JP5016208B2 - Surface treatment method for optical element - Google Patents

Description

  The present invention relates to a surface treatment method for an optical element used in a laser system.

  Lasers are being put to practical use as fundamental technologies in various fields ranging from chemical technology to industrial technology due to their distinctive characteristics as light. Therefore, a laser system that generates and outputs laser light is required to have various characteristics such as generation of high-density energy and specific wavelength output in a region extending from ultraviolet to infrared. However, if an attempt is made to generate laser light having an extremely high energy density, very powerful laser light passes through optical elements such as lenses and reflecting mirrors in the laser system. In this case, the surface of the optical element or the inside thereof is damaged by the laser, and the damage causes the problem that it is difficult to increase the intensity of the laser beam. As can be seen from the above, it is no exaggeration to say that characteristics such as laser output and lifetime are determined by the laser damage resistance of the optical element in the laser system.

  On the other hand, the laser damage resistance of the optical element is lower on the surface than on the inside due to the influence of the surface-modified layer generated by cutting or optical polishing. Therefore, creation of an optical element surface having a high laser damage resistance is an indispensable element for extending the life and performance of a laser system.

  Here, the surface altered layer of the optical element will be briefly described. FIG. 7 is a diagram schematically showing the state of the surface altered layer. As shown in FIG. 7, the surface altered layer 101 includes a polishing impurity layer 102, a defect layer 103, and a strained layer 104. In particular, the polishing impurity layer 102 is a layer generated by burying polishing abrasive grains in the surface layer in the polishing process, and is approximately 100 nm to 1000 nm deep from the surface of the optical element, depending on the polishing method.

  Conventionally, in order to improve the laser damage resistance of an optical element, a technique for removing abrasive grains buried in the polishing impurity layer has been developed. For example, there is a conventional surface treatment method that removes a surface layer having residual abrasive grains by irradiating a surface of a mechanically polished crystal optical element with an ion beam (see, for example, Patent Document 1).

  Another conventional surface treatment method involves chemically treating the surface of the optical element with a chemical solution that reacts with the abrasive to remove the remaining abrasive without affecting the surface of the optical element. (For example, refer to Patent Document 2).

The method of chemically treating the surface of the optical element with a chemical solution that reacts with the material of the optical element has not been used so far because it degrades the surface state of the optical element.
JP 2000-271664 A JP 2002-286902 A

  However, in the surface treatment method using an ion beam described in Patent Document 1 or the like, defects may be generated due to the atomic bonds on the surface of the optical element being broken by the irradiated ions. As a result, there is a problem that the surface of the laser beam absorbs more and the laser damage resistance is lowered.

  Further, in the surface treatment method by chemical treatment described in Patent Document 2 or the like, when the layer in which the abrasive grains are buried is removed, the deterioration of the surface state is inevitable. There was a case of decline. This was more noticeable for materials other than quartz glass.

  Furthermore, in recent years, utilization of laser beams in the high energy deep ultraviolet region is expected, and when these laser beams are used, a reduction in laser damage resistance of optical elements in the laser system is a major obstacle. Therefore, there is a strong demand for an optical element having a surface with high laser damage resistance.

  The present invention has been made to solve the above-described problems, and removes abrasive grains embedded in the surface layer of an optical element without reducing the laser damage resistance of the optical element that has been optically polished. An object of the present invention is to provide a method for surface treatment of an optical element.

  An optical element surface treatment method according to the present invention is an optical element surface treatment method for treating the surface of an optical element that has been optically polished, and is an ion etching step in which etching is performed by irradiating the surface of the optical element with an ion beam. And a chemical etching step of performing etching by immersing and dissolving the ion-etched surface in a predetermined solution that reacts with the material of the optical element. Hereinafter, this optical element surface treatment method is referred to as “first optical element surface treatment method”.

  Preferably, a depth at which the optical element is etched in the ion etching step is deeper than a depth at which the optical element is etched in the chemical etching step. Hereinafter, this optical element surface treatment method is referred to as “second optical element surface treatment method”.

  Preferably, in the second optical element surface treatment method, the optically polished optical element has a polishing impurity layer in which polishing abrasive grains are buried in the surface layer. The ion etching step is a step of removing the polishing impurity layer. Hereinafter, this optical element surface treatment method is referred to as “third optical element surface treatment method”.

  Preferably, in the third optical element surface treatment method, the polishing impurity layer is present to a depth of 1000 nm or less from the optically polished surface of the optical element. Hereinafter, this optical element surface treatment method is referred to as “fourth optical element surface treatment method”.

  Preferably, in any one of the first to fourth optical element surface treatment methods, the optical element is made of any one of quartz glass, calcium fluoride, and aluminum oxide.

  According to the surface treatment method of an optical element according to the present invention, an ion etching step for performing etching by irradiating the surface of the optical element with an ion beam, and a predetermined solution that reacts the ion-etched surface with the material of the optical element. And a chemical etching step of performing etching by immersing and dissolving in the optical element, so that the abrasive grains buried in the surface layer of the optical element can be removed without lowering the laser damage resistance.

Embodiments of the present invention will be described below with reference to the accompanying drawings.
(Embodiment 1)
In the surface treatment method for an optical element according to the first embodiment of the present invention, first, an ion beam is irradiated to the optically polished surface, and the abrasive grains buried in the surface layer are removed. Furthermore, chemical etching is performed on the surface whose state has deteriorated due to the energy of the ion beam, and the deteriorated portion is removed. Here, the optical element is an element arranged in the optical path of the laser beam in the laser system, and examples thereof include a lens, a window material, a prism, a polarizer, a mirror substrate, a wavelength conversion element, and a Q switch. Examples of the material of the optical element include optical glass and ceramic materials used for lenses and the like, and nonlinear crystals used for wavelength conversion elements and Q switches. In the optical element surface treatment method according to the first embodiment, chemical etching is performed after ion etching, whereby the surface state deteriorated by ion etching can be improved and the laser damage resistance can be improved. Therefore, an optical material such as calcium fluoride (CaF ₂ ) or oxidation that hardly causes improvement in laser damage resistance due to physical or chemical defects caused by irradiation ion energy during ion etching and deterioration of the surface state. It is more effective when applied to aluminum (Al ₂ O ₃ ) or the like.

In the surface treatment method of an optical element according to the first embodiment, first, an ion beam is irradiated to an optical material such as CaF ₂ or Al ₂ O ₃ whose surface state deteriorates when an ion beam is irradiated. Polishing abrasive grains are removed. Naturally, the surface state deteriorates due to the energy of ions. In the case of CaF ₂ crystal, the transmittance decreases due to the influence. The laser damage resistance is reduced or slightly improved as compared to the case where the optical polishing is performed without surface treatment (hereinafter referred to as “untreated”). In the energy region used by the inventor, paying attention to the fact that the surface state deterioration by the ion beam is about several atomic layers, the chemical etching slightly melts the surface to reduce the number of atoms deteriorated by the ion beam. Remove. In this case, the chemical etching process can be performed in a very short time, so that the surface state does not deteriorate. Due to this chemical etching treatment, the reduced transmittance returns to the state of the untreated state. Furthermore, the laser damage resistance is improved up to twice that of the untreated case.

  Hereinafter, the surface treatment method of the optical element according to the first embodiment will be described in detail. FIG. 1 is a flowchart for explaining a surface treatment method for an optical element according to Embodiment 1 of the present invention. As shown in FIG. 1, first, ion etching is performed by irradiating the surface of an optical element that has been optically polished with an ion beam (step S1). Thereby, the abrasive grains embedded in the surface layer of the optical element can be removed. The acceleration voltage of the ion beam is, for example, 10 eV to 1000 eV. By this ion etching, the polishing impurity layer in which the polishing abrasive grains are buried is removed. The depth from the surface of the polishing impurity layer is, for example, 100 nm to 1000 nm.

However, the energy of irradiated ions causes chemical and / or physical defects and deteriorates the surface state. Thereby, for example, when the optical element is made of CaF ₂ , the transmittance of the optical element is lowered. Therefore, chemical etching is performed on the ion-etched surface, and the surface is melted (step S2), thereby removing the surface deteriorated by the ion beam. At that time, the etching solution used is a solution that reacts with the material of the optical element, and is about 0.1 to 60 nm by chemical reaction without deteriorating the surface state such as the surface roughness of the optical element and the composition of the material surface. It is a solution of acid or alkali that can remove the extreme surface layer. For example, when the optical element is made of CaF ₂ soluble in acid, phosphoric acid (H ₃ PO ₄ ) or the like is used. This is because acid such as hydrochloric acid and sulfuric acid has a very low concentration, and if it is not etched for a long time, the reactivity is too strong and the surface state deteriorates. On the other hand, phosphoric acid has a very high reaction even at a relatively high concentration. The reason is to proceed gently. By the surface treatment method described above, the polishing impurity layer constituting the surface altered layer of the optical element can be removed.

Below, the measurement method and measurement result of the laser damage tolerance of the optical element processed with the above-mentioned surface treatment method are demonstrated. FIG. 2 is a diagram showing a configuration example of a laser damage resistance measuring apparatus. As shown in FIG. 2, the laser damage tolerance measuring apparatus 1 includes an Nd: YAG pulse laser 2, a CsLiB ₆ O ₁₀ (CLBO) crystal 3, a prism 4, a beam damper 5, a reflection mirror 6, an attenuator 7, A power meter 8 and a short focus lens 9 are provided. The attenuator 7 includes a half-wave plate 10, a polarizer 11, and a beam splitter 12. The CLBO crystal 3 is a wavelength conversion element that converts the wavelength of laser light from 532 nm to 266 nm, the beam damper 5 is an element that blocks off incident laser light with wavelengths of 1064 nm and 532 nm, and the attenuator 7 is incident laser light. It is an element that attenuates. Using this measuring apparatus 1, a laser beam emitted from an Nd: YAG second harmonic pulse laser (wavelength 532 nm) is irradiated to the CLBO crystal 3 which is a nonlinear optical crystal, and a laser having a wavelength of 266 nm which is a fourth harmonic. The light is generated, and the laser light having a wavelength of 266 nm is condensed by the short focal lens 9 having a focal length of, for example, 400 mm via the attenuator 7, and the collected laser light is irradiated to the optical element 13 to be measured. To do. Here, the irradiation of the laser beam to the optical element 13 is performed while changing the irradiation position while changing the intensity of the laser beam by the attenuator 7. Specifically, the intensity of the laser beam is changed by changing the direction of polarization of the laser beam by rotating the half-wave plate 10 of the attenuator 7. The intensity of the laser beam is monitored by a power meter 8. Thereby, a plurality of traces irradiated with different laser intensities exist on the surface of the optical element after the laser irradiation.

  Thereafter, the surface of the optical element irradiated with the laser light is observed with a Nomarski microscope. When laser damage occurs, plasma emission and irreversible damage marks occur on the damaged part, and the laser intensity corresponding to the position of the damage marks can be regarded as the limit of the laser intensity that causes damage to the optical element. . Therefore, the laser intensity per unit area and unit time is calculated from the incident energy intensity of the laser beam when the damage occurs, the beam diameter and the pulse width, and the laser intensity is used as the damage threshold. That is, the laser damage resistance is proportional to the calculated damage threshold, and can be expressed using the damage threshold.

FIG. 3 shows the measurement results of the laser damage resistance of each optical element when the optical element made of quartz glass and the optical element made of CaF ₂ were respectively processed using the surface treatment method according to the first embodiment. It is a graph. Here, an optical element made of quartz glass having a size of 40 mmφ whose surface is optically polished with high accuracy and an optical element made of CaF ₂ are prepared, and the optically polished surface of each optical element is subjected to the present embodiment. Surface treatment was performed using the surface treatment method according to mode 1, and etching was performed to a depth of 100 nm from the surface to remove the polishing impurity layer. Further, the surface after the surface treatment was irradiated with an ultraviolet laser beam having a wavelength of 266 nm using the measuring apparatus shown in FIG. 2, and the laser damage resistance was measured.

The vertical axis of the graph in FIG. 3 represents the relative value of the laser damage resistance when the laser damage resistance of the quartz glass is “1” when the surface treatment is not performed after optical polishing. . As shown in FIG. 3, when the surface treatment method according to the first embodiment is used, both the optical element made of quartz glass and the optical element made of CaF ₂ are in the deep ultraviolet region as compared with the case of untreated. It was possible to improve the laser damage resistance at about 2 times. Further, for CaF ₂ , in addition to the case where the surface is treated by the surface treatment method according to the first embodiment, that is, the method in which ion etching and chemical etching are combined, and the case where the surface is not treated, Laser damage resistance was also measured when only ion etching was performed. When only ion etching is performed, as shown in FIG. 3, it is clear that the laser damage resistance is larger than that of the unprocessed case, but the laser damage resistance is smaller than that of the case where chemical etching is combined. became. That is, it has been found that the laser damage resistance is dramatically improved by combining ion etching and chemical etching.

In the surface treatment method for an optical element according to the first embodiment, first, the abrasive grains are removed by performing ion etching on the polished surface of the optical element, and then the surface deteriorated by ion etching is chemically treated. It is dissolved and removed by etching. In the surface treatment method according to the first embodiment, the deterioration of the surface state caused by the ion beam irradiation is caused by several atoms on the surface of the optical element. To improve the deterioration of the surface condition. In this case, compared with the conventional method in which the entire polishing impurity layer is removed by chemical etching, the chemical etching process can be performed in a very short time, so that the surface state hardly deteriorates. By this chemical etching, the transmittance of an optical element made of, for example, CaF ₂ returns to the state before the ion beam etching is performed.

Note that the depth from the surface of the polishing impurity layer in the optical element varies depending on the optical polishing method and conditions, and for example, the surface-modified layer may hardly occur as in the case of performing ultraprecision polishing. In such a case, the depth etched using the surface treatment method according to the first embodiment may be less than 100 nm from the surface. For example, FIG. 4 shows the etching depth and laser damage tolerance when optical polishing is performed on a CaF ₂ optical element by a certain method and then surface treatment is performed by using the method according to the first embodiment. It is the graph which showed this relationship. The vertical axis of this graph represents the relative value of the laser damage resistance when the laser damage resistance when the optical element made of CaF ₂ is unprocessed is set to “1”. In the ion etching, the acceleration voltage of the ion beam was set to 400 eV. Chemical etching was performed for 10 minutes using phosphoric acid as an etching solution. As shown in FIG. 4, when the etching was performed from the optically polished surface to a depth of 40 nm, the laser damage resistance was improved more than twice as compared with the case of no treatment. That is, depending on the optical polishing method and conditions, even if the depth etched using the processing method according to the first embodiment is less than 100 nm, the laser damage resistance can be improved by about twice. . Note that black circles and white circles in FIG. 4 indicate the measurement results of the laser damage resistance of each surface when the surface treatment is performed on the (100) plane and the (111) plane of the crystal constituting the optical element, respectively. . Further, as indicated by a dotted line in FIG. 4, the average relative value corresponding to each etching depth is about 2.2.

Next, the relationship between the etching depth of chemical etching and the laser damage resistance will be described. FIG. 5 shows the etching depth and laser damage resistance of chemical etching when surface treatment is performed on the optical element made of CaF ₂ using the method according to the first embodiment as described with reference to FIG. It is the graph which showed the relationship. The horizontal axis of this graph shows the etching depth in chemical etching after ion etching, and the vertical axis shows the laser in the case of an unprocessed optical element made of CaF ₂ as in the graph and vertical axis of FIG. The relative value of the laser damage resistance when the damage resistance is "1" is shown. In the chemical etching, phosphoric acid was used as an etching solution as described above. As shown in FIG. 5, when the etching was performed from the ion-etched surface to a depth of 13 mm (1.3 nm), the laser damage resistance was improved more than twice as compared with the untreated case. That is, in the surface treatment method according to the first embodiment, the deterioration of the surface state due to ion etching is about several atomic layers, and if the several atomic layers are removed by chemical etching, a desired laser damage resistance can be obtained. all right.

