WO2008122410A2

WO2008122410A2 - Optical correction element and method for the correction of temperature-induced imaging aberrations in optical systems, projection objective and projection exposure apparatus for semiconductor lithography

Info

Publication number: WO2008122410A2
Application number: PCT/EP2008/002693
Authority: WO
Inventors: Daniel Kraehmer; Olaf Conradi; Aurelian Dodoc; Ulrich Loering
Original assignee: Carl Zeiss Smt Ag
Priority date: 2007-04-05
Filing date: 2008-04-04
Publication date: 2008-10-16
Also published as: DE102008000968A1; TW200903184A; WO2008122410A3

Abstract

The invention relates to a method for the correction of temperature-induced imaging aberrations of an optical system. In this case, the correction is effected using a liquid layer (24) arranged in the optical system in such a way that the imaging aberrations are compensated for by an inhomogeneous temperature distribution being formed in the liquid layer (24) through absorption of optical useful radiation. Furthermore, the invention relates to an optical correction element (21) for application in said method, and to a projection objective (7) and a projection exposure apparatus (1) having the optical correction element (21) according to the invention.

Description

Optical correction element and method for the correction of temperature-induced imaging aberrations in optical systems, projection objective and projection exposure apparatus for semiconductor lithography

The invention relates to an optical correction element for an optical system, to a method for the correction of temperature-induced imaging aberrations in optical systems, and to a projection exposure apparatus for semiconductor lithography in which the optical correction element and the method are used.

Figure 1 illustrates a projection exposure apparatus 1 for semiconductor lithography according to the prior art. This apparatus serves for the exposure of structures onto a substrate coated with photosensitive materials, which substrate is generally predominantly composed of silicon and is referred to as wafer 2, for the production of semiconductor components, such as e.g. computer chips.

In this case, the projection exposure apparatus 1 essentially comprises an illumination device 3, a device 4 for receiving and exactly positioning a mask provided with a structure, a so-called reticle 5, which is used to determine the later structures on the wafer 2, a device 6 for the mounting, movement and exact positioning of precisely said wafer 2, and an imaging device, namely a projection objective 7, with a plurality of optical elements 8 which are mounted by means of mounts 9 in an objective housing 10 of the projection objective 7.

In this case, the basic functional principle provides for imaging the structures introduced into the reticle 5 onto the wafer 2; the imaging is generally implemented in demagnifying fashion. After exposure has been effected, the wafer 2 is moved further in the arrow direction, such that a multiplicity of individual fields, each having the structure prescribed by the reticle 5, are exposed on the same wafer 2. On account of the step-by-step advancing movement of the wafer 2 in the projection exposure apparatus 1, the latter is often also referred to as a stepper.

The illumination device 3 provides a projection beam 11 required for the imaging of the reticle 5 on the wafer 2, for example light or a similar electromagnetic radiation. A laser or the like can be used as a source for this radiation. The radiation is shaped in the illumination device 3 by means of optical elements in such a way that the projection beam 11, upon impinging on the reticle 5, has the desired properties with regard to diameter, polarization, shape of the wavefront and the like.

By means of the beams 11, an image of the reticle 5 is generated and transferred to the wafer 2 in correspondingly demagnified fashion by the projection objective 7, as has already been explained above. The projection objective 7 has a multiplicity of individual refractive, diffractive and/or reflective optical elements such as e.g. lenses, mirrors, prisms, terminating plates and the like.

The optical elements 8 which are formed e.g. as lenses and are arranged in the projection exposure apparatus heat up during the lithography process, such that image aberrations occur which can make it more difficult or even impossible to effect imaging of the desired structures. The heating of the lenses is caused by the absorption of the useful radiation having a specific wavelength, used for the exposure, within the lens material and the layer materials, e.g. within antireflection layers. In order that despite the described heating of the optical elements 8 used, it is possible to effect imaging of the desired structures with the required precision, a correction by means of manipulators such as e.g. manipulator lenses is usually performed. Said lenses can be for example displaced, tilted or else deformed. This causes an active alteration of the lenses, such that a correction effect arises as a result of the altered position of said lenses or the altered lens geometry within the optical design of the projection objective. Manipulators which lead to a correction effect as a result of an altered refractive index within the lens are also known.

However, only specific image aberrations can be corrected by means of the active correction. Wavefront aberrations which can be described by functions with higher radial and azimuthal polynomials are generally difficult to correct. The mechanical deformations can generally only be established for lower orders, such that a comprehensive correction of the wavefront is impossible. Wavefront aberration here denotes the phase deviation from an ideal spherical wave, which in the ideal case converges at the desired image point.

One approach for solving the problem described is addressed in the international patent application WO 2006/053751 A2, attributed to the applicant. Said document describes various approaches for correcting in particular also the thermally induced image aberrations described above.

The object of the present invention is to specify a method and devices by means of which thermally induced imaging aberrations in optical systems, in particular in projection objectives for semiconductor lithography, can be corrected rapidly, simply and efficiently. - A -

This object is achieved by means of the method described in Claim 1, and also by means of the devices having the features presented in Claims 15, 33 and 37. The subclaims relate to advantageous variants or embodiments of the invention.

