follows.

BRIEF DESCRIPTION OF THE DRAWINGS

[0016] Embodiments of the invention will now be described, by way of example only, with reference to the accompanying schematic drawings in which corresponding reference symbols indicate corresponding parts, and in which:

[0017] Figure 1 depicts schematically a lithographic system according to an embodiment of the invention;

[0018] Figure 2 is a more detailed view of the system of Figure 1 and shows a novel monitoring and control system for an EUV radiation source; [0019] Figure 3 schematically shows a cross-section of an embodiment of a system according to the present invention;

[0020] Figures 4a - 4c schematically show cross-sectional views of test radiation sub-systems as can be applied in the present invention;

[0021] Figures 5a - 5 f schematically show cross-sectional views of sensor sub-systems as can be applied in the present invention;

[0022] Figure 6 schematically shows a cross-section of another embodiment of a system according to the present invention;

[0023] Figure 7 schematically shows a radiation limiter sub-system as can be applied in a system according to the present invention;

[0024] Figure 8 schematically shows a cross-section of a system according to the present invention in a calibration position;

[0025] Figure 9 schematically shows a cross-section of yet another embodiment of a system according to the present invention;

[0026] Figure 10 schematically shows part of a collector mirror as can be inspected by the system according to the present invention.

[0027] Figure 11 schematically shows a system according to the invention on a whiteboard together with the inventor.

[0028] Figure 12 schematically shows a cross-section of an embodiment of a system according to the present invention for testing or inspecting a mirror;

[0029] Figure 13 schematically shows a cross-section of another embodiment of a system according to the present invention for testing or inspecting a mirror;

DETAILED DESCRIPTION

[0030] Figure 1 schematically depicts a lithographic system 100 according to an embodiment of the present invention, the lithographic system comprising a lithographic apparatus and an EUV radiation source configured for generating EUV radiation, e.g. 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 lithographic scanning apparatus comprises: an illumination system (illuminator) IL configured to condition a radiation beam B (e.g. EUV radiation); a support structure (e.g. a mask table) MT constructed to support a patterning device (e.g. a mask or a reticle) MA and connected to a first positioner PM configured to accurately position the patterning device; a substrate table (e.g. a wafer table) WT constructed to hold a substrate (e.g. a resist-coated wafer) W and connected to a second positioner PW configured to accurately position the substrate; and a projection system (e.g. a reflective projection system) PS configured to project a pattern imparted to the radiation beam B by patterning device MA onto a target portion C (e.g. comprising one or more dies) of the substrate W.

[0031] The illumination system IL may include various types of optical components, such as refractive, reflective, magnetic, electromagnetic, electrostatic or other types of optical components, or any combination thereof, for directing, shaping, or controlling radiation.

[0032] The support structure MT holds the patterning device MA in a manner that depends on the orientation of the patterning device, the design of the lithographic apparatus, and other conditions, such as for example whether or not the patterning device is held in a vacuum environment. The support structure can use mechanical, vacuum, electrostatic or other clamping techniques to hold the patterning device. The support structure may be a frame or a table, for example, which may be fixed or movable as required. The support structure may ensure that the patterning device is at a desired position, for example with respect to the projection system.

[0033] The term“patterning device” should be broadly interpreted as referring to any device that can be used to impart a radiation beam with a pattern in its cross-section such as to create a pattern in a target portion of the substrate. The pattern imparted to the radiation beam may correspond to a particular functional layer in a device being created in the target portion, such as an integrated circuit.

[0034] The patterning device may be transmissive or reflective. Examples of patterning devices include masks, programmable mirror arrays, and programmable LCD panels. Masks are well known in lithography, and include mask types such as binary, alternating phase-shift, and attenuated phase- shift, as well as various hybrid mask types. An example of a programmable mirror array employs a matrix arrangement of small mirrors, each of which can be individually tilted so as to reflect an incoming radiation beam in different directions. The tilted mirrors impart a pattern in a radiation beam which is reflected by the mirror matrix.

[0035] The projection system, like the illumination system, may include various types of optical components, such as refractive, reflective, magnetic, electromagnetic, electrostatic or other types of optical components, or any combination thereof, as appropriate for the exposure radiation being used, or for other factors such as the use of a vacuum. It may be desired to use a vacuum for EUV radiation since other gases may absorb too much radiation. A vacuum environment may therefore be provided to the whole beam path with the aid of a vacuum wall and vacuum pumps.

[0036] As here depicted, the apparatus is of a reflective type (e.g. employing a reflective mask).

[0037] The lithographic apparatus may be of a type having two (dual stage) or more substrate tables (and/or two or more mask tables). In such“multiple stage” machines the additional tables may be used in parallel, or preparatory steps may be carried out on one or more tables while one or more other tables are being used for exposure.

[0038] Referring to Figure 1, the illuminator IL receives an extreme ultra violet radiation beam from the source collector module SO of the EUV radiation source. Methods to produce EUV light include, but are not necessarily limited to, converting a material into a plasma state that has at least one element, e.g., xenon, lithium or tin, with one or more emission lines in the EUV range. In one such method, often termed laser produced plasma ("LPP") the required plasma can be produced by irradiating a fuel, such as a droplet, stream or cluster of material having the required line-emitting element, with a laser beam. The source collector module SO may be part of an EUV radiation system including a laser, not shown in Figure 1, for providing the laser beam exciting the fuel. The resulting plasma emits output radiation, e.g., EUV radiation, which is collected using a radiation collector, disposed in the source collector module. The laser and the EUV radiation source may be separate entities, for example when a CO2 laser is used to provide the laser beam for fuel excitation.

[0039] In such cases, the laser is not considered to form part of the lithographic system and the radiation beam is passed from the laser to the source collector module with the aid of a beam delivery system comprising, for example, suitable directing mirrors and/or a beam expander. In other cases the source may be an integral part of the source collector module, for example when the source is a discharge produced plasma EUV generator, often termed as a DPP source.

[0040] The illuminator IL may comprise an adjuster for adjusting the angular intensity distribution of the radiation beam. Generally, at least the outer and/or inner radial extent (commonly referred to as s-outer and s-inner, respectively) of the intensity distribution in a pupil plane of the illuminator can be adjusted. In addition, the illuminator IL may comprise various other components, such as facetted field and pupil mirror devices. The illuminator may be used to condition the radiation beam, to have a desired uniformity and intensity distribution in its cross-section.

[0041] The radiation beam B is incident on the patterning device (e.g., mask) MA, which is held on the support structure (e.g., mask table) MT, and is patterned by the patterning device. After being reflected from the patterning device (e.g. mask) MA, the radiation beam B passes through the projection system PS, which focuses the beam onto a target portion C of the substrate W. With the aid of the second positioner PW and position sensor PS2 (e.g. an interferometric device, linear encoder or capacitive sensor), the substrate table WT can be moved accurately, e.g. so as to position different target portions C in the path of the radiation beam B. Similarly, the first positioner PM and another position sensor PS1 can be used to accurately position the patterning device (e.g. mask) MA with respect to the path of the radiation beam B. Patterning device (e.g. mask) MA and substrate W may be aligned using mask alignment marks Ml, M2 and substrate alignment marks PI, P2.

[0042] The depicted apparatus could be used in at least one of the following modes:

1. In step mode, the support structure (e.g. mask table) MT and the substrate table WT are kept essentially stationary, while an entire pattern imparted to the radiation beam is projected onto a target portion C at one time (i.e. a single static exposure). The substrate table WT is then shifted in the X and/or Y direction so that a different target portion C can be exposed. 2. In scan mode, the support structure (e.g. mask table) MT and the substrate table WT are scanned synchronously while a pattern imparted to the radiation beam is projected onto a target portion C (i.e. a single dynamic exposure). The velocity and direction of the substrate table WT relative to the support structure (e.g. mask table) MT may be determined by the (de-)magnification and image reversal characteristics of the projection system PS.

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

[0043] Combinations and/or variations on the above described modes of use or entirely different modes of use may also be employed. The embodiments to be illustrated involve scanning, as in the modes 2 and 3 just mentioned.

[0044] Although specific reference may be made in this text to the use of lithographic apparatus in the manufacture of ICs, it should be understood that the lithographic apparatus described herein may have other applications, such as the manufacture of integrated optical systems, guidance and detection patterns for magnetic domain memories, flat-panel displays, liquid-crystal displays (LCDs), thin-film magnetic heads, etc. The skilled artisan will appreciate that, in the context of such alternative applications, any use of the terms“wafer” or“die” herein may be considered as synonymous with the more general terms“substrate” or“target portion", respectively. The substrate referred to herein may be processed, before or after exposure, in for example a track (a tool that typically applies a layer of resist to a substrate and develops the exposed resist), a metrology tool and/or an inspection tool. Where applicable, the disclosure herein may be applied to such and other substrate processing tools. Further, the substrate may be processed more than once, for example in order to create a multi-layer IC, so that the term substrate used herein may also refer to a substrate that already contains multiple processed layers.

[0045] Figure 2 shows the system 100 in more detail, including the EUV radiation source comprising the source collector module SO and the lithographic scanning apparatus comprising the illumination system IL, and the projection system PS. The source collector module SO of the EUV radiation source is constructed and arranged such that a vacuum environment can be maintained in an enclosing structure 220 of the source collector module SO. The systems IL and PS are likewise contained within vacuum environments of their own. An EUV radiation emitting plasma 210 may be formed by a laser produced LPP plasma source. The function of source collector module SO is to deliver EUV radiation beam 20 from the plasma 210 such that it is focused in a virtual source point. The virtual source point is commonly referred to as the intermediate focus (IF), and the source collector module is arranged such that the intermediate focus IF is located at or near an aperture 221 in the enclosing structure 220. The virtual source point IF is an image of the radiation emitting plasma 210

[0046] From the aperture 221 at the intermediate focus IF, the radiation traverses the illumination system IL, which in this example includes a facetted field mirror device 22 and a facetted pupil mirror device 24. These devices form a so-called“fly’s eye” illuminator, which is arranged to provide a desired angular distribution of the radiation beam 21, at the patterning device MA, as well as a desired uniformity of radiation intensity at the patterning device MA. Upon reflection of the beam 21 at the patterning device MA, held by the support structure (mask table) MT, a patterned beam 26 is formed and the patterned beam 26 is imaged by the projection system PS via reflective elements 28, 30 onto a substrate W held by the wafer stage or substrate table WT.

