Projection exposure apparatus including at least one mirror

A microlithographic projection exposure apparatus includes: a projection lens for imaging mask structures via an exposure radiation including at least one optical element and at least one manipulator; a read-in device for reading in application-specific structure information defining at least one property of an angular distribution of the exposure radiation upon entering the projection lens; and a travel establishing device for establishing a travel command defining a change to be made in an optical effect of the at least one optical element by manipulation of a property of the optical element via the at least one manipulator along a travel. The travel establishing device is configured to establish the travel command in an at least two-stage optimization.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims benefit under 35 U.S.C. § 119 to the German Patent Application No. 10 2015 220 537.1 filed on Oct. 21, 2015. The entire disclosure of this patent application is incorporated into the present application by reference.

FIELD

The disclosure relates to a microlithographic projection exposure apparatus and to a method for controlling such a projection exposure apparatus. A microlithographic projection exposure apparatus, in the production of semiconductor components, serves for producing structures on a substrate in the shape of a semiconductor wafer. For this purpose, the projection exposure apparatus includes a projection lens for imaging mask structures onto the wafer during an exposure process, the projection lens having at least one optical element, in particular a plurality of optical elements.

BACKGROUND

A projection lens having the smallest possible wavefront aberrations is often desirable to ensure that the mask structures are imaged onto the wafer as precisely as possible. Projection lenses are therefore equipped with manipulators which make it possible to correct wavefront aberrations via a change in state of individual optical elements of the projection lens. Examples of such a change in state include: a change of position in one or more of the six rigid-body degrees of freedom of the relevant optical element, application of heat and/or cold to the optical element, and a deformation of the optical element.

To correct aberrations of the projection lens occurring over the course of time, the travels of the manipulators are regularly readjusted. For this purpose, a travel establishing device establishes a travel command which contains travel settings for the individual manipulators of the projection lens. The readjustment of the manipulator travels is generally effected in different adjustment stages. A first of the adjustment stages includes a maintenance adjustment performed at relatively long intervals, such as of approximately a few weeks or approximately one month. Another of the adjustment stages includes an operating adjustment performed during exposure operation of the projection exposure apparatus with a high cycle rate of at least one repetition per second, in particular of at least 1000 repetitions per second.

During the operating adjustment, the aberration characteristic of the projection lens is usually measured regularly and, if appropriate, changes in the aberration characteristic between the individual measurements are determined by simulation. In this regard, for example, lens element heating effects can be taken into account computationally. In this context, a “lens element heating” is understood to mean not only the heating of a transmission lens element but also the heating of a mirror. The manipulator changes to be performed for correcting the aberration characteristic are calculated via a travel generating optimization algorithm, which is also designated as “manipulator change model” or “lens element model”. Such optimization algorithms are described for example in WO 2010/034674 A1.

The angular distribution of the exposure radiation that is present upon entering the projection lens is influenced by the so-called illumination mode used during the exposure process and also the structure type of the mask structures imaged in this case. Illumination mode and structure type of the imaged mask structures are application-specific and are often designated as “UseCase”. Information regarding illumination mode and/or structure type is designated below as application-specific structure information.

Taking account of the application-specific structure information for calculating the travel command in the context of the operating adjustment conventionally involves the use of time-consuming “fundamental” optimization algorithms for establishing the travel command, which has the effect that the cycle rate achievable in this case is regarded as insufficient for the operating adjustment. Therefore, “fast” optimization algorithms, such as, for instance, optimization algorithms based on Tikhonov regularization, are often used. Such “fast” optimization algorithms, compared with the “fundamental” algorithms, at the expense of accuracy, are often simplified or based on simplified assumptions, such that they can proceed in a shorter time. In the case of the “fast” optimization algorithms used here, it is conventional practice to dispense with taking account of the application-specific structure information on account of the time losses associated therewith. In this regard, in the case of a Tikhonov-regularized optimization algorithm, for instance, taking account of the illumination mode involves subsequent optimization of weighting parameters explained in greater detail below, which in light of the prior art is not automatable, but rather involves a qualified engineer. Therefore, an adaptation of the optimization result to the illumination mode is usually dispensed with in the prior art.

In the case of the maintenance adjustment performed at relatively long intervals, a so-called standard setup is carried out, which is very complex and generally takes up several hours. All the manipulators of the projection lens are preferably used in this case. The manipulators generally include so-called semiactive manipulators alongside conventional manipulators, the driveability of which is not subject to any restrictions. Semiactive manipulators can implement only a very limited number of drivings over their lifetime. The semiactive manipulators include e.g. manipulators for decentrization of lens elements and/or mirrors orthogonally with respect to the optical axis of the projection lens. The degrees of freedom assigned to the semiactive manipulators are also designated as partly active manipulator degrees of freedom in the context of this application. The semiactive manipulators have only a limited influence on the possible performance of the lens and primarily serve for extending the range of the manipulator system of the projection lens over the lifetime of the lens.

On account of the high expenditure of time for carrying out a standard setup, it is conventional practice here to optimize the projection lens without taking account of the application-specific structure information defined by the “UseCase”. Rather, the optimization aims to uniformly minimize the wavefront deviations. The blame for dispensing with taking account of the application-specific structure information in the conventional standard setup lies with practice in semiconductor production, according to which in exposure operation the “UseCase” set at the projection exposure apparatus is changed frequently, e.g. within one day and/or within one week. If the application-specific structure information were taken into account in the standard setup, the standard setup would have to be repeated upon each change of the “UseCase”, which would result in each case in a production outage of at least several hours.

SUMMARY

The disclosure seeks to provide a projection exposure apparatus and a method for controlling such a projection exposure apparatus with which the problems mentioned above are solved and, in particular, a fast establishment of a travel command adapted to the application-specific structure information is made possible. This applies firstly to the establishment of a travel command in the context of an operating adjustment, that is to say during exposure operation, and secondly to the establishment of a travel command in the context of a maintenance adjustment.