The method according to the invention for the correction of temperature-induced imaging aberrations of an optical system achieves the above-described correction using a liquid layer arranged in the optical system. In contrast to the methods known from the prior art, the compensation of the imaging aberrations is achieved by virtue of an inhomogeneous temperature distribution being formed in the liquid layer through absorption of optical useful radiation. In this case, optical useful radiation is understood to mean that optical radiation which passes through the optical elements associated with the optical system when the optical system is used as intended. By way of example, the optical useful radiation of a projection objective in semiconductor lithography is the ultraviolet radiation used for imaging the structures of the reticle onto the wafer.

By virtue of the fact that the optical useful radiation itself is used for the correction, a self-compensation in particular of the temperature-induced imaging aberrations of the optical system can be achieved in a simple and efficient manner. Assuming that the intensity distribution of the optical useful radiation used in the liquid layer corresponds to the intensity distribution in most of the rest of the optical elements of the system and that the temperature distribution generated by the absorption in the liquid layer thus substantially corresponds to the intensity distribution of the optical useful radiation, the assumption can be made that precisely in the regions in which the temperature-induced imaging aberrations are maximal, the correction effect of the liquid layer is also maximal.

The correction effect of the liquid layer is essentially based on the fact that on account of the temperature change in the liquid layer, the refractive index also changes locally. This change in the refractive index results in a local change in the course of the wavefront of the incident optical radiation, which precisely compensates for the induced deviations from the ideal wavefront given a suitable choice of the properties of the liquid layer.

In this case, the liquid layer can be laterally extended substantially perpendicular to the optical axis of the system and be formed in particular as a liquid plane-parallel plate.

In an alternative embodiment of the invention, the liquid layer can be formed as a liquid lens having at least one curved surface.

The thickness of the plane-parallel plate or the geometrical parameters of the liquid lens can be kept constant during the operation of the optical system since the compensation of the imaging aberrations is not achieved by means of a change in the geometry, but rather, as already described above, by means of a change in the distribution of the refractive index in the liquid. In other words, e.g. two plane-parallel plates used as delimiting elements are arranged in a manner fixed with respect to one another.

Water, in particular, has proved worthwhile as a liquid for forming the liquid layer according to the invention. Water has a large negative temperature coefficient of the refractive index dn/dT in the region of -100*10^~6/K at a wavelength of 193 nm. This value has a magnitude approximately five times as large as in the case of quartz, but exhibits an opposite sign. The aforementioned differences in the material constants can thus advantageously be utilized for the compensation of the imaging aberrations in an optical system. Since the absorption of electromagnetic radiation having the aforementioned wavelength within the water is more than one order of magnitude greater than in the case of quartz, a local heating within the water will also be rapidly established.

Other liquids are also appropriate as an alternative to the use of water; thus, e.g. interesting candidates are the so-called high-index liquids. Examples that may be mentioned include cyclohexane having a 193-nm refractive index of 1.57 (close to the refractive index of quartz glass) , or decalin having a refractive index of approximately 1.65.

In one advantageous variant of the invention, the optical properties of the liquid layer can be set within a certain range. In this way it is possible to compensate for fluctuations of the absorption in optical elements such as e.g. lenses or layers which can adversely influence the effectiveness of a self- compensating liquid layer. One advantageous possibility for such setting consists in altering the absorption properties of the liquid layer by adding e.g. salts such as e.g. sodium chloride, calcium chloride or potassium iodide, all of which have a very good solubility in water. In general, the absorption will rise with the concentration of the salts, such that it is advantageous always to begin with a certain starting concentration and, during a calibration step for the system, to alter the absorption of the liquid layer until the self-compensation is set optimally.

The liquid layer formed as a liquid plane-parallel plate can be arranged in particular between two substantially plane-parallel plates which contain quartz glass or CaF₂ or comprise the materials mentioned. The two substances mentioned are established and well-controlled materials which are widely used in optical systems and the properties of which are well known .

For the case where the plane-parallel plates contain quartz glass, it is advantageous if the ratio of the thickness of the liquid layer to the summed thickness of the two substantially plane-parallel plates lies between 0.2 and 1.0, preferably between 0.25 and 0.75, particularly preferably between 0.3 and 0.5.

If the plane-parallel plates contain CaF₂ it is advantageous if the ratio of the thickness of the liquid layer to the summed thickness of the two substantially plane-parallel plates lies between 0.1 and 2.0, preferably between 0.3 and 1.5, particularly preferably between 0.5 and 1.

In this case, the summed thickness of the two substantially plane-parallel plates can lie in particular within a range of between 1.5 mm and 15 mm, preferably between 2 mm and 12 mm.

In order to improve or ensure the functionality of the optical correction element according to the invention, it may be advantageous to reduce or prevent the convection of the liquid within the liquid layer. The convection would have the effect of levelling out the temperature differences along the lateral extent of the liquid layer, whereby the correction effect would be reduced. In order to reduce the disadvantageous effect described, additional elements can be arranged in the liquid layer. In this case, the aforementioned additional elements create in particular honeycomb-shaped or rectangular partial spaces which are closed off relative to one another and which subdivide the liquid layer.

In one advantageous variant of the invention, the refractive index of the liquid is adapted to the refractive index of the surrounding material in the cold state. What is thereby achieved is that the structure formed by the additional elements, in the cold state, has an optically homogeneous appearance and does not cause any disturbing optical path differences.

