DE102022211909A1 - OPTICAL SYSTEM, LITHOGRAPHY SYSTEM AND METHOD OF OPERATING AN OPTICAL SYSTEM - Google Patents

Abstract

An optical system (100) for a lithography system (1), having a mirror (101) with an optically active surface (103) for reflecting a working light (104) of the lithography system (1), a light source (106), in particular an infrared light source, for emission a heating radiation (107), and a reflective element (108) for reflecting the emitted heating radiation (107) onto the optically active surface (103) of the mirror (101), the reflective element (108) in an image plane (110) of the optical system (100) is arranged.

Description

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

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

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.

A problem that arises is that the mirrors heat up as a result of absorption of EUV radiation. The resulting temperature fluctuations in the mirror and the associated thermal deformations of the mirror result as a function of the current heat flow distribution of the mirror and the material-dependent thermal expansion coefficient. Temperature fluctuations and thermal deformation of the mirrors can lead to wavefront aberrations and thus impair the imaging properties of the mirrors.

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

In order to avoid high temperatures and large temperature fluctuations, conventional mirror modules in a lithography system are cooled, for example. The cooling takes place, for example, through cooling lines in the mirror carrier.

In addition, it is known to use other optical elements to correct aberrations caused by temperature fluctuations within the mirrors.

Furthermore, conventional mirror modules of a lithography system are heated locally, for example, in order to avoid large temperature fluctuations. This is, for example, a preheating that takes place before a mirror is illuminated with a useful radiation (e.g. EUV radiation) from the lithography system. The heating takes place, for example, in the form of infrared radiation of individual mirrors. Typically, a mirror is irradiated with an intensity profile that modifies the expected or existing temperature profile in this mirror in such a way that the wavefront aberrations caused are reduced. For this purpose, heating devices are installed, for example, in the vicinity of the mirror to be irradiated, but away from the useful beam path. Preheating is particularly desirable when the ambient temperature drops below the intended operating temperature as a result of exposure pauses or operational pauses. Deviations from the operating temperature can, for example, be up to 5°C in absolute terms.

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

• einen Spiegel mit einer optisch aktiven Fläche zum Reflektieren eines Arbeitslichts der Lithographieanlage,
• eine Lichtquelle, insbesondere Infrarotlichtquelle, zum Aussenden einer Heizstrahlung, und
• ein reflektierendes Element zum Reflektieren der ausgesendeten Heizstrahlung auf die optisch aktive Fläche des Spiegels, wobei das reflektierende Element in einer Bildebene des optischen Systems angeordnet ist.

Accordingly, an optical system for a lithography system is proposed. The optical system features:

• a mirror with an optically active surface for reflecting a working light of the lithography system,
• a light source, in particular an infrared light source, for emitting heating radiation, and
• a reflective element for reflecting the emitted heating radiation onto the optically active surface of the mirror, the reflective element being arranged in an image plane of the optical system.

Due to the fact that the reflecting element is arranged in the image plane of the optical system, ie in an easily accessible area, the reflecting element can be arranged easily. For example, a wafer used in an exposure with working light (eg EUV light) can be a easily exchanged for the reflective element. As a result, the mirror can be heated, for example during a pause in the exposure of the working light, by means of the infrared light source and the reflecting element. For example, the reflecting element can also be easily exchanged for another reflecting element. This means that a reflecting element that is particularly suitable for a specific heating process can be used.

The mirror can be heated by irradiating the optically active surface of the mirror with the heating radiation. For example, the mirror can be heated locally in order to avoid large temperature fluctuations in the mirror. For example, the mirror can be preheated, i.e. before the mirror is exposed to the working light (e.g. EUV radiation) of the lithography system. For example, the mirror is heated by means of the radiant heat during a break in exposure or during a break in operation, in which the mirror is not exposed to the light required to work. This can, for example, prevent the mirror from cooling down due to an ambient temperature that is below the intended operating temperature. The (infrared) light source and the reflecting element together form in particular a heating device for (e.g. local) heating of the mirror. In particular, the heating device is used for contactless heating of the mirror.

Temperature fluctuations in the mirror can be reduced by locally heating the mirror, so that thermal deformation of the mirror is avoided. Consequently, wavefront aberrations and aberrations can be suppressed.

For example, during the heating, the mirror is irradiated with an intensity profile which modifies an expected or existing temperature profile in this mirror in such a way that the wavefront aberrations caused are reduced.

