Method for highly accurately determining the location of a structure, apparatus for carrying out the method and metrology system for measuring lithography masks

Abstract

To determine the position of at least one structure of a carrier (2), a plurality of measurement values are generated, wherein the carrier (2) is displaced between the detections of the position of the at least one structure for generating the plurality of measurement values.

Description

[0001] Method for highly accurately determining the location of a structure

[0002] The content of the German patent application DE 10 2024 203 890.3 is incorporated herein by reference.

[0003] The invention relates to a method for highly accurately determining the location of a structure. In particular, the structure can be a line-type structure whose width is smaller than its length.

[0004] This structure is, in particular, a mark on a lithography mask. The structure may also be a mask defect. It can also be a structure which is intended to be imaged.

[0005] The invention further relates to an apparatus for carrying out the method and to a metrology system for measuring lithography masks.

[0006] Special metrology systems and measurement methods can be used to measure the location of the structure. Such a system and method is described, for example, in EP 1 191 481 A2.

[0007] There is a constant demand for improving a method for determining the location of structures, in particular on lithography masks, and an apparatus for carrying out such a method and a metrology system for measuring lithography masks.

[0008] These objects are achieved by the subject matter of the invention on which the present application is based. According to the invention, the position of at least one structure is determined multiple times, wherein the relative position of the carrier of the structure with respect to the beam path of the illumination radiation used for detecting the former is changed between the measurements. The carrier of the structure may in particular be a lithography mask.

[0009] For the sake of simplicity, the carrier of the structure is also referred to below as "the carrier".

[0010] This allows systematic errors, which can result from a specified illumination of the structure, to be at least partially, in particular largely, eliminated.

[0011] In particular, it has been recognized that an error sensitivity, which can result for non-telecentric and / or non-symmetric, in particular oblique, illumination of the mask, can be at least partially, in particular completely, eliminated.

[0012] The method leads in particular in the case of non-telecentric and / or non- symmetric, in particular oblique, illumination of the mask and / or when using a non-telecentric optical unit to improvements, in particular regarding the accuracy and / or the robustness.

[0013] The method leads to improvements in particular for a location of the entrance pupil in the finite and / or for a location of the entrance pupil which is not centred around an axis, oriented perpendicular to the object plane, through a central object field point, in particular regarding the accuracy and / or the robustness. According to one aspect, at least one, in particular exactly one, two or all of the following method steps can be performed between the repeated operations of detecting the position of the at least one structure: rotating the carrier of the structure in the carrier plane, in particular by 90°, displacing the carrier of the structure in the direction parallel to the chief ray direction of the illumination radiation, displacing the carrier of the structure in the direction perpendicular to the carrier plane.

[0014] In general, it is possible that the carrier of the structure is rotated in the carrier plane by an angle a, to which the following applies: 75° < a < 105°, in particular 80° < a < 100° and in particular 85° < a < 95°.

[0015] The "chief ray direction" is the direction of a centroid ray of the illumination radiation. In particular, it may be the direction of a ray from a centre of gravity, in particular a geometric centre of gravity, of the pupil to a central point in the object field.

[0016] A straight line is parallel to the chief ray direction if it either coincides with the straight line representing the chief ray direction or has a constant distance therefrom. A direction vector of the chief ray direction and a direction vector of the straight line parallel to the chief ray direction form in particular an angle of 180° or 0° relative to each other. The direction vectors are in particular linearly dependent.

[0017] In particular, the carrier may be displaced in a direction which is substantially parallel to the chief ray direction. In particular, it is possible that the direction vectors enclose an angle 0, to which the following applies: 0 < 10°, in particular 0 < 7°, in particular 0 < 5° and in particular 0 < 3°.

[0018] A direction is in particular perpendicular to the carrier plane if the direction can be represented by a straight line having a direction vector which is oriented parallel to a surface normal of the carrier plane.