In a further embodiment of the invention, additional temperature-regulating elements are present for modifying the temperature distribution in the liquid layer. In this way, the liquid layer can be heated in a targeted and spatially local fashion for the purpose of fine adaptation and for the purpose of enhancement of flexibility. This can be done in a manner adapted to the respective conditions of use during the operation of the optical correction element. In this case, it has in particular proved worthwhile to effect heating electrically via the resistance of thin wires as resistance heating elements which are fitted in the form of a grid or in an alternative geometrical arrangement on the inner side of the surfaces of the delimiting elements which adjoin the liquid layer. Depending on the number of wires, it is thus possible to establish a different spatial resolution of the temperature distribution that can be set.

An alternative possibility with regard to heating is to utilize infrared irradiation. In this case, it should be taken into consideration that e.g. water is sufficiently transparent only in a relatively narrow spectral range for optical applications and is very highly absorbent in the spectral range above approximately 2 μm. For optimized coupling of the IR radiation into the water, the surrounding material should not be too highly absorbent at the wavelength under consideration.

In this case, the IR radiation can be coupled in by means of a plurality of optical systems fitted laterally with respect to the correction element. Furthermore, direct coupling in using optical waveguides such as e.g. fibres is conceivable.

The method described above and the optical correction element likewise described can be applied particularly advantageously in a projection exposure apparatus for semiconductor lithography.

An advantageous choice for the location of the correction element according to the invention in the projection exposure apparatus in this case consists in arranging the optical correction element at a distance from a pupil plane which corresponds to a subaperture ratio of greater than 0.7.

For an optical system which images an object field having a maximum object height onto an image field under a given aperture, the subaperture ratio is divided by

\R - H\ / ( \R - H\ + \H\ ) ,

where, proceeding from an object point of maximum object height, R is the marginal ray height and H is the principal ray height, and these ray heights are measured in a given plane which is parallel to a pupil plane of the optical system.

The subaperture ratio assumes values of between 0 and 1. The subaperture ratio has the value 1 in each pupil plane of the optical system and the value 0 in each field plane of the optical system.

This definition is applied in particular for projection optics for semiconductor lithography since these are corrected for a maximum object height and maximum aperture, depending on the projection optic. As a result, a maximum object height and an aperture are naturally assigned to such projection optics as optical systems.

In general, the greatest contributions to the aberrations of a projection exposure apparatus originate from so-called pupil aberrations, which can be corrected the most effectively in the region of the pupil. Furthermore, the region of the pupil plane is a particularly favourable location for performing an image aberration correction by means of optical correction elements since, through measures in the pupil plane, it is possible to achieve identical types of modifications of the imaging in each location of an image plane.

As an alternative or in addition, a further correction element according to the invention can be arranged near the field or intermediately between field and pupil, for example, in order to realize an additional correction effect.

The correction potential of the method described and of the optical element according to the invention is manifested particularly in projection exposure apparatuses having illumination devices suitable for generating a dipole illumination. In these cases, the assumption can be made of a particularly inhomogeneous distribution of the absorbed optical power in the optical elements of the projection exposure apparatus. The aforementioned correction potential can be increased even further by virtue of the optical useful radiation radiating through the correction element according to the invention a number of times, in particular twice.

Basic considerations with respect to the invention and also some exemplary embodiments are presented below with reference to the figures.

In the figures:

Figure 1 shows a projection objective for semiconductor lithography according to the prior art (cf. introduction to the description) ;

Figure 2 shows an exemplary optical correction element;

Figure 3 shows a clear illustration of the principle underlying the invention;

Figure 4 shows an illustration of the corrected wavefront (field centre) in figure parts 4a) and 4b) ;

Figure 5 shows a first exemplary embodiment of a projection objective in which two correction elements according to the invention are arranged;

Figure 6 shows an alternative design of a projection objective in which two correction elements according to the invention are likewise used;

Figure 7 shows a further example of a projection objective in which the correction elements according to the invention are applied;

Figure 8 shows the correction potential of different arrangements of the correction elements in figure parts 8a) and 8b) ;

Figure 9 shows an illustration of the required thicknesses of the liquid layer as a function of the thickness of the delimiting elements;

Figure 10 shows two possible variants for the configuration of the correction element in a plan view;

Figure 11 shows a projection exposure apparatus in which a correction element according to the invention is arranged in the projection objective;

Figure 12 shows a further variant of the invention, in which the delimiting elements exhibit non- plane surfaces;

Figure 13 shows an optical correction element according to the invention with temperature-regulating elements;

Figure 14 shows an additional variant for suppressing convection;

Figure 15 shows a further possibility for suppressing convection, and

Figure 16 shows a further variant of the invention in which convection is avoided.

Figure 2 shows an exemplary optical correction element 21, having the two delimiting elements 22 and 23 formed as plane-parallel plates. The liquid layer 24, which is realized as a layer of water in the present example, is arranged between the two delimiting elements 22 and 23, respectively. It goes without saying that liquids other than water, e.g. a high-index liquid such as e.g. cyclohexane or decalin, are also conceivable for the liquid layer 24. The delimiting elements 22 and 23 can comprise quartz glass or CaF₂ or else some other material. For adapting the optical properties of the liquid layer 24, the latter can contain in particular salts such as e.g. sodium chloride, calcium chloride or potassium iodide.

