Extreme ultraviolet mask inspection – laser interferometer optical alignment

Abstract

A method for interferometer alignment in an apparatus for sample inspection with an inspection means and a sample stage, wherein at least a portion of at least one interferometer for monitoring a sample stage position is mechanically coupled to the inspection means, comprises aligning the interferometer relative to an optical alignment means coupled to the inspection means.

Description

[0001] Extreme ultraviolet mask inspection — laser interferometer optical alignment

[0002] 1. Field of the invention

[0003] The present invention relates to, inter alia, apparatuses for sample inspection as well as methods, optical alignment means, and computer programs for interferometer alignment in such apparatuses for sample inspection.

[0004] 2. Technical background

[0005] Currently, apparatuses for sample inspection are used for the inspection of samples like substrates, wafers, or objects for lithography such as lithography masks, etc.

[0006] As a result of the constantly increasing integration density in microelectronics, objects for lithography, in particular lithography masks, must be able to image ever smaller structural elements in a photoresist layer of a wafer. In order to meet these requirements, the exposure wavelength is being shifted to ever shorter wavelengths, down to the extreme ultraviolet (EUVj wavelength range (e.g., to nm to 15 nm) and to corresponding EUV photolithography masks, herein also called EUV masks. Objects for lithography, especially lithography masks, cannot always be produced without visible or printable defects on a wafer due to the ever smaller dimensions of the structure or pattern elements. A defect in a lithography mask multiplies with each exposure process and is found on the respective exposed wafer. Therefore, apparatuses have been developed for both, repairing masks (e.g., repair tools / apparatuses for sample repair) and for inspecting masks (e.g. apparatuses for sample inspection, e.g., by imaging the sample).

[0007] In typical examples for apparatuses for sample inspection, the CCD camera is operated in a so-called time delay integration (TDI) mode and / or another pixel- / pixel-line-wise image acquisition mode. TDI relates to a special readout method for CCD. In TDI imaging, an x-axis may define (e.g., vertical) columns of the CCD sensor, and the y-axis (e.g., orthogonal to the x-axis) may define the direction of motion of the sample, e.g., an EUV photolithography mask. The CCD is configured to accumulate charges by converting incoming photons (light) into electric charge. In TDI, this may occur in a (CCD pixel-)line-wise fashion: As the sample moves along the y-axis, the charge maybe shifted / transferred along the x-axis during charge readout and in synchronization with said motion. By synchronizing this transfer timing with the movement of the object, exposures can be integrated equal to the number of lines of the CCD pixels. Therein, each shift corresponds to a new exposure. By accumulating the signal along the x-axis (columns) over multiple exposures, TDI may thus effectively integrate the signal and reduces noise, resulting in a high-quality image representing the motion of the sample along the y-axis.

[0008] Both, the short wavelengths at which such apparatuses for sample inspection operate and the underlying concept of image capture, e.g., TDI, require a high degree of control of the sample position and / or at least knowledge thereof. However, sample stages configured to move samples within the apparatus in a controlled way may exhibit a positional error: Such error, despite correct electric control, could stem from mechanical imperfections such as backlash, hysteresis, or mechanical play within the components of the stage. Such errors may lead to deviations in the actual position of the stage compared to the commanded position, resulting in inaccuracies in the final positioning. Additionally, factors like thermal expansion or external vibrations might contribute to minute positional discrepancies, e.g., despite accurate electric control.

[0009] These problems maybe further exacerbated by further requirements regarding positional stability of the apparatus. This may require mounting different components of the apparatus on separate vibration damping systems. This may, e.g., allow for relative motion of, e.g., an inspection means of the apparatus, e.g., on a first vibration damping system, and the sample stage, e.g., on a second vibration damping system.

[0010] Conventional apparatuses therefore use interferometers that may be configured to measure a position of the sample stage in one or more dimensions. Typically, at least three interferometers may be used to measure the position in all three dimensions and optionally, three further interferometers may be used to measure the orientation in terms of rotation about the three axes of the three dimensions. Other configurations are possible in which the position and / or orientation in fewer dimensions maybe measured. Independently from the exact measurement, the apparatuses rely in their accuracy of determining the sample position and / or orientation on the initial alignment of the interferometers.

[0011] Current approaches rely on interferometers utilizing reflections from the sample stage. During alignment, the reflections are aligned to be centered on the respective detectors. However, the points on the sample stage that reflect the light back are typically distant from the point of interest (POI) where the inspection means, e.g., comprising a light source and / or a projection optics, interacts with the sample, e.g., by directing a (e.g., focused) light beam thereon and / or imaging at least a portion of the sample. This may add an Abbe error to the interferometer measurement. Further, the inspection means and the sample stage typically may move (“float”) relative to one another due to being mechanically decoupled. The alignment with reference to a moveable reference point may introduce further deviations from an ideal alignment.

[0012] Thus, there is therefore a need to further improve apparatuses for sample inspection, methods, optical alignment means, and computer programs for interferometer alignment in such apparatuses for sample inspection and related aspects.

[0013] 3. Summary

[0014] The aspects of the present invention meet the above need at least in part.

[0015] A first aspect of the present invention relates to a method for interferometer alignment in an apparatus for sample inspection with an inspection means and a sample stage. At least a portion of at least one interferometer for monitoring a sample stage position is mechanically coupled to the inspection means. The method comprises aligning the interferometer relative to an optical alignment means coupled to the inspection means.

[0016] Mechanically coupling at least a portion of the at least one interferometer to the inspection means may advantageously improve the accuracy of the operation of the apparatus. In detail, conventional approaches typically rely on mechanically coupling one or more interferometers to the sample stage. This approach appears natural as it allows to measure the errors in sample positioning as described herein, e.g., caused by mechanical imperfections, e.g., despite correct electric control. However, the inventors understood that it maybe advantageous to measure the stage position relative to the inspection means to account for variations / errors thereof. Such variations / errors may possibly be the result of (unwanted) vibrations, errors in stage control, mechanical impacts etc.

[0017] The approach for accurate alignment, as described herein, of the at least one interferometer of the apparatus, allows to align the at least one interferometer, even in this scenario, without issues of the reference point “floating” relative to the at least one interferometer. Even further, the method described herein may not only circumvent this “floating” problem, it may optimize alignment approaches even further: By providing the optical alignment means, a suitable reference point maybe chosen, e.g., closer to the POI than it would be possible when aligning the at least one interferometer relative to the sample stage. In detail, the sample stage typically has a predetermined size that is large enough as to receive the entire sample. This results in the (reflective) sides of the sample stage that may be used for alignment in the prior art to be spaced apart from the POI by a (relatively long) predetermined distance. Such size requirements do not exist for an optical alignment means as described herein. For example, the optical alignment means maybe provided with one or more reflective surfaces much closer, or even directly at, the coordinates of the POI. The alignment to the POI may yield various advantages: Firstly, it allows control means for moving the sample and / or the sample stage carrying the sample in a coordinate system in relation to the POI without any errors related thereto. Secondly, measurement errors of the interferometer(s) are prevented: E.g., a rotation of the sample stage about the POI (or another point) may, when the interferometer would not be aligned to the POI, cause the interferometers, e.g., positioned as described herein in reference to Figs. 2a and 2b, to detect a rotation and additionally - by error - a translation, which is prevented by the alignment to the POI as described herein.

[0018] In some examples, the apparatus for sample inspection may, e.g., as described in reference to Fig. la, comprise one or more components that may be vibrationally decoupled from their surrounding and / or stabilized by a vibration damping system / (active or passive) vibration isolation system. Herein, when two components are mounted to different elements between which a vibration isolation system / a vibration damping system is arranged, they may be seen as mechanically decoupled. As a consequence, mechanically decoupled components may move relative to one another. E.g., the sample / the sample stage may move relative to the inspection means, and the least a portion of the interferometer may move relative to the sample stage.

