Projection exposure method, projection lens and microlithographic projection exposure apparatus

Abstract

In a projection exposure method for exposing a 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 radiation from a wavelength range around a design wavelength of < 260 nm, a substrate (SUB) is coated with a relatively thick radiation-sensitive photoresist layer (RS) and exposed with the aid of a projection lens (PO) which produces a focus in a design focus position (FOC0) at the design wavelength and offset focus positions from an axially extended focus region (ΔFOC) around the design focus position (FOC0) for other wavelengths from the wavelength range. The projection lens is configured as a single-waist system or two-convexity system. During an exposure time interval, different wavelengths from the wavelength range around the design wavelength are used in such a way that an axial extent of the focus region is at least equal to the magnitude of the layer thickness. The projection lens is optically corrected with regard to the transverse chromatic aberration CHV in such a way that in the region of the design wavelength, the imaging scale is substantially independent of the wavelength such that in the case of a change in the wavelength by 1 picometre, a position of an image point associated with an object point varies by at most 2 nm for all image points within an image field.

Description

[0001] Projection exposure method, projection lens and microlithographic projection exposure apparatus

[0002] The following disclosure is based on German patent application 102024111454.1 filed on April 24, 2024, which is incorporated into this application by reference.

[0003] FIELD OF APPLICATION AND PRIOR ART

[0004] The invention relates to a projection exposure method for exposing a 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, and to a projection exposure apparatus suitable for carrying out the method and to a projection lens usable in said apparatus.

[0005] Microlithographic projection exposure methods are predominantly used nowadays for producing semiconductor components and other finely structured components, such as, for example, structured components for microsystems engineering. Highly integrated semiconductor components typically comprise a plurality of layers, of which only some layers are structured very finely, e.g. on the scale of a few dozen nanometres, while other layers have significantly coarser structures. The former layers realize in particular the actual main function of the semiconductor component, such as e.g. calculations and storage of data, while the latter layers serve e.g. for addressing and power supply. Structures having relatively coarse typical dimensions are also found in the field of microsystems engineering, such as e.g. in microelectromechanical systems (MEMS) or in microoptoelectromechanical systems (MOEMS). Semiconductor components are typically produced from a substrate of a semiconductor, while other substrate materials, in particular metals and vitreous substances, are also used in microsystems engineering.

[0006] Microlithographic projection exposure generally involves the use of masks (reticles) bearing the pattern of a structure to be imaged, e.g. a line pattern of a layer of a semiconductor component. A mask is positioned in a projection exposure apparatus between an illumination system and a projection lens in the region of the object plane of the projection lens and is illuminated with an illumination radiation provided by the illumination system. The radiation altered by the mask and the pattern passes as projection radiation through the projection lens, which images the pattern of the mask onto the substrate to be exposed. The substrate can be e.g. a semiconductor wafer. The substrate to be exposed bears a radiation-sensitive (i.e. photosensitive) layer composed of photoresist or resist on its side to be structured. Said layer is referred to as a photoresist layer or else as a resist layer. One of the aims in the development of projection exposure apparatuses is to produce lithographically structures with smaller and smaller dimensions on the substrate. The size of the structures that can be produced depends crucially on the resolving power of the utilized projection lens, and the latter can be increased, firstly, by reducing the wavelength of the projection radiation used for the projection and, secondly, by increasing the image-side numerical aperture NA of the projection lens used in the process. Many modern projection lenses operate at design wavelengths of less than 260 nm in the deep ultraviolet (DUV) or vacuum ultraviolet (VUV) range.

[0007] Projection lenses are optically corrected for their respective design wavelength and produce a focus in a design focus position at the design wavelength. If the longitudinal chromatic aberration CHL (referred to as: axial chromatic aberration AX) of the systems is not fully corrected, deviations from the design wavelength result in axially offset focus positions from a focus region around the design focus position.

[0008] In order that an image of the pattern that is as faithful to the original as possible is transferred to the substrate during the exposure process, the radiation-sensitive layer on the substrate surface should lie in the image-side focus region of the projection lens during the exposure time interval. In particular, the layer arranged on the substrate surface should lie in the region of the depth of focus (DOF) of the projection lens. According to one common definition, the depth of focus specifies the distance relative to the plane of best focus for which the intensity of a point image is at least 80% of the intensity in the plane of best focus. This is equivalent to the condition that the diameter of the point image practically does not change. The depth of focus amounts to half of the Rayleigh unit Rll, which is defined as Rll = X / NA2, wherein X is the operating wavelength of the projection exposure apparatus and NA is the image-side numerical aperture of the projection lens; the region in which the depth of focus condition is met accordingly has a total thickness equal to the Rayleigh unit Rll. In general, the depth of focus becomes smaller, the higher the resolving power of the projection lens.

[0009] Further miniaturization of the feature sizes on computer chips is becoming more and more difficult technically and physically and more and more costly. As an alternative to better utilization of the area of the wafer, it is possible to utilize the third dimension by producing structures not just on or at the surface of the wafer but into the wafer in the depth direction.

[0010] The third dimension is used, for example, to create a three-dimensional stacking of flash memory structures, e.g. in the production of 3D NAND flash memories. 3D NAND is a type of non-volatile flash memory in which the memory cells are stacked vertically in multiple layers. When producing 3D NAN Ds, relatively thick resist layers have to be exposed. The depth of focus of customary standard scanners is generally not sufficient for this purpose.

[0011] There are already proposals for methods for exposing relatively thick photoresist layers.

[0012] The application US 2007 / 0013889 A1 describes a projection exposure apparatus having an illumination system for generating a beam, an arrangement of individually controllable elements which serve to impart a pattern to the beam in its cross-section, a substrate table for supporting a substrate, and a projection system for projecting the patterned beam onto a target portion of the substrate. The beam comprises a plurality of beam components. These include a first beam component having a first frequency spectrum around a first frequency and at least a second beam component having a second frequency spectrum around a second frequency. The second frequency is different from the first frequency. The projection system focuses the first and second beam components at different heights relative to the substrate table.

[0013] In the projection exposure method and the projection exposure apparatus in US 4,937,619 A, a substrate is exposed with illumination light of different wavelengths, wherein the longitudinal chromatic aberration of the projection lens changes the position of the focus surface relative to the substrate surface with the wavelength.

[0014] In the projection exposure method and the projection exposure apparatus in US 5,303,002, a substrate is exposed simultaneously with narrowband pulses of closely adjacent different wavelengths. On account of the longitudinal chromatic aberration of the projection lens, an effective magnification of the depth of focus is attained.

[0015] PROBLEM AND SOLUTION

[0016] One problem addressed by the invention is that of providing a projection exposure method which allows the exposure of a relatively thick radiation-sensitive photoresist layer on a substrate to be exposed such that an exposed volume having sharp contours positioned correctly is producible over the entire thickness of the layer. A further problem addressed is that of providing a projection exposure apparatus suitable for carrying out the method and a projection lens usable therefor.

[0017] In order to solve this problem, the invention provides a projection exposure method having the features of Claim 1. Furthermore, a dioptric projection lens according to Claim 9 and a projection exposure apparatus according to Claim 14 are provided, which can be used for carrying out the projection exposure method. Advantageous developments are specified in the dependent claims. The wording of all the claims is incorporated by reference in the content of the description.

[0018] In the context of carrying out the projection exposure method, a substrate to be exposed is coated with a radiation-sensitive photoresist layer having a layer thickness.