Figure 3 clearly illustrates the principle underlying the invention. In its left-hand figure part a) , Figure 3 shows the disturbed wavefront of an inhomogeneously heated lens element 25 near the pupil. In this case, the inhomogeneous heating of the lens element 25 may be attributable to a dipole illumination. For comparison, the right-hand figure part b) illustrates the disturbed wavefront of the optical correction element 21 according to the invention having the liquid layer 24 and the delimiting elements 22 and 23. Figure 3 clearly reveals that the disturbance of the wavefronts is virtually identical apart from the respective sign. Consequently, a correction effect can be obtained through a suitable choice of the thickness of the liquid layer 24.

Figure 4 shows an illustration of the corrected wavefront (field centre) in figure parts 4a) and 4b) . In this case, subfigure 4a) shows the wavefronts after a correction by means of exclusively active manipulators according to the prior art, whereas Figure 4b) illustrates the wavefront after compensation by means of the correction element according to the invention. The considerably better correction of the wavefront by the correction element according to the invention can clearly be discerned.

Figure 5 shows a first exemplary embodiment of a catadioptric projection objective 7 having two folding mirrors, in which two correction elements 21 according to the invention are arranged. In this case, the correction elements 21 each comprise two plane plates not designated in any greater detail in the figure, the spacing between the plane plates being filled with water. The compensation of the wavefront altered by heating of the surrounding optical elements, not designated in any greater detail in Figure 5, is achieved in this way. It goes without saying that the correction elements 21 according to the invention can be arranged at any desired positions within the projection objective 7.

The parameters of the design illustrated in Figure 5 are summarized below in tabular form. In this case, in the usual way the consecutive numbering designates the sequence of the respective surfaces of optical elements in the system under consideration.

The following aspheric formula holds true for the parameters indicated in the table "aspheric constants": +c₂h^< ₊...

In this case, P is the sagitta of the relevant surface parallel to the optical axis, h is the radial distance from the optical axis, r is the radius of curvature of the relevant surface, K is the conic constant and Cl, C2, ... are the aspheric constants presented in the table. 03. 07. 2008

ASPHERIC CONSTANTS

Figure 6 shows an alternative design of a projection 7 in which three correction elements 21 according to the invention are used. The design of the projection objective 7 as illustrated in Figure 6 is distinguished by the fact that the beam path is led in such a way that the optical useful radiation radiates twice through the correction element 21' situated in the vertical branch of the beam path. In this case, in the course of passing through the projection objective 7, the projection beam is deflected by 90° from the original direction via the reflective elements 31, 32. In other words, the projection beam exhibits a T-shaped course when passing through the projection objective 7. It goes without saying that courses that deviate more or less from the T-shape are also conceivable. For the deflection of the projection beam it is also possible to use optical elements whose deflecting effect is not based on reflection but rather for example on diffraction effects or other optical effects.

The parameters of the design illustrated in Figure 6 are compiled below in tabular form in a manner analogous to the preceding example:

ASPHERIC CONSTANTS

Figure 7 shows a further example of a projection objective 7 in which the correction elements 21 according to the invention are applied. In this case, the correction element 21 is situated in the region of a pupil plane of the projection objective 7, said pupil plane not being designated in the drawing. In addition or as an alternative, it is possible for an optical correction element to be arranged not (only) in the region of the upper pupil plane, that is to say in the region of that pupil which is situated in the vicinity of the reticle, but rather in the region of a lower or the bottommost pupil plane, that is to say in the vicinity of the wafer. In Figure 7, in particular the region directly upstream or downstream of one of the three biconvex lenses 26 is appropriate for this. The advantage of choosing this location in the projection objective for the optical correction element 21 according to the invention is that the beam divergence is smaller in the region of the lower pupil plane than in the region of the upper pupil plane. That is to say that the angular scattering of the individual beam bundles of the projection light is smaller in the region of the lower pupil plane than in the region of the upper pupil plane. This is accompanied by a smaller variance of the optical path of the projection beam bundles through the optical correction element 21 according to the invention and, consequently, an improved manipulator effect or compensation effect. In the region of the lower pupil plane, although an overall larger thickness of the liquid layer 24 will be necessary for ensuring the compensation effect, the associated convection problems can be effectively reduced in particular with the measures described below in particular with reference to Figures 13 and 14.

The parameters of the design illustrated in Figure 7 are summarized below in tabular form:

ASPHERIC CONSTANTS

Figure 8 shows, in figure part 8a) , the correction potential of the arrangement shown in Figure 7; Figure 8b) illustrates the correction potential of an arrangement in which one correction element 21 is arranged in the vicinity of a pupil plane and another correction element is arranged in the vicinity of a field plane of the projection objective. Here the correction elements 21 in both cases each comprise two 5 mm thick SiO₂ plates with a layer of water with a thickness of 4 mm arranged between the plates.

The relation of the thicknesses of the liquid layer and of the delimiting elements for an optimum compensation effect of the correction element according to the invention will be considered by way of example below. The fundamental stipulation here is that the temperature coefficient of the refractive index (dn/dT) of the liquid used exhibits a different sign from that of the surrounding material, in particular of the material of the optical elements used in the projection objective. The relation between the thickness of the liquid layer and the summed thickness of the delimiting elements follows the following formula:

dn_glass η lens water dT 1 + A AT₁

A T¹ dⁿwater Σ ι=\ 'lass water dT

The quantity D_water/Dgiass is the relation between liquid thickness and thickness of the surrounding glass material. Furthermore, dn_gi_ass/dT and dn_water/dT designate the temperature coefficients of the refractive index of the glass material and, respectively, of the liquid used. The quantity Di*ΔTi is equal to the product of individual lens thickness and temperature rise within the lens element i. The sum over all i (with the exception of the correction element itself) identifies the contributions of all the lens elements to the overall effect.

