FIELD OF THE INVENTION

The invention relates to an arrangement for an EUV lithography apparatus, comprising a reflective optical element having an optically effective surface configured to reflect incident EUV radiation, and comprising a filament arrangement for producing a reagent that cleans the optically effective surface, wherein the filament arrangement comprises at least one filament configured as a glow or heating element.

BACKGROUND

An arrangement of the type mentioned in the introduction is known from U.S. 2010/0051827 A1.

A lithography apparatus generally serves for producing microstructured semiconductor components, for example. Specifically, with the lithography apparatus, projection radiation is used to image a pattern of a mask onto a wafer. During the lithographic production of semiconductor components, the resolution with which a structure can be imaged onto the substrate (wafer) is sometimes limited by the wavelength of the projection radiation. In order to obtain integrated circuits having a very high structure density and small structure dimensions, it has therefore been proposed to use projection radiation in the extreme ultraviolet spectral range (EUV). EUV radiation can have e.g. a wavelength of approximately 13 nm.

The EUV radiation source is typically a plasma source. In order to produce a plasma, often a so-called target is irradiated with laser light having a long wavelength, wherein tin (Sn), for example, is used as the target. The plasma produced then emits the EUV radiation.

In EUV lithography apparatuses, exclusively reflective optical elements are used as optical elements since there are no lens element materials which transmit radiation in the EUV range. The reflective optical elements present in an EUV lithography apparatus have to have the highest possible reflectivity in order to ensure a sufficiently high total reflectivity of the optical system of the EUV lithography apparatus. The reflectivity and the lifetime of the reflective optical elements can be reduced undesirably by contaminations of the optically effective surfaces of the reflective optical elements. In this case, the collector mirror arranged closest to the target is particularly burdened by contaminations. The collector mirror captures the EUV radiation emanating from the plasma and collects it in order to feed the EUV radiation into the further beam path. In particular, deposits of the target material, for example Sn, and of chemical compounds thereof can form at the optically effective surface of the collector mirror. However, other deposits such as carbon compounds can also deposit on the collector mirror and other reflective optical elements of the lithography apparatus.

In order to clean away contaminations of the optically effective surfaces of reflective optical elements in EUV lithography apparatuses, it is known to bring the optical surfaces into contact with activated hydrogen, i.e. hydrogen that is present for example in the form of H*, H+and/or H2+. It is thereby possible to convert contaminations on the optically effective surfaces into readily volatile compounds that no longer adhere to the optically effective surface. A reagent within the meaning of the present invention can be activated hydrogen, for example.

It is desirable for the optically effective surfaces of the optically reflective elements to be able to be cleaned, for example with the aid of activated hydrogen, also during exposure operation of the lithography apparatus. Various approaches are known in the prior art for this purpose. The document U.S. 2010/0051827 A1 cited above describes a nested collector mirror comprising a plurality of mirror shells nested in one another, wherein the optically effective surfaces of the individual mirror shells are operated with grazing incidence. For cleaning away contaminations during operation, it is proposed, inter alia, to arrange a plurality of filaments configured as glow wires on a rear-side surface of the individual mirror shells, i.e. an optically non-effective surface facing away from the optically effective surface of the respective mirror shell, such that the glow wires are situated in the shadow region of the EUV radiation. This is intended to have no adverse effect on the optical performance of the collector mirror. However, this approach is not suitable, or is suitable only to a lesser extent, for non-nested mirrors, in particular for collector mirrors having only one optically effective surface for reflecting the EUV radiation. This is because an arrangement of the filaments on the mirror rear side would not have a sufficient cleaning effect on the optically effective surface.

Other approaches in the prior art, as described in DE 10 2008 040 720 A1, for example, provide one or more cleaning heads comprising a feed for a cleaning gas, for example molecular hydrogen, a glow wire and a discharge for atomic and/or molecular hydrogen, wherein the outlet of the cleaning device is positioned such that it is arranged outside the reflection region of the reflective optical element to be cleaned.

U.S. 2017/0064800 A1 proposes, in the case of a collector mirror configured for normal incidence of the EUV radiation, arranging a plurality of filaments in the form of glow wires outside the optically effective surface in direct proximity to the outer edge region of the optically effective surface of the collector mirror symmetrically around the center of the EUV collector mirror. In that case, the hot glow wires convert molecular hydrogen into activated hydrogen, for example into hydrogen radicals, which etch away Sn that has deposited on the optically effective surface.

Despite the numerous approaches for cleaning optically effective surfaces in EUV lithography apparatuses, there is still a need for an arrangement of the type mentioned in the introduction which is improved with regard to the cleaning effect on the optically effective surface of the reflective optical element.

SUMMARY

With regard to the arrangement mentioned in the introduction, this object is achieved by virtue of arranging at least one filament in direct proximity to and along the optically effective surface of the reflective optical element, wherein a thickness and/or positioning of the at least one filament are/is chosen so as to minimize an optical influence of the at least one filament at least in the far field of the EUV radiation reflected by the optically effective surface.

It was not recognized in the prior art that the etching rate of the reagent produced at a hot filament for cleaning the optically effective surface decreases greatly with increasing distance from the filament. Therefore, the present invention departs from the concept of arranging the filament arrangement outside the optically effective surface. According to the invention, the filament arrangement is arranged in direct proximity to the optically effective surface, which is to be cleaned, of the reflective optical element. The cleaning effect of the filament arrangement is thus significantly improved. Since the filament(s) in the arrangement according to the invention is/are situated in the reflection region of the optically effective surface, in the arrangement according to the invention provision is furthermore made for minimizing the optical influence of the at least one filament through suitable selection of the thickness and/or positioning of the at least one filament at least in the far field of the EUV radiation reflected by the optically effective surface. Positioning here should be understood also to mean the orientation of the at least one filament. “Far field of the EUV radiation reflected by the optically effective surface” should be understood to mean the radiation field or the wavefront of the EUV radiation far away from the reflective optical element. What is crucial is not whether the filament(s) has/have an optical influence on the reflected EUV radiation in direct proximity to the reflective optical element, but rather whether a possible disturbance of the wavefront becomes apparent in the image plane of the imaging system of the EUV lithography apparatus, i.e. in the plane of the wafer to be exposed. Suitable selection of thickness and/or positioning of the at least one filament along the optically effective surface makes it possible to minimize such detrimental optical influences on the reflected EUV radiation to an extent such that the wavefront of the EUV radiation in the wafer plane is within a specification required for the respective case of exposure.

The filament arrangement can comprise a single filament having such a long length that it can be arranged in a manner distributed over the optically effective surface, for example in a meandering fashion. However, the filament arrangement can also comprise a plurality of individual filaments arranged in a manner distributed along the surface, wherein the filaments or a portion of the filaments can be connected to one another.

The arrangement according to the invention is particularly advantageous in the case where the reflective optical element is a collector mirror of an EUV light source, wherein such a collector mirror is, in particular, a collector mirror designed for normal incidence.

In one preferred embodiment, the thickness of the at least one filament is less than 3 mm, preferably less than 2 mm, more preferably less than 1 mm.

The smaller the thickness of the at least one filament, the smaller the optical influence thereof in the far field of the EUV radiation reflected by the optically effective surface.

In a further preferred embodiment, the at least one filament is arranged at a distance in a range of 5 mm to 50 mm from the optically effective surface

Spacing apart the at least one filament from the optically effective surface firstly has the advantage of a smaller optical influence on the reflected EUV radiation, and secondly the thermal influence of the at least one filament on the optically effective surface is reduced as a result. The thermal influence can be further reduced or eliminated by suitably cooling the optical element. The spacing apart can be in a range of 5 mm to 30 mm, furthermore in a range of 5 mm to 20 mm.

In a further preferred embodiment, the filament arrangement comprises a plurality of filaments arranged in a manner distributed uniformly along the optically effective surface

In this case, it is advantageous that a uniform cleaning effect is established over the optically effective surface. Secondly it is thereby possible also to avoid thermal influences of the filaments on the optically effective surface that are distributed nonuniformly over the optically effective surface.

Preferred embodiments are described below with regard to the positioning of the at least one filament along the optically effective surface.

In one embodiment, the at least one filament is positioned in a manner extending at least substantially radially between a center of the optically effective surface and a radially outer edge region of the optically effective surface.

If the filament arrangement comprises a plurality of filaments, the filament arrangement can be embodied preferably centrally radially symmetrically with respect to the center of the optically effective surface.

In a further embodiment, the at least one filament is positioned in a manner extending at least substantially circularly or partly circularly around the center of the optically effective surface.

A filament arrangement that is rotationally symmetrical with respect to the center can be provided as a result.

In yet another embodiment, the at least one filament is positioned in a manner extending at least substantially tangentially with respect to the center of the optically effective surface.

In connection with the abovementioned embodiment and/or the embodiment mentioned further above of radial positioning, in the case where the reflective optical element is arranged upstream of a facet mirror during operation in a lithography apparatus, said facet mirror comprising facets having a larger dimensioning in a first direction than in a second direction perpendicular to the first direction, it is advantageous if the at least one filament extends in a direction at least substantially parallel to the first direction of the facets along the optically effective surface.

It has been found that the optical influence in the case of rectilinear positioning of the at least one filament is particularly small or negligible if the filament(s) extend(s) in the direction of the longer dimension of the facets of the facet mirror.

The optical influence can also be minimized by the at least one filament extending along a region of the optically effective surface which reflects a part of the EUV radiation that is not used in the far field.

This measure can make use of the fact that the EUV radiation reflected by the optically effective surface does not contribute in its entirety to the imaging into the wafer plane. This is the case, for example, if there are arranged in the further beam path of the EUV radiation segmented reflective optical elements, such as the facet mirrors already mentioned, for example, which comprise non-reflective regions, such as gaps between mirror segments, for example, such that that part of the EUV radiation which is incident on the non-reflective regions is not guided further in the direction of the wafer plane. By tracing back the beam path from the non-reflective regions to the optical element—in particular collector mirror—to be cleaned, it is possible to determine the regions on the optically effective surface to be cleaned in which the filament(s) can be arranged, without a disturbing optical influence being manifested in the far field.

In further embodiments, the at least one filament is configured as a resistance heating element.

The configuration of the at least one filament as a resistance heating element with a corresponding voltage and/or current supply has the advantage over other ways of heating the filament(s), such as with thermal radiation, for example, in that the filaments can be heated to very high temperatures and the reagent, e.g. activated hydrogen, can thus be produced effectively. Moreover, stray heat effects as a result of external heating systems are avoided. The filament(s) can be operated with DC voltage or with AC voltage.

What may be considered to be suitable material for the filaments is, in particular, a material that does not evaporate, or evaporates only little, at high temperatures, such as tungsten, for example.

It is furthermore advantageous if the at least one filament is connected to at least one electrical supply conductor for supplying the at least one filament with electric current, wherein the at least one supply conductor is preferably configured as a holder for the at least one filament.

In this case, the electrical supply conductor(s) can have a greater thickness than the at least one filament. By way of example, a filament can have a thickness of 1 mm, and the electrical supply conductor can have a thickness of up to 3 mm. The supply conductor(s) can advantageously be configured as wires.

In a further preferred embodiment, the at least one supply conductor extends along the optically effective surface radially between a center and a radially outer edge region of the optically effective surface.

This arrangement of the at least one supply conductor can advantageously be chosen for one of the abovementioned embodiments of the positioning of the at least one filament according to which the filament(s) likewise extend(s) radially, or in which the filament(s) extend(s) circularly or partly circularly around the center of the optically effective surface.

The polarities of the electrical voltage between respectively adjacent supply conductors can alternate, which has the advantage that electric fields caused by the supply conductors cancel one another out, or the polarities of the electrical voltage can be identical among the supply conductors, which has the advantage that a connection of identical polarity can be used for a plurality of supply conductors, which simplifies the construction because the number of required connections decreases.

In one of the abovementioned embodiments with regard to the positioning of the at least one filament in which the filament(s) extend(s) along the optically effective surface linearly, that is to say in a straight fashion, the supply conductor(s) preferably also extend(s) along the optically effective surface in a straight fashion, but perpendicularly to the filaments.

However, in contrast to the filaments, the electrical supply conductors can also be arranged completely outside the optically effective surface, as provided in a further preferred embodiment. In this embodiment, the electrical supply conductors have no optical influence whatsoever on the EUV radiation that is reflected by the optically effective surface. In connection with the embodiment mentioned above, one or more additional electrically insulating or electrically nonconductive holding elements for holding the at least one filament in position can be arranged along the optically effective surface. The additional holding element(s) serve(s) for stabilizing the arrangement and positioning the at least one filament.

Preferably, the filament(s) is/are operated with a bias voltage, which prevents charged particles, in particular ions, from being accelerated toward the optically effective surface, which could result in the optically effective surface being damaged.

An EUV lithography apparatus according to the invention comprises an arrangement according to one or more of the embodiments mentioned above.

Further advantages and features are evident from the following description and the attached drawing.

The aforementioned features and those yet to be explained below can be used not only in the combination specified in each case but also in other combinations or on their own, without departing from the scope of the present invention.

Exemplary embodiments of the invention are illustrated in the drawing and described in more detail with reference thereto. In the Figures:

