WO2004036316A1

WO2004036316A1 - Optical arrangement and method of specifying and performing a deformation of an optical surface of an optical element being part of the optical arrangement

Info

Publication number: WO2004036316A1
Application number: PCT/EP2002/011492
Authority: WO
Inventors: Jean-Noel Fehr; Andreas Frommeyer
Original assignee: Carl Zeiss Smt Ag
Priority date: 2002-10-15
Filing date: 2002-10-15
Publication date: 2004-04-29
Also published as: AU2002346911A1

Abstract

An optical arrangement (12), in particular a microlithographic projection exposure system, in particular having rotational asymmetric illumination, e.g. having a slit shaped illumination field, includes at least one optical element (16, 18, 20) having at least one optical surface (3) being shaped to correct the imaging properties of the optical arrangement (12). Included is a surface deformation device (6) co-operating with the optical element (16, 18, 20) which is arranged to produce a specific thermal deformation of at least a region of the optical surface (3) of the optical element (16, 18, 20). This region has a shape being specifically adapted to the imaging properties to be corrected. Thus, the correction of specific aberrations is possible. The deformation of an optical surface of an optical element being part of the optical arrangement to be generated by the surface deformation device is specified by measuring the aberrations of the desired optical properties of the optical arrangement in a field plane of the optical arragemment, by shifting the measured aberration or a model function representing the measured aberration by adding a positive constant to get only positive values to obtain a desired deformation pattern of the optical surface, by determining a relative thermal energy profile to be generated by the surface deformation device and by determining the absolute thermal energy profile using material parameters of part of the optical element dedicated to be illuminated by the correction light.

Description

Optical arrangement and method of specifying and performing a deformation of an optical surface of an optical element being part of the optical arrangement

The present invention relates to an optical arrangement according to the preamble of claim 1 and to a method of specifying and performing a deformation of an optical surface of an optical element being part of the optical arrangement .

Optical arrangements with optical correction surfaces are known in the art of projection objectives being part of projection exposure systems. These known correction surfaces are e. g. aspheric surfaces polished so as to correct measured or calculated aberrations of the optical arrangements. Furthermore, it is known, for example from EP 0 678 768 A2 to heat parts of optical elements of a projection objective asymmetrically in order to to compen- sate asymmetric lens heating due to residual absorption of the projection light. Furthermore, it is known from EP 0 823 662 A2 to use correction light beams guided parallel to the projection light beam through a projection objective in order to influence the imaging properties of the projection objective.

These known approaches suffer from certain drawbacks :

In the case of polished aspheres, no flexibility in the optical properties of the correction optical element can be achieved. Thus, drifts or residual errors in the optical properties of the projection objective cannot be compensated. On the other hand, the known schemes of additional heating of optical elements by heating devices or by correction light beams allow the correction of aberrations with low order symmetrical properties only without the possibility of a precise control of the imaging properties of the projection objective.

Therefore it is the object of the present invention to provide an optical arrangement capable of a specific and flexible correction of occurring aberrations.

This object is achieved by an optical arrangement having the features of claim 1. By this approach, not only low order symmetrical contributions of the aberrations can be corrected but the overall optical properties give the basis for determining the shape of the region of the optical surface of the optical element to be deformed. The optical element is thermally deformed and thermally influenced only to the extent that is necessary for compensating the aberration. Therfore, a thermal overload of the thermally influenced optical element is avoided.

Furthermore, the optical arrangement of claim 1 is capable of compensating long term as well as short term aberration effects. "Long term" means a time period which is comparable to the life time of the overall system as is the case for the effect called "compaction", for instance. "Short term" means a time period which is very much smaller than the lifetime of the system, especially one or several minutes, hours or days. This system, if used to compensate for heating, will have to be efficient in the first minutes or in the first hour of lens exposure.

Optical arrangements according to claims 2 and 3 give the possibility of a direct thermal influence to the optical surface of the respective optical element. This enables a local thermally induced deformation with high spatial resolution. An optical arrangement according to claim 4 avoids an interference between the projection beam path and the surface deformation device.

With a surface deformation device according to claim 5, a fine adjustment of the overall thermal load to be deposited in the optical element is possible.

Heating or cooling elements according to claim 6 give the possibility of a rugged and inexpensive surface deformation device .

A surface deformation device according to claim 7 can be flexible, as the deposit of thermal energy to the optical element can be controlled via the wavelength, the shape, the intensity of the correction light beam and also via the type of thermal linkage between the correction light beam and t e' optical element. This thermal linkage may be a direct one, i. e. direct exposure of the optical element by the correction light beam, or an indirect one, i. e. exposure of an intermediate medium being in thermal contact with the optical element, providing a transfer of thermal energy from the surface deformation device to the optical element.

A surface deformation device according to claim 8 avoids the generation of potentially disturbing ghost reflexes of correction light within the optical arrangement.

A beam shaping element according to claim 9 gives the possibility of forming the correction light beam in a flexible way. Alternatively, the correction light beam may be formed by directly influencing the correction light source A filter element according to claim 10 and/or an aperture stop according to claim 11 lead to a relatively inexpensive way of forming the correction light beam.

Preferably, the beam shaping element includes at least one diftractive element as is the subject matter of claim 12.

A beam shaping element with movable parts according to claims 13 and 14 is flexible.

Preferably, the movable parts have the form of ellipsoid rings or ring sectors as is the subject matter of claim 15.

Using a surface deformation device according to claim 16 the correction light beam can be formed without absorbing or blocking light.

An array according to claim 17 is well adapted to the symmetrical behaviour of most common types of aberration. Therefore, this kind of surface deformation device may operate using known mathematical schemes to approximate wave front aberrations .

Light emitting diodes according to claim 18 are available in a variety of output powers, beam qualities and wavelengths .

In some cases, the radius of curvature has to be changed only in order to compensate an occurring aberration. In this case, an optical arrangement according to claim 19 is well adapted to provide this compensation.

