JP2016206425A - Optical module and manufacturing method thereof - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an optical module that is monolithic integration of a silicon optical waveguide and a quartz-based optical waveguide and its performance is not deteriorated, and its manufacturing method.SOLUTION: An optical module comprises: a lower clad layer 101 made of silicon oxide; a core 102 formed on the lower clad layer 101; an upper clad layer 103 formed on the lower clad layer 101, covering the core 102; and a protective film 104 formed covering the upper clad layer 103. The core 102 is made of one of SiO, SiONand SiN, the upper clad layer 103 is made of silicon oxide, and the protective film 104 is made of silicon nitride.SELECTED DRAWING: Figure 1

Description

  The present invention relates to an optical module composed of an optical waveguide element used in the field of optical communication and a method for manufacturing the same.

  With the rapid spread of cloud computing and mobile environments, the volume of information circulation continues to increase, and at present, there is a strong demand for lowering the cost and power consumption as well as increasing the capacity and speed of optical networks. To achieve this, it is important to reduce the size of an optical device for wavelength-division multiplexing (WDM).

  Against this background, in recent years, optical devices and optical circuits based on optical waveguides with silicon cores, which can be miniaturized, are excellent in economy and energy saving, and are excellent in fusion with electrical devices, have been developed. It attracts attention and is actively researching and developing. However, since silicon has a high refractive index of about 3.48, it has a high light confinement and is advantageous for downsizing, but has a problem that device characteristics are sensitive to processing errors.

  In addition, there is a problem rooted in the optical nonlinearity of silicon induced by high power transmission that occurs with the increase in wavelength. In communication network applications that require polarization independence and a high dynamic range, optical cores with silicon cores are used. It has become clear that it is not easy to meet the required performance with only the waveguide.

  On the other hand, in application to communication networks, especially high-density wavelength division multiplexing (DWDM), research and development of optical devices using silica-based optical waveguides with a small relative refractive index difference between the core and the clad has preceded, and already high-performance quartz. Based optical waveguide devices have been developed and used in major optical networks. However, silica-based optical devices have high stability with low loss, but because they cannot theoretically be downsized, densely integrated, or operated at high speed, the high performance requirements of next-generation networks can be realized using only silica-based devices. It is still difficult to do.

  Due to the above-mentioned background, optical functional elements composed of silicon optical waveguides and optical functional elements composed of silica optical waveguides are integrated and integrated, making the most of the features of silicon devices and silica based devices. There has been proposed an optical device that is intended to be realized (see Patent Document 1).

  In this technology, as shown in FIG. 5, an optical device in which an optical functional element using a silicon optical waveguide and an optical functional element using a silica-based waveguide are integrated and developed has been developed. FIG. 5 is an optical micrograph of an optical module comprising a silicon fine wire optical waveguide type VOA (Variable Optical Attenuator) and a silica-based AWG (Arrayed Waveguide Grating). This optical module includes a carrier injection variable optical attenuator (VOA) 502 manufactured using a silicon thin wire optical waveguide and an arrayed waveguide diffraction grating (AWG) 503 manufactured using a silica-based optical waveguide. A region 504 constituted by a silicon fine wire optical waveguide and a region 505 constituted by a silica-based optical waveguide are provided.

  Each output port of the AWG 503 is connected to a silicon fine wire optical waveguide constituting the VOA 502 via the SSC 501a of the corresponding channel. In this configuration, the mode field diameter of the optical waveguide is reduced by the SSC 501a. Thereby, the optical signal of each output port demultiplexed by the AWG 503 is input to the input port of the VOA 502 of the corresponding channel, and the intensity adjustment of the optical signal of each channel can be performed independently for each channel.

  Each output port of the VOA 502 is connected to a silica-based optical waveguide (not shown) via the SSC 501b of the corresponding channel. In this configuration, the mode field diameter of the waveguide is expanded by the SSC 501b. In addition, the silica-based optical waveguide of each channel is coupled to an optical fiber outside the device, so that an output signal can be obtained.

  The optical device described above is small by using a silicon optical waveguide for a small modulation device that requires high-speed operation, and a silica-based waveguide for a wavelength filter that requires low loss and high wavelength separation characteristics. And high-performance devices.

  First, a commercially available silicon-on-insulator (SOI) substrate is prepared. This SOI substrate is provided with a surface silicon layer having a thickness of about 200 nm on a silicon substrate portion serving as a support via a buried insulating layer having a thickness of 3 μm.

  Next, the surface silicon layer of the SOI substrate is patterned by a lithography technique and an etching technique to form a silicon core of SSC 501a and SSC 501b and a silicon core of VOA 502, and further, ion implantation and electrode production are performed to produce a VOA 502.

  Subsequently, on the buried insulating layer (undercladding) of the SOI substrate, a quartz film serving as a core of the AWG 503 constituted by the quartz optical waveguide is deposited by plasma CVD. At this time, it is important to deposit the quartz-based film under a low temperature condition of about 300 ° C. or lower so that the already formed silicon core is not deteriorated or destroyed by heat. For this reason, a quartz film is formed by a plasma CVD method capable of forming a film at a low temperature. Next, the quartz film is patterned by a photolithography technique and an etching technique, and a quartz optical waveguide core of the AWG 503 is manufactured.

After each core is formed as described above, a silicon oxide (SiO ₂ ) film is deposited to form an overclad, thereby completing an optical module in which silicon optical elements and quartz optical elements are monolithically integrated. Since the silicon optical waveguide and the quartz-based waveguide have different mode field sizes, they are connected with low loss by forming a spot size converter between them. This silicon-quartz-based fusion device has an excellent feature that it is possible to achieve both miniaturization and high functionality and high performance.

JP 2012-42849 A

  However, it has been clarified that the device according to the above-described technology has the following problems in the reliability test for practical use, which has been a practical problem.

  In integration with silicon optical waveguide elements, it is necessary to deposit a quartz-based material without damaging the silicon optical waveguide. Therefore, the quartz-based material and the material for the upper cladding are quartz-based films formed by low-temperature film formation by plasma CVD. Is used. However, a quartz-based film formed at a low temperature is slightly inferior to a conventional quartz-based film formed at a high temperature of 1000 ° C. or higher, and therefore gradually absorbs moisture in the atmosphere. As a result, the absorption of water having a peak near 1450 nm gradually increased, and the loss gradually increased with time in the communication wavelength band in the 1550 nm region.

  The present invention has been made to solve the above-described problems, and an object of the present invention is to prevent the deterioration of the characteristics of an optical module in which a silicon optical waveguide and a silica-based optical waveguide are monolithically integrated.

An optical module according to the present invention includes a lower clad layer made of silicon oxide, a core formed on the lower clad layer and made of any one of SiO _x , SiO _x N _y , and SiN, and a lower clad layer And an upper clad layer formed of silicon oxide and covering the core, and a protective film formed of silicon nitride and covering the upper clad layer.

In the optical module, a first silicon core made of silicon formed on the lower cladding layer in the first region and having the same width in the waveguide direction, and a lower cladding layer in the second region following the first region A second silicon core made of silicon with a taper shape that is formed continuously on the first silicon core and gradually decreases in width toward the tip, and covers the first silicon core on the lower cladding layer in the first region. An intermediate clad layer formed of any one of SiO _x , SiO _x N _y , and SiN, and the core is formed on the lower clad layer in the second region so as to cover the second silicon core. A core, and a second core formed continuously from the first core on the lower cladding layer of the third region following the second region, the first core being formed continuously from the intermediate cladding layer, The cladding layer is During the cladding layer, the first core is formed on the second core. Note that the upper surface of the upper cladding layer is preferably planarized.

The method for manufacturing an optical module according to the present invention includes a lower clad forming step of forming a lower clad layer made of silicon oxide, and a core made of any one of SiO _x , SiO _x N _y , and SiN as a lower portion. A core forming step formed on the cladding layer, an upper cladding layer forming step of forming an upper cladding layer made of silicon oxide on the lower cladding layer so as to cover the core, and a protective film made of silicon nitride And a protective film forming step of covering the upper clad layer.

In the optical module manufacturing method, before the core formation step, the first silicon core made of silicon having the same width in the waveguide direction on the lower cladding layer in the first region, and the first region following the first region. A silicon core forming step of forming a second silicon core made of tapered silicon, the width of which gradually decreases toward the tip of the first silicon core on the lower cladding layer of the two regions, An intermediate cladding layer made of any one of SiO _x , SiO _x N _y , and SiN covering the first silicon core on the lower cladding layer in the first region, and a second silicon core on the lower cladding layer in the second region A first core continuous with the intermediate cladding layer and a core composed of the second core continuous with the first core are formed on the lower cladding layer in the third region following the second region. The layer formation step, intermediate cladding layer, the first core to form an upper cladding layer on the second core. In addition, it is good to provide the planarization process which planarizes the upper surface of an upper clad layer before a protective film formation process, and it is good to form a protective film on the planarized upper clad layer in a protective film formation process.