[0019] In particular, the carrier can be displaced along a direction that is substantially perpendicular to the carrier plane. It is possible that the carrier plane and the direction enclose an angle y, to which the following applies: 75° < y < 105°, in particular 80° < y < 100°, in particular 85° < y < 95°.

[0020] The chief ray angle indicates the inclination of the chief ray relative to a surface normal to the carrier plane, in particular relative to a surface normal to the object field.

[0021] The carrier plane is understood to be a plane which characterizes the orientation of the area on which the structures are arranged, in particular to the best possible approximation.

[0022] The carrier of the structure can have one or more structures.

[0023] From the position of the structures, both the distance between structures and the position with respect to a reference point on the carrier can be determined.

[0024] The metrology system for detecting the position of the structures has a radiation source for generating illumination radiation, an illumination optical unit for illuminating the carrier having the structures with illumination radiation, and a measurement apparatus for spatially resolved detection of the position of the structures, in particular relative to the respective masks.

[0025] The metrology system may also have, in particular, an imaging optical unit for projecting an image of the carrier, in particular of the structures, onto the measurement apparatus. In particular, the image of the carrier can be imaged into an image field. The image field can be in a measurement plane. The measurement plane does not necessarily have to be absolutely planar. It can also be curved. However, a fully planar measurement plane can be beneficial.

[0026] The displacement of the carrier is used to record a focus stack. The focus stack may comprise two or more, in particular exactly or at least three, four, five, six or seven images of the structures, in particular of the marks on a lithography mask.

[0027] A smaller number of images allows the method to be carried out more quickly.

[0028] A larger number of images allows a more precise determination of the best focus position and / or the exact position of the structures on the carrier.

[0029] It may be particularly advantageous to perform a combination of a displacement of the carrier and a rotation of the carrier in the carrier plane, in particular by 90°, for generating a plurality of measurement values for characterizing the position of the structure. A "measurement value" is the representation of the detected position of the structure, for example in the form of one or more Cartesian coordinates of a selected reference system. In particular, it is possible that a measurement value comprises a pair of Cartesian coordinates representing a point in a Cartesian coordinate system. In particular, a measurement value is a displayed result, which is generated from a detection of the position of the structure on the carrier.

[0030] A "plurality of measurement values" can then be understood to mean a quantity comprising at least two measurement values. In particular, the plurality of measurement values can also be generated without the carrier being displaced between the generation of individual measurement values. It is also conceivable that the carrier is displaced between the generation of individual measurement values.

[0031] In particular, it is possible that the plurality of measurement values comprises exactly two measurement values, wherein one measurement value is generated before a displacement of the carrier and one measurement value is generated after a displacement of the carrier.

[0032] In particular, a defocus-dependent variation of the magnification and / or a defocus-dependent displacement of the image of the structure can be determined, in particular eliminated, by way of a displacement of the carrier. This allows a systematic error, which can occur with a non-telecentric and / or non-symmetric illumination of the mask, to be eliminated at least partially, in particular largely, preferably completely. A possible residual error can be compensated for, in particular be eliminated, by curve fitting, in particular a linear or cubic function of the description of the position of the structure as a function of the defocus position, and / or a rotation of the carrier in the carrier plane, in particular by 90°, and redetermination of the position of the structure.

[0033] According to one aspect, the carrier can be illuminated with illumination radiation with a chief ray angle (CRA) of greater than 0° for determining the position of the structure. In other words, the structure can be illuminated obliquely.

[0034] The "chief ray angle" is understood in particular to mean the angle that the chief ray encloses with a surface normal to the object plane or to the carrier plane.

[0035] The chief ray angle can be in particular at least 1°, in particular at least 5°, in particular at least 10°.

[0036] Oblique illumination corresponds to a decentred entrance pupil. This is also referred to as non-symmetric illumination.

[0037] In particular, the illumination of the structure can be non-telecentric.