A surface deformation device according to claim 20 uses a so-called "bi-metal" -effect for inducing a deformation of the optical surface of the optical element . Dependent on the shape of the deformation component, different modes of deformation may be induced to the optical surface,

In a first approach according to claim 21, the surface deformation is induced by a thermal deformation of the deformation component. Therefore, this deformation can be adapted via the thermal linkage to the deformation component to meet the specific compensation requirements.

In an alternative approach according to claim 22, the deformation is induced by the thermal deformation of the optical element itself. This leads to the possibility of a passive compensation scheme, where the residual absorption of projection light leading to the occurrence of an aberration simultaneously results in a "bi-metal" -deformation compensating this aberration without additional thermal input .

A deformation component according to claim 23 does not require any constructive change of the optical element .

Using a separate part as a deformation component according to claim 24 gives the possibility to influence the shape of deformation via the shape of the deformation component. The latter then can be homogeneously thermalized which is in most cases easier to achieve than thermalizing an integral deformation component of a given shape.

An arrangement of deformation components according to claim 25 is well adapted to compensate for occurring astigmatism.

Tapered deformation components according to claim 26 provide a progressive deformation influence from the center or the optical axis of the optical element to its circumferential portion. This deformation is adapted to other kinds of aberrations occurring in projection exposure systems.

The specific shapes of the region of the optical surface being deformed according to claims 27 through 32 are adapted to specific kinds of aberrations, too.

An optical arrangement according to claim 33 avoids an undesired bending of an optical element due to the "bimetal " -effect .

A deformation scheme according to claim 34 gives the possibility of a specific and highly spatially resolved deformation of the optical surface of an optical element without the need of exposing this optical surface to correction light .

A selection of the optical element to be deformed according to claim 35 facilitates the calculation of a deformation pattern of an optical surface which is required to compensate for a specific aberration.

It is another object of the present invention to provide a method of specifying and performing a deformation of an optical surface of an optical element being part of an optical arrangement to compensate for a given aberration. This is achieved by a method according to claim 36. The mathematical and/or computational requirements to implement such a method are well known in the art .

The advantages of the subject matters of claims 37 and 38 correspond to those of the respective optical arrange- ments .

A decomposing step according to claim 39 facilitates the mathematical handling when deriving the desired thermal energy profile starting from the measured aberration.

Where the desired thermal energy profile leading to correspondent surface deformation is not a direct image of the measured wave front aberration, a method according to claim 40 comprising the intermediate steps of analyzing the effects of the heating of selected basic modes of the surface deformation device helps to derive the desired thermal energy profile starting from the measured aberration.

Selected modes according to claim 41 facilitate the mathematical handling of such a method.

A method according to claim 42 takes into account the loss in spatial resolution due to the extended deformation of the optical surface resulting from absorption of the emission of a single light element.

Singular value decomposition according to claim 43 is a well known mathematical tool for facilitating the expenditure of the computation.

The methods described in claims 44 to 47 are suitable where transient effects in the deformations of the optical elements cannot be neglected.

In the following, preferred embodiments of the invention will be described. In the drawings:

figure 1 is a schematic side view of a trans issive optical element illuminated by a correction light beam generated by a surface deformation device and also illuminated by a projection light beam;

figure 2 is a schematic sectional view taken along a meridional plane of a projection objective comprising mirrors with optical surfaces being illuminated by correction light beams generated by surface deformation devices being which are similar to the surface deformation device of figure 1;

figure 3 is a top view of the optical surface of an optical element subdivided into separate regions to be illuminated by a correction light device;

figure 4 is a schematic view taken along the beam path of a correction light beam and showing an aperture stop along the correction light beam optical path of an elliptical off-axis mirror;

figure 5 is the same view as figure 4, a ring segment of the aperture stop being removed;

figure 6 is a top view of the optical surface of a mirror showing four regions being exposed to projection light;

figure 7 is similar to figure 6, showing the optical surface of the mirror being exposed both to projection light and to correction light;

figure 8 is a flow chart of a method of specifying a deformation of an optical surface to be generated by the surface deformation device;

figure 9 is a schematic sectional view taken along a meridional plane of a projection objective similar to that of fig. 2 comprising mirrors with back surfaces being illuminated by correction light beams generated by surface deformation devices which are similar to the surface deformation device of figure 1;

figure 10 is a perspective view of a mirror, the optical surface being disposed to projection light and the back surface being exposed to correction light;

figure 11 is a sectional view taken along line XXI-XXI of figure 10;

figure 12 is a sectional view of a mirror according to another embodiment being exposed to a correction light beam;

figure 13 is a schematical view of a correction light device which includes an array of light emitting elements being arranged as a plurality of concentric rings each consisting of a plurality of light emitting diodes;

figure 14 is a flow chart of another method of specifying a deformation of an optical surface to be generated by the surface deformation device;

figure 15 is a calculated representation of the deformation of an optical surface generated by illumination with a single light emitting diode; figure 16 is a schematic top view of a back surface of a mirror carrying a surface deformation device which includes eight metal bars as deformation components in a wheel-spoke arrangement ;

figure 17 is a schematic side view of two joined material layers representing a mirror and a deformation component;

figure 18 is a view similar to figure 17, the two material layers having both a temperature being different from that of figure 17;

figure 19 is a view similar to that of figure 16, the surface deformation components being, however, an integral part of the back surface of the mirror;

figure 20 is a view similar to that of figure 17 representing a sectional view of a mirror of the type of figure 19 having an integral layer of material serving as a surface deformation component;

figure 21 is a view similar to that of figure 20, the layer serving as a surface deformation component being at a different temperature than the rest of the mirror;

figure 22 is a view similar to that of figures 16 and

19, showing the back surface of an elliptical mirror having another arrangement of surface deformation components; figures 23 and 24 are schematic views of arrangements of heating elements located on the back surface of a mirror;

figure 25 is a schematic view of an arrangement of cooling elements located on the back surface of a mirror.

