Method and device for determining alignment of photomask on sample stage which is displaceable along at least one axis and rotatable about at least one axis

Abstract

To provide a method and device for improving determination of an alignment of a photomask.SOLUTION: The present invention relates to a method (1700, 1800) for determining an alignment of a photomask (270, 410, 700) on a sample stage (100) which is displaceable along at least one axis that is parallel to a chuck surface (195, 265) of the sample stage (100), and is rotatable about at least one axis (250) that is perpendicular to the chuck surface (195, 265). The method (1700, 1800) comprises the following step: rotating the sample stage (100) by a predefined angle and measuring a height change (1570) of the photomask (270, 410, 700) during rotation at a predetermined, non-vanishing distance (490) with respect to a rotation axis (1450) for the purpose of determining the alignment of the photomask (270, 410, 700) on the sample stage (100).SELECTED DRAWING: Figure 14

Description

[Technical field]

[0001] This patent application claims priority from German Patent Application No. DE102020209638.4 entitled "Patent application for a patentable material containing a photomask, which is suitable for use in a photo-taking and photo-taking application" filed on July 30, 2020 with the German Patent and Trademark Office. German Patent Application No. DE102020209638.4 is incorporated by reference in its entirety into this patent application.

[0002] The present invention relates to a method and apparatus for determining the alignment of a photomask on a sample stage that is displaceable along at least one axis and rotatable about at least one axis, in particular, the present invention relates to a method and apparatus for determining the alignment of a photomask on a sample stage that is displaceable along at least one axis parallel to a chuck surface and rotatable about at least one axis perpendicular to a chuck surface of the sample stage. [Background technology]

[0003] As a result of the increasing integration density in the semiconductor industry, photolithography masks have to image smaller and smaller structures onto the wafer. To generate the small feature dimensions to be imaged onto the wafer, photolithography masks or templates for nanoimprint lithography with ever smaller features or pattern elements are required. Thus, the processes for manufacturing photolithography masks and templates for nanoimprint lithography become more and more complex and therefore more time consuming and ultimately more expensive as well. Due to the small feature sizes of the pattern elements of photolithography masks or templates, defects during mask or template manufacturing cannot be eliminated. These defects must be repaired as far as possible.

[0004] Defects or defects in photolithography masks, photomasks, exposure masks, or simply masks are often repaired by supplying one or more process or precursor gases to the repair location and scanning the defect with, for example, an electron beam. The electron beam usually causes a local chemical reaction, which, depending on the precursor gas used, results in a local etching process that can be used to remove locally excess material from the photomask or template for nanoimprint lithography. Alternatively, in the presence of the corresponding precursor gas, the electron beam causes a local chemical deposition reaction that locally deposits material on the photomask, thus replacing locally missing material of the mask.

[0005] A further source of defects in photolithography masks are particles that deposit on the mask, for example as a result of the handling of the mask. These particles that interfere with the imaging of the mask must be removed from the mask as well. The interfering particles can first be removed from the photomask using a local particle beam induced etching process. Furthermore, it is possible to use micromanipulators, for example in the form of a scanning probe microscope, to remove excess material from the photomask by mechanically processing the particles, for example particles present on the mask.

[0006] Due to the ever-smaller structures of photomasks and the shorter actinic wavelengths to which the masks are exposed, ever-smaller defects and / or ever-smaller particles have the effect of disturbing the imaging behavior of the photomask. In this regard, for masks for the extreme ultraviolet (EUV) wavelength range, for example, the actinic wavelengths are in the range of about 10 nm to 15 nm. This means that ever-better tools are needed to handle defects in photolithographic masks. Moreover, this development has the consequence that the demands on the precision with which identified defects must be able to be approached for repair purposes are likewise increased.

[0007] Firstly, due to the increasing requirements for precision and secondly due to the expanding mobility of the sample stage, the accurate alignment or calibration of the mask to the sample stage or mask plate becomes more and more complicated and costly. As a result, the alignment or calibration of the mask to be repaired on the sample stage increasingly affects the repair time of defective masks. As a result, said repair time becomes longer or the throughput of the repaired masks becomes lower.

[0008] Therefore, the problem addressed by the present invention is to define a method and apparatus for improving the determination of photomask alignment. Summary of the Invention

[0009] According to an exemplary embodiment of the present invention, this problem is at least partially solved by the subject matter of the independent claims of the present application. Exemplary embodiments are set forth in the dependent claims.

[0010] A first embodiment relates to a method for determining the alignment of a photomask on a sample stage displaceable along at least one axis parallel to a chuck surface of the sample stage and rotatable about at least one axis perpendicular to the chuck surface, the method comprising rotating the sample stage by a predefined angle and measuring the height change of the photomask during the rotation at a predefined non-vanishing distance relative to the axis of rotation for the purpose of determining the alignment of the photomask on the sample stage.

[0011] Sample stages for photomasks typically have three translation axes perpendicular to each other. A sample stage carrying the photomask to be repaired and displaceable in three directions, but also rotatable about an axis perpendicular to the surface of the sample stage, extends the possibilities of defect repair by the fact that defects can be imaged and / or processed from different sides. Furthermore, the combined rotational and translational movement allows defects with a specific contour to be tracked to a processing position in a simpler form compared to a movement composed of two mutually orthogonal displacements.

[0012] However, the inventors have found that rotating the sample stage around the z-axis perpendicular to the chuck surface can cause height changes at processing locations located outside the rotation axis. The height changes of the photomask are caused by a rotation axis that is not aligned perpendicular to the surface of the mask. A rotation axis that is not perpendicular to the mask results in a rocking motion of the photomask during the photomask rotation. As a result, the tool that processes the defect cannot be correctly positioned in the z-direction, i.e. perpendicular to the chuck surface. In the worst case, as a result, during the rotational movement of the sample stage, the repair tool may damage a non-defective mask location as a result of unintended mechanical contact.

[0013] The method according to the invention determines the height change caused by the rotation of the sample stage at the defect processing location on the photomask. The defect processing location can be localized at any desired location on the mask. As a result, the method according to the invention fully exploits the possibilities for improved defect repair resulting from a rotatable sample stage. Furthermore, the method according to the invention avoids unintentional damage to the photomask during the rotation process.

[0014] The sample stage can be rotated through an angle of 60° or more, preferably 90° or more, more preferably 180° or more, and most preferably 360° or more.

[0015] As the rotation angle increases, the accuracy with which it is possible to determine the height change or height profile of the photomask during rotation increases. It is therefore advantageous to select a rotation angle as large as possible, i.e., about 360°. Rotating with a larger rotation angle will only improve the statistics of the data recording.

[0016] The photomask may comprise a transmissive or reflective photomask. The transmissive mask may comprise any conventional photomask, such as a binary mask, a phase-shift mask or a mask for multiple exposure. The reflective photomask may comprise a mask for the extreme ultraviolet (EUV) wavelength range, in particular a binary mask or a phase-shift mask. However, the photomask may also comprise a template for nanoimprint lithography.

[0017] The sample stage can be a sample stage of a repair tool. In particular, the repair tool can induce localized etching and / or deposition processes. Furthermore, the repair tool can include one or more scanning probe microscopes that can be used as manipulators or micromanipulators for processing the defects.

[0018] The height change may be measured by at least one height sensor.

[0019] The method according to the invention may include the additional step of predetermining a non-vanishing distance for the axis of rotation.Furthermore, the method according to the invention may include the additional step of determining the coordinates of the axis of rotation on the surface of the photomask.

[0020] The step of determining the coordinates of the rotation axis may include: (a) measuring a first set of coordinates of at least two markings on the photomask by a particle beam of at least one particle beam source without rotating the photomask; (b) rotating the photomask by an angle of 0°<θ<180° or 180°<θ<360°; and (c) measuring a second set of coordinates of the at least two markings on the rotated photomask by a particle beam of the particle beam source.

[0021] A photomask typically has a number of markings, usually applied at regular intervals on the side of the mask that is structured by the pattern. Each of the markings generally has a reference point. The mask coordinates (u,v), typically specified by the mask manufacturer, refer to said points. The coordinates (u,v) of a defect on the mask D , v D ) also refer to these reference points. In order to maximize the measurement accuracy when measuring the two reference points of the marking, it is advantageous to use markings that are at the greatest possible distance from each other on the photomask.

[0022] Based on the two measured sets of coordinates of the two markings, the photomask can be rotated before measuring the second set, and the coordinates of the rotation axis of the photomask can be ascertained.

[0023] Furthermore, the step of determining the coordinates of the rotation axis may include lowering the sample stage before approaching the at least two markings for the purpose of measuring the first and / or second set of coordinates and / or before rotating the sample stage.

[0024] This measure can prevent the particle beam source and / or one or more scanning probe microscopes or their micromanipulators from unintentionally contacting the surface of the photomask during movement of the photomask.

