Patent Application
20180364492
The disclosure relates to an optical system, in particular for microscopy. The disclosure is able to be advantageously used in a wide variety of application fields, for example in microscopy applications in the material sciences field, in biology or various other basic research. A further possible use of the disclosure is a mask inspection system for inspecting reticles or masks for use in a microlithographic projection exposure apparatus.
In brightfield reflected-light microscopy, the object to be investigated is illuminated using a beam splitter, which is tilted relative to the illumination light that is incident from a light source and which deflects the light onto the object to be investigated. To increase the attainable resolution, a transition to increasingly lower operating wavelengths is desirable in this case.
In optical systems designed for the EUV range, i.e. at wavelengths of less than 30 nm (e.g. approximately 13 nm or approximately 7 nm), owing to the lack of availability of suitable light-transmissive refractive materials, mirrors are used as optical components for the imaging process. This is also true for systems which are designed for shortwave VUV radiation (e.g. wavelengths below 150 nm), because such systems are also preferably designed as mirror systems.
In the above-mentioned applications, beam splitters are used which transmit and reflect proportions of the respective illumination light in order to, on the one hand, direct the relevant electromagnetic radiation onto a sample to be investigated (e.g. arranged in the object plane of a microscope objective) and, on the other hand, to supply it to a detector. The desired properties that exist here in practice also generally include—in addition to a minimization of occurring absorption and scattering losses—that the proportions which are separated at the beam splitter (i.e. the transmitted proportion and the reflected proportion of the electromagnetic radiation) are, if possible, identical in terms of intensity (what is known as a “50/50 beam splitter”).
To minimize absorption losses in the respective operating wavelength, in particular the implementation of beam splitters with dielectric layer systems is known, which have a sequence of individual layers made of materials having a different refractive index.
However, in practice frequently the issue can arise that, to cover a larger wavelength range, in principle a large multiplicity of different dielectric layers is desired, which in turn involves increasing absorption losses due to the multiplicity of interfaces with increasing stray light proportions and in particular at low wavelengths of e.g. less than 150 nm.
The present disclosure seeks to provide an optical system, in particular for microscopy, wherein beam splitting over a comparatively large wavelength range is made possible while avoiding the previously described problems.
In accordance with one aspect of the disclosure, an optical system, in particular for microscopy, includes: a beam splitter having a light entrance surface and a light exit surface; wherein the beam splitter absorbs, for a specified operating wavelength range of the optical system, less than 20% of electromagnetic radiation that is incident on the light entrance surface; and wherein the beam splitter is arranged in the optical system such that the angles of incidence which occur during operation of the optical system at the light entrance surface and/or at the light exit surface, with reference to the respective surface normal, are at least 70°.
The disclosure involves the concept of a passage through at least one interface of a beam splitter, located in the optical beam path, with relatively high angles of incidence (with reference to the respective surface normal) in an optical system, such as a microscope, with the consequence that even without using a coating, such as a dielectric layer system, at the beam splitter a relatively high reflectivity is realized and, as a result, a high throughput is able to be attained (which may be comparable to beam splitters in the visible spectral range and close to the theoretical ideal value of 25%).
Due to the omission of the (e.g. dielectric) coating or structuring of the beam splitter according to the disclosure, the problems of layer degradation which typically occur in such layer systems can be avoided, as a result of which the production effort and costs can be significantly reduced. Furthermore, due to the omission of a layer system which is formed from a multiplicity of dielectric individual layers, absorption and scatter losses can be minimized.
Due to the functional principle of the beam splitter according to the disclosure, a beam split with high broadbandedness with respect to the possible operating wavelength range is already “intrinsically” attained, wherein, depending on the embodiment, operating wavelengths of below 120 nm (in particular also in the EUV range, i.e. less than 30 nm, in particular less than 15 nm) and into the infrared spectral range are realizable.
According to an embodiment, the beam splitter is arranged in the optical system in such a way that the angles of incidence occurring at the light entrance surface and/or at the light exit surface during operation of the optical system, with reference to the respective surface normal, are at least 75°, in particular at least 80°.
In accordance with one embodiment, the beam splitter has a plane-parallel geometry. It can have in this case in particular a thickness of less than 1 mm, more particularly less than 0.5 mm. As a result, a comparatively low or minimum optical path within the respective material of the beam splitter can be realized, with the consequence that absorption losses, an unavoidable beam offset between the light proportions which are reflected at the two interfaces of the beam splitter, and also chromatic aberrations can be minimized.
In accordance with a further embodiment, the beam splitter has at least one component with wedge-shaped or wedge-section-shaped geometry. An implementation of this type has in particular the advantage that, after multiple reflections within the beam splitter, light proportions that exit with a beam offset can be relatively easily blocked out due to the exit angles that differ from the exiting “used light,” and thus a disturbing influence of such light proportions on the imaging result can be avoided.
