DE102022204833A1 - OPTICAL SYSTEM, LITHOGRAPHY EQUIPMENT AND METHOD OF MANUFACTURING AN OPTICAL SYSTEM - Google Patents

Abstract

An optical system (200) for a lithography system (1), having a mirror (201) with an optically active surface (202) and a mirror substrate (203), wherein the mirror substrate (203) comprises at least two sections (205, 206) which in are arranged at different distances (d1, d2) from the optically active surface (202), and a zero crossing temperature (ZCT1, ZCT2) of a coefficient of thermal expansion (CTE1, CTE2) is configured differently in the two sections (205, 206).

Description

The present invention relates to an optical system, a lithography system with such an optical system and a method for producing such an optical system.

Microlithography is used to produce microstructured components such as integrated circuits. The microlithography process is carried out using a lithography system which has an illumination system and a projection system. The image of a mask (reticle) illuminated by the illumination system is projected by the projection system onto a substrate coated with a light-sensitive layer (photoresist) and arranged in the image plane of the projection system, for example a silicon wafer, in order to place the mask structure on the light-sensitive coating of the substrate transferred to.

By using EUV radiation, ie radiation at wavelengths of, for example, approximately 13.5 nm, very small structures can be produced on the substrate using a lithography system. In this way, for example, a structure reduction in the production of semiconductor components can be further promoted.

In lighting and projection systems designed for the EUV range, mirrors are used as optical components for the imaging process due to the lack of availability of suitable transparent materials. A problem that arises is that the mirrors heat up as a result of absorption of the radiation emitted by the EUV light source. If the temperature rises sharply, the mirrors can thermally deform. The thermal deformation results as a function of the heat flow distribution and the material-dependent thermal expansion coefficient (thermal expansion coefficient). Furthermore, an optical coating of the mirror can degrade due to an increase in temperature. Both thermal deformation of the mirrors and damage to their optical coatings can impair the imaging properties of the mirrors.

By increasing the radiation power of the EUV radiation source, more semiconductor components can be manufactured per time. However, as the radiation power of the EUV radiation source increases, the energy density on the mirror surface increases further, so that the problem of the temperature increase in the mirrors of the lithography system becomes even greater.

To avoid impermissibly high temperatures, conventional mirror modules in a lithography system can be cooled by cooling lines in the mirror substrate. However, it was found that active cooling can lead to a jump in the temperature distribution depending on the distance from the optically active surface. The temperature of the mirror substrate can drop rapidly, particularly in the area of the cooling channels, which can result in deformations and thus a deterioration in the imaging performance.

Incidentally, the problem of cooling mirrors is not limited either to lithography systems in general or to EUV lithography systems in particular. Rather, it is a problem that occurs wherever incident (high-energy) radiation heats a mirror above a permissible temperature.

Against this background, an object of the present invention is to provide an improved optical system, a lithography system with such an optical system and a method for producing such an optical system.

Accordingly, an optical system for a lithography system is proposed. The optical system has a mirror with an optically active surface and a mirror substrate. The mirror substrate comprises at least two sections, which are arranged at different distances from the optically active surface. A zero crossing temperature of a coefficient of thermal expansion is designed differently in the two sections.

Due to the different design of the zero crossing temperature of the coefficient of thermal expansion in the two sections, the material properties of the mirror substrate in the two sections can be specifically designed for a temperature or temperature distribution prevailing there when the mirror is used.

As a result, deformations caused by heat input into the mirror (eg by irradiation with EUV light) and associated deterioration in the imaging properties can be reduced or avoided.

For example, the at least two sections (e.g. directly) adjoin one another.

The coefficient of thermal expansion (thermal expansion coefficient) gives a change in geometric shape and dimensions of a material with a change in temperature. The thermal expansion coefficient is, for example, a linear thermal expansion coefficient that indicates a change in length of a material as a function of a temperature change. The coefficient of thermal expansion is itself temperature dependent, ie a temperature dependent function.

At its zero crossing temperature ("zero crossing temperature"), the thermal expansion coefficient shows a zero crossing in its temperature dependence, in the vicinity of which there is no or only negligible thermal expansion of the mirror substrate material. The zero crossing temperature of the coefficient of thermal expansion can be adjusted in particular during the manufacture of the mirror substrate. In particular, the zero crossing temperature of the coefficient of thermal expansion is set depending on an expected operating temperature of the respective section of the mirror substrate.

The material of the two sections of the mirror substrate is in particular a material with a low coefficient of thermal expansion. For example, the coefficient of thermal expansion at a desired operating temperature is within a range of +/- 20 ppb/K (parts per billion per Kelvin), +/- 15 ppb/K, +/- 10 ppb/K, and/or +/- 5 ppb /K However, the coefficient of thermal expansion can also lie within a different range. Such an ultra-low thermal expansion material (e.g., a substrate material sold under the designation "ULE" for "Ultra-Low Expansion" by Corning Inc.) exhibits very little change in geometric shape and dimensions due to changes in temperature on. In particular, such a low coefficient of thermal expansion material is ideally applied in the vicinity of its (e.g., default) zero-crossing temperature.