[0025] Determining a coordinate of the axis of rotation may include determining the coordinate of the axis of rotation from measured first and second sets of coordinates of the at least two markings.

[0026] Choosing a rotation angle of approximately 90° optimizes the accuracy in determining the rotation axis or in determining the coordinates of the rotation axis, and therefore it is advantageous to choose an angle of approximately 90° for rotating the photomask.

[0027] The measurement of the at least two markings can be performed by at least one particle beam source configured to generate at least one focused particle beam from the group of: photon beams, electron beams, ion beams, atomic beams, and molecular beams.

[0028] To measure the markings or for reference points on the markings, the electron beam of a scanning electron microscope (SEM) or modified SEM is often used. The electron beam can produce a very small spot (D S<1 nm). This allows a very high lateral resolution in determining the reference point of the marking. Furthermore, imaging of the marking with an electron beam causes no or only very little damage to the marking and thus to the entire photomask. The marking can be imaged manually and / or in an automated manner. By way of example, this can be done by centering a particular structure of the marking, for example its reference point, in the SEM image. After centering the electron beam on the marking, a measurement transducer of the sample stage indicates the desired coordinate values.

[0029] Predetermining the non-annihilation distance relative to the axis of rotation can include determining coordinates of an incidence point on the photomask of a particle beam of the at least one height sensor. The particle beam of the at least one height sensor can include a massless particle beam, e.g., a photon beam or a light beam, and / or a massed particle beam, e.g., an electron beam.

[0030] To accurately ascertain the height change of the photomask during the rotation of the sample stage, it is necessary to know the distance between the coordinates of the rotation axis and the coordinates of the point of incidence of the particle beam of the height sensor on the photomask. By measuring this distance, it is possible to determine the radius of the circle or circle segment of the point of incidence of the particle beam of the height sensor on the photomask during the rotation of the sample stage, and thus of the photomask, whose height change is being measured.

[0031] Determining coordinates of a point of incidence of the particle beam on the at least one height sensor may include scanning the sample stage with the objective of centering the at least one height sensor over at least one of the at least two markings.

[0032] Scanning the sample stage may include a scan path adapted to one of the at least two markings.

[0033] Scanning the sample stage can include displacing the sample stage without rotating the sample stage. Scanning the sample stage can include (a) performing a first scan of the sample stage in an oblique direction relative to the marking being scanned, (b) displacing the scan path a predefined distance perpendicular to the first scan path for a second scan, and (c) repeating step (b) until the repeated scans meet a reference point on the scanned marking.

[0034] Additionally, scanning the sample stage may include rotating the sample stage such that the scanned circle segment covers at least a portion of at least one of the at least two markings.

[0035] Additionally, scanning the sample stage may include performing the scan such that a scan time for centering the at least one height sensor achieves a minimum value.

[0036] Identifying the reference point of the marking is a complex process. In contrast to the particle beam of the particle beam source, the particle beam of the height sensor cannot be scanned over the photomask. Rather, a large part of the sample stage carrying the mask must be moved, including the photomask itself, to determine the position at which the reference point of the marking of the photomask is located under the point of incidence of the particle beam of the height sensor. It is therefore highly advantageous to select the scanning of the sample stage so as to minimize the length of the required scanning path and therefore the time required for scanning.

[0037] Measuring the height change can include displacing the axis of rotation below a point of incidence on the photomask of a particle beam of at least one particle beam source.

[0038] Measuring the height change may include displacing the coordinate of the axis of rotation below the point of incidence of the particle beam on the photomask.

[0039] By determining the coordinates of the rotation axis on the photomask, as explained above, it is possible to ascertain the distance between the height sensor and the rotation axis, and therefore the radius of the circle or circle segment that the incidence point of the particle beam of the height sensor moves around the rotation axis during measurement of height changes.

[0040] The method according to the invention may further comprise the step of interpolating and / or extrapolating the measured height change of the photomask to values ​​deviating from the predetermined non-annihilation distance. The step of interpolating and / or extrapolating the measured height change may comprise the step of fitting the measured height change to a mathematical model. The step of fitting the measured height change to a mathematical model may comprise fitting to a functional curve.

[0041] The method may further include interpolating and / or extrapolating the measured height change of the photomask for distances that do not correspond to a distance between a point of incidence of the at least one particle beam of the particle beam source and a point of incidence of the particle beam of the at least one height sensor for purposes of determining a height change at any location on the photomask. The interpolation and / or extrapolation may include linear interpolation and / or linear extrapolation.

[0042] Measuring the height change may further include determining an orientation of the rotation axis of the sample stage relative to a normal to the mask and / or determining a maximum height change during rotation of the sample stage.

[0043] The normal to the chuck surface of the sample stage corresponds to the z-axis of the coordinate system of the sample stage. A deviation in the orientation of the rotation axis with respect to the normal to the chuck surface, or a tilt of the rotation axis with respect to the z-axis, does not in itself result in a height change measured by the at least one height sensor during rotation of the photomask as a result of the rotation of the sample stage. Rather, a height change of the photomask as the rotation of the sample stage occurs is caused only if the rotation axis of the sample stage is not aligned perpendicular to the surface of the photomask, i.e., the rotation axis has a tilt with respect to the normal to the mask.

[0044] Additionally, measuring the height change may include setting at least one height sensor at a center position of its measurement area prior to rotating the sample stage, thereby ensuring that the height sensor detects the entire height change during rotation of the sample stage.

[0045] The method according to the invention may further comprise the step of measuring the z-coordinate of the photomask on the axis of rotation.Furthermore, the method according to the invention may comprise the step of measuring the z-coordinate of the photomask in the coordinates of the axis of rotation.

[0046] The sample stage may be displaceable along at least two axes parallel to the chuck surface, and the method according to the invention may further comprise the steps of (a) measuring coordinates of the at least two markings by a particle beam of at least one particle beam source without rotating the photomask together with the sample stage, and (b) determining an affine transformation between a coordinate system of the photomask and a coordinate system of the sample stage. The coordinate system associated with the sample stage may be a coordinate system related to the chuck surface.

[0047] The method for determining the alignment of the photomask can be divided into at least two steps. The order of the two steps can be selected as desired. In the following, it is assumed that the two steps do not influence each other. In one step, the mask coordinates u, v assigned to the mask design, typically made available by the mask manufacturer, thus forming a coordinate system assigned to the photomask, are placed in a one-to-one relationship with a coordinate system (x, y) associated with the sample stage. This is preferably a coordinate system related to the chuck surface of the sample stage. A further step comprises, as explained above, checking the influence of a rotation axis that is not oriented perpendicular to the surface of the mask. For this purpose, the photomask is rotated by the sample stage by a predefined angle and the height changes occurring during the process are measured. From the measurement data, the height changes of the mask during the mask rotation and / or mask displacement can be checked.

[0048] After carrying out the method according to the invention, the coordinates u of the defect, typically specified in mask coordinates, D , v D is expressed as a function of the rotation angle θ in the coordinate system x D , y D , z D In particular, the height position of the defect on the mask or its repair position can be ascertained. The sample stage can bring the photomask positioned thereon to the supposed repair position of the defect. Furthermore, a repair tool can be positioned with high precision to treat the defect.

[0049] The method according to the invention may further comprise the step of determining an affine transformation between the coordinate system of the photomask and the coordinate system of the sample stage from a first set of coordinates of the at least two markings of the photomask.

[0050] For reasons of the economy of the method, it is expedient to reuse the first set of measured coordinates of the two markings measured to determine the coordinates of the rotation axis for determining the affine transformation. However, it is also possible to select two new markings of the photomask and measure these with the particle beam of at least one particle beam source.

[0051] Determining the affine transformation may include determining translation, scaling and rotation parameters of the coordinate system of the photomask and the coordinate system of the sample stage relative to each other based on the measured coordinates of the at least two markings.

[0052] Based on the two measured reference points x1, y1 and x2, y2 of the two markings and the two reference points u1, v1 and u2, v2 predefined in the mask coordinate system by the mask manufacturer, four parameters can be determined, namely two parameters of displacement (a, b), one parameter of rotation (α) and one parameter of scaling (s).

[0053] Determining the affine transformation may include measuring the coordinates of at least three markings of the photomask and determining the translation, scaling, rotation, shear, and parallel stretching parameters of the photomask coordinate system and the sample stage coordinate system. Any combination of the listed affine transformations can be described using a 2×3 matrix. Determining the six matrix coefficients requires six equations that can be verified by measurements of at least three markings.

[0054] Using the six equations, six parameters can be determined: these are the shear and parallel stretching parameters in addition to the four parameters already determined above.

[0055] The at least one height sensor and / or the at least one particle beam source may be in a known relationship to a coordinate system of the sample stage.

[0056] The sample stage may be displaceable along at least two axes parallel to the chuck surface of the sample stage, and the method according to the present invention may further include a step of measuring the height differences of at least three points on the photomask that are not on a straight line by at least one height sensor without rotating the sample stage.

