Patent Application
20240255767
Embodiments of the present invention relate to a method and a device for generating laser pulses.
Using laser pulses to generate a plasma for generating extreme ultraviolet (EUV) radiation for EUV lithography is known. The EUV radiation can in particular be generated in a target material irradiated using laser pulses, preferably in the form of a tin droplet.
The laser pulses are generated for this purpose in a seed source and amplified in the amplifiers of an amplifier unit. The laser pulses amplified in the amplifier unit are guided via a beam guiding unit to a focusing unit, which focuses the laser pulses in a target area in which the target material is provided. The target material is typically provided in the form of a droplet by a droplet source, which emits droplets at a fixed time interval. These droplets are irradiated using the laser pulses and as a result emit (EUV) radiation. Each droplet is struck by at least one laser pulse, so that the frequency at which the droplet source is operated corresponds to the frequency at which the seed source emits the laser pulses.
The power of the radiation emitted by the target material is dependent here, among other things, on the pulse duration and the energy of the laser pulses with which the target material is irradiated. The greater the pulse energy of the laser pulses, the higher the power is of the EUV radiation emitted by the target material. The pulse duration also influences the EUV yield. Shorter pulses tend to be advantageous here.
The laser pulses generated in the seed source enter an amplifier of the amplifying unit and are amplified therein. The amplifier comprises an amplification medium, such as CO2 or a solid.
If the amplifier is operated in saturation, the energy level which was emptied by stimulated emission can be refilled by adjacent energy levels which have not participated in the laser transition. This takes place on a time scale characteristic for the respective amplification medium.
If an incident pulse is now observed, the power of which is sufficient to completely empty the excited energy level by stimulated emission and which is short enough that this energy level cannot be refilled during the pulse duration by adjacent energy levels, the following is observed: The first part of the time curve of the pulse is strongly exaggerated and then first drops off rapidly and then slower and slower. The first part of the pulse thus already saturates the amplifier, so that no power is still available for the rear (later) part of the pulse.
On the other hand, if a pulse incident in the amplifier is observed, the power of which is sufficient to completely empty the excited energy level by stimulated emission and which is sufficiently long that this energy level can be refilled during its duration by adjacent energy levels, the pulse power then first strongly rises as in the above described short pulse, in order to then drop again. However, the falling flank continues over a longer period of time, since the energy level is continuously refilled from the adjacent levels. The first part of the pulse does saturate the amplifier, but the emptied energy level is then refilled by adjacent energy levels.
The energy yield is thus limited with short pulses, since with short pulses the first part of the laser pulse already saturates the amplifier, so that power no longer remains for the later part of the laser pulse.
Embodiments of the present invention provide a method for generating laser pulses. The method includes generating first laser pulses and second laser pulses using at least one laser source, and amplifying the first laser pulses and second laser pulses using an optical amplifier. Each respective second laser pulse passes through the optical amplifier offset in time in relation to a respective first laser pulse by a time offset. The method further includes separating the first laser pulses from the second laser pulses using an optical beam splitter based on at least one beam property, in which the first laser pulses differ from the second laser pulses, and passing the first laser pulses through a retardation unit. A time duration for each respective first laser pulse to pass through a retardation section of the retardation unit corresponds to the time offset in relation to the respective second laser pulse. The second laser pulses do not pass through the retardation unit. The method further includes superimposing the first laser pulses with the second laser pulses using a superposition unit to form superimposed laser pulses.
Subject matter of the present disclosure will be described in even greater detail below based on the exemplary figures. All features described and/or illustrated herein can be used alone or combined in different combinations. The features and advantages of various embodiments will become apparent by reading the following detailed description with reference to the attached drawings, which illustrate the following:
Embodiments of the present invention provide a method and a device which enable high-energy short laser pulses to be generated.
According to embodiments of the present invention, a method for generating laser pulses comprising the following method steps:
If not indicated otherwise, the lists are not to be viewed as exhaustive, but rather as “at least”. In particular, in addition to the first laser pulses and second laser pulses, further laser pulses (for example third laser pulses, third and fourth laser pulses, etc.) can be used.
In order to generate a short laser pulse of high power, two short pulses, in particular of equal length, are introduced into the same amplifier in a time interval in which the excited energy level participating in the laser transition is at least partially refilled from adjacent energy levels. The laser pulses, before they are amplified, can have the same or also different maximum powers. Depending on the time interval, the excited energy level participating in the laser transition is completely or partially refilled upon introduction of the second laser pulse into the amplifier.
