Vibration-monitored laser lens system

Abstract

The invention relates to a method (12) for monitoring a laser lens system (16). At least one sensor (24) measures the vibrations, the structure-borne sound and / or the sound emitted by the laser lens system (16). The measurement can be limited to a modulation frequency of a laser beam (18) directed through the laser lens system (16). Alternatively or additionally, the measurement can be compared with a measurement carried out during optimal operation. The invention further relates to a laser apparatus (10), in particular for carrying out such a method (12).

Description

[0001] Vibration-monitored laser optics

[0002] Background of the invention

[0003] The invention relates to a method for monitoring laser optics of a laser system. The invention further relates to a laser system for implementing such a method.

[0004] In optics for high-power laser applications, defects that arise during operation, e.g., burn-ins or increased absorption due to contamination, are a frequent cause of unplanned maintenance or damage to machines. The invention serves to detect slowly accumulating defects (e.g., continuously increasing degree of contamination) and / or isolated incidents (e.g., the occurrence of burn-ins on coatings and optics due to pulse peaks or particle burn-off) as early as possible. This can prevent machine failures at an early stage. The applicant is aware of monitoring the operation of laser optics by controlling the coolant temperature. If a defect occurs in the laser optics, it heats up, which can be detected by monitoring the coolant temperature. However, an increase in the coolant temperature can often only be detected after a significant delay.

[0005] Furthermore, the applicant is aware of monitoring laser optics using a light source and photodiode. However, such monitoring is susceptible to flashes from particles passing through the laser optics, but does not lead to defects in the laser optics.

[0006] Furthermore, the applicant is aware of monitoring the laser system's output variables, for example, comparing the target and actual laser power values. However, even in this case, defects are only detected relatively late.

[0007] The processes known to the applicant have not necessarily become public knowledge.

[0008] Object of the invention

[0009] The object of the invention, therefore, is to provide a method that allows for rapid and reliable monitoring of a laser system in a structurally simple manner. It is also an object of the invention to provide a laser system for implementing such a method.

[0010] This object is achieved according to the invention by a method according to claim 1 and a laser system according to claim 7. The dependent claims represent preferred embodiments.

[0011] The object of the invention is thus achieved by a method for monitoring laser optics of a laser system with the method steps:

[0012] C) Passing a laser beam through the laser optics; D) Detecting the vibrations, structure-borne noise, and / or sound generated in the laser optics using a sensor;

[0013] F) Generating an output based on the measured vibrations, structure-borne noise and / or sound.

[0014] A defect causes / occurs laser-induced vibration / structure-borne sound and / or sound. Laser power is absorbed at the defect, leading to a temperature increase and thus to thermal expansion of the laser optics, and structure-borne sound, sound, and / or vibrations are generated in the laser optics. Such a defect can arise, for example, through ablation of laser optics material.

[0015] If a sudden temperature increase occurs, e.g., due to particle burn-off, breakage of the laser optics, damage to the coatings of the laser optics due to pulse peaks, plasma breakthroughs in the ambient medium or on the laser optics or their coating itself, a measurable shock wave may be generated in the laser optics. If the laser optics is located in an atmosphere, a sound wave is also emitted, which is also detectable.

[0016] Based on the temporal progression of the recorded vibrations, the recorded sound and / or structure-borne sound, e.g. the occurrence of peaks in the recorded signal, such an event can be inferred.

[0017] In pulsed or power-modulated lasers, modulation of the laser power leads to a modulation of the absorbed power and thus to a pulsating heat source, which in turn causes pulsating thermal expansion and thus vibrations, structure-borne sound, and sound emission. Therefore, a sound, vibration, or structure-borne sound source is expected to be localized around the absorbing defect, the frequency of which corresponds to the modulation frequency of the incident laser beam (e.g., the pulse repetition rate in pulsed lasers or the modulation / control frequency in power-controlled continuous lasers). Thus, an increase in amplitude in the relevant frequency range can be measured in the frequency spectrum of the emitted sound, structure-borne sound, and / or vibrations. The intensity and thus the amplitude of this sound, structure-borne sound, and / or vibrations is proportional to the average absorbed power.Thus, the absorbed power can be directly determined from the increase in amplitude.

[0018] The sensors are particularly preferably attached to the laser optics.

[0019] Particularly preferably, the vibrations, structure-borne noise, and / or sound generated in the laser optics are recorded using at least three sensors. In the event of a defect, the at least three sensors record sound, structure-borne noise, and / or vibrations at different times. Triangulation can then be used to determine the position of the defect within the laser optics volume from time-of-flight measurements (in the case of a shock wave) or phase differences (in the case of a frequency-based evaluation).

[0020] By monitoring vibrations, structure-borne sound, and / or noise, discrete defect events can be identified in the time domain, while analyzing the amplitude distribution in the frequency domain enables the detection of slower events, such as increased absorption due to accumulated contamination. Both evaluation methods are complementary and can be used alternatively or simultaneously.

[0021] By detecting the vibrations, structure-borne noise, and / or sound emitted by the laser optics, defects occurring in the laser optics can be easily and reliably detected early. In other words, the method according to the invention enables efficient monitoring of laser optics while requiring minimal installation space.

[0022] The output can be provided, preferably optically and / or acoustically, on an output device. The output can include an alarm if the output exceeds predefined threshold values. Preferably, the measurement in method step D) is reduced to a modulation frequency range of the laser beam, in particular to a pulse frequency of the laser beam. The limitation can be achieved by a bandpass filter. The bandpass filter can be in the form of hardware and / or software. The modulation frequency range of the laser beam is preferably between 10 kHz and 150 kHz, in particular between 20 kHz and 130 kHz, particularly preferably between 30 kHz and 120 kHz.

[0023] In a preferred embodiment of the method, the method additionally comprises the following method steps:

[0024] A) The laser beam passes through the laser optics in a defect-free state, i.e. in optimal operation;

[0025] B) Detecting the vibrations, structure-borne noise and / or sound generated in the laser optics during optimal operation using the sensor and storing the recorded data;

[0026] E) Comparing the data stored in process step B) with the data acquired in process step D) and generating an output in process step F) in the event of a discrepancy in the data.

[0027] This allows the vibrations generated during optimal operation, the structure-borne noise generated during optimal operation, and the sound generated during optimal operation to be ignored, allowing unforeseen deviations to be easily detected. In a preferred embodiment of the invention, the output is only generated when the deviation between the two measurements exceeds a predefined threshold.

[0028] Particularly preferably, the vibrations, structure-borne noise, and / or sound generated in the laser optics are recorded during optimal operation as a function of the laser power. In this way, signal fluctuations resulting from a deliberate increase in laser power are calibrated out. Preferably, the measurement in method step B), in particular as in method step D), is reduced to a modulation frequency range of the laser beam, in particular to a pulse frequency of the laser beam. This allows background vibrations, structure-borne noise, and / or background sound to be masked out.

[0029] In the method according to the invention, a MEMS vibrometer and / or a laser triangulation sensor can be used as the sensor. Alternatively or additionally, the following sensor(s) can be provided: a) a laser vibrometer; b) a piezo microphone, in particular with sensitivity in the acoustic and / or ultrasonic range; and / or c) a microphone, in particular in the form of an optical microphone.

[0030] In a particularly preferred embodiment of the invention, the method comprises the following method step:

[0031] G) Generating EUV radiation by irradiating target material, in particular in the form of a tin droplet, with the laser beam.

[0032] The object of the invention is further achieved by a laser system, in particular for carrying out a method described here, with a laser optics for guiding a laser beam, wherein the laser system has a sensor for monitoring the laser optics, wherein the sensor is in the form of

[0033] - a vibration sensor;

[0034] - a structure-borne sound sensor; and / or

[0035] - a sound sensor.

[0036] The laser system can have a target material that can be irradiated with the laser beam to generate EUV radiation. To avoid repetition, the process features mentioned for carrying out the process described here also apply to the device (laser system), and vice versa.

[0037] Further advantages of the invention will become apparent from the description and the drawings. Likewise, the above-mentioned and further-described features can be used individually or in combination in any desired manner. The embodiments shown and described are not intended to be exhaustive, but rather are exemplary in nature for describing the invention.

[0038] Detailed description of the invention and drawing

[0039] Fig. 1 shows schematically the structure of a laser system according to the invention for carrying out a method according to the invention, wherein vibrations emanating from a laser optics are measured.

[0040] Fig. 2a shows the amplitude of measured oscillations of the laser optics from Fig. 1 over time, with a defect in the laser optics increasing.

[0041] Fig. 2b shows the oscillations measured for Fig. 2a after filtering at approximately 50 kHz.

[0042] Fig. 2c shows the amplitude of measured oscillations of the laser optics from Fig. 1 over time during optimal operation.

[0043] Fig. 2d shows the amplitude of measured oscillations of the laser optics from Fig. 1 over time in normal operation after subtracting the amplitude measured in optimal operation.

[0044] Fig. 1 shows a laser system 10 for carrying out a method 12. The laser system 10 has a laser source 14 and laser optics 16, wherein the laser optics 16 can have one or more mirrors and / or one or more lenses. A laser beam 18 emerging from the laser optics 16 irradiates a target material 20, here in the form of a tin droplet. This excites the target material 20 to emit EUV radiation 22. A sensor 24 is arranged on the laser optics 16, e.g. on a lens or a mirror. The sensor 24 is designed to measure vibrations, structure-borne sound and / or sound. The sensor 24 is connected wirelessly and / or by cable to an output device 26. The data measured by the sensor 24 are processed and / or output unprocessed in the output device 26.

[0045] Fig. 2a shows the vibration, structure-borne sound, or sound amplitude measured with sensor 24 in arbitrary units over time. Since laser beam 18 is pulsed at 50 kHz, higher amplitudes are measured primarily at intervals of 20 ps. The amplitudes increase, which suggests an increasing defect in laser optics 16.

[0046] Fig. 2b shows the signal measured in Fig. 2a after passing through a bandpass filter in the 50 kHz range. From Fig. 2b, it is even clearer than from Fig. 2a that the vibrations, structure-borne noise, and / or sound attributable to the pulsed laser beam 18 are increasing.

[0047] Fig. 2c shows a measurement corresponding to Fig. 2a during optimal operation of the laser system 10. The measurement shown in Fig. 2c during optimal operation can be stored, in particular in the output device 26.

[0048] Fig. 2d shows a measurement corresponding to Fig. 2c in normal operation after subtracting the measurement in optimal operation. Optimal operation is represented by the dashed line. Normal operation is represented by the dashed and solid lines. The actually output difference measurement (solid line) shows the signals measured in normal operation after subtracting the signals measured in optimal operation. In Fig. 2d, an additional peak results for normal operation compared to optimal operation. This additional peak indicates a suddenly occurring defect, e.g. a break in a previously damaged laser optics that occurs between two laser pulses. The amplitude of the peaks following this additional peak is higher than the amplitude of the peaks preceding this additional peak, since the laser optics then has a defect.Since the acoustic signal is excited at the frequency and phase of the laser beam impinging on the laser optics, a defect that increases over time, for example, results in a pattern similar to that shown in Fig. 2a during normal operation, i.e., the amplitude of the respective peaks increases continuously. If the defect occurs when the laser beam impinges on the laser optics, no additional peak is created as shown in Fig. 2d. Compared to optimal operation, only the amplitude of the individual peaks increases. By focusing on the deviation from optimal operation, spontaneously occurring defects can be easily detected.

[0049] Taking a summary of all figures of the drawing, the invention relates to a method 12 for monitoring a laser optics system 16. At least one sensor 24 measures the vibrations, structure-borne noise, and / or sound emanating from the laser optics system 16. The measurement can be limited to a modulation frequency of a laser beam 18 guided through the laser optics system 16. Alternatively or additionally, the measurement can be compared with a measurement during optimal operation. The invention further relates to a laser system 10, in particular for implementing such a method 12.

[0050] List of reference symbols

[0051] 10 Laser system

[0052] 12 Process 14 Laser source

[0053] 16 Laser optics

[0054] 18 laser beam

[0055] 20 Target material

[0056] 22 EUV radiation 24 Sensor

[0057] 26 Output device

Claims

Patent claims 1. Method (12) for monitoring a laser optic (16) of a Laser system (10) with the following process steps: C) Guiding a laser beam (18) through the laser optics (16); D) Measuring the vibrations, structure-borne sound and / or sound generated in the laser optics (16) by means of a sensor (24); F) Generating an output of the measured vibrations, structure-borne sound and / or sound; and / or an output of the change in the measured vibrations, structure-borne sound and / or sound.

2. Method according to claim 1, wherein in process step D) a measurement of the vibrations, structure-borne sound and / or sound is reduced to a modulation frequency range of the laser beam (18).

3. Method according to one of the preceding claims comprising the following method steps: A) Guiding a laser beam (18) through the laser optics (16) in a defect-free state; B) Measuring the vibrations, structure-borne sound and / or sound generated in the defect-free laser optics (16) by means of a sensor (24) and storing this measurement; E) Comparing the measurement stored in process step B) with the measurement in process step D) and generating an output in process step F) if there is a deviation between the two measurements.

4. Method according to claim 3 in conjunction with claim 2, wherein in process step B) a measurement of the vibrations, structure-borne sound and / or sound is reduced to a modulation frequency range of the laser beam (18). A method according to any one of the preceding claims, wherein a) a sensor (24) in the form of a laser vibrometer is used to measure the vibrations of the laser optics (16); b) a sensor (24) in the form of a piezoelectric microphone with sensitivity in the acoustic and / or ultrasonic range is arranged on the laser optics (16) to measure the structure-borne sound of the laser optics; and / or c) a microphone and / or optical microphone is used to measure the sound of the laser optics (16), wherein the laser optics (16) is arranged in a gas atmosphere. A method according to any one of the preceding claims comprising the following method step: G) Generating EUV radiation (22) by irradiating target material (20) with the laser beam (18). Laser system (10) for carrying out a method (12) according to one of the preceding claims, comprising a laser optic (16) for guiding a laser beam (18), wherein the laser system (10) has a sensor (24) for monitoring the laser optic (16), the sensor (24) being in the form - a vibration sensor for measuring the vibrations of the laser optics (16); - a structure-borne sound sensor for measuring the structure-borne sound of the laser optics (16); and / or - a sound sensor for measuring the sound emitted by the laser optics (16). Laser system according to claim 7, wherein the laser system (10) has a target material (20) that can be irradiated with the laser beam (18) for generating EUV radiation (22).

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