WO2013050198A1

WO2013050198A1 - Method for setting the intensity distribution in an optical system of a microlithographic projection exposure apparatus, and optical system

Info

Publication number: WO2013050198A1
Application number: PCT/EP2012/065827
Authority: WO
Inventors: Jörg ZIMMERMANN
Original assignee: Carl Zeiss Smt Gmbh
Priority date: 2011-10-07
Filing date: 2012-08-13
Publication date: 2013-04-11
Also published as: DE102011084152A1

Abstract

The invention relates to a method for setting the intensity distribution in an optical system of a microlithographic projection exposure apparatus, and to an optical system. A method according to the invention comprises the following steps: producing a transmission-influencing layer (140, 240, 340) on at least one surface of an optical element (122, 223, 322, 508, 509, 512, 716, 816) arranged in the optical system, and partially eliminating said layer (140, 240, 340) by irradiating a part of the layer in such a way that the optical element (122, 223, 322, 508, 509, 512, 716, 816) has a predetermined transmission distribution owing to the remaining portion (140a, 240a, 340a) of the layer.

Description

Method for setting the intensity distribution

in an optical system of a microlithographic projection exposure apparatus, and optical system

CROSS REFERENCE TO RELATED APPLICATIONS

This application claims priority of German Patent Application DE 10 201 1 084 152.0 and US 61/544,345, both filed on October 7, 201 1. The content of these applications is hereby incorporated by reference.

BACKGROUND OF THE INVENTION

Field of the invention

The invention relates to a method for setting the intensity distribution in an optical system of a microlithographic projection exposure apparatus, and to an optical system.

Prior art

Microlithography is used for producing microstructured components, such as, for example, integrated circuits or LCDs. The microlithography process is carried out in a so-called projection exposure apparatus comprising an illumination device and a projection lens. The image of a mask (= reticle) illuminated by means of the illu- mination device is in this case projected by means of the projection lens onto a substrate (e.g. a silicon wafer) coated with a light-sensitive layer (photoresist) and arranged in the image plane of the projection lens, in order to transfer the mask structure to the light-sensitive coating of the substrate. In the illumination device and also in the projection lens, a targeted setting of defined illumination settings and/or of different apodizations may be desired for different reasons. In this case, here and hereinafter an illumination setting is under- stood to mean the intensity distributions in a pupil plane of the illumination device. Apodization is understood to mean the transmission distribution in a pupil plane of the projection lens.

One example of application of such a targeted setting is so-called "Tool-to-Tool matching", which is performed in order to match the imaging properties of different projection exposure apparatuses to one another to the effect that the combination of illumination device, reticle and projection lens at the wafer level ideally produces the same imaging result in the event of exchange between the two systems, and corresponding results are thus obtained during exposure at the wafer level. Fur- ther examples are the optimization of the depth of focus or of the imaging contrast by providing the best possible illumination setting with respect to the process window, or the best possible apodization.

For the targeted setting of defined illumination settings, diverse approaches are known, for example by using diffractive optical elements (so-called DOEs) or else mirror arrangements comprising a multiplicity of micromirrors that can be set independently of one another, as disclosed e.g. in WO 2005/026843 A2. Furthermore, e.g. the use of transmission filters is known in order to set a desired apodization in the projection lens.

Since the parameters relevant to the illumination setting respectively obtained and also the apodization and thus the imaging properties can change over time, there is a need to be able to set their setting as well in a flexible manner depending on the variable conditions. SUMMARY OF THE INVENTION

It is an object of the present invention to provide a method for setting the intensity distribution in an optical system of a microlithographic projection exposure appara- tus and an optical system which enable an increased flexibility with regard to the intensity distributions that can be set in the projection exposure apparatus.

This object is achieved in accordance with the features of the independent patent claims.

A method according to the invention for setting the intensity distribution in an optical system of a microlithographic projection exposure apparatus comprises the following steps:

- Producing a transmission-influencing layer on at least one surface of an op- tical element arranged in the optical system; and

- Partially eliminating said layer by irradiating a part of the layer in such a way that the optical element has a predetermined transmission distribution owing to the remaining portion of the layer. The invention is based on the concept, in particular, of - for the purpose of setting intensity distribution - setting a desired transmission distribution by firstly covering a surface of an optical element arranged in the optical system with a transmission- influencing layer or contamination and subsequently selectively freeing it again of said transmission-influencing layer or contamination, with the result that the de- sired transmission distribution arises on account of the remaining portion of the layer.

According to the invention, the step of partially eliminating the transmission- influencing layer by irradiating a part of the layer is preferably performed "in situ", or while the optical element is arranged in the optical system of the microlithographic projection exposure apparatus, respectively. Further, the light source used for partially eliminating the transmission-influencing layer by irradiating a part of the layer can be a part of the optical system of the microlithographic projection exposure apparatus. Furthermore, the (preceding) step of producing a transmission- influencing layer on at least one surface of the optical element is also preferably performed while the optical element is arranged in the optical system of the micro- lithographic projection exposure apparatus.

The step of partially eliminating the layer (also designated hereinafter as "contamination layer") by irradiating a part of the contamination layer with UV light is also designated as "burning free". However, the invention is not restricted to irradiation with UV light. In further embodiments, irradiation with electrons can also be effected for partially eliminating the layer.

The procedure according to the invention firstly takes account of the fact that the setting of the desired transmission distribution, which can be effected, in particular, by introducing an inorganic or organic chemical compound, for example a hydrocarbon compound, into a region adjoining the surface of the optical element, is generally not directly possible by means of selective application of the contamination, since exact control of the deposition of contamination particles is not possible or the contamination particles firstly deposit substantially homogeneously on the relevant surface.

Secondly, the procedure according to the invention leads to a high flexibility with regard to the setting of different transmission distributions in so far as the steps of producing and subsequently partially eliminating the contamination layer can be carried out repeatedly - and merely with variation of the respectively irradiated region of the contamination layer - such that other transmission profiles can be set after renewed "contamination" by means of corresponding irradiation.

The approach according to the invention is based on the insight that for the partial elimination of the contamination layer that is effected in the method of the invention, typically required intensities of the UV light used for this purpose generally significantly exceed the intensities occurring during the actual lithography process. Consequently, the deliberately set, remaining regions of the contamination layer are not already significantly degraded during the actual lithography process itself (i.e. the "normal" operation of the projection exposure apparatus), rather a stable operation of the projection exposure apparatus with the transmission profile respectively set (that is to say stably set contamination layer) is possible.

In accordance with one embodiment, producing a contamination layer is effected by introducing an inorganic or organic chemical compound, more particularly a hydrocarbon compound, into a region adjoining the surface of the optical element. In accordance with one embodiment, the projection exposure apparatus comprises a light source and an illumination device, wherein UV light generated by the light source enters into the illumination device, and wherein irradiating a part of the contamination layer with UV light is effected by means of light from said light source. The invention can thus be realized with little outlay since the light source present anyway in the projection exposure apparatus can be used for partially eliminating the contamination layer. Here in each case during the introduction of the contamination particles or the application of the contamination layer, no mask is provided in the object plane of the projection lens. In accordance with another embodiment, the projection exposure apparatus comprises a first light source and an illumination device, wherein UV light generated by the light source enters into the illumination device, and wherein irradiating a part of the contamination layer with UV light is effected by means of light from a second light source, which is provided in addition to the first light source. The use of a second light source provided in addition to the first light source has the advantage that, if appropriate, other wavelength ranges and/or other (in particular higher) intensities can be used for the "burning free".

In accordance with one embodiment, the step of partially eliminating the contami- nation layer comprises the following step: setting a predetermined intensity distribution of the light impinging on the contamination layer produced, wherein said intensity distribution is set in a manner dependent on the predetermined transmission distribution. ln accordance with one embodiment, setting the predetermined intensity distribution in this way is effected using a mirror arrangement comprising a plurality of mutually adjustable mirror elements.

In accordance with one embodiment, setting the predetermined intensity distribution is effected using a diffractive optical element (DOE), which is designed specifically for the process of "burning free" according to the invention. Such a DOE can, in particular, also be provided in a changer having further DOEs designed for the process of "burning free" and/or with DOEs designed for the actual lithography process.

In accordance with one embodiment, the steps according to the invention of producing and of subsequently partially eliminating the contamination layer are car- ried out repeatedly, wherein at least two mutually different transmission distributions of the optical element are set.

In this case, in particular, setting the mutually different transmission distributions can be effected by using different diffractive optical elements.

In accordance with one embodiment, the method furthermore comprises the following step: determining an intensity distribution in a pupil plane of the optical system, wherein partially eliminating the contamination layer in the manner according to the invention is effected in such a way that the intensity distribution determined approximates to a desired intensity distribution. In this case, determining the intensity distribution can be effected directly on the basis of a spatially resolved measurement of light coupled out from said pupil plane. Furthermore, determining the intensity distribution can also be effected indirectly on the basis of an angularly resolved measurement of light coupled out from an object plane or an image plane of the microlithographic projection exposure apparatus.

The invention furthermore relates to an optical system of a microlithographic projection exposure apparatus, comprising: - an optical element;

- a source for producing a transmission-influencing layer on at least one surface of the optical element; and

- a radiation source for partially eliminating said layer by irradiating a part of the layer.

In accordance with one embodiment, the optical element is arranged in a pupil plane of the optical system. In accordance with a further embodiment, the optical element is arranged outside a pupil plane of the optical system.

In accordance with one embodiment, the radiation source is a UV light source. In accordance with one embodiment, the projection exposure apparatus comprises a first light source and an illumination device, wherein UV light generated by the light source enters into the illumination device, and wherein the UV light source for partially eliminating the contamination layer by irradiating a part of the contamination layer with UV light is a second light source, which is provided in addition to the first light source.

In accordance with one embodiment, the optical system furthermore comprises a changer for exchanging the optical element. For further preferred configurations and advantages of the optical system, reference is made to the above explanations in connection with the method.

The invention furthermore relates to a microlithographic projection exposure apparatus comprising an optical system according to the invention having the features described above. Further configurations of the invention can be gathered from the description and the dependent claims. The invention is explained in greater detail below on the basis of exemplary embodiments illustrated in the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

In the figures: Figures 1 a-c show schematic illustrations for elucidating the sequence of a method according to the invention in a first embodiment;

Figures 2-3 show schematic illustrations for elucidating further embodiments of the method according to the invention;

Figures 4a-d show schematic illustrations for elucidating exemplary transmission profiles or intensity distributions that can be set by means of the method according to the invention; and Figures 5-8 show schematic illustrations in each case of a projection exposure apparatus in which the invention is realized.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

Firstly, the sequence of a method according to the invention in a first embodiment is explained below with reference to Fig. 1 a-c. Fig. 1 a shows, in a simplified illustration, a microlithographic projection exposure apparatus comprising an illumination device 1 10 (symbolized merely by a single lens element 1 1 1 ) and a projection lens 120 (symbolized merely by two lens elements 121 , 122), wherein the illumination device 1 10 illuminates a mask (reticle) 1 15 situated in an object plane of the projection lens 120, and wherein the projection lens 120 images said object plane onto an image plane, in which a wafer 125 having a light-sensitive layer is situated during the lithography process.

In accordance with Fig. 1 a, the optical system illustrated furthermore comprises a contamination source 130, by means of which contamination particles in the form of an inorganic or organic chemical compound, e.g. a hydrocarbon compound (CxHy), are introduced, by means of a nozzle provided with a valve, into a region adjoining the surface of an optical element of the optical system, the light entrance surface of the lens element 122 in the exemplary embodiment.

Suitable hydrocarbon compounds which "can be burned free" by means of UV radiation are (without the invention being restricted to them) readily volatile compounds having a boiling point below 150°C, for example acetone (C3H₆0), 2- methylpentane (CeHi ), acetic acid (C2H 02), methyl ethyl ketone (2-butanone, C₄H₈0), m-/p-xylene (CsHio), benzene (C6H₆), toluene (C7H8), styrene (CsHs). Low-volatility compounds such as e.g. phenol (C6H₆0) or benzyl alcohol (C7H8O) are likewise conceivable in principle, but are able to be "burned free" with UV radiation less successfully. During the "burning-free" process, the long hydrocarbon chains are decomposed, and the short-chain radicals evaporate. The burning-free process can be supported by the presence of oxygen (O2). Comparatively less well suited are silicon-containing compounds or compounds which contain plasticizers, since this can lead to irreversible salt formation.

