Optical system for a metrology system and metrology system with such an optical system

Abstract

Optical system (21) for a metrology system (1) for measuring an object (2) in the form of a lithography mask, - with an object holder (14) for holding the object (2) in an object plane (13), - with a transmissive optical focusing component (11) arranged in the beam path of illumination light (4) between a light source (5) of the metrology system (1) and an object field (12) in the object plane (13), to generate an illumination focus (16) in the beam path of the illumination light (4) after the transmissive optical focusing component (11), - wherein the transmissive optical focusing component (11) has a focal length (f1) that is less than 5 mm, - with a detection device (6) for detecting the illumination light (4) in the beam path after the object field (12), - with an imaging optic (22) for imaging the illumination focus (16) generated by the transmissive optical focusing component (11) into a further illumination focus (16') in the area of ​​the object field (12).

Description

[0001] The invention relates to an optical system for a metrology system for measuring an object. Furthermore, the invention relates to a metrology system for measuring an object using such an optical system.

[0002] A metrology system of the type mentioned above is known, for example, from US 2012 / 0 008 123 A1. Further systems for measuring lithography masks are known from the articles by Na J. et al., "Application of actinic mask review system for the preparation of HVM EUV lithography with defect free mask," Proc. of SPIE Vol. 10145, 101450M-1; by Goldberg K. et al., "Actinic mask imaging: recent results and future directions from the SHARP EUV microscope," Proc. of SPIE Vol. 9049, 90480Y-1; and by Naulleau et al., "Electro-optical system for scanning microscopy of extreme ultraviolet masks with a high harmonic generation source," Optics Express, Vol. 22, 20144, 2014. Another metrology system is known from US 9,904,060 B2. US 2010 / 0 294 949 A1 discloses a scanning microscope device.

[0003] It is an object of the present invention to further develop an optical system for a metrology system in such a way that its handling is made easier, particularly with regard to the object arrangement.

[0004] This problem is solved according to the invention by an optical system with the features specified in claim 1.

[0005] According to the invention, it has been found that the use of a short focal length transmissive optical focusing component does not require a correspondingly small working distance between this transmissive optical focusing component and the lithography mask as the object. The imaging optics interposed for imaging the illumination focus generated by the transmissive optical focusing component, whose function corresponds to that of a relay optics, allow for an increase in the working distance. The working distance can be greater than 20 mm, greater than 25 mm, greater than 50 mm, and even greater than 100 mm.The working distance is the distance between the object field and the nearest component of the optical system, which is typically a component of the imaging optics used to image the illumination focus generated by the transmissive optical focusing component into the further illumination focus within the object field. The working distance can be measured as the actual distance between the nearest points of the object field and the corresponding nearest component of the optical system, or as the pure z-distance between the object field and a component of the optical system that overlaps it in the x / y direction and is located further away in the z-direction.

[0006] The imaging scale of the imaging optics for imaging the illumination focus generated by the transmissive optical focusing component into the further illumination focus within the illumination field can be in the range of 1. In this case, the imaging optics serve primarily or exclusively to increase the working distance, particularly between the object field and the nearest component of the optical system. The numerical aperture in the illumination focus generated by the optical focusing component and the numerical aperture in the further illumination focus within the object field are then equal. Alternatively, the imaging scale can be less than 1.This can be used to reduce the necessary refractive power of the transmissive optical focusing component, which in turn can serve to reduce an unwanted chromatic error of the optical system, in particular a longitudinal chromatic aberration of the transmissive optical focusing component.

[0007] When the transmissive optical focusing component is designed as a zone plate, also known as a zone lens, the advantages of the optical system are particularly evident.

[0008] A working distance according to claim 3 enables non-critical handling of the object without it coming into unwanted contact with components of the optical system.

[0009] An imaging optic according to claim 4 has proven effective in practice. The imaging optic can be designed as a catoptric or catadioptric optic.