[0019] In the projection exposure method, the substrate covered with the photoresist layer and arranged in the region of the image plane of a projection lens is exposed with at least one image of a pattern arranged in the region of the object plane of the projection lens, the object plane being optically conjugate to the image plane. A mask (reticle) bearing the pattern is generally used. It is also possible to use a controllable pattern producing device to produce the pattern to be imaged.

[0020] Radiation from a wavelength range around a design wavelength of less than 260 nm (nanometres) is used for exposure purposes. The term “design wavelength” denotes that wavelength for which the optical set-up of the projection lens is designed in regard to optical aberrations.

[0021] Projection lenses of the type considered in this application operate at design wavelengths of less than 260 nm in the deep ultraviolet (DUV) or vacuum ultraviolet (VUV) range. Powerful laser light sources are available here, in particular KrF excimer lasers having a wavelength of approximately 248 nm and ArF lasers having a wavelength of approximately 193 nm. Moreover, relatively high resolutions are attainable at these wavelengths.

[0022] In the projection exposure apparatus and respectively in the projection exposure method, a projection lens is used which is optically corrected with regard to the longitudinal chromatic aberration CHL in such a way that a focus in a design focus position is produced at the design wavelength and offset focus positions from an axially extended focus region around the design focus position are produced for other wavelengths from a wavelength range around the design wavelength. The projection lens thus has finite values for the longitudinal chromatic aberration CHL.

[0023] The substrate is arranged in relation to the projection lens in such a way that the design focus position lies in the region of the photoresist layer.

[0024] In an exposure operation, the substrate coated with the photoresist layer is exposed with at least one image of the pattern by means of the projection lens. In this case, under the control of a control unit within an exposure time interval, different wavelengths from the wavelength range around the design wavelength are generated and used in such a way that an axial extent of the used focus region is at least equal to the magnitude of the layer thickness. This exploits the fact that the projection lens has a finite value for the longitudinal chromatic aberration CHL.

[0025] What arises as a desired effect as a result is an axial magnification of the effectively used focus region, which corresponds to a magnification of the effective depth of focus (DOF). As a result, it becomes possible to expose even relatively thick photoresist layers over their entire depth with sufficiently focused radiation.

[0026] According to one formulation of the claimed invention, the projection lens used is a dioptric projection lens having a reducing imaging scale B. The projection lens is designed for a design wavelength of < 260 nm and is configured as a single-waist system or as a two-convexity system. The projection lens has along an optical axis between the object plane and the image plane a first lens element group having negative refractive power and directly following the object plane; a second lens element group having positive refractive power and directly following the first lens element group; a third lens element group having negative refractive power and directly following the second lens element group in order to generate a waist around a region of minimal marginal rays heights between the object plane and the image plane; a fourth lens element group having positive refractive power and directly following the third lens element group; and a fifth lens element group having positive refractive power and directly following the fourth lens element group, wherein an aperture stop is mounted between the fourth and fifth lens element groups. The first lens element group has more lens elements having negative refractive power than lens elements having positive refractive power.

[0027] By way of example, the first lens element group can be free of lens elements having positive refractive power and can be formed just by a single negative lens element or can have exactly two lens elements having negative refractive power.

[0028] 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 is configured such that no further lens element groups are present apart from the aforementioned five lens element groups. If a projection lens is designed as a single-waist system, the Petzval correction can be ensured in conjunction with overall compact structural dimensions.

[0029] The inventor’s proposal provides that the projection lens should be optically corrected with regard to the transverse chromatic aberration CHV in such a way that in the region of the design wavelength, the imaging scale is substantially independent of the wavelength in such a way that in the case of a change in the wavelength by 1 picometre, a lateral position of an image point associated with an object point varies by at most 2 nm for all image points within an image field.

[0030] According to another formulation, in the case of such projection lenses, for the transverse chromatic aberration of the projection lens in the entire image field, in particular also at the edge or in a corner of the imaged image field, the following condition holds true: nm \CHV\ < 2 — pm

[0031] The transverse chromatic aberration CHV is often also referred to as transverse chromatic aberration or chromatic magnification aberration and causes partial images to be imaged with different sizes for different wavelengths. The transverse chromatic aberration thus results in a dependence of the imaging scale on the wavelength. This is also referred to as the chromatic difference of magnification (CHV) or as a wavelength-dependent distortion. The transverse chromatic aberration is thus manifested in the dependence of the image height of an off-axis image point on the wavelength. This has the consequence that the transverse chromatic aberration has no effect in the exposure process on the optical axis (image height = 0), but is more and more noticeable towards the edge of the image field. To put it another way, a field dependence arises regarding the effects on the image generation.

[0032] On account of a particularly good correction state in the case of the transverse chromatic aberration CHV, what is achieved is that no significant change in the effective imaging scale arises when the wavelength changes. To put it in yet another way, when the wavelength changes, the focused image of a structure element of the pattern will shift within the photoresist layer, on account of the presence of the longitudinal chromatic aberration, substantially parallel to the optical axis of the projection lens or substantially perpendicularly to the substrate surface and practically without lateral drifting, specifically not just in the region of the optical axis but over the entire image field.

[0033] It is thus possible, for example, by means of a targeted change in the wavelength of the projection light, with the aid of a relatively thick photoresist layer, in the entire image field, to produce holes and / or other structures, such as e.g. trenches, which extend substantially perpendicularly to the substrate surface. By virtue of the transverse chromatic aberration being reduced to the greatest possible extent, the projection lens is suitable particularly for methods of multi-focal imaging, which attains a depth variation of the focus position by changing the wavelength of the used radiation.

[0034] In many exemplary embodiments, the longitudinal chromatic aberration is more than 150 nm / pm; the condition CHL > 300 nm / pm can also hold true. This depends on the wavelength range and also the dispersion of the (one or more) lens element materials used. The axial focus position can thus be varied to a sufficient extent by wavelength variation.

[0035] In some processes, the substrate is coated with a photoresist layer having a layer thickness of more than 1 pm, wherein preferably the layer thickness is in the range of 5 pm to 20 pm, optionally also above that.

[0036] According to one development, the wavelength of the radiation is varied around the design wavelength by at least ± 20 pm during an exposure time interval, in order that, with the given longitudinal chromatic aberration, even relatively thick photoresist layers are exposed with sharp contours and positionally correctly over the complete thickness direction.

[0037] If a projection lens of the type presented here is used in multi-focal imaging, which projection lens has a finite value for the longitudinal chromatic aberration but is corrected to the greatest possible extent with regard to the transverse chromatic aberration, it is possible to implement multi-focal imaging without the axial position of the substrate to be exposed being changed during the exposure time interval. In preferred variants of the projection exposure method, accordingly an axial position of the pattern and / or an axial position of the substrate remain(s) constant during the exposure time interval. Corresponding manipulators that would allow an axial displacement of the substrate can thus be dispensed with.

[0038] Various approaches can be used to optimize the suitability of a projection lens for the methods described.

[0039] Some embodiments are distinguished by the fact that all transparent optical parts, i.e. in particular all lens elements and if appropriate plate-shaped elements additionally present, are fabricated from the same optical material, in particular from synthetic fused silica. Such “single-material systems” have production engineering advantages, inter alia, because all the optical surfaces can be processed using similar processes. Moreover, in the case of single-material systems, it is relatively easily possible to compensate for changes in the imaging properties on account of air pressure fluctuations solely by way of slight changes in the wavelength, such that the optical imaging performance can be kept relatively stable even under variable ambient conditions. In the case of a single-material system composed of synthetic fused silica, an additional factor is that this material is available with very high quality at tenable costs and can be processed well, such that manufacturing faults can be minimized.

