Device for Monitoring the Alignment of a Laser Beam, and EUV Radiation Generating Apparatus having such a Device

Abstract

This disclosure relates to a device for monitoring the alignment of a laser beam, comprising: a detector having an opening for passage of the laser beam, at least two temperature sensors which are mounted on the detector, and a temperature monitoring device which is connected to the at least two temperature sensors, for monitoring the alignment of the laser beam relative to the opening. The at least two temperature sensors have a temperature-dependent resistance which either increases as the temperature increases or decreases as the temperature increases, and the at least two temperature sensors are connected in series with the temperature monitoring device. This disclosure relates also to an EUV radiation generating apparatus which has at least one device as described above for monitoring the alignment of a laser beam.

Claims

1. A system for monitoring the alignment of a laser beam, the system comprising: a detector defining an opening for passage of the laser beam; a plurality of temperature sensors mounted on the detector; and a temperature monitoring device connected to the plurality of temperature sensors, wherein the temperature monitoring device is configured to monitor the alignment of the laser beam relative to the opening, wherein each temperature sensor of the plurality of temperature sensors has a temperature-dependent resistance that either increases as a temperature of the temperature sensor increases or decreases as the temperature of the temperature sensor increases, and wherein the plurality of temperature sensors are connected in series with the temperature monitoring device.

2. The system according to claim 1, wherein the plurality of temperature sensors comprise one or more PTC resistors.

3. The system according to claim 1, wherein the detector comprises a base body surrounding the opening annularly, and wherein the plurality of temperature sensors are integrated with the base body.

4. The system according to claim 3, wherein the base body is composed of metal.

5. The system according to claim 3, further comprising an absorber mounted on the base body.

6. The system according to claim 3, wherein the detector comprises a cooling body connected to the base body via a thermal bridge.

7. The system according to claim 6, wherein the cooling body defines at least one cooling channel, and wherein the at least one cooling channel is configured to receive a cooling medium.

8. The system according to claim 1, wherein the plurality of temperature sensors is distributed evenly about a periphery of the opening.

9. The system according to claim 1, wherein the temperature monitoring device comprises a first switching element, wherein the first switching element is configured to switch from a respective first switching state of the first switching element into a second switching state of the first switching element when a first switching threshold is exceeded, wherein the first switching threshold corresponds to a first temperature threshold value.

10. The system according to claim 9, wherein the temperature monitoring device further comprises a second switching element, wherein the second switching element is configured to switch from a first switching state of the second switching element into a second switching state of the second switching element when a second switching threshold is exceeded, wherein the second switching threshold corresponds to a second temperature threshold value, and wherein the first temperature threshold value is different than the second temperature threshold value.

11. The system according to claim 10, wherein the first switching element and the second switching element each comprise a respective Zener diode.

12. The system according to claim 10, wherein the temperature monitoring device further comprises an adjustment device configured to adjust at least one of the first temperature threshold value or the second temperature threshold value.

13. The system according to claim 10, wherein at least one of the first switching element and the second switching element is connected in series with the plurality of temperature sensors.

14. An EUV radiation generating apparatus, the apparatus comprising: a vacuum chamber defining a vacuum environment, wherein the vacuum chamber is configured to receive a target material in a target region of the vacuum environment; a beam guiding device configured to guide a laser beam into the target region to generate EUV radiation; and a system for monitoring the alignment of a laser beam, the system comprising: a detector defining an opening for passage of the laser beam; a plurality of temperature sensors mounted on the detector; and a temperature monitoring device connected to the plurality of temperature sensors, wherein the temperature monitoring device is configured to monitor the alignment of the laser beam relative to the opening, wherein each temperature sensor of the plurality of temperature sensors has a temperature-dependent resistance that either increases as a temperature of the temperature sensor increases or decreases as the temperature of the temperature sensor increases, and wherein the plurality of temperature sensors are connected in series with the temperature monitoring device.

15. The EUV radiation generating apparatus according to claim 14, further comprising: a laser beam generating device configured to generate the laser beam, wherein the system is configured to switch off the laser beam generating device upon determining that a temperature threshold value is exceeded.

Description

DESCRIPTION OF DRAWINGS

[0025] FIG. 1 is a schematic representation of an EUV radiation generating apparatus having a detector.

[0026] FIG. 2 is a circuit diagram of a temperature monitoring device for monitoring the temperature of the detector shown in FIG. 1.

[0027] FIG. 3 is a detailed representation of an embodiment of the detector of FIG. 1.

DETAILED DESCRIPTION

[0028] In the following description of the drawings, identical reference numerals are used for components that are the same or have the same function.

[0029] FIG. 1 shows an EUV radiation generating apparatus 1 which has a laser beam generating device 2 (also called a driver laser device), a beam guiding chamber 3, and a vacuum chamber 4. In a vacuum environment 4a formed in the vacuum chamber 4 there is arranged a focusing device in the form of a focusing lens 6 for focusing a CO.sub.2 laser beam 5 in a target region B. The EUV radiation generating apparatus 1 shown in FIG. 1 corresponds substantially to the structure as described in U.S. Publication No. 2011/0140008 A1, which is incorporated herein by reference in its entirety.

[0030] The laser beam generating device 2 includes a CO.sub.2 beam source and a plurality of amplifiers for generating a laser beam 5 with high radiation power (>1 kW). For a detailed description of examples of possible configurations of the laser beam generating device 2, reference is made to U.S. Publication No. 2011/0140008 A1. From the laser beam generating device 2, the laser beam 5 is deflected via a plurality of deflecting mirrors 7 to 11 of the beam guiding chamber 3 and a further deflecting mirror 12 in the vacuum chamber 4 onto the focusing lens 6, which focuses the laser beam 5 in the target region B in which tin is arranged as target material 13.

[0031] The target material 13 is struck by the focused laser beam 5 and thereby converted into a plasma state, which serves to generate EUV radiation 14. The target material is supplied to the target region B by means of a supply device (not shown), which guides the target material along a predetermined path which crosses the target region B. For details of the supply of the target material, reference is likewise made to U.S. Publication No. 2011/0140008 A1.

[0032] In a beam guiding space of the beam guiding chamber 3 there is provided a device 15 for enlarging the beam diameter of the laser beam 5, which has a first off-axis parabolic mirror 16 with a first, convex reflecting surface and a second off-axis parabolic mirror 17 with a second, concave reflecting surface. The reflecting surfaces of an off-axis parabolic mirror 16, 17 in each case form the off-axis segments of an elliptic paraboloid. The term off-axis means that the reflecting surfaces do not contain the axis of rotation of the paraboloid (and hence also not the vertex of the paraboloid).

[0033] The optical elements 7 to 11, 16, 17, 12, 6 together form a beam guiding device 18 for guiding the laser beam 5 into the target region B. For monitoring the beam path of the laser beam 5 there is arranged in the beam guiding space of the beam guiding chamber 3 a device 20 for monitoring the alignment of the laser beam 5. The device 20 includes a detector 21 having an opening 22 for the passage of the laser beam 5, and a temperature monitoring device 23. In the example shown, four temperature sensors 24a-d are integrated into the detector 21, of which two are shown in FIG. 1. The temperature sensors 24a-d are electrically connected to the temperature monitoring device 23 in order to monitor the alignment of the laser beam 5 relative to the opening 22. The four temperature sensors 24a-d are in the form of PTC resistors, for example PT100 or PT500, and therefore have a temperature-dependent resistance which increases as the temperature increases.

[0034] When the laser beam 5 is aligned correctly, the laser beam axis coincides with the central axis, or the mid-point, of the circular opening 22, such that the laser beam 5 strikes the detector 21 acting as a diaphragm only in a narrow edge region with low radiation power. If, on the other hand, the laser beam 5 is maladjusted, that is to say at an angle or displaced relative to the central axis of the detector 21, the proportion of the radiation power which strikes the detector 21 increases. The more pronounced the misalignment, the more the temperature of the detector 21 increases. Accordingly, the alignment of the laser beam 5 can be monitored on the basis of the temperature of the detector 21 monitored by means of the temperature sensors 24a-d.

[0035] FIG. 2 shows a circuit diagram of the temperature monitoring device 23 and of the temperature sensors 24a-d. As shown in FIG. 2, the four temperature sensors 24a-d are connected in series in order to be able to detect the temperature rise of the detector 21 as quickly as possible. As can likewise be seen in FIG. 2, the temperature sensors 24a-d are distributed evenly along the periphery of the opening 22, that is to say adjacent temperature sensors 24a-d are aligned at an angle of 90 to one another in the peripheral direction. In the example shown, the temperature sensors 24a-d each have a resistance of approximately 500 K at room temperature, such that the total resistance 26 of the series connection of the temperature sensors 24a-d shown on the right in FIG. 2 at room temperature is approximately 2 M.