If the excited energy level is already completely refilled, a maximum possible amplification thus results for the second laser pulse. If the excited energy level is not yet completely refilled, a lesser amplification results. In the case of incident laser pulses of equal maximum power, the maximum power of the second laser pulse is less in this case after its amplification than that of the first laser pulse.
After the two laser pulses have passed through the amplifier (or amplifiers), they are chronologically superimposed again. A laser pulse results, the pulse form and duration of which essentially correspond to the pulse form and duration of the two short laser pulses. The maximum power of the superimposed laser pulse or its energy is, however, up to twice as high as the power or energy of the individual laser pulses before they are superimposed. Superimposed laser pulses having high power or energy and short pulse duration can thus be achieved by the amplification of individual short laser pulses in a laser pulse train and the subsequent superposition thereof.
The pulse duration of a short laser pulse and thus the pulse duration of the first and second laser pulses is between 10 ns and 200 ns here, preferably between 20 ns and 100 ns. The pulse duration of a long laser pulse is greater than 200 ns here. The amplification medium is preferably CO2.
The superimposed laser pulses are preferably output at a rate between 10 kHz and 500 kHz, in particular between 20 kHz and 200 kHz, preferably between 30 kHz and 100 kHz. The rate of the laser pulses is preferably oriented here to the rate of a droplet source, thus of a target material in a target area.
The first laser pulses preferably differ from the generated second laser pulses due to their wavelength and/or their polarization. This enables the first laser pulses to be separated from the second laser pulses. The first laser pulses separated from the second laser pulses can then pass through a retardation section, through which the second laser pulses do not pass.
The first and second laser pulses can already have different wavelengths or polarizations directly upon their generation. In particular, the first and second laser pulses can be generated by two separate beam sources, which each generate laser pulses having different beam properties, such as wavelength or polarization. These can in particular be two CO2 lasers, which are operated at different wavelengths.
The different beam properties, such as wavelength or polarization, can also be applied to the first and/or second laser pulses after they are generated. In this case, the first and second laser pulses are preferably initially generated in a single beam source having the same beam properties. However, the first and second laser pulses can also be generated in different beam sources having the same beam properties. The different beam properties are applied to them (in both cases) in a following method step. The polarization of the first or second laser pulses can be rotated here with the aid of a retardation plate (such as a half-wave plate) or a Pockels cell, in particular with the aid of an electro-optical modulator (EOM). Alternatively, the frequency of the first or second laser pulses can also be shifted with the aid of a frequency shifting unit, for example an acousto-optical modulator.
The second laser pulses can be generated with a time delay to the generation of the first laser pulses. In particular a control unit can be used to set the time interval of the first laser pulses in relation to the second laser pulses. The first and second laser pulses can be generated here in different or also in a single laser source.
The time interval between first and second laser pulses can however in particular also be set in that the second laser pulses pass through a longer route after they are generated than the first laser pulses. The first and second laser pulses can be generated here in different laser sources. A control unit can be used, which controls the laser sources so that the laser pulses meet in method step E) to form superimposed laser pulses.
In a further preferred embodiment of the invention, the second laser pulses follow the first laser pulses at a constant time interval.
The time interval between the first laser pulses and the second laser pulses is preferably between 10 ns and 1500 ns, in particular between 50 ns and 1000 ns, preferably between 80 ns and 500 ns. The maximum possible amplification of the second laser pulse is dependent here on the time interval between the first and second laser pulses. The greater the time interval is between the first and the second laser pulses, the more completely the excited energy level participating in the laser transition is refilled by adjacent energy levels and the more strongly the second laser pulse can be amplified.
On the other hand, the rate of the superimposed laser pulses which strike the droplet is predetermined by the rate of the droplet source. Since sufficient energy also has to be present again in the amplifier for the next pulse sequence made up of first and second laser pulse, there is a maximum time interval between the first and second laser pulse from which—depending on the clock rate of the droplet source—the following pulse sequence can no longer be optimally amplified.
It is ensured by the above-mentioned values that both the following pulse sequence made up of first and second laser pulse and the second laser pulse of the observed pulse sequence can be amplified at a predetermined clock rate of between 10 kHz and 500 kHz, in particular between 20 kHz and 200 kHz, preferably between 30 kHz and 100 kHz.
The above-mentioned values moreover take into consideration that the length of the retardation section which the second laser pulse has passed through assumes a technically reasonable value (for example between 3 m and 450 m).