[0040] According to the inventor’s insights, in the case of such projection lenses, a correction of the transverse chromatic aberration CHV can then be achieved particularly well if the diameters of the convexities formed upstream and downstream of the waist differ significantly from one another. In this case, on the one hand, the first convexity, nearer the object, should have a somewhat smaller diameter than the second convexity, nearer the image plane. On the other hand, the diameters should differ from one another neither too much nor too little. It is particularly advantageous if, in the case of a projection lens having an object height OBH and an image-side numerical aperture NA, the second lens element group has a maximum optically free lens element diameter D2and the fourth and fifth lens element groups have a maximum optically free lens element diameter D4 5, and for a normalized diameter ratio

[0041] A = 100 • the following condition holds true:

[0042] 1.05 < A < 1.34

[0043] This teaching is based, inter alia, on the following insights. In the case of a microlithographic projection lens, the Petzval sum necessarily has to be corrected in order to be able to ensure the required correction of the aberrations across the entire image field. If a projection lens is designed as a single-waist system or as a two-convexity system, an important contribution to the Petzval correction can be made in conjunction with overall compact structural dimensions.

[0044] For single-material systems, it can be shown that in the case of a two-convexity system whose Petzval sum is intended to be corrected sufficiently well, a change in diameter of one of the convexities causes an opposite change in diameter of the respective other convexity. Details will also be explained in conjunction with the exemplary embodiments. If, under this constraint, the projection lens is then also intended to be corrected very well with regard to the transverse chromatic aberration, this is possible in principle by way of a reduction of the chief ray height in the first convexity, nearer the object. This reduces the prismatic effects of lens elements having positive refractive power of the first convexity, nearer the object, on the chief ray and thus leads to a reduction of the transverse chromatic aberration CHV. As a consequence, it should be established here as a teaching that the lens element diameters in the near-object first convexity should be less than those in the near-image second convexity.

[0045] It should furthermore be noted that the maximum optically free lens element diameter D4 5of the near-image convexity is crucially driven by the numerical aperture NA and therefore increases if the NA increases. In order to be able to compare systems having different numerical apertures, it is therefore expedient to divide the respective diameter value by the numerical aperture. Analogously thereto, the maximum optically free lens element diameter of the near-object convexity (£>2) is greatly driven by the object height OBH, and so here, too, a normalization to the object height is expedient. Therefore, the parameter

[0046] D2

[0047] OBH _NA D? P4.5 OBH ' D4 5NA is introduced, which multiplied by the value 100 yields the normalized diameter ratio A.

[0048] In order then to correct the transverse chromatic aberration to the greatest possible extent, diameters should differ from one another neither too much nor too little. According to the inventor’s insights, best results are attainable if the condition

[0049] 1.05 < A < 1.34 is complied with. If the lower limit is significantly undershot, then a disadvantageously great overcorrection of CHV results. On the other hand, if the upper limit is significantly exceeded, such that the maximum diameters become approximately equal in magnitude or even the diameter of the first convexity becomes greater than that of the second convexity, then that regularly results in an excessively great undercorrection.

[0050] According to the inventor’s experiments, it may be the case in particular that the transverse chromatic aberration can be corrected sufficiently well if the normalized diameter ratio A is in the range of between 1.10 and 1.30 or even in a still narrower range of between 1.15 and 1.25, such that the conditions 1.10 < A < 1.30 and / or 1.15 < A < 1.25 hold(s) true.

[0051] It has been found that it is advantageous in many cases if the first lens element group has at least two lens elements having negative refractive power. The beam expansion for forming the first convexity can thereby be attained with the aid of four optically effective lens element surfaces, whereby inter alia excessively large angles of incidence and attendant imaging aberrations can be minimized.

[0052] Preferably, a further criterion concerning the so-called waist distance should be satisfied. The waist distance is the distance measured parallel to the optical axis between the object plane and a constriction location of minimal marginal ray height in the region of the waist. For a distance ratio AT / L between the waist distance AT and the object-image distance L of the projection lens, the condition 0.4 < AT / L < 0.5 should preferably be satisfied in order to obtain a set-up which is advantageous with regard to the correction of transverse chromatic aberrations. In particular, the condition 0.4 < AT / L < 0.45 should be satisfied.

[0053] The projection lens is preferably designed as a dry lens, i.e. as a projection lens having an imageside numerical aperture NA < 1. In preferred embodiments, the image-side numerical aperture NA is in the range of 0.6 to 0.95. As a result, a good compromise between sufficiently fine resolution and sufficiently large depth of focus can be achieved, which has been found to be advantageous for multi-focal imaging.

[0054] It is also possible to fabricate lens elements of the projection lens from at least two or exactly two different materials, for example by the majority of the lens elements being produced from synthetic fused silica and a smaller number, e.g. one or two or three lens elements, being produced from an optical material having lower dispersion. If a material having lower dispersion is used in one or more converging lens elements in the near-object first convexity, for example, the effect on the chromatic splitting of the chief ray in these lens elements is less pronounced than in materials having higher dispersion. Accordingly, the diameter of the object-side convexity can become larger and as a result the diameter of the image-side convexity can become smaller with the same Petzval correction. This leads overall to a system in which the maximum diameter that occurs is minimal, which is beneficial for the compactness of the overall system.

[0055] Some embodiments provide for at least all lens elements of the first lens element group, of the third lens element group, of the fourth lens element group and of the fifth lens element group to consist of the same optical material, in particular of synthetic fused silica. A second material, for example calcium fluoride (or fluorite), can be used to fabricate one converging lens element, two converging lens elements or more converging lens elements in the second lens element group therefrom.

[0056] According to another formulation of the use of a plurality of materials, preferably a projection lens is provided wherein the majority of the lens elements consist of a first material having an Abbe number v-, and at least one lens element consists of a second material having an Abbe number v2> v-,, and wherein Hj is the maximum ray height of the chief ray at the location of the i-th lens element, wherein for all lens elements j of the second material the absolute value of the chief ray height, i.e. Hj, is greater than 0.75 x OBH. If this condition is complied with, what can be achieved is that the diameters of the first and second convexities can become closer to one another and optionally become equal in magnitude again, which is beneficial for the compactness.

[0057] According to one development, in some embodiments it is provided that the maximum occurring angle of incidence imaxof a light beam on the lens element directly upstream of the stop position and the lens element directly downstream of the stop position is the condition 2 sin imax< NA. To put it another way, strong menisci in the stop space should be avoided as far as possible. Material is saved since large angles of incidence in the collimated part always cause greatly hollow surfaces, which leads to material-intensive menisci. Furthermore, there is a resulting gain in sensitivity for the lens elements, e.g. as far as the centration sensitivity of the lens element is concerned.

[0058] The invention also relates to a projection exposure apparatus suitable for carrying out the method, and a projection lens usable therein. Properties of preferred embodiments have already been described above in connection with the projection exposure method, to which reference is made here.

[0059] BRIEF DESCRIPTION OF THE DRAWINGS

[0060] 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.

[0061] Figure 1 shows an exemplary embodiment of a microlithographic projection exposure apparatus which is suitable for multi-focal imaging (MFI) and can be used therefor, and also details for elucidating a sinusoidal wavelength variation (detail I) and for image generation in a thick resist layer (detail II);

[0062] Figure 2 shows a reference system from the prior art;

[0063] Figures 3A to 3C schematically show beam paths for different wavelengths;

[0064] Figures 4A to 4C schematically show beam paths in a two-convexity system; Figure 5A shows a wavelength-time diagram (X / t diagram) with a temporal sequence of individual pulses whose centroid wavelengths vary substantially according to a sinusoidal time function;

[0065] Figure 5B schematically shows effects of the transverse chromatic aberration on the image size when the wavelength is changed;

[0066] Figures 6 to 11 show various exemplary embodiments (first to sixth exemplary embodiments) for dioptric projection lenses with little transverse chromatic aberration.

