Method and measuring setup for topography measurement on a surface

Abstract

The invention relates to a method and a measuring arrangement for topography measurement on a surface. In this process, the surface is scanned line by line in a scanning operation carried out in a measuring arrangement to generate an image composed of a plurality of scan lines, wherein, during this generation of the image, a phase shift between adjacent scan lines is set depending on at least one determined disturbance frequency of a disturbance occurring in the measuring arrangement during the scanning operation.

Description

BACKGROUND OF THE INVENTION Area of ​​the invention

[0001] The invention relates to a method and a measuring arrangement for topography measurement on a surface. State of the art

[0002] Microlithography is used to manufacture microstructured components, such as integrated circuits or LCDs. The microlithography process is carried out in a projection exposure system, which includes an illumination unit and a projection lens. The image of a mask (= reticulum) illuminated by the illumination unit is projected by the projection lens onto a substrate (e.g., a silicon wafer) coated with a photosensitive layer (photoresist) and positioned in the image plane of the projection lens. This transfers the mask structure onto the photosensitive coating of the substrate.

[0003] In projection imaging systems designed for the EUV range, i.e. at wavelengths below 15 nm (e.g. about 13.5 nm or about 7 nm), mirrors are used as optical components for the imaging process due to the lack of availability of suitable translucent refractive materials.

[0004] Approaches to increasing the image-side numerical aperture to improve resolution (NA) involve enlarging the mirror surface area. At the same time, there is a practical need to adjust the effect of the optical element on the system wavefront within the respective optical system as precisely as possible, ideally according to a predefined specification. In addition to locally long-wavelength surface defects of the optical element that affect the system wavefront, locally short-wavelength surface defects can also be significant with regard to scattered light losses.

[0005] In particular, the existing conditions for surface finishing in microlithography applications typically require highly precise machining with accuracies in the nanometer or even picometer range. During mirror manufacturing, a roughness assessment is typically carried out, for example, using interferometric or microscopic measurements (e.g., with an atomic force microscope, AFM), to determine whether further surface finishing is necessary.

[0006] This roughness assessment presents a significant challenge with increasing dimensions, such as those of EUV mirrors (e.g., with diameters on the order of 1 meter or more), given the existing high accuracy requirements. As the size or mass of the mirror to be measured with regard to its surface topography increases, so does the required size of the suitable measuring setup (e.g., the atomic force microscope), which in turn leads to increased sensitivity to unwanted, but usually unavoidable, vibration disturbances that occur during topography measurement.

[0007] In practice, the problem arises that it is not easy to distinguish between the aforementioned disruptive influences on the measurement process, e.g., due to vibrations, on the one hand, and the influence of the topography of the test object or mirror to be measured on the measurement result, with the consequence that the measurement result is ultimately distorted by the aforementioned disruptive influences.

[0008] However, if the topography measurement on the surface of the test object or mirror is faulty, the processing carried out based on this topography measurement is ultimately also incorrect, which in turn impairs the optical properties of the mirror and the optical system containing this mirror (e.g. the microlithographic projection exposure system). SUMMARY OF THE INVENTION

[0009] It is an object of the present invention to provide a method and a measuring arrangement for topography measurement on a surface which enables comparatively fast and reliable characterization even for optical elements that have comparatively large dimensions or are designed for optical systems with high numerical aperture.

[0010] This problem is solved by the method and the measuring arrangement according to the features of the dependent claims.

[0011] In an inventive method for topography measurement on a surface, - the surface is scanned line by line in a scanning process carried out in a measuring setup to produce an image composed of a plurality of scan lines; - wherein, in generating this image, a phase shift between adjacent scan lines is set depending on at least one determined disturbance frequency of a disturbance occurring in the measurement setup during the scanning process.