The amplifier unit is preferably designed in the form of an amplifier chain. Efficient amplification of the laser pulses can take place in this way.
The laser source and the amplifier unit preferably involve CO2 lasers and amplifiers, respectively.
The splitting of the laser pulses is preferably performed using a dichroic mirror. This preferably takes place when the laser pulses differ in their wavelength. Alternatively thereto, the splitting can be performed using a polarization mirror. This preferably takes place when the laser pulses differ in their polarization.
The retardation unit is preferably designed in the form of a retardation section. The retardation section can have multiple reflective optical elements. The retardation can thus take place in a simply designed manner.
The combination of the laser pulses after the retardation unit is preferably carried out using a dichroic mirror. This preferably takes place when the laser pulses differ in their wavelength. Alternatively thereto, the combination can be carried out using a polarization mirror.
Further preferably, the combined laser pulses are used to generate EUV radiation.
Embodiments of the invention also relate to a device for generating laser pulses, in particular for carrying out a method described here, comprising the following features:
The generation of the first laser pulses and second laser pulses can take place in different laser sources. The device can comprise a control unit which is designed to control the laser sources so that the first laser pulses are superimposed with the second laser pulses in the superposition unit to form superimposed laser pulses.
The amplifier unit is preferably designed in the form of an amplifier chain, in particular made up of CO2 amplifiers.
The separation unit and/or the superposition unit can be designed in the form of a dichroic mirror. Alternatively thereto, the superposition unit can be designed in the form of a polarization mirror.
The retardation unit can be designed in the form of a retardation section.
Further preferably, the output unit can have an EUV generating unit. The EUV generating unit has the above-described features for generating the superimposed laser pulses here. The superimposed laser pulses are preferably directed onto a target material that can be generated in the EUV generating unit (“EUV system”), in particular in the form of a tin droplet. A plasma can then arise in the target material, which emits the EUV radiation. The EUV generating unit can have a vacuum chamber, into which the target material can be introduced with the aid of a target source in a target area, a focusing unit for focusing the superimposed laser pulses in the target area, and a beam guiding unit for guiding the superimposed laser pulses to the focusing unit.
Further advantages of the invention may be found from the description and the drawing. Likewise, according to the invention the features mentioned above and those yet to be explained further may respectively be used individually or together in any desired combinations. The embodiments shown and described should not be understood as an exhaustive list, but rather are of an exemplary character for describing the invention.
First laser pulses 16a, 16b and second laser pulses 18a, 18b are generated here in a laser source 14, in the form of a seed source here. The first laser pulses 16a, b differ from the second laser pulses 18a, b, in particular in the wavelength and/or the polarization of their laser radiation.
The laser pulses 16a, b, 18a, b are amplified in an optical amplifier unit 20, in the form of an amplifier chain here.
According to embodiments of the invention, various laser pulses 16a, b, 18a, b are introduced into the amplifier unit 20, which can be amplified arriving in rapid succession in particular by using the energy from adjacent energy levels. The differentiation of the laser pulses 16a, b, 18a, b can contribute to the time interval between a first laser pulse 16a, b and a second laser pulse 18a, b not being limited by the filling of an energy level.
The time interval A between first laser pulses 16a, b and second laser pulses 18a, b is in particular equal. It is preferably between 10 ns and 1500 ns.
After the amplifier unit 20, the laser pulses 16a, b, 18a, b pass through a separation unit 22, here in the form of a dichroic mirror or a polarization mirror, where they are separated from one another on the basis of their wavelength or polarization.
The first laser pulses 16a, b then pass through a retardation unit 24, in the form of a retardation section here. The second laser pulses 18a, b do not pass through the retardation unit 24. Because the first laser pulses 16a, b pass through the retardation unit 24, the first laser pulses 16a, b can be chronologically superimposed with the second laser pulses 18a, b. At an exemplary interval A of 100 ns between the respective first laser pulses 16a, b and the respective second laser pulses 18a, b, the retardation section in air has a length of approximately 30 m. In order to design the retardation unit 24 compactly, it can have multiple deflection mirrors 26a, 26b, 26c, 26d.
The device 10 has a superposition unit 28, here in the form of a dichroic mirror or a polarization mirror. Due to the retardation unit 24, the respective first laser pulses 16a, b are combined with the respective second laser pulses 18a, b in the superposition unit 28 to form superimposed laser pulses 30a, 30b.
