JP2012147022A - Extreme ultraviolet light source apparatus - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an extreme ultraviolet light source apparatus which prevents damage to a driver laser by reflected light (return light), which returns to the driver laser after being reflected by a target body or plasma.SOLUTION: An extreme ultraviolet (EUV) light source apparatus comprises: a target supply section which supplies a target in a chamber; a laser condensing optical system which generates plasma by condensing a laser beam outputted from a driver laser to the target; an EUV condensing optical system which condenses EUV light emitted from the plasma and outputs the same; a retarder which reflects the laser beam which has passed a predetermined optical path while converting a first linear polarization component thereof into a circular polarization component, and reflects the return light, which is generated by the condensed laser beam being reflected by the target or the plasma, while converting a circular polarization component thereof into a second linear polarization component; and a coating mirror which is arranged on the optical path of the laser beam, reflects the first linear polarization component and absorbs the second linear polarization component.

Description

  The present invention relates to an extreme ultra violet (EUV) light source device used as a light source of an exposure apparatus.

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

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

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

  In such an LPP EUV light source device, a short pulse laser is generally used as a driving light source (driver laser). This is because the short pulse laser is suitable for obtaining high CE (conversion efficiency: conversion efficiency from irradiation laser light to EUV light) in the LPP type EUV light source device.

  FIG. 22 is a schematic diagram showing a configuration example of a driver laser used in the LPP type EUV light source device. The driver laser 13 includes an oscillator 13a that generates laser light and an amplifier 13b that amplifies the laser light generated by the oscillator 13a. A laser having such a configuration is called an oscillation amplification type laser.

Here, when the amplifier 13b does not have an optical resonator, the laser system having such a configuration is called a MOPA (Master Oscillator Power Amplifier) system. The amplifier 13b includes carbon dioxide (CO ₂ ), nitrogen (N ₂ ), helium (He), and, if necessary, CO containing hydrogen (H ₂ ), carbon monoxide (CO), xenon (Xe), and the like. _{2 It} has the discharge device which excites laser gas by discharge. Note that, unlike the amplifier 13b shown in FIG. 22, when a resonator is provided in the amplification stage, laser oscillation by the amplification stage alone is possible. A laser system having such a configuration is called a MOPO (Master Oscillator Power Oscillator) system.

  The laser beam having the energy A emitted from the oscillator 13a is amplified to the laser beam having the desired energy B by the amplifier 13b. The laser beam having this energy B is condensed by a laser condensing optical system and irradiated onto the target material. Here, in FIG. 22, only one stage of amplifier is provided in order to amplify the laser energy A to the laser energy B. However, when desired laser energy B cannot be obtained, a plurality of stages of amplifiers are used. May be.

Next, a configuration example of a short pulse CO ₂ laser used as the oscillator 13a will be described. Patent Document 1 discloses a configuration of a short pulse RF (Radio Frequency excitation) -CO ₂ laser (FIG. 5 of Patent Document 1). In this RF-CO ₂ laser, high repetition operation of laser pulses is possible up to about 100 kHz. In practical use, it is necessary to obtain 100 W class EUV emission. However, when the CE by the CO ₂ laser is estimated to be 0.5% and the propagation loss is estimated to be 70%, the output required for the CO ₂ laser is about 60 kW. Become. In order to achieve an output of 60 kW in a short pulse laser, a repetition frequency of about 50 kHz to 100 kHz is required in consideration of durability of an optical element or the like. Note that the pulse width of the laser light emitted from the oscillator is desirably 100 ns or less.

The reason is as follows. The output of the CO ₂ laser is E _total , the pulse oscillation repetition frequency is f _i (i = 1, 2, 3,...), And the optical energy of one pulse is E _pj (j = 1, 2, 3,...). Then, there is a relationship of E _total = f ₁ × E _p1 = f ₂ × E _p2 . Here, when _Ep is large, the damage given to the optical element through which the laser light is transmitted increases, so that the optical element is rapidly deteriorated. Therefore, _{E p} is smaller is desirable. Therefore, to reduce the _{E p} in order to obtain the desired _{E total,} may be increased repetition frequency f.

In order to realize such a high repetition frequency, it is appropriate to use an RF (Radio Frequency excitation) -CO ₂ laser. The reason is, as the pulsed CO ₂ lasers, but this addition also has TEA (Transverse Excitation Atmospheric) -CO ₂ laser, the state of the art, 2 kHz about repetitive operation is because the limit.

