Method for calibrating an optical position sensor for position detection of a static measuring object and calibration unit

Abstract

The invention relates to a method for calibrating an optical position sensor (101) for position detection of a static measuring object (102), comprising the steps: a. Acquiring a first calibration value at a first relative distance d between the static measuring object (102) and the optical position sensor (101), b. At least once changing the relative distance to at least one further relative distance between the optical position sensor (101) and the static measuring object (102) that differs from the first relative distance, and recording a further calibration value at the further relative distance, and c. Storing at least the first calibration value at the first relative distance and the further calibration value at the next relative distance in a memory as a calibration curve. The invention also relates to a calibration unit and an inspection system.

Description

[0001] The invention relates to a method for calibrating an optical position sensor for detecting the position of a static measurement object, in particular in a lithography machine, preferably in a microlithographic projection exposure system, or in an inspection system, preferably in an inspection system for masks or EUV masks. The invention further relates to a calibration unit and an inspection system.

[0002] Projection exposure systems are used to create extremely fine structures, particularly on semiconductor devices or other microstructured components. The operating principle of these systems is based on creating extremely fine structures down to the nanometer range by means of a generally reduced-size image of structures on a mask, a so-called reticle, on an element to be structured, a so-called wafer, which is provided with photosensitive material. The minimum dimensions of the created structures depend directly on the wavelength of the light used. This light is shaped in an illumination optics for optimal illumination of the reticle. Recently, light sources with an emission wavelength in the range of a few nanometers, for example between 1 nm and 120 nm, particularly in the range of 13.5 nm, have been increasingly used. This wavelength range is also referred to as the EUV range.

[0003] In addition to systems operating in the EUV range, the microstructured components are also manufactured using the market-established DUV systems with a wavelength between 100 nm and 300 nm, particularly 193 nm. With the demand for ever smaller structures, the requirements for optical correction in the systems have also increased. With each new generation of projection exposure systems operating in the EUV or DUV range, throughput is increased to increase cost-effectiveness.

[0004] Lithography machines can have static measurement objects. The term "static" here means that the measurement object is immobile, in particular actuator-free, i.e., cannot be actuated. In particular, the measurement object can be stationary and arranged immobile on an object, in particular on a frame structure. The measurement objects can have cooling lines or other elements that transmit vibrations to the measurement object. With increasing demands on measurement accuracy, the vibrations of a stationary measurement object must also be determinable. Typically, the position of the measurement object is determined using a plurality of position sensors for all six rigid body degrees of freedom (i.e., the three translational degrees of freedom and the three rotational degrees of freedom). In order to determine the position correctly, calibration is necessary beforehand.For this purpose, the measurement object typically has at least one actuator, so that the relative position between the position sensor and the measurement object is changed by moving the measurement object toward or away from the position sensor via the actuation of the actuator. For various relative distances between the position sensor and the measurement object, the measurement signal from the position sensor is recorded and stored in a memory as a calibration, along with the associated relative distance.

[0005] Examples of such calibration methods are US 2005 / 0280563 A1, DE 10 2020 123 482 A1, “Experimental Investigation of Linear Encoder's Subdivisional Errors under Different Scanning Speeds”, D. Gurauskis et al.; Appl. Science; “Auto-Calibration and Noise Reduction for the Sinusoidal Signals of Magnetic Encoders”; Ha Xuan Nguyen et al., IEEE 2017 and the “Encoder Calibration Method for High Precision Servo Systems With a Sinusoidal Encoder”, Youngwoo Lee et al.; IEEE Transactions on industrial electronics, Vol. 69, No. 1;.

[0006] It is therefore the object of the present invention to provide a method for calibrating an optical position sensor, a calibration unit and an inspection system which enable a calibration of a position sensor with respect to a static measuring object.

[0007] The problem concerning the method is solved by a method according to the features of claim 1, and the problem concerning the calibration unit is solved by a calibration unit according to the features of claim 9. The problem concerning the inspection system is solved by an inspection system with the features of claim 14. Advantageous embodiments with expedient further developments are specified in the subclaims.

[0008] The method for calibrating an optical position sensor for detecting the position of a static measurement object, in particular in a lithography machine, preferably in a microlithographic projection exposure system, or in an inspection system, preferably in an inspection system for masks or EUV masks, comprises in particular the following steps: • Recording a first calibration value at a first relative distance between the static measuring object and the optical position sensor, • Changing the relative distance at least once to at least one further relative distance between the optical position sensor and the static measuring object that differs from the first relative distance and recording a further calibration value at the further relative distance, and • Storing at least the first calibration value at the first relative distance and the further calibration value at the further relative distance in a memory as a calibration curve.