The disclosure includes a microlithographic projection exposure apparatus including a projection lens for imaging mask structures via an exposure radiation. The projection lens has at least one optical element and at least one manipulator. Furthermore, the projection exposure apparatus includes a read-in device for reading in application-specific structure information which defines at least one property of an angular distribution of the exposure radiation upon entering the projection lens, and a travel establishing device for establishing a travel command which defines a change to be made in an optical effect of the at least one optical element by manipulation of a property of the optical element via the at least one manipulator along a travel. The travel establishing device is configured to establish the travel command in an at least two-stage optimization. In this case, a first stage of the optimization is configured to establish an approximation of the travel command from a state characterization of the projection lens via a first optimization algorithm, which is based on a predefined standard angular distribution of the exposure radiation upon entering the projection lens. A second stage of the optimization is configured to establish an optimization result of the travel command, via a second optimization algorithm, from the approximation of the travel command taking account of the application-specific structure information.

The angular distribution of the exposure radiation should be understood to mean an angle-resolved intensity distribution of the exposure radiation. The read-in device can be embodied as an input device for manually inputting the application-specific structure information or as a data transmission interface for automatically reading in the application-specific structure information.

The use of the designation “first stage of the optimization” and “second stage of the optimization” means that the second stage of the optimization temporarily succeeds the first stage of the optimization; however, this does not necessarily need to occur directly, that is to say that it is not excluded thereby to effect, if appropriate, a further (intermediate) stage of the optimization between the first stage and the second stage of the optimization.

The travel establishing device according to the disclosure can be configured for establishing the travel command in the context of an operating adjustment in accordance with one embodiment and for establishing the travel command in the context of a maintenance adjustment in accordance with a further embodiment.

In the embodiment of the travel establishing device configured for the operating adjustment, the two-stage optimization according to the disclosure makes it possible to establish a travel command adapted to the application-specific structure information with a cycle rate that is high enough for the purposes of the operating adjustment. On account of the use of a predefined standard angular distribution of the exposure radiation that is implemented according to the disclosure in the first stage of the optimization, a “fast” optimization algorithm, e.g. based on Tikhonov regularization, can be used for the first optimization algorithm used here. Since the assumed angular distribution of the exposure radiation thus does not vary in the first optimization stage, for instance a subsequent optimization of weighing parameters of the Tikhonov regularization is not necessary, for which reason the first optimization stage can proceed fully automatically at high speed.

Since the second stage of the optimization according to the disclosure can proceed from the approximation generated in the first stage, the optimization complexity for establishing the optimization result taking account of the application-specific structure information is comparatively low. Thus, the second optimization stage, e.g. via a “fundamental” optimization algorithm which would be too time-consuming in a conventional one-stage optimization, can establish the optimization result in a comparatively short time.

Furthermore, the two-stage optimization according to the disclosure enables the number of manipulator degrees of freedom on which the second optimization stage is based to be kept smaller than would be possible in the case of a one-stage optimization. In particular, the number of manipulator degrees of freedom on which the second optimization stage is based can be kept smaller than the manipulator degrees of freedom on which the first optimization stage is based. In this regard, for example, the second optimization stage can be based only on so-called overlay degrees of freedom, explained in greater detail below.

A reduced number of manipulator degrees of freedom on which the second optimization stage is based also makes it possible to use a “fast” optimization algorithm for the second optimization stage, such as, for instance, an optimization algorithm in a Tikhonov regularization, in which the weighting parameters are then chosen comparatively robustly, i.e. with comparatively high values which allow only small deflections of the assigned manipulator degrees of freedom.

A reduced number of manipulator degrees of freedom on which the second optimization stage is based furthermore makes it possible to dispense with the step of advance calculation of a pseudo-inverse of the so-called normal equation, which step is absolutely necessary in many major optimization problems, such as, for example, in the optimization of a merit function in a Tikhonov regularization using singular value decomposition (SVD). Dispensing with this step furthermore makes it possible to perform the second optimization stage at high speed.

In the embodiment of the travel establishing device configured for the maintenance adjustment, the two-stage optimization according to the disclosure makes it possible to take account of the application-specific structure information, defined for example by the “UseCase”. In this regard, the splitting into a first optimization stage based on a standard angular distribution and a second optimization stage, which processes further the result of the first optimization stage taking account of the application-specific structure information, allows the second optimization stage to be performed separately for the case where the application-specific structure information, in particular the “UseCase”, has changed.

The separate performance of the second optimization stage can also be designated as “application-specific fast setup”. This “application-specific fast setup” can be carried out in a short period of time and thus affords the possibility of rapidly establishing a travel command adapted to the application-specific structure information. In accordance with one embodiment, the second optimization algorithm is configured in such a way that, in the embodiment configured for the maintenance adjustment, the second optimization stage can be carried out in a period of time which is less than 20%, in particular less than 10%, of the period of time for carrying out the first optimization stage. The period of time for carrying out the second optimization stage can be for example less than 1 hour, less than 10 minutes, less than 1 minute or less than 1 second.

As already mentioned above, in accordance with one embodiment, the first optimization algorithm is based on a merit function in a Tikhonov regularization which contains implicit constraints described with the aid of weighting parameters. Such a merit function in a Tikhonov regularization is described for example on page 42 of WO 2010/034674A1 under (a′″). In accordance with a further embodiment, the first optimization algorithm is configured to establish the solution of the optimization of the merit function, which can be configured in a Tikhonov regularization or else in some other form, on the basis of a singular value decomposition.