US Pat. No. 6,697,408 (FIG. 5)

  As described above, by irradiating the target material with laser light from the driver laser, the target material is excited and turned into plasma, and EUV light is emitted from this plasma. Here, it is conceivable that the laser light emitted from the driver laser is reflected by the target material or plasma, and the reflected light returns to the driver laser, thereby being amplified in the amplifier 13b and causing a failure of the oscillator 13a. .

  Therefore, in view of the above points, the present invention provides an extreme ultraviolet light source device capable of preventing a driver laser from being broken by reflected light (returned light) reflected by a target material or plasma and returning to the driver laser. Objective.

  In order to solve the above problems, an extreme ultraviolet light source according to one aspect of the present invention is an extreme ultraviolet light source device that generates extreme ultraviolet light by irradiating a target with laser light, and generates extreme ultraviolet light. A laser collector that includes a chamber, a target supply unit that supplies a target to a predetermined position in the chamber, and at least one optical element, and that generates plasma by condensing laser light emitted from the driver laser onto the target. An optical optical system, an EUV condensing optical system that condenses and emits extreme ultraviolet light emitted from plasma, and a first linearly polarized component of laser light emitted from a driver laser and passing through a predetermined optical path The circularly polarized component of the return light that is reflected while being converted into the polarized component and reflected by the focused laser beam by the target or plasma is converted into the second component. A retarder that reflects while converting to a linearly polarized component, and a coating mirror that is disposed in the optical path of the laser light emitted from the driver laser and reflects the first linearly polarized component and absorbs the second linearly polarized component To do.

  According to the present invention, it is possible to prevent failure of the driver laser by reducing the reflected light (returned light) that is reflected by the target material or plasma and returns to the driver laser.

It is a side view which shows the internal structure of the EUV light source device which concerns on the 1st Embodiment of this invention. It is a top view which shows the internal structure of the EUV light source device which concerns on the 1st Embodiment of this invention. It is a figure which shows the relationship between the position of a condensing lens, and return light. It is a figure which shows the apparatus used in the experiment for measuring a return light. It is a figure which shows the relationship between the return light quantity measured by experiment, and EUV light quantity. It is a figure which shows the result of having measured the change of the return light quantity by changing the angle of the target with respect to the laser incident direction. It is a figure which shows the laser beam reflected by a target in the state which inclined the target 30 degrees with respect to the incident direction of a laser beam. It is a figure which shows the laser beam reflected and scattered by plasma. It is a figure which shows the waveform of incident laser beam and return light. It is a figure which shows the experimental value and calculated value of a return light quantity in the state whose incident angle of the laser beam with respect to a target is 0 degree | times. It is a figure which shows the calculated value of the return light quantity in the state whose incident angle of the laser beam with respect to a target is 0 degree | times, 30 degree | times, and 45 degree | times. It is a figure which shows the return light quantity reduction effect at the time of using a spatial filter as a filter part shown in FIG.1 and FIG.2. It is a figure which shows the structure using the combination of a spatial filter and a collimating optical system as a filter part shown in FIG.1 and FIG.2. It is a figure which shows the return light reduction means in the 2nd Embodiment of this invention. It is a figure which shows the return light reduction means in the 3rd Embodiment of this invention. It is a figure which shows the return light reduction means in the 4th Embodiment of this invention. It is a figure which shows the return light reduction means in the 5th Embodiment of this invention. It is a figure which shows the return light reduction means in the 6th Embodiment of this invention. It is a figure which shows the return light reduction means in the 7th Embodiment of this invention. It is a figure for demonstrating operation | movement of the retarder shown in FIG. It is a figure which shows the structure of the optical system in the extreme ultraviolet light source device which concerns on the 8th Embodiment of this invention. It is the schematic which shows the structural example of the driver laser used in a LPP type EUV light source device.

Hereinafter, embodiments for carrying out the present invention will be described in detail with reference to the drawings. The same constituent elements are denoted by the same reference numerals, and the description thereof is omitted.
FIG. 1 is a side view showing the internal structure of the EUV light source apparatus according to the first embodiment of the present invention, and FIG. 2 is a plan view showing the internal structure of the EUV light source apparatus according to the first embodiment of the present invention. FIG. The EUV light source apparatus according to the present embodiment employs a laser excitation plasma (LPP) system that generates EUV light by irradiating a target material with laser light to excite it.

  As shown in FIGS. 1 and 2, this EUV light source device includes a vacuum chamber 10 in which EUV light is generated, a target supply device 11 that supplies a target 1, and excitation laser light 2 that is irradiated to the target 1. A laser beam that generates a plasma 3 by focusing the excitation laser beam 2 generated by the driver laser 13 on the target 1, and a driver laser 13 that generates a laser beam and an optical element such as at least one lens or mirror The optical system 14, the EUV collector mirror 15 that collects and emits the EUV light 4 emitted from the plasma 3, the target collection device 16 that collects the target 1, and at least the laser collection optical system 14 include And a position adjusting mechanism 20 that adjusts the position of one optical element.

  Further, the EUV light source device includes a filter unit 30 that reduces the amount of return light incident on the driver laser 13 when the laser beam collected by the laser focusing optical system 14 is reflected by the target 1 and the plasma 3; A return light detector 40 for detecting return light at a predetermined position, a multilayer mirror 51 and an EUV light detector 52 for detecting EUV light at a light emitting point without passing through the EUV collector mirror 15, and a return light detector; 40 and / or a control unit 60 for controlling the driver laser 13 and / or the position adjusting mechanism 20 based on the detection result of the EUV light detector 52.

  The vacuum chamber 10 is provided with an introduction window 18 for introducing the excitation laser light 2 and a lead-out window 19 for leading EUV light emitted from the plasma to the exposure device. Note that the inside of the exposure device is also kept in a vacuum or a reduced pressure state, similarly to the inside of the vacuum chamber 10. The target supply device 11 supplies the target 1 to a predetermined position in the vacuum chamber 10 while adjusting the position of the target 1.

  The driver laser 13 is a laser light source capable of pulse oscillation at a high repetition frequency (for example, a pulse width of about several nanoseconds to several tens of nanoseconds and a frequency of about 1 kHz to 100 kHz). The laser condensing optical system 14 includes at least one lens or mirror, and condenses the laser light emitted from the driver laser 13. The laser beam 2 focused by the laser focusing optical system 14 irradiates the target 1 supplied by the target supply device 11 at a predetermined position in the vacuum chamber 10. As a result, a part of the target 1 is excited and turned into plasma, and various wavelength components are emitted from the light emitting point. Here, the light emitting point means a position where the plasma 3 is generated.

  The EUV collector mirror 15 condenses light by selectively reflecting a predetermined wavelength component (for example, EUV light in the vicinity of 13.5 nm) out of various wavelength components emitted from the plasma 3. It is a system. The EUV collector mirror 15 has a concave reflecting surface. For example, molybdenum (Mo) and silicon (Si) for selectively reflecting EUV light having a wavelength of about 13.5 nm are reflected on the reflecting surface. The multilayer film is formed.

  In FIG. 1, the EUV light is reflected in the right direction by the EUV collector mirror 15, collected at the EUV intermediate condensing point, and then output to the exposure unit. The EUV light condensing optical system is not limited to the EUV collector mirror 15 shown in FIG. 1, and may be configured by using a plurality of optical components. However, in order to suppress the EUV light absorption, It is necessary to.

  The target recovery device 16 is disposed at a position facing the target supply device 11 with the light emission point interposed therebetween, and recovers the target that has not been turned into plasma. The collected target may be returned to the target supply device 11 again by the target circulation device and reused.

  The multilayer mirror 51 is formed with, for example, a multilayer film of molybdenum and silicon having a high reflectance with respect to a wavelength near 13.5 nm. The EUV photodetector 52 is constituted by, for example, a zirconium (Zr) filter and a photodiode. The zirconium filter cuts light having a wavelength of 20 nm or more. The photodiode outputs a detection signal corresponding to the intensity or energy of incident light.

In the present embodiment, as shown in FIG. 22, as the driver laser 13, an oscillator 13a configured by a CO ₂ laser capable of generating light having a relatively long wavelength, and a laser beam generated by the oscillator 13a are amplified. An oscillation amplification type laser composed of at least one amplifier 13b is used. By using a long wavelength and short pulse laser, generation of a low-temperature and high-density plasma region that does not contribute to generation of EUV light and serves as a heat source for generating debris is suppressed. Neutral particles emitted from and adhering to the reflective surface of the EUV collector mirror 15 are greatly reduced. On the other hand, since fast ions are also emitted from the plasma 3, the multilayer film formed on the reflective surface of the EUV collector mirror 15 is scraped off.

  As the target 1, solid tin (Sn) or lithium (Li) is used. In that case, since the neutral particles generated from the target are very small, the amount (deposition amount) of the neutral particles adhering to the reflecting surface of the EUV collector mirror 15 is determined under the predetermined conditions. It was verified that the amount of the multilayer film (sputtering amount) scraped from the reflecting surface of the mirror 15 can be balanced or the deposition amount can be made smaller than the sputtering amount. Thereby, the problem that debris adheres to the reflective surface of the EUV collector mirror 15 can be solved.

