JP2008270549A - Driver laser for extreme ultraviolet light source - Google Patents

Abstract

A driver laser for an extreme ultraviolet light source capable of achieving a large output while suppressing energy per pulse or unit time of pulse laser light in a single-row configuration.
The driver laser includes (i) a laser discharge tube that excites laser light using CO ₂ as a laser medium, and a total reflection mirror and a partial reflection mirror that constitute an optical resonator with the laser discharge tube interposed therebetween. An oscillation stage laser device having an acoustooptic device disposed in a laser beam path between the total reflection mirror and the partial reflection mirror, and (ii) a pulse train of laser light having a predetermined repetition frequency in the oscillation stage laser device A laser control unit that controls the acoustooptic device so that is generated in a burst with a predetermined period; and (iii) an amplification stage device including at least one amplifier that amplifies laser light output from the oscillation stage laser device; It comprises.
[Selection] Figure 2

Description

  The present invention relates to a driver laser that emits light to a target in an extreme ultra violet (EUV) light source used as a light source of an exposure apparatus.

  In recent years, along with the miniaturization of semiconductor processes, miniaturization in photolithography has rapidly progressed, and in the next generation, fine processing of 100 nm to 70 nm, and further fine processing of 50 nm or less will be required. Therefore, for example, in order to meet the demand for fine processing of 50 nm or less, development of an exposure apparatus that combines an EUV light source with a wavelength of about 13 nm and a reduced projection reflective optics is expected.

  As an EUV light source, an LPP (laser produced plasma) light source (hereinafter also referred to as “LPP type EUV light source device”) using plasma generated by irradiating a target with laser light, and by discharge There are three types: a DPP (discharge produced plasma) light source using generated plasma and an SR (synchrotron radiation) light source using orbital radiation. Among these, since the LPP light source can considerably increase the plasma density, extremely high luminance close to that of black body radiation can be obtained, and light emission only in a necessary wavelength band is possible by selecting a target material. EUV requires a large amount of power because it is a point light source with a typical angular distribution and there is no structure such as an electrode around the light source, and it is possible to secure a very large collection solid angle of 2πsteradian. It is considered to be a powerful light source for lithography.

  Here, the principle of EUV light generation by the LPP method will be described. By irradiating the target material supplied into the vacuum chamber with laser light, the target material is excited and turned into plasma. Various wavelength components including EUV light are radiated from this plasma. Therefore, EUV light is reflected and collected using an EUV collector mirror that selectively reflects a desired wavelength component (for example, a component having a wavelength of 13.5 nm), and is output to the exposure unit.

In such an LPP type EUV light source device, particularly when a solid target is used, there is a problem of the influence of neutral particles and ions emitted from plasma. Since the EUV collector mirror is installed in the vicinity of the plasma, neutral particles emitted from the plasma adhere to the reflective surface of the EUV collector mirror and reduce the reflectivity of the mirror. On the other hand, the ions emitted from the plasma scrape off the multilayer film formed on the reflective surface of the EUV collector mirror. Note that the scattered matter from the plasma containing neutral particles and ions and the debris of the target material are called debris. However, recently, by using a CO ₂ laser as a driving light source (driver laser) and using solid tin (Sn) as a target material, the amount of debris generated from tin by laser light irradiation is greatly reduced. It was confirmed that

  By the way, the output currently required for the EUV light source is about 140 W. From the viewpoint of improving productivity, it is highly likely that output will continue to be improved. In the case of the LPP method, the output that can be extracted as EUV light used for exposure is about 1 to 2% of the driver laser output. Therefore, when the output required for the EUV light source is 140 W, for example, a driver laser output of about 10 kW is required.

  As in the case of the excimer laser for lithography, the output value required for the light source device is expected to gradually increase due to the demand for improvement in the number of wafers processed per unit time. For example, in an excimer laser, an output value required for a light source device is about 10 W at the beginning of introduction, but is currently 40 W or more. Similarly, with an EUV light source, it can be easily predicted that an output value that is four times or more the current value is required. At that time, the driver laser requires an output of 40 kW or more. This output value may further increase.

When a tin (Sn) target is irradiated with a laser beam using a CO ₂ laser, 2% or more of CE (intensity) is about 10 ¹⁰ W / cm ² depending on the literature. conversion efficiency (conversion efficiency from irradiation laser light to EUV light) is possible, and it is becoming clear that CE becomes lower at higher intensity.

Intensity is defined by the following equation (1).
I = E / (τ · D) (1)
Here, each code is defined as follows.
I: Intensity (W / cm ² )
E: Laser energy (J)
τ: Laser pulse width (s)
D: Laser spot diameter (cm)

EUV light sources also have limitations on etendue. This is a performance limited by the optical system of the exposure apparatus at the subsequent stage. In the LPP type EUV light source, it depends on the EUV emission diameter and the condensing solid angle. For example, typically required Etendue values are less than 1 mm ² · steradian, and to achieve this, the EUV emission size is 0. 2 mm diameter when the collection solid angle of the EUV collector mirror is 2πsteradian. It must be 3 mm or less. Here, if EUV emission diameter≈condensing spot diameter, a condensing spot diameter of up to 0.3 mm is acceptable.