The working light of the lithography system is, in particular, radiation that is used to expose a mask and/or a substrate, e.g. a silicon wafer. The working light is, for example, EUV radiation from an EUV lithography system. In other examples, the working light can also be DUV radiation from a DUV lithography system.

The light source, in particular an infrared light source, is arranged, for example, in the vicinity of the mirror to be irradiated, but away from the beam path of the working light of the lithography system.

The heating radiation emitted by the light source is, in particular, infrared radiation. The light source is, for example, a laser source, e.g., an infrared laser source. The radiation emitted by the light source, i.e. the heating radiation, is monochromatic radiation, for example.

The reflective element serves in particular to reflect the heating radiation in the direction of the optically active surface of the mirror and to radiate the reflected heating radiation onto the optically active surface of the mirror.

The reflective element is made of, for example, a reflective metal such as aluminum. The reflective element can also have a reflective layer system, for example.

The mirror of the optical system is, for example, a mirror of the lithography system. The mirror of the optical system is, for example, a mirror of a projection system of the lithography system. The mirror of the optical system is, for example, the last mirror before the wafer plane of the projection system of the lithography system.

The mirror of the optical system is set up to be exposed during an exposure operation of the lithography system with mandatory work (e.g. EUV light). For example, the mirror is set up to reflect the working light onto an object arranged in the image plane of the optical system, for example a wafer arranged in the image plane of the optical system. The mirror of the optical system is also set up to be irradiated with the heating radiation at the same time as or with a time offset to an exposure to the working light.

In particular, during a pause in the exposure of the lithography system in relation to the working light of the lithography system (e.g. when changing the reticle or during maintenance), instead of the working light, heating radiation structured by means of the reflecting element can be radiated onto the optically active surface of the mirror. In this way, after a break in the exposure of the working light, it can be exposed again immediately with an acceptable aberration level, or a necessary thermalization time of the mirror is shortened.

The image plane of the optical system is, for example, an image plane of a projection system of the lithography system. The image plane of the optical system is, for example, a wafer plane of the lithography system replaceable reflective element are formed.

According to one embodiment, the mirror has an opening and the light source is set up to radiate the heating radiation through the opening onto the reflecting element.

As a result, the heating device comprising the light source and the reflective element can also be used in an optical system with limited installation space.

The opening of the mirror is in particular a through opening. The opening is, for example, a central opening and/or an obscuration opening of the mirror. In particular, the mirror has obscured optics.

According to a further embodiment, the light source is arranged behind the mirror with respect to the optically active surface.

In particular, the light source is arranged on a side of the mirror which is remote from the optically active surface.

Due to the fact that the light source is arranged behind the mirror in relation to the optically active surface of the mirror, the light source can be used in front of the optically active surface of the mirror despite a limited installation space.

According to a further embodiment, the reflective element has a diffraction grating, in particular a phase grating, a reflection grating, a reflection phase grating, an amplitude grating and/or a reflection amplitude grating.

wobei α den Einfallswinkel, β den Beugungswinkel, n die Beugungsordnung, λ die Wellenlänge und g die Gitterkonstante (Gitterperiode) bezeichnet. Weiterhin wurde bei der Gleichung von einer Vorzeichendefinition der Ein- bzw. Ausfallswinkel ausgegangen, bei der Ein- und Ausfallswinkel nullter Ordnung entgegengesetzte Vorzeichen aufweisen, wodurch sich das Pluszeichen auf der linken Seite der Gleichung ergibt.A diffraction grating is an optical grating for diffracting incident radiation. A diffraction grating has in particular a regular arrangement of structures (eg surface structures) by which the amplitude of an incident radiation is changed or its phase is changed by changing the optical path length, so that the radiation is diffracted and interfered. The lattice structures have, for example, linear webs and grooves. The lattice constant describes the periodicity of the lattice structures. The lattice equation applies to the direction of the incident and the diffracted radiation $$\text{sin}\mathrm{a}+\text{sin}\beta =\text{n}\mathrm{\lambda /}\text{G}\text{,}$$

where α denotes the angle of incidence, β the angle of diffraction, n the order of diffraction, λ the wavelength and g the grating constant (grating period). Furthermore, the equation was based on a sign definition of the angles of incidence and reflection, in which the angles of incidence and reflection have opposite signs of the zeroth order, which results in the plus sign on the left-hand side of the equation.

A monochromatic radiation that strikes a diffraction grating at an angle α is deflected in different directions according to the diffraction orders. Larger deflection angles correspond to higher orders of diffraction.