[0057] The method according to the invention may further comprise the step of interpolating and / or extrapolating height differences at positions of the photomask deviating from the three measurement points from the measured height differences. The interpolation and / or extrapolation may comprise linear interpolation and / or linear extrapolation.

[0058] By means of an interpolation and / or extrapolation step, the height of each point on the photomask due to the oblique or tilted position of the mask can be calculated from the measurement data of the three measurement points.

[0059] The method for determining the alignment of the photomask can thus be divided into three parts. The order of performing the individual parts of the method can be selected as desired. However, it is expedient to perform the last-mentioned part first. Based on the measured height difference, it is possible, as already mentioned above, to ascertain the tilted or oblique position of the photomask on the sample stage. This position can be taken into account when performing measurements for the further parts of the method. It is thus possible to reliably prevent damage to said mask when performing translational movements of the mask, which are performed to bring the mask into the position required for the measurements.

[0060] After carrying out the method according to the present invention, the coordinates of the defect, typically specified in mask coordinates (u D , v D ) in the coordinate system of the sample stage (x D , y D , zD ) can be converted into a height image. In particular, the height position of the defect on the mask or at its repair position can be ascertained. The sample stage can bring the photomask positioned thereon to the expected repair position of the defect. Furthermore, a repair tool can be positioned with high precision to treat the defect.

[0061] The z-coordinate or height of a point on the front side of the mask, i.e. the side carrying the pattern, is composed of three contributions: (a) the height of the photomask at the position of the rotation axis, (b) the height change of the point under consideration with respect to the rotation axis due to the tilted position of the mask, and (c) the height change of the point under consideration with respect to the rotation due to the rotation axis not pointing perpendicular to the surface of the mask. The actual z-coordinate of the point under consideration is obtained by adding these three contributions. The three contributions are additive and depend on the rotation angle θ.

[0062] In a first embodiment, an apparatus for determining an alignment of a photomask on a sample stage that is displaceable along at least one axis parallel to a chuck surface of the sample stage and rotatable about at least one axis perpendicular to the chuck surface comprises at least one height sensor configured to measure height changes while rotating the sample stage through a predefined angle, the at least one height sensor being at a predetermined non-vanishing distance relative to the axis of rotation for the purpose of determining the alignment of the photomask on the sample stage.

[0063] The height sensor may include a chromatic confocal sensor. The height sensor may further include at least one element from the group of an optical microscope, a capacitive distance sensor, an interferometer, and a scanning probe microscope. The scanning probe microscope may include an atomic force microscope (AFM), a scanning tunneling microscope (STM), a magnetic force microscope, a scanning near-field optical microscope, and a scanning near-field acoustic microscope. Additionally, the height sensor may include a scanning electron microscope (SEM).

[0064] The apparatus may be configured to carry out the steps of the method according to the invention. The apparatus may be configured to carry out the method according to the invention in an automated manner. In particular, the apparatus may be configured to automatically carry out different aspects of the method according to the invention, alone or in combination with other aspects.

[0065] Finally, the computer program may contain instructions which, when executed by a computer system, cause the computer system to perform the method steps of the methods described above.

[0066] The following detailed description describes presently preferred exemplary embodiments of the invention with reference to the drawings. [Brief description of the drawings]

[0067] [Figure 1] FIG. 2 is a schematic diagram of an exemplary sample stage having three translational axes and one rotational axis. [Diagram 2] FIG. 2 is a schematic cross-sectional view of the exemplary sample stage of FIG. 1. [Diagram 3] FIG. 1 illustrates various coordinates within an exemplary sample stage coordinate system and a coordinate system associated with a photomask. [Figure 4] FIG. 2 illustrates the oblique position of the photomask on the sample stage of FIG. 1 and shows diagrammatically a particle beam source for scanning the markings of the photomask and a height sensor for measuring the z coordinate of the photomask. [Diagram 5] 11A-11C are diagrams illustrating schematic diagrams of z-coordinate measurements by a height sensor for various mask patterns. [Figure 6] FIG. 2 is a plan view showing one type of marking with its associated reference point. [Figure 7] FIG. 2 is a plan view of an exemplary photomask having three markings. [Figure 8] FIG. 7 shows a first example of a scan of a height sensor within the marking of FIG. 6. [Figure 9a]FIG. 9 shows a first part of a flow diagram of scanning a height sensor according to the first example of FIG. 8; [Figure 9b] FIG. 9 is a diagram reproducing the second part of the flow diagram of the scanning of the height sensor according to the first example of FIG. 8; [Figure 10] FIG. 7 is a diagram illustrating a second example of scanning of a height sensor within the marking of FIG. 6. [Figure 11] FIG. 8 is a plan view of the photomask in FIG. 7 rotated by 90 degrees. [Figure 12] FIG. 8 is a plan view of the photomask of FIG. 7 after being rotated by an arbitrary rotation angle. [Figure 13] FIG. 2 is a schematic diagram illustrating translation caused by rotation. [Figure 14] FIG. 13 illustrates an oblique position of the photomask on the sample stage, depicting the rotation axis not oriented perpendicular to the mask, thus resulting in a rocking motion of the photomask during rotation. [Figure 15] The upper partial image is a plan view of the mask of FIG. 7, where the height change and height profile during rotation of the mask are measured at a distance d0 from the rotation axis, and the lower partial image shows the deviation of the orientation of the rotation axis from the perpendicular to the photomask. [Figure 16] FIG. 13 shows an example of a height profile as a function of rotation angle for a rotation axis that is not aligned perpendicular to the mask. [Figure 17] FIG. 2 is a flow diagram of a method according to the present invention for determining the alignment of a photomask on a sample stage. [Figure 18a] FIG. 18 shows a first part of the flow diagram of FIG. 17, with details specified in more detail. [Figure 18b] FIG. 18 shows a second part of the flow diagram of FIG. 17, with details specified in more detail. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0068] A currently preferred embodiment of the method according to the invention and the device according to the invention is described below. The method according to the invention is described in detail on the basis of determining the alignment of a photomask on a sample stage which, besides three translation axes, also has one rotation axis. To apply the method described herein, it is not necessary for the sample stage to have three translation axes; one displacement axis parallel to the sample receiving surface and one rotation axis aligned perpendicular to the sample receiving surface are sufficient for this. Furthermore, the method according to the invention and the device according to the invention are not limited to the alignment of a photomask on a sample stage. Rather, they can be used not only for aligning templates for nanoimprint lithography, but also for aligning any components in microsystem technology, provided that said components have markings.

[0069] FIG. 1 shows a schematic diagram of an exemplary sample stage 100 with three translational axes and one rotational axis. The translational axes are substantially perpendicular to each other and thus form a Cartesian coordinate system. In the following, the sample stage 100 is also referred to as mask stage 100 or stage 100. The base plate 110 of the sample stage 100 has a rail 120 for displacing the chuck surface of the sample stage 100 in the y direction. The slide 130 of the sample stage 100 can move along the y axis on the rail 120. The slide 130 carries on its upper surface a rail 140 that can displace a second slide 150 of the sample stage 100 along the x direction. The second slide 150 forms a base plate 160 for a displacement unit 170 in the z direction. A rotational axis 180 is arranged on the displacement unit 170 of the sample stage 100 for the z direction, said rotational axis being oriented or aligned parallel to the z direction. The rotating shaft 180 carries a chuck 190, a mask holder 190, or a mask plate 190. In the following, the chuck surface 195 of the chuck 190 refers to the sum of the points at which a photomask without a pattern on its underside is placed on the chuck 190.

[0070] Here, and elsewhere in this specification, the term "substantially" refers to a measured variable within the limits of error of a prior art measurement device when used to measure the variable.

[0071] The diagram 200 in Fig. 2 shows a schematic cross-section of the sample stage 100 of Fig. 1. On the left side, the coordinate system of the sample stage 100 is further shown. The coordinate system x b , y b , z b relates to the base plate 205 of the sample stage 100 of FIG.

[0072] The base plate 205 carries a rail 210 for displacing a slide 215 in the y direction oriented perpendicular to the plane of the drawing. The slide 215 has on its upper surface a rail 220 for displacing a second slide 225 in the x direction. The second slide 225 forms a base plate for a displacement unit 235 in the z direction. The displacement unit 235 is fixed to the second slide 225 by means of a fixing pin 230. By displacing the wedge 237, the upper plate 245 can be displaced in the z direction.

[0073] A rotation shaft 250 is fixed to an upper plate 245 mounted on the displacement unit 235 by using a pin 255. The coordinate system associated with the upper surface of the upper plate 245 is hereinafter expressed as x w , y w , z w 1 is oriented in the z direction. As already explained above, the rotation axis 250 carries a chuck 260 or chuck surface 265. The chuck 260 may, however, include a three-point support. The chuck 260 may also fix the photomask 270 on the chuck 260 using electrostatic forces (electrostatic chuck) or by generating a vacuum (vacuum chuck). The coordinates associated with the top surface of the chuck 260 are denoted in the following as x ch , y ch , z ch It is called.

[0074] The chuck 260 or mask plate 260 carries a photomask 270. The mask 270 can be a transmissive or reflective mask. The photomask 270 typically includes at least a substrate 275 and a pattern 280. The substrate 275 can include a quartz substrate and / or a material with a low thermal expansion coefficient (LTE (Low Thermal Expansion) substrate). The pattern 280 can be a pattern of a binary photomask 270. In this case, the pattern 280 can include an absorber structure and can include, for example, chrome. However, the pattern 280 can also include a pattern 280 of a phase-shift photomask 270. The phase-shift mask 270 can be manufactured, for example, by etching a corresponding pattern 280 into the substrate 275 of the mask 270. Furthermore, the pattern 280 can include structural elements that shift the phase of the actinic radiation relative to the radiation incident on the substrate 275 and absorb a portion of the light of the actinic radiation wavelength incident on the pattern 280. An example of this is the OMOG (opaque MoSi (molybdenum silicide) on glass) mask.

[0075] The reflective mask 270 includes a photomask for the extreme ultraviolet (EUV) wavelength range, which typically has actinic wavelengths in the range of 10 nm to 15 nm. The EUV mask can be embodied as a binary mask and / or a phase shift mask.

[0076] The right-pointing arrow u represents the u coordinate in the mask coordinate system of FIG. ch and y ch can be measured by the particle beam 285 of the particle beam source 290 and is expressed in the coordinate system x b , y b , z b The particle beam source 290 is in a known relationship to the base plate 110, 205 of the sample stage 100. This relationship is represented in FIG.

[0077] Diagram 300 of Figure 3 again diagrammatically illustrates the coordinate systems of various intermediate stages within sample stage 100 up to the coordinate system of photomask 270. As indicated in block 310, the coordinate system associated with base plate 205 of sample stage 100 is x b , y b , z b 3, where the letter b stands for "base." The double arrow 320 in FIG. 3 represents the transition from the base plate 205 of the sample stage 100 to the upper surface of the top plate 245 on which the rotation axis 250 is mounted. As shown in block 330, the associated coordinate system is x w , y w , z w The subscript w stands for "wedge". w , y w , z w can be translated in all three spatial directions.

[0078] The double arrow 340 describes the transition from the coordinate system associated with the top plate 245 to the coordinate system of the chuck 260, which has, relative to the coordinate system of the base plate, three translation coordinates as well as a rotation angle θ that describes the rotation of the sample stage 100, and therefore the photomask 270, about its axis of rotation 250: x ch , y ch , z ch , θ. The subscript "ch" stands for "chuck." This coordinate system is shown in block 350 of FIG. 3. The coordinate system associated with the chuck 260 can have a rotation θ about the axis of rotation 250 relative to a coordinate system coupled to the top plate 245.

[0079] Finally, the double arrow 360 represents the transition from the coordinate system of the specimen receiver or chuck 190, 260 to the coordinate system associated with the photomask 270: u, v, w, θ. As explained above, the association of the various coordinate systems within the specimen stage 100 is known. This means that the position of the particle beam 285 of the particle beam source 290 has a known relationship to the coordinate system of the base plate 110, 205 of the specimen stage 100.

[0080] The reference point of the focused electron beam 285 of the SEM 290 can be chosen such that u=0, v=0, w=0, θ=0. After corresponding adjustments, the electron beam 285 of the SEM 290 is typically circular and therefore rotationally invariant. If no rotation occurs around the rotation axis 250, the coordinate system of the mask 270 and mask holder 260 is u, v, w and x ch , y ch , z ch Only the abscissas u, v of the mask coordinate system u, v, w are important for describing the structural elements of the pattern, as well as possible defects. Therefore, mask manufacturers typically specify only the u and v coordinates of the marking for the reference points of the marking.

[0081] Diagram 400 of FIG. 4 illustrates the determination of a tilted or oblique position of the photomask 410 on the chuck 190, 260. The tilted position of the mask 410 may be caused by the chuck 190, 260 being tilted. It is also possible that the chuck 190, 260 and / or the lower surface 420 of the photomask 310 have one or more particles that result in a tilted position of the mask 410 (not shown in FIG. 4). It goes without saying that a tilted position of the mask 410 may also be caused by a combination of the chuck 190, 260 being tilted and one or more particles between the chuck surface 195, 265 and the lower surface of the photomask 410.

[0082] FIG. 4 represents an apparatus 400 including a modified Scanning Electron Microscope (SEM) 440 as an example of a particle beam source 440. In the following, the particle beam source 440 is always realized as a modified SEM. From the output of a column 445 of the SEM 440, an electron beam 450 can be scanned over the top surface 430 or the front surface 430 of the photomask 410. As already explained above, the electron beam 450 can be focused to a very small spot so that its lateral spread is very small during the scanning of the mask 410. For this reason, the focused electron beam 450 can induce laterally limited chemical etching reactions (EBIE, electron beam induced etching) or locally limited chemical deposition reactions (EBID, electron beam induced deposition) in the presence of suitable precursor gases (not shown in FIG. 4).

[0083] As an alternative or in addition to the electron beam 450, it is also possible to use an ion beam, for example a beam of gallium ions, to scan the photomask 270, 410 or to scan the markings of the mask 270, 410. Similarly, atomic and / or molecular beams, for example a helium beam, can be used for this purpose. Finally, a photon beam can also be used to scan the photomask 270, 410 or its markings. When using photons, it is advantageous to use any particle beam source with the shortest possible wavelength, for example a particle beam source for the EUV wavelength range. The shorter the wavelength of the photon beam used, the better the resolution of the light beam source. In an alternative exemplary embodiment, a digital camera can be used to image the markings of the mask 270, 410.

[0084] However, the beam direction resolution of the focused electron beam 450 is limited due to the beam direction profile of said beam. For this reason, the apparatus 400 includes a height sensor 460 that can be used to detect the height or z coordinate of the photomask 410, 460. The height sensor 460 directs a particle beam in the form of a light beam 470 to the upper or front surface 430 of the photomask 410 and determines the distance between the height sensor 460 and the upper surface 430 of the photomask 410 based on the radiation reflected from the front surface 430, said radiation not shown in FIG. 4. The height sensor 460 can therefore be used to bring the photomask 410, 460 into the working distance of the electron beam 450.

[0085] The height sensor 460 can be embodied in the form of an interferometer, for example as a Fabry-Perot interferometer or as a white light interferometer. However, the height sensor 460 can also be realized as a confocal point sensor, as a chromatic confocal sensor or as a confocal laser scanning microscope. The height sensor 460 typically has a resolution in the submicron range. To increase the resolution range of the height sensor 460, the height sensor 460 can include two or more sensors with different measurement ranges and resolutions. It is thus possible to measure the distance between the upper surface 430 of the mask 410 and the height sensor 460 with a coarse resolution in a first step and then with a progressively higher resolution in one or more subsequent steps.

[0086] The height sensor 460 checks the distance between the height sensor 460 and the upper surface 430 of the photomask 410 for at least three points on the surface of the mask that are not on a straight line. For this purpose, the sample stage 100 displaces the photomask 410 along the x and y directions. In order to minimize the measurement errors when determining the height difference from the three measurement points, it is advantageous to select the measurement points such that they are at the largest possible distance from each other. The large distance between the at least three measurement points of the height sensor 460 can be, for example, D>5 cm. Furthermore, in order to minimize the measurement errors when measuring the height difference, it is advantageous if the three measurement points straddle a Cartesian coordinate system.

[0087] Additionally, the apparatus 400 includes an exemplary sample stage 100, which is not shown in FIG. 4. Additionally, a coordinate system x b , y b , z b 4. and the coordinate system u, v associated with the photomask 410 are shown in FIG. w represents the height change of the photomask 410 due to the tilted or skewed position of the photomask 410.

[0088] The upper partial image 500 of FIG. 5 illustrates measurement data for ascertaining the tilt position of the mask from the height difference of at least three measurement points, the mask 410 having only a sparse pattern 520. The data measured by the height sensor 460 represents the overlay of the tilt position of the mask 410 with the geometric shape of the pattern. This relationship is shown in the diagram 530 placed under the mask 410. The diagram 540 presents the variation of the light intensity detected by the height sensor 460 when it is scanned over a pattern element 520, the substrate 510 and then a second pattern element 520. The height or thickness of the pattern element 520 is, for example, in the range of 70 nm to 100 nm for a binary mask. This means that in the example shown in FIG. 5, the resolution of the height sensor 460 is about 10 nm.

