Magnifying imaging optics for metrology system for examining objects

Abstract

To develop a magnifying image formation optical unit in such a way that an excellent image formation result satisfying stringent requirements of a metrology system can be obtained for a given manufacturing outlay.SOLUTION: A magnifying image formation optical unit (20c) is part of a metrology system for examining objects. The magnifying image formation optical unit has at most four mirrors (M1 to M4) that image-form an object field (4) within an object plane (17) on an image field (21) within an image plane (22). According to one aspect, an entrance pupil of the magnifying image formation optical unit has: a boundary shape that deviates from an ellipse; and an aspect ratio not equal to 1. According to another aspect, reflection surfaces of small-area mirrors (M2 and M3) to be used for guiding image formation light (3) along an image formation beam route deviate from a spherical shape by at most 10 μm. As a result, the magnifying image formation optical unit is achieved that can obtain an image formation result satisfying stringent requirements of a metrology system for a given manufacturing outlay.SELECTED DRAWING: Figure 4

Description

Technical Field

[0001] The content of German Patent Application No. 102023213267.2 is incorporated herein by reference.

[0002] The present invention relates to a magnifying imaging optical unit for a measurement system for inspecting an object. Further, the present invention relates to an illumination optical unit for a measurement system adapted to such an imaging optical unit, an optical system comprising such an imaging optical unit and an illumination optical unit, and further to a measurement system comprising such an optical system.

Background Art

[0003] Mask inspection systems are known from US Patent No. 8,842,284, US Patent Application Publication No. 2013 / 0250428, WO2016 / 012426A1, US Patent No. 10,042,248B2, DE10220815A1, and WO2012 / 101269A1. DE102010029050A1 and DE102011084255A1 each disclose a magnifying imaging optical unit and further a measurement system comprising such an imaging optical unit. DE102010029049A1 discloses an illumination system for a measurement system and further a measurement system comprising such an illumination optical unit.

Prior Art Documents

Patent Documents

[0004]

Patent Document 1

Patent Document 2

Patent Document 3

Patent Document 4

Patent Document 5

Patent Document 6

Patent Document 7

Patent Document 8

Patent Document 9

Patent Document 10

Patent Document 11

Non-Patent Document

[0005]

Non-Patent Document 1

Summary of the Invention

[0006] An object of the present invention is to develop a magnifying imaging optical unit so that a good imaging result satisfying the strict requirements of a measurement system can be obtained for a given manufacturing cost.

[0007] According to a first aspect, this object is achieved by a magnifying imaging optical unit according to claim 1 in accordance with the present invention.

[0008] According to the present invention, a non-elliptical entrance pupil having an aspect ratio not equal to 1 provides an additional possibility to adapt the guidance of the imaging beam path within the magnifying imaging optical unit, specifically the beam path of the illumination light irradiating the object field, and it has been recognized that this can improve the imaging quality and / or light throughput of the imaging optical unit. The boundary shape of the entrance pupil can be adapted to the imaging requirements, for example, different typical object structure sizes in two mutually perpendicular field dimensions. Alternatively or additionally, it is possible to adapt to the diffraction effect of the object structure and / or the illumination pupil of the illumination optical unit of the measurement system, and the illumination pupil has a boundary shape with an aspect ratio not equal to 1 for that part.

[0009] The x:y aspect ratio of the entrance pupil of the magnifying imaging optical unit can be in the range between 1.1:1 and 5:1, where the x coordinate can be perpendicular to the meridian plane of the magnifying imaging optical unit. The x:y aspect ratio can be, for example, 2:1.

[0010] The resulting minimum object-side numerical aperture of the magnifying imaging optical unit can be greater than 0.1, greater than 0.12, for example 0.125, or 0.135.

[0011] The boundary shape of the entrance pupil according to claim 2 can be well adapted to the structural requirements, specifically the spatial requirements of the imaging beam path. As a result, a correspondingly high light throughput can be obtained.

[0012] Correspondingly, the same applies to the entrance pupil with a notch according to claim 3. The notch can take into account the obscuration by at least one mirror of the magnifying imaging optical unit.

[0013] The advantages of an entrance pupil having an aspect ratio other than equal to 1 are particularly well - revealed in the case of the configuration according to claim 4. At least one mirror whose boundary corresponds to the boundary of the entrance pupil can be a near - pupil mirror. This can include the first mirror of the magnifying imaging optical unit and optionally also a second mirror.

[0014] According to another aspect, the object described in the introduction part is achieved by a magnifying imaging optical unit having the features specified in claim 5 according to the present invention.

[0015] According to the present invention, it has been recognized that it is possible to implement a magnifying imaging optical unit with small - area mirrors that deviate only slightly or not at all from a spherical shape without an undesirable degradation of imaging quality. This facilitates the manufacture of the small - area mirrors of the magnifying imaging optical unit and provides corresponding manufacturing advantages.

[0016] The deviation of the reflecting surface from a spherical shape is measured with respect to the spherical shape that best fits each mirror reflecting surface. Such a best fit can be determined by square error minimization when comparing each reflecting surface with a sphere.

