TDI camera synchronization for mask inspection system

Abstract

An apparatus for inspection of a sample, the apparatus comprises a sample stage, an inspection means configured to inspect the sample on the sample stage in a line-wise scanning pattern, a position measurement means configured to measure a relative position of the inspection means and the sample stage while scanning, and a synchronization means configured to repeatedly provide a synchronization signal for synchronizing the inspection means, while scanning, based at least partly on the measured relative position.

Description

[0001] TDI camera synchronization for mask inspection system

[0002] 1. Field of the invention

[0003] The present invention relates to, inter alia, apparatuses, methods, and computer programs for sample inspection, in particular mask 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 extreme ultraviolet (EUV) 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 EUV wavelength range (e.g., to nm to 15 nm) and to corresponding EUV photolithography masks, herein also called EUV masks. Such samples, 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).

[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 -wise or 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 may be 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 that are 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 reduce 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.

[0009] Conventional apparatuses typically rely on a global clock that may be used to assign each snapshot of the camera with a corresponding stage position. However, there are multiple disadvantages: Firstly, time delays occur which may result in an erroneous assignment of inspection data (e.g., image data) to the position data. Thus, such time delays might reduce the degree of synchronization between sample stage and camera. Secondly, conventional position data are provided by means of the inputs into the sample stage controller which may typically exhibit a control error that is not accounted for. Third, the better the respective mechanical decoupling / vibration damping of the respective components relative to one another become, the stronger the negative effects on the accuracy of the inspection become. This situation seems to pose a disadvantageous situation in which neither the mechanical decoupling / vibration damping, nor the inspection may be optimized in full.

[0010] Thus, there is therefore a need to further improve apparatuses, methods, and computer programs for sample inspection.

[0011] 3. Summary

[0012] Aspects of the present disclosure improve the aforementioned and other known concepts and meet the above need at least in part. A first aspect relates to an apparatus for inspection of a sample, the apparatus comprising: a sample stage, an inspection means configured to inspect the sample on the sample stage in a line-wise scanning pattern, a position measurement means configured to measure a relative position of the inspection means and the sample stage while scanning, and a synchronization means configured to repeatedly provide a synchronization signal for synchronizing the inspection means, while scanning, based at least partly on the measured relative position.

[0013] Firstly, said solution may be independent from time delays that might otherwise result in an erroneous assignment of inspection data (e.g., image data) to the position data. Thus, a higher degree of synchronization may be achieved. There may not be a need for clocks of the respective components of the apparatus and / or for synchronization of such clocks.

[0014] Secondly, it may account for control errors that occur during operation of a sample stage controller. For example, 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. Synchronization based at least partly on the measured relative position may remove all these sources of error.

[0015] Third, it may resolve issues caused by requirements regarding positional stability of the apparatus. This may require mounting different components of the apparatus on separate vibration damping systems. And 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. While in the prior art, the better the respective mechanical decoupling / vibration damping of the respective components relative to one another become, the stronger the negative effects on the accuracy of the inspection become. The invention, however, allows avoiding said difficulty and allows optimization of both, the mechanical decoupling / vibration damping, and / or the inspection in full (and, e.g., independently from one another.

[0016] Thereby, the principle is independent of the sample stage speed (as, e.g., in conventional line-wise scanning approaches). Rather, the synchronization signal concept allows for synchronization for any (e.g., variable) sample stage speed. Further, the measuring of the relative position allows to account for any relative motion (e.g., induced by vibrations, mechanical impact, etc. that may result in a relative motion of sample stage and inspection means that cannot be accounted for in cases where, e.g., only a sample stage position relative to the pedestal it is mounted to is measured.

[0017] Generally, the inspection means may, e.g., comprise a projection optic, wherein, in some examples, the projection optics may, e.g., be configured to direct light onto a certain point of the sample, e.g., as described herein. Further, the inspection means may, e.g., comprise a camera, e.g., configured to collect light from the sample, e.g., at least a part of the light directed onto the sample by the projection optics and reflected from the sample.

