The following disclosure is based on German patent application 10 2022 205 700.7 filed on June 3, 2022, which is incorporated into this application by reference.

FIELD OF APPLICATION AND PRIOR ART

The invention relates to a dioptric projection lens for imaging a pattern arranged in an object plane of the projection lens into an image plane of the projection lens by means of electromagnetic radiation at an operating wavelength λ _o in the ultraviolet range from 300 nm to 450 nm, a projection exposure apparatus equipped with the projection lens and a projection exposure method that is carried out with the aid of the projection lens.

Microlithographic projection exposure methods are predominantly used nowadays for producing semiconductor components and other finely structured components, for example masks for photolithography. Here, use is made of masks (reticles) or other pattern generating devices, which carry or form the pattern of a structure to be imaged, for example a line pattern of a layer of a semiconductor component. The pattern is positioned in the region of the object plane of the projection lens between an illumination system and a projection lens in a projection exposure apparatus and illuminated by illumination radiation provided by the illumination system. The radiation modified by the pattern travels through the projection lens as projection radiation, said projection lens imaging the pattern onto the substrate to be exposed. The surface of the substrate is arranged in the image plane of the projection lens, which image plane is optically conjugate to the object plane. The substrate is generally coated with a radiation-sensitive layer (resist, photoresist).

Typically, the demands of the semiconductor component manufacturers are different for the exposure of critical and non-critical structures. Currently, critical structures, which is to say fine structures, are predominantly produced using dioptric or catadioptric immersion systems that operate with operating wavelengths in the deep ultraviolet range (DUV), in particular at approximately 193 nm. By now, finest structures are exposed using EUV systems. These are projection exposure apparatuses constructed using reflective components only, which operate at operating wavelengths in the extreme ultraviolet range (EUV) between approximately 5 nm and 20 nm, for example at approx. 13.4 nm. For the purposes of producing mid-critical or non-critical layers with typical structure dimensions of significantly more than 150 nm, work is conventionally carried out using projection exposure apparatuses that are designed for operating wavelengths in the ultraviolet range from approx. 300 nm to approx. 450 nm. In this wavelength range, use is usually made of dioptric (refractive) projection lenses, the production of which is easily controllable on account of their rotational symmetry about the optical axis.

Projection exposure apparatuses for operating wavelengths at 365.5 nm ± 2 nm (so-called i-line systems) are used particularly frequently for these applications. They use the i-line of a mercury vapour lamp, with the natural full bandwidth thereof being restricted with the aid of a filter, or in any other way, to a narrower used bandwidth Δλ, for example of approx. 2 nm. During the projection, ultraviolet light of a relatively broad wavelength band is used such that the projection lens must be corrected well in relation to chromatic aberrations in order to ensure low-aberration imaging at the sought-after resolution, even with a broadband projection light. The longitudinal chromatic aberration (CHL), in particular, must be corrected in order to attain a sufficient imaging quality.

For the general understanding of the chromatic correction, consider the imaging with light from a broadband radiation source, the latter emitting light with different wavelengths around a central wavelength A, wherein the radiation distribution is characterizable by a spectral bandwidth Δλ (full width at half maximum). Let the three wavelength components include the wavelengths λ ₁ < λ ₂ < A3. The size of the longitudinal chromatic aberration CHL then corresponds to the maximum length of the focal range along the optical axis into which the different wavelengths are focused. The distance between the closest focal position of the wavelengths and the focal position of the wavelengths furthest away corresponds to the size of the longitudinal chromatic aberration of the imaging system for the broadband light source. The focal position of the central wavelength along the optical axis can be considered to be the image plane within the focal range. The variation of the paraxial focus position with the wavelength can be expanded in a power series. In this case, the linear portion is referred to as "primary spectrum", and the quadratic portion is referred to as "secondary spectrum"; moreover, it is also possible to define a "tertiary spectrum" by way of the cubic portion.

The primary spectrum can be corrected by combining converging and diverging lens elements that consist of different optical materials with different dispersion. In other words, it is possible to correct the longitudinal chromatic aberration in such a way that the paraxial focal planes for two different wavelengths, for example the minimum and the maximum wavelength of the spectral range, coincide on the optical axis. Such optical imaging systems are also referred to as "achromatized" or as an "achromat" in this application.

As a rule, a longitudinal chromatic aberration remainder remains for other wavelengths that are not captured by the correction. This longitudinal chromatic aberration remainder is usually the "secondary spectrum".

In some cases, it is also possible to correct the secondary spectrum by a suitable choice of optical materials, lens element dimensions, distances and refractive powers, etc. The secondary spectrum may possibly be corrected to such an extent that the focal positions of all three wavelengths λ ₁ , λ ₂ and λ ₃ of the considered wavelength range lie at the same axial position; only the "tertiary spectrum" remains in turn. In this application, an optical system where the secondary spectrum is also corrected is also referred to as "apochromatically corrected" or as an "apochromat".

In the case of dioptric projection lenses operated in broadband fashion, different lens element materials with sufficiently different dispersion properties are used for colour correction (i.e., for the correction of chromatic aberrations), said lens element materials being distributed within the projection lens into regions with different ray height ratios. Transparent materials used in the typical i-line projection lenses include, in particular, synthetic fused silica (SiO ₂ ) and the specialist glasses made commercially available under the designations FK5, LF5 and LLF1 by SCHOTT, Mainz, Germany. In these optical glasses, the synthetic fused silica and the FK5 glass are typical representatives of glasses with relatively low dispersion (crown glasses), while the glasses LF5 and LLF1 are typical representatives of glasses with relatively high dispersion (flint glasses). Other manufacturers use different names for their types of glasses. Lens elements made of a crown glass are referred to as "crown lens elements" and lens elements made of a flint glass are also referred to as "flint lens elements" within this application.

A good example for consistent implementation of the conventional correction principles is disclosed in US 6,806,942 B2, especially in the eighth exemplary embodiment.

OBJECT AND SOLUTION

It is an object of the invention to provide a projection lens, a projection exposure apparatus and a projection exposure method which operate with UV radiation at an operating wavelength in the ultraviolet range from 300 nm to 450 nm, in particular with UV radiation of the mercury i-line, and which offer a high imaging quality. In order to solve this problem, the invention provides a dioptric projection lens having the features of Claim 1. Furthermore, a projection exposure apparatus having the features of Claim 15 and a projection exposure method having the features of Claim 17 are provided. Advantageous developments are specified in the dependent claims. The wording of all the claims is incorporated by reference in the content of the description.

According to phrasing of the claimed invention, a dioptric projection lens is provided, which is embodied with the aid of electromagnetic radiation at an operating wavelength λ ₀ in the ultraviolet range from 300 nm to 450 nm to image a pattern arranged in the object plane of the projection lens into the image plane of the projection lens with a reducing imaging scale (|βΙ| < 1) in the case of an image-side numerical aperture NA. Consequently, this relates to a reduction lens. This is linked, inter alia, to advantages in terms of costs and advantages in the precision of the structures to be produced since it is possible to work using masks in which the typical structure dimensions are larger than the typical structure dimensions of the target structures.

All optical elements equipped with refractive power are lens elements, which is to say refractive optical elements. The lenses are arranged between the object plane and the image plane along an optical axis and are embodied in their totality to bring about this imaging. A stop position suitable for attaching an aperture stop is located between the object plane and the image plane, a chief ray of the imaging intersecting the optical axis at said stop position. The actual image-side NA used can be specified with the aid of an aperture stop. The lens elements comprise at least one flint lens element made of a first material with a relatively low Abbe number and at least one crown lens element made of a second material with a higher Abbe number relative to the first material. Consequently, the chromatic correction is assisted by the use of different lens element materials.

A peculiarity of the claimed invention can be found in the fact that the projection lens is designed so that for a parameter SSP with the condition SSP < 0.1 nm' ² applies. Here, C ₂ is the square component of a third order function which describes the dependence of the paraxial image position on the wavelength in the region of the operating wavelength λ ₀ . ndition is satisfied, it is possible not only to obtain a good imaging quality in the central region of the image field in the vicinity of the optical axis, but also to obtain a good and at the same time largely uniform imaging quality over the entire image field.