[0017] The deviation of the reflecting surface of each small - area mirror from a spherical shape can be at most 5 μm or at most 1 μm. Such a small deviation enables processing during mirror manufacture in a small number of processing cycles. The small - area mirror can be implemented as a nano - asphere where the deviation of the reflecting surface from a spherical shape is at most 10 times the used wavelength. Such a nano - asphere can be measured by measurement techniques for measuring spherical reflecting surfaces.

[0018] The intermediate image can be located between the first mirror and the second mirror in the imaging beam path of the magnifying imaging optical unit. Such an intermediate image can be used to improve the imaging aberration correction effect. Furthermore, this can be used to create a particularly compact beam path in the region of the mirror arranged near the intermediate focus or near-field.

[0019] The parameter P defined in WO2009 / 024164A1 can be used to characterize such a "near-field" property. When the parameter P is less than 0.5, specifically less than 0.4, less than 0.3, less than 0.25, or less than 0.2, the mirror is regarded as a near-field. In an actual mirror, this parameter P is usually greater than 0.05.

[0020] The last mirror of the magnifying imaging optical unit in the imaging beam path can be implemented as a near-field.

[0021] The first mirror, and optionally further the second mirror, in the imaging beam path of the magnifying imaging optical unit can be implemented as a near pupil. In such a near pupil mirror, the parameter P can be greater than 0.5, greater than 0.6, greater than 0.7, and even greater than 0.8. In an actual near pupil mirror, the parameter P is usually less than 0.95.

[0022] The magnifying imaging optical unit can be designed to be used with EUV imaging light having a wavelength in the range of specifically 5 nm to 30 nm, for example 13.5 nm. The magnifying imaging optical unit can specifically have a high-reflection coating implemented for the corresponding EUV wavelength.

[0023] The structural length of the magnifying imaging optical unit can be at most 1250 mm, and as a result, a compact optical unit is obtained.

[0024] The mirrors of the magnifying imaging optical unit can be implemented such that none of the mirrors has a diameter of the reflecting surface used to guide the imaging light along the imaging beam path that is longer than 400 mm. As a result, a compact optical unit is also obtained.

[0025] In the case of the embodiment according to claim 6, the reflecting surfaces of all the mirrors deviate only slightly, i.e., by at most 25 μm, from a spherical shape, i.e., from a mirror having a diameter of at least 50 mm, which is also called a large-area mirror below. As a result, corresponding manufacturing advantages are obtained for all the mirrors of the magnifying imaging optical unit. The deviation of the reflecting surface mirror from the spherical shape can be at most 20 μm, at most 15 μm, or for all, the mirror can also be of the same size as discussed above in relation to at least one small-area mirror.

[0026] The features of the magnifying imaging optical unit according to the two aspects discussed above can also exist in combination with each other.

[0027] The distance ratio according to claim 7 has the result that imperfections and / or contamination on the last mirror in the imaging beam path of the magnifying imaging optical unit have no undesirable effect on the imaging quality of the optical unit. This is particularly true when the last mirror is implemented as a near-field lens. The distance between the last mirror and the image plane can be more than 65% of the distance between the object plane and the image plane. This distance is usually less than the distance between the object plane and the image plane.

[0028] Assuming a corresponding embodiment of the magnifying imaging optical unit, the distance between the last mirror and the third-to-last mirror in the imaging beam path of the magnifying imaging optical unit along the coordinate perpendicular to the image plane can be less than 15%, less than 12%, or less than 10% of the distance between the object plane and the image plane. This distance between the last mirror and the third-to-last mirror is usually more than 1% of the distance between the object plane and the image plane.

[0029] The magnification ratio according to claim 8 has been proven to be actually valuable. Such a magnification ratio can be adapted to the spatial pixel size of the spatial resolution detection device of the measurement system that captures the image field.

[0030] The magnifying imaging optical unit may have an object field having a range of two object field dimensions in the range of 0.1 mm to 1 mm in each case, for example 0.1 mm 2 ~0.5 mm 2 and may have an area of. A typical object field range is 0.3 mm × 0.6 mm, or 0.5 mm × 0.5 mm.

[0031] The incident angles according to claims 9 and 10 have been proven to be actually valuable, and as a result, advantageous reflection conditions or good imaging conditions can be obtained. The incident angle with respect to the mirror can be at most 13° in each case, or even smaller.

[0032] As a result of the RMS wavefront aberration according to claim 11, good imaging quality can be obtained. The Petzval radius on the image field side of the magnifying imaging optical unit can be more than 500 mm.

[0033] The minimum value for the deviation of the mirror reflecting surface from the spherical shape according to claim 12 has been proven to be actually valuable. At a wavelength of use of 13.5 nm, this corresponds to the minimum deviation, that is, a deviation lower limit of 25 nm. Depending on the embodiment of the magnifying imaging optical unit, a plurality of mirrors, for example two or three mirrors, or exactly one of the mirrors may satisfy this criterion of the deviation lower limit. In another embodiment, all the mirrors of the magnifying imaging optical unit satisfy the deviation lower limit that is twice the wavelength of use.

[0034] The advantages of the irradiation optical unit according to claim 13 correspond to those already described above in relation to various aspects of the magnifying imaging optical unit.

[0035] The boundary shape of the illumination pupil can be at least approximately elliptical, can be at least approximately stadium-shaped, and can further be at least approximately semi-circular.

