Device and method for characterizing a mask for microlithography

Abstract

Device for characterizing a mask for microlithography, with • at least one light source (105, 405, 605) which emits coherent light; • an illumination optics (110, 410, 610) which has a zone plate (111, 411, 611) and generates a diffraction-limited light spot on the mask (120, 320, 420, 620) from the coherent light of the at least one light source (105, 405, 605) in a predetermined diffraction order; • a scanning device via which a scanning movement of the diffraction-limited light spot relative to the mask (120, 320, 420, 620) can be carried out; • a sensor unit (130, 430, 630); and • an evaluation unit for evaluating the light incident on the sensor unit (130, 430, 630) from the mask (120, 320, 420, 620); characterized in that the device is configured such that light emanating from the zone plate (111, 411, 611) in a diffraction order that does not correspond to the predetermined diffraction order is eliminated by at least 90% from the useful beam path leading to the sensor unit (130, 430, 630); wherein the zone plate (411, 511) is arranged in an off-axis configuration such that a beam bundle running from the light source (405) to the zone plate (411, 511) only impinges on a partial area (512) of the optically usable surface of the zone plate (411, 511).

Description

BACKGROUND OF THE INVENTIONField of the invention

[0001] The invention relates to a device and a method for characterizing a mask for microlithography. 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 so-called projection exposure system, which has an illumination device and a projection lens. The image of a mask (= reticle) illuminated by the illumination device is projected by the projection lens onto a substrate (e.g., a silicon wafer) coated with a light-sensitive layer (photoresist) and arranged in the image plane of the projection lens, in order to transfer the mask structure to the light-sensitive coating of the substrate.

[0003] In the lithography process, unwanted defects on the mask have a particularly detrimental effect, as they can be reproduced with each exposure step. To minimize mask defects and achieve successful mask repair, a direct analysis of the imaging effect of potential defect positions is desirable. Therefore, there is a fundamental need to measure and qualify the mask quickly and easily, ideally under the same conditions as those actually encountered in the projection exposure system.

[0004] There are different approaches to emulating the projection exposure system during mask inspection (i.e. measuring the mask, if possible, under conditions analogous to those of the projection exposure system).

[0005] First, it is known to capture and evaluate an aerial image of a section of the mask in a mask inspection system. To capture the aerial image, the structures on the mask to be measured are illuminated with magnifying illumination optics, and the light coming from the mask is projected onto a detector unit via imaging optics and detected. It is also known to illuminate the mask in the mask inspection system in a manner identical to that used in the projection exposure system, in particular by setting the same wavelength, the same numerical aperture, and the identical (possibly polarized) illumination setting in the mask inspection system.

[0006] Furthermore, an approach is also known in which a device for characterizing a mask is designed as a scanning microscope to emulate the conditions present in the projection exposure system. According to the schematic representation of Fig. 7, on the one hand, an illumination optics 710 for illuminating the mask 720 to be characterized with fully coherent light from a light source 705 is designed to emulate the projection optics of the microlithographic projection exposure system. On the other hand, an image capture or sensor arrangement 730 with a plurality of pixels 730-1,...730-n of this scanning microscope is designed to emulate the illumination optics of the microlithographic projection exposure system. Another possibility is to use a non-spatially resolved or non-pixellated intensity detector and to use a physical aperture to emulate the illumination pattern.

[0007] In principle, the emulation of projection optics or illumination optics of the microlithographic projection exposure system described above is accompanied by a relatively high expenditure on equipment and also on manufacturing technology, which must be taken into account in the corresponding device for mask characterization.However, a possible approach to designing the beam-shaping illumination optics with a zone plate to replace an EUV mirror, which is comparatively complex in terms of manufacturing technology and installation space, leads in practice to the further problem that, in addition to the (typically first) diffraction order used to focus the useful light onto the mask to be characterized in the scanning device for mask characterization, further (positive and negative) diffraction orders of the light generated by the light source also pass from the zone plate via the mask to be characterized to the sensor arrangement or detector unit, which in turn results in a reduction in contrast. A further complicating factor is that the disruptive influence of chromatic aberrations is generally particularly pronounced in the higher diffraction orders mentioned.As a result, the performance of the mask characterization device is impaired, which in turn leads to incorrect characterization and ultimately to errors in the lithography process performed using the mask.