[0047] Each system IL and PS is arranged within its own vacuum or near-vacuum environment, defined by enclosing structures similar to enclosing structure 220. More elements than shown may generally be present in illumination system IL and projection system PS. Further, there may be more mirrors present than those shown in the Figures. For example there may be one to six additional reflective elements present in the illumination system IL and/or the projection system PS, besides those shown in Figure 2. The United States patent application publications referred to above show three additional elements in the illumination system, for example.

[0048] Considering source collector module SO in more detail, laser energy source comprising laser 223 is arranged to deposit laser energy 224 into a fuel, such as xenon (Xe), tin (Sn) or lithium (Li), creating the highly ionized plasma 210 with electron temperatures of several 10's of eV. Higher energy EUV radiation may be generated with other fuel materials, for example Tb and Gd. The energetic radiation generated during de-excitation and recombination of these ions is emitted from the plasma, collected by a near- normal incidence collector mirror CO and focused on the aperture 221. The plasma 210 and the aperture 221 are located at first and second focal points of collector or collector mirror CO, respectively.

[0049] To deliver the fuel, which for example is liquid tin, a droplet generator 226 is arranged within the enclosure 220, arranged to fire a high frequency stream 228 of droplets towards the desired location of plasma 210. In operation, laser energy 224 is delivered in a synchronism with the operation of droplet generator 226, to deliver impulses of radiation to turn each fuel droplet into a plasma 210. The frequency of delivery of droplets may be several kilohertz, for example 50 kHz. In practice, 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, in order to condition the droplet for receipt of a main pulse, e.g., by shaping the droplet as 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 trap 230 is provided on the opposite side of the enclosing structure 220, to capture fuel that is not, for whatever reason, turned into plasma.

[0050] Numerous additional components in the source collector module and the lithographic apparatus are present in a typical lithographic system, though not illustrated here. These include arrangements for reducing or mitigating the effects of contamination within the enclosed vacuum, for example to prevent deposits of fuel material damaging or impairing the performance of collector mirror CO and other optics. Also, one or more spectral purity filters may be included in the source collector module SO and/or illumination system IL. These filters are for eliminating as much as possible radiation of unwanted wavelengths, that is generated by the laser and/or the plasma 210, in addition to the wanted wavelengths of the EUV radiation. The spectral purity filter(s) may 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, for example downstream of the virtual source point IF. Multiple filters can be deployed. The skilled person is familiar with the need for these measures, and the manner in which they may be implemented, and further detail is not required for the purposes of the present disclosure.

[0051] Referring to laser 223 from Figure 2 in more detail, the laser in the presented embodiment is of the MOPA (Master Oscillator Power Amplifier) type. This consists of a“master” laser or“seed” laser, labeled MO in the diagram, followed by a power amplifier (PA). A beam delivery system 240 is provided to deliver the laser energy 224 into the module SO. In practice, the pre-pulse element of the laser energy will be delivered by a separate laser, not shown separately in the diagram. Faser 223, fuel source (i.e. the droplet generator) 226 and other components may e.g. be controlled by a source control module 242.

[0052] As one of ordinary skill in the art will know, reference axes X, Y and Z may be defined for measuring and describing the geometry and behavior of the lithographic system, its various components, and the radiation beams 20, 21, 26. At each part of the lithographic system, a local reference frame of X, Y and Z axes may be defined. The Z axis broadly coincides with the direction optical axis O at a given point in the system, and is generally normal to the plane of a patterning device (reticle) MA and normal to the plane of substrate W. In the source collector module, the X axis coincides broadly with the direction of fuel stream (228, described below), while the Y axis is orthogonal to that, pointing out of the page as indicated in Figure 2. On the other hand, in the vicinity of the support structure MT that holds the reticle MA, the X axis is generally transverse to a scanning direction aligned with the Y axis. For convenience, in this area of the schematic diagram Figure 2, the X axis points out of the page, again as marked. These designations are conventional in the art and will be adopted herein for convenience. In principle, any reference frame can be chosen to describe the lithographic system and its behavior. [0053] Referring in a little more detail to the illumination system, faceted field mirror device 22 comprises an array of individual facets, so that the EUV radiation beam 20 is divided into a number of sub beams, of which one is labeled 260 in the diagram. Each sub beam is directed towards an individual facet on the faceted pupil mirror device 24. The facets of pupil mirror device 24 are arranged to direct their individual sub beams onto a target which is a slit-shaped area of patterning device MA. The division into sub beams 260 and the combination into a single beam 21 is designed to create highly uniform illumination over the slit area, when the illumination arriving from the source collector module is highly non-uniform in its angular distribution. As is also known, the facets of the devices 22 and/or 24 may be steerable and/or maskable, in order to implement different illumination modes.

The conditioned EUV radiation beam 21 is delivered to patterning device MA through a conditioning and masking module 262. This module includes a masking unit, also referred to as the reticle mask (REMA) which may have movable blades defining the extent of an illumination slit in X and Y directions. Typically, the illumination slit as applied in EUV-type lithographic apparatuses may be curved.

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

[0054] To expose a target portion C on substrate W, pulses of radiation are generated on substrate table WT and masked table MT perform synchronized movements 266, 268 to scan the pattern on patterning device MA through the slit of illumination.

[0055] Examples of illumination systems including REMA and UNICOM functions are described in United States Patent Application Publication Nos. 2005/0274897A1 and

2011/0063598A.

[0056] Many 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 from the source collector module SO. In systems based on LPP sources, control of alignment is generally achieved by controlling the location of the plasma 210, rather than by moving the collector mirror CO. The collector mirror, the exit aperture 221 and the illuminator IL are aligned accurately during a set-up process, so that aperture 221 is located at the second focal point of collector mirror. However, the exact location of the virtual source point IF formed by the EUV radiation at the exit of the source optics is dependent on the exact location of the plasma 210, relative to the first focal point of the collector mirror. To fix this location accurately enough to maintain sufficient alignment generally requires active monitoring and control.

[0057] For this purpose, source control module (controller) 242 in this example controls the location of the plasma 210 (the source of the EUV radiation), by controlling the injection of the fuel, and also for example the timing of energizing pulses from laser. In a typical example, energizing pulses of laser radiation 224 are delivered at a rate of 50 kHz (period 20 ps), and in bursts lasting anything from, say, 20 ms to 20 seconds. The duration of each main laser pulse may be around 1 ps, while the resulting EUV radiation pulse may last around 2 ps. By appropriate control, it is maintained that the EUV radiation beam is focused by collector mirror CO precisely on the aperture 221. If this is not achieved, all or part of the beam will impinge upon surrounding material of the enclosing structure.

[0058] The source control module 242 is supplied with monitoring data from one or more arrays of sensors (not shown (which provide a first feedback path for information as to the location of the plasma. The sensors may be of various types, for example as described in Unites States Patent Application Publication No. 2005/0274897A1, mentioned above. The sensors may be located at more than one position along the radiation beam path. They may for example be located around and/or behind the field mirror device 22, purely for the sake of example. The sensor signals just described can be used for control of the optical systems of the illuminator IL and projection system PS. They can also be used, via 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 can be processed for example to determine the observed location of the virtual source IF, and this is extrapolated to determine, indirectly, the location of the EUV source. If the virtual source location drifts, as indicated by the sensor signals, corrections are applied by control module 242 to re-center the beam in the aperture 221.

[0059] Rather than rely entirely on the signals from the illuminator sensors, additional sensors and feedback paths may generally be provided in the source collector module SO itself, to provide for more rapid, direct and/or self-contained control of the radiation source. Such sensors may include one or more cameras, for example, monitoring 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.

[0060] In order to ensure that the substrate W is provided with the appropriate dosage of radiation, it is important to ensure that the collector or collector mirror CO sufficiently reflects the generated EUV radiation. In particular, it is desirable that that collector mirror CO has a sufficiently high reflectivity and that the reflectivity is as uniform as possible.

[0061] In this respect, it can be pointed out that during the generation of the EUV radiation as described above, debris may be generated due to the interaction of the stream of fuel droplets 228 with the laser pulses. Such debris may cause a contamination of the collector mirror CO, thus adversely affecting the reflectivity and the uniformity of the reflectivity. Also, 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 e.g. include droplets or particles or contaminants that fall down from components of the EUV source that are arranged above the collector mirror CO or contamination caused by spitting of surfaces such as vane surfaces or a tin catcher. [0062] In order to reduce the adverse effect of contamination, a collector mirror such as collector mirror CO shown in Figure 2 may be subjected to a cleaning process to remove the contamination. Such a cleaning process may e.g. include a carbon-dioxide snow cleaning process.

[0063] In order to check that the cleaning process has been effective, it is preferred to subject the collector mirror, after the cleaning process, to an inspection, whereby the reflectivity of the collector mirror is assessed, e.g. at a plurality of locations on the mirror.

[0064] The present invention provides a system for performing such an inspection or testing of a collector mirror of an EUV lithographical system and a method of inspecting or testing a collector mirror of an EUV lithographic system.

[0065] More general, the present invention provides a system for performing an 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 such as applied in EUV radiation sources, lithographic projection systems, illumination systems as used in lithographic apparatuses, etc. With reference to Figure 2, the present invention may e.g. be applied for the inspection or testing of the collector mirror CO, any of the mirrors as applied in the illuminator IL such as the field facet device 22 or the pupil mirror device 24, any of the mirrors as applied in the projection system PS, such as reflective elements 28 or 30.

