Method for determining generated aerial images of a measured object

Abstract

To determine a generated aerial image of the measurement object as a result of illumination and imaging under illumination and imaging conditions of the optical generation system, first, a measurement aerial image of the measurement object is captured (17). This is performed under illumination and imaging conditions of the optical measurement system, including a predetermined measurement illumination setting. Measurement aerial image data is generated during this capturing. An object structure of the measurement object is reconstructed from the captured measurement aerial image data by a reconstruction algorithm (18). Data of the reconstructed object structure is generated during this reconstruction. A generated aerial image is simulated (19) from the reconstructed object structure data under illumination and imaging conditions of the optical generation system, including a generation illumination setting different from the measurement illumination setting. This results in a determination method with relaxed requirements for the optical measurement system used in the determination method, even under harsh illumination and imaging conditions of the optical generation system.

Description

[Technical field]

[0001] The content of German Patent Application No. 10 2019 208 552.0 is incorporated herein by reference.

[0002] The invention relates to a method for determining a production aerial image of an object to be measured as a result of illumination and imaging under illumination and imaging conditions of an optical production system, and further to a metrology system comprising an optical measurement system for carrying out the method. [Background technology]

[0003] Measuring systems are known from US 2017 / 0131528 (corresponding document: WO 2016 / 0124425 A2) and US 2017 / 0132782. WO 2017 / 207297 A1 discloses a method for predicting the imaging result of a lithography mask. Summary of the Invention

[0004] The aim of the present invention is to develop such a determination method such that, even under the harsh illumination and imaging conditions of the optical production system, the requirements for the optical measurement system used in the determination method as described in the introduction are relaxed.

[0005] This object is achieved according to the invention by a determination method having the features defined in claim 1.

[0006] According to the invention, it is recognized that the determination of the generated aerial image can be performed using an optical measurement system in which a measurement illumination setting different from the generation illumination setting is used, even for difficult illumination and imaging conditions of the optical generation system, and the measurement illumination setting can be configured in such a way that it is easier to realize in the optical measurement system, resulting in a reduced requirement for the overall optical measurement system. In particular, it is possible to simulate a generated aerial image in the measurement setup that is impossible or difficult to realize, for example because it requires an aperture structure that is impossible or difficult to manufacture.

[0007] Based on the results of the simulation-generated aerial image, i.e. the determination method, the object structure of the measured object can be optimized until the simulation-generated aerial image corresponds to the predetermined aerial image. The determination method can thus be part of an iterative process of optimizing the object structure until it is optimized for the illumination and imaging conditions of an optical generation system that generates the image structure, which in turn is the starting point for the generation of correspondingly structured semiconductor components with very high resolution, for example if the optical generation system is used for the generation of micro- or nanostructured semiconductor components.

[0008] The measurement illumination setting includes a predetermined illumination aperture of the optical measurement system, the predetermined illumination aperture being predefined by an edge contour of an illumination pupil.

[0009] In this determination method, the object structure reconstruction method known from the technical papers “Method for Retrieval of the Three-Dimensional Object Potential by Inversion of Dynamical Electron Scattering” by Van den Broek et al., Phys.Rev.Lett 109, 245502 (2012) and WO2017 / 207297A1 can be used.

[0010] In optimizing object structures, as an example, object defects can be identified and optionally repaired.

[0011] It has proven to be advantageous to predefine the measurement illumination setting by means of a setting stop as claimed in claim 2.

[0012] A generated illumination setting having an elliptical edge profile of the illumination pupil according to claim 3 is an example of a stringent generated illumination setting.

[0013] The same applies mutatis mutandis to a free-form or SMO (Source Mask Optimization) illumination setup as claimed in claim 3. Such free-form illumination setups cannot be described by any of the standardized illumination setups "conventional", "annular", "dipole" or "multipole", but rather are identified by the free shaping of the arrangement of pupil regions where illumination light impinges in the illumination pupil. For the SMO method, reference is made to the technical paper "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).

[0014] The generated lighting setting with varying lighting intensity according to claim 5 is another example of a strict generated lighting setting. The minimum lighting intensity can be greater than 1% or 10% of the maximum lighting intensity. The minimum lighting intensity can be less than 50% of the maximum lighting intensity.