For the better understanding of the significance of this parameter and the insights underlying the invention, the intention is to initially explain further fundamentals of the correction of the longitudinal chromatic aberration CHL.

The longitudinal chromatic aberration CHL (alternatively "axial chromatic aberration" AX or simply "axial chromatic") is understood to mean the wavelength dependence of the paraxial image position. In the case of dioptric systems, the longitudinal chromatic aberration can only be corrected by the use of at least two or else more material types. In the process, each material is characterized by three parameters, specifically

- the refractive index n of the material at the principal wavelength (e.g., 365.5 nm in this case), the dispersion given by the Abbe number v, which is calculated here in exemplary fashion for the wavelengths of the g-, h- and i-line of a mercury vapour lamp as follows: _g the relative partial dispersion where the parameters n,, nh and n _g specify the refractive indices at the wavelengths of the i-line, the h-line and the g-line, respectively. The contribution of a lens element to the correction of the longitudinal chromatic aberration is proportional to the refractive power φ of the lens element, to the dispersion (i.e. , to the reciprocal of the Abbe number v) and to the square of the marginal ray height y ² marginal ^l at th loecation of the lens element. Consequently, the lens elements located at large marginal ray heights predominantly contribute to the longitudinal chromatic aberration. For many systems, this is the case in the vicinity of the aperture stop. In the following explanations, the simplifying assumption that all lens elements relevant to the correction of the longitudinal chromatic aberration have the same marginal ray height is made. Further, the assumption is made that this relates to tightly standing "thin lenses".

Then, the achromatization condition (correction of the primary spectrum) is given by where <pt, v _t are the refractive powers and Abbe numbers of the lens elements involved. Analogously, the apochromatization condition (correction of the secondary spectrum) is

The correction of the longitudinal chromatic aberration can be influenced by various measures. In this case, the quality of this correction is characterized by the curve of the paraxial image position with the wavelength. In the region around the mercury i-line at 2 ₀ = 365.5nm, this curve can be approximated very well by a third order function:

The second-order fit coefficient C ₂ then reproduces the quadratic component of particular interest here, as it describes the "secondary spectrum".

It was recognized that, in practice, it is generally not expedient within the scope of the measures for correcting chromatic aberrations when designing optical imaging systems for projection lithography to concentrate exclusively on the greatest possible correction of the primary spectrum and the secondary spectrum. Instead, further application-relevant criteria should be included in the assessment of correction measures. According to the teaching of this invention, this applies in particular to the depth of focus (DOF) obtainable by an imaging system. If this is relatively large in comparison with the secondary spectrum, it may be the case that the imaging quality is sufficiently good for a given application despite an incompletely corrected secondary spectrum. On the other hand, it may be the case that the remaining secondary spectrum is so large even in the case of, in absolute terms, relatively small values in comparison with the depth of focus that the imaging quality is perceptibly impaired. Thus, in order to be able to assess the effect of the longitudinal chromatic aberration on the imaging quality, it is advantageous according to the insights of the inventor to relate this to the Rayleigh unit which provides a measure for the depth of focus. Hence, the variable SSP for the secondary spectrum arises as a characterization for the correction of the secondary spectrum

This novel parameter consequently describes a criterion, relevant in practice to the field of projection lithography, for designing optical imaging systems intended to be used with broadband radiation. Various options are described in the context of the exemplary embodiments as to how projection lenses with an imaging quality suitable in practice can be constructed in view of this parameter.

In this application, the mercury i-line at λ _o = 365.5 nm is assumed to be the principal wavelength (operating wavelength). Both the Abbe numbers and the partial dispersions are calculated with λ _g = 436 nm, = 406 nm, = 365.5 nm at the wavelengths of the g-, h- and i-line. In principle, the results are transferable to other wavelengths ranging from approx. 300 nm to approx. 450 nm and/or other light sources suitable to this end.

In some embodiments, the correction is optimized to such an extent that the condition SSP < 0.05 nm' ² or even the condition SSP < 0.01 nm' ² is satisfied.

According to a development, the projection lens is embodied as a one-waist system. This comprises a first lens element group with negative refractive power close to the object plane; a second lens element group with positive refractive power following the first lens element group; a third lens element group with negative refractive power following the second lens element group, for producing a waist around a region of minimal marginal ray heights between the object plane and the image plane; a fourth lens element group with positive refractive power following the third lens element group, between the third lens element group and the stop position; and a fifth lens element group with positive refractive power, between the stop position and the image plane. Hence, a refractive power sequence N-P-N-P-P is realizable, where "P" represents a lens element group with positive refractive power overall and "N" represents a lens element group with negative refractive power overall. The projection lens can be embodied so that no further lens element groups are present apart from the aforementioned five lens element groups. Then, the first lens element group immediately follows the object plane and the further lens element groups follow one another in immediate succession. If a projection lens is designed as a one-waist system, an important contribution can be made to the Petzval correction in the case of an overall compact building mass.

Preferably the projection lens has an imaging scale of more than 1 :4 in terms of absolute value, with the imaging scale preferably being 1 :2 (|P| = 0.50). Alternatively or in addition, the imageside numerical aperture NA can be less than 0.4, with preferably the condition 0.1 < NA < 0.4 applying.

A class of projection lenses with properties that are particularly suitable in practice is characterized in that a projection lens is embodied as a one-waist system with a refractive power sequence N-P-N-P-P, an image-side numerical aperture NA in the range of 0.2 < NA < 0.4 and an imaging scale of the order of 2: 1 (|P| = 0.50) or less.

It is possible therewith to be able to expose a large field on the wafer with moderate resolution. However, advantages of the invention can also be used in the case of different combinations of P and NA, for example also in the case of (|β| = 0.25) and/or 0.4 < NA < 0.8.

In some embodiments, an essential contribution to the chromatic correction is brought about by virtue of at least one of the lens elements consisting of a material, in particular a glass, which has a refractive index n > 1.61 and an Abbe number v < 50 at the operating wavelength. On account of the relatively low Abbe number, the material in this respect has similar properties to conventional flint materials. The lens element made of this material preferably has negative refractive power.

Alternatively or in addition, provision can be made for at least one lens element to consist of a material, in particular a glass, which has an Abbe number v < 50 and a relative partial dispersion in the range from 0.645 to 0.650 at the operating wavelength. Consequently, the relative partial dispersion of such a material is significantly closer to the relative partial dispersions of typical crown materials than to the relative partial dispersions of conventional flint materials such as LLF1 and LF5, for example. The use of such a material in combination with conventional crown glasses can consequently make a significant contribution to the correction of the secondary spectrum. Such a material can be used as an alternative to conventional flint materials, or in addition thereto, in order to assist the chromatic correction. What this can achieve in some embodiments is that the first material and the second material have an Abbe number difference Av of 10 or more (i.e., Av > 10), and a partial dispersion difference Ad is less than 0.007. Consequently, a significant contribution to the correction of the secondary spectrum can also be provided by lens elements with moderate individual refractive powers.

According to one development, provision is made for the projection lens to comprise at least one lens element made of a material which has an anomalous partial dispersion in the wavelength range of 300 nm to 450 nm. It has been established that the relative partial dispersion of most optical glasses depends approximately linearly on the Abbe number v, in accordance with what is known as the normal line. If the relative partial dispersion deviates significantly from the normal line, then this is referred to as anomalous partial dispersion. The position of the "conventional" normal line, on which the plurality of optical glasses are approximately located, is defined on the basis of value pairs of the glass types K7 and F2 (SCHOTT catalogue "Optisches Gias", 2014).

Since the glasses K7 and F2 play no role in the field of producing lens elements for optical systems for operating wavelengths in the region of UV light in the wavelength range from 300 nm to 450 nm, a "problem-adjusted normal line" d _N p is defined for the purposes of this application, the latter being a line of best fit through the value pairs of the practically relevant glasses FK5, LF5, LLF1 and SIO ₂ , which to a good approximation are located in the vicinity of, or on, this line of best fit. If the partial dispersion of a material is on, or in the vicinity of, this problem-adjusted normal line SNP, then this is a material with normal partial dispersion. By contrast, if there is a significant offset Δ$ P = &- #NP, then the material exhibits anomalous partial dispersion.