From the above, from the viewpoint of a driver laser of an EUV light source, the laser spot diameter is 0.3 mm or less, the intensity is 10 ⁹ W / cm ² to 10 ¹¹ W / cm ² , preferably 5 × It is important to satisfy the design condition of 10 ⁹ W / cm ^{2 to} 3 × 10 ¹⁰ W / cm ² , more preferably approximately 1 × 10 ¹⁰ W / cm ² .

On the other hand, as for the pulse width of the laser light, in a commercially available CO ₂ laser that is easily available, the pulse width is typically about 20 ns to 30 ns. As a laser that achieves such a pulse width, for example, there is a TEA laser or a slab laser. Alternatively, the above pulse width may be obtained from an axial flow type continuous wave laser by a cavity dump or the like. Since the energy of laser pulses output from these lasers is typically several μJ, when trying to obtain energy of several tens of mJ necessary for generating EUV light, an amplifier using a CO ₂ laser as an oscillator A laser system such as a MOPA (master oscillator power amplifier) combined with the above is required. When assuming a CO ₂ laser as an oscillator, for example, if the pulse width is 20 ns and the above-described design conditions are taken into consideration, it is possible to estimate that the energy per one pulse of the laser pulse is about 150 mJ or less as a driver laser by MOPA. it can.

  That is, the restriction that the energy per pulse is about 150 mJ or less is imposed due to the restriction of the optimum intensity, the spot size as an exposure apparatus, and the pulse width of an easily available oscillator. However, as described above, a laser output of 40 kW or more is expected, and if this is achieved at a typical repetition rate of 100 kHz, the laser energy will be 400 mJ.

  As a related technique, the following Patent Document 1 discloses a driver laser system for an EUV light source device capable of generating EUV light stably with high efficiency. This driver laser system is configured in accordance with the MOPA system, and is arranged in parallel by controlling the pulse width of the laser light so as to generate the laser light and shorten the pulse width of the laser light to a predetermined value. A laser system that amplifies the laser light in a plurality of laser amplification systems, and a laser system control device that controls the operation timing of the laser system so that the laser light is sequentially emitted from the plurality of laser amplification systems.

  However, according to the configuration in which a plurality of oscillators and amplifiers are arranged in parallel as described above, the number of oscillators and amplifiers increases, resulting in an increase in cost. The cost composition ratio of the laser system in the EUV light source apparatus is very large, and in some cases exceeds 80% of the apparatus price. Therefore, in the laser system of Patent Document 1, the EUV light source device becomes very expensive.

Patent Document 2 below discloses a technique for forming an arbitrary pulse waveform by synchronizing a plurality of laser sequences and superimposing output light from these laser sequences. According to this, there is a possibility that an arbitrary pulse width can be realized by combining commercially available lasers. However, as described above, since the cost of the laser device in the EUV light source is very high, when a large number of oscillators are used, only a high-cost laser system can be realized.
JP 2006-128157 A (first page, FIG. 1) US Patent Application Publication No. 2005/0117620 (first page, FIG. 1)

  Therefore, in view of the above points, the present invention provides a driver laser for an extreme ultraviolet light source that can achieve a large output while suppressing energy per pulse or unit time of pulse laser light with a single-row configuration. Objective.

In order to solve the above problems, a driver laser according to one aspect of the present invention is a driver laser used in an extreme ultraviolet light source device that generates extreme ultraviolet light by irradiating a laser beam onto the target to convert the target into plasma. (I) a laser discharge tube that excites laser light using CO ₂ as a laser medium, a total reflection mirror and a partial reflection mirror that constitute an optical resonator across the laser discharge tube, a total reflection mirror, and a partial reflection An oscillation stage laser device having an acousto-optic element disposed in a laser beam path between the mirror and the mirror; and (ii) a pulse train of laser light having a predetermined repetition frequency in the oscillation stage laser device is bursty at a predetermined cycle. A laser control unit that controls the acousto-optic element so as to be generated, and (iii) amplifies the laser light output from the oscillation stage laser device Comprising an amplifier stage device that includes at least one amplifier.

    ADVANTAGE OF THE INVENTION According to this invention, the driver laser for extreme ultraviolet light sources which can achieve a big output, suppressing the energy per pulse or unit time of a pulse laser beam with a single-row structure can be provided. This technology can cope with an increase in required output for future EUV light sources, and can realize an inexpensive EUV light source for a long period of time.

Hereinafter, the best mode for carrying out the present invention will be described in detail with reference to the drawings. In addition, the same reference number is attached | subjected to the same component and description is abbreviate | omitted.
FIG. 1 is a schematic diagram showing a configuration of an EUV light source apparatus to which a driver laser according to the present invention is applied. This EUV light source apparatus employs a laser-excited plasma (LPP) system that generates EUV light by irradiating a target material with laser light and exciting it.

  As shown in FIG. 1, this EUV light source device includes a vacuum chamber 10 in which EUV light is generated, a target supply device 11 that supplies a target 1, and a pulse laser beam 2 for excitation that is irradiated onto the target 1. The driver laser 12 to be generated, the laser condensing optical system 14 for generating the plasma 3 by condensing the pulse laser beam 2 generated by the driver laser 12 and irradiating the target 1, and the EUV emitted from the plasma 3 An EUV collector mirror 15 that collects and emits the light 4 and a target recovery device 16 that recovers the target 1 are provided.

