JPH09146132A - Short-wavelength solid-state laser device for processing - Google Patents

Abstract

PROBLEM TO BE SOLVED: To make the stable output of a laser beam for processing of a short wavelength possible and to obtain a short-wavelength solid-state laser device for processing of high output which does not generate color centers, etc., by using lithium tetraborate (Li2 B4 O7 ) as a wavelength conversion single crystal. SOLUTION: This short-wavelength solid-state laser device for processing has a wavelength conversion element 21 consisting of the lithium tetraborate single crystal. Namely, the lithium tetraborate is used as the wavelength conversion element. The lithium tetraborate is capable of creating the light of the wavelength of quadruple wave (266nm) and quintuple wave (213nm) of high coherence from, for example, an Nd:YG laser (wavelength 1064nm). This quintuple wave (213nm) is shorter in the wavelength than a KrF excimer laser (248nm). In such a case, the wavelength conversion element 21 consisting of the rod- shaped lithium tetraborate single crystal is arranged in front of the YAG laser 22 and the double wave and quadruple wave generating unit 23.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a short wavelength solid-state laser device for processing.

[0002]

The excimer laser was first realized in 1970 by Basov et al. Of the Soviet Union by a method of exciting liquid Xe with an electron beam. Furthermore, in 1976, it succeeded in oscillating by discharge excitation, and came to be used as an industrial laser. The discharge excitation type excimer laser is a pulse repetitive oscillation laser of ultraviolet rays, and includes ArF (193 nm), KrF (248 nm),
Ultraviolet light emitted by a compound such as XeCl (308 nm) was amplified by an optical resonator and extracted as laser light.

A feature of processing with short wavelength laser light is ablation. This means that when a short-wavelength laser beam is applied to a work piece, the work piece is decomposed and scattered instantaneously along with strong light emission and impact sound, and evaporation processing without heat influence can be performed. This phenomenon was described in 1982 by R. Srinivas
It was discovered by An et al. and is called optical ablation. Polymer materials such as polyimide and polymers, ceramics and glass processing (ablation processing), surface modification, marking or thin film formation with evaporated materials, pharmaceutical manufacturing, isotope separation, laser radar, etc. Is being researched.

However, the disadvantages of the excimer laser are 1)
There is a short pulse property that the light emission time is short. For example, in the case of a pulse laser of several hundreds of pps (pulse per second), only a pulsed light of 10 ⁻⁹ seconds is generated every 10 ⁻² seconds, and the laser emission time is remarkably shorter than the interval. It is an obstacle to film formation.

Other disadvantages of the excimer laser are 2) short life of the medium gas, 3) difficulty in downsizing the device, 4) complicated maintenance, and 5) high operating cost. There is. On the other hand, a short-wavelength solid-state laser using a wavelength conversion single crystal may overcome the above disadvantages in the wavelength band of the excimer laser, but its low output has been a problem.

As a conventional wavelength conversion single crystal element, KTP
(KTiOPO ₄ ) and BBO (β-BaB ₂ O ₄ ) are known.

[0007]

However, the wavelength conversion element made of KTP has a KT for the wavelength of the laser incident light.
The transparent area of P is as wide as 0.25 to 4.5 μm, but 1 μ
Phase matching is not performed below m. That is, there is a problem that only the second harmonic can be output. Therefore, there is a problem that it is difficult to shorten the wavelength of the processing short-wavelength solid-state laser device. Further, it is difficult to increase the size of the crystal, and the refractive index changes inside the crystal. Therefore, even elements cut out from one crystal have different phase matching angles because of different refractive indexes. In addition, there is a problem that so-called “soo” easily enters the crystal.

In BBO, the laser damage resistance is KT.
Although it is larger than P, it has a problem that it is slightly soluble in water, has deliquescent properties, has a difficulty in handling, and that it is difficult to prepare large crystals. Further, there is a problem that when ultraviolet light is incident on the BBO, a color center is generated due to deterioration of the crystal. The color center is a point defect inside a transparent crystal detected by the generation of an absorption band in the single crystal.

Thus, although a processing short-wavelength solid-state laser device using a wavelength conversion element has been proposed, it does not provide a wavelength conversion element suitable for this processing short-wavelength solid-state laser device. The present invention has been made in view of the above circumstances, and provides a high-power short-wavelength solid-state laser device for processing, which can stably output a short-wavelength processing laser beam and does not generate a color center or the like. The purpose is to

[0010]

DISCLOSURE OF THE INVENTION The inventors of the present invention have diligently studied a method of increasing the output of the generated laser beam by eliminating the above disadvantages of the excimer laser in the excimer laser wavelength region, and as a result, lithium tetraborate ( Below, Li ₂ B
₄ O ₇ ) is also used as a wavelength conversion single crystal, and a short wavelength solid-state laser incorporating the same can replace the excimer laser, whereby it can be used as a new laser light source for processing that overcomes the disadvantages of the excimer laser. Found,
The present invention has been completed.

That is, the processing short-wavelength solid-state laser device according to the present invention is a processing short-wavelength solid-state laser device having a wavelength conversion element made of lithium tetraborate single crystal. In the short wavelength solid-state laser device for processing according to the present invention, lithium tetraborate is used as the wavelength conversion element.

According to the discovery of the present inventors, lithium tetraborate is used, for example, in Nd: YAG laser (wavelength 1064n).
m), the fourth harmonic (266 nm) with high coherence,
It is possible to generate light having a wavelength of a fifth harmonic (213 nm). The fifth harmonic (213 nm) has a shorter wavelength than the KrF excimer laser (248 nm).

If light having a wavelength of a fourth harmonic or a fifth harmonic can be produced, an infrared laser for which a high-power device has already been developed can be used to easily produce laser light in the ultraviolet region or a region close to it. It is possible to perform ablation by using this laser light as a processing laser.

Since lithium tetraborate can grow a large-diameter single crystal, the wavelength conversion efficiency of a crystal of the same size is inferior to that of a BBO single crystal.
Since the wavelength conversion efficiency is proportional to the square of the laser input power and the square of the crystal length, it is possible to use a larger crystal body of lithium tetraborate capable of growing a large single crystal, resulting in a wavelength conversion efficiency Can be increased.

Further, since lithium tetraborate has a laser damage resistance which is 10 times or more larger than that of BBO, it has an advantage that a laser having a large power can be incident. Furthermore, BB
When O is irradiated with ultraviolet rays for a long time, a color center is generated, but in lithium tetraborate, the crystal is more resistant to deterioration by ultraviolet rays than BBO, and the element lasts longer. Furthermore, even if the lithium tetraborate is irradiated with ultraviolet rays, color centers are hardly generated.

Therefore, the laser device using the wavelength conversion element made of lithium tetraborate can be suitably used as a short wavelength solid-state laser device for processing.

[0017]

Embodiments of the present invention will be described below with reference to the drawings. FIG. 1 is a sectional view showing a pulling device used in an embodiment of the present invention, and FIG. 2 is a schematic view of a short wavelength solid-state laser device for processing according to the present invention.

Example 1 FIG. 1 shows a lithium tetraborate single crystal pulling apparatus 10 used in this example, which has a platinum crucible 1 having a diameter of 90 mm and a height of 100 mm in which lithium tetraborate is melted. . Around the platinum crucible 1, a heater 4 (for example, a resistance heater) for melting lithium tetraborate in the crucible is provided via heat insulating materials 2 and 3. On the other hand, in the upper part of the platinum crucible 1, double heat insulating walls 5 and 6 are provided, and a pulling shaft 7 to which a seed crystal is attached,
It penetrates through the heat insulating walls 5 and 6.

First, a lithium tetraborate single crystal was grown using such a pulling apparatus 10. That is, lithium tetraborate (Li ₂ B ₄ O ₇ ) polycrystalline body 13
After filling 00g in a platinum crucible and melting with a heater,
A lithium tetraborate single crystal having a diameter of 2 inches or more was pulled in the [001], [110], and [100] directions.

As the growth conditions at this time, the temperature gradient between the surface of the melt and 10 mm directly above the melt is 80 ° C., the temperature gradient above it is 30 ° C./cm, and the straight body part of the single crystal is pulled up. The speed was 0.5 mm / hour, and the rotation speed of the seed crystal was 2 rpm. Next, the grown single crystal was tilted by 79 degrees from the C-axis and cut into a rod shape having a length × width of 20 × 20 mm and a length of 50 mm, and the input / output surfaces on both end faces thereof were optically polished.

Next, as shown in FIG. 2, the wavelength conversion element 21 made of the rod-shaped lithium tetraborate single crystal obtained in this manner was used to output 1.5 J, 10 Hz, 10 nsec.
It is arranged in front of the YAG laser 22 and the second and fourth harmonic generation unit 23. A laser beam having a wavelength of 1064 nm and an output of 1.5 J generated from a YAG laser is passed through a second harmonic / fourth harmonic generation unit 23 to generate a wavelength 4
ω (266 nm) and 110 mJ light, and wavelength ω (1
064 nm) and 400 mJ of light are generated. A wavelength conversion element 21 for converting those lights into lithium tetraborate.
By passing the two light through (sum frequency), 5
Ultraviolet light (70 mJ) of ω (213 nm) was generated. Wavelengths other than the desired wavelength are separated using a filter such as a prism, and only the desired light (5ω) is cut off with a glass of 1 mm thickness.
It was irradiated to 4.

It was confirmed that a plume was formed on the portion of the glass 24 irradiated with the light of the fifth harmonic wave and the glass was ablated. Also, a few minutes later, that 5
The glass was penetrated by the light of the overtone. At that time, no abnormality was observed in the wavelength conversion element made of lithium tetraborate single crystal.

In this embodiment, the antireflection coating is not applied to the polished surface of the lithium tetraborate single crystal. Therefore, it is expected that the output of the fifth harmonic wave will be even stronger when this reflection coating is applied. Comparative Example 1 A laser device having the configuration shown in FIG. 2 was constructed in the same manner as in Example 1 except that a rod-shaped BBO having a length × width of 5 × 5 mm and a length of 5 mm was used as the wavelength conversion element. Then, the same ablation process as in Example 1 was performed.

As output light, ultraviolet light of 5 m of 100 mJ was observed, but BBO self-heats because it has a small angle of phase matching and a small allowable temperature range and further has ultraviolet absorption.
It is very difficult to stably generate high power 4ω, 5ω light for a long time. Then, it was confirmed that the color center was generated and the crystal was deteriorated in the test for 100 hours. That is, it has been confirmed that a short-wavelength solid-state laser device for processing that requires long-term output stability has a problem when BBO is used as a wavelength conversion element.

Since BBO is grown by the flux method, impurities are easily taken in and the yield is low. Therefore, the manufacturing cost increases. The present invention is not limited to the above-mentioned embodiments, but can be variously modified within the scope of the present invention.

For example, the light source used as the short-wavelength solid-state laser device for processing is not limited to the above embodiment, and a ruby laser, a glass laser, an alexandrite laser, a garnet laser, a sapphire laser, a semiconductor laser or the like can be used.

[0027]

As described above, according to the short wavelength solid-state laser device for processing of the present invention, an infrared laser, etc. for which a high-power device has already been developed is used as a light source, and its light is emitted from tetraboric acid. By shortening the wavelength with the wavelength conversion element made of lithium, it is possible to easily produce laser light in the ultraviolet region or a region close thereto. Therefore, ablation can be performed by using this laser light as a processing laser.

Since lithium tetraborate can grow a large-diameter single crystal, the wavelength conversion efficiency of a crystal of the same size is inferior to that of a BBO single crystal.
Since the wavelength conversion efficiency is proportional to the square of the laser input power and the square of the crystal length, it is possible to use a larger crystal body of lithium tetraborate capable of growing a large single crystal, resulting in a wavelength conversion efficiency Can be increased.

Further, since lithium tetraborate has laser damage resistance 10 times or more larger than that of BBO, it also has an advantage that a laser of high power can be incident. Furthermore, BB
When O is irradiated with ultraviolet rays for a long time, a color center is generated, but in lithium tetraborate, the crystal is more resistant to deterioration by ultraviolet rays than BBO, and the element lasts longer. Furthermore, even if the lithium tetraborate is irradiated with ultraviolet rays, color centers are hardly generated.

Therefore, the laser device using the wavelength conversion element made of lithium tetraborate can be suitably used as a short wavelength solid-state laser device for processing which does not have the various drawbacks of the excimer laser.

[Brief description of the drawings]

FIG. 1 is a cross-sectional view showing a pulling device used in an example of the present invention.

FIG. 2 is a schematic configuration diagram of a short wavelength solid-state laser device for processing according to the present invention.

[Explanation of symbols]

 1 ... Platinum crucible 2, 3 ... Heat insulating material 4 ... Heater 5, 6 ... Heat insulating wall 7 ... Lifting shaft 10 ... Lifting device 21 ... Wavelength conversion element 22 ... YAG laser 23 ... Second harmonic / fourth harmonic generating unit 24 ... Glass

Claims (1)

[Claims] 1. A short-wavelength solid-state laser device for processing having a wavelength conversion element made of lithium tetraborate single crystal.