Examples of the material of the mirror substrate in either or both portions include a glass material of TiO ₂ ·SiO ₂ in which the ultra-low thermal expansion coefficient is realized by varying the concentration of TiC ₂ . Another example is a Li ₂ O-Al ₂ O ₃ -SiO ₂ glass ceramic (distributed under the name "Zerodur" by Schott) with a crystalline phase, in which the ultra-low coefficient of thermal expansion is achieved by uniformly distributed nanocrystals in a residual glass phase.

In accordance with one embodiment, the mirror substrate has cooling channels which are arranged between the two sections, in the section arranged closer to the optically active surface and/or in the section arranged further away from the optically active surface.

This allows the mirror to be actively cooled. For example, during operation, a coolant, such as water, is conducted at a specific temperature through the cooling channels.

Furthermore, by suitably arranging the two sections of the mirror substrate depending on the configuration of the cooling channels and/or by suitably designing the material, in particular the zero crossing temperature, thermal deformations of the two sections (e.g. in the region of the cooling channels) can be reduced or avoided. For example, the zero crossing temperature of the coefficient of thermal expansion above the cooling channels, i.e. between the optically active surface and the cooling channels, can be configured differently than below the cooling channels.

In particular, the at least two sections of the mirror substrate have a first section arranged closer to the optically active surface and a second section arranged further away from the optically active surface. For example, the cooling channels are arranged in an area of the first section adjacent to the second section and/or in an area of the second section adjacent to the first section.

According to a further embodiment, the zero crossing temperature of the thermal expansion coefficient is configured differently in the two sections in relation to a level of a value of the zero crossing temperature, a homogeneity of a spatial distribution of the zero crossing temperature and/or a slope of a temperature dependency of the thermal expansion coefficient adjacent to the zero crossing temperature.

The level of the value of the zero crossing temperature is set in particular as a function of the operating temperature expected in each section when the mirror is used (eg in an EUV lithography system). For example, the level of the zero crossing temperature value is set to the expected (e.g. average) operating temperature in the respective sections.

In reality, due to an inhomogeneous material composition, the mirror substrate material can have a spatial distribution of the zero crossing temperature of the coefficient of thermal expansion (in particular a point-to-point variation of the zero crossing temperature). In this case, under the amount of the value of Zero crossing temperature in a respective section of the mirror substrate, in particular, a representative value, for example an average value, of the spatial distribution of the zero crossing temperature in this section.

Furthermore, the homogeneity of the spatial distribution of the zero crossing temperature in the two sections of the mirror substrate can also be set differently in a targeted manner. As a result, for example, different temperature gradients in the sections can be taken into account.

According to a further embodiment, the zero crossing temperature of the coefficient of thermal expansion is set to a higher value in the section arranged closer to the optically active surface than in the section arranged further from the optically active surface.

As a result, the material of the section of the mirror substrate arranged closer to the optically active surface can be designed specifically for the higher operating temperature near the optically active surface. Furthermore, the material of the section of the mirror substrate arranged further away from the optically active surface can be designed further away from the optically active surface for the lower operating temperature.

According to a further embodiment, the spatial distribution of the zero crossing temperature is more homogeneous in the section arranged closer to the optically active surface than in the section arranged further from the optically active surface.

High temperatures and large temperature gradients in the section of the mirror substrate arranged closer to the optically active surface place higher demands on the material homogeneity, in particular the homogeneity of the zero crossing temperature. Due to the fact that the material of the section arranged closer to the optically active surface is produced more homogeneously in relation to a point-to-point variation of the zero crossing temperature, thermal deformation of the mirror substrate can nevertheless be kept within limits or prevented.

In contrast, the material of the section of the mirror substrate that is arranged further away from the optically active surface, in which lower temperatures and smaller temperature gradients are present, can be produced with greater tolerances with regard to material homogeneity. This allows the mirror substrate to be manufactured more simply and cost-effectively.

According to a further embodiment, the slope of the temperature dependency of the coefficient of thermal expansion adjacent to the zero crossing temperature is lower in the section arranged closer to the optically active surface than in the section arranged further from the optically active surface.

According to a further embodiment, the design of the zero crossing temperature changes continuously in a border area of the two sections.

Mechanical overload due to discontinuous coefficients of expansion can be avoided by a continuous transition in the design of the zero crossing temperature of the coefficient of thermal expansion from one of the two sections to the other of the two sections of the mirror substrate.

For example, the first section (out of bounds) has a first value of zero crossing temperature and the second section (out of bounds) has a second value of zero crossing temperature. Furthermore, the value of the zero-crossing temperature changes continuously from the first value to the second value within the limit range.

According to a further aspect, a lithography system with an optical system as described above is proposed.

The lithography system is, for example, an EUV lithography system. EUV stands for "extreme ultraviolet" and refers to a wavelength of the working light between 0.1 nm and 30 nm, in particular 13.5 nm. The lithography system can also be a DUV Act lithography system. DUV stands for "deep ultraviolet" and describes a working light wavelength between 30 and 250 nm.

In an EUV or DUV lithography system, EUV or DUV radiation is directed onto a photomask (“reticle”) by means of a beam shaping and illumination system. The photomask has a structure which is imaged on a wafer or the like in reduced form by means of a projection system of the lithography system.

The mirror of the optical system can be any of the mirrors of the EUV or DUV lithography system.