  The vacuum chamber 10 has an introduction window 18 for introducing the pulsed laser light 2 for excitation, and unnecessary light among the EUV light emitted from the plasma (an electromagnetic wave having a shorter wavelength than the EUV light or a wavelength shorter than that of the EUV light). An SPF (spectral purity filter) 19 that removes long ultraviolet rays and the like and passes a desired wavelength component (for example, a component having a wavelength of 13.5 nm) through the exposure device is provided. Note that the inside of the exposure device is also kept in a vacuum or a reduced pressure state, similarly to the inside of the vacuum chamber 10.

  The driver laser 12 has a single-row configuration based on the MOPA method, and an oscillation stage laser device (OSC: oscillator) 121 that outputs pulse laser light, and an N stage that amplifies the pulse laser light output from the oscillation stage laser device 121. The amplifier stage device 122 includes the amplifiers AMP (1) to AMP (N), and the laser control unit 123 that controls the operation of the oscillation stage laser device 121. Here, N is a natural number, and the amplification stage device 122 includes at least one stage amplifier. The driver laser 12 gradually amplifies the pulsed laser light output from the oscillator OSC by sequentially passing through the amplifiers AMP (1), AMP (2),..., And the EUV generation laser light from AMP (N). Is output.

A preferred embodiment of the driver laser 12 shown in FIG. 1 will be described below.
FIG. 2 is a schematic diagram showing a configuration of an oscillation stage laser device used in the driver laser according to the first embodiment of the present invention. In the first embodiment, a CO ₂ laser capable of pulse oscillation at a high repetition frequency is used as the oscillation stage laser device 121. For example, in order to achieve an output of 40 kW, if the repetition frequency is 400 kHz, the energy per pulse may be 100 mJ. Thus, by performing high repetition in the oscillation stage laser device 121, the number of oscillators and amplifiers constituting the driver laser 12 can be minimized.

The oscillation stage laser device 121 includes a CO ₂ laser discharge tube 20 that excites laser light using CO ₂ as a laser medium, and a total reflection mirror 21 and a partial reflection mirror 22 that constitute a resonator with the laser discharge tube 20 interposed therebetween. And an acoustooptic device 23 such as an AOQ switch (Acousto-Optic Q-switch) that switches light under the control of the laser controller 123 (FIG. 1). The pulse repetition frequency f in the mode-locked laser is expressed by the following equation (2). Here, c represents the speed of light, and L represents the resonator length.
f = c / (2L) (2)

According to Equation (2), when L = 1 m, the pulse repetition frequency is 150 MHz. As apparent from the equation (2), the repetition frequency f can be changed by arbitrarily selecting the resonator length L. When mode lock is used for a CW (continuous wave) laser using CO ₂ as a laser medium, the pulse width is about 10 ns to 20 ns, and mode lock is applied to a TEA (Transversely Excited Atmospheric) laser using CO ₂ as a laser medium. When used, the pulse width is about 1 ns.

The mode lock includes active mode locking (Active Mode Locking) and passive mode locking (Passive Mode Locking). In the active mode lock, as shown in FIG. 2, a modulator is inserted into the resonator, and the modulator turns resonance on / off. In the wavelength range of the CO ₂ laser, an acoustooptic device 23 using a crystal such as germanium (Ge) is used as a modulator. The acoustooptic element 23 induces an acoustooptic effect in the crystal by applying an acoustic wave to the crystal and changes the refractive index of the crystal. This change in refractive index cuts out the laser beam in the resonator, and a pulsed laser beam is output from the laser resonator.

  The frequency of the acoustic wave applied to the crystal is c / (2L). However, if a repetition frequency practical for an EUV light source, for example, 400 kHz is realized, the resonator length L becomes 375 m. Since this is not feasible, in this embodiment, acoustic waves are generated in bursts. For example, the resonator length L is set to 1 m, the laser pulse repetition frequency is set to 150 MHz, and a laser pulse train having a repetition frequency of 150 MHz is generated every 2.5 μs to form a burst waveform as shown in FIG.

  In that case, the pulse laser beam having a repetition frequency of 150 MHz generated in one burst can be regarded as almost continuous light in plasma generation, so that a laser beam having a repetition frequency of 400 kHz can be substantially obtained. By changing the burst interval or changing the number of pulses in one burst, high repetition can be easily achieved, and the substantial pulse width can be arbitrarily changed. Arbitrary repetition frequency and pulse width can be obtained by configuring the oscillator in this way.

Next, a second embodiment of the present invention will be described.
FIG. 4 is a schematic diagram showing a configuration of an oscillation stage laser device used in the driver laser according to the second embodiment of the present invention. The oscillation stage laser device shown in FIG. 4 is a passive mode-locked laser by changing the acoustooptic element 23 to a saturable absorber 24 in the oscillation stage laser device shown in FIG.

  In a passive mode-locked laser, a saturable absorber 24 having a short upper level relaxation time is inserted in a laser resonator. The supersaturated absorber 24 has a higher transmittance as the laser intensity increases, and functions as an optical gate. That is, in the expression (2), a strong longitudinal mode that is an integral multiple is selected to form a pulse, and the laser resonator performs an oscillation operation. As the supersaturated absorber 24, SF6 or a mixed gas of SF6 or the like is used. In that case, the repetition frequency is adjusted by adjusting the mixing ratio and gas pressure of the supersaturated absorber 24.