Figure 1 shows a first embodiment of the invention. A planoconvex lens 1 being schematically depicted serves to guide and to form a projection light beam 2. The lens 1 is made of optical glass, e. g. SiO~, but also other optical materials may be used.

The lens 1 is part of a projection objective of a micro- lithographic projection exposure system. In addition to the projection light beam 2, an optical surface 3 of the lens 1 is illuminated by a correction light beam 4 generated by a correction light source 5. The correction light beam 4 has a wavelength in the mid infrared region (3 to 30 μm) which is absorbed by the material of the lens 1. The overall intensity of the correction light beam 4 should be in the region of 500 mW and should have a typical

2 maximum light density of 0,05 mW/mm .

The correction light source 5 is a laser diode. Other light sources, e. g. a HeNe laser at 3,391 μm or a tunable frequency doubled CO laser (with a typical tuning range between 4,6 and 5,8 μm) are available in this wavelength region. The correction light source 5 may be a pulsed or continuous light source.

The correction light beam 4 emitted by the correction light source 5 is formed by a beam shaping element 6 (see below) so as to generate a defined correction light intensity distribution at the optical surface 3. The intensity distribution generated by the beam shaping element 6 results in a corresponding heat load distribution in a surface deformation region 7 beneath the optical surface 3 of the lens 1. The surface deformation region 7 is shown hatched in figure 1. The deformation pattern resulting from the thermal expansion of the surface deformation region 7 according to the heat load distribution is shown strongly exaggerated. In fact, the maximal deviation of the deformed optical surface from the initial surface is typically smaller than 10 nm. The local temperature within the surface deformation region 7 is typically not increased by more than 20 .

The depth of the surface deformation region 7 depends on the absorption coefficient of the material of the lens 1 at the wavelength of the correction light beam 4. Due to the thermal expansion of the optical material of the lens 1 resulting from absorption of correction light, the optical surface 3 of the lens 1 illuminated by the correc- tion light beam 4 is deformed. The local amplitude of the deformation pattern is proportional to the local heat deposit. The deformed optical surface 8 is indicated by a dotted line in figure 1.

Typical sizes of structure of the deformation pattern

2 induced by the deformation are bigger than 2 mm in cross section.

A small fraction of the intensity of the correction light beam 4 is not absorbed by the lens 1 but is reflected by the optical surface 3. The correction light source 5 and the beam shaping element 6 are arranged such that this reflected light beam 4 ' does not interfere with other components of the projection objective. The beam shaping element 6 includes an imaging optics 9 which generates an image of the intensity distribution of the correction light beam 4 generated by other parts of the beam shaping element 6 on the optical surface 3. These other parts of the beam shaping element 6 comprise a filter element 10, e. g. a neutral filter, including a plurality of filter parts being movable relative to each other and an aperture stop 11, also including a plurality of aperture stop elements being movable relative to each other. Other details of the beam shaping element 6 will be described below with respect to another embodiment of the invention .

In the following, further embodiments according to the inventions are described. Components which have already been described with respect to previous embodiments are referred to using the same reference numbers and will not be described again in detail.

Figure 2 shows a projection objective 12 of a micro- lithographic projection exposure system generating an image of a reticle plane 13 in a wafer plane 14. The projection objective 12 includes six mirrors 15 to 20 guiding and shaping the projection light beam 2. In order to indicate the beam path of the projection light beam 2, three individual projection light rays 2', 2'', 2 are illustrated in figure 2.

The mirrors 16, 18 and 20, being the second, the fourth and the sixth mirror, respectively, and guiding the projection light beam 2 in the projection objective 12 after passing the reticle plane 13 are illuminated by individual correction light beams 4 shaped by beam shaping elements 6. The correction light sources generating the correction light beams 4 are not shown in figure 2. In order to adapt the surface deformation of the optical surfaces of the mirrors 16, 18, 20 to measured imaging properties of the projection objective 12, in particular to measured wave front aberrations induced in the projection objective 12 by the projection light beam 2, the optical surfaces of the mirrors 16, 18, 20 are subdivided into individual target regions for correction light . An example for such a subdivision is shown in figure 3 which illustrates a subdivision being typically applicable to mirrors 16 and 20. The optical surface 3 is subdivided into a center region 21 surrounded by five concentric ring-shaped regions 22 to 26. The inner ring-shaped region

22 adjacent to the center region 21 is subdivided into four equally shaped sectors. The four outer ring-shaped regions

23 to 26 are subdivided into eight equally shaped sectors. Each of these target regions resulting from this subdivision, i. e. the center region 21 and the sectors of the ring-shaped regions 22 to 26, can be illuminated indepen- dently by beam parts of the correction light beam 4. The intensities of these independent parts of the correction light beam 4 can be controlled individually using the beam shaping element 6.

Due to this individual control of the parts of the correction light beam, individual heat loads can be deposited in surface deformation regions on and/or beneath the optical surface 3 or the mirror carrier carrying this optical surface 3, respectively, in order to generate a heating distribution resulting in a correspondent distribution of deformations. The shape of these surface deformation regions depends on the materials of the optical surface of the mirrors 16, 18, 20 which may be an individual reflective layer, and of the mirror carrier and additionally depends on the wavelength of the correction light being used and, of course, on the type of aberration to be corrected.

Using the division of the optical surface 3 into the center region 21 and the five ring-shaped regions 22 to 26, i.e. six concentric illumination regions, a deformation of the optical surface 3 corresponding to Zernike-functions up to Z36 is possible. Making further use of the additional division into sectors gives the possibility to address even higher Zernike-functions .

