WO2022149191A1 - Optical circuit board and method for forming same - Google Patents

Abstract

For the purpose of application in a light amplifier having a thin-film-layered structure, the present invention provides: a solid solution thin film of a bismuth oxide (Bi₂O₃) doped with erbium (Er) and silica (SiO₂); and a method for forming a material of the thin film. According to the present invention, an oxide thin film resulting from 4 at% or more of Er³⁺ being doped in δ-Bi₂O₃, serving as a matrix, is deposited on nondoped SiO₂ by a sputtering method, and interdiffusion is then caused between Er-doped δ-Bi₂O₃ and SiO₂ by performing post annealing in an oxidizing atmosphere, whereby a solid solution thin film that is deposited on SiO₂ and in which Er is doped in a solid solution of δ-Bi₂O₃ and SiO₂ is provided.

Description

Optical circuit board and its formation method

The present invention relates to an optical circuit substrate and a method for forming the same, which comprises a solid solution thin film of bismuth oxide and silica doped with Er ³⁺ , for the purpose of application to the field of optoelectronics such as an optical amplifier.

An optical amplifier is an element that can amplify signal light in the state of direct light without converting it into an electric signal or the like. Due to these properties, the optical amplifier can suppress the loss of signal light due to scattering, etc., and enables long-distance transmission of signal light. Therefore, it is an indispensable optical component in modern optical communication. be.

The principle of the optical amplifier is that when there are many electrons excited in an orbit with a high energy level, the electrons in the excited state transition to a lower order due to an electromagnetic wave (stimulated light) emitted from the outside. , The phenomenon that light of the same phase is amplified and emitted (stimulated emission) is used. An optical fiber is a typical example of an optical amplifier. Taking this optical fiber as an example, an oxide doped with a rare earth element at a low concentration is applied to a portion called a core through which signal light passes, and the electrons of this rare earth element are excited by the excitation light in a high energy order. When the excited electrons transition to the ground state by irradiation with signal light, light having the same phase as the signal light is amplified and emitted. In particular, an optical amplifier using erbium (Er) as a dopant is called an erbium-Doped Fiber Amplifier (hereinafter referred to as EDFA), which has high transmission efficiency and is generally used in long-distance optical communication. Since it is suitable for amplifying signals of the wavelength used (1.55 μm band), it is an optical amplifier that is being widely put into practical use.

On the other hand, in recent years, in order to reduce the size of optical communication systems, the structure in which optical components are laminated as a thin film on a substrate is becoming mainstream in optical network circuits. In an optical network circuit with such a structure, when an Er-doped oxide such as EDFA is used as the core of an optical amplifier, it cannot be applied in the fiber state. Therefore, the Er-doped signal amplification section is used as a thin layer. There is no choice but to set it as.

Generally, an optical waveguide has a structure in which a clad layer is arranged around a core layer through which signal light passes, and the intensity of the signal light is increased by confining the signal light in the core layer by utilizing the difference in refractive index between the two. Also, the transmission efficiency is improved. The larger the difference in refractive index between the core layer and the clad layer, the higher the effect of confining light due to reflection, and the higher the transmission efficiency of signal light.

In such an optical waveguide, the core layer has silica (SiO ₂ ), bismuth oxide (Bi ₂ O ₃ ), and germanium oxide (GeO ₂ ) from the viewpoint of signal light transmission and the difference in refractive index from the core layer. , Boron oxide (B ₂ O ₃ ) and gallium oxide (Ga ₂ O ₃ ) are mainly used, and these can be formed by a sputtering method or the like. Further, SiO ₂ , GeO ₂ , B ₂ O ₃ , etc. can be a core layer with high transmission efficiency because the refractive index can be increased by mixing Bi ₂ O ₃ in the state of an amorphous network structure.

As a conventional technique for forming such an oxide thin film having a high refractive index, a method of adding zinc (hereinafter referred to as Zn) to a lithium-niobate composite oxide (hereinafter referred to as LiNbO ₃ ) is an example. Can be mentioned. This is a method of increasing the effective refractive index of the LiNbO ₃ surface by depositing Zn on LiNbO ₃ and thermally diffusing the Zn element toward the LiNbO ₃ side. However, few are known of other materials applying similar processes.

At present, there is a problem in the oxide thin film material used for the core layer in realizing a laminated structure having high transmission efficiency in an optical network circuit. For example, Bi ₂ O ₃ absorbs light in the visible region, but is transparent to near-infrared light (wavelength: 1.3 to 1.5 μm) used in optical communication, so it is useful as a core layer in optical waveguides. Material. In this case, if SiO ₂ is used for the clad layer, the difference in refractive index between the two is extremely large, so that it is possible to strongly confine the signal light in Bi ₂ O ₃ which is the core layer, and an optical waveguide with high transmission efficiency. Can be. However, when Bi ₂ O ₃ and SiO ₂ are laminated, the refractive index changes rapidly at the interface, so there is a problem that the light loss due to scattering becomes very large.

In addition, in order to achieve high transmission efficiency, it is necessary to dope Er ³⁺ to the core layer to impart an optical amplification effect by stimulated emission, as in EDFA. However, for the thin film material selected as the core layer, The proper concentration of Er ³⁺ that effectively produces photoamplification has not been clarified. If the amount of Er ³⁺ doped is greater than appropriate, there is concern that the solid solution limit to the luminescent site will be exceeded, ErO ₆ units will be generated at the interstitial positions, and Er ³⁺ that does not contribute to luminescence will increase. .. On the contrary, when the amount of Er is smaller than the appropriate amount, the amount of Er that contributes to light emission is reduced, so that there is a problem that the transmission efficiency is lowered.

In addition, there is also a problem that the method of forming a core layer having an appropriate amount of Er on the clad layer has not been determined.

To solve such problems, the present invention provides a thin film material having high transmission efficiency, which is useful as a core layer of an optical waveguide, and a method for forming the thin film material.

An object of the present invention is to provide a thin film material that realizes an optical amplifier having high transmission efficiency in an optical network circuit having a structure in which optical components are laminated on a substrate, and a method for forming the thin film material.

In one embodiment of the present invention, a thin film doped with erbium (Er) is formed on a substrate (SiO ₂ ) with an oxide obtained by dissolving silica (SiO ₂ ) in bismuth oxide (Bi ₂ O ₃ ). It is a method of forming an optical circuit board.
The first step of forming a thin film containing Bi ₂ O ₃ on the substrate by a sputtering method, and
A second step of post-annealing, which causes mutual diffusion between the thin film formed in the first step and the substrate to change the thin film material into a solid solution thin film.
It is a method of forming an optical circuit board.

It is a schematic diagram which shows the appearance that Bi ₂ O ₃ -SiO ₂ : Er which becomes a core layer is formed on SiO ₂ which becomes a clad layer. It is a figure which shows the crystal structure analysis result obtained by the X-ray diffraction after post-annealing with respect to the sample A which Er concentration is 1.5 at%. It is a figure which shows the crystal structure analysis result obtained by the X-ray diffraction after post-annealing with respect to the sample B which Er concentration is 4 at%. It is a figure which shows the element distribution in the depth direction of O, Si, Bi, Er acquired by the secondary ion mass spectrometry after post-annealing with respect to the sample A which Er concentration is 1.5 at%. It is a figure which shows the element distribution in the depth direction of O, Si, Bi, Er acquired by the secondary ion mass spectrometry after post-annealing with respect to the sample B which Er concentration is 4 at%. It is a figure which shows the emission spectrum obtained by the photoluminescence method after post-annealing with respect to the sample A which Er concentration is 4 at%. It is a figure which shows the emission spectrum obtained by the photoluminescence method after post-annealing with respect to the sample B which Er concentration is 4 at%.

In the embodiment of the present invention, a thin film material (hereinafter referred to as Bi ₂ O ₃ -SiO ₂ ) doped with Er ³⁺ of 4 at% or more is added to a solid solution in which SiO ₂ is dissolved in δ-Bi ₂ O ₃ (hereinafter referred to as Bi 2 O 3 -SiO 2). Hereinafter, Bi ₂ O ₃ -SiO ₂ : Er) is provided as a thin film material for the core layer. Further, a method of forming this Bi ₂ O ₃ -SiO ₂ : Er by a sputtering method and then post-annealing is provided.

The above-mentioned SiO ₂ is also a useful material as a clad layer, but since the refractive index is increased by adding Bi or germanium (hereinafter referred to as Ge), it can be a useful material as a core layer. In particular, when Bi is added to make Bi ₂ O ₃ -SiO ₂ , the refractive index increases remarkably, so that it can be a core layer that realizes high transmission efficiency.

Furthermore, by using Bi ₂ O ₃ -SiO ₂ : Er _, which is doped with Er ³⁺ at a concentration of ₄ at.% Or _more , the optical amplification effect by stimulated emission is effective. Can be generated in.
On the other hand, for the clad layer, select non-doped SiO ₂ which has a large difference in refractive index from Bi ₂ O ₃ -SiO ₂ : Er.

