Optical modulator

Abstract

To provide an optical modulator that can improve the surface flatness of a resin portion.SOLUTION: An optical modulator comprises: a substrate; a first Mach-Zehnder modulator portion including a first semiconductor mesa portion provided on the substrate, a first mesa waveguide provided on the first semiconductor mesa portion, and a second mesa waveguide provided on the first semiconductor mesa portion; a second Mach-Zehnder modulator portion including a second semiconductor mesa portion provided on the substrate, a third mesa waveguide provided on the second semiconductor mesa portion, and a fourth mesa waveguide provided on the second semiconductor mesa portion; at least one protruding portion provided on a bottom surface of a trench formed between the first semiconductor mesa portion and the second semiconductor mesa portion on the substrate; and a resin portion provided in the trench and embedding the at least one protruding portion.SELECTED DRAWING: Figure 3

Description

[Technical Field]

[0001] The present disclosure relates to optical modulators. [Background technology]

[0002] Patent Document 1 discloses a Mach-Zehnder modulator having two mesa waveguides provided on a substrate, and a resin portion is provided on the substrate to embed the two mesa waveguides. [Prior art documents] [Patent documents]

[0003] [Patent Document 1] Japanese Patent Publication No. 2022-37930 Summary of the Invention [Problem to be solved by the invention]

[0004] When multiple Mach-Zehnder modulators are provided on a substrate, grooves are formed on the substrate between the multiple Mach-Zehnder modulators to electrically isolate the multiple Mach-Zehnder modulators from each other. When a resin portion is formed in the groove, a depression may be formed in the surface of the resin portion above the groove due to the difference in height between the top surface of the mesa waveguide and the bottom surface of the groove.

[0005] The present disclosure provides an optical modulator that can improve the flatness of the surface of the resin part. [Means for solving the problem]

[0006] An optical modulator according to one aspect of the present disclosure includes: a substrate; a first Mach-Zehnder modulation section including a first semiconductor mesa section provided on the substrate, a first mesa waveguide provided on the first semiconductor mesa section, and a second mesa waveguide provided on the first semiconductor mesa section; a second Mach-Zehnder modulation section including a second semiconductor mesa section provided on the substrate, a third mesa waveguide provided on the second semiconductor mesa section, and a fourth mesa waveguide provided on the second semiconductor mesa section; at least one protrusion section provided on a bottom surface of a groove formed on the substrate between the first semiconductor mesa section and the second semiconductor mesa section; and a resin section provided in the groove and burying the at least one protrusion section. [Effects of the Invention]

[0007] According to the present disclosure, an optical modulator capable of improving the flatness of the surface of the resin part is provided. [Brief explanation of the drawings]

[0008] [Figure 1] FIG. 1 is a plan view schematically showing an optical modulator according to an embodiment. [Figure 2] FIG. 2 is a cross-sectional view taken along line II-II in FIG. [Figure 3] FIG. 3 is a cross-sectional view taken along line III-III in FIG. [Figure 4] FIG. 4 is a cross-sectional view showing an example of a part of an optical modulator. [Figure 5] FIG. 5 is a cross-sectional view showing another example of a part of an optical modulator. [Figure 6] FIG. 6 is a cross-sectional view schematically showing a part of an optical modulator according to another embodiment. [Figure 7] FIG. 7 is a cross-sectional view schematically showing a part of an optical modulator according to another embodiment. [Figure 8] FIG. 8 is a cross-sectional view schematically showing a part of an optical modulator according to another embodiment. [Figure 9] FIG. 9 is a graph showing an example of the surface position of the resin part. [Figure 10]FIG. 10 is a cross-sectional view of an optical modulator without a protrusion, corresponding to FIG. [Figure 11] FIG. 11 is a cross-sectional view of an optical modulator without a protrusion, corresponding to FIG. DETAILED DESCRIPTION OF THE INVENTION

[0009] [Description of the embodiments of the present disclosure] First, embodiments of the present disclosure will be listed and described.

[0010] (1) An optical modulator includes: a substrate; a first Mach-Zehnder modulation unit including a first semiconductor mesa portion provided on the substrate, a first mesa waveguide provided on the first semiconductor mesa portion, and a second mesa waveguide provided on the first semiconductor mesa portion; a second Mach-Zehnder modulation unit including a second semiconductor mesa portion provided on the substrate, a third mesa waveguide provided on the second semiconductor mesa portion, and a fourth mesa waveguide provided on the second semiconductor mesa portion; at least one protrusion portion provided on a bottom surface of a groove formed on the substrate between the first semiconductor mesa portion and the second semiconductor mesa portion; and a resin portion provided in the groove and burying the at least one protrusion portion.

[0011] In the optical modulator, the surface of the resin portion is elevated above the groove by at least one protrusion, thereby improving the flatness of the surface of the resin portion.

[0012] (2) In the above (1), the at least one protrusion may include a plurality of protrusions, and the plurality of protrusions may be arranged two-dimensionally.

[0013] (3) In the above (1) or (2), a conductor pattern may be further provided on the resin portion, and the conductor pattern may be arranged so as to at least partially overlap the at least one protrusion portion when viewed from a direction perpendicular to the main surface of the substrate.

