Spot Size Converter

Abstract

A spot size converter (100) capable of controlling the spot size in the vertical direction is obtained. The spot size converter (100) comprises a core layer (120) and a cladding layer (110). The cladding layer (110) surrounds the core layer (120). The core layer (120) includes a first core layer (121). The refractive index of the cladding layer (110) is lower than the refractive index of the first core layer (121). The core layer (120) includes a high refractive index region (131). The high refractive index region (131) has a refractive index higher than the refractive index of the first core layer (121).

Description

[Technical field]

[0001] The present disclosure relates to spot size converters. [Background technology]

[0002] Conventionally, a spot size converter used in a semiconductor laser is used to convert the spot size of light propagating through an optical waveguide. When waveguides with different spot sizes are joined together, the light coupling efficiency decreases. By arranging a spot size converter between the optical waveguides to be joined and adjusting the spot size, the light can be propagated efficiently. In order to improve the light coupling efficiency, the spot size can be adjusted in the vertical direction by changing the thickness of the core layer of the spot size converter. For example, the spot size converter disclosed in JP 2015-206969 A (Patent Document 1) includes a cladding layer and a core layer. [Prior art documents] [Patent documents]

[0003] [Patent Document 1] JP 2015-206969 A Summary of the Invention [Problem to be solved by the invention]

[0004] However, there is a problem that the manufacturing process becomes complicated when the core layer is thickened as described above in order to adjust the spot size in the vertical direction, so there is room for improvement in the spot size converter with respect to the adjustment of the spot size in the vertical direction.

[0005] The present disclosure has been made to solve the above-mentioned problems, and an object of the present disclosure is to provide a spot size converter capable of controlling the spot size in the vertical direction. [Means for solving the problem]

[0006] A spot size converter according to the present disclosure includes a core layer and a cladding layer. The cladding layer surrounds the core layer. The core layer includes a first core layer. The cladding layer has a refractive index lower than the refractive index of the first core layer. The core layer includes a high refractive index region. The high refractive index region has a refractive index higher than the refractive index of the first core layer. Effect of the Invention

[0007] Based on the above, it is possible to obtain a spot size converter capable of controlling the spot size in the vertical direction. [Brief description of the drawings]

[0008] [Figure 1] FIG. 1 is a perspective view of a spot size converter according to a first embodiment. [Diagram 2] 1 is a plan view of a spot size converter according to a first embodiment. [Diagram 3] 3 is a cross-sectional view of the spot size converter shown in FIG. 2 along line III-III. [Figure 4] 4 is a cross-sectional view of the spot size converter shown in FIG. 2 along line IV-IV. [Diagram 5] 4 is a flowchart of a method for manufacturing the spot size converter according to the first embodiment. [Figure 6] 3 is a cross-sectional view showing a step of a manufacturing method of the spot size converter according to the first embodiment. [Figure 7] 3 is a cross-sectional view showing a step of a manufacturing method of the spot size converter according to the first embodiment. [Figure 8] 3 is a cross-sectional view showing a step of a manufacturing method of the spot size converter according to the first embodiment. [Figure 9] 3 is a cross-sectional view showing a step of a manufacturing method of the spot size converter according to the first embodiment. [Figure 10] 3 is a cross-sectional view showing a step of a manufacturing method of the spot size converter according to the first embodiment. [Figure 11] 3 is a plan view showing a step of a manufacturing method of the spot size converter according to the first embodiment. FIG. [Figure 12] 3 is a cross-sectional view showing a step of a manufacturing method of the spot size converter according to the first embodiment. [Figure 13] 3 is a plan view showing a step of a manufacturing method of the spot size converter according to the first embodiment. FIG. [Figure 14] 14 is a cross-sectional view of the spot size converter shown in FIG. 13 along line XIV-XIV. [Figure 15] 3 is a plan view showing a step of a manufacturing method of the spot size converter according to the first embodiment. FIG. [Figure 16] 16 is a cross-sectional view of the spot size converter shown in FIG. 15 along line XVI-XVI. [Figure 17] 3 is a plan view showing a step of a manufacturing method of the spot size converter according to the first embodiment. FIG. [Figure 18] 18 is a cross-sectional view of the spot size converter shown in FIG. 17 along line XVIII-XVIII. [Figure 19] 3 is a plan view showing a step of a manufacturing method of the spot size converter according to the first embodiment. FIG. [Figure 20] 20 is a cross-sectional view of the spot size converter shown on line XX-XX in FIG. 19. [Figure 21] FIG. 1 is a perspective view of a spot size converter in accordance with Comparative Example 1. [Figure 22] FIG. 13 is a distribution diagram showing light intensity in the vertical direction. [Diagram 23] FIG. 11 is a perspective view of a spot size converter according to a second embodiment. [Figure 24] FIG. 11 is a plan view of a spot size converter according to a second embodiment. [Diagram 25] 25 is a cross-sectional view of the spot size converter shown by line XXV-XXV in FIG. 24. [Figure 26] 26 is a cross-sectional view of the spot size converter shown in FIG. 24 along line XXVI-XXVI. [Figure 27] 10 is a flowchart of a method for manufacturing a spot size converter according to the second embodiment. [Figure 28] 10 is a cross-sectional view showing a step of a manufacturing method of the spot size converter according to the second embodiment. [Figure 29] 10 is a cross-sectional view showing a step of a manufacturing method of the spot size converter according to the second embodiment. [Diagram 30] 10 is a cross-sectional view showing a step of a manufacturing method of the spot size converter according to the second embodiment. [Diagram 31] 10 is a cross-sectional view showing a step of a manufacturing method of the spot size converter according to the second embodiment. [Diagram 32] 10 is a cross-sectional view showing a step of a manufacturing method of the spot size converter according to the second embodiment. [Diagram 33] 11 is a plan view showing a step of a manufacturing method of the spot size converter according to the second embodiment. FIG. [Diagram 34] 11 is a plan view showing a step of a manufacturing method of the spot size converter according to the second embodiment. FIG. [Diagram 35] 35 is a cross-sectional view of the spot size converter shown in FIG. 34 along line XXXV-XXXV. [Diagram 36] 11 is a plan view showing a step of a manufacturing method of the spot size converter according to the second embodiment. FIG. [Figure 37] 37 is a cross-sectional view of the spot size converter shown in FIG. 36 along line XXXVII-XXXVII. [Figure 38] 10 is a cross-sectional view showing a step of a manufacturing method of the spot size converter according to the second embodiment. [Figure 39] 10 is a cross-sectional view showing a step of a manufacturing method of the spot size converter according to the second embodiment. [Diagram 40] 11 is a plan view showing a step of a manufacturing method of the spot size converter according to the second embodiment. FIG. [Diagram 41] 11 is a plan view showing a step of a manufacturing method of the spot size converter according to the second embodiment. FIG. [Diagram 42] 42 is a cross-sectional view of the spot size converter shown in FIG. 41 along line XLII-XLII. [Diagram 43] 43 is a cross-sectional view of the spot size converter shown in FIG. 41 along line XLIII-XLIII. [Diagram 44]11 is a plan view showing a step of a manufacturing method of the spot size converter according to the second embodiment. FIG. [Diagram 45] 45 is a cross-sectional view of the spot size converter shown in FIG. 44 along line XLV-XLV. [Diagram 46] 45 is a cross-sectional view of the spot size converter shown in FIG. 44 along line XLVI-XLVI. [Figure 47] 10 is a cross-sectional view showing a step of a manufacturing method of the spot size converter according to the second embodiment. [Figure 48] 11 is a plan view showing a step of a manufacturing method of the spot size converter according to the second embodiment. FIG. [Figure 49] 49 is a cross-sectional view of the spot size converter shown in FIG. 48 along line XLIX-XLIX. [Figure 50] 11 is a plan view showing a step of a manufacturing method of the spot size converter according to the second embodiment. FIG. [Figure 51] 51 is a cross-sectional view of the spot size converter shown along line LI-LI in FIG. 50. [Figure 52] 11 is a plan view showing a step of a manufacturing method of the spot size converter according to the second embodiment. FIG. [Diagram 53] 53 is a cross-sectional view of the spot size converter shown in FIG. 52 along line LIII-LIII. [Figure 54] FIG. 11 is a perspective view of a spot size converter according to Comparative Example 2. [Figure 55] FIG. 13 is a distribution diagram showing light intensity in the vertical direction. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0009] Hereinafter, embodiments of the present disclosure will be described. Note that unless otherwise specified, the same or corresponding parts in the following drawings are given the same reference numerals, and the description thereof will not be repeated.