In accordance with a further embodiment, the first optimization algorithm and/or the second optimization algorithm are/is based on a merit function in a Tikhonov regularization which contains implicit constraints described with the aid of weighting parameters, wherein the values of the weighting parameters are left unchanged in each case when the first optimization algorithm and/or the second optimization algorithm are/is executed. A fine setting of the Tikhonov weighting parameters is therefore not effected during the respective optimization stage. As already mentioned above, such a merit function in a Tikhonov regularization is described for example on page 42 of WO 2010/034674A1 under (a′″). The weight matrix G contained therein contains the weighting parameters mentioned above. The weighting parameters serve to counteract a deviation of a travel setting of a relevant travel from a basic setting during the execution of the optimization algorithm. This counteracting is effected by virtue of the fact that in the case of an increasing deviation of the travel setting of a travel from the basic setting thereof, the value of a penalty term progressively increases.

In accordance with a further embodiment, the application-specific structure information includes an indication regarding the illumination mode used during the imaging of the mask structures and/or an indication regarding a structure type of the mask structures.

In accordance with a further embodiment, the travel command includes travel settings which are assigned to a multiplicity of manipulator degrees of freedom of the at least one manipulator. Those travel settings which are assigned to a first set of the manipulator degrees of freedom serve as optimization variables in the first stage of the optimization, and those travel settings which are assigned to a second set of the manipulator degrees of freedom, the second set not being identical to the first set of the manipulator degrees of freedom, serve as optimization variables in the second stage of the optimization.

In accordance with a further embodiment, the first set of the manipulator degrees of freedom and the second set of the manipulator degrees of freedom are disjoint. In accordance with a further embodiment, the second set contains fewer manipulator degrees of freedom than the first set.

In accordance with a further embodiment, the second set of manipulator degrees of freedom includes overlay degrees of freedom of the projection lens which are selected in such a way that a manipulation via the at least one manipulator along one of the overlay degrees of freedom or along a combination of a plurality of the overlay degrees of freedom brings about a change in an overlay aberration of the projection lens. Overlay aberrations indicate local image position displacements of imaged mask structures relative to the setpoint positions thereof on the substrate. The overlay aberration can include an image position displacement of an imaged mask structure at one or a plurality of field points. Overlay aberrations which can be changed by manipulation along one or a plurality of the overlay degrees of freedom can correlate with deviations in the Zernike coefficients Z2, Z3, Z7and Z8. A combination of a plurality of overlay degrees of freedom is also designated as virtual manipulator, as described for example in WO 2015/036002 A1.

A manipulator having at least one overlay degree of freedom can be realized for example by a deformable plate arranged in a near-field position. In the case where mirrors are used as optical elements of the projection lens, such manipulators can also be realized by active mirror surfaces. In the case of the latter, local shape defects in the mirror surface can be actively corrected. In one embodiment of an optical element having an active mirror surface, the optical element has a carrying structure and an optical surface structure that is deformable with respect to the carrying structure. The top side of the optical surface structure serves as mirror surface and thus for reflecting the exposure radiation. The surface structure is supported by supporting elements at a multiplicity of points. An actuation location with an actuator for raising or lowering a corresponding section of the surface structure is in each case arranged between the supporting elements. The actuators can be pneumatic, electrostatic, magnetic or piezotechnology-based actuators.

Furthermore, a manipulator having overlay degrees of freedom can be realized by a thermal manipulator arranged in a pupil plane of the projection lens. Such a thermal manipulator is described for example in WO 2008/034636 A2. This involves a current-operated thermal manipulator having a plane-parallel quartz plate. The plate contains a two-dimensional matrix of heating zones which can be individually heated via conductor tracks and resistive structures. By setting the electrical power introduced, it is possible to set an individual temperature and thus a specific refractive index for each zone.

In accordance with a further embodiment, the state characterization of the projection lens includes field-resolved overlay aberration parameters characterizing the imaging quality of the projection lens with respect to overlay aberrations, and the overlay degrees of freedom are selected in such a way that one of the overlay degrees of freedom or a combination of the overlay degrees of freedom is suitable for correcting a field profile of at least one of the overlay aberration parameters. The field profile of the at least one of the aberration parameters should be understood to mean the profile of the aberration parameter when the aberration parameter is measured at different locations in the image field of the projection lens. The combination of a plurality of overlay degrees of freedom for correcting the field profile of at least one of the aberration parameters should be understood to mean an actuation of the at least one manipulator along a combination of the manipulator degrees of freedom identified as overlay degrees of freedom such that the field profile of the at least one overlay aberration parameter is corrected.

In accordance with one embodiment variant, the field profile of the at least one overlay aberration parameter which is provided for correction via the overlay degrees of freedom is one of the following field profiles: the field profile of third order of the Zernike coefficients Z2(Z2_3) or of a higher order of the Zernike coefficient Z2, the field profile of fourth order of the Zernike coefficient Z3(Z3_4) or of a higher order of the Zernike coefficient Z3. In this context, the order of the field profile is understood to mean the radial order of the field profile.

In accordance with a further embodiment, the state characterization of the projection lens includes aberration parameters characterizing the imaging quality of the projection lens, and the second stage of the optimization is effected on the basis of a subset of the aberration parameters whose elements in each case relate to an overlay aberration of the projection lens.

In accordance with a further embodiment, the travel establishing device is configured to establish the travel command in less than one second. In this case, the travel establishing device is configured for the operating adjustment. In particular, the travel establishing device is configured to establish the travel command in a period of time of less than 100 milliseconds, in particular of less than 50 milliseconds or less than 20 milliseconds. Such a fast establishment of the travel command can also be designated as real-time establishment. An updating rate of the state characterization is correspondingly adapted to the clock rate of the travel establishing device.