[0018] In some examples, the relative position of the inspection means and the sample stage while scanning may be measured by measuring the relative position of the sample stage and the projection optics of the inspection means. Synchronizing the inspection means, while scanning, based at least partly on the measured relative position may comprise synchronizing the camera of the inspection means. The relative position of the inspection means and the sample stage may herein relate to the position of the inspection means relative to the sample stage and / or the relative position of the sample stage relative to the inspection means. For example, in an exemplary scenario, the inspection means (and / or any first means) maybe located at a first absolute position (xi, yi, Zi) and the sample stage (and / or any second means) maybe located at a second absolute position (xs, ys, zs), wherein x, y, and z may relate to three mutually orthogonal coordinates. In other examples, other coordinates maybe used. The relative position may thus be calculated as, e.g., the position of the inspection means relative to the sample stage (xi, y;, z;)-(xs, ys, zs) = (x;-xs, y;-ys, z;-zs) and / or the relative position of the sample stage relative to the inspection means (xs, ys, zs)-(x;, y;, Zi) = (xs-xi, ys-yi, zs-zi). In some examples, the relative position may comprise only one or more dimensions (e.g., in the example above, one or more of: x;-xs, y;-ys, z;-zs, xs-x;, ys-y;, and zs-z;). This understanding of the relative position may apply analogously to a relative position of any two means described herein.

[0019] The synchronization means may, e.g., be integrated into and / or coupled to either of the other means, preferably into the position measurement means.

[0020] In some examples, the apparatus may be configured to control an inspection operation of the inspection means at least in part based on the synchronization signal. For example, this may allow to synchronize the inspection data (e.g., image data) with the actual sample motion.

[0021] In some examples, the synchronization means may be configured to synchronize a camera of the inspection means to a motion of the sample stage. E.g., the motion of the sample stage maybe expressed as a change in the relative position of the inspection means and the sample stage.

[0022] In some examples, the synchronization means may further be configured to provide the synchronization signal to the inspection means. The inspection means may then be configured to use the synchronization signal appropriately for controlling the inspection operation.

[0023] For example, at least a portion of the position measurement means may be mechanically coupled to the inspection means. Mechanically coupling at least a portion of the position measurement means to the inspection means may advantageously improve the accuracy of the operation of the apparatus. In detail, conventional approaches at most rely on mechanically coupling position measurement means to (the pedestal of 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 may be advantageous to measure the stage position relative to the inspection means to account for variations / errors thereof as well. Such variations / errors may possibly be the result of (unwanted) vibrations, errors in stage control, mechanical impacts etc. In some examples, the synchronization means may further be configured to provide the synchronization signal when the relative position corresponds to a characteristic unit of the inspection according to a resolution of the inspection means, preferably a pixel. This may in a particularly advantageous way increase the degree of synchronization between the sample stage and the inspection means.

[0024] In some examples, the synchronization means may further be configured to provide the synchronization signal when the relative position corresponds to a pixel. The pixel may for example be a pixel of a camera of the inspection means.

[0025] In one example, the inspection means comprises imaging optics that may, e.g., image a portion of the sample of a predetermined size (e.g., a window with sub-micrometer side lengths, e.g. below 200 nm x 200 nm size, below 50 nm x 50 nm size, such as about 10 nm x 10 nm size) to one pixel of the inspection means (which may comprise side lengths of 5 pm x 5 pm). Thus, whenever the relative position of the sample stage to the inspection means has moved by the length of one window (e.g. 50 nm), a synchronization signal may be provided informing the inspection means that at this point a neighboring pixel may from now on image said portion of the sample. To give another example, e.g., with a resolution four times as high, the imaging optics may, e.g., image a portion of the sample of a predetermined size of about 12.5 nm x 12.5 nm to one pixel of the inspection means in the second example. Thus, in this example, the synchronization signal maybe provided whenever the relative position of the sample stage to the inspection means has moved by 12.5 nm (instead of 50 nm in the first example). Thus, the synchronization signal may be adapted to the resolution of the inspection means (e.g., including the influence like magnification etc. of imaging optics comprised therein and / or coupled thereto).

[0026] In the example of TDI (described herein) the charge transfer may be performed upon providing said synchronization signal, accordingly.

[0027] In general, the measured relative position may be used to determine an absolute position and / or a change in position. The absolute position may match with a pixel in a (virtual) pixel grid on the sample (wherein a virtual pixel on the sample may correspond to the portion that is imaged to one pixel of the inspection means). Further, the position change (e.g., by a predetermined magnitude in one or more dimensions) may correspond to a change to a next pixel of the inspection means.

[0028] In some examples, the inspection means may comprise an EUV camera and / or an EUV light source and / or an EUV projection optics, wherein, e.g., the projection optics may comprise a reflection-based optics, e.g., a mirror.

[0029] The sample may comprise and / or be a mask and / or an object for EUV photolithography, preferably for EUV photolithography. The mask may generally be a photolithography mask. The photolithography mask may have an aspect ratio of between i:i and 1:4, preferably between 1:1 and 1:2, most preferably of 1:1 or 1:2. The photolithography mask may have a nearly rectangular shape. The photolithography mask may be preferably 5 to 7 inch long and wide, most preferably 6 inch long and wide. Alternatively, the photolithography mask may be 5 to 7 inch long and 10 to 14 inch wide, preferably 6 inch long and 12 inch wide.

[0030] The projection optics may, e.g., be configured to direct the light of the EUV light source onto a certain point of the sample, e.g., the POI described herein. Therefore, the inspection means and / or the projection optics may, e.g., further comprise at least one (reflective) optical element, e.g., a mirror, configured to direct the light emitted by the EUV light source to the sample, e.g., in a collimated, focused and / or aerial way. In some examples, the inspection means may thus be distributed throughout the apparatus as described herein. In some examples, the EUV light source may comprise a laser-pulsed tin plasma.

[0031] When such distributed inspection means are present, the relative position between the inspection means and the sample stage may, e.g., be determined in reference to at least one portion of the inspection means and / or a point of interest (POI) of the inspection means and the sample stage, wherein the POI may, e.g., be the point that the inspection means interacts with the sample, e.g., by directing a (e.g., focused) light beam thereon and / or which the inspection means (e.g., comprising a camera) is imaging. In such example the POI may e.g., correspond to a characteristic position in the field of view of the camera, e.g., the center or a predetermined corner thereof. Especially in high-resolution EUV-based apparatuses, the synchronization concept allows for inspection operations at resolutions that are in principle possible by using light in the EUV wavelength range.

[0032] In some examples, the position measurement means may comprise an optical sensor, preferably an interferometer. Other possible examples for the optical sensor may, e.g., comprise a light detection and ranging (Lidar) system, an infrared distance sensor, a time-of-flight camera, and / or a stereo vision system.

[0033] Especially the use of an interferometer allows to achieve extremely high resolutions sufficient to ensure the high inspection (e.g., image) quality described herein.

[0034] 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 may be 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. In result, up to six (or potentially even more) interferometers may be used to measure the relative position in six dimensions (e.g., three translations and three rotations).

[0035] The at least one interferometer may, e.g., be based on heterodyne, grating shearing, and / or Michelson interferometry.

[0036] In the special case of the at least one interferometer, at least a portion thereof 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 may act as one end mirror of the interferometer.

[0037] 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 may be 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.

[0038] The at least one interferometer may, e.g., be aligned to be in a fixed and / or predetermined position relative to at least one further interferometer. Thus, it may be ensured 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 interferometer(s) 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] 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, e.g., upon reflection of its measurement beam at a first reflective surface of the sample stage) 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, e.g., upon reflection of its measurement beam at a second reflective surface of the sample stage). 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.

[0040] In some examples, the sample stage may comprise a third reflective surface, wherein a normal vector of the third reflective surface may form an angle between io° and 8o°, preferably between io° and 40°, with the normal vector of the first and / or second surface. A third interferometer may, analogously to the first and / or the second interferometer, measure the relative position in the z-direction.

[0041] The same applies analogously to rotations about at least one axis, preferably about up to three exes (e.g., x-, y-, and z-axis). 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 and / or perpendicular to the scanning motions).

[0042] In some examples, the position measurement means may further be configured to measure the relative position at a rate of 1 MHz or more, preferably 10 MHz or more, more preferably loo MHz or more.

[0043] At reasonable scanning speeds, such high rates may ensure that the relative position updates may be acquired so quickly one after another, that the position changes between two subsequent measurements are at or even below the resolution threshold of the apparatus such that any relative position update rate-dependent error maybe negligible or at least reduced.

[0044] In some examples, the position measurement means may further be configured to measure the relative position with a resolution of 50 nm or less, 20 nm or less, preferably 10 nm or less, or even 1 nm or less.

[0045] In typical exemplary embodiments, this resolution is sufficient to ensure a high inspection resolution.

