A cavity-pumped co2 laser for an EUV light source and a control method thereof

Abstract

The application relates to the technical field of lasers, and provides a cavity emptying CO2 laser for an EUV light source and a control method thereof. The laser comprises a resonant cavity formed by a first concave mirror and a second concave mirror; a CO2 radio frequency waveguide arranged in the resonant cavity; a quarter-wave plate arranged on one side of the CO2 radio frequency waveguide and used for changing the polarization state of passing light; a Pockels cell arranged on the side, opposite to the quarter-wave plate, of the CO2 radio frequency waveguide and configured to not change the polarization state of the passing light when not powered and to change the polarization state of the passing light when powered; and a thin film polarizer arranged on the side, away from the CO2 radio frequency waveguide, of the Pockels cell. When the Pockels cell is not powered, the resonant cavity is in a high-loss state, so as to promote the inversion of the particle number accumulation of the CO2 radio frequency waveguide; and when the Pockels cell is powered, the light beams propagating back and forth in the CO2 radio frequency waveguide are orthogonal circularly polarized light. The application can output high-repetition-frequency narrow-pulse-width laser.

Description

Technical Field

[0001] This application relates to the field of laser technology, and more specifically, to a cavity-emptied CO2 laser for EUV light sources and its control method. Background Technology

[0002] Extreme ultraviolet (EUV) lithography machines are currently the only semiconductor manufacturing equipment capable of achieving nodes of 5nm and below, and are hailed as the crown jewel of the semiconductor industry. Currently, industrial-scale EUV lithography machines employ a laser-produced plasma (LPP)-EUV light source technology. In a vacuum environment, a high-repetition-rate, high-pulse-energy, narrow-pulse-width CO2 main-pump laser bombards a liquid-droplet tin target, generating high-intensity plasma. This plasma undergoes energy level transitions to generate EUV light radiation with a wavelength of approximately 13.5nm, which is then reflected and concentrated by a collecting mirror before being transferred to the subsequent exposure system.

[0003] The power of EUV light directly determines the chip processing speed. To achieve a processing speed of 100 12-inch wafers per hour, at least 550mW of EUV power needs to be provided at the end of the exposure system. Considering the high loss of the EUV coupling system, at least 200W of EUV light needs to be generated at the intermediate focus (IF). The corresponding main pump CO2 laser power needs to be greater than 20kW, which cannot be directly generated by a single laser oscillator. To meet this power requirement, the main pump laser needs to adopt a master oscillator power amplifier (MOPA) structure.

[0004] The MOPA structure enhances the average power by coupling the seed light into the gain medium of the amplification stage, while maintaining a relatively stable repetition rate and pulse width during amplification. The output parameters of the seed laser significantly influence the output power and power stability of the main pump laser. Therefore, constructing a high-repetition-rate, narrow-pulse-width seed laser is of great importance to the development of LPP-EUV light sources. Summary of the Invention

[0005] This application provides a cavity-emptied CO2 laser for EUV light sources and a control method thereof, to solve at least one of the technical problems mentioned above. Specifically:

[0006] Some embodiments of this application provide a cavity-emptied CO2 laser for EUV light sources, including:

[0007] A resonant cavity formed by a first concave mirror and a second concave mirror;

[0008] A CO2 radio frequency waveguide is disposed within the resonant cavity;

[0009] A quarter-wave plate is disposed on one side of the CO2 radio frequency waveguide to change the polarization state of the transmitted light;

[0010] A Pockel cell is located on the side of the CO2 radio frequency waveguide opposite to the quarter-wave plate and is configured to not change the polarization state of the transmitted light when no power is applied, and to change the polarization state of the transmitted light when power is applied.