[0008] DE 10 2020 207 566 A1 discloses, among other things, a device for characterizing a mask in a microlithographic projection exposure system, comprising, among other things, at least one light source emitting coherent light, an illumination optics system generating a diffraction-limited light spot on the mask from the coherent light of the at least one light source, a scanning device via which a scanning movement of the diffraction-limited light spot relative to the mask can be carried out, a sensor unit, and an evaluation unit for evaluating the light incident from the mask onto the sensor unit. Furthermore, an output coupling element is provided for outputting a portion of the coherent light emitted by the light source to an intensity sensor. In one embodiment, this output coupling element is formed by a reflective region of a zone plate arranged in the illumination optics system.

[0009] UT Sanli et al.: “Multilayer Fresnel Zone Plates for X-ray Microscopy”, Microscopy and Microanalysis 21 (Suppl 3) (2015), pp. 1987-1988, doi:10.1017 / S1431927615010715 discloses, among other things, an exemplary STXM setup with a zone lens located in the illumination optics with an OSA component (“order-sorting aperture”) located in front of the sample for filtering out higher diffraction orders.

[0010] B. Rösner et al.: “Soft x-ray microscopy with 7nm resolution”, Optica, Vol. 7, No. 11, (2020), pp. 1602-1608, also discloses, among other things, the combined use of a zone plate and an “OSA component” in an STXM setup.

[0011] For further information on the state of the art, reference is made, by way of example, to DE 10 2010 063 337 B4 and the publication M. Benk et al.: "Upgrade to the SHARP EUV mask microscope", Proc. SPIE 10957, Extreme Ultraviolet (EUV) Lithography X, 109570V (2019), doi: 10.1117 / 12.2516387. SUMMARY OF THE INVENTION

[0012] It is an object of the present invention to provide a device and a method for characterizing a mask for microlithography, which enable reliable mask characterization with reduced equipment and manufacturing expenditure while at least partially avoiding the problems described in the introduction.

[0013] This object is achieved by the device and the method according to the features of the independent patent claims.

[0014] A device according to the invention for characterizing a mask for microlithography comprises: - at least one light source emitting coherent light; - an illumination optics which has a zone plate and generates a diffraction-limited light spot on the mask from the coherent light of the at least one light source in a predetermined diffraction order; - a scanning device via which a scanning movement of the diffraction-limited light spot relative to the mask can be carried out; - a sensor unit; and - an evaluation unit for evaluating the light incident from the mask onto the sensor unit; - wherein the device is configured such that light emanating from the zone plate in a diffraction order that does not correspond to the predetermined diffraction order is eliminated by at least 90% from the useful beam path leading to the sensor unit; and - wherein the zone plate is arranged in an off-axis configuration or without rotational symmetry to the optical system axis of the illumination optics, so that a beam bundle running from the light source to the zone plate only impinges on a partial area of ​​the optically usable surface of the zone plate.

[0015] According to one embodiment, the device is configured such that light emanating from the zone plate in a diffraction order that does not correspond to the predetermined diffraction order is eliminated from the useful beam path leading to the sensor unit by at least 95%, in particular by at least 98%.

[0016] The invention is initially based on the approach (known as such from DE 10 2010 063 337 B4) in which, in order to emulate the conditions prevailing in the projection exposure apparatus, a device for characterizing a mask is designed as a scanning microscope, wherein, on the one hand, the illumination optics of this scanning microscope are designed such that they emulate the projection optics of the microlithographic projection exposure apparatus, and wherein, on the other hand, the image recording or sensor unit of this scanning microscope is designed such that it emulates the illumination optics of the microlithographic projection exposure apparatus. In other words, the imaging optics and the illumination optics in the device according to the invention for characterizing a mask exchange roles with regard to the emulation of the microlithographic projection exposure apparatus. With regard to the basic functioning of such a device orFor the mask inspection system, reference is made to the above-mentioned patent specification DE 10 2010 063 337 B4.

[0017] Based on this approach, the invention now includes the concept of, on the one hand, reducing the equipment and manufacturing effort required for focusing the useful light onto the mask to be characterized by using a zone plate within the beam-shaping illumination optics compared to an EUV mirror (which is comparatively complex in terms of production and cost) and, on the other hand, at least partially eliminating the disturbing background that generally accompanies the use of such a zone plate.

[0018] The said cleaning can be realized, as described below using different embodiments, by using one or more suitably placed aperture stops and / or by a suitable off-axis configuration of the zone plate, whereby the disturbing diffraction orders responsible for the undesired background are either "cut off" from the actual useful light tube (in the case of the use of aperture stops) or (in the case of the said off-axis configuration) are directed in such a way that the light emanating from the zone plate in these undesired diffraction orders does not reach the sensor arrangement or detector unit via the mask to be characterized.

[0019] According to the invention, the use of a zone plate within the beam-shaping illumination optics deliberately accepts increased light loss due to the absorbing or reflecting regions of the zone plate in order to achieve, in return, the advantages described above in terms of equipment and manufacturing technology. Furthermore, the invention takes advantage of the fact that the provision of the full angular bandwidth of the numerical aperture in the illumination optics, as offered in a scanning system according to the concept described above and known, for example, from DE 10 2010 063 337 B4, is still ensured in the embodiments according to the invention.

[0020] According to one embodiment, the predetermined diffraction order is the (+1)-th diffraction order or the (-1)-th diffraction order.

[0021] According to one embodiment, the device has at least one aperture stop for at least partially eliminating the light emanating from the zone plate in a diffraction order that does not correspond to the predetermined diffraction order.

[0022] According to one embodiment, at least one aperture stop is arranged between the zone plate and the mask for at least partially eliminating the light emanating from the zone plate in a diffraction order that does not correspond to the predetermined diffraction order, relative to the optical beam path.

[0023] According to one embodiment, at least one aperture stop is arranged in the optical beam path between the mask and the sensor arrangement for at least partially eliminating the light emanating from the zone plate in a diffraction order that does not correspond to the predetermined diffraction order.

[0024] According to the invention, the zone plate is arranged in an off-axis configuration or without rotational symmetry to the optical system axis of the illumination optics, so that a beam bundle running from the light source to the zone plate only impinges on a partial area of ​​the optically usable surface of the zone plate.

[0025] According to one embodiment, the center beam of a first beam section extending from the light source to the zone plate and the center beam of a second beam section extending from the zone plate to the mask in the predetermined diffraction order are not parallel to each other.

[0026] According to one embodiment, the device further comprises an energy sensor, wherein the light eliminated from the useful beam path at least partially impinges on this energy sensor. This configuration is advantageous in that the light eliminated from the useful beam path for the background removal according to the invention is still partially used, for example, to detect beam intensity fluctuations and correct them if necessary.

[0027] According to one embodiment, the light source has an operating wavelength of less than 30 nm, in particular less than 15 nm.

[0028] The invention further relates to a method for characterising a mask for microlithography, - wherein a diffraction-limited light spot is generated on the mask from coherent light generated by at least one light source in a predetermined diffraction order via an illumination optics which has a zone plate; - wherein a scanning movement of the diffraction-limited light spot is carried out relative to the mask; - wherein light incident from the mask onto a sensor unit is evaluated; and - wherein light emanating from the zone plate in a diffraction order that does not correspond to the specified diffraction order is eliminated by at least 90% from a useful beam path extending from the light source to the sensor unit; and - wherein the zone plate is arranged in an off-axis configuration or without rotational symmetry to the optical system axis of the illumination optics, so that a beam bundle running from the light source to the zone plate only impinges on a partial area of ​​the optically usable surface of the zone plate.

[0029] For advantages and preferred embodiments of the method, reference is made to the above statements in connection with the device according to the invention.

[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 embodiments shown in the attached figures. BRIEF DESCRIPTION OF THE DRAWINGS

[0032] They show: Fig. 1 is a schematic representation to explain the possible structure of a device according to the invention for characterizing a mask in a first embodiment; Fig. 2 a schematic representation of a zone plate used in a device according to the invention in an exemplary embodiment; Fig. 3a-3c are schematic representations illustrating the inventive concept of partially eliminating unwanted diffraction orders emanating from a zone plate from the useful light beam path; Fig. 4 a schematic representation to explain the possible structure of a device according to the invention for characterizing a mask in a further embodiment; Fig. 5 is a schematic diagram illustrating the possible use of a zone plate in a device according to the invention in an off-axis configuration according to a further embodiment; Fig. 6a-6c are schematic representations illustrating an inventive elimination of undesired diffraction orders from the useful light beam path in a further embodiment; and Fig. 7 is a schematic diagram illustrating a conventional structure of a mask characterization apparatus. DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

