Devices for examining the surface of a mask

Abstract

Device (100) for examining a surface (120) of a mask (110), comprising: a. a probe (130) that interacts with the surface (120) of the mask (110); and b. a measuring device (140) for determining a reference distance (145) of the mask (110) from a reference point (147), wherein c. the measuring device (140) measures the reference distance (145) of the mask (110) in a measuring area (148) of the mask (110) which is located on a side (125) of the mask (110) facing away from the surface (120).

Description

1. Technical field

[0001] The present invention relates to devices for examining the surface of a test object, such as a lithography mask. 2. State of the art

[0002] Modern microscopic techniques such as atomic force microscopy or scanning tunneling microscopy allow the examination of surfaces of test objects with an accuracy in the nanometer range.

[0003] However, one problem with surface investigations at such scales is that the devices used for this purpose may be exposed to external disturbances which can affect and disrupt the microscopy probes used for the investigation so strongly that the desired measurement accuracy can no longer be achieved.

[0004] To address this problem, a scanning probe microscope with the ability to inherently suppress disturbances is described, for example, in US 2006 / 0033024 A1 and in the publication by AW Sparks and SR Manalis, "Atomic force microscopy with inherent disturbance suppression for nanostructure imaging," Nanotechnology 17 (2006), pp. 1574-1579, February 21, 2006, doi:10.1088 / 0957-4484 / 17 / 6 / 007. A device for measuring a property of a test object's surface using scanning probe microscopy comprises a localized probe that detects the surface property and a delocalized sensor that is mechanically coupled to and positioned next to the localized probe. This setup allows the scanning probe microscope's susceptibility to disturbances to be reduced.

[0005] The publication by G. Schitter and A. Stemmer, “Eliminating mechanical perturbations in scanning probe microscopy”, Nanotechnology 13 (2002), p. 663 ff., 20.09.2002, doi:10.1088 / o957-4484 / 13 / 5 / 324 discloses a method for removing mechanical vibrations in scanning probe microscopes by detecting the vibrations with a distance sensor and subsequently removing the vibrations from the measured topology signal.

[0006] However, a disadvantage of the methods and devices known from the prior art is that the delocalized (distance) sensor is always located next to the probe, which is used for the actual measurement of the test surface's topography. Therefore, the sensor and the probe measure different areas or points of the test surface, which can lead to inaccuracies in computation or suppression of mechanical disturbances. Due to design limitations, however, the sensor cannot be positioned arbitrarily close to the probe.

[0007] Furthermore, it can happen that, for example, the sensor's measuring range extends beyond the edge of the test surface while the probe is still measuring the test surface, or vice versa. This means that with methods and devices known from the prior art, the entire test surface of a test object cannot be measured, and that a marginal strip of the surface always remains that cannot be simultaneously captured by the sensor and the probe. Therefore, no direct compensation for mechanical disturbances can be achieved in this marginal strip area.

[0008] The present invention is therefore based on the objective of providing improved methods and devices for investigating the surface properties of a test object such as a lithography mask, which allow for higher measurement accuracy and also enable the compensation of external mechanical disturbances over the largest possible areas of the test surface.

[0009] The article “Calibrated scanning force microscope with capabilities in the subnanometre range,” by K. Hasche et al., in Surf. Interface Anal. 2002, Vol. 33, pp. 71–74, refers to an atomic force microscope (SFM) and distance measurements traceable to measurement standards. Traceability to the unit of length is achieved by calibrating several thousand selected and sufficiently defined reference positions in the three-dimensional measurement range using three miniature laser interferometers and their output signals at distances of λ / 2 (λ corresponds to the wavelength of the He / Ne laser radiation). The expanded measurement uncertainty U of the laser interferometer output signals is estimated to be ≤ 1 nm.The results reported in this article relate to measurement uncertainties in the sub-nanometer range, obtained by comparing measured periods of sinusoidal one-dimensional gratings using optical diffractometry with expanded measurement uncertainties ≤ 0.1 nm, as well as to SFM measurement results from scanning probe microscopes, such as the thickness of a thin film or the grating spacing. The current state of the work allows for the estimation of an expanded measurement uncertainty of ≤ 0.4 nm, and there is hope of achieving a measurement uncertainty value close to the picometer range. The application-oriented objective of this microscopy technique is to use it for the evaluation (calibration) of spacing parameters for objects in semiconductor technology and other dimensions of micro- and nanostructures.

[0010] The publication “A metrological scanning force microscope used for coating thickness and other topographical measurements,” by M. Bienias et al., in Appl. Phys. A, Vol. 66, pp. S837–S842 (1998), describes a commercial atomic force microscope (SFM) modified by the integration of three miniature laser interferometers. New calibration procedures were implemented to better meet the requirements for traceability of measurement results to primary measurement standards. The resulting improvement lies in the precise positioning of the probe relative to the points to be scanned on the surface. The publication provides a brief introduction to scanning probe microscopes and interferometers, the three-dimensional calibration of the SFM, selected calibration results, and the compensation of calibration errors by the SFM's control software.SFM is used to determine the thickness of a coating layer with corresponding artifacts.

[0011] German patent application DE 43 44 499 A1 proposes an interferometric data acquisition method for a scanning probe microscope with a detector probe capable of detecting the surface structure of objects down to atomic resolution. The detector probe is mounted on a probe holder and is movable relative to the object surface. A structure signal characterizing the surface structure is transmitted from the detector probe to a recording and display device, whereby a noise signal representing the relative movement between the probe holder and the object is determined interferometrically. The noise signal is superimposed on the structure signal in such a way that the display device reproduces only the structure signal. An interferometer independent of the structure signal is used to determine the noise signal.In an atomic force microscope with a bending spring as a detector probe, two laser beam interferometers are used, one of which measures the deflection of the bending spring, the other the interference signal.

[0012] US patent US 7,381,975 B1 describes a microscope that generates magnified images of a sample. An observation component generates image data of the sample, and a motion component generates relative motion between the sample and the observation component. A sensor independently generates position data directly from the sample. A controller receives image data from the observation component and position data from the sensor and uses the position data to correct specific relationships between the image data. 3. Summary of the invention

[0013] This problem is at least partially solved by a device for examining a surface of a mask according to claim 1. In one embodiment, the device comprises a probe that interacts with the surface of the mask, and a measuring device for determining a reference distance of the mask from a reference point, wherein the measuring device measures the reference distance of the mask in a measuring area of ​​the mask that is arranged on a side of the mask facing away from the surface.

[0014] In this embodiment, the measuring area is therefore not located on the surface of the mask that is being examined by the probe. Rather, the measuring area is located on a different surface of the mask, for example, on a surface of the mask opposite the surface being examined.

[0015] The interaction of the probe with the surface can be characterized, for example, by a force or by a tunneling current. For instance, the interaction of the probe with the surface can be based on or include van der Waals forces between the probe and the surface. Other interaction possibilities are known to those skilled in the art.