[0067] DETAILED DESCRIPTION OF THE EXEMPLARY EMBODIMENTS

[0068] Various aspects of the technical problems addressed in this application and the solution to these problems are explained below on the basis of the use of purely refractive (dioptric) projection lenses at the wavelengths of 248 nm (DU ) and 193 nm (VUV).

[0069] Figure 1 shows an example of a microlithographic projection exposure apparatus WSC which is usable in the production of semiconductor components and other finely structured components and which operates with light or electromagnetic radiation from the deep ultraviolet (DUV) range in order to obtain resolutions down to fractions of micrometres.

[0070] The projection exposure apparatus as a whole is intended to be able to be used as necessary to expose a relatively thick radiation-sensitive photoresist layer RES on a substrate W to be exposed such that within the entire exposed image field over the entire thickness of the layer, an exposed volume can be produced with positionally correct, sharp contours.

[0071] As light source LS, use is made of a tunable KrF excimer laser having a main operating wavelength of approximately 248 nm. Other UV laser light sources or operating wavelengths are likewise possible, e.g. an ArF laser which emits at approximately 193 nm. The light source LS operates in pulsed operation and emits pulsed radiation during operation, i.e. a temporal sequence of pulses PS (see double-T lines in detail I), with a pulse frequency in the kilohertz range (i.e. at at least one kHz, e.g. at approximately 6 kHz). Each pulse can be characterized by its centroid wavelength and its spectral bandwidth or spectral pulse width. The centroid wavelength is that wavelength which is present with maximum intensity in the pulse. The spectral bandwidth of a pulse is given by the full width at half maximum (FWHM) of the intensity distribution around the centroid wavelength of the pulse. Here the term “light source” is intended to encompass not only the primary light source that emits light, but also devices possibly present, such as filters, stops, bandwidth narrowing modules, wavelength varying modules or the like, which serve to alter the characteristic of the light emitted by the primary light source before said light enters the illumination system.

[0072] The light source can emit pulses of different wavelengths from a wavelength range WLB of at least ± 20 pm (picometres) around its main operating wavelength. For this purpose, the light source LS has a resonator and adjustable devices which alter optical properties of the resonator. A wavelength setting can be achieved e.g. by virtue of a rotatable grating or etalon being mounted in the resonator and / or the resonator length being varied. As a result, only light in a narrow frequency range can be reflected back into the resonator and contribute to the induced emission. Light of other wavelengths is reflected out of the resonator. As a result of the rotation of grating or etalon, different frequencies are successively fed back, and the wavelength (centroid wavelength) of the pulses of the laser light can be tuned.

[0073] The change between different centroid wavelengths within the wavelength range can take place relatively rapidly, such that ten or more pulses, possibly even one hundred or more pulses, of different centroid wavelengths from the wavelength range can be emitted within a fraction of a second.

[0074] In 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 can be switched for example between conventional on-axis illumination with different degrees of coherence c and off-axis illumination. By way of example, the off-axis illumination modes comprise annular illumination or dipole illumination or quadrupole illumination or any other multi-polar illumination.

[0075] Those optical components which receive the light from the light source LS and shape illumination radiation from this light, which illumination radiation is directed to the illumination field lying in the exit plane ES or to the reticle M, are part of the illumination system ILL of the projection exposure apparatus.

[0076] 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 PAT arranged at the reticle lies in the region of 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. For the purposes of scanner operation, the mask is movable parallel to this plane in a scanning direction (y-direction) perpendicular to the optical axis OA (z-direction) with the aid of a scanning drive.

[0077] The device RS comprises an integrated lifting device for linearly displacing the mask in relation to the object plane in the z-direction, i.e. perpendicular to the object plane, and an integrated tilting device for tilting the mask about a tilt axis extending in the x-direction.

[0078] Following downstream of the reticle plane OS is the projection lens PO, which acts as a reduction lens and, given an image-side numerical aperture NA < 1 , images an image of the pattern arranged at the mask M with a reduced scale, for example with the scale of 1 :4 (|P| = 0.25) or 1 :5 (|P| = 0.20), onto a substrate W coated with a photoresist layer RES, the light-sensitive substrate surface SS of which lies in the region of the image plane IS of the projection lens PO.

[0079] 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).

[0080] The device WS furthermore comprises a lifting device for linearly displacing the substrate in relation to the image plane in the z-direction and a tilting device for tilting the substrate about a tilt axis extending in the x-direction.

[0081] 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 Oil of the projection exposure apparatus.

[0082] 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 with respect to the optical axis OA (on-axis field).

[0083] The effective image field IF in the image surface IS, which is optically conjugate to the effective object field, has the same shape and the same aspect ratio between height B and width A as the effective object field, but, in the case of projection lenses with a reducing action (with | p | < 1), the absolute field dimension is reduced by the imaging scale p of the projection lens, i.e. A = | | A* and B = | p | B*.

[0084] In the rotationally symmetrical system, the circle which is centred with respect to 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 parametrized 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 and is therefore also referred to as object height OBH.

[0085] The object field point of maximum field height is imaged into an image field point which has a corresponding field height corresponding to the maximum field height in the image field IF.

[0086] The projection lens and the projection exposure apparatus as a whole are intended to be able to be used, inter alia, for producing holes in the entire effective image field during the production of 3D structures using relatively thick photoresist layers, which holes are formed exactly vertically in the photoresist (resist).

[0087] The projection lens has an uncorrected longitudinal chromatic aberration (CHL, referred to as: axial chromatic aberration AX). That has the consequence that the focus of the projection lens moves along the optical axis if the wavelength of the radiation used for exposure purposes changes.

[0088] During nominal operation, exposure is normally effected using light from a narrowband laser. The latter has a central wavelength (main wavelength) and a finite bandwidth. In this case, the bandwidth is usually very small, mostly significantly less than one picometre (pm).

[0089] As a consequence of the laser bandwidth and the uncorrected longitudinal chromatic aberration, a fully focused image of the main wavelength with the increasingly defocused images of the secondary wavelengths are thus superimposed in the photoresist (resist). These have the effect that the contrast of the imaged structures is slightly decreased, but the pure resolution is not detrimentally affected thereby.

[0090] The photoresist layer or the resist in which the structures are intended to be imaged has a finite layer thickness. The latter is often greater than the so-called depth of focus (DOF) of the projection lens, and so the image can be significantly defocused at the top side and the underside of the resist layer by comparison with the centre. This effect becomes all the more pronounced, the thicker the resist and the greater the image-side numerical aperture NA of the projection lens, since the depth of focus decreases as NA increases.

[0091] An additional factor is that in some present-day applications, relatively thick photoresist layers are employed and properly perpendicular “holes” are intended to be exposed into these layers.

[0092] One possibility for exposing such holes into the photoresist would be to move the wafer progressively through the focus during the exposure process, i.e. during an exposure time interval, such that the best focusing plane becomes located firstly at the top side, then in the centre and finally on the underside of the resist layer. However, this requires a high-frequency mechanical lifting movement of the wafer, which can be realized only with high technical complexity.