[0015] It has been found to be advantageous in an optical production system to have a generating illumination setting as claimed in claim 6. The individual illuminated areas in the illumination pupil can be arranged separately from one another.

[0016] It is practically or completely impossible to predefine such resulting illumination settings by aperture settings, since the production costs of such aperture settings would be prohibitively high, if they were even possible at all.

[0017] It has likewise proven advantageous to predefine the resulting light settings with individual area configurations or individual area arrangements as claimed in claims 7 to 9. The individual areas may have circular boundaries, but this is not essential.

[0018] The advantages of the measurement system as claimed in claim 10 correspond to those already explained above with reference to the convergence method according to the invention.

[0019] This metrology system is capable of measuring lithography masks which are subjected to projection exposure to produce semiconductor components with a very high structure resolution, for example less than 30 nm, in particular even less than 10 nm, which may be possible.

[0020] The measuring system according to claim 11 is flexibly usable: it can be equipped with a number of variable setting apertures which are interchangeable in an automated manner by means of changing mounts.

[0021] An exemplary embodiment of the present invention will now be described in more detail with reference to the drawings. [Brief description of the drawings]

[0022] [Figure 1] FIG. 2 shows a schematic representation of a metrology system for determining an aerial image of a measurement object in the form of a lithographic mask, the metrology system comprising an illumination system, an imaging optical unit and a spatially resolved detection device. [Diagram 2] 2 illustrates a sequence of main method steps of a method of using the measurement system described in FIG. 1 to determine an aerial image of a measurement object as a result of illumination and imaging under illumination and imaging conditions of the optical generation system. [Diagram 3] FIG. 2 shows a more detailed yet schematic representation of the imaging of a lithographic mask by an imaging optical unit of the metrology system according to FIG. 1; [Figure 4] FIG. 2 shows a schematic diagram of input variables influencing the determined generated aerial image, i.e. in particular data relating to the optical generation system that images the measured object, data of the reconstructed object structure, and data relating to the illumination conditions of the optical generation system with a generation illumination setting that differs from the measurement illumination setting of the measurement system described in FIG. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0023] 1 is a cross-sectional view corresponding to a meridional section showing the optical path of EUV illumination light or imaging light 1 in a measurement system 2. The illumination light 1 is generated by an EUV light source 3.

[0024] In order to facilitate the display of positional relationships, an xyz Cartesian coordinate system is used below. In Fig. 1, the x-axis extends perpendicular to the paper surface. The y-axis extends to the right in Fig. 1. The z-axis extends upward in Fig. 1.

[0025] The light source 3 can be a laser plasma source (laser produced plasma (LPP)) or a discharge source (discharge produced plasma (DPP)). In principle, a synchrotron-based light source (for example a free electron laser (FEL)) can also be used. The used wavelength of the illumination light 1 can be in the range of 5 nm to 30 nm. In principle, in the case of a variant of the projection exposure apparatus 2, it is also possible to use a light source with some other used light wavelength (for example a used wavelength of 193 nm).

[0026] In an illumination optical unit (not shown in more specific details) of the illumination system of the measurement system 2, to which the light source 3 also belongs, the illumination light 1 is adjusted in order to provide a specific illumination setting 5, i.e. a specific illumination angle distribution, of the illumination. Said illumination setting 5 corresponds to a specific intensity distribution of the illumination light 1 in the illumination pupil of the illumination optical unit of the illumination system 4.

[0027] In Figure 1, an example of an illumination setup 5 is shown diagrammatically as it exists in the plane of the page as an annular illumination setup having a total of four illumination poles 6 with webs and shaped approximately as quadrants. The illumination pupil in which the illumination setup 5 exists is in fact oriented perpendicular to the plane of the page of Figure 1 and the direction of propagation of the illumination light 1 through the illumination pupil.

[0028] In the illumination pupil, there is in each case a defined illumination intensity at the location of the illumination poles 6 and no illumination intensity elsewhere. The illumination setting 5 can be predefined by a setting diaphragm 7 which transmits the illumination light 1 at the location of the illumination poles 6 and blocks the illumination light around the illumination poles 6. An example of such a setting diaphragm 7 is a metal sheet with a passage opening whose shape exactly corresponds to the shape of the illumination poles 6. The setting diaphragm is arranged in the pupil plane of the illumination optical unit of the measurement system 2.

