WO2021004785A1

WO2021004785A1 - Illumination system for an euv apparatus and method for restoring it

Info

Publication number: WO2021004785A1
Application number: PCT/EP2020/067624
Authority: WO
Inventors: Florian Baumer; André ORTHEN
Original assignee: Carl Zeiss Smt Gmbh
Priority date: 2019-07-09
Filing date: 2020-06-24
Publication date: 2021-01-14
Also published as: DE102019210092A1; TW202109205A

Abstract

An illumination system for an EUV apparatus is configured to receive, during operation of the EUV apparatus, EUV radiation of an EUV radiation source at a source position in an entrance plane and to shape, from at least one portion of the received EUV radiation, illumination radiation that is directed into an illumination field in an exit plane of the illumination system and in the illumination field fulfils an illumination specification. The illumination system comprises a plurality of mirror modules which are installed at assigned installation positions and which define an illumination beam path leading from the source position to the illumination field. The mirror modules comprise a first facet mirror (FAC1) and a second facet mirror (FAC2), wherein the first facet mirror (FAC1) comprises first facets (F1) configured for reflecting EUV radiation and arranged in or near a field plane of the illumination system, said field plane being optically conjugate with respect to the plane of the illumination field (BF), and the second facet mirror comprises second facets configured for reflecting EUV radiation and arranged in or near a pupil plane of the illumination system. At least one of the first facets (F1) comprises at least one alignment marking (JM) which, during use of the illumination system as intended, is not able to be imaged into the illumination field by the EUV radiation and is able to be imaged into the illumination field upon incidence of measurement light (ML) originating from a different wavelength range than the EUV radiation.

Description

Illumination system for an EUV apparatus and method for restoring it

FIELD OF APPLICATION AND PRIOR ART

The invention relates to an illumination system for an EUV apparatus, to a method for restoring an illumination system installed in an EUV apparatus, and to a detector module.

The EUV apparatus can be, for example, a projection exposure apparatus for EUV microlithography or a mask inspection apparatus, employing EUV radiation, for inspecting masks (reticles) for EUV microlithography.

Lithographic projection exposure methods are predominantly used nowadays for producing semiconductor components and other finely structured components, such as, for example, masks for photolithography. In this case, use is made of masks (reticles) or other patterning devices that bear or form the pattern of a structure to be imaged, for example a line pattern of a layer of a semiconductor component. The pattern is positioned in a projection exposure apparatus between an illumination system and a projection lens in the region of the object plane of the projection lens and is illuminated with an illumination radiation shaped by the illumination system. The radiation modified by the pattern travels through the projection lens as projection radiation, said projection lens imaging the pattern with a reduced scale onto the substrate to be exposed. The surface of the substrate is arranged in the image plane of the projection lens, which image plane is optically conjugate to the object plane. The substrate is generally coated with a radiation-sensitive layer (resist, photoresist).

One of the aims in the development of projection exposure apparatuses is to lithographically produce structures having smaller and smaller dimensions on the substrate, for example to obtain greater integration densities in semiconductor components. One approach consists in working with shorter wavelengths of the electromagnetic radiation. By way of example, optical systems have been developed which use electromagnetic radiation from the extreme ultraviolet range (EUV), in particular having operating wavelengths in the range of between 5 nanometres (nm) and 30 nm, in particular of 13.5 nm.

The illumination system is configured for receiving EUV radiation of an EUV radiation source and for shaping illumination radiation from at least one portion of the received EUV radiation. The illumination radiation is directed into an illumination field in an exit plane of the illumination system during exposure operation, wherein the exit plane of the illumination system and the object plane of the projection lens advantageously coincide. The illumination radiation is characterized by specific illumination parameters and is incident on the pattern within the illumination field with a defined position, shape and size at defined angles. The EUV radiation source, which may be a plasma source, for example, is arranged in a source module separate from the illumination system, said source module generating a secondary radiation source at a source position in an entrance plane of the illumination system.

Arranged in a housing of an illumination system of the type considered here are a plurality of mirror modules, which are each located in the final installed state at installation positions that are provided for the mirror modules. The mirror modules or reflective mirror surfaces of the mirror modules define an illumination beam path extending from the source position to the illumination field.

The mirror modules include a first mirror module having a first facet mirror at a first installation position of the illumination system and a second mirror module having a second facet mirror at a second installation position of the illumination system. A mirror module of this type has a main body acting as a carrier, on which facet elements with facets that reflect EUV radiation are mounted individually or in groups in accordance with a specific local distribution. Further mirror modules (one or more) can be provided in the illumination beam path.

The reflective facets of the first facet mirror are arranged in or near a field plane of the illumination system, said field plane being conjugate with respect to the exit plane or with respect to the illumination field, for which reason the first facet mirror is often also referred to as a“field facet mirror”. Correspondingly, the second facet mirror is often also referred to as a “pupil facet mirror” because its facets that reflect EUV radiation are situated in or near a plane that is Fourier-transformed in relation to the exit plane.

The two facet mirrors contribute in the illumination system of the EUV apparatus to the homogenization or mixing of the EUV radiation.

An EUV projection exposure apparatus comprising an illumination system of this type is known e.g. from patent US 7 473 907 B2.

In the course of initial production (new production) of an illumination system by the manufacturer and also later during possible repair or restoration of an illumination system installed in an EUV apparatus for the end customer, it is endeavoured to systematically optimize the mirror positions after installation in the context of an alignment, such that the required optical performance can be attained in a tenable time.

WO 2019/081555 A1 (corresponding to DE 10 2017 219 179 B3) describes a method for restoring an illumination system installed in an EUV apparatus. The method can be used e.g. during mirror exchange at an illumination system at the location of its use. In a manner similar to the prior art from WO 2017/153165 A1 , the method comprises a swap operation of a mirror module, in which one of the mirror modules is disassembled from its installed position and removed from the illumination system, a mirror module with nominally the same design is installed in the installation position in place of the removed mirror module and the installed mirror module is adjusted in rigid body degrees of freedom in the installation position while changing the relative orientation of the installed mirror module.

Here, before the swap operation, a reference measurement is performed in order to capture a reference state that represents the adjustment state before the start of the swap operation. A comparison measurement is performed after finishing the swap operation. The alignment state or adjustment state measured in the process is compared to the reference state in order to restore the adjustment state before the swap operation. The method is thus based on a relative measurement or on a comparison between the result of a reference measurement and the result of a comparison measurement, performed in the same way, at a later time. The intention here is to capture changes in the illumination system in the interim time between the reference measurement and comparison measurement.

In the relative measurement technique for the mirror exchange, the assumption is made that, initially (prior to the exchange of a mirror module), a sufficiently well adjusted illumination system is present. During the reference measurement, which is carried out after components of a measuring system (measurement light source module and detector module) have been fitted, a small number of system measurement variables are then enough for sufficient characterization of the alignment state. One meaningful system measurement variable is the “position of the illumination field” (also referred to as “field position”). Other system measurement variables are e.g. the“spatial distribution of measurement light in the pupil plane” (corresponding to“telecentricity”) and the“position of the measurement light spots on the facets of the second mirror module” (corresponding to“spot centration).

According to the disclosure of WO 2019/081555 A1 , during the reference measurement and the comparison measurement for determining the“field position”, the positions of a first end section and of a second end section, opposite the first end section, of the illumination field are measured, wherein the position of the illumination field in an intermediate section lying between the first and second end sections is not measured. Moreover, a spatial distribution of measurement light in a pupil plane that is Fourier-transformed in relation to the exit plane is measured for a field point lying in the intermediate section. For the simultaneous measurement of these measurement variables, it is possible to use a detector module having no movable parts and/or no controllable travel axes for the movement of optical components.

PROBLEM AND SOLUTION

One problem addressed by the invention is to provide an illumination system for an EUV apparatus which exhibits particular ease of maintenance and in particular enables simple alignment after a mirror exchange. A further problem addressed is to provide a method for restoring an illumination system installed in an EUV apparatus, which method can be carried out rapidly and reliably. Furthermore, the intention is to provide a detector module which is usable at the illumination system and in the method and has a simple construction.

In order to solve these problems, the invention provides an illumination system having the features of Claim 1 , a method for restoring an illumination system installed in an EUV apparatus having the features of Claim 11 , and a detector module having the features of Claim 14. Furthermore, the invention relates to a mirror module having the features of Claim 15. Advantageous developments are specified in the dependent claims. The wording of all the claims is incorporated by reference in the content of the description.

In accordance with a first aspect of the invention, an illumination system of the type mentioned in the introduction is provided, wherein at least one of the facets of the first facet mirror comprises at least one alignment marking which, during use of the illumination system as intended, cannot be imaged into the illumination field by the EUV radiation used in the process, but can be imaged into the illumination field upon incidence of measurement light originating from a different wavelength range than the EUV radiation. Since the alignment marking is not imaged into the illumination field with the aid of the EUV radiation during use of the illumination system as intended, the alignment marking does not disturb intended operation of the EUV apparatus. The illumination intensity distribution in the illumination field is thus not locally altered, or is locally altered only to a small, undisturbing extent, by the alignment marking. However, if measurement light originating from a different wavelength range than the EUV radiation is radiated into the illumination system, the same alignment marking is imaged into the illumination field. As a result it is possible, by measuring the positions of the imaging of the alignment marking in the exit plane of the illumination system, to draw conclusions about the position of the illumination field in the exit plane, that is to say about the “field position”. Consequently, the parameter“field position” can be determined with the aid of a measurement technique which is set up in a relatively simple manner and which, firstly, does not rely on the use of EUV radiation and, secondly, merely has to be able to determine the position of the imaging of the alignment marking in the exit plane with sufficient measurement accuracy.

In comparison with the prior art in WO 2019/081555 A1 , an advantage afforded, inter alia, is that for carrying out the measurement the field edges in the region of the mutually opposite end sections of the illumination field are not required for the measurement. Said field edges can thus remain free for some other use, for example for fitting stops that trim the field edges, for example in order to obtain a sharp intensity decrease in the region of the field edges.

An alignment marking should permit an optical position determination that is as accurate as possible on the basis of an imaging of the alignment marking in the exit plane. An alignment marking can have e.g. a shape that is round in a quasi-punctiform fashion, or a cruciform shape. In principle, many different geometries are possible for the alignment marking. Geometries that permit a position determination that is as accurate as possible, such as e.g. circles, annuli, crosses, chequered patterns or the like, are particularly advantageous. Specific markers from photogrammetry that have a particularly sharp autocorrelation function are also usable. What is crucial for the geometry is an as accurately determinable position as possible of the imaging of the alignment marking in the exit plane of the illumination system, e.g. with the aid of a detector (e.g. CCD/CMOS chip, position-sensitive diode (PDS), 4-quadrant diode) in the exit plane.

Measurement light from a wavelength range that is in the visible spectral range (VIS) or in the adjacent UV spectral range or IR spectral range is preferably used. Cost-effective measurement light sources and detectors are available for this. Therefore, the alignment marking is preferably designed for measurement light whose wavelength is in the visible spectral range (VIS) or in the adjacent UV spectral range or IR spectral range. Consequently, with the measurement light of this type it is possible to generate a detectable imaging in the exit plane.

The particular properties of the alignment marking with regard to the different capability of imaging with the use of EUV radiation, on the one hand, and with the use of measurement light, on the other hand, can be brought about in various ways.

One particularly elegant possibility for realization arises in the case of illumination systems which are constructed in such a way that the first facet mirror comprises one or a plurality of partly illuminated first facets. A partly illuminated first facet is a first facet comprising a first surface section, which lies within the illumination beam path, and a second surface section, which lies outside the illumination beam path. In this case, at least one alignment marking can be arranged within the second surface section, that is to say in a region of the first facet which is not reached by the EUV radiation during operation as intended, that is to say does not lie in the EUV used region of the first facet. If such an alignment marking is intended to be used for the measurement, then the measurement light should be radiated in such that it propagates at least partly outside the illumination beam path of the EUV radiation and reaches that region of the second surface section of the first facet which is provided with the alignment marking, and also the illumination field.

EUV illumination systems comprising partly illuminated first facets are disclosed for example in US 2008/278704 A 1 corresponding to DE 10 2006 036 064 A1 in the name of the present applicant. The disclosure content of these documents with regard to the construction of the illumination system is in this respect incorporated by reference in the content of the present description.

At least one partly illuminated first facet bearing an alignment marking can lie in the outer edge region of the illuminated region at the first facet mirror, such that the first facet bearing the alignment marking partly projects out of the region of the illumination towards the outside.

In some embodiments, the illumination beam path has in the field plane a ring shape with an illumination-free central region. In these cases, it is possible that there exists at least one partly illuminated first facet comprising a first surface section, which lies within the illumination beam path and a second surface section which lies in the central region, wherein at least one alignment marking is arranged in the second surface section. This often affords the possibility of applying an alignment marking in the central region between lateral edges of the first facet, such that the end regions remain free and an image of the alignment marking can be generated in the region of the field centre. This can be advantageous for a reliable measurement.

It is possible to apply one or a plurality of alignment markings outside the outer boundary of the illuminated region and also one or a plurality of alignment markings in the illumination-free central region.

If at least one alignment marking is arranged outside the illumination beam path on a second surface section of a first facet, then there are a particularly large number of possibilities with regard to the configuration of the alignment marking since there is no need to ensure that the alignment marking is not visible or not able to be imaged for EUV radiation.

The alignment marking can comprise for example a marking structure having an absorbing effect for the measurement light. Alternatively or additionally, the alignment marking can comprise a marking structure having a diffractive effect for the measurement light. The structure dimensions of a diffractive marking structure should be designed such that the diffracted measurement light is not incident in the illumination field and as a result does not adversely affect the“visibility” of the imaging of the alignment marking for the measurement process.

It is also possible for an alignment marking to comprise a marking structure having a reflecting surface inclined in comparison with the surrounding mirror surface of the first facet in such a way that measurement light incident on the reflective surface is reflected out of the beam path of the measurement light, such that the imaging of the alignment marking in the exit plane appears at least partly dark.

Alternatively or additionally, under specific prerequisites it is also possible to apply at least one alignment marking in that region of a first facet which is reached by the EUV radiation during operation, that is to say lies within the illumination beam path. In this case, the alignment marking should be configured in such a way that it is not“visible” to the EUV radiation in the sense that despite irradiation with EUV radiation no imaging (that is detectable or disturbing for operation of the illumination system with EUV radiation) of the alignment marking is generated in the illumination field by means of the EUV radiation. Such an alignment marking applied in the region of the EUV used area of the facet can comprise a marking structure having a reflective effect for EUV radiation in substantially exactly the same way as the region in direct proximity, but outside the alignment marking, but having an absorbing and/or diffractive effect for the measurement light. The marking structure can be embodied as a grating structure, for example, the typical structure sizes of which in comparison with the wavelength of the EUV radiation are so macroscopic that no diffraction of the EUV radiation takes place, wherein the structure sizes are dimensioned, on the other hand, such that a diffractive effect is present for the measurement light. If measurement light from the visible wavelength range (VIS) is used, for example, the marking structure can be configured for example as a grooved grating or echelon grating having grating structure widths in the range of 1 pm to 100 pm. If the alignment marking is intended to comprise a marking structure having an absorbing effect for the measurement light, this can be effected for example by way of a coating with a coating material which is substantially transparent to EUV radiation but absorbs the measurement light to such a significant extent that an imaging of the alignment marking arises in the illumination field if measurement light is irradiated onto the region of the alignment marking.

In principle, it is sufficient if just a single alignment marking is provided at the first facet mirror. The position of the sole first facet provided with alignment marking then has the effect of being representative of the position of the entire mirror module. Particularly small position tolerances of the first facets should then be taken into consideration.

In variants in which the position of an individual channel in the illumination system can have a relatively large (statistical) position tolerance, measurements of the position of a plurality of facets can be advantageous. In some embodiments, therefore, at the first facet mirror two or more mutually identical or different alignment markings are applied at different locations, in particular at different first facets. As a result, the measurement accuracy can be increased, if appropriate.

It can be advantageous and in some embodiments it is provided that the alignment marking is arranged at a distance from the lateral edges of the first facet, that is to say outside a first end section and an opposite second end section of the first facet, such that the imaging of said alignment marking does not lie in the vicinity of the lateral field edges of the illumination field

The measurement technique is suitable inter alia for EUV apparatuses that are designed for scanner operation and accordingly have a slot-shaped illumination field with a relatively low height in the scanning direction (y-direction) and a width that is relatively large in comparison with the height in the cross-scan direction (x-direction) perpendicular or orthogonal thereto. The illumination field can be rectangular or curved in arcuate fashion. The term“end section” in this case relates to the two end sections of the illumination field in the cross-scan direction. What can be achieved in this way is that the measurement cannot be adversely affected by elements that act as a stop, for example, in order to accurately define the course of the outer field edge of the illumination field. As a result, a field position measurement is possible which functions without the measurement of the edge regions of the illumination field, such that said edge regions can be used for other purposes. The width of the end sections that should remain free of an alignment marking can be e.g. at least 10% or at least 20% of the total width in the cross scan direction.

The invention also relates to a method for restoring an illumination system installed in an EUV apparatus and comprising at its first facet mirror at least one first facet having at least one alignment marking which, during use of the illumination system as intended, is not able to be imaged into the illumination field by the EUV radiation and is able to be imaged into the illumination field upon incidence of measurement light originating from a different wavelength range than the EUV radiation.

In a manner similar to the prior art from WO 2019/081555 A1 , the method comprises a swap operation of a mirror module, in which one of the mirror modules is disassembled from its installed position and removed from the illumination system, a mirror module with nominally the same design is installed in the installation position in place of the removed mirror module and the installed mirror module is adjusted in rigid body degrees of freedom in the installation position while changing the relative orientation of the installed mirror module. Here, before the swap operation, a reference measurement is performed in order to capture a reference state that represents the adjustment state before the start of the swap operation. A comparison measurement is performed after finishing the swap operation. The alignment state or adjustment state measured in the process is compared to the reference state in order to restore the adjustment state before the swap operation.

During the alignment after installation of the swapped-in mirror module, it is optionally possible to likewise use the alignment degrees of freedom of other, non-replaced mirror modules of the illumination system in order to optimize the performance and bring this as close as possible to the reference state again. Thus, it is possible to perform a system alignment with a post adjustment of a plurality of optical elements.

The position of an image of at least one alignment marking in the illumination field is in each case measured during the reference measurement and the comparison measurement. The position of the illumination field in the exit plane can be determined therefrom, such that a relatively simple measurement of the“field position” is possible with the aid of the alignment marking. If the position of the imaging of the alignment marking from the comparison measurement does not correspond to the position of the imaging during the reference measurement within the scope of the measurement accuracy, then the adjustment operation can be continued until there is sufficient correspondence.

As in the prior art in WO 2019/081555 A1 , in addition to determining the field position, it is also possible to measure a spatial distribution of measurement light in a pupil plane that is Fourier- transformed in relation to the exit plane for a field point lying at a distance from the image of the alignment marking and/or a position of a measurement light spot on a facet of the second facet mirror.

The invention also relates to a detector module for a measurement system for measuring system measurement variables in an illumination field in an exit plane of an illumination system of an EUV apparatus. The detector module comprises a field sensor arrangement for capturing the position of an image of an alignment marking, wherein a capture region of the field sensor arrangement is arranged such that it lies outside a first end section and a second end section, opposite the first end section, of the illumination field. Moreover, provision is made of a pupil sensor arrangement for capturing a spatial distribution of measurement light in a pupil plane that is Fourier-transformed in relation to the exit plane for a field point. Said field point does not lie in the capture region of the field sensor arrangement, but rather outside said region. A simultaneous measurement of field position and pupil illumination (corresponding to the spatial distribution of measurement light in a pupil plane of the illumination system that is Fourier- transformed in relation to the exit plane) is thus possible. The field point can lie centrally between the end sections, but this is not mandatory, and so an eccentric arrangement is also possible. If the capture region of the field sensor arrangement is chosen such that the end sections do not lie in the capture region, said end sections can be used for other purposes, without the measurement being adversely affected by the other use.

Furthermore, the invention relates to a mirror module comprising a first facet mirror of the type described in this application for use in an illumination system of an EUV apparatus. By virtue of the fact that at least one of the first facets of the first facet mirror comprises at least one alignment marking which, during use of the illumination system as intended, is not able to be imaged into the illumination field by the EUV radiation and is able to be imaged into the illumination field upon incidence of measurement light originating from a different wavelength range than the EUV radiation, the installation of such a mirror module at the installation position provided therefor can considerably simplify the alignment of the illumination system.

BRIEF DESCRIPTION OF THE DRAWINGS

Further advantages and aspects of the invention are evident from the claims and from the following description of preferred exemplary embodiments of the invention, which are explained below with reference to the figures.

Fig. 1 schematically shows optical components of an EUV microlithographic projection exposure apparatus comprising a detector module in accordance with one embodiment of the invention;

Fig. 2 shows a number of ray trajectories in a mirror arrangement having two facet mirrors;

Fig. 3 shows a schematic plan view of the first facet mirror, wherein some of the first facets lie only partly in the illumination beam path;

Fig. 4 schematically shows one exemplary embodiment of a measurement light source module; Fig. 5 schematically shows one exemplary embodiment of a detector module; Fig. 6 schematically shows the shape of an arcuately curved illumination field in which an imaging of an alignment marking lies in the capture region of a field sensor;

Fig 7 A to 7C show alignment markings that function according to different principles; and

Figs 8A, 8B schematically show an alignment marking in a section of a partially illuminated field facet (8A), which section is not able to be reached by EUV radiation, and a corresponding imaging of the alignment marking in the exit plane of the illumination system, wherein the position of the imaging is captured (8B) by a 4-quadrant sensor.

DETAILED DESCRIPTION OF THE EXEMPLARY EMBODIMENTS

Figure 1 shows by way of example optical components of an EUV microlithographic projection exposure apparatus WSC. The EUV microlithographic projection exposure apparatus serves during operation for exposing a radiation-sensitive substrate W, which is arranged in the region of an image plane IS of a projection lens PO, with at least one image of a pattern of a reflective mask (here also alternatively referred to as reticle), said pattern being arranged in the region of an object plane OS of the projection lens. In the case of the example, the substrate is a wafer composed of semiconductor material that is coated with a light-sensitive resist layer.

To facilitate understanding of the description, a Cartesian system coordinate system SKS is indicated, which reveals the respective orientation relationship of the components illustrated in the figures. The projection exposure apparatus WSC is of the scanner type. The x-axis extends perpendicularly into the plane of the drawing in Figure 1. The y-axis extends towards the right. The z-axis extends downward in the plane of the drawing. The object plane OS and the image plane IS both extend parallel to the x-y-plane. During the operation of the projection exposure apparatus, the mask and the substrate are moved synchronously or simultaneously in the y- direction (scanning direction) during a scan operation and are thereby scanned.

The apparatus is operated with the radiation from a primary radiation source RS. An illumination system ILL serves for receiving the radiation from the primary radiation source and for shaping illumination radiation directed onto the pattern. The projection lens PO serves for imaging the pattern onto the light-sensitive substrate.

The primary radiation source RS may be, inter alia, a laser plasma source or a gas discharge source or a synchrotron-based radiation source or a free electron laser (FEL). Such radiation sources generate a radiation RAD in the EUV range, in particular having wavelengths of between 5 nm and 15 nm, preferably 13.5 nm. The illumination system and the projection lens are constructed with components that are reflective to EUV radiation in order that they can operate in this wavelength range.

The primary radiation source RS is situated in a source module SM, which is separate from the illumination system ILL and which also comprises, inter alia, a collector COL for collecting the primary EUV radiation. The source module SM generates during exposure operation a secondary radiation source SLS at a source position SP in an entrance plane ES of the illumination system ILL. The secondary radiation source SLS is the optical interface between the EUV radiation source or the source module SM and the illumination system ILL.

The illumination system comprises a mixing unit MIX and a planar deflection mirror GM (also referred to as G mirror GM), which is operated under grazing incidence. The illumination system shapes the radiation and thereby illuminates an illumination field BF situated in the exit plane of the illumination system, said exit plane being situated in the object plane OS of the projection lens PO or in the vicinity thereof. In this case, the shape and size of the illumination field determine the shape and size of the effectively used object field in the object plane OS. During operation of the apparatus, the reflective reticle is arranged in the region of the object plane OS.

The mixing unit MIX substantially consists of two facet mirrors FAC1 , FAC2. The first facet mirror FAC1 is arranged in a plane of the illumination system which is optically conjugate with respect to the object plane OS. Therefore, it is also referred to as a field facet mirror. The second facet mirror FAC2 is arranged in a pupil plane of the illumination system that is optically conjugate with respect to a pupil plane of the projection lens. Therefore, it is also referred to as a pupil facet mirror.

With the aid of the pupil facet mirror FAC2 and the optical assembly disposed downstream in the beam path and comprising the deflection mirror GM operated with grazing incidence, the individual mirroring facets (individual mirrors) of the first facet mirror FAC1 are imaged into the illumination field.

The spatial (local) illumination intensity distribution at the field facet mirror FAC1 determines the local illumination intensity distribution in the illumination field. The spatial (local) illumination intensity distribution at the pupil facet mirror FAC2 determines the illumination angle intensity distribution in the illumination field OF.

The shape of the illumination field is determined substantially by the shape of the facets of the field facet mirror FAC1 , the images of which fall into the exit plane of the illumination system. The illumination field can be a rectangular field or else a curved field (ring field). The beam-guiding region optically between the source position SP and the exit plane (plane of the image field) is the illumination beam path, in which the EUV radiation is successively incident during operation on the first facet mirror FAC1 , the second facet mirror FAC2 and the deflection mirror GM.

For further explanation, Figure 2 schematically illustrates a mirror arrangement SA, having a first facet mirror FAC1 and a second facet mirror FAC2.

The first facet mirror FAC1 has a multiplicity of first facets F 1 , which are in the form of elongate arcs in the exemplary embodiment shown. This shape of the first facets, however, should be understood to be merely exemplary. Only a few of the facets are shown. The number of first facets in practice is typically significantly higher and can be more than 100 or even more than 300.

The second facet mirror FAC2 has a multiplicity of second facets F2, which are in the form of small dies in the exemplary embodiment shown, which again should be understood to be merely an example.

The first facets F1 are arranged on a first main body B1 of the first facet mirror FAC1. The first main body forms, together with the first facets it carries and any further components, e.g. attachment means, actuators etc., a first mirror module SM1.

The first mirror module SM1 can be installed in its entirety at the installation position that is provided therefor on an associated first carrier structure TS1 of the illumination system or else be demounted again in its entirety and removed. The orientation of the first mirror module SM1 in space, or relative to a reference coordinate system (e.g. the SKS of the housing of the illumination system), can be defined by way of the first module coordinate system MKS1.

The second facets F2 are analogously arranged on a second main body B2 of the second facet mirror, as a result of which a completely installable and replaceable second mirror module SM2 is formed. The orientation of the second mirror module SM2 in space, or relative to a reference coordinate system, can be defined by way of the second module coordinate system MKS2.

The relative orientation or position of a mirror module with respect to the associated carrier structures (frame structure of the illumination system) or to the system coordinate system that is linked thereto can be continuously variably or incrementally set with great accuracy in six degrees of freedom. Suitable adjustment means are provided herefor, which can also be referred to as tilt manipulators. Possible configurations for adjustment means are described in WO 2019/081555 A1 , to which reference is made in this respect.

Depicted in Fig. 2 by way of example are a few rays ST illustrating the EUV illumination beam path when the mirror arrangement is installed in an optical system and in operation. The rays ST start here from a first field plane FE1 (intermediate focus), and are then reflected by the facets F1 of the first facet mirror FAC1 onto the facets F2 of the second facet mirror FAC2. The rays are directed by the facets F2 of the second facet mirror FAC2 into a second field plane FE2, corresponding to the exit plane of the illumination system. Images IM of the facets of the first facet mirror FAC1 are thereby produced in the second field plane FE2, wherein, in more precise terms, the images of all first facets F1 are produced in the field plane FE2 by mutual superimposition. The superimposed images IM together form the illuminated illumination field BF.

Between the facets F1 of the first facet mirror FAC1 and the facets F2 of the second facet mirror FAC2 there is a unique assignment. That means that each facet F1 of the first facet mirror FAC1 is assigned a specific facet F2 of the second facet mirror FAC2. In Fig. 2, this is shown for a facet F1-A and a facet F1-B of the first facet mirror FAC1 and a facet F2-A and a facet F2-B of the second facet mirror FAC2. In other words, those rays ST which are reflected by the facet F1-A are incident exactly on the facet F2-A, and those used light rays which are reflected by the facet F1-B are incident on the facet F2-B etc. In this case, there is a 1 :1 assignment between the facets F1 of the first facet mirror FAC1 and the facets F2 of the second facet mirror FAC2.

In deviation from a 1 :1 assignment between the facets F1 and F2, it is also possible, however, that each facet F1 is assigned more than one facet of the facets F2. This is the case if the facets F1 are tiltable, that is to say can assume various tilted states, with the result that, in a first tilted state, each facet F1 is assigned a specific facet of the second facets F2, and, accordingly, in a different tilted state, is assigned a different facet of the second facets F2. Generally possible is a 1 :n assignment (n being a natural number) between the first facets F1 and the second facets F2, depending on how many states the first facets F1 can assume.