The maximum temperature change within a plane lens element is approximately given by

This holds true for a plane element with conventional illumination, i.e. circular illumination centred about the optical axis of the element. In this case, P denotes the irradiation power, a denotes the absorption coefficient to the base e, λ denotes the thermal conductivity of the material, Hm denotes the radius of the conventional illumination, and H_lens denotes the lens height.

In the case of the liquid layer it can be assumed that the temperature within the liquid is approximately equal to the temperature of the delimiting glass surfaces. In this case, the temperature of the liquid decreases as the thermal conductivity increases (e.g. quartz -> CaF₂) . As a result, the required thickness of the liquid layer increases as the thermal conductivity increases .

A layer of water with two delimiting glass plates can be represented approximately as a homogeneous element having effective absorption a_eff and thermal conductivity λ_efff where the following hold true:

_ ' s

Figure 9 illustrates the required thicknesses of the layer of water as a function of a thickness of the delimiting elements. In this case, the glass types quartz and CaF₂ are assumed for the material of the delimiting elements. The values used are in this case Hin/Hi_en_s = 0.2, P=I W, a_water = 0.0829/cm and α_gIass = 0 (negligible in comparison with water) . The curve parameter in Figure 9 is the wavefront aberration to be compensated for in the overall objective (200 nm and 150 nm) . This can be calculated by means of the temperature change and the thicknesses of the optical elements, in particular of the lenses in the projection objective.

The relationships represented hold true for the material system quartz/water/quartz and CaF2/water/CaF2, respectively. The delimiting elements have plane surfaces in this case.

A generalization of the relationships represented above can be achieved by curved surfaces of the delimiting elements. In this case, the projection objective can be modelled as an effective average lens. This effective lens can be determined by the optical path lengths on the optical axis and along a marginal ray. From the point of view of the heating effect, the overall objective behaves equivalently to a lens having positive refractive power. In order to be able to compensate for the heating effect of a lens of this type, a liquid lens having a larger centre thickness than edge thickness should be used. This lens results from equivalent considerations to the above representation. Figure 12 illustrates some exemplary embodiments of an optical correction element 21 having delimiting elements 22 and 23 having non-plane surfaces .

A possible problem in the realization of the correction element according to the invention might arise from the fact that the desired temperature distribution is destroyed on account of convection in the liquid layer. In this case, it is advantageous for the liquid layer to be structured e.g. in honeycomb-shaped fashion substantially orthogonally with respect to the direction of the optical useful radiation, that is to say that one of the delimiting elements is structured in such a way that the liquid is "filled" into individual cells and closed off as it were by the second delimiting element.

The heat exchange via convection is greatly suppressed in this way. It is furthermore advantageous to adapt the refractive index of the liquid to the refractive index of the surrounding material of the delimiting elements. This ensures that the honeycomb structure, in the cold state, has an optically homogeneous appearance and does not cause any disturbing optical path differences .

Figure 10 shows two possible variants for the configuration of the correction element in a plan view. In this case, additional elements 33 are provided which subdivide the liquid layer (not designated in Figure 10) into individual partial spaces 34. The partial spaces 34 are formed in honeycomb-shaped fashion in the variant illustrated in Figure 10a) , whereas they are configured as rectangles, in particular squares, in the variant illustrated in Figure 10b) .

Figure 11 illustrates a projection exposure apparatus 1 in which a correction element 21 according to the invention is arranged in the projection objective 7. Furthermore, the illumination device 3 of the projection exposure apparatus 1 is designed in such a way that a dipole illumination is realized by means of the illumination device 3. In a projection exposure apparatus 1 configured in this way, the correction element 21 according to the invention exhibits its maximum correction potential on account of the extremely inhomogeneous illumination distribution.

Figure 13 shows a further variant of a correction element 21 according to the invention, which exhibits as temperature-regulating elements 40 heating wires which are distributed over the correction element 21 in grid-shaped fashion and which, upon suitable contact- connection, can generate a desired temperature distribution across the correction element 21. Figure 13 furthermore illustrates an infrared source embodied as IR laser 41, by means of which source a delimited, directional infrared beam 42 can be directed onto any desired points on the correction element 21, which leads to local heating of the correction element 21. Local cooling (e.g. using a cooling, directed fluid stream) is also conceivable.

As an alternative or in addition (not illustrated) , the additional elements 33 can also be produced as conductive elements e.g. made from a metallic material or be provided with a conductive layer running in the direction of the optical axis. In this way, given a suitable contact-connection of the additional elements 33, in a double functionality, it is possible both to reduce the convection within the liquid layer 24 and to set a desired temperature distribution in the case of using the conductive regions of the additional elements 33 in the manner of a resistance heating system.