[0010] Embodiment 1 <Configuration of spot size converter> Fig. 1 is a perspective view of spot size converter 100 according to the first embodiment. Fig. 2 is a plan view of spot size converter 100 according to the first embodiment. Fig. 3 is a cross-sectional view of spot size converter 100 shown along line III-III in Fig. 2. Fig. 4 is a cross-sectional view of spot size converter 100 shown along line IV-IV in Fig. 2. Note that in Figs. 1 and 2, the boundary between high refractive index region 131 and core layer 120 is indicated by a dotted line.

[0011] The spot size converter 100 shown in FIG. 1 to FIG. 4 is used in, for example, a semiconductor laser, and mainly includes a substrate 101, a cladding layer 110, and a core layer 120. The core layer 120 that propagates light includes a first core layer 121. As described later, the refractive index n1a of the first core layer 121 is higher than the refractive index n2a of the cladding layer 110. The cladding layer 110 and the core layer 120 function as an optical waveguide. The cladding layer 110 is connected onto the substrate 101. The cladding layer 110 surrounds the core layer 120. The core layer 120 propagates light. As shown in FIG. 1, the direction in which the core layer 120 is arranged as viewed from the substrate 101 is defined as a first direction X. The direction in which light propagates in the core layer 120 is defined as a second direction Y. The second direction Y is perpendicular to the first direction X. A direction perpendicular to the first direction X and the second direction Y is defined as a third direction Z.

[0012] As shown in FIG. 1, the shape of the core layer 120 is a quadrangular prism extending in the second direction Y. That is, the cross-sectional shape of the core layer 120 in a cross section perpendicular to the second direction Y is a quadrangle. The core layer 120 has a first end face 121s1, a second end face 121s2, and a side face 121s3. The first end face 121s1 is, for example, an emission face. The emission face is an end face from which light can be emitted. The second end face 121s2 is, for example, an incidence face. The incidence face is an end face into which light can be incident. The first end face 121s1 is a face on the opposite side of the second end face 121s2. The side face 121s3 is a face that connects the second end face 121s2 and the first end face 121s1. That is, the side face 121s3 is a face other than the first end face 121s1 and the second end face 121s2.

[0013] The cladding layer 110 surrounds the core layer 120. The cladding layer 110 is connected to the core layer 120 only at the side surface 121s3. In other words, the cladding layer 110 is not connected to the second end surface 121s2 and the first end surface 121s1. Specifically, the cladding layer 110 includes a first upper cladding layer 113, a first lower cladding layer 111, and a pair of first side cladding layers 112. The first lower cladding layer 111 is connected to the substrate 101, the core layer 120, and the first side cladding layer 112. The first lower cladding layer 111 is disposed so as to be sandwiched between the core layer 120 and the substrate 101 in the first direction X.

[0014] The pair of first side cladding layers 112 are disposed apart from each other in the third direction Z. The pair of first side cladding layers 112 are disposed to sandwich the core layer 120 and the first upper cladding layer 113. The shape of the first side cladding layer 112 as viewed from the second direction Y is L-shaped. Specifically, the first side cladding layer 112 has a protruding portion 112a. The protruding portion 112a is connected to the first lower cladding layer 111. When viewed from the surface where the first side cladding layer 112 is connected to the core layer 120, the protruding portion 112a is disposed on the opposite side to the region where the core layer 120 is disposed.

[0015] The first upper cladding layer 113 is connected to the upper surface of the core layer 120. That is, in the first direction X, the core layer 120 is sandwiched between the first lower cladding layer 111 and the first upper cladding layer 113. In the third direction Z, the core layer 120 is sandwiched between a pair of first side cladding layers 112. In this manner, the cladding layer 110 surrounds the core layer 120. Note that the width of the first upper cladding layer 113 in the third direction Z is the same as the width of the core layer 120 in the third direction Z. The width of the first lower cladding layer 111 in the third direction Z is larger than the width of the core layer 120 in the third direction Z. In this manner, as shown in FIG. 1 and FIG. 3, the spot size converter 100 according to the first embodiment has a striped mesa structure.

[0016] Here, the refractive index of the core layer 120 is n1a. Specifically, the refractive index of the first core layer 121 is n1a. The refractive index of the cladding layer 110 is n2a. The refractive index n1a of the core layer 120 is greater than the refractive index n2a of the cladding layer 110. Note that, when the cladding layer 110 is composed of a plurality of members, the refractive index n2a of the cladding layer 110 corresponds to the largest refractive index among the refractive indexes of the plurality of members. When light travels from a medium with a high refractive index to a medium with a low refractive index, if the angle of incidence of the light to the medium with a low refractive index exceeds the critical angle, the light does not travel to the medium with a low refractive index. In other words, if the angle of incidence of the light to the medium with a low refractive index exceeds the critical angle, total reflection occurs in which all of the light is reflected to the medium with a high refractive index. In this way, light can be propagated by surrounding the core layer 120 with the cladding layer 110.

[0017] Here, problems with the conventional spot size converter 100 will be described. The second end face 121s2 is connected to another optical device (for example, a semiconductor modulator, a silicon optical waveguide, an optical fiber, etc.). When optical waveguides with different spot sizes are coupled to each other, the optical coupling efficiency decreases. In particular, when the spot size is small, it is necessary to mount the optical device with high precision. In other words, when the spot size is small, high mounting technology is required, and as a result, high manufacturing costs are required. Therefore, it is preferable that the spot size is large. The spot size converter 100 adjusts the spot size of the light incident from the second end face 121s2 so that the spot size of the light is matched to the spot size of the optical device to which it is coupled. Specifically, the spot size converter 100 can increase the spot size of the light emitted from the first end face 121s1.

[0018] The spot size is determined by the thickness and width of the core layer 120. That is, the spot size is adjusted by changing the thickness and width of the core layer 120. Alternatively, the light can be made to seep into the cladding layer 110 by reducing the difference between the refractive index of the core layer 120 and the refractive index of the cladding layer 110. The width of the core layer 120 in the third direction Z can be adjusted relatively easily by general lithography and dry processing techniques.

[0019] On the other hand, when the thickness of the core layer 120 in the first direction X is increased, there is a problem that the manufacturing process becomes complicated. Specifically, when a manufacturing process for a semiconductor device is applied to the manufacturing process for the spot size converter 100, the crystal film to become the core layer 120 is thickened by epitaxial growth. Then, dry etching is performed by RIE (Reactive Ion Etching) or the like. The depth of the dry etching needs to be controlled with high precision. In order to control the depth of the dry etching with high precision, it is necessary to form an ESL (Etch-Stop Layer) in advance in the optical device. Therefore, when the thickness of the core layer 120 in the first direction X is increased, the manufacturing process becomes complicated.

[0020] The spot size converter 100 is not used alone. The spot size converter 100 is used by being monolithically integrated with an optical device such as a semiconductor laser. Therefore, when the optical device to be coupled is a semiconductor laser, if the core layer 120 is made thicker, the upper cladding layer of the optical device to be coupled must be made thicker. If the upper cladding layer is made thicker, the element resistance of the semiconductor laser increases. As a result, the characteristics of the semiconductor laser deteriorate. It is also possible to thicken only the cladding layer 110 of the spot size converter 100 by selective growth technology. However, in this case, advanced selective growth technology is required. In other words, thickening the core layer 120 may cause restrictions in terms of characteristics. For the above reasons, there is room for improvement in the spot size converter 100 with respect to adjusting the spot size in the vertical direction.

[0021] Here, the feature of the spot size converter 100 according to the present embodiment is that the core layer 120 includes a high refractive index region 131 as shown in FIG. 1 to FIG. 4. The refractive index of the high refractive index region 131 is n3a. The refractive index n3a is larger than the refractive index n1a of the first core layer 121. In the first direction X, the high refractive index region 131 may be disposed closer to the first upper cladding layer 113 than the center of the thickness of the core layer 120 in the first direction X, but is preferably disposed closer to the substrate 101. In this way, the light propagating in the core layer 120 spreads in the direction in which the high refractive index region 131 is disposed. In particular, by disposing the high refractive index region 131 on the substrate 101 side, the light spreads to the substrate 101 on the first lower cladding layer 111 side shown in FIG. 1. As a result, the spot size can be enlarged along the first direction X while maintaining the performance of the optical device without thickening the cladding layer of the optical device to be coupled.

[0022] As shown in FIG. 2, in a plan view seen from the first direction X, the shape of the high refractive index region 131 may be, for example, triangular. The width of the high refractive index region 131 in the third direction Z at the first end face 121s1 is the same as the width of the core layer 120 in the third direction Z. The width of the high refractive index region 131 in the third direction Z becomes narrower as it approaches the second end face 121s2. From a different perspective, the high refractive index region 131 includes a first high refractive index region 131a and a second high refractive index region 131b. In the second direction Y, the first high refractive index region 131a is disposed at a position farther from the second end face 121s2 than the second high refractive index region 131b. The width of the second high refractive index region 131b in the third direction Z is smaller than the width of the first high refractive index region 131a in the third direction Z. That is, in a plan view, the high refractive index region 131 has a tip portion 131t where two side surfaces of the high refractive index region 131 meet at an acute angle. In a plan view, the tip portion 131t of the high refractive index region 131 is a portion of the high refractive index region 131 that is disposed closest to the second end face 121s2. The base of the high refractive index region 131 is disposed on the first end face 121s1. In this manner, the width of the high refractive index region 131 in the third direction Z increases from the second end face 121s2 toward the first end face 121s1, so that the spot size adiabatically expands along the light propagation direction.

[0023] In addition, in a plan view, the shape of the high refractive index region 131 does not necessarily have to be a triangle. In a plan view, the shape of the high refractive index region 131 may adopt any shape. For example, in a plan view, the periphery of the high refractive index region 131 may be configured with a curve. Specifically, the sides connecting both ends of the high refractive index region 131 to the tip portion 131t in the first end face 121s1 may be curved. For example, in a plan view, the shape of the high refractive index region 131 may be a rectangle. Specifically, the width of the high refractive index region 131 in the third direction Z may be uniformly the same as the width of the core layer 120 from the first end face 121s1 to the second end face 121s2. In this way, in a plan view, the shape of the high refractive index region 131 may be a polygon.

[0024] <Method of manufacturing spot size converter> Fig. 5 is a flowchart of a method for manufacturing spot size converter 100 according to the first embodiment. As shown in Figs. 5 and 6, the method for manufacturing spot size converter 100 according to the present embodiment first performs a step (S1a) of preparing substrate 101. Substrate 101 may be, for example, an InP (indium phosphide) substrate 101. The conductivity type of substrate 101 may be p-type. The conductivity type of substrate 101 may be n-type. Substrate 101 may be a semi-insulating substrate.

[0025] Next, a step (S2a) of forming the cladding layer 110 is performed. In this step (S2a), as shown in FIG. 7, a first lower cladding layer 111 is formed on the substrate 101. The first lower cladding layer 111 is formed by a crystal growth method such as metal organic chemical vapor deposition (MOCVD). The first lower cladding layer 111 is made of a material having a refractive index n2a lower than a refractive index n1a of the core layer 120 described below. The material constituting the first lower cladding layer 111 may be any material that is lattice-matched with the core layer 120, and may be, for example, InP or InGaAsP.

[0026] As described later, the light propagating in the core layer 120 may seep into the first lower cladding layer 111. By making the difference between the refractive index of the first lower cladding layer 111 and the refractive index n1a of the core layer 120 smaller than the difference between the refractive index of the first upper cladding layer 113 or the first side cladding layer 112 and the refractive index n1a of the core layer 120, the light propagating in the core layer 120 can seep into the first lower cladding layer 111.

[0027] Next, a step (S3a) of forming the core layer 120 is performed. In this step (S3a), as shown in FIG. 8, the core layer 120 (first core layer 121) is formed on the first lower cladding layer 111. The core layer 120 may be formed by using metal organic chemical vapor deposition, similar to the method of forming the first lower cladding layer 111. The material constituting the core layer 120 may be, for example, InP. From the viewpoint of light absorption rate, the core layer 120 composed of InP is preferably undoped. The core layer 120 may be semi-insulating. The material of the core layer 120 may be InGaAsP, AlGaInAs, or InGaAs. The core layer 120 may be n-type InP or p-type InP. The core layer 120 may be composed of only a single material. The core layer 120 may be composed of a plurality of materials each having a different refractive index.

[0028] Next, a step (S4a) of forming the high refractive index region 131 is performed. In this step (S4a), as shown in FIG. 9, the high refractive index region 131 is formed on the core layer 120. The high refractive index region 131 may be formed by using metal organic chemical vapor deposition, similar to the method of forming the first lower cladding layer 111. The material constituting the high refractive index region 131 is a material having a refractive index n3a higher than the refractive index n1a of the core layer 120. When the material constituting the core layer 120 is undoped InP, the material constituting the high refractive index region 131 may be, for example, InGaAsP, AlGaInAs, or InGaAs. When the material constituting the core layer 120 is n-type InP, the material constituting the high refractive index region 131 may be, for example, undoped InP, semi-insulating InP, or p-type InP. In this way, the plasma effect can be utilized. When core layer 120 is composed of a plurality of materials each having a different refractive index, it is necessary to select the materials so that the refractive index n3a of high refractive index region 131 is higher than the effective refractive index n1a of core layer 120. In this manner, first lower cladding layer 111, core layer 120, and high refractive index region 131 may be formed in this order on substrate 101 by metal organic chemical vapor deposition.

[0029] Next, an etching step (S5a) is performed. In this step (S5a), as shown in FIG. 10, a first processing mask 141 is formed on the high refractive index region 131. The first processing mask 141 is, for example, either a SiO2 film or a SiN film. The SiO2 film or the SiN film is formed, for example, by chemical vapor deposition (CVD). The first processing mask 141 may be patterned into a desired shape using photolithography and etching techniques. As shown in FIG. 11, the shape of the first processing mask 141 may be triangular in plan view. In plan view, the shape of the first processing mask 141 does not necessarily have to be triangular. For example, in plan view, the sides connecting both ends of the first processing mask 141 on the first end face 121s1 to the tip portion on the second end face 121s2 side may be curved. For example, in a plan view, the shape of the first processing mask 141 may be a rectangle. Specifically, the width of the first processing mask 141 in the third direction Z may be the same from the first end face 121s1 to the second end face 121s2, and may be the same as the width of the core layer 120, for example.

[0030] Next, the high refractive index region 131 is dry-etched using the first processing mask 141 as a mask. Specifically, the high refractive index region 131 in the region where the first processing mask 141 is arranged in a plan view is not etched. On the other hand, the high refractive index region 131 in the region where the first processing mask 141 is not arranged in a plan view is etched. As a result, as shown in FIG. 12, a part of the high refractive index region 131 is etched so that a part of the core layer 120 is exposed. At this time, a part of the core layer 120 may be etched in the first direction X. The etching may be wet etching. After etching the high refractive index region 131, the first processing mask 141 is removed by wet etching. In this manner, the high refractive index region 131 having a desired shape is formed.

[0031] Next, a step (S6a) of forming core layer 120 is performed. In this step (S6a), as shown in Figures 13 and 14, core layer 120 is formed on high refractive index region 131 and on exposed parts of core layer 120. Figure 14 is a cross-sectional view of spot size converter 100 shown along line segment XIV-XIV in Figure 13. Similar to the method of forming first lower cladding layer 111, core layer 120 may be formed using metal-organic chemical vapor deposition.

[0032] The position of the high refractive index region 131 in the first direction X within the core layer 120 is determined by the thickness of the core layer 120 in the first direction X in the step (S3a) of forming the core layer 120 and the thickness of the core layer 120 in the first direction X in the step (S6a) of forming the core layer 120.

[0033] For example, in the step (S3a) of forming core layer 120, core layer 120 is formed to a thickness of 1 μm in first direction X. In the step (S4a) of forming high refractive index region 131, high refractive index region 131 is formed to a thickness of 100 nm in first direction X. In the step (S6a) of forming core layer 120, core layer 120 is formed to a thickness of 1 μm in first direction X. In this case, high refractive index region 131 is disposed at the center of the thickness of core layer 120 in first direction X.

[0034] For example, in the step (S3a) of forming core layer 120, core layer 120 is formed to a thickness of 500 nm in first direction X. In the step (S4a) of forming high refractive index region 131, high refractive index region 131 is formed to a thickness of 100 nm in first direction X. In the step (S6a) of forming core layer 120, core layer 120 is formed to a thickness of 1500 nm in first direction X. In this case, in the first direction X, high refractive index region 131 is positioned closer to substrate 101 than the center of the thickness of core layer 120 in first direction X.

[0035] For example, the step (S3a) of forming the core layer 120 may not be performed. In other words, the step (S4a) of forming the high refractive index region 131 may be performed after the step (S2a) of forming the cladding layer 110. In the step (S4a) of forming the high refractive index region 131, the high refractive index region 131 is formed to a thickness of 100 nm in the first direction X. In the step (S6a) of forming the core layer 120, the core layer 120 is formed to a thickness of 2 μm in the first direction X. In this case, the high refractive index region 131 is disposed at a position in the core layer 120 closest to the first lower cladding layer 111 in the first direction X.