According to a further aspect, a method for producing an optical system for proposed a lithography system. The optical system has a mirror with an optically active surface and a mirror substrate. The method includes the step of producing the mirror substrate with at least two sections, wherein a zero crossing temperature of a coefficient of thermal expansion is configured differently in the two sections. In addition, the two sections are designed to be arranged at different distances from the optically active surface.

According to one embodiment of the method, the two sections of the mirror substrate are manufactured individually, or manufactured by separating at least one mirror substrate ingot.

According to a further embodiment of the method, the method includes the step of joining the two sections together.

In particular, the two sections are joined together physically and/or chemically. For example, the two sections are joined in such a way that the thermal expansion properties change continuously in a border area of the two sections.

According to a further embodiment of the method, cooling channels are formed, in particular milled, in one or both sections.

For example, the cooling channels are formed in one or both sections before the two sections are joined together.

According to a further embodiment of the method, the two sections of the mirror substrate for forming the different design of the zero crossing temperature of the coefficient of thermal expansion are produced from different material compositions and/or by varying degrees of homogenization of one or more material compositions.

For example, in a glass material of TiO ₂ -SiO ₂ in which the ultra-low coefficient of thermal expansion is realized by varying the concentration of TiO ₂ , greater homogeneity of the zero-crossing temperature of the coefficient of thermal expansion is achieved by more uniform distribution of the TiO ₂ . For example, in the case of a Li ₂ O-Al ₂ O ₃ -SiO ₂ glass ceramic (Zerodur), in which the ultra-low coefficient of thermal expansion is achieved by nanocrystals in a residual glass phase, greater homogeneity of the zero crossing temperature of the coefficient of thermal expansion is achieved through a more even distribution of the nanocrystals.

According to a further embodiment of the method, at least one of the two sections of the mirror substrate is subsequently treated, in particular heat-treated or tempered, to form the different design of the zero crossing temperature of the thermal expansion coefficient.

"A" is not necessarily to be understood as being limited to exactly one element. Rather, a plurality of elements, such as two, three or more, can also be provided. Any other count word used here should also not be understood to mean that there is a restriction to precisely the stated number of elements. Rather, numerical deviations upwards and downwards are possible, unless otherwise stated.

The embodiments and features described for the optical system apply correspondingly to the lithography system and the proposed method and vice versa.

Further possible implementations of the invention also include combinations of features or embodiments described above or below with regard to the exemplary embodiments that are not explicitly mentioned. The person skilled in the art will also add individual aspects as improvements or additions to the respective basic form of the invention.

• 1 zeigt schematisch im Meridionalschnitt eine Projektionsbelichtungsanlage für die EUV- Projektionslithografie;
• 2 zeigt eine Querschnittsansicht eines optischen Systems der Projektionsbelichtungsanlage aus 1 gemäß einer Ausführungsform;
• 3 veranschaulicht einen Temperaturverlauf eines Wärmeausdehnungskoeffizienten eines Spiegelsubstratmaterials eines Spiegels des optischen Systems aus 2 gemäß einer Ausführungsform;
• 4 zeigt ein Flussablaufdiagramm eines Verfahrens zum Herstellen des optischen Systems aus 2 gemäß einer Ausführungsform;
• 5 veranschaulicht einen Herstellungsschritt des Verfahrens nach 4; und
• 6 veranschaulicht einen weiteren Herstellungsschritt des Verfahrens nach 4.

Further advantageous refinements and aspects of the invention are the subject matter of the dependent claims and of the exemplary embodiments of the invention described below. The invention is explained in more detail below on the basis of preferred embodiments with reference to the enclosed figures.

• 1 shows a schematic meridional section of a projection exposure system for EUV projection lithography;
• 2 FIG. 12 shows a cross-sectional view of an optical system of the projection exposure apparatus from FIG 1 according to one embodiment;
• 3 FIG. 12 illustrates a temperature profile of a thermal expansion coefficient of a mirror substrate material of a mirror of the optical system 2 according to one embodiment;
• 4 FIG. 12 shows a flow chart of a method for manufacturing the optical system 2 according to one embodiment;
• 5 illustrates a manufacturing step of the method according to FIG 4 ; and
• 6 illustrates a further manufacturing step of the method 4 .

Elements that are the same or have the same function have been provided with the same reference symbols in the figures, unless otherwise stated. Furthermore, it should be noted that the representations in the figures are not necessarily to scale.

The following are first with reference to the 1 the essential components of a projection exposure system 1 for microlithography are described as an example. The description of the basic structure of the projection exposure system 1 and its components should not be understood as limiting here.

One embodiment of an illumination system 2 of the projection exposure system 1 has, in addition to a light or radiation source 3, illumination optics 4 for illuminating an object field 5 in an object plane 6. In an alternative embodiment, the light source 3 can also be provided as a separate module from the rest of the illumination system. In this case the lighting system does not include the light source 3 .

A reticle 7 arranged in the object field 5 is exposed. The reticle 7 is held by a reticle holder 8 . The reticle holder 8 can be displaced in particular in a scanning direction via a reticle displacement drive 9 .