[0046] The apparatus may, in an exemplary embodiment, further comprise a decoupling means configured to mechanically decouple the sample stage and the inspection means. The decoupling means may, e.g., comprise a mechanical isolation system, preferably a vibration damping system and / or a vibration control system. The position measurement means and / or the inspection means may, in some exemplary embodiments, be mounted to the decoupling means.

[0047] Any of the means described herein may, e.g., be mounted on common or separate decoupling means ensuring high mechanical stability, even when the apparatus itself and / or its surrounding experiences mechanical impacts or vibrations that might otherwise disturb the inspection. Such disturbances may be reduced significantly. In some examples, the sample stage may further be configured to change the relative position, preferably at a constant speed and / or in the scanning pattern.

[0048] When operating the scanning at a constant speed, despite not being necessarily required by the present invention, ensures that essentially the same intensity of signal maybe acquired in each inspection step, e.g., acquiring one TDI snapshot as described herein. This may improve uniformity and reliability of such approaches.

[0049] In some examples, the inspection means may comprise a time delay and integration (TDI) camera, e.g., comprising a plurality of pixel lines. The TDI camera maybe a TDI camera for the EUV wavelength range, i.e., the TDI camera may be configured to detect EUV radiation. E.g., the inspection means may comprise at least one line-sensor.

[0050] Thereby, the present invention achieves to combine the common advantages of TDI: E.g., as the line-by-line displacement in the TDI camera maybe synchronized with the movement of the image on the (sensor of the) TDI camera, it is achieved that the individual object points can be integrated for a correspondingly longer time. This may result in a better signal -to-noise ratio (even at high speeds of scanning and / or in low light conditions). TDI may further achieve high sensitivity by so-called “back thinning”. As described herein, CCD image sensors are usually read out serially via a single output which may limit the maximum readout speed to a few io M pixel / s. Higher speeds are achieved by TDI by, e.g., equipping the readout register with several taps ("ports"). For example, a TDI camera with 128 lines may output 50,000 lines / s through 4 ports, whereby the exposure time may, e.g., be not 20 ps (1 s / 50,000) but 2.56 ms (1 s / 50,000 x 128). However, all of these advantages rely on reliable and accurate synchronization which may be realized by the present invention as a critical requirement for the efficient use of TDI in an apparatus for sample inspection as described herein.

[0051] In some examples, the TDI camera may be configured to transfer charges collected by a pixel line based at least partly on the synchronization signal line-by-line.

[0052] In some examples, the TDI camera may be configured to transfer charges collected by a pixel based at least partly on the synchronization signal pixel-by-pixel. Synchronizing said charge transfer with the relative motion of the sample stage relative to the inspection means provides a well-suited synchronization concept for TDI, which, in the context of the present invention, allows to exploit all TDI advantages despite the challenges described herein, e.g., in reference to the impact of vibrations and / or mechanical decoupling of selected components of the apparatus for sample inspection.

[0053] The underlying concept of TDI is described herein in reference to Fig. 4.

[0054] In some examples, the position measurement means may further be configured to store position data comprising the measured relative position in a position data buffer storage and / or the inspection means may further be configured to store inspection data comprising a line-wise snapshot in an inspection data buffer storage.

[0055] In some examples, the position measurement means may further be configured to store position data comprising the measured relative position in a position data buffer storage and / or the inspection means may further be configured to store inspection data comprising a pixel-wise snapshot in an inspection data buffer storage.

[0056] Storing data in a buffer may, despite high scanning speed and / or the generation of large amounts of inspection data (e.g., image data), allow the apparatus to execute all required functionalities at the required speed without slowing down the inspection process unnecessarily.

[0057] In some examples, the apparatus may further comprise a post-processing means configured to access the data from the position data buffer storage and / or the inspection data buffer storage to generate an image.

[0058] Any read-out from the buffers may occur simultaneously and / or at least partly after scanning and may therefore allow to operate the different tasks at their own optimized speed and accuracy. This may improve the overall performance of the apparatus as described herein. In other embodiments, the post-processing means may also acquire the inspection data (e.g., image data) from elsewhere, e.g., receive them directly from the inspection means, i.e., not via the buffer.

[0059] In some examples, the apparatus may further comprise a means for providing an initiation signal, wherein the initiation signal may, e.g., be configured to initiate an operation of the inspection means, of the position measurement means and / or of the synchronization means.