[0093] Another possibility for this is so-called “multi-focal imaging” (MFI). This makes use precisely of the fact that the CHL of the projection lens is not corrected. Typical values for CHL can be e.g. of the order of magnitude of less than 100 nm / pm. If, during the exposure, the laser wavelength is shifted within a wavelength range around the design wavelength of the projection lens slightly towards shorter or longer wavelengths, then the image likewise moves through the resist, entirely analogously to a mechanical lifting movement of the wafer. This allows holes to be exposed into the photoresist, without the axial position of the wafer being changed.

[0094] For illustration, the detail II in Figure 1 shows the image generation in a thick resist layer RES. The projection lens PO produces a focus in a design focus position FOCO at its design wavelength (here approximately 248 nm) and offset focus positions from an axially extended focus region AFOC around the design focus position FOCO for other wavelengths from the wavelength range WLB.

[0095] If the holes are intended to be produced in the resist as exactly perpendicularly to the substrate surface as possible, then it must be ensured that the so-called transverse chromatic aberration (CHV, referred to as lateral colour LAT), i.e. the change in the imaging scale with the wavelength, is corrected as well as possible. This is because if the top side of the photoresist is exposed in conjunction with a slight detuning of the laser wavelength towards the short-wave end, the centre is exposed at the main wavelength and the underside is exposed in conjunction with a detuning towards the long-wave end of the wavelength range, then a significant CHV would have the effect that the image on the top side, the centre and the underside of the photoresist layer would have different sizes. “Skew” holes in the photoresist would be an inevitable consequence thereof.

[0096] In this application, systematic approaches are presented and implemented for providing projection lenses in which the transverse chromatic aberration is corrected to the greatest possible extent.

[0097] Firstly, with reference to Figure 2 to Figure 5B, an explanation is given of how the chromatic difference of magnification of refractive systems arises.

[0098] Figure 2 illustrates a representative dioptric lithographic lens from the prior art (WO 2003 / 075096 A1 , first exemplary embodiment), which serves here as a reference system REF. This involves a projection lens designed for 193 nm and having an image-side NA of 0.85. The lens has two convexities (lens elements L3 to L8 and lens elements L17 to L23) composed primarily of converging lens elements and a strong waist (lens elements L9 to L12) composed primarily of diverging lens elements. All lens elements consist of the same material, this also being referred to here as a “single-material system”. The material here is synthetic fused silica.

[0099] The chief ray CR of the field edge is depicted in bold in the lens element sectional view. Furthermore, the lens elements L5, L11 and L20 (counted from the object plane OS) are depicted in bold.

[0100] Lens element L5 is a converging lens element in the first convexity of the lens. The chief ray here has a large height (i.e. lateral distance from the optical axis). The refraction of the chief ray by the lens element can be imagined to be replaced by the refraction at a prism, the latter also being depicted in bold with hatching in the drawing (cf. Figure 3A).

[0101] As a direct consequence of the dispersion of the lens element material quartz, the “blue” chief ray “B” is refracted more strongly and the “red” chief ray “R” is refracted more weakly than the “green” chief ray “G”. The same happens at the adjacent, converging optical parts, in particular L4, L6, L7. (“Blue” denotes the light at shorter wavelengths than the main wavelength (“green”); analogously, “red” is intended to denote the longer-wave light.)

[0102] The splitting of the chief ray by the individual lens elements is all the greater, (i) the greater the refractive power of the lens element, (ii) the higher the chief ray height at the location of the lens element; and (iii) the greater the dispersion of the lens element material (or the smaller the Abbe number). As a consequence, downstream of the first convexity, the “blue” chief ray B passes through the design “below” the “green” chief ray G, and the “red” chief ray R passes through the design “above” said "green" chief ray.

[0103] The effect is partly corrected by the diverging lens elements of the waist, as illustrated on the basis of the example of the lens element L11 , at which lens elements once again the “blue” chief ray B is deflected more strongly and the “red” chief ray R is deflected more weakly than the “green” chief ray G.

[0104] Analogously, the converging lens elements downstream of the aperture stop AS, where the chief ray passes through the optical axis, lead to a compensation of the chromatic splitting of the converging lens elements of the first convexity, as illustrated in exemplary fashion on the basis of the example of the lens element L20. However, the chief ray height at the location of the lens element L20 is already so small that the converging lens element has only a small deflecting effect on the chief ray and, consequently, has only very little compensating effect.

[0105] It transpires that the chromatic splitting of the chief ray height in the first convexity can no longer be completely compensated for by the lens elements of the waist and the lens elements downstream of the stop. Therefore, the “blue” chief ray B passes through the image plane at a greater distance from the optical axis than the “green” chief ray G. Analogously, the “red” chief ray R impinges on the image plane at a smaller distance from the optical axis. The consequence is that the “blue” image is somewhat larger, and the “red” image somewhat smaller, than the “green” image. A chromatic difference of magnification, the so-called CHV, thus arises (cf. Figure 5B). The chromatic difference of magnification CHV can be specified in a height difference of the chief ray intersection points for each wavelength interval in the image space, for instance in nm / pm.

[0106] In the case of the projection lens considered in Figure 2, the result is a CHV of 8.5 nm / pm at the outermost field edge (half field diagonal). The half field diagonal for the imaging at the short wavelengths is thus 8.5 nm / pm more than at the “green” or central wavelength. Assuming that the photoresist had a thickness of 1 pm, then given a CH L of around 400 nm / pm the wavelength would have to be changed by around 2.5 pm in order to shift the image from the upper edge of the resist to the lower edge. The size of the image (full diagonal) would then differ by approximately 2.5 • 17 nm « 42 nm between the top side and the underside of the photoresist layer. This would lead to significantly skew holes in the corner of the exposure field and would not be acceptable. In order to afford a further understanding of the solutions below, the relationship between the diameters of the individual lens element groups and the correction of the Petzval sum will also be discussed here.

[0107] In the case of a microlithographic projection system, the Petzval sum necessarily has to be corrected in order to be able to ensure the required correction of the aberrations across the entire image field.

[0108] Figures 4A to 4C schematically illustrate a projection system as a two-convexity system, namely comprising a converging lens element L1 , a diverging lens element L2 and a further converging lens element L3, for instance in the form of a simple triplet. The individual refractive powers of the lens elements shall be selected such that the Petzval sum of the system is corrected.

[0109] The intention then is to reduce the diameter of the first convexity. This can be achieved by the refractive power of the first group being significantly increased (Figure 3B). However, this hugely influences the correction of the Petzval sum, which will then be undercorrected since too much positive refractive power is present in the system.

[0110] The only possibility for correcting the Petzval sum again in this simplified mode of consideration is to reduce the refractive power of the third lens element group. However, this necessitates increasing the diameter of this lens element group in order to be able to make the same contribution to the total refractive power of the system. That is illustrated schematically in Figure 4C.

[0111] Consequently, a change in diameter of one of the two convexities causes an opposite change in diameter of the other convexity.

[0112] A teaching derivable therefrom states that the CHV is correctable only if the diameter of the front convexity, nearer the object, is relatively small. This has the consequence that the diameter of the back convexity, nearer the image, is greater than that of the front convexity. To put it another way: If both convexities (in the present case here of NA 0.8 or 0.85) have identical diameters, the CHV is undercorrected. Reducing the diameter of the first convexity results in a situation where the CHV is corrected. Further reduction means that the CHV will be overcorrected.