[0014] (4) In any one of the above (1) to (3), the height of the at least one protrusion may be equal to or greater than the height of the first semiconductor mesa portion.

[0015] (5) In any one of the above (1) to (4), the at least one protrusion may contain the same semiconductor material as the semiconductor material contained in the first semiconductor mesa portion.

[0016] (6) In any one of the above (1) to (5), the depth of the groove may be 2 μm or more.

[0017] (7) In any one of (1) to (6) above, the resin part may be a first resin part, and the optical modulator may further include a second resin part provided on the first resin part.

[0018] [Details of the embodiments of the present disclosure] Hereinafter, embodiments of the present disclosure will be described in detail with reference to the accompanying drawings. In the description of the drawings, the same or equivalent elements are designated by the same reference numerals, and duplicate explanations will be omitted. In the drawings, mutually intersecting X-axis, Y-axis, and Z-axis directions are shown as necessary. For example, the X-axis, Y-axis, and Z-axis directions are orthogonal to each other.

[0019] FIG. 1 is a plan view schematically showing an optical modulator according to an embodiment. FIG. 2 is a cross-sectional view taken along line II-II in FIG. 1. FIG. 3 is a cross-sectional view taken along line III-III in FIG. 1. FIGS. 2 and 3 are cross-sections including the Y-axis direction and the Z-axis direction. The optical modulator 10 shown in FIG. 1 is, for example, a Mach-Zehnder modulator. The optical modulator 10 can modulate the intensity or phase of light in, for example, optical communications, to generate a modulated signal. The optical modulator 10 can attenuate light by, for example, adjusting the intensity of the light.

[0020] The optical modulator 10 includes a substrate 12, a first Mach-Zehnder modulation section MZ1, and a second Mach-Zehnder modulation section MZ2. The optical modulator 10 may include three or more Mach-Zehnder modulation sections.

[0021] The first Mach-Zehnder modulation section MZ1 includes a first semiconductor mesa section SM1 provided on the substrate 12, a first mesa waveguide M1 provided on the first semiconductor mesa section SM1, and a second mesa waveguide M2 ​​provided on the first semiconductor mesa section SM1. The first mesa waveguide M1 and the second mesa waveguide M2 ​​extend in the X-axis direction and have a height in the Z-axis direction. The first mesa waveguide M1 and the second mesa waveguide M2 ​​are optically coupled to each other at the input end and the output end of the first Mach-Zehnder modulation section MZ1, respectively. The second mesa waveguide M2 ​​is spaced apart from the first mesa waveguide M1.

[0022] The second Mach-Zehnder modulation section MZ2 includes a second semiconductor mesa section SM2 provided on the substrate 12, a third mesa waveguide M3 provided on the second semiconductor mesa section SM2, and a fourth mesa waveguide M4 provided on the second semiconductor mesa section SM2. The second semiconductor mesa section SM2 is spaced apart from the first semiconductor mesa section SM1. The third mesa waveguide M3 and the fourth mesa waveguide M4 extend in the X-axis direction and have a height in the Z-axis direction. The third mesa waveguide M3 and the fourth mesa waveguide M4 are optically coupled to each other at the input end and the output end of the second Mach-Zehnder modulation section MZ2, respectively. The fourth mesa waveguide M4 is spaced apart from the third mesa waveguide M3.

[0023] The first to fourth mesa waveguides M1 to M4 are arranged in order in opposite directions along the Y-axis. The second mesa waveguide M2 ​​is arranged between the first mesa waveguide M1 and the third mesa waveguide M3. The third mesa waveguide M3 is arranged between the second mesa waveguide M2 ​​and the fourth mesa waveguide M4.

[0024] The optical modulator 10 may include an input port P1 into which light is input and an output port P2 from which light is output. The input port P1 is located at a first edge of the substrate 12. The output port P2 is located at a second edge of the substrate 12 opposite the first edge. The first and second edges of the substrate 12 extend in the Y-axis direction. The optical modulator 10 may further include a phase adjustment unit between the first and second Mach-Zehnder modulation units MZ1 and MZ2 and the output port P2.

[0025] The input port P1 is connected to the input end of a mesa waveguide W1. The mesa waveguide W1 is provided on a semiconductor mesa portion WM1 provided on the substrate 12. The output end of the mesa waveguide W1 is optically coupled to the input end of an optical demultiplexer C1. The optical demultiplexer C1 is a multi-mode interference (MMI) coupler such as a 1x2 multi-mode interference coupler. The output end of the optical demultiplexer C1 is connected to the input end of a mesa waveguide W2 and the input end of a mesa waveguide W3. The mesa waveguide W2 is provided on a semiconductor mesa portion WM2 provided on the substrate 12. The mesa waveguide W3 is provided on a semiconductor mesa portion WM3 provided on the substrate 12. The output end of the mesa waveguide W2 is connected to the input end of a first Mach-Zehnder modulator MZ1. The output end of the mesa waveguide W3 is connected to the input end of the second Mach-Zehnder modulator section MZ2.

[0026] The input end of a mesa waveguide W4 is connected to the output end of the first Mach-Zehnder modulator MZ1. The mesa waveguide W4 is provided on a semiconductor mesa portion WM4 provided on the substrate 12. The output end of the mesa waveguide W4 is optically coupled to the input end of an optical multiplexer C2. The optical multiplexer C2 is, for example, a multimode interference coupler such as a 2×1 multimode interference coupler. The input end of a mesa waveguide W5 is connected to the output end of the second Mach-Zehnder modulator MZ2. The mesa waveguide W5 is provided on a semiconductor mesa portion WM5 provided on the substrate 12. The output end of the mesa waveguide W5 is optically coupled to the input end of the optical multiplexer C2. The input end of a mesa waveguide W6 is connected to the output end of the optical multiplexer C2. The mesa waveguide W6 is provided on a semiconductor mesa portion WM6 provided on the substrate 12. The output end of the mesa waveguide W6 is connected to the output port P2.

[0027] The mesa waveguides W1 to W6 may have the same semiconductor material and layer structure as the first to fourth mesa waveguides M1 to M4. The semiconductor mesas WM1 to WM6 may have the same semiconductor material and layer structure as the first and second semiconductor mesas SM1 and SM2.

[0028] The optical modulator 10 includes at least one protrusion PR. The optical modulator 10 may include multiple protrusions PR. The multiple protrusions PR may be spaced apart from each of the first semiconductor mesa portion SM1 and the second semiconductor mesa portion SM2. A gap may be provided between adjacent protrusions PR. The multiple protrusions PR may be arranged periodically. The multiple protrusions PR may be arranged two-dimensionally. The multiple protrusions PR may be arranged in a lattice pattern.

[0029] The top surface of each protrusion PR may have a rectangular shape or another shape. The area of ​​the top surface of each protrusion PR is 2500 μm 2 The height HPR of each protrusion PR may be equal to or greater than the height HSM1 of the first semiconductor mesa portion SM1 or the height of the second semiconductor mesa portion SM2. The height HPR of each protrusion PR is the distance from the main surface 12a of the substrate 12 to the top surface of the protrusion portion PR. Each protrusion portion PR may have the same semiconductor material and layered structure as the first semiconductor mesa portion SM1 or the second semiconductor mesa portion SM2. The protrusion portion PR may be formed together with the first semiconductor mesa portion SM1 and the second semiconductor mesa portion SM2 by photolithography and etching.

[0030] The plurality of protrusions PR are provided on a bottom surface TRb of a groove TR formed on the substrate 12 between the first semiconductor mesa portion SM1 and the second semiconductor mesa portion SM2. The groove TR is defined by a side surface SM1s of the first semiconductor mesa portion SM1, a side surface SM2s of the second semiconductor mesa portion SM2, and the main surface 12a of the substrate 12. The side surface SM1s of the first semiconductor mesa portion SM1 is a side surface close to the second semiconductor mesa portion SM2. The side surface SM2s of the second semiconductor mesa portion SM2 is a side surface close to the side surface SM1s of the first semiconductor mesa portion SM1. The depth of the groove TR may be 2 μm or more. The depth of the groove TR may be the same as the height HSM1 of the first semiconductor mesa portion SM1 or the height of the second semiconductor mesa portion SM2.

[0031] A plurality of protrusions PR may be further disposed between the first semiconductor mesa portion SM1 and a third edge of the substrate 12. In this case, the first semiconductor mesa portion SM1 is disposed between the plurality of protrusions PR and the second semiconductor mesa portion SM2. A plurality of protrusions PR may be further disposed between the second semiconductor mesa portion SM2 and a fourth edge of the substrate 12. In this case, the second semiconductor mesa portion SM2 is disposed between the plurality of protrusions PR and the first semiconductor mesa portion SM1. The third and fourth edges of the substrate 12 extend in the X-axis direction.

[0032] The optical modulator 10 includes a first resin portion R1 that embeds the multiple protrusions PR. The first resin portion R1 is provided in the groove TR and embeds the multiple protrusions PR. The first resin portion R1 may embed the first and second semiconductor mesas SM1 and SM2, or the first to fourth mesa waveguides M1 to M4. The surface position of the first resin portion R1 may be at the same level as or lower than the top surfaces of the first to fourth mesa waveguides M1 to M4. The optical modulator 10 may further include a second resin portion R2 provided on the first resin portion R1. Each of the first resin portion R1 and the second resin portion R2 may contain benzocyclobutene (BCB). Each of the first resin portion R1 and the second resin portion R2 may be formed by applying a resin solution to the substrate 12 by spin coating and then curing the resin solution. By adjusting the rotation speed of the spin coater, the resin solution remaining on the top surfaces of the first to fourth mesa waveguides M1 to M4 can be removed.

[0033] An insulating film 20 may be provided between the substrate 12 and the first resin portion R1. The insulating film 20 covers the substrate 12, the protrusion portion PR, the first and second semiconductor mesa portions SM1 and SM2, and the first to fourth mesa waveguides M1 to M4. An insulating film 22 may be provided between the first resin portion R1 and the second resin portion R2. An insulating film 24 may be provided on the second resin portion R2. The insulating films 20, 22, and 24 may be silicon oxynitride (SiON) films.

[0034] The optical modulator 10 may include upper electrodes UE1 to UE4, as shown in FIGS. 1 and 3. The upper electrode UE1 is connected to the top surface of the first mesa waveguide M1. The upper electrode UE2 is connected to the top surface of the second mesa waveguide M2. The upper electrode UE3 is connected to the top surface of the third mesa waveguide M3. The upper electrode UE4 is connected to the top surface of the fourth mesa waveguide M4. The upper electrodes UE1 to UE4 may extend along the first to fourth mesa waveguides M1 to M4, respectively. The upper electrodes UE1 to UE4 may be provided in openings provided in the insulating films 20 and 22. The upper electrodes UE1 to UE4 may be a stack including a platinum (Pt) layer, a titanium (Ti) layer, a platinum (Pt) layer, and a gold (Au) layer.

[0035] Conductor patterns CP11 to CP14 may be connected to the upper electrodes UE1 to UE4, respectively. The conductor patterns CP11 to CP14 are provided on the first resin portion R1 and the insulating film 22 and extend on the insulating film 22. The conductor patterns CP11 to CP14 may extend along the first to fourth mesa waveguides M1 to M4, respectively. The conductor patterns CP11 to CP14 may be gold (Au) layers.

[0036] The connecting conductors CN1 to CN4 may be connected to the conductor patterns CP11 to CP14, respectively. The connecting conductors CN1 to CN4 penetrate the second resin part R2 and the insulating film 24 in the Z-axis direction.

[0037] Conductor patterns CP1 to CP4 may be connected to the connection conductors CN1 to CN4, respectively. The conductor patterns CP1 to CP4 are provided on the first resin part R1. The conductor patterns CP1 to CP4 may be provided on the second resin part R2 and the insulating film 24 and extend on the insulating film 24. The conductor patterns CP1 to CP4 may include first portions extending along the first and fourth mesa waveguides M1 and M4, respectively, and second portions connecting the first portions to the connection conductors CN1 to CN4, respectively. The conductor patterns CP1 to CP4 may be gold (Au) layers. When viewed from the Z-axis direction (a direction perpendicular to the main surface 12a of the substrate 12), the conductor patterns CP1 to CP4 may be arranged to at least partially overlap the protrusions PR.

[0038] Conductor pattern CP1 is connected to upper electrode UE1 by connecting conductor portion CN1 and conductor pattern CP11. Conductor pattern CP2 is connected to upper electrode UE2 by connecting conductor portion CN2 and conductor pattern CP12. Conductor pattern CP3 is connected to upper electrode UE3 by connecting conductor portion CN3 and conductor pattern CP13. Conductor pattern CP4 is connected to upper electrode UE4 by connecting conductor portion CN4 and conductor pattern CP14.

[0039] 1 and 2, the optical modulator 10 may include bottom electrodes LE1 and LE2. The bottom electrode LE1 is connected to an upper surface of the first semiconductor mesa portion SM1. The bottom electrode LE2 is connected to an upper surface of the second semiconductor mesa portion SM2. The bottom electrodes LE1 and LE2 may be provided in an opening provided in the insulating film 20. The bottom electrodes LE1 and LE2 may be a laminate including a gold germanium (AuGe) layer, a nickel (Ni) layer, and a gold (Au) layer.

[0040] The lower electrodes LE1 and LE2 may be connected to connection conductors CNa and CNb, respectively. The connection conductors CNa and CNb penetrate the first resin portion R1 and the insulating film 22 in the Z-axis direction.

[0041] Conductor patterns CP15 and CP16 may be connected to the connection conductor portions CNa and CNb, respectively. The conductor patterns CP15 and CP16 are provided on the first resin portion R1 and the insulating film 22 and extend on the insulating film 22. The conductor patterns CP15 and CP16 are provided between the first resin portion R1 and the second resin portion R2. The conductor patterns CP15 and CP16 may be gold (Au) layers.

[0042] The connecting conductors CN5 and CN6 may be connected to the conductor patterns CP15 and CP16, respectively. The connecting conductors CN5 and CN6 penetrate the second resin part R2 and the insulating film 24 in the Z-axis direction.

[0043] Conductor patterns CP5 and CP6 may be connected to the connection conductors CN5 and CN6, respectively. The conductor patterns CP5 and CP6 may be provided on the second resin portion R2 and the insulating film 24 and extend on the insulating film 24. The conductor patterns CP5 and CP6 may extend along the first and fourth mesa waveguides M1 and M4, respectively. The conductor patterns CP5 and CP6 may be gold (Au) layers. When viewed from the Z-axis direction, the conductor patterns CP5 and CP6 may be arranged to at least partially overlap the protrusion portion PR.

[0044] The conductor pattern CP5 is connected to the lower electrode LE1 by the connection conductor portion CN5, the conductor pattern CP15, and the connection conductor portion CNa. The conductor pattern CP6 is connected to the lower electrode LE2 by the connection conductor portion CN6, the conductor pattern CP16, and the connection conductor portion CNb.

[0045] The refractive index of the first mesa waveguide M1 can be changed by applying a voltage between the lower electrode LE1 and the upper electrode UE1. The refractive index of the second mesa waveguide M2 ​​can be changed by applying a voltage between the lower electrode LE1 and the upper electrode UE2. The refractive index of the third mesa waveguide M3 can be changed by applying a voltage between the lower electrode LE2 and the upper electrode UE3. The refractive index of the fourth mesa waveguide M4 can be changed by applying a voltage between the lower electrode LE2 and the upper electrode UE4.

[0046] FIG. 4 is a cross-sectional view showing an example of a portion of an optical modulator. FIG. 4 is an enlarged view of a portion of FIG. 2 or FIG. 3. FIG. 4 shows a substrate 12, a first semiconductor mesa portion SM1, and a first mesa waveguide M1. The second semiconductor mesa portion SM2 may have the same structure as the first semiconductor mesa portion SM1. The second to fourth mesa waveguides M2 to M4 may have the same structure as the first mesa waveguide M1.

[0047] The first semiconductor mesa portion SM1 may include a semiconductor layer 30 provided on the substrate 12, and a semiconductor layer 32 provided on the semiconductor layer 30. The first mesa waveguide M1 may include a semiconductor layer 34 provided on the semiconductor layer 32, a core layer 36 provided on the semiconductor layer 34, a semiconductor layer 38 provided on the core layer 36, and a semiconductor layer 40 provided on the semiconductor layer 38.

[0048] The substrate 12 is, for example, a semi-insulating semiconductor substrate. The substrate 12 includes a III-V compound semiconductor doped with an insulating dopant. The substrate 12 includes, for example, InP doped with iron (Fe). The dopant concentration of the substrate 12 is 1×10 17 cm -3 More than 1×10 18 cm -3 It may be the following:

[0049] The semiconductor layer 30 includes a III-V compound semiconductor doped with a p-type dopant. The semiconductor layer 30 includes, for example, InGaAs or InP doped with zinc (Zn). The semiconductor layer 30 has a dopant concentration greater than the dopant concentration of the semiconductor layer 32. The dopant concentration of the semiconductor layer 30 may be 10 times or more greater than the dopant concentration of the semiconductor layer 32. The dopant concentration of the semiconductor layer 30 is 5×10 18 cm -3 May be more than 1 x 10 19 cm -3 The thickness of the semiconductor layer 30 is, for example, 0.5 μm or more and 2.0 μm or less.

[0050] The semiconductor layer 32 includes a III-V compound semiconductor doped with a p-type dopant. The semiconductor layer 32 may include a semiconductor material different from the semiconductor material of the semiconductor layer 30. The semiconductor layer 32 includes, for example, InP doped with Zn. The dopant concentration of the semiconductor layer 32 is 5×10 17 cm -3 Over 2×10 18 cm -3 The semiconductor layer 34 may contain the same semiconductor material as the semiconductor layer 32. The total thickness of the semiconductor layer 32 and the semiconductor layer 34 may be greater than the thickness of the semiconductor layer 30, for example, 1.0 μm or more and 3.0 μm or less.

[0051] The core layer 36 is an i-type semiconductor layer, i.e., an undoped semiconductor layer. The core layer 36 may have a multiple quantum well structure. The core layer 36 includes, for example, an AlGaInAs-based III-V compound semiconductor. The width of the core layer 36 is, for example, 1.5 μm or less.

[0052] The semiconductor layer 38 includes a III-V compound semiconductor doped with an n-type dopant. The semiconductor layer 38 includes, for example, InP doped with Si. The dopant concentration of the semiconductor layer 38 is 5×10 17 cm -3 Over 2×10 18 cm -3The thickness of the semiconductor layer 38 is, for example, not less than 1.0 μm and not more than 3.0 μm.

[0053] The semiconductor layer 40 includes a III-V compound semiconductor doped with an n-type dopant. The semiconductor layer 40 may include a semiconductor material different from that of the semiconductor layer 38. The semiconductor layer 40 includes, for example, InGaAs or InP doped with Si. The semiconductor layer 40 has a dopant concentration greater than the dopant concentration of the semiconductor layer 34. The dopant concentration of the semiconductor layer 40 is 5×10 18 cm -3 May be more than 1 x 10 19 cm -3 The thickness of the semiconductor layer 40 is, for example, 0.1 μm or more and 0.5 μm or less.

[0054] Fig. 5 is a cross-sectional view showing another example of a portion of an optical modulator. Fig. 5 is a cross-sectional view corresponding to Fig. 4. The substrate 12, the first semiconductor mesa portion SM1, and the first mesa waveguide M1 may have the structure shown in Fig. 5 instead of the structure shown in Fig. 4. In this structure, the first semiconductor mesa portion SM1 includes a semiconductor layer 132 provided on the substrate 12. The first mesa waveguide M1 includes a semiconductor layer 134 provided on the semiconductor layer 132, a core layer 36 provided on the semiconductor layer 134, a semiconductor layer 138 provided on the core layer 36, and a semiconductor layer 140 provided on the semiconductor layer 138.

[0055] The semiconductor layer 132 and the semiconductor layer 134 include the same semiconductor material as the semiconductor layer 38 in FIG. 4. The semiconductor layer 132 and the semiconductor layer 134 include, for example, InGaAs or InP doped with Si. The semiconductor layer 138 includes the same semiconductor material as the semiconductor layer 32 and the semiconductor layer 34 in FIG. 4. The semiconductor layer 138 includes, for example, InP doped with Zn. The semiconductor layer 140 includes the same semiconductor material as the semiconductor layer 30 in FIG. 4. The semiconductor layer 140 includes, for example, InGaAs or InP doped with Zn.

[0056] In the optical modulator having the structure shown in FIG. 4, a deeper groove TR can be formed compared to the optical modulator having the structure shown in FIG. 5, and therefore the volume of the first resin portion R1 is larger.

[0057] FIG. 10 is a cross-sectional view of an optical modulator that does not have a protrusion, corresponding to FIG. 2. FIG. 11 is a cross-sectional view of an optical modulator that does not have a protrusion, corresponding to FIG. 3. As shown in FIGS. 10 and 11, an optical modulator 100 that does not have a protrusion PR includes first and second resin parts R101 and R102. Deep recesses are formed in the surfaces of the first and second resin parts R101 and R102 above the groove TR. Therefore, the surfaces of the first and second resin parts R101 and R102 have low flatness.

[0058] On the other hand, according to the optical modulator 10 of this embodiment, as shown in FIGS. 2 and 3, the protrusions PR raise the surface of the first resin portion R1 above the grooves TR. This improves the flatness of the surface of the first resin portion R1. The surface of the first resin portion R1 may be flat, or a shallow depression may be formed in the surface of the first resin portion R1 above the grooves TR. Furthermore, the protrusions PR raise the surface of the second resin portion R2 above the grooves TR. This improves the flatness of the surface of the second resin portion R2. The surface of the second resin portion R2 may be flat, or a shallow depression may be formed in the surface of the second resin portion R2 above the grooves TR.

[0059] Improving the flatness of the surfaces of the first and second resin portions R1 and R2 reduces variations in the thickness of the first and second resin portions R1 and R2 near openings formed in the first and second resin portions R1 and R2 by, for example, dry etching. As a result, variations in the height of the connection conductors CNa, CNb, CN5, and CN6 can be reduced. This prevents poor connections between the lower electrode LE1 and the connection conductor portion CNa, between the connection conductor portion CNa and the conductor pattern CP15, between the conductor pattern CP15 and the connection conductor portion CN5, and between the connection conductor portion CN5 and the conductor pattern CP5. Similarly, it prevents poor connections between the lower electrode LE2 and the connection conductor portion CNb, between the connection conductor portion CNb and the conductor pattern CP16, between the conductor pattern CP16 and the connection conductor portion CN6, and between the connection conductor portion CN6 and the conductor pattern CP6.

[0060] Furthermore, the optical modulator 10 can reduce the volume of the first resin portion R1, thereby preventing the insulating film 20 from peeling off from the side surfaces of the first to fourth mesa waveguides M1 to M4 due to stress from the first resin portion R1. This reduces the reverse current value when a voltage is applied to the first to fourth mesa waveguides M1 to M4.

[0061] FIG. 6 is a cross-sectional view schematically illustrating a portion of an optical modulator according to another embodiment. The optical modulator 10A illustrated in FIG. 6 has a model structure corresponding to the optical modulator 10 illustrated in FIGS. 1 to 3. FIG. 6 is a cross-sectional view corresponding to FIG. 3. As illustrated in FIG. 6, the optical modulator 10A includes, in the Y-axis direction (a direction perpendicular to the side surface of the third mesa waveguide M3), a first region AR1 extending from the side surface of the third mesa waveguide M3 to a position beyond the end of the conductor pattern CP3, and a second region AR2 farther from the side surface of the third mesa waveguide M3 than the first region AR1. In the Y-axis direction, the distance from the side surface of the third mesa waveguide M3 to the end of the conductor pattern CP3 is X1. In the Y-axis direction, the distance from the end of the conductor pattern CP3 to the end of the first region AR1 is X2. Therefore, the length of the first region AR1 in the Y-axis direction is X1+X2. X1 is, for example, 30 to 100 μm. X2 is, for example, 30 μm or more. The optical modulator 10A may include a third region extending in the opposite direction of the Y-axis from the side surface of the second mesa waveguide M2 ​​to a position beyond the end of the conductor pattern CP2, as shown in FIG. 3. The third region is defined in the same manner as the first region AR1. The second region AR2 is disposed between the third region and the first region AR1.

[0062] In the optical modulator 10A, protrusions PR are provided in the first and second regions AR1 and AR2. The protrusions PR may also be provided in the third region. The filling factor (FF) (%), which indicates the degree of filling of the protrusions PR, is expressed by the following formula: FF=B 2 / (A+B) 2 ×100 In the formula, A represents the distance between adjacent protrusions PR in the X-axis direction and the Y-axis direction. B represents the dimensions of the protrusions PR in the X-axis direction and the Y-axis direction. When B is 0, i.e., when no protrusions PR are provided, FF is 0%. When A and B are both 10 μm, FF is 25%. When A is 0 μm, i.e., when there is no gap between the protrusions PR, FF is 100%. Both A and B may be greater than 0 μm. In the first region AR1, B may be 20 μm or less, and FF may be 50% or less. In the second region AR2, B may be 0 μm or more, and FF may be 0 to 100%.

[0063] In the first region AR1, a gap may be provided between adjacent protrusions PR. In this case, eddy current loss caused by a magnetic field passing through the protrusions PR due to a current flowing through the conductor pattern CP3 can be reduced compared to when there is no gap between adjacent protrusions PR. In the second region AR2, a gap may be provided between adjacent protrusions PR, or no gap may be provided.

[0064] FIG. 7 is a cross-sectional view schematically illustrating a portion of an optical modulator according to another embodiment. FIG. 7 is a cross-sectional view corresponding to FIG. 6. The optical modulator 10B illustrated in FIG. 7 has the same structure as the optical modulator 10A illustrated in FIG. 6, except that it includes a terrace portion PR2 instead of the protrusion portion PR in the second region AR2. The terrace portion PR2 is provided on a bottom surface TRb of a groove TR formed on the substrate 12 between the first semiconductor mesa portion SM1 and the second semiconductor mesa portion SM2. In the optical modulator 10B, there is no gap between adjacent protrusion portions PR in the second region AR2. The terrace portion PR2 has a larger dimension in the Y-axis direction than the protrusion portion PR. The terrace portion PR2 extends along the main surface 12a of the substrate 12 throughout the entire second region AR2. The terrace portion PR2 may have the same semiconductor material and layered structure as the first and second semiconductor mesa portions SM1 and SM2. The terrace portion PR2 can be formed together with the first semiconductor mesa portion SM1 and the second semiconductor mesa portion SM2 by photolithography and etching.

[0065] According to the optical modulator 10B, the flatness of the surfaces of the first and second resin parts R1 and R2 can be further improved compared to the optical modulator 10A. Furthermore, compared to the optical modulator 10A, the volume of the first resin part R1 can be reduced, thereby reducing the stress on the first resin part R1.

[0066] FIG. 8 is a cross-sectional view schematically illustrating a portion of an optical modulator according to another embodiment. FIG. 8 is a cross-sectional view corresponding to FIG. 6. The optical modulator 10C illustrated in FIG. 8 has the same structure as the optical modulator 10B illustrated in FIG. 7, except that the second region AR2 includes a terrace portion PR3 instead of the terrace portion PR2. The terrace portion PR3 has the same structure as the terrace portion PR2, except for its height. The height of the terrace portion PR3 is greater than the height of the terrace portion PR2. The height of the terrace portion PR3 may be equal to the sum of the heights of the first and second semiconductor mesa portions SM1 and SM2 and the heights of the first to fourth mesa waveguides M1 to M4. The lower portion of the terrace portion PR3 may have the same semiconductor material and layered structure as the first and second semiconductor mesa portions SM1 and SM2. The lower portion of the terrace portion PR3 may be formed together with the first and second semiconductor mesa portions SM1 and SM2 by photolithography and etching. The upper portion of the terrace portion PR3 may have the same semiconductor material and layered structure as the first to fourth mesa waveguides M1 to M4, and may be formed together with the first to fourth mesa waveguides M1 to M4 by photolithography and etching.

[0067] According to the optical modulator 10C, the flatness of the surfaces of the first and second resin parts R1 and R2 can be further improved compared to the optical modulator 10B. Furthermore, compared to the optical modulator 10B, the volume of the first resin part R1 can be reduced, thereby reducing the stress on the first resin part R1.

[0068] The following describes various experiments that were conducted to evaluate the optical modulators 10, 10A, 10B, and 10C, but the experiments described below do not limit the present invention.

[0069] (First experiment) An optical modulator with the same structure as the optical modulator 10A in Figure 6 was fabricated. Multiple protrusions PR were periodically arranged two-dimensionally in the X-axis and Y-axis directions. The spacing A between the protrusions PR and the dimension B of each protrusion PR were both 10 μm. Each protrusion PR had a square top surface with sides of 10 μm. Therefore, the FF was 25%. The height of the protrusion PR was 2.7 μm. The heights of the first and second semiconductor mesa portions SM1 and SM2 were 2.7 μm. The heights of the first to fourth mesa waveguides M1 to M4 were 2.7 μm. In the Y-axis direction, the distance between the side of the second mesa waveguide M2 ​​and the side of the first mesa waveguide M1 was 340 μm. The first and second resin portions R1 and R2 were formed by spin-coating a BCB solution onto the substrate 12 and then curing it.

[0070] (Second experiment) An optical modulator was fabricated in the same manner as in the first experiment, except that the protrusions PR were not provided. FF was 0%.

[0071] (First experiment results) For the optical modulators of the first and second experiments, cross sections corresponding to the cross section in Figure 6 were observed using a scanning electron microscope (SEM). The surface positions of the first and second resin parts R1 and R2 were measured from the SEM images. The measurement results are shown in Figure 9.

[0072] FIG. 9 is a graph showing an example of the surface position of the resin portion. In the graph of FIG. 9, the horizontal axis represents the distance (μm) from the side of the mesa waveguide. 0 μm on the horizontal axis corresponds to the position of the side of the second mesa waveguide M2 ​​(the side closest to the first mesa waveguide M1) in the Y-axis direction of FIG. 3. The vertical axis represents the surface position (μm) of the resin portion. 0 μm on the vertical axis corresponds to the surface position of the resin portion at the highest position. The surface position of the resin portion at the highest position is approximately the same as the position of the top surfaces of the first and second mesa waveguides M1 and M2. In the graph, surface position 2R101 represents the surface position of the first resin portion R1 in the second experiment. Surface position 2R102 represents the surface position of the second resin portion R2 in the second experiment. Surface position 1R1 represents the surface position of the first resin portion R1 in the first experiment. Surface position 1R2 indicates the surface position of the second resin part R2 in the first experiment.

[0073] The difference between the surface position 1R1 of the first resin portion R1 at the lowest position in the first experiment and the surface position 2R101 of the first resin portion R1 at the lowest position in the second experiment was 1.03 μm. The difference between the surface position 1R2 of the second resin portion R2 at the lowest position in the first experiment and the surface position 2R102 of the second resin portion R2 at the lowest position in the second experiment was 0.32 μm. Therefore, it can be seen that the depth of the depressions formed on the surface of the resin portion in the first experiment is shallower than in the second experiment. Therefore, it can be seen that the flatness of the surface of the resin portion is improved in the first experiment compared to the second experiment.

[0074] Although the exemplary embodiments of the present invention have been described in detail above, the present invention is not limited to the above embodiments.

[0075] The embodiments disclosed herein should be considered to be illustrative in all respects and not restrictive. The scope of the present invention is defined by the claims, not by the above meaning, and is intended to include all modifications within the meaning and scope of the claims. [Explanation of symbols]

[0076] 1R1…Surface position 1R2…Surface position 2R101…Surface position 2R102…Surface position 10...Optical modulator 10A...Optical modulator 10B...Optical modulator 10C...Optical modulator 12... Circuit board 12a…main surface 20...Insulating film 22...insulating film 24...Insulating film 30...Semiconductor layer 32...Semiconductor layer 34...Semiconductor layer 36...Core layer 38...Semiconductor layer 40...Semiconductor layer 100...Optical modulator 132...Semiconductor layer 134...Semiconductor layer 138...Semiconductor layer 140...Semiconductor layer AR1: First Area AR2: Second Region C1…Optical demultiplexer C2…Optical multiplexer CN1: Connecting conductor CN5: Connecting conductor CN6: Connecting conductor CNa: Connecting conductor CNb: Connecting conductor CP1...Conductor pattern CP2: Conductor pattern CP3...Conductor pattern CP4...Conductor pattern CP5...Conductor pattern CP6...Conductor pattern CP11...Conductor pattern CP12...Conductor pattern CP13...Conductor pattern CP14...Conductor pattern CP15...Conductor pattern CP16...Conductor pattern LE1: Lower electrode LE2: Lower electrode M1: First mesa waveguide M2: Second mesa waveguide M3: Third mesa waveguide M4: Fourth mesa waveguide MZ1: First Mach-Zehnder modulation section MZ2: Second Mach-Zehnder modulation section P1...input port P2: Output port PR…Protrusion PR2…Terrace area PR3…Terrace section R1: First resin part R2: Second resin part R101: First resin part R102: Second resin part SM1: First semiconductor mesa SM1s…side SM2: Second semiconductor mesa SM2s…Side TR…Groove UE1...upper electrode UE2...upper electrode UE3...Top electrode UE4…Top electrode W1...Mesa waveguide W2...Mesa waveguide W3...Mesa waveguide W4...Mesa waveguide W5...Mesa waveguide W6...Mesa waveguide WM1: Semiconductor mesa part WM2: Semiconductor mesa part WM3: Semiconductor mesa part WM4: Semiconductor mesa part WM5: Semiconductor mesa part WM6: Semiconductor mesa part

Claims

1. A substrate; a first Mach-Zehnder modulation section including a first semiconductor mesa section provided on the substrate, a first mesa waveguide provided on the first semiconductor mesa section, and a second mesa waveguide provided on the first semiconductor mesa section; a second Mach-Zehnder modulation section including a second semiconductor mesa section provided on the substrate, a third mesa waveguide provided on the second semiconductor mesa section, and a fourth mesa waveguide provided on the second semiconductor mesa section; at least one protrusion provided on a bottom surface of a groove formed on the substrate between the first semiconductor mesa portion and the second semiconductor mesa portion; a resin portion provided in the groove and embedding the at least one protrusion portion; An optical modulator comprising:

2. the at least one protrusion comprises a plurality of protrusions; The optical modulator according to claim 1 , wherein the plurality of protrusions are arranged two-dimensionally.

3. The resin portion further includes a conductive pattern provided on the resin portion, 3. The optical modulator according to claim 1, wherein the conductor pattern is disposed so as to at least partially overlap the at least one protrusion when viewed from a direction perpendicular to the main surface of the substrate.

4. 3. The optical modulator according to claim 1, wherein the height of the at least one protrusion is equal to or greater than the height of the first semiconductor mesa portion.

5. 3. The optical modulator according to claim 1, wherein the at least one protrusion portion includes the same semiconductor material as that included in the first semiconductor mesa portion.

6. 3. The optical modulator according to claim 1, wherein the depth of said groove is 2 [mu]m or more.

7. the resin portion is a first resin portion, 3. The optical modulator according to claim 1, further comprising a second resin portion provided on the first resin portion.

Images