Accordingly, using two concentric illumination regions, a deformation corresponding to Z4 , using three concentric illumination regions a deformation corresponding to Z9, using four concentric illumination regions a deformation corresponding to Z16 and using five concentric illumination regions a deformation corresponding to Z25 may be obtained.

Figures 4 and 5 show views taken along the beam path of the correction light beam 4 which serves for illuminating the mirror 18 of the projection objective 12 of figure 2. In order to reach the optical surface of the mirror 18, the correction light beam 4 has to pass the beam shaping element 6. Part of the beam shaping element 6 is, as described above, the aperture stop 11. In the case of the mirror 18 , the aperture stop 11 includes three ring segments 27 to 29 being arranged concentrically with respect to the optical axis of the off-axis mirror 18. The ring segments 27 to 29 are carried by a ring segment holder 30.

With respect to the beam path of the correction light beam 4, the ring segments 27 to 29 are arranged such that the whole aperture of the mirror 18 is covered by the ring segments 27 to 29 so that the whole aperture of the mirror 18 may be blocked. This configuration is shown in figure 4. In figure 5, the middle ring segment 28, which in the configuration of figure 4 is arranged between the inner ring segment 27 and the outer ring segment 29, is removed from the beam path of the correction light beam 4. This removal can be done e. g. by rotating the ring segment 28 with respect to an axis being coincident with the optical axis by 180 or by pivoting the ring segment 28 with respect to an axis lying in the plane of the ring segment holder and being perpendicular to the optical axis or by lateral translation of the ring segment 28 with respect to the beam path of the correction light beam 4. The number of rings depends, among others, on the type of aberration to be corrected.

Due to the removal of ring segment 28, a correspondingly ring segment shaped correction light beam 4 is able to reach the optical surface of the mirror 18 thereby depositing a certain heat load by absorption and inducing a respective surface deformation pattern.

Also, the filter element 10 can include filter elements being arranged e.g. similar to the ring segments 27 to 29.

Another example of an illumination scheme of the optical surfaces of the mirrors 16, 18, 20 is shown in figures 6 and 7. Depending on the structure located in the reticle plane 13 to be projected by the projection objective 12, e. g. a mask of a micro-processor, a particular projection light pattern is projected onto each mirror 15 to 20 of the projection objective 12. For example, this projection light pattern may have the shape of four quadratic regions 31 on the optical surface of the mirror 16, as is shown in figure 6. In this case, the correction light beam 4 illuminating the optical surface of the mirror 16 may be shaped complementary to the projection light pattern so as to produce a homogeneous heating distribution across the optical surface of the mirror 16 due to the combined absorption of the respective patterns of the protection light beam 2 and the correction light beam 4. As a result, no disturbing aberration of the optical surface of the mirror 16 due to residual absorption of projection light will occur. Border effects can be taken into account by departing somewhat from the above idealized configuration.

In the following, a method of specifying the deformation of an optical surface to be generated by the surface deformation device 5, 6 is described with reference to figure 8.

In a measuring step 32, the aberrations with respect to the desired optical properties of the projection objective 12 are measured in the wafer plane 14. This may be done by conventional wave front analysis. Next, in a data processing step 33, the measured aberrations or a model function representing the measured aberrations derived by a mathematical evaluation of the measured aberrations, are shifted by adding a positive constant so as to obtain only positive values . This constant shift may be compensated by a corresponding focus shift of the projection objective, i. e. by a Z-manipulation of the wafer stage.

Preferably, the mirror to be illuminated by the correction light beam is selected such that a deformation of the optical surface resulting in a specific heat distribution, and, thus, deformation pattern of the optical surface leads to a similar distribution of changes in the optical properties of the optical arrangement. In case of the projection objective 12 of figures 2 and 9, mirror 16 should be chosen to achieve this result. In this case a model function, i. e. a function derived from the measured aberrations for calculating the desired deformation pattern of the optical surface, of the aberration is obtained.

In a subsequent determination step 34, a relative light intensity profile across the correction light beam 4 according to the desired deformation pattern is determined. Finally, in a second determination step 35, the absolute light intensity profile is determined, using material parameters, e. g. absorption coefficient and thermal expansion coefficient of the regions of the optical element which are to be illuminated by the correction light beam.

Figure 9 shows another embodiment of a projection objective 12 including a surface deformation device according to the invention. The projection objective 12 of figure 9 is similar to that of figure 2 with respect to the configuration of the reticle plane 13 , the wafer plane 14 and the configuration and optical design of the mirrors

15 to 20. In figure 9, the back surfaces of mirrors 16, 18 and 20 are illuminated by individual correction light beams 4 shaped by independent beam shaping elements 6. By illuminating the back surfaces of the mirrors 16, 18 and 20 the form of the optical surfaces of these mirrors is influenced, as will be described below.

Figure 10 and 11 show an example of illuminating the mirror 16 with the projection light beam 2 and the correc- tion light beam 4 in a schematic perspective view. The projection light beam 2 illuminates a rectangular projection illumination region 36. Due to residual absorption of the projection light beam 2, the optical surface 3 of the mirror 16 and the mirror carrier 37 is heated in a first surface deformation region 7' (see figure 11) . The correction light beam 4 illuminates a correction illumination region 38 which is the counterpart of the projection illumination region 36 on a back surface 39 of the mirror 16. Accordingly, a second surface defor- mation region 7¹' is heated by absorption of the correction light beam 4 opposite to the surface deformation region 7 ' . The intensity of the correction light beam 4 is adapted to the intensity of the projection light beam 2 so that the heating loads in the surface deformation regions 7' , 7' ' are equal. Therefore, a bending of the optical surface 3 of the mirror 16 due to lateral expansion of the first surface deformation region 7 ^■ is prevented by an equal lateral expansion of the second surface deformation region 7' ' .