In the 1 a Cartesian xyz coordinate system is drawn in for explanation. The x-direction runs perpendicularly to the plane of the drawing and into the plane of the drawing. The y-direction is horizontal and the z-direction is vertical to the drawing plane. The scanning direction is in the 1 along the y-direction. The z-direction runs perpendicular to the object plane 6.

The projection exposure system 1 includes projection optics 10. The projection optics 10 are used to image the object field 5 in an image field 11 in an image plane 12. The image plane 12 runs parallel to the object plane 6. Alternatively, there is also an angle other than 0° between the object plane 6 and the Image plane 12 possible.

A structure on the reticle 7 is imaged onto a light-sensitive layer of a wafer 13 arranged in the region of the image field 11 in the image plane 12 . The wafer 13 is held by a wafer holder 14 . The wafer holder 14 can be displaced in particular along the y-direction via a wafer displacement drive 15 . The displacement of the reticle 7 via the reticle displacement drive 9 on the one hand and the wafer 13 on the other hand via the wafer displacement drive 15 can be synchronized with one another.

The radiation source 3 is an EUV radiation source. The radiation source 3 emits in particular EUV radiation 16, which is also referred to below as useful radiation, illumination radiation, illumination light or working light. In particular, the useful radiation has a wavelength in the range between 5 nm and 30 nm. The radiation source 3 can be a plasma source, for example an LPP source (laser produced plasma, plasma generated with the aid of a laser) or a DPP Source (Gas Discharged Produced Plasma). It can also be a synchrotron-based radiation source. The radiation source 3 can be a free-electron laser (free-electron laser, FEL).

The illumination radiation 16 emanating from the radiation source 3 is bundled by a collector 17 . The collector 17 can be a collector with one or more ellipsoidal and/or hyperboloidal reflection surfaces. The at least one reflection surface of the collector 17 can be exposed to the illumination radiation 16 in grazing incidence (Grazing Incidence, GI), i.e. with angles of incidence greater than 45°, or in normal incidence (Normal Incidence, NI), i.e. with angles of incidence less than 45° will. The collector 17 can be structured and/or coated on the one hand to optimize its reflectivity for the useful radiation and on the other hand to suppress stray light.

After the collector 17, the illumination radiation 16 propagates through an intermediate focus in an intermediate focal plane 18. The intermediate focal plane 18 can represent a separation between a radiation source module, comprising the radiation source 3 and the collector 17, and the illumination optics 4.

The illumination optics 4 comprises a deflection mirror 19 and a first facet mirror 20 downstream of this in the beam path. The deflection mirror 19 can be a plane deflection mirror or alternatively a mirror with an effect that influences the bundle beyond the pure deflection effect. Alternatively or additionally, the deflection mirror 19 can be designed as a spectral filter, which separates a useful light wavelength of the illumination radiation 16 from stray light of a different wavelength. If the first facet mirror 20 is arranged in a plane of the illumination optics 4 which is optically conjugate to the object plane 6 as the field plane, it is also referred to as a field facet mirror. Of the First facet mirror 20 includes a multiplicity of individual first facets 21, which are also referred to below as field facets. Of these facets 21 are in the 1 only a few shown as examples.

The first facets 21 can be embodied as macroscopic facets, in particular as rectangular facets or as facets with an arcuate or part-circular edge contour. The first facets 21 can be embodied as planar facets or alternatively as convexly or concavely curved facets.

Like for example from the DE 10 2008 009 600 A1 is known, the first facets 21 themselves can each also be composed of a large number of individual mirrors, in particular a large number of micromirrors. The first facet mirror 20 can be embodied in particular as a microelectromechanical system (MEMS system). For details refer to the DE 10 2008 009 600 A1 referred.

The illumination radiation 16 runs horizontally between the collector 17 and the deflection mirror 19, ie along the y-direction.

A second facet mirror 22 is arranged downstream of the first facet mirror 20 in the beam path of the illumination optics 4. If the second facet mirror 22 is arranged in a pupil plane of the illumination optics 4, it is also referred to as a pupil facet mirror. The second facet mirror 22 can also be arranged at a distance from a pupil plane of the illumination optics 4 . In this case, the combination of the first facet mirror 20 and the second facet mirror 22 is also referred to as a specular reflector.

Specular reflectors are known from US 2006/0132747 A1 , the EP 1 614 008 B1 and the U.S. 6,573,978 .

The second facet mirror 22 includes a plurality of second facets 23. In the case of a pupil facet mirror, the second facets 23 are also referred to as pupil facets.

The second facets 23 can also be macroscopic facets, which can have round, rectangular or hexagonal borders, for example, or alternatively facets composed of micromirrors. In this regard, also on the DE 10 2008 009 600 A1 referred.

The second facets 23 can have plane or alternatively convexly or concavely curved reflection surfaces.

The illumination optics 4 thus forms a double-faceted system. This basic principle is also known as a honeycomb condenser (Fly's Eye Integrator).

It can be advantageous not to arrange the second facet mirror 22 exactly in a plane which is optically conjugate to a pupil plane of the projection optics 10 . In particular, the pupil facet mirror 22 can be arranged tilted relative to a pupil plane of the projection optics 7, as is the case, for example, in FIG DE 10 2017 220 586 A1 is described.

