METHOD OF OPERATING SAME

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention generally relates to a microlithographic projection exposure apparatus, and in particular to such appara- tus having a projection objective that contains a wavefront correction device.

2. Description of Related Art

Microlithography (also called photolithography or simply lithography) is a technology for the fabrication of integrated circuits, liquid crystal displays and other microstructured devices. The process of microlithography, in conjunction with the process of etching, is used to pattern features in thin film stacks that have been formed on a substrate, for example a silicon wafer. At each layer of the fabrication, the wafer is first coated with a photoresist which is a material that is sensitive to radiation, such as deep ultraviolet (DUV) , vacuum ultraviolet (VUV) or extreme ultraviolet (EUV) light. Next, the wafer with the photoresist on top is exposed to projection light through a mask in a projection exposure ap- paratus. The mask contains a circuit pattern to be projected onto the photoresist. After exposure the photoresist is developed to produce an image corresponding to the circuit pattern contained in the mask. Then an etch process transfers the circuit pattern into the thin film stacks on the wafer.. Finally, the photoresist is removed. Repetition of this proc- ess with different masks results in a multi-layered micro- structured component.

A projection exposure apparatus typically includes an illumination system, a mask alignment stage for aligning the mask, a projection objective and a wafer alignment stage for aligning the wafer coated with the photoresist. The illumination system illuminates a field on the mask that may have the shape of a rectangular slit or a narrow ring segment, for example . In current projection exposure apparatus a distinction can be made between two different types of apparatus. In one type each target portion on the wafer is irradiated by exposing the entire mask pattern onto the target portion in one go; such an apparatus is commonly referred to as a wafer stepper. In the other type of apparatus, which is commonly referred to as a step-and-scan apparatus or simply scanner, each target portion is irradiated by progressively scanning the mask pattern under the projection light beam in a given reference direction while synchronously scanning the substrate parallel or anti-parallel to this direction. The ratio of the velocity of the wafer and the velocity of the mask is equal to the magnification β of the projection lens. A typical value for the magnification is β=±1/4. The present invention is only concerned with such step-and-scan apparatus. It is to be understood that the term "mask" (or reticle) is to be interpreted broadly as a patterning means. Commonly used masks contain transmissive or reflective patterns and may be of the binary, alternating phase-shift, attenuated phase-shift or various hybrid mask type, for example. One of the essential aims in the development of projection exposure apparatus is to be able to lithographically produce structures with smaller and smaller dimensions on the wafer. Small structures lead to high integration densities, which generally has a favorable effect on the performance of the microstructured components produced with the aid of such apparatus. Furthermore, the more devices can be produced on a single wafer, the higher is the throughput of the apparatus.

The size of the structures which can be generated depends primarily on the resolution of the projection objective being used. Since the resolution of projection objectives is inversely proportional to the wavelength of the projection light, one way of increasing the resolution is to use projection light with shorter and shorter wavelengths. The shortest wavelengths currently used are 248 nm, 193 nm or 157 nm and thus lie in the deep or vacuum ultraviolet spectral range. Also apparatus using EUV light having a wavelength of about 13 nm are meanwhile commercially available. Future apparatus will probably use EUV light having a wavelength as low as 6.9 nm.

The correction of image errors (i.e. aberrations) is becoming increasingly important for projection objectives with very high resolution. Different types of image errors usually re- quire different correction measures.

The correction of rotationally symmetric image errors is comparatively straightforward. An image error is referred to as being rotationally symmetric if it is invariant against a rotation of the optical system. Rotationally symmetric image errors can be corrected, for example, at least partially by moving individual optical elements along the optical axis.

Correction of those image errors which are not rotationally symmetric is more difficult. Such image errors occur, for example, because lenses and other optical elements heat up ro- tationally asymmetrically. One image error of this type is astigmatism. A major cause for rotationally asymmetric image errors is a rotationally asymmetric, in particular slit-shaped, illumination of the mask, as is typically encountered in projection exposure apparatus of the scanner type. The slit-shaped illu- minated field causes a non-uniform heating of those optical elements that are arranged in the vicinity of field planes. This heating results in deformations of the optical elements and, in the case of lenses and other elements of the refractive type, in changes of their refractive index. If the mate- rials of refractive optical elements are repeatedly exposed to the high energetic projection light, also permanent material changes are observed. For example, a compaction of the materials exposed to the projection light sometimes occurs, and this compaction results in local and permanent changes of the refractive index. In the case of mirrors the reflective multi-layer coatings may be damaged by the high local light intensities so that the reflectance is locally altered.

The heat induced deformations, index changes and coating damages alter the optical properties of the optical elements and thus cause image errors. Heat induced image errors sometimes have a twofold symmetry. However, image errors with other symmetries, for example threefold or fivefold, are also frequently observed in projection objectives.

Another major cause for rotationally asymmetric image errors are certain asymmetric illumination settings in which the pupil plane of the illumination system is illuminated in a rotationally asymmetric manner. Important examples for such settings are dipole settings in which only two poles are illuminated in the pupil plane. With such a dipole setting, also the pupil planes in the projection objective contain two strongly illuminated regions. Consequently, lenses or mirrors arranged in or in the vicinity of such an objective pupil plane are exposed to a rotationally asymmetric intensity distribution which gives rise to rotationally asymmetric image errors. Also quadrupole settings often produce rotationally asymmetric image errors, although to a lesser extent than di- pole settings.

In order to correct rotationally asymmetric image errors, US 6,338,823 Bl proposes a lens which can be selectively deformed with the aid of a plurality of actuators distributed along the circumference of the lens. The deformation of the lens is determined such that heat induced image errors are at least partially corrected. A more complex type of such a wavefront correction device is disclosed in US 2010/0128367 Al.

US 7,830,611 B2 discloses a similar wavefront correction device. In this device one surface of a deformable plate contacts an index matched liquid. If the plate is deformed, the deformation of the surface adjacent the liquid has virtually no optical effect. Thus this device makes it possible to obtain correcting contributions from the deformation not of two, but of only one optical surface. A partial compensation of the correction effect, as it is observed if two surfaces are deformed simultaneously, is thus prevented.

However, the deformation of optical elements with the help of actuators has also some drawbacks. If the actuators are arranged at the circumference of a plate or a lens, it is possible to produce only a restricted variety of deformations with the help of the actuators. This is due to the fact that both the number and also the arrangement of the actuators are fixed. In particular it is usually difficult or even impossible to produce deformations which may be described by higher order Zernike polynomials, such as Z ₁₉ , Z ₃₆ , Z ₄₀ or Z ₆ . US 2010/0201958 Al and US 2009/0257032 Al disclose a wave- front correction device that also comprises two refractive optical elements, for example glass plates, that are separated from each other by a liquid layer. However, in contrast to the device described in the aforementioned US 7,830,611 B2, a wavefront correction of light propagating through the refractive optical elements is not produced by deforming them, but by changing their refractive index locally. To this end one refractive optical element may be provided with heating wires that extend over the surface through which projection light passes. With the help of the heating wires a temperature distribution inside the refractive optical element can be produced that causes, via the dependency dn/dT of the refractive index n on the temperature T, the desired distribution of the refractive index. If this wavefront correction device is arranged in a pupil plane of the projection objective, the density of the heating wires may be higher at regions that are exposed to projection light. The liquid en- sures that the average temperatures of the optical elements are kept constant. Although even higher order wavefront errors can be corrected very well, this device has a complex structure and is therefore expensive.

WO 2011/116792 Al discloses a wavefront correction device in which a plurality of fluid flows emerging from outlet apertures enter a space through which projection light propagates during operation of the projection exposure apparatus. A temperature controller sets the temperature of the fluid flows individually for each fluid flow. The temperature distribu- tion is determined such that optical path length differences caused by the temperature distribution correct wavefront errors .

Unpublished international patent application PCT/EP2011/ 004859 (Zellner et al.) discloses a wavefront correction de- vice in which a plurality of heating light beams are directed towards a circumferential rim surface of a refractive optical element. After entering the refractive optical element, the heating light beams are partially absorbed inside the element. In this manner almost arbitrary temperature distribu- tions with steep temperature gradients can be produced inside the refractive optical element, but without a need to arrange heating wires in the projection light path that absorb, reflect, diffract and/or scatter projection light to an albeit small, but not negligible extent.

All wavefront correction devices using a plurality of heating paths - irrespective whether these are formed by heating wires, fluid flows or heating light beams - tend to be very complex and require very sophisticated control algorithms. Additionally, in particular if heating wires are used, light losses and an increased level of scattering are inevitable.

SUMMARY OF THE INVENTION

It is therefore an object of the present invention to provide a microlithographic projection exposure apparatus comprising a wavefront correction device which is capable of correcting even higher order wavefront deformations, but which has nevertheless a reduced complexity as compared to the prior art devices .

This object is achieved, in accordance with the present in- vention, by a microlithographic projection exposure apparatus comprising a mask stage that is configured to move a mask along a scan direction. A substrate stage is configured to move a substrate. A projection objective is configured to image the mask on the substrate. The apparatus is adapted to a scan operation in which the mask is moved along the scan direction while it is imaged on the substrate. The projection objective has an object plane in which the mask is supported by the mask stage, and an image plane in which the substrate is supported by the substrate stage. At least one pupil plane is arranged between the object plane and the image plane. A wavefront correction device is arranged between the object plane and the image plane, but outside the at least one pupil plane, and comprises a solid body having an optical surface on which projection light is incident during operation of the apparatus. Furthermore, the wavefront correction device has a plurality of heating paths along which heat can be individually generated. The heating paths are arranged inside a cor- rection volume of the solid body, through which volume projection light passes, with a varying non-zero heating path density. The heating path density is, along at least one, and preferably along a plurality or even along any arbitrary, line that is parallel to the scan direction, higher in a cen- ter of the correction volume than at a margin thereof.

The invention is based on the perception that in apparatus of the scanner type the irradiance distribution on the solid body, which may be a refractive optical element in the case of transparent devices or a mirror in the case of reflective devices, is, in the absence of the mask and assuming conventional illumination, rotationally symmetrical only if the solid body is arranged exactly in a pupil plane. However, the closer the wavefront correction device is arranged to a field plane, the more will the irradiance distribution on the solid body approximate the irradiance distribution which is produced on the mask by the illumination system, less a scaling factor. In an apparatus of the scanner type, the field illuminated on the mask has approximately the shape of a rectangular or curved slit. The irradiance usually has its maximum value in the center of the slit if seen along the scan direction, and drops off to zero towards at least one margin, but usually towards both margins. This irradiance profile is usually produced over the entire length of the slit.

Consequently, also the irradiance at the center of the cor- rection volume of the solid body, if seen along the scan direction, is higher than at one or usually both margins thereof. Since only a reduced amount of projection light is incident on portions close to the margins of the correction volume, the potential of these portions to correct wavefront deformations is reduced, too. Therefore the heating path density can be reduced at those portions without significantly affecting the device's ability to correct wavefront deformations . One way of increasing the heating path density may be to provide a sub-volume, which may be contiguous to the margin, of the correction volume in which all heating paths extend perpendicular to the scan direction, i.e. there are no heating paths extending along the scan direction. This makes use of the fact that stronger gradients of the temperature distribution are often required only along the scan direction, but not in a perpendicular direction. Then the wavefront correction device is still, if it is arranged sufficiently close to a field plane, capable to reduce fading effects occurring if the image slightly moves along the scan direction or perpendicular thereto so that images are superimposed at different locations or focus positions.

In some embodiments the heating path density is uniform along a direction that is perpendicular to the scan direction and a normal on the optical surface of the solid body. This ensures that any adverse effects associated with the heating paths, such as increased absorption or scattering, are at least field independent. In other words, different points in the image plane will than be exposed to the same amount of scat- tered or diffracted light.

In some embodiments the solid body is a refractive optical element through which projection light passes during opera- . tion of the apparatus. However, as mentioned above, the solid body may also be a mirror. In that case the temperature pro- file produced in the solid body does not modify the refractive index distribution, but the shape of the mirror substrate. Examples of such mirror substrates are disclosed in US 2010/0200777 Al, although heating paths are there provided only in order to produce a uniform temperature distribution. The present invention is therefore also applicable to EUV projection exposure apparatus, for example.

In some embodiments heating wires extend along the heating paths. In other embodiments the wavefront correction device comprises at least one heating light source which is configured to produce one or more heating light beams propagating through the solid body and defining a heating path during operation of the wavefront correction device. The heating path density may, along the at least one line, be at least two times, and preferably at least 100 times, higher in the center of the correction volume than at the margin.

Generally the apparatus may comprise an illumination system which is configured to produce on a portion of the mask an irradiance distribution which drops off, if seen along the scan direction, towards one of the margins.

If this irradiance distribution has, along a line parallel to the scan direction, its highest irradiance in a center of the distribution so that the irradiance drops off not only to- wards one, but towards both margins, the heating path density may, along the at least one line parallel to the scan direction, also be higher in a center of the correction volume than at both opposite margins thereof.

Subject of the present invention is also a method of operat- ing a microlithographic projection exposure apparatus comprising the following steps: a) providing a microlithographic projection exposure apparatus comprising a mask stage that is configured to move a mask along a scan direction, a substrate stage configured to move a substrate, and a projection objective configured to image the mask on the substrate. The apparatus is adapted to a scan operation in which the mask is moved along the scan direction while it is imaged on the substrate. The projection objective has an object plane in which the mask is supported by the mask stage, and an image plane in which the substrate is supported by the substrate stage. A pupil plane is arranged between the object plane and the image plane. A wavefront correction device is arranged either between the object plane and the pupil plane or between the pupil plane and the image plane and comprises a solid body having an optical surface on which projection light is incident during operation of the apparatus. The wavefront correction device further has a plurality of heating paths along which heat can be individually generated. The heating paths are arranged inside a correction volume of the solid body with a varying heating path density. The heating path density is, along a line parallel to the scan direction, higher in a center of the correction volume than at a margin thereof. The projection objective further comprises a manipulator that is configured to change the optical properties of an optical element contained in the projection obj ective; b) determining field dependent imaging aberrations, in particular by measurement and/or simulation; c) determining a first effect that modifies an optical wave- front and is produced by the wavefront correction device; d) determining a second effect that modifies the optical

wavefront and is produced by the manipulator, such that the superposition of the first effect and the second effect at least partially reduces the imaging aberrations determined in step b) ; e) controlling the wavefront correction device such that it produces the first effect; f) controlling the manipulator such that it produces the second effect.

This method is based on the perception that a simplified wavefront correction device having regions with a reduced heating path density may be capable only of modifying the wavefront deformations in such a manner that other manipulators are able to correct the residual wavefront deformation to such an extend that it approximates an ideal wavefront.

The manipulator may be configured to modify the magnifica- tion of the projection objective. For example, it has been found that the distortion produced by an astigmatic deformation of lenses in the vicinity of a field plane can effectively be corrected by changing the magnification and by producing a temperature distribution in the solid body hav- ing approximately a bell-shaped profile along the scan direction .

DEFINITIONS

The term "light" denotes any electromagnetic radiation, in particular visible light, UV, DUV and VUV and also EUV light.

The term "light ray" is used herein to denote light whose path of propagation can be described by a line.

The term "light beam" is used herein to denote a plurality of light rays. A light beam usually has an irradiance profile across its diameter that may vary along the propagation path.

The term "surface" is used herein to denote any planar or curved surface in the three-dimensional space. The surface may be part of a body or may be completely separated therefrom. The term "optically conjugate" is used herein to denote the imaging relationship between two points or two surfaces. Imaging relationship means that a light bundle emerging from a point converges at the optically conjugate point. The term "field plane" is used herein to denote a plane that is optically conjugate to the mask plane.

The term "pupil plane" is used herein to denote a plane in which all light rays, which converge or diverge under the same angle in a field plane, pass through the same point. As usual in the art, the term "pupil plane" is also used if it is in fact not a plane in the mathematical sense, but is slightly curved so that, in a strict sense, it should be referred to as pupil surface.

The term "wavefront correction" is used to denote any modifi- cation of a wavefront such that the wavefront is either a better approximation to an ideal undisturbed wavefront, or such that the wavefront can be more easily modified by other means such that it finally approximates better an ideal undisturbed wavefront. 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 drawings in which: FIG. 1 is a schematic perspective view of a projection exposure apparatus in accordance with the present invention;

FIG. 2 is schematic meridional section through the apparatus shown in FIG. 1; is a top view on a wavefront correction device according to a first embodiment that is contained in a projection objective being part of the apparatus shown in FIGS. 1 and 2; is a schematic cross section through the wavefront correction device shown in FIG. 3 along line IV-IV; is an enlarged top view on the illuminated region of the refractive optical element shown in FIG. 3 in which the arrangement of heating wires can be better discerned; illustrates the irradiance distribution inside the illuminated region on the refractive optical element; is an exaggerated perspective view of an astigmatic deformation of an optical surface in the vicinity to a field plane; exemplarily illustrates the distortion which is produced in the image plane by the deformed surface shown in FIG. 7; illustrates the effect of an increase of the magnification of the projection objective; illustrates the residual distortion resulting from a superposition of the distortions shown in FIGS. 8 and 9; is a top view on a wavefront correction device according to an alternative embodiment in which heating paths are formed by heating light beams;

FIG. 12 is a cross-section through the wavefront correction device shown in FIG. 11 along line XII-XII. DESCRIPTION OF PREFERRED EMBODIMENTS

I.

General Construction of Projection Exposure Apparatus

FIG. 1 is a perspective and highly simplified view of a pro- jection exposure apparatus 10 in accordance with the present invention. The apparatus 10 comprises an illumination system 12 which produces projection light. The latter illuminates a field 14 on a mask 16 containing a pattern 18 of fine features 19. In this embodiment the illuminated field 14 has a rectangular shape. However, other shapes of the illuminated field 14, for example ring segments, are contemplated as well .

A projection objective 20 having an optical axis OA and containing a plurality of lenses LI to L4 images the pattern 18 within the illuminated 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 lo- cated in an image plane of the projection objective 20. The mask 16 is positioned by means of a mask stage (not shown) in an object plane of the projection objective 20. Since the latter has a magnification β with |β|< 1, a reduced image 18' of the pattern 18 within the illuminated field 14 is pro- jected onto the light sensitive layer 22.

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 illuminated field 14 then scans over the mask 16 so that patterned areas larger than the illumi- nated field 14 can be continuously imaged. The ratio between the velocities of the substrate 24 and the mask 16 is equal to the magnification β of the projection objective 20. If the projection objective 20 does not invert the image (β >0) , the mask 16 and the substrate 24 move along the same direction, as this is indicated in FIG. 1 by arrows Al and A2. However, the present invention may also be used with catadioptric projection objectives 20 having off-axis object and image fields.

FIG. 2 is a schematic meridional section through the apparatus 10 shown in FIG. 1. In this section also a mask stage 26, which supports and moves the mask 16 in an object plane 28 of the projection objective 20, and a wafer stage 32, which sup- ports and moves the substrate 24 in an image plane 30 of the projection objective 20, are shown.

Inside the projection objective 20 two manipulators Ml and M2 are arranged that are configured to individually displace the lenses LI and L2, respectively, along an optical axis OA of the projection objective 20. This makes it possible to change the magnification β of the projection objective 20 within a certain range.

In this embodiment the projection objective 20 has an intermediate image plane 34. The image of the features 18 formed in the intermediate image plane may be substantially blurred and/or distorted as a result of various aberrations. In particular, the intermediate image plane 34 may be strongly curved.

A first pupil plane 36 is arranged between the object plane 28 and the intermediate image plane 34, and a second pupil plane 38 is arranged between the intermediate image plane 34 and the image plane 30 of the projection objective 20. In the first and second pupil plane 36, 38 all light rays converging or diverging under the same angle from any of the field planes, i. e. the object plane 28, the intermediate image plane 34 and the image plane 30, pass through the same point, as this is illustrated in FIG. 2. This implies that all light rays intersecting a field plane parallel to the optical axis OA, such as light ray 40 indicated as a broken line, intersect the optical axis OA in the first and second pupil plane 36, 38.

In the intermediate image plane 34 a wavefront correction de- vice 42 for correcting wavefront errors is arranged. This device will be described in more detail below in the following section .

II.

Wavefront Correction Device Referring first to FIG. 2, the wavefront correction device 42 includes a solid body formed by a refractive optical element 44 having a first optical surface 46 at one side and a second optical surface 48 at an opposite side. Through a portion of the refractive optical element 44, which portion shall be re- ferred to in the following as correction volume, projection light passes when the mask 16 is imaged on the light sensitive surface 22. The refractive optical element 44 has a circumferential rim surface 50 extending between the two optical surfaces 46, 48. In this embodiment the optical sur- faces 46, 48 of the refractive optical element 44 are planar and parallel to each other, and the rim surface 50 is cylindrical. Thus the refractive optical element 44 has the shape of a planar circular disk. However, the refractive optical element 44 may also have other shapes; in particular it may form a lens having at least one curved surface.

The refractive optical element 44 contains a plurality of thin buried heating wires 52. The constitution and arrangement of the heating wires will be explained in the following in more detail with reference to FIGS. 3 and 4 which show a top view on the wavefront correction device _. 42 and a section through it along line IV-IV. During exposure operation of the apparatus 10, it is assumed that an approximately rectangular surface region 54 indicated by a dotted line is illuminated on the upper surface 46 of the refractive optical element 44 by projection light PL. In the absence of the mask 16, the illuminated region 54 is an intermediate image of the rectangular field 14 which is illuminated on the mask 16 by the illumination system 12. Since the lenses LI and L2 usually do not produce a perfect image of the field 14, the illuminated region 54 has not exactly the geometry of the illuminated field 14.

Inside the illuminated region 54 the heating wires 52 are arranged within the correction volume, through which the projection light PL passes, in a manner that can better be discerned in FIG. 5. This figure is an enlarged cut-out of FIG. 3 showing only the illuminated surface region 54, which confines the correction volume on one side, and the heating wires 52. In this embodiment the heating wires 52 comprise first heating wires 52X extending parallel to the X direction, and second heating wires 52Y extending parallel to the Y direction which coincides with the scan direction of the apparatus 10. The second heating wires 52Y are significantly shorter than the first heating wires 52X and are present only in a central portion 56 of the illuminated surface region 54 if seen along the Y direction. At the adjacent marginal por- tions 58, 60, if seen along the Y direction, there are only first heating wires 52X, but no second heating wires 52Y. Thus the correction volume comprises a sub-volume, which is contiguous to the margins, in which all heating wires extend perpendicularly to the scan direction Y. Consequently, the heating path density is, along at least one line that is parallel to the scan direction Y, higher in the central portion 56 of the correction volume than at a margin thereof.

In this embodiment both the first heating wires 52X and also the second heating wires 52Y are equidistant, i. e. the dis- tance between adjacent first heating wires 52X is equal for any pair of adjacent first heating wires 52X. The same also applies for the second heating wires 52Y. Thus the density of the heating wires is uniform along the X direction which is perpendicular to the scan direction Y and a normal on the optical surfaces 46, 48 of the refractive optical element 44. Furthermore, in this specific embodiment, the distances between the first heating wires 52X on the one hand and the second heating wires 52Y on the other hand are also equal. Generally, the distances between adjacent first and second heating wires 52X, 52Y may also be different.

Referring back to FIG. 3, each first and second heating wire 52X, 52Y is connected via conductor lines 62X and 62Y, respectively, to its own voltage supply. Thus it is possible to apply individually to each first and second heating wire 52X, 52Y a voltage. The respective electronic circuitry is distributed, in the embodiment shown, among voltage supply units 64 which are arranged at the end of the conductor lines 62X, 62Y. The supply units 64 are received in an annular support structure 66 comprising, as shown in FIG. 4, two rings 66a,

66b. The voltage supply units 64 are, in turn, connected to a control unit 68 via signal lines 70, as this is shown in FIG. 2.

As can best be seen in the cross section of FIG. 4, the first heating wires 52X and the second heating wires 52Y on the other hand are arranged in different planes XY within the refractive optical element 44. To this end the refractive optical element 44 is formed by two disc-shaped plates 44a, 44b which are cemented together by a thin intermediate cement layer 71. The first and second heating wires 52X, 52Y are received in grooves which are formed on the opposing surfaces of the plates 44a, 44b being in contact with the cement layer 71. In this embodiment the heating wires 52X, 52Y, and thus also the grooves, are assumed to have a square cross section, but other geometries are envisaged as well.

The cement layer 71 levels out remaining bumps or depressions on the surfaces of the plates 44a, 44b facing each other, and also electrically isolates the heating wires 52X, 52Y. If the latter are provided with surrounding installation, the cement layer 71 may also be dispensed with.

Other configurations how first and second heating wires 52X, 52Y extending along different directions can be applied to the refractive optical element 44 are described in US

2009/0257032 Al whose full disclosure is incorporated herein by reference.

III.

Function The wavefront correction device 42 is used to correct, or generally speaking to modify, aberrations that occur in the projection objective 20. These aberrations may be the result of the initial design of the projection objective 20. However, the wavefront correction device 42 is specifically adapted to modify the optical wavefronts passing through the projection objective 20 if optical properties of optical elements of the projection objective 20 change. Such changes may be the result of ageing phenomena (long term changes) or the result of varying temperature distributions (short term changes) that are caused by the absorption of projection light PL inside the lenses LI to L4, for example. The temperature distributions sometimes also vary depending on the arrangement and the density of the features 18 on the mask 16 and the angular light distribution (also referred to as illu- mination setting) of the projection light when it illuminates the mask 16. In order to avoid a degradation of the image quality, the latter is repeatedly determined either by measurements or by simulations. If the imaging quality tends to degrade to such an extent that it would eventually not meet the specifica- tions any more, corrective measures have to be taken. To this end the wavefront correction device 42 produces a temperature distribution, and resulting therefrom a refractive index distribution, inside the refractive optical element 44 such that an optical wavefront passing through the refractive optical element 44 will be subjected to a wavefront modification. Sometimes the wavefront modification may be described as a wavefront correction. Often, however, the wavefront is not corrected in the strict sense of the term, but is only modified so that it can be corrected more effectively with other correction means provided in the projection objective, for example using the manipulators Ml, M2 that individually displace the lenses LI, L2 along the optical axis OA.

The desired temperature distribution inside the refractive optical element 44 is produced with the help of the heating wires 52X, 52Y. Each heating wire 52X, 52Y dissipates a certain amount of electrical energy into heat as a result of the resistance of the heating wires 52X, 52Y. The amount of the heat dissipated by the heating wires 52X, 52Y can be controlled by varying the applied voltage. It is thus possible to produce a wide variety of different temperature distributions inside the refractive optical element 44.

Since the wavefront correction device 42 is arranged in or in close proximity to the intermediate image plane 34, it is particularly suited to correct, or more generally to modify, aberrations that have a field dependency. This means that the aberration is not uniform over the field, but varies so that the type and/or degree of aberrations are different for different points in the image plane. A typical example of such a field dependent aberration is distortion. This is an aberra- tion in which points in an object plane are sharply imaged onto the image plane, but the spatial relationship between the points is disturbed so that only a distorted image is obtained. Distortion often increases with increasing distance from the optical axis OA, and therefore the corrective potential of the wavefront correction device 42 must be equally good at the margins of the field.

As it has been explained above with reference to FIG. 5, the density of the heating wires 52X, 52Y is, if seen along at least one line parallel to the scan direction Y, higher at the center of the illuminated surface region 54 than at its opposite margins 58, 60. It has been found that in spite of the reduced density of the heating wires 52X, 52Y at the margins, an equally good correction or modification of field de- pendent aberrations can be achieved with the help of the wavefront correction device 42. The reason for this is associated with the scan operation of the apparatus 10. In projection exposure apparatus of the scanner type, the irradi- ance distribution of the field 14 illuminated by the illumi- nation system 12 on the mask 16 usually has a distribution where the highest irradiance is, if seen along the scan direction Y, obtained at the center. Towards the opposite margins, the irradiance drops continuously to zero. Along the X direction, i. e. perpendicular to the scan direction Y, the irradiance does not vary. This profile of the irradiance distribution along the scan direction Y is used to avoid illumination dose fluctuations that are caused by the interaction of the scanning process with the pulsed emission of the projection light PL by the illumination system 12, as this is described, for example, in US 7,551,263 B2 (Gruner et al . ) .

In the absence of the mask 16, a (usually blurred) image of this irradiance distribution is obtained in the intermediate image plane 34 in which the wavefront correction device 42 is arranged. FIG. 6 illustrates the illuminated surface region 54 on the refractive optical element 44. A solid line 72 represents the profile of the irradiance distribution along the scan direction Y.

As a result of this approximately bell-shaped profile 72 the irradiance in the center portion 56 of the illuminated surface region 54 is always significantly higher than at the marginal portions 58, 60. Therefore also the corrective potential of the wavefront correction device 42 is significantly higher in the central portion 56. For that reason it suffices to have a high density of heating wires 52X, 52Y in this central portion 56 of the illuminated surface region 54. Thus only in this central portion 56 it is possible to produce a temperature distribution with high gradients. In the marginal portions 58, 60, the density of the heating wires 52X, 52Y is smaller, and consequently also the gradients of the temperature distribution that can be

achieved there will be smaller. In other words, the "resolution" of the temperature distribution is greatest at those portions of the refractive optical element 44 where the high- est irradiances occur.

The reduced density of the heating wires 52X, 52Y in the marginal portions 58, 60 has the advantage that the complexity of the wavefront correction device 42 is significantly reduced and its control is simplified. Furthermore, each heat- ing wire 52X, 52Y contributes to light losses and scattering, and also from that perspective it is advantageous to keep the number of heating wires as small as possible.

IV.

Example of Distortion Correction In the following it will be described with reference to FIGS. 7 to 10 how a particular distortion may be corrected in the projection objective 20. FIG. 7 illustrates, in an exaggerated manner, an astigmatic deformation of one of the surfaces of the last lens L4 which is arranged close to the image plane 30 of the projection objective 20. Such an astigmatic deformation is usually caused by a non-rotationally symmetric temperature distribution, which is, in turn, a result of the slit-like geometry of the field 14 which is illuminated on the mask 16.

The astigmatic deformation of the lens L4 is assumed to cause a distortion in the image plane 30 as it is illustrated in FIG. 8. The length and direction of the arrows indicate the direction and amount of the displacement of image points in the image plane 30. As can be seen, the distortion vanishes at the center of the field and increases with increasing distance from the center. FIG. 9 shows the distortion which is associated with an increased magnification β of the projection objective 20. It can be seen that for a line y = 0 the distortion shown in FIG. 9 can completely compensate the distortion shown in FIG. 8. As mentioned further above, it is assumed that an in- creased magnification β can be achieved with the help of the manipulators Ml, M2 by suitably displacing the lenses LI, L2.

FIG. 10 shows the resulting distortion after the magnification β of the projection objective 20 has been increased in this manner. The arrows indicating the distortion are now di- rected exclusively parallel or anti-parallel to the Y direction. This means that the image is magnified or stretched only along the scan direction Y.

Such a magnification along the scan direction Y can be easily corrected with the help of the wavefront correction device 42. A suitable temperature profile which will result in an increased magnification only along the scan direction Y may also be bell-shaped along that direction. Since the bell- shaped irradiance distribution 72 shown in FIG. 6 already re- suits in such a temperature profile (at least if the feature density on the mask 16 is approximately uniform) , this temperature profile may only need to be adjusted to some extent in order to achieve the desired quantitative effect. If the feature density on the mask 16 is approximately uniform, the temperature distribution inside the refractive optical element 44 does not have to vary along the X direction. On the other hand, if, for example, no projection light passes the mask 16 along a central stripe extending along the scan direction Y, it may be necessary to additionally heat up the central portion of the refractive optical element 44 with the help of those second heating wires 52Y that extend through the center of the illuminated surface region 54.

Any residual offset may be compensated for by other manipula- tors provided in the apparatus 10.

V.

Alternative Embodiments

FIGS. 11 and 12 show a wavefront correction device 142 according to a further embodiment in a top view and a sectional view similar to FIGS. 3 and 4. Identical or corresponding parts are be designated with the same reference numerals augmented by 100 and are not explained again in detail.

In this embodiment heating paths are not formed by heating wires 52X, 52Y, but by paths along which heating light beams propagate. To this end the wavefront correction device 142 comprises a first LED bar 164X and a second LED bar 164Y. The LED bars 164X, 164Y each comprise a plurality of LEDS that are arranged in a common XY plane. Each LED in the first LED bar 164X produces a first heating light beam 152X that ex- tends along the X direction. Each LED in the second LED bar 164Y produces a second heating light beam 152Y that extends along the Y direction. In the correction volume, which is partly confined by the illuminated region 154 on the refractive optical element 144, the heating light beams 152X, 152Y intersect. As a matter of course, the heating light beams 152X, 152Y may also propagate in two or more different planes.

Also in this embodiment the density of the heating light beams 152X, 152Y is, if seen along the scan direction Y, higher in the center of the illuminated region 52 than at its margin, as can be clearly seen in FIG. 11. In contrast to the first embodiment, the density of heating light beams 152X,

152Y decreases almost continuously towards the margin if seen along the Y direction.

This embodiment further differs from the first embodiment in that the wavefront correction device 142 is not arranged in the intermediate image 34, but close to the mask plane 28, as this is shown with broken lines in FIG. 1. Such an arrangement may be more advantageous particularly in those cases in which the quality of the intermediate image formed in the intermediate image plane 34 is so poor that a proper correction of field dependent aberrations becomes difficult.