  As described above, according to the present invention, since the protective film made of silicon nitride is formed, the characteristics of the optical module in which the silicon optical waveguide and the silica-based optical waveguide are monolithically integrated are not deteriorated. The effect will be obtained.

FIG. 1 is a configuration diagram showing a configuration of an optical module manufactured by an optical module manufacturing method according to an embodiment of the present invention. FIG. 2A is a cross-sectional view showing a partial configuration of an optical module manufactured by an optical module manufacturing method according to an embodiment of the present invention. FIG. 2B is a cross-sectional view showing a partial configuration of the optical module manufactured by the optical module manufacturing method according to the embodiment of the present invention. FIG. 2C is a cross-sectional view showing a partial configuration of the optical module manufactured by the optical module manufacturing method according to the embodiment of the present invention. FIG. 3A is a cross-sectional view showing a partial configuration of an optical module manufactured by the optical module manufacturing method according to the embodiment of the present invention. FIG. 3B is a cross-sectional view showing a partial configuration of the optical module manufactured by the optical module manufacturing method according to the embodiment of the present invention. FIG. 3C is a cross-sectional view showing a partial configuration of the optical module manufactured by the method of manufacturing an optical module according to the embodiment of the present invention. FIG. 4A is a cross-sectional view showing the state of each step for explaining the method for manufacturing the optical module in the embodiment of the present invention. FIG. 4B is a cross-sectional view showing the state of each step for explaining the method for manufacturing the optical module in the embodiment of the present invention. FIG. 4C is a cross-sectional view showing the state of each step for explaining the method for manufacturing the optical module in the embodiment of the present invention. FIG. 4D is a cross-sectional view showing the state of each step for explaining the method of manufacturing the optical module in the embodiment of the present invention. FIG. 4E is a cross-sectional view showing the state of each step for explaining the method of manufacturing the optical module in the embodiment of the present invention. FIG. 4F is a cross-sectional view showing the state of each step for explaining the method of manufacturing the optical module in the embodiment of the present invention. FIG. 4G is a cross-sectional view showing the state of each step for explaining the method for manufacturing the optical module in the embodiment of the present invention. FIG. 5 is an optical micrograph of an optical module composed of a silicon fine wire optical waveguide type VOA and a silica-based AWG.

  Hereinafter, embodiments of the present invention will be described with reference to the drawings. First, a configuration example of an optical module manufactured by the method for manufacturing an optical module of the present invention will be described with reference to FIGS. 1 and 2A to 2C. FIG. 1 is a plan view showing a configuration of an optical module according to an embodiment of the present invention. 2A to 2C are cross-sectional views illustrating the configuration of the optical module. 2A shows a cross section taken along the line aa 'in FIG. 1, FIG. 2B shows a cross section taken along the line bb' in FIG. 1, and FIG. 2C shows a cross section taken along the line cc 'in FIG.

This optical module includes a lower clad layer 101 made of silicon oxide, a core 102 formed on the lower clad layer 101, and an upper clad layer 103 formed on the lower clad layer 101 so as to cover the core 102. And a protective film 104 formed to cover the upper clad layer 103. The core 102 is made of any of SiO _x , SiO _x N _y , and SiN, the upper cladding layer 103 is made of silicon oxide, and the protective film 104 is made of silicon nitride.

  In the first region 131, the first silicon core 105 made of silicon is formed on the lower cladding layer 101, and in the second region 132 following the first region 131, the silicon is formed on the lower cladding layer 101. A second silicon core 106 is formed. The first silicon core 105 has the same width in the waveguide direction. The second silicon core 106 is formed continuously with the first silicon core 105 and has a tapered shape in which the width gradually decreases toward the tip 106a. The first silicon core 105 and the second silicon core 106 are composed of the same layer and are integrally formed.

Further, an intermediate cladding layer 107 formed so as to cover the first silicon core 105 is provided on the lower cladding layer 101 in the first region 131. The intermediate cladding layer 107 is composed of any one of SiO _x , SiO _x N _y , and SiN.

