Method for operating a microlithographic projection exposure apparatus, and projection exposure apparatus

Abstract

The invention relates to a method for operating a microlithographic projection exposure apparatus (1). In the method according to the invention, during a first exposure phase with the aid of an illumination optical unit (4) illumination radiation (16) according to a first illumination setting is formed and the illumination radiation (16) is fed to an image field (11) of a projection optical unit (10) with the aid of the projection optical unit (10). During a second exposure phase, with the aid of the illumination optical unit (4) illumination radiation (16) according to a second illumination setting, which differs from the first illumination setting, is formed and the illumination radiation (16) is fed to the image field (11) of the projection optical unit (10) with the aid of the projection optical unit (10). During an exposure pause between the first exposure phase and the second exposure phase, with the aid of the illumination optical unit (4) illumination radiation (16) according to the second illumination setting is formed. This illumination radiation (16) is incident on an optical element (M2) of the projection optical unit (10), but is not fed to the image field (11) of the projection optical unit (10).

Description

[0001] Method for operating a microlithographic projection exposure apparatus, and projection exposure apparatus

[0002] The invention relates to a method for operating a microlithographic projection exposure apparatus. Furthermore, the invention relates to a projection exposure apparatus.

[0003] Projection exposure apparatuses are used in particular in the production of semiconductors and generally have an illumination optical unit and a projection optical unit. Projection exposure apparatuses are operated with radiation generated by a radiation source. This radiation is also referred to as illumination radiation. The illumination optical unit generates from the illumination radiation a desired illumination radiation distribution, which is also referred to as illumination setting, for illuminating a reticle, which is also referred to as a mask. The projection optical unit is used to image the reticle onto a radiation-sensitive (lightsensitive) material, which has been applied, for example, on a wafer or on some other substrate, in particular made from a semiconductor material. In this way, the radiationsensitive material is exposed in a structured manner with a pattern predefined by the reticle. Since the reticle has tiny structural elements, which are intended to be transferred to the substrate with high precision, it is required that the illumination optical unit forms a desired illumination setting precisely and reproducibly and the imaging by the projection optical unit takes place precisely and reproducibly.

[0004] The illumination optical unit and the projection optical unit have a number of optical elements, such as lenses or mirrors, for example, which deflect the illumination radiation in a predefined manner. In general, a portion of the illumination radiation is in each case absorbed by the respective optical element, and so the optical elements are heated up. The thermal expansion of the optical elements associated with the heating up results in a change in the optical properties of the affected optical elements, which ultimately influences the radiation deflection of the optical elements. This has particularly negative consequences for the optical elements of the projection optical unit, since this leads to imaging errors (aberrations) and thus limits the precision achievable with the projection optical unit.

[0005] Projection exposure apparatuses are being supplied by increasingly powerful radiation sources, resulting in the optical elements of the projection optical unit becoming heated up to a greater and greater extent. In addition, the ongoing miniaturization in semiconductor fabrication requires ever greater precision for the imaging by the projection optical unit. Both exacerbate the problem associated with the heating up of the optical elements.

[0006] The invention is based on the object of enabling structures of a reticle to be transferred to a substrate with as consistent precision as possible using a projection exposure apparatus.

[0007] This object is achieved by the combinations of features of the alternative independent claims.

[0008] In the method according to the invention for operating a microlithographic projection exposure apparatus, during a first exposure phase with the aid of an illumination optical unit illumination radiation according to a first illumination setting is formed and the illumination radiation is fed to an image field of a projection optical unit with the aid of the projection optical unit. During a second exposure phase, with the aid of the illumination optical unit, illumination radiation according to a second illumination setting, which differs from the first illumination setting, is formed and the illumination radiation is fed to the image field of the projection optical unit with the aid of the projection optical unit. During an exposure pause between the first exposure phase and the second exposure phase, with the aid of the illumination optical unit, illumination radiation according to the second illumination setting is formed. This illumination radiation is incident on an optical element of the projection optical unit, but is not fed to the image field of the projection optical unit.

[0009] The invention has the advantage that it allows structures of a reticle to be transferred to a substrate with consistent precision during operation of a projection exposure apparatus. Already during the exposure pause, the temperature of at least one optical element of the projection optical unit is regulated by illumination radiation according to the second illumination setting, so that, upon the transition to the subsequent exposure phase with the second illumination setting, severe temperature fluctuations can be avoided and the changes in the aberrations caused by the projection optical unit are kept within limits.

[0010] The illumination radiation can be influenced downstream of the first optical element in the beam path of the projection optical unit in such a way that it does not reach the image field of the projection optical unit. In particular, the illumination radiation is influenced downstream of the last optical element in the beam path of the projection optical unit in such a way that it does not reach the image field of the projection optical unit. The illumination radiation can be deflected and / or absorbed so that it does not reach the image field of the projection optical unit. The second exposure phase can temporally succeed the first exposure phase.

[0011] During the first exposure phase and the second exposure phase, a reticle can in each case be arranged in the region of an object field of the projection optical unit. In particular, during the first exposure phase and the second exposure phase, different reticles can be arranged in the region of the object field of the projection optical unit. Likewise, it is also possible that, during the first exposure phase and the second exposure phase, the same reticle is arranged in the region of the object field of the projection optical unit. Furthermore, during the first exposure phase and the second exposure phase, the same substrate can be arranged in the region of the image field of the projection optical unit. Likewise, it is also possible that, during the first exposure phase and the second exposure phase, different substrates are arranged in the region of the image field of the projection optical unit.

[0012] The invention furthermore relates to a microlithographic projection exposure apparatus, comprising an illumination optical unit and a projection optical unit. The illumination optical unit forms illumination radiation according to a first illumination setting or according to a second illumination setting, which differs from the first illumination setting, and illuminates an object field of the projection optical unit with the respective illumination setting. The projection optical unit directs the illumination radiation from the object field to an image field of the projection optical unit. The projection optical unit has an element which is temporarily arranged in the beam path of the projection optical unit when the object field of the projection optical unit is illuminated with the second illumination setting, such that during this time no illumination radiation reaches the image field of the projection optical unit.

[0013] The element that prevents illumination radiation from reaching the image field of the projection optical unit can be a deflection element or an absorption element. The element can be arranged closer to a field plane than to a pupil plane in the beam path of the projection optical unit. Furthermore, the element can be arranged between the last optical element and the image field in the beam path of the projection optical unit. This ensures that the temperature of all optical elements of the projection optical unit is regulated by the illumination radiation in the exposure pause.

[0014] In addition, the invention relates to a method for operating a microlithographic projection exposure apparatus, wherein during an exposure phase illumination radiation is directed with the aid of a projection optical unit from an object field of the projection optical unit to an image field of the projection optical unit, during an exposure pause no illumination radiation is directed to the image field of the projection optical unit, during the exposure pause at least one optical element of the projection optical unit is heated in such a way that a temperature distribution of this optical element that is formed during the exposure phase is replicated and the heating during the exposure pause in each case replicates the heating effect of the illumination radiation during the exposure phase and an additional heating, which acts on the at least one optical element in addition to the illumination radiation during the exposure phase.

[0015] This variant of the invention has the further advantage that the temperature of the at least one optical element of the projection optical unit can be regulated without illumination radiation. In addition, there are greater freedoms in the heating of the at least one optical element, on account of not being restricted to the heating effect of the illumination radiation.

[0016] The at least one optical element of the projection optical unit can be heated with the additional heating during the exposure phase in such a way that the temperature distribution of this optical element has a smaller deviation from a rotationally symmetrical distribution or a homogeneous distribution than without the additional heating. This generally reduces the aberrations caused by the optical element and facilitates at least partial compensation of the remaining aberrations.

[0017] The at least one optical element of the projection optical unit can be heated during the exposure pause in such a way that a temperature distribution of this optical element that is formed by the illumination radiation during the exposure phase is replicated. This allows changes in the temperature distribution and associated changes in the aberrations caused by the optical element to be kept low.

[0018] The at least one optical element of the projection optical unit can be heated during the exposure pause in such a way that the temperature distribution of this optical element that is formed during the preceding exposure phase is replicated. Likewise, it is possible for the at least one optical element of the projection optical unit to be heated during the exposure pause in such a way that the temperature distribution of this optical element that is formed during the subsequent exposure phase is replicated. In this context, the heating effect of the illumination radiation during the exposure phase can in each case be simulated by the heating during the exposure pause.

[0019] The invention further relates to a microlithographic projection exposure apparatus, wherein the projection exposure apparatus has a projection optical unit, which during an exposure phase directs illumination radiation from an object field of the projection optical unit to an image field of the projection optical unit, the projection optical unit has a heating element, which heats an optical element of the projection optical unit during an exposure pause, in which the projection optical unit does not direct illumination radiation to the image field of the projection optical unit, such that a temperature distribution of the optical element that is formed during the exposure phase is replicated and the heating during the exposure pause in each case replicates the heating effect of the illumination radiation during the exposure phase and an additional heating, which acts on the at least one optical element in addition to the illumination radiation during the exposure phase.

[0020] The heating element can be formed as a radiation source, which emits heating radiation having a different wavelength than the illumination radiation. Likewise, the heating element can also be formed in such a way that it transfers thermal energy to the optical element to be heated by means of heat conduction.

[0021] The invention furthermore relates to a method for operating a microlithographic projection exposure apparatus, wherein illumination radiation according to a first illumination setting and subsequently illumination radiation according to a second illumination setting, which differs from the first illumination setting, are generated, the way in which the second illumination setting is formed depends on the first illumination setting, the illumination radiation is directed in each case with the aid of a projection optical unit from an object field of the projection optical unit to an image field of the projection optical unit and the formation of the second illumination setting takes account of how a thermal state of the projection optical unit changes upon the transition from the first illumination setting to the second illumination setting. It is advantageous that precise information about the first illumination setting is generally available without additional effort.

[0022] The second illumination setting can be selected from a plurality of potential second illumination settings. That enables the formation of the second illumination setting to be carried out efficiently. The selection of the second illumination setting from the potential second illumination settings can be carried out according to one rule or according to a plurality of rules. The selection of the second illumination setting from the potential second illumination settings can be carried out taking into consideration at least one permissible sequence constituted by a first illumination setting and a second illumination setting. This can be realized with little effort. Additionally or alternatively, the selection of the second illumination setting from the potential second illumination settings can be carried out taking into consideration at least one impermissible sequence constituted by a first illumination setting and a second illumination setting. This procedure has the advantage that the selection options when selecting the second illumination setting are comparatively slightly restricted, so that a suitable second illumination setting can generally be found.

[0023] For different first illumination settings, at least one illumination setting which can be selected as a second illumination setting can in each case be stored in a memory. Alternatively or additionally, for different first illumination settings, at least one illumination setting which is not permitted to be selected as a second illumination setting can in each case be stored in a memory.

