Optical modulator

Abstract

An optical modulator (10) is provided with an optical waveguide (1) containing a material having an electro-optical effect, a first electrode (2), and a second electrode (3). The first electrode (2) contains a semiconductor material. The second electrode (3) contains a metal material and is disposed so as to form a potential difference with the first electrode (2) and apply an electric field to the optical waveguide (1).

Description

Technical Field

[0001] The present disclosure relates to light modulators. Background Art

[0002] Due to the popularity of mobile terminals and cloud, the amount of Internet traffic has increased significantly. Therefore, the demand for optical communications has expanded. In optical communications, optical transceivers are required to convert optical signals and electrical signals. In optical transceivers, optical modulators are the main components. Optical modulators are responsible for converting electrical signals into optical signals.

[0003] Conventional optical modulators are described in, for example, Patent Document 1 and Non-Patent Document 1. The optical modulator of Patent Document 1 has a core having a slit waveguide structure. The core has an upper high refractive index layer, a lower high refractive index layer, and a low refractive index layer provided in a gap (slit) between these high refractive index layers. The refractive index of the upper and lower high refractive index layers is greater than the refractive index of the low refractive index layer. The upper and lower high refractive index layers each have a contact region. Metal electrodes are connected to the contact regions.

[0004] The optical modulator of non-patent document 1 includes an optical waveguide and two metal electrodes. The optical waveguide is arranged between the two metal electrodes. For example, one metal electrode, the optical waveguide and the other metal electrode are arranged in parallel. Alternatively, one metal electrode is stacked on the optical waveguide, and the other metal electrode is stacked on the optical waveguide on the opposite side thereof. The two electrodes are arranged to apply an electric field to the optical waveguide.

[0005] Prior Art Literature

[0006] Patent Literature

[0007] Patent Document 1: Japanese Patent Application Publication No. 2020-034610

[0008] Non-patent literature

[0009] Non-patent literature 1: Integrated lithium niobate electro-optic modulators: when performance meets scalability, Mian Zhang et al., Optica Vol.8, Issue 5, pp.652-667 (2021) Summary of the invention

[0010] Problem that the invention aims to solve

[0011] In order to apply a broadband and high-frequency signal to the electrode while suppressing power consumption, it is preferred that the thickness of the electrode is large. However, when the electrode is formed of a metal material as in the conventional optical modulator, if the thickness of the electrode is increased, a large internal stress is generated in the electrode. Due to this internal stress, cracks are easily generated in the electrode. In particular, when two metal electrodes are stacked and arranged on both sides of the optical waveguide, internal stress is generated in each electrode, which causes the two electrodes to warp and deform in the same direction. Therefore, the optical waveguide warps and deforms together with the two electrodes, and cracks are also easily generated in the optical waveguide.

[0012] An object of the present disclosure is to provide an optical modulator capable of suppressing the occurrence of cracks and applying an electrical signal to an electrode while suppressing power consumption.

[0013] Technical solutions to solve problems

[0014] The optical modulator of the present disclosure includes an optical waveguide, a first electrode, and a second electrode. The optical waveguide includes a material having an electro-optical effect. The first electrode includes a semiconductor material. The second electrode includes a metal material and is configured to form a potential difference with the first electrode to apply an electric field to the optical waveguide.

[0015] Effects of the Invention

[0016] According to the optical modulator according to the present disclosure, it is possible to suppress the occurrence of cracks and to apply an electrical signal to the electrode while suppressing power consumption. BRIEF DESCRIPTION OF THE DRAWINGS

[0017] Figure 1 It is a cross-sectional view showing a schematic structure of the optical modulator according to the first embodiment.

[0018] Figure 2 It is a cross-sectional view showing a schematic structure of a light modulator according to the second embodiment.

[0019] Figure 3 It is a cross-sectional view showing a schematic structure of an optical modulator according to a third embodiment.

[0020] Figure 4 It is a cross-sectional view showing a schematic structure of an optical modulator according to a fourth embodiment.

[0021] Figure 5 It is a cross-sectional view showing a schematic structure of an optical modulator according to the fifth embodiment.