[0089] The lower partial image 550 of Figure 5 shows the scanning of the height sensor 460 over the mask 410 having a dense pattern 570. Diagrams 580 and 590 represent the measured height profile and the intensity profile detected by the height sensor 460 while it is scanning over the dense pattern 570. The measured height over the dense pattern 570 is a combination of the height of the substrate 510 and the height or distance of the top surface of the pattern 570 from the height sensor 460.

[0090] In order to avoid degradation of the resolution when measuring the height difference of at least three points distributed on the mask 410, the measurement points lying within the dense pattern 570 should be ignored as much as possible when determining the height difference of the at least three measurement points. In this case, the measured intensity profile 540, 590 of the height sensor 460 can be used to determine whether the measurement point is within the dense pattern 570 or not. If it is determined that the measurement point of the height sensor 460 is in an area of ​​the dense pattern 570, this measurement point should be discarded and replaced by a measurement point that ends up being located either on the large area pattern element 520 or on the substrate 510 of the photomask 410.

[0091] From the height difference of at least three measurement points measured by the height sensor, it is possible to determine the plane in which the top surface 430 of the photomask 410 is embedded, which spans the at least three measurement points. As a result, the tilt position of the mask 410 can be determined. The variation in height of the top surface 430 of the photomask relative to any point of the mask can be determined by identifying this point on the surface of the mask. Of course, it is also possible to determine the height or z-coordinate of any point on the surface of the mask by interpolating and / or extrapolating the height difference of at least three points measured by the height sensor.

[0092] As soon as the tilt position of the photomask 410 is known, this tilt position can be taken into account when translational movements are performed by the sample stage 100. As a result, it is possible to reliably prevent the translational movements of the sample stage 100 from causing unintended damage to the tilted photomask 410 and / or the SEM 440. Besides the SEM 440, the device for identifying and repairing the photomask 410 can include one or more scanning probe microscopes, not shown in FIG. 4. Said scanning probe microscopes are often mounted at a certain distance from the SEM 440, close to the top surface 430 of the photomask 410. If the mask is mounted at an angle, translational movements over a large distance can easily lead to the probe and / or micromanipulator of the scanning probe microscope coming into unintended contact with the photomask 410. After the tilt position of the photomask 410 has been identified, this problem can be avoided.

[0093] FIG. 6 shows an example of a marking 600 or fiducial mark 600. The marking 600 is applied by the mask manufacturer on the photomask 270, 410, typically at regular or irregular intervals. The exemplary marking 600 has the shape of a square with a side length 640 of 350 μm. The marking 600 is divided into four quadrants by two crosspieces 610 and 620. In the upper right quadrant 630, the marking 600 has a small square 660. The upper right corner of the square 660 defines the reference point 650 of the marking 600.

[0094] The white or clear portion 630 of the marking 600 may be the substrate 275, 510 of the photomask 270, 410. The cross bars 610, 620 and the small black squares 660 may include, for example, an absorber material of the binary photomask 270, such as, for example, chrome.

[0095] The marking 600 can be scanned by the electron beam 450 of the SEM 440, and the reference points 650 of the fiducial marks 600 can be ascertained from the scanned data.

[0096] FIG. 7 shows a schematic plan view of a photomask 700. The exemplary photomask 700 has three markings 710, 730, 750 in the form of right angles at three corners. Reference points 720, 740, 760 of the exemplary markings 710, 730, 750 form the intersection points inside the right angles. On the mask 700, a coordinate system u, v associated with the photomask 700 is shown. Reference points u1, v1, u2, v2 and u3, v3 in the mask coordinate system u, v are provided by the mask manufacturer to the user of the mask 700. The markings 710, 730, and 750 often have the form of the markings 600 of FIG. 6.

[0097] At least two of the three markings 710, 730, 750 are scanned by the electron beam 450 of the SEM 440 to determine the reference points 720, 740, 760 of at least two of the three markings 710, 730, 750. To position the markings 710, 730, 750 under the electron beam 450 of the SEM 440, the sample stage 100 performs exclusively translational movements of the chucks 190, 260 in the x- and y-directions. No rotational movements of the sample stage 100 are performed to determine the reference points 720, 740, 760. As soon as one of the markings 710, 730, 750 is positioned under the electron beam 450, the electron beam 450 scans the markings 710, 730, 750 to confirm their reference points 720, 740, 760.

[0098] After scanning two markings, e.g. markings 710, 730, the reference points 720, 740 of these markings in the coordinate system x, y of the sample stage 110 are known to be x1, y1 and y1, y2. Using the two measured reference points x1, y1 and x2, y2 and the reference points u1, v1 and u2, v2 provided by the mask manufacturer, the displacement, rotation and scaling of the two coordinate systems relative to each other can be determined. The affine transformation links the measured reference points x1, y1 and x2, y2 to the reference points u1, v1 and u2, v2 provided by the mask manufacturer. If only two markings are used, a general affine transformation is underdetermined uniquely. Such an embodiment limits the affine transformation to displacement (offset), scaling and rotation.

[0099] The established description of affine coordinates is given in matrix form using homogeneous coordinates.

number

[0100] When aligning two coordinate systems relative to each other based on two reference points, for example reference points 720 and 740, the general vector equation above is modified as follows:

number

[0101] In this case the parameters are a, b representing the displacement or offset, s the scaling, and α the rotation angle of the two coordinate systems relative to each other.

[0102] By inserting the measured reference point 720 and the predefined reference point 740, a system of equations is obtained that can be solved using methods of linear algebra. If the reference points 720, 740, 760 of the three markings 710, 730, 750 are measured for the photomask 700 and inserted into a general vector equation, the parameters of shear m and rotational stretch can be further determined in addition to the displacements a, b, scaling s, and rotation α.

[0103] Identified defects D , v D The coordinates of are typically given in u,v coordinates in a coordinate system linked to the photomask 270, 410, 700. After identifying the parameters of the affine transformation, these coordinates can be transformed into the coordinates of the sample stage 100. As already explained above, the SEM 440 operates in the coordinates of the sample stage 100.

[0104] It will now be described how the height change of the photomask 270, 410, 700 as a result of the rocking motion of the mask 270, 410, 700 caused by a rotation axis that is not oriented perpendicular to the mask 410 or the upper surface 430 of the mask 410 is determined. The first described step of determining the distance 490 between the SEM 440 and the height sensor 460, or the point of incidence on the photomask 270, 410, 700 of the particle beam 450 of the SEM 440 and the particle beam 470 of the height sensor 460, is only necessary if this distance is not known. This is the case, for example, after a service operation has been performed on the apparatus 400 that has changed this distance.

[0105] The distance 490 is ascertained in two sub-steps. In a first sub-step, the height sensor 460 is positioned below the reference points 720, 740, 760 of the markings 710, 730, 750 already measured by the electron beam 440 of the SEM 450. If only the two markings 710 and 730 have been measured by the electron beam 440 of the SEM 450, one of the markings 710 or 730 is selected for this purpose.

[0106] This process can be very time consuming if the sample stage 100 scans the markings in a similar manner to the electron beam 450 of the SEM 440. In contrast to the electron beam 450 of the SEM 440, the particle beam 470 of the height sensor 460 is stationary. Therefore, in order to position the particle beam 470 of the height sensor 460 under a reference point 720, 740, 760 of one of the markings 710, 730, 750, as opposed to the case during the scanning of the electron beam 450, the photomask 270, 410, 700 must be moved under the particle beam 470 of the height sensor 460. This requires moving the mass of the photomask 270, 410, 700 and most of the mass of the sample stage 100. For this reason, centering the particle beam 470 of the height sensor 460 under one of the reference points is a time consuming process. Therefore, it is not really effective to simply scan the photomask 270, 410, 700 by performing a translational movement of the sample stage 100. Rather, it is very advantageous to select a scanning path of the sample stage such that a reference point 720, 740, 760 of one of the markings 710, 730, 750 is located under the particle beam 470 of the height sensor 460 after the shortest possible scanning time, i.e. after as few translational movements as possible of the sample stage 100. The scanning paths for which this is applicable depend on the type of marking 500, 710 present on the photomask 270, 410, 700.

[0107] FIG. 8 shows, for the exemplary marking 600 of FIG. 6, a schematic of a convenient scan path that allows the reference point 650 of the marking 600 to be positioned or centered under the particle beam 470 of the height sensor 460 after a short scan time. The flow diagram 900 of FIGS. 9a and 9b shows various steps for centering the particle beam 470 of the height sensor 460 under the reference point 650. The method starts at 900. Step 905 involves scanning the sample stage 100 in the marking 600 in an oblique direction relative to the cross bars 610 and 620. This first scan is represented by a straight line 810 in the left partial image of FIG. 8. The length of the first scan 810 is selected so that the particle beam 470 of the height sensor 460 "sees" or images the cross bars 610 and 620 of the marking 600. The intensity signal detected by the height sensor 460 during the first scan 810 is shown in the top right partial image of FIG. 8. At the locations of the cross bars 610 and 620, the absorber material has the effect that the intensity signal reflected from the marking is reduced or eliminated.