[0012] The invention is based in particular on the concept of setting a phase shift between adjacent scan lines in a scanning process carried out during a topography measurement on a surface (e.g., an EUV mirror), in which the surface is scanned line by line and an image composed of a plurality of scan lines is generated, with the result that a disturbance acting on the scanning process in the measuring arrangement used (e.g., vibration) does not disappear as such, but its effect on the measurement result is reduced (i.e., the disturbance is at least partially “concealed”).

[0013] The invention is based on the understanding that, if a disturbance is present in the measuring setup affecting the scanning process or occurring during the scanning process, the image composed of multiple scan lines typically exhibits a wave or fringe pattern that interferes with or overlays the evaluation of the topography measurement. This pattern, in turn, is associated with one or more pronounced peaks at specific spatial wavelengths in a 2D ISO PSD calculated to determine the scattered light intensity (as explained in more detail below). The radial profile is considered in the calculated 2D Fourier transform as the 2D ISO PSD, based on isotropic scattered light generation.

[0014] However, this is a measurement artifact insofar as the scattered light generation by the surface or mirror corresponding to this peak in the 2D Fourier transformation does not actually occur in the relevant scattering angle range, but is only falsely "simulated".

[0015] In other words, according to the invention, the topography measurement is carried out by introducing the said phase shift in such a way that this topography measurement is ideally “blind” to the disturbance occurring during the scanning process.

[0016] According to one embodiment, the topography measurement is performed as a roughness measurement. In other embodiments, however, the topography measurement can also include other measurements, e.g., determinations of the grid height or width of grids or diffractive structures.

[0017] According to one embodiment, the interference frequency is determined in advance before the topography measurement is carried out. The invention particularly includes the approach of determining the interference frequency relevant as the basis for the phase shift according to the invention based on prior knowledge, e.g., from a previous measurement or simulation (i.e., before the actual scanning process during the topography measurement), whereby the phase shift setting according to the invention can then be set uniformly for the entire image.

[0018] According to one embodiment, the interference frequency is determined after the topography measurement has begun.

[0019] According to one embodiment, this determination of the interference frequency is carried out repeatedly, in particular after each scan line has been recorded.

[0020] This embodiment of the method according to the invention has the advantage – although it involves increased measurement and, if necessary, control engineering effort – that the determination of the disturbance frequency and the setting of a suitable phase shift can be carried out dynamically during the ongoing operation of the measuring arrangement, which in particular allows temporal fluctuations of the disturbance occurring in the measuring arrangement during the scanning process to be taken into account by means of a suitable adjustment of the phase shift according to the invention.

[0021] According to one embodiment, the interference frequency is determined as the dominant interference frequency of the disturbance occurring in the measuring arrangement during the scanning process.

[0022] According to one embodiment, the phase shift is adjusted such that a predetermined peak in the 2D-PSD spectrum is reduced compared to an analogous scenario without adjusting the phase shift.

[0023] According to one embodiment, the measuring arrangement is an atomic force microscope. However, the invention is not limited to this, but can also be advantageously applied in other measuring arrangements (e.g., another microscope or a camera).

[0024] According to one embodiment, the disturbance occurring in the measuring arrangement during the scanning process is a vibration disturbance or a disturbance of electromagnetic coupling.

[0025] According to one embodiment, the disturbance occurring in the measuring arrangement during the scanning process has a frequency in the range of 100 Hz to 250 Hz.

[0026] According to one embodiment, the topography measurement is performed on the surface of an optical element, particularly for microlithography. However, the invention is also fundamentally applicable to the characterization of other surfaces.

[0027] According to one embodiment, the optical element is a mirror. However, the invention is not limited to this; in other applications, the optical element could also be, for example, a lens.

[0028] According to one embodiment, the optical element is designed for an operating wavelength of less than 30 nm, in particular less than 15 nm. However, the invention is not limited to this, and in further applications the optical element can also be designed for a different operating wavelength.

[0029] The invention further relates to a measuring arrangement for topography measurement on a surface, wherein the measuring arrangement is configured to carry out a method with the features described above.

[0030] Further embodiments of the invention can be found in the description and the dependent claims.