[0016] By combining the probe's measurement signal and the measured reference distance of the mask from the reference point, the surface features of interest during the examination can be separated from effects caused by external disturbances acting on the device. An exemplary procedure for achieving this is described in the aforementioned publication "Atomic force microscopy with inherent disturbance suppression for nanostructure imaging" by AW Sparks and SR Manalis.

[0017] Since the measuring device measures the reference distance of the mask in a measuring area of ​​the mask which is not located on the surface of the mask which carries the structures and properties to be investigated and which is measured by the probe, the measuring device can be positioned in such a way as to optimize the compensation of external disturbances without the arrangement of the measuring device hindering the desired arrangement of the probe or vice versa.

[0018] The measuring area is located on one side of the mask facing away from the surface.

[0019] For example, the probe can be used to examine an upper surface of the mask, while the measuring device's measuring area is located on a lower surface, or vice versa. If the probe and measuring device are positioned on opposite surfaces of the mask, there is a particularly high degree of flexibility in their placement.

[0020] If, for example, the mask rests on a possibly movable support table during the examination (see below), the support table can allow measurement "from below," for instance, by being made of a transparent material (possibly with a known refractive index), thus permitting optical distance measurement through it. Alternatively, the support table could be designed as a kind of frame with cutouts in the areas of the mask to be examined, through which the distance can be measured. Further possibilities will become apparent to a person skilled in the art based on their expertise.

[0021] The measuring area and a measuring point of the probe can be essentially opposite each other, particularly on different sides of the mask.

[0022] "Essentially" here can mean insofar as this can be precisely adjusted from a structural engineering perspective and taking into account the extent of the measuring range. Assuming that external disturbances do not lead to variations in the thickness of the mask under investigation—which proves to be an excellent approximation in practice—positioning the measuring range of the measuring device directly opposite the probe's measuring point can enable particularly good compensation of external disturbances, since propagation effects of the disturbances within the device or the mask can be largely neglected. Furthermore, such an arrangement can allow (approximately) the entire surface of the mask to be examined, as it prevents situations where, for example, the measuring device has already moved beyond the edge of the mask's surface while the probe is still measuring above the mask.

[0023] The reference distance of the mask from the reference point can be a distance averaged over the measuring range. The measuring range of the measuring device can, for example, have an extent that is significantly larger than the surface structures being examined by the probe. Here and in the following, a direction from the surface of the mask towards the probe is referred to as the z-direction. The xy-plane to which the z-direction is perpendicular can form a tangent plane to the mask at the probe's measuring point. For a planar mask, the xy-plane can also essentially coincide with the surface of the mask being examined, at least on "macroscopic scales," i.e., on scales that are significantly larger than the surface structures being examined by the probe (for example, on scales in the micrometer or millimeter range).

[0024] Because the reference distance is an averaged distance, the measurement of the reference distance is particularly sensitive to disturbances in the z-direction, which can negatively affect the measurement result of the surface investigations to a particularly large extent.

[0025] The measuring device can operate on the principle of optical interference and / or capacitive distance measurement. Such an arrangement can be advantageous from a structural and cost perspective. Furthermore, such measuring devices can be easily adjusted and calibrated, and they allow measurements within the desired accuracy range. The measuring device can also operate according to another measuring principle known to those skilled in the art.

[0026] The reference point can be a point on the measuring device. The reference distance can therefore be, for example, a distance between the measuring device and the mask. If, for instance, an optical interferometer is used as the measuring device, the reference point can be, for example, a point on the exit lens of the interferometer, a point on the sensor of the interferometer, or any other arbitrary point on the interferometer.

[0027] The measuring device can be mechanically coupled to the probe in such a way that a movement of the probe relative to the mask leads to a movement of the measuring device relative to the mask.

[0028] Such a mechanical coupling of the probe to the measuring device can further improve the accuracy of compensating for external disturbances. The coupling can be rigid or largely rigid, so that the movements of the probe and the measuring device are highly correlated. A very rigid coupling, for example, can ensure that the probe and the measuring device react to external disturbances with essentially identical movements. Depending on the degree of coupling, however, more or less pronounced time delays may occur between the movement of the probe and the movement of the measuring device, and / or the amplitudes of the movements may differ. This may be desirable, for example, to obtain further information about the nature of the disturbances, or it may be undesirable to keep the evaluation as simple as possible. Depending on the requirements, the person skilled in the art will therefore design the coupling appropriately.

[0029] The measuring device and the probe can, for example, be designed as a single structural unit.

[0030] According to a further aspect of the invention, the problem mentioned at the outset, which underlies the present invention, is solved at least partially by a device for examining a surface of a mask, which in one embodiment has a probe that interacts with the surface of the mask, as well as a first measuring device for determining a first reference distance of the mask in a first measuring area of ​​the mask from a first reference point and a second measuring device for determining a second reference distance of the mask in a second measuring area of ​​the mask from a second reference point, wherein the first measuring area and / or the second measuring area are part of the surface of the mask with which the probe interacts.

[0031] By using a first and a second measuring device, a larger area of ​​the mask's surface can be measured with the device, and little or no extrapolation to the surface's edges is required. For example, the first and second measuring devices can be positioned on opposite sides of the probe. It is also possible for the device to have a third, a fourth, and possibly further measuring devices.

[0032] The first and second reference points can also coincide. For example, the first and second measuring devices can each measure a reference distance to the mask from a common reference point, whereby the two measuring devices can be directed at different measuring areas on the mask.

[0033] It is possible that the first measuring range of the first measuring device and the second measuring range of the second measuring device do not completely overlap.

[0034] Because the measuring ranges do not completely overlap, and the two measuring devices can therefore cover different areas of the mask's surface, it can be achieved, as mentioned above, that in every (or at least almost every) measuring position of the probe, at least one of the two measuring ranges still captures the mask's surface. This allows for the largest possible area of ​​the mask's surface to be examined directly with interference compensation, and extrapolation, for example into the mask's edge regions, can be avoided or reduced.

[0035] The first reference distance of the mask from the first reference point can be a distance averaged over a first measuring range of the first measuring device, and / or the second reference distance of the mask from the second reference point can be a distance averaged over a second measuring range of the second measuring device. Regarding possible dimensions and shapes of the measuring ranges, reference is made to the corresponding explanations above in the discussion of the measuring device with a measuring range that is not located on the surface of the mask being examined by the probe, which are also applicable to the device according to the invention described here.

[0036] The first measuring device and / or the second measuring device can operate according to the principle of optical interference and / or the principle of capacitive distance measurement. The first measuring device and / or the second measuring device can also operate according to another measuring principle known to those skilled in the art.

[0037] The first reference point can be a point on the first measuring device and / or the second reference point can be a point on the second measuring device.

