JP2019168477A - Soft x-ray light source, exposure apparatus, and microscope - Google Patents

Abstract

To provide a soft X-ray light source, in which the conversion efficiency of laser light to soft X-rays is increased.SOLUTION: Provided is a soft X-ray light source, comprising: a laser device 36 in which by irradiating a target 32 with a laser beam 60, a plasma containing element contained in the target is generated, and a soft X-ray 62 is emitted from the plasma; and a chamber 30 in which the target is accommodated inside, and when the laser device irradiates the target with the laser light, the gas pressure of the gas in the inside is set at 10 Pa or more and less than atmospheric pressure.SELECTED DRAWING: Figure 12

Description

  The present invention relates to a soft X-ray light source, an exposure apparatus, and a microscope. For example, the present invention relates to a soft X-ray light source, an exposure apparatus, and a microscope that emit soft X-rays by irradiating a target with laser light.

  Laser-produced plasma (LPP) light sources are known as extreme ultraviolet (EUV) light or soft X-ray light sources (for example, Patent Documents 1 to 4). The LPP light source is a light source that utilizes extreme ultraviolet light or soft X-rays emitted from the plasma by irradiating the target material with laser light to form plasma.

  In an EUV light source, it is known that scattered particles called debris are prevented from contaminating or damaging an optical system by introducing a buffer gas into the chamber (for example, Patent Document 5). It is known that by introducing a buffer gas into a chamber during plasma discharge, ions generated by the plasma are delayed to prevent damage to the optical system or the like (for example, Patent Document 6).

Special Table 2000-509190 JP 2001-35688 A JP 2013-251100 A JP 2013-93308 A JP 2008-41391 A Special table 2014-510404 gazette

  An LPP light source is required to have a high output. For this reason, it is required to increase the conversion efficiency indicating the energy of the soft X-rays emitted with respect to the energy of the irradiated laser beam. Patent Documents 5 and 6 describe introducing a buffer gas into the chamber in order to suppress contamination and damage to the optical system and the like. However, it does not describe a method for increasing the output of soft X-rays radiated from plasma.

  The present invention has been made in view of the above problems, and an object of the present invention is to increase the conversion efficiency from laser light to soft X-rays in a soft X-ray light source.

  The present invention generates a plasma containing an element contained in the target by irradiating the target with a laser beam, and emits soft X-rays from the plasma; and the target is housed inside the laser device. A soft X-ray light source comprising: a chamber in which a gas pressure of the internal gas is 10 Pa or more and less than an atmospheric pressure when the target is irradiated with the laser beam.

  The said structure WHEREIN: The gas pressure of the said inside gas can be set as the structure which is 10 Pa or more and 1000 Pa or less.

  The said structure WHEREIN: The gas pressure of the said inside gas can be set as the structure which is a gas pressure that the plasma sphere formed by the said plasma becomes smaller than the plasma sphere in a vacuum.

  In the above structure, the wavelength of the soft X-ray may be 13 nm or more and 14 nm or less, and the gas may be at least one of a gas containing silicon, a gas containing molybdenum, a helium gas, and a hydrogen gas.

  In the above structure, the wavelength of the soft X-ray may be 6 nm or more and 7 nm or less, and the gas may be at least one of a gas containing carbon, a gas containing boron, a helium gas, and a hydrogen gas.

  In the above configuration, the wavelength of the soft X-ray is 4.5 nm or more and 5 nm or less, and the gas is at least one of a gas containing carbon, a gas containing nitrogen, a helium gas, and a hydrogen gas. it can.

  In the above configuration, the wavelength of the soft X-ray is 2.3 nm or more and 4.5 nm or less, and the gas is at least one of a gas containing oxygen, a gas containing nitrogen, a helium gas, and a hydrogen gas. be able to.

  The present invention is an exposure apparatus including the soft X-ray light source.

  The present invention is a microscope including the soft X-ray light source.

  ADVANTAGE OF THE INVENTION According to this invention, in the soft X-ray light source, the conversion efficiency from a laser beam to a soft X-ray can be improved.

FIG. 1A and FIG. 1B are diagrams showing the relative light emission intensity with respect to the wavelength in Experiment 1. FIG. FIG. 2 is a diagram showing the relative light emission intensity with respect to the relative pressure in Experiment 1. FIG. FIG. 3A and FIG. 3B are diagrams showing the light emission intensity with respect to the wavelength in Experiment 2. FIG. FIG. 4 is a graph showing the light emission intensity with respect to the N ₂ gas pressure in Experiment 2. FIGS. 5A and 5B are diagrams showing the emission intensity with respect to the wavelength in Experiment 3. FIG. FIG. 6 is a graph showing the relative light emission intensity with respect to the wavelength after the absorption correction in Experiment 3. FIGS. 7A and 7B are average images of plasma spheres in Experiment 4. FIG. FIGS. 8A and 8B are diagrams showing gray values for pixels at AB in the images of FIGS. 7A and 7B, respectively. FIG. 9A is a diagram showing the number of charged particles in the Faraday cup with respect to time in Experiment 5, and FIG. 9B is a diagram in which the time in FIG. 9A is converted into energy. FIG. 10A and FIG. 10B are schematic diagrams showing the diffusion process of charged particles from the considered plasma in vacuum. FIG. 11A to FIG. 11C are schematic diagrams showing the diffusion process of charged particles from the plasma in the gas considered. FIG. 12A and FIG. 12B are block diagrams of the soft X-ray light source according to the first embodiment. FIG. 13A and FIG. 13B are diagrams (part 1) showing the line intensity of UTA emission with respect to the wavelength of each element. FIGS. 14A and 14B are diagrams (part 2) showing the line intensity of UTA emission with respect to the wavelength of each element. FIG. 15A and FIG. 15B are diagrams (part 1) showing the transmittance of soft X-rays with respect to the wavelength. FIG. 16A and FIG. 16B are diagrams (part 2) showing the transmittance of soft X-rays with respect to the wavelength. FIG. 17 is a diagram (part 3) illustrating the transmittance of soft X-rays with respect to wavelength. FIG. 18 is a block diagram of an exposure apparatus according to the second embodiment. FIG. 19 is a block diagram of the microscope according to the third embodiment.

First, experiments conducted by the inventors will be described.
The experimental procedure is as follows.
A target is placed in the chamber.
A gas is introduced into the chamber.
The target is irradiated with laser light.
The spectrum of soft X-rays emitted from the target is measured using a spectroscope and a soft X-ray detector.

[Experiment 1]
The conditions of Experiment 1 are shown below.
Laser equipment: Geko XII (Osaka University Laser Energy Research Center)
Laser light wavelength: 1053 nm
Pulse energy: 120J
Pulse width: 500 picoseconds Target: Au (gold) or Al (aluminum) Thin film with a film thickness of 100 μm Chamber gas: N ₂ (nitrogen) gas Spectrometer: Flat-field type grazing incidence spectrometer (grating groove spacing) 2400 / mm)
Distance from target to spectrometer entrance window: 800mm
Distance from spectrometer to detector: 800mm
The target to the detector is filled with gas at a constant pressure.

FIG. 1A and FIG. 1B are diagrams showing the relative light emission intensity with respect to the wavelength in Experiment 1. FIG. The targets in FIGS. 1A and 1B are Au and Al, respectively. The relative light emission intensity is normalized by the light emission intensity at which the gas pressure at which no gas is introduced into the chamber is 0 Pa after correction of soft X-ray absorption by N ₂ gas (hereinafter, absorption correction). The absorption correction was calculated from the soft X-ray transmittance of N ₂ gas when the distance from the target to the detector was 1600 mm. The wavelength was calibrated with the K-shell absorption edge NK (approximately 3.0 nm) attributed to N ₂ gas and the K-shell absorption edge CK (approximately 4.4 nm) attributed to the spectroscopic carbon.

As shown in FIG. 1A, when the target is Au, the relative emission intensity increases as the pressure of N ₂ gas increases at a wavelength of 3 to 6 nm. In particular, the relative emission intensity is large on the long wavelength side of the NK end and the CK end, and the relative emission intensity is about 5 when the gas pressure is 400 Pa. As shown in FIG. 1B, when the target is Al, the gas pressure is 267 Pa, and the relative emission intensity on the long wavelength side at the NK end is 12.

  FIG. 2 is a diagram showing the relative light emission intensity with respect to the relative pressure in Experiment 1. FIG. The relative emission intensity shown is the emission intensity at a wavelength of 3.2 nm. The relative pressure is such that the gas pressure is 67 Pa. As shown in FIG. 2, the relative emission intensity increases linearly with respect to the gas pressure regardless of the type of target.

[Experiment 2]
The conditions of Experiment 2 are shown below.
Laser device: Nd: YAG laser Laser light wavelength: 1064 nm
Pulse energy: less than 1 J Pulse width: 7 nanoseconds Target: Au
Gas in chamber: N ₂ gas Spectroscope: Flat-field type oblique incidence spectroscope (diffraction grating groove interval 2400 lines / mm)
Distance from target to spectrometer and detector: 1000mm
The target to the detector is filled with gas at a constant pressure.

FIG. 3A and FIG. 3B are diagrams showing the light emission intensity with respect to the wavelength in Experiment 2. FIG. FIGS. 3A and 3B show the emission intensities before and after absorption correction, respectively. The emission intensity is an arbitrary unit. The emission intensity after the absorption correction was calculated from the soft X-ray transmittance of N ₂ gas when the distance from the target to the detector was 1000 mm.

  As shown in FIG. 3A, when the wavelength is 2 nm to 4.5 nm and the gas pressure is 50 Pa to 99 Pa, the emission intensity is smaller than that of 0 Pa. When the gas pressure is 125 Pa or higher, the emission intensity is higher than that when the gas pressure is 0 Pa. When the wavelength is 4.5 nm to 7 nm, the emission intensity at a gas pressure of 0 Pa increases as the wavelength increases. As shown in FIG. 3B, after the absorption correction, the emission pressure becomes greater than 0 Pa when the gas pressure is 99 Pa or more. As the gas pressure increases, the emission intensity increases. As described above, when the absorption correction is performed, the emission intensity becomes larger than the gas pressure of 0 Pa when the gas pressure is 100 Pa or more at 3 nm to 6 nm.

FIG. 4 is a graph showing the light emission intensity with respect to the N ₂ gas pressure in Experiment 2. The emission intensity is an integrated value (arbitrary unit) at a wavelength of 2.3 nm to 4.4 nm. The same experiment was performed three times, and the results of the three experiments are indicated by white squares, black circles, and white circles, respectively. As the gas pressure is increased compared to 0 Pa, the emission intensity increases. In particular, when the gas pressure is 50 Pa or more, the emission intensity increases linearly with respect to the gas pressure. In order to increase the emission intensity, the gas pressure is preferably 100 Pa or more, more preferably 150 Pa or more, and further preferably 200 Pa or more.

[Experiment 3]
The conditions of Experiment 3 are shown below.
Laser device: Nd: YAG laser Laser light wavelength: 1064 nm
Pulse energy: less than 0.85 J Pulse width: 8 nanoseconds to 12 nanoseconds Laser beam diameter: 22 μm
Laser light irradiation intensity: 2.0 × 10 ¹³ ± 5 × 10 ¹¹ W / cm ²
Target: W (tungsten) Thin film with a film thickness of 0.3 mm Gas in chamber: He (helium) gas Spectrometer: 166 ° constant angle spectroscope (diffraction grating groove interval 600 / mm)
Distance from target to spectrometer: 1500mm
Distance from target to the entrance slit of the spectrometer: 1000mm
Distance from spectrometer to exit slit: 250mm
A detector (photodiode: EUVUV100 manufactured by IRD) having a sensitivity particularly at a wavelength of 1 nm to 30 nm is provided downstream of the exit slit.
The gas from the target to the entrance slit of the spectroscope is filled with a gas having a substantially constant pressure.

  FIGS. 5A and 5B are diagrams showing the emission intensity with respect to the wavelength in Experiment 3. FIG. FIG. 5A and FIG. 5B show the emission intensities before and after absorption correction, respectively. FIG. 6 is a diagram showing the relative light emission intensity with respect to the wavelength after the absorption correction. The emission intensity is an integrated value of 100 pulses and is an arbitrary unit. The emission intensity after absorption correction was calculated from the soft X-ray transmittance of He gas when the distance from the target to the entrance slit of the spectrometer was 1000 mm.

  As shown in FIG. 5 (a), the emission intensity decreases as the gas pressure increases in the wavelength range of 8 nm to 30 nm. This is because soft X-rays are absorbed by He. As shown in FIG. 5B, when the absorption correction is performed, the emission intensity increases as the gas pressure increases in the wavelength range of 8 nm to 30 nm. The behavior is different when the wavelength is 17 nm or less and 17 nm or more. A broad emission peak is observed at a wavelength of 8 nm to 17 nm. When the gas pressure is 30 Pa or more, the emission intensity in this wavelength region increases linearly with an increase in gas pressure. When the wavelength is from 17 nm to 30 nm, no clear peak structure is observed. The emission intensity in this wavelength region increases exponentially with increasing gas pressure.

  As shown in FIG. 6, when the gas pressure in Experiment 3 is normalized with the emission intensity of 0 Pa, peaks P1 to P4 are observed at wavelengths of about 8.5 nm, 9.5 nm, 10.5 nm, and 20 nm. . As the gas pressure increases, the intensity of peaks P1 to P4 also increases.

The light emission having a wavelength of 12 nm to 17 nm in FIG. 5B is shown in T. Putterich et al., Plasma Phys. Control. Fusion 50 (2008) 085016, from W ²⁴⁺ to W ²⁷⁺ when the plasma temperature is 1 keV. It is considered that the light emission. On the other hand, peaks P1 to P3 in FIG. 6 are respectively W ²⁰⁺ , W ¹⁹⁺ , and W ¹⁸⁺ described in CS Harte, et al., J. Phys. B: At. Mol. Opt. It is considered that the light emission. The peak P4 is considered to be W ²⁰⁺ emission described in C. Suzuki, et al., J. Phys. B: At. Mol. Opt. Phys., 44 175004. Since light emission having a wavelength of 17 nm or more has a different behavior from light emission of 17 nm or less, the light emission mechanism is considered to be different. Light emission having a wavelength of 17 nm or more is considered to be, for example, bremsstrahlung of electrons.

Summarizing the results of Experiment 1 to Experiment 3, when the target in the gas atmosphere is irradiated with laser light, the intensity of the emitted soft X-rays increases. That is, the conversion efficiency is increased. The same phenomenon occurs in Au, Al and W as targets. A similar phenomenon occurs in N ₂ and He as gases. The same phenomenon occurs when the wavelength is in the range of 2 nm to 30 nm.

  The following experiment was conducted in order to investigate the mechanism of the increase in emission intensity due to the introduction of gas into the chamber.

[Experiment 4]
Using the experimental apparatus of Experiment 3, the shutter was opened for 0.1 seconds and the plasma sphere was photographed. 85 photographs were taken for gas pressures of 0 Pa and 60 Pa, respectively. PCA (Principal Component Analysis) processing was performed, and each average image was calculated.

  FIGS. 7A and 7B are average images of plasma spheres in Experiment 4. FIG. 7A and 7B, the left circular white pattern is a plasma sphere. The white pattern on the right is a reflection image of the plasma sphere. In FIG. 7A and FIG. 7B, the gas pressure is 0 Pa and 60 Pa, respectively. When FIG. 7A is compared with FIG. 7B, the plasma sphere appears to be smaller in FIG. 7B.

  FIGS. 8A and 8B are diagrams showing gray values for pixels at AB in the images of FIGS. 7A and 7B, respectively. In FIG. 8A and FIG. 8B, the pixel position where the gray value is half that of white (0.0045) is indicated by a broken line. Assume that the interval between the broken lines corresponds to the diameter of the plasma sphere. The diameter of the plasma sphere with a gas pressure of 60 Pa is about 80% of the gas pressure of 0 Pa. Thus, it can be seen that the plasma sphere contracts due to the introduction of gas.

[Experiment 5]
Using the experimental apparatus of Experiment 3, the velocity of charged particles flying from the plasma was measured when no gas was introduced into the chamber (gas pressure was 0 Pa). A laser beam is incident along the normal direction of the target, and a Faraday cup is arranged at a position with a distance of 115 mm from the irradiation point on a straight line at an angle θ with respect to the normal extending from the laser irradiation point on the target surface. . The time response immediately after laser beam irradiation of incoming charged particles was measured in synchronization with the laser beam pulse.

  FIG. 9A is a diagram showing the number of Faraday cup charged particles with respect to time in Experiment 5. FIG. FIG. 9A shows the number of charged particles obtained from the measured value of the Faraday cup as an absolute value. FIG. 9B is a diagram in which the time in FIG. 9A is converted into energy. The results show that the angle θ is 23.7 °, 43.7 °, 53.7 °, and 76.0 °.

As shown in FIGS. 9A and 9B, four peaks Pa, Pb, Pc, and Pd are observed. The peak symbols Pb, Pc, and Pd in FIGS. 9A and 9B represent the corresponding peaks, respectively. Calculation of the velocity of charged particles from the time of each peak is as follows.
Pa: 3.2 × 10 ⁵ m / sec Pb: 2.5 × 10 ⁴ m / sec Pc: 8.9 × 10 ³ m / sec Pd: 3.3 × 10 ³ m / sec

  When peaks Pa, Pb, Pc and Pd are considered as signals from W ions and converted to energy, they are 50000 eV, 620 eV, 70 eV and 12 eV, respectively. The energy of the peak Pa is about two orders of magnitude higher than the black body radiation temperature peak value (180 eV to 600 eV) expected from the input energy, and is too high for the input energy. This signal is not considered. On the other hand, when the peak Pa is considered as a signal due to electrons and converted to energy, it becomes 0.3 eV, which is a value that can be taken sufficiently as electrons in plasma. From the above, it is considered that the peak Pa corresponds to electrons, and the peaks Pb, Pc and Pd correspond to W ions having different valences.

[Experimental considerations]
In Experiment 1 to Experiment 3, the mechanism by which the emission intensity increases due to the introduction of gas into the chamber is not clear, but based on the results of Experiment 4 and Experiment 5, for example, it is considered as follows.

  From the result of Experiment 5, the diffusion of charged particles from the plasma in vacuum can be considered as follows. FIG. 10A and FIG. 10B are schematic diagrams showing the diffusion process of charged particles from the considered plasma in vacuum. As shown in FIG. 10A, when the target 10 is irradiated with the laser beam 12, a part 10a of the target 10 is melted. A plasma sphere 18 of the material of the target 10 (for example, W) is generated. The plasma sphere 18 includes electrons 14 and target material ions 16. When about picoseconds have elapsed from the irradiation of the laser beam 12, the soft X-ray 20 is emitted from the plasma sphere 18.

  As shown in FIG. 10B, when about microseconds have elapsed from the irradiation of the laser beam 12, charged particles that have obtained energy from the laser beam scatter in a vacuum. When the energy of the laser beam is equally distributed to the charged particles, some of the electrons 14 that have left the constraints of the ions 16 a to 16 d scatter the fastest, and ions 16 a to 16 d having different valences scatter later than the electrons 14. The ions 16a to 16d have different valences, and the speed varies depending on the valence. Thus, the outermost side becomes the electron 14 and the ions 16a to 16d become the inner side.

  Next, from the result of Experiment 4, it is considered that the plasma sphere 18 in the gas contracts as follows. FIG. 11A to FIG. 11C are schematic diagrams showing the diffusion process of charged particles from the plasma in the gas considered. As shown in FIG. 11A, gas molecules 22 exist in the chamber. When the target 10 is irradiated with the laser beam 12, a plasma sphere 18 is generated as in FIG. As shown in FIG. 11B, soft X-rays 20 are radiated from the plasma sphere 18 when about picoseconds have elapsed from the irradiation of the laser beam 12. As shown in FIG. 11C, when nanoseconds elapse after the soft X-rays 20 are emitted, the gas molecules 22 are irradiated to the soft X-rays 20, and the gas molecules 22 are ionized by the photoelectric effect.

  Ions 22 a and electrons 22 b are generated by ionization of the gas molecules 22. Coulomb interaction works between the ions 22 a and the electrons 22 b and the electrons 14. Through this, the electrons 14 are cooled, and the scattering speed of the electrons 14 is suppressed. Similarly, scattering of ions 16 in the plasma sphere 18 is also suppressed. Thereby, as in Experiment 4, it is considered that the plasma sphere 18 contracts in the gas atmosphere. At this time, the electrons 14 and the ions 16 move so as to satisfy the principle of minimum heat generation. As a result, the electrons 14 release energy in the form of light or the like to decrease the movement speed and increase the electron density in the plasma sphere 18. On the other hand, since the ions 16 cannot emit energy in the form of light, a part of the kinetic energy is converted into internal energy and the valence is increased. As a result, the density of the ions 16 increases. In other words, in the compressed plasma sphere 18, it is considered that the electron density and the ion density increase simultaneously.

比例係数ｂは、イオンの振動子強度ｆと電子温度Ｔ_ｅによって定まる。放射する光のエネルギーをΔＥとすると、比例係数ｂは数式２で表される。

プラズマ中での波長λの軟Ｘ線を発生するイオン密度をＮ_{ｉ，ｅｍｉｔ}、レーザ光の持続時間（パルス時間）をｔ_{ｌａｓｅｒ}とおくと、プラズマから放射される波長λの軟Ｘ線のエネルギーＥ（λ）は数式３となる。ｈはプランク定数、νは軟Ｘ線の振動数である。

Consider the energy of soft X-rays 20 emitted from the expanding plasma sphere 18. One ions photon flux _{p rad} to radiation is proportional to the electron density _{n e.} Using the proportionality coefficient b, the photon flux p _rad is expressed by Equation 1.

Proportional coefficient b is determined by the oscillator strength f and the electron temperature T _e of the ion. When the energy of the emitted light is ΔE, the proportional coefficient b is expressed by Equation 2.

When the density of ions that generate soft X-rays with a wavelength λ in plasma is _Ni , _emit , and the duration (pulse time) of laser light is t _laser , the energy of soft X-rays with wavelength λ emitted from the plasma E (λ) is expressed by Equation 3. h is Planck's constant, and ν is the frequency of soft X-rays.

The plasma generated by the pulsed laser beam expands and loses energy. At this time, the radial width L of the plasma emission region is inversely proportional to the plasma absorption coefficient α. The absorption coefficient alpha using proportional coefficient a, degree of ionization Z and electron density _{n ^e} α = _{aZn e} ²
In other words, the width L of the light emitting region is the reciprocal of the absorption coefficient α. Therefore, L is expressed by Equation 4.

数式５の最大値はｔ_{ｌａｓｅｒ}＝ｔ_ｅｘｐのときで、このときは数式３となる。数式３および５におけるイオン密度Ｎ_{ｉ，ｅｍｉｔ}と電子密度ｎ_ｅが増大し、軟Ｘ線の発光強度を増大させるためには、プラズマの膨張を抑制し、イオン密度Ｎ_{ｉ，ｅｍｉｔ}と電子密度ｎ_ｅを増大させればよいことがわかる。 When the plasma expands at the speed V _exp , the soft X-ray emission region expands to double the time t _exp = L / V _exp . For this reason, the amount of light emitted from the ions corresponding to the expansion of the light emitting region loses energy and the number thereof decreases. When the decrease in intensity accompanying the decrease in the number of ions is simply substituted into Equation 2 as t _laser / t _exp , the energy E (λ) of the soft X-ray becomes Equation 5.

The maximum value of Equation 5 is when t _laser = t _exp , and in this case, Equation 3 is obtained. Ion density _{N i} in Equation 3 and _5, increases the _emit an electron density _{n e,} to increase the emission intensity of the soft X-rays, inhibit the expansion of the plasma ion density _{N i, emit} an electron density n It can be seen that _e should be increased.

As described with reference to FIGS. 10A to 11C, the size of the plasma sphere 18 contracts when the target 10 is in a gas atmosphere. This increases the ion density and the electron density. Therefore, the ion density N _i in Equation 3 and _5, increases the _emit an electron density n _e, it is considered that the emission intensity of the soft X-rays is increased. According to this consideration, it can be seen that the increase in emission intensity due to gas does not depend on the type of target, the type of gas, and the type of laser light.

  The embodiment of the present invention will be described based on the above Experiment 1 to Experiment 5 and the consideration thereof.

[Embodiment 1]
FIG. 12A and FIG. 12B are block diagrams of the soft X-ray light source according to the first embodiment. FIG. 12A shows a case where the target is solid, and FIG. 12B shows a case where the target is liquid.

  As shown in FIG. 12A, the soft X-ray light source 100 includes a chamber 30, a target 32, an optical system 34, a laser device 36, a gas supply unit 38, and a vacuum pump 40. The chamber 30 is provided with windows 31a and 31b. The target 32 is installed in the chamber 30. The laser device 36 emits a laser beam 60. The laser beam 60 is applied to the target 32 through the window 31a. Soft X-rays 62 emitted from the target 32 are collected by the optical system 34. The soft X-ray 62 is emitted from the chamber 30 through the window 31b. The gas supply unit 38 supplies gas into the chamber 30 and maintains the gas pressure in the chamber 30 constant. The vacuum pump 40 exhausts the gas in the chamber 30.

  As shown in FIG. 12B, in the soft X-ray light source 102, the target supply unit 42 supplies droplets of the target 32 into the chamber 30. The target 32 falls due to gravity. The laser beam 60 is applied to the target 32 while the target 32 is falling. The target recovery unit 44 recovers the dropped target 32. Other configurations are the same as those in FIG.

  According to the first embodiment, the laser device 36 generates plasma including an element included in the target 32 by irradiating the target 32 with the laser light 60 and emits soft X-rays 62 from the plasma. The chamber 30 accommodates the target 32 therein, and when the laser device 36 irradiates the target 32 with the laser beam 60, the gas pressure of the internal gas is set to 10 Pa or more and less than atmospheric pressure. That is, the gas supply unit 38 supplies the gas so that the periphery of the target 32 has a gas pressure of 10 Pa or more and less than atmospheric pressure when the target 32 is irradiated with the laser beam 60. Thereby, in the soft X-ray light sources 100 and 102, the conversion efficiency from the laser beam 60 to the soft X-ray 62 can be increased.

  Although the example of solid or liquid was demonstrated as the target 32, the target 32 may be gas. When the target 32 is a gas, the gas pressure of the gas inside the chamber containing the target gas may be 10 Pa or more and atmospheric pressure or less. It is preferable that the partial pressure of the gas not including the target gas is 10 Pa or more. Alternatively, plasma may be generated by irradiating a solid or liquid target with laser light in a pre-pulse, making a part or all of the target into plasma, and further irradiating the plasma target with laser light.

The wavelength of the laser beam 60 is not particularly limited as long as it generates plasma. The wavelength of the laser beam 60 can be, for example, about several hundred nm to several tens of μm. As the laser device 36, a YAG laser device or a CO ₂ laser device capable of high output can be used. The laser beam 60 is a pulsed laser beam. The pulse width is preferably longer than the time for the soft X-rays 20 to reach the gas molecules 22 in FIG. Assuming that the range in which the gas molecules 22 exist is about 30 cm, the pulse width is preferably 1 nanosecond or more, more preferably 2 nanoseconds or more, and further preferably 5 nanoseconds or more. The pulse width is preferably 500 nanoseconds or less, and more preferably 100 nanoseconds or less.

  The wavelength of the soft X-ray 62 is generally 0.3 nm to 200 nm. In Experiment 1 to Experiment 3, the wavelength of the soft X-ray is 2 nm to 30 nm. The wavelength range of 2 nm to 30 nm is also called extreme ultraviolet light. The wavelength of the soft X-ray can be arbitrarily set according to the purpose.

  Consider a preferred material for the target 32. The light emission mechanism from plasma is roughly classified into three. The first is blackbody radiation. Blackbody radiation does not depend on the target type. The emission intensity can be increased by increasing the plasma temperature and / or plasma density. The second is light emission by braking radiation. Braking radiation is generated when acceleration is applied to charged particles by Coulomb force between charged particles, such as when electrons in plasma pass around ions. Braking radiation does not depend on the type of target.

  The third is light emission by electronic transition. Light emission by electronic transition is most easily used because of its high intensity even at short wavelengths. Since the ionization energy differs depending on the atomic number and the valence of ions, a plurality of emission lines can be observed in the soft X-rays emitted from the plasma. The energy of the emission lines from the K-shell, L-shell and M-shell in monovalent ions from Li (lithium) with atomic number 3 to 94 Am (americium) is, for example, A. Thompson et al., "X-ray Data Booklet "(Lawrence Berkely National Laboratory, Univ. Of Cal., Berkeley, CA 94720 (2009)) Section 1.2. When the atomic number is increased, a large number of transitions are generated from multivalent ions, and a wide emission called UTA (Unresolved Transition Array) is exhibited. In order to increase the emission intensity, UTA emission is preferably used.

  UTA luminescence from each element is described in T Higashiguchi, et al., J. Physics: Conf. Series 463 (2013) 012024. FIG. 13A to FIG. 14B are diagrams showing the line intensity of UTA emission with respect to the wavelength of each element described in this document. FIG. 13A shows In (indium), Sn (tin), Sb (antimony), Te (tellurium), I (iodine), Xe (xenon), Cs (cesium) having atomic numbers of 49 to 59, It is a figure which shows the linear intensity | strength in the range whose wavelength in Ba (barium), La (lanthanum), Ce (cerium), and Pr (praseodymium) is 7 nm to 26 nm. FIG. 13B shows Nd (neodymium), Pm (promethium), Sm (samarium), Eu (europium), Gd (gadolinium), Tb (terbium), Dy (dysprosium) having atomic numbers of 60 to 70, It is a figure which shows the linear intensity | strength in the range whose wavelength in Ho (holmium), Er (erbium), Tm (thulium), and Yb (ytterbium) is 4 nm to 13 nm.

  FIG. 14A shows Lu (lutetium), Hf (hafnium), Ta (tantalum), W (tungsten), Re (rhenium), Os (osmium), Ir (iridium) having atomic numbers of 71 to 81, It is a figure which shows the linear intensity | strength in the range whose wavelength in Pt (platinum), Au (gold), Hg (mercury), and Tl (thallium) is 3 nm to 9 nm. FIG. 14B shows Pb (lead), Bi (bismuth), Po (polonium), At (astatin), Rn (radon), Fr (francium), Ra (radium) having atomic numbers of 82 to 92, It is a figure which shows the linear intensity | strength in the range whose wavelengths in Ac (actinium), Th (thorium), Pa (protoactinium), and U (uranium) are 1 nm to 8 nm.

  As shown in FIGS. 13 (a) to 14 (b), the UTA emission wavelength decreases as the atomic number increases. The material of the target 32 can be appropriately selected according to the emission wavelength. The target 32 is preferably solid or liquid in the chamber 30. From the viewpoint of the wavelength of UTA emission and the target 32 being solid or liquid, the material of the target 32 is preferably as follows.

  As an application using a soft X-ray light source, there is EUVL (Extreme ultraviolet lithography). For EUVL, soft X-ray having a wavelength of around 13.5 nm, for example, 13 nm to 14 nm is used. In FIG. 13A, the wavelength of 13.5 nm is indicated by a solid line, and the wavelengths of 13 nm and 14 nm are indicated by dotted lines. As a target material having a wavelength range of 13 nm to 14 nm, In, Sn, Sb, Te, I or the like is preferable.

  Further, as the next generation EUVL, a wavelength of 6 nm to 7 nm has been studied. In FIG. 13B, wavelengths 6 nm and 7 nm are indicated by dotted lines. Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb or the like is preferable as a target material having a wavelength range of 6 nm to 7 nm.

  Furthermore, the range of 4.5 nm to 5 nm is called a carbon window. This range is a range on the long wavelength side of the K-shell absorption edge of carbon. Because of the longer wavelength than the K-shell absorption edges of oxygen, nitrogen and carbon, it is relatively transparent to all biological elements. For this reason, it becomes a soft X-ray suitable for observation of light element compounds such as cells and polymers. In FIG. 14A, the wavelength of 4.5 nm and the wavelength of 5.0 nm are shown by a one-dot chain line. As a target material in this wavelength range, Lu, Hf, Ta, W, Re, Os, Ir, Pt, Au, Hg, Tl, or the like is preferable.

  Furthermore, the range from 2.3 nm to 4.5 nm is called the water window. This range is the range between the K-shell absorption edges of carbon and oxygen. Since it has a shorter wavelength than the K-shell absorption edge of oxygen, it is transparent to water and opaque to carbon. For this reason, it becomes a soft X-ray suitable for observation of cells or the like. The wavelengths 2.3 nm and 4.5 nm are shown by dotted lines in FIGS. As a target material in this wavelength range, Lu, Hf, Ta, W, Re, Os, Ir, Pt, Au, Hg, Tl, Pb, Bi, or the like is preferable.

  Next, a material preferable as a gas will be considered. As described above [Experimental considerations], the increase in emission intensity does not depend on the type of gas. However, a gas that absorbs soft X-rays is not preferable. The transmittance increases on the long wavelength side of the absorption edge. For this reason, it is preferable to select the gas so that the absorption edge is on the short wavelength side of the soft X-ray wavelength.

Further, as shown in [Experimental considerations], electrons scattered from the plasma are cooled by the gas and the plasma sphere 18 is compressed, so that the emission intensity increases. For this reason, when the electrons are cooled more strongly, the plasma sphere 18 is compressed smaller. The degree of electron cooling is proportional to the internal energy of the gas. For example, when comparing N ₂ are He and diatomic molecule gas is a single atomic gases, poor towards N ₂ is the spatial symmetry of the molecule, a large energy due to the rotation of the reason molecules, the internal energy as a result growing. Therefore, it is considered that at a wavelength where He and N ₂ have the same transmittance, electrons scattered from the plasma are cooled more strongly by N ₂ and, as a result, the emission intensity is increased. For this reason, it is preferable to select a gas composed of molecules with poorer spatial symmetry, in addition to the absorption edge being on the short wavelength side of the soft X-ray wavelength. From the viewpoint of spatial symmetry, a polyatomic gas is preferable to a monoatomic gas such as a rare gas. In addition, a gas containing a plurality of types of elements is preferable to a single element gas such as H ₂ gas and N ₂ gas.

  The gases shown in terms of the absorption edge and spatial symmetry can be mixed and used when they are not reactive with each other. As a result, when a gas that can be expected to have a stronger electron cooling effect cannot be used for safety reasons, it can be used by changing to a safer state by mixing.

  In the wavelength range of 15 nm or more, He without an absorption edge or Kr (krypton) having an M-shell absorption edge of about 13.2 nm is preferable.

In the wavelength range of about 12.3 nm to 15 nm, a gas containing Si (silicon) having an L shell absorption edge of 12.4 nm is preferable. Examples of the gas containing Si include SiCl ₄ , SiF ₄ , SiH ₄ , SiH ₃ (CH ₃ ), SiH ₂ (CH ₃ ) ₂ , SiH (CH ₃ ) ₃ , Si ₂ H ₆ , SiHCl ₃ , SiH ₂ Cl _2. Or a silane-based gas such as Si (OCH ₃ ) ₄ .

In the wavelength range of 10 nm to 12.3 nm, a gas containing P (phosphorus) having an L-shell absorption edge of about 9.2 nm is preferable. The gas containing P is, for example, PF ₃ , PF ₅ , PCl ₃ , PCl ₅ , PH _3, or POCl ₃ .

In the wavelength range of 9 nm to 12 nm, a gas containing Sn (tin) having an N-shell absorption edge of 9.0 nm is preferable. The gas containing Sn is, for example, SnH ₄ or SnCl ₄ .

In the wavelength range of 7.6 nm to 10 nm, a gas containing S (sulfur) having an L-shell absorption edge of about 7.6 nm is preferable. The gas containing S is, for example, S ₂ H, SF ₆ or SO ₂ .

In the wavelength range of 6.6 nm to 7.6 nm, a gas containing B (boron) having a K-shell absorption edge of 6.6 nm is preferable. The gas containing B is, for example, BBr ₃ , BCl ₃ , BF ₃ or B ₂ H ₆ .

In a wavelength range from 5.4 nm to 20 nm, a gas containing Mo (molybdenum) having an N-shell absorption edge of 5.4 nm is preferable. The gas containing Mo is, for example, MoF ₆ or MoCl ₅ .

In the wavelength range of 4.5 nm to 6.6 nm, a gas containing C (carbon) having a K-shell absorption edge of about 4.4 nm is preferable. Gas containing C, for example _{CH 4, C 2 H 2,} C 2 H 4, C 2 H 8, C 3 H 6, C 4 H 6, C 4 H 10, C 3 H 8, C 3 H 6, C ₆ H ₆ , (CH ₃ ) ₂₀ , CO ₂ or fluorocarbons.

In the wavelength range of 3.4 nm to 4.5 nm, a gas containing N (nitrogen) having a K-shell absorption edge of about 3.0 nm is preferable. The gas containing N is, for example, N ₂ , ND ₃ , NF ₃ , NH ₃ , NO, N ₂ O, NO _2, or N (CH ₃ ) ₃ .

In the wavelength range of 2.3 nm to 3.4 nm, a gas containing O (oxygen) having a K-shell absorption edge of about 2.3 nm is preferable. The gas containing O is, for example, CO ₂ , NO, N ₂ O, NO ₂ , SO ₂ or fluorocarbons.

Below 2.3 nm, the transmittance is high for many gases. For example, an inert gas can be used. The inert gas is, for example, He (helium), Ne (neon), argon (Ar), Kr (krypton), Xe (xenon), Rn (radon), N ₂ , CO ₂ or fluorocarbons.

  A gas containing an element having no clear absorption edge can be used regardless of the wavelength of soft X-rays. An element that does not have a clear absorption edge is, for example, He or H (hydrogen).

  From the above, when the wavelength range of soft X-rays is 13 nm or more and 14 nm or less, the gas is preferably at least one of a gas containing silicon, a gas containing Mo, a helium gas, and a hydrogen gas.

  When the soft X-ray wavelength range is 6 nm or more and 7 nm or less, the gas is preferably at least one of a gas containing carbon, a gas containing boron, a helium gas, and a hydrogen gas.

  When the soft X-ray wavelength range is 4.5 nm or more and 5 nm or less, the gas is preferably at least one of a gas containing carbon, helium gas, and hydrogen gas.

  When the wavelength range of soft X-rays is 2.3 nm or more and 4.5 nm or less, the gas is preferably at least one of a gas containing oxygen, a gas containing nitrogen, helium gas, and hydrogen gas.

  Next, a preferable gas pressure will be considered. The lower limit of the gas pressure is such a gas pressure that the light emission intensity of soft X-rays is higher than in vacuum. Such a gas pressure is realized by setting the gas pressure so that the plasma sphere formed by the plasma is smaller than the plasma sphere in the vacuum, as shown in FIGS. it can. The size (diameter) of the plasma sphere in the gas atmosphere is preferably 95% or less, more preferably 90% or less of the plasma sphere in vacuum.

  From FIG. 1A, FIG. 4 and FIG. 6, the gas pressure is preferably 10 Pa or more, more preferably 50 Pa or more, and even more preferably 100 Pa or more.

The upper limit of the gas pressure is in a range where soft X-ray absorption is small. The intensity I (λ, p) of the soft X-ray after passing through the gas is a function of the wavelength λ of the soft X-ray and the gas pressure p. If the emission intensity I ₀ (λ, 0) in vacuum, the amplification factor α (λ, p) of the emission intensity due to the introduction of gas, and the transmittance T (λ, p) of soft X-rays in the gas, the intensity I (λ, p) is expressed by the following equation.
I (λ, p) = I ₀ (λ, 0) × α (λ, p) × T (λ, p)
From Experiment 1 to Experiment 3, assuming that α (λ, p) is about 10, in order to make I (λ, p) larger than I ₀ (λ, 0), T (λ, p) is 0.1. The above is preferable. In order to further increase I (λ, p), it is preferable that T (λ, p) is 0.6 or more.

  The transmittance T (λ, p) depends on the distance that the soft X-ray passes through the gas. In order to prevent soft X-rays from being absorbed by gas, it is effective to shorten the distance that soft X-rays pass through the gas. However, considering the optical system and the like, this distance is at least about 10 cm. Therefore, the transmittance when soft X-rays passed through 10 cm of gas was calculated by changing the gas pressure of various gases.

FIG. 15A to FIG. 17 are diagrams illustrating the soft X-ray transmittance with respect to the wavelength, and are diagrams illustrating the soft X-ray transmittances of N ₂ , CHClF ₂ , SiH ₄ , Kr, and He, respectively. N ₂ , CHClF ₂ , SiH ₄ and Kr are examples of gas having an absorption edge, and He is an example having no absorption edge.

As shown in FIG. 15A, in N ₂ , the vicinity of 3.0 nm is the N K-shell absorption edge. In order to set the transmittance to 0.1 or more and 0.6 or more in almost all wavelength ranges, the gas pressure of N ₂ gas is about 100 Pa or less and about 30 Pa or less, respectively. When the gas pressure is 1000 Pa, the wavelength range where the transmittance is 0.1 or more is 1.5 nm or less, or 3 nm to 5 nm. When the gas pressure is 5000 Pa, the wavelength range where the transmittance is 0.1 or more is 1 nm or less, or 3 nm to 3.6 nm.

  The gas pressure is preferably about 1000 Pa or less and about 700 Pa or less, respectively, in order to set the transmittance to 0.1 or more and 0.6 or more in the vicinity of the water window on the long wavelength side of the N K-shell absorption edge. In order to set the transmittance to 0.1 or more and 0.6 or more in the wavelength range of 6 nm to 7 nm, the gas pressure is preferably about 800 Pa or less and about 500 Pa or less, respectively. In order to set the transmittance to 0.1 or more and 0.6 or more in the wavelength range of 13 nm to 14 nm, the gas pressure is preferably about 300 Pa or less and about 80 Pa or less, respectively.

As shown in FIG. 15B, in CHClF ₂ , the K-shell absorption edge of F is about 1.8 nm, the K-shell absorption edge of C is about 4.4 nm, and the L-shell absorption edge of Cl is about 6.2 nm. Exists. In order to set the transmittance to 0.1 or more and 0.6 or more in almost all wavelength ranges, the gas pressure of the CHClF ₂ gas is about 50 Pa or less and about 10 Pa or less, respectively. The gas pressure is preferably about 200 Pa or less and about 50 Pa or less, respectively, in order to make the transmittance in the vicinity of the water window 0.1 or more and 0.6 or more. In order to set the transmittance to 0.1 or more and 0.6 or more in the wavelength range of 6 nm to 7 nm, the gas pressure is preferably about 200 Pa or less and about 100 Pa or less, respectively. In order to set the transmittance to 0.1 or more and 0.6 or more in the wavelength range of 13 nm to 14 nm, the gas pressure is preferably about 100 Pa or less and about 30 Pa or less, respectively.

As shown in FIG. 16A, in SiH ₄ , the vicinity of 12.4 nm is the L shell absorption edge of Si. In order to set the transmittance to 0.1 or more and 0.6 or more in the wavelength range of 13 nm to 14 nm, the gas pressure of SiH ₄ is preferably about 2000 Pa or less and about 500 Pa or less, respectively.

  As shown in FIG. 16B, in Kr, the vicinity of 13.2 nm is the M shell absorption edge. In order for the transmittance to be 0.1 or more and 0.6 or more in the wavelength range of 14 nm to 20 nm, the gas pressure is preferably about 1000 Pa or less and about 400 Pa or less, respectively.

  As shown in FIG. 17, there is no clear absorption edge in He. In the water window, the gas pressure is 5000 Pa or less and the transmittance is 0.6 or more. In the wavelength range of 6 to 7 nm, the transmittance is 0.1 or more at 5000 Pa or less. In the wavelength range of 13 nm to 14 nm, the gas pressure is 500 Pa or less and the transmittance is 0.6 or more.

  As shown in FIGS. 15 (a) to 16 (b), in a gas having an absorption edge, when the wavelength of soft X-rays is located on the long wavelength side of the absorption edge, the gas pressure is preferably 1000 Pa or less, more preferably 500 Pa or less. Preferably, 200 Pa or less is more preferable. As shown in FIG. 17, in a gas having no K-shell absorption edge, the gas pressure may be less than atmospheric pressure in the vicinity of the water window. The gas pressure may be 1000 Pa or less even in the wavelength range of 13 nm to 14 nm.

  From the above, the gas pressure around the target 32 is 10 Pa or more and less than atmospheric pressure, preferably 10 Pa or more and 1000 Pa or less, more preferably 50 Pa or more and 500 Pa or less.

[Embodiment 2]
The second embodiment is an exposure apparatus using the soft X-ray light source of the first embodiment. FIG. 18 is a block diagram of an exposure apparatus according to the second embodiment. As shown in FIG. 18, the exposure apparatus 104 includes a soft X-ray light source 100 or 102, a chamber 50, and optical systems 52 and 54. The chamber 50 is maintained in a vacuum having a degree of vacuum that does not absorb soft X-rays. The optical system 52 condenses the soft X-rays 62 emitted from the soft X-ray light source 100 or 102 on the mask 56. The optical system 54 focuses the soft X-rays reflected by the mask 56 on the wafer 58. As a result, the pattern on the mask 56 is transferred to the wafer 58.

  For example, 250 W is required for the output of the soft X-ray light source in EUVL using a wavelength of 13.5 nm. By using the soft X-ray light source according to the first embodiment, the conversion efficiency can be increased and the output of the soft X-ray light source can be increased.

  As in the second embodiment, the soft X-ray light source 100 or 102 according to the first embodiment can be used in an exposure apparatus.

[Embodiment 3]
The third embodiment is a microscope using the soft X-ray light source of the first embodiment. FIG. 19 is a block diagram of the microscope according to the third embodiment. As shown in FIG. 19, the microscope 105 includes a soft X-ray light source 100 or 102, a chamber 70, optical systems 72 and 74, and a two-dimensional detector 76. The inside of the chamber 70 is maintained in a vacuum having a degree of vacuum that does not absorb soft X-rays. The optical system 72 condenses the soft X-rays 62 emitted from the soft X-ray light source 100 or 102 onto the sample 78 that is desired to be observed as the soft X-rays 63. The soft X-ray 64 transmitted or reflected by the sample 78 is collected by the optical system 74 on the two-dimensional detector 76. Thereby, a transmission image or a reflection image of the sample is enlarged and projected onto the two-dimensional detector 76.

  Microscopes using soft X-rays have a spatial resolution that is more than an order of magnitude higher than conventional visible light microscopes, and the element distribution of substances can be obtained without staining by the inner shell absorption edge. It is requested. However, since a strong soft X-ray light source is required, a method using radiant light as a light source has been mainly used. As in the third embodiment, the soft X-ray light source 100 or 102 according to the first embodiment can be used in a soft X-ray microscope.

  The soft X-ray light source 100 or 102 according to the first embodiment can also be used for measuring instruments such as an emission spectroscopic device, an absorption spectroscopic meter, or a photoelectron spectroscopic device in addition to the exposure apparatus and the soft X-ray microscope. For example, biological cells can be observed and analyzed by applying the soft X-ray light source of Embodiment 1 to the wavelength of a water window or a carbon window.

  The preferred embodiments of the present invention have been described in detail above. However, the present invention is not limited to the specific embodiments, and various modifications and changes can be made within the scope of the gist of the present invention described in the claims. It can be changed.

10, 32 Target 12, 60 Laser light 20, 62 Soft X-ray 30 Chamber 34 Optical system 36 Laser device 38 Gas supply unit 40 Vacuum pump

Claims (9)

A laser device that irradiates a target with laser light to generate a plasma containing an element contained in the target, and emits soft X-rays from the plasma;
A chamber in which the target is housed and the gas pressure of the internal gas is 10 Pa or more and less than atmospheric pressure when the laser device irradiates the target with the laser beam;
A soft X-ray light source characterized by comprising:   2. The soft X-ray light source according to claim 1, wherein a gas pressure of the internal gas is 10 Pa or more and 1000 Pa or less.   3. The soft X-ray light source according to claim 1, wherein a gas pressure of the gas inside the gas is such that a plasma sphere formed by the plasma is smaller than a plasma sphere in a vacuum. The wavelength of the soft X-ray is 13 nm or more and 14 nm or less,
The soft X-ray light source according to any one of claims 1 to 3, wherein the gas is at least one of a gas containing silicon, a gas containing molybdenum, a helium gas, and a hydrogen gas. The wavelength of the soft X-ray is 6 nm or more and 7 nm or less,
The soft X-ray light source according to any one of claims 1 to 3, wherein the gas is at least one of a gas containing carbon, a gas containing boron, a helium gas, and a hydrogen gas. The wavelength of the soft X-ray is 4.5 nm or more and 5 nm or less,
The soft X-ray light source according to any one of claims 1 to 3, wherein the gas is at least one of a gas containing carbon, a gas containing nitrogen, a helium gas, and a hydrogen gas. The wavelength of the soft X-ray is 2.3 nm or more and 4.5 nm or less,
The soft X-ray light source according to any one of claims 1 to 3, wherein the gas is at least one of a gas containing oxygen, a gas containing nitrogen, a helium gas, and a hydrogen gas.   An exposure apparatus including the soft X-ray light source according to claim 1. A microscope including the soft X-ray light source according to any one of claims 1 to 7.

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