The invention relates to a method for approximating imaging properties of an optical production system to imaging properties of an optical measurement system. Further, the invention relates to a metrology system having a measurement system for performing such a method.
A metrology system is known from US 2017/0131 528 A1 (parallel document WO 2016/0124 425 A2) and from US 2017/0132782 A1.
It is an aspect of the present invention to improve an accuracy of an approximation of imaging properties of an optical production system to imaging properties of an optical measurement system, which can be part of a metrology system, in particular.
According to the invention, this aspect is achieved by an approximation method having the features specified in claim 1.
According to the invention, it was recognized that, for the purposes of approximating imaging properties of the optical production system to those of the optical measurement system, there is an improvement in the accuracy if it is not a wavefront difference between the two optical systems that is minimized but if the focus is on minimizing a deviation of the transfer functions of the two optical systems. In addition to the wavefront, the respective transfer function also includes, in particular, the illumination setting during the object illumination, i.e., an illumination angle distribution during the object illumination. Taking account of the illumination setting in the approximation method improves the imaging property approximation. In particular, the imaging property approximation can be undertaken object-independently such that, in any case for a certain class of objects, an adjustment position of the at least one adjustment component, which arises on account of the approximation method, leads to the desired approximation of the imaging properties for all objects of this class. In particular, such objects can be real objects, i.e., objects with a real mask transmission function, and/or weak objects, i.e., objects whose diffraction spectrum is dominated by the zero order of diffraction such that the zero order of diffraction makes up more than 90%, for example, of the diffraction intensity in a certain diffraction angle range.
The target transfer function can be an optimal transfer function, i.e., in particular, an aberration-free transfer function. Alternatively, it is also possible to work with a given wavefront aberration of the optical production system when specifying the target transfer function. The optical production system, firstly, and the optical measurement system, secondly, can be two different optical systems. In principle, however, it is also possible for the optical production system and the optical measurement system to be a system with the same structure.
Using the respectively found adjustment position of the at least one adjustment component, in which the deviation of the transfer functions from one another is minimized, it is then possible, in particular, to generate or emulate a 3D aerial image of the object with the aid of the optical measurement system. For each z-coordinate of the aerial image, i.e., for each coordinate perpendicular to the image plane, it is then possible to choose a different adjustment position of the at least one adjustment component, which different adjustment position has respectively emerged during the approximation method when minimizing the transfer function deviation taking account of the wavefront of the production system corresponding to this z-coordinate.
Adjustable degrees of freedom can be those of translation and/or those of rotation. As an alternative or in addition thereto, it is possible to deform an adjustment component for adjustment purposes.
An adjustment of a plurality of degrees of freedom of one and the same adjustment component according to claim 2 increases the options of the approximation method for minimizing the transfer function deviation.
This applies accordingly if, according to claim 3, use is made of a plurality of adjustable adjustment components. This plurality of adjustment components, too, can in turn be adjustable in more than one degree of freedom.
A method according to claim 4 increases the use possibilities of the approximation method and, as a consequence, of an aerial image emulation by the measurement system, brought in line with the production system in the case of the corresponding illumination setting.
Usable illumination settings can be a conventional illumination setting, an annular illumination setting with a small or a large illumination angle, a dipole illumination setting, a multi-pole illumination setting, in particular a quadrupole illumination setting. Poles of such a multi-pole illumination setting can have different edge contours, for example leaflet or lens-element-shaped edge contours.
By way of example, the method according to claim 5 facilitates a specification of adjustment positions of the at least one adjustment component for the purposes of emulating 3D aerial images.
The use of a lookup table according to claim 6 simplifies an aerial image emulation for various illumination settings.
In the case of the specified illumination setting, the measurement system can then be brought into the assigned adjustment position of the adjustment components, for example following a query of the manipulator positions from the lookup table. Subsequently, imaging with the measurement system can then be performed for a given object, said imaging yielding, e.g., a 2D value contribution for a 3D aerial image of the production system to be emulated.
The advantages of a metrology system according to claim 7 correspond to those that have already been explained above with reference to the approximation method according to the invention.
The metrology system can be used to measure a lithography mask provided for projection exposure for producing semiconductor components with very high structure resolution, which is better than 30 nm, for example, and which can be better than 10 nm, in particular.
An exemplary embodiment of the invention is explained in greater detail below with reference to the drawing. In said drawing:
The illumination system 4 contains an EUV light source and an illumination optical unit, neither of which are illustrated in detail. The light source can be a laser plasma source (LPP; laser produced plasma) or a discharge source (DPP; discharge produced plasma). In principle, a synchrotron-based light source may also be used, e.g., a free electron laser (FEL). A used wavelength of the illumination light 1 can lie in the range between 5 nm and 30 nm. In principle, in a variant of the projection exposure apparatus 2, a light source for another used light wavelength can also be used, for example for a used wavelength of 193 nm.