[0011] A thin-film polarizer is disposed on the side of the Pockel cell away from the CO2 radio frequency waveguide;

[0012] When the Pockel cell is not powered, the quarter-wave plate, the Pockel cell, and the thin-film polarizer work together to put the resonant cavity in a high-loss state to promote the accumulation of reversed particle number in the CO2 radio frequency waveguide; when the Pockel cell is powered, the quarter-wave plate, the Pockel cell, and the thin-film polarizer work together to make the beam of light propagating back and forth in the CO2 radio frequency waveguide orthogonally circularly polarized.

[0013] In some embodiments, the CO2 radio frequency waveguide has a waveguide aperture of 3 mm, a total waveguide length of 2.0 m, and output windows with high transmittance to 10.6 μm beams at both ends.

[0014] In some embodiments, the Pockel cell is a cadmium telluride Pockel cell, with antireflection coatings on both sides for 10.6μm light beams.

[0015] In some embodiments, the first and second concave mirrors have a radius of curvature of 658 mm and are coated with a high-reflectivity film for 10.6 μm light beams.

[0016] In some embodiments, the laser outputs a pulsed laser with a repetition frequency of 80 kHz and a pulse width of 25 ns.

[0017] In some embodiments, the pulse single longitudinal mode retention rate of the laser output is >99.5%, and the pulse width RMS instability is <2%.

[0018] According to an embodiment of this application, a pulse waveform control method for a CO2 laser according to any one of the above claims is also provided, comprising:

[0019] An excitation voltage is applied to the CO2 radio frequency waveguide to cause it to generate a fluorescent beam;

[0020] When no voltage is applied to the Pockel cell, the fluorescent beam along the first direction maintains its beam state after passing through the Pockel cell. After passing through the thin-film polarizer, the fluorescent beam is converted into p-polarized light. The p-polarized fluorescence is reflected by the second concave mirror and propagates along the second direction. The p-polarized light along the second direction is converted into right-hand circularly polarized light by the quarter-wave plate. The right-hand circularly polarized light is reflected by the first concave mirror and propagates along the first direction. After passing through the quarter-wave plate along the first direction, it becomes s-polarized light. The s-polarized light is reflected out of the resonant cavity by the thin-film polarizer and cannot form effective oscillation, causing the resonant cavity to be in a high-loss state and the number of inverted particles to continue to accumulate.

[0021] When the number of inverted particles reaches saturation, a voltage is applied to the Pockel cell. The p-polarized light along the second direction becomes left-handed circularly polarized light after passing through the Pockel cell. The left-handed circularly polarized light enters the CO2 radio frequency waveguide and becomes s-polarized light after passing through the quarter-wave plate. The s-polarized light is reflected by the first concave mirror and propagates along the first direction. After passing through the quarter-wave plate along the first direction, it becomes right-handed circularly polarized light. The right-handed circularly polarized light is restored to p-polarized light after passing through the Pockel cell, matching the transmission direction of the thin-film polarizer to form a low-loss resonant cavity. The accumulated photon energy is amplified within the resonant cavity.

[0022] When the Pockel cell voltage is removed, the resonant cavity returns to a high-loss state, generating a high-repetition-rate, narrow-pulse-width laser, which is then output through the thin-film polarizer.

[0023] In some embodiments, it also includes:

[0024] The Pockel cell is kept in a de-energized state. Once the number of inverted particles is exhausted, the particle accumulation phase of the next cycle begins.

[0025] In some embodiments, the voltage applied by the Pockel box is 3000V.

[0026] In some embodiments, the high-repetition-rate, narrow-pulse-width output laser is a pulsed laser with a repetition frequency of 80 kHz and a pulse width of 25 ns.

