Thin film search method for YAG single crystal joint surface

Abstract

To provide a thin film search method for a YAG single crystal joint surface that makes efficient identification on a proper thin film on a joint surface when manufacturing a joint body of a YAG single crystal which has an extremely few voids in a joint boundary surface.SOLUTION: A thin film search method for a YAG single crystal joint surface follows: a thin film formation procedure for forming a thin film which has continuously changed in composition over from one end to the other end on one surface of one sample substrate between a sample substrate of a doped YAG single crystal and a sample substrate of an undoped YAG single crystal; an arraying procedure for arraying the other sample substrate smaller in area than the one sample substrate over the thin film from one end to the other end; a heat processing procedure for performing heat processing after the arraying procedure; and a determination procedure for determining the quality of the thin film based upon the joint state between the one sample substrate and the other sample substrate.SELECTED DRAWING: Figure 1

Description

[Technical field]

[0001] The present invention relates to a method for detecting thin films on a bonding interface between YAG (yttrium aluminum garnet) single crystals. [Background technology]

[0002] In fields such as laser processing, there is a demand for small, high-power, high-beam quality lasers. Recently, solid-state lasers using laser diodes (LDs) as excitation light sources have become widespread. In particular, yttrium aluminum garnet (hereinafter referred to as "YAG") crystals doped with neodymium (Nd) or ytterbium (Yb) are increasingly being used as solid-state laser materials.

[0003] In order to realize a high-power output laser beam from a solid-state laser, the dissipation of heat generated in the solid, which is the laser gain material, is a major problem. As a method for effectively dispersing localized heat generation, it has been proposed to join a laser crystal doped with an optically active substance to an undoped crystal, and the diffusion bonding method and direct bonding method have been devised (Patent Document 1).

[0004] Research into the structure of laser media has also progressed, and a method using a disk-shaped laser gain material is known. By forming the laser gain material into a thin disk shape, the light receiving surface of the external excitation light can be made large, and uniform cooling can be achieved over the entire disk surface. Among these, the reflective (active mirror) structure has attracted attention because it allows cooling by a heat sink or fluid by applying a reflective film to one surface of the thin film disk. Normally, active mirror structures improve mechanical strength and heat dissipation by joining a thin film disk doped with an optically active element to a material not doped with an optically active element (Patent Document 2).

[0005] As a means for cooling the laser medium, many techniques have been proposed for bonding with a heat transfer member. For example, Patent Document 3 discloses a technique in which the bonding surface is treated with oxygen plasma, and then irradiated with an atomic beam of an inert gas to perform surface activation bonding. According to the technique disclosed therein, it is also possible to bond the YAG laser medium, which is an oxide, to sapphire, which is an oxide and serves as a heat transfer member. In addition, among these, it is shown that when the laser medium is polycrystalline, the laser medium is easily destroyed by electric field concentration occurring near the interface between the laser medium and space, and the optical damage threshold is lower than that of a single crystal, and as a countermeasure, it is possible to increase the optical damage threshold by surface activation bonding a transparent single crystal to the end face (Patent Document 3,

[0027] ).

[0006] Various bonding techniques have also been disclosed in the field of electronic components. For example, Patent Document 4 proposes an atomic diffusion bonding method in which an amorphous oxide thin film is formed in a vacuum and chemical bonds are generated at the bonding interface through atomic diffusion. [Prior art documents] [Patent documents]

[0007] [Patent Document 1] Patent No. 4374415 [Patent Document 2] Patent No. 5330801 [Patent Document 3] Patent No. 6245587 [Patent Document 4] Patent No. 7131778 Summary of the Invention [Problem to be solved by the invention]

[0008] YAG single crystals and polycrystalline YAG ceramics are used as media for solid-state lasers. Although YAG ceramics have the advantage of being easy to manufacture in large sizes and relatively inexpensively, it is preferable to use YAG single crystals for high-power lasers because of the presence of grain boundaries, which lowers the laser damage threshold on the surface, and because YAG ceramics have lower thermal conductivity than single crystals, especially at low temperatures.

[0009] In addition, as a technique for joining a laser medium and a heat dissipating member, in the case of a polycrystalline body, a method has been proposed in which the raw powder of the laser medium and the heat dissipating material are brought into contact and sintered at the same time, but in the case of a laser medium using a YAG single crystal, it is necessary to process and join a YAG single crystal doped with an optically active substance and an undoped YAG single crystal after crystal growth. For this reason, it is necessary to join the joining interface by polishing it to a flat mirror finish, but voids may remain at the interface after joining. Since it is thought that these tiny voids may have an effect on high-power lasers, a joined single crystal without voids at the joining interface is desired.