[0010] An imaging optic according to claim 5 can, in particular, comprise exactly one mirror and can then be designed with low reflection losses. Alternatively, the imaging optic can comprise two mirrors or even more than two mirrors, thus ensuring imaging with low aberrations. Ideally, the imaging optic can correct or compensate for aberrations introduced by the transmissive optical focusing component.

[0011] A folding mirror according to claim 6 enables the optical system to be adapted to structural conditions.

[0012] An aspherical mirror according to claim 7 enables good aberration control during imaging. The imaging optics can include at least one mirror whose mirror surface is designed as a freeform surface. This eliminates limitations that would exist due to a rotational symmetry axis of a spherical or aspherical mirror surface.

[0013] A main beam angle according to claim 8 enables illumination of the object with low shadowing effects and a correspondingly high-quality measurement of the object.

[0014] With an actuator according to claim 9, adjustment of the object perpendicular to the object plane is possible. Additionally, the object holder can also be displaced in at least one direction parallel to the object plane, and in particular in two independent directions parallel to the object plane, by means of corresponding actuators. With the actuator for displacing the object holder perpendicular to the object plane, a 3D aerial image can be measured, in particular, by capturing a so-called focus stack. Here, an object image is measured at various z-positions of the object holder and thus of the object.

[0015] With a suitable actuator for moving the object holder to the object plane, it is also possible to ensure separate focusing for different wavelength components of the illumination light, i.e., to ensure that the object is sharply imaged into an arrangement or detection plane of the detection device for the corresponding wavelength component.

[0016] The advantages of a metrology system according to claim 10 correspond to those already explained above with reference to the optical system. A spectral width Δλ / λ (FWHM, full width half max) of the illuminating light produced by the light source can be at least 5 x 10 -4 , at least 1 × 10 -3 , at least 3 × 10 -3 , at least 5 × 10 -3 , at least 1 x 10 -2 the value and can, for example, be in the range between 1 / 250 and 1 / 300.

[0017] An EUV light source according to claim 11 enables actinic measurement, in particular of an EUV lithography mask as the object. The EUV light source can be a plasma light source.

[0018] Exemplary embodiments of the invention are explained in more detail below with reference to the drawing. This drawing shows: Fig. 1. Schematic representation of a metrology system for measuring an object; Fig. 2 a top view of a zone plate as a transmissive optical focusing component for generating an illumination focus in the area of ​​an object field of an optical system of the metrology system; Fig. 3 illumination foci in the area of ​​an object plane of the optical system, for different wavelength components of illumination light from a light source of the metrology system in a beam path after the zone plate; Fig. 4. A design of an illumination light beam path of the optical system after the zone plate up to a spectrally sensitive design of the detection device of the optical system; Fig. 5 schematically a usage variant of the detection device according to Fig. 4 using an object holder which can be moved perpendicular to the object plane by means of an actuator; Fig. 6 in one to Fig. 4. Similar representation shows another embodiment of a spectrally sensitive detection device of the optical system, wherein a grating is designed as a bandpass filter for filtering at least one selected wave luminous component from the illumination luminous material; Fig. 7. Another embodiment of a beam path of the optical system between the zone plate and the object field using imaging optics to image the illumination focus generated by the zone plate into a further illumination focus in the area of ​​the object field; and Fig. 8 an enlargement of detail VIII in Fig. 7.

[0019] Fig. Figure 1 shows a highly schematic metrology system 1 for measuring an object 2. An example of the object 2 to be measured is a lithography mask for projection lithography for the production of micro- or nanostructured semiconductor devices. The diagram depicts the beam path of a main beam 3 of illumination light 4 between a light source 5 and a detection device 6 of the metrology system 1.

[0020] The light source 5 is an EUV light source for generating the EUV illumination light 4 with a central useful wavelength in the range between 5 nm and 30 nm, in particular 13.5 nm. The spectral width Δλ / λ (FWHM, full width half max) of the EUV illumination light 4 used for illuminating the object 2 is at least 1 x 10 -4 and can, for example, be in the range between 1 / 250 and 1 / 300. Light source 5 can be a plasma light source or an HHG light source.