[0038] The probe can be mechanically coupled to the first measuring device in such a way that a movement of the probe relative to the mask leads to a movement of the first measuring device relative to the mask and / or the probe can be mechanically coupled to the second measuring device in such a way that a movement of the probe relative to the mask leads to a movement of the second measuring device relative to the mask.

[0039] The first and second measuring devices can also be mechanically coupled to each other.

[0040] The first measuring device and the probe can, for example, be designed as a single structural unit. The second measuring device and the probe can also be designed as a single structural unit. Finally, all three components can also be designed as a single structural unit.

[0041] The advantages of these design options were already discussed above in the discussion of the measuring device with a measuring range that is not located on the surface of the mask examined by the probe, and the considerations made there can be transferred to the first and second measuring devices discussed here.

[0042] It is even possible that the first and / or the second measuring device are designed like the measuring device described above, which measures a reference distance of the mask in a measuring area of ​​the mask that is not located on the surface of the mask being examined by the probe. Therefore, all features described above in relation to such a measuring device can also apply to the first and / or second measuring device, and vice versa.

[0043] According to a further aspect of the invention, the problem mentioned at the outset, which underlies the present invention, is solved at least partially by a device for examining a surface of a mask, which in one embodiment has a probe that interacts with the surface of the mask, as well as a measuring device for determining a reference distance of the mask from a reference point and a correlation unit configured to correlate a measurement signal of the probe and a measurement signal of the measuring device.

[0044] As explained above, the effect of disturbances on a device for surface examination of a mask is generally a physically complex process. External disturbances will generally not only act in the z-direction but also have components in the xy-direction. Furthermore, such disturbances will only have a finite propagation speed, meaning they will propagate through the device and affect different areas of the device at different times and with varying intensity. Additionally, the degree of coupling between the probe, the measuring device, and, if applicable, the mask, can influence how the probe and the measuring device react to the external disturbances and how they move relative to the mask. This must be taken into account during the evaluation.

[0045] To account for these various influencing factors, a correlation unit can be used, which makes it possible to consider, for example, the finite propagation speed, the influence of the coupling between the probe and the measuring device, or the attenuation of disturbances as they pass through the device. Such a correlation unit can allow for the detection and modeling of significantly more complex disturbances, such as temporally and spatially resolved disturbances with components in all three spatial directions, thus enabling improved compensation of external disturbances.

[0046] Furthermore, improved modeling of the influence of external disturbances can allow for improved extrapolation of disturbance compensation into the edge regions of the mask when using only a single measuring device arranged next to the probe. This can increase the measurement accuracy of the device according to the invention.

[0047] According to the invention, the correlation unit has a phase analyzer configured to determine a phase difference between the measurement signal of the probe and the measurement signal of the measuring device.

[0048] By determining a phase difference between the probe signal and the signal from the measuring device, the finite propagation speed of interference can be taken into account. This, in turn, can allow the probe and measuring device to be positioned at a distance from each other within the device while still achieving good compensation for external interference—even in the edge regions of the mask. This can simplify the structural design of such a device.

[0049] The measuring device of the device with correlation unit can be configured as a measuring device as described above, which measures a reference distance of the mask in a measuring area of ​​the mask that is not located on the surface of the mask measured by the probe. All features described in relation to such a measuring device can therefore also be transferred to the measuring device of the device with correlation unit, and vice versa.

[0050] The measuring device of the device with correlation unit can also be configured as the first and / or second measuring device of a device as described above. In this case, multiple correlation units can be used. All features described in relation to such a first and / or second measuring device can therefore also be applied to the measuring device of the device with correlation unit described here, and vice versa.

[0051] According to a further aspect of the invention, the aforementioned problem underlying the present invention is at least partially solved by a device for examining the surface of a mask, which in one embodiment comprises a probe that interacts with the surface of the mask, and a measuring device for determining a reference distance of the mask from a reference point. The measuring device measures the reference distance of the mask within a measuring area of ​​the mask, wherein a measuring point of the probe lies within the measuring area.

[0052] In this embodiment, the measuring area on the surface of the mask used to determine the reference distance therefore includes the measuring point of the probe, so that interference compensation is always possible, at least for the "immediate vicinity" of the probe's measuring point. In particular, this prevents the measuring area of ​​the measuring device from having already extended completely beyond the edge of the mask while the probe's measuring point is still above the mask surface.

[0053] In particular, it is possible that the measuring device has a beam path and the probe is arranged in the beam path of the measuring device.

[0054] For example, the probe could be a cantilever of an atomic force microscope. If the beam path and the extent of the measuring range of the measuring device are selected such that the light reflected by the cantilever, or the "shadow" cast by the cantilever into the measuring range of the measuring device, is negligible compared to the light reflected by the mask surface in the unshaded measuring range, then a sufficiently high measurement accuracy with respect to the reference distance can be achieved despite the cantilever being positioned within the beam path. The mention of a cantilever of an atomic force microscope is merely an example, and those skilled in the art will recognize that the principle just described can also be applied to other types of probes for surface examination.

[0055] The beam path of the measuring device can, for example, include a reflective element which projects a collimated beam of light onto the measuring area.

[0056] Imaging a collimated light beam onto the measuring area can increase the measurement accuracy of the reference distance measurement or facilitate the measurement.

[0057] It can also facilitate averaging the measurement of the reference distance across the measuring range. The use of a reflective element can, for example, save installation space or otherwise simplify the design of the device.

[0058] The reflecting element can be designed, for example, as a plane mirror. It can also be designed as a concave mirror, particularly a parabolic mirror. Other mirror shapes are also conceivable.

[0059] To illustrate this point, let's return to the example of a cantilever in an atomic force microscope. Using a plane mirror, which can be significantly larger than the cantilever, can mean that the light reflected or blocked by the cantilever is negligible compared to the light reaching and reflected from the mask surface. A similar effect can occur when using a concave or parabolic mirror: such a mirror can focus or even concentrate the collimated light beam onto the measurement area on the mask surface in a narrowing cone. The diameter of the light cone in the cantilever region can be similar to that of a plane mirror, making the amount of light reflected or blocked by the cantilever negligible.Nevertheless, focusing on a measuring area with a small diameter can be achieved if desired, and a sufficiently large amount of light can be directed onto this measuring area. For example, the larger the opening angle of the light cone, the less significant the vignetting caused by the cantilever becomes.

[0060] The measuring device of the device described here can also be configured as the first and / or second measuring device of a device as described above. All features described in relation to such a first and / or second measuring device can therefore also be applied to the measuring device of the device described here, and vice versa.

[0061] Furthermore, the measuring device of the device described here can also be configured as a measuring device of a device with a correlation unit. All features described in relation to such a measuring device can therefore also be transferred to the measuring device of the device described here, and vice versa.

[0062] A device according to the invention can further comprise a source of interference which is designed to expose the device to controllable disturbances.