[0060] Providing such initiation signal may on the one hand provide a useful automation means that may increase user satisfaction. On the other hand, it may ensure that a subsequent synchronization signal may be initiated thereby such that from the start, the synchronization may work as planned.

[0061] The initiation signal may, e.g., only be provided once while the synchronization signal may be provided repeatedly such that they may be understood as different signals, herein.

[0062] A second aspect of the present invention relates to a method for inspection of a sample, the method comprising: inspecting, by an inspection means, the sample on a sample stage in a line-wise scanning pattern, measuring, by a position measurement means, a relative position of the inspection means and the sample stage while scanning, and providing repeatedly, by a synchronization means, a synchronization signal for synchronizing the inspection means, while scanning, based at least partly on the measured relative position.

[0063] A third aspect of the present invention relates to a computer program comprising instructions for carrying out the steps of the method as described herein when the instructions are executed.

[0064] The method and the computer program essentially achieve the advantages described herein in reference to the apparatus. Any means of the apparatus and / or any functionality described in reference thereto may be implemented as a step of the method according to the second aspect and / or as an instruction of the computer program according to the third aspect, and vice versa.

[0065] 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.

[0066] In some examples, the inspection means may comprise a detector, such as a camera, e.g. a camera sensitive to EUV radiation. But the concepts as described herein may also be used for other detectors, such as particle detectors (e.g., electron detectors), etc.

[0067] In some implementations, each of the inspection means, the camera, etc. can include a particle or radiation source, e.g. to generate radiation (e.g., in the EUV wavelength range as described herein), an image sensor (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 / camera.

[0068] In some implementations, the apparatus can include a data processor and a storage device. The data processor in the apparatus can e.g., execute instructions of a computer program as described herein. The storage device can store inspection data and optionally relative position data. 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.

[0069] The processing of data described in this document, such as combining the snapshots acquired in a line-wise scanning approach, e.g., TDI, 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.

[0070] 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.

[0071] For example, the one or more computers can be configured to be suitable for the execution of a computer program and can include, byway 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.

[0072] 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.

[0073] 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.

[0074] In some examples, the synchronization means may be implemented as hardware, firmware, software or a combination thereof, e.g., using the components described herein.

[0075] 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 contraiy - should also not be understood such that the feature is essential or indispensable for the function of the embodiment.

[0076] 4. Short description of the figures

[0077] Fig. la shows an exemplary apparatus for sample inspection.

[0078] Fig. lb shows an exemplary EUV photolithography mask.

[0079] Fig. ic shows a first exemplary synchronization concept.

[0080] Fig. id shows a second exemplaiy synchronization concept.

[0081] Fig. le shows a third exemplaiy synchronization concept.

[0082] Fig. 2a shows a plurality of interferometers of an exemplary apparatus for sample inspection configured to measure a position of an exemplary sample stage.

[0083] Fig. 2b shows a schematic illustration of the working principle of a plurality of interferometers for position and orientation measurements.

[0084] Fig. 3 shows a schematic representation of an exemplary interferometer of an exemplary apparatus for sample inspection. Fig. 4 illustrates an example of sample inspection by acquiring sample images in a TDI mode.

[0085] 5. Detailed description

[0086] 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 or more position measurement means, such as interferometers 1021, 1023. The projection optics 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 may be 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 projection optics 1040 maybe 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 may be 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 projection optics 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.

[0087] 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 noo and / or a volume comprising the sample stage 1010, the projection optics 1040, and / or the at least one interferometer 1021, 1023) may, e.g., be below i-io2mbar, below 1 mbar, or below i-io-2mbar and / or said internal pressure may be above i-io-9, above i-io_7or above i-io-6.

[0088] The chamber 1100 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. The camera 1080 may, e.g., be understood as a part of the inspection means mentioned herein. Both, the image acquisition unit 1090 and the camera 1080 may, in some examples, be understood as components of the projection optics 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 projection optics 1040 mounted fixedly to further frame 1060 as well as relative to the at least one interferometer 1021, 1023, a further sensor 1070 (e.g., configured to emit a measurement beam 1071 for measuring a relative position of the camera 1080 and the projection optics 1040) is provided to measure the relative position of the camera 1080 relative to the projection optics 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.