[0022] Figure 6 A diagram showing a modified example of the optical modulator according to the fifth embodiment.

[0023] Figure 7 It is a cross-sectional view showing a schematic structure of an optical modulator according to the sixth embodiment.

[0024] Figure 8 This is a cross-sectional view showing a schematic structure of an optical modulator according to the seventh embodiment.

[0025] Fig. 9 It is a cross-sectional view showing a schematic structure of an optical modulator according to the eighth embodiment.

[0026] Fig.10 This is a cross-sectional view showing a schematic structure of an optical modulator according to the ninth embodiment.

[0027] Fig.11 It is a cross-sectional view showing a schematic structure of an optical modulator according to a tenth embodiment.

[0028] Fig.12 A diagram showing a modified example of the optical modulator according to the tenth embodiment. DETAILED DESCRIPTION

[0029] Hereinafter, embodiments of the present disclosure will be described. In addition, in the following description, embodiments of the present disclosure will be described by way of example, but the present disclosure is not limited to the examples described below. In the following description, specific numerical values ​​and specific materials are sometimes exemplified, but the present disclosure is not limited to these exemplifications.

[0030] The optical modulator according to the embodiment of the present disclosure includes an optical waveguide, a first electrode, and a second electrode. The optical waveguide includes a material having an electro-optical effect. The first electrode includes a semiconductor material. The second electrode includes a metal material and is configured to form a potential difference with the first electrode to apply an electric field to the optical waveguide (a first structure).

[0031] In the first structure, of the first electrode and the second electrode for applying an electric field to the optical waveguide, the first electrode is a semiconductor electrode containing a semiconductor material. Compared with a metal electrode, the semiconductor electrode can suppress internal stress and be formed thick. By forming the semiconductor electrode thick, power consumption can be suppressed, and a broadband and high-frequency signal can be applied to the semiconductor electrode. In addition, the semiconductor electrode can be formed while suppressing internal stress, so that the generation of cracks caused by internal stress can be suppressed.

[0032] In the first structure, of the first electrode and the second electrode for applying an electric field to the optical waveguide, the second electrode is a metal electrode made of a metal material. The metal material constituting the second electrode has a higher electrical conductivity than a semiconductor material. Therefore, compared with the case where both the first electrode and the second electrode are semiconductor electrodes, electrical loss can be suppressed.

[0033] The optical modulator of the first structure may further include a low dielectric constant layer having a refractive index smaller than that of the optical waveguide. In this case, at least the first electrode and the second electrode may be arranged with a gap between them and the optical waveguide, and the low dielectric constant layer may be arranged in the gap (second structure). Alternatively, the first electrode and the second electrode may be arranged with a gap between them and the optical waveguide, and the low dielectric constant layer may be arranged in each gap (third structure).

[0034] In the second structure, at least the first electrode as a semiconductor electrode is arranged with a gap between it and the optical waveguide. In this case, the first electrode does not contact the optical waveguide. Furthermore, a low dielectric constant layer having a refractive index smaller than that of the optical waveguide is provided in the gap between the first electrode and the optical waveguide. As a result, light passing through the optical waveguide becomes less likely to leak to the first electrode, and light loss can be suppressed.

[0035] In the third structure, the first electrode and the second electrode are arranged with a gap between the first electrode and the optical waveguide. Furthermore, a low dielectric constant layer having a refractive index smaller than that of the optical waveguide is provided in the gap between the first electrode and the optical waveguide and in the gap between the second electrode and the optical waveguide. As a result, light passing through the optical waveguide becomes less likely to leak to the first electrode and the second electrode, respectively, and light loss can be further suppressed.

[0036] In the optical modulator of the second or third structure, preferably, the size of the gap is greater than or equal to 0.750 μm and less than or equal to 1.675 μm (fourth structure).

[0037] Although it is weak, there is still light leaking from the optical waveguide to the low dielectric constant layer. This leaking light is called evanescent light. If the size of the gap between the first electrode and / or the second electrode and the optical waveguide is greater than 0.750 μm as in the fourth structure, the evanescent light becomes less likely to contact the first electrode and / or the second electrode, which can further suppress the loss of light.

[0038] In the fourth structure, the size of the gap between the first electrode and / or the second electrode and the optical waveguide is less than 1.675 μm. In this case, the distance between the first electrode and / or the second electrode and the optical waveguide does not become too large, and the size of the electric field on the optical waveguide can be ensured even if the voltage applied between the first electrode and the second electrode is not increased.

[0039] In any of the optical modulators of the second to fourth structures, the semiconductor material may be a silicon semiconductor material in which impurities are doped in silicon, and the main component of the low dielectric constant layer may be SiO2 (the fifth structure). Because the semiconductor material is a silicon semiconductor material, when a low dielectric constant layer is provided in the gap between the first electrode and the optical waveguide, a low dielectric constant layer of SiO2 can be formed on the first electrode of the silicon semiconductor material by a thermal oxidation method. In the film formation using the thermal oxidation method, the low dielectric constant layer has good adhesion to the first electrode, and foreign matter is not easy to enter the interface between the first electrode and the low dielectric constant layer. Therefore, at the interface between the first electrode and the low dielectric constant layer, electrical loss can be suppressed. In addition, the aggregation of foreign matter can be suppressed at the interface between the first electrode and the low dielectric constant layer, so that the reliability and life of the optical modulator can be improved.

[0040] In any one of the optical modulators of the first to fifth structures, preferably, the first electrode is stacked on the optical waveguide, and the second electrode is stacked on the optical waveguide on the opposite side of the first electrode (sixth structure). In this case, in the stacking direction of the first electrode, the optical waveguide, and the second electrode, the optical waveguide exists between the first electrode and the second electrode. Therefore, the electric field generated by the first electrode and the second electrode can be efficiently applied to the optical waveguide.

[0041] In the optical modulator of the sixth structure, the first electrode may include a convex portion. The convex portion is provided on the surface located on the optical waveguide side in the stacking direction of the first electrode, the optical waveguide, and the second electrode, and protrudes toward the optical waveguide (the seventh structure). In this case, the convex portion can concentrate the electric field on the optical waveguide. Therefore, the voltage applied between the first electrode and the second electrode can be reduced, and the power consumption can be further suppressed.

[0042] In the optical modulator of the seventh structure, when observed in a cross section perpendicular to the direction in which the optical waveguide extends, the length of the convex portion in the direction perpendicular to the stacking direction may be smaller as it approaches the optical waveguide (the eighth structure). For example, when the convex portion has a rectangular shape in a cross section, the side surface of the convex portion is continuous at a right angle to the other portion of the surface on the optical waveguide side of the first electrode. On the other hand, when the length of the convex portion in the direction perpendicular to the stacking direction of the electrode and the optical waveguide is smaller as it approaches the optical waveguide, as in the eighth structure, the side surface of the convex portion can be made to be continuous relatively smoothly to the other portion of the surface on the optical waveguide side of the first electrode. Thus, it is possible to prevent electrical loss from being generated at the boundary between the convex portion and the other portion.

[0043] In any one of the sixth to eighth structures of the optical modulator, preferably, when viewed in a cross section perpendicular to the direction in which the optical waveguide extends, the length of the surface of the first electrode on the optical waveguide side in a direction perpendicular to the stacking direction of the first electrode, the optical waveguide, and the second electrode is greater than the length of the optical waveguide (ninth structure). In this case, an electric field can be applied to the entire region of the optical waveguide.

[0044] In any one of the optical modulators of the first to ninth structures, preferably, the semiconductor material in the first electrode is a silicon semiconductor material obtained by doping silicon with impurities (the tenth structure).

[0045] In the optical modulator of the tenth structure, preferably, the concentration of impurities in the first electrode is 1.0×10 17 cm -3 Above and 1.0×10 22 cm -3 As shown below (11th structure). In the first electrode, as the impurity increases, the resistivity decreases and the conductivity increases. If the impurity concentration is 1.0×10 17 cm -3 If the concentration of impurities is 1.0×10 22 cm -3 The following can prevent the precipitation of impurities.

