DE102022209397A1 - Lithography system and optical element with flow-through channels - Google Patents

Abstract

One aspect of the invention relates to a lithography system, comprising: a radiation source for generating radiation, in particular for generating EUV radiation (16), at least one optical element (25), comprising: a substrate (26) with an optical surface (27) , to which a reflective coating (28) for reflecting radiation, in particular EUV radiation (16), is applied, wherein a plurality of channels (29) through which a fluid (30) can flow runs in the substrate (26), the are spaced from the optical surface (27). A distance (d) of at least one channel (29) from the optical surface (27) and/or a lateral distance (p) between at least two of the channels (29) depends on a power density impinging on the optical surface (27). Radiation (16) set the radiation source. Another aspect of the invention relates to an optical element (25).

Description

Background of the Invention

The invention relates to a lithography system, comprising: a radiation source for generating radiation, in particular EUV radiation, and at least one optical element, comprising: a substrate with an optical surface on which a reflective coating for reflecting radiation, in particular EUV Radiation, is applied, wherein a plurality of channels through which a fluid can flow run in the substrate and are spaced apart from the optical surface. The invention also relates to an optical element for reflecting radiation, in particular for reflecting EUV radiation, comprising: a substrate having an optical surface to which a reflective coating for reflecting radiation, in particular EUV radiation, is applied, wherein in a plurality of channels through which a fluid can flow run along the substrate and are spaced apart from the optical surface.

The lithography system can be a lithography system for exposing a wafer or another optical arrangement for lithography. In particular, it can be an EUV lithography system, i.e. a lithography system that uses EUV radiation. For example, it can be an EUV inspection system, e.g. for the inspection of masks, wafers or the like used in EUV lithography. The optical element with the reflective coating can in particular be a mirror, which is why the optical surface is also referred to below as a mirror surface.

With increasing power of the radiation source of lithography systems, thermally induced deformations of mirror surfaces and associated aberrations increase. One way of counteracting these effects is direct cooling of the mirrors. With direct cooling, channels or channel structures are introduced into the volume of the substrate through which a (cooling) fluid can flow. The cooling fluid is usually water. The channel structures or the channels have sections that run below the optical surface in order to effectively dissipate the thermal energy released when the radiation hits the optical surface, in order to set the absolute temperature level on the optical surface in a targeted manner and to adjust the local temperature gradients to reduce the optical surface. The design and the arrangement of the channels have a significant influence on the performance of the cooled optical element.

Depending on the (local) power density of the radiation impinging on the optical surface, high temperature differences between the optical surface and the channels through which the flow passes, as well as high temperature gradients along the optical surface, can also occur in the case of directly cooled optical elements. The main problem here is the spatially concentrated high power peaks on the optical surface, which locally lead to high temperatures and temperature gradients.

In the DE 102011005778 A1 describes an optical element for a projection exposure system for semiconductor lithography, which has an optically active surface and at least one cooling component for cooling the optical element, the cooling component being connected to at least two separate cooling circuits and being designed in such a way that the optically active surface is at least a sub-area can be cooled more than in another sub-area. The optical element can have a mirror carrier which is provided with a heat sink or designed as a heat sink and has cooling ducts which are arranged in different cooling zones for cooling individual partial areas.

In the DE102018202687A1 or in the DE102019200750A1 describes a manufacturing method for a cavity structure in a component of a projection exposure system, in which a hollow structure is produced in the component, for example in a mirror body, by selective laser etching. In the method, at least one of a plurality of accesses for an etching attack to create the hollow structure is formed as an auxiliary etching access which is not produced by selective laser etching.

The DE102016221878A1 describes a projection exposure system for semiconductor lithography with a plurality of components, in which at least one component has a structure produced by means of selective laser etching, for example a temperature control channel.

In the WO2021115643A1 describes an optical element for reflecting radiation, which has a substrate with a surface on which a reflective coating is applied. At least one channel through which a cooling medium can flow is formed in the substrate. The channel has a length of at least 10 cm and a cross-sectional area of the channel does not vary by more than +/- 20% over the length of the channel. The distance of the channels to the surface, the measured between the midpoint of the cross-sectional area of the respective cooling channel and the surface can be around 1 cm. The distance of at least one channel to the surface may be between 1 and 3 times the distance between adjacent channels. The cross-sectional area of the channel may have a height to width ratio of less than 5:1, for example the height to width ratio may be >1.0 or >0.9.

object of the invention

The object of the invention is to provide a lithography system and an optical element in which the arrangement and/or the geometry of the channels is selected in such a way that temperature gradients on the optical surface are reduced.

subject of the invention

This object is achieved according to a first aspect by a lithography system, in particular by an EUV lithography system, of the type mentioned in which a distance of at least one channel from the optical surface and / or a lateral distance between at least two of the channels depending on a Power density of the radiation of the radiation source impinging on the optical surface is fixed.