  Here, in this example, the core 102 includes a first core 102a and a second core 102b. The first core 102 a is formed on the lower cladding layer 101 in the second region 132 so as to cover the second silicon core 106. The first core 102 a is formed continuously from the intermediate cladding layer 107. In addition, the second core 102 b is formed continuously on the lower cladding layer 101 in the third region 133 following the second region 132 so as to be continuous with the first core 102 a. The intermediate cladding layer 107, the first core 102a, and the second core 102b are composed of the same layer and are integrally formed.

  The upper cladding layer 103 is formed on the intermediate cladding layer 107, the first core 102a, and the second core 102b. In the first region 131, the intermediate cladding layer 107 and the upper cladding layer 103 function as an upper cladding with respect to the first core 102a. In the third region 133, the upper cladding layer 103 functions as a cladding above the second core 102b.

  In this optical module, a silicon fine wire optical waveguide element composed of a first silicon core 105 covered with an intermediate cladding layer 107 in a first region 131 and a second silicon core 106 in a second region 132 are covered with a first core 102a. And a silica-based optical waveguide element formed by the second core 102b of the third region 133.

  The silicon thin-line optical waveguide device in the first region 131 is composed of a first silicon core 105 having dimensions such as a width and a height of a cross section perpendicular to the optical waveguide direction. On the other hand, the silica-based optical waveguide element in the third region 133 is composed of the second core 102b whose dimensions such as the width and height of the cross section perpendicular to the optical waveguide direction are on the order of several microns. In order to optically connect these two regions with low loss, mode field size conversion is performed in the spot size conversion portion of the second region 132.

  Next, the dimension and refractive index of each component will be exemplified. First, the lower cladding layer 101 has a layer thickness of 3 μm and a refractive index of 1.45. It is important that the thickness of the lower clad layer 101 is such that the light propagating through the silica-based optical waveguide device by the core 102 (second core 102b) leaks below the lower clad layer 101 and the loss does not increase. is there. The layer thickness of the lower cladding layer 101 is not limited to 3 μm, and may be set as appropriate depending on the refractive index structure of the silica-based optical waveguide element.

  The first silicon core 105 and the second silicon core 106 have a refractive index of 3.48. The first silicon core 105 and the second silicon core 106 have a thickness (height) of 200 nm. The width of the first silicon core 105 is 400 nm. The width of the start portion of the second silicon core 106 connected to the first silicon core 105 is 400 nm. However, the width becomes narrower toward the tip 106a, and the width of the tip 106a is 80 nm.

  The intermediate cladding layer 107, the first core 102a, and the second core 102b have a refractive index of 1.50 and a thickness (height) of 3 μm. The first core 102a and the second core 102b (core 102) have a width of 3 μm. The upper cladding layer 103 has a layer thickness of about 7 μm and a refractive index of 1.47. The protective film 104 has a thickness of about 0.2 μm and a refractive index of 1.97.

Here, the intermediate cladding layer 107, the first core 102a, and the second core 102b may be made of silicon oxide (SiO _x ) having a lower oxygen composition ratio than the stoichiometric composition of SiO ₂ . In addition, the intermediate cladding layer 107, the first core 102a, and the second core 102b can be made of SiO _x N _y or SiN with adjusted refractive index. However, it is desirable that the shape and refractive index of the silicon fine wire optical waveguide element and the silica-based optical waveguide element are adjusted so as to satisfy the single mode condition.

  By the way, in the structure described above, a step is formed on the surface of the upper cladding layer 103 by the first core 102a and the second core 102b. In such a state, a problem such as a crack occurring in the stepped portion is likely to occur due to a stress change caused by a thermal history applied in the manufacturing process. Therefore, the protective film 104 is preferably formed in a flat state.

  For example, as shown in FIGS. 3A to 3C, the upper surface of the upper cladding layer 103 may be flattened, and the protective film 104 may be formed thereon. 3A shows a cross section taken along the line aa 'in FIG. 1, FIG. 3B shows a cross section taken along the line bb' in FIG. 1, and FIG. 3C shows a cross section taken along the line cc 'in FIG.

  Hereinafter, the manufacturing method of the optical module in embodiment of this invention is demonstrated using FIG. 4A-FIG. 4F. First, a well-known silicon-on-insulator (SOI) substrate is prepared. The buried insulating layer of this SOI substrate is the lower cladding layer 101, and the surface silicon layer is patterned to form the first silicon core 105 and the second silicon core 106 as shown in FIG. 4A. For example, a predetermined resist pattern is formed on the surface silicon layer by photolithography. Next, the surface silicon layer is etched by known reactive ion etching using the resist pattern as a mask to form the first silicon core 105 and the second silicon core 106. Thereafter, the resist pattern is removed. In FIGS. 4A to 4F, the substrate on which the lower cladding layer 101 is formed is not shown.

Next, as shown in FIG. 4B, a film 301 made of a material having a refractive index higher than that of SiO ₂ having a stoichiometric composition is formed on the lower cladding layer 101 on which the first silicon core 105 and the second silicon core 106 are formed. Form. For example, the film 301 may be made of SiO _x having a lower oxygen composition ratio than that of SiO ₂ having a stoichiometric composition. The film 307 may be formed by depositing the SiO _x by, for example, electron cyclotron resonance (ECR) plasma CVD. According to the ECR plasma CVD method, SiO _x can be deposited under a low temperature condition of 200 ° C. or lower, and a film 301 with good film quality can be formed. Therefore, the film 301 can be formed without changing the thickness and dimensions of the first silicon core 105 and the second silicon core 106 due to oxidation or the like.

  Next, by patterning the film 301, as shown in FIG. 4C, the first core 102a is formed in the second region 132 on the second silicon core 106 side from the boundary between the first silicon core 105 and the second silicon core 106. Then, the second core 102b is formed in the third region 133 that follows. Further, the intermediate cladding layer 107 is formed. For example, a predetermined resist pattern is formed on the film 301 by photolithography. Next, the film 301 is etched by known reactive ion etching using the resist pattern as a mask to form the intermediate cladding layer 107, the first core 102a, and the second core 102b.

  Here, the first core 102 a and the second core 102 b are formed in a larger cross-sectional shape than the first silicon core 105 and the second silicon core 106. Further, the second silicon core 106 is covered with the first core 102a. With this configuration, the second region 132 becomes a spot size conversion unit. The first region 131 covering the first silicon core 105 with the intermediate cladding layer 107 is a silicon fine wire optical waveguide element. Further, following the spot size converter, the third region 133 formed by the second core 102b in a state where there is no core made of silicon is a silica-based optical waveguide device.

Next, as shown in FIG. 4D, a film 302 is formed on the intermediate cladding layer 107, the first core 102a, and the second core 102b. For example, the film 302 may be formed by depositing SiO ₂ by plasma CVD. The surface (upper surface) of the film 302 formed in this manner is in a state where irregularities are formed reflecting the steps of the first core 102a, the second core 102b, and the like.

Next, the step on the surface of the film 302 is flattened to form an upper clad layer lower portion 103a having a flattened surface as shown in FIG. 4E. For example, the surface of the film 302 may be planarized by grinding and polishing using a well-known chemical mechanical polishing (CMP) method. Subsequently, for example, SiO ₂ is deposited on the lower clad layer lower portion 103a by, for example, a plasma CVD method, thereby forming an upper clad layer upper portion 103b as shown in FIG. 4F. Layer 103 is formed. By forming the upper clad layer upper portion 103b, the upper clad layer 103 has a predetermined thickness for the waveguide design.

  Next, as shown in FIG. 4F, a protective film 104 is formed by depositing silicon nitride on the upper cladding layer 103 by, for example, plasma CVD. More preferably, silicon nitride is deposited by ECR plasma CVD. In the already formed optical waveguide, there is a concern that the refractive index changes due to post-treatment at a high temperature. On the other hand, according to the ECR plasma CVD method, deposition can be performed under a low temperature condition of 200 ° C. or less, and the above-described influence is less likely to occur.

  As described above, an optical module including the silicon thin-line optical waveguide device in the first region 131, the spot size conversion unit in the second region 132, and the silica-based optical waveguide device in the third region 133 is obtained. In the silicon fine line optical waveguide device in the first region 131, the core 102 and the upper clad layer 103 function as an upper clad.

  As described above, according to the present invention, since the protective film made of silicon nitride is provided, moisture absorption in the atmosphere in the upper cladding layer is suppressed, and the silicon optical waveguide and the silica-based optical waveguide The characteristics of the optical module in which the monolithic integration is integrated will not be deteriorated.

  The effect of suppressing moisture absorption will be described with reference to Table 1. Table 1 shows the results of testing changes in loss in an optical module placed in an environment of high temperature and high humidity conditions of 85 ° C. and 85%. As shown in Table 1, when the protective film was not formed, an increase in loss of 0.62 dB / cm was measured for the optical module. In contrast, when the protective film was formed, the loss increase was reduced to 0.34 dB / cm. Further, when the protective film is formed after planarization, no increase in loss is observed. This indicates that a protective film made of SiN has a moisture-proof effect to prevent water from entering the optical waveguide, and that a greater effect can be obtained if the protective film is formed in a flat state without a step structure. ing. Thus, according to the present invention, a high-quality and high-performance optical module can be realized.

  The present invention is not limited to the embodiment described above, and many modifications and combinations can be implemented by those having ordinary knowledge in the art within the technical idea of the present invention. It is obvious.

The gist of the present invention is to provide a structure capable of maintaining the loss of an optical module monolithically integrated with a silicon optical waveguide and a silica-based optical waveguide while maintaining a low loss, and a method for manufacturing the structure. SiO _x , SiO _x N _y , The core composed of any one of SiN and the silicon core need only be connected and arranged, and the core composed of any one of SiO _x , SiO _x N _y , and SiN overlaps with the silicon core. There is no need. Further, the tip of the silicon core does not need to be tapered. For example, the silicon core may have a shape that becomes thicker toward the tip.

Here, the connection means that a core composed of any one of SiO _x , SiO _x N _y , and SiN and a silicon core can be optically coupled. Therefore, in the region to be connected, the end of the silicon core and the end of the quartz-based core do not need to be in contact with each other, and may be separated as long as optical coupling is possible. However, considering the coupling efficiency between the two, it is better to configure the spot size conversion unit as described above.

  DESCRIPTION OF SYMBOLS 101 ... Lower clad layer, 102 ... Core, 102a ... First core, 102b ... Second core, 103 ... Upper clad layer, 104 ... Protective film, 105 ... First silicon core, 106 ... Second silicon core, 106a ... Tip 107, intermediate cladding layer, 131, first region, 132, second region, 133, third region.

Claims (6)

A lower cladding forming step of forming a lower cladding layer composed of silicon oxide;
Forming a core made of any of SiO _x , SiO _x N _y , and SiN on the lower cladding layer;
An upper clad layer forming step of forming an upper clad layer made of silicon oxide on the lower clad layer so as to cover the core;
And a protective film forming step of forming a protective film made of silicon nitride so as to cover the upper clad layer. In the manufacturing method of the optical module of Claim 1,
Before the core forming step, a first silicon core made of silicon having the same width in the waveguide direction on the lower cladding layer in the first region, and the lower portion of the second region following the first region A silicon core forming step of forming a second silicon core made of tapered silicon on the cladding layer, the width of which gradually becomes narrower toward the tip of the first silicon core;
In the core formation step, an intermediate clad layer made of any one of SiO _x , SiO _x N _y , and SiN covering the first silicon core on the lower clad layer in the first region, A first core that covers the second silicon core on the lower cladding layer and that continues to the intermediate cladding layer, and a second core that continues to the first core on the lower cladding layer in a third region following the second region. Forming the core consisting of a core,
In the upper clad layer forming step, the upper clad layer is formed on the intermediate clad layer, the first core, and the second core. In the manufacturing method of the optical module of Claim 1 or 2,
A flattening step of flattening the upper surface of the upper clad layer before the protective film forming step;
In the protective film forming step, the protective film is formed on the planarized upper clad layer. A lower cladding layer composed of silicon oxide;
A core formed on the lower cladding layer and made of any one of SiO _x , SiO _x N _y , and SiN;
An upper clad layer formed on the lower clad layer and covering the core and made of silicon oxide;
An optical module comprising: a protective film formed over the upper clad layer and made of silicon nitride. The optical module according to claim 4,
A first silicon core made of silicon formed on the lower cladding layer in the first region and having the same width in the waveguide direction;
A second silicon core made of tapered silicon formed continuously on the first silicon core on the lower cladding layer in the second region following the first region and gradually narrowing toward the tip;
An intermediate clad layer formed on the lower clad layer in the first region so as to cover the first silicon core and composed of any one of SiO _x , SiO _x N _y , and SiN;
The core is
A first core formed on the lower cladding layer in the second region and covering the second silicon core;
A second core formed continuously on the lower cladding layer in the third region following the second region and continuously with the first core;
The first core is formed continuously from the intermediate cladding layer;
The upper clad layer is formed on the intermediate clad layer, the first core, and the second core. An optical module, wherein: The optical module according to claim 4 or 5,
An optical module, wherein the upper surface of the upper cladding layer is flattened.