[0036] The temperature sensors 24a-d, or their total or equivalent resistor 26, are connected in series in the temperature monitoring device 23 with a parallel circuit including two switching elements in the form of Zener diodes 27a, 27b. In a respective branch, an LED 29a, 29b and a suitably dimensioned series resistor 30a, 30b is connected in series with the Zener diode 27a, 27b. A potentiometer is connected in series with the temperature sensors 24a-d. The potentiometer serves as a device 28 for changing temperature threshold values T.sub.1, T.sub.2, as will be described in greater detail below.

[0037] When the laser beam 5 is aligned correctly, the detector 21 is heated only insignificantly, that is to say the temperature of the detector 21 is only slightly higher than room temperature. The temperature monitoring device 23 is operated with a constant voltage of 24 V, and the series resistors 24a-d are matched to the resistance values of the temperature sensors 24a-d in such a manner that, when the laser beam 5 is aligned correctly, a sufficiently great voltage falls at the two LEDs 29a, 29b to illuminate them. Through the two active LEDs 29a, 29b, an operator can recognize that the laser beam 5 is aligned correctly.

[0038] If the temperature of the detector 21 increases, the temperature sensors 24a-d are heated and their resistance increases. The voltage VT falling at the total or equivalent resistor 26 also increases accordingly. If the voltage VT falling at the temperature sensors 24a-d becomes too great as a result of the increase in the temperature of the detector 21, the voltage falling at the two Zener diodes serving as switching elements 27a, 27b falls. If the voltage at the Zener diodes 27a, 27b falls below the breakdown voltage V.sub.Z1, V.sub.Z2 (which in the example shown is V.sub.Z1=5.6 V in the case of the first Zener diode 27a and V.sub.Z2=2.7 V in the case of the second Zener diode 27b), the respective Zener diode 27a, 27b blocks, that is to say the corresponding branch of the switching circuit is open. Thus, a voltage no longer falls at the associated LED 29a, 29b and it is no longer illuminated.

[0039] As has been described above, the breakdown voltages V.sub.Z1, V.sub.Z2 of the two Zener diodes 27a, 27b are different in the example shown, such that they are reached at different temperature threshold values T.sub.1, T.sub.2. The first Zener diode 27a, which has a greater breakdown voltage V.sub.Z1, is thereby switched at a smaller temperature threshold value T.sub.1 from a first switching state, in which the Zener diode 27a does not block, to a second switching state, in which the Zener diode 27a blocks the current flow. Correspondingly, the second Zener diode 27b only switches from the first switching state into the second switching state and blocks the current flow to the associated second LED 29b at a higher temperature threshold value T.sub.2>T.sub.1.

[0040] The first Zener diode 27a serving as a first switching element can accordingly be used to inform the operator that the temperature at the detector 21 is unusually high and, for example, has exceeded a temperature threshold value T.sub.1 of 35 C. If the temperature of the detector 21 continues to increase and exceeds the second temperature threshold value T.sub.2, which can be, for example, 50 C., the second Zener diode 27b blocks and the second LED 29b is no longer illuminated, which indicates to an operator that the laser beam generating device 2 should be switched off or the alignment of the laser beam 5 should be suitably corrected.

[0041] It will be appreciated that, when the second temperature threshold value T.sub.2 is exceeded, an acoustic notification can be given as an alternative or in addition to a visual notification. Alternatively or in addition, the device 20, or the temperature monitoring device 23, can in particular be designed to transmit a signal to the laser beam generating device 2 in order to switch off the laser beam 5 and thus protect the detector 21 and further components arranged in the region of the beam path of the laser beam 5 from damage. By means of the series connection of the temperature sensors 24a-d with the switching elements 27a, 27b, the switching circuit of the temperature monitoring device 23 is also opened in the case of an interruption, for example if one of the temperature sensors 24a-d burns out, such that this fault can also be detected.

[0042] In order to adjust the temperature threshold values T.sub.1, T.sub.2 at which the Zener diodes serving as switching elements 27a, 27b switch from the first switching state into the second switching state, there is arranged in the temperature monitoring device 23 an adjustable device in the form of a potentiometer 28. The resistance of the potentiometer 28, and thus the voltage V.sub.P falling thereat, can be adjusted by an operator. Because the voltage that falls at the Zener diodes 27a, 27b is reduced by the amount of the voltage V.sub.P that falls at the potentiometer 28, the temperature threshold values T.sub.1, T.sub.2 at which the switching elements 27a, 27b switch from the first switching state into the second switching state can be adjusted by adjusting the voltage V.sub.P. The voltage V.sub.P can be adjusted on the basis of the temperature-dependent characteristic curves of the temperature sensors 24a-d in the form of PTC resistors. If the two temperature threshold values T.sub.1, T.sub.2 are to be adjusted independently of one another, it is possible to use two potentiometers which are arranged in a respective branch of a Zener diode 27a, 27b.

[0043] There are several possibilities for the configuration of the detector 21, one of which is shown by way of example in FIG. 3. As shown in FIG. 3, the detector 21 has a base body 31 with an annular inside geometry enclosing the opening 22, which is circular in a plan view of the detector 21. The base body 31 is made of a material having high thermal conductivity. Suitable materials for the base body 31 are in particular metals, for example copper. The temperature sensors 24a, 24b are integrated into the base body 31; in the example shown, they are mounted in cavities in the form of bores and are in flat contact with the base body 31 or are embedded by means of a heat conducting paste or a heat conducting adhesive in order to produce a good heat transfer. In a region of the base body 31 facing the incident laser beam 5 there is mounted an absorber 32 which is likewise annular and is made of an absorber material. In the example shown, the absorber material is hard anodized aluminum. Due to the hard layer, the underlying aluminum is also sheathed against melting, such that peaks in the absorber material are maintained for longer. The absorption is also increased as a result of the hard anodization. Depressions are formed in the absorber 32, the radial cross-section of which depressions in each case has the shape of an acute triangle in order to achieve good absorption of the laser radiation 5. It will be appreciated that materials other than those described here can be used for the base body 31 and for the absorber 32.

[0044] The base body 31 is connected to a cooling body 33 made of aluminum via a thermal bridge 34 which is likewise annular and is in the form of a web. Otherwise, the cooling body 33 is separated from the base body 31 by a gap and is thus thermally insulated. When the laser beam 5 is aligned correctly, a comparatively small thermal load is generated by the laser radiation in the edge region of the laser beam 5, which is cropped by the detector 21 acting as a diaphragm. This thermal load is dissipated via the thermal bridge 34 provided in the vicinity of the periphery of the opening 22.

[0045] When the laser beam 5 is aligned incorrectly, the thermal load on the base body 31 increases sharply, such that it can no longer be dissipated sufficiently via the thermal bridge 34. This leads to heating of the base body 31 and thus also of the temperature sensors 24a-d integrated therein. The temperature sensors 24a-d may also be mounted on the outside of the base body 31, provided there is flat contact and thus good heat transfer.

[0046] In the plate-like cooling body 35 there is provided a cooling channel 35 through which there flows a cooling medium, for example cooling water. The base body 31 arranged in front of the cooling body 33 in the beam direction of the laser beam 5 absorbs the impinging laser radiation and prevents the detector 21 from melting through to the cooling channel 35, such that the cooling medium can be prevented from escaping into the beam path of the laser beam 5 or into the surroundings.

[0047] In summary, temperature rises caused by an incorrect alignment of the laser beam 5 can be detected quickly in the manner described above, and the detector 21 and further components can thus be protected from damage by the high-energy laser radiation. The location at which the incorrectly aligned laser beam 5 strikes the detector 21 is virtually irrelevant on account of the rapid heat transfer, or the high thermal conduction coefficient of the base body 31, and a sufficiently large number of temperature sensors 24a-d. Instead of temperature sensors 24a-d in the form of PTC resistors, it is also possible to use temperature sensors in the form of NTC resistors or in the form of other electronic components whose resistance is highly dependent on the temperature or which have at least one other temperature-dependent property which can be detected by the temperature monitoring device 23. The device 20 can of course also be used in other optical devices or systems in which a high-energy laser beam is used, for example in a laser processing machine or the like.

[0048] A number of embodiments have been described. Nevertheless, it will be understood that various modifications may be made without departing from the spirit and scope of the invention. Accordingly, other embodiments are within the scope of the following claims.