The inventors have recognized that an optimal arrangement of the channels, in particular their distance from the optical surface and their lateral distance, should be selected depending on the power density of the radiation impinging on the optical surface in order to minimize temperature gradients on the optical surface as much as possible.

In the context of this application, the distance from the optical surface is understood to mean the minimum distance that the respective channel, more precisely its peripheral lateral surface, has from the optical surface. If the distance in the longitudinal direction of the channel varies, the distance is understood to be the arithmetic mean of the distances in the longitudinal direction of the channel from the optical surface. The lateral distance between adjacent channels is understood to mean the distance between the centers of the cross-sectional areas of adjacent channels. If the lateral distance varies in the longitudinal direction of the channels, the lateral distance is understood to be the arithmetic mean of the lateral distances in the longitudinal direction of a respective pair of adjacent channels.

The smaller the distance between the channels and the optical surface, the smaller the temperature difference between the optical surface and the fluid that flows through the channels for cooling the optical element. In principle, the temperature gradients on the optical surface are reduced the smaller the distance between the channels and the optical surface is, regardless of the power distribution of the radiation impinging on the surface. However, the distance between the channels and the optical surface cannot be chosen to be arbitrarily small due to manufacturing reasons. If the distance between the channels and the optical surface is small, the structure or the arrangement of the channels can be seen in the temperature distribution on the optical surface in the form of a pattern corresponding to the arrangement of the channels, which should be avoided.

In one embodiment, a maximum power density of the radiation at the optical surface is less than 2 mW/mm ² and the distance of at least one channel, in particular all channels, from the optical surface is 5 mm or less. The maximum power density is the power density (also: heat flow density) at that point on the optical surface where the power density is maximum. The maximum power density at the optical surface depends, among other things, on the lighting settings of the lithography system. The maximum power density refers to the maximum possible power density at the optical surface that can be generated with the lithography system (under operating conditions) at all available illumination settings.

The temperature gradients at the optical surface typically increase with increasing maximum power density of the radiation at the optical surface. In order to efficiently reduce the temperature gradients, smaller distances between the channels and the surface are therefore usually required with a larger maximum power density. However, for the reasons described above, it does not always make sense to choose a minimum distance between the surface and the channels. It has been shown that with the maximum power density specified above, a (maximum) distance between the channels and the optical surface of 5 mm or less is sufficient to largely compensate for the temperature gradients on the optical surface.

In an alternative embodiment, a maximum power density of the radiation at the optical surface is between 2 mW/mm ² and 3 mW/mm ² and the distance of at least one channel, in particular all channels, from the optical surface is 3 mm or less. It has showed that in the specified maximum power density interval, a (maximum) distance of 3 mm or less is sufficient to effectively suppress the temperature gradients at the optical surface.

In a further alternative embodiment, a maximum power density of the radiation at the optical surface is more than 3 mW/mm ² and the distance of at least one channel, in particular all channels, from the optical surface is 2 mm or less. It has been shown that with a maximum power density of more than 3 mW/mm ² the distance of a respective channel from the optical surface should generally be 2 mm or less, possibly 1 mm or less, in order to accommodate the temperature gradients of the optical surface to a sufficient extent.

In one embodiment, the distance of at least one channel, in particular all channels, from the optical surface is defined as a function of a local power density of the radiation at the position of the respective channel. In this case, the distance between a respective channel and the optical surface is determined depending on the local power density and thus depending on the power distribution on the optical surface, i.e. the local cooling by means of the fluid is adapted to the power distribution on the optical surface. The distance between the channels and the optical surface is reduced in areas or at positions with a high power density in order to locally improve the cooling effect and to dampen temperature peaks. The channels are typically located at different distances from the optical surface in this embodiment. The local power density, which represents the arithmetic mean of all possible illumination settings, is used to determine the distance between a respective channel and the optical surface.