Figure 14 shows a further variant for the suppression of convection. In this case, the liquid layer 24 is subdivided into the two partial layers 24a and 24b via the separating element 50. In this case, the partial layers 24a and 24b are arranged one behind another relative to the light path through the optical correction element 21. In this case, the respective thicknesses of the two partial layers 24a and 24b are reduced by comparison with the examples outlined above; in particular, they lie within a range of less than 3 mm, thicknesses of 2 or else 1 mm are advantageous in this case. The subdivision of the liquid layer into a plurality of thinner partial layers has the effect of reducing the convection in the partial layers 24a and 24b also in the direction of the layer plane. In this case, the correction effect of the correction element 21 is practically completely retained on account of the unchanged summed total thickness of the liquid layer 24. In this case, the variant presented has the advantage that the optical effect of the convection-reducing measures can be kept small; thus, by way of example, a plane-parallel quartz glass plate having a small optical influence can be used as the separating element 50.

Figure 15 shows a further possibility for reducing convection. In this case, a reduction of the layer thickness of the liquid layer 24 is made possible by virtue of the fact that the latter is thermally insulated from the delimiting elements 22 and 23 by means of the insulating layers 51. As has already been shown above, the required minimum thickness of the liquid layer 24 also depends, inter alia, on the extent to which there is thermal coupling between the liquid layer 24 and the delimiting elements 22 and 23, respectively. It has essentially been found that, the better the thermal coupling between the liquid layer 24 and the delimiting elements 22 and 23, respectively, an increased thickness of the liquid layer 24 becomes necessary. On account of their thermal insulation effect, the insulating layers 51 reduce the heat transfer between the liquid layer 24 and the delimiting elements 22 and 23, respectively, and thus enable a thinner liquid layer 24. In this way it becomes possible to dimension the liquid layer 24 of the order of magnitude of the values presented in connection with Figure 14 and thus, on account of the reduced thickness, largely to prevent convection also in the layer direction. In this case, the insulating layer 51 can be configured in such a way that gas bubbles are introduced in that region of the delimiting elements 22 and 23 which faces the liquid layer, the diameter of said gas bubbles being chosen such that the optical influence of the gas bubbles is largely reduced. Depending on the position of the optical correction element in the objective, bubble classes up to 5 x 0.25 shall be permitted, for example. This corresponds to a total area of 1.25 mm². According to ISO 1010-3 it is permitted to distribute this area among more bubbles of an equivalent total area as long as no accumulation occurs in this case.

A further variant - illustrated in Figure 16 - for reducing the disturbing influences caused by the convection consists in the liquid that forms the liquid layer 24 being replaced at regular intervals, and therefore in providing for a homogenization of the temperature distribution in the liquid layer 24. For this purpose, the correction element 21 is provided with the inlets and outlets 55, which ensured that the liquid layer 24 is at least partly replaced. In other words, the period of time in which the temperature distribution in the liquid layer 24 first forms without the convection exceeding a disturbing degree is utilized for the manipulator effect of the optical correction element. In this case, that time window in which, although a compensating temperature distribution has already formed in the liquid layer 24, the convection has not yet exceeded a disturbing degree can advantageously be used for the use of the manipulator. Essential parameters that influence the position and length of said time window are viscosity and thermal conductivity of the liquid and also the form of the temperature distribution in the liquid layer 24. In addition - as already explained with reference to Figure 13 - there is the possibility of influencing the temperature distribution and thus the convection in the liquid layer by means of temperature-regulating elements, that is to say by means of targeted local cooling or heating. In this respect, Figure 16 illustrates by way of example an infrared source embodied as IR laser 41, by means of which source a delimited, directional infrared beam 42 can be directed onto any desired points on the correction element 21 or the liquid layer 24, which leads to local heating of the correction element 21 or the liquid layer 24.

Claims

Patent Claims

1. Method for the correction of temperature-induced imaging aberrations of an optical system, the correction being effected using a liquid layer (24) arranged in the optical system, characterized in that the imaging aberrations are compensated for by an inhomogeneous temperature distribution being formed in the liquid layer (24) through absorption of optical useful radiation.

2. Method according to Claim 1, characterized in that the liquid layer (24) is formed as a liquid plane- parallel plate.

3. Method according to Claim 1, characterized in that the liquid layer (24) is formed as a liquid lens having at least one curved surface.

4. Method according to any of the preceding claims, characterized in that the geometrical parameters of the liquid layer (24) are kept constant during the operation of the optical system.

5. Method according to any of the preceding claims, characterized in that water or a high-index liquid such as e.g. cyclohexane or decalin is used as the liquid.

6. Method according to Claim 2, characterized in that the liquid layer (24) formed as a liquid plane-parallel plate is arranged between two delimiting elements (22, 23) formed as substantially plane-parallel plates.

7. Method according to any of the preceding claims, characterized in that the delimiting elements (22, 23) contain quartz glass or CaF₂.

8. Method according to Claim 6, characterized in that the plane-parallel plates contain quartz glass and the ratio of the thickness of the liquid layer (24) to the summed thickness of the two substantially plane- parallel plates lies between 0.2 and 1.0, preferably between 0.25 and 0.75, particularly preferably between 0.3 and 0.5.

9. Method according to Claim 6, characterized in that the plane-parallel plates contain CaF₂ and the ratio of the thickness of the liquid layer (24) to the summed thickness of the two substantially plane-parallel plates lies between 0.1 and 2.0, preferably between 0.3 and 1.5, particularly preferably between 0.5 and 1.