In the illumination optical unit of the illumination system 4, the illumination light 1 is conditioned so that a certain illumination setting of the illumination, i.e., a specific illumination angle distribution, is provided. A specific intensity distribution of the illumination light 1 in an illumination pupil of the illumination optical unit of the illumination system 4 corresponds to this illumination setting.
In order to facilitate the presentation of positional relationships, a Cartesian xyz-coordinate system is used hereinafter. In
The illumination light 1 illuminates an object field 5 of an object plane 6 of the projection exposure apparatus 2 with the respectively set illumination setting, for example with one of the illumination settings according to
The illumination light 1 is reflected by the lithography mask 7, as illustrated schematically in
The illumination light or imaging light 1 propagates between the entrance pupil plane 9 and an exit pupil plane 10 within the imaging optical unit 3. A circular exit pupil 11 of the imaging optical unit 3 is located in the exit pupil plane 10. The imaging optical unit 3 is anamorphic and generates the circular exit pupil 11 from the elliptical entrance pupil 8.
The imaging optical unit 3 images the object field 5 into an image field 12 in an image plane 13 of the projection exposure apparatus 2. Below the image plane 13,
A wavefront aberration φ, illustrated schematically in
The imaging light intensities Iscanner (x, y, zw) at the various z-values around the image plane 13 are also referred to as a 3D aerial image of the projection exposure apparatus 2. The projection exposure apparatus 2 is embodied as a scanner. Firstly, the lithography mask 7 and, secondly, a wafer disposed in the image plane 13 are scanned, synchronously with respect to one another, during the projection exposure. As a result, the structure on the lithography mask 7 is transferred onto the wafer.
Components and functions, which have already been explained above with reference to
In contrast to the anamorphic imaging optical unit 3 of the projection exposure apparatus 2, a measurement imaging optical unit 15 of the metrology system 14 is embodied as an isomorphic optical unit, i.e., as an optical unit with an isomorphic imaging scale. Apart from a global imaging scale, an entrance measurement pupil 16 is converted in this case, true to form, into an exit measurement pupil 17. Together with the illumination system 4, the measurement imaging optical unit 15 of the metrology system 14 forms an optical measurement system for object imaging.
The metrology system 14 has an elliptical aperture stop 16a in the entrance pupil plane 9. The embodiment of such an elliptical aperture stop 16a in a metrology system is known from WO 2016/012 426 A1. This elliptical aperture stop 16a generates the elliptical entrance measurement pupil 16 of the measurement imaging optical unit 15. Here, the inner edge of the aperture stop 16a specifies the external contour of the entrance measurement pupil 16. This elliptical entrance measurement pupil 16 is converted into the elliptical exit measurement pupil 17. An aspect ratio of the elliptical entrance measurement pupil 16 can be just as large as that of the elliptical entrance pupil 8 of the imaging optical unit 3 of the projection exposure apparatus 2. In respect of the metrology system, reference is also made to WO 2016/012 425 A2.
The measurement imaging optical unit 15 has at least one displaceable and/or deformable measurement optical unit adjustment component. Such a measurement optical unit adjustment component is illustrated schematically at Mi as a mirror in
{right arrow over (a)} A displaceability or manipulability of the displaceable and/or deformable measurement optical unit adjustment component Mi is indicated schematically in
A spatially resolving detection device 20, which could be a CCD camera, is disposed in a measurement plane 19 of the metrology system 14, which represents an image plane of the measurement imaging optical unit 15. In a manner similar to
As a rule, the imaging optical unit 3 of the optical production system differs from the measurement imaging optical unit 15 of the optical measurement system, which is elucidated in the example above by the difference between anamorphic imaging by the production system and isomorphic imaging by the measurement system. Other and/or additional differences between the imaging optical units of the production system and of the measurement system which lead to the imaging of the imaging optical unit of the optical production system differing from that of the optical measurement system are also possible.
The object of the approximation or convergence method explained below is that of bringing the imaging properties of the optical measurement system in line with the imaging properties of the optical production system of the projection exposure apparatus 2 by way of an adjustment displacement of the at least one measurement optical unit adjustment component Mi in such a way that a correspondence between the aerial images IScanner of the optical production system and Imeasured of the optical measurement system that is as good as possible arises for different objects to be imaged in the case of the resultant adjustment of the measurement imaging optical unit. Here, it was recognized that an optimization of such an approximation of the imaging properties can be improved by virtue of the goal not being minimization of the wavefront difference but that, in fact, minimization of the deviation of illumination setting-dependent transfer functions leads to a better result.
In the imaging property approximation method according to the invention, the difference between the wavefronts of the optical units 3, 15 is not optimized independently of the set illumination setting; instead, there is an illumination setting-dependent minimization of the difference between the transfer functions of, firstly, the optical production system of the projection exposure apparatus 2 (transfer function TP) and, secondly, the measurement system of the metrology system 14 (transfer function TM).