In a development of this embodiment, the product of the local power density on the optical surface and the distance of a respective channel from the optical surface is essentially constant for all channels. In order to generate the most homogeneous possible temperature distribution on the optical surface, it is beneficial if the following relationship between the local power density q(x,y) (heat flow density) on the optical surface and the distance d(x,y) of a respective channel from the optical surface is maintained: $$\text{q}\left(\text{x},\text{y}\right)\times \text{i.e}\left(\text{x},\text{y}\right)=cOnst.$$

In the example described here, it is assumed that the optical surface is a flat surface whose Cartesian coordinates x, y designate the respective position of the local power density. The distance to the respective channel is measured in a direction z perpendicular to the optical surface. The position x,y, at which the distance to the optical surface or the local power density is determined, corresponds to the center of a respective channel or the cross section of the respective channel. The distance d(x,y) in the z-direction is measured as above from the top of the lateral surface of the channel. In case the optical surface is curved, the direction in which the distance is determined corresponds to the normal direction of the optical surface at the respective position of the channel.

In the context of this application, “essentially constant” means that the maximum value c _max and the minimum value c _min of the product of the local power density q(x,y) and the respective distance d(x,y) are: 0 .9 < c _max / c _min < 1.1. The closer the ratio between the maximum value c _max and the minimum value c _min is to 1, the more homogeneous is typically the temperature distribution on the optical surface.

Complex geometries of the channels or channel structures can result from the requirement that the local distance of a respective channel from the optical surface should vary according to the local power density of the incident radiation. If, for example, the power maximum of the incident radiation occurs predominantly at the edges of the optical surface, the local distance of the channels from the optical surface can be reduced in these areas, while a larger local distance from the optical surface is defined in the center of the optical surface.

In a further embodiment, the lateral spacing between at least two adjacent channels, in particular between all channels, is defined as a function of a local power density of the radiation impinging on the optical surface. In this case, the density of the channels in the lateral direction is determined as a function of the power distribution of the radiation impinging on the surface. In areas of high power density, the density, ie the lateral distance between adjacent channels, can be increased in order to locally improve the cooling effect of the fluid flowing through the channels and to dampen temperature peaks. In areas of low power density, the distances between adjacent channels can be increased accordingly. If, for example, the maximum power occurs predominantly at the edges of the optical surface, the channel density in these areas can be increased, while in the center of the optical surface the channel density is reduced in the lateral direction.

In a further embodiment, at least two of the channels, in particular all channels, have a lateral distance of 4.5 mm or less, preferably 3 mm or less, particularly preferably 2 mm or less from one another and/or at least one of the channels, in particular all channels have a channel width which is greater than 1.1 times, preferably greater than 1.5 times, in particular greater than 2 times and preferably no greater than 5 times a channel height.

In this embodiment, the density of the channels below the optical surface is maximized or selected as large as possible. Such maximization of the density of channels below the optical surface typically results in a more spatially uniform cooling effect. Regardless of the power distribution at the optical surface, the surface temperature is reduced in this way and spatial temperature gradients are attenuated. The increase in the lateral density of the channels can be achieved by reducing the lateral distance between the cooling structures (e.g. the lateral distance between two adjacent channels) and/or by widening the individual channels in the lateral direction (e.g. increasing the width of the channels in relation to their height). It goes without saying that the channels do not necessarily have to meet the condition specified above that the channel width is greater than 1.1 times the channel height. In particular, the channel width can correspond to the channel height or be smaller than the channel height.

The height and the width of the channels refer to the cross-sectional area of the channels in a plane perpendicular to the optical surface. The height is measured in the cross-sectional area perpendicular to the optical surface, the width parallel to the optical surface. The channels described here do not necessarily have a square or rectangular cross-section, rather the cross-sectional geometry of the channels is basically arbitrary, e.g. polygonal, circular, elliptical, oval, ... The height and width in the respective cross-section refer to the maximum extension of the channels perpendicular or parallel to the optical surface.

Increasing the density of the cooling channels is particularly beneficial if the channels are very close to the optical surface, e.g. 2 mm, 1 mm or less, since otherwise the pattern of the channels is visible in the form of irregularities in the temperature distribution on the optical surface (so.). By reducing the lateral distance between the channels, the temperature profile can be made more uniform even if the distance between the channels and the optical surface is small, e.g. 1 mm. For this purpose, for example, when the distance between the channels and the optical surface is reduced from approx. 2 mm to approx. 1 mm, the lateral distance between the channels can be reduced from 4.5 mm to 3 mm or less.

Another aspect of the invention relates to an optical element of the type mentioned in which at least one channel, in particular all channels, has a distance of 5 mm or less, particularly preferably 3 mm or less, in particular 2 mm or less, from the optical Have surface of the substrate.