[0019] In the special case of the at least one interferometer, most of its components may, e.g., be mounted to the inspection means while, in a typical operation, it directs its measurement beam onto one reflective surface of the sample stage which acts as one end mirror of the interferometer. Herein, such arrangement may be understood as the at least one interferometer and the sample stage as to be mechanically decoupled. Generally, it may be sufficient when a reference beam path of the at least one interferometer is fully mechanically decoupled from the sample stage for said at least one interferometer (cf. Fig. 5) to be seen as mechanically decoupled from the sample stage.

[0020] For example, the sample stage position may comprise a relative position of the inspection means and the sample stage (e.g., to one another), e.g., in at least one dimension. The at least one dimension may, e.g., comprise up to three translational dimensions and / or up to three rotational dimensions. For example, the sample stage and the inspection means maybe configured to be movable with respect to one another. Preferably, the sample stage is arranged to be movable, but in some examples also the inspection means may be movable.

[0021] The sample inspection described herein may, e.g., comprise viewing and / or imaging the sample, e.g., by capturing an aerial image of at least a part of the sample.

[0022] The sample may, e.g., comprise an EUV photolithography mask - e.g., as described in reference to Fig ib.

[0023] For example, the at least one interferometer may comprise a first interferometer configured to emit a first measurement beam and a second interferometer. Aligning the at least one interferometer may, in some examples, comprise aligning the first interferometer relative to the optical alignment means by guiding the first measurement beam at least partly through the second interferometer. This exemplary alignment realized by guiding the first measurement beam at least partly through the second interferometer, e.g., through a retroreflector of the second interferometer, may, e.g., achieve a particularly accurate and reliable alignment of the apparatus, as described herein.

[0024] Analogously, the second interferometer may be configured to emit a second measurement beam. Aligning the at least one interferometer may, in some examples, comprises aligning the second interferometer relative to the optical alignment means by guiding the second measurement beam at least partly through the first interferometer, e.g., through a retroreflector of the first interferometer.

[0025] A second aspect of the present invention relates to a method for interferometer alignment in an apparatus for sample inspection with an inspection means and a sample stage, wherein at least a portion of at least one interferometer for monitoring sample stage position is mechanically coupled to the inspection means. The at least one interferometer comprises a first interferometer configured to emit a first measurement beam and a second interferometer. The method comprises aligning the first interferometer by guiding the first measurement beam at least partly through the second interferometer.

[0026] Aligning the first interferometer by guiding the first measurement beam at least partly through the second interferometer allows multiple advantageous technical effects: It directly couples the alignment of the first and second interferometer without the need of any further means and / or steps of the method. This may thus result in a more accurate alignment and thus a more accurate operation of the aligned apparatus.

[0027] Generally, the methods described herein may be performed prior to the final assembly of the apparatus described herein. E.g., the interferometers may be aligned as an isolated interferometer system before apparatus assembly and then mounted to the apparatus as a pre-aligned system and / or the alignment as described herein may be performed before mounting the sample stage to the apparatus as described herein.

[0028] The method described herein may, e.g., further comprise aligning the sample stage relative to the interferometer(s), wherein the interferometer(s) may, e.g., be aligned as described herein. Aligning the sample stage may, e.g., comprise moving the sample stage (translationally) in at least one dimension and / or rotate it about at least one axis with high precision.

[0029] The (at least one) interferometer described herein may, e.g., have a resolution in the picometer range, preferably between i and too pm, more preferably between i and io pm (typically of approximately 2.4 pm).

[0030] In some examples, aligning the first interferometer may comprise aligning the first interferometer relative to an optical alignment means coupled to the inspection means and / or aligning the second interferometer may comprise aligning the second interferometer relative to an optical alignment means coupled to the inspection means. This may, e.g., be performed one after another or alternatingly, e.g., by (e.g., repeatedly) switching between aligning the first and the second interferometer.

[0031] This allows to realize the advantageous technical effects described herein in reference to the first aspect of the invention.

[0032] In some examples, the first interferometer may be configured to measure a position of the sample stage in a first dimension and the second interferometer may be configured to measure a position of the sample stage in a second dimension, wherein the first dimension and the second dimension may be essentially perpendicular to one another.

[0033] An essentially perpendicular orientation may advantageously allow for fully independent position measurements which simplifies any correction approach based at least partly on the corresponding interferometer measurements.

[0034] In one example, the first interferometer may, e.g., be configured to measure a first position of the sample stage along an x-axis (along the (i,o,o)-direction) and the second interferometer may, e.g., be configured to measure a second position of the sample stage along a y-axis (along the (o,i,o)-direction). In this example, the optical alignment means may, e.g., comprise a surface with a surface normal in the (i,i,o)-direction that may thus be configured to reflect the first measurement beam in a 90°-angle to the second interferometer. This way, the first and second interferometer may thus be aligned orthogonally to one another. This may advantageously decouple the measurements in the x- and y-direction. A corresponding example is described in more detail in reference to Fig. 6.

[0035] The method may, e.g., further comprise coupling the optical alignment means rigidly to the inspection means, wherein the optical alignment means is configured to reflect the first measurement beam.

[0036] Coupling the optical alignment means rigidly to the inspection means may, e.g., comprise a direct or indirect coupling, e.g., via suitable coupling means like a holder, an arm, a frame, etc.. The holder may, e.g., comprise means for moving / translating the optical alignment means in at least one dimension and / or rotate it about at least one axis with high precision (e.g., comparable to the sample stage).

[0037] In an exemplary embodiment, coupling the optical alignment means rigidly to the inspection means may comprise positioning the optical alignment means in a predetermined relative position and orientation relative to the apparatus, preferably at the POL

[0038] The predetermined relative position and orientation relative to the apparatus may ensure that the reference point relative to which the at least one interferometer may be aligned to is in a fixed and / or predetermined position relative to the at least one interferometer. Further, it may ensure that the direction in which light from the at least one interferometer may be reflected and / or redirected leads along a fixed and / or predetermined direction, preferably along the directions of motion of the sample stage. In a particularly preferred embodiment, the interferometers may thus measure the position(s) (e.g., in (orthogonal) x-, y-, and / or z-direction(s)) of the sample stage in precisely the directions along which the sample stage may move. This allows for an advantageous situation in which, when one interferometer detects a position error along a first dimension (e.g., along the x-axis) the sample stage may be controlled as to move along that dimension (e.g., along the x-axis) in order to correct for that error.

[0039] The POI may, e.g., comprise a point, a line, an area and / or a volume addressed by the inspection means. E.g., when the inspection means comprises a beam source providing an inspection beam to the sample, the POI may comprise the region whereto the beam maybe focused. This may, e.g., define a certain volume relating to the (e.g., x-y-) position determined by the lateral positioning of the sample relative to the beam and the height (e.g., in z-direction) determined, e.g., by the focus of the beam and / or the height-positioning of the sample.

[0040] In some examples, the second interferometer may comprise a second retroreflector and guiding the first measurement beam at least partly through the second interferometer may comprise guiding the first measurement beam to a first detector of the first interferometer via the optical alignment means and the second retroreflector. Vice versa, in some examples, the first interferometer may comprise a first retroreflector and guiding the second measurement beam at least partly through the first interferometer may comprise guiding the second measurement beam to a second detector of the second interferometer via the optical alignment means and the first retroreflector.

[0041] The first and / or second retroreflector may, e.g., comprise a prism and / or a corner cube prism. They may be configured to reflect light back towards its source with minimal scattering.