  Furthermore, in the present embodiment, under the control of the control unit 60, the position adjustment mechanism 20 is configured so that the position of the focusing point of the laser focusing optical system 14 deviates from the emission point of the plasma 3 by a predetermined distance. The position of at least one optical element included in the laser focusing optical system 14 is adjusted. This will be described below by taking as an example a case where a condensing lens is used as the laser condensing optical system 14.

  FIG. 3 is a diagram illustrating the relationship between the position of the condenser lens and the return light. FIG. 3A shows the best focus state. The return light is collimated to have a high directivity and can propagate a long distance, and is incident on the driver laser 13. The amount of return light to be maximized. FIG. 3B shows a state of the defocus 1 in which the position of the condenser lens approaches the plasma 3 side, and the return light has a large divergence angle and is incident on the driver laser 13. The amount of return light is reduced. FIG. 3C shows a state of the defocus 2 in which the position of the condenser lens is far from the plasma 3 side, and the driver laser 13 has a large divergence angle after the return light converges once. The amount of return light incident on the is reduced. The same applies to the case where a condensing mirror is used as the laser condensing optical system 14.

FIG. 4 is a diagram showing an apparatus used in an experiment for measuring return light. In this experiment, a CO ₂ laser that generates laser light having a wavelength of 10.6 μm was used as the driver laser, and only the P-polarized component was incident on the apparatus shown in FIG. Moreover, solid (flat plate) tin (Sn) was used as a target. As the condenser lens, a meniscus lens having a focal length of 60 mm was used.

  A thin film polarizer (TFP) arranged in the optical path of the return light transmits the P-polarized component and reflects the S-polarized component. The P-polarized laser light incident from the left side of FIG. 4 passes through the thin film polarizer and enters the quarter-wave plate. P-polarized laser light becomes circularly polarized light by passing through the quarter-wave plate. The laser beam condensed by the condenser lens is irradiated onto the target in the vacuum vessel, and the target is turned into plasma.

  The circularly polarized return light reflected by the target or plasma is collimated by the condenser lens and passes through the quarter-wave plate to become S-polarized light. The return light that has become S-polarized light is reflected in the direction of the photodetector by the thin-film polarizer. Here, the diameter of the quarter-wave plate is 10 mm, and the peripheral portion of the quarter-wave plate is fixed by a holder that does not transmit laser light having a diameter of about 50 mm. Acts as a spatial filter. The return light reflected by the thin film type polarizer enters the photodetector, and the amount of return light is measured. The relationship between the amount of return light (light intensity) measured by this experiment and the amount of EUV light (light intensity) measured simultaneously is shown in FIG.

  FIG. 5 is a diagram showing changes in the return light amount and the EUV light amount with respect to the position of the condenser lens. In FIG. 5 and the like, the best focus position is 0 μm, and the closer to the target is a negative value. As shown in FIG. 5, it has been clarified that the amount of return light greatly depends on the position of the condenser lens and is maximized at the best focus position. Further, by setting the position of the condensing lens as the defocus position, the amount of return light is greatly reduced. On the other hand, it can be seen that the EUV light quantity becomes maximum at the defocus position, not at the best focus position. Accordingly, by setting the position of the condenser lens as the defocus position, both CE (conversion efficiency from irradiation laser light to EUV light) can be increased and the amount of return light can be reduced. Can do.

  By the way, as components of the return light, there are a component reflected on the surface of the target before plasma generation and a component reflected and scattered in the plasma after plasma generation. Therefore, an experiment was conducted to determine which of these components is the main component. In this experiment, using a tin (Sn) flat plate as a target, the angle of the target (incident angle) with respect to the laser incident direction was changed to 0 degrees, 30 degrees, and 45 degrees, and the change in the amount of return light was measured. . The result is shown in FIG.

  FIG. 6 is a diagram illustrating a change in the amount of return light with respect to the position of the condenser lens with the incident angle as a parameter. FIG. 7 is a diagram showing laser light reflected by the target in a state where the target is tilted 30 degrees with respect to the incident direction of the laser light. Since the target is considered to behave like a mirror with respect to the laser beam, when the target is tilted by 30 degrees with respect to the incident direction of the laser beam as shown in FIG. Cannot return to the laser incident direction. However, as shown in FIG. 6, a considerable amount of return light was observed even when the target was tilted 30 or 60 degrees with respect to the laser incident direction. From this result, it is considered that the component reflected in the surface of the target in the return light is very small.

  FIG. 8 is a diagram showing laser light reflected and scattered by plasma. The cause of the change in the amount of return light by tilting the target with respect to the incident direction of the laser light is that the plasma generation direction is mainly the normal of the target surface, as shown in FIGS. Since it has directivity in the direction, it is considered that the amount of return light changes when the amount of light reflected and scattered from the plasma in the incident direction of the laser light changes.

  FIG. 9 is a diagram illustrating waveforms of incident laser light and return light. These waveforms are measured by arranging a measuring instrument capable of observing the laser pulse waveform at the position of the photodetector shown in FIG. As shown in FIG. 9, the return light is mainly generated in the second half of the laser pulse, and there is little component reflected on the surface of the target before plasma generation (corresponding to the first half of the laser pulse), and plasma generation is performed. It can be seen that the components (corresponding to the latter half of the laser pulse) reflected and scattered later by the plasma are the main components.

  Next, the dependence of the return light on the condenser lens position was calculated by performing an optical simulation. This optical simulation reproduces the experimental apparatus shown in FIG. 4 and represents reflection / scattering by plasma with a laser light reflecting surface as a multi-lens array having a curvature radius of 0.1 mm.

  FIG. 10 is a diagram showing experimental values and calculated values of the amount of return light when the incident angle of the laser beam with respect to the target is 0 degree, and the results of the optical simulation almost coincide with the experimental results. FIG. 11 is a diagram illustrating calculated values of the amount of return light when the incident angle of the laser beam on the target is 0 degree, 30 degrees, and 45 degrees. From these results, it can be seen that the amount of return light greatly depends on the position of the condenser lens, that is, the arrangement of the condenser optical system.

From the above experiment, the following became clear.
(1) Return light is mainly reflected light from plasma rather than the surface of the target.
(2) The return light from the plasma greatly depends on the position of the condenser lens (or condenser mirror). When the condensing lens is at the best focus position, the amount of return light incident on the driver laser becomes the largest, so the collimating effect of the return light by the condensing lens has a great influence. Therefore, the amount of return light incident on the driver laser can be reduced by placing the condenser lens at the defocus position so as not to collimate the return light.
(3) When the intensity of the laser beam is sufficient, as shown in FIG. 3B, the CE (irradiation laser) is brought into the defocused 1 state where the position of the condenser lens approaches the plasma 3 side. The conversion efficiency from light to EUV light is maximized. Therefore, it is possible to achieve both reduction of the amount of return light and realization of high CE.

  Thus, since the return light has a large divergence angle when the position of the condenser lens is in a defocused state, the amount of return light is reduced by using a spatial filter that transmits the laser light but blocks the return light. It becomes possible.

  FIG. 12 is a diagram illustrating a return light amount reduction effect when a spatial filter is used as the filter unit illustrated in FIGS. 1 and 2. In the best focus state shown in FIG. 12A, the effect of reducing the amount of returned light cannot be obtained even if the spatial filter 31 is provided, but the state of defocus 1 shown in FIG. In the state of defocus 2 shown in c), the light quantity reduction effect can be obtained by providing the spatial filter 31. Further, as shown in FIG. 12B, when the position of the laser condensing optical system (condensing lens) 14 is in a defocused state 1 approaching the plasma 3 side, CE (irradiated laser light to EUV light). Conversion efficiency) is maximized (see FIG. 5).

  FIG. 13 is a diagram illustrating a configuration using a combination of a spatial filter and a collimating optical system as the filter unit illustrated in FIGS. 1 and 2. As shown in FIG. 13, the collimated laser beam having a good beam quality is condensed by the condenser lens 32 to a size equal to or smaller than the pinhole, passes through the spatial filter 33 in which the pinhole is formed, and then collimated lens 34. Is collimated again. On the other hand, since the return light has a large divergence angle, it is difficult to focus the light on the size of a pinhole or less, and the spatial filter 33 largely blocks the return light.

  As shown in FIG. 5, there is a correlation between the amount of return light and the position of the laser focusing optical system 14 that the amount of return light increases as the laser focusing optical system 14 approaches the best focus state with respect to the target. Therefore, the positional information of the laser condensing optical system 14 can be obtained by monitoring the amount of return light using this correlation. Therefore, the control unit 60 shown in FIG. 1 controls the position adjustment mechanism 20 based on the amount of return light detected by the return light detector 40, thereby maintaining the defocused state of the laser focusing optical system 14. be able to. For example, the control unit 60 performs feedback control of the position adjustment mechanism 20 so as to keep the amount of return light detected by the return light detector 40 at a set value. Thereby, even when the target position fluctuates, it is possible to control the laser condensing optical system 14 to be arranged at an appropriate position.

  Alternatively, the control unit 60 controls the position adjusting mechanism 20 based on the amount of return light detected by the return light detector 40 and the amount of extreme ultraviolet light detected by the EUV light detector 52. May be. Thereby, more highly accurate control can be performed. For example, the control unit 60 determines that the amount of return light detected by the return light detector 40 approaches the first set value, and the amount of extreme ultraviolet light detected by the EUV light detector 52 is the second setting. The position adjustment mechanism 20 is feedback-controlled so as to approach the value.

  The control unit 60 controls the output of the driver laser 13 and refers to the value of the lookup table stored in the storage unit provided in the control unit 60 according to the output of the driver laser 13. Setting values (first and second setting values) may be set.

  Next, a second embodiment of the present invention will be described. In the following embodiments, in addition to reducing the amount of return light by a spatial filter so that the position of the laser focusing optical system is defocused, return light reduction means for further reducing the amount of return light is provided. It has been.

  FIG. 14 is a diagram showing return light reducing means in the second embodiment of the present invention. In the second embodiment, the driver laser 13 (see FIG. 1) emits a laser beam mainly composed of a P-polarized component, transmits the P-polarized component in the optical path of the laser beam, and reflects the S-polarized component. An element 71 and a quarter-wave plate 72 that also serves as a spatial filter are arranged. As the polarization separation element 71, a thin film polarizer (TFP), a polarization beam splitter, an uncoated window substrate, or the like can be used. The polarization separation element 71 transmits the P-polarized component of the incident laser light, and the quarter-wave plate 72 converts the P-polarized laser light into circularly polarized light. Thereby, circularly polarized laser light is irradiated onto the target 1 via the laser focusing optical system 14, and plasma 3 is generated.

  Although the return light from the plasma 3 is circularly polarized, it is converted to S-polarized light by the quarter-wave plate 72, and the S-polarized return light is reflected by the polarization separation element 71 and disposed in the optical path of the reflected light. The light enters the return light monitor 73. Thereby, the return light incident on the driver laser 13 is reduced. The return light monitor 73 can observe the waveform and generation timing of the return light.

  FIG. 15 is a diagram showing return light reducing means in the third embodiment of the present invention. In the third embodiment, the driver laser 13 (see FIG. 1) emits a laser beam mainly composed of an S-polarized component, transmits the P-polarized component in the optical path of the laser beam, and reflects the S-polarized component. An element 71 and a quarter-wave plate 72 that also serves as a spatial filter are arranged. The polarization separation element 71 reflects the S-polarized component of the incident laser light in the direction of the quarter-wave plate 72, and the quarter-wave plate 72 converts the S-polarized laser light into circularly polarized light. Thereby, circularly polarized laser light is irradiated onto the target 1 via the laser focusing optical system 14, and plasma 3 is generated.

  Although the return light from the plasma 3 is circularly polarized light, it is converted to P-polarized light by the quarter-wave plate 72, and the P-polarized return light is transmitted through the polarizing element 71 and arranged in the optical path of the transmitted light. The light enters the optical monitor 73. Thereby, the return light incident on the driver laser 13 is reduced.

  FIG. 16 is a diagram showing return light reducing means in the fourth embodiment of the present invention. In the fourth embodiment, the driver laser 13 (see FIG. 1) emits a laser beam mainly composed of P-polarized component light, and transmits a P-polarized component and reflects an S-polarized component in the optical path of the laser beam. And a quarter-wave plate 72 that also functions as a spatial filter are disposed. As a result, the S-polarized light component that has been slightly transmitted through the first polarization separation element 71 is reflected by the second, third, or fourth polarization separation element 71, so that the return light amount Is further reduced. Further, by arranging two polarization separation elements 71 as a pair and arranging each pair of polarization separation elements 71 in a square shape, the optical path difference of laser light can be eliminated.

  FIG. 17 is a diagram showing return light reducing means in the fifth embodiment of the present invention. In the fifth embodiment, the driver laser 13 (see FIG. 1) emits a laser beam mainly composed of an S-polarized component, and a coating that reflects S-polarized light and absorbs P-polarized light is applied to the optical path of the laser light. A coating mirror 74 and a quarter-wave plate 72 that also serves as a spatial filter are disposed. As the coating mirror 74, an ATFR coating mirror manufactured by II-VI can be used. The coating mirror 74 reflects the S-polarized component of the incident laser light in the direction of the quarter-wave plate 72, and the quarter-wave plate 72 converts the S-polarized laser light into circularly polarized light. Thereby, circularly polarized laser light is irradiated onto the target 1 via the laser focusing optical system 14, and plasma 3 is generated.

  Although the return light from the plasma 3 is circularly polarized, it is converted to P-polarized light by the quarter wavelength plate 72, and the P-polarized return light is absorbed by the coating mirror 74. Thereby, the return light returning to the driver laser 13 is reduced.

  FIG. 18 is a diagram showing return light reducing means in the sixth embodiment of the present invention. In the sixth embodiment, the optical path of the laser light emitted from the driver laser 13 (see FIG. 1) includes a polarizer 75 such as a Brewster window, and a retarder 76 that functions as a quarter wavelength plate and a mirror. And are arranged. The polarizer 75 receives laser light (S-polarized light) emitted from the incident laser light, transmits the partially polarized component, enters the retarder 76, and reflects the plane-polarized component. The retarder 76 converts the partially polarized light component into circularly polarized light when it is reflected. Thereby, circularly polarized laser light is irradiated onto the target 1 via the laser focusing optical system 14, and plasma 3 is generated.

  Although the return light from the plasma 3 is circularly polarized light, it is converted into plane polarized light when reflected by the retarder 76, and the plane polarized return light is reflected by the polarizer 75 and arranged in the optical path of the reflected light. The light enters the return light monitor 73. Thereby, the return light returning to the driver laser 13 is reduced.

  FIG. 19 is a diagram showing return light reducing means in the seventh embodiment of the present invention. In the seventh embodiment, the retarder 76 in the sixth embodiment shown in FIG. 18 is changed to a concave retarder 77. The retarder 76 shown in FIG. 18 has a flat reflecting surface, whereas the concave retarder 77 shown in FIG. 19 has a curved reflecting surface. Thereby, the concave retarder 77 can also serve as the laser condensing optical system 14.

  FIG. 20 is a diagram for explaining the operation of the retarder shown in FIG. As shown in FIG. 20, when the retarder 76 reflects incident light linearly polarized at 45 ° with respect to the incident surface, the retarder 76 converts the incident light into circularly polarized light. On the contrary, when the circularly polarized light is incident, the retarder 76 converts the light into linearly polarized light when it is reflected.

  FIG. 21 is a diagram showing a configuration of an optical system in the extreme ultraviolet light source apparatus according to the eighth embodiment of the present invention. In the eighth embodiment, two mirrors 13c having different reflectances depending on the polarization are arranged between the oscillator 13a and the amplifier 13b. For example, the mirror 13c absorbs the P-polarized component and reflects the S-polarized component with a high reflectance. Here, the laser light supplied from the oscillator 13a needs to be aligned with S-polarized light, but this can be easily performed by the conventional technique. For example, laser oscillation of only P-polarized light can be obtained by arranging the window glass surface of the laser tube at a Brewster angle with respect to the optical axis. This can be easily converted to S-polarized light using a wave plate or a mirror. Further, a similar mirror 13d is arranged on the outlet side of the amplifier 13b.

  A half-wave plate 13e and two polarization separation elements 71 are disposed in the optical path of the laser light reflected by the mirror 13d. The half-wave plate 13e converts the S-polarized laser light reflected by the mirror 13d into P-polarized light. The polarization separation element 71 transmits P-polarized light and reflects S-polarized light. The P-polarized laser light that has passed through the polarization separation element 71 is condensed by the condenser lens 32, passes through the bin hole formed in the spatial filter 33, is collimated by the collimator lens 34, and enters the retarder 76. The retarder 76 converts P-polarized laser light into circularly-polarized light when reflecting the laser light.

  The laser light reflected by the retarder 76 passes through the spatial filter 31, is condensed by the laser condensing optical system 14, and is irradiated onto the target, and plasma 3 is generated. Since the laser focusing optical system 14 is disposed closer to the target by a predetermined distance than the best focus position, the laser focusing optical system 14 is in a defocused state, and the return light has a spread.

However, in order to obtain the maximum CE in the defocused state, the laser intensity (laser intensity) at the time of best focus needs to exceed the optimum value as the laser light irradiation condition. Here, the laser intensity is expressed by the following equation.
Laser intensity (W / cm ² ) = Laser pulse energy (J)
/ (Pulse width (s) × spot size (cm ² ))
That is, there are three methods for increasing the laser intensity: (1) increasing the pulse energy of the laser light, (2) shortening the pulse width of the laser light, and (3) reducing the spot size. . Among these, the parameter that can be changed by the laser focusing optical system 14 is the spot size. The laser intensity becomes maximum in the best focus state with the smallest spot size, and decreases as the spot size increases in the defocus state. Therefore, in the present embodiment, by using an optical system capable of condensing laser light to a spot size smaller than the spot size that provides the maximum CE in the best focus state, as the laser condensing optical system 14. Even in the defocus state, CE is maximized.