[0036] In this manner, the position of the high refractive index region 131 in the first direction X within the core layer 120 can be easily controlled.

[0037] Next, an etching step (S7a) is performed. In this step (S7a), as shown in FIG. 15 and FIG. 16, a second processing mask 142 is formed on the core layer 120. FIG. 16 is a cross-sectional view of the spot size converter 100 shown on the line segment XVI-XVI in FIG. 15. The second processing mask 142 is, for example, either a SiO2 film or a SiN film. The method for forming the SiO2 film or the SiN film is, for example, a chemical vapor deposition method. The second processing mask 142 may be patterned into a desired shape using a photolithography technique and an etching technique. As shown in FIG. 15, the shape of the second processing mask 142 is a rectangle in a plan view. The second processing mask 142 is formed so as to extend from the first end face 121s1 to the second end face 121s2. The second processing mask 142 is formed at the center of the width of the core layer 120 in the third direction Z so as to expose the edge of the core layer 120. In other words, the width of the second processing mask 142 in the third direction Z is uniformly smaller than the width of the core layer 120.

[0038] Next, the core layer 120 is dry-etched using the second processing mask 142 as a mask. Specifically, the core layer 120 in the region where the second processing mask 142 is arranged in a plan view is not etched. On the other hand, the core layer 120 in the region where the second processing mask 142 is not arranged in a plan view is etched. As a result, only the core layer 120 located on both sides of the second processing mask 142 in the third direction Z is etched. At this time, as shown in FIG. 17 and FIG. 18, the core layer 120 may be etched in the first direction X so that a part of the first lower clad layer 111 is exposed. Also, it is sufficient that the core layer 120 is etched in the first direction X to a position lower than the high refractive index region 131. Therefore, for example, the etching may reach the substrate 101. The etching may be wet etching, but dry etching is preferable from the viewpoint of controlling the shape of the core layer 120. After etching the core layer 120, the second processing mask 142 is removed by wet etching. In this manner, an optical waveguide having a mesa structure 151 of a desired shape is formed, as shown in FIG.

[0039] Next, a step (S8a) of forming a cladding layer 110 is performed. In this step (S8a), as shown in FIG. 19 and FIG. 20, the cladding layer 110 is formed on the core layer 120 and on the exposed part of the first lower cladding layer 111. FIG. 20 is a cross-sectional view of the spot size converter 100 shown on the line segment XX-XX in FIG. 19. Specifically, as shown in FIG. 20, a first upper cladding layer 113 is formed on the core layer 120. A pair of first side cladding layers 112 are formed on the first lower cladding layer 111 so as to sandwich the core layer 120. The material constituting the first upper cladding layer 113 and the first side cladding layer 112 may be a material having a refractive index n2a lower than the refractive index n1a of the core layer 120. When the material constituting the core layer 120 is InP, the material constituting the first upper cladding layer 113 and the first side cladding layer 112 may be, for example, at least one of a SiO2 film and a SiN film. The method for forming the SiO2 film or the SiN film is, for example, chemical vapor deposition. When the material constituting the core layer 120 is, for example, at least one of InGaAsP and AlGaInAs, the material constituting the first upper cladding layer 113 and the first side cladding layer 112 may be InP. The method for forming InP is, for example, metalorganic vapor phase epitaxy.

[0040] It is sufficient that the core layer 120 is in contact with a medium having a refractive index lower than the refractive index n1a of the core layer 120. Therefore, at least one of the first upper cladding layer 113 and the first side cladding layer 112 may be air. In other words, the side surface 121s3 of the core layer 120 may be exposed. In this case, after the etching step (S7a) is performed, a SiO2 film may be formed as a protective film for the mesa structure 151, but nothing may be formed.

[0041] The refractive index n2a of the first upper cladding layer 113 and the first side cladding layer 112 may be smaller than the refractive index of the first lower cladding layer 111. This allows the light propagating through the core layer 120 to seep into the substrate 101 without seeping into the first upper cladding layer 113 and the first side cladding layer 112. In this manner, the material constituting the first upper cladding layer 113 and the first side cladding layer 112 may be different from the material constituting the first lower cladding layer 111.

[0042] In this way, a spot size converter 100 with vertically controllable spot size can be obtained, as shown in FIGS.

[0043] <Operation> Next, the operation of the spot size converter 100 will be described.

[0044] The spot size converter 100 is connected to an optical device such as a light source or a modulator. For example, assume that the spot size converter 100 is connected to a semiconductor laser, which is a light source. The spot size converter 100 is connected to the semiconductor laser at the second end face 121s2. Light propagates from the semiconductor laser. The light enters the core layer 120. As described above, the core layer 120 is surrounded by the cladding layer 110, which has a refractive index lower than that of the core layer 120. Therefore, the light is confined in the core layer 120. The light propagates in the core layer 120 along the second direction Y. The light spreads in the direction in which the high refractive index region 131 is arranged. In particular, since the high refractive index region 131 is arranged on the substrate 101 side, the light spreads in the vertical direction (the first direction X shown in FIG. 1). The light is output from the first end face 121s1. In this manner, the light is output to the outside of the semiconductor device including the spot size converter 100.

[0045] <Action and effect> A spot size converter 100 according to the present disclosure comprises a core layer 120 and a cladding layer 110. The cladding layer 110 surrounds the core layer 120. The core layer 120 includes a first core layer 121. The refractive index of the cladding layer 110 is lower than the refractive index of the first core layer 121. The core layer 120 includes a high refractive index region 131. The high refractive index region 131 has a higher refractive index than the refractive index of the first core layer 121.

[0046] In this way, the light propagating in the core layer 120 spreads in the direction in which the high refractive index regions 131 are arranged. In particular, by appropriately changing the locations in the first direction X where the high refractive index regions 131 are arranged, the spot size can be adjusted in the vertical direction (the first direction X shown in FIG. 1).

[0047] The spot size converter 100 further includes a substrate 101. The substrate 101 is connected to the cladding layer 110. A direction in which the core layer 120 is viewed from the substrate 101 is defined as a first direction X. The high refractive index region 131 is disposed closer to the substrate 101 than the center of the thickness of the core layer 120 in the first direction X. In this manner, light spreads in the vertical direction. The light spread in the vertical direction can seep out toward the substrate 101.

[0048] In the spot size converter 100, the high refractive index region 131 includes a first high refractive index region 131a and a second high refractive index region 131b. In the second direction Y, the first high refractive index region 131a is farther from the second end face 121s2 than the second high refractive index region 131b. The width of the second high refractive index region 131b in a direction perpendicular to the second direction Y (third direction Z) is smaller than the width of the first high refractive index region 131a. In this way, the spot size is expanded adiabatically along the light propagation direction. As a result, the spot size is expanded efficiently.

[0049] FIG. 21 is a perspective view of the spot size converter 300 according to the comparative example 1. FIG. 21 corresponds to FIG. 1. The spot size converter 300 shown in FIG. 21 basically has the same configuration as the spot size converter 300 shown in FIG. 1 to FIG. 4, but is different in that the core layer 120 does not include the high refractive index region 131. FIG. 22 is a distribution diagram showing the light intensity in the vertical direction at the first end face 121s1. The distribution diagram shows the light intensity distribution in a near field pattern (NFP). The solid line in FIG. 22 shows the light intensity distribution in the example 1. The example 1 basically has the same configuration as the spot size converter 100 according to the first embodiment. In FIG. 22, the dotted line shows the light intensity distribution in the comparative example 1. In FIG. 22, the horizontal axis shows the height of the core layer in the vertical direction, specifically, the distance (unit: μm) from the first lower cladding layer 111. In FIG. 22, the vertical axis shows the relative light intensity (unit: -) in the vertical direction. In addition, the horizontal axis of Fig. 22 uses the surface where the first lower cladding layer 111 is connected to the core layer 120 as the reference (=0 µm). The direction in which the first upper cladding layer 113 is located as viewed from the first lower cladding layer 111 is the positive direction on the horizontal axis. On the other hand, the direction in which the substrate 101 is located as viewed from the first lower cladding layer 111 is the negative direction on the horizontal axis. The distribution diagram shown in Fig. 22 is normalized by the maximum value of the light intensity in the first direction X (vertical direction).

[0050] As can be seen from FIG. 22, there are two distances from the first lower cladding layer 111 where the light intensity is 0.5. The difference between the distances from the first lower cladding layer 111 at these two points is the half-width. The half-width in Comparative Example 1 is 1.87 μm. The half-width in Example 1 is 2.13 μm. In other words, the half-width in Example 1 is larger than the half-width in Comparative Example 1. From the above results, it can be seen that the core layer 120 includes the high refractive index region 131, thereby increasing the spot size in the first direction X.

[0051] Embodiment 2 <Structure of Spot Size Converter> FIG. 23 is a perspective view of spot size converter 200 according to the second embodiment. FIG. 23 corresponds to FIG. 1. FIG. 24 is a plan view of spot size converter 200 according to the second embodiment. FIG. 24 corresponds to FIG. 2. FIG. 25 is a cross-sectional view of spot size converter 200 shown along line segment XXV-XXV in FIG. 24. FIG. 25 corresponds to FIG. 3. FIG. 26 is a cross-sectional view of spot size converter 200 shown along line segment XXVI-XXVI in FIG. 24. FIG. 26 corresponds to FIG. 4. Note that in FIGS. 23 and 24, the boundary between high refractive index region 231 and second core layer 241 is indicated by a dotted line.

[0052] The spot size converter 200 shown in FIGS. 23 to 26 basically has the same configuration as the spot size converter 100 shown in FIGS. 1 to 4, but is different in that the core layer 220 includes a region having a higher refractive index than the other regions of the core layer 220, in addition to the high refractive index region 231. Specifically, the core layer 220 includes a first core layer 221 and a second core layer 241. The first core layer 221 corresponds to the core layer 220 of the first embodiment. The refractive index n2b of the second core layer 241 is higher than the refractive index n1b of the first core layer 221. In this way, the light propagates through the second core layer 241 with low loss.

[0053] In particular, it is more preferable that the refractive index n2b of second core layer 241 is higher than the refractive index n3b of high refractive index region 231. The refractive index of first core layer 221 is n1b. The refractive index of second core layer 241 is n2b. The refractive index of high refractive index region 231 is n3b. The refractive index of cladding layer 250 is n4b. The spot size converter 200 satisfies the relationship n2b>n3b>n1b>n4b. In this way, light can be efficiently propagated from second core layer 241 to first core layer 221.

[0054] For ease of explanation, as shown in FIG. 24, a region in the second direction Y where the high refractive index region 231 is not arranged in a plan view is defined as a first region 281. Specifically, the first region 281 is a region in the second direction Y from the first end face 221s1 to the tip portion 231t of the high refractive index region 231. On the other hand, a region in the second direction Y where the high refractive index region 231 is arranged is defined as a second region 282. Specifically, the second region 282 is a region in the second direction Y from the first end face 221s1 to the tip portion 231t of the high refractive index region 231.

[0055] As shown in FIG. 23 and FIG. 24, the second core layer 241 is disposed at the center of the core layer 220 in the third direction Z. The second core layer 241 also extends in the second direction Y from the second end face 221s2 to a region overlapping with the high refractive index region 231. Specifically, as shown in FIG. 24, in a plan view, the second core layer 241 extends from the second end face 221s2 to the second region 282. In this way, a part of the second core layer 241 is disposed in the region in the second direction Y where the high refractive index region 231 is disposed. In a plan view, the shape of the second core layer 241 in the first region 281 is a rectangle. Specifically, in the first region 281, the width of the second core layer 241 in the third direction Z is uniform. In the first region 281, the width of the second core layer 241 in the third direction Z is smaller than the width of the first core layer 221 in the third direction Z in the second region 282.

[0056] In a plan view, the shape of the second core layer 241 in the second region 282 is a triangle. In the second region 282, the width of the second core layer 241 in the third direction Z gradually decreases toward the first end face 221s1. Specifically, the second core layer 241 includes a first core region 241a and a second core region 241b. In the second direction Y, the second core region 241b is farther from the second end face 221s2 than the first core region 241a. In the third direction Z, the width of the second core region 241b is smaller than the width of the first core region 241a. In the second region 282, the second core layer 241 has a tip portion 241t. In the second core layer 241, the tip portion 241t is disposed at a position closest to the first end face 221s1 in the second direction Y. In the second region 282, the base of the second core layer 241 is disposed at the boundary 283 between the first region 281 and the second region 282. The sides connecting both ends of the base of the second core layer 241 to the tip portion 241t are straight lines. Thus, in a plan view, the shape of the second core layer 241 is a pentagon that combines a rectangle and a triangle.

[0057] By doing this, as the light propagating through the second core layer 241 approaches the first end face 221s1, the second core layer 241 is no longer able to confine the light. The light gradually seeps out from the second core layer 241 into the first core layer 221. When the width of the second core layer 241 in the third direction Z becomes sufficiently small, all of the light transfers from the second core layer 241 to the first core layer 221.

[0058] In addition, in a plan view, the shape of the second core layer 241 does not necessarily have to be a pentagon. In a plan view, the shape of the second core layer 241 may be any shape. The shape of the second core layer 241 may be a polygon such as a triangle or a rectangle. For example, in a plan view, the sides connecting both ends of the base of the second core layer 241 and the tip portion 241t may be curved.

[0059] 25, the high refractive index region 231 may be disposed at a position closer to the substrate 201 than the second core layer 241 in the first direction X. Specifically, in the first direction X, the second core layer 241 may be disposed at a position sandwiched between the high refractive index region 231 and the first upper cladding layer 251. In this way, the light leaking out from the second core layer 241 spreads in the vertical direction (the first direction X shown in FIG. 25).

[0060] In plan view, the high refractive index region 231 may have any shape. The shape of the high refractive index region 231 in the spot size converter 200 according to the second embodiment is a shape combining a rectangle and two triangles, as shown in FIG. 24. Specifically, the high refractive index region 231 extends from the first end face 221s1 to the boundary 283 between the first region 281 and the second region 282. In plan view, the rectangular portion of the high refractive index region 231 is disposed on the first end face 221s1 side in the second direction Y. In the second direction Y, the rectangular portion of the high refractive index region 231 is disposed between the first end face 221s1 and the triangular portion of the high refractive index region 231. The width of the rectangular portion of the high refractive index region 231 in the third direction Z is uniform from the first end face 221s1 to the region where the triangular portion of the high refractive index region 231 is disposed. The width in the third direction Z of the quadrangular portion of the high refractive index region 231 matches the width in the third direction Z of the first core layer 221 .

[0061] The triangular portion of the high refractive index region 231 is disposed between the boundary 283 between the first region 281 and the second region 282 and the rectangular portion of the high refractive index region 231. The width of the triangular portion of the high refractive index region 231 in the third direction Z gradually decreases from the rectangular portion of the high refractive index region 231 toward the second end face 221s2. Specifically, the triangular portion of the high refractive index region 231 includes a first high refractive index region 231a and a second high refractive index region 231b. In the second direction Y, the first high refractive index region 231a is farther from the second end face 221s2 than the second high refractive index region 231b. The width of the second high refractive index region 231b in the third direction Z is smaller than the width of the first high refractive index region 231a. As shown in FIG. 24, when two triangular portions in the high refractive index region 231 are arranged in the third direction Z, the width of the high refractive index region 231 is the sum of the widths of the two triangular portions in the third direction Z. The triangular portion in the high refractive index region 231 has a tip portion 231t. The tip portion 231t is disposed at the boundary 283 between the first region 281 and the second region 282. The triangular portion in the high refractive index region 231 and the square portion in the high refractive index region 231 are connected at the base of the triangular portion in the high refractive index region 231. The sides connecting both ends of the base of the triangular portion in the high refractive index region 231 and the tip portion 231t are straight lines. The sides connecting both ends of the base of the triangular portion in the high refractive index region 231 and the tip portion 231t may be curved lines.

[0062] As shown in FIG. 24, in a plan view, the tip portion 241t of the second core layer 241 is disposed at the same position as a point p1 where two triangular portions in the high refractive index region 231 are connected. Specifically, the two triangular portions in the high refractive index region 231 are separated from each other by the same distance from the center of the width of the first core layer 221 in the third direction Z. That is, the point p1 is disposed at the center of the width of the first core layer 221 in the third direction Z. The tip portion 241t of the second core layer 241 is disposed at the center of the width of the first core layer 221 in the third direction Z in the third direction Z. In the second direction Y, the tip portion 241t of the second core layer 241 is disposed at a position where the base of the triangular portion in the high refractive index region 231 is disposed.

[0063] The function of the core layer 220 in the first region 281 is different from the function of the core layer 220 in the second region 282. The refractive index n2b of the second core layer 241 is higher than the refractive index n1b of the first core layer 221. Therefore, in the first region 281, the first core layer 221 connected to the second core layer 241 functions as a cladding layer for the second core layer 241.