Next, a third embodiment of the present invention will be described. In the third embodiment, the first embodiment or the second embodiment described above is improved.
FIG. 5 is a schematic diagram showing a configuration around an oscillation stage laser device used in a driver laser according to a third embodiment of the present invention. In the oscillation stage laser apparatus shown in FIG. 2, when the gain of the CO ₂ laser discharge tube 20 is high, laser light may be continuously generated even when the acoustooptic device 23 is in a burst pause. In order to prevent this, the polarization beam splitters 25 and 26 and the Pockels cell 27 that switches light by the electro-optic effect are combined to prevent the laser light generated during the burst pause from being output to the subsequent amplifier. ing.

  The Pockels cell 27 emits light by rotating the phase of incident light by λ / 2 when a predetermined voltage is applied. Here, λ represents the wavelength of light. Thereby, the P-polarized laser light becomes S-polarized light, and the S-polarized laser light becomes P-polarized light. For example, when the polarization beam splitters 25 and 26 transmit P-polarized laser light, the P-polarized laser light is incident on the Pockels cell 27 and a predetermined voltage is not applied to the Pockels cell 27. P-polarized pulsed laser light emitted from the Pockels cell 27 passes through the polarization beam splitter 26. On the other hand, in a state where a predetermined voltage is applied to the Pockels cell 27, the S-polarized pulsed laser light emitted from the Pockels cell 27 is reflected by the polarization beam splitter 26 and emitted in the direction of the arrow.

  The operation timing of the Pockels cell 27 is synchronized with the operation timing of the acoustooptic device 23, and the laser control unit 123 (FIG. 1) turns on the Pockels cell 27 while the acoustooptic device 23 is turned on in a burst manner. As a result, only the necessary pulse laser beam is output to the subsequent amplifier. Note that the same configuration can be adopted when the saturable absorber 24 is used instead of the acousto-optic element 23.

Next, a fourth embodiment of the present invention will be described. In the fourth embodiment, the oscillation stage laser device has a configuration in which a solid-state laser and a plurality of nonlinear crystals are combined.
FIG. 6 is a schematic diagram showing the configuration of an oscillation stage laser device used in a driver laser according to the fourth embodiment of the present invention. As shown in FIG. 6, this oscillation stage laser device 121 includes a solid-state laser 31 capable of outputting pulsed laser light at a high repetition frequency, a nonlinear crystal 32 (nonlinear crystal A), and a nonlinear crystal 33 (nonlinear crystal). B) and a spectral matching unit 34.

As the solid-state laser 31, for example, a titanium sapphire laser that oscillates in a wavelength region near 660 nm to 900 nm is used. Further, as the nonlinear crystal A, for example, BBO (beta-barium-borite: βBaB ₂ O ₄ ) is used. This nonlinear crystal A has a wavelength λ ₂ from the nonlinear crystal A according to the relationship 1 / λ ₁ = 1 / λ ₂ + 1 / λ ₃ by phase matching with the oscillation wavelength λ ₁ of the solid-state laser 31. A wavelength component (wavelength component λ ₂ ) and a wavelength component having a wavelength λ ₃ (wavelength component λ ₃ ) are output.

By phase matching the nonlinear crystal B with respect to these wavelength components λ ₂ and λ ₃ , a wavelength component (wavelength component λ ₄ ) having a wavelength λ ₄ corresponding to the difference frequency between the wavelength component λ ₂ and the wavelength component λ ₃ is obtained. ₄ ) is obtained. As the nonlinear crystal B, for example, AgGaS ₂ or HgGa ₂ S ₂ is used, and thereby a broadband laser beam having a wavelength of 9 μm to 12 μm is obtained as the wavelength component λ ₄ . The spectrum matching unit 34 matches the spectrum of the laser beam generated by the nonlinear crystal B with the amplification characteristic of the amplification stage device 122 (FIG. 1), and outputs the laser beam.

  As the spectrum matching unit 34, for example, a regenerative amplifier is used. A broadband laser beam can be obtained as the output of the nonlinear crystal B, but even if it is incident on the amplification stage device 122, high amplification efficiency cannot be obtained. This is because the laser medium (gain medium) of the amplification stage device 122 has a discrete gain spectrum. Therefore, for example, by using a regenerative amplifier, a spectral component that matches the discrete gain spectrum of the amplification stage device 122 is grown from the broadband spectrum by passing the laser medium through the broadband laser beam many times. . By making the laser light thus obtained incident on the amplification stage device 122, high amplification efficiency can be obtained.

This embodiment can be realized when a laser device capable of broadband oscillation such as a titanium sapphire laser is used. The titanium sapphire laser can output laser light at a repetition frequency of about several tens of kHz to several tens of MHz. With this configuration, the oscillation stage laser device can be highly repeated. For example, when λ ₁ = 950 nm, λ ₄ = 10600 nm (10.6 μm) can be obtained by setting λ ₂ = 1400.88 nm and λ ₃ = 1614.21 nm.

  In the following embodiments, in order to achieve an output of 40 kW or more in the driver laser, the energy per unit time is suppressed by extending the pulse width at or near the oscillator while using an easily available oscillator. And design constraints are relaxed. For example, if the pulse width is 100 ns, the laser energy is allowed up to 750 mJ. Since only one oscillator is required and the amplification system connected to the oscillator is a single row, the laser cost can be kept very low. As means for extending the pulse width, there are an optical path delay method, a method of arranging an optical switch element inside or outside the laser resonator, and a method of combining them. By using these techniques, it is possible to construct a driver laser system at a minimum cost while extending the pulse width and relaxing the laser energy limitation.

