Method of operating a microlithographic projection apparatus and illumination system of such an apparatus

Abstract

An illumination system of a microlithographic projection apparatus includes a spatial light modulator having a modulation surface including a plurality of micromirrors. Each micromirror includes a mirror surface having an orientation that can be changed individually for each micromirror. For at least one of the micromirrors, at least one parameter that is related to the mirror surface is measured. The orientation of the mirror surfaces is controlled depending on the at least one measured parameter. A light pattern is produced on the modulation surface, and an image of the light pattern is formed on an optical integrator that has a plurality of light entrance facets. Images of the light entrance facets are superimposed on a mask.

Claims

1. A method of operating an illumination system of a microlithographic projection apparatus, the illumination system comprising a spatial light modulator comprising a modulation surface comprising a plurality of micromirrors arranged in a modulation plane, each micromirror comprising a mirror surface having an individually changeable orientation, the method comprising: for at least one of the micromirrors, measuring a parameter related to the mirror surface to provide a measured parameter; controlling the orientation of the mirror surface depending on the measured parameter; producing a light pattern on the modulation surface; forming an image of the light pattern on a raster field plane of an optical integrator comprising a plurality of light entrance facets arranged in the raster field plane, the raster field plane being optically conjugate to the modulation plane; and superimposing images of the light entrance facets on a mask.

2. The method of claim 1, wherein the measured parameter is selected from the group consisting of a position of the mirror surface and an orientation of the mirror surface.

3. The method of claim 1, wherein measuring the parameter comprises measuring an angular irradiance distribution at least at one field point in a mask plane or a plane that is optically conjugate to the mask plane.

4. The method of claim 3, wherein the measured parameter comprises the orientation of the mirror surface, and the method comprises measuring the angular irradiance distribution at a plurality of field points in the mask plane or a plane that is optically conjugate to the mask plane.

5. The method of claim 4, comprising simultaneously measuring the angular irradiance distribution at the plurality of field points.

6. The method of claim 5, wherein the illumination system comprises a plurality of position resolving light sensors arranged at positions that are associated with the plurality of field points, and the method comprises using each light sensor to measure the angular irradiance distribution for the associated field point.

7. The method of claim 1, wherein measuring the parameter comprises measuring an irradiance at least at one field point in a mask plane or a plane that is optically conjugate to the mask plane.

8. The method of claim 7, wherein the measured parameter is the orientation of the mirror surface, and the method comprises measuring irradiance at a plurality of field points in the mask plane or a plane that is optically conjugate to the mask plane.

9. The method of claim 1, wherein measuring the parameter comprises computing the based on a shape of the mirror surfaces, a spatial arrangement of the modulation surface relative to the light entrance facets, and a shape of the light entrance facets.

10. The method of claim 1, wherein the parameter is measured while controlling the orientations of the mirror surfaces so that adjacent mirror surfaces do not superimpose light on the mask.

11. The method of claim 1, wherein the parameter is measured while detecting a position of a marker point that is fixedly arranged with respect to the spatial light modulator.

12. The method of claim 1, wherein the modulation surface is observed by a camera which detects light that has been reflected or scattered by the micromirrors or by structures between the micromirrors.

13. The method of claim 1, wherein controlling the orientation of the mirror surface comprises laterally shifting a control pattern when the mirror surface has been measured as being laterally displaced.

14. A method of operating an illumination system of a microlithographic projection apparatus comprising a spatial light modulator comprising a modulation surface comprising a plurality of micromirrors arranged in a modulation plane, each micromirror comprising a mirror surface having an individually changeable orientation, the method comprising: for at least one of the micromirrors, measuring a parameter related to the mirror surface to provide a measured parameter; controlling the orientation of the mirror surface depending on the measured parameter; producing a light pattern on the modulation surface; and forming an image of the light pattern on a field plane which is optically conjugate to the modulation plane.

15. An illumination system, comprising: a spatial light modulator comprising a modulation surface which comprises a plurality of micromirrors arranged in a modulation plane, each micromirror comprising a mirror surface having an individually changeable orientation; a measuring device configured to measure, for at least one of the micromirrors, a parameter that is related to the mirror surface to provide a measured parameter; a control unit configured to control the orientation of the micromirrors depending on the measured parameter; an optical integrator comprising a plurality of light entrance facets arranged in a raster field plane which is optically conjugate to the modulation plane; and an imaging optical system configured to provide an image of the modulation surface on the raster field plane.

16. An illumination system, comprising: a spatial light modulator comprising a modulation surface comprising a plurality of micromirrors arranged in a modulation plane, each micro-mirror comprising a mirror surface having an individually changeable orientation; a measuring device configured to measure, for at least one of the micromirrors, a parameter related to the mirror surface to provide a measured parameter; a control unit configured to control the orientation of the micromirrors depending on the measured parameter; and an imaging optical system configured to form an image of the modulation surface on a field plane that is optically conjugate to the modulation plane.

17. The illumination system of claim 16, further comprising a pupil forming unit which comprises a mirror array comprising a plurality of mirrors, wherein, for each mirror of the mirror array of the pupil forming unit, the mirror is configured to illuminate a spot on the modulation plane at a position that is variable by changing a deflection angle produced by the mirror.

18. The illumination system of claim 15, further comprising a pupil forming unit which comprises a mirror array comprising a plurality of mirrors, wherein, for each mirror of the mirror array of the pupil forming unit, the mirror is configured to illuminate a spot on the modulation plane at a position that is variable by changing a deflection angle produced by the mirror.

19. The method of claim 14, wherein the method comprises imaging the mirror surface of each micromirror onto the raster field plane.

20. The method of claim 14, wherein: the illumination system further comprises a pupil forming unit which comprises a mirror array comprising a plurality of mirrors; and the method further comprises illuminating a spot on the modulation plane at a position that is variable by changing a deflection angle produced by a mirror of the pupil forming unit.

21. The method of claim 1, wherein the method comprises imaging the mirror surface of each micromirror onto the raster field plane.

22. The method of claim 1, wherein: the illumination system further comprises a pupil forming unit which comprises a mirror array comprising a plurality of mirrors; and the method further comprises illuminating a spot on the modulation plane at a position that is variable by changing a deflection angle produced by a mirror of the pupil forming unit.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) Various features and advantages of the present disclosure may be more readily understood with reference to the following detailed description taken in conjunction with the accompanying drawings in which:

(2) FIG. 1 is a schematic perspective view of a projection exposure apparatus in accordance with one embodiment of the present disclosure;

(3) FIG. 2A is an enlarged perspective view of the mask to be projected by the projection exposure apparatus shown in FIG. 1, illustrating local variations of the angular irradiance distribution on the mask;

(4) FIG. 2B is an enlarged perspective view similar to FIG. 2A illustrating different angular irradiance distributions that are adapted to different mask patterns;

(5) FIG. 3 is a meridional section through an illumination system being part of the apparatus shown in FIG. 1;

(6) FIG. 4 is a perspective view of a first mirror array contained in the illumination system shown in FIG. 3;

(7) FIG. 5 is a perspective view of a second mirror array contained in the illumination system shown in FIG. 3;

(8) FIG. 6 is a perspective view of an optical integrator contained in the illumination system shown in FIG. 3;

(9) FIG. 7 is a schematic meridional section through the first and the second mirror array shown in FIGS. 4 and 5;

(10) FIG. 8 is a perspective view on the second mirror array shown in FIG. 5, but illuminated with two poles;

(11) FIG. 9 is a perspective view of the optical integrator shown in FIG. 6, but illuminated with two poles;