[0064] <Method of manufacturing spot size converter> FIG. 27 is a flowchart of a method for manufacturing spot size converter 200 according to the second embodiment. FIG. 28 to FIG. 53 are cross-sectional views or plan views for explaining a method for manufacturing spot size converter 200 according to the second embodiment shown in FIG. 27. As shown in FIG. 27 and FIG. 28, in the method for manufacturing spot size converter 200 according to the second embodiment, first, a step (S1b) of preparing substrate 201 is performed. Substrate 201 may be, for example, InP (indium phosphide) substrate 201. The conductivity type of substrate 201 may be p-type. The conductivity type of substrate 201 may be n-type. Substrate 201 may be a semi-insulating substrate.

[0065] Next, a step (S2b) of forming a cladding layer 250 is performed. In this step (S2b), as shown in FIG. 29, a first lower cladding layer 253 is formed on the substrate 201. The first lower cladding layer 253 is formed by a crystal growth method such as metal-organic chemical vapor deposition. The material constituting the first lower cladding layer 253 is a material having a refractive index lower than the refractive index n1b of the first core layer 221, as described later. The material constituting the first lower cladding layer 253 may be any material that is lattice-matched with the core layer 220, and may be, for example, InP or InGaAsP.

[0066] As described later, the light propagating in the first core layer 221 may seep into the first lower clad layer 253. In this case, by making the difference between the refractive index of the first lower clad layer 253 and the refractive index n1b of the first core layer 221 smaller than the difference between the refractive index of the first upper clad layer 251 or the first side clad layer 252 and the refractive index n1b of the core layer 220, the light propagating in the core layer 220 can seep into the first lower clad layer 221.

[0067] Next, a step (S3b) of forming the core layer 220 is performed. In this step (S3b), as shown in FIG. 30, the first core layer 221 constituting the core layer 220 is formed on the first lower cladding layer 253. The first core layer 221 may be formed by using metal organic chemical vapor deposition, similar to the method of forming the first lower cladding layer 253. The material constituting the first core layer 221 may be, for example, InP. From the viewpoint of light absorption, the first core layer 221 made of InP is preferably undoped. The first core layer 221 may be semi-insulating. The material of the first core layer 221 may be InGaAsP, AlGaInAs, or InGaAs. The material constituting the first core layer 221 may be n-type InP or p-type InP. The first core layer 221 may be made of only a single material. The first core layer 221 may be made of a plurality of materials each having a different refractive index. The first lower cladding layer 253 and the first core layer 221 may be made of the same material. That is, the first lower cladding layer 253 and the first core layer 221 may be formed simultaneously by metal organic chemical vapor deposition. However, the first core layer 221 in the first region 281 has a different function from the first core layer 221 in the second region 282. That is, the first core layer 221 in the first region 281 functions as a cladding layer for the second core layer 241 (see FIG. 23). The first core layer 221 in the second region 282 functions as a core layer that propagates light that seeps out from the second core layer 241.

[0068] Next, a step (S4b) of forming the high refractive index region 231 is performed. In this step (S4b), as shown in FIG. 31, the high refractive index region 231 is formed on the first core layer 221. The high refractive index region 231 may be formed by using a metal organic chemical vapor deposition method, similar to the method of forming the first lower cladding layer 253. The material constituting the high refractive index region 231 is a material having a refractive index n3b higher than the refractive index n1b of the first core layer 221. When the material constituting the first core layer 221 is undoped InP, the material constituting the high refractive index region 231 may be, for example, InGaAsP, AlGaInAs, or InGaAs. When the material constituting the first core layer 221 is n-type InP, the material constituting the high refractive index region 231 may be, for example, undoped InP, semi-insulating InP, or p-type InP. In this way, the plasma effect can be utilized. When first core layer 221 is composed of a plurality of materials each having a different refractive index, it is necessary to select the materials so that the refractive index n3b of high refractive index region 231 is higher than the effective refractive index n1a of first core layer 221. In this manner, first lower cladding layer 253, first core layer 221, and high refractive index region 231 may be formed in this order on substrate 201 by metal organic chemical vapor deposition.

[0069] Next, an etching step (S5b) is performed. In this step (S5b), as shown in FIG. 32, a first processing mask 261 is formed on the high refractive index region 231. The first processing mask 261 is, for example, either a SiO2 film or a SiN film. The method for forming the SiO2 film or the SiN film is, for example, a chemical vapor deposition method. As shown in FIG. 33, the first processing mask 261 may be patterned into a desired shape using a photolithography technique and an etching technique. As shown in FIG. 33, in a plan view, the shape of the first processing mask 261 may be a combination of a triangle and a rectangle. In a plan view, the sides connecting both ends of the base of the triangular portion of the first processing mask 261 to the tip may be curved. For example, in a plan view, the shape of the first processing mask 261 may be a rectangle. Specifically, the width of the first processing mask 261 in the third direction Z may be uniformly the same as the width of the first core layer 221 from the first end face 221s1 (see FIG. 24) to the second end face 221s2 (see FIG. 24).

[0070] Next, the high refractive index region 231 is dry-etched using the first processing mask 261 as a mask. Specifically, the high refractive index region 231 in the region where the first processing mask 261 is arranged in a plan view is not etched. On the other hand, the high refractive index region 231 in the region where the first processing mask 261 is not arranged in a plan view is etched. As a result, as shown in FIG. 34 and FIG. 35, only the high refractive index region 231 is etched so that a part of the first core layer 221 is exposed. At this time, a part of the first core layer 221 may be etched in the first direction X. The etching may be wet etching. After the high refractive index region 231 is etched, the first processing mask 261 is removed by wet etching. In this manner, the high refractive index region 231 having a desired shape is formed.

[0071] Next, a step (S6b) of forming core layer 220 is performed. In this step (S6b), first core layer 221 is formed on high refractive index region 231 and a part of first core layer 221, as shown in Figures 36 and 37. Figure 37 is a cross-sectional view of spot size converter 200 shown along line segment XXXVII-XXXVII in Figure 36. Similar to the method of forming first lower cladding layer 253, first core layer 221 may be formed using metal-organic chemical vapor deposition.

[0072] Next, as shown in FIG. 38, the second core layer 241 is formed on the first core layer 221. The material constituting the second core layer 241 is a material having a refractive index n2b higher than the refractive index n1b of the first core layer 221. The material constituting the second core layer 241 is more preferably a material having a refractive index n2b higher than the refractive index n3b of the high refractive index region 231. When the second core layer 241 is composed of a plurality of materials each having a different refractive index, it is preferable that the above-mentioned plurality of materials are selected so that the effective refractive index n2b of the second core layer 241 is higher than the effective refractive indexes of the first core layer 221 and the high refractive index region 231. The material constituting the second core layer 241 may be, for example, InGaAsP or AlGaInAs.

[0073] 39, the first core layer 221 is formed on the second core layer 241. The first core layer 221 may be formed by metal organic chemical vapor deposition, similar to the method of forming the first lower cladding layer 253. At this time, the first core layer 221 in the first region 281 is formed as the second upper cladding layer 213.

[0074] Next, an etching step (S7b) is performed. In this step (S7b), as shown in FIG. 40, a second processing mask 262 is formed on the first core layer 221. The second processing mask 262 is, for example, either a SiO2 film or a SiN film. The method for forming the SiO2 film or the SiN film is, for example, a chemical vapor deposition method. The second processing mask 262 is patterned into a desired shape using a photolithography technique and an etching technique. As shown in FIG. 40, in a plan view, the shape of the second processing mask 262 is, for example, a pentagon. The second processing mask 262 is formed so as to extend from the second end face 221s2 to the second region 282. The second processing mask 262 is formed at the center of the width of the first core layer 221 in the third direction Z. The width of the second processing mask 262 in the third direction Z is smaller than the width of the core layer 220.

[0075] Next, the first core layer 221 and the second core layer 241 are dry-etched. Specifically, the first core layer 221 and the second core layer 241 in the region where the second processing mask 262 is arranged in a plan view are not etched. On the other hand, the first core layer 221 and the second core layer 241 in the region where the second processing mask 262 is not arranged in a plan view are etched. In this dry etching process, the first core layer 221 is etched to a position between the second core layer 241 and the high refractive index region 231 in the first direction X. That is, as shown in FIG. 41 and FIG. 42, the first core layer 221 may be etched in the first direction X so that a part of the first core layer 221 (the first core layer 221 in the second region 282 and the part of the first core layer 221 having the function as the second lower cladding layer 211 in the first region 281) is exposed. Fig. 42 is a cross-sectional view of spot size converter 200 shown along line segment XLII-XLII in Fig. 41. As shown in Fig. 42, the side surface of second core layer 241 is exposed. High refractive index region 231 is not exposed and is surrounded by first core layer 221. Although wet etching may be used as the etching, dry etching is preferable from the viewpoint of controlling the shape of core layer 220. In this manner, an optical waveguide with a ridge structure having a desired shape is formed, as shown in Fig. 43.