In accordance with a further embodiment, the travel establishing device is configured to carry out the two-stage optimization in the context of a maintenance adjustment and to perform the second stage of the optimization, in each case proceeding from the approximation of the travel command, for different items of application-specific structure information. In other words, the travel establishing device is configured to establish from the approximation of the travel command established by the first optimization stage, by multiple performance of the second optimization stage, different optimization results respectively adapted to a different item of application-specific structure information. The first performance of the second optimization stage can be effected in the context of a so-called “standard setup” on the basis of a forthcoming “UseCase”. The second performance of the second optimization stage can then be effected in the context of a so-called “fast setup”, in which the result of the first optimization stage performed in the context of the “standard setup” is then adapted to a new “UseCase”.

In accordance with a further embodiment, each of the manipulator degrees of freedom is allocated an expected lifetime performance which specifies an expected maximum number of travel adjustments along the relevant manipulator degree of freedom that are able to be performed over the lifetime of the projection lens. The manipulator degrees of freedom include at least one partly active manipulator degree of freedom and at least one fully active manipulator degree of freedom, wherein the lifetime performance of the fully active manipulator degree of freedom is greater than the lifetime performance of the partly active manipulator degree of freedom by at least a factor of 100, and wherein the at least one partly active manipulator degree of freedom is included by the first set of the manipulator degrees of freedom and is not included by the second set of the manipulator degrees of freedom. In particular, the respective lifetime performance of the fully active manipulator degrees of freedom is greater than the respective lifetime performance of the partly active manipulator degrees of freedom by at least a factor of 100, in particular by at least a factor of 1000. The partly active manipulator degrees of freedom should be understood to mean, in particular, the degrees of freedom of movement that are able to be implemented via the semi-active manipulators mentioned above.

In accordance with a further embodiment, an algorithm generator is furthermore provided, which is configured to generate the second optimization algorithm on the basis of the application-specific structure information. In accordance with one embodiment variant, the second optimization algorithm generated by the algorithm generator includes a merit function and, in particular, at least one constraint described outside the merit function, also designated as external constraint. In this case, the number of external constraints can be greater than 100, greater than 1000, greater than 10 000 or greater than 100 000.

In accordance with a further embodiment, the second optimization algorithm is based on a merit function containing at least one implicit constraint. The implicit constraint can be a limit for an image aberration selected from a group of image aberrations, such as, for instance, odd Zernike coefficients, or a combination of specific image aberrations, such as e.g. a root mean square (RMS) of Zernike coefficients which belong in particular to a predetermined group of Zernike coefficients.

In accordance with a further embodiment, the at least one implicit constraint contains a limit for one lithographic image aberration or a combination of a plurality of lithographic image aberrations, wherein a lithographic image aberration is determinable on the basis of at least one image of the mask structures that is generated lithographically via the projection lens. “Lithographic aberrations” are understood to mean aberrations of the projection lens which are directly measurable in the lithographic image, i.e. in the areal image present in the substrate plane, or in the structure produced by the lithographic imaging in the photoresist on the substrate. Such lithographic aberrations are also designated as imaging size aberrations and are in contrast to wavefront aberrations, which cannot be measured directly in the lithographic image. One example of such a lithography aberration is a so-called “overlay aberration”. As already mentioned above, overlay aberrations indicate local image position displacements of imaged mask structures relative to the setpoint position thereof on the substrate.

In accordance with a further embodiment, the lithographic image aberration includes an overlay aberration, a focal position aberration and/or a fading aberration. The fading aberration relates to a projection exposure apparatus embodied as a step-and-scan exposure apparatus. In this case, during the imaging of a mask onto a wafer, the mask and the wafer move relative to one another. A fading aberration should be understood to mean an indication of how an image aberration changes in the scanning direction, i.e. in the direction of the relative movement between mask and wafer during the exposure. A focal position aberration is a deviation of the focus of the mask structures to be imaged from the setpoint focal position thereof. The focus that is crucial here is often also designated as “best focus”.

The disclosure includes a method according to the disclosure for controlling a microlithographic projection exposure apparatus including a projection lens for imaging mask structures. The projection lens once again includes at least one optical element and at least one manipulator. The method according to the disclosure includes the following steps: reading in application-specific structure information which defines at least one property of an angular distribution of the exposure radiation upon entering the projection lens, and establishing a travel command which defines a change to be made in an optical effect of the at least one optical element by manipulation of a property of the optical element via the at least one manipulator along a travel, in an at least two-stage optimization. In a first stage of the optimization, an approximation of the travel command is established from a state characterization of the projection lens via a first optimization algorithm, which is based on a predefined standard angular distribution of the exposure radiation upon entering the projection lens. In a second stage of the optimization, via a second optimization algorithm, an optimization result of the travel command is established from the approximation of the travel command taking into account the application-specific structure information.

In accordance with one embodiment of the method according to the disclosure, the two-stage optimization is repeated at time intervals of at least one week, in particular at time intervals of at least one month, in the context of a standard setup.

In accordance with a further embodiment, before a repetition of the two-stage optimization, the second stage of the optimization is carried out separately at least once. Carrying out the second optimization stage separately should be understood to mean carrying out the second stage without again carrying out the first optimization stage. Carrying out the second optimization stage separately can be effected for example in the context of the “fast setup” mentioned above.

The features indicated with regard to the abovementioned embodiments, exemplary embodiments or embodiment variants, etc., of the projection exposure apparatus according to the disclosure can be correspondingly applied to the control method according to the disclosure, and vice versa. These and other features of the embodiments according to the disclosure are explained in the description of the figures and the claims. The individual features can be realized either separately or in combination as embodiments of the disclosure. Furthermore, they can describe advantageous embodiments which are independently protectable and protection for which is claimed if appropriate only during or after pendency of the application.

DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS

In the exemplary embodiments or embodiments or embodiment variants described below, elements which are functionally or structurally similar to one another are provided with the same or similar reference signs as far as possible. Therefore, for understanding the features of the individual elements of a specific exemplary embodiment, reference should be made to the description of other exemplary embodiments or the general description of the disclosure.

