System for generating multi-spectral-line CO2 laser pulse seed

Abstract

The invention discloses a system for generating a multi-spectral-line CO2 laser pulse seed, which belongs to the technical field of CO2 lasers and comprises a resonant cavity comprising a reflecting element group, a first grating and a gain medium; the first grating is used for spatially separating light beams with different wavelengths in the gain medium to realize simultaneous oscillation of multiple spectral lines; the reflecting element group is used for reflecting each spectral line beam split by the first grating back to the gain medium in parallel; the beam selecting and expanding mechanism comprises an aperture-adjustable diaphragm group and a beam expanding lens group and is used for controlling the transverse mode and spatial distribution of each spectral line; and the cavity emptying structure is used for outputting all the light beams in the resonant cavity when the photons are accumulated to the maximum value so as to form a multi-spectral-line CO2 laser pulse seed. The pulse seed generation device can be applied to the LPP extreme ultraviolet lithography technology to generate pulse seeds for driving laser, and has the advantages of being low in cost, stable in output and good in pulse quality.

Description

TECHNICAL FIELD

[0001] The present application belongs to the technical field of CO2 laser, more particularly, relates to a system for generating multi-spectral CO2 laser pulse seed. BACKGROUND

[0002] LPP (Laser Produced Plasma) extreme ultraviolet technology is an advanced technology for generating extreme ultraviolet light, which is usually applied to the photolithography process in semiconductor manufacturing for producing 7-nanometer and more advanced process chips. In the LPP technology, a high-energy laser is used to hit a droplet composed of materials such as tin (Sn), and a plasma is formed at the time of hitting. The energy and radiation generated by this plasma is extreme ultraviolet light (EUV).

[0003] At present, the most advanced extreme ultraviolet (EUV) lithography light source adopts a high-power, high-repetition-rate short-pulse CO2 laser as its driving light source. In order to achieve the single-pulse energy and average power required for EUV generation, the laser driver is required to generate a short-pulse CO2 laser with a duration of 5-20 ns, a repetition rate of more than 100 kHz, and an average laser power of more than 20 kW. The structure of Master Oscillator Power Amplifier (MOPA) is the mainstream scheme to achieve this goal. This scheme is realized by injecting a short-pulse CO2 laser seed into a multi-stage laser amplifier.

[0004] Many documents have pointed out that if the pulse seed is a multi-spectral seed, the seed can extract more energy in the amplifier, thereby significantly improving the power of the driving pulse, thereby obtaining higher extreme ultraviolet conversion efficiency (CE). The existing scheme for obtaining a multi-spectral seed mainly uses a quantum cascade laser (QCL), but this scheme has the problems of high manufacturing cost, complex system, easy to be affected by temperature, and poor stability. SUMMARY

[0005] In view of the above defects or improvement needs of the prior art, the present application provides a system for generating a multi-spectral CO2 laser pulse seed, thereby solving the technical problems of high cost, complex system, poor stability, and great temperature influence caused by the multi-stage quantum cascade laser used in the structure of the traditional system.

[0006] To achieve the above-mentioned purpose, according to one aspect of the present application, a system for generating a multi-spectral CO2 laser pulse seed is provided, comprising: A resonant cavity, comprising a first total reflection mirror, a gain medium, a reflective element component, a first grating, a cavity emptying structure, and a second total reflection mirror arranged sequentially. The gain medium is located between the first total reflection mirror and the reflective element assembly. The gain medium is used to discharge and generate a light beam. After being reflected by the second total reflection mirror, the light beam is incident on the first grating and exits at different diffraction angles. It is then reflected by the reflective element assembly into the gain medium so that the light beams of different spectral lines are independently distributed in the gain medium, thereby realizing the simultaneous oscillation of multi-spectral line light beams between the first total reflection mirror and the first grating. The cavity emptying structure is used to output all the beam in the resonant cavity when the photons accumulate to the maximum value during the oscillation of the multi-spectral beam, forming a multi-spectral CO2 laser pulse seed.

