WO2008131930A1

WO2008131930A1 - Mirror matrix for a microlithographic projection exposure apparatus

Info

Publication number: WO2008131930A1
Application number: PCT/EP2008/003371
Authority: WO
Inventors: Jürgen Fischer; Daniel Benz; Horst Stacklies; Florian Bach; Norbert MÜHLBERGER
Original assignee: Carl Zeiss Smt Ag
Priority date: 2007-04-25
Filing date: 2008-04-25
Publication date: 2008-11-06

Abstract

A mirror matrix (1), particularly for use in an illumina tion system of a microlithographic exposure apparatus, comprises at least two mirror elements (10), which are arranged next to one another on a support (2). Each mirror element has a reflective surface (11), an installation volume (50) associated with the mirror element, a bearing (20) defining a rotational axis (21) and an electromagnetic drive (30). The drive comprises a stator unit (31) and a rotor unit (30) that is connected to the mirror substrate (12). The electromagnetic drive is configured to tilt the mirror substrate (12) about the rotational axis (21) such that the stator unit (31) is always arranged inside the installation volume (50), and such that the rotor unit (32) does not enter the installation volume (150) of a neighbouring mirror element (110) when the mirror substrate (12) has reached its maximum tilting angle.

Description

MIRROR MATRIX FOR A MICROLITHOGRAPHIC PROJECTION EXPOSURE APPARATUS

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a mirror matrix comprising at least two mirror elements arranged next to one another on a support. More particularly, the invention relates to mirror matrices that are suitable to be used in an illumination system of a microlithographic projection exposure apparatus.

2. Description of Related Art

It has been proposed to used mirror matrices in illumination systems of microlithographic projection exposure apparatus, see, for example, WO 2005/026843 A2. A mirror matrix in an illumination system makes it possible to produce with great flexibility and minimal light losses different angular distribution of the projection light which impinges on the mask to be illuminated.

The mirror matrices are usually constructed as micro- electro-mechanical systems (MEMS) and comprise several thousand individual mirrors whose reflective surfaces may have a side length of 1 mm or even 10 μm. From a co-pending international application entitled "ILLUMINATION SYSTEM FOR ILLUMINATING A MASK IN A MICRO- LITHOGRAPHIC EXPOSURE APPARATUS", which has been filed on the same day as the present application by the same applicant and which claims priority of provisional US patent application 60/913,962 filed on April 25, 2007, an illumination system is known in which similar advantages are achieved, but with a mirror matrix having a substantially smaller number of mirror elements, for example less than 100 or even less than 50 mirror elements.

This is made possible by imaging a diffractive optical element, which has several zones with differing diffractive structures, on the mirror matrix, or alternatively by providing the reflective surfaces of the mirror ele- ments with diffractive structures. To each mirror element a far field intensity distribution is then associated which is produced by the respective zone of the diffractive optical element and may have almost any arbitrary shape, for example hexagonal or triangular. The intensity distribution in the pupil surface of the illumination system, which determines the angular distribution of light in the mask plane, is then not merely a combination of small dots, but of (larger) intensity distributions that have more complex contours and may be assembled without or with only small gaps to form poles, an annulus or a disc in the pupil surface. SUMMARY OF THE INVENTION

It is an object of the present invention to provide a mirror matrix which, with a reduced number of mirror elements, has a simple construction and simple drives.

This object is achieved by a mirror matrix comprising at least two mirror elements, which are arranged next to one another on a support. Each mirror element has a reflective surface applied on a mirror substrate which is separated from a support. Each mirror element further has an installation volume associated with the mirror element, wherein the installation volume is confined by the reflective surface of the mirror element, all surface normals on the support that intersect a boundary of the surface, and an area enclosed by the normals on the support. Each mirror element has a bearing defining a rotational axis and an electromagnetic drive, which comprises a stator unit and a rotor unit that is connected to the mirror substrate. The electromagnetic drive is configured to tilt the mirror substrate about the rotational axis such that the stator unit is always arranged inside the installation volume, and such that the rotor unit does not enter the installation space of a neighbouring mirror element when the mirror substrate has reached its maximum tilting angle.

The advantage of the mirror matrix according to the invention is that the electromagnetic drive is arranged in the installation volume such that, even with maximum tilting angles of the mirror substrate about a rotational axis, no component of the electromagnetic drive enters the installation volume of the neighbouring mirror ele- ment . This makes it possible to construct a mirror matrix in which the individual mirror substrates next to another can be arranged very closely together. Consequently, the ratio of the total reflective surface of the mirror matrix to the total surface of the matrix (here also referred to as filling ratio) is very large and may significantly exceed 70%. The electromagnetic drive of the mirror substrate of a mirror element furthermore ensures a precise and reliable forward or close-loop control of the tilting angles.

The installation volumes of the mirror elements may be identical when there is an equal alignment of the reflective surfaces with respect to the area enclosed by the normals on the support. This allows a modular construction of the mirror matrix. As a result, the mirror ele- ments may be interchanged with one another, for example. Also maintenance and replacement of failing mirror elements is simplified with a modular structure.

The mirror elements may be arranged one-dimensionally along a line or two-dimensionally in a surface which may also be curved. Since the mirror substrates of mirror elements are intended to be tiltable about at least one rotational axis, a gap having a gap area has to be left between the reflective surfaces of neighbouring mirror elements. This allows contactless tilting of neighbouring mirror substrates. In order to ensure a high efficiency in respect of the light reflected by the mirror matrix (i. e. a high filling ratio) , the ratio between the sum of the reflective surfaces of the mirror elements and this sum plus the sum of the gap areas should be more than 0.70.

The support of the mirror matrix may be flat or may have a curvature along at least one direction so that, in the case of identically constructed mirror elements, the reflected surfaces of the mirror substrates (in the undeflected state) follow the direction of the curvature of the support. It may furthermore be advantageous for the reflective surfaces of the mirror elements to have equal distances from the support, or for this distance to follow a predetermined functional profile which, for example, defines the distance as a function of the position coordinates of the mirror elements on the support.

The reflective surfaces of the mirror elements may be flat in order to obtain a maximum surface coverage of the reflective surface for a given gap area. However, alter- natively some or all reflective surfaces may have a curvature along at least one direction. The contour of the reflective surfaces may in either case be rectangu- lar, square, triangular, lunulate or hexagonal, for example. The total number of mirror elements contained in the mirror matrix may be as low as 50. However, there are no technical constraints in this respect, and therefore it is also considered to have mirror matrices with more than 4000 or even 5000 mirror elements.

Each mirror element may have a second rotational axis which is preferably arranged in the vicinity of the reflective surface and orthogonal to the first rotational axis in order to allow linearly independent tilting of the mirror substrate about the two rotational axes. In this case each mirror element should comprise a second electromagnetic drive, which comprises a stator unit and a rotor unit that is connected to the mirror substrate. The electromagnetic drive is configured to tilt the mirror substrate about the second rotational axis such that the stator unit is always arranged inside the installation volume, and such that the rotor unit does not enter the installation volume of a neighbouring mirror element when the mirror substrate has reached its maximum tilting angle.

With such a configuration it is possible to construct mirror matrices which allow the mirror substrate to be tilted about any arbitrary tilting axis. Nevertheless the individual mirror substrates can be arranged very close together so that a high filling ratio of the mirror matrix can be achieved. At least one rotor unit may remain inside the installation volume of the mirror element when the mirror substrate has reached its maximum tilting angle. Then mirror elements arranged next to one another can be tilted in mutually opposite directions without the risk of neighbouring electromagnetic drives or parts thereof impeding one another.

The reflective surface of the mirror elements may have an area between 9 mm² and 400 mm², preferably between 100 mm² and 225 mm².

The stator unit may comprise a ferromagnetic yoke having at least one energizable coil or a permanent magnet. The rotor unit may comprise at least one permanent magnet or an energizable coil.

The at least one coil or permanent magnet may have a longitudinal axis that is aligned at least approximately tangential with regard to a sphere having a centre of curvature that coincides with the rotational axis. This makes it possible to define a "strong" balanced position of the rotor unit to which the rotor unit returns if the at least one coil is not energized.

The bearing may be formed by at least one solid-state articulation. To this end the bearing may comprise a spherical cap formed in the mirror substrate or provided therein. A counter bearing connected to the support then engages in the spherical cap. Instead of a spherical cap, a convex spherical surface which engages in a corresponding recess in a counter bearing may be envisaged. This has the advantage that with suitable dimensioning of the radius of the convex spherical surface the rotational axis can be placed on the reflective surface.

Each mirror element may comprise two rotor and stator units arranged mutually orthogonally, the mirror substrate being held in position by the force of permanent magnets. This is assisted by the bearing friction. An advantage of such an embodiment is that the mirror substrate is very easy to replace.

In another embodiment the rotor unit of the electromagnetic drive comprises at least one sensor unit for posi- tion determination of the rotor unit.

The reflective surface of the mirror elements may be designed to reflect light having a wavelength of less than 350 nm.

In another embodiment the reflective surface of each mirror element comprises diffractive structures. This makes it possible to realize complex intensity distribution in a pupil surface of the illumination system with very few mirror elements. According to another embodiment the reflective surface of the at least one mirror element is configured to be rotatable about a further axis that extends at least substantially perpendicular to the support. Particularly in conjunction with mirror elements comprising diffrac- tive structure or being non-axisymmetrically curved, this makes it possible to rotate the far field intensity distributions produced by the mirror element. It may then be preferred that the reflective surface of the mirror elements has a circular contour.

BRIEF DESCRIPTION OF THE DRAWINGS

Various features and advantages of the present invention may be more readily understood with reference to the following detailed description taken in conjunction with the accompanying drawing in which:

Fig. Ia is a perspective cut-out of a mirror matrix according to the invention;

Fig. Ib is a schematic sectional view along line A-A of the mirror matrix of Fig. Ia showing two neighbouring mirror elements;

Fig. Ic is a plan view of a mirror matrix having a recurring pattern of differently large reflective surfaces of the mirror elements in accordance with another embodiment; Fig. 2 is a schematic section through a mirror element showing only the bearing arrangement of the mirror substrate according to one embodiment;

Fig. 3 is a plan view of a mirror element with elec- tromagnetic drive arranged at 90° to one another;

Fig. 4a and 4b are sections through mirror elements according to another embodiment in two different tilting positions;

Fig. 5 is a sectional cut-out of the bearing arrangement of a mirror substrate according to another embodiment;

Fig. 6 is a perspective view on a solid-state articulation of a mirror substrate according to one embodiment;

Fig. 7 is a top view on a bridge according to an alternative embodiment for use in the articulation shown in Fig. 6;

Fig. 8 is a perspective view of a solid-state articu- lation according to still another embodiment;

Fig. 9a is a perspective view of a solid-state articulation according to a still further embodiment; Fig. 9b is a side view on a solid-state articulation according to one further embodiment;

Figs. 10a and 10b are schematic illustrations showing a gear for tilting the mirror substrates using four articulations;

Fig. 11 is a schematic sectional view similar to Fig. Ib for a mirror element having a third rotational degree of freedom;

Fig. 12 is an enlarged cut-out of the mirror element shown in Fig. 11;

Fig. 13 is a schematic sectional view similar to Fig. 11 for a mirror element having a third rotational degree of freedom according to another embodiment;

Fig. 14 is an enlarged cut-out of the mirror element shown in Fig. 13;

Fig. 15 is a meridional section through an illumination system comprising a mirror matrix in accordance with the present invention. DESCRIPTION OF PREFERRED EMBODIMENTS

1. Basic Setup of Mirror Matrix

Fig. Ia schematically shows a portion of a mirror matrix 1 comprising a plurality of mirror elements 10 which have, in the embodiment shown, planar reflective surfaces. All mirror elements have at least one adjacent neighbouring mirror element; any arbitrary pair of adjacent mirror elements is denoted in the following by 10 and 110.

The mirror matrix 1 furthermore comprises a support 2, on which the mirror elements 10 are arranged. In the embodiment shown in Fig. Ia, the mirror elements 10 are two- dimensionally arranged in linear rows and columns on the support 2, and the reflective surfaces all have the same quadratic geometry. In alternative embodiments, the support 2 may be designed to be curved along at least one direction. Furthermore, instead of linearly in rows and columns, the mirror elements 10 may also be arranged on curved lines . Having a curved support 3 may be advanta- geous when the mirror matrix 1 is arranged in a curved (i.e. distorted) image field of an objective (see the embodiment described below with reference to FIG. 15) . In this case, unlike the embodiment represented in Fig. Ia, the size of the reflective surface of the mirror elements 10 may be depend on the position of the mirror element 10 inside the mirror matrix 1. Furthermore, the mirror elements may be configured with reflective surfaces which, for example, are triangular. This is exemplarily shown in Fig. Ia for two mirror element Ilia, 111b. Alternatively, the geometries of the reflective surfaces of the mirror elements 10 may be rectangular or hexagonal . Maximal surface coverage by reflective surfaces 11 can thus be obtained for a given minimum distance between adjacent mirror elements 10, 110. The surface coverage or filling factor is defined as the ratio of the total reflective area of a mirror matrix to its total area, i.e. the sum of the reflective area and the area covered by gaps 80 formed between adjacent mirror elements.

Other arrangements in respect of the size of the individ- ual mirror elements are likewise possible. For example, a recurring pattern of differently large reflective surfaces 10 ', 110' may be envisaged, as is shown schematically in the plan view of Fig. Ic.

2. Mirror Elements

Fig. Ib is a sectional view along line A-A indicated in Fig. Ia, showing two neighbouring mirror elements 10, 110. The mirror element 10 on the left hand side of Fig. Ib is illustrated in more detail, while for the neighbouring mirror element 110 only some parts are shown and denoted by reference numerals. The mirror element 10 comprises a mirror substrate 12 on which the reflective surface denoted by 11 is applied. An installation volume 50 is assigned to the mirror element 10. The installation volume 50 is confined by the the reflective surface 11 of the mirror element 10, all surface normals 51, 52 on the support 3 that intersect the boundary of the surface 11, and an area 53 enclosed by the normals 51, 52 on the support 2. The mirror element 10 comprises at least one bearing 20, which is arranged close to its reflective surface 11 and defines at least one rotational axis 21 in its vicinity.

The mirror element 10 furthermore comprises an electromagnetic drive 30, which is associated with the rotational axis 21 and is used to tilt the mirror substrate 12 with its reflective surface 11 about this rotational axis 21. The electromagnetic drive 30 comprises a stator unit 31 which is arranged inside the installation volume 50, comprises a soft-iron yoke 41 and is connected indirectly or directly to the support 2 and which supports coils 36. The electromagnetic drive 30 further comprises a rotor unit 32, which is connected indirectly or directly to the mirror substrate 12. The rotor unit 32 is designed in respect of its shape or in respect of its connection to the mirror substrate 12 so that, when there is maximal tilting of the mirror substrate 12 by means of the electromagnetic drive 30 about the rotational axis 21 assigned to the rotor unit 32, the rotor unit 32 does not enter the installation space 150 of the neighbouring mirror element 110.

The coils 36 have a longitudinal axis that is aligned at least approximately tangential with regard to a sphere SP having a centre of curvature that coincides with the rotational axis 21. This defines a "strong" balanced position of the rotor unit 32 to which the rotor 32 unit returns if the coils 36 are not energized. In this case the soft-iron yoke 41 will magnetize, as a result of magnetic forces exerted by the permanent magnets of the rotor unit 32, so as to keep the permanent magnets in the balanced position shown in Fig. Ib.

Applications of the mirror matrix 1 in an illumination system of a microlithographic projection exposure appara- tus, as is exemplarily shown in Fig. 15 to be discussed further below, entail for instance the following requirements: The tilt angle of the mirror substrates should lie between +/-10° and +/-3° around a predefined resting position. This should be achieved with a resolution of more than 10 bits (this allows encoding of 2¹⁰ = 1024 levels), i.e. with a resolution by a factor less than 1/1000. The number of mirror elements in the precision mechanical embodiment of the drive according to the invention should be less than 1000, preferably less than 200. Even with mirror matrices having about 50 mirror elements, very good results in respect of pupil formation can be achieved in an illumination system as is described in the above mentioned co-pending international patent application entitled "ILLUMINATION SYSTEM FOR ILLUMINATING A MASK IN A MICROLITHOGRAPHIC EXPOSURE APPARATUS", which has been filed on the same day as the present application by the same applicant and which claims priority of provisional US patent application 60/913,962 filed on April 25, 2007. For a square arrangement of mirror elements 10, edge lengths of each mirror support 12 of about 3 mm to 10 mm are favoured.

As has already been mentioned above, the bearing 20 of the mirror substrate 12 is preferably configured so that the rotational axis is located as close as possible to the reflective surface. In the embodiment shown this is achieved by forming the bearing 20 as a ruby bearing centrally positioned on the side of the mirror substrate 12 which does not support the reflective surface 11. The bearing 20 comprises a recess 22, in which a counter- bearing 23 connected to the support 2 engages. The recess 22 may be designed linearly as shown in Figs. Ib and 2, so that the rotational axis 21 is defined by the recess 22 and the engaging counter-bearing 23. The counter- bearing 23 engaging in the bearing 20 is preferably made of hardened steel .

As an alternative to the linear recess, the recess 22 may also be designed axisymmetrically or form a bearing seat in which a corresponding counter-bearing, for example a spherical or conical surface, engages. If the bearing arrangement is designed axisymmetrically in respect of the bearing 20 and the counter-bearing 23, then a spherical head of the counter-bearing 23 engages in the spherical cap-shaped recess 22 of the ruby bearing 20. In such a configuration of the invention, the tilting axes of the substrate 12 are no longer established by the bearing itself. They may be defined by means of the electromagnetic drive 30, for example, the rotor unit 32 and the stator unit 31 allowing a rotational movement of the rotor in a plane so that the rotational axis is formed perpendicularly to this plane.

This alternative is schematically shown in Fig. 3, which is a schematic plan view of a mirror element 10. The bearing 20, a first electromagnetic drive 30 and a second electromagnetic drive 60 are located on the underside of the mirror substrate 12 and are therefore indicated with broken lines. As already described with reference to Fig. Ib, the first electromagnetic drive 30 comprises the stator 31 and the rotor 32, wherein the rotor 32 can rotate about the rotational axis 33, which extends through the midpoint 21 of the cap-shaped recess 22 of the bearing 20 and is arranged perpendicularly to a plane defined by the drive 30. The second electromagnetic drive unit 60, arranged rotated by 90° with respect to the first electromagnetic drive 30, is constructed similarly as the drive 30 so that a second rotational axis 63 is formed, which likewise extends through the midpoint 21 of the cap-shaped recess 22 in the ruby bearing 20 and is oriented perpendicularly to the surface defined by the second drive unit 60. Owing to this arrangement of the electromagnetic drives 30, 60, it is possible to tilt the mirror substrate 12, and thus the reflective surface 11, independently around two rotational axes 33, 63 by desired tilting angles by means of the pair of drives 30, 60.

Figs 4a and 4b schematically show an embodiment of a mirror element 10 in sectional views similar to Fig. Ib in two different tilting positions. Identical or corresponding elements are denoted by the same reference numerals. The main difference to the embodiment shown in Fig. Ib is that the bearing 20 is configured so that the rotational axis is positioned directly on the reflective surface 11. This is achieved by a spherically convex ruby bearing 20, whose centre of curvature 21 coincides with a point on the reflective surface 11. A corresponding arrangement of the drive 30, 60 according to Fig. 3 results in two rotational axes 33 and 63 lying orthogonal to one another, which intersect at the centre of curvature 21 on the mirror surface 11.

The counter-bearing 23 is guided through a recess 29 in the rotor unit 31. Also in this embodiment the stator unit 31 comprises at least one coil body 36, which can be energized with an electrical current in order to set up a magnetic field between the stator unit 31 and the rotor unit 32. The rotor unit 32 comprises at least one perma- nent magnet 35 which, by deflection, follows the magnetic field which can be modified by the stator unit 31. The drive 30 therefore reacts somewhat like a linear electromagnetic drive unit for small angle deflections.

As an alternative, the coil bodies 36 and permanent magnets 35 may be interchanged, so that for example at least one coil body is fitted on the rotor unit. Combinations of permanent magnets and coil bodies are also envisaged. If a variable magnetic field is set up at the stator, and if the rotor merely possesses permanent magnets, as is shown in Figs Ib, 4a and 4b, then this has the advantage that no electrical energy supply is required for the moving parts, i.e. the rotor unit 32. For larger deflections on the other hand, magnetic screening of the mirror element 10 from its neighbouring element

110 may be necessary, depending on the geometrical extent of the rotor unit 32 and the arrangement of the permanent magnets 35. This may for example be carried out by means of a Ni-Fe alloy with high permeability, for instance a Mu-metal . In this embodiment, it is advantageous for the permanent magnets 35 to be arranged at 45° to the support body 2 when the rotor unit 32 is undeflected, as can be seen in Fig. Ib. If the rotor unit 32 comprises only energizable coils 36, then this advantageously results in a smaller perturbation of the neighbouring mirror elements 110 by magnetic fields. It does, however, entail the disadvantage that the electrical connections for the coils 36 must be fed to the rotor unit 32 via resilient articulations .

A sensor system 17, 18, which determines the deflection of the mirror substrate 12, is accommodated in the mirror element 10 shown in Figs. 4a and 4b. The sensor system may for example comprise a permanent magnet 18 guided over a Hall sensor 17, the permanent magnet being connected rigidly to the rotor unit 32 or the mirror substrate 12. Based on the sensor data, the current in the coils 36 is regulated, for example via a closed-loop control, in order to adjust the desired tilting angle.

Other kinds of sensors that may be used for measuring the position of angle of individual mirror elements 10 are capacitive sensors, inductive sensors or optical encoding sensors. Components of the sensors may be fixed directly to the mirror substrate 12 or to structures between the mirror substrate 12 and the electromagnetic drive 30. Furthermore, the sensors may be integrated into an electromagnetic drive 30 or arranged below the drive.

3. Alternative Bearings and Articulations

As an alternative to the ruby bearing shown in Figs . 2 and 3, a ball bearing may be used, as is shown in the simplified sectional view of Fig. 5. In this embodiment the mirror substrate 12 is connected to a bearing 20 having the shape of a spherical cap. The counter-bearing 23 is provided with a spherical recess 22 which accommodates the bearing 20 and a plurality of balls 40. The balls are distributed within a gap remaining between the recess 22 and the bearing 20 so as to allow for articu- lated movement of the mirror substrate 12 with respect to the fixed counter-bearing 23. Since the ball bearing shown in Fig. 5 is axisymmetrically, the tilting axes have to be established by other means, for example the electromagnetic drive, as has been explained above with reference to Figs. 3 and 4a, 4b.

Fig. 6 is a perspective view on a solid-state articulation that may be used as an alternative to the bearings described above. The articulation is made of a single metal plate 42 in which elongated openings 44 are punched out. The openings 44 have the shape of two concentric stripes each of which substantially following the contour of a square. In each square a pair of bridges 46a, 46b and 48a, 48b, respectively, remain that allow for articulation of the inner portion surrounded by the respective stripes around a tilt axis X and Y, respectively. The square area confined within the inner bridges 48a, 48b supports the reflective surface 11. The solid-state articulation shown in Figs. 6 thus has a cardanic function with intersecting tilt axes X, Y.

The bridges 46a, 46b and 48a, 48b in this embodiment are bars having a rectangular cross-section. In other embodiments the bridges 46a, 46b and 48a, 48b have more complex geometries. For example, some or all bridges 46a, 46b and 48a, 48b may have a meandering shape as shown for a bridge 4βa' in Fig. 7, and the cross-section may be elliptical or may vary along the length of the bridges 46a, 46b and 48a, 48b.

Fig. 8 is a perspective view of a solid-state articulation which is axisymmetric and thus has not a cardanic function. The articulation consists of a single member 70 having a substantially cylindrical shape and a central contraction 72 in which the diameter of the member 70 is substantially reduced. The material and the diameter of the member 70 at the contraction 72 is determined such that the opposite ends of the member 70 may be freely articulated with respect to each other about arbitrary axes. The cross-section of the contraction 72 may have a circular shape if axisymmetric bending stress is desired. However, in some cases it may be more advantageous to have different bending stresses for tilting movements about the x and y axis. The cross-section of the contrac- tion 72 may then be elliptical or rectangular, for example. The articulation shown in Fig. 8 may be used in conjunction with two electromagnetic drives 30, 60 arranged as shown in Fig. 3 and defining two rotational axes .

Fig. 9a is a perspective view of a solid-state articulation according to a still further embodiment which is configured very similarly to the articulation shown in Fig. 7 and thus also has a cardanic function. The plates 62, 64 of the embodiment shown in Fig. 7 are replaced by cylindrical members 62', 64'. The first member 62' is connected via a ridge 68' to a third member 69 on which the mirror substrate 12 may be mounted, and via a ridge 66' to the second member 64'. Also in this embodiment the ridges 66', 68' extend along orthogonal directions so as to achieve a cardanic function.

Fig. 9b is a side view of a similar articulation connect- ing a substrate 2 to the mirror support 12. The articulation shown in Fig. 9b differs from the articulation shown in Fig. 9a in that ridges 66'', 68 '' have not the shape of parallel plates, but of elements having the shape of biconcave cylinder sections. It has to be noted that the articulation shown in Fig. 9b is shown in an orientation in which it is, as compared to the articulation shown in Fig. 9a, rotated by 90° around its longitudinal axis.

In all solid-state articulation the solid-material has resilient properties so that a restoring action of the mirror substrate 12 takes place after actuation of the electromagnetic drive 30, 60. Such solid-state articulations have the advantage that almost no friction occurs, and thus an extremely high angular resolution can be achieved when the mirror substrates are tilted. A disad- vantage, however, of a solid-state articulation is that for a particular tilting angle an actuating force must always act against the resilient force exerted by the articulation. This makes it necessary to energize the electromagnets in the electromagnetic drives almost constantly. As a result a larger amount of heat is produced in the mirror elements 10 which has to be removed by cooling means.

Figs. 10a and 10b are schematic illustrations of a four- articulation gear in two different configurations. The gear comprises four articulations 73a, 73b, 73c, 73d that may be formed by solid-state articulations as described above with reference to Figs. 6 to 9. Two articulations 73a, 73b are fixed to the mirror substrate 12, and the other two articulations 73c, 73d are fixed to a structure fixed to the support 2 of the mirror array 1. Pairs of opposing articulations 73a, 73c and 73b, 73d are con- nected by rigid connector members 75 and 77, respectively. One drive or, in this embodiment, two drives 30a, 30b may act on the connector members 75, 77, or the articulations 73c, 73d may be integrated into the drives 30a, 30b, as is indicated with dotted lines in Figs. 10a, 10b.

Such a four-articulation gear gives more flexibility with regard to the position of the rotational axis: as becomes clear from comparing Figs. 10a and 10b, the rotational axis is positioned far away from the mirror substrate 12 and also the gear that enables this rotation. With a gear as shown in Fig. 10a, 10b, an additional bearing for supporting the mirror substrate 12 may be completely dispensed with.

4. Third Rotational Axis

Some or all mirror elements 10 may have a third rota- tional degree of freedom so that the reflective surface 12 is rotatable about an axis that extends at least substantially perpendicular to the support 2. Such an embodiment is shown in Figs. 11 and 12 in a sectional view similar to Fig. Ib and an enlarged cut-out, respec- tively. Identical or corresponding parts are denoted by the same reference numerals.

The mirror element 10 shown in Figs. 11 and 12 differs from the mirror element shown in Fig. Ib mainly in that the stator unit 31 is configured such that it can rotate about the counter-bearing 23, as is indicated by an arrow A in Fig. 11. To this end one or, in this embodiment, two ball bearings 82, 83 and a DC motor 84 are arranged coaxially with the counter-bearing 23. As is shown in more detail in the enlarged cut-out of Fig. 12, the counter-bearing 23 is provided with a central stepped bore 85 having a wider section 86 that receives the two ball bearings 82, 83 and the DC motor 84. A snap ring 93 fixes the ball bearings 82, 83 and the DC motor 84 in place . An electrical connection between leads 87 connected to the coils 36 on the one hand and leads 88 fixed to the counter-bearing 23 are established in this embodiment by sliding contacts 89 extending from the stator unit 31 and brushing over contacts 99 fixed to the counter-bearing 23. Similar sliding contacts (not shown) may be provided for the voltage supply of the DC motor 84.

By actuating the DC motor 84 , the stator unit 31 rotates about the fixed counter-bearing 23 and thus about a rotational axis 90 which is at least substantially perpendicular to the base 2. Due to the magnetic forces prevailing between the coils 36 and the permanent magnets fixed to the rotor unit 32, the rotor unit 32 follows rotations of the stator unit 31 about the third rota- tional axis 90. By energizing the coils 36 the mirror substrate 12 can additionally be tilted around the rotational axis 21. If the mirror element comprises a second electromagnetic drive 60 similar to the embodiment shown in Fig. 3, the mirror substrate 12 can also be tilted by an orthogonal rotational axis intersecting both the rotational axis 21 and the rotational axis 90. The mirror substrate 12 then has three independent rotational degrees of freedom.

A sensor system (not shown) which determines the deflec- tion of the mirror substrate should be mounted such that its components rotate together about the third rotational axis 90. For example, a permanent magnet could be fixed to the rotor unit 32, and a Hall sensor could be fixed to the yoke 41. An additional sensor system may be provided to measure rotations of the yoke 41 about the third rotational axis 90.

Figs. 13 and 14 are illustrations similar to Figs. 11 and 12 showing another embodiment of a mirror element 10 that also has a third rotational degree of freedom. In this embodiment, the counter-bearing 23 is rigidly fixed to the stator unit 31 which again is configured to be ro- tatable around a rotational axis 90 which extends at least substantially perpendicular to the base 2. If the bearing 20 of the mirror substrate 12 is configured such that it can rotate only around a single rotational axis 21, a rotation of the counter-bearing 23 about the rota- tional axis 90 will force the bearing 20, together with the mirror substrate 12, to rotate also about the rotational axis 90. Oscillations that may be prominent in the embodiment shown in Figs. 11 and 12 are reduced because the rotational position is not determined by the magnetic forces prevailing between the coils 36 and the permanent magnets of the rotor unit 32.

For enabling the stator unit 31 with the fixed counter- bearing 23 to rotate about the rotational axis 90, the stator unit 31 is rotatably supported on a pivot pin 91 with the help of ball bearings 82, 83 and a DC motor 84, similar to the embodiment shown in Figs. 11 and 12. With regard to the details shown in Figs. 14, the only differ- ence to the embodiment shown in Fig.12 is that two additional snap rings 93b, 93c are provided to keep the bearings 82, 83 and the DC motor 84 in place.

Instead of ball bearings other kinds of bearings, in particular solid-state articulations, may be used. For example, the stator unit 31 may be connected via three, four or more flexible webs to a member extending in a plane and fixed to the support 2 (counter-bearing 23 in the embodiment shown in Figs. 11 and 12, or pivot pin 91 in the embodiment shown in Figs. 13 and 14) .

Instead of sliding contacts 89 flat spiral springs may be used, or the electrical connection may be established directly by means of webs of a solid-state articulation.

Instead of a DC motor 84 other actuating means may be provided. For example, piezo-electric elements or bimetal drives may be used, particularly in conjunction with solid-state articulations .

The mirror matrix 1 may be designed as a modular system in which a plurality of modules comprising one or more mirror elements 10 are assembled on the support 2. The modules may be connected to the support 2 using plug-in connectors, by soldering or by bonding, for example. A modular design facilitates maintenance, increases the flexibility and makes it possible to upgrade the mirror matrix 1, for example by adding additional modules if there is a desire for an increased total reflective surface. Apart from that significant cost reductions can be achieved by a modular system because the individual modules may be manufactured at large scales for different kinds of end products.

5. Illumination System

FIG. 15 is detailed meridional section through an illumination system 112 in which the mirror matrix 1 may be used. For the sake of clarity, the illustration of FIG. 15 is considerably simplified and not to scale. This particularly implies that different optical units are represented by very few optical elements only. In reality, these units may comprise significantly more lenses and other optical elements .

The illumination system 112 includes a housing 128 and a light source that is, in the embodiment shown, realized as an excimer laser 130. The excimer laser 130 emits projection light that has a wavelength of about 193 nm. Other types of light sources and other wavelengths, for example 248 nm or 157 nm, are also contemplated.

In the embodiment shown, the projection light emitted by the excimer laser 130 enters a beam expansion unit 132 in which the light bundle is expanded without altering the geometrical optical flux. The beam expansion unit 132 may comprise several lenses as shown in FIG. 15, or may be realized as a mirror arrangement. After passing through the beam expansion unit 132, the projection light impinges on an optical raster element 134.

The optical raster element 134 comprises a plurality of adjacent zones Zi_j that form, in the embodiment shown, a regular grid-like array in a plane in which the optical raster element 134 extends. Each zone Z^ contains a diffractive optical element which produces a carefully designed intensity distribution in the far field. In the far field the distance, at which an intensity distribution produced by a diffractive optical element is observed, is large in comparison to the typical width of the diffractive structures contained in the element. In this case the far field intensity distribution is given by the Fourier transform of the amplitude function which describes the geometry of the diffracting structures. For that reason it is possible to design almost for any arbitrary desired far field intensity distribution with the help of a suitable diffractive optical element. The diffractive optical element has an amplitude function which is given by the inverse Fourier transform of the desired far field intensity distribution. Diffractive optical elements of this kind are often referred to as "computer generated holograms" (CGH) and are readily available from various suppliers of optical technology.

Alternatively, the zones Zi_j of the optical raster element 134 may contain a plurality of microlenses, for example spherical, aspherical, cylindrical or prismatic microlenses. Spherical and cylindrical microlenses produce, for example, far field intensity distributions having the geometry of a circular disc or a rectangular strip, respectively.

The plane in which the optical raster element 134 extends is an object plane 136 of an optical imaging system 138 which is represented, in the simplified illustration of FIG. 15, by two positive lenses 140 and 141. The optical imaging system 138 images the object plane 136 to an image plane 140, thereby achieving an optical conjugation between the object plane 136 and the image plane 140. Thus each light bundle diverging from a particular object point in the object plane 136 converges to an associated image point in the image plane 140. In FIG. 15 this is indicated by dotted lines MR that represent marginal rays of a light bundle that emerges from an on-axis point in the object plane 136.

In this specific embodiment the optical imaging system 138 contains a plane folding mirror 142 which reduces the overall length of the illumination system 112. The folding mirror 142 is arranged in a pupil plane 144 so that the far field distributions produced by the zones Zij of the optical raster element 134 are formed on the folding mirror 142. However, the folding mirror 142 may be completely dispensed with, or it may also be arranged outside the pupil plane of the optical imaging system 38. In the image plane 140 of the optical imaging system 138 the mirror matrix 1 is arranged. As has been explained above, the mirror matrix 1 comprises a plurality of small individual mirror elements 10 that can be tilted, inde- pendently from each other, by two tilt axes that are preferably aligned perpendicularly to each other. The total number of mirror elements 10 is preferably less than 100, and even more preferably less than 50. The reflecting surfaces of the mirror elements 10 may be plane, but could also be curved if an additional reflective power is desired.

The tilting movements of the individual mirror elements 10 are controlled by a mirror control unit 150 which is connected to an overall system control 152 of the illumi- nation system 112. Actuators that are used to set the desired tilt angles of the mirror elements 10 receive control signals from the mirror control unit 50 such that each individual mirror element 10 is capable of reflecting an impinging light ray by a reflection angle that is variable in response to the control signal. In the embodiment shown there is a continuous range of tilt angles, and therefore reflection angles, at which the individual mirror elements 10 can be arranged. In other embodiments, the actuators are configured such that only a limited number of discrete tilting angles can be set. An embodiment with only two different tilting angles will be described further below. The illumination system 12 further comprises a zoom lens system 158 having a variable focal length. The zoom lens system 158 is represented in FIG. 15 by a single lens which is displaceable along an optical axis of the illu- mination system 112, as is indicated by double arrow 162.

Behind the zoom lens system 158 a pair 164 of axicon elements 166, 168 having opposing conical surfaces is arranged. If both axicon elements 166, 168 are in immediate contact, the axicon pair 164 has only the effect of a plane parallel plate. If both axicon elements 166, 168 are moved apart, as is indicated in FIG. 15 by a double arrow 169, the spacing between the axicon elements 166, 168 causes a shift of light energy radially outward. Since axicon elements are known as such in the art, these will not be explained here in further detail.

Reference numeral 170 denotes a pupil surface of the illumination system 112 that substantially defines the angular distribution of the light impinging on a mask 116. The pupil surface 170 is usually plane or slightly curved and is arranged in or in immediate vicinity of an optical integrator 172 which produces a plurality of secondary light sources. The optical integrator 172 is realized, in the embodiment shown, as a fly's eye lens comprising two substrates 174, 176 that each includes two orthogonal arrays of parallel cylindrical microlenses.

The optical integrator 172 increases the range of angles formed between the light rays and an optical axis OA of the illumination system 112. As the angular distribution in the pupil surface 170 directly translates into an intensity distribution in a subsequent field plane, the optical integrator 172 substantially determines the geometry of an illuminated field 114 on the mask 116. Since the optical integrator 172 increases the range of angles considerably more in the X direction then in the Y direction, the illuminated field 114 has larger dimensions along the X direction than along the Y direction (i.e. the scan direction) .

The projection light emerging from the secondary light sources produced by the optical integratorl72 enters a condenser 178 that is represented in FIG. 15 by a single lens only for the sake of simplicity. The condenser 162 ensures a Fourier relationship between the pupil surface 170 and a subsequent intermediate field plane 180 in which a field stop 182 is arranged. The condenser 178 superimposes the light bundles, which are produced by the secondary light sources, in the intermediate field plane 180, thereby achieving a very homogenous illumination of the intermediate field plane 180. The field stop 182 may comprise a plurality of movable blades and ensures sharp edges of the illuminated field 114 on the mask 116.

A field stop objective 184 provides optical conjugation between the intermediate field plane 180 and the mask plane 186 in which the mask 16 is arranged. The field stop 182 is thus sharply imaged by the field stop objective 184 onto the mask 116.

The above description of the preferred embodiments has been given by way of example. From the disclosure given, those skilled in the art will not only understand the present invention and its attendant advantages, but will also find apparent various changes and modifications to the structures and methods disclosed. The applicant seeks, therefore, to cover all such changes and modifica- tions as fall within the spirit and scope of the invention, as defined by the appended claims, and equivalents thereof .

Claims

1. Mirror matrix (1) comprising at least two mirror elements (10), which are arranged next to one another on a support (2), wherein each mirror element has

- a reflective surface (11) applied on a mirror substrate (12) which is separated from the support (2) ,

an installation volume (50) associated with the mirror element, wherein the installation volume is confined by the reflective surface

(11) of the mirror element (10), all surface normals (51, 52) on the support (3) that intersect a boundary of the surface (11) , and an area (53) enclosed by the normals (51, 52) on the support (2),

a bearing (20) defining a rotational axis (21),

an electromagnetic drive (30), which comprises a stator unit (31) and a rotor unit (30) that is connected to the mirror substrate (12), the electromagnetic drive being configured to tilt the mirror substrate (12) about the rotational axis (21) such that the stator unit (31) is always arranged inside the installation volume (50), and such that

the rotor unit (32) does not enter the installation volume (150) of a neighbouring mirror element (110) when the mirror substrate (12) has reached its maximum tilting angle.

2. Mirror matrix according to Claim 1, wherein the installation volumes (50, 150) of the mirror elements (10, 110) are identical when there is an equal alignment of the reflective surfaces with respect to the area (53) enclosed by the normals (51, 52) on the support (2) .

3. Mirror matrix according to one of the preceding claims, wherein the mirror elements (10, 110) are arranged one-dimensionally along a line.

4. Mirror matrix according to one of Claims 1 to 3, wherein the mirror elements (10, 110) are arranged two-dimensionally in a surface.

5. Mirror matrix according to one of the preceding claims, wherein a gap having a gap area is left between the reflective surfaces (11) of neighbouring mirror elements (10, 110).

6. Mirror matrix according to Claim 5, wherein the ratio between the sum of the reflective surfaces (11) of the mirror elements (10, 110) and this sum plus the sum of the gap areas is more than 0.70.

7. Mirror matrix according to one of the preceding claims, wherein the support (2) is flat or has a curvature along at least one direction.

8. Mirror matrix according to one of the preceding claims, wherein the reflective surfaces (11) of the mirror elements (10, 110) are have equal distances from the support (2) .

9. Mirror matrix according to one of the preceding claims, wherein the reflective surfaces (11) of the mirror elements (10, 110) are flat and have rectan- gular, square, triangular or hexagonal contours.

10. Mirror matrix according to one of the preceding claims, wherein each mirror element (10, 110) has a second rotational axis, which is preferably arranged in the vicinity of the reflective surface (11) .

11. Mirror matrix according to Claim 10, wherein the two rotational axes are orthogonal.

12. Mirror matrix according to one of Claims 10 and 11, wherein each mirror element comprises a second electromagnetic drive (60), which comprises a sta- tor unit (61) and a rotor unit (62) that is con- nected to the mirror substrate (12), the electromagnetic drive (60) being configured to tilt the mirror substrate (12) about the second rotational axis such that

the stator unit (61) is always arranged inside the installation volume (50), and such that

the rotor unit does not enter the installation space (150) of a neighbouring mirror element (110) when the mirror substrate (12) has reached its maximum tilting angle.

13. Mirror matrix according to one of the preceding claims, wherein at least one rotor unit (32) remains inside the installation volume (50) of the mirror element when the mirror substrate (12) has reached its maximum tilting angle.

14. Mirror matrix according to one of the preceding claims, wherein the reflective surface (11) of the mirror element (10) has an area between 9 mm² and 400 mm².

15. Mirror matrix according to one of the preceding claims, wherein the stator unit (31) comprises a ferromagnetic yoke (41) having at least one energi- zable coil (36) or a permanent magnet.

16. Mirror matrix according to Claim 15, wherein the at least one coil (36) or permanent magnet has a lon- gitudinal axis that is aligned at least approximately tangential with regard to a sphere (SP) having a centre of curvature that coincides with the rotational axis (21).

17. Mirror matrix according to one of the preceding claims, wherein a rotor unit (32) comprises at least one permanent magnet (35) or an energizable coil .

18. Mirror matrix according to one of the preceding claims, wherein the bearing (20) is formed by at least one solid-state articulation.

19. Mirror matrix according to one of Claims 1 to 17, wherein the bearing (20) comprises a spherical cap formed in the mirror substrate (12) or provided therein .

20. Mirror matrix according to Claim 19, wherein a counter-bearing (23) connected to the support (2) engages in the spherical cap.

21. Mirror matrix according to Claim 20, comprising a plurality of balls (40) that are arranged between the spherical cap and the counter-bearing (23) .

22. Mirror matrix according to one of the preceding claims, wherein each mirror element (10) comprises two rotor and stator units arranged mutually orthogonally, and the mirror substrate (12) is held in position by the force of permanent magnets.

23. Mirror matrix according to one of the preceding claims, wherein the rotor unit (32) comprises at least one sensor unit (17, 18) for position determination of the rotor unit.

24. Mirror matrix according to one of the preceding claims, wherein the reflective surface (11) of the at least one mirror element (10) is designed to reflect light having a wavelength of less than 350 nm.

25. Mirror matrix according to one of the preceding claims, wherein the reflective surface (11) of each mirror element (10) comprises diffractive structures .

26. Mirror matrix according to any of the preceding claims, wherein the reflective surface (11) of the at least one mirror element (10) is configured to be rotatable about a further axis that extends at least substantially perpendicular to the support (2) .

27. Mirror matrix according to Claim 26, wherein the reflective surface has a circular contour.

28. Mirror matrix according to Claim 26 or 27, wherein the stator unit (31) is rotatably mounted relative to the base (2) about the further axis, and wherein actuating means (84) are provided to rotate the stator unit (31) about the further axis.

29. Mirror matrix according to any of Claims 26 to 28, wherein the mirror substrate (12) is rotated about the further axis by magnetic forces produced by the electromagnetic drive (30, 60).

30. Mirror matrix according to any of Claims 26 to 28, wherein the mirror substrate (12) is rotated about the further axis by mechanical forces exerted by the bearing (20) .