[0029] A change holder 7a shown in FIG. 1 allows the setting aperture 7 to be exchanged for an exchange setting aperture which changes the respective measurement illumination setting.

[0030] Instead of the shown quadrupole illumination setting 5, it is also possible to predefine other illumination settings in the measurement system 2, such as, for example, a conventional illumination setting in which practically all illumination angles are used for object illumination (in particular with the exception of illumination angles close to normal or mean incidence on the object to be illuminated), an annular illumination setting with small illumination angles overall, i.e. illumination angles close to normal or mean incidence which themselves can then be omitted, or a dipole illumination setting in which the individual illumination poles can each have an edge contour that approximately corresponds to the outline of a "leaflet", i.e. a section through a biconvex lens element, by using correspondingly different shapes and / or distributions of passing apertures.

[0031] The illumination system 4 together with an imaging or projection optical unit 8 forms an optical measurement system 9 of the metrology system 2 .

[0032] With the respective illumination settings 5, the illumination light 1 illuminates an object field 10 in an object plane 11 of the measurement system 2. A lithographic mask 12, also called a reticle, is arranged as a reflective object in the object plane 11. The object plane 11 extends parallel to the xy-plane.

[0033] FIG. 3, which shows in more detail the guidance of the imaging light 1 through the projection optical unit 8 of the measurement system 2, shows the object structure 13 of the object to be imaged facing the projection optical unit 8 as a line structure extending perpendicular to the plane of the paper of FIG. 3.

[0034] As shown diagrammatically in Figure 1, illumination light 1 is reflected by a lithographic mask 12 and enters the entrance pupil of an imaging optical unit 8 in an entrance pupil plane. The entrance pupil of an imaging optical unit 8 used in this way may have circular or elliptical boundaries.

[0035] In the imaging optical unit 8, the illumination light or imaging light 1 propagates between an entrance pupil plane and an exit pupil plane. A circular exit pupil of the imaging optical unit 8 is present in the exit pupil plane.

[0036] The imaging optical unit 8 images the object field 10 into an image field 14 in an image plane 15 of the measurement system 2. The imaging by the projection optical unit 8 has an imaging magnification of more than 500. Depending on the embodiment of the projection optical unit 8, the imaging magnification can be more than 100, more than 200, more than 250, more than 300, more than 400 or even much more than 500. The imaging magnification of the projection optical unit 8 is typically less than 2000.

[0037] The projection optical unit 8 serves to image a section of the object 12 onto an image plane 15 .

[0038] Located in the image plane 15 is a spatially resolving detection device 16 of the measurement system 2, which may include a CCD camera.

[0039] The metrology system 2 with the optical measurement system 9 is used to perform a method for determining an aerial image of a measured object 12 as a result of illumination and imaging under illumination and imaging conditions of an optical production system of an EUV projection exposure apparatus (not shown). The aerial image of the object 12 generated by the optical production system of the production projection exposure apparatus can thus be simulated or emulated by the metrology system 2.

[0040] The main steps of this method are explained below with reference to FIGS.

[0041] In a capturing step 17, the metrology system 2 captures a measurement space image I(x,y) of the measurement object 12 under illumination and imaging conditions of the optical measurement system 9. In this case, the measurement space image is captured with a predefined measurement illumination setting (e.g. illumination setting 5). Intensity data I(x,y) of the measurement space image is generated during this capturing.

[0042] In a subsequent reconstruction step 18 of the method, a reconstruction algorithm is used to derive from the acquired measurement space image data I(x,y) a transfer function T Mask An object structure 13 of the form (x,y) is reconstructed. Data of the reconstructed object structure 13 are generated during this reconstruction step 18. Such an object structure reconstruction algorithm from captured measurement aerial image data is described in the technical paper "Method for Retrieval of the Three-Dimensional Object Potential by Inversion 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 regard, reference is made to WO2017 / 207297A1.

[0043] In a subsequent simulation step 19 of the determination method, data T Mask from the electric field E of the generated aerial image, i.e. the aerial image obtained by the optical generation system of the generated projection exposure apparatus. I (x,y) is simulated. The illumination and imaging conditions of the optical production system include a production illumination setting 19a (see FIG. 4) which differs from the measurement illumination system 5.