The individual first facets 21 are imaged in the object field 5 with the aid of the second facet mirror 22 . The second facet mirror 22 is the last beam-forming mirror or actually the last mirror for the illumination radiation 16 in the beam path in front of the object field 5.

In another embodiment of the illumination optics 4 that is not shown, transmission optics can be arranged in the beam path between the second facet mirror 22 and the object field 5 , which particularly contributes to the imaging of the first facets 21 in the object field 5 . The transmission optics can have exactly one mirror, but alternatively also have two or more mirrors, which are arranged one behind the other in the beam path of the illumination optics 4 . The transmission optics can in particular comprise one or two mirrors for normal incidence (NI mirror, normal incidence mirror) and/or one or two mirrors for grazing incidence (GI mirror, gracing incidence mirror).

The illumination optics 4 has the version in which 1 shown, exactly three mirrors after the collector 17, namely the deflection mirror 19, the field facet mirror 20 and the pupil facet mirror 22.

In a further embodiment of the illumination optics 4, the deflection mirror 19 can also be omitted, so that the illumination optics 4 can then have exactly two mirrors after the collector 17, namely the first facet mirror 20 and the second facet mirror 22.

The imaging of the first facets 21 by means of the second facets 23 or with the second facets 23 and transmission optics in the object plane 6 is generally only an approximate imaging.

The projection optics 10 includes a plurality of mirrors Mi, which are numbered consecutively according to their arrangement in the beam path of the projection exposure system 1 .

At the in the 1 example shown, the projection optics 10 includes six mirrors M1 to M6. Alternatives with four, eight, ten, twelve or another number of mirrors Mi are also possible. The projection optics 10 are doubly obscured optics. The penultimate mirror M5 and the last mirror M6 each have a passage opening for the illumination radiation 16. The projection optics 10 has an image-side numerical aperture which is, for example, greater than 0.5 and which can also be greater than 0.6 and which, for example, is 0 can be 7 or 0.75.

Reflection surfaces of the mirrors Mi can be designed as free-form surfaces without an axis of rotational symmetry. Alternatively, the reflection surfaces of the mirrors Mi can be designed as aspherical surfaces with exactly one axis of rotational symmetry of the reflection surface shape. Just like the mirrors of the illumination optics 4, the mirrors Mi can have highly reflective coatings for the illumination radiation 16. These coatings can be designed as multilayer coatings, in particular with alternating layers of molybdenum and silicon.

The projection optics 10 has a large object-image offset in the y-direction between a y-coordinate of a center of the object field 5 and a y-coordinate of the center of the image field 11. This object-image offset in the y-direction can be something like this be as large as a z-distance between the object plane 6 and the image plane 12.

The projection optics 10 can in particular be anamorphic. In particular, it has different image scales βx, βy in the x and y directions. The two image scales βx, βy of the projection optics 10 are preferably at (βx, βy)=(+/−0.25, /+−0.125). A positive image scale β means an image without image reversal. A negative sign for the imaging scale β means imaging with image inversion.

The projection optics 10 thus leads to a reduction in the ratio 4:1 in the x-direction, ie in the direction perpendicular to the scanning direction.

The projection optics 10 lead to a reduction of 8:1 in the y-direction, ie in the scanning direction.

Other imaging scales are also possible. Image scales with the same sign and absolutely the same in the x and y directions, for example with absolute values of 0.125 or 0.25, are also possible.

The number of intermediate image planes in the x-direction and in the y-direction in the beam path between the object field 5 and the image field 11 can be the same or, depending on the design of the projection optics 10, can be different. Examples of projection optics with different numbers of such intermediate images in the x and y directions are known from U.S. 2018/0074303 A1 .

In each case one of the pupil facets 23 is assigned to precisely one of the field facets 21 in order to form a respective illumination channel for illuminating the object field 5 . In this way, in particular, lighting can result according to Köhler's principle. The far field is broken down into a large number of object fields 5 with the aid of the field facets 21 . The field facets 21 generate a plurality of images of the intermediate focus on the pupil facets 23 assigned to them.

The field facets 21 are each imaged by an associated pupil facet 23 superimposed on the reticle 7 for illuminating the object field 5 . In particular, the illumination of the object field 5 is as homogeneous as possible. It preferably has a uniformity error of less than 2%. Field uniformity can be achieved by superimposing different illumination channels.

The illumination of the entrance pupil of the projection optics 10 can be defined geometrically by an arrangement of the pupil facets. The intensity distribution in the entrance pupil of the projection optics 10 can be set by selecting the illumination channels, in particular the subset of the pupil facets that guide light. This intensity distribution is also referred to as illumination setting or illumination pupil filling.

A likewise preferred pupil uniformity in the area of defined illuminated sections of an illumination pupil of the illumination optics 4 can be achieved by redistributing the illumination channels.

Further aspects and details of the illumination of the object field 5 and in particular the entrance pupil of the projection optics 10 are described below.

The projection optics 10 can in particular have a homocentric entrance pupil. This can be accessible. It can also be inaccessible.