[0063] By introducing controllable disturbances, as opposed to random disturbances such as those that can occur during the normal operation of the device, the device's response to the controllable disturbances can be precisely investigated, thus achieving a very accurate calibration of the device.

[0064] A device according to the invention can further comprise a placement table for placing the mask and an actuator configured to move the placement table in at least one direction.

[0065] An actuator can be understood as a complex structural arrangement that allows the probe table to be moved as desired. The actuator can comprise multiple individual components. For example, it can have several piezoelectric sensors that can move the probe table in one or more spatial directions. The actuator can be designed, for instance, to move the probe table relative to the probe in such a way that the strength of the interaction between the probe and the surface remains essentially constant. The actuator can also be designed to move the probe table so that the surface under investigation is scanned by the probe.

[0066] For example, the actuator can move the platform in the z-direction during scanning such that a force, e.g., a van der Waals force, remains constant between the probe tip and the mask surface under investigation, allowing the device to operate as an atomic force microscope (AFM). Alternatively, the actuator can move the platform in the z-direction during scanning such that a tunneling current remains constant between the probe tip and the mask surface under investigation, allowing the device to operate as a scanning tunneling microscope (STM). However, it is clear to those skilled in the art that these are merely two examples. Other methods known in the prior art for examining a mask surface can also be applied, and the present invention is not limited to the examples mentioned here.

[0067] A device according to the invention can also include a position transmitter for determining the position of the support table within a plane defined by the support table. In principle, such a position transmitter can also provide a position of the support table within another predefined plane.

[0068] For example, one or more optical interferometers and / or capacitive distance sensors can be used as position transmitters to determine the position of the support table within the xy-plane discussed above (or another predefined plane). This can help to better detect and eliminate components of external disturbances acting perpendicular to the z-direction from the measurement or the measurement results. Thus, such a position transmitter can further increase the achievable measurement accuracy of the device.

[0069] Alternatively or additionally to a movable support table, a device according to the invention can have a displacement unit configured to move the probe in at least one direction. The displacement unit can, for example, be configured to move the probe in such a way that the strength of the interaction between the probe and the surface of the mask is kept essentially constant, analogous to the corresponding descriptions of the movable support table above. The displacement unit can also serve to move the probe in the plane of the mask in order to scan the surface to be examined.

[0070] In general, the same statements apply to the displacement unit as to the actuator for moving the application table. For example, the displacement unit can have one or more piezo controllers that can move the probe in one or more spatial directions.

[0071] A device according to the invention can also include a probe position sensor for determining the position of the probe within a predefined plane. For example, the probe position sensor can provide a position of the probe within a plane defined by the support table or a plane defined by the surface of the mask. Here, too, one or more optical interferometers and / or capacitive distance sensors can be used, for example.

[0072] Moving the support table and moving the probe thus represent two possible operating modes of a device according to the invention, which can each be used alone or in combination with each other.

[0073] In the event that the probe is actively moved, the device according to the invention can further be designed such that the measuring device, or the first and / or second measuring device, follows the displacements of the probe, i.e., it is moved together with the probe. For this purpose, for example, a rigid mechanical coupling between the probe and the respective measuring device (see above) can be provided. The probe and the respective measuring device can, for example, be designed as a single structural unit.

[0074] A device according to the invention can also be configured to be calibrated by a method of the placement table and / or a method of the probe along a predetermined sequence of movements.

[0075] By moving the support table and / or probe along a predetermined sequence of movements, it is possible, in particular, to determine the scale of all sensors, probes, and measuring devices present in the apparatus and to normalize the scales of these components relative to each other. If such normalization is omitted, artifacts could appear in the measurement results that seem like external interference but are actually due to differing measurement scales of the components.

[0076] For calibration, it is also possible to set the scan size of the device according to the invention to zero, i.e., the probe tip remains stationary at a specific position, while all other operating parameters are selected as they will be during the subsequent image acquisition. This yields data that reflects only (or at least predominantly) the disturbances. Subsequently, one or more adjustment parameters can be optimized so that the data corrected by these parameter adjustments is free of disturbances. Such a procedure could, for example, be carried out before each image acquisition to further improve measurement accuracy.

[0077] The procedure of the application table and / or the probe along a predefined sequence of movements and the determination of such adaptation parameters can be used alternatively or in combination with each other.

[0078] A device according to the invention can include a compensation unit for compensating for external disturbances by applying a compensation signal to control the device. The applied signal can serve to control the actuator and / or the displacement unit.

[0079] The compensation unit can therefore allow a kind of "noise cancelling", so that the influence of external disturbances does not have to be subsequently subtracted from the received measurement signals, or at most only partially, but the external disturbances are suppressed directly during the measurement by countermeasures.

[0080] The study of the surface can include an examination of the surface topography.

[0081] For this purpose, for example, the aforementioned principles of atomic force microscopy or scanning tunneling microscopy, or other principles and methods suitable for this purpose from the prior art, can be used.

[0082] It should also be mentioned here that the various functional components, devices, and units discussed within the scope of the invention do not necessarily have to represent separate structural components. Rather, several or even all functional components, devices, and units can be combined in a single structural component. Furthermore, it is also possible for individual, several, or all functional components to be implemented by software, which, when executed, causes corresponding structural components to provide the respective functionality.

[0083] Another aspect of the invention is a method for examining the surface of a mask, wherein an embodiment of a device according to the invention is used. 4. Brief description of the characters

[0084] The following detailed description describes possible embodiments of the invention with reference to the following figures: Fig. 1: An embodiment of a device according to the invention for examining a surface of a test object with a measuring device which measures on a side facing away from the surface, like the probe; Fig. 2a-b: An embodiment of a device according to the invention for examining a surface of a test object with a probe and two measuring devices; Fig. 3a-c: An embodiment of a device according to the invention for examining a surface of a test object with a correlation unit; and Fig. 4a-b: An embodiment of a device according to the invention for examining a surface of a test object with a measuring device in whose measuring range the measuring point of the probe lies. 5. Detailed description of possible embodiments

[0085] Possible embodiments of the present invention are described in detail below. However, it is emphasized that the present invention is not limited to these embodiments. Rather, the design possibilities described in relation to these specific embodiments can be further modified and combined in other ways within the scope of the invention, and individual optional features can also be omitted if they appear unnecessary. To avoid redundancies, particular reference is made to the explanations in the preceding sections, which also remain valid for the detailed description that follows.

[0086] Furthermore, it is mentioned that, for the sake of simplicity, the following descriptions always refer to the examination of lithography mask surfaces, but the invention is not limited to this. Rather, the surfaces of other types of test objects can also be examined within the scope of the invention.

[0087] Fig. Figure 1 shows an embodiment of a device 100 for examining a surface 120 of a mask 110. In the case shown here, the mask 110 is a planar or flat mask 110. In addition to the surface 120, whose properties are to be examined, the mask 110 therefore has a second surface on the opposite side 125.

