WO2016184743A1

WO2016184743A1 - Pupil facet mirror

Info

Publication number: WO2016184743A1
Application number: PCT/EP2016/060535
Authority: WO
Inventors: Thomas Fischer
Original assignee: Carl Zeiss Smt Gmbh
Priority date: 2015-05-20
Filing date: 2016-05-11
Publication date: 2016-11-24
Also published as: TW201702637A; TWI761304B; DE102015209175A1; CN107567598A; KR20180008494A; KR102611719B1; CN107567598B

Abstract

It is an object of the present invention to improve a pupil facet mirror for an illumination optical unit of a projection exposure apparatus. The essence of the invention is to form the pupil facet mirror (20) with pupil facets (29) of different sizes. The pupil facets (29) may in particular also have different forms. At least a subset of the pupil facets may have an irregular form. They have in particular side edges of different lengths. They may in particular have the form of an irregular polygon.

Description

Pupil facet mirror

The content of German Patent Application DE 10 2015 209 175.9 is incorporated by reference herein.

The invention relates to a pupil facet mirror for an illumination optical unit of a projection exposure apparatus. The invention also relates to a method for determining the design of a pupil facet mirror. Furthermore, the invention relates to an illumination optical unit for a projection exposure appa- ratus with a corresponding pupil facet mirror, to an illumination system and to an optical system with such an illumination optical unit and to a projection exposure apparatus with a corresponding illumination optical unit. Finally, the invention relates to a method for producing a microstructured or nanostructured component and to a component produced according to the method.

An illumination optical unit with a facet mirror is known for example from US 201 1/0001947 Al, US 2013/0335720 Al and US 6,859,328 B2. It is an object of the present invention to improve a pupil facet mirror for an illumination optical unit of a projection exposure apparatus. This object is achieved by a pupil facet mirror according to Claim 1. The essence of the invention is to form the pupil facet mirror with pupil facets of different sizes. The pupil facets may in particular also have different forms. At least a subset of the pupil facets may have an irregular form. They have in particular side edges of different lengths. They may in particular have the form of an irregular polygon. In the case of forming a pupil facet as an n-gon, that is to say with an n- cornered reflection surface, the form of the reflection surface has an m-fold rotational symmetry, where: m < n. The form of the reflection surface may in particular merely have a trivial, one-fold rotational symmetry. In the case of hexagonal pupil facets, the form may in particular have a one-fold, a two-fold or a three-fold rotational symmetry.

According to one aspect of the invention, the subset of the pupil facets with such an irregular form comprises in particular at least 10%, in particular at least 20%, in particular at least 30%, in particular at least 40%, in particular at least 50%, in particular at least 60%, in particular at least 70%, in particular at least 80%, in particular at least 90% of the pupil facets. It is also possible to form all of the pupil facets with an irregular form. It is also possible to prescribe an upper limit for the number of pupil facets with an ir- regular form. The upper limit may be for example 90%, 80%, 70%, 60%, 50%, 40%, 30%, 20% or 10%.

According to the invention, it has been recognized that an overexposure of the facets of the pupil facet mirror can be reduced, in particular minimized, in particular avoided, by an adaptation of the size and/or form of the pupil facets to the spot form, that is to say the form of an image of the radiation source that is produced by the facets of a first facet mirror on the facets of the pupil facet mirror. This allows the transmission of the illumination system to be maximized. It is also possible in this way to improve the stability of the illumination system, in particular with regard to a possible drift during operation.

The term pupil facet mirror serves primarily to differentiate from the first facet mirror of the illumination optical unit, arranged ahead in the path of the beam of the illumination radiation, which is also referred to as the field facet mirror. The first facet mirror is preferably arranged in a field plane of the illumination optical unit conjugate with respect to the object field. It may however also be arranged at a distance from such a field plane. It is preferably arranged in the vicinity of a corresponding field plane.

The pupil facet mirror, which is referred to generally as the second facet mirror, is preferably arranged in a pupil plane of the illumination optical unit. It may also be arranged at a distance from such a pupil plane. It is however preferably arranged pupil-near. For a more precise, quantitative definition of the term pupil-near, reference should be made to DE 10 2012 216 502 Al .

The mirror elements of the pupil facet mirror, which are also referred to as pupil facets, are in particular arranged rigidly. There is no need for an actuating mechanism for displacing the mirror elements. The structure of the pupil facet mirror is simplified considerably as a result.

According to one aspect of the invention, the sizes of at least two of the mirror elements of the pupil facet mirror differ by a factor of at least 1.05, in particular at least 1.1 , in particular at least 1.15, in particular at least 1.2. The size of the mirror elements should be understood here as meaning in particular the area content of their reflection surface. The size of the mirror elements preferably differs at most by a factor of at most 2, in particular at most 1.5, in particular at most 1.3.

The mirror elements of the pupil facet mirror have in particular polygonal individual reflection surfaces. Adjacent mirror elements preferably have reflection surfaces that have side edges running parallel to one another. In particular, the side edges running adjacent to one another of two adjacent mirror elements run parallel. This allows the degree of filling of the pupil facet mirror to be increased.

According to a further aspect of the invention, the pupil facet mirror has a high degree of filling. The degree of filling is also referred to as the degree of integration. It is in particular at least 0.7°, in particular at least 0.8°, in particular at least 0.9°. A high degree of filling has the effect of reducing, in particular avoiding, transmission losses.

