JP2013044764A - Laser device, method for suppressing photorefractive effect of quasi-phase-matching wavelength conversion optical element, exposure device, and inspection device - Google Patents

Abstract

The present invention provides a technique capable of suppressing the photorefractive effect in a quasi phase matching type wavelength conversion optical element, and provides a laser device and the like that can solve the problems caused by the effect.
A laser device LS includes a laser light output unit 1 that outputs laser light, and a wavelength conversion unit 3 that converts the wavelength of laser light output from the laser light output unit and outputs the laser light. The wavelength conversion unit 3 includes a quasi-phase matching type wavelength conversion optical element 31 that converts a wavelength λ laser light into a wavelength λ / 2 laser light, and includes an incident surface and an output surface of the wavelength conversion optical element 31. At least one of them is provided with a reflectance reduction coating that reduces the reflectance of light having a wavelength of λ to λ / n (n ≧ 3).
[Selection] Figure 1

Description

  The present invention relates to a method for suppressing a photorefractive effect in a quasi phase matching type wavelength conversion optical element, and a laser device, an exposure apparatus, and an inspection apparatus including the quasi phase matching type wavelength conversion optical element.

  As a laser apparatus provided with a quasi phase matching type wavelength conversion optical element, for example, an all-solid-state laser apparatus suitable for use in an exposure apparatus, various inspection apparatuses, phototherapy apparatuses, and the like is known. Such a laser apparatus includes a laser beam output unit that outputs a fundamental laser beam having a predetermined wavelength, and a wavelength conversion unit that converts the wavelength of the fundamental laser beam output from the laser beam output unit ( For example, see Patent Document 1 and Patent Document 2.)

  The wavelength conversion unit is provided with a wavelength conversion optical element, for example, configured to sequentially convert the wavelength of the fundamental laser beam in the infrared region by a plurality of wavelength conversion optical elements and output the laser beam in the ultraviolet region. The In recent years, Quasi Phase Matching (QPM) type is used as a wavelength conversion optical element because it has high conversion efficiency that is difficult to obtain with bulk crystals and high beam quality that does not cause walk-off can be obtained. Wavelength conversion optical elements have been used (see, for example, Patent Document 3 and Patent Document 4).

As a quasi phase matching type wavelength conversion optical element that converts a laser beam having a wavelength of λ into a laser beam having a wavelength of λ / 2, for example, PPLN (Periodically Poled LiNbO ₃ ), PPLT (Periodically Poled LiTaO ₃ ), PPKTP (Periodically Poled KTiOPO ₄ ), quartz quasi-phase matched crystal, and the like are known. A quasi phase matching crystal, such as PPLN, is formed by forming a domain-inverted structure with a predetermined period in a LiNbO ₃ crystal having a large nonlinear optical constant. The quasi phase matching type wavelength conversion optical element is also referred to as a QPM crystal or a QPM element. In the present specification, the QPM crystal will be described as appropriate.

JP 2008-122785 A JP 2010-93210 A JP 2002-350914 A JP 2002-122898 A

  However, among these QPM crystals, the PPLN crystal and the PPLT crystal have been found to significantly generate a photorefractive effect when the power density of incident light increases. The photorefractive effect is a phenomenon in which a refractive index distribution according to the spatial intensity distribution of incident light is induced in the crystal, and is generally expressed in a region below visible light (short wavelength side). When the photorefractive effect occurs, the beam pattern of the light passing through the crystal is distorted, and the beam profile of the emitted light changes.

  Therefore, when the light wavelength-converted by the QPM crystal is sequentially incident on the optical element and propagated, for example, when the light is sequentially focused and incident on the wavelength conversion optical element in the subsequent stage, the light collecting spot position Further, there is a problem that the wavelength conversion efficiency and the output beam change due to changes in the spot size and the like, which is a factor that hinders high output.

  The present invention has been made in view of the problems as described above, and provides a technique capable of suppressing the photorefractive effect in the QPM crystal, and a laser apparatus and an exposure apparatus capable of solving the problems caused by the photorefractive effect. It is an object to provide an optical system.

  In order to solve the above-mentioned problems, the inventor conducted extensive research on the photorefractive effect generated in the QPM crystal, and obtained the following knowledge.

  The inventor uses a PPLN crystal that converts incident light with a wavelength of 1547 nm to a wavelength of 774 nm as a representative example of a QPM crystal, and focuses a pulsed fundamental wave with a wavelength of 1547 nm on the PPLN crystal for generating the second harmonic. The incident light was incident and the output light was observed.

The light emitted from the PPLN crystal is the power density of incident light (in the case where the incident light is pulsed light, the value obtained by dividing the peak power of the pulsed light by the condensing cross-sectional area. In this specification, it is called peak intensity. In the state of kW / cm ² order, a second harmonic (second harmonic) 2ω of wavelength 774 nm generated by wavelength conversion and a fundamental wave ω of wavelength 1547 nm transmitted without wavelength conversion were seen. .

However, when the power of the incident light is increased and the power density (peak intensity) of the incident light is about several MW / cm ² , in addition to the fundamental wave and the second harmonic wave, the third harmonic wave having a wavelength of 516 nm ( (3rd harmonic) 3ω can be seen. When the power of the incident light is further increased and the power density becomes about 4 to 5 MW / cm ² or more, in addition to the fundamental wave, the second harmonic wave, and the third harmonic wave, the fourth harmonic wave (fourth harmonic wave) having a wavelength of 389 nm. (Wave) 4ω was also found.

Next, the inventor examined the wavelength dependence of the photorefractive effect. The results are as follows. With respect to the second harmonic wave having a wavelength of 774 nm, the power density (peak intensity) of the second harmonic wave emitted from the PPLN crystal is several tens MW / cm ² on the time axis scale of several hundred hours, The beam pattern gradually changed. At this time, the degree of deformation of the beam pattern was slight. On the other hand, for the third harmonic wave having a wavelength of 516 nm, the power of the third harmonic wave emitted from the PPLN crystal (peak intensity) is several MW / cm ² on a time axis scale of several tens of hours. The beam pattern changed suddenly and greatly. From this, it was found that the photorefractive effect generated in the QPM crystal has a strong wavelength dependency and is highly sensitive to light having a short wavelength (high-order harmonics).

That is, when the power density (peak intensity) of incident light incident on the QPM crystal becomes a high power density state of the order of MW / cm ² , third and higher order higher harmonics are generated. The refractive effect was noticeable.

  By the way, a wavelength conversion optical element in the infrared to visible region generally forms a thin film called an AR coating (Anti Reflection coating) on the incident surface and the exit surface of the element. Used. The AR coating is performed for the purpose of reducing the reflection loss on the incident / exit surface of the light before wavelength conversion incident on the wavelength conversion optical element and the light after wavelength conversion generated by the wavelength conversion optical element. For example, in a wavelength converting optical element for generating a second harmonic that is incident on a fundamental wave of wavelength λ and generates a second harmonic of wavelength λ / 2, the wavelength is in the vicinity of λ and in the vicinity of λ / 2. AR coating with reduced reflectivity is applied to the entrance surface and the exit surface. This is the same whether the wavelength conversion optical element is a bulk crystal or a QPM crystal.

  In other words, the AR coat formed on the input and output surface of the second harmonic generation QPM crystal is near the fundamental wave having a wavelength of λ and the light in the vicinity of the second harmonic having a wavelength of λ / 2. Although the reflectance is sufficiently reduced, it is not defined for light in other wavelength bands. From the reflectance characteristics data of the AR coating published by the wavelength conversion optical element manufacturer, the reflectance is not reduced in the wavelength band of the third harmonic of the wavelength λ / 3 and the fourth harmonic of the wavelength λ / 4. It is thought that it is getting higher.

  Then, the ratio of the third harmonic and the fourth harmonic generated in the QPM crystal is reflected at the exit surface and the entrance surface of the QPM crystal, and multiple reflections are made between the entrance and exit surfaces as shown in FIG. This increases the power density of each component light. This is considered to be a factor that causes the photorefractive effect due to higher harmonics to be generated more efficiently (with high sensitivity).

  Based on the above findings, the inventor completed the invention shown below.

  A first aspect illustrating the present invention is a laser device. The laser device includes a laser light output unit that outputs laser light, and a wavelength conversion unit that converts the wavelength of the laser light output from the laser light output unit and outputs the laser light. The wavelength conversion unit includes a quasi phase matching type wavelength conversion optical element (for example, the wavelength conversion optical elements 31 and 34 in the embodiment) that converts the incident laser light having the wavelength λ into laser light having the wavelength λ / 2 and emits the laser light. ), And at least one of the entrance surface and the exit surface of the wavelength conversion optical element, in addition to the light with the wavelengths λ and λ / 2, the reflectance for the light with the wavelength λ / n (n ≧ 3) Further, it is configured by applying a reflectance reduction coating that also reduces the reflectance.

  The reflectance-reducing coating can have a reflectance of 1% or less for light of wavelength λ and wavelength λ / 2, a reflectance of 10% or less for light of wavelength λ / 3, and a wavelength λ / The reflectance with respect to the light of 4 can be 10% or less.

Further, the laser light having a wavelength λ incident on the quasi phase matching type wavelength conversion optical element can be configured such that the power density is on the order of MW / cm ² . The quasi phase matching type wavelength conversion optical element may be a PPLN crystal or a PPLT crystal.

  In addition, the wavelength converting unit further converts the wavelength of the laser light having the wavelength λ / 2 emitted from the quasi phase matching type wavelength converting optical element (for example, the wavelength converting optical elements 32 and 33 in the embodiment). , 35, 36), and the laser beam in the ultraviolet region can be output from the wavelength converter.

  A second aspect illustrating the present invention is a method for suppressing the photorefractive effect of a quasi phase matching type wavelength conversion optical element. This method uses a quasi-phase matching type wavelength conversion optical element (for example, the wavelength conversion optical elements 31 and 34 in the embodiment) that converts an incident laser beam having a wavelength λ into a laser beam having a wavelength λ / 2. At least one of the entrance surface and the exit surface is provided with a reflectance reduction coating that reduces the reflectance of light having a wavelength of λ / n (n ≧ 3) in addition to the light of wavelengths λ and λ / 2, A high-order harmonic having a wavelength that is incidentally generated in the process of wavelength conversion and shorter than λ / 2 is emitted to the outside of the element.

  The reflectance-reducing coating can have a reflectance of 1% or less for light of wavelength λ and wavelength λ / 2, a reflectance of 10% or less for light of wavelength λ / 3, and a wavelength λ / The reflectance with respect to the light of 4 can be 10% or less.

Further, the laser light having a wavelength λ incident on the quasi phase matching type wavelength conversion optical element can be configured such that the power density is on the order of MW / cm ² . The quasi phase matching type wavelength conversion optical element may be a PPLN crystal or a PPLT crystal.

  In addition, the wavelength converting unit further converts the wavelength of the laser light having the wavelength λ / 2 emitted from the quasi phase matching type wavelength converting optical element (for example, the wavelength converting optical elements 32 and 33 in the embodiment). , 35, 36), and the laser beam in the ultraviolet region can be output from the wavelength converter.

  A third aspect illustrating the present invention is an exposure apparatus. An exposure apparatus includes a laser device according to the first aspect, a mask support unit that holds a photomask on which a predetermined exposure pattern is formed, and an exposure object support unit that holds an exposure object (for example, an exposure object in the embodiment). An object support table 105), an illumination optical system for irradiating the photomask held by the mask support with the laser beam output from the laser device, and an exposure held by the exposure object support for the light transmitted through the photomask. And a projection optical system that projects onto the object.

  A third aspect illustrating the present invention is an inspection apparatus. The inspection apparatus irradiates the test object held by the test object support unit with the laser apparatus of the first aspect, the test object support part that holds the test object, and the laser beam output from the laser apparatus. An illumination optical system and a detector (for example, the TDI sensor 125 in the embodiment) that detects light from the test object are configured.

  The laser device of the first aspect includes a quasi-phase matching type wavelength conversion optical element that converts a wavelength λ laser light into a wavelength λ / 2 laser light, and includes an incident surface and an output surface of the wavelength conversion optical element. At least one of them is provided with a reflectance reduction coating that reduces the reflectance of light with wavelengths λ / n (n ≧ 3) in addition to light with wavelengths λ and λ / 2. According to such a configuration, even if a higher-order harmonic higher than the third harmonic is generated in the quasi phase matching type wavelength conversion optical element (QPM crystal) for generating the second harmonic, the generated higher-order harmonic is not generated. The light is emitted to the outside of the element without reciprocating between the incident and outgoing surfaces. Thereby, the photorefractive effect which was a problem in the QPM crystal can be suppressed, and a laser apparatus capable of solving various problems resulting from the manifestation of the effect can be provided.

  The photorefractive effect suppressing method for a quasi phase matching type wavelength conversion optical element according to the second aspect is a quasi phase matching type wavelength conversion optical that converts a wavelength λ laser light into a laser light of wavelength λ / 2 and outputs it. A reflectance-reducing coating that reduces the reflectance of light having a wavelength of λ / n (n ≧ 3) in addition to light of wavelengths λ and λ / 2 on at least one of the incident surface and the exit surface of the element The high-order harmonic having a wavelength incidentally generated in the wavelength conversion process and shorter than λ / 2 is emitted to the outside of the element. According to such a configuration, even if a higher-order harmonic more than the third harmonic is generated in the quasi-phase matching type wavelength conversion optical element (QPM crystal) for generating the second harmonic, the generated higher-order harmonic is not generated. The light is emitted to the outside of the element without reciprocating between the incident and outgoing surfaces. Thereby, the photorefractive effect which has been a problem in the QPM crystal can be suppressed, and means capable of solving various problems resulting from the manifestation of the effect can be provided.

  The exposure apparatus according to the third aspect includes the laser apparatus according to the first aspect. For this reason, the photorefractive effect which was a problem in the exposure apparatus which has a QPM crystal | crystallization in the wavelength conversion part of a laser apparatus can be suppressed, and the exposure apparatus which can solve the various problems resulting from the expression of the effect is provided. be able to.

  The inspection apparatus according to the fourth aspect includes the laser apparatus according to the first aspect. For this reason, the photorefractive effect which was a problem in the inspection apparatus which has a QPM crystal | crystallization in the wavelength conversion part of a laser apparatus can be suppressed, and the inspection apparatus which can solve the various problems resulting from the expression of the effect is provided. be able to.

It is a schematic diagram of the laser apparatus illustrated as an aspect of the present invention. It is a schematic block diagram which shows the structural example of the wavelength conversion part in the said laser apparatus. It is a schematic block diagram of the exposure apparatus shown as a 1st application example of the system provided with the said laser apparatus. It is a schematic block diagram of the test | inspection apparatus shown as the 2nd application example of the system provided with the said laser apparatus. It is explanatory drawing which shows typically a mode that a high order harmonic carries out multiple reflection at the entrance / exit surface of a QPM crystal.

Hereinafter, embodiments for carrying out the present invention will be described with reference to the drawings. A laser device LS exemplified as an aspect of the present invention is shown in FIG. The laser device LS includes a laser light output unit 1 that outputs laser light having a predetermined wavelength, a wavelength conversion unit 3 that converts the wavelength of the laser light output from the laser light output unit 1, and outputs the laser light. And a control unit 8 that controls the operation of the wavelength conversion unit 3. In the present embodiment, the laser beam output unit 1 outputs the fundamental laser beam La (La ₁ , La ₂ , La ₃ ) in the infrared to visible region, and the wavelength conversion unit 3 converts the wavelength to the laser beam Lv in the ultraviolet region. A configuration for conversion and output will be described as an example.

In the illustrated laser system LS, a laser beam output unit 1, the second laser for outputting a first laser beam output unit 1a for outputting a first fundamental wave laser light La _1, the second fundamental wave laser light La ₂ The optical output unit 1b and the third laser beam output unit 1c that outputs the _third fundamental laser beam La ₃ are configured. Here, the wavelength of the _first fundamental laser beam La ₁ output from the first laser beam output unit ₁ a, the wavelength of the second fundamental laser beam La ₂ output from the second laser beam output unit 1 b, and the third The wavelength of the _third fundamental laser beam La ₃ output from the laser beam output unit 1 c can be appropriately set according to the wavelength of the laser beam Lv output from the laser device LS and the configuration of the wavelength conversion unit 3.

Further, FIG. 1 shows a configuration in which the laser light output unit 1 includes a laser light generation unit 10 that outputs seed light and an optical amplification unit 20 that amplifies the seed light output from the laser light generation unit 10. . The laser light generator 10 includes a first laser light source 11 that generates a first fundamental wave seed light Ls ₁ , a second laser light source 12 that generates a second fundamental wave seed light Ls ₂ , and a third fundamental wave. A third laser light source 13 for generating seed light Ls ₃ is provided.

The optical amplifying unit 20 includes a first fiber optical amplifier 21 that amplifies the first fundamental wave seed light Ls ₁ output from the first laser light source 11, and a second fundamental wave seed output from the second laser light source 12. A second fiber optical amplifier 22 that amplifies the light Ls ₂ and a third fiber optical amplifier 23 that amplifies the third fundamental wave seed light Ls ₃ output from the third laser light source 13 are provided.

That is, in the laser light output unit 1 of this configuration, the first fundamental light seed light Ls ₁ output from the first laser light source 11 is amplified by the first fiber light amplifier 21 in the first laser light output unit 1a. The amplified first fundamental laser beam La ₁ is output to the wavelength converter 3. Further, in the second laser beam output unit 1b, the seed light Ls ₂ of the second fundamental wave output from the second laser light source 12 is amplified by the second fiber amplifier 22, a second fundamental wave laser light amplified La ₂ is output to the wavelength converter 3. Similarly, in the third laser light output unit 1c, a third fundamental seed beam Ls ₃ output from the third laser light source 13 is amplified by the third fiber optical amplifier 23, a third fundamental laser amplified The light La ₃ is output to the wavelength conversion unit 3.

Each of the first, second, and third fiber optical amplifiers 21, 22, and 23 can be configured by a single-stage fiber optical amplifier, or a plurality of fiber optical amplifiers can be connected in series to form a plurality of stages. It can also be configured as a group of fiber optical amplifiers. In addition, when there is one of the first, second, and third fundamental laser beams La ₁ , La ₂ , and La ₃ having the same wavelength, a laser light source that generates seed light is shared, and the laser light source The fundamental wave seed light output from the optical fiber may be split in parallel by an optical splitter or the like and made incident on a plurality of fiber optical amplifiers. For example, when the wavelengths of the _first to third fundamental laser beams La _{1 to} La ₃ are all the same, the fundamental wave seed light output from a single laser light source (for example, output from the second laser light source 12). The second fundamental wave seed light Ls ₂ ) can be split into three in parallel by an optical splitter or the like and incident on the first to third fiber optical amplifiers 21 to 23.

In this configuration, the first, second, and third laser light sources 11, 12, and 13 are laser light sources that generate seed light Ls (Ls ₁ , Ls ₂ , Ls ₃ ) in the infrared region having a wavelength of 1547 nm. 11 to 13 are used to amplify by the corresponding fiber optical amplifiers 21 to 23 and output a fundamental laser beam having a wavelength of 1547 nm.

  At this time, as the first to third laser light sources 11 to 13, a DFB (Distributed Feedback) semiconductor laser having an oscillation wavelength of 1.5 μm can be suitably used. The DFB semiconductor laser can generate seed light having a single wavelength of 1547 nm by oscillating in a state in which the temperature is controlled by a temperature controller using a Peltier element or the like. The DFB semiconductor laser can be oscillated or pulsed at an arbitrary intensity by controlling the waveform of the excitation current. The laser light generator 10 is provided with an external modulator such as an EOM (Electro Optic Modulator), and the output light of the CFB-oscillated DFB semiconductor laser is pulse-modulated by the external modulator. May be output.

  As the first to third fiber optical amplifiers 21 to 23 for amplifying infrared light having a wavelength of 1547 nm, an erbium-doped fiber optical amplifier (EDFA) having a core doped with erbium (Er) can be suitably used. The operations of the first to third laser light sources 11 to 13 and the first to third fiber optical amplifiers 21 to 23 are controlled by the control unit 8.

The first fundamental laser beam La ₁ emitted from the first fiber optical amplifier 21, the second fundamental laser beam La ₂ emitted from the second fiber optical amplifier 22, and the third fiber optical amplifier 23. The third fundamental laser beam La ₃ is output from the laser beam output unit 1 (1a, 1b, 1c) and input to the wavelength conversion unit 3.

  In the above, the laser beam output unit 1 is configured by the laser beam generator 10 (first to third laser light sources 11 to 13) and the optical amplifier 20 (first to third fiber optical amplifiers 21 to 23). However, the laser beam output unit 1 may be configured by a fiber laser (first to third fiber lasers).

The wavelength conversion unit 3 is provided with a wavelength conversion optical system 30 including a plurality of wavelength conversion optical elements and mirrors. A schematic configuration of the wavelength conversion optical system 30 is shown in FIG. In FIG. 2, the ellipses shown on the optical path are collimator lenses and condenser lenses, and their descriptions are omitted. In the figure, p-polarized light whose polarization plane is parallel to the paper surface is indicated by up and down arrows, and s-polarized light whose polarization surface is perpendicular to the paper surface is indicated by ◯ marks with dots. The fundamental wave is represented by ω, the second harmonic is represented by 2ω, and the third and higher order nth harmonics are represented by nω. In this configuration, the first fundamental laser beam La ₁ and the second fundamental laser beam La ₂ are p-polarized, and the third fundamental laser beam La ₃ is s-polarized. The second fundamental wave laser beams La ₁ and La ₂ may be s-polarized light, and the third fundamental laser beam La ₃ may be p-polarized light.

The wavelength conversion optical system 30 of this configuration example is emitted from the first series I and second fiber optical amplifier 22 through which the first fundamental laser beam La ₁ emitted from the first fiber optical amplifier 21 enters and propagates. The second series II in which the second fundamental wave laser beam La ₂ is incident and propagated, and the third series III in which the third fundamental wave laser beam La ₃ emitted from the third fiber optical amplifier 23 is incident and propagated. , And the fourth series IV in which the laser beams propagated through the first, second, and third series are superimposed and propagated, and are mainly composed of six wavelength conversion optical elements 31 to 36 provided in each series. . First, an outline of the wavelength conversion optical system 30 will be described.

The first fundamental laser beam La ₁ having a wavelength of 1547 nm and an angular frequency ω incident on the first series I is wavelength-converted sequentially from ω → 2ω → 3ω → 5ω by the wavelength conversion optical elements 31 to 33 provided in this series. Then, the generated fifth harmonic 5ω is incident on the fourth series IV. The second fundamental wave laser beam La ₂ having a wavelength of 1547 nm and an angular frequency ω incident on the second series II is wavelength-converted from ω → 2ω by the wavelength conversion optical element 34 provided in this series, and the generated double wave Enters the fourth series IV. The third fundamental laser beam La ₃ having a wavelength of 1547 nm and an angular frequency ω incident on the third series III enters the fourth series IV without being wavelength-converted.

In the fourth series IV, the wavelength conversion optical element 35 generates the seventh harmonic wave 7ω by the sum frequency generation of the fifth harmonic wave 5ω generated in the first series and the second harmonic wave 2ω generated in the second series. In the element 36, the sum frequency generation of the 7th harmonic wave 7ω and the fundamental wave ω (third fundamental wave laser beam La ₃ ) incident from the third series III generates an angular frequency of 8 times the fundamental wave ω and a fundamental wavelength. An eighth harmonic wave 8ω having a wavelength of 193 nm, which is 1/8 of the wave, is generated. Hereinafter, a detailed configuration of the wavelength conversion optical system 30 including each series of wavelength conversion optical elements will be described.

In the first series I, wavelength conversion optical elements 31, 32, and 33 are arranged. The first fundamental laser beam La ₁ incident on the first series is focused and incident on the wavelength conversion optical element 31, and the angular frequency is twice that of the fundamental wave by the second harmonic generation in the wavelength conversion optical element 31. Generates a second harmonic wave 2ω of ½ (774 nm). The p-polarized double wave 2ω generated by the wavelength conversion optical element 31 and the p-polarized fundamental wave ω transmitted through the wavelength conversion optical element 31 are condensed and incident on the wavelength conversion optical element 32, and the fundamental frequency is generated by sum frequency generation. A triple wave 3ω having three times the wave and a wavelength of 1/3 (516 nm) is generated. A quasi phase matching (QPM) crystal is suitably used as the wavelength conversion optical element 31 for generating the second harmonic wave, and an LBO (LiB ₃ O ₅ ) crystal is preferably used as the wavelength conversion optical element 32 for generating the third harmonic wave.

The QPM crystal 31 for generating a second harmonic is a PPLN (Periodically Poled) in which a periodically poled structure is formed in a ferroelectric crystal having a high nonlinear optical constant with respect to light in this wavelength band, for example, a LiNbO ₃ crystal or a LiTaO ₃ crystal. LiNbO ₃ ) crystal, PPLT (Periodically Poled LiTaO ₃ ) crystal and the like can be suitably used. In addition, PPKTP (Periodically Poled KTiOPO ₄ ) crystal can be used as another QPM crystal, and LBO crystal can be used as a bulk crystal.

  The third wave 3ω of s-polarized light generated in the wavelength conversion optical element 32 and the second wave 2ω of p-polarized light transmitted through the wavelength conversion optical element 32 are transmitted through the two-wavelength wavelength plate 45 and only the second wave 2ω is s. Convert to polarized light. As the two-wavelength wave plate, for example, a wave plate made of a uniaxial crystal flat plate cut in parallel to the optical axis of the crystal is used. This wave plate rotates the plane of polarization with respect to light of one wavelength (second harmonic) and prevents the rotation of the plane of polarization with respect to light of the other wavelength (third harmonic). It is configured by cutting the thickness so as to be an integral multiple of λ / 2 for light of one wavelength and the integral multiple of λ for light of the other wavelength.

The second harmonic wave 2ω and the third harmonic wave 3ω, both of which are s-polarized light, are condensed and incident on the wavelength conversion optical element 33, and by sum frequency generation, the angular frequency is 5 times that of the fundamental wave, and the wavelength is 1/5 (309 nm). A double wave 5ω is generated. For example, a BBO (β-BaB ₂ O ₄ ) crystal is suitably used as the wavelength converting optical element 33 for generating the fifth harmonic wave. At this time, the phase matching is Type I angular phase matching. Note that a CLBO (CsLiB ₆ O ₁₀ ) crystal can also be used as the wavelength conversion optical element 33. Since the beam cross section of the fifth harmonic wave 5ω emitted from the BBO crystal is elliptical due to the walk-off, the beam cross section is shaped into a circle by the two cylindrical lenses 46v and 46h and is incident on the mirror 41. Let

  The mirror 41 has a wavelength selectivity for transmitting laser light in the wavelength band of the fundamental wave ω and the second harmonic wave 2ω and reflecting laser light in the wavelength band of the fifth harmonic wave 5ω, and this mirror ( The fifth polarized wave 5ω of the p-polarized light reflected by the dichroic mirror) 41 is incident on the wavelength conversion optical element 35 of the fourth series IV.

A wavelength converting optical element 34 is disposed in the second series II. The second fundamental laser beam La ₂ incident on the second series is focused and incident on the wavelength conversion optical element 34 to generate a double wave 2ω of p-polarized light. A QPM crystal (such as a PPLN crystal) or a bulk crystal (such as an LBO crystal) similar to the wavelength conversion optical element 31 can be used as the wavelength converting optical element 34 for generating the second harmonic wave. The p-polarized double wave 2ω generated in the wavelength conversion optical element 34 is incident on the mirror 42.

  The mirror 42 has a wavelength selectivity that transmits laser light in the wavelength band of the fundamental wave ω and reflects laser light in the wavelength band of the second harmonic 2ω, and is reflected by the mirror (dichroic mirror) 42. The second polarized wave 2ω of the p-polarized light is transmitted through the mirror 41, superimposed on the same axis as the fifth harmonic 5ω, and enters the fourth series IV wavelength conversion optical element 35.

The third series III is not provided with a wavelength conversion optical element. The third fundamental laser beam La ₃ incident on the third series is incident on the mirror 43 without being wavelength-converted. The s-polarized fundamental wave ω reflected by the mirror 43 passes through the mirror 42 and the mirror 41 and is coaxial with the second harmonic 2ω reflected by the mirror 42 and the fifth harmonic 5ω reflected by the mirror 41. The light is superimposed and incident on the fourth series IV wavelength conversion optical element 35.

  In the fourth series IV, a wavelength conversion optical element 35 and a wavelength conversion optical element 36 are disposed. In addition, in each of the optical paths of the fundamental wave ω, the second harmonic 2ω, and the fifth harmonic 5ω described above, the wavelength conversion optical elements 35 and 36 are set so that light of each wavelength is condensed and incident on the predetermined spot size. A lens is provided. In the wavelength conversion optical element 35, the sum frequency is generated by the fifth polarized wave 5ω of the p-polarized light incident from the first series I and the second harmonic wave 2ω of the p-polarized light incident from the second series II, and the angular frequency is the fundamental wave. And a seventh harmonic wave 7ω having a wavelength of 1/7 (221 nm) is generated. The wavelength conversion optical element 35 for generating the seventh harmonic wave uses a CLBO crystal for Type I angle phase matching.

  The 7th harmonic wave 7ω of s-polarized light generated by the wavelength conversion optical element 35 and the fundamental wave ω of s-polarized light incident from the third wavelength series III and transmitted through the wavelength conversion optical element 35 are incident on the wavelength conversion optical element 36. The sum frequency generation generates an eighth harmonic wave 8ω having an angular frequency of 8 times the fundamental wave and a wavelength of 1/8 (193 nm). As the wavelength conversion optical element 36 for generating the eighth harmonic wave, a CLBO crystal is used for Type I angle phase matching.

  Then, the laser light Lv having a wavelength of 193 nm generated in the wavelength conversion optical element 36 is output from the wavelength conversion unit 3.

  In the laser device LS configured as described above, the fundamental wavelength ω having a wavelength of 1547 nm is focused and incident, and the wavelength conversion optical elements 31 and 34 that generate the second harmonic 2ω having the wavelength of 774 nm by the second harmonic generation are PPLN A QPM crystal such as a crystal or a PPLT crystal is preferably used.

  The light emitted from the wavelength conversion optical elements 31 and 34 is basically a second harmonic wave having a wavelength of 774 nm generated by wavelength conversion and a fundamental wave having a wavelength of 1547 nm that is transmitted without wavelength conversion. However, when the power density (peak intensity) of the incident fundamental wave is increased, a third harmonic wave having a wavelength of 516 nm and a fourth harmonic wave having a wavelength of 387 nm are generated even though the relative power level is low. Specifically, when a PPLN crystal is used as the wavelength conversion optical element 31, the power level of the third harmonic is about three orders of magnitude lower than the second harmonic, and the power level of the fourth harmonic is higher than the second harmonic. It was about 4 digits lower.

  On the other hand, as described above, the photorefractive effect has a strong wavelength dependence, and even when the photorefractive effect is not observed at the second harmonic, the power of the third harmonic is about 1/10 of the second harmonic. A remarkable photorefractive effect is observed at the density (peak intensity). For this reason, when the 3rd and 4th harmonic waves generated by the wavelength conversion optical elements 31 and 34 are multiple-reflected at the light incident / exit surfaces of these elements, it causes a photorefractive effect.

  Therefore, in the laser device LS, light having a wavelength of λ / n (n ≧ 3) of the fundamental wave from λ to n-fold wave is reflected on at least one of the emission surface and the incident surface of the wavelength conversion optical elements 31 and 34. Broadband reflectance reduction coating (AR coating) is applied to reduce the rate. That is, it is configured to actively emit higher harmonics of the third harmonic or higher that can be generated by the wavelength conversion optical elements 31 and 34 to the outside of the element.

  By applying the AR coating as described above to either the exit surface or the entrance surface of the QPM crystal, multiple reflections of higher harmonics can be prevented and the photorefractive effect can be suppressed. By applying the AR coating to both, the generated high-order harmonics can be immediately emitted to the outside, and the effect of suppressing the photorefractive effect can be further improved.

  In addition, by applying an AR coating that reduces the reflectance of light in the wavelength band of the fundamental wave to the third harmonic, the third harmonic with the highest power among the higher-order harmonics incidentally generated in the QPM crystal. The photorefractive effect can be suppressed by releasing it to the outside, and the above effect can be further enhanced by setting the reflectance reduction band to the fundamental wave to the fourth harmonic or more.

  In other words, a broadband AR coat having a wavelength λ to λ / n (n ≧ 3) is applied to at least one of the entrance and exit surfaces of the wavelength conversion optical elements 31 and 34 to emit from the wavelength conversion optical elements 31 and 34. Output beam can be stabilized to a high power range. As a result, various problems resulting from the photorefractive effect can be solved, and high output of the laser device can be realized.

  In the above description, the configuration in which the broadband AR coat for reducing the reflectance with respect to the light having the wavelength of λ to n-fold wave of λ / n (n ≧ 3) is illustrated, but the wavelength is λ, λ. / 3 wavelength reduction type AR coating with reduced reflectivity for light of λ / 2, λ / 3, 4 wavelength reduction type with reduced reflectivity for light of wavelength λ, λ / 2, λ / 3, λ / 4 It may be an AR coat or the like.

  The AR coating as described above has a reflectance of 1% or less (for example, about 0.5%) for the fundamental wave and the second harmonic wave when the wavelength for reducing the reflectance is changed from the fundamental wave to the third harmonic wave. It is preferable that the reflectance with respect to the harmonic wave is 10% or less (for example, about 5%). In addition, when the wavelength for reducing the reflectance is changed from the fundamental wave to the fourth harmonic, it is preferable that the reflectance for the fourth harmonic is 10% or less in addition to the above.

  The AR coat is formed by a thin multilayer coating. For this reason, if the reflectance reduction band is wide and the reflectance is lowered, it becomes difficult to form a thin film (high cost), and the light resistance is also lowered. By setting the reflectance reduction characteristics of the AR coating applied to the wavelength conversion optical elements 31 and 34 as described above, the AR coating can be applied using a general thin film forming facility, and high light resistance is achieved. The effect of suppressing the photorefractive effect can be exhibited while holding.

  As described above, the wavelength output from the laser light output unit 1 is an example of a quasi phase matching type wavelength conversion optical element (QPM crystal) that converts the laser light having the wavelength λ into the laser light having the wavelength λ / 2. A configuration in which the fundamental wave of λ = 1547 nm is applied to the wavelength conversion optical elements 31 and 34 that convert the wavelength of the fundamental wave to the second harmonic of the wavelength λ / 2 = 774 (773.5) nm is shown. However, as is apparent from the above description, the present technology is not limited to the above-described wavelength, and can be applied to a QPM crystal that generates second harmonics in an arbitrary wavelength region, and obtains the same effect. Can do.

  In addition, as an example of a laser device including a QPM crystal in the wavelength conversion unit 3, a fundamental wave having a wavelength of 1547 nm is output from the laser light output unit 1, and the wavelength conversion optical elements 31 and 34 that are QPM crystals and other wavelength conversion optics are output. The configuration is shown in which the wavelength is sequentially converted by the elements 32, 35, and 36 and laser light in the ultraviolet region (deep ultraviolet region) having a wavelength of 193 nm is output. However, the present technology can be applied if the wavelength conversion unit includes a QPM crystal. The wavelength of the laser light incident on the wavelength conversion unit and the configuration of the wavelength conversion unit (the number and arrangement of wavelength conversion optical elements, etc.) The wavelength of the laser beam output from the wavelength conversion unit can be appropriately set according to the application or function of the laser device (for example, using the configuration disclosed in the above-described patent document). .

  Furthermore, as an example of the QPM crystal, a PPLN crystal and a PPLT crystal are illustrated. However, depending on the wavelength, power density, etc. of laser light incident on the crystal, an appropriate ferroelectric crystal is polarized with a period matching the conversion wavelength. A QPM crystal having an inverted structure can be used.

  The laser device LS as described above is small and light and easy to handle, optical processing devices such as exposure devices and stereolithography devices, inspection devices such as photomasks and wafers, observation devices such as microscopes and telescopes, The present invention can be suitably applied to a measuring device such as a length measuring device or a shape measuring device, and a system such as a phototherapy device.

  Next, as a first application example of a system including the laser device LS, an exposure apparatus used in a photolithography process for manufacturing a semiconductor or a liquid crystal panel will be described with reference to FIG. The exposure apparatus 100 is in principle the same as photolithography, and a device pattern precisely drawn on a quartz glass photomask 113 is applied to an exposure object 115 such as a semiconductor wafer or glass substrate coated with a photoresist. Optically project and transfer.

  The exposure apparatus 100 includes the laser apparatus LS, the illumination optical system 102, the mask support table 103 that holds the photomask 113, the projection optical system 104, and the exposure object support table 105 that holds the exposure object 115. And a driving mechanism 106 that moves the exposure object support table 105 in a horizontal plane. The illumination optical system 102 includes a plurality of lens groups, and irradiates the photomask 113 held on the mask support 103 with the laser light output from the laser device LS. The projection optical system 104 is also composed of a plurality of lens groups, and projects the light transmitted through the photomask 113 onto the exposure object 115 on the exposure object support table.

  In the exposure apparatus 100 having such a configuration, the laser light output from the laser apparatus LS is input to the illumination optical system 102, and the laser light adjusted to a predetermined light flux is applied to the photomask 113 held on the mask support 103. Irradiated. The light that has passed through the photomask 113 has an image of a device pattern drawn on the photomask 113, and this light of the exposure object 115 held on the exposure object support table 105 via the projection optical system 104. A predetermined position is irradiated. As a result, the image of the device pattern on the photomask 113 is image-exposed at a predetermined magnification on the exposure object 115 such as a semiconductor wafer or a liquid crystal panel.

  According to such an exposure apparatus 100, the photorefractive effect, which has been a problem in the exposure apparatus having the QPM crystal in the wavelength conversion section of the laser apparatus, is suppressed, has high output and beam stability, and has high output. An apparatus can be provided.

  Next, as a second application example of the system including the laser device LS, FIG. 4 showing a schematic configuration of an inspection device used in an inspection process of a photomask, a liquid crystal panel, a wafer, or the like (test object). The description will be given with reference. The inspection apparatus 200 illustrated in FIG. 4 is preferably used for inspecting a fine device pattern drawn on a light-transmitting object 213 such as a photomask.

  The inspection apparatus 200 includes a laser device LS, an illumination optical system 202, a test object support table 203 that holds the test object 213, a projection optical system 204, and a TDI that detects light from the test object 213. The sensor 215 and a drive mechanism 206 that moves the test object support table 203 in a horizontal plane are provided. The illumination optical system 202 includes a plurality of lens groups, and adjusts the laser light output from the laser device LS to a predetermined light flux and irradiates the test object 213 held on the test object support base 203. The projection optical system 204 is also composed of a plurality of lens groups, and projects the light transmitted through the test object 213 onto a TDI (Time Delay Integration) sensor 215.

  In the inspection apparatus 200 having such a configuration, a laser beam output from the laser apparatus LS is input to the illumination optical system 202, and a laser beam adjusted to a predetermined light flux is held on the object support table 203. The test object 213 such as is irradiated. The light from the object 213 (transmitted light in this configuration example) has an image of a device pattern drawn on the object 213, and this light is transmitted to the TDI sensor 215 via the projection optical system 204. Projected and imaged. At this time, the horizontal movement speed of the test object support table 203 by the drive mechanism 206 and the transfer clock of the TDI sensor 215 are controlled in synchronization.

  Therefore, an image of the device pattern of the test object 213 is detected by the TDI sensor 215, and by comparing the detection image of the test object 213 detected in this way with a predetermined reference image set in advance, The defect of the fine pattern drawn on the test object is extracted.

  According to such an inspection apparatus 200, the photorefractive effect, which has been a problem in an exposure apparatus having a QPM crystal in the wavelength conversion section of the laser apparatus, is suppressed, and it has high output and beam stability, and has a wide inspection area. It is possible to provide an inspection apparatus capable of inspection at high speed and high accuracy. In the case where the test object 213 does not have optical transparency like a wafer or the like, the reflected light from the test object is incident on the projection optical system 204 and guided to the TDI sensor 215 in the same manner. can do.

LS laser apparatus 1 Laser light output unit 3 Wavelength conversion unit 10 Laser light generation unit 20 Optical amplification unit 30 Wavelength conversion optical system 31 to 36 Wavelength conversion optical element (31, 34 pseudo phase matching type wavelength conversion optical element)
DESCRIPTION OF SYMBOLS 100 Exposure apparatus 102 Illumination optical system 103 Mask support stand 104 Projection optical system 105 Exposure object support table 113 Photomask 115 Exposure object 200 Inspection apparatus 202 Illumination optical system 203 Test object support stand 204 Projection optical system 213 Test object 215 TDI sensor (detector)

Claims (13)

A laser beam output unit that outputs a laser beam, and a wavelength conversion unit that wavelength-converts and outputs the laser beam output from the laser beam output unit,
The wavelength conversion unit includes a quasi-phase matching type wavelength conversion optical element that converts the wavelength of the incident laser beam having the wavelength λ into a laser beam having the wavelength λ / 2 and emits the converted laser beam.
Reflection that reduces the reflectance of light having a wavelength of λ / n (n ≧ 3) in addition to light having a wavelength of λ and λ / 2 on at least one of the incident surface and the exit surface of the wavelength conversion optical element. A laser device, characterized in that a rate-reducing coating is applied. 2. The reflectance reducing coating according to claim 1, wherein the reflectance with respect to light of wavelength λ and wavelength λ / 2 is 1% or less, and the reflectance with respect to light of wavelength λ / 3 is 10% or less. The laser apparatus described. The laser device according to claim 2, wherein the reflectance reduction coating has a reflectance of 10% or less with respect to light having a wavelength λ / 4. 4. The laser device according to claim 1, wherein the laser light having a wavelength λ incident on the quasi-phase matching type wavelength conversion optical element has a power density on the order of MW / cm ^{2. 5.} . 5. The laser device according to claim 1, wherein the quasi phase matching type wavelength conversion optical element is a PPLN crystal or a PPLT crystal. The wavelength conversion unit further includes a wavelength conversion optical element that further converts the wavelength of the laser light having the wavelength λ / 2 emitted from the quasi phase matching type wavelength conversion optical element, and the laser beam in the ultraviolet region from the wavelength conversion unit. The laser apparatus according to any one of claims 1 to 5, wherein the laser apparatus is configured to output. At least one of the incident surface and the emission surface of the quasi phase matching type wavelength conversion optical element that converts the wavelength of the incident laser beam having the wavelength λ into a laser beam having the wavelength λ / 2 and emits the converted laser beam,
In addition to light with wavelengths λ and λ / 2, a reflectance reduction coating that reduces the reflectivity for light with wavelengths λ / n (n ≧ 3) is applied,
A photorefractive of a quasi phase matching type wavelength conversion optical element characterized in that a high-order harmonic having a wavelength incidentally generated in the wavelength conversion process shorter than λ / 2 is emitted outside the element. Effect suppression method. 8. The reflectance reducing coating according to claim 7, wherein the reflectance with respect to light of wavelength λ and wavelength λ / 2 is 1% or less, and the reflectance with respect to light of wavelength λ / 3 is 10% or less. A method for suppressing a photorefractive effect of a quasi phase matching type wavelength conversion optical element as described. 9. The method for suppressing a photorefractive effect of a quasi phase matching type wavelength conversion optical element according to claim 8, wherein the reflectance reduction coating has a reflectance of 10% or less with respect to light having a wavelength [lambda] / 4. 10. The pseudo phase according to claim 7, wherein the laser light having a wavelength λ incident on the pseudo phase matching type wavelength conversion optical element has a power density on the order of MW / cm ^2. A method for suppressing a photorefractive effect of a matched wavelength conversion optical element. The photorefractive effect suppression of the quasi phase matching type wavelength conversion optical element according to any one of claims 7 to 10, wherein the quasi phase matching type wavelength conversion optical element is a PPLN crystal or a PPLT crystal. Method. A laser device according to any one of claims 1 to 6;
A mask support for holding a photomask on which a predetermined exposure pattern is formed;
An exposure object support for holding the exposure object;
An illumination optical system for irradiating the photomask held by the mask support with the laser beam output from the laser device;
An exposure apparatus comprising: a projection optical system that projects light transmitted through the photomask onto an exposure target held by an exposure target support. A laser device according to any one of claims 1 to 6;
An object support for holding the object;
An illumination optical system for irradiating a test object held by the test object support unit with laser light output from the laser device;
An inspection apparatus comprising: a detector that detects light from the test object.

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