(12) FIG. 10 is a schematic meridional section through a portion of the illumination system in which only a mirror array, a condenser and an array of optical raster elements are shown;

(13) FIGS. 11A and 11B are top views on the second mirror array and the optical integrator shown in FIG. 3;

(14) FIG. 12 illustrates an irradiance distribution on a light entrance facet of the optical integrator;

(15) FIG. 13 is a graph showing the scan integrated irradiance distribution along the X direction produced by the light entrance facet shown in FIG. 12;

(16) FIG. 14 illustrates another irradiance distribution on a light entrance facet of the optical integrator;

(17) FIG. 15 is a graph showing the scan integrated irradiance distribution along the X direction produced by the light entrance facet shown in FIG. 14;

(18) FIG. 16 is an enlarged cutout from FIG. 3 illustrating two pupil measuring units that measure the pupil (i.e. angular irradiance distribution) in the mask plane of the illumination system;

(19) FIG. 17 illustrates how a pupil measurement at different field positions provides information with regard to the position and the orientation of mirror surfaces;

(20) FIGS. 18A and 18B illustrate how the orientation of mirror surfaces can be determined with the help of the pupil measuring units shown in FIG. 16;

(21) FIG. 19 is an enlarged cutout from FIG. 3 illustrating two irradiance sensors that measure the irradiance in the mask plane;

(22) FIG. 20 illustrates how the irradiance measurement at different field positions provides information with regard to the position and the orientation of mirror surfaces;

(23) FIG. 21A to 21D illustrate how the orientations of mirror surfaces can be determined with the help of the irradiance measuring units shown in FIG. 19;

(24) FIG. 22 is a schematic cross-section through a measuring device according to an embodiment in which the positions of marker points are used to measure the relative position between three modulator units;

(25) FIG. 23 is a perspective schematic view on a measuring device according to an embodiment in which marker points on the spatial light modulator are imaged on detectors located on the optical integrator;

(26) FIG. 24 is a perspective schematic view on a measuring device according to an embodiment in which a camera detects measuring light that has been reflected or scattered from the micromirrors or structures between the micromirrors;

(27) FIG. 25A is a top view similar to FIG. 11A on the micromirror array of the spatial light modulator;

(28) FIG. 25B illustrates the irradiance distribution in the illumination field 14;

(29) FIG. 26A is a top view similar to FIG. 25A after a drift movement of the spatial light modulator;

(30) FIG. 26B illustrates the effect of the drift movement on the illumination field 14;

(31) FIG. 27 is a flow diagram of a method in accordance with the present disclosure.

DESCRIPTION OF PREFERRED EMBODIMENTS

I. General Construction of Projection Exposure Apparatus

(32) FIG. 1 is a perspective and highly simplified view of a projection exposure apparatus 10 in accordance with the present disclosure. The apparatus 10 includes a light source 11 that may be realized as an excimer laser, for example. The light source 11 in this embodiment produces projection light having a center wavelength of 193 nm. Other wavelengths, for example 157 nm or 248 nm, are envisaged as well.

(33) The apparatus 10 further includes an illumination system 12 which conditions the projection light provided by the light source 11 in a manner that will be explained below in further detail. The projection light emerging from the illumination system 12 illuminates an illumination field 14 on a mask 16. The mask 16 contains a pattern 18 formed by a plurality of small features 19 that are schematically indicated in FIG. 1 as thin lines. In this embodiment the illumination field 14 has the shape of a rectangle. However, other shapes of the illumination field 14, for example a ring segment, are also contemplated.

(34) A projection objective 20 including lenses L1 to L6 images the pattern 18 within the illumination field 14 onto a light sensitive layer 22, for example a photoresist, which is supported by a substrate 24. The substrate 24, which may be formed by a silicon wafer, is arranged on a wafer stage (not shown) such that a top surface of the light sensitive layer 22 is precisely located in an image plane of the projection objective 20. The mask 16 is positioned via a mask stage (not shown) in an object plane of the projection objective 20. Since the latter has a magnification with ||<1, a minified image 18 of the pattern 18 within the illumination field 14 is projected onto the light sensitive layer 22.

(35) During the projection the mask 16 and the substrate 24 move along a scan direction which corresponds to the Y direction indicated in FIG. 1. The illumination field 14 then scans over the mask 16 so that patterned areas larger than the illumination field 14 can be continuously imaged. The ratio between the velocities of the substrate 24 and the mask 16 is equal to the magnification of the projection objective 20. If the projection objective 20 does not invert the image (>0), the mask 16 and the substrate 24 move along the same direction, as this is indicated in FIG. 1 by arrows A1 and A2. However, the present disclosure may also be used in stepper tools in which the mask 16 and the substrate 24 do not move during projection of the mask.

II. Field Dependent Angular Irradiance Distribution

(36) FIG. 2A is an enlarged perspective view of the mask 16 containing another exemplary pattern 18. For the sake of simplicity it is assumed that the pattern 18 is uniform, i.e. it includes only identical features 19 that extend along the Y direction and are spaced apart by the same distance. It is further assumed that these features 19 are best imaged on the light sensitive layer 22 with an X dipole illumination setting.

(37) An exit pupil 26a associated with one of three exemplary light bundles is represented by a circle. The light bundle converges towards a field point that is located at a certain X position of the illumination field 14 at a first time during a scan cycle. In the exit pupil 26a two poles 27a and 27a, which are spaced apart along the X direction, represent directions from which projection light PL propagates towards this field point. The light energies concentrated in each pole 27a, 27a are assumed to be equal. Thus the projection light impinging from the +X direction has the same energy as the projection light PL impinging from the X direction. Since the features 19 are assumed to be uniformly distributed over the pattern 18, this X dipole illumination setting should be produced at each field point on the mask 16.

(38) Another exit pupil denoted by 26b is associated with a light bundle that converges towards a field point being located at another X position of the illumination field 14 at a later time of the scan cycle. The light energies concentrated in each pole 27b, 27 are again equal. However, as it becomes clear by a comparison with the ideal pole 27a, the pole 27b is slightly displaced. This means that the field point receives the same amount of projection light PL, but the directions from which the projection light PL impinges on the field point are not ideal for imaging the features 19 on the light sensitive layer 22.

(39) A further exit pupil denoted by 26c is associated with a point in the illumination field 14 that is located at still another X position at a still later time of the scan cycle. Here it is assumed that the directions from which the projection light PL impinges on the field point are again ideal for imaging the features 19. Therefore the poles 27c, 27c have the ideal position, and consequently also the light cones associated with these poles 27c, 27c have the same cone angle and orientation as the cones associated with the ideal exit pupil 26a. However, the poles 27c, 27c are not balanced, i.e. the light energies concentrated in the poles 27c, 27c differ from one another, as this is represented in FIG. 2A by different grey shadings. Thus the projection light PL impinging from the +X direction has less to energy than the projection light PL impinging from the X direction.

(40) From the foregoing it becomes clear that the ideal angular irradiance distribution represented by the exit pupil 26a is not obtained at each X position in the illumination field 14. The angular irradiance distribution is therefore field-dependent, i.e. at different field points the angular irradiance distribution is different.

(41) A field dependence may not only occur along the X direction, but also along the Y direction within the illumination field 14. Then one point on the mask 16 experiences different angular irradiance distributions while it passes through the illumination field 14 during a scan cycle. If a field dependence along the Y direction (i.e. the scan direction) occurs, it has to be taken into account that the total effect for a particular field point is obtained by integrating the different angular irradiance distributions.

(42) There is a wide variety of further field-dependent deviations of a real angular irradiance distribution from the ideal one. For example, the poles in the exit pupil associated with some field points may be deformed, blurred or may not have a desired non-uniform irradiance distribution.

