The contents of the German Patent Application DE 10 2019 208 552.0 is incorporated by reference herein.

The invention relates to a method for determining a production aerial im age of an object to be measured as a result of an illumination and imaging with illumination and imaging conditions of an optical production system. Furthermore, the invention relates to a metrology system comprising an op tical measurement system for carrying out the method.

A metrology system is known from US 2017/0131 528 A1 (parallel docu ment WO 2016/0124 425 A2) and from US 2017/0132782 Al. WO

2017/207 297 Al discloses a method for predicting an imaging result of a lithography mask.

It is an object of the present invention to develop a determining method of the type mentioned in the introduction in such a way that the requirements directed to an optical measurement system used in the determining method, even under demanding illumination and imaging conditions of the optical production system, are relaxed.

This object is achieved according to the invention by means of a determin ing method having the features specified in Claim 1.

According to the invention, it has been recognized that it is possible to carry out a determination of a production aerial image even for demanding illumination and imaging conditions of the optical production system using an optical measurement system in which a different measurement illumina tion setting in comparison with a production illumination setting is used. The measurement illumination setting can then be configured such that it is able to be realised more simply within the optical measurement system, with the result that the requirements directed to the optical measurement system overall are relaxed. It is possible, in particular, to simulate produc tion aerial images which, in a measurement set-up, would not be able to be realised, or would be able to be realised only with difficulty, since this would require for example a stop structure not able to be fabricated, or able to be fabricated only with difficulty, in the measurement set-up.

On the basis of the simulated production aerial image, that is to say on the basis of the result of the determining method, the object structure of the measured object can be optimized until the simulated production aerial im age corresponds to a predefined aerial image. The determining method can thus be part of an iterative process for optimizing an object structure until the object structure has been optimized for the illumination and imaging conditions of the optical production system for generating an image struc ture, which in turn, for example if the optical production system is used for producing micro- or nanostructured semiconductor components, is a start ing point for the production of correspondingly structured semiconductor components with extremely high resolution.

The measurement illumination setting includes a predefined numerical illu mination aperture of the optical measurement system. Said predefined illu mination aperture is predefined by an edge contour of the illumination pu pil. In the determining method, it is possible to use a reconstruction method for an object structure known from the technical article "Method for Retrieval of the Three-Dimensional Object Potential by Inversion of Dynamical Electron Scattering" by Van den Broek et ah, Phys.Rev.Lett 109, 245502 (2012) and also from WO 2017/207 297 Al.

In the optimization of the object structure, by way of example, object de fects can be identified and optionally repaired.

A predefinition of a measurement illumination setting by a setting stop ac cording to Claim 2 has proved worthwhile.

A production illumination setting having an elliptic edge contour of an illu mination pupil according to Claim 3 is one example of a demanding pro duction illumination setting.

The same applies, mutatis mutandis, to a freeform or SMO (Source Mask Optimization) illumination setting according to Claim 3. Such a freeform illumination setting cannot be described by any of the standardized illumi nation settings "conventional", "annular", "dipole" or "multipole", but ra ther is distinguished by a free shaping of the arrangement of pupil regions on which illumination light impinges within the illumination pupil. With regard to the SMO methodology, reference is made to the technical article "Source mask optimization methodology (SMO) and application to real full chip optical proximity corrections" by D. Zhang et al., Proceedings SPIE 8326, Optical Microlithography XXV, 83261V (13 March 2012). A production illumination setting with varying illumination intensity ac cording to Claim 5 is a further example of a demanding production illumi nation setting. The minimum illumination intensity can be greater than 1% or else greater than 10% of the maximum illumination intensity. The mini mum illumination intensity can be less than 50% of the maximum illumina tion intensity.

A production illumination setting according to Claim 6 has proved worth while within an optical production system. The illuminated individual re gions within the illumination pupil can be arranged separately from one an other.

It is virtually impossible or not possible at all for such a production illumi nation setting to be predefined by way of a setting stop since a production outlay for a setting stop of this type, if such a setting stop were producible in the first place, would be enormously high.

Individual region configurations or individual region arrangements accord ing to Claims 7 to 9 have likewise proved worthwhile when predefining a production illumination setting. The individual regions can have a circular boundary; however, this is not mandatory.

The advantages of a metrology system according to Claim 10 correspond to those that have already been explained above with reference to the conver gence method according to the invention. The metrology system can measure a lithography mask provided for pro jection exposure for producing semiconductor components with an ex tremely high structure resolution, which for example is better than 30 nm and which in particular can be better than 10 nm.

A metrology system according to Claim 11 is flexibly usable. The metrol ogy system can comprise a plurality of changeable setting stops, which can be exchanged for one another in an automated manner by way of a chang ing mount.