In this way, the sputtering method is used to form a laminate in which Bi ₂ O ₃ -SiO ₂ : Er is applied to the core layer and SiO ₂ is applied to the clad layer. Bi ₂ O ₃ (hereinafter referred to as Bi ₂ O ₃ : Er) doped with Er ³⁺ is formed on SiO ₂ that functions as a clad layer by a sputtering method, and Bi ₂ O ₃ : Er is the upper layer, SiO ₂ To form a laminated body with the lower layer. By post-annealing this laminate, Bi ₂ O ₃ : Er atoms in Bi 2 O 3: Er and Si atoms in SiO ₂ are mutually diffused, resulting in Bi ₂ O ₃ : Er. Changes to Bi ₂ O ₃ -SiO ₂ : Er. If the film thickness of Bi ₂ O ₃ : Er is considerably thinner than that of SiO ₂ , the reaction with SiO ₂ proceeds over the entire area of Bi ₂ O ₃ : Er film, and Bi ₂ O ₃ : All Er changes to Bi ₂ O ₃ -SiO ₂ : Er. Since SiO ₂ remains underneath, as a result, it is possible to obtain a structure in which Bi ₂ O ₃ -SiO ₂ : Er as a core layer and SiO ₂ as a clad layer are laminated.

Furthermore, at the interface between the two layers formed by the mutual diffusion of atoms, the concentration of each element in the film thickness direction has a distribution that gradually changes. That is, since a sudden change in the refractive index at the above-mentioned interface is suppressed, high signal light confinement efficiency can be obtained.

In the thin film material thus formed, Bi ₂ O ₃ -SiO ₂ : Er is an optical amplifier that functions as a core layer and SiO ₂ as a clad layer. Further, since the thin film laminated structure in which the optical waveguide is provided on the substrate is realized, the size can be reduced.

Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings.

An embodiment of the present invention will be described in detail below. In this embodiment, Bi ₂ O ₃ containing Er ³⁺ is formed on SiO ₂ as a clad layer by a two-source simultaneous discharge sputtering method, and then post-annealed at 400 ° C. to obtain atomic atoms. This is a method of causing mutual diffusion and forming Bi ₂ O ₃ -SiO ₂ : Er as a core layer on SiO ₂ .

FIG. 1 shows an aspect in which Bi ₂ O ₃ -SiO ₂ : Er 13 as a core layer is formed on SiO ₂ 11 which is a clad layer. Then, FIG. 1 (a) shows a state immediately after film formation by the sputtering method, and FIG. 1 (b) shows a state in which Bi ₂ O ₃ : Er 12 is changed to Bi ₂ O ₃ -SiO ₂ : Er 13 by post-annealing. Shows.

In the present embodiment, Bi ₂ O ₃ : Er 12 is formed by two-source simultaneous discharge sputtering using two targets. A Bi ₂ O ₃ target, an Er ₂ O ₃ target, and a Si (100) single crystal substrate on which SiO ₂ 11 having a thickness of 1 μm is formed are installed in the chamber of the sputtering apparatus, respectively. At this time, the board is installed at the opposite positions of the targets. Then, by generating electric discharges on the surfaces of the two sputtering targets at the same time, Bi ₂ O ₃ : Er 12 is formed on SiO ₂ 11 by the deposition and reaction of the sputter particles emitted from the targets.

Two types of sputter gas, argon (Ar) and oxygen (O ₂ ), are used and flow into the chamber at flow rates of 8 sccm and 6 sc cm, respectively. Then, an electron cyclotron resonance (hereinafter referred to as ECR) discharge is simultaneously generated for the Bi ₂ O ₃ target, and a high frequency (Radio Frequency: hereinafter referred to as RF) magnetron discharge is simultaneously generated for the Er ₂ O ₃ target. As a result, a laminated body of Bi ₂ O ₃ : Er 12 and SiO ₂ 11 as shown in FIG. 1 (a) can be obtained.

The macro wave power for generating ECR plasma in the Bi ₂ O ₃ target was 500 W, and the RF power for applying the bias voltage to the target was 500 W. On the other hand, the RF output in the Er ₂ O ₃ target was set to two conditions of 30 W and 60 W in order to compare and evaluate the effect of Er concentration on the stimulated emission effect.

In this embodiment configured in this way, when the RF output is 30 W, the Er concentration is 1.5 at% Bi ₂ O ₃ : Er12 (hereinafter referred to as sample A), and when the RF output is 60 W, Er. Bi ₂ O ₃ : Er 12 (hereinafter referred to as sample B) having a concentration of 4 at% was formed on SiO ₂ 11 of the substrate, respectively. In all the samples, the film thickness of Bi ₂ O ₃ : Er12 was about 500 nm.

Then, Sample A and Sample B were post-annealed in an O ₂ atmosphere at 1 atm. The purpose of this step is to cause mutual diffusion between SiO ₂ 11 and Bi ₂ O ₃ : Er 12 to change Bi ₂ O ₃ : Er 12 to Bi ₂ O ₃ -SiO ₂ : Er 13. The heat treatment temperature is 400 ° C. and the heat treatment time is 10 minutes.

FIG. 2 shows the crystal structure analysis results obtained by X-Ray diffraction (hereinafter referred to as XRD) with respect to the sample A after post-annealing. The peaks detected in the diffraction pattern were all identified as belonging to α-Bi ₂ O ₃ which is a stable phase at room temperature, and the surface layer of sample A was composed of Bi ₂ O ₃ of α phase having monoclinic crystals. It turned out to be a phase.

FIG. 3 shows the crystal structure analysis results obtained by XRD for the sample B after post-annealing. Similar to FIG. 2, when the detected peaks were identified, they all belonged to δ-Bi ₂ O ₃ , so that the sample B was based on the δ-phase Bi ₂ O ₃ having a cubic crystal. It turned out to be a phase.

In general, in the δ phase of Bi ₂ O ₃ like sample B, a part of the oxygen site, which is an anion site, is an empty site, and the cation Bi ³⁺ is a face-centered cubic lattice (Face-Centered Cubic:). It is known to have a structure exhibiting fcc). The reason why sample B exhibited such a crystal structure is that the high concentration of Er ³⁺ , which easily binds to oxygen, causes oxygen to selectively bind to Er ³⁺ , and the oxidation of Bi ₂ O ₃ in the parent phase. It is probable that this was due to the decrease in degree. It is generally well known that Bi ₂ O ₃ is a material that is easily reduced (for example, Non-Patent Document 1).

FIG. 4 shows the distribution in the depth direction for each element of O, Si, Bi, and Er obtained by the secondary ion mass spectrometry (hereinafter referred to as SIMS) of sample A. Since the secondary ionic strength of each element changes sharply in the region of about 0.5 μm in depth, it is recognized that there is a clear interface between Bi ₂ O ₃ : Er 12 and SiO ₂₁₁ . rice field. Further, since the secondary ionic strength of Bi and Er is the noise level in the SiO ₂ region deeper than about 0.7 μm, it can be seen that Bi and Er are not contained in SiO ₂ 11. Furthermore, in the region of Bi ₂ O ₃ -SiO ₂ : Er13, the secondary ionic strength of Si is more than 3 orders of magnitude lower than that of SiO 211, and Si is also included in Bi ₂ O ₃ -SiO ₂ : _Er13 . You can also see that it is not. That is, in sample A, almost no mutual diffusion occurred between Bi ₂ O ₃ : Er 12 and SiO ₂ 11, and Bi ₂ O ₃ : Er 12 did not change to Bi ₂ O ₃ -SiO ₂ : Er 13. Conceivable. Further, since each layer has a structure separated by a clear interface, sample A may cause light loss due to scattering or the like at the interface.

FIG. 5 shows the distribution in the depth direction for each element of O, Si, Bi, and Er obtained by SIMS of sample B. Since the secondary ions of Si are detected at high intensity in the region with a depth of about 0.5 μm from the outermost surface, it can be seen that Si is diffused in Bi ₂ O ₃ : Er12. On the other hand, in the region deeper than about 0.5 μm, secondary ions of Bi and Er are detected at constant values, and it can be seen that Bi and Er are also contained in SiO ₂₁₁ . In addition, pile-up was observed in the secondary ion spectra of Bi and Er in the region of 0.7 to 1.2 μm in depth. From these results, it is considered that the Si elements of SiO ₂₁₁ and the Bi and Er elements of Bi ₂ O ₃ : Er 12 are mutually diffused by post-annealing. In addition, the secondary ionic strength of each element at a depth of about 0.5 μm changes more slowly than that of sample A, and it can be seen that the element concentration does not change sharply at the interface. From this, it is considered that the sample B does not cause a sudden change in the refractive index at the interface, and high transmission efficiency can be realized.