[0024] That potential second illumination setting can be selected for which the change in the thermal state of the projection optical unit is the least or is below a predefined value upon the transition from the first illumination setting to the second illumination setting.

[0025] The second illumination setting can be selected depending on the aberrations caused by the projection optical unit after switching to the second illumination setting. That potential second illumination setting can be selected for which the change in the aberrations caused by the projection optical unit is the least or is below a predefined value upon the transition from the first illumination setting to the second illumination setting.

[0026] The invention furthermore relates to a method for operating a microlithographic projection exposure apparatus, wherein a first reticle is illuminated with illumination radiation according to an illumination setting and is imaged with the aid of a projection optical unit, after the first reticle a second reticle, which differs from the first reticle, is illuminated with illumination radiation according to the same illumination setting and is imaged with the aid of the projection optical unit, the illumination setting is adapted to the second reticle in such a way that the second reticle is imaged with a higher precision than the first reticle. That has the further advantage that, even after the reticle change, the illumination situation remains approximately the same and thus, beyond the effects of a possible temporary switchoff of the illumination radiation, there are no appreciable changes in the thermal load on the projection optical unit due to the reticle change.

[0027] The illumination setting can be adapted to the second reticle in such a way that the second reticle is imaged as optimally as possible or with a predefined minimum precision. The first reticle and the second reticle can be selected in such a way that the first reticle has structures for which an imaging is permissible with a lower precision than for the structures of the second reticle. What can be achieved in this way is that despite a use of the same illumination setting for the first and second reticles in both cases the predefined standards in the imaging precision can be adhered to.

[0028] The invention furthermore relates to a method for operating a microlithographic projection exposure apparatus, wherein during a first exposure phase an object field of a projection optical unit is illuminated with illumination radiation according to a first illumination setting, during a second exposure phase the object field of the projection optical unit is illuminated with illumination radiation according to a second illumination setting, which differs from the first illumination setting, and during an exposure pause between the first exposure phase and the second exposure phase at least one optical element of the projection optical unit is heated depending on the configuration of the second illumination setting, and the at least one optical element is heated during the exposure pause such that the thermal deformation of the optical element that results from the heating replicates the deformation of the optical element that is expected during the second exposure phase.

[0029] That has the further advantage that, for each envisaged configuration of the second illumination setting, an associated heating for the exposure pause can already be uniquely determined and stored in advance. During operation, for a given second illumination setting, the associated heating scheme can then in each case be read out from the memory and applied during the exposure pause. Replicating the expected deformation enables the transition to this future thermal deformation to be brought forward in time, so that at the beginning of the second exposure phase the at least one optical element is not as far from its thermal equilibrium as it would be without this measure. The deformation of the optical element that is expected during the second exposure phase can be determined by means of a simulation. The optical element can be heated during the exposure pause in such a way that the deformation of the optical element during the second exposure phase is reduced. In particular, the deformation of the optical element during the second exposure phase can be minimized. Furthermore, the optical element can be heated during the exposure pause in such a way as to reduce changes in aberrations that occur during the second exposure phase and are caused by the optical element.

[0030] The invention also relates to a method for operating a microlithographic projection exposure apparatus, wherein during an exposure phase illumination radiation is directed with the aid of a projection optical unit from an object field of the projection optical unit to an image field of the projection optical unit and a substrate arranged in the region of the image field of the projection optical unit is exposed, and during an exposure pause no illumination radiation is guided to the image field of the projection optical unit and at least one optical element of the projection optical unit is heated depending on aberrations of the projection optical unit that are expected for the exposure phase following the exposure pause.

[0031] The invention furthermore relates to a method for operating a microlithographic projection exposure apparatus, wherein during a first exposure phase an object field of a projection optical unit is illuminated with illumination radiation according to a first illumination setting and the projection optical unit is heated by the illumination radiation, during a second exposure phase the object field of the projection optical unit is illuminated with illumination radiation according to a second illumination setting, which differs from the first illumination setting, and the projection optical unit is heated by the illumination radiation and during the second exposure phase changes in the aberrations of the projection optical unit are caused by the elimination of the heating effect of the illumination radiation according to the first illumination setting and / or by the heating effect of the illumination radiation according to the second illumination setting and these changes in the aberrations of the projection optical unit are at least partially compensated for with the aid of at least one manipulator of the projection optical unit. That has the further advantage that aberrations of the projection optical unit that are caused by thermal effects can be reduced without lead time with immediate effect in each case. This measure can also be combined with the measures already described above.

[0032] The at least one manipulator can be a mechanical manipulator. The at least one manipulator can be controlled depending on a simulation of the changes in the aberrations of the projection optical unit that are expected during the second exposure phase. The at least partial compensation of the aberrations of the projection optical unit can take place as an alternative or in addition to the heating of at least one optical element of the projection optical unit in an exposure pause between the first exposure phase and the second exposure phase. An additive realization has the advantage that a greater reduction of the aberrations can be achieved and / or smaller travel paths are sufficient for the at least one manipulator.

[0033] The invention will be explained in more detail below on the basis of the exemplary embodiments illustrated in the drawing, in which:

[0034] Figure 1 schematically shows one exemplary embodiment of a projection exposure apparatus for EUV projection lithography in a meridional section,

[0035] Figure 2 shows one exemplary embodiment of a projection exposure apparatus for DUV projection lithography in a schematic illustration,

[0036] Figure 3 shows a diagram for elucidating the temporal profile of imaging aberrations caused by the thermal loading of one of the optical elements of a projection optical unit of the projection exposure apparatus,

[0037] Figure 4 shows, in an illustration corresponding to Figure 3, a diagram for elucidating the temporal profile of the imaging aberrations caused by an optical element of the projection optical unit in the event that measures are taken according to one variant of the invention, Figure 5 shows a wafer-side portion of one exemplary embodiment of the projection optical unit in a schematic illustration,

[0038] Figure 6 shows, in an illustration corresponding to Figure 3, a diagram for elucidating the temporal profile of the imaging aberrations caused by an optical element of the projection optical unit in the event that measures are taken according to a further variant of the invention,

[0039] Figure 7 shows a detail from one exemplary embodiment for the projection optical unit in a schematic illustration,

[0040] Figure 8 shows, in an illustration corresponding to Figure 3, a diagram for elucidating the temporal profile of the imaging aberrations caused by an optical element of the projection optical unit in the event that measures are taken according to a further variant of the invention, and

[0041] Figure 9 shows an optical element of the projection optical unit in a schematic illustration.

[0042] Figure 1 schematically shows one exemplary embodiment of a projection exposure apparatus 1 for EUV projection lithography in a meridional section.

[0043] The essential component parts of a microlithographic projection exposure apparatus 1 are described in exemplary fashion below, initially with reference to Figure 1. The description of the basic set-up of the projection exposure apparatus 1 and the component parts thereof should be understood here to be non-limiting.

[0044] One embodiment of an illumination system 2 of the projection exposure apparatus 1 has, in addition to a light or radiation source 3, an illumination optical unit 4 for illuminating an object field 5 in an object plane 6. In an alternative embodiment, the radiation source 3 can also be provided as a module separate from the rest of the illumination system. In this case, the illumination system does not comprise the radiation source 3. A reticle 7 arranged in the object field 5 is exposed. The reticle 7 is held by a reticle holder 8. The reticle holder 8 is displaceable by way of a reticle displacement drive 9 in particular in a scanning direction.

[0045] A Cartesian xyz-coordinate system is depicted in Figure 1 for explanation purposes. The x- direction runs perpendicularly into the plane of the drawing. The y-direction runs horizontally, and the z-direction runs vertically. The scanning direction runs along the y- direction in Figure 1. The z-direction runs perpendicularly to the object plane 6.

[0046] The projection exposure apparatus 1 comprises a projection optical unit 10. The projection optical unit 10 serves for imaging the object field 5 into an image field 11 in an image plane 12. The image plane 12 runs parallel to the object plane 6. Alternatively, an angle between the object plane 6 and the image plane 12 that differs from 0° is also possible.

[0047] A structure on the reticle 7 is imaged onto a light-sensitive layer of a wafer 13 arranged in the region of the image field 11 in the image plane 12 or of some other substrate. The wafer 13 is held by a wafer holder 14. The wafer holder 14 is displaceable by way of a wafer displacement drive 15 in particular along the y-direction. The displacement, firstly, of the reticle 7 by way of the reticle displacement drive 9 and, secondly, of the wafer 13 by way of the wafer displacement drive 15 can be synchronized with one another.

[0048] The radiation source 3 is an EUV radiation source. The radiation source 3 emits illumination radiation 16, which is also referred to below as used radiation or illumination light. In the exemplary embodiment illustrated, the illumination radiation 16 has a wavelength in the EUV range, in particular in the range of between 5 nm and 30 nm. The radiation source 3 can be a plasma source, for example an LPP (laser produced plasma) source or a GDPP (gas discharge produced plasma) source. It can also be a synchrotron-based radiation source. Likewise, the radiation source 3 can be a free electron laser (FEL).

[0049] The illumination radiation 16 emerging from the radiation source 3 is focused by a collector 17. The collector 17 can be a collector with one or with a plurality of ellipsoidal and / or hyperboloidal reflection surfaces. The illumination radiation 16 can be incident on the at least one reflection surface of the collector 17 with grazing incidence (GI), i.e. at angles of incidence of greater than 45°, or with normal incidence (NI), i.e. at angles of incidence of less than 45°. The collector 17 can be structured and / or coated, on the one hand for optimizing its reflectivity for the illumination radiation 16 and on the other hand for suppressing extraneous light.

[0050] Downstream of the collector 17, the illumination radiation 16 propagates through an intermediate focus in an intermediate focal plane 18. The intermediate focal plane 18 can constitute a separation between a radiation source module, comprising the radiation source 3 and the collector 17, and the illumination optical unit 4.

[0051] The illumination optical unit 4 comprises a deflection mirror 19 and, disposed downstream thereof in the beam path, a first facet mirror 20. The deflection mirror 19 can be a plane deflection mirror or alternatively a mirror with a beam-influencing effect going beyond the pure deflection effect. Alternatively or additionally, the deflection mirror 19 can be embodied as a spectral filter separating a used light wavelength of the illumination radiation 16 from extraneous light having a wavelength that deviates therefrom. If the first facet mirror 20 is arranged in a plane of the illumination optical unit 4 which is optically conjugate to the object plane 6 as a field plane, it is also referred to as a field facet mirror. The first facet mirror 20 comprises a multiplicity of individual first facets 21, which are also referred to below as field facets. Only some of these first facets 21 are illustrated in Figure 1 by way of example.

