DE102022204580A1 - METHOD OF MAKING OR OPERATION OF A MIRROR IN A LITHOGRAPHY PLANT - Google Patents

Abstract

A method is disclosed for producing a mirror (M3) for use in a lithography system (1) and/or for operating a mirror (M3) in a lithography system (1), the mirror (M3) having a first material (G200). a first thermal expansion coefficient (α300) which has a zero crossing (304, 304'), and second material (G202) with a second thermal expansion coefficient (α302) which has no zero crossing, with in particular the following steps: Changing (S608) a or several of the following depending on a simulated wavefront error: (i) a material composition of the first and/or second material (G200, G202) to adapt the first and/or second thermal expansion coefficient (α300, α302),(ii) a preheating temperature of the mirror (M3 ), and/or(iii) a flow temperature of a coolant with which the mirror (M3) is cooled; andmanufacturing (S610) the mirror (M3) with the changed material composition and/or operating the mirror (M3) with the changed preheating temperature and/or changed advance temperature in the exposure mode (306, 306').

Description

The present invention relates to a method for producing a mirror for use in a lithography system and/or for operating a mirror in a lithography 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.

Driven by the striving for ever smaller structures in the manufacture of integrated circuits, EUV lithography systems are currently being developed which use light with a wavelength in the range from 0.1 nm to 30 nm, in particular 13.5 nm. Since most materials absorb light of this wavelength, reflective optics, ie mirrors, must be used in such EUV lithography systems instead of—as before—refractive optics, ie lenses.

Nevertheless, mirrors also absorb a not inconsiderable proportion of the incident EUV light, for example 30% depending on the angle of incidence. This leads to a corresponding heating of the mirrors. This is accompanied by a thermal deformation of the mirrors.

This in turn regularly leads to wavefront errors on the exposed substrate. Accordingly, it is known in the prior art to take measures that minimize such thermal deformations.

One such measure consists in manufacturing uncooled mirrors, in particular, from a material with a very low coefficient of thermal expansion. Glass, ceramics or metal alloys are known as examples of such materials. In particular, a titanium silicate glass from the Corning company is known under the brand name “ULE” (Ultra-Low Expansion Glass). Another example is SiSiC, which is a silicon carbide material, i.e. a ceramic. Furthermore, the nickel-iron alloy "Invar" is known from the prior art, which consists of 64% iron and 36% nickel.

The aforementioned materials (hereinafter also “first material”) have a temperature-dependent coefficient of thermal expansion that is designed in such a way that it is as small as possible at the expected operating temperature, i.e. the temperature at which the material is used. As a result, relative changes in length at the operating temperature can be reduced to a minimum.

However, the mirrors in lithography systems also include components made of other materials (hereinafter also “second material”). This applies, for example, to sockets, adhesive spots or the optical coating, which has the optically effective area ("optical footprint"). These materials expand linearly and make a non-negligible contribution to the deformation of the mirror, in particular to the deformation of its optically effective surface.

Furthermore, it is known to preheat mirrors before the actual exposure operation. This has the effect that the mirror only deforms insignificantly during the actual exposure (e.g. with a dipole or quadrupole). A change in the thermal deformation of the mirror over time can also be counteracted by this measure.

Finally, cooled mirrors are known. In these, the mirror substrate in particular is cooled using a coolant, for example water. With this solution, the heat is dissipated so that it cannot even bring about a thermal expansion of the mirror substrate.

Against this background, it is an object of the present invention to provide an improved approach.

1. a) Simulieren einer Temperaturverteilung auf dem Spiegel in einem Belichtungsbetrieb der Lithographieanlage;
2. b) Simulieren einer Deformation des Spiegels in Abhängigkeit der simulierten Temperaturverteilung sowie des ersten und zweiten Wärmeausdehnungskoeffizienten;
3. c) Simulieren eines Wellenfrontfehlers in Abhängigkeit der simulierten Deformation;
4. d) Ändern eines oder mehrerer der Folgenden in Abhängigkeit des simulierten Wellenfrontfehlers:
   1. (i) einer Materialzusammensetzung des ersten und/oder zweiten Materials zur Anpassung des ersten und/oder zweiten Wärmeausdehnungskoeffizienten,
   2. (ii) einer Vorheiztemperatur des Spiegels, und/oder
   3. (iii) einer Vorlauftemperatur eines Kühlmittels, mit welchem der Spiegel gekühlt wird; und
5. e) Herstellen des Spiegels mit der geänderten Materialzusammensetzung und/oder Betreiben des Spiegels mit der geänderten Vorheiztemperatur und/oder geänderten Vorlauftemperatur in dem Belichtungsbetrieb.