(43) If field dependent deviations from the ideal angular irradiance distribution occur, this generally has a negative impact on the quality of the pattern image that is formed on the light sensitive layer 22. In particular, the dimensions of the structures that are produced with the help of the apparatus 10 may vary inadvertently, and this may compromise the function of the devices containing these structures. Therefore it is generally desired to eliminate any field dependence of the illumination setting in the illumination field 14. To this end the illumination system 12 has to be capable to influence the angular irradiance distribution in a field dependent manner. Since some of the causes for undesired field dependences of the angular irradiance distribution may change very rapidly, it is usually desirable to change the field dependence very rapidly, toosometimes even within a single scan cycle.

(44) Occasionally, however, it is desirable to deliberately introduce a field dependence of the angular irradiance distribution. This may be expedient, for example, if the projection objective 20 or the mask 16 have field depending properties that affect the image of the pattern 18 on the light sensitive layer 22. Variations of the imaging properties of the projection objective 20 may occur as a result of manufacturing tolerances, aging phenomena or non-uniform temperature distributions, for example. A field dependence of the mask 16 often occurs as a result of features that have different orientations or dimensions, for example.

(45) This is illustrated in FIG. 2B which is an enlarged perspective view of the mask 16 similar to FIG. 2A. For the sake of simplicity, it is assumed that the pattern 18 on the mask 16 includes three first identical pattern areas 181a, 181b, 181c which are arranged one behind the other along the scan direction Y. It is further assumed that the features 19 of the first pattern areas 181a, 181b, 181c are straight lines extending along the scan direction Y.

(46) The pattern 18 further includes three identical second pattern areas 182a, 182b, 182c which are also arranged one behind the other along the scan direction Y, but laterally displaced from the first pattern areas 181a, 181b, 181c so that the first pattern areas 181a, 181b, 181c and the second pattern areas 182a, 182b, 182c have no common X coordinate. It is assumed that the second pattern areas 182a, 182b, 182c contain features 19 extending along the X direction and features 19 extending along the Y direction.

(47) The mask 16 is assumed to be used in a manufacturing step in which two different dies are exposed simultaneously and will be subjected to the same subsequent manufacturing steps such as etching. The dies are small enough so that they can be positioned next to each other within the image field of the projection objective 20. Within one complete scanning cycle three dies of a first type associated with the first pattern areas 181a, 181b, 181c, and three dies of a second type associated with the second pattern areas 182a, 182b, 182c can be exposed. Then the scanning direction is either reversed, or the mask 16 is returned to its original position without any illumination, and a further scanning cycle is performed. In this way two rows of different dies can be exposed simultaneously on the substrate 24.

(48) Generally different patterns involve different angular irradiance distributions at mask level if an optimum image quality is desired. In this embodiment it is assumed that the features 19 extending along the Y direction are best imaged on the light sensitive layer 22 with an X dipole illumination setting. In FIG. 2B the pupil 261 associated with a light bundle that converges towards a field point located in one of the first pattern areas is indicated by a broken circle. In the pupil 261 two poles 271, 271, which are spaced apart along the X direction, represent directions from which light propagates towards the field point. Since the patterns are assumed to be uniform over the first pattern areas 181a, 181b, 181c, it is thus desirable for this X dipole illumination setting to be produced at each field point in the first pattern areas 181a, 181b, 181c.

(49) For the features 19 in the second pattern areas 182a, 182b, 182c it is assumed that a conventional illumination setting results in the best image quality. FIG. 2B indicates a central pole 272 which is illuminated in the pupil 262 associated with a light bundle that converges towards a field point in one of the second pattern areas 182a, 182b, 182c. Again, this conventional illumination setting shall be produced at each field point in the second pattern areas 182a, 182b, 182c.

(50) This implies that the illumination system 12 is desirably capable of producing two different illumination settings simultaneously and side by side within the illumination field 14.

(51) In the following a design of the illumination system 12 which is capable of produce angular irradiance distributions with almost any arbitrary field dependency will be described in more detail with reference to FIGS. 3 to 15.

III. General Construction of Illumination System

(52) FIG. 3 is a meridional section through the illumination system 12 shown in FIG. 1. For the sake of clarity, the illustration of FIG. 3 is considerably simplified and not to scale. This particularly implies that different optical units are represented by one or very few optical elements only. In reality, these units may include significantly more lenses and other optical elements.

(53) In the embodiment shown, the projection light PL emitted by the light source 11 enters a beam expansion unit 32 which outputs an expanded and almost collimated light beam 34. To this end the beam expansion unit 32 may include several lenses or may be realized as a mirror arrangement, for example.

(54) The projection light beam 34 then enters a pupil forming unit 36 that is used to produce variable spatial irradiance distributions in a subsequent plane. To this end the pupil forming unit 36 includes a first mirror array 38 of very small mirrors 40 that can be tilted individually about two orthogonal axes with the help of actuators. FIG. 4 is a perspective view of the first mirror array 38 illustrating how two parallel light beams 42, 44 are reflected into different directions depending on the tilting angles of the mirrors 40 on which the light beams 42, 44 impinge. In FIGS. 3 and 4 the first mirror array 38 includes only 66 mirrors 40; in reality the first mirror array 38 may include several hundreds or even several thousands mirrors 40.

(55) The pupil forming unit 36 further includes a prism 46 having a first plane surface 48a and a second plane surface 48b that are both inclined with respect to an optical axis OA of the illumination system 12. At these inclined surfaces 48a, 48b impinging light is reflected by total internal reflection. The first surface 48a reflects the impinging light towards the mirrors 40 of the first mirror array 38, and the second surface 48b directs the light reflected from the mirrors 40 towards an exit surface 49 of the prism 46. The angular irradiance distribution of the light emerging from the exit surface 49 can thus be varied by individually tilting the mirrors 40 of the first mirror array 38. More details with regard to the pupil forming unit 36 can be gleaned from US 2009/0116093 A1.

(56) The angular irradiance distribution produced by the pupil forming unit 36 is transformed into a spatial irradiance distribution with the help of a first condenser 50. The condenser 50, which may be dispensed with in other embodiments, directs the impinging light towards a digital spatial light modulator 52 that is configured to reflect impinging light in a spatially resolved manner. To this end the digital spatial light modulator 52 includes a second mirror array 54 of micromirrors 56 that are arranged in a modulation plane 57 and can be seen best in the enlarged cut-out C of FIG. 3 and the enlarged cut-out C of FIG. 5. In contrast to the mirrors 40 of the first mirror array 38, however, each micromirror 56 of the second mirror array 54 has only two stable operating states, namely an on state, in which it directs impinging light via a first objective 58 towards an optical integrator 60, and an off state, in which it directs impinging towards a light absorbing surface 62.

(57) The second mirror array 54 may be realized as a digital mirror device (DMD), as they are commonly used in beamers, for example. Such devices may include up to several million micromirrors that can be switched between the two operating states many thousands times per second.

(58) Similar to the pupil forming unit 36, the spatial light modulator 52 further includes a prism 64 having an entrance surface 65 that is arranged perpendicular to the optical axis OA and a first plane surface 66a and a second plane surface 66b that are both inclined with respect to the optical axis OA of the illumination system 12. At these inclined surfaces 66a, 66b impinging light is reflected by total internal reflection. The first surface 66a reflects the impinging light towards the micromirrors 56 of the second mirror array 54, and the second surface 66b directs the light reflected from the micromirrors 56 towards a surface 68 of the prism 64.