A straight-line equation of the problem-adjusted normal line is given by

Within the context of this application, a material has an anomalous relative partial dispersion if the offset A$NP =$- SNP in terms of absolute value is greater than 0.002, which is to say if the condition |A5NP| > 2*1 O’ ³ applies.

According to one development, provision is made for the projection lens to comprise lens elements made of at least three different materials. In particular, it may be the case that the projection lens comprises, in addition to the at least one flint lens element made of a first material with the relatively low first Abbe number and the at least one crown lens element made of a second material with the higher second Abbe number relative to the first material, at least one further lens element made of a third material which has a third Abbe number located between the first and the second Abbe number, with the first material and the second material being located substantially on the problem-adjusted normal line of the relative partial dispersion (i.e., having a normal relative partial dispersion in this respect) in the region of the operating wavelength and the third material having an anomalous relative partial dispersion, or being located in a manner deviating significantly from the problem-adjusted normal line, in the region of the operating wavelength.

The inventor has recognized that such a combination of materials allows several preconditions for a good correction of the secondary spectrum to be met better than in the case of material combinations which only use the conventional lens element materials. Thus, for example, it should theoretically be possible to correct the secondary spectrum by combining two materials, for example glasses, which are sufficiently transparent at the operating wavelength and which have the same relative partial dispersion but different Abbe numbers. These preconditions cannot be satisfied well for typical conventionally used combinations. For example, if the combination of a crown glass such as KF5 with a flint glass such as LF5 or LLF1 is considered, then the differences in the relative partial dispersion are relatively large. By contrast, if combinations such as FK5/CaF ₂ or SIO ₂ /CaF ₂ are considered, it is possible from the view of similar relative partial dispersions to obtain virtually complete apochromatization as the relative partial dispersions of these material combinations are located relatively close together. In the aforementioned conventional combinations of FK5/LF5 or FK5/LLF1 , the remaining secondary spectrum will be greater in the first case than in the second since, in the case of a comparable difference of the Abbe numbers, the difference of the relative partial dispersions is smaller in the case of the second combination.

By contrast, if a third material of the above-described type is used, an even further reduced secondary spectrum, for example, should be expected if the crown material FK5 is combined with the third material.

According to insights of the inventor, a second option of obtaining an apochromatization to a good approximation can be obtained if a third material which satisfies the condition above is used. Vividly, this can be expressed as follows: an apochromatization is practically not possible when using three materials with different Abbe numbers and relative partial dispersions if, when a functional relationship between the Abbe number and the relative partial dispersion is considered in an appropriate diagram, all three materials are located substantially on, or in the vicinity of, a joint connecting line. By contrast, if the three materials are chosen so that the third material, which is located between the first and the second material in relation to the Abbe number, is located at a distance from a connecting line through the other two materials, then an apochromatization may be successful. The greater the included triangular area, the better the result. By way of example, if a combination of the conventional glasses FK5, LF5 and LLF1 (or glasses of corresponding similar specifications from different manufacturers) is considered, then this opens up no realistic option for apochromatization as all three glasses are virtually located on one joint connecting line, with the result that an included triangular area virtually vanishes. By contrast, if a first, second and third material are combined according to the condition above, then an apochromatization is possible under certain circumstances if the area of the triangle is sufficiently large. This is because two materials with normal relative partial dispersion are combined with a third material with anomalous relative partial dispersion in this case. This can be used to reduce the secondary spectrum to the greatest possible extent.

As already mentioned, provision can be made for the lens elements of the projection lens to comprise at least one lens element made of a material with anomalous relative partial dispersion. In particular, it may be the case that the fourth lens element group LG4 and/or the fifth lens element group LG5 contains at least one lens element made of a material with anomalous partial dispersion.

In some embodiments, crystalline calcium fluoride (CaF ₂ ) material is used as the material with anomalous partial dispersion. In combination with a glass with a smaller Abbe number, this material can serve as crown material.

Alternatively or in addition, provision can be made for the material with anomalous relative partial dispersion to deviate from the problem-adjusted normal line in the negative direction and moreover have an Abbe number of less than 50. This material can preferably be used like a flint material, with the result that preferably at least one negative lens element consists of this material.

In particular, use can be made of a specialist glass recently developed by Schott of Mainz, Germany, which is commercially available as N-SSK20. At 365 nm, this material has a refractive index of n = 1 .6468, an Abbe number of v = 41 .23 and a relative partial dispersion of Specialist glasses from other manufacturers which have optical characteristics of the order of these values can likewise be used.

Some embodiments are distinguished by environmentally polluting components being completely dispensed with in the glasses used. In some embodiments, all lens elements consist of lead-free material, and hence all glass lens elements consist of lead-free glass. In this case, the term "lead- free" means that, in particular, lead or lead ions may at best occur as a residual contaminant, which for example is rendered attainable by virtue of dispensing with the use of PbO in the raw material during the manufacturing process. PbO is frequently used to produce glasses which have a relative partial dispersion that deviates negatively from the normal line. This approach is dispensed with here for the benefit of the environment. In particular, it consequently may be the case that all flint lens elements with negative refractive power consist of a lead-free glass. All negative flint lens elements may have the same lead-free glass or may have different lead-free glasses. The material may be the aforementioned N-SSK20 glass or a comparable glass from a different manufacturer.

On the basis of specific examples, the application discloses for the first time that the use of lead- free materials, in particular lead-free glasses, is possible in principle for all lens elements of a dioptric projection lens for wavelengths in the range from approx. 300 nm to approx. 450 nm, in particular for the mercury i-line at λo =365.5 nm as a principal wavelength (operating wavelength), and discloses for the first time how projection lenses with an excellent correction of chromatic aberrations can be constructed under these conditions. This is considered to be a separate invention, which should be considered to be independent of features of other inventions disclosed within this application and which is also usable, for example, in projection lenses with a unit scale (imaging scale 1 :1).

In many embodiments, the projection lens is designed so that the lens element with the greatest optically clear diameter is located within the second convexity. By contrast, the first convexity is at least exactly as thick or high as the second convexity in some embodiments, with the first convexity preferably also being able to have a greater extension than the second convexity. In particular, the condition D ₂ >A max(D ₄ , Ds) may apply, where D ₂ is a maximum diameter within the second lens element group LG2, D ₄ is a maximum diameter within the fourth lens element group LG4, Ds is a maximum diameter within the fifth lens element group LG5 and the condition A = 1.0 or even A = 1.1 applies for the parameter A. What can be achieved by this design is that large converging lenses which are able to make a contribution to the Petzval correction are available despite advantageous, relatively small marginal ray heights in the region of the fourth and fifth lens element group.

The invention also relates to a projection exposure apparatus for exposing a radiation-sensitive substrate arranged in the region of an image plane of a projection lens with at least one image of a pattern arranged in the region of an object plane of the projection lens, with the projection exposure apparatus comprising a projection lens according to the claimed invention. Preferably, the projection exposure apparatus and hence also the projection lens are designed for radiation at the i-line of a mercury vapour lamp (central operating wavelength of approx. 365.5 nm, optionally with a restricted bandwidth of a few nm). An i-line system operates with a mercury vapour lamp as a light source and only uses i-line radiation for imaging purposes. Suitable coatings for lenses and good photoresists, inter alia, are well established for these powerful light sources, and so it is possible in this respect to build on earlier developments. However, other light sources and/or other UV operating wavelengths can also be used. By way of example, two or three lines of a mercury vapour lamp (g-, h- and i-line at approx. 436 nm, approx. 405 nm and approx. 365 nm) can be used simultaneously if the projection lens is corrected in corresponding broadband fashion. Alternatively, a frequency tripled Nd:YAG laser at approximately 355 nm, for example, could be used. Furthermore, there are various LED sources which emit wavelengths between 360 and 400 nm. Optionally, the emission range may also be slightly adapted.

BRIEF DESCRIPTION OF THE DRAWINGS

Further advantages and aspects of the invention are evident from the claims and from the description of exemplary embodiments of the invention, which will be explained below with reference to the figures.

