The disclosure relates to an EUV illumination device and to a method for operating a microlithographic projection exposure apparatus designed for operation in the EUV.
Microlithography is used for producing microstructured components, such as for example integrated circuits or LCDs. The microlithography process is conducted in what is called a projection exposure apparatus, which comprises an illumination device and a projection lens. The image of a mask (=reticle) illuminated via the illumination device is projected here via the projection lens onto a substrate (e.g. a silicon wafer) that is coated with a light-sensitive layer (photoresist) and arranged in the image plane of the projection lens, in order to transfer the mask structure onto the light-sensitive coating of the substrate.
In projection lenses designed for the EUV range, i.e. at wavelengths of, e.g., approximately 13 nm or approximately 7 nm, mirrors are used as optical components for the imaging process owing to the general lack of availability of suitable light-transmissive refractive materials.
During the operation of a projection exposure apparatus it is typically desirable to set specific polarization distributions in the pupil plane and/or in the reticle in a targeted manner in the illumination device for the purpose of optimizing the imaging contrast and also to be able to carry out a change in the polarization distribution during the operation of the projection exposure apparatus. Thus, the use of s-polarized radiation may be advantageous for the purposes of obtaining the highest possible image contrast especially in the case of a projection exposure apparatus for imaging specific structures when the so-called vector effect in the case of relatively large values of the numerical aperture (NA) is taken into account.
However, scenarios where the use of unpolarized radiation rather than an operation with polarized radiation is advantageous also occur in practice during the operation of a projection exposure apparatus. By way of example, this may be the case even for high values of the numerical aperture (NA) if the structures to be imaged within the scope of the lithography process are not linear structures or structures that otherwise define a preferred orientation but structures without a preferred orientation (e.g. contact holes). In the latter case, the use of linearly polarized radiation might not only fail to yield an advantage but might be found to be disadvantageous as a consequence of an induced unwanted asymmetry.
Further relevant circumstances are given by the fact that the initial production of unpolarized radiation by the utilized EUV source (e.g. a plasma source), as is conventional, is typically accompanied by a loss of radiant flux—specifically as a consequence of the desired output coupling of the respective unwanted polarization component—when polarized radiation is provided, which in turn impairs the performance of the projection exposure apparatus.
Consequently, if the aforementioned aspects are taken into account, it can be desirable in practice to be able to switch between an operating mode with polarized radiation and an operating mode with unpolarized radiation, depending on the operating scenario of the projection exposure apparatus—and for example depending on the structures to be imaged in each case.
However, the implementation of such a switchover might made more difficult in a projection exposure apparatus designed for operation in EUV because, firstly, it is generally desirable for the beam geometry applicable with respect to the beam entry into the illumination device or the beam exit from the illumination device to be maintained from practical points of view but, secondly, in general no suitable transmissive polarization-optical components such as beam splitters are available in the relevant EUV wavelength range. However, the polarization manipulation on the basis of a reflection below the Brewster angle, as is available in the EUV range, is usually accompanied by the introduction of one or more additional beam deflections and hence can involve a significant light loss if an unchanging beam geometry is ensured at the same time.
Reference is made, purely by way of example, to DE 10 2008 002 749 A1, DE 10 2018 207 410 A1 and publication M. Y. Tan et al.: “Design of transmission multilayer polarizer for soft X-ray using a merit function”, OPTICS EXPRESS Vol. 17, No. 4 (2009), pp. 2586-2599.
The present disclosure seeks to provide an EUV illumination device of a microlithographic projection exposure apparatus designed for operation in the EUV and a method for operating a microlithographic projection exposure apparatus designed for operation in the EUV, which can help facilitate a flexible switchover without transmission losses between an operation with polarized radiation and an operation with unpolarized radiation.
According to an aspect, the disclosure provides an EUV illumination device of a microlithographic projection exposure apparatus designed for operation in the EUV including:
Within the meaning of the present application, an illumination device is understood to mean an optical system which illuminates a reticle with a defined spatial and angular distribution by virtue of the radiation of a real or virtual light source being suitably reshaped. In embodiments, for example, the EUV illumination device according to the disclosure can receive the radiation of a plasma (i.e. a real light source) via a collector. In further embodiments, the EUV illumination device can also receive the radiation from an intermediate focus (i.e. a virtual light source).
For example, the disclosure involves realizing a flexible switchover between a polarized operating mode and an unpolarized operating mode in an EUV illumination device, depending on the application scenario and depending on the structures to be imaged in the lithography process in each case, which switchover avoids additional beam deflections, by virtue of exchanging a reflective component situated in the optical beam path of the illumination device for another reflective component with an identical surface geometry but with a different reflection layer system.
According to the disclosure, the provision of two different reflective components which are exchangeable for one another and, as explained below, differ in terms of their spectral reflection profiles for s-polarized and p-polarized radiation but otherwise correspond to one another with respect to their surface geometry can have the consequence that the overall geometry of the beam path within the illumination device remains unchanged even after an exchange of one component for the other component taking place for the purpose of a switchover between polarized and unpolarized operation (i.e. a change between a polarizing and an unpolarizing illumination device) and hence that no additional beam deflections, which are accompanied by an unwanted light loss, are present.