[0046] In any of the optical modulators of the first to eleventh structures, the refractive index of the first electrode is less than 3 (the twelfth structure). In this case, for example, the refractive index of the first electrode becomes less than 3 according to the concentration (doping amount) of the impurity in the eleventh structure.

[0047] In any one of the optical modulators of the tenth to twelfth structures, preferably, the first electrode is a silicon single crystal substrate (the thirteenth structure).

[0048] In any of the optical modulators of the tenth to twelfth structures, the first electrode may be an active layer of the SOI substrate (fourteenth structure).

[0049] In any of the optical modulators of the first to fourteenth structures, preferably, the main component of the metal material is a noble metal (the fifteenth structure). In this case, the metal material constituting the second electrode is mainly composed of a noble metal that is not easily chemically reacted, so the modulation characteristics of the optical modulator are stable. Furthermore, if the main component of the metal material constituting the second electrode is a noble metal, the resistance value of the second electrode can be suppressed, and the power consumption can be suppressed.

[0050] In any of the optical modulators of the first to fifteenth structures, preferably, when viewed in a cross section perpendicular to the direction in which the optical waveguide extends, the area of ​​the first electrode is larger than the area of ​​the second electrode (the sixteenth structure). The second electrode includes a metal material, so that the resistance value is sufficiently small even if the cross-sectional area is not increased. On the other hand, the first electrode includes a semiconductor material having a lower electrical conductivity than the metal material, so the resistance value can be reduced by making the cross-sectional area larger than the second electrode. In this regard, in the sixteenth structure, the cross-sectional area of ​​the first electrode is larger than the cross-sectional area of ​​the second electrode, so the resistance value of the first electrode becomes smaller, and power consumption can be suppressed.

[0051] In any one of the optical modulators of the first to sixteenth structures, preferably, the second electrode is a signal electrode and the first electrode is a ground electrode (the seventeenth structure). That is, the second electrode, which is a metal electrode, is used as a signal electrode, and the first electrode, which is a semiconductor electrode, is used as a ground electrode. The second electrode includes a metal material having high conductivity and low attenuation of high-frequency signals. If the second electrode is set as a signal electrode, the driving voltage can be suppressed.

[0052] Any optical modulator in the first to seventeenth structures may also include a metal thin layer thinner than the first electrode. The metal thin layer is disposed on the surface of the optical waveguide side of the first electrode (the eighteenth structure). The metal thin layer has a large electrical conductivity and a small attenuation of high-frequency signals. If the metal thin layer is disposed on the surface of the optical waveguide side of the first electrode, the driving voltage can be suppressed. Furthermore, in the first electrode, due to the skin effect, the high-frequency signal is transmitted more through the surface layer, so the electrical conductivity near the surface layer is preferably greater. In this regard, if the metal thin layer is disposed on the surface of the optical waveguide side of the first electrode, the resistivity can be reduced and the attenuation of the signal can be suppressed.

[0053] In any one of the optical modulators of the first to eighteenth structures, preferably, the surface layer on the optical waveguide side of the first electrode is doped with impurities at a higher concentration than other portions of the first electrode (the nineteenth structure). In this case, in the first electrode, a region with high electrical conductivity can be locally present near the optical waveguide, and attenuation of high-frequency signals can be suppressed by the skin effect.

[0054] Hereinafter, embodiments of the present disclosure will be described with reference to the accompanying drawings. In each of the drawings, the same or corresponding structures are denoted by the same reference numerals, and the description thereof will not be repeated.

[0055] <First embodiment>

[0056] [Structure of light modulator]

[0057] Figure 12 is a cross-sectional view showing a schematic structure of an optical modulator 10 according to the first embodiment. The optical modulator 10 includes an optical waveguide 1, a first electrode 2, and a second electrode 3. Figure 1 , a cross section perpendicular to the direction in which the optical waveguide 1 extends is shown. In this specification, a cross section means a cross section perpendicular to the direction in which the optical waveguide 1 extends, unless otherwise specified.

[0058] like Figure 1 As shown, the optical waveguide 1 may have a substantially rectangular cross section. The optical waveguide 1 includes a material having an electro-optical effect. The optical waveguide 1 functions as a light transmission path. As the optical waveguide 1, LiNbO3 (lithium niobate), LiTaO3 (lithium tantalate), PLZT (lead lanthanum zirconate titanate), KTN (potassium tantalate niobate), or BaTiO3 (barium titanate) may be used. As the optical waveguide 1, an electro-optic polymer (EO polymer) may also be used.

[0059] The first electrode 2 and the second electrode 3 function as control electrodes for controlling light passing through the optical waveguide 1. The first electrode 2 and the second electrode 3 may each have a substantially rectangular cross section. The first electrode 2 and the second electrode 3 are arranged to form a potential difference between each other and apply an electric field to the optical waveguide 1. The optical waveguide 1 is arranged between the first electrode 2 and the second electrode 3.

[0060] In the example of this embodiment, the first electrode 2 is stacked on the optical waveguide 1. The second electrode 3 is stacked on the optical waveguide 1 on the opposite side of the first electrode 2. From another point of view, the first electrode 2 and the second electrode 3 are arranged to sandwich the optical waveguide 1. In this embodiment, the optical waveguide 1 is directly stacked on the first electrode 2, and the first electrode 2 is in contact with the optical waveguide 1. Furthermore, the second electrode 3 is directly stacked on the optical waveguide 1, and the second electrode 3 is in contact with the optical waveguide 1.

[0061] The first electrode 2 includes a semiconductor material. That is, the first electrode 2 is a semiconductor electrode. The semiconductor material used for the first electrode 2 is typically a silicon semiconductor material in which impurities are doped in Si (silicon). As the semiconductor material, for example, other single element semiconductors using Ge (germanium) or compound semiconductors such as GaAs (gallium arsenide) can also be used. The impurity can be either a p-type impurity or an n-type impurity. For example, when the semiconductor material is a silicon semiconductor material, a Group 3 element such as boron can be used as a p-type impurity, and a Group 5 element such as phosphorus, arsenic, and antimony can be used as an n-type impurity.

[0062] When the semiconductor material used for the first electrode 2 is a silicon semiconductor material, the concentration (doping amount) of impurities in the first electrode 2 is preferably 1.0×10 17 cm -3 Above and 1.0×1022 cm -3 As the doping amount of impurities increases, the resistivity of the semiconductor material decreases and the conductivity increases. If the doping amount is 1.0×10 17 cm -3 As described above, the first electrode 2 can effectively function as an electrode. 22 cm -3 Then, the precipitation of impurities can be prevented by the solid solubility limit of impurities in the silicon semiconductor material. As the doping amount increases, the refractive index of the first electrode 2 decreases. For example, the refractive index of the first electrode 2 is less than 3.

[0063] The above-mentioned doping amount range is described in more detail below. The upper limit of the doping amount is preferably 1.0×10 22 cm -3 The reason is based on the solid solubility limit of impurities in silicon semiconductor materials. In the first electrode 2, if the doping amount exceeds 1.0×10 22 cm -3 , impurities are deposited, and the reliability of the first electrode 2 and the optical modulator 10 is reduced. On the other hand, the lower limit of the doping amount is preferably 1.0×10 17 cm -3 The reason is as follows. As an indicator for designing the thickness and width of the electrode, there is skin depth. If the thickness and width of the electrode are smaller than the skin depth, the resistance value increases. Therefore, it is preferable to set the thickness and width of the electrode to be greater than the skin depth. In the case of considering the use of an optical modulator that processes a frequency signal of 1 GHz or more, when the doping amount is 1.0×10 17 cm -3 When the conductivity is 1000 S / m, the skin depth becomes 500 μm. For example, if we consider forming an electrode using a silicon semiconductor material by microfabrication, the thickness of the electrode is limited to about 500 μm. In this way, from the perspective of ensuring the performance of the electrode, in order to obtain an electrode with a conductivity of more than 1000 S / m, it is sufficient to set the doping amount to 1.0×10 17 cm -3 That’s all.

[0064] The first electrode 2 is, for example, a silicon single crystal substrate. Impurities are, for example, pre-doped into a mother substrate of silicon single crystal that is a raw material of the first electrode 2. The mother substrate is arranged on another substrate and patterned (etched, cut, etc.), thereby forming the first electrode 2. The first electrode 2 may also be an active layer of an SOI (Silicon on Insulator) substrate. In this case, the first electrode 2 can be formed by patterning (etched, cut, etc.) the active layer of the SOI substrate. For the first electrode 2 formed in this way, impurities may be further introduced by a thermal diffusion method or an ion implantation method.

[0065] The first electrode 2 may also be a semiconductor silicon layer formed on a substrate. For example, a silicon layer may be formed on a substrate by sputtering, vapor deposition, or CVD. Impurities may be introduced into the silicon layer by thermal diffusion or ion implantation to form a semiconductor silicon layer serving as the first electrode 2.

[0066] The second electrode 3 includes a metal material. That is, the second electrode 3 is a metal electrode. Regarding the metal material used for the second electrode 3, for example, its main component is a noble metal. The noble metal is, for example, Au (gold). As the noble metal, Ag (silver), Pt (platinum), etc. can also be used. The metal material can also contain trace amounts of other metal elements such as Cr and Ti. As the metal material, copper, aluminum, or alloys thereof can also be used.

[0067] The second electrode 3 can be stacked on the optical waveguide 1 and the first electrode 2, for example, as follows. First, a material substrate having an electro-optical effect is arranged on the first electrode 2, and the material substrate and the first electrode 2 are bonded. Then, photolithography and etching are performed on the material substrate to form the optical waveguide 1. Next, a metal layer is formed on the optical waveguide 1 by sputtering, evaporation, etc. The formed metal film is patterned by photolithography, and the second electrode 3 is formed by etching.

[0068] The second electrode 3 is used as a signal electrode, and the first electrode 2 is used as a ground electrode. Conversely, the first electrode 2 may be used as a signal electrode, and the second electrode 3 may be used as a ground electrode.

[0069] exist Figure 1 In the example shown, the optical waveguide 1 has a thickness t1 corresponding to the length in the stacking direction of the optical waveguide 1 and the electrodes 2 and 3, and a width w1 corresponding to the length in the direction perpendicular to the stacking direction. The first electrode 2 has a thickness t2 corresponding to the length in the stacking direction, and a width w2 corresponding to the length in the direction perpendicular to the stacking direction. The second electrode 3 has a thickness t3 corresponding to the length in the stacking direction, and a width w3 corresponding to the length in the direction perpendicular to the stacking direction.

[0070] When viewed in a cross section perpendicular to the direction in which the optical waveguide 1 extends, the area of ​​the first electrode 2 is larger than the area of ​​the second electrode 3. Specifically, the cross-sectional area of ​​the first electrode 2, which is a semiconductor electrode, is larger than the cross-sectional area of ​​the second electrode 3, which is a metal electrode. It is preferable to set the cross-sectional area of ​​the first electrode 2 and the cross-sectional area of ​​the second electrode 3 so that the resistance value of the first electrode 2 is substantially the same as the resistance value of the second electrode 3. That is, the performance of the first electrode 2 as an electrode is preferably equivalent to that of the second electrode 3. For example, the cross-sectional area of ​​the first electrode 2 can be set to "(conductivity of the second electrode 3 / conductivity of the first electrode 2)×cross-sectional area of ​​the second electrode 3".

[0071] Under the condition that the width w2 of the first electrode 2 and the width w3 of the second electrode 3 are the same, in order to make the resistance value of the first electrode 2 and the resistance value of the second electrode 3 consistent, the product of the conductivity and the thickness t2 in the first electrode 2 and the product of the conductivity and the thickness t3 in the second electrode 3 are consistent. For example, when the metal material in the second electrode 3 is Au, usually, the thickness t3 of the second electrode 3 is set to be greater than 0.1 μm and less than 2.0 μm, and its conductivity is 4.3×10 7 On the other hand, since the electrical conductivity of the first electrode 2 is smaller than that of the second electrode 3, the thickness t2 of the first electrode 2 is larger than the thickness t3 of the second electrode 3. The electrical conductivity of the first electrode 2 changes depending on the doping amount of the impurity.

[0072] For example, when the first electrode 2 is made of a silicon semiconductor material, when the doping amount of the impurity is 1.0×10 22 cm -3 When the conductivity of the first electrode 2 becomes 1×10 7 At this time, the value obtained by dividing the conductivity of the second electrode 3 by the conductivity of the first electrode 2 is 4.3, and the thickness t2 of the first electrode 2 can be set to 4.3 times the thickness t3 of the second electrode 3. On the other hand, when the doping amount of impurities is 1.0×10 17 cm -3 , the conductivity of the first electrode 2 becomes 1000 S / m. At this time, the value obtained by dividing the conductivity of the second electrode 3 by the conductivity of the first electrode 2 is 4.3×10 3 The thickness t2 of the first electrode 2 can be set to 4.3×10 of the thickness t3 of the second electrode 3. 3 times.

[0073] Therefore, the lower limit of the thickness t2 of the first electrode 2 can be set to 4.3 times the lower limit of the thickness t3 of the second electrode 3, that is, 0.1 μm. In other words, the thickness t2 of the first electrode 2 can be set to 0.43 μm or more. On the other hand, the upper limit of the thickness t2 of the first electrode 2 can be set to 4.3×10 times the upper limit of the thickness t3 of the second electrode 3, that is, 2.0 μm.3 That is, the thickness t2 of the first electrode 2 can be set to 8600 μm (8.6 mm) or less.

[0074] However, the thickness t2 of the first electrode 2 is determined in consideration of, for example, processability in addition to the conductivity of each of the electrodes 2 and 3 and the thickness t3 of the second electrode 3. For example, when the first electrode 2 is formed using a silicon semiconductor material, the thickness t2 of the first electrode 2 is preferably 500 μm or less from the viewpoint of processability.

[0075] From another point of view, when a high-frequency current is passed through the first electrode 2, the thickness t2 required for the first electrode 2 can be estimated based on the skin effect. The following formula (1) is a formula for calculating the skin depth of a conductor.

[0076] [Mathematical formula 1]

[0077] (1)

[0078] d: Skin depth [m]

[0079] ρ: Resistivity of conductor [Ω·m] = 1 / σ

[0080] ω: angular frequency of alternating current [rds / s] = 2πf

[0081] μ: Magnetic permeability of conductor [H / m]

[0082] σ: Conductor conductivity [S / m]

[0083] f: frequency of alternating current [Hz]

[0084] According to formula (1), the required thickness t2 of the first electrode 2 can be determined. More specifically, by making the thickness t2 of the first electrode 2 larger than the skin depth calculated using formula (1), the resistance of the first electrode 2 can be reduced, and unnecessary electrical loss can be suppressed. The higher the conductivity of the first electrode 2, the better. However, there is a solid solubility limit for the doping amount of impurities. In addition, if the doping amount is close to the solid solubility limit, the impurities will cluster and become inert as carriers. Therefore, if the doping amount exceeds a certain amount, the conductivity of the first electrode 2 will be saturated. If the solid solubility limit of the doping amount is 1.0×10 22 cm -3 , then the conductivity of the first electrode 2 becomes 1×10 7 S / m, the skin depth of a 1 GHz electrical signal is 5 μm. However, it is conceivable that the actual electrical conductivity decreases by one order of magnitude to 1×10 6Therefore, if the electric signal is given a bandwidth and is used to process a signal of 0.5 GHz or higher, the thickness t2 of the first electrode 2 is preferably 25 μm or more.

[0085] The thickness t2 of the first electrode 2 as a semiconductor electrode can be measured, for example, by the following method. The first method is a measurement method based on SEM observation. In this method, the optical modulator 10 is cut by FIB (focused ion beam) to collect a sample. The cross section of the collected sample can be photographed by SEM, and the thickness t2 of the first electrode 2 can be measured from the obtained image. The second method is an optical measurement method. In this method, the thickness t2 of the first electrode 2 can be directly measured by interference spectroscopy. Regardless of which method is used, the measurement results are substantially the same.

[0086] The thickness t3 of the second electrode 3 as a metal electrode can be measured, for example, by the following method. The first method is the