[0031] The invention is explained in more detail below with reference to an embodiment shown in the accompanying figures. BRIEF DESCRIPTION OF THE DRAWINGS

[0032] They show: Fig. 1 a schematic and simplified diagram to illustrate a stripe pattern occurring in a topography measurement carried out in a conventional manner and without the phase shift according to the invention; Fig. 2 a schematic and simplified diagram to illustrate the effect of the phase shift according to the invention on the result of the topography measurement; Fig. 3a-3b Diagrams to illustrate the effect of the phase shift according to the invention on a 2D-ISO-PSD determined in the evaluation of the topography measurement, wherein Fig. 3a a scenario without the phase shift adjustment according to the invention and Fig. 3b corresponds to a scenario with phase shift adjustment; and Fig. 4 A schematic representation to illustrate the possible setup of a microlithographic projection exposure system designed for operation in the EUV. DETAILED DESCRIPTION OF PREFERRED EXECUTION FORMS

[0033] The following refers to the information in Fig. 1, Fig. 2 and Fig. Diagrams 3a-3b illustrate an exemplary sequence of a method according to the invention for topography measurement on the surface of an optical element (such as an EUV mirror) and describe the mode of operation of this method. Reference is made here—without limiting the invention to this—to a measuring arrangement in the form of an atomic force microscope and a roughness measurement performed with it. Furthermore, it is assumed, also by way of example—and without limiting the invention to this—that the disturbance occurring during the operation of the measuring arrangement is a vibration disturbance.

[0034] Fig. Figure 1 initially shows, in a purely schematic and highly simplified representation, a typical striped pattern obtained during a topography measurement, which is attributable to a vibration disturbance occurring in the measuring arrangement during a scanning process. It should be noted that, for the purpose of a black-and-white-friendly representation, the grayscale levels that more precisely describe the wave pattern characteristic of the vibration disturbance have been omitted; however, the concept according to the invention is still valid even in the simplified (black-and-white) representation. Fig. 1-2 becomes clear.

[0035] The evaluation of the roughness measurement performed with the measuring setup or the AFM, or of the image acquired in the process (composed of multiple scan lines), is carried out in a known manner with the aim of predicting scattered light in specific directions by calculating a two-dimensional Fourier transform. Depending on the specific application scenario, the scattered light prediction can be simplified by assuming isotropic scattering and thus using the radial profile (so-called 2D-ISO-PSD) in the calculated 2D Fourier transform for the scattered light prediction.

[0036] The aforementioned vibration disturbance occurring during the scanning process when capturing the image during topography measurement, or the associated stripe or wave pattern in the image, now leads to a pronounced interference peak in the aforementioned 2D-ISO-PSD, which ultimately distorts the scattered light prediction.

[0037] To take this into account, according to the present invention, a phase shift between adjacent scan lines is set during the generation of the image while performing the topography measurement (typically by setting a suitable waiting time at the beginning of each line scan), wherein the corresponding suitable phase delay is in turn set depending on a determined disturbance frequency of said disturbance occurring during the scanning process.

[0038] The phase shift can be, for example, 180° or π (or a corresponding value plus an integer multiple of 2π). However, the invention is not limited to these values; other phase shift values ​​are also intended to reduce, if necessary only partially, the striped or wave pattern or the corresponding interference peak in the 2D ISO PSD caused by vibration disturbance. In further embodiments, a statistically distributed phase shift between adjacent scan lines, ranging from 0° to 360°, can also be set.

[0039] As a result, the phase shift adjustment according to the invention leads to the following: Fig. 2. The characteristic striped or wave pattern of the disturbance is hardly recognizable anymore. Furthermore, in the calculation of the two-dimensional Fourier transform or the 2D ISO PSD, which is relevant for scattered light prediction, the phase delay setting according to the invention manifests itself in such a way that the above-described and in Fig. 3a the clearly recognizable interference peak almost completely disappears and instead a comparatively high-frequency noise occurs at comparatively short spatial wavelengths (« 500 nm), whereby the latter effect lies outside the specified range relevant for scattered light prediction and is therefore uncritical.

