JP2010054547A - Ultraviolet laser device - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an ultraviolet laser device by fifth harmonic generation of quasi-continuous oscillation, which is usable for mask inspection. <P>SOLUTION: The ultraviolet laser device 10 includes a mode clock laser device 1 which outputs a basic wave having a pulse width of 50 psec or less; a second harmonic generation crystal 2 to which the basic wave is incident to generate a second harmonic; a beam splitter BS1 which separates the second harmonic and residual basic wave emitted from the crystal 2 from each other; a fourth harmonic generation crystal 3 to which the second harmonic is incident to generate a fourth harmonic; a beam combiner BS2 which makes the fourth harmonic emitted from the crystal 3 coaxial to the residual basic wave from the crystal 2; and a fifth harmonic generation crystal 4 which generates a fifth harmonic by summing and mixing the coaxial fourth harmonic and the residual basic wave. <P>COPYRIGHT: (C)2010,JPO&INPIT

Description

  The present invention relates to an ultraviolet laser device, and more particularly to an ultraviolet laser device capable of quasi-continuous oscillation.

  Ultraviolet light sources are used for illumination in photomask inspection apparatuses for semiconductor exposure that are becoming increasingly finer. As the light source wavelength, it is useful to use exposure wavelengths such as 248 nm and 193 nm as disclosed in Patent Document 1. However, the excimer laser that emits light of these wavelengths can only generate a low repetitive pulse oscillation of several kHz at most, and there is a problem that throughput does not increase when used as an inspection light source.

  A xenon lamp or the like can obtain continuous light of 248 nm, but has a problem of low brightness. Patent Document 2 proposes the use of a 193 nm solid-state light source. However, the light source described in Patent Document 2 uses five stages of wavelength conversion, and there is a problem that the apparatus is complicated and difficult to maintain.

  Under such circumstances, a continuous output laser light source is often used as the illumination light source of the inspection apparatus even if it is different from the exposure wavelength. A frequently used light source is the second harmonic (244 nm to 257 nm) of an argon ion laser. However, an argon ion laser is a gas laser that uses a discharge by a high-voltage electric field and has problems such as extremely low electro-optical conversion efficiency, a large-sized device, a short life, and a long downtime due to frequent maintenance.

  Further, Patent Document 3 proposes an apparatus that generates 198.5 nm light by mixing 244 nm argon laser with 1064 nm light. However, in the apparatus described in Patent Document 3, problems such as an increase in the size of the apparatus and frequent maintenance are increasing. In the case of 266 nm light, which is the fourth harmonic of a 1064 nm solid-state laser, continuous oscillation is possible with a highly efficient and small configuration compared to a gas laser. However, in order to obtain a practical output for mask inspection, it is necessary to convert the external resonator using a second harmonic (532 nm) having a single frequency with a narrow spectral width, and the generated 266 nm is also narrowed. End up.

  As the illumination light source of the inspection apparatus, the narrower the spectral width, the less adverse effects caused by the chromatic aberration of the optical system. However, the problem that interference noise occurs in each optical component of the illumination system and the illumination effect is significantly deteriorated is a major obstacle. Patent Document 4 proposes a method for solving this problem. However, two laser devices are required, and there is a new problem that the device becomes large and complicated.

  In the fifth harmonic generation of a conventionally known 1064 nm solid-state laser, deep ultraviolet light having a wavelength of 213 nm can be obtained from one laser device by mixing the fundamental wave 1064 nm with the fourth harmonic 266 nm. The fifth harmonic is characterized in that deep ultraviolet light can be obtained from a single laser device with a relatively simple configuration. In general, a Q-switched laser having a pulse width of nanosecond order is used.

However, in the prior art, as for the fifth harmonic (213 nm), a pulse oscillation with a low repetition rate of several kHz is commercially available, but there is no proven report on a quasi-continuous oscillation 213 nm light source. As the operation of 213 nm light, continuous oscillation is possible as disclosed in Patent Document 5, but there is a problem of interference noise as in the case of 266 nm light.
JP-A-8-94338 JP 2001-85306 A Japanese Patent No. 3939928 JP 2006-72139 A JP 2006-343786 A

  The present invention has been made in view of such circumstances, and an object of the present invention is to provide an ultraviolet laser light source by quasi-continuous oscillation fifth harmonic generation that can be used for mask inspection.

  An ultraviolet laser device according to a first aspect of the present invention includes a mode-locked laser device that outputs a fundamental wave having a pulse width of 50 psec or less, and a first nonlinear optical crystal that receives the fundamental wave and generates a second harmonic. A beam splitter that separates the second harmonic wave and the remaining fundamental wave emitted from the first nonlinear optical crystal; a second nonlinear optical crystal that receives the second harmonic wave and generates a fourth harmonic wave; A beam combiner in which the fourth harmonic emitted from the second nonlinear optical crystal and the residual fundamental wave from the first nonlinear optical crystal are coaxial, and a sum frequency mixing of the coaxial fourth harmonic and the residual fundamental wave And a third nonlinear optical crystal that generates the fifth harmonic. Thereby, the fifth harmonic of quasi-continuous oscillation can be generated.

  In the ultraviolet laser device according to the second aspect of the present invention, in the above-described ultraviolet laser device, a fourth harmonic pulse precedes the third nonlinear optical crystal in comparison with the remaining fundamental wave, and 3 The 1st optical means for passing through the inside of a nonlinear optical crystal substantially simultaneously is further provided. Thereby, the conversion efficiency of the fifth harmonic of quasi-continuous oscillation can be improved.

  An ultraviolet laser apparatus according to a third aspect of the present invention is the above ultraviolet laser apparatus, wherein the first optical means is at least one parallel provided on the optical path of the remaining fundamental wave after passing through the beam splitter. It is a flat transparent substrate. Thereby, it is possible to improve the conversion efficiency of the fifth harmonic of quasi-continuous oscillation with a simple configuration.

  An ultraviolet laser device according to a fourth aspect of the present invention is characterized in that, in the above-mentioned ultraviolet laser device, the first optical means is provided with an antireflection film. Thereby, loss of beam power due to reflection can be reduced.

  An ultraviolet laser device according to a fifth aspect of the present invention is the ultraviolet laser device described above, wherein the first optical means is formed by adjusting a thickness of at least one of the beam splitter and the beam combiner. It is characterized by this. Thereby, the conversion efficiency of the fifth harmonic of quasi-continuous oscillation can be improved without increasing the number of members.

  An ultraviolet laser device according to a sixth aspect of the present invention is the ultraviolet laser device described above, wherein any of the residual fundamental wave, the second harmonic wave, and the fourth harmonic wave between the beam splitter and the beam combiner. A second optical means for adjusting the output or the polarization direction is provided on the road. As a result, the output of the fifth harmonic can be stabilized.

  An ultraviolet laser device according to a seventh aspect of the present invention includes a mode-locked laser device or a Q-switched laser device that outputs a fundamental wave, and a first nonlinear optical crystal that generates the second harmonic upon incidence of the fundamental wave. A beam splitter that separates the second harmonic wave and the remaining fundamental wave emitted from the first nonlinear optical crystal; a second nonlinear optical crystal that receives the second harmonic wave and generates a fourth harmonic wave; A beam combiner in which the fourth harmonic emitted from the second nonlinear optical crystal and the residual fundamental wave from the first nonlinear optical crystal are coaxial, and a sum frequency mixing of the coaxial fourth harmonic and the residual fundamental wave A third nonlinear optical crystal that generates a fifth harmonic, and an output thereof on an optical path of any of the remaining fundamental wave, the second harmonic, and the fourth harmonic between the beam splitter and the beam combiner. Or partial In which and a second optical means for adjusting the direction. As a result, the output of the fifth harmonic can be stabilized.

  An ultraviolet laser device according to an eighth aspect of the present invention is the ultraviolet laser device described above, wherein the second optical means includes a half-wave plate. Thereby, the polarization direction of the remaining fundamental wave, the second harmonic, or the fourth harmonic can be adjusted, and the output of the fifth harmonic can be stabilized.

  An ultraviolet laser apparatus according to a ninth aspect of the present invention is the ultraviolet laser apparatus described above, wherein the second optical means further includes a polarizer. Thereby, the output of a residual fundamental wave, a 2nd harmonic, or a 4th harmonic can be adjusted, and it becomes possible to stabilize the output of a 5th harmonic.

  An ultraviolet laser apparatus according to a tenth aspect of the present invention is the above-described ultraviolet laser apparatus, wherein a photodetector for detecting a part of the fifth harmonic and an output of the photodetector are substantially constant. And a control mechanism that rotationally drives the half-wave plate. As a result, the output of the fifth harmonic can be automatically stabilized.

  An ultraviolet laser device according to an eleventh aspect of the present invention is the above ultraviolet laser device, wherein the wavelength of the fundamental wave emitted from the mode-locked laser device is 1020 to 1100 nm, and the wavelength of the fifth harmonic is 204. It is characterized by being -220 nm.

  An ultraviolet laser device according to a twelfth aspect of the present invention is the ultraviolet laser device described above, wherein the mode-locked laser device comprises a mode-locked laser oscillator and an amplifier that amplifies the output light. Is.

  ADVANTAGE OF THE INVENTION According to this invention, the ultraviolet laser light source by the 5th harmonic generation of the quasi-continuous oscillation which can be utilized for a mask test | inspection can be provided.

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

Embodiment 1 FIG.
The configuration of the ultraviolet laser apparatus according to Embodiment 1 of the present invention will be described with reference to FIG. FIG. 1 is a diagram showing a configuration of an ultraviolet laser device 10 according to the present embodiment. As shown in FIG. 1, the ultraviolet laser device 10 includes a mode-locked laser device 1, a second harmonic generation crystal (first nonlinear optical crystal) 2, and a fourth harmonic generation crystal (second nonlinear optical crystal) 3. , Fifth harmonic generation crystal (third nonlinear optical crystal) 4, condenser lenses L1, L3, collimating lenses L2, L4, re-condensing lenses L5, L6, lens L7, beam splitters BS1, BS3, beam combiner BS2 And folding mirrors M1 and M2.

The mode-locked laser device 1 emits laser light having a wavelength of 1064 nm with a pulse width of 50 psec or less. The 1064 nm light from the mode-locked laser device 1 is condensed on the second harmonic generation crystal 2 by the condenser lens L1, and a part thereof is converted into the second harmonic having a wavelength of 532 nm. As the second harmonic generation crystal 2, LBO, BBO, KTP, KDP, LiNbO ₃ , PPLN, or the like is used. The conversion efficiency depends on the beam quality, pulse width, crystal quality, and the like of 1064 nm light, but is usually 40 to 60%.

  The generated 532 nm light is collimated by the collimator lens L2, reflected by the beam splitter BS1, and incident on the fourth harmonic generation crystal 3 through the folding mirror M1 and the condenser lens L3. A part of the 532 nm light is converted to 266 nm which is the fourth harmonic by the fourth harmonic generation crystal 3. As the fourth harmonic generation crystal 3, BBO, CLBO, LB4, or the like is used.

  The 266 nm light emitted from the fourth harmonic generating crystal 3 is reflected by the beam combiner BS2 through the collimating lens L4 and the re-condensing lens L5, and is incident on the fifth harmonic generating crystal 4. As the fifth harmonic generation crystal 4, BBO, CLBO, LB4, or the like can be used.

  On the other hand, the remaining 1064 nm light that has not been converted by the second harmonic generation crystal 2 passes through the beam splitter BS1. The 1064 nm light transmitted through the beam splitter BS1 is reflected by the folding mirror M2, and passes through the beam combiner BS2 through the re-condensing lens L6. The 1064 nm light transmitted through the beam combiner BS2 is returned to the same axis as the 266 nm light, is incident on the fifth harmonic generation crystal 4, and a fifth harmonic of quasi-continuous oscillation with a wavelength of 213 nm is generated.

  The fifth harmonic has a pulse width of less than 50 psec and a spectrum width of about 0.001 nm. This spectrum width is a moderate spread that does not cause the problem of interference noise in the mask inspection apparatus and does not cause the problem of chromatic aberration.

  The fifth harmonic wave having a wavelength of 213 nm is reflected by the beam splitter BS3, passes through the lens L7, and is output. As described above, according to this embodiment, a quasi-continuous oscillation ultraviolet laser device that can be used for mask inspection can be realized with a simple configuration.

Embodiment 2. FIG.
An ultraviolet laser apparatus according to Embodiment 2 of the present invention will be described with reference to FIG. FIG. 2 is a diagram showing a configuration of the ultraviolet laser apparatus 10 according to the present embodiment. The present embodiment is different from the first embodiment in that an optical path length compensation element 5 is provided. 2, the same components as those in FIG. 1 are denoted by the same reference numerals, and description thereof is omitted.

  The present inventor performed quasi-continuous oscillation of 213 nm light using a 1064 nm mode-locked laser apparatus, and found the following points in order to improve the generation efficiency of 213 nm light in quasi-continuous oscillation. For example, the pulse width of the laser beam by the mode-locked laser device 1 used in the experiment was 10 psec, and the propagation distance in air was as short as 3 mm. In this case, the temporal overlap of the 1064 nm and 266 nm light pulses incident on the fifth harmonic generation crystal 4 may shift due to group delays in optical components that do not cause a problem in the third and fourth harmonics.

  In order to improve the efficiency of the fifth harmonic generation by the mode-locked laser, the present inventor is effective to match the temporal overlap of the 1064 nm and 266 nm light pulses incident on the fifth harmonic generation crystal 4. I found out.

  Here, the group delay in the ultraviolet laser device 10 will be described with reference to FIG. As shown in FIG. 3, consider the case where the 1064 nm light and the 532 nm light after the second harmonic generation are separated by the beam splitter BS1 and recombined with the 266 nm light converted from the 532 nm light and the beam combiner BS2.

  In this configuration, the 1064 nm light is transmitted twice through the beam splitter (BS1, BS2), so that the transit time is long. Further, since the 1064 nm light is bent upward by the beam splitters BS1 and BS2, it becomes longer than the optical path length of 532 nm and 266 nm light. For example, it is assumed that the beam splitters BS1 and BS2 are made of synthetic quartz having a group refractive index of 1.46 and have a thickness of 5 mm. In this case, the total delay time is 31.5 psec. Further, the group delay of 1064 nm in the re-condensing lens L6 is 7.7 psec if it is made of quartz having a thickness of 5 mm.

  On the other hand, the fourth harmonic generation crystal 3 is a CLBO crystal having a length of 10 mm, and the lenses L3, L4, and L5 are quartz lenses having a thickness of 5 mm. The group delay of the 532 nm light by the condenser lens L3 and the group delay of the 266 nm light accompanying the fourth harmonic generation crystal 3, the collimating lens L4, and the re-condensing lens L5 is 49.1 psec in total.

  Therefore, in this example, the 1064 nm light pulse that arrives at the fifth harmonic generation crystal 4 precedes the 266 nm light by 9.9 psec. In the present embodiment, the optical path length compensation element 5 is provided so that the 1064 nm light and the 266 nm light overlap in time in the fifth harmonic generation crystal 4. By making the optical path length of 1064 nm light substantially the same as the optical path lengths of 532 nm light and 266 nm light, the conversion efficiency to 213 nm light can be increased.

  In general, a laser optical path length delay method is often realized by combining four plane mirrors M11, M12, M13, and M14 as shown in FIG. 11 (for example, Japanese Patent Publication No. 3686777). However, such a configuration is complicated and large, and is inappropriate for adjusting a slight delay time of about 10 psec (3 mm) as in the present embodiment.

  As shown in FIG. 2, in the present embodiment, the optical path length compensation element 5 is provided on the optical path of the remaining fundamental wave 1064 nm light after passing through the beam splitter BS1. The optical path length compensation element 5 is provided to make the optical path lengths of the 1064 nm light and the 266 light substantially coincide with each other and improve the conversion efficiency of the pulsed light from the mode-locked laser device 1. As the optical path length compensation element 5, at least one parallel plane transparent substrate that transmits 1064 nm light can be used.

  In addition, since the group delay in the transmission of the fifth harmonic generation crystal 4 is also generated, it is preferable that the 1064 nm light pulse is incident with a slight delay from the 266 nm light pulse. Thereby, two pulses overlap in the vicinity of the center of the fifth harmonic generation crystal 4 and the conversion efficiency of 213 nm light can be maximized.

  As described above, in the example illustrated in FIG. 3, when the optical path length compensation element 5 is not provided, the fifth harmonic generation crystal 4 is about 9.9 psec ahead of the pulse of 1064 nm light with respect to the pulse of 266 nm light. Is incident on. In the present embodiment, for example, a parallel flat plate made of synthetic quartz (group refractive index 1.46 at 1064 nm) having a thickness of 8.5 mm is used as the optical path length compensation element 5.

  In this case, it takes an extra time of 8.5 mm × (1.46-1.00) ÷ light velocity = 13.0 psec as compared with the case where 1064 nm light passes through the air. Accordingly, the pulse of 266 nm light can be incident on the fifth harmonic generation crystal 4 ahead of (13−9.9) = 3.1 psec as compared with the pulse of 1064 nm light. When the fifth harmonic generation crystal 4 is a CLBO crystal having a length of 10 mm, the group refractive index of 266 nm is 1.69 with respect to the group refractive index of 1.64 of 1064 nm. For this reason, the 1064 nm light pulse catches up with the 266 nm light pulse in the vicinity of the center of the fifth harmonic generating crystal 4, and the sum frequency mixing can be performed most efficiently.

  FIG. 4 shows the conversion efficiency to the fifth harmonic when the optical path length compensation element 5 is provided and when the optical path length compensation element 5 is not provided. In the example shown in FIG. 4, a mode-locked oscillation Nd: YVO 4 laser having a maximum output of 9.5 W is used as the mode-locked laser device 1. When the present invention is not used, the fifth harmonic output is only 40 mW. However, by using the present invention, it was possible to obtain an output 10 times as large as 400 mW. Thus, the conversion efficiency can be greatly improved by inserting the optical path length compensation element 5 into the optical path length of 1064 nm light.

  In order to select the total thickness of the optical path length compensation element 5 so that the output of 213 nm light is maximized, it is preferable to provide a plurality of optical path length compensation elements 5. By using a parallel plate transparent substrate as the optical path length compensation element 5, even if a plurality of optical path length compensation elements 5 are appropriately inserted on the optical path of 1064 nm light, it is easily optimized without changing the optical axis angle. can do. This is an extremely useful feature when manufacturing a plurality of devices.

  In addition, it is also possible to change suitably to the optical path length compensation element 5 in which thickness differs. Moreover, it is preferable to apply an antireflection film to the incident surface and the exit surface of the optical path length compensation element 5. Thereby, power loss can be suppressed. It is also possible to compensate for the optical path length by adjusting the thickness of at least one of the beam splitter BS1 and the beam combiner BS2. For example, in the above example, the thickness of the beam splitter BS1 and the beam combiner BS2 can be 9 mm or more. Thereby, the optical path length can be compensated without increasing the number of components.

  FIG. 5 shows another configuration example of the optical path length compensation element 5 used in the ultraviolet laser apparatus according to the present embodiment. As shown in FIG. 5, the two optical path length compensation elements 5a and 5b are arranged in a symmetrical arrangement close to the Brewster angle. By entering the optical path length compensation element 5a so as to be horizontally polarized, substantially the same effect can be obtained without providing an antireflection film on the optical path length compensation element 5.

Further, as the optical path length compensation element 5, any transparent optical material that transmits 1064 nm light can be used instead of synthetic quartz. For example, a glass such as BK7 or a crystal such as sapphire, CaF ₂ , or BBO may be used.

  Here, with reference to FIG. 6 and FIG. 7, another configuration example of the ultraviolet laser device according to the present embodiment will be described. As shown in FIG. 6, beam splitters BS1 and BS2 that reflect 1064 nm light and transmit 532 nm light may be used. In this case, since the 532 nm light is delayed by the transmission of the beam splitter, a larger optical path length compensating element 5 for adjusting the optical path length of the 1064 nm light is required.

  In addition, as shown in FIG. 7, it is possible to use a beam splitter BS1 that reflects 1064 nm light and transmits 532 nm light, and a beam combiner BS2 that asks for 1064 nm light or reflects 532 nm light. is there.

  As described above, according to the present embodiment, one mode-locked laser device can be used for mask inspection with a relatively small configuration having an appropriate spectrum width and no problem of interference noise using a fundamental wave as a fundamental wave. It is possible to realize a high-power deep ultraviolet laser light source by quasi-continuous oscillation fifth harmonic generation.

  In the present embodiment, the optical path length compensation element 5 is provided on the optical path of 1064 nm light. However, the present invention is not limited to this. For example, it may be provided on the optical path of the second harmonic after being reflected by the beam splitter BS1 or on the optical path of the fourth harmonic after being converted by the fourth harmonic generation crystal 3.

Embodiment 3 FIG.
An ultraviolet laser apparatus according to Embodiment 3 of the present invention will be described with reference to FIG. FIG. 8 is a diagram showing a configuration of the ultraviolet laser device 20 according to the present embodiment. The ultraviolet laser device 20 according to the present embodiment has a function of stabilizing the output of the fifth harmonic.

  Conventionally, providing an APC (Automatic Power Control) circuit to stabilize the output of generated wavelength converted light has been performed in a harmonic generation type light source. However, as a result of studies by the present inventors, it has been found that stable control is very difficult even if an APC circuit is provided as in the prior art.

  That is, when the wavelength conversion process of three stages is performed as in the case of the fifth harmonic, there is no linearity of the output of the fifth harmonic with respect to the pumping light output. Further, as shown in FIG. 10, it was found that the output of the fifth harmonic depends on the fifth power with respect to the output fluctuation of the fundamental wave. Therefore, when the fundamental wave output varies, the output of the fifth harmonic varies more greatly.

  In addition, since wavelength conversion is performed using as many as three nonlinear optical crystals, the temperature due to light absorption in each crystal changes when the output of the fundamental wave is changed. For this reason, a change in phase matching condition is induced, and there is a phenomenon that hysteresis is large. For this reason, it is desirable that the light output incident on the nonlinear optical crystal at each stage and the generated wavelength conversion output are not changed as much as possible.

  Therefore, in the present embodiment, an ultraviolet laser device described below has been devised for the purpose of providing an ultraviolet laser device using fifth harmonic generation having an output stabilization mechanism with good controllability by simple means. .

  As shown in FIG. 8, the ultraviolet laser device 20 according to the present embodiment includes a mode-locked laser device 1, a second harmonic generation crystal 2, a fourth harmonic generation crystal 3, and a fifth harmonic generation crystal. 4, rotary wave plate 6, polarizer 7, photodetector 8, APC circuit 9, condenser lenses L1, L3, collimating lenses L2, L4, re-condensing lenses L5, L6, lens L7, beam splitters BS1, BS3 , BS4, beam combiner BS2, and folding mirrors M1 and M2. Here, an example in which an Nd: YAG laser having a wavelength of 1064 nm and an output of 10 W is used as the mode-locked laser device 1 will be described.

The 1064 nm light from the mode-locked laser device 1 is condensed on the second harmonic generation crystal 2 by the condenser lens L1, and a part thereof is converted into the second harmonic having a wavelength of 532 nm. As the second harmonic generation crystal 2, LBO, BBO, KTP, KDP, LiNbO ₃ , PPLN, or the like is used. The conversion efficiency depends on the beam quality, pulse width, crystal quality, etc. of 1064 nm light, but is usually about 50% and 5 W.

  The generated 532 nm light is collimated by the collimator lens L2, reflected by the beam splitter BS1, and incident on the fourth harmonic generation crystal 3 through the folding mirror M1 and the condenser lens L3. A part of the 532 nm light is converted to 266 nm which is the fourth harmonic by the fourth harmonic generation crystal 3. As the fourth harmonic generation crystal 3, BBO, CLBO, LB4, or the like is used. From 5 W of 532 nm light, a maximum of about 2 W of 266 nm light is generated.

  The 266 nm light emitted from the fourth harmonic generating crystal 3 is reflected by the beam combiner BS2 through the collimating lens L4 and the re-condensing lens L5, and is incident on the fifth harmonic generating crystal 4. As the fifth harmonic generation crystal 4, BBO, CLBO, LB4, or the like can be used.

  On the other hand, the remaining 1064 nm light that has not been converted by the second harmonic generation crystal 2 is about 5 W. The remaining 1064 nm light passes through the beam splitter BS1, is reflected by the folding mirror M2, passes through the re-condensing lens L6, and passes through the beam combiner BS2. The 1064 nm light transmitted through the beam combiner BS2 is returned to the same axis as the 266 nm light and is incident on the fifth harmonic generation crystal 4.

  In the optimum state, a fifth harmonic (213 nm) of about 1 W at maximum is generated from 2W 266 nm light and 5 W 1064 nm light. Actually, it is often a little smaller due to a loss in the optical components on the way.

  At this time, the rotary wave plate 6 and the polarizer 7 are installed in the optical path of the remaining fundamental wave separated by the beam splitter BS1. The remaining fundamental wave passes through the polarizer 7 by the rotation of the rotary wave plate 6. Thereby, the output of the remaining 1064 nm light can be easily adjusted. The output of the fifth harmonic is proportional to the product of the residual fundamental wave and the output of the fourth harmonic. Therefore, as shown in FIG. 9, the output of the fifth harmonic is proportional to the output fluctuation of the fundamental wave. Therefore, if the output of the remaining fundamental wave is adjusted, the output of the fifth harmonic is proportional to the adjusted output of the remaining fundamental wave while keeping the outputs of the second and fourth harmonics substantially constant. Since it can be controlled, it can be controlled easily and stably.

  The fifth harmonic is reflected by the beam splitter BS3 and is incident on the beam splitter BS4 which is an optical branching unit. The beam splitter BS4 reflects part of the fifth harmonic toward the photodetector 8 and transmits the rest. A part of the fifth harmonic is detected by the photodetector 8. The APC circuit 9 controls the output of the 1064 nm light and keeps the output of the fifth harmonic at a predetermined value by rotationally driving the rotary wave plate 6 based on the light detection signal output from the light detector 8. .

The output of the 1064 nm light is proportional to (sin θ) ² with respect to the rotation angle θ of 0 to 45 ° of the half-wave plate. The rotary wave plate 6 has a configuration in which a half-wave plate is attached to a rotary device driven by a step motor. If the APC circuit 9 controls the detection value of the 213 nm light, which is the fifth harmonic, by the photodetector 8 to be constant, automatic constant control (APC) of the output of the 213 nm light can be realized.

  According to the present embodiment, it is possible to easily realize the APC function of controlling the output of the fifth harmonic by the mode-locked laser device 1 by compensating for the output change due to the deterioration of the crystal or the optical component or the like. This is because the output of 213 nm light can be adjusted by adjusting only the output of 1064 nm light or the polarization plane without changing the 266 nm output in which the hysteresis effect due to thermal influence is significant. As a result, it is possible to stably control the stability of about ± 2%, which has conventionally been secured only about ± 5%. For this reason, it can be used for a light source for a photomask defect inspection apparatus or for processing.

In the present embodiment, a combination of a half-wave plate and a polarizer is used, but the same effect can be obtained even if the polarizer is removed. Since the polarization direction of the 1064 nm light that can be mixed with the 266 nm light is constant in the fifth harmonic generation crystal 4, if the polarization plane is rotated by the half wavelength plate, the component that contributes to the fifth harmonic generation is still the half wavelength plate. If the rotation angle is θ, it depends on (sin θ) ² .

  In the above-described embodiment, the rotary wave plate 6 and the polarizer 7 are arranged on the optical path of the remaining fundamental wave. However, the present invention is not limited to this. For example, optical means for adjusting the output or polarization direction may be provided on the second harmonic optical path after the beam splitter BS1 and on the fourth harmonic optical path after the fourth harmonic generating crystal 3. Is possible.

  Further, in the present embodiment, the mode-locked laser device 1 is used, but a Q-switched laser can be used instead. Furthermore, the APC function and the optical path length compensation function can be realized at the same time by combining the second and third embodiments.

1 is a diagram illustrating a configuration of an ultraviolet laser device according to a first embodiment. FIG. 6 is a diagram showing a configuration of an ultraviolet laser device according to a second embodiment. 6 is a diagram for explaining group delay of light of each wavelength in the ultraviolet laser apparatus according to Embodiment 2. FIG. 6 is a graph showing a result of fifth harmonic conversion of the ultraviolet laser device according to the second embodiment. It is a figure which shows the other structural example of the optical path length compensation element used for the ultraviolet laser apparatus which concerns on Embodiment 2. FIG. It is a figure which shows the other structural example of the ultraviolet laser apparatus which concerns on Embodiment 2. FIG. It is a figure which shows the other structural example of the ultraviolet laser apparatus which concerns on Embodiment 2. FIG. 6 is a diagram showing a configuration of an ultraviolet laser device according to Embodiment 3. FIG. 10 is a graph showing the relationship between the fifth harmonic and the fundamental wave of the ultraviolet laser device according to Embodiment 3. It is a graph which shows the relationship between the 5th harmonic of the conventional ultraviolet laser apparatus, and a fundamental wave. It is a figure for demonstrating the delay method of the conventional laser optical path length.

DESCRIPTION OF SYMBOLS 1 Mode-locked laser apparatus 2 Crystal for 2nd harmonic generation 3 Crystal for 4th harmonic generation 4 Crystal for 5th harmonic generation 5 Optical path length compensation element 6 Rotating wave plate 7 Polarizer 8 Photo detector 9 APC circuit 10 , 20 Ultraviolet laser device L1, L3 Condensing lens L2, L4 Collimating lens L5, L6 Recondensing lens L7 Lens M1, M2 Folding mirror M11, M12, M13, M14 Plane mirror BS1, BS3, BS4 Beam splitter BS2 Beam combiner

Claims (12)

A mode-locked laser device that outputs a fundamental wave having a pulse width of 50 psec or less;
A first nonlinear optical crystal that receives the fundamental wave and generates a second harmonic;
A beam splitter that separates the second harmonic and the residual fundamental wave emitted from the first nonlinear optical crystal;
A second nonlinear optical crystal that receives the second harmonic and generates a fourth harmonic;
A beam combiner coaxially with a fourth harmonic wave emitted from the second nonlinear optical crystal and a residual fundamental wave from the first nonlinear optical crystal;
An ultraviolet laser device comprising: a third nonlinear optical crystal that generates a fifth harmonic by mixing the sum of the coaxial fourth harmonic and the residual fundamental wave.   The first optical means for arriving at the third nonlinear optical crystal with a fourth harmonic pulse ahead of the residual fundamental wave and passing through the third nonlinear optical crystal substantially simultaneously. 2. An ultraviolet laser device according to 1.   3. The ultraviolet laser device according to claim 2, wherein the first optical unit is at least one parallel plane transparent substrate provided on an optical path of a residual fundamental wave after passing through the beam splitter.   The ultraviolet laser device according to claim 2, wherein the first optical unit is provided with an antireflection film.   The light source device according to claim 2, wherein the first optical unit is formed by adjusting a thickness of at least one of the beam splitter and the beam combiner.   A second optical means for adjusting the output or polarization direction of the residual fundamental wave, the second harmonic, or the fourth harmonic is provided between the beam splitter and the beam combiner. The ultraviolet laser device according to claim 1 or 2. A mode-locked laser device or a Q-switched laser device that outputs a fundamental wave;
A first nonlinear optical crystal that receives the fundamental wave and generates a second harmonic;
A beam splitter that separates the second harmonic and the residual fundamental wave emitted from the first nonlinear optical crystal;
A second nonlinear optical crystal that receives the second harmonic and generates a fourth harmonic;
A beam combiner coaxially with a fourth harmonic wave emitted from the second nonlinear optical crystal and a residual fundamental wave from the first nonlinear optical crystal;
A third nonlinear optical crystal that generates a fifth harmonic by sum-frequency mixing the coaxial fourth harmonic and the residual fundamental wave;
An ultraviolet laser apparatus comprising: a second optical means for adjusting an output or a polarization direction on an optical path of any one of a residual fundamental wave, a second harmonic, and a fourth harmonic between the beam splitter and the beam combiner. .   The ultraviolet laser device according to claim 6, wherein the second optical unit includes a half-wave plate.   The ultraviolet laser apparatus according to claim 8, wherein the second optical unit further includes a polarizer. A photodetector for detecting a part of the fifth harmonic;
A control mechanism that rotationally drives the half-wave plate so that the output of the photodetector is substantially constant;
The ultraviolet laser device according to any one of claims 6 to 9, further comprising:   The wavelength of the fundamental wave emitted from the mode-locked laser device is 1020 to 1100 nm, and the wavelength of the fifth harmonic is 204 to 220 nm. Ultraviolet laser equipment.   The ultraviolet laser apparatus according to claim 1, wherein the mode-locked laser apparatus includes a laser oscillator that performs a mode-locking operation and an amplifier that amplifies the output light.

Images