[0010] In a method for producing such a bond between YAG single crystals, for example, when a thin film is interposed between the bonding surfaces, identification of the optimal thin film is extremely complicated because it is necessary to prepare and test samples for each material used in the thin film.

[0011] The present invention has been made in consideration of these problems, and aims to provide a method for searching for thin films on the bonding surface of a YAG single crystal, which efficiently identifies suitable thin films on the bonding surface in order to produce a bonded YAG single crystal with extremely few voids at the bonding interface. [Means for solving the problem]

[0012] In order to achieve the above object, the present invention provides a method for searching for a thin film on a YAG single crystal joining surface, for joining a surface of a doped YAG single crystal, which is an yttrium-aluminum-garnet single crystal doped with an optically active substance, to a surface of an undoped YAG single crystal, which is an yttrium-aluminum-garnet single crystal not doped with an optically active substance, the method being for searching for a thin film to be formed on at least one of the surfaces of the doped YAG single crystal and the undoped YAG single crystal, and comprising: forming a thin film of a composition of a first substance on one side of one of the sample substrates of the doped YAG single crystal and the undoped YAG single crystal from one end to the other end; a thin film formation step of forming a thin film having a different composition from the one end to the other end, in which the composition of the first substance changes continuously and the composition of the second substance changes continuously inversely to the change in the composition of the first substance from the other end to the one end; an alignment step of arranging a plurality of other sample substrates, of the doped YAG single crystal sample substrate and the undoped YAG single crystal sample substrate, each of which has an area smaller than that of the one sample substrate, on the thin film from the one end to the other end; a heat treatment step of performing a heat treatment after the alignment step; and a judgment step of judging the quality of the thin film based on the bonding state between the one sample substrate and the other sample substrate.

[0013] The above-mentioned thin film searching method for the YAG single crystal bonding surface is characterized in that the thin film is an oxide thin film containing Si.

[0014] The above-mentioned method for searching for a thin film on a YAG single crystal bonding surface is characterized in that the thin film is an oxide thin film containing Ca and / or Mg.

[0015] In the above-mentioned method for searching for a thin film on a YAG single crystal bonding surface, the heat treatment is performed in air at a temperature of 1200° C. to 1600° C., and then O 2 The method is characterized in that the hot isostatic pressing is performed in an atmosphere containing

[0016] Regarding a method for manufacturing a laser medium by bonding a YAG single crystal doped with an optically active substance such as Nd, Yb, or cerium (Ce) (doped YAG single crystal) with an undoped YAG single crystal (undoped YAG single crystal), it has been discovered that a bonded body with few voids at the bonded interface can be produced by coating at least one of the surfaces with an oxide containing tetravalent ions such as Si and divalent ions such as Mg, Ca, Fe, or Mn by a method such as sputtering, and then bonding the bonded interface and subjecting it to heat treatment. By coating the YAG single crystal with a thin film containing Si, Si diffuses into the YAG single crystal as ions during heat treatment, activating the movement of atoms near the bonding interface and reducing voids at the interface. Regarding the composition of the coating, when considering a material containing tetravalent Si ions, it is necessary to consider a composition containing divalent ions in order to maintain charge neutrality. There are several candidates for divalent ions, including Mg, Ca, Fe, and Mn, but for the thin film (hereinafter also referred to as the "buffer material") that bonds to the YAG single crystal, it is expected that a good quality bond can be obtained by using not only a single divalent ion but a combination of multiple types of ions.

[0017] As a method for searching for a thin film on the YAG single crystal bonding surface according to the present invention, a highly efficient method for searching for a buffer material for YAG single crystal bonding uses combinatorial sputtering deposition, and one side of one surface of a 30 mm x 30 mm YAG single crystal substrate is coated with SiCaO 3 , and on the other side SiMgO 3 A thin film of Si(Ca) is coated on the substrate, and in the middle, Ca at the Ca site is replaced linearly with Mg from one side to the other. That is, the composition of the thin film on the substrate becomes Si(Ca 1-x Mg x )O 3 A combinatorial sample was used in which a thin film was formed in which x changed continuously from 0 to 1. At each composition point, a YAG single crystal of 3 mm × 3 mm, which is smaller than the substrate, was bonded, and Si(Ca 1-x Mg x )O 3This is a material exploration method characterized by identifying the value of x that achieves high-quality junctions from a single film formation experiment.