As described above, regardless of the power distribution of the radiation impinging on the optical surface, minimizing the distance between the channels and the optical surface minimizes the temperature difference between the cooling fluid and the optical surface. The small distance specified above reduces local temperature gradients on the optical surface. Depending on the local power density (see above), the distances between a respective channel and the surface can be chosen to be of different sizes. In the embodiment described here, none of the channels are located more than 5 mm, 3 mm or 2 mm from the optical surface. As described above, the greater the maximum power density of the radiation impinging on the optical surface, the smaller the distance of the channels from the optical surface should be.

In one embodiment, the plurality of channels are at a constant distance from the optical surface. In this case, all channels are equidistant from the optical surface, i.e. the distances between the channels and the optical surface are not fixed depending on the local power density at the optical surface.

A further aspect of the invention, which can in particular be combined with the aspects described above, relates to an optical element of the type mentioned initially, in which at least two of the channels, in particular all channels, preferably have a lateral spacing of 4.5 mm or less of 3 mm or less, more preferably 2 mm or less from each other and/or in which at least one of the channels, in particular all channels, has a channel width which is greater than 1.1 times, preferably greater than 1.5 times, in particular greater than 2 times and preferably no greater than is 5 times a channel height.

In this aspect of the invention, the density of the channels beneath the optical surface and hence the heat exchanging surface area into the fluid is maximized. As described above, this is particularly favorable when the channels have a very small distance from the optical surface, which can be, for example, 2 mm or less or 1 mm or less, in order to avoid that the pattern of the channels in the Temperature distribution on the optical surface becomes visible. It goes without saying that the channels do not necessarily have to meet the condition that the channel width is greater than 1.1 times the channel height. In particular, the channel width can correspond to the channel height or be smaller than the channel height.

In another embodiment, the substrate is monolithic. In this case, the channels can be introduced into the substrate, for example, by selective laser etching, as is the case in the document cited at the outset WO2021115643A1 is described. The channels can also be created in the substrate in other ways, for example by laser ablation.

In a further embodiment, the substrate has a first part-body and a second part-body which are assembled at an interface, with the plurality of channels running in the substrate in the region of the interface between the two part-bodies. In this case, the substrate is composed of two or more partial bodies, which are connected to one another at the interface by a suitable bonding method. The cross section of a respective cooling channel can be divided between the two partial bodies. In this case, a groove is typically milled into both part bodies. However, it is also possible for a groove to be milled only into the first partial body or only into the second partial body and the respective other partial body to cover the groove in the manner of a cover in order to form the cross section of the cooling channel. In both cases, the boundary surface runs within or at the edge of the cross section of a respective cooling channel. The cover, which covers the cooling channel, is typically the first partial body on which the surface with the reflective coating is formed.

Further features and advantages of the invention result from the following description of exemplary embodiments of the invention, with reference to the figures of the drawing, which show details essential to the invention, and from the claims. The individual features can each be realized individually or together in any combination in a variant of the invention.

character list

• 1 schematisch im Meridionalschnitt eine Projektionsbelich-tungsanlage für die EUV-Projektionslithografie,
• 2 eine schematische Schnittdarstellung eines EUV-Spiegels mit einem Substrat, das eine Mehrzahl von mit einem Kühlfluid durchströmbaren Kanälen aufweist,
• 3a-c schematische Darstellungen des Spiegels von 2 mit einer Mehrzahl von Kanälen, die jeweils in konstantem Abstand von einer optischen Oberfläche und in einem konstanten lateralen Abstand zueinander angeordnet sind,
• 4a,b zwei Diagramme, welche die Abhängigkeit des maximalen Temperaturgradienten vom Abstand der Kanäle zur optischen Oberfläche bzw. vom lateralen Abstand zwischen benachbarten Kanälen zeigen, sowie
• 5a,b eine Darstellung eines optischen Elements, bei dem der Abstand zwischen den Kanälen und der optischen Oberfläche in Abhängigkeit von der lokalen Leistungsdichte der auf die optische Oberfläche auftreffenden Strahlung festgelegt ist.

Exemplary embodiments are shown in the schematic drawing and are explained in the following description. It shows

• 1 a schematic meridional section of a projection exposure system for EUV projection lithography,
• 2 a schematic sectional view of an EUV mirror with a substrate having a plurality of channels through which a cooling fluid can flow,
• 3a-c schematic representations of the mirror from 2 with a plurality of channels, each of which is arranged at a constant distance from an optical surface and at a constant lateral distance from one another,
• 4a,b two diagrams showing the dependence of the maximum temperature gradient on the distance between the channels and the optical surface and on the lateral distance between adjacent channels, and
• 5a,b a representation of an optical element in which the distance between the channels and the optical surface is determined as a function of the local power density of the radiation impinging on the optical surface.

In the following description of the drawings, identical reference symbols are used for identical or functionally identical components.