[0044] As an example, the generating light setting 19a shown in Figure 4 has a circular edge contour 20. Alternatively, generating light settings having edge contours that deviate from a circular shape (eg, an elliptical edge contour) can be used.

[0045] The generating illumination setting 19a shown in Fig. 4 is a free-form illumination setting. Such a free-form illumination setting cannot be represented by any of the standardized illumination settings "conventional", "annular", "dipole" or "multipole". The free-form generating illumination setting 19a has a multiplicity of illuminated individual regions 21 within the edge contour 20 of the generating illumination pupil. The individual regions 21 are positioned at selected lattice points of a point grid that completely cover the illumination pupil within the edge contour.

[0046] Each of the illuminated individual regions 21 has the same typical diameter, which can be in the range of 0.5% to 10% of the total pupil area.

[0047] The illuminated individual regions 21 may have a circular boundary. The illuminated individual regions 21 are arranged in such a manner that they are irregularly distributed on (across) the illumination pupil within the edge contour 20. The illuminated individual regions 21 are arranged in such a manner that they are distributed with a varying surface density on (across) the illumination pupil within the edge contour 20.

[0048] In the case of the illumination setting 19a, all illumination individual regions are illuminated with the same illumination intensity. In the case of the other generative illumination settings, the illumination intensity can in particular vary continuously over the illuminated region of the illumination pupil in a range between a minimum illumination intensity greater than 0 and a maximum illumination intensity.

[0049] On the other hand, in the case of a generating illumination setting corresponding to the generating illumination setting 19a, this variation in illumination intensity can be realized by different individual regions 21 being illuminated with different illumination intensities. In this case, certain individual regions of the individual regions 21 can be illuminated with a maximum illumination intensity, while other individual regions 21 can be illuminated with a lower illumination intensity (for example, 90%, 80%, 70%, 60%, 50%, 40%, 30%, 20%, 10%, 5%, 1% of the maximum illumination intensity). In this case, different individual regions 21 can be illuminated with different stages of illumination intensity. Alternatively, a continuous variation of illumination intensity is possible. By using stages of illumination intensity, two, three, four, five, six, seven, eight, nine, ten or more stages can be used.

[0050] The simulation of the resulting aerial image I(x,y) is also influenced by data relating to the imaging conditions of the projection optical unit 22 of the production projection exposure apparatus. The production projection optical unit 22, which overall differs significantly from the measurement projection optical unit 8 of the metrology system 2, is shown diagrammatically at the left edge of Figure 4. Alternatively, it is also possible to use a measurement projection optical unit that corresponds to the production projection optical unit.

[0051] The image-side numerical aperture of the production projection optical unit 22 can be in the range of 0.3 to 0.9 (for example 0.33, 0.4, 0.45, 0.5, 0.55, 0.6, 0.65, 0.7). The imaging magnification of the production projection optical unit 22 can be less than 1, so that the production projection optical unit 22 demagnifies and images the object structure 13 into the image field 26 of the production projection exposure tool. This demagnification can be, for example, 4 times, so that the imaging magnification is 0.25. Other imaging magnifications in the range of 0.1 to 0.5 are also possible.

[0052] In addition to the production illumination settings 19a, the simulation step 19 is influenced by further illumination conditions of the optical production system of the production projection exposure apparatus, in particular by the apparatus functions of the illumination system 23 of the production projection exposure apparatus. Said apparatus functions are also influenced by data of the EUV light source and data of the illumination projection optical unit of the production projection exposure apparatus. Such data are for example data on the illumination uniformity, i.e. data that represent a measure of how the actual illumination intensity on the illuminated object field corresponds to the desired illumination intensity. These data may further include data on the photon noise of the light source 3.

[0053] Furthermore, the simulation step 19 may also be influenced by certain further properties of the coating of the object to be measured and / or the substrate on which the object is imaged by the production system. Corresponding optical data may be absorption coefficients of absorbing layers and / or multilayers.

[0054] FIG. 4 reveals the input variables that influence the simulation of the resulting aerial image I. These include the transfer function T POB , the transfer function T of the object structure 13 determined in the reconstruction step 18 Mask , and the lighting conditions E of the generating lighting system 23 including the generating lighting settings 19a illu (u,v), where u,v are frequency domain coordinates. The simulation is illustrated diagrammatically by box 27 in FIG.