In order to facilitate the description, a Cartesian xyz-coordinate system is indicated in the drawing, from which system the respective positional relationship of the components illustrated in the figures is evident. InFIG. 1, the x-direction runs perpendicular and into the plane of the drawing, the z-direction toward the right, and the y-direction upwardly.

FIG. 1shows an embodiment according to the disclosure of a microlithographic projection exposure apparatus10. The present embodiment is designed for operation in the UV wavelength range, i.e. with electromagnetic radiation of, for example, 365 nm, 248 nm or 193 nm. However, the disclosure is not limited to projection exposure apparatus in the UV wavelength range. Further embodiments according to the disclosure are designed for example for operating wavelengths in the EUV wavelength range, i.e. with electromagnetic radiation having a wavelength of less than 100 nm, in particular a wavelength of approximately 13.5 nm or approximately 6.7 nm. In the case of an operating wavelength in the EUV range, all the optical elements are embodied as mirrors.

The projection exposure apparatus10in accordance withFIG. 1includes an exposure radiation source12for generating exposure radiation14. The exposure radiation14firstly passes through an illumination system16and is directed by the latter onto a mask18. The illumination system16is configured to generate different angular distributions of the exposure radiation14incident on the mask18. Depending on an illumination mode desired by the user, also called illumination setting, the illumination system16configures the angular distribution of the exposure radiation14incident on the mask18. Examples of illumination mode settings that can be chosen include a so-called dipole illumination, annular illumination and quadrupole illumination.

The mask18has mask structures for imaging onto a substrate24in the form of a wafer and is mounted displaceably on a mask displacing stage. The mask18is configured as a transmission mask in the present case. Particularly in the case of EUV lithography, the mask can also be embodied as a reflection mask. In the embodiments in accordance withFIG. 1, the exposure radiation14passes through the mask18and thereupon passes through a projection lens22configured to image the mask structures onto the substrate24. The exposure radiation14is guided within the projection lens22via a multiplicity of optical elements E1to E18. The substrate24is mounted displaceably on a substrate displacing stage26. The projection exposure apparatus10can be embodied as a so-called scanner or as a so-called stepper.

The projection lens22has 18 optical elements E1to E18in the embodiment in accordance withFIG. 1. The optical elements E1to E7and E10to E18are configured as transmission lens elements, and the optical elements E8and E9are configured as mirrors. A respective manipulator M1to M6is assigned to the optical elements E2, E3, E6, E8, Ell and E16. While the manipulators M1to M4are so-called fully active manipulators (FA), the drivability of which is not subject to any restrictions, the manipulators M5and M6are so-called semi-active or partly active manipulators (SA). As already mentioned above, semi-active manipulators can implement only a very limited number of drivings over their lifetime. These manipulators have only a limited influence on the possible lens performance and primarily serve for extending the range of the manipulator system of the projection lens22over the lifetime of the lens.

The manipulator M1assigned to the optical element E6, the manipulator M2assigned to the optical element E16and the manipulator M6allocated to the optical element Ell enable in each case a displacement of the assigned optical elements E6, E16and Ell, respectively, in the z-direction and thus substantially perpendicular to the plane in which the respective optical surfaces of the optical elements lie. The manipulators M1, M2and M6are thus respectively allocated a manipulator degree of freedom, the travel setting of which is designated by x1, x2and x6, respectively.

The manipulator M3is configured to apply inwardly directed pressure to the edge of the assigned optical element at at least two opposite points, such that the curvature of the optical element E2is intensified. In other words, the manipulator M3enables bending of the optical element E2by compression at the edge side. While the compression direction can be oriented differently in principle, hereinafter for the sake of simplicity the manipulator M3is allocated only one manipulator degree of freedom (compression in the y-direction in accordance withFIG. 1), the travel setting of which is designated by x3. The manipulator M3serves for correcting so-called “overlay aberrations” of the projection lens22. The manipulator degree of freedom defined by the travel setting x3is therefore designated as overlay degree of freedom. As already mentioned above, overlay aberrations indicate local image position displacements of mask structures imaged onto the substrate24relative to the setpoint position thereof on the substrate24.

The manipulator M4assigned to the optical element E8embodied as a mirror serves for the active deformation of the mirror surface of E8by the targeted actuation of one or more points of the mirror surface in a direction arranged transversely with respect to the mirror surface. A mirror provided with such a manipulator is also known as a deformable mirror. While the degrees of freedom of movement of the different actuation points of the mirror surface can be regarded as separate degrees of freedom of the manipulator M4, for the sake of simplicity manipulator M4is allocated only one manipulator degree of freedom, the travel setting of which is designated by x4. The manipulator M4, too, serves for correcting overlay aberrations of the projection lens22. Therefore, the manipulator degree of freedom defined by the travel setting x4is also designated as overlay degree of freedom.

The manipulator M5enables a displacement of the optical element E3assigned to it in the x- and y-directions and thus substantially parallel to the plane in which the optical surface of the optical element E3lies. That is to say that the manipulator M5has two degrees of freedom, namely a displacement in the x-direction and a displacement in the y-direction. For the sake of simplicity, hereinafter the manipulator M5is allocated only one manipulator degree of freedom, designated by the travel setting x5.

Alternatively or additionally, it is also possible to provide manipulators configured to perform some other type of alteration of a state variable of the assigned optical element by corresponding actuation of the manipulator. In this regard, an actuation can for example also be effected by a specific temperature distribution being applied to the optical element. In this case, the travel can be manipulated by an alteration of the temperature distribution.

The projection exposure apparatus10furthermore includes a central control unit30for controlling the exposure process, including the mask displacing stage and the substrate displacing stage26. Via a read-in device28, in the form either of a manual input device or of a data transmission interface, a mask selection indication20and application-specific structure information32including an illumination mode indication36and a mask structure indication38assigned to the mask selection indication20are read in.