[0027] Compared with related technologies, the above-described solutions of this application have at least the following beneficial effects:

[0028] This application proposes a novel cavity-emptied CO2 laser oscillator stage scheme for EUV light sources. By placing a quarter-wave plate and a Pockel cell at opposite ends of the CO2 gain medium, the beam propagating back and forth in the gain medium becomes orthogonally circularly polarized light, and the optical field is uniformly distributed in the light transmission direction. This eliminates the spatial hole-burning effect, effectively suppresses multi-longitudinal-mode oscillations, and achieves single-longitudinal-mode output while ensuring output power, thereby improving the pulse waveform and providing a high-performance seed laser for EUV light sources. Attached Figure Description

[0029] The accompanying drawings, which are incorporated in and form part of this specification, illustrate embodiments consistent with this application and, together with the description, serve to explain the principles of this application. It is obvious that the drawings described below are merely some embodiments of this application, and those skilled in the art can obtain other drawings based on these drawings without any inventive effort. In the drawings:

[0030] Figure 1 The diagram shows a cavity-emptied CO2 laser structure for EUV light sources, provided for some embodiments of this application.

[0031] Figure 2 A flowchart illustrating a cavity-emptied CO2 laser waveform control method for EUV light sources, provided in some embodiments of this application.

[0032] Figure 3 The waveform diagram is generated using existing technology.

[0033] Figure 4 The waveform diagrams are generated by the waveform control methods provided in some embodiments of this application.

[0034] Explanation of reference numerals in the attached figures:

[0035] 1. First concave mirror; 2. Quarter-wave plate; 3. CO2 radio frequency waveguide; 4. Pockel cell; 5. Thin-film polarizer; 6. Second concave mirror. Detailed Implementation

[0036] To make the objectives, technical solutions, and advantages of this application clearer, the application will be further described in detail below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of this application, and not all embodiments. Based on the embodiments in this application, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of this application.

[0037] The terminology used in the embodiments of this application is for the purpose of describing particular embodiments only and is not intended to limit the application. The singular forms “a,” “said,” and “the” used in the embodiments of this application and the appended claims are also intended to include the plural forms, and “multiple” generally includes at least two unless the context clearly indicates otherwise.

[0038] It should be understood that the term "and / or" used in this article is merely a description of the relationship between related objects, indicating that three relationships can exist. For example, A and / or B can represent: A existing alone, A and B existing simultaneously, and B existing alone. Additionally, the character " / " in this article generally indicates that the preceding and following related objects have an "or" relationship.

[0039] It should be understood that although the terms first, second, third, etc., may be used to describe embodiments of this application, these terms should not be used in isolation. These terms are only used to distinguish between different terms. For example, first may also be referred to as second without departing from the scope of embodiments of this application, and similarly, second may also be referred to as first.

[0040] It should also be noted that the terms "comprising," "including," or any other variations thereof are intended to cover non-exclusive inclusion, such that an article or device that comprises a list of elements includes not only those elements but also other elements not expressly listed, or elements inherent to such an article or device. Without further limitation, an element defined by the phrase "comprising one" does not exclude the presence of other identical elements in the article or device that includes said element.

[0041] Currently, industrial EUV light sources mainly employ quantum cascade laser technology and extracavity modulation technology to achieve high repetition rate and narrow pulse width seed laser output. Both suffer from low pulse peak power, making amplification difficult. Electro-optic cavity emptying CO2 laser technology can also achieve high repetition rate and narrow pulse width seed laser output, but it is prone to multi-longitudinal mode oscillations, leading to deteriorated pulse waveforms and even pulse multi-peak phenomena.

[0042] For LPP-EUV light sources, the optimal pulse width of the main pump laser is on the order of 10 ns. Currently, the main technical means to achieve this target are electro-optic cavity emptying CO2 laser technology, quantum cascade laser technology, and extracavity modulation technology.

[0043] The pulse width of the output laser of an electro-optic cavity empty laser is mainly determined by the length of the resonant cavity and the switching speed of the Q switch. To achieve a pulse width of more than 10 ns, the length of the resonant cavity is usually more than 1 m. The longitudinal mode spacing is inversely proportional to the length of the resonant cavity, which easily leads to multi-longitudinal mode output. The beat frequency phenomenon between different longitudinal modes will further lead to multi-peak phenomenon in the pulse waveform, making it unsuitable for LPP-EUV light sources.

[0044] Quantum cascade laser technology outputs laser light based on the electronic transitions between subbands of semiconductor-coupled quantum wells. By adjusting the thickness of the active quantum well, the wavelength of the output laser can be tuned to 10.6 μm, matching the wavelength of a CO2 laser amplifier. Through modulation techniques, pulse widths on the order of 10 ns can be achieved. However, its output power is low, with the peak power of the laser pulse only on the order of W. In contrast, the peak power of the main pump CO2 laser used in LPP-EUV light sources needs to reach around 10 MW. The low output power of quantum cascade laser technology exacerbates the amplification pressure on the laser amplification stage.

[0045] Extracavity modulation technology modulates continuous or wide-pulse CO2 lasers into narrow-pulse laser outputs using Pockels cells or acousto-optic modulators. However, it also suffers from the problem of low peak power of the output pulses, requiring high amplification capability from the laser amplification stage.

[0046] Based on this, this application proposes a cavity-emptied CO2 laser for EUV light sources, comprising: a resonant cavity formed by a first concave mirror 1 and a second concave mirror 6; a CO2 radio frequency waveguide 3 disposed within the resonant cavity; a quarter-wave plate 2 disposed on one side of the CO2 radio frequency waveguide 3 for changing the polarization state of the transmitted light; and a Pockels cell 4 disposed on the side of the CO2 radio frequency waveguide 3 opposite to the quarter-wave plate 2, configured to not change the polarization state of the transmitted light when not powered, and to change the polarization state of the transmitted light when powered. Polarization state; thin-film polarizer 5, disposed on the side of the Pockel cell 4 away from the CO2 radio frequency waveguide 3; wherein, when the Pockel cell 4 is not powered, the quarter-wave plate 2, the Pockel cell 4 and the thin-film polarizer 5 cooperate to put the resonant cavity in a high-loss state to promote the accumulation of reversed particle number in the CO2 radio frequency waveguide 3; when the Pockel cell 4 is powered, the quarter-wave plate 2, the Pockel cell 4 and the thin-film polarizer 5 cooperate to make the beam of light propagating back and forth in the CO2 radio frequency waveguide 3 orthogonally circularly polarized light.

[0047] This application proposes a novel cavity-emptied CO2 laser oscillator stage scheme for EUV light sources. By placing a quarter-wave plate and a Pockel cell at opposite ends of the CO2 gain medium, the beam propagating back and forth in the gain medium becomes orthogonally circularly polarized light, and the optical field is uniformly distributed in the light transmission direction. This eliminates the spatial hole-burning effect, effectively suppresses multi-longitudinal-mode oscillations, and achieves single-longitudinal-mode output while ensuring output power, thereby improving the pulse waveform and providing a high-performance seed laser for EUV light sources.

[0048] The embodiments of this application will now be described in detail with reference to the accompanying drawings.

[0049] like Figure 1 As shown, some embodiments of this application provide a cavity-emptied CO2 laser for EUV light sources, used to provide the high-performance seed laser required by EUV light sources.

[0050] The CO2 laser includes a resonant cavity formed by a first concave mirror 1 and a second concave mirror 6. The radii of curvature of the first concave mirror 1 and the second concave mirror 6 are 658 mm, which can effectively control the beam size within the cavity. Furthermore, the inner surfaces of the first concave mirror 1 and the second concave mirror 6 are coated with a 10.6 μm high-reflectivity film, which causes the laser to oscillate back and forth within the resonant cavity.

[0051] The CO2 laser also includes a CO2 radio frequency waveguide 3, which is disposed within the resonant cavity. The distances from each end of the CO2 radio frequency waveguide 3 to the first concave mirror 1 and the second concave mirror 6 are 329 mm, respectively. The waveguide aperture of the CO2 radio frequency waveguide 3 is 3 mm, and the total waveguide length is 2.0 m, meaning the total length of the folded-back optical path of the CO2 radio frequency waveguide 3 is 2.0 m. Both ends are provided with output windows that allow high transmission of 10.6 μm beams. Multiple reflectors are arranged between the two output windows to form a gain optical path. The CO2 radio frequency waveguide 3 can be a D-shaped waveguide, a circular waveguide, a rectangular waveguide, etc., and is not limited thereto.