Another scheme for a controlled optical surface deformation by back surface illumination of a mirror with a correction light beam 4 is shown in figure 12. Here, the mirror 16' to be illuminated with correction light comprises a thin mirror carrier 37' . The correction light beam 4 having a specific intensity distribution generated by a beam shaping element (not shown in figure 12) is absorbed in a surface deformation region 7 having the form of a channel starting from the back surface 39 and ending at the optical surface 3. Due to the thermal expansion of the optical material of the mirror 16' throughout this channel, a surface deformation of the back surface 39 and a mirrorlike surface deformation of the optical surface 3 corresponding to the intensity distribution across the correc- tion light beam 4 occur.

Preferably, with respect to the optical arrangements shown in figures 2 and 9, pupil errors contributing to the overall aberration measured in the wafer plane 14 should be corrected by mirror 16 and field errors should be corrected by deforming the optical surfaces of mirrors 18 and 20, respectively.

Preferably, more than 80 % of the correction light beam 4 should be absorbed by the lens 1 in the arrangement of figure 1 or by the mirrors 16, 18, 20 in the arrangements of figures 2 and 9.

Another embodiment of a surface deformation device accor- ding to the invention is shown in figure 13. Here, the correction light source 5 includes an array 40 of light emitting diodes 41. The light emitting diodes 41 are arranged in three concentric diode rings 42 to 44 surrounding a central diode 45. The inner diode ring 42 consists of eight light emitting diodes, the middle diode ring 43 consists of sixteen light emitting diodes and the outer diode ring consists of twenty-four light emitting diodes. The array 40 is connected via a control line 46 to a diode control device 47. Using the diode control device 47, the diodes constituting the array 40 can be controlled separately and individually. With the help of the array 40, similar shapes of a correction light beam 4 may be realized as is described above with respect to the target region subdivision of figures 3, 4 and 5. With the three diode rings 42 to 44 surrounding the central diode 45, deformations can be produced by illuminating an optical surface with the array 40 which can be described by Zernike- polynomials up to Z16. Making further use of the possibility to control the emission of the diodes constituting each of the diode rings 42 to 44 separately gives the possibility to address even higher Zernike-functions .

The distance between neighbouring light emitting diodes 41 is chosen such, that, taken into account the imaging of the array onto the optical surface of the mirror, the distance between the images of the individual light emitting diodes is not greater than 6 mm.

A more detailed method of specifying the deformation of an optical surface to be generated by a surface deformation device including the array 40 will now be described with reference to figures 14 and 15 :

After the measuring step 32, the measured aberrations are decomposed in a basis of orthonormal polynomials, e. g. Zernike polynomials. This is done in a decomposing step 48. Subsequently, in an analysing step 49, the effect of the illumination of the mirrors in selected modes, i.e. selected beam shapes, of the correction light beam 4 on the imaging properties of the projection objective is measured and the measured aberration is approximated by a model function obtained of the imaging property effects previously measured in the selected modes of the mirror illumination. During this approximation, the contributions of the several selected modes are determined.

Next, the data processing step 33 is performed on the obtained model function to obtain the desired deformation pattern.

Subsequently, the relative light intensity profile of the light emitting diodes across the array is determined in the course of a convolution step 50. This convolution step 50 starts with the determination of the shape of the deformation of the optical surface of the mirror caused by an illumination of the mirror by a single light emitting diode 41 of the array 40.

Such punctual deformation is depicted in figure 15. Here, a mirror carrier 37 made of Zerodur is illuminated by a single light emitting diode across a surface of 1,8 mm 2 with a power of 17,6 W. The wavelength of the light emitting diode is chosen to be absorbed by Zerodur. The mirror carrier 37 has a radius of 250 mm and a thickness of 50 mm.

According to model calculations, Zerodur as optical material requires an array 40 with an illumination power of 1 W on a surface cross section of 10 4 mm2 to produce a surface deformation of 1 nm.

A deformation of the optical surface of the mirror carrier 37 with a radius of 12 mm and a maximum depth of 0,11 nm is obtained. In combination with the chosen distance between the individual light emitting diodes this leads to the possibility of controlling the global deformation of the mirror.

Subsequently, during the convolution step 50, an inverse convolution of the deformation pattern for this obtained punctual deformation is performed. This gives the desired relative light intensity profile of the light emitting diodes 41 of the array 40 to be adjusted by the diode control device 47 in order to generate the desired surface deformation pattern.

Subsequently, the determination steps 34 and 35 are performed.

Another embodiment of an optical arrangement including a surface deformation device according to the present invention is shown in figures 16 to 18.

Figure 16 shows a top view of the back surface of a mirror used in the projection objective 12, e. g. mirror

16. The back surface 39 of the mirror 16 carries eight metal bars 51 which are joined to the mirror carrier 37. The metal bars 51 are configured in a wheel-spoke arrangement. The connection between the mirror carrier 37 and the metal bars 51 can be realized e. g. by gluing the metal bars 51 to the mirror carrier 37.

The function of the metal bars 51 can be seen from the schematic illustration of figures 17 and 18 and can be understood as being a "bi-metal" effect. For simplification, in figures 17 and 18 only one metal bar 51 connected to the mirror carrier 37 is indicated. In figure

17, the metal bar 51 and the mirror carrier 37 have a same first temperature T_ . At this temperature, no stress is present at the back surface 39 of the mirror 16 being the joint surface between the metal bar 51 and the mirror carrier 37. In figure 18, the metal bar 51 and the mirror carrier 37 both are at a second temperature T_ different from T_n . Due to the different thermal expansion coefficients of the metal bar 51 and the material of the mirror carrier 37, stress is induced across the back surface 39. In the example given in figure 18, - is lower than T . As the metal bar 51 has a higher thermal expansion coefficent than the glass material of the mirror carrier 37, the greater absolute contraction of the metal bar 51 compared to the contraction of the mirror carrier 37 leads to a bending of the complete structure resulting in a convex shape of the optical surface 3 of the mirror 16.