[0055] Furthermore, the determination method (schematically identified by reference symbol 28 in FIG. 4) may also be affected by system-specific effects, i.e. data relating to aberrations, uniformity data and photon noise, measured during the preparation of said determination method during system adjustment and system calibration, which are deduced from data of the measurement aerial image I(x,y) and optionally data accompanying the calculation. The step involving said system-specific effects is indicated at 24 in FIG. 4. Overall, the determination aerial image I=Σ|E of the object to be measured 12 is calculated. I 2| results as a result of illumination and imaging under the illumination and imaging conditions of the optical projection system with the generation projection optical unit 22 according to the following formula:

number

[0056] In this case, FT stands for Fourier transform, and FT -1 denotes the inverse Fourier transform. u and v denote the pupil coordinates of the producing illumination setting 19a and the producing projection system 23, respectively, in the frequency domain.

[0057] This determination method allows for example to perform an aerial image determination of much more complex generated illumination settings, such as illumination setting 19a (schematically identified by reference symbol 29 in Fig. 4), by means of a measurement illumination setting 5 which can be realized by a relatively low-cost generateable set aperture 7. In this way, the design requirements for the set aperture 7 of the measurement system 2 are reduced.

[0058] The measurement space image I is captured in three dimensions. For this purpose, the object 12 is displaced stepwise in the z direction by the object displacement device 12a, which is shown diagrammatically in FIG. 1, and the z-step z i For multiple 2D aerial images (x, y, z i ) is generated. A 3D measurement aerial image (I(x,y,z)) then results from the multiple 2D aerial images. Image plane 15 is also referred to as the measurement plane.

Claims

1. 1. A method for determining a generated aerial image (I(x,y)) of a measurement object (12) as a result of illumination and imaging under illumination and imaging conditions of an optical generation system (22, 23), comprising: - capturing (17) a measurement space image (I(x,y)) of the measurement object (12) under illumination and imaging conditions of an optical measurement system (9), including a predetermined measurement illumination setting (5), wherein data of the measurement space image (I(x,y)) is generated during the capturing (17); A step (18) of reconstructing an object structure (13) of the measurement object (12) from the data of the captured measurement space image (I(x, y, z)) by a reconstruction algorithm, and data (T Mask ) is generated during said reconstructing step (18); The illumination and imaging conditions (T) of the optical generation system (22, 23) include a generation illumination setting (19a) different from the measurement illumination setting (5). POB , E illu ), and the data (T Mask simulating said generated aerial image (I(x,y)) from the Including, the optical generating system (22, 23) on the one hand and the optical measuring system (9) on the other hand are different optical systems; method.

2. 2. The method according to claim 1, characterized in that the measurement illumination setting (5) is predefined by a set aperture (7).

3. 3. The method according to claim 1 or 2, characterized in that the generated illumination setting (19a) has an elliptical edge contour of an illumination pupil.

4. The method according to any one of claims 1 to 3, characterized in that the generating illumination setting (19a) is a freeform or source mask optimized (SMO) illumination setting.

5. 5. The method according to claim 1, wherein the generating illumination setting (19a) has an illumination intensity on an illumination area of ​​an illumination pupil varying in a range between a minimum illumination intensity greater than 0 and a maximum illumination intensity.

6. The method according to any one of the preceding claims, characterized in that the generated illumination setting (19a) comprises a number of illuminated individual regions (21) in an illumination pupil.

7. 7. The method of claim 6, wherein the illuminated individual areas (21) each have the same typical diameter.

8. 8. Method according to claim 6 or 7, characterized in that the illuminated individual areas (21) are arranged in such a way that they are unevenly distributed on the illumination pupil.

9. Method according to any one of claims 6 to 8, characterized in that the illuminated individual areas (21) are arranged in such a way that they are distributed with a varying surface density on the illumination pupil.

10. A metrology system (2) comprising an optical measurement system (9) for carrying out the method according to any one of claims 1 to 9, an illumination system (4) comprising an illumination optical unit for illuminating the object (12) under test with a predetermined illumination setting (5), an imaging optical unit (8) for imaging a portion of the object (12) onto a measurement plane (15), a spatially resolved detection device (16) arranged in the measurement plane (15), Measurement system (2).

11. 11. The measurement system according to claim 10, characterized by a variable setting diaphragm (7) for predefining the measurement illumination setting (5).

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