[0009] The static measurement object is preferably an optical element, in particular a mirror or a lens. The term static in the context of the present invention means that the measurement object is immobile, i.e. stationary and in particular fixed in position and / or actuator-free, i.e. in particular free of a solid-state actuator, piezo actuator, or Lorenz actuator, or a capacitive or electromagnetic actuator. In other words, the measurement object is not movable by an actuator and is not directly connected to an actuator. The measurement object can be rigidly and in particular stationary attached to an object, wherein the object is preferably a frame structure, in particular a sensor frame or a force frame. The method thus makes it possible to create a calibration in order to be able to carry out the most accurate position detection possible of an immobile measurement object.

[0010] It is particularly advantageous if the relative distance between the static measurement object and the optical position sensor is changed by moving the optical position sensor. In other words, the position sensor alone moves toward or away from the static measurement object.

[0011] On the one hand, it is preferred if the relative distance between the static measurement object and the optical position sensor is changed by means of an actuator associated with the optical position sensor. The actuator can be designed as a solid-state actuator, in particular as a piezo actuator, or as an electrostrictive, magnetostrictive, or ferroelectric actuator. Alternatively, the actuator can also be designed as a capacitive actuator or as an electromagnetic actuator, in particular as a Lorenz actuator, although the list of possible actuators is not exhaustive.

[0012] Within the scope of the method, it is further preferred if the optical position sensor is designed as an encoder or as an interferometer. In a simple embodiment, the interferometer preferably comprises a light source, a beam splitter, and, in addition to the (movable) mirror attached to the measurement object as a measurement target, another mirror, and a detector. The light emitted by the light source is split by means of the beam splitter, with the first reflected light beam striking the mirror of the measurement target and the second reflected light beam striking the fixed additional mirror, and each being reflected back to a detector, thereby generating interference. Relative position determination is then possible via the change in the interference, with detection preferably taking place using an intensity measurement, a phase shift measurement, or a frequency measurement.The optical position sensor can be formed as any type of interferometer. Alternatively, the optical position sensor can also be formed as a frequency-based position sensor. The frequency-based position sensor preferably comprises at least one optical resonator, wherein this resonator has a stationary first resonator mirror, a stationary second resonator mirror, and a retroreflector or plane mirror formed as a first measurement target. The second resonator mirror is preferably formed as a deflection mirror, which reflects a measurement beam coming from the first measurement target back into itself. In such a frequency-based position sensor, a standing wave is generated in the resonator, i.e., in an optical cavity. The frequency of the Fabry-Perot resonances generated in this way shifts depending on a changing distance change in the resonator.The basic principle of the described frequency-based position sensor is explained in WO 2019 / 223968 A1, which is incorporated by reference.

[0013] Furthermore, the optical position sensor can also be designed as an optical encoder, in particular as an interferometric encoder based on a diffraction grating. In a particularly simple embodiment, optical encoders comprise a beam splitter that splits the light emitted by the light source into two beams. The two beams impinge on the diffraction grating at two different positions and are diffracted there. The first beam is then projected via a λ / 4 plate onto a first mirror, and the second beam is projected via another λ / 4 plate onto a second mirror, where they are each reflected back onto the diffraction grating, diffracted there, and directed back onto the beam splitter. Changes in the interference pattern can be detected by means of at least one or more photodetectors, in particular four photodetectors, and thus position changes can be detected.A suitable optical encoder is described, for example, in EP 1 286 139 B1.

[0014] Furthermore, it is alternatively preferred if the relative distance between the static measurement object and the optical position sensor is changed by changing an optical path length of the optical position sensor by means of an actuator assigned to one of the mirrors of the optical position sensor. If the position sensor is designed as an interferometric encoder, it is preferred if the actuator is assigned to one of the mirrors from which the light beam is reflected back onto the diffraction grating. In particular, it is preferred if the actuator is arranged on the mirror that is arranged in the measurement beam path of the encoder and from which the light is reflected back onto the grating. Alternatively, the actuator can also be arranged on one of the mirrors of the reference beam path. The actuator is preferably designed as a solid-state actuator, in particular as a piezo actuator.

[0015] If the position sensor is formed as an interferometer, it is advantageous if at least one actuator is additionally assigned to or arranged on one of the mirrors of the interferometer, which actuator is configured to change the position of the mirror relative to another mirror or a beam splitter in such a way that the optical path length is adjusted.

[0016] Furthermore, it is advantageous if the measuring object has at least one line, for example a cooling line or a heating line, and that a movement of the measuring object relative to the optical position sensor is induced by adjusting a pressure in the at least one line from a first pressure to a second pressure deviating from the first pressure.

[0017] Furthermore, it is also possible that objects to which the measurement object is connected, such as a frame or a scanner or a wafer stage, which have an actuator, are used to indirectly move the measurement object, which itself is actuator-free, i.e. has no actuator.