[0033] The embodiments of a device for characterizing a mask for microlithography described below initially have in common that - based on the principle described in DE 10 2010 063 337 B4 and DE 10 2020 207 566 A1 - the device or mask inspection system is designed as a scanning microscope, wherein completely coherent light from a light source is directed via an illumination optics onto the mask to be characterized in such a way that only a single diffraction-limited light spot is illuminated on the mask.The emulation of the illumination optics of the microlithographic projection exposure system can be carried out in the device according to the invention in a manner known per se on the imaging side by appropriately selecting the pixels contributing to the image in the sensor unit following the mask in the beam path. Said pixel selection can be carried out in different ways, in particular according to the embodiments described in DE 10 2010 063 337 B4 or also according to the further embodiments described in DE 10 2020 207 566 A1. Furthermore, a non-spatially resolved or non-pixellated intensity detector can also be used in combination with a physical aperture to emulate the illumination pattern.

[0034] The embodiments described below also share the common feature that the use of a zone plate in the beam-shaping illumination optics in a device for mask characterization is combined with the at least partial elimination of unwanted diffraction orders from the useful light beam path. "Unwanted diffraction orders" are understood to mean those diffraction orders that do not correspond to the predefined diffraction order used to focus on the mask to be characterized and that, without appropriate countermeasures, thus cause an interfering background and a concomitant reduction in the contrast achievable with the device.

[0035] Fig. 1 shows a purely schematic representation of a device according to the invention in a first embodiment. According to Fig. 1, the device comprises a light source 105 for generating fully coherent light, wherein the light source 105 can be configured, in particular, as an HHG laser. In further embodiments, other coherent sources (e.g., a synchrotron or a free-electron laser) can also be used.

[0036] Coherent light emanating from the light source 105 is first collimated by a collimator (not shown separately) and hits according to Fig. 1 to an illumination optics 110, which focuses the illumination light onto a diffraction-limited light spot on a mask 120 to be characterized. The illumination optics 110 represents a beam-shaping optics and, according to the invention, has a zone plate 111.

[0037] To check the imaging effect of the mask 120, a scanning movement of the diffraction-limited light spot relative to the mask 120 is carried out, wherein this scanning process can be realized by moving only the illumination optics 120 or the component generating the diffraction-limited light spot, by moving the illumination optics 110 and sensor unit 130 while the mask is held fixed, or by moving only the mask 120 while the illumination optics 110 and sensor unit 130 are held fixed.

[0038] For pixel selection on the sensor unit 130 side for the purpose of emulating the illumination device of the microlithographic projection exposure system, the sensor unit 130 can be configured as a spatially resolved sensor arrangement (e.g., a CCD camera), analogous to the aforementioned DE 10 2010 063 337 B4. Depending on the respective illumination setting to be emulated, a finite proportion of the pixels can then be disregarded when evaluating the light incident on the sensor unit 130. However, the invention is not limited thereto. Thus, in further embodiments, a non-spatially resolved sensor (e.g., a simple photodiode) can also be used to detect the light emanating from the mask to be characterized, in which case the said pixel selection is then carried out in another way (e.g., using arrangements of independently adjustable optical elements described in DE 10 2020 207 566 A1).

[0039] Although in the embodiments described below, the predetermined diffraction order used for focusing on the mask is the (+1)th diffraction order, the invention is not limited to this. Thus, depending on the specific design of the device for mask characterization or the corresponding beam path, a different diffraction order of the light emanating from the zone plate (e.g., the (+3)th diffraction order) can also be used for focusing on the mask, so that, according to the invention, other diffraction orders that do not correspond to this diffraction order can then be at least partially eliminated from the useful light beam path.

[0040] With further reference to Fig. 1, in the illustrated first embodiment of the device for mask characterization, the zone plate 111 present in the illumination optics 110 is used in a so-called “on-axis” configuration and by realizing a substantially rotationally symmetrical arrangement of the zone plate with respect to the system axis of the illumination optics 110, wherein such a zone plate in Fig. 2 is only shown schematically and is labelled “211”.

