Optical modulator and method for manufacturing the same

Abstract

In an optical modulator in which a Mach-Zehnder modulator and a semiconductor optical amplifier are monolithically integrated on a substrate, the thickness of the overcladding layers of the Mach-Zehnder modulator and the semiconductor optical amplifier can be set independently. [Solution] The optical modulator includes a substrate and a monolithically integrated semiconductor optical amplifier and Mach-Zehnder modulator on the substrate, wherein the difference between the thickness of the overclad layer of the semiconductor optical amplifier and the thickness of the overclad layer of the Mach-Zehnder modulator is set to be greater than the difference in height between the upper surface of the core layer of the semiconductor optical amplifier and the upper surface of the core layer of the Mach-Zehnder modulator.

Description

[Technical Field]

[0001] This disclosure relates to an optical modulator in which a semiconductor optical amplifier and a Mach-Zehnder modulator are monolithically integrated on a substrate, and to a method for manufacturing the same. [Background technology]

[0002] Saturation output P of a semiconductor optical amplifier (hereinafter also referred to as "SOA") 3dB It is generally represented by the following formula (1) (Non-Patent Document 1).

[0003] P 3dB =A eff I 3dB = 0.7·(dw / Γ)·(hf / A g τ)····(1) Here, P 3dB θ is the saturation power, Aeff is the mode cross-section, I3dB is the saturation light intensity, d is the active layer thickness, w is the active layer width, Γ is the light confinement coefficient, h is Planck's constant, f is the light frequency, Ag is the differential gain, and τ is the carrier lifetime.

[0004] Equation (1) shows that reducing the optical confinement coefficient Γ in the active layer improves the saturation output of the SOA. One effective way to reduce the optical confinement coefficient Γ in the core layer is to reduce the thickness of the SOA core layer.

[0005] When the core layer is composed of multiple quantum wells (hereinafter also referred to as "MQW"), reducing the thickness of the core layer means reducing the number of quantum wells. In this case, the volume of the active layer (corresponding to dw in equation (1) above) also becomes smaller, but if the optical confinement coefficient Γ becomes smaller than dw, a larger saturation power can be obtained as a result.

[0006] On the other hand, reducing the optical confinement coefficient Γ of the core will cause light to leak into the upper and lower cladding layers. When fabricating an SOA on an InP substrate, the upper contact layer, which is formed of p-type doped InGaAs or the like and located above the upper overcladding layer (hereinafter also referred to as "OC"), has a large optical absorption coefficient. Therefore, if the OC thickness is insufficient when the optical confinement coefficient Γ of the core is reduced, it will lead to an increase in absorption loss in the upper contact layer.

[0007] InP Mach-Zehnder modulators (hereinafter also referred to as "MZM") are well known (see Non-Patent Document 2), and attempts have been made to monolithically integrate SOA into these MZMs (see Non-Patent Document 3).

[0008] (Conventional manufacturing method) The following describes a conventional method for manufacturing an optical modulator in which SOA and MZM are monolithically integrated on a substrate, with reference to the drawings.

[0009] Optical modulators with monolithic integration of SOA and MZM on a substrate are manufactured using butt joint regrowth technology. Butt joint regrowth technology is a technique that enables the monolithic integration of optical circuits consisting of different waveguide cores and cladding layers by etching away a portion of the surface and regrowing layers with different compositions and material systems.

[0010] Figure 1 is an illustrative diagram showing the manufacturing process using this butt joint regrowth technology.

[0011] As shown in Figure 1(a), a wafer 101 for MZM is prepared. Then, as shown in Figure 1(b), the MZM-MQW layer in the SOA formation region 102 on the wafer 101 for MZM, which is the region where SOAs are to be integrated, is replaced with an SOA active layer that will serve as the core of the SOA, and the MZM-OC layer is replaced with an SOA-OC layer. (For the sake of clarity, the "SOA active layer" will be referred to as the "SOA core layer" below.) After that, SOA waveguides, MZM waveguides, etc. are formed in the SOA region and the MZM region to complete the optical modulators 103 and 104 in which SOA and MZM are monolithically integrated (Figure 1(c)).

[0012] Note that Figure 1 shows an image of two typical polarization-multiplexed IQ-MZM103 and 104 units being formed for clarity. In reality, the number of optical modulators formed on a single wafer is determined by the size of the area where the optical modulator chips are formed.

[0013] Referring to Figures 2 to 10, the specific steps for manufacturing an optical modulator in which SOA and MZM are monolithically integrated on a substrate using the butt joint regrowth technique shown in Figure 1 are illustrated. Figures 2 to 10 show a section of region 105 enclosed by the dashed line in Figure 1(c). Although Figures 2 to 10 explain the process assuming that only one MZM is formed for clarity, it is clear that the same process can be used to manufacture the device even when two MZMs are formed, as shown in region 105 of Figure 1.

[0014] Figure 2 is a schematic diagram showing the layer structure of region 105 of the MZM wafer 101 in Figure 1(a), i.e., the optical modulator formation region. The MZM substrate 101 in Figure 2 shows the schematic configuration of the layer structure of the MZM wafer 101 in Figure 1.

[0015] First, as a substrate (wafer) for forming the optical circuit, an MZM substrate 101 is prepared, on a semi-insulating InP substrate 201 as shown in Figure 2, in which an n-InP layer 202 which will be the lower cladding layer, a p-type cladding layer 203, an MZM-MQW layer 204 which will be the core layer, an MZM-OC layer 205 which will be the overcladding layer, and a contact layer 206 are formed in that order from bottom to top.

[0016] Alternatively, instead of preparing a substrate 101 for the MZM with each layer pre-formed, it may be created by sequentially forming each layer on a semi-insulating InP wafer substrate.

[0017] Figure 2(b) is a top view of the optical modulator formation region extracted from the wafer and viewed from the Y-axis direction, and Figure 2(a) is a cross-sectional view showing the schematic configuration of the cross section IIa-IIa in the top view.

[0018] Note that the thicknesses of each layer shown in the drawings are given a certain degree of dimension for ease of understanding, and therefore the ratio of the thicknesses of each layer in the drawings does not necessarily correspond to the actual thickness of each layer or the ratio of the thicknesses between each layer (the same applies to the following drawings).

[0019] In this example, the materials and thicknesses of each layer are as follows: the p-type cladding layer uses InAlAs with a thickness of 0.05 μm, the MZM-MQW layer uses InGaAlAs / InAlAs with a thickness of 0.5 μm, and the MZM-OC layer uses n-InP with a thickness of 1.5 μm. The contact layer uses InGaAs with a thickness of 0.2 μm.

[0020] (First step) The first step is to remove the MZM-MQW layer, which forms the core layer of the MZM in the SOA formation region, and the MZM-OC layer, which forms the overcladding layer of the MZM. The first step is positioned as a pre-step to replace the MZM-MQW layer, which forms the core layer of the MZM in the SOA formation region, with the SOA core layer, and the MZM-OC layer, which forms the overcladding layer of the MZM, with the SOA-OC layer, which forms the overcladding layer of the SOA.

[0021] In the first step, the p-type cladding layer 203, the MZM-MQW layer 204, the MZM-OC layer 205, and the contact layer 206 in the SOA formation region 102 are removed by etching, and a regrowth groove 301 of the SOA core layer is formed. FIG. 3 shows a state in which the regrowth groove 301 of the SOA core layer is formed in the SOA formation region of the MZM substrate 101 by etching in the first step.

[0022] For the etching of the MZM-OC layer 205, wet etching using a mixed solution of hydrochloric acid and phosphoric acid is used, and for the etching of the contact layer 206, the MZM-MQW layer 204, and the p-type cladding layer 203, a diluted solution of sulfuric acid and hydrogen peroxide water (so-called piranha) is used.

[0023] FIG. 3(b) is a top view of the optical modulator formation region on the wafer as viewed from the Y-axis direction, and FIG. 3(a) is a cross-sectional view showing a schematic configuration of the cross-section at IIIa-IIIa of the top view. Since up to the p-type cladding layer 203 has been removed in the SOA formation region 102 in this step, in FIG. 3(b), the top surface of the SOA formation region is the n-InP layer 202, and the top surfaces of the other regions are the contact layer 206.

[0024] (Second step) The second step is a step of regrowing the SOA core layer by epitaxial growth. The second step is positioned as a step of replacing the MZM core layer with the SOA core layer.

[0025] FIG. 4 is a diagram showing a state in which the SOA core layer, which is the second step, is regrown. In the second step, the n-InP layer 401, the SOA core layer 402, and the p-InP layer 403 are regrown by the MOCVD method, and the SOA core layer 402 is formed in the SOA formation region. Since the layer thickness of the core layer of the SOA is designed to be thinner than the layer thickness of the MZM-MQW layer forming the MZM core layer, in the regrowth process of the SOA core layer 402, the n-InP layer 401 forming the lower cladding layer of the SOA and a part of the overcladding layer of the SOA (the p-InP layer 403) are regrown to a predetermined thickness. As will be described later, the overcladding layer of the SOA is formed by the p-InP layer 403 regrown in the second step and the SOA-OC layer regrown in the fifth step. Therefore, in this specification, the overcladding layer (OC layer) of the SOA means the layer formed by the p-InP layer 403 and the SOA-OC layer.

[0026] FIG. 4(b) is a top view of the optical modulator formation region of the wafer as viewed from the Y-axis direction, and FIG. 4(a) is a cross-sectional view showing a schematic configuration of the cross-section at IVa-IVa of the top view. By this process, since each layer is formed in the order of the n-InP layer 401, the SOA core layer 402, and the p-InP layer 403, in FIG. 4(b), the upper surfaces of the SOA formation region 102 and other regions are both the p-InP layer 403.

[0027] Illustrating the materials and thicknesses of each layer in this example, the thickness of the n-InP layer 401 is 0.25 μm, the SOA core layer 402 is composed of InGaAsP, and its layer thickness is 0.1 μm. Note that the SOA core layer may be a multiple quantum well of InGaAsP. Also, the thickness of the p-InP layer 403 is 0.2 μm.

[0028] (Third step, fourth step) The third step is a step of removing unnecessary n-InP layers, SOA core layers, and P-InP layers formed outside the SOA formation region 102. The fourth step is a step of removing unnecessary MZM-OC layers remaining in regions other than the MZM modulation region 501.

[0029] Figure 5 shows the state in which, in the third step, unnecessary n-InP layer 401, SOA core layer 402, and p-InP layer 403 on the region other than the SOA formation region 102 (circuit formation region) are removed, and a selective growth mask layer 502 such as SiN is formed in the MZM modulation region 501, and in the fourth step, unnecessary MZM-OC layer 205 and contact layer 206 on the region other than the MZM modulation region 501 are removed.

[0030] Figure 5(b) is a top view of the optical modulator formation region of the wafer as seen from the Y-axis direction, and Figure 5(a) is a cross-sectional view showing the schematic configuration of the Va-Va cross section of the top view. In Figure 5(b), the top surface of the SOA formation region is a p-InP layer 403, and the top surface of the MZM modulation region 501 is a selective growth mask layer. The top surface of the MZM region other than these regions is an MZM-MQW layer 204.

[0031] In the third step, after removing unnecessary n-InP layer 401, SOA core layer 402, and p-InP layer 403 other than the SOA formation region 102, a selective growth mask 502 is formed on the MZM modulation region. In the fourth step, by using the selective growth mask 502 as an etching mask, unnecessary MZM-OC layer 205 and contact layer 206 in regions other than the MZM modulation region 501 are removed by etching.

[0032] (Step 5) The fifth step is to epitaxially grow the SOA-OC layer, which, together with the p-InP layer 403, constitutes the OC layer of the SOA. In the fifth step, as shown in Figure 6, the SOA-OC layer 601 and the SOA contact layer 602 are regrown and formed by the MOCVD method. Figure 6(b) is a top view of the optical modulator formation region of the wafer as seen from the Y-axis direction, and Figure 6(a) is a cross-sectional view showing the schematic configuration of the cross section VIa-VIa in the top view. In this step, the SOA-OC layer 601 and the contact layer 602 are sequentially formed in the region other than the MZM modulation region 501 where the selective growth mask layer 502 is formed. Therefore, in Figure 6(b), the top surface of the region other than the MZM modulation region is the contact layer 602, and the top surface of the MZM modulation region is the selective growth mask layer 502.

[0033] As an example of the materials and layer thicknesses used to form the SOA-OC layer 601 and contact layer 602 in this process, the SOA-OC layer 601 is formed of p-InP with a thickness of 1.5 μm, and the contact layer 602 is formed of InGaAs with a thickness of 0.2 μm.

[0034] (Step 6) The sixth step is to remove unnecessary SOA-OC layers outside the SOA formation region. In the sixth step, as shown in Figure 7, a selective growth mask 701 is formed in the SOA formation region 102, and the SOA-OC layer 601 and contact layer 602 formed outside the SOA formation region 102 are removed by etching. Figure 7(b) is a top view of the optical modulator formation region of the wafer viewed from the Y-axis direction, and Figure 7(a) is a cross-sectional view showing the schematic configuration of the cross section from VIIa-VIIa in the top view. As a result of this step, unnecessary SOA-OC layers 601 and contact layers 602 in areas other than the SOA formation region 102 where the selective growth mask layer 701 is formed are removed. Therefore, in Figure 7(b), the top surfaces of the SOA formation region 102 and the MZM modulation region 501 are the selective growth mask 701 and the selective growth mask 502, respectively, while the top surface of the other areas is the MZM-MQW layer 204. Therefore, the overcladding layer (OC layer) 702 of the SOA is formed by the SOA-OC layer 601 and the p-InP layer 403.

[0035] (Step 7) The seventh step is to regrow the semi-insulating OC layer, which will serve as the overcladding layer for input waveguides other than the SOA and MZM included in the optical modulator, output waveguides, waveguides connecting the SOA and MZM, splitters, multiplexers, etc.

[0036] In the seventh step, as shown in Figure 8, a semi-insulating OC layer 801 is formed by MOCVD in the areas other than the SOA formation region 102 and the MZM modulation region 501 where the selective growth mask is formed. The thickness of the semi-insulating OC layer is formed to be the same as the thickness of the MZM-OC layer. Figure 8(b) is a top view of the optical modulator formation region of the wafer as seen from the Y-axis direction, and Figure 8(a) is a cross-sectional view showing the schematic configuration of the cross section along line VIIIa-VIIIa in the top view. In this step, a semi-insulating OC layer is formed in the areas other than the SOA formation region 102 where the selective growth mask layer 701 is formed and the MZM modulation region 501 where the selective growth mask layer 502 is formed. Therefore, in Figure 8(b), the top surfaces of the SOA formation region 102 and the MZM modulation region 501 are the selective growth mask 701 and the selective growth mask 502, respectively, while the top surface of the other areas is the semi-insulating OC layer 801.

[0037] As an example of the material and layer thickness used to form the semi-insulating OC layer 801 in this process, Fe-doped InP is used, with a thickness of 1.5 μm.

[0038] (Step 8) The eighth step is the process of forming the MZM and SOA waveguides. In the eighth step, as shown in Figure 9, ridge waveguides are formed in the MZM modulation region 501 and the SOA formation region 102.

[0039] Figure 9(b) is a top view of the optical modulator formation region of the wafer as seen from the Y-axis direction, and Figure 9(a) is a cross-sectional view showing the schematic configuration of the cross section IXa-IXa in the top view. Furthermore, Figure 9(c) is a cross-sectional view showing the schematic configuration of the cross section IXc-IXc in the top view of Figure 9(b), and Figure 9(d) is a cross-sectional view showing the schematic configuration of the cross section IXd-IXd in the top view.

[0040] In this process, the contact layer 602, SOA-OC layer 601, and p-InP layer 403 are removed from the SOA formation region 102, except for the portion that forms the ridge waveguide 901. Therefore, in Figure 9(b), the upper surface of the SOA formation region 102, excluding the portion that forms the ridge waveguide 901, is the SOA core layer, and the upper surface of the portion that forms the ridge waveguide 901 is the contact layer. Similarly, in the MZM modulation region 501, the contact layer 206 and MZM-OC layer 205 are removed from the portions that form the ridge waveguides 902 and 903. Therefore, in Figure 9(b), the upper surface of the MZM modulation region 501, excluding the portions that form the ridge waveguides 902 and 903, is the MZM-MQW layer 204, and the upper surface of the portion that forms the ridge waveguides 902 and 903 is the contact layer 206.

[0041] Ridge waveguides can be formed, for example, by reactive ion etching (RIE) using methane gas plasma and wet etching with a mixture of hydrochloric acid and phosphoric acid.

[0042] (Step 9) The ninth step is the process of forming input waveguides, output waveguides, waveguides connecting the SOA and MZM, splitters, multiplexers, etc., in the optical modulator formation region other than the SOA formation region and the MZM modulation region (the so-called MZM passive region).

[0043] In the ninth step, as shown in Figure 10, a high mesa waveguide is formed in the optical modulator formation region (the so-called MZM passive region) other than the SOA formation region and the MZM modulation region. Figure 10(b) is a top view of the optical modulator formation region of the wafer as seen from the Y-axis direction, and Figure 10(a) is a cross-sectional view showing the schematic configuration of the cross section along line Xa-Xa in the top view. Figure 10(c) is a cross-sectional view showing the schematic configuration of the cross section along Xc-Xc in the top view of Figure 10(b).

[0044] The high-mesa waveguide formed in this ninth step has the layer structure shown in Figure 10(c). In this step, the semi-insulating OC layer 801, MZM-MQW layer 204, and p-type cladding layer 203 that were present in areas other than the SOA formation region 102 and the MZM modulation region 501 are removed in areas other than the high-mesa waveguide formation portion (the portion depicted by the lines in Figure 10(b)). Therefore, in Figure 10(b), the upper surface of the area other than the SOA formation region 102 and the MZM modulation region 501, excluding the portion where the high-mesa waveguide is formed, is the n-InP layer 202, and the upper surface of the portion where the high-mesa waveguide is formed is the semi-insulating OC layer 801. The high-mesa waveguide can be formed, for example, by RIE.

[0045] Subsequently, in the next step, an insulating layer is formed, followed by electrode formation, cleavage, polishing, and AR to complete the optical modulator with monolithic integration of SOA and MZM. [Prior art documents] [Non-patent literature]

[0046] [Non-Patent Document 1] Ikuo Mito, "Semiconductor Laser Amplifier," Optics, Vol. 25, No. 11 (1996). [Non-Patent Document 2] Y. Ogiso et al., "80-GHz Bandwidth and 1.5-V Vπ InP-Based IQ Modulator," Journal of Lightwave Technology, vol. 38, no. 2, pp. 249-255, 15 Jan.15, 2020. [Non-Patent Document 3] Y. Ueda et al., "Partial Regrowth of Optical-Gain Section for Improved Wafer Process Flexibility of InP Photonic Integrated Circuits," Journal of Lightwave Technology, vol. 40, no. 8, 2022. [Overview of the project] [Problems that the invention aims to solve]

[0047] In the method for manufacturing an optical modulator with monolithic integration of SOA on an MZM substrate using the conventional back-joint regrowth method described above, in the fifth step, the SOA-OC layer 601 and the contact layer are regrowed together with the P-InP layer 403 regrowed in the second step to form the SOA overcladding layer (OC layer) 702. However, the upper surface of the contact layer 602 could not be grown to a height greater than the lower surface of the selective growth mask 502 layer of the MZM modulation region 501.

[0048] In the fifth step, if the thickness of the SOA-OC layer 601 is increased to further increase the thickness of the SOA's OC layer 702, manufacturing defects such as the selective growth mask layer 502 of the MZM modulation region being embedded in the SOA-OC layer 601 and impairing the flatness of the wafer will occur. This makes it difficult to remove the SOA-OC layer near the selective growth mask layer 502 in the subsequent sixth step, causing a deterioration in manufacturing yield.

[0049] Therefore, conventionally, the thickness of the SOA's OC layer 702 (the sum of the thicknesses of the p-InP layer 403 and the SOA-OC layer 601) was determined by the thickness of the MZM's OC layer (usually the thickness of the MZM's OC layer is suitable to be around 1.0 to 2.0 μm) and the thickness of the p-InP layer 403, while the thickness of the SOA-OC layer 601 was determined by the thickness of the MZM-OC layer 205. In other words, conventionally, it was not possible to design the thickness of the SOA's OC layer 702 independently of the thickness of the MZM's OC layer (i.e., the MZM-OC layer 205). As a result, it was not possible to optimize the thickness of the MZM's OC layer and the SOA's OC layer, leading to the problem of insufficient thickness of the SOA's OC layer and a decrease in the SOA's saturation output.

[0050] This disclosure has been made in view of the above issues, and its purpose is to enable the independent setting of the OC layer of the SOA and the OC layer of the MZM in an optical modulator that monolithically integrates an SOA and an MZM.

[0051] Furthermore, this disclosure aims to provide an optical modulator that monolithically integrates SOA and MZM, wherein the thickness of the OC layers of MZM and SOA are set independently, that is, the difference in the thickness of the OC layers of MZM and SOA is greater than or equal to the thickness of the p-InP layer 403 (the difference in height between the upper surface of the core layer of SOA, i.e., the SOA core layer 401, and the upper surface of the core layer of MZM, i.e., the MZM-MQW layer 204). [Means for solving the problem]

[0052] To solve the above problems, one aspect of the present disclosure is a method for manufacturing an optical modulator that monolithically integrates a semiconductor optical amplifier and a Mach-Zehnder modulator, comprising the steps of: removing a portion of the core layer and overcladding layer of the semiconductor substrate in the semiconductor optical amplifier formation region of the semiconductor substrate, on a semiconductor substrate having a sacrificial layer on its surface and including at least a core layer and an overcladding layer; epitaxially growing the core layer of the semiconductor optical amplifier; and epitaxially growing the overcladding layer of the semiconductor optical amplifier to at least the sacrificial layer.

[0053] Furthermore, one aspect of this disclosure is an optical modulator comprising a substrate and a monolithically integrated semiconductor optical amplifier and Mach-Zehnder modulator on the substrate, wherein the difference between the thickness of the overclad layer of the semiconductor optical amplifier and the thickness of the overclad layer of the Mach-Zehnder modulator is greater than the difference between the height of the top surface of the core layer of the semiconductor optical amplifier and the height of the top surface of the core layer of the Mach-Zehnder modulator. [Effects of the Invention]

[0054] According to this disclosure, the SOA-OC layer, which is the OC layer of the SOA, can be regrown at least to the sacrificial layer. By setting the thickness of the sacrificial layer, the thickness of the SOA's OC layer can be made sufficiently thick, regardless of the thickness of the MZM's OC layer. This reduces absorption loss in the contact layer and enables the realization of an optical modulator with a high SOA saturation output. [Brief explanation of the drawing]

[0055] [Figure 1] This is an illustrative diagram of a method for manufacturing an optical modulator that monolithically integrates a semiconductor optical amplifier and a Mach-Zehnder modulator on a substrate using butt joint regrowth technology. [Figure 2] This figure shows a schematic configuration of the layer structure of a substrate for a Mach-Zehnder modulator used in a conventional manufacturing method. [Figure 3] This is a diagram illustrating the first step of a conventional manufacturing method. [Figure 4] This is a diagram illustrating the second step of the conventional manufacturing method. [Figure 5] This diagram illustrates the third and fourth steps of the conventional manufacturing method. [Figure 6] This is a diagram illustrating the fifth step of the conventional manufacturing method. [Figure 7] This is a diagram illustrating the sixth step of the conventional manufacturing method. [Figure 8] This is a diagram illustrating the seventh step of the conventional manufacturing method. [Figure 9] This is a diagram illustrating the eighth step of the conventional manufacturing method. [Figure 10] This is a diagram illustrating the ninth step of the conventional manufacturing method. [Figure 11] This figure shows a schematic configuration of the layer structure of a substrate for a Mach-Zehnder modulator used in the manufacturing method of the first embodiment of this disclosure. [Figure 12] This is a diagram illustrating the first step of the manufacturing method according to the first embodiment of the present disclosure. [Figure 13]This is a diagram illustrating a second step of the manufacturing method according to the first embodiment of the present disclosure. [Figure 14] This diagram illustrates the third and fourth steps of the manufacturing method according to the first embodiment of this disclosure. [Figure 15] This is a diagram illustrating the fifth step of the manufacturing method according to the first embodiment of this disclosure. [Figure 16] This is a diagram illustrating the sixth step of the manufacturing method according to the first embodiment of this disclosure. [Figure 17] This is a diagram illustrating the seventh step of the manufacturing method according to the first embodiment of the present disclosure. [Figure 18] This is a diagram illustrating the eighth step of the manufacturing method according to the first embodiment of the present disclosure. [Figure 19] This is a diagram illustrating the ninth step of the manufacturing method according to the first embodiment of the present disclosure. [Figure 20] This is a flowchart of the manufacturing method according to the first embodiment of the present disclosure. [Figure 21] This figure shows a schematic configuration of an optical modulator according to the second embodiment of the present disclosure. [Figure 22] This figure shows a schematic configuration of the waveguide of an optical modulator according to the second embodiment of this disclosure. [Figure 23] This figure shows the optical loss at the contact layer of the SOA for the optical modulator of the embodiment of this disclosure and the optical modulator of the comparative example. [Figure 24] This figure shows a schematic configuration of the layer structure of a substrate for a Mach-Zehnder modulator used in the manufacturing method of the second embodiment of this disclosure. [Figure 25] This figure illustrates the third and fourth steps of the manufacturing method according to the second embodiment of the present disclosure. [Modes for carrying out the invention]

[0056] The embodiments of this disclosure will be described in detail below with reference to the drawings. Note that the following description is illustrative, and embodiments with some configurations modified are possible without departing from the gist of this disclosure.

[0057] Identical or similar symbols indicate identical or similar elements, and repeated explanations may be omitted. Numerical values ​​in the following descriptions are illustrative, and other numerical values ​​may be used to implement this disclosure, provided that they do not deviate from the essence of this disclosure. In this specification, the direction of the Y-axis arrow shown in the figure is defined as upward, and the distance in the Y-axis direction from the top surface of the substrate on which the semiconductor optical amplifier and Mach-Zehnder modulator are integrated is described as height.

[0058] One embodiment of the present disclosure is a manufacturing method for an optical modulator that monolithically integrates a semiconductor optical amplifier (SOA) and a Mach-Zehnder modulator (MZM), wherein the thickness of the OC layer of the SOA can be designed independently of the thickness of the OC layer of the MZM, thereby enabling optimization of the thickness of each OC layer.

[0059] The present disclosure is a method for manufacturing an optical modulator that monolithically integrates a semiconductor optical amplifier, characterized by using a semiconductor substrate having a sacrificial layer on its surface and including at least a core layer and an overcladding layer, specifically a wafer for a Mach-Zehnder modulator having a sacrificial layer on its surface, and using a butt joint regrowth technique to replace a part of the semiconductor structure for the Mach-Zehnder modulator, and epitaxially growing (regrowing) an SOA-OC layer, which will be the core layer of the SOA, up to at least the sacrificial layer.

[0060] According to the manufacturing method of this disclosure, the thickness of the OC layer of the SOA can be set independently of the thickness of the OC layer of the semiconductor substrate by setting the thickness of the sacrificial layer. As a result, it is possible to realize an optical modulator with monolithically integrated SOAs in which the thickness of the OC layer of the SOA is sufficient to reduce the optical absorption loss in the contact layer formed on the OC layer of the SOA.

[0061] Furthermore, the manufacturing method of the present disclosure can also be used to manufacture an optical modulator that monolithically integrates a semiconductor optical amplifier (SOA) and a Mach-Zehnder modulator (MZM), which includes a substrate and a monolithically integrated semiconductor optical amplifier (SOA) and Mach-Zehnder modulator (MZM) on the substrate, wherein the difference in thickness between the thickness of the OC layer of the SOA and the thickness of the OC layer of the MZM is greater than the difference in height between the height of the top surface of the core layer of the SOA and the height of the top surface of the core layer of the MZM.

[0062] In this disclosure, the conducted type of the Mach-Zehnder modulator is assumed to be nipn, but it may also be pin type.

[0063] (First embodiment) Hereinafter, with reference to Figures 11 to 19, a method for manufacturing an optical modulator with a monolithically integrated semiconductor optical amplifier (SOA) according to the first embodiment of this disclosure will be described.

[0064] In the manufacturing method of this embodiment, as a method for monolithically integrating SOA on a semiconductor substrate having a sacrificial layer on its surface and at least a core layer and an overcladding layer, a butt joint regrowth technique is used, similar to conventional manufacturing methods, in which a part of the core layer and overcladding layer in the SOA formation region is replaced with an SOA core layer which becomes the core layer of the SOA and an SOA-OC layer which becomes the overcladding layer of the SOA. In other words, the manufacturing method of this embodiment is a method for manufacturing an optical modulator in which a semiconductor optical amplifier and the Mach-Zehnder modulator are monolithically integrated, comprising the steps of: removing a part of the core layer and overcladding layer of the semiconductor substrate in the SOA formation region, on a semiconductor substrate having a sacrificial layer on its surface and having at least a core layer and an overcladding layer; epitaxially growing the core layer of the SOA; and epitaxially growing the overcladding layer of the SOA to at least the sacrificial layer. In the following, we will describe a semiconductor substrate having a sacrificial layer on its surface and at least a core layer and an overcladding layer, using as an example a substrate for MZM described in the prior art, with a sacrificial layer formed on its surface.

[0065] In Figures 11 to 19, as in Figures 2 to 10 which describe the conventional manufacturing method, the region 105 enclosed by the dashed line in Figure 1(c) is shown as an excerpt. As in Figures 2 to 10, for clarity, only one MZM is formed in the explanation, but as shown in region 105 in Figure 1, it is clear that the same process can be used to manufacture two MZMs.

[0066] Figure 11 is a schematic diagram showing the layer structure of a semiconductor substrate 1101 used in this embodiment, which has a sacrificial layer on its surface and at least a core layer and an overcladding layer. In Figure 11, an MZM substrate is shown as a specific example of the semiconductor substrate 1101, and in this embodiment, the MZM substrate 1101 has a sacrificial layer formed on its uppermost surface. The thickness of this sacrificial layer is set according to the thickness of the OC layer of the SOA. As shown in Figure 11, the MZM substrate 1101 used in this embodiment has an n-InP layer 202, a p-type cladding layer 203, an MZM-MQW layer 204, an MZM-OC layer 205, a contact layer 206, and a sacrificial layer 1102 formed in order from bottom to top on a semi-insulating InP substrate (wafer) 201.

[0067] The manufacturing method of this embodiment, which uses an MZM substrate 1101 having a sacrificial layer 1102 on its surface as shown in Figure 11, is a method for manufacturing an optical modulator in which SOA and MZM are monolithically integrated, and includes a step of using a butt joint regrowth technique in which the MZM-MQW layer, which becomes the core layer of MZM, is replaced with an SOA core layer in the SOA formation region, and the MZM-OC layer, which becomes the overcladding layer of MZM, is replaced with an SOA-OC layer, which becomes the overcladding layer of SOA.

[0068] Alternatively, instead of preparing a substrate 1101 for MZM with each layer pre-formed, a step of sequentially forming each layer on a semi-insulating InP wafer substrate may be added. Furthermore, a step of forming a sacrificial layer 1102 may be added to the conventional MZM substrate 101 described above.

[0069] Figure 11(b) is a top view of the optical modulator formation region of the wafer as seen from the Y-axis direction, and Figure 11(a) is a cross-sectional view showing the schematic configuration of the cross section XIa-XIa in the top view.

[0070] As an example of the materials and thicknesses of each layer here, the p-type cladding layer 203 is InAlAs with a thickness of 0.05 μm, the MZM-MQW layer 204 is InGaAlAs / InAlAs with a thickness of 0.5 μm, the MZM-OC layer 205 is n-InP with a thickness of 1.5 μm, and the contact layer 206 is InGaAs with a thickness of 0.2 μm.

[0071] The sacrificial layer 1102 can be appropriately set according to the difference in thickness between the OC layer of the MZM being designed and the OC layer of the SOA. As an example of material and thickness, n-InP is used and the layer thickness is 0.85 μm. Undoped InP may also be used for the sacrificial layer 1102.

[0072] (First step) The first step is to remove the MZM-MQW layer, which forms the core layer of the MZM in the SOA formation region, and the MZM-OC layer, which forms the overcladding layer of the MZM. The first step is positioned as a pre-step to replace the MZM-MQW layer, which forms the core layer of the MZM in the SOA formation region, with the SOA core layer, and the MZM-OC layer, which forms the overcladding layer of the MZM, with the SOA-OC layer, which forms the overcladding layer of the SOA.

[0073] In the first step, the p-type cladding layer 203, MZM-MQW layer 204, MZM-OC layer 205, contact layer 206, and sacrificial layer 1102 of the SOA formation region 102 are removed by etching to form the regrowth groove 301 of the SOA core layer. Figure 12 shows the state in which the regrowth groove 301 of the SOA core layer has been formed in the SOA formation region of the MZM substrate 1101 by etching in the first step.

[0074] Wet etching using a mixture of hydrochloric acid and phosphoric acid was used for etching the MZM-OC layer 205 and the sacrificial layer 1102, while diluted solutions of sulfuric acid and hydrogen peroxide (so-called piranha solution) were used for etching the contact layer 206, MZM-MQW 204, and the p-type cladding layer 203.

[0075] Figure 12(b) is a top view of the optical modulator formation region on the wafer as seen from the Y-axis direction, and Figure 12(a) shows the schematic configuration of the cross section along line XIIa-XIIa in the top view. In this process, the p-type cladding layer 203 is removed in the SOA formation region 102, so in Figure 12(b), the top surface of the SOA formation region is the n-InP layer 202, and the top surface of the other regions is the sacrificial layer 1102.

[0076] (Second step) The second step is the process of epitaxially growing the SOA core layer. The second step is positioned as the process of replacing the MZM core layer with the SOA core layer.

[0077] Figure 13 shows the state after the SOA core layer has been regrowthed in the second step. In the second step, the n-InP layer 401, SOA core layer 402, and p-InP layer 403 are regrowthed by the MOCVD method, and the SOA core layer 402 is generated in the SOA formation region. Since the core layer of the SOA is designed to be thinner than the thickness of the MZM-MQW layer that forms the MZM core layer, in the regrowth process of the SOA core layer 402, the n-InP layer 401 that forms the lower cladding layer of the SOA and the p-InP layer that becomes part of the OC layer of the SOA are regrowthed to a predetermined thickness. The overcladding layer of the SOA is formed by the p-InP layer 403 regrowthed in the second step and the SOA-OC layer regrowthed in the fifth step, as will be described later. Therefore, in this specification, the overcladding layer of the SOA means the layer formed by the p-InP layer 403 and the SOA-OC layer.

[0078] Figure 13(b) is a top view of the optical modulator formation region of the wafer as seen from the Y-axis direction, and Figure 13(a) is a cross-sectional view showing the schematic configuration of the cross section at XIIIa-XIIIa in the top view. In this process, the n-InP layer 401, SOA core layer 402, and p-InP layer 403 are formed in that order, so in Figure 13(b), the top surface of the SOA formation region 102 and the top surface of the other regions are both p-InP layer 403.

[0079] In the manufacturing method of this embodiment, the difference from the conventional manufacturing method is that, on the upper surface of other regions, an n-InP layer 401 and a p-InP layer forming the upper cladding (OC) layer are formed on top of the sacrificial layer 1102.

[0080] In this example, the materials and thicknesses of each layer are as follows: the n-InP layer 401 has a thickness of 0.25 μm, and the SOA core layer 402 is made of InGaAsP with a thickness of 0.1 μm. The SOA core layer may also be a multiple quantum well made of InGaAsP. The p-InP layer 403 has a thickness of 0.2 μm.

[0081] (Third and fourth steps) The third step is to remove any unnecessary SOA core layers and p-InP layers formed outside the SOA formation region 102. The fourth step is to remove any unnecessary MZM-OC layers remaining in regions other than the MZM modulation region 501.

[0082] Figure 14 shows the state in which, in the third step, unnecessary SOA core layers 402 and p-InP layers 403 on the MZM regions (circuit formation regions) other than the SOA formation region 102 are removed, and a selective growth mask layer 502 such as SiN is formed in the MZM modulation region 501, and in the fourth step, unnecessary MZM-OC layers 205, contact layers 206, sacrificial layers 1102 and n-InP layers 401 in the MZM regions other than the MZM modulation region 501 are removed.

[0083] Figure 14(b) is a top view of the optical modulator formation region of the wafer as seen from the Y-axis direction, and Figure 14(a) is a cross-sectional view showing the schematic configuration of the cross section from XIVa to XIVa in the top view. In Figure 14(b), the top surface of the SOA formation region is a p-InP layer 403, and the top surface of the MZM modulation region 501 is a selective growth mask layer 502. The top surface of the MZM region other than these regions is an MZM-MQW layer 204.

[0084] The third step of this embodiment involves removing unnecessary SOA core layers 402 and p-InP layers 403 other than the SOA formation region 102, and then forming a selective growth mask 502 on the MZM modulation region. This is because, since the sacrificial layer 1102 and the n-InP layer 401 formed on the MZM substrate 1101 are made of the same material, the sacrificial layer 1102 and the n-InP layer 401 are used as essentially a single sacrificial layer to set the thickness of the SOA-OC layer, which becomes the OC layer of the SOA.

[0085] If the sacrificial layer 1102 is formed of a different material from the n-InP layer 401, the unnecessary n-InP layer 401 may be removed in a third step to form a selective growth mask 502 on the sacrificial layer, as described later.

[0086] In the fourth step, the selective growth mask 502 is also used as an etching mask to remove the unnecessary MZM-OC layer 205, contact layer 206, sacrificial layer 1102, and n-InP layer 401 in the MZM region other than the MZM modulation region 501 by etching. (Step 5) The fifth step is to epitaxially grow the SOA-OC layer 1501, which will become the OC layer of the SOA together with the p-InP layer 403. In the fifth step, as shown in Figure 15, the SOA-OC layer 1501 and the SOA contact layer 1502 are regrown and formed by the MOCVD method. Figure 15(b) is a top view of the optical modulator formation region of the wafer as seen from the Y-axis direction, and Figure 15(a) is a cross-sectional view showing the schematic configuration of the cross section from XVa to XVa in the top view.

[0087] In the manufacturing method of this embodiment, the thickness of the MZM modulation region is set to be larger than that of the conventional MZM modulation region in the fifth step by the thickness of the sacrificial layer 1102 and the n-InP layer 401. Therefore, the SOA-OC layer 1501 and the contact layer 1502 can be regrown up to the height of the sacrificial layer 1102 and the n-InP layer 401 without the risk of causing defects. Thus, in the manufacturing method of this embodiment, by utilizing the sacrificial layer 1102, the SOA-OC layer 1501, which will become the OC layer of the SOA, is epitaxially grown to at least the sacrificial layer, making it possible to set the thickness of the OC layer of the SOA according to the height of the sacrificial layer 1102.

[0088] In this process, the SOA-OC layer 1501 and the contact layer 1502 are sequentially formed in the regions other than the MZM modulation region 501 where the selective growth mask layer 502 is formed. Therefore, in Figure 15(b), the upper surface of the regions other than the MZM modulation region 501 is the contact layer 1502, and the upper surface of the MZM modulation region 501 is the selective growth mask layer 502.

[0089] As an example of the materials and thicknesses of the SOA-OC layer 1501 and contact layer 1502 formed in this process, the SOA-OC layer 1501 is formed of p-InP with a thickness of 2.8 μm, and the contact layer 1502 is formed of InGaAs with a thickness of 0.2 μm.

[0090] Thus, in the manufacturing method of this embodiment, by epitaxially growing the SOA-OC layer 1501, which will be the OC layer of the SOA, to at least the sacrificial layer, it is possible to regrow it to a height greater than that of the MZM-OC layer. Furthermore, the thickness of the SOA-OC layer 1501 can be appropriately set by the thickness of the sacrificial layer formed on the MZM substrate 1101.

[0091] (Step 6) The sixth step is to remove unnecessary SOA-OC layers outside the SOA formation region. In the sixth step, as shown in Figure 16, a selective growth mask 701 is formed in the SOA formation region 102, and the SOA-OC layer 1501 and contact layer 1502 formed outside the SOA formation region 102 are removed by etching. Figure 16(b) is a top view of the optical circuit formation region 101 of the wafer viewed from the Y-axis direction, and Figure 16(a) is a cross-sectional view showing the schematic configuration of the cross section at XVIa-XVIa in the top view. This step removes unnecessary SOA-OC layers 1501 and contact layers 1502 in areas other than the SOA formation region 102 where the selective growth mask layer 701 is formed. Therefore, in Figure 16(b), the top surfaces of the SOA formation region 102 and the MZM modulation region 501 are the selective growth mask 701 and the selective growth mask 502, respectively, while the top surface of the other areas is the MZM-MQW layer 204. Therefore, in this embodiment, the overcladding layer (OC layer) 1601 of the SOA is composed of an SOA-OC layer 1501 and a p-InP layer 403.

[0092] (Step 7) The seventh step is to regrow the semi-insulating OC layer, which will serve as the overcladding layer for input waveguides other than the SOA and MZM included in the optical modulator, output waveguides, waveguides connecting the SOA and MZM, splitters, multiplexers, etc.

[0093] In the seventh step, as shown in Figure 8(b), a semi-insulating OC layer 801 is formed by MOCVD in the areas other than the SOA formation region 102 and the MZM modulation region 501 where the selective growth mask is formed. The thickness of the semi-insulating OC layer is formed to be the same as the thickness of the MZM-OC layer. Figure 17(b) is a top view of the optical modulator formation region of the wafer as seen from the Y-axis direction, and Figure 17(a) is a cross-sectional view showing the schematic configuration of the cross section along line XVIIa-XVIIa in the top view. As a result of this step, a semi-insulating OC layer is formed in the areas other than the SOA formation region 102 where the selective growth mask layer 701 is formed and the MZM modulation region 501 where the selective growth mask layer 502 is formed. Therefore, in Figure 8(b), the top surfaces of the SOA formation region 102 and the MZM modulation region 501 are the selective growth mask 701 and the selective growth mask 502, respectively, while the top surface of the other areas is the semi-insulating OC layer 801.

[0094] As an example of the material and layer thickness used to form the semi-insulating OC layer 801 in this process, Fe-doped InP is used, with a thickness of 1.5 μm.

[0095] (Step 8) The eighth step is the process of forming the MZM and SOA waveguides. In the eighth step, as shown in Figure 18, ridge waveguides are formed in the MZM modulation region 501 and the SOA formation region 102.

[0096] Figure 18(b) is a top view of the optical modulator formation region of the wafer as seen from the Y-axis direction, and Figure 18(a) is a cross-sectional view showing the schematic configuration of the cross section at XVIIIa-XVIIIa in the top view. Furthermore, Figure 18(c) is a cross-sectional view showing the schematic configuration of the cross section at XVIIIc-XVIIIc in the top view of Figure 18(b), and Figure 18(d) is a cross-sectional view showing the schematic configuration of the cross section at XVIIId-XVIIId in the top view.

[0097] In this process, the contact layer 1502, SOA-OC layer 1501, and p-InP layer 403 are removed from the SOA formation region 102 except for the portion that forms the ridge waveguide 1801. Therefore, in Figure 18(b), the upper surface of the SOA formation region 102 except for the portion that forms the ridge waveguide 1801 is the SOA core layer, and the upper surface of the portion that forms the ridge waveguide 1801 is the contact layer. Similarly, in the MZM modulation region 501, the contact layer 206 and MZM-OC layer 205 are removed except for the portions that form the ridge waveguides 1802 and 1803. Therefore, in Figure 18(b), the upper surface of the MZM modulation region 501 except for the portions that form the ridge waveguides 1802 and 1803 is the MZM-MQW layer 204, and the upper surface of the portion that forms the ridge waveguides 1802 and 1803 is the contact layer 206.

[0098] Ridge waveguides can be formed, for example, by reactive ion etching (RIE) using methane gas plasma and wet etching with a mixture of hydrochloric acid and phosphoric acid.

[0099] (Step 9) The ninth step is the process of forming input waveguides, output waveguides, waveguides connecting the SOA and MZM, splitters, multiplexers, etc., in the optical modulator formation region other than the SOA formation region and the MZM modulation region (the so-called MZM passive region).

[0100] In the ninth step, as shown in Figure 19, a high mesa waveguide is formed in the optical modulator formation region (the so-called MZM passive region) other than the SOA formation region 102 and the MZM modulation region 501. Figure 19(b) is a top view of the optical modulator formation region of the wafer as seen from the Y-axis direction, and Figure 19(a) is a cross-sectional view showing the schematic configuration of the cross section from XIXa to XIXa in the top view. Figure 19(c) is a cross-sectional view showing the schematic configuration of the cross section from XIXc to XIXc in the top view of Figure 19(b).

[0101] The high-mesa waveguide formed in this ninth step has the layer structure shown in Figure 19(c). In this step, the semi-insulating OC layer 801, MZM-MQW layer 204, and p-type cladding layer 203 that were present in areas other than the SOA formation region 102 and the MZM modulation region 501 are removed in areas other than the high-mesa waveguide formation portion (the portion depicted by the lines in Figure 19(b)). Therefore, in Figure 19(b), the upper surface of the area other than the SOA formation region 102 and the MZM modulation region 501, excluding the portion forming the high-mesa waveguide, is the n-InP layer 202, and the upper surface of the portion forming the high-mesa waveguide is the semi-insulating OC layer 801. The high-mesa waveguide can be formed, for example, by RIE.

[0102] Subsequently, in the next step, an insulating layer is formed, followed by electrode formation, cleavage, polishing, and AR to complete the optical modulator chip with monolithic integration of SOA and MZM.

[0103] (flowchart) Figure 20 shows a flowchart of the manufacturing method of this embodiment. Each step in the flowchart is as described in Figures 11 to 19. In the manufacturing method of this embodiment, a semiconductor substrate 1101 for MZM, which has a sacrificial layer on its surface as shown in Figure 11 and includes at least a core layer and an overcladding layer, is subjected to steps 1 through 9 in sequence. After that, as a subsequent step, an insulating layer is formed, followed by electrode formation, cleavage, polishing, and AR to complete the optical modulator chip.

[0104] (Second embodiment) Next, as a second embodiment, an optical circuit that can be manufactured by the manufacturing method of the first embodiment will be described.

[0105] The optical circuit according to the second embodiment includes a substrate and a monolithically integrated semiconductor optical amplifier (SOA) and Mach-Zehnder modulator (MZM) on the substrate, wherein the difference in thickness between the OC layer thickness of the SOA and the OC layer thickness of the MZM is greater than the difference in height between the top surface of the core layer of the SOA and the top surface of the core layer of the MZM.

[0106] In the optical circuit of this embodiment, the thickness of the overclad layer 1601 of the semiconductor optical amplifier is set to be thicker than the thickness of the overclad layer (MZM-OC layer 205) of the Mach-Zehnder modulator, thereby significantly reducing losses in the contact layer of the semiconductor optical amplifier.

[0107] Figure 21 shows a schematic configuration of an optical modulator with monolithic integration of SOA as an example of an optical circuit according to the second embodiment of this disclosure. The optical modulator with monolithic integration of SOA according to the second embodiment of this disclosure can be manufactured, for example, by the manufacturing method of the first embodiment. The optical circuit of this embodiment will be described below with reference to Figures 21 to 23.

[0108] The optical modulator 2100 is an optical modulator that monolithically integrates a semiconductor optical amplifier (SOA) and a Mach-Zehnder modulator (MZM) on a semi-insulating InP substrate. Figure 21(b) is a top view of the schematic configuration of the optical modulator 2100, and Figure 21(a) is a cross-sectional view showing the schematic configuration of the cross section XXIa-XXIa in Figure 21(b). Furthermore, Figure 22(a) is a cross-sectional view showing the schematic configuration of the cross section XXIIa-XXIIa in the top view of Figure 21(b), Figure 22(b) is a cross-sectional view showing the schematic configuration of the cross section XXIIb-XXIIb in the same top view, and Figure 22(c) is a cross-sectional view showing the schematic configuration of the cross section XXIIc-XXIIc in the same top view. Note that in Figures 21 and 22, insulating layers and electrodes have been omitted to make the relationship between the thickness of the SOA-OC layer 1501 and the p-InP layer 403, which constitute the OC layer 1601 of the SOA of the optical modulator 2100, and the thickness of the MZM-OC layer easier to understand.

[0109] As shown in Figure 21(a), the SOA region 102 of the optical modulator 2100 is formed on a semi-insulating InP substrate 201 by n-InP layers 202 and 401 and an SOA core layer 402. There, as shown in Figure 22(a), a ridge waveguide 901 is formed, comprising an overcladding layer 1601 made of p-InP (SOA-OC layer 1501 and p-InP layer 403) and a contact layer 1502.

[0110] Furthermore, the MZM region 501 of the optical modulator 2100 is formed on a semi-insulating InP substrate 201 by an n-InP layer 202, a p-type cladding layer 203, and an MZM-MQW layer 204. There, as shown in Figure 22(b), ridge waveguides 902 and 903 are formed, each comprising an overcladding layer (MZM-OC layer) 205 made of n-InP and a contact layer 206.

[0111] Furthermore, the optical modulator 2100 has an input waveguide for inputting light, a waveguide connecting the SOA and MZM, and an output waveguide for outputting light, all formed on a semi-insulating InP substrate 201 using high-mesa waveguides comprising an n-InP layer 202, a p-type cladding layer 203, an MZM-MQW layer 204, and a semi-insulating OC layer 801, as shown in Figure 22(c).

[0112] In the optical modulator 2100, the difference in thickness between the combined thickness (t1) of the SOA-OC layer 1501 and the p-InP layer 403 that constitute the OC layer 1601 of the SOA and the thickness (t2) of the MZM-OC layer 205 that constitutes the OC layer of the MZM is set to be considerably larger than the difference in height between the top surface of the SOA core layer 402 and the top surface of the MZM core layer, i.e., the MZM-MQW layer 204 (the distance α in the Y-axis direction from the top surface of the SOA core layer 402 to the top surface of the MZM-MQW layer 204, which corresponds to the thickness of the p-InP layer 403).

[0113] In the optical modulator 2100 of this embodiment, the thickness of the OC layer 1601 of the SOA is set to be considerably larger than the thickness of the OC layer of the MZM. Therefore, the thickness of the SOA overcladding is made sufficient, reducing absorption loss in the contact layer and enabling the realization of an optical modulator with a large SOA saturation output. Although the conducted type of the modulator in the optical modulator 2100 shown in Figure 21 is nipn type, it may also be pin type.

[0114] (Examples) Next, examples and comparative examples of this embodiment will be described.

[0115] In the optical modulator manufactured as an example of this embodiment, the p-type cladding layer 203 was made of InAlAs with a thickness of 0.05 μm, the MZM-MQW layer 204 was made of InGaAlAs / InAlAs with a thickness of 0.5 μm, the MZM-OC layer 205 was made of n-InP with a thickness of 1.5 μm, and the contact layer 206 was made of InGaAs with a thickness of 0.2 μm.

[0116] The n-InP layer 401 has a thickness of 0.25 μm, and the SOA core layer 402 is made of InGaAsP with a thickness of 0.1 μm. The p-InP layer 403 has a thickness of 0.2 μm. The SOA-OC layer 1501 uses p-InP with a thickness of 2.8 μm, and the contact layer 1502 is made of InGaAs with a thickness of 0.2 μm. The semi-insulating OC layer 801 uses Fe-doped InP with a thickness of 1.5 μm.

[0117] In the first embodiment, the thickness of the OC layer of MZM is 1.5 μm, and the thickness of the OC layer 1601 of SOA is the sum of the thicknesses of the SOA-OC layer 1501 and the p-InP layer 403, which is 3 μm. In the first embodiment, the difference in thickness between the thickness of the OC layer 1601 of SOA and the thickness of the OC layer of MZM is 1.5 μm, which is greater than the difference in height between the height of the top surface of the SOA core layer 402 and the height of the top surface of the MZM core layer (MZM-MQW layer 204) (thickness of the p-InP layer 403), which is 0.2 μm.

[0118] As a comparative example, an optical modulator was created in which the thickness of the OC layer of the MZM manufactured by the conventional manufacturing method was set to 1.5 μm, and the thickness of the OC layer 702 of the SOA was set to 1.7 μm (i.e., the difference in thickness between the OC layer of the SOA and the OC layer of the MZM was the same as the difference in height between the top surface of the core layer of the SOA and the height of the core of the MZM). Furthermore, as a second embodiment of this model, an optical modulator was created in which the thickness of the OC layer 1601 of the SOA was set to 2.5 μm, and as a third embodiment, an optical modulator was created in which the thickness of the OC layer 1601 of the SOA was set to 2.0 μm. The materials and dimensions of each layer other than the thickness of the OC layer of the SOA were set to be the same as in the first embodiment.

[0119] As shown in Fig. 23, in the comparative example where the thickness of the OC layer 702 of the SOA was formed to be 1.7 μm, the absorption loss in the contact layer was 5.1 cm -1 On the other hand, in the first embodiment where the thickness of the OC layer 1601 of the SOA was formed to be 3 μm, it was 0.054 cm -1 In the second embodiment where the thickness of the OC layer 1601 of the SOA was formed to be 2.5 μm, it was 0.27 cm -1 In the third embodiment where the thickness of the OC layer 1601 of the SOA was formed to be 2 μm, it was 1.5 cm -1 It was thus.

[0120] From the above results, compared with the comparative example, the optical modulators of the embodiments of the present embodiment were all able to greatly reduce the loss in the contact layer. In particular, in the first embodiment where the thickness of the OC layer of the SOA was 3 μm, it was shown that the loss in the contact layer was reduced to 1 / 94 compared with the comparative example formed with 1.7 μm.

[0121] In the optical modulator in which the SOA and the MZM in which the thickness of the OC layer of the SOA of the present embodiment was set larger than that of the conventional one independently of the thickness of the OC layer of the MZM were monolithically integrated, the loss in the contact layer could be greatly reduced.

[0122] (Third Embodiment) Next, as the third embodiment, a manufacturing method in which a part of the manufacturing method described in the first embodiment is changed will be described. The process of the present embodiment uses a semiconductor substrate having a sacrificial layer on the surface and including at least a core layer and an overclad layer, similar to the manufacturing method described in the first embodiment, but is characterized in that the sacrificial layer formed on the surface, which is the uppermost surface of the semiconductor substrate, is composed of a plurality of layers. Also in the present embodiment, a case where a sacrificial layer is formed on the surface of the MZM substrate as in the first embodiment will be described as an example.

[0123] Figure 24 is a schematic diagram showing the layer structure of the MZM substrate 2401 used in the manufacturing method of this embodiment. The MZM substrate 2401 in Figure 24 is cut out from the region 105 enclosed by the dashed line in Figure 1(c), similar to the MZM substrate 1101 described in Figure 11 and Figures 2 to 10 which describe the conventional manufacturing method. Figure 24(b) is a top view of the optical modulator formation region of the wafer viewed from the Y-axis direction, and Figure 24(a) is a cross-sectional view showing the schematic configuration of the cross section XXIVa-XXIVa in the top view.

[0124] The manufacturing method of this embodiment is characterized by using an MZM substrate 2401 as the substrate for forming the optical modulator, which is formed by sequentially forming an n-InP layer 202, a p-type cladding layer 203, an MZM-MQW layer 204, an MZM-OC layer 205, a contact layer 206, and a sacrificial layer 2402 from bottom to top on a semi-insulating InP substrate (wafer) 201 shown in Figure 24.

[0125] In this embodiment, the sacrificial layer 2402 is formed by sequentially stacking an n-InP layer 2403 and an InGaAsP layer 2404. The InGaAsP layer 2404 may be a layer containing three or more elements from In, Ga, As, P, and Al, such as InGaAs.

[0126] In this embodiment, by using a sacrificial layer 2402 formed by multiple layers including an InGaAsP layer 2404, it is possible to utilize the InGaAsP layer 2404 as an etching stopper layer.

[0127] In the third step of the manufacturing method of the first embodiment, as shown in Figure 14, unnecessary SOA core layer 402 and p-InP layer 403 other than the SOA formation region 102 were removed, and then a selective growth mask 502 was formed on the MZM modulation region. In contrast, in the third step of the manufacturing method of this embodiment, as shown in Figure 25, unnecessary n-InP layer 401, SOA core layer 402, and p-InP layer 403 other than the SOA formation region 102 were removed, and then a selective growth mask 502 was formed on the MZM modulation region. Furthermore, in the manufacturing method of this embodiment, similar to the manufacturing method of the first embodiment, in the fifth step, it is possible to regrow the SOA-OC layer 1501, which will become the OC layer 1601 of the SOA, up to at least the sacrificial layer 2404. In this embodiment, in the fifth step, the SOA-OC layer 1501, which will become the OC layer of the SOA, and the contact layer are grown up to the upper surface of the sacrificial layer 2404.

[0128] Thus, in the manufacturing method of this embodiment, in the regrowth process of the SOA-OC layer 1501 and the contact layer 1502, the thickness to be regrowthed is set not by the thickness of the sacrificial layer 1102 and the n-InP layer 401, as in the first embodiment, but only by the thickness of the sacrificial layer 2402 (the thickness of the n-InP layer 2403 and the InGaAsP layer 2404) that is formed in advance on the MZM substrate 2401.

[0129] The other steps of the manufacturing method of this embodiment are substantially the same as those of the manufacturing method of the first embodiment, so their description is omitted here. [Explanation of symbols]

[0130] 101, 1101, 2401... Substrates for MZM (MZM wafers) 102... SOA formation area, SOA area 201·············Semi-insulating InP substrate 202·············n-InP layer 203·············p-type cladding layer 204...MZM-MQW layer 205...MZM-OC layer 206············Contact layer 301···········Regrowth groove 401·············n-InP layer 402·············SOA core layer (SOA active layer) 403································p-InP layer 501·············MZM modulation region, MZM region 502,701·········Selection Growth Mask 601,1501········SOA-OC layer 602,1502········Contact layer 702,1601········Overclad layer (OC layer) of SOA 801·············Semi-insulating OC layer 901, 902, 903... Ridge waveguide 1102,2401········Victims 1801, 1802, 1803... Ridge waveguides 2100············Optical modulator 2403·············n-InP layer 2404...InGaAsP layer

Claims

1. circuit board and A semiconductor optical amplifier and a Mach-Zehnder modulator monolithically integrated on the aforementioned substrate, Includes, The difference between the thickness of the overcladding layer of the semiconductor optical amplifier and the thickness of the overcladding layer of the Mach-Zehnder modulator is greater than the difference in height between the top surface of the core layer of the semiconductor optical amplifier and the top surface of the core layer of the Mach-Zehnder modulator. Optical modulator.

2. The optical modulator according to claim 1, characterized in that the thickness of the overcladding layer of the semiconductor optical amplifier is twice the thickness of the overcladding layer of the Mach-Zehnder modulator.

3. The optical modulator according to claim 1 or claim 2, wherein the semiconductor optical amplifier is configured with a ridge-type waveguide.

4. A semiconductor substrate having a sacrificial layer on its surface and comprising at least a core layer and an overcladding layer, In the semiconductor optical amplifier formation region of the semiconductor substrate, a step is made to remove a portion of the core layer and overcladding layer of the semiconductor substrate, The process of epitaxially growing the core layer of a semiconductor optical amplifier, A step of epitaxially growing the overcladding layer of a semiconductor optical amplifier to at least the sacrificial layer, A method for manufacturing an optical modulator that monolithically integrates the semiconductor optical amplifier and a Mach-Zehnder modulator, including the above.

5. The method for manufacturing an optical modulator according to claim 4, characterized in that the thickness of the overcladding layer of the semiconductor optical amplifier is set by setting the thickness of the sacrificial layer.

6. The method for manufacturing an optical modulator according to claim 4 or 5, characterized in that the sacrificial layer is formed of InP.

7. The method for manufacturing an optical modulator according to claim 4 or 5, characterized in that the sacrificial layer is formed of a layer containing three or more elements from In, Ga, As, P, and Al in its composition and a layer made of InP.

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