[0088] For examining the surface 120, the device 100 has a probe 130. The probe 130 interacts with the surface 120 of the mask 110 at a measuring point 135. The interaction of the probe 130 with the surface 120 of the mask 110, indicated by the double arrow 170, can be characterized, for example, by a force or by a tunnel current between the tip of the probe 130 and the measuring point 135 of the surface 120 of the mask 110. For example, the interaction of the probe 130 with the surface 120 of the mask 110 can be based on or include van der Waals forces between the probe 130 and the surface 120. Other possibilities are obvious to those skilled in the art. To examine the surface 120 of the mask 110, the mask 110 can, for example, be moved or shifted relative to the probe 130 such that the strength of the interaction 170 between the probe 130 and the surface 120 is essentially (i.e.is kept constant (within the measurement error).

[0089] For this purpose, the mask 110 can, for example, be placed on a support table (not shown here), and the support table can be connected to an actuator (also not shown here) configured to move the support table in one or more spatial directions. Alternatively or additionally, the probe 130 can also be actively moved. For this purpose, the device 100 can have a displacement unit (not shown) configured to move the probe 130 in one or more spatial directions.

[0090] Basic methods for examining a mask surface 120 using the interaction 170 between the tip of a probe 130 and the mask surface 120 are known to those skilled in the art. Therefore, this point will not be discussed in further detail here.

[0091] The device 100 further comprises a measuring device 140, which serves to determine a reference distance 145 of the mask 110 from a reference point 147. The measuring device 140 measures the reference distance 145 of the mask 110 in a measuring area 148 of the mask 110, which is not located on the surface 120 of the mask 110 that is being examined by the probe 130. Rather, in the case shown here, the measuring area 148 is located on the side 125 of the mask 110 that faces away from the surface 120 to be examined. The measuring point 135, with which the tip of the probe 130 interacts 170, and the measuring area 148 of the measuring device 140 are located on essentially opposite sides of the mask 110."Essentially" here can mean, insofar as this can be precisely adjusted from a structural engineering perspective and taking into account the extent of the measuring range 148 (for example, a center point of the measuring range 148 can be opposite the measuring point 135, or any point of the measuring range 148 can be opposite the measuring point 135).

[0092] The measuring device 140 can, for example, operate on the principle of optical interference and / or on the principle of capacitive distance measurement. Other measuring principles known to those skilled in the art can also be used in the measuring device 140. For example, the measuring device 140 could be a laser interferometer.

[0093] The measuring device 140 can be configured such that the reference distance 145 of the mask 110 is averaged from the reference point 147 over the measuring range 148. The measuring range 148 can, for example, have a round, oval, or other shape. The reference point 147, from which the reference distance 145 of the mask 110 is measured, can, for example, be any point on the measuring device 140. If the measuring device 140 is, for example, a laser interferometer, then the reference point 147 can, for example, be a point on the exit lens of the interferometer or a point on the sensor of the interferometer, etc.

[0094] The measuring device 140 can be mechanically coupled to the probe 130 such that a movement of the probe 130 relative to the mask 110 leads to a movement of the measuring device 140 relative to the mask 110. In the illustration of the Fig. Figure 1 indicates such a mechanical coupling 160. For example, if the probe 130 can be actively moved by a displacement unit, this coupling can be designed such that the probe 130 and the measuring device 140 are moved together as a single unit by the displacement unit. For example, the probe 130 and the measuring device 140 can be designed as a structural unit.

[0095] By analyzing the measurement signal of the probe 130 and the measured reference distance 145, the device 100 can distinguish the influences of external disturbances on the device 100 from structural information regarding the surface 120 of the mask 110, and the disturbing influences of the external disturbances can thus be at least partially compensated or calculated out of the measurement results.

[0096] The Fig. Figure 2a shows an embodiment of a further device 200 according to the invention for examining a surface 120 of a mask 110. The device 200 has a probe 130 that interacts with the surface 120 of the mask 110. Regarding the mask 110, its surfaces / sides 120, 125, the probe 130, and its interaction 170 with a measuring point 135 on the surface 120 of the mask 110, reference is made to the corresponding explanations in the discussion of Fig. 1 indicated which can be transferred to the embodiment 200 shown here.

[0097] Explicitly shown is in Fig. 2a A mounting table 180 on which the mask 110 rests. The mounting table 180 is connected to an actuator 185, which allows the mounting table 180 to be moved in at least one spatial direction. The actuator 185 can, for example, have one or more piezo controllers and / or a spindle drive, etc. Alternatively or additionally, the probe 130 could also be actively moved by a displacement unit, as discussed above.

[0098] In Fig. Figure 2a shows a coordinate system 290 in the upper right corner. As can be seen from the coordinate system 290, a direction from the measuring point 135 on the surface 120 of the mask 110 towards the probe 130 is designated as the z-direction. This direction is perpendicular to a plane located in the xy-direction, which here coincides with a plane defined by the support table 180. However, it would also be conceivable that the z-direction lies obliquely on a plane defined by the support table 180, meaning that the table plane does not coincide with the xy-plane. These statements regarding the x, y, and z directions can also be applied to all other embodiments of the invention described herein, and therefore the coordinate system 290 is shown only once in Figure 2a. Fig. 2a is marked.

[0099] The actuator 185 can be configured to move the support table 180 in such a way that the strength of the interaction 170 between the probe 130 and the surface 120 is kept essentially constant, thus enabling, for example, an investigation of the topography or other properties of the surface 120. Alternatively or additionally, if the probe 130 is actively moved by a displacement unit, the displacement unit could also bring about the necessary displacements of the probe 130 relative to the mask 120, or the actuator 185 and the displacement unit could work together for this purpose.

[0100] The device 200 comprises a first measuring device 240 for determining a first reference distance 245 of the mask 110 from a first reference point 247 and a second measuring device 250 for determining a second reference distance 255 of the mask 110 from a second reference point 257. Although not shown, it is also possible that the device 200 comprises further measuring devices for determining further reference distances.

[0101] In the Fig. In the embodiment shown in Figure 2a, the first measuring device 240 and the second measuring device 250 are arranged on opposite sides next to the probe 130. The measuring point 135 lies between the first measuring range 248 of the first measuring device 240 and the second measuring range 258 of the second measuring device 250. In the embodiment shown here, the first measuring range 248 and the second measuring range 258 do not overlap at all. In principle, however, it is also possible for the first measuring range 248 and the second measuring range 258 to partially overlap. Because the first measuring range 248 and the second measuring range 258 do not completely overlap, a larger area of ​​the surface 120 of the mask 110 can be covered by using two measuring devices 240 and 250 than would be possible with only one measuring device.

