DEVICE AND METHOD FOR MEASURING THE BEAM ANGLE OF A LIGHT BEAM GUIDED BY A BEAM GUIDING OPTICS

Description

[0001] This application claims priority from German patent application DE 10 2018 124 342.1, filed on October 2, 2018. BACKGROUND OF THE INVENTION Gebiet der Erfindung

[0002] The invention relates to a control device for a light beam and a method for measuring the beam angle of a light beam guided by a beam guiding optics.

[0003] The invention is particularly advantageous for monitoring an excitation laser beam in an EUV plasma source. However, applications of the invention are not limited to this, but include, in particular, all applications in which the fastest possible position detection and control of a light beam or laser beam is desired, for example, in material processing or determining the position of objects relative to the focal position of an imaging optics. Stand der Technik

[0004] Known approaches for measuring the beam angle or detecting the propagation direction of a light beam include, in addition to camera-based approaches, in particular the use of light intensity sensor arrangements in the form of commercially available quadrant detectors.

[0005] Fig. 7 shows a purely schematic representation of a fundamentally possible measurement setup. A light beam, whose propagation direction or beam angle relative to the optical system axis is to be measured, is focused via a focusing unit 75 onto a quadrant detector 70 arranged in the image-side focal plane of this focusing unit 75. The quadrant detector 70 is composed of four light intensity sensors 71-74, with the desired beam angle or propagation direction of the light beam being determined by calculating the light intensities measured with these light intensity sensors 71-74. The arrangement of the quadrant detector 70 at the focal length distance of the focusing unit 75 has the advantage that the measurement signal is insensitive to a lateral displacement of the laser beam to be measured, and thus only the beam propagation direction is actually measured.In principle, semiconductor-based detectors, such as those used in quadrant detectors, can achieve measurement bandwidths up to the GHz range, whereas camera-based detectors in the infrared range only achieve measurement bandwidths of 1 kHz or less.

[0006] Although the measuring principle described above fundamentally enables relatively fast angle and position detection of the light beam compared to camera-based approaches, a disadvantage of the described measuring principle in practice results from the comparatively small measuring range limited by the spot size of the light beam on the quadrant detector. In particular, the focusing of the light beam results in comparatively small average beam diameters or spot sizes of the order of less than 100 µm on the quadrant detector, so that even with comparatively small changes in the beam angle, the beam migrates from the center of the quadrant detector, and ultimately, no measurement signal usable for position determination is available as soon as the spot is located on only one of the light intensity sensors 71-74.

[0007] Approaches to overcoming the problem described above by means of diffraction-based widening of the light beam through the use of an aperture stop have the disadvantage that only a comparatively small proportion of the light beam (e.g. 10% of the total power) contributes to the actual measurement signal. As a result, this proportion may no longer be sufficiently representative of the entire light beam and the measurement signal may instead be dominated by, for example, local wavefront inclinations. A further disadvantage of using an aperture stop is that the arrangement becomes sensitive to a lateral shift of the laser beam. In other words, it is no longer possible to easily distinguish whether a spot movement on the quadrant detector is caused by a change in the direction of beam propagation or by a change in the lateral position of the laser beam.

[0008] For the prior art, reference is made to DE 10 2014 203 141 A1 and DE 10 2012 212 354 A1. DE 10 2010 050947 A1 discloses a control device for a laser beam in a system for generating EUV radiation. The control device comprises a beam focusing unit, an adjustable mirror, a beam splitter, and a quadrant detector. SUMMARY OF THE INVENTION

[0009] The object of the present invention is to provide a control device and a method for measuring the beam angle of a light beam guided by a beam guiding optics, which enable the most accurate beam angle measurement possible over the largest possible measuring range while avoiding the disadvantages described above.

[0010] This problem is solved according to the features of the independent patent claims.

[0011] The invention is based on the concept of keeping the measured light beam essentially at a predetermined position of the light intensity sensor array at all times during beam angle measurement of a light beam by actively tracking a component located in the optical beam path of the light beam (which can be the light intensity sensor array itself, the focusing unit, or even a deflection optic located in the beam path). This avoids measurement range limitations resulting from the problem described above of the light beam or laser spot "wandering out," for example, from the center of a quadrant detector. The control signal required for said tracking or for "centering the spot position" can be used as the measurement signal ultimately relevant for beam angle determination.

[0012] Due to the tracking described above, the inventive concept basically accepts mechanically induced limitations with regard to the control bandwidth of the tracking (with an upper limit for the mechanical tracking being in the range of 100 Hz to 1 kHz).

[0013] However, the invention advantageously makes use of the fact that the disturbances in the position or beam direction of the light beam which are ultimately to be measured or "corrected" in a beam guidance unit typically correspond to a "1 / f α< -behavior" (α ≥ 1), so that comparatively small disturbances or amplitudes occur at relatively high frequencies and vice versa. This circumstance has the consequence that, on the one hand, disturbances of large amplitudes can be detected via the tracking mechanism due to the comparatively low frequencies, and on the other hand, for disturbances of high frequencies due to the then comparatively small amplitudes, the (as described above relatively small) measuring range of a light intensity sensor arrangement in the form of a quadrant detector itself can be used, so that in the end the entire required measuring range can be covered over a high bandwidth (which is only limited by the measuring bandwidth of the quadrant detectors used, which can be in the GHz range, for example). With the additional use of the measuring range of the

[0014] Light intensity sensor arrangement, ie the quadrant detector, then results in the measurement signal ultimately relevant for the beam angle measurement from the control signal required for tracking or "centering the spot position" on the light intensity sensor arrangement, additionally taking into account the (residual) deviation of the spot on the said light intensity sensor arrangement or the quadrant detector from the respective center.

[0015] According to one embodiment, the light intensity sensor arrangement comprises a beam splitting device for splitting the light beam between the light intensity sensors. The beam splitting device can, in particular, comprise a prism and, for example, be designed as a pyramid-shaped prism in conjunction with a quadrant detector.

[0016] As will be described in more detail below, this design allows for the fact that, due to manufacturing reasons, gaps remain between the adjacent light intensity sensors in a quadrant detector, whereby the dimensions of these gaps can typically be in the range of the order of (50-100) µm, with the result that, without further measures, a significant proportion of light falls into said gap and thus does not contribute to the measurement.

[0017] Through the inventive use of the aforementioned beam splitting device or the prism, which can itself be machined or polished to a sharp edge, it can be achieved that the relevant light components are distributed from the outset among the individual light intensity sensors of the light intensity sensor arrangement and do not even fall onto the slits, whereby a lateral change in the point of incidence of the light beam on the beam splitting device leads to a change in the intensity values ​​measured by the respective light intensity sensors. In the embodiment described above, the pyramid tip of the pyramid-shaped prism is then selected as the predetermined location on the light intensity sensor arrangement with respect to which the intensity center of the light beam is to maintain a substantially constant relative position, for example.

[0018] The invention further relates to the use of a control device having the features described above for monitoring an excitation laser beam in an EUV plasma source.

[0019] Furthermore, the invention also relates to a method for beam angle measurement according to claim 8.

[0020] Further embodiments of the invention can be found in the description and the dependent claims. The invention is explained in more detail below with reference to exemplary embodiments illustrated in the accompanying drawings. BRIEF DESCRIPTION OF THE DRAWINGS

[0021] They show: Figure 1 shows a schematic representation of the basic possible structure of a control device for a light beam; Figures 2a-2b show schematic representations to explain the structure and functioning of a device for beam angle measurement in a first embodiment; Figures 3-6 show schematic representations to explain further embodiments of a device; Figure 7 shows a schematic representation to explain a conventional device for beam angle measurement; and Figure 8 shows a schematic representation of the basic structure of an EUV plasma source. DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

[0022] Fig. 1 shows first the basic structure of a control device for a light beam, which is used, for example, to monitor and control an excitation laser beam in a with reference to Fig. 8 EUV plasma source described below.