[0040] As a result, the disturbance occurring during the scanning process in topography measurement is not eliminated as such, but its effect on the scattered light prediction according to the invention is “disguised”, since the result of the topography measurement is no longer distorted by said disturbance.

[0041] Fig. Figure 4 shows, as an application example of an optical element or mirror characterized within the scope of the invention, a schematic representation of a projection exposure system 101 designed for operation in EUV. The invention can be used for surface characterization during or after the fabrication of any optical element of this projection exposure system. However, the invention is not limited to its implementation in the manufacture of optical elements for operation in EUV or to optical elements in general. In particular, the invention can also be implemented in the topography measurement of optical elements for other operating wavelengths (e.g., in the VUV range or at wavelengths smaller than 250 nm) as well as for use in other optical systems (not intended for microlithography).

[0042] According to Fig. Figure 4 of the projection exposure system 101 comprises a lighting device 102 and a projection lens 110. The lighting device 102 serves to illuminate an object field 105 in an object plane 106 with radiation from a radiation source 103 via a lighting optic 104. A reticule 107 arranged in the object field 105 is exposed. The reticule 107 is held by a reticule holder 108. The reticule holder 108 can be moved, particularly in a scanning direction, via a reticule displacement drive 109. Fig. Figure 4 shows a Cartesian xyz coordinate system for illustrative purposes. The x-direction runs perpendicular to the plane of the drawing. The y-direction runs horizontally, and the z-direction runs vertically. The scan direction runs in Fig. 4 along the y-direction. The z-direction runs perpendicular to the object plane 106.

[0043] The projection lens 110 serves to image the object field 105 onto an image field 111 in an image plane 112. A structure on the reticulum 107 is imaged onto a light-sensitive layer of a wafer 113 located in the image field 111 within the image plane 112. The wafer 113 is held by a wafer holder 114. The wafer holder 114 can be displaced, particularly along the y-direction, via a wafer displacement drive 115. The displacement of the reticulum 107 via the reticulum displacement drive 109 and of the wafer 113 via the wafer displacement drive 115 can be synchronized.

[0044] Radiation source 103 is an EUV radiation source. Specifically, radiation source 103 emits EUV radiation, which is also referred to as useful radiation or illumination radiation. This useful radiation has a wavelength in the range between 5 nm and 30 nm. Radiation source 103 could be, for example, a plasma source, a synchrotron-based radiation source, or a free-electron laser (FEL). The illumination radiation 116, emanating from the radiation source 103, is focused by a collector 117 and propagated through an intermediate focus in an intermediate focal plane 118 into the illumination optics 104. The illumination optics 104 has a deflecting mirror 119 and, downstream in the beam path, a first faceted mirror 120 (with schematically indicated facets 121) and a second faceted mirror 122 (with schematically indicated facets 123).

[0045] The projection lens 110 has a plurality of mirrors Mi (i = 1, 2, ...) which are numbered according to their arrangement in the beam path of the projection exposure unit 101. In the Fig. In the example shown, the projection lens 110 has six mirrors M1 to M6. Alternatives with four, eight, ten, twelve, or any other number of mirrors Mi are also possible. The penultimate mirror M5 and the last mirror M6 each have a passage for the illumination radiation 116. The projection lens 110 is a double-obscured optical system. The projection lens 110 has an image-side numerical aperture, which, for example, can be larger than 0.3, and in particular larger than 0.5, and furthermore, larger than 0.6.

[0046] Even though the invention has been described with reference to specific embodiments, numerous variations and alternative embodiments are apparent to the person skilled in the art, for example, through the combination and / or exchange of features of individual embodiments. Accordingly, it is understood to the person skilled in the art that such variations and alternative embodiments are included in the present invention, and that the scope of the invention is limited only to the extent of the appended claims and their equivalents.

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