[0021] In the beam path of the illumination light 4 from the light source 5, an intermediate focus plane 7 is arranged, in which an intermediate focus diaphragm 8 is located. The intermediate focus diaphragm 8 serves to separate the useful illumination light 4 from, in particular, unwanted debris. Downstream of the intermediate focus diaphragm 8, a stray light filter can be arranged in the beam path of the illumination light 4 to separate the useful illumination light 4 from unwanted wavelength components carried in the beam path.

[0022] After the light source 5, the illumination light 4 is guided by an optical system 9 of the metrology system 1.

[0023] To clarify the positional relationships between components of the metrology system, the following is shown in the Fig. 1. A Cartesian xyz coordinate system is drawn. The x-direction runs in the Fig. 1 to the right. The y-direction runs in the Fig. 1 perpendicular to the drawing plane into it. The z-direction runs in the Fig. 1 up.

[0024] In the variant of optical system 9, which is in the Fig. As shown in Figure 1, a folding mirror 10 for the illumination light 4 is arranged in the beam path of the illumination light 4 after the intermediate focus aperture 8. A zone plate 11 of the optical system 9 is arranged in the beam path after the folding mirror 10. Fig. 2 is shown in plan view. The zone plate 11 represents a transmissive optical focusing component, which is arranged in the beam path of the illumination light 4 between the light source 5 and an object field 12 in an object plane 13 of the optical system 9.

[0025] An object holder 14 of the optical system serves to hold object 2 in the object plane 13, such that a section of object 2 lies within the object field 12. The object holder 14 can be moved perpendicular to the object plane 13 via an actuator 15, as shown in the Fig. 1 is illustrated by a displacement double arrow Δz.

[0026] The zone plate 11 creates an illumination focus 16 (see also Fig. 4) in the area of ​​object field 12.

[0027] A principal beam angle α (compare Fig. 1), with which the illumination light 4 falls into the object field 12, can be smaller than 6°.

[0028] An object-side numerical aperture of the illumination light beam path can be in the range of 0.1.

[0029] Object 2 is designed as a reflective object. Illumination light 4 reflected from object 2 is guided as detection light from the optical system 9 to the detection device 6. In the design according to Fig. 1 In the beam path of the detection light, a further folding mirror 17 is arranged between the object 2 and the detection device 6.

[0030] Fig. Figure 3 illustrates focusing conditions in the area of ​​the object plane 13 due to the dispersion of the zone plate 11. Different wavelength components 41 to 45 are present in the Fig. Figure 3 shows the wavelengths split in the x-direction for illustration. Wavelength component 41 is, by way of example, the component with the longest wavelength, and wavelength component 45 is the component with the shortest wavelength within the spectral width of the illumination light used. Due to the dispersion of the zone plate 11, wavelength components 41 to 45 are focused at different z-positions into the illumination focus 16 in the region of the object plane 13.

[0031] Fig. Figure 4 shows a configuration for guiding the illumination light 4 towards the object 2 and the detection device 6. A dispersive optical component 18, in the form of a grating, is arranged in the beam path of the detection light 4 between the object field 12 and the detection device 6. The grating 18 spatially separates the different wavelength components 41 to 45 of the detection light 4. These wavelength components 41 to 45 of the detection light 4 are at least partially spatially separated in the beam path following the grating 18.

[0032] The detection device 6 is arranged in an arrangement or detection plane 19, in which the wavelength components 41 to 45 are at least partially spatially separated. The detection device 6 is designed as a sensor array with, in the illustrated embodiment, five sensor elements 61 to 65 for the at least partially separate detection of the wavelength components 41 to 45 of the illumination or detection light 4 in the beam path to the object field 12. Depending on the embodiment, the detection device can have two, three, five, ten, or even more sensor elements 6. i The detection device 6 can be designed as a sensor line or as a two-dimensional sensor array, for example in the form of a CCD or CMOS array.

[0033] Using the detection device 6 according to Fig. 4. With a z-position of the object 2, information about the object 2 from different z-heights of the object structures there can be resolved via the different wavelength components 41 to 45 and can be acquired without z-displacement of the object 2 (single-shot) in z-resolved form using spectrally sensitive detection via the grating 18 and the detection device 6.

[0034] Alternatively or additionally, the z-actuator 15 can be used in combination with spectrally sensitive detection according to Fig. 4 can be used, as shown by the Fig. 5 schematically illustrated.

[0035] The first column of the Fig. Figure 5 illustrates a total of five different z-positions of object 2, which can be controlled via actuator 15 with object holder 14. These z-positions are numbered -2, -1, 0, +1 and +2.

[0036] Fig. Figure 5 schematically shows in the second column a measurement result of the sensor line detection device 6 according to Fig. 4 at these different z-positions of object 2. At z-position -2, the signal at sensor element 61 is strongest, since the object plane 13 coincides there with the illumination focus 16 of the wavelength component 41. Accordingly, the maximum detection intensity, which is measured with the sensor line detection device 6, shifts at the further z-positions -1, 0, +1 and +2 to sensor elements 62, 63, 64 and 65, as shown in the second column of the Fig. 5 each by an “X” at the respective sensor element 6 i clarifies.

[0037] Using a pre-generated deconvolution matrix M, which is located in the Fig. 5 in the third column after an unfolding operator, the measurement result, for example for the z-position “z = 0” of the object plane 13, is unfolded into a signal which contains only signal contributions at this z-displacement position of the object 2 by the object holder 14, as exemplified in the last row of the Fig. Figure 5 illustrates this. Further deconvolution matrices M can be used to generate the corrected detection signals for the other z-values ​​-2, -1, +1, and +2. The deconvolution matrices M incorporate properties of the optical system 9, in particular pre-measured channel crosstalk information between the sensor elements 6. i the sensor line detection device 6.

[0038] Fig. Figure 6 shows another application of a detection arrangement similar to that of Fig. 4. Here, the grating 18 is not used for single-shot detection of the different z-object structure heights, but as a bandpass filter to filter out at least one selected wavelength component from the used spectral width of the illumination or detection light. Fig. Figure 6 shows a position of the grating 18 for utilizing the wavelength component 44, which illuminates the sensor element 64. The other wavelength components 41 to 43 and 45 do not contribute to illuminating the sensor elements 6 in this position of the grating 18. i at.

[0039] For use as a bandpass filter, the grating 18 is equipped with an actuator 20 for pivoting the grating 18 and thus for selecting the wavelength component 4 used for detection. i in combination, which is in the Fig. 6 is represented by a double arrow Δλ.

[0040] Fig. Figure 7 shows a beam path of a variant of an optical system 21 for the metrology system 1. Components and functions, which are described above in connection with the Fig. 1, Fig. 2, Fig. 3, Fig. 4, Fig. 5 to Fig. Items 6, which have already been explained, bear the same reference numbers and will not be discussed again in detail.

[0041] Fig. Figure 7 shows a variant of the beam path of the illumination light 4 using five selected individual beams between the zone plate 11 and the object field 12. In contrast to the beam path according to Fig. 1. The beam path of the illumination light 4 runs through the zone plate 11 along the x-direction. In this embodiment of the beam path according to Fig. Therefore, the folding mirror 10 is omitted.

[0042] Zone plate 11 has a focal length f1 (compare Fig. 8), which is smaller than 5 mm, which can be smaller than 2 mm, which can be smaller than 1 mm and, in the illustrated version, is in the range of 0.5 mm.

[0043] To image the illumination focus 16 generated by the zone plate 11 into a further illumination focus 16' in the area of ​​the object field 12, an imaging optic 22 of the optical system 21 is used. Fig. 7. The imaging optics 22 are designed as mirror optics. In the design according to Fig. The imaging optics 22 has two mirrors, namely a first mirror M1 in the beam path of the illumination light 4 after the zone plate 11 and a further, subsequent mirror M2.

[0044] When executed according to Fig.In section 7, mirror M1 is designed as a planar folding mirror. Alternatively, mirror M1 can also have an imaging effect. Mirror M2 is designed as an aspherical mirror. Alternatively, mirror M2 can also be designed as a spherical mirror. In particular, mirror M2 can be designed as a freeform surface mirror.

[0045] The working distance between the zone plate 11 and the object field 12 can be significantly greater than the focal length f1 due to the interposed imaging optics 22, and can be, for example, greater than 10 mm, greater than 20 mm, greater than 50 mm, and 100 mm or more. The working distance is the distance between the object field and the nearest component of the optical system, which is typically a component of the imaging optics used to image the illumination focus generated by the transmissive optical focusing component into the further illumination focus in the area of ​​the object field.The working distance can be measured as the actual distance between the nearest points of the object field and the corresponding nearest component of the optical system, or as the pure z-distance between the object field and a component of the optical system that overlaps it in the x / y direction and is located further away in the z direction.

[0046] The interposed imaging optics 22 make it possible, in particular, to set a desired dispersion in the design, independent of the required working distance, for example, with the aim of being particularly suitable for combination with the detection device 6. It is advantageous here if the dispersion between adjacent sensor elements 6 i the spectral detection device 6 leads to an offset Δz of, for example, 50 nm - 200 nm, since this can correspond to a z-interval in a z-stack or image stack recorded by the metrology system 1.

[0047] The mapping scale for the mapping of the object field 12 into an image field in the area of ​​the arrangement plane 19 can be larger than 10, can be larger than 25, can be larger than 50, can be larger than 100, can be larger than 250, can be larger than 300 and can, for example, be in the range of 500 or 1000.

[0048] To measure the structure of object 2, an image of the object structure in object field 12 is captured by the detection device 6. Depending on the measurement method, either a single image is captured or a stack of images (aerial image) is captured in several z-positions, in which case object 2 is moved to corresponding z-positions by means of the object holder 14 and the actuator 15.

Claims

[1] Optical system (21) for a metrology system (1) for measuring an object (2) in the form of a lithography mask, - with an object holder (14) for holding the object (2) in an object plane (13), - with a transmissive optical focusing component (11) arranged in the beam path of illumination light (4) between a light source (5) of the metrology system (1) and an object field (12) in the object plane (13), to generate an illumination focus (16) in the beam path of the illumination light (4) after the transmissive optical focusing component (11), - wherein the transmissive optical focusing component (11) has a focal length (f1) that is less than 5 mm, - with a detection device (6) for detecting the illumination light (4) in the beam path after the object field (12), - with an imaging optic (22) for imaging the illumination focus (16) generated by the transmissive optical focusing component (11) into a further illumination focus (16') in the area of ​​the object field (12). [2] Optical system according to claim 1, characterized by , that the transmissive optical focusing component (11) is designed as a zone plate. [3] Optical system according to claim 2, characterized by a working distance between the zone plate (11) and the object field (12) that is greater than 10 mm. [4] Optical system according to any one of claims 1 to 3, characterized by , that the imaging optics (22) are designed as mirror optics. [5] Optical system according to any one of claims 1 to 4, characterized by that the imaging optics have at least one mirror (M1, M2). [6] Optical system according to claim 5, characterized by , that the imaging optics (22) has at least one folding mirror (M1). [7] Optical system according to claim 5 or 6, characterized by , that the imaging optics (22) has at least one aspherical mirror (M2). [8] Optical system according to any one of claims 1 to 7, characterized by a principal beam angle (α) of the illumination light (4) that falls into the object field (12) which is less than 6°. [9] Optical system according to any one of claims 1 to 8, characterized by an actuator (15) for relocating the object holder (14) perpendicular to the object plane (13). [10] Metrology system (1) for measuring an object (2), - with an optical system (21) according to one of claims 1 to 9, - with a light source (5) for generating illumination light (4). [11] Metrology system according to claim 10, characterized by , that the light source (5) is an EUV light source.

Images