The entrance pupil of the projection optics 10 cannot regularly be illuminated exactly with the pupil facet mirror 22 . When imaging the projection optics 10, which telecentrically images the center of the pupil facet mirror 22 onto the wafer 13, the aperture rays often do not intersect at a single point. However, a surface can be found in which the distance between the aperture rays, which is determined in pairs, is minimal. This surface represents the entrance pupil or a surface conjugate to it in position space. In particular, this surface shows a finite curvature.

The projection optics 10 may have different positions of the entrance pupil for the tangential and for the sagittal beam path.

In this case, an imaging element, in particular an optical component of the transmission optics, should be provided between the second facet mirror 22 and the reticle 7 . With the help of this optical element, the different positions of the tangential entrance pupil and the sagittal entrance pupil can be taken into account.

At the in the 1 In the arrangement of the components of the illumination optics 4 shown, the pupil facet mirror 22 is arranged in a surface conjugate to the entrance pupil of the projection optics 10 . The field facet mirror 20 is arranged tilted to the object plane 6 . The first facet mirror 20 is tilted relative to an arrangement plane that is defined by the deflection mirror 19 .

The first facet mirror 20 is tilted relative to an arrangement plane that is defined by the second facet mirror 22 .

By irradiating the mirrors 19, 20, 22, M1-M6 or other mirrors of the illumination optics 4 and the projection optics 10 with working light 16, these mirrors heat up due to absorption of the radiation. This can result in thermal deformations, which impair the imaging properties.

2 12 shows a cross-sectional view of an optical system 200 of the lithography system (projection exposure system) 1 from FIG 1 according to one embodiment.

The optical system 200 has a mirror 201 . The mirror 201 can be any of the mirrors 19, 20, 22, M1-M6 of the projection exposure system 1 ( 1 ) or around others in 1 not shown mirrors of the illumination optics 4 and the projection optics 10 of the projection exposure system 1 act. However, the invention is not limited to use in an EUV projection exposure system. In particular, the invention can also be used in a DUV projection exposure system or in another optical system.

The mirror 201 comprises an optically active surface 202 and a mirror substrate 203. The mirror substrate 203 is made, for example, from a TiO ₂ —SiO ₂ glass material or from a Li ₂ O—Al ₂ O ₃ —SiO ₂ glass ceramic (Zerodur). Furthermore, the mirror 201 includes, for example, a reflective layer system 204 to provide the optically active surface 202.

During operation of the projection exposure system 1, the mirror 201 ( 2 ) with working light 16 ( 1 ) illuminates and heats up.

The mirror substrate 203 comprises at least two sections 205, 206, which are arranged at different distances d1, d2 from the optically active surface 202. In other words, a first section 205 is arranged closer (distance d1) to the optically active surface 202, and a second section 206 is arranged further away (distance d2) from the optically active surface 202. For example, the first section 205 is arranged directly adjacent to the reflection layer system 204, which provides the optically active surface 202. For example, the second section 206 is arranged directly adjacent to the first section 205 .

Furthermore, a material of the two sections 205, 206 is manufactured and/or processed in such a way that the thermal expansion properties of the sections 205, 206 for the irradiation of the mirror 201, e.g. B. with EUV light 16 in the projection exposure system 1 ( 1 ), In the sections 205, 206 occurring temperatures and temperature gradients are particularly suitable. In particular, a zero crossing temperature ZCT of a thermal expansion coefficient (Coefficient of Temperature Expansion) CTE is set differently in the two sections 205 , 206 .

3 illustrates an example of a temperature profile of a coefficient of thermal expansion CTE ₁ of a material of the first section 205 and a coefficient of thermal expansion CTE ₂ of a material of the second section 206. The temperature at which the temperature-dependent function of the coefficient of thermal expansion CTE ₁ or CTE ₂ intersects the zero line is called the zero crossing temperature ZCT ₁ or ZCT ₂ of the coefficient of thermal expansion of the respective section 205, 206. As in 3 illustrated, in the example shown, the material has the closer section 205 arranged on the optically active surface 202 has a higher zero crossing temperature ZCT ₁ than the material of the section 206 arranged further from the optically active surface 202 (ZCT ₁ >ZCT ₂ ).

Furthermore, the zero crossing temperature changes, for example, in a limit area 207 ( 2 ) of the two sections 205, 206 continuously from the value ZCT ₁ to the value ZCT ₂ in order to avoid mechanical stress due to a discontinuous zero crossing temperature of the thermal expansion coefficient.

The mirror substrate 203 can have cooling channels 208 for actively cooling the mirror 201 . In 2 two of the cooling channels are provided with a reference number by way of example. In the example of 2 the cooling channels 208 are arranged in the second section 206 of the mirror substrate 203 . In particular, the cooling channels 208 are arranged in a region 209 of the second section 206 which is adjacent to the first section 205 . In other examples, the cooling channels 208 can also be arranged in the first section 205, in particular in an area 210 of the first section 205 that is adjacent to the second section 206. Furthermore, the cooling channels 208 can also be arranged in both and/or between the two sections 205, 206.

In addition to or instead of setting different zero crossing temperatures ZCT ₁ and ZCT ₂ in the two sections 205, 206 of the mirror substrate 203, a homogeneity of a spatial distribution of the zero crossing temperature in the two sections 205, 206 can also be set differently. In particular, the spatial distribution of the zero crossing temperature can be more homogeneous in the first section 205, which is arranged closer to the optically active surface 202, than in the second section 206, which is arranged further away from the optically active surface 202.