The illumination mode indication36defines the illumination mode to be used in the next forthcoming exposure. As mentioned above, the illumination mode, often also called “illumination setting”, defines the angular distribution of the exposure radiation14incident on the mask structures during the imaging operation of the projection exposure apparatus. The central control unit30communicates the illumination mode indication36to the illumination system16for setting the corresponding illumination mode.

The mask selection indication20designates the mask18to be used in the next forthcoming exposure. The central control unit30communicates the mask selection indication20to a mask loading unit for instigating the arrangement of the corresponding mask18in the mask plane. As already mentioned above, the mask structure indication38designates a structure type of the mask structures on the mask18selected via the mask selection indication20. On the basis of the structure type, the mask structures to be imaged are classified with regard to their effect on the angular distribution of the exposure radiation14after interaction with the mask structures, i.e. the angular distribution of the exposure radiation upon entering the projection lens22. This classification can distinguish e.g. between dense structures, such as e.g. grating structures, and sparsely arranged structures, such as, e.g. isolated lines, or between line structures and hole structures.

Both the illumination mode indication36and the mask structure indication38thus influence the angular distribution of the exposure radiation14upon entering the projection lens22and therefore define in each case at least one property of the angular distribution. From the knowledge of both indications36and38, the angular distribution upon entering the projection lens22can be calculated at least approximately. The application-specific structure information32containing the two indications36and38, which information also designates a so-called “UseCase” in this application, is communicated to the travel establishing device40by the central control unit30.

The projection exposure apparatus10furthermore includes a manipulator controller34for controlling the manipulators M1to M6. The manipulation controller34in turn includes a state generator54and the travel establishing device40. The state generator54transfers current state characterizations64and64aof the projection lens22to the travel establishing device40, which generates a travel command50therefrom. The travel command50includes travels xi, in the case shown the travels x1to x6. The travels x1to x6serve for controlling the manipulators M1to M6along the manipulator degrees of freedom assigned thereto, as described in greater detail below.

As already mentioned, the travel command50generated by the travel establishing device40includes changes to be made by the manipulators M1to M6in the form of travels xiof corresponding state variables of the associated optical elements. In this case, a distinction is drawn between an operating adjustment and a maintenance adjustment. In the operating adjustment only the settings of the fully active manipulators M1to M4are optimized, while in the maintenance adjustment the settings of all manipulators, i.e. both of the fully active manipulators M1to M4and of the semi-active manipulators M5and M6, are optimized. The established travels xiare communicated to the individual manipulators M1to M6via travel signals and predefine for them respective correction travels to be formed. In the operating adjustment, the travels x5and x6are not included by the travel command50, or have the value zero. The correction travels define corresponding displacements of the assigned optical elements for correcting wavefront aberrations of the projection lens22that have occurred or are expected.

In order to establish the travels xi, the travel establishing device40receives from the state generator54respectively updated state characterizations in the form of aberration parameters of the projection lens22. The respectively updated state characterizations are generally extrapolated state characterizations64ain the case of the operating adjustment and measured state characterizations64in the case of the maintenance adjustment.

In the operating adjustment, the travel establishing device40in accordance with one embodiment generates updated travels xiin periods of time of less than one second. By way of example, the travels xican be updated in periods of time of less than 200 milliseconds and thus in real time. An updating of the travels that is carried out in less than one second makes it possible, for example, to readjust the manipulators after each field exposure.

The aberration parameters included by the state characterization64or64acan include for example Zernike coefficients characterizing the wavefront. In the present application, as described for example in paragraphs [0125] to [0129] of US 2013/0188246A1, the Zernike functions known from e.g. Chapter 13.2.3 of the textbook “Optical Shop Testing”, 2ndEdition (1992) by Daniel Malacara, publisher John Wiley & Sons, Inc. Zmnare designated by Zjin accordance with so-called fringe sorting, in which case cjare then the Zernike coefficients assigned to the respective Zernike polynomials (also referred to as “Zernike Functions”). Fringe sorting is illustrated for example in Table 20-2 on page 215 of “Handbook of Optical Systems”, Vol. 2 by H. Gross, 2005 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim. A wavefront deviation W(ρ,Φ) at a point in the image plane of the projection lens is expanded depending on the polar coordinates (ρ, Φ) in the pupil plane as follows:

While the Zernike polynomials are designated by Zj, i.e. with a subscripted index j, in the context of this application the Zernike coefficients cj, as customary among those skilled in the art, are designated by Zj, i.e. with a normally positioned index, such as Z5and Z6for astigmatism, for example.

In accordance with one embodiment, the state generator54has a memory56and a simulation unit58. State characterizations64in the form of aberration parameters that were established via a wavefront measurement at the projection lens22are stored in the memory56. These measurement results can be collected via an external wavefront measuring device. Alternatively, however, the state characterizations64can also be measured by a wavefront measuring unit52integrated in the substrate displacing stage26. For instance, such a measurement can be carried out regularly after each exposure of a wafer or respectively after the exposure of a complete wafer set. Alternatively, a simulation or a combination of simulation and reduced measurement can also be performed instead of a measurement.

For the purpose of the maintenance adjustment, the state characterization is forwarded directly to the travel establishing device40. In the context of the operating adjustment, by contrast, the measured values of the state characterization64in the form of aberration parameters, the measured values being stored in the mirror56, are adapted by the simulation unit58to respective updated conditions during the exposure process. In accordance with one embodiment variant, for this purpose the current irradiation intensity62is regularly communicated to the simulation unit58by the central control unit30.

The simulation unit58calculates therefrom changes in the aberration parameters brought about on account of lens element heating on the basis of the respective illumination mode indication36and/or the respective mask structure indication38. Furthermore, the simulation unit continuously receives measured values from a pressure sensor60that monitors the ambient pressure of the projection exposure apparatus10. Effects of changes in the ambient pressure on the aberration parameters are taken into account by the simulation unit58.

The functioning of the travel establishing device40is illustrated inFIG. 2for the case of the operating adjustment and inFIG. 3for the case of the maintenance adjustment. Referring toFIG. 2, firstly the functioning of the optimization performed at short time intervals in the context of the operating adjustment is explained below. The optimization is effected in each case in two stages. In the first optimization stage, identified by “I” inFIG. 2, an approximated travel command74is established via a first optimization algorithm70from the state characterization provided by the state generator54, specifically directly after a wavefront measurement firstly from the measured state characterization64and thereupon respectively from the extrapolated state characterization64a.

The first optimization algorithm70serves for optimizing a merit function72, also called objective function. In accordance with one embodiment, the optimization problem solved here reads as follows:
min(∥Mx−b∥22+∥Gx∥22)  (2)

The merit function ∥Mx−b∥22+∥Gx∥22that is minimized in this case is a Tikhonov-regularized merit function and contains ∥Mx−b∥22as main term and ∥Gx∥22as so-called penalty term. In this case, ∥ ∥2denotes the Euclidean norm. As already mentioned, optimization algorithms based on Tikhonov regularization are time-optimized, that is to say that they yield fast optimization results in comparison with conventional optimization algorithms. The travels of the approximated travel command74to be generated by the optimization algorithm70are described by a travel vector x, the vector components of which in the present embodiment are the travel settings x1to x4assigned to the fully active manipulators M1to M4in accordance withFIG. 1. The current state characterization64or64ais described by a state vector b. The sensitivities of the manipulators NI, in the present case the manipulators M1to M4, with regard to the degrees of freedom thereof in the case of a state change are described via a sensitivity matrix M in accordance with one embodiment variant. In this case, the sensitivity matrix M describes the relationship between an adjustment of the manipulator Miby a standard travel xi0and a resultant change in the state vector b of the projection lens22.

The weight matrix G contained in the penalty term contains weighting parameters for each of the travel settings xi, the weighting parameters serving to counteract a deviation of the travel setting xiof the relevant travel from a basic setting during the execution of the optimization algorithm70. To put it generally, this counteracting is brought about by the weight matrix G by virtue of the fact that in the case of an increasing deviation of the travel setting xiof a travel from the basic setting thereof, the value of the penalty term progressively increases. Without further measures, the use of such a weight matrix G leads to “soft limit values” for the relevant travel settings xi. Specifically, the weight matrix G acts like rubber bands on the travel settings to be adopted by the manipulators Mi, which prevent an excessive deviation from the initial or basic settings thereof, but do not predefine rigid limits for the deviation. With regard to the fundamental configuration of such a weight matrix G, reference is made to the indications given in the document WO 2010/034674 A1, in particular pages 42 and 43 of the document, in connection with the description of a Tikhonov regularization.

The weight matrix G is set to a predefined standard angular distribution of the exposure radiation14when entering the projection lens22, that is to say that the angular distribution on which the weight matrix G is based is effected independently of the current application-specific structure information32and preferably remains unchanged from optimization to optimization. Subsequent optimization of the weighting parameters contained in the weight matrix G, also referred to as weighting parameters of the Tikhonov regularization in the context of this application, on the basis of the current application-specific structure information32is therefore not necessary. The first optimization stage can thus proceed fully automatically at high speed. As a result the first optimization stage yields the approximated travel command74including approximations of the travel settings x1to x4.

In the second optimization stage, designated by “II” inFIG. 2, the final travel command50ais established via a second optimization algorithm76, proceeding from the approximated travel command74, the final travel command then being used as travel command50for driving the manipulators M1to M4according toFIG. 1in the context of the operating adjustment. The second optimization algorithm76serves for minimizing a merit function78which, in the present embodiment, does not differ from the merit function72in terms of the fundamental construction, that is to say that it is likewise a Tikhonov-regularized merit function in the present exemplary embodiment. The merit function78differs in the choice of the weighting parameters contained in the weight matrix G, however in so far as the weighting parameters in the merit function78are chosen more robustly in relation to the merit function72, that is to say that they have comparatively high values which allow only smaller deflections of the assigned manipulator degrees of freedom.

The second optimization algorithm76is designed in a targeted manner to adapt the approximated travel command74established in the first optimization stage to the currently present angular distribution—defined by the application-specific structure information32—of the exposure radiation14when entering the projection lens22. In other words, the second optimization stage is configured in a targeted manner for taking account of the application-specific structure information32. In the present embodiment of the merit function78, this is effected by targeted selection of the weighting parameters in the weight matrix G, of the travel vector x and of the state vector b.

In this way, a recognition that a changed angular distribution in the exposure radiation14principally influences overlay aberrations is used to the effect that the travel vector x used in the second optimization stage is allocated only the travel settings x3and x4assigned to the manipulators M3and M4. Thus, only the overlay degrees of freedom of the manipulator system are optimized in the second optimization stage. In a manner adapted thereto, the state vector b used in the second optimization stage includes in a targeted manner Zernike coefficients characterizing overlay aberrations of the projection lens22, such as, for instance, the Zernike coefficients Z2, Z3, Z7and Z8. In the present embodiment, the state vector b includes the field-resolved overlay aberration parameter Z2_3(field profile of third order of the Zernike coefficient Z2) and field profiles of higher orders of the Zernike coefficient Z2and furthermore the field-resolved overlay aberration parameter Z3_4(field profile of fourth order of the Zernike coefficient Z3) and field profiles of higher orders of the Zernike coefficient Z3. In this context, the order of the field profile is understood to mean the radial order of the field profile.