To this end, a production transfer function TP of the imaging by the production system is initially determined as a target transfer function, with the production transfer function TP being dependent on a certain, selected target illumination setting for an object illumination, for example for the illumination setting according to
What is exploited here is that, depending on the spatial frequency coordinates k and depending on the component degrees of freedom a of the components of the associated imaging optical unit, a spectrum F of an aerial image, i.e., a Fourier transform of the aerial image, can be approximately described as follows:
F({right arrow over (k)}, {right arrow over (a)})≈F0+F1(T0(σ, A)−iT1u(φ, σ, A)+T2g(φ, σ, A))=F0+F1T (1)
This approximate relationship applies to real masks, i.e., to masks without an imaginary part of a mask transmission function. Moreover, this relationship applies to weak masks, i.e., to objects whose object spectrum is dominated by the zero order of diffraction.
Here, F0 is a constant diffraction background of the mask. F1 is a spatial frequency-dependent factor, which depends only on the mask and not on properties of the imaging optical unit. T0, T1 and T2 are contributions to the transfer function T, which depend only on the properties of the imaging system and not on the mask.
Here, the following applies:
T
0({right arrow over (k)})=2σ*A (2)
Here, σ is the specified illumination setting. A({right arrow over (k)}) is an amplitude apodization form of the respective imaging optical unit (1 within the available numerical aperture; 0 outside). * denotes a convolution operator.
T
1
u({right arrow over (k)})=T1({right arrow over (k)})−T1(−{right arrow over (k)}) (3)
Here, T1 ({right arrow over (k)})=σφ*A−σ*φ (4)
Here, φ is the respective wavefront of the imaging optical unit which, in the case of the measurement imaging optical unit 15, is dependent on the respective position {right arrow over (a)} of the at least one measurement optical unit adjustment component.
T
2
g({right arrow over (k)})=T1({right arrow over (k)})−T1(−{right arrow over (k)}) (5)
Here, T2({right arrow over (k)})=σφ* φ−½σφ2*A−½σ*φ2 (6)
Determining an optical transfer function of an imaging optical unit for weak objects is described, for example, in the article “High-resolution transport-of-intensity quantitative phase microscopy with annular illumination” by C. Zuo et al., Scientific Reports, 7:7654/DOI: 10.1038/s41598-017-06837-1 (www.nature.com/scientificreports), published on Aug. 9, 2017.
Thus, in the case of weak real masks, a minimization of the difference between the transfer functions T for, firstly, the optical production system and, secondly, the optical measurement system leads to a minimization of the difference between the spectra and, as a consequence, to the desired minimization of the aerial images.
By inserting the determinable values for the illumination setting σ, the apodization function A and the wavefront φ, it is possible to determine the transfer functions TP, TM for, firstly, the optical production system (production transfer function) and, secondly, the optical measurement system (measurement transfer function).
By way of the wavefront φ, the measurement transfer function TM depends on the respective adjustment position {right arrow over (a)} of the at least one measurement optical unit adjustment component Mi. Now, using a numerical optimization method, a minimum of the deviation of the production transfer function TP from the measurement transfer function TM is searched for by varying the adjustment degrees of freedom of the at least one measurement optical unit adjustment component.
Once again, this minimization can be implemented as an RMS minimization, and so the following expression is minimized:
|TP({right arrow over (k)})−TM({right arrow over (k)})|2 (7)
Examples of mask structures of the lithography mask 7 which were found to be suitable for this approximation method are line structures with a critical dimension (CD) ranging between 8 nm and 30 nm and a pitch ranging between 1:1 and 1:2. Here, it is possible to resolve defects with a typical size ranging between 2×2 nm2 and 5×5 nm2. Here, the defects on the lithography mask 7 may occur as elevations or as cutouts. Defocus values ranging up to 30 nm, for example +/−22 nm, can be taken into account here during the approximation method in the imaging properties of the optical production system. What emerges for these boundary conditions is that the minimization of the deviation of the transfer functions, as explained above, leads to better approximation results than a pure minimization of the deviation of the wavefronts, as explained above on the basis of
The production transfer function TP can be determined for various relative image positions, which deviate from an ideal relative image position (defocus equal to 0) in the image field of the projection system.
Depending on the illumination setting, a specific set of adjustment values arises for the measurement optical unit adjustment component or for the measurement optical unit adjustment components. The associated manipulator positions can be assigned to the respective illumination settings and stored in a lookup table. Then, if an optimum approximated aerial image of the optical measurement system should be produced in the case of a certain illumination setting, the set of manipulator settings matching the chosen illumination setting can be queried, and set, by querying the values of the lookup table.
Number | Date | Country | Kind |
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102019206648.8 | May 2019 | DE | national |
This application is a continuation of and claims priority under 35 U.S.C. § 120 from PCT Application No. PCT/EP 2020/062 836, filed on May 8, 2020, which claims priority from German Application No. 10 2019 206 648.8, filed on May 8, 2019. The entire contents of the above applications are incorporated herein by reference.
Number | Date | Country | |
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Parent | PCT/EP2020/062836 | May 2020 | US |
Child | 17519906 | US |