(59) If all micromirrors 56 of the second mirror array 54 are in their on state, the second mirror array 54 has substantially the effect of a plane beam folding mirror. However, if one or more micromirrors 56 are switched to their off state, the spatial irradiance distribution of the light emerging from the modulation plane 57 is modified. This can be used, in a manner that will be explained in more detail further below (cf. section IV), to produce a field dependent modification of the angular irradiance distribution on the mask 16.

(60) As it already has been mentioned above, the light emerging from the prism 64 passes through the first objective 58 and impinges on the optical integrator 60. Since the light passing through the first objective 58 is almost collimated, the first objective 58 may have a very low numerical aperture (for example 0.01 or even below) and thus can be realized with a few small spherical lenses. The first objective 58 images the modulation plane 57 of the spatial light modulator 52 onto the optical integrator 60.

(61) The optical integrator 60 includes, in the embodiment shown, a first array 70 and a second array 72 of optical raster elements 74. FIG. 6 is a perspective view of the two arrays 70, 72. Each array 70, 72 includes, on each side of a support plate, a parallel array of cylinder lenses extending along the X and the Y direction, respectively. The volumes where two cylinder lenses cross form optical raster elements 74. Thus each optical raster element 74 may be regarded as a microlens having cylindrically curved surfaces. The use of cylinder lenses is advantageous particularly in those cases in which the refractive power of the optical raster elements 74 shall be different along the X and the Y direction. A different refractive power is involved if the square irradiance distribution on the optical integrator 60 shall be transformed into a slit-shaped illumination field 14, as this is usually the case. The surface of the optical raster elements 74 pointing towards the spatial light modulator 52 will be referred to in the following as light entrance facet 75.

(62) The optical raster elements 74 of the first and second array 70, 72 respectively, are arranged one behind the other in such a way that one optical raster element 74 of the first array 70 is associated in a one to one correspondence with one optical raster element 74 of the second array 72. The two optical raster elements 74, which are associated with each other, are aligned along a common axis and define an optical channel. Within the optical integrator 60 a light beam which propagates in one optical channel does not cross or superimpose with light beams propagating in other optical channels. Thus the optical channels associated with the optical raster elements 74 are optically isolated from each other.

(63) In this embodiment a pupil plane 76 of the illumination system 12 is located behind the second array 72; however, it may equally be arranged in front of it. A second condenser 78 establishes a Fourier relationship between the pupil plane 76 and a field stop plane 80 in which an adjustable field stop 82 is arranged.

(64) The field stop plane 80 is optically conjugated to a raster field plane 84 which is located within or in close proximity to the light entrance facets 75 of the optical integrator 60. This means that each light entrance facet 75 in the raster field plane 84 is imaged onto the entire field stop plane 80 by the associated optical raster element 74 of the second array 72 and the second condenser 78. The images of the irradiance distribution on the light entrance facet 75 within all optical channels superimpose in the field stop plane 80, which results in its very uniform illumination of the mask 16. Another way of describing the uniform illumination of the mask 16 is based on the irradiance distribution which is produced by each optical channel in the pupil plane 76. This irradiance distribution is often referred to as secondary light source. All secondary light sources commonly illuminate the field stop plane 80 with projection light from different directions. If a secondary light source is dark, no light impinges on the mask 16 from a (small) range of directions that is associated with this particular light source. Thus it is possible to set the desired angular irradiance distribution on the mask 16 by simply switching on and off the secondary light sources formed in the pupil plane 76. This is accomplished by changing the irradiance distribution on the optical integrator 60 with the help of the pupil forming unit 36.

(65) The field stop plane 80 is imaged by a second objective 86 onto a mask plane 88 in which the mask 16 is arranged with the help of a mask stage (not shown). The adjustable field stop 82 is also imaged on the mask plane 88 and defines at least the short lateral sides of the illumination field 14 extending along the scan direction Y.

(66) The pupil forming unit 36 and the spatial light modulator 52 are connected to a control unit 90 which is, in turn, connected to an overall system control 92 illustrated as a personal computer. The control unit 90 is configured to control the mirrors 40 of the pupil forming unit 36 and the micromirrors 56 of the spatial light modulator 52 in such a manner that the angular irradiance distribution in the mask plane 88 has the intended field dependency. In the following section IV the function and control of the illumination system 12 will be described.

(67) The illumination system further includes a measuring device 200 that is configured to measure the position of the second mirror array 54 and the orientation of its micromirrors 56. In the embodiment shown the measuring device 200 includes two pupil measuring units 202a, 202b that are rigidly connected by a first connecting structure 203 and are configured to measure the angular irradiance distribution simultaneously at two different field point in the mask plane 88.

(68) Additionally, or preferably alternatively, the measuring device 200 includes two irradiance measuring units 204a, 204b that are also rigidly connected by a second connecting structure 205 and are configured to measure the irradiance simultaneously at two different field point in the mask plane 88.

(69) The function of the pupil measuring units 202a, 202b and of the irradiance measuring units 204a, 204b will be explained in more detail further below with reference to FIGS. 16 to 21.

IV. Function and Control of the Illumination System

(70) 1. Pupil Forming

(71) FIG. 7 schematically illustrates how the pupil forming unit 36 produces an irradiance distribution on the micromirrors 56 of the spatial light modulator 52. For the sake of simplicity the prisms 46, 64 are not shown.

(72) Each mirror 40 of the first mirror array 38 is configured to illuminate a spot 94 on the modulation plane 57 of the spatial light modulator 52 at a position that is variable by changing a deflection angle produced by the respective mirror 40. Thus the spots 94 can be freely moved over the modulation plane 57 by tilting the mirrors 40 around their tilt axes. In this way it is possible to produce a wide variety of different irradiance distributions on the modulation plane 57. The spots 94 may also partly or completely overlap, as this is shown at 95. Then also graded irradiance distributions may be produced.

(73) FIG. 8 is a perspective view, similar to FIG. 5, on the second mirror array 54 contained in the spatial light modulator 52. Here it is assumed that the pupil forming unit 36 has produced an irradiance distribution on the second mirror array 54 that consists of two square poles 27 each extending exactly over 66 micromirrors 56. The poles 27 are arranged point-symmetrically along the X direction.

(74) The objective 58 forms an image of this irradiance distribution on the light entrance facets 75 of the optical integrator 60, as this is shown in FIG. 9. Here it is assumed that all micromirrors 56 are in the on-state so that the irradiance distribution formed on the second mirror array 54 is identically reproduced (apart from a possible scaling due to a magnification of the objective 58) on the light entrance facets 75 of the optical integrator 60. The regular grid shown on the light entrance facets 75 represent an image of the borderlines of the micromirrors 56, but this image does not appear outside the poles 27 and is shown in FIG. 9 for illustrative reasons only.

(75) 2. Field Dependency

(76) Since the light entrance facets 75 are located in the raster field plane 84, the irradiance distribution on the light entrance facets 75 is imaged, via the optical raster elements 74 of the second array 72 and the second condenser 78, on the field stop plane 80.

(77) This will now be explained with reference to FIG. 10 which is an enlarged and not to scale cut-out from FIG. 3. Here only two pairs of optical raster elements 74 of the optical integrator 60, the second condenser 78 and the intermediate field stop plane 80 are shown schematically.

(78) Two optical raster elements 74 that are associated with a single optical channel are referred to in the following as first microlens 101 and second microlens 102. The microlenses 101, 102 are sometimes referred to as field and pupil honeycomb lenses. Each pair of microlenses 101, 102 associated with a particular optical channel produces a secondary light source 106 in the pupil plane 76. In the upper half of FIG. 10 it is assumed that converging light bundles L1a, L2a and L3a illustrated with solid, dotted and broken lines, respectively, impinge on different points of the light entrance facet 75 of the first microlens 101. After having passed the two microlenses 101, 102 and the condenser 78, each light bundle L1a, L2a and L3a converges to a focal point F1, F2 and F3, respectively. From the upper half of FIG. 10 it becomes clear that points, where light rays impinge on the light entrance facet 75, and points where these light rays pass the field stop plane 80 (or any other conjugated field plane), are optically conjugate.