[0042] Thus, the back-reflection of the retroreflector(s) may allow for an essentially ideal reflection behavior that allows for the relative interferometer alignment described herein. In detail, the first and second interferometers may be aligned in a plane and orthogonally to one another.

[0043] Notably, guiding the measurement beam of one interferometer at least partly through another interferometer deviates from the natural measurement path of the interferometers described herein. The natural measurement path of each interferometer may be defined as the interferometer path that is passed by the measurement beam and / or the reference beam of the respective interferometer when reflected a respective reference beam end mirror, respectively. The natural beam path of an interferometer may, e.g., particularly be seen as a beam path that does not overlap with the beam path of any other interferometer and / or not lead through another interferometer. IO

[0044] In some examples, the optical alignment means may comprise at least one element, preferably a first reflective surface. Therein, the optical alignment means and / or the at least one element may be configured to redirect the first measurement beam to the first detector by an essentially orthogonal deflection.

[0045] Thereby, the optical alignment means differs from a conventional mirror that might be positioned such that it reflects the first measurement beam to the first detector. However, a conventional mirror cannot do so under essentially orthogonal deflection. The orthogonal deflection may, e.g., advantageously allow to guide the measurement beam at least partly through another interferometer, e.g., in a 90°-angle relative to the fist interferometer.

[0046] In some examples, the aligning the at least one interferometer may comprise optimizing an intensity detected by the first detector by adjusting the position and / or rotation of the at least one interferometer.

[0047] For example, adjusting the position and / or rotation of the at least one interferometer may comprise adjusting the settings of a 6-degrees-of-freedom (6D0F) stage on which the at least one interferometer may be mounted. In some examples, two or more interferometers may, e.g., be placed on separate (e.g., 6D0F) stages.

[0048] The method may, e.g., further comprise aligning the second interferometer, preferably by guiding the first measurement beam at least partly through the first interferometer.

[0049] The as-achieved mutual alignment may, e.g., result in a particularly good adjustment of the two interferometers relative to one another. This may result in a precise and accurate operation of the as-aligned apparatus.

[0050] In some examples, the optical alignment means may further be configured to allow for alignment of at least two interferometers with the optical alignment means in the same position. Thereby, no repositioning of the optical alignment means may be required which may simplify and accelerate the method. Saving time maybe particularly advantageous when re-alignment may be required in the apparatus described herein. As typically, such apparatuses are expensive and their demand may be high, saving time may result in high cost savings.

[0051] In some examples, the optical alignment means may further comprise a coupling means for coupling to the apparatus, preferably to the inspection means, in a predetermined relative position and orientation and / or at least one reflective surface (e.g., a first, second, third, and / or fourth surface as described herein) configured to reflect light of the at least one interferometer.

[0052] Thereby, the at least one surface may, e.g., be positioned and / or oriented relative to the apparatus and thus the at least one interferometer resulting in a high accuracy achieved through the alignment described herein.

[0053] A third aspect of the present invention relates to a computer program comprising instructions for executing the steps of the method described herein, e.g., by an apparatus for sample inspection, e.g., as described herein.

[0054] A computer program can be written in any form of programming language, including compiled or interpreted languages, and it can be deployed in any form, including as a stand-alone program or as a module, component, subroutine, or other unit suitable for use in a computing environment.

[0055] A fourth aspect of the present invention relates to an optical alignment means for interferometer alignment in an apparatus comprising an inspection means and at least one interferometer, wherein the optical alignment means is configured for use in a method for interferometer alignment in an apparatus for sample inspection as described herein.

[0056] Such optical alignment means provides the advantages described herein in reference to the respective method as it poses the central component therein that allows for the reliably and accurate alignment of the involved interferometer(s). For example, other sources than light sources may be possible in other embodiments, e.g., (charged or uncharged) particle (e.g., electrons, ions, neutrons, ...) sources.

[0057] In some examples, the optical alignment means may comprise a first reflective surface, a second reflective surface, and a third reflective surface. A normal vector of the second reflective surface and a normal vector of the third reflective surface may form an angle of about 900(e.g., with an accuracy of ± o.i°, preferably 0.05°, most preferably of ± o.oi°) and / or a normal vector of the first reflective surface may form an angle of about 45° (e.g., with an accuracy of ± o.i°, preferably 0.05°, most preferably of ± o.oi°) with the normal vector of the second reflective surface and / or the third reflective surface.

[0058] Such optical alignment means is particularly advantageous at it allows for full x-y- alignment as well as full orientational alignment. A corresponding exemplary embodiment of the optical alignment means is described in reference to Fig. 3.

[0059] In some examples, the optical alignment means may comprise a fourth reflective surface, wherein a normal vector of the fourth reflective surface may form an angle between io° and 8o°, preferably between io° and 40°, with the normal vector of the second surface.

[0060] Thereby, the optical alignment means can reflect light of an interferometer configured to measure a position of the sample stage in the z-direction (i.e., the direction perpendicular to the sample surface).

[0061] The first, second, third, and / or fourth reflective surfaces described herein may comprise essentially planar portions, e.g., described by the normal vectors herein. The essentially planar portions of different surfaces may be separated by edges of the optical alignment means and / or be connected by, e.g., curved portions of the optical alignment means.

[0062] A fifth aspect of the present invention relates to an apparatus for sample inspection.

[0063] The apparatus comprises an inspection means, a sample stage, at least one interferometer for monitoring a sample stage position, wherein at least a portion of the at least one interferometer is mechanically coupled to the inspection means, and an optical alignment means for aligning the at least one interferometer, wherein the optical alignment means is coupled to the inspection means.

[0064] The apparatus may thus execute the method as described herein and by being aligned accordingly, it may exhibit the advantages described herein. Conventional apparatuses may not be able to receive an according optical alignment tool as typically the sample stage which may be used for (less accurate and / or reliable) alignment may block the volume required to place the optical alignment means within the (conventional) apparatus. E.g., the apparatus described herein may therefore comprise means for displacing the sample stage such as to allow for introducing the optical alignment means into the apparatus as described herein. This may improve the operation accuracy and / or reliability of the apparatus.

[0065] A sixth aspect of the present invention relates to an apparatus for sample inspection, comprising an inspection means, a sample stage, at least one interferometer for monitoring a sample stage position, wherein at least a portion of the at least one interferometer is mechanically coupled to the inspection means and comprises a first interferometer configured to emit a first measurement beam and a second interferometer, and an optical alignment means guiding the first measurement beam at least partly through the second interferometer for aligning the at least one interferometer.

[0066] By being able to be aligned by guiding the first measurement beam at least partly through the second interferometer, the apparatus achieves the advantageous effects described herein in reference to the corresponding method for interferometer alignment. This may improve the operation accuracy and / or reliability of the apparatus.

[0067] A seventh aspect of the present invention relates to an apparatus for sample inspection aligned according to a method for interferometer alignment according to any of the aspects of the invention as described herein. The advantageous alignment of the apparatus may improve its operation accuracy and / or reliability. In some examples, the apparatus may further comprise a receptacle for coupling the optical alignment means, as described herein, to the apparatus in a predetermined relative position and orientation.

[0068] This may improve the reliability of the alignment depending on the optical alignment means and / or coupling the optical alignment means to the apparatus may be accelerated and simplified. Further, it ensures that no (potentially disturbing) motion between the optical alignment means and the POI occurs.

[0069] The receptacle may, e.g., be configured to couple to the optical alignment means in a space-locking, force-locking, and / or friction-locking way. In some examples, the optical alignment means and the receptacle may comprise at least one surface each, that are mutually adapted such that when the two surfaces contact each other, they may be restricted in their relative motion (e.g., comprising relative translation and rotation).