[0102] The first reference distance 245 of the mask 110 from the first reference point 247 can be a distance averaged over the first measuring range 248. Likewise, the second reference distance 255 of the mask 110 from the second reference point 257 can be a distance averaged over the second measuring range 258. To allow such an averaged measurement, the first measuring device 240 and / or the second measuring device 250 can, for example, operate according to the principle of optical interference and / or the principle of capacitive distance measurement; they can, for example, be laser interferometers. Other measuring principles known to those skilled in the art can also be used. Regarding possible shapes and dimensions of the measuring ranges 248 and 258, reference is made to the corresponding descriptions of measuring range 148 above.

[0103] The first reference point 247 can be a point of the first measuring device 240, as shown here. Similarly, the second reference point 257 can be a point of the second measuring device 250.

[0104] In Fig. Figure 2a further shows a first mechanical coupling 260 of the first measuring device 240 with the probe 130 and a second mechanical coupling 265 of the second measuring device 250 with the probe 130. While a direct mechanical coupling of the first measuring device 240 with the second measuring device 250 is not shown, the person skilled in the art understands that such a coupling may also exist.

[0105] The first mechanical coupling 260 can cause a movement of the probe 130 relative to the mask 110 to result in a movement of the first measuring device 240 relative to the mask 110. The second mechanical coupling 265 can cause a movement of the probe 130 relative to the mask 110 to result in a movement of the second measuring device 250 relative to the mask 110. As explained above, the design of the first mechanical coupling 260 and the second mechanical coupling 265 can affect the correlation of the measurement signals from the probe 130 and the first and second measuring devices 240 and 250, respectively. Using such correlations for interference suppression can increase the measurement accuracy achievable with the device 200. The possibility of a mechanical coupling such that the respective measuring device 240 and 250 is moved together with the probe 130 when it is actively moved has already been discussed.For example, the first measuring device 240 and / or the second measuring device 250 together with the probe 130 can be designed as a structural unit.

[0106] As in Fig. As shown in Figure 2b, it is also possible that one of the two measuring devices 240, 250 – or even both – is designed as a measuring device 140, whose measuring range 148 is arranged on a side 125 of the mask 110 opposite the surface 120. Regarding possible design options for such a measuring device 140, reference is made to the discussion of Fig. 1 above, and the statements made there can be applied to the statements in Fig. The embodiment shown in 2b can be transferred. Fig. 2b the first measuring device 240 from Fig. 2a is replaced by such a “measuring from below” measuring device 140. However, this is only an example.

[0107] The Fig. Figure 3a shows an embodiment of a further device 300 according to the invention for examining a surface 120 of a mask 110. The device 300 has a probe 130 that interacts with the surface 120 of the mask 110. Regarding the mask 110, its surfaces / sides 120, 125, the probe 130, and its interaction 170 with a measuring point 135 on the surface 120 of the mask 110, reference is again made to the corresponding explanations in the discussion of Fig. 1 indicated which can be directly transferred to the embodiment 300 shown here.

[0108] The device 300 further comprises a measuring device 340 for determining a reference distance 345 of the mask 110 from a reference point 347. The measuring device 340 measures the reference distance within a measuring range 348.

[0109] Regarding possible configurations of the measuring device 340 and further details concerning the reference distance 345, the measuring range 348 and the position of the reference point 347, reference is made to the corresponding explanations in the Fig. 1 and Fig. Reference is made to the devices 100 and 200 shown in 2a-b, which can be applied analogously to the device 300 shown here.

[0110] Thus, the measuring device 340 can be used, as in Fig. Figure 3a explicitly shows, for example, a mechanical coupling 360 to the probe 130, which causes a movement of the probe 130 relative to the mask to result in a movement of the measuring device 340 relative to the mask. Such a mechanical coupling 360 can, for example, influence how strongly the movements of the probe 130 and the movements of the measuring device 340 are correlated with each other in response to external disturbances. It can also serve to ensure that when the probe 130 is actively moved, the measuring device 340 is moved together with it. For example, the measuring device 340 and the probe 130 can be designed as a single structural unit.

[0111] The device 300 includes a correlation unit 390 configured to correlate a measurement signal from the probe 130 and a measurement signal from the measuring device 340. The correlation unit 390 includes a phase analyzer configured to determine a phase difference between the signal from the probe 130 and the signal from the measuring device 340. Such a phase difference can, for example, provide information about how external disturbances propagate through the device 300. The correlation unit 390 can also, for example, compare the amplitudes of the measurement signal from the probe 130 and the measurement signal from the measuring device 340. Further possibilities for correlating the two measurement signals are readily apparent to those skilled in the art.

[0112] The measuring device 340 can also be configured as a measuring device 140, which measures a reference distance 145 of the mask 110 from a reference point 147 in a measuring area 148 of the mask 110, which is not located on the surface 120 of the mask 110 that is examined by the probe 130. Such an embodiment of the device 300 is described in Fig. 3b explicitly shown. All aspects relating to the measuring device 140 can be discussed in the context of embodiment 100. Fig. The statements made in point 1 can also be applied to the case shown here.

[0113] Furthermore, it is also possible that the device 300 has several measuring devices instead of one measuring device 340, some or all of which are coupled to the probe 130 via a correlation unit 390. Each of the measuring devices in question can be coupled to the probe 130 via its own correlation unit. Or several or all of the measuring devices in question can be coupled to the probe 130 via a common correlation unit 390. Fig. Figure 3c shows an exemplary embodiment of the device 300, which includes the two measuring devices 240 and 250, as described in connection with Fig. 2a were combined with a common correlation unit of 390. All within the context of the discussion of Fig. The statements made in 2a-b can therefore also be applied to the statements in Fig. The case shown in 3c will be applied.

[0114] The Fig. Figures 4a-b show embodiments of a further device 400 according to the invention for examining a surface 120 of a mask 110. The device 400 has a probe 130 which interacts with the surface 120 of the mask 110 at a measuring point 135. Regarding the mask 110, its surface 120, the probe 130, and its interaction with the measuring point 135 on the surface 120 of the mask 110, reference is made to the corresponding explanations in the discussion of Fig. 1 indicated which can be directly transferred to the embodiment 400 shown here.

[0115] The device 400 includes a measuring device 440 for determining a reference distance of the mask 110 from a reference point. The reference point is located in the Fig. 4a-b is not explicitly shown. It can, for example, be located on a sensor of the measuring device 440 or be defined as another point of the measuring device 440. For further details regarding the reference distance and the position of the reference point, please refer to the corresponding explanations above.

[0116] The measuring device 440 measures the reference distance of the mask in a measuring range 448 (on the surface 120) of the mask 110. The measuring point 135 of the probe 130 lies within the measuring range 448. The measuring range 448 used to determine the reference distance therefore includes the measuring point 135 of the probe 130, so that interference compensation is always possible, at least for the "immediate vicinity" of the measuring point 135 of the probe 130.

[0117] In order to allow the probe 130 to be measured within the measuring range 448 from a structural perspective, the measuring device 440 shown here has a beam path 441 within which the probe 130 is arranged.

[0118] As in the Fig. As illustrated in Figures 4a-b, probe 130 can, for example, be a cantilever of an atomic force microscope. The beam path 441 and the extent of the measuring range 448 in which the measuring device 440 measures can be selected such that the light reflected by the cantilever, or the "shadow" cast by the cantilever, into the measuring range 448 is negligible compared to the light reflected by the mask surface 120 in the unshaded parts of the measuring range 448.

[0119] The beam path 441 of the measuring device 440 can in particular include a reflecting element 442 which projects an (approximately, i.e., as far as technically feasible) collimated light beam 443 onto the measuring area 448. This can, for example, allow for a structurally compact implementation of the device 400.

[0120] In Fig. 4a The reflecting element 442 is a concave or parabolic mirror. This mirror focuses the collimated light beam 443 onto the measuring area 448 in a narrowing light cone. In the area of ​​the probe 130, for example, the cantilever of an atomic force microscope, the diameter of the narrowing light cone can still be sufficiently large that the amount of light reflected or blocked by the probe 130 can be negligible for measuring the reference distance, even though the measuring area 448 may be highly focused, i.e., have a very small extent (e.g., compared to the diameter of the light beam 443). The larger, for example, the opening angle of the light cone, the less significant the shadowing by the probe 130 may be.

[0121] In the Fig. In the embodiment of the device 400 shown in Figure 4b, the reflecting element 442 is a plane mirror. This directs the collimated light beam 443 as a still (approximately) parallel light beam onto the measuring area 448. This can particularly effectively minimize the influence of shadowing by the probe 130 and further improve the (averaged) distance measurement of the reference distance.

[0122] Finally, it should be mentioned that the measuring device 440 described here can also be referred to as the first measuring device 240 and / or second measuring device 250 of the devices described in the Fig. The device 200 described in 2a-b can be configured as follows. The measuring device 440 described here can also be configured as measuring device 340 as described in the Fig. The device 300 described in sections 3a-c is configured with a correlation unit 390. The design features described in relation to the measuring devices 240, 250 and 340 can therefore also be transferred to the measuring device 440 and vice versa.

[0123] From the statements made above, it is evident to the person skilled in the art that the devices 100, 200, 300 and 400 described herein can be combined with one another in many ways within the scope of the invention - insofar as they are (structurally) technically compatible - and that the embodiments explicitly shown herein are merely examples intended to promote the understanding of the invention.

[0124] In addition to the ones in the Fig. 1, Fig. 2a-b, Fig. 3a-c and Fig. In addition to the components explicitly shown in 4a-b, devices 100, 200, 300 and 400 may also have further additional components.

[0125] A device 100, 200, 300, 400 according to the invention can, for example, have a source of interference which is designed to shield the device 100, 200, 300, 400 from controllable interferences on the side facing away from such interferences. This can serve to investigate the influence of such external interferences on the device 100, 200, 300, 400 under controllable conditions and thus contribute to an improvement in interference compensation.

[0126] A device 100, 200, 300, 400 according to the invention can also have a placement table 180 for placing the mask 110, as shown in Fig.Figure 2a shows that a device 100, 200, 300, or 400 according to the invention can also include an actuator 185 configured to move the support table 180 in at least one direction, for example, in the x, y, and / or z directions. The actuator 185 can be configured to move the support table 180 such that the strength of the interaction 170 between the probe 130 and the surface 120 is kept essentially constant. For further details regarding the support table 180 and the actuator 185, reference is made to the corresponding descriptions elsewhere.

[0127] A device 100, 200, 300, 400 according to the invention can also include one (or more) position sensors that serve to determine the position of the support table 180 within a plane defined by the support table. As already mentioned, the support table can, for example, be arranged in the xy-plane such that the z-direction, which represents the measuring direction of the probe 130, is perpendicular to the plane defined by the support table 180. Such a position sensor can, for example, be designed as a laser interferometer and / or a capacitive distance sensor, which is arranged next to the support table 180 and whose measuring range is located on the vertical edge of the support table 180. Such a position sensor can provide additional information regarding the response of the device 100, 200, 300, 400 to external disturbances and thus improve disturbance compensation.

[0128] Alternatively or additionally to a movable support table 180, a device 100, 200, 300, 400 according to the invention can have a displacement unit (not shown) configured to displace the probe 130 in at least one direction, for example in the x, y, or z direction. The displacement unit can be configured to displace the probe 130 such that the strength of the interaction 170 between the probe 130 and the surface 120 of the mask 110 is kept essentially constant.

[0129] In general, the same statements apply to the displacement unit as to the actuator 185 for moving the mounting table 180. For example, the displacement unit can have one or more piezo controllers which can move the probe 130 in one or more spatial directions.

[0130] A device 100, 200, 300, 400 according to the invention can also include a probe position sensor (not shown) for determining the position of the probe 130 within a predefined plane. For example, the probe position sensor can provide a position of the probe 130 within a plane defined by the support table 180 or within a plane defined by the surface 120 of the mask 110. These two planes (the plane defined by the support table 180, the plane defined by the surface 120) can coincide, for example, for a planar mask 110. Or they can be different, for example, for a non-planar mask 110. For example, one or more optical interferometers and / or capacitive distance sensors can be used as probe position sensors.

[0131] The movement of the support table 180 and the movement of the probe 130 thus represent two possible operating modes of a device 100, 200, 300, 400 according to the invention, which can each be used alone or in combination with each other.

[0132] A device 100, 200, 300, 400 according to the invention can also be configured to be calibrated by moving the mounting table 180 and / or the probe 130 along a predetermined sequence of movements. Alternatively or additionally, data acquisition can also be performed at image size zero, i.e., at a fixed position of the probe 130, as already described.

[0133] A device 100, 200, 300, 400 according to the invention can include a compensation unit for compensating external disturbances by applying a compensation signal to control the device 100, 200, 300, 400. The disturbance compensation can therefore be at least partially active by means of noise cancelling. The signal applied with the compensation signal can, for example, be used to control the actuator 185 of the loading table 180 and / or the transfer unit.

[0134] Finally, as an exemplary application of a device 100, 200, 300, 400 according to the invention, the examination of the topography of the surface 120 should be mentioned. However, this is only one possibility, and the invention is not limited to this. Furthermore, the invention also includes methods for examining a surface 120 of a mask 110 – or another test object – using a device 100, 200, 300, 400 according to the invention.

Claims

[1] Device (100) for examining a surface (120) of a mask (110), comprising: a. a probe (130) that interacts with the surface (120) of the mask (110); and b. a measuring device (140) for determining a reference distance (145) of the mask (110) from a reference point (147), wherein c. the measuring device (140) measures the reference distance (145) of the mask (110) in a measuring area (148) of the mask (110) which is located on a side (125) of the mask (110) facing away from the surface (120). [2] Device (110) according to the preceding claim, wherein the measuring area (148) and a measuring point (135) of the probe (130) are located substantially opposite each other on different sides of the mask (110). [3] Device (100) according to claim 1 or 2, wherein the reference distance (145) of the mask (110) from the reference point (147) is a distance averaged over the measuring range (148). [4] Device (100) according to any one of the preceding claims 1-3, wherein the measuring device (140) operates according to the principle of optical interference and / or the principle of capacitive distance measurement. [5] Device (100) according to any one of the preceding claims 1-4, wherein the reference point (147) is a point of the measuring device (140). [6] Device (100) according to any one of the preceding claims 1-5, wherein the measuring device (140) is mechanically coupled to the probe (130) such that a movement of the probe (130) relative to the mask (110) results in a movement of the measuring device (140) relative to the mask (110). [7] Device (200) for examining a surface (120) of a mask (110), comprising: a. a probe (130) that interacts with the surface (120) of the mask (110); b. a first measuring device (240) for determining a first reference distance (245) of the mask (110) in a first measuring range (148, 248) of the mask (110) from a first reference point (247); and c. a second measuring device (250) for determining a second reference distance (255) of the mask (110) in a second measuring area (258) of the mask (110) from a second reference point (257), wherein the first measuring area (148, 248) and / or the second measuring area (258) are part of the surface (120) of the mask (110) with which the probe (130) interacts. [8] Device (200) according to the preceding claim 7, wherein a first measuring range (248) of the first measuring device (240) and a second measuring range (258) of the second measuring device (250) do not completely overlap. [9] Device (200) according to one of the preceding claims 7 or 8, wherein the first reference distance (245) of the mask (110) from the first reference point (247) is a distance averaged over a first measuring range (248) of the first measuring device (240) and / or the second reference distance (255) of the mask (110) from the second reference point (257) is a distance averaged over a second measuring range (258) of the second measuring device (250). [10] Device (200) according to any one of the preceding claims 7-9, wherein the first measuring device (240) and / or the second measuring device (250) operate according to the principle of optical interference and / or the principle of capacitive distance measurement. [11] Device (200) according to any one of the preceding claims 7-10, wherein the first reference point (247) is a point of the first measuring device (240) and / or the second reference point (257) is a point of the second measuring device (250). [12] Device (200) according to any one of the preceding claims 7-11, wherein the probe (130) is mechanically coupled to the first measuring device (240) such that a movement of the probe (130) relative to the mask (110) leads to a movement of the first measuring device (240) relative to the mask (110) and / or the probe (130) is mechanically coupled to the second measuring device (250) such that a movement of the probe (130) relative to the mask (110) leads to a movement of the second measuring device (250) relative to the mask (110). [13] Device (200) according to any one of the preceding claims 7-12, wherein the first measuring device (240) and / or the second measuring device (250) is configured as a measuring device (140) according to any one of claims 1-6. [14] Device (300) for examining a surface (120) of a mask (110), comprising: a. a probe (130) that interacts with the surface (120) of the mask (110); b. a measuring device (340) for determining a reference distance (345) of the mask (110) from a reference point (347); and c. a correlation unit (390) configured to correlate a measurement signal of the probe (130) and a measurement signal of the measuring device (340), wherein the correlation unit (390) has a phase analyzer configured to determine a phase difference between the measurement signal of the probe (130) and the measurement signal of the measuring device (340). [15] Device (300) according to the preceding claim 14, wherein the measuring device (340) is configured as a measuring device (140) according to any one of claims 1-6. [16] Device (300) according to the preceding claim 14, wherein the measuring device (340) is configured as a first measuring device (240) and / or as a second measuring device (250) according to any one of claims 7-13. [17] Device (400) for examining a surface (120) of a mask (110), comprising: a. a probe (130) that interacts with the surface (120) of the mask (110); and b. a measuring device (440) for determining a reference distance of the mask (110) from a reference point, wherein c. the measuring device (440) measures the reference distance of the mask (110) in a measuring range (448) of the mask (110), and d. wherein a measuring point (135) of the probe (130) lies within the measuring range (448). [18] Device (400) according to the preceding claim 17, wherein the measuring device (440) has a beam path (441) and wherein the probe (130) is arranged in the beam path (441) of the measuring device (440). [19] Device (400) according to the preceding claim 18, wherein the beam path (441) of the measuring device (440) has a reflection element (442) which maps a collimated light beam (443) onto the measuring area (448). [20] Device (400) according to the preceding claim 19, wherein the reflection element (442) is designed as a plane mirror or as a concave mirror. [21] Device (400) according to any one of the preceding claims 17-20, wherein the measuring device (440) is configured as a first measuring device (240) and / or as a second measuring device (250) according to any one of claims 7-13. [22] Device (400) according to one of the preceding claims 17-20, wherein the measuring device (440) is configured as a measuring device (340) according to one of claims 14 or 15. [23] Device (100; 200; 300; 400) according to any one of the preceding claims 1-22, further comprising a source of disturbance configured to subject the device (100; 200; 300; 400) to controllable disturbances. [24] Device (100; 200; 300; 400) according to any one of the preceding claims 1-23, further comprising a placement table (180) for placing the mask (110) and an actuator (185) configured to move the placement table (180) in at least one direction (x; y; z). [25] Device (100; 200; 300; 400) according to the preceding claim 24, wherein the actuator (185) is configured to move the support table (180) such that the strength of the interaction (170) between the probe (130) and the surface (120) is kept substantially constant. [26] Device (100; 200; 300; 400) according to one of the preceding claims 24 or 25, further comprising a position transmitter for determining a position of the support table (180) within a plane (xy-plane) defined by the support table. [27] Device (100; 200; 300; 400) according to any one of the preceding claims 1-26, further comprising a displacement unit configured to displace the probe (130) in at least one direction (x; y; z). [28] Device (100; 200; 300; 400) according to the preceding claim 27, wherein the displacement unit is configured to displace the probe (130) such that the strength of the interaction (170) between the probe (130) and the surface (120) is kept substantially constant. [29] Device (100; 200; 300; 400) according to one of the preceding claims 27 or 28, further comprising a probe position sensor for determining a position of the probe (130) within a predefined plane (xy-plane). [30] Device (100; 200; 300; 400) according to any one of the preceding claims 24-29, wherein the device (100; 200; 300; 400) is configured to be calibrated by a method of the placement table (180) and / or a method of the probe (130) along a predetermined sequence of movements. [31] Device (100; 200; 300; 400) according to any one of the preceding claims 1-30, further comprising a compensation unit for compensating external disturbances by applying a signal to control the device (100; 200; 300; 400) with a compensation signal. [32] Device (100; 200; 300; 400) according to the preceding claim 31 in combination with one of the preceding claims 24-29, wherein the signal serves to control the actuator (185) and / or the displacement unit. [33] Device (100; 200; 300; 400) according to any one of the preceding claims 1-32, wherein the investigation of the surface (120) comprises an investigation of a topography of the surface (120).

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