  The fifth and sixth embodiments of the present invention will be described. According to the fifth and sixth embodiments, in the driver laser 12 shown in FIG. 1, a pulse extension optical system is inserted between the oscillation stage laser device 121 and the amplification stage device 122 to reduce the pulse width of the laser light. By stretching, the limitation of laser energy per pulse can be relaxed.

  FIG. 7 is a schematic diagram showing a configuration of a pulse stretching optical system used in a driver laser according to a fifth embodiment of the present invention. The pulse extension optical system 124 shown in FIG. 7 includes two beam splitters 41 and 42 and four mirrors 43 to 46 as optical elements. In such an optical system 124, part of the laser light incident on the beam splitter 41 is transmitted through the beam splitter 41 and the other part is reflected in the direction of the mirror 43. The reflected light reflected by the beam splitter 41 is delayed by passing through a delay optical path that is longer than the transmitted light in terms of distance, and is combined with the transmitted light at the beam splitter 42. As the delay optical path, for example, the beam splitter 41 → mirror 43 → mirror 44 → beam splitter 42, or the beam splitter 41 → mirror 43 → mirror 44 → mirror 45 → mirror 46 → mirror 43 → mirror 44 → beam splitter 42 There is an optical path.

  Further, part of the laser light incident on the beam splitter 42 from the beam splitter 41 is transmitted through the beam splitter 42, and the other part is reflected in the direction of the mirror 45 and is delayed by passing through the delay optical path. For example, the optical path of the beam splitter 42 → mirror 45 → mirror 46 → mirror 43 → mirror 44 → beam splitter 42 exists as the delay optical path. When such a process is repeated, the optically delayed light is superimposed on the transmitted light with a time delay, and as a result, a laser beam having a long pulse width is obtained. Here, an arbitrary pulse width can be obtained by adjusting the distance between the optical elements in the direction of the arrow. Further, the pulse waveform can be adjusted to some extent by adjusting the reflectivity of the beam splitter.

  FIG. 8 is a schematic diagram showing a configuration of a pulse stretching optical system used in a driver laser according to the sixth embodiment of the present invention. A pulse stretching optical system 126 shown in FIG. 8 is obtained by combining an amplifier 47 with the pulse stretching optical system 124 in the fifth embodiment. By using the amplifier 47, pulse shaping can be performed at the same time as the pulse expansion, so that there is an advantage that the degree of freedom is higher than in the fifth embodiment.

  Next, a seventh embodiment of the present invention will be described. In the seventh embodiment, laser light output from one oscillation stage laser device is branched into a plurality of optical paths, and a plurality of laser lights having different delays due to optical path differences are synthesized. .

  FIG. 9 is a diagram showing a configuration of a driver laser according to the seventh embodiment of the present invention. The driver laser 12 includes an oscillation stage laser device (OSC: oscillator) 121 that outputs a pulse laser beam, and a beam splitter 51a as a beam branching unit that branches the pulse laser beam output from the oscillation stage laser device 121 into a plurality of optical paths. ˜51c and the mirror 52, beam synthesizers 53a to 53c for synthesizing a plurality of pulse laser beams having different delays depending on the optical path, and an N-stage amplifier for amplifying the pulse laser beams output from the beam synthesizer 53c And an amplification stage device 122 including AMP (1) to AMP (N). Here, N is a natural number.

  As shown in FIG. 10A, the pulse width of the laser light output from the oscillation stage laser device 121 is about 30 ns, and pulses are observed at the observation points 1A to 1D at sequentially delayed timings. A plurality of pulse laser beams respectively output from the beam splitters 51a to 51c and the mirror 52 are sequentially synthesized by the beam synthesizers 53a to 53c, so that the observation is made at the observation points 2 to 4 as shown in FIG. Can be obtained. Further, by amplifying the pulse laser beam output from the beam combiner 53c by the amplification stage device 122, a rectangular pulse having a pulse width of the order of 100 ns with a substantially constant intensity is obtained at the observation point 5.

  In consideration of the amplification characteristics of the amplifiers AMP (1) to AMP (N), it is desirable to set the pulse waveform at the observation point 4 shown in FIG. 10B at the input of the amplifier AMP (1). This can be realized by giving an intensity difference as shown in FIG. 10A to the pulse intensities at the observation points 1A to 1D. In order to set such an intensity difference, the reflectivity of the beam splitters 51a to 51c may be gradually lowered.

  For example, assuming that the reflectivity of the beam splitter 51a is 95%, the reflectivity of the beam splitter 51b is 85%, and the reflectivity of the beam splitter 51c is 55%, the energy at the observation point 1A is output from the oscillation stage laser device 121. 5% of the energy, the energy at the observation point 1B is 10% of the energy output from the oscillation stage laser device 121, the energy at the observation point 1C is 30% of the energy output from the oscillation stage laser device 121, Since the energy at the observation point 1D is 55% of the energy output from the oscillation stage laser device 121, the intensity ratio as shown in FIG. According to this embodiment, it is easy to shape the waveform of the laser light based on the optical path length for delay and the reflectivity of the beam splitter, and the waveform after shaping is easy to grasp.

  Next, an eighth embodiment of the present invention will be described. In the eighth embodiment, in the driver laser, the pulse width of the laser light is extended by utilizing a saturation phenomenon in laser amplification. That is, when light having an intensity of a predetermined value or more is input to the amplification stage device, a phenomenon in which the amplification factor gradually approaches a constant value occurs, and this is used for pulse shaping.

  FIG. 11 is a schematic diagram showing the configuration of a driver laser according to the eighth embodiment of the present invention. The driver laser 12 has an oscillation stage laser device (OSC: oscillator) 127 that outputs a pulse laser beam whose intensity increases with time, and a pulse laser beam output from the oscillation stage laser device 127 at an amplification factor that decreases with time. And an amplification stage device 128 including N stages of amplifiers AMP (1) to AMP (N) to be amplified. Here, N is a natural number.

  For example, the oscillation stage laser device 127 oscillates a pulse having a pulse width (FWHM) of about 30 ns, a base of about 100 ns, and an intensity that increases with time, like a waveform at the observation point 1 shown in FIG. This can be realized relatively easily by adjusting the rise time of a laser that performs pulse cutting using an optical switch element such as a cavity dump. For example, such a pulse waveform can be easily formed by adjusting a voltage waveform applied to a crystal such as Cd / Te.

  When a pulse having such a waveform is passed through an amplifier whose excitation intensity is adjusted so as to achieve saturation amplification, the first half of the pulse is amplified with a high gain because the energy is low, and the amplifier is saturated in the second half of the pulse. Therefore, it is amplified with a low amplification factor. By utilizing this phenomenon, it is possible to selectively amplify the skirt portion of the pulse. If this is repeated in the amplifiers AMP (1) to AMP (N), the pulse width sequentially increases as in the observation points 2 to 4, and finally a pulse having a pulse width of the order of 100 ns can be obtained. it can. According to this embodiment, the capacity of the amplifier can be reduced, the number of control elements is small, and a low-cost laser system with a relatively simple configuration becomes possible.

Next, a ninth embodiment of the present invention will be described. In the ninth embodiment, the pulse width is extended by arranging an optical switch element outside the oscillation stage laser device.
FIG. 13 is a schematic diagram showing a configuration around an oscillation stage laser device used in a driver laser according to a ninth embodiment of the present invention. As shown in FIG. 13, this oscillation stage laser device 121 includes a laser discharge tube 28 of CW excitation or pulse excitation, and a total reflection mirror 21 and a partial reflection mirror 22 constituting a resonator with the laser discharge tube 28 interposed therebetween. Contains. Further, in the oscillation stage laser device 121, polarization beam splitters 25 and 26 and a Pockels cell 27 are combined.

  The Pockels cell 27 emits light by rotating the phase of incident light by λ / 2 when a predetermined voltage is applied. Here, λ represents the wavelength of light. Thereby, the P-polarized laser light becomes S-polarized light, and the S-polarized laser light becomes P-polarized light. For example, when the polarization beam splitters 25 and 26 transmit P-polarized laser light, the P-polarized pulse laser light is incident on the Pockels cell 27 and a predetermined voltage is not applied to the Pockels cell 27. The P-polarized pulsed laser light emitted from the Pockels cell 27 passes through the polarization beam splitter 26. On the other hand, in a state where a predetermined voltage is applied to the Pockels cell 27, the S-polarized pulsed laser light emitted from the Pockels cell 27 is reflected by the polarization beam splitter 26 and emitted in the direction of the arrow.

  As a result, laser light can be extracted from the CW-excited laser oscillator using polarized light, and an arbitrary pulse waveform can be obtained. Since the pulse width may be limited by the length of the resonator, reserve a sufficient resonator length for the desired pulse width. The light oscillated from the CW oscillator can be extracted in the direction of the arrow only when the Pockels cell 27 is turned on by the polarization beam splitters 25 and 26 and the Pockels cell 27. The pulse width of the laser light is adjusted by this time interval.

Next, a tenth embodiment of the present invention will be described. In the tenth embodiment, the pulse width is extended by arranging an optical switch element in the oscillation stage laser device.
FIG. 14 is a schematic diagram showing a configuration of an oscillation stage laser device used in the driver laser according to the tenth embodiment of the present invention. As shown in FIG. 14, the oscillation stage laser device 121 includes a CW-excited or pulse-excited laser discharge tube 28, total reflection mirrors 21a and 21b constituting a resonator with the laser discharge tube 28 interposed therebetween, and a polarization beam splitter. 25, a Pockels cell 27a, a λ / 4 wave plate 29, and a mirror 30.

  When a predetermined voltage is applied, the Pockels cell 27a rotates the phase of incident light by λ / 4 and emits it. Here, λ represents the wavelength of light. For example, when the polarization beam splitter 25 transmits P-polarized laser light and the predetermined voltage is applied to the Pockels cell 27a, the P-polarized laser light emitted from the polarization beam splitter 25 is Pockels. By reciprocating between the cell 27 a and the λ / 4 wavelength plate 29, it becomes P-polarized light again and passes through the polarization beam splitter 26.

  On the other hand, in a state where a predetermined voltage is not applied to the Pockels cell 27a, the P-polarized laser light emitted from the polarization beam splitter 25 becomes S-polarized by reciprocating the λ / 4 wavelength plate 29, and the polarized beam The light is reflected by the splitter 26 and emitted in the direction of the arrow. Thus, while the Pockels cell 27a is turned on, the light reciprocates inside the resonator, but when the Pockels cell 27a is turned off, the laser light is extracted to the outside. With this time interval, the pulse width of the laser light can be changed. Unlike the ninth embodiment, the tenth embodiment is high in energy efficiency because less laser light is discarded.

Next, an eleventh embodiment of the present invention will be described. In the eleventh embodiment, the pulse width of the laser light is extended using an amplifier.
FIG. 15 is a schematic diagram showing a configuration of an amplifier used in the driver laser according to the eleventh embodiment of the present invention. In the amplifier shown in FIG. 1 (for example, AMP (1)), as shown in FIG. 15, a high reflection mirror 63 is disposed in the vicinity of the incident window 61 provided in the laser discharge tube 60, and in the vicinity of the emission window 62. Since the partial reflection mirror 64 is disposed in the laser beam, the laser light transmitted through the partial reflection mirror 64 and the light reflected by the partial reflection mirror 64 and the high reflection mirror 63 are emitted in the same direction. A plurality of pulse laser beams delayed by time are combined, and the pulse width is extended. The driver laser according to this embodiment has a relatively simple and space-saving configuration.

  The present invention can be used in a driver laser that irradiates light to a target in an EUV light source used as a light source of an exposure apparatus.

It is the schematic which shows the structure of the EUV light source device to which the driver laser concerning this invention is applied. It is the schematic which shows the structure of the oscillation stage laser apparatus used in the driver laser which concerns on the 1st Embodiment of this invention. It is a figure which shows the output waveform of the oscillation stage laser apparatus used in the driver laser which concerns on the 1st Embodiment of this invention. It is the schematic which shows the structure of the oscillation stage laser apparatus used in the driver laser which concerns on the 2nd Embodiment of this invention. It is the schematic which shows the structure of the oscillation stage laser apparatus periphery used in the driver laser which concerns on the 3rd Embodiment of this invention. It is the schematic which shows the structure of the oscillation stage laser apparatus used in the driver laser which concerns on the 4th Embodiment of this invention. It is the schematic which shows the structure of the pulse expansion | extension optical system used in the driver laser which concerns on the 5th Embodiment of this invention. It is the schematic which shows the structure of the pulse expansion | extension optical system used in the driver laser which concerns on the 6th Embodiment of this invention. It is a figure which shows the structure of the driver laser which concerns on the 7th Embodiment of this invention. It is a figure which shows the timing and waveform of several pulses in the 7th Embodiment of this invention. It is the schematic which shows the structure of the driver laser which concerns on the 8th Embodiment of this invention. It is a figure which shows the waveform of the pulse in the 8th Embodiment of this invention. It is the schematic which shows the structure of the periphery of the oscillation stage laser apparatus used in the driver laser which concerns on the 9th Embodiment of this invention. It is the schematic which shows the structure of the oscillation stage laser apparatus used in the driver laser which concerns on the 10th Embodiment of this invention. It is the schematic which shows the structure of the amplifier used in the driver laser which concerns on the 11th Embodiment of this invention.

Explanation of symbols

  DESCRIPTION OF SYMBOLS 1 ... Target, 2 ... Laser beam, 3 ... Plasma, 4 ... EUV light, 10 ... Vacuum chamber, 11 ... Target supply apparatus, 12 ... Driver laser, 14 ... Laser condensing optical system, 15 ... EUV collector mirror, 16 ... Target recovery device, 18 ... introduction window, 19 ... SPF, 20, 28, 60 ... laser discharge tube, 21, 21a, 21b ... total reflection mirror, 22 ... partial reflection mirror, 23 ... acoustic optical element, 24 ... supersaturated absorber , 25, 26 ... Polarizing beam splitters, 27, 27a ... Pockels cell, 29 ... λ / 4 wave plate, 30 ... Mirror, 31 ... Solid state laser, 32, 33 ... Non-linear crystal, 34 ... Spectral matching unit, 41, 42 ... Beam splitter, 43 to 46, 52 ... mirror, 47 ... amplifier, 51a to 51c ... beam splitter, 53a to 53c ... beam combiner, 61 ... Emission window, 62 ... Emission window, 63 ... High reflection mirror, 64 ... Partial reflection mirror, 121, 127 ... Oscillation stage laser device, 122, 128 ... Amplification stage device, 123 ... Laser control unit, 124, 126 ... Pulse extension optics system

Claims (10)

A driver laser used in an extreme ultraviolet light source device that generates extreme ultraviolet light by irradiating the target with laser light to generate plasma.
A laser discharge tube that excites laser light using CO ₂ as a laser medium, a total reflection mirror and a partial reflection mirror that constitute an optical resonator across the laser discharge tube, and the total reflection mirror and the partial reflection mirror An oscillation stage laser device having an acousto-optic element disposed in the path of the laser light between,
A laser controller that controls the acoustooptic device so that a pulse train of laser light having a predetermined repetition frequency is generated in a burst manner at a predetermined period in the oscillation stage laser device;
An amplification stage device including at least one amplifier for amplifying laser light output from the oscillation stage laser device;
A driver laser comprising: A driver laser used in an extreme ultraviolet light source device that generates extreme ultraviolet light by irradiating the target with laser light to generate plasma.
A laser discharge tube that excites laser light using CO ₂ as a laser medium, a total reflection mirror and a partial reflection mirror that constitute an optical resonator across the laser discharge tube, and the total reflection mirror and the partial reflection mirror An oscillation stage laser device that generates a pulse train of laser light having a predetermined repetition frequency in a burst with a predetermined period, and a saturable absorber disposed in the path of the laser light between
An amplification stage device including at least one amplifier for amplifying laser light output from the oscillation stage laser device;
A driver laser comprising: A first polarization beam splitter that receives laser light output from the oscillation stage laser device and emits laser light having first polarization in a predetermined direction;
An optical switch element that changes the polarization of the laser light emitted from the first polarization beam splitter to a second polarization when a predetermined voltage is applied;
A second polarization beam splitter that receives laser light emitted from the optical switch element and emits laser light having a second polarization to the amplification stage device;
The laser control unit controls the optical switch element so that a pulse train of laser light having a predetermined repetition frequency is emitted to the amplification stage device in a burst manner at a predetermined cycle. Or the driver laser of 2. A driver laser used in an extreme ultraviolet light source device for generating extreme ultraviolet light by irradiating a target with laser light to generate the extreme ultraviolet light.
An oscillation stage laser device that generates pulsed laser light having a first wavelength at a predetermined repetition frequency, and a second wavelength longer than the first wavelength based on the laser light generated by the oscillation stage laser device, and A first nonlinear crystal that generates laser light having a third wavelength, and a fourth wavelength that is longer than the second wavelength and the third wavelength, based on the laser light generated by the first nonlinear crystal. A second nonlinear crystal that generates a laser beam having the above and a spectrum matching unit that matches the spectrum of the laser beam generated by the second nonlinear crystal with the amplification characteristics of the subsequent stage and outputs the laser beam An oscillation stage laser device;
An amplification stage device including at least one amplifier for amplifying laser light output from the oscillation stage laser device;
A driver laser comprising: A driver laser used in an extreme ultraviolet light source device for generating extreme ultraviolet light by irradiating a target with laser light to generate the extreme ultraviolet light.
An oscillation stage laser device that generates pulsed laser light using CO ₂ as a laser medium;
Upon receiving the laser beam generated by the oscillation stage laser device, the laser beam having the first polarization is transmitted in the first direction, and the laser beam having the second polarization is not parallel to the first direction. A first polarizing beam splitter that reflects in the direction of 2;
Upon receiving the laser light transmitted through the first polarization beam splitter, the laser light having the first polarization is transmitted in the first direction, and the laser light having the second polarization is not parallel to the first direction. A second polarizing beam splitter that reflects in a third direction;
The laser beam reflected by the first or second polarization beam splitter is reflected a plurality of times and is incident on the second polarization beam splitter, and the laser beam is emitted from the second polarization beam splitter in a first direction. A plurality of mirrors that extend the pulse width of the laser light by emitting,
An amplification stage device including at least one amplifier for amplifying laser light emitted from the second polarization beam splitter;
A driver laser comprising:   6. The driver laser of claim 5, further comprising an amplifier inserted between the first or second polarizing beam splitter and one of the plurality of mirrors. A driver laser used in an extreme ultraviolet light source device that generates extreme ultraviolet light by irradiating the target with laser light to generate plasma.
An oscillation stage laser device that generates pulsed laser light using CO ₂ as a laser medium;
Beam branching means for branching laser light output from the oscillation stage laser device into a plurality of optical paths;
Beam combining means for extending the pulse width of the laser beam by combining a plurality of laser beams with different delays according to each optical path;
An amplification stage device including at least one stage of amplifier for amplifying the laser beam output from the beam combining means;
A driver laser comprising: A driver laser used in an extreme ultraviolet light source device for generating extreme ultraviolet light by irradiating a target with laser light to generate the extreme ultraviolet light.
An oscillation stage laser device that generates pulsed laser light whose intensity increases with time using CO ₂ as a laser medium;
An amplification stage apparatus including at least one amplifier for extending the pulse width of the laser light by amplifying the pulsed laser light output from the oscillation stage laser apparatus with an amplification factor that decreases with time;
A driver laser comprising: A driver laser used in an extreme ultraviolet light source device that generates extreme ultraviolet light by irradiating the target with laser light to generate plasma.
An oscillation stage laser device for generating laser light;
A first polarization beam splitter that receives laser light generated by the oscillation stage laser device and emits laser light having a first polarization in a first direction;
An optical switch element that changes the polarization of the laser light emitted from the first polarization beam splitter to a second polarization when a predetermined voltage is applied;
A second polarization beam splitter that receives laser light emitted from the optical switch element and emits light having a second polarization in a second direction;
An amplification stage device including at least one amplifier for amplifying laser light emitted from the second polarization beam splitter;
A laser controller that controls the optical switch element so that laser light having a predetermined pulse width is emitted to the amplification stage device;
A driver laser comprising: A driver laser used in an extreme ultraviolet light source device for generating extreme ultraviolet light by irradiating a target with laser light to generate the extreme ultraviolet light.
An oscillation stage laser device for generating pulsed laser light;
A laser discharge tube that excites laser light output from the oscillation stage laser device, and a total reflection mirror and a partial reflection mirror that constitute an optical resonator across the laser discharge tube; An amplification stage device including at least one stage amplifier that amplifies the output laser beam and extends the pulse width of the laser beam;
A driver laser comprising: