Method for calibrating a particle sensor, particle sensor and device having a particle sensor - Patents.com

Abstract

The invention relates to a method for calibrating a particle sensor (2), comprising the steps of focusing a light beam, in particular a laser beam (8), on a calibration plane (KE) in order to generate a calibration intensity distribution (10), in particular a calibration focus, in said calibration plane (KE), a calibration plate (1) being arranged in said calibration plane (KE) and forming thereon a contrast area (2) for modulating the intensity (I) of the light beam, in particular the laser beam (8), moving the calibration plate (1) and / or the calibration focus (10) in the calibration plane (KE) and measuring at least one intensity signal (I T ;I R ), and recording at least one intensity signal (I T ;I R The invention also relates to a particle sensor (2) for carrying out this method and to an apparatus comprising at least one such particle sensor (2).

Description

[Technical field]

[0001] The present invention relates to a method for calibrating a particle sensor, to a particle sensor and to a device comprising at least one such particle sensor. [Background technology]

[0002] Particle sensors serve to characterize particles, for example to determine their position, their velocity, their acceleration, their trajectory and / or their size. Particle characterization is of great relevance in many fields, for example in the chemical, pharmaceutical or semiconductor industries.

[0003] Particle sensors based on optical measurement methods, especially using laser radiation, have found widespread use in monitoring the purity of air and liquids. However, the accuracy of such particle sensors quickly deteriorates if contamination accumulates on surfaces in the optical path of the laser beam or if there are changes in the parameters of the light beam or the laser parameters. This requires complex recalibration of the system, which is not always feasible during field use.

[0004] US Pat. No. 5,399,433 describes a calibration device for automatically calibrating an analytical system for measuring particle velocities. The calibration device comprises a plate having a contrast area on its surface for the purpose of modulating light intensity. The contrast area is moved to simulate the movement of a particle introduced into a measurement cell during a measurement operation of the analytical system. The plate or the contrast area can be introduced into an observation beam path of the analytical system for the purpose of calibrating the analytical system. The plate defines a calibration plane located in the vicinity of a surface on which an image of the particles is generated. [Prior art documents] [Patent documents]

[0005] [Patent Document 1] U.S. Pat. No. 3,885,415 [Patent Document 2] DE 102019209213.6 Summary of the Invention [Problem to be solved by the invention]

[0006] The invention is based on the object of providing a method, a particle sensor and a device comprising a particle sensor which allow a very accurate calibration of the particle sensor. [Means for solving the problem]

[0007] This object is achieved by a method for calibrating a particle sensor, the method comprising the steps of directing, in particular focusing, a light beam, in particular a laser beam, on a calibration plane in order to generate a calibration intensity distribution, in particular a calibration focus, in the calibration plane, where a calibration plate is arranged in the calibration plane and forms contrast areas thereon for modulating the intensity of the light beam, in particular a laser beam, moving the calibration plate and / or the calibration intensity distribution in the calibration plane, recording at least one intensity signal of the light beam, in particular a laser beam, after passing through the calibration plane, and calibrating the particle sensor, usually by evaluating the at least one intensity signal taking into account the (known) properties of the contrast area.

[0008] In the method according to the invention, the contrast area on the calibration plate is used for the purpose of calibrating the particle sensor with the contrast area having known properties, such as for example a known size or a known diameter. The contrast area serves to simulate particles or form calibration particles that are detected by the particle sensor during the calibration operation and differ from the surroundings of the contrast area on the calibration plate with respect to at least one (optical) property. By way of example, the surroundings of the contrast area may be formed by the substrate of the calibration plate. The light beam or laser beam may for example be at least partially scattered, absorbed, diffracted or reflected by the contrast area. The surroundings of the contrast area, for example in the form of a substrate, may differ as substantially as possible from the optical properties of the contrast area and may for example be (substantially completely) transparent to the light beam or laser beam. A large difference between the contrast area and the surroundings occurs, for example, when the contrast area reflects the light beam or laser beam while the surroundings of the contrast area absorb the light beam or laser beam.

[0009] The calibration exploits that the contrast regions have known properties, e.g. known size / area or shape, and known optical properties in terms of absorption, reflectance, scattering behavior, etc. The movement of the calibration plate or the calibration intensity distribution in the calibration plane is also defined during the calibration and is therefore known. Thus, by moving the calibration plate or the calibration intensity distribution, it is possible to move a calibration particle with known properties along a known trajectory. Thus, the particle sensor can be calibrated with respect to measurement variables, e.g. particle size, particle material, particle trajectory, particle velocity, particle acceleration, etc.

[0010] In the sense of this application, calibration of a particle sensor is also understood to mean checking an existing calibration of the particle sensor. If the existing calibration turns out to be inaccurate during the check, as may be the case for example due to contamination of surfaces in the beam path of the light or laser beam or changes in the parameters of the light or laser beam (see above), a recalibration of the particle sensor can be performed.

[0011] The particle sensor characterizes the calibration particles in the same way as the particles passing through the measurement volume of the particle sensor, i.e. the calibration plate simulates the passage of the particles through the particle sensor. When a light or laser beam is incident on the calibration particles or contrast areas during the movement of the calibration plate or the calibration intensity distribution, this usually results in an attenuation of the recorded intensity signal. Usually, the attenuation of the intensity signal increases with the increase of the area of ​​the calibration particles. In particular, the attenuation of the intensity signal may be proportional to the area of ​​the calibration particles.

[0012] The light or laser beam is directed onto a calibration plane. The light or laser beam that generates the calibration intensity distribution in the calibration plane can be, for example, a collimated light or laser beam. If particles with smaller particle diameters are to be characterized, it is advantageous to focus the light or laser beam into the calibration plane. In this case, the calibration intensity distribution forms a calibration focus in the calibration plane.