The metal bars 51 can be thermalized separately using heating or cooling elements known in the art. Examples for such elements are resistance heaters, heaters including sources of electromagnetic waves, e. g. IR-sources as disclosed above, heaters utilizing the Joule effect, Peltier coolers or fluid coolers.

Figures 19 to 21 show another embodiment of an optical arrangement including a surface deformation device according to the present invention which resembles the arrangement described above with respect to figures 16 to 18. In the embodiment of figures 19 to 21, no separate metal bars are provided. Instead, integral surface deforming regions 52 of the mirror 16 serve as parts of the surface deformation device.

The deformation function of the integral surface deforming regions 52 is described with respect to figures 20 and 21. Similar to the schematic approach of figures 17, 18 above, in figures 20 and 21 only one integral surface deforming region 52 is depicted.

In figure 20, the integral surface deforming region 52 has the same temperature than the rest of the mirror carrier 37. Therefore, no deformation is induced to the optical surface 3. In figure 21, the integral surface deforming region 52 has been cooled. This may be done by bringing the integral surface deforming region 52 in thermal contact e. g. with a Peltier cooling element or by fluid cooling the integral surface deforming region 52 via a thermal linkage element containing cooling channels. As result of the cooling, the integral surface deforming region 52 contracts, therefore deforming the optical surface 3 of the mirror carrier 37 to a convex shape .

By individually heating or cooling the metal bars 51 of the embodiment of figure 16 or of the integral surface deforming region 52 of the embodiment of figure 19, specific aberrations, e. g. astigmatism or coma may be corrected. Also, by applying equal thermalization to the metal bars 51 or the integral surface deforming region 52, a global change of the radius of curvature of mirror 16 may be realized.

According to an alternative application of these latter embodiments, the metal bars 51 or the surface deforming regions 52 are not thermalized and the deformation of the optical surface 3 is induced by the thermal deformation of the mirror 16 itself. Provided an appropriate choice of the shape, dimension and material of the metal bars 51 or the surface deformation regions 52, this gives a passive compensation scheme, where the residual absorption of protection light being the source of an aberration simultaneously results in the "bi-metal"-deformation compensating this aberration without additional thermal input.

Instead of localized deforming regions like the metal bars 51 or the integral surface deforming regions 52, a homogeneous deforming coating can be applied to the back surface 39 of the mirror carrier 37. The effect of such coating can be understood from the description of figures 17 and 18 under the assumption, that in these figures the whole mirror is shown.

Another configuration which may be adopted for the metal bars 51 of the embodiment of figure 16 or for the integral surface deforming regions 52 of the embodiment of figure 19 is shown in figure 22. Here, the elliptical mirror 18 of the projection objective 12 is shown. This alternative configuration is also a kind of wheel-spoke arrangement adapted to the elliptical shape of the mirror 18. Instead of rectangular deforming regions as in the case of the metal bars 51 or the integral surface deforming regions 52, triangular deforming regions 53 are arranged in the embodiment of figure 22. Each deforming region 53 is tapered towards the center of the mirror 18. With this shape of the deforming regions 53, a stronger effect in the circumferential region than in the central region of the mirror 18 can be realized when thermalizing the deforming regions 53.

In order to realize a typical magnitude of aberration correction, a temperature difference between the deforming regions 53 and the mirror carrier 37 of 0,1 may be sufficient. This also holds with respect to the metal bars 51 and the integral surface deforming regions 52.

Finally, configurations of heating elements (figures

23 and 24) or cooling elements (figure 25) are described with respect to figures 23 to 25. These thermalizing elements are in thermal contact to the back surface 39 of a mirror, e. g. of mirror 16.

Figure 23 shows a heating element configuration including three concentric ring elements 54 to 56, each being subdivided into eight sectors 57. Each of these sectors 57 can be heated individually.

Figure 24 shows a heating element configuration including a circular center heating element 58 surrounded by two concentric ring elements 59, 60. Each of the ring elements 59, 60 is subdivided into four heating sectors. The borders 61 between the heating sectors of the outer ring element 60 are angularly shifted by 45 in the circumferential direction with respect to the borders 62 between the heating elements of ring element 59.

Figure 25 shows an example of a configuration of cooling elements. This configuration includes two concentric cooling ring elements 63, 64. The ring elements 63, 64 are subdivided into four sectors each.

Instead of heating or cooling the complete ring elements it is also possible to heat or cool at the intersection points of the lines defining the ring segments.

Using such heating or cooling devices, surface deformations according to the principles described above can be realized.

In the above description, it was silently assumed that the whole system was always in thermal equilibrium so that no transient effects take place. This, however, is frequently not the case. In some optical arrangements, the time constants of the system characterizing the time needed to generate a compenstion of aberrations by deformation are too big. This is to say that the time necessary for deforming the optical element does not follow the transient change of the aberrations and, thus, the remaining aberrations are beyond the tolerable limits .

Also, it is difficult to realize enough consecutive messurements of the aberrations over the time.

In order to deal with this problem a method can be applied which anticipates the deformation of the optical elements making use of a feed forward approach as follows :

Firstly, the impact of the projection light on the optical elements is analyzed and the resulting deformations of the optical elements are studied. The aberrations caused by these deformations are calculated. Then, the transient behaviour of each aberration is determined by computation. One or more influence functions for the optical elements are calculated based upon the aberrations induced by a particular and controlled deformation of the optical surface of one or more optical element.

As the deformation behaviour is monotonous, a feed forward strategy based upon the knowledge of the system can be found. In some cases, the only parameter required in this respect is a time constant defining the mirror deforma- tion which can be measured or calculated. In order to minimize the aberrations at all times the time constant of the deformation of the optical element induced by projection light is synchronized with the time constant of the deformation induced by a correction light beam. In other, more complicated cases the transient behaviour of the mirror is determined experimentally in advance and stored in a look-up-table and in use of the projection objective the light intensity is adapted to the stored values .

As a result, the compensating deformation induced by the correction light beam is able to follow the deformation caused by the projection light and a compensation can be achieved at all times without the need of a large number of consecutive measurements.

Claims

1. Optical arrangement, in particular micro-lithographic projection exposure system having in particular a rotationally asymmetric illumination field, especially a slit or ringfield segment shaped illumination field, including at least one optical element having at least one optical surface being shaped to correct the imaging properties of the optical arrangement,

characterized in that,

a surface deformation device (5, 6; 54 to 56; 59, 60;

63, 64; 51; 52; 53) co-operating with the optical element

(1; 16, 20; 16') is provided, the surface deformation device (5, 6; 54 to 56; 59, 60; 63, 64; 51; 52; 53) being arranged to produce a specific thermal deformation (8) of at least one region of the optical surface (3) of the optical element (1; 16, 20; 16'), said region having a shape being specifically adapted to the imaging properties to be corrected.

2. Optical arrangement according to claim 1, wherein the optical element (1) is a trans issive optical element .

3. Optical arrangement according to claim 1 or 2 , wherein the optical element is a mirror (16, 20; 16'), the optical surface (3) of which is deformed by the surface deformation device (5, 6) .

4. Optical arrangement according to any of the preceding claims, wherein the optical element is a mirror (16,

20; 16') the back surface (39) of which is deformed by the surface deformation device (5, 6; 54 to 56; 59, 60; 63, 64; 51; 52; 53) .

5. Optical arrangement according to any of the preceding claims, wherein the surface deformation device (5, 6) includes a source (5) of electromagnetic waves is producing the deformation of the at least one region (7) of the optical surface (3) of the optical element (1; 16, 20; 16' ) .

6. Optical arrangement according to any of the preceding claims, wherein the surface deformation device includes heating (54 to 56; 58 to 60) or cooling (63, 64) elements, a heat transfer taking place between the heating or cooling elements and the at least one region (39) of the optical surface (3) or of the optical element (16, 20) .

7. Optical arrangement according to any of the preceding claims, wherein the surface deformation device (5, 6) includes a correction light device having a correction light source (5) generating a correction light beam (4) guiding energy of the correction light to the optical element (1; 16, 20; 16') in order to correct the imaging properties of the optical element (1; 16, 20; 16') by deforming the optical element (1; 16, 20; 16').

8. Optical arrangement according to claim 7, wherein the correction light beam (4) of the correction light device (5, 6) is projected onto the optical element (1; 16, 20) such that reflected correction light (4') does not interfere with other elements (15, 17, 19) of the optical arrangement.

9. Optical arrangement according to claim 7 or 8, wherein the correction light device (5, 6) comprises a beam shaping element (6) for forming a correction light beam (4) emitted by the correction light source (5) corresponding to the at least one region (7) of the optical surface (3) to be deformed by the surface deformation device (5, 6) .

10. Optical arrangement according to claim 9, wherein the beam shaping element (6) includes at least one filter element (10) , especially a gray filter.

11. Optical arrangement according to claim 9 or 10, wherein the beam shaping element (6) includes at least one aperture stop (11) .

12. Optical arrangement according to any of claims 9 to 11, wherein the beam shaping element includes at least one diffractive element.

13. Optical arrangement according to claim 10 or 11, wherein the filter element (10) and/or the aperture stop (11) include a plurality of movable parts (27 to 29) in order to form the shape of the correction light beam (4) .

14. Optical arrangement according to claim 13, wherein the movable parts (27 to 29) have the shape of concen- trie rings or ring sectors being arranged to be rotatable with respect to an axis passing through the center of the ring or the ring sectors .

15. Optical arrangement according to claim 13, wherein the movable parts have the shape of elliptical rings or ring segments .

16. Optical arrangement according to any of claims 7 to 15, wherein the surface deformation device (5, 6) includes an array (40) of light emitting elements (41) for forming a correction light beam (4) emitted by the correction light source (5) corresponding to the at least one region (7) of the optical surface (3) to be deformed by the surface deformation device (5, 6) .

17. Optical arrangement according to claim 16, wherein the array (40) includes a plurality of concentric rings (42 to 44) , each ring (42 to 44) including a plurality of single light elements (41) .

18. Optical arrangement according to claim 17, wherein the rings are elliptical rings .

19. Optical arrangement according any of claims 16 to 18, wherein the single light elements (41) are light emitting diodes, in particular laser diodes.

20. Optical arrangement according to any of claims 7 to 19, wherein the optical element (16, 20) is a mirror and the back surface (39) of the mirror (16, 20) is illuminated by a correction light beam (4) having a homogeneous energy distribution.

21. Optical arrangement according to any of the preceding claims, wherein the optical element (16, 20) having the optical surface (3) to be deformed is a mirror and the surface deformation device (51; 52; 53) includes at least one deformation component (51; 52; 53) cooperating with the optical element (16, 20) by deforming the surface (3) of the optical element (16, 20) via a relative thermal dimensional change of the deformation component (51; 52; 53) relative to the optical element (16, 20).

22. Optical arrangement according to claim 21, wherein the relative thermal dimensional change is the result of a thermal change of the dimension of the deformation component (51; 52; 53;).

23. Optical arrangement according to claim 21 or 22, wherein the relative thermal dimensional change is the result of the thermal change of the dimension of the optical element (16, 20) .

24. Optical arrangement according to any of claims 21 to 23, wherein the deformation component (52; 53) is an integral part of the optical element .

25. Optical arrangement according to any of claims 21 to 23, wherein the deformation component (51) is a separate part being connected to the optical element (16, 20) .

26. Optical arrangement according to any of claims 21 to 25, wherein the surface deformation device includes a plurality of deformation components (51; 52; 53) in a wheel-spoke arrangement.

27. Optical arrangement according to any of claims 21 to 25, wherein the surface deformation device includes a plurality of deformation components (51; 52; 53) in form of concentric elliptical rings.

28. Optical arrangement according to claim 26, wherein each of the deformation components (53) has a trian- gular shape tapering towards the center or the optical axis of the optical element (16, 20) .

29. Optical arrangement according to any of the preceding claims, wherein the at least one region (7) of the optical surface (3) to be deformed by the surface deformation device (5, 6; 54 to 56; 59, 60; 63, 64; 51; 52; 53) is ring-shaped.

30. Optical arrangement according to claim 29, wherein the center (54 to 56; 58 to 60; 63, 64; 21 to 26) of the ring-shaped region coincides with the optical axis of the optical element (1; 16, 20) .

31. Optical arrangement according to claim 29 or 30, wherein the at least one region of the optical surface (3) to be deformed by the surface deformation device (5, 6; 54 to 56; 59, 60; 63, 64; 51; 52; 53) includes a plurality of concentric ring-shaped regions (54 to 56; 58 to 60; 63, 64; 21 to 26) .

32. Optical arrangement according to any of claims 29 to 31, wherein the at least one region of the optical surface

(3) to be deformed by the surface deformation device (5, 6; 54 to 56; 59, 60; 63, 64; 51; 52; 53) includes a plurality of sectors (57) of the at least one ring-shaped region (54 to 56; 59,^' 60; 63, 64; 22 to 26) .

33. Optical arrangement according to any of claims 29 to 32, wherein the at least one ring shaped region is an elliptical ring shaped region.

34. Optical arrangement according to any of the preceding claims, wherein the at least one region of the optical surface (3) to be deformed by the surface deformation device (5, 6) has a shape which is complementary to another region (31) of the optical surface which is deformed by the energy of projection light (2) being guided by the optical element (16) .

35. Optical arrangement according to any of claims 4 to 34, wherein the at least one region (7'') of the back surface (39) to be deformed by the surface deformation device (5, 6) has a shape which is similar to a region (7') of the optical surface (3) which is deformed by the energy of projection light (2) being guided by the optical element (16) .

36. Optical arrangement according to any of claims 4 to 34, wherein the at least one region (8') of the back surface (39) to be deformed by the surface deformation device (5, 6) has a shape which is similar to the desired deformation (8) of the optical surface (3) of the optical element (16 ' ) .

37. Optical arrangement according to any of the preceding claims, including a plurality of optical elements (15 to 20) , wherein the optical element (16) having the optical surface (3) to be deformed is selected such that a deformation of the optical surface (3) resulting in a specific height distribution of the optical surface leads to a similar distribution of relative changes in the optical properties of the optical arrangement .

38. Method of specifying and performing a deformation of an optical surface (3) to be generated by a surface deformation device according to any of the preceding claims including the steps of :

a) measuring (32) aberrations from the desired optical properties of the optical arrangement in a field plane (14) of the optical arrangement;

b) determining (34) a relative thermal energy profile to be generated by the surface deformation device (5, 6; 54 to 56; 59, 60; 63, 64; 51; 52; 53);

c) determining (35) the absolute thermal energy profile using material parameters of the region (7) of the optical element (1; 16, 20; 16') which is to be thermalized;

d) thermalizing the optical element (1; 16, 20; 16') according to the determined absolute thermal energy profile .

39. Method according to claim 38, wherein the thermalizing step includes an illumination of the optical element (1; 16, 20; 16') by a correction light beam (4) .

40. Method according to claim 39, wherein the step of illuminating the optical element (1; 16, 20; 16') by a correction light beam (4) includes the generation of the correction light beam (4) using an array of single light elements (41) which are controlled individually.

41. Method according to any of claims 38 to 40, wherein the measured (32) aberrations are decomposed (48) in a basis of orthonormal polynomials.

42. Method according to any of claims 38 to 40, wherein the imaging property effect of thermalizing selected regions of the optical element (1; 16, 20; 16') by activating selected modes of the surface deformation device (5, 6; 54 to 56; 59, 60; 63, 64; 51; 52; 53) is measured and the measured aberration is approximated (49) by a model function obtained by a superposition of the imaging property effects previously measured in the selected modes of the suface deformation device (5, 6; 54 to 56; 59, 60; 63, 64; 51; 52; 53), determining the corresponding contri- butions of the selected modes and activating the selected modes accordingly.

43. Method according to claim 42, wherein the selected modes are members of orthonormal polynomials .

44. Method according to any of claims 40 to 43, wherein the relative thermal energy profile represented by a relative light intensity profile of the single light elements (41) across the array (40) is determined (50) by determining the shape of a punctual deformation, e. g. a deformation induced on the optical surface (3) by a single light element (41) and by inverse convolution of the deformation pattern showing the punctual deformation,

45. Method according to any of claims 36 to 44, wherein the determination of the thermal energy profile is done by singular value decomposition (SVD) .

46. Method according to any of claims 38 to 45, wherein the transient behaviour of the heat induced aberration is taken into account by anticipating the deformation of the optical element (1; 16, 20; 16').

47. Method according to claim 46, wherein the transient behaviour of an aberration and the transient behaviour of the deformation of the optical element (1; 16, 20; 16 ' ) induced by heat are determined and wherein the transient behaviour of the aberration and the transient behaviour of the deformation of the optical element (1; 16, 20) induced by heat are synchronized by varying the intensity of a correction light source used for heating the optical element (1; 16, 20; 16').

48. Method according to claim 45, wherein the transient correction comprises the following steps:

a) analysis and characterization of exposure light heat up;

b) determination of influence function for external heat sources heat up;

c) conception of a feed forward algorithm;

d) synchronization of exposure light heat up with influence functions;

e) intensity variation of influence functions.

49. Method according to claim 47, wherein the transient behaviours of the aberration and of the deformation of the optical element (1; 16, 20; 16') induced by heat are determiend by experiment .