[0018] The calibration unit for calibrating an optical position sensor with respect to a measurement object, in particular for a lithography machine with a static, in particular stationary, measurement object and with an optical position sensor, is characterized in particular by the fact that a movement unit is provided which is configured to change a relative distance between the optical position sensor and the static measurement object and with a readout unit which is configured to record a measurement signal of the optical position sensor at at least two different relative distances and to store them in a memory as calibration values. The calibration unit thus enables the calibration of a position sensor with respect to an immobile, i.e. stationary and in particular stationary, measurement object. The measurement object is preferably formed free of an actuator, i.e., not actuatable.The static measurement object is preferably an optical element, in particular a mirror or a lens. The term static in the context of the present invention means that the measurement object is immobile, i.e. stationary and in particular fixed in position and / or actuator-free, i.e. in particular free of a solid-state actuator, piezo actuator, or Lorenz actuator, or a capacitive or electromagnetic actuator. In other words, the measurement object is not movable by an actuator and is not directly connected to an actuator. The measurement object can be rigidly and in particular stationary attached to an object, wherein the object is preferably a frame structure, in particular a sensor frame or a force frame. The method thus makes it possible to create a calibration in order to be able to carry out the most accurate position detection possible of an immobile measurement object.

[0019] Furthermore, it is advantageous if the optical position sensor is designed as an encoder or as an interferometer. In a simple embodiment, the interferometer preferably comprises a light source, a beam splitter, and, in addition to the (movable) mirror attached to the measurement object as a measurement target, another mirror, and a detector. The light emitted by the light source is split by the beam splitter, with the first reflected light beam striking the mirror of the measurement target and the second reflected light beam striking the fixed additional mirror, and each being reflected back to a detector, thereby generating interference. Relative position determination is then possible via the change in the interference, with detection preferably taking place using an intensity measurement, a phase shift measurement, or a frequency measurement.The optical position sensor can be formed as any type of interferometer. Alternatively, the optical position sensor can also be formed as a frequency-based position sensor. The frequency-based position sensor preferably comprises at least one optical resonator, wherein this resonator has a stationary first resonator mirror, a stationary second resonator mirror, and a retroreflector or plane mirror formed as a first measurement target. The second resonator mirror is preferably formed as a deflection mirror, which reflects a measurement beam coming from the first measurement target back into itself. In such a frequency-based position sensor, a standing wave is generated in the resonator, i.e., in an optical cavity. The frequency of the Fabry-Perot resonances generated in this way shifts depending on a changing distance change in the resonator.The basic principle of the described frequency-based position sensor is explained in WO 2019 / 223968 A1, which is incorporated by reference.

[0020] Furthermore, the optical position sensor can also be designed as an optical encoder, in particular as an interferometric encoder based on a diffraction grating. In a particularly simple embodiment, optical encoders comprise a beam splitter that splits the light emitted by the light source into two beams. The two beams impinge on the diffraction grating at two different positions and are diffracted there. The first beam is then projected via a λ / 4 plate onto a first mirror, and the second beam is projected via another λ / 4 plate onto a second mirror, where they are each reflected back onto the diffraction grating, diffracted there, and directed back onto the beam splitter. Changes in the interference pattern can be detected by means of at least one or more photodetectors, in particular four photodetectors, and thus position changes can be detected.A suitable optical encoder is described, for example, in EP 1 286 139 B1.

[0021] Particularly preferably, the movement unit is formed as an actuator associated with the optical position sensor, in particular indirectly or directly connected to the position sensor. The position sensor can be connected to a housing or to a frame, in particular to a sensor frame, via an intermediate element. In one embodiment, it is preferred if the intermediate element has the at least one actuator, i.e., the at least one actuator is arranged on or in the intermediate element. The intermediate element can be designed to position the position sensor relative to the measurement object.

[0022] The actuator can be designed as a solid-state actuator, in particular as a piezo actuator, or as an electrostrictive, magnetostrictive, or ferroelectric actuator. Alternatively, the actuator can also be designed as a capacitive actuator or as an electromagnetic actuator, in particular as a Lorenz actuator, although the list of possible actuators is not exhaustive.

[0023] In one embodiment, it is preferred if the actuator is assigned to one of the mirrors of the position sensor and is configured to adjust the position of the mirror such that the optical path length is adjusted. If the position sensor is designed as an interferometric encoder, it is preferred if the actuator is assigned to one of the mirrors from which the light beam is reflected back onto the diffraction grating. In particular, it is preferred if the actuator is arranged on the mirror that is arranged in the measuring beam path (not in the reference beam path) of the encoder and from which the light is reflected back from the grating to the grating.