In accordance with a further embodiment, the beam splitter has a prism-shaped geometry. Such an implementation has in particular the advantage that, with integration of the beam splitter in the optical (overall) system, a generally desired realization of 90° deflections between incident and transmitted beam is possible without additional folding or deflection mirror and thus with a reduction of the total number of optical components or mirrors.
According to one embodiment, the beam splitter is made from a material selected from the group including magnesium fluoride (MgF2), lithium fluoride (LiF), aluminum fluoride (AlF3), calcium fluoride (CaF2) and barium fluoride (BaF2).
In accordance with one embodiment, the beam splitter consists only of this material.
In accordance with one embodiment, the beam splitter has at least one uncoated component having the light entrance surface and/or the light exit surface. In other words, the beam splitter preferably has no (e.g. dielectric) coating whatsoever, with the result that in particular no layer degradation can occur either.
According to an embodiment, the optical system is designed for an operating wavelength of less than 150 nm, in particular less than 120 nm.
According to an embodiment, the optical system is designed for an operating wavelength of less than 30 nm, more particularly less than 15 nm.
According to an embodiment, the optical system is a microscope.
According to one embodiment, the optical system is a mask inspection system for inspecting reticles or masks for use in a microlithographic projection exposure apparatus.
Further configurations of the disclosure can be gathered from the description and the claims.
The disclosure is explained in greater detail below on the basis of exemplary embodiments illustrated in the accompanying figures, in which:
In the design of a brightfield reflected-light microscope shown schematically in
The disclosure is not limited to the realization in such a microscope. For example, the disclosure, or the beam splitter having a design in accordance with the disclosure, can in further applications also be realized e.g. in a mask inspection system for inspecting reticles or masks for use in a projection exposure apparatus of a mask inspection apparatus or in another optical system.
Various embodiments of a beam splitter according to the disclosure will be described below with reference to the schematic illustrations of
In
To minimize absorption losses, the beam splitter 100 preferably has a thickness of less than 1 mm, in particular less than 0.5 mm. Furthermore, the beam splitter 100 is produced from a material which is sufficiently transmissive or light-transmissive in the respective operating wavelength range. The material and thickness of the beam splitter is preferably selected such that the beam splitter absorbs, for a specified operating wavelength range of the optical system, less than 20% of electromagnetic radiation that is incident on the light entrance surface. At operating wavelengths in the region around 120 nm or below, e.g. magnesium fluoride (MgF2) is a suitable material.
In further embodiments, the beam splitter 100 can also be produced with an even lower thickness (e.g. including as a thin film made of silicon (Si)). Advantageous is a thickness, which is as low as possible due to the absorption losses, of less than 1 μm, with further preference a thickness of less than 100 nm.
One embodiment for ensuring sufficient stability or avoidance of undesired impairment of the imaging quality due to any surface deformations of the beam splitter is illustrated merely schematically in
Once again with reference to
In order to simplify elimination of such light proportions which have a disturbing effect for highly precise imaging, the beam splitter according to the disclosure can also have, as shown in
Since in the implementation of the beam splitter 200 described above with wedge-shaped or wedge-section-shaped geometry one of the interfaces (specifically the first interface 200a) does not contribute to the reflection proportion, the transmission proportion at this interface is preferably as great as possible. To this end, the angle of incidence at the relevant interface is preferably significantly smaller than at the other (reflective) interface, wherein the angle of incidence at the relevant interface which does not contribute to the reflection proportion preferably can be selected to be smaller than 65°. In further embodiments, it is possible, as indicated in
In the previously described implementation of the beam splitter according to the disclosure as a prism, the above statements relating to the highest possible transmission proportion of the interface which does not contribute to the reflection proportion or the preferably performed selection of correspondingly lower angles of incidence at the relevant interface analogously apply.
According to
Furthermore, due to the use of a second subelement in the beam splitter according to the disclosure or owing to an asphere which is formed on an interface of the beam splitter, an astigmatic wavefront error can also be corrected.
In further embodiments, the beam splitter according to the disclosure can also be arranged in the optical system such that the optical beam path is in each case reversed as compared to the previously described embodiments.
Even though the disclosure has been described on the basis of specific embodiments, numerous variations and alternative embodiments are apparent to a person skilled in the art, for example by combination and/or exchange of features of individual embodiments. Accordingly, it goes without saying for a person skilled in the art that such variations and alternative embodiments are concomitantly encompassed by the present disclosure, and the scope of the disclosure is restricted only within the meaning of the accompanying patent claims and the equivalents thereof.
| Number | Date | Country | Kind |
|---|---|---|---|
| 10 2016 203 749.8 | Mar 2016 | DE | national |
The present application is a continuation of, and claims benefit under 35 USC 120 to, international application PCT/EP2017/053598, filed Feb. 17, 2017, which claims benefit under 35 USC 119 of German Application No. 10 2016 203 749.8, filed Mar. 8, 2016. The entire disclosure of these applications are incorporated by reference herein.
| Number | Date | Country | |
|---|---|---|---|
| Parent | PCT/EP2017/053598 | Feb 2017 | US |
| Child | 16113585 | US |
