METHOD FOR CALIBRATING A PARTICLE SENSOR, PARTICLE SENSOR AND DEVICE WITH A PARTICLE SENSOR

Description

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

[0002] Particle sensors are used to characterize particles, for example, to determine particle position, particle velocity, particle acceleration, particle trajectory, and / or particle size. Particle characterization is highly relevant for many industries, such as the chemical, pharmaceutical, and semiconductor industries.

[0003] Optical particle sensors, particularly those using laser radiation, are widely used in monitoring air and liquid purity. However, the accuracy of such particle sensors is compromised as soon as contaminants accumulate on surfaces in the laser beam's optical path or the beam or laser parameters change. This necessitates a complex recalibration of the system, which is not always feasible in the field.

[0004] US Patent 3,885,415 describes a calibration device for the automatic calibration of an analysis system for measuring particle velocities. The calibration device comprises a disk with contrast areas on its surface for modulating light intensity. These contrast areas are moved to simulate the movement of particles introduced into a measurement cell during the analysis system's operation. The disk, or the contrast areas, can be placed in an observation beam path of the analysis system for calibration purposes. The disk defines a calibration plane located near a surface where an image of the particles is generated. Another calibration device is known from US Patent 4,135,821A. Aufgabe der Erfindung

[0005] The invention is based on the objective of providing a method, a particle sensor and a device with a particle sensor that enable calibration of the particle sensor with high accuracy. Gegenstand der Erfindung

[0006] This problem is solved by a method according to claim 1.

[0007] In the method according to the invention, contrast areas on a calibration disk are used to calibrate the particle sensor. These contrast areas have known properties, such as a known size or diameter. They serve to simulate particles or constitute calibration particles that are detected by the particle sensor during calibration operation and differ from the surroundings of the contrast areas on the calibration disk by at least one (optical) property. The surroundings of the contrast areas can, for example, be formed by the substrate of the calibration disk. The contrast areas can, for example, at least partially scatter, absorb, diffract, or reflect the light or laser beam. The surroundings of the contrast areas, for example, in the form of the substrate, differ as much as possible from the optical properties of the contrast areas and can influence the light or laser beam.For example, a laser beam can be transmitted (almost completely). A strong difference between the contrast areas and the surroundings also occurs, for example, when the contrast areas reflect the light or laser beam, while the surroundings absorb it.

[0008] Calibration utilizes the fact that the contrast areas have known properties, such as a known size / area or geometry and known optical properties with regard to absorption, reflectivity, scattering behavior, etc. The movement of the calibration disk or the calibration intensity distribution within the calibration plane is also predetermined during calibration and is therefore known. By moving the calibration disk or the calibration intensity distribution, a calibration particle with known properties can be moved along a known trajectory. Calibration of the particle sensor can therefore be performed, for example, with regard to the measured parameters particle size, particle material, particle trajectory, particle velocity, particle acceleration, etc.

[0009] For the purposes of this application, calibration of the particle sensor also includes the verification of an existing calibration of the particle sensor. If the verification reveals that the existing calibration is faulty, for example due to contamination of surfaces in the beam path of the light or laser beam or to a change in the parameters of the light or laser beam, the particle sensor can be recalibrated.

[0010] The particle sensor characterizes the calibration particle in the same way as a particle passing through a measurement volume of the particle sensor; that is, the passage of a particle through the particle sensor is simulated using the calibration disk. If, during the movement of the calibration disk or the calibration intensity distribution, the light or laser beam strikes a calibration particle or a contrast area, this typically causes an attenuation of the detected intensity signal. The attenuation of the intensity signal is typically greater the larger the area of ​​the calibration particle. In particular, the attenuation of the intensity signal can be proportional to the area of ​​the calibration particle.

[0011] The light or laser beam is aligned with the calibration plane. The light or laser beam that generates the calibration intensity distribution in the calibration plane can, for example, be a collimated light or laser beam. If particles with a relatively small diameter are to be characterized, it is advantageous to focus the light or laser beam in the calibration plane. In this case, the calibration intensity distribution in the calibration plane forms a calibration focus.

[0012] The calibration method described above can be used with a variety of particle sensors that operate on the basis of optical radiation, in particular laser radiation. For example, the calibration method can be used with a sensor arrangement or particle sensor as described in DE 10 2019 209 213.6, which is incorporated in its entirety by reference into this application. In the particle sensor described therein, a mode-conversion device is used to generate a laser beam field distribution that exhibits a different combination of local intensity and local polarization direction at each position within the field distribution. An analyzer optic serves to determine polarization-dependent intensity signals of the laser beam field distribution as it traverses a measurement range.The particles are characterized based on their polarization-dependent intensity signals, exploiting the fact that the field distribution, or rather the optical modes of the field distribution, exhibit a unique correlation between the local intensity distribution and the local polarization direction. It should be understood, however, that other types of particle sensors, which rely on optical radiation (e.g., laser radiation) or the attenuation of optical radiation (e.g., laser radiation) as it passes through a measurement volume, can also be calibrated using the calibration procedure described here.

[0013] In a variant not part of the claimed invention, the calibration disc is arranged in a calibration plane within a measuring volume through which the particles flow during the particle sensor's operation. In this variant, the calibration disc is inserted into the measuring volume from the outside, through which the particles flow during operation. In this case, calibration is performed outside of the measuring operation to calibrate the particle sensor before commissioning or during maintenance. This variant requires that the measuring volume be accessible from the outside to position the calibration disc, which is not always possible.

[0014] According to the invention, the calibration intensity distribution, in particular a calibration focus, is superimposed in the calibration plane, and a measurement intensity distribution, in particular a measurement focus, is superimposed in a measurement plane. The measurement plane is formed in a measurement volume, which the particles traverse during the measurement operation of the particle sensor. In this case, the calibration disk is not placed in the measurement volume but is arranged at a distance from the measurement volume in the beam path of the light or laser beam. The light or laser beam can be focused in the measurement plane so that the measurement intensity distribution forms a measurement focus, but this is not strictly necessary. The superimposition of the calibration intensity distribution or the calibration focus and the measurement intensity distribution or the measurement focus can be achieved using a beam shaping device in the form of an imaging optic.The imaging optics generate an intensity distribution or a dual focus with comparable beam parameters (beam radius, etc.). The calibration disk, for example, can be placed in one of the two foci (calibration focus), while the second focus (measurement focus) is located in the measurement volume through which the particles to be characterized flow. Knowing the beam radii of the calibration intensity distribution or calibration focus and the measurement intensity distribution or measurement focus, the particle size in the measurement volume can be calculated after calibration using the calibration particles or the contrast areas of the calibration disk. The particle size is defined as the cross-sectional area of ​​a particle in the measurement plane.

[0015] Therefore, the calibration disk can be integrated into the particle sensor, allowing the calibration to be checked and repeated during field use. During measurement operation, the calibration disk remains stationary and is either removed from the path of the light or laser beam or positioned to prevent excessive attenuation. For example, the calibration disk can be positioned so that the light or laser beam passes through a transparent area of ​​the substrate where no contrast zone is formed.

[0016] Therefore, not only does the measurement intensity distribution or the measurement focus in the measurement plane form a kind of "virtual sensor area" that enables remote characterization of the particles without having to position the sensor at the particle location, but the calibration intensity distribution or the calibration focus in the calibration plane also enables calibration without introducing the calibration particles into the measurement plane or the measurement volume. This variant thus enables remote diagnostics or remote calibration of the particle sensor.

[0017] In this advanced training, the calibration disc is arranged in a housing separate from the measuring volume. It is advantageous to arrange the calibration disc in a volume hermetically separated from the measuring volume to protect it from contamination by particles flowing through the measuring volume.

[0018] In another variant, the contrast areas on the calibration disk have different surface areas. These varying surface areas or sizes of the contrast areas simulate calibration particles with different, pre-known particle sizes or diameters. The size of the contrast areas is adapted to the type of particles to be characterized or to the type of particle sensor. Calibration with differently sized contrast areas or calibration particles on the calibration disk enables high accuracy in subsequent particle measurements across a wide size range.

[0019] In another variant, the contrast areas are formed by microstructures on the surface of a substrate, particularly a wafer. In this case, the calibration disk can have a substrate made of, for example, glass or a crystal, on which contrast areas are formed in the form of microstructures with known structure sizes, acting like calibration particles. The microstructures can be applied, for example, by microlithography to the surface of a transparent substrate, such as a glass wafer. The contrast areas can form structures that are at least partially absorbing, reflecting, or scattering the light or laser beam. Due to the microstructuring, both the shape and the size or area of ​​the contrast areas are known with high precision, so that they can be used as calibration standards. The microstructuring can be applied to a layer, e.g.,Microstructuring can be performed on a metal layer applied to the substrate, but it is also possible to microstructure the substrate itself. To adjust the size of the contrast areas generated by the microstructuring, the calibration, which is carried out using the calibration disk, can be verified using standard particles of defined size, for example, polystyrene spheres of known size.

[0020] In an alternative variant, the contrast areas are formed by calibration particles, particularly those with a predetermined particle size. The calibration particles preferably comprise at least two groups with different optical properties, and the calibration particles of the at least two groups are made of different materials. These different optical properties can, for example, be different (complex) refractive indices, causing the calibration particles to absorb, reflect, scatter, and / or diffract light or laser radiation differently. The use of calibration particles made of different materials allows for the calibration of the particle sensor for the detection of particles made of different materials.to perform material classes without needing different calibration discs for this purpose.

[0021] The materials from which the calibration particles of a given group are formed can include, for example, metals (e.g., steel), sand (e.g., SiO₂), plastics (e.g., polystyrene or latex), ceramics, etc., as these materials typically differ significantly in their optical properties. The three material classes—metals, sand, and plastics—are typical components of dusts.

[0022] In this case, the calibration particles can be applied to the calibration disk with a predefined distribution or pattern (or in predefined surface areas or fields). Alternatively, the calibration particles of the different groups can be applied in a statistically distributed manner (su). It is also possible, of course, to apply only one type of calibration particle to the calibration disk, possibly with different predefined particle sizes.

[0023] In a further development of this variant, the calibration particles are applied to the surface of a substrate in a statistically distributed manner and / or the calibration particles are formed by self-structuring on the surface of the substrate.

[0024] In the first case, the size of the calibration particles is known, but their exact distribution on the surface is not known in advance. Instead, the calibration particles are statistically distributed when applied to the surface. In this case, the calibration particles, i.e., particles of known size, can be suspended in low concentrations in liquid, transparent adhesive and applied as a thin layer to the calibration disk or substrate. After the adhesive dries, the calibration particles are statistically distributed across the calibration disk. For this variant, spherical, monodisperse particles of known size are particularly suitable, such as polystyrene latex spheres, which are established as a standard for particle size measurement and are available with a narrow size distribution. Such calibration particles are used, for example,Used for the calibration of particle counters according to the ISO 21501 standard, see the link "de.wikipedia.org / wiki / ISO_21501".

[0025] In the second case, the adhesive or photoresist is applied to the disc-shaped substrate using a dispenser. Before curing (specifically with UV light or in an oven), this photoresist or adhesive forms spherical segments up to hemispheres due to surface tension, depending on its viscosity. The three-dimensional structure of the calibration particles generated in this way, i.e., through self-structuring, allows for an 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 produced in this way depends on the process parameters, including those during the curing of the photoresist or adhesive, and can therefore be assumed to be known for given process parameters.The disc-shaped substrate can additionally be cleaned, coated or laser-structured before the adhesive is applied, resulting in a hydrophilic or hydrophobic surface for the adapted shaping of the droplets.

[0026] In another variant, the calibration disk is automatically rotated and / or automatically moved when moving in the calibration plane.

[0027] The calibration disk can be rotated around a predefined axis to move it within the calibration plane and simulate the movement of a particle. Automated rotation of the calibration disk can be achieved, for example, using an electric motor. With a suitably sized calibration disk, the calibration particles move approximately linearly through the light or laser beam of the particle sensor, causing an attenuation of the beam's intensity, which is generally proportional to the area of ​​the contrast zone or the calibration particle. In this case, the calibration disk is typically circular and can have a diameter between 10 mm and 100 mm. The calibration disk can be rotated at a frequency between 0.1 Hz and 200 Hz. For example, [various devices] can be attached to the calibration disk.Between 1 and 400 calibration particles per revolution may be applied. These calibration particles can be, for example, round and applied using a lithographic process. The light or laser beam at the calibration intensity distribution or calibration focus can have a diameter between 20 µm and 200 µm. The diameters of the calibration particles are typically between 0.05% and 100% of the focus or beam diameter.

[0028] The calibration disk can also be moved linearly within the calibration plane. For automated movement, a piezoelectric drive, a piezoelectric crystal, or a linear actuator can be used. Using a piezoelectric drive to move the calibration disk has proven advantageous because it operates without wear and therefore does not introduce measurement errors due to abrasion that could otherwise accumulate on the disk. The frequency of the calibration disk's movement can be, for example, on the order of 1 Hz to 100 Hz. In this case, the calibration disk can, for example, have a rectangular shape.

[0029] Alternatively or additionally, the calibration intensity distribution or the calibration focus of the light or laser beam can also be moved within the calibration plane. To move the calibration intensity distribution, the light or laser beam can perform a scanning motion, for example, by being deflected using mirror scanners or scanner mirrors, an acousto-optic modulator, etc., to vary the position of the calibration intensity distribution within the focal plane. In this case, the calibration disk can be fixed in place, but this is not mandatory; i.e., a superimposed movement of the calibration disk and the calibration intensity distribution is also possible.

[0030] Another aspect of the invention relates to a particle sensor according to claim 8.

[0031] As described above in connection with the procedure, the particle sensor's calibration mode uses contrast ranges to simulate particle movement. Based on the detected intensity signal(s), and provided the size of the contrast ranges or the calibration particles is known, the particle sensor can be calibrated. During calibration, the detected intensity signals can be stored as characteristic of a specific particle size, velocity, and / or trajectory and serve as a calibration standard. In the particle sensor's measurement mode, this calibration standard can be used to characterize particles passing through a measurement volume.

[0032] With the particle sensor described here, particle characterization can be performed in transmission, meaning the light or laser beam is emitted from the transmitter and passes through the measurement volume in transmission. However, it is also possible for the particle sensor to characterize the particles through scattering and / or absorption of the light or laser beam by the particles. In these latter two cases, the reflected or scattered light or laser beam is detected by the receiver's detector.

[0033] In an embodiment not part of the claimed invention, the calibration disk is arranged in a calibration plane located within a measuring volume through which the particles pass during a measurement operation of the particle sensor. As described above, in this case, the calibration disk is typically only located in the measuring volume during calibration operation. During the measurement operation of the particle sensor, the calibration disk is removed from the measuring volume either automatically or manually.

[0034] According to the invention, the optics of the particle sensor comprise an imaging optic for imaging the calibration intensity distribution in the calibration plane and a measurement intensity distribution, in particular a measurement focus, in a measurement plane, wherein the measurement plane is formed in a measurement volume through which the particles traverse during a measurement operation of the particle sensor. In this embodiment, the calibration disk is generally permanently integrated into the particle sensor, typically into the transmitter of the particle sensor. In this case, the calibration disk can optionally be automatically inserted into the beam path of the light or laser beam before the start of the calibration operation, but it is also possible for the calibration disk to be permanently arranged in the beam path of the laser beam. In this case, the calibration disk is positioned so that it interferes with the execution of the measurement operation as little as possible.For example, in this case, the position at which the light or laser beam hits the calibration disk may be located where there is no contrast area.

[0035] In another embodiment, the transmitter has a housing with an exit window, and the receiver has a housing with an entry window, between which the measuring volume is formed. In this case, the calibration disk is typically located in the housing of the transmitter or the receiver and hermetically sealed from the measuring volume. This is advantageous to prevent contamination of the calibration disk by particles passing through the measuring volume. Arranging the calibration disk in the receiver is preferred because contamination of the entry or exit window can lead to a reduction in the power of the light or laser beam. In this case, calibration can be performed with the already reduced power and with the potential optical disturbances of the light or laser beam caused by the dirt on the entry or exit window.

[0036] The calibration disk can be designed in the manner described above in connection with the method. For example, the contrast areas can have different surface areas. The contrast areas can also be designed to at least partially scatter, absorb, or reflect the light or laser beam. The contrast areas on the calibration disk can be formed by microstructuring. It is also possible that the contrast areas are formed by calibration particles with a predetermined particle size, which may be statistically distributed on the surface of a substrate of the calibration disk, or that groups of calibration particles with different properties are provided on the calibration disk.

[0037] In another embodiment, the movement device for rotating the calibration disk in the calibration plane is designed. In this case, the movement device can, for example, be designed as an electric motor which acts on a rotary axis to which the calibration disk is attached.

[0038] In another embodiment, the movement device is designed to shift the calibration disc within the calibration plane. In this case, the movement device can, for example, be designed as a piezo actuator that engages, for instance, a lateral edge of the calibration disc to shift it within the calibration plane. The piezo actuator enables wear-free movement of the calibration disc within the calibration plane.

[0039] Another aspect of the invention relates to a device, in particular an EUV radiation generation device, comprising: a measuring chamber to which particles, for example in a target area, can be fed, and a particle sensor, which is designed as described above, for characterizing the particles in the measuring chamber.

[0040] As described above, the particle sensor can be used in a wide variety of applications to characterize solid, liquid, or gaseous particles or particle streams. The measuring chamber can be designed, for example, to hold or allow flow of a gas or liquid. In the first case, the device can be used, for instance, to measure air purity, such as to characterize soot particles or to measure fine dust. Characterizing powders, e.g., with regard to their powder or particle size distribution, is also possible, for example, with powders used in additive manufacturing (3D printing) or with building materials, such as cement. Characterizing particles in liquids is also possible, e.g., characterizing bacteria in milk. In all these applications, the sensor...Machine learning can be used in devices to characterize the particles or to perform calibration. In machine learning, test data is generated and a suitable AI, e.g., in the form of a neural network or similar, is trained.

[0041] The particle sensor can also be used in the semiconductor industry. For example, it can be used to characterize particles or particle streams in a vacuum chamber within an EUV radiation generation device. Such an EUV radiation generation device typically includes a driver laser array for generating a driver laser beam and a beam delivery system for feeding the driver laser beam to the vacuum chamber described above. The driver laser beam is focused in a target area of ​​the vacuum chamber, into which a target material in the form of tin particles or tin droplets is introduced or fed. Upon irradiation with the driver laser beam, each particle transitions into a plasma state and emits EUV radiation.The particles of the target material that are guided to the target area, as well as the particles that are generated when the laser beam hits the target material (during atomization of the target material), can be characterized using the particle sensor described above.

[0042] Further advantages of the invention will become apparent from the description and the drawing. Likewise, the features mentioned above and those listed below can be used individually or in any combination. The embodiments shown and described are not to be understood as an exhaustive list, but rather serve as examples illustrating the invention.

[0043] They show: Fig. 1a,b schematic representations of a calibration disk used for calibrating a particle sensor, designed for rotation or displacement in a calibration plane; Fig. 2a,b schematic representations of the calibration disk of Fig. 1a,b , which is arranged in a calibration plane in a measurement volume of the particle sensor, Fig. 3 a schematic representation of a particle sensor which has an imaging optic for imaging a calibration focus in a calibration plane and a measurement focus in a measurement plane onto each other, wherein the measurement plane is formed in a measurement volume of the particle sensor, and Fig. 4 a schematic representation of an EUV radiation generation device which has a particle sensor for characterizing particles in a vacuum chamber.

[0044] In the following description of the drawings, identical reference symbols are used for identical or functionally equivalent components.

[0045] Fig. 1a und Fig. 1b Each shows a calibration disk 1 for calibrating a particle sensor 2, which is located in Fig. 2a , b and in Fig. 3 The calibration disk 1 has a transparent substrate 3 in the form of a crystal wafer, a glass wafer, or a plastic disk. In the example shown, circular contrast areas 4 are formed on the substrate 3, more precisely on a planar surface of the substrate 3. The contrast areas 4 have a known size and geometry and serve as calibration particles for calibrating the particle sensor 2.

[0046] The in Fig. 2a,b or in Fig. 3 The particle sensor 2 shown has a transmitter 5 and a receiver 6. The transmitter 5 includes a laser source 7, which serves to generate a laser beam 8. Instead of a laser source 7, another type of light source can optionally be used, which generates a light beam or optical radiation that is not laser radiation. The light source could, for example, be an LED or the like.

[0047] In the example shown, the laser source 7 is a diode laser with a laser wavelength selected from a range between approximately 250 nm and approximately 1600 nm, depending on the application. The laser beam 8 is focused by means of an optical system in the form of a focusing optic 9 onto a calibration focus 10 in a calibration plane KE, in which the calibration disk 2 is located. The calibration focus 10 forms a calibration intensity distribution in the calibration plane KE. As the laser beam 8 passes through the calibration disk 1, more precisely the contrast areas 4, it encounters the contrast areas 4 and its intensity I is modulated. The modulation of the intensity I can be achieved, for example, by reducing the transparency of the contrast areas 4 compared to the substrate 3, by modifying the refractive index of the contrast areas 4 relative to the substrate 3, etc.

[0048] At the in Fig. 1a In the example shown, the contrast areas 4 are formed from a material that absorbs the laser beam 8, while the substrate 3 of the calibration disk 1 is formed from a material transparent to the wavelength of the laser beam 8. When the laser beam 8 strikes a respective contrast area 4, the intensity I of the laser beam 8 decreases as it passes through the calibration disk 1.

[0049] The in Fig. 1a The calibration disk 1 shown, or the calibration plane KE, is located in a measuring volume 11 of the [unclear text]. Fig. 2a The particle sensor 2 shown is arranged. The receiver 6 of the particle sensor 2 of Fig. 2a The device is arranged along the propagation direction Z (Z-direction of an XYZ coordinate system) along a line of sight to the laser source 7 or the receiver 6 and has a detector 12, which serves to detect an intensity signal IT of the laser beam 8 transmitted by the calibration disk 1. An evaluation unit 13 serves to evaluate the intensity signal IT of the laser beam 8.

[0050] In contrast to the one in Fig. 1a the in Fig. 1b In the calibration disk 1 shown, the contrast areas 4 are formed from a material that reflects the wavelength of the laser beam 8. The in Fig. 1b The calibration disk 1 shown is in a calibration plane KE of the in Fig. 2b particle sensor 2 shown is arranged, which differs from the one in Fig. 2a The particle sensor 2 shown differs essentially 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, so that the detector 12 detects an intensity signal IR of the laser beam 8 reflected at the contrast areas 4. It is also possible that the detector 12 of the particle sensor 2 Fig. 2b It serves to detect scattered radiation of the laser beam 8 scattered at the contrast areas 4.

[0051] The in Fig. 1a,b The focusing of the laser beam 8 shown in the calibration plane KE serves to generate a small beam diameter for the laser beam 8 in the calibration plane KE. Focusing can be omitted if comparatively large particles are to be detected with the particle sensor 2. In this case, the laser beam 8 can be collimated or even directed divergently onto the calibration plane KE. Instead of the focusing optics 9, a different type of optic, e.g., collimating optics, is used in this case.

[0052] In order to create the most accurate possible calibration standard for the calibration of particle sensor 2, the following are necessary in the Fig. 1a In the example shown, the contrast areas 4 are formed in the form of microstructures. These microstructures are created by microlithographically structuring a metal layer that was applied to the surface of the substrate 3 and subsequently removed from the surface of the calibration disk 1 during the microstructuring process, with the exception of the circular contrast areas 4. This microstructuring allows for precise control of the size of the contrast areas 4, or the calibration particles, and their spacing, thereby increasing the accuracy of the particle sensor 2 calibration. In the example shown, the contrast areas 4 have different surface areas or diameters to simulate particles of varying sizes and thus enable the particle sensor 2 to achieve accuracy across a wide range of particle sizes.As an alternative to producing the contrast areas 4 by microstructuring, these can also be formed by ultrashort pulse processing, etc. on the calibration disk 1.

[0053] At the in Fig. 1b In the example shown, the contrast areas are formed by calibration particles 4, which are applied to the surface of the substrate 3 with a predetermined distribution (in a grid). The calibration particles 4 from adjacent columns of the grid arrangement differ from each other in their optical properties, in the example shown by their (complex) refractive index; that is, they form different groups of calibration particles 4. Fig. 1b The groups of calibration particles 4 are formed from different materials. In Fig. 1b As an example, a first group 4a of calibration particles 4 is shown, which is made of a metallic material, specifically steel; a second group 4b of calibration particles 4, which is made of sand (SiO₂); and a third group 4c of calibration particles 4, which is made of a plastic (e.g., polystyrene or latex). The different materials of the calibration particles 4 allow the particle sensor 2 to be calibrated for characterizing particles made of different particle materials.

[0054] It is also possible that the contrast areas 4 on the calibration disk 1 are formed by calibration particles that are statistically distributed on the surface of the substrate 3. In this case, the contrast areas 4 are calibration particles, i.e., particles of known size. To apply the calibration particles 4, they can, for example, be suspended in a low concentration in liquid, transparent adhesive and applied as a thin layer to the calibration disk 1 or to the substrate 3. After the adhesive has dried, the calibration particles 4 are statistically distributed across the calibration disk 1. In this case, spherical, monodisperse particles of known size, such as polystyrene latex spheres, are particularly suitable as calibration particles 4.

[0055] The adhesive or photoresist can also be applied to the disc-shaped substrate 3 using a dispenser. Before curing (specifically with UV light or in an oven), this photoresist or adhesive forms spherical segments up to hemispheres due to surface tension, depending on its viscosity. These segments constitute the calibration particles 4. The three-dimensional structure of the calibration particles 4 generated in this way allows for an approximation of the scattering behavior of particles with a spherical surface and is therefore particularly suitable for a particle sensor 2 based on the principle of scattered light measurement. The structure size of the calibration particles 4 generated in this way depends on the process parameters, including those during the curing of the photoresist or adhesive, and can therefore be assumed to be known for given process parameters.The disc-shaped substrate 3 can additionally be cleaned, coated or laser-structured before the application of the adhesive, resulting in a hydrophilic or hydrophobic surface for the adapted shaping of the droplets.

[0056] The particle sensor 2 from Fig. 2a,b and from Fig. 3 It also features a movement device 14a, 14b to move the calibration disk 1 in the calibration plane KE during calibration. In the case of the Fig. 2a In the particle sensor 2 shown, the movement device 14a is an electric motor which has a motor hub 15 that serves as the axis of rotation for the calibration disk 1. Fig. 1a serves. The in Fig. 1a The circular calibration disk 1 shown is rotatably mounted and has a central bore into which the motor hub 15 of the electric motor 14a engages.

[0057] For the calibration of the particle sensor 2, the calibration disc 1 is used. Fig. 1a The electric motor 14a sets the contrast areas 4 in rotation, whereby they move approximately linearly through the calibration focus 10. The calibration disc 1 of Fig. 1a The calibration disk 1 has a diameter that can range from approximately 10 mm to approximately 100 mm. With one rotation of the calibration disk 1, only a single contrast area 4 may be traversed. However, typically multiple contrast areas 4 are arranged circumferentially around the calibration disk 1; in particular, several hundred contrast areas 4 can be attached to the calibration disk 1 and traverse the calibration focus 10 with each rotation of the calibration disk 1. The calibration disk 1 can be rotated, for example, at a frequency between 0.1 Hz and 200 Hz. A typical order of magnitude for the diameter of the laser beam 8 at the calibration focus 10 is between approximately 10 µm and approximately 5 mm, for example, between approximately 40 µm and approximately 100 µm.

[0058] At the in Fig. 2b In the example shown, the motion device 14b is a piezo actuator located on a lateral edge of the calibration disk 1. Fig. 1b attacks in order to move it along a displacement direction Y in the calibration plane KE. The calibration disk 1 of Fig. 2b To simplify displacement within the calibration plane KE, the calibration disk 1 has a rectangular geometry. A particle trajectory of the contrast areas 4 or the calibration particles can also be simulated when the calibration disk 1 is moved within the calibration plane KE. The piezo actuator 14a can, for example, move the calibration disk 1 within the calibration plane KE at an (adjustable) frequency between 1 Hz and 100 Hz, thereby deflecting the calibration particles or the contrast areas 4. The movement can be specifically targeted within a particular group 4a-c of calibration particles 4 or along a specific slit of the grid pattern on the calibration disk 1.

[0059] Alternatively or additionally to moving the calibration disk 1 in the calibration plane KE, the calibration focus 10 can also be moved in the calibration plane KE. For this purpose, the transmitter 5 can have a movement device, e.g., in the form of a scanner device, which includes one or more scanner mirrors, an acousto-optic modulator, a polygon mirror, etc., for moving the laser beam 8 and thus also the calibration focus 10 in the calibration plane KE.

[0060] At the in Fig. 2a,b In the particle sensor 2 shown, which is not part of the claimed invention, the calibration disc KE is arranged outside of the measurement operation in the measurement volume 11, which is traversed by particles during the measurement operation. For this purpose, the calibration disc KE can be inserted into the measurement volume 11 automatically or, if necessary, manually.

[0061] At the in Fig. 3 The particle sensor 2 shown can be calibrated during operation (measuring operation). In contrast to the one in Fig. 2a,b The particle sensor 2 shown is located in the Fig. 3 In the particle sensor 2 shown, the calibration plane KE is not located within the measurement volume 11, which is traversed by the particles P to be characterized during measurement operation. Instead, in the example shown, it is spaced apart in the direction of the laser beam 8 from a measurement plane ME that is formed within the measurement volume 11. To image the calibration focus 10 in the calibration plane KE and a measurement intensity distribution in the form of a measurement focus 16 in the measurement plane ME, the particle sensor 2, or more precisely the receiver 6 of the particle sensor 2, has an imaging optic 17. The imaging optic 17 serves to generate a double focus with comparable beam parameters (beam radius, etc.) in the calibration plane KE and in the measurement plane ME.For this purpose, the imaging optics 17 can, for example, include a collimation device and a focusing device, which can be designed, for example, in the form of lenses or (curved) mirrors.

[0062] At the in Fig. 3 In the particle sensor 2 shown, a focusing optic 9 of the transmitter 5 serves to focus the laser beam 8 at the measurement focus 16 in the measurement plane ME. In principle, it is possible to distinguish the role of the calibration plane KE and the measurement plane ME in the process described in Fig. 3 to swap the particle sensor 2 shown. In this case, the focusing optics 9 serve to focus the laser beam 8 onto a calibration focus 10 in the calibration plane KE. The calibration disk 1 is located in the transmitter 5 and not in the receiver 6, which has proven to be rather disadvantageous for the calibration of the particle sensor 2.

[0063] In principle, the calibration disk 1 can remain permanently in the beam path of the laser beam 8 to allow calibration during ongoing measurement. This is useful if the measurement (e.g., for clumping in a fluid) should not be interrupted, so that a 100% inspection can be performed. In this case, it is necessary to distinguish the particles P passing through the measurement volume 11 from the calibration particles 4 during the ongoing measurement. This is possible, for example, if the calibration particles 4 differ from the particles P in the measurement volume 11 in at least one optical property. Such a distinction is also possible with identical optical properties, provided the arrangement of the calibration particles 4 on the calibration disk 1 is known, and provided that there are not too many coincidences of particles P in the measurement volume 11 and on the calibration disk 1.The pattern in which the calibration particles 4 are arranged on the calibration disk 1 can be specifically chosen so that the temporal sequence of the intensity signals caused by the calibration particles 4 can be distinguished from the intensity signals of the particles P to be measured during evaluation in the evaluation device 13.

[0064] Alternatively, the calibration disk 1 can be automatically inserted into the beam path of the laser beam 8 to perform the calibration, and the calibration disk can be removed from the beam path of the laser beam once the calibration is complete. The motion device 14b can be used for inserting and removing the calibration disk 1 from the beam path, but it is also possible that another motion device is provided for this purpose in the transmitter 5.

[0065] Knowing the beam radii of the laser beam 8, the calibration intensity distribution or calibration focus 10, and the measurement intensity distribution or measurement focus 16, the particle size P in the measurement volume 11 can be calculated after calibration. It is understood that, in addition to the particle size P, other measured parameters of the particles P can also be calibrated. These measured parameters can include, for example, the particle position, particle velocity, etc. Calibration of these measured parameters is possible because the calibration disk 1 is moved in the calibration plane KE by means of the respective motion device 14a, 14b.

[0066] The characterization of the particles P (and the calibration particles 4) takes place in the evaluation unit 13 of the receiver 6 of the particle sensor 2. The detector 12 for detecting the intensity signals IT, IR of the laser beam 8 can be a spatially resolved detector, for example a CCD detector, but this is not mandatory. For example, the detector 12 can be configured for the non-spatially resolved detection of several polarization-dependent intensity signals, which enable the characterization of the particles P, as described in the patent application DE 10 2019 209 213.6 cited above. In this case, the detector 12 can, for example, have several photodiodes.

[0067] The one related to Fig. 2a,b and Fig. 3 The particle sensor 2 described can be used to characterize particles P in a variety of different applications. The following is an example using the following examples: Fig. 4 Such an application is described in more detail using an EUV radiation generation device 30. The EUV radiation generation device 30 comprises a beam source 31, an amplifier arrangement 32 with three optical amplifiers or amplifier stages 33a-c, a beam guidance device 34 (not shown 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 in which particles P are introduced. The particles P, or a single tin droplet, serve as the target material and are irradiated by the driver laser beam 31a. The tin droplet transitions into a plasma state and emits EUV radiation, which is focused by means of a collector mirror 37. In the Fig. 4 In the example shown, the collector mirror 37 has an opening for the passage of the laser beam 31a. The beam source 31, in the example shown, has two CO₂ lasers to generate a pre-pulse and a main pulse, which are amplified together in the amplifier arrangement 32 and focused onto the target area 36. The beam source 31, together with the amplifier arrangement 32, forms a driver laser arrangement 39 of the EUV radiation generation device 30.

[0068] As in Fig. 4 As can also be seen, the transmitter 5 and receiver 6 of the particle sensor 2 are attached to the vacuum chamber 38, so that the vacuum chamber 38 forms a measuring chamber for the particle sensor 2. The measuring volume 11, in which the measuring plane ME is formed, extends through the target area 36 containing the particles P in the form of tin droplets. With the aid of the particle sensor 2, the particles P and their movement towards the target area 36 can be examined, and their movement or trajectory can be determined. The size of the particles P, or the size of smaller particles generated when each tin droplet is atomized by the driver laser beam 31a, can also be determined with the aid of the particle sensor 2. The trajectories or the velocity of the particles P generated during atomization can also be detected by means of the particle sensor 2.

[0069] As in Fig. 4 As can also be seen, the transmitter 5 is shielded from the environment by a housing 23. Similarly, the receiver 6 is also shielded from the environment by a housing 24. An exit window 21a is formed on the housing 23 of the transmitter 2 for the exit of the laser beam 8. Correspondingly, an entry window 21b is formed on the housing 24 of the receiver 6 for the entry of the laser beam 8 into the receiver 6 after passing through the measuring volume 11. The windows 21a and 21b make it possible to shield the transmitter 5 and the receiver 6 from the environment, so that the particle sensor 2 can be used for the detection of various liquid, gaseous, or solid media. Fig. 4 The particle sensor 2 shown thus also shields the calibration disc 1 from the housing 23 of the transmitter 5 in relation to the measuring volume 11 and is therefore protected from the particles P that pass through the measuring volume 11.

Claims

1. Method for calibrating a particle sensor (2), comprising: aligning, in particular focusing, a light beam, in particular a laser beam (8), onto a calibration plane (KE) to generate a calibration intensity distribution (10), in particular a calibration focus, in the calibration plane (KE), a calibration disc (1) being arranged in the calibration plane (KE), on which disc contrast regions (2) are formed to modulate the intensity (I) of the light beam, in particular the laser beam (8), moving the calibration disc (1) and / or the calibration intensity distribution (10) in the calibration plane (KE), detecting at least one intensity signal (IT; IR) of the light beam, in particular the laser beam (8), after passing through the calibration plane (KE), and calibrating the particle sensor (2) by evaluating the at least one intensity signal (IT; IR), characterized in that the calibration intensity distribution (10) in the calibration plane (KE) and a measurement intensity distribution (16), in particular a measurement focus, in a measurement plane (ME) are mapped onto each other, the measurement plane (ME) being arranged in a measurement volume (11) which the particles (P) pass through during the measurement operation of the particle sensor (2).

2. Method according to claim 1, in which the calibration disc (1) is arranged in a housing (23) separated from the measurement volume (11).

3. Method according to claim 1 or 2, in which the contrast regions (4) on the calibration disc (1) have different surface areas.

4. Method according to any of the preceding claims, in which the contrast regions are formed by microstructures (4) on the surface of a substrate (3), in particular a wafer.

5. Method according to any of claims 1 to 4, in which the contrast regions are formed by calibration particles (4), wherein the calibration particles (4) preferably have at least two groups (4a-c) of calibration particles (4) which have different optical properties and are formed in particular from different materials.

6. Method according to claim 5, in which the calibration particles (4) are applied statistically distributed on the surface of a substrate (3), in particular a wafer, and / or in which the calibration particles (4) are formed by self-structuring on the surface of the substrate (3).

7. Method according to any of the preceding claims, in which the calibration disc (1) is automatically rotated and / or automatically moved when moving in the calibration plane (KE).

8. Particle sensor (2) comprising: a transmitter (5) which has a light source, in particular a laser source (7), for generating a light beam, in particular a laser beam (8), an optical unit (9; 9, 17) for aligning, in particular focusing, the light beam, in particular the laser beam (8), onto a calibration plane (KE) to generate a calibration intensity distribution (10), in particular a calibration focus, in the calibration plane (KE), a calibration disc (1) being arranged in the calibration plane (KE), on which disc contrast regions (4) are formed to modulate the intensity (I) of the light beam, in particular the laser beam (8), a movement device (14a; 14b) for moving the calibration disc (1) and / or the calibration intensity distribution (10) in the calibration plane (KE), a receiver (6) which has a detector (12) for detecting at least one intensity signal (IT; IR) of the light beam, in particular the laser beam (8), after passing through the calibration plane (KE), and an evaluation device (13) for evaluating the at least one intensity signal (IT; IR) to calibrate the particle sensor (2) in a calibration operation of the particle sensor (2), characterized in that the optical unit (9, 17) comprises an imaging optics (17) for imaging the calibration intensity distribution (10) in the calibration plane (KE) and a measurement intensity distribution (16), in particular a measurement focus, in a measurement plane (ME), the measurement plane (ME) being formed in a measurement volume (11) which the particles (P) pass through during the measurement operation of the particle sensor (2).

9. Particle sensor according to claim 8, in which the transmitter (5) has a housing (23) having an exit window (21a) and in which the receiver (6) has a housing (24) having an entry window (21b), between which the measurement volume (11) is formed.

10. Particle sensor according to any of claims 8 or 9, in which the movement device (14a) is designed to rotate the calibration disc (1) in the calibration plane (KE).

11. Particle sensor according to any of claims 8 to 10, in which the movement device (14b) is designed to rotate the calibration disc (1) in the calibration plane (KE).

12. Apparatus, in particular EUV radiation generating apparatus (30), comprising: a measurement chamber (38) to which particles (P) can be supplied, and a particle sensor (1) according to any of claims 8 to 11 for characterizing the particles (P) in the measurement chamber (38).