[0070] In some examples, the at least one interferometer may comprise a plurality of interferometers, preferably six interferometers, configured to determine a position and orientation of a sample in six degrees of freedom. The six degrees of freedom may, e.g., relate to three translations (e.g., perpendicular to one another and / or in x-, y-, and z- direction) and to three rotations (e.g., perpendicular to one another and / or about the x- , y-, and z-axes).

[0071] Thereby, a full 6D0F determination of the (relative) sample stage position (and thus the sample position) may be achieved. This allows to account for all position errors that might occur during operation of the apparatus. In other examples, e.g., when rotations may be excluded, the at least one interferometer may comprise three interferometers, e.g., configured to measure x-, y-, and z-positions.

[0072] In some examples, the at least one interferometer may comprise a light source configured to emit a beam, a beam splitter configured to split the beam into a reference beam and a measurement beam, a retroreflector, and a detector configured to receive the reference beam and the measurement beam. For example, the light source, the beam splitter, the retroreflector, and / or the detector may be mechanically coupled to the inspection means such that at least a portion of at least one interferometer is mechanically coupled to the inspection means. This may, e.g., apply particularly to the first and / or interferometer described herein, e.g., in the scenario in which the first measurement beam may be guided at least partly through the second interferometer (e.g., via the second retroreflector) and / or the second measurement beam may be guided at least partly through the first interferometer (e.g., via the first retroreflector).

[0073] In some examples, the inspection means may comprise an (EUV) camera, a projection optics configured to image at least a part of a sample on the sample stage (and optionally to project it onto the (EUV) camera) and / or an (EUV) light source. Herein, the (EUV) wavelength range may, e.g., be between io nm and 15 nm, preferably at about 13.5 nm.

[0074] An eight aspect of the present invention relates to an interferometer for an apparatus for sample inspection with an inspection means and a sample stage. Therein, the interferometer is configured for monitoring a sample stage position, when at least a portion of the at least one interferometer is mechanically coupled to the inspection means.

[0075] In some examples, the interferometer may further be configured for being aligned by an optical alignment means for aligning the at least one interferometer, wherein the optical alignment means may, e.g., be coupled to the inspection means.

[0076] In some examples, the interferometer may comprise a first interferometer configured to emit a first measurement beam and a second interferometer. For example, the interferometer may be configured such that an optical alignment means can guide the first measurement beam at least partly through the second interferometer for aligning the interferometer.

[0077] A ninth aspect of the present invention relates to an interferometer aligned according to a method as described herein. In some examples, the interferometer may further comprise a plurality of interferometers, preferably six interferometers, configured to determine a position and orientation of a sample in six degrees of freedom.

[0078] In some examples, the interferometer may comprise at least one interferometer comprising: a light source configured to emit a beam, a beam splitter configured to split the beam into a reference beam and a measurement beam, a retroreflector, and / or a detector configured to receive the reference beam and the measurement beam.

[0079] Such interferometers may, e.g., achieve the advantageous technical effects described herein in reference to the apparatus for sample inspection which may comprise at least one corresponding interferometer.

[0080] Generally, any functionality described herein in reference to the apparatus and / or the optical alignment means maybe implemented as a step of a method and / or and instruction of a computer, and vice versa.

[0081] In some implementations, the apparatus and / or the inspection means can comprise a light or radiation source to generate light or radiation, an image sensor / camera (e.g., CCD or CMOS (complementary metal oxide semiconductor) sensor) having an array of individually addressable sensing elements for capturing images of a sample, and optics (e.g., one or more lenses, mirrors or reflecting surfaces, filters, and / or image stops) to direct and / or focus light or radiation from the one or more light or radiation source to the sample, and from the sample to the image sensor. In some implementations, the apparatus can include one or more computers that include one or more data processors configured to execute one or more programs that include a plurality of instructions according to the principles described above. Each data processor can include one or more processor cores, and each processor core can include logic circuitry for processing data. For example, a data processor can include an arithmetic and logic unit (ALU), a control unit, and various registers. Each data processor can include cache memory. Each data processor can include a system-on-chip (SoC) that includes multiple processor cores, random access memory, graphics processing units, one or more controllers, and one or more communication modules. Each data processor can include millions or billions of transistors. The processing of data described in this document, such as capturing an aerial image, can be carried out using one or more computers, which can include one or more data processors for processing data, one or more storage devices for storing data, and / or one or more computer programs including instructions that when executed by the one or more computers cause the one or more computers to carry out the processes. The one or more computers can include one or more input devices, such as a keyboard, a mouse, a touchpad, and / or a voice command input module, and one or more output devices, such as a display, and / or an audio speaker.

[0082] In some implementations, the one or more computing devices can include digital electronic circuitry, computer hardware, firmware, software, or any combination of the above. The features related to processing of data can be implemented in a computer program product tangibly embodied in an information carrier, e.g., in a machine- readable storage device, for execution by a programmable processor; and method steps can be performed by a programmable processor executing a program of instructions to perform functions of the described implementations. Alternatively or in addition, the program instructions can be encoded on a propagated signal that is an artificially generated signal, e.g., a machine-generated electrical, optical, or electromagnetic signal, that is generated to encode information for transmission to suitable receiver apparatus for execution by a programmable processor.

[0083] For example, the one or more computers can be configured to be suitable for the execution of a computer program and can include, by way of example, both general and special purpose microprocessors, and any one or more processors of any kind of digital computer. Generally, a processor will receive instructions and data from a read-only storage area or a random access storage area or both. Elements of a computer system include one or more processors for executing instructions and one or more storage area devices for storing instructions and data. Generally, a computer system will also include, or be operatively coupled to receive data from, or transfer data to, or both, one or more machine-readable storage media, such as hard drives, magnetic disks, solid state drives, magneto-optical disks, or optical disks. Machine-readable storage media suitable for embodying computer program instructions and data include various forms of non-volatile storage area, including by way of example, semiconductor storage devices, e.g., EPROM, EEPROM, flash storage devices, and solid state drives; magnetic disks, e.g., internal hard disks or removable disks; magneto-optical disks; and CD- ROM, DVD-ROM, and / or Blu-ray discs.

[0084] In some implementations, the processes described above can be implemented using software for execution on one or more mobile computing devices, one or more local computing devices, and / or one or more remote computing devices (which can be, e.g., cloud computing devices). For instance, the software forms procedures in one or more computer programs that execute on one or more programmed or programmable computer systems, either in the mobile computing devices, local computing devices, or remote computing systems (which may be of various architectures such as distributed, client / server, grid, or cloud), each including at least one processor, at least one data storage system (including volatile and non-volatile memory and / or storage elements), at least one wired or wireless input device or port, and at least one wired or wireless output device or port.

[0085] In some implementations, the software may be provided on a medium, such as CD- ROM, DVD-ROM, Blu-ray disc, a solid state drive, or a hard drive, readable by a general or special purpose programmable computer or delivered (encoded in a propagated signal) over a network to the computer where it is executed. The functions can be performed on a special purpose computer, or using special-purpose hardware, such as coprocessors. The software can be implemented in a distributed manner in which different parts of the computation specified by the software are performed by different computers. Each such computer program is preferably stored on or downloaded to a storage media or device (e.g., solid state memory or media, or magnetic or optical media) readable by a general or special purpose programmable computer, for configuring and operating the computer when the storage media or device is read by the computer system to perform the procedures described herein. The inventive system can also be considered to be implemented as a computer-readable storage medium, configured with a computer program, where the storage medium so configured causes a computer system to operate in a specific and predefined manner to perform the functions described herein. The embodiments of the present invention that are described in this specification and the optional features and properties respectively mentioned in this regard should also be understood to be disclosed in all combinations with one another. In particular, in the present case, the description of a feature comprised by an embodiment - unless explicitly explained to the contrary - should also not be understood such that the feature is essential or indispensable for the function of the embodiment.

