CN112041752A

CN112041752A - System for testing mirrors, such as collector mirrors, and method of testing mirrors, such as collector mirrors

Info

Publication number: CN112041752A
Application number: CN201980028418.4A
Authority: CN
Inventors: 约翰尼斯·克里斯汀·里昂纳多斯·弗兰肯
Original assignee: ASML Netherlands BV
Current assignee: ASML Netherlands BV
Priority date: 2018-04-26
Filing date: 2019-04-17
Publication date: 2020-12-04
Also published as: TW201946499A; EP3785078A1; WO2019206767A1; NL2022961A; KR20210003113A

Abstract

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

Description

System for testing mirrors, such as collector mirrors, and method of testing mirrors, such as collector mirrors

Cross Reference to Related Applications

This application claims priority from european application 18169481.1 filed 2018, 4, 26, the entire contents of which are incorporated herein by reference.

Technical Field

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

Background

A lithographic apparatus is a machine that applies a desired pattern onto a substrate, usually onto a target portion of the substrate. Lithographic apparatus can be used, for example, in the manufacture of Integrated Circuits (ICs). In that case, 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. Such a pattern can be transferred onto a target portion (e.g., a portion that includes a die, 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) disposed on the substrate. Typically, a single substrate will contain a network of adjacent target portions that are successively patterned.

Photolithography is widely recognized as a critical step in the manufacture of ICs and other devices and/or structures. However, as the dimensions of features manufactured using photolithography become smaller and smaller, photolithography is becoming a more important factor for enabling the manufacture of small ICs or other devices and/or structures.

The theoretical estimate of the limit of pattern printing can be given by the rayleigh resolution criterion, as shown in equation (1):

where λ is the wavelength of the radiation used, NA is the numerical aperture of the projection system used to print the pattern, k₁Is a process dependent adjustment factor, also known as the rayleigh constant, and CD is the feature size (or critical dimension) of the printed feature. From equation (1), the reduction in the minimum printable size of a feature can be obtained in three ways: by shortening the exposure wavelength λ, by increasing the numerical aperture NA, or by decreasing k₁The value of (c).

In order to shorten the exposure wavelength and thus reduce the minimum printable size, it has been proposed to use an Extreme Ultraviolet (EUV) radiation source. EUV radiation is electromagnetic radiation having a wavelength in the range of 5nm to 20nm, for example in the range of 13nm to 14 nm. It has further been proposed that EUV radiation having a wavelength of less than 10nm may be used, for example in the range of 5nm to 10nm, such as 6.7nm or 6.8 nm. Possible sources include Laser Produced Plasma (LPP) sources, but other types of sources are possible.

An example of the current progress in the development of LPP sources for EUV Lithography is described in Benjamin Szu-Min Lin, David Brandt, Nigel Farrar, paper "High power LPP Source system maintenance status", SPIE conference records volume 7520, Lithogrphy Asia 2009, 2009-12 months (SPIE digital library reference DOI: 10.1117/12.839488). In a lithographic system, the source apparatus will typically be contained within its own vacuum enclosure, while a small exit aperture is provided to couple the EUV radiation beam into the optical system in which the radiation is to be used.

For high resolution patterning for lithography, the EUV radiation beam must be conditioned to obtain desired parameters, such as uniformity of intensity distribution and angular distribution, when it reaches the reticle. Examples of illumination systems are described in U.S. patent application publication Nos. 2005/0274897A1 (Carl Zeiss/ASML) and 2011/0063598A (Carl Zeiss). An example system includes a "fly's eye" illuminator that transforms the highly non-uniform intensity profile of the EUV source into a more uniform and controllable source.

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

Known methods for determining the reflectivity are considered to be rather time consuming and expensive. Such as EUV radiation sources or other mirrors applied in a lithographic apparatus may also require the aforementioned inspection and testing. Similarly, known methods for determining the reflectivity of such mirrors can also be time consuming and expensive.

Disclosure of Invention

Various aspects of embodiments of the present invention are directed to providing an alternative system and method of testing collector mirrors of an EUV source.

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

According to another aspect of the invention there is provided a method of testing a collector mirror, the collector mirror having a first focus and a second focus, the method comprising: projecting test radiation from the second focal point onto the collector mirror; receiving, by a sensor, test radiation reflected from the collector mirror toward the first focal point; and limiting the test radiation received by the sensor to test radiation reflected from the limited portion of the collector mirror; controlling movement of a radiation limiting system along a series of different positions, thereby limiting the test radiation received by the sensor to test radiation reflected from a corresponding series of different limited portions of the collector mirror.

According to yet another aspect of the present invention, there is provided a system configured to test a mirror, the system comprising:

a test radiation subsystem operable to project test radiation onto the mirror;

a sensor subsystem operable to receive test radiation reflected from the mirror; and

a radiation limiter subsystem operable to limit the test radiation as received by the sensor subsystem to test radiation reflected from the limited portion of the mirror;

a control subsystem operable to control movement of the radiation limiter subsystem along a series of different positions, thereby limiting the test radiation received by the sensor subsystem to test radiation reflected from a corresponding series of different limited portions of the mirror.

These aspects of the invention, as well as various alternative features and embodiments thereof, will be understood by those skilled in the art from the following description of examples.

Drawings

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:

FIG. 1 schematically depicts a lithographic system according to an embodiment of the invention;

FIG. 2 is a more detailed view of the system of FIG. 1 and illustrates a new monitor control system for an EUV radiation source;

fig. 3 schematically shows a cross-section of an embodiment of the system according to the invention;

fig. 4a to 4c schematically show cross-sectional views of a test radiation subsystem as can be applied in the present invention;

5 a-5 f schematically illustrate cross-sectional views of a sensor subsystem as can be applied in the present invention;

FIG. 6 schematically shows a cross-section of another embodiment of a system according to the invention;

FIG. 7 schematically illustrates a radiation limiter subsystem as can be applied in a system according to the invention;

fig. 8 schematically shows a cross-section of a system according to the invention in a calibration position;

fig. 9 schematically shows a cross-section of a further embodiment of the system according to the invention;

fig. 10 schematically shows a portion of a collector mirror as can be inspected by the system according to the invention.

Fig. 11 schematically shows a system according to the invention on a whiteboard and the inventors.

FIG. 12 schematically shows a cross-section of an embodiment of a system according to the invention for testing or inspecting a mirror;

fig. 13 schematically shows a cross-section of another embodiment of a system according to the invention for testing or inspecting a mirror.

Detailed Description

Fig. 1 schematically depicts a lithographic system 100 according to an embodiment of the invention, the lithographic system comprising a lithographic apparatus and an EUV radiation source configured to generate EUV radiation, e.g. an EUV radiation beam. In the embodiment as shown, the EUV radiation source comprises a source collector module SO. In an embodiment as shown, a 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 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.

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

The support structure MT holds the patterning device MA in a manner that depends on the orientation of the patterning device MA, the design of the lithographic apparatus, and other conditions, such as for example whether 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, for example, a frame or a table, 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.

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.

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.

Similar to the illumination system, the projection system may include various types of optical components, such as refractive, reflective, magnetic, electromagnetic, electrostatic or other types of optical components, or any combination thereof, as appropriate for the exposure radiation being used, or for other factors such as the use of a vacuum. It may be desirable to use vacuum for EUV radiation because other gases may absorb too much radiation. Thus, a vacuum environment may be provided to the entire beam path by means of the vacuum wall and the vacuum pump.

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

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.

