Illumination optical unit

An illumination optical unit for an EUV projection exposure apparatus has a diaphragm comprising a radiation-transmissive region having a discrete symmetry group. The form of the diaphragm is adapted to the form of the facets of a pupil facet mirror or to the form of the radiation source. The diaphragm is preferably arranged in the region of an intermediate focal plane.

The contents of German patent application DE 10 2011 076 297.3 and U.S. 61/488,901 are incorporated by reference.

The invention relates to an illumination optical unit and an illumination system comprising a diaphragm. Moreover, the invention relates to an EUV projection exposure apparatus comprising an illumination system of this type, a method for producing a micro- or nanostructured component, and a component produced by the method.

It is known that the optical quality of an exposure apparatus can be improved by suitable arrangement of one or a plurality of diaphragms. An EUV projection exposure apparatus comprising a diaphragm is known from US 2007/242799 A1, for example.

Circular diaphragms are known from the prior art. According to the invention, it has been recognized that this does not constitute the optimum design of a diaphragm.

Therefore, it is an object of the invention to improve an illumination optical unit for an EUV projection exposure apparatus.

This object is achieved by an illumination optical unit for illuminating an object field that can be imaged by an imaging optical unit with radiation emitted from an EUV radiation source. The illumination optical unit includes a pupil facet mirror having a multiplicity of facets of a specific form, and a diaphragm. The diaphragm includes a first region which is transmissive to impinging EUV radiation, and a second region which is opaque to impinging EUV radiation. The regions define a diaphragm plane. At least one of the regions has a discrete symmetry group in the diaphragm plane. The diaphragm is arranged in the region of an intermediate focal plane of the EUV radiation source. At least one of the regions of the diaphragm has a form which is adapted to the form of the facets of the pupil facet mirror or to the form of the radiation source.

According to the invention, it has been recognized that the form of the diaphragm also has a significant influence on the imaging quality of the projection exposure apparatus. It has surprisingly been established that a circular diaphragm form is often not optimal, rather that a diaphragm embodied e.g. in polygonal fashion leads to improved transmission properties and thus to an improved imaging quality. The diaphragm has a radiation-transmissive region having, in particular, a discrete symmetry group in the diaphragm plane. The discrete symmetry is, in particular, a non-trivial n-fold rotational symmetry, where n>2, in particular 2<n<10, in particular n=2 or n=4. In this case, the diaphragm can be embodied as a circumferential diaphragm. It circumferentially delimits the beam path. The frame of the diaphragm can also have a direct symmetry group. The latter corresponds, in particular, to the symmetry group of the diaphragm opening.

According to an advantageous embodiment, the form of the diaphragm is adapted to the form of the facets of a facet element of the illumination optical unit. Advantageously, the form of the diaphragm is adapted, in particular, to the form of the pupil facets. In the case of rectangular facets, the diaphragm advantageously likewise has a rectangular form. In this case, the aspect ratio of the diaphragm advantageously corresponds exactly to the aspect ratio of the facets. In particular, a square embodiment of the diaphragm is advantageous in the case of square facets.

According to a further advantageous embodiment, the form of the diaphragm is adapted to the form of the radiation source. This is advantageous particularly in the case of an arrangement of the diaphragm in the region of an intermediate image of the radiation source. As a result, the stability of the illumination of the object field can be improved further.

A rectangular embodiment of the diaphragm in which at least one of the regions of the diaphragm is embodied in rectangular fashion makes it possible for the form of the diaphragm to be adapted particularly well to the embodiment of the projection exposure apparatus and/or of the object to be imaged. As a special case the rectangular embodiment of the diaphragm can be embodied, in particular, in square fashion.

According to an embodiment in which the first region of the diaphragm is marginally surrounded completely by the second region, the diaphragm is embodied as an aperture stop. This is advantageous, in particular, if the frame of the diaphragm has further, in particular mechanical, functions.

The diaphragm is in particular embodied as an intermediate focus diaphragm, that is to say that it is arranged in the region of an intermediate focal plane of the radiation source.

The form of the diaphragm is in particular adapted to the intensity distribution of the EUV radiation coming from the EUV radiation source. In this case, in particular the embodiment of the EUV radiation source and the structural details of the collector can be taken into account. Such an adaptation of the form of the diaphragm, in particular of the diaphragm opening, makes it possible to optimize, in particular, the transmission properties, for example the total intensity, of the EUV radiation emerging from the source unit, under the secondary condition that the diaphragm opening is as small as possible, in order to achieve a good vacuum separation between the radiation source and the illumination optical unit.

A further object of the invention consists in improving an illumination system of an EUV projection exposure apparatus.

This object is achieved by an illumination system for illuminating an object field that can be imaged by an imaging optical unit with radiation emitted from an EUV radiation source. The illumination system includes a source unit and an illumination optical unit as described above. The diaphragm is arranged in the region of an intermediate focal plane of the EUV radiation source.

The advantages of an illumination system of this type correspond to those which have been described above for the illumination optical unit.

A vacuum separation may be provided between the source unit and the illumination optical unit, and the diaphragm may be arranged at a boundary between the source unit and the illumination optical unit. The diaphragm is arranged at the boundary between the source unit and the illumination optical unit. The vacuum separation between the source unit and the illumination optical unit can be improved as a result.

Further objects of the invention are to specify a projection exposure apparatus comprising the illumination optical unit according to the invention, a method for producing a component using the projection exposure apparatus, and a component produced by the method.

These objects are achieved according to the invention by a projection exposure apparatus according to including an illumination system described above and and imaging optical unit for imaging the object field into an image field, a production method that includes using such a projection exposure apparatus, and by a component according to such a method.

The advantages of these subjects correspond to those which have already been discussed above.

FIG. 1schematically shows in a meridional section a projection exposure apparatus1for microlithography. An illumination system2of the projection exposure apparatus1has, besides a radiation source3, an illumination optical unit4for exposing an object field5in an object plane6. A recticle7arranged in the object field5, the reticle being held by a recticle holder8illustrated merely as an excerpt is exposed in this case. A projection optical unit9serves for imaging the object field5into an image field10into an image plane11. A structure on the recticle7is imaged onto a light-sensitive layer of a wafer12arranged in the region of the image field10in the image plane11, the wafer being held by a wafer holder13likewise illustrated schematically.

The radiation source3is an EUV radiation source having an emitted used radiation in the range of between 5 nm and 30 nm. This can involve a plasma source, for example a GDPP (gas discharge-produced plasma) source or an LPP (laser-produced plasma) source. A radiation source placed on a synchrotron can also be used for the radiation source3. Information about a radiation source of this type can be found by the person skilled in the art for example from U.S. Pat. No. 6,859,515 B2. EUV radiation14emerging from the radiation source3is concentrated by a collector15. Downstream of the collector15, the EUV radiation14propagates through an intermediate focal plane16before it impinges on a field facet mirror17. The field facet mirror17is arranged in a plane of the illumination optical unit4which is optically conjugate with respect to the object plane6.

The EUV radiation14is also designated hereinafter as illumination light or as imaging light.

Downstream of the field facet mirror17, the EUV radiation14is reflected from a pupil facet mirror18having a multiplicity of pupil facets. In particular, each field facet images the intermediate focus onto a pupil facet assigned to it. The assignment between the field facets and the pupil facets can be switchable. The pupil facets can be embodied in rectangular fashion, in particular in square fashion.FIG. 3shows a pupil facet mirror18with rectangular pupil facets18a.FIG. 4shows a pupil facet mirror18with rectangular pupil facets18a. For details in this regard, reference should be made to U.S. Pat. No. 6,452,661, in particular FIG. 15 and FIG. 23. The pupil facet mirror18is arranged in a pupil plane of the illumination optical unit4which is optically conjugate with respect to a pupil plane of the projection optical unit9. With the aid of the pupil facet mirror18and an imaging optical assembly in the form of a transfer optical unit19having mirrors20,21and22designated in the order of the beam path, field facets of the field facet mirror17are imaged into the object field5. The field facets have a form adapted to the form of the object field5. They are embodied, in particular, in rectangular or arcuate fashion. The last mirror22of the transfer optical unit19is a mirror for grazing incidence (“grazingincidence mirror”). The pupil facet mirror18and the transfer optical mirror19form a subsequent optical unit for transferring the illumination light14into the object field5. The transfer optical unit19can be dispensed with particularly when the pupil facet mirror18is arranged in an entrance pupil of the projection optical unit9. For further details of the illumination optical unit4, reference should be made to U.S. Pat. No. 6,452,661.

For simpler description of positional relationships, a Cartesian xyz coordinate system is depicted inFIG. 1. The x-axis runs perpendicularly to the plane of the drawing into the latter inFIG. 1. The y-axis runs toward the right. The z-axis runs downward. The object plane6and the image plane11both run parallel to the xy plane.

The recticle holder8is displaceable in a controlled manner such that, during the projection exposure, the recticle7can be displaced in a displacement direction in the object plane6parallel to the y-direction. The wafer holder13is correspondingly displaceable in a controlled manner such that the wafer12is displaceable in a displacement direction in the image plane11parallel to the y-direction. As a result, the reticle7and the wafer12can be scanned firstly through the object field5and secondly through the image field10. The displacement direction is also designated as the scanning direction. The displacement of the reticle7and of the wafer12in the scanning direction can preferably be effected synchronously with one another.

A diaphragm23is arranged in the intermediate focal plane16. A schematic illustration of the diaphragm23, from which further details can be gathered, is represented inFIG. 2. The diaphragm23comprises a first region, which is embodied, in particular, as an opening24, and a second region, which is embodied as a frame25. The first region is transmissive to impinging EUV radiation. In this case, transmissive should be understood to mean that the first region24has a transmittance of at least 80%, of at least 90%, of at least 95%, of at least 98%, or even of more than 99%, for the impinging EUV radiation14. The opaque second region25correspondingly has a transmittance of at most 5%, of at most 1%, or of at most 0.1%.

The regions24,25define a diaphragm plane, which coincides with the intermediate focal plane16in the exemplary embodiment illustrated inFIG. 1. The diaphragm plane is oriented, in particular, perpendicularly to an optical axis of the projection exposure apparatus1.

In the exemplary embodiment illustrated, the regions24,25are embodied in rectangular fashion, in particular in square fashion. However, they have rounded corners. Expressed generally, the regions24,25are embodied in polygonal fashion. In particular, at least one of the regions24,25has a discrete symmetry group in the diaphragm plane. In other words, the diaphragm23has a non-trivial, n-fold rotational symmetry, but no circular symmetry. Consequently, n>2 holds true. In particular 2<n<10, in particular n=2 or n=4 holds true. By way of example, the square diaphragm23illustrated inFIG. 2has a four-fold rotational symmetry, while a rectangular diaphragm having an aspect ratio not equal to 1 has only a two-fold, but not a four-fold rotational symmetry. The aspect ratio of the diaphragm23corresponds, in particular, to the aspect ratio of the pupil facets on the pupil facet mirror18.

In the exemplary embodiment illustrated inFIG. 2, the first region24is marginally surrounded completely by the second region25. The diaphragm23is thus embodied as a circumferential diaphragm.

The diaphragm23can be part of a source unit26. The source unit26additionally comprises the radiation source3and the collector15. Since the diaphragm23is arranged in the intermediate focal plane16, the radiation-transmissive first region24can be made very small. It has, in particular, a maximum diameter dmaxin the range of 1 mm to 50 mm, in particular in the range of 3 mm to 30 mm, in particular in the range of 5 mm to 15 mm. The smaller the transmissive region24, the better the source unit26can be separated, in particular vacuum-separated, from the illumination optical unit4. A vacuum separation between the source unit26and the illumination optical unit4is important, in particular, if an atmosphere that is harmful to the illumination optical unit4is present in the source unit26.

In order to transfer the EUV radiation14emerging from the radiation source3from the source unit26to the illumination optical unit4in a manner as free from losses as possible, it is advantageous if the radiation-transmissive first region24has a form which is adapted to the intensity distribution of the EUV radiation14at the position of the diaphragm23, in particular in the region of the intermediate focal plane16.

The diaphragm23can also be part of the illumination optical unit4. It is advantageous if the radiation-transmissive first region24of the diaphragm23has a form which is adapted to the form of the pupil facets of the pupil facet mirror18.

Particularly in the case of a square embodiment of the pupil facets, a square embodiment of the diaphragm23, in particular of the first region24of the diaphragm23, is advantageous. The diaphragm23has, in particular, the same symmetry properties as the pupil facets.

When the projection exposure apparatus1is used, the reticle7and the wafer12, which bears a coating light-sensitive to the illumination light14, are provided. Afterward, at least one section of the recticle7is projected onto the wafer12with the aid of the projection exposure apparatus1. During the projection of the recticle7onto the wafer12, the recticle holder8and/or the wafer holder13can be displaced in a direction parallel to the object plane6and/or parallel to the image plane11, respectively. The displacement of the recticle7and of the wafer12can preferably be effected synchronously with one another. The final step involves developing the light-sensitive layer exposed via the illumination light14on the wafer12. A micro- or nanostructured component, in particular a semiconductor chip, is produced in this way.

It has surprisingly been established that the stability of the uniformity U of the illumination of the object field5for a predefined size of the radiation-transmissive first region24of the diaphragm23is improved via a square embodiment of the region in comparison with a circular embodiment. This was attributable to the fact that disturbances in the uniformity which can be caused by an incorrect positioning of the radiation source3are smaller in the case of a square embodiment of the radiation-transmissive first region24of the diaphragm23than in the case where a round diaphragm is used. The diaphragm23according to the invention thus contributes to an improved imaging quality of the projection exposure apparatus1.

It was furthermore established that the advantages brought about by the diaphragm23according to the invention are also dependent, inter alia, on the concrete configuration of the radiation source3. In one particular advantageous embodiment, the design of the radiation-transmissive first region24of the diaphragm23is adapted to the concrete form of the radiation source3. This adaptation can be as an alternative or in addition to the adaptation to the form of the pupil facets on the pupil facet mirror18. An adaptation of the diaphragm23to the form of the radiation source3is advantageous, in particular, in the case of an arrangement of the diaphragm23in the intermediate focal plane16, since an intermediate image of the radiation source3is present here. A roundish embodiment of the radiation source3will lead to a roundish intermediate image. Correspondingly, an oval embodiment of the radiation source3leads to an oval intermediate image and a rather rectangular, in particular square, embodiment of the radiation source3leads to a corresponding intermediate image.FIG. 5shows a rectangular radiation source.FIG. 6shows a square radiation source3. According to the invention, provision can be made, in the case of the radiation source3being replaced, for the diaphragm23also to be replaced by a new diaphragm23adapted to the new radiation source3.

An improved uniformity of the illumination of the object field5leads to a greater stability of the illumination. This is advantageous, in particular, if, during the exposure of the wafer12, the radiation source3is not absolutely stationary, for example wobbles somewhat. This is relevant, in particular, when a point source in the intermediate focal plane16is not imaged exactly onto a point on the pupil facets of the pupil facet mirror18by the field facets of the field facet mirror17, rather the position on the pupil facets correlates with the position on the field facet and thus with the position in the recticle7.