The following are referring to 1 the essential components of an optical arrangement for EUV lithography in the form of a projection exposure system 1 for microlithography are described by way of example. The description of the basic structure of the projection exposure system 1 and of its components is not to 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 illuminated. 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 1 a Cartesian xyz coordinate system is drawn in for explanation. The x-direction runs perpendicular to the plane of the drawing. The y-direction is horizontal and the z-direction is vertical. 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 comprises a projection system 10. The projection system 10 is used to image the object field 5 in an image field 11 in an image plane 12. A structure on the reticle 7 is imaged on a light-sensitive layer of a wafer arranged in the region of the image field 11 in the image plane 12 13. 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 or illumination 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 mirror 17 . The collector mirror 17 can be a collector mirror with one or more ellipsoidal and/or hyperboloidal reflection surfaces. The at least one reflection surface of the collector mirror 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° become. The collector mirror 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 mirror 17, the illumination radiation 16 propagates through an intermediate focus in an intermediate focus plane 18. The intermediate focus plane 18 can represent a separation between a radiation source module, comprising the radiation source 3 and the collector mirror 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. 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. A second facet mirror 22 is arranged downstream of the first facet mirror 20 in the beam path of the illumination optics 4. The second facet mirror 22 comprises a plurality of second facets 23.

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

The projection system 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 illustrated example, the projection system 10 comprises six mirrors M1 to M6. Alternatives with four, eight, ten, twelve or another number of mirrors Mi are also possible. The penultimate mirror M5 and the last mirror M6 each have a passage opening for the illumination radiation 16. The projection system 10 involves doubly obscured optics. The projection optics 10 has an image-side numerical aperture that is greater than 0.4 or 0.5 and which can also be greater than 0.6 and which can be, for example, 0.7 or 0.75.

Like the mirrors of the illumination optics 4, the mirrors Mi can have a highly reflective coating for the illumination radiation 16.

2 FIG. 1 shows a sectional representation of a mirror 25, which can be, for example, one of the mirrors Mi of the projection optics 10 or another reflective optical element of the EUV lithography system 1. The mirror 25 has a substrate 26 which is formed from a material with a low coefficient of thermal expansion, for example from Zerodur® or from Clearceram®. The substrate 26 has an optical surface 27 which is planar in the example shown, but which can also deviate from a planar geometry and, for example, can be concavely or convexly curved. A reflective coating 28 is applied to the optical surface 27 . In the example shown, the reflective coating 28 has a plurality of pairs of layers made of materials, each with a different real part of the refractive index, which can be formed from Si and Mo, for example, at a wavelength of 13.5 nm for the EUV radiation 16 impinging on the optical surface 27 .

As in 2 As can be seen, the substrate 26 has a plurality of channels 29 through which a fluid 30 flows. In the example shown, the fluid 30 is water, but it can also be another liquid or possibly a gas. In the example shown, the channels 29 run in a straight line along a longitudinal direction which corresponds to the x-direction of an xyz coordinate system. All of the channels 29 are arranged at a constant distance d from the optical surface 27 of the mirror 25 . The channels 29 have a square cross-section with a height h measured in the z-direction perpendicular to the optical surface 27 and a width b measured in the y-direction perpendicular to the longitudinal direction (x-direction) of the channels 29 . In the example shown, the following applies: h = b = 2 mm. The lateral distance p between the centers of the cross-sectional areas of two adjacent channels 29 is the same for all channels 29 . In the example shown, the following applies: p = 4 mm.

As in 3a As can be seen, the fluid 30 is fed to a respective channel 29 formed in the substrate 26 via an inlet opening 31a on the circumferential, cylindrical lateral surface of the substrate 26 and is discharged from the substrate 26 via an opposite outlet opening 31b in the longitudinal direction.

For supplying the fluid 30 to the respective inlet opening 31a and for removing the fluid 30 from the respective outlet opening 31b, the projection exposure system 1 has a cooling device 32, which is shown schematically in 1 is shown. In the example shown, the cooling device 32 is used to supply the fluid 30 in the form of cooling water to the channels 29 (hereinafter: cooling channels) and for this purpose has a supply line (not shown) which is connected in a fluid-tight manner to the respective inlet openings 31a. The cooling device 32 also has a discharge line, not illustrated, in order to discharge the fluid 30 from the respective outlet openings 31b.

