KR101408483B1 - Method of manufacturing a projection objective and projection objective manufactured by that method - Google Patents

Abstract

A method for manufacturing a projection objective comprises defining an initial design for a projection objective and using a merit function having a plurality of merit function components AB, IRRAD EFP each reflecting a particular quality parameter, Lt; / RTI > One of the advantageous function components is a normalized effective irradiance value representing the effective irradiance AB, IRRAD EFF normalized to the effective irradiance at the image surface of the projection objective is directly adjacent to the image surface of the projection objective Defines a maximum radial illumination requirement that does not exceed a predetermined illuminance threshold IRR TV on each optical surface of the projection objective except for the last optical surface. Whereby very small effective sub-apertures on the optical surfaces and / or optical surfaces located in the corrosive areas are systematically avoided.

Description

BACKGROUND OF THE INVENTION 1. Field of the Invention [0001] The present invention relates to a method for manufacturing a projection objective and a projection objective manufactured by the method,

The present invention is directed to a method of manufacturing a projection objective comprising defining an initial design for a projection objective and optimizing the design using an advantage function. The method is used to fabricate projection objectives that can be used in microlithography processes to produce miniaturized devices. The invention also relates to a projection objective produced by such a method.

Microlithography processes are commonly used in the manufacture of miniaturized devices such as integrated circuits, liquid crystal devices, micro-pattern structures and micro-mechanical elements. In such a process, the projection objective serves to project the pattern of the patterning structure (typically a photomask (mask, reticle)) onto a substrate (typically a semiconductor wafer). The substrate is coated with a photosensitive layer (resist) that is exposed to the image of the patterning structure using the projected radiation.

In order to form finer structures, it is all sought to increase the image side numerical aperture (NA) of the projection objective and to use ultraviolet radiation having a shorter wavelength, preferably less than about 260 nm. Consequently, higher demands are placed on the complexity of the projection objective. The projection objective typically has at least 10 or 20 or even more than 25 multiple optical elements, such as lenses, curved mirrors, and the like. Each single optical element, as well as the entire structure comprising a plurality of optical elements arranged in any manner, must be designed and manufactured with high accuracy to provide imaging of the patterning structure onto the substrate within a large image field and to have a low degree of aberrations do.

Creating a new design of the projection objective is a complex task involving optimization of the structural parameters and quality parameters of the projection objective. The structural parameters include the refractive index of the material forming the lenses, the surface form parameters of the lenses and mirrors (if applicable), the distance between the first and second surfaces of each lens, the distance between the surfaces of different optical elements The distance between the object plane of the projection objective and the incident surface of the object-side front element of the projection objective, the distance between the image plane and the exit surface of the image-side front element of the projection objective, , A refractive index of the medium disposed between the object plane and the object-side front element, and between the image plane and the image-side front element.

The quality parameters include parameters indicative of the optical performance of the projection objective in items such as, for example, selected aberrations, image side numerical aperture, magnification of the projection objective, and the like.

Optimizing the design to meet the desired details of the optical performance and other quality characteristics of the projection objective is now being carried out using computational methods such as tracking the beam to optimize the parameters of the projection objective while observing certain boundary conditions . CODE V, a lens analysis and design program sold by Optical Research Associates, Inc., is a commonly used software tool employed for that purpose. The optimization includes minimizing or maximizing the appropriately selected merit function according to the parameters of the design. Typically, the composition of the benefit function is achieved by using a number of benefit function components that can represent the optical aspect, composition aspect, and other aspects that describe the optimization goal of the particular design.

Due to the high number of parameters of the design, the solution space of the optimization process has a high dimension, and the smallest number of local minimums in the solution space, where computational methods are confined, yielding results far from the design satisfying the required details And maxima. Therefore, the optical designer designing the projection objective for microlithography must meet the meticulous task in observing certain boundary conditions based on its intuitive force and / or to determine the principles of a new design suitable for a particular application. Designers will therefore embody the "initial design" that serves as a potentially successful "starting point" for computer-based optimization and then improve the design based on it by computational optimization. Typically, one or more results will still be insufficient for the desired overall detail, and many efforts should be attempted until a satisfactory solution is found. Therefore, in terms of computational manufacturing, the cost of the new design will be high.

Once an appropriate design has been found, the optical elements of the projection objective must be manufactured and assembled to obtain the actual product of the manufacturing process. During this manufacturing step, care must be taken to avoid surface defects of optical surfaces such as scratches and fouling as much as possible. Such as a projection objective designed for immersion operation in an aperture range with an image side numerical aperture NA > 1.0, while low or medium aperture systems can be relatively tolerant to such artificial defects. In the manufacturing process of numerical aperture lithographic projection objectives, very stringent details are generally applied to surface defects.

At least one concave mirror and at least one flat folding mirror disposed to deflect radiation emitted from the concave mirror toward the image surface from the concave mirror, Lt; RTI ID = 0.0 > refraction < / RTI > Representative examples are shown, for example, in WO 2004/019128 A2 or JP 2005-003982 A.

Surface defects on optical surfaces can fundamentally affect the optical performance of the projection objective by causing light loss due to different damage mechanisms. For example, a relatively rough surface area on an ideal smooth surface will typically result in stray light, negatively impacting optical performance. In addition, transparent surface defects act as phase objects, resulting in light loss due to interference. Opacity contamination can block the area of the projection beam, thereby causing heterogeneity of the intensity distribution in the image field. It is required to have a projection objective that is relatively insensitive to performance degradation caused by surface defects on the optical surfaces.

It is an object of the present invention to provide a method of manufacturing a projection objective so as to produce a complex projection objective for microlithography in a cost effective manner while maintaining a high standard for optical performance.

It is another object of the present invention to provide a method of manufacturing a projection objective so as to produce a projection objective for microlithography having an optical performance that is relatively insensitive to surface defects present on the optical surfaces in the projection objective .

It is another object of the present invention to provide a method of manufacturing a projection objective having a field variation of a low degree of intensity.

It is another object of the present invention to provide a projection objective that is relatively insensitive to surface defects caused by contaminations and other effects on optical surfaces within the projection objective.

As a solution to these and other objects, the present invention relates to a method, according to one approach, for defining a projection, including defining an initial design for a projection objective and optimizing the design using a merit function There is provided a method of manufacturing an objective lens, comprising:

Defining a plurality of advantageous functional components each reflecting a specific quality parameter, wherein one of the advantageous functional components comprises a normalized effective value representing a normalized effective irradiance for the effective irradiance at the image surface of the projection objective Defines a maximum radiance requirement that requires that the irradiance value does not exceed a predetermined irradiance threshold on each optical surface of the projection objective other than the last optical surface directly adjacent to the image surface of the projection objective ;

Calculating a numerical value for each of the advantageous functional components based on a corresponding feature of the preliminary design of the projection objective;

Calculating, from the advantageous functional components, a total advantageous function that can be represented by numerical items reflecting quality parameters;

Continuously varying at least one structural parameter of the projection objective until the resulting overall advantage function reaches a predetermined acceptable value and recalculating the resulting overall advantage function for each successive change;

Obtaining structural parameters of an optimized projection objective having a predetermined acceptable value for the resulting overall advantage function; And

And implementing the parameters to produce a projection objective.

The term " irradiance "generally refers to the power of electromagnetic radiation at one surface per unit area. Specifically, the term "irradiance" refers to the power of electromagnetic radiation incident on the surface. The term "effective irradiance, " as used herein, refers to the contribution to the overall irradiance incident on the optical surface, a contribution originating from the radiation emitted from a single object field point. Instead, the effective irradiance can be referred to as "pinhole irradiance" in the present application.

If a large value of effective irradiance is avoided on the optical surfaces, the optical performance of the projection objective may become relatively insensitive to surface defects present on the optical surfaces in the projection objective.

In many cases, the "last optical surface ", i.e., the optical surface of the projection objective closest to the image surface, is subject to special conditions requiring that the last optical surface be excluded from the optimization process. The spatial distribution of the incident points of the radiation rays over the last optical surface is heavily influenced by the concept of the associated projection process (dry projection or immersion projection) and depends on the image side working space The space between the surfaces). For example, it may be useful to provide a narrow gap (typically 1 millimeter or more in width) between the last optical surface and the image surface to introduce immersion liquid for immersion exposure. In view of the inflow and outflow of the immersion medium within the image space, it is often desired to have a substantially flat last optical surface. Under these conditions, the intensity load of the last optical surface (expressed, for example, as the effective irradiance) is basically dependent on the image side working distance, the size of the effective image field, the refractive index of the medium behind the last optical element, Side numerical aperture NA. Therefore, the structural parameters indicating the position and surface shape of the last optical surface are not free parameters and should be excluded from the optimization process.

Various conditions can result or contribute to the local maximum of the effective irradiance on the optical surfaces. For example, large effective irradiance values may occur where the optical surface lies within the corrosive zone. In one embodiment, such conditions include: calculating the position and extent of potential corrosive areas within the projection objective; And by optimizing the structural parameters of the projection objective so that no optical surface is located within the corrosive region.

Alternatively or additionally, the optical surfaces can be important where the size of the actual sub-apertures of the ray bundles becomes significantly smaller. Thus, one embodiment of the method includes the steps of: defining a plurality of representative field points; Computing light beams originating from the field points and intersecting regions of the light beams with optical surfaces, wherein a crossing region between the light beam and the optical surface is defined by an area of the crossing region, defining a real sub-aperture having a sub-aperture size; Defining a sub-aperture size threshold; And for all optical surfaces of the projection objective other than the last optical surface directly adjacent to the image surface of the projection objective, the actual sub-aperture size for selected field points falls below the sub-aperture size threshold , Optimizing the structural parameters of the projection objective.

Embodiments of the method may include operations that take into account one or more causes of very high effective irradiance concentration. For example, one advantage function component may define a maximum radiance requirement, as described above, and the other advantage function component may be one that has the size of the actual sub-apertures on all of the optical surfaces (except the last optical surface) A minimum actual sub-aperture requirement may be defined such that the resulting optical design will automatically have only those optical surfaces that lie above a certain real sub-aperture size threshold.

The method is particularly suitable for the design of various types of optical systems, especially for projection objectives used in microlithography with relatively high image-side numerical apertures, such as those found primarily in projection objectives for immersion lithography where NA > Can be applied in design. A detailed analysis of a large number of conventional projection objectives may be performed by using at least one concave mirror and at least one intermediate image as well as a beam flux traveling towards the concave mirror from a beam bundle reflected from the concave mirror It has been shown that small effective and / or actual sub-apertures and / or erosion conditions can occur particularly in a catadioptric projection objective with at least one flat deflecting mirror for separation. Projection objectives of this type may be used in a wide range of applications, such as those of the present invention, such that there is no optical surfaces (except for the last optical surface adjacent to the image surface) where erosion conditions occur and / or very small effective and / or actual sub- May be designed according to embodiments.

In some embodiments, the projection objective comprises a plurality of optical elements arranged to image an off-axis object field disposed at an object surface onto a non-contiguous image field disposed at an image surface of the projection objective, Said optical elements comprising:

A first articulated objective lens part forming a first intermediate image from the radiation coming from an object surface and including a first pupil surface;

A second objective lens unit including at least one concave mirror for forming the first intermediate image into a second intermediate image and including a second pupil plane optically conjugate to the first pupil plane; And

A third articulated objective lens part forming an image of the second intermediate image on the image surface and including a third pupil surface optically conjugate to the first and second pupil surfaces is formed.

Embodiments may have exactly two intermediate images.

The second objective lens portion can have exactly one concave mirror and the projection objective has a first folding mirror for deflecting the radiation from the object surface in the direction of the concave mirror and a second folding mirror for deflecting the radiation coming from the concave mirror onto the image surface Lt; RTI ID = 0.0 > a < / RTI > second folding mirror. The deflection mirrors may all be flat. The projection objective may be designed for immersion lithography with NA> 1.

Figure 1 shows a detailed view of a conventional catadioptric projection objective with exactly two intermediate images and one concave mirror wherein relatively small actual sub-apertures of the light beams occur at optical surfaces close to the intermediate image (FIG. 1A), folding mirrors are located in the corrosive zone (FIG. 1B).

Figure 2 shows a schematic view of the axial direction of the lens surface located at the pupil plane and illuminated by dipole illumination where contamination is present in one region of the poles.

Fig. 3 shows a schematic flow chart showing a preferred embodiment of a manufacturing method for avoiding optical surfaces having a very large concentration of radiant illumination.

Figure 4 illustrates a pupil raster of pupil points for dividing a pupil plane into raster fields having substantially the same raster field area in accordance with an embodiment of the present invention.

Fig. 5 shows the intersections of the rays of the pupil raster shown in Fig. 4 on the selected optical surface far from the pupil plane, wherein the region of relatively high effective irradiance (or pinhole illumination intensity) is shown.

Fig. 6 shows the intersections of the rays of the pupil raster shown in Fig. 4 on the selected optical surface far from the pupil plane, wherein the region having the corrosion condition is shown.

Figure 7 shows one embodiment of a catadioptric projection objective with two intermediate images, one concave mirror and two flat deflecting mirrors wherein corrosion conditions are systematically avoided on optical surfaces near the intermediate images have.

Figure 8 shows the traces of selected field points around the edge of a rectangular field on the first and second folding mirrors of the embodiment of Figure 7;

As an introduction to some of the problems solved by the preferred embodiments of the present invention, consider a high-aperture reflective refractive objective lens suitable for immersion lithography with NA > 1. In many cases, such designs exhibit rather small actual sub-apertures on the optical surfaces. The term "real sub-aperture ", as used herein, refers to the" real sub-aperture " of a ray bundle originating from a specific object point (i.e., a field point in an object surface) Mark "(intersection area). The small actual sub-apertures comprise at least one concave mirror and at least one intermediate phase, as well as at least one flat deflection mirror with at least one flat deflection mirror for separating the beam traveling towards the concave mirror from the beam reflected from the concave mirror. This can occur particularly in projection objectives.

For purposes of explanation, FIG. 1A shows a detailed view of a conventional catadioptric projection objective taken from FIG. 2 of WO 2004/019128 A2. The projection objective lens has a first articulated objective portion OP1 for imaging an object field from a flat object surface OS into a first intermediate image IMI1, a second intermediate image IMI2, And a concave mirror (CM) for imaging the second intermediate image (IMI2) onto a flat image surface parallel to the object surface (OS) And a third articulated objective portion OP3 (only partially shown). The first flat folding mirror FM1 deflects the radiation coming from the object surface towards the concave mirror CM. A second flat folding mirror FM2, which is orthogonal to the first folding mirror, deflects the radiation coming from the concave mirror toward the image surface.

The light beam RB originating from the off-axis field point FP1 of the effective object field located completely out of the optical axis AX is located within the intersecting zones of a size varying with the position of the optical surface in the optical system Lt; / RTI > intersects the optical surfaces. The intersection zones are denoted as "actual sub-apertures" in the present application. Some representative intersection areas are highlighted in bold lines in FIG. The first actual sub-opening SA1 on the incidence side of the plane-parallel plate immediately following the object surface is closer to the field surface (object surface) and relatively small. The size of the second actual sub-aperture SA2 near the first pupil plane P1 on the concave incidence side of the meniscus lens substantially corresponds to the size of the pupil at that position and is relatively large. All actual sub-apertures are substantially superimposed on the pupil surface. The actual sub-apertures are arranged so that, as can be seen for the actual sub-aperture (SA3) on the concave exit side of the positive meniscus lens immediately upstream of the first folding mirror (FM1) As the optical surfaces are closer to the first intermediate image IMI1, as can be seen by the actual sub-aperture SA4 formed on the first folding mirror FM1 on the upstream side. On the contrary, the fifth actual sub-opening SA5 on the convex surface of the negative meniscus lens just in front of the concave mirror CM is substantially parallel to the size of the second pupil P2 in which the concave mirror CM is located As shown in FIG.

In order to obtain images without vignetting and pupil obscuration, non-axis object fields and image fields are used in this design. When the stock field is used, the effort required to correct the imaging aberrations increases with increasing the distance between the non-axis object field and the optical axis to increase the size of the "design object field ". The "design object field" includes all field points of an object surface that can be projected by a projection objective with sufficient imaging accuracy for the intended lithography process. Within the design object field, all imaging aberrations must be sufficiently corrected for the intended projection purpose, while for field points outside the design object field at least one of the aberrations is greater than the desired threshold. Thus, in order to facilitate correction, it may be required to keep the size of the design object field small, which also requires minimizing the offset between the optical axis and the non-axis object field. Efforts to minimize such offsets often lead to designs with lens surfaces and / or mirror surfaces that are relatively close to the field surface so that the corresponding actual sub- The openings become smaller. For example, the lens surface immediately upstream of the first intermediate image IMI1 and the actual sub-apertures SA3 and SA4 on the mirror surface are relatively small.

Small actual sub-apertures on optical surfaces generally require stringent requirements for cleanliness of the manufacturing process and the resulting optical surfaces, which are substantially free of severe surface defects. Since the energy emitted from the selected field point is concentrated in a relatively small area in the area of the small real sub-aperture, defects present in that area of the small real sub-aperture may block some of the light intensity present in the light beam Thereby causing optical loss. In the case where the corresponding actual sub-aperture is small, surface defects of a given size result in a greater light loss effect than in areas with large real sub-apertures, because the area of the defect resulting in the perturbation and the area of the actual sub- And the ratio between the area (the local area) becomes simply undesirable. The localized light loss caused by defects can result in a uniformity error with a direct problem with respect to the critical dimension of the structures made with the projection objective. In particular, an undesirable change in the critical dimension (CD variation) may be caused in relation to the sensitivity of the photosensitive coating (photoresist) on the substrate.

In addition, the localized light loss may also result in telecentric errors resulting from the shift of the energy center of the pupil generally in the direction of the unperturbed regions. As shown, Fig. 2 shows a schematic view of the axial direction of the lens surface LS located on the circular pupil plane in the projection objective lens. The projection exposure apparatus is operated as bipolar illumination, for example, to improve the depth of focus and / or contrast when printing densely packed unidirectional lines. The corresponding intensity distribution of the projected radiation is characterized by two "poles," ie, a first pole (PO1) and a second pole (PO2) on both sides of the optical axis (AX) It is built. There is no light intensity on the optical axis. Surface contamination CON exists on the lens surface LS within the region of the first pole PO1. Since the contaminant CON blocks a significant portion of the light energy present at the first pole PO1, the energy center of the pupil will shift towards the unperturbed second pole PO2. This eccentric energy center in the pupil corresponds to the direction of travel of the intensity center of the image in the image space in which the wafer is located. Due to contamination (CON), the traveling direction deviates from the optical axis and is directed at a finite angle with respect to the optical axis. This results in an oblique orientation of the aerial image, for example if the position of the substrate surface locally changes due to a non-flat substrate surface and / or due to positional changes caused by the wafer stage, then telecentricity - Can induce induced distortion.

Assuming that the actual sub-apertures are substantially homogeneously illuminated so that there is no or little difference in localized radiation energy input between different locations within the actual sub-aperture, then the actual sub- The size of the apertures is considered as an excellent indicator of which optical surfaces will be critical to surface defects.

A substantially homogeneous distribution of irradiance in the actual sub-apertures may be an excellent approach for many purely refractive optical systems. However, there may be significant variations in the local irradiance in the actual sub-opening such that significant differences in the local irradiance may occur within the actual sub-apertures. It has been found useful to define an "effective sub-aperture" in order to take into account possible inhomogeneities with respect to the local irradiance in the actual sub-apertures, To take into account local changes in the environment. As mentioned above, defects on the optical surfaces can be significant if a relatively large synchrotron radiation energy is concentrated in the region of the defect. Thus, within the actual sub-aperture, such areas may be particularly severe if the local irradiance has a relatively large value when compared to other portions of the actual sub-aperture. In order to substantially avoid the problems associated with the local concentration of the radiation energy, it may be useful to analyze the area of the actual sub-aperture to identify the area (or areas) where the maximum value for the effective irradiance occurs. In a geometric description, the local irradiance may be characterized by a specific value for the "geometric light density" of rays originating from a single field point in the object plane, where the maximum value for the effective irradiance is the geometric ray Occurs where the maximum value for density occurs.

Now consider a system optimization that takes into account such local maxima of the geometric light density (or effective irradiance). The tolerance for the system layout can have a particular value, assuming that each actual sub-aperture is illuminated homogeneously. However, the tolerance must be more stringent when localized concentration of the radiation energy in the actual sub-aperture occurs. This effect can be considered by defining "effective sub-apertures" corresponding to sub-apertures representing positions with maximum geometric light density (or maximum effective radial intensity). Within this concept, the magnitude of the "effective sub-aperture" is equal to the magnitude of the "actual sub-aperture" in systems where the actual sub- aperture is uniformly illuminated. However, if localized localization of the synchrotron radiation occurs in the actual sub-aperture, it is represented by the size of the effective sub-aperture which is smaller than the size of the actual sub-aperture.

By way of illustration, FIG. 1B shows a detail view of another conventional catadioptric projection objective taken from Example 5 of WO 2004/019128 A2. The same or similar elements are denoted by the same reference numerals as in FIG. One ray of light RB emerges from the selected field point FP1 on the object surface OS. The opening angle of the light beam on the object surface is determined by the object side numerical aperture NA _OBJ . The trajectories of various selected rays including rays Rl and R2 are shown. As is apparent from the figure, the geometric light beam density defined by the rays of light RB is transmitted to the area of the large positive meniscus lens ML, which is on the upstream side of the first folding mirror FM1 across the optical surfaces Substantially homogeneous. However, as the rays approach the first folding mirror FM1, the local density of the rays (i.e., the geometric light beam density) increases to the outside of the light beam (furthest away from the optical axis AX) do. In other words, the geometric light density becomes non-homogeneous in the region of the first folding mirror at a small distance upstream from the first intermediate image IMI1. In particular, the rays R1 and R2 originating from the field point FP1 with different aperture values intersect in an area at or near the first folding mirror FM1. (Note that such intersection of single rays occurs particularly in the areas of the first folding mirror FM1 and the second folding mirror FM2 optically close to the intermediate images IMI1 and IMI2, respectively. The intersection of rays originating from a common field point with a " light " indicates the presence of a "caustic" condition. It is evident from FIG. 1B that both the first folding mirror FM1 and the second folding mirror FM2 are located in the regions where the rays of light RB intersect, i.e. in the "caustic regions ".

As noted above, the geometric light density corresponding to a particular field point can be viewed as representing the contribution of a particular field point to the total irradiance on the optical surface. This contribution is referred to in this application as "effective irradiance ". In an accident experiment, consider an illumination pinhole (representing one single field point) in the object plane of the optical system. A ray of light originates from the field point. The "effective irradiance" corresponding to the field point is the contribution of the irradiance of the light beam originating from its field point at the selected optical surface. This contribution to the total irradiance entering the optical surface is a function of the position of the pinhole in the object surface as well as a function of the position (or location) on the optical surface. The maximum value of all values of the effective irradiance (which is the irradiance corresponding to a single object field point) is not intended to exceed the predetermined "irradiance threshold ".

In the corrosive region, that is, in the region where the corrosive condition exists, the effective irradiance (pinhole irradiance) can lead to substantially directing the concentration of serious light energy on a relatively small surface area. From the point of view of ray propagation, if different rays emitted from object points with different numerical apertures intersect on or near one optical surface, a corrosion condition is given on the optical surface. Surface defects on the optical surface where the corrosive area is located affect the light rays emitted from the object point at different aperture angles, thereby substantially further degrading the imaging quality than defects located outside the corrosive area .

In one embodiment of the method of manufacturing a projection objective according to the present invention, the occurrence of erosion conditions on the optical surfaces and / or the generation of too small effective sub-apertures on the optical surfaces causes the optical elements of the projection objective But are systematically avoided by corresponding sub-routines in software used in computer-based optical design leading to typical layouts. In the event that such corrosion conditions are systematically avoided, the requirements for cleanliness of the process can be mitigated without substantially jeopardizing the optical performance of the resulting projection objective.

In this embodiment, one of the merit function components used in the computational process is that the effective irradiance that occurs on each optical surface (except for the last optical surface closest to the image surface) is equal to the maximum effective IRRDD _EFF Lt; RTI ID = 0.0 > IRRTV. ≪ / RTI >

As used herein, the term "irradiance" refers to the power of electromagnetic radiation at one surface per unit area. Specifically, the term "irradiance" refers to the power of electromagnetic radiation incident on the surface. SI units for irradiance are in watts per square meter (W / ㎡). Radiance is sometimes called intensity, but it should not be confused with radiant intensity with different units. The term "effective irradiance " (also referred to as" pinhole irradiance ") refers to the contribution to total irradiance incident on one optical surface, originating from the radiant light coming from one single object field point. From the light ray progression, the "effective irradiance" corresponds to the "geometric light ray density" of different rays originating from a common field point with equidistant aperture steps. The effective irradiance can be homogeneous on one surface. In general, the effective irradiance is defined to define an area of maximum effective irradiance (corresponding to a large local geometric light density) and an area of smaller values for effective irradiance (corresponding to a smaller geometric light density) It may also vary across a surface.

If relatively large values of effective irradiance occur on the optical surface, such optical surfaces can be compromised for surface defects such as scratches and contamination. Relatively large values of effective irradiance can be caused, for example, by corrosion conditions on the optical surface and / or by too small effective sub-apertures on the optical surface.

One embodiment of an optimization routine implemented in a software program used to calculate the optical design (layout) of the projection objective is now described with reference to the schematic flow chart shown in FIG. In the first step S1 (optimization for aberrations, OPT AB), the basic layout of the optical design is optimized with respect to aberrations to obtain the optimized design D1. To this end, conventional subroutines of appropriate optical design programs may be used to change one or more structural parameters of the projection objective and to calculate the resulting overall aberrations of the design. As a result, the optimization design D1 may or may not have optical surfaces that occur locally, with a large concentration of effective irradiance on one or more of the optical surfaces.

In a second step S2, a normalized effective irradiance value IRRAD _EFF representing the effective irradiance normalized to the effective irradiance at the image surface of the projection objective is obtained (the last optical power directly adjacent to the image surface of the projection objective The optimized design is analyzed to determine whether or not a predetermined irradiance threshold IRRTV on each optical surface of the projection objective (except the surface) is exceeded.

If the normalized effective irradiance value IRRAD _EFF does not exceed the irradiance threshold IRRTV on any optical surface (except when the last optical surface is possible), then the parameters representing the optimization design D1 are the aberration level, AB is input to the aberration determining step S3 to determine whether it is above or below the predetermined aberration threshold value ATV. If the aberration degree is below the aberration threshold, the optimization design (D2) is output as a result of the optimization process. The final design D2 will be the same as the optimization design D1 that serves as input to the emission intensity determination in the second step S2.

If the irradiance determination in the second step S2 determines that the normalized effective irradiance value IRRAD of the optimization design D1 exceeds the irradiance threshold value IRRTV for at least one optical surface (excluding the last optical surface) , The calculation routine proceeds to an irradiance optimization step S4 (OPT IRRAD), where the optimization design D1 is again optimized for effective irradiance to reduce the local concentration of irradiance on the critical optical surfaces do. To this end, at least one structural parameter of the projection objective is modified and an overall advantage function is employed, including an advantage function component that defines the maximum irradiance requirement. The resulting re-optimization design D1 'then determines the illuminance intensity through step S1 to determine whether the normalized illuminance value is now below the illuminance threshold value IRRTV for each of the considered optical surfaces Is input to step S2. Repetitions involving more than two re-optimization steps may be used for this purpose.

If the radiation illuminance determining step S2 determines that the resulting re-optimization design D1 'has been optimized with respect to the local maximum occurrence of the radiation illuminance, re-optimization of the irradiance illuminates one or more critical aberrations The optimized structure is input to the aberration determining step S3 to determine whether to be above the threshold value. If the re-optimization design is still acceptable for aberrations, the final design (D2 ') is obtained and output in step S5.

If the design needs to be optimized with respect to aberrations, in order to modify the optical design (which has already been optimized with respect to the irradiance) also with respect to aberrations, the structural parameters received from step S2, ). One or more iterations may be needed to re-optimize the design with respect to aberrations and still obtain a normalized irradiance value below the critical irradiance threshold IRRTV for all critical optical surfaces. The final design (D2 ') is the result of the re-optimization process.

In some embodiments, the subroutine relating to the irradiance that determines the determination step S2 allows for avoiding too small sub-apertures on all optical surfaces, except for the last optical surface closest to the image surface, Thereby avoiding the occurrence of erosive conditions on the surfaces.

One embodiment of a computational routine designed to systematically avoid the occurrence of corrosion conditions on the optical surfaces is now described with reference to Figures 4-6.

In the first step of the computational part of the manufacturing method, a number of representative field points are defined. Ray tracing is performed on the rays of rays of light originating from the representative field points.

In a second step, a pupil raster is defined, in which the pupil raster represents an array of raster points in the pupil plane of the projection objective, the raster points being arranged in a two- Distance away from each other.

In a third step, a ray trajectory of the rays originating from representative field points and passing through the raster points of the pupil raster is calculated for each representative field point. Commercially available design programs such as CODE V, OSLO or ZEMAX may be used for this purpose.

에 따라 계단형으로 증가하는데, 여기서 i = 0, 1, ..., n 이고, NA는 투영 대물렌즈의 이미지측 개구수이며, β는 물체 필드와 이미지 필드 사이의 배율이다. 그렇게 함으로써, 동공면은 실질적으로 동일한 래스터 필드 면적을 갖는 래스터 필드(또는 래스터 셀)들로 재분할된다.The pupil raster typically surrounds the entire usable aperture of the projection objective, wherein the aperture determines the aperture angle of the light beam originating from each representative field point. In the embodiment of the pupil raster shown schematically in Figure 4, the coordinates of the raster points in the pupil plane are such that the neighboring raster points (i.e., raster points directly adjacent to each other at a predetermined distance) have the same distance in the azimuthal direction It is given in polar coordinates. In this embodiment, the angular step width between neighboring raster points in the circumferential direction (azimuth direction) is 10/3 degrees. The coordinates of the raster points in the radial direction (radial direction) correspond to the aperture angles included between the respective rays and the optical axis. The angle is also referred to as the "pupil angle" in the present application. The absolute value of the sine of the sinusoid is defined as the square root function between angle 0 and maximum angle k _max = NA -

, Where n is the image-side numerical aperture of the projection objective, and? Is the magnification between the object field and the image field. By doing so, the pupil plane is subdivided into raster fields (or raster cells) having substantially the same raster field area.

In a fourth step, the intersections of the optical surfaces and the selected rays in the projection objective are calculated for each optical surface (potentially excluding the last optical surface immediately adjacent to the image surface in some cases).

In the following steps, pairs of directly neighboring raster points in the pupil plane are considered. Directly neighboring raster points are defined such that the two raster points of the pair have the same azimuth coordinate or the same dynamic coordinate, while each of the other coordinates differs by one coordinate step in each coordinate direction according to the preselected step width . For each pair of neighboring raster points, the following difference quotient is computed:

Where f represents the number of each optical surface and i, j represents the index of neighboring pupil raster points. In Equation (1), the variables x and k are vectors. The components of vector x represent the coordinates of a point on the optical surfaces in real space. The components of vector k are direction sign values indicating the x, y, z coordinates of the unit vector indicating the direction of travel of the light in the entrance pupil of the optical system (i.e., in the pupil space). Thus, the quotient in Equation (1) is a measure that represents the relationship between the step having a predetermined step width (variable k) in the pupil space and the corresponding step width (variable x) in the actual space on the optical surface. In other words, the gradient parameter g ^f _ij defined by the numerator of equation (1) represents the degree of change in intersection points on each surface for a given difference in pupil coordinates. (The differential quotient of equation (1) is an approximation of the differential quotient, which indicates that the finite step width is typically used in numerical calculations. And becomes a derivative share.

Note that the numerical criterion given in equation (1) above is defined using linear gradients. More precisely, the ratio between the areas of one grid mesh element on the corresponding one and the corresponding one in the orientation space could be controlled. Practically, however, it is shown that controlling the linear gradients in nearly orthogonal directions is sufficient in most cases to avoid local peaks of light density.

In a further step, a gradient threshold value is defined that represents the minimum acceptable acceptance between neighboring intersections on the optical surfaces. For example, a gradient threshold g ^f _ij (min) = 10 mm may be defined.

5 further illustrates the intersection points of the rays of the pupil raster shown in Fig. 4 on the selected optical surface for the selected representative field point. The local density of the intersections in the high irradiance area (HIRAD) at the lower right corner of the optical surface is considerably larger than in other parts of the optical surface, such as in the upper left, diametrically opposed to the higher irradiance area It is clear, therefore, that the distribution of the intersections and, consequently, the distribution of the irradiance across the optical surface is not uniform. However, no erosion conditions are given on the optical surface, because (in spite of the fact that the step width between neighboring intersections representing the amount of irradiance in each area varies greatly across the optical surface) Because the series of intersections in the pupil plane is the same as in the pupil plane. The situation shown in Fig. 5 corresponds to a value of 4.7 _mm for the gradient parameter g ^f _ij .

In the next step, the minimum value of the gradient parameter is calculated for each of the optical surfaces (optionally, except for the last optical surface). If the minimum value computed for a particular optical surface is found to be less than the gradient threshold, then the structural parameters of the projection objective are optimized such that the minimum gradient for each optical surface increases until it is greater than or equal to the gradient threshold do. For example, the optimization process may include increasing the distance between each optical surface and a neighboring field surface (such as an object surface, intermediate surface, or image surface). Alternatively or additionally, the local tilt of the optical surface within the minimum gradient area may be changed to increase the calculated gradient for neighboring intersections.

The corresponding radius R _SUBEFF of the "effective sub-opening" can be calculated according to the following equation (2).

Here, Min (g ^f _ij ) is the minimum value of the difference quotient in Equation (1) and NA _OBJ is the image side numerical aperture. Note that the area having the minimum value for the gradient parameter corresponds to the area having the maximum geometric light density (and the maximum effective irradiance).

As a final result of the optimization process, all of the optical surfaces of the projection objective are positioned and shaped such that the gradient parameter is equal to or greater than the gradient threshold. In view of the radiance, this requirement is satisfied if the normalized irradiance value IRRAD, which represents the normalized irradiance for the irradiance at the image surface of the projection objective, is greater than the normalized irradiance value IRRAD for each optical surface of the projection objective (Except for the last optical surface directly adjacent to the image surface) does not exceed the predetermined illuminance threshold IRRTV. If the irradiance does not exceed the acceptable maximum value for the normalized irradiance, the optical performance of the projection objective becomes relatively insensitive to potential defects on the optical surfaces, as described above.

For comparison, FIG. 6 shows the intersections of the rays of the pupil raster on the "virtual" system surface within the region where the corrosion condition occurs for the field point. At the upper left of the hypothetical surface, the intersection points corresponding to the pupil coordinates at the outer edge of the pupil are located at the intersection of the axial ray, rather than the intersection points corresponding to raster points located somewhere between the optical axis and the outer edge of the pupil It is clear that they are closer. In other words, the rays originating from a certain field point with different numerical apertures (represented by different radial coordinates in the pupil raster) may be arranged such that the light rays corresponding to the larger aperture value in the pupil plane correspond to smaller aperture values Intersect on or near each optical surface so as to intersect the optical surface closer to the axis of the ray than to the rays of light. In this description, the gradient parameter g ^f _ij obtains a minimum value of 0 mm in the region of the optical surface lying in the caustic region (CAUSTIC) (upper left).

Adopting this method systematically leads to optical design in which no optical surfaces are located in areas with corrosive and / or very small effective sub-apertures. In the event that such optical surfaces are avoided in the optical system, details regarding surface quality and / or contamination can be mitigated, thereby facilitating the fabrication of the optical system.

FIG. 7 illustrates one embodiment of a catadioptric projection objective 100 that follows such conditions. The projection objective is designed for a nominal UV-operating wavelength of? = 193 nm. The details are given in Tables 1 and 1a. The projection objective 100 can project an image of a pattern on a reticle disposed on a flat object surface OS (object plane) onto a reduced scale, while accurately forming two intermediate intermediate images IMI1 and IMI2 , E.g., 4: 1, onto a flat image surface IS (image plane). The rectangular effective object field OF and the image field IF are off-axis completely outside the optical axis AX. The first articulated objective lens unit OP1 is designed to image the pattern on the object surface to the first intermediate image IMI1 on an enlarged scale. The second refraction type refracting / reflecting type objective lens unit OP2 images the first intermediate image IMI1 into the second intermediate image IMI2 at a magnification close to 1: (- 1). The third articulated objective section OP3 images the second intermediate image IMI2 at a strong reduction ratio onto the image surface IS.

The path of the principal ray CR of the outer field point of the reserved object field OF is shown in bold in Fig. 7 so as to be easily traced along the beam path of the projection beam. For purposes of the present application, the term "principal ray " (also known as the main ray) refers to the ray that travels from the outermost field point (farthest from the optical axis) of the object field OF used effectively to the center of the incident pupil . Due to the rotational symmetry of the system, the principal ray can be selected from equivalent field points within the meridional plane as shown in the figure for illustrative purposes. In projection objective lenses that are substantially telecentric at the object side, the principal ray emerges from the object surface at a very small angle or parallel to the optical axis. The imaging process is also characterized by the trajectory of the ambient light. As used herein, "ambient light" is a ray traveling from the on-axis object field point (the field point on the optical axis) to the edge of the aperture stop. The ambient light may not contribute to image formation due to vignetting when the stockpile effective object field is used. The principal ray and the peripheral ray are selected to characterize the optical properties of the projection objective. The angles included between such selected rays and the optical axis at a given axial location are referred to as "principal ray angle" (CRA) and "ambient ray angle" (MRA), respectively. The radial distance between such selected rays and the optical axis at a given axial location is referred to as the " principal ray height "(CRH) and the" ambient ray height "(MRH), respectively.

Three mutually conjugate pupil surfaces P1, P2 and P3 are formed at positions where the principal ray CR crosses the optical axis. The first pupil plane P1 is formed in the first objective lens part between the object plane and the first intermediate image and the second pupil plane P2 is formed in the second objective lens part between the first intermediate image and the second intermediate image , And a third pupil plane P3 are formed in the third objective lens portion between the second intermediate image and the image surface IS.

The second objective lens unit OP2 includes a single concave mirror CM. The first flat folding mirror FM1 is disposed optically close to the first intermediate image IMI1 at an angle of 45 degrees with respect to the optical axis AX so as to reflect the radiation coming from the object surface in the direction of the concave mirror CM do. A second folding mirror FM2 having a flat mirror surface arranged at right angles to the flat mirror surface of the first folding mirror reflects the radiation coming from the concave mirror CM in the direction of the image surface parallel to the object surface.

The folding mirrors FM1 and FM2 are respectively located in the optical vicinity of the intermediate image, and the result is that the etendue (geometrical flux) is kept small. The intermediate images are preferably not located on the flat mirror surfaces, resulting in a finite minimum distance between the optically closest mirror surface and the intermediate image. This ensures that any defects in the mirror surface, such as scratches or impurities, are not sharply imaged onto the image surface.

The first objective lens unit OP1 includes two lens groups LG1 and LG2 each having a positive refractive power on both sides of the first pupil plane P1. The first lens group LG1 is designed to function in the manner of a Fourier lens group that performs a single Fourier transform by imaging the telecentric incident pupil of the projection objective on the first pupil plane Pl. This Fourier transform leads to a relatively small maximum principal ray angle CRA _P1 of about 17 degrees at the first pupil plane. As a result, the optical freedom diameter of the first pupil is relatively large, as indicated by the diameter D ₁ = 145 mm of the radiation beam at the first pupil plane, according to the Lagrange invariant.

A relatively small principal ray angle with a large pupil diameter corresponds to a relatively large axial extension of the pupil space (PS). For the purposes of the present application, the pupil space is defined by the condition RHR < B &gt; for the ray height ratio RHR = CRH / << 1 is satisfied, the peripheral ray height MRH is defined as an area sufficiently larger than the principal ray height CRH. The upper limit B of the beam height ratio may be, for example, less than 0.4, or less than 0.3, or less than 0.2. If the above condition is met, the correction applied to the pupil space will have a substantially field-constant effect. The axial expansion of the pupil space increases as the principal ray angle in each pupil decreases. In this embodiment, the condition RHR &lt; 0.3 is satisfied.

In the present embodiment, the pupil space PS includes a first lens L1-6 (biconvex lens) of LG2 immediately downstream of the pupil plane, and the first lens L1-6 (biconvex lens) on the image side of the pupil plane To the positive convex positive lens L1-5 which is on the upstream side of the first pupil plane to the lens L1-7. (FS1 (= 41 mm) and FS2 (= 62 mm)) having an axial extension of at least 40 mm so as to position one or more thin correction elements in the free space is formed on the pupil plane P1 in the pupil space PS. I.e., optically close to the pupil plane. Thus, this embodiment enables to interpose one or more correction elements optically close to the first pupil plane Pl to obtain substantially the same correction effect (constant correction to the field) for all field points of the field do.

A parallel plate PP is arranged on the first pupil plane P1 where the condition RHR ~ 0 is satisfied in the pupil space PS. The parallel plate may serve as a placeholder for a correction element which may be substantially formed as a plane parallel plate having the same thickness and material, which is part of the initial design of the projection objective, wherein at least one The surface has an aspherical shape.

As mentioned above, for high-aperture projection objective lenses commonly used in microlithography, relatively small actual and / or effective sub-apertures of the light beams may be used for certain optical surfaces, RTI ID = 0.0 &gt; optical surfaces. &Lt; / RTI &gt; For example, in the embodiment of FIG. 7, both the first folding mirror FM1 and the second folding mirror FM2 are spaced apart from each other so that each small actual sub-aperture is formed on the folding mirrors, And IMI2, respectively. Generally, the irradiance on the optical surface increases as the size of the sub-apertures decreases. If a relatively large value of irradiance occurs on the optical surface, the optical surface may be critical to surface defects such as scratches and contamination. Corrosion conditions may also occur on some optical surfaces in the optical system, particularly near field planes. If the optical surface is in the corrosive zone, the irradiance may be divergent.

Due to these effects, the details of surface quality and contamination must be kept particularly strict in optical systems having optical surfaces in areas of small sub-openings and / or in areas where corrosion conditions occur. On the other hand, if such surfaces are avoided in an optical system, the details of surface quality and / or contamination can be mitigated, thereby facilitating the manufacture of optical systems.

In the fabrication of the embodiment of FIG. 7, it is necessary to correct the optical path such that the first and second folding mirrors FM1 and FM2 are sufficiently far away from the intermediate phase to obtain a relatively large sub- There was a special emphasis on controlling the corrosion conditions so that no corrosion conditions would occur in FM1 and FM2. This is qualitatively illustrated in FIG. 8, which shows the traces of the 18 selected field points around the edge of the rectangular field on the first folding mirror FM1 and the second folding mirror FM2. In each of the traces, the light beams are shown as ten equidistant opening stairs. It is clear that the substantially elliptical lines corresponding to the light beams of the different apertures originating from the same field point do not intersect and are fitted without intersection of the folding mirrors. This indicates that both the first and second folding mirrors are in regions that are free of erosion conditions, i.e., "corrosion free" regions. Also, a biconvex positive lens, which is geometrically located in the dual-path region between the folding mirrors (FM1, FM2) and the concave mirror, and which is optically located relatively close to the first and second intermediate images, There is no area. As a result, the embodiment of FIG. 7 is relatively tolerant of surface defects and / or contamination on optical surfaces close to the intermediate images IMI1, IMI2.

The above description of the preferred embodiments has been given in an illustrative manner. The individual features may be implemented singly or in combination as embodiments of the present invention, or may be implemented in other fields of application. They may also represent advantageous embodiments that may be protected from the right to claim protection in the present application as filed or to claim protection for the duration of the present application. It will be apparent to those skilled in the art, upon reading the present disclosure, that various changes and modifications will be apparent to those skilled in the art without departing from the invention and its advantages, as well as with the structure and method disclosed. The Applicant therefore seeks to cover all such variations and modifications as are within the spirit and scope of the present invention as defined by the appended claims and their equivalents.

The contents of all claims are hereby incorporated by reference.

Claims (14)

Defining an initial design for a projection objective and optimizing the design using an advantage function, the method comprising: Defining a plurality of advantageous functional components each reflecting a specific quality parameter, wherein one of the advantageous functional components comprises a normalized effective value representing a normalized effective irradiance for the effective irradiance at the image surface of the projection objective Defines a maximum radiance requirement that requires that the irradiance value does not exceed a predetermined irradiance threshold on each optical surface of the projection objective other than the last optical surface directly adjacent to the image surface of the projection objective ; Calculating numerical values for each of the advantageous function components based on a corresponding feature of the preliminary design of the projection objective; Calculating, from the advantageous functional components, a total advantageous function that can be represented by numerical items reflecting quality parameters; Continuously varying at least one structural parameter of the projection objective and recalculating the resulting overall advantage function for each successive change until the resulting overall advantage function reaches a predetermined acceptable value; Obtaining structural parameters of an optimized projection objective having a predetermined acceptable value for the resulting overall advantage function; And And implementing the parameters to produce a projection objective. The method according to claim 1, Calculating the position and extent of potential corrosive areas in the projection objective; And And optimizing structural parameters of the projection objective such that no optical surface is located within the corrosive region. 3. The method according to claim 1 or 2, Defining a plurality of representative field points; Defining a pupil raster representing an array of spaced apart raster points within the pupil plane of the projection objective; Calculating, for each representative field point, a ray trajectory of light rays originating from the representative field points and passing through raster points of the pupil raster; Calculating, for each optical surface, the intersections of the optical surface and the rays; Computing, for each optical surface, a number of gradient parameters representing respective gradients between intersections corresponding to neighboring raster points arranged directly adjacent to each other; Defining a gradient threshold value that represents an acceptable minimum gradient between neighboring intersections; And Optimizing the structural parameters of the projection objective such that the gradient parameter is not below the gradient threshold for each optical surface of the projection objective except for the last optical surface. The method of claim 3, Wherein the pupil raster is defined such that the pupil plane is subdivided into raster fields having substantially the same raster field area. The method of claim 3, The pupil raster is defined as a polar coordinate so that neighboring raster points have the same distance in the azimuthal direction, and the pupil angle k is between 0 and k _max = NA 占

, Where i = 0, 1, ..., n, NA is the image side numerical aperture of the projection objective, and [beta] is a magnification between the object field and the image field. 3. The method according to claim 1 or 2, Defining a plurality of representative field points; Calculating the intersection areas of the light beams originating from the field points and the optical surfaces and the optical surfaces, wherein the intersection area between the light beam and the optical surface is the actual sub-aperture size defined by the area of the intersection area Defining a real sub-aperture having a first sub-aperture; Defining a sub-aperture size threshold; For all optical surfaces of the projection objective except the last optical surface directly adjacent to the image surface of the projection objective, the actual sub-aperture size for the selected field points is not below the sub-aperture size threshold And optimizing the structural parameters of the projection objective. 6. A projection objective produced according to the method of claims 1 or 2, characterized in that the projection objective comprises a plurality of optical elements configured to image the pattern provided on the object surface of the projection objective onto the image surface of the projection objective lens. 8. The method of claim 7, Wherein the projection objective is a catadioptric projection objective designed to image a stock object field disposed at an object surface into a stock image field disposed at an image surface, At least one concave mirror; At least one intermediate phase; And And at least one folding mirror disposed to deflect the radiation from the object surface to the concave mirror or to deflect the radiation from the concave mirror to the image surface. 9. The method of claim 8, Said optical elements comprising: A first articulated objective lens unit forming a first intermediate image from the radiation coming from the object surface and including a first pupil plane; A second objective lens unit including at least one concave mirror for forming the first intermediate image into a second intermediate image and including a second pupil plane optically conjugate to the first pupil plane; And And a third refractive objective lens portion forming an image of the second intermediate image on the image surface and including a third pupil surface optically conjugate to the first and second pupil surfaces. 10. The method of claim 9, The projection objective has precisely two intermediate images or the second objective part has exactly one concave mirror or the projection objective has exactly two intermediate images and the second objective part is exactly one Lt; RTI ID = 0.0 &gt; mirror, Wherein the projection objective has a first folding mirror for deflecting the radiation from the object surface in the direction of the concave mirror and a second folding mirror for deflecting the radiation from the concave mirror toward the image surface, The objective lens is designed for immersion lithography with NA > 1, or the projection objective has a first folding mirror for deflecting the radiation from the object surface in the direction of the concave mirror and a second folding mirror for deflecting the radiation coming from the concave mirror in the direction And wherein the projection objective is designed for immersion lithography with NA > 1. CLAIMS What is claimed is: 1. A projection objective comprising a plurality of optical elements having optical surfaces configured to image a non-shrink object field disposed at an object surface into a non-contiguous image field disposed at an image surface of the projection objective, At least one concave mirror; At least one intermediate phase; And At least one folding mirror disposed to deflect the radiation from the object surface to the concave mirror or to deflect the radiation from the concave mirror to the image surface, Wherein the structural parameters of the projection objective are adjusted so that no optical surface is located within the corrosive region. 12. The method of claim 11, Said optical elements comprising: A first articulated objective lens unit forming a first intermediate image from the radiation coming from the object surface and including a first pupil plane; A second objective lens unit including at least one concave mirror for forming the first intermediate image into a second intermediate image and including a second pupil plane optically conjugate to the first pupil plane; And And a third refractive objective lens portion forming an image of the second intermediate image on the image surface and including a third pupil surface optically conjugate to the first and second pupil surfaces. 13. The method of claim 12, The projection objective has precisely two intermediate images or the second objective part has exactly one concave mirror or the projection objective has exactly two intermediate images and the second objective part is exactly one Lt; RTI ID = 0.0 &gt; mirror, Wherein the projection objective has a first folding mirror for deflecting the radiation from the object surface in the direction of the concave mirror and a second folding mirror for deflecting the radiation from the concave mirror toward the image surface, The objective lens is designed for immersion lithography with NA > 1, or the projection objective has a first folding mirror for deflecting the radiation from the object surface in the direction of the concave mirror and a second folding mirror for deflecting the radiation coming from the concave mirror in the direction And wherein the projection objective is designed for immersion lithography with NA > 1. 14. The method according to claim 11, 12 or 13, Wherein the at least one folding mirror is a flat mirror.

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