FIELD

The disclosure relates to an optical arrangement, for example a lithography system, in particular an EUV or DUV lithography system, including: a first component, in particular a carrying frame, a second component, which is movable in relation to the first component, in particular a mirror or a housing, and also at least one stop with at least one stop face for limiting the movement of the second component in relation to the first component. The disclosure also relates to a method for repairing such an optical arrangement following a shock load.

BACKGROUND

For the purposes of this application, a lithography system is understood as meaning an optical system or an optical arrangement that can be used in the field of lithography. Apart from a lithography apparatus, which serves for the production of semiconductor components, the optical system may be for example an inspection system for the inspection of a photomask used in a lithography apparatus (hereinafter also referred to as a reticle), for the inspection of a semiconductor substrate to be structured (hereinafter also referred to as a wafer), or a metrology system, which is used for measuring a lithography apparatus or parts thereof, for example for measuring a projection system. The optical arrangement or the lithography system may be in particular an EUV lithography system, which is designed for used radiation at wavelengths in the EUV wavelength range of between about 5 nm and about 30 nm.

Reference is often made hereinafter to a movable (second) component or a moved/movable object in the form of an (EUV) mirror. In principle, however, the movable component may also be any other object or any other component or subassembly, for example an optical element such as a lens or a prism, a wafer stage, a further carrying frame or a further carrying structure for optical or non-optical components, etc.

The first component may be for example a carrying frame (known as a “force frame”) of the optical arrangement, which substantially absorbs all of the forces acting on the optical arrangement. The movable component is typically spring-mounted with respect to the carrying frame, or mechanically decoupled from it, so that ideally no forces or vibrations are transmitted from the carrying frame to the movable component. The first component may, however, also be some other component, for example a carrier component part, which is spring-mounted with respect to the carrying frame of the optical arrangement or mechanically decoupled from it.

In the case of an EUV lithography apparatus, specifically in the case of an EUV lens for imaging of a mask onto a light-sensitive substrate, the sensitivity of the quality of the imaging with respect to the deformation of optical surfaces of EUV mirrors is particularly great. Therefore, during the operation of an EUV lithography apparatus or an EUV lens, EUV mirrors are set in a suspended state, so that the forces and moments acting on the EUV mirror become as small as possible. This state of suspension or of being mechanically decoupled from the environment has the consequence that the EUV mirrors can in effect move freely between their end positions, which are typically defined by (end) stops or the positions of the stop faces thereof, and may collide with the (end) stops.

A collision of the EUV mirrors with the (end) stops typically only occurs when there is an unwanted shock load, such as occurs in an earthquake or possibly during transportation of the EUV lithography system. If the stops for limiting the travel are embodied as hard mechanical stops, the kinetic energy of the accelerated EUV mirror is not absorbed under such a shock load and a high energetic impulse is introduced into the optical arrangement, which may lead to collisions of the EUV mirrors with optics modules or optics components within the optical arrangement. The collisions can lead to mechanical deformations or damage to the EUV mirrors or to other component parts of the optical arrangement.

The second component can also be a housing or any other component part or a system or an interconnected group of component parts, in particular a complete system consisting of a plurality of component parts, for example a lithography illumination system, which is mounted or arranged on a carrying frame or on a carrying structure (base frame). In this case, the second component can be, for example, a housing in which the optical elements and further component parts of the illumination system are accommodated. The second component has a very large mass in this case, which is why a very complex damping system involving much installation space, for example hydraulic damping, which additionally has to be sealed against the atmosphere, is typically used to protect against a shock load.

SUMMARY

The disclosure seeks to provide an optical arrangement in which undesired damage to component parts in the case of a shock load is avoided, and to provide a method for repairing such an optical arrangement following a shock load.

In one aspect, the disclosure provides an optical arrangement in which the stop has a metal foam for absorbing the kinetic energy (or as much of the kinetic energy as possible) of the second component when striking against the stop face. Such a stop enables damping by energy dissipation.

The metal foam is typically a molded body which is plastically deformed or compressed when the second component strikes the stop face. The compression occurs substantially along the direction of force of the force generated by the movable component when striking against the stop face. The use of a stop including a metal foam is advantageous over the use of stops including elastomers, e.g., rubber, for damping or absorbing the kinetic energy in that an element made of metal foam has almost the same stiffness in the installed state as an element of a comparable size made of a solid material. On account of the comparatively high stiffness, the desired precision for observing the installation tolerances when installing the stop in the optical arrangement can be observed.

According to the book “Cellular solids: Structure and properties” by L. J. Gibson and M. F. Ashby, Cambridge University Press, 1997, the solid foams belong to the so-called cellular materials, which in turn are defined by a relative density of less than 30%. The relative density is understood to mean the ratio of the density of the cellular material to the density of the material from which the cell walls of the cellular material were produced. Materials with a relatively high relative density are called “porous solids”. In the present application, a foam is understood to be a solid which is defined as described above, i.e., which has a relative density of less than 30%.

Metal foams can be produced in a comparatively simple manner, cf., for example, the article “Metallschaum für die Wirbelsäule” by S. von der Weiden, WELT Print Wissen, Sep. 24, 2009. In principle, the type of process for producing metal foams makes it possible to produce metal foams of very different porosity, pore size and metal web thickness, which are adapted to the respective desired properties, cf., for example, the article “Innovative Werkstoffkunde mit dem Spielzeug aus der Kindheit” by Dipl.-Ing. (F. H.) A. M. Matz, Konturen 2011, pages 68-71, Pforzheim University. The properties of the metal foam can therefore be adapted to the desired properties of the stop at a particular installation position by way of a suitable choice of the parameters during its production. The structure and processes for the production of open-cell aluminum foams also emerge from the dissertation “Mechanische Eigenschaften and Versagensmechanismen offenzelliger Aluminiumschaum-Strukturen” by Paul Schüler, Univ. Berlin, 2016. In respect of the characterization of energy absorption properties of metal foams, reference is made to the dissertation “Zur Einsetzbarkeit von Metallschäumen in der Verkehrstechnik” by J. Burzer, Univ. Bayreuth, 2000.

In one embodiment, the stop is detachably connected to the first component, preferably via a screw connection. When the metal foam is compressed by the second component striking against the stop face, the metal foam is plastically deformed or compressed, with the volume of the metal foam decreasing. Following the mechanical deformation or compression, the metal foam thus no longer assumes its original volume, and so the stop typically has to be replaced after a shock load. Such a replacement of the stop is simplified by the—direct or indirect—detachable connection of the stop or of the metal foam to the first component.

In a further embodiment, the stop has a solid connecting component part, which is preferably metallic, via which the stop is connected to the first component. Within the scope of this application, a solid component part is understood to mean a component part in which at least 99% of the volume consists of the respective material, e.g., metallic material, and only less than 1% of the volume consists of air or cavities.

In principle, it is possible to mechanically process the metal foam and, for example, to drill holes in the metal foam in order to connect the latter to the first component. In order to avoid damage to the metal foam, it was found to be advantageous if the metal foam is connected to the first component via a solid connecting component part. In particular, one or more bores for fastening the stop using a screw connection can be provided in such a solid connecting component part. There may be female threads in the bores, although this is not mandatory. The connecting component part itself may also have a geometry that facilitates an interlocking and/or force fit connection to the first component.

In one embodiment, the stop has a solid stop component part, which is preferably metallic, on which the stop face is formed. It was found to be advantageous if the metal foam of the stop does not come into direct contact with the second component when the second component strikes against the stop face. The stop component part and/or the connecting component part can form a cover layer for the metal foam and can be formed, for example, from a metallic material, e.g., from aluminum or from steel, but possibly also from a non-metallic material, e.g., from plastic or from carbon fibers.

In a further embodiment, the metal foam is attached between the stop component part and the connecting component part, i.e., the stop component part and the connecting component part extend at least in part along surfaces of the metal foam that lie opposite one another. The use of a stop in such a sandwich construction, which has a core made of metal foam, which is attached between the stop component part and the connecting component part, facilitates a particularly compact construction of the stop. When the second component strikes against the stop face, the metal foam is typically compressed along a direction of force which (locally) extends at an angle, in particular perpendicular, to the stop face.

It was found to be advantageous if the connecting component part and the stop component part, between which the metal foam is disposed, have the same geometry.

In one embodiment, the stop component part and the connecting component part have a cylindrical embodiment. In this case, the metal foam is introduced into the interspace between the radially inner connecting component part and the radially outer stop component part (or vice versa). In this case, the stop is disposed relative to the second component in such a way that the force exerted by the second component against the stop face upon impact acts substantially with a radial force component on the stop, i.e., the metal foam is substantially compressed in the radial direction.

In this case, the cylindrical connecting component part forms a sleeve which can be pushed onto a suitably shaped, typically rod-shaped component part of the first component. The rod-shaped component part can project beyond the stop or beyond the connecting component part, e.g., in the axial direction, and can be provided with a thread, at least in the projecting section, it being possible to screw a nut onto the thread in order to fasten the stop to the first component in the axial direction. When fastening in the axial direction, the lock nut substantially presses against the end face of the rigid connecting component part and not against the metal foam, as the latter would deform plastically. It is understood that there can be connections other than a screw connection for detachably connecting the stop to the first component, for example a latching connection.

In an alternative embodiment, the stop component part and the connecting component part have a slab-shaped embodiment, i.e., the stop component part and the connecting component part are embodied as plane plates. In this case, the connecting component part can project for example laterally beyond the metal foam in order to (releasably) connect the connection component part to the first component. In this case, the force which is exerted by the second component on the first component upon impact acts substantially perpendicular to the flat stop component part, and so the volume of the metal foam located between the stop component part and the connecting component part is compressed in the longitudinal direction.

In a further embodiment, the stop component part and/or the connecting component part are/is connected to the metal foam in an integral manner, in particular via a metallurgical integral connection. In this case, the metal foam forms a metal foam composite with the stop component part and the connecting component part. The integral connection between the metal foam and the stop component part or the connecting component part is preferably realized via a metallurgical connection, i.e., via a connection in which the respective (generally metallic) stop or connecting component part is heated together with the metal foam to a temperature at which a permanent connection is established with the metal foam without an additional joining agent being used for this purpose. Where desired, the integral connection can also be established using a joining agent, for example an adhesive. However, realizing the integral connection using an adhesive is problematic due to the fact that adhesives generally outgas substances that are undesirably released into the environment. This can be particularly problematic in EUV lithography apparatuses as high demands are placed there on the cleanliness of the vacuum environment under which the optical elements are operated.

In a further embodiment, the metal foam contains aluminum. In particular, the metal foam can consist of aluminum or of an aluminum-containing alloy. For the purposes of selecting an aluminum foam that is suitable for the present application, reference can be made, for example, to the specialist book “Taschenbuch für Aluminiumschäume” by T. Hipke et al., Beuth-Verlag, 2011, which provides a systematic summary of the most important information on the use of aluminum foams. However, it is understood that the metal foam can also contain or consist of other metallic materials, for example nickel, etc.

A so-called hybrid foam can possibly also be used to absorb relatively large loads; here, the aluminum frame of the metal foam is coated by a very thin layer, for example made of nickel, as described in the article “Stabiler Metallschaum für Leichtbau und Aufprallschutz” in the specialist journal METALL 2013 (www.bauforum.at/metall/stabiler-metallschaum-fuer-Leichtbau-und-aufprallschutz-19861). As can be gathered from the dissertation “Mechanische Eigenschaften und Versagensmechanismen offenzelliger Aluminiumschaum-Strukturen” by Paul Schiller, Univ. Berlin, 2016 (cf. FIG. 88 therein), cited at the outset, a mass-specific energy absorption of the order of more than approximately 1.5 kJ/kg can be obtained for aluminum foams in the case of a degree of compression (i.e., a length reduction in the direction of compression) of more than 50%.

The stop is desirably designed or dimensioned in such a way that as much kinetic energy of the second component as possible is absorbed by the metal foam in the case of a shock load caused by an earthquake, for example. Assuming an accelerated mass of the second component of approximately 2000 kg, as may be the case for a second component in the form of a housing of an optical system, for example an illumination system containing one or more mirrors, and an acceleration of approx. 40 m/s2(corresponding to approximately four times the acceleration due to gravity g) over a time interval of 0.020 s, this corresponds to a speed v of 0.8 m/s. In the present case, the formula for the kinetic energy E=½ mv2yields E=½*2000 kg*(0.8 m/s)2=640 J. With a mass-specific energy absorption of approximately 1.5 kJ/kg for the metal foam, the mass m of the metal foam arises as 640 J/(1500 J/kg)=0.42 kg. A metal foam with such a weight has manageable dimensions in terms of its volume, and so, despite little installation space available, the use of a metal foam facilitates the realization of a stop that absorbs a considerable proportion of the kinetic energy of the second component.

A further aspect of the disclosure relates to a method for repairing an optical arrangement, embodied as described above, after a shock load, the method including: replacing at least one stop, in which the metal foam was compressed by the shock load, by a stop (another stop), in which the metal foam is not compressed (has not yet been compressed). As described above, a substantial portion of the kinetic energy of the second component can be absorbed by the stop by virtue of the metal foam being compressed. This prevents optical or mechanical components of the optical arrangement other than the stop from being damaged or deformed in the event of a shock load and having to be possibly replaced following the shock load. Therefore, following the replacement of the stop or, where desired, a plurality of stops, the optical arrangement can be put back into operation without the optomechanics having been damaged.

Further features and advantages of the disclosure are evident from the following description of exemplary embodiments of the disclosure, with reference to the figures of the drawing, which show certain details of the disclosure, and from the claims. The individual features can be realized in each case individually by themselves or as a plurality in any desired combination in a variant of the disclosure.