[0036] The advantages of the optical system according to claim 14 or 15 and the measurement system according to claim 16 correspond to those already described above with reference to the magnifying imaging optical unit and the illumination optical unit.

[0037] The light source of the measurement system can be an EUV light source.

[0038] The detection device can have at least one TDI camera.

[0039] The measurement system can be implemented as a mask inspection system or a wafer inspection system.

[0040] The inspection system can include an object holder that functions to hold an object that is inspected and mechanically coupled to an object displacement drive so that a scanning displacement of the object during irradiation is possible.

[0041] The inspection system can be an actinic mask or a system for wafer inspection.

[0042] An exemplary embodiment of the present invention will be described in more detail below with reference to the drawings.

Brief Description of the Drawings

[0043]

Figure 1

Figure 2

Figure 3

Figure 4

Figure 5

Figure 6

Figure 7

DETAILED DESCRIPTION OF THE INVENTION

[0044] The irradiation optical unit 1 is a component of the optical system 2 of the mask inspection system 2a used together with the EUV irradiation light 3. For the irradiation optical unit 1 in FIG. 1, the beam path of the irradiation light 3 is indicated by marginal rays and chief rays. The illumination field 4 of the mask inspection system is irradiated by the irradiation light 3.

[0045] The irradiation light 3 is generated within the light source region 6 by the EUV light source 5. The light source 5 can generate EUV used radiation within a wavelength range of 2 nm to 30 nm, for example, within a range of 2.3 nm to 4.4 nm, or within a range of 5 nm to 30 nm, for example, 13.5 nm.

[0046] The light source 5 is implemented as a plasma light source. As an example, this can be a laser-produced plasma (LPP) or a discharge-produced plasma (DPP). In principle, such a plasma light source is known as a light source for EUV projection exposure apparatuses. Alternatively, the light source 5 can also be implemented as a harmonic EUV source. The pulse frequency of the light source 5 can be within the kHz range.

[0047] To facilitate the positional relationship, a Cartesian xyz coordinate system is used hereinafter. The x-axis is perpendicular to the plane of the drawing of FIG. 1 and intersects that plane. The y-axis proceeds horizontally to the left in FIG. 1, and the z-axis proceeds vertically upward in FIG. 1.

[0048] After emission by the light source 5, the irradiation light 3 passes through a used light filter 8 arranged at an operating position between the source volume 6 of the irradiation optical unit 1 and the first elliptical mirror IL1 in the beam path of the irradiation light 3. The used light filter 8 can be, for example, one of a plurality of filters kept available in the filter magazine of the measurement system 2a. Another used light filter can be arranged at a standby position outside the irradiation beam path of the irradiation optical unit 1. The used light filters 8 can have the same transmission characteristics, in which case a switch between the used light filters can be carried out if deterioration of the filter effect of the used light filter 8 during operation is detected. Alternatively, the used light filters can also have different filter characteristics, for example, can transmit different used light wavelength ranges into the downstream irradiation beam path, or can be optimized to remove different external light components.

[0049] The used light filter can be implemented to be induced along the irradiation beam path and remove specific pump light used during the generation of the used light in the source volume 6.

[0050] Downstream of the filter 8 and the mirror IL1, the irradiation light 3 first passes through an aperture stop 9 that defines the boundary of the edge of the beam of the irradiation light 3. Thereafter, the irradiation light beam 3 is transferred towards the beam homogenizing element 11 of the irradiation optical unit 1. In this case, the mirror IL1 acts as an input coupling optical unit 10 for coupling the irradiation light 3 into the beam homogenizing element 11.

[0051] Between the source volume 6 and the beam homogenizing element 11, generally downstream of the first mirror IL1 of the irradiation optical unit 1, the irradiation light 3 passes through an opening in the wall of the vacuum chamber VK, which is shown between the mirror IL1 and the irradiation light aperture stop 9 in the irradiation light beam path of FIG. 1.

[0052] The aperture stop 9 limits the numerical aperture of the irradiation light beam 3 emitted by the light source region 6 to a value of the numerical aperture in the range of 0.02 to 0.2, for example in the range of 0.07 to 0.15, or in the range of 0.05 to 0.08. As an alternative to the aperture stop 9, or in addition to the aperture stop 9, an aperture-limiting stop can be arranged between the beam homogenizing element 11 and the downstream optical component of the irradiation optical unit 1, as shown by 9a in FIG. 1. Such another aperture stop can also be arranged in the beam path of the irradiation light 3 between two downstream optical components of the irradiation optical unit 1 on the downstream side of the beam homogenizing element 11.

[0053] The elliptical mirror IL1 serves to image the light source region 6 of the EUV light source 5 into the entrance aperture 12 within the entrance plane 13 of the beam homogenizing element 11. Accordingly, the first focus of the elliptical mirror IL1 is arranged within the light source region 6, and the second focus of the elliptical mirror IL1 is arranged within the entrance aperture 12. The elliptical mirror IL1 is used to focus the irradiation light beam 3 into the entrance aperture 12 within the entrance plane 13 of the beam homogenizing element 11. The entrance-side numerical aperture of the irradiation light beam 3 upon entering the entrance aperture 12 can be in the range of 0.02 to 0.2, for example about 0.05.