In addition, a slope of a temperature dependency of the coefficient of thermal expansion CTE ₁ , CTE ₂ adjacent to the zero crossing temperature ZCT1 , ZTC2 can also be different in the two sections 205 , 206 .

The following is related to the 4 until 6 discloses a method for manufacturing the optical system 200 2 described according to one embodiment.

In a first step S1, S1' of the method, a mirror substrate 203 ( 2 ) with at least two sections 205, 206, wherein a zero crossing temperature ZCT ₁ , ZTC ₂ a thermal expansion coefficient CTE ₁ , CTE ₂ ( 3 ) is designed differently in the two sections 205, 206.

In the first step of the method S1, S1', the two sections 205, 206 of the mirror substrate 203 can be produced individually (step S1 in 4 ).

Alternatively, as in 5 1, at least one mirror substrate ingot 211 can be manufactured (step S11'). The at least one mirror substrate ingot 211 is then divided into two sections 205', 206' (corresponding to sections 205, 206 in 2 ) separately (step S12').

If the two sections 205, 206 of the mirror substrate 203 are produced individually in step S1, then the two sections 205, 206 for setting a different zero crossing temperature ZCT ₁ , ZCT ₂ of the thermal expansion coefficient CTE ₁ , CTE ₂ can be produced from different material compositions. In addition or instead, the two sections 205, 206 can also be subsequently heat-treated in different ways (eg by tempering) to set a different zero crossing temperature ZCT ₁ , ZCT ₂ of the thermal expansion coefficient CTE ₁ , CTE ₂ . In particular, in step S1 a zero crossing temperature ZCT ₁ of the first section 205 is set higher than a zero crossing temperature ZCT ₂ of the second section 206. For example, a zero crossing temperature ZCT ₁ of the first section 205 is set to 28° C. and a zero crossing temperature ZCT ₂ of the second section 206 is set 22°C. However, the zero crossing temperatures ZCT ₁ , ZCT ₂ of the two sections 205, 206 can also, for example depending on the operating temperature to be expected (eg average operating temperature) during the irradiation of the mirror 201 ( 2 ) with light (e.g. EUV light 16, 1 ), can be set to other temperature values.

Furthermore, the sections 205, 206 can also be produced in step S1 in such a way that their (respective) material composition, in particular the spatial distribution of their (respective) zero crossing temperature ZCT ₁ , ZCT ₂ , has a different degree of homogeneity. For example, a material composition of the first section 205 is made more homogeneous (eg better mixed) than a material composition of the second section 206, so that the first section 205 can be exposed to higher temperatures and temperature gradients without being severely deformed.

If, on the other hand, the two sections 205', 206' of the mirror substrate 203' are made from a mirror substrate ingot 211 ( 5 ) by separating (e.g. cutting), then the different zero crossing temperatures ZCT ₁ , ZCT ₂ of the coefficient of thermal expansion CTE ₁ , CTE ₂ of the two sections 205 ', 206' can be adjusted to different temperature values by subsequent treatment, in particular heat treatment (annealing). to be set.

In a variant of step S1', two or more mirror substrate ingots - similar to ingot 211 in 5 , however, the two or more ingots have different material compositions from each other, in particular different zero crossing temperatures of the coefficient of thermal expansion - two or more first sections (corresponding to section 205') and two or more second sections (corresponding to section 206') produced by cutting. Corresponding to a mutual combination of the corresponding sections, mirror substrates (similar to the mirror substrate 203, 203′) can be produced in this way, which have a different design of the zero crossing temperature in the two sections.

In a second optional step S2 of the method, cooling channels 208 can be formed, in particular milled, in one or both sections 205, 206. The cooling channels 208 can, as in 2 shown only be milled in the second section 206 of the mirror substrate 203 . Alternatively, they can also only be milled into the first section 205 of the mirror substrate 203 (not shown). 6 Figure 12 illustrates another alternative in which the cooling channels 208' are milled into both sections 205", 206".

In a third step S3 of the method, the two sections 205, 206 (or 205', 206' or 205'', 206'') are joined together. In particular, the two sections 205, 206 (or 205', 206' or 205'', 206'') are joined together physically and chemically. For example, by heating a boundary area (see 207 in 2 ) assembled in such a way that the thermal expansion properties, in particular the zero-crossing temperature, change continuously from the first section 205 (205', 205'') to the second section 206 (206', 206'').

Although the present invention has been described using exemplary embodiments, it can be modified in many ways.

Reference List

1

projection exposure system

2

lighting system

3

light source

4

lighting optics

5

object field

6

object level

7

reticle

8th

reticle holder

9

reticle displacement drive

10

projection optics

11

image field

12

picture plane

13

wafers

14

wafer holder

15

Wafer displacement drive

16

illumination radiation

17

collector

18

intermediate focal plane

19

deflection mirror

20

first facet mirror

21

first facet

22

second facet mirror

23

second facet

200

optical system

201

optical element

202

optically active surface

203, 203'

mirror substrate

204

reflective layer system

205, 205', 205''

section

206, 206', 206''

section

207

border area

208, 208'

cooling channel

209

area

210

area

211

ingot

CTE

coefficient of thermal expansion

CTE1, CTE2

coefficient of thermal expansion

d1

Distance

d2

Distance

M1-M6

mirror

S1-S3

process steps

S1', S11', S21'

process steps

ZCT1, ZCT2

zero crossing temperature

QUOTES INCLUDED IN DESCRIPTION

This list of the documents cited by the applicant was generated automatically and is included solely for the better information of the reader. The list is not part of the German patent or utility model application. The DPMA assumes no liability for any errors or omissions.