In this case, the disclosure can involve the insight obtained by the inventor on the basis of comprehensive simulation investigations that the spectral reflection profiles which are respectively applicable to s-polarized and p-polarized radiation and which are provided by the respective reflection layer systems of the reflective components that have been exchanged for one another according to the disclosure can be shifted in a targeted manner by way of a suitable adaptation (e.g. thickness scaling of the individual layers forming the layer stack of the reflection layer system) relative to the relevant “transmission interval” of the entire optical system (i.e. in particular, the downstream optical components of the illumination device in the beam path).
This targeted adaptation or shift of the spectral reflection profiles applicable to the s-polarized and p-polarized radiation can in turn be implemented, for example, so that, for the reflective component used in the “polarized operation” of the illumination device or projection exposure apparatus, the spectral reflection profile applicable to s-polarized radiation but not the spectral reflection profile with the respective maximum reflectivity values applicable to p-polarized radiation is located within the transmission range of the optical system. By contrast, the targeted adaptation or shift of the spectral reflection profiles applicable to the s-polarized and p-polarized radiation can be implemented for the reflective component used in the “unpolarized operation” of the illumination device or projection exposure apparatus in such a way that the maximum reflectivity values of both spectral reflection profiles (i.e. both the spectral reflection profile for p-polarized radiation and the spectral reflection profile for s-polarized radiation) are located within the transmission range.
According to an embodiment, a wavelength λ0 exists as mean wavelength in a specified wavelength interval [(λ0−Δλ0/2), (λ0+Δλ0/2)] of width Δλ0 such that the first reflection layer system satisfies the following conditions:
where, in the reflection profiles (r1s(λ), r1p(λ)) of the first reflection layer system, λ1sl and λ1pl denote the shortest wavelength and λ1sr and λ1pr denote the longest wavelength for which in each case s-polarized and p-polarized radiation, respectively, is reflected with a reflectivity of at least 50% of the maximum reflectivity.
According to an embodiment, a wavelength λ0 exists as mean wavelength in a specified wavelength interval [(λ0−Δλ0/2), (20+Δλ0/2)] of width Δλ0 such that the second reflection layer system satisfies the following conditions:
where, in the reflection profiles (r2s(λ), r2p(λ)) of the second reflection layer system, λ2sl and λ2pl denote the shortest wavelength and λ2sr and λ2pr denote the longest wavelength for which in each case s-polarized and p-polarized radiation, respectively, is reflected with a reflectivity of at least 50% of the maximum reflectivity.
Analogously to the aforementioned considerations, it is also possible for the EUV illumination device for a wavelength interval [(−
);(
+
)] to be defined in which the transmissivity is at least 50% of the maximum transmissivity of the EUV illumination device. According to one embodiment, Δλ0 lies between
and
The stated transmission range [(λ0−Δλ0/2), (λ0+Δλ0/2)] of the projection exposure apparatus differs from the transmission range [(−
);(
+
)] of the pure illumination device because a transmission range becomes narrower the more reflections take place at mirrors. The width of the transmission range falls approximately with the square root of the number of reflections. In a typical scenario, a fraction of between ½ and ¼ of the total number of reflections takes place in the illumination device, with the result that the width of the stated transmission range lies between 1/√{square root over (2)} and ½ of the width of the transmission range of the illumination device.
In embodiments of the disclosure, both the first and the second reflective component can be a facet mirror, such as a pupil facet mirror having a plurality of pupil facets or a field facet mirror having a plurality of field facets. In further embodiments, both the first and the second reflective components can also comprise at least one mirror facet of a facet mirror each, such as a pupil facet mirror or a field facet mirror.
In further embodiments, the first and the second reflective component can also each comprise at least one micromirror of a specular reflector.
In further embodiments, the first and the second reflective component can each be a collector mirror.
The disclosure furthermore also relates to a method of operating a microlithographic projection exposure apparatus designed for operation in the EUV, wherein an object plane of a projection lens is illuminated using an illumination device and wherein the object plane is imaged with the projection lens into an image plane of the projection lens, wherein a first reflective component with a first reflection layer system located in the optical beam path of the illumination device is exchanged for a second reflective component with a second reflection layer system for switching between a polarized operating mode and an unpolarized operating mode, and wherein a polarization degree, defined as a ratio between the reflectivities for s-polarized and p-polarized radiation, is greater for the first reflective component by a factor of at least 1.5 than for the second reflective component.
Further configurations of the disclosure are evident from the description and the dependent claims.
The disclosure is explained in greater detail below on the basis of exemplary embodiments illustrated in the accompanying figures.