[0023] According to Fig. 1 The light beam to be characterized and controlled with respect to its beam angle is first directed by a beam guiding device 11 onto a beam splitter 12, at which a measuring beam is branched off and fed to a device 10 for beam angle measurement. Embodiments of this device 10 are described below with reference to the schematic illustrations of Fig. 2 bis Fig. 6 described.

[0024] The measurement signal provided by the device 10, which is characteristic of the beam angle of the light beam, is fed back to the beam guiding device 11 as a control signal in order to correct any disturbances in the position or beam angle of the light beam. Fig. 1 the portion of the light beam not branched off via the beam splitter 12 is incident as a working beam via a focusing optics 13 onto a target 14 (e.g. in the form of a tin droplet in the EUV plasma source of Fig. 8 ).

[0025] In the following, possible embodiments of a device according to the invention for beam angle measurement corresponding to the device 10 of Fig. 1 based on Fig. 2 bis Fig. 6 described.

[0026] The device has the same characteristics as the conventional arrangement of Fig. 7 a light intensity sensor arrangement with a plurality of light intensity sensors, each of which is in turn designed as a quadrant detector (e.g. four-quadrant diode).

[0027] What the embodiments have in common is that the "spot position" or the intensity center of the light beam on said light intensity sensor arrangement is tracked or kept substantially constant relative to a predetermined location on the light intensity sensor arrangement (in particular relative to the center of the respective quadrant detector).

[0028] According to Fig. 2a-2b has a device in the conventional construction of Fig. 7 analogously, a light intensity sensor arrangement 20 in the form of a quadrant detector, which is arranged at a distance of the focal length of a focusing unit 21. "22" denotes a control unit or tracking unit, which in the embodiment of Fig. 2a-2b in response to a change in the beam angle by manipulating the position of the light intensity sensor arrangement 20 (in particular its displacement in a direction lateral to the direction of light propagation, ie within the xy plane in the drawn-in coordinate system), the relative position between the spot or point of incidence of the light beam on the light intensity sensor arrangement 20 on the one hand and its center (corresponding to the "point of incidence" of the individual light intensity sensors of the quadrant detector) is maintained up to a maximum deviation which corresponds to half the average beam diameter upon incidence on the light intensity sensor arrangement.

[0029] In this way, a conventional structure of Fig. 7 This prevents the spot from drifting out of the measuring range of the quadrant detector, which occurs even with slight beam angle changes. Furthermore, the control signal of the tracking unit 22 required for said tracking is used as the measurement signal characteristic of the beam angle change. In addition, when determining this measurement signal, a (residual) deviation of the spot on the light intensity sensor arrangement 20, ie of the quadrant detector, can also be taken into account, since, as already described above, corresponding high-frequency interference with a comparatively low amplitude cannot be detected via the tracking, but can be detected via the measuring range of the quadrant detector itself.

[0030] Fig. 3 and Fig. 4 show in schematic representation further possible embodiments of a device, wherein in comparison to Fig. 2a-2b analogous or essentially functionally equivalent components are designated by reference numerals increased by "10" or "20".

[0031] The design differs from Fig. 3 from that of those from Fig. 2a-2b by the fact that according to Fig. 3 the position of the focusing unit 31 is manipulated via the tracking unit 32, as a result of which the relative position between the intensity center of gravity or spot on the one hand and the center of the light intensity sensor arrangement 30, ie the quadrant detector, on the other hand, can be kept essentially constant.

[0032] According to Fig. 4 the device has an additional deflection optics 43 in the optical beam path, e.g. in the form of a deflection mirror, wherein the tracking unit 42 is designed here to manipulate the position of this deflection optics 43.

[0033] Piezoelectric ultrasonic drives are particularly suitable for implementing the above-described position manipulations via the respective tracking unit due to their comparatively small mechanical dimensions, low weight, and advantageous dynamic properties with regard to achievable speeds and accelerations. However, the invention is not limited to this, so that other actuators for actively tracking the respective element (i.e., the light intensity sensor arrangement, the focusing unit, and / or a deflection device) can also be used for position manipulation, particularly in the form of spindle drives or electromagnetic drives with linear motors, Lorenz actuators, etc.

[0034] In further embodiments, a suitable design of the light intensity sensor arrangement takes into account the presence of manufacturing-related gaps between the individual light intensity sensors and the fact that, without further measures, a significant portion of the light beam to be characterized with regard to its beam angle hits these gaps and does not contribute to the measurement signal. The schematic representation of Fig. 5 , wherein the remaining gap area between the light intensity sensors 51-54 of a light intensity sensor arrangement 50 in the form of a quadrant detector is designated by "55". The typical gap dimensions can be in the range of (50-100)µm and, as in Fig. 5 indicated have a size comparable to the size of the spot of the light beam 56.

[0035] To overcome this problem, a light intensity sensor arrangement 60 in the embodiment of Fig. 6 a beam splitting device 61 arranged upstream in the optical beam path, which in the specific embodiment is designed as a pyramid-shaped prism and splits an incident light beam, represented as a beam tube 65, into the individual light intensity sensors 62. Since the edges of the prism forming the beam splitting device 61 can be polished to a sharp edge (typically to edge sharpnesses of less than 1 µm), the light losses described above are avoided and the entire intensity of the light beam is utilized in the measurement according to the invention.

[0036] Fig. 8 shows as a possible application of the invention a conventional structure of a laser plasma source, e.g. for use in lithography to generate the required EUV light of a projection exposure system designed for the EUV range (e.g. at wavelengths of e.g. about 13nm or about 7nm).

[0037] This EUV light source has a high-energy laser (not shown), e.g., for generating infrared radiation 81 (e.g., a CO2 laser with a wavelength of λ≈ 10.6µm), which is focused via focusing optics, passes through an opening 83 in an ellipsoidal collector mirror 82, and is directed as an excitation beam onto a target material 86 (e.g., tin droplets) generated by a target source 84 and fed to a plasma ignition position 85. The infrared radiation 81 heats the target material 86 located in the plasma ignition position 85 such that it enters a plasma state and emits EUV radiation. This EUV radiation is focused via the collector mirror 82 onto an intermediate focus IF (= "Intermediate Focus") and enters through this into a subsequent illumination device, the border 87 of which is only indicated and which has a free opening 88 for the light entry.

[0038] Of key importance for the dose stability or temporal stability of the EUV emission characteristics achievable in an EUV light source or laser plasma source and the realizable EUV light yield is that the tin droplets "flying" into the laser plasma source very quickly with increasing light demand (e.g., with an injection rate in the range of 100 kHz or at a time interval of, for example, 10 µs) are individually hit with high precision (e.g., with an accuracy of less than 1 µm) and reproducibly by the laser beam atomizing the droplets. In the above-mentioned setup, this in turn requires a highly precise adjustment of the droplet position as well as a highly precise tracking of the infrared radiation 83 generated, for example, by the CO 2 laser. The control device according to the invention can thus be used for monitoring or beam angle measurement of the corresponding excitation beam generated by the CO 2 laser in the system already described on the basis of Fig. 1 described structure can be used.

Claims

1. Control device for a light beam, comprising • a beam guiding device (11) for guiding a light beam; • a beam splitter (12) for splitting this light beam into a diverted measurement beam and a non-diverted working beam; and • an apparatus (10) for beam angle measurement, wherein the beam splitter (12) is configured to feed the diverted measurement beam to the apparatus (10), wherein the apparatus (10) for beam angle measurement comprises: - a light intensity sensor arrangement (20, 30, 40, 50, 60) comprising a plurality of light intensity sensors; - a focusing unit (21, 31, 41) for focusing the diverted measurement beam at a specified location on the light intensity sensor arrangement (20, 30, 40, 50, 60); - wherein the light intensity sensor arrangement (20, 30, 40, 50, 60) is a quadrant detector, with the specified location on the light intensity sensor arrangement (20, 30, 40, 50, 60) corresponding to the centre of this quadrant detector; and - an adjustment unit (22, 32, 42); characterized in that - the adjustment unit (22, 32, 42) is configured, by active adjustment of a component of the apparatus (10) that is situated downstream of the beam splitter (12) in the beam path of the diverted measurement beam, on the basis of light intensities measured by the light intensity sensors of the light intensity sensor arrangement (20, 30, 40, 50, 60), by way of an adjustment of the relative position of the intensity centroid of the diverted measurement beam in relation to the specified location on the light intensity sensor arrangement (20, 30, 40, 50, 60), to keep this relative position constant up to a specified maximum deviation when there is a change in the beam angle of the diverted measurement beam present upon entry of the measurement beam in the apparatus (10), said maximum deviation corresponding to half the mean beam diameter of the measurement beam upon incidence on the light intensity sensor arrangement (20, 30, 40, 50, 60); and - wherein the apparatus (10) is configured to supply a control signal respectively required for adjustment purposes as a measurement signal characteristic of the beam angle of the light beam and to feed said control signal to the beam guiding device (11) as a closed-loop control signal, such that disturbances in the beam angle of the light beam can be removed by closed-loop control.

2. Control device according to Claim 1, characterized in that the adjustment unit (22, 32, 42) comprises at least one manipulator for manipulating the position of the light intensity sensor arrangement (20, 30, 40, 50, 60) and / or of the focusing unit (21, 31, 41).

3. Control device according to Claim 1 or 2, characterized in that the apparatus (10) comprises at least one deflection optical unit (43) for deflecting the diverted measurement beam prior to the incidence on the light intensity sensor arrangement (40), wherein the adjustment unit (42) comprises at least one manipulator for manipulating the tilt angle and / or the position of this deflection optical unit (43).

4. Control device according to any of the preceding claims, characterized in that the specified location on the light intensity sensor arrangement (20, 30, 40, 50, 60) in each case adjoins these light intensity sensors.

5. Control device according to any of the preceding claims, characterized in that the light intensity sensor arrangement (60) comprises a beam splitter device (61) for splitting the measurement beam among the light intensity sensors.

6. Control device according to Claim 5, characterized in that the beam splitter device (61) comprises at least one prism.

7. Use of a control device according to any of the preceding claims for monitoring and controlling an excitation laser beam in a plasma source for generating light in the extreme ultraviolet (EUV) range.

8. Method for beam angle measurement, - wherein a light beam guided by a beam guiding device (11) is split via a beam splitter (12) into a diverted measurement beam and a non-diverted working beam; - wherein the diverted measurement beam is fed to an apparatus (10) for beam angle measurement and is focused onto a specified location of a light intensity sensor arrangement (20, 30, 40, 50, 60) present in this apparatus (10), wherein the light intensity sensor arrangement (20, 30, 40, 50, 60) comprises a plurality of light intensity sensors; - wherein the light intensity sensor arrangement (20, 30, 40, 50, 60) is a quadrant detector, with the specified location on the light intensity sensor arrangement (20, 30, 40, 50, 60) corresponding to the centre of this quadrant detector; characterized in that - a relative position of the intensity centroid of the diverted measurement beam in relation to a specified location on the light intensity sensor arrangement (20, 30, 40, 50, 60), when there is a change in the beam angle present upon entry of the diverted measurement beam in the apparatus (10), is adjusted by active adjustment of a component of the apparatus (10) that is situated downstream of the beam splitter (12) in the beam path of the diverted measurement beam, - wherein a control signal respectively required for adjustment purposes is used as a measurement signal that is relevant for the beam angle measurement and supplied by the apparatus (10).