Typical layer thicknesses of the contamination layer can be (without the invention being restricted to them) in the range of 10 nm to 50 nm.

In this way, as illustrated schematically in Fig. 1 a, a contamination layer 140 is produced on the relevant surface of the lens element 122. Typically, the region adjoining e.g. the light entrance surface of the lens element 122 substantially consti- tutes a closed-off volume, such that the contamination particles can be introduced there. Since, in principle, contamination layers can form on all surfaces within the closed- off volume, wherein all surfaces in the beam path are freed of the contamination during the burning-free process, the exit surface of the preceding lens element should also be taken into consideration, under certain circumstances. If the lens element respectively preceding the relevant surface (e.g. the light entrance surface of the lens element 122) in the beam path is arranged sufficiently far away from the preferred lens element surface and accordingly no longer in a pupil plane, it can be expedient for the volume within which the contamination according to the invention is effected to be kept as small as possible by means of a thin transparent (glass) plate.

In the exemplary embodiment in Fig. 1 , the optical element provided with the contamination layer 140 is a lens element 122 of the projection lens 120. However, the invention is not restricted thereto. Rather, in other embodiments, as illustrated schematically in Fig. 2, the optical element provided with the contamination layer 240 can also be an optical element 223 which is introduced into the optical system especially for this purpose - and moreover is not required for the imaging process (e.g. has no refractive power or is configured as a plane-parallel plate). In this case, in Fig. 2 elements which are analogous or substantially functionally identical to Fig. 1 a-c are designated by corresponding reference numerals higher by 100.

In accordance with Fig. 1 b, a further step, as illustrated in Fig. 1 b, involves partially eliminating the contamination layer 140 by irradiating a part of the contamination layer 140 with UV light 150. In this case, those regions for which the transmission is intended to be maximal in the transmission distribution ultimately desired are illuminated with corresponding illumination distribution. In the exemplary embodiment, the UV light 150 used for this purpose is generated by means of the same light source which also generates the UV light which is required for the actual lithography process and is coupled into the illumination device 1 10. However, the invention is not restricted thereto. Rather, in other embodiments, as illustrated schematically in Fig. 3, the UV light 350 used for partially eliminating the contamination layer 340 can also be generated by means of an additionally provided ("external") light source. Thus, if appropriate, other wavelength ranges and/or other (in particular higher) intensities can be used for the "burning-free" process. Typical wavelengths that can be used for the "burning-free" process are e.g. approximately 157 nm (from an F₂ laser as light source), approximately 193 nm (from an ArF excimer laser as light source), approximately 248 nm (from a KrF excimer la- ser as light source) or else approximately 365 nm (from a mercury high-pressure lamp as light source).

In Fig. 3, elements which are analogous or substantially functionally identical to Fig. 1 a-c are designated by corresponding reference numerals higher by 200. In the examples which are illustrated in Fig. 1 -3 and in which the optical element provided with the contamination layer 140, 240 and 340, respectively, is situated within the projection lens, no mask is provided in the object plane of the projection lens in each case during the introduction of the contamination particles or the application of the contamination layer. Consequently, the same intensity distribution as in the pupil plane of the illumination device 1 10, 210 and 310, respectively, arises in the pupil plane of the projection lens 120, 220 and 320, respectively.

The result of the irradiation of the optical element 122 illustrated in Fig. 1 b is illustrated schematically in Fig. 1 c. Accordingly, owing to the irradiation with the UV light 150, a region 140b free of contamination particles has arisen between a residual portion 140a of the contamination layer 140. Since the intensities of the UV light 150 which are typically required for partially eliminating the contamination layer in accordance with Fig. 1 b significantly exceed the intensities occurring during the actual lithography process, the residual portion 140a of the contamination layer 140 is maintained substantially stably during the subsequent lithography process. Consequently, for the lithography process, a defined transmission profile dependent on the intensity distribution used during the irradiation with UV light 150 in accordance with Fig. 1 b was set at the location of the optical element 122. The setting of the intensity distribution used during the irradiation with UV light 150 in accordance with Fig. 1 b - and thus also of the transmission profile ultimately resulting at the location of the optical element 122 - can be effected, for example, by positioning a diffractive optical element (DOE) that is suitable, or is coordinated with said intensity distribution, in the illumination device. In further exemplary embodiments, the flexible setting of the intensity distribution used during the irradiation with UV light 150 in accordance with Fig. 1 b can also be effected using a mirror arrangement comprising a multiplicity of mirror elements that can be set inde- pendently of one another.

Exemplary profiles of transmission profiles that can be set by means of the method described above are illustrated in Fig. 4a-d. This involves in accordance with Fig. 4a a rotationally symmetrical profile having an obscuration (transmission = 0) within a predefined radius, in accordance with Fig. 4b a rotationally symmetrical profile having a transmission that is attenuated (e.g. by a few, in the example five, percent) outside a predefined radius, a rotationally symmetrical profile with a gradual radial profile, in accordance with Fig. 4c a freeformed transmission distribution (e.g. having regions having a transmission T=1 and further regions having trans- mission values ti , t₂ and t₃, respectively) or in accordance with Fig. 4d a transmission profile that is parameterized, e.g. described by a Zernike polynomial.

Concrete exemplary embodiments of a projection exposure apparatus in which the invention is in each case realized in the illumination device are respectively ex- plained below with reference to Fig. 5-8.

The projection exposure apparatus illustrated schematically in Fig. 5 comprises a light source 501 , an illumination device 510 and a projection lens 520. The illumination device 510 comprises downstream of the light source 501 in the light propagation direction successively a beam shaping unit 502, a deflection mirror

503, a diffractive optical element (DOE) 504, a zoom lens 505, an optical integrator or a light mixing system 506 in the form of a fly's eye lens, a converging lens 507 and a masking or diaphragm system 516, which is imaged by means of a REMA lens 512 onto a mask (reticle) 515 situated in an object plane of the projec- tion lens 520. Structures situated on the mask 515 are imaged in the lithography process into the image plane of the projection lens 520, in which a wafer 525 having a light-sensitive layer is arranged. In Fig. 5, the projection lens 520 is symbolized merely by two lens elements 521 , 522 for the sake of simplicity. A realization of the invention in the projection exposure apparatus in accordance with Fig. 5 can be effected, in particular, by an optical element 512, as indicated in a hatched manner in Fig. 5, being positioned within the pupil plane of the REMA lens 51 1 , wherein firstly a contamination layer is then applied to said element 512 in accordance with the method according to the invention and then said element is partly freed again of said contamination layer by means of UV irradiation in order to set a desired transmission distribution. Furthermore, it is also possible, as indicated in Fig. 6, for the contamination layer to be applied to the light exit surface 616b of a lens element 616 and the light entrance surface 617a of the adjacent lens 617 and for these surfaces then to be partly freed again of said contamination layer by means of UV irradiation in order to set a desired transmission distribution, wherein use is made of the fact that the successive lens elements 616, 617 in the beam path already enclose a substantially closed interspace into which the contamination particles can be introduced.

In accordance with a further embodiment, it is also possible for two optical elements 508, 509, as likewise indicated in a hatched manner in Fig. 5, to be ar- ranged in positions outside a pupil plane and, in an analogous manner, firstly for a contamination layer to be applied to said optical elements and then for the latter to be partly freed again of said contamination layer by means of UV irradiation in order to set a desired transmission distribution. By virtue of the fact that the optical elements 508, 509 are arranged in positions outside a pupil plane, field-dependent effects of the intensity distribution can be set or compensated for. In the case where the optical elements 508, 509 are used, different transmission profiles are preferably set on the relevant surfaces of the elements 508, 509. In the concrete exemplary embodiment, this can also be effected using the (system-inherent) light source 501 , specifically by virtue of the fact that after the production of a contami- nation layer on both elements 508, 509 in a two-stage UV irradiation, firstly with diaphragm system 516 open both elements 508, 509 are subjected to a UV irradiation in order to set the transmission distribution desired for the second element 509, then the diaphragm system 507 is closed and then with the diaphragm sys- tem closed only the first element 508 is subjected to a UV irradiation in order to set the transmission distribution desired for said first element 508.

A projection exposure apparatus illustrated schematically in Fig. 7 comprises a light source 701 , an illumination device 710 and a projection lens 720. The illumination device 710 comprises downstream of the light source 701 in the light propagation direction successively a beam shaping unit 702, a deflection mirror 703, a diffractive optical element (DOE) 704 and an afocal optical unit 705, the front focal point of which lies on the DOE 704 and the back focal point of which lies in the plane depicted in a dashed manner. A compensation filter 706 is situated in the pupil plane within the afocal optical unit 705 or in proximity to said pupil plane.

Downstream of the afocal optical unit 705 along the light propagation direction there follow a zoom lens 707, an optical integrator or a light mixing system 708 in the form of a fly's eye lens, a converging lens 709 and a masking or diaphragm system 71 1 , which is imaged by means of a REMA lens 712 (comprising two lens elements 712a and 712b and a deflection mirror 713) onto a mask (reticle) 715 situated in an object plane of the projection lens 720. Structures situated on the mask 715 are imaged in the lithography process into the image plane of the pro- jection lens 720, in which a wafer 725 having a light-sensitive layer is arranged.

The realization of the invention in the projection exposure apparatus in accordance with Fig. 7 can be effected, in particular, by an optical element 716, as indicated in a hatched manner in Fig. 7, being positioned in direct proximity to the optical inte- grator or light mixing system 708 (which is situated in a pupil plane), wherein firstly a contamination layer is then applied to said element 716 in accordance with the method according to the invention and said element is then partly freed again of said contamination layer by means of UV irradiation in order to set a desired transmission distribution.

In the system shown in Fig. 7, too, depending on the distance between the optical element 716 and the light mixing system 708, the setting of the transmission distri- bution - as described with reference to Fig. 5 - can be effected with or without field-dependent effects.

Fig. 8 shows a further possible exemplary embodiment, wherein in comparison with the construction from Fig. 7 analogous or substantially functionally identical components are designated by reference numerals increased by "100".

In accordance with Fig. 8, an optical element 816, to which firstly a contamination layer is applied in accordance with the method according to the invention and which is then partly freed again of said contamination layer by means of UV irradiation in order to set a desired transmission distribution, is positioned, as indicated in a hatched manner, within the pupil plane of the REMA lens. In accordance with Fig. 8, the intensity distribution respectively generated in the illumination device is furthermore measured, for which purpose a beam splitter 818 for coupling out illumination light and a detector unit 830 for intensity measurement are provided in accordance with the exemplary embodiment. On the basis of the measurement result of the detector unit 830 the originally desired or predetermined transmission distribution - to be set by "burning free" by means of the method according to the invention - of the optical element 816 can in turn be ad- justed and corrected if necessary.

In the case of the above-described embodiments in Fig. 5-8, the concept of "burning free" according to the invention, as in the case of the embodiments described above with reference to Fig. 1 -4, has the advantage, in particular, that a high flexi- bility with regard to the setting of different transmission distributions can be obtained in so far as the steps of producing and subsequently partially eliminating the contamination layer can be carried out repeatedly - and merely with variation of the respectively irradiated region of the contamination layer - such that other transmission profiles can be set after renewed "contamination" by means of corre- sponding irradiation.

Even though the invention has been described on the basis of specific embodiments, numerous variations and alternative embodiments, e.g. by combination and/or exchange of features of individual embodiments, are evident to the person skilled in the art. Accordingly, it goes without saying for the person skilled in the art that such variations and alternative embodiments are concomitantly encompassed by the present invention, and the scope of the invention is restricted only within the meaning of the accompanying patent claims and the equivalents thereof.

Claims

Method for setting the intensity distribution in an optical system of a micro- lithographic projection exposure apparatus, wherein the method comprises the following steps: a) Producing a transmission-influencing layer (140, 240, 340) on at least one surface of an optical element (122, 223, 322, 508, 509, 512, 716, 816) arranged in the optical system; and b) partially eliminating said layer (140, 240, 340) by irradiating a part of the layer in such a way that the optical element (122, 223, 322, 508, 509, 512, 716, 816) has a predetermined transmission distribution owing to the remaining portion (140a, 240a, 340a) of the layer.