(79) The lower half of FIG. 10 illustrates the case when collimated light bundles L1b, L2b and L3b impinge on different regions of the light entrance facet 75 of the first microlens 101. This is the more realistic case because the light impinging on the optical integrator 60 is usually substantially collimated. The light bundles L1b, L2b and L3b are focused in a common focal point F located in the second microlens 102 and then pass, now collimated again, the field stop plane 80. Again it can be seen that, as a result of the optical conjugation, the region where a light bundle L1b, L2b and L3b impinges on the light entrance facet 75 corresponds to the region which is illuminated in the field stop plane 80. As a matter of course, these considerations apply separately for the X and the Y direction if the microlenses 101, 102 have refractive power both along the X and Y direction.

(80) Therefore each point on a light entrance facet 75 directly corresponds to a conjugated point in the intermediate field stop plane 80 (and hence in the illumination field 14 on the mask 16). If it is possible to selectively influence the irradiance on a point on a light entrance facet 75, it is thus possible to influence the irradiance of a light ray that impinges on the conjugated point in the illumination field 14 from a direction that depends on the position of the light entrance facet 75 with respect to the optical axis OA of the illumination system. The larger the distance between the light entrance facet 75 from the optical axis OA is, the larger is the angle under which the light ray impinges on the point on the mask 16.

(81) 3. Modifying Irradiance on Light Entrance Facets

(82) In the illumination system 12 the spatial light modulator 52 is used to modify the irradiance on points on the light entrance facets 75. In FIG. 9 it can be seen that each pole 27 extends over a plurality of small areas that are images of the micromirrors 56. If a micromirror is brought into an off state, the conjugated area on the light entrance facet 75 will not be illuminated, and consequently no projection light will impinge on a conjugated area on the mask from the (small) range of directions that is associated with this particular light entrance facet 75.

(83) This will be explained in more detail with reference to FIGS. 11A and 11B which are top views on the micromirrors 56 of the spatial light modulator 52 and on the light entrance facets 75 of the optical integrator 60, respectively.

(84) The thick dotted lines on the second mirror array 54 divide its modulation plane 57 into a plurality of object areas 110 each including 33 micromirrors 56. The objective 58 forms an image of each object area 110 on the optical integrator 60. This image will be referred to in the following as image area 110. Each image area 110 completely coincides with a light entrance facet 75, i.e. the image areas 110 have the same shape, size and orientation as the light entrance facets 75 and are completely superimposed on the latter. Since each object area 110 includes 33 micromirrors 56, the image areas 110 also include 33 images 56 of micromirrors 56.

(85) In FIG. 11A there are eight object areas 110 that are completely illuminated by the pupil forming unit 36 with projection light. These eight object areas 110 form the two poles 27. It can be seen that in some of the object areas 110 one, two or more micromirrors 56d represented as black squares have been controlled by the control unit 90 such that they are in an off-state in which impinging projection light is not directed towards the objective 58, but towards the light absorbing surface 62. By switching micromirrors between the on and the off state it is thus possible to variably prevent projection light from impinging on corresponding regions within the image areas 110 on the light entrance facets 75, as this is shown in FIG. 11B. These regions will be referred to in the following as dark spots 56d.

(86) As has been explained above with reference to FIG. 10, the irradiance distribution on the light entrance facets 75 is imaged on the field stop plane 80. If a light entrance facet 75 contains one or more dark spots 56d, as this is illustrated in the upper portion of FIG. 12, the irradiance distribution produced in the mask plane 88 by the associated optical channel will have dark spots at certain X positions, too. If a point on a mask passes through the illumination field 14, the total scan integrated irradiance will thus depend on the X position of the point in the illumination field 14, as this is shown in the graph of FIG. 13. Points in the middle of the illumination field 14 will experience the highest scan integrated irradiance, because they do not pass through dark spots, and points at the longitudinal ends of the illumination field 14 will receive total irradiances that are reduced to different extents. Thus the field dependency of the angular irradiance distribution on the mask 16 and also the spatial irradiance distribution can be modified by selectively bringing one or more micromirrors 56 of the spatial light modulator 52 from an on-state into the off-state.

(87) In a foregoing it has to be assumed that each object area 110, which is imaged on one of the light entrance facets 75, contains only 33 micromirrors 56. Thus the resolution along the cross-scan direction X that can be used to modify the field dependency of the angular irradiance distribution is relatively coarse. If the number of micromirrors 56 within each object area 110 is increased, this resolution can be improved.

(88) FIG. 14 illustrates a top view on one of the light entrance facets 75 for an embodiment in which 2020 micromirrors 56 are contained in each object area 110. Then more complicated scan integrated irradiance distributions along the X direction can be achieved on the mask 16, as this is illustrated in the graph shown in FIG. 15.

V. Measuring the Position and Orientation of the Micromirrors

(89) The ability of the illumination system 12 to produce an angular irradiance distribution in a field dependent manner crucially depends on an accurate imaging of the second mirror array 54 on the optical integrator 60 in the manner illustrated in FIGS. 11A and 11B. As expressed further above, the image areas 110 have to cover exactly one light entrance facet 75 of the optical integrator 60. Otherwise the field dependency of the angular irradiance distribution obtained in the mask plane 88 will deviate significantly from what is intended. For example, if a micromirror 56 is imaged not on the intended, but on the adjoining light entrance facet 75, it will finally be imaged on the opposite side of the illumination field 14. Thus a minute shift of an image area 110 may translate into a shift of light energy to a very distant portion of the illumination field 14.

(90) If the second mirror array 54 of the digital spatial light modulator 52 performs, during the operation of the illumination system 12, minute drift movements relative to the optical integrator 60, the proper imaging illustrated in FIGS. 11A and 11B will usually be disturbed. For example, if the entire mirror array 54 shown in FIG. 11A drifts along the +X direction by a distance d which is assumed to be equal to the width of one micromirror 56 (see FIG. 11A), the poles 27 illuminated on the mirror array 54 will cover one or more wrong micromirrors 56, for example micromirrors that are in the off state.

(91) Similar considerations also apply if a micromirror 56 does not reflect the impinging reflection light into the desired direction. This may happen if the micromirror 56 is stuck in one of the tilting positions, for example, or if its reflective coating forming the mirror surface has been damaged by the high energy projection light.

(92) In the following different approaches will be described how the position and/or the orientation of the micromirrors 56 can be efficiently measured.

(93) 1. Pupil Measurement

(94) FIG. 16 is an enlarged schematic cutout from the bottom portion of the illumination system 12. The illumination system 12 is shown in a measuring state in which the mask 16 has been removed and the two pupil measuring units 202a, 202b of the measuring device 200 shown in FIG. 3 have been moved into a measuring position with the help of a drive (not shown).

(95) Each pupil measuring unit 202a, 202b includes a housing 208a, 208b having a cover that is formed by a diaphragm 210a, 210b that has a central pinhole 212a, 212b. The pinholes 212a, 212b are arranged exactly in the mask plane 88 of the illumination system 12.

(96) Each housing 208a, 208b contains a condenser 214a, 214b having a focal plane that coincides with the mask plane 88. Thus light entering the housing 208a, 208b through the pinhole 212a, 212b leaves the condenser 214a, 214b as collimated light, i.e. light that propagates parallel to the optical axis 216a, 216b of the pupil measurement units 202a, 202b.