[0089] The sample stage 1010 maybe configured to hold and / or move a sample, e.g., a EUV photolithography mask. The sample stage 1010 maybe 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 / 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 may be connected via a supply link 1140, e.g., comprising electrical connections, sensing capabilities, and / or other supply means, e.g., for cooling, heating, etc. The apparatus 1000 may comprise an external control means 1160 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 projection optics 1040, a cooling system configured to cool the apparatus and / or a part thereof, etc.

[0090] 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.

[0091] The exemplaiy mask 100 comprises a backside coating 110 applied onto a substrate 120. The backside coating may, e.g., comprise TaB and / or have a thickness between 10 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.

[0092] The mask too 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 about 4 nm as shown in the example of Fig. lb. The second layer 132 may, e.g., comprise Mo and / or have a thickness of about 3 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 may be 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 maybe between 3 nm and 11 nm. Other exemplary masks may, e.g., comprise three or more alternating layers. 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 may be 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.

[0093] Figs, ic-ie show a first, second and third exemplary synchronization concept. In detail, they show selected components of an apparatus 1000 as described herein and how they interact with each other in order to ensure that the camera 1080 and the sample stage are synchronized with one another. In all three Figs, ic-ie, the apparatus 1000 further comprises an external control means 1160, in these examples, e.g., comprising means for image processing and / or image reconstruction, e.g., according to a TDI mode as described herein. Therein, the position measurement means 1020, e.g., comprising one or more interferometers, may comprise and / or may be coupled to a control circuitry 1020a (e.g., a field programmable gate array (FPGA) configured for a high frequent relative position measurement update, e.g., at a rate of 10 MHz or more), a real-time interface 1020b (e.g., operating at a frequency of ca. 5 kHz or more), and / or a buffer storage 1020c (e.g., configured to store the updated relative position data frequency of 10 MHz or more). As illustrated by the dotted lines, the presence of said buffer storage 1020c and / or providing said relative position data 1025, e.g., to the external control means 1160 may be rendered obsolete by the invention described herein. Further, the camera 1080 maybe configured to provide the inspection data 1081, comprising, e.g., a line-wise snapshot of the sample, in an inspection data buffer storage and / or to provide them to the external control means 1160.

[0094] In Fig. ic and Fig. id, the position measurement means 1020 configured to measure a relative position of the inspection means 1080 and the sample stage while scanning, e.g., by means of the control circuitry 1020a, in the example of Fig. ic, provides a synchronization signal 1024 for synchronizing the inspection means 1080, while scanning, based at least partly on the relative position measured by the position measurement means 1020. Thereby, the control circuitry may comprise and / or act as a synchronization means 1020a, e.g., as described herein. In the examples of Figs, id and le, the apparatus 1000 further comprises a means ion for providing an initiation signal 1012, wherein the initiation signal 1012 is configured to initiate an operation of the inspection means 1080, of the position measurement means 1020 and / or of the synchronization means 1020a. Therein, the means 1011 may, e.g., comprise or be comprised in a controller of the sample stage. Said stage may, e.g., receive input 1071 (e.g., in a closed loop for position control) from the sensor 1070. In one example, the initiation signal 1012 maybe provided when the sample stage starts scanning (e.g., upon initiation by the sample stage controller 1011).

[0095] Fig. le shows an additional feature to the concept of Fig. id, namely the position measurement means 1020, e.g., via its interface 1020b providing feedback (e.g., in a closed loop) to the sample stage controller 1011, e.g., to computationally correct for positional errors measured by the position measurement means 1020, e.g., by assigning a corresponding time-stamp to the images 1081 and the the relative position data 1025, the acquisition of both of which may be triggered by the initiation signal 1012. Said error may relate to a difference between the set position and the actual position of the sample stage. The control means 1160 may then computationally correct the images 1081 according to the relative position data 1025 based at least partly on the corresponding time-stamps.

[0096] 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.

[0097] Fig. 2b shows a schematic illustration of the working principle of a plurality of interferometers i02iy, i02irx, i02irz for position and orientation measurements.

[0098] 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 maybe translated into a position information of the sample stage, depending on which of these is positioned in the measurement beam(s).

[0099] 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.

[0100] 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, maybe derived from comparing the displacements measured by interferometers i02iy, i02irx with the displacement measured by the interferometer i02irz.

[0101] 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.

[0102] 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.

[0103] The concept explained in reference to Fig. 2b may be transferred to any other plurality of interferometers and / or other interferometer configurations.