[0024] Within the scope of the invention, it is particularly preferred if the measurement object is an optical element, in particular a mirror or a lens. The measurement object can be arranged rigidly, in particular stationary, on an object, in particular on a frame, for example a force frame or a sensor frame, and is designed without an actuator, so that the optical element is non-adjustable.

[0025] The calibration unit is particularly designed to carry out the method according to the invention.

[0026] The inspection system according to the invention for inspecting a substrate for semiconductor technology, in particular a wafer, a wafer stage, a reticle, or a reticle stage, has at least one static measurement object and at least one optical position sensor for detecting the distance between the static measurement object and a reference. Furthermore, at least one calibration unit is provided for calibrating the optical position sensor with respect to the static measurement object. A movement unit is also provided, which is configured to change a relative distance d between the optical position sensor and the static measurement object, wherein a readout unit is configured to detect a measurement signal from the optical position sensor at at least two differing relative distances and to store it in a memory as calibration values.The inspection system preferably comprises an evaluation unit that compares the detected positions or distances, in particular of structures of the object, with predetermined distances or positions of the structures of the object and initiates measures if there is a deviation by a predetermined limit. The inspection system can preferably be configured to detect defects on a mask, in particular an EUV mask. The inspection system can have a camera, in particular a TDI camera (time delay and integration camera). The camera can be configured to image at least parts of the mask, in particular the EUV mask, onto the camera. The camera is preferably configured for defect detection, with at least part of the mask being imaged by the camera. The static measurement object is preferably an optical element, in particular a mirror (for example an EUV mirror) or a lens.Alternatively, the measurement object can also be a support structure, a mask, or a wafer. The advantages and embodiments mentioned for the calibration unit also apply to the inspection system comprising a calibration unit. An example of an inspection system for reticle or wafer inspection (without the calibration unit according to the invention) is known from document DE 10 2012 205 181 A1, the entire content of which is incorporated into the present application by reference.

[0027] The term "static" in the context of the present invention means that the measurement object is immobile, i.e., stationary, and in particular fixed in place and / or actuator-free, i.e., in particular, it is free of a solid-state actuator, piezo actuator, Lorenz actuator, or a capacitive or electromagnetic actuator. In other words, the measurement object is not movable by an actuator and is not directly connected to an actuator.

[0028] Furthermore, it is advantageous if the optical position sensor is designed as an encoder or as an interferometer. In a simple embodiment, the interferometer preferably comprises a light source, a beam splitter, and, in addition to the (movable) mirror attached to the measurement object as a measurement target, another mirror, and a detector. The light emitted by the light source is split by the beam splitter, with the first reflected light beam striking the mirror of the measurement target and the second reflected light beam striking the fixed additional mirror, and each being reflected back to a detector, thereby generating interference. Relative position determination is then possible via the change in the interference, with detection preferably taking place using an intensity measurement, a phase shift measurement, or a frequency measurement.The optical position sensor can be formed as any type of interferometer. Alternatively, the optical position sensor can also be formed as a frequency-based position sensor. The frequency-based position sensor preferably comprises at least one optical resonator, wherein this resonator has a stationary first resonator mirror, a stationary second resonator mirror, and a retroreflector or plane mirror formed as a first measurement target. The second resonator mirror is preferably formed as a deflection mirror, which reflects a measurement beam coming from the first measurement target back into itself. In such a frequency-based position sensor, a standing wave is generated in the resonator, i.e., in an optical cavity. The frequency of the Fabry-Perot resonances generated in this way shifts depending on a changing distance change in the resonator.The basic principle of the described frequency-based position sensor is explained in WO 2019 / 223968 A1, which is incorporated by reference.

[0029] Furthermore, the optical position sensor can also be designed as an optical encoder, in particular as an interferometric encoder based on a diffraction grating. In a particularly simple embodiment, optical encoders comprise a beam splitter that splits the light emitted by the light source into two beams. The two beams impinge on the diffraction grating at two different positions and are diffracted there. The first beam is then projected via a λ / 4 plate onto a first mirror, and the second beam is projected via another λ / 4 plate onto a second mirror, where they are each reflected back onto the diffraction grating, diffracted there, and directed back onto the beam splitter. Changes in the interference pattern can be detected by means of at least one or more photodetectors, in particular four photodetectors, and thus position changes can be detected.A suitable optical encoder is described, for example, in EP 1 286 139 B1.

[0030] Particularly preferably, the movement unit is formed as an actuator associated with the optical position sensor, in particular indirectly or directly connected to the position sensor. The position sensor can be connected to a housing or to a frame, in particular to a sensor frame, via an intermediate element. In one embodiment, it is preferred if the intermediate element has the at least one actuator, i.e., the at least one actuator is arranged on or in the intermediate element. The intermediate element can be designed to position the position sensor relative to the measurement object.