[0052] The first facets 21 can be embodied as macroscopic facets, in particular as rectangular facets or as facets with an arcuate or partly circular edge contour. The first facets 21 can be embodied as plane facets or alternatively as convexly or concavely curved facets.

[0053] As is known from DE 10 2008 009 600 Al, for example, the first facets 21 themselves can each also be composed of a multiplicity of individual mirrors, in particular a multiplicity of micromirrors. The first facet mirror 20 can be embodied in particular as a microelectromechanical system (MEMS system). For details, reference is made to DE 10 2008 009 600 Al.

[0054] Between the collector 17 and the deflection mirror 19, the illumination radiation 16 travels horizontally, i.e. along the y-direction. In the beam path of the illumination optical unit 4, a second facet mirror 22 is arranged downstream of the first facet mirror 20. If the second facet mirror 22 is arranged in a pupil plane of the illumination optical unit 4, it is also referred to as a pupil facet mirror. The second facet mirror 22 can also be arranged at a distance from a pupil plane of the illumination optical unit 4. In this case, the combination of the first facet mirror 20 and the second facet mirror 22 is also referred to as a specular reflector. Specular reflectors are known from US 2006 / 0132747 Al, EP 1 614 008 Bl and US 6,573,978.

[0055] The second facet mirror 22 comprises a plurality of second facets 23. In the case of a pupil facet mirror, the second facets 23 are also referred to as pupil facets.

[0056] The second facets 23 can likewise be macroscopic facets, which can for example have a round, rectangular or else hexagonal boundary, or can alternatively be facets composed of micromirrors. In this regard, reference is likewise made to DE 10 2008 009 600 Al.

[0057] The second facets 23 can have plane or alternatively convexly or concavely curved reflection surfaces.

[0058] The illumination optical unit 4 thus forms a doubly faceted system. This basic principle is also referred to as a fly's eye condenser (fly's eye integrator).

[0059] It can be advantageous to arrange the second facet mirror 22 not exactly in a plane that is optically conjugate to a pupil plane of the projection optical unit 10. In particular, the second facet mirror 22 can be arranged so as to be tilted in relation to a pupil plane of the projection optical unit 10, as described for example in DE 10 2017 220 586 Al.

[0060] The individual first facets 21 are imaged into the object field 5 with the aid of the second facet mirror 22. The second facet mirror 22 is the last beam-shaping mirror or else actually the last mirror for the illumination radiation 16 in the beam path upstream of the object field 5.

[0061] In a further embodiment (not illustrated) of the illumination optical unit 4, a transfer optical unit contributing in particular to the imaging of the first facets 21 into the object field 5 can be arranged in the beam path between the second facet mirror 22 and the object field 5. The transfer optical unit can comprise exactly one mirror, or else alternatively two or more mirrors arranged one behind another in the beam path of the illumination optical unit 4. The transfer optical unit can in particular comprise one or two normal-incidence mirrors (NI mirrors) and / or one or two grazing-incidence mirrors (GI mirrors).

[0062] In the embodiment shown in Figure 1, the illumination optical unit 4 has exactly three mirrors downstream of the collector 17, specifically the deflection mirror 19, the first facet mirror 20 and the second facet mirror 22.

[0063] In a further embodiment of the illumination optical unit 4, the deflection mirror 19 can also be omitted, and so the illumination optical unit 4 can then have exactly two mirrors downstream of the collector 17, specifically the first facet mirror 20 and the second facet mirror 22.

[0064] The imaging of the first facets 21 into the object plane 6 by means of the second facets 23 or using the second facets 23 and a transfer optical unit is regularly only approximate imaging.

[0065] The projection optical unit 10 comprises a plurality of mirrors Mi, which are consecutively numbered in accordance with their arrangement in the beam path of the projection exposure apparatus 1.

[0066] In the example illustrated in Figure 1, the projection optical unit 10 comprises six mirrors Ml to M6. Alternatives with four, eight, ten, twelve or any other number of mirrors Mi are likewise possible. The projection optical unit 10 is a doubly obscured optical unit. The penultimate mirror M5 and the last mirror M6 in each case have a through-opening, through which, during the exposure of the wafer 13, the radiation contributing to the exposure passes on its way from the reticle 7 to the wafer 13. The projection optical unit 10 has an image-side numerical aperture that is greater than 0.5 and can also be greater than 0.6, and can be for example 0.7 or 0.75.

[0067] The reflection surfaces of the mirrors Mi can be embodied as freeform surfaces without an axis of rotational symmetry. Alternatively, the reflection surfaces of the mirrors Mi can be designed as aspherical surfaces with exactly one axis of rotational symmetry of the reflection surface shape. Just like the mirrors of the illumination optical unit 4, the mirrors Mi can have highly reflective coatings for the illumination radiation 16. These coatings can be designed as multilayer coatings, in particular with alternating layers of molybdenum and silicon. The projection optical unit 10 has a large object-image offset in the y-direction between a y- coordinate of a centre of the object field 5 and a y-coordinate of the centre of the image field 11. This object-image shift in the y-direction may be of approximately the same magnitude as a z-di stance between the object plane 6 and the image plane 12.

[0068] The projection optical unit 10 can be embodied in particular in anamorphic fashion. In particular, it has different imaging scales Px, Py in the x- and y-directions. The two imaging scales Px, Py of the projection optical unit 10 are preferably (Px, Py) = (+ / -0.25, + / -0.125). A positive imaging scale P means imaging without image inversion. A negative sign for the imaging scale P means imaging with image inversion.

[0069] The projection optical unit 10 thus leads to a reduction in size with a ratio of 4: 1 in the x- direction, i.e. in a direction perpendicular to the scanning direction.

[0070] The projection optical unit 10 leads to a reduction in size of 8: 1 in the y-direction, i.e. in the scanning direction.

[0071] Other imaging scales are also possible. Imaging scales with the same signs and the same absolute values in the x- and y-directions, for example with absolute values of 0.125 or 0.25, are also possible.

[0072] The number of intermediate image planes in the x-direction and in the y-direction in the beam path between the object field 5 and the image field 11 can be the same or can be different, depending on the embodiment of the projection optical unit 10. Examples of projection optical units 10 with different numbers of such intermediate images in the x-direction and y- direction are known from US 2018 / 0074303 Al.

[0073] In each case one of the second facets 23 is assigned to exactly one of the first facets 21 in order to form a respective illumination channel for illuminating the object field 5. In particular, this can result in illumination according to the Kohler principle. The far field is decomposed into a multiplicity of object fields 5 with the aid of the first facets 21. The first facets 21 generate a plurality of images of the intermediate focus on the second facets 23 respectively assigned to them. The first facets 21 are each imaged onto the reticle 7 by an assigned second facet 23 with images overlaid over one another for the purpose of illuminating the object field 5. The illumination of the object field 5 is in particular as homogeneous as possible. It preferably has a uniformity error of less than 2%. Field uniformity can be achieved by overlaying different illumination channels.

[0074] The illumination of the entrance pupil of the projection optical unit 10 can be defined geometrically by an arrangement of the second facets 23. The intensity distribution in the entrance pupil of the projection optical unit 10 can be set by selecting the illumination channels, in particular the subset of the second facets 23 that guide light. This intensity distribution is also referred to as illumination setting or illumination pupil filling.

[0075] A likewise preferred pupil uniformity in the region of portions of an illumination pupil of the illumination optical unit 4 which are illuminated in a defined manner can be achieved by a redistribution of the illumination channels.

[0076] Further aspects and details of the illumination of the object field 5 and in particular of the entrance pupil of the projection optical unit 10 are described below.

[0077] The projection optical unit 10 can comprise in particular a homocentric entrance pupil. The latter can be accessible. It can also be inaccessible.

[0078] The entrance pupil of the projection optical unit 10 regularly cannot be exactly illuminated using the second facet mirror 22. In the case of imaging by the projection optical unit 10 which telecentrically images the centre of the second facet mirror 22 onto the wafer 13, the aperture rays often do not intersect at a single point. However, it is possible to find an area in which the spacing of the aperture rays that is determined in pairs becomes minimal. This area constitutes the entrance pupil or an area conjugate thereto in real space. In particular, this area exhibits a finite curvature.

[0079] It may be the case that the projection optical unit 10 has different positions of the entrance pupil for the tangential beam path and for the sagittal beam path. In this case, an imaging element, in particular an optical component of the transfer optical unit, should be provided between the second facet mirror 22 and the reticle 7. With the aid of this optical element, the different position of the tangential entrance pupil and the sagittal entrance pupil can be taken into account.

[0080] In the arrangement of the components of the illumination optical unit 4 illustrated in Figure 1, the second facet mirror 22 is arranged in an area conjugate to the entrance pupil of the projection optical unit 10. The first facet mirror 20 is arranged so as to be tilted with respect to the object plane 6. The first facet mirror 20 is arranged so as to be tilted with respect to an arrangement plane defined by the deflection mirror 19.

[0081] The first facet mirror 20 is arranged so as to be tilted with respect to an arrangement plane defined by the second facet mirror 22.

[0082] Figure 2 shows one exemplary embodiment of a projection exposure apparatus 1 for DUV projection lithography in a schematic illustration. In this case, DUV stands for “deep ultraviolet”. In particular, the projection exposure apparatus 1 can be designed for operation at a wavelength of 193 nm.

[0083] The projection exposure apparatus 1 has an illumination optical unit 4 and a projection optical unit 10. The internal set-up of the illumination optical unit 4 and the internal set-up of the projection optical unit 10, which can each comprise for example optical components, sensors, manipulators etc., are not illustrated in detail. In the case of the projection optical unit 10, a mirror M is indicated as representative of its optical components. The mirror M can be cooled with the aid of a cooling medium, which is provided by a cooling device 24. The cooling medium is a fluid, for example water. Additionally or alternatively, the illumination optical unit 4 can have a cooled mirror M and an associated cooling device 24. The projection optical unit 10 and / or the illumination optical unit 4 can also have a plurality of cooled mirrors M and cooling devices 24. In the case of the illumination optical unit 4 and in the case of the projection optical unit 10, lenses and further mirrors - cooled or uncooled - can for example be present as further optical components.

[0084] By analogy, at least one cooling device 24, which can for example be connected to the mirror M3, can also be provided in the exemplary embodiment of the projection exposure apparatus 1 illustrated in Figure 1. The radiation required for the operation of the projection exposure apparatus 1 is generated by a radiation source 3. The radiation source 3 can be in particular an excimer laser, for example an argon fluoride laser, which generates illumination radiation 16 of the wavelength 193 nm.

[0085] Arranged between the illumination optical unit 4 and the projection optical unit 10 is a reticle holder 8, fixed on which is a reticle 7, also referred to as a mask. The reticle holder 8 has a reticle displacement drive 9. Arranged downstream of the projection optical unit 10, seen in the direction of radiation, is a wafer holder 14, which carries a wafer 13 or some other substrate and has a wafer displacement drive 15.