[0038] Oblique illumination allows the use of a reflective, in particular a purely reflective, metrology system. In particular, the metrology system may have only reflective optical elements. The metrology system can be configured in particular as a mirror system. The carrier of the structures may be a reflective carrier. In particular, it can be a mask for EUV lithography.

[0039] The illumination radiation can be in the EUV wavelength range in particular. In particular, it may have a wavelength of 13.5 rnn or less. This results in a particularly high resolution and thus in a particularly accurate determination of the position of the structures on the mask. It can also allow for a greater working distance and a greater depth of field.

[0040] According to one aspect, the chief ray direction of the illumination radiation for illuminating the carrier and a surface normal to the carrier plane define a plane which is parallel to one of the sides of the carrier. This plane is also referred to in particular as the plane of incidence of the illumination radiation.

[0041] Two planes are called parallel to each other when their surface normals are parallel to each other.

[0042] In particular, it is also possible that the chief ray direction and the surface normal of the carrier plane and the one side of the carrier enclose an angle 5, to which the following applies: 5 < 10°, in particular 5 < 7°, in particular 5 < 4°, and in particular 5 < 1°.

[0043] This can ensure that the illumination radiation is oblique, in particular non- telecentric, in particular non-symmetric, only with respect to one component of a Cartesian coordinate system of the carrier. With respect to the other component, the illumination radiation may be perpendicular. With respect to the second component, the illumination radiation may in particular be symmetric. This makes it possible to determine a component of the position of the structures without the systematic dependence on the defocus position resulting from non-telecentric, in particular oblique, non-symmetric illumination.

[0044] According to one aspect, the entrance pupil of the illumination radiation for illuminating the carrier of the structure may lie in infinity.

[0045] The structures can thus be illuminated with a parallel beam. This can eliminate a dependence of the magnification on the focus position.

[0046] For the realization of an entrance pupil which lies in infinity, the entrance pupil may be arranged in particular in a focal point of the illumination optical unit. In particular, it is possible that the deviation of the position of the entrance pupil from the focal point of the illumination optical unit deviates by not more than 5% of a focal length of the illumination optical unit, in particular by not more than 4%, in particular by not more than 3%, in particular by not more than 2% and in particular by not more than 1%.

[0047] The illumination pupil may have an elliptical envelope. The latter may have an eccentricity of at least 1.1, in particular at least 1.5, in particular at least 2.

[0048] The imaging optical unit can also be catoptric.

[0049] In particular, the imaging optical unit may have an anamorphic design. The imaging optical unit may have different imaging scales in particular in two mutually perpendicular directions, which may be aligned in particular parallel to the sides of the carrier. The imaging scales can have a ratio of at least 1.5: 1, in particular at least 2: 1.

[0050] According to one aspect, the carrier can be rotated by 90° between the two detections of the position and the generation of the plurality of measurement values of the at least one structure on the carrier.

[0051] This is what enables - save for a sign change - the x-direction and y-direc- tion of a Cartesian coordinate system of the carrier to be swapped. This makes it in particular possible to swap the directions which are illuminated obliquely and which are illuminated perpendicularly.

[0052] In particular, it is possible to combine the plurality of the measurement values generated by means of the two detections for a value for identifying the position of the at least one structure into a combined measurement value, wherein, for generating the combined measurement value from the measurement values of the plurality of measurement values, in each case the component which indicates the measurement value in the direction perpendicular to the plane of incidence of the illumination radiation is used.

[0053] It is also possible that in each case the component which indicates the measurement value in a direction substantially perpendicular to the plane of incidence of the illumination radiation is used. Here, the direction can enclose with the plane of incidence of the illumination radiation an angle £, to which the following applies: 75° < & < 105°, in particular 80° < & < 100°, in particular 85° < e < 95°. According to a further aspect, a best focus position can be determined from the measurement values of the position of the structure.

[0054] For this purpose, the dependence of the sharpness on the defocus position can be described by a function, in particular a Gaussian function. The function, in particular the Gaussian function, can be adapted to the measurements.