(97) The collimated light impinges on a position resolving light sensor 218a, 218b that may be formed by a CCD chip, for example. Since the angles of incidence of the projection light rays 220a, 220b in the mask plane 88 translate into positions in the Fourier related plane of the position resolving light sensor 218a, 218b, the latter effectively detects the angular irradiance distribution of the projection light rays 220a, 220b at the position of the pin hole 212a, 212b.

(98) The position resolving light sensors 218a, 218b are connected to the control unit 90 (see FIG. 3) which computes, from the spatial irradiance distribution measured in the plane of the position resolving light sensors 218a, 218b, the angular irradiance distribution of the projection light rays 220a, 220b in the mask plane 88. Since two pupil measuring units 202a, 202b are provided, it is possible to measure the angular irradiance distribution in the mask plane 88 simultaneously at two different field positions. By accurately displacing the two pupil measuring units 202a, 202b, which are connected by the connecting structure 203, parallel to the mask plane 88, it is possible to quickly measure the angular irradiance distribution at a large number of different field points in a short time. As a matter of course, also more than two, for example nine pupil measuring units may be combined so that a correspondingly large number of measurements can be performed simultaneously. This reduces the time that to measure the angular irradiance distributions at a number of field points being equal to the number of micromirrors 56 in each one object area 110 that is completely imaged on one light entrance facet 75.

(99) In the following it will be described with reference to FIGS. 17 and 18A-18B how the pupil measuring unit 202a, 202b can be used to measure the position and the orientation of the mirror surfaces of the micromirrors 56.

(100) FIG. 17 shows on the left hand side 4 object areas 110-1 to 110-4 of the spatial light modulator 52 that are delimited by triple lines for the sake of illustration. The first object area 110-1 includes 33 mirror surfaces 224-1-1 to 224-1-9, the second object area 110-2 33 mirror surfaces 224-2-1 to 224-2-9, and so on. Here it is assumed that the micromirrors 56 are controlled such that a chessboard-like light pattern is produced in a modulation surface 222 that is formed by all mirror surfaces 224. The light pattern produced in each object area 110-1 to 110-4 is imaged on one light entrance facet 75 of the optical integrator 60. For the sake of simplicity it is assumed that the total number of light entrance facets 75 is only 4; in reality this number may be much larger, for example between 10.sup.2 and 10.sup.6.

(101) At the right hand side of FIG. 17 the illumination field 14 (here assumed to have a square shape) is schematically illustrated. Because each object area 100 includes 33 mirror surfaces 224 that are imaged on one light entrance facet 75, and because the latter is then imaged on the entire illumination field 14, 9 image areas 226-1 to 226-9 are formed in the illumination field 14. Since the images of the light entrance facets 75 superimpose in the mask plane 88, at each image area 226-1 to 226-9 images of 4 mirror surfaces superimpose. For example, at the first image area 226-1 the images of the mirror surfaces 224-1-1, 224-2-1, 224-3-1 and 224-4-1 superimpose. Similarly, only the mirror surfaces 224-1-2, 224-2-2, 224-3-2 and 224-4-2 of each object area 110-1 to 110-4 contribute to the illumination of the second image area 226-2, and so on.

(102) In the middle portion of FIG. 17 the pupils P1 and P2 of light bundles that converge to a point in the image areas 226-1 and 226-2, respectively, are schematically illustrated. The pupil of each light bundle is determined by the irradiance distribution on the optical integrator 60 that is produced by the spatial light modulator 52. The irradiance distribution on the optical integrator 60 is an image of the chessboard-like pattern shown on the left hand side of FIG. 17.

(103) On the left hand side of FIG. 17 it can be seen that from the four mirror surfaces 224-1-1, 224-2-1, 224-3-1 and 224-4-1 that are imaged on the first image area 226-1, the two mirror surfaces 224-1-1 and 224-4-1 are in an off-state illustrated by black color, and two mirror surfaces 224-2-1 and 224-3-1 are in an on state which is illustrated by white color. Thus only the two mirror surfaces 224-2-1 and 224-3-1, from which arrows indicated with dotted lines start, in fact illuminate the first image area 226-1. In the pupil P1 of the light bundles that form the first image area 226-1, the images of these two illuminated mirror surfaces 224-2-1 and 224-3-1 on the optical integrator 60 can be seen.

(104) Thus, if one of the pupil measuring units 202a, 202b is positioned underneath the illumination system 12 so that its pinhole 212a, 212b is within the first image area 226-1, its position resolving light sensors 218a, 218b will detect an irradiance distribution which corresponds to the pupil P1 shown in the middle portion of FIG. 17. As a matter of course, the white lines in the pupil P1 are shown here only for illustrative purposes; in reality the pupil P1 will be completely dark with the exception of the two illuminated regions 224-2-1 and 224-3-1 that are images of the mirror surfaces 224-2-1 and 224-3-1.

(105) If the modulation surface 222 is laterally displaced as a result of a drift movement of the entire mirror array 54, also the illuminated regions 224-2-1 and 224-3-1 will be displaced in the pupil P1 that is measured by the position resolving light sensor 202a or 202b. Therefore it is possible to compute, on the basis of the position of the illuminated regions 224-2-1 and 224-3-1 that are detected on the position resolving light sensors 218a, 218b, where the mirror surfaces 224 of the spatial light modulator 52 are located. In this computation other quantities, in particular the shape of the mirror surfaces 224, the spatial arrangement of the modulation surface 222 relative to the light entrance facets 75, and the shape of the light entrance facets 75 usually have to be taken into account.

(106) Since drift movements between adjacent mirror surfaces 224 are unlikely to occur to an extent that is significant, usually it suffices to measure the irradiance distribution only at one field point in the mask plane 88. If drift movement occur between mirror surfaces 224 within a single object area 110-1 to 110-4, it might be desirable to perform the measurement of the angular irradiance distribution at a larger number of field points.

(107) The pupil P2 associated with light bundles that converge to a point in second image area 226-2 is also shown in the middle portion of FIG. 17. It can be seen that the location of the illuminated regions 224-1-2 and 224-4-2 have changed, because now only the mirror surfaces 224-1-2 and 224-4-2 contribute to the illumination of the second image area 226-2. If needed, the pupils associated with all 9 image areas 226-1 to 226-9 can be measured in this way.

(108) It is also possible to measure whether the mirror surfaces 224 contributing to the illumination of the image areas 226-to 226-9 have the desired orientation. For example, if the mirror surface 224-2-1 is assumed to be stuck in an off state, it would be dark, and correspondingly the illuminated region 224-2-1 in the first pupil P1 would be missing. Thus, with a measurement of the angular irradiance distribution at one field point within the first image area 226-1 it is possible to measure not only the position, but also the orientation of all four mirror surfaces 224-1-1, 224-2-1, 224-3-1 and 224-4-1 that are imaged on the first image area 226-1. If the orientation of all mirror surfaces 224 shall be measured, it is desirable to perform the measurement of the angular irradiance distribution for all nine image areas 226-1 to 226-9.

(109) From FIG. 17 it also becomes clear why the chessboard-like irradiance distribution produced by the pupil forming unit 36 on the modulation surface 222 is advantageous. Even if the images of individual mirror surfaces 224 on the optical integrator 60 are blurred due to diffraction and aberrations, the illuminated regions 224-2-1, 224-3-1, 224-1-2 and 224-4-2 are still spaced apart from each other by distances that ensure that they can be properly and accurately detected by the position resolving light sensors 218a, 218b.