[0007] Preferably, the system further includes a beam selection mechanism and a beam expander mechanism. The beam selection mechanism is disposed between the first total reflection mirror and the gain medium. The beam selection mechanism includes an adjustable aperture stop group for selecting transverse modes and controlling the power of individual spectral line beams. The beam expander mechanism is disposed between the gain medium and the reflective element group. The beam expander mechanism includes a beam expander lens group for adjusting the beam diameter of each spectral line in the multi-spectral line system.

[0008] Preferably, the cavity voiding structure is an electro-optic cavity voiding structure, including a Brewster window and an electro-optic crystal, wherein the Brewster window is placed at a Brewster angle.

[0009] Preferably, the cavity emptying structure is an all-optical cavity emptying structure, including a control laser and a semiconductor crystal wafer. The control laser is used to irradiate the semiconductor crystal wafer to change its reflectivity, thereby achieving cavity emptying.

[0010] Preferably, the control laser includes a picosecond Nd:YAG laser and a nanosecond Nd:YAG laser.

[0011] Preferably, the semiconductor crystal wafer is a germanium crystal wafer.

[0012] Preferably, the control laser includes a picosecond Nd:YAG laser and a nanosecond Nd:YAG laser.

[0013] Preferably, the first grating is a blazed grating with 110-180 lines, exhibiting only 0th and +1st order diffraction.

[0014] Preferably, the reflective element group includes at least two total reflection mirrors.

[0015] Preferably, the reflective element group includes a second grating, and the second grating is placed parallel to the first grating.

[0016] Preferably, the multi-spectral CO2 laser pulse seed output by the system is used as the driving light source for LPP-EUV lithography.

[0017] In summary, compared with the prior art, the above-described technical solutions conceived by this invention can achieve the following beneficial effects: 1. The system for generating multi-spectral-line CO2 laser pulse seeds proposed in this invention adopts a spatial separation oscillation mechanism and a cavity emptying structure. That is, the beams of different wavelengths are spatially separated in the gain medium through a grating to achieve simultaneous oscillation of multiple spectral lines. The cavity emptying structure achieves high repetition rate and short pulse output, which has the advantages of low cost, stable output and good pulse quality.

[0018] 2. The system for generating multi-spectral CO2 laser pulse seeds proposed in this invention can be configured with either an electro-optic cavity emptying structure or a fully optical cavity emptying structure, making the system structure flexible.

[0019] 3. The system for generating multi-spectral line CO2 laser pulse seeds proposed in this invention is used as a driving light source for LPP-EUV lithography to improve energy extraction efficiency and conversion efficiency. Attached Figure Description

[0020] Figure 1 This is a schematic diagram of an embodiment of the system for generating multi-spectral-line CO2 laser pulse seeds according to the present invention; Figure 2 This is a schematic diagram of an embodiment of the system for generating multi-spectral-line CO2 laser pulse seeds according to the present invention; Figure 3 This is a schematic diagram of an embodiment of the system for generating multi-spectral-line CO2 laser pulse seeds according to the present invention.

[0021] In all the accompanying drawings, the same reference numerals are used to denote the same elements or structures, wherein: 1, first total reflection mirror; 2, second total reflection mirror; 3, first adjustable aperture stop; 4, second adjustable aperture stop; 5, third adjustable aperture stop; 6, gain medium; 7, third total reflection mirror; 8, fourth total reflection mirror; 9, fifth total reflection mirror; 10, first grating; 11, Brewster window; 12, electro-optic crystal; 13, first beam expander group; 14, second beam expander group; 15, third beam expander group; 16, second grating; 17, picosecond Nd:YAG laser; 18, nanosecond Nd:YAG laser; 19, germanium crystal sheet. Detailed Implementation

[0022] To make the objectives, technical solutions, and advantages of this invention clearer, the invention will be further described in detail below with reference to the accompanying drawings and embodiments. It should be understood that the specific embodiments described herein are merely illustrative and not intended to limit the invention. Furthermore, the technical features involved in the various embodiments of this invention described below can be combined with each other as long as they do not conflict with each other.