[0013] The calibration method described above can be used with a number of particle sensors operating on the basis of optical radiation, in particular laser radiation. By way of example, the calibration method can be used for a sensor arrangement or particle sensor described in US Pat. No. 6,399,363, which is incorporated in its entirety by reference into the content of the present application. In the case of the particle sensor described in this document, a mode converter is used to generate a field distribution of a laser beam with a different combination of local intensity and local polarization direction of the laser beam at each position of the field distribution. The analyzer optical unit serves to determine the polarization-dependent intensity signal of the field distribution of the laser beam that has passed through the measurement volume. The particles are characterized on the basis of the polarization-dependent intensity signal, exploiting the fact that the field distribution or the optical mode of the field distribution has an inherent correlation between the local intensity distribution and the local polarization direction. However, it is understood that other types of particle sensors based on optical radiation, e.g. laser radiation, or the attenuation of optical radiation, e.g. laser radiation, when passing through a measurement volume can also be calibrated using the calibration method described herein.

[0014] In a variant, the calibration plate is placed in a calibration plane in the measurement volume through which the particles pass during the measurement operation of the particle sensor. In this variant, the calibration plate is introduced from the outside into the measurement volume through which the particles flow during the measurement operation. In this case, the calibration is implemented outside the measurement operation in order to calibrate the particle sensor before it is operated or during operation. This variant requires that the measurement volume is accessible from the outside in order to position the calibration plate, which is not possible in all cases.

[0015] In an alternative variant, the calibration intensity distribution in the calibration plane, in particular the calibration focus, and the measurement intensity distribution in the measurement plane, in particular the measurement focus, are imaged on one another. The measurement plane is formed in a measurement volume through which the particles pass during the measurement operation of the particle sensor. In this case, the calibration plate is not introduced into the measurement volume but is arranged at a predetermined distance from the measurement volume in the beam path of the light or laser beam. It is possible, but not necessary, for the light or laser beam to be focused in the measurement plane so that the measurement intensity distribution forms the measurement focus. The imaging of the calibration intensity distribution or the calibration focus and the measurement intensity distribution or the measurement focus on one another can be implemented with a beam shaping device in the form of an imaging optical unit. The imaging optical unit generates intensity distributions or double focuses with comparable beam parameters (such as beam radius). By way of example, the calibration plate can be introduced at the other of the two focuses (calibration focus), with the second focus (measurement focus) being located in the measurement volume through which the particles to be characterized flow. Knowledge of the calibration intensity distribution or calibration focus and the beam radius of the measurement intensity distribution or measurement focus allows the use of the calibration particles or contrast areas of the calibration plate to calculate the size of the particle in the measurement volume after calibration, where the particle size of a particle is understood to mean the cross-sectional area of ​​the particle in the measurement plane.

[0016] In this variant, the calibration plate may be integrated into the particle sensor, so that the calibration can be checked and repeated during use of the particle sensor in the field. The calibration plate does not move during the measurement operation of the particle sensor, and is removed from the beam path of the light or laser beam, or positioned in such a way that the attenuation of the light or laser beam is not too significant. As an example, the calibration plate may be positioned such that the light or laser beam passes through it in transparent areas of the substrate where no contrast areas are formed.

[0017] In this variant, not only does the measurement intensity distribution or measurement focus in the measurement plane form a certain type of "virtual sensor surface" that allows remote characterization of the particle without the need to position a sensor at the particle's location, but the calibration intensity distribution or calibration focus in the calibration plane allows calibration without the introduction of calibration particles into the measurement plane or measurement volume, thus enabling remote diagnosis or calibration of the particle sensor in this variant.

[0018] In a development of this variant, the calibration plate is arranged in a housing that is separated from the measurement volume. In order to protect the calibration plate from contamination by particles flowing through the measurement volume, it is advantageous to arrange the calibration plate in a volume that is hermetically separated from the measurement volume.

[0019] In a further variant, the contrast regions on the calibration plate have different surface areas. The different surface areas or sizes of the contrast regions simulate calibration particles with different, previously known particle sizes and particle diameters. The size of the contrast regions is adapted to match the type of particle or type of particle sensor to be characterized. Calibration with different sized contrast regions or calibration particles on the calibration plate allows high accuracy during subsequent particle measurements over a large range of sizes.

[0020] In a further variant, the contrast areas are formed by microstructures on the surface of a substrate, in particular a wafer. In this case, the calibration plate may have a substrate, for example made from glass or crystal, on which contrast areas are formed in the form of microstructures with known structure dimensions, these contrast areas acting like calibration particles. The microstructures may be applied, for example by microlithography, to the surface of a transparent substrate, for example in the form of a glass wafer. The contrast areas may form structures that at least partially absorb, or reflect, or scatter a light or laser beam. Both the shape and the size or surface area of ​​the contrast areas are known with great precision as a result of the microstructuring, and these contrast areas can therefore be used as calibration standards. The microstructuring may be implemented, for example, on a layer, such as a metal layer, applied to the substrate, but it is also possible to microstructure the substrate itself. With a view to comparing the size of the contrast areas generated by the microstructuring, the calibration carried out with the calibration plate may optionally be verified with standard particles of defined size, for example polystyrene spheres of known size.

[0021] In an alternative variant, the contrast area is formed by calibration particles with a specifically defined particle size, the calibration particles preferably comprising at least two groups of calibration particles with different optical properties, in particular formed from different materials. By way of example, the different optical properties may be different (complex) refractive indices, which result in the calibration particles absorbing, reflecting, scattering and / or diffracting light or laser radiation in different ways. The use of calibration particles formed from different materials allows a calibration of the particle sensor to be performed for the detection of particles of different materials or material classes, without different calibration plates being required for this purpose.

[0022] The materials from which the calibration particles of the individual groups are formed may be, for example, metal (e.g. steel), sand (e.g. SiO2), plastic (e.g. polystyrene or latex), ceramics, etc., since these materials usually differ significantly with respect to their optical properties. The three material classes metal, sand and plastic are common constituents of dust.

[0023] In this case, the calibration particles may for example be applied to the calibration plate with a defined distribution or with a defined pattern (or within a defined surface area or field), although different groups of calibration particles may optionally be applied with a statistical distribution (see below). Of course, it is also possible to optionally apply only one type of calibration particles with different defined particle sizes to the calibration plate.

[0024] In a development of this variant, the calibration particles are applied to the surface of the substrate in a statistical distribution and / or the calibration particles are formed on the surface of the substrate by self-structuring.

[0025] The size of the calibration particles is known in the first case, but the exact distribution on the surface is not known beforehand, rather the calibration particles are statistically distributed during application to the surface. In this case, the calibration particles, i.e. particles of known size, can be suspended in low concentration, for example, in a liquid transparent adhesive and applied as a thin layer to the calibration plate or substrate. When the adhesive dries, the calibration particles are statistically distributed on the calibration plate. In particular, spherical monodisperse particles of known size, such as polystyrene latex balls, which are established as a standard for particle size measurement and are available with a narrow size distribution, are suitable calibration particles for this variant. As an example, such calibration particles are used to calibrate particle counters according to the ISO21501 standard, see the link "de.wikipedia.org / wiki / ISO_21501".

[0026] In the second case, the adhesive or photoresist is applied to the plate-shaped substrate with the aid of a dispenser. Before curing (in particular with UV light or in an oven), this photoresist or adhesive forms spherical segments, which are at most hemispherical as a result of surface tension, depending on the viscosity of the photoresist or adhesive. The three-dimensional structure of the calibration particles thus generated, i.e. by self-structuring, allows the approximation of the scattering behavior of particles with a spherical surface and is therefore particularly suitable for particle sensors based on the principle of scattered light measurement. The structure size of the calibration particles thus generated depends, inter alia, on the process parameters during the curing of the photoresist or adhesive and can be assumed as known for the given process parameters. In addition, the plate-shaped substrate can be cleaned, coated or laser-structured before the application of the adhesive, so that a hydrophobic or hydrophilic surface results for the adapted shaping of the droplets.

[0027] In a further variant, the calibration plate is rotated in an automated manner and / or displaced in an automated manner within its range of movement in the calibration plane.

[0028] The calibration plate can be pivoted or rotated around a given axis of rotation with the aim of moving in a calibration plane and simulating the movement of particles. By way of example, the automated rotation of the calibration plate can be implemented by means of an electric motor. In the case of a calibration plate appropriately dimensioned with respect to its diameter, the calibration particles move in an approximately straight line through the light or laser beam of the particle sensor here and result in an attenuation of the intensity of the light or laser beam that is generally proportional to the area of ​​the contrast area or the calibration particle. In this case, the calibration plate is usually designed to be circular and can have a diameter of, for example, 10 mm to 100 mm. By way of example, the calibration plate can be rotated with a frequency of 0.1 Hz to 200 Hz. By way of example, 1 to 400 calibration particles / revolution can be applied to the calibration plate. By way of example, the calibration particles can be round and can be applied by lithographic methods. The light or laser beam can have a diameter of, for example, 20 μm to 200 μm at the calibration intensity distribution or calibration focus. The diameter of the calibration particle is typically between 0.05% and 100% of the focal diameter or beam diameter.

[0029] The calibration plate can also be displaced linearly in the calibration plane. In this case, for example, a piezoelectric drive or a piezoelectric crystal or a linear drive can be used for the automated displacement. The use of a piezoelectric drive for displacing the calibration plate has been found to be advantageous, since it operates without wear and therefore does not generate any measurement errors that may otherwise accumulate on the calibration plate as a result of product wear. By way of example, the frequency of the displacement of the calibration plate can be at the level of 1 Hz to 100 Hz. In this case, the calibration plate can have, for example, a rectangular shape.

[0030] Alternatively or additionally, the calibration intensity distribution or the calibration focal point of the light or laser beam can also be moved in the calibration plane. To move the calibration intensity distribution, the light or laser beam can perform a scanning movement, for example by the light or laser beam being deflected by using a mirror scanner or a scanner mirror, an acousto-optical modulator, etc., in order to change the position of the calibration intensity distribution in the focal plane. In this case, the calibration plate can be arranged in a stationary manner, but this is not necessary, i.e. it is also possible that there is a superimposed movement of the calibration plate and the calibration intensity distribution.

[0031] A further aspect of the invention relates to a particle sensor comprising an emitter having a light source for generating a light beam, in particular a laser source for generating a laser beam, an optical unit for directing, in particular focusing, the light beam, in particular the laser beam, on a calibration plane in order to generate a calibration intensity distribution, in particular a calibration focus, in the calibration plane, where a calibration plate is arranged in the calibration plane, a movement device for moving the calibration plate and / or the calibration intensity distribution in the calibration plane, a receiver comprising a detector for recording at least one intensity signal of the light beam, in particular the laser beam, after passing through the calibration plane, and an evaluation device for evaluating the at least one intensity signal during a calibration operation of the particle sensor in order to calibrate the particle sensor.

[0032] As further described above in connection with the method, the movement of the particles is simulated using the contrast region during a calibration operation of the particle sensor. If the size of the contrast region or the calibration particle is known, a calibration of the particle sensor can be performed based on the recorded intensity signal or signals. During the calibration, each recorded intensity signal can be saved as a characteristic for a particular particle size or a particular particle velocity and / or particle trajectory and can serve as a calibration standard. This calibration standard can be used to characterize the particles passing through the measurement volume during a measurement operation of the particle sensor.

[0033] In the case of the particle sensors described here, the characterization of the particles can be implemented in transmission, i.e. a light or laser beam is emitted by an emitter and passes through the measurement volume in transmission. However, the particle sensor can also perform the characterization of the particles as a result of scattering and / or absorption of the light or laser beam on the particles. In the last mentioned two cases, the reflected or scattered light or laser beam is detected at a detector of the receiver.

[0034] In one embodiment, the calibration plate is placed in a calibration plane located within the measurement volume through which particles pass during a measurement operation of the particle sensor. As further described above, the calibration plate in this case is typically placed within the measurement volume only during a calibration operation. During a measurement operation of the particle sensor, the calibration plate is removed from the measurement volume in an automated or manual manner.

[0035] In an alternative embodiment, the optical unit of the particle sensor comprises an imaging optical unit for imaging the calibration intensity distribution in the calibration plane and the measurement intensity distribution in the measurement plane, in particular the measurement focus, onto each other, the measurement plane being formed in a measurement volume through which the particles pass during the measurement operation of the particle sensor. In this embodiment, the calibration plate is generally permanently integrated into the particle sensor, typically the emitter of the particle sensor. In this case, the calibration plate is optionally introduced into the beam path of the light or laser beam in an automatic manner before the calibration operation is started, but the calibration plate may be permanently placed in the beam path of the laser beam. In this case, the calibration plate is positioned in a manner that interferes as little as possible with the implementation of the measurement operation. By way of example, in this case, the position at which the light or laser beam is incident on the calibration plate may be a place where no contrast area is located.

[0036] In a further embodiment, the emitter has a housing with an exit window and the receiver has a housing with an entrance window, a measurement volume being formed between the exit and entrance windows. In this case, the calibration plate is usually arranged in the housing of the emitter or receiver and is hermetically separated from the measurement volume. This is advantageous for the prevention of contamination of the calibration plate by particles passing through the measurement volume. The arrangement of the calibration plate in the receiver is preferred, since the power of the light or laser beam can be reduced due to contamination of the entrance or exit window. In this case, the calibration can be implemented already with reduced power and with possible optical interference of the light or laser beam as a result of dust on the entrance or exit window.

[0037] The calibration plate can be constructed in the manner described above in connection with the method. By way of example, the contrast areas can have different surface areas. In addition, the contrast areas can also be designed to at least partially scatter, absorb or reflect the light beam or laser beam. The contrast areas on the calibration plate can be formed by microstructuring. The contrast areas can optionally be formed by calibration particles with a defined particle size applied in a statistical distribution to the surface of the substrate of the calibration plate, or groups of calibration particles with different properties can be provided on the calibration plate.

[0038] In a further embodiment, the displacement device is designed to rotate the calibration plate in the calibration plane, in which case the displacement device can for example be designed as an electric motor which engages with a rotating shaft to which the calibration plate is fixed.

[0039] In a further embodiment, the displacement device is designed to displace the calibration plate in the calibration plane. In this case, the displacement device can be designed as a piezoelectric actuator, which for example engages with a lateral edge of the calibration plate in order to displace the calibration plate in the calibration plane. The piezoelectric actuator allows a non-abrasive displacement of the calibration plate in the calibration plane.

[0040] A further aspect of the invention relates to an apparatus, in particular an EUV radiation generating apparatus, comprising a measurement chamber, in which particles can be supplied to the measurement chamber, e.g. a target area, and a particle sensor designed as described above for characterising particles in the measurement chamber.

[0041] Moreover, as mentioned above, particle sensors can be used in multiple applications with the aim of characterizing solid, liquid or gas particles or particle flows. By way of example, the measurement chamber can be designed to receive a gas or liquid or to allow said gas or liquid to pass through. In the first case, the device can serve for example to measure the purity of air, for example to characterize soot particles or for particulate matter measurement. Characterization of powders is also possible, for example in the case of powders used for additive manufacturing (3D printing) or in the case of building materials such as cement, for example in view of the size distribution of the powder or grains. Characterization of particles in liquids is also possible, for example characterization of particles in the form of bacteria in milk. Machine learning can be used in all these applications and devices with the aim of characterizing particles or performing calibrations. In the case of machine learning, test data is generated and a suitable AI, for example in the form of a neural network or similar, is trained.

[0042] Particle sensors may also be used in the semiconductor industry. By way of example, particle sensors may be used to characterize particles or particle flows in a measurement chamber in the form of a vacuum chamber of an EUV radiation generating device. Typically, such an EUV radiation generating device comprises a driver laser arrangement for generating a driver laser beam and a beam supply device for supplying the driver laser beam to the aforementioned vacuum chamber. The driver laser beam is focused in a target area of ​​the vacuum chamber, in which a target material in the form of tin particles or tin droplets has already been introduced or supplied. The individual particles transition to a plasma state during irradiation by the driver laser beam and in the process emit EUV radiation. Particles of the target material guided to the target area and particles generated during the incidence of the laser beam on the target material (in case of evaporation of the target material) can be characterized using the aforementioned particle sensor.

[0043] Further advantages of the invention are evident from the description and the drawings. Likewise, the features mentioned above and those not yet presented can in each case be used alone or in multiples in any desired combination. The illustrated and described embodiments should not be understood as an exhaustive list, but rather as exemplary features for outlining the invention. [Brief description of the drawings]

[0044] [Figure 1] 1a and 1b show schematic diagrams of a calibration plate serving to calibrate a particle sensor and designed for rotation or displacement in a calibration plane. [Diagram 2] 2a and 2b show schematic diagrams of the calibration plate of FIGS. 1a and 1b positioned in a calibration plane within the measurement volume of a particle sensor. [Diagram 3] FIG. 3 shows a schematic diagram of a particle sensor having an imaging optical unit for imaging a calibration focus in a calibration plane and a measurement focus in a measurement plane onto one another, the measurement plane being formed in a measurement volume of the particle sensor. [Figure 4]FIG. 4 shows a schematic diagram of an EUV radiation producing apparatus having a particle sensor for characterizing particles in a vacuum chamber. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0045] In the following description of the drawings, the same reference numbers are used for identical or functionally identical components.

[0046] Figures 1a and 1b show a calibration plate 1 for calibrating a particle sensor 2 depicted in Figures 2a, 2b and 3 respectively. The calibration plate 1 comprises a transparent substrate 3 in the form of a crystal wafer, a glass wafer or a plastic plate. In the example shown, a circular contrast area 4 is formed on the substrate 3, more precisely on the planar surface of the substrate 3. The contrast area 4 has a size and shape that is known beforehand and serves as a calibration particle for calibrating the particle sensor 2.

[0047] The particle sensor 2 shown in Figure 2a, 2b or 3 comprises an emitter 5 and a receiver 6. The emitter 5 comprises a laser source 7 serving to generate a laser beam 8. A different type of light source generating a light beam or optical radiation other than laser radiation can optionally also be used instead of the laser source 7. By way of example, the light source can be an LED or similar.

[0048] In the illustrated example, the laser source 7 is a diode laser with a laser wavelength selected from the wavelength range of about 250 nm to about 1600 nm depending on the application. By means of an optical unit in the form of a focusing optical unit 9, the laser beam 8 is focused onto a calibration focus 10 in a calibration plane KE, in which the calibration plate 1 is arranged. The calibration focus 10 forms a calibration intensity distribution in the calibration plane KE. When passing through the calibration plate 1, and more precisely the contrast area 4, the laser beam 8 is incident on the contrast area 4 and is modulated with respect to its intensity I. The modulation of the intensity I can be realized, for example, by the transparency of the contrast area being reduced with respect to the substrate 3, the refractive index of the contrast area 4 being changed with respect to the substrate 3, etc.

[0049] In the example shown in Fig. 1a, the contrast areas 4 are formed from a material that absorbs the laser beam 8, whereas the substrate 3 of the calibration plate 1 is formed from a material that is transparent to the wavelength of the laser beam 8. When the laser beam 8 is incident on an individual contrast area 4, the intensity I of the laser beam 8 is reduced as it passes through the calibration plate 1.

[0050] The calibration plate 1 or calibration plane KE shown in Fig. 1a is placed in the measurement volume 11 of the particle sensor 2 shown in Fig. 2a. The receiver 6 of the particle sensor 2 in Fig. 2a is arranged along the propagation direction Z (Z direction of the XYZ coordinate system) along the collimation line to the laser source 7 or receiver 6, and the intensity signal I T The evaluation device 13 detects the intensity signal I of the laser beam 8. T It plays a role in evaluating the

[0051] In contrast to the calibration plate 1 shown in Figure 1a or 1b, the contrast area 4 is made of a material that reflects the wavelength of the laser beam 8. The calibration plate 1 shown in Figure 1b is placed in a calibration plane KE of the particle sensor 2 shown in Figure 2b, which differs substantially from the particle sensor 2 shown in Figure 2a in that the receiver 6 is arranged at an angle to the propagation direction of the laser beam 8 generated by the laser source 7, which allows the detector 12 to measure the intensity signal I R The detector 12 of the particle sensor 2 of FIG. 2b can also serve to detect the scattered radiation of the laser beam 8 scattered in the contrast area 4.

[0052] The focusing of the laser beam 8 in the calibration plane KE shown in Figures 1a, 1b serves to generate a small beam diameter of the laser beam 8 in the calibration plane KE. Focusing can optionally be omitted if larger particles are intended to be detected by the particle sensor 2. In this case, the laser beam 8 can optionally be collimated or possibly emitted in a divergent manner onto the calibration plane KE. In this case, instead of the focusing optical unit 9, a different type of optical unit is used, such as, for example, a collimating optical unit.

[0053] The contrast areas 4 are formed as microstructures in the example shown in FIG. 1a in order to form the most accurate calibration standard possible for the calibration of the particle sensor 2. The microstructures are formed by microlithographic structuring of a metal layer that is applied to the surface of the substrate 3 and is removed again from the surface of the calibration plate 1 during the microstructuring except for the circular contrast areas 4. The microstructuring allows for an accurate definition of the size of the contrast areas 4 or of the calibration particles and their spacing in order to increase the accuracy when calibrating the particle sensor 2. In the example shown, the contrast areas 4 have surface areas of different sizes or different diameters in order to simulate particles of different sizes and thus allow an accuracy of the particle sensor 2 for different particle sizes over a particularly wide size range. These can also be formed by ultrashort pulse machining or the like on the calibration plate 1 as an alternative to the creation of the contrast areas 4 by microstructuring.

[0054] In the example shown in FIG. 1b, the contrast area is formed by calibration particles 4 applied to the surface of the substrate 3 in a defined distribution (as an array). The calibration particles 4 of adjacent rows of an array-type configuration differ from each other with respect to their optical properties, in particular their (complex) refractive index in the example shown, i.e. they form different groups of calibration particles 4. In FIG. 1b, the groups of calibration particles 4 are formed from different materials. By way of example, FIG. 1b shows a first group 4a of calibration particles 4 formed from a metallic material, more precisely steel, a second group 4b of calibration particles 4 formed from sand (SiO2) and a third group 4c of calibration particles 4 formed from a plastic (e.g. polystyrene or latex). As a result of the different materials of the calibration particles 4, the particle sensor 2 can be calibrated for the characterization of particles manufactured from different particle materials.

[0055] Similarly, the contrast areas 4 on the calibration plate 1 can also be formed by calibration particles applied to the surface of the substrate 3 in a statistical distribution. In this case, the contrast areas 4 are calibration particles, i.e. particles with a known size. To apply the calibration particles 4, they can be suspended in low concentration, for example, in a liquid transparent glue and applied as a thin layer to the calibration plate 1 or to the substrate 3. When the glue dries, the calibration particles 4 are statistically distributed on the calibration plate 1. In particular, spherical monodisperse particles of known size, such as, for example, polystyrene latex balls, are themselves calibration particles 4 in this case.

[0056] An adhesive or photoresist can also be applied to the plate-shaped substrate 3 with the aid of a dispenser. Before curing (especially with UV light or in an oven), this photoresist or adhesive molds spherical segments, up to a hemisphere, which, depending on the viscosity of the photoresist or adhesive, form calibration particles 4 as a result of surface tension. The three-dimensional structure of the calibration particles 4 thus generated allows the approximation of the scattering behavior of particles with a spherical surface and is therefore particularly suitable for particle sensors 2 based on the principle of scattered light measurement. The structure size of the calibration particles 4 thus generated depends, inter alia, on the process parameters for curing the photoresist or adhesive and can therefore be assumed to be known for given process parameters. In addition, before the application of the adhesive, the plate-shaped substrate 3 can be cleaned, coated or laser structured, so that a hydrophobic or hydrophilic surface results for the adapted shaping of the droplets.

[0057] The particle sensor 2 of Figures 2a, 2b and 3 also comprises a displacement device 14a, 14b for displacing the calibration plate 1 in the calibration plane KE during calibration. In the particle sensor 2 shown in Figure 2a, the displacement device 14a is an electric motor with a motor hub 15, which serves as a rotation shaft for the calibration plate 1 of Figure 1a. The circular calibration plate 1 shown in Figure 1a is rotatably mounted and has a central bore in which the motor hub 15 of the electric motor 14a engages.

[0058] For the calibration of the particle sensor 2, the calibration plate 1 of FIG. 1a is rotated by means of an electric motor 14a, and the contrast area 4 moves in an approximately straight line through the calibration focus 10. The calibration plate 1 of FIG. 1a has a diameter that may range from about 10 mm to about 100 mm. In the case of the rotation of the calibration plate 1, optionally only a single contrast area 4 can be passed. However, in general, a plurality of contrast areas 4 are arranged in a distributed manner on the calibration plate 1 in the circumferential direction, and in particular several hundred contrast areas 4 can be applied to the calibration plate 1 and pass through the calibration focus 10 during one rotation of the calibration plate 1. By way of example, the calibration plate 1 can be rotated at a frequency of 0.1 Hz to 200 Hz. The usual level of the size of the diameter of the laser beam 8 at the calibration focus 10 is, for example, about 10 μm to about 5 mm, such as about 40 μm to about 100 μm.

[0059] In the case of the example shown in FIG. 2b, the displacement device 14b is a piezoelectric actuator engaged on a lateral edge of the calibration plate 1 of FIG. 1b in order to displace the calibration plate 1 along a displacement direction Y in the calibration plane KE. To simplify the displacement in the calibration plane KE, the calibration plate 1 of FIG. 2b has a rectangular shape. It is also possible to simulate the particle trajectories of the contrast areas 4 or the calibration particles during the displacement of the calibration plate 1 in the calibration plane KE. The piezoelectric actuator 14a can displace the calibration plate 1 in the calibration plane KE, for example at a frequency of (adjustable) 1 Hz to 100 Hz, and in the process deflect the calibration particles or the contrast areas 4. The displacement can be implemented in a targeted manner, particularly within a specific group 4a-c of the calibration particles 4, or along a specific row of an array-type pattern on the calibration plate 1.

[0060] As an alternative or in addition to moving the calibration plate 1 in the calibration plane KE, the calibration focal point 10 can also be moved in the calibration plane KE. For this purpose, the emitter 5 can have a movement device, for example in the form of a scanner device, which includes one or more scanner mirrors, acousto-optical modulators, polygon mirrors, etc., in order to move the laser beam 8 and thus also the calibration focal point 10 in the calibration plane KE.

[0061] In the case of the particle sensor 2 shown in Figures 2a, 2b, a calibration plate KE is arranged outside the measurement operation in a measurement volume 11 through which the particles pass during the measurement operation. For this purpose, the calibration plate KE can be introduced into the measurement volume 11 in an automated manner or optionally manually.

[0062] In the case of the particle sensor 2 shown in Fig. 3, the calibration can be implemented during operational operation (in a measurement operation). In contrast to the particle sensor 2 shown in Fig. 2a, 2b, the calibration plane KE of the particle sensor 2 shown in Fig. 3 is not arranged in the measurement volume 11 through which the particles P to be characterized pass during the measurement operation, but instead, in the illustrated example, said plane is spaced apart in the beam direction of the laser beam 8 from the measurement plane ME formed in the measurement volume 11. In order to image the measurement intensity distributions in the form of the calibration focus 10 in the calibration plane KE and the measurement focus 16 in the measurement plane ME onto each other, the particle sensor 2, more precisely the receiver 6 of the particle sensor 2, comprises an imaging optical unit 17. The imaging optical unit 17 serves to generate a double focus with comparable beam parameters (such as beam radius) in the optical plane KE and in the measurement plane ME, respectively. For this purpose, for example, the imaging optical unit 17 may comprise a collimation device and a focusing device, which may have the form of, for example, lenses or (curved) mirrors.

[0063] In the case of the particle sensor 2 shown in Fig. 3, the focusing optical unit 9 of the emitter 5 serves to focus the laser beam 8 onto a measurement focus 16 in the measurement plane ME. In principle, in the case of the particle sensor 2 depicted in Fig. 3, it is possible to interchange the roles of the calibration plane KE and the measurement plane ME. In this case, the focusing optical unit 9 serves to focus the laser beam 8 onto a calibration focus 10 in the calibration plane KE. In this case, the calibration plate 1 is not arranged in the receiver 6 but in the emitter 5, which has been found to be rather disadvantageous for the calibration of the particle sensor 2.

[0064] In principle, the calibration plate 1 can remain permanently in the beam path of the laser beam 8 in order to enable calibration during the active measurement operation. This is advantageous if the measurement must not be interrupted (e.g. due to agglomeration in the fluid) and thus a 100% test can be performed. In this case, the particles P passing through the measurement volume 11 must be differentiated from the calibration particles 4 during the active measurement. By way of example, this is possible if the calibration particles 4 differ from the particles in the measurement volume 11 with respect to at least one optical property. Such a differentiation is also possible in the case of identical optical properties, if the particles P in the measurement volume 11 do not coincide too frequently with the particles on the calibration plate 1 and if the arrangement of the calibration particles 4 on the calibration plate 1 is known. The pattern in which the calibration particles 4 are arranged on the calibration plate 1 can in particular be selected in such a way that the sequence of intensity signals over time generated by the calibration particles 4 can be differentiated within the scope of the evaluation in the evaluation device 13 from the intensity signals of the particles P to be measured.

[0065] Alternatively, the calibration plate 1 can be introduced in an automated manner into the beam path of the laser beam 8 in order to carry out the calibration, and the calibration plate can be removed again from the beam path of the laser beam once the calibration is completed. The moving device 14b is used to introduce and remove the calibration plate 1 respectively into and out of the beam path, although it is also possible to provide a further moving device in the emitter 5 for this purpose.

[0066] The size of the particles P in the measurement volume 11 can be calculated after calibration by knowledge of the beam radius of the laser beam 8 at the calibration intensity distribution or calibration focal point 10 and the measurement intensity distribution or measurement focal point 16. It is understood that in addition to the size of the particles P, a calibration of other measurement variables of the particles P can also be performed. By way of example, these measurement variables can be particle position, particle velocity, etc. Calibration of these measurement variables is possible because the calibration plate 1 is moved in the calibration plane KE by means of the respective movement devices 14a, 14b.

[0067] The characterization of the particle P (and the calibration particle 4) is carried out in the evaluation device 13 of the receiver 6 of the particle sensor 2. The intensity signal I T , I R The detector 12 that records P may be a detector with spatial resolution, such as, for example, a CCD detector, but this is not essential. By way of example, the detector 12 can be designed to record a number of polarization-dependent intensity signals without spatial resolution, which intensity signals allow the characterization of the particle P as described in the above-cited patent application (Patent Document 2). In this case, the detector 12 may, for example, have a number of photodiodes.

[0068] The particle sensor 2 described in relation to Figures 2a, 2b and 3 can be used to characterize particles P in a number of different applications. One such application is described in more detail below on the basis of an EUV radiation generating device 30, the description being for illustrative purposes on the basis of Figure 4. The EUV radiation generating device 30 comprises a beam source 31, an amplifier arrangement 32 with three optical amplifiers or amplifier stages 33a-c, a beam guiding device 34, not further illustrated in detail, and a focusing device 35. The focusing device 35 serves to focus a driver laser beam 31a, generated by the beam source 31 and amplified by the amplifier arrangement 32, onto a target area 36 in a vacuum chamber 38 into which the particles P are introduced. The particles P or individual tin droplets or droplets serve as target material and are irradiated with the aid of the driver laser beam 31a. In this process, the tin droplets transition into a plasma state and emit EUV radiation, which is focused with the aid of a collector mirror 37. 4, the collector mirror 37 has an opening for the passage of the laser beam 31a. In the example shown, the beam source 31 comprises two CO2 lasers generating a pre-pulse and a main pulse, which are together amplified in an amplifier arrangement 32 and focused on a target area 36. The beam source 31 together with the amplifier arrangement 32 form a driver laser arrangement 39 of the EUV radiation generating device 30.

[0069] Similarly, as can be seen in Fig. 4, the emitter 5 and receiver 6 of the particle sensor 2 are mounted in a vacuum chamber 38, which thus forms a measurement chamber for the particle sensor 2. The measurement volume 11, in which the measurement plane ME is formed, passes through a target area 36 with particles P in the form of tin droplets. With the particle sensor 2, it is possible to inspect the particles P or their movement towards the target area 36 and to determine their movement or trajectory. The size of the particles P or the size of smaller particles generated during the evaporation of the individual tin droplets by the driver laser beam 31a can also be determined with the particle sensor 2. The trajectory or velocity of the particles P generated during the evaporation can also be detected with the aid of the particle sensor 2.

[0070] Similarly, as can be observed in Fig. 4, the emitter 5 is shielded from the environment by the housing 23. The receiver 6 is therefore also shielded from the environment by the housing 24. An exit window 21a is formed in the housing 23 of the emitter 5 and serves as an exit for the laser beam 8. An entrance window 21b is therefore formed in the housing 24 of the receiver 6 for the entry of the laser beam 8 into the receiver 6 after passing through the measurement volume 11. The windows 21a, 21b allow the emitter 5 and the receiver 6 to be shielded from the environment, so that the particle sensor 2 can be used to detect different liquid, gaseous or solid media. Thus, in the case of the particle sensor 2 shown in Fig. 4, the calibration plate 1 is also shielded from the measurement volume 11 by the housing 23 of the emitter 5 and is thus protected from particles P passing through the measurement volume 11. [Explanation of symbols]

[0071] 1 Calibration Plate 2 Particle Sensor 3 Base material 4 Contrast Area 4a First Group 4b Second Group 4c Third group 5 Emitter 6 Receiver 7 Laser Source 8. Laser Beam 9 Focusing optical unit 10 Calibration Focus 11 Measurement volume 12 Detector 13 Evaluation equipment 14a Mobile device 14b Mobile device 15 Motor hub 16 Measurement focus 17 Imaging optical unit 21a Exit window 21b Entrance window 23 Housing 24 Housing 30 EUV radiation generator 31 Beam Source 31a Driver laser beam 32 Amplifier Configuration 33 Amplifier Stage 34 Beam guide device 35 Focusing device 36 Target Area 37 Collector mirror 38 Vacuum Chamber 39 Driver Laser Configuration I strength IR Strength Signal IT strength signal KE Calibration Plane ME measurement plane P particles

Claims

1. A method for calibrating a particle sensor (2), comprising the steps of: - focusing a light or laser beam (8) on a calibration plane (KE) in order to generate a calibration intensity distribution (10) or a calibration focus in said calibration plane (KE), a calibration plate (1) being arranged in said calibration plane (KE) and forming thereon a contrast area (4) for modulating the intensity (I) of said light or laser beam (8); - moving the calibration plate (1) and / or the calibration intensity distribution (10) in the calibration plane (KE); - recording at least one intensity signal (IT; IR) of said light beam or said laser beam (8) after passing said calibration plane (KE); calibrating said particle sensor (2) by evaluating said at least one intensity signal (IT; IR), the calibration intensity distribution (10) or the calibration focus in the calibration plane (KE) and the measurement intensity distribution (16) or the measurement focus in a measurement plane (ME), respectively, are imaged, the measurement plane (ME) being arranged in a measurement volume (11) through which particles (P) pass during a measurement operation of the particle sensor (2); The method, wherein said contrast region is formed by calibration particles (4), said calibration particles (4) comprising at least two groups (4a-c) of calibration particles (4) having different optical properties, said groups (4a-c) being made of different materials.

2. 2. The method of claim 1, wherein the calibration plate (1) is arranged in a housing (23) that is separated from the measurement volume (11).

3. 3. The method according to claim 1 or 2, wherein the contrast areas are formed by calibration particles (4), the calibration particles (4) having different surface areas.

4. The method according to any one of claims 1 to 3, wherein the contrast areas are formed by microstructures (4) on the surface of the substrate (3).

5. 5. The method according to claim 1, wherein the calibration particles (4) are applied to the surface of a substrate (3) with a statistical distribution and / or the calibration particles (4) are formed on the surface of the substrate (3) by self-structuring.

6. The method according to any one of claims 1 to 5, wherein the calibration plate (1) is rotated in an automated manner and / or displaced in an automated manner within the range of movement in the calibration plane (KE).

7. A particle sensor (2), an emitter (5) having a light source or laser source (7) for generating a light beam or laser beam (8); an optical unit (9; 9, 17) for focusing the light beam or the laser beam (8) on a calibration plane (KE) in order to generate a calibration intensity distribution (10) or a calibration focus in said calibration plane (KE), a calibration plate (1) being arranged in said calibration plane (KE) and forming thereon a contrast area (4) for modulating the intensity (I) of the light beam or the laser beam (8); a displacement device (14a; 14b) for displacing the calibration plate (1) and / or the calibration intensity distribution (10) within the calibration plane (KE), a receiver (6) having a detector (12) for recording at least one intensity signal (IT, IR) of said light beam or said laser beam (8) after passing said calibration plane (KE); an evaluation device (13) for evaluating said at least one intensity signal (IT, IR) for the purpose of calibrating said particle sensor (2) during a calibration operation of said particle sensor (2), the optical unit (9, 17) comprises an imaging optical unit (17) for imaging the calibration intensity distribution (10) or a calibration focus in the calibration plane (KE) and the measurement intensity distribution (16) or a measurement focus in a measurement plane (ME), the measurement plane (ME) being formed in a measurement volume (11) through which particles (P) pass during a measurement operation of the particle sensor (2), The contrast region is formed by calibration particles (4), the calibration particles (4) including at least two groups (4a-c) of calibration particles (4) having different optical properties and formed from different materials.

8. 8. The particle sensor according to claim 7, wherein the emitter (5) has a housing (23) having an exit window (21 a) and the receiver (6) has a housing (24) having an entrance window (21 b), and a measuring volume (11) is formed between the exit window (21 a) and the entrance window (21 b).

9. 9. Particle sensor according to claim 7 or 8, wherein the displacement device (14a) is designed to rotate the calibration plate (1) in the calibration plane (KE).

10. Particle sensor according to any one of claims 7 to 9, wherein the displacement device (14b) is designed to displace the calibration plate (1) in the calibration plane (KE).

11. An apparatus comprising: a measurement chamber (38) to which particles (P) can be supplied; A particle sensor (2) according to any one of claims 7 to 10 for characterising the particle (P) in the measurement chamber (38), An apparatus comprising:

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