Fig. 1 shows a schematic illustration of a projection exposure apparatus according to one exemplary embodiment;

Fig. 2 shows an Abbe diagram with values of the Abbe number for representative available materials;

Fig. 3 shows a diagram of the relative partial dispersions of the materials from Fig. 2;

Fig. 4 shows a schematic meridional lens element section of a projection lens in accordance with the first exemplary embodiment in partial figure 4A, the curve of the longitudinal chromatic aberration (CHL) in micrometres [μm] plotted against the wavelength in the region around the operating wavelength of 365.5 nm in partial figure 4B and a corresponding diagram for a reference system REF from the prior art (US 6,806,942 B2, eighth exemplary embodiment) in Fig. 4C.

Fig. 5 to Fig. 12 show respective schematic meridional lens element sections of a projection lens in accordance with the second to ninth exemplary embodiment in partial figures 5Ato 12A, the respective curve of CHL ([μm]) plotted against the wavelength in the region around the operating wavelength of 365.5 nm in partial figures 5B to 12B and the longitudinal aberration of the field point on the optical axis for all three wavelengths in the case of the sixth exemplary embodiment in Fig. 9C.

DETAILED DESCRIPTION OF THE EXEMPLARY EMBODIMENTS

Fig. 1 shows an example of a microlithographic projection exposure apparatus WSC, which is utilizable in the production of semiconductor components and other finely structured components and which operates with light or electromagnetic radiation from the ultraviolet (UV) range in order to obtain resolutions down to fractions of micrometres. A mercury vapour lamp serves as primary radiation source or as light source LS. Said lamp emits a broad spectrum with emission lines of relatively high intensity I in wavelength ranges with centroid wavelengths at approx. 436 nm (visible light, blue, g-line), approx. 405 nm (visible light, purple, h-line) and approx. 365.5 nm (near ultraviolet, UV-A, i-line). This part of the spectrum is illustrated in the schematic l(X) diagram.

The projection exposure apparatus is an i-line system which only uses the light from the i-line, which is to say UV light around a central operating wavelength Xo of approx. 365.5 nm. The natural full bandwidth of the i-line is restricted to a narrower used bandwidth AX, for example of approx. 2.5 nm, with the aid of a filter, or in any other way.

At its exit surface ES, an illumination system ILL disposed downstream of the light source LS generates a large, sharply delimited and substantially homogeneously illuminated illumination field, which is adapted to the requirements of telecentricity of the projection lens PO arranged downstream thereof in the light path. The illumination system ILL has devices for setting different illumination modes (illumination settings) and, for example, can be switched between conventional on-axis illumination with different degrees of coherence o and off-axis illumination.

Those optical components which receive the light from the light source LS and form illumination radiation from the light, which illumination radiation is directed to the reticle M, are part of the illumination system ILL of the projection exposure apparatus.

Arranged downstream of the illumination system is a device RS for holding and manipulating the mask M (reticle) in such a way that the pattern arranged at the reticle lies in the object plane OS of the projection lens PO, which coincides with the exit plane ES of the illumination system and which is also referred to here as reticle plane OS. Following downstream of the reticle plane OS is the projection lens PO, which is to say an imaging system, which images an image of the pattern arranged at the mask M with a defined imaging scale, for example with a reducing scale 1 :2 (|β| = 0.50), onto a substrate W coated with a photoresist layer, the light-sensitive substrate surface SS of which lies in the region of the image plane IS of the projection lens PO.

The substrate to be exposed, which is a semiconductor wafer W in the exemplary case, is held by a device WS which comprises a scanner drive in order to move the wafer synchronously with the reticle M perpendicular to the optical axis OA in a scanning direction (y-direction). The device WS, which is also referred to as "wafer stage", and the device RS, which is also referred to as "reticle stage", are constituent parts of a scanner device which is controlled by way of a scan control device which, in the embodiment, is integrated in the central control device CU of the projection exposure apparatus.

The illumination field generated by the illumination system ILL defines the effective object field OF used during the projection exposure. In the exemplary case, the latter is rectangular, has a height A* measured parallel to the y-direction and has a width B* < A* measured perpendicular thereto (in the x-direction). The effective object field lies centred to the optical axis (on-axis field).

The effective image field in the image surface IS, which is optically conjugate to the effective object field, has the same form and the same aspect ratio between the height B and width A as the effective object field, but, in the case of projection lenses with a reducing action (with|β| < 1 ), the absolute field dimension is reduced by the imaging scale β of the projection lens, which is to say A =|β| A* and B - 1 p | B*.

In the rotationally symmetric system, the circle which is centred around the optical axis OA, which encloses the effective object field OF and which touches the corners thereof specifies the size of the object field within which the correction of the optical aberrations must meet the specification at all field points. This then also applies to all field points in the effective object field. The correction of aberrations becomes more complicated the larger this object field has to be. In this case, the size of the circle is parameterized by the object field radius OBH or half the object field diameter OBH, which simultaneously corresponds to the maximum field height of an object field point.

To be able to better illustrate how individual correction measures may affect the imaging quality of a projection lens, all exemplary embodiments have a uniform imaging scale β = -0.5 and an image-side numerical aperture NA = 0.35. The object field radius OBH is 59 mm in all cases, with the result that an effective image field with dimensions of 52 mm * 28 mm can be produced on the substrate (wafer). Stipulating these technical parameters serves the better comparability of the exemplary embodiments among themselves. The application of the discoveries of this invention is not limited to imaging systems with these technical parameters.

To obtain an overview of some relevant properties of available lens materials, a few representative available materials are intended to be initially characterized by way of their position in the Abbe diagram (Fig. 2) and in the diagram of the relative partial dispersions (Fig. 3).

The Abbe diagram (Fig. 2) provides an overview of the dispersion of the materials. The Abbe number v plotted on the abscissa (labelled "Abbe") enables a characterization of the dispersion properties of a material within a wavelength range of interest. The ordinate (labelled "Index") specifies the refractive index n at 365.5 nm. In general, low Abbe numbers represent materials with relatively strong dispersion and high Abbe numbers represent materials with relatively weak dispersion. Therefore, the Abbe number is sometimes also referred to as "reciprocal relative dispersion". The potential for the correction of the primary spectrum can be estimated with the aid of the Abbe numbers.

Fig. 2 depicts the Abbe diagram of a few potentially usable materials. The Abbe numbers are calculated for the wavelengths at the g-, h- and i-line. Equation (1) above applies. It is readily possible to correct the primary spectrum longitudinally (CHL) if materials with the greatest possible difference in their Abbe numbers are combined. In this context, crown glasses with a large Abbe number are used in the converging lens elements and flint glasses with a small Abbe number are used in the diverging lens elements.

The conventional combination of FK5 (v = 62.2) with LF5 (v = 31.2) in this case has a sufficiently large difference in the Abbe numbers to allow achromatization without individual refractive powers that are too large.

A combination of CaF2 (crystalline material) and LF5, for example, would be even more advantageous in this case. However, as a rule, this is not used since CaF2 is a significantly more expensive material, the availability of which is also significantly restricted in relation to FK5.

A further conventional combination is FK5 and LLF1 (v = 35.9). The difference of the Abbe numbers is slightly smaller in this case, and this has a more disadvantageous effect on the achromatization. As a consequence, work is carried out with greater individual refractive powers of the lens elements. Moreover, it is evident from the diagram that fused silica (SiO ₂ ) and FK5 have virtually the same Abbe number, and consequently have a very comparable behaviour in view of achromatization.

Additionally, a glass labelled N-SSK20 is also plotted; it will be described in more detail hereinafter. The glass N-SSK20 is an example of a lens element material (glass) which has a refractive index n > 1.61 and an Abbe number v < 50 at the operating wavelength Xo.

For N-SSK20, it is evident that the Abbe number difference in relation to FK5 becomes smaller yet again. Thus, a further increase in the individual refractive powers of the lens elements used for achromatization should be expected.

Whether a lens element material acts as a crown glass or as a flint glass in a specific combination also depends on the combination partner. By way of example, in combination with a material with v < 40 (for example LF5, LLF1 , LLF6), synthetic fused silica (SiO ₂ ) acts as a relative crown material. By contrast, if synthetic fused silica is combined with a material with v > 65, for example CaF ₂ , synthetic fused silica acts as a relative flint material.