[0031] The actuator can be designed as a solid-state actuator, in particular as a piezo actuator, or as an electrostrictive, magnetostrictive, or ferroelectric actuator. Alternatively, the actuator can also be designed as a capacitive actuator or as an electromagnetic actuator, in particular as a Lorenz actuator, although the list of possible actuators is not exhaustive.

[0032] In one embodiment, it is preferred if the actuator is assigned to one of the mirrors of the position sensor and is configured to adjust the position of the mirror such that the optical path length is adjusted. If the position sensor is designed as an interferometric encoder, it is preferred if the actuator is assigned to one of the mirrors from which the light beam is reflected back onto the diffraction grating. In particular, it is preferred if the actuator is arranged on the mirror that is arranged in the measuring beam path (not in the reference beam path) of the encoder and from which the light is reflected back from the grating to the grating.

[0033] The calibration unit of the inspection system is particularly designed to carry out the method according to the invention.

[0034] Further features, properties, and advantages of the present invention are described in more detail below using embodiments with reference to the accompanying figures. All features described so far and below are advantageous both individually and in any combination. The embodiments described below are merely examples and do not limit the subject matter of the invention. They show: Fig. 1a a schematic representation of a microlithographic projection exposure system designed for operation in the EUV, Fig. 1b a schematic representation of a microlithographic projection exposure system designed for operation in DUV, Fig. 2 a schematic representation of a first embodiment of a sensor with localization unit, Fig. 3 a schematic representation of a second embodiment of the calibration unit for a measurement object and Fig. 4 a schematic representation of a third embodiment of the calibration unit for a measurement object.

[0035] Fig. Figure 1a shows a schematic representation of an exemplary projection exposure system 600 designed for operation in the EUV spectrum, in which the present invention can be implemented. However, the invention can also be used in other nanopositioning systems.

[0036] According to Fig. 1a, an illumination device in a projection exposure system 600 designed for EUV comprises a field facet mirror 603 and a pupil facet mirror 604. The light from a light source unit, which comprises a plasma light source 601 and a collector mirror 602, is directed onto the field facet mirror 603. A first telescope mirror 605 and a second telescope mirror 606 are arranged in the light path downstream of the pupil facet mirror 604. A deflection mirror 607 is arranged downstream in the light path, which deflects the radiation incident upon it onto an object field in the object plane of a projection lens comprising six mirrors 651-656. At the location of the object field, a reflective structure-bearing mask 621 is arranged on a mask table 620, which is imaged by means of the projection lens into an image plane in which a substrate 661 coated with a light-sensitive layer (photoresist) is located on a wafer table 660.

[0037] The invention can be used in a DUV system as well as in Fig. 1b. A DUV system is basically like the EUV system described above from the Fig. 1a, whereby mirrors and lenses can be used as optical elements in a DUV system and the light source of a DUV system emits useful radiation in a wavelength range from 100 nm to 300 nm.

[0038] The Fig. The DUV lithography system 700 shown in Figure 1b has a DUV light source 701. An ArF excimer laser, for example, can be provided as the DUV light source 701, which emits radiation 702 in the DUV range at, for example, 193 nm. A beam-shaping and illumination system 703 directs the DUV radiation 702 onto a photomask 704. The photomask 704 is designed as a transmissive optical element and can be arranged outside the systems 703. The photomask 704 has a structure that is imaged in a reduced size onto a wafer 706 or the like by means of the projection system 705. The projection system 705 has a plurality of lenses 707 and / or mirrors 708 for imaging the photomask 704 onto the wafer 706. Individual lenses 707 and / or mirrors 708 of the projection system 705 can be arranged symmetrically to the optical axis 709 of the projection system 705.It should be noted that the number of lenses 707 and mirrors 708 of the DUV lithography system 700 is not limited to the number shown. More or fewer lenses 707 and / or mirrors 708 can also be provided. In particular, the beam shaping and illumination system 703 of the DUV lithography system 700 has a plurality of lenses 707 and / or mirrors 708. Furthermore, the mirrors are usually curved at their front side for beam shaping. An air gap 710 between the last lens 707 and the wafer 706 can be replaced by a liquid medium having a refractive index > 1. The liquid medium can be, for example, ultrapure water. Such a setup is also referred to as immersion lithography and has an increased photolithographic resolution.

[0039] Fig. 2 shows a schematic representation of a first embodiment of a calibration unit 100 for calibrating an optical position sensor 101 with respect to a measurement object 102 for a lithography machine. The measurement object 102 is statically formed, meaning it is immobile, i.e., stationary and in particular fixed in position and / or in particular actuator-free, and is rigidly arranged immobile on an object 108, in particular on a frame, for example a force frame or a sensor frame, i.e., rigidly connected to it. The measurement object 102 is formed here as an optical element, in particular as a mirror or a lens, but can be any other object of the lithography machine. The optical element can be exposed to vibrations by other elements indirectly connected to it, or can comprise, for example, cooling lines, the coolant flowing therein can also cause the optical element to vibrate.In this case, the optical element is part of a lithography machine; in particular, the optical element is part of a microlithographic projection exposure system 600, 700, for example, part of its projection lens. To increase imaging accuracy, such vibrations must be detected, or the position of the measurement object 102 / optical element must be detected. For this purpose, a plurality of position sensors 101 are provided, preferably six position sensors for all rigid body degrees of freedom of the measurement object 102. In . Fig. 2 shows one of the position sensors 101 as an example, whereby any number of position sensors 101 can be present.

[0040] The at least one optical position sensor 101 is preferably attached to a frame 109, for example to a sensor frame or a force frame of the lithography machine. The static measurement object 102 and the at least one optical position sensor 101 can both be attached to the same object 108, i.e., both can be attached, for example, to the sensor frame. Furthermore, a movement unit 106 is provided, which is configured to change a relative distance d between the optical position sensor 101 and the static measurement object 102. Furthermore, a readout unit (not shown in detail) is provided, which is configured to detect a measurement signal from the optical position sensor 101 at at least two, preferably a plurality of differing relative distances d and to store it as calibration values ​​in a memory (likewise not shown in detail).The calibration unit 100 thus enables the calibration of an optical position sensor 101 with respect to an immobile, i.e. stationary and in particular fixed and / or actuator-free measuring object 102.

[0041] Particularly preferably, the movement unit 106 is formed as an actuator 103 associated with the optical position sensor, in particular indirectly or directly connected to the position sensor 101.

[0042] The actuator 103 can be arranged at any location on the position sensor 101, but it is particularly advantageous if the actuator 103 is arranged at least in part on or around a measuring head of the position sensor 101, or at least partially surrounds it.

[0043] The position sensor 101 can be connected to a housing or to a frame 109, in particular to a sensor frame, via an intermediate element 110. In one embodiment, it is preferred if the intermediate element 110 has the at least one actuator 103, i.e., the at least one actuator 103 is arranged on or in the intermediate element 110. Furthermore, the intermediate element 110 can be configured to position the position sensor 101 relative to the measurement object 102.

[0044] The actuator 103 can be designed as a solid-state actuator, in particular as a piezo actuator, or as an electrostrictive, magnetostrictive, or ferroelectric actuator. Alternatively, the actuator 103 can also be designed as a capacitive actuator or as an electromagnetic actuator, in particular as a Lorenz actuator, although the list of possible actuators 103 is not exhaustive.

[0045] Furthermore, it is advantageous if the optical position sensor 101 is formed as an encoder 104 or as an interferometer 105. An embodiment of an interferometer 105 is described in the Fig. 3. In this simple embodiment, the interferometer 105 comprises a light source 111, a beam splitter 112, and, in addition to the (movable) mirror 113 attached to the measurement object as a measurement target, another mirror 114, and a detector 115. The light emitted by the light source 111 is split by the beam splitter 112, with the first reflected light beam 116 striking the mirror 113 of the measurement target and the second reflected light beam 117 striking the fixed additional mirror 114, and each being reflected back to a detector 115, thereby generating interference. A relative position determination is then possible via the change in the interference, with detection preferably taking place using an intensity measurement, a phase shift measurement, or a frequency measurement. The optical position sensor can be formed as any type of interferometer.In one embodiment, the further mirror 114 of the interferometer 105 can additionally be assigned at least one actuator 103, which is configured to change the position of the mirror relative to the mirror 113 / beam splitter 112 in such a way that the optical path length is adjusted. Calibration is then performed at different optical path lengths, which are adjusted by adjusting the mirror 114.

[0046] Furthermore, the optical position sensor 101 can also be formed as an optical encoder 104, in particular as an interferometric encoder based on a diffraction grating. Such an encoder 104 is described, for example, in Fig. 4 and comprises a beam splitter 112, which splits the light emitted by the light source 111 into two beams. The two beams impinge on the diffraction grating 116 at two different positions and are diffracted there. The first beam is then projected via a λ / 4 plate onto a first mirror 117, and the second beam is projected via another λ / 4 plate onto a second mirror 118, where they are each reflected back onto the diffraction grating 116, diffracted there, and directed back onto the beam splitter 112. Changes in the interference pattern can be detected by means of at least one or more photodetectors, in particular by means of four photodetectors, and thus position changes can be detected.

