JP2005242257A - Device and method for generating highly efficient coherent ultraviolet ray - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a coherent ultraviolet ray generating device that converts an infrared-ray laser oscillator output capable of tuning a small-sized coherent UV light source with high efficiency and a solid laser oscillation output of fixed laser wavelength into secondary higher harmonics of two kinds of wavelength in high-efficiency wavelength converting methods by using nonlinear optical crystals and outputting coherent ultraviolet rays of 210 to 260 nm in wavelength with high coversion efficiency through a sum frequency mixing nonlinear optical crystal. <P>SOLUTION: Provided are at least one or more solid laser resonators for excitation, a means of generating coherent beams of two wavelengths by exciting two kinds of solid laser media with laser beam from the laser resonators for excitation, a means of guiding the coherent beams to nonlinear optical crystals matched with their wavelengths and generating secondary higher harmonic coherent beams by the wavelengths, an optical means of coaxially arranging the coherent beams of the secondary higher harmonics, a means of guiding the coaxially arranged coherent beams to the nonlinear crystal for sum frequency mixing, and the high-efficiency coherent ultraviolet-ray generating device which generates a sum frequency beam through the nonlinear optical crystal for sum frequency mixing. <P>COPYRIGHT: (C)2005,JPO&NCIPI

Description

  The present invention relates to an apparatus for generating coherent light having an ultraviolet wavelength of 210 nm to 260 nm by an all-solid-state laser apparatus.

  Conventionally, in the method of irradiating laser light to detect surface defects and dirt on semiconductor wafers, etc., and measuring scattering from the surface, the oscillation wavelength of 488 nm of the argon ion laser is converted to a harmonic by a nonlinear optical crystal, and the wavelength Fine defects and dirt are detected by using a 244 nm ultraviolet laser. On the other hand, in place of the laser beam having an excimer laser wavelength of 248 nm used in the semiconductor lithography apparatus, it is possible to reduce the size and extend the life of the light source device by oscillating ultraviolet light with a solid-state laser, thereby expanding the use. For example, not only the lithography exposure light source but also the inspection of masks of related parts can be used to reduce the equipment cost.

  Therefore, a configuration as shown in FIG. 1 is considered as a highly efficient coherent ultraviolet ray generation method. A solid-state laser Nd: YAG3 excited by a pumping light 2 from a semiconductor laser LD1 oscillates a fundamental oscillation wavelength 4 having a wavelength of 1.06 μm, and this light is introduced into the solid-state laser resonator or outside the resonator based on a well-known method. The green light 6 is obtained by converting into the second harmonic by the formed resonance type resonator with built-in nonlinear optical crystal or the nonlinear optical crystal 5 installed outside. This green light is used as an optical excitation light source of a tunable laser using titanium-added sapphire (Ti: Sa) 7 as an oscillation medium, and a titanium sapphire laser oscillating at an oscillation wavelength of 750 nm is optically excited.

  Using this light, the second harmonic wave having a wavelength of 375 nm is generated in the second harmonic wave generating nonlinear crystal 9. On the other hand, the solid-state laser 13 is optically excited by the oscillation output 12 from the semiconductor laser diode 11, and this output light 14 is incident on the nonlinear optical crystal 15 to generate green light 16. The titanium-added sapphire 17 is photoexcited with green light 16 to oscillate at a wavelength of 750 nm to obtain an output 18. The beam having a wavelength of 750 nm is reflected by the mirror 19 and is aligned by the dichroic mirror 20 with the same optical axis as the second harmonic output beam 10 of the solid-state laser. As a result, a beam 21 with two wavelengths superimposed is formed, the beam 21 is guided to a nonlinear optical crystal 22 for sum frequency mixing, and coherent ultraviolet rays around 250 nm, which is a sum frequency mixing frequency of two wavelengths, are generated from this crystal. To do.

  The conversion efficiency from the electricity to the light output by the ultraviolet ray generator having such a configuration is that each wavelength of the beam for performing the final stage sum frequency light mixing passes through the oscillation stage of the tunable solid-state laser. A decrease in oscillation efficiency in stages is included, resulting in an efficiency of less than 1% at most with respect to the output of the entire system.

  As a method for generating ultraviolet light having a wavelength of about 250 nm of continuous oscillation output by wavelength conversion, for example, Optics Letter, Vol.25, No.19, p.1457, 2000, the second harmonic of titanium sapphire laser light (373 nm ) And near-infrared light (780 nm) from a semiconductor laser is reported to be sum-frequency mixed with a BBO crystal to obtain continuous output ultraviolet light (252 nm). This is a wavelength conversion method using an external resonator that increases the wavelength conversion efficiency by increasing the light intensity using the resonance effect, and uses a method called two-wavelength resonance that resonates both lights that are sum-frequency mixed. It is.

  In Japanese Patent Laid-Open No. 2000-171843, the Nd: YAG laser fundamental wave (1064 nm) is re-mixed with the second harmonic (532 nm) of a Nd: YAG laser, etc., and the titanium sapphire laser light (approximately 700 nm) is mixed in the sum frequency ) To obtain light having a wavelength of about 248 nm by sum frequency mixing. Furthermore, Applied Optics, Vol.39, No.30, p.5505, 2000 has a pulse oscillation of 242nm wavelength by the sum frequency mixing of Nd: YLF laser third harmonic (349nm) and titanium sapphire laser light (780nm). It has been reported that ultraviolet light was obtained.

  In the first conventional example, the second harmonic (373 nm) of the titanium sapphire laser light and the near-infrared light (780 nm) from the semiconductor laser are sum-frequency mixed by the BBO crystal, as shown in the above example. An output of 50 mW at a wavelength of 252 nm is the maximum reported at present, and it is considered difficult to obtain an output of 100 mW or more for industrial use. This is because the maximum output of both the titanium sapphire laser and semiconductor laser used for sum frequency mixing is at most several watts, and mixing cannot be performed without using the BBO crystal, which has the effect of limiting the wavelength conversion efficiency called the walk-off effect. Due to

In another example, the method disclosed in Japanese Patent Application Laid-Open No. 2000-171843 has a complicated configuration by performing two steps of sum frequency mixing, so it is not easy to apply it to generation of ultraviolet light with continuous output. There are no reported cases.
JP 2000-171843 A Japanese Patent Laid-Open No. 2003-50412 U.S. Pat. No. 4,826,283 US Pat. No. 5,835,513 US Pat. No. 5,850,407 US Pat. No. 5,898,717 US Pat. No. 5,936,983 US Pat. No. 6,002,697 US Pat. No. 6,061,370 US Pat. No. 6,229,829 US Pat. No. 6,532,100 US Pat. No. 6,584,134 US Pat. No. 6,587,487 Optics Letter, Vol.25, No.19, p.1457, 2000 Applied Optics, Vol.39, No.30, p.5505, 2000

  As laser applications, short-wavelength laser light sources are increasingly required in the fields of semiconductor, communication, medical, microfabrication, and science, especially in the microfabrication process, inspection process and apparatus in the semiconductor field. Therefore, the problem to be solved by the present invention is that a highly efficient and small coherent UV light source uses a tunable infrared laser oscillator output and a solid-state laser oscillation output of a fixed laser wavelength, both using a nonlinear optical crystal. To provide a coherent ultraviolet ray generator that outputs a coherent ultraviolet ray having a wavelength in the range of 210 nm to 260 nm with high conversion efficiency through a nonlinear optical crystal for sum frequency mixing using a second wavelength by a simple wavelength conversion method. .

The present invention includes at least one excitation solid-state laser resonator;
Means for generating two wavelengths of coherent beams by exciting two types of solid-state laser media with a laser beam from the excitation laser resonator;
Means for directing the coherent beam to a nonlinear optical crystal matched to each wavelength and generating a respective second harmonic coherent beam for each wavelength;
Optical means for coaxially arranging the second harmonic coherent beam;
Means for guiding the coherent beam arranged coaxially to a nonlinear crystal for sum frequency light mixing;
The above problem is solved by a high-efficiency coherent ultraviolet ray generator that generates a sum frequency beam from the sum frequency mixing nonlinear optical crystal.

  The present invention also generates a semiconductor laser-excited solid-state laser output wavelength λ1 at the second harmonic, excites a tunable laser (for example, titanium-doped sapphire) crystal with the above output, and has an oscillation wavelength range of 750 to 1000 nm. The laser beam λ2 is oscillated and the output is converted into a first coherent beam having a wavelength λ2 / 2 of 350 nm to 500 nm by second harmonic generation, while the solid-state laser output wavelength λ3 of the semiconductor laser is excited. A second coherent light having a wavelength λ3 / 2, which is a second harmonic output, is generated, and the first coherent light having a wavelength λ2 / 2 and the second coherent light having a wavelength λ3 / 2 are coaxially arranged to form a nonlinear optical crystal. Provides a high-efficiency coherent ultraviolet ray generator having a third wavelength λ4 (λ4 = λ2 · λ3 / {2 (λ2 + λ3)}) of 210 to 260 nm by sum frequency mixing.

  As an effect of the present invention, two types of coherent beams using a second harmonic generation method inside a resonator having high conversion efficiency from fundamental waves of two types of solid-state laser oscillators are arranged on the same axis, A sum frequency output beam having a third wavelength can be obtained with high conversion efficiency from the wavelength beam using sum frequency mixing using a nonlinear optical crystal, and coherent ultraviolet light with a wavelength of 210 nm to 260 nm is highly efficient and compact. This can be realized with a solid-state device.

  Specifically, according to the apparatus of the present invention, the remarkable effects (1) to (3) can be achieved.

  (1) As the first laser, a titanium sapphire laser that can be excited by the second harmonic of the Nd: YAG laser can be applied. An output of several watts can be easily obtained with an all-solid configuration, and the second harmonic generated by the nonlinear optical crystal can be about several hundreds mW to 1 W, enabling a high-power ultraviolet light source.

  (2) As the second laser, an Nd: YAG laser having a wavelength of 1064 nm capable of generating the maximum output as a solid laser can be used. The fundamental wave is converted to the second harmonic by a nonlinear optical crystal such as KTP or LBO, and continuous output visible laser light with a wavelength of 532 nm or more of about 10 W or more can be easily generated to increase the output of the device. Can do.

  (3) As the sum frequency mixing crystal, a CLBO crystal can be used instead of a BBO crystal. The CLBO crystal has a smaller walk-off effect than the BBO crystal and has a short absorption edge at a short wavelength, so that it can generate higher-power ultraviolet light.

  The present invention generates a semiconductor laser-excited solid-state laser output wavelength λ1 at the second harmonic, excites a tunable laser (e.g., titanium-doped sapphire) crystal with the above output, and a laser having an oscillation wavelength range of 750 to 1000 nm. The light λ2 is oscillated, and the output is converted into a first coherent light having a wavelength λ2 / 2 of a wavelength of 350 nm to 500 nm by second harmonic generation, while the second harmonic of the solid-state laser output wavelength λ3 of the semiconductor laser excitation. A second coherent light having a wavelength λ3 / 2, which is a wave output, is generated, and the first coherent light having a wavelength λ2 / 2 and the second coherent light having a wavelength λ3 / 2 are arranged coaxially and summed in a nonlinear optical crystal. A high-efficiency coherent ultraviolet ray generator having a third wavelength λ4 (λ4 = λ2 · λ3 / {2 (λ2 + λ3)}) of 210 nm to 260 nm is provided by frequency mixing.

  With such a configuration, high second harmonic conversion and high sum frequency mixing phenomenon can generate small and highly efficient coherent ultraviolet light, and the 244nm or KrF excimer laser from the second harmonic generator of the conventional argon ion laser It is possible to realize an all-solid-state laser that is much smaller and more efficient than the 248 nm generator. Hereinafter, an embodiment of the present invention will be described with reference to FIG.

  A schematic configuration of an embodiment of the present invention is shown in FIG. An infrared laser beam from a solid-state laser oscillator in which a solid-state laser medium is excited by an output light 32 of a semiconductor laser diode LD31 for exciting a solid-state laser is placed in a resonator, and the wavelength is 532 nm by using a nonlinear optical crystal. Light 34 having a wavelength ω1 (= 1 / λ1) in the vicinity of the green wavelength region is generated. A well-known technique can be used for the configuration of this type of solid-state laser oscillator 33, and the nonlinear optical crystal is placed inside or outside the resonator of the solid-state laser oscillator 33.

  The green light 34 is used as excitation light, for example, guided to a laser medium installed in a tunable laser resonator to excite a laser medium made of titanium-added sapphire, and tunable with a frequency ω2 (= 1 / λ2). The laser 36 is oscillated. A second harmonic output 37 having a frequency of 2ω2 is generated from the laser oscillation output of the tunable fundamental wave 36 using a nonlinear optical crystal 37.

  If necessary, a titanium sapphire and a nonlinear optical crystal can be installed in the same laser resonator, and the nonlinear phenomenon can be effectively exhibited under a high electric field to obtain a high conversion efficiency. The nonlinear optical crystal 37 can also be installed outside the tunable laser resonator.

  On the other hand, the beam 40 from the semiconductor laser diode 39 is used to excite the laser medium of the solid-state laser oscillator 41 and oscillates an infrared oscillation light that is a fundamental wave of a solid-state laser having a frequency ω3 (= 1 / λ3). The laser light uses a nonlinear optical crystal 43 that generates a second harmonic having a frequency of 2ω3, and generates a second harmonic 44 having a frequency of 2ω3 that is green light. This is green light near the wavelength of 532 nm.

  The nonlinear optical crystal 43 may be installed inside or outside the oscillator of the solid-state laser oscillator 41. The second harmonic 44 of the frequency 2ω3 is optically coaxially arranged with the second harmonic 38 of the frequency 2ω2 of the tunable laser, and a beam 48 in which two wavelength beams of at least frequencies 2ω2 and 2ω3 travel on the same axis. Each optical axis is adjusted so that Therefore, the total reflection mirror 45 has a high reflectance with respect to the frequency 2ω3, and the dichroic mirror 47 used to superimpose two kinds of beams having different wavelengths has a high transmittance with respect to the wavelength 2ω2, and the frequency 2ω3 Has a high reflectance characteristic, and a beam is guided as a coaxial beam 48 by the dichroic mirror 47 for each of the two wavelengths.

  The two-wavelength superimposed beam 48 is incident on a nonlinear optical crystal 49 for sum frequency mixing under phase matching conditions, and a part of incident beam energy is converted into coherent light having a frequency of 2 (ω 2 + ω 3). A solid-state laser medium such as Nd: YAG, Nd: YVO4, or Nd: YLF is used as the solid-state laser medium with the above configuration. Examples of the tunable solid-state laser medium include titanium-added sapphire, Cr-added alexandrite, and other solid-state laser media in which the lower level of the atomic oscillation level is a vibration level. As the nonlinear optical crystal, KTP, LBO, BBO, LiNbO3, LiIO3, KDP, KD * P BaNa (NbO3) and other known nonlinear optical crystals can be used.

  In the configuration according to the present invention, one of the two wavelengths is composed of the second harmonic of the solid-state laser excited by the semiconductor laser, so that not only high output but also high oscillation efficiency is achieved. In order to contribute to the electric field formation, the conversion efficiency of the sum frequency light mixing is increased. The wavelength conversion of this all solid-state system is about 5%, and high conversion efficiency can be obtained. The second harmonic of an Nd: YAG laser can be used to excite a titanium sapphire laser as a solid-state laser, the oscillation wavelength can be set to 700 nm to 1000 nm, and the second harmonic can be obtained as a beam 38 from 350 nm to 500 nm. I can do it.

  On the other hand, when a Nd: YAG laser is used as the solid-state laser having the frequency ω2, the wavelength is fixed at 532 nm, and the sum frequency thereof is 211 nm to 257 nm. This wavelength range includes 244 nm, which is the second harmonic of an argon ion laser, and 248 nm, which is an excimer laser of a KrF laser. A laser oscillator that is smaller, lighter, and more efficient than a gas laser laser oscillator can be realized.

  The green laser beam 34 for tunable laser excitation and the beam 44 with the frequency λ3 for sum frequency mixing may be used by dividing the output beam from the same solid-state laser. In this case, two types of solid laser oscillation media can be used: a tunable laser oscillation medium and a fundamental oscillation laser medium for generating green light.

  FIG. 3 shows a configuration diagram of Embodiment 2 of the present invention. A near-infrared laser 51 having a single longitudinal mode oscillation continuous output, such as a titanium sapphire laser device, oscillates at a frequency ω2. The output light 52 is incident on the inside of a ring-type external resonator composed of four mirrors 53, 54, 55, and 56 via a mirror 53 that forms one of the four mirrors. Part of the light exits from the incident mirror 53 while rotating.

  A piezo for driving the mirror 55 by detecting the relative phase difference of the two beams 57 in which the emitted light and the reflected light directly reflected from the output light 52 by the mirror 3 are combined by the first error signal detector 58. By feedback control of the optical path length of the ring resonator to an integral multiple of the laser beam wavelength by an actuator 59 such as an element, the electric field in the resonator that can obtain optical resonance in the resonator is adjusted to a condition that can be enhanced.

  Thereby, since the light intensity in the resonator increases, the second harmonic of the frequency 2ω2 is efficiently generated by the nonlinear optical crystal 60, and the second harmonic output 61 is generated outside the ring resonator via the output mirror 56. It is emitted. If the wavelength of the titanium sapphire laser beam 52 is 900 nm, the wavelength of the second harmonic 61 is 450 nm. The second harmonic is incident on a second ring-type external resonator composed of mirrors 62, 63, 64, 65, and forms a beam 75 having an electric field enhanced from the outside while circulating inside.

  The difference between the phase of the reflected light of the second harmonic 61 from the mirror 62 and the phase of the beam transmitted around the second ring resonator and transmitted from the mirror 62 to the outside, that is, the error signal is the second. Resonance is obtained by controlling the resonator of the laser 1 by feeding back the wavelength of the titanium sapphire laser 51 so that the phase error signal is minimized.

  The Nd: YAG laser 67 having a frequency ω3 of single longitudinal mode oscillation is a generator of the laser beam 70 obtained by converting the fundamental wave of the Nd: YAG laser into the second harmonic of the frequency 2ω3 using the nonlinear optical crystal 69. The output light 70 having a wavelength of 532 nm is incident on the ring-type external resonator via the mirror 63. This light is driven by the second error signal detector 71 and the piezo element 72 by the feedback of the error signal from the 450 nm light 61 having the wavelength 2ω2, and the resonance condition is maintained. In this way, the circular beam 76 is formed by resonating both the beam 450 nm of the beam 61 incident on the second external resonator and the wavelength 532 nm, which is the frequency 2ω3 of the beam 70, in the same resonance resonator. Therefore, the sum frequency mixing action by the nonlinear optical crystal 66 is efficiently realized, and the sum frequency light 73 having a wavelength of 244 nm having a frequency 2 (ω2 + ω3) is generated. In addition to BBO, CLBO can be applied as a nonlinear optical crystal for sum frequency mixing of light with wavelengths of 532 nm and 450 nm.

  When an argon ion laser, for example, is used as the first laser oscillator, a CLBO crystal can be used as the first nonlinear optical crystal in addition to BBO.

  As mentioned above, although preferable embodiment of this invention was described, this invention is not limited to the form and wavelength which were mentioned above. For example, if the wavelength of the titanium sapphire laser is 930 nm, the sum frequency wavelength according to this configuration is 248 nm which is widely used for semiconductor exposure and the like, and is effective as an irradiation light source for a semiconductor inspection apparatus. Instead of the Nd: YAG laser, an Nd: YLF laser, a Yb: YAG laser, a Yb: GLASS laser, or the like may be used.

  As an application example of the present invention, an output of pulse laser oscillation or CW oscillation can be obtained as an all-solid high-efficiency coherent ultraviolet light source. Conventionally, this type of device has a low oscillation efficiency and requires a large device, but according to the present invention, it has been realized by a small, lightweight, high-efficiency and long-life device. This is due to the high density of recording, high integration of semiconductor devices, and fiber Bragg in fields that require coherent ultraviolet light sources such as DVD, semiconductor wafer inspection, lithography, biochemistry, and optical communication component processing, which are laser applications. Great innovation in the manufacturing process and fine processing of the writing process of grating.

Configuration diagram of conventional technology Configuration diagram of Example 1 Configuration diagram of Example 2

Explanation of symbols

31, 39: Pumping semiconductor diode laser
33: Second harmonic generation solid-state laser
35: Tunable solid-state laser
37, 43: Nonlinear optical crystals for second harmonic generation
41: Solid state laser,
47… Dichroic mirror
49: Nonlinear optical crystal for sum frequency mixing
51: Near-infrared laser (titanium sapphire laser)
58: First error signal detector
59: Piezo actuator
60: Nonlinear optical crystal
53, 54, 55, 56: Ring resonator mirror
62, 63, 64, 65: Second ring resonator mirror
66: Second error signal detector
67: Nd: YAG laser
69: Nonlinear optical crystal
72: Piezo element
66: Nonlinear optical crystal for sum frequency mixing
74: Mirror

Claims (3)

At least one pumping solid state laser resonator;
Means for generating two wavelengths of coherent beams by exciting two types of solid-state laser media with a laser beam from the excitation laser resonator;
Means for directing the coherent beam to a nonlinear optical crystal matched to each wavelength and generating a respective second harmonic coherent beam for each wavelength;
Optical means for coaxially arranging the second harmonic coherent beam;
Means for guiding the coherent beam arranged coaxially to a nonlinear crystal for sum frequency light mixing;
A high-efficiency coherent ultraviolet ray generator that generates a sum frequency beam from the nonlinear optical crystal for sum frequency mixing. Semiconductor laser excitation solid laser output wavelength λ1 output is generated in the second harmonic, tunable laser crystal is excited with the above output, and laser light λ2 in the oscillation wavelength range of 750 to 1000 nm is oscillated, and this output is output in the second harmonic. Converted to the first coherent light of wavelength λ2 / 2 by the wave generation,
On the other hand, the second coherent light of wavelength λ3 / 2 which is the second harmonic output of the solid-state laser output wavelength λ3 of the semiconductor laser excitation is generated, and the first coherent light of wavelength λ2 / 2 and the wavelength λ3 / 2 are generated. A high-efficiency coherent ultraviolet ray generator having a second wavelength λ4 (λ4 = λ2 · λ3 / {2 (λ2 + λ3)} of 210 nm to 260 nm, in which the second coherent light is arranged coaxially and is mixed in a nonlinear optical crystal. Semiconductor laser excitation solid laser output wavelength λ1 output is generated in the second harmonic, tunable laser crystal is excited with the above output, and laser light λ2 in the oscillation wavelength range of 750 to 1000 nm is oscillated, and this output is output in the second harmonic. Converted to the first coherent light of wavelength λ2 / 2 by the wave generation,
On the other hand, the second coherent light of wavelength λ3 / 2 which is the second harmonic output of the solid-state laser output wavelength λ3 of the semiconductor laser excitation is generated, and the first coherent light of wavelength λ2 / 2 and the wavelength λ3 / 2 are generated. A high-efficiency coherent ultraviolet ray generation method of 210 nm to 260 nm of the third wavelength λ4 (λ4 = λ2 · λ3 / {2 (λ2 + λ3)} by sum frequency mixing in a non-linear optical crystal by arranging the second coherent light coaxially.

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