[0018] A thin film consisting of tetravalent Si ions and divalent ions diffuses by replacing the trivalent yttrium (Y) and aluminum (Al) ions in the YAG crystal during heat treatment, and it is believed that the coexistence of divalent Mg, Ca, etc. compensates for the charge, making the replacement easier, accelerating diffusion, and reducing voids. These divalent ions and Si diffuse into the YAG crystal to form a garnet structure, which is thought to reduce the optically inhomogeneous phase.

[0019] In the present invention, the heat treatment is preferably carried out in air at a temperature of 1200°C or higher and 1600°C or lower. Since oxygen deficiency may occur when the YAG single crystal is heat treated in a reducing atmosphere, it is preferable to heat treat the YAG single crystal in an oxidizing atmosphere, that is, in air. This is because ions of Si, Mg, Ca, etc. do not diffuse sufficiently at heat treatment temperatures below 1200°C, and voids may grow conversely at temperatures above 1600°C. Furthermore, it is more preferable to carry out hot isostatic pressing (HIP) treatment after the heat treatment in air. The HIP treatment is preferably carried out in an atmosphere that prevents the reduction of YAG, and O 2 atmosphere containing, for example, Ar+O 2 The pressure and temperature can be the same as those normally used, for example, 1500°C and 200 MPa. By performing HIP treatment, the voids at the bonding interface are further reduced. Effect of the Invention

[0020] The present invention has made it possible to efficiently identify thin films (buffer materials) for producing single crystal materials without voids at the bonding interface. As a by-product, it has become possible to provide optical components such as strong and dense laser medium bonding bodies. [Brief description of the drawings]

[0021] [Figure 1]Photo of a bonding sample of SiCaO3 ⇔ MgSiO3 composition gradient film [Diagram 2] STEM-ABF image of the bonded interface between (111) YAG single crystals bonded together without a buffer material (Comparative example) [Diagram 3] STEM-ABF image of the bonded interface between (111) YAG single crystals bonded together using SiCa0.5Mg0.5O3 as a buffer material (Example 1) [Figure 4] STEM-ABF image of the area where the crystal lattice is continuously connected at the interface between (111) YAG single crystals bonded with SiCa0.5Mg0.5O3 as a buffer material [Diagram 5] Bonding of SiCaO3 ⇔ SiO2 composition gradient films (Example 2) [Figure 6] MgSiO3 ⇔ SiO2 Composition gradient film bonding (Example 3) DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0022] The method for searching for a thin film on a YAG single crystal bonding surface according to an embodiment of the present invention includes at least a thin film forming step, an alignment step, a heat treatment step, and a determination step.

[0023] In the thin film formation procedure, a thin film is formed on one side of at least one of the doped YAG single crystal sample substrate and the undoped YAG single crystal sample substrate. That is, there are a mode in which a thin film is formed on one side of the doped YAG single crystal sample substrate (a thin film is not formed on the undoped YAG single crystal sample substrate), a mode in which a thin film is formed on one side of the undoped YAG single crystal sample substrate (a thin film is not formed on the doped YAG single crystal sample substrate), and a mode in which a thin film is formed on both one side of the doped YAG single crystal sample substrate and one side of the undoped YAG single crystal sample substrate. In the thin film, the composition of the first substance changes continuously from one end to the other end of the sample substrate, and the composition of the second substance changes continuously inversely to the change in the composition of the first substance from the other end to the one end, so that the composition is different from one end to the other end. The surface of the sample substrate is appropriately mirror-polished.

[0024] The thin film may be, for example, a thin oxide film containing Si and another material, a thin oxide film containing Si and a thin oxide film containing Ca and Mg, a thin oxide film containing Si and a thin oxide film containing Ca, a thin oxide film containing Si and a thin oxide film containing Mg, a thin oxide film containing Ca and Mg and another material, etc. The first material may be, for example, SiCaO 3 , CaSiO 3 , MgSiO 3 etc., and the second material is, for example, MgSiO 3 , SiO 2 In addition, since the thin film is only required to be an oxide thin film containing Si, Ca, Mg, etc., the first substance and the second substance are arbitrary as long as they satisfy the composition of the thin film.

[0025] In the alignment procedure, one of the doped YAG single crystal sample substrate and the undoped YAG single crystal sample substrate is arranged on the thin film of the other sample substrate (hereinafter referred to as the "non-film-formed sample substrate") on which a thin film is formed. The non-film-formed sample substrate has a smaller area than the film-formed sample substrate. The non-film-formed sample substrate is aligned on the thin film of the film-formed sample substrate in the direction in which the composition of the thin film changes. The alignment procedure may also be performed between film-formed sample substrates.

[0026] The heat treatment procedure is as follows: heat treatment in air at 1200°C to 1600°C, then O 2 The hot isostatic pressing treatment is carried out in an atmosphere containing

[0027] In the judgment procedure, the quality of the thin film is judged based on the bonding state between the film-formed sample substrate and the non-film-formed sample substrate. The judgment criteria are, for example, whether the two sample substrates do not separate when moderate vibration is applied, whether there is an interference pattern on the sample substrate when visually inspected, and whether the two sample substrates are connected to the extent that the thin film cannot be confirmed at the bonding interface by STEM-ABF images, etc. Hereinafter, the embodiment of the present invention will be described in further detail.

[0028] A buffer material with a composition that changes continuously from one side to the other side of the substrate is deposited on a (111) undoped YAG single crystal substrate with a size of 30 mm x 30 mm and a thickness of 1 mm by using a combinatorial deposition technique. The composition of the material that changes continuously is Si(Ca 1-x Mg x )O 3 A thin film expressed according to the composition formula, where x changes continuously from 0 to 1, is obtained on one substrate. This results in a film-formed sample substrate. On this substrate, a (111) undoped or doped YAG single crystal substrate with an area of ​​3 mm × 3 mm and a thickness of 1 mm is placed in a lattice pattern as a non-film-formed sample substrate (Fig. 1), and then heat treatment is performed to bond the interface between the 30 mm square YAG substrate and the 3 mm square substrate.

[0029] The bonding is achieved by heat treatment in air at 1200°C to 1600°C. Below 1200°C, the elements of the coated thin film cannot diffuse sufficiently, but at temperatures above approximately 1300°C, a liquid phase is generated and diffusion is activated. Conversely, at temperatures above 1600°C, voids may grow, so high-temperature heat treatment is not necessary. During heat treatment, it is a good idea to apply a load of about the same as a weight to ensure that the bonding surfaces are tightly attached. High pressures such as hot pressing are not necessary; a light load of 10kPa or less is sufficient.

[0030] It is more preferable to carry out HIP treatment after bonding by heat treatment. By coating with a thin film before bonding, it is possible to reduce voids at the interface. The conditions for HIP treatment are preferably an oxidizing atmosphere. This is because YAG will develop oxygen deficiencies if treated in a reducing atmosphere, so additional oxidation treatment is necessary. Specifically, Ar+O 2 A gas atmosphere is preferable. The temperature should be 1500°C or less. A general-purpose HIP device can be used, and 1500°C is a sufficient temperature for the diffusion of thin film components. The pressure can be the standard condition of the device, for example, 200 MPa.

[0031] By carrying out the above bonding experiment using a substrate with a coating material of varying composition on a 30 mm square YAG substrate, the Si(Ca 1-x Mg x )O 3 We will evaluate which composition provides good bonding with the 3 mm square substrate, and identify which composition is suitable for bonding.

[0032] <Example> Examples will be described below. The surfaces to be bonded on the undoped YAG single crystal (111) substrate, measuring 30 mm × 30 mm and 1 mm thick, were mirror-polished and precision-cleaned. Then, Si(Ca 1-x Mg x )O 3A thin film is deposited. The thickness of the thin film is 50 nm, and it is deposited to have a uniform thickness. From one end of the 30 mm square YAG substrate to the other end, the composition of the thin film is SiCaO 3 From SiMgO 3 The value changes continuously to

[0033] On the other hand, prepare 40 undoped YAG single crystal (111) substrates measuring 3 mm × 3 mm and 1 mm thick. These 3 mm square substrates are mirror-polished and arranged at equal intervals in a 5 × 8 matrix with the polished surface facing the coated 30 mm square YAG (111) substrate. The bonding is achieved by heat treatment in air at 1500°C for 10 hours.

[0034] The appearance of the sample after bonding is shown in Figure 1. The 3 mm square YAG substrates, originally arranged in a 5 x 8 array, are slightly misaligned due to vibrations during the experimental process, but it is possible to determine whether the 3 mm square substrates are bonded to the 30 mm square substrate. In some places, they are not bonded, and come off when the sample is removed from the heat treatment furnace.

[0035] In Figure 1, the 3 mm square substrate that appears white and the substrate on which interference patterns are visible indicate that the bonding is not dense and that there are gaps. On the other hand, the 3 mm square substrate that appears the same color tone as the 30 mm square substrate shows that it is well bonded. The composition on the substrate is SiCaO 3 From SiMgO 3 Considering that the temperature changes continuously, good bonding (i.e., no misalignment with the 30 mm square substrate, no interference pattern, and the same color tone as the 30 mm square substrate) is obtained with SiCa 0.5 Mg 0.5 O 3 (around the center in Fig. 1), and for bonding of YAG single crystals, SiCa 0.5 Mg 0.5 O 3 is suitable as a buffer material.

[0036] Figure 2 shows a STEM-ABF image of the interface when a YAG single crystal substrate was bonded without using a buffer material (comparative example), and Figure 3 shows a SiCa 0.5 Mg 0.5 O 3 The figure shows a STEM-ABF image of the interface when a YAG single crystal substrate coated with a buffer material having the composition of the following is bonded (Example 1). In this example, 0.5 Mg 0.5 O 3 The white dashed lines in Fig. 3 indicate the areas where the buffer material cannot be observed at the interface and both YAG single crystals appear to be connected continuously. 0.5 Mg 0.5 O 3 An enlarged image of this area is shown in Figure 4. Figure 4 is a STEM ABF image of a portion of the bonding interface where the crystal lattice is continuously connected.

[0037] SiCa 0.5 Mg 0.5 O 3 At the interface where no SiCa buffer material is present, the boundary between the upper and lower substrates is not visible, and the garnet structure of the YAG is continuously connected. 0.5 Mg 0.5 O 3 However, by receiving Al ions from the YAG substrate, 1.5 Mg 1.5 Al 2 (SiO 4 ) 3 This is thought to be because it has a garnet structure such as the above.

[0038] As another example, SiCaO 3 ⇔SiO 2 Combinatorial deposition and MgSiO 3 ⇔SiO 2 It is also possible to perform combinatorial film formation such as the following. The respective results are shown in Figures 5 (Example 2) and 6 (Example 3).

[0039] SiCaO 3 ⇔ SiO2 In the case of bonding of gradient composition films, SiCaO 3 Only SiO 2 It can be seen that the bonding is better when the ratio of MgSiO 3 ⇔ SiO 2 In the case of bonding of the gradient composition films, good bonding was obtained overall. 3 and MgSiO 3 The difference in the melting points of MgO and CaO and the difference in the ionic radii of Y, Ca, and Mg when forming the garnet structure are considered to be the causes of the difference.

[0040] In this way, by using combinatorial deposition, it is possible to screen for the possibility of bonding with high efficiency, and by combining analytical techniques such as STEM and SIMS for the bonding of each composition, it becomes possible to make more detailed comparisons and analyze the bonding state and ion distribution, etc.

[0041] Although the embodiment of the present invention has been described in detail above, the present invention is not limited to the above embodiment, and various design modifications can be made to the present invention without departing from the scope of the claims.

Claims

1. A method for searching for a thin film on a YAG single crystal joining surface, the method being for searching for a thin film to be formed on at least one of a surface of a doped YAG single crystal, which is an yttrium aluminum garnet single crystal doped with an optically active substance, and a surface of an undoped YAG single crystal, which is an yttrium aluminum garnet single crystal not doped with an optically active substance, the method comprising: a thin film formation step in which, on one surface of one of the doped YAG single crystal sample substrate and the undoped YAG single crystal sample substrate, a composition of a first substance changes continuously from one end to the other end, and a composition of a second substance changes continuously from the other end to the one end in a manner opposite to the change in composition of the first substance, thereby forming a thin film having a different composition from the one end to the other end; an alignment step of arranging a plurality of other sample substrates, each of which has an area smaller than that of one of the doped YAG single crystal sample substrates and the undoped YAG single crystal sample substrates, on the thin film from the one end to the other end; a heat treatment step of performing a heat treatment after the alignment step; a judgment step of judging the quality of the thin film based on the bonding state between the one sample substrate and the other sample substrate.

2. The method for searching for a thin film on a YAG single crystal bonding surface according to claim 1, A method for searching for a thin film on a YAG single crystal bonding surface, characterized in that the thin film is an oxide thin film containing Si.

3. 3. A method for searching for a thin film on a YAG single crystal bonding surface according to claim 2, comprising the steps of: A method for searching for a thin film on a YAG single crystal bonding surface, wherein the thin film is an oxide thin film containing Ca and / or Mg.

4. A method for searching for a thin film on a YAG single crystal bonding surface according to any one of claims 1 to 3, comprising the steps of: In the heat treatment, the heat treatment is performed in air at 1200° C. or more and 1600° C. or less, and then O 2 A method for searching for a thin film on a YAG single crystal bonding surface, comprising the steps of: subjecting the YAG single crystal to hot isostatic pressing in an atmosphere containing

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