In the case where the optical element is made of CaF ₂ , even if the surface treatment method according to the first embodiment is used, the transmittance is not lowered as compared with the case where it is not treated. This is because the transmittance, which has been lowered by the deterioration of the surface state by ion etching, is improved by the improvement of the surface state by the subsequent chemical etching. FIG. 6 shows the measurement results of the transmittance of the optical element after ion etching and after chemical etching when surface treatment is performed on the optical element made of CaF ₂ using the method according to the first embodiment. It is a graph. Here, the transmittance was measured using a spectrophotometer. The horizontal axis of the graph indicates the wavelength of light irradiated on the optical element by the spectrophotometer, and the vertical axis indicates the measurement result of the transmittance when the light of each wavelength is irradiated. In FIG. 6, solid line A indicates the transmittance after ion etching and chemical etching, solid line B indicates the transmittance when untreated, and solid line C indicates after ion etching only. That is, the transmittance before chemical etching is shown. As shown in FIG. 6, it was found that the transmittance of the CaF ₂ crystal was lowered by ion etching, but was restored to the transmittance in the case of untreated by the subsequent chemical etching.

Since quartz glass and CaF ₂ can be used as optical materials for deep ultraviolet, if the surface is treated using the surface treatment method according to the first embodiment, high-power laser light is emitted in the deep ultraviolet region. Application to the deep ultraviolet laser system is expected.

It is a flowchart for demonstrating the surface treatment method of the optical element by Embodiment 1 of this invention. It is the figure which showed the structural example of the measuring apparatus of a laser damage tolerance. 5 is a graph showing measurement results of laser damage resistance when the surface of an optical element is processed using the surface treatment method according to the first embodiment. 5 is a graph showing the relationship between etching depth and laser damage resistance when surface treatment is performed on the optical element made of CaF ₂ using the method according to the first embodiment. 5 is a graph showing the relationship between the etching depth in chemical etching and the laser damage resistance when surface treatment is performed on the optical element made of CaF ₂ using the method according to the first embodiment. The optical element made of CaF _2,, is a graph showing the measurement results of the transmittance of the optical element after the ion etching and chemical etching in the case of performing a surface treatment using the method of the first embodiment . It is the figure which showed the state of the surface alteration layer.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 Laser damage tolerance measuring apparatus 2 Nd: YAG pulse laser 3 CLBO crystal 4 Prism 5 Beam damper 6 Reflecting mirror 7 Attenuator 8 Power meter 9 Short focus lens 10 1/2 wavelength plate 11 Polarizer 12 Beam splitter 13 Optical element

Claims (4)

An optical element surface treatment method for treating a surface of an optical element that has been optically polished,
An ion etching step in which etching is performed by irradiating the surface of the optical element with an ion beam in order to remove the polishing impurity layer embedded in the abrasive grains, which is present on the surface layer of the optically polished optical element ;
Chemical etching is performed by immersing and dissolving the ion-etched surface in a predetermined solution that reacts with the material of the optical element in order to remove the deteriorated portion of the surface that has deteriorated due to the energy of the ion beam. An optical element surface treatment method.   The optical element surface treatment method according to claim 1, wherein a depth at which the optical element is etched in the ion etching step is deeper than a depth at which the optical element is etched in the chemical etching step. The optical element surface treatment method according to claim 1 , wherein the polishing impurity layer is present to a depth of 1000 nm or less from the optically polished surface of the optical element. The optical element, the optical element surface processing method according to any one of claims 1 to 3, wherein quartz glass, calcium fluoride, and in that it consists either of aluminum oxide.

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