For verification, the in-plane distribution of elements with respect to sample B was obtained by SIMS. As a result, in the region of Bi ₂ O ₃ -SiO ₂ : Er13, Bi and Er were uniform in the plane, while Si was partially observed to have high intensity. This indicates that a compound of Bi ₂ O ₃ and SiO ₂ containing a high concentration of Si is formed, and it is considered that a part of Si derived from the substrate is diffused in Bi ₂ O ₃ : Er 12. Conceivable. Bi _{2SiO 5} is known as a main compound of Bi ₂ O ₃ and SiO ₂ (for example, Non-Patent Document 2). When the analysis was advanced to a deeper region, Bi and Er were observed at a significant intensity in SiO ₂₁₁ . From this, it can be concluded that the diffusion of Si from the substrate to Bi ₂ O ₃ : Er 12 is the main process, but the diffusion of Bi and Er from Bi ₂ O ₃ : Er 12 to the substrate also proceeded at the same time.

FIG. 6 shows an emission spectrum acquired by photoluminescence (hereinafter referred to as PL). In the measurement, the sample was excited with a solid-state laser (output: 50 mW) with a wavelength of 532 nm, and the emission in the wavelength range was acquired at once by the CCD detector. In the emission spectrum of sample A, the structure of crystal field splitting consisting of eight peaks is observed. This is because α-Bi ₂ O ₃ is a crystal belonging to the monoclinic system, and when Er ³⁺ replaces the Bi ³⁺ site, the symmetry around Er ³⁺ has a low symmetry such as C2v. is doing. That is, it was confirmed that Er ³⁺ , in which luminescence was observed, was a substitutional solid solution at the Bi ³⁺ site of the α-Bi ₂ O ₃ crystal.

FIG. 7 shows the emission spectrum of sample B obtained by the PL method. From this result, it is found that sample B does not have a clear crystal field splitting microstructure, suggesting that the luminescent Er ³⁺ is contained in the amorphous phase. Since the Bi ₂ O ₃ -SiO ₂ compound having an indefinite ratio composition becomes amorphous until the crystallization temperature is reached, it is considered that Er ³⁺ bound to the Bi ₂ O ₃ -SiO ₂ compound emits light.

From the above, the Bi ₂ O ₃ -SiO ₂ : Er13 formed according to the present embodiment has a higher refractive index than SiO ₂₁₁ , and therefore can be used as a core layer for transmitting the signal light of the optical network circuit. can. Further, since this Bi ₂ O ₃ -SiO ₂ : Er 13 is formed by mutual diffusion between SiO ₂ 11 and Bi ₂ O ₃ : Er 12, the change in the element concentration in the vicinity of the interface is gradual and steep in the relevant part. Since there is no change in the refractive index, it is possible to have high signal light confinement efficiency. And this mutual diffusion occurs remarkably when the concentration of Er ³⁺ to be doped is 4 at% or more.

In the Bi ₂ O ₃ -SiO ₂ : Er13 thus formed, the doped Er ³⁺ exhibits strong light emission, so that it is imparted with an optical amplification effect by stimulated emission and can be applied as an optical amplifier. .. Moreover, since the emission spectrum in FIG. 7 is wide, a sufficiently wide bandwidth can be secured.

As an optical amplifier with high transmission efficiency, it is expected to be applied to an optical waveguide having a structure in which thin films are laminated.

Claims (8)

1. A method for forming an optical circuit substrate in which a thin film doped with erbium (Er) is formed on an oxide obtained by dissolving silica (SiO ₂ ) in bismuth oxide (Bi ₂ O ₃ ).
   The first step of forming a thin film containing Bi ₂ O ₃ on the substrate by a sputtering method, and
   A second step of post-annealing, which causes mutual diffusion between the thin film formed in the first step and the substrate and changes the thin film into a solid solution thin film.
   A method for forming an optical circuit board.
2. The method for forming an optical circuit board according to claim 1, wherein the sputtering method is used.
   Electronic sacron resonance discharge to Bi ₂ O ₃ target,
   High frequency magnetron discharge to Er ₂ O ₃ target,
   A method for forming an optical circuit board, which is a two-source simultaneous discharge type sputtering method.
3. The method for forming an optical circuit board according to claim 1, wherein the post-annealing is a heat treatment at 400 ° C. for 1 hour in an oxygen atmosphere of 1 atm.
4. The method for forming an optical circuit board according to claim 1, 2 or 3, wherein the substrate is SiO ₂ .
5. A solid solution thin film in which Er is doped in an oxide obtained by dissolving silica SiO ₂ in Bi ₂ O ₃ to bring about a photoamplifying action.
   The solid solution thin film is formed on a substrate having a large difference in refractive index.
   Optical circuit board.
6. The optical circuit board according to claim 5, wherein the Bi ₂ O ₃ has a δ phase.
7. The optical circuit board according to claim 5, wherein the Er is doped with 4 at% or more.
8. The optical circuit board according to claim 5, wherein the substrate is SiO ₂ .

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