(110) As a matter of course, also other light patterns may be formed on the modulation surface 222. Even if all mirror surfaces of the modulation surface 222 are in the on state, the illuminated regions in the pupils P1, P2 cannot overlap, but then the distance between these regions becomes somewhat smaller, as this is illustrated in FIG. 18A. Then in each pupil P1, P2 there are not only two, but four illuminated regions 224-1-1, 224-2-1, 224-3-1, 224-4-1 and 224-1-2, 224-2-2, 224-3-2, 224-4-2.

(111) In FIG. 18B it is assumed that one mirror surface 224-4-2 is damaged. This can be detected by measuring the angular irradiance distribution at a field point in the second image area 226-2. In the pupil P2 associated with this field point and measured by the position resolving light sensor 218a, 218b this can be detected by a black spot at a region 224-4-2 on which the damaged mirror surface 224-4-2 is imaged.

(112) 2. Field Measurement

(113) Instead of, or in addition to, the measurement of the angular irradiance distribution at one or more points in the illumination field 14, only the irradiance may be measured.

(114) FIG. 19 is the same illustration enlarged schematic cutout from the bottom portion of the illumination system 12 as FIG. 16. However, here the two pupil measuring units 202a, 202b have been brought into a parking position, and the two irradiance measuring units 204a, 204b briefly mentioned above with reference to FIG. 3 are now in their measuring position.

(115) Each irradiance measuring unit 204a, 204b includes a housing 229a, 229b including a diaphragm 230a, 230b that contains a small pinhole 232a, 232b. Each irradiance measuring unit 204a, 204b further includes a light sensor 234a, 234b that measures the irradiance of the light that has passed through the pinholes 232a, 232b. The light sensor 234a, 234b is not position resolving and thus integrates the irradiance for all angles of incidence. Therefore each light sensor 234a, 234b detects the total irradiance in the mask plane 88 at the position of the pinhole 232a, 232b.

(116) FIG. 20 illustrates how relative drift movements between the spatial light modulator 52 and the optical integrator 60 can be measured with the help of the irradiance measuring units 204a, 204b. It is assumed that the spatial light modulator 52 is controlled in such manner that an illuminated stripe 240 consisting of the image areas 226-1, 226-5 and 226-8 is produced in the illuminated field 14. This involves having the mirror surfaces 224 in the middle columns of all four object areas 110-1 to 110-4 are in the on state and all other mirror surfaces 224 are in the off state.

(117) The position of the stripe 240 is accurately measured with the help of the irradiance measuring units 204a, 204b. If the relative position between the spatial light modulator 52 and the optical integrator 60 changes, the position of the stripe 240 in the mask plane 88 changes correspondingly. Therefore, it is possible, by measuring a light pattern in the mask plane 88 with the help of the irradiance measuring unit 204a, 204b, to compute a change of the relative position between the spatial light modulator 52 and the optical integrator 60.

(118) If only the micromirrors of one object area are in the on state, it is possible to measure relative drift movements between the object areas 110-1 to 110-4.

(119) The irradiance measuring units 204a, 204b can also be used to measure the orientation of the mirror surfaces 224. This is illustrated in FIGS. 21A to 21D that show on the left hand side the light patterns produced on the modulation surface 222 and on the right hand side the irradiance distribution in the mask plane 88 that is measured by the irradiance measuring units 204a, 204b.

(120) In FIG. 21A it is illustrated how a particular irradiance distribution is produced on the object area 110-1 and thus on the light entrance facet 75 on which the object area 110-1 is imaged. All other light entrance facets 75 are not illuminated. Therefore the irradiance distribution measured in the illumination field 14 directly corresponds to the light pattern shown on the right hand side of FIG. 21A.

(121) The measurement may be performed in the following manner:

(122) One of the irradiance measuring units 204a, 204b is positioned such that its pinhole 232a, 232b is within the first image area 226-1. The mirror surface 224-1-1 is brought into the on state and all other mirror surfaces are brought into the off state, as this is shown in FIG. 21B. If the mirror surface 224-1-1 functions properly, the irradiance measuring unit 204a will detect the full irradiance. If the tilting angle is not correct, or the reflective coating is damaged, only a reduced amount of projection light will impinge on the irradiance measuring unit 204a, 204b.

(123) Then the mirror surface 224-1-1 is brought into the off state as well. No irradiance should now be detected by the irradiance measuring units 204a, 204b, as this is shown in FIG. 21C.

(124) In a next step, the mirror surface 224-2-1 from the adjoining object area 110-2 is brought into the on state. The irradiance measuring unit 204a, 204b, which is still at the same position as before, should now detect the full irradiance, because also the mirror surface 224-2-1 is imaged on the first image area 226-1. Then the mirror surface 224-2-1 is brought into the off state, and the irradiance measuring unit 204a, 20b should detect no light.

(125) These steps are repeated until the orientation of all four mirror surfaces 224 that are imaged on the first image area 226-1 has been measured. Then the irradiance measuring unit 204a, 204b moves to a next measuring point in the second image area 226-2. Now the four mirror surfaces 224-1-2, 224-2-2, 224-3-2 and 224-4-2, which are imaged on the second image area 226-12, are successively brought from an off state to an on state. Since the switching of the mirror surfaces 224 is quite fast, the measurements for each image area 226-1 to 226-9 can be completed very quickly. In total the irradiance measuring unit 204a, 204b has to be displaced N=9 times, wherein N is the number of mirror surfaces 224 that are imaged on one light entrance facet 75. Since the measuring device includes two irradiance measuring units 204a, 204b, the measuring time can be reduced by approximately 50%.

(126) 3. Marker Points

(127) Another approach for measuring the absolute position of the mirror array 54 of the spatial light modulator 52 involves the use of one or more marker points that are fixedly arranged directly on the spatial light modulator 52 or a structure that is rigidly fixed to the spatial light modulator 52.

(128) a) Marker Points on Back Side of Micromirror Array

(129) FIG. 22 is a schematic cross-section through an embodiment of a spatial light modulator 52 that includes three modulator units 52a, 52b and 52c. Each modulator unit 52a, 52b, 52c includes a support 152a, 152b, 152c that supports a digital micromirror array 54a, 54b, 54c. The micromirror arrays 54a, 54b, 54c are configured to deflect impinging projection light PL towards two different directions, namely a first direction along which the projection light travels towards the optical integrator 60, and a second direction along which the projection light travels towards the light absorbing surface 62, depending on whether the micromirrors 56 are in the on or in the off position.

(130) Even if the support plates 152a, 152b, 152c are connected to a common support structure (not shown), the support plates 152a, 152b, 152c may perform minute drift movements during operation of the illumination system 12. These drift movements may be caused by the thermal expansion of elements contained in the illumination system 12. As explained further above, such drift movements may severely compromise the field dependency of the angular irradiance distribution if no counter-measures are taken.

(131) The illumination system 12 therefore includes a measuring device 150 which includes, in the embodiment shown, three pairs of marker points 160a, 160b, 160c which are fixedly arranged on the back side of the support plates 152a, 152b, 152c, i.e. on the side which is opposite the micromirror array 54a, 54b, 54c. The measuring device 150 further includes a detector 162 which is configured to accurately detect the position of the marker points 160a, 160b, 160c. There is a wide variety of measuring principles that may be used to this end. For example, the detector 162 may emit a light beam that is reflected from the marker points 160a, 160b, 160c, and the distance information is encoded in the wavelength or the phase of the reflected light. Since position measuring systems are known in the art as such, they will not be described in further detail here.