Referring to FIG. 1, the illuminator IL receives an extreme ultraviolet radiation beam from a source collector module SO of an EUV radiation source. Methods to produce EUV light include, but are not necessarily limited to, converting a material into a plasma state having at least one element (e.g., xenon, lithium, or tin) using one or more emission lines in the EUV range. In one such method, often referred to as laser produced plasma ("LPP"), the desired plasma may be produced by irradiating a fuel, such as a droplet, stream or cluster of material having the desired line emitting elements, with a laser beam. The source collector module SO may be part of an EUV radiation system comprising a laser, not shown in fig. 1, which is used to provide a laser beam that excites the fuel. The resulting plasma emits output radiation, e.g., EUV radiation, using radiation disposed in a source collector moduleA radiation collector to collect the radiation. For example, when CO is used₂Where the laser is to provide a laser beam for fuel excitation, the laser and the EUV radiation source may be separate entities.

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 by means 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-producing plasma EUV generator, often referred to as a DPP source.

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

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 reflection 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 M1, M2 and substrate alignment marks P1, P2.

The depicted apparatus can 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 magnification (de-magnification) and image reversal characteristics of the projection system PS.

3. In another mode, the support structure (e.g. a 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 used 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.

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

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. It will be appreciated by those of ordinary skill in the art 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 substrates referred to herein may be processed, before or after exposure, in for example a coating and development system (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.

Fig. 2 shows in more detail the system 100 comprising an EUV radiation source comprising a source collector module SO, a lithographic scanning apparatus comprising an illumination system IL, and a 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 the enclosing structure 220 of the source collector module SO. The systems IL and PS are likewise contained within their own vacuum environment. The EUV radiation-emitting plasma 210 may be formed by a laser-produced LPP plasma source. The function of the source collector module SO is to pass the EUV radiation beam 20 from the plasma 210 such that it is focused in a virtual source point. The virtual source point is often referred to as an Intermediate Focus (IF), and the source collector module is arranged such that the intermediate focus IF is located at or near the aperture 221 in the enclosure 220. The virtual source point IF is an image of the radiation-emitting plasma 210.

The radiation traverses the illumination system IL, which in this example comprises a facet field mirror device 22 and a facet pupil mirror device 24, from an aperture 221 at an intermediate focus IF. These devices form what are known as "fly-eye" illuminators, which are arranged to provide a desired angular distribution of the radiation beam 21 at the patterning device MA, and a desired uniformity of radiation intensity at the patterning device MA. After the beam 21 has been reflected at the patterning device MA, which is 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 or substrate table WT.

Each system IL and PS is disposed within its own vacuum or near-vacuum environment defined by an enclosure similar to enclosure 220. There may generally be more elements in the illumination system IL and the projection system PS than shown. Furthermore, there may be more mirrors than those shown in the figures. For example, in addition to the reflective elements shown in FIG. 2, there may be one to six additional reflective elements in the illumination system IL and/or the projection system PS. For example, the above-mentioned U.S. patent application publication shows three additional elements in the illumination system.

Considering the source collector module SO in more detail, the laser energy source comprising a laser 223 is arranged to deposit laser energy 224 into a fuel such as xenon (Xe), tin (Sn) or lithium (Li) to generate a highly ionized plasma 210 with electron temperatures of tens of eV. Other fuel materials (e.g., Tb and Gd) may be utilized to generate higher energy EUV radiation. The high energy radiation generated during the de-excitation and recombination or recombination of these ions is emitted from the plasma, collected by the near normal incidence collector mirror CO and focused on the aperture 221. The plasma 210 and the aperture 221 are located at a first focus and a second focus of the collector or collector mirror CO, respectively.

In order to deliver fuel, for example liquid tin, a droplet generator 226 is arranged within the enclosure 220, the droplet generator 226 being arranged to emit a stream of high frequency droplets 228 towards a desired location of the plasma 210. In operation, laser energy 224 is delivered in synchronization with the operation of droplet generator 226 to deliver pulses of radiation to turn each fuel droplet into plasma 210. The delivery frequency of the droplet may be a few kilohertz, for example 50 kHz. In practice, the laser energy 224 is delivered in at least two pulses: a pre-pulse and a main pulse. The pre-pulse delivers a finite amount of energy to the droplet before it reaches the plasma site, so that the droplet is conditioned for receiving the main pulse, for example by shaping the droplet into a cake (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 plasma 210. The trap 230 is disposed on the opposite side of the enclosing structure 220 to trap fuel that does not become plasma for whatever reason.

There are numerous additional components in the source collector module and the lithographic apparatus in a typical lithographic system, but are not illustrated here. These components include arrangements for reducing or mitigating the effects of contamination within the enclosed vacuum, for example to prevent deposits of fuel material from damaging or impairing the performance of the collector mirror CO and other optics. Furthermore, one or more spectral purity filters may be included in the source collector module SO and/or the illumination system IL. In addition to EUV radiation of the desired wavelength, these filters also serve to eliminate as much as possible radiation of the undesired wavelength generated by the laser and/or plasma 210. The spectral purity filter may be positioned near the virtual source point or at any point between the collector mirror CO and the virtual source point. The filter may be placed at other locations in the radiation path, for example downstream of the virtual source point IF. Multiple filters may be deployed. Those skilled in the art are familiar with the needs for such measures and the manner in which they can be implemented and other details are not required for the purposes of this disclosure.

Referring in more detail to the laser 223 from fig. 2, the laser in the embodiment presented is of the master controlled oscillator power amplifier (MOPA) type. This type consists of a "master" laser or "seed" laser, labeled MO in the illustration, followed by a Power Amplifier (PA). The beam delivery system 240 is configured to deliver laser energy 224 into the module SO. In practice, the pre-pulse elements of laser energy will be delivered by a separate laser, not separately shown in the illustration. The laser 223, fuel source (i.e., droplet generator) 226, and other components may be controlled, for example, by a source control module 242.

Those of ordinary skill in the art will appreciate that reference axes X, Y and Z may be defined to measure and describe the geometry and behavior of the lithography system, its various components, and the radiation beams 20, 21, 26. At each part of the lithography system, X, Y local reference coordinate systems and Z-axis may be defined. The Z axis is substantially coincident with the direction of the optical axis O at a given point in the system and is substantially orthogonal to the plane of the patterning device (reticle) MA and to the plane of the substrate W. In the source collector module, the X-axis is approximately coincident with the direction of fuel flow (228, described below), while the Y-axis is orthogonal to the direction, pointing out of the page, as indicated in fig. 2. On the other hand, in the vicinity of the support structure MT holding the reticle MA, the X-axis is substantially transverse to the scan direction aligned with the Y-axis. For convenience, in this region of the schematic fig. 2, the X-axis is pointing out of the page, again as labeled. These designations are conventional in the art and will be employed herein for convenience. In principle, any reference coordinate system may be chosen to describe the lithography system and its behavior.

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

The adjusted EUV radiation beam 21 is delivered to the patterning device MA by adjusting the shutter module 262. Such modules comprise a mask unit, also called reticle shield (REMA), which may have movable blades that define the extent of the illumination slit in the X-direction and the Y-direction. Typically, the illumination slit as applied in an EUV type lithographic apparatus may be curved.

In front of the REMA may also be a radiation uniformity correction module (UNICOM).

To expose a target portion C on the substrate W, pulses of radiation are generated on the substrate table WT and the masked table MT performs

synchronous movements

266, 268 to scan the pattern on the patterning device MA through an illumination aperture.

Examples of illumination systems that include REMA and UNICOM functions are described in U.S. patent application publication nos. 2005/0274897a1 and 2011/0063598 a.

A number of measures are applied in the source controller 242. Such measures include monitoring to ensure that the virtual source point IF is aligned with the aperture 221 at the exit of the source collector module SO. In LPP source based systems, control of the alignment is typically achieved by controlling the location of the plasma 210 rather than by moving the collector mirror CO. The collector mirror, exit aperture 221 and illuminator IL are accurately aligned during the setup process so that aperture 221 is located at the second focal point of the collector mirror. However, the exact location of the virtual source point IF formed by the EUV radiation at the exit of the source optics depends on the exact location of the plasma 210 with respect to the first focus of the collector mirror. Securing such sites with sufficient accuracy to maintain adequate alignment typically requires active monitoring and control.

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

The source control module 242 is supplied with monitoring data from one or more arrays of sensors (not shown) that provide a first feedback path for information about the location of the plasma. The sensors may be of various types, such as described in U.S. patent application publication No. 2005/0274897a1, referenced above. The sensor may be located at more than one position along the path of the radiation beam. For example only, the sensors may be located around and/or behind the field mirror device 22, for example. The sensor signals just described may be used to control the illuminator IL and the optical system of the projection system PS. The sensor signals may also be used to assist the control module 242 of the source collector module SO via a feedback path to adjust the intensity and position of the EUV plasma source 210. The sensor signals may be processed, for example, to determine an observed location of the virtual source IF, and to extrapolate such a location to indirectly determine the location of the EUV source. If the virtual source site drifts, as indicated by the sensor signal, a correction is applied by the control module 242 to re-center the beam in the hole 221.

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

To ensure that the substrate W is provided with a suitable dose of radiation, it is important to ensure that the collector or collector mirror CO adequately reflects the EUV radiation produced. In particular, it is desirable to have a sufficiently high reflectivity of the collector mirror CO, and to make the reflectivity as uniform as possible.

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

To reduce the adverse effects of contamination, a collector mirror, such as the collector mirror CO shown in fig. 2, may be subjected to a cleaning process to remove contamination. Such cleaning processes may, for example, include carbon dioxide snow cleaning processes.

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

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

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

Fig. 3 schematically shows an embodiment of a system according to the invention for testing collector mirrors. The collector mirror 300 has a first focal point (focus/focal point)310 and a second focal point 320. Focal point 310 may, for example, generally correspond to a location where radiation-emitting plasma 210 is formed as shown in fig. 2, while focal point 320 corresponds, for example, to an intermediate focal point IF as shown in fig. 2.

The system according to the invention comprises a test radiation subsystem 330 operable to project test radiation 330.1 from the second focal point 320 to the collector mirror 300. Within the meaning of the present invention, test radiation refers to radiation used during testing or inspection of a mirror, such as the illustrated collector mirror 300.

In an embodiment of the invention, the test radiation as applied has a wavelength or spectrum of wavelengths selected based on the radiation used under normal operating conditions of the collector mirror. In particular, if a collector mirror is to be used in an EUV lithography system, the wavelength or wavelength spectrum of the test radiation may be selected based on the EUV radiation applied in the lithographic apparatus. Thus, in embodiments of the invention, the test radiation may comprise radiation having a wavelength in the range of 10nm to 20 nm. In an embodiment, the test radiation as applied may have a wavelength spectrum of 13.5nm +/-1nm full width at half maximum (FWHM). In an embodiment, a test radiation subsystem as applied in a system according to the invention may be configured to apply radiation of different wavelengths or different wavelength ranges to inspect or test the mirrors. In such embodiments, the test radiation subsystem may include multiple radiation sources and/or multiple filters to filter the generated radiation to achieve the desired test radiation. As an example, it may be useful to evaluate the reflectivity of a mirror, such as collector mirror 300, at different wavelengths or wavelength ranges. It is possible, for example, to evaluate the reflectivity of the collector mirror at the aforementioned spectrum of 13.5nm +/-1nm, but also at a spectrum that encompasses or includes Infrared (IR) radiation.

In an embodiment, the test radiation subsystem is configured to generate radiation spanning an angle α₀So that in principle the entire collector mirror 300 can be irradiated. Note that the illustration of fig. 3 represents the collector mirror 300 in a two-dimensional cross-section and, in practice, the test radiation is typically emitted in three dimensions. Thus, the angle α in FIG. 3₀Is the angle subtended by the three-dimensional test radiation pattern when projected onto a two-dimensional plane having the cross-section of the collector mirror 300. In an embodiment, the test radiation subsystem is configured to produce radiation spanning less than an angle α₀The test radiation of the angle of (a). In such embodiments, the entire collector mirror 300 may still be scanned or irradiated, for example by rotating the test radiation subsystem 330 or a portion thereof.

The system according to the present invention further comprises a sensor subsystem, schematically indicated by reference numeral 340, operable to receive test radiation reflected from collector mirror 300 towards first focal point 310, e.g. test radiation indicated by arrow 342. In an embodiment, as will be discussed in more detail below, the sensor subsystem 340 may include one or more sensors or detectors for measuring the test radiation reflected from the mirror (i.e., collector mirror 300).

In accordance with the present invention, the system for testing the collector mirrors further includes a radiation limiter subsystem 360. The radiation limiter subsystem 360 is configured to limit the test radiation received by the sensor subsystem 340 to test radiation reflected from the limited portion of the collector mirror 300. Thus, the radiation measured or detected by the sensor subsystem will only involve radiation reflected from a limited portion of collector mirror 300. In the embodiment as shown, the restricted portion of the collector mirror is indicated by mirror segment 300.1. By limiting the test radiation received by the sensor subsystem to test radiation reflected from a limited portion of the collector mirror, the performance of that particular portion of the collector mirror can be assessed, for example assessing the reflectivity of that portion.

In the embodiment as shown, the radiation limiter subsystem 360 is configured to pass only a small portion of the generated radiation 330.1 towards the collector mirror. A small fraction of the radiation 330.1 subtends a specific angle a₀A much smaller angle. To accomplish this, the radiation limiter subsystem 360 includes a shielding member 360.1 that blocks the test radiation, whereby the shielding member 360.1 includes an aperture, such as a tubular aperture 360.2, through which a small portion of the test radiation 330.1 can pass. The radiation limiter subsystem 360 thus enables only a limited portion (e.g. the portion 300.1 of the collector mirror 300) to be irradiated by the test radiation generated by the test radiation subsystem 330.

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

In an embodiment of the invention, a set of differently restricted portions covers substantially the entire collector mirror 300.

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

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

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

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

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

Preferably, as already indicated above, for example when the collector mirror is used in a lithography system as illustrated in fig. 2, 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. By doing so, the reflectivity of the collector mirror can be determined more accurately for the relevant wavelength or wavelength spectrum. To achieve this situation, different options exist.

In an embodiment of the present invention, the test radiation subsystem 330 includes a radiation source and a multilayer mirror assembly for spectrally filtering the generated radiation to achieve a desired test radiation, i.e., a test radiation having an appropriate wavelength or spectrum of wavelengths. As is known, multilayer mirrors, also called Bragg mirrors, are one type of reflective optical element consisting of a plurality of layers with alternating thicknesses and refractive indices. The thickness is tuned to manage constructive interference of radiation of a desired wavelength reflected from subsequent layers. In an embodiment, the radiation source of the test radiation subsystem 330 comprises a Xe EUV source. In such embodiments, the multilayer mirror arrangement may be configured to filter the radiation into a wavelength spectrum of, for example, 13.5nm +/-1nm full width at half maximum (FWHM).

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

Figure 4a schematically shows a radiation source 420, for example a radiation source for generating EUV radiation. In the embodiment as shown, the aperture 410 is arranged to allow a portion of the generated radiation, indicated by arrow 420.1, to impinge on the 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 arrow 420.2. Second mirror 440 is configured to reflect the received radiation toward aperture 450, as indicated by arrow 420.3. The aperture may be, for example, an outlet of the aperture tube 460 that is configured to receive the radiation 420.3 reflected from the mirror 440. In the embodiment as shown, mirrors 430 and 440 may be used to filter undesired wavelengths or wavelength components from the radiation generated by source 420. Alternatively or additionally, mirrors 430 and 440 may form a beam expander or a beam reducer for controlling the width, and thus the intensity, of the beam formed by the test radiation impinging on collector 300.