10. Method according to any of the preceding Claims 6-9, characterized in that the summed thickness of the two substantially plane-parallel plates lies within a range of 1.5 mm - 15 mm, preferably within a range of 2 mm - 12 mm.

11. Method according to any of the preceding claims, characterized in that soluble substances, in particular salts such as sodium chloride, calcium chloride or potassium iodide, are added to the liquid layer (24) for modifying its optical properties.

12. Method according to any of the preceding claims, characterized in that the temperature distribution in the liquid layer (24) is modified by additional temperature-regulating elements (40).

13. Method according to Claim 12, characterized in that resistance heating elements or infrared radiation sources are used as the temperature-regulating elements (40) .

14. Method according to any of the preceding claims, characterized in that the liquid layer (24) is at least partly replaced as soon as the convection that occurs in the liquid layer (24) impairs the correction effect of the liquid layer (24) beyond a certain degree.

15. Optical correction element (21) for the compensation of temperature-induced imaging aberrations in an optical system, the correction element (21) containing a liquid layer (24) formed as a liquid plane-parallel plate and arranged between two delimiting elements (22, 23) having planar surfaces, characterized in that the two delimiting elements (22, 23) are arranged in a manner fixed with respect to one another.

16. Optical correction element (21) according to Claim

15, characterized in that the two delimiting elements (22, 23) are formed as substantially plane-parallel plates .

17. Optical correction element (21) according to either of the preceding Claims 15 or 16, characterized in that the two delimiting elements (22, 23) contain quartz glass or CaF2-

18. Optical correction element (21) according to Claim

16, characterized in that the plane-parallel plates contain quartz glass and the ratio of the thickness of the liquid layer (24) to the summed thickness of the two substantially plane-parallel plates lies between 0.2 and 1.0, preferably between 0.25 and 0.75, particularly preferably between 0.3 and 0.5.

19. Optical correction element (21) according to Claim 16, characterized in that the plane-parallel plates contain CaF₂ and the ratio of the thickness of the liquid layer (24) to the summed thickness of the two substantially plane-parallel plates lies between 0.1 and 2.0, preferably between 0.3 and 1.5, particularly preferably between 0.5 and 1.

20. Optical correction element (21) according to either of the preceding Claims 18 or 19, characterized in that the summed thickness of the two substantially plane-parallel plates lies within a range of 1.5 mm - 15 mm, preferably within a range of 2 mm - 12 mm.

21. Optical correction element (21) according to any of the preceding Claims 15-20, characterized in that the liquid layer (24) contains water or a high-index liquid such as e.g. cyclohexane or decalin.

22. Optical correction element (21) according to any of the preceding Claims 15-21, characterized in that additional elements (33, 50, 51) for reducing convection in the liquid layer (24) are arranged between the delimiting elements (22, 23).

23. Optical correction element (21) according to Claim 22, characterized in that the additional elements (33) create in particular honeycomb-shaped or rectangular partial spaces (34) which are closed off relative to one another and which subdivide the liquid layer (24) .

24. Optical correction element (21) according to Claim 22, characterized in that the additional element is at least one separating element (50) which subdivides the liquid layer (24) into at least two partial layers (24a, 24b) which are arranged one behind another in the light path through the optical correction element (21).

25. Optical correction element (21) according to Claim 24, characterized in that the partial layers

(24a, 24b) have an average thickness of less than 3 mm, preferably less than 2 mm, particularly preferably less than 1 mm.

26. Optical correction element (21) according to Claim 22, characterized in that the additional element is at least one insulating layer (51) for thermally insulating the liquid layer (24) from at least one delimiting element (22, 23).

27. Optical correction element (21) according to Claim 26, characterized in that the insulating layer (51) is formed by gas bubbles in that region of the delimiting element (22, 23) which faces the liquid layer (24).

28. Optical correction element (21) according to any of the preceding Claims 15-27, characterized in that the refractive index of the liquid is adapted to the refractive index of the surrounding material in the cold state.

29. Optical correction element (21) according to any of the preceding Claims 15-28, characterized in that the liquid layer (24) contains soluble substances, in particular salts such as sodium chloride, calcium chloride or potassium iodide, for modifying its optical properties .

30. Optical correction element (21) according to any of the preceding Claims 15-29, characterized in that additional temperature-regulating elements (40) are present for modifying the temperature distribution in the liquid layer (24) .

31. Optical correction element (21) according to Claim 30, characterized in that the temperature-regulating elements (40) are resistance heating elements or infrared radiation sources.

32. Optical correction element (21) according to any of the preceding Claims 15-31, characterized in that the optical correction element (21) has inlets and outlets (55) for at least partly replacing the liquid layer (24) .

33. Projection objective (7) for semiconductor lithography, characterized in that the projection objective (7) contains at least one optical correction element (21) according to any of Claims 15-32.

34. Projection objective (7) for semiconductor lithography according to Claim 33, characterized in that at least one optical correction element (21) is arranged in the region of a pupil plane at a distance from the pupil plane which corresponds to a subaperture ratio of greater than 0.7.

35. Projection objective (7) for semiconductor lithography according to either of Claims 33 and 34, characterized in that the optical useful radiation radiates through at least one of the correction elements (21') a number of times, in particular twice.