[0104] Fig. 3 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. 3 may correspond to a natural beam path / natural measurement path mentioned herein.

[0105] 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. 3 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.

[0106] The reference beam (dotted line) follows the following path through the interferometer 3000:

[0107] The light deflection unit 3200 further comprises two quarter wave plates 3230, 3240 (in the representation of Fig. 3 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. 3 upper side of the polarizing beam splitter (cube) 3210).

[0108] 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.

[0109] The light deflection unit 3200 further comprises a retroreflector 3220 (in the representation of Fig. 3 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.

[0110] The measurement beam (dashed line) follows the following path through the interferometer 3000:

[0111] The transmitted (horizontally polarized) portion of the light passes the quarter wave plate 3230 a first time, is reflected by the sample stage 1010, 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.

[0112] 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.

[0113] There, the measurement beam 3110b and the reference beam 3120b overlap and may yield the interferometric signal as in conventional (generally known) interferometry. Fig. 4 illustrates an example of sample inspection by acquiring sample images in a TDI mode: In principle, a TDI camera may behave similarly to a pure line scan camera (with one pixel line) and is typically used when an object moves transversely and constantly in relation to a camera. The significant difference to such conventional line scan cameras is that TDI cameras have not just one line of pixels, but a number (e.g., 128), which may, e.g., make them a so-called area scan camera.

[0114] In conventional TDI scanning imaging, image data maybe collective continuously as the sample (e.g., with the sample stage) moves relative to the camera 1080, preferably at an essentially constant velocity. In the example of Fig. 4, the sample stage moves along the arrow indicating the sample stage motion while the camera 1080 remains stationary. However, in other examples, the camera may move relative to the sample (e.g., in the opposite direction) additionally or alternatively to the sample motion.

[0115] The camera 1080 in the example of Fig. 4 comprises 10x10 = 100 pixels 1082. This small number is merely shown for illustrative purposes. Conventional cameras 1080 typically may have more pixels, typically by multiple orders of magnitude. A pixel, e.g., a CCD sensor pixel, maybe configured to accumulate a response to light impinging on said pixel by converting said incoming light into electrons. For TDI, the pixels may logically be grouped into (in Fig. 3 horizontal) pixel lines orthogonal to the motion direction.

[0116] At a certain moment in time, light from a single point on the sample is imaged onto a certain pixel, as illustrated by the small dot in the fifth pixel from the left in the uppermost pixel line. The radius of the dot illustrates the number of electrons accumulated thereby. As the sample moves past the camera 1080, the synchronization signal may be used to trigger a vertical transfer 1085a when the above point of the sample is imaged onto the fifth pixel from the left in the second pixel line from the top. The transfer 1085a may, e.g., comprise transferring the electrons from the light of that point of the sample may be transferred by transfer 1085a, and further light from that point results in a further accumulation of electrons in the respective pixel. This process is repeated multiple times, as illustrated by the transfers io85b-io85i until the electrons reach the lowest pixel line (shaded), also called the register. Thereby, a virtual pixel builds up (illustrated by increasing radius of black dot transferred from the top pixel line to the bottom pixel line of Fig. 4) that crosses the length of the camera detector.

[0117] The accumulated signal (illustrated by the largest black dot), e.g., once it has reached the register, may be extracted as follows: The pixels in the register, one after another, may provide their electrons (which may correspond to the signal / image intensity) rightward (in the example of Fig. 3) pixel by pixel, with the rightmost pixel providing its electrons to the read-out unit 1084. The read-out unit 1084 may comprise, e.g., a read amplifier configured to read out the number of electrons of each pixel and / or an A / D- converter configured to convert the (e.g., analog) output from the read amplifier into a digital signal that may, e.g., be processed by a computer etc.

Claims

Claims1. An apparatus (1000) for inspection of a sample, the apparatus (1000) comprising: a sample stage (1010); an inspection means (1040, 1080) configured to inspect the sample on the sample stage (1010) in a line-wise scanning pattern; a position measurement means (1020) configured to measure a relative position of the inspection means (1040, 1080) and the sample stage (1010) while scanning; and a synchronization means configured to repeatedly provide a synchronization signal (1024) for synchronizing the inspection means (1040, 1080), while scanning, based at least in part on the measured relative position.