In an embodiment of the present invention, the mirror assembly includes

mirrors

430 and 440, and the aperture tube 460 may be used as a radiation limiter subsystem as discussed above. In such embodiments, the mirror assembly and the aperture tube may, for example, be configured to be displaceable relative to the radiation source 420 so as to change the direction of the radiation 420.3 emitted from the aperture. By appropriately sizing the mirrors or mirror assemblies, the radiation beam 420.3 emitted from aperture 450 may be a collimated beam 420.4, i.e. a parallel beam of radiation which may impinge on a restricted portion of the collector mirror. In order to reflect collimated beam 420.4 from the collector mirror toward the first focal point of the mirror, collimated beam 420.4 needs to be emitted from the second focal point. This can be achieved, for example, by: the mirror assembly and the bore tube are arranged such that the second focus is within the bore tube, for example at location 470.

Fig. 4b schematically shows another embodiment of a mirror assembly that may be applied to filter the radiation from the radiation source. Fig. 4b schematically shows a radiation source 520, for example a radiation source for generating EUV radiation. In the embodiment as shown, the radiation source 520 is configured to emit a portion of the generated radiation, indicated by arrow 520.1, to impinge on a first mirror 530, e.g. a multilayer mirror, shown here in cross-section. The first mirror 530 is configured to reflect the received radiation towards a second mirror 540 (e.g. a multilayer mirror), indicated by arrow 520.2. The second mirror 540 is configured to reflect the received radiation toward a focal point 550. As discussed above, mirrors 530 and 540 may be used to filter the radiation generated by radiation source 520 to obtain test radiation having an appropriate wavelength or spectrum. Forward from focal point 550, filtered radiation 520.3 may be used as test radiation for scanning a collector mirror as discussed above with reference to fig. 3, for example. In such embodiments, the generated test radiation may be limited, for example, by a radiation limiter 360, the radiation limiter 360 being configured to limit the generated test radiation, for example radiation 520.3, to impinge on the limited portion of the collector mirror. To ensure that the emitted test radiation 520.3, or the part thereof impinging on the collector mirror, is reflected from the collector mirror (e.g. collector mirror 300) towards the first focal point of the mirror, it is necessary to emit the test radiation 520.3 from the second focal point of the collector mirror. This can be achieved, for example, by: the

mirror assemblies

530, 540 are arranged such that the second focal point of the collector mirror is substantially coincident with the focal point 550.

The mirror

arrangement comprising mirrors

530 and 540 is called a Schwarzschild objective lens.

Fig. 4c schematically shows a further embodiment of a mirror assembly which may be applied to filter the radiation from the radiation source, said mirror assembly also comprising a schwarsier

objective comprising mirrors

630 and 640. Fig. 4c schematically shows a radiation source 620, for example a radiation source for generating EUV radiation. In the embodiment as shown, the radiation source 620 is configured to emit a portion of the generated radiation indicated by arrow 620.1 to impinge on a first mirror 630, e.g. a multilayer mirror, shown here in cross-section. First mirror 630 is configured to reflect the received radiation toward second mirror 640 (e.g., a multilayer mirror), as indicated by arrow 620.2. Second mirror 640 is configured to reflect the received radiation as a collimated beam 620.3 through aperture 660 of first mirror 630. Those skilled in the art will appreciate that by appropriately sizing and positioning mirrors 630 and 640 of the schwarzenith objective lens, it can be ensured that the emitted radiation is shaped into a collimated beam 620.3, rather than being focused onto focal point 550 as shown in fig. 4 b. In such an arrangement, the

mirror arrangements

630, 640 may thus both act as filters for the generated test radiation of the appropriate wavelength or wavelength spectrum and shape the test radiation so as to obtain a test radiation beam 620.3 configured to impinge on the restricted portion of the collector mirror under examination. Thus, the mirror arrangement may be considered to act as a radiation limiter that limits the test radiation to impinge on a limited portion of the collector mirror.

Again, to ensure that radiation is reflected from the collector mirror (e.g. collector mirror 300) towards the first focal point of the mirror, it is necessary to emit test radiation 620.3 from the second focal point of the collector mirror. This may be achieved, for example, by arranging a second mirror 640 at or near the second focal point of the collector mirror being inspected.

In a similar manner as discussed above, the

multilayer mirror assembly

630, 640 may be configured to be displaceable to scan at least a portion of the surface of the collector mirror with the test radiation 620.3 in order to determine the reflectivity of the mirror.

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

In embodiments of the present invention, source sensors (i.e., sensors configured to generate source measurement data representative of radiation emitted by the source of the test radiation subsystem or test radiation) may also be disposed in a radiation limiter subsystem, such as radiation limiter subsystem 360 shown in fig. 3. In such embodiments, the source sensor may be disposed at or near or in the aperture tube 360.2 of the radiation limiter subsystem 360 in order to capture the portion of the radiation irradiated by the test radiation subsystem 330.

As already indicated above, the system according to the invention comprises a sensor subsystem, such as the sensor subsystem 340 shown in fig. 3. Various embodiments of such a sensor subsystem as applied herein are discussed in fig. 5.

Fig. 5a schematically shows a first arrangement of a sensor subsystem 700 as may be applied in a system according to the invention, and a collector mirror 300, the reflectivity of which is to be examined. The sensor subsystem 700 includes a sensor 700.1 having an active area 700.2 facing the collector mirror 300. In the embodiment as shown, the active region 700.2 is considered to be located at or near the first focal point of the collector mirror 300 in order to receive the test radiation emitted from the second focal point and then reflected from the mirror 300.

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

In the arrangement as shown, the sensor subsystem is configured to remain stationary relative to the collector mirror 300. Thus, depending on the angle of the emitted radiation, the angle of incidence of the test radiation reflected from collector mirror 300 may vary with respect to the optical axis O, 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.

Alternatively, as schematically shown in fig. 5b, a sensor subsystem 750 may be applied, comprising a plurality of sensors 750.1, 750.2, 750.3 having different directions with respect to the optical axis O. By doing so, depending on the angle of incidence of the received radiation, the most sensitive sensor can be selected, i.e. the sensor whose active area preferably faces the received radiation.

Fig. 5c schematically shows a further alternative sensor subsystem 760 and collector mirror 300, sensor subsystem 760 comprising a sensor 760.1 with an active area 760.2. The sensor subsystem further comprises an actuator or actuator arrangement 770 configured to displace the sensor 760.1 with respect to the optical axis O. In particular, the actuator arrangement may be configured to rotate the sensor 760.1 about an axis 780 that is parallel to the indicated Z-direction and passes through the first focus of the collector mirror, and about an axis that is perpendicular to the drawing and passes through the first focus of the collector mirror. In such an arrangement, the sensor 760.1, in particular the active region 760.2 of the sensor 760.1, may be directed to face the test radiation reflected towards the first focal point, e.g. the reflected beam facing the radiation beam 710.2.

In an embodiment, as schematically shown in fig. 5d, the sensor 790.1 of the sensor subsystem 790 as applied in a system according to the invention may be mounted inside a tube, e.g. inside the bore 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 to face in the direction reflected from the collector mirror, for example to face the reflected beam of the radiation beam 710.2. By disposing the sensor 790.1 inside the bore tube 795 or at the distal end of the bore tube 795, the sensor 790.1 may sense only radiation received by the bore tube 795. By doing so, measurement disturbances due to stray radiation may be mitigated or avoided.

The sensor subsystem as schematically shown in fig. 5a to 5d is configured to receive the reflected radiation directly, i.e. without applying spectral filtering of the reflected radiation. As discussed above, it may be preferred to inspect or test the collector mirror using test radiation having the same or similar wavelength as applied during normal operation, since it is of particular interest to characterize the collector mirror for that particular wavelength or spectrum of wavelengths. For such an evaluation using the sensor subsystems of fig. 5a to 5d, it may be advantageous to apply these systems in combination with a test radiation subsystem configured to filter the test radiation (e.g. the test radiation subsystem illustrated in fig. 4a to 4 c), wherein the first mirror and/or the second mirror may be implemented with a spectral purity filter.