An exemplary embodiment of the invention is explained in greater detail below with reference to the drawing. In said drawing:

Fig. 1 schematically shows a metrology system for determining an aerial image of an object to be measured in the form of a lithography mask, comprising an illumination system, an imaging optical unit and a spatially resolving detection device; Fig. 2 shows a sequence of main method steps of a method in which the metrology system according to Fig. 1 is used, for determining an aerial image of an object to be meas ured as a result of an illumination and imaging with illu mination and imaging conditions of an optical production system;

Fig. 3 shows in greater detail, but still schematically, an imag ing of the lithography mask with the imaging optical unit of the metrology system according to Figure 1; and Fig. 4 shows a schematic visualization of input variables that in fluence a determined production aerial image, namely in particular data concerning an optical production system which images the object to be measured, data of a recon structed object structure and also data concerning illumi nation conditions of the optical production system with a production illumination setting, which differs from a measurement illumination setting of the metrology sys- tern according to Figure 1.

Figure 1 shows, in a sectional view corresponding to a meridional section, a beam path of EUV illumination light or imaging light 1 in a metrology system 2. The illumination light 1 is produced by an EUV light source 3.

In order to facilitate the presentation of positional relationships, a Cartesian xyz-coordinate system is used hereinafter. In Fig. 1, the x-axis runs perpen dicularly to the plane of the drawing and out of the latter. The y-axis runs towards the right in Fig. 1. The z-axis runs upwards in Fig. 1.

The light source 3 can be a laser plasma source (LPP; laser produced plasma) or a discharge source (DPP; discharge produced plasma). In princi ple, a synchrotron-based light source can also be used, for example a free electron laser (FEL). A used wavelength of the illumination light 1 can be in the range of between 5 nm and 30 nm. In principle, in the case of a vari ant of the projection exposure apparatus 2, it is also possible to use a light source for some other used light wavelength, for example for a used wave length of 193 nm. In an illumination optical unit (not illustrated in more specific detail) of an illumination system of the metrology system 2, to which the light source 3 also belongs, the illumination light 1 is conditioned such that a specific il lumination setting 5 of the illumination is provided, that is to say a specific illumination angle distribution. Said illumination setting 5 corresponds to a specific intensity distribution of the illumination light 1 in an illumination pupil of the illumination optical unit of the illumination system 4.

One example of the illumination setting 5 is indicated in Figure 1 schemati cally in a manner lying in the plane of the drawing as an annular illumina tion setting provided with webs and having a total of four illumination poles 6 shaped approximately as quadrants. The illumination pupil in which the illumination setting 5 is present is actually arranged perpendicu larly to the plane of the drawing in Figure 1 and perpendicularly to the di rection of propagation of the illumination light 1 through the illumination pupil.

In the illumination pupil there is a predefined illumination intensity in each case at the location of the illumination poles 6, otherwise no illumination intensity. The illumination setting 5 can be predefined by a setting stop 7, which is transmissive to the illumination light 1 at the location of the illu mination poles 6 and blocks the illumination light in the surroundings of the illumination poles 6. One example of such a setting stop 7 is a metal sheet having passage openings, the shape of which corresponds exactly to the shape of the illumination poles 6. The setting stop is arranged in a pupil plane of the illumination optical unit of the metrology system 2. With the aid of a changing holder 7a indicated in Figure 1, the setting stop 7 can be exchanged for an exchange setting stop for changing the respec tive measurement illumination setting.

Instead of the quadmpole illumination setting 5 illustrated, by use corre spondingly with differently shaped and/or distributed passage openings, it is also possible to predefine other illumination settings within the metrol ogy system 2, for example a conventional illumination setting in which practically all illumination angles are used for object illumination, in par ticular with the exception of illumination angles close to perpendicular or average incidence on the object to be illuminated, an annular illumination setting with small illumination angles overall, that is to say illumination an gles close to perpendicular or average incidence, which itself can in turn be omitted, or dipole illumination settings, wherein the individual poles can each have a "leaflet" contour, that is to say an edge contour that corre sponds approximately to the section through a biconvex lens element.

Together with an imaging optical unit or projection optical unit 8, the illu mination system 4 constitutes an optical measurement system 9 of the me trology system 2.

With the illumination setting 5 respectively set, the illumination light 1 il luminates an object field 10 of an object plane 11 of the metrology system 2. A lithography mask 12, also referred to as a reticle, is arranged as a re flective object in the object plane 11. The object plane 11 runs parallel to the x-y-plane. In Figure 3, which shows more specific details for guiding the imaging light 1 through the projection optical unit 8 of the metrology system 2, ob ject structures 13 on the object which are to be imaged, facing the projec tion optical unit 8, are indicated as line structures running perpendicularly to the plane of the drawing in Figure 3.