(132) By measuring the position of each marker point 160a, 160b, 160c, the detector 162 can accurately detect even the slightest drift movements of the support plates 152a, 152b, 152c to which the micromirror arrays 54a, 54b, 54c are fixedly attached. In this manner the position of the micromirrors 56 of the micromirror arrays 54a, 54b, 54c can be commonly measured.

(133) b) Marker Points on Front Side

(134) FIG. 23 schematically illustrates an embodiment in which a common board 164 supports on its front side a plurality of micromirror arrays 54-1 to 54-9 and three marker points 166a to 166c. The marker points 166a to 166c are formed by laser diodes or similar light emitting elements and are part of the measuring device 150. The latter further includes three position resolving light detectors 168a, 168b, 168c that are attached to a front surface of the optical integrator 60 outside the circular region 169 on which projection light impinges.

(135) The arrangement of the three marker points 166a, 166b, 166c is imaged by the first objective 58, together with the micromirror arrays 54-1 to 54-9 of the spatial light modulator 52, on the optical integrator 60 so that the images of the marker points 166a, 166b, 166c lie within the position resolving light detectors 168a, 168b and 168c, respectively.

(136) If the relative position between the board 164 of the spatial light modulator 52 and the optical integrator 60 changes as a result of minute drift movements, the images of the marker points 166a, 166b, 166c move over the position resolving light detectors 168a, 168b, 168c. By carefully analyzing the position changes of the images of the marker points 166a, 166b, 166c on the position resolving light detectors 168a, 168b, 168c, it is possible to measure arbitrary movements of the board 164 in all six degrees of freedom.

(137) 4. Measuring Reflections and Scattered Light

(138) FIG. 24 illustrates a different approach for in measuring the position and also the orientation of the micromirrors 56. According to this approach, measuring light that is distinct from the projection light and has been reflected or scattered from the spatial light modulator 52 is detected.

(139) Similar to the embodiment shown in FIG. 23, the spatial light modulator 52 includes a board 164 that supports nine micromirror arrays 54-1 to 54-9 that are spaced apart by gaps and define the modulation surface 222. The latter is imaged by the first objective 58 on the optical integrator 60. Grey dotted lines represent the projection light PL that first impinges on the spatial light modulator 52 and is then reflected by those micromirrors 56 being in the on state towards the first objective 58 and the optical integrator 60.

(140) The measuring device 150 in this embodiment includes a measuring light source 170 which directs measuring light ML on the spatial light modulator 52. The measuring light ML is reflected, either from the micromirrors 56 that are in the on or from the micromirrors 56 that are in the off state, towards a camera 171 that is configured to observe the modulation surface 222 of the spatial light modulator 52. The camera 171 detects light that has been reflected or scattered by the micromirrors 56 or by structures in the gaps between the micromirrors 56. The camera 171 includes an objective 172 and a position resolving light detector 174 on which an image of the modulation surface 222 is formed.

(141) In order to avoid that the measuring light ML reaches the optical integrator 60, the projection light PL impinges on the spatial light modulator 52 basically parallel to an XZ plane, whereas the measuring light ML impinges on the spatial light modulator 52 basically parallel to an orthogonal YZ plane.

(142) From the location of the images of the micromirrors 56 on the position resolving light detector 174 it is possible to detect whether the board 164 that supports the micromirrors 56 has drifted, and also to quantitatively determine the amount of the drift movement. It is even possible to detect relative drift movements between adjacent micromirrors arrays 54.

(143) The camera 171 may also detect the orientation of the mirror surfaces of the micromirrors 56. Preferably the measuring light ML is reflected towards the camera 171 by micromirrors 56 that are either in the on or the off position. Even if the measuring light ML is not directly reflected towards the camera 171 in any of the orientations of the mirror surfaces, the position resolving light detector 172 may detect some of the light that has been scattered by the micromirrors 56 or by structures arranged on the board 164 between the mirror surfaces.

(144) It may also be envisaged to illuminate the spatial light modulators from different directions with measuring light ML. For example, the measuring light source 170 may be fixed on a curved rail so that it can be guided around the spatial light modulator 52, or a plurality of measuring light sources 170 may be arranged at different positions with regard to the spatial light modulator 52.

(145) 5. Compensation of Drift Movements

(146) In the foregoing it has been described in detail how relative drift movements between the spatial light modulator 52 and the optical integrator 60 can be measured. In the following it will be described how the control of the spatial light modulator 52 may be modified for compensating the adverse effects associated with the measured drift movements.

(147) FIG. 25A is a schematic top view similar to FIG. 11A on the modulation surface 222 of the micromirror array 54 of the spatial light modulator 52. Here it is assumed that the micromirror 56d shall be in an off state so that the image area 226-4 in the illumination field 14 receives less light from pole 27 than from pole 27, as this is shown in FIG. 25B.

(148) FIGS. 26A and 26B illustrate the same situation after the micromirror array 54 has been displaced as a result of a thermal drift movement by a distance d along the +X direction. The distance d is again assumed to be equal to the width of one micromirror 56.

(149) The position of the poles 27, 27 in absolute terms does not change, but relative to the micromirror array 54 the poles 27, 27 shift by the width of one micromirror 56 towards the X direction. Since the imaging on the optical integrator 60 is not affected by the shift of the micromirror array 54, different groups of micromirrors 56 will be imaged on the light entrance facets 75. The grid G represents the borderlines of the object areas 110 on the micromirror array 54. It can be seen that the micromirror 56d being in the off state is now centered in an object area 110. Consequently, the position in the illuminated field 14 where less projection light is received from the pole 27 than from the pole 27, has changed to the image area 226-5, as this is shown in FIG. 26B. Since the position of the poles 27a, 27b relative to the optical integrator 60 has not changed, the angular irradiance distribution is not affected otherwise.

(150) From FIG. 26A it becomes clear how an inadvertent change of the field dependency can be compensated. It is only desirable to control the micromirror array 54 such that not the micromirror 56d, but the micromirror 56d, which is the next micromirror adjoining the micromirror 56d along the X direction, is brought into the off state. Then the same situation is obtained as shown in FIGS. 25A and 25B.

(151) This simple example demonstrates that almost any arbitrary relative drift movements between the spatial light modulator 52 and the optical integrator 60 can be compensated for by an appropriate control of the micromirrors 56 of the spatial light modulator 52. Even if the spatial light modulator 52 is not displaced by a distance d or an integer multiple thereof, a modified control of the micromirrors 56 makes it usually possible to reduce adverse effects associated with drift movements to a minimum.

VI. EUV

(152) In the foregoing the disclosure has been described with reference to the projection exposure apparatus 10 which uses VUV projection light. However, it is also possible to use the concepts outlined above in EUV projection apparatus.

(153) WO 2009/100856 A1 describes an EUV illumination system which makes it possible to produce a desired field dependency of the irradiance and the angular irradiance distribution. Also in that case small mirrors have to be controlled individually in order to achieve the desired field dependencies.

VII. Important Method Steps

(154) Important method steps of the present disclosure will now be summarized with reference to the flow diagram shown in FIG. 27.

(155) In a first step S1 a spatial light modulator is provided that includes a modulation surface which is formed by a plurality of micromirrors. Each micromirror includes a mirror surface having an orientation that can be changed individually for each micromirror.

(156) In a second step S2 at least one parameter that is related to the mirror surface is measured.

(157) In a third step S3 the orientation of the micromirrors is controlled depending on the at least one parameter.

(158) In a fourth step S4 a light pattern on the modulation surface is produced.

(159) In a fifth step S5 an image of the light pattern is formed on an optical integrator that has a plurality of light entrance facets.

(160) In a sixth step S6 images of the light entrance facets are superimposed on a mask.