Method according to Claim 1 , characterized in that producing a transmission- influencing layer (140, 240, 340) in step a) is effected by introducing an inorganic or organic chemical compound, more particularly a hydrocarbon compound, into a region adjoining the surface of the optical element (122, 223, 322, 508, 509, 512, 716, 816).

Method according to Claim 1 or 2, characterized in that partially eliminating said layer (140, 240, 340) is effected by irradiation with UV light (150, 350).

Method according to Claim 3, characterized in that the projection exposure apparatus comprises a light source and an illumination device, wherein UV light generated by the light source enters into the illumination device, and wherein irradiating a part of the layer (140, 240) with UV light in step b) is effected by means of light (150) from said light source.

Method according to Claim 3, characterized in that the projection exposure apparatus comprises a first light source and an illumination device, wherein UV light generated by the light source enters into the illumination device, and wherein irradiating a part of the layer (340) with UV light in step b) is effected by means of light (350) from a second light source, which is provided in addition to the first light source.

Method according to any of the preceding claims, characterized in that step b) of partially eliminating said layer comprises the following step:

- Setting a predetermined intensity distribution of the light (150, 350) impinging on the layer (140, 240, 340) produced in step a), wherein said intensity distribution is set in a manner dependent on the predetermined transmission distribution.

Method according to Claim 6, characterized in that setting said predetermined intensity distribution is effected using a mirror arrangement comprising a plurality of mutually adjustable mirror elements.

Method according to Claim 6, characterized in that setting said predetermined intensity distribution is effected using a diffractive optical element (DOE).

Method according to any of the preceding claims, characterized in that steps a) and b) are carried out repeatedly, wherein at least two mutually different transmission distributions of the optical element (122, 223, 322, 508, 509, 512, 716, 816) are set.

Method according to Claim 9, characterized in that setting the mutually different transmission distributions is effected by using different diffractive optical elements.

Method according to any of Claims 1 to 10, characterized in that the optical system is an illumination device of the microlithographic projection exposure apparatus.

12. Method according to any of Claims 1 to 10, characterized in that the optical system is a projection lens of the microlithographic projection exposure apparatus.

13. Method according to any of the preceding claims, characterized in that said method furthermore comprises the following step:

- Determining an intensity distribution in a pupil plane of the optical system;

- wherein partially eliminating the layer in step b) is effected in such a way that the intensity distribution determined approximates to a desired intensity distribution.

14. Method according to Claim 13, characterized in that determining the intensity distribution is effected directly on the basis of a spatially resolved measurement of light coupled out from said pupil plane.

15. Method according to Claim 13, characterized in that determining the intensity distribution is effected indirectly on the basis of an angularly resolved measurement of light coupled out from an object plane or an image plane of the microlithographic projection exposure apparatus.

16. Optical system of a microlithographic projection exposure apparatus, comprising:

• an optical element (122, 223, 322, 508, 509, 512, 716, 816);

• a source (130, 230, 330) for producing a transmission-influencing layer (140, 240, 340) on at least one surface of the optical element (122, 223, 322, 508, 509, 512, 716, 816); and

• a radiation source for partially eliminating said layer by irradiating a part of the layer.

17. Optical system according to Claim 16, characterized in that the radiation source is a UV light source.

18. Optical system according to Claim 16 or 17, characterized in that the optical element is arranged in a pupil plane of the optical system. 19. Optical system according to any of Claims 16 to 18, characterized in that the projection exposure apparatus comprises a first light source and an illumination device, wherein UV light generated by the light source enters into the illumination device, and wherein the radiation source for partially eliminating the layer by irradiating a part of the layer is a second light source, which is provided in addition to the first light source.

20. Optical system according to any of Claims 16 to 19, characterized in that said optical system furthermore comprises a changer for exchanging the optical element (122, 223, 322, 508, 509, 512, 716, 816).

21 . Optical system according to any of Claims 16 to 20, characterized in that the optical system is an illumination device of the microlithographic projection exposure apparatus. 22. Optical system according to any of Claims 16 to 20, characterized in that the optical system is a projection lens of the microlithographic projection exposure apparatus.

23. Microlithographic projection exposure apparatus, characterized in that it comprises an optical system according to any of Claims 16 to 22.