The illumination light 1 is reflected from the lithography mask 12, as illus trated schematically in Fig. 1, and enters an entrance pupil of the imaging optical unit 8 in an entrance pupil plane. The used entrance pupil of the im aging optical unit 8 can have a circular or else elliptic boundary.

Within the imaging optical unit 8, the illumination or imaging light 1 prop agates between the entrance pupil plane and an exit pupil plane. A circular exit pupil of the imaging optical unit 8 lies in the exit pupil plane.

The imaging optical unit 8 images the object field 10 into an image field 14 in an image plane 15 of the metrology system 2. A magnifying imaging scale during the imaging by the projection optical unit 8 is greater than 500. Depending on the embodiment of the projection optical unit 8, the magnifying imaging scale can be greater than 100, can be greater than 200, can be greater than 250, can be greater than 300, can be greater than 400 and can also be significantly greater than 500. The imaging scale of the projection optical unit 8 is regularly less than 2000.

The projection optical unit 8 serves for imaging a section of the object 12 into the image plane 15.

A spatially resolving detection device 16 of the metrology system 2 is ar ranged in the image plane 15. This may involve a CCD camera. The metrology system 2 having the optical measurement system 9 is used for carrying out a method for determining an aerial image of the object 12 to be measured as a result of illumination and imaging with illumination and imaging conditions of an optical production system of an EUV projec tion exposure apparatus (not illustrated). The aerial image of the object 12 generated by the optical production system of the production projection ex posure apparatus can thus be simulated or emulated with the aid of the me trology system 2.

Main steps of this method are explained below with the aid of Figures 2 and 4 as well.

In a capturing step 17, the metrology system 2 captures a measurement aer- ial image I (x, y) of the object 12 to be measured with the illumination and imaging conditions of the optical measurement system 9. In this case, the measurement aerial image is captured with a predefined measurement illu mination setting, for example with the illumination setting 5. Intensity data I (x, y) of the measurement aerial image are generated during this captur- ing.

A subsequent reconstruction step 18 of the determining method involves reconstructing an object structure 13 in the form of a transfer function T _Mask (x, y) of the object 12 to be measured from the data I (x, y) of the captured measurement aerial image by means of a reconstruction algorithm. Data of the reconstructed object structure 13 are generated during this reconstruc tion step 18. Such an object structure reconstruction algorithm from cap tured measurement aerial image data is described in the technical article "Method for Retrieval of the Three-Dimensional Object Potential by Inver sion of Dynamical Electron Scattering" by Van den Broek et al.,

Phys.Rev.Lett 109, 245502 (2012). This reconstruction algorithm can also be applied to lithography masks. In this context, reference is made to WO 2017/207 297 Al.

In a subsequent simulation step 19 of the determining method, an electric field Ei (x, y) of a production aerial image, that is to say of an aerial image obtained by means of the optical production system of the production pro jection exposure apparatus, is simulated from the data T _Mask of the recon structed object structure 13 with the illumination and imaging conditions of the optical production system. Said illumination and imaging conditions of the optical production system include a production illumination setting 19a (cf. Figure 4), which differs from the measurement illumination system 5.

The production illumination setting 19a, which is illustrated by way of ex ample in Figure 4, has a circular edge contour 20. Alternatively, a produc tion illumination setting having an edge contour deviating from the circular shape, for example having an elliptic edge contour, can be used.

The production illumination setting 19a illustrated in Figure 4 is a freeform illumination setting. Such a freeform illumination setting cannot be de scribed by any of the standardized illumination settings "conventional", "annular", "dipole" or "multipole". The freeform production illumination setting 19a has a multiplicity of illuminated individual regions 21 within the edge contour 20 of the production illumination pupil. The individual re gions 21 are arranged in the manner of selected grid points of a point grid that completely covers the illumination pupil within the edge contour. Each of the illuminated individual regions 21 has the same typical diame ter. The typical diameter of the individual regions 21 can be in the range of between 0.5% and 10% of the total pupil area.

The illuminated individual regions 21 can have a circular boundary. The il luminated individual regions 21 are arranged in a manner distributed irreg ularly over the illumination pupil within the edge contour 20. The illumi nated individual regions 21 are arranged in a manner distributed over the il lumination pupil with varying surface density within the edge contour 20.

In the case of the illumination setting 19a, all the illuminated individual re gions are illuminated with the same illumination intensity. In the case of an alternative production illumination setting, an illumination intensity can vary in particular continuously over illuminated regions of the illumination pupil in the range between a minimum illumination intensity and a maxi mum illumination intensity, wherein the minimum illumination intensity is greater than 0.