[0055] The best focus position can then be determined by interpolation. This allows the best focus position to be ascertained with better accuracy, in particular with sub-pixel accuracy and / or an accuracy better than the depth of field of the metrology system.

[0056] According to a further aspect, a dependence of the position of the at least one structure on a defocus position can be ascertained from the measurement values.

[0057] For this purpose, curve fitting, in particular a linear or a cubic function, may be provided.

[0058] In particular, this makes it possible to calculate the position of the structure in the carrier plane, in particular relative to the carrier, in the best focus position. This further improves the accuracy of the method.

[0059] According to a further aspect, the chief ray direction, in particular the actual chief ray direction, can be measured. In particular, it can be ascertained from or during one of the measurements to determine the position of the structures. Alternatively, it is possible to specify the nominal chief ray direction. In this case, the actual chief ray direction can be assumed to be known. This simplifies the method.

[0060] According to a further aspect, the position of the at least one structure can be detected in different ways at least three times, in particular at least four times, in particular at least five times, in particular at least seven times, for generating a plurality of measurement values. In this case, the carrier can be displaced between repeated detections, in particular along the chief ray direction or in the direction perpendicular to the carrier plane. In particular, it cannot be rotated in the process. The carrier plane is also known as the mask plane.

[0061] Furthermore, the carrier can be rotated in the carrier plane between two other of the detections. In particular, it cannot be displaced in the process, in particular not along the chief ray direction or in the direction perpendicular to the carrier plane.

[0062] The displacement of the carrier along the chief ray direction or in the direction perpendicular to the carrier plane can be used to eliminate systematic errors resulting from the non-telecentric and / or non-symmetric illumination.

[0063] The rotation of the carrier in the carrier plane can be used to eliminate residual errors.

[0064] According to a further aspect, the metrology system may be configured to identify at least one defect of a lithography mask. The method is also suitable in particular for the analysis of defects.

[0065] According to a further aspect, the metrology system may be configured to detect a position of the defect on the lithography mask.

[0066] The method is also suitable in particular for localizing defects.

[0067] An apparatus for carrying out the described method comprises an illumination optical unit for illuminating a carrier with illumination radiation and a displaceable carrier holder, wherein the carrier holder allows at least a selection from the following displacements: rotation of the carrier in a carrier plane, displacement of the carrier in the direction parallel to the chief ray direction, and displacement of the carrier in the direction perpendicular to the carrier plane.

[0068] In particular, the carrier holder can allow one, two or all three of these displacements.

[0069] A metrology system for measuring lithography masks, in particular for determining the position of structures of the lithography mask, comprises a corresponding apparatus.

[0070] The structures can be in particular marks, structures to be imaged or mask defects. In particular, the metrology system can determine the locations of the structures on the mask relative to one another, determine the locations of the structures relative to a reference on the mask, or both.

[0071] The metrology system may also comprise a radiation source. The radiation source may be, in particular, an EUV radiation source, i.e. a radiation source for generating illumination radiation in the EUV range. The radiation source can in particular generate illumination radiation with a wavelength of 13.5 nm or less.

[0072] The metrology system may also comprise an imaging optical unit. For details not mentioned in the previous description, reference is made to the state of the art. An EUV metrology system for imaging and measuring structures is, inter alia, described in EP 1 455 365 A2. Methods for determining the structure position and the position of marks are described e.g. in DE 10 2006 059 432 At

[0073] According to a further aspect, the metrology system may be designed to identify defects of a lithography mask.

[0074] The apparatus for carrying out the method and the metrology system are also suitable in particular for analysing defects.

[0075] According to a further aspect, the metrology system may be designed to detect positions of defects on the lithography mask.