Regardless of whether the test radiation subsystem applies filtering of the test radiation, filtering of the test radiation reflected from the collector mirror may be applied in a similar manner. Such an embodiment is schematically illustrated in fig. 5e to 5 f.

Fig. 5e schematically shows the sensor subsystem 800, as well as the collector mirror 300, as may be applied in a system according to the invention. The sensor subsystem 800 as schematically shown comprises a sensor 800.1, and the multi-layer mirror assembly comprises a pair of

mirrors

810, 820, the

mirrors

810, 820 being configured to reflect test radiation 830.1 reflected from the collector mirror 300 towards the sensor 800.1 according to the indicated arrow 830.2. In the embodiment as shown, the incident radiation 830.1 is received by an aperture tube 840, the aperture tube 840 being arranged to receive reflected radiation towards a first focal point 850 of the collector mirror 300. In embodiments of the present invention, the sensor subsystem 800 may also be equipped with an actuator or actuator arrangement for adjusting at least one of the aperture tube, the multilayer mirror assembly and the sensor to receive radiation reflected from different restricted portions of the collector mirror.

In an embodiment of the invention, the sensor subsystem as applied in a system according to the invention may use a schwarsier objective to spectrally filter any incident radiation (i.e. test radiation reflected from collector mirror 300). Such an embodiment is schematically illustrated in fig. 5 f. Fig. 5f schematically shows a sensor subsystem 900, as may be applied in a system according to the invention, and a collector mirror 300. The sensor subsystem 900 as schematically shown includes a sensor 900.1, and the multi-layer mirror assembly includes a pair of

mirrors

910, 920 shown in cross-section, the

mirrors

910, 920 being configured to reflect test radiation 930.1 reflected from the collector mirror 300 towards the sensor 900.1 according to the indicated arrow 930.2. It can be seen that mirrors 910 and 920 form a schwarsier objective lens configured to receive the test radiation beam 930.1 reflected from collector mirror 300 towards the first focal point of the collector mirror. In the embodiment as shown, the first focal point of the collector mirror may, for example, be arranged to substantially coincide with the first mirror 910. In the embodiment as shown, the incident radiation 930.1 is received via an aperture tube 940, the aperture tube 940 being arranged to receive reflected radiation towards the first focus of the collector mirror 300, i.e. at the location of the mirror 910. In embodiments of the present invention, the sensor subsystem 900 may also be equipped with an actuator or actuator arrangement for adjusting at least one of the aperture tube, the multilayer mirror assembly and the sensor in order to receive radiation reflected from different restricted portions of the collector mirror 300. It may be noted that the

mirror arrangement

910, 920 may be configured in a similar way as the mirror arrangement as shown in fig. 4c, but used in the opposite way; that is, in the embodiment of fig. 4c, the schwarzenship objective is used to generate a collimated beam from the radiation source, while in the embodiment of fig. 5f, the schwarzenship objective is configured to receive and project the collimated beam 930.1 onto the sensor.

In the embodiments of the sensor subsystem shown in fig. 5e or 5f, a mirror arrangement is used to filter the test radiation received by the sensor. When using such an embodiment, filtering of the radiation generated by the source may be omitted.

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

Such an embodiment is schematically illustrated in fig. 6. Fig. 6 schematically shows the same features and components as fig. 3. In addition, the embodiment includes another radiation limiter subsystem 390, the other radiation limiter subsystem 390 being configured to limit the test radiation received by the sensor subsystem 340 to radiation reflected from the restricted portion 300.1 of the collector mirror 300. In the embodiment as shown, another radiation limiter subsystem 390 includes a shield member 390.1 that projects from an orifice tube 390.2. Stray light effects may be mitigated or avoided due to the inclusion of the radiation limiter subsystem in both the optical path between the collector mirror and the first focal point and the optical path between the second focal point and the collector mirror. Those skilled in the art will appreciate that the two radiation limiter subsystems are shifted in synchronism, since the two radiation limiter subsystems need to be oriented towards the same restricted part, e.g. part 300.1 of collector mirror 300.

In an embodiment of the invention, the radiation limiter subsystem includes a hemispherical shielding member and an aperture tube protruding through an apex of the hemisphere. Such an embodiment is schematically illustrated in fig. 7. Fig. 7 schematically illustrates a radiation limiter subsystem comprising a hemispherical shielding member 950 and an aperture tube 960 protruding through the apex of the hemisphere. The radiation limiter subsystem further comprises an actuator arrangement 970, the actuator arrangement 970 comprising a first actuator 970.1 and a second actuator 970.2, the first actuator 970.1 being configured to rotate the shielding member 950 and the bore tube 960 about an axis parallel to the indicated X-axis, the second actuator 970.2 being configured to rotate the shielding member 950 and the bore tube 960 about an axis parallel to the indicated Z-axis. The radiation limiter subsystem as schematically shown in fig. 7 may, for example, be applied as the radiation limiter subsystem 360 shown in fig. 6, the radiation limiter subsystem 390 shown in fig. 6, or both. The actuator assembly 970 as schematically shown may also be used to displace any of the test radiation subsystems shown in fig. 4a to 4c and/or to displace any of the sensor subsystems shown in fig. 5c to 5 f.

In an embodiment of the invention, the radiation limiter subsystem as schematically shown in fig. 7 is applied such that the center 975 of a portion of a sphere formed by a hemisphere approximately corresponds to the location of the first or second focal point.

Before a mirror, such as collector mirror 300, is inspected or tested by a system according to the present invention, it may be advantageous to calibrate the system. In an embodiment of the invention, a calibration method is proposed, whereby the test radiation emitted by the test radiation subsystem is emitted directly towards the sensor subsystem. Fig. 8 schematically shows a system according to the invention arranged in a calibration position. The system as schematically shown generally corresponds to the system shown in fig. 6, which includes a test radiation subsystem 330, a sensor subsystem 340, a control subsystem 370, and a radiation limiter subsystem, including a first radiation limiter subsystem 360 and a second or further radiation limiter subsystem 390. In contrast to the situation as depicted in fig. 6,

radiation limiter subsystems

360 and 390 as shown in fig. 8 are arranged (i.e., rotated) such that apertures 360.2 and 390.2 point toward each other. In such a position, also referred to as a calibration position, test radiation beam 1000 may be emitted directly into aperture 390.2 via aperture 360.2 for reception by sensor subsystem 340, in particular by the sensors of sensor subsystem 340. The system may be calibrated by comparing source measurement data (i.e., data indicative of the amount of radiation emitted by the test radiation subsystem 330) with measurement data obtained from the sensor subsystem 340 in a calibration position, i.e., determining the relationship between the emitted test radiation and the received test radiation in the absence of the collector mirror. In fig. 8, reference numerals 370.2 and 370.3 may indicate the obtained measurement data and the source measurement data, respectively, as provided to the control unit 370. In an embodiment of the invention, prior to determining the measurement or test sequence of the reflectivity of the collector mirror, a ratio of the amount of received radiation to the amount of emitted radiation, or a difference between the amount of received radiation and the amount of emitted radiation, may be determined.

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

With respect to the calibration method as discussed, it can be noted that the same method can also be applied when omitting either the first radiation limiter subsystem 360 or the second radiation limiter subsystem 390.

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

In an embodiment of the invention, the mirror to be inspected or tested is mounted at such a frame in an orientation with respect to gravity substantially corresponding to the orientation of said mirror during normal operation. Such an embodiment is schematically illustrated by collector mirror 1100 in fig. 9.

Fig. 9 schematically shows a collector mirror 1100 mounted inside a system 1200 according to an embodiment of the invention. In the embodiment as shown, the system includes a first vacuum chamber or vessel 1210, in which first vacuum chamber or vessel 1210 collector mirror 1100 can be disposed on, for example, frame 1220. In the embodiment as shown, container 1210 is mounted to external frame 1230 at an angle, for example selected such that collector mirror 1100 is disposed at approximately the same angle as applied during normal operation when mounted to frame 1220. By doing so, it may be assumed that any deformation of collector mirror 1100, e.g. due to gravity, will substantially correspond to the deformation of mirror 1100 during normal operation (i.e. when mounted, e.g., in an EUV source of a lithography system such as the system shown in fig. 2). The system 1200 further comprises: a test radiation subsystem 1240 arranged to emit test radiation 1240.1 from the second focal point 1250 of the collector mirror; and a sensor subsystem 1260 arranged to receive test radiation 1260.1 reflected from collector mirror 1100 towards first focal point 1255 of collector mirror 1100. In a similar manner as shown in fig. 6, in the embodiment as shown, system 1200 further comprises a radiation limiter subsystem comprising a first radiation limiter subsystem 1270 and a second or further radiation limiter subsystem 1280 configured to limit test radiation received by the sensors to test radiation reflected from the limited portion of collector mirror 1100. Arrow 1285 schematically illustrates the possible displacement of

radiation limiter subsystems

1270 and 1280 during testing of the collector mirror. In the embodiment as shown, the first radiation limiter subsystem 1270 includes a shield member 1270.1 and an aperture tube 1270.2. In the embodiment as shown, the test radiation subsystem 1240 is disposed in a second vacuum chamber or container 1290, the first vacuum chamber 1210 and the second vacuum chamber 1290 being separated by a wall 1300 comprising a pair of generally parallel wall portions 1300.1 and 1300.2 having apertures in the wall portions 1300.1 and 1300.2 to allow the test radiation to pass towards the collector mirror. In the embodiment as shown, wall portions 1300.1 and 1300.2 are separated by a gap 1310, gap 1310 being configured to receive shield member 1270.1 of first radiation limiter subsystem 1270. In the embodiment as shown, the apertures in the wall 1300 are configured such that the test radiation as emitted can reach all parts of the collector mirror 1100. By arranging the shield member 1270.1 of the first radiation limiter subsystem between the two wall portions 1300.1 and 1300.2 that separate the walls of both

vacuum chambers

1210 and 1290, a labyrinth seal (labyrinth) is created that will block or impede the propagation of any debris generated by the test radiation subsystem 1240 into the vacuum chamber 1210 containing the collector mirror. To further impede or block contamination of the vacuum chamber 1210, systems according to the invention can further include a purge gas subsystem configured to introduce a flow of purge gas toward the second vacuum chamber 1290 in order to impede propagation of debris to the first vacuum chamber 1210. Reference numeral 1320 refers to such a purge gas subsystem. It may also be noted that the purge gas subsystem may be used to purge any of the aperture tubes applied in embodiments of the present invention in order to prevent debris from propagating towards the collector mirror.

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

Typically, the collector mirror as applied in an EUV radiation source is equipped with a grating arranged to reduce IR radiation reflected towards the second focus of the collector mirror (e.g. the intermediate focus IF shown in fig. 2). Such a grating is schematically shown in fig. 10. Fig. 10 schematically shows a portion 1400 of a collector mirror, comprising a grating 1410 having a period P. FIG. 10 further shows a test radiation beam 1420 configured to impinge on a restricted portion of collector mirror portion 1400. The radiation beam 1420 has a cross-section Pb corresponding to the size of the spot or restricted portion of the irradiated collector mirror.

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

Fig. 11 schematically shows a cross-sectional view of a system according to the invention, drawn on a whiteboard 1500 by inventor HF.

In an embodiment of the system according to the invention as described above, the system according to the invention is described as an inspection or testing system for a collector mirror, in particular a collector mirror having a first focus and a second focus. When the mirror to be tested has a first focus and a second focus, it is advantageous, as described above, to arrange the test radiation subsystem at one of the first focus and the second focus and to arrange the sensor subsystem at the other of the first focus and the second focus. By doing so, the properties of a mirror that radiation emitted from a first focus will reach a second focus (or vice versa) can be used, regardless of the angle at which the radiation is emitted. However, the invention may also be implemented to inspect or test other mirrors, i.e. mirrors without a first focus and/or a second focus.

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

A system according to the invention comprising such measures is schematically illustrated in fig. 12.

Fig. 12 schematically illustrates a system 1100 configured to inspect a mirror 1110, the mirror 1110 having a reflective surface 1110.1, in accordance with the present invention. The reflective surface 1110.1 may generally have any shape. The mirror 1110 may be, for example, a substantially flat mirror, a parabolic mirror, or a free-form mirror. A system according to an embodiment of the invention includes a test radiation subsystem 1120, a sensor subsystem 1130, and a radiation limiter subsystem, including a first radiation limiter subsystem 1140.1 and a second radiation limiter subsystem 1140.2. The subsystems have substantially the same functions as the subsystems described above. In the embodiment as shown, the system further includes an actuator subsystem 1150 configured to displace the sensor subsystem 1130 and the radiation limiter subsystem 1140.2. Specifically, the actuator subsystem 1150 is configured to displace the sensor subsystem 1130 and the radiation limiter subsystem 1140.2 along the X-axis.

The system 1100 according to the invention comprises a test radiation subsystem 1120, the test radiation subsystem 1120 being operable to project test radiation 1120.1 onto the mirror 1100. Similar considerations as described above apply with respect to the type of test radiation, in particular the wavelength or wavelength range of the radiation. In an embodiment, the test radiation subsystem 1120 is configured to generate a spanning angle α₀So that in principle the whole mirror 1100 can be irradiated. Note that the illustration of fig. 12 represents the collector mirror 300 in a two-dimensional cross-section, and in practice the test radiation is typically emitted in three dimensions. In an embodiment, the test radiation subsystem 1120 is configured to produce radiation spanning less than an angle α₀The test radiation of the angle of (a). In such an embodimentStill, the entire mirror 1100 may be scanned or irradiated, for example, by rotating the test radiation subsystem 1120, or portions thereof.

The system 1100 according to the present invention further comprises a sensor subsystem 1130, the sensor subsystem 1130 being operable to receive test radiation, such as indicated by arrow 1142, reflected from the mirror 1100 towards the sensor subsystem 1130. In an embodiment, sensor subsystem 1130 may include one or more sensors or detectors for measuring test radiation reflected from mirror 1110, as described, for example, above.

In accordance with the present invention, system 1100 for testing mirror 1110 further includes a radiation limiter subsystem comprising a first radiation limiter subsystem 1140.1 and a second radiation limiter subsystem 1140.2. The radiation limiter subsystems 1140.1, 1140.2 are configured to limit the test radiation received by the sensor subsystem 1130 to test radiation reflected from the limited portion of the mirror 1110. By doing so, the radiation measured or detected by the sensor subsystem 1130 will only involve radiation reflected from a limited portion of the mirror 300. In the embodiment as shown, the restricted portion of the mirror is indicated by mirror segment 1110.2. By limiting the radiation received by the sensor subsystem 1130 to radiation reflected from the limited portion 1110.2 of the mirror, the performance of that particular portion of the mirror can be assessed, for example by assessing the reflectivity of that portion.

In the embodiment as shown, radiation limiter subsystem 1140.1 is configured to pass only a small portion 1141 of generated radiation 1120.1 toward mirror 1110. A small fraction 1141 of the radiation 1120.1 subtends a specific angle a₀A much smaller angle. To achieve this, the radiation limiter subsystem 1140.1 includes a shielding member with an aperture, such as a tubular aperture through which a small portion of the test radiation 1120.1 can pass. The radiation limiter subsystem 1140.1 thus enables only a limited portion (e.g., portion 1110.2 of the mirror) to be irradiated by the test radiation generated by the test radiation subsystem 330. In a similar manner, radiation limiter subsystem 1140.2 is configured to allow only a limited portion of sensor subsystem from mirror 1110Test radiation is received. It should be noted that the first radiation limiter subsystem 1140.1 and the second radiation limiter subsystem 1140.2 may thus have substantially the same functionality and structure as the radiation limiter subsystems as described in fig. 3 and 6. It should be noted that in a similar manner as described in fig. 3, either of the first and second radiation limiter subsystems 1140.1, 1140.2 may be omitted.

In accordance with the present invention, the system further includes a control subsystem 1170, the control subsystem 1170 configured to control movement of the radiation limiter subsystem, for example, by providing control signals 1170.1 and 1170.2 to the first and second radiation limiter subsystems 1140.1 and 1140.2. In particular, the control subsystem 1170 is configured to control the position of the radiation limiter subsystem such that the limited portion of the mirror irradiated by the test radiation can be changed. Thus, control subsystem 1170 may, for example, be configured to control movement of the radiation limiter subsystem along a series of different positions, thereby limiting test radiation received by sensor subsystem 1130 to test radiation reflected from a corresponding series of different limited portions of mirror 1110. By doing so, the performance of the different restricted portions of the mirror 1110 can be evaluated, for example, the reflectivity of those portions.

In an embodiment of the present invention, a set of different restricted portions covers substantially the entire mirror 1110.

In embodiments of the present invention, the radiation limiter subsystem may include one or more actuators to change the direction of the test radiation 1141 projected onto the mirror 1110 and to change the direction of the test radiation 1142 received by the sensor subsystem 1130.

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

In an embodiment, the processing unit 1180 may be configured to process the measurement data in order to determine a spatial reflectivity distribution of at least a portion of the collector mirror. In such embodiments, measurement data representing test radiation reflected from different confined portions of mirror 1110 can be compared to data representing test radiation generated or emitted to determine the reflectivity of the different confined portions of the mirror. An example of a measure of reflectivity is the ratio of the amount of test radiation received to the amount of test radiation generated or emitted, the difference between (on the one hand) the amount of test radiation generated or emitted and (on the other hand) the amount of test radiation received. Thus, in embodiments of the present invention, the measurement data obtained from the sensor subsystem 1130 may be supplemented with active measurement data, which represents the amount of test radiation emitted or generated during the scanning process. Such source measurement data may be provided to the processing unit 1180 of the control subsystem 1170, e.g., via a data channel.

The embodiment of the system according to the invention as schematically shown in fig. 12 further comprises an actuator subsystem 1150 configured to displace the sensor subsystem 1130 and the radiation limiter subsystem 1140.2. The goal of such an actuator subsystem 1150 is to ensure that test radiation, such as that indicated by reference numeral 1142, reflected from the mirror is captured by the sensor subsystem 1130. In the absence of the first and second focal points, it will be clear to the skilled person that test radiation directed towards different locations on the reflector will also be reflected to different locations. Thus, in embodiments of the present invention, the fact that test radiation directed toward different locations on the reflector will be reflected to different locations is anticipated by providing a displacement of the sensor subsystem 1130 and the radiation limiter subsystem 1140.2.

Such a required shift is schematically illustrated in fig. 12. Specifically, in fig. 12, dashed arrow 1143 represents test radiation directed toward confined portion 1110.3 of reflector 1110, confined portion 1110.3 being at a different location on the mirror than confined portion 1110.2. Dashed arrow 1144 represents the test radiation reflected from restricted portion 1110.3. It can be seen that the reflected test radiation 1144 is directed at a different location and at a different angle than the reflected test radiation 1142. To capture the reflected test radiation 1144, the sensor subsystem 1130 and the radiation limiter subsystem 1140.2 may be displaced to a location schematically indicated by dashed line 1190. Dashed line 1190 thus schematically represents the location of sensor subsystem 1130 and the direction of radiation limiter subsystem 1140.2 that enables capture of test radiation 1144. In an embodiment of the invention, the control unit 1170 may be configured to determine for a particular limited portion of the mirror at which the test radiation is aimed, where the reflected test radiation is to be directed, based on the known shape or predetermined shape information of the mirror to be inspected, and the known relative positions of the test radiation subsystem and the mirror. Based on such determined direction, the control unit 1170 may then be configured to control the actuator subsystem 1150, e.g., by providing the appropriate control signal 1170.4 to the actuator subsystem 1150, to displace the sensor subsystem 1130 and radiation limiter subsystem 1140.2 to the appropriate locations to capture the reflected radiation.

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

In the embodiment as shown, the test radiation subsystem 1120 is mounted to the frame 1200, and the sensor subsystem 1130 and the radiation limiter subsystem 1140.2 may be displaced relative to the frame 1200 by the actuator subsystem 1150. In such an embodiment, the relative positions of the test radiation subsystem to be inspected and the mirror may remain fixed during inspection.

As an alternative to displacing the sensor subsystem 1130 and the radiation limiter subsystem 1140.2 by the actuator subsystem 1150, the system according to the invention may also comprise an actuator subsystem configured to displace the mirror 1110 relative to the test radiation subsystem 1120.

Such an embodiment is schematically depicted in fig. 13. The system 1300 as schematically shown in fig. 13 generally corresponds to the system 1100 shown in fig. 12, except as follows. In the embodiment as shown in fig. 13, the test radiation subsystem 1120 and the sensor subsystem 1130 are mounted to a common frame 1200 along with the radiation limiter subsystem. In the embodiment as shown, the test radiation subsystem 1120 and the sensor subsystem 1130 are therefore considered to have a fixed relative position. To ensure that the test radiation reflected from the mirror 1110 is reflected towards the sensor subsystem 1130, the system is provided with an actuator subsystem 1155 configured to displace the mirror 1110. In particular, actuator subsystem 1155 is configured to displace the mirror such that reflected radiation 1145 reflected from restricted portion 1110.3 is directed toward the same location as reflected radiation 1142. In particular, in the embodiment as shown, actuator subsystem 1155 is configured to tilt or rotate mirror 1110 such that test radiation 1143 is reflected as radiation 1145 towards a location of sensor subsystem 1130. It should be noted that in order for the sensor subsystem 1130 to capture the reflected radiation 1145, it may be necessary to rotate the radiation limiter subsystem 1140.2. In a similar manner as described with reference to fig. 12, the control unit 1170 may be configured to determine where the reflected test radiation is to be directed for a particular restricted portion of the mirror at which the test radiation is aimed based on the known shape or predetermined shape information of the mirror to be inspected and the known relative positions of the test radiation subsystem and the mirror. Based on such determined direction, the control unit 1170 may then be configured to control the actuator subsystem 1155 to displace the mirror to an appropriate location or position, e.g., by providing an appropriate control signal to the actuator subsystem 1155, to reflect the test radiation received by the mirror to the sensor subsystem.

An alternative to displacing the sensor subsystem 1130 and the radiation limiter subsystem 1140.2 by the actuator subsystem 1150, or displacing the mirror 1110 relative to the test radiation subsystem by the actuator subsystem 1155, is to apply an actuator subsystem for displacing the test radiation subsystem 1120 relative to the mirror 1110, thereby arranging for reflected radiation to reach the sensor subsystem 1130 regardless of which limited portion of the mirror is irradiated.

In the embodiments as described with reference to fig. 12 and 13, the actuator subsystem is used to ensure that an optical path for the test radiation is established between the test radiation subsystem and the sensor subsystem. In such embodiments, the actuator subsystem is therefore used to ensure that the test radiation subsystem, mirror and sensor subsystem are in the appropriate relative positions and orientations, such that a particular portion of the mirror being inspected or tested is illuminated, and the test radiation reflected from the mirror is captured by the sensor subsystem. According to the invention, the optical path of the test radiation will also pass through the radiation limiter subsystem or several radiation limiter subsystems. As described above, the system may also be positioned or displaced by the actuator subsystem.

In an embodiment of the invention, an actuator subsystem is also applied to control the angle at which the test radiation is incident on the mirror. As can be seen in fig. 12, the incident angle of test radiation 1141 is different from the incident angle of test radiation 1143. This may be undesirable. In particular, it may be desirable to test the mirror, for example to determine the reflectivity of the mirror, such that the mirror is illuminated in substantially the same way (i.e. at the same angle of incidence) as the applied test radiation, as occurs during normal use of the mirror. For certain applications, the radiation beam may illuminate the mirror at approximately the same angle across the entire mirror surface. To test such mirrors, it may therefore be advantageous to ensure that the test radiation strikes the mirror at the appropriate angle, regardless of which part of the mirror is irradiated. Such an arrangement may be ensured, for example, by controlling the relative positions and/or orientations of the test radiation subsystem and the mirror. Such control may be established, for example, by applying an actuator subsystem configured to control the relative position and/or orientation of the test radiation subsystem and the mirror. Such an actuator subsystem may for example be controlled by a control subsystem as described above, whereby the control subsystem is configured to generate control signals for controlling the actuator subsystem based on the required angle of incidence information of the mirror under test or inspection.

Thus, in embodiments of the invention, the actuator subsystem or several actuator subsystems as applied may serve a dual purpose:

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

-ensuring that the test radiation impinges on the mirror tested at the appropriate or desired angle of incidence.

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

In embodiments of the invention, the actuator subassembly may for example comprise a 5 or 6 degree of freedom robotic arm to displace the test radiation subsystem and/or the sensor subsystem.

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

In such embodiments, it may for example be advantageous to assign the required degrees of freedom that need to be actuated on the different elements.

Referring to the embodiment of FIG. 12, it may be advantageous, for example, to arrange an actuator subsystem to scan mirror 1110, the actuator subsystem being configured to displace the sensor subsystem along the X-axis and the mirror along the Y-axis. Thus, by using two linear motors or actuators, a two-dimensional scan of the mirror surface can be obtained.

As mentioned, the system as described above with reference to fig. 12 and 13 enables inspection or testing of mirrors without a first focus and a second focus. The system can therefore be applied to evaluate the reflectivity of mirrors of arbitrary shape or mirrors with a single focal point.

Typically, as will be apparent from the examples given above, it will be necessary to apply an actuator subsystem, which in turn enables displacement or rotation in at least two degrees of freedom (applied to the same or different elements of the system) in order to scan or inspect the surface of the mirror. However, in the case of a mirror having an optical axis (i.e. an axis along which rotational symmetry exists), it may be sufficient to displace any element of the system (i.e. the mirror or the test radiation subsystem or the sensor subsystem) in only one degree of freedom (e.g. a translational degree of freedom along the optical axis). To achieve this, both the test radiation subsystem and the sensor subsystem should be arranged on the optical axis, as shown for example in fig. 3 and 6. In such an arrangement, the parabolic mirror may be tested, for example, using a system as shown in fig. 3 and/or 6, with the addition of an actuator subsystem configured to translate the sensor subsystem along the optical axis to ensure capture of reflected radiation.

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 could be defined in large part by a computer program containing one or more sequences of machine-readable instructions for carrying out the steps of the methods as disclosed above, or by a data storage medium such as a semiconductor memory, magnetic or optical disk on which such a computer program is stored. The above description is intended to be illustrative and not restrictive. 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

1. A system configured to test a collector mirror having a first focal point and a second focal point, the system comprising:

a test radiation subsystem operable to project test radiation from the second focal point onto the collector mirror;

a sensor subsystem operable to receive test radiation reflected from the collector mirror toward the first focal point; and

a radiation limiter subsystem operable to limit the test radiation received by the sensor subsystem to test radiation reflected from the limited portion of the collector mirror;

a control subsystem operable to control movement of the radiation limiter subsystem along a series of different positions, thereby limiting the test radiation received by the sensor subsystem to test radiation reflected from a corresponding series of different limited portions of the collector mirror.

2. The system of claim 1, wherein the control subsystem comprises a processing unit configured to:

-receiving measurement data from the sensor subsystem, the measurement data representing test radiation reflected from the corresponding series of different restricted portions of the collector mirror;

-processing the measurement data to determine a spatial reflectivity profile of at least a portion of the collector mirror.

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

4. The system of claim 3, wherein the test radiation subsystem comprises an EUV source for producing EUV radiation.

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

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

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

8. The system of any preceding claim, wherein the sensor subsystem comprises a sensor operable to receive the test radiation reflected from the collector mirror towards the first focal point and to generate a measurement signal representative of the received test radiation.

9. The system of claim 8, wherein the sensor subsystem comprises a multi-layer mirror assembly for filtering the test radiation received by the sensor.

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

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

12. The system of any preceding claim, wherein the radiation limiter subsystem comprises a shielding member having an aperture therein, the shielding member being arranged in the optical path of the test radiation.

13. The system of claim 12, wherein the aperture is formed by an aperture tube of the radiation limiter subsystem.

14. The system of claim 12 or 13, wherein the radiation limiter subsystem is arranged in an optical path of the test radiation between the second focal point and the collector mirror.

15. The system of claim 12 or 13, wherein the radiation limiter subsystem is arranged in an optical path of the test radiation between the second focal point and the collector mirror.

16. The system of claim 12, wherein the radiation limiter subsystem comprises:

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

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

17. The system of claim 16, wherein the control subsystem is configured to control movement of the first and second shield members to establish an optical path for the test radiation between the second focus and the first focus via both the first and second apertures.

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

-positioning both the first and second apertures along an optical axis passing through the first and second focal points;

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

-receiving a measurement signal of the sensor subsystem representing the received test radiation;

-receiving a source measurement signal of the test radiation subsystem representing the test radiation projected by the test radiation subsystem; and

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

19. The system of any preceding claim, wherein the test radiation subsystem comprises a Xe, Li, or Sn radiation source.

20. A system configured to test a mirror, the system comprising:

a test radiation subsystem operable to project test radiation onto the mirror;

a radiation limiter subsystem operable to limit the test radiation received by the sensor to test radiation reflected from the limited portion of the mirror;

a control subsystem operable to control movement of the radiation limiter subsystem along a series of different positions, thereby to limit the test radiation received by the sensor to test radiation reflected from a corresponding series of different limited portions of the mirror.

21. The system of claim 20, further comprising:

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

22. The system of claim 21, wherein the control subsystem is configured to control the actuator subsystem to establish an optical path for the test radiation between the test radiation subsystem and the sensor subsystem.

23. A method of testing a collector mirror having a first focal point and a second focal point, the method comprising:

projecting test radiation from the second focal point onto the collector mirror;

receiving, by a sensor, test radiation reflected from the collector mirror toward the first focal point;

limiting the test radiation received by the sensor to test radiation reflected from the limited portion of the collector mirror; and

controlling movement of a radiation limiting subsystem along a series of different positions, thereby limiting the test radiation received by the sensor to test radiation reflected from a corresponding series of different limited portions of the collector mirror.

24. The method of 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 focal point and the collector mirror; and

a second shielding member having a second aperture therein is arranged in the optical path of the test radiation between the collector mirror and the first focal point.

25. The method of claim 24, wherein the step of controlling movement comprises:

controlling movement of the first and second shield members to establish an optical path for the test radiation between the second focus and the first focus via both the first and second apertures.

26. The method of claim 25, further comprising a calibration step of the test method, the calibration step comprising:

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

-receiving a measurement signal representative of the received test radiation;

-receiving a source measurement signal representative of the emitted test radiation; and

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