[0086] 4. Short description of the figures

[0087] Fig. la shows an exemplary apparatus for sample inspection.

[0088] Fig. lb shows an exemplary EUV photolithography mask.

[0089] Fig. 2a shows a plurality of interferometers of an exemplary apparatus for sample inspection configured to measure a position of an exemplary sample stage.

[0090] Fig. 2b shows a schematic illustration of the working principle of a plurality of interferometers for position and orientation measurements.

[0091] Fig. 3 shows a three-dimensional view of a plurality of interferometers of an exemplary apparatus for sample inspection during alignment with an optical alignment means.

[0092] Fig. 4 shows a top view of a plurality of interferometers of an exemplary apparatus for sample inspection during alignment with an optical alignment means.

[0093] Fig. 5 shows a schematic representation of an exemplary interferometer of an exemplary apparatus for sample inspection.

[0094] Fig. 6 shows a schematic illustration of an interferometer alignment comprising guiding the first measurement beam at least partly through the second interferometer.

[0095] 5- Detailed description Fig. la shows an exemplary apparatus for sample inspection 1000. The exemplary apparatus 1000 comprises a sample stage 1010 that may move in up to six dimensions (e.g., translational motion in x-, y-, and / or z-direction). The sample stage position may be measured by one ore more interferometers 1021, 1023. The inspection means 1040 may, in the example of Fig. la, comprise a means to emit and / or deflect a light beam 1030, e.g., a EUV beam, preferably of ca. 13.5 nm wavelength. Said light beam 1030 maybe directed onto the sample at a point of interest POI, e.g., as described herein. Both, the one or more interferometers 1021, 1023 and the inspection means 1040 may be mounted to the same frame 1060, as shown in Fig. la. Said frame 1050 in turn, may e.g., be mounted to a stabilization system, e.g., comprising a further frame 1060 that maybe vibrationally decoupled from its surrounding and / or stabilized by a vibration damping system / (active) vibration isolation system (AVIS) 1061, 1062. Said AVIS 1061, 1062 may allow mounting the further frame 1060 to a chamber 1100 of the apparatus 1000 without exposing the inspection means 1040 and / or the one or more interferometers 1021, 1023 to the vibrations thereof that would possibly impede their functionalities. The further frame 1060 and / or the frame 1050 may thus not be rigidly coupled to the environment but rather, e.g., comprise a dynamic link 1170 to the frame. Said dynamic link 1170 may, e.g., comprise electrical connections and / or other supply means, e.g., for cooling, heating, etc. The dynamic link 1170, however, is dynamic in such that it essentially does not couple the frame 1060 and the chamber 110 mechanically.

[0096] The chamber 1100 may, e.g., be equipped with a working atmosphere control means 1110, e.g., comprising a vacuum pump configured to generate a vacuum and / or flushing means configured to control the pressure and / or composition of the chamber-internal atmosphere, e.g., by flushing it with a certain gas, like e.g., H2, Ar, N2, or mixtures thereof. Thereby, the working atmosphere within the chamber 1100 may be controlled accordingly. Generally, also in other examples than the one shown in Fig. la, an internal pressure within at least a part of the apparatus (e.g., the chamber 1100 and / or a volume comprising the sample stage 1010, the inspection means 1040, and / or the at least one interferometer 1021, 1023) may, e.g., be below i-io2mbar, below 1 mbar, or below i-io2mbar and / or said internal pressure may be above i-io-?, above i-icr 7 or above i-io-6. The chamber noo may, e.g., further provide an image acquisition unit 1090, e.g., comprising a camera 1080 configured to acquire images 1081 of a sample provided at the sample stage 1010 and providing said images 1081 to the image acquisition unit 1090. Both, the image acquisition unit 1090 and the camera 1080 may, in some examples, be understood as components of the inspection means 1040. As the camera 1080 (and the image acquisition unit 1090) may thus move relative to the further frame 1060 and thus the components of inspection means 1040 mounted fixedly to further frame 1060 as well as relative to the at least one interferometer 1021, 1023, a further sensor is provided to measure the relative position of the camera 1080 relative to the inspection means 1040, the further frame 1060, and / or the at least one interferometer 1021, 1023 / ... . The chamber 1100 of the apparatus 1000 may, e.g., rest on a pedestal 1150, e.g., in a mechanically damped way.

[0097] The sample stage 1010 may be configured to hold and / or move a sample, e.g., a EUV photolithography mask. The sample stage 1010 may be mounted on a carrier short stroke 1110, which in turn may be mounted onto a long stroke 1120, either of which may, e.g., comprise means for moving, translating, and / or rotating the sample stage 1010 as described herein. Additionally or alternatively, the sample stage 1010 itself may execute said moving moving / translating / rotating / ... of itself and thus the sample mounted thereon. Said long stroke may, e.g., in the embodiment of Fig. la be mounted onto an adjustment plate 1130, wherein the adjustment plate 1130 and the long stroke 1120 maybe connected via a supply link 1140, e.g., comprising electrical connections, sensing capabilities, and / or other supply means, e.g., for cooling, heating, etc.

[0098] The apparatus 1000 may comprise an external control means 1060 which may comprise one or more elements, e.g., a computer for control and / or read-out, a light source configured to provide the light beam to the inspection means 1040, a cooling system configured to cool the apparatus and / or a part thereof, etc..

[0099] Fig. lb shows an EUV photolithography mask 100 comprising a multilayer system 130 The mask 100 may comprise a layered architecture. In the example of Fig. 1, the layers may extend in the horizontal plane. The layers may typically have lateral dimensions in the horizontal plane that extend the thickness in the vertical direction (i.e., the stacking direction of the layers) by many orders of magnitude. The exemplary mask 100 comprises a backside coating no applied onto a substrate 120. The backside coating may, e.g., comprise TaB and / or have a thickness between io nm and 500 nm, preferably between 50 nm and 100 nm. The substrate may, e.g., comprise a low thermal expansion material and / or have a thickness in the mm range, preferably between 1 mm and 10 mm, more preferably between 6 mm and 7 mm. The three dots illustrate that the drawing of Fig. ib is not scaled but the substrate may be much thicker than shown here.

[0100] The mask 100 may further comprises a multilayer system 130 comprising two alternatingly stacked layers, a first layer 131 and a second layer 132. For example, the first layer 131 and the second layer 132 may comprise materials with different refractive indices. This allows them to form a reflective Bragg mirror, e.g., as described herein. The first layer 131 may, e.g., comprise Si and / or have a thickness of ca. 4.0 nm as shown in the example of Fig. lb. The second layer 132 may, e.g., comprise Mo and / or have a thickness of ca. 2.9 nm as shown in the example of Fig. ib. Again, the three dots illustrate that the drawing of Fig. ib is not scaled but the multilayer structure 130 may be much thicker than shown here. Typically, multilayer systems 130 may comprise between 10 and 200 bilayers (e.g., each comprising one first layer 131 and one second layer 132), e.g., resulting in thicknesses of the multilayer system 130 in the nanometer range. The multilayer system maybe terminated by a final layer 133. In the example of Fig. ib, the final layer 133 may comprise Si and / or have a thickness that is different from the first and / or second layer 131, 132 and / or may be between 3 nm and 11 nm. Other exemplaiy masks may, e.g., comprise three or more alternating layers.

[0101] The mask 100 may, e.g., further comprise a cap layer 140. The cap layer 140 may, e.g., comprise ruthenium (Ru). The multilayer system 130 may thus be sandwiched between the substrate 120 and the cap layer 140. The cap layer 140 maybe attached to the multilayer system 130 on the one side and to an absorber layer 150 on the other side. The absorber layer 150 may be patterned such as to allow it to be used in a photolithography mask.

[0102] A plurality of interferometers, for example as shown in Fig. 2a, may, e.g., by comprising grating-based interferometers, allow for 6D0F position (or called displacement) and orientation (or called angle) measurements. The portion shown in Fig. 2a may comprise three identical interferometers, e.g., sharing the same light source or comprising a separate light source each. Each detection part / interferometer i02iy, i02irx, i02irz may, e.g., be based on heterodyne, grating shearing, and / or Michelson interferometry.

[0103] Fig. 2b shows a schematic illustration of the working principle of a plurality of interferometers i02iy, i02irx, i02irz for position and orientation measurements.

[0104] Position information in the three perpendicular directions (x, y, z) may be sensed, e.g., simultaneously, by each of the interferometers i02iy (and interferometers 1021X and 1021Z of Fig. 2a). In this example, only the position information of the interferometer i02iy may be translated into a position information of the sample stage and / or the optical alignment means, depending on which of these is positioned in the measurement beam(s).

[0105] The other two interferometers i02irx, i02irz may relate to orientation / angle information (0X, 0y, 0Z) as illustrated in the three scenarios of Fig. 2b. The orientation information may be obtained by comparing the displacement measurement results between two corresponding interferometers i02iy, i02irx, i02irz.

[0106] In the first scenario (left panel of Fig. 2b), the reflective surface (illustrated by black, grid-like structure) is rotated about the z-axis by an angle 0Z. The shaded surface illustrates the reference position of the surface for all three scenarios of Fig. 2b. In the exemplary first scenario, the interferometers i02iy, i02irx measure the same displacement relative to the tilted surface and the interferometer i02irz measures a different displacement relative to the tilted surface. Thus, one can derive that the rotation must have occurred about the z-axis. The angle information, e.g., the angle 0Z, may be derived from comparing the displacements measured by interferometers i02iy, i02irx with the displacement measured by the interferometer i02irz.

[0107] In the second scenario (central panel of Fig. 2b), the reflective surface (illustrated by black, grid-like structure) is rotated about the x-axis by an angle 0X. In the exemplary second scenario, the interferometers i02iy, i02irz measure the same displacement relative to the tilted surface and the interferometer i02irx measures a different displacement relative to the tilted surface. Thus, one can derive that the rotation must have occurred about the x-axis. The angle information, e.g., the angle 0X, may be derived from comparing the displacements measured by interferometers i02iy, i02irz with the displacement measured by the interferometer i02irx.

[0108] In the third scenario (right panel of Fig. 2b), the reflective surface (illustrated by black, grid-like structure) is rotated about the y-axis by an angle 0y. As it can be seen from Fig. 2b, none of the interferometers i02iy, i02irx, i02irz measures a displacement induced by the rotation of the sample about the y-axis as such rotation is exactly perpendicular to the measurement beam paths of all three interferometers i02iy, i02irx, i02irz. Thus, another plurality of interferometers (e.g., interferometers 1022X, i022ry of Fig. 2a) are required to measure said angle 0y.

[0109] The concept explained in reference to Fig. 2b may be transferred to any other plurality of interferometers and / or other interferometer configurations.

[0110] Fig. 3 shows a three-dimensional view of a plurality of interferometers i02iy, i02irx, i02irz, 1022X, i022ry, 1023Z of an exemplary apparatus (e.g., as shown in Fig. la) for sample inspection during alignment with an optical alignment means 2000.

[0111] In the exemplary embodiment of Fig. 3, the interferometers i02iy, i02irx, i02irz, 1022X, i022ry, 1023Z are comprised in three interferometer units 1021, 1022, 1023. The first interferometer unit 1021 comprises the interferometers i02iy, i02irx, i02irz, the second interferometer unit 1022 comprises the interferometers 1022X, i022ry and the third interferometer unit 1023 comprises the interferometer 1023Z.

[0112] The optical alignment means 2000 comprises a first reflective surface 2001 with a surface normal in the (i,i,o)-direction. It is thus configured to redirect the measurement beam from the interferometer i02iy back to the interferometer i02iy (in detail to its detector) by an essentially orthogonal deflection which guides the as- deflected measurement beam at least partly through the interferometer 1022X (and from there back to the interferometer i02iy). Thereby, the interferometers i02iy, 1022X are coupled to one another in the example of Fig. 3. The measurement beams of all the other interferometers i02irx, i02irz, i022ry, 1023Z are all essentially reflected directly back to the respective interferometers i02irx, i02irz, i022ry, 1023Z by the respective surfaces 2002, 2003, 2004 of the optical alignment means 2000. In detail, the second surface 2002 with a surface normal in the (i,o,o)-direction may, e.g., as shown in Fig. 3, be configured to reflect the light from the interferometers i02irx, i02irz of the first interferometer unit 1021. The third surface 2003 with a surface normal in the (o,i,o)-direction may, e.g., as shown in Fig. 3, be configured to reflect the light from the interferometer i022ry of the second interferometer unit 1022. The fourth surface 2004 with a surface normal in the (cos(a),o,sin(a))-direction may, e.g., as shown in Fig. 3, be configured to reflect the light from the interferometer 1023Z of the third interferometer unit 1023. Therein, a indicates the angle between the fourth surface 2004 and the second surface 2002. For 0=90°, the surface would be entirely horizontal. Typically, as shown in Fig. 3, the fourth surface 2004 may have an angle a that is neither o° nor 90°. In such examples, the z-resolution may be worse than the x- or y- resolution. However, typically, the z- resolution is less decisive for the final accuracy of the operation of the apparatus.

[0113] In some examples, each of the interferometer units 1021, 1022, 1023 may comprise a 6D0F stage for repositioning and / or rotating the interferometer units 1021, 1022, 1023 each as a whole. The same may apply to any situation with lass, more and / or differently arranged interferometers.

[0114] Fig. 4 shows a top view of a plurality of interferometers i02iy, i02irx, i02irz, 1022X, i022ry, 1023Z of an exemplary apparatus for sample inspection during alignment with an optical alignment means. Essentially, Fig. 4 shows the same situation as Fig. 3. Therein, the exact beam path, especially between the coupled interferometers i02iy, 1022X can be seen.

[0115] Fig. 5 shows a schematic representation of an exemplary interferometer 3000 of an exemplary apparatus for sample inspection. In detail, the exemplary interferometer 3000 comprises three sub-components: a light source 3100, a light deflection unit 3200, and a detector 3300. The light path shown in Fig. 5 may correspond to a natural beam path / natural measurement path mentioned herein. The light source 3100 may, e.g., be configured to emit a light beam comprising unpolarized light that may logically be separated into a horizontally polarized portion, which may form the measurement beam 3110a, 3110b (dashed line), and a vertically polarized portion, which may form the reference beam 3120a, 3120b (dotted line). The light deflection unit 3200 of Fig. 5 comprises a polarizing beam splitter 3210 comprising a beam splitting surface 3211 that is oriented in a 450angle relative to the incoming light 3110a, 3120a such that the horizontally polarized portion 3110a is transmitted through the beam splitting surface 3211 and the vertically polarized portion 3120a is reflected in a 90° angle.

[0116] The reference beam (dotted line) follows the following path through the interferometer 3000:

[0117] The light deflection unit 3200 further comprises two quarter wave plates 3230, 3240 (in the representation of Fig. 5 on the upper and right side of the polarizing beam splitter (cube) 3210) and a mirror coating 3250 (on the in the representation of Fig. 5 upper side of the polarizing beam splitter (cube) 3210).

[0118] Thus, the reflected (vertically polarized) portion of the light passes the quarter wave plate 3240 a first time, is reflected by mirror 3250, and passes the quarter wave plate 3240 a second time. By passing the quarter wave plate 3240 twice, its polarization is turned by 90° from vertical to horizontal. Thus, it then passes the beam splitting surface 3211 when it impinges thereon.

[0119] The light deflection unit 3200 further comprises a retroreflector 3220 (in the representation of Fig. 5 on the lower side of the polarizing beam splitter (cube) 3210). Thus, the reference beam (dotted line) is reflected by the retroreflector 3210, passes the quarter wave plate 3240 a third time, is reflected by mirror 3250, and passes the quarter wave plate 3240 a fourth time. By passing the quarter wave plate 3240 twice (again), its polarization is turned by 90° from horizontal to vertical. Thus, when it impinges on the beam splitting surface 3211, again, it is reflected thereby to the detector 3300. The measurement beam (dashed line) follows the following path through the interferometer 3000:

[0120] The transmitted (horizontally polarized) portion of the light passes the quarter wave plate 3230 a first time, is reflected by the optical alignment means 2000 (the explanation of Fig. 5, however, analogously applies to any other reflecting surface, as well), and passes the quarter wave plate 3230 a second time. By passing the quarter wave plate 3230 twice, its polarization is turned by 90° from horizontal to vertical. Thus, it is then reflected by the beam splitting surface 3211 (in a 90° angle) when it impinges thereon and is redirected to the retroreflector 3220.

[0121] Then, the measurement beam (dashed line) is reflected by the retroreflector 3220, which guides the measurement beam back to the beam splitting surface 3211 which reflects it again (in a 90° angle). The as-reflected beam then passes the quarter wave plate 3230 a third time, is reflected by mirror 3250, and passes the quarter wave plate 3230 a fourth time. By passing the quarter wave plate 3230 twice (again), its polarization is turned by 90° from vertical to horizontal. Thus, when it impinges on the beam splitting surface 3211, again, it is transmitted thereby to the detector 3300.

[0122] There, the measurement beam 3110b and the reference beam 3120b overlap and may yield the interferometric signal as in conventional (generally known) interferometry.

[0123] This concept may, e.g., directly be applied to the interferometers i02irx, i02irz, i022ry, 1023Z when their light is reflected by the respective surfaces 2002, 2003, 2004 of the optical alignment means 2000 in Figs. 3 and 4. The (in the example of Figs 3 and 4) coupled interferometers i02iy, 1022X may, however, interact slightly differently as described in reference to Fig. 6:

[0124] Fig. 6 shows a schematic illustration of an interferometer alignment comprising guiding the first measurement beam 3110a, 3110b at least partly through the second interferometer 4000, in the example of Fig. 6 by orthogonal reflection at the first surface 2001 of the optical alignment means 2000 of Figs. 3 and 4. In detail, the measurement beam 3120a, 3120b takes the exact same path as described in reference to Fig. 5. The measurement beam 3110a, 3110b (dashed line), however, takes a substantially different path:

[0125] The horizontally polarized portion of the light that is transmitted by the beam splitting surface 3211 passes the quarter wave plate 3230 (such that the initially horizontally polarized light is the circularly polarized) and is reflected by the surface 2001 such that it is guided to the further interferometer 4000. The further interferometer 4000 essentially comprises the same components as the interferometer 3000. The reflected (now circularly polarized) measurement beam passes the quarter wave plate 4230. By passing the quarter wave plate 4230, its polarization turns from circularly polarized to vertically polarized. Thus, it is then reflected by the beam splitting surface 4211 (in a 90° angle) when it impinges thereon and is redirected to the retroreflector 4220 of the further interferometer 4000.

[0126] Then, the measurement beam (dashed line) is reflected by the retroreflector 4220, which guides the measurement beam back to the beam splitting surface 4211 which reflects it again (in a 90° angle). The reflected beam then passes the quarter wave plate 4230 a third time, is reflected by mirror 3250, and passes the quarter wave plate 3230 (such that the initially horizontally polarized light is the circularly polarized) and is reflected by the surface 2001 such that it is guided back to the interferometer 3000. There, by passing the quarter wave plate 3230 (again), its polarization turns from circularly polarized to horizontal. Thus, when it impinges on the beam splitting surface 3211, again, it is transmitted thereby to the detector 3300.

[0127] There, the measurement beam 3110b and the reference beam 3120b overlap and may yield the interferometric signal as in conventional (generally known) interferometry.

[0128] Essentially, the coupling in this example may utilize the retroreflector 4220 of the further interferometer 4000 instead of the retroreflector 3220 of the interferometer 3000. Apart from that, the fundamental physics remains unchanged compared to the natural beam path illustrated in Fig. 5.

[0129] 6. Embodiments The present invention provides the following embodiments:

[0130] 1. A method for interferometer alignment in an apparatus for sample inspection with an inspection means and a sample stage, wherein at least a portion of at least one interferometer for monitoring a sample stage position is mechanically coupled to the inspection means, wherein the method comprises: aligning the interferometer relative to an optical alignment means coupled to the inspection means.

[0131] 2. Method of embodiment i, wherein the at least one interferometer comprises a first interferometer configured to emit a first measurement beam and a second interferometer; and aligning the at least one interferometer comprises aligning the first interferometer relative to the optical alignment means by guiding the first measurement beam at least partly through the second interferometer.

[0132] 3. A method for interferometer alignment in an apparatus for sample inspection with an inspection means and a sample stage, wherein at least a portion of at least one interferometer for monitoring sample stage position is mechanically coupled to the inspection means and comprises a first interferometer configured to emit a first measurement beam and a second interferometer, wherein the method comprises: aligning the first interferometer by guiding the first measurement beam at least partly through the second interferometer.

[0133] 4. Method of embodiment 3, wherein aligning the first interferometer comprises aligning the interferometer relative to an optical alignment means coupled to the inspection means.

[0134] 5. Method of any of the embodiments 2-4, wherein: the first interferometer is configured to measure a position of the sample stage in a first dimension; the second interferometer is configured to measure a position of the sample stage in a second dimension; and the first dimension and the second dimension are essentially perpendicular to one another. Method of any of embodiments 2-5, further comprising coupling the optical alignment means rigidly to the inspection means; wherein the optical alignment means is configured to reflect the first measurement beam. Method of embodiment 6, wherein the coupling the optical alignment means rigidly to the inspection means comprises positioning the optical alignment means in a predetermined relative position and orientation relative to the apparatus, preferably at a point of interest. Method of any of embodiments 2-7, wherein the second interferometer comprises a second retroreflector; and guiding the first measurement beam at least partly through the second interferometer comprises guiding the first measurement beam to a first detector of the first interferometer via the optical alignment means and the second retroreflector. Method of any of embodiments 2-8, wherein the optical alignment means comprises at least one element, preferably a first reflective surface, configured to redirect the first measurement beam to the first detector by an essentially orthogonal deflection. Method of any of the previous embodiments, wherein the aligning the at least one interferometer comprises optimizing an intensity detected by the first detector by adjusting the position and / or rotation of the at least one interferometer. 11. Method of any of embodiments 2-10, further comprising aligning the second interferometer; preferably by guiding the first measurement beam at least partly through the first interferometer.

[0135] 12. Method of any of the previous embodiments, wherein the optical alignment means is further configured to allow for alignment of at least two interferometers with the optical alignment means in the same position.