[0076] The apparatus for carrying out the method and the metrology system are also suitable in particular for localizing defects. Further details and advantages of the invention will become apparent from the description of exemplary embodiments with reference to the figures. In the figures:

[0077] Fig. 1 schematically shows a metrology system for measuring carriers,

[0078] Figures 2A and 2B schematically show an alignment of the illumination radiation relative to a focus plane,

[0079] Fig. 2C schematically shows the illumination pupil corresponding to the illumination according to Figures 2A and 2B,

[0080] Figures 3A and 3B show examples of a structure in two arrangements of the carrier of the structure which are rotated by 90° relative to each other,

[0081] Fig. 4 schematically shows the effect of displacing the carrier in the direction perpendicular to the carrier plane for illustration purposes,

[0082] Figures 5A to 5C show exemplary images of a structure in the case of a displacement of the carrier perpendicular to the carrier plane according to Fig. 4,

[0083] Fig. 6 schematically shows the effect of displacing the carrier in the direction of the chief ray direction for illustration purposes, Figures 7A to 7C show exemplary images of a structure in the case of a displacement of the carrier in the direction parallel to the chief ray direction according to Fig. 6.

[0084] Fig. 1 shows highly schematically a metrology system 1 for examining a carrier 2, in particular in the form of a lithography mask 2 for EUV projection lithography. The metrology system 1, which is also known as APMI (Actinic Patterned Mask Inspection), can be used in particular to examine defects on the carrier 2 and their effects on imaging in EUV projection lithography. The carrier 2, here a lithography mask 2, can be checked in particular for structuring errors. The structuring error can then be examined by means of an analysis of what is known as an aerial image (aerial image metrology system, AIMS). AIMS systems are known from DE 102 20 815 Al. The metrology system 1 is used for the examination of a reflective carrier 2, in particular a reflective lithography mask 2.

[0085] The metrology system 1 can also be used in particular for determining the positioning of a structure 11. These structures 11 can be marks in particular. Such marks serve as anchor points for the precise alignment of the carrier 2, in particular a lithography mask. Unless otherwise indicated in the description, the structures 11 may also be structures to be imaged or mask defects.

[0086] In order to simplify the representation of locational relationships, a Cartesian xyz-coordinate system will be used hereinafter. The x-axis in Fig. 1 runs perpendicularly to the plane of the drawing and out of the latter. The y-axis in Fig. 1 runs towards the right. The z-axis in Fig. 1 runs upwards. The metrology system 1 has an EUV light source 3 for generating illumination and imaging light 4. The EUV light source 3 can be a plasma source, such as a laser-produced plasma (LPP) source or a gas discharge-produced plasma (GDP) source. The EUV light source 3 can also be an EUV laser. The latter can be realized, for example, by multiplying the frequency of longer-wave laser radiation. The EUV light source 3 emits usable illumination and imaging light 4 with a wavelength of 13.5 nm. Other wavelengths in the range between 5 nm and 100 nm, in particular in the range between 5 nm and 30 nm, can also be used as illumination and imaging light 4 with a corresponding design of the EUV light source 3.

[0087] An illumination optical unit 5 is used for transferring the illumination and imaging light 4 from the EUV light source 3 to an object field 6, in which a section of the reflective carrier 2, in particular of the one lithography mask 2, is arranged.

[0088] An imaging optical unit 7 having a strong magnification factor, for example of 500, images the object field 6 via an imaging beam path 8 into an image field 9. A spatially resolving detection device in the form of a CCD sensor 10 captures an intensity distribution of the illumination and imaging light 4 over the field of view 9. A CCD chip of the CCD sensor 10 may be a time delay integrated CCD (TDI CCD) chip. A TDI CCD chip can be used in particular for the examination of a lithography mask 2 moving through the object field 6. A displacement direction of the lithography mask 2 can run along the y-direction.