[0113] Figures 5A and 5B elucidate the relationships once again in a different way. Figure 5A shows, in a wavelength-time diagram (l / t diagram), the temporal sequence of individual pulses, represented by vertical double-T lines. The centroid wavelengths of the individual pulses vary substantially according to a sinusoidal time function. In accordance with the pulse frequency fPof the laser, there is a temporal pulse spacing 1 / fPbetween directly successive pulses. The spectral bandwidth of the pulses is approximately 0.35 nm and is represented more than proportionally by the line length. The spectral bandwidth AA of the wavelength range WLB in which the pulse wavelengths vary is approximately 35 pm and is thus greater than the spectral pulse width by two orders of magnitude. The spectral bandwidth AA corresponds to the amplitude of the illustrated sine function. The wavelengths of the pulses vary periodically with a wavelength alternating frequency fww. the reciprocal value 1 / fww of which corresponds to the period length of the sine function on the time axis.

[0114] Figure 5B schematically illustrates a cross-section through the resist on the wafer. At the (“green”) main wavelength (solid lines), the best focusing plane lies approximately in the centre of the resist, the “red” focusing plane (long dashed lines) is on the underside of the resist, and the “blue” (short dashed lines) is on the top side. At the field edge, an uncorrected CHV results in “skew” holes. As an acceptable limit value for the correction of the CHV, what is applicable in this application is approximately the order of magnitude

[0115] In some cases it may possibly be sufficient if the condition CHV < 3 pm / nm holds true, and it is particularly preferred if CHV < 1 pm / nm.

[0116] Hereinafter, with reference to Figures 6 to 11 , an explanation is given of various exemplary embodiments for projection lenses which can be used in the context of the invention claimed here in order to enable multi-focal imaging (MFI). In MFI by way of wavelength shifting, the longitudinal chromatic aberration of projection systems is used to shift the best focusing plane through a “thick” photoresist layer during exposure by means of detuning the laser wavelength.

[0117] In the following description of projection lenses, the term "optical axis" OA 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, i.e. in the region of a stop position suitable for mounting 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 is taken to mean the paraxial marginal ray height and the paraxial chief ray height, respectively (not discernible in drawings). 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.

[0118] 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.

[0119] Tables 6, 6A to 11 , 11 A summarize the specification of the respective design in tabular form. The "SURF" column indicates the number of a refractive surface or surface with a different characteristic, the "RADIUS" column indicates the radius r of the surface (in mm), the "THICKNESS" column indicates the distance d of the surface from the following surface (in mm), and the "MATERIAL" column indicates the material of the optical components. Columns "INDEX1", "INDEX2" and "INDEX3" indicate the refractive index of the material at the wavelengths XO (INDEX1), XO - 1 nm (INDEX2) und XO + 1 nm (INDEX3) ( O = design wavelength). The "SEMIDIAM" column shows the usable free radii or the half free optical diameters of the lens elements (in mm) or 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: p(h)=[((1 / r)h2) / (1+SQRT(1-(1+K)(1 / r)2h2))]+C1*h4+C2*h6+....

[0120] The reciprocal value (1 / r) of the radius indicates the surface curvature, and h indicates the distance of a surface point from the optical axis (i.e. the ray height). Thus, p(h) indicates the sagittal height, i.e. the distance of the surface point from 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".

[0121] 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 order from the object plane OS to the image plane IS, and so, for example, the lens element L1 is the first lens element directly following the object plane. Not all lens elements have been provided with a reference sign for reasons of clarity.

[0122] First exemplary embodiment (N802; A=193)

[0123] Figure 6 shows a schematic meridional lens element sectional view of a first exemplary embodiment of a dioptric projection lens PO-1 (designation N802) 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.

[0124] The projection lens is provided as an imaging system with a reducing effect, at a design wavelength of approximately 193 nm, 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, i.e. without producing an intermediate image, and with a reduced scale, specifically with the scale of -1 :4 (imaging scale p = -0.25).

[0125] This is a pure quartz design with NA 0.85. The wafer-side field size corresponds to a typical scanner field of 26 x 8 mm2. Given an imaging scale of ft = —0.25 this means that the object circle has a semidiameter of OBH = 54.41 mm. Within this object circle, the aberrations of the imaging are sufficiently corrected for the lithographic exposure process.

[0126] Between the object plane and the image plane, the only pupil plane of the imaging system lies where the chief ray OR of the optical imaging intersects the optical axis OA. The aperture stop AS of the system is mounted in the region of the pupil plane. Therefore, the position suitable for mounting the aperture stop is also referred to as stop position BP here.

[0127] A stop region extends around the stop position, the condition | CRH I 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 here is greater than the chief ray height. The optical set-up can be characterized as follows. A first lens element group LG1 with negative refractive power, formed by two lens elements L1 and L2 in the example, directly follows the object plane OS. Lens element L1 is a biconcave negative lens element and L2 is a further biconcave negative lens element. The first lens element group thus has more lens elements having negative refractive power (namely two) than lens elements having positive refractive power (namely zero). By increasing the divergence, the first lens element group 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.

[0128] A second lens element group LG2 with positive refractive power directly follows the first lens element group LG1. This second lens element group comprises the four lens elements L3 to L6, which each 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.

[0129] A third lens element group LG3 with negative refractive power directly follows the second lens element group LG2. This third lens element group comprises the four lens elements L7 to L10 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. Lens elements L7, L8 and L10 are each biconcave negative lens elements, whereas lens element L9 is a biconvex positive lens element.

[0130] The third lens element group LG3 is followed by a fourth lens element group LG4 having positive refractive power and two lens elements L11 to L12, which are each positive meniscus lens elements having concave entrance surfaces. The lens elements of the fourth lens element group are arranged between the third lens element group LG3 and the stop position suitable for mounting an aperture stop AS.

[0131] In the design, a plane plate PP having no refractive power is provided between the third and fourth lens element groups, and imaging properties can possibly be manipulated with the aid of said plane plate.

[0132] 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 L13 to L16 exhibiting positive refractive power and embodied in the form of meniscus lens elements which are concave on the image side. A plane-parallel plate PL almost or completely without refractive power is provided, as an optical and mechanical termination of the projection lens, between the last lens element L16 with significant refractive power 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 negative third lens element group LG3 between a near-object first convexity (at LG2) and a near-image second convexity (at LG4 and LG5). This design as a single-waist system contributes to the Petzval correction. The first convexity lies in LG2, and the second convexity lies in LG4 and LG5; therefore, the diameters are designated by D2and D4 5

[0133] All the lens elements consist of synthetic fused silica (SiO2), and so a single-material system is present.

[0134] The projection lens contains seven aspherical rotationally symmetrical lens element surfaces (aspheres) that assist in attaining the necessary aberration level, namely the lens element surfaces SRF 2, 4, 13, 20, 25, 33 and 35.

[0135] In all the exemplary embodiments, a photoresist layer composed of a resist material with the hypothetical refractive index n = 1.70 is present in the image plane. The image then arises at a depth of 500 nm in this material and the defocus of ± 500 nm takes place exclusively in this material. This is important in particular in order to be able to correctly assess the spherical aberration that arises at the defocus.

[0136] The maximum optically free diameter of the first convexity is D2= 233.4 mm, and that of the second convexity is D4 5= 300.9 mm. Both convexities thus have a significantly different diameter. For the normalized diameter ratio

[0137] A = 100 the following thus holds true: A = 1.21.

[0138] Ray tracing shows that the transverse chromatic aberration at the edge of the object circle is nm

[0139] CHV = —0.93— i.e. the intersection point of the chief ray through the image plane varies maximally by just less than one nm in terms of height given a detuning of the laser by 1 pm. Furthermore, the variation of the wavefront tilt (Zernike Z3) with the wavelength can be calculated d.Z3nm nmas well. Said tilt, too, with — UA = 0.15— P'ILlies significantly below the threshold of 1~ P'IL.