[0052] The CO2 laser also includes a quarter-wave plate 2, which is disposed on one side of the CO2 radio frequency waveguide 3, for example, between the first concave mirror 1 and the CO2 radio frequency waveguide 3, to change the polarization state of the transmitted light.

[0053] The CO2 laser also includes a Pockel cell 4, which is disposed on the side of the CO2 radio frequency waveguide 3 opposite to the quarter-wave plate 2, for example, between the second concave mirror 6 and the CO2 radio frequency waveguide 3. It is configured to not change the polarization state of the transmitted light when no power is applied, and to change the polarization state of the transmitted light when power is applied. The Pockel cell 4 is a cadmium telluride Pockel cell, with anti-reflection coatings for 10.6μm beams on both sides.

[0054] The CO2 laser also includes a thin-film polarizer 5, disposed on the side of the Pockel cell 4 away from the CO2 radio frequency waveguide 3; for example, disposed between the second concave mirror 6 and the Pockel cell 4. When the thin-film polarizer 5 is a p-polarized thin-film polarizer, only p-polarized light is allowed to pass through; when the thin-film polarizer 5 is an s-polarized thin-film polarizer, only s-polarized light is allowed to pass through.

[0055] When the Pockel cell 4 is not powered, the quarter-wave plate 2, the Pockel cell 4, and the thin-film polarizer 5 work together to prevent light propagating along the first direction from passing through the thin-film polarizer 5, thereby placing the resonant cavity in a high-loss state to promote the accumulation of inverted particle number in the CO2 RF waveguide 3. When the accumulated inverted particle number in the CO2 RF waveguide 3 reaches a threshold, when the Pockel cell 4 is powered, the quarter-wave plate 2, the Pockel cell 4, and the thin-film polarizer 5 work together to allow light propagating along the first direction to pass through the thin-film polarizer 5, thereby causing the light to oscillate between the first concave mirror 1 and the second concave mirror 6. The beam of light propagating back and forth within the CO2 RF waveguide 3 is orthogonally circularly polarized light. For clarity, the first direction and the second direction are as follows: Figure 1 As shown.

[0056] In some embodiments, the high-repetition-rate, narrow-pulse-width output laser is a pulsed laser with an output repetition frequency of 80 kHz and a pulse width of 25 ns.

[0057] In some embodiments, the output laser has a pulse single-mode retention rate >99.5% and a pulse width RMS instability <2%. This single-mode, high-repetition-rate, narrow-pulse-width laser specification fully meets the requirements for seed lasers for EUV light sources.

[0058] According to embodiments of this application, such as Figure 2 As shown, a pulse waveform control method for a cavity-emptied CO2 laser oriented towards an EUV light source, as described above, is also provided, comprising the following steps:

[0059] Step S102: Apply an excitation voltage to the CO2 radio frequency waveguide 3 to generate a fluorescent beam.

[0060] Step S104: No voltage is applied to the Pockel cell 4. The fluorescent beam along the first direction remains in beam state after passing through the Pockel cell 4. The fluorescent beam is converted into P-polarized light after passing through the thin-film polarizer 5. The P-polarized fluorescence is reflected by the second concave mirror 6 and propagates along the second direction. The P-polarized light along the second direction is converted into right-hand circularly polarized light by the quarter-wave plate 2. The right-hand circularly polarized light is reflected by the first concave mirror 1 and propagates along the first direction. After passing through the quarter-wave plate 2 along the first direction, it becomes S-polarized light. The S-polarized light is reflected out of the resonant cavity by the thin-film polarizer 5 and cannot form effective oscillation, causing the resonant cavity to be in a high-loss state and the number of inverted particles to continue to accumulate.