[0089] Illumination and detection of the illumination and imaging light 4 emanating from the object field 6 can be done in different ways. In the metrology system 1 according to Fig. 1, illumination is carried out with a numerical aperture NA of, for example, 0.25. The imaging optical unit 7 can cover this numerical aperture completely or partially, depending on the design. Assuming a perfectly reflective carrier 2, in particular a lithography mask 2, the entire reflected illumination and imaging light 4, or a part thereof, of the lithography mask 2 which is assumed to be perfectly reflective, can thus be captured by the imaging optical unit 7. Such illumination is also known as brightfield illumination. Dark field illumination is also possible, in which only components of the illumination and imaging light 4 which are scattered or diffracted by the carrier 2, in particular a lithography mask 2, are detected by the CCD sensor 10.

[0090] As can be seen schematically from Fig. 1, the carrier 2, in particular the lithography mask 2, is illuminated under oblique incidence of the illumination radiation 4. The chief ray 12 of the illumination radiation 4 runs in particular in the yz-plane. The telecentric vector thus has a component in the y-direction. Its component in the x-direction is zero.

[0091] The illumination of the carrier 2, in particular of the lithography mask 2, is in other words non-telecentric. The entrance pupil of the illumination is asymmetric with respect to a normal through a central point of the object field 6, which normal is oriented parallel to the z-direction.

[0092] Preferably, the entrance pupil of the illumination can lie in infinity. In principle, the illumination optical unit 5 may also have a finite entrance pupil.

[0093] The profile of the illumination radiation 4 in the region of the carrier 2, in particular the lithography mask 2, is again schematically illustrated in Figures 2A and 2B. The corresponding illumination pupil is shown schematically in Fig. 2C as an example. Furthermore, different defocus positions zi to ZN of the carrier 2 are shown as examples in Fig. 2A. The best focus position is marked using z*.

[0094] Since the chief ray 12 is parallel to the yz-plane, in particular perpendicular to the x-axis, a telecentric measurement can be emulated in that two consecutive measurements are carried out, wherein the carrier 2 is rotated in the carrier plane by 90° between the two measurements.

[0095] This is shown schematically by way of example in Figures 3 A and 3B for the case of a beam path that is telecentric only in the x-direction.

[0096] Fig. 3A schematically shows a carrier 2 in a first measurement position. In the first measurement position, a first measurement step is performed to determine position data of the structure 11. The position data can be acquired in particular in the form of a two-dimensional vector. Coordinates on the carrier 2 are specified in particular by means of the first Cartesian xi’yi‘- coordinate system shown in Fig. 3 A. This coordinate system is also referred to as the first carrier coordinate system.

[0097] Furthermore, a Cartesian xy-coordinate system is shown in Fig. 3A. This coordinate system is also referred to as a measurement coordinate system. The measurement coordinate system is in particular identical to the first carrier coordinate system.

[0098] The carrier 2 has a structure 11 in the form of a T. When determining the position of the structure 11, the position data of the structure 11 acquired in the measurement coordinate system can be transferred to the first carrier coordinate system by means of identical mapping and thus stored for the carrier under consideration. In particular, a multiplicity of position data of the structure 11 can be determined in this maimer, all of which can be present in the form of two-dimensional vectors in each case. The position of the structure 11 on the carrier can be determined via the totality of these vectors. In particular, position data about the edges of the structure 11 can be determined with such a measurement step. By means of these position data, in particular, the distances between structures 11 and also the distances of the structures 11 from the edges of the carrier 2 can be ascertained.

[0099] Fig. 3B shows the same carrier 2 as Fig. 3A, but in a second measurement position. In comparison with the first measurement position, the carrier 2 has been rotated clockwise by 90° within a carrier plane. The alignment of the measurement coordinate system is invariant with respect to this rotation. The first carrier coordinate system is rotated by way of this rotation to the second Cartesian X2‘y2‘ -coordinate system, shown in Fig. 3B. This coordinate system is also referred to as the second carrier coordinate system. In particular, the origin of the first carrier coordinate system under this rotation is invariant. The origin of the first carrier coordinate system coincides with the origin of the second carrier coordinate system. In particular, all three coordinate systems have the same origin.