The illumination beam path is composed of many individual illumination channels, wherein an illumination channel extends in each case from the source position or from the intermediate focus FE1 , via a first facet F1 and a second facet F2 that is currently assigned to the first facet, into the illumination field.

In the exemplary embodiment shown of the mirror arrangement, the first facet mirror FAC1 is conjugate to the field plane FE2 and is therefore also referred to as a field facet mirror. By contrast, the second facet mirror FAC2 is conjugate to a pupil plane and is therefore also referred to as a pupil facet mirror.

In the case that the mirror arrangement is used in an illumination system of a projection exposure apparatus, the field plane FE2 is that plane in which the reticle is arranged, the pattern of which is intended to be imaged onto a wafer. In the case that the mirror arrangement SA is used in a mask inspection apparatus, the field plane FE2 is that plane in which the mask to be inspected is arranged.

In the exemplary embodiment of Figure 1 , the illumination system comprises, in addition to a mirror arrangement having two facet mirrors FAC1 and FAC2, which acts as a mixing unit MIX, also the field-forming mirror FFM, which is operated under grazing incidence and is situated between the second facet mirror FAC2 and the exit plane or the object plane of the projection lens. This additional mirror can be favourable for reasons of structural space. In other exemplary embodiments, the illumination system has, in addition to the two facet mirrors FAC1 and FAC2, no further mirrors or else one or more further imaging or non-imaging mirrors in the illumination beam path.

A special feature of the illumination system under consideration here will be explained in greater detail with reference to fig. 3. Fig. 3 shows a schematic plan view of the first facet mirror FAC1 from the side of the reflective first facets F1 , only some of the first facets being shown. The figure furthermore shows the EUV used region of the EUV radiation in the region of the first facet mirror, that is to say a sectional view through the illumination beam path in the region of the first facets. The illuminated region in the field plane is substantially ring-shaped, outwardly delimited by an outer edge RA and inwardly delimited by an inner edge Rl. The inner edge encloses an illumination-free central region MB, that is to say a substantially circular central region, in which no EUV radiation is incident during the use of the illumination system as intended. This central obscuration results from the construction of the source module, in which the primary radiation source and the collector for collecting the primary EUV radiation are situated. Most of the first facets lie completely within the illumination beam path, such that the reflective surfaces are completely illuminated and contribute completely to the intensity in the illumination field. In order to be able to use the highest possible proportion of the primary EUV radiation for the illumination, there are furthermore numerous first facets F1-T which lie only partly in the illumination beam path and are accordingly referred to as“partly illuminated first facets” F1-T or as partially illuminated first facets.

A partly illuminated or partially illuminated first facet is distinguished by the fact that it comprises a first surface section A1 , which lies within the illumination beam path, and a second surface section A2, which lies outside the illumination beam path. The second surface section A2 is thus not reached by the EUV radiation during operation. It nevertheless has the same structural and optical properties as the illuminated first surface section. Examples of EUV illumination systems comprising partially illuminated field facets are disclosed e.g. in US 2008/278704 A1 corresponding to DE 10 2006 036 064 A 1 The disclosure content of these documents in this respect is incorporated by reference in the content of this description.

A special feature of some of the partly illuminated first facets F1-T shall already be pointed out at this juncture. In the case of the partly illuminated first facet F1-T visible at the top left, an alignment marking JM, symbolized by a cross in the case of the example, is situated on the mirror surface that is reflective for EUV radiation outside the illumination beam path, that is to say in the unilluminated region or in the second surface section A2. Further partly illuminated first facets F1-T are situated in the region of the central obscuration. One of these partly illuminated field facets has centrally between its outer edges an alignment marking JM in the unilluminated region, that is to say within the second surface section A2. Since the alignment markings in this case lie outside the region illuminated with EUV radiation, their existence within the illumination field, which is situated in a manner optically conjugate with respect to the field plane of the first facets, is not visible during EUV operation. To put it another way, the illumination intensity distribution in the illumination field is not adversely affected by the existence of the alignment markings since the latter, although indeed lying on reflective surfaces of first facets, lie outside the illumination beam path of the EUV radiation.

In the illumination system ILL from Fig. 1 , all three mirror modules, i.e. the first field facet mirror FAC1 , the second field facet mirror FAC2 and the deflection mirror GM, are in each case replaceable in their entirety. That is to say, after releasing corresponding attachment means, they can be removed from their respective installation positions and be replaced by other components, for example components which are nominally of the same construction, without completely disassembling the illumination system. This has been instituted, inter alia, because it may be possible for the mirror properties to degrade after prolonged irradiation with high-energy EUV radiation to the point where the intended overall performance of the illumination system can no longer be ensured. Replacement of a mirror module may be appropriate for other reasons as well, for example if, in a facet mirror having facets which are resettable by way of actuators, facet resetting fails.

The replacement of a mirror module should be able to be performed within a short period of time and, after the mirror exchange, the illumination system should once again fulfil its desired function. In particular, the position of the illumination field in the exit plane should be situated sufficiently close to its desired position and the radiation should again be incident on the illumination field with the same angle distribution at a given illumination setting as before the mirror exchange.

Since it is not possible, despite narrow manufacturing tolerances, to ensure that the optical performance of the illumination system after replacement of a mirror module systematically corresponds again to the desired performance before the mirror exchange, auxiliary means are provided in the illumination system of the exemplary embodiment that permit the systematic optimization of the mirror positions after installation such that the required optical performance can be achieved within an acceptable time period. The devices make possible a targeted adjustment of the illumination system at the site of its use, that is to say for example at the place of manufacture of semiconductor chips.

For this reason, inter alia, the illumination system of the exemplary embodiment is equipped with components of a measurement system MES which permits optical acquisition of information for determining the orientations of the mirror modules in the respective installation positions that are associated with the mirror modules, with the result that the adjustment can be systematically made on the basis of the measurement values which are obtained by the measurement system. The measurement system MES of the exemplary embodiment has the following components.

A measurement light source module MSM includes a measurement light source MLS for emitting measurement light from the visible spectral range. The measurement light source used can be, for example, a light-emitting diode (LED) or a laser diode. The measurement light source module MSM is arranged at the housing H of the illumination system outside the evacuable interior by way of first interface structures IF1 , can be mounted for measurement purposes and, if needed, removed again and may optionally be used for measurement purposes at a different location. In some embodiments, the position of the measurement light source module with respect to the housing can be changed using positioning drives in terms of multiple axes and both parallel to the central radiation direction and perpendicular thereto.

One exemplary embodiment of a measurement light source module that can be used in the measurement system in Fig. 1 will be explained in greater detail in association with Fig. 4. A primary measurement light source MLS, for example in the form of an LED, is arranged in an entrance plane E1. Arranged within the housing are lens elements L1 , L2 of a 2f imaging system, with which an image of the primary measurement light source MLS, that is to say a secondary measurement light source SMLS, is produced in an exit plane E2, which is optically conjugate to the entrance plane E1. Situated between the entrance plane and the exit plane is a pupil plane PE, which is a plane that is Fourier-transformed in relation to the entrance plane and exit plane. Located in the region of the pupil plane PE is an (optional) stop CS having an aperture MO, through which a selected portion of the measurement light can pass. The position of the aperture MO is freely selectable in two dimensions within the pupil plane. The displaceable stop CS can thus be used to select a specific portion of the measurement light for emission. The site of the through-opening in the pupil plane here determines the angle of incidence of the measurement light, which was allowed to pass, at the site of the secondary measurement light source SMLS and consequently also the emission angle of the measurement light from the measurement light source module. In this way, various individual channels or channel groups of the illumination system can be selected for a measurement.

An embodiment of a measurement light source module, not illustrated pictorially, has a substantially less complex structure; by way of example, it has no two-dimensionally adjustable stop and therefore does not allow a channel-resolved measurement. Such a simple embodiment may suffice, particularly in the case of the relative measurements described here. Therein, it is not necessary to measure the relative orientation of individual channels. It is possible to measure the relative orientation of the superimposition of all channels at one time. Then, it is possible to use a very simple design, for example, an LED in the position of the secondary measurement light source SMLS.

A special feature of the measurement light source module MSM is that the latter generates a larger beam cone (larger cone angle) than the EUV source, with the result that measurement light ML can also propagate outside the EUV illumination beam path and is incident on the second surface sections - not illuminated by EUV radiation - of partially illuminated first facets (cf. dashed lines ML in Fig. 1 and Fig. 3)

A switchable input coupling device IN is provided for coupling measurement light emitted by the measurement light source module MSM into the illumination beam path at an input coupling position upstream of the first facet mirror FAC1. The input coupling device comprises a plane mirror, which serves as the input coupling mirror MIN and which can be panned between a neutral position (illustrated in dashed lines) outside the illumination beam path and the input coupling position (illustrated in solid lines) using an electric drive. In the case of the example, the measurement light source module generates an image of the measurement light source MLS at the site of the source position SP (intermediate focus of the EUV radiation). The input coupling mirror MIN can be panned such that the measurement light beam is coupled into the illumination beam path at the site of the source position SP as if the measurement light source MLS were located at the site of the source position SP. With this arrangement, it is thus possible to imitate or simulate the source beam present in EUV operation by way of measurement light. Situated behind the last mirror module of the illumination beam path, that is to say in the example of Fig. 1 downstream of the deflection mirror GM, in the region between the deflection mirror GM and the exit plane of the illumination system (object plane OS of the projection lens), is a switchable output coupling device OUT for coupling measurement light out of the illumination beam path, wherein the measurement light is coupled out after the measurement light has been reflected at each of the mirror modules of the illumination beam path. The switchable output coupling device comprises a plane mirror, which is used as the output coupling mirror MOUT and which can be panned between the neutral position (illustrated in dashed lines) outside the illumination beam path and the output coupling state (illustrated in solid lines) using an electric drive. In the output coupling state, the output coupling mirror reflects the measurement light coming from the deflection mirror GM in the direction of a detector module position, in which a detector module DET is arranged.

The measurement light source module MSM is designed such that a portion of the measurement light ML can propagate outside the EUV illumination beam path in such a way that measurement light is incident on the second sections A2 - lying outside the EUV illumination beam path - of the partly illuminated first facets F1-T and is reflected by the latter in the direction of the illumination field BF. In Fig. 1 , a beam of the measurement light ML is illustrated in a dashed manner. In Fig. 3, the outer boundary of the measurement light beam path is shown by the dashed line ML. Said beam path also includes the second surface sections A2 of the partially illuminated first facets F1-T at the outer edge of the EUV illumination beam path and in the central region MB.

The detector module DET is embodied as a compact, portable detector module that, when necessary, can be secured to the envisaged detector position for measurement purposes and that can be disassembled again without much outlay in the case of non-use.

With the aid of second interface structures IF2, the removable or interchangeable detector module DET is secured with its detector position being stationary (i.e., immobile or in a fixed position) to the outer side of the housing H of the illumination system. Where necessary, the detector position can be set exactly with the aid of alignment screws or the like (see crossed double-headed arrows). However, there are no electrically controllable positioning drives or positioning drives that are controllable in any other way in order to adjust the position of the detector module in relation to the housing H. Travel axes for displacing the detector module in relation to the housing H of the illumination system and/or travel axes for displacing optical components within the detector module are not provided. One exemplary embodiment will be explained in greater detail in association with Fig. 5. All controllable components of the measurement system MES are connected in signal- transmitting fashion to the control unit SE of the measurement system in the ready-for-operation assembled state of the measurement system. Also situated in the control unit is an evaluation unit for evaluating the measurement values obtained using the measurement light, which measurement values represent the alignment state of the mirror modules within the illumination system.

In order to explain a possible measurement principle, schematic Fig. 6 shows the shape of an illumination field BF that is curved in arcuate fashion (“ring field”), the height of which in the scanning direction (y-direction) is multiple times larger than the width of the illumination field BF in the cross-scan direction (x-direction) perpendicular to the scanning direction. The illumination field that is curved in arcuate fashion can be imagined to be subdivided into a first end section END1 , which can be identified on the left in Fig. 6, a second end section END2, which lies opposite the first end section in the x-direction, and an intermediate portion ZW, lying between the end sections END1 , END2, in which the central plane MT lies, said central plane being a plane of mirror symmetry of the illumination field in the exemplary case. The illumination field is delimited by an edge RD, at which there is, over a short distance, a great intensity decrease between a relatively high illumination intensity within the illumination field and a very low intensity outside the illumination field.

During the operation of the illumination system, the illumination intensity within the intermediate region is relatively constant, while the intensity of the illumination light within the end portions END1 , END2 decreases somewhat in the lateral direction towards the outer lateral edges RD1 , RD2 (i.e., in the x-direction), and so there is in each an intensity gradient in the x-direction adjacent to the outer edges RD1 , RD2 within the illumination field. These intensity gradients result from the above-described superimposition of the images of numerous field facets of the field facet mirror and represent a desired property of the intensity distribution within the illumination field.

In the measurements described here, an interchangeable detector module DET of a measurement system MES is used, wherein the detector module, in principle, can be constructed as illustrated in Fig. 5, for example. The detector module DET comprises a field sensor arrangement FSA, which makes it possible to capture the spatial distribution of the intensity of the illumination field in a rectangular capture region EB (cf. Fig. 6). The latter lies within the intermediate region ZW and includes neither the first end section END1 nor the second end section END2. As a result, possible shading effects in the end regions do not affect the measurement. The position of the capture region EB is chosen such that the imaging JM’ of an alignment marking lies within the capture region. The position of the imaging of the alignment marking within the exit plane can be determined exactly by means of evaluation of the intensity distribution in the capture region EB. This enables conclusions to be drawn about the“field position”, that is to say about the position of the illumination field within the exit plane.

Moreover, a pupil sensor arrangement PSA is integrated; it can be used to capture the spatial distribution of measurement light in a pupil plane that is Fourier-transformed in relation to the exit plane of the illumination system for at least one field point lying outside the capture region EB.

The field sensor arrangement FSA comprises a field sensor FS having a rectangular capture region EB. The field sensor comprises an optoelectronic transducer WD1 that is sensitive to measurement light, and associated electronics. The transducer can comprise a CCD or CMOS sensor, for example, in order to be able to record the intensity distribution in the associated capture region with a sufficiently high spatial resolution. The light-sensitive entrance surface of the field sensor FS lies in the entrance plane EE of the detector module DET.

A stop BL that is non-transmissive to the measurement light, with an aperture BO lying in the entrance plane EE, is arranged next to the transducer WD1. The stop BL belongs to the pupil sensor arrangement PSA. The aperture is embodied as a slit opening that is decentred with respect to the central plane, said slit opening extending completely in the y-direction beyond the edges of the illumination field. A pinhole stop having a circular or square aperture is also possible. The pupil sensor arrangement PSA includes a sensor (pupil sensor PS) with an areal extent, which is set back in relation to the entrance plane EE. A Fourier optical unit FO is arranged between the aperture BO and the light-sensitive entrance surface at the transducer WD2 of the pupil sensor PS, and so the light-sensitive entrance surface of the pupil sensor PS is situated in a plane that is Fourier-transformed in relation to the entrance plane EE. The measurement light coming from the illumination system is guided through the aperture BO in the entrance plane EE and through the Fourier optical unit FO onto the entrance surface of the pupil sensor PS. With the aid of the Fourier optical unit, a pupil imaging is created, by means of which the spatial distribution of the illumination intensity in the pupil plane of the illumination system for a field point in the region of the aperture BO is imaged onto the light-sensitive entrance surface of the pupil sensor PS.

Within the scope of the embodiments described here, it is possible to perform alignment methods which permit the optimization of the mirror position (orientation of a mirror module in its installation position) after installation, such that the required optical performance of the entire illumination system can be reliably achieved. In many method variants, the measurement system is used to capture three system measurement variables or performance measurement variables, specifically:

(i) the position of the illumination field at the reticle level or in the exit plane of the illumination system (corresponding to the object plane OS of the projection lens). This system measurement variable is determined with the aid of the at least one alignment marking.

(ii) the spatial distribution of measurement light in a pupil plane of the illumination system that is Fourier-transformed in relation to the exit plane, said spatial distribution determining the telecentricity at the reticle level or in the exit plane. The pupil sensor arrangement is used for this purpose.

(iii) a luminous spot deposition on pupil facets, i.e. the position of a measurement light spot on a facet of the second facet mirror FAC2.

The benefit of such measurements is described in WO 2017/153165 A1 in the name of the present applicant in association with the use of a moveable detector module with a single field sensor and a single pupil sensor and also in WO 2019/081555 A1 in association with the use of a detector module fitted in a stationary manner. Analogous measurement and evaluation methods can be used in the context of the present invention. With regard to the measurement and evaluation methods, the disclosure content of WO 2017/153165 A1 and of WO 2019/081555 A1 is incorporated by reference in the content of the present description.

Independently of the formulated problem primarily addressed here, that of developing a system measurement technique for mirror exchange which manages without using the lateral field edges, it may be advantageous, in principle to provide some or all regions of field facets that are not illuminated by the EUV radiation with alignment markings designed for example to be absorbing and/or diffractive and/or reflective for visible light or infrared radiation. In the course of a system alignment, the field position of the channels provided with alignment markings can be determined with higher accuracy, which may be advantageous for the system alignment and for specific system tests.

With reference to Figures 7A, 7B, etc., schematically selected configuration possibilities for alignment marking will now be explained.

Fig. 7 A shows one example of a possible configuration of an alignment marking JM fashioned in the form of a binary phase grating having a diffractive effect for measurement light ML from the visible spectral range (VIS). The figure shows a sectional view perpendicularly through a multilayer structure of a first facet F1 , which is reflective for EUV radiation, perpendicular to the reflective mirror surface. The multilayer structure has a multiplicity of successive layers - extending parallel to one another - of high refractive index and, relative thereto, low refractive index material for forming the EUV-reflecting multilayer. In order to form the alignment marking JM, the mirror surface is macroscopically structured and has in each case front regions V and rear regions H, which are aligned parallel to one another and are offset relative to one another by a predefined offset d in the direction of their surface normals. In the case of the example, the front regions V and the rear regions H have identical widths in a direction perpendicular to their surface normals. The width of the front regions V is also referred to as ridge width, and the width of the rear regions H is also referred to as groove width. A multiplicity of such regions are provided in the region of the alignment marking, in such a way that the regions form a grating structure having a grating period p, which is also referred to as grating constant or pitch. In the case of the example, the grating period p is chosen such that the measurement light from the visible spectral range, upon impinging on the alignment marking, is diffracted away from the measurement light beam path leading into the image field, such that the imaging of the alignment marking in the exit plane appears dark. The grating period p can be for example in the range of 1 micrometre to 100 micrometres.

An alignment marking of this type can be applied both outside the illumination beam path that is able to be reached by EUV radiation in a second surface section of a first facet F1 and within the illumination beam path, since in the latter case the grating period of the grating structure in comparison with the wavelength of the EUV radiation is so great that the EUV radiation is reflected practically without being influenced by the alignment marking. Instead of the grating structure having steplike steep transitions between front and rear regions, oblique sidewalls can also be provided in the region of the transitions, which may be advantageous for reasons of producibility, for example. What are crucial here are primarily the large grating periods p of the grating structure in comparison with the EUV radiation wavelength.

Fig. 7B shows a perpendicular sectional view through a first facet F1 in the region of an alignment marking JM having an absorbing effect for the measurement light ML. For this purpose, a coating CT having the outer shape of the alignment marking formed is applied to the multilayer structure of the first facet F1. The coating material and the layer thickness are chosen such that the coating has an absorbing effect for visible light, with the result that at most a very small fraction of the incident measurement light ML is reflected and the remaining portion of said measurement light is absorbed in the alignment marking. Therefore, the imaging of the alignment marking in the exit plane appears dark. Such an alignment marking can be applied for example outside the region illuminated by EUV radiation in a second surface section of the first facet F1. If appropriate, it is also possible to apply such an alignment marking in a first surface section of the first facet. Consideration should then be given to ensuring that the coating material, although having an absorbing effect for measurement light, is as transparent as possible for EUV radiation, such that no image of the alignment marking appears in the exit plane upon illumination with EUV radiation.

Fig. 7C schematically shows one example of an alignment marking JM having a marking structure having a reflective effect for measurement light ML in such a way that a large portion of the measurement light ML incident on the alignment marking is reflected out of the illumination beam path for the measurement light. An alignment marking of this type can be produced for example such that firstly at the envisaged location for the alignment marking, using a spherical or conical milling tool, for example, a small indentation or a spherical recess is milled into the surface of the facet substrate. A customary multilayer coating that is reflective for EUV radiation and has a reflective effect for visible light (measurement light) as well is then applied to the facet. As shown in Fig. 7C, measurement light ML incident on the sidewalls of the indentation that are directed obliquely with respect to the main direction of incidence is reflected out of the beam path for measurement light, such that the image of the alignment marking in the illumination field or in the exit plane appears dark.

Depending on the embodiment of the alignment markings, a light-sensitive diode or some other light-sensitive sensor unit can also be used instead of a two-dimensionally resolving sensor arrangement for determining the field position of these channels. In the example in Fig. 8A, an alignment marking JM is applied in a second section A2 of a partially illuminated field facet F1- T, said second section not being able to be reached by EUV radiation, said alignment marking being designed as a bright point in a square region that appears dark. Fig. 8B schematically shows a corresponding imaging JM‘ of the alignment marking in the exit plane of the illumination system. The field sensor arrangement here comprises a 4 quadrant diode 4Q having four light- sensitive square sensor fields Q1 , Q2, Q3, Q4 that can be evaluated independently of one another. A faster measurement with higher temporal resolution can thereby be carried out in comparison with measurement using a camera.

Claims

Patent claims

1. Illumination system for an EUV apparatus, which illumination system is configured to receive, during operation of the EUV apparatus, EUV radiation (LR) of an EUV radiation source (LS) at a source position in an entrance plane and to shape, from at least one portion of the received EUV radiation, illumination radiation that is directed into an illumination field (BF) in an exit plane (ES) of the illumination system and in the illumination field fulfils an illumination specification, wherein:

the illumination system (ILL) comprises a plurality of mirror modules which are installed at assigned installation positions and which define an illumination beam path leading from the source position to the illumination field;

the mirror modules comprise a first facet mirror (FAC1) and a second facet mirror (FAC2),

the first facet mirror (FAC1) comprises first facets (F1) configured for reflecting EUV radiation and arranged in or near a field plane of the illumination system, said field plane being optically conjugate with respect to the plane of the illumination field (BF), and the second facet mirror (FAC2) comprises second facets (F2) configured for reflecting EUV radiation and arranged in (or near) a pupil plane of the illumination system, characterized in that

at least one of the first facets (F1) comprises at least one alignment marking (JM) which, during use of the illumination system as intended, is not able to be imaged into the illumination field by the EUV radiation and is able to be imaged into the illumination field upon incidence of measurement light originating from a different wavelength range than the EUV radiation.

2. Illumination system according to Claim 1 , characterized in that the alignment marking is designed for measurement light whose wavelength is in the visible spectral range (VIS) or in the adjacent UV spectral range or IR spectral range.

3. Illumination system according to Claim 1 or 2, characterized in that the first facet mirror (FAC1) comprises one or a plurality of partly illuminated first facets (F1-T) comprising a first surface section (A1), which lies within the illumination beam path, and a second surface section (A2), which lies outside the illumination beam path, wherein at least one alignment marking is arranged in the second surface section (A2).

4. Illumination system according to Claim 3, characterized in that at least one partly illuminated first facet (F1-T) bearing an alignment marking (JM) lies in the outer edge region of the illuminated region at the first facet mirror (FAC1), such that the first facet bearing the alignment marking (JM) partly projects out of the region of the illumination beam path towards the outside,

and/or

in that the illumination beam path has in the field plane a ring shape with an illumination- free central region, there exists at least one partly illuminated first facet (F1-T) comprising a first surface section (A1), which lies within the illumination beam path, and a second surface section (A2), which lies in the central region, and at least one alignment marking (JM) is arranged in the second surface section (A2).

5. illumination system according to any of the preceding claims, characterized in that at least one alignment marking comprises a marking structure having an absorbing effect for the measurement light, and/or in that at least one alignment marking comprises a marking structure having a diffractive effect for the measurement light and/or at least one alignment marking comprises a marking structure having a reflective surface inclined in comparison with the surrounding mirror surface of the first facet (F1) in such a way that measurement light incident on the reflective surface is reflected out of the beam path of the measurement light.

6. Illumination system according to any of the preceding claims, characterized in that at least one alignment marking (JM) is arranged in that region of a first facet (F1) which is reached by the EUV radiation during operation, wherein the alignment marking is configured in such a way that no imaging of the alignment marking is able to be generated in the illumination field by means of the EUV radiation.

7. Illumination system according to Claim 6, characterized in that at least one alignment marking (JM) comprises a marking structure having a reflective effect for EUV radiation in substantially exactly the same way as a region in direct proximity, but outside the alignment marking, and having an absorbing effect for the measurement light, wherein preferably the alignment marking comprises a coating which is substantially transparent to EUV radiation but absorbs measurement light in such a way that an imaging of the alignment marking arises in the illumination field if measurement light is radiated onto the region of the alignment marking.

8. Illumination system according to Claim 6 or 7, characterized in that at least one alignment marking (JM) comprises a marking structure embodied as a grating structure, wherein a typical structure size of the grating structure, in comparison with the wavelength of the EUV radiation, is so macroscopic that no diffraction of the EUV radiation takes place, wherein the structure size is dimensioned such that a diffractive effect is present for the measurement light, wherein preferably the marking structure is designed for the use of measurement light from the visible wavelength range (VIS) and comprises grating structure widths in the range of 1 pm to 100 pm.

9. Illumination system according to any of the preceding claims, characterized in that at the first facet mirror (FAC1) two or more mutually identical or different alignment markings (JM) are applied at different locations, in particular at different first facets.

10. Illumination system according to any of the preceding claims, characterized in that an alignment marking (JM) is arranged at a distance from the lateral edges of the first facet outside a first end section (END1) and an opposite second end section (END2) of the first facet (F1), such that an imaging (JM‘) of said alignment marking does not lie in the vicinity of a lateral field edge of the illumination field (BF).

11. Method for restoring an illumination system installed in an EUV apparatus, said illumination system being embodied according to any of Claims 1 to 10,

wherein the method comprises a swap operation of one of the mirror modules, in which the mirror module is disassembled from its installed position and removed from the illumination system, a mirror module with nominally the same design is installed in the installation position in place of the removed mirror module and the installed mirror module is adjusted in rigid body degrees of freedom in the installation position while changing the relative orientation of the installed mirror module;

wherein, before the swap operation, a reference measurement is performed in order to capture a reference state that represents the adjustment state before the start of the swap operation and wherein, after the swap operation has finished, a comparison measurement is performed and the adjustment state measured in the process is compared to the reference state in order to restore the adjustment state from before the swap operation,

characterized in that

the position of an image (JM‘) of at least one alignment marking (JM) in the illumination field (BF) is measured during the reference measurement and the comparison measurement.

12. Method according to Claim 11 , characterized in that measurement light is used which originates from a different wavelength range than the EUV radiation which is utilized during the intended use of the illumination system, wherein a wavelength range of the measurement light is preferably in the visible spectral range or the adjacent UV spectral range or IR spectral range.

13. Method according to Claim 11 or 12, characterized in that measurement light is radiated in such that it propagates at least partly outside the illumination beam path of the EUV radiation.

14. Detector module (DET) for a measurement system for measuring system measurement variables in an illumination field (BF) in an exit plane of an illumination system of an EUV apparatus, comprising:

a field sensor arrangement (FSA) for capturing the positions of an image (JM‘) of an alignment marking (JM), wherein a capture region (EB) of the field sensor arrangement is arranged such that it lies outside a first end section (END1) and a second end section (END2), opposite the first end section, of the illumination field (BF), and

a pupil sensor arrangement (PSA) for capturing a spatial distribution of measurement light in a pupil plane that is Fourier-transformed in relation to the exit plane for a field point lying outside the capture region of the field sensor arrangement (FSA).

15. Mirror module comprising a first facet mirror (FAC1) for use in an illumination system of an EUV apparatus, wherein the illumination system is configured to receive, during operation of the EUV apparatus, EUV radiation (LR) of an EUV radiation source (LS) at a source position in an entrance plane and to shape, from at least one portion of the received EUV radiation, illumination radiation that is directed into an illumination field (BF) in an exit plane (ES) of the illumination system and in the illumination field fulfils an illumination specification,

wherein the first facet mirror (FAC1) comprises first facets (F1) configured for reflecting EUV radiation and arrangeable in or near a field plane of the illumination system, said field plane being optically conjugate with respect to the plane of the illumination field (BF),

characterized in that

at least one of the first facets (F1) comprises at least one alignment marking (JM) which, during use of the illumination system as intended, is not able to be imaged into the illumination field (BF) by the EUV radiation and is able to be imaged into the illumination field upon incidence of measurement light originating from a different wavelength range than the EUV radiation.