[0108] After the first scan 810, in step 915, the signal detected by the height sensor 460 is analyzed and the distance d1 between the two cross bars 610 and 620 is determined. Then, in step 920, the sample stage 100 is displaced a predefined distance perpendicular to the diagonal scan direction. The next step 925 includes performing a second scan 820, as shown in FIG. 8. Then, in step 930, for the second scan 820, the distance d2 between the two cross bars 610 and 620 of the marking 600 is ascertained. This is illustrated diagrammatically in the center-left partial image of FIG. 8. The wider area where the signal disappears indicates that the second scan 820 has partially passed through the square 660 of the marking 600.

[0109] Decision block 935 then involves determining whether distance d1 is greater than d2. If this is the case, then the displacement of the sample stage 100 between the first scan 810 and the second scan 820 has been made in the correct direction, i.e., towards the reference point 650 of the marking 600. As shown in block 945, this is towards the bottom left. Step 950 then involves identifying the offset or displacement of the sample stage 100 required to cause the third scan 830 to pass diagonally through the square 660 and thus the reference point 650 of the marking 600. In step 955, the sample stage 100 is displaced vertically relative to the second scan 820 by the identified offset.

[0110] In step 960, a third scan 830 is performed using the sample stage 100. The third scan 830 is performed along the diagonal of the square 660 as shown in FIG. 8. Then, in step 965, the third scan 830 is analyzed and a reference point 650 of the marking 600 is determined from the third scan 830. This is shown diagrammatically in the lower left partial image of FIG. 8. The desired reference point 650 of the marking 600 is the point in the upper right corner of the square 660 of the marking 600 and at the right edge of the area where the detection signal of the third scan 830 disappears. Finally, the method ends in step 970.

[0111] If in decision block 935 it is determined that the distance d2 is greater than d1, then the direction of the displacement of the sample stage 100 for the next scan is reversed relative to the displacement made between the first scan 810 and the second scan 820. This is performed in block 940 of Figure 9. Step 950 involves ascertaining the offset required to make the third scan 830 pass through the diagonal of the square 660 and thus the reference point 650 of the marking 600. Further steps are then performed as already described in the previous section.

[0112] 10 shows a schematic diagram of an alternative method for centering the reference points 720, 740, 760 of the markings 710, 730, 750 of the photomask 270, 410, 700 under the particle beam 470 of the height sensor 460. Instead of the translational motion of the sample stage 100 described above, it is also possible to position the particle beam 470 of the height sensor 460 under its reference points 720, 740, 760 using a rotation of the sample stage 100 about a defined rotation axis or pivot point inside one of the markings 710, 730, 750.

[0113] Fig. 10 represents a first scan 1010 within the marking 600. The top right sub-image of Fig. 10 shows the intensity profile detected by the height sensor 460 during the rotation of the sample stage 100. The two dips in the detected light intensity of the first scan 1010 represent the scanning of the particle beam 470 of the height sensor 460 under the cross bars 610 and 620. The wider area where the reflected light intensity vanishes represents the particle beam 470 of the height sensor 460 scanning over a portion of the square 660 of the marking 600.

[0114] After displacing the pivot point of the rotation axis in the marking 600, a second scan 1020 is performed by rotating the rotation axis 250 of the sample stage 100. The result is shown in the center-left partial image of FIG. 10. It is again shown that the particle beam 470 of the height sensor 460 is scanning over the two cross bars 610 and 620 as well as the square 660 of the marking 600. From the two scans 1010 and 1020 and the displacement of the rotation axis in the marking 600 made between the two scans as well as the change in the radius of the circular rotation movement, the pivot point and the radius of the circular scan are identified that make the third scan 1030 pass through the reference point 650 of the marking 600. The bottom-left partial image represents the intensity profile detected by the height sensor 460 during the circular scan. A reference point 650 of the desired marking 600 can be determined from the scans 1010, 1020, and 1030 as the upper right corner 650 of the square 660 of the marking 600. Determining the reference point 650 of the marking 600 by the circular or crescent scans 1010, 1020, and 1030 is more complicated compared to the linear scans 810, 820, 830 described with reference to FIG.

[0115] After centering the particle beam 470 of the height sensor 460, the coordinates of the particle beam 470 in the coordinate system of the sample stage 100 are known. As explained above, the marking 710, 730 or 750 selected for centering the particle beam 470 of the height sensor 460 has already been scanned by the electron beam 450 of the SEM 440. The desired distance 490 between the particle beam 450 of the SEM 440 and the particle beam 470 of the height sensor 460 is given by the difference between the coordinates of the reference point 650 of the marking 710, 730 or 750 measured by the SEM 440 and the coordinates of the reference point 710, 730, 750 of the marking 710, 730 or 750 measured by the height sensor 460.

[0116] The three steps described below serve to determine the location of the rotation axis on the photomask 270, 410, 700. These calibration steps must be performed once for a new or modified apparatus 400. Additionally, these calibration steps must be performed if the coordinates of the electron beam 450 of the SEM 440 relative to the coordinate system of the sample stage 100 are lost.

[0117] In a first sub-step to ascertain the position of the rotation axis, the photomask 270, 410, 700 is rotated by 90° together with the sample stage 100. Figure 11 shows the photomask 700 of figure 7 rotated by 90° counterclockwise around the z-axis of the coordinate system of the sample stage 100. Rotating the photomask 270, 410, 700 by a rotation angle of 90° is advantageous, since the selection of this angle makes it possible to determine with minimal error the position of the rotation axis relative to the photomask 270, 410, 700. However, it is also possible to select a rotation angle in the range 0<θ<180°.

[0118] Since the possible rocking motions due to the rotation are not yet known at the time of rotating the photomask 700, it is advantageous to increase the distance between the output of the column 445 of the SEM 440 and the probe or micromanipulators of the scanning probe microscope, if present, before performing the rotation in order to prevent damage to the photomask 700, the SEM 440 or the scanning probe microscope and / or the gas injection system. This can be done, for example, by lowering the chuck 260 of the sample stage 100. A gas injection system, not shown in FIG. 4, can be used to supply, for example, one or more corresponding precursor gases at the location where the EBIE or EBID process is performed.

[0119] In a next step, the reference points 720, 740, 760 of at least two of the three markings 710, 730, 750 of the rotated photomask 700 are again measured by the electron beam 450 of the SEM 440. The performance of these measurements has been explained above in the context of the discussion of Fig. 6 and Fig. 7. As also already explained above, during the determination of the reference points 720, 740, 760 of at least two of the three markings 710, 730, 750, no rotation by the rotation axis 250 of the sample stage 100 is allowed to be performed. Furthermore, care should be taken to ensure that between the performance of the rotation movement by the sample stage 100 and the measurement by the electron beam 450 of the SEM 400 of the reference points 720, 740, 760 of the rotated photomask 700 of the sample stage 100, the device 400 does not perform any tasks that could change, firstly, the position of the sample stage 100 and, secondly, the position of the electron beam 450 of the SEM 440.

[0120] If the photomask 270, 410, 700 is placed on the chuck 190, 260 such that its centre point is located directly above the axis of rotation 250 of the sample stage 100, then rotation of the photomask 270, 410, 700 about the axis of rotation 250 of the sample stage 100 has the effect that the markings 710, 730, 750, as well as their reference points 720, 740, 760, circulate on a known circular path around the axis of rotation 250 of the sample stage 100. However, in general, a photomask 270, 410, 700 fixed by a mask holder will not satisfy this condition since in the general case the position of the axis of rotation 250, and therefore the centre point of the circular path of the reference points 720, 740, 760, is unknown.

[0121] As already explained above, a combined translational and rotational motion can be described by an affine transformation. Figure 12 illustrates, in a schematic manner, a photomask 700 rotated by an arbitrary angle θ about a rotation axis perpendicular to the surface of said photomask. The coordinates of the reference points 720, 740 of the markings 710, 730 before the rotation are x 1i , y 1i and x 2i , y2i where the subscript "i" represents the original coordinates of the reference points 720, 740 of the markings 710, 730. After rotation, the reference points 720, 740 have the coordinate x 1r , y 1r and x 2r , y 2r where the subscript "r" represents the coordinates of the reference points 720, 740 of the markings 710, 730 after rotation. As can be inferred from FIG. 12, the change in coordinates of the reference points 720, 740 can be expressed as: Δx i =x 2i -x 1i、 Δ yi =y 2i -y 1i , Δx r =x 2r -x 1r , and Δy r =y 2r -y 1r Furthermore, the following relationship: tan(θ i )=Δy i / Δx i , tan(θ r )=Δy r / Δx r , θ=θ r -θ i Therefore, the rotation angle θ can be determined from the reference points 720, 740 of the measured markings 710, 730.