[0041] For the inventive elimination of undesired diffraction orders, ie those which do not correspond to the (+1)-th diffraction order for focusing on the mask 120 to be characterized, in the example of Fig. 1 a first aperture stop 115 and a second aperture stop 116. The first aperture stop 115 is located between the zone plate 111 and the mask 120 with respect to the optical beam path and thus effects an at least partial elimination of undesired diffraction orders (e.g. the third diffraction order) even before the light emanating from the zone plate 111 strikes the mask 120. The second aperture stop 116, on the other hand, is located between the mask 120 and the sensor arrangement 130 with respect to the optical beam path and thus serves to at least partially eliminate light which has still reached the mask 120 from the zone plate 111 in undesired diffraction orders despite the presence of the first aperture stop 115 and has been directed by the latter in the direction of the evaluation unit 130.

[0042] The basic operation of an aperture stop to eliminate unwanted diffraction orders is shown in the schematic diagrams of Fig. 3a-3c illustrates, where compared to Fig. 1 analogous or essentially functionally equivalent components are designated by reference numerals increased by “200”.

[0043] In this case, the diffraction order used for focusing on the mask 320 - without the invention being limited to this - is in accordance with Fig. 3b by the (+1)-th diffraction order, whereby due to the effect of the aperture stop 315, light which is emitted by the zone plate 311 in the zeroth diffraction order (cf. Fig. 3a) or the (+3)-th diffraction order (cf. Fig. 3c), is eliminated to a significant or predominant extent from the useful light beam path. The light incident on the mask 320 in the said undesired diffraction orders despite the presence of this aperture stop 315 still causes a disturbing background (although significantly reduced due to the aperture stop 315), which, as described above with reference to Fig. 1 can be further reduced by using an additional aperture stop in the beam path between the mask and the sensor arrangement or detector unit.

[0044] Furthermore, it should be noted that in principle the illumination beam path and the detection or imaging beam path can be separated by a relative tilt of the mask and the illumination.

[0045] As described below for further embodiments based on Fig. 4, Fig. 5 and Fig. 6a-6c, in addition to or as an alternative to the use of aperture diaphragms, the inventive, at least partial elimination of the light emanating from the zone plate in undesired diffraction orders from the useful light beam path is achieved by using the zone plate in a so-called "off-axis configuration" and using an asymmetrical structure of the illumination optics in such a way that the light emanating from the zone plate in undesired diffraction orders (i.e. not used for focusing on the mask) does not even hit the optically used area of ​​the mask (but for example passes this mask) or does not reach the sensor arrangement via the mask.

[0046] Fig. Figure 5 serves to illustrate the above principle, whereby only a partial area 512 of a zone plate 511 (corresponding to a section within the dashed circle) is used to achieve the above-described effect. This partial use can in turn be achieved either by a corresponding off-axis placement of the zone plate 511 in the optical beam path within the illumination optics or by manufacturing the corresponding zone plate only in said section 512 from the outset.

[0047] Fig. 4 now shows in to Fig. 1 analogous schematic representation of the possible structure of a device according to the invention for mask characterization, wherein in comparison to Fig. 1 analogous or essentially functionally equivalent components are designated by reference numerals increased by “300”.

[0048] Furthermore, Fig. 6a-6c are schematic diagrams illustrating the operation of a zone plate used in the off-axis configuration described above, wherein, in comparison to Fig. 3a-3c, analogous or essentially functionally identical components are designated by reference numerals increased by “300”. Both in the embodiment of Fig. 4 as well as in the schematic representations of Fig. 6a-6c, the inventive elimination of undesired (ie not those used for focusing on the mask) diffraction orders from the useful light beam path is based to a substantial extent on the off-axis configuration of the zone plate 411 or 611 used in each case, but is, as can be seen from Fig. 6 and Fig. 6a-6c, the optical path is additionally supported by aperture stops 415, 416, and 615-617, respectively. The use of such aperture stops is optional depending on the specific design of the optical beam path, so that the inventive elimination of unwanted diffraction orders can also be achieved solely by a suitable off-axis configuration of the zone plate.

[0049] As in Fig. 4, an energy sensor 440 can optionally be used to partially capture light emanating from the zone plate in undesired diffraction orders (e.g., the third diffraction order) and thus, for example, to detect beam intensity fluctuations and, if necessary, to correct them.

Claims

[1] Device for characterizing a mask for microlithography, with • at least one light source (105, 405, 605) which emits coherent light; • a lighting optic (110, 410, 610) which has a zone plate (111, 411, 611) and generates a diffraction-limited light spot on the mask (120, 320, 420, 620) from the coherent light of the at least one light source (105, 405, 605) in a predetermined diffraction order; • a scanning device by which a scanning movement of the diffraction-limited light spot relative to the mask (120, 320, 420, 620) can be performed; • a sensor unit (130, 430, 630); and • an evaluation unit for evaluating the light incident on the sensor unit (130, 430, 630) from the mask (120, 320, 420, 620); characterized by, that the device is configured such that light emanating from the zone plate (111, 411, 611) in a diffraction order that does not correspond to the specified diffraction order is eliminated from the useful beam path leading to the sensor unit (130, 430, 630) by at least 90%; wherein the zone plate (411, 511) is arranged in an off-axis configuration, so that a beam of light passing from the light source (405) to the zone plate (411, 511) only strikes a partial area (512) of the optically usable area of ​​the zone plate (411, 511). [2] Device according to claim 1, characterized by , that the device is configured such that light emanating from the zone plate (111, 411, 611) in a diffraction order that does not correspond to the specified diffraction order is eliminated from the useful beam path leading to the sensor unit (130, 430, 630) by at least 95%. [3] Device according to claim 1, characterized by, that the device is configured such that light emanating from the zone plate (111, 411, 611) in a diffraction order that does not correspond to the specified diffraction order is eliminated from the useful beam path leading to the sensor unit (130, 430, 630) by at least 98%. [4] Device according to any one of claims 1 to 3, characterized by , that the given diffraction order is the (+1)th diffraction order or the (-1)th diffraction order. [5] Device according to any one of the preceding claims, characterized by , that this has at least one aperture diaphragm (115, 116, 315, 415, 416, 615, 616, 617) to at least partially eliminate the light emanating from the zone plate (111, 411, 611) in a diffraction order that does not correspond to the specified diffraction order. [6] Device according to claim 5, characterized by, that at least one aperture diaphragm (115, 315, 415, 615, 616) is arranged for at least partial elimination of the light emanating from the zone plate (111, 411, 611) in a diffraction order not corresponding to the specified diffraction order with respect to the optical beam path between zone plate (111, 411, 611) and mask (120, 320, 420, 620). [7] Device according to claim 5 or 6, characterized by , that at least one aperture diaphragm (116, 416, 616, 617) is arranged to at least partially eliminate the light emanating from the zone plate (111, 411, 611) in a diffraction order not corresponding to the specified diffraction order in the optical beam path between mask (120, 420, 620) and sensor arrangement (130, 430, 630). [8] Device according to any one of the preceding claims, characterized by, that the central ray of a first beam section running from the light source (405) to the zone plate (411, 511) and the central ray of a second beam section running from the zone plate (411, 511) to the mask (420) in the specified diffraction order are not parallel to each other. [9] Device according to any one of the preceding claims, characterized by , that it further comprises an energy sensor (440), wherein the light eliminated from the useful beam path at least partially hits this energy sensor (440). [10] Device according to any one of the preceding claims, characterized by , that the light source (105, 405, 605) has a working wavelength of less than 30 nm. [11] Device according to any one of the preceding claims, characterized by , that the light source (105, 405, 605) has a working wavelength of less than 15 nm. [12] Method for characterizing a mask for microlithography, - wherein a diffraction-limited light spot on the mask (120, 320, 420, 620) is generated on a diffraction-limited light spot on the mask (120, 320, 420, 620) from coherent light generated by at least one light source (105, 405, 605) in a predetermined diffraction order via an illumination optic (110, 410, 610) which has a zone plate (111, 411, 611); - wherein a scan movement of the diffraction-limited light spot is performed relative to the mask (120, 320, 420, 620); and - wherein light incident on a sensor unit (130, 430, 630) from the mask (120, 320, 420, 620) is evaluated; characterized by, that light emanating from the zone plate (111, 411, 611) in a diffraction order that does not correspond to the specified diffraction order is eliminated by at least 90% from a useful beam path running from the light source (105, 405, 605) to the sensor unit (130, 430, 630); wherein the zone plate (411, 511) is arranged in an off-axis configuration, such that a beam of light running from the light source (405) to the zone plate (411, 511) only strikes a partial area (512) of the optically usable area of ​​the zone plate (411, 511).

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