[0054] The angle of incidence of the central principal ray of the irradiation light beam 3 with respect to the input coupling mirror IL1 can be in the range between 10° and 20°. The elliptical mirror IL1 can be a normal incidence (NI) mirror, but can also be implemented as a grazing incidence (GI) mirror.

[0055] The entrance aperture 12 and the exit aperture 14 of the beam homogenizing element 11 are each square or rectangular, and typical dimensions are in the range of 0.5 mm to 5 mm, for example, between 0.5 mm and 2 mm, or between 0.5 mm and 1 mm. The aspect ratio of the entrance aperture 12 of the beam homogenizing element 11 and the exit aperture 14 of the same size for the irradiated light 3 at the exit plane 15 is between 0.5 and 2. Typical sizes of the entrance aperture 12 and the exit aperture 14 of the beam homogenizing element 11 are, for example, 0.5 mm × 1.0 mm, 0.75 mm × 0.75 mm, 1.0 mm × 2.0 mm, 1.5 mm × 2.0 mm.

[0056] The beam homogenizing element 11 can be implemented as a hollow waveguide.

[0057] The beam homogenizing element 11 has a typical length in the range of 50 mm to 500 mm, for example, in the range of 50 mm to 150 mm, specifically in the range of 50 mm to 100 mm, perpendicular to the planes 13 and 15, that is, along the main beam direction of the irradiated light 3.

[0058] The angle between the normal to the entrance plane 13 of the beam homogenizing element 11 and the principal ray of the irradiated light beam 3 incident on the entrance aperture 12 can be 0°, or alternatively, different from 0°, for example, between 0° and 1.5°, for example, between 0.25° and 0.75°, specifically about 0.5°.

[0059] The ratio of the distance between the entrance plane 13 and the exit plane 15 to the size or typical diameter of the entrance aperture 12 and the exit aperture 14 is in the range of 50 to 1000, for example, in the range of 50 to 200.

[0060] Located downstream of the beam homogenizing element 11, an imaging output coupling mirror optical unit 16 having two mirrors IL2, IL3 images an exit aperture 14 located within the exit plane 15 of the beam homogenizing element 11 into the illumination field of view 4 within the object plane 17. This imaging may have an image-side numerical aperture in the range between 0.05 and 0.2.

[0061] In the illustrated embodiment, the output coupling mirror optical unit 16 has exactly two mirrors, namely mirrors IL2 and IL3. The aforementioned optional aperture stop downstream of the beam homogenizing element 11 can be arranged between the beam homogenizing element 11 and the mirror IL2, or between the mirrors IL2 and IL3.

[0062] The output coupling mirror optical unit 16 is implemented in the style of a Wolter telescope, i.e., in the style of a type I Wolter optical unit. Such Wolter optical units are described in J. D. Mangus, J. H. Underwood "Optical Design of a Glancing Incidence X-ray Telescope", Applied Optics, Vol. 8, 1969, page 95, and the references cited therein. In such Wolter optical units, hyperboloids can also be used instead of paraboloids. Such a combination of an elliptical mirror and a hyperbolic mirror also constitutes a type I Wolter optical unit.

[0063] An exemplary embodiment of the output coupling mirror optical unit 16 is described in U.S. Patent No. 10,042,248B2.

[0064] The imaging factor β1 of the input coupling mirror optical unit 10 can be in the range between 0.1 and 50, that is, its operation can vary from a reduction of one-tenth to an enlargement of 50 times. The imaging factor β2 of the output coupling mirror optical unit 16 can be in the range between 0.02 and 10, that is, its operation can vary from a reduction of one-fiftieth to an enlargement of 1 time. In the case of the irradiation optical unit 1, the product of the two imaging factors β1 and β2 can be in the range between 0.25 and 10.

[0065] The reticle 18 to be inspected, held by the reticle holder 19, is arranged in the object plane 17 as an object to be inspected or a mask to be inspected. The reticle holder 19 is mechanically operably coupled to the reticle displacement drive 20, and the reticle 18 is displaced along the object displacement direction y by the reticle displacement drive 20 during mask inspection. In this way, the scanning displacement of the reticle 18 in the object plane 17 is enabled.

[0066] The irradiation field of view 4 has typical dimensions in the object plane 17 of less than 1.5 mm. In the illustrated embodiment, the range of the irradiation field of view 4 is 1 mm in the x direction and 0.5 mm in the y direction.

[0067] The x / y aspect ratio of the irradiation field of view 4 corresponds to the x / y aspect ratio of the exit aperture 14.

[0068] The irradiation field of view 4, or a part of the irradiation field of view 4 that then constitutes the object field of view, is imaged into the image field of view 21 in the image plane 22 by the projection optical unit 20a. The size of the image field of view 21 can be in the range of 150 mm × 300 mm. The shorter image field of view range extends along the operation direction y.

[0069] The projection optical unit 20a has a magnification ratio of 500 for imaging the object field of view or the irradiation field of view 4 into the image field of view 21. Depending on the embodiment of the projection optical unit 20a, the magnification ratio can be in the range between 250 and 500.

[0070] The projection optical unit 20a has mirrors M1, M2, M3 that are successively numbered in the imaging beam path of the projection optical unit 20a, that is, it includes a total of three mirrors. Depending on the embodiment of the projection optical unit 20a, the number of mirrors can also be four or more. Another embodiment that can be used instead of the projection optical unit 20a will also be described with reference to FIG. 2 and later.