Within the scope of this application, first materials with a relatively low Abbe number are also referred to as "relative flint materials" and second materials with a relatively higher Abbe number are also referred to as "relative crown materials". To simplify matters, lens elements made of a relative crown material are also referred to as "crown lens elements" for short and lens elements made of a relative flint material are also referred to as "flint lens elements" for short in a more specific exemplary embodiment in this application.

A further selection criterion relates to the relative partial dispersion (also referred to as "relative dispersion", "partial dispersion") of the materials. The relative partial dispersion & of optical glass and other optical materials denotes the strength of the dispersion of the material in two different wavelength ranges. Thus, it is a measure for the difference in the dispersion in these two wavelength ranges. Fig. 3 shows the diagram of the relative partial dispersion for the same materials as in Fig. 2. The potential for correcting the secondary spectrum can be estimated on the basis of the diagram of the relative partial dispersion. It is evident that most materials conventionally used in i-line systems (e.g., FK5, LF5, LLF1 and optionally SIO2) are located in the vicinity of, or on, a line of best fit -SNP through precisely these four glasses. Within the scope of this application, this solid line is referred to as "problem-adjusted" normal line SNP. The straight- line equation of the problem-adjusted normal line is given by It is evident that LF5 and LLF1 are situated practically exactly on this problem-adjusted normal line. FK5 and fused silica (SiO ₂ ) deviate slightly upwardly. In this context, materials located on, or in the vicinity of, the "problem-adjusted" normal line are referred to as materials with normal relative partial dispersion.

For reference, the "conventional" normal line SN is additionally plotted using a dashed line. The latter is defined by the position of the glasses F2 and K7, which play no role within the scope of designing projection lenses for the wavelength range of the mercury lines.

In the diagram of the relative partial dispersion there are materials (especially the two materials N-SSK20 and CaF ₂ ), which are located significantly away from the problem-adjusted normal line •SNP (and also away from the conventional normal line). The glass N-SSK20 deviates slightly downwardly while the crystalline material CaF ₂ is located significantly above the normal line (i.e., A3NP > 1 ). These materials are examples of materials with "anomalous relative partial dispersion". Within the context of this application, a material has an anomalous relative partial dispersion if the offset AONP =5- 5NP in terms of absolute value is greater than 0.002, which is to say if the condition |A5NP| > 2*10 ^-3 applies. It will be shown how these can be used to play a significant role for the correction of the secondary spectrum.

As mentioned, the (vertical) distance A^NP of the glasses from the line of best fit or the "problem- adjusted" normal line ONP can be specified for the purpose of characterizing the two materials. This is 8.1 10 for CaF ₂ and 3.5 IO for N SSK20.

In the following description of preferred embodiments of projection lenses, the term "optical axis" denotes a straight line through the centres of curvature of the curved lens element surfaces. In the examples, the object is a mask (reticle) with the pattern of an integrated circuit; it may also relate to a different pattern, for example of a grating. In the examples, the image is projected onto a wafer provided with a photoresist layer, said wafer acting as a substrate. Other substrates are also possible, for example elements for liquid crystal displays or substrates for optical gratings.

Some peculiarities can be elucidated on the basis of the profiles and the relationships between chief rays and marginal rays of the imaging. In this case, a chief ray CR refers to a ray which starts from an edge point of the object field and intersects the optical axis in the region of a pupil plane, which is to say in the region of a stop position suitable for attaching an aperture stop (AS). A marginal ray MR within the meaning of the present application leads from the centre of the object field to the edge of the aperture stop. The perpendicular distance of these rays from the optical axis yields the corresponding ray height. To the extent that this application refers to a "marginal ray height" (MRH) or a "chief ray height" (CRH), this refers to the paraxial marginal ray height and the paraxial chief ray height, respectively.

The term "stop region" denotes a region around the stop position (i.e., upstream and downstream of the stop position), in which a ray height ratio | CRH/MRH | between the chief ray height CRH and the marginal ray height MRH of the imaging is less than one. Consequently, relatively large marginal ray heights occur in the stop region.

The specifications of the projection lenses shown in the figures of the drawing are indicated in the tables compiled at the end of the description, the numbering of which tables respectively corresponds to the numbering of the corresponding figure of the drawing.

Tables 4 to 12 summarize the specification of the respective design in tabular form. In this case, column "SURF" indicates the number of a refractive surface or surface distinguished in some other way, column "RADIUS" indicates the radius r of the surface (in mm), column "THICKNESS" indicates the distance d - designated as thickness - between the surface and the subsequent surface (in mm) and column "MATERIAL" indicates the material of the optical components. Columns "INDEX1", "INDEX2" and "INDEX3" indicate the refractive index of the material at the wavelengths 365.5 nm (INDEX1), 364.5 nm (INDEX2) and 366.5 nm (INDEX3). Column "SEMIDIAM" indicates the usable, free radii or the free optical semidiameters of the lens elements (in mm) or of the optical elements. The radius r=0 (in the column "RADIUS") corresponds to a plane surface. Some optical surfaces are aspherical. Tables with appended "A" indicate the corresponding asphere data, wherein the aspherical surfaces are calculated according to the following specification:

In this case, the reciprocal (1/r) of the radius indicates the surface curvature and h indicates the distance between a surface point and the optical axis (i.e., the beam height). Consequently, p(h) indicates the sagittal height, which is to say the distance between the surface point and the surface vertex in the z-direction (direction of the optical axis). The coefficients K, C1 , C2, ... are represented in the tables with appended "A".

Tables 13 and 14 assemble further parameters of the exemplary embodiments in an overview.

In the following description of exemplary embodiments, the same reference signs are used in all figures for the same or corresponding features. Lens elements are numbered in their sequence from the object plane to the image plane, and so, for example, the lens element L1 is the first lens element immediately following the object plane. Not all lens elements have been provided with a reference sign for reasons of clarity.

Figure 4A shows a schematic meridional lens element sectional view of a first exemplary embodiment of a dioptric projection lens PO-1 (designation N467) with selected beams for elucidating the imaging beam path or the projection beam path of the projection radiation passing through the projection lens during operation.

The projection lens is provided as an imaging system with a reducing effect, for imaging a pattern of a mask arranged in its object plane OS onto its image plane IS aligned parallel to the object plane directly, which is to say without producing an intermediate image, and with a reduced scale, specifically in the scale of -1 :2 (imaging scale β = -0.5).

Between the object plane and the image plane, the only pupil plane of the imaging system lies where the chief ray CR of the optical imaging intersects the optical axis OA. The aperture stop AS of the system is attached in the region of the pupil plane. Therefore, the position suitable for attaching the aperture stop is also referred to as stop position BP here.

A stop region BB extends around the stop position, the condition |CRH/MRH| < 1 applying to a ray height ratio between the chief ray height CRH and the marginal ray height MRH of the imaging in said stop region. Thus, the marginal ray height is higher than the chief ray height. The optical structure can be characterized as follows:

A first lens element group LG1 with negative refractive power, formed by exactly one lens element L1 in the example, immediately follows the object plane OS. Lens element L1 is a negative meniscus lens element with a convex entrance surface and a concave exit surface. By increasing the divergence, it prepares the formation of a convexity in the subsequent beam path. Such a negative group in the direct vicinity of the object plane enables the formation of a subsequent convexity at an axially short length, and is consequently conducive to a compact structural shape.

A second lens element group LG2 with positive refractive power immediately follows the first lens element group LG1. This second lens element group comprises the three lens elements L2, L3 and L4, which all have positive refractive power. The second lens element group collects the rays coming from the first lens element group and, as a result, forms, at least approximately, a convexity in the projection beam path. A third lens element group LG3 with negative refractive power immediately follows the second lens element group LG2. This third lens element group comprises the three lens elements L5, L6 and L7 and produces a waist around a local minimum of the marginal ray height between the object plane OS and the image plane IS in the projection beam path. To this end, each of the three lens elements (biconcave negative lens element L5, object-side convex negative meniscus lens element L6 and the biconcave negative lens element L6) has negative refractive power.

A fourth lens element group LG4 with positive refractive power and a total of four lens elements L8 to L11 immediately follows the third lens element group LG3. The lens elements of the fourth lens element group are arranged between the third lens element group LG3 and the stop position suitable for attaching an aperture stop AS. They comprise three positive lens elements (L8, L9 and L11 ) and an object-side concave negative meniscus lens element L10, between which and the stop position only one positive lens element (L11) is arranged.

A fifth lens element group LG5 with positive refractive power overall is situated between the stop position and the image plane IS. The fifth lens element group comprises four lens elements L12 to L15 with refractive power, in the refractive power sequence P-N-P-P, where P represents positive refractive power and N represents negative refractive power.

A plane parallel plate PL without refractive power is provided, as an optical and mechanical termination of the projection lens, between the last lens element with refractive power (positive lens element L15) and the image plane.

Consequently, the projection lens is characterized by the refractive power sequence N-P-N-P-P, where "P" represents a lens element group with positive refractive power and "N" represents a lens element group with negative refractive power. There is only a single pronounced waist in the region of the third negative lens element group LG3 between an object-near convexity (at LG2) and an image-near convexity (at LG4 and LG5). This design as a one-waist system contributes to the Petzval correction.

All positive lens elements with the exception of the last lens element L15 closest to the image consist of the glass FK5 (Abbe number v ~ 52), while the last lens element L15 consists of fused silica (SiO ₂ , Abbe number v = 60).

The lens elements with negative refractive power from the first lens element group LG1 and third lens element group LG3 are manufactured from fused silica with a low refractive index. The use of optical material with a low refractive power in the diverging lens elements of the waist is advantageous for the correction of the Petzval sum (field curvature).

The projection lens contains aspherical lens element surfaces (aspheres). The lens elements L1 , L7, L8 and L15 are aspherical lenses, in each of which one of the lens element surfaces is rotationally symmetrically aspherical while the other one has a spherical design. All aspherical lens element surfaces are located on lens elements made of synthetic fused silica, which is inter alia advantageous for precise manufacturing. Expressed differently, all aspherical lens element surfaces are located on lens elements made of crown material with v > 55.

The biconcave negative lens element L10 in the region of a large marginal ray height in the stop region upstream of the stop position BP and the biconcave negative lens element L13 in the region of a large marginal ray height in the stop region downstream of the stop position, by contrast, both are flint lens elements; they each consist of flint glass with the designation LF5, which is to say a lens element material with a relatively low Abbe number of v ~ 31.

A significant contribution to the correction of the longitudinal chromatic aberration can be made with the aid of a combination of at least one crown lens element with positive refractive power and at least one flint lens element with negative refractive power. This lens element pair should be arranged in a region of the optical imaging system in which the marginal ray of the imaging has a ray height (marginal ray height) that is as large as possible. As a rule, this is the case in a stop region in the vicinity of the system stop.

The projection lens only contains two flint lens elements, respectively one in the fourth and the fifth lens element group. The first, second and third lens element group do not contain any flint lens elements with v < 55. Consequently, flint material is used very sparingly. More precisely, there is in each case a combination of at least one crown lens element with positive refractive power (e.g., L9 or L12) and at least one flint lens element with negative refractive power (in this case L10 or L13) immediately upstream and immediately downstream of the stop position. These lens element pairs are each arranged in the stop region, which is to say where the marginal ray of the imaging has the greatest possible beam height (marginal beam height). In particular, the beam height ratio |CRH/MRH| being less than 0.2 applies to the flint lens elements in this exemplary embodiment.

In this respect, the principle of achromatization is similar to the prior art. An essential contribution is made by the two diverging flint lens elements L10 and L13 which are arranged in the region around the aperture stop AS. The intention is to now compare this embodiment with the prior art. The eighth exemplary embodiment in patent document US 6,806,942 B2 is considered to be a particularly relevant reference example (REF) in view of the quality of the correction of the primary spectrum and secondary spectrum. This relates to a projection lens with three convexities and two waists, designed for the mercury i-line (365.5 nm). The numerical aperture NA is 0.70; the object field radius is 54.41 mm. An image field radius of approximately 13.6 mm arises with an imaging scale of ft = -0.25. This renders possible the exposure of a rectangular slot field of 26 x 8 mm ² on the wafer.

In this prior art, the correction of the longitudinal chromatic aberration (primary spectrum) is achieved by three diverging lens elements made of LF5, which are arranged in the region of large marginal ray heights in the vicinity of the stop position. The primary spectrum is corrected virtually in full, and all that still remains is the component of the secondary spectrum (cf. Fig. 4C). The coefficient of the secondary spectrum is determined as Normalized in relation to the Rayleigh unit, a value of

In the first exemplary embodiment, the primary spectrum is likewise fully corrected longitudinally (CHL) (cf. Fig. 4B), and what remains is the secondary spectrum with a value of

At first glance, this value appears significantly more disadvantageous than in the aforementioned prior art. However, a relatively good imaging quality is in fact attained. This can be understood as follows: According to theory, the contribution of a lens element to the longitudinal chromatic aberration is proportional to the dispersion of the lens element (i.e., inversely proportional to the Abbe number) and to the square of the marginal ray height (see above).

If the prior art (US 6, 806, 942 B2, eighth exemplary embodiment) is stopped down to a numerical aperture of 0.35, then this yields a marginal ray height of approximately 125 mm in the region of the stop. By contrast, the present exemplary embodiment has a marginal ray height of 190 mm in the case of this numerical aperture (NA = 0.35). If a ratio is formed of squares of the two This virtually perfectly reflects the ratio of the two coefficients of the secondary spectrum If the coefficient of the secondary spectrum is normalized to the Rayleigh unit of the system, then this nevertheless already yields, for the first exemplary embodiment (design designation N467) with a substantial improvement over the aforementioned prior art. Expressed differently: the effect of the secondary spectrum on the imaging quality is significantly less pronounced than in the prior art.

For illustrative purposes, Fig. 4B shows the associated diagram of the chromatic correction, in which the wavelength in the region around the operating wavelength of 365 nm (at the value "0") is plotted along the x-axis and the longitudinal chromatic aberration CHL ("axial chromatic") is plotted in [μm] along the y-axis.

The first exemplary embodiment has further characteristics, inter alia the following: There are few lens elements in the region in front of the wafer (e.g., only a single lens element in the last 30% of the length of the system). The flint lens elements with negative refractive power have greater curvature on the side facing away from the stop than on the side facing the stop, which is to say the flint lens element in front of the stop has the more strongly curved surface facing the reticle (i.e. , the object plane) and the flint lens element after the stop has this surface facing the wafer. There are even menisci in this exemplary embodiment. This property can also be found like this in further exemplary embodiments. Aspheres are all located on concave surfaces. There are no flint lens elements outside of the stop region, in particular no flint lens elements in the near-field region.

The same reference signs are used for corresponding or similar features in the following exemplary embodiments, without these being mentioned separately again, for reasons of clarity. Together with the teaching of the first exemplary embodiment, these can be used to illustrate different correction approaches in detail.

Figure 5A shows a lens element section of a projection lens PO-2 according to a second exemplary embodiment; Fig. 5B illustrates the longitudinal chromatic aberration (CHL). It was already possible to show on the basis of the first exemplary embodiment that the marginal ray height in the region of the system stop or aperture stop AS has a significant influence on the strength of the longitudinal chromatic aberration and hence, following the correction of the primary spectrum, on the strength of the secondary spectrum. This effect can be used to further reduce the secondary spectrum.

The second exemplary embodiment (Figs 5, 5A, projection lens PO-2, designation N528P) is distinguished in relation to the first exemplary embodiment by virtue of, inter alia, the marginal ray height at the location of the aperture stop AS being reduced to approximately 140 mm. As a result, the first convexity (lens elements L4, L5 and L6) becomes significantly more pronounced in order nevertheless to ensure the correction of the Petzval sum. The coefficient of the secondary spectrum is reduced to

In this case, the second lens element group LG2 is provided by the lens elements L4, L5 and L6, the fourth lens element group LG4 is provided by the lens elements L11 and L12 and the fifth lens element group LG5 is provided by the lens elements L13, L14, L15, L16 and L17. The maximum optically dear diameter of the second lens element group LG2 is 215 mm, the maximum optically dear diameter of the fourth lens element group LG4 is 141 mm and the maximum optically clear diameter of the fifth lens element group is around 183 mm. Hence, the following ratio arises:

If the first convexity is at least just as pronounced as or even more pronounced than the second convexity, then a sufficient correction of the image field curvature can be obtained even in the case of a moderate marginal ray height in the stop region.

Figure 6A shows a lens element section of a projection lens PO-3 (designation N526) according to a third exemplary embodiment; Fig. 6B illustrates the longitudinal chromatic aberration (CHL). Starting point for the third exemplary embodiment is, once again, the first exemplary embodiment PO-1 (N467). In this case, the longitudinal correction of the primary spectrum (i.e., CHL) is implemented by diverging lenses L10 and L14 made of LLF1 instead of LF5. In the stop region BB, these are arranged upstream and downstream of the stop position, respectively, and are each combined with positive crown lens elements. Since the difference of the Abbe numbers between FK5 and LLF1 is smaller than that between FK5 and LF5, this yields diverging lens elements with greater refractive powers than in the first exemplary embodiment. On the other hand, however, the partial dispersions of FK5 and LLF1 are doser together than those of FK5 and LF5. This leads (of. Fig. 6B) to the remaining secondary spectrum being able to be improved by approximately 10% - in the case of comparable diameters in the stop region - in relation to the starting point of the first exemplary embodiment, specifically to

Fig. 7A shows a lens element section of a projection lens PO-4 (designation N527) according to a fourth exemplary embodiment; Fig. 7B illustrates the longitudinal chromatic aberration (CHL).

The converging crown lens elements in the stop region (in particular L11 to L13 and L15) lead to chromatic under-correction, which is corrected by the diverging flint lens elements L10 and L14.

If use is now made of lens elements made of materials with a large difference in terms of the Abbe numbers, then it is possible to implement achromatization with significantly smaller individual refractive powers. Following the correction of the primary spectrum, this leads to a smaller secondary spectrum.

Therefore, the converging lens elements in the stop region BB which provide the greatest contribution to the chromatic under-correction (lens elements L11 , L12, L13, L15) were replaced by lens elements made of fluorspar (CaF2) in the fourth exemplary embodiment. Hence, the difference of the Abbe numbers of the two materials grows significantly vis-a-vis the first exemplary embodiment. As a consequence, the diverging LLF1 lens elements (lens elements 10 and 14) exhibit a significantly smaller refractive power than in the third exemplary embodiment for the purpose of fully correcting the primary spectrum. The consequence of this is that the remaining secondary spectrum is hence also significantly smaller (cf. Fig. 7B), with the coefficients

Fig. 8A shows a lens element section of a projection lens PO-5 (designation N474) according to a fifth exemplary embodiment; Fig. 8B illustrates the longitudinal chromatic aberration (CHL).

This example illustrates an option for the use of the glass material N-SSK20 as flint material in a negative lens element. This material is an exemplary representative of materials with anomalous relative partial dispersion, with a negative deviation AS from the conventional normal line and from the problem-adjusted normal line #NG and with an Abbe number v from the range of approx. 40 to approx. 55 (cf. Figs 2 and 3). Further properties can also be summarized as set forth below. At the operating wavelength Xo , the glass material has a refractive index n > 1.61 and an Abbe number v < 50. Furthermore, the glass material has a relative partial dispersion & in the range from 0.645 to 0.650.

Although the difference of the Abbe numbers between FK5 and N-SSK20 is smaller than between FK5 and LLF1 or LF5, which is to say stronger individual refractive powers are required for the correction of the primary spectrum, the spacing of the partial dispersions between the glasses is smaller in turn. This may have an advantageous effect on the correction of the secondary spectrum.

In this exemplary embodiment, achromatization is implemented with three rather than two diverging lens elements (lens elements L9, L12, L16) made of N-SSK20, which is to say with one more than in the previous exemplary embodiments. Moreover, the individual refractive powers of the diverging lens elements are stronger than in the preceding designs. However, in line with expectations, the coefficient of the remaining secondary spectrum has reduced again, to

By way of example, the fifth exemplary embodiment demonstrates an i-line projection lens in which all lens elements consist of lead-free material. From an environmental point of view, this is considered to be a significant advantage over the prior art.

Fig. 9A shows a lens element section of a projection lens PO-6 (designation N476) according to a sixth exemplary embodiment, Fig. 9B illustrates the longitudinal chromatic aberration (CHL), and Fig. 9C shows the longitudinal spherical aberration for the three wavelengths of the mercury lines.

The projection lens PO-6 is an example of a design in which use is made of the fact that the glasses FK5, LF5 and N-SSK20 are no longer located on, or very close to, a line (line of best fit, problem-adjusted normal line $ _NG ) in the diagram of the partial dispersion (Fig. 3), but instead span a triangular area DF (hatched in Fig. 3). This creates the conditions required to be able to obtain a full correction of the secondary spectrum by joint use of all three materials. The sixth exemplary embodiment (designation N476) is such a system. In this case, the following materials are used in the region of the aperture stop AS, which is to say between the waist TL of the system and the image plane IS:

- FK5 in the converging lens elements L11, L13, L14, L16, L19, L22, L24, L25 and L26

- N-SSK20 in the diverging lens elements L12, L15, L17, L20 and L23, and

- LF5 in the converging lens elements L18 and L21.

This material mixture leads precisely to a vanishing secondary spectrum, as is clearly evident in Fig. 9B: With

G,N476 — SSP _N476 — — the coefficient of the secondary spectrum plays virtually no role anymore.

From the chromatic correction diagrams, it is evident that a not insignificant longitudinal primary spectrum remains. In principle, this could be corrected by way of a slight adjustment of the individual refractive powers. However, the primary spectrum here is set in this manner in order to compensate the Gaussian aberration ("spherochromatism") of the overall system. This is evident from Fig. 9C, where the longitudinal aberration of the field point on the optical axis is displayed for all three wavelengths.

Fig. 10A shows a lens element section of a projection lens PO-7 (designation N531) according to a seventh exemplary embodiment; Fig. 10B illustrates the longitudinal chromatic aberration (OHL).

If the diagram of the partial dispersions (Fig. 3) is looked at in detail, it is evident that fluorspar (CaFa) and fused silica (SiOz) or FK5 have virtually the same partial dispersions with a finite difference in the Abbe numbers, which is to say it should be possible to use these materials to obtain a system with a fully corrected primary and secondary spectrum.

Precisely this is what is demonstrated in the seventh exemplary embodiment. In this case, all converging lens elements (with the exception of the lens element L18) of the aperture convexity are consequently made of fluorspar, while all diverging lens elements are made of fused silica. Thus, fluorspar acts as a (relative) crown material, and fused silica acts as a (relative) flint material.

A small residual amount of the secondary spectrum would remain in the illustrated form of the design. This can still be corrected in full by virtue of adding to the design a converging lens with a weak refractive power made of a further material, the said material again spanning a triangular area with fused silica and fluorspar in the diagram of the relative partial dispersions. In the example here, this is the lens element L18 made of the material LF5. With practically the same effect it would also be possible to use a lens element made of LLF1 or else N-SSK20.

However, the use of a "conventional" flint lens element with weak refractive power made of LF5 or LLF1 would reintroduce lead into the system. However, it is also possible to dispense with this lens element, which has as a consequence that there is a minimal remaining secondary spectrum in a once again lead-free system.

A lead-free variant can be constructed using lens elements made out of the materials CaFz, SiOz and FK5.

The result is a projection lens with a quite complicated structure, the installation length of which has risen to 1100 mm vis-a-vis the previous lenses of 1000 mm and in which positive lens elements and negative lens elements with extreme radii of curvature alternate. There are two causes for this, namely (i) the difference in the Abbe numbers is significantly lower than in the previous designs, which is why the correction of the primary spectrum requires greater individual refractive powers and (ii) the refractive index of the materials used is significantly smaller than previously (at least in respect of the "flint lens elements"), with the result that the increased refractive powers lead to yet again more extreme geometries (more curved surface radii). In conclusion, the coefficients of the chromatic correction of the secondary spectrum are

In this case, too, a remaining portion of the primary spectrum compensates the spherochromatism of the overall system.

Fig. 11A shows a lens element section of a projection lens PO-8 (designation N549) according to an eighth exemplary embodiment; Fig. 11 B illustrates the longitudinal chromatic aberration (CHL). Even more effort was put into the correction within the scope of this exemplary embodiment in order to be able to cover a much larger spectral range. Here, the aberrations for the g-, h- and i- lines were corrected. However, the aberrations for the wavelengths in between have not been corrected (cf. Fig. 11 B).

The corrections in this design were essentially carried out using the combination of FK5 and N- SSK20, with the aid of LF5. Further assistance was provided by the fact that two further lens elements were formed from fluorspar (CaF ₂ ).

The coefficients of the chromatic correction are no longer very meaningful anymore since there is a zero of the chromatic aberration near the i-line and the third order function is dearly visible in the longitudinal chromatic aberration.

Fig. 12A shows a lens element section of a projection lens PO-9 (designation N572) according to a ninth exemplary embodiment; Fig. 12B illustrates the longitudinal chromatic aberration (CHL).

The exemplary embodiment is designed as a projection lens completely without lead. This exemplary embodiment is derived from the seventh exemplary embodiment (Figs 10, 10A), which is already largely without lead.

The seventh exemplary embodiment consists virtually completely of (lead-free) FK5, fused silica and fluorspar lens elements. To take account of the small difference in the relative partial dispersion of the two materials, a little refractive power (lens element L18) made of a third material, specifically an LF5 lens element, was additionally "admixed" in that case.

The ninth exemplary embodiment, in which the LF5 lens element was removed in comparison with the seventh example, was developed from this. Further, all FK5 lens elements were replaced by fused silica lens elements; this could be implemented without significant influence on the correction of the system, especially on the chromatic correction as well.

Strict apochromatization is no longer given, but nevertheless the longitudinal chromatic aberration is still corrected extremely well. The following values were determined for the parameters relevant in this application:

The structure of the system, which is to say for example the division into lens element groups, practically does not differ from the seventh exemplary embodiment in this case. The main differences are in the fifth lens element group LG5 - caused by the omission of the thin lens element L18 from Fig. 10A.

A few teachings of the invention can be summarized as follows.

It appears advantageous to design the projection lens as a one-waist system with the refractive power sequence N-P-N-P-P. This means that there is only a single pronounced waist in the region of the third negative lens element group LG3 between an object-near convexity (at LG2) and an image-near convexity (at LG4 and LG5). The first lens element group immediately following the object plane may be formed by a single negative lens element (for example, in the first or third exemplary embodiment) or else may comprise a plurality of lens elements, for example two or three lens elements (second exemplary embodiment). In the case of a plurality of lens elements, the first lens element L1 immediately following the object plane may be a positive lens element (cf., e.g., PO-5, PO-6 or PO-8). In any case, it appears advantageous if the first lens element surface immediately following the object plane (entrance surface of the lens element L1) is convexly arched towards the object plane OS.

In respect of the suitability of material combinations for achromatization, it is possible in summary to inter alia learn the following.

It is readily possible to correct the primary spectrum longitudinally (CHL) if glasses or transparent materials with the greatest possible difference in their Abbe numbers are combined. In this context, crown glasses with a large Abbe number are used in the converging lens elements and flint glasses with a small Abbe number are used in the diverging lens elements. The conventional combination of FK5 (v = 62. 2) with LF5 (v = 31.2) in this case has a sufficiently large difference in the Abbe numbers to allow achromatization without individual refractive powers that are too large. For example, a combination of CaF ₂ and LF5 or a combination with FK5 and LLF1 (v = 35.9) is also possible. Optionally, FK5 can be produced by SiO2.

In view of the relative partial dispersion, it is possible to attempt to find a good compromise between a plurality of optimization concepts. Firstly, the secondary spectrum can be corrected well if two glasses with the same or a very similar relative partial dispersion (i.e., located level in the diagram of Fig. 3) are available. The secondary spectrum can also be corrected if at least three glasses are available, the connecting lines of which include a finite, non-vanishing area (cf. triangular area DF in Fig. 3).

For the combination with conventional crown glasses (such as FK5 or S1O2, for example) and conventional flint glasses (such as LF5 and/or LLF1 , for example), suitable third material coming into question appears to be predominantly that which deviates from the problem-adjusted normal line by at least 0.002. The specialist glass N-SSK20 is a representative for this material class.

AM. 67.6!

69.4 92.4

94.6 96.0

95.1 83.8 74.5! 68.4

53.7 51.0

49.8 49.8

60.3 77.7

82.3 88.0

88.9 87.9!

94.0 95.4

96.7

95 9 98.3!

98.3 96.0

92.4 95.9

95.3 74.6

71.6 41.8

41.0

29.5 Table 5 (N528P)

SURF RADIUS THICKNESS MATERIAL INDEX1 INDEX2 INDEX3 SEMIDIAM.

0 0 30.998382

1 240.605915 30.783167 LF5 1.619068 1.619457 1.618683 67.1

' 2' -206.696904 1.005432! ! ( ' ' 67.2!

3 -1435.171215 10.000000 SIO2 1.474477 1.474623 1.474332 64.9

4 86.176482 46.337697; j ! i '! 60.8'

5 -89.686873' 10.000000! LLF1 1.579164 1.579477 1 578854 61.0

6 1015.276739 17.812356! 75.0

7 -333.111277 44.226818 FK5 1.503934 1.504084. 1.503784 79.2

8 -137.492051 0.999407 90.4

9 -3402.459643 50.445120 FK5 1.503934 1.504084 1.503784 104.7

10 -158.457240 1.174386 107.5

11 170.752829 44.283238 FK5 1.503934 1.504084 1.503784 107.4

12 1477.797124 78.232376 104.9

13 148.060941 46.738707 FK5 1.503934 1.504084 1.503784 71.8

14 170.068434 21.786964 56.0

15 -199.942284s 8.000000 LLF1 1.579164 1.579477 1.578854 53.8^

16 127.178403 25.198740' 49.9

17 -102.567740 8.000000 LLF1 1.579164 1.579477 1.578854 50.0;

18 868.694705 0.999046 54.5;

19 394.273423 30.504675 SiO2 1.474477 1.474623 1.474332' 55.8;

20 1710.678324 13.563633 61.3

21 4531.941468 27.774771 FK5 1.503934; 1.504084 1.503784 65.4

22 -140.712699 7.330849' 67.6

23 448.786916; 25.675074 FK5 1.503934 1.504084 1.503784 69.6

24 -240.4424041 -6.571454 69.5

25 0.000000 98.720808 68.9'

26 268.117926 35.155007 FK5 1.503934 1.504084 1.503784 81.9'

27 -280.452333 2.269341 81.6.

28 -275.488838 10.000000 LF5 1.619068 1.619457 1.618683 81.2'

29 199.878331 40.409608 80.9

30 358.142081 37.372582 FK5 1.503934 1.504084 1.503784 91.3

31 -281.736753 15.384871 92.3

32 151.795520 39.276903 FK5 1.503934 1.504084 1.503784 90.4

33 6983 685683 53.689455! j ' ' ' 87.8!

34 88.721432 19.766659 SiO2 1.474477 1.474623 1.474332 58.5

35! 79.032097 57.657733' j ' ' ' 49.9

! 36 0.000000 3.000000 5102 ! 1.474477' 1.474623! 1.474332; 34.9!

37 0.000000 12.000000; f ! ' ! 34.1!

38 0.000000 0.000000' ! ! ! ' 29.5!

Table 5A (N528P)

SRF 4 20 35

K 0 0 0

Cl -2.794179E-07 9.564778E-08 2.750227E-08

C2 -1.266970E-11 -3.221812E-12 1.256872E-11

C3 -5.972407E-16 -3.441566E-16 1.729595E-15

C4 -3.690270E-19 1.093536E-19 8.386768E-19

C5 7.822037E-23 -2.255906E-23 4.064781E-22

C6 -1.507972E-26 2.706503E-27 -2.100703E-25

C7 5.583562E-31 -1.303048E-31 6.200241E-29