JP2014016452A - Optical branching element and optical semiconductor integrated circuit device - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an optical branching element and an optical semiconductor integrated circuit device having a multimode interference waveguide, in which distribution of distortion caused by an insulating film is suppressed to decrease deviation.SOLUTION: A layered structure of a semiconductor clad layer, a semiconductor waveguide core layer and a semiconductor clad layer is prepared, and at least the underside semiconductor clad layer is partially etched to form a mesa type multimode interference waveguide. An upper surface of the obtained waveguide is covered with a first insulating film while a side surface is covered with a second insulating film having a coefficient of thermal expansion in an opposite magnitude relation with the coefficients of thermal expansion of the semiconductor clad layer and of the semiconductor waveguide core layer, to the magnitude relation between the first insulating film and the above layers.

Description

  The present invention relates to an optical branching device and an optical semiconductor integrated circuit device. For example, for a multi-mode interference (MMI) waveguide having a high-mesa waveguide structure in which a waveguide side surface is not embedded with a semiconductor material. The present invention relates to an optical branching device (optical coupler) and an optical semiconductor integrated circuit device using the same.

  With the recent enhancement of functionality of optical communication devices, optical semiconductor integrated circuit devices in which a plurality of semiconductor lasers, semiconductor optical modulators, and photodiodes (PDs) are monolithically integrated on a single semiconductor substrate have come to be used. ing. In these optical semiconductor integrated circuit devices, an optical branching device (optical coupler) that couples signal light from a semiconductor laser / semiconductor optical modulator or branches the signal light toward a plurality of PDs is essential. .

  Among them, an optical branching unit (optical coupler) using a multimode interference (MMI) waveguide made of a semiconductor is easy to monolithically integrate with a semiconductor laser, a semiconductor optical modulator, and a PD, and There is an advantage that it is small and insertion loss is small. In the MMI waveguide, a phenomenon in which the light intensity distribution at the input end is reproduced at the output end of the waveguide by generating a plurality of transverse higher-order modes inside the wide waveguide and propagating it for a certain distance, ie, self Imaging occurs. An optical branching device can be formed using this self-imaging (see, for example, Non-Patent Document 1).

  FIG. 13 is a conceptual plan view of an optical semiconductor integrated circuit device including a conventional MMI optical branching device, in which an input waveguide 61, a 4 × 4 MMI optical branching device 62, an output waveguide 63, and four PDs 64 are sequentially connected. (Non-Patent Document 3). In this optical semiconductor integrated circuit device, an example is shown in which 4 × 4 MMI is used as an optical branching device for dividing incident light into four PDs into four equal parts, but 4 × 4 MMI is 4 on the input side. It can be used as an optical coupler that couples light incident on one waveguide to one waveguide on the output side.

Figure 14 is a schematic plan view of a 4 × 4 MMI optical splitter comprises an MMI waveguide 62 ₂ consisting of a wide waveguide portion having a width, the four input ports 62 ₁ and four output ports 62 ₃ ing.

  When the MMI optical branching device is formed of a semiconductor, the side surface of the waveguide core layer is exposed from the viewpoint of reducing loss and variation in output light intensity among a plurality of output ports, that is, suppressing the deviation, or A high-mesa waveguide covered with a material other than a semiconductor is preferable.

  This is because the lateral optical confinement becomes stronger, so that a higher-order lateral higher-order mode is generated and self-imaging occurs more ideally. Further, since the effective width of the multimode interference waveguide is narrowed, the length of the multimode interference waveguide proportional to the square of the effective width is shortened, which is advantageous from the viewpoint of miniaturization.

In a high-mesa type semiconductor waveguide, generally, an insulating film such as SiO ₂ or SiN covers the top, side and bottom surfaces of the waveguide mesa structure, that is, a flat surface connected to the mesa for protecting the waveguide ( For example, see Patent Document 1).

  FIG. 15 is an explanatory diagram of a conventional 4 × 4 MMI optical splitter, FIG. 15 (a) is a plan view, and FIG. 15 (b) is an alternate long and short dash line connecting AA ′ in FIG. 15 (a). FIG. The high mesa type 4 × 4 MMI optical splitter 70 sequentially deposits a core layer 72 and an upper clad layer 73 on a lower clad layer 71, and a high mesa structure MMI waveguide 75 formed by mesa etching, and a high mesa structure. It consists of four input ports 74 and four output ports 76 having a high mesa structure. Further, the top surface, side surface, and bottom surface (surfaces on both sides of the mesa) of each high mesa structure are covered with an insulator film 77.

JP 2010-226062 A

Soldano et. al. , J .; LIGHTWAVE TECHNOLOGY, VOL. 13, NO. 4, APRIL 1995 pp. 615-627 Optoelectronic and Microelectronic Materials and Devices, 2004 Conference on, pp. 97-100 H. G. Bach et. al. , Waveguide-integrated Components Based 100 Gb / s Photoreceivers: From Direct to Coherent Detection, 2010 International Conference on Indium Phosphoretics. 445-448

  However, since the insulator film formed as the protective film is generally formed at a high temperature and has a different thermal expansion coefficient from that of the semiconductor, distortion occurs at room temperature. In particular, in the high mesa structure, since an insulator film is formed on the upper surface and side surfaces of the waveguide, there is a problem that a strain distribution is likely to occur inside the waveguide. This situation will be described with reference to FIG.

  FIG. 16 is an explanatory diagram of the strain distribution in the lateral direction of the conventional MMI waveguide, and shows the strain indicated by the thick broken line in FIG. 15B, that is, the strain at each position from the center toward one end face. ing. When a SiN film having a thermal expansion coefficient smaller than that of a semiconductor is used as the protective film, the deposition temperature of the SiN film is generally 200 ° C. or higher, which is higher than room temperature. It shrinks more and is pulled by the SiN film that is less shrunk due to the temperature drop.

  In a high-mesa multimode interference waveguide structure, a horizontal tensile strain is generated on the top surface of the waveguide mesa. On the other hand, a tensile strain in the vertical direction is generated on the side surface, and a compressive strain is generated in the horizontal direction on the top surface of the waveguide mesa. As a result, the horizontal strain in the core layer of the multimode interference waveguide is in the tensile direction (+ direction in the graph) near the waveguide center (position 0 μm), and in the compression direction (graph 11) near the waveguide end (position 11 μm). In the negative direction), and the difference in strain state between the end and the center increases.

  Since such a strain distribution becomes a refractive index distribution of the semiconductor waveguide, the characteristics of the MMI waveguide are deteriorated and the deviation is increased. As described above, in the conventional high mesa waveguide, the strain distribution generated by the insulating film serving as the protective film cannot be sufficiently suppressed, and thus there is a problem that the deviation cannot be sufficiently reduced.

  Accordingly, an object of the present invention is to suppress the strain distribution caused by the insulator film and reduce the deviation in the high-mesa multimode interference waveguide.

  From one aspect disclosed, a substrate, a semiconductor waveguide core layer provided above the substrate, a semiconductor clad layer disposed above and below the semiconductor waveguide core layer and having a refractive index lower than that of the semiconductor waveguide core layer, A mesa-shaped multimode interference waveguide having a stacked structure of: a first insulator film provided on the upper surface of the multimode interference waveguide; and a second insulator provided on a side surface of the multimode interference waveguide A thermal expansion coefficient of the semiconductor waveguide core layer and the cladding layer is a value between the thermal expansion coefficient of the first insulator film and the thermal expansion coefficient of the second insulator film. An optical branching device is provided.

  From another viewpoint to be disclosed, a semiconductor waveguide core layer provided above the substrate on the substrate, and disposed above and below the semiconductor waveguide core layer, the refractive index is lower than that of the semiconductor waveguide core layer. A mesa-shaped multimode interference waveguide having a laminated structure with a semiconductor cladding layer, a first insulator film provided on the top surface of the multimode interference waveguide, and a first insulator film provided on a side surface of the multimode interference waveguide Two insulating films, and the thermal expansion coefficient of the semiconductor waveguide core layer and the cladding layer is between the thermal expansion coefficient of the first insulating film and the thermal expansion coefficient of the second insulating film. An optical semiconductor integrated circuit device comprising: an optical branching device that is a value; and at least one optical semiconductor element of a semiconductor laser, a semiconductor optical amplifier, a semiconductor optical modulator, or a photodiode are monolithically integrated Is provided.

  According to the disclosed optical branching device and the optical semiconductor integrated circuit device, it is possible to reduce the deviation by suppressing the strain distribution caused by the insulator film.

It is explanatory drawing of the optical branching device of embodiment of this invention. It is explanatory drawing of the film-forming temperature dependence of the thermal expansion coefficient of a SiN film | membrane. It is explanatory drawing of the 4x4 MMI optical splitter of Example 1 of this invention. It is explanatory drawing of the distortion distribution of the 4x4 MMI optical splitter of Example 1 of this invention. It is explanatory drawing to the middle of the manufacturing process of the 4x4 MMI optical splitter of Example 1 of this invention. It is explanatory drawing to the middle after FIG. 5 of the manufacturing process of the 4 * 4 MMI optical splitter of Example 1 of this invention. It is explanatory drawing after FIG. 6 of the manufacturing process of the 4 * 4 MMI optical splitter of Example 1 of this invention. It is explanatory drawing of the 4x4 MMI optical splitter of Example 2 of this invention. It is explanatory drawing to the middle of the manufacturing process of the 4x4 MMI optical splitter of Example 2 of this invention. It is explanatory drawing after FIG. 9 of the manufacturing process of the 4 * 4 MMI optical splitter of Example 2 of this invention. It is explanatory drawing of the 4x4 MMI optical splitter of Example 3 of this invention. It is a top view of the optical semiconductor integrated circuit device of Example 4 of this invention. It is a conceptual top view of the optical semiconductor integrated circuit device containing the conventional MMI optical branching device. It is a notional top view of a 4x4 MMI optical splitter. It is explanatory drawing of the conventional 4x4 MMI optical splitter. It is explanatory drawing of the distortion distribution of the horizontal direction of the conventional MMI waveguide.

  Here, with reference to FIG.1 and FIG.2, the optical branching device of embodiment of this invention is demonstrated. FIG. 1 is an explanatory diagram of an optical branching device according to an embodiment of the present invention, FIG. 1 (a) is a plan view, and FIG. 1 (b) is an alternate long and short dash line connecting AA 'in FIG. 1 (a). FIG.

  An optical branching device according to an embodiment of the present invention includes a semiconductor waveguide core layer 11 and semiconductor clad layers 12 and 13 disposed above and below the semiconductor waveguide core layer 11 and having a refractive index lower than that of the semiconductor waveguide core layer 11. An MMI waveguide 14 having a mesa structure formed by etching the laminated structure to at least a part of the lower semiconductor clad layer 12, an input waveguide 17, and an output waveguide 18 are provided. The width perpendicular to the waveguide direction of the propagation light of the MMI waveguide 14 is made wider than the cut-off width of the transverse secondary mode of the propagation light.

  The upper surface of the MMI waveguide 14 is covered with a first insulator film 15, and the side surface of the MMI waveguide 14 is covered with a second insulator film 16. The first insulator film 15 and the second insulator film 16 are such that the thermal expansion coefficient of the semiconductor waveguide core layer 11 and the semiconductor cladding layers 12 and 13 is the same as that of the first insulator film 15 and the second coefficient of thermal expansion. A material having a value between the thermal expansion coefficients of the insulating film 16 is selected. Note that the bottom surface (planes on both sides of the mesa) may also be covered with the second insulator film 16.

  The first insulator film 15 may extend to the side surface and the bottom surface of the mesa structure so as to cover the second insulator film 16, or the second insulator film 16 may be the first insulator film 16. You may extend on the upper surface of a mesa so that the insulator film 15 may be covered.

The first insulator film 15 is typically SiO ₂ , and the second insulator film 16 is typically SiN. For example, the thermal expansion coefficient of a SiO ₂ film formed at a film forming temperature of 340 ° C. using a low pressure chemical vapor deposition method (LP-CVD method) is 7.97 × 10 ⁻⁶ K ⁻¹ . On the other hand, the thermal expansion coefficient of the SiN film formed at 280 ° C. using the plasma CVD method is 4 × 10 ⁻⁶ K ⁻¹ . Since the thermal expansion coefficient of InP is 4.5 × 10 ⁻⁶ K ⁻¹ , the relationship SiO ₂ >InP> SiN is obtained.

Insulating films having a smaller thermal expansion coefficient than InP include quartz (SiO ₂ ), and insulating films having a larger thermal expansion coefficient than InP include LiF, MgF, CaF ₂ , BaF ₂ , quartz (SiO ₂₎ . ₂₎ include _Al 2 _{O 3.} Note that the thermal expansion coefficient of SiN varies depending on the film forming conditions, and is larger or smaller than the thermal expansion coefficient of InP.

FIG. 2 is an explanatory diagram of the film formation temperature dependence of the thermal expansion coefficient of the SiN film (see Non-Patent Document 2). The higher the film formation temperature, the smaller the thermal expansion coefficient, for example, 5 × at 250 ° C. It becomes about 10 ⁻⁶ K ⁻¹ and becomes larger than the thermal expansion coefficient of InP.

  As described above, in the embodiment of the present invention, two types of insulator films having opposite thermal expansion coefficient magnitude relationships with respect to the waveguide core layer 11 are formed between the top surface and the side surface of the mesa of the MMI waveguide 14. Therefore, the distribution of strain applied to the waveguide core layer 11 can be reduced.

  For example, the upper surface of the mesa may have a compressive strain in the horizontal direction, and the side surface may have a tensile strain in the vertical direction. The strain in the horizontal direction due to the insulating film on the side surface becomes a compressive strain, and the compressive strain enters in the horizontal direction of the entire waveguide, so that the strain distribution is suppressed. As a result, the refractive index distribution inside the waveguide can be reduced, and the deviation of the multimode interference waveguide can be reduced.

  Further, when the first insulator film 15 extends to the side surface and the bottom surface of the mesa structure so as to cover the second insulator film 16, or the second insulator film 16 is used as the first insulation film. Even if it extends to the upper surface of the mesa so as to cover the body film 15, the insulating film on the extending side may be made relatively thin.

  Note that the relationship between the thermal expansion coefficients may be reversed. In this case, the upper surface of the mesa has a tensile strain in the horizontal direction, and the side surface has a compressive strain in the vertical direction. The semiconductor material constituting the multimode interference waveguide is not limited to the InGaAsP / InP system, but can be applied to other semiconductor materials such as AlGaInAs / InP system, InGaAs / GaAs system, and AlGaAs / GaAs system. is there. In that case, the material and film formation temperature of the first insulator film and the second insulator film may be selected in accordance with the thermal expansion coefficient of the semiconductor material to be used.

  Next, a 4 × 4 MMI optical splitter according to the first embodiment of the present invention will be described with reference to FIGS. 3 to 7. FIG. 3 is an explanatory diagram of the 4 × 4 MMI optical splitter according to the first embodiment of the present invention, FIG. 3A is a plan view, and FIG. 3B is AA ′ in FIG. It is sectional drawing along the dashed-dotted line which connects.

  The i-type InGaAsP core layer 23 is made of InGaAsP having a thickness of 0.5 μm and a composition wavelength of 1.05 μm. The i-type InP buffer layer 22 serving as a lower clad and the i-type InP buffer layer 22 serving as an upper clad are 1.0 μm thick. A waveguide is configured by being sandwiched between the type InP cladding layers 24. The MMI waveguide of the 4 × 4 MMI optical splitter forms a high-mesa waveguide structure by removing a part of the i-type InP cladding layer 24, the i-type InGaAsP core layer 23, and the i-type InP buffer layer 22. Yes.

  The mesa structure MMI waveguide has a width of 22 μm and a length of 450 μm. A 300 nm thick SiO 2 film 29 is formed on the top surface of the mesa structure, and a 300 nm thick SiN film is formed on the side surface. 31 is formed. The width of the MMI waveguide is set to a value wider than the cutoff width of the transverse secondary mode so that at least three transverse modes are generated. The length of the MMI waveguide is a value adjusted so that the desired self-imaging occurs.

The thermal expansion coefficient of an insulator film such as SiO ₂ and SiN varies depending on the film formation conditions. Here, the relationship between the thermal expansion coefficients of SiO ₂ , InP (InGaAsP), and SiN is SiO ₂ > InP (InGaAsP)> SiN is used. In such a relationship of the thermal expansion coefficient, when the temperature is lowered to room temperature from the time of film formation, the portion where SiO ₂ is formed has compressive strain in the direction parallel to the film on the semiconductor side, and the portion where the SiN film is formed Has tensile strain in the direction parallel to the film on the semiconductor side.

Thus, two types of insulator films of SiO ₂ and SiN having different thermal expansion coefficients are formed in the MMI waveguide. Therefore, the horizontal strain applied to the waveguide by the SiO ₂ film 29 from the upper surface of the MMI waveguide and the horizontal strain applied to the waveguide by the SiN film 31 from the side of the waveguide are both in the direction of compressive strain. It becomes possible to do.

As a result, the strain distribution inside the waveguide can be suppressed as compared with the conventional case where the entire surface of the waveguide is uniformly covered with an insulator film such as a SiN film. FIG. 4 shows the result of calculating the horizontal strain at the center of the i-type InGaAsP core layer 23 in the conventional structure and the structure of the first embodiment. In the calculation, the thermal expansion coefficients of InP, SiN, and SiO ₂ were assumed to be 4.5 × 10 ⁻⁶ , 2.5 × 10 ^−6, and 11.6 × 10 ⁻⁶ , respectively. FIG. 4 also shows the strain distribution of Example 2 described later.

  As shown in FIG. 4, in the conventional structure, tensile strain (+ strain) is applied near the center of the waveguide, and compressive strain (-strain) is applied near the side surface of the waveguide, and the distribution is large. On the other hand, in the structure of Example 1 of the present invention, it can be seen that compressive strain is applied to both the central portion of the waveguide and the vicinity of the side surface, and the strain distribution is reduced. Thereby, the refractive index distribution inside the waveguide can be reduced, the characteristics of the 4 × 4 MMI waveguide can be improved, and the deviation can be reduced.

  Next, a manufacturing method of the 4 × 4 MMI optical splitter according to the first embodiment of the present invention will be described with reference to FIGS. In each figure, the left figure is a plan view, and the right figure is a cross-sectional view taken along the alternate long and short dash line connecting AA 'in the left figure. First, as shown in FIG. 5A, an i-type InP buffer layer 22 having a thickness of 1.0 μm and a i-type InGaAsP core having a thickness of 0.5 μm are formed on a semi-insulating InP substrate 21. A layer 23 and an i-type InP cladding layer 24 having a thickness of 1.0 μm are sequentially deposited.

Next, as shown in FIG. 5B, after an SiO ₂ film is formed on the entire surface, an SiO ₂ mask 25 is formed on a portion where the MMI waveguide and its input / output waveguide are formed by a normal photolithography process and etching process. Form. Next, dry etching is performed using the SiO ₂ mask as a mask to form the high-mesa MMI waveguide 26, the input waveguide 27, and the output waveguide 28. Here, the size of the MMI waveguide is 22 μm × 450 μm, and the stripe width of the input waveguide 27 and the output waveguide 28 is 2.5 μm.

  Next, as shown in FIG. 6C, after removing the SiO 2 mask, a 300 nm thick SiO 2 film 29 is formed on the entire surface by using a low pressure chemical vapor deposition method (LP-CVD method). Next, as shown in FIG. 6D, the exposed portion of the SiO 2 film 29 is removed using buffered hydrofluoric acid using the resist pattern 30 as a mask. At this time, the distance between the end portion of the SiO 2 film 29 and the end portion of the MMI waveguide 26 is set to 1.0 μm in consideration of accuracy such as resist patterning. The upper surface of the MMI waveguide 26 only needs to be covered with the SiO2 film as the first insulating film (for example, 90% or more), and a part of the end of the upper surface of the MMI waveguide 26 is thus covered. You don't have to be.

Next, as shown in FIG. 7E, an SiN film 31 having a thickness of 300 n is formed on the entire surface by plasma CVD. Next, as shown in FIG. 7F, the 4 × 4 MMI optical branching of the first embodiment of the present invention is performed by removing the SiN film 31 on the upper surface of the SiO ₂ film 29 by performing etch back by dry etching. The basic structure of the vessel is completed. Note that the SiN film 31 may remain on the bottom surface. Further, a part of the upper portion of the SiN film 31 formed on the side surface of the waveguide may be etched by etch back, and the side surface of the waveguide core layer and the vicinity thereof are covered with the SiN film 31 which is the second insulating film. It should be.

In Example 1 described above, after removing the SiO ₂ mask serving as a mesa etching mask, a new SiO ₂ film is provided as a protective film. However, the SiO ₂ mask is etched again into a predetermined pattern and remains as it is. It may be used as a protective film. In that case, a similar structure can be obtained by forming a SiN film over the entire surface. Further, the SiO ₂ film 29 and the SiN film 31 deposited on the mesa bottom surface are not essential, and may be removed as necessary.

  Next, a 4 × 4 MMI optical splitter according to a second embodiment of the present invention will be described with reference to FIGS. FIG. 8 is an explanatory diagram of a 4 × 4 MMI optical splitter according to a second embodiment of the present invention, FIG. 8A is a plan view, and FIG. 8B is AA ′ in FIG. It is sectional drawing along the dashed-dotted line which connects.

  The i-type InGaAsP core layer 23 is made of InGaAsP having a thickness of 0.5 μm and a composition wavelength of 1.05 μm. The i-type InP buffer layer 22 serving as a lower clad and the i-type InP buffer layer 22 serving as an upper clad are 1.0 μm thick. A waveguide is configured by being sandwiched between the type InP cladding layers 24. The optical coupler portion of the 4 × 4 MMI waveguide forms a high-mesa waveguide structure by removing a part of the i-type InP cladding layer 24, the i-type InGaAsP core layer, and the i-type InP buffer layer.

The mesa structure MMI waveguide has a width of 22 μm and a length of 450 μm. A 300 nm thick SiN film 32 is formed on the side surface and bottom surface of the mesa structure, and the SiN film 32 is covered on the top surface. Thus, the SiO ₂ film 34 having a thickness of 150 nm is formed. Again, the relationship between the thermal expansion coefficients of SiO ₂ , InP (InGaAsP), and SiN is set to satisfy SiO ₂ >InP> SiN.

In the structure of Example 2, since SiO ₂ is formed on the mesa upper surface of the MMI waveguide, the horizontal strain direction from the mesa upper surface by the insulator film is a compressive strain. On the other hand, since the relatively thin SiO ₂ film on the outside of the relatively thick SiN film is formed on a side surface, the direction according distortion is dominated by the thermal expansion coefficient of the SiN film, tensile strain in the vertical direction Therefore, when viewed from the horizontal direction, the strain is compressive with respect to the MMI waveguide.

  As a result, since the compressive strain is applied to the entire waveguide in the horizontal direction, the strain distribution inside the waveguide can be reduced, and the refractive index distribution inside the waveguide is suppressed, and the characteristics of the MMI waveguide are reduced. Can be improved and the deviation can be suppressed.

  FIG. 4 shows the result of calculating the horizontal distortion in the central portion of the waveguide core layer in the structure of the second embodiment. As in the structure of the first embodiment, both the central portion of the waveguide and the vicinity of the side surface are shown. It can be seen that compression strain is applied and the strain distribution is reduced. This confirms that the refractive index distribution inside the waveguide is reduced, the characteristics of the MMI waveguide are improved, and the deviation is reduced.

  Next, with reference to FIG.9 and FIG.10, the manufacturing process of the 4 * 4 MMI optical splitter of Example 2 of this invention is demonstrated. First, as shown in FIG. 9A, a high-mesa MMI waveguide is formed in the same manner as in the first embodiment. Next, as shown in FIG. 9B, a SiN film 32 having a thickness of 300 nm is formed on the entire surface by plasma CVD.

Next, as shown in FIG. 10C, the exposed portion of the SiN film 32 is removed by etching with buffered hydrofluoric acid using the resist pattern 33 as a mask. At this time, in consideration of the accuracy of resist patterning, the SiN film 32 is left at about 1 μm at the end of the upper surface of the MMI waveguide. Next, as shown in FIG. 10 (d), after removing the resist pattern 33, a SiO ₂ film 34 having a thickness of 150 nm is formed on the entire surface, so that the 4 × 4 MMI light of Example 2 of the present invention is obtained. The basic structure of the turnout is completed.

In Example 2 of the present invention, the strain distribution is reduced by utilizing the difference in film thickness between the SiN film covering the side surface of the mesa and the SiO ₂ film. Since the back process is not necessary, the manufacturing process is simplified. In addition, since the SiO ₂ film 34 is formed on the entire surface and etching is not performed thereafter, the entire waveguide is protected by the insulating film, which is preferable from the viewpoint of device reliability.

  Next, the 4 × 4 MMI optical branching device according to the third embodiment of the present invention will be described with reference to FIG. 11. The basic manufacturing process is exactly the same as that of the first embodiment, and the last etch back process is performed. Since it is only not performed, description of a manufacturing process is abbreviate | omitted. FIG. 11 is an explanatory diagram of a 4 × 4 MMI optical splitter according to a third embodiment of the present invention, FIG. 11A is a plan view, and FIG. 11B is an AA ′ in FIG. It is sectional drawing along the dashed-dotted line which connects.

  The i-type InGaAsP core layer 23 is made of InGaAsP having a thickness of 0.5 μm and a composition wavelength of 1.05 μm. The i-type InP buffer layer 22 serving as a lower clad and the i-type InP buffer layer 22 serving as an upper clad are 1.0 μm thick. A waveguide is configured by being sandwiched between the type InP cladding layers 24. The MMI waveguide of the 4 × 4 MMI optical splitter forms a high-mesa waveguide structure by removing a part of the i-type InP cladding layer 24, the i-type InGaAsP core layer, and the i-type InP buffer layer.

The mesa structure MMI waveguide has a width of 22 μm and a length of 450 μm. A 300 nm thick SiO 2 film 29 is formed on the top surface of the mesa structure, and a 150 nm SiN film on the side and bottom surfaces. 31 is formed so as to cover the SiO ₂ film. Again, the relationship between the thermal expansion coefficients of SiO ₂ , InP (InGaAsP), and SiN is set to satisfy SiO ₂ > InP (InGaAsP)> SiN.

In Example 3, two layers of insulator films are formed on the top surface of the mesa of the MMI waveguide, and one layer of insulator film is formed on the side and bottom surfaces. The SiO ₂ film is formed on the top surface of the mesa. A SiN film exists. Since the SiO ₂ film is relatively thick, the thermal expansion coefficient of the SiO ₂ film becomes dominant, and the horizontal strain due to the insulator film on the upper surface of the mesa becomes a compressive strain. On the other hand, since the SiN film is formed on the side surface of the mesa, the strain in the vertical direction due to the insulator film on the side surface becomes tensile strain, and becomes compressive strain in the horizontal direction.

  As in the case of the first embodiment or the second embodiment described above, the strain distribution is suppressed because the overall strain distribution is a compressive strain in the horizontal direction. As a result, the refractive index distribution inside the waveguide is suppressed, and the characteristics of the MMI waveguide can be improved.

In the third embodiment of the present invention, the strain distribution is reduced by utilizing the difference in film thickness between the SiO ₂ film and the SiN film covering the upper surface of the mesa. Since the back process is not necessary, the manufacturing process is simplified.

  Next, an optical semiconductor integrated circuit device according to Example 4 of the present invention will be described with reference to FIG. Here, an example in which the 4 × 4 MMI optical branching device of the first embodiment is applied to a photodiode integrated 90 ° hybrid element in which the phase is shifted by 90 ° between the ports will be described.

  FIG. 12 is a plan view of an optical semiconductor integrated circuit device according to a fourth embodiment of the present invention. The input waveguide section 41 includes four waveguides, the 4 × 4 MMI optical branching device 42, and the four waveguides. The output waveguide section 43 includes a PD section 44 including four photodiodes.

For example, after the phase-modulated signal light is incident on the first waveguide from the left of the input waveguide 41 and the local oscillation light is incident on the third waveguide and branched into four equal parts in the 4 × 4 MMI optical splitter 42, respectively. The light is input to the photodiodes 44 _{1 to} 44 ₄ of the PD unit 44 through the output waveguide 63. In the photodiodes 44 _{1 to} 44 ₄ , a current is output in proportion to the intensity of the input light.

The intensity of light input to the photodiode 44 _{1-44 4,} the phase-modulated optical signal and determined by the phase relationship between the local oscillation light, the photodiode 44 ₁ and the differential and the photodiode 44 ₃ and the photo of the photodiode 44 _{and second} output current by monitoring the difference between the output current of the diode 44 _4, it is possible to detect the phase state of the phase-modulated optical signal. If the phase-modulated signal light and local oscillation light to the photodiodes 44 _{1 to} 44 ₄ are not accurately divided into four equal parts (if there is a deviation) in the 4 × 4 MMI optical branching unit 42, the difference signal is shifted and phase modulation is performed. The phase state of the signal light cannot be accurately detected.

In addition, each of the photodiodes 44 _{1 to} 44 ₄ mesa-etches an n-type InP buffer layer, an i-type InGaAsP light absorption layer, a p-type InP cladding layer, and a p-type InGaAs contact layer that are sequentially formed on a semi-insulating InP substrate. It is formed by doing. In addition, each photodiode ₄₄ 1 ~ 44 _4, n-side electrodes ₄₅ 1 to 45 ₄ and the p-side electrode ₄₆ 1-46 ₄ are provided, respectively.

The input waveguide and the output waveguide have the same stacked structure as the 4 × 4 MMI optical splitter, and the lower cladding layer, the core layer, and the mesa structure photodiodes 44 _{1 to} 44 ₄ are butt-jointed to each other. After the upper clad layer is laminated, it is etched to form a high mesa structure. Here, since only the upper surface of the MMI waveguide is covered with the SiO ₂ film 29 and the other side surfaces and the bottom surface of the MMI waveguide are covered with the SiN film 31, the passivation film for the photodiodes 44 _{1 to} 44 ₄ is also the SiN film 31. Form. As the passivation film of the PD portion 44, SiN is more suitable than the SiO ₂ film, and therefore, the SiN film is formed directly on the portion other than the upper surface of the MMI waveguide, out of the two insulating films covering the MMI waveguide. The above structure is preferable.

Also in the fourth embodiment, the 4 × 4 MMI optical branching unit can relax the strain distribution of the 4 × 4 MMI waveguide and suppress the deviation of the MMI as in the first embodiment. As a result, it possible to suppress the deviation of the photodiode 44 ₁ and the photodiode 44 _second difference signal and the difference signal of the photodiode 44 ₃ and the photodiode 44 ₄ by deviations, to accurately detect the phase state of the phase-modulated optical signal It becomes possible.

  In each of the above embodiments, the MMI optical branching device has been described as a 4 × 4 MMI optical branching device having four input ports and four output ports, but a 2 × 4 MMI optical branching device having two input ports, The present invention is also applicable to MMI optical branching devices having other configurations.

  In the fourth embodiment, the optical semiconductor element in which the optical branching device and the PD are integrated has been described. Instead of the PD, another optical semiconductor element such as a semiconductor laser, a semiconductor optical amplifier, or a semiconductor optical modulator is used as the MMI. It may be monolithically integrated with the waveguide. At that time, the insulating film on the upper surface of the MMI waveguide and other MMI waveguides are formed so that the insulating film covering the semiconductor laser, the semiconductor optical amplifier, and the semiconductor optical modulator becomes a more suitable insulating film as a passivation film of each element. It is desirable to select an insulating film that covers the side and bottom surfaces of the waveguide. Specifically, among the two insulating films covering the MMI waveguide, it is desirable that the insulating film covering the side and bottom surfaces of the MMI waveguide is an insulating film suitable as a passivation film for each element.

Here, the following supplementary notes are attached to the embodiments of the present invention including Examples 1 to 4.
(Additional remark 1) Laminated structure of a substrate, a semiconductor waveguide core layer provided above the substrate, and a semiconductor clad layer disposed above and below the semiconductor waveguide core layer and having a refractive index lower than that of the semiconductor waveguide core layer A mesa-shaped multimode interference waveguide having: a first insulator film provided on an upper surface of the multimode interference waveguide; and a second insulator film provided on a side surface of the multimode interference waveguide. The coefficient of thermal expansion of the semiconductor waveguide core layer and the cladding layer is a value between the coefficient of thermal expansion of the first insulator film and the coefficient of thermal expansion of the second insulator film. Optical branching device.
(Supplementary note 2) The optical branching device according to supplementary note 1, wherein the first insulating film further covers the second insulating film.
(Supplementary note 3) The optical branching device according to supplementary note 2, wherein the first insulating film is thinner than the second insulating film.
(Supplementary note 4) The optical branching device according to supplementary note 1, wherein the first insulating film is further covered with the second insulating film.
(Supplementary note 5) The optical branching device according to supplementary note 4, wherein the second insulating film is thinner than the first insulating film.
(Appendix 6) At least one of the first insulating film that directly covers and covers the semiconductor upper surface of the multimode interference waveguide and the second insulating film that covers and directly contacts the semiconductor upper surface intersect the semiconductor upper surface and the side surface. 6. The optical branching device according to any one of appendices 1 to 5, wherein the optical splitter is not in direct contact with the semiconductor surface in the vicinity of the corner.
(Appendix 7) Lamination of a semiconductor waveguide core layer provided above the substrate and a semiconductor clad layer disposed above and below the semiconductor waveguide core layer and having a refractive index lower than that of the semiconductor waveguide core layer on the substrate A mesa-shaped multimode interference waveguide having a structure, a first insulator film provided on an upper surface of the multimode interference waveguide, and a second insulator film provided on a side surface of the multimode interference waveguide; An optical branching device in which the thermal expansion coefficient of the semiconductor waveguide core layer and the cladding layer is a value between the thermal expansion coefficient of the first insulator film and the thermal expansion coefficient of the second insulator film And a semiconductor laser, a semiconductor optical amplifier, a semiconductor optical modulator, or at least one optical semiconductor element of a photodiode are monolithically integrated.
(Additional remark 8) The optical semiconductor integrated circuit of Additional remark 7 characterized by the above-mentioned. The passivation film which coat | covers the said optical semiconductor element is either the said 1st insulator film or the said 2nd insulator film.
(Supplementary Note 9) The optical semiconductor element is a photodiode, the photodiode, a connection waveguide connecting the photodiode and the optical branching device, the optical branching device, and an input connected to the optical branching device. 9. The optical semiconductor integrated circuit device according to appendix 7 or appendix 8, wherein waveguides are arranged in order.

DESCRIPTION OF SYMBOLS 11 Semiconductor waveguide core layers 12 and 13 Semiconductor clad layer 14 MMI waveguide 15 1st insulator film 16 2nd insulator film 17 Input waveguide 18 Output waveguide 21 Semi-insulating InP substrate 22 i-type InP buffer layer 23 i-type InGaAsP core layer 24 i-type InP clad layer 25 SiO ₂ mask 26 MMI waveguide 27 input waveguide 28 output waveguide 29, 34 SiO ₂ film 30, 33 resist pattern 31, 32 SiN film 41 input waveguide section 42 4 × 4 MMI optical splitter 43 Output waveguide portion 44 PD portion 44 _{1 to} 44 ₄ Photo diode 45 _{1 to} 45 ₄ n-side electrode 46 _{1 to} 46 ₄ p-side electrode 61 Input waveguide 62 4 × 4 MMI optical splitter 62 ₁ Input port 62 ₂ MMI waveguide 62 ₃ Output port 63 Output waveguide 64 PD
70 4 × 4 MMI optical splitter 71 Lower cladding layer 72 Core layer 73 Upper cladding layer 74 Input port 75 MMI waveguide 76 Output port 77 Insulator film

Claims (4)

A substrate,
A mesa-shaped multimode having a laminated structure of a semiconductor waveguide core layer provided above the substrate and a semiconductor clad layer disposed above and below the semiconductor waveguide core layer and having a refractive index lower than that of the semiconductor waveguide core layer An interference waveguide;
A first insulator film provided on an upper surface of the multimode interference waveguide;
A second insulator film provided on a side surface of the multimode interference waveguide,
The light whose thermal expansion coefficient of the said semiconductor waveguide core layer and a clad layer is a value between the thermal expansion coefficient of the said 1st insulator film, and the thermal expansion coefficient of the said 2nd insulator film, Turnout.   The optical branching device according to claim 1, wherein the first insulating film further covers the second insulating film.   The optical branching device according to claim 1, wherein the second insulating film further covers the first insulating film. On the board
A mesa-shaped multimode having a laminated structure of a semiconductor waveguide core layer provided above the substrate and a semiconductor clad layer disposed above and below the semiconductor waveguide core layer and having a refractive index lower than that of the semiconductor waveguide core layer The semiconductor waveguide core, comprising: an interference waveguide; a first insulator film provided on an upper surface of the multimode interference waveguide; and a second insulator film provided on a side surface of the multimode interference waveguide. An optical branching device in which a coefficient of thermal expansion of the layer and the cladding layer is a value between a coefficient of thermal expansion of the first insulator film and a coefficient of thermal expansion of the second insulator film;
An optical semiconductor integrated circuit device, wherein at least one kind of optical semiconductor element of a semiconductor laser, a semiconductor optical amplifier, a semiconductor optical modulator, or a photodiode is monolithically integrated.

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