[0071] The aperture stop 9b is arranged within the entrance pupil plane EP of the projection optical unit 20a, and the entrance pupil plane EP is arranged between the reflective reticle 18 and the first mirror M1 in the imaging beam path of the irradiation light or imaging light. The aperture stop 9b can also serve to predefine the possible internal obscuration of the projection optical unit 20a.

[0072] The mirrors M1 and M2 of the projection optical unit 20a are implemented as illumination NI mirrors, and the incident angles of the illumination light and the imaging light 3 are less than 45°. The maximum incident angle of the individual light rays of the imaging light 3 with respect to the mirrors M1 and M2 is about 14°.

[0073] The illumination light or the imaging light 3 enters the image field of view 21 at an incident angle of less than 5°.

[0074] The mirror M1 has a boundary of the reflective surface used to guide the imaging light or the illumination light 3 along the imaging beam path, which corresponds to the boundary of the entrance pupil EP predefined by the aperture stop 9b.

[0075] At least one of the mirrors M1 and M2 can be implemented as an aspherical mirror. The projection optical unit 20a can have one aspherical mirror or two aspherical mirrors.

[0076] For that part, the illumination optical unit 1 of the measurement system 2a has an illumination pupil predefined by the aperture stop 9b that is adapted to the entrance pupil EP. This illumination pupil of the illumination optical unit 1 can have a boundary shape deviating from an ellipse and an aspect ratio not equal to 1. The illumination pupil can have a boundary shape that is approximately elliptical, approximately stadium-shaped, or approximately semi-circular, having a corresponding aspect ratio deviating from 1. The aspect ratio of the illumination pupil can correspond to the aspect ratio of the entrance pupil EP of the imaging optical unit 20a.

[0077] Another embodiment of the projection optical unit 20b that can be used instead of the projection optical unit 20a in the mask inspection system 2a will be described below with reference to FIGS. 2 and 3. Components and functions corresponding to those already described above with reference to FIG. 1 specifically have the same reference numerals and will not be discussed in detail again.

[0078] The projection optical unit 20b has a total of four mirrors M1, M2, M3, and M4 between the object field or illumination field 4 and the image field 21 in the imaging beam path, and the mirrors are also successively numbered in the order in which the illumination light or imaging light 3 strikes them. In FIG. 2, the imaging beam path of the projection optical unit 20b is shown by marginal rays emerging from two mutually spaced object field points. In the display according to FIG. 3, the number of mutually spaced field points is three.

[0079] At least one of the mirrors M1 to M4 can be implemented as an aspherical mirror. The projection optical unit 20b can have one aspherical mirror, can have two aspherical mirrors, or can have three aspherical mirrors. It is also possible that all four mirrors of the projection optical unit 20b are implemented as aspherical.

[0080] The entrance pupil plane where the aperture stop 9b is arranged is located between the object field 4 and the mirror M1.

[0081] The boundary shape of the entrance pupil EP of the projection optical unit 20b, predefined by the inner boundary of the aperture stop 9b, is semi-circular and corresponds to the shape of the entrance pupil that will also be described below in connection with another embodiment of the projection optical unit, as shown in FIGS. 4 and 5.

[0082] This semi-circular diameter range of the entrance pupil EP extends parallel to the x coordinate. In the region of the corresponding diameter boundary section that extends along the x coordinate and penetrates the entrance pupil EP, the boundary of the entrance pupil EP is defined by the obscuration caused by the mirror M2.

[0083] In the case of the projection optical unit 20b, the x:y aspect ratio of the entrance pupil EP is 2:1. Depending on the embodiment of the projection optical unit 20b, the aspect ratio can be in the range between 5:1 and 1.1:1. The numerical aperture of the projection optical unit 20b on the object side is approximately 0.125 (NAy = 0.125) in the yz plane according to FIG. 2, and approximately 0.25 (NAx = 0.25) in the xz plane perpendicular to the yz plane. Depending on the embodiment of the projection optical unit 20b, NAx can be between 0.1 and 0.5, and NAy can be between 0.05 and 0.25.

[0084] The intermediate image 24 is located between the mirrors M1 and M2 in the imaging beam path of the projection optical unit 20b.

[0085] On the mirrors M1 and M2, individual light rays related to different field points but related to the same irradiation angle are at relatively short distances from each other, at most one quarter of the total used reflection surface diameter of each mirror. Characterizing the field of view or pupil proximity of each mirror, the parameter P defined in WO2009 / 024164A1 has a value of P > 0.5 for each of the mirrors M1 and M2. Therefore, the mirrors M1 and M2 are proximity pupils. Specifically, the mirror M1 is a proximity pupil mirror.

[0086] On mirrors M3 and M4, individual light rays associated with the same viewing point but different irradiation angles are, relative to each other, at a distance of at most one quarter of the full-use reflection surface diameter of each respective mirror, and the parameter P (again, see the definition in WO2009 / 024164A1) is less than 0.5 for each of mirrors M3 and M4. Thus, mirrors M3 and M4 are mirrors arranged in the near field in the imaging beam path of the projection optical unit 20b. Specifically, mirror M4 is a near-field mirror.