[0122] Parameter t x and t y represents the translation or displacement of the markings 710 and 730 caused by the rotation. Thus, we obtain the following result for the affine transformation describing the rotation:

number

[0123] Rotating the photomask by the angle θ should not result in a scaling of the mask 700. Therefore, for the parameter s, we can assume to a good approximation that s=1. Thus, any deviation in determining s that is greater than the errors in checking the other parameters gives an indication of the error in determining the position of the rotation axis. The translation parameter t x and t y For , the above determinant gives the following result: t x =x r -cosθ x i +sinθ y i t y =y r -sinθ x i -cosθ y i

[0124] The goal is then to find the coordinate x of the rotation axis such that c , y c The key is to find out. x r =x i y r =y i

[0125] The coordinate of the rotation axis is x c , y c where "c" represents the center. The markings 710, 730, 750 and their respective reference points 720, 740, 760 move on a circular path around the axis of rotation 250. The coordinate x c , y c If the sample stage 110 is moved in the x and y directions such that the coordinates of the incident point of the electron beam 450 of the SEM 440 coincide with the coordinates of the incident point of the electron beam 450 of the SEM 440, the image recorded by the SEM 440 will rotate about its center point, i.e., the point of the SEM image will undergo a pure rotational motion.

[0126] Apart from this special case, i.e. when the axis of rotation 250 does not coincide with the axis of the electron beam 450, every point observed by the SEM 440 during the rotation of the mask 700 experiences a combined rotational and translational motion. For example, the coordinate x D , y D If a defect having x is within the area scanned by electron beam 450 of SEM 440, the defect will typically move out of the scan field or field of view of the SEM during rotation of photomask 700 about rotation axis 250. Compensating translations of specimen stage 110 in the x and y directions will cause defect x to move out of the scan field or field of view of the SEM again after the rotation of mask 700 is completed. D , y D can be displaced into the field of view or scan area of ​​the SEM 440. The combination of rotation of the sample stage 110 about the axis of rotation 250 and the compensating translational motion of the sample stage 110 causes the sample stage 110 to D , y D It will rotate around the center.

[0127] The coordinate system x of the chuck surface 265 of the chuck 190, 260 c , y c , z c The origin of is typically selected such that the electron beam 450 of the SEM 440 is located at the coordinate origin.

[0128] After some transformations, x i , x r and y i , y r Using the conditions specified above for the coordinate x of the central rotation axis c , y c Let t be the translation x and t y and as a function of the rotation angle θ.

number

[0129] In this case, the translation parameter t x and ty is determined from the formula specified above, and the measured coordinates of the reference points 720, 740, 760 of the markings 710, 730, 750 before and after rotation are inserted into said formula.

[0130] Parameter t x , t y , and θ are determined, the axis of rotation is coordinate x c , y c When the specimen stage 100 is displaced so as to have the following relationship, the scanning area of ​​the electron beam 450 of the SEM 440 does not displace even if the chucks 190, 260 are rotated.

[0131] FIG. 13 shows the rotation of the point x by performing a rotation Δθ in which the coordinates of the rotation axis do not coincide with the coordinates of the incident point of the electron beam 440 on the photomask 270, 410, 700. i , y i Therefore, as explained above, the rotation of Δθ gives the point x i , y i This translation is identified by the subscript "it". i , y i can be expressed as a function of the original coordinates and the rotation angle Δθ. x it =(x i -x c )·cosΔθ-(y i -y c )·sinΔθ+x c -x i y it =(x i -x c )·sinΔθ+(y i -y c )·cosΔθ+y c -y i

[0132] To counterbalance the translation caused by the rotation of the electron beam 450 of the SEM 440 relative to the scan area, the specimen stage 100 must be displaced in the opposite direction by the amount specified above.

[0133] Diagram 1400 of FIG. 14 illustrates various possible defects or imperfections of various components of the sample stage 100. As shown in FIG. 14, the second slide 225 of the sample stage 100, which moves the mask plate 190, 260 along the x-direction, can be tilted by an angle α with respect to the base plate 205 of the sample stage 100. Furthermore, the second slide 225 of the sample stage 100 can be rotated by an angle β with respect to the chuck 190, 260 or the mask plate 190, 260. These two imperfections of the sample stage 100 result in a tilted or oblique position of the photomask 270. It has already been explained in detail above how the oblique position of the photomask 270 can be ascertained and taken into account when performing the translation of the sample stage 100.

[0134] However, a tilted position of the photomask 270 does not result in a rocking motion of the mask 270 as long as the rotation axis 1450 of the sample stage 100 is aligned perpendicular to the mask 270, 410, 700 or its top surface 430. In contrast, an orientation of the rotation axis 1450 that deviates from perpendicular to the photomask 270, 410, 700 results in a height change during rotation of the mask 270, 410, 700 by the sample stage 100. The height change of the photomask 270, 410, 700 during rotation is referred to below as a rocking motion of the mask 270, 410, 700.

[0135] FIG. 14 shows diagrammatically that the rotation axis 1450 has an angle δ with respect to the z-axis of the base plate 205 of the sample stage 100. However, the angular deviation δ does not result in a rocking movement of the photomask 270, 410, 700 during rotation of the photomask, as long as the rotation axis 1450 is oriented perpendicular to the mask 270, 410, 700, i.e., γ=90°. In the exemplary diagram 1400, the angle γ, which is the angle between the surface 430 of the mask and the orientation of the rotation axis 1450, has a value that deviates from 90°. An angle γ≠90°, however, results in a height change detected by the particle beam 470 of the height sensor 460 during rotation of the mask 270, 410, 700 about the rotation axis 1450.

[0136] 15 shows a plan view of the mask 700 in the upper partial image. The sample stage 100 is arranged so that the coordinate 1510 of the rotation axis 1450 on the photomask 700 coincides with the incident point x c , y c 15. The point of incidence of particle beam 470 on height sensor 460 is at distance 490 or d0 from the point of incidence of electron beam 450 on SEM 440. What is sought is the height change, height profile, or rocking motion of point 1520 when the rotation axis is not oriented perpendicular to photomask 700 as shown in the lower partial image of FIG.

[0137] Now assume that height sensor 460 detects the maximum height change during rotation of photomask 700 about rotation axis 1450 for the rotation angle shown in FIG. max It is called θ max If θ = ±90°, then γ = 90° holds and the height change as a result of the swing motion disappears. max = ±180°, the change in height of the photomask 700 detected by the height sensor 460 reverses its sign.

[0138] From the lower partial image of FIG. 15, the height change Δh detected by the height sensor 460 for the maximum height changemax 1570 can be determined. Δh max = d0 tan(δ+γ-90°)

[0139] For the height profile h(θ, d0) measured by height sensor 460 as a function of rotation angle θ during rotation of photomask 700 about rotation axis 1450, we therefore obtain the following: h(θ, d0) = cos(θ-θ max )·d0·tan(δ+γ-90°)+h off or h(θ, d0) = Δh max cos(θ-θ max )+h off Here, the parameter θ max , Δh max and h off is initially unknown.

[0140] Graph 1600 in FIG. 16 shows an example of a measured height profile h(θ, d0) as a function of rotation angle θ in radians. The height change or height profile is expressed in micrometers. Points 1610 in FIG. 16 represent the measurement points of the height sensor 460 during the rotation process. The solid curve 1620 represents a function fitted to the measurement points 1610. For example, a numerical optimization algorithm such as the Levenberg-Marquardt algorithm can be used to fit the measurement points 1610. The parameter θ max , Δh max and h off is determined as a result of fitting the measured data 1610 to the above function.

[0141] The height change at any point on the photomask 700, for example at point 1520, can be ascertained by linear interpolation and / or linear extrapolation of the height profile h(θ, d0).

number

[0142] 17 again presents exemplary steps of a process for determining the alignment of a photomask 270, 410, 700 on a sample stage 100 that is displaceable along at least one axis parallel to the chuck surface 195, 265 of the sample stage 100 and rotatable about at least one axis 250, 1450 perpendicular to the chuck surface 195, 265. The method begins at step 1710.

[0143] The next step 1720 involves determining the tilt position of the photomask 270, 410, 700. For this purpose, the height sensor 460 measures the z-coordinates of at least three points on the mask, the three measured points not lying on a straight line.

[0144] Step 1730 comprises determining parameters of an affine transformation that connects the coordinate system of the sample stage 100 with the coordinate system of the photomask 270, 410, 700. For this purpose, the coordinates of at least two markings 710, 730, 750 of the photomask 270, 410, 700 are measured by the particle beam 450 of the particle beam source 440.

[0145] Both steps 1720 and 1730 are optional steps and therefore are represented in FIG. 17 by dashed boxes around them.