[0023] like Figures 1-3 As shown, this invention proposes a system for generating multi-spectral-line CO2 laser pulse seeds, the system comprising: The resonant cavity includes a first total reflection mirror 1, a gain medium 6, a reflective element assembly, a first grating 10, a cavity emptying structure, and a second total reflection mirror 2 arranged sequentially. The gain medium 6 is located between the first total reflection mirror 1 and the reflective element assembly. The gain medium 6 is used to generate a light beam through discharge. After being reflected by the second total reflection mirror 2, the light beam is incident on the first grating 10 and exits at different diffraction angles. It is then reflected by the reflective element assembly into the gain medium 6 so that the light beams of different spectral lines are independently distributed in the gain medium 6, thereby realizing the simultaneous oscillation of multi-spectral line light beams between the first total reflection mirror 1 and the first grating 10. The cavity emptying structure is used to output all the light beams in the resonant cavity when the photons accumulate to a maximum value during the oscillation of multi-spectral line light beams, forming a multi-spectral line CO2 laser pulse seed.

[0024] The technical solution of the present invention will be further illustrated below through specific embodiments.

[0025] Example 1 like Figure 1 As shown, this system is a cavity-emptied laser system. The main optical path structure consists of a first total reflection mirror 1, a second total reflection mirror 2, a first grating 10, a third total reflection mirror 7, a fourth total reflection mirror 8, and a fifth total reflection mirror 9. This optical path structure, together with the gain medium 6, forms the resonant cavity structure. The first adjustable aperture stop 3, the second adjustable aperture stop 4, and the third adjustable aperture stop 5 are used for transverse mode selection and power control of individual spectral lines. The first beam expander group 13, the second beam expander group 14, and the third beam expander group 15 are used to adjust the beam diameter of each spectral line to better match the amplifier gain. The Brewster window 11 and the electro-optic crystal 12 constitute the cavity-emptied structure.

[0026] To further explain, the first grating 10 is a blazed grating with a line count controlled between 110 and 180 lines, so that the diffraction order only exists in the 0th and +1st orders, thereby obtaining higher diffraction efficiency.

[0027] To further explain, the first grating 10 is used to spatially separate light of different wavelengths in the gain medium 6, and the third total reflection mirror 7, the fourth total reflection mirror 8, and the fifth total reflection mirror 9 are used in conjunction with the first grating 10 to parallelly incident light beams of different spectral lines into the gain medium.

[0028] To further explain, the laser working medium is a gain dielectric discharge tube, which can be used in waveguide lasers to obtain higher gain.

[0029] To further explain, since the gain of different spectral lines in the resonant cavity is different and uncontrollable, the first adjustable aperture stop 3, the second adjustable aperture stop 4, and the third adjustable aperture stop 5 can be used in conjunction with the first beam expander group 13, the second beam expander group 14, and the third beam expander group 15 to change the spot size of each spectral line in the output beam, so that the seed finally output by the oscillator can be distributed in the amplifier with a spatial structure of higher extraction efficiency.

[0030] To further clarify, the Brewster window 11 needs to be placed at a Brewster angle.

[0031] To further explain, the total cavity length of the resonant cavity should be controlled within 3 meters in order to obtain cavity emptying seed light pulse output of less than 20 nanoseconds.

[0032] In this embodiment, the system includes, Optical path structure: Due to mode competition within the resonant cavity, a laser can typically only stably output a beam with one spectral line at a time, or output seed beams of multiple spectral lines in an uncontrollable and unpredictable manner. Therefore, to achieve stable output of multiple spectral line seeds, this system employs a spatial separation oscillation method. A grating is essential in this system to achieve spatial separation oscillation.

[0033] According to the grating equation:

[0034] In the formula, d The grating constant is i Angle of incidence The diffraction angle, m For diffraction series, Lambda Let λ be the wavelength of light. As can be seen from the formula, light of different spectral lines is incident from the second total reflection mirror 2 onto the first grating 10. After passing through the grating, it exits at different diffraction angles and is then reflected into the gain medium 6 by the third total reflection mirror 7, the fourth total reflection mirror 8, and the fifth total reflection mirror 9, respectively. It can be seen that at this time, light of different spectral lines has its own independent spatial distribution in the gain medium 6.

[0035] For a 10.59 μm beam, if a 150-line grating is used, only the 0th and +1st orders exist. The system uses +1st order diffraction to achieve spatial separation and oscillation of different spectral line seeds. For 10.59 and 10.57 μm seeds, under these conditions, a spatial separation of 5 mm can be achieved in the gain medium 6 at an incident angle of 48°, thereby enabling simultaneous oscillation of the two spectral line seeds.

[0036] Beam selection and expansion structure: The first adjustable aperture stop 3, the second adjustable aperture stop 4, and the third adjustable aperture stop 5 are used to select the transverse mode and control the power of individual spectral lines.

[0037] Since different spectral lines have different gains in the resonant cavity and are difficult to control directly, the strongest spectral line (10.59 μm) generally produces a larger seed spot and higher beam intensity than other spectral lines when not affected by external factors. The amplifier's amplification efficiency is affected by the space-filling efficiency (spot size) of the seed light. This results in only the central part of the amplifier amplifying multi-spectral line seed light, while the edge parts only amplify strong line seed light, ultimately underutilizing the amplifier's efficiency. Therefore, the spatial distribution of the seed light is controlled by using the first adjustable aperture stop 3, the second adjustable aperture stop 4, the third adjustable aperture stop 5, and the first beam expander group 13, the second beam expander group 14, and the third beam expander group 15, maximizing the space-filling efficiency of different spectral line seed spots in the amplifier.

[0038] Cavity emptying structure: When the system is working, the initial gain medium 6 begins to discharge, and no voltage is applied to the electro-optic crystal 12. After the beam passes through the Brewster window, only the p-polarized energy can oscillate in the cavity. Thereafter, photons continue to accumulate. When the photons accumulate to the maximum value, the center wavelength working voltage is applied to the electro-optic crystal. After the beam passes through the electro-optic crystal, it passes through the mirror and then passes through the electro-optic crystal again. When it returns to the Brewster window, it has become s-polarized light. The Brewster window performs total internal reflection on s-polarized light. At this time, the beam in the cavity is completely emptied, realizing cavity emptying output of multi-spectral line seed.

[0039] The cavity emptying pulse width depends on the resonant cavity length, which is the time it takes for a beam of light to travel back and forth in the cavity, which is 2L / c. For a resonant cavity length of one meter, the pulse width is usually on the order of nanoseconds. Since extreme ultraviolet light driving light sources require pulses with a pulse width of 5-20ns, this system is feasible for use in extreme ultraviolet light driving light sources.

[0040] The photons in the resonant cavity typically need to travel back and forth 5 times within the cavity to accumulate to their maximum value, resulting in an accumulation time of 10 L / c. For a one-meter discharge tube, the output pulse repetition frequency can reach 30 MHz. Furthermore, extreme ultraviolet light driving light sources require a repetition frequency of over 100 kHz. Therefore, this system is feasible for the use of extreme ultraviolet light driving light sources.

[0041] Example 2 like Figure 2 As shown, in this embodiment, the third total reflection mirror 7, the fourth total reflection mirror 8, and the fifth total reflection mirror 9 in Embodiment 1 are replaced with the second grating 16, which can also achieve parallel distribution of seeds with different spectral lines in the gain medium.

[0042] To further clarify, the second grating 16 and the first grating 10 have exactly the same parameters.

[0043] To further explain, the second grating 16 needs to be placed parallel to the first grating 10.

[0044] To further explain, an aperture stop needs to be added to the beam combining path to prevent oscillations parallel to the main optical path from occurring.

[0045] To further explain, since the main optical components constituting the optical path structure in the system are only four elements—the first total reflection mirror 1, the second total reflection mirror 2, the first grating 10, and the second grating 16—this embodiment is simpler than the mechanism in Embodiment 1, the optical path is easier to adjust, and it also has certain advantages in stability.

[0046] In this embodiment, two gratings are used as internal components of the resonant cavity. Since the gratings will generate some loss in the 0th order diffraction, its output power will be lower than that of the system structure in Embodiment 1.

[0047] In this embodiment, the optical path structure is as follows: Based on grating equation

[0048] When light of different spectral lines is incident from mirror 2 onto the first grating 10, the angle of incidence is... The diffraction angles are respectively Then, the light is incident on the second grating 16 along its diffraction angle. Since the second grating 16 and the first grating 10 are placed parallel to each other, their incident angles are respectively... Substituting into the grating equation, we can calculate that its exit diffraction angle from grating 16 is... This achieves spatial separation and parallel distribution of light with different spectral lines in the gain medium 6. Similarly, when light with different spectral lines is incident from the first total reflection mirror 1 onto the second grating 16 and then onto the first grating 10 along its diffraction angle, its incident angles are respectively... It can be calculated that its exit diffraction angle from the first grating 10 is... Furthermore, it achieves beam combining of beams with different spectral lines in spatial distribution.

[0049] The beam selection and expansion mechanism and the cavity emptying structure are the same as the system structure in Example 1.

[0050] Example 3 like Figure 3 As shown, in the system structure of Embodiment 1, the cavity emptying structure is replaced by an all-optical cavity emptying structure composed of a Brewster window 11 and an electro-optic crystal 12.

[0051] To further clarify, the single-pulse power of the picosecond Nd:YAG laser 17 needs to be higher than 1.5 mJ / cm². 3 The pulse width is less than 50 picoseconds to achieve a higher response speed.

[0052] To further clarify, the nanosecond Nd:YAG laser 18 needs to have a power density below 100 mJ / cm². 3 The pulse width is higher than 20 nanoseconds to avoid excessive temperature of germanium crystal sheet 19 while achieving seed light output with a pulse width of 20 nanoseconds.

[0053] To further explain, the germanium crystal sheet 19 needs to be placed in a Brewster angle structure, so the output beam is linearly polarized light.

[0054] Compared to electro-optic cavity emptying, all-optical cavity emptying has the obvious advantages of a wider operating spectral range and faster response speed. All-optical cavity emptying can achieve multi-band cavity emptying at 10.6 micrometers and 9.6 micrometers.

[0055] In this embodiment, the optical path structure and beam selection and expansion mechanism are the same as the system structure in Embodiment 1, except that... Cavity emptying structure: Under natural conditions, the generation and recombination of charge carriers inside a semiconductor maintain a dynamic equilibrium, and the semiconductor is in a state of thermal equilibrium. When a semiconductor is irradiated with a laser whose single-photon energy is greater than its bandgap, the generation of charge carriers exceeds recombination, and a large number of non-equilibrium charge carriers instantaneously appear on the semiconductor surface, expanding outward to form a dense, electrically neutral plasma. When the illumination stops, the number of non-equilibrium charge carriers on the semiconductor surface is far greater than that in the thermal equilibrium state, so the recombination rate is much greater than the generation rate, and the non-equilibrium charge carrier density in the semiconductor surface plasma is much higher than that in the interior, thus spontaneously diffusing inward, eventually causing the non-equilibrium charge carriers to gradually disappear, and the semiconductor returns to a state of thermal equilibrium. For germanium semiconductors, when the generated charge carrier density exceeds a critical value... (approximately 1.7*10) 19 cm -3When the reflectivity of the light beam in the infrared band is high, it will jump from around 0 to over 93%.

[0056] When a control beam with photon energy greater than the bandgap of a germanium sheet is irradiated, the sheet rapidly excites a large number of charge carriers. This rapid increase in charge carrier density alters the reflection and transmission state of infrared light beams. Simultaneously, each charge carrier in the germanium sheet possesses an intrinsic frequency, which depends on the carrier density; the higher the density, the higher the intrinsic frequency. Furthermore, charge carriers reflect photons with frequencies lower than their intrinsic frequency. When the charge carrier concentration reaches a certain level, the intrinsic frequency rises to a level higher than that of infrared light. At this point, infrared light is reflected by the charge carriers, thus enabling cavity emptying of multi-spectral, multi-band CO2 lasers.

[0057] In this invention, two Nd:YAG lasers output control beams to irradiate a germanium sheet to control the state of the germanium sheet. Among them, the picosecond Nd:YAG laser 17 outputs picosecond-level pulses, and the nanosecond Nd:YAG laser 18 outputs nanosecond pulses.

[0058] When the control beam is not irradiating, the germanium crystal 19 is in a high-transmission state, and self-excited oscillation occurs within the resonant cavity, continuously accumulating photons. When the photons accumulate to a maximum value, the picosecond Nd:YAG laser 17 and nanosecond Nd:YAG laser 18 output control beams to irradiate the germanium crystal 19. At this point, the germanium crystal 19 changes from a high-transmission state to a high-reflection state, and all photons within the cavity are reflected and output from the germanium crystal 19, achieving cavity emptying.

[0059] It should be noted that the germanium semiconductor described in this embodiment is only one implementation scheme, and other semiconductors can be replaced according to actual needs.

[0060] It should be noted that the Nd:YAG laser controlled in this embodiment is only one implementation method, and other controlled lasers with single-photon energies greater than the semiconductor bandgap can be used as needed.

[0061] Those skilled in the art will readily understand that the above description is merely a preferred embodiment of the present invention and is not intended to limit the present invention. Any modifications, equivalent substitutions, and improvements made within the spirit and principles of the present invention should be included within the scope of protection of the present invention.

Claims

1. A system for generating a multiline CO2 laser pulse seed, comprising: The application relates to a multi-spectrum CO2 laser pulse seed system. The resonant cavity comprises a first total reflection mirror (1), a gain medium (6), a reflection element group, a first grating (10), a cavity emptying structure and a second total reflection mirror (2) arranged in sequence. The gain medium (6) is arranged between the first total reflection mirror (1) and the reflection element group, and is used for discharging to generate a light beam; the light beam is reflected by the second total reflection mirror (2) and then enters the first grating (10) to be emitted at different diffraction angles; the light beam is reflected by the reflection element group to the gain medium (6) so that the light beams of different spectrum lines are independently distributed in the gain medium (6), thereby realizing simultaneous oscillation of the multi-spectrum light beam between the first total reflection mirror (1) and the first grating (10). The cavity emptying structure is used for outputting all the light beams in the resonant cavity when the photon accumulation reaches the maximum value during the oscillation of the multi-spectrum light beam, thereby forming a multi-spectrum CO2 laser pulse seed.

2. The system for generating a multiline CO2 laser pulse seed according to claim 1, wherein, The system further comprises a beam selection mechanism and a beam expansion mechanism; the beam selection mechanism is arranged between the first total reflection mirror (1) and the gain medium (6) and comprises an adjustable aperture diaphragm group, which is used for selecting a transverse mode and controlling the power of a single spectrum line light beam; the beam expansion mechanism is arranged between the gain medium (6) and the reflection element group and comprises a beam expansion mirror group, which is used for adjusting the beam diameter of each spectrum line in the multi-spectrum light beam.

3. A system for generating a multiline CO2 laser pulse seed according to claim 2, wherein, The cavity emptying structure is an electro-optic cavity emptying structure, which comprises a Brewster window and an electro-optic crystal, and the Brewster window is arranged at a Brewster angle.

4. The system for generating a multiline CO2 laser pulse seed according to claim 2, wherein, The cavity emptying structure is a full-optical cavity emptying structure, which comprises a control laser and a semiconductor crystal sheet; the control laser is used for irradiating the semiconductor crystal sheet to change the reflectivity of the semiconductor crystal sheet, thereby realizing cavity emptying.

5. A system for generating a multiline CO2 laser pulse seed according to claim 4, wherein, The semiconductor crystal sheet is a germanium crystal sheet.

6. A system for generating a multiline CO2 laser pulse seed as defined in claim 5, wherein The control laser comprises a picosecond Nd:YAG laser (17) and a nanosecond Nd:YAG laser (18).

7. The system for generating a multiline CO2 laser pulse seed according to claim 1, wherein, The first grating (10) is a blazed grating with a line number of 110-180 lines, and only 0th order and +1st order diffractions exist.

8. The system for generating a multiline CO2 laser pulse seed according to claim 2, wherein, The reflection element group comprises at least two total reflection mirrors.

9. The system for generating a multiline CO2 laser pulse seed according to claim 2, wherein, The reflection element group comprises a second grating (16), and the second grating (16) is arranged in parallel with the first grating (10).

10. The system for generating a multiline CO2 laser pulse seed according to claim 1, wherein, The multi-spectrum CO2 laser pulse seed output by the system is used for an LPP-EUV photolithography driving light source.

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