Patent Literature Cited

• DE 102008009600 A1 [0063, 0068]
• US 2006/0132747 A1 [0066]
• EP 1614008 B1 [0066]
• US6573978 [0066]
• DE 102017220586 A1 [0071]
• US 2018/0074303 A1 [0085]

Claims (14)

Optical system (200) for a lithography system (1), comprising a mirror (201) with an optically active surface (202) and a mirror substrate (203), wherein the mirror substrate (203) comprises at least two sections (205, 206) which are arranged at different distances (d1, d2) from the optically active surface (202), and a zero crossing temperature (ZCT ₁ , ZCT ₂ ) of a coefficient of thermal expansion (CTE ₁ , CTE ₂ ) in the two sections (205, 206) is configured differently. Optical system after claim 1 , wherein the mirror substrate (203) has cooling channels (208) which are arranged between the two sections (205, 206), in the section (205) closer to the optically active surface (202) and/or in the section further from the optically active surface (202) remotely located portion (206). Optical system after claim 1 or 2 , wherein the zero crossing temperature (ZCT ₁ , ZCT ₂ ) of the thermal expansion coefficient (CTE ₁ , CTE ₂ ) in the two sections (205, 206) with respect to a height of a value of the zero crossing temperature (ZCT ₁ , ZCT ₂ ), a homogeneity of a spatial Distribution of the zero crossing temperature (ZCT ₁ , ZCT ₂ ) and/or a slope of a temperature dependence of the coefficient of thermal expansion (CTE ₁ , CTE ₂ ) adjacent to the zero crossing temperature (ZCT ₁ , ZCT ₂ ) is configured differently. Optical system after claim 3 , wherein the zero-crossing temperature (ZCT ₁ , ZCT ₂ ) of the coefficient of thermal expansion (CTE ₁ , CTE ₂ ) in the section (205) closer to the optically active surface (202) is set to a higher value (ZCT ₁ ) than in the section further from the optically active surface (202) arranged section (206). Optical system after claim 3 or 4 , wherein the spatial distribution of the zero crossing temperature (ZCT ₁ , ZCT ₂ ) is more homogeneous in the section (205) arranged closer to the optically active surface (202) than in the section (206) arranged further from the optically active surface (202). Optical system according to one of claims 3 - 5 , wherein the slope of the temperature dependence of the coefficient of thermal expansion (CTE ₁ , CTE ₂ ) adjacent to the zero crossing temperature (ZCT ₁ , ZCT ₂ ) is smaller in the section (205) closer to the optically active surface (202) than in the section (205) further from the optically active one Area (202) arranged section (206). Optical system according to one of Claims 1 - 6 , wherein the configuration of the zero crossing temperature (ZCT ₁ , ZCT ₂ ) changes continuously in a border area (207) of the two sections (205, 206). Lithography system (1) with an optical system (200) according to one of Claims 1 - 7 , Method for producing an optical system (200) for a lithography system (1), wherein the optical system has a mirror (201) with an optically active surface (202) and a mirror substrate (203), and the method includes the step of producing (S1 , S1 ') of the mirror substrate (203, 203') with at least two sections (205, 206, 205', 206'), wherein a zero crossing temperature (ZCT ₁ , ZCT ₂ ) of a thermal expansion coefficient (CTE ₁ , CTE ₂ ) in the the two sections (205, 206, 205', 206') is designed differently, and the two sections (205, 206, 205', 206') are set up to be at different distances (d1, d2) from the optically active surface (202 ) to be arranged. procedure after claim 9 , wherein the two sections (205, 206, 205', 206') of the mirror substrate (203, 203') are produced individually (S1), or by separating (S12') at least one mirror substrate ingot (211) (S1 '). procedure after claim 9 or 10 , comprising a step of assembling (S3) the two sections (205, 206, 205', 206'). Procedure according to one of claims 9 - 11 , cooling channels (208, 208') being formed, in particular milled, in one or both sections (205, 206, 205", 206") (S2). Procedure according to one of claims 9 - 12 , wherein the two sections (205, 206, 205 ', 206') of the mirror substrate (203, 203 ') to form the different configuration of the zero crossing temperature (ZCT ₁ , ZCT ₂ ) of the thermal expansion coefficient (CTE ₁ , CTE ₂ ) from different material compositions and/or are produced by varying degrees of homogenization of one or more material compositions. Procedure according to one of claims 9 - 13 , wherein at least one of the two sections (205, 206, 205 ', 206') of the mirror substrate (203, 203 ') to form the different configuration of the zero crossing temperature (ZCT ₁ , ZCT ₂ ) of the thermal expansion coefficient (CTE ₁ , CTE ₂ ) subsequently treated, in particular heat-treated or tempered.

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