[0140] Second exemplary embodiment (N803): The second exemplary embodiment (projection lens PO-2) in Figure 7 differs only little from the first exemplary embodiment. It has the same parameters (A = 193 nm, OBH = 54.41 mm, NA = 0.85) as the first exemplary embodiment. A significant difference is that the aperture stop has been shifted further in the direction of the image plane by one lens element. As a result, the stop lies in the collimated part of the imaging beam path and is thus less sensitive as far as the tolerances are concerned. In this regard, for instance, a gravity-dictated sag of the stop blades affects the numerical aperture to a significantly lesser extent.

[0141] The maximum optical free diameters of the lens elements are D = 224.9 mm in the first convexity and D45= 297.4 mm in the second convexity. Both convexities thus have a significantly different diameter. For the normalized diameter ratio A, the following thus holds true: A = 1.18. The nm transverse chromatic aberration is CHV = —0.59“ and the variation of the wavefront tilt with the dZ3nmwavelength is — UA = -0.34— P,IL

[0142] Third exemplary embodiment (N804):

[0143] The third exemplary embodiment (Figure 8) arose from the first example. The stop position is situated in the distinctly non-collimated part of the design.

[0144] Overall, the third exemplary embodiment has fewer optical parts than the first exemplary embodiment. Inter alia, the plane-parallel plate between LG3 and LG4 was dispensed with.

[0145] In order to obtain more degrees of freedom of design, the almost plane-parallel plate of the last optical part in the first exemplary embodiment (Figure 6) was split into a negative lens element with strong refractive power and a positive lens element with strong refractive power. This has made it possible to save a further lens element in the first convexity of the design.

[0146] Moreover, this design has a so-called “bi-aspherical” lens element L1. Here both lens element surfaces are aspherical. However, this property has no influence on the correction of the transverse chromatic aberration.

[0147] The maximum optical free diameters of the lens elements are D = 233.0 mm and in the second convexity D4 5= 300.9 mm. Both convexities thus have a significantly different diameter. For the normalized diameter ratio A, it holds true that A = 1.21. The transverse chromatic aberration is nm Z3nm

[0148] CHV = -0.90— Pf,Land the variation of the wavefront tilt with the wavelength is — CIA = 0.34“ Pf,L Fourth exemplary embodiment (N801):

[0149] The fourth exemplary embodiment (Figure 9) is designed for the wavelength A = 248 nm.

[0150] For the same object circle OBH = 54.41 mm the fourth exemplary embodiment has a somewhat smaller numerical aperture of NA = 0.80. This is achieved with 17 optical parts (16 lens elements and one plane plate), divided once again among five lens element groups. This is a pronounced single-waist system or two-convexity system. The aperture stop lies in the collimated part of the design.

[0151] In the design, a plane plate for manipulating imaging properties during operation is again present between LG3 and LG4.

[0152] The maximum optically free diameters of the lens elements are somewhat smaller than in the preceding, higher-aperture exemplary embodiments and are

[0153] £>2 = 216.4 mm in the first convexity and D4 5= 265.0 mm in the second convexity. For the normalized diameter ratio A, the following holds true: A = 1.20. The transverse chromatic nm dZ3 aberration is CHV = -0.32— and the variation of the wavefront tilt with the wavelength is — CIA nm

[0154] = -0.62-

[0155] Fifth exemplary embodiment (N805):

[0156] The exemplary embodiments up to this point have always involved single-material systems in which the correction of the transverse chromatic aberration was achieved by the reduction of the chief ray height in the converging lens elements of the first convexity and hence the reduction of their diameters. In order to meet the Petzval condition, for this purpose it was essential for the diameter of the second convexity to increase.

[0157] A different variant of the correction shall be presented in the following two exemplary embodiments. For this purpose, optical material having a lower dispersion is used in the converging lens elements of the first convexity which are responsible for the transverse chromatic aberration. In particular, fluorite (CaF2) is suitable here. The latter has a lower dispersion than fused silica and, consequently, with the same refractive power of the lens element, leads to less pronounced splitting of the “blue” and “red” chief rays in the first convexity. Figure 10 illustrates the fifth exemplary embodiment PO-5. The design emerged from the second exemplary embodiment. It has the same specifications as far as wavelength (A = 193 nm), object circle (OBH = 54.41 mm), numerical aperture (NA 0.85) and stop position are concerned.

[0158] In the first convexity, the quartz has been replaced by fluorite in the lens element L5 with the strongest refractive power, this lens element simultaneously having the largest chief ray height. The fluorite lens element is illustrated with hatching in the lens element sectional view.

[0159] This measure makes it possible to slightly increase the diameter of the first convexity, without causing the CHV to increase. The lower dispersion of fluorite compared with quartz now again allows a larger chief ray height, without adversely affecting the CHV.

[0160] With an increase in the diameter of the first convexity, the diameter of the second convexity and thus the maximum lens element diameter of the design can then be significantly reduced.

[0161] This results in the two maximum optically free diameters D2= 240.5 mm (first convexity) and D4 5= 265.9 mm (second convexity). For the normalized diameter ratio A, the following holds true: A = 1.41.

[0162] It is evident that here the value for A lies outside that range which should be complied with for single-material systems in order to obtain sufficiently small values for CHV. Nevertheless, the nm dZ3nmprojection lens is suitable for MFI. The resulting values are CHV = —0.47— and — = —0.39—

[0163] Sixth exemplary embodiment (N806):

[0164] The measures of the fifth exemplary embodiment have already made it possible to reduce the maximum lens element diameter very greatly from D2= 300 mm to D2= 26 mm, but this diameter is still significantly greater than the maximum diameter in the first convexity. A further adjustment would once again lead to a larger diameter of the first convexity and thus to an increase in the CHV.

[0165] That is unless a further converging lens element in the first convexity is configured as a fluorite lens element. This is precisely what has been carried out in the sixth exemplary embodiment (Figure 11). What has now been possible to achieve is that both convexities have approximately the same and thus globally minimal diameter. This results in the following parameters: D2= 252.8 mm, D4 5= 255.9 mm. For the normalized nm diameter ratio A, the following holds true: A = 1.54. This results in the values CHV = —0.88— and

[0166] — d = -0.1

[0167] The use of the second material in the fifth and sixth exemplary embodiments does make it possible to attain the chromatic magnification aberration in conjunction with smaller maximum lens element diameters; on the other hand, disadvantages arise elsewhere. In this regard, for example, the compensation of a change in air pressure which leads to aberrations in the imaging is no longer attainable solely by way of a detuning of the exposure wavelength, but rather would require a complete manipulator concept with displaceable lens elements (Z-manipulators).

[0168] Furthermore, the use of fluorite as material is disadvantageous as far as costs and processability of the optical parts are concerned. All this needs to be weighed against the advantage of the more compact design.

[0169] The use of the second material only indirectly affects the longitudinal chromatic aberration. Owing to the possibility of a more compact second convexity and hence the stop region, the longitudinal chromatic aberration that arises there is significantly smaller than in the case of the first exemplary embodiments. Some properties of the exemplary embodiments are compiled in an overview in Table 1 below. Tables 6, 6A to 11 , 11A show the design data of the exemplary embodiments.