[0047] In one embodiment, the actuator 103 can be assigned to one of the mirrors 117, 118 of the optical encoder 104 and configured to adjust the position of this mirror relative to the diffraction grating 116. In particular, it is preferred if the actuator is arranged on the mirror 117 that is located in the measurement beam path (not in the reference beam path) of the encoder, and from which the light diffracted by the diffraction grating is reflected back onto the diffraction grating.

[0048] In addition, the measurement object can also have at least one cooling line (not shown in detail), such that a movement of the measurement object 102 relative to the optical position sensor 101 is induced by adjusting a pressure in the at least one cooling line from a first pressure to a second pressure that differs from the first pressure.

[0049] Furthermore, it is also possible that objects to which the measurement object 102 is connected, such as a frame or a scanner or a wafer stage, which have an actuator, are used to move the measurement object, which itself is actuator-free, i.e., does not have an actuator.

[0050] Alternatively, the described calibration unit 100 can also be used for or in an inspection system for inspecting a substrate for semiconductor lithography, wherein the inspection system comprises a static measurement object 102, in particular an optical element, for example a mirror or an EUV mirror or a lens.

[0051] The described calibration unit 100 is in particular configured to carry out the method described below.

[0052] The method for calibrating an optical position sensor 101 for detecting the position of a static measurement object 102, preferably in a lithography machine or in an inspection system, preferably in an inspection system for masks or EUV masks, comprises in particular the following steps: a. Detecting a first calibration value at a first relative distance d between the static measurement object 102 and the optical position sensor 101, b. Changing the relative distance d at least once to at least one further relative distance between the optical position sensor 101 and the static measurement object 102 that differs from the first relative distance and detecting a further calibration value at the further relative distance d, and c. Storing at least the first calibration value at the first relative distance and the further calibration value at the further relative distance in a memory as a calibration curve.

[0053] The static measurement object 102 is preferably an optical element, in particular a mirror or a lens. The term "static" in the context of the present invention means that the measurement object is immobile, i.e., stationary and in particular fixed in position and / or actuator-free, i.e., in particular, free from a solid-state actuator, in particular a piezo actuator, a Lorenz actuator, or a capacitive or electromagnetic actuator. In other words, the measurement object 102 is not movable by an actuator and is not directly connected to an actuator. The measurement object 102 can be rigidly and in particular stationary attached to an object 108, wherein the object 108 is preferably a frame structure, in particular a sensor frame or a force frame. The method thus enables the most accurate possible position detection of an immobile measurement object 102.

[0054] The change in the relative distance d between the static measurement object 102 and the optical position sensor 101 is preferably carried out by means of a movement unit 106, wherein the movement unit 106 is assigned to the optical position sensor 101 or is formed as such. In other words, preferably the position sensor 101 alone moves toward the static measurement object 102 or away from the static measurement object 102.

[0055] In the embodiment according to the Fig. 2, the relative distance between the static measurement object 102 and the optical position sensor 101 is changed by means of at least one actuator 103 assigned to or arranged on the optical position sensor 101. The actuator 103 can be designed as a solid-state actuator, in particular as a piezo actuator or as an electrostrictive, magnetostrictive, or ferroelectric actuator. Alternatively, the actuator can also be designed as a capacitive actuator or as an electromagnetic actuator, in particular as a Lorenz actuator, whereby the list of possible actuators is not exhaustive.

[0056] The optical position sensor 101 is in this case formed as an encoder 104 or as an interferometer 105.

[0057] Furthermore, it is alternatively preferred if the relative distance between the static measurement object 102 and the optical position sensor 101 is changed by changing an optical path length of the optical position sensor 101 by means of an actuator 103 assigned to one of the mirrors of the optical position sensor 101. This is the case, for example, in the embodiments of Fig. 3 and Fig. 4. The adjustment of the various relative distances is achieved by setting different optical path lengths by adjusting a mirror 117, 114 in the interferometer 105 or in the optical encoder 104. LIST OF REFERENCE SYMBOLS 100 calibration units 101 optical position sensor 102 static measuring object 103 Actuator 104 encoders 105 interferometers 106 movement units 108 objects 109 frames 110 Intermediate element 111 Light source 112 beam splitters 113 Mirror (measurement target) 114 more mirrors 115 Detector 116 diffraction gratings (encoders) 117 first mirror (encoder) 118 second mirror (encoder) 600 projection exposure system 601 plasma light source 602 collector mirror 603 field facet mirror 604 Pupillary facet mirror 605 first telescope mirror 606 second telescope mirror 607 Deflecting mirror 620 Mask table 621 Mask 651 Mirror (projection lens) 652 mirror (projection lens) 653 Mirror (projection lens) 654 mirror (projection lens) 655 Mirror (projection lens) 656 mirrors (projection lens) 660 wafer table 661 coated substrate 700 DUV lithography system 701 DUV light source 702 DUV radiation / beam path 703 Beam Forming and Illumination System (DUV) 704 Photomask 705 projection system 706 wafers 707 lens 708 mirrors 709 optical axis QUOTES CONTAINED IN THE DESCRIPTION