36. Projection objective (7) for semiconductor lithography according to any of Claims 33-35, characterized in that the projection objective (7) is a catadioptric objective having at least two folding mirrors .

37. Projection exposure apparatus (1) for semiconductor lithography having an optical correction element (21) having a liquid layer (24) for the correction of imaging aberrations, the liquid layer (24) being arranged at least partly in the beam path of the projection exposure apparatus, characterized in that the projection exposure apparatus has an illumination device (3) suitable for generating an inhomogeneous illumination, in particular a dipole illumination of a reticle (5) arranged in the beam path .

38. Projection exposure apparatus (1) according to Claim 37, characterized in that the liquid layer (24) is formed as a liquid plane-parallel plate arranged between two delimiting elements (22, 23) having planar surfaces, said delimiting elements being arranged in a manner fixed with respect to one another.

39. Projection exposure apparatus (1) according to Claim 38, characterized in that the two delimiting elements (22, 23) are formed as substantially plane- parallel plates.

40. Projection exposure apparatus (1) according to either of the preceding Claims 38 and 39, characterized in that the two delimiting elements (22, 23) contain quartz glass or CaF₂.

41. Projection exposure apparatus (1) . according to Claim 40, characterized in that the plane-parallel plates contain quartz glass and the ratio of the thickness of the liquid layer (24) to the summed thickness of the two substantially plane-parallel plates lies between 0.2 and 1.0, preferably between 0.25 and 0.75, particularly preferably between 0.3 and 0.5.

42. Projection exposure apparatus (1) according to Claim 40, characterized in that the plane-parallel plates contain CaF₂ and the ratio of the thickness of the liquid layer (24) to the summed thickness of the two substantially plane-parallel plates lies between 0.1 and 2.0, preferably between 0.3 and 1.5, particularly preferably between 0.5 and 1.

43. Projection exposure apparatus (1) according to either of the preceding Claims 41 or 42, characterized in that the summed thickness of the two substantially plane-parallel plates lies within a range of 1.5 mm - 15 mm, preferably within a range of 2 mm - 12 mm.

44. Projection exposure apparatus (1) according to any of the preceding Claims 37-43, characterized in that the liquid layer (24) contains water or a high-index liquid such as e.g. cyclohexane or decalin.

45. Projection exposure apparatus (1) according to any of the preceding Claims 37-44, characterized in that additional elements (33) for reducing convection in the liquid layer (24) are arranged between the delimiting elements (22, 23) .

46. Projection exposure apparatus (1) according to Claim 45, characterized in that the additional elements

(33) create in particular honeycomb-shaped or rectangular partial spaces (34) which are closed off relative to one another and which subdivide the liquid layer (24) .

47. Projection exposure apparatus (1) according to Claim 45, characterized in that the additional element is at least one separating element (50) which subdivides the liquid layer (24) into at least two partial layers (24a, 24b) which are arranged one behind another in the light path through the optical correction element (21) .

48. Projection exposure apparatus (1) according to Claim 47, characterized in that the partial layers (24a, 24b) have an average thickness of less than 3 mm, preferably less than 2 mm, particularly preferably less than 1 mm.

49. Projection exposure apparatus (1) according to Claim 37, characterized in that the additional element is at least one insulating layer (51) for thermally insulating the liquid layer (24) from at least one delimiting element (22, 23) .

50. Projection exposure apparatus (1) according to Claim 49, characterized in that the insulating layer (51) is formed by gas bubbles in that region of the delimiting element (22, 23) which faces the liquid layer (24) .

51. Projection exposure apparatus (1) according to any of the preceding Claims 37-50, characterized in that the refractive index of the liquid is adapted to the refractive index of the surrounding material in the cold state.

52. Projection exposure apparatus (1) according to any of the preceding Claims 37-51, characterized in that the liquid layer (24) contains soluble substances, in particular salts such as sodium chloride, calcium chloride or potassium iodide, for modifying its optical properties .

53. Projection exposure apparatus (1) according to any of the preceding Claims 37-52, characterized in that additional temperature-regulating elements (40) are present for modifying the temperature distribution in the liquid layer (24) .

54. Projection exposure apparatus (1) according to Claim 53, characterized in that the temperature- regulating elements (40) are resistance heating elements or infrared radiation sources.

55. Projection exposure apparatus (1) according to any of the preceding Claims 37-54, characterized in that the optical correction element (21) is arranged in a projection objective (7) of the projection exposure apparatus (1) .

56. Projection exposure apparatus (1) according to Claim 55, characterized in that at least one optical correction element (21) in the projection objective (7) is arranged in the region of a pupil plane at a distance from the . pupil plane which corresponds to a subaperture ratio of greater than 0.7.

57. Projection exposure apparatus (1) according to either of Claims 55 and 56, characterized in that the optical useful radiation radiates through at least one of the correction elements (21' ) a number of times, in particular twice.

58. Projection exposure apparatus (1) according to any of Claims 55-57, characterized in that the projection objective (7) is a catadioptric objective having at least two folding mirrors.

59. Projection exposure apparatus (1) according to any of Claims 55-58, characterized in that at least one optical correction element (21) is arranged in the region of a pupil plane near the wafer.

60. Projection exposure apparatus (1) according to any of the preceding Claims 37-59, characterized in that the optical correction element (21) has inlets and outlets (55) for at least partially replacing the liquid layer (24) .