In the case of a production illumination setting which otherwise corre sponds to the production illumination setting 19a, this variation of the illu mination intensity can be achieved by the different individual regions 21 being illuminated with different illumination intensities. In this case, cer tain individual regions from among the individual regions 21 can be illumi nated with the maximum illumination intensity and other individual regions 21 can be illuminated with lower illumination intensity, for example with 90%, 80%, 70%, 60%, 50%, 40%, 30%, 20%, 10%, 5%, 1% of the maxi mum illumination intensity. In this case, the different individual regions 21 can be illuminated with differently gradated illumination intensity or else a continuous variation of the illumination intensity is possible. With the use of a gradated illumination intensity, two steps, three steps, four steps, five steps, six steps, seven steps, eight steps, nine steps, ten steps or even more steps can be used.

The simulation of the production aerial image I (x, y) is also influenced by data concerning the imaging conditions of a projection optical unit 22 of the production projection exposure apparatus. The production projection optical unit 22, which generally differs greatly from the measurement pro jection optical unit 8 of the metrology system 2, is illustrated schematically on the far left in Figure 4. Alternatively, it is also possible to use a meas urement projection optical unit that corresponds to the production projec tion optical unit.

An image-side numerical aperture of the production projection optical unit 22 can be in the range of between 0.3 and 0.9, for example 0.33, 0.4, 0.45, 0.5, 0.55, 0.6, 0.65, 0.7. An imaging factor of the production projection op tical unit 22 can be less than 1, such that the production projection optical unit 22 images the object structure 13 in a reduced fashion into an image field 26 of the production projection exposure apparatus. This reduction can be four-fold, for example, thus resulting in an imaging scale of 0.25. Other imaging scales in the range of between 0.1 and 0.5 are also possible.

Besides the production illumination setting 19a, the simulation step 19 is also influenced by even further illumination conditions of the optical pro duction system of the production projection exposure apparatus, in particu lar an apparatus function of an illumination system 23 of the production projection exposure apparatus. Said apparatus function is influenced by data of the EUV light source and also data of the illumination and projec tion optical unit of the production projection exposure apparatus. Such data are e.g. data concerning the uniformity of the illumination, that is to say data representing a measure of how well an actual illumination intensity over the object field to be illuminated corresponds to a desired illumination intensity. These data can furthermore include data concerning the photon noise of the light source 3.

Moreover, the simulation step 19 can additionally be influenced by specific further properties of a coating of the object to be measured and/or of a sub strate onto which the object is imaged by the production system. Corre sponding optical data may be absorption coefficients of an absorption layer and/or of a multilayer.

Figure 4 elucidates the input variables that influence the simulation of the production aerial image I. They include a transfer function TPOB of the pro duction projection optical unit 22, the transfer function T _Mask of the object structure 13 determined during the reconstruction step 18, and also the illu mination conditions Em _u (u, v) of the production illumination system 23 in cluding the production illumination setting 19a. In this case, u, v are coor dinates in the frequency domain. The simulation is illustrated schematically by the box 27 in Fig. 4.

Moreover, the determining method (identified schematically by the refer ence sign 28 in Fig. 4) can additionally be influenced by system-specific effects, namely aberrations, measured during preparation for the determin ing method during the adjustment of the system and during a calibration of the system, uniformity data and data concerning photon noise, which are estimated from the data of the measurement aerial image I (x, y) and op tionally concomitantly included in the calculation. This step of including system- specific effects is illustrated at 24 in Figure 4. Overall, the deter mined aerial image of the object 12 to be measured arises as a re sult of the illumination and imaging with illumination and imaging condi tions of the optical projection system with the production projection optical unit 22 in accordance with the following formula:

Ei (x, y) = FT ¹ [TpoB x FT [T _MaSk (c', y') x FT ¹ [E _u (u, v)]]]

In this case, FT denotes Fourier transformation, FT ¹ denotes inverse Fou rier transformation u and v denote pupil coordinates of the production illu mination setting 19a and of the production projection system 23, respec tively, in the frequency domain.

The determining method makes it possible, for example, with the aid of a measurement illumination setting 5 which can be realised with a setting stop 7 producible with comparatively low outlay, to carry out an aerial im age determination (identified schematically by the reference sign 29 in Fig. 4) for a significantly more complex production illumination setting in the manner of the illumination setting 19a. The design requirements in respect of the setting stops 7 of the metrology system 2 are thus reduced.

The measurement aerial image I is captured in three dimensions. For this purpose, the object 12 is displaced step by step in the z-direction with the aid of an object displacement device 12a, which is illustrated schematically in Figure 1, such that on the basis of the imaging transfer of this z-displace- ment from the object plane 11 into the image plane 15, a plurality of 2D aerial images I (x, y, Zi) are generated for z-steps Zi in the region around the image plane 15. A 3D measurement aerial image (I (x, y, z)) then arises from the plurality of 2D aerial images. The image plane 15 is also referred to as measurement plane.