[0100] During a position measurement of the structure 11 according to a measurement position shown in Fig. 3B, the measurement data are still recorded in the measurement coordinate system. The position data obtained in this way, which can be present in particular in the form of a two-dimensional vector, can be transferred to the second carrier coordinate system by means of rotational mapping, the representation of which in two dimensions can correspond in particular to a rotation matrix. Such mapping inverts in particular the entries of the vector, wherein the first entry of the vector additionally undergoes a sign change. The position data can be stored accordingly for the carrier 2.

[0101] Finally, the obtained position data of the first measurement step and the second measurement step can be merged. In particular, of each of the two measurement steps, only the respective position data which have been recorded in the x-direction with respect to the measurement coordinate system are retained. These position data are stored with respect to the carrier 2, for example in the form of an additional vector. The first entry of the vector corresponds to the x-position data of the first measurement step, while the second entry of the vector corresponds to the x-position data of the second measurement step.

[0102] In addition, aberrations, in particular astigmatism and / or spherical aberration and also other symmetric wavefront aberrations, can also lead to an impairment of position measurements. This is particularly the case if these aberrations lead to the fact that there is no uniquely identifiable first focus position.

[0103] In order to avoid influences due to astigmatism, it is advantageous if the structure used as a marker is insensitive to astigmatism. For example, it may be symmetric, in particular have no preferential orientation. In particular, it can be axially symmetric or circular.

[0104] In order to avoid any influence of spherical aberration, the structure may preferably not have any special spatial frequencies. It can also be advantageous to use different structures in combination.

[0105] Fig. 4 illustrates as an example that the carrier 2, in particular a lithography mask, can be displaced perpendicular to the carrier plane. The corresponding displacement vector 13 has only one component in the z-direction.

[0106] Exemplary images of a structure 11 used as a mark in different defocus positions z-i, zo and zi are shown in Figures 5A, 5B and 5C. As is shown as an example, the precise position of the structure 11 depends on the defocus position z.

[0107] The direction of the chief ray 12 can be determined from the position dependence of the structure 11 on the defocus position z.

[0108] In principle, the chief ray direction can also be specified or measured in other ways.

[0109] If the carrier 2, in particular the lithography mask 2, is displaced in the direction parallel to the chief ray 12, which is shown as an example in Fig. 6, the position of the structure 11 remains constant for the different defocus positions z-i, zo and zi (illustrated by way of example in Figures 7A, 7B and 7C).

[0110] By aligning the displacement vector 13 parallel to the direction of the chief ray 12, the so-called "line of sight" error (or telecentric error), which is caused by the oblique illumination, can thus be eliminated.

[0111] Any residual errors can be eliminated by fitting the curve of a function to the measurement values to describe the position of the structure 11. To determine the best focus position z*, a metric for characterizing the sharpness of the image of the structure 11 can be defined and its dependence on the defocus position z can be determined. By fitting a function, in particular a Gaussian function, to the curve of sharpness as a function of the defocus position z, the best focus position z* can be determined with a sub-pixel accuracy, in particular an accuracy better than the depth of field of the imaging.

[0112] Curve fitting can also be used to determine the actual position of the structure 11. Here, in particular, a linear or cubic function to the curve which represents the position of the structure 11 as a function of the defocus position z can be used. This can also improve the determination of the position of the structure 11. Such curve fitting allows in particular the ascertainment of the nominal position of the structure 11 with sub-pixel / sub-depth-of- field accuracy.

[0113] It is also possible to eliminate residual errors, which can remain in particular after a displacement of the carrier 2, in particular the lithography mask 2, in the direction parallel to the chief ray direction, by way of a rotation of the carrier 2, in particular the lithography mask, previously described with reference to Figures 3A and 3B and repeated measurement.

Claims

Claims:

1. Method for determining the location of at least one structure on a carrier (2), comprising the following steps:1.

1. provide a metrology system (1) comprising1.1.

1. a radiation source (3) for generating illumination radiation (4),1.1.

