DE102023102206A1 - Contrast measurement for laser pulse adjustment - Google Patents

Abstract

The invention relates to a method and a measuring device (10) for characterizing a laser pulse (28). The laser pulse (28) is evenly scattered over at least two, in particular identical, detectors (16a, 16b) and the measurement signals of the detectors (16a, 16b) are superimposed. The amplitude of one measurement signal is resolved more finely than the amplitude of the other measurement signal. The measurement signal with the finer resolution can be used to determine a prepulse (34), and the measurement signal with the lower resolution can be used to measure a pulse (36), i.e. a main pulse. A contrast (26) of the laser pulse (28) can be determined by dividing the areas of the prepulse (34) and the pulse (36).

Description

The invention relates to a method for characterizing a laser pulse. The invention further relates to a measuring device for carrying out such a method.

State of the art

It is known to analyze the temporal course of the amplitude of a laser pulse.

From the WO 2020/169941 A1 A method and a device for measuring a pulse signal have become known. It has also become known to divide a signal between several detectors. However, it is described as a disadvantage that several oscilloscope channels and detectors have to be used for this.

The CN109596228A discloses a device for analyzing a laser pulse in the nanosecond range. The laser beam is split into several laser beams, the individual laser beams are attenuated and each is directed to a semiconductor photodetector.

The known methods and devices require a comparatively complex optical structure.

Task

The invention is therefore based on the object of providing a method and a measuring device which are structurally significantly simpler and yet enable a precise characterization of a laser pulse.

Solution

This object is achieved according to the invention by a method according to patent claim 1 and a measuring device according to patent claim 13. The dependent claims give preferred embodiments.

• A) Streuung eines Laserpulses;
• B) Messung der Amplitude des gestreuten Laserpulses mit wenigstens zwei Detektoren;
• D) Ausgabe der gemessenen Detektionssignale mit verschiedenen Skalierungen.

The object according to the invention is therefore achieved by a method comprising the following method steps:

• A) Scattering of a laser pulse;
• B) measuring the amplitude of the scattered laser pulse with at least two detectors;
• D) Output of the measured detection signals with different scalings.

The method according to the invention enables the precise characterization of a laser pulse in a particularly simple structural manner. By outputting the amplitude of the laser pulse with two different scales (two different magnifications), both the rise range of the laser pulse, which has a low amplitude, and the maximum amplitude of the laser pulse can be recorded in a meaningful manner.

In the present description, the term “laser pulse” refers to the laser light of the laser pulse.

In a further preferred design, at least one diffusing disc is used in method step A). Preferably, several diffusing discs arranged one behind the other are used.

Furthermore, the central part (middle part, main part) of the laser pulse is preferably blocked so that the scattered part of the laser pulse is distributed particularly evenly across the at least two detectors. Furthermore, it can be provided that the optical signal is attenuated by absorption or reflection.

The detectors used can be of the same design.

Alternatively or in addition, detectors in the form of photodiodes can be used to simplify the design.

At least one detection signal can be amplified in a method step C) provided between method steps B) and D). Preferably, both detection signals are amplified.

Before method step D), in particular between method steps B) and D) or C) and D), signal processing of the at least two detector signals of the detectors can take place.

In a particularly preferred embodiment of the method, the detection signals are output on an oscilloscope. The laser pulse can thus be reproduced particularly precisely and with high resolution.

• E) Bestimmung eines Hauptanstiegszeitpunkts des Laserpulses;
• F) Bestimmung der Fläche unter einem ersten Detektionssignal bis zum Hauptanstiegszeitpunkt;
• G) Bestimmung der Fläche unter einem zweiten Detektionssignal ab dem Hauptanstiegszeitpunkt;
• H) Bestimmung des Kontrastes mittels Division der Flächen.

The process may include the following further steps:

• E) Determination of a main rise time of the laser pulse;
• F) Determination of the area under a first detection signal up to the main rise time;
• G) Determination of the area under a second detection signal from the main rise time;
• H) Determination of contrast by dividing the areas.

From the ratio of the two aforementioned areas, key characteristics of the laser pulse can be determined with a single size.

The main rise time can be determined by an algorithm. Alternatively or additionally, the detection signals can be evaluated by an algorithm, in particular the same algorithm.

The method according to the invention can further comprise the adjustment of a laser source for generating the laser pulse on the basis of the detection signals.

A laser resonator of the laser source can be adjusted. The laser resonator can be adjusted in particular by changing the polarization in the laser resonator using a lambda plate.

A first laser pulse can be divided into a characterized laser pulse described here and a main laser pulse. In the method described here, only a part of the first laser pulse is characterized, and the main laser pulse can be used for an application.

The main laser pulse can be used in particular to generate EUV radiation by irradiating a target material or for laser lift-off. When generating EUV radiation, target material in the form of tin droplets can be irradiated. Laser lift-off is preferably used in display production.

The object according to the invention is further achieved by a measuring device for carrying out a method described here. The measuring device has a scattering unit for scattering the laser pulse, at least two detectors for detecting the scattered laser pulse and an output unit for outputting the detection signals with differently scaled amplitudes. The control unit can be set up to attenuate the laser pulse. The detectors can be adapted to a predetermined wavelength.

The features described for the method apply analogously to the measuring device and vice versa.

Further features and advantages of the invention emerge from the description, the claims and the drawing. According to the invention, the features mentioned above and those described below can each be used individually or in combination in any convenient way. The embodiments shown and described are not to be understood as an exhaustive list, but rather are exemplary in nature for describing the invention.

Character list

The invention is explained in more detail below using an advantageous embodiment shown in the figures. However, the invention is not limited to this embodiment.

• 1: schematisch den Aufbau einer erfindungsgemäßen Messvorrichtung zur Durchführung eines erfindungsgemäßen Verfahrens,
• 2: eine Schnittansicht einer Streueinheit der Messvorrichtung mit Detektoren zur Messung von Streulicht,
• 3: oben eine Messung eines vergleichsweise hohen Kontrastes eines Laserpulses und unten die Auftragung des Kontrastes über mehrere Messungen, und
• 4: oben eine Messung eines vergleichsweise niedrigen Kontrastes eines Laserpulses und unten die Auftragung des Kontrastes über mehrere Messungen zum Auffinden eines Justage-Optimums.

Show it:

• 1 : schematically shows the structure of a measuring device according to the invention for carrying out a method according to the invention,
• 2 : a sectional view of a scattering unit of the measuring device with detectors for measuring scattered light,
• 3 : above a measurement of a comparatively high contrast of a laser pulse and below the plot of the contrast over several measurements, and
• 4 : above a measurement of a comparatively low contrast of a laser pulse and below the plot of the contrast over several measurements to find an adjustment optimum.

1 shows a measuring device 10 for carrying out a method according to the invention for characterizing a laser pulse. The measuring device 10 has a scattering unit 14 with a first detector 16a and a second detector 16b. The two detectors 16a, 16b are preferably designed identically, here in particular in the form of photodiodes.

The detection signals measured with the detectors 16a, 16b are preferably amplified in an amplifier 18 and output on an output unit 20, here in particular in the form of an oscilloscope. The output takes place on a first channel 22a and a second channel 22b of the output unit 20.

Thus, the essentially identical signal can be output twice on the output unit 20, but with different scaling or magnification. The output unit 20 can have a computer or calculator 24 which is used to determine and output a contrast 26 of a laser pulse 28 radiating into the scattering unit 14 (see 2 ) is set up.

2 shows a sectional view of the scattering unit 14 for the laser pulse 28. The scattering unit 14 preferably has several scattering disks, of which the scattering disks 30a, 30b are provided with a reference number. The scattering unit 14 further preferably has a blocking element 32 in order to block the central part of the laser pulse 28. As a result, evenly scattered light components of the laser pulse 28 reach the detectors 16a, 16b.

3 shows an output from the output unit 20 at the top. The amplitude of the laser pulse 28 is shown with two different scales. One scale was chosen so that the amplitude of the laser pulse 28 can be displayed in full. The other scale was chosen so that the amplitude of the laser pulse 28 is cut off at the top, but the rise of the laser pulse 28, the prepulse 34, is clearly visible. The prepulse 34 is followed by the pulse 36. The transition between the prepulse 34 and the pulse 36, the main rise time, occurs in the present example at time t = 0.

The ratio of the area of the prepulse 34 to the area of the pulse 36 results in the contrast 26 (in the 3 and 4 not provided with reference symbols). In the present example, the contrast 26 is relatively high at 1.413%, which is due to the already quite significant increase in the prepulse 34.

For some applications, it should be avoided that the prepulse 34 is already so significantly pronounced, since this can, for example, lead to undesirable preheating of a target (not shown). A low contrast 26 can therefore be seen in some applications as a high quality of the measuring device 10 (see 1 ) can be interpreted.

3 shows below a plot of the contrast 26 for successive measurements. From this plot it can be seen that from the 60th measurement onwards the contrast 26 increases sharply and a readjustment of the measuring device 10 (see 1 ) should be carried out.

4 corresponds to the representation from 3 , in the upper illustration with a pulse 36, but with a prepulse 34 of lower amplitude and an associated lower contrast 26 (0.445%) of the laser pulse 28.

The illustration below shows an adjustment of the measuring device 10 (see 1 ), with an optimum being found at measuring point 80.

Taking a look at all the figures in the drawing, the invention relates in summary to a method and a measuring device 10 for characterizing a laser pulse 28. The laser pulse 28 is evenly scattered over at least two, in particular identical, detectors 16a, 16b and the measurement signals from the detectors 16a, 16b are superimposed. The amplitude of one measurement signal is stretched more than the amplitude of the other measurement signal. The more magnified measurement signal can be used to determine a prepulse 34, the less magnified measurement signal can be used to measure a pulse 36, i.e. a main pulse. A contrast 26 of the laser pulse 28 can be determined by dividing the areas of the prepulse 34 and the pulse 36.

List of reference symbols

10

Measuring device

14

Spreading unit

16a

first detector

16b

second detector

18

amplifier

20

Output unit

22a

first channel

22b

second channel

24

calculator

26

contrast

28

Laser pulse

30a

Diffuser

30b

Diffuser

32

Blocking element

34

Prepuls

36

Pulse

QUOTES INCLUDED IN THE DESCRIPTION

This list of documents listed by the applicant was generated automatically and is included solely for the better information of the reader. The list is not part of the German patent or utility model application. The DPMA accepts no liability for any errors or omissions.

Cited patent literature

• WO 2020/169941 A1 [0003]
• CN109596228A [0004]

Claims (13)

Method for characterizing a laser pulse (28) with the following method steps: A) Scattering the laser pulse (28); B) Detecting the scattered laser pulse (28) with at least two detectors (16a, 16b); D) Outputting at least two detection signals from the detectors (16a, 16b) with different scalings of the amplitude of the detection signals. Procedure according to Claim 1 , in which at least one scattering disk (30a, 30b) is used for scattering the laser pulse (28) in method step A). Procedure according to Claim 1 or 2 , in which the central part of the laser pulse (28) is blocked during the scattering of the laser pulse (28) in method step A). Method according to one of the preceding claims, in which the detectors (16a, 16b) used in method step B) are of identical design. Method according to one of the preceding claims, in which detectors (16a, 16b) in the form of photodiodes are used in method step B). Method according to one of the preceding claims with the following further method step between method steps B) and D): C) amplification of the at least two detection signals. Method according to one of the preceding claims, in which the output of the at least two detection signals in method step D) takes place on an oscilloscope. Method according to one of the preceding claims with the following further method steps after method step D): E) determining a main rise time of the laser pulse (28) based on at least one of the at least two detection signals; F) determining the area of the detected laser pulse (28) under a first of the at least two detection signals up to the main rise time; G) determining the area of the detected laser pulse under a second of the at least two detection signals from the main rise time; H) determining a contrast (26) of the laser pulse (28) by dividing the area determined in method step F) by the area determined in method step G). Procedure according to Claim 8 , in which the main rise time is determined by an algorithm. Method according to one of the preceding claims with the following further method step after method step D), in particular after method step H): I) Adjustment of a laser source for generating the laser pulse (28). Method according to one of the preceding claims, in which a first laser pulse is divided into a main laser pulse and the laser pulse (28). Procedure according to Claim 11 , in which the main laser pulse is used to generate EUV radiation by irradiating a target material or for laser lift-off. Measuring device (10) for carrying out a method according to one of the preceding claims, wherein the measuring device (10) has the following: a) a scattering unit (14) for scattering the laser pulse (28); b) at least two detectors (16a, 16b) for detecting the scattered laser pulse (28); c) an output unit (20) for outputting the at least two detection signals with different scalings of the amplitudes of the detection signals.

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