[0066] Figure 3 schematically shows an embodiment of a system according to the present invention for testing a collector mirror. The collector mirror 300 has a first focus or focal point 310 and a second focus or focal point 320. The focal point 310 may e.g. substantially correspond to the location where the radiation emitting plasma 210 as shown in Figure 2 is formed, whereas the focal point 320 e.g. corresponds to the intermediate focus IF as shown in Figure 2.

[0067] The system according to the present invention comprises a test radiation sub-system 330 that is operative to project test radiation 330.1 from the second focus 320 to the collector mirror 300. Within the meaning of the present invention, test radiation refers to the radiation that is used during the testing or inspecting of the mirror, e.g. the collector mirror 300 as shown.

In an embodiment of the present invention, the test radiation as applied has a wavelength or wavelength spectrum that is selected based on the radiation that is used under normal operating conditions of the collector mirror. In particular, in case the collector mirror is to be used in a EUV lithographic system, the wavelength or wavelength spectrum of the test radiation may be selected based on the EUV radiation applied in the lithographic apparatus. As such, in an embodiment of the present invention, the test radiation may comprise radiation having a wavelength in the range from 10 to 20 nm. In an embodiment, the test radiation as applied may have a wavelength spectrum of 13.5 nm +/- 1 nm, FWHM, full width half maximum. In an embodiment, the test radiation sub-system as applied in the system according to the present invention may be configured to apply radiation of different wavelengths or different wavelength ranges for inspecting or testing a mirror. In such embodiment, the test radiation sub-system may comprises multiple radiation sources and/or multiple filters to filter the generated radiation, so as to arrive at the desired test radiation. As an example, it may be useful to assess the reflectivity of a mirror such as a collector mirror 300 at different wavelengths or wavelength ranges. It may e.g. be useful to assess the reflectivity of the collector mirror at the aforementioned spectrum of 13.5 nm +/- 1 nm but also at a spectrum encompassing or including infrared (IR) radiation.

In an embodiment, the test radiation sub-system is configured to generate test radiation that spans an angle ao, such that, in principle, the entire collector mirror 300 can be irradiated. Please note that the diagram of Fig.3 represents the collector mirror 300 in a two-dimensional cross-section and that, in practice, the test radiation is generally emitted in three dimensions. The angle ao in Fig.3 is therefore the angle subtended by the three-dimensional test radiation pattern when projected onto the two- dimensional plane with the cross-section of the collector mirror 300. In an embodiment, test radiation sub-system is configured to generate test radiation that spans an angle that is smaller than the angle ao. In such embodiment, the entire collector mirror 300 may still be scanned or irradiated, e.g. by rotating the test radiation sub-system 330 or a portion thereof.

[0068] The system according to the present invention further comprises a sensor sub-system, schematically indicated by reference number 340, that is operative to receive test radiation, e.g. test radiation indicated by the arrow 342, that is reflected off the collector mirror 300 towards the first focus 310. In an embodiment, as will be discussed in more detail below, the sensor sub-system 340 may comprise one or more sensors or detectors for measuring the test radiation that is reflected off the mirror, i.e. the collector mirror 300.

[0069] In accordance with the present invention, the system for testing a collector mirror further comprises a radiation limiter sub-system 360. The radiation limiter sub-system 360 is configured to limit the test radiation as received by the sensor sub-system 340 to test radiation that is reflected off a limited portion of the collector mirror 300. By doing so, the radiation that is measured or detected by the sensor sub-system will involve only radiation that is reflected off the limited portion of the collector mirror 300. In the embodiment as shown, the limited portion of the collector mirror is indicated by a mirror segment 300.1. By limiting the radiation as received by the sensor sub-system to radiation that is reflected off the limited portion of the collector mirror, one can make an assessment of the performance of that particular portion of the collector mirror, e.g. an assessment of the reflectivity of that portion.

[0070] In the embodiment as shown, the radiation limiter sub-system 360 is configured to only let a small portion of the generated radiation 330.1 pass towards the collector mirror. The small portion of the radiation 330.1 subtends a much smaller angle than the angle ao. In order to realize that, the radiation limiter sub-system 360 comprises a shielding member 360.1 that blocks the test radiation, whereby the shielding member 360.1 comprises an aperture, e.g. a tubular shaped aperture 360.2, through which the small portion of the test radiation 330.1 can pass. The radiation limiter sub system 360 thus enables that only a limited portion, e.g. portion 300.1 of the collector mirror 300 is irradiated by the test radiation generated by the test radiation sub-system 330.

[0071] In accordance with the present invention, the system further comprises a control sub system 370 that is configured to control a movement of the radiation limiter sub-system 360, e.g. by providing a control signal 370.1 to the radiation limiter sub-system 360. In particular, the control sub system 370 is configured to control a position of the radiation limiter sub-system 360 such that the limited portion of the collector mirror that is irradiated by test radiation can be changed. As such, the control sub-system may e.g. be configured to control a movement of the radiation limiter sub-system along a sequence of different positions, thereby limiting the test radiation as received by the sensor sub-system 340 to the test radiation that is reflected off a respective sequence of different limited portions of the collector mirror 300. By doing so, an assessment of the performance of the different limited portions of the collector mirror, e.g. an assessment of the reflectivity of these portions, can be made.

[0072] In an embodiment of the present invention, a set of the different limited portions substantially covers the entire collector mirror 300.

[0073] In an embodiment of the present invention, the radiation limiter sub-system comprises one or more actuators to displace the aperture tube 360.2, e.g. together with the shielding member 360.1, so as to change the limited portion of the collector mirror 300 that is irradiated by the test radiation. In such embodiment, the aperture tube 360.2 may e.g. be rotated about an axis perpendicular to the plane of the drawing and e.g. through the second focus 320. By doing so, the angle a defining at which angle the test radiation impinges on the collector mirror, can be modified.

In a similar manner, the aperture tube 360.2 may e.g. be rotated about an axis that is parallel to the indicated Z-axis and e.g. through the second focus 320. By combining both rotational movements, the entire surface of the collector mirror may be selectively scanned by test radiation. This embodiment will be explained in more detail below.

[0074] As will be appreciated by the skilled person, other types of actuators such as linear actuators or motors may also be applied to displace the radiation limiter sub-system, thereby controlling which limited portion of the collector mirror is being irradiated.

[0075] In an embodiment of the present invention, the test radiation sub-system 330 or the radiation limiter sub-system 360 are configured to shape the generated test radiation into a beam. In such embodiment, the test radiation sub-system 330 and/or the radiation limiter sub-system 360 are configured to project the test radiation as a beam onto the collector mirror 300. In such embodiment, the control sub-system 370 may be configured to direct the beam or beam-shaped test radiation onto a plurality of different limited portions of the collector mirror, one limited portion after the other. Such a sequential projection of the test radiation beam on different limited portions of the collector mirror may also be referred to as scanning the collector mirror with a test radiation beam. During such a scanning process, whereby a part of the collector mirror 300 or the entire mirror is scanned, the sensor sub-system 340 may generate a set of measurement data representative of test radiation reflected off the different limited portions of the collector mirror 300. Such measurement data may e.g. be provided to the control sub-system 370 of the system according to the present invention via a data channel, as indicated by the arrow 370.2. In an embodiment, the measurement data 370.2 as received may e.g. be processed by a processing unit 380 of the control sub-system 370. Such a processing unit may e.g. be embodied as a processor, a microprocessor, computer or the like. Such a processing unit 380 may e.g. comprises a memory unit for storing the measurement data.

[0076] In an embodiment, the processing unit 380 may be configured to process the

measurement data so as to determine a spatial reflectivity distribution of at least part of the collector mirror. In such embodiment, the measurement data representing the test radiation reflected off the different limited portions of the collector mirror 300 can be compared with data representative of the generated or emitted test radiation to determine the reflectivity of the different limited portions of the collector mirror. Examples of a measure for the reflectivity are the ratio of the amount of received test radiation to the amount of the generated or emitted test radiation, and a difference between, on the one hand, the amount of generated or emitted test radiation and, on the other hand, the amount of received test radiation. As such, in an embodiment of the present invention, the measurement data obtained from the sensor sub-system 340 can be complemented with source measurement data, the source measurement data representing the amount of test radiation emitted or generated during the scanning process. Such source measurement data may e.g. be provided to the processing unit 380 of the control sub-system 370 via a data channel as indicated by the arrow 370.3. In such embodiment, the test radiation sub-system 330 may also comprise a sensor, referred to as the source sensor, that is configured to generate a source measurement signal representative of the test radiation projected by the test radiation sub-system 330. By combining the measurement data of the sensor sub-system with the source measurement data of the test radiation sub-system, a more accurate assessment of the reflectivity across the collector mirror 300 can be made, because, based on the source measurement data, any fluctuations in the test radiation as generated and emitted during the scanning process can be taken into account.

[0077] As an alternative to the application of a source sensor in the test radiation sub-system 330, it may be worth mentioning that a sensor may be applied in the aperture tube 360.2 as well, in order to obtain a measure for the amount of test radiation that is emitted towards the collector mirror 300. In a similar manner, measurement data of such an aperture sensor may be combined with measurement data obtained from a sensor of the sensor sub-system 340 to determine a spatial reflectivity map of at least part of the collector mirror 300. [0078] Preferably, as already indicated above, the wavelength or wavelength spectrum of the test radiation as applied preferably corresponds to the wavelength or wavelength spectrum as used during normal operation of the collector mirror, e.g. when the collector mirror is used in a lithographic system as illustrated in Figure 2. By doing so, one can more accurately determined the reflectivity of the collector mirror for the relevant wavelength or wavelength spectrum. In order to realize this, different options exist.

In an embodiment of the present invention, the test radiation sub-system 330 comprises a radiation source and a multilayer mirror assembly for spectrally filtering the generated radiation in order to arrive at the require test radiation, i.e. test radiation having the appropriated wavelength or wavelength spectrum. As known, a multilayer mirror, also referred to as Bragg mirror, is a type of reflective optical element composed of multiple layers of alternating thickness and refractive index. The thicknesses are tuned to govern constructive interference of radiation of a desired wavelength reflected off subsequent layers. In an embodiment, the radiation source of the test radiation sub system 330 comprises a Xe EUV source. In such embodiment, the multilayer mirror arrangement can be configured to filter the radiation to a wavelength spectrum of e.g. of 13.5 nm +/- 1 nm, FWHM, full width half maximum.

[0079] Figure 4 schematically shows three possible multilayer mirror arrangements for generating the appropriate test radiation, which can e.g. be applied in a test radiation sub-system 330 of a system according to the invention.

[0080] Figure 4a schematically shows a radiation source 420, e.g. a radiation source for generating EUV radiation. In the embodiment as shown, an aperture 410 is provided to allow part of the generated radiation, indicated by the arrow 420.1, to impinge on a first mirror 430, e.g. a multilayer mirror. The first mirror 430 is configured to reflect the received radiation towards a second mirror 440, e.g. a multilayer mirror, as indicated by the arrow 420.2. the second mirror 440 is configured to reflect the received radiation towards an aperture 450, as indicated by the arrow 420.3. The aperture may e.g. be an outlet of an aperture tube 460, configured to receive the radiation 420.3 reflected off the mirror 440. In the embodiment as shown, the mirrors 430 and 440 may serve to filter undesired wavelengths or wavelength components from the radiation as generated by the source 420. Alternatively, or in addition, the mirrors 430 and 440 may form a beam expander or a beam compressor for control of the width, and therefore of the intensity, of the beam formed by the test radiation impinging on the collector 300.

[0081] In an embodiment of the present invention, the mirror assembly comprising the mirrors 430 and 440 and the aperture tube 460 may serve as radiation limiter sub-system as discussed above. In such an embodiment, the mirror assembly and the aperture tube may e.g. be configured to be displaceable relative to the radiation source 420, so as to change the orientation of the radiation 420.3 emitted from the aperture. By proper dimensioning of the mirrors or of the mirror assembly, the radiation beam 420.3 emitted from the aperture 450 may be a collimated beam 420.4, i.e. a parallel beam of radiation which can impinge on a limited portion of the collector mirror. In order for the collimated beam 420.4 to reflect from the collector mirror towards the first focal point or first focus of the mirror, the collimated beam 420.4 needs to be emitted from the second focal point or second focus. This can e.g. be realized by arranging the mirror assembly and aperture tube in such manner that the second focus lies within the aperture tube, e.g. at location 470.

[0082] Figure 4b schematically shows another embodiment of a mirror assembly that can be applied to filter radiation from a radiation source. Figure 4b schematically shows a radiation source 520, e.g. a radiation source for generating EUV radiation. In the embodiment as shown, the radiation source 520 is configured to emit part of the generated radiation, indicated by the arrow 520.1, to impinge on a first mirror 530, e.g. a multilayer mirror, shown here in cross-section. The first mirror 530 is configured to reflect the received radiation as indicated by the arrow 520.2, towards a second mirror 540, e.g. a multilayer mirror,. The second mirror 540 is configured to reflect the received radiation towards a focal point 550. As discussed above, the mirrors 530 and 540 may serve to filter the radiation as generated by the radiation source 520 to obtain test radiation with the appropriate wavelength or spectrum. From the focal point 550 onwards, the filtered radiation 520.3 may be used as test radiation for e.g. scanning a collector mirror as discussed above, with reference to Figure 3. In such embodiment, the generated test radiation may e.g. be limited by a radiation limiter 360 that is configured to limit the generated test radiation, e.g. radiation 520.3, to impinge on a limited portion of the collector mirror. In order to ensure that the emitted test radiation 520.3, or the portion thereof that impinges on the collector mirror, is reflected from the collector mirror, e.g. collector mirror 300, towards the first focal point or first focus of the mirror, the test radiation 520.3 needs to be emitted from the second focal point or second focus of the collector mirror. This can e.g. be realized by arranging the mirror assembly 530, 540 in such manner that the second focus of the collector mirror substantially coincides with the focal point 550.

[0083] The mirror arrangement including the mirrors 530 and 540 is known as a Schwarzschild objective.

[0084] Figure 4c schematically shows yet another embodiment of a mirror assembly that can be applied to filter radiation from a radiation source, the mirror assembly also including a Schwarzschild objective including mirrors 630 and 640. Figure 4c schematically shows a radiation source 620, e.g. a radiation source for generating EUV radiation. In the embodiment as shown, the radiation source 620 is configured to emit part of the generated radiation, indicated by the arrow 620.1, to impinge on a first mirror 630, e.g. a multilayer mirror, shown here in cross-section. The first mirror 630 is configured to reflect the received radiation, as indicated by the arrow 620.2, towards a second mirror 640, e.g. a multilayer mirror. The second mirror 640 is configured to reflect the received radiation as a collimated beam 620.3 through an aperture 660 of the first mirror 630. As will be appreciated by the skilled person, by appropriate dimensioning and positioning of the mirrors 630 and 640 of the Schwarzschild objective, one can ensure that the emitted radiation is shaped as a collimated beam 620.3, rather than being focused onto a focal point 550 as shown in Figure 4b. In such an arrangement, the mirror arrangement 630, 640 may thus serve both as a filter to generated test radiation having the appropriate wavelength or wavelength spectrum and shaping the test radiation so as to obtain a beam of test radiation 620.3 configured to impinge on a limited portion of a collector mirror that is inspected. As such, the mirror arrangement can be considered to serve as a radiation limiter, limiting the test radiation to impinge on a limited portion of the collector mirror.

Again, in order to ensure that the radiation that is reflected off the collector mirror, e.g. collector mirror 300, towards the first focal point or first focus of the mirror, the test radiation 620.3 needs to be emitted from the second focal point or second focus of the collector mirror. This can e.g. be realized by arranging the second mirror 640 at or near the second focus of the collector mirror that is inspected.

In a similar manner as discussed above, the multilayer mirror assembly 630, 640 can be configured to be displaceable so as to scan at least part of the surface of the collector mirror with the test radiation 620.3, so as to determine the reflectivity of the mirror.

[0085] In an embodiment of the present invention, the test radiation sub-system as applied further comprises a sensor for sensing the radiation as emitted by the source. Such a sensor, also referred to as the source sensor, may e.g. be configured to directly receive the radiation as emitted by the source. In Figure 4c, such a sensor 650 is schematically shown, the sensor 650 being arranged to receive radiation 620.4 as emitted by the source. 620. Alternatively, the source sensor 650 may be arranged to receive part of the test radiation that is reflected off of a mirror, e.g., first mirror 630 or second mirror 640, of the mirror arrangement as applied in an embodiment of the present invention. In such arrangement, the source sensor may thus measure radiation having the same wavelength or wavelength spectrum as the test radiation as received by the collector mirror.

[0086] In an embodiment of the present invention, a source sensor, i.e. a sensor configured to generate source measurement data representative of the radiation or test radiation emitted by a source of the test radiation sub-system, may also be arranged in a radiation limiter sub-system such as radiation limiter sub-system 360 as shown in Figure 3. In such embodiment, the source sensor may be arranged at or near or in an aperture tube 360.2 of the radiation limiter sub-system 360 so as to capture part of the radiation that is irradiated by the test radiation sub-system 330.

[0087] As already indicated above, the system according to the present invention comprises a sensor sub-system, e.g. sensor sub-system 340 as shown in Figure 3. Various different embodiment of such a sensor sub-system as applied in the present invention are discussed in Figure 5.

[0088] Figure 5a schematically shows a first arrangement of a sensor sub-system 700, together with a collector mirror 300 whose reflectivity is to be checked, as can be applied in a system according to the present invention. The sensor sub-system 700 comprises a sensor 700.1 having an active area 700.2 facing the collector mirror 300. hi the embodiment as shown, the active area 700.2 is deemed to be located at or near the first focus of the collector mirror 300, so as to receive test radiation that is emitted from the second focus and then reflected off of the mirror 300.

In an embodiment, the sensor 700.1 may e.g. comprise an EUV photodiode. The sensor sub-system 700 further comprises an output 700.3 for outputting a measurement signal 700.4 representative of the received radiation.

[0089] In the arrangement as shown, the sensor sub-system is configured to remain stationary with respect to the collector mirror 300. As such, the angle of incidence of the test radiation reflected off of the collector mirror 300 may vary, depending on the angle of the emitted radiation, 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 can be taken into account.

[0090] As an alternative, as schematically shown in Figure 5b, a sensor sub-system 750 may be applied which comprises a plurality of sensors 750.1, 750.2, 750.3 having a different orientation relative to the optical axis O. By doing so, depending on the angle of incidence of the receive radiation, one can select the sensor that is most sensitive, i.e. the sensor who’s active area faces the received radiation the best.

[0091] Figure 5c schematically shows yet an alternative sensor sub-system 760, together with a collector mirror 300, the sensor sub-system 760 comprising a sensor 760.1 having an active area 760.2. The sensor sub-system 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 configure to rotate the sensor 760.1 about an axis 780, the axis 780 being parallel to the indicated Z- direction and crossing the first focus of the collector mirror, and about an axis perpendicular to the drawing and crossing the first focus of the collector mirror. In such an arrangement, the sensor 760.1, in particular the active area 760.2 of the sensor 760.1 may be directed to face the test radiation reflected towards the first focus, e.g. facing the reflected beam of the radiation beam 710.2.

[0092] In an embodiment, as schematically shown in Figure 5d, a sensor 790.1 of a sensor sub system 790 as applied in a system according to the present invention, may be mounted inside a tube, e.g. an aperture tube 795. In the embodiment as shown, the sensor 790.1 and the aperture tube 795 may be displaced by an actuator or actuator arrangement 770 as discussed above, so as to face reflected off of the collector mirror, e.g. facing the reflected beam of the radiation beam 710.2. By arranging the sensor 790.1 inside or at the far end of the aperture tube 795, the sensor 790.1 can only sense radiation that is received by the aperture tube 795. By doing so, measurement disturbances due to stray radiation can be mitigated or avoided.

[0093] The sensor sub- systems as schematically shown in Figured 5a - 5d are configured to directly receive the reflected radiation, i.e. no spectral filtering of the reflected radiation is applied. As discussed above, it may be preferred to inspect or test the collector mirror using test radiation having the same or similar wavelength as applied during normal operation, because one is in particular interested in characterizing the collector mirror for that specific wavelength or wavelength spectrum. In order to make such an assessment using the sensor sub-systems of Figures 5a - 5d, it may be advantageous to apply these systems in combination with a test radiation sub-system which is configured to filter the test radiation, e.g. test-radiation sub-systems as illustrated in Figures 4a - 4c, wherein the first mirror and/or the second mirror may be implemented with a spectral purity filter.

[0094] Irrespective of whether a filtering of the test radiation is applied by the test radiation sub system, one may, in a similar manner apply a filtering of the test radiation reflected off of the collector mirror. Such embodiments as schematically shown in Figures 5e - 5f.

[0095] Figure 5e schematically shows a sensor sub-system 800, as can be applied in a system according to the present invention, together with a collector mirror 300. The sensor sub-system 800 as schematically shown comprises a sensor 800.1 and a multilayer mirror assembly comprises a pair of mirrors 810, 820 that are configured to reflect test radiation 830.1 reflected off of the collector mirror 300 towards the sensor 800.1, in accordance with the indicated arrows 830.2. In the embodiment as shown, the incoming radiation 830.1 is received by an aperture tube 840 arranged to receive the reflected radiation towards the first focus 850 of the collector mirror 300. In an embodiment of the present invention, the sensor sub-system 800 may further be equipped with an actuator or actuator arrangement for adjusting at least one of the aperture tube, the multilayer mirror assembly and the sensor so as to receive reflected radiation off of different limited portions of the collector mirror.

[0096] In an embodiment of the present invention, the sensor sub-system as applied in a system according to the present invention can make use of a Schwarzschild objective for spectrally filtering any incoming radiation, i.e. test radiation reflected off of the collector mirror 300. Such an embodiment is schematically shown in Figure 5f. Figure 5f schematically a sensor sub-system 900, as can be applied in a system according to the present invention, together with a collector mirror 300. The sensor sub-system 900 as schematically shown comprises a sensor 900.1 and a multilayer mirror assembly comprises a pair of mirrors 910, 920, shown in cross-section, that are configured to reflect test radiation 930.1 reflected off of the collector mirror 300 towards the sensor 900.1, in accordance with the indicated arrows 930.2. As can be seen, the mirrors 910 and 920 form a Schwarzschild objective configured to receive a beam of test radiation 930.1 reflected off of the collector mirror 300, towards the first focus of the collector mirror. In the embodiment as shown, the first focus of the collector mirror may e.g. be arranged to substantially coincide with the first mirror 910. In the embodiment as shown, the incoming radiation 930.1 is received via an aperture tube 940 arranged to receive the reflected radiation towards the first focus of the collector mirror 300, i.e. where the mirror 910 is located. In an embodiment of the present invention, the sensor sub-system 900 may further be equipped with an actuator or actuator arrangement for adjusting at least one of the aperture tube, the multilayer mirror assembly and the sensor so as to receive reflected radiation off of different limited portions of the collector mirror 300. It can be pointed out that the mirror arrangement 910, 920 can be configured in a similar manner as the mirror arrangement as shown in Figure 4c, but is used in an inverted manner; i.e. in the embodiment of Figure 4c, the Schwarzschild objective is used to generate a collimated beam from a radiation source, whereas in the embodiment of Figure 5f, the

Schwarzschild objective is configured to receive a collimated beam 930.1 and project it onto a sensor.

[0097] In the embodiments of the sensor sub-systems shown in Figures 5e or 5f, the test radiation as received by the sensor is filtered using a mirror arrangement. When such embodiments are used, a filtering of the radiation as generated by the source may be omitted.

[0098] In the arrangement as shown in Figure 3, a radiation limiter sub-system 360 is arranged in the beam path between the test radiation sub-system 330 and the collector mirror 300. As such, only a limited portion 300.1 of the collector mirror 300 is irradiated at a time. Alternatively, the radiation limiter sub-system can be implemented in the radiation path between the collector mirror 300 and the sensor sub-system 340. In such an embodiment, the test radiation emitted towards the collector mirror 300 need not be spatially limited to impinge on only a limited portion of the collector mirror, since the limitation of the test radiation as received by the sensor is limited by a radiation limiter sub-system arranged in the radiation path between the collector mirror 300 and the sensor sub-system 340. In an embodiment of the present invention, the radiation limiter sub-system comprises a first radiation limiter sub-system arranged in a path between the test radiation sub-system and the collector mirror and a second radiation limiter sub-system arranged in a path between the collector mirror and the sensor sub-system. In such a radiation limiter sub-system the first radiation limiter sub-system may e.g. comprise a first shielding member having a first aperture therein, the first shielding member being arranged in an optical path of the test radiation between the second focus and the collector mirror, whereas the second radiation limiter sub-system may e.g. comprise a second shielding member having a second aperture therein, the second shielding member being arranged in an optical path of the test radiation between the collector mirror and the first focus. Note that, within the meaning of the present invention, optical path may refer to the trajectory followed by the test radiation, even though the radiation applied may not be visible.

[0099] Such an embodiment is schematically shown in Figure 6. Figure 6 schematically shows the same features and components as Figure 3. In addition, the embodiment comprises a further radiation limiter sub-system 390 that is configured to limit test radiation as received by the sensor sub-system 340 to radiation reflected off of a limited portion 300.1 of the collector mirror 300. In the embodiment as shown, the further radiation limiter sub-system 390 comprises a shielding member 390.1 protruded by an aperture tube 390.2. As a result of including radiation limiter sub-systems in both the optical path between the collector mirror and the first focus and the optical path between the second focus and the collector mirror, stray light effects can be mitigated or avoided. As will be appreciated by the skilled person, displacements of both radiation limiter sub-systems are synchronised in that both radiation limiter sub-systems need to be oriented towards the same limited portion, e.g. portion 300.1 of the collector mirror 300.

[00100] In an embodiment of the present invention, the radiation limiter sub-system comprises a hemisphere shaped shielding member and a aperture tube protruding through an apex of the hemisphere. Such an embodiment is schematically shown in Figure 7. Figure 7 schematically shows a radiation limiter sub-system comprising a hemispherical shielding member 950 and an aperture tube 960 protruding through the apex of the hemisphere. The radiation limiter sub-system further comprises an actuator arrangement 970 comprises a first actuator 970.1 configured to rotate the shielding member 950 and the aperture tube 960 about an axis parallel to the indicated X-axis and a second actuator 970.2 configured to rotate the shielding member 950 and the aperture tube 960 about an axis parallel to the indicated Z-axis. The radiation limiter sub-system as schematically shown in Figure 7 may e.g. applied as the radiation limiter sub-system 360 shown in Figure 6, the radiation limiter sub-system 390 shown in Figure 6, or both. The actuator assembly 970 as schematically shown may also be used to displace either one of the test radiation sub-systems shown in Figures 4a - 4c and/or to displace either one of the sensor sub-systems shown in Figures 5c - 5f.

[00101] In an embodiment of the present invention, the radiation limiter sub-system as schematically shown in figure 7 is applied such that a centre 975 of the sphere, of which the hemisphere forms a part, substantially corresponds to a position of either the first focus or the second focus.

[00102] Prior to inspecting or testing a mirror such as collector mirror 300 by a system according to the present invention, it may be advantageous to calibrate the system. In an embodiment of the present invention, a calibration method is proposed whereby test radiation, emitted by the test radiation sub-system, is emitted directly towards the sensor sub-system. Figure 8 schematically shows a system according to the present invention arranged in a calibration position. The system as schematically shown substantially corresponds to the system shown in Figure 6, the system comprising a test radiation sub-system 330, a sensor sub-system 340, a control sub-system 370 and a radiation limiter sub-system comprising a first radiation limiter sub-system 360 and a second or further radiation limiter sub-system 390. Compared to the situation as depicted in Figure 6, the radiation limiter sub-systems 360 and 390 as shown in Figure 8 are arranged, i.e. rotated, such that the apertures 360.2 and 390.2 point towards each other. In such position, also referred to as the calibration position, a beam of test radiation 1000 can be emitted via the aperture 360.2 directly into the aperture 390.2, so as to be received by the sensor sub-system 340, in particular by a sensor of the sensor sub system 340. By comparing, in the calibration position, source measurement data, i.e. data indicative of the emitted amount of radiation by the test radiation sub-system 330 with measurement data obtained from the sensor sub-system 340, one can calibrate the system, i.e. determine a relationship between the emitted test radiation and the received test radiation in the absence of the collector mirror. In Figure 8, reference numbers 370.2 and 370.3 may respectively indicate the obtained measurement data and source measurement data as provided to the control unit 370. In an embodiment of the present invention, 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 the emitted radiation, may be determined prior to the measurement or test sequence of determining the reflectivity of the collector mirror.

In an embodiment of the present invention, the ratio of the received radiation to the emitted radiation, or the difference, may be again be determined after the measurement or test sequence of determining the reflectivity of the collector mirror. By determining the mentioned ratio or difference before and after the measurement sequence, any drift or deterioration of either the test radiation sub-system 330 or the sensor sub-system 340 may be detected and taken into account.

[00103] With respect to the calibration method as discussed, it can be pointed out that the same method may be applied as well, when either the first radiation limiter sub-system 360 or the second radiation limiter sub-system 390 is omitted.

[00104] In an embodiment of the present invention, the mirror that is to be inspected or tested, e.g. the collector mirror 300, is mounted at a substantially stationary frame during the inspection or testing.

[00105] In an embodiment of the present invention, the mirror that is to be inspected or tested is mounted at such a frame at an orientation with respect to gravity substantially corresponding to the orientation of the mirror during normal operation. Such an embodiment is schematically shown in Figure 9 for a collector mirror 1100.

[00106] Figure 9 schematically shows a collector mirror 1100 mounted inside a system 1200 according to an embodiment of the present invention. In the embodiment as shown, the system comprises a first vacuum chamber or vessel 1210 in which the collector mirror 1100 can be arranged, e.g. on a frame 1220. In the embodiment as shown, the vessel 1210 is mounted to an external frame 1230 at an angle, the angle e.g. being selected such that the collector mirror 1100 is arranged, when mounted to the frame 1220, at substantially the same angle as applied during normal operation. By doing so, one can assume that any deformations of the collector mirror 1100, e.g. due to gravity, will substantially correspond to deformations of the mirror 1100 during normal operation, i.e. when e.g. mounted in an EUV source of a lithographical system, such as the system shown in Figure 2. The system 1200 further comprises a test radiation sub-system 1240 arranged to emit test radiation 1240.1 from a second focus 1250 of the collector mirror, and a sensor sub-system 1260 arranged to receive test radiation 1260.1 reflected off of the collector mirror 1100 towards the first focus 1255 of the collector mirror 1100. In the embodiment as shown, the system 1200 further comprises, in a similar manner as shown in Figure 6, a radiation limiter sub-system comprising a first radiation limiter sub- system 1270 and a second or further radiation limiter sub-system 1280, the radiation limiter sub system being configured to limit the test radiation limit the test radiation as received by the sensor to test radiation reflected off a limited portion of the collector mirror 1100. Arrows 1285 schematically illustrate possible displacements of the radiation limiter sub-systems 1270 and 1280 during testing of the collector mirror. In the embodiment as shown, the first radiation limiter sub-system 1270 comprises a shielding member 1270.1 and an aperture tube 1270.2. In the embodiment as shown, the test radiation sub-system 1240 is arranged in a second vacuum chamber or vessel 1290, the first and second vacuum chambers 1210 and 1290 are separated by a wall 1300, the wall comprising a pair of substantially parallel wall portions 1300.1 and 1300.2 having an aperture therein to allow the test radiation to pass towards the collector mirror. In the embodiment as shown, the wall portions 1300.1 and 1300.2 are separated by a gap 1310 configured to receive a shielding member 1270.1 of the first radiation limiter sub-system 1270. In the embodiment as shown, the aperture in the wall 1300 is configured such that test radiation as emitted can reach all portions of the collector mirror 1100. By arranging the shielding member 1270.1 of the first radiation limiter sub-system in between the two wall portions 1300.1 and 1300.2 of the wall separating both vacuum chambers 1210 and 1290, a labyrinth is created that blocks or hinders propagation of any debris that is generated by the test radiation sub-system 1240 into the vacuum chamber 1210 containing the collector mirror. In order to further hinder or block a contamination of the vacuum chamber 1210, the system according to the present invention may further comprise a purge gas sub-system that is configured to introduce a flow of purge gas towards the second vacuum chamber 1290, so as hinder debris to propagate to the first vacuum chamber 1210. Reference number 1320 refers to such a purge-gas sub-system. It may also be pointed out that a purge-gas sub-system may be used to purge any of the aperture tubes applied in embodiments of the present invention, in order to stop debris from propagating towards the collector mirror.

[00107] As already discussed above, the system for inspection or testing a collector mirror, e.g. a collector mirror of an EUV radiation source, is configured to, in an embodiment of the present invention, irradiate a limited portion of the collector mirror by test radiation, e.g. a beam of test radiation. Phrased differently, the test radiation sub-system 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 spot that is irradiated corresponding to the size of the radiation beam, i.e. the beam of test radiation as applied. In principle, the size of the spot as applied may be selected at random. As will be understood by the skilled person, the smaller the spot-size, i.e. corresponding to the limited portion of the collector mirror irradiation per measurement, the higher the resolution of the reflectivity map as obtained. Also, the smaller the spot-size, the longer the measurement of the entire collector mirror may take and the smaller a signal to noise ration of the measurement may be.

[00108] Typically, a collector mirror as applied in an EUV radiation source is equipped with a grating arranged to reduce IR radiation reflected towards the second focus, e.g. the intermediate focus IF shown in Figure 2, of the collector mirror. Such a grating is schematically shown in Figure 10. Figure 10 schematically shows a portion 1400 of a collector mirror, including a grating 1410 having a periodicity P. Figure 10 further shows a test radiation beam 1420 configured to impinge on a limited portion of the collector mirror portion 1400. The beam of radiation 1420 has a cross-section Pb corresponding to the size of a spot or limited portion of the collector mirror that is irradiated.

It has been devised by the inventor that the edges 1410.1 of the grating may cause disturbances of the measurements as performed. In order to avoid such disturbances, it has been found that it may be advantageous to select the size of the spot that is irradiated by the radiation beam 1420 as an integer times the periodicity of the grating, i.e. selecting Pb = n x P, n being an integer number.

[00109] Figure 11 schematically shows a cross-sectional view of a system according to the present invention, as drawn on a white board 1500 by the inventor HF.

[00110] In the embodiments of the system according to the invention as described above, the system according to the invention was described as an inspection or testing system for a collector mirror, in particular a collector mirror having a first focus and a second focus. When the mirror that is to be tested has a first focus and a second focus, it is, as described above, advantageous to arrange the test radiation sub-system in one of the first focus and the second focus and arrange the sensor sub system in the other of the first focus and the second focus. By doing so, use can be made of the mirror property that radiation emitted from the first focus will arrive at the second focus (or vice versa), irrespective of the angle at which the radiation is emitted. The present invention may however also be implemented to inspect or test other mirrors, i.e. mirrors who do not have a first focus and/or a second focus.

[00111] In the absence of a first focus and/or a second focus, appropriate measures may need to be taken to ensure that the test radiation as emitted by the test radiation sub-system and reflected by the mirror is captured by the sensor sub-system of the system according to the invention. Such measures may e.g. include the application of one or more actuators or motors which can displace the sensor sub system and/or the test radiation sub-system and/or the radiation limiter sub-system and/or the mirror that is being tested or inspected.

[00112] A system according to the present invention in which such measures are included is schematically illustrated in Figure 12.

[00113] Figure 12 schematically illustrates a system 1100 according to the present invention, the system being configured to inspect a mirror 1110, said mirror 1110 having a reflective surface 1110.1. the reflective surface 1110.1 may in general have an arbitrary shape. The mirror 1110 may e.g. be a substantially flat mirror, a parabolic mirror or a freeform mirror. The system according to an embodiment of the present invention comprises a test radiation sub-system 1120, a sensor sub-system 1130 and a radiation limiter sub-system comprising a first radiation limiter sub-system 1140.1 and a second radiation limiter sub-system 1140.2. Said sub-systems substantially having the same functionality as the sub-systems described above. In the embodiment as shown, the system further comprises a actuator sub-system 1150 that is configured to displace the sensor sub-system 1130 and the radiation limiter sub-system 1140.2. In particular, the actuator sub-system 1150 is configured to displace the sensor sub-system 1130 and the radiation limiter sub-system 1140.2 along the X-axis.

[00114] The system 1100 according to the present invention comprises a test radiation sub-system 1120 that is operative to project test radiation 1120.1 to the mirror 1100. Regarding 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 sub-system 1120 is configured to generate test radiation that spans an angle ao, such that, in principle, the entire mirror 1100 can be irradiated. Please note that the diagram of Fig.12 represents the collector mirror 300 in a two- dimensional cross-section and that, in practice, the test radiation is generally emitted in three dimensions. In an embodiment, test radiation sub-system 1120 is configured to generate test radiation that spans an angle that is smaller than the angle ao. In such embodiment, the entire mirror 1100 may still be scanned or irradiated, e.g. by rotating the test radiation sub-system 1120 or a portion thereof.

[00115] The system 1100 according to the present invention further comprises a sensor sub system 1130 that is operative to receive test radiation, e.g. test radiation indicated by the arrow 1142, that is reflected off the mirror 1100 towards the sensor sub-system 1130. In an embodiment, as e.g. described above, the sensor sub-system 1130 may comprise one or more sensors or detectors for measuring the test radiation that is reflected off the mirror 1110.

[00116] In accordance with the present invention, the system 1100 for testing a mirror 1110 further comprises a radiation limiter sub-system comprising a first radiation limiter sub-system 1140.1 and a second radiation limiter sub-system 1140.2. The radiation limiter sub-system 1140.1, 1140.2 is configured to limit the test radiation as received by the sensor sub-system 1130 to test radiation that is reflected off a limited portion of the mirror 1110. By doing so, the radiation that is measured or detected by the sensor sub-system 1130 will involve only radiation that is reflected off the limited portion of the mirror 300. In the embodiment as shown, the limited portion of the mirror is indicated by a mirror segment 1110.2. By limiting the radiation as received by the sensor sub-system 1130 to radiation that is reflected off the limited portion 1110.2 of the mirror, one can make an assessment of the performance of that particular portion of the mirror, e.g. an assessment of the reflectivity of that portion.

[00117] In the embodiment as shown, the radiation limiter sub-system 1140.1 is configured to only let a small portion 1141 of the generated radiation 1120.1 pass towards the mirror 1110. The small portion 1141 of the radiation 1120.1 subtends a much smaller angle than the angle ao. hi order to realize that, the radiation limiter sub-system 1140.1 comprises a shielding member with an aperture, e.g. a tubular shaped aperture through which the small portion of the test radiation 1120.1 can pass. The radiation limiter sub-system 1140.1 thus enables that only a limited portion, e.g. portion 1110.2 of the mirror is irradiated by the test radiation generated by the test radiation sub-system 330. In a similar manner, the radiation limiter sub-system 1140.2 is configured to only allow the sensor sub-system to receive test radiation from a limited portion of the mirror 1110. Note that the first radiation limiter sub-system 1140.1 and a second radiation limiter sub-system 1140.2 may thus have substantially the same functionality and structure as the radiation limiter sub-systems as described in Figures 3 and 6. Note that, in a similar manner as described in Figure 3, either one of the first radiation limiter sub-system 1140.1 and a second radiation limiter sub-system 1140.2 may be omitted.

[00118] In accordance with the present invention, the system further comprises a control sub system 1170 that is configured to control a movement of the first radiation limiter sub-system 1140.1 and a second radiation limiter sub-system 1140.2, e.g. by providing control signals 1170.1 and 1170.2 to the radiation limiter sub-systems. In particular, the control sub-system 1170 is configured to control a position of the radiation limiter sub-systems such that the limited portion of the mirror that is irradiated by test radiation can be changed. As such, the control sub-system 1170 may e.g. be configured to control a movement of the radiation limiter sub-systems along a sequence of different positions, thereby limiting the test radiation as received by the sensor sub-system 1130 to the test radiation that is reflected off a respective sequence of different limited portions of the mirror 1110. By doing so, an assessment of the performance of the different limited portions of the mirror 1110, e.g. an assessment of the reflectivity of these portions, can be made.

[00119] In an embodiment of the present invention, a set of the different limited portions substantially covers the entire mirror 1110.

[00120] In an embodiment of the present invention, the radiation limiter sub-systems may comprise one or more actuators so as to change an orientation of the test radiation 1141 that is projected onto the mirror 1110 and to change an orientation of the test radiation 1142 as received by the sensor sub-system 1130.

[00121] In an embodiment, the control sub-system 1170 may be configured to direct a beam or beam-shaped test radiation onto a plurality of different limited portions of the mirror 1110, one limited portion after the other. Such a sequential projection of the test radiation beam on different limited portions of the mirror 1110 may also be referred to as scanning the mirror with a test radiation beam. During such a scanning process, whereby a part of the mirror 1110 or the entire mirror is scanned, the sensor sub-system 1130 may generate a set of measurement data representative of test radiation reflected off the different limited portions of the collector mirror 1110. Such measurement data may e.g. be provided to the control sub-system 1170 of the system according to the present invention via a data channel, as indicated by the arrow 1170.3. In an embodiment, the measurement data 1170.3 as received may e.g. be processed by a processing unit 1180 of the control sub-system 1170. Such a processing unit may e.g. be embodied as a processor, a microprocessor, computer or the like. Such a processing unit 1180 may e.g. comprises a memory unit for storing the measurement data. [00122] In an embodiment, the processing unit 1180 may be configured to process the measurement data so as to determine a spatial reflectivity distribution of at least part of the collector mirror. In such embodiment, the measurement data representing the test radiation reflected off the different limited portions of the mirror 1110 can be compared with data representative of the generated or emitted test radiation to determine the reflectivity of the different limited portions of the mirror. Examples of a measure for the reflectivity are the ratio of the amount of received test radiation to the amount of the generated or emitted test radiation, and a difference between, on the one hand, the amount of generated or emitted test radiation and, on the other hand, the amount of received test radiation. As such, in an embodiment of the present invention, the measurement data obtained from the sensor sub-system 1130 can be complemented with source measurement data, the source measurement data representing the amount of test radiation emitted or generated during the scanning process. Such source measurement data may e.g. be provided to the processing unit 1180 of the control sub-system 1170 via a data channel.

[00123] The embodiment of the system according to the invention as schematically shown in Figure 12 further comprises an actuator sub-system 1150 that is configured to displace the sensor sub system 1130 and the radiation limiter sub-system 1140.2. The objective of this actuator sub-system 1150 is to ensure that the test radiation that is reflected off the mirror, e.g. the radiation indicated by the reference number 1142, is captured by the sensor sub-system 1130. In the absence of a first focus and a second focus, it will be clear to the skilled person that test radiation that is directed towards different locations on the mirror will be reflected to different locations as well. As such, in an embodiment of the present invention, the fact that test radiation that is directed towards different locations on the mirror will be reflected to different locations is anticipated by providing a displacement of the sensor sub-system 1130 and the radiation limiter sub-system 1140.2.

[00124] In Figure 12, such a required displacement is schematically illustrated. In particular, in Figure 12, the dotted arrow 1143 represents test radiation that is directed towards a limited portion 1110.3 of the mirror 1110, the limited portion 1110.3 being at a different location on the mirror, compared to the limited portion 1110.2. The dotted arrow 1144 represent the reflected test radiation off the limited portion 1110.3. As can be seen, the reflected test radiation 1144 is directed at a different location and at a different angle, compared to the reflected test radiation 1142. In order to capture the reflected test radiation 1144, the sensor sub-system 1130 and the radiation limiter sub system 1140.2 can be displaced to the location schematically indicated by the dotted line 1190. The dotted line 1190 thus schematically represents a location of the sensor sub-system 1130 and an orientation of the radiation limiter sub-system 1140.2 that enables the capturing of the test radiation 1144. In an embodiment of the present invention, the control unit 1170 may be configured to determine, based on a known shape, or predetermined shape information, of the mirror that is to be inspected, an a known relative position of the test radiation sub-system and the mirror, where the reflected test radiation will be directed at, for a particular limited portion of the mirror where test radiation is aimed at. Based on this determined direction, the control unit 1170 may then be configured to control the actuator sub-system 1150 to displace the sensor sub-system 1130 and the radiation limiter sub-system 1140.2 to the appropriate location to capture the reflected radiation, e.g. by providing an appropriate control signal 1170.4 to the actuator sub-system 1150.

[00125] In an embodiment, the actuator sub-system 1150 may e.g. comprise one or more actuators such as electromagnetic actuators or piezo-electric actuators and/or one or more linear or planar motors for displacing the sensor sub-system 1130 and the radiation limiter sub-system 1140.2. In an embodiment, the actuator sub-system 1150 may be configured to displace the sensor sub-system 1130 and the radiation limiter sub-system 1140.2 in multiple degrees of freedom. In the embodiment as shown, the actuator sub-system 1150 may e.g. be configured to displace the sensor sub-system 1130 and the radiation limiter sub-system 1140.2 in the X-direction and the Y-direction, the Y-direction being perpendicular to the XZ-plane as indicated.

[00126] In the embodiment as shown, the test radiation sub-system 1120 is mounted to a frame 1200, the sensor sub-system 1130 and the radiation limiter sub-system 1140.2 being displaceable relative to the frame 1200 by the actuator sub-system 1150. In such embodiment, the relative position of the test radiation sub-system and the mirror that is to be inspected may remain fixed during the inspection.

[00127] As an alternative to displacing the sensor sub-system 1130 and the radiation limiter sub system 1140.2 by the actuator sub-system 1150, the system according to the present invention may also comprise an actuator sub-system that is configured to displace the mirror 1110 relative to the test- radiation sub-system 1120.

[00128] Such an embodiment is schematically depicted in Figure 13. The system 1300 as schematically shown in Figure 13 substantially corresponds to the system 1100 shown in Figure 12, apart from the following. In the embodiment as shown in Figure 13, the test radiation sub-system 1120 and the sensor sub-system 1130, together with the radiation limiter sub-systems are mounted to a common frame 1200. In the embodiment as shown, the test radiation sub-system 1120 and the sensor sub-system 1130 are thus considered to have a fixed relative position. In order to ensure that test radiation that is reflected off the mirror 1110 is reflected towards the sensor sub-system 1130, the system is provided with an actuator sub-system 1155 that is configured to displace the mirror 1110. In particular, the actuator sub-system 1155 is configured to displace the mirror in such manner that a reflected radiation 1145, reflected off the limited portion 1110.3, is directed towards the same location as the reflected radiation 1142. In particular, in the embodiment as shown, the actuator sub-system 1155 is configured to tilt or rotate the mirror 1110 in such manner that the test radiation 1143 is reflected, as radiation 1145 towards the location of the sensor sub-system 1130. Note that, in order for the sensor sub-system 1130 to capture the reflected radiation 1145, the radiation limiter sub-system 1140.2 may need to be rotated. In a similar manner as described with reference to Figure 12, the control unit 1170 may be configured to determine, based on a known shape, or predetermined shape information, of the mirror that is to be inspected, and a known relative position of the test radiation sub-system and the mirror, where the reflected test radiation will be directed at, for a particular limited portion of the mirror where test radiation is aimed at. Based on this determined direction, the control unit 1170 may then be configured to control the actuator sub-system 1155 to displace the mirror to the appropriate location or position to reflect the test radiation as received by the mirror to the sensor sub-system, e.g. by providing an appropriate control signal to the actuator sub-system 1155.

[00129] Yet another alternative to displacing the sensor sub-system 1130 and the radiation limiter sub-system 1140.2 by the actuator sub-system 1150 or to displacing the mirror 1110 relative to the test radiation sub-system by the actuator sub-system 1155 is to apply an actuator sub-system for displacing the test radiation sub-system 1120 relative to the mirror 1110, thereby arranging that the reflected radiation arrives at the sensor sub-system 1130, irrespective of which limited portion of the mirror is irradiated.

[00130] In the embodiments as described with reference to Figures 12 and 13, an actuator sub system is used to ensure that an optical path is established between the test radiation sub-system and the sensor sub-system for the test radiation. In such embodiments, the actuator sub-system is thus used to ensure that the test radiation sub-system, the mirror and the sensor sub-system are in appropriate relative positions and orientations such that test radiation that impinges a particular portion of the mirror that is inspected or tested and that is reflected off the mirror, is captured by the sensor sub system. In accordance with the present invention, the optical path of the test radiation will also pass the radiation limiter sub-system or sub-systems. Said systems may also, as described above, be positioned or displaced by the actuator sub-system.

[00131] In an embodiment of the present invention, the actuator sub-system is further applied to control the angle of incidence of the test radiation onto the mirror. As can be seen in Figure 12, the angle of incidence of the test radiation 1141 differs from the angle of incidence of the test radiation 1143. This may be undesirable. In particular, it may be desirable to test a mirror, e.g. to determine the reflectivity of the mirror, in such manner that the test radiation as applied impinges the mirror in substantially the same manner, i.e. at the same angle of incidence, as occurring during normal use of the mirror. For a particular application, a radiation beam may impinge the mirror at substantially the same angle, across the entire mirror surface. To test such a mirror, it may thus be advantageous to ensure that the test radiation, irrespective of which portion of the mirror is irradiated, impinges the mirror at the appropriate angle. Such an arrangement can e.g. be ensured by controlling the relative position and/or orientation of the test radiation sub-system and the mirror. Such control can e.g. be established by applying an actuator sub-system configured to control a relative position and/or orientation of the test radiation sub-system and the mirror. Such actuator sub-system may e.g. be controlled by a control sub-system as described above, whereby the control sub-system is configured to generate control signals for controlling the actuator sub-system, based on required angle-of- incidence information of the mirror that is tested or inspected.

[00132] As such, in an embodiment of the present invention, the actuator sub-system or actuator sub-systems as applied may serve a dual purpose:

To provide an optical path for the test radiation between the test radiation sub-system and the sensor sub- system, and

To ensure that the test radiation impinges on the mirror that is tested at the appropriate or desired angle of incidence.

In order to meet both requirements, movement or displacement in additional or multiple degrees of freedom may be required.

In an embodiment of the present invention, the actuator sub-assembly may e.g. comprise a 5 or 6 degrees of freedom robot arm for displacing the test radiation sub-system and/or the sensor sub system.

[00133] It can be pointed out that above mentioned embodiments of the system according to the invention may also be combined. In such combined embodiment, one could e.g. apply an actuator sub-system that is configure displace both the mirror and the test radiation sub-system, or both the mirror and the sensor sub- system or any other combination.

[00134] In such embodiment, it may e.g. be advantageous to distribute the required degrees of freedom that need to be actuated over different components.

[00135] Referring to the embodiment of Figure 12, it may e.g. be advantageous to, in order to scan the mirror 1110, arrange for an actuator sub-system that is configured to displace the sensor sub system along the X-axis and to displace the mirror along the Y-axis. As such, by using two linear motors or actuators, a two-dimensional scan of the mirror surface can be obtained.

[00136] As mentioned, the systems as described above with reference to Figures 12 and 13 enable to inspect or test mirrors which do not have a first and second focal point. Said systems may thus be applied to assess the reflectivity of arbitrary shaped mirrors or mirrors having a single focal point.

[00137] Typically, as will be apparent from the examples given above, one would require the application of an actuator sub-system then enables a displacement or rotation in at least two degrees of freedom (either applied to the same component or to different components of the system) in order to scan or inspect the surface of a mirror. However, in case of a mirror having an optical axis, i.e. an axis along which there is a rotational symmetry, it may be sufficient to displace any of the components of the system, i.e. either the mirror or the test radiation sub-system or the sensor sub system in only one degree of freedom, e.g. a translational degree of freedom along the optical axis. In order to realize this, both the test radiation sub-system and the sensor sub-system should be arranged on the optical axis, as e.g. shown in Figures 3 and 6. In such an arrangement, one could e.g. test a parabolic mirror using a system as shown in Figure 3 and/or Figure 6, with the addition of an actuator sub-system that is configured to translate the sensor sub-system along the optical axis to ensure capturing of the reflected radiation.

[00138] While specific embodiments of the invention have been described above, it will be appreciated that the invention may be practiced otherwise than as described. The behavior of the system may be defined in large part by a computer program containing one or more sequences of machine-readable instructions for implementing certain steps of a method as disclosed above, or a data storage medium (e.g. semiconductor memory, magnetic or optical disk) having such a computer program stored therein. The descriptions above are intended to be illustrative, not limiting. Thus it will be apparent to one skilled in the art that modifications may be made to the invention as described without departing from the scope of the claims set out below.

Claims

WHAT IS CLAIMED IS:

1. A system configured for testing a collector mirror having a first focus and a second focus, the system comprises:

a test radiation sub-system operative to project test radiation from the second focus onto the collector mirror;

a sensor sub-system operative to receive test radiation reflected off the collector mirror towards the first focus; and

a radiation limiter sub-system operative to limit the test radiation as received by the sensor sub-system to test radiation reflected off a limited portion of the collector mirror; a control sub-system operative to control a movement of the radiation limiter sub system along a sequence of different positions, thereby limiting the test radiation as received by the sensor sub-system to test radiation reflected off a respective sequence of different limited portions of the collector mirror.

2. The system according to claim 1, wherein the control sub-system comprises a processing unit configured to:

receive measurement data from the sensor sub-system, the measurement data being representative of test radiation reflected off the respective sequence of different limited portions of the collector mirror;

process the measurement data so as to determine a spatial reflectivity distribution of at least part of the collector mirror.

3. The system according to claim 1 or 2, wherein the test radiation comprises EUV radiation.

4. The system according to claim 3, wherein the test radiation sub-system comprises an EUV source for generating the EUV radiation.

5. The system according to claim 3 or 4, wherein the test radiation sub-system comprises a multilayer mirror assembly for filtering the EUV radiation.

6. The system according to claim 5, wherein the multilayer mirror assembly comprises a pair of multilayer mirrors for spectral shaping a spectrum of the EUV radiation projected from the second focus by the test radiation sub-system.

7. The system according to claim 6, wherein the pair of multilayer mirrors are arranged as a Schwarzschild objective.

8. The system according to any of the preceding claims, wherein the sensor sub-system comprises a sensor operative to receive the test radiation reflected off the collector mirror towards the first focus and to generate a measurement signal representative of the test radiation as received.

9. The system according to claim 8, wherein the sensor sub-system comprises a multilayer mirror assembly for filtering the test radiation as received by the sensor.

10. The system according to claim 8 or 9, wherein the sensor comprises an EUV photodiode.

11. The system according to any of the preceding claims, wherein the test radiation sub-system comprise a source sensor configured to generate a source measurement signal representative of the test radiation projected by the test radiation sub-system.

12. The system according to any of the preceding claims, wherein the radiation limiter sub

system comprises a shielding member having an aperture therein, the shielding member being arranged in an optical path of the test radiation.

13. The system according to claim 12, wherein the aperture is formed by an aperture tube of the radiation limiter sub-system.

14. The system according to claim 12 or 13, wherein the radiation limiter sub-system is arranged in an optical path of the test radiation between the second focus and the collector mirror.

15. The system according to claim 12 or 13, wherein the radiation limiter sub-system is arranged in an optical path of the test radiation between the second focus and the collector mirror.

16. The system according to claim 12, wherein the radiation limiter sub-system comprises:

a first shielding member having a first aperture therein, the first shielding member being arranged in an optical path of the test radiation between the second focus and the collector mirror;

a second shielding member having a second aperture therein, the second shielding member being arranged in an optical path of the test radiation between the collector mirror and the first focus.

17. The system according to claim 16, wherein the control sub-system is configured to control a movement of the first shielding member and of the second shielding member so as to establish an optical path for the test radiation between the second focus and the first focus via both the first aperture and the second aperture.

18. The system according to claim 16 or 17, wherein the control sub-system is configured to perform a calibration of the system by:

positioning both the first aperture and the second aperture along an optical axis through the first and second focus;

controlling the test radiation sub-system to emit test radiation directly towards the sensor sub-system through both the first aperture and the second aperture;

receiving a measurement signal of the sensor sub- system representative of the test radiation as received;

receiving a source measurement signal of the test radiation sub-system representative of the test radiation projected by the test radiation sub-system, and

calibrating the system based on the measurement signal and the source measurement signal.

19. The system according to any of the preceding claims, wherein the test radiation sub-system comprises a Xe, Li or Sn radiation source.

20. A system configured for testing a mirror, the system comprises:

a sensor sub-system operative to receive test radiation reflected off the mirror; and a radiation limiter sub-system operative to limit the test radiation as received by the sensor to test radiation reflected off a limited portion of the mirror;

a control sub-system operative to control a movement of the radiation limiter sub system along a sequence of different positions, thereby limiting the test radiation as received by the sensor to test radiation reflected off a respective sequence of different limited portions of the mirror.

21. The system according to claim 20, further comprising:

an actuator sub-system configured to displace at least one of the test radiation sub system, the sensor sub-system, the radiation limiter sub-system and the mirror.

22. The system according to claim 21, wherein the control sub-system is configured to control the actuator sub-system so as to establish an optical path for the test radiation between the test radiation sub-system and the sensor sub-system.

23. Method of testing a collector mirror having a first focus and a second focus, the method comprises: projecting test radiation from the second focus onto the collector mirror; receiving test radiation reflected off the collector mirror towards the first focus by a sensor; and limiting the test radiation as received by the sensor to test radiation reflected off a limited portion of the collector mirror; controlling a movement of a radiation limiting sub-system along a sequence of different positions, thereby limiting the test radiation as received by the sensor to test radiation reflected off a respective sequence of different limited portions of the collector mirror.

24. The method according to claim 23, wherein the step of limiting the test radiation comprises: arranging a first shielding member having a first aperture therein, in an optical path of the test radiation between the second focus and the collector mirror, and;

arranging a second shielding member having a second aperture therein, in an optical path of the test radiation between the collector mirror and the first focus.

25. The method according to claim 24, wherein the step of controlling a movement comprises: controlling a movement of the first shielding member and of the second shielding member so as to establish an optical path for the test radiation between the second focus and the first focus via both the first aperture and the second aperture.

26. The method according to claim 25, further comprising a calibration step for the test method, the calibration step comprising:

controlling test radiation to be emitted directly towards the first focus through both the first aperture and the second aperture;

receiving a measurement signal representative of the test radiation as received; receiving a source measurement signal representative of the test radiation emitted, and

calibrating the test method based on the measurement signal and the source measurement signal.