[0061] Step S106: When the number of inverted particles reaches saturation, a voltage is applied to the Pockel cell 4. The p-polarized light along the second direction becomes left-handed circularly polarized light after passing through the Pockel cell 4. The left-handed circularly polarized light enters the CO2 radio frequency waveguide 3 and becomes s-polarized light after passing through the quarter-wave plate 2. The s-polarized light is reflected by the first concave mirror 1 and propagates along the first direction. After passing through the quarter-wave plate 2 along the first direction, it becomes right-handed circularly polarized light. The right-handed circularly polarized light is restored to p-polarized light after passing through the Pockel cell 4, which matches the transmission direction of the thin-film polarizer 5 to form a low-loss resonant cavity. The accumulated photon energy is amplified in the resonant cavity.

[0062] Step S108: Remove the voltage of the Pockel cell 4, and the resonant cavity returns to a high-loss state, forming a high-repetition-rate, narrow-pulse-width laser and outputting it through the thin-film polarizer 5.

[0063] In some embodiments, it also includes:

[0064] Step S110: Continue to keep the power off of the Pockel box 4. When the number of inverted particles is exhausted, enter the particle accumulation stage of the next cycle.

[0065] As described above, this application is a novel cavity-emptied CO2 laser for EUV light sources. The Pockel cell 4 and the quarter-wave plate 2 are located at both ends of the CO2 radio frequency waveguide 3, respectively. The polarization state of the beam can be controlled by adjusting the voltage applied to the Pockel cell 4.

[0066] In step S104, when no voltage is applied to the Pockel cell 4, the polarization state of the fluorescent beam radiated by the CO2 RF waveguide 3 remains unchanged when passing through the Pockel cell 4. When passing through the thin-film polarizer 5, the s-polarized fluorescent beam is reflected out of the resonant cavity, and the p-polarized beam is transmitted to the second concave mirror 6. After being reflected by the concave mirror 6, it passes through the thin-film polarizer 5, the Pockel cell 4, and the CO2 RF waveguide 3 in sequence, still maintaining p-polarization. After passing through the quarter-wave plate 2, the fluorescent beam becomes circularly polarized light. After being reflected by the first concave mirror 1, it passes through the quarter-wave plate 2 again, becoming linearly polarized light, but with its polarization direction rotated by 90° to s-polarization. It passes through the CO2 RF waveguide 3 and the Pockel cell 4 again, with no change in polarization state. After being transmitted to the thin-film polarizer 5, it is reflected out of the resonant cavity. Due to the combined effect of the quarter-wave plate 2 and the thin-film polarizer 5, the beam cannot form reciprocating oscillations within the resonant cavity, and the number of inverted particles in the CO2 RF waveguide 3 accumulates.

[0067] In step S106, as Figure 1In the direction of the arrow, when the number of inverted particles reaches saturation, a voltage is applied to the Pockel cell 4. The p-polarized light, along the second direction, smoothly passes from region D through the thin-film polarizer 5 to region C. After passing through the Pockel cell 4, the p-polarized light becomes left-handed circularly polarized light (region B). This left-handed circularly polarized light enters the CO2 RF waveguide 3 and, after passing through the quarter-wave plate 2, becomes s-polarized light (region A). The s-polarized light, after being reflected by the first concave mirror 1, propagates along the first direction and, after passing through the quarter-wave plate 2, becomes right-handed circularly polarized light, entering region B from region A. In the CO2 RF waveguide 3, it encounters the left-handed circularly polarized light in the second direction. The right-handed circularly polarized light, after passing through the Pockel cell 4, reverts to p-polarized light (region C), matching the transmission direction of the thin-film polarizer 5, and smoothly passes from region C through the thin-film polarizer 5 to region D, repeating the cycle. This process forms a low-loss resonant cavity, where the accumulated photon energy is amplified.