[0170] Table 1 Table 6 (N802) Table 6A (N802)

[0171] Table 7A (N803)

[0172] Table 8A (N804) Table 7 (N803) Table 8 (N804) Table9(N801) Table 9A (N801)

[0173] Table 10A (N805)

[0174] Table 11A (N806) Table 10 (N805) Table 11 (N806)

Claims

Patent Claims1 . Projection exposure method for exposing a 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 radiation from a wavelength range around a design wavelength, comprising: coating the substrate (SUB) with a radiation-sensitive photoresist layer (RS) having a layer thickness (SD); providing a projection lens (PO) which produces a focus in a design focus position (FOCO) at the design wavelength and offset focus positions from an axially extended focus region (AFOC) around the design focus position (FOCO) for other wavelengths from the wavelength range, arranging the substrate in relation to the projection lens in such a way that the design focus position lies in the region of the photoresist layer; exposing the substrate coated with the photoresist layer with the image of the pattern by means of the projection lens (PO), wherein within an exposure time interval under the control of a control unit, different wavelengths from the wavelength range around the design wavelength are used in such a way that an axial extent of the focus region is at least equal to the magnitude of the layer thickness, wherein the projection lens used is a dioptric projection lens (PO) designed for a design wavelength of < 260 nm and having a reducing imaging scale B, which lens is configured as a single-waist system and has along an optical axis (AX) between the object plane and the image plane: a first lens element group (LG1) having negative refractive power and directly following the object plane; a second lens element group (LG2) having positive refractive power and directly following the first lens element group; a third lens element group (LG3) having negative refractive power and directly following the second lens element group in order to generate a waist (TL) around a region of minimal marginal rays heights between the object plane (OS) and the image plane (IS); a fourth lens element group (LG4) having positive refractive power and directly following the third lens element group; and a fifth lens element group (LG5) having positive refractive power and directly following the fourth lens element group, wherein the first lens element group (LG1) has more lens elements having negative refractive power than lens elements having positive refractive power and wherein an aperture stop is mounted between the fourth and fifth lens element groups; wherein the projection lens is optically corrected with regard to a transverse chromatic aberration CHV in such a way that in the region of the design wavelength, the imaging scale is substantially independent of the wavelength in such a way that in the case of a change in the wavelength by 1 picometre, a position of an image point associated with an object point varies laterally by at most 2 nm for all image points within an image field.

2. Projection exposure method according to Claim 1 , characterized in that the substrate is coated with a photoresist layer having a layer thickness of more than 1 pm, wherein preferably the layer thickness is in the range of 5 pm to 20 pm.

3. Projection exposure method according to Claim 1 or 2, characterized in that the wavelength of the radiation is varied around the design wavelength by at least ± 20 pm during the exposure time interval.

4. Projection exposure method according to any of the preceding claims, characterized in that an axial position of the pattern and / or an axial position of the substrate remain(s) constant during the exposure time interval.

5. Projection exposure method according to any of the preceding claims, characterized in that a projection lens having an object height OBH and an image-side numerical aperture NA is used, wherein(i) all transparent optical parts are fabricated from the same optical material, in particular from synthetic fused silica;(ii) the second lens element group (LG2) has a maximum optically free lens element diameter D2and the fourth and fifth lens element groups (LG4, LG5) have a maximum optically free lens element diameter D4 5and(iii) for a normalized diameter ratiothe following condition holds true:1.05 < A < 1.

346. Projection exposure method according to any of the preceding claims, characterized in that the condition 1.10 < A < 1.30, in particular the condition 1.15 < A < 1.25, holds true.

7. Projection exposure method according to any of Claims 1 to 4, characterized in that a projection lens is used wherein lens elements of the projection lens are fabricated from at least two or exactly two different materials, wherein preferably a majority of the lens elements are produced from synthetic fused silica and a smaller number, in particular one or two or three lens elements, are produced from an optical material having lower dispersion, in particular calcium fluoride, wherein preferably the second lens element group has one or more converging lens elements composed of a material having a lower dispersion than fused silica.

8. Projection exposure method according to Claim 7, characterized in that a majority of the lens elements consist of a first material having an Abbe number v-, and at least one lens element consists of a second material having an Abbe number v2> v^ wherein Hj is the maximum ray height of the chief ray at the location of the i-th lens element, wherein for all lens elements j of the second material the absolute value of the chief ray height, i.e. Hj, is greater than 0.75 x OBH.

9. Dioptric projection lens for imaging a pattern arranged in the region of an object plane of the projection lens into an image plane of the projection lens, the image plane being optically conjugate to the object plane, with radiation from a wavelength range around a design wavelength of < 260 nm on a reducing imaging scale B, wherein the projection lens produces a focus in a design focus position (FOCO) at the design wavelength and offset focus positions from an axially extended focus region (AFOC) around the design focus position (FOCO) for other wavelengths from the wavelength range, wherein wherein the projection lens is configured as a single-waist system having an object height OBH and an image-side numerical aperture NA and has along an optical axis (AX) between the object plane and the image plane: a first lens element group (LG1) having negative refractive power and directly following the object plane; a second lens element group (LG2) having positive refractive power and directly following the first lens element group; a third lens element group (LG3) having negative refractive power and directly following the second lens element group in order to generate a waist (TL) around a region of minimal marginal ray heights between the object plane (OS) and the image plane (IS); a fourth lens element group (LG4) having positive refractive power and directly following the third lens element group; and a fifth lens element group (LG5) having positive refractive power and directly following the fourth lens element group, wherein the first lens element group (LG1) has more lens elements having negative refractive power than lens elements having positive refractive power and wherein an aperture stop is mounted between the fourth and fifth lens element groups; characterized in that(i) all transparent optical parts are fabricated from the same optical material, in particular from synthetic fused silica;(ii) the second lens element group (LG2) has a maximum optically free lens element diameter D2and the fourth and fifth lens element groups (LG4, LG5) have a maximum optically free lens element diameter D4 5and(iii) for a normalized diameter ratioA = 100 •the following condition holds true:1.05 < A < 1.3410. Projection lens according to Claim 9, characterized in that the condition 1.10 < A < 1.30, in particular the condition 1.15 < A < 1.25, holds true.

11. Projection lens according to Claim 9 or 10, characterized in that the projection lens is optically corrected with regard to a transverse chromatic aberration CHV in such a way that in the region of the design wavelength, the imaging scale is substantially independent of the wavelength in such a way that in the case of a change in the wavelength by 1 picometre, a position of an image point associated with an object point, for all image points within an image field, varies laterally by at most 2 nm within the entire image field.

12. Projection lens according to either of Claims 10 and 11 , characterized in that the projection lens has an image-side numerical aperture NA < 1 , wherein preferably the condition 0.6 < NA < 0.95 holds true.

13. Projection lens according to any of Claims 10 to 12, characterized in that the first lens element group (LG1) has at least two lens elements (L1 , L2) having negative refractive power.

14. Projection exposure apparatus for exposing a substrate arranged in the region of an image plane of a projection lens with at least one image of a pattern of a mask arranged in the region of an object plane of the projection lens with radiation from a wavelength range around a design wavelength of < 260 nm, comprising: a wavelength-variable light source (LS) for emitting a sequence of radiation of different wavelengths from the wavelength range around the design wavelength; an illumination system (ILL) for receiving the radiation and for generating an illumination radiation directed onto the mask (M); a projection lens (PO) for generating an image of the pattern in the region of the image surface (IS) of the projection lens, characterized in that the light source is configured to emit different wavelengths from the wavelength range around the design wavelength under the control of a control unit within an exposure time interval, and the projection lens (PO) is configured according to any of Claims 1 to 13.

Images