[0087] The boundary of the reflection surface of mirror M1 used to guide the imaging light 3 along the imaging beam path corresponds to the boundary of the entrance pupil EP. This also applies approximately to mirror M2. In the case of mirror M2, the boundary shape is specularly reflected about the xz plane compared to the boundary of mirror M1 and further compared to the boundary of the entrance pupil EP.

[0088] The boundary of the reflection surface of mirror M4 used to guide the imaging light 3 along the imaging beam path corresponds to the boundary of the image field 21, and the boundary of the image field 21 is implemented as a rectangle or a square. In a similar manner, this also applies to mirror M3.

[0089] Mirror M4, i.e., the last mirror in the imaging beam path, is at a distance A from the image plane 22, and the distance A is more than 60% of the distance B between the object plane 17 and the image plane 22.

[0090] The distance C between mirror M4 and M2, i.e., the distance between the last mirror and the third-to-last mirror along the coordinate perpendicular to the image plane 22, is less than 15% of the distance B between the object plane 17 and the image plane 22.

[0091] The distance D between the second-to-last mirror M3 in the imaging beam path of the projection optical unit 20b and the image plane 22 is more than 20%, more than 25%, more than 30%, and further 35% of the distance B between the field planes 17, 22.

[0092] The projection optical unit 20b also has, in this case, a magnification ratio for imaging the object field 4 onto the image field 21 within the range between 250 and 500.

[0093] In the imaging beam path in the case of the projection optical unit 20b, each ray has an incident angle of at most 13° with respect to the mirrors M1 to M4 in each case. The incident angle of each ray of the imaging light 3 with respect to the image field 21 is at most 5° in the case of the projection optical unit 20b.

[0094] Another embodiment of the projection optical unit 20c, which can be used instead of the aforementioned projection optical unit within the mask inspection system 2a, will be described below with reference to FIGS. 4 and 5. Components and functions corresponding to those already described above with reference to FIGS. 1 to 3 have specifically the same reference signs and will not be discussed in detail again.

[0095] The imaging beam path of the projection optical unit 20c corresponds in principle to the imaging beam path of the projection optical unit 20b.

[0096] The distance A between the mirror M4 and the image plane 22 is approximately 69% of the distance B between the object plane 17 and the image plane 22. The distance C between the mirrors M4 and M2 is approximately 13% of the distance B.

[0097] In the case of the projection optical unit 20c, the distance D between the second last mirror M3 in the imaging beam path and the image plane 22 is approximately 35% of the distance B between the field planes 17, 22.

[0098] The object field numerical aperture NAx is 0.27 in the case of the projection optical unit 20c. The object field numerical aperture NAy is 0.135. The object-side field size is 0.74 mm × 0.28 mm, and the field offset is 0.06 mm in the y direction.

[0099] The projection optical unit 20c has a magnification ratio of 435.

[0100] In the case of the projection optical unit 20c, the distance A has a magnitude such that shading of the pixel dimension within the image field of view 21 does not occur due to imperfections on the reflecting surface of the mirror M4 having a typical size of 0.16 mm.

[0101] Individual light rays in the imaging beam path of the projection optical unit 20c do not have an incident angle greater than 13° with respect to one of the mirrors M1 to M4.

[0102] In the case of the projection optical unit 20c, the imaging light 3 enters the image field of view 21 at an incident angle of less than 5°.

[0103] In the case of the projection optical unit 20c, the wavefront aberration RMS across the image field of view 21 is at most 20 mλ, specifically 10 mλ in the case of the exemplary embodiment of the projection optical unit 20c. In the case of the projection optical unit 20c, the Petzval radius on the image field side is greater than 500 mm. The strain on the image field side is 1 nm.

[0104] The mirrors M1 and M2 each have a boundary of a reflecting surface used to guide the imaging light or the illumination light 3 along the imaging beam path, corresponding to the boundary of the entrance pupil EP. In the case of the mirror M2, the boundary shape is specularly reflected about the xz plane compared to the boundary shape of the entrance pupil EP shown in FIG. 5.

[0105] The optical design data of the projection optical unit 20c is summarized in Table 1a / b below.

[0106] The first column of Table 1a shows each optical surface starting from the object field of view 4.

[0107] The second column of Table 1a shows the radius of curvature of each optical surface.

[0108] The subsequent columns of Table 1a show the radius of curvature of the sphere that fits the optical surface.

[0109] The fourth column of Table 1a shows the z - distance with respect to each preceding surface.

[0110] The fifth column of Table 1a shows the optical effect of such a surface, if an optical effect exists. In the case of mirrors M1 to M4, this optical effect is "REFL", i.e., reflectivity.

[0111] The first column of Table 1b shows the maximum value of the incident height (distance perpendicular to the optical axis) of each surface description of the optical surface in mm.

[0112] The second column of Table 1b shows the maximum deviation from the best-fitting sphere of each aspherical optical surface, also in mm.

[0113] Furthermore, for the mirror surfaces of mirrors M1, M2, and M4, Table 2 below also shows the coefficients K, C1, C2, and C3 that follow the following aspherical formula.

[0114] p(h)=[((l / r)h 2 ) / (1+SQRT(1-(1+K)(l / r) 2 h 2 ))]+C1·h 4 +C2·h 6 +C3·h 8 .... In this case, p is the sagittal height, h is the incident height, r is the radius of curvature, K is the conic constant, and C1, C2, and C3 are the first three even coefficients of the aspherical correction polynomial.

[0115] The deviation from the best-fitting sphere results from the difference between the sagittal height according to the surface formula of the aspherical surface and the best-fitting sphere. [Table 1]

[0116] [Table 2]

[0117]

Table 3

[0118] Therefore, in the case of the projection optical unit 20c, the mirrors M1, M2, and M4 are implemented as aspherical surfaces. The mirror M3 is a spherical mirror.

[0119] In FIG. 5, the obscuration effect of the mirror M2 with respect to the entrance pupil EP is also indicated by OBS. The mirror M2 is implemented such that the obscuration OBS does not overlap with the semi-circular entrance pupil EP.

[0120] Another embodiment of the projection optical unit 20d that can be used in place of the aforementioned projection optical unit within the mask inspection system 2a will be described below with reference to FIGS. 6 and 7. Components and functions corresponding to those already described above with reference to FIGS. 1 to 5 have specifically the same reference numerals and will not be discussed in detail again.

[0121] The object field-side numerical aperture NAx is 0.25 in the case of the projection optical unit 20d. The object field-side numerical aperture NAy is 0.125. The object-side field size is 0.56 mm × 0.36 mm, and the field offset is 0.06 mm in the y direction.

[0122] The projection optical unit 20d also has four mirrors M1 to M4. In the case of the projection optical unit 20d, in the figure, the beam path is specularly reflected about the xz plane compared to the beam path of the projection optical unit 20c. In other respects, the beam path within the projection optical unit 20d corresponds in principle to the beam path within the projection optical unit 20c.

[0123] In the case of the projection optical unit 20d, the following holds for the ratio of the distances A and B.

Equation

[0124] Another ratio of distances is

Number

Number

[0125] Figure 7 shows the edge contour of the entrance pupil EP, and the edge contour simultaneously corresponds to the inner boundary contour of the aperture stop 9b. The entrance pupil EP of the projection optical unit 20d has a boundary shape having a semicircular boundary portion 25 and a diameter boundary portion 26. Along the diameter boundary portion 26 that generates the entire boundary of the entrance pupil EP together with the semicircular boundary portion 25, a notch portion 27 exists on the boundary of the entrance pupil EP. The central notch of this notch portion 27 extends parallel to the diameter boundary portion 26 in a form displaced in the positive y direction with respect to the diameter boundary portion 26. This central notch of the notch portion 27 merges into the diameter boundary portion 26 via two diagonal boundary portions 28, 29.

[0126] The y distance between the central notch portion 27 and the diameter boundary portion 26 is less than 15% of the y range of the entire entrance pupil EP. Therefore, the entire notch of the diameter boundary portion has an area that can be ignored compared to the semicircular envelope around the entrance pupil EP and even compared to the area of the entrance pupil EP itself.

[0127] The notch portion 27 is caused by the obscuration of the entrance pupil EP caused by the mirror M2 of the projection optical unit 20d.

[0128] The magnification ratio is 435 in the case of the projection optical unit 20d.

[0129] In the case of the projection optical unit 20d, the RMS wavefront aberration over the image field 21 is 15 mλ.

[0130] In the case of the projection optical unit 20d, the Petzval radius on the image field side is about 20.000 mm.

[0131] Optical design data regarding the projection optical unit 20d is again summarized below in two tables, and its structure corresponds to the tables for the projection optical unit 20c in FIGS. 4 and 5. [Table 4]

[0132] [Table 5]

[0133] [Table 6]

[0134] The aspherical mirrors of the projection optical unit 20a, and further the aspherical mirrors M1, M2, and M4 of the projection optical units 20c and 20d, have reflecting surfaces that deviate from the spherical shape by at most 25 μm. The small-area mirrors M2 and M3, also called small mirrors having a reflecting surface diameter of less than 50 mm, deviate from the spherical shape by at most 5 μm with respect to the range of the reflecting surface. In the cases of the projection optical units 20c and 20d, the respective spherical mirror M3 does not deviate from the spherical shape at all.

[0135] The irradiation light beam path of the irradiation light 3 for irradiating the reticle 18 and the imaging light beam path of the projection optical unit 20a for imaging the object field 4 within the image field 21 intersect with each other in the intersection region. This intersection region can be within the region of the entrance pupil plane EP of the projection optical unit 20a. Here, the imaging light beam path intersects with the irradiation light beam path between the exit aperture 14 of the irradiation optical unit 1 and the mirror IL2, and further between the mirrors IL2 and IL3 of the irradiation optical unit 1.

[0136] The image field of view 21 is captured by a detection device 23, for example, one or a plurality of CCD cameras. For details regarding imaging onto the image field of view, reference is made to the references cited in U.S. Patent No. 10,042,248 B2 and U.S. Patent No. 10,042,248 B2. The detection device 23 can also be implemented as a TDI (Time Delay Integration) detection device equipped with a plurality of TDI detectors.

[0137] The detection device 23 is implemented in a spatially resolved manner. The detection device 23 can include sensor pixels having a typical pixel size of at most 20 μm × 20 μm. This pixel size can be smaller, for example, 15 μm × 15 μm or 10 μm × 10 μm. The pixel dimensions along the image field of view coordinates x and / or y can be in the range of 1 μm to 20 μm.

[0138] For example, inspection of the structure of the reticle 18 is possible by the mask inspection system 2a.

Explanation of Reference Signs

[0139] 1 Illumination optical unit 2 Optical system 2a Mask inspection system 3 Illumination light, EUV illumination light, illumination light beam, imaging light 4 Illumination field of view 5 Light source 6 Light source region, source volume 8 Used optical filter 9 Aperture stop 9a Aperture limiting stop 9b Aperture stop 10 Input coupling optical unit 11 Beam homogenizing element 12 Entrance aperture 13 Entrance plane 14 Exit aperture 15 Exit plane 16 Imaging output coupling mirror optical unit 17 Object plane 18 Reticle 19 Reticle holder 20 Reticle Displacement Drive 20a Projection Optical Unit 20b Projection Optical Unit 20c Projection Optical Unit 20d Projection Optical Unit 21 Image Field of View 22 Image Plane 23 Detection Device 24 Intermediate Image 25 Semi-circular Boundary Portion 26 Diameter Boundary Portion 27 Notch Portion, Central Notch Portion 28 Oblique Boundary Portion 29 Oblique Boundary Portion EP Entrance Pupil IL1 Mirror IL2 Mirror IL3 Mirror M1 Mirror M2 Mirror M3 Mirror M4 Mirror OBS Obscuration VK Vacuum Chamber

Claims

1. An enlarged imaging optical unit (20a; 20b; 20c; 20d) for a measurement system (2a) for inspecting an object (18), Comprising at most four mirrors (M1, M2; M3; M1, M2, M3, M4) that image an object field of view (4) in an object plane (17) along an imaging beam path into an image field of view (21) in an image plane (22), An enlarged imaging optical unit comprising an entrance pupil (EP) having a boundary shape deviating from an ellipse and an aspect ratio not equal to 1.

2. The enlarged imaging optical unit according to claim 1, characterized in that the entrance pupil (EP) has a boundary shape having a semi-circular boundary portion (25).

3. The enlarged imaging optical unit according to claim 2, characterized in that a notch portion (27) is present in the boundary along a diameter boundary portion (26) that generates the entire boundary of the entrance pupil (EP) together with the semi-circular boundary portion (25).

4. The enlarged imaging optical unit according to any one of claims 1 to 3, characterized in that at least one mirror (M1; M1, M2) comprises a boundary of a reflecting surface used to guide imaging light (3) along the imaging beam path corresponding to the boundary of the entrance pupil (EP).

5. An enlarged imaging optical unit (20a; 20b; 20c; 20d) for a measurement system (2a) for inspecting an object (18), Comprising at most four mirrors (M1, M2, M3; M1, M2, M3, M4) that image an object field of view (4) in an object plane (17) into an image field of view (21) in an image plane (22), wherein at least one of the mirrors is a small-area mirror (M2, M3) having a diameter of the reflecting surface of less than 50 mm, and the reflecting surface of the small-area mirror (M2, M3) deviates from a spherical shape by at most 10 μm. An enlarged imaging optical unit.

6. The enlarged imaging optical unit according to claim 5, characterized in that the reflecting surfaces of all the mirrors deviate from a spherical shape by at most 25 μm.

7. The enlarged imaging optical unit according to any one of claims 1 to 6, characterized in that the last mirror (M3; M4) in the imaging beam path is at a distance more than 60% of the distance between the object plane (17) and the image plane (22) from the image plane (22).

8. The enlarged imaging optical unit according to any one of claims 1 to 7, characterized by a magnification ratio in the range between 250 and 500.

9. The enlarged imaging optical unit according to any one of claims 1 to 8, characterized in that each individual ray in the imaging beam path does not have an incident angle greater than 14° with respect to one of the mirrors (M1, M2; M1 to M4).

10. The enlarged imaging optical unit according to any one of claims 1 to 9, characterized in that the imaging beam path is implemented such that the imaging light (3) enters the image field of view (21) at an incident angle of less than 5°.

11. The enlarged imaging optical unit according to any one of claims 1 to 10, characterized by a maximum RMS wavefront aberration of 50 mλ.

12. The enlarged imaging optical unit according to any one of claims 1 to 11, characterized in that at least one of the reflecting surfaces of the mirror deviates from a spherical shape by at least twice the operating wavelength.

13. An illumination optical unit (1) for a measurement system (2a) for inspecting an object (18), comprising an illumination pupil adapted to the entrance pupil (EP) of the enlarged imaging optical unit according to any one of claims 1 to 12, the illumination optical unit (1) having a boundary shape deviating from an ellipse and an aspect ratio not equal to 1.

14. An optical system (2) comprising an imaging optical unit (20a; 20b; 20c; 20d) according to any one of claims 1 to 12 and an illumination optical unit (1) for illuminating the object field of view (4) with illumination light (3).

15. An optical system (13) comprising the illumination optical unit according to claim 13.

16. Comprising the optical system according to claim 14 or 15, comprising a light source (5), a measurement system (2a) comprising a spatially resolving detection device (22) for capturing the image field of view (21).

Images