At the in 2 In the mirror 25 shown, the substrate 26 is composed of two partial bodies 26a, 26b along an interface 33 which forms a joining surface. The cooling channels 29 are formed by grooves which are made in the second part-body 26b adjacent to the interface 33 . The first partial body 26a is plate-shaped and covers the grooves or the cooling channels 29 in the second partial body 26b over the entire surface. It goes without saying that part of the cross section of the cooling channels 29 can also be formed in the first part body 26a. In addition, it is not absolutely necessary for the substrate 25 to be composed of two or more sub-bodies 26a, b; the substrate 25 can rather also be designed in one piece (monolithic).

The EUV radiation 16 strikes at the in 2 and 3a-c example shown with a location-dependent varying power distribution on the optical surface 27, which is a dipole distribution that has two poles along the y-direction with a local, maximum power density q _max . Without cooling through the cooling channels 29, the locally different power density of the EUV radiation 16 impinging on the optical surface 27 leads to an inhomogeneous temperature distribution on the optical surface 27, in which high temperature gradients occur in particular in the vicinity of the two poles of the dipole distribution. as in 3a-c is indicated by dashed lines. In order to minimize temperature gradients on the optical surface 27 as far as possible, it is necessary to select the arrangement or the geometry of the cooling channels 29 in a suitable manner, as is described in more detail below.

wobei für T_max gilt: T_max= T_max,os - T_ref, wobei T_max,os die maximale Temperatur an der optischen Oberfläche 27 und T_ref die Umgebungstemperatur (22°C) bezeichnet, welche der minimalen Temperatur an der optischen Oberfläche 27 entspricht. T_max,b entspricht der Differenz zwischen der maximalen Temperatur an der optischen Oberfläche 27 bezogen auf die Umgebungstemperatur T_ref für den Fall, dass der Abstand d eines jeweiligen Kühlkanals 29 zur optischen Oberfläche 27 bei 5 mm liegt, was beispielhaft als Referenzwert festgelegt wurde. Der Wert von T_max,b ist abhängig von der Wärmestromdichteverteilung und hängt u.a. von der Anordnung des Spiegels 25 in der Projektionsbelichtungsanlage 1 ab. T_max entspricht dem PV-Wert der Temperaturverteilung an der optischen Oberfläche 27. 4a shows the dependence of the (relative) temperature gradient ΔT _max,rel on the distance d of a respective cooling channel 29 from the optical surface 27. The temperature gradient ΔT _max,rel is defined as follows: $$\mathrm{\Delta }{\text{T}}_{\text{Max}\text{,rel}}={\text{T}}_{\text{Max}}/{\text{T}}_{\text{Max}\text{, b}}-1\text{,}$$

where T _max is: T _max = T _max,os - T _ref , where T _max,os is the maximum temperature at the optical surface 27 and T _ref is the ambient temperature (22°C) which is the minimum temperature at the optical surface 27 corresponds. T _max,b corresponds to the difference between the maximum temperature on the optical surface 27 in relation to the ambient temperature T _ref in the event that the distance d of a respective cooling channel 29 from the optical surface 27 is 5 mm, which was specified as a reference value by way of example. The value of T _max,b depends on the heat flow density distribution and depends, among other things, on the arrangement of the mirror 25 in the projection exposure system 1 . T _max corresponds to the PV value of the temperature distribution on the optical surface 27.

As in 4a can be seen, it is fundamentally favorable for the reduction of the temperature gradient if the cooling channels 29 have the smallest possible distance d from the optical surface 27, which is 5 mm (reference in 4a) or less, 3 mm or less, or 2 mm or less. It has been shown that the distance d between the cooling channels 29 and the optical surface 27 should be selected to be smaller, the greater the maximum power density q _max at the optical surface 27 is.

In the event that the maximum power density q _max of the EUV radiation 16 on the optical surface 27 is less than 2 mW/mm ² , the distance d of at least one cooling channel 29, in particular all cooling channels 29, from the optical surface 27 should be 5mm or less. In the event that the maximum power density q _max of the EUV radiation 16 on the optical surface 27 is between 2 mW/mm ² and 3 mW/mm ² , the distance d of at least one cooling channel 29, in particular all cooling channels 29, should be optical surface 27 is 3 mm or less. In the event that a maximum power density q _max of the radiation 16 on the optical surface 27 is more than 3 mW/mm ² , the distance d of at least one cooling channel 29, in particular all cooling channels 29, from the optical surface 27 should be 2 mm or less lie.

How based on 4b As can be seen, the (relative) temperature gradient ΔT _max,rel on the optical surface 27 is also dependent on the lateral distance p between two adjacent cooling channels 29 and decreases with decreasing lateral distance p. The lateral distance p between each two adjacent cooling channels 29 can be 2 shown example, for example, 4.5 mm or less, 3 mm or less or 2 mm or less. As a reference for the in 4b In the representation shown, a lateral distance p of 6 mm was assumed between two adjacent cooling channels 29 in each case.