[0068] Among them, after the light beam passes through the thin-film polarizer 5, the light beam is p-polarized light with electric vector E 1. Along the principal axis of the parallel waveplate and the direction perpendicular to the principal axis of the waveplate, it can be decomposed into:

[0069] (1)

[0070] in E 0 represents the magnitude of the electric vector. k For wave vectors, z This represents the beam propagation distance. After passing through Pockel cell 4, the beam becomes circularly polarized, and the electric vector... E 2 can be represented as

[0071] (2)

[0072] After passing through quarter-wave plate 2, the light becomes linearly polarized. After reflection, it passes through quarter-wave plate 2 again and becomes circularly polarized once more. The electric vector... E 3 can be represented as

[0073] (3)

[0074] in L Let be the optical length of the resonant cavity. The light intensity distribution within the CO2 RF waveguide 3 can then be expressed as:

[0075] (4)

[0076] As can be seen from equation (4), the light intensity distribution and position within the CO2 radio frequency waveguide 3 are related. zSince it is unrelated to spatial hole burning, there is no spatial hole burning phenomenon, which can effectively suppress multi-mode oscillation. After the beam passes through the Pockel cell 4 again, it becomes p-polarized light, which can be transmitted to the concave mirror through the thin-film polarizer 5, and oscillate back and forth in the resonant cavity to form laser. When the inverted particle number is fully extracted, the quarter-wave voltage on the Pockel cell is removed, and the beam propagation law is consistent with the description in the previous paragraph. The s-polarized laser is output by the thin-film polarizer.

[0077] In traditional structures, the polarization state of the oscillating beam in the RF waveguide is p-polarized light, resulting in severe spatial hole burning effects, which easily leads to multi-mode output. Inter-mode beat frequencies cause degradation of the pulse waveform, such as... Figure 3 As shown.

[0078] This application eliminates the spatial burning effect without affecting the output power and pulse width, effectively suppressing multi-longitudinal-mode oscillations and forming a single-longitudinal-mode output, thereby improving the pulse waveform and enabling its application in EUV light sources, such as... Figure 4 As shown.

[0079] In some embodiments, in step S102, the voltage applied by the Pockel box 4 is 3000V.

[0080] In some embodiments, the high-repetition-rate, narrow-pulse-width output laser is a pulsed laser with a repetition frequency of 80 kHz and a pulse width of 25 ns.

[0081] Finally, it should be noted that the various embodiments in this specification are described by way of example, and each embodiment focuses on the differences from other embodiments. Similar or identical parts between embodiments can be referred to interchangeably. For the systems or apparatus disclosed in the embodiments, since they correspond to the methods disclosed in the embodiments, the descriptions are relatively simple, and relevant parts can be referred to the method section.

[0082] The above embodiments are only used to illustrate the technical solutions of this application, and are not intended to limit them. Although this application has been described in detail with reference to the foregoing embodiments, those skilled in the art should understand that modifications can still be made to the technical solutions described in the foregoing embodiments, or equivalent substitutions can be made to some of the technical features. Such modifications or substitutions do not cause the essence of the corresponding technical solutions to deviate from the spirit and scope of the technical solutions of the embodiments of this application.

Claims

1. A cavity-dump CO2 laser for EUV light sources, characterized in that The application relates to a CO2 laser resonator. The resonator cavity is formed by a first concave mirror (1) and a second concave mirror (6). A CO2 radio frequency waveguide (3) is arranged in the resonator cavity. A quarter wave plate (2) is arranged on one side of the CO2 radio frequency waveguide (3) and used for changing the polarization state of the passing light. A Pockels cell (4) is arranged on the side of the CO2 radio frequency waveguide (3) opposite to the quarter wave plate (2) and configured to not change the polarization state of the passing light when not powered and to change the polarization state of the passing light when powered. A thin film polarizer (5) is arranged on the side of the Pockels cell (4) away from the CO2 radio frequency waveguide (3). When the Pockels cell (4) is not powered, the quarter wave plate (2), the Pockels cell (4) and the thin film polarizer (5) cooperate to make the resonator cavity in a high-loss state, so as to promote the inversion of the particle number accumulation of the CO2 radio frequency waveguide (3); when the Pockels cell (4) is powered, the quarter wave plate (2), the Pockels cell (4) and the thin film polarizer (5) cooperate to make the light beam propagating back and forth in the CO2 radio frequency waveguide (3) be right-handed circularly polarized light.