On account of the robustly chosen weighting parameters of the weight matrix G, subsequent optimization of the weighting parameters is not necessary and the second optimization stage can likewise proceed fully automatically at high speed. As optimization algorithm76of the second optimization stage, instead of the above-described optimization algorithm in a Tikhonov regularization, it is also possible to use a different optimization algorithm, for instance a so-called “fundamental” optimization algorithm having external constraints.

Referring toFIG. 3, the functioning of the optimization performed in the context of the maintenance adjustment is explained below. The complete maintenance adjustment, also referred to as standard setup, is usually performed at intervals of more than one week, e.g. once a month. All the manipulators of the projection lens22, that is to say not only the fully active manipulators M1to M4but also the semi-active manipulators M5and M6serving for extending the range of the manipulator system, are included in the maintenance adjustment. The optimization according to the disclosure in the context of the maintenance adjustment, as illustrated inFIG. 3, is likewise effected in two stages.

In the first optimization stage, identified by “I”, an approximated travel command84is established via a first optimization algorithm80from the measured state characterization64provided by the state generator54. This is effected, for example, by the optimization of a merit function82taking account of external constraints90. In accordance with one embodiment, the optimization problem solved here reads as follows:
min∥Mx−b∥22
where
xi≦speci(3)

In this case, spec′ denotes the constraints allocated to the respective travel settings xi. The travel vector x used here includes as vector components the travel settings of all manipulator degrees of freedom of the projection lens22, that is to say the travel settings x1to x6in the present case. The first optimization algorithm80of the maintenance adjustment, like the first optimization algorithm70of the operating adjustment as well, is designed for a predefined standard angular distribution of the exposure radiation14when entering the projection lens22, that is to say that the current application-specific structure information32is disregarded in the first optimization stage.

The first optimization algorithm80is designed for optimum accuracy and is therefore very time-consuming in its execution. In this regard, performing the first optimization stage may take a number of hours. As a result the first optimization stage yields the approximated travel command84including approximations of the travel settings x1to x6, in the present case.

The travel establishing device40includes an algorithm generator94, which is configured to generate a second optimization algorithm86on the basis of the application-specific structure information32defining the currently present angular distribution of the exposure radiation14when entering the projection lens22, the second optimization algorithm serving to generate the final travel command from the approximated travel command84.

In the second optimization stage, designated by “II” inFIG. 3, the final travel command50bis established via the second optimization algorithm86, proceeding from the approximated travel command84, the final travel command then being used as travel command50for driving the manipulators M1to M4according toFIG. 1in the context of the maintenance adjustment. The second optimization algorithm86serves for minimizing a merit function88, in particular taking account of external constraints92. The number of external constraints92can be greater than 100, in particular greater than 1000 or greater than 10 000. Furthermore, the merit function88in accordance with one embodiment also includes implicit constraints. In accordance with one embodiment variant, the merit function88reads as follows:
∥Mx−b∥22+Hf+Hovl+Hrms+Hbf(4)

In this case, Hfdenotes a term of the merit function88which contains a constraint with regard to so-called fading-aberrations. A fading-aberration should be understood to mean an indication of how an image aberration changes in the scanning direction of a projection exposure apparatus, i.e. in the direction of the relative movement between mask and wafer, during the exposure. Hovldescribes a constraint with regard to overlay aberration or of summation values of overlay aberrations of the projection lens22. Hrmscontains grouped RMS values of the Zernike coefficients bjas constraint. As is known to the person skilled in the art, an RMS value of coefficients should be understood to mean the root of the sum of the squares of the coefficients. Furthermore, grouped RMS values and weighted sums of selected Zernike coefficients can be incorporated in the merit function88. Hbfdescribes a constraint which predefines specifications with regard to summation values of so-called focal position aberrations, also referred to as “best focus” sums. As already mentioned above, a focal position aberration is a deviation of the focus of the mask structures to be imaged from the setpoint focal position thereof. The focus that is crucial here is often also designated as “best focus”.

Analogously to the second optimization algorithm76of the operating adjustment, the second optimization algorithm86of the maintenance adjustment is designed in a targeted manner to adapt the approximated travel command74established in the first optimization stage to the currently present angular distribution—defined by the application-specific structure information32—of the exposure radiation14when entering the projection lens22. In other words, the second optimization stage is configured in a targeted manner for taking account of the application-specific structure information32. Furthermore, in the context of the maintenance adjustment, the second optimization stage is intended to proceed considerably faster, in particular faster by at least the factor5or10, in comparison with the first optimization stage. In this regard, the period of time for carrying out the second optimization stage can be, for example, less than 1 hour, less than 10 minutes, less than 1 minute or less than 1 second.

In order to achieve these stipulations, the travel vector x used in the second optimization stage is restricted to the degrees of freedom of the fully active manipulators, that is to say in the present case to the degrees of freedom assigned to the travel settings x1to x4.

On account of the desired short time of the second optimization stage, for the case where the “UseCase” changes during the production operation of the projection exposure apparatus10, the second optimization stage can be repeated on the basis of the changed application-specific structure information32, but still proceeding from the approximated travel command84established beforehand, and the new final travel vector50bestablished in this case can be taken as a basis for further production operation. The repeated performance of the second optimization stage can be designated as “fast setup”, in the case of which the result of the “standard setup” is then adapted to a new “UseCase”.

The above description of exemplary embodiments should be understood to be by way of example. The disclosure effected thereby firstly enables the person skilled in the art to understand the present disclosure and the advantages associated therewith, and secondly encompasses alterations and modifications of the described structures and methods that are also obvious in the understanding of the person skilled in the art. Therefore, all such alterations and modifications, in so far as they fall within the scope of the disclosure in accordance with the definition in the accompanying claims, and equivalents are intended to be covered by the protection of the claims.

LIST OF REFERENCE SIGNS