[0086] Furthermore, Figure 2 also illustrates a control device 25, which is connected to the illumination optical unit 4, the projection optical unit 10, the cooling device 24, the radiation source 3, the reticle holder 8 or the reticle displacement drive 9 and the wafer holder 14 or the wafer displacement drive 15. By analogy, the projection exposure apparatus 1 in Figure 1 can likewise have a control device 25, which can be connected to corresponding components.

[0087] The projection exposure apparatus 1 serves the purpose of imaging the reticle 7 onto the wafer 13 with high precision. For this purpose, the reticle 7 is illuminated with the aid of the illumination optical unit 4 and the illuminated reticle 7 is imaged onto the wafer 13 with the aid of the projection optical unit 10. Specifically, the following procedure is adopted:

[0088] The illumination optical unit 4 transforms the illumination radiation 16 generated by the radiation source 3 in an exactly defined way by means of its optical components and directs it onto the reticle 7. Depending on the embodiment, the illumination optical unit 4 can be formed in such a way that it illuminates the entire reticle 7 or only a partial region of the reticle 7. The illumination optical unit 4 is capable of illuminating the reticle 7 in such a way that there are almost identical illumination conditions at each illuminated point of the reticle 7. In particular, the intensity and the angular distribution of the incident illumination radiation 16 are almost identical for each illuminated point of the reticle 7.

[0089] The illumination optical unit 4 is capable of illuminating the reticle 7 optionally with illumination radiation 16 of a multiplicity of different angular distributions. These angular distributions of the illumination radiation 16 are also referred to as illumination settings. The desired illumination setting is generally selected in dependence on the structure elements formed on the reticle 7. Used relatively often for example are dipole or quadrupole illumination settings, in the case of which the illumination radiation 16 is incident on each illuminated point of the reticle 7 from two different directions or from four different directions, respectively. Depending on the form of the illumination optical unit 4, the different illumination settings can be produced for example by means of different diffractive optical elements in combination with a zoom axicon optical unit or by means of mirror arrays, which have in each case a multiplicity of small mirrors that are arranged next to one another and are individually settable with respect to their angular position.

[0090] The reticle 7 can be formed for example as a glass plate, which is transparent to the illumination radiation 16 fed by the illumination optical unit 4 and applied to which are opaque structures, for example in the form of a chromium coating.

[0091] The projection exposure apparatus 1 can be formed in such a way that the entire reticle 7 is illuminated at the same time by the illumination optical unit 4 and is imaged completely onto the wafer 13 by the projection optical unit 10 in a single exposure step.

[0092] Alternatively, the projection exposure apparatus 1 can also be formed in such a way that only a partial region of the reticle 7 is illuminated at the same time by the illumination optical unit 4 and the reticle displacement drive 9 is controlled by the control device 25 in such a way that, during the exposure of the wafer 13, the reticle 7 is moved in relation to the illumination optical unit 4 and, as a result, the illuminated partial region migrates over the reticle 7 as a whole. The wafer 13 is moved synchronously by coordinated control of the wafer displacement drive 15, in which the imaging properties of the projection optical unit 10 are also taken into account, and so the respectively illuminated partial region of the reticle 7 is imaged onto a partial region of the wafer 13 provided for it. This movement of the reticle 7 and of the wafer 13 is also referred to as scanning.

[0093] In order to be able to transfer the latent image produced by the exposure of the wafer 13 in both embodiments of the projection exposure apparatus 1 into a physical structure, a lightsensitive layer is applied to the wafer 13. The image of the reticle 7 is formed in this lightsensitive layer by exposure and a permanent structure can be produced from it on the wafer 13 with the aid of subsequent chemical processes. The reticle 7 is generally imaged onto the wafer 13 not only once, but multiple times next to one another. For this purpose, after each imaging of the reticle 7 onto the wafer 13, the wafer holder 14 is displaced laterally in a way corresponding to the size of the image of the reticle 7 on the wafer 13. The imaging of the reticle 7 can be performed here in each case as a whole or sequentially by scanning. After the interaction with the reticle 7, the illumination radiation 16 is sometimes also referred to as exposure radiation, since it exposes the wafer 13 or more precisely the light-sensitive layer thereof with the image of the reticle 7. The chemical treatment of the wafer 13 is only started when the desired number of imagings of the reticle 7 onto the wafer 13 have been carried out.

[0094] The imaging of the reticle 7 onto the wafer 13 or some other substrate can be detrimentally affected by numerous influences that have a negative effect on the image quality achieved. One of these influences is based on the fact that the illumination radiation 16 used to image the reticle 7 onto the wafer 13 thermally loads, i.e. heats up, the optical elements of the projection optical unit 10. The thermal loading changes the optical properties of the optical elements of the projection optical unit 10.

[0095] A number of measures that can be used to limit negative effects caused by the thermal loading of the optical elements of the projection optical unit 10 are described in greater detail below.

[0096] Figure 3 shows a diagram for elucidating the temporal profile of imaging errors caused by the thermal loading of one of the optical elements of the projection optical unit 10. The designation aberrations is also equally used hereinafter for these imaging errors. In Figure 3, the aberrations (abbreviated as Aberr.) caused by this optical element are plotted against time t. The sum of a plurality of aberrations or a particularly characteristic aberration can optionally be used for the representation. The optical element can be a mirror or a lens, for example. The following description is based on a mirror by way of example. This mirror can be e.g. the mirror M2 from Figure 1 or another of the mirrors of the projection optical unit 10 illustrated there. An analogous profile also results for the aberrations of the entire projection optical unit 10 upon thermal loading, in the event that the corrective measures that will be discussed below are not implemented. The curve illustrated in Figure 3 starts at a time to with a very low value for the aberrations. Before the time to, the radiation source 3 is switched off or blocked out, so that no illumination radiation 16 reaches the object field 5 of the projection optical unit 10 and the reticle 7 arranged there is thus not illuminated. Accordingly, no illumination radiation 16 is directed by the projection optical unit 10 from the reticle 7 to the image field 11 of the projection optical unit 10 and the wafer 13 arranged there. Consequently, the mirror M2 used for Figure 3 is not thermally loaded by the illumination radiation 16 before the time to and therefore, at the time to, has a temperature close to the value for which it is designed. That means that the optical surface of the mirror M2 has a shape that differs only little from the shape formed during the production of the mirror M2, which causes only small aberrations.

[0097] Starting from the time to, illumination radiation 16 reaches the object field 5 of the projection optical unit 10, so that the reticle 7 is illuminated with the illumination radiation 16. The illumination radiation 16 is directed with the aid of the projection optical unit 10 from the reticle 7 to the image field 11 of the projection optical unit 10 and exposes the wafer 13 arranged there with the image of the reticle 7. On the way from the reticle 7 to the wafer 13, the illumination radiation 16 is reflected and partially absorbed inter alia at the mirror M2 used for Figure 3. The absorbed portion of the illumination radiation 16 is converted into heat, and so the mirror M2 is heated up. This means that at the time to there is an abrupt increase in the heating power introduced into the mirror M2. Under otherwise identical conditions, the greater the radiation power of the radiation source 3 used, the greater the increase in the heating power. In order to achieve the highest possible throughput with the projection exposure apparatus 1, i.e. to expose as many wafers 13 as possible per unit time, as powerful a radiation source 3 as possible is striven for. Accordingly, the better this requirement is met, the greater the increases in the heating power that occur. As explained in greater detail below, the abrupt increase in the heating power introduced into the mirror M2 leads to a rise in the aberrations caused by the mirror M2. As the heating power increases, the associated aberrations become more and more prominent.

[0098] As the temperature of the mirror M2 used for Figure 3 rises, the mirror surface, in particular also the optical surface of the mirror M2, is deformed to an increasingly greater extent. That results in a rise in the aberrations caused by the deformation. The curve illustrated in Figure 3 thus rises at the time to first sharply and then at ever lower rates. This trend continues until a time ti, at which the illumination of the reticle 7 with the illumination radiation 16 is stopped. The illumination of the reticle 7 can be stopped by switching off or blocking out the radiation source 3. That may happen for example in connection with a wafer change in which a previously exposed wafer 13 is exchanged for a new wafer 13. In an analogous manner, the illumination of the reticle 7 can be stopped during a batch change in which a previously exposed batch of wafers 13 is exchanged for a new batch of wafers 13. A batch comprises a plurality of wafers 13, for example 20 to 25 items. However, it is also possible for the batch to comprise more or fewer wafers 13.

[0099] As soon as the illumination of the reticle 7 is stopped, illumination radiation 16 is also no longer directed by the projection optical unit 10 from the object field 5 of the projection optical unit 10 to the image field 11 of the projection optical unit 10 and thus onto the wafer 13. Consequently, illumination radiation 16 is no longer incident on the optical elements of the projection optical unit 10 and in particular on the mirror M2 used for Figure 3, so that the heating up of the mirror M2 caused by a partial absorption of the illumination radiation 16 is stopped. At the time ti, therefore, the heating power introduced into the mirror M2 drops abruptly to zero. However, this does not mean that the mirror M2 will remain at a constant temperature starting from the time ti. Instead, the mirror M2 cools down since heat is radiated via the surface of the mirror M2 and dissipated via the mount of the mirror M2. The cooling down of the mirror M2 continues until a time t2, at which the illumination of the reticle 7 is resumed and there is thus again an abrupt increase in the heating power introduced into the mirror M2.

[0100] The cooling down of the mirror M2 that abruptly commences at the time ti has the effect that the deformation of the mirror M2, previously caused by the heating up, is again partially withdrawn or changed. In this case, the deformation generally does not decrease 1 : 1 according to the previous heating up, since the cooling down influences not only the average temperature of the mirror, but also the temperature distribution. In this respect, the illustration in Figure 3 is greatly simplified, and merely indicates that with the cooling down the deformation of the mirror M2 is again partially withdrawn and, accordingly, the aberrations resulting from the deformation likewise decrease. The interruption of the illumination of the reticle 7 at the time ti could thus be regarded as positive, since the aberrations caused by the mirror M2 are subsequently reduced. However, the curve profile in Figure 3 shows only the proportion of the aberrations constituted by the mirror M2 used for this figure, and not the aberrations caused by the projection optical unit 10 as a whole. The overall resulting aberrations have a different profile from that illustrated in Figure 3, since measures are provided to reduce the aberrations caused by the projection optical unit 10 as a whole. These measures work well if the aberrations change slowly and continuously with the most consistent tendency possible. An abrupt change in the aberrations on a short time scale, caused for example by a sudden change in the heating up of the mirror M2 used for Figure 3, is much more difficult to compensate for. Even if this change causes a reduction in the contribution by the mirror M2 to the aberrations, the overcompensation of the envisaged corrective measures initially results in higher aberrations overall. Consequently, the cooling down of the mirror M2 in the period from ti to t2 and its influence on the aberrations do not constitute a negligible or even desired effect, but rather a disturbance of the imaging by the projection optical unit 20 that cannot easily be rectified. This disturbance takes place on a short time scale of a few seconds.

[0101] The time interval from to to ti, during which the wafer 13 is exposed, is also referred to below as exposure phase, and the time interval from ti to t2, during which the wafer 13 is not exposed, is referred to as exposure pause. Further exposure phases extend over the time intervals from t2 to t3, from t4 to ts and from teto t?, and further exposure pauses extend over the time intervals from t3 to , from ts to te and starting from t?. Exposure phases and exposure pauses thus succeed one another in alternation. On the basis of the relatively long exposure phases, the relatively short exposure pauses and the heating and cooling powers on which the example illustrated is based, overall there occurs a rise in the heating up of the mirror M2, which is modulated by the exposure pauses and gradually attains saturation, and a corresponding profile of the aberrations caused by the mirror M2.

[0102] The aberrations arising in the manner described above can be counteracted with various measures, which are described in more detail below. The configuration of these measures is influenced in each case by insights regarding the cause of the aberrations, so that a highly effective and reliable reduction of the aberrations resulting overall for the projection optical unit 10 is possible.

[0103] Figure 4 shows, in an illustration corresponding to Figure 3, a diagram for elucidating the temporal profile of the imaging aberrations (aberrations) caused by an optical element of the projection optical unit 10 in the event that measures are taken according to one variant of the invention. The optical element can once again be a mirror or a lens, the following description being based by way of example on a mirror as optical element, in particular the mirror M2 analogously to Figure 3.

[0104] The curve depicted in the diagram starts at the time to with very small aberrations, analogously to Figure 3. Before the time to, the radiation source 3 is switched off or blocked out, so that no illumination radiation 16 reaches the object field 5 of the projection optical unit

[0105] 10 and the reticle 7 arranged there is not illuminated. Accordingly, no illumination radiation 16 either is directed by the projection optical unit 10 from the object field 5 to the image field

[0106] 11 and the wafer 13 arranged there. The mirror M2 used for Figure 4 is not thermally loaded and therefore, at the time to, has a temperature close to the value for which it is designed, and causes only small aberrations.

[0107] Analogously to Figure 3, starting from the time to, illumination radiation 16 reaches the object field 5 of the projection optical unit 10, so that the reticle 7 is illuminated with the illumination radiation 16. The illumination radiation 16 is directed with the aid of the projection optical unit 10 from the reticle 7 to the image field 11 of the projection optical unit 10 and exposes the wafer 13 arranged there with the image of the reticle 7. On the way from the reticle 7 to the wafer 13, the illumination radiation 16 is reflected and partially absorbed at the mirror M2, so that the mirror M2 is heated up. In the variant according to Figure 4, too, this means that at the time to there is an abrupt increase in the heating power introduced into the mirror M2.

[0108] As the temperature of the mirror M2 rises, the mirror surface is deformed to an increasingly greater extent, and so the aberrations caused thereby rise. Up to the time ti, the curve profile illustrated in Figure 4 is identical to Figure 3. In contrast to Figure 3, in the variant illustrated in Figure 4, however, the illumination of the reticle 7 is not stopped at the time ti, but rather is maintained unchanged after the time ti as well. Accordingly, the illumination radiation 16 is directed from the projection optical unit 10 in the direction of wafer 13 after the time ti as well. Nevertheless, the exposure phase started at the time to ends at the time ti and an exposure pause follows starting from the time ti, that is to say that starting from the time ti the illumination radiation 16 required for the exposure of the wafer 13 no longer reaches the image field 11 of the projection optical unit 10. Specifically, the exposure pause is caused by the fact that the illumination radiation 16 is deflected and / or absorbed upstream of the image field 11 of the projection optical unit 10, and so it cannot be incident on the wafer 13. This deflection and / or absorption occurs in the beam path of the projection optical unit 10 downstream of the mirror M2 used for Figure 4 and upstream of the image field 11 of the projection optical unit 10 and is illustrated in Figure 5.

[0109] Figure 5 shows a wafer-side portion of one exemplary embodiment of the projection optical unit 10 in a schematic illustration.

[0110] Between a wafer-side last optical element, in particular the mirror M6 of the projection optical unit 10, and the wafer 13 arranged in the region of the image field 11 of the projection optical unit 10, an absorption element 26 is positioned, which absorbs the illumination radiation 16 deflected by the wafer-side last optical element, so that it does not reach the image field 11 of the projection optical unit 10 and the wafer 13 is thus not exposed. The absorption element 26 is of movable design and is moved out of the beam path of the projection optical unit 10 when the wafer 13 is to be exposed. When the wafer 13 is not to be exposed, the absorption element 26 is moved into the beam path of the projection optical unit 10. Up to incidence on the absorption element 26, the illumination radiation 16 is deflected by the projection optical unit 10 in a manner identical to that in exemplary embodiments without an absorption element 26. Accordingly, all optical elements which are arranged upstream of the absorption element 26 in the beam path of the projection optical unit 10 are heated up by the illumination radiation 16 during the exposure pause in the same manner as during the exposure phase. The mirror M2 used for Figure 4 is also arranged upstream of the absorption element 26 in the beam path of the projection optical unit 10. This means that the mirror M2 continues to heat up continuously after the time ti as well. This is manifested in a continuous increase in the aberrations in Figure 4. In contrast to Figure 3, therefore, in Figure 4 an abrupt decrease in the aberrations caused by the mirror M2 does not occur at the time ti.

[0111] At the time t2, the absorption element 26 is moved out of the beam path of the projection optical unit 10, so that the illumination radiation 16 is no longer blocked by the absorption element 26, but rather is incident on the wafer 13 and exposes the latter. At the time t2, the exposure pause thus ends and a new exposure phase begins. For the mirror M2 used for Figure 4, however, nothing changes at the time t2 either. Since the reticle 7 is still illuminated, the mirror M2 is still heated up by the illumination radiation 16. Accordingly, the trend continues unchanged for the aberrations illustrated in Figure 4 and these continue to rise continuously. This also applies to the further exposure phases from t4 to ts and from te to t? as well as to the further exposure pauses from t3 to and from ts to te. However, the gradient of the curve illustrated in Figure 4 decreases more and more. That results from the fact that the heating power transferred to the mirror M2 remains approximately constant over time. As the temperature of the mirror M2 increases, however, more and more thermal energy is lost for example through emission and heat conduction, so that over the course of time a dynamic equilibrium forms in conjunction with a constant temperature or temperature distribution of the mirror M2. Consequently, the aberrations caused by the mirror M2 gradually attain saturation.

[0112] There are good correction options for the profile of the aberrations that is illustrated in Figure 4, and so with the outlined continuous temperature regulation of the mirror M2 used for Figure 4 and optionally also of further optical elements of the projection optical unit 10 by means of the illumination radiation 16 overall it is possible to achieve very small resulting aberrations for the projection optical unit 10.

[0113] The absorption element 26 can also be arranged downstream of the first optical element in the beam path of the projection optical unit 10 at a different location from that illustrated in Figure 5, so that the illumination radiation 16 is incident on, and heats up, only a subset of the optical elements. In addition, the absorption element 26 can be arranged outside the projection optical unit 10 in the region between the projection optical unit 10 and the image field 11 and thus the wafer 13. Instead of the absorption element 26, a deflection element can also be provided, which deflects the illumination radiation 16 such that it does not reach the wafer 13. For example, the illumination radiation 16 can be deflected to a suitable location and absorbed there.

[0114] In one development of the embodiment described above, different illumination settings are used. For example, the wafers 13 of one or more batches are exposed with illumination radiation 16 according to a first illumination setting. For this purpose, during an exposure phase a first reticle 7a is illuminated with illumination radiation 16 according to the first illumination setting and the illumination radiation 16 is directed to the image field 11 with the aid of the projection optical unit 10 in order to expose a wafer 13 of the batch. The exposure phase is followed by an exposure pause, during which a wafer change is carried out. During the exposure pause, the first reticle 7a is still illuminated and the illumination radiation 16 is thereafter deflected and / or absorbed before it reaches the image field 11. That is then followed again by an exposure phase, in which a further wafer 13 of the batch is exposed. The same procedure can be adopted for all wafers 13 of the batch.

[0115] If the first illumination setting is to be used for a further batch, it is possible to switch to the further batch during an exposure pause. During this exposure pause, too, the first reticle 7a is still illuminated and the illumination radiation 16 is thereafter deflected and / or absorbed before it reaches the image field 11. The length of the exposure pause can be adapted to the time required for the batch change. The exposure of the wafers 13 of the further batch can be effected in a manner analogous to that already described.

[0116] This procedure can be repeated until a second illumination setting, which differs from the first illumination setting, is to be used. The switching from the first illumination setting to the second illumination setting takes place in an exposure pause, for example after the exposure of the last wafer 13 of the last batch with illumination radiation 16 according to the first illumination setting. If the first reticle 7a is retained, the first reticle 7a is illuminated with illumination radiation 16 according to the second illumination setting immediately at the beginning of the exposure pause. Shortly before or at the same time as the start of the illumination of the first reticle 7a with the second illumination setting, the absorption element 26 is moved into the beam path of the projection optical unit 10, so that the illumination radiation 16 cannot reach the image field 11.

[0117] In general, however, with the switching from the first illumination setting to the second illumination setting, the first reticle 7a is also exchanged for a second reticle 7b. This exchange occurs at the beginning of the exposure pause. Immediately after the exchange of the first reticle 7a and still during the exposure pause, the second reticle 7b is illuminated with illumination radiation 16 according to the second illumination setting. In an analogous manner, in this case, shortly before or at the same time as the start of the illumination of the second reticle 7b, the absorption element 26 is moved into the beam path of the projection optical unit 10, so that the illumination radiation 16 cannot reach the image field 11.

[0118] The illumination of the first or second reticle 7a, 7b during the exposure pause with illumination radiation 16 according to the second illumination setting has the effect that the optical elements of the projection optical unit 10 which are arranged in the beam path between the reticle 7a or 7b and the absorption element 26 are each irradiated with the same illumination radiation distribution as in the subsequent exposure phase. Accordingly, these optical elements are already heated up during the exposure pause in a manner analogous to that during the subsequent exposure phase. In this case, the use of the second illumination setting in the exposure pause leads to better results in the subsequent exposure phase compared with a use of the first illumination setting in the exposure pause. That is attributable to the fact that the optical elements of the projection optical unit 10 in the case of an illumination with illumination radiation 16 according to the second illumination setting during the exposure pause, at the beginning of the subsequent exposure phase, already have basic characteristics of the temperature distributions, which become increasingly more pronounced during the exposure phase, and therefore there is no abrupt and thus difficult-to-correct change in the temperature distributions at the beginning of the exposure phase. The manifestation of this positive effect depends on the time duration during which the optical elements of the projection optical unit 10 are preheated with illumination radiation 16 according to the second illumination setting in the exposure pause. If the effect is otherwise too small, the time duration of the exposure pause and thus the time duration of the illumination of the first or second reticle 7a, 7b with illumination radiation 16 according to the second illumination setting can be extended accordingly. That does reduce the throughput of wafers 13 a little. In return, a more precise exposure of the wafers 13 is possible at the beginning of the exposure phase. The exposure of the wafers 13 and optionally of one or more batches of wafers 13 with illumination radiation 16 according to the second illumination setting is carried out in a manner analogous to that described for the first illumination setting.

[0119] In principle, it is also possible to expose the same wafer 13 with illumination radiation 16 according to different illumination settings.

[0120] In addition, it is possible, at the beginning of the exposure pause for the wafer change, to switch off the radiation source 3 or to block out illumination radiation 16 generated by the radiation source 3 before reaching the reticle 7. In this modification, the absorption element 26 is situated in the beam path of the projection optical unit 10 only during the exposure pauses in which the illumination setting is changed and optionally the reticle 7 is changed. In exposure pauses in which exclusively a wafer change is carried out, it is then not necessary to position the absorption element 26 in the beam path of the projection optical unit 10, since in this modification no illumination radiation 16 passes into the projection optical unit 10 in any case in such exposure pauses.

[0121] According to a further embodiment, the correctability of the aberrations caused by one or more optical elements of the projection optical unit 10 is improved by the exposure pauses being shortened and the cooling down of the respective optical element during the exposure pauses thereby being reduced. This is explained in greater detail below with reference to Figure 6.

[0122] Figure 6 shows, in an illustration corresponding to Figure 3, a diagram for elucidating the temporal profile of the imaging errors (aberrations) caused by an optical element of the projection optical unit 10 in the event that measures are taken according to a further variant of the invention. The optical element can once again be a mirror or a lens, the following description being based by way of example on a mirror as optical element, in particular the mirror M2.

[0123] The curve illustrated in Figure 6 shows a similar profile to the curve in Figure 3. However, the exposure pauses, i.e. the distances between the times ti and t2 and the corresponding times t3 and t4, or ts and te, are shorter for Figure 6 and the corresponding decreases in the aberrations are less than for Figure 3. For the rest, Figure 6 corresponds to Figure 3, and so the descriptive explanations of Figure 3 can also be used for Figure 6.

[0124] The somewhat smoother profile of the curve in Figure 6 compared with the curve in Figure 3 has the consequence that the aberrations in the embodiment of Figure 6 are smaller than at Figure 3, at which the exposure pauses are not shortened. That applies to all optical elements of the projection optical unit 10 which cause appreciable aberrations and are heated up by the illumination radiation 16 to such an extent that the aberrations caused by them change significantly.

[0125] A shortening of the exposure pauses can be achieved by reducing the measurement accuracy for measurements that are carried out during the exposure pauses and accordingly allowing a shorter measurement time to be scheduled. The measurements can be for example measurements of the alignment between reticle 7 and wafer 13. In this case, for example, the position of the image of the reticle 7 on the wafer 13 can be measured and the reticle 7 and the wafer 13 can be positioned relative to one another depending on the measurement results. Since these alignment measurements crucially determine the length of the exposure pauses, shortening the measurement time required therefor has a direct effect on the exposure pauses.

[0126] Additionally or alternatively, a shortening of the exposure pauses can be achieved by parallelizing as comprehensively as possible as many as possible of the measurements carried out during the exposure pauses. That can be achieved for example by a simultaneous measurement at a plurality of field points of the image field 11 of the projection optical unit 10.

[0127] Shortening the exposure pauses has furthermore the positive additional effect that more wafers 13 can be exposed per time and thus the wafer throughput increases.

[0128] According to a further embodiment, the correctability of the aberrations caused by an optical element of the projection optical unit 10 is improved by heating power being fed to the optical element during the exposure pauses in a manner other than by the illumination radiation 16, and the cooling down of the optical element during the exposure pauses thereby being reduced or prevented. This procedure can equally also be applied to a plurality of optical elements of the projection optical unit 10. This is explained in greater detail below with reference to Figures 7 and 8.

[0129] Figure 7 shows a detail from one exemplary embodiment for the projection optical unit 10 in a schematic illustration. The illustration in Figure 7 relates to a time during an exposure pause.

[0130] Figure 7 illustrates the mirror M2 of the projection optical unit 10 and a radiant heater 27. The radiant heater 27 generates heating radiation 28 in the infrared range and thereby irradiates the surface of the mirror M2. The wavelength of the heating radiation 28 is chosen such that, on the one hand, it does not contribute to the exposure of the wafer 13, i.e. does not cause a reaction in the light-sensitive layer applied to the wafer 13, and, on the other hand, it is absorbed by the mirror M2 to a sufficient extent that the latter heats up. In particular, an areal irradiation of the mirror surface is carried out using the radiant heater 27. An intensity profile can be chosen which replicates the heating effect of the illumination radiation 16 incident on the mirror M2 during the exposure phases. That means that the mirror surface is not just homogeneously heated with the heating radiation 28, but depending on the illumination setting and the arrangement of the mirror M2 in the beam path of the projection optical unit 10, for example, with a dipole distribution, a quadrupole distribution, etc. That has the effect that the mirror M2 behaves thermally during the exposure pauses in a manner similar to that during the exposure phases, if the mirror surface is heated with the heating radiation 28 during the exposure pauses in the aforementioned manner. Consequently, fluctuations in the thermal behaviour of the mirror M2 caused by the exposure pauses can be largely avoided. This is explained in greater detail with reference to Figure 8.

[0131] Figure 8 shows, in an illustration corresponding to Figure 3, a diagram for elucidating the temporal profile of the imaging aberrations (aberrations) caused by an optical element of the projection optical unit 10 in the event that measures are taken according to a further variant of the invention. The optical element can once again be a mirror or a lens, the following description being based by way of example on the mirror M2 illustrated in Figure 7 as optical element.

[0132] The curve depicted in the diagram starts at the time to with very small aberrations, analogously to Figure 3. Before the time to, the radiation source 3 is switched off or blocked out, so that the reticle 7 is not illuminated and, accordingly, no illumination radiation 16 is directed from the reticle 7 to the image field 11 by the projection optical unit 10. Therefore, at the time to, the mirror M2 used for Figure 8 has a temperature close to the value for which it is designed, and causes only small aberrations.

[0133] Analogously to Figure 3, the reticle 7 is illuminated starting from the time to and, accordingly, illumination radiation 16 is directed by the projection optical unit 10 from the reticle 7 to the image field 11 and the wafer 13 is exposed with the image of the reticle 7. On the way from the reticle 7 to the wafer 13 arranged in the region of the image field 11, the illumination radiation 16 is reflected and partially absorbed at the mirror M2, so that the mirror M2 is heated up. In the variant according to Figure 8, too, this means that at the time to there is an abrupt increase in the heating power introduced into the mirror M2 and subsequently a gradual rise in the temperature of the mirror M2. As the temperature of the mirror M2 rises, the mirror surface and in particular also the optical surface are deformed to an increasingly greater extent, and so the aberrations caused thereby rise. Up to the time ti, the curve profile illustrated in Figure 8 is identical to Figures 3, 4 and 6. At the time ti, the illumination of the reticle 7 is stopped, and so the associated heating of the mirror M2 drops abruptly to zero. At the same time, the radiant heater 27 is switched on and the mirror M2 is irradiated with heating radiation 28 which in terms of its intensity and its local distribution manifests a heating effect which replicates the heating effect of the illumination radiation 16 during the exposure phase. Nevertheless, the exposure phase started at the time to ends at the time ti and an exposure pause follows starting from the time ti, since the heating radiation 28 has a different wavelength than the illumination radiation 16 and does not contribute to the exposure of the wafer 13. Depending on the geometric and optical conditions, the heating radiation 28 does not reach the wafer 13 at all and then for this reason, too, cannot cause any exposure of the wafer 13.

[0134] Up to the time t2, with the aid of the radiant heater 27 the mirror M2 is continuously further heated up and deformed to a greater and greater extent, such that the profile of the aberrations illustrated in Figure 8 approximately corresponds to the profile illustrated in Figure 4, in which the mirror M2 is subjected to the illumination radiation 16 in the exposure pause as well. However, the two profiles are not absolutely identical, since the heating effect of the illumination radiation 16 cannot in practice be replicated with arbitrary accuracy by the heating radiation 28. However, the deviations are tolerable. It is important that the abrupt change in the aberrations caused by the exposure pause can be prevented and instead there is only a continuous change in the context of a constant trend. This continuous change can be corrected with good success by dedicated manipulators of the projection optical unit 10, thereby achieving an acceptable amount of aberrations resulting overall for the projection optical unit 10.

[0135] At the time t2, the radiant heater 27 is switched off and the illumination of the reticle 7 with illumination radiation 16 is resumed. As a result, the exposure pause is ended and a new exposure phase is started. For the reasons already mentioned, there is hardly any change in the heating power introduced into the mirror M2, so that the mirror M2 continues to be heated up continuously.

[0136] The described sequence of exposure phases, in which the mirror M2 is heated up by illumination radiation 16, and exposure pauses, in which the mirror M2 is heated up by heating radiation 28, is continued until all the wafers 13 provided for this have been exposed. The temperature rise of the mirror M2 and in association therewith the profile of the aberrations caused by the mirror M2 continue further and further, but increasingly level off and finally attain saturation. According to Figure 8, this involves successively going through the further exposure pauses from ts to and from ts to te as well as the further exposure phases from to ts and from te to t?. At the time t?, the illumination of the reticle 7 with illumination radiation 16 is stopped and there is no alternative irradiation of the mirror M2 with heating radiation 28. Accordingly, the mirror M2 cools down to a great extent from the time t? and subsequently the aberrations caused by the mirror M2 also decrease.

[0137] In a modification of the described variant, the mirror M2 is heated not only during the exposure pauses but also during the exposure phases by means of the radiant heater 27. In this case, the heating profile of the radiant heater 27 during the exposure phases is adapted to the heating effect of the illumination radiation 16 in such a way that in total a more symmetrical, in particular a more rotationally symmetrical, or a more homogeneous, or other heating of the mirror M2 results, which causes better correctable aberrations compared with when the illumination radiation 16 solely takes effect. During the exposure pauses, the illumination of the reticle 7 is stopped in each case and the mirror M2 is heated exclusively with the heating radiation 28. In this case, the radiant heater 27 is controlled in such a way that it replicates the total heating power introduced during the exposure phase, i.e. the proportion of the illumination radiation 16 and the proportion of the heating radiation 28. That is generally associated with a considerable change in the local distribution of the heating radiation 28 generated by the radiant heater 27. Alternatively, the radiant heater 27 could continue to be operated during the exposure pause without any change compared with during the exposure phase and additionally a further radiant heater 27 could be switched on, which compensates for the heating power of the illumination radiation 16 that has been lost during the exposure pause.

[0138] In this modification, overall a curve similar to that illustrated in Figure 8 would result for the aberrations of the mirror M2, but the curve would generally have a flatter profile, since a higher symmetry and / or homogeneity of the temperature distribution usually leads to less pronounced aberrations.

[0139] As an alternative to an areal heating using the radiant heater 27, a rather punctiform heating can also be provided, in which only a small spot of the mirror surface is heated approximately homogeneously and the spot is scanned over the surface to be heated of the mirror M2. In addition, it is possible to use a plurality of radiant heaters 27, each areally heating a partial region of the mirror M2 or generating a spot that is scanned over the respective partial region. As an alternative or in addition to one radiant heater 27 or a plurality of radiant heaters 27, one or a plurality of differently embodied heating elements can also be used. For example, the heating elements can be formed as electrical heating elements which are in thermal contact with the mirror M2.

[0140] The variants described above can also be combined with one another. For example, the exposure pauses can be shortened and a radiant heater 27 or some other heating element can additionally bring about heating in the exposure pauses. Alternatively the shortening of the exposure pauses can be realized by reduction of the measurement accuracy and an additional parallelization of the measurement, etc.

[0141] In all variants, remaining drifts of the aberrations during the exposure phases can be further reduced with the aid of manipulators. For example, mirrors or lenses of the projection optical unit 10 can be displaced or rotated or tilted or deformed in such a way that the aberrations of the projection optical unit 10 overall are reduced and in particular kept within a desired range. It is possible to estimate the expected aberrations by means of a prediction model and to control the manipulators on this basis. The manipulators can be controlled synchronously in time with the scan or step movement during the exposure of the wafer 13.

[0142] According to a further embodiment, the aberrations caused by the projection optical unit 10 are kept as small as possible by the temporal sequence of illumination settings being adapted to one another in such a way that there is as little change as possible in the spatial temperature distribution at one optical element or at a plurality of optical elements of the projection optical unit 10 when the illumination setting is changed.

[0143] For example, illumination of the reticle 7 with illumination radiation 16 according to an illumination setting in the form of a horizontal dipole has the consequence that some optical elements of the projection optical unit 10 are supplied with heating power with a spatial distribution which is similar to this horizontal dipole. Illumination of the reticle 7 with illumination radiation 16 according to an illumination setting in the form of a vertical dipole has the consequence that some optical elements of the projection optical unit 10 are supplied with heating power with a spatial distribution which is similar to this vertical dipole. This effect as well as the resulting consequences and possible countermeasures are explained in greater detail with reference to Figure 9.

[0144] Figure 9 shows an optical element of the projection optical unit 10 in a schematic illustration. The optical element is a mirror, for example the mirror M2 of the projection optical unit 10 illustrated in Figure 1. However, the statements made below also apply, analogously, to the other optical elements of the projection optical unit 10 illustrated in Figure 1 and also to the optical elements of the projection optical unit 10 illustrated in Figure 2. Figure 9 illustrates a plan view of the optical surface of the mirror M2 and depicts two first heating zones 29 and two second heating zones 30.

[0145] The first heating zones 29 are formed as two ellipses, which are illustrated using solid line depiction and are arranged at a horizontal distance from one another. The first heating zones 29 arise during an illumination of the reticle 7 with illumination radiation 16 according to the illumination setting in the form of a horizontal dipole. Such an illumination setting leads to a corresponding local concentration of the intensity of the illumination radiation 16 in portions of the beam path of the projection optical unit 10 which are not too far away from the pupil. Optical elements of the projection optical unit 10 which are arranged within such a portion, such as the mirror M2, for example, are thus irradiated with a dipole-like intensity distribution of the illumination radiation 16. In this case, the intensity distribution of the illumination radiation 16 entails a corresponding distribution of the heating power, so that the first heating zones 29 illustrated in Figure 9 form on the optical surface of the mirror M2. The first heating zones 29 do not correspond exactly to the illumination setting, since the intensity distribution of the illumination radiation 16 at the location of the mirror M2 does not correspond exactly to the illumination setting. The temperature distribution on the optical surface of the mirror M2 resulting from the distribution of the heating power deviates even more from the form of the illumination setting, since the temperature distribution changes owing to emission and heat conduction effects. Nevertheless, the temperature distribution still has the basic characteristics of the form of the illumination setting.

[0146] The second heating zones 30 are formed as two ellipses, which are illustrated using dashed line depiction and are arranged at a vertical distance from one another. The second heating zones 30 arise during an illumination of the reticle 7 with illumination radiation 16 according to the illumination setting in the form of a vertical dipole. The second heating zones 30 are formed in a manner analogous to that described for the first heating zones 29 by the effect of the illumination radiation 16 on the mirror M2 and lead to a corresponding temperature distribution in the region of the optical surface of the mirror M2.

[0147] The illustration in Figure 9 reveals that, when the illumination setting changes from a horizontal dipole to a vertical dipole, completely different regions of the mirror M2 are heated up. Since the heating up of the mirror M2 gives rise to a deformation dependent on the heating up, the aforementioned illumination settings lead to greatly different deformations of the mirror M2. Consequently, a change in the illumination setting from a horizontal dipole to a vertical dipole entails a great change in the deformation state of the mirror M2 and in particular the optical surface of the mirror M2. Accordingly, the aberrations caused by the mirror M2 also change very significantly when the illumination setting is changed in this way.

[0148] By means of a suitable sequence of illumination settings, the change in the local distribution of the heating power acting on the mirror M2 and the resulting deformation and thus the change in the aberrations caused by the mirror M2 can be kept within reasonable limits. In other words, not every sequence of illumination settings is permitted, rather only such changes of the illumination setting are carried out which do not entail an impermissibly large deformation of the mirror M2 and thus an impermissibly large change in the aberrations caused by the mirror M2. This will generally be the case with a sequence of two illumination settings that are similar to one another. Accordingly, a table listing all permissible sequences of two illumination settings can be created for all envisaged illumination settings. If this is associated with excessively large restrictions, such that an economically viable process sequence would no longer be possible, only sequences of two illumination settings that are particularly unfavourable with regard to the associated aberrations can instead be entered in the table, and these sequences are then avoided during operation of the projection exposure apparatus 1. It is also possible to combine both procedures.

[0149] In general, illumination settings are selected depending on the reticle 7 which is to be illuminated with illumination radiation 16 according to the illumination setting. A limitation of the sequence of illumination settings therefore leads to a corresponding limitation of the sequence of reticles 7. The latter then generally represents the process-limiting factor, since there is a specification regarding which structures are to be transferred to the wafers 13 and which reticles 7 are thus to be used. The reticles 7 to be used then in turn specify which appropriate illumination settings are required.

[0150] If the combination of reticles 7 to be used for production to expose wafers 13 with desired structures and of the sequence of illumination settings permitted for production has the effect that the wafers 13 cannot be exposed with all the desired structures, the procedure according to a further embodiment can be adopted.

[0151] In this further embodiment, compromises in the selection of an optimal illumination setting for the respective reticle 7 are made, where that is tenable. The procedure is such that an exposure of wafers 13 with the image of a first reticle 7a takes place first. The first reticle 7a has less critical structures and is illuminated with illumination radiation 16 in the form of a first illumination setting. An exposure of wafers 13 with the image of a second reticle 7b takes place subsequently. The second reticle 7b has particularly critical structures and is illuminated with illumination radiation 16 in the form of a second illumination setting. The second illumination setting is selected in such a way that it enables the structures of the second reticle 7b to be imaged onto the wafers 13 as optimally as possible.

[0152] When selecting the first illumination setting for the first reticle 7a with the less critical structures, a deviation from the optimum is permitted with regard to the imaging of the structures. Instead, the first illumination setting is selected so that it is sufficiently similar to the second illumination setting. Accordingly, the structures of the first reticle 7a are transferred to the wafers 13 respectively provided therefor with reasonable but not maximum possible precision.

[0153] When there is a change from the first reticle 7a to the second reticle 7b, a change from the first illumination setting to the second illumination setting takes place. Since the first illumination setting and the second illumination setting are similar, the change from the first illumination setting to the second illumination setting results in only a moderate change in the heating effect of the illumination radiation 16 for the optical elements of the projection optical unit 10. Consequently, the change of the illumination setting has only minor effects on the aberrations of the projection optical unit 10, so that shortly after the change of the illumination setting the critical structures of the second reticle 7b can already be transferred with high precision to the wafers 13 respectively provided therefor. In one development of this embodiment, the second illumination setting, which is optimized for the critical structures of the second reticle 7b, is used not only for the illumination of the second reticle 7b, but also for the illumination of the first reticle 7a. In this development, when there is a change from the first reticle 7a to the second reticle 7b, a change of the illumination setting does not occur, and so for the optical elements of the projection optical unit 10 there is no change in the heating effect of the illumination radiation 16 and the structures of the second reticle 7b are transferred with optimum precision to the wafers 13 respectively provided therefor. Strictly speaking, despite the same illumination setting, there is actually a slight change in the heating effect of the illumination radiation 16, since the different structures of the second reticle 7b are transferred by the projection optical unit 10 with the illumination radiation 16. However, this effect is generally small in comparison with the use of a different illumination setting.

[0154] The variants just described assume that there is sufficient freedom regarding the selection of the reticles 7 and the illumination settings. If the freedom in the selection is severely restricted or there is no selection at all, the variants described below can be used in a change from a first illumination setting for the illumination of a first reticle 7a to a second illumination setting for the illumination of a second reticle 7b, in order to keep the aberrations small. These variants can equally also be used if a sufficient number of reticles 7 and illumination settings are available.

[0155] It is important that information is available both about the first illumination setting currently used and about the second illumination setting following the first illumination setting. From this information it is possible to determine with the aid of a model how the optical properties of one or more optical elements of the projection optical unit 10 change as a result of the change in the thermal loading upon the transition from the first illumination setting to the second illumination setting. In addition, the resulting aberrations can be predicted. On the basis of these data, measures can be taken which make it possible to reduce the aberrations upon the transition from the first illumination setting to the second illumination setting.