  The return light is incident on the spatial filter 31 via the laser condensing optical system 14, but has a spread, so that the amount of return light is reduced by the spatial filter 31. The return light that has passed through the spatial filter 31 is converted from circularly polarized light to S polarized light when reflected by the retarder 76. The return light reflected by the retarder 76 passes through the collimator lens 34, the spatial filter 33, and the condensing lens 32. At this time, the amount of return light is further reduced. Since the remaining return light is S-polarized light, most of it is reflected by the two polarization separation elements 71 and enters the return light monitor 73. Although two return light monitors 73 are shown in FIG. 21, one of them may be replaced with a damper that absorbs the return light.

  The remaining return light enters the half-wave plate 13e, is converted from S-polarized light to P-polarized light, is absorbed by the mirror 13d, and is incident on the amplifier 13b to be amplified so that the oscillator 13a is not damaged. Reduced. Further, the return light slightly returned from the amplifier 13b is absorbed by the mirror 13c, and almost no return light is incident on the oscillator 13a. In the above, the number and arrangement location of the optical elements constituting the filter such as the spatial filter 31 are arbitrary, but by arranging an optical element that reduces the amount of return light between the amplifier 13b and the oscillator 13a, Safety can be further increased.

  The present invention can be used in an extreme ultraviolet light source device used as a light source of an exposure apparatus.

  DESCRIPTION OF SYMBOLS 1 ... Target, 2 ... Laser beam, 3 ... Plasma, 4 ... EUV light, 10 ... Vacuum chamber, 11 ... Target supply apparatus, 13 ... Driver laser, 13a ... Oscillator, 13b ... Amplifier, 13c, 13d ... Mirror, 13e ... Half-wave plate, 14 ... laser focusing optical system, 15 ... EUV collector mirror, 16 ... target recovery device, 18 ... introduction window, 19 ... lead-out window, 20 ... position adjusting mechanism, 30 ... filter section, 31, 33 DESCRIPTION OF SYMBOLS ... Spatial filter, 32 ... Condensing lens, 34 ... Collimating lens, 40 ... Return light detector, 51 ... Multilayer mirror, 52 ... EUV photodetector, 60 ... Control part, 71 ... Polarization separation element, 72 ... 1 / 4 wavelength plate, 73 ... Return light monitor, 74 ... Coating mirror, 75 ... Polarizer, 76 ... Retarder, 77 ... Concave retarder

Claims (6)

An extreme ultraviolet light source device that generates extreme ultraviolet light by irradiating a target with laser light,
A chamber in which extreme ultraviolet light is generated;
A target supply unit for supplying a target to a predetermined position in the chamber;
A laser condensing optical system including at least one optical element and generating plasma by condensing laser light emitted from a driver laser on the target;
An EUV condensing optical system that condenses and emits extreme ultraviolet light emitted from the plasma;
Reflected while converting the first linearly polarized light component of the laser light emitted from the driver laser and passing through a predetermined optical path into a circularly polarized light component, and the collected laser light is reflected by the target or the plasma. A retarder that reflects and converts the circularly polarized component of the returned light into a second linearly polarized component;
A coating mirror disposed in the optical path of the laser light emitted from the driver laser and reflecting the first linearly polarized component and absorbing the second linearly polarized component;
An extreme ultraviolet light source device comprising: A return light detector that detects the return light reflected by the target or the plasma and incident on the driver laser after the laser light collected by the laser focusing optical system;
A position adjusting mechanism for adjusting the position of at least one optical element included in the laser focusing optical system;
A controller that controls the position adjustment mechanism based on the amount of return light detected by the return light detector;
The extreme ultraviolet light source device according to claim 1, further comprising: Further comprising an extreme ultraviolet light detector for detecting extreme ultraviolet light emitted from the plasma;
The control unit controls the position adjusting mechanism based on an amount of return light detected by the return light detector and an amount of extreme ultraviolet light detected by the extreme ultraviolet light detector. 2. The extreme ultraviolet light source device according to 2.   The extreme ultraviolet light source device according to any one of claims 1 to 3, wherein the driver laser emits a laser beam mainly composed of a first linearly polarized light component.   The extreme ultraviolet light source device according to any one of claims 1 to 4, wherein the retarder has a curved reflecting surface to serve as the laser condensing optical system. The extreme ultraviolet light source device according to claim 1, wherein the driver laser includes an oscillator configured by a CO ₂ laser and at least one amplifier that amplifies laser light generated by the oscillator.

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