[0136] 13. Method of any of the previous embodiments, wherein the optical alignment means further comprises a coupling means for coupling to the apparatus in a predetermined relative position and orientation; and / or at least one reflective surface configured to reflect light of the at least one interferometer.

[0137] 14. Method of any of the previous embodiments, wherein the inspection means comprises an extreme ultraviolet (EUV) camera and / or an EUV light source.

[0138] 15. An optical alignment means for interferometer alignment, in an apparatus comprising an inspection means and at least one interferometer, wherein the optical alignment means is configured for use in a method according to any of embodiments 1-14.

[0139] 16. Optical alignment means of embodiment 15, wherein the optical alignment means comprises a first reflective surface, a second reflective surface, and a third reflective surface; wherein a normal vector of the second reflective surface and a normal vector of the third reflective surface form an angle of about 90°; and / or wherein a normal vector of the first reflective surface forms an angle of about 450with the normal vector of the second reflective surface and / or the third reflective surface.

[0140] 17. Optical alignment means of embodiment 16, wherein the optical alignment means comprises a fourth reflective surface; wherein optionally a normal vector of the fourth reflective surface forms an angle between io° and 8o°, preferably between io° and 40°, with the normal vector of the second surface.

[0141] 18. An apparatus for sample inspection comprising: an inspection means; a sample stage; at least one interferometer for monitoring a sample stage position, wherein at least a portion of the at least one interferometer is mechanically coupled to the inspection means; and an optical alignment means for aligning the at least one interferometer, wherein the optical alignment means is coupled to the inspection means.

[0142] 19. An apparatus for sample inspection, comprising: an inspection means; a sample stage; at least one interferometer for monitoring a sample stage position, wherein at least a portion of the at least one interferometer is mechanically coupled to the inspection means and comprises a first interferometer configured to emit a first measurement beam and a second interferometer; and an optical alignment means guiding the first measurement beam at least partly through the second interferometer for aligning the at least one interferometer.

[0143] 20. Apparatus for sample inspection aligned according to a method according to any of embodiments 1-14.

[0144] 21. Apparatus of any of embodiments 18-20, further comprising: a receptacle for coupling the optical alignment means to the apparatus in a predetermined relative position and orientation.

[0145] 22. Apparatus of any of embodiments 18-21, wherein the at least one interferometer comprises a plurality of interferometers, preferably six interferometers, configured to determine a position and orientation of a sample in six degrees of freedom. 23. Apparatus of any of embodiments 18-22, wherein the at least one interferometer comprises: a light source configured to emit a beam; a beam splitter configured to split the beam into a reference beam and a measurement beam; a retroreflector; and a detector configured to receive the reference beam and the measurement beam. 24. Apparatus of any of embodiments 18-23, wherein the inspection means comprises an extreme ultraviolet (EUV) camera and / or an EUV light source.

Claims

Claims1. A method for interferometer alignment in an apparatus for sample inspection with an inspection means and a sample stage, wherein at least a portion of at least one interferometer for monitoring a sample stage position is mechanically coupled to the inspection means, wherein the method comprises: aligning the interferometer relative to an optical alignment means coupled to the inspection means.

2. Method of claim 1, wherein the at least one interferometer comprises a first interferometer configured to emit a first measurement beam and a second interferometer; and aligning the at least one interferometer comprises aligning the first interferometer relative to the optical alignment means by guiding the first measurement beam at least partly through the second interferometer.

3. A method for interferometer alignment in an apparatus for sample inspection with an inspection means and a sample stage, wherein at least a portion of at least one interferometer for monitoring a sample stage position is mechanically coupled to the inspection means and comprises a first interferometer configured to emit a first measurement beam and a second interferometer, wherein the method comprises: aligning the first interferometer by guiding the first measurement beam at least partly through the second interferometer.

4. Method of claim 2 or 3, wherein: the first interferometer is configured to measure a position of the sample stage in a first dimension; the second interferometer is configured to measure a position of the sample stage in a second dimension; and the first dimension and the second dimension are essentially perpendicular to one another.

5. Method of any of claims 1, 2 or 4 referred back to claim 2, further comprising coupling the optical alignment means rigidly to the inspection means; wherein the optical alignment means is configured to reflect the first measurement beam.

6. Method of claim 5, wherein coupling the optical alignment means rigidly to the inspection means comprises positioning the optical alignment means in a predetermined relative position and orientation relative to the apparatus, preferably at a point of interest.

7. Method of any of claims 2-6, wherein the second interferometer comprises a second retroreflector; and guiding the first measurement beam at least partly through the second interferometer comprises guiding the first measurement beam to a first detector of the first interferometer via the optical alignment means and the second retroreflector.

8. Method of any of claims 1, 2, or 4-7, wherein the optical alignment means comprises at least one element, preferably a first reflective surface, configured to redirect the first measurement beam to the first detector by an essentially orthogonal deflection.

9. Method of claim 7 or 8, wherein the aligning the at least one interferometer comprises optimizing an intensity detected by the first detector by adjusting the position and / or rotation of the at least one interferometer.

10. Method of any of claims 2-10, further comprising aligning the second interferometer; preferably by guiding the first measurement beam at least partly through the first interferometer.

11. Method of any of the previous claims, wherein the optical alignment means is further configured to allow for alignment of at least two interferometers with the optical alignment means in the same position.

12. Method of any of claims 1, 2, or 4-12 referred back to claim 1 or 2, wherein the optical alignment means further comprises a coupling means for coupling to the apparatus in a predetermined relative position and orientation.

13. Method of any of claims 1, 2, or 4-12 referred back to claim 1 or 2, wherein the optical alignment means further comprises at least one reflective surface configured to reflect light of the at least one interferometer.

14. An optical alignment means, wherein the optical alignment means is configured for use in a method according to any of claims 1, 2, or 4-13 referred back to claim 1 or 2-.

15. Optical alignment means of claim 14, wherein the optical alignment means comprises a first reflective surface and a third reflective surface; wherein a normal vector of the second reflective surface and a normal vector of the third reflective surface form an angle of about 90°.

16. Optical alignment means of claim 14 or 15, wherein the optical alignment means comprises a first reflective surface, a second reflective surface, and a third reflective surface; wherein a normal vector of the first reflective surface forms an angle of about 450with the normal vector of the second reflective surface and the third reflective surface.

17. Optical alignment means of claim 15 or 16, wherein the optical alignment means comprises a fourth reflective surface; wherein optionally a normal vector of the fourth reflective surface forms an angle between io° and 8o°, preferably between io° and 40°, with the normal vector of the second surface.

18. An interferometer for an apparatus for sample inspection with an inspection means and a sample stage;wherein the interferometer is configured for monitoring a sample stage position, and at least a portion of the at least one interferometer is configured to be mechanically coupled to the inspection means.

19. The interferometer according to claim 18, wherein the interferometer is further configured for being aligned by an optical alignment means for aligning the at least one interferometer, wherein the optical alignment means is coupled to the inspection means.

20. The interferometer according to claim 18 or 19, wherein the interferometer comprises a first interferometer configured to emit a first measurement beam and a second interferometer; and the interferometer is configured such that an optical alignment means can guide the first measurement beam at least partly through the second interferometer for aligning the interferometer.

21. An interferometer aligned according to a method according to any of claims 1- 13-22. Interferometer of any of claims 18-21, further comprising a plurality of interferometers, preferably six interferometers, configured to determine a position and orientation of a sample in six degrees of freedom.

23. Interferometer of any of claims 18-22, comprising: a light source configured to emit a beam; a beam splitter configured to split the beam into a reference beam and a measurement beam; a retroreflector; and a detector configured to receive the reference beam and the measurement beam.

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