As below based on 3a-c is described, it can be advantageous to reduce the lateral distance p between adjacent cooling channels 29 if the distance d to the optical surface 27 is reduced: In the case of the in 3a In the example shown, the distance d between the cooling channels 29 and the optical surface 27 is 2 mm, the lateral distance p is 4.5 mm and the width b (corresponding to the height h) of the cooling channels 29 is 2 mm. At the in 3b In the example shown, the distance d between the cooling channels 29 and the optical surface 27 was reduced to 1 mm and the lateral distance p of 4.5 mm was retained. As in 3b is indicated by the dashed, star-shaped temperature distribution in the area of the maximum power density q _max of the incident EUV radiation 16, the structure or the pattern of the cooling channels 29 is visible in the temperature distribution on the optical surface 27. This is not desirable since the temperature distribution on the optical surface 27 should be as homogeneous as possible. 3c shows the case in which the distance d to the optical surface 27 is also 1 mm, but the lateral distance p between two adjacent cooling channels 29 has been reduced to 3 mm. As in 3c can be seen, a temperature distribution that is radially symmetrical around the respective pole with the maximum power density q _max occurs on the optical surface 27, as is also the case in 3a the case is.

In particular in the event that the distance d of the cooling channels 29 from the optical surface 27 is 2 mm or less or 1 mm or less, it is therefore favorable if the density of the cooling channels 29 under the optical surface 29 is selected as large as possible , in order to obtain a cooling effect that is as uniform as possible. In addition or as an alternative to reducing the lateral distance p between adjacent channels 29, it is also possible to widen the cooling channels 29 in the lateral direction or to increase their aspect ratio b/h, in which case the following can apply in particular: b/h>1.1, preferably >1.5, in particular >2, where the following preferably applies: b/h<5.

5a,b show an example of a mirror 25 with an optical surface 27 on which EUV radiation with a power density q(x,y) that varies depending on the location impinges, with a greater power density resulting from a greater length of the in 5a shown arrows is indicated. the inside 5a,b The mirror 25 shown differs from that in 3a-c shown mirror 25 in that the cooling channels 29 are not constant, but rather have a location-dependent varying distance d(x,y) from the optical surface 27. The distance d (x,y) of a respective cooling channel 29 to the optical surface 27 is in the sectional view of 5a measured at that point in the y direction which corresponds to the center of the cross section of the respective cooling channel 29 in the zy plane.

The following applies to the distance d(x,y) from the optical surface 27 and the local power density q(x,y) at the position of the respective cooling channel 29 in 5a,b shown example: $$\text{q}\left(\text{x},\text{y}\right)\times \text{i.e}\left(\text{x},\text{y}\right)=cOnst.$$

This selection of the location-dependent varying distance d(x,y) of the cooling channels 29 from the optical surface 27 can generally produce a homogeneous temperature distribution on the optical surface 27. The equation given above applies to the in 3a-c shown dipole power distribution q(x,y) of the incident EUV radiation 16, at the two poles of which large temperature gradients arise on the optical surface 27.

During operation of the EUV lithography system 1, different power density distributions q(x,y) can occur on the optical surface 27 depending on the illumination settings. The equation given above can be satisfied for that lighting setting at which the temperature gradients are at a maximum. As a rule, however, the arithmetic mean is formed from several location-dependent power distributions q(x,y) with different lighting settings and used for the above formula.

At the in 5a,b shown example, the lateral distance p between adjacent cooling channels 29 is constant. However, it is also possible, in addition to or as an alternative to varying the distance d(x,y) of the cooling channels 29 from the optical surface 27, to change the density of the cooling channels 29 as a function of the local power distribution 27. In this case, the density of the cooling channels 29 is increased in areas with a high power density q(x,y) in order to improve the cooling effect locally and to dampen temperature peaks. For this purpose, the lateral distance p between two adjacent cooling channels 29 can be fixed or varied depending on the local power density q(x,y) of the incident EUV radiation 16 and/or the width b or the aspect ratio b/h of the cooling channels 29 can be fixed or varied depending on the local power density q(x,y).

In the examples described above, a respective cooling channel 29 runs in a straight line and at a constant distance below the entire optical surface 27. Contrary to what is shown in the above examples, several cooling channels 29 are usually connected via a common distributor with a common inlet opening 31a of the Substrate 26 connected and these are connected via a common collector with a common outlet opening 31b of the substrate 26. In this case, the cooling channels 29 shown are sections of a channel structure which has a more complex geometry than is the case in connection with FIG 3a-c and in 5a,b was described. The requirements described above for the arrangement and/or the geometry of the cooling channels 29 also apply if these form sections of a complex channel structure.

QUOTES INCLUDED IN DESCRIPTION

This list of 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 102011005778 A1 [0005]
• DE 102018202687 A1 [0006]
• DE 102019200750 A1 [0006]
• DE 102016221878 A1 [0007]
• WO 2021115643 A1 [0008, 0033]

Claims (13)

Lithography system (1), comprising: a radiation source (3) for generating radiation, in particular for generating EUV radiation (16), at least one optical element (25), comprising: a substrate (26) with an optical surface (27) , to which a reflective coating (28) for reflecting radiation, in particular EUV radiation (16), is applied, wherein a plurality of channels (29) through which a fluid (30) can flow runs in the substrate (26), the are spaced from the optical surface (27), characterized in that a distance (d, d(x,y)) of at least one channel (29) from the optical surface (27) and/or a lateral distance (p) between at least two of the channels (29) as a function of a power density (q _max , q(x,y)) of the radiation (16) of the radiation source (3) impinging on the optical surface (27). lithography system claim 1 , in which a maximum power density (q _max ) of the radiation (16) on the optical surface (27) is less than 2 mW/mm ² and the distance (d) of at least one channel (29), in particular all channels (29) , from the optical surface (27) is 5 mm or less. lithography system claim 1 , in which a maximum power density (q _max ) of the radiation (16) on the optical surface (27) is between 2 mW/mm ² and 3 mW/mm ² and the distance (d) of at least one channel (29), in particular all of them Channels (29) from the optical surface is 3mm or less. lithography system claim 1 , in which a maximum power density (q _max ) of the radiation (16) on the optical surface (27) is more than 3 mW/mm ² and the distance (d) of at least one channel (29), in particular all channels (29) , from the optical surface is 2 mm or less. Lithography system according to one of the preceding claims, in which the distance (d(x,y)) of at least one channel (29), in particular all channels (29), from the optical surface (27) as a function of a local power density (q(x ,y)) of the radiation (16) at the position (x,y) of the channel (29). lithography system claim 5 , in which the product (q(x,y) xd(x,y)) of the local power density (q(x,y)) at the optical surface (27) and the distance (d(x,y)) of a respective Channel (29) from the optical surface (27) is substantially constant for all channels (29). Lithography system according to one of the preceding claims, in which at least two of the channels (29), in particular all channels (29), have a lateral distance (p) of 4.5 mm or less, preferably 3 mm or less, particularly preferably 2 mm or less of one another, and/or in which at least one of the channels (29), in particular all channels (29), have a channel width (b) which is greater than 1.1 times, preferably greater than 1.5 times times, in particular greater than 2 times and preferably not greater than 5 times a channel height (h). Lithography system according to one of the preceding claims, in which the lateral distance (p) between at least two adjacent channels (29), in particular between all channels (29), depending on a local power density (q (x, y)) on the optical Surface (27) incident radiation (16) is fixed. Optical element (25), comprising: a substrate (26) having an optical surface (27) to which a reflective coating (28) for reflecting radiation, in particular for reflecting EUV radiation (16), is applied, wherein in a plurality of channels (29) through which a fluid (30) can flow run along the substrate (26) and are spaced apart from the optical surface (27), characterized in that at least one channel (29), in particular all channels (29), have a distance (d) of 5 mm or less, particularly preferably 3 mm or less, in particular 2 mm or less, from the optical surface (27) of the substrate (26). Optical element after claim 9 , wherein the plurality of channels (29) has a constant distance (d) from the optical surface (27). Optical element according to the preamble of claim 9 , especially after one of the claims 9 or 10 in which at least two of the channels (29), in particular all channels (29), have a lateral distance (p) of 4.5 mm or less, preferably 3 mm or less, particularly preferably 2 mm or less, from one another, and / or at least one of the channels (29), in particular all channels (29), have a channel width (b) which is greater than 1.1 times, preferably greater than 1.5 times, in particular greater than 2 times is times and preferably not more than 5 times a channel height (h). Optical element according to one of claims 9 until 11 , in which the substrate (26) is monolithic. Optical element according to one of claims 9 until 12 , in which the substrate (26) has a first part-body (26a) and a second part-body (26b) which are assembled at an interface (33), the plurality of channels (29) in the substrate (26) in the region of the Interface (33) between the two part-bodies (26a, 26b) runs.

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