WO2022003765A1

WO2022003765A1 - Optical waveguide and method for manufacturing same

Info

Publication number: WO2022003765A1
Application number: PCT/JP2020/025491
Authority: WO
Inventors: 泰土澤; 卓磨相原
Original assignee: 日本電信電話株式会社
Priority date: 2020-06-29
Filing date: 2020-06-29
Publication date: 2022-01-06
Also published as: JPWO2022003765A1

Abstract

In the present invention, in a first step S101, a lower clad layer (102) made of silicon oxide is formed on a substrate (101), and in a second step S102, a silicon nitride layer (103) is formed on the lower clad layer (102) using a plasma CVD method that uses a nitrogen raw material and a silicon raw material constituted of a compound containing deuterium and silicon. The silicon raw material can be SiD₄. In this step, the silicon nitride layer (103) is formed under a condition that calls for less nitrogen raw material than a condition for producing silicon nitride having a stoichiometric composition by the plasma CVD method.

Description

Optical waveguide and its manufacturing method

The present invention relates to an optical waveguide having a core made of silicon nitride and a method for manufacturing the same.

Due to the rapid spread of cloud and mobile environments, the amount of information distributed continues to increase, and at present, there is a strong demand for larger capacity, higher speed, and lower power consumption of optical networks. To achieve this, it is important to improve the performance, power consumption, and cost of optical devices that support Wavelength-division multiplexing (WDM) communication, which is a feature of optical communication. Against this background, in recent years, each optical device such as an optical waveguide device, a light emitting device, and a light receiving device has become smaller and has higher performance, and each optical device has been integrated on the same substrate to improve the functionality of the device. Research and development for cost reduction is becoming active.

In order to realize such an optical integrated device, a low-loss optical waveguide, which is a basic element of optical transmission, is required, but the device can be miniaturized, is economical, and has a wide range of refractive index selectivity. Expectations are high for optical waveguides with Si, SiN, and SiON as cores and SiO _{2 as clads, which have a high degree of design freedom.}

Optical waveguides with Si, SiN, and SiON as cores have long been known as optical waveguides with a higher refractive index difference between the core and clad than quartz-based waveguides, and have been studied for communication network applications. However, in order to manufacture an optical waveguide with a small propagation loss using these materials with a high refractive index as a core, there is a problem that a technique for forming a film of a high-quality core material and a high-precision processing technique are required. Due to the difficulty of manufacturing, a practical device that can be used in a communication network has not been realized. However, today, advances in film formation technology and processing technology have made it possible to manufacture devices at a practical level.

In particular, in the film forming technology for forming SiN and SiON films, the NH group and OH group in the core film, which absorbs in the communication wavelength band by dehydrogenating the gas used for film formation and hinders the reduction of loss. As a result, an optical waveguide with SiN and SiON as the core and a silicon oxide film as a clad can be realized with low loss and excellent performance applicable to communication networks. (See Patent Document 1).

As shown in FIG. 5, when forming a layer of silicon nitride serving as a core, as compared to when using SiH _4, by using the SiD ₄ gas, generated NH groups having strong absorption in the 1.5μm vicinity However, it is possible to realize a practical level low-loss optical waveguide even in the communication wavelength band.

Japanese Unexamined Patent Publication No. 2016-405440

However, in integrated devices for next-generation communication networks, higher performance is required, and the refractive index of the core is also required to be higher.

The present invention has been made to solve the above problems, and an object thereof is to further increase the refractive index of a core composed of silicon nitride.

The method for producing an optical waveguide according to the present invention comprises a first step of forming a lower clad layer made of silicon oxide on a substrate, and a compound containing heavy hydrogen and silicon on the lower clad layer. A second step of forming a silicon nitride layer by a plasma CVD method using a silicon raw material and a nitrogen raw material, and a third step of forming a core on a lower clad layer by patterning the silicon nitride layer. A fourth step of covering the core on the lower clad layer to form an upper clad layer by depositing silicon oxide by a plasma CVD method is provided, and the second step is plasma CVD of silicon nitride having a chemical quantitative composition. The silicon nitride layer is formed under the condition that the amount of the nitrogen raw material is less than the condition to be produced by the method.

Further, the optical waveguide according to the present invention includes a lower clad layer made of silicon oxide formed on a substrate, a core made of silicon nitride containing heavy hydrogen formed on the lower clad layer, and a core. It is provided with an upper clad layer made of silicon oxide, which is covered and formed on the lower clad layer, and the core is under the condition that the nitrogen raw material is less than the condition for producing silicon nitride having a stoichiometric composition by the plasma CVD method. It is composed of silicon nitride formed of.

As described above, according to the present invention, the silicon nitride layer is formed to form the core under the condition that the nitrogen raw material is less than the condition for producing silicon nitride having a stoichiometric composition by the plasma CVD method. , The refractive index of the core composed of silicon nitride can be further increased.

FIG. 1 is an explanatory diagram for explaining a method for manufacturing an optical waveguide according to an embodiment of the present invention. FIG. 2 is a configuration diagram showing the configuration of the ECR plasma CVD apparatus. FIG. 3 is a characteristic diagram showing the relationship between the flow rate of _{N 2 and the refractive index of the formed silicon nitride layer.} FIG. 4A is a characteristic diagram showing the results of measuring the wavelength dependence of the propagation loss of the optical waveguide by the core made of the silicon nitride layer prepared at a flow rate of _{N 2 gas of 100 sccm.} FIG. 4B is a characteristic diagram showing the results of measuring the wavelength dependence of the propagation loss of the optical waveguide by the core made of the silicon nitride layer made of the silicon nitride layer made at the flow rate of _{N 2 gas of 40 sccm.} FIG. 5 is a characteristic diagram showing the propagation loss of the optical waveguide formed by the silicon nitride core formed by using _{SiH 4} _{and the optical waveguide formed by the silicon nitride core formed by using SiD 4.}

Hereinafter, a method for manufacturing an optical waveguide according to an embodiment of the present invention will be described with reference to FIG.

First, in the first step S101, as shown in FIG. 1A, a lower clad layer 102 made of silicon oxide is formed on the substrate 101. The substrate 101 is made of, for example, single crystal silicon. The lower clad layer 102 is formed to have a layer thickness of 3 μm or more.

For example, the lower clad layer 102 can be formed by thermally oxidizing the surface of the substrate 101 made of silicon. Further, the lower clad layer 102 can be formed by depositing silicon oxide on the substrate 101 by a known CVD (Chemical Vapor Deposition) method. However, when another device is already formed on the substrate 101, it is important to form the lower clad layer 102 at a low temperature of about 200 ° C. In this case, the lower clad layer 102 can be formed by depositing silicon oxide by an electron cyclotron resonance (ECR) plasma CVD method capable of forming a high-quality film at a low temperature.

Next, in the second step S102, as shown in FIG. 1 (b), a silicon raw material composed of a compound containing deuterium and silicon and a nitrogen raw material were used on the lower clad layer 102. The silicon nitride layer 103 is formed by the plasma CVD method. The silicon raw material can be _{SiD 4.} In this step, it is important to form the silicon nitride layer 103 under the condition that the nitrogen raw material is less than the condition for producing silicon nitride having a stoichiometric composition by the plasma CVD method. The silicon nitride layer 103 is formed, for example, to have a thickness of about 0.5 to 1 μm. The silicon nitride layer 103 is a layer for forming a core, and the thickness of the silicon nitride layer 103 can be determined in correspondence with a desired core thickness. Details of the formation of the silicon nitride layer 103 by the plasma CVD method will be described later.

Next, in the third step S103, the silicon nitride layer 103 is patterned to form the core 104 on the lower clad layer 102 as shown in FIG. 1 (c). The core 104 can have a width of about 1 to 2 μm and a height of about 0.5 to 1 μm in a cross-sectional view.
In this step, first, a layer of photosensitive photoresist is formed on the silicon nitride layer 103. Next, a resist pattern is formed on the silicon nitride layer 103 by layer patterning the photoresist using a known lithography technique.

Next, for example, _{by reactive ion etching using an etching gas containing carbon and fluorine such as carbon tetrafluoride (CF 4} ) and sulfur hexafluoride (SF ₆ ), the silicon nitride layer with the resist pattern as a mask is used. The core 104 can be formed by etching 103. After forming the core 104 in this way, the resist pattern is removed by dissolving it in an organic solvent or the like.

Next, in the fourth step S104, silicon oxide is deposited by the plasma CVD method to form the upper clad layer 105 by covering the core 104 on the lower clad layer 102 as shown in FIG. 1 (d). do. For example, similar to the formation of the lower clad layer 102 described above, the upper clad layer 105 can be formed by depositing silicon oxide by the ECR plasma CVD method capable of forming a high-quality film at a low temperature.

By the above manufacturing method, the lower clad layer 102 made of silicon oxide formed on the substrate 101, the core 104 made of silicon nitride containing deuterium formed on the lower clad layer 102, and the core 104. An optical waveguide including an upper clad layer 105 made of silicon oxide formed on the lower clad layer 102 is obtained.

Here, as will be described later, it is important that the core 104 is composed of silicon nitride formed under the condition that the nitrogen raw material is less than the condition for producing silicon nitride having a stoichiometric composition by the plasma CVD method. .. Further, the hydrogen contained in the core 104 contains deuterium as a main component. According to the optical waveguide configured in this way, the refractive index of the core 104 can be made higher, and a lower loss optical waveguide can be realized.

Here, the formation (deposition) of the silicon nitride layer by the above-mentioned ECR plasma CVD method will be described. In the ECR plasma CVD method, the ECR plasma CVD apparatus shown in FIG. 2 is used. This apparatus includes a plasma generation chamber 201, a film forming chamber 202, a substrate base 203, a magnetic coil 204, a waveguide 205, a quartz window 206, a gas introduction pipe 207, a gas introduction pipe 208, and a gas introduction pipe 210.

In this apparatus, a magnetic field (875 gauss) satisfying the ECR condition is generated in an appropriate region inside the plasma generation chamber 201 by a magnetic coil 204 arranged around the plasma generation chamber 201, and plasma is generated in the film formation chamber 202. A divergent magnetic field for drawing ions is formed in the form of a stream 211.

First, the N ₂ gas is introduced into the plasma generation chamber 201 through the gas introduction pipe 207 installed in the film forming chamber 202 or the gas introduction pipe 210 installed in the plasma generation chamber 201, and the pressure in the plasma generation chamber 201 is reduced to 0. After setting the value to about 1 Pa, a 2.45 GHz microwave (1200 to 1800 W) is introduced from the waveguide 205 into the plasma generation chamber 201 via the quartz window 206 to generate a plasma of nitrogen gas. When it is confirmed that plasma is stably generated, SiD ₄ gas is supplied to the film forming chamber 202 by a gas introduction tube 208 closer to (near) the substrate 209 to obtain N ₂ / SiD ₄ mixed plasma, and N _{2 is obtained.} Induces a plasma reaction between and SiD ₄ and a surface reaction on the substrate 209, and silicon nitride is deposited on the surface of the substrate 209 to form a silicon nitride layer.

In this film formation, ions accelerated by the electric field generated in the plasma flow 211 enter the surface of the substrate 209 and give an impact, and this energy promotes the formation reaction of the silicon nitride layer, resulting in dense high-quality silicon nitride. Layers are formed.

ECR plasma is characterized by high gas decomposition efficiency, stable and difficult to decompose N ₂ can be efficiently decomposed, and ions in ECR plasma are incident on the substrate 209 with appropriate energy. Therefore, the use of ECR plasma has an advantage that high-quality silicon nitride can be deposited even if the substrate 209 is kept at a low temperature of 200 ° C. or lower. Further, _{by supplying the SiD 4} gas to a location (near) closer to the substrate 209, the raw material can be decomposed at a location closer to the substrate 209, and the film-forming material can reach the surface of the substrate 209 more efficiently. This makes it possible to carry out film formation and refractive index control more efficiently.

_{Next, the relationship between the flow rate of N 2} and the refractive index of the formed silicon nitride layer in the above-mentioned deposition of silicon nitride will be described with reference to FIG. Here, the refraction of the silicon nitride layer formed when the flow rate of the _{SiD 4} gas supplied to the plasma generation chamber 201 (deposition chamber 202) is _{fixed at 40 sccm and the flow rate of the N 2 supplied is reduced from 100 sccm.} The rate is being measured. Note that sccm is a unit of flow rate, and indicates that ^{a fluid at 0 ° C. and 1013 hPa flows 1 cm 3 per minute.}

As shown in FIG. 3, when _{the flow rate of N 2} is reduced from 100 sccm, the refractive index is almost constant at about 1.88 up _{to 70 sccm, but when the flow rate of N 2} is less than 70 sccm, the refractive index increases. .. Further, when the N ₂ flow rate is made smaller than 40 sccm, the refractive index rises sharply. Consequently, first, with respect to the flow rate of SiD ₄ gas, in a state where N ₂ is sufficiently supplied, to react all SiD ₄ being fed at the same time, the silicon nitride layer with a refractive index 1.89 is formed To. It is considered that this is because a silicon nitride layer having a stoichiometric composition was formed.

On the other hand, if the supply amount of _{N 2} _{is small and N 2} is insufficient in the reaction with _{SiD 4,} it is considered that a Si-rich silicon nitride layer is formed as a result and the refractive index increases. In this way, by forming the silicon nitride layer under the condition that the nitrogen raw material is less than the condition for producing silicon nitride having a stoichiometric composition by the plasma CVD method, a silicon nitride layer having a higher refractive index can be formed. .. This phenomenon indicates that a silicon nitride layer having a selected refractive index can be formed in the range of 1.88 to 2.1 by adjusting the flow rate ratio of _{SiD 4} and N _2.

Here, in the formation of silicon nitride by the plasma CVD method as described above, the layer of silicon nitride to be formed is formed even if the supply amount of each raw material gas is a condition for forming silicon nitride having a stoichiometric composition. , It does not become a stoichiometric composition. On the other hand, as described above, the refractive index of the formed silicon changes depending on the supply amount of the nitrogen raw material, and when the supply amount is reduced beyond a certain condition, the refractive index is in a state of sharply increasing. The singularity of the supply amount of the nitrogen raw material differs depending on the manufacturing apparatus. It is considered unrealistic to specify the composition of the silicon nitride thus formed to obtain the effect of the present invention. Therefore, regarding the state of the core of the optical waveguide of the present invention, it is considered impractical to directly specify the core by its structure or characteristics, such as by clearly specifying the composition that contributes to the effect of the present invention. Will be.

By the way, in the explanation using FIG. 5, by using SiD ₄ gas for the production of silicon nitride, it is possible to prevent the generation of NH groups having strong absorption in the vicinity of 1.5 μm, and it is practical even in the communication wavelength band. It was explained that a low-loss optical waveguide can be realized. However, when these results were confirmed in detail, a peak of absorption by the NH group was still observed, although it was weak at around 1.5 μm. This absorption is considered to have a non-negligible effect on the characteristics of optical devices that require high performance for the next generation.

The cause of this weak absorption peak due to the NH group is that the NH group is caused by hydrogen remaining in the device such as _{water (H 2} O) adsorbed on the inner wall of the film forming chamber of the device forming the silicon nitride layer. It is considered that this is because it is generated in the silicon nitride layer in a small amount. Even if the degree of vacuum is increased, it is not easy to completely remove the hydrogen remaining in the apparatus. On the other hand, as shown below, it was found that NH in the silicon nitride layer can be reduced by reducing the flow rate of the _{N 2 gas as described above.}

First, two optical waveguide samples were prepared with a core made of two silicon nitride layers made at _{different N 2 gas flow rates.} The results of measuring the wavelength dependence of the propagation loss of these two samples are shown in FIGS. 4A and 4B. FIG. 4A shows the result of an optical waveguide using a core made of a silicon nitride layer made under the condition of _{N 2 flow rate of 100 sccm.} FIG. 4B shows the result of an optical waveguide using a core made of a silicon nitride layer made under the condition of _{N 2 flow rate of 40 sccm.} In either case, the flow rate of the _{SiD 4 gas is 40 sccm.}

In either _{case, NH absorption is suppressed by using SiD 4} gas, and a sufficiently low loss is obtained as a value. However, as shown in FIG. 4A, when the N ₂ flow rate is 100 sccm, a weak absorption peak is still seen near the wavelength of 1510 nm. It is considered that this is due to NH generated in the silicon nitride layer mainly by adsorbing to the inner wall of the apparatus by plasma CVD and using H _{2 O remaining in the apparatus as a hydrogen source.} Although this NH is a trace amount, it causes a loss in the optical waveguide, and the loss is 0.9 dB / cm at the peak of 1510 nm. Although it is minute, if the loss differs depending on the wavelength, the characteristics will be affected in the next-generation devices that require high characteristics and cannot be ignored.

On the other hand, as shown in FIG. 4B, in _{the case of N 2} flow rate of 40 sccm, the peak near 1510 nm disappears, and the loss is about 0.4 dB / cm at any wavelength. This indicates that the formed silicon nitride contains almost no NH. This factor, that with a reduced supply of N _2, remains in SiD ₄ is a device that can not react with N _2, which is attached to the inner wall of the apparatus, resulting in from the inner wall as a source of NH This is thought to be because it has the effect of effectively suppressing the supply of hydrogen. The refractive index of the silicon nitride layer formed under the condition of N _{2 flow rate of 40 sccm is 1.92.}

By reducing the N ₂ flow rate and forming a core from a Si-rich silicon nitride layer to fabricate an optical waveguide, it is possible to reduce the loss at 1510 nm and realize a wavelength-independent waveguide of loss. As described above, " _{reducing the N 2} flow rate" is "a condition in which the nitrogen raw material is less than the condition for producing silicon nitride having a stoichiometric composition by the plasma CVD method".

As described above, according to the present invention, the silicon nitride layer is formed to form a core under the condition that the nitrogen raw material is less than the condition for producing silicon nitride having a stoichiometric composition by the plasma CVD method. Therefore, the refractive index of the core composed of silicon nitride can be further increased.

It should be noted that the present invention is not limited to the embodiments described above, and many modifications and combinations can be carried out by a person having ordinary knowledge in the art within the technical idea of the present invention. That is clear.

101 ... substrate, 102 ... lower clad layer, 103 ... silicon nitride layer, 104 ... core, 105 ... upper clad layer.

Claims

The first step of forming a lower clad layer made of silicon oxide on the substrate,
A second step of forming a silicon nitride layer on the lower clad layer by a plasma CVD method using a silicon raw material composed of a compound containing deuterium and silicon and a nitrogen raw material.
A third step of forming a core on the lower clad layer by patterning the silicon nitride layer, and
A fourth step of covering the core on the lower clad layer to form an upper clad layer by depositing silicon oxide by a plasma CVD method is provided.
The second step is a method for producing an optical waveguide, characterized in that the silicon nitride layer is formed under the condition that the nitrogen raw material is less than the condition for producing silicon nitride having a stoichiometric composition by a plasma CVD method.
In the method for manufacturing an optical waveguide according to claim 1,
A method for producing an optical waveguide, characterized in that the silicon raw material is SiD 4.
A lower clad layer made of silicon oxide formed on the substrate,
A core made of silicon nitride containing deuterium formed on the lower clad layer,
It comprises an upper clad layer made of silicon oxide, which covers the core and is formed on the lower clad layer.
The core is an optical waveguide characterized in that it is composed of silicon nitride formed under conditions where the nitrogen raw material is less than the condition for producing silicon nitride having a stoichiometric composition by a plasma CVD method.
In the optical waveguide according to claim 3,
The hydrogen contained in the core is an optical waveguide characterized by containing deuterium as a main component.