[0076] Next, a step (S8b) of forming core layer 220 is performed. In this step (S8b), as shown in FIG. 44, core layer 220 is formed on second lower cladding layer 211 and first core layer 221. FIG. 45 is a cross-sectional view of spot size converter 200 shown on line segment XLV-XLV in FIG. 44. FIG. 46 is a cross-sectional view of spot size converter 200 shown on line segment XLVI-XLVI in FIG. 44. As shown in FIG. 45, first core layer 221 is formed up to the position of the lower surface of second processing mask 262 in the first direction X. That is, first core layer 221 is formed so as to cover second core layer 241. Second processing mask 262 is exposed. Similar to the method of forming first lower cladding layer 253, first core layer 221 may be formed using metal organic chemical vapor deposition. 44, the first core layer 221 in the first region 281 is formed as the second side cladding layer 212. After the first core layer 221 is formed, the second processing mask 262 is removed by wet etching.

[0077] Considering that the second side cladding layer 212 in the first region 281 is connected to an optical device such as a semiconductor laser, it is preferable that the second side cladding layer 212 has a current confinement function. Therefore, the second side cladding layer 212 may have a three-layer structure in which, for example, a p-type InP layer, an n-type InP layer, and a p-type InP layer are stacked in this order. The second side cladding layer 212 may have a three-layer structure in which, for example, a p-type InP layer, a semi-insulating InP layer, and an n-type InP layer are stacked in this order. The second side cladding layer 212 may have a two-layer structure in which a semi-insulating InP layer and an n-type InP layer are stacked in this order. The second side cladding layer 212 may have a structure other than the above combination as long as it has a current confinement function.

[0078] Next, a step (S9b) of forming core layer 220 is performed. In this step (S9b), as shown in Fig. 47, first core layer 221 is formed on second upper cladding layer 213, second side cladding layer 212, and first core layer 221. Similar to the method of forming first lower cladding layer 253, first core layer 221 may be formed by metal organic chemical vapor deposition. At this time, as shown in Fig. 47, first core layer 221 in first region 281 is formed as second upper cladding layer 213.

[0079] Considering that the second upper cladding layer 213 in the first region 281 is coupled to an optical device such as a semiconductor laser, the upper surface of the first core layer 221 formed in this step (S9b) may become a low-resistance contact layer for ohmic contact with the second upper cladding layer 213 and the electrode. A material having a large absorption coefficient, such as InGaAs, is used as a material for forming the contact layer. Therefore, the presence of the contact layer on the waveguide causes a loss for the propagating light. After the contact layer is formed, only the contact layer of the spot size converter 200 may be selectively etched. When InGaAs is used as a material for forming the contact layer, the contact layer can be selectively removed by wet etching using a mixed solution containing tartaric acid, for example. The mixed solution may contain tartaric acid, hydrogen peroxide, water, and the like.

[0080] The position of the high refractive index region 231 in the first direction X within the core layer 220 is determined by the thickness in the first direction X of the first core layer 221 in the step (S3b) of forming the core layer 220, the thickness in the first direction X of the first core layer 221 in the step (S6b) of forming the core layer 220, the thickness in the first direction X of the first core layer 221 in the step (S8b) of forming the core layer 220, and the thickness in the first direction X of the first core layer 221 in the step (S9b) of forming the core layer 220.

[0081] For example, in the step (S3b) of forming the core layer 220, the first core layer 221 is formed to a thickness of 1 μm in the first direction X. In the step (S4b) of forming the high refractive index region 231, the high refractive index region 231 is formed to a thickness of 100 nm in the first direction X. In the step (S6b) of forming the core layer 220, the first core layer 221 is formed to a thickness of 1 μm in the first direction X. In the step (S8b) of forming the core layer 220, the first core layer 221 is formed to a thickness of 500 nm in the first direction X. In the step (S9b) of forming the core layer 220, the first core layer 221 is formed to a thickness of 2 μm in the first direction X. In this case, the thickness of the core layer 220 in the first direction X is 4 μm. The high refractive index region 231 is disposed at a position 1 μm away from the first lower cladding layer 253 in the first direction X. In this manner, in the first direction X, the high refractive index area 231 is disposed closer to the substrate 201 than the center of the thickness of the core layer 220 in the first direction X.

[0082] In this way, the position of the high refractive index region 231 in the first direction X within the core layer 220 can be easily controlled.

[0083] Next, an etching step (S10b) is performed. In this step (S10b), as shown in FIG. 48 and FIG. 49, a third processing mask 263 is formed on the first core layer 221. FIG. 49 is a cross-sectional view of the spot size converter 200 shown on the line segment XLIX-XLIX in FIG. 48. The third processing mask 263 is, for example, either a SiO2 film or a SiN film. The method for forming the SiO2 film or the SiN film is, for example, a chemical vapor deposition method. The third processing mask 263 may be patterned into a desired shape using a photolithography technique and an etching technique. As shown in FIG. 48, the shape of the third processing mask 263 is a rectangle in a plan view. The third processing mask 263 is formed so as to extend from the first end surface 221s1 to the second end surface 221s2. The third processing mask 263 is formed at the center of the width of the first core layer 221 in the third direction Z so as to expose the edge portion of the first core layer 221. In other words, the width of the third processing mask 263 in the third direction Z is uniformly smaller than the width of the first core layer 221.

[0084] Next, the core layer 220 is dry-etched. Specifically, the first core layer 221 in the region where the third processing mask 263 is arranged in a plan view is not etched. On the other hand, the first core layer 221 in the region where the third processing mask 263 is not arranged in a plan view is etched. As a result, only the edge portion in the third direction Z of the first core layer 221 is etched. At this time, as shown in FIG. 50 and FIG. 51, the first core layer 221 may be etched in the first direction X so that a part of the first lower cladding layer 253 is exposed. Alternatively, the first core layer 221 and the first lower cladding layer 253 may be etched in the first direction X so that a part of the substrate 201 is exposed. The etching may be wet etching, but dry etching is preferable from the viewpoint of controlling the shape of the core layer 220. After etching the core layer 220, the third processing mask 263 is removed by wet etching. In this way, as shown in FIG. 51, an optical waveguide of a desired shape of the mesa structure 254 is formed.

[0085] Next, a step (S11b) of forming a cladding layer 250 is performed. In this step (S11b), as shown in Figs. 52 and 53, a cladding layer 250 (see Fig. 23) is formed on the core layer 220 and on a part of the exposed first lower cladding layer 253. Specifically, as shown in Fig. 53, a first upper cladding layer 251 is formed on the first core layer 221. A pair of first side cladding layers 252 is formed on the first lower cladding layer 253. The pair of first side cladding layers 252 are disposed so as to sandwich the core layer 220. The material constituting the first upper cladding layer 251 and the first side cladding layer 252, which are components of the cladding layer 250, may be a material having a refractive index n4b lower than the refractive indexes n1b and n2b of the core layer 220. When the material constituting the core layer 220 is InP, the material constituting the first upper cladding layer 251 and the first side cladding layer 252 may be, for example, at least one of a SiO2 film and a SiN film. The method for forming the SiO2 film or the SiN film is, for example, chemical vapor deposition. When the material constituting the core layer 220 is, for example, at least one of InGaAsP and AlGaInAs, the material constituting the first upper cladding layer 251 and the first side cladding layer 252 may be InP. The method for forming the InP film is, for example, metalorganic vapor phase epitaxy.

[0086] It is sufficient that the core layer 220 is in contact with a medium having a refractive index lower than that of the core layer 220. Therefore, at least one of the first upper cladding layer 251 and the first side cladding layer 252 may be air. In other words, the side surface 221s3 of the core layer 220 may be exposed. In this case, after the etching step (S7a) is performed, a SiO2 film may be formed as a protective film for the mesa structure 254, but nothing may be formed.

[0087] The refractive index of the first upper cladding layer 251 and the first side cladding layer 252 may be smaller than the refractive index of the first lower cladding layer 253. In this way, the light propagating through the core layer 220 can seep out to the substrate 201 without seeping out to the first upper cladding layer 251 and the second side cladding layer 212. In this way, a material different from that of the first lower cladding layer 253 may be used as the material constituting the first upper cladding layer 251 and the first side cladding layer 252.

[0088] In this way, a spot size converter 200 with a vertically controllable spot size can be obtained, as shown in FIGS.

[0089] <Operation> Next, the operation of spot size converter 200 will be described.

[0090] The spot size converter 200 is connected to an optical device such as a light source or a modulator. Here, it is assumed that the spot size converter 200 is connected to a semiconductor laser, which is a light source. The semiconductor laser has, for example, a buried hetero structure. The buried hetero structure is a structure in which the active layer (core layer) of the semiconductor laser is sandwiched between materials with different band gaps from above and below, and the active layer of the semiconductor laser is sandwiched between layers with different band gaps and current confinement functions from the side. The spot size converter 200 is connected to the semiconductor laser at the second end face 221s2. Light propagates from the semiconductor laser. The light is incident on the second core layer 241. The second core layer 241 has a buried hetero structure like the semiconductor laser. Therefore, the light propagating from the semiconductor laser propagates to the second core layer 241 with the propagation mode changed as little as possible. As a result, the light propagates to the second core layer 241 with low loss. The second core layer 241 is surrounded by the second upper cladding layer 213, the second side cladding layer 212, and the second lower cladding layer 211 (corresponding to the first core layer 221), which have a refractive index n1b lower than the refractive index n2b of the second core layer 241. Therefore, light is confined in the second core layer 241 in the first region 281.

[0091] In the second core layer 241, the light reaches the second region 282. In the second region 282, the width of the second core layer 241 in the third direction Z gradually decreases toward the first end face 221s1. Therefore, as the light propagating through the second core layer 241 approaches the first end face 221s1, the second core layer 241 is no longer able to confine the light. The light gradually seeps out from the second core layer 241 to the first core layer 221. When the width of the second core layer 241 in the third direction Z becomes sufficiently small, all of the light moves from the second core layer 241 to the first core layer 221.

[0092] The light propagates through the first core layer 221 along the second direction Y. The light spreads in the direction in which the high refractive index regions 231 are arranged. In particular, since the high refractive index regions 231 are arranged on the substrate 201 side, the light spreads in the vertical direction (first direction X shown in FIG. 23). The light is emitted from the first end face 221s1. In this manner, the light is emitted to the outside of the semiconductor device including the spot size converter 200.

[0093] <Action and effect> In the above-described spot size converter 200, the core layer 220 includes a second core layer 241. The refractive index n2b of the second core layer 241 is higher than the refractive index n1b of the first core layer 221. In this way, light propagates through the second core layer 241 with low loss and without changing the propagation mode.

[0094] In the spot size converter 200, the high refractive index area 231 is disposed at a position closer to the substrate 201 side than the second core layer 241 in the first direction X. In this way, the light leaking out from the second core layer 241 spreads in the vertical direction (first direction X).

[0095] In the spot size converter 200, the core layer 220 has a first end face 221s1 and a second end face 221s2. A direction in which the first end face 221s1 is viewed from the second end face 221s2 is defined as a second direction Y. A part of the second core layer 241 is disposed in a region in the second direction Y in which the high refractive index region 231 is disposed. The second core layer 241 includes a first core region 241a and a second core region 241b. In the second direction Y, the second core region 241b is farther from the second end face 221s2 than the first core region 241a. The width of the second core region 241b in a direction perpendicular to the second direction Y is smaller than the width of the first core region 241a.

[0096] In this way, as the light propagating through the second core layer 241 approaches the first end face 221s1, the second core layer 241 is no longer able to confine the light. The light can gradually seep out from the second core layer 241 into the first core layer 221. When the width of the second core layer 241 in the third direction Z becomes sufficiently small, all of the light can transfer from the second core layer 241 to the first core layer 221.

[0097] In the above spot size converter 200, the refractive index of first core layer 221 is n1b. The refractive index of second core layer 241 is n2b. The refractive index of high refractive index region 231 is n3b. The refractive index of cladding layer 250 is n4b. The spot size converter 200 satisfies the relationship n2b>n3b>n1b>n4b. In this way, light can be efficiently propagated from second core layer 241 to first core layer 221.

[0098] FIG. 54 is a perspective view of the spot size converter 400 according to the comparative example 2. FIG. 54 corresponds to FIG. 23. The spot size converter 400 shown in FIG. 54 basically has the same configuration as the spot size converter 200 shown in FIG. 23 to FIG. 26, but is different in that the core layer 220 does not include the high refractive index region 231. FIG. 55 is a distribution diagram showing the light intensity in the vertical direction at the first end face 221s1. FIG. 55 corresponds to FIG. 22. In FIG. 55, the solid line shows the light intensity distribution in the example 2. The example 2 basically has the same configuration as the spot size converter 200 according to the second embodiment. In FIG. 55, the dotted line shows the light intensity distribution in the comparative example 2. In FIG. 55, the horizontal axis shows the height of the core layer in the vertical direction, specifically, the distance (unit: μm) from the first lower cladding layer 253. In FIG. 55, the vertical axis shows the relative light intensity (unit: -) in the vertical direction. In addition, on the horizontal axis of Fig. 55, the surface where first lower cladding layer 253 is connected to first core layer 221 is set as the reference (=0 μm). The direction in which first core layer 221 is located as viewed from first lower cladding layer 253 is the positive direction on the horizontal axis. On the other hand, the direction in which substrate 201 is located as viewed from first lower cladding layer 253 is the negative direction on the horizontal axis. The distribution diagram shown in Fig. 55 is normalized by the maximum value of light intensity in first direction X (vertical direction).

[0099] 55, the half width in Comparative Example 2 is 2.67 μm. The half width in Example 2 is 3.37 μm. That is, the half width in Example 2 is larger than the half width in Comparative Example 2. From the above results, it is found that the spot size is increased by including high refractive index region 231 in core layer 220.

[0100] The embodiments disclosed herein should be considered to be illustrative and not restrictive in all respects. Unless inconsistent, at least two of the embodiments disclosed herein may be combined. The basic scope of the present disclosure is indicated by the claims, not the above description, and is intended to include all modifications within the meaning and scope of the claims. [Explanation of symbols]

[0101] 100,200,300,400 Spot size converter, 101,201 Substrate, 110,250 Cladding layer, 111,253 First lower cladding layer, 112,252 First side cladding layer, 112a Protrusion, 113,251 First upper cladding layer, 120,220 Core layer, 121,221 First core layer, 121s2,221s2 Second end surface, 121s1,221s1 First end surface, 121s3,221s3 Side surface, 131,231 High refractive index region, 131a,231a First high refractive index region, 131b,231b Second high refractive index region, 131t,231t,241t Tip, 141,261 First processing mask, 142,262 Second processing mask, 151, 254 mesa structure, 211 second lower cladding layer, 212 second side cladding layer, 213 second upper cladding layer, 241 second core layer, 241a first core region, 241b second core region, 263 third processing mask, 281 first region, 282 second region, 283 boundary, X first direction, Y second direction, Z third direction, n1a, n1b, n2a, n2b, n3b, n3a, n4b refractive index, p1 point.

Claims

1. A spot size converter that converts a spot size of light incident on an incident surface and emits the light from an exit surface, comprising: A core layer; a clad layer surrounding the core layer; The core layer includes a first core layer, the refractive index of the clad layer is lower than the refractive index of the first core layer, the core layer includes a high refractive index region having a refractive index higher than a refractive index of the first core layer, The high refractive index area is disposed away from the entrance surface, A spot size converter, wherein a width of the high refractive index region becomes narrower from the exit surface to the entrance surface.

2. Further comprising a substrate; the substrate is connected to the cladding layer; When a direction in which the core layer is viewed from the substrate is defined as a first direction, The spot size converter according to claim 1 , wherein the high refractive index region is disposed closer to the substrate than the center of the thickness of the core layer in the first direction.

3. the core layer comprises a second core layer, The spot size converter of claim 2 , wherein the refractive index of the second core layer is higher than the refractive index of the first core layer.

4. The spot size converter according to claim 3 , wherein the high refractive index region is disposed at a position closer to the substrate side than the second core layer is in the first direction.

5. the core layer has a first end surface and a second end surface; When a direction in which the first end surface is viewed from the second end surface is defined as a second direction, a portion of the second core layer is disposed in a region in the second direction in which the high refractive index region is disposed, the second core layer includes a first core region and a second core region, In the second direction, the second core region is farther from the second end face than the first core region, The spot size converter according to claim 3 or 4, wherein a width of the second core region in a direction perpendicular to the second direction is smaller than a width of the first core region.

6. The refractive index of the first core layer is n1b, The refractive index of the second core layer is n2b, The refractive index of the high refractive index region is n3b, If the refractive index of the cladding layer is n4b, 5. The spot size converter according to claim 3, wherein the relationship n2b>n3b>n1b>n4b is satisfied.

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