2. The laser of claim 1, wherein, The waveguide aperture of the CO2 radio frequency waveguide (3) is 3 mm, the total length of the waveguide is 2.0 m, and both ends are provided with output windows with high transmittance to 10.6-micron light beams.

3. The laser of claim 1, wherein, The Pockels cell (4) is a cadmium telluride Pockels cell, and both sides are coated with an antireflection film for 10.6-micron light beams.

4. The laser of claim 1, wherein, The curvature radius of the first concave mirror (1) and the second concave mirror (6) is 658 mm, and the high-reflection film for 10.6-micron light beams is coated.

5. The laser of claim 1, wherein, The laser outputs pulsed laser with a repetition frequency of 80 kHz and a pulse width of 25 ns.

6. The laser of claim 1, wherein, The pulse single longitudinal mode retention rate of the laser output laser is greater than 99.5%, and the pulse width RMS instability is less than 2%.

7. A method of controlling the pulse shape of a CO2 laser as claimed in any one of claims 1 to 6, characterized in that The application further relates to a method for operating the CO2 laser resonator. An excitation voltage is applied to the CO2 radio frequency waveguide (3) to generate a fluorescent light beam. The Pockels cell (4) does not apply voltage, and the fluorescent light beam remains in the light beam state after passing through the Pockels cell (4) in the first direction; the fluorescent light beam is converted into p-polarized light after passing through the thin film polarizer (5), and the p-polarized fluorescent light propagates in the second direction after being reflected by the second concave mirror (6); the p-polarized light is converted into right-handed circularly polarized light after passing through the quarter wave plate (2) in the second direction, and the right-handed circularly polarized light propagates in the first direction after being reflected by the first concave mirror (1); the s-polarized light is reflected out of the resonator cavity by the thin film polarizer (5) after passing through the quarter wave plate (2) in the first direction, and cannot form effective oscillation, so that the resonator cavity is in a high-loss state, and the inversion of the particle number is continuously accumulated. When the inversion population reaches saturation, the Pockels cell (4) applies a voltage, the p-polarized light becomes left-handed circularly polarized light after passing through the Pockels cell (4) in the second direction, the left-handed circularly polarized light enters the CO2 radio frequency waveguide (3) and becomes s-polarized light after passing through the quarter-wave plate (2); the s-polarized light propagates along the first direction after being reflected by the first concave mirror (1), becomes right-handed circularly polarized light after passing through the quarter-wave plate (2) in the first direction; the right-handed circularly polarized light becomes p-polarized light after passing through the Pockels cell (4), matches the transmission direction of the film polarizer (5), forms a low-loss resonant cavity, and the accumulated photon energy is amplified in the resonant cavity; The voltage of the Pockels cell (4) is removed, and the resonant cavity returns to a high-loss state, forming a high-repetition-rate, narrow-pulse-width laser and outputting through the film polarizer (5).

8. The method of claim 7, wherein, Further comprising: The Pockels cell (4) is continuously kept in the off state, and when the inversion population is exhausted, it enters the next cycle of population accumulation stage.

9. The method of claim 7, wherein, The voltage applied by the Pockels cell (4) is 3000V.

10. The method of claim 7, wherein, The output high-repetition-rate, narrow-pulse-width laser is a pulsed laser with a repetition frequency of 80kHz and a pulse width of 25ns.

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