JP2009145791A - Wavelength conversion device, inspection device, and wavelength conversion method - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a wavelength conversion device for supplying continuous deep ultraviolet laser beams having sufficient power usable for an inspection light source of a mask or the like. <P>SOLUTION: The wavelength conversion device 10 comprises a first light source 11 for generating light of first wavelength; a second light source 12 for generating light of second wavelength shorter than the first wavelength; a resonator 13 for resonating the light of first wavelength; a first wavelength conversion crystal 14 arranged in the resonator 13; and a second wavelength conversion crystal 15 arranged adjoining the emission side of the first wavelength conversion crystal 14. The emission plane of the first wavelength conversion crystal 14 and the incidence plane of the second wavelength conversion crystal 15 are arranged to face so as to be parallel with each other. The first wavelength conversion crystal 14 generates light of third wavelength by the generation of total frequency of the first wavelength light resonated by the resonator 13 and the second wavelength light, and the second wavelength conversion crystal 15 generates continuous waves of fourth wavelength by the generation of total frequency of the first wavelength light resonated by the resonator 13 and the third wavelength light. <P>COPYRIGHT: (C)2009,JPO&INPIT

Description

  The present invention relates to a wavelength conversion device, an inspection device, and a wavelength conversion method, and in particular, a wavelength conversion device that generates a deep ultraviolet continuous wave using a wavelength conversion technology using a nonlinear optical effect, and a mask inspection device using the same. And a wavelength conversion method.

  In recent years, various deep ultraviolet laser devices that generate deep ultraviolet laser light have been proposed for application to microfabrication techniques such as semiconductor manufacturing processes (see, for example, Patent Documents 1 to 8). Such a deep ultraviolet laser device is configured to generate a deep ultraviolet laser beam having a target wavelength by generating a harmonic or a sum frequency by applying a wavelength conversion technique using a nonlinear optical effect.

  Patent Document 1 describes a wavelength converter that generates a laser beam having a wavelength of 198.5 nm by sum frequency generation using a titanium sapphire laser having a wavelength of 1064 nm and an argon ion laser having a wavelength of 244 nm. In Patent Document 1, sum frequency generation is performed by a wavelength conversion crystal disposed in a resonator. The wavelength conversion crystal is a CLBO crystal, and is arranged so that the incident surface and the emission surface have a Brewster angle with respect to the resonance light.

  Patent Documents 2 to 4 describe laser devices that use a BBO crystal to generate laser light having a desired wavelength by sum frequency generation. Patent Documents 5 and 6 describe means for generating deep ultraviolet laser light by generating harmonics and sum frequency of laser light output in pulses. In Patent Document 7, two wavelength conversion crystals arranged adjacent to each other are provided in a resonator. In the first wavelength conversion crystal, laser light having a wavelength of 224 nm is generated by the sum frequency generation of the fourth harmonic of the 1064 nm laser light and the 1410 nm laser light. Further, in the next wavelength conversion crystal, a deep ultraviolet laser beam having a wavelength of 193.5 nm can be obtained by generating a sum frequency of a laser beam having a wavelength of 224 nm generated by the first wavelength conversion crystal and a laser beam having a wavelength of 1410 nm.

  In addition, a mask inspection apparatus for inspecting a mask on which a pattern is formed, such as a photomask used for manufacturing a semiconductor, is known. With the progress of semiconductor technology, that is, miniaturization, the defect size required to be detected is decreasing year by year. In order to increase the defect detection sensitivity, it is necessary to shorten the wavelength of the inspection light source. Therefore, detection resolution is improved by using deep ultraviolet light (DUV) as illumination light.

Japanese Patent No. 3939928 Special table 2002-54632 gazette Japanese Patent Laid-Open No. 10-341054 JP 2001-337354 A JP 2007-86104 A JP 2001-337356 A JP 2007-86108 A

  However, when a conventional deep ultraviolet laser device is applied as an inspection light source of a mask inspection apparatus, there are the following problems. That is, in the mask inspection apparatus, it is desired to increase the output of the deep ultraviolet continuous wave in order to improve the inspection sensitivity. However, for example, as in Patent Document 2, when a laser beam having a wavelength of 193 nm cannot be generated unless a BBO crystal is used, the BBO crystal has a large absorption at a wavelength of 193 nm. Cannot be realized.

  Further, even when a laser other than the BBO crystal can be used for generation of laser light having a wavelength of 193 nm as in Patent Document 5, a proposal that is difficult to realize has been made such as the light source wavelength used is 1410 nm. In Patent Document 6, since the number of wavelength conversions is too large, a practical output cannot be obtained unless pulse oscillation is used, and continuous wave deep ultraviolet light cannot be generated.

  In Patent Document 7, two wavelength conversion crystals are inserted in the ring resonator, but an AR coat (antireflection film) for a total of three wavelengths is required. For this reason, it is difficult to realize an AR coating that is resistant to damage and has a high yield strength.

  The present invention has been made against the background of the above circumstances, and an object of the present invention is to perform wavelength conversion for supplying continuous deep ultraviolet laser light having sufficient power that can be used for an inspection light source such as a mask. An apparatus, a wavelength conversion method, and an inspection apparatus using the same.

  The wavelength converter according to the first aspect of the present invention includes a first light source that generates light having a first wavelength, a second light source that generates light having a second wavelength shorter than the first wavelength, and the first light source. A resonator for resonating light of a wavelength; a first wavelength conversion crystal disposed in the resonator; and a second wavelength conversion crystal disposed adjacent to an emission side of the first wavelength conversion crystal; The exit surface of the first wavelength conversion crystal and the entrance surface of the second wavelength conversion crystal are arranged to face each other in parallel, and the first wavelength conversion crystal is resonated by the resonator. The third wavelength light is generated by the sum frequency generation of the first wavelength light and the second wavelength light, and the second wavelength conversion crystal is the first wavelength light resonated by the resonator. Then, a continuous wave of the fourth wavelength is generated by generating the sum frequency of the light of the third wavelength. Thereby, a continuous wave of high output can be obtained efficiently.

  The wavelength conversion device according to a second aspect of the present invention is the above wavelength conversion device, wherein the incident surface of the first wavelength conversion crystal and the emission surface of the second wavelength conversion crystal are resonated by the resonator. It is arranged so as to have a Brewster angle with respect to the light of the first wavelength. Thereby, it is not necessary to provide an AR coat on the incident surface of the first wavelength conversion crystal and the emission surface of the second wavelength conversion crystal, and a reduction in loss can be realized.

  In the wavelength conversion device according to a third aspect of the present invention, in the above wavelength conversion device, the emission surface of the first wavelength conversion crystal and the incident surface of the second wavelength conversion crystal are resonated by the resonator. It is arranged so as to have a Brewster angle with respect to the light of the first wavelength. Thereby, it is not necessary to provide an AR coat on the emission surface of the first wavelength conversion crystal and the incident surface of the second wavelength conversion crystal, and a reduction in loss can be realized.

  In the wavelength conversion device according to a fourth aspect of the present invention, in the above wavelength conversion device, the first wavelength light has a wavelength of 1550 to 1560 nm, and the second wavelength light has a wavelength of 257 to 258 nm. The fourth wavelength light generated in the second wavelength conversion crystal has a wavelength of 193 to 194 nm. Thereby, it becomes possible to generate deep ultraviolet light of 193 to 194 nm efficiently.

  A wavelength conversion device according to a fifth aspect of the present invention is characterized in that, in the above wavelength conversion device, the light of the first wavelength is generated by an erbium-doped fiber laser or an amplifier. The present invention is particularly effective in such a case.

  In the wavelength conversion device according to a sixth aspect of the present invention, in the above wavelength conversion device, the light of the second wavelength is a second harmonic of an argon ion laser or a fourth harmonic of a Yb-added solid-state laser. It is characterized by being. The present invention is particularly effective in such a case.

  The wavelength conversion device according to a seventh aspect of the present invention is the above wavelength conversion device, wherein the first wavelength conversion crystal is a Type-2 phase-matching nonlinear optical crystal, and the second wavelength conversion crystal is Type. It is a −1 phase matching nonlinear optical crystal. The present invention is particularly effective in such a case.

  A wavelength conversion device according to an eighth aspect of the present invention is characterized in that, in the above wavelength conversion device, the first wavelength conversion crystal and the second wavelength conversion crystal are any one of LBO, CBO, and CLBO. To do. Thereby, the wavelength conversion efficiency can be increased, and a high-power continuous wave can be obtained.

  An inspection apparatus according to a ninth aspect of the present invention includes any one of the wavelength conversion apparatuses described above. Thereby, inspection sensitivity can be improved.

  An inspection apparatus according to a tenth aspect of the present invention generates light having a first wavelength of 1550 to 1560 nm, generates light having a second wavelength of 257 to 258 nm, and resonators the light having the first wavelength. The third wavelength light is generated by the sum frequency generation of the resonated light of the first wavelength and the light of the second wavelength by the first wavelength conversion crystal disposed in the resonator. The second wavelength conversion crystal is disposed adjacent to the emission side of the first wavelength conversion crystal so that the emission surface of the first wavelength conversion crystal and the incident surface of the second wavelength conversion crystal are parallel to each other. The fourth wavelength light is generated by the sum frequency generation of the first wavelength light and the third wavelength light resonated by the resonator. Thereby, high-power continuous deep ultraviolet light can be efficiently generated.

  According to the present invention, it is possible to provide a wavelength conversion device, a wavelength conversion method, and an inspection device using the wavelength conversion device for supplying continuous deep ultraviolet laser light having sufficient power that can be used as an inspection light source such as a mask. .

  Embodiments of the present invention will be described below with reference to the drawings. The following description shows preferred embodiments of the present invention, and the scope of the present invention is not limited to the following embodiments. In the following description, the same reference numerals denote the same contents.

  A wavelength converter according to an embodiment of the present invention will be described with reference to FIG. FIG. 1 is a diagram illustrating a configuration of a wavelength conversion device 10 according to the present embodiment. As shown in FIG. 1, the wavelength conversion device 10 includes a first light source 11, a second light source 12, a resonator 13, and a first wavelength conversion crystal 14. The wavelength conversion device 10 according to the present embodiment is suitable for an inspection light source for inspecting a photomask or the like.

  The first light source 11 generates laser light in the infrared region having a continuous oscillation wavelength of 1550 to 1560 nm. As the first light source 11, an erbium-doped fiber laser (EDFA: Erbium Doped Fiber Laser) that has already been put into practical use and marketed, or an amplifier can be used. In the present embodiment, the first wavelength light generated from the first light source 11 is described as 1556 nm.

  The second light source 12 generates laser light in the ultraviolet region having a continuous oscillation wavelength of 257 to 258 nm. As the second light source 12, an argon ion laser or a Yb-added solid laser can be used. When an argon ion laser that generates laser light having a wavelength of 515 nm is used as the second light source 12, laser light having a wavelength of 257.5 nm can be obtained by the second harmonic generation. In addition, when a Yb-added solid-state laser that generates a laser beam having a wavelength of 1030 nm is used as the second light source 12, a laser beam having a wavelength of 257.5 nm can be obtained by the fourth harmonic generation. In the present embodiment, the second wavelength light generated from the second light source 12 is described as 257.5 nm.

  The resonator 13 is a ring type optical resonator and includes a plurality of mirrors. In the present embodiment, the resonator 13 includes four mirrors 13a, 13b, 13c, and 13d. That is, the four mirrors 13a, 13b, 13c, and 13d constitute a resonator 13 having a ring-shaped (actually polygonal) optical path. The resonator 13 resonates the laser light generated by the first light source 11 and increases the light intensity. The number of mirrors is not limited to this.

A first wavelength conversion crystal 14 and a second wavelength conversion crystal 15 are arranged in the resonator 13. The first wavelength conversion crystal 14 and the second wavelength conversion crystal 15 are nonlinear optical crystals, and any one of LBO (LiB ₃ O ₅ ), CBO (CsB ₃ O ₅ ), and CLBO (CsLiB ₆ O ₁₀ ) is used. Can do. These nonlinear optical crystals have smaller absorption at wavelengths of 193 to 194 nm than BBO crystals. For this reason, the wavelength conversion efficiency can be increased, and a high-power continuous wave can be obtained. The second wavelength conversion crystal 15 is disposed adjacent to the emission side of the first wavelength conversion crystal 14. Further, the emission surface of the first wavelength conversion crystal 14 and the incident surface of the second wavelength conversion crystal 15 are arranged to face each other so as to be parallel to each other.

  The first wavelength conversion crystal 14 generates light of the third wavelength by generating a sum frequency of the laser light of the first wavelength resonated by the resonator 13 and the laser light of the second wavelength generated by the second light source 12. . The second wavelength conversion crystal 15 generates a continuous wave of the fourth wavelength by generating a sum frequency of the laser light of the first wavelength resonated by the resonator 13 and the laser light of the third wavelength. The fourth wavelength light generated in the second wavelength conversion crystal 15 has a wavelength of 193 to 194 nm.

  In the present embodiment, the first wavelength conversion crystal 14 is generated by the sum frequency generation of the laser light (wavelength 1556 nm) resonated by the resonator 13 and the laser light (wavelength 257.5 nm) generated by the second light source 12. Laser light having a wavelength of 220.9 nm is generated. The second wavelength conversion crystal 15 generates a continuous wave of 193.5 nm by generating a sum frequency of the laser light (wavelength 1556 nm) resonated by the resonator 13 and the laser light having a wavelength of 220.9 nm. Thus, according to the present invention, it is possible to efficiently generate a high-power 193-194 nm deep ultraviolet light continuous wave.

  Here, the arrangement of the first wavelength conversion crystal 14 and the second wavelength conversion crystal 15 will be described with reference to FIG. FIG. 2 is a diagram illustrating an arrangement example of the first wavelength conversion crystal 14 and the second wavelength conversion crystal 15. In the example shown in FIG. 2A, each of the first wavelength conversion crystal 14 and the second wavelength conversion crystal 15 is a parallelogram-shaped nonlinear optical crystal. Further, the incident surface and the exit surface of the first wavelength conversion crystal 14, and the entrance surface and the exit surface of the second wavelength conversion crystal 15 all have a Brewster angle with respect to the laser light resonated by the resonator 13. Is arranged. Thereby, it is not necessary to provide an AR coat on the incident surface and the exit surface of the first wavelength conversion crystal 15, the incident surface and the exit surface of the second wavelength conversion crystal, and a reduction in loss can be realized. In addition, the aberration in the ring resonator 13 can be corrected.

  In the example shown in FIG. 2B, both the first wavelength conversion crystal 14 and the second wavelength conversion crystal 15 are trapezoidal nonlinear optical crystals. Further, the incident surface and the exit surface of the first wavelength conversion crystal 14, and the entrance surface and the exit surface of the second wavelength conversion crystal 15 all have a Brewster angle with respect to the laser light resonated by the resonator 13. Is arranged. Thereby, similarly to the above, it is not necessary to provide an AR coat, a reduction in loss can be realized, and aberrations in the ring resonator 13 can be corrected. In the example shown in FIGS. 2A and 2B, the first wavelength laser beam emitted from the first wavelength conversion crystal 14 and the third wavelength laser beam may be separated. For this reason, it is preferable that the first wavelength conversion crystal 14 and the second wavelength conversion crystal 15 be as close as possible.

  Moreover, you may arrange | position the trapezoidal 1st wavelength conversion crystal 14 and the 2nd wavelength conversion crystal 15 like FIG.2 (c). That is, the incident surface of the first wavelength conversion crystal 14 is arranged so as to have a Brewster angle with respect to the laser light resonated by the resonator 13. Further, the emission surface of the second wavelength conversion crystal 15 is arranged so as to have a Brewster angle with respect to the laser light resonated by the resonator 13. The exit surface of the first wavelength conversion crystal 14 and the entrance surface of the second wavelength conversion crystal 15 are surfaces perpendicular to the optical axes of the first wavelength laser light and the third wavelength laser light emitted from the first wavelength conversion crystal 14. It has become.

  Therefore, the first wavelength laser beam and the third wavelength laser beam emitted from the first wavelength conversion crystal 14 are parallel and enter the second wavelength conversion crystal 15 perpendicularly. Thereby, the mixing of the first wavelength laser beam and the third wavelength laser beam in the second wavelength conversion crystal 15 is easy. Note that it is preferable to provide an AR coat on the exit surface of the first wavelength conversion crystal 14 and the entrance surface of the second wavelength conversion crystal 15. In the case of FIG. 2C, a ring type resonator can be constituted by two mirrors, and the device configuration can be simplified.

  Moreover, it is also possible to arrange | position the 1st wavelength conversion crystal 14 and the 2nd wavelength conversion crystal 15 of parallelogram shape like FIG.2 (d). Also in this case, a ring resonator can be formed by two mirrors, and the apparatus configuration can be simplified.

  The first wavelength conversion crystal 14 is a Type-2 phase matching nonlinear optical crystal. Here, a Type-2 phase-matching nonlinear optical crystal will be described with reference to FIG. FIG. 3 is a diagram for explaining a Type-2 phase-matching nonlinear optical crystal. In FIG. 3, black circles indicate vertical polarization with respect to the first wavelength conversion crystal 14, and arrows indicate horizontal polarization with respect to the first wavelength conversion crystal 14. The solid line is a laser beam having the first wavelength generated and resonated with the first light source 11, the dotted line is the laser beam having the second wavelength generated with the second light source 12, and the one-dot chain line is the first wavelength conversion crystal. 14 is a third wavelength laser beam generated at 14.

  The Type-2 phase-matching nonlinear optical crystal is capable of generating a sum frequency when the polarization states of two incident lights are orthogonal. As shown in FIG. 3, the first wavelength laser light is horizontally polarized with respect to the first wavelength conversion crystal 14, and the second wavelength laser light is vertically polarized with respect to the first wavelength conversion crystal 14. Therefore, in the first wavelength conversion crystal 14, the third wavelength laser beam is generated by the sum frequency generation of the first wavelength laser beam and the second wavelength laser beam. That is, the first wavelength conversion crystal 14 generates laser light (λ3 = 1 / (1 / λ1 + 1 / λ2)) having the wavelength λ3 by the sum frequency mixing of the resonance light (wavelength λ1) and the external incident light (wavelength λ2). .

  Specifically, when the first wavelength (λ1) is 1556 nm and the second wavelength (λ2) is 257.5 nm, θ = 56.6 ° and φ = 90 ° with respect to the optical axis as the first wavelength conversion crystal 14. When the LBO crystal cut into is used, Type-2 sum frequency mixing becomes possible, and laser light having a wavelength λ3 = 220.9 nm is generated.

  Further, since the laser light of the first wavelength is incident on the first wavelength conversion crystal 14 at the Brewster angle, there is no reflection loss. Since the second wavelength laser beam is vertically polarized, there is some reflection loss, but this is not a big problem in practice.

  The second wavelength conversion crystal 15 is a Type-1 phase matching nonlinear optical crystal. Here, with reference to FIG. 4, the nonlinear optical crystal of Type-1 phase matching will be described. FIG. 4 is a diagram for explaining a type-1 nonlinear optical crystal of phase matching. Also in FIG. 4, black circles indicate vertical polarization with respect to the first wavelength conversion crystal 14, and arrows indicate horizontal polarization with respect to the first wavelength conversion crystal 14. The solid line is the laser beam having the first wavelength generated by the first light source 11 and resonated, the alternate long and short dash line is the laser beam having the third wavelength generated by the first wavelength conversion crystal 14, and the dotted line is the second laser beam. This is a fourth wavelength laser beam generated in the wavelength conversion crystal 15.

  Type-1 phase-matching nonlinear optical crystal is capable of generating sum frequency when the polarization states of two incident lights are parallel. As shown in FIG. 4, the first wavelength laser light is horizontally polarized with respect to the second wavelength conversion crystal 15, and the third wavelength laser light is also horizontally polarized with respect to the second wavelength conversion crystal 15. Therefore, in the second wavelength conversion crystal 15, the fourth wavelength laser beam is generated by the sum frequency generation of the first wavelength laser beam and the third wavelength laser beam. That is, the second wavelength conversion crystal 15 generates a laser beam (λ4 = 1 / (1 / λ1 + 1 / λ3)) having a wavelength λ4 by the sum frequency mixing of the resonant light (wavelength λ1) and the incident light (wavelength λ3). Note that the second wavelength conversion crystal 15 does not participate in the laser light of the second wavelength (λ2) that has left the first wavelength conversion crystal 14.

  Specifically, when the first wavelength (λ1) is 1556 nm and the third wavelength (λ3) is 220.3 nm, the second wavelength conversion crystal 15 is cut to θ = 90 ° and φ = 77 ° with respect to the optical axis. When the LBO crystal is applied, Type-1 sum frequency mixing is possible, and laser light having a wavelength λ3 = 193.4 nm is generated.

  Further, since the laser light of the first wavelength is incident on the first wavelength conversion crystal 14 at the Brewster angle, there is no reflection loss. The laser light of the third wavelength is also horizontally polarized light and is not strictly a Brewster angle, but has almost no reflection loss because it is almost close to the Brewster angle.

  As described above, according to the present invention, a commercially available commercial laser light source (wavelength 1550 to 1560 nm) and a laser light source with a wavelength of 257.5 nm are used, and high-power 193 to 194 nm is efficiently produced by sum frequency mixing. It is possible to generate a continuous wave of deep ultraviolet light. For sum frequency mixing, LBO, CBO, and CLBO crystals having sufficient transmittance even with a laser beam of 193 to 194 nm can be used. Further, by arranging the two wavelength conversion crystals in the resonator as a Brewster type, two-stage sum frequency generation can be performed with high efficiency.

  Further, the wavelength conversion device according to the present invention can be suitably used for an inspection device for inspecting a mask or the like. In the wavelength converter according to the present invention, laser light having a wavelength of 193 to 194 nm can be obtained, so that it can be matched with the exposure wavelength of 193.36 nm, and inspection sensitivity can be improved.

  That is, the mask inspection apparatus to which the present invention is applied is intended for a mask inspection apparatus using a next-generation DUV laser beam having a wavelength of 200 nm or less, and is used for defect inspection at the time of manufacturing a next-generation mask by a mask manufacturer. Not only can it be used, but it can also be used for mask quality control in improving semiconductor manufacturing.

It is a figure which shows the structure of the wavelength converter which concerns on embodiment. It is a figure for demonstrating arrangement | positioning of the wavelength conversion crystal which concerns on embodiment. It is a figure for demonstrating the characteristic of the 1st wavelength conversion crystal which concerns on embodiment. It is a figure for demonstrating the characteristic of the 2nd wavelength conversion crystal which concerns on embodiment.

Explanation of symbols

DESCRIPTION OF SYMBOLS 10 Wavelength conversion apparatus 11 1st light source 12 2nd light source 13 Resonator 13a, 13b, 13c, 13d Mirror 14 1st wavelength conversion crystal 15 2nd wavelength conversion crystal

Claims (10)

A first light source that generates light of a first wavelength;
A second light source that generates light having a second wavelength shorter than the first wavelength;
A resonator for resonating light of the first wavelength;
A first wavelength conversion crystal disposed in the resonator;
A second wavelength conversion crystal disposed adjacent to the emission side of the first wavelength conversion crystal,
The exit surface of the first wavelength conversion crystal and the entrance surface of the second wavelength conversion crystal are arranged to face each other so as to be parallel to each other,
The first wavelength conversion crystal generates light of a third wavelength by sum frequency generation of the light of the first wavelength resonated by the resonator and the light of the second wavelength,
The second wavelength conversion crystal is a wavelength conversion device that generates a continuous wave of a fourth wavelength by generating a sum frequency of the light of the first wavelength resonated by the resonator and the light of the third wavelength.   The entrance surface of the first wavelength conversion crystal and the exit surface of the second wavelength conversion crystal are disposed so as to have a Brewster angle with respect to the light of the first wavelength resonated by the resonator. Item 2. The wavelength converter according to Item 1.   The exit surface of the first wavelength conversion crystal and the incident surface of the second wavelength conversion crystal are arranged so as to have a Brewster angle with respect to the light of the first wavelength resonated by the resonator. Item 3. The wavelength conversion device according to Item 1 or 2. The first wavelength light has a wavelength of 1550 to 1560 nm,
The second wavelength light has a wavelength of 257 to 258 nm,
4. The wavelength conversion device according to claim 1, wherein the fourth wavelength light generated in the second wavelength conversion crystal has a wavelength of 193 to 194 nm.   The wavelength converter according to claim 1, wherein the light having the first wavelength is generated by an erbium-doped fiber laser or an amplifier.   The wavelength conversion according to any one of claims 1 to 4, wherein the light of the second wavelength is a second harmonic of an argon ion laser or a fourth harmonic of a Yb-added solid-state laser. apparatus. The first wavelength conversion crystal is a Type-2 phase matching nonlinear optical crystal,
The wavelength conversion device according to claim 1, wherein the second wavelength conversion crystal is a Type-1 phase matching nonlinear optical crystal.   The wavelength converter according to claim 1, wherein the first wavelength conversion crystal and the second wavelength conversion crystal are any one of LBO, CBO, and CLBO.   An inspection apparatus provided with the wavelength converter of any one of Claims 1-8. Generating light having a first wavelength of 1550 to 1560 nm;
Generating light having a second wavelength of 257 to 258 nm,
Resonating light of the first wavelength with a resonator;
The first wavelength conversion crystal disposed in the resonator generates the third wavelength light by the sum frequency generation of the resonated light of the first wavelength and the light of the second wavelength,
By the second wavelength conversion crystal disposed adjacent to the emission side of the first wavelength conversion crystal so that the emission surface of the first wavelength conversion crystal and the incident surface of the second wavelength conversion crystal are parallel to each other. A wavelength conversion method for generating light of a fourth wavelength by generating a sum frequency of the light of the first wavelength and the light of the third wavelength resonated by the resonator.

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