JP2007287886A - Laser, and tester using the laser - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a laser which changes the number of longitudinal modes to be generated without changing the wavelength conversion efficiency. <P>SOLUTION: The laser 1 has a laser oscillator 10 for generating a laser beam of a wavelength resonant to a first cavity resonator and an external resonator type wavelength converter 20 for converting the wavelength of the laser beam taken out of the laser oscillator 10 to emit a laser beam of a wavelength resonant to a second cavity resonator 24. The first cavity resonator has a resonance wavelength not greater than 1/15 times that of the second cavity resonator 24. <P>COPYRIGHT: (C)2008,JPO&INPIT

Description

  The present invention relates to a laser device and an inspection apparatus using the laser device. In particular, the present invention relates to a configuration of a wavelength conversion laser and an inspection apparatus using the wavelength conversion laser.

  Along with the miniaturization of semiconductors accompanying the progress of semiconductors, the size required for defect detection in semiconductors has been decreasing year by year. Therefore, it is necessary to shorten the wavelength of the inspection light source in order to increase the defect detection sensitivity. Therefore, in order to shorten the wavelength of the inspection light source, a wavelength conversion technique may be used. As an example, wavelength conversion is performed using a YAG laser having a fundamental wave of 1064 nm, and an ultraviolet laser beam having a wavelength of 266 nm of the fourth harmonic is obtained.

  Currently, a continuously oscillating laser (CW laser) may be used for defect inspection of the mask substrate surface (for example, Non-Patent Document 1). Therefore, it has become necessary to convert the wavelength of laser light oscillated by a CW laser. With the miniaturization of semiconductors, it is necessary to condense the laser light used for the inspection light source and reduce the spot diameter. In order to reduce the spot diameter, it is considered preferable to use a single laser beam transverse mode and longitudinal mode.

  However, if both the transverse mode and longitudinal mode of the laser beam are set to a single mode, the coherence becomes very high, and a speckled mottled pattern called speckle is generated, which becomes noise on the image in the inspection apparatus. There was a problem of end. Therefore, there is a method for solving this problem by setting only the transverse mode related to the light collecting property to a single mode and the longitudinal mode to a multimode.

  For these reasons, there is a need for a wavelength conversion laser for wavelength conversion of laser light having a multi-longitudinal mode. A general wavelength conversion laser is an internal wavelength conversion type in which a wavelength conversion crystal is disposed inside a resonator (for example, Patent Document 1). In this internal wavelength conversion type laser, the wavelength of the CW laser can be converted efficiently.

  However, in order to efficiently convert the wavelength of a multi-longitudinal mode CW laser beam having a plurality of longitudinal modes, there is a restriction on the wavelength converter that a standing wave is generated between the resonators in all longitudinal modes. Necessary. This is different from a simple external resonator in the case of a single longitudinal mode. For this reason, when a fundamental wave of a multi-longitudinal mode is generated by an internal wavelength conversion type laser, problems such as a light intensity beat occur in the second harmonic extracted outside (for example, non-patent document) 2).

  For these reasons, it is preferable to perform wavelength conversion on the multi-longitudinal mode fundamental laser once generated outside the resonator. However, it is necessary to apply an external resonator in order to increase the power of the fundamental wave in the wavelength conversion crystal.

  Here, the configuration of a conventional general wavelength conversion laser apparatus will be briefly described. A conventional wavelength conversion type laser device 900 shown in FIG. 6 includes a laser oscillator 901 and an external resonator type wavelength converter 902, and the external resonator type wavelength converter 902 has a shape called a bow tie type. Yes.

  The laser oscillator 901 uses a YAG crystal 903 as a laser medium, and an output mirror 904 and a total reflection mirror 905 form a resonator. The external resonator type wavelength converter 902 includes a partial transmission mirror 907, two mirrors 908a and 908b, and a dichroic mirror 909.

  The fundamental laser beam oscillated in the laser oscillator 901 is extracted from the output mirror 904. The laser light passes through the lens 906 and enters the resonator from the partial transmission mirror 907 in the external resonator type wavelength converter 902. An SHG crystal 910 is disposed at a position where the beam is focused. Here, SHG refers to the second harmonic, and the SHG crystal is also called a nonlinear optical crystal, a nonlinear optical element, a wavelength conversion crystal, or a wavelength conversion element. The generated second harmonic laser light is extracted from the dichroic mirror 909 to the outside.

However, in order to efficiently convert the wavelength of multi-longitudinal mode laser light, it is necessary to adjust the resonator length of the external resonator to an integral multiple of the wavelength in all longitudinal modes. The reason for this will be described below. The exact wavelength of each longitudinal mode of the fundamental laser beam oscillated in the resonator is a value obtained by dividing the resonator length L1 of the fundamental wave by an integer, considering that a standing wave is formed between the resonators. . Therefore, each mode can be distinguished by the number of standing waves. For example, if an integer is m, it can be distinguished by m, (m + 1), (m + 2). Therefore, the exact wavelengths λ _m , λ _{m + 1} , λ _{m + 2} ... Of each longitudinal mode can be obtained by L1 / m, L1 / (m + 1), L1 / (m + 2).

When the resonator length L2 of the external resonator is an integral multiple of the wavelength of each longitudinal mode of these fundamental waves, each mode efficiently converts the wavelength. That is, L2 needs to be an integral multiple of the wavelengths λ _m , λ _{m + 1} , λ _{m + 2} ... Of each longitudinal mode, and L2 / L1 · m, L2 / L1 · (m + 1), L2 / L1 · (m + 2 ) ... must be an integer. That is, if the value of L2 / L1 becomes an integer, L2 becomes an integral multiple of the wavelength of each longitudinal mode without depending on the value of m. Therefore, L2 may be an integer multiple of L1.
USP 5,696,780 Journal of Optical Society of America, B, Vol.3, No.9, pp.1175-1180, 1986. Proceedings of SPIE Vol.5256, pp.556-565, 2003.

  However, in the conventional wavelength conversion method for multi-longitudinal mode laser light, it is necessary to change the resonator length of the external resonator when the resonator length of the fundamental laser light is changed. There is a problem that must be redesigned. This is because the number of longitudinal modes of the fundamental laser beam may be changed when the number of longitudinal modes desired to be generated is changed for reasons such as eliminating speckles. At this time, it is necessary to change the resonator length of the fundamental laser beam.

  In particular, as can be seen from the shape of the external resonator type wavelength converter 902 of the wavelength conversion type laser device 900 shown in FIG. 6 as a conventional example, it is difficult to easily change the resonator length, and the mirror to be configured Since all or at least the mirrors on both sides of the SHG crystal 910 need to be concave, when changing the resonator length, it is necessary to change not only the position of each mirror but also the curvature of those concave surfaces. is there.

  The present invention has been made to solve such a problem, and an object of the present invention is to provide a wavelength conversion laser capable of easily changing the number of longitudinal modes to be generated.

  In the laser device according to the first aspect of the present invention, a laser oscillator that oscillates the laser light resonated by the first resonator, and used to convert the wavelength of the laser light extracted from the laser oscillator, A laser device having an external resonator type wavelength converter for emitting laser light resonated by a second resonator, wherein the resonator length of the second resonator is the resonator of the first resonator; The length is 1/15 or less. In this laser apparatus, even if the number of modes to be generated is changed by reducing the resonator length of the second resonator to 1/15 or less of the resonator length of the first resonator, the wavelength conversion efficiency is improved. Wavelength conversion can be performed without lowering.

  The laser device according to the second aspect of the present invention is the above laser device, wherein the second resonator in the external resonator type wavelength converter includes two opposing mirrors and the two pieces. And a nonlinear crystal that performs wavelength conversion. The laser apparatus according to the third aspect of the present invention is the laser apparatus according to the second aspect, wherein one of the two opposing mirrors has a partially transmissive film that transmits a part of light to the concave mirror. The first mirror is provided, and the other of the two opposing mirrors is a second mirror in which a concave mirror is provided with a dichroic film having different reflectivity according to the wavelength. By doing so, a very compact external resonator can be formed.

  A laser device according to a fourth aspect of the present invention is the laser device according to the second aspect, wherein the external resonator type wavelength converter includes two semi-convex lenses and the semi-convex lens. The one half-convex lens is provided with a partially transmissive film that transmits part of light on the lens surface, and the other half-convex lens is disposed on the lens surface. A dichroic film having a different reflectance according to a predetermined wavelength is provided. By doing so, a very compact external resonator can be formed, and the laser device can be made very small.

  The laser device according to a fifth aspect of the present invention is the laser device according to the second aspect, wherein the external resonator wavelength converter includes the nonlinear crystal, and the nonlinear crystal faces each other. The two end faces have a convex shape, a partial transmission film that transmits part of light is provided on one end face of the two end faces having the convex shape, and the other end face has a wavelength according to the wavelength. A dichroic film having different reflectance is provided. By doing so, a very compact external resonator can be formed. Further, since the external resonator is provided with a film on a nonlinear crystal, the cost can be reduced without the need for another lens or mirror.

  The laser device according to the sixth aspect of the present invention is the laser device according to the second aspect, wherein the external resonator wavelength converter is a lens in contact with the nonlinear crystal and an end face of the nonlinear crystal. The incident surface and the exit surface of the laser light in the nonlinear crystal are acute angles with respect to the end surface with which the lens is in contact, and a partial transmission film that transmits a part of the light on the incident surface A dichroic film having a different reflectance according to the wavelength is provided on the exit surface, and a reflective film that totally reflects light is provided on the lens surface of the lens. By doing so, it is possible to eliminate the reflection of the laser beam on the incident side.

  A laser device according to a seventh aspect of the present invention is the above laser device, wherein the laser oscillator is a YAG laser, and the resonator length of the laser oscillator is 0.3 m or more. To do.

  The inspection apparatus according to the eighth aspect of the present invention is an inspection apparatus using the above laser device as a light source. By using this inspection apparatus, speckle noise can be eliminated, so that highly sensitive defect detection can be performed.

  According to the laser apparatus and the mask inspection apparatus using the laser apparatus according to the present invention, the wavelength conversion efficiency is reduced by setting the resonator length of the external resonator to 1/15 or less of the resonator length of the fundamental laser oscillator. The number of vertical modes can be changed without dropping the. In addition, since wavelength conversion can be performed with the longitudinal mode as the multi-mode, a light source that does not cause speckles that adversely affect the defect detection sensitivity in the inspection device even if the spot diameter is reduced by focusing is provided. can do.

Embodiment 1 FIG.
Hereinafter, specific embodiments to which the present invention is applied will be described in detail with reference to the drawings. In this embodiment, the present invention is applied to a laser apparatus and a mask inspection apparatus using the laser apparatus. The laser device according to the present embodiment includes a laser oscillator and a resonator-type wavelength converter provided outside thereof, and the resonator length of the resonator provided in the resonator-type wavelength converter is determined by the laser oscillator. The resonator length is 1/15 or less.

  For this reason, we offer a multi-longitudinal mode laser device that can freely change the number of longitudinal modes of laser light that is wavelength-converted without changing the wavelength conversion efficiency by changing the resonator length of the laser oscillator. it can. Moreover, in the laser apparatus in this Embodiment, since the resonator length provided in the wavelength converter can be shortened, it is possible to make a laser apparatus compact. In this embodiment, a YAG laser is described as an example of the laser oscillator, but the present invention is not limited to this.

  In FIG. 1, the schematic block diagram of the laser apparatus 1 which concerns on this Embodiment is shown. The laser device 1 includes a laser oscillator 10 and an external resonator type wavelength converter 20. In FIG. 1, a mirror 30 is used to make light incident from the laser oscillator 10 to the external resonator type wavelength converter 20.

  The laser oscillator 10 includes a laser light source LD 11, an optical fiber 12, a collimating lens 13, a reflecting mirror 14 with a hole, a YAG crystal 15, an output mirror 16, a total reflecting mirror 17, and a quarter wavelength plate 18. ing. The LD 11 uses, for example, a semiconductor laser having a wavelength of 808 nm as an excitation light source for the YAG crystal 15.

  Laser light with a wavelength of 808 nm oscillated from the LD 11 passes through the optical fiber 12. The light emitted from the optical fiber 12 travels while being collected by the collimator lens 13, is reflected by the reflecting mirror with a hole, and is collected and incident on the end surface of the YAG crystal 15. As a result, laser light having a fundamental wave of 1064 nm is generated in the resonator formed by the output mirror 16 and the total reflection mirror 17. The quarter-wave plate 18 disposed in the resonator is for correcting polarization disturbance due to heat of the YAG crystal 15.

  The fundamental laser beam extracted from the output mirror 16 is reflected by the mirror 30. The light reflected by the mirror 30 enters the external resonator type wavelength converter 20. The laser light incident on the external resonator type wavelength converter 20 is reflected by the polarization beam splitter 21. The laser beam reflected by the polarization beam splitter 21 passes through the rotator 22 that can rotate the polarization direction of the incident laser beam by 45 degrees, and proceeds while being narrowed down through the lens 23. Then, the light is condensed in the SHG crystal 241 disposed in the external resonator 24 constituted by the partial transmission mirror 242 and the dichroic mirror 243.

  The partial transmission mirror 242 has a reflectance of about 80% with respect to the fundamental wave wavelength of 1064 nm, and the dichroic mirror 243 has a reflectance of 99% or more with respect to the fundamental wave of 1064 nm. The dichroic mirror 243 is provided with a PZT (piezo) element 244 so that the position of the dichroic mirror 243 in the optical axis direction can be finely adjusted with an accuracy of several nanometers.

  On the other hand, when the fundamental laser beam that has entered the external resonator 24 is returned from the partial transmission mirror 242 to the lens 23, it passes through the rotator 22 again, so that the polarization direction is further rotated 45 degrees and incident from the mirror 30. It becomes a laser beam having a polarization direction rotated 90 degrees from the polarization direction of the laser beam. Therefore, the light passes through the polarization beam splitter 21 and enters the photodetector 25.

  Further, a signal 26 is sent to the PZT element 244 to finely adjust the position of the dichroic mirror 243 in the optical axis direction so that the laser power measured by the photodetector 25 is minimized. As a result, the fundamental laser beam forms a standing wave in the external resonator 24.

  The partial transmission mirror 242 has a high reflectance of 99% or more with respect to the wavelength of 532 nm, and the dichroic mirror 243 has a high transmittance of 99% or more with respect to the wavelength of 532 nm. As a result, the second harmonic generated in the SHG crystal 241 is extracted from the external resonator 24 to the outside.

  In the laser device 1 according to the present embodiment, the resonator length of the external resonator 24 constituted by the partial transmission mirror 242 and the dichroic mirror 243 in the external resonator type wavelength converter 20 is set in the laser oscillator 10. The resonator length of the resonator composed of the output mirror 16 and the total reflection mirror 17 is 1/15 or less. The reason will be described below.

First, the resonator length of the resonator in the laser oscillator 10 is L1, and the resonator length of the resonator in the external resonator type wavelength converter 20 is L2. For each longitudinal mode of the fundamental wave, the condition that the second harmonic resonates is expressed by L2 = m · L1 using an integer m. The reason will be described below. First, the wavelength of laser light in modes m, m + 1, m + 2 is
λ _m = L1 / m, λ _{m + 1} = L1 / (m + 1), λ _{m + 2} = L1 / (m + 2)
It becomes.

The condition of L2 _{is, L2 / λ m, L2 /} λ m + 1, and L2 / λ _{m + 2} may if an integer. In order to satisfy the above-mentioned condition of L2 for any m, L2 needs to be an integral multiple of L1. However, it is necessary not only to adjust L2 to an integral multiple of L1, but also to finely adjust the wavelength level. When the resonator length L2 of the external resonator deviates from an integral multiple of half of the wavelength λ of the fundamental laser beam incident on the external resonator, assuming that deviation is ΔL (assumed to be an absolute value), ΔL is divided by λ. The value becomes a value from 0 to 1.

Here, when the resonator length L2 of the external resonator deviates from an integral multiple of half the wavelength λ of the fundamental laser beam incident thereon, the deviation is set to ΔL. For a certain longitudinal mode m, the deviation ΔL _m from the standing wave of the resonator length L2 is
ΔL _m = L2-INT (L2 / λ _m ) · λ _m
The resonator length L2 is finely adjusted so that ΔLm becomes zero. In the longitudinal mode m at this time, a relationship of L2 = n · λ _m (n: integer) is established.

At this time, the deviation ΔL _{m + 1} from the standing wave in the longitudinal mode _{m + 1} is
ΔL _{m + 1} = L2-INT (L2 / λ _{m + 1} ) · λ _{m + 1}
= L2- [n + INT (n / m)] · λ _{m + 1}
= [L2 / L1-INT (n / m)] · λ _{m + 1}
It becomes. Here, INT is a function for converting to an integer, and is a function that rounds off the number after the decimal point of the number in parentheses after INT to make an integer. That is, L2 / L1-INT (n / m) represents the number of parts after the decimal point of L2 / L1. The number of parts after the decimal point of L2 / L1 is a ratio to the wavelength λm + 1 of the deviation from the standing wave at the end face of the resonator length L2 in the longitudinal mode m + 1.

  Further, the required accuracy with respect to the resonator length of the external resonator 24 is expressed using finesse F indicating the sharpness of interference fringes in the resonator. That is, if the number of the fractional part of L2 / L1 in the above formula is smaller than 1 / F, this resonator can oscillate the longitudinal mode laser light. This is because the resonator has a width that is allowed at the wavelength at which laser light is oscillated, and the peak width divided by the wavelength width between modes is finesse. That is, the resonator length of the external resonator 24 is 2 by the external resonator 24 even if there is an error of 1 / F of the resonator length L1 in an integral multiple of the resonator length L1 of the resonator in the laser oscillator 10. The harmonic can be resonated.

  Furthermore, in the external resonator 24 in the external resonator type wavelength converter 20, the half mirror on which the fundamental wave first enters has a reflectance of 80% or more with respect to the fundamental wave. A mirror that totally reflects the fundamental wave is used. As a result, the finesse for the fundamental wave in the external resonator is 14 or more.

  Therefore, in the laser device 1 according to the present embodiment, the resonator length L2 of the external resonator 24 is set to 1/15 or less of the resonator length of the resonator of the laser oscillator 10. This is because the resonator length L2 of the external resonator 24 is the length of the above-described allowable deviation. That is, the allowable range of the resonator length L2 of the external resonator 24 is a range within a value obtained by increasing / decreasing the resonator length L1 by 1 / F from an integer multiple of the resonator length L1 of the resonator in the laser oscillator 10. Therefore, the resonator length L2 of the external resonator 24 in the laser apparatus 1 according to the present embodiment is obtained by adding 1 / F of the resonator length L1 to 0 times the resonator length L1 of the resonator in the laser oscillator 10. Value. Therefore, the resonator length L2 of the external resonator 24 in the laser apparatus 1 according to the present embodiment is set to be 1/15 or less of the resonator length of the resonator of the laser oscillator 10.

  By using the laser device 1 having such a configuration, since the resonator length L2 in the external resonator type wavelength converter 20 does not depend on the resonator length L1 of the laser oscillator 10, the longitudinal mode to be generated can be increased or decreased. Even if this is the case, it is not necessary to change the resonator length L2 in the external resonator type wavelength converter 20.

  The above will be described with reference to FIG. In FIG. 2, the resonator length L1 of the laser oscillator 10 is 1 m, and the phases of the longitudinal modes m and m + 1 with respect to the length of L2 are shown. When the resonator length L2 of the external resonator deviates from an integral multiple of half of the wavelength λ of the fundamental laser beam incident on the external resonator, assuming that deviation is ΔL (assumed to be an absolute value), ΔL is divided by λ. The obtained value becomes the phase of each wave, and becomes a value from 0 to 1. It is considered that the longitudinal mode can resonate if the length is L2 when this phase is in the vicinity of 0 or 1. Therefore, if the phase of both waves of the longitudinal modes m and m + 1 can exist in the vicinity of 0 or 1 by finely adjusting L2, both waves can be resonated. In FIG. 2, the vicinity of 0 or 1 is indicated by hatched hatching.

  FIG. 2A is a diagram when L2 = 100 cm. If the external resonator 24 has a resonator length L2 that is substantially the same as the resonator length L1 of the laser oscillator 10, the waves of the longitudinal modes m and m + 1 have substantially the same phase. Therefore, by setting the resonator length L2 of the external resonator 24 so that the phase of the wave of the longitudinal mode m is at the hatched hatched position, the external resonator 24 that can resonate the waves of the adjacent mode is provided. Can be formed.

  FIG.2 (b) is a figure when L2 = 50 cm vicinity. In this case, the phases of the waves of the longitudinal modes m and m + 1 are opposite to each other and cannot resonate in adjacent modes. Therefore, the resonator length L2 of the external resonator cannot be set near 0.5 m.

  FIG. 2C is a diagram when L2 = 101 cm. In this case, the phases of the longitudinal modes m and m + 1 are out of phase. However, even in the case of this deviation, the length of the phase of the waves of modes m and m + 1 that fall within the hatched hatched position can be the resonator length of the external resonator. That is, the external resonator 24 that can include a plurality of longitudinal modes can be formed.

  From the above, it is possible to form the external resonator 24 that can resonate a plurality of longitudinal modes, regardless of which is smaller than an integral multiple of the resonator length L1 of the laser oscillator 10. The small one that can resonate a plurality of longitudinal modes is about 1 / F. Therefore, in the laser device 1 according to the present embodiment, the resonator length of the external resonator 24 is set to the laser oscillator 10. 1 / F of the resonator length L1. Since the reflectance of the mirror in the external resonator that is generally used is about 80%, the resonator length L2 of the external resonator 24 according to the present embodiment is set to 1 / of the resonator length L1 of the laser oscillator 10. 15 or less.

Here, the resonator length L1 of the laser oscillator 10 is set to 1 m as an example. The longitudinal mode interval Δλ around the fundamental wave 1064.14 nm of the YAG laser is Δλ≈λ ² /2·L1≈0.566 pm
It becomes. Since the gain width of the YAG laser is about 0.01 nm, 17 longitudinal modes can oscillate simultaneously.

On the other hand, the resonator length L2 of the external resonator 24 is 4 mm. As a result, L2 / L1 = 250. That is, when the external resonator 24 forms a standing wave in a certain mode m, the rate at which the adjacent longitudinal mode m + 1 deviates from the standing wave is ΔL _{m + 1} / λ _{m + 1} = 1/250. As a result, for the longitudinal mode separated by 17 pieces, the ratio of deviation from the standing wave is ΔL _{m + 16} / λ _{m + 16} = 16/250 to 1/16. Therefore, since the amount of shift in the light in the longitudinal mode separated by 17 becomes 1/15 or less, all of the 17 longitudinal modes can be wavelength-converted efficiently.

  Although the laser apparatus 1 according to the present embodiment has been described as generating the second harmonic of the YAG laser, an external resonator type wavelength converter is further added to generate the fourth harmonic. Also in the case of the configuration, if the resonator length of the external resonator for the fourth harmonic is the same as that described above, all the longitudinal modes of the second harmonic can be efficiently wavelength-converted. Thus, the present invention is not limited to generating the second harmonic of the YAG laser.

  Next, with respect to the external resonator 24 in the laser apparatus 1, another structural example of an external resonator that can realize a similarly short resonator is shown in FIG. FIG. 3A shows a standard external resonator 40 similar to the external resonator 24 in the laser apparatus 1 shown in FIG. The structure is such that a pair of concave mirrors 42 a and 42 b are arranged on both sides of the SHG crystal 41. Further, the partially permeable film 43 and the dichroic film 44 are coated on the inside of the concave mirrors 42a and 42b. That is, the fundamental laser beam is incident from the surface coated with the partial transmission film 43, and the second harmonic laser beam is emitted from the surface coated with the dichroic film 44. However, it is necessary to apply antireflection coating (AR coating) for both the fundamental wave and the second harmonic on both end faces of the SHG crystal 41.

  In the external resonator 50 shown in FIG. 3B, the semi-convex lenses 52a and 52b are in close contact with the end face of the SHG crystal 51 by optical contact. The surface of the semi-convex lenses 52a and 52b opposite to the surface in close contact with the SHG crystal 51 is coated with a partial transmission film 53 and a dichroic film 54. The fundamental laser beam is incident from the partially transmissive film 53 side, and the second harmonic laser beam is emitted from the dichroic film 54 side. The feature of this structure is that, unlike the external resonator 40 shown in FIG. 3A, it is not necessary to apply an AR coating that tends to cause damage to both end faces of the SHG crystal 51.

  In the external resonator 60 shown in FIG. 3C, a partial transmission film 62 and a dichroic film 63 are provided on an SHG crystal 61. Both end faces of the SHG crystal 61 are convex. The partially permeable film 62 and the dichroic film 63 are coated on the surfaces thereof. In this structure, a fundamental laser beam is incident from the partially transmissive film 62 side, and a second harmonic laser beam is emitted from the dichroic film 63 side.

  The feature of this structure is that the resonator length can be particularly shortened. However, regarding the control of the resonator length, since the SHG crystal 61 itself cannot expand and contract, it can be resonated by slightly changing the installation angle (the direction in the horizontal direction or the vertical direction). Alternatively, since the refractive index has temperature dependency, resonance can also be controlled by controlling the temperature of the SHG crystal 61.

  The external resonator 70 shown in FIG. 3D has a structure in which an optical path of laser light forms a loop like a triangle in the SHG crystal 71. The end surface of the SHG crystal 71 is a surface that refracts the laser light so that the optical path of the laser light is triangular. The partially permeable film 72 and the dichroic film 73 are provided on the end face of the SHG crystal 71, and both are flat. Further, the lens 74 is brought into close contact with an end face different from the end face provided with the partial transmission film 72 of the SHG crystal 71 and the dichroic film 73. The outer surface of the lens 74 is a convex mirror and is coated with a reflective film 75. The end surface to which the lens 74 is closely attached and the end surface on which the partial transmission film 72 and the dichroic film 73 are provided have an acute angle.

As a result, the fundamental laser beam incident on the SHG crystal 71 forms a resonator by the partially transmissive film 72 and the dichroic film 73, and a second harmonic is generated. The optical path of the laser light at this time reaches the dichroic film 73 after passing through the lens 74 from the partially transmitting film 72 and reflected by the reflecting film 75. The feature of this structure is that the laser light path in the SHG crystal 71 is made into a loop shape, so that the fundamental laser light returned from the SHG crystal 71 to the outside again is in the opposite direction, that is, the direction of the fundamental laser oscillator. Will not be returned to.

  From the above, in the laser device according to the present embodiment, even if the resonator length of the laser oscillator is changed or the number of longitudinal modes to be used is increased or decreased, the wavelength conversion efficiency is not reduced without reducing the wavelength conversion efficiency. Wavelength conversion can be performed without changing the resonator length. In the laser device according to the present embodiment, the length of the resonator provided in the external resonator can be shortened, so that the laser device can be made compact. Furthermore, since the laser apparatus according to the present embodiment can generate laser light having a short wavelength in an arbitrary number of longitudinal modes, it is optimal for an application field that needs to be focused on a minute spot and in which interference is to be suppressed. Therefore, the present invention can be applied not only to a device for detecting minute defects like the mask inspection device of the present invention but also to optical recording.

Embodiment 2. FIG.
In the second embodiment, a laser device that uses a laser oscillator as an argon ion laser and generates the second harmonic of the argon ion laser will be described. As is widely known, an argon ion laser has a large number of oscillation lines in the green region. However, each oscillation line, for example, only a line having a wavelength of 514.5 nm, can be simultaneously used in several to a dozen longitudinal modes. Oscillates. The present invention relates to a laser apparatus capable of simultaneously converting the wavelength of the plurality of longitudinal mode lights. Components and operating principles similar to those of the first embodiment are omitted.

  FIG. 4 shows a configuration diagram of the laser apparatus 100 according to the present embodiment. The laser apparatus 100 according to the present embodiment has an argon ion laser 110. The argon ion laser 110 has a laser tube 111, an output mirror 113, and a total reflection mirror 114 having Brewster windows 112a and 112b attached to both ends. Argon gas is sealed inside the laser tube 112. Laser oscillation occurs by discharging the laser tube 112. In this laser oscillation, laser light having a wavelength of 514.5 nm is generated and resonates between the output mirror 113 and the total reflection mirror 114.

  The laser light extracted from the output mirror 113 passes through the polarization beam splitter 121 and the rotator 122. The light that has passed through the rotator 122 travels while being squeezed through the lens 123. The light focused by the lens 123 enters the external resonator 130.

  The external resonator 130 has an SHG crystal 131, a partial transmission film 132, and a dichroic film 133. Similar to the SHG crystal 61 shown in FIG. 2C, the SHG crystal 131 has a convex shape on both ends. The end face of the SHG crystal 131 is directly coated with a partial transmission film 132 and a dichroic film 133.

  The second harmonic generated in the external resonator 130 is extracted from the dichroic film 133. An unconverted fundamental wave in the external resonator is emitted to the partial transmission film 132 side and the dichroic film 133 side. The light emitted to the partial transmission film 132 side travels in the opposite direction through the lens 123 and the rotator 122, is reflected by the polarization beam splitter 121, and is received by the photodetector 140.

  On the other hand, in the laser apparatus 100, the PZT element 115 is attached to the output mirror 113. The PZT element 115 finely adjusts the position of the output mirror 113 so that the amount of light received by the photodetector 140 is minimized. That is, since the resonator length of the SHG crystal 131 cannot be finely adjusted, the resonator length of the fundamental wave is finely adjusted. Accordingly, the wavelength of the generated fundamental laser beam is finely adjusted so that the second harmonic resonates in the external resonator.

  Laser apparatus 100 according to the present embodiment does not require fine adjustment of the resonator length of the second harmonic, so that the external resonator length can be extremely shortened. Further, the SHG crystal 131 directly coated with the reflection films 132 and 133 to be resonated can be used.

  From the above, in the laser device according to the present embodiment, even if the resonator length of the laser oscillator is changed or the number of longitudinal modes to be used is increased or decreased, the wavelength conversion efficiency is not reduced without reducing the wavelength conversion efficiency. Wavelength conversion can be performed without changing the resonator length. In the laser device according to the present embodiment, the length of the resonator provided in the external resonator can be shortened, so that the laser device can be made compact.

Embodiment 3 FIG.
Embodiment 1 and Embodiment 2. In the present embodiment, the laser apparatus has been described. And Embodiment 2. An inspection apparatus using the laser apparatus will be described. Embodiment 1 in terms of components and operating principles And Embodiment 2. Items similar to are omitted.

  A schematic structural diagram of the inspection apparatus 200 according to the present embodiment is shown in FIG. The inspection apparatus 200 uses the laser apparatus 1 shown in FIG. 1 and a laser apparatus using an external resonator provided as a light source. That is, the laser device that generates the fourth harmonic is used as the light source 210. Therefore, ultraviolet laser light having a wavelength of 266 nm including a number of longitudinal modes can be obtained and used as a light source.

  The inspection apparatus 200 includes a light source 210 and an inspection unit 220. The transverse mode in the ultraviolet laser light incident on the inspection unit 220 from the light source 210 is a single mode. The ultraviolet laser light is incident in a polarization direction that is reflected by the polarization beam splitter 221 in the inspection unit 220.

  The ultraviolet laser light is reflected by the polarization beam splitter 221 and reflected by the polygon mirror 222. The laser light reflected by the polygon mirror 222 passes through the relay lens 223 and passes through the quarter wavelength plate 224. The laser light that has passed through the quarter-wave plate 224 is incident on the objective lens 225 and is condensed on the mask substrate 230.

  The laser light reflected by the mask substrate 230 passes through the objective lens 225, the quarter wavelength plate 224, the relay lens 223, and the polygon mirror 222 in this order, and enters the polarization beam splitter 221. Since the light incident on the polarization beam splitter 221 passes through the quarter-wave plate 224 twice, the polarization direction is rotated by 90 degrees. Therefore, the light is transmitted through the polarization beam splitter 221. The laser light transmitted through the polarization beam splitter 221 passes through the projection lens 226 and enters the photodetector 227.

  If there is a defect on the mask substrate 230, the intensity of the reflected light of the laser beam condensed on the defect is changed, and is identified by the photodetector 227. That is, it is necessary to be able to collect light with a small spot on the mask substrate 230. For this reason, the ultraviolet laser light used is in a single transverse mode with high light collecting performance. However, if the longitudinal mode is also single, the coherence becomes very high, so that the intensity of the reflected light may change even when there is no defect on the mask substrate 230. Therefore, it is necessary to use laser light having as low an interference as possible.

  Therefore, in the inspection apparatus 200 according to the present embodiment, since the laser apparatus of the present invention is used as a light source, a single transverse mode laser beam including a number of longitudinal modes can be used. Therefore, the light can be condensed with a small spot on the mask substrate, and the coherence is low, so that the probability of occurrence of pseudo defects can be greatly reduced.

  It should be noted that the present invention is not limited to the above-described embodiments, and various modifications can be made without departing from the scope of the present invention. For example, when an ultraviolet semiconductor laser (UV-LD) having a wavelength of 405 nm is used as the fundamental laser, ultraviolet light having a wavelength of 202.5 nm is obtained by the second harmonic. At this time, a compact multi-longitudinal mode ultraviolet light source can be realized if the external resonator can oscillate in a number of longitudinal modes.

1 is a configuration diagram of a laser device according to Embodiment 1. FIG. It is a figure which shows the shift | offset | difference from a standing wave with respect to the resonator length of a multi longitudinal mode SHG external resonator. FIG. 3 is a structural example diagram of an external resonator used in the first embodiment. 6 is a configuration diagram of a laser apparatus according to Embodiment 2. FIG. It is a block diagram of the mask inspection apparatus which concerns on Embodiment 3. FIG. It is a block diagram of the conventional wavelength conversion type laser apparatus.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 Laser apparatus 10 Laser oscillator 11 LD 12 Optical fiber 13 Collimate lens 14 Reflection mirror 15 YAG crystal 16 Output mirror 17 Total reflection mirror 18 1/4 wavelength plate 20 External resonator type wavelength converter 21 Polarizing beam splitter 22 Rotator 23 Lens 24 External resonator 241 SHG crystal 242 Partial transmission mirror 243 Dichroic mirror 244 PZT element 25 Photo detector 26 Signal 30 Mirror 40 External resonator 41 SHG crystal 42 Concave mirror 43 Partial transmission film 44 Dichroic film 50 External resonator 51 SHG crystal 52 Semi-convex Type lens 53 Partial transmission film 54 Dichroic film 60 External resonator 61 SHG crystal 62 Partial transmission film 63 Dichroic film 70 External resonator 71 SHG crystal 72 Partial transmission film 73 Dichroic film 74 Lens 75 Anti Film 100 Laser device 110 Argon ion laser 111 Laser tube 112 Brewster window 113 Output mirror 114 Total reflection mirror 115 PZT element 121 Polarizing beam splitter 122 Rotator 123 Lens 130 External resonator 131 SHG crystal 132 Partial transmission film 133 Dichroic film 140 Optical detection 200 Inspection device 210 Light source 220 Inspection unit 221 Polarization beam splitter 222 Polygon mirror 223 Relay lens 224 1/4 wavelength plate 225 Objective lens 227 Projection lens 228 Photo detector 230 Mask substrate 900 Wavelength conversion laser device 901 Laser oscillator 902 External resonance Type wavelength converter 903 YAG crystal 904 Output mirror 905 Total reflection mirror 906 Lens 907 Partial transmission mirror 908 Mirror 909 Dichroic Kkumira 910 SHG crystal

Claims (8)

A laser oscillator that oscillates a laser beam resonated by a first resonator;
A laser device having an external resonator type wavelength converter that is used to convert the wavelength of laser light extracted from the laser oscillator and emits laser light resonated by a second resonator;
The laser device characterized in that the resonator length of the second resonator is 1/15 or less of the resonator length of the first resonator.   The second resonator in the external resonator type wavelength converter includes two opposing mirrors and a nonlinear crystal that performs wavelength conversion and is positioned between the two opposing mirrors. The laser device according to claim 1. One of the two opposing mirrors is a first mirror provided with a partial transmission film that transmits a part of light to a concave mirror;
3. The laser device according to claim 2, wherein the other of the two opposing mirrors is a second mirror in which a concave mirror is provided with a dichroic film having a reflectivity different depending on a wavelength. The external resonator type wavelength converter has two semi-convex lenses and the nonlinear crystal sandwiched by the semi-convex lenses,
The one half-convex lens is provided with a partial transmission film that transmits a part of light on a lens surface;
The laser device according to claim 2, wherein the other half-convex lens is provided with a dichroic film having a different reflectance according to a predetermined wavelength on a lens surface. The external resonator type wavelength converter has the nonlinear crystal,
The nonlinear crystal has a convex shape at two opposite end surfaces, and a partial transmission film that transmits part of light is provided on one end surface of the two end surfaces having the convex shape. 3. The laser device according to claim 2, wherein a dichroic film having a different reflectance according to the wavelength is provided on the end face. The external resonator wavelength converter has the nonlinear crystal and a lens in contact with an end face of the nonlinear crystal,
The incident surface and the exit surface of the laser beam in the nonlinear crystal are acute with respect to the end surface with which the lens is abutted, and a partial transmission film that transmits a part of the light is provided on the entrance surface. Is provided with a dichroic film with different reflectivity according to the wavelength,
The laser apparatus according to claim 2, wherein a reflection film that totally reflects light is provided on a lens surface of the lens.   The laser apparatus according to any one of claims 1 to 6, wherein the laser oscillator is a YAG laser, and a resonator length of the laser oscillator is 0.3 m or more.   An inspection apparatus using the laser device according to any one of claims 1 to 7 as a light source.

Images