[0156] One possible measure is to heat at least one optical element of the projection optical unit 10 during the exposure pause between the first illumination setting and the second illumination setting. In this case, the radiant heater 27 illustrated in Figure 7 and already mentioned or some other heating element can be used. The heated optical element can be for example a mirror, in particular once again the mirror M2. Heating can influence the shape of the mirror M2, and in particular the shape of its optical surface. In this case, the mirror M2 can be heated during the exposure pause in such a way that, when the illumination of the second reticle 7b with the second illumination setting is started, the deformation of the mirror M2 caused by the heating effect of the illumination radiation 16 is as small as possible. Without this measure, a large deformation is to be expected, especially if the first illumination setting and the second illumination setting differ greatly from one another. The heating of the mirror M2 by means of the radiant heater 27 can replicate for example the heating effect of the illumination radiation 16 in the case of an illumination of the second reticle 7b with the second illumination setting. It is likewise possible to heat the mirror M2 during the exposure pause with a heating distribution which, in contrast to the procedures described previously, neither replicates the illumination radiation 16 of the second illumination setting nor is formed as symmetrically or homogeneously as possible. For this purpose, predictive data for the mirror M2 are determined with the aid of the aforementioned model for the effect of the thermal loading on the optical properties. These predictive data are taken as a basis for determining a suitable heating distribution. A heating distribution determined computationally in this way will generally lead to better results than a viewpoint-based procedure. However, the implementation of the prediction and the calculation of the heating distribution based thereon require a certain amount of effort.

[0157] A further possible measure is to determine, on the basis of the predictive data, predefined values for the control of manipulators which can be used to influence the aberrations of the projection optical unit 10. With these manipulators, optical elements of the projection optical unit 10 can be mechanically influenced and thereby displaced, rotated, tilted or deformed, for example. Since this influencing takes effect instantaneously, it is sufficient if the manipulators are controlled with the determined predefined values at the start of the illumination of the second reticle 7b with illumination radiation 16 according to the second illumination setting, i.e. at the beginning of the exposure phase with the second illumination setting.

[0158] This further measure of controlling manipulators can also be combined with the abovedescribed measure of heating during the exposure pause. This makes it possible to achieve better results than with each measure by itself and / or to reduce the travel paths of the manipulators and / or the heating power introduced in the exposure pause. Many further measures from among those described can also be combined with one another, thereby affording a large number of possibilities for reducing the effects of exposure pauses and changes in the illumination setting on the aberrations caused by the projection optical unit 10 to an acceptable degree.

[0159] Reference signs

[0160] 1 Projection exposure apparatus

[0161] 2 Illumination system

[0162] 3 Radiation source

[0163] 4 Illumination optical unit

[0164] 5 Object field

[0165] 6 Object plane

[0166] 7 Reticle

[0167] 7a First reticle

[0168] 7b Second reticle

[0169] 8 Reticle holder

[0170] 9 Reticle displacement drive

[0171] 10 Proj ecti on opti cal unit

[0172] 11 Image field

[0173] 12 Image plane

[0174] 13 Wafer

[0175] 14 Wafer holder

[0176] 15 Wafer displacement drive

[0177] 16 Illumination radiation

[0178] 17 Collector

[0179] 18 Intermediate focal plane

[0180] 19 Deflection mirror

[0181] 20 First facet mirror

[0182] 21 First facet

[0183] 22 Second facet mirror

[0184] 23 Second facet

[0185] 24 Cooling device

[0186] 25 Control device

[0187] 26 Absorption element

[0188] 27 Radiant heater

[0189] 28 Heating radiation 29 First heating zone

[0190] 30 Second heating zone

[0191] M Mirror

[0192] Ml Mirror

[0193] M2 Mirror

[0194] M3 Mirror M4 Mirror

[0195] M5 Mirror

[0196] M6 Mirror

Claims

Patent Claims1. Method for operating a microlithographic projection exposure apparatus (1), wherein- during a first exposure phase with the aid of an illumination optical unit (4) illumination radiation (16) according to a first illumination setting is formed and the illumination radiation (16) is fed to an image field (11) of a projection optical unit (10) with the aid of the projection optical unit (10),- during a second exposure phase with the aid of the illumination optical unit (4) illumination radiation (16) according to a second illumination setting, which differs from the first illumination setting, is formed and the illumination radiation (16) is fed to the image field (11) of the projection optical unit (10) with the aid of the projection optical unit (10),- during an exposure pause between the first exposure phase and the second exposure phase with the aid of the illumination optical unit (4) illumination radiation (16) according to the second illumination setting is formed and the illumination radiation (16) is incident on an optical element of the projection optical unit (10), but is not fed to the image field (11) of the projection optical unit (10).

2. Microlithographic projection exposure apparatus (1), comprising an illumination optical unit (4) and a projection optical unit (10), wherein- the illumination optical unit (4) forms illumination radiation (16) according to a first illumination setting or according to a second illumination setting, which differs from the first illumination setting, and illuminates an object field (5) of the projection optical unit (10) with the respective illumination setting,- the projection optical unit (10) directs the illumination radiation (16) from the object field (5) to an image field (11) of the projection optical unit (10), and- the projection optical unit (10) has an element (26) which is temporarily arranged in the beam path of the projection optical unit (10) when the object field (5) of the projection optical unit (10) is illuminated with the second illumination setting, such that during this time no illumination radiation (16) reaches the image field (11) of the projection optical unit (10).

3. Projection exposure apparatus (1) according to Claim 2, wherein the element (26) is a deflection element or an absorption element.

4. Method for operating a microlithographic projection exposure apparatus (1), wherein- during an exposure phase illumination radiation (16) is guided with the aid of a projection optical unit (10) from an object field (5) of the projection optical unit (10) to an image field (11) of the projection optical unit (10),- during an exposure pause no illumination radiation (16) is guided to the image field(11) of the projection optical unit (10),- during the exposure pause at least one optical element (M2) of the projection optical unit (10) is heated in such a way that a temperature distribution of this optical element (M2) that is formed during the exposure phase is replicated and- the heating during the exposure pause replicates the heating effect of the illumination radiation during the exposure phase and an additional heating, which acts on the at least one optical element in addition to the illumination radiation during the exposure phase.

5. Method according to Claim 4, wherein the at least one optical element (M2) of the projection optical unit (10) is heated with the additional heating during the exposure phase in such a way that the temperature distribution of this optical element (M2) has a smaller deviation from a rotationally symmetrical distribution or a homogeneous distribution than without the additional heating.

6. Method according to either of Claims 4 and 5, wherein the at least one optical element (M2) of the projection optical unit (10) is heated during the exposure pause in such a way that the temperature distribution of this optical element (M2) that is formed during the preceding exposure phase is replicated.

7. Microlithographic projection exposure apparatus (1), wherein- the projection exposure apparatus (1) has a projection optical unit (10), which during an exposure phase directs illumination radiation (16) from an object field (5) of the projection optical unit (10) to an image field (11) of the projection optical unit (10),- the projection optical unit (10) has a heating element (27), which heats an optical element (M2) of the projection optical unit (10) during an exposure pause, in which the projection optical unit (10) does not direct illumination radiation (16) to the image field (11) of the projection optical unit (10), such that a temperature distribution of the optical element (M2) that is formed during the exposure phase is replicated and- the heating during the exposure pause replicates the heating effect of the illuminationradiation during the exposure phase and an additional heating, which acts on the at least one optical element in addition to the illumination radiation during the exposure phase.

8. Projection exposure apparatus (1) according to Claim 7, wherein the heating element(27) is formed as a radiation source, which emits heating radiation (28) having a different wavelength than the illumination radiation (16).

9. Method for operating a microlithographic projection exposure apparatus (1), wherein- illumination radiation (16) according to a first illumination setting and subsequently illumination radiation (16) according to a second illumination setting, which differs from the first illumination setting, are generated,- the way in which the second illumination setting is formed depends on the first illumination setting,- the illumination radiation (16) is directed with the aid of a projection optical unit (10) from an object field (5) of the projection optical unit (10) to an image field (11) of the projection optical unit (10) and the formation of the second illumination setting takes account of how a thermal state of the projection optical unit (10) changes upon the transition from the first illumination setting to the second illumination setting.

10. Method according to Claim 9, wherein the second illumination setting is selected from a plurality of potential second illumination settings.

11. Method according to Claim 10, wherein the selection of the second illumination setting from the potential second illumination settings is carried out taking into consideration at least one permissible sequence and / or at least one impermissible sequence constituted by a first illumination setting and a second illumination setting.

12. Method for operating a microlithographic projection exposure apparatus (1), wherein- a first reticle (7a) is illuminated with illumination radiation (16) according to an illumination setting and is imaged with the aid of a projection optical unit (10),- after the first reticle (7a) a second reticle (7b), which differs from the first reticle (7a), is illuminated with illumination radiation (16) according to the same illumination setting and is imaged with the aid of the projection optical unit (10),- the illumination setting is adapted to the second reticle (7b) in such a way that the second reticle (7b) is imaged with a higher precision than the first reticle (7a).

13. Method according to Claim 12, wherein the first reticle and the second reticle are selected in such a way that the first reticle has structures for which an imaging is permissible with a lower precision than for the structures of the second reticle.

14. Method for operating a microlithographic projection exposure apparatus (1), wherein- during a first exposure phase an object field (5) of a projection optical unit (10) is illuminated with illumination radiation (16) according to a first illumination setting,- during a second exposure phase the object field (5) of the projection optical unit (10) is illuminated with illumination radiation (16) according to a second illumination setting, which differs from the first illumination setting,- during an exposure pause between the first exposure phase and the second exposure phase at least one optical element (M2) of the projection optical unit (10) is heated depending on the configuration of the second illumination setting, and- the optical element (M2) is heated during the exposure pause such that the thermal deformation of the at least one optical element (M2) that results from the heating replicates the deformation of the optical element (M2) that is expected during the second exposure phase.

15. Method for operating a microlithographic projection exposure apparatus (1), wherein- during a first exposure phase an object field (5) of a projection optical unit (10) is illuminated with illumination radiation (16) according to a first illumination setting and the projection optical unit (10) is heated by the illumination radiation (16),- during a second exposure phase the object field (5) of the projection optical unit (10) is illuminated with illumination radiation (16) according to a second illumination setting, which differs from the first illumination setting, and the projection optical unit (10) is heated by the illumination radiation (16) and- during the second exposure phase changes in the aberrations of the projection optical unit (10) are caused by the elimination of the heating effect of the illumination radiation (16) according to the first illumination setting and / or by the heating effect of the illumination radiation (16) according to the second illumination setting and these changes in the aberrations of the projection optical unit (10) are at least partially compensated for with the aid of at least one manipulator of the projection optical unit (10).

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