According to a first aspect, a method for producing a mirror for use in a lithography system and/or for operating a mirror in a lithography system is provided. The mirror has a first material with a first coefficient of thermal expansion, which has a zero crossing. Furthermore, the mirror has a second material with a second coefficient of thermal expansion, which does not have a zero crossing. The procedure includes the following steps:

1. a) simulating a temperature distribution on the mirror in an exposure operation of the lithography system;
2. b) simulating a deformation of the mirror as a function of the simulated temperature distribution and the first and second coefficients of thermal expansion;
3. c) simulating a wavefront error depending on the simulated deformation;
4. d) changing one or more of the following depending on the simulated wavefront error:
   1. (i) a material composition of the first and/or second material to adjust the first and/or second coefficient of thermal expansion,
   2. (ii) a preheat temperature of the mirror, and/or
   3. (iii) an inlet temperature of a coolant with which the mirror is cooled; and
5. e) producing the mirror with the changed material composition and/or operating the mirror with the changed preheating temperature and/or changed flow temperature in the exposure mode.

One finding of the inventor is that contributions of the second material to the surface deformation of the mirror and thus to the wavefront error cannot be reduced or can hardly be reduced. A change in the second material, for example the mirror coating, is a comparatively large intervention in the architecture of the mirror and should preferably be avoided. At the same time, the expansion of the optical coating alone, especially in the case of a water-cooled mirror, can lead to the surface deformation exceeding the specification. A further problem in this context is that material parameters, for example the optical coating, are often only insufficiently known. The deformation contributions of the optical coating can become apparent in particular in the case of spatially concentrated, high power peaks on the optically effective surface.

One idea on which the present invention is based is therefore to compensate for the contributions of the second material to thermal expansion and thus to the wavefront error. This can be done in different ways. What the approaches have in common is that the temperature distribution on the mirror, the deformation of the mirror and finally the wavefront error resulting from the deformation are first simulated. Depending on this simulation, the material composition of the first or the second material is then adapted, for example. This with the aim of changing the first and/or second coefficient of thermal expansion.

In preferred embodiments, only the material composition of the first material is adjusted and not that of the second. The adaptation of the material composition includes in particular a change in the chemical composition of the material. In addition, the geometry of the material could be adjusted, for example by providing bores etc., in order to influence the thermal expansion. For example, in the case of a first material which comprises silicon oxide doped with titanium oxide, the proportion of titanium oxide can be increased or decreased. The change in the material composition leads in particular to a shift in the zero crossing. "Zero crossing" means the temperature at which the first coefficient of thermal expansion is zero (Zero-Crossing Temperature - ZCT). Through a suitable choice of the zero crossing, the contribution of the first material to the surface deformation of the mirror and thus to the wavefront error can be adjusted in such a way that it compensates the corresponding contribution of the second material as well as possible.

In other embodiments, the preheating temperature of the mirror or a pre-flow temperature of a coolant with which the mirror is cooled is changed depending on the simulated wavefront error.

The preheating temperature of the mirror can be set in particular by one or more radiant heaters, which heat the optically effective surface in particular with infrared light. The “preheating temperature” specifically refers to a temperature of the mirror just before the exposure process.

In the present case, the exposure process or exposure operation generally means, i.e. without assuming a reference to possible preheating, the state of the lithography system in which working or useful light falls on the wafer to be exposed. The exposure mode can in particular include a number of “steps” in a stepper mode of the lithography system. The exposure operation ends (temporarily) when the fully exposed wafer (or a plurality thereof) is removed. This can be done, for example, by moving out the wafer, which is arranged on a wafer table below the projection objective. Subsequently (this period of time also no longer belongs to the exposure process), a new wafer (or several new wafers) is arranged on the wafer table below the projection lens.

The "supply temperature" means the initial temperature (ie before the cooling process) of a cooling fluid, in particular a liquid or a gas, which is used for mirror cooling. The "forward" means the section of the coolant that is in front of the mirror in terms of flow located. Water, in particular, comes into consideration as a coolant. The flow temperature can be between 15 and 60 degrees Celsius, for example.

By changing the preheating temperature or the flow temperature, the mean temperature of the first and/or second material is changed. Due to the fact that the coefficients of thermal expansion of the first and second material behave differently depending on the temperature, these measures can also be used to adjust the respective contributions of the first and second material to the surface deformation of the mirror and thus to the wavefront error in such a way that these compensate.

The aforementioned measures can be combined with one another in order to keep the wavefront error as small as possible.

In embodiments, the mirror can be another optical element, such as a lens, an optical grating, or a lambda plate. Such lenses can be used in DUV lithography systems, for example.

The first coefficient of thermal expansion may have a linear or substantially linear dependence on temperature. The second coefficient of thermal expansion may be temperature independent or substantially temperature independent.

According to one embodiment, changing the material composition of the preheating temperature and/or the flow temperature according to step d) is such that a sum of a relative change in length of the first material and a relative change in length of the second material during exposure operation is reduced.

Relative length change is commonly expressed as the ratio of length change (ΔL) to total length (L). The "reduction" refers here to a state before step d) as a reference or comparison point, ie to a state in the simulation model of the mirror. The change according to step d) can initially mean a change in the simulation model. This simulation model can then in turn run through steps a) to d) (also repeatedly) in order to determine whether the change is (actually) accompanied by a reduction in the wavefront error. Alternatively, the mirror is manufactured and/or put into operation with the modified material composition.

The relative change in length of the first and/or second material here refers to at least one point within the respective material or an axis (along which the change in length is viewed) that intersects both the first and the second material, namely the second material in particular in an area in which the optically effective surface of the mirror is located. In particular, the axis can be oriented perpendicular to the surface. Alternatively or additionally, a change in length in one plane can be considered, in particular in the plane of the coating or optically effective surface. This is because the expansion of the coating in its main extension plane x, y can lead to a bimetal-like bending of the entire body (mirror).

According to a further embodiment, the relative changes in length of the first and second material have opposite signs.

As a result, the relative changes in length of the first and second material advantageously compensate each other, so that their sum is smaller than the sum of the absolute value of the change in length of the first material and the absolute value of the relative change in length of the second material.

According to a further embodiment, the changing of the material composition, the preheating temperature and/or the flow temperature according to step d) is such that the first coefficient of thermal expansion is negative over the entire exposure operation.

This results in favorable compensation for the relative changes in length of the first and second material.

According to a further embodiment, it is provided that the sum of the first and second thermal expansion coefficients has a zero crossing in a temperature range corresponding to the exposure mode.

• aa) Erfassen von Messdaten, insbesondere eines Wellenfrontfehlers; und
• bb) Ändern eines oder mehrerer der Folgenden in Abhängigkeit der erfassten Messdaten:
  1. (i) einer Vorheiztemperatur des Spiegels, und/oder
  2. (ii) einer Vorlauftemperatur eines Kühlmittels, mit welcher der Spiegel gekühlt wird;

wobei das Ändern in Schritt bb) derart beschaffen ist, dass sich eine Summe aus einer relativen Längenänderung des ersten Materials und einer relativen Längenänderung des zweiten Materials reduziert, die relativen Längenänderungen des ersten und zweiten Materials ein entgegengesetztes Vorzeichen aufweisen, und/oder der erste Wärmeausdehnungskoeffizient über den gesamten Belichtungsbetrieb negativ ist.According to a further embodiment, the following steps are carried out in the exposure operation of the lithography system:

• aa) acquisition of measurement data, in particular a wavefront error; and
• bb) Changing one or more of the following depending on the recorded measurement data:
  1. (i) a preheat temperature of the mirror, and/or
  2. (ii) an inlet temperature of a coolant with which the mirror is cooled;

wherein the change in step bb) is such that a sum of a relative change in length of the first material and a relative change in length of the second material is reduced, the relative changes in length of the first and second material have opposite signs, and/or the first coefficient of thermal expansion is negative over the entire exposure operation.

This embodiment is based on the idea, even during operation of the lithography system, i. H. in exposure mode, to control the preheating temperature and/or the flow temperature in such a way that the thermal expansion contributions of the first and second material compensate each other. This happens depending on measurement data. These include, in particular, the wavefront error. In addition, the measurement data can also include, for example, an actual temperature of a mirror surface, in particular the optically effective area, a partial pressure within the vacuum chamber of the lithography system in which the mirror is housed, or an exposure pattern or a value associated therewith. For example, if the exposure pattern includes a quadrupole instead of a dipole, this speaks for a higher energy input into the mirror, so that countermeasures against a potential surface deformation with the help of the changed preheating temperature or changed flow temperature can be indicated.

According to a further embodiment, steps a) to d) and/or aa) and bb) are repeated until the simulated or detected wavefront error falls below a predetermined threshold value.

According to a further embodiment, the first material has glass, a ceramic and/or a metal alloy. In particular, ULE, SiSiC or Invar come into consideration. The first material is preferably in the form of a mirror substrate. According to a further embodiment, the second material is designed as a mirror coating. The mirror coating is preferably arranged on the mirror substrate.

According to a further embodiment, the zero crossing is in a temperature range between 15 and 50 degrees Celsius.

This corresponds to a typical value in the field of lithography.

According to a further embodiment, the mirror is intended for use in a projection objective of the lithography system. Alternatively or additionally, it is used there.

• aa) Erfassen von Messdaten, insbesondere eines Wellenfrontfehlers; und
• bb) Ändern eines oder mehrerer der Folgenden in Abhängigkeit der erfassten Messdaten:
  1. (i) einer Vorheiztemperatur des Spiegels, und/oder
  2. (ii) einer Vorlauftemperatur, mit welcher der Spiegel gekühlt wird;

wobei das Ändern in Schritt bb) derart beschaffen ist, dass sich eine Summe aus einer relativen Längenänderung des ersten Materials und einer relativen Längenänderung des zweiten Materials reduziert, die relativen Längenänderungen des ersten und zweiten Materials ein entgegengesetztes Vorzeichen aufweisen, und/oder der erste Wärmeausdehnungskoeffizient über den gesamten Belichtungsbetrieb negativ ist.According to a second aspect, a method is provided for operating a mirror in a lithography system in an exposure mode, the mirror comprising a first material with a first coefficient of thermal expansion, which has a zero crossing, and a second material, with a second coefficient of thermal expansion, which has no zero crossing. with the following steps:

• aa) acquisition of measurement data, in particular a wavefront error; and
• bb) Changing one or more of the following depending on the recorded measurement data:
  1. (i) a preheat temperature of the mirror, and/or
  2. (ii) a flow temperature with which the mirror is cooled;

wherein the change in step bb) is such that a sum of a relative change in length of the first material and a relative change in length of the second material is reduced, the relative changes in length of the first and second material have opposite signs, and/or the first coefficient of thermal expansion is negative over the entire exposure operation.

The lithography system is in particular an EUV or DUV lithography system. EUV stands for "Extreme Ultraviolet" and designates a working light wavelength between 0.1 nm and 30 nm. The projection exposure system can also be a DUV lithography system. DUV stands for "Deep Ultraviolet" and describes a working light wavelength between 30 nm and 250 nm.

"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 advantages and features described for the first aspect apply correspondingly to the second aspect 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 see individual aspects as improvements or add supplements to the respective basic form of the invention.

• 1 zeigt einen schematischen Meridionalschnitt einer Projektionsbelichtungsanlage bzw. Lithographieanlage für die EUV-Projektionslithographie;
• 2 zeigt einen Ausschnitt aus der Lithographieanlage gemäß 1 im Bereich der Projektionsoptik;
• 3 bis 5 zeigen jeweils in einem Diagramm die relative Längenänderung als Funktion der Temperatur;
• 6 zeigt ein erstes Flussdiagramm; und
• 7 zeigt ein zweites Flussdiagramm.

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 or lithography system for EUV projection lithography;
• 2 shows a section of the lithography system according to FIG 1 in the field of projection optics;
• 3 until 5 each show the relative change in length as a function of temperature in a diagram;
• 6 shows a first flow chart; and
• 7 shows a second flowchart.

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.

1 shows an embodiment of a projection exposure system 1 (lithography system), in particular an EUV lithography system. 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 2. In this case, the lighting system 2 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 via a reticle displacement drive 9, in particular in a scanning direction.

In the 1 a Cartesian coordinate system with an x-direction x, a y-direction y and a z-direction z is drawn in for explanation. The x-direction x runs perpendicularly into the plane of the drawing. The y-direction y runs horizontally and the z-direction z runs vertically. The scanning direction is in the 1 along the y-direction y. The z-direction z 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 y 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 light source 3 is an EUV radiation source. The light source 3 emits in particular EUV radiation 16, which is also referred to below as useful radiation, illumination radiation or illumination light. The useful radiation 16 has in particular a wavelength in the range between 5 nm and 30 nm. The light source 3 can be a plasma source, for example an LPP source (Engl .: Laser Produced Plasma, plasma generated with the help of a laser). or a DPP (Gas Discharged Produced Plasma) source. It can also be a synchrotron-based radiation source. The light source 3 can be a free-electron laser (FEL).

The illumination radiation 16 emanating from the light 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 in grazing incidence (Engl.: Grazing Incidence, GI), i.e. with angles of incidence greater than 45°, or in normal incidence (Engl.: Normal Incidence, NI), i.e. with incidence angles smaller than 45° , are acted upon by the illumination radiation 16 . 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 focus plane 18. The intermediate focus plane 18 can be a separation between a radiation source module comprising the light source 3 and the collector 17, and the illumination optics 4 represent.

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. The first facet mirror 20 includes a multiplicity of individual first facets 21, which can also be referred to as field facets. Of these first 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 y.

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 (English: 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 second facet mirror 22 can be arranged tilted relative to a pupil plane of the projection optics 10, 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, grazing 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 first facet mirror 20 and the second 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 greater than 0.5 and which can also be greater than 0.6 and which, for example, is 0.7 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 y 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 y can be about 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 x, y. The two image scales βx, βy of the projection optics 10 are preferably at (βx, βy)=(+/−0.25, /+−0.125). A positive imaging scale 6 means imaging without image reversal. A negative sign for the imaging scale 6 means imaging with image inversion.

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

The projection optics 10 lead to a reduction of 8:1 in the y-direction y, 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 x, y, for example with absolute values of 0.125 or 0.25, are also possible.

The number of intermediate image planes in the x and y directions x, y 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 x, y are known from U.S. 2018/0074303 A1 .

In each case one of the second facets 23 is assigned to precisely one of the first 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 first facets 21 . The first facets 21 generate a plurality of images of the intermediate focus on the second facets 23 assigned to them.

The first facets 21 are each imaged onto the reticle 7 by an associated second facet 23 in a superimposed manner 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 arranging the second facets 23 . 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 second facets 23 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 second facet mirror 22 . When imaging the projection optics 10, which telecentrically images the center of the second 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 shown arrangement of the components of the illumination optics 4, the second facet mirror 22 is arranged in a conjugate to the entrance pupil of the projection optics 10 surface. The first 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 .

2 shows a section from the lithography system 1 according to FIG 1 . The section shows the mirror M3 in detail 1 (ie in the area of the projection lens 10) partially.

The mirror M3 comprises—in each case viewed in section—a mirror substrate 200 on which a coating 202 is applied. The mirror substrate 200 is made from a material G ₂₀₀ , such as a glass, in particular silicon oxide doped with titanium oxide, a ceramic, in particular SiSiC, or a metal alloy, in particular Invar. The coating 202 comprises a second material G ₂₀₂ such as a multilayer of molybdenum and silicon. The coating 202 forms the optically effective surface (optical footprint) of the mirror M3. The illumination radiation 16 hits this (see also 1 ) and is reflected there. A part of the illumination radiation 16 is absorbed by the coating 202 and the mirror substrate 200 and converted into heat there. In principle, this heating leads to a thermal deformation in the area of the optically effective surface. This in turn can have a negative effect on the wafer 13 (see 1 ) affect the impinging wavefront or sometimes cause wavefront errors in this way.

The mirror M3 can have a cooling device. For this purpose, the mirror substrate 200 can comprise one or more cooling channels 204, for example. The fluid flowing through the cooling channels 204, in particular water, cools the mirror substrate 200 and the coating 202 and in the process dissipates the heat introduced by the illumination radiation 16. Furthermore, additionally or alternatively, the lithography system 1 can have an in 2 have the heating device shown, which comprises an infrared radiator 206 according to the exemplary embodiment. The infrared radiator 206 heats the mirror M3 with the help of the radiation it generates. The infrared radiator 206 is arranged outside the beam path.

3 12 illustrates a relative change in length dL/L as a function of the temperature T. A graph is denoted by 300 which shows the change in length dL/L as a function of the temperature T for the mirror substrate 200. FIG 2 indicates. The change in length dL/L refers, for example, to a change in length in the in 2 designated z-direction, that is to say perpendicular to the main extension plane of the mirror M3 or in its thickness direction (ie also essentially perpendicular to the optically active surface).

The graph 300 has a gradient α ₃₀₀ which corresponds to the coefficient of thermal expansion (here also “first coefficient of thermal expansion”) of the mirror substrate 200 . How 3 As can be seen, the coefficient of thermal expansion α ₃₀₀ changes with the temperature T. In particular, the coefficient of thermal expansion α ₃₀₀ has a linear dependence on the temperature T. Accordingly, the graph is 300 a parabola shape. A zero crossing of the coefficient of thermal expansion α ₃₀₀ is denoted by 304 .

The graph 302 describes the relative change in length dL/L of the coating 202 as a function of the temperature T. The relative change in length also relates here to the z-direction, as explained above in connection with the graph 300 . The graph 302 has a gradient α ₃₀₂ which corresponds to the coefficient of thermal expansion (here also “second coefficient of thermal expansion”) of the coating 202 . The coefficient of thermal expansion α ₃₀₂ is constant, ie independent of the temperature T and without crossing zero.

Furthermore shows 3 an operating window 306 (here also referred to as “exposure operation”) of the lithography system 1. This means that the temperature in the mirror substrate 200 and in the coating 202, in particular the spatially and temporally average temperature, varies between the temperatures T1 and T2 during the exposure process . It is in 3 recognizing that the window of operation 306 includes the zero crossing 304 .

6 shows in a flow chart several method steps in a method according to an embodiment.

In a step 600, a simulation model of the lithography system 1 or of parts thereof is created. The simulation model includes, in particular, the mirror M3 with its mirror substrate 200 and associated coefficient of thermal expansion α ₃₀₀ and its coating 202 with associated coefficient of thermal expansion α ₃₀₂ . At this point in time, a corresponding lithography system 1 with the mirror M3 can already exist, but does not have to.

In a step S602, a temperature distribution on the mirror M3 in an exposure mode 306 of the lithography system 1 is now simulated. The temperature distribution is influenced in many ways. For example, this depends on the respective lighting setting or pattern. For example, a dipole leads to a different heat input than a quadrupole. Furthermore, the temperature distribution depends on whether or not the mirror M3 has been preheated by the heater 206 before the exposure operation. The temperature distribution also depends on whether a cooling device 204 is provided and on the respective flow temperature. In principle, the present method can run for cases in which no heating device 206 and also no cooling device 204 or only one of them or both are provided. The temperature distribution can, for example, be simulated in the simulation model using a finite element method.

In step S604, the deformation of the mirror M3 is simulated as a function of the simulated temperature distribution and the thermal expansion coefficients α ₃₀₀ , α ₃₀₂ . In particular, the deformation of the optically effective surface can be simulated. In addition to the factors mentioned, other factors can also be included for the deformation of the mirror. The other factors include, for example, a rigidity of the mirror substrate 200 and the coating 202.

In a further step S608, a wavefront error is simulated, in particular calculated. This happens depending on the simulated deformation. For example, the wavefront error can be determined as the deviation of an RMS (Route Mean Square) value of a simulated wavefront from the RMS value of a target wavefront.

In step S608, depending on the simulated wavefront error, three different changes can be made to the simulation model in particular, which is done by feedback 610 in 6 is indicated. Alternatively, the changes according to step S608 without feedback 610 are implemented directly in step S612 (in reality) to be described later.

Returning to step S608, a change in a (simulated) material composition of the first and/or second material G ₂₀₀ , G ₂₀₂ is determined there in the event of a return 610 . This is with the aim of adjusting the first and/or second coefficient of thermal expansion α ₃₀₀ , α ₃₀₂ in order to thereby reduce the wavefront error.

Alternatively or additionally, the preheating temperature of the mirror M ₃₀₀ can be changed in the simulation model with the aid of the (simulated) heating device 206 .

Furthermore, as an alternative or in addition, the flow temperature with which the mirror M3 is cooled can be changed with the aid of the (simulated) cooling device 204 .

The method then runs through again with the adapted simulation model (step S600) according to steps S602 to S608. If the simulated wavefront error falls below a defined threshold value (this can, for example, be determined empirically and specified in step S600), step S612 follows.

In step S612, the mirror 300 is manufactured. In any case, this is the case when a changed material composition was determined in step S608. In this case, the mirror M3 is manufactured with the changed material composition. For example, a chemical composition of the first material is defined, which deviates from that initially specified for the simulation model in step S600. The determined composition can, for example, have a larger proportion of titanium oxide doping in the silicon oxide base material.

However, the mirror M3 does not have to be newly manufactured in step S612. If this already exists, for example, in a way that is integrated in the lithography system 1, a changed preheating temperature or a changed flow temperature in the exposure mode can be set in the (real) lithography system on the basis of the simulation (steps S600 to S608). This means that the (real) heating device 206 provides a changed preheating temperature (according to the simulation) and/or that the (real) cooling device 204 is operated with a changed flow temperature (according to the simulation) and cools the mirror M3 accordingly.

It is just as conceivable to produce the mirror M3 with the altered material composition and then to operate it in the exposure mode with the altered preheating temperature and altered flow temperature. In this case, several measures would be combined in order (in reality with the help of the previous simulation) to minimize the (real) wavefront error.

Now returning to 3 there is a graph 300' shown. This has a slope α ₃₀₀ '. In this case, a changed material composition of the first material G ₂₀₀ was determined in step S608 and optionally modeled again (feedback 610) and/or implemented in reality (step S612). 3 shows the adjusted simulation model or reality alike. The effect of the changed material composition is that the zero crossing 304 of the mirror substrate 200 has shifted. The new zero crossing is denoted by 304'. The new zero crossing 304′ is offset to the right outside of the operating window 306. This has the effect that the derivative or gradient α ₃₀₀ ′ in the operating window is always negative. This can have several advantages. On the one hand, this simplifies the regulation of the lithography system 1, and in particular the prediction of the effect of a temperature change within the operating window 306 on the deformation of the optically effective surface of the mirror M3. Furthermore, it is thus ensured that the relative change in length dL/L of the material G ₂₀₀ has a negative sign and thus compensates for the (positive) change in length dL/L of the material G ₂₀₂ .

4 12 shows a diagram according to another embodiment. In contrast to 3 a graph 400 is shown, which represents the sum of the relative changes in length dL/L of the first and second material G ₂₀₀ , G ₂₀₂ and ultimately a summation of the graphs 300 and 302 3 is equivalent to. α ₄₀₀ ' is the combined coefficient of thermal expansion of the first and second materials G ₂₀₀ , G ₂₀₂ . It can be seen that the graph 400 does not have a zero crossing 404 within the window of operation 306 . In step 608 (see 6 ) the material composition is now changed such that the new zero crossing 404' is shifted to the right and within the operating window 306 as shown for the graph 400'. The graph 400′ shows the sum of the change in length dL/L for the first and second material G ₂₀₀ , G ₂₀₂ according to the changed material composition. The changed material composition can affect both the first and the second material (that is, only one or both of them). The optimization according to 4 thus aims at the sum of the relative changes in length.

5 Figure 12 illustrates an embodiment in which the material composition is not adjusted (although this could be done additionally). Rather, the window of operation 306 is shifted (toward the new window of operation 306'). The new operating window 306' is delimited by the temperatures T ₃ and T ₄ which deviate from the temperatures T ₁ and T ₂ and/or can lie entirely outside the temperature interval T ₁ to T ₂ . Alternatively, the temperature intervals T ₁ to T ₂ and T ₃ to T ₄ may partially (but not entirely) overlap.

The shifting of the operating window 306 is caused by a changed preheating temperature or a changed flow temperature in step S608 (see 6 ) causes. The change then has an effect optionally in the simulation model, but in any case in reality when operating the mirror M3 or the lithography system 1 with the changed preheating temperature and/or changed flow temperature in exposure mode (within the new operating window 306').

As in 5 As can be seen, the zero crossing 304 of the coefficient of thermal expansion α ₃₀₀ before the change according to step S608 is within the operating window 306. After the change In contrast, the zero crossing 304 lies to the right outside of the operating window 306′. Consequently, the coefficient of thermal expansion α ₃₀₀ has an exclusively negative gradient within the entire new operating window 306'.

Now referring again to 6 a method step S614 is also shown there. This relates to the exposure operation of the lithography system 1 following the production step in step S612 and/or the operation of the lithography system 1 with the changed preheating temperature or changed flow temperature according to step S612.

In contrast to step S612, which can contain unique default values determined in the simulation model for the preheating temperature and/or flow temperature in real operation of the lithography system 1 or can make use of these default values, in step S614 measurement data of the lithography system 1, in particular of the mirror M3 , recorded. This can include, for example, a detected mirror temperature, mirror position and/or orientation, a partial pressure within the vacuum chamber, and so on. A detected or derived wavefront error preferably also belongs to this measurement data. In step S614, this measurement data is acquired. In the subsequent step S616, the preheating temperature of the mirror M3 or the flow temperature, with which the mirror M3 is cooled, are changed depending on the measured data acquired. This in turn with the aim of compensating for the relative changes in length of the material G ₂₀₀ and G ₂₀₂ , as already shown on the basis of FIG 5 explained.

The zero crossing 304 and/or 304' can be in a temperature range between 15 and 50 degrees Celsius, for example.

7 shows a method according to a further embodiment. In this case, the previous simulation according to steps S600 to S608 (see 6 ) renounced. The mirror M3 is operated in the lithography system 1, with measurement data being acquired according to step S700 and the preheating temperature of the mirror M3 or the flow temperature of a coolant with which the mirror M3 is cooled being adjusted in step S702. Steps S700 and S702 ultimately correspond to steps S614 and S616 6 , so that the statements made there apply accordingly here. In the method according to 7 a compensation of the relative changes in length of the first and second material G ₂₀₀ , G ₂₀₂ should accordingly be achieved.

Aspects of the present invention have been described above in relation to a coating 202 with the second material G ₂₀₂ . Instead of the coating 202, it could just as well be a bushing, a layer of adhesive or any other component that influences the thermal expansion of the mirror M3.

Although explained above on the basis of an EUV lithography system, the aspects described above can also be applied to lenses or other optical elements, for example in a DUV lithography system or in another device in the field of lithography. “Projection exposure system” and “lithography system” have the same meaning here.

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

mirror substrate

202

coating

204

cooling device

206

heating device

300

graph

300'

graph

302

graph

304

zero crossing

304'

zero crossing

306

operating window

306'

operating window

400

graph

400'

graph

G200

first material

G202

second material

M1

Mirror

M2

Mirror

M3

Mirror

M4

Mirror

M5

Mirror

M6

Mirror

S600 - S616

process steps

S700, S702

process steps

T1-T4

temperatures

e.g

z direction

dL/L

relative length change

α300 - α400'

coefficient of thermal expansion

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 102008009600 A1 [0056, 0060]
• US 2006/0132747 A1 [0058]
• EP 1614008 B1 [0058]
• US6573978 [0058]
• DE 102017220586 A1 [0063]
• US 2018/0074303 A1 [0077]

Claims (10)

Method for producing a mirror (M3) for use in a lithography system (1) and/or for operating a mirror (M3) in a lithography system (1), the mirror (M3) comprising a first material (G ₂₀₀ ) with a first coefficient of thermal expansion (α ₃₀₀ ) which has a zero crossing (304, 304'), and second material (G ₂₀₂ ) with a second thermal expansion coefficient (α ₃₀₂ ) which does not have a zero crossing, with the following steps: a) simulating (S602) a temperature distribution on the mirror (M3) in an exposure mode (306, 306') of the lithography system (1); b) simulating (S604) a deformation of the mirror (M3) depending on the simulated temperature distribution and the first and second thermal expansion coefficients (α ₃₀₀ , α ₃₀₂ ); c) simulating (S606) a wavefront error as a function of the simulated deformation; d) changing (S608) one or more of the following depending on the simulated wavefront error: (i) a material composition of the first and/or second material (G ₂₀₀ , G ₂₀₂ ) to adjust the first and/or second coefficient of thermal expansion (α ₃₀₀ , α ₃₀₂ ), (ii) a preheating temperature of the mirror (M3), and/or (iii) a flow temperature of a coolant with which the mirror (M3) is cooled; and e) producing (S610) the mirror (M3) with the changed material composition and/or operating the mirror (M3) with the changed preheating temperature and/or changed advance temperature in the exposure mode (306, 306'). procedure after claim 1 , wherein the changing of the material composition, the preheating temperature and/or the flow temperature according to step d) is such that a sum of a relative change in length (dL/L) of the first material (G200) and a relative change in length (dL/L) of the second material (G ₂₀₂ ) is reduced in exposure mode (306, 306'). procedure after claim 2 , wherein the relative changes in length (dL/L) of the first and second materials (G ₂₀₀ , G ₂₀₂ ) have opposite signs. procedure after claim 2 or 3 , wherein the changing of the material composition, the preheating temperature and/or the flow temperature according to step d) is such that the first coefficient of thermal expansion (α _300' ) is negative over the entire exposure operation (306, 306'). Procedure according to one of Claims 1 - 4 , wherein the following steps are carried out in the exposure operation (306, 306') of the lithography system (1): aa) acquisition (S614) of measurement data, in particular a wavefront error; and bb) changing (S616) one or more of the following depending on the recorded measurement data: (i) a preheating temperature of the mirror (M3), and/or (ii) a flow temperature of a coolant with which the mirror (M3) is cooled; wherein the change in step bb) is such that a sum of a relative change in length (dL/L) of the first material (G ₂₀₀ ) and a relative change in length (dL/L) of the second material (G ₂₀₂ ) is reduced, the relative changes in length (dL/L) of the first and second material (G ₂₀₀ , G ₂₀₂ ) have an opposite sign, and/or the first coefficient of thermal expansion (α _{300 '} ) is negative over the entire exposure operation (306, 306'). Procedure according to one of Claims 1 - 5 , wherein steps a) to d) and/or aa) and bb) are repeated (S610) until the simulated or detected wavefront error falls below a predetermined threshold value. Procedure according to one of Claims 1 - 6 , wherein the first material (G ₂₀₀ ) comprises glass, a ceramic and/or a metal alloy and/or is designed as a mirror substrate (200) and/or wherein the second material (G ₂₀₂ ) is designed as a mirror coating (202). Procedure according to one of Claims 1 - 7 , wherein the zero crossing (304, 304 ') is in a temperature range between 15 and 50 degrees Celsius. Procedure according to one of Claims 1 - 8th , wherein the mirror (M3) is intended for use in a projection lens (10) of the lithography system (1) or is used there. Method for operating a mirror (M3) in a lithography system (1) in an exposure mode (306, 306'), the mirror (M3) comprising a first material (G ₂₀₀ ) with a first thermal expansion coefficient (α ₃₀₀ ) which has a zero crossing ( 304, 304') and a second material (G ₂₀₂ ) with a second coefficient of thermal expansion (α ₃₀₂ ) which does not have a zero crossing, comprising the following steps: aa) acquiring (700) measurement data, in particular a wavefront error; and bb) changing (702) one or more of the following depending on the acquired measurement data: (i) a preheating temperature of the mirror (M3), and/or (ii) a flow temperature of a coolant with which the mirror (M3) is cooled; wherein the change in step bb) is such that a sum of a relative change in length (dL/L) of the first material (G ₂₀₀ ) and a relative change in length (dL/L) of the second material (G ₂₀₂ ) is reduced, the relative changes in length (dL/L) of the first and second material (G ₂₀₀ , G ₂₀₂ ) have an opposite sign, and/or the first coefficient of thermal expansion (α _{300 '} ) is negative over the entire exposure operation (306, 306').

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