The superimposed laser pulses 30a, b are output in an output unit 32. In the present case, the output unit 32 has an extreme ultraviolet (EUV) light generating unit 34. A plasma for emitting EUV radiation 38 is generated in the EUV light generating unit 34 in a target material 36, in particular in the form of a tin droplet.
The EUV light generating unit 34 has a beam guiding unit 40 and a focusing unit 42. The focusing unit 42 is used to focus the superimposed laser pulses 30a, b in a target area 44, into which the target material 36 is introduced. The target material 36 enters a plasma state upon the irradiation using the laser pulses 30a, b and emits the EUV radiation 38, which is collected by means of a collector mirror 46.
In the example shown, the collector mirror 46 has an opening for the passage of the laser pulses 30a, b and the focusing unit 42 separates a vacuum chamber 48, in which the target material 36 is arranged, from the beam guiding unit 40.
The physical background of the embodiments of the invention will be described hereinafter on the basis of
The rectangular pulse 16a is amplified in the amplifier 20; an amplified pulse 16b results. Its pulse power rises nearly vertically, to then drop off initially steeply, then slowly, and finally nearly vertically. The first part of the pulse 16a thus already saturates the amplifier 20, so that only a little power is still available for the rear part of the pulse. The nearly vertical rise and drop of the pulse power is attributed here to the rectangular shape of the incoming pulse 16a.
Due to the steep drop of the pulse power of the incoming rectangular pulse 16a, the pulse power again drops nearly vertically. The first part of the pulse 16a does saturate the amplifier, but the emptied energy levels are then refilled by adjacent energy levels.
Two short rectangular pulses 16a, 18a at a time interval are shown in
The pulse duration of a short rectangular pulse 16a, 16b, 18a, 18b is typically 50 ns, the pulse duration of a long rectangular pulse 16a, 16b, 18a, 18b is typically 500 ns. The time interval between the two rectangular pulses 16a, 16b, 18a, 18b is typically 100 ns. The pulse durations of the amplified pulses 16b, 18b approximately correspond to the pulse durations of the pulses 16a, 18a, which are not yet amplified.
As described above, embodiments of the invention relate to a method 12 for generating superimposed laser pulses 30a, b, which are each composed at least of first laser pulses 16a, b and second laser pulses 18a, b. The first and second laser pulses 16a, b, 18a, b are amplified using different properties. After the amplification, the first laser pulses 16a, b are retarded in relation to the second laser pulses 18a, b in order to superimpose them on the second laser pulses 18a, b. The superimposed laser pulses 30a, b are preferably used to generate extreme ultraviolet (EUV) radiation 38. Embodiments of the invention also relate to a device 10 for generating superimposed laser pulses 30a, b.
While subject matter of the present disclosure has been illustrated and described in detail in the drawings and foregoing description, such illustration and description are to be considered illustrative or exemplary and not restrictive. Any statement made herein characterizing the invention is also to be considered illustrative or exemplary and not restrictive as the invention is defined by the claims. It will be understood that changes and modifications may be made, by those of ordinary skill in the art, within the scope of the following claims, which may include any combination of features from different embodiments described above.
The terms used in the claims should be construed to have the broadest reasonable interpretation consistent with the foregoing description. For example, the use of the article “a” or “the” in introducing an element should not be interpreted as being exclusive of a plurality of elements. Likewise, the recitation of “or” should be interpreted as being inclusive, such that the recitation of “A or B” is not exclusive of “A and B,” unless it is clear from the context or the foregoing description that only one of A and B is intended. Further, the recitation of “at least one of A, B and C” should be interpreted as one or more of a group of elements consisting of A, B and C, and should not be interpreted as requiring at least one of each of the listed elements A, B and C, regardless of whether A, B and C are related as categories or otherwise. Moreover, the recitation of “A, B and/or C” or “at least one of A, B or C” should be interpreted as including any singular entity from the listed elements, e.g., A, any subset from the listed elements, e.g., A and B, or the entire list of elements A, B and C.
| Number | Date | Country | Kind |
|---|---|---|---|
| PCT/EP2021/075775 | Sep 2021 | WO | international |
This application is a continuation of International Application No. PCT/EP2021/075775 (WO 2023/041180 A1), filed on Sep. 20, 2021, and claims benefit to PCT/EP2021/075775, filed on filed on Sep. 20, 2021. The aforementioned application is hereby incorporated by reference herein.
| Number | Date | Country | |
|---|---|---|---|
| Parent | PCT/EP2021/075775 | Sep 2021 | WO |
| Child | 18605858 | US |