A phase grating is a diffraction grating in which the phase of incident radiation is changed by changing the optical path length. A phase grating has the advantage that the incident radiation largely contributes to the reflected radiation and radiation losses are low.

A reflection grating is a diffraction grating in which (in contrast to a transmission grating) the incident radiation is reflected.

A reflection phase grating is a phase changing reflection grating.

An amplitude grating, also called an absorption grating, is an amplitude-changing diffraction grating. A reflection amplitude grating is an amplitude varying reflection grating.

The diffraction grating can also have a blazed grating.

In other examples, the reflective element can also be a simple mirror, a Fresnel element and/or a structured diffuser.

According to a further embodiment, the heating radiation reflected by the reflecting element has a predetermined spatial intensity distribution for heating the mirror according to the predetermined spatial intensity distribution, and
the predetermined spatial intensity distribution is based on a determined heat flow distribution of the mirror due to exposure to the working light.

For example, when exposed to the working light, only part of the optically active surface of the mirror is irradiated with the working light and only this part has therefore heated up. An example of a partial exposure of a mirror surface is an exposure in the form of a pattern (eg dipole pattern, X-dipole, Y-dipole and/or DRAM profile). In this In this case, there is an uneven heat flow distribution over the optically active surface due to exposure to the working light.

By heating the mirror using the reflected heating radiation, which has the predetermined spatial intensity distribution, the mirror can be heated in a targeted manner, for example locally. For example, the heating of the mirror using the reflected heating radiation can maintain a heat flow distribution due to exposure to the working light even when the working light is not exposed to light. Alternatively, the heating of the mirror using the reflected heating radiation can also be designed to specifically cool the heated areas.

The heat flow distribution of the mirror due to exposure to the working light is, for example, an instantaneous heat flow distribution of the mirror due to exposure to the working light and/or an expected heat flow distribution of the mirror when exposed to the working light.

The reflected heating radiation with the predetermined spatial intensity distribution is structured radiation and/or structured light, for example.

According to a further embodiment, the reflective element is set up to generate the reflected heating radiation with the predetermined spatial intensity distribution from unstructured heating radiation emitted by the light source.

The reflective element has, for example, a diffraction grating that includes a plurality of grating regions that each have different grating parameters. The plurality of grating areas differ from one another, for example, by different grating periods, different duty cycles of the periodic grating structures and/or grating structures of different heights. A diffraction angle (deflection angle) can be varied by changing the grating period. The intensity of the reflected radiation can be varied by changing a pulse duty factor of the periodic grating structures. In particular, in the case of almost perpendicular incidence on the reflecting element, the specular reflection can be suppressed in that a height difference between a ridge and a groove of the grating area divided by the cosine of the angle of incidence is close to a quarter of the wavelength, so that specularly "reflected" waves interfere destructively at ridges and grooves.

For example, the reflecting element is a reflection phase grating with linear ridges and grooves. For example, the multiple grating areas differ from one another by different periodicities of the ridges and grooves (different grating periods), by different ratios of the width of the ridges to the grating period (different duty cycles of the periodic grating structures) and/or by different height differences between ridge and groove (grating structures of different heights).

In other examples, the reflective element can also have a plurality of diffraction grating structures that can be selected by means of a wavelength of the light source (multiple grating coding).

According to a further embodiment, the optical system has at least one temperature sensor for detecting at least one temperature value of the mirror. In addition, the at least one recorded temperature value is used to determine a heat flow distribution of the mirror based on exposure to the working light.

For example, the reflective element is selected based on the at least one sensed temperature value. For example, the reflective element is used to generate a spatial intensity distribution of the reflected heating radiation that corresponds to the at least one recorded temperature value.

The at least one temperature sensor includes, for example, a contact temperature sensor, such as a resistance sensor, a thermistor, and/or a thermocouple. The at least one temperature sensor can, for example, also have a non-contact temperature sensor, such as an infrared camera.

The at least one temperature sensor is used, for example, to record a number of temperature values for a number of spatially distributed areas of the mirror. The at least one temperature sensor is used, for example, to record a temperature distribution of the mirror.

According to a further embodiment, the reflective element is set up to generate reflected heating radiation with a first predetermined spatial intensity distribution. In addition, the optical system has at least one further reflective element for reflecting the heating radiation emitted by the light source onto the optically active surface of the mirror. The at least one further reflective element is set up to generate reflected heating radiation with at least one second predetermined spatial intensity distribution, which differs from the first predetermined spatial intensity distribution. Furthermore the optical system has a device for selectively arranging the reflective element and the at least one further reflective element in the beam path of the emitted heating radiation.

As a result, a reflective element that is particularly suitable for a specific heating process can be used.

According to a second aspect, a lithography system, in particular an EUV lithography system, with an optical system as described above is proposed.

The lithography system can be an EUV lithography system. EUV stands for "Extreme Ultraviolet" and describes a working light wavelength between 0.1 nm and 30 nm. The lithography 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.

The mirror of the optical system is, for example, a mirror of a projection system of the lithography system. The mirror of the optical system is, for example, the last mirror of the projection system of the lithography system in front of the object to be exposed (e.g. the silicon wafer).

According to one embodiment of the second aspect, the image plane is a wafer plane of the lithography system.

For example, the image plane of the optical system is an image plane of a projection system of the lithography system.

1. a) Aussenden einer Heizstrahlung, insbesondere Infrarotstrahlung, und
2. b) Reflektieren der ausgesendeten Heizstrahlung durch ein in einer Bildebene des optischen Systems angeordnetes reflektierendes Element auf die optisch aktive Fläche des Spiegels.

According to a third aspect, a method for operating an optical system for a lithography system is proposed. The optical system has a mirror with an optically active surface for reflecting a working light of the lithography system. The procedure has the steps:

1. a) emitting a heating radiation, in particular infrared radiation, and
2. b) Reflecting the emitted heat radiation onto the optically active surface of the mirror by a reflecting element arranged in an image plane of the optical system.

According to an embodiment of the third aspect, the mirror has an opening and the heating radiation is radiated through the opening onto the reflective element.

According to a further embodiment of the third aspect, steps a) and b) are carried out in an exposure pause of the lithography system in relation to the working light of the lithography system, and/or
steps a) and b) are carried out during exposure of the mirror to the working light.

The exposure pause of the lithography system in relation to the working light is, for example, an exposure pause due to a reticle change and/or maintenance of the lithography system.

When the heating process of steps a) and b) is carried out while the mirror is being exposed to working light, the reflected heating radiation can, for example, only be radiated onto outer regions of the optically active surface which are not used for useful radiation (i.e. the working light).

• Ermitteln einer Wärmestromverteilung des Spiegels aufgrund einer Belichtung mit dem Arbeitslicht, und
• Auswählen eines reflektierenden Elements basierend auf der ermittelten Wärmestromverteilung mindestens einen erfassten Temperaturwert.

According to a further embodiment of the third aspect, the method has the steps:

• determining a heat flow distribution of the mirror due to exposure to the working light, and
• Selecting a reflective element based on the determined heat flow distribution at least one detected temperature value.

If the reflecting element has a multiple grating coding, instead of selecting the reflecting element, a wavelength of the heating radiation can also be selected for selectively activating a diffraction grating structure from a plurality of diffraction grating structures of the reflecting element.

According to a further embodiment of the third aspect, the heat flow distribution of the mirror is determined by detecting at least one temperature value of the mirror using at least one temperature sensor, and/or
the heat flow distribution of the mirror is determined by running a simulation.

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

The embodiments and features described for the optical system apply accordingly and reciprocally to the proposed lithography system and the proposed method.

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

• 1 zeigt einen schematischen Meridionalschnitt einer Projektionsbelichtungsanlage für die EUV-Projektionslithographie;
• 2 zeigt ein optisches System für die Projektionsbelichtungsanlage aus 1 gemäß einer ersten Ausführungsform;
• 3 zeigt ein Beispiel einer Wärmestromverteilung eines Spiegels des optischen Systems aus 2;
• 4 zeigt ein weiteres Beispiel einer Wärmestromverteilung eines Spiegels des optischen Systems aus 2;
• 5 zeigt ein reflektierendes Element des optischen Systems aus 2;
• 6 zeigt ein optisches System für die Projektionsbelichtungsanlage aus 1 gemäß einer zweiten Ausführungsform; und
• 7 zeigt ein Flussablaufdiagramm eines Verfahrens zum Betreiben des optischen Systems aus 2 oder 6.

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

• 1 shows a schematic meridional section of a projection exposure system for EUV projection lithography;
• 2 shows an optical system for the projection exposure apparatus 1 according to a first embodiment;
• 3 FIG. 12 shows an example of a heat flux distribution of a mirror of the optical system 2 ;
• 4 FIG. 12 shows another example of a heat flow distribution of a mirror of the optical system 2 ;
• 5 shows a reflective element of the optical system 2 ;
• 6 shows an optical system for the projection exposure apparatus 1 according to a second embodiment; and
• 7 FIG. 12 shows a flow chart of a method for operating the optical system 2 or 6 .

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 comprises 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, an angle different from 0° between the object plane 6 and the image plane 12 is also possible.

A structure on the reticle 7 is imaged on 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, illumination light or working 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 source (Engl.: Gas Discharged Produced Plasma, plasma generated by means of gas discharge). 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 exposed to the illumination radiation 16 in grazing incidence (GI), i.e. with angles of incidence greater than 45°, or in normal incidence (Engl.: Normal Incidence, NI), i.e. with angles of incidence less than 45°. 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 represent a separation between a radiation source module, comprising the light source 3 and the collector 17, and the illumination optics 4.

The illumination optics 4 comprises a deflection mirror 19 and a first facet mirror 20 downstream of this in the beam path. The deflection mirror 19 can be a plane deflection mirror or alternatively a mirror with an effect that influences the bundle beyond the pure deflection effect. Alternatively or additionally, the deflection mirror 19 can be designed as a spectral filter, which separates a useful light wavelength of the illumination radiation 16 from stray light of a different wavelength. If the first facet mirror 20 is arranged in a plane of the illumination optics 4 which is optically conjugate to the object plane 6 as the field plane, it is also referred to as a field facet mirror. The first facet mirror 20 includes a large number 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 multiplicity of individual mirrors, in particular a multiplicity 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, gracing incidence mirror).

The illumination optics 4 has the version in which 1 shown, exactly three mirrors after the collector 17, namely the deflection mirror 19, the 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 can be 0.7 or 0.75, for example.

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 approximately 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, By in the x and y directions x, y. The two image scales βx, By of the projection optics 10 are preferably at (6x, βy)=(+/−0.25, +/-0.125). A positive imaging scale 6 means imaging without image reversal. A negative sign for the imaging scale B 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 .

By irradiating the optical elements 19, 20, 22, M1-M6 or other optical elements of the illumination optics 4 and the projection optics 10 with working light 16, these optical elements heat up due to absorption of the radiation. As a result, temperature gradients can occur in these optical elements, which can lead to thermal deformation and wavefront aberrations.

2 12 shows an optical system 100 according to a first embodiment. The optical system 100 is an optical system for the projection exposure system 1 (lithography system). 1 .

The optical system 100 has a mirror 101 . The mirror 101 can be any of the mirrors 19, 20, 22, M1-M6 of the projection exposure system 1 ( 1 ) be. The mirror 101 is preferably the last mirror M6 of the projection optics 10 of the projection exposure system 1.

The mirror 101 has a mirror body 102 and an optically active surface 103 for reflecting a working light 104 of the lithography system 1 . The working light 104 corresponds, for example, to the working light 16 in 1 . The working light 104 is an EUV light, for example. The mirror 101 also has a passage opening 105 for the working light 104 . In other words, the mirror 101 is obscure optics.

The optical system 100 also has a light source 106 for emitting heating radiation 107 . Light source 106 is, in particular, an infrared light source for emitting infrared radiation 107. Light source 106 is, for example, an infrared laser for emitting monochromatic infrared radiation 107.

The optical system 100 also has a reflective element 108 for reflecting the emitted heating radiation 107 . In particular, the infrared light source 106 radiates the heating radiation 107 onto the reflective element 108 . Furthermore, the reflective element 108 reflects the incident heating radiation 107 in the form of reflected radiation 109 in the direction of the optically active surface 103 of the mirror 101.

The reflective element 108 is arranged in an image plane 110 of the optical system 100 . The image plane 110 corresponds, for example, to the image plane 12 of the lithography system 1 1 . In other words, the reflective element 108 is arranged in a wafer plane 12 of the lithography system 1, for example.

Infrared light source 106 and reflective element 108 together form, in particular, a heating device 111 for local heating of mirror 101. In particular, heating device 111 is used for non-contact heating of mirror 101. For example, heating device 111 is used for heating mirror 101 during a pause in the exposure of working light 104.

During operation of the lithography system 1, as above with reference to 1 described, work light 104 (work light 16 in 1 ) onto a substrate 112 (substrate 13 in 1 ) is projected to transfer a mask pattern onto the photosensitive coating of the substrate 112. The substrate 112 is a silicon wafer, for example. The wafer 112 is in particular in the image plane 110 (image plane 12 in 1 ) of the projection system 10 ( 1 ) arranged.