[0146] In step 1740, for purposes of determining the alignment of the photomask 270, 410, 700, the sample stage 100 is rotated a predefined angle and the height change of the photomask 270, 410, 700 is measured during rotation at a non-vanishing distance relative to the rotation axis 250, 1450.

[0147] The method ends in step 1750.

[0148] Finally, flow diagram 1800 of Figures 18a and 18b reproduces the method described in more detail in Figure 17. The method begins at step 1805.

[0149] In a next step 1810, at least one height sensor 460 measures the height differences of at least three non-linear points on the photomask 270, 410, 700. In step 1815, the tilt position of the photomask 270, 410, 700 is determined from the measured height differences.

[0150] Then, in step 1820 , a first set of coordinates of at least two markings 710 , 730 , 750 of the photomask 270 , 410 , 700 are determined by the particle beam 450 of the at least one particle beam source 440 .

[0151] In step 1825, parameters of an affine transformation connecting the coordinate system of the photomask 270, 410, 700 with the coordinate system of the sample stage 100 are determined based on the measured first set of coordinates and the coordinates of at least two markings 710, 730, 750 provided by the mask manufacturer.

[0152] Four steps 1810-1825 are optional steps and are therefore shown in FIG. 18 with dashed boxes around them.

[0153] In step 1830, the height sensor 460 is centered over one of the at least two markings 710, 730, 750 measured by the particle beam 450 of the at least one particle beam source 440. Then, in step 1835, a distance 490 between the point of incidence of the particle beam 450 of the at least one particle beam source 440 and the point of incidence of the particle beam 470 of the height sensor 460 is determined.

[0154] Steps 1830 and 1835 must be performed, for example, only if distance 490 is not known, which situation is indicated by the dotted boxes around the two optional steps.

[0155] In step 1840, the photomask 270, 410, 700 is rotated by an angle 0°<α<180°. Preferably, the rotation is performed by an angle of 90°.

[0156] Then, in step 1845, a second set of coordinates of the at least two markings 710, 730, 750 of the photomask 270, 410, 700 are measured by the particle beam 450 of the at least one particle beam source 440. Thereafter, in step 1850, the coordinates of the rotation axis 1450 on the photomask 270, 410, 700 are determined.

[0157] Steps 1840-1855 are optional steps and are therefore indicated by dotted boxes around them in Fig. 18. These steps can only be performed if, for example, the position of the rotation axis 1450 on the photomask 270, 410, 700 is unknown. This is the case, for example, when the coordinates of the particle beam 450 of at least one particle beam source 440 in the coordinate system of the sample stage 100 are lost.

[0158] In step 1855 , the axis of rotation 1450 is optionally displaced using the sample stage 100 such that its coordinates coincide with the coordinates of the point of incidence of the particle beam 450 of the at least one particle beam source 440 .

[0159] In the next step 1860, the sample stage 100 is rotated through a predefined angle and the height change h(θ, d0) of the photomask 270, 410, 700 is measured during the rotation.

[0160] The wobble motion of the photomask can then be determined from the measured height change h(θ,d0) in step 1865. The method finally ends at 1870.

Claims

1. 1. A method (1700, 1800) for determining an alignment of a photomask (270, 410, 700) on a sample stage (100) displaceable along at least one axis parallel to a chuck surface (195, 265) of the sample stage (100) and rotatable about at least one axis (250) perpendicular to the chuck surface (195, 265), comprising: rotating the sample stage (100) through a predefined angle and measuring the height change (1570) of the photomask (270, 410, 700) during rotation at a predefined non-vanishing distance (490) relative to the axis of rotation (1450) for the purpose of determining the alignment of the photomask (270, 410, 700) on the sample stage (100); A method (1700, 1800) comprising:

2. The method (1700, 1800) of claim 1, wherein the sample stage (100) is rotated through an angle of at least 60°, preferably at least 90°, more preferably at least 180°, and most preferably at least 360°.

3. The method (1700, 1800) of claim 1 or 2, comprising the additional step of predetermining the non-vanishing distance relative to the axis of rotation (1450).

4. The method (1700, 1800) of any one of claims 1 to 3, comprising the additional step of determining the coordinates of the axis of rotation (1450) on the surface (430) of the photomask (270, 410, 700).

5. The step of determining the coordinates of the axis of rotation (1450) comprises: a. measuring a first set of coordinates of at least two markings (710, 730, 750) of said photomask (270, 410, 700) by a particle beam (450) of a particle beam source (440) without rotating said photomask (270, 410, 700); b. rotating the photomask (270, 410, 700) by an angle of 0°<θ<180° or 180°<θ<360°; c. measuring a second set of coordinates of the at least two markings (710, 730, 750) of the rotated photomask (270, 410, 700) with the particle beam (450) of the particle beam source (440); The method of claim 4 , comprising:

6. 6. The method (1700, 1800) of claim 5, wherein determining the coordinates of the axis of rotation (1450) comprises determining the coordinates of the axis of rotation (1450) from the measured first and second sets of coordinates of the at least two markings (710, 730, 750).

7. 7. The method (1700, 1800) of claim 5 or 6, wherein the step of measuring the at least two markings (710, 730, 750) is performed by at least one particle beam source (440) configured to generate at least one focused particle beam (450) from the group of photon beams, electron beams, ion beams, atomic beams, and molecular beams.

8. The method (1700, 1800) according to any one of claims 3 to 7, wherein the step of predetermining the non-annihilation distance relative to the axis of rotation (1450) comprises a step of determining coordinates of an incidence point on the photomask (270, 410, 700) of a particle beam (470) of at least one height sensor (460).

9. The method (1700, 1800) of claim 8, wherein the step of determining the coordinates of the point of incidence of the particle beam (470) on the at least one height sensor (460) comprises a step of scanning the sample stage (100) for the purpose of centering the at least one height sensor (460) over at least one of the at least two markings (710, 730, 750) of claim 5.

10. 10. The method (1700, 1800) of claim 9, wherein the step of scanning the sample stage (100) comprises a scanning path (810, 820, 830, 1010, 1020, 1030) adapted to the at least two markings (710, 730, 750).

11. The method (1700, 1800) of any one of claims 1 to 10, wherein the step of measuring the height change (1570) comprises displacing the axis of rotation (1450) below a point of incidence of a particle beam (450) of a particle beam source (440) on the photomask (270, 410, 700).

12. The method (1700, 1800) of any one of claims 1 to 11, further comprising the step of interpolating and / or extrapolating the measured height change (1570) of the photomask (270, 410, 700) to values ​​that deviate from the predetermined non-annihilation distance (490).

13. The method (1700, 1800) of any one of claims 1 to 12, further comprising the step of measuring the z-coordinate of the photomask (270, 410, 700) on the axis of rotation (1450).

14. the sample stage (100) is displaceable along at least two axes parallel to the chuck surface (195, 265); a. measuring coordinates of at least two markings (710, 730, 750) of the photomask (270, 410, 700) by a particle beam (450) of a particle beam source (440) without rotating the photomask (270, 410, 700) together with the sample stage (100); b. determining an affine transformation between the coordinate system of the photomask (270, 410, 700) and the coordinate system of the sample stage (100) from the measured coordinates; The method (1700, 1800) of any one of claims 1 to 13, further comprising:

15. The method (1700, 1800) of any one of claims 5 to 14, further comprising the step of determining an affine transformation between a coordinate system of the photomask and a coordinate system of the sample stage from the first set of coordinates.

16. 16. The method (1700, 1800) of claim 14 or 15, wherein determining the affine transformation comprises determining translation, scaling and rotation parameters of the coordinate system of the photomask and the coordinate system of the sample stage relative to each other.

17. The method (1700, 1800) of any one of claims 1 to 16, wherein the sample stage (100) is displaceable along at least two axes parallel to a chuck surface (195, 265) of the sample stage (100), and further comprising a step of measuring height differences of at least three points on the photomask (270, 410, 700) that are not on a straight line by the at least one height sensor (460) without rotating the sample stage (100).

18. 20. The method (1700, 1800) of claim 17, further comprising the step of interpolating and / or extrapolating the height differences at positions of the photomask (270, 410, 700) that deviate from the three measurement points from the measured height differences.

19. 1. An apparatus (400) for determining an alignment of a photomask (270, 410, 700) on a sample stage (100), the sample stage (100) being displaceable along at least one axis parallel to a chuck surface (195, 265) of the sample stage (100) and rotatable about at least one axis perpendicular to the chuck surface (195, 265), the apparatus (400) comprising: An apparatus (400) comprising at least one height sensor (460) configured to measure height changes (1570) while rotating the sample stage (100) through a predefined angle, the at least one height sensor (460) being at a predetermined non-vanishing distance (490) relative to the axis of rotation (1450) for the purpose of determining the alignment of the photomask (270, 410, 700) on the sample stage (100).

20. A computer program comprising instructions which, when executed by a computer system, cause said computer system to carry out the method steps of any one of claims 1 to 18.

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