2. an illumination optical unit (5) for illuminating a carrier (2) which has at least one structure (11), is arranged on a displaceable carrier holder and extends in a carrier plane with the illumination radiation (4), and1.1.

3. a measurement apparatus for detecting the position of the at least one structure (11) of the carrier (2) in a spatially resolved manner,1.

2. illuminating the carrier (2) with the illumination radiation (4),1.2.

1. wherein the illumination radiation (4) has a chief ray direction,1.

3. detecting the position of the at least one structure (11) of the carrier (2) in a spatially resolved manner by means of the measurement apparatus,1.

4. wherein the position of the at least one structure (11) of the carrier (2) is detected at least twice in different ways for generating a plurality of measurement values,1.

5. wherein the carrier (2) is displaced between the two detections to generate the plurality of measurement values.

2. Method according to Claim 1, characterized in that the displacing of the carrier (2) comprises at least one of the following method steps:2.

1. rotating the carrier (2) in the carrier plane,2.

2. displacing the carrier (2) in the direction parallel to the chief ray direction of the illumination radiation,2.

3. displacing the carrier (2) in the direction perpendicular to the carrier plane.

3. Method according to either of the preceding claims, characterized in that the carrier (2) is illuminated with illumination radiation (4) at a chief ray angle (CRA) greater than 0°.

4. Method according to any of the preceding claims, characterized in that the chief ray direction (12) when illuminating the carrier (2) and a surface normal to the carrier plane define a plane oriented parallel to one of the sides of the carrier (2).

5. Method according to any of the preceding claims, characterized in that the entrance pupil of the illumination radiation (4) when illuminating the carrier (2) lies in infinity.

6. Method according to any of the preceding claims, characterized in that the carrier (2) is rotated by 90° between the two detections of the position of the at least one structure (11).

7. Method according to Claim 6, characterized in that the plurality of the measurement values generated by means of the two detections are combined to a combined measurement value for identifying the location of the at least one structure (11), wherein the component which indicates the measurement value in the direction perpendicular to theplane of incidence of the illumination radiation (4) is used for generating the combined measurement value from the measurement values of the plurality of measurement values.

8. Method according to any of the preceding claims, characterized in that a best focus position (zO) is ascertained from the measurement values.

9. Method according to any of the preceding claims, characterized in that a dependence of the position of the at least one structure (11) on a defocus position (z) is ascertained from the measurement values.

10. Method according to Claim 9, characterized in that the position of the at least one structure (11) in a best focus position (zO) is ascertained from the dependence of the position of the at least one structure (11) on a defocus position (z).

11. Method according to any of the preceding claims, characterized in that the chief ray direction (12) is measured.

12. Method according to any of the preceding claims, characterized in that the position of the at least one structure (11) of the carrier (2) is detected at least three times in different ways for generating a plurality of measurement values,12.

1. wherein the carrier (2) is displaced along the chief ray direction or in the direction perpendicular to the carrier plane between two of the detections, and12.

2. wherein the carrier (2) is rotated in the carrier plane between two other of the detections.

13. Method according to any of the preceding claims, characterized in that the metrology system (1) is configured to identify at least one defect of a lithography mask.

14. Method according to any of the preceding claims, characterized in that the metrology system (1) is configured to detect a position of the defect on the lithography mask.

15. Apparatus for carrying out the method according to any of the preceding claims, comprising15.

1. an illumination optical unit (5) for illuminating a carrier (2) having at least one structure (11) with illumination radiation (4), and15.

2. a displaceable carrier holder.

16. Apparatus according to Claim 15, characterized in that the carrier holder allows at least a selection from the following displacements:16.

1. rotation of the carrier (2) in a carrier plane,16.

2. displacement of the carrier (2) in the direction parallel to the chief ray direction,16.

3. displacement of the carrier (2) in the direction perpendicular to the carrier plane.

17. Metrology system (1) for measuring carriers (2), in particular lithography masks (2), comprising an apparatus according to Claim 15 or 16.

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