[0000] This list of documents submitted by the applicant was generated automatically and is included solely for the convenience of the reader. This list is not part of the German patent or utility model application. The DPMA assumes no liability for any errors or omissions. Cited patent literature

[0000] US 2005 / 0280563 A1

[0005] DE 10 2020 123 482 A1

[0005] WO 2019 / 223968 A1 [0012, 0019, 0028] EP 1 286 139 B1 [0013, 0020, 0029] DE 10 2012 205 181 A1

[0026] Zitierte Nicht-Patentliteratur

[0000] D. Gurauskis et al.; Appl. Science; „Auto-Calibration and Noise Reduction for the Sinusoidal Signals of Magnetic Encoders

[0005] Ha Xuan Nguyen et al., IEEE 2017 und die „Encoder Calibration Method for High Precision Servo Systems With a Sinusoidal Encoder

[0005] Youngwoo Lee et al.; IEEE Transactions on industrial electronics, Vol. 69, No. 1

[0005]

Claims

[1] Method for calibrating an optical position sensor (101) for detecting the position of a static measurement object (102), comprising the steps: a. detecting a first calibration value at a first relative distance d between the static measurement object (102) and the optical position sensor (101), b. Changing the relative distance at least once to at least one further relative distance between the optical position sensor (101) and the static measurement object (102) that differs from the first relative distance and detecting a further calibration value at the further relative distance, and c. Storing at least the first calibration value at the first relative distance and the further calibration value at the further relative distance in a memory as a calibration curve. [2] Method according to claim 1, characterized bythat the relative distance between the static measuring object (102) and the optical position sensor (101) is changed by means of a movement of the optical position sensor (101). [3] Method according to claim 1 or 2, characterized by that the relative distance between the static measuring object (102) and the optical position sensor (101) is changed by means of at least one actuator (103) assigned to the optical position sensor (101). [4] Method according to one of claims 1 to 3, characterized by that the optical position sensor (101) is formed as an optical encoder (104) or as an interferometer (105). [5] Method according to one of claims 3 or 4, characterized by that the actuator is formed as a solid-state actuator or as a Lorenz actuator or as a capacitive actuator. [6] Method according to one of claims 1 to 5, characterized bythat the change in the relative distance d between the static measuring object (102) and the optical position sensor (101) takes place by changing an optical path length of the optical position sensor (101) by means of an actuator (103) assigned to one of the mirrors of the optical position sensor (101). [7] Method according to claim 1, characterized by in that the static measuring object (102) has at least one line, and in that a movement of the measuring object (102) relative to the optical position sensor (101) is induced by adjusting a pressure in the at least one line from a first pressure to a second pressure deviating from the first pressure. [8] Calibration unit (100) for calibrating an optical position sensor (101) with respect to a measurement object (102) with a static measurement object (102) and with an optical position sensor (102), wherein a movement unit (106) is provided which is configured to change a relative distance d between the optical position sensor (101) and the static measurement object (102) and with a readout unit which is configured to detect a measurement signal of the optical position sensor (101) at at least two mutually differing relative distances and to store it in a memory as calibration values. [9] Calibration unit according to claim 8, characterized by that the optical position sensor (101) is formed as an encoder (104) or as an interferometer (105). [10] Calibration unit (100) according to claim 8 or 9, characterized in that the movement unit (106) is formed as an actuator (103) associated with the optical position sensor (101). [11] Calibration unit (100) according to claim 10, characterized by that the actuator (103) is assigned to one of the mirrors of the optical position sensor (101) and is configured to change the optical path length. [12] Calibration unit (100) according to claim 10 or 11, characterized by that the actuator (103) is formed as a solid-state actuator or as a Lorenz actuator or as a capacitive actuator. [13] Calibration unit (100) according to one of claims 8 to 12, characterized by that the static measuring object (101) is an optical element. [14] Inspection system for inspecting a substrate for semiconductor technology, with at least one static measurement object and with at least one optical position sensor (101) for detecting the distance between the static measurement object (102) and a reference, and with at least one calibration unit (100) for calibrating the optical position sensor (101) with respect to the measurement object (102), wherein a movement unit (106) is provided which is designed to change a relative distance d between the optical position sensor (101) and the static measurement object (102) and with a readout unit which is designed to detect a measurement signal of the optical position sensor (101) at at least two mutually differing relative distances and to store it in a memory as calibration values.

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