US20150351209A1 - System and method for generating extreme ultraviolet light - Google Patents
System and method for generating extreme ultraviolet light Download PDFInfo
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- US20150351209A1 US20150351209A1 US14/807,875 US201514807875A US2015351209A1 US 20150351209 A1 US20150351209 A1 US 20150351209A1 US 201514807875 A US201514807875 A US 201514807875A US 2015351209 A1 US2015351209 A1 US 2015351209A1
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- H01S3/091—Processes or apparatus for excitation, e.g. pumping using optical pumping
- H01S3/094—Processes or apparatus for excitation, e.g. pumping using optical pumping by coherent light
- H01S3/094026—Processes or apparatus for excitation, e.g. pumping using optical pumping by coherent light for synchronously pumping, e.g. for mode locking
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- H01S3/00—Lasers, i.e. devices using stimulated emission of electromagnetic radiation in the infrared, visible or ultraviolet wave range
- H01S3/10—Controlling the intensity, frequency, phase, polarisation or direction of the emitted radiation, e.g. switching, gating, modulating or demodulating
- H01S3/10061—Polarization control
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- H01S3/10—Controlling the intensity, frequency, phase, polarisation or direction of the emitted radiation, e.g. switching, gating, modulating or demodulating
- H01S3/11—Mode locking; Q-switching; Other giant-pulse techniques, e.g. cavity dumping
- H01S3/1106—Mode locking
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- H01S3/00—Lasers, i.e. devices using stimulated emission of electromagnetic radiation in the infrared, visible or ultraviolet wave range
- H01S3/10—Controlling the intensity, frequency, phase, polarisation or direction of the emitted radiation, e.g. switching, gating, modulating or demodulating
- H01S3/13—Stabilisation of laser output parameters, e.g. frequency or amplitude
- H01S3/131—Stabilisation of laser output parameters, e.g. frequency or amplitude by controlling the active medium, e.g. by controlling the processes or apparatus for excitation
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- H01S3/00—Lasers, i.e. devices using stimulated emission of electromagnetic radiation in the infrared, visible or ultraviolet wave range
- H01S3/14—Lasers, i.e. devices using stimulated emission of electromagnetic radiation in the infrared, visible or ultraviolet wave range characterised by the material used as the active medium
- H01S3/16—Solid materials
- H01S3/163—Solid materials characterised by a crystal matrix
- H01S3/1631—Solid materials characterised by a crystal matrix aluminate
- H01S3/1636—Al2O3 (Sapphire)
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- H01S3/00—Lasers, i.e. devices using stimulated emission of electromagnetic radiation in the infrared, visible or ultraviolet wave range
- H01S3/14—Lasers, i.e. devices using stimulated emission of electromagnetic radiation in the infrared, visible or ultraviolet wave range characterised by the material used as the active medium
- H01S3/16—Solid materials
- H01S3/163—Solid materials characterised by a crystal matrix
- H01S3/164—Solid materials characterised by a crystal matrix garnet
- H01S3/1643—YAG
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- H01S3/00—Lasers, i.e. devices using stimulated emission of electromagnetic radiation in the infrared, visible or ultraviolet wave range
- H01S3/14—Lasers, i.e. devices using stimulated emission of electromagnetic radiation in the infrared, visible or ultraviolet wave range characterised by the material used as the active medium
- H01S3/22—Gases
- H01S3/223—Gases the active gas being polyatomic, i.e. containing two or more atoms
- H01S3/2232—Carbon dioxide (CO2) or monoxide [CO]
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- H05G2/001—X-ray radiation generated from plasma
- H05G2/003—X-ray radiation generated from plasma being produced from a liquid or gas
- H05G2/005—X-ray radiation generated from plasma being produced from a liquid or gas containing a metal as principal radiation generating component
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- H01S3/00—Lasers, i.e. devices using stimulated emission of electromagnetic radiation in the infrared, visible or ultraviolet wave range
- H01S3/23—Arrangements of two or more lasers not provided for in groups H01S3/02 - H01S3/22, e.g. tandem arrangements of separate active media
- H01S3/2308—Amplifier arrangements, e.g. MOPA
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- H01S3/00—Lasers, i.e. devices using stimulated emission of electromagnetic radiation in the infrared, visible or ultraviolet wave range
- H01S3/23—Arrangements of two or more lasers not provided for in groups H01S3/02 - H01S3/22, e.g. tandem arrangements of separate active media
- H01S3/2383—Parallel arrangements
Definitions
- This disclosure relates to a system and a method for generating extreme ultraviolet (EUV) light.
- EUV extreme ultraviolet
- microfabrication with feature sizes at 60 nm to 45 nm and further, microfabrication with feature sizes of 32 nm or less will be required.
- an exposure apparatus is needed in which a system for generating EUV light at a wavelength of approximately 13 nm is combined with a reduced projection reflective optical system.
- LPP Laser Produced Plasma
- DPP Discharge Produced Plasma
- SR Synchrotron Radiation
- a system may include a chamber, a laser beam apparatus configured to generate a laser beam to be introduced into the chamber, a laser controller for the laser beam apparatus to control at least a beam intensity and an output timing of the laser beam, and a target supply unit configured to supply a target material into the chamber.
- the target material may be irradiated with the laser beam for generating extreme ultraviolet light.
- a system may include a chamber, a laser beam apparatus configured to output a laser beam into the chamber, a laser controller for the laser beam apparatus to control energy of the laser beam to achieve a predetermined fluence, and a target supply unit configured to supply a target material into the chamber.
- the target material may be irradiated with the laser beam for generating extreme ultraviolet light.
- a method according to yet another aspect of this disclosure for generating extreme ultraviolet light in a system that includes a laser beam apparatus, a laser controller, a chamber, and a target supply unit may include supplying a target material into the chamber in a form of a droplet, irradiating the target material with a pre-pulse laser beam from the laser beam apparatus, and irradiating the target material having been irradiated with the pre-pulse laser beam with a main pulse laser beam from the laser beam apparatus in a range of 0.5 ⁇ s to 3 ⁇ s after the target material is irradiated with the pre-pulse laser beam.
- FIG. 1 schematically illustrates an exemplary configuration of an EUV light generation system according to one embodiment of this disclosure.
- FIG. 2 is a conceptual diagram showing a droplet being irradiated with a pre-pulse laser beam.
- FIGS. 3A through 3C show simulation results of diffusion when a molten tin droplet is irradiated with a pre-pulse laser beam.
- FIG. 3D is a photograph capturing a molten tin droplet being irradiated with a pre-pulse laser beam.
- FIG. 4A schematically shows a molten tin droplet being irradiated with a pre-pulse laser beam, as viewed in the direction perpendicular to the beam axis.
- FIG. 4B schematically shows a molten tin droplet being irradiated with a pre-pulse laser beam, as viewed in the direction of the beam axis.
- FIGS. 5A through 5H show the simulation results of diffusion when a molten tin droplet having a diameter of 60 ⁇ m is irradiated with a pre-pulse laser beam.
- FIG. 5I shows the spot size of a main pulse laser beam.
- FIG. 6 shows a diffusion diameter of a diffused target generated when a molten tin droplet having a diameter of 60 ⁇ m is irradiated with a pre-pulse laser beam and a conversion efficiency (CE) corresponding to a timing at which the diffused target is irradiated with a main pulse laser beam.
- CE conversion efficiency
- FIGS. 7A through 7H show the simulation results of diffusion when a molten tin droplet having a diameter of 10 ⁇ m is irradiated with a pre-pulse laser beam.
- FIG. 7I shows the spot size of a main pulse laser beam.
- FIG. 8 shows a diffusion diameter of a diffused target generated when a molten tin droplet having a diameter of 10 ⁇ m is irradiated with a pre-pulse laser beam and a conversion efficiency (CE) corresponding to a timing at which the diffused target is irradiated with a main pulse laser beam.
- CE conversion efficiency
- FIG. 9 schematically illustrates an exemplary configuration of an EUV light generation system according to a first embodiment.
- FIG. 10 schematically illustrates an exemplary configuration of an EUV light generation system according to a second embodiment.
- FIG. 11 schematically illustrates an exemplary configuration of an EUV light generation system according to a third embodiment.
- FIGS. 12A through 12F show a droplet being irradiated with a first pre-pulse laser beam and a diffused target being irradiated with a second pre-pulse laser beam.
- FIG. 13 schematically illustrates an exemplary configuration of an EUV light generation system according to a modification of the third embodiment.
- FIG. 14 schematically illustrates an exemplary configuration of an EUV light generation system according to a fourth embodiment.
- FIG. 15 schematically illustrates an exemplary configuration of a Ti:sapphire laser configured to output a pre-pulse laser beam in an EUV light generation system according to a fifth embodiment.
- FIG. 16 schematically illustrates an exemplary configuration of a fiber laser configured to output a pre-pulse laser beam in an EUV light generation system according to a sixth embodiment.
- FIG. 17A is a table showing irradiation conditions of a pre-pulse laser beam in the EUV light generation system of any one of the embodiments.
- FIG. 17B is a table showing irradiation conditions of a main pulse laser beam in the EUV light generation system of any one of the embodiments.
- FIG. 18 schematically illustrates an exemplary configuration of an EUV light generation system according to a seventh embodiment.
- FIG. 19A is a conceptual diagram showing a droplet being irradiated with a linearly-polarized pre-pulse laser beam.
- FIG. 19B shows the simulation result of diffusion of the droplet.
- FIG. 20A is a conceptual diagram showing a droplet being irradiated with a linearly-polarized pre-pulse laser beam.
- FIG. 20B shows the simulation result of diffusion of the droplet.
- FIG. 21 is a graph showing absorptivity of a P-polarization component and an S-polarization component of a laser beam by a molten tin droplet.
- FIGS. 22A through 22F show a droplet being irradiated with a circularly-polarized pre-pulse laser beam and a diffused target being irradiated with a main pulse laser beam according to a seventh embodiment.
- FIGS. 23A through 23F show a droplet being irradiated with an unpolarized pre-pulse laser beam and a diffused target being irradiated with a main pulse laser beam according to the seventh embodiment.
- FIGS. 24A through 24F show a droplet being irradiated with a radially-polarized pre-pulse laser beam and a diffused target being irradiated with a main pulse laser beam according to the seventh embodiment.
- FIGS. 25A through 25F show a droplet being irradiated with an azimuthally-polarized pre-pulse laser beam and a diffused target being irradiated with a main pulse laser beam according to the seventh embodiment.
- FIGS. 26A and 26B are diagrams for discussing a method for measuring the degree of linear-polarization.
- FIG. 27 shows a first example of a polarization converter in the seventh embodiment.
- FIGS. 28A through 28C show a second example of a polarization converter in the seventh embodiment.
- FIGS. 29A and 29B show a third example of a polarization converter in the seventh embodiment.
- FIG. 30 shows a fourth example of a polarization converter in the seventh embodiment.
- FIG. 31 schematically illustrates an exemplary configuration of an EUV light generation system according to an eighth embodiment.
- FIGS. 32A through 32C schematically illustrates an exemplary configuration of a laser apparatus configured to output a pre-pulse laser beam in an EUV light generation system according to a ninth embodiment.
- FIGS. 33A through 33C schematically illustrates an exemplary configuration of a laser apparatus configured to output a pre-pulse laser beam in an EUV light generation system according to a modification of the ninth embodiment.
- FIGS. 34A and 34B show an example of a polarization converter in the ninth embodiment.
- FIG. 35 is a graph on which the obtained conversion efficiency (CE) in accordance with a fluence of a pre-pulse laser beam is plotted.
- FIG. 36 is a graph showing the result of an experiment for generating a diffused target in an EUV light generation system.
- FIG. 37 is a graph on which the obtained conversion efficiency (CE) for the corresponding delay time since a droplet is irradiated with a pre-pulse laser beam until a diffused target is irradiated by a main pulse laser beam is plotted for differing diameters of the droplet.
- FIG. 1 schematically illustrates an exemplary configuration of an EUV light generation system according to an embodiment of this disclosure.
- the EUV light generation system of this embodiment may be of an LPP type.
- the EUV light generation system may include a chamber 1 , a target supply unit 2 , a driver laser 3 , an EUV collector mirror 5 , and an EUV light generation controller 7 .
- the chamber 1 may be a vacuum chamber, and the EUV light is generated inside the chamber 1 .
- the chamber 1 may be provided with an exposure apparatus connection port 11 and a window 12 .
- the EUV light generated inside the chamber 1 may be outputted to an external processing apparatus, such as an exposure apparatus (reduced projection reflective optical system), through the exposure apparatus connection port 11 .
- a laser beam outputted from the driver laser 3 may enter the chamber 1 through the window 12 .
- the target supply unit 2 may be configured to supply a target material, such as tin (Sn) and lithium (Li), used to generate the EUV light, into the chamber 1 at a timing specified by a droplet controller 8 .
- the target material inside the target supply unit 2 may be outputted through a target nozzle 13 in the form of droplets DL.
- the droplet DL may, for example, be 10 ⁇ m to 100 ⁇ m (inclusive) in diameter. Of a plurality of droplets DL supplied into the chamber 1 , ones that are not irradiated with a laser beam may be collected into a target collection unit 14 .
- the driver laser 3 is configured to output a laser beam used to excite the target material.
- the driver laser 3 may be a master oscillator power amplifier type laser apparatus.
- the laser beam from the driver laser 3 may be a pulse laser beam with a pulse duration of a few to a few tens of nanoseconds and a repetition rate of 10 kHz to 100 kHz.
- the driver laser 3 may be configured to output a pre-pulse laser beam and a main pulse laser beam.
- a combination of a Yttrium Aluminum Garnet (YAG) laser apparatus for outputting a pre-pulse laser beam and a CO 2 laser apparatus for outputting a main pulse laser beam may be used.
- YAG Yttrium Aluminum Garnet
- CO 2 laser apparatus for outputting a main pulse laser beam
- this embodiment is not limited thereto, and any suitable laser apparatus may be used.
- Each of the pre-pulse laser beam and the main pulse laser beam from the driver laser 3 may be reflected by a laser beam focusing optical system that includes a high-reflection mirror 15 a and an off-axis paraboloidal mirror 15 b , and enter the chamber 1 through the window 12 .
- a laser beam focusing optical system that includes a high-reflection mirror 15 a and an off-axis paraboloidal mirror 15 b , and enter the chamber 1 through the window 12 .
- each of the pre-pulse laser beam and the main pulse laser beam may be focused in a plasma generation region PS.
- the droplet DL When the droplet DL is irradiated with the pre-pulse laser beam, the droplet DL may be diffused into fine particles.
- a target material in a state where fine particles of a droplet DL are diffused may be referred to as a diffused target.
- the diffused target may be irradiated with the main pulse laser beam.
- the target material constituting the diffused target may be excited by the energy of the main pulse laser beam. With this, the target material may be turned into plasma, and rays of light at various wavelengths including the EUV light may be emitted from the plasma.
- the EUV collector mirror 5 may be configured to selectively reflect light at a predetermined wavelength (e.g., EUV light at a central wavelength of approximately 13.5 nm) among rays of light at various wavelengths emitted from the plasma.
- the EUV collector mirror 5 may have a spheroidal concave surface on which a multilayer reflective film formed of a molybdenum (Mo) layer and a silicon (Si) layer laminated alternately is formed.
- Mo molybdenum
- Si silicon
- the EUV light generation controller 7 may be configured to output an oscillation trigger signal and a laser beam intensity setting signal to the driver laser 3 . With this, the EUV light generation controller 7 may control the beam intensity and the generation timing of the pre-pulse laser beam such that a droplet supplied into the chamber 1 is transformed into a desired diffused target. Further, the EUV light generation controller 7 may control the beam intensity and the generation timing of the main pulse laser beam such that plasma in a desired condition may be generated from the diffused target upon being irradiated with the main pulse laser beam.
- the oscillation trigger signal may be outputted based on an oscillation trigger detection signal from an exposure apparatus controller 9 , and the generation timing of the laser beams by the driver laser 3 may be controlled accordingly.
- the laser beam intensity setting signal may be outputted based on the oscillation trigger detection signal from the exposure apparatus controller 9 and an EUV pulse energy detection signal from either an EUV light detector 16 or the exposure apparatus controller 9 .
- the laser beam intensity setting signal may be outputted to the driver laser 3 in order to control the beam intensity of the laser beams.
- the EUV light generation controller 7 may include a trigger counter 7 a and a timer 7 b , and may count the number of oscillation trigger detection signals per unit time.
- the laser beam intensity setting signal may be outputted based on the EUV pulse energy detection signal and the number of counted oscillation trigger detection signals.
- FIG. 2 is a conceptual diagram showing a droplet being irradiated with a pre-pulse laser beam.
- the droplet is viewed in a direction perpendicular to the beam axis (Z-direction) of the pre-pulse laser beam.
- the pre-pulse laser beam when the pre-pulse laser beam is focused on the droplet DL, laser ablation may occur at a surface of the droplet DL that has been irradiated with the pre-pulse laser beam.
- a shock wave may occur from the surface of the droplet DL irradiated with the pre-pulse laser beam toward the interior of the droplet DL due to the energy by the laser ablation.
- This shock wave may propagate throughout the droplet DL.
- the droplet DL may not be broken up when the beam intensity of the pre-pulse laser beam is weak. However, when the beam intensity of the pre-pulse laser beam is equal to or greater than a first predetermined value (e.g., 1 ⁇ 10 9 W/cm 2 ), the droplet DL may be broken up by this shock wave.
- a first predetermined value e.g., 1 ⁇ 10 9 W/cm 2
- FIGS. 3A through 3C show the simulation results of diffusion of a molten tin droplet being irradiated with a pre-pulse laser beam.
- FIG. 3D is a photograph capturing a molten tin droplet being irradiated with a pre-pulse laser beam under the condition that is identical to that in the simulation shown in FIG. 3C .
- the droplet is viewed in a direction perpendicular to the beam axis of the pre-pulse laser beam (Z-direction).
- the spot size of the main pulse laser beam and the beam intensity of the pre-pulse laser beam striking the droplet DL are indicated.
- a diffusion diameter Dd of the diffused target and an irradiation spot size Dm of the main pulse laser beam are indicated.
- the droplet is hardly diffused.
- the beam intensity of the pre-pulse laser beam is 6.4 ⁇ 10 8 W/cm 2
- the droplet is hardly diffused.
- the beam intensity of the pre-pulse laser beam is 1.6 ⁇ 10 9 W/cm 2 (2.5 times greater than the beam intensity in the simulation shown in FIG. 3A )
- the droplet is broken up.
- the broken-up droplet is turned into numerous minute particles and forms a diffused target. These minute particles may be diffused in a disc-shape as viewed in the Z-direction. Further, as shown in FIG.
- the droplet is diffused in the irradiation spot of the main pulse laser beam, whereby a large portion of the main pulse laser beam may be used to generate plasma.
- a diffused target may have a greater total surface area than a single droplet. As shown below, when a single droplet is broken into n 3 smaller pieces, the radius of a smaller piece may become (1/n) of the radius of the original droplet. Here, the total surface area of the smaller pieces may be n times greater than the surface area of the original droplet.
- a volume V 1 of the undiffused droplet may be expressed in Expression (1) below.
- V 1 4 ⁇ r 3 /3 (1)
- a total volume V 2 of n 3 smaller pieces each having a radius (r/n) may be expressed in Expression (2) below.
- V 2 n 3 ⁇ 4 ⁇ ( r/n ) 3 /3 (2)
- a surface area S 1 of the undiffused droplet having the radius r may be expressed in Expression (3) below.
- a total surface area S 2 of n 3 smaller pieces each having the radius (r/n) may be expressed in Expression (4) below.
- the total surface area S 2 of n 3 smaller pieces each having the radius (r/n) is n times greater than the surface area S 1 of the undiffused droplet having the radius r.
- the droplet may be diffused, and the total surface area may be increased.
- the energy of the main pulse laser beam may be absorbed efficiently by the diffused small particles.
- a larger portion of the diffused small particles may be turned into plasma, and EUV light with higher energy may be obtained. Accordingly, a high CE may be obtained.
- the diffused target has such a shape that the length in the direction of the beam axis of the pre-pulse laser beam is shorter than the length in the direction perpendicular to the beam axis of the pre-pulse laser beam.
- the diffused target having such a shape may be irradiated with the main pulse laser beam traveling substantially along the same path as the pre-pulse laser beam. Since the diffused target may be irradiated with the main pulse laser beam more uniformly, the main pulse laser beam may be absorbed efficiently by the target material.
- the diffusion diameter Dd of the diffused target may be equal to or smaller than the irradiation spot size Dm of the main pulse laser beam. Because of this size, the entire diffused target may be irradiated with the main pulse laser beam, and thus a larger portion of the diffused target may be turned into plasma. As a result, generation of debris of the target material may be suppressed.
- the diffusion diameter Dd of the diffused target may be equal to or closer to the irradiation spot size Dm of the main pulse laser beam. With this, a larger portion of the energy of the main pulse laser beam may be absorbed by the diffused target, whereby a higher CE may be obtained.
- FIG. 3B shows that the position of the beam waist of the main pulse laser beam substantially coincides with the position of the diffused target, this disclosure is not limited thereto. That is, the position of the beam waist of the main pulse laser beam and the position of the diffused target do not necessarily have to coincide with each other.
- the irradiation spot size Dm may be interpreted as a diameter of a cross-section of the main pulse laser beam at or around the position at which the diffused target is irradiated with the main pulse laser beam.
- a cross-section area of the main pulse laser beam may be larger than a maximum cross-section area of the diffused target.
- FIGS. 4A and 4B schematically show a molten tin droplet having been irradiated with the pre-pulse laser beam.
- the diffused target is viewed in a direction perpendicular to the beam axes of the pre-pulse laser beam and the main pulse laser beam (Z-direction).
- the diffused target is viewed in a direction of the beam axes of the pre-pulse laser beam and the main pulse laser beam.
- an outer diameter Dout of a torus-shaped diffused target and the irradiation spot size Dm of the main pulse laser beam are indicated.
- the droplet DL when the pre-pulse laser beam is focused on the droplet DL, laser ablation may occur at the surface of the droplet DL.
- a second predetermined value e.g., 6.4 ⁇ 10 9 W/cm 2
- the droplet DL may be broken up, and a torus-shaped diffused target as shown in FIGS. 4A and 4B may be formed.
- the torus-shaped diffused target may be diffused symmetrically about the beam axis of the pre-pulse laser beam and into a torus-shape.
- the beam intensity of the pre-pulse laser beam may be in the range of 6.4 ⁇ 10 9 W/cm 2 to 3.2 ⁇ 10 10 W/cm 2 (inclusive), and the diameter of the droplet may be in the range of 12 ⁇ m and 40 ⁇ m (inclusive).
- a torus-shaped diffused target may be formed in 0.5 ⁇ s to 2.0 ⁇ s after a droplet is irradiated with a pre-pulse laser beam. Accordingly, the diffused target may preferably be irradiated with the main pulse laser beam in the above time span after the droplet is irradiated with the pre-pulse laser beam.
- the torus-shaped diffused target has such a shape that the length in the direction of the beam axis of the pre-pulse laser beam is shorter than the length in the direction perpendicular to the beam axis of the pre-pulse laser beam.
- the diffused target having such a shape may be irradiated with the main pulse laser beam traveling substantially along the same path as the pre-pulse laser beam. With this, the diffused target may be irradiated with the main pulse laser beam more efficiently, and thus the main pulse laser beam may be absorbed efficiently by the target material. Accordingly, the CE in the LPP type EUV light generation system may be improved. The CE of approximately 3% was obtained through an experiment under the above conditions.
- torus-shape means an annular shape, but the diffused target does not necessarily have to be perfectly annular in shape, and may be substantially annular in shape.
- the irradiation spot size Dm of the main pulse laser beam may preferably be in the following relationship with the outer diameter Dout of the torus-shaped diffused target.
- the entire torus-shaped diffused target may be irradiated with the main pulse laser beam, and a larger portion of the diffused target may be turned into plasma. As a result, generation of debris of the target material may be reduced.
- FIGS. 5A through 5H show the simulation result of diffusion when a molten tin droplet having a diameter of 60 ⁇ m is irradiated with a pre-pulse laser beam.
- the droplet or the diffused target is viewed in a direction (X-direction) perpendicular to the beam axis of the pre-pulse laser beam (Z-direction).
- FIGS. 5A through 5D respectively show the states of the target material at timings where a time T is 0 ⁇ s, 0.4 ⁇ s, 0.8 ⁇ s, and 1.4 ⁇ s after the droplet DL is irradiated with the pre-pulse laser beam.
- a time T is 0 ⁇ s, 0.4 ⁇ s, 0.8 ⁇ s, and 1.4 ⁇ s after the droplet DL is irradiated with the pre-pulse laser beam.
- FIGS. 5E through 5H respectively show the states of the target material at timings where a time T is 0 ⁇ s, 0.4 ⁇ s, 0.8 ⁇ s, and 1.4 ⁇ s after the droplet DL is irradiated with the pre-pulse laser beam.
- FIG. 5I shows the irradiation spot size of the main pulse laser beam at a position where the diffused target is irradiated with the main pulse laser beam.
- the beam intensity of the pre-pulse laser beam is 1.5 ⁇ 10 9 W/cm 2 .
- a large portion of the diffused target may be irradiated with the main pulse laser beam in approximately 0.4 ⁇ s after a droplet is irradiated with the pre-pulse laser beam. Accordingly, generation of debris may be reduced if the diffused target is irradiated with the main pulse laser beam at the above timing.
- a droplet having a diameter of 60 ⁇ m may be broken into small particles and diffused upon being irradiated with a pre-pulse laser beam.
- the maximum value and the minimum value of a diameter of a small particle in the diffused target are indicated.
- the maximum value of a diameter of a small particle in the diffused target is 48.0 ⁇ m. That is, the droplet has not been broken up sufficiently by the pre-pulse laser beam, and a large portion of the diffused target may not be turned into plasma even when the diffused target is irradiated with the main pulse laser beam. This may suggest that a large amount of debris may be generated.
- the minimum value of a diameter of a small particle in the diffused target is 3.7 ⁇ m in 0.4 ⁇ s, 3.5 ⁇ m in 0.8 ⁇ s, and 3.1 ⁇ m in 1.4 ⁇ s, respectively, after a droplet is irradiated with a pre-pulse laser beam. This suggests that the more the time T elapses after a droplet is irradiated with a pre-pulse laser beam, the smaller the diameter of a small particle becomes, and the number of small particles may increase.
- FIG. 6 shows a change over time in the diffusion diameter Dd of the diffused target when a molten tin droplet having a diameter of 60 ⁇ m is irradiated with the pre-pulse laser beam and a conversion efficiency when the diffused target is irradiated with a main pulse laser beam at a given point in time.
- the diffusion diameter Dd of the diffused target may substantially coincide with the irradiation spot size of the main pulse laser beam in approximately 0.4 ⁇ s after the droplet is irradiated with the pre-pulse laser beam.
- a high CE may be obtained if the diffused target is irradiated with the main pulse laser beam in approximately 3 ⁇ s after the droplet is irradiated with the pre-pulse laser beam (see a white arrow B in FIG. 6 ).
- a preferable delay time for the main pulse laser beam from the irradiation with the pre-pulse laser beam to reduce generation of debris may differ from a preferable delay time to obtain a high CE. That is, when a molten tin droplet having a diameter of 60 ⁇ m is irradiated sequentially with a pre-pulse laser beam and then a main pulse laser beam, it may be difficult to reduce debris and obtain a high CE at the same time.
- FIGS. 7A through 7H show the simulation results of diffusion when a molten tin droplet having a diameter of 10 ⁇ m is irradiated with the pre-pulse laser beam.
- the droplet or the diffused target is viewed in a direction (X-direction) perpendicular to the beam axis of the pre-pulse laser beam (Z-direction).
- FIGS. 7A through 7D respectively show the states of the target material at timings where a time T is 0 ⁇ s, 0.1 ⁇ s, 0.25 ⁇ s, and 0.5 ⁇ s after the droplet is irradiated with the pre-pulse laser beam.
- a time T is 0 ⁇ s, 0.1 ⁇ s, 0.25 ⁇ s, and 0.5 ⁇ s after the droplet is irradiated with the pre-pulse laser beam.
- FIGS. 7E through 7H respectively show the states of the target material at timings where a time T is 0 ⁇ s, 0.1 ⁇ s, 0.25 ⁇ s, and 0.5 ⁇ s after the droplet is irradiated with the pre-pulse laser beam.
- FIG. 7I shows the irradiation spot size of the main pulse laser beam at a position where the diffused target is irradiated with the main pulse laser beam.
- the beam intensity of the pre-pulse laser beam is 1.5 ⁇ 10 9 W/cm 2 .
- the maximum value of a diameter of a small particle in a diffused target is 2.2 ⁇ m in 0.1 ⁇ s, 1.1 ⁇ m in 0.25 ⁇ s, and 1.1 ⁇ s in 0.5 ⁇ s after the droplet is irradiated with the pre-pulse laser beam. This suggests that the maximum value of a diameter of a small particle in a diffused target becomes constant in 0.25 ⁇ s after the droplet is irradiated with the pre-pulse laser beam.
- the minimum value of a diameter of a small particle in the diffused target is 0.2 ⁇ m in 0.1 ⁇ s, 0.2 ⁇ m in 0.25 ⁇ s, and 0.2 ⁇ m in 0.5 ⁇ s after the droplet is irradiated with the pre-pulse laser beam.
- a small particle in a diffused target is sufficiently small in 0.1 ⁇ s after the droplet is irradiated with the pre-pulse laser beam.
- a higher CE may be obtained if the diffused target is irradiated with the main pulse laser beam in 0.1 ⁇ s after the droplet is irradiated with the pre-pulse laser beam.
- FIG. 8 shows a change over time in the diffusion diameter Dd of the diffused target when a molten tin droplet having a diameter of 10 ⁇ m is irradiated with the pre-pulse laser beam and a conversion efficiency when the diffused target is irradiated with the main pulse laser beam at a given point in time.
- the diffusion diameter Dd of the diffused target may substantially coincide with the irradiation spot size of the main pulse laser beam in 0.1 ⁇ s after the droplet is irradiated with the pre-pulse laser beam. Accordingly, generation of debris may be reduced if the diffused target is irradiated with the main pulse laser beam in 0.1 ⁇ s after the droplet is irradiated with the pre-pulse laser beam (see a white arrow A in FIG. 8 ).
- FIG. 8 With reference to FIG.
- a high CE may be obtained if the diffused target is irradiated with the main pulse laser beam in approximately 0.15 ⁇ s after the droplet is irradiated with the pre-pulse laser beam (see a white arrow B in FIG. 8 ).
- the simulation results suggest that a gap between a preferable delay time for the main pulse laser beam to reduce debris and a preferable delay time for the main pulse laser beam to obtain a high CE is relatively small. That is, when a molten tin droplet having a diameter of 10 ⁇ m is irradiated sequentially with the pre-pulse laser beam and then the main pulse laser beam, it may be possible to reduce debris and obtain a high CE at the same time.
- a molten tin droplet having a diameter of 10 ⁇ m may be referred to as a mass-limited target since it is a target with a minimum mass required for generating desired EUV light.
- FIG. 9 schematically illustrates an exemplary configuration of an EUV light generation system according to a first embodiment.
- a beam path of a pre-pulse laser beam from a YAG pulse laser apparatus 3 a and a beam path of a main pulse laser beam from a CO 2 pulse laser apparatus 3 b may be made to substantially coincide with each other by a beam combiner 15 c . That is, in the first embodiment, the pre-pulse laser beam and the main pulse laser beam are guided into the chamber 1 along substantially the same path.
- an EUV light emission signal may be inputted to the EUV light generation controller 7 from the exposure apparatus controller 9 .
- the EUV light generation controller 7 may be configured to output a YAG laser beam intensity setting signal to the YAG pulse laser apparatus 3 a . Further, the EUV light generation controller 7 may be configured to output a CO 2 laser beam intensity setting signal to the CO 2 pulse laser apparatus 3 b.
- the EUV light generation controller 7 may be configured to output an EUV light emission trigger signal to a trigger controller 17 .
- the trigger controller 17 may be configured to output a droplet output signal to a droplet controller 8 .
- the droplet controller 8 may input the droplet output signal to the target supply unit 2 , and upon receiving the droplet output signal, the target supply unit 2 may output a droplet DL through the target nozzle 13 .
- the trigger controller 17 may be configured to output a YAG laser oscillation trigger signal to the YAG pulse laser apparatus 3 a .
- the YAG laser oscillation trigger signal may be outputted such that the droplet DL is irradiated with the pre-pulse laser beam at a timing at which the droplet DL reaches the plasma generation region PS.
- the trigger controller 17 may be configured to output a CO 2 laser oscillation trigger signal to a master oscillator 3 d in the CO 2 pulse laser apparatus 3 b .
- the CO 2 laser oscillation trigger signal may be outputted such that the diffused target is irradiated with the main pulse laser beam after a delay time T from the timing at which the droplet DL is irradiated with the pre-pulse laser beam.
- the delay time T is a time required for a desired diffused target to be formed.
- the YAG pulse laser apparatus 3 a may be configured to output the pre-pulse laser beam at a first wavelength based on the YAG laser beam intensity setting signal from the EUV light generation controller 7 and the YAG laser oscillation trigger signal from the trigger controller 17 .
- the pre-pulse laser beam from the YAG pulse laser apparatus 3 a may be expanded in diameter by a beam expander 4 and then be incident on the beam combiner 15 c.
- the CO 2 pulse laser apparatus 3 b may include the master oscillator 3 d , a preamplifier 3 h , a main amplifier 3 j , and relay optical systems 3 g , 3 i , and 3 k respectively disposed downstream from the master oscillator 3 d , the preamplifier 3 h , and the main amplifier 3 j .
- the master oscillator 3 d may be configured to output a seed beam at a second wavelength based on the CO 2 pulse laser oscillation trigger signal.
- the seed beam from the master oscillator 3 d may be amplified to desired beam intensity by the preamplifier 3 h and the main amplifier 3 j based on the CO 2 laser beam intensity setting signal.
- the amplified laser beam may be outputted from the CO 2 pulse laser apparatus 3 b as the main pulse laser beam and be incident on the beam combiner 15 c.
- the beam combiner 15 c may be configured to transmit the pre-pulse laser beam at the first wavelength (e.g., 1.06 ⁇ m) and reflect the main pulse laser beam at the second wavelength (e.g., 10.6 ⁇ m). More specifically, the beam combiner 15 c may include a diamond substrate on which a multilayer film having the aforementioned reflection/transmission properties for the pre-pulse laser and the main pulse laser is formed. Accordingly, the beam combiner 15 c may serve to make the beam path of the pre-pulse laser beam and the beam path of the main pulse laser beam coincide with each other and supply the pre-pulse laser beam and the main pulse laser beam into the chamber 1 along the same path. Alternatively, a beam combiner configured to reflect the pre-pulse laser beam at the first wavelength and transmit the main pulse laser beam at the second wavelength may be used to make the respective beam paths coincide with each other.
- a beam combiner configured to reflect the pre-pulse laser beam at the first wavelength and transmit the main pulse laser beam at the second wavelength
- the droplet controller 8 , the YAG pulse laser apparatus 3 a , and the CO 2 pulse laser apparatus 3 b may operate in synchronization with one another based on the various signals from the trigger controller 17 .
- the YAG pulse laser apparatus 3 a may output the pre-pulse laser beam in synchronization with the timing at which the droplet supplied into the chamber 1 from the target supply unit 2 reaches a predetermined region.
- the CO 2 pulse laser apparatus 3 b may output the main pulse laser beam in synchronization with the timing at which a desired diffused target is formed after the droplet is irradiated with the pre-pulse laser beam.
- the pre-pulse laser beam and the main pulse laser beam may be guided to the plasma generation region PS in substantially the same direction (substantially the same path).
- a through-hole formed in the EUV collector mirror 5 may be made small and need not be formed in plurality.
- the wavelength (e.g., 1.06 ⁇ m) of the pre-pulse laser beam from the YAG pulse laser apparatus 3 a is equal to or shorter than one-tenth of the wavelength (e.g., 10.6 ⁇ m) of the main pulse laser beam from the CO 2 pulse laser apparatus 3 b .
- the wavelength of the pre-pulse laser beam is sufficiently shorter than the wavelength of the main pulse laser beam, the following advantages may be speculated.
- the absorptivity of the pre-pulse laser beam by the target material, such as tin, may be higher than that of the main pulse laser beam.
- the irradiation spot size of the pre-pulse laser beam focused on the droplet may be reduced. As a result, a small droplet DL may be irradiated efficiently with the pre-pulse laser beam having small pulse energy and be diffused.
- FIG. 10 schematically illustrates an exemplary configuration of an EUV light generation system according to a second embodiment.
- the pre-pulse laser beam from the YAG pulse laser apparatus 3 a and the main pulse laser beam from the CO 2 pulse laser apparatus 3 b are guided into the chamber 1 along separate beam paths.
- the pre-pulse laser beam outputted from the YAG pulse laser apparatus 3 a may be reflected by a high-reflection mirror 15 e and an off-axis paraboloidal mirror 15 g . Then, the pre-pulse laser beam may pass through a through-hole formed in the EUV collector mirror 5 , and be focused on a droplet inside the chamber 1 to form a diffused target.
- the main pulse laser beam outputted from the CO 2 pulse laser apparatus 3 b may be reflected by a high-reflection mirror 15 d and an off-axis paraboloidal mirror 15 f . Then, the main pulse laser beam may pass through another through-hole formed in the EUV collector mirror 5 , and be focused on the diffused target inside the chamber 1 .
- the pre-pulse laser beam and the main pulse laser beam may be guided through separate optical systems to the plasma generation region PS. Accordingly, each of the pre-pulse laser beam and the main pulse laser beam may be focused to have a desired beam spot with ease. Further, an optical element, such as a beam combiner, for making the beam paths of the pre-pulse laser beam and the main pulse laser beam need not be used. Still, the pre-pulse laser beam and the main pulse laser beam may strike the droplet DL and the diffused target respectively in substantially the same direction.
- FIG. 11 schematically illustrates an exemplary configuration of an EUV light generation system according to a third embodiment.
- a first pre-pulse laser beam from the YAG pulse laser apparatus 3 a and a second pre-pulse laser beam and the main pulse laser beam from the CO 2 pulse laser apparatus 3 b may be guided into the chamber 1 .
- the CO 2 pulse laser apparatus 3 b may include the master oscillator 3 d configured to output the seed beam of the main pulse laser beam and a master oscillator 3 e configured to output a seed beam of the second pre-pulse laser beam.
- the seed beam of the second pre-pulse laser beam from the master oscillator 3 e may be amplified by the preamplifier 3 h and the main amplifier 3 j to desired beam intensity.
- the amplified seed beam may be outputted from the CO 2 pulse laser apparatus 3 b as the second pre-pulse laser beam, and then be incident on the beam combiner 15 c .
- the seed beam of the main pulse laser beam from the master oscillator 3 d may also be amplified by the preamplifier 3 h and the main amplifier 3 j to desired beam intensity.
- the amplified seed beam may be outputted from the CO 2 pulse laser apparatus 3 b as the main pulse laser beam, and then be incident on the beam combiner 15 c.
- Each of the master oscillators 3 d and 3 e may be a semiconductor laser configured to oscillate in a bandwidth that can be amplified by a CO 2 gain medium. More specifically, each of the master oscillators 3 d and 3 e may include a plurality of quantum cascade lasers (QCL).
- QCL quantum cascade lasers
- FIGS. 12A through 12F show a droplet DL being irradiated with a first pre-pulse laser beam and a diffused target being irradiated with a second pre-pulse laser beam in the third embodiment.
- the droplet or the diffused target is viewed in a direction (X-direction) perpendicular to the beam axes of the first and second pre-pulse laser beams (Z-direction).
- FIGS. 12D through 12F the droplet or the diffused target is viewed in the direction of the beam axes of the first and second pre-pulse laser beams (Z-direction).
- the droplet When a droplet of the target material shown in FIGS. 12A and 12D is irradiated with the first pre-pulse laser beam, the droplet may be diffused as shown in FIGS. 12B and 12E so that a first diffused target may be formed.
- the first diffused target may be irradiated with the second pre-pulse laser beam when the first diffused target is diffused to a desired size that is substantially the same as or smaller than the irradiation spot size of the second pre-pulse laser beam.
- the first diffused target When the first diffused target is irradiated with the second pre-pulse laser beam, the first diffused target may be broken into even smaller particles and be diffused to form a second diffused target.
- the second diffused target may be irradiated with the main pulse laser beam when the second diffused target is diffused to a desired size that is substantially the same as or smaller than the irradiation spot size of the main pulse laser beam.
- the second diffused target which includes smaller particles than those in the first diffused target, is irradiated with the main pulse laser beam, the energy of the main pulse laser beam may be absorbed by the second diffused target efficiently. Because a large portion of the second diffused target may be turned into plasma, a high CE may be obtained. Further, by controlling the irradiation spot size of the main pulse laser beam to substantially coincide with the diffusion diameter of the second diffused target, a high CE and debris reduction may both be achieved.
- a mass limited target e.g., a molten tin droplet having a diameter of 10 ⁇ m
- a mass limited target e.g., a molten tin droplet having a diameter of 10 ⁇ m
- the target material is irradiated with the first and second pre-pulse laser beams, and then the diffused target is irradiated with the main pulse laser beam.
- this disclosure is not limited thereto, and the target material may be irradiated with three or more pre-pulse laser beams.
- the first pre-pulse laser beam is outputted from the YAG pulse laser apparatus 3 a
- the second pre-pulse laser beam and the main pulse laser beam are outputted from the CO 2 pulse laser apparatus 3 b .
- this disclosure is not limited thereto, and all the laser beams may be outputted, for example, from a CO 2 laser apparatus.
- the first and second pre-pulse laser beams may be outputted from a first laser apparatus, and the main pulse laser beam may be outputted from a second laser apparatus.
- the first laser apparatus may be a YAG laser apparatus or a fiber laser apparatus
- the second laser apparatus may be a CO 2 laser apparatus.
- FIG. 13 schematically illustrates an exemplary configuration of an EUV light generation system according to a modification of the third embodiment.
- the EUV light generation system shown in FIG. 13 may include a first YAG pulse laser apparatus 3 m , a second YAG pulse laser apparatus 3 n , and a beam combiner 3 p.
- the first and second YAG pulse laser apparatuses 3 m and 3 n may each receive the YAG laser beam intensity setting signal from the EUV light generation controller 7 and the YAG laser oscillation trigger signal from the trigger controller 17 .
- the first YAG pulse laser apparatus 3 m may be configured to output the first pre-pulse laser beam, and the first pre-pulse laser beam may be incident on the beam combiner 3 p .
- the second YAG pulse laser apparatus 3 n may be configured to output the second pre-pulse laser beam, and the second pre-pulse laser beam may also be incident on the beam combiner 3 p .
- the beam combiner 3 p may be positioned to make the beam paths of the first and second pre-pulse laser beams coincide with each other and output the first and second pre-pulse laser beams toward the beam expander 4 .
- the first and second pre-pulse laser beams and the main pulse laser beam may be guided into the chamber 1 .
- the first and second pre-pulse laser beams may respectively be outputted from first and second fiber laser apparatuses.
- FIG. 14 schematically illustrates an exemplary configuration of an EUV light generation system according to a fourth embodiment.
- FIG. 14 shows a sectional view taken along XIV-XIV plane in any of FIGS. 9 through 11 and 13 .
- An EUV light generation system according to the fourth embodiment may be similar in configuration to any one of the first through third embodiments but may differ in that the EUV light generation system of the fourth embodiment may further include magnets 6 a and 6 b .
- a magnetic field may be generated with the magnets 6 a and 6 b inside the chamber 1 and ions generated inside the chamber 1 may be collected by the magnetic field.
- Each of the magnets 6 a and 6 b may be an electromagnet that includes a coil winding and a cooling mechanism of the coil winding.
- a power source 6 c that is controlled by a power source controller 6 d may be connected to each of the magnets 6 a and 6 b .
- the power source controller 6 d may regulate current to be supplied to the magnets 6 a and 6 b from the power source 6 c so that a magnetic field in a predetermined direction may be generated in the chamber 1 .
- a superconductive magnet for example, may be used as each of the magnets 6 a and 6 b . Although two magnets 6 a and 6 b are used in this embodiment, a single magnet may be used. Alternatively, a permanent magnet may be provided in the chamber 1 .
- Plasma generated when a target material is irradiated with a main pulse laser beam may include positive ions and negative ions (or electrons).
- the positive and negative ions moving inside the chamber 1 may be subjected to Lorentz force in the magnetic field, and thus the ions may move in spiral along magnetic lines of force.
- the ionized target material may be trapped in the magnetic field and collected into ion collection units 19 a and 19 b provided in the magnetic field. Accordingly, debris inside the chamber 1 may be reduced, and deterioration in optical element, such as the EUV collector mirror 5 , due to the debris adhering to the optical element may be suppressed.
- the magnetic field is in the direction shown by an arrow, but a similar function can be achieved even when the magnetic field is oriented in the opposite direction.
- a mitigation technique for reducing debris adhering to the optical element is not limited to the use of the magnetic field.
- a substance deposited onto the EUV collector mirror 5 may be etched using an etching gas. Debris may be made to react with hydrogen gas (H 2 ) or a hydrogen radical (H) in the magnetic field, and the debris may be removed as a vaporized compound.
- FIG. 15 schematically illustrates an exemplary configuration of a Ti:sapphire laser configured to output the pre-pulse laser beam in an EUV light generation system according to a fifth embodiment.
- a Ti:sapphire laser 50 a of the fifth embodiment may be provided outside the chamber 1 as a driver laser for outputting the pre-pulse laser beam in any one of the first through fourth embodiments.
- the Ti:sapphire laser 50 a may include a laser resonator formed by a semiconductor saturable absorber mirror 51 a and an output coupler 52 a .
- a concave mirror 53 a , a first pumping mirror 54 a , a Ti:sapphire crystal 55 a , a second pumping mirror 56 a , and two prisms 57 a and 58 a are provided in this order from the side of the semiconductor saturable absorber mirror 51 a in the optical path in the laser resonator.
- the Ti:sapphire laser 50 a may include a pumping source 59 a for introducing a pumping beam into the laser resonator.
- the first pumping mirror 54 a may be configured to transmit the pumping beam from the outside of the laser resonator with high transmittance and reflect the laser beam inside the laser resonator with high reflectance.
- the Ti:sapphire crystal 55 a may serve as a laser medium that undergoes stimulated emission with the pumping beam.
- the two prisms 57 a and 58 a may selectively transmit a laser beam at a predetermined wavelength.
- the output coupler 52 a may transmit a part of the laser beam amplified in the laser resonator and output the amplified laser beam from the laser resonator, and reflect the remaining part of the laser beam back into the laser resonator.
- the semiconductor saturable absorber mirror 51 a may have a reflective layer and a saturable absorber layer laminated thereon. A part of an incident laser beam of low beam intensity may be absorbed by the saturable absorber layer, and another part of the incident laser beam of high beam intensity may be transmitted through the saturable absorber layer and reflected by the reflective layer. With this, the pulse duration of the incident laser beam may be shortened.
- a semiconductor pumped Nd:YVO 4 laser may, for example, be used as the pumping source 59 a .
- the second harmonic wave from the pumping source 59 a may be introduced into the laser resonator through the first pumping mirror 54 a .
- the position of the semiconductor saturable absorber mirror 51 a may be adjusted so as to adjust the resonator length for a predetermined longitudinal mode.
- a picosecond pulse laser beam may be outputted through the output coupler 52 a .
- the pulse laser beam may be amplified by a regenerative amplifier.
- a target material may be irradiated with a picosecond pulse laser beam or a pulse laser beam having a shorter pulse duration.
- thermal diffusion at the irradiation portion may be made extremely small. Accordingly, energy that may be diffused can be used for the ablation effect.
- a droplet may be diffused with smaller pulse energy.
- FIG. 16 schematically illustrates an exemplary configuration of a fiber laser configured to output the pre-pulse laser beam in an EUV light generation system according to a sixth embodiment.
- a fiber laser 50 b of the sixth embodiment may be provided outside the chamber 1 as a driver laser for outputting the pre-pulse laser beam in any one of the first through fourth embodiments.
- the fiber laser 50 b may include a laser resonator formed by a high-reflection mirror 51 b and a semiconductor saturable absorber mirror 52 b .
- a grating pair 53 b , a first polarization maintenance fiber 54 b , a multiplexer 55 b , a separation element 56 b , a second polarization maintenance fiber 57 b , and a focusing optical system 58 b may be provided in this order from the side of the high-reflection mirror 51 b in the beam path in the laser resonator.
- the fiber laser 50 b may include a pumping source 59 b for introducing a pumping beam into the laser resonator.
- the multiplexer 55 b may be configured to introduce the pumping beam from the pumping source 59 b to the first polarization maintenance fiber 54 b and may transmit a laser beam traveling back and forth between the first polarization maintenance fiber 54 b and the second polarization maintenance fiber 57 b .
- the first polarization maintenance fiber 54 b may be doped with ytterbium (Yb), and may undergo stimulated emission with the pumping beam.
- the grating pair 53 b may selectively reflect a laser beam at a predetermined wavelength.
- the semiconductor saturable absorber mirror 52 b may be similar in configuration and function to the semiconductor saturable absorber mirror 51 b in the fifth embodiment.
- the separation element 56 b may separate a part of the laser beam amplified in the laser resonator and output the separated laser beam from the laser resonator and return the remaining part of the laser beam back into the laser resonator. This configuration may lead to mode-locking of the fiber laser 50 b .
- a picosecond pulse laser beam may be outputted through the separation element 56 b.
- the target material may be irradiated with the pre-pulse laser beam with high precision since the pre-pulse laser beam is introduced through an optical fiber.
- the M 2 value that expresses deviation from an ideal Gaussian distribution of the laser beam intensity distribution is approximately 1.2.
- the M 2 value being closer to 1 means that the focusing performance is high. Accordingly, when a fiber laser is used, a small target may be irradiated with a pre-pulse laser beam with high precision.
- the absorptivity may increase with the second harmonic wave (a wavelength of 532 nm), further with the third harmonic wave (a wavelength of 355 nm), and even further with the fourth harmonic wave (a wavelength of 266 nm).
- a picosecond pulse laser beam is used.
- similar effects can be obtained even with a femtosecond pulse laser beam.
- a droplet can be diffused even with a nanosecond pulse laser beam.
- a fiber laser with such specifications as a pulse duration of approximately 15 ns, a repetition rate of 100 kHz, pulse energy of 1.5 mJ, a wavelength of 1.03 ⁇ m, and the M 2 value of below 1.5 may be used as a pre-pulse laser apparatus.
- FIGS. 17A and 17B are tables showing irradiation conditions of the laser beams in the EUV light generation system in any one of the embodiments.
- irradiation pulse energy is E (J)
- a pulse duration is T (s)
- an irradiation spot size is Dm (m)
- beam intensity W (W/m 2 ) of the laser beam may be expressed in Expression 5 below.
- FIG. 17A shows four examples (case 1 through case 4 ) of irradiation conditions of the pre-pulse laser beam.
- the diameter of a molten tin droplet is 60 ⁇ m.
- the irradiation conditions for diffusing such a droplet and generating a desired diffused target may be as follows. For example, when the irradiation spot size Dm is 100 ⁇ m, the beam intensity W of the laser beam at 1.6 ⁇ 10 9 W/cm 2 is required. In that case, the irradiation pulse energy E may be set to 1.9 mJ, and the pulse duration T may be set to 15 ns. With such a pre-pulse laser beam, a diffused target as shown in FIG. 3B may be generated.
- the diameter of a molten tin droplet is 10 ⁇ m (i.e., a mass-limited target).
- the irradiation conditions for diffusing such a droplet and generating a desired diffused target may be as follows. For example, when the irradiation spot size Dm is 30 ⁇ m, the beam intensity W of the laser beam at 1.6 ⁇ 10 9 W/cm 2 is required. In that case, the irradiation pulse energy E may be set to 0.17 mJ, and the pulse duration T may be set to 15 ns. With such a pre-pulse laser beam, a diffused target as shown in FIG. 7B may be generated.
- the laser apparatus as shown in FIG. 15 or 16 is used for outputting the pre-pulse laser beam. Further, in the cases 3 and 4 , the droplet is a mass-limited target, and the beam intensity W of the laser beam at 1 ⁇ 10 10 W/cm 2 is required.
- FIG. 17B shows four examples (case 1 through case 4 ) of irradiation conditions of the main pulse laser beam.
- the diffusion diameter of a diffused target is 250 ⁇ m.
- Irradiation conditions for turning such a diffused target into plasma may be as follows. For example, when the irradiation spot size Dm is 250 ⁇ m, the beam intensity W of the laser beam at 1.0 ⁇ 10 10 W/cm 2 is required. In that case, the irradiation pulse energy E may be set to 100 mJ, and the pulse duration T may be set to 20 ns. Accordingly, energy required to turn the diffused target into plasma may be supplied to the diffused target.
- the diffusion diameter of the diffused target, the irradiation spot size Dm, and the beam intensity W of the laser beam are the same as in the case 1 shown in FIG. 17B .
- the irradiation pulse energy E may be set to 150 mJ
- the pulse duration T may be set to 30 ns. With this, energy required to turn the diffused target into plasma may be supplied to the diffused target.
- the diffusion diameter of a diffused target is 300 ⁇ m.
- Irradiation conditions for turning such a diffused target into plasma may be as follows. For example, when the irradiation spot size Dm is 300 ⁇ m, the beam intensity W of the laser beam at 1.1 ⁇ 10 10 W/cm 2 is required. In that case, the irradiation pulse energy E may be set to 200 mJ, and the pulse duration T may be set to 25 ns. Thus, energy required to turn the diffused target into plasma may be supplied to the diffused target.
- the diffusion diameter of a diffused target is 200 ⁇ m.
- Irradiation conditions for turning such a diffused target into plasma may be as follows. For example, when the irradiation spot size Dm is 200 ⁇ m, the beam intensity W of the laser beam at 1.2 ⁇ 10 10 W/cm 2 is required. In that case, the irradiation pulse energy E may be set to 200 mJ, and the pulse duration T may be set to 50 ns. With this, energy required to turn the diffused target into plasma may be supplied to the diffused target.
- the beam intensity of the pre-pulse laser beam and the main pulse laser beam may be set by setting the irradiation pulse energy E and the pulse duration T of the laser beam.
- FIG. 18 schematically illustrates an exemplary configuration of an EUV light generation system according to a seventh embodiment.
- the polarization state of the pre-pulse laser beam from a fiber laser apparatus 31 may be controlled by a polarization converter 20 .
- the polarization converter 20 may be configured to change the polarization state of the pre-pulse laser beam into a state other than the linear polarization.
- the polarization converter 20 may be provided at a predetermined position in a beam path between the driver laser and the plasma generation region PS.
- a polarization retarder is also included in the polarization converter.
- the fiber laser apparatus 31 may include a fiber laser controller 31 a and the fiber laser 50 b described with reference to FIG. 16 (the sixth embodiment).
- a CO 2 pulse laser apparatus 32 may include a CO 2 laser controller 32 a , the master oscillator 3 d , the preamplifier 3 h , the main amplifier 3 j , and the relay optical systems 3 g , 3 i , and 3 k as described with reference to FIG. 9 (the first embodiment).
- the EUV light generation controller 7 may output a fiber laser beam intensity setting signal to the fiber laser controller 31 a . Further, the EUV light generation controller 7 may output a CO 2 laser beam intensity setting signal to the CO 2 laser controller 32 a.
- the trigger controller 17 may output a fiber laser oscillation trigger signal to the fiber laser 50 b . Further, the trigger controller 17 may output a CO 2 laser oscillation trigger signal to the master oscillator 3 d.
- the fiber laser 50 b may be configured to output a pre-pulse laser beam at a first wavelength based on the fiber laser oscillation trigger signal.
- the fiber laser controller 31 a may be configured to control the output intensity of the fiber laser 50 b based on the fiber laser beam intensity setting signal.
- the pre-pulse laser beam from the fiber laser 50 b may be expanded in diameter by the beam expander 4 . Thereafter, the polarization state of the pre-pulse laser beam may be changed by the polarization converter 20 , and then the pre-pulse laser beam may be incident on the beam combiner 15 c.
- the master oscillator 3 d may be configured to output a seed beam at a second wavelength based on the CO 2 laser oscillation trigger signal.
- the CO 2 laser controller 32 a may be configured to control the output intensity of the preamplifier 3 h and the main amplifier 3 j based on the CO 2 laser beam intensity setting signal.
- the seed beam from the master oscillator 3 d may be amplified by the preamplifier 3 h and the main amplifier 3 j to desired beam intensity.
- the fiber laser 50 b is used to output the pre-pulse laser beam.
- This disclosure is not limited thereto.
- a YAG laser or a Ti:sapphire laser may be used to output the pre-pulse laser beam.
- the first pre-pulse laser beam may be outputted from a fiber laser apparatus capable of achieving a small spot
- the second pre-pulse laser beam may be outputted from a YAG laser apparatus or a Ti:sapphire laser apparatus capable of outputting ultrashort pulse laser beam.
- the main pulse laser beam may be outputted from a CO 2 laser apparatus capable of achieving high power laser beam. That is, a desired number of pre-pulse laser beams may be outputted from a plurality of separate laser apparatuses. Further, in accordance with the state of the diffused target at the time of being irradiated with the second pre-pulse laser beam, the diffused target may be irradiated with a plurality of pre-pulse laser beams respectively at different wavelengths, and with difference spot sizes, energy, and pulse durations.
- FIGS. 19A and 20A are conceptual diagrams showing a droplet being irradiated with a linearly-polarized pre-pulse laser beam.
- FIGS. 19B and 20B show the simulation result of a droplet being irradiated with a linearly-polarized pre-pulse laser beam.
- the droplet is viewed in a direction (X-direction) perpendicular to the polarization direction of the pre-pulse laser beam.
- the droplet is viewed in a direction of the beam axis (Z-direction) of the pre-pulse laser beam.
- the droplet may be diffused, and as shown in FIGS. 19B and 20B , a diffused target may be generated.
- the simulation result reveals that the diffused target is diffused further in a direction (X-direction) perpendicular to the polarization direction (Y-direction) of the pre-pulse laser beam.
- the diffused target diffused as such is irradiated with the main pulse laser beam traveling along substantially the same path as the pre-pulse laser beam, as shown in FIGS. 19B and 20B
- the shape of the diffused target may differ largely from the cross-sectional shape of the main pulse laser beam. Accordingly, a large portion of the main pulse laser beam may not be used to generate plasma.
- FIG. 21 is a graph showing absorptivity of a P-polarization component and an S-polarization component of a laser beam incident on the surface of a molten tin droplet.
- the wavelength of the laser beam is 1.06 ⁇ m.
- the absorptivity of the laser beam may depend on the angle of incidence and the polarization state of the laser beam.
- the absorptivity of the P-polarization component of an incident laser beam is at the highest when the angle of incidence of the laser beam is 80 to 85 degrees, and gradually decreases as the angle of incidence shifts from that angle range.
- the absorptivity of the S-polarization component is substantially the same as that of the P-polarization component when the laser beam is incident on the surface of the molten tin droplet at substantially 0 degree (i.e., substantially normal incidence), and decreases as the angle of incidence increases. For example, when the angle of incidence is equal to or greater than 80 degrees, the absorptivity of the S-polarization component approximates to 0%.
- laser ablation region areas toward the edges of the irradiation surface in the Y-direction. That is, the absorptivity of the laser beam is high in these areas, and strong laser ablation may occur.
- a shock wave may propagate toward the inside of the droplet from the laser ablation regions. This shock wave may propagate toward the edges of the droplet in the X-direction as shown in FIG. 20A , and the droplet may be diffused in the X-direction as shown in FIG. 20B .
- the polarization state of the pre-pulse laser beam may be changed into a polarization state other than the linear polarization using the polarization converter 20 .
- the spot size of the pre-pulse laser beam may be equal to or greater than the diameter (e.g., 40 ⁇ m) of the droplet, the entire irradiation surface of the droplet may be irradiated with the pre-pulse laser beam.
- the droplet may be diffused symmetrically about the beam axis of the pre-pulse laser beam, and the diffused target may be irradiated with the main pulse laser beam efficiently.
- the polarization converter 20 may be configured to change the pre-pulse laser beam into a substantially circularly-polarized laser beam, a substantially unpolarized laser beam, a substantially radially-polarized laser beam, a substantially azimuthally-polarized laser beam, and so forth.
- FIGS. 22A and 22B show a droplet being irradiated with a circularly-polarized pre-pulse laser beam.
- FIGS. 22C and 22D show a diffused target generated when the droplet is irradiated with the pre-pulse laser beam being irradiated with a main pulse laser beam.
- FIGS. 22E and 22F schematically show plasma generated when the diffused target is irradiated with the main pulse laser beam.
- the polarization vector draws a circle on a plane (X-Y plane) perpendicular to the beam axis of the laser beam. Further, the polarization state of the pre-pulse laser beam is circular at any position along the X-Y plane (see FIGS. 22A and 22B ). In the circularly-polarized laser beam, the ratio of an X-direction polarization component and a Y-direction polarization component is substantially 1:1.
- the distribution of absorptivity of the pre-pulse laser beam in the surface of the droplet may be symmetrical about the center axis of the droplet in the irradiation direction of the laser beam.
- the diffusion state of the droplet may be symmetrical about the center axis of the droplet, and the shape of the diffused target may become disc-like (see FIGS. 22C and 22D ). This allows the shape of the diffused target to substantially coincide with the cross-sectional shape of the main pulse laser beam so that the main pulse laser beam may be absorbed efficiently by the diffused target.
- FIGS. 23A and 23B show a droplet being irradiated with an unpolarized pre-pulse laser beam.
- FIGS. 23C and 23D show a diffused target generated when the droplet is irradiated with the pre-pulse laser beam being irradiated with a main pulse laser beam.
- FIGS. 23E and 23F schematically show plasma generated when the diffused target is irradiated with the main pulse laser beam.
- the pre-pulse laser beam shown in FIG. 23B is substantially unpolarized.
- the ratio of the X-direction polarization component and the Y-direction polarization component is substantially 1:1.
- the distribution of absorptivity of the pre-pulse laser beam in the surface of the droplet may be symmetrical about the center axis of the droplet in the irradiation direction of the laser beam.
- the diffusion state of the droplet may be symmetrical about the center axis of the droplet, and the shape of the diffused target may, for example, become disc-like. Accordingly, the main pulse laser beam may be absorbed by the diffused target efficiently.
- FIGS. 24A and 24B show a droplet being irradiated with a radially-polarized pre-pulse laser beam.
- FIGS. 24C and 24D show a diffused target generated when the droplet is irradiated with the pre-pulse laser beam being irradiated with a main pulse laser beam.
- FIGS. 24E and 24F schematically show plasma generated when the diffused target is irradiated with the main pulse laser beam.
- the distribution of absorptivity of the pre-pulse laser beam in the surface of the droplet may be symmetrical about the beam axis of the pre-pulse laser beam.
- the beam axis of the pre-pulse laser beam preferably coincides with the center axis of the droplet.
- the diffusion state of the droplet may be symmetrical about the beam axis of the pre-pulse laser beam, and the shape of the diffused target may, for example, become disc-like. Accordingly, the main pulse laser beam may be absorbed by the diffused target efficiently.
- the spot size of the pre-pulse laser beam is controlled to be equal to or greater than the diameter (e.g., 40 ⁇ m) of the droplet
- the entire irradiation surface of the droplet may be irradiated with the pre-pulse laser beam incident thereon mostly as the P-polarization component. Accordingly, the absorptivity of the pre-pulse laser beam may be increased, and the energy required to generate a desired diffused target may be kept small.
- FIGS. 25A and 25B show a droplet being irradiated with an azimuthally-polarized pre-pulse laser beam.
- FIGS. 25C and 25D show a diffused target generated when the droplet is irradiated with the pre-pulse laser beam being irradiated with a main pulse laser beam.
- FIGS. 25E and 25F schematically show plasma generated when the diffused target is irradiated with the main pulse laser beam.
- the distribution of absorptivity of the pre-pulse laser beam in the surface of the droplet may be symmetrical about the beam axis of the pre-pulse laser beam.
- the beam axis of the pre-pulse laser beam preferably coincides with the center axis of the droplet.
- the diffusion state of the droplet may be symmetrical about the beam axis of the pre-pulse laser beam, and the shape of the diffused target may, for example, become disc-like. Accordingly, the main pulse laser beam may be absorbed by the diffused target efficiently.
- the distribution of the absorptivity of the pre-pulse laser beam in the surface of the droplet is made symmetrical about the center axis of the droplet and/or the beam axis of the pre-pulse laser beam by controlling the polarization state of the pre-pulse laser beam.
- this disclosure is not limited thereto.
- the distribution of the absorptivity of the pre-pulse laser beam in the surface of the droplet need not be perfectly symmetrical about the beam axis, but may be substantially symmetrical. Accordingly, the polarization state of the pre-pulse laser beam may, for example, be elliptical as well.
- FIG. 26A schematically illustrates an exemplary configuration of a device for measuring the degree of linear polarization.
- the device may include a polarization prism and a beam intensity detector.
- FIG. 26B shows the relationship between the rotation angle of the polarization prism and the detection result of the beam intensity detector.
- a linearly-polarized pre-pulse laser beam from the fiber laser 50 b may be changed into an elliptically-polarized laser beam by the polarization converter 20 .
- This elliptically-polarized laser beam may be focused by a focusing optical system 41 and made to be incident on the polarization prism 42 .
- the beam intensity of the laser beam outputted from the polarization prism 42 may be detected by the beam intensity detector 43 .
- the polarization prism 42 may be formed by bonding two refractive crystals such as calcite.
- the polarization prism 42 may be used to extract a laser beam of a predetermined polarization direction as an output laser beam from an input beam in accordance with the orientation of the bonding surface of the prism. As the polarization prism 42 is rotated about the beam axis of the pre-pulse laser beam, the polarization prism 42 may transmit a laser beam polarized in a direction corresponding to the rotation angle. In the description below, it is assumed that the polarization prism 42 may be an ideal prism having a sufficiently high extinction factor.
- the beam intensity of the output beam from the polarization prism 42 may change periodically as the polarization prism 42 is rotated by 180 degrees.
- the degree of linear polarization P may be obtained from a maximum value Imax and a minimum value Imin of the beam intensity.
- the degree of linear polarization P measured by the device shown in FIG. 26A may be substantially 0% for a laser beam of a polarization state that is substantially symmetrical about the beam axis (e.g., circularly-polarized laser beam, unpolarized laser beam, radially-polarized laser beam, azimuthally-polarized laser beam).
- the degree of linear polarization P may be substantially 100% for a linearly-polarized laser beam.
- the diffused target may be formed in a desired shape (e.g., disc-shape).
- FIG. 27 shows a first example of a polarization converter in the seventh embodiment.
- a quarter-wave plate 21 for converting a linearly-polarized laser beam into a circularly-polarized laser beam may be used as the polarization converter.
- the transmissive quarter-wave plate 21 may be a refractive crystal that provides a phase difference of ⁇ /2 between a polarization component parallel to the optic axis of the crystal and a polarization component perpendicular to the optic axis of the crystal.
- a linearly-polarized laser beam may be converted into a circularly-polarized laser beam when the linearly-polarized laser beam is incident on the quarter-wave plate 21 such that the polarization direction thereof is inclined by 45 degrees with respect to the optic axis of the quarter-wave plate 21 .
- the polarization direction of the linearly-polarized laser beam is inclined by 45 degrees in the other direction, the rotation direction of the circular polarization is reversed.
- This disclosure is not limited to the transmissive quarter-wave plate 21 , and a reflective quarter-wave plate may be used as well.
- FIGS. 28A through 28C show a second example of a polarization converter in the seventh embodiment.
- FIG. 28A is a front view of the polarization converter
- FIG. 28B is an enlarged fragmentary sectional view of the polarization converter taken along a radial direction plane
- FIG. 28C shows one mode for the use of the polarization converter.
- a random phase plate 22 for converting a linearly-polarized laser beam into an unpolarized laser beam may be used as the polarization control apparatus.
- the transmissive random phase plate 22 may be a transmissive optical element having a diameter D, on whose input or output surface minute square regions each having a length d on each side are formed by randomly arranged recesses and protrusions.
- the random phase plate 22 may divide an input beam having the diameter D into small square beams each having the length d on each side. With this configuration, the random phase plate 22 may provide a phase difference of n between a small beam transmitted through a protrusion 22 a and a small beam transmitted through a recess 22 b .
- the phase difference n may be provided by setting a step ⁇ t between the protrusion 22 a and the recess 22 b as in Expression (7) below, where the wavelength of the incident laser beam is A, and the refractive index of the random phase plate 22 is n 1 .
- the transmissive random phase plate 22 may, for example, be provided between the pre-pulse laser apparatus and the focusing optical system 15 .
- a linearly-polarized laser beam may be incident on the random phase plate 22 , and the laser beam transmitted through the random phase plate 22 may become unpolarized.
- Laser beams polarized in directions perpendicular to each other do not interfere. Accordingly, when this laser beam is focused by the focusing optical system 15 , the cross-sectional beam intensity distribution at the focus may not be Gaussian but may be closer to the top-hat distribution.
- the droplet When a droplet is irradiated with such a pre-pulse laser beam, the droplet may be diffused substantially symmetrically about the center axis of the droplet. Accordingly, the diffused target may become disc-shaped, and the main pulse laser beam may be absorbed by the diffused target efficiently.
- the transmissive random phase plate 22 may be used instead.
- the protrusion 22 a and the recess 22 b may be in any other polygonal shapes, such as a hexagonal shape, a triangular shape.
- FIGS. 29A and 29B show a third example of a polarization converter in the seventh embodiment.
- FIG. 29A is a perspective view of the polarization converter
- FIG. 29B is a front view of the polarization converter.
- FIGS. 29A and 29B show an n-divided wave plate 23 for converting a linearly-polarized laser beam into a radially-polarized laser beam.
- the n-divided wave plate 23 may be a transmissive optical element in which n triangular half-wave plates 231 , 232 , . . . , 23 n are arranged symmetrically about the beam axis of the laser beam.
- Each of the half-wave plates 231 , 232 , . . . , 23 n may be a refractive crystal that provides a phase difference of n between a polarization component parallel to the optic axis of the crystal and a polarization component perpendicular to the optic axis of the crystal.
- the laser beam When a linearly-polarized laser beam is incident on such a half-wave plate perpendicularly such that the polarization direction is inclined by an angle ⁇ with respect to the optic axis of the half-wave plate, the laser beam may be outputted from the half-wave plate with its polarization direction being rotated by 2 ⁇ .
- the half-wave plate 231 and the half-wave plate 233 may be arranged so that their respective optic axes make an angle of 45 degrees. Then, the polarization direction of the linearly-polarized laser beam transmitted through the half-wave plate 231 and the polarization direction of the linearly-polarized laser beam transmitted through the half-wave plate 233 may differ by 90 degrees. In this way, the polarization direction of the incident laser beam may be changed in accordance with an angle formed by the optic axis of the half-wave plate and the polarization direction of the incident laser beam. With this, the polarization directions of the laser beams transmitted through the respective half-wave plates may be changed to predetermined polarization directions.
- the re-divided wave plate 23 may convert a linearly-polarized laser beam into a radially-polarized laser beam. Further, by changing the arrangement of the half-wave plates in the n-divided wave plate 23 , a linearly-polarized laser beam may be converted into an azimuthally-polarized laser beam as well.
- FIG. 30 shows a fourth example of a polarization converter in the seventh embodiment.
- FIG. 30 shows a phase compensator 24 a , a polarization rotator 24 b , and a theta cell 24 c for converting a linearly-polarized laser beam into a radially-polarized laser beam.
- the theta cell 24 c may be an optical element into which a twisted nematic (TN) liquid crystal is injected, and the liquid crystal molecules are arranged so as to be twisted from the input side toward the output side.
- a linearly-polarized laser beam incident on the theta cell 24 c may be rotated along the twist of the alignment of the liquid crystal molecules, and a laser beam linearly-polarized in a direction inclined with respect to the polarization direction of the input beam may be outputted from the theta cell 24 c .
- the theta cell 24 c may convert a linearly-polarized input beam into a radially-polarized output beam.
- the beam intensity may be decreased at a boundary between an upper half and a lower half of the laser beam outputted from the theta cell 24 c . Accordingly, a phase of the upper half of the laser beam may be shifted by n by the phase compensator 24 a prior to the laser beam being incident on the theta cell 24 c .
- the arrows indicate that the phases of the input beam are opposite between the upper and lower halves of the laser beam.
- the upper half of the phase compensator 24 a may include a TN liquid crystal in which the alignment of the liquid crystal molecules is twisted by 180 degrees from the input side toward the output side.
- a linearly-polarized laser beam in which the phases of the upper and lower halves are opposite is made to be incident on the theta cell 24 c , laser beams of the same phase may be outputted around the boundary between the upper and lower halves of the output laser beam. With this, the beam intensity may be prevented from being decreased at the boundary between the upper and lower halves of the laser beam outputted from the theta cell 24 c.
- the polarization rotator 24 b may be configured to rotate the polarization direction of the linearly-polarized input beam by 90 degrees.
- the theta cell 24 c may convert the linearly-polarized laser beam into an azimuthally-polarized laser beam.
- the polarization rotator 24 b may be formed of a TN liquid crystal in which the alignment of the liquid crystal molecules is twisted by 90 degrees from the input side toward the output side.
- the conversion of the polarization state may be achieved relatively freely by using the phase compensator 24 a , the polarization rotator 24 b , and the theta cell 24 c .
- the wavelength of the laser beam of which the polarization direction is changed may differ depending on the thickness of the wave plate.
- the polarization direction of an input beam of a relatively broad bandwidth may be changed. Accordingly, using the theta cell 24 c may make it possible to change the polarization direction even when the bandwidth of the pre-pulse laser beam is broad.
- FIG. 31 schematically illustrates the configuration of an EUV light generation system according to an eighth embodiment.
- the polarization state of a pre-pulse laser beam from the fiber laser apparatus 31 may be controlled by the polarization converter 20 , and this pre-pulse laser beam may be guided into the chamber 1 along a beam path that is different from that of the main pulse laser beam.
- FIGS. 32A through 32C schematically illustrates an exemplary configuration of a laser apparatus configured to output a pre-pulse laser beam in an EUV light generation system according to a ninth embodiment.
- a laser apparatus 60 a of the ninth embodiment may be provided outside the chamber 1 (see, e.g., FIG. 1 ) as a driver laser for outputting a pre-pulse laser beam in any one of the first through fourth embodiments.
- the laser apparatus 60 a may include a laser resonator that includes a reflective polarization converter 61 a and a front mirror 62 .
- a laser medium 63 may be provided in the laser resonator. Stimulated emission light may be generated from the laser medium 63 with a pumping beam from a pumping source (not shown). The stimulated emission light may travel back and forth between the polarization converter 61 a and the front mirror 62 and be amplified by the laser medium 63 . Thereafter, an amplified laser beam may be outputted from the laser apparatus 60 a.
- the polarization converter 61 a may be configured to reflect with high reflectance a laser beam of a predetermined polarization direction in accordance with the input position on the polarization converter 61 a .
- a radially-polarized laser beam shown in FIG. 32B or an azimuthally-polarized laser beam shown in FIG. 32C may be amplified in the laser resonator. A part of the amplified laser beam may be transmitted through the front mirror 62 and outputted as the pre-pulse laser beam.
- a polarization converter may be used as a part of the resonator of the driver laser. With this, a polarization converter need not be provided in a beam path between the driver laser and the plasma generation region PS as in the seventh embodiment.
- FIGS. 33A through 33C schematically illustrates the exemplary configuration of a laser apparatus configured to output a pre-pulse laser beam in an EUV light generation system according to a modification of the ninth embodiment.
- a laser apparatus 60 b of this modification may include a laser resonator that includes a rear mirror 61 and a reflective polarization converter 62 a .
- a radially-polarized laser beam shown in FIG. 33B or an azimuthally-polarized laser beam shown in FIG. 33C may be amplified in the laser resonator.
- a part of the amplified laser beam may be transmitted through the polarization converter 62 a and outputted as the pre-pulse laser beam.
- FIGS. 34A and 34B show an example of a polarization converter in the ninth embodiment.
- FIG. 34A is a perspective view of the polarizer
- FIG. 34B is an enlarged fragmentary sectional view of a diffraction grating portion of the polarization converter, taken along the radial direction plane.
- the reflective polarization converter 61 a may be a mirror on which a concentric circular diffraction grating 611 is formed.
- a multilayer film 612 may be formed on a glass substrate 613
- the diffraction grating 611 may be formed on the multilayer film 612 .
- the polarization converter 61 a configured as such (here, the polarization direction is substantially parallel to the direction of the grooves in the diffraction grating 611 )
- the azimuthally-polarized laser beam may be transmitted through the diffraction grating 611 and propagated to the multilayer film 612 .
- the radially-polarized laser beam may not be transmitted through the diffraction grating 611 and may be reflected thereby.
- using the polarization converter 61 a configured as such in the laser resonator may make it possible to output a radially-polarized laser beam.
- the polarization converter 61 a may reflect an azimuthally-polarized laser beam with high reflectance. In this case, the azimuthally-polarized laser beam may be outputted. Further, forming the diffraction grating 611 on the polarization converter 62 a of the modification (see FIGS. 33A through 33C ) of the ninth embodiment may make it possible to output a radially-polarized laser beam or an azimuthally-polarized laser beam.
- FIG. 35 is a graph on which the obtained conversion efficiency (CE) in accordance with a fluence (energy per unit area of a beam cross-section at its focus) of a pre-pulse laser beam is plotted.
- the measuring conditions are as follows.
- a molten tin droplet having a diameter of 20 ⁇ m is used as a target material.
- a laser beam with a pulse duration of 5 ns to 15 ns outputted from a YAG pulse laser apparatus is used as a pre-pulse laser beam.
- a laser beam with a pulse duration of 20 ns outputted from a CO 2 pulse laser apparatus is used as a main pulse laser beam.
- the beam intensity of the main pulse laser beam is 6.0 ⁇ 10 9 W/cm 2
- the delay time for the irradiation with the main pulse laser beam is 1.5 ⁇ s after the irradiation with the pre-pulse laser beam.
- the horizontal axis of the graph shown in FIG. 35 shows a value in which the irradiation conditions of the pre-pulse laser beam (pulse duration, energy, spot size) are converted into a fluence. Further, the vertical axis shows the CE in the case where the diffused target generated in accordance with the irradiation conditions of the pre-pulse laser beam is irradiated with the above main pulse laser beam.
- the measurement results shown in FIG. 35 reveal that increasing the fluence of the pre-pulse laser beam may improve the CE (approximately 3%). That is, at least in a range where the pulse duration of the pre-pulse laser beam is 5 ns to 15 ns, there is a correlation between the fluence and the CE.
- the EUV light generation controller 7 may be configured to control the fluence, instead of the beam intensity, of the pre-pulse laser beam.
- the measurement results shown in FIG. 35 reveal that the fluence of the pre-pulse laser beam may preferably be in the range of 10 mJ/cm 2 to 600 mJ/cm 2 .
- the range of 30 mJ/cm 2 to 400 mJ/cm 2 is more preferable.
- the range of 150 mJ/cm 2 to 300 mJ/cm 2 is even more preferable.
- FIG. 36 is a graph showing the result of an experiment for generating a diffused target in an EUV light generation system.
- the pre-pulse laser beam may be converted into a circularly-polarized laser beam by the polarization converter 20 .
- the horizontal axis in FIG. 36 shows a time that has elapsed since a droplet is irradiated with a pre-pulse laser beam.
- the vertical axis shows a diffusion radius of a diffused target generated when the droplet is irradiated with the pre-pulse laser beam.
- the diffusion radius is a radius of a space where a particle of a predetermined diameter exists.
- Changes over time in the diffusion radius after the irradiation with the pre-pulse laser beam are plotted for the droplets respectively having diameters of 12 ⁇ m, 20 ⁇ m, 30 ⁇ m, and 40 ⁇ m.
- the diffusion radius has low dependency on the droplet diameter.
- the changes over time in the diffusion radius are relatively gradual in 0.3 ⁇ s to 3 ⁇ s after a droplet is irradiated with a pre-pulse laser beam. It is speculated that the variation in the diffusion radius for each droplet is small during this time period. Accordingly, if the diffused target is irradiated with a main pulse laser beam during this time period, the variation in generated EUV energy may be small between pulses.
- FIG. 37 is a graph on which the obtained conversion efficiency (CE) for the corresponding delay time since a droplet is irradiated with a pre-pulse laser beam until a diffused target is irradiated by a main pulse laser beam is plotted for differing diameters of the droplet.
- the measuring conditions are as follows. Molten tin droplets respectively having diameters of 12 ⁇ m, 20 ⁇ m, 30 ⁇ m, and 40 ⁇ m are used as the target material.
- a laser beam with a pulse duration of 5 ns outputted from a YAG pulse laser apparatus is used as a pre-pulse laser beam.
- the fluence of the pre-pulse laser beam is 490 mJ/cm 2 .
- a laser beam with a pulse duration of 20 ns outputted from a CO 2 pulse laser apparatus is used as a main pulse laser beam.
- the beam intensity of the main pulse laser beam is 6.0 ⁇ 10 9 W/cm 2 .
- the measurement results shown in FIG. 37 reveal that the delay time for the irradiation with the main pulse laser beam may preferably be in a range of 0.5 ⁇ s to 2.5 ⁇ s after the irradiation with the pre-pulse laser beam. However, it is found that the optimum range of the delay time for the irradiation with the main pulse laser beam to obtain a high CE differs depending on the diameter of the droplet.
- the delay time for the irradiation with the main pulse laser beam may preferably be in a range of 0.5 ⁇ s to 2 ⁇ s after the irradiation with the pre-pulse laser beam.
- the range of 0.6 ⁇ s to 1.5 ⁇ s is more preferable.
- the range of 0.7 ⁇ s to 1 ⁇ s is even more preferable.
- the delay time for the irradiation with the main pulse laser beam may preferably be in a range of 0.5 ⁇ s to 2.5 ⁇ s after the irradiation with the pre-pulse laser beam.
- the range of 1 ⁇ s to 2 ⁇ s is more preferable.
- the range of 1.3 ⁇ s to 1.7 ⁇ s is even more preferable.
- the delay time for the irradiation with the main pulse laser beam may preferably be in a range of 0.5 ⁇ s to 4 ⁇ s after the irradiation with the pre-pulse laser beam.
- the range of 1.5 ⁇ s to 3.5 ⁇ s is more preferable.
- the range of 2 ⁇ s to 3 ⁇ s is even more preferable.
- the delay time for the irradiation with the main pulse laser beam may preferably be in a range of 0.5 ⁇ s to 6 ⁇ s after the irradiation with the pre-pulse laser beam.
- the range of 1.5 ⁇ s to 5 ⁇ s is more preferable.
- the range of 2 ⁇ s to 4 ⁇ s is even more preferable.
- the driver laser 3 corresponds to a laser beam generation apparatus configured to output a pre-pulse laser beam and a main pulse laser beam.
- the YAG pulse laser apparatus 3 a (see FIGS. 9 through 11 ) and the fiber laser apparatus 31 (see FIGS. 18 and 31 ) correspond to a first pulse laser apparatus.
- the CO 2 pulse laser apparatus 3 b (see FIGS. 9 through 11 ) and the CO 2 pulse laser apparatus 32 (see FIGS. 18 and 31 ) correspond to a second pulse laser apparatus.
- the EUV light generation controller 7 corresponds to a laser controller.
Abstract
A system includes a chamber, a laser beam apparatus configured to generate a laser beam to be introduced into the chamber, a laser controller for the laser beam apparatus to control at least a beam intensity and an output timing of the laser beam, and a target supply unit configured to supply a target material into the chamber, the target material being irradiated with the laser beam for generating extreme ultraviolet light.
Description
- The present application is a continuation of U.S. patent application Ser. No. 13/492,0687, filed on Jun. 8, 2012, which is a continuation-in-part of International Application No. PCT/JP2011/052767, with an international filing date of Feb. 9, 2011, which claims priority from Japanese Patent Application No. 2010-034889 filed Feb. 19, 2010, Japanese Patent Application No. 2010-265789 filed Nov. 29, 2010, and Japanese Patent Application No. 2011-015691 filed Jan. 27, 2011. The present application further claims priority from Japanese Patent Application No. 2011-133111 filed Jun. 15, 2011.
- 1. Technical Field
- This disclosure relates to a system and a method for generating extreme ultraviolet (EUV) light.
- 2. Related Art
- In recent years, semiconductor production processes have become capable of producing semiconductor devices with increasingly fine feature sizes, as photolithography has been making rapid progress toward finer fabrication. In the next generation of semiconductor production processes, microfabrication with feature sizes at 60 nm to 45 nm, and further, microfabrication with feature sizes of 32 nm or less will be required. In order to meet the demand for microfabrication with feature sizes of 32 nm or less, for example, an exposure apparatus is needed in which a system for generating EUV light at a wavelength of approximately 13 nm is combined with a reduced projection reflective optical system.
- Three kinds of systems for generating EUV light are known in general, which include a Laser Produced Plasma (LPP) type system in which plasma is generated by irradiating a target material with a laser beam, a Discharge Produced Plasma (DPP) type system in which plasma is generated by electric discharge, and a Synchrotron Radiation (SR) type system in which orbital radiation is used.
- A system according to one aspect of this disclosure may include a chamber, a laser beam apparatus configured to generate a laser beam to be introduced into the chamber, a laser controller for the laser beam apparatus to control at least a beam intensity and an output timing of the laser beam, and a target supply unit configured to supply a target material into the chamber. The target material may be irradiated with the laser beam for generating extreme ultraviolet light.
- A system according to another aspect of this disclosure may include a chamber, a laser beam apparatus configured to output a laser beam into the chamber, a laser controller for the laser beam apparatus to control energy of the laser beam to achieve a predetermined fluence, and a target supply unit configured to supply a target material into the chamber. The target material may be irradiated with the laser beam for generating extreme ultraviolet light.
- A method according to yet another aspect of this disclosure for generating extreme ultraviolet light in a system that includes a laser beam apparatus, a laser controller, a chamber, and a target supply unit may include supplying a target material into the chamber in a form of a droplet, irradiating the target material with a pre-pulse laser beam from the laser beam apparatus, and irradiating the target material having been irradiated with the pre-pulse laser beam with a main pulse laser beam from the laser beam apparatus in a range of 0.5 μs to 3 μs after the target material is irradiated with the pre-pulse laser beam.
-
FIG. 1 schematically illustrates an exemplary configuration of an EUV light generation system according to one embodiment of this disclosure. -
FIG. 2 is a conceptual diagram showing a droplet being irradiated with a pre-pulse laser beam. -
FIGS. 3A through 3C show simulation results of diffusion when a molten tin droplet is irradiated with a pre-pulse laser beam. -
FIG. 3D is a photograph capturing a molten tin droplet being irradiated with a pre-pulse laser beam. -
FIG. 4A schematically shows a molten tin droplet being irradiated with a pre-pulse laser beam, as viewed in the direction perpendicular to the beam axis. -
FIG. 4B schematically shows a molten tin droplet being irradiated with a pre-pulse laser beam, as viewed in the direction of the beam axis. -
FIGS. 5A through 5H show the simulation results of diffusion when a molten tin droplet having a diameter of 60 μm is irradiated with a pre-pulse laser beam. -
FIG. 5I shows the spot size of a main pulse laser beam. -
FIG. 6 shows a diffusion diameter of a diffused target generated when a molten tin droplet having a diameter of 60 μm is irradiated with a pre-pulse laser beam and a conversion efficiency (CE) corresponding to a timing at which the diffused target is irradiated with a main pulse laser beam. -
FIGS. 7A through 7H show the simulation results of diffusion when a molten tin droplet having a diameter of 10 μm is irradiated with a pre-pulse laser beam. -
FIG. 7I shows the spot size of a main pulse laser beam. -
FIG. 8 shows a diffusion diameter of a diffused target generated when a molten tin droplet having a diameter of 10 μm is irradiated with a pre-pulse laser beam and a conversion efficiency (CE) corresponding to a timing at which the diffused target is irradiated with a main pulse laser beam. -
FIG. 9 schematically illustrates an exemplary configuration of an EUV light generation system according to a first embodiment. -
FIG. 10 schematically illustrates an exemplary configuration of an EUV light generation system according to a second embodiment. -
FIG. 11 schematically illustrates an exemplary configuration of an EUV light generation system according to a third embodiment. -
FIGS. 12A through 12F show a droplet being irradiated with a first pre-pulse laser beam and a diffused target being irradiated with a second pre-pulse laser beam. -
FIG. 13 schematically illustrates an exemplary configuration of an EUV light generation system according to a modification of the third embodiment. -
FIG. 14 schematically illustrates an exemplary configuration of an EUV light generation system according to a fourth embodiment. -
FIG. 15 schematically illustrates an exemplary configuration of a Ti:sapphire laser configured to output a pre-pulse laser beam in an EUV light generation system according to a fifth embodiment. -
FIG. 16 schematically illustrates an exemplary configuration of a fiber laser configured to output a pre-pulse laser beam in an EUV light generation system according to a sixth embodiment. -
FIG. 17A is a table showing irradiation conditions of a pre-pulse laser beam in the EUV light generation system of any one of the embodiments. -
FIG. 17B is a table showing irradiation conditions of a main pulse laser beam in the EUV light generation system of any one of the embodiments. -
FIG. 18 schematically illustrates an exemplary configuration of an EUV light generation system according to a seventh embodiment. -
FIG. 19A is a conceptual diagram showing a droplet being irradiated with a linearly-polarized pre-pulse laser beam. -
FIG. 19B shows the simulation result of diffusion of the droplet. -
FIG. 20A is a conceptual diagram showing a droplet being irradiated with a linearly-polarized pre-pulse laser beam. -
FIG. 20B shows the simulation result of diffusion of the droplet. -
FIG. 21 is a graph showing absorptivity of a P-polarization component and an S-polarization component of a laser beam by a molten tin droplet. -
FIGS. 22A through 22F show a droplet being irradiated with a circularly-polarized pre-pulse laser beam and a diffused target being irradiated with a main pulse laser beam according to a seventh embodiment. -
FIGS. 23A through 23F show a droplet being irradiated with an unpolarized pre-pulse laser beam and a diffused target being irradiated with a main pulse laser beam according to the seventh embodiment. -
FIGS. 24A through 24F show a droplet being irradiated with a radially-polarized pre-pulse laser beam and a diffused target being irradiated with a main pulse laser beam according to the seventh embodiment. -
FIGS. 25A through 25F show a droplet being irradiated with an azimuthally-polarized pre-pulse laser beam and a diffused target being irradiated with a main pulse laser beam according to the seventh embodiment. -
FIGS. 26A and 26B are diagrams for discussing a method for measuring the degree of linear-polarization. -
FIG. 27 shows a first example of a polarization converter in the seventh embodiment. -
FIGS. 28A through 28C show a second example of a polarization converter in the seventh embodiment. -
FIGS. 29A and 29B show a third example of a polarization converter in the seventh embodiment. -
FIG. 30 shows a fourth example of a polarization converter in the seventh embodiment. -
FIG. 31 schematically illustrates an exemplary configuration of an EUV light generation system according to an eighth embodiment. -
FIGS. 32A through 32C schematically illustrates an exemplary configuration of a laser apparatus configured to output a pre-pulse laser beam in an EUV light generation system according to a ninth embodiment. -
FIGS. 33A through 33C schematically illustrates an exemplary configuration of a laser apparatus configured to output a pre-pulse laser beam in an EUV light generation system according to a modification of the ninth embodiment. -
FIGS. 34A and 34B show an example of a polarization converter in the ninth embodiment. -
FIG. 35 is a graph on which the obtained conversion efficiency (CE) in accordance with a fluence of a pre-pulse laser beam is plotted. -
FIG. 36 is a graph showing the result of an experiment for generating a diffused target in an EUV light generation system. -
FIG. 37 is a graph on which the obtained conversion efficiency (CE) for the corresponding delay time since a droplet is irradiated with a pre-pulse laser beam until a diffused target is irradiated by a main pulse laser beam is plotted for differing diameters of the droplet. - Hereinafter, selected embodiments of this disclosure will be described in detail with reference to the accompanying drawings. The embodiments to be described below are merely illustrative in nature and do not limit the scope of this disclosure. Further, the configuration(s) and operation(s) described in each embodiment are not all essential in implementing this disclosure. Note that like elements are referenced by like reference numerals and characters, and duplicate descriptions thereof will be omitted herein.
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FIG. 1 schematically illustrates an exemplary configuration of an EUV light generation system according to an embodiment of this disclosure. The EUV light generation system of this embodiment may be of an LPP type. As shown inFIG. 1 , the EUV light generation system may include achamber 1, atarget supply unit 2, adriver laser 3, anEUV collector mirror 5, and an EUVlight generation controller 7. - The
chamber 1 may be a vacuum chamber, and the EUV light is generated inside thechamber 1. Thechamber 1 may be provided with an exposureapparatus connection port 11 and awindow 12. The EUV light generated inside thechamber 1 may be outputted to an external processing apparatus, such as an exposure apparatus (reduced projection reflective optical system), through the exposureapparatus connection port 11. A laser beam outputted from thedriver laser 3 may enter thechamber 1 through thewindow 12. - The
target supply unit 2 may be configured to supply a target material, such as tin (Sn) and lithium (Li), used to generate the EUV light, into thechamber 1 at a timing specified by adroplet controller 8. The target material inside thetarget supply unit 2 may be outputted through atarget nozzle 13 in the form of droplets DL. The droplet DL may, for example, be 10 μm to 100 μm (inclusive) in diameter. Of a plurality of droplets DL supplied into thechamber 1, ones that are not irradiated with a laser beam may be collected into atarget collection unit 14. - The
driver laser 3 is configured to output a laser beam used to excite the target material. Thedriver laser 3 may be a master oscillator power amplifier type laser apparatus. The laser beam from thedriver laser 3 may be a pulse laser beam with a pulse duration of a few to a few tens of nanoseconds and a repetition rate of 10 kHz to 100 kHz. In this embodiment, thedriver laser 3 may be configured to output a pre-pulse laser beam and a main pulse laser beam. As thedriver laser 3, a combination of a Yttrium Aluminum Garnet (YAG) laser apparatus for outputting a pre-pulse laser beam and a CO2 laser apparatus for outputting a main pulse laser beam may be used. However, this embodiment is not limited thereto, and any suitable laser apparatus may be used. - Each of the pre-pulse laser beam and the main pulse laser beam from the
driver laser 3 may be reflected by a laser beam focusing optical system that includes a high-reflection mirror 15 a and an off-axis paraboloidal mirror 15 b, and enter thechamber 1 through thewindow 12. Inside thechamber 1, each of the pre-pulse laser beam and the main pulse laser beam may be focused in a plasma generation region PS. - When the droplet DL is irradiated with the pre-pulse laser beam, the droplet DL may be diffused into fine particles. In this specification, a target material in a state where fine particles of a droplet DL are diffused may be referred to as a diffused target. The diffused target may be irradiated with the main pulse laser beam. Upon being irradiated with the main pulse laser beam, the target material constituting the diffused target may be excited by the energy of the main pulse laser beam. With this, the target material may be turned into plasma, and rays of light at various wavelengths including the EUV light may be emitted from the plasma.
- The
EUV collector mirror 5 may be configured to selectively reflect light at a predetermined wavelength (e.g., EUV light at a central wavelength of approximately 13.5 nm) among rays of light at various wavelengths emitted from the plasma. TheEUV collector mirror 5 may have a spheroidal concave surface on which a multilayer reflective film formed of a molybdenum (Mo) layer and a silicon (Si) layer laminated alternately is formed. TheEUV collector mirror 5 may be positioned such that a first focus of the spheroidal surface lies in the plasma generation region PS and a second focus thereof lies in an intermediate focus region IF. With this, the EUV light reflected by theEUV collector mirror 5 may be focused at the second focus, and outputted to an external exposure apparatus. - The EUV
light generation controller 7 may be configured to output an oscillation trigger signal and a laser beam intensity setting signal to thedriver laser 3. With this, the EUVlight generation controller 7 may control the beam intensity and the generation timing of the pre-pulse laser beam such that a droplet supplied into thechamber 1 is transformed into a desired diffused target. Further, the EUVlight generation controller 7 may control the beam intensity and the generation timing of the main pulse laser beam such that plasma in a desired condition may be generated from the diffused target upon being irradiated with the main pulse laser beam. - The oscillation trigger signal may be outputted based on an oscillation trigger detection signal from an
exposure apparatus controller 9, and the generation timing of the laser beams by thedriver laser 3 may be controlled accordingly. The laser beam intensity setting signal may be outputted based on the oscillation trigger detection signal from theexposure apparatus controller 9 and an EUV pulse energy detection signal from either anEUV light detector 16 or theexposure apparatus controller 9. The laser beam intensity setting signal may be outputted to thedriver laser 3 in order to control the beam intensity of the laser beams. The EUVlight generation controller 7 may include atrigger counter 7 a and atimer 7 b, and may count the number of oscillation trigger detection signals per unit time. The laser beam intensity setting signal may be outputted based on the EUV pulse energy detection signal and the number of counted oscillation trigger detection signals. - Diffusion of a droplet upon being irradiated with a pre-pulse laser beam will now be discussed.
FIG. 2 is a conceptual diagram showing a droplet being irradiated with a pre-pulse laser beam. InFIG. 2 , the droplet is viewed in a direction perpendicular to the beam axis (Z-direction) of the pre-pulse laser beam. - As shown in
FIG. 2 , when the pre-pulse laser beam is focused on the droplet DL, laser ablation may occur at a surface of the droplet DL that has been irradiated with the pre-pulse laser beam. As a result, a shock wave may occur from the surface of the droplet DL irradiated with the pre-pulse laser beam toward the interior of the droplet DL due to the energy by the laser ablation. This shock wave may propagate throughout the droplet DL. The droplet DL may not be broken up when the beam intensity of the pre-pulse laser beam is weak. However, when the beam intensity of the pre-pulse laser beam is equal to or greater than a first predetermined value (e.g., 1×109 W/cm2), the droplet DL may be broken up by this shock wave. -
FIGS. 3A through 3C show the simulation results of diffusion of a molten tin droplet being irradiated with a pre-pulse laser beam.FIG. 3D is a photograph capturing a molten tin droplet being irradiated with a pre-pulse laser beam under the condition that is identical to that in the simulation shown inFIG. 3C . In each ofFIGS. 3A through 3D , the droplet is viewed in a direction perpendicular to the beam axis of the pre-pulse laser beam (Z-direction). Further, inFIGS. 3A through 3C , the spot size of the main pulse laser beam and the beam intensity of the pre-pulse laser beam striking the droplet DL are indicated. InFIG. 3B , a diffusion diameter Dd of the diffused target and an irradiation spot size Dm of the main pulse laser beam are indicated. - As shown in
FIG. 3A , when the beam intensity of the pre-pulse laser beam is 6.4×108 W/cm2, the droplet is hardly diffused. On the other hand, as shown inFIG. 3B , when the beam intensity of the pre-pulse laser beam is 1.6×109 W/cm2 (2.5 times greater than the beam intensity in the simulation shown inFIG. 3A ), the droplet is broken up. The broken-up droplet is turned into numerous minute particles and forms a diffused target. These minute particles may be diffused in a disc-shape as viewed in the Z-direction. Further, as shown inFIG. 3C , when the beam intensity of the pre-pulse laser beam is 5.5×109 W/cm2 (8.6 times greater than the beam intensity in the simulation shown inFIG. 3A ), the droplet is broken up, and the minute particles of the broken-up droplet may be diffused in a dish-shape. As can been seen from the comparison betweenFIG. 3C andFIG. 3D , the state of the actual diffusion of the minute particles were similar to the simulation result. - In the case shown in
FIG. 3A , it is speculated that even when the droplet is irradiated with the main pulse laser beam, a large portion of the energy of the main pulse laser beam is not absorbed by the droplet, whereby a high CE may not be obtained. That is, with respect to the size of the target material after being irradiated with the pre-pulse laser beam, the irradiation spot size of the main pulse laser beam is too large. Accordingly, a large portion of the main pulse laser beam may not strike the droplet and may not be used to generate plasma. On the other hand, in the cases shown inFIGS. 3B and 3C , the droplet is diffused in the irradiation spot of the main pulse laser beam, whereby a large portion of the main pulse laser beam may be used to generate plasma. Further, a diffused target may have a greater total surface area than a single droplet. As shown below, when a single droplet is broken into n3 smaller pieces, the radius of a smaller piece may become (1/n) of the radius of the original droplet. Here, the total surface area of the smaller pieces may be n times greater than the surface area of the original droplet. - When the radius of an undiffused droplet is r, a volume V1 of the undiffused droplet may be expressed in Expression (1) below.
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V 1=4πr 3/3 (1) - A total volume V2 of n3 smaller pieces each having a radius (r/n) may be expressed in Expression (2) below.
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V 2 =n 3×4π(r/n)3/3 (2) - The total volume V2 of n3 smaller pieces each having the radius (r/n) may be equal to the volume V1 of the undiffused droplet having the radius r (V2=V1).
- A surface area S1 of the undiffused droplet having the radius r may be expressed in Expression (3) below.
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S 1=4πr 2 (3) - A total surface area S2 of n3 smaller pieces each having the radius (r/n) may be expressed in Expression (4) below.
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S 2 =n 3×4π(r/n)2 =n×4πr 2 (4) - Accordingly, the total surface area S2 of n3 smaller pieces each having the radius (r/n) is n times greater than the surface area S1 of the undiffused droplet having the radius r.
- In this way, in the cases shown in
FIGS. 3B and 3C , the droplet may be diffused, and the total surface area may be increased. As a result, the energy of the main pulse laser beam may be absorbed efficiently by the diffused small particles. With this, a larger portion of the diffused small particles may be turned into plasma, and EUV light with higher energy may be obtained. Accordingly, a high CE may be obtained. - In either of the cases shown in
FIGS. 3B and 3C , the diffused target has such a shape that the length in the direction of the beam axis of the pre-pulse laser beam is shorter than the length in the direction perpendicular to the beam axis of the pre-pulse laser beam. The diffused target having such a shape may be irradiated with the main pulse laser beam traveling substantially along the same path as the pre-pulse laser beam. Since the diffused target may be irradiated with the main pulse laser beam more uniformly, the main pulse laser beam may be absorbed efficiently by the target material. - The diffusion diameter Dd of the diffused target may be equal to or smaller than the irradiation spot size Dm of the main pulse laser beam. Because of this size, the entire diffused target may be irradiated with the main pulse laser beam, and thus a larger portion of the diffused target may be turned into plasma. As a result, generation of debris of the target material may be suppressed.
- Further, the diffusion diameter Dd of the diffused target may be equal to or closer to the irradiation spot size Dm of the main pulse laser beam. With this, a larger portion of the energy of the main pulse laser beam may be absorbed by the diffused target, whereby a higher CE may be obtained. Although
FIG. 3B shows that the position of the beam waist of the main pulse laser beam substantially coincides with the position of the diffused target, this disclosure is not limited thereto. That is, the position of the beam waist of the main pulse laser beam and the position of the diffused target do not necessarily have to coincide with each other. In this disclosure, the irradiation spot size Dm may be interpreted as a diameter of a cross-section of the main pulse laser beam at or around the position at which the diffused target is irradiated with the main pulse laser beam. - Although a case where the main pulse laser beam has a circular cross-section and the cross-section of the diffused target is circular has been described, this disclosure is not limited thereto. For example, a cross-section area of the main pulse laser beam may be larger than a maximum cross-section area of the diffused target.
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FIGS. 4A and 4B schematically show a molten tin droplet having been irradiated with the pre-pulse laser beam. InFIG. 4A , the diffused target is viewed in a direction perpendicular to the beam axes of the pre-pulse laser beam and the main pulse laser beam (Z-direction). InFIG. 4B , the diffused target is viewed in a direction of the beam axes of the pre-pulse laser beam and the main pulse laser beam. InFIG. 4B , an outer diameter Dout of a torus-shaped diffused target and the irradiation spot size Dm of the main pulse laser beam are indicated. - As described with reference to
FIG. 2 , when the pre-pulse laser beam is focused on the droplet DL, laser ablation may occur at the surface of the droplet DL. Here, when the beam intensity of the pre-pulse laser beam is equal to or greater than a second predetermined value (e.g., 6.4×109 W/cm2), the droplet DL may be broken up, and a torus-shaped diffused target as shown inFIGS. 4A and 4B may be formed. The torus-shaped diffused target may be diffused symmetrically about the beam axis of the pre-pulse laser beam and into a torus-shape. - For example, for generating a torus-shaped diffused target, the beam intensity of the pre-pulse laser beam may be in the range of 6.4×109 W/cm2 to 3.2×1010 W/cm2 (inclusive), and the diameter of the droplet may be in the range of 12 μm and 40 μm (inclusive).
- Irradiation of the torus-shaped diffused target with the main pulse laser beam will now be described. A torus-shaped diffused target may be formed in 0.5 μs to 2.0 μs after a droplet is irradiated with a pre-pulse laser beam. Accordingly, the diffused target may preferably be irradiated with the main pulse laser beam in the above time span after the droplet is irradiated with the pre-pulse laser beam.
- Further, as shown in
FIGS. 4A and 4B , the torus-shaped diffused target has such a shape that the length in the direction of the beam axis of the pre-pulse laser beam is shorter than the length in the direction perpendicular to the beam axis of the pre-pulse laser beam. The diffused target having such a shape may be irradiated with the main pulse laser beam traveling substantially along the same path as the pre-pulse laser beam. With this, the diffused target may be irradiated with the main pulse laser beam more efficiently, and thus the main pulse laser beam may be absorbed efficiently by the target material. Accordingly, the CE in the LPP type EUV light generation system may be improved. The CE of approximately 3% was obtained through an experiment under the above conditions. - For example, it is speculated that when a torus-shaped diffused target is irradiated with a main pulse laser beam of a Gaussian beam intensity distribution, plasma is emitted cylindrically from the torus-shaped diffused target. Then, the plasma diffused toward the inner portion of the cylinder may be trapped therein. Accordingly, high-temperature, high-density plasma may be generated, and the CE may be improved. Here, “torus-shape” means an annular shape, but the diffused target does not necessarily have to be perfectly annular in shape, and may be substantially annular in shape.
- Further, the irradiation spot size Dm of the main pulse laser beam may preferably be in the following relationship with the outer diameter Dout of the torus-shaped diffused target.
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Dm≧Dout - With this relationship, the entire torus-shaped diffused target may be irradiated with the main pulse laser beam, and a larger portion of the diffused target may be turned into plasma. As a result, generation of debris of the target material may be reduced.
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FIGS. 5A through 5H show the simulation result of diffusion when a molten tin droplet having a diameter of 60 μm is irradiated with a pre-pulse laser beam. In each ofFIGS. 5A through 5D , the droplet or the diffused target is viewed in a direction (X-direction) perpendicular to the beam axis of the pre-pulse laser beam (Z-direction).FIGS. 5A through 5D respectively show the states of the target material at timings where a time T is 0 μs, 0.4 μs, 0.8 μs, and 1.4 μs after the droplet DL is irradiated with the pre-pulse laser beam. In each ofFIGS. 5E through 5H , the droplet or the diffused target is viewed in the direction (Z-direction) of the beam axis of the pre-pulse laser beam.FIGS. 5E through 5H respectively show the states of the target material at timings where a time T is 0 μs, 0.4 μs, 0.8 μs, and 1.4 μs after the droplet DL is irradiated with the pre-pulse laser beam.FIG. 5I shows the irradiation spot size of the main pulse laser beam at a position where the diffused target is irradiated with the main pulse laser beam. Here, the beam intensity of the pre-pulse laser beam is 1.5×109 W/cm2. - With reference to the simulation results shown in
FIGS. 5A through 5H along with the irradiation spot size of the main pulse laser beam shown inFIG. 5I , the following can be found. A large portion of the diffused target may be irradiated with the main pulse laser beam in approximately 0.4 μs after a droplet is irradiated with the pre-pulse laser beam. Accordingly, generation of debris may be reduced if the diffused target is irradiated with the main pulse laser beam at the above timing. - A droplet having a diameter of 60 μm may be broken into small particles and diffused upon being irradiated with a pre-pulse laser beam. In each of
FIGS. 5A through 5D , the maximum value and the minimum value of a diameter of a small particle in the diffused target are indicated. With the beam intensity of the pre-pulse laser beam in this simulation, the maximum value of a diameter of a small particle in the diffused target is 48.0 μm. That is, the droplet has not been broken up sufficiently by the pre-pulse laser beam, and a large portion of the diffused target may not be turned into plasma even when the diffused target is irradiated with the main pulse laser beam. This may suggest that a large amount of debris may be generated. The minimum value of a diameter of a small particle in the diffused target is 3.7 μm in 0.4 μs, 3.5 μm in 0.8 μs, and 3.1 μm in 1.4 μs, respectively, after a droplet is irradiated with a pre-pulse laser beam. This suggests that the more the time T elapses after a droplet is irradiated with a pre-pulse laser beam, the smaller the diameter of a small particle becomes, and the number of small particles may increase. This in turn suggests that in the case where a molten tin droplet having a diameter of 60 μm is irradiated with a pre-pulse laser beam, if a diffused target is irradiated with a main pulse laser beam within a range where the time T after the droplet is irradiated with the pre-pulse laser beam is between 0.4 μs and 1.4 μs, the CE may be improved further with a longer time T. -
FIG. 6 shows a change over time in the diffusion diameter Dd of the diffused target when a molten tin droplet having a diameter of 60 μm is irradiated with the pre-pulse laser beam and a conversion efficiency when the diffused target is irradiated with a main pulse laser beam at a given point in time. As shown inFIGS. 5F and 6 , the diffusion diameter Dd of the diffused target may substantially coincide with the irradiation spot size of the main pulse laser beam in approximately 0.4 μs after the droplet is irradiated with the pre-pulse laser beam. Accordingly, generation of debris may be reduced if the diffused target is irradiated with the main pulse laser beam in 0.4 μs after the droplet is irradiated with the pre-pulse laser beam (see a white arrow A inFIG. 6 ). On the other hand, with reference toFIG. 6 , a high CE may be obtained if the diffused target is irradiated with the main pulse laser beam in approximately 3 μs after the droplet is irradiated with the pre-pulse laser beam (see a white arrow B inFIG. 6 ). This simulation results suggest that a preferable delay time for the main pulse laser beam from the irradiation with the pre-pulse laser beam to reduce generation of debris may differ from a preferable delay time to obtain a high CE. That is, when a molten tin droplet having a diameter of 60 μm is irradiated sequentially with a pre-pulse laser beam and then a main pulse laser beam, it may be difficult to reduce debris and obtain a high CE at the same time. -
FIGS. 7A through 7H show the simulation results of diffusion when a molten tin droplet having a diameter of 10 μm is irradiated with the pre-pulse laser beam. In each ofFIGS. 7A through 7D , the droplet or the diffused target is viewed in a direction (X-direction) perpendicular to the beam axis of the pre-pulse laser beam (Z-direction).FIGS. 7A through 7D respectively show the states of the target material at timings where a time T is 0 μs, 0.1 μs, 0.25 μs, and 0.5 μs after the droplet is irradiated with the pre-pulse laser beam. In each ofFIGS. 7E through 7H , the droplet or the diffused target is viewed in the direction of the beam axis of the pre-pulse laser beam (Z-direction).FIGS. 7E through 7H respectively show the states of the target material at timings where a time T is 0 μs, 0.1 μs, 0.25 μs, and 0.5 μs after the droplet is irradiated with the pre-pulse laser beam.FIG. 7I shows the irradiation spot size of the main pulse laser beam at a position where the diffused target is irradiated with the main pulse laser beam. Here, the beam intensity of the pre-pulse laser beam is 1.5×109 W/cm2. - With reference to the simulation results shown in
FIGS. 7A through 7H along with the irradiation spot size of the main pulse laser beam shown inFIG. 7I , it can be said that a large portion of the diffused target may be irradiated with the main pulse laser beam in 0.1 μs after the droplet is irradiated with the pre-pulse laser beam. Accordingly, generation of debris may be reduced if the diffused target is irradiated with the main pulse laser beam at the above timing. - As shown in
FIGS. 7A through 7D , the maximum value of a diameter of a small particle in a diffused target is 2.2 μm in 0.1 μs, 1.1 μm in 0.25 μs, and 1.1 μs in 0.5 μs after the droplet is irradiated with the pre-pulse laser beam. This suggests that the maximum value of a diameter of a small particle in a diffused target becomes constant in 0.25 μs after the droplet is irradiated with the pre-pulse laser beam. The minimum value of a diameter of a small particle in the diffused target is 0.2 μm in 0.1 μs, 0.2 μm in 0.25 μs, and 0.2 μm in 0.5 μs after the droplet is irradiated with the pre-pulse laser beam. This suggests that a small particle in a diffused target is sufficiently small in 0.1 μs after the droplet is irradiated with the pre-pulse laser beam. This in turn suggests that a higher CE may be obtained if the diffused target is irradiated with the main pulse laser beam in 0.1 μs after the droplet is irradiated with the pre-pulse laser beam. -
FIG. 8 shows a change over time in the diffusion diameter Dd of the diffused target when a molten tin droplet having a diameter of 10 μm is irradiated with the pre-pulse laser beam and a conversion efficiency when the diffused target is irradiated with the main pulse laser beam at a given point in time. - As shown in
FIGS. 7F and 8 , the diffusion diameter Dd of the diffused target may substantially coincide with the irradiation spot size of the main pulse laser beam in 0.1 μs after the droplet is irradiated with the pre-pulse laser beam. Accordingly, generation of debris may be reduced if the diffused target is irradiated with the main pulse laser beam in 0.1 μs after the droplet is irradiated with the pre-pulse laser beam (see a white arrow A inFIG. 8 ). On the other hand, with reference toFIG. 8 , a high CE may be obtained if the diffused target is irradiated with the main pulse laser beam in approximately 0.15 μs after the droplet is irradiated with the pre-pulse laser beam (see a white arrow B inFIG. 8 ). The simulation results suggest that a gap between a preferable delay time for the main pulse laser beam to reduce debris and a preferable delay time for the main pulse laser beam to obtain a high CE is relatively small. That is, when a molten tin droplet having a diameter of 10 μm is irradiated sequentially with the pre-pulse laser beam and then the main pulse laser beam, it may be possible to reduce debris and obtain a high CE at the same time. A molten tin droplet having a diameter of 10 μm may be referred to as a mass-limited target since it is a target with a minimum mass required for generating desired EUV light. -
FIG. 9 schematically illustrates an exemplary configuration of an EUV light generation system according to a first embodiment. In the EUV light generation system according to the first embodiment, a beam path of a pre-pulse laser beam from a YAGpulse laser apparatus 3 a and a beam path of a main pulse laser beam from a CO2pulse laser apparatus 3 b may be made to substantially coincide with each other by abeam combiner 15 c. That is, in the first embodiment, the pre-pulse laser beam and the main pulse laser beam are guided into thechamber 1 along substantially the same path. - First, an EUV light emission signal may be inputted to the EUV
light generation controller 7 from theexposure apparatus controller 9. The EUVlight generation controller 7 may be configured to output a YAG laser beam intensity setting signal to the YAGpulse laser apparatus 3 a. Further, the EUVlight generation controller 7 may be configured to output a CO2 laser beam intensity setting signal to the CO2pulse laser apparatus 3 b. - In addition, the EUV
light generation controller 7 may be configured to output an EUV light emission trigger signal to atrigger controller 17. Thetrigger controller 17 may be configured to output a droplet output signal to adroplet controller 8. Thedroplet controller 8 may input the droplet output signal to thetarget supply unit 2, and upon receiving the droplet output signal, thetarget supply unit 2 may output a droplet DL through thetarget nozzle 13. Thetrigger controller 17 may be configured to output a YAG laser oscillation trigger signal to the YAGpulse laser apparatus 3 a. The YAG laser oscillation trigger signal may be outputted such that the droplet DL is irradiated with the pre-pulse laser beam at a timing at which the droplet DL reaches the plasma generation region PS. Further, thetrigger controller 17 may be configured to output a CO2 laser oscillation trigger signal to amaster oscillator 3 d in the CO2pulse laser apparatus 3 b. The CO2 laser oscillation trigger signal may be outputted such that the diffused target is irradiated with the main pulse laser beam after a delay time T from the timing at which the droplet DL is irradiated with the pre-pulse laser beam. Here, the delay time T is a time required for a desired diffused target to be formed. - The YAG
pulse laser apparatus 3 a may be configured to output the pre-pulse laser beam at a first wavelength based on the YAG laser beam intensity setting signal from the EUVlight generation controller 7 and the YAG laser oscillation trigger signal from thetrigger controller 17. The pre-pulse laser beam from the YAGpulse laser apparatus 3 a may be expanded in diameter by abeam expander 4 and then be incident on thebeam combiner 15 c. - The CO2
pulse laser apparatus 3 b may include themaster oscillator 3 d, apreamplifier 3 h, amain amplifier 3 j, and relayoptical systems master oscillator 3 d, thepreamplifier 3 h, and themain amplifier 3 j. Themaster oscillator 3 d may be configured to output a seed beam at a second wavelength based on the CO2 pulse laser oscillation trigger signal. The seed beam from themaster oscillator 3 d may be amplified to desired beam intensity by thepreamplifier 3 h and themain amplifier 3 j based on the CO2 laser beam intensity setting signal. The amplified laser beam may be outputted from the CO2pulse laser apparatus 3 b as the main pulse laser beam and be incident on thebeam combiner 15 c. - The
beam combiner 15 c may be configured to transmit the pre-pulse laser beam at the first wavelength (e.g., 1.06 μm) and reflect the main pulse laser beam at the second wavelength (e.g., 10.6 μm). More specifically, thebeam combiner 15 c may include a diamond substrate on which a multilayer film having the aforementioned reflection/transmission properties for the pre-pulse laser and the main pulse laser is formed. Accordingly, thebeam combiner 15 c may serve to make the beam path of the pre-pulse laser beam and the beam path of the main pulse laser beam coincide with each other and supply the pre-pulse laser beam and the main pulse laser beam into thechamber 1 along the same path. Alternatively, a beam combiner configured to reflect the pre-pulse laser beam at the first wavelength and transmit the main pulse laser beam at the second wavelength may be used to make the respective beam paths coincide with each other. - The
droplet controller 8, the YAGpulse laser apparatus 3 a, and the CO2pulse laser apparatus 3 b may operate in synchronization with one another based on the various signals from thetrigger controller 17. With this, the YAGpulse laser apparatus 3 a may output the pre-pulse laser beam in synchronization with the timing at which the droplet supplied into thechamber 1 from thetarget supply unit 2 reaches a predetermined region. Then, the CO2pulse laser apparatus 3 b may output the main pulse laser beam in synchronization with the timing at which a desired diffused target is formed after the droplet is irradiated with the pre-pulse laser beam. - According to the first embodiment, the pre-pulse laser beam and the main pulse laser beam may be guided to the plasma generation region PS in substantially the same direction (substantially the same path). Thus, a through-hole formed in the
EUV collector mirror 5 may be made small and need not be formed in plurality. - Further, the wavelength (e.g., 1.06 μm) of the pre-pulse laser beam from the YAG
pulse laser apparatus 3 a is equal to or shorter than one-tenth of the wavelength (e.g., 10.6 μm) of the main pulse laser beam from the CO2pulse laser apparatus 3 b. When the wavelength of the pre-pulse laser beam is sufficiently shorter than the wavelength of the main pulse laser beam, the following advantages may be speculated. - (1) The absorptivity of the pre-pulse laser beam by the target material, such as tin, may be higher than that of the main pulse laser beam.
(2) The irradiation spot size of the pre-pulse laser beam focused on the droplet may be reduced.
As a result, a small droplet DL may be irradiated efficiently with the pre-pulse laser beam having small pulse energy and be diffused. -
FIG. 10 schematically illustrates an exemplary configuration of an EUV light generation system according to a second embodiment. In the EUV light generation system according to the second embodiment, the pre-pulse laser beam from the YAGpulse laser apparatus 3 a and the main pulse laser beam from the CO2pulse laser apparatus 3 b are guided into thechamber 1 along separate beam paths. - The pre-pulse laser beam outputted from the YAG
pulse laser apparatus 3 a may be reflected by a high-reflection mirror 15 e and an off-axis paraboloidal mirror 15 g. Then, the pre-pulse laser beam may pass through a through-hole formed in theEUV collector mirror 5, and be focused on a droplet inside thechamber 1 to form a diffused target. - The main pulse laser beam outputted from the CO2
pulse laser apparatus 3 b may be reflected by a high-reflection mirror 15 d and an off-axis paraboloidal mirror 15 f. Then, the main pulse laser beam may pass through another through-hole formed in theEUV collector mirror 5, and be focused on the diffused target inside thechamber 1. - According to the second embodiment, the pre-pulse laser beam and the main pulse laser beam may be guided through separate optical systems to the plasma generation region PS. Accordingly, each of the pre-pulse laser beam and the main pulse laser beam may be focused to have a desired beam spot with ease. Further, an optical element, such as a beam combiner, for making the beam paths of the pre-pulse laser beam and the main pulse laser beam need not be used. Still, the pre-pulse laser beam and the main pulse laser beam may strike the droplet DL and the diffused target respectively in substantially the same direction.
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FIG. 11 schematically illustrates an exemplary configuration of an EUV light generation system according to a third embodiment. In the EUV light generation system according to the third embodiment, a first pre-pulse laser beam from the YAGpulse laser apparatus 3 a and a second pre-pulse laser beam and the main pulse laser beam from the CO2pulse laser apparatus 3 b may be guided into thechamber 1. - The CO2
pulse laser apparatus 3 b may include themaster oscillator 3 d configured to output the seed beam of the main pulse laser beam and amaster oscillator 3 e configured to output a seed beam of the second pre-pulse laser beam. The seed beam of the second pre-pulse laser beam from themaster oscillator 3 e may be amplified by thepreamplifier 3 h and themain amplifier 3 j to desired beam intensity. The amplified seed beam may be outputted from the CO2pulse laser apparatus 3 b as the second pre-pulse laser beam, and then be incident on thebeam combiner 15 c. The seed beam of the main pulse laser beam from themaster oscillator 3 d may also be amplified by thepreamplifier 3 h and themain amplifier 3 j to desired beam intensity. The amplified seed beam may be outputted from the CO2pulse laser apparatus 3 b as the main pulse laser beam, and then be incident on thebeam combiner 15 c. - Each of the
master oscillators master oscillators -
FIGS. 12A through 12F show a droplet DL being irradiated with a first pre-pulse laser beam and a diffused target being irradiated with a second pre-pulse laser beam in the third embodiment. In each ofFIGS. 12A through 12C , the droplet or the diffused target is viewed in a direction (X-direction) perpendicular to the beam axes of the first and second pre-pulse laser beams (Z-direction).FIGS. 12A through 12C respectively show the states of the target material at delay times T=0, T=t2, and T=tm (where, 0<t2<tm) after the droplet is irradiated with the first pre-pulse laser beam. In each ofFIGS. 12D through 12F , the droplet or the diffused target is viewed in the direction of the beam axes of the first and second pre-pulse laser beams (Z-direction).FIGS. 12D through 12F respectively show the states of the target material at delay times T=0, T=t2, and T=tm (where, 0<t2<tm) after the droplet is irradiated with the first pre-pulse laser beam. - When a droplet of the target material shown in
FIGS. 12A and 12D is irradiated with the first pre-pulse laser beam, the droplet may be diffused as shown inFIGS. 12B and 12E so that a first diffused target may be formed. The first diffused target may be irradiated with the second pre-pulse laser beam when the first diffused target is diffused to a desired size that is substantially the same as or smaller than the irradiation spot size of the second pre-pulse laser beam. - When the first diffused target is irradiated with the second pre-pulse laser beam, the first diffused target may be broken into even smaller particles and be diffused to form a second diffused target. The second diffused target may be irradiated with the main pulse laser beam when the second diffused target is diffused to a desired size that is substantially the same as or smaller than the irradiation spot size of the main pulse laser beam.
- Since the second diffused target, which includes smaller particles than those in the first diffused target, is irradiated with the main pulse laser beam, the energy of the main pulse laser beam may be absorbed by the second diffused target efficiently. Because a large portion of the second diffused target may be turned into plasma, a high CE may be obtained. Further, by controlling the irradiation spot size of the main pulse laser beam to substantially coincide with the diffusion diameter of the second diffused target, a high CE and debris reduction may both be achieved.
- Note that, in the third embodiment, a mass limited target (e.g., a molten tin droplet having a diameter of 10 μm) may preferably be used.
- In the third embodiment, the target material is irradiated with the first and second pre-pulse laser beams, and then the diffused target is irradiated with the main pulse laser beam. However, this disclosure is not limited thereto, and the target material may be irradiated with three or more pre-pulse laser beams.
- Further, in the third embodiment, the first pre-pulse laser beam is outputted from the YAG
pulse laser apparatus 3 a, and the second pre-pulse laser beam and the main pulse laser beam are outputted from the CO2pulse laser apparatus 3 b. However, this disclosure is not limited thereto, and all the laser beams may be outputted, for example, from a CO2 laser apparatus. - Alternatively, the first and second pre-pulse laser beams may be outputted from a first laser apparatus, and the main pulse laser beam may be outputted from a second laser apparatus. Here, the first laser apparatus may be a YAG laser apparatus or a fiber laser apparatus, and the second laser apparatus may be a CO2 laser apparatus.
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FIG. 13 schematically illustrates an exemplary configuration of an EUV light generation system according to a modification of the third embodiment. The EUV light generation system shown inFIG. 13 may include a first YAGpulse laser apparatus 3 m, a second YAGpulse laser apparatus 3 n, and abeam combiner 3 p. - The first and second YAG
pulse laser apparatuses light generation controller 7 and the YAG laser oscillation trigger signal from thetrigger controller 17. The first YAGpulse laser apparatus 3 m may be configured to output the first pre-pulse laser beam, and the first pre-pulse laser beam may be incident on thebeam combiner 3 p. The second YAGpulse laser apparatus 3 n may be configured to output the second pre-pulse laser beam, and the second pre-pulse laser beam may also be incident on thebeam combiner 3 p. Thebeam combiner 3 p may be positioned to make the beam paths of the first and second pre-pulse laser beams coincide with each other and output the first and second pre-pulse laser beams toward thebeam expander 4. - Even with this configuration, as in the third embodiment described with reference to
FIG. 11 , the first and second pre-pulse laser beams and the main pulse laser beam may be guided into thechamber 1. Here, the first and second pre-pulse laser beams may respectively be outputted from first and second fiber laser apparatuses. -
FIG. 14 schematically illustrates an exemplary configuration of an EUV light generation system according to a fourth embodiment.FIG. 14 shows a sectional view taken along XIV-XIV plane in any ofFIGS. 9 through 11 and 13. An EUV light generation system according to the fourth embodiment may be similar in configuration to any one of the first through third embodiments but may differ in that the EUV light generation system of the fourth embodiment may further includemagnets magnets chamber 1 and ions generated inside thechamber 1 may be collected by the magnetic field. - Each of the
magnets power source 6 c that is controlled by apower source controller 6 d may be connected to each of themagnets power source controller 6 d may regulate current to be supplied to themagnets power source 6 c so that a magnetic field in a predetermined direction may be generated in thechamber 1. A superconductive magnet, for example, may be used as each of themagnets magnets chamber 1. - Plasma generated when a target material is irradiated with a main pulse laser beam may include positive ions and negative ions (or electrons). The positive and negative ions moving inside the
chamber 1 may be subjected to Lorentz force in the magnetic field, and thus the ions may move in spiral along magnetic lines of force. With this, the ionized target material may be trapped in the magnetic field and collected intoion collection units chamber 1 may be reduced, and deterioration in optical element, such as theEUV collector mirror 5, due to the debris adhering to the optical element may be suppressed. InFIG. 14 , the magnetic field is in the direction shown by an arrow, but a similar function can be achieved even when the magnetic field is oriented in the opposite direction. - A mitigation technique for reducing debris adhering to the optical element is not limited to the use of the magnetic field. Alternatively, a substance deposited onto the
EUV collector mirror 5 may be etched using an etching gas. Debris may be made to react with hydrogen gas (H2) or a hydrogen radical (H) in the magnetic field, and the debris may be removed as a vaporized compound. -
FIG. 15 schematically illustrates an exemplary configuration of a Ti:sapphire laser configured to output the pre-pulse laser beam in an EUV light generation system according to a fifth embodiment. A Ti:sapphire laser 50 a of the fifth embodiment may be provided outside thechamber 1 as a driver laser for outputting the pre-pulse laser beam in any one of the first through fourth embodiments. - The Ti:
sapphire laser 50 a may include a laser resonator formed by a semiconductorsaturable absorber mirror 51 a and anoutput coupler 52 a. Aconcave mirror 53 a, afirst pumping mirror 54 a, a Ti:sapphire crystal 55 a, asecond pumping mirror 56 a, and twoprisms saturable absorber mirror 51 a in the optical path in the laser resonator. Further, the Ti:sapphire laser 50 a may include apumping source 59 a for introducing a pumping beam into the laser resonator. - The
first pumping mirror 54 a may be configured to transmit the pumping beam from the outside of the laser resonator with high transmittance and reflect the laser beam inside the laser resonator with high reflectance. The Ti:sapphire crystal 55 a may serve as a laser medium that undergoes stimulated emission with the pumping beam. The twoprisms output coupler 52 a may transmit a part of the laser beam amplified in the laser resonator and output the amplified laser beam from the laser resonator, and reflect the remaining part of the laser beam back into the laser resonator. The semiconductorsaturable absorber mirror 51 a may have a reflective layer and a saturable absorber layer laminated thereon. A part of an incident laser beam of low beam intensity may be absorbed by the saturable absorber layer, and another part of the incident laser beam of high beam intensity may be transmitted through the saturable absorber layer and reflected by the reflective layer. With this, the pulse duration of the incident laser beam may be shortened. - A semiconductor pumped Nd:YVO4 laser may, for example, be used as the pumping
source 59 a. The second harmonic wave from the pumpingsource 59 a may be introduced into the laser resonator through thefirst pumping mirror 54 a. The position of the semiconductorsaturable absorber mirror 51 a may be adjusted so as to adjust the resonator length for a predetermined longitudinal mode. With this mode-locking of the Ti:sapphire laser 50 a, a picosecond pulse laser beam may be outputted through theoutput coupler 52 a. Here, when the pulse energy is small, the pulse laser beam may be amplified by a regenerative amplifier. - According to the fifth embodiment, a target material may be irradiated with a picosecond pulse laser beam or a pulse laser beam having a shorter pulse duration. When the target material is irradiated with a short pulse laser beam, thermal diffusion at the irradiation portion may be made extremely small. Accordingly, energy that may be diffused can be used for the ablation effect. As a result, according to the fifth embodiment, compared to the nanosecond pulse laser beam, a droplet may be diffused with smaller pulse energy.
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FIG. 16 schematically illustrates an exemplary configuration of a fiber laser configured to output the pre-pulse laser beam in an EUV light generation system according to a sixth embodiment. Afiber laser 50 b of the sixth embodiment may be provided outside thechamber 1 as a driver laser for outputting the pre-pulse laser beam in any one of the first through fourth embodiments. - The
fiber laser 50 b may include a laser resonator formed by a high-reflection mirror 51 b and a semiconductorsaturable absorber mirror 52 b. Agrating pair 53 b, a firstpolarization maintenance fiber 54 b, amultiplexer 55 b, aseparation element 56 b, a secondpolarization maintenance fiber 57 b, and a focusingoptical system 58 b may be provided in this order from the side of the high-reflection mirror 51 b in the beam path in the laser resonator. Further, thefiber laser 50 b may include apumping source 59 b for introducing a pumping beam into the laser resonator. - The
multiplexer 55 b may be configured to introduce the pumping beam from the pumpingsource 59 b to the firstpolarization maintenance fiber 54 b and may transmit a laser beam traveling back and forth between the firstpolarization maintenance fiber 54 b and the secondpolarization maintenance fiber 57 b. The firstpolarization maintenance fiber 54 b may be doped with ytterbium (Yb), and may undergo stimulated emission with the pumping beam. Thegrating pair 53 b may selectively reflect a laser beam at a predetermined wavelength. The semiconductorsaturable absorber mirror 52 b may be similar in configuration and function to the semiconductorsaturable absorber mirror 51 b in the fifth embodiment. Theseparation element 56 b may separate a part of the laser beam amplified in the laser resonator and output the separated laser beam from the laser resonator and return the remaining part of the laser beam back into the laser resonator. This configuration may lead to mode-locking of thefiber laser 50 b. When the pumping beam from the pumpingsource 59 b is introduced into themultiplexer 55 b through an optical fiber, a picosecond pulse laser beam may be outputted through theseparation element 56 b. - According to the sixth embodiment, in addition to the effect similar to that of the fifth embodiment, the target material may be irradiated with the pre-pulse laser beam with high precision since the pre-pulse laser beam is introduced through an optical fiber. Further, generally, in a fiber laser, the M2 value that expresses deviation from an ideal Gaussian distribution of the laser beam intensity distribution is approximately 1.2. The M2 value being closer to 1 means that the focusing performance is high. Accordingly, when a fiber laser is used, a small target may be irradiated with a pre-pulse laser beam with high precision.
- The shorter the wavelength of a laser beam, the higher the absorptivity of the laser beam by tin. Accordingly, when the priority is placed on the absorptivity of the laser beam by tin, a laser beam at a shorter wavelength may be advantageous. For example, compared to the fundamental harmonic wave outputted from an Nd:YAG laser apparatus at a wavelength of 1064 nm, the absorptivity may increase with the second harmonic wave (a wavelength of 532 nm), further with the third harmonic wave (a wavelength of 355 nm), and even further with the fourth harmonic wave (a wavelength of 266 nm).
- Here, an example where a picosecond pulse laser beam is used is shown. However, similar effects can be obtained even with a femtosecond pulse laser beam. Further, a droplet can be diffused even with a nanosecond pulse laser beam. For example, a fiber laser with such specifications as a pulse duration of approximately 15 ns, a repetition rate of 100 kHz, pulse energy of 1.5 mJ, a wavelength of 1.03 μm, and the M2 value of below 1.5 may be used as a pre-pulse laser apparatus.
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FIGS. 17A and 17B are tables showing irradiation conditions of the laser beams in the EUV light generation system in any one of the embodiments. When irradiation pulse energy is E (J), a pulse duration is T (s), and an irradiation spot size is Dm (m), beam intensity W (W/m2) of the laser beam may be expressed inExpression 5 below. -
W=E/(T(Em/2)2π) (5) -
FIG. 17A shows four examples (case 1 through case 4) of irradiation conditions of the pre-pulse laser beam. In thecase 1, the diameter of a molten tin droplet is 60 μm. The irradiation conditions for diffusing such a droplet and generating a desired diffused target may be as follows. For example, when the irradiation spot size Dm is 100 μm, the beam intensity W of the laser beam at 1.6×109 W/cm2 is required. In that case, the irradiation pulse energy E may be set to 1.9 mJ, and the pulse duration T may be set to 15 ns. With such a pre-pulse laser beam, a diffused target as shown inFIG. 3B may be generated. - In the
case 2 shown inFIG. 17A , the diameter of a molten tin droplet is 10 μm (i.e., a mass-limited target). The irradiation conditions for diffusing such a droplet and generating a desired diffused target may be as follows. For example, when the irradiation spot size Dm is 30 μm, the beam intensity W of the laser beam at 1.6×109 W/cm2 is required. In that case, the irradiation pulse energy E may be set to 0.17 mJ, and the pulse duration T may be set to 15 ns. With such a pre-pulse laser beam, a diffused target as shown inFIG. 7B may be generated. - In the
cases FIG. 17A , the laser apparatus as shown inFIG. 15 or 16 is used for outputting the pre-pulse laser beam. Further, in thecases -
FIG. 17B shows four examples (case 1 through case 4) of irradiation conditions of the main pulse laser beam. In thecase 1, the diffusion diameter of a diffused target is 250 μm. Irradiation conditions for turning such a diffused target into plasma may be as follows. For example, when the irradiation spot size Dm is 250 μm, the beam intensity W of the laser beam at 1.0×1010 W/cm2 is required. In that case, the irradiation pulse energy E may be set to 100 mJ, and the pulse duration T may be set to 20 ns. Accordingly, energy required to turn the diffused target into plasma may be supplied to the diffused target. - In the
case 2 shown inFIG. 17B , the diffusion diameter of the diffused target, the irradiation spot size Dm, and the beam intensity W of the laser beam are the same as in thecase 1 shown inFIG. 17B . In that case, the irradiation pulse energy E may be set to 150 mJ, and the pulse duration T may be set to 30 ns. With this, energy required to turn the diffused target into plasma may be supplied to the diffused target. - In the
case 3 shown inFIG. 17B , the diffusion diameter of a diffused target is 300 μm. Irradiation conditions for turning such a diffused target into plasma may be as follows. For example, when the irradiation spot size Dm is 300 μm, the beam intensity W of the laser beam at 1.1×1010 W/cm2 is required. In that case, the irradiation pulse energy E may be set to 200 mJ, and the pulse duration T may be set to 25 ns. Thus, energy required to turn the diffused target into plasma may be supplied to the diffused target. - In the
case 4 shown inFIG. 17B , the diffusion diameter of a diffused target is 200 μm. Irradiation conditions for turning such a diffused target into plasma may be as follows. For example, when the irradiation spot size Dm is 200 μm, the beam intensity W of the laser beam at 1.2×1010 W/cm2 is required. In that case, the irradiation pulse energy E may be set to 200 mJ, and the pulse duration T may be set to 50 ns. With this, energy required to turn the diffused target into plasma may be supplied to the diffused target. - As described above, the beam intensity of the pre-pulse laser beam and the main pulse laser beam may be set by setting the irradiation pulse energy E and the pulse duration T of the laser beam.
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FIG. 18 schematically illustrates an exemplary configuration of an EUV light generation system according to a seventh embodiment. In the EUV light generation system according to the seventh embodiment, the polarization state of the pre-pulse laser beam from afiber laser apparatus 31 may be controlled by apolarization converter 20. Thepolarization converter 20 may be configured to change the polarization state of the pre-pulse laser beam into a state other than the linear polarization. Thepolarization converter 20 may be provided at a predetermined position in a beam path between the driver laser and the plasma generation region PS. In this disclosure, a polarization retarder is also included in the polarization converter. - In the seventh embodiment, the
fiber laser apparatus 31 may include afiber laser controller 31 a and thefiber laser 50 b described with reference toFIG. 16 (the sixth embodiment). A CO2pulse laser apparatus 32 may include a CO2 laser controller 32 a, themaster oscillator 3 d, thepreamplifier 3 h, themain amplifier 3 j, and the relayoptical systems FIG. 9 (the first embodiment). - The EUV
light generation controller 7 may output a fiber laser beam intensity setting signal to thefiber laser controller 31 a. Further, the EUVlight generation controller 7 may output a CO2 laser beam intensity setting signal to the CO2 laser controller 32 a. - The
trigger controller 17 may output a fiber laser oscillation trigger signal to thefiber laser 50 b. Further, thetrigger controller 17 may output a CO2 laser oscillation trigger signal to themaster oscillator 3 d. - The
fiber laser 50 b may be configured to output a pre-pulse laser beam at a first wavelength based on the fiber laser oscillation trigger signal. Thefiber laser controller 31 a may be configured to control the output intensity of thefiber laser 50 b based on the fiber laser beam intensity setting signal. The pre-pulse laser beam from thefiber laser 50 b may be expanded in diameter by thebeam expander 4. Thereafter, the polarization state of the pre-pulse laser beam may be changed by thepolarization converter 20, and then the pre-pulse laser beam may be incident on thebeam combiner 15 c. - The
master oscillator 3 d may be configured to output a seed beam at a second wavelength based on the CO2 laser oscillation trigger signal. The CO2 laser controller 32 a may be configured to control the output intensity of thepreamplifier 3 h and themain amplifier 3 j based on the CO2 laser beam intensity setting signal. The seed beam from themaster oscillator 3 d may be amplified by thepreamplifier 3 h and themain amplifier 3 j to desired beam intensity. - In the seventh embodiment, the
fiber laser 50 b is used to output the pre-pulse laser beam. This disclosure, however, is not limited thereto. For example, a YAG laser or a Ti:sapphire laser may be used to output the pre-pulse laser beam. Alternatively, in a configuration where two-stage irradiation with the first and second pre-pulse laser beams is employed, the first pre-pulse laser beam may be outputted from a fiber laser apparatus capable of achieving a small spot, and the second pre-pulse laser beam may be outputted from a YAG laser apparatus or a Ti:sapphire laser apparatus capable of outputting ultrashort pulse laser beam. Then, the main pulse laser beam may be outputted from a CO2 laser apparatus capable of achieving high power laser beam. That is, a desired number of pre-pulse laser beams may be outputted from a plurality of separate laser apparatuses. Further, in accordance with the state of the diffused target at the time of being irradiated with the second pre-pulse laser beam, the diffused target may be irradiated with a plurality of pre-pulse laser beams respectively at different wavelengths, and with difference spot sizes, energy, and pulse durations. -
FIGS. 19A and 20A are conceptual diagrams showing a droplet being irradiated with a linearly-polarized pre-pulse laser beam.FIGS. 19B and 20B show the simulation result of a droplet being irradiated with a linearly-polarized pre-pulse laser beam. InFIGS. 19A and 19B , the droplet is viewed in a direction (X-direction) perpendicular to the polarization direction of the pre-pulse laser beam. InFIGS. 20A and 20B , the droplet is viewed in a direction of the beam axis (Z-direction) of the pre-pulse laser beam. - With reference to
FIGS. 19A and 20A , a case where a droplet is irradiated with a linearly-polarized pre-pulse laser beam will be discussed. In this case, the droplet may be diffused, and as shown inFIGS. 19B and 20B , a diffused target may be generated. The simulation result reveals that the diffused target is diffused further in a direction (X-direction) perpendicular to the polarization direction (Y-direction) of the pre-pulse laser beam. When the diffused target diffused as such is irradiated with the main pulse laser beam traveling along substantially the same path as the pre-pulse laser beam, as shown inFIGS. 19B and 20B , the shape of the diffused target may differ largely from the cross-sectional shape of the main pulse laser beam. Accordingly, a large portion of the main pulse laser beam may not be used to generate plasma. - Here, a reason why the diffused target is diffused largely in a direction (X-direction) perpendicular to the polarization direction of the linearly-polarized pre-pulse laser beam will be considered.
FIG. 21 is a graph showing absorptivity of a P-polarization component and an S-polarization component of a laser beam incident on the surface of a molten tin droplet. In the case shown inFIG. 21 , the wavelength of the laser beam is 1.06 μm. As shown in the graph, the absorptivity of the laser beam may depend on the angle of incidence and the polarization state of the laser beam. - The absorptivity of the P-polarization component of an incident laser beam is at the highest when the angle of incidence of the laser beam is 80 to 85 degrees, and gradually decreases as the angle of incidence shifts from that angle range. On the other hand, the absorptivity of the S-polarization component is substantially the same as that of the P-polarization component when the laser beam is incident on the surface of the molten tin droplet at substantially 0 degree (i.e., substantially normal incidence), and decreases as the angle of incidence increases. For example, when the angle of incidence is equal to or greater than 80 degrees, the absorptivity of the S-polarization component approximates to 0%.
- Based on such absorptivity properties, it is speculated that energy of the laser beam is absorbed the most where a linearly-polarized laser beam is incident on the surface of the droplet as the P-polarization component at a degree within a range of 80 to 85 degrees. Portions of the droplet where the laser beam is incident thereon as the P-polarization component at an angle within the above range are areas toward the edges of the irradiation surface in the Y-direction (hereinafter, referred to as “laser ablation region”). That is, the absorptivity of the laser beam is high in these areas, and strong laser ablation may occur. As a result of the reaction of the laser ablation in the laser ablation regions, a shock wave may propagate toward the inside of the droplet from the laser ablation regions. This shock wave may propagate toward the edges of the droplet in the X-direction as shown in
FIG. 20A , and the droplet may be diffused in the X-direction as shown inFIG. 20B . - Accordingly, in the seventh embodiment, the polarization state of the pre-pulse laser beam may be changed into a polarization state other than the linear polarization using the
polarization converter 20. Further, by controlling the spot size of the pre-pulse laser beam to be equal to or greater than the diameter (e.g., 40 μm) of the droplet, the entire irradiation surface of the droplet may be irradiated with the pre-pulse laser beam. With this, the droplet may be diffused symmetrically about the beam axis of the pre-pulse laser beam, and the diffused target may be irradiated with the main pulse laser beam efficiently. - The
polarization converter 20 may be configured to change the pre-pulse laser beam into a substantially circularly-polarized laser beam, a substantially unpolarized laser beam, a substantially radially-polarized laser beam, a substantially azimuthally-polarized laser beam, and so forth. -
FIGS. 22A and 22B show a droplet being irradiated with a circularly-polarized pre-pulse laser beam.FIGS. 22C and 22D show a diffused target generated when the droplet is irradiated with the pre-pulse laser beam being irradiated with a main pulse laser beam.FIGS. 22E and 22F schematically show plasma generated when the diffused target is irradiated with the main pulse laser beam. - In a circularly-polarized laser beam, the polarization vector draws a circle on a plane (X-Y plane) perpendicular to the beam axis of the laser beam. Further, the polarization state of the pre-pulse laser beam is circular at any position along the X-Y plane (see
FIGS. 22A and 22B ). In the circularly-polarized laser beam, the ratio of an X-direction polarization component and a Y-direction polarization component is substantially 1:1. When a droplet is irradiated with the circularly-polarized pre-pulse laser beam, the distribution of absorptivity of the pre-pulse laser beam in the surface of the droplet may be symmetrical about the center axis of the droplet in the irradiation direction of the laser beam. As a result, the diffusion state of the droplet may be symmetrical about the center axis of the droplet, and the shape of the diffused target may become disc-like (seeFIGS. 22C and 22D ). This allows the shape of the diffused target to substantially coincide with the cross-sectional shape of the main pulse laser beam so that the main pulse laser beam may be absorbed efficiently by the diffused target. -
FIGS. 23A and 23B show a droplet being irradiated with an unpolarized pre-pulse laser beam.FIGS. 23C and 23D show a diffused target generated when the droplet is irradiated with the pre-pulse laser beam being irradiated with a main pulse laser beam.FIGS. 23E and 23F schematically show plasma generated when the diffused target is irradiated with the main pulse laser beam. - The pre-pulse laser beam shown in
FIG. 23B is substantially unpolarized. In such an unpolarized laser beam, the ratio of the X-direction polarization component and the Y-direction polarization component is substantially 1:1. When a droplet is irradiated with the unpolarized pre-pulse laser beam, the distribution of absorptivity of the pre-pulse laser beam in the surface of the droplet may be symmetrical about the center axis of the droplet in the irradiation direction of the laser beam. As a result, the diffusion state of the droplet may be symmetrical about the center axis of the droplet, and the shape of the diffused target may, for example, become disc-like. Accordingly, the main pulse laser beam may be absorbed by the diffused target efficiently. -
FIGS. 24A and 24B show a droplet being irradiated with a radially-polarized pre-pulse laser beam.FIGS. 24C and 24D show a diffused target generated when the droplet is irradiated with the pre-pulse laser beam being irradiated with a main pulse laser beam.FIGS. 24E and 24F schematically show plasma generated when the diffused target is irradiated with the main pulse laser beam. - When a droplet is irradiated with the radially-polarized pre-pulse laser beam, the distribution of absorptivity of the pre-pulse laser beam in the surface of the droplet may be symmetrical about the beam axis of the pre-pulse laser beam. Here, the beam axis of the pre-pulse laser beam preferably coincides with the center axis of the droplet. As a result, the diffusion state of the droplet may be symmetrical about the beam axis of the pre-pulse laser beam, and the shape of the diffused target may, for example, become disc-like. Accordingly, the main pulse laser beam may be absorbed by the diffused target efficiently.
- Further, when the spot size of the pre-pulse laser beam is controlled to be equal to or greater than the diameter (e.g., 40 μm) of the droplet, the entire irradiation surface of the droplet may be irradiated with the pre-pulse laser beam incident thereon mostly as the P-polarization component. Accordingly, the absorptivity of the pre-pulse laser beam may be increased, and the energy required to generate a desired diffused target may be kept small.
-
FIGS. 25A and 25B show a droplet being irradiated with an azimuthally-polarized pre-pulse laser beam.FIGS. 25C and 25D show a diffused target generated when the droplet is irradiated with the pre-pulse laser beam being irradiated with a main pulse laser beam.FIGS. 25E and 25F schematically show plasma generated when the diffused target is irradiated with the main pulse laser beam. - When a droplet is irradiated with the azimuthally-polarized pre-pulse laser beam, the distribution of absorptivity of the pre-pulse laser beam in the surface of the droplet may be symmetrical about the beam axis of the pre-pulse laser beam. Here, the beam axis of the pre-pulse laser beam preferably coincides with the center axis of the droplet. As a result, the diffusion state of the droplet may be symmetrical about the beam axis of the pre-pulse laser beam, and the shape of the diffused target may, for example, become disc-like. Accordingly, the main pulse laser beam may be absorbed by the diffused target efficiently.
- In the seventh embodiment, the distribution of the absorptivity of the pre-pulse laser beam in the surface of the droplet is made symmetrical about the center axis of the droplet and/or the beam axis of the pre-pulse laser beam by controlling the polarization state of the pre-pulse laser beam. However, this disclosure is not limited thereto. The distribution of the absorptivity of the pre-pulse laser beam in the surface of the droplet need not be perfectly symmetrical about the beam axis, but may be substantially symmetrical. Accordingly, the polarization state of the pre-pulse laser beam may, for example, be elliptical as well.
-
FIG. 26A schematically illustrates an exemplary configuration of a device for measuring the degree of linear polarization. The device may include a polarization prism and a beam intensity detector.FIG. 26B shows the relationship between the rotation angle of the polarization prism and the detection result of the beam intensity detector. - As shown in
FIG. 26A , a linearly-polarized pre-pulse laser beam from thefiber laser 50 b may be changed into an elliptically-polarized laser beam by thepolarization converter 20. This elliptically-polarized laser beam may be focused by a focusingoptical system 41 and made to be incident on thepolarization prism 42. The beam intensity of the laser beam outputted from thepolarization prism 42 may be detected by thebeam intensity detector 43. Thepolarization prism 42 may be formed by bonding two refractive crystals such as calcite. Thepolarization prism 42 may be used to extract a laser beam of a predetermined polarization direction as an output laser beam from an input beam in accordance with the orientation of the bonding surface of the prism. As thepolarization prism 42 is rotated about the beam axis of the pre-pulse laser beam, thepolarization prism 42 may transmit a laser beam polarized in a direction corresponding to the rotation angle. In the description below, it is assumed that thepolarization prism 42 may be an ideal prism having a sufficiently high extinction factor. - As shown in
FIG. 26B , the beam intensity of the output beam from thepolarization prism 42 may change periodically as thepolarization prism 42 is rotated by 180 degrees. Here, as shown in Expression (6), the degree of linear polarization P may be obtained from a maximum value Imax and a minimum value Imin of the beam intensity. -
P=(Imax−Imin)/(Imax+Imin)×100(%) (6) - The degree of linear polarization P measured by the device shown in
FIG. 26A may be substantially 0% for a laser beam of a polarization state that is substantially symmetrical about the beam axis (e.g., circularly-polarized laser beam, unpolarized laser beam, radially-polarized laser beam, azimuthally-polarized laser beam). On the other hand, the degree of linear polarization P may be substantially 100% for a linearly-polarized laser beam. Here, when the degree of linear polarization P is in the following ranges, the diffused target may be formed in a desired shape (e.g., disc-shape). - 0%≦P<30% (preferable range)
- 0%≦P<20% (more preferable range)
- 0%≦P<10% (most preferable range)
- These ranges may be adjusted with the extinction factor of the actually-used
polarization prism 42 taken into consideration. -
FIG. 27 shows a first example of a polarization converter in the seventh embodiment. InFIG. 27 , a quarter-wave plate 21 for converting a linearly-polarized laser beam into a circularly-polarized laser beam may be used as the polarization converter. - The transmissive quarter-
wave plate 21 may be a refractive crystal that provides a phase difference of π/2 between a polarization component parallel to the optic axis of the crystal and a polarization component perpendicular to the optic axis of the crystal. As shown inFIG. 27 , a linearly-polarized laser beam may be converted into a circularly-polarized laser beam when the linearly-polarized laser beam is incident on the quarter-wave plate 21 such that the polarization direction thereof is inclined by 45 degrees with respect to the optic axis of the quarter-wave plate 21. When the polarization direction of the linearly-polarized laser beam is inclined by 45 degrees in the other direction, the rotation direction of the circular polarization is reversed. This disclosure is not limited to the transmissive quarter-wave plate 21, and a reflective quarter-wave plate may be used as well. -
FIGS. 28A through 28C show a second example of a polarization converter in the seventh embodiment.FIG. 28A is a front view of the polarization converter,FIG. 28B is an enlarged fragmentary sectional view of the polarization converter taken along a radial direction plane, andFIG. 28C shows one mode for the use of the polarization converter. InFIGS. 28A through 28C , arandom phase plate 22 for converting a linearly-polarized laser beam into an unpolarized laser beam may be used as the polarization control apparatus. - The transmissive
random phase plate 22 may be a transmissive optical element having a diameter D, on whose input or output surface minute square regions each having a length d on each side are formed by randomly arranged recesses and protrusions. Therandom phase plate 22 may divide an input beam having the diameter D into small square beams each having the length d on each side. With this configuration, therandom phase plate 22 may provide a phase difference of n between a small beam transmitted through aprotrusion 22 a and a small beam transmitted through arecess 22 b. The phase difference n may be provided by setting a step Δt between theprotrusion 22 a and therecess 22 b as in Expression (7) below, where the wavelength of the incident laser beam is A, and the refractive index of therandom phase plate 22 is n1. -
Δt=λ/2(n 1−1) (7) - As shown in
FIG. 28C , the transmissiverandom phase plate 22 may, for example, be provided between the pre-pulse laser apparatus and the focusingoptical system 15. A linearly-polarized laser beam may be incident on therandom phase plate 22, and the laser beam transmitted through therandom phase plate 22 may become unpolarized. Laser beams polarized in directions perpendicular to each other do not interfere. Accordingly, when this laser beam is focused by the focusingoptical system 15, the cross-sectional beam intensity distribution at the focus may not be Gaussian but may be closer to the top-hat distribution. When a droplet is irradiated with such a pre-pulse laser beam, the droplet may be diffused substantially symmetrically about the center axis of the droplet. Accordingly, the diffused target may become disc-shaped, and the main pulse laser beam may be absorbed by the diffused target efficiently. - This disclosure is not limited to the transmissive
random phase plate 22, and a reflective random phase plate may be used instead. Further, theprotrusion 22 a and therecess 22 b may be in any other polygonal shapes, such as a hexagonal shape, a triangular shape. -
FIGS. 29A and 29B show a third example of a polarization converter in the seventh embodiment.FIG. 29A is a perspective view of the polarization converter, andFIG. 29B is a front view of the polarization converter.FIGS. 29A and 29B show an n-dividedwave plate 23 for converting a linearly-polarized laser beam into a radially-polarized laser beam. - The n-divided
wave plate 23 may be a transmissive optical element in which n triangular half-wave plates wave plates - For example, the half-
wave plate 231 and the half-wave plate 233 may be arranged so that their respective optic axes make an angle of 45 degrees. Then, the polarization direction of the linearly-polarized laser beam transmitted through the half-wave plate 231 and the polarization direction of the linearly-polarized laser beam transmitted through the half-wave plate 233 may differ by 90 degrees. In this way, the polarization direction of the incident laser beam may be changed in accordance with an angle formed by the optic axis of the half-wave plate and the polarization direction of the incident laser beam. With this, the polarization directions of the laser beams transmitted through the respective half-wave plates may be changed to predetermined polarization directions. As a result, there-divided wave plate 23 may convert a linearly-polarized laser beam into a radially-polarized laser beam. Further, by changing the arrangement of the half-wave plates in the n-dividedwave plate 23, a linearly-polarized laser beam may be converted into an azimuthally-polarized laser beam as well. -
FIG. 30 shows a fourth example of a polarization converter in the seventh embodiment.FIG. 30 shows aphase compensator 24 a, apolarization rotator 24 b, and atheta cell 24 c for converting a linearly-polarized laser beam into a radially-polarized laser beam. - The
theta cell 24 c may be an optical element into which a twisted nematic (TN) liquid crystal is injected, and the liquid crystal molecules are arranged so as to be twisted from the input side toward the output side. A linearly-polarized laser beam incident on thetheta cell 24 c may be rotated along the twist of the alignment of the liquid crystal molecules, and a laser beam linearly-polarized in a direction inclined with respect to the polarization direction of the input beam may be outputted from thetheta cell 24 c. Accordingly, by setting the twisted angle of the alignment of the liquid crystal molecules in thetheta cell 24 c so as to differ in accordance with the azimuth angle direction, thetheta cell 24 c may convert a linearly-polarized input beam into a radially-polarized output beam. - However, when a linearly-polarized laser beam is converted into a radially-polarized laser beam only with the
theta cell 24 c, the beam intensity may be decreased at a boundary between an upper half and a lower half of the laser beam outputted from thetheta cell 24 c. Accordingly, a phase of the upper half of the laser beam may be shifted by n by thephase compensator 24 a prior to the laser beam being incident on thetheta cell 24 c. InFIG. 30 , the arrows indicate that the phases of the input beam are opposite between the upper and lower halves of the laser beam. The upper half of thephase compensator 24 a may include a TN liquid crystal in which the alignment of the liquid crystal molecules is twisted by 180 degrees from the input side toward the output side. In this way, when a linearly-polarized laser beam in which the phases of the upper and lower halves are opposite is made to be incident on thetheta cell 24 c, laser beams of the same phase may be outputted around the boundary between the upper and lower halves of the output laser beam. With this, the beam intensity may be prevented from being decreased at the boundary between the upper and lower halves of the laser beam outputted from thetheta cell 24 c. - The
polarization rotator 24 b may be configured to rotate the polarization direction of the linearly-polarized input beam by 90 degrees. When a laser beam of which the polarization direction is rotated by 90 degrees is made to be incident on thetheta cell 24 c, thetheta cell 24 c may convert the linearly-polarized laser beam into an azimuthally-polarized laser beam. Thepolarization rotator 24 b may be formed of a TN liquid crystal in which the alignment of the liquid crystal molecules is twisted by 90 degrees from the input side toward the output side. In this case, by controlling the DC voltage applied to thepolarization rotator 24 b so as to switch between a state where the alignment of the liquid crystal molecules are twisted and a state where the alignment is not twisted, switching between a radially-polarized output beam and an azimuthally-polarized output beam may be achieved. - In this way, the conversion of the polarization state may be achieved relatively freely by using the
phase compensator 24 a, thepolarization rotator 24 b, and thetheta cell 24 c. Further, as described with reference toFIGS. 27 through 29B , when the polarization direction is to be changed using a wave plate (phase plate), the wavelength of the laser beam of which the polarization direction is changed may differ depending on the thickness of the wave plate. However, as described with reference toFIG. 30 , when thetheta cell 24 c is used, the polarization direction of an input beam of a relatively broad bandwidth may be changed. Accordingly, using thetheta cell 24 c may make it possible to change the polarization direction even when the bandwidth of the pre-pulse laser beam is broad. -
FIG. 31 schematically illustrates the configuration of an EUV light generation system according to an eighth embodiment. In the EUV light generation system according to the eighth embodiment, the polarization state of a pre-pulse laser beam from thefiber laser apparatus 31 may be controlled by thepolarization converter 20, and this pre-pulse laser beam may be guided into thechamber 1 along a beam path that is different from that of the main pulse laser beam. -
FIGS. 32A through 32C schematically illustrates an exemplary configuration of a laser apparatus configured to output a pre-pulse laser beam in an EUV light generation system according to a ninth embodiment. Alaser apparatus 60 a of the ninth embodiment may be provided outside the chamber 1 (see, e.g.,FIG. 1 ) as a driver laser for outputting a pre-pulse laser beam in any one of the first through fourth embodiments. - As shown in
FIG. 32A , thelaser apparatus 60 a may include a laser resonator that includes areflective polarization converter 61 a and afront mirror 62. Alaser medium 63 may be provided in the laser resonator. Stimulated emission light may be generated from thelaser medium 63 with a pumping beam from a pumping source (not shown). The stimulated emission light may travel back and forth between thepolarization converter 61 a and thefront mirror 62 and be amplified by thelaser medium 63. Thereafter, an amplified laser beam may be outputted from thelaser apparatus 60 a. - The
polarization converter 61 a may be configured to reflect with high reflectance a laser beam of a predetermined polarization direction in accordance with the input position on thepolarization converter 61 a. In accordance with the reflective properties of thepolarization converter 61 a, a radially-polarized laser beam shown inFIG. 32B or an azimuthally-polarized laser beam shown inFIG. 32C may be amplified in the laser resonator. A part of the amplified laser beam may be transmitted through thefront mirror 62 and outputted as the pre-pulse laser beam. - According to the ninth embodiment, a polarization converter may be used as a part of the resonator of the driver laser. With this, a polarization converter need not be provided in a beam path between the driver laser and the plasma generation region PS as in the seventh embodiment.
-
FIGS. 33A through 33C schematically illustrates the exemplary configuration of a laser apparatus configured to output a pre-pulse laser beam in an EUV light generation system according to a modification of the ninth embodiment. Alaser apparatus 60 b of this modification may include a laser resonator that includes arear mirror 61 and areflective polarization converter 62 a. In accordance with the reflective properties of thepolarization converter 62 a, a radially-polarized laser beam shown inFIG. 33B or an azimuthally-polarized laser beam shown inFIG. 33C may be amplified in the laser resonator. A part of the amplified laser beam may be transmitted through thepolarization converter 62 a and outputted as the pre-pulse laser beam. -
FIGS. 34A and 34B show an example of a polarization converter in the ninth embodiment.FIG. 34A is a perspective view of the polarizer, andFIG. 34B is an enlarged fragmentary sectional view of a diffraction grating portion of the polarization converter, taken along the radial direction plane. As shown inFIG. 34A , thereflective polarization converter 61 a may be a mirror on which a concentriccircular diffraction grating 611 is formed. Further, as shown inFIG. 34B , in thepolarization converter 61 a, amultilayer film 612 may be formed on aglass substrate 613, and thediffraction grating 611 may be formed on themultilayer film 612. - When an azimuthally-polarized laser beam is incident on the
polarization converter 61 a configured as such (here, the polarization direction is substantially parallel to the direction of the grooves in the diffraction grating 611), the azimuthally-polarized laser beam may be transmitted through thediffraction grating 611 and propagated to themultilayer film 612. On the other hand, when a radially-polarized laser beam is incident on thepolarization converter 61 a configured as such (here, the polarization direction is substantially perpendicular to the direction of the grooves in the diffraction grating 611), the radially-polarized laser beam may not be transmitted through thediffraction grating 611 and may be reflected thereby. In the ninth embodiment (seeFIGS. 32A through 32C ), using thepolarization converter 61 a configured as such in the laser resonator may make it possible to output a radially-polarized laser beam. - Here, when the grooves in the
diffraction grating 611 are formed radially, thepolarization converter 61 a may reflect an azimuthally-polarized laser beam with high reflectance. In this case, the azimuthally-polarized laser beam may be outputted. Further, forming thediffraction grating 611 on thepolarization converter 62 a of the modification (seeFIGS. 33A through 33C ) of the ninth embodiment may make it possible to output a radially-polarized laser beam or an azimuthally-polarized laser beam. -
FIG. 35 is a graph on which the obtained conversion efficiency (CE) in accordance with a fluence (energy per unit area of a beam cross-section at its focus) of a pre-pulse laser beam is plotted. - The measuring conditions are as follows. A molten tin droplet having a diameter of 20 μm is used as a target material. A laser beam with a pulse duration of 5 ns to 15 ns outputted from a YAG pulse laser apparatus is used as a pre-pulse laser beam. A laser beam with a pulse duration of 20 ns outputted from a CO2 pulse laser apparatus is used as a main pulse laser beam. The beam intensity of the main pulse laser beam is 6.0×109 W/cm2, and the delay time for the irradiation with the main pulse laser beam is 1.5 μs after the irradiation with the pre-pulse laser beam.
- The horizontal axis of the graph shown in
FIG. 35 shows a value in which the irradiation conditions of the pre-pulse laser beam (pulse duration, energy, spot size) are converted into a fluence. Further, the vertical axis shows the CE in the case where the diffused target generated in accordance with the irradiation conditions of the pre-pulse laser beam is irradiated with the above main pulse laser beam. - The measurement results shown in
FIG. 35 reveal that increasing the fluence of the pre-pulse laser beam may improve the CE (approximately 3%). That is, at least in a range where the pulse duration of the pre-pulse laser beam is 5 ns to 15 ns, there is a correlation between the fluence and the CE. - Accordingly, in the above embodiments, the EUV
light generation controller 7 may be configured to control the fluence, instead of the beam intensity, of the pre-pulse laser beam. The measurement results shown inFIG. 35 reveal that the fluence of the pre-pulse laser beam may preferably be in the range of 10 mJ/cm2 to 600 mJ/cm2. The range of 30 mJ/cm2 to 400 mJ/cm2 is more preferable. The range of 150 mJ/cm2 to 300 mJ/cm2 is even more preferable. - From the measurement results where the CE is improved when the fluence of the pre-pulse laser beam is controlled as above, it is speculated that a droplet is diffused in a disc-shape, a dish-shape, or a torus-shape under the above conditions. That is, it is speculated that when a droplet is diffused, the total surface area is increased, the energy of the main pulse laser beam is absorbed efficiently by the diffused target, and as a result, the CE is improved.
-
FIG. 36 is a graph showing the result of an experiment for generating a diffused target in an EUV light generation system. In this experiment, the EUV light generation system of the eighth embodiment is used. The pre-pulse laser beam may be converted into a circularly-polarized laser beam by thepolarization converter 20. The horizontal axis inFIG. 36 shows a time that has elapsed since a droplet is irradiated with a pre-pulse laser beam. The vertical axis shows a diffusion radius of a diffused target generated when the droplet is irradiated with the pre-pulse laser beam. The diffusion radius is a radius of a space where a particle of a predetermined diameter exists. Changes over time in the diffusion radius after the irradiation with the pre-pulse laser beam are plotted for the droplets respectively having diameters of 12 μm, 20 μm, 30 μm, and 40 μm. As seen fromFIG. 36 , the diffusion radius has low dependency on the droplet diameter. Further, the changes over time in the diffusion radius are relatively gradual in 0.3 μs to 3 μs after a droplet is irradiated with a pre-pulse laser beam. It is speculated that the variation in the diffusion radius for each droplet is small during this time period. Accordingly, if the diffused target is irradiated with a main pulse laser beam during this time period, the variation in generated EUV energy may be small between pulses. -
FIG. 37 is a graph on which the obtained conversion efficiency (CE) for the corresponding delay time since a droplet is irradiated with a pre-pulse laser beam until a diffused target is irradiated by a main pulse laser beam is plotted for differing diameters of the droplet. - The measuring conditions are as follows. Molten tin droplets respectively having diameters of 12 μm, 20 μm, 30 μm, and 40 μm are used as the target material. A laser beam with a pulse duration of 5 ns outputted from a YAG pulse laser apparatus is used as a pre-pulse laser beam. The fluence of the pre-pulse laser beam is 490 mJ/cm2. A laser beam with a pulse duration of 20 ns outputted from a CO2 pulse laser apparatus is used as a main pulse laser beam. The beam intensity of the main pulse laser beam is 6.0×109 W/cm2.
- The measurement results shown in
FIG. 37 reveal that the delay time for the irradiation with the main pulse laser beam may preferably be in a range of 0.5 μs to 2.5 μs after the irradiation with the pre-pulse laser beam. However, it is found that the optimum range of the delay time for the irradiation with the main pulse laser beam to obtain a high CE differs depending on the diameter of the droplet. - When the diameter of the droplet is 12 μm, the delay time for the irradiation with the main pulse laser beam may preferably be in a range of 0.5 μs to 2 μs after the irradiation with the pre-pulse laser beam. The range of 0.6 μs to 1.5 μs is more preferable. The range of 0.7 μs to 1 μs is even more preferable.
- When the diameter of the droplet is 20 μm, the delay time for the irradiation with the main pulse laser beam may preferably be in a range of 0.5 μs to 2.5 μs after the irradiation with the pre-pulse laser beam. The range of 1 μs to 2 μs is more preferable. The range of 1.3 μs to 1.7 μs is even more preferable.
- When the diameter of the droplet is 30 μm, the delay time for the irradiation with the main pulse laser beam may preferably be in a range of 0.5 μs to 4 μs after the irradiation with the pre-pulse laser beam. The range of 1.5 μs to 3.5 μs is more preferable. The range of 2 μs to 3 μs is even more preferable.
- When the diameter of the droplet is 40 μm, the delay time for the irradiation with the main pulse laser beam may preferably be in a range of 0.5 μs to 6 μs after the irradiation with the pre-pulse laser beam. The range of 1.5 μs to 5 μs is more preferable. The range of 2 μs to 4 μs is even more preferable.
- In the above description, the driver laser 3 (see
FIG. 1 ) corresponds to a laser beam generation apparatus configured to output a pre-pulse laser beam and a main pulse laser beam. The YAGpulse laser apparatus 3 a (seeFIGS. 9 through 11 ) and the fiber laser apparatus 31 (seeFIGS. 18 and 31 ) correspond to a first pulse laser apparatus. The CO2pulse laser apparatus 3 b (seeFIGS. 9 through 11 ) and the CO2 pulse laser apparatus 32 (seeFIGS. 18 and 31 ) correspond to a second pulse laser apparatus. The EUV light generation controller 7 (seeFIG. 1 ) corresponds to a laser controller. - The above-described embodiments and the modifications thereof are merely examples for implementing this disclosure, and this disclosure is not limited thereto. Making various modifications according to the specifications or the like is within the scope of this disclosure, and other various embodiments are possible within the scope of this disclosure. For example, the modifications illustrated for particular ones of the embodiments can be applied to other embodiments as well (including the other embodiments described herein).
- The terms used in this specification and the appended claims should be interpreted as “non-limiting.” For example, the terms “include” and “be included” should be interpreted as “including the stated elements but not limited to the stated elements.” The term “have” should be interpreted as “having the stated elements but not limited to the stated elements.” Further, the modifier “one (a/an)” should be interpreted as “at least one” or “one or more.”
Claims (13)
1-11. (canceled)
12. A system comprising:
a laser beam apparatus configured to generate a laser beam; and
a target supply unit configured to supply a target material to be irradiated with the laser beam for generating extreme ultraviolet light, wherein
the laser beam apparatus is configured to:
generate a first pre-pulse laser beam with which the target material is irradiated, the first pre-pulse laser beam having a pulse duration of less than 1 ns,
generate a second pre-pulse laser beam with which the target material irradiated with the first pre-pulse laser beam is irradiated, and
generate a main pulse laser beam with which the target material irradiated with the second pre-pulse laser beam is irradiated.
13. The system according to claim 12 , wherein the laser beam apparatus includes a CO2 laser apparatus.
14. The system according to claim 12 , wherein the laser beam apparatus comprises:
a first laser apparatus configured to generate the first pre-pulse laser beam with which the target material is irradiated;
a second laser apparatus configured to generate the second pre-pulse laser beam with which the target material irradiated with the first pre-pulse laser beam is irradiated; and
a third laser apparatus configured to generate the main pulse laser beam with which the target material irradiated with the second pre-pulse laser beam is irradiated.
15. The system according to claim 14 , wherein
the first and second laser apparatuses are configured to generate laser beams each having a first wavelength, and
the third laser apparatus is configured to generate a laser beam having a second wavelength longer than the first wavelength.
16. The system according to claim 15 , wherein
the first laser apparatus is a first YAG laser apparatus;
the second laser apparatus is a second YAG laser apparatus; and
the third laser apparatus is a CO2 laser apparatus.
17. The system according to claim 12 , wherein the laser beam apparatus comprises:
a first laser apparatus configured to generate the first pre-pulse laser beam with which the target material is irradiated and the second pre-pulse laser beam with which the target material irradiated with the first pre-pulse laser beam is irradiated; and
a second laser apparatus configured to generate the main pulse laser beam with which the target material irradiated with the second pre-pulse laser beam is irradiated.
18. The system according to claim 17 , wherein
the first laser apparatus is configured to generate a laser beam having a first wavelength, and
the second laser apparatus is configured to generate a laser beam having a second wavelength longer than the first wavelength.
19. The system according to claim 18 , wherein
the first laser apparatus is a YAG laser apparatus; and
the second laser apparatus is a CO2 laser apparatus.
20. The system according to claim 18 , wherein
the first laser apparatus is a fiber laser apparatus; and
the second laser apparatus is a CO2 laser apparatus.
21. The system according to claim 12 , wherein the laser beam apparatus includes a mode-locked laser apparatus.
22. The system according to claim 21 , wherein the mode-locked laser apparatus is a Ti:sapphire laser.
23. The system according to claim 21 , wherein the mode-locked laser apparatus is a fiber laser.
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PCT/JP2011/052767 WO2011102277A1 (en) | 2010-02-19 | 2011-02-09 | Extreme-ultraviolet light source device and method for generating extreme-ultraviolet light |
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JP5739099B2 (en) * | 2008-12-24 | 2015-06-24 | ギガフォトン株式会社 | Target supply device, control system thereof, control device thereof and control circuit thereof |
NL2003777A (en) * | 2009-01-08 | 2010-07-13 | Asml Netherlands Bv | Laser device. |
US9265136B2 (en) | 2010-02-19 | 2016-02-16 | Gigaphoton Inc. | System and method for generating extreme ultraviolet light |
JP2013004258A (en) * | 2011-06-15 | 2013-01-07 | Gigaphoton Inc | Extreme ultraviolet light generation device and extreme ultraviolet light generation method |
JP6047573B2 (en) | 2011-09-02 | 2016-12-21 | エーエスエムエル ネザーランズ ビー.ブイ. | Radiation source |
US9366967B2 (en) | 2011-09-02 | 2016-06-14 | Asml Netherlands B.V. | Radiation source |
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US9113540B2 (en) | 2015-08-18 |
US9167678B2 (en) | 2015-10-20 |
US20140183379A1 (en) | 2014-07-03 |
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