Optical hollow waveguide assembly, illumination optical unit, and inspection apparatus

Abstract

To provide an optical hollow waveguide assembly.SOLUTION: The optical hollow wave guide assembly (11b) comprises an optical hollow wave guide (11a) for guiding the illumination light (3) with a body (21) with a continuous wave guide cavity (22) with wave guide inputs (12) and wave guide outputs (14), wherein the wave guide cavity inner walls of the wave guide cavity (22) are designed to be highly reflective for the illumination light (3) at least under grazing incidence. The hollow wave guide assembly (11b) further comprises wave guide purging devices (26, 34, 42) for purging the wave guide cavity (22) with a purge gas (29), comprising purge gas connectors (27, 35, 43, 54) and at least one nozzle (28) arranged fluidically downstream of the purge gas connectors (27) for supplying the purge gas (29) at least indirectly to the wave guide outputs (14) for generating a purge gas flow from the wave guide outputs (14) to the wave guide inputs (12).SELECTED DRAWING: Figure 2

Description

[Technical Field]

[0001] The content of German Patent Application No. 10 2024 207 074.2 is incorporated herein by reference.

[0002] The present invention relates to an optical hollow waveguide assembly, an illumination optical unit comprising such an optical hollow waveguide assembly, and an inspection device comprising such an illumination optical unit. [Background technology]

[0003] A hollow waveguide assembly is known, for example, from WO 2015 / 003903. [Prior art documents] [Patent documents]

[0004] [Patent Document 1] International Publication No. 2015 / 003903 Summary of the Invention

[0005] One problem addressed by the present invention is to improve optical hollow waveguide assemblies, in particular to provide optical hollow waveguide assemblies that have reliable purging and that are optimized in particular with regard to the utilization of installation space within an illumination optical unit.

[0006] This problem is solved by an optical hollow waveguide assembly having the features set out in claim 1.

[0007] According to the invention, it has been recognized that by generating a purge gas flow, in particular a continuous purge gas flow, from the waveguide output to the waveguide input, it is possible to provide an optical hollow waveguide assembly with a particularly compact design, the arrangement of further components of the hollow waveguide assembly and / or of the optical system comprising the hollow waveguide assembly being hardly, in particular not at all, affected adversely.

[0008] A further purge gas connector in the body, via which purge gas is supplied to the waveguide cavity, is omitted as a result of which a purge gas flow is generated from the waveguide output to the waveguide input. The hollow waveguide body can be particularly simply manufactured as a result.

[0009] Moreover, further openings in the waveguide cavity, through which illumination light may escape and / or through which additional contaminants may enter the waveguide cavity, are omitted. The waveguide cavity, in particular its walls and / or wall coatings, can be manufactured with particular precision.

[0010] At the same time, unidirectional purging of the waveguide cavity from the waveguide output to the waveguide input has been found to be sufficient to reliably clean the waveguide cavity and protect it from contamination.

[0011] Contamination can enter the waveguide cavity from other components of the illumination optical unit, for example from the EUV source, through the waveguide output and / or the waveguide input, thereby contaminating the waveguide cavity.

[0012] These contaminations can be eliminated by purging, in particular by continuous purging, using a purge gas, in particular in this case hydrogen gas can be used as the purge gas.

[0013] In particular, it is possible to purge, in particular continuously purge, the waveguide cavity during operation of the upper inspection device, whereby hydrogen gas can interact with the illumination light, in particular DUV or EUV light, supplied to the waveguide cavity, thereby generating free charge carriers that remove contamination.

[0014] The waveguide cavity is purged with a purge gas flowing counter to the flow. This should be understood to mean that the propagation direction of the illumination light, in particular the chief ray of the illumination light, is oriented opposite, in particular antiparallel, to the flow direction of the purge gas. Antiparallel orientation of two directions should be understood to mean that the scalar product of the normalized direction vector of the first direction and the normalized direction vector of the second direction is equal to −1.

[0015] The hollow waveguide can be specifically designed to homogenize the illumination light. For example, the integrity of the hollow waveguide is of great importance to the field of mask inspection, since inspection of photolithography masks and / or wafers can be performed particularly accurately if the illumination light is as homogenous as possible. Maintaining this integrity is possible with the hollow waveguide assembly according to the invention, along with optimal utilization of the available installation space.

[0016] The hollow waveguide, in particular the waveguide cavity, may extend in particular linearly, preferably along a central longitudinal axis of the hollow waveguide, in particular of the waveguide cavity. The central longitudinal axis of the hollow waveguide may in particular be parallel to the optical axis of the upper illumination optical unit.

[0017] The hollow waveguide, in particular the waveguide cavity, may have a length in the range of 10 mm to 500 mm, for example, 20 mm to 500 mm, 20 mm to 300 mm, or otherwise in the range of 20 mm to 80 mm. The length may in particular be the distance between the waveguide input and the waveguide output in the direction of the central longitudinal axis.

[0018] The ratio of the length of the waveguide cavity perpendicular to its central longitudinal axis to its diameter may in particular be in the range of 10-1000, for example 10-500, 30-500, 30-300, 30-80, or 200-500.

[0019] The cross section of the waveguide cavity may in particular be polygonal, for example rectangular or square. The size and / or shape of the cross section of the waveguide cavity perpendicular to the central longitudinal axis may preferably be constant along the central longitudinal axis, i.e. in particular from the waveguide input to the waveguide output.

[0020] The body of the hollow waveguide may surround the waveguide cavity, particularly circumferentially, in a direction perpendicular to the central longitudinal axis of the hollow waveguide.

[0021] The body of the hollow waveguide may be particularly rigid. The body may be formed in one piece or in multiple pieces. The body may, for example, consist of glass and / or metal. The body may preferably be made of an EUV-reflective material, for example ruthenium and / or molybdenum, and / or may be coated with an EUV-reflective material in the region of the cavity inner wall.

[0022] The hollow waveguide assembly may in particular comprise a cover at least partially enclosing the hollow waveguide. The cover may in particular serve to stabilize and / or protect the body. The waveguide purging device may be at least partially integrated into the cover and / or formed between the cover and other components of the hollow waveguide assembly, for example between the cover and an illumination light guide arranged downstream of the hollow waveguide.

[0023] For purging, particularly continuous purging, of the waveguide cavity, a purge gas is at least indirectly supplied to the waveguide output via the purging device, and may in particular be provided by an external purge gas source fluidly connected to a purge gas connector of the purging device.

[0024] The purge gas is at least indirectly supplied to the waveguide output by using a nozzle. Indirect supply here should be understood to mean that the purge gas from the nozzle does not flow directly into the waveguide output or into the waveguide cavity. The purge gas may flow indirectly, for example, due to the fact that further flow-effective components may be fluidly arranged between the nozzle and the waveguide output. For example, the pressure and / or flow rate of the purge gas in the waveguide cavity may be influenced, and in particular controlled, by the further flow-effective components.

[0025] The nozzle may in particular be formed as a rotationally symmetrical component part. The nozzle may in particular be a ring nozzle.

[0026] The optical hollow waveguide according to claim 2 is of particular practical importance. EUV sources can lead to contamination of hollow waveguides, which can be reliably removed by using a waveguide purging device. The EUV range may correspond in particular to a wavelength range between 2 nm and 30 nm, for example between 2.3 nm and 4.4 nm or between 5 nm and 30 nm, for example 13.5 nm. Particularly high demands are placed on optical components, particularly for guiding and reflecting illumination light, in this wavelength range.

[0027] In particular, the inner walls of the waveguide cavity may be designed to be reflective, in particular highly reflective, to the illumination light, at least under grazing incidence, where in particular the inner walls can be coated with a coating that is highly reflective to light in the EUV wavelength range, for example made of ruthenium.

[0028] An optical hollow waveguide assembly according to claim 3 allows the pressure and / or velocity of the purge gas in the waveguide cavity to be set particularly precisely.

[0029] In the purge gas connector, the purge gas has a relatively high pressure and / or a relatively high velocity. Hereinafter, the pressure at which the purge gas is provided through the purge gas connector is also referred to as a first pressure. Hereinafter, the velocity at which the purge gas is provided through the purge gas connector is also referred to as a first velocity. In this context, velocity should be understood to mean the flow rate of the gas.

[0030] The first pressure of the purge gas may be in particular in the range of 20 Pa to 70 Pa, in particular in the range of 25 Pa to 60 Pa, and in particular in the range of 30 Pa to 50 Pa.

[0031] The first speed may in particular be in the range of 700 m / s to 1000 m / s, in particular 750 m / s to 950 m / s, and in particular 800 m / s to 900 m / s.

[0032] The first pressure and / or the first velocity may result in significant eddies in the purge gas and / or mechanical loading, in particular damage, of the optical component parts, in particular the hollow waveguides and their bodies.

[0033] By using a pressure reducer, the first pressure and / or first velocity can be intentionally reduced to a second pressure and / or second velocity at which undesirable vortices can be avoided at least to the greatest extent possible and at which the kinetic energy of the purge gas does not become significant.

[0034] The second pressure may in particular be in the range of 10 Pa to 30 Pa, in particular in the range of 12 Pa to 28 Pa, and in particular in the range of 15 Pa to 25 Pa.

[0035] The second speed may in particular be in the range of 10 m / s to 40 m / s, in particular 15 m / s to 35 m / s, and in particular 20 m / s to 30 m / s.

[0036] In particular, a third pressure and a third velocity of the purge gas may be established within the waveguide cavity.

[0037] In this case, the third pressure may be in the range of, in particular, 8 Pa to 17 Pa, in particular 9 Pa to 16 Pa, and in particular 10 Pa to 15 Pa.

[0038] The third speed may in particular be in the range of 3 m / s to 12 m / s, in particular 4 m / s to 11 m / s, and in particular 5 m / s to 10 m / s.

[0039] The third velocity is directed specifically towards the waveguide input so that the purge gas propagates at least substantially through the waveguide cavity towards the waveguide input.

[0040] At the waveguide input, the purge gas may have, inter alia, a fourth pressure and / or a fourth velocity.

[0041] In this case, the fourth pressure is in particular less than 15 Pa, in particular less than 13 Pa, and in particular less than 10 Pa.

[0042] This increases the pressure difference between the second pressure and the fourth pressure, which may in particular be in the range of 8 Pa to 17 Pa, in particular 9 Pa to 16 Pa, and in particular 10 Pa to 15 Pa. This pressure difference draws the purge gas further in the direction of the waveguide input.

[0043] The fourth speed may be in the range of 2 m / s to 8 m / s, in particular 3 m / s to 7 m / s, and in particular 4 m / s to 6 m / s.

[0044] The optical hollow waveguide assembly according to claim 4 is particularly easy to realize. By using a pressure chamber, the purge gas can be accumulated so that the second pressure and / or second velocity can be set according to the Bernoulli equation and / or the continuity equation. The pressure chamber is simple to realize in terms of production technology.

[0045] The pressure chamber may be located immediately downstream of the purge gas connector.

[0046] It is also possible for the pressure chamber to be located indirectly downstream of the purge gas connector, in which case the purge gas connector may be fluidly connected to the pressure chamber, for example, by using an additional purge gas line.

[0047] The pressure chamber may in particular be formed in a ring shape around the beam path of the illumination light, which allows the purge gas to be evenly distributed and adapted in terms of pressure and / or velocity, even when supplied unidirectionally via a purge gas connector.

[0048] By using an optical hollow waveguide assembly as claimed in claim 5, the second pressure and / or the second velocity can be set even more accurately.

[0049] The intermediate gap may in particular have a cross-sectional area that is smaller than the cross-sectional area of ​​the pressure chamber. A cross-sectional narrowing may be formed at the transition from the pressure chamber to the intermediate gap.

[0050] An enlarged cross-sectional area may be formed at the transition from the intermediate gap to the nozzle.

[0051] The optical hollow waveguide assembly according to claim 6 is firstly particularly robust and secondly particularly compact. By using a cover, the hollow waveguide can be particularly well shielded from the surroundings. As a result, adverse environmental influences on the hollow waveguide can be shielded.

[0052] The cover is particularly designed to enclose the lateral sides of the hollow waveguide. The cover may be particularly designed to be fluid-tight at least in the region of the waveguide output. This improves the dynamic pressure setting and thus the targeted guidance of the purge gas flow. The waveguide input may be particularly formed on the bottom surface of the hollow waveguide. The waveguide output may be particularly formed on the top surface of the hollow waveguide.

[0053] It is also possible for the cover to at least partially surround the top surface of the hollow core waveguide.

[0054] As a result of the at least partial integration of the hollow waveguide purging device, the optical hollow waveguide assembly has a particularly efficient design in terms of installation space due to the spatial synergy between the cover and the hollow waveguide purging device.

[0055] The waveguide purging device may in particular be completely integrated into the cover. The hollow waveguide purging device may in particular form part of the cover in the region of the waveguide output.

[0056] The optical hollow waveguide assembly according to claim 7 can be particularly efficiently integrated into higher-level optical systems, such as, for example, an illumination optical unit. By using the illumination light guide, the illumination light can be deliberately transmitted to further components of the illumination optical unit, in particular to an output-coupling optical unit.

[0057] By using an illumination light guide, the illumination light can be guided in a way that protects it from external influences. It is also possible to intentionally influence the shape and / or direction of the illumination light, in particular the wavefront of the illumination light. This can further contribute to an efficient design of the optical hollow waveguide assembly in terms of installation space.

[0058] The illumination light guide may in particular comprise a conductor base by means of which the illumination light guide is arranged, in particular fixed, on the hollow waveguide. A conductor input may be arranged on the conductor base, and illumination light enters the illumination light guide at said conductor input. The conductor input may in particular be arranged coplanar with the waveguide output.

[0059] The illumination light guide may in particular comprise a hollow, for example tubular, conductor body, which may influence the shape and / or direction of the illumination light.

[0060] The illumination light guide may in particular comprise a conductor cover plate which may be arranged opposite the conductor base in relation to the conductor body, and a conductor output may be formed in the conductor cover plate, and the illumination light emerges from said conductor output.

[0061] The conductor body, the conductor base and / or the conductor cover plate may in particular be integrally formed.

[0062] The illumination light guide is specifically designed to guide illumination light in the EUV range.

[0063] The optical hollow waveguide assembly according to claim 8 is embodied in a particularly compact manner. By at least partially integrating the waveguide purging device into the illumination light guide, installation space can be saved.

[0064] The waveguide purging device may in particular be arranged on the conductor base. The waveguide purging device may in particular be arranged to be at least partially integrated into the conductor base.

[0065] The waveguide purging device may be fully integrated into the illumination light guide, in particular into the conductor base. In such a case, the illumination light guide, in particular the conductor base, may be at least partially, in particular fully, integrated into the cover. The illumination light guide may in particular form part of the cover, in particular the part of the cover in the region of the waveguide output.

[0066] By using an optical hollow waveguide assembly as claimed in claim 9, a purge gas can be used to substantially, in particular completely, purge the waveguide cavity. By using a nozzle, the flow properties, in particular the pressure and / or velocity, of the purge gas in the waveguide cavity can be intentionally influenced.

[0067] Since the illumination light is transmitted to further components of the illumination optical unit after passing through the waveguide cavity, in particular by using an illumination light guide, the area around the waveguide output cannot be made completely fluid-tight to the surroundings, in particular to the illumination light guide.

[0068] In order to avoid diffusion losses of the purge gas to the greatest possible extent, and in particular completely, the purge gas is introduced directly into the waveguide cavity via the waveguide output.

[0069] By using an optical hollow waveguide assembly according to claim 10, the second pressure and / or the second velocity can be established particularly precisely. The purge gas is supplied to the waveguide output indirectly via the accumulation chamber. Any adverse effects on the hollow waveguide cavity and / or the hollow waveguide output and / or the beam path of the illumination light as a result of the direct introduction of the purge gas are minimized.

[0070] As a result of introducing the purge gas into the accumulation chamber, intentional turbulence can be introduced into the accumulation chamber. The turbulence can be introduced particularly near the waveguide output. This can particularly result in an accumulation point coinciding with the waveguide output where the purge gas has a desired pressure (second pressure) and / or a desired velocity (second velocity). Because this turbulence can be intentionally controlled, the second pressure and / or second velocity can therefore be precisely settable.

[0071] The accumulation chamber may in particular be formed between the cover and the hollow waveguide.

[0072] The accumulation chamber may in particular be formed between the illumination light guide, in particular the conductor base, and the hollow waveguide.

[0073] The optical hollow waveguide assembly according to claim 11 further contributes to optimal use of purge gas.

[0074] By using a plurality of nozzle openings arranged symmetrically with respect to the waveguide output, the velocity component extending perpendicular to the central longitudinal axis of the waveguide cavity when introduced directly into the waveguide cavity can be reduced, in particular settled to an average value, which results in a purge gas that exhibits a particularly laminar flow and can be substantially, in particular completely, used.

[0075] Even when introducing into an accumulation chamber, multiple nozzle openings have proven advantageous, as this allows for more precise control of the turbulence around the accumulation point.

[0076] By using an optical hollow waveguide assembly as claimed in claim 12, the purge gas utilization can be further optimized. A vacuum generator assists the purge gas flow from the waveguide output to the waveguide input. The surroundings of the waveguide input can be evacuated by the vacuum generator. In particular, a vacuum can be created in the region of the waveguide input.

[0077] The vacuum generator may serve to evacuate higher level optics such as an illumination optical unit.

[0078] For example, a vacuum pump may be used as the reduced pressure generator.

[0079] A further problem addressed by the present invention is that of improving the illumination optical unit.

[0080] This problem is solved by an illumination optical unit having the features set out in claim 13.

[0081] The illumination optical unit may comprise further optical components and optics, such as a diaphragm and mirrors, to focus the illumination light onto the object in the object plane.

[0082] Of particular practical importance is an illumination optical unit as claimed in claim 14. The illumination optical unit may in particular be used to prove masks for lithography, in particular microlithography or nanolithography. The masks may be proved in the context of a lithographic process and / or in the context of mask inspection.

[0083] A further problem addressed by the present invention is that of improving inspection equipment.

[0084] This problem is solved by an inspection device having the features presented in claim 15.

[0085] In addition to the illumination optical unit, the inspection apparatus may also include a projection optical unit that converges illumination light reflected by an object in the object field, particularly in the EUV range, into the image field to generate an image of the object in the image field.

[0086] The object to be inspected may in particular be a photolithography mask and / or a photolithography wafer. The mask and / or the wafer may be structured or unstructured.

[0087] Exemplary embodiments of the invention are described in more detail below with reference to the accompanying drawings. [Brief explanation of the drawings]

[0088] [Figure 1] 1 is a schematic diagram illustrating an optical system having an illumination optical unit for a mask inspection system for use with EUV illumination light. [Figure 2] 1 is a schematic cross-sectional view of a first exemplary embodiment of a hollow waveguide assembly having a waveguide purging device and an illumination light guide. [Figure 3] 10 is a schematic cross-sectional view of a further exemplary embodiment of a hollow waveguide assembly having a waveguide purging device and an illumination light guide. [Figure 4]10 is a schematic cross-sectional view of a third exemplary embodiment of a hollow waveguide assembly having a waveguide purging device and an illumination light guide. [Figure 5] 1 is a perspective view of an exemplary embodiment of an illumination light guide. [Figure 6] 6 is a cross-sectional view of the illumination light guide according to FIG. 5; [Figure 7] 6 is a schematic diagram of a conductor input of the illumination light guide according to FIG. 5; DETAILED DESCRIPTION OF THE INVENTION

[0089] The illumination optical unit 1 is a component of an illumination system 2 of a mask inspection system for use with EUV illumination light 3. In the drawing, the beam path of the illumination light 3 is indicated by marginal rays. An illumination field or object field 4 of the mask inspection system is illuminated by the illumination light 3.

[0090] The illumination light 3 is generated by an EUV light source 5 in a source region or source volume 6. The light source 5 may generate EUV radiation within a wavelength range of 2 nm to 30 nm, for example in the range of 2.3 nm to 4.4 nm or in the range of 5 nm to 30 nm, for example 13.5 nm.

[0091] The light source 5 may be embodied as a plasma light source. For example, the light source can be a laser plasma source (LPP; Laser Produced Plasma) or else a discharge source (DPP; Discharge Produced Plasma). It is also possible to use harmonic EUV sources. In principle, such plasma sources are known light sources of EUV projection exposure apparatus.

[0092] For ease of orientation, a Cartesian xyz coordinate system is used hereafter. The x-axis is perpendicular to the drawing surface of Figure 1. The y-axis extends horizontally to the right in Figure 1, and the z-axis extends vertically upwards in Figure 1.

[0093] The source region 6 has a generally elliptical shape and has a maximum extent, also referred to as the main extension direction, parallel to the y-axis. The main emission direction of the illumination light 3 from the source region 6 extends along this main extension direction, i.e., along the longest axis of the elliptical source region 6 when approximating an ellipse.

[0094] After emission of illumination light by the light source 5 , the illumination light 3 first passes through an aperture stop 9 which bounds the end of the beam of illumination light 3 .

[0095] The aperture stop 9 can be designed to be exchangeable. For example, an aperture wheel may be provided for this purpose, which stores various aperture stop embodiments that can be used alternately in the beam path of the illumination light 3. Different input apertures of the illumination light 3 can be specified by such an exchangeable aperture stop design.

[0096] The aperture stop 9 may be embodied so as to be exchangeable and / or adjustable and / or configurable with respect to its diaphragm edge. As a result, different diaphragm geometries of the aperture stop 9 can be realized and / or configured. For example, the specifiable diaphragm geometries may be circles with selectable diameters and / or ellipses with selectable ellipse sizes and, optionally, selectable ellipse semiaxial ratios. Such an ellipse semiaxial ratio specifiable by the aperture stop 9 may be 2:1.

[0097] Downstream of the aperture stop 9, the illumination light beam 3 is transmitted from an input coupling mirror 10 of the illumination optical unit 1 to a beam homogenizing device 11. As will be explained in more detail below, the input coupling mirror 10 may also be part of the beam homogenizing device 11. A beam homogenizing element, for example, in this case, a hollow waveguide 11a, may also be part of the beam homogenizing device 11. In other exemplary embodiments, the beam homogenizing device 11 may instead or in addition also comprise at least one facet mirror that serves to split the EUV illumination light 3 into multiple individual beams that are superimposed on one another for the purpose of homogenized mixing. In this case, the beam homogenizing device may also comprise, for example, two facet mirrors arranged in series.

[0098] The aperture stop 9 constrains the numerical aperture of the illumination light beam 3 emitted by the source region 6 to a numerical aperture value in the range of 0.02 to 0.3, for example, in the range of 0.02 to 0.1 or 0.05 to 0.08. A numerical aperture greater than 0.1, i.e., in the range of 0.1 to 0.3, as specified by the aperture stop 9, allows for a greater light yield in the illumination light beam path between the source volume 6 and the illuminated field 4.

[0099] An incoherent illumination setting may be used.

[0100] As an alternative to or in addition to the aperture stop 9, an aperture-limiting stop may be arranged between the hollow waveguide 11a and a downstream optical component of the illumination optical unit 1. It is also possible that such a further aperture stop is arranged in the beam path of the illumination light 3, downstream of the hollow waveguide 11a, between two downstream optical components of the illumination optical unit 1.

[0101] For example, the input coupling mirror 10 is embodied as an ellipsoidal mirror, which serves to image the source region 6 of the EUV light source 5 onto the waveguide input 12 in the entrance face 13 of the hollow waveguide 11a. Thus, a first focal point of the ellipsoidal mirror 10 is located within the source region 6, and a second focal point of the ellipsoidal mirror 10 is located within or in the region of the waveguide input 12. The ellipsoidal mirror 10 is used to focus the illumination light beam 3 onto the waveguide input 12 in the entrance face 13 of the hollow waveguide 11a. The entrance numerical aperture of the illumination light beam 3 when it enters the waveguide input 12 may be in the range of 0.02 to 0.2, and may be, for example, about 0.15, or about 0.05, or about 0.1.

[0102] In the embodiment of the illumination optical unit 1 according to FIG. 1, the ellipsoidal mirror 10 represents a mirror for grazing incidence (GI).

[0103] Depending on the embodiment of the input coupling optical unit, the input coupling optical unit may have exactly one input coupling mirror as depicted in FIG. 1, using the example of input coupling mirror 10, or may otherwise have multiple input coupling mirrors, for example two or three input coupling mirrors.

[0104] The waveguide input 12 and waveguide output 14 of the hollow waveguide 11a are in either a square or rectangular shape, with typical diameters ranging from 0.5 mm to 5 mm. The aspect ratio of the waveguide input 12 of the hollow waveguide 11a and the identically sized waveguide output 14 for the illumination light 3 in the exit face 15 is 0.25 to 4, for example 0.5 to 2. Typical dimensions of the waveguide input 12 and waveguide output 14 of the hollow waveguide 11a are 0.75 mm x 0.75 mm, 1.0 mm x 2.0 mm, or 1.5 mm x 2.0 mm.

[0105] The inner wall of the waveguide cavity of hollow waveguide 11a is provided with a highly reflective coating, for example a ruthenium coating, for the illumination light 3. The waveguide cavity is a rectangular parallelepiped according to the rectangular waveguide input 12 and waveguide output 14. The hollow waveguide 11a has a typical length in the beam direction of the illumination light 3 in the range of 10 to 500 mm, for example, 20 mm to 500 mm, 20 mm to 300 mm, or otherwise in the range of 20 mm to 80 mm.

[0106] The angle of incidence of the illumination light 3 on the inner wall of the waveguide cavity of the hollow waveguide 11a is, for example, greater than 60°. The illumination light 3 impinges on the inner wall at a small angle of incidence.

[0107] The angle between the longitudinal axis of the hollow waveguide 11a and the chief ray of the illumination light beam 3 incident on the waveguide input 12 may be 0° or, alternatively, may be different from 0°, for example in the range of 0° to 1.5°, for example 0.25° to 0.75°, in particular around 0.5°.

[0108] The ratio of the length of the hollow waveguide 11a, i.e., the distance between the entrance face 13 and the exit face 15, to the typical diameter of the hollow waveguide 11a, i.e., the typical dimension or typical diameter of the waveguide input 12 or the waveguide output 14, is in the range of 10 to 1000, and may be, for example, 10 to 500, 30 to 500, 30 to 300, or otherwise 30 to 80 or 200 to 500.

[0109] Such hollow waveguides 11a may be exposed to contamination as a result of material removal and / or external contamination. A purge gas device (not depicted in FIG. 1) serves to clean the hollow waveguides 11a and will be described in detail below on the basis of exemplary embodiment variants with reference to FIGS. 2 to 4.

[0110] The illumination light 3 can be transmitted to the upper optics by using an illumination light guide (not depicted in FIG. 1). Exemplary illumination light guides are described in detail below with reference to FIGS. 5-7.

[0111] An imaging output coupling mirror optical unit 16, depicted schematically in Figure 1 and positioned downstream of hollow waveguide 11a, images waveguide output 14, located in exit face 15 of hollow waveguide 11a, onto an illuminated field 4 in object plane 17. This imaging may have an image-side numerical aperture in the range of 0.1 to 0.3.

[0112] For example, two or more mirrors of the output-coupling mirror optical unit 16 may be embodied as mirrors for small angle incidence of the illumination light 3 .

[0113] The above-mentioned optional aperture stop downstream of the hollow waveguide 11a may be arranged between the hollow waveguide 11a and the first mirror of the output coupling mirror optical unit 16, or else between different mirrors of the output coupling mirror optical unit 16.

[0114] The output-coupling mirror optical unit 16 may be embodied in the style of a Wolter telescope, in particular in the style of a Type I Wolter optical unit. Such a Wolter optical unit is described in JD Mangus and JH Underwood, "Optical Design of a Glancing Incidence X-ray Telescope," Applied Optics, Vol. 8, 1969, p. 95, and references cited therein. In such a Wolter optical unit, a hyperbolic surface may also be used instead of a parabolic surface. Such a combination of an elliptical mirror and a hyperbolic mirror also constitutes a Type I Wolter optical unit.

[0115] Further exemplary embodiments of the output-coupling mirror optical unit 16 are described in U.S. Patent No. 10,042,248. Alternatively, the mirrors of the output-coupling mirror optical unit 16 may also include reflective surfaces in the form of freeform surfaces.

[0116] A reticle 18 to be inspected, held by a reticle holder 19, is arranged in the object plane 17. The reticle holder 19 is mechanically operatively connected to a reticle displacement drive 20, by means of which the reticle 18 is displaced in an object displacement direction y during mask inspection. In this way, a scanning displacement of the reticle 18 in the object plane 17 is possible.

[0117] The illuminated field 4 has typical dimensions in the object plane 17 that are less than 1 mm and may be less than 0.5 mm. In the illustrated embodiment, the illuminated field 4 extends 0.5 mm in the x direction and 0.5 mm in the y direction.

[0118] The x / y aspect ratio of the illumination field 4 may correspond to the x / y aspect ratio of the waveguide output 14 .

[0119] Using a projection optical unit, not depicted in FIG. 1, the illumination field 4 is imaged into an image field in an image plane.

[0120] The image file is detected by a detection device, such as a CCD camera or multiple CCD cameras. For details on imaging into the image field, reference is made to U.S. Patent No. 10,042,248 and the documents designated therein and in U.S. Patent No. 10,042,248.

[0121] For example, inspection of structures on reticle 18 is possible using a mask inspection system.

[0122] The imaging magnification β1 of the input coupling mirror optical unit 10 may be in the range of 0.1 to 50, i.e., the effect of the input coupling mirror optical unit may vary from a reduction by a factor of 10 to an enlargement by a factor of 50. The imaging magnification β2 of the output coupling mirror optical unit 16 may be in the range of 0.02 to 10, i.e., the effect of the output coupling mirror optical unit may vary from a reduction by a factor of 50 to an enlargement by a factor of 10. In the case of the illumination optical unit 1, the product of the two imaging magnifications β1, β2 may range from 0.25 to 10.

[0123] Three exemplary embodiments of an optical hollow waveguide assembly 11b having a hollow waveguide 11a with a purge gas device and an illumination light guide will be described in detail below with reference to Figures 2 to 4. First, the exemplary embodiment according to Figure 2 will be described in detail. Identical components of the exemplary embodiments according to Figures 3 and 4 described below retain the same reference numerals and will not be described in detail again.

[0124] The hollow waveguide 11a has a body 21. In particular, the body 21 may be formed from multiple parts. With regard to possible embodiments of the body 21 of the hollow waveguide 11a, reference is made to DE 10 2014 219 112 A1, in particular FIG. 3 and the relevant description therein, the disclosure of which is incorporated by reference in its entirety into the present application.

[0125] A waveguide cavity 22 is formed inside the body 21. In particular, the waveguide cavity 22 has an inner surface that is coated with a coating that is highly reflective to light in the EUV or DUV range, for example made from ruthenium.

[0126] The main body 21 is surrounded by a cover 23. The cover 23 mainly serves to protect the main body 21.

[0127] Moreover, the hollow waveguide assembly 11b comprises a waveguide purging device 26 having a purge gas connector 27 and a nozzle 28. The waveguide purging device 26 serves to purge the hollow waveguide 11a, particularly the waveguide cavity 22, with a purge gas 29. In particular, the hollow waveguide 11a can be continuously purged with the purge gas 29. In particular, hydrogen gas can be used as the purge gas 29.

[0128] 2, the cover 23 is attached to the body 21 such that an accumulation chamber 24 is formed in the area of ​​the waveguide output 14. An accumulation point 25 is formed within the accumulation chamber and substantially coincides with the waveguide output 14.

[0129] Purge gas 29 is first introduced into pressure chamber 30 of waveguide purging device 26 via purge gas connector 27. Pressure chamber 30 serves to reduce a first pressure P1 and a first velocity V1 at which the purge gas flows through purge gas connector 27.

[0130] In particular, the pressure P1 may be in the range of 30 Pa to 50 Pa. In particular, the velocity V1 may be in the range of 800 m / s to 900 m / s. An intermediate gap 31, through which the purge gas 29 flows out of the pressure chamber and into the nozzle 28 and is thus introduced into the accumulation chamber 24, is arranged between the pressure chamber 30 and the nozzle 28. As a result of the vortex generated when the purge gas 29 is introduced, the second pressure P2 and the second velocity V2 at the accumulation point 25 can be intentionally set. In particular, the second pressure P2 is in the range of 15 Pa to 25 Pa. In particular, the second velocity V2 is in the range of 20 m / s to 30 m / s.

[0131] As a result of pressure P2 at accumulation point 25, purge gas 29 flows into waveguide cavity 22 and exits waveguide cavity 22 at waveguide input 12. As it passes through waveguide cavity 22, purge gas 29 experiences a pressure loss ΔP as a result of friction, the inertia of purge gas 29, and the interaction of purge gas 29 with illumination light 3. Purge gas 29 has a third velocity V3 and a third pressure P3 within the waveguide cavity. Specifically, third velocity V3 may be in the range of 5 m / s to 10 m / s. Specifically, third pressure P3 may be in the range of 10 Pa to 15 Pa.

[0132] In particular, the waveguide input 12 and the waveguide output 14 may be located at different heights in the Earth's gravitational field, which may affect, and in particular increase, the pressure loss ΔP3. In particular, the pressure loss ΔP3 may be in the range of 10 Pa to 15 Pa.

[0133] At the waveguide input, a fourth pressure P4 and / or a fourth velocity V4 is established. In particular, the fourth pressure P4 may be less than 10 Pa. In particular, the fourth velocity V4 is in the range of 4 m / s to 6 m / s.

[0134] As a result, it is possible to purge, in particular continuously purge, the hollow waveguide 11 a against the flow, in this context meaning that the propagation direction of the purge gas 29 extends antiparallel to the propagation direction of the illumination light 3.

[0135] A sensor 33 is attached to the waveguide input 12. In particular, the sensor 33 is capable of detecting pressure P4 and / or velocity V4. The sensor 33 may also measure properties of the illumination light 3, such as the intensity and / or energy of the illumination light. The sensor 33 may also take the form of a purge gas sensor 33 for detecting the flow rate of the purge gas 29. In particular, the waveguide purging device 26 allows the waveguide 11a, and in particular the waveguide cavity 22 of the waveguide, to be purged without compromising the positioning and / or function of the sensor 33 at the waveguide input 12.

[0136] Arranged at the waveguide output 14 is an illumination light guide 32 that serves to transmit the illumination light 3 from the hollow waveguide to the output coupling mirror optical unit 16. An exemplary illumination light guide 32 is described in detail below with reference to Figures 5-7.

[0137] Nozzle 28 is used to introduce purge gas 29 into waveguide output 14 indirectly via accumulation chamber 24. In the exemplary embodiment shown, at least parts of waveguide purging device 26, in particular pressure chamber 30, intermediate gap 31 and nozzle 28, are formed between the cover and a base of illumination light guide 32 facing hollow waveguide 11 a and the side of cover 23 facing illumination light guide 32. For example, suitable structures may be introduced into cover 23 and the base of illumination light guide 32 that form corresponding parts of waveguide purging device 26 when illumination light guide 32 is fixed to cover 23.

[0138] For example, the nozzle 28 may be formed between the cover 23 and the illumination light guide 32. As a result, the nozzle may be formed, for example, as a ring nozzle. It is also possible for multiple nozzles or nozzle openings to be formed between the cover and the illumination light guide.

[0139] In other exemplary embodiments, at least portions of the waveguide purging device 26, particularly the purge gas connector, the pressure chamber, the intermediate gap and / or one or more nozzles, may be fully integrated into the illumination light guide 32, particularly into the base of the illumination light guide.

[0140] As a result of introducing purge gas 29 into accumulation chamber 24, turbulence and / or vortices of the purge gas may be intentionally introduced into the accumulation chamber. This turbulence and / or these vortices may be particularly pronounced at accumulation point 25. This may increase the dynamic pressure at accumulation point 25, thereby improving purge gas flow through waveguide cavity 22.

[0141] 3 shows a further exemplary embodiment of an optical hollow waveguide assembly 11c in which purge gas 29 is introduced indirectly into the waveguide cavity 22 by using an accumulation chamber 40 and an accumulation point 41. Also depicted is a waveguide purging device 42 having a purge gas connector 43 and a nozzle 44 with multiple nozzle openings 45. The waveguide purging device 42 comprises a pressure chamber 47 and an intermediate gap 48.

[0142] By using two or more nozzle openings 45, it is possible to particularly deliberately influence the vortex flow in the accumulation chamber 40. As a result, the second pressure P2 and / or the second velocity V2 at the accumulation point 41 can be set particularly precisely.

[0143] The nozzle 44 may in particular comprise two nozzle openings 45, in particular at least three nozzle openings 45, in particular at least four nozzle openings 45. In particular, the nozzle openings 45 may be arranged symmetrically around the accumulation point 41.

[0144] The waveguide purging device 42 may be designed as part of the cover 23 or may be at least partially integrated into the cover 23 .

[0145] The optical hollow waveguide assembly 11c comprises an illumination light guide 49 that is formed as a separate component part from the waveguide purging device 42. In particular, the illumination light guide 49 may be connected to the waveguide purging device 42, and in particular fastened to the waveguide purging device 42. It is also possible for the individual components of the waveguide purging device to be integrated into the illumination light guide 42 and / or for these components to be formed between the cover and the illumination light guide 42.

[0146] 4 depicts an optical hollow waveguide assembly 11d having a waveguide purging device 34 with a purge gas connector 35 and a nozzle 36. Also according to the exemplary embodiment according to FIG. 4, the waveguide purging device 34 comprises a pressure chamber 37 and an intermediate gap 38.

[0147] 4 differs from the above-described exemplary embodiment mainly in that the purge gas 29 is introduced directly into the waveguide cavity 22. It is possible to omit the accumulation chamber 24 and the associated accumulation point 25. The nozzle 36 is arranged on the peripheral side of the cross section of the waveguide output 14 so that the purge gas 29 flows directly into the waveguide output 14.

[0148] FIG. 4 further depicts an illumination light guide 39 which is also designed as a separate component part from the waveguide purging device 34 .

[0149] 5 to 7 show exemplary embodiments of an illumination light guide 50. For example, the illumination light guide 50 may be used in the exemplary embodiments described above, in particular in the exemplary embodiment according to FIG.

[0150] In this case, the illumination light guide 50 comprises a conductor body 51 , a conductor base 52 and a conductor cover plate 53 .

[0151] The conductor base 52 serves to connect, in particular to the hollow waveguide 11a, and in particular to the hollow waveguide cover 23, in a fluid-tight manner. The conductor cover plate 53 serves to connect, in particular to a flange connection, the illumination light guide 50 to downstream components in the beam path of the illumination light 3.

[0152] The above-described waveguide purging device may be at least partially integrated into the conductor base 52. In the illustrated exemplary embodiment, the conductor base 52 is designed so that a fluid channel for forming the purging device is introduced on the side facing outward from the body 51. Following the fluid-tight connection of the conductor base 52 to the cover 27 of the hollow waveguide 11a, a fluid channel, e.g., a pressure chamber 30, is formed between the conductor base 52 and the cover. A purge gas connector 54 is integrated into the conductor base 52.

[0153] In another exemplary embodiment (not depicted in the drawings), the waveguide purging device is fully integrated into the conductor base 52. In particular, a nozzle is formed in the conductor base 52, which allows the purge gas 29 to flow directly or indirectly into the waveguide output 14. In particular, the conductor base 52 can act as part of the cover 23 of the hollow waveguide. In that case, it is possible to omit a separate cover, in particular in the region of the waveguide output 14.

[0154] In still further exemplary embodiments, the waveguide purging device may be separate from the illumination light guide 50. For example, the conductor base 52 may serve only to mechanically connect the illumination light guide 50 to the cover 23 in which the waveguide purging device is formed, or to a separate waveguide purging device.

[0155] Conductor base 52 allows illumination light guide 50 to be attached to hollow waveguide 11a such that conductor input 55 is arranged flush with waveguide output 14. Conductor input 55 serves as the entry point for illumination light 3 to exit waveguide output 14 of hollow waveguide 11a.

[0156] The conductor body 51 extends in particular parallel to the direction of propagation of the illumination light 3 and has a longitudinal axis L that in particular coincides with the direction of propagation of the illumination light 3 .

[0157] In that case, conductor input 55 and conductor output 56 lie in a plane perpendicular to longitudinal axis L and are located at respective ends of conductor body 51. Conductor body 51 takes the form of a hollow body enclosing an interior through which illumination light 3 is guided from conductor input 55 to conductor output 56. Conductor input 55 is formed in conductor base 52 and conductor output 56 is formed in conductor cover plate 53.

[0158] The interior has a cross-section defined perpendicular to the longitudinal axis L. The cross-section has a cross-sectional area and a cross-sectional shape. In this context, cross-sectional shape should be understood to mean the shape of the cross-sectional contour, regardless of the circumference and / or surface area of ​​the cross-sectional area of ​​the interior. In particular, different cross-sectional areas with the same surface area can have different cross-sectional shapes. Moreover, it is possible that two cross-sections with different cross-sectional areas can have the same cross-sectional shape, for example, a circular or square cross-sectional shape.

[0159] The conductor input 55 has a different cross-sectional shape than the conductor output 56. At the conductor input 55 and the conductor output 56, the interior in each case has the same cross-sectional shape as the conductor input 55 and the conductor output 56, respectively. The cross-sectional shape of the interior cross section varies along the longitudinal axis L between the conductor input 55 and the conductor output 56. The cross-sectional shape varies particularly continuously along the longitudinal axis L. The cross-sectional shape of the interior can vary particularly smoothly along the longitudinal axis L, i.e., without corners and / or kinks. It is also possible for the cross-sectional shape of the interior to vary discontinuously.

[0160] In the exemplary embodiment shown, input conductor 55 has a circular cross-sectional shape, and output conductor 56 has an elliptical cross-sectional shape.

[0161] In other exemplary embodiments (not shown), conductor input 55 and conductor output 56 may also have polygonal cross-sectional shapes, particularly rectangular cross-sectional shapes. For example, the cross-sectional shape of conductor input 55 may be a regular polygon, particularly a square. The cross-sectional shape of conductor output 56 may be an irregular polygon, particularly a rectangle. Combinations of circular and polygonal cross-sectional shapes are also possible.

[0162] Upon emerging from the conductor output 56, the illumination light 3 has, in particular, a cross-sectional shape of the beam cross-section that corresponds to the cross-sectional shape of the conductor output 56.

[0163] 5-7, conductor output 56 has a larger cross-sectional area than conductor input 55. Conductor body 52 and the interior of conductor body 52 are conical in form. The cross-section of the interior increases particularly monotonically in cross-sectional area along longitudinal axis L, while simultaneously changing such that the initially circular cross-sectional shape of the interior eventually transitions to an elliptical cross-sectional shape.

[0164] In particular, the cross-sectional shape of the conductor input 55 has an input side ratio S1 substantially equal to 1. As a result of the input side ratio S1 being substantially equal to 1, the illumination light guide 50 can be particularly easily attached to the hollow waveguide 11a. The symmetrical area of ​​the conductor input 55 simplifies manufacturing. The purge gas supply is also improved and, in particular, becomes particularly uniform.

[0165] In particular, the input side ratio S1 may be defined as the ratio of the length to the width of the smallest rectangle forming the envelope of the cross section of the conductor input 55, i.e., the smallest rectangle whose area completely encompasses the cross section of the conductor input 55. For a circular cross section, such an enveloping rectangle takes the form of a square, resulting in an input side ratio S1 of 1. The diameter of the conductor input 55 having a circular cross section may in particular be in the range of 10 mm to 20 mm, and in particular may be in the order of about 15 mm.

[0166] The conductor output 56 has an output side ratio S2 that is not equal to 1. For the conductor output, the output side ratio S2 is defined analogously to the input side ratio S1 of the conductor input 55. For an elliptical cross-sectional shape, the output side ratio S2 corresponds to the quotient of the major axis and the minor axis of the ellipse. In particular, the major axis of the ellipse may have a length in the range of 30 mm to 60 mm, in particular about 45 mm. In particular, the minor axis of the ellipse may have a length in the range of 20 mm to 40 mm, in particular about 30 mm. In particular, for the output side ratio S2, 1.1

Claims

1. An optical hollow waveguide assembly (11b, 11c, 11d) comprising: an optical hollow waveguide (11a) for guiding the illumination light (3), --having a body (21) with a continuous waveguide cavity (22) having a waveguide input (12) and a waveguide output (14); an optical hollow waveguide (11a), the inner cavity walls of which are designed to be reflective to the illumination light; a waveguide purging device (26, 34, 42) for purging said waveguide cavity (22) with a purge gas (29), -- purge gas connectors (27, 35, 43, 54), and a waveguide purging device (26, 34, 42) comprising at least one nozzle (28, 36, 44, 45) arranged fluidly downstream of said purge gas connector (27, 35, 43, 54) for supplying said purge gas (29) at least indirectly to said waveguide output (14) to generate a purge gas flow from said waveguide output (14) to said waveguide input (12); An optical hollow waveguide assembly (11b, 11c, 11d) comprising:

2. 2. The optical hollow waveguide assembly (11b, 11c, 11d) according to claim 1, characterized in that the optical hollow waveguide (11a) is configured to guide illumination light (3) in the EUV range.

3. The pressure (P) of the supplied purge gas (20) 1 ) and / or velocity (V 1 3. The optical hollow waveguide assembly (11b, 11c, 11d) according to claim 1 or 2, characterized in that a pressure reducer for reducing the pressure of the purge gas is arranged between the purge gas connector (27, 35, 43, 54) and the nozzle (28, 36, 44, 45).

4. 4. Optical hollow waveguide assembly (11b, 11c, 11d) according to claim 3, characterized in that said pressure reducer comprises at least one pressure chamber (30, 37, 47, 57) for accumulating a purge gas (29).

5. 5. The optical hollow waveguide assembly (11b, 11c, 11d) according to claim 4, characterized in that the at least one pressure chamber (30, 37, 47, 57) is connected to the at least one nozzle (30, 36, 44, 45) via an intermediate gap (31, 38, 48).

6. The optical hollow waveguide assembly (11b, 11c, 11d) according to any one of claims 1 to 5, characterized in that the hollow waveguide assembly comprises a cover (23) at least partially enclosing the optical hollow waveguide (11a), and the hollow waveguide purging device (26, 34, 42) is at least partially integrated in the cover (23).

7. The optical hollow waveguide assembly (11b, 11c, 11d) according to any one of claims 1 to 6, characterized by an illumination light guide (32, 39, 49, 50) arranged downstream of the waveguide output (14) in the propagation direction of the illumination light (3).

8. 8. The optical hollow waveguide assembly (11b, 11c, 11d) according to claim 7, characterized in that the waveguide purging device (26, 34, 42) is at least partially integrated into the illumination light guide (32, 50).

9. The optical hollow waveguide assembly (11d) according to any one of claims 1 to 8, characterized in that the nozzle (36) introduces the purge gas (29) directly into the waveguide output (14).

10. 9. The optical hollow waveguide assembly (11b, 11c) according to claim 1, characterized by an accumulation chamber (24, 40) arranged in the region of the waveguide output (14), and wherein the at least one nozzle (28, 44, 45) introduces the purge gas (29) into the accumulation chamber (24, 40).

11. 11. The optical hollow waveguide assembly (11b, 11c, 11d) according to any one of claims 1 to 10, characterized in that the waveguide purging device (26, 34, 42) comprises at least two nozzles (44, 45) arranged symmetrically with respect to a central longitudinal axis of the waveguide cavity (22).

12. Optical hollow waveguide assembly (11b, 11c, 11d) according to any one of claims 1 to 11, characterized by a reduced pressure generator arranged in the region of the waveguide input (12).

13. An illumination optical unit (1) comprising an optical hollow waveguide assembly (11b, 11c, 11d) according to any one of claims 1 to 12.

14. 14. Illumination optical unit (1) according to claim 13, characterized in that the illumination optical unit (1) is designed for illumination by using illumination light (3) in the EUV range.

15. Inspection apparatus comprising an illumination optical unit (1) according to claim 13 or 14.

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