In the figures:
What is common to the embodiments of the disclosure described below is the basic concept of providing two reflective optical components with differing spectral reflection profiles in a manner such that, for a specified wavelength interval, one of the two components is suitable for a polarized operating mode and the other of the two components is suitable for an unpolarized operating mode. In this case, the aforementioned wavelength interval can be in particular a transmission interval of the respective optical system (e.g. the illumination device of a microlithographic projection exposure apparatus) for which the reflective optical components according to the disclosure are intended and which is typically determined by the reflection profile of the remaining optical components present in the optical system (in particular, the downstream optical components in relation to the optical beam path).
Below, the principle underlying the aforementioned targeted adjustment of the respective reflection layer systems of the reflective optical components according to the disclosure for the polarized and unpolarized operation, respectively, is initially explained with reference to the diagrams in
In general, a given reflection layer system for a specified angle of incidence and a specified wavelength spectrum of the electromagnetic radiation comprises a specific value Is for the reflectivity of s-polarized radiation and a specific value rp for the reflectivity of p-polarized radiation. Consequently, according to
For given materials of the individual layers within the reflection layer system, the values for rs and rp are, in turn, dependent on the respective layer thicknesses, and so reflection layer systems with different value pairs (rs, rp) can be provided by varying these layer thicknesses. As a result, the provision of a multiplicity of corresponding reflection layer systems with different value pairs (rs, rp) in each case allows coverage of a specific region in the rs-rp diagram, for example in accordance with
Accordingly, a corresponding union of the relevant obtainable regions arises according to
Hence, in general, the suitable selection of a defined point in the rs-rp diagram, which in turn corresponds to a uniquely defined layer structure, can be made depending on the intended use or operating mode and the correspondingly produced reflective optical component can be exchanged where desired following the simulation of a multiplicity of reflection layer systems or reflective optical components formed thereby. Once again, depending on the use scenario, this selection can alternatively be made either to maximize the total reflectance provided by the reflection layer system or to provide a specific degree of polarization (corresponding to a ratio of the reflectivities respectively obtained for s-polarized radiation and p-polarized radiation).
What can be observed in this context is that the ultimately practice-oriented or preferred value pairs (rs, rp) are located on the respective edge of the obtainable regions, for example according to
The reflection layer systems used according to the disclosure can be both periodic and aperiodic layer systems. To provide different spectral reflection profiles both for s-polarized and for p-polarized radiation, the corresponding layer designs are now suitably varied, with the consequence that the wavelength-dependent profile of the respective reflectivities Is and rp in the relevant transmission interval ultimately has the respective suitable shape for the polarized or unpolarized operation.
As is evident from
As a consequence, the reflection layer system according to
The realization of the above-described concept according to the disclosure in reflection layer systems in the form of aperiodic multiple layer systems now allows the influencing of the two parameters of width and position of the respective peak in the wavelength-dependent reflectivity profile independently of one another by changing the layer design. The corresponding values for s-polarization and p-polarization are correlated for a given layer design, and so width and position of the peaks for s-polarization and p-polarization cannot be chosen completely independently of one another. However, as already explained on the basis of
Tables 1-4 represent aperiodic layer designs in exemplary fashion, to be precise for systems made of molybdenum silicon (MoSi) or ruthenium silicon (RuSi). For fixed rs=0.7, the tables in each case specify the layer designs that have a maximum and minimum rp, respectively.
For exemplary angles of incidence,
The thickness of silicon for minimum rp is represented by a dash-dotted line. The thickness of molybdenum or ruthenium for minimum rp is represented by a line with a dash followed by two dots.
Exchanging at least one reflective component located in the optical beam path for a component that corresponds with respect to its surface geometry but differs with respect to the reflection layer system present, for the purposes of changing the operating mode between “polarized” and “unpolarized”, can generally be realized for different components of the optical system or of the illumination device.
The disclosure is not restricted to the structure of the illumination device as illustrated in
Possible implementations of the “component exchange” according to the disclosure are explained below with reference to the merely schematic illustrations of
With reference to
In a further embodiment, elucidated in
To the extent that reference is made to a pupil facet mirror in the embodiments described above, there can be an analogous realization for the field facet mirror as well.
In a further variant, the reflection layer systems can be attached to the collector mirror 803, reference having made to
Even though the disclosure has been described on the basis of specific embodiments, numerous variations and alternative embodiments will be apparent to a person skilled in the art, for example through combination and/or exchange of features of individual embodiments. Accordingly, it will be apparent to a person skilled in the art that such variations and alternative embodiments are also encompassed by the present disclosure, and the scope of the disclosure is restricted only within the scope of the appended patent claims and the equivalents thereof.
Number | Date | Country | Kind |
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10 2021 210 492.4 | Sep 2021 | DE | national |
The present application is a continuation of, and claims benefit under 35 USC 120 to, international application No. PCT/EP2022/074741, filed Sep. 6, 2022, which claims benefit under 35 USC 119 of German Application No. 10 2021 210 492.4, filed Sep. 21, 2021. The entire disclosure of each these applications is incorporated by reference herein.
Number | Date | Country | |
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Parent | PCT/EP2022/074741 | Sep 2022 | WO |
Child | 18605591 | US |