2. Apparatus (1000) of claim 1, wherein the synchronization means is configured to synchronize a camera of the inspection means (1040, 1080) to a motion of the sample stage.

3. Apparatus (1000) of claim 1 or 2, wherein the synchronization means is further configured to provide the synchronization signal (1024) to the inspection means (1040, 1080); wherein the inspection means (1040, 1080) is configured to control an inspection operation at least in part based on the synchronization signal (1024).

4. Apparatus (1000) of any of claims 1-3, wherein the synchronization means is further configured to provide the synchronization signal (1024) when the relative position corresponds to a characteristic unit of the inspection according to a resolution of the inspection means (1040, 1080).

5. Apparatus (1000) of any of claims 1-4, wherein the synchronization means is further configured to provide the synchronization signal (1024) when the relative position corresponds to a pixel.

6. Apparatus (1000) of any of claims 1-5, wherein the inspection means (1040, 1080) comprises an extreme ultraviolet, EUV, camera (1080) and / or an EUV projection optics (1040).

7. Apparatus (1000) of any of claims 1-6, wherein the position measurement means (1020) comprises an optical sensor, preferably an interferometer (1021, 1022, 1023).

8. Apparatus (1000) of any of claims 1-7, wherein the position measurement means (1020) is further configured to measure the relative position at a rate of 1 MHz or more, preferably 10 MHz or more, more preferably 100 MHz or more.

9. Apparatus (1000) of any of claims 1-8, wherein the position measurement means (1020) is further configured to measure the relative position with a resolution of 50 nm or less, preferably 20 nm or less.

10. Apparatus (1000) of any of claims 1-9, further comprising a decoupling means configured to mechanically decouple the sample stage (1010) and the inspection means (1040, 1080); wherein the decoupling means comprises a mechanical isolation system, preferably a vibration damping system and / or a vibration control system; wherein preferably the position measurement means (1020) and / or the inspection means (1040, 1080) is mounted to the decoupling means.

11. Apparatus (1000) of any of claims 1-10, wherein the sample stage (1010) is further configured to change the relative position, preferably at a constant speed.

12. Apparatus (1000) of any of claims 1-11, wherein the inspection means (1040, 1080) comprises a time delay and integration, TDI, camera (1080) comprising a plurality of pixel lines.13- Apparatus (1000) of claim 12, wherein the TDI camera is configured to transfer charges collected by a pixel line based at least partly on the synchronization signal (1024) line-by-line.

14. Apparatus (1000) of claim 12 or 13, wherein the TDI camera is configured to transfer charges collected by a pixel based at least partly on the synchronization signal (1024) pixel-by-pixel.

15. Apparatus (1000) of any of claims 1-14, wherein the position measurement means (1020) is further configured to store position data comprising the measured relative position in a position data buffer storage (1020c); and / or wherein the inspection means (1040, 1080) is further configured to store inspection data comprising a line-wise snapshot in an inspection data buffer storage.

16. Apparatus (1000) of any of claims 1-15, wherein the position measurement means (1020) is further configured to store position data comprising the measured relative position in a position data buffer storage (1020c); and / or wherein the inspection means (1040, 1080) is further configured to store inspection data comprising a pixel-wise snapshot in an inspection data buffer storage.

17. Apparatus (1000) of claim 15 or 16, further comprising a post-processing means (1160) configured to access the data from the position data buffer storage (1020c) and / or the inspection data buffer storage to generate an image.

18. Apparatus (1000) of any of claims 1-17, further comprising a means for providing an initiation signal (1012), wherein the initiation signal (1012) is configured to initiate an operation of the inspection means (1040, 1080), and / or wherein the initiation signal (1012) is configured to initiate an operation of the position measurement means (1020), and / or wherein the initiation signal (1012) is configured to initiate an operation of the synchronization means.

19. Method for inspection of a sample, the method comprising:inspecting, by an inspection means (1040, 1080), the sample on a sample stage (1010) in a line-wise scanning pattern; measuring, by a position measurement means (1020), a relative position of the inspection means (1040, 1080) and the sample stage (1010) while scanning; and providing repeatedly, by a synchronization means, a synchronization signal(1024) for synchronizing the inspection means (1040, 1080), while scanning, based at least partly on the measured relative position.

20. Computer program comprising instructions for carrying out the steps of the method of claim 19 when the instructions are executed.

Images