According to a further aspect of the invention, the individual reflection surfaces of the mirror elements have in each case a form that respectively evolves from, or has evolved from, or is obtainable from a basic form se- lected from a group of at most five, in particular at most four, in particular at most three, in particular at most two, different basic forms with at most twelve, in particular at most ten, in particular at most eight, in particular at most six side edges by parallel displacement of at least one of the side edges. It is in particular possible that all of the individual reflection surfaces have a form that respectively evolves from the same form by parallel displacement of at least one of the side edges.

Convex basic forms, polygons in particular or polygons in general, which have circular arc section-shaped edges, serve as the basic form. In particu- lar, equilateral polygons, in particular regular polygons, may serve as basic forms. The polygons may be in particular triangles, rectangles, pentagons, hexagons or octagons. The basic forms are in particular selected in such a way that a parqueting of a plane with them is possible. This may take the form of a general parqueting of any kind or in particular a demiregular, semiregular or regular parqueting of the plane.

The forms of the individual reflection surfaces evolve in particular into a displacement of one or more of their edges in the direction of the respective centre perpendicular of these edges or, in the case of arcuate edges, in the direction of a centre perpendicular through a line joining their two corner points. In other words, the internal angles of the basic forms are preserved during the displacement. As a result, on the one hand the design of the pu- pil facet mirror is facilitated. Also, the production and/or handling of the pupil facets can be facilitated as a result. The occurrence of undesired interspaces between the individual mirrors can also be avoided as a result.

A displacement of one edge of one of the mirror elements may at the same time lead to a displacement of an adjacent edge of an adjacent mirror element. This will be explained in still more detail below.

According to a further aspect of the invention, at least some of the individual reflection surfaces are formed hexagonally. It may in particular be en- visaged to form all of the individual reflection surfaces hexagonally. A hexagonal formation of the individual reflection surfaces makes parqueting possible substantially without any gaps.

The internal angles of the individual reflection surfaces may in particular be in each case 120°. Alternatives are similarly possible. For example, the individual reflection surfaces may also be formed in the form of parallelograms. A combination of different forms, for example parallelograms and pentagons, is also possible. According to a further aspect of the invention, the mirror elements are arranged on grid points of a rectangular grid, in particular a hexagonal grid. The mirror elements are in particular arranged in such a way that the geometrical centroids of their reflection surfaces before the adaptation of the same lie on grid points of a rectangular hexagonal grid.

According to an alternative embodiment, the mirror elements are in particular arranged in such a way that the geometrical centroids of their reflection surfaces before the adaptation of the same lie on grid points of a grid systematically distorted along one direction, in particular a hexagonal grid systematically distorted along one direction.

In other words, the pupil facets are arranged on the pupil facet mirror in particular on the basis of the densest packing of circles, that is to say a hex- agonal grid. For the adaptation of the pupil facets to the spot form, it may be envisaged in particular to vary the form and size of adjacent pupil facets on the basis of this arrangement by paired displacement of side edges that are parallel to one another. This will be described in still more detail below. According to one aspect of the invention, it is provided in particular that the sizes of the individual reflection surfaces have a schematic scaling in dependence on the position of the mirror elements on the pupil facet mirror and/or a paired individual variation of adjacent individual reflection surfaces.

A systematic scaling of the sizes of the individual reflection surfaces allows a tilting of the pupil facet mirror in relation to a plane running parallel to the object plane to be taken into account. The systematic scaling relates in particular to the basic forms of the individual reflection surfaces of the mirror elements.

The size L of one of the individual reflection surfaces along the distorting direction may be characterized in particular by the following estimate: 0.9(d/dref)² < L : L_ref < l .l(d/d_ref)², in particular 0.95(d/d_ref)² < L : L_ref < 1.05(d/dref)², in particular 0.97(d/d_ref)² < L : L_ref < 1.03(d/d_ref)², in particular 0.99(d/dref)² < L : L_ref < 1.01(d/d_ref)², in particular 0.995(d/d_ref)² < L : L_ref < 1.005(d/dref)². Here, d denotes the distance, in particular the optical path, of the respective facet from the reticle. L_ref and d_ref denote any desired reference facet, for example the smallest facet.

The specific form of the individual reflection surfaces on their own may be influenced by the paired individual variation of adjacent individual reflec- tion surfaces. The displacement of two parallel edges of adjacent mirror elements that is provided according to the invention has the effect in particular that the size of the individual reflection surfaces of the one mirror element is increased at the expense of the size of the individual reflection surface of the other mirror element, respectively. As a result, an improve- ment in the transmission, in particular a maximization of the transmission, is possible.

A further object of the invention is to improve a method for determining the design of a pupil facet mirror. This object is achieved by a method comprising the following steps: prescribing basic forms selected from a group of at most five different basic forms with at most 12 side edges for forms of the individual re- flection surfaces of the mirror elements of a pupil facet mirror, in particular according to Claim 1 ,

adapting the size and/or the form of the individual reflection surfaces to improve the transmission and/or system stability,

- a systematic scaling and/or a paired individual variation of adjacent individual reflection surfaces being provided for the adaptation of the size and/or form of the individual reflection surfaces.

The adaptation of the size and/or the form of the individual reflection sur- faces allows the transmission and/or system stability to be improved.

A paired variation of adjacent individual reflection surfaces should be understood as meaning in particular - as described above - that the size of an individual reflection surface is increased at the expense of the size of an adjacent individual reflection surface by displacing one of its side edges.

According to a further aspect of the invention, it is envisaged to displace parallel edges of adjacent mirror elements in pairs for the adaptation of the size of the individual reflection surfaces. The other mirror elements may be respectively left unchanged by this.

According to a further aspect of the invention, it is envisaged to take into account in the adaptation of the size of the individual reflection surfaces the intensity distribution of illumination radiation in the region of the indi- vidual reflection surfaces and/or the arrangement of the mirror device in an illumination optical unit.

According to the invention, it has been recognized that the imaging properties of the collector, and possibly spectral-filtering surface structures on the collector, can cause an ellipticity of the illumination spot on the pupil facet mirror on account of an anisotropy of the radiation source, in particular an anisotropy of the plasma. This may involve in particular varying the orientation of the illumination spots over the far field. The position, size and form of the illumination spots may be determined by simulation or experimentally. It may be determined, in particular calculated, from the data of the radiation source and/or the illumination optical unit.

It has also been recognized that a distortion of the grid for the arrangement of the pupil facets may be caused by a tilted arrangement of the pupil facet mirror in relation to a plane parallel to the object plane. This may be taken into account in the design of the pupil facet mirror.

Further objects of the invention are to improve an illumination optical unit for a projection exposure apparatus, an illumination system for a projection exposure apparatus and an optical system for a projection exposure apparatus and also a corresponding projection exposure apparatus.

These objects are respectively achieved by a pupil facet mirror according to the description above.

The advantages are evident from those of the pupil facet mirror.

According to a further aspect of the invention, an EUV radiation source, that is to say a radiation source that emits illumination radiation in the EUV range, in particular in the wavelength range from 5 nm to 30 mm, serves as the radiation source. Further objects of the invention are to improve a method for producing a microstructured or nanostructured component and to improve such a component. These objects are achieved by providing a projection exposure apparatus according to the invention. The advantages are evident from those described above.

The component can be produced with extremely high structural resolution. In this way it is possible, for example, to produce a semiconductor chip having an extremely high integration or storage density.

Further advantages, details and particulars of the invention will become apparent from the description of exemplary embodiments with reference to the drawings. In the figures: Figure 1 schematically shows a meridional section through a projection exposure apparatus for EUV projection lithography;

Figures 2 and 3 show variants of the arrangement of field facet mirrors that may be configured with monolithic field facets but may also have field facets that are respectively constructed from a plurality of individual mirrors;

Figure 4 schematically shows a plan view of a partial region of a pupil facet mirror which, together with the field facet mirror, is part of an illumination optical unit of the projection exposure apparatus:

Figure 5 shows an exemplary representation of a variant of a pupil facet that can be used in the case of the pupil facet mirror shown in Figure 4, an edge contour of a partial beam of the illumination light which impinges on the pupil facet by way of precisely one of the field facets and a prescribed illumination channel being represented on the pupil facet, a field- dependent centroid profile of illumination light subbeams that emanate from different points on the associated field facet during the imaging of the light source being represented in addition to the edge contour of the illumination light partial beam;

Figure 6 shows a schematic, simplified representation of a detail of the beam path in the illumination optical unit to illustrate a systematic distortion from the viewpoint of an object field point of equidistant directions on the pupil facet mirror that is in- duced by a tilting of the pupil facet mirror in relation to a plane parallel to the object field plane, shows a schematic representation to explain the paired individual variation of the size of adjacent individual reflection surfaces that is provided according to the invention and

Figure 8 schematically shows a plan view of a partial region of a pupil facet mirror according to an alternative embodiment.

Figure 1 schematically shows a microlithographic projection exposure apparatus 1 in a meridional section. The projection exposure apparatus 1 includes a light or radiation source 2. An illumination system 3 of the projection exposure apparatus 1 has an illumination optical unit 4 for exposing an illumination field coinciding with an object field 5 in an object plane 6. The illumination field may also be larger than the object field 5. In this case, an object in the form of a reticle 7 that is arranged in the object field 5 and is held by an object or reticle holder 8 is exposed. The reticle 7 is also referred to as lithography mask. The object holder 8 is displaceable along an object displacement direction by means of an object displacement drive 9. A projection optical unit 10, which is shown highly schematically, serves for imaging the object field 5 into an image field 1 1 in an image plane 12. A structure on the reticle 7 is imaged onto a light-sensitive layer of a wafer 13 arranged in the region of the image field 1 1 in the image plane 12. The wafer 13 is held by a wafer holder 14. The wafer holder 14 is displaceable parallel to the object displacement direction in a manner synchronized with the object holder 8 by means of a wafer displacement drive 15. The radiation source 2 is an EUV radiation source having an emitted used radiation in the range of between 5 nm and 30 nm. This can be a plasma source, for example a GDPP (gas discharge-produced plasma) source or an LPP (laser-produced plasma) source. A radiation source based on a synchrotron or on a free electron laser (FEL) can also be used for the radiation source 2. Information about such a radiation source can be found by the person skilled in the art for example from US 6,859,515 B2. EUV radiation 16 that emanates from the radiation source 2, in particular the used illumination light illuminating the object field 5, is focused by a collector 17. A corresponding collector is known from EP 1 225 481 A. Downstream of the collector 17, the EUV radiation 16 propagates through an intermediate focus 18a in an intermediate focal plane 18 before being incident on a field facet mirror 19. The field facet mirror 19 is a first facet mirror of the illumination optical unit 4. The field facet mirror 19 has a plurality of reflective field facets 25, which are only shown very schematically in Figure 1. The field facet mirror 19 is arranged in a field plane of the illumination optical unit 4, which is optically conjugate with respect to the object plane 6. The EUV radiation 16 is also referred to hereinafter as illumination radiation, illumination light or as imaging light.

Downstream of the field facet mirror 19, the EUV radiation 16 is reflected by a pupil facet mirror 20. The pupil facet mirror 20 is a second facet mir- ror of the illumination optical unit 4. The pupil facet mirror 20 is arranged in a pupil plane of the illumination optical unit 4, which is optically conjugate with respect to the intermediate focal plane 18 and with respect to a pupil plane of the illumination optical unit 4 and the projection optical unit 10 or coincides with this pupil plane. The pupil facet mirror 20 has a plu- rality of reflective pupil facets 29, which are only shown very schematically in Figure 1. The pupil facets 29 have in each case a reflection surface 33, which is also referred to as an individual reflection surface. For simplicity, the reflection surface 33 itself is also referred to as pupil facet 29. With the aid of the pupil facets 29 of the pupil facet mirror 20 and a downstream imaging optical assembly in the form of a transfer optical unit 21 having mirrors 22, 23 and 24 designated in the order of the beam path, the field facets of the field facet mirror 19 are imaged into the object field 5 superimposed on one another. The last mirror 24 of the transfer optical unit 21 is a grazing incidence mirror. Depending on the integration of the illumina- tion optical unit 4, it is also possible to dispense entirely or partially with the transfer optical unit 21.

Illumination light 16, which is for example guided in the object plane 6 towards greater absolute x values than the x dimension of the object field 5, may be guided towards a number of energy or dose sensors (of which one dose sensor 24a is schematically shown in Figure 1) with the aid of a corresponding optical unit that is not shown. The dose sensor 24a is signal- connected to a central control device 24b in a way that is not shown. The dose sensor 24a generates an input signal for controlling the light source 2 and/or the object displacement drive 9 and/or the wafer displacement drive 15. By this means, a dose adaptation of an exposure of the wafer 13 in the image field 1 1 can be achieved by adaptation on the one hand of a power of the light source 2 and/or on the other hand a scanning speed.

The control device 24b is signal-connected inter alia to tilting actuators for the field facets 25 of the field facet mirror 19.

In order to simplify the description of positional relationships, Figure 1 plots a Cartesian xyz-coordinate system as a global coordinate system for the description of the positional relationships of components of the projection exposure apparatus 1 between the object plane 6 and the image plane 12. The x-axis runs perpendicularly to the plane of the drawing into the latter in Figure 1. The y-axis runs towards the right and parallel to the dis- placement direction of the object holder 8 and of the wafer holder 14 in

Figure 1. The z-axis runs downwards in Figure 1 , that is to say perpendicularly to the object plane 6 and to the image plane 12.

The x-dimension over the object field 5 or the image field 1 1 is also re- ferred to as the field height. The object displacement direction runs parallel to the y-axis.

In the other figures, local Cartesian xyz-coordinate systems are plotted. The x-axes of the local coordinate systems run parallel to the x-axis of the global coordinate system shown in Figure 1. The xy planes of the local coordinate systems represent arrangement planes of the component respectively shown in the figure. The y- and z-axes of the local coordinate systems are correspondingly tilted by a specific angle about the respective x- axis.

Figures 2 and 3 show examples of various facet arrangements for the field facet mirror 19. Each of the field facets 25 represented there may be constructed as a group of individual mirrors comprising a plurality of individu- al mirrors, as known for example from WO 2009/100 856 Al . Each of the individual mirror groups then has the function of a facet of a field facet mirror such as is disclosed for example in US 6,438,199 Bl or US

6,658,084 B2. In terms of actuation, the field facets 25 may be configured such that they can tilt between a plurality of tilting positions.

The field facet mirror 19 shown in Figure 2 has a multiplicity of arcuate ly configured field facets 25. These are arranged in groups in field facet blocks 26 on a field facet carrier 27. Altogether, the field facet mirror 19 shown in Figure 2 has twenty six field facet blocks 26, for which three, five or ten of the field facets 25 are combined in groups.

Between the field facet blocks 26 there are interspaces 28.

The field facet mirror 19 shown in Figure 3 has rectangular field facets 25, which in turn are arranged in groups as field facet blocks 26, between which there are interspaces 28. Figure 4 schematically shows a plan view of a detail of the pupil facet mirror 20. Pupil facets 29 of the pupil facet mirror 20 are arranged in the region of an illumination pupil of the illumination optical unit 4. The number of pupil facets 29 is in reality greater than the number of field facets 25 and may be a multiple of the number of the field facets 25. The pupil facets 29 are arranged on a pupil facet carrier 30 of the pupil facet mirror 20. A distribution within the illumination pupil of pupil facets 29 impinged by the illumination light 16 by way of the field facets 25 provides an illumination pupil, that is to say an actual illumination angle distribution in the object field 5.

The pupil facets 29 all have a hexagonal form. In particular, they exclusively have internal angles of 120°. Each of the field facets 25 serves for transferring part of the illumination light 16, that is to say an illumination light partial beam 16i, from the light source 2 to one of the pupil facets 29.

It is assumed hereafter in a description of illumination light partial beams 16i that the associated field facet 25 is respectively illuminated to the maximum, that is to say over its entire reflection surface. In this case, an edge contour of the illumination light partial beam 16i coincides with an edge contour of the illumination channel, for which reason the illumination channels are also denoted hereinafter by 16i. The respective illumination channel 16i represents a possible light path of an illumination light partial beam 16i, by way of the further components of the illumination optical unit 4 that illuminates the associated field facet 25 to the maximum. The transfer optical unit 21 has respectively for each of the illumination channels 16i one of the pupil facets 29 for transferring the illumination light partial beam 16i from the field facet 25 towards the object field 5. In each case one illumination light partial beam 16i, of which two illumination light partial beams 16i (i = 1 , ... , N; N: number of field facets) are shown schematically in Figure 1 , is guided between the light source 2 and the object field 5 by way of precisely one of the field facets 25 and by way of precisely one of the pupil facets 29 by way of in each case one illumina- tion channel.

Figure 5 shows one of the pupil facets 29 that can be used in the case of the pupil facet mirror 20. The pupil facet 29 shown in Figure 5 has a hexagonal edge contour with side edges 32. The facet 29 shown in Figure 5 has the form of a regular hexagon. For the pupil facet mirror 20 that is shown as a detail in Figure 4, this serves as a basic form for all of the pupil facets 29. Such an edge contour makes it possible to cover the pupil facet carrier 30 densely, or at least as densely as possible, with the pupil facets 29. The pupil facet mirror 20 has in particular a degree of filling of at least 0.6, in par- ticular at least 0.7, in particular at least 0.8, in particular at least 0.9. Such an edge contour makes it possible to cover the pupil facet carrier 30 densely, or at least as densely as possible, with the pupil facets 29.

The pupil facet 29 shown in Figure 5 is impinged with the illumination light partial beam 16i by an arcuate field facet 25 of the field facet mirror 19 shown in Figure 2.

In the case of the arrangement shown in Figure 5, an entire cross section of the illumination light partial beam 16i lies on the pupil facet 29 such that the illumination light partial beam 16i is not cut into peripherally by the edge of the pupil facet 29. An edge contour of the cross section of the illumination light partial beam 16i on the pupil facet 29 has an approximately arc-, bean- or kidney-shaped form and may be understood as a convolution of the arcuate field facets 25 shown in Figure 2 with a round source area of the light source 2. This convolution is caused by the fact that an image of the light source 2 occurs for different portions on the associated field facet 25, that is to say field-dependently, at different image locations, and moreover generally at an image location that lies at a distance from the pupil facet 29 along the illumination channel 16i, in the beam path therefore either ahead of or after the pupil facet 29.

The arc-shaped edge contour of the illumination light partial beam 16i on the pupil facet 29 represents a light spot of the illumination light partial beam 16i. The light spot of the illumination light partial beam 16i on the pupil facet 29 is also referred to as an illumination spot, and the form that is delimited by its edge contour is also referred to as the spot form.

Depicted by dashed lines in the edge contour of the illumination light par- tial beam 16i on the pupil facet 29 are a number of subbeams Ιόί¹, 16i², ... 16i^x. The illumination light partial beam 16i is made up of a multiplicity of such subbeams 16j^j. If the optical parameters of the illumination are known, the illumination light partial beam 16i on the respective pupil facet 29 can be calculated, for example with the aid of an optical design program, and is also referred to in this connection as a "point-spread function".

The illumination light 16 of these subbeams Ιόί¹ to 16i^x emanates from different points 25¹ of the associated field facet 25. In Figure 2, emanating points 25 ^l, 25² and 25^x are depicted by way of example on one of the field facets 25.

A kernel of an edge contour of the respective illumination light partial beam 16i on each pupil facet 29 is represented by a field-dependent centroid profile 31i of all subbeams 16i^j emanating from the associated field facet 25. This centroid profile 31 i is individual to each illumination channel 16i and depends inter alia on the geometrical profile of the illumination channel 16i between the light source 2 and the respective pupil facet 29 by way of the associated field facet 25.

Figure 5 shows here an idealized field-dependent centroid profile 31i.

Further aspects of the pupil facet mirror 20 are described below.

As shown by way of example in Figure 5, the illumination spot is at different distances from the side edges 32 of the pupil facets 29.

According to the invention, it has been recognized that it is advantageous for achieving the highest possible resolution if the illumination pupil has a lowest possible degree of filling. Here it is advantageous to make the pupil facets 29 as small as possible. On the other hand, the pupil facets 29 must not become too small, since there could otherwise be an overexposure, and consequently an undesired transmission loss. In order to reduce, in particu- lar minimize, transmission losses, the pupil facets 29 are arranged as densely packed as possible.

As described below, it is envisaged according to the invention to adapt the size and/or form of the pupil facets 29 to the spot form of the respective illumination light partial beam 16i. As a result, an overexposure of the pupil facets 29 can be reduced, in particular minimized, in particular avoided, and consequently the transmission of the illumination system 3 can be increased, in particular maximized. The maximum overexposure of the pupil facets 29 is in particular at most 20%, in particular at most 10%, in particular at most 5%. It depends inter alia on the details of the radiation source 2.

Furthermore, the degree of filling can be reduced, in particular minimized, and the resolution thereby increased, by the adaptation of the size and/or form of the pupil facets 29 to the spot form of the respective illumination light partial beam 16i.

For determining the size and/or form of the pupil facets 29, it is envisaged to arrange the pupil facets 29 densely on the pupil facet mirror 20 on the basis of the densest packing of circles, that is to say on the basis of an arrangement on a hexagonal grid. On the basis of such an arrangement of the pupil facets 29, which is uniform, in particular regular, the form and/or size of the individual pupil facets 29, in particular their reflection surfaces 33, is adapted. In the adaptation of the form and/or size of the reflection surfaces 33 of the pupil facets 29, account is taken in particular of the spot form of the respective partial beam 16i with illumination radiation 16, that is to say the intensity distribution of the illumination radiation 16 in the region of the reflection surfaces 33 and/or the arrangement of the pupil facet mirror 20 in the beam path of the illumination radiation 16, in particular its ar- rangement in the illumination optical unit 4, in particular with regard to the alignment in relation to the object plane 6.

For adapting the size and/or form of the reflection surfaces 33, a systematic scaling is provided in particular, for taking into account the distortion of the grid as a consequence of an angle of convolution of the pupil facet mirror 20 with respect to an optical axis, in particular a tilting of the pupil facet mirror 20 in relation to the object plane 6. As an alternative or in addition to this, the individual spot form may be taken into account in the adap- tation of the size and/or form of the reflection surfaces 33. This will be described in still more detail below.

Usually, an image of the radiation source 2 in the region of the intermediate focus is directionally dependent in form and size. A three-dimensional plasma leads in particular also to a three-dimensional plasma image in the intermediate focus.

The directional dependence of the plasma image may be attributable to an anisotropy of the plasma, to imaging properties of the collector 17 and to spectral-filtering surface structures on the collector 17. As a result, the directional dependence of the plasma image may lead to an ellipticity of the illumination spots corresponding to individual field points. The orientation of the spots may vary here over the far field. The spots may have any orientation. The lengths of their semiaxes may differ from one another, in par- ticular in the range from 10% to 40%.

The field facets 25 form the image of the radiation source 2 in the intermediate focus 18a on different pupil facets 29. This is shown by way of example in Figure 6.

In the projection of the image of the radiation source 2 in the intermediate focus 18a onto the different pupil facets 29, there are channel-individual imaging scales. Furthermore, on account of different image widths in relation to different pupil facets 29, which cannot be avoided in particular in the case of switchable field facets 25, there may be imperfections in the point projection of the image of the radiation source 2 by the field facets 25.

Overall, the size, form and orientation of the illumination spots on the pupil facet mirror 20 depends essentially on the field facet 25 that is assigned to the respective illumination channel. The illumination of the individual pupil facets 29 is the result of superimposing the point projection of the field facet 25 respectively assigned thereto and of the actual spot form, in particular from the viewpoint of a field point. This is graphically shown in Figure 5. The form of the illumination spots is the result of the envelope of the images of the illumination light partial beam 16} . As shown exaggerated in Figure 6 for purposes of illustration, a tilting of the pupil facet mirror 20 in relation to a plane running parallel to the object plane 6 leads to a distortion, in particular a systematic distortion, of the regular grid on which the pupil facets 29 are arranged. The tilting of the pupil facet mirror 20 has in particular the effect that, from the viewpoint of the reticle 7, equidistant directions correspond to non-equidistant positions of the individual pupil facets 29. This can be taken into account by a systematic scaling of the size and/or form of the reflection surfaces 33 of the pupil facets 29. Different pupil facets 29 of the pupil facet mirror 20 may in particular have different forms and/or sizes. This statement relates at least to a subset of the pupil facets 29. It goes without saying that it is also possible to form a subset or a number of subsets of pupil facets 29 with identical sizes and forms. In addition or as an alternative to a systematic scaling of the form and/or size of the reflection surfaces 33 to take into account a tilting of the pupil facet mirror 20, the sizes of individual pupil facets 29, in particular indi- vidual pairs of adjacent pupil facets 29, may be individually varied in pairs, by the boundaries, that is to say the side edges 32 running parallel to one another of adjacent pupil facets 29, being displaced in pairs. This is schematically indicated in Figures 4, 7 and 8 by double-headed arrows 34. The form and/or size of the adjacent pupil facets 29 can be adapted to the actual spot form and size by such a paired individual variation. In this way it is possible in particular for a loss of radiation due to overexposure of the pupil facets 29 to be reduced, in particular minimized, preferably avoided completely. As a result, the transmission of the illumination system 3 is increased. Furthermore, the system stability, in particular with regard to drift, can be improved as a result.

Furthermore, the illumination spots on the adapted pupil facets 29 can be displaced, in order to improve further the transmission of the illumination system 3 and/or the system stability. A displacement of the illumination spots on the pupil facets 29 can be achieved by a suitable tilting of the field facets 25. This can be achieved by corresponding working of the reflection surface of the field facets 25 and/or an adjustment of the same and/or by an actuating mechanism.

For determining the exact position of all the side edges 32, an optimization algorithm may be provided. In this way it is possible in particular to take into account the actual intensity distribution of the illumination radiation 16 in the region of the pupil facet mirror 20, in particular in the region of the reflection surfaces 33 and/or the arrangement of the pupil facet mirror 20 in the illumination optical unit 4, in particular its tilting in relation to the object plane 6. The concept of the paired individual variation of the form and size of the pupil facets 29 is explained below on the basis of the schematic representation in Figure 7. In Figure 7, two adjacent pupil facets 29 with in each case an illumination spot 16i are shown by way of example. The illumination spot 16i shown represents here a region with a certain minimum intensity of the illumination radiation 16. Corresponding intensity profiles 35 of the illumination radiation 16 in the region of the pupil facets 29 are shown by way of example in the lower part of Figure 7. As shown by way of example in Figure 7, there may be an overexposure of the pupil facets 29. Here, part of the illumination radiation 16, which is not incident on the correct one of the pupil facets 29, is not used for illuminating the reticle 7 in the object field 5. This therefore constitutes a radiation loss 36, which is identified in the lower part of Figure 7 by hatching. This radiation loss 36, in particular total radiation loss with regard to the two adjacent pupil facets 29, can be reduced, in particular minimized, by a parallel displacement of adjacent side edges 32 of the reflection surface 33.

In Figure 7, the positions of the side edges 32^* before the displacement are shown for purposes of illustration. The form of the pupil facets 29 with the side edges 32^* corresponds precisely to the basic form of the respective pupil facets 29. In the case of the exemplary embodiment shown in Figures 4, 5 and 7, a regular hexagon serves in each case as the basic form for the pupil facets 29. Other basic forms are similarly possible. The basic forms may in particular be selected from a group of various basic forms. They may in particular be selected from a group of at most five, in particular at most four, in particular at most three, in particular at most two different basic forms. The basic forms may in particular have at most twelve, in particular at most ten, in particular at most eight, in particular at most six, in particular at most five, in particular at most four, in particular at most three side edges 32. Polygons in particular or polygons in general, that is to say polygons with circular arc section-shaped edges 32, come into consideration in particular as basic forms. Equilateral, in particular regular, polygons may serve in particular as basic forms. The basic forms are in particular selected in such a way that a plane can be parqueted with them. This may take the form of a general parqueting or in particular a demiregular, sem- iregular or regular parqueting.

The actual intensity distribution of the illumination radiation 16 in the region of the pupil facet mirror 20 may be determined by simulation or ex- perimentally. It may in particular be determined, in particular calculated, from data of the radiation source 2 and/or of the illumination optical unit 4.

The angles, in particular the internal angles, between the side edges 32 of the individual pupil facets 29 are left constant in each case by the parallel displacements. In the case of hexagonal pupil facets 29 with internal angles of 120°, opposite side edges 32 of the pupil facets 29 in particular remain parallel. Their lengths are however changed by the parallel displacements. The side edges 32 of one and the same pupil facet 29 may in particular have lengths that are different by up to a factor of two. In particular in the case of pupil facets 29 with an irregular form, the side edges 32 of one and the same pupil facet 29 may also deviate even more from one another.

It may be envisaged to prescribe a maximum value by which adjacent pupil facets 29 are permitted to differ in their size. Adjacent pupil facets 29 may in particular have a size ratio of at most 1.2, in particular at most 1.1. This may be prescribed as a boundary condition for the determination of the design of the pupil facet mirror 20. In the foregoing description of an exemplary embodiment of the pupil facet mirror 20 on the basis of Figures 4 to 7, it has been assumed that the individual pupil facets 29 have hexagonal reflection surfaces 33. This is not absolutely necessary. The method according to the invention for adapting the form and/or size of the reflection surfaces 33 can also be used in the case of other facet packings. For example, a Cartesian packing or a packing with pupil facets 29 of different sizes and different forms may be provided as the starting arrangement or starting packing of the pupil facet mirror 20. A corresponding example is shown by way of example in Figure 8. In the case of this exemplary embodiment, the basic forms of the pupil facets 29 have been selected from two different basic forms. A first subset of the pupil facets 29 has parallelogram-shaped reflection surfaces 33. A second subset of the pupil facets 29 has pentagonal reflection surfaces 33. In each case two of the parallelogram-shaped reflection surfaces 33 and two of the pentagonal reflection surfaces 33 together have a parallelogram-shaped smallest convex envelope. In the case of this embodiment, it may be prescribed as a boundary condition for the adaptation of the form and/or size, that is to say for the displacement of the side edges 32, that this envelope is preserved for in each case two of the parallelogram-shaped reflection surfaces 33 and two of the pentagonal reflection surfaces 33. In this case, exclusively side edges 32 that do not lie in the circumferential region of this envelope are displaced.

Individual aspects of the forming of the pupil facet mirror 20 according to the invention and of the method for its design are described once again below. The individual pupil facets 29 are arranged rigidly, that is to say non- displaceably.

At least two of the pupil facets 29 are of sizes that differ by a factor of at least 1.1. An upper limit for the maximum difference in size of two pupil facets 29 may be prescribed. The upper limit is for example at most two, in particular at most 1.5.

The pupil facets 29 may respectively have a form that evolves from a basic form. The basic form may for its part be selected from a group of at most five, in particular at most four, in particular at most three, in particular at most two, in particular precisely one basic form. The pupil facet mirror 20 may in other words have at most five, in particular at most four, in particular at most three, in particular at most two different types of pupil facets. The pupil facets 29 may also all come from the same group. It is in particular possible to form all of the pupil facets 29 hexagonally.

The design of the pupil facet mirror 20 may in particular be based on a regular arrangement of the pupil facets 29. A systematic scaling and/or a paired individual variation of adjacent reflection surfaces 33 may be provided for adapting the size and/or form of the individual reflection surfaces 33.

The projection exposure apparatus 1 is provided for producing a micro- structured or nanostructured component. With the aid of the projection exposure apparatus 1 , at least part of the reticle 7 is imaged on a region of a light-sensitive layer on the wafer 13. This serves for the lithographic production of a microstructured or nanostructured component, in particular a semiconductor component, for example a microchip. Depending on the configuration of the projection exposure apparatus 1 as a scanner or as a stepper, the reticle 7 and the wafer 13 are moved continuously in scanning mode or step-by-step in stepper mode in a time-synchronized manner in the y direction. Finally, the light-sensitive layer exposed by the illumination radiation 16 on the wafer 13 is developed.

Claims

Patent claims:

1. Pupil facet mirror (20) for an illumination optical unit (4) of a projection exposure apparatus (1) comprising

1.1. a plurality of mirror elements (29) with polygonal individual reflection surfaces (33),

1.2. at least two of the individual reflection surfaces (33) having different forms and/or sizes, and

1.3. at least a subset of the mirror elements (29) having an irregular form.

2. Pupil facet mirror (20) according to Claim 1, characterized in that the individual reflection surfaces (33) of the mirror elements (29) have in each case a form that respectively has evolved from a basic form selected from a group of at most five different basic forms with at most 12 side edges (32) by parallel displacement of at least one of the side edges (32).

3. Pupil facet mirror (20) according to one of the preceding claims, characterized in that the forms and/or sizes of the individual reflection surfaces (33) have a systematic scaling in dependence on the position of the mirror elements (29) on the pupil facet mirror (20) and/or a paired individual variation of adjacent individual reflection surfaces (33).

4. Pupil facet mirror (20) according to one of the preceding claims, characterized in that at least some of the individual reflection surfaces (33) are formed hexagonally.

5. Pupil facet mirror (20) according to one of the preceding claims, characterized in that the mirror elements (29) are arranged on grid points of a regular grid.

6. Pupil facet mirror (20) according to one of Claims 1 to 4, characterized in that the mirror elements (29) are arranged on grid points of a regular grid distorted along one direction.

7. Method for determining the design of a pupil facet mirror (20) accord- ing to one of Claims 1 to 6, comprising the following steps:

7.1. prescribing basic forms selected from a group of at most five different basic forms with at most 12 side edges (32) for forms of individual reflection surfaces (33),

7.2. adapting the size of the individual reflection surfaces (33) to im- prove the transmission and/or system stability,

7.3. a systematic scaling and/or a paired individual variation of adjacent individual reflection surfaces (33) being provided for the adaptation of the size of the individual reflection surfaces (33).

8. Method according to Claim 7, characterized in that it is envisaged to displace parallel edges (32) of adjacent mirror elements (29) in pairs for the adaptation of the size of the individual reflection surfaces (33).

9. Method according to either of Claims 7 and 8, characterized in that the intensity distribution of illumination radiation (16) in the region of the individual reflection surfaces (33) and/or the arrangement of the pupil facet mirror (20) in an illumination optical unit (4) is taken into account in the adaptation of the size of the individual reflection surfaces (33).

10. Illumination optical unit (4) for a projection exposure apparatus (1) comprising

10.1. a first facet mirror ( 19) with a plurality of first facets (25) and 10.2. a second facet mirror in the form of a pupil facet mirror (20) according to one of Claims 1 to 6.

1 1. Illumination system (3) for a projection exposure apparatus (1), comprising

1 1.1. an illumination optical unit (4) according to Claim 10 and

1 1.2. a radiation source (2) for producing illumination radiation

(16).

12. Optical system for a projection exposure apparatus (1) comprising 12.1. an illumination optical unit (4) according to Claim 10 and

12.2. a projection optical unit (10) for transferring illumination radiation (16) from an object field (5) into an image field (1 1).

13. Microphotographic projection exposure apparatus (1) comprising

13.1. an illumination optical unit (4) according to Claim 10,

13.2. a projection optical unit (10) for transferring illumination ra diation (16) from an object field (5) into an image field (1 1) and

13.3. a radiation source (2) for producing illumination radiation

(16).

14. Method for producing a microstmctured or nanostmctured component comprising the following steps:

14.1. providing a projection exposure apparatus (1) according to Claim 13,

14.2. providing a substrate (13), to which a layer of a light- sensitive material has been at least partially applied,

14.3. providing a reticle (7), which has structures to be imaged,

14.4. projecting at least part of the reticle (7) onto a region of the light-sensitive layer of the substrate (13) with the aid of the projection exposure apparatus (1).

Component, produced by a method according to Claim 14.