DEVICE AND METHOD FOR MEASURING THE BEAM ANGLE OF A LIGHT BEAM GUIDED BY A BEAM GUIDING OPTICAL UNIT - Patent application

Abstract

The present invention relates to an apparatus and method for measuring the beam angle of a light beam guided by a beam guidance optical unit. The apparatus according to the invention comprises a light intensity sensor device (20, 30, 40, 50, 60), a focusing unit (21, 31, 41) for focusing the light beam at a specified position on the light intensity sensor device, and an adjusting unit (22, 32, 42) for adjusting the relative position of the intensity centroid of the light beam with respect to the specified position on the light intensity sensor device in the presence of a change in the beam angle as it enters the apparatus, wherein the adjusting unit (22, 32, 42) is configured to keep the relative position of the intensity centroid of the light beam with respect to the specified position on the light intensity sensor device (20, 30, 40, 50, 60) constant up to a specified maximum deviation, which maximum deviation corresponds to half the average beam diameter at incidence on the light intensity sensor device (20, 30, 40, 50, 60).

Description

[Technical Field]

[0001] This application claims priority from German Patent Application No. DE 10 2018 124 342.1, filed October 2, 2018, the contents of which are incorporated by reference into the body of the present application.

[0002] The present invention relates to an apparatus and method for measuring the beam angle of a light beam directed by a beam directing optical unit.

[0003] The present invention can be used to advantage in particular when monitoring excitation laser beams in EUV plasma sources, but its applications are not limited to such applications and include all applications in which the fastest possible position capture and monitoring of a light or laser beam is desirable, in particular when processing materials or determining the position of an object relative to the focus position of an imaging optical unit. [Background technology]

[0004] In addition to camera-based methods, known methods for measuring beam angles or capturing the propagation direction of a light beam include the use of light intensity sensor arrangements, particularly in the form of commercially available quadrant detectors.

[0005] FIG. 7 shows, in a purely schematic view, a possible measurement setup in principle. Here, a light beam to be measured in terms of its propagation direction or beam angle relative to the axis of the optical system is focused by a focusing unit 75 onto a quadrant detector 70, which is located in the image-side focal plane of the focusing unit 75. The quadrant detector 70 is assembled from four light intensity sensors 71-74, and the desired beam angle or propagation direction of the light beam is determined from a computational combination of the light intensities measured by these light intensity sensors 71-74. Positioning the quadrant detector 70 one focal distance from the focusing unit 75 is advantageous in that the measurement signal is insensitive to lateral displacements of the laser beam being measured, and therefore only the beam propagation direction is actually measured. In principle, semiconductor-based detectors, as used in quadrant detectors, can be used to achieve measurement bandwidths up to the gigahertz range, whereas camera-based detectors only reach measurement bandwidths of 1 kHz or less in the infrared range.

[0006] Although the aforementioned measurement principle, in principle, facilitates relatively fast capture of the angle or position of the light beam compared to, for example, camera-based methods, a drawback of the aforementioned measurement principle in practice lies in the relatively small measurement area, which is limited by the spot size of the light beam on the quadrant detector. In particular, focusing the light beam to a relatively small average beam diameter or spot size of the order of less than 100 μm on the quadrant detector means that even a relatively small change in beam angle will cause the beam to move from the center of the quadrant detector, and the spot will soon still be incident on only one of the light intensity sensors 71-74, ultimately resulting in no usable measurement signal being available that can be evaluated to determine the position.

[0007] The method for overcoming the aforementioned problems by diffraction-based expansion of the light beam through the use of an aperture stop has the disadvantage that only a relatively small component of the light beam (e.g., up to 10% of the total power) contributes to the actual measurement signal, with the result that this component is not sufficiently representative of the entire light beam; for example, the measurement signal may instead be dominated by local wavefront tilt. A further drawback of using an aperture stop is that the instrument becomes sensitive to lateral displacements of the laser beam; in other words, it is not possible to easily distinguish whether a movement of the quadrant detector spot is due to a change in the beam propagation direction or a change in the lateral position of the laser beam.

[0008] With regard to the prior art, reference is made, by way of example, to DE 10 2014 203 141 A1 and DE 10 2012 212 354 A1. Summary of the Invention

[0009] The object of the present invention is to provide an apparatus and method for measuring the beam angle of a light beam guided by a beam guidance optical unit, thereby facilitating the measurement of the beam angle as accurately as possible over as large a measurement area as possible while avoiding the aforementioned drawbacks.

[0010] This object is achieved according to the features of the independent claims.

[0011] The device for measuring the beam angle is a light intensity sensor device; a focusing unit for focusing the light beam at a designated position on the light intensity sensor device; - an adjustment unit for adjusting the relative position of the intensity centroid of the light beam with respect to a designated position on the light intensity sensor device when there is a change in the beam angle as it enters the device; - the adjusting unit is configured to maintain constant a relative position of the intensity centroid of the light beam with respect to a specified position on the light intensity sensor device up to a specified maximum deviation; - This maximum deflection corresponds to half the average beam diameter at the entrance to the light intensity sensor device.

[0012] In particular, the invention is based on the idea that during the measurement of the beam angle of a light beam, by active adjustment of a component located in the optical beam path of the light beam (this component can be the light intensity sensor device itself, a focusing unit or a diffractive optical unit located in the beam path), the light beam is kept substantially always measured at a specified position of the light intensity sensor device, in this way avoiding, for example, limitations in the measurement range resulting from the initially mentioned problem of the light beam or laser spot "moving" from the center of the quadrant detector. Here, the control signals required for said adjustment or "centering the spot position", respectively, can finally be used as measurement signals relevant for determining the beam angle.

[0013] Due to the aforementioned adjustments, mechanically induced limitations on the control bandwidth of the adjustments are in principle accepted in the concept according to the invention (in this respect with an upper limit of the mechanical adjustment in the range of 100 Hz to 1 kHz).

[0014] However, in this respect, the present invention recognizes that the disturbances in the position or beam direction of the light beam, which are ultimately to be measured or "removed by closed-loop control" in the beam steering unit according to the present invention, are typically "1 / f αThe present invention advantageously utilizes the situation where the frequency of a light intensity sensor device follows a "α≧1" behavior (α≧1), where α is greater than or equal to 1, so that relatively small disturbances or amplitudes occur at relatively high frequencies, and vice versa. Such a situation has the consequence that, firstly, due to the relatively low frequencies in this respect, the effects of disturbances with large amplitudes can be captured by an adjustment mechanism in accordance with the invention, and secondly, due to the relatively small amplitudes, the (as initially indicated, relatively small) measurement area itself of the light intensity sensor device, for example in the form of a quadrant detector, can be used for the effects of disturbances at higher frequencies, so that the entire required measurement range can be covered over a wide bandwidth (limited only by the measurement bandwidth of the utilized quadrant detector, which can for example be in the GHz range).

[0015] In the case of the aforementioned additional use of a measurement area of ​​a light intensity sensor device or quadrant detector, the measurement signal ultimately related to the measurement of the beam angle results from the control signal required for adjusting or "centering the spot position" on the light intensity sensor device, further taking into account the (residual) deviation of the spot from its center on the aforementioned light intensity sensor device or quadrant detector.

[0016] According to one embodiment, the light intensity sensor apparatus comprises a beam splitter device for splitting the light beam onto the light intensity sensor. In particular, the beam splitter device may comprise a prism, for example configured as a pyramidal prism together with a quadrant detector.

[0017] As will be explained in detail below, this configuration can take into account the fact that for manufacturing reasons, gaps remain between adjacent light intensity sensors in light intensity sensor instruments consisting of multiple light intensity sensors, such as quadrant detectors. The dimensions of these gaps can typically be on the order of (50-100) μm, with the result that without further measures, a large portion of the light falls into such gaps and therefore does not contribute to the measurement.

[0018] By using the aforementioned beam splitter device or prism (the prism itself can be machined or polished to have sharp edges) according to the present invention, it can be achieved that the relevant light components are dispersed in advance to the individual light intensity sensors of the light intensity sensor instrument, without falling into gaps, so that lateral changes in the point of incidence of the light beam on the beam splitter device lead to changes in the intensity values ​​measured by each light intensity sensor. In the aforementioned configuration, for example, the tip of the pyramid of a pyramidal prism is then selected as the designated position on the light intensity sensor instrument, with respect to which tip the intensity centroid of the light beam should maintain a substantially constant relative position.

[0019] The invention further relates to the use of an apparatus having the above characteristics for monitoring an excitation laser beam in an EUV plasma source.

[0020] Furthermore, the invention also relates to a method for measuring the beam angle of a light beam guided by a beam guidance optical unit, in particular using a device having the aforementioned characteristics, - a light beam is focused at a specified position of the light intensity sensor device; - actively adjusting the relative position of the intensity centroid of the light beam with respect to a specified position on the light intensity sensor device when there is a change in the beam angle as it enters the device; The control signals respectively required for the adjustment are used as measurement signals relevant for determining the beam angle.

[0021] Further features of the invention can be gleaned from the description and the dependent claims.In the following, the invention will be explained in more detail on the basis of example embodiments shown in the attached drawings. [Brief explanation of the drawings]

[0022] [Figure 1] 1 shows a schematic diagram of a possible basic structure of a control device for a light beam. [Figure 2]1 a) and 1b) are schematic diagrams illustrating the structure and function of a device according to the invention for measuring beam angles in a first embodiment; [Figure 3] 10A and 10B show schematic diagrams for explaining other embodiments of the device according to the present invention. [Figure 4] 10A and 10B show schematic diagrams for explaining other embodiments of the device according to the present invention. [Figure 5] 10A and 10B show schematic diagrams for explaining other embodiments of the device according to the present invention. [Figure 6] 10A and 10B show schematic diagrams for explaining other embodiments of the device according to the present invention. [Figure 7] FIG. 1 is a schematic diagram illustrating a conventional device for measuring a beam angle. [Figure 8] 1 is a diagram showing a schematic diagram of the basic structure of an EUV plasma source. DETAILED DESCRIPTION OF THE INVENTION

[0023] First, FIG. 1 shows the basic structure of a control device for a light beam, which can be used, for example, to monitor and control an excitation laser beam in an EUV plasma source, which is further described with reference to FIG. 8.

[0024] According to Fig. 1, a light beam, which is characterized and controlled in terms of beam angle and comes from a beam directing device 11, first hits a beam splitter device 12, where a measurement beam is redirected and fed to an apparatus 10 according to the invention for measuring the beam angle. In the following, an embodiment will be described with respect to the apparatus 10 with reference to the schematic diagrams of Figs. 2 to 6.

[0025] In order to compensate for disturbances present in the position or beam angle of the light beam, a characteristic of the measurement signal of the beam angle of the light beam provided by the apparatus 10 is returned as a control signal to the beam steering device 11. According to Figure 1, the component of the light beam that has not been redirected by the beam splitter device 12 impinges on a target 14 (for example in the form of tin droplets in the EUV plasma source of Figure 8) as an actuation beam via a focusing optical unit 13.

[0026] In the following, possible embodiments of the device according to the invention for measuring beam angles, corresponding to the device 10 of FIG. 1, are explained on the basis of FIGS.

[0027] Here, in any case similar to the conventional device of FIG. 7, an apparatus according to the present invention comprises a light intensity sensor device including a plurality of light intensity sensors, each configured as a quadrant detector (e.g., a four-quadrant diode), although the present invention is not limited to such a configuration.

[0028] Common to all of the embodiments according to the present invention is that the "spot position" or intensity centroid of the light beam on the aforementioned light intensity sensor device is adjusted or maintained substantially constant relative to a specified position on the light intensity sensor device (particularly relative to the center of each quadrant detector).

[0029] 2a-2b, the device according to the invention comprises a light intensity sensor arrangement 20 in the form of a quadrant detector, arranged one focal length from a focusing unit 21, in a manner similar to the conventional structure of FIG. 7. "22" denotes a control or adjustment unit which, in the example embodiment of FIGS. 2a-2b, firstly maintains the relative position between the spot or point of incidence of the light beam on the light intensity sensor arrangement 20 and the centre of the light intensity sensor (corresponding to the "confluence" of the individual light intensity sensors of the quadrant detector) by manipulating the position of the light intensity sensor arrangement 20 (in particular the displacement of this position transversely to the direction of light propagation (i.e. in the xy plane in the coordinate system shown)) in response to changes in the beam angle, up to a maximum deviation which preferably corresponds to half the average beam diameter at incidence on the light intensity sensor arrangement.

[0030] This prevents the spot from moving out of the measurement area of ​​the quadrant detector, which in the conventional configuration of Fig. 7 already occurs for small changes in beam angle. Furthermore, the control signal of the adjustment unit 22 required for the aforementioned adjustment can be used as a characteristic of the measurement signal for changes in beam angle. Furthermore, as mentioned above, corresponding high-frequency disturbances with relatively low amplitude may not be captured by the adjustment, but can be captured on the measurement area of ​​the quadrant detector, so that the (residual) deviation of the spot on the light intensity sensor device 20 or the quadrant detector can be taken into account when determining this measurement signal.

[0031] Figures 3 and 4 show schematic diagrams of other possible embodiments of the device according to the invention, in which components similar to or having substantially the same functions as the components of Figures 2a-2b are designated with reference symbols increased by "10" and "20", respectively.

[0032] Here, the embodiment of Figure 3 differs from the embodiment of Figures 2a-2b in that, according to Figure 3, the position of the focusing unit 31 is manipulated by the adjustment unit 32, so that the overall relative position between firstly the intensity centroid or spot and secondly the center of the light intensity sensor device 30 or quadrant detector can be kept substantially constant.

[0033] According to FIG. 4, the device comprises an additional deflection optical unit 43 in the optical beam path, for example in the form of a deflection mirror, in which case the adjustment unit 42 is designed to manipulate the position of this deflection optical unit 43 .

[0034] Piezoelectric ultrasonic drives are particularly suitable for realizing the aforementioned positional manipulations by the respective adjustment units due to their relatively small mechanical dimensions, low weight, and advantageous dynamic properties in terms of achievable speeds and accelerations. However, the present invention is not limited to piezoelectric ultrasonic drives, so that the positional manipulations can also use other actuators, in particular in the form of spindle drives or electromagnetic drives using linear motors, Lorentz actuators, etc., to actively adjust the respective elements (i.e., the light intensity sensor device, the focusing unit, and / or the deflection device).

[0035] In other embodiments of the present invention, the presence of manufacturing-related gaps between individual light intensity sensors and the situation where, without further measures, a significant component of the light beam, characterized in terms of beam angle, falls on those gaps and does not contribute to the measurement signal, are taken into account by appropriate configuration of the light intensity sensor device. The schematic diagram of Figure 5 helps to illustrate such a situation, where gap regions remain between light intensity sensors 51-54 of a light intensity sensor device 50 in the form of a quadrant detector, indicated by "55." Typical gap dimensions may be on the order of (50-100) μm and may have a size comparable to the size of the spot of light beam 56, as shown in Figure 5.

[0036] To overcome this problem, the light intensity sensor apparatus 60 according to the invention, in the embodiment of Fig. 6, comprises a beam splitter device 61 arranged upstream in the optical beam path, said beam splitter device being configured in a particular example embodiment as a pyramidal prism, which splits the incoming light beam, represented as beam tube 65, to the individual light intensity sensors 62. The edges of the prism forming the beam splitter device 61 can be polished to have sharp edges (typically an edge sharpness of less than 1 µm), so that the aforementioned light losses are prevented and the entire intensity of the light beam is utilized in measurements according to the invention.

[0037] As a possible application of the present invention, FIG. 8 shows a conventional structure of a laser plasma source, for example for use in lithography, to generate the EUV light required for projection exposure apparatus designed for the EUV region (e.g. wavelengths of about 13 nm or about 7 nm).

[0038] The EUV light source comprises, for example, a high-energy laser (not shown) for generating infrared radiation 81 (e.g., a CO2 laser with a wavelength λ of approximately 10.6 μm). The infrared radiation is focused by a focusing optical unit, passes through an aperture 83 present in a collector mirror 82 embodied as an ellipsoid, and is directed as an excitation beam to a target material 86 (e.g., tin droplets) generated by a target source 84 and provided at a plasma ignition position 85. The infrared radiation 81 heats the target material 86 at the plasma ignition position 85 in such a way that the target material transitions to a plasma state and emits EUV radiation. The EUV radiation is focused by the collector mirror 82 at an intermediate focus IF and passes through it to a downstream illumination device. Only an edge 87 of the illumination device is shown; the illumination device has a free aperture 88 for the light to enter.

[0039] For the dose and time stability of the EUV radiation characteristics achievable in EUV light sources or laser-plasma sources, and for the feasible EUV emission efficiency, it is crucial that the tin droplets "flying" into the laser-plasma source very quickly (e.g., at injection rates in the 100 kHz range or, for example, at time intervals of 10 μs) are individually struck by a laser beam that atomizes the droplets with high precision (e.g., greater than 1 μm) and in a reproducible manner. In the aforementioned configuration, this then requires very precise setting of the droplet position and very precise tracking of the infrared radiation 83 generated, for example, by a CO laser. Therefore, the device according to the present invention can be used to monitor or measure the beam angle of the corresponding excitation beam generated by a CO laser in the configuration already described based on FIG. 1.

[0040] Although the present invention has been described based on particular embodiments, numerous variations and alternative embodiments will be apparent to those skilled in the art, for example, by combining and / or substituting features of the particular embodiments. Accordingly, it will be apparent to those skilled in the art that such variations and alternative embodiments are encompassed by the present invention, and that the scope of the present invention is limited only as provided by the appended claims and equivalents thereof.

Claims

1. 1. An apparatus for measuring a beam angle, which is a direction of propagation of a light beam steered by a beam steered optical unit relative to an axis of an optical system, said apparatus comprising: a beam splitter (12) for splitting the light beam into a redirected measurement beam and an unredirected action beam; a light intensity sensor device (20, 30, 40, 50, 60); a focusing unit (21, 31, 41) for focusing the light beam at a designated position on the light intensity sensor device (20, 30, 40, 50, 60); an adjustment unit (22, 32, 42) for adjusting the relative position of the intensity centroid of the light beam with respect to a designated position on the light intensity sensor device (20, 30, 40, 50, 60) when there is a change in the beam angle as it enters the device; the adjusting unit (22, 32, 42) is configured to maintain the relative position of the intensity centroid of the light beam with respect to the specified position on the light intensity sensor device (20, 30, 40, 50, 60) constant up to a specified maximum deviation; the adjusting unit (22, 32, 42) is configured to actively adjust a component located downstream of a beam splitter (12) in an optical beam path of the redirected measurement beam, The apparatus, wherein said maximum deflection corresponds to half the average beam diameter at incidence on said light intensity sensor device (20, 30, 40, 50, 60).

2. 2. The apparatus according to claim 1, wherein the adjusting unit (22, 32, 42) comprises at least one manipulator for manipulating the position of the light intensity sensor device (20, 30, 40, 50, 60) and / or the focusing unit (21, 31, 41).

3. 3. The apparatus according to claim 1 or 2, wherein the apparatus comprises at least one deflection optical unit (43) for deflecting the light beam before it enters the light intensity sensor device (40), and the adjustment unit (42) comprises at least one manipulator for manipulating the tilt angle and / or position of the deflection optical unit (43).

4. The device according to any one of claims 1 to 3, wherein the light intensity sensor device (20, 30, 40, 50, 60) comprises a plurality of light intensity sensors.

5. 5. The apparatus of claim 4, wherein the designated location on the light intensity sensor device (20, 30, 40, 50, 60) is adjacent to the light intensity sensor.

6. 6. The apparatus of claim 1, wherein the light intensity sensor device (20, 30, 40, 50, 60) is a quadrant detector, and the specified position on the light intensity sensor device (20, 30, 40, 50, 60) corresponds to the center of the quadrant detector.

7. Apparatus according to any one of claims 4 to 6, wherein the light intensity sensor equipment (60) comprises a beam splitter device (61) for splitting the light beam onto the light intensity sensor.

8. 8. The apparatus of claim 7, wherein the beam splitter device (61) comprises at least one prism.

9. The apparatus according to any one of claims 1 to 8, wherein the light beam is a laser beam.

10. The apparatus according to any one of claims 1 to 8, wherein the light beam is a laser beam having a wavelength in the infrared region.

11. Use of the device according to any one of claims 1 to 10 for monitoring an excitation laser beam in an EUV plasma source.

12. A method for measuring the beam angle of a light beam directed by a beam directing optical unit using an apparatus according to any one of claims 1 to 10, comprising: the light beam is focused at a designated position of a light intensity sensor device (20, 30, 40, 50, 60); actively adjusting the relative position of the intensity centroid of the light beam with respect to a designated position on the light intensity sensor device (20, 30, 40, 50, 60) when there is a change in the beam angle as it enters the device; A method in which the control signals respectively required for adjustment are used as measurement signals relevant to determining said beam angle.

Images