It is noted that the wafer 112 in 2 is shown shifted out of focus of mirror 101. In order to transfer the mask structure to the light-sensitive coating of the wafer 112, it is necessary to arrange the wafer 112 in the focus of the mirror 101 (similar to the wafer 13 in 1 located at the focus of mirror M6).

The working light 104 is in 2 shown with a dashed line throughout. As in 2 shown, when the wafer 112 is exposed, the working light 104 is emitted, for example, via a mirror 113 (e.g. similar to the mirror M4 in 1 ) in the direction of the through hole 105 of the mirror 101 deflected. The working light 104 then strikes a further mirror 114, which reflects the working light 104 onto the optically active surface 103 of the mirror 101. The other mirror 114 is in 2 represented as a non-obscured mirror. However, the further mirror 114 can also be used in a manner similar to the mirror M5 in 1 have a passage opening for the working light 104 . The working light 104 reflected by the optically active surface 103 of the mirror 101 is then projected onto the wafer 112 (in a state in which the wafer 112 is in the focus of the mirror 101).

Depending on the mask structure that is transferred to the light-sensitive coating of the wafer 112, the exposure of the mirror 101 to the working light 104 can take place unevenly over the optically active surface 103. Due to the exposure of the mirror 101 to the working light 104, an uneven heat flow distribution on the mirror 101 can thus arise.

3 shows the mirror 101 in a plan view of its optically active surface 103. In 3 An example of a heat flow distribution 115 of the mirror 101 is also illustrated. The heat flow distribution 115 has a pattern in the form of a dipole 116, in particular an X-dipole 116. The reference numeral 117 designates, for example, an outer area of the optically active surface 103 of the mirror 101 or designates an optically inactive surface of the mirror 101.

4 shows the mirror 101 in a plan view of its optically active surface 103 with a further example of a heat flow distribution 118 of the mirror 101. The heat flow distribution 118 has a pattern 119 (DRAM pattern 119), which arises during the exposure of the wafer 112 for the production of a DRAM semiconductor component. A DRAM semiconductor device is in particular a memory device with random access (engl. Dynamic Random Access Memory, DRAM).

The 3 and 4 show two examples of a heat flow distribution 115, 118 of the mirror 101 merely for illustration purposes. However, a heat flow distribution of the mirror 101 due to exposure to the working light 104 can assume any form, in particular any heat flow pattern. In particular, the heat flow distribution of the mirror 101 is dependent on the lithography system 1 ( 1 ) each manufactured semiconductor component.

During a pause in the exposure of the working light 104, the mirror 101 can be heated locally by means of the heating device 111, which includes the infrared source 106 and the reflecting element 108.

As in 5 In particular, as shown, the reflective element 108 includes a diffraction grating 120 . The diffraction grating 120 is, for example, a reflection phase grating. The diffraction grating 120 has, for example, periodic grating structures 121 in the form of linear ridges 122 and grooves (furrows) 123 . Furthermore, the diffraction grating 120 comprises a plurality of grating regions 124, 125 5 three grid areas 124, 125 are shown. However, the diffraction grating 120 can also have a different number of grating areas 124, 125, differently shaped grating areas 124, 125 and/or differently designed grating areas 124, 125 have. The lattice areas 124, 125 differ from one another by various lattice parameters. For example, the two lattice regions 124 have a first lattice constant (periodicity of the webs 122 and grooves 123) and the lattice region 125 has a second lattice constant which is smaller than the first lattice constant.

In particular, the diffraction grating 120 (reflective element 108) is designed such that the heating radiation 109 ( 2 ) has a predetermined spatial intensity distribution for local heating of the mirror 101. For example, the diffraction grating 120 is configured such that the heating radiation 109 reflected by the diffraction grating 120 and impinging on the optically active surface 103 of the mirror 101 serves to locally heat the mirror 101 according to a determined heat flow distribution. For example, the diffraction grating 120 is designed in such a way that the heating radiation 109 reflected by the diffraction grating 120 heats up the mirror 101 according to the 3 shown heat flow distribution 115, the in 4 shown heat flow distribution 118 or according to any other heat flow distribution.

The optical system 100 also includes, for example, one or more temperature sensors 126 ( 2 ) for detecting at least one temperature value of the mirror 101. In 2 five temperature sensors 126 are shown as an example, two of which are provided with a reference number. However, the optical system 100 can also have more or fewer than five temperature sensors 126 . As shown, the temperature sensors 126 are disposed, for example, within the mirror body 102 . However, the temperature sensors 126 can also be configured differently than in 2 shown. For example, the temperature sensors 126 can also be non-contact temperature sensors, such as infrared cameras (not shown). The temperature sensors 126 are used to determine a heat flow distribution (e.g. the heat flow distribution 115, 118 in 3 and 4 ) of mirror 101 due to exposure to working light 104.

Additionally or alternatively, the heat flow distribution 115, 118 of the mirror 101 can also be determined based on a simulation. For example, a heat flow distribution can be determined before a specific exposure of the mirror 101 to the working light 104 .

As in 2 shown, the optical system 100 can have at least one further reflective element 127 for reflecting the heating radiation 107 emitted by the light source 106 onto the optically active surface 103 of the mirror 101.

For example, the wafer 112, the reflective element 108 and the at least one further reflective element 127 are held by a holder 128 (e.g. similar to the wafer holder 14 in 1 ) held. The holder 128 is, for example, via a displacement drive 129 (e.g. similar to the wafer displacement drive 15 in 1 ) Displaceable in particular along the y-direction.

With the help of the displacement drive 129 and the holder 128, the further reflecting element 127 can be placed in the beam path of the heating radiation 107 (in the positive y-direction in 2 ) can be moved. As a result, the further reflecting element 127 can be used to reflect the heating radiation 107 and thus to heat up the mirror 101 . In particular, the reflective element 108 has such a diffractive structure ( 5 ) that the reflective element 108 can generate a reflected heating radiation 109 with a first predetermined spatial intensity distribution. In addition, the further reflective element 127 has, in particular, such a diffraction structure that the further reflective element 127 can generate a reflected heating radiation 109 with a second predetermined spatial intensity distribution.

The displacement drive 129 allows the reflective element 108 or the further reflective element 127 to be used for local heating of the mirror 101 as desired. In particular, the reflective element 108 or the second reflective element 127 can optionally be arranged in the beam path of the emitted heating radiation 107 .

in the in 2 In the example shown, the infrared light source 106 is set up to radiate the heating radiation 107 through the passage opening 105 of the mirror 101 onto the reflective element 108 . In particular, in this example the light source 106 is arranged on a side 130 of the mirror 101 facing away from the optically active surface 103 .

As in 6 shown, in another embodiment of the optical system 100', a light source 106' can also be arranged on the side of the optically active surface 103' of a mirror 101'. In this case, the heating radiation 107' is radiated onto the reflecting element 108' without passing through an obscuring opening 105' of the mirror 101'.

The following is related to 7 a method for operating an optical system 100 is described.

In a first step S1 of the method, a heat flow distribution 115, 118 ( 3 and 4 ) of a mirror 101 due to an exposure with a working light 104 ( 2 ) determined.

For example, the heat flow distribution 115, 118 of the mirror 101 by detecting at least one temperature value of the mirror 101 using at least one temperature sensor 126 ( 2 ) determined. Additionally or alternatively, the heat flow distribution 115, 118 of the mirror 101 can also be determined by running a simulation.

In a second step S2 of the method, a reflective element 108 ( 2 ) based on the determined heat flow distribution 115, 118 selected.

Furthermore, in step S2 the selected reflecting element 108 is arranged in an image plane 110 of the optical system 100 and in a beam path of a heating radiation 107 .

In a third step S3 of the method, the heating radiation 107, in particular infrared radiation 107, is emitted.

In a fourth step S4 of the method, the emitted heating radiation 107 is reflected onto the optically active surface 103 of the mirror 101 by the selected reflecting element 108 arranged in the beam path of the heating radiation 107 .

Steps S3 and S4 can be carried out in particular when the working light 104 is not exposed to light. Alternatively, steps S3 and S4 can also be carried out while the mirror 101 is being exposed to the working light 104 . When the heating process of steps S3 and S4 is carried out while the mirror 101 is being exposed to the working light 104, the reflected heating radiation 109 can, for example, only affect outer areas 117 ( 3 ) of the optically active surface 103 are irradiated, which are not used for useful radiation (ie the working light 104).

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

100, 100'

optical system

101, 101'

Mirror

102

mirror body

103, 103'

optically active surface

104, 104'

work light

105, 105'

opening

106, 106'

light source

107, 107'

heat radiation

108, 108'

reflective element

109

reflected radiation

110, 110'

picture plane

111

heating device

112

substrate (wafer)

113

Mirror

114

Mirror

115

heat flow distribution

116

dipole

117

Area

118

heat flow distribution

119

Pattern (DRAM Pattern)

120

diffraction grating

121

lattice structure

122

web

123

groove

124

grid area

125

grid area

126

temperature sensor

127

reflective element

128

bracket

129

displacement drive

130

Page

M1

Mirror

M2

Mirror

M3

Mirror

M4

Mirror

M5

Mirror

M6

Mirror

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 [0082, 0086]
• US20060132747A1 [0084]
• EP 1614008 B1 [0084]
• US6573978 [0084]
• DE 102017220586 A1 [0089]
• US20180074303 A1 [0103]

Claims (15)

Optical system (100) for a lithography system (1), having a mirror (101) with an optically active surface (103) for reflecting a working light (104) of the lithography system (1), a light source (106), in particular an infrared light source, for emitting heating radiation (107), and a reflective element (108) for reflecting the emitted heating radiation (107) onto the optically active surface (103) of the mirror (101), the reflective element (108) being arranged in an image plane (110) of the optical system (100). Optical system after claim 1 , wherein the mirror (101) has an opening (105) and the light source (106) is set up to radiate the heating radiation (107) through the opening (105) onto the reflective element (108). Optical system after claim 2 , wherein the light source (106) is arranged behind the mirror (101) with respect to the optically active surface (103). Optical system according to one of Claims 1 until 3 , wherein the reflective element (108) has a diffraction grating (120), in particular a phase grating, a reflection grating, a reflection phase grating, an amplitude grating and/or a reflection amplitude grating. Optical system according to one of Claims 1 until 4 , wherein the heating radiation (109) reflected by the reflecting element (108) has a predetermined spatial intensity distribution for heating the mirror (101) according to the predetermined spatial intensity distribution, and the predetermined spatial intensity distribution is based on a determined heat flow distribution (115, 118) of the mirror (101) due to exposure to the working light (104). Optical system after claim 5 , The reflective element (108) being set up to generate the reflected heating radiation (109) with the predetermined spatial intensity distribution from an unstructured heating radiation (107) emitted by the light source (106). Optical system according to one of Claims 1 until 6 , Having at least one temperature sensor (126) for detecting at least one temperature value of the mirror (101), wherein the at least one detected temperature value is used to determine a heat flow distribution (115, 118) of the mirror (101) due to exposure to the working light (104). Optical system according to one of Claims 1 until 7 , wherein the reflective element (108) is set up to generate reflected heating radiation (109) with a first predetermined spatial intensity distribution, the optical system has at least one further reflective element (127) for reflecting the heating radiation (107) emitted by the light source (106) onto the optically active surface (103) of the mirror (101), the at least one further reflective element (127) being set up to emit reflected heating radiation (1 09) with at least one second predetermined spatial intensity distribution, which differs from the first predetermined spatial intensity distribution, and the optical system has a device (129) for selectively arranging the reflecting element (108) and the at least one further reflecting element (127) in the beam path of the emitted heating radiation (107). Lithography system (1), in particular EUV lithography system, with an optical system (100) according to one of Claims 1 until 8th . lithography system claim 9 , wherein the image plane (110) is a wafer plane (12) of the lithography system. Method for operating an optical system (100) for a lithography system (1), wherein the optical system (100) has a mirror (101) with an optically active surface (103) for reflecting a working light (104) of the lithography system (1), and the method has the steps: a) emitting (S3) heating radiation (107), in particular infrared radiation, and b) Reflecting (S4) the emitted heating radiation (107) by a reflective element (108) arranged in an image plane (110) of the optical system (100) onto the optically active surface (103) of the mirror (101). procedure after claim 11 , wherein the mirror (101) has an opening (105) and the heating radiation (107) is radiated through the opening (105) onto the reflective element (108). procedure after claim 11 or 12 , wherein steps a) and b) are performed in an exposure pause of the lithography system (1) in relation to the working light (104, 16) of the lithography system (1), and/or steps a) and b) during a Exposure of the mirror (101) with the working light (104, 16) are performed. Procedure according to one of Claims 11 until 13 Having the steps: determining (S1) a heat flow distribution (115, 118) of the mirror (101) due to exposure to the working light (104), and selecting (S2) a reflecting element (108, 127) based on the determined heat flow distribution (115, 118). procedure after Claim 14 , wherein the heat flow distribution (115, 118) of the mirror (101) is determined by detecting at least one temperature value of the mirror (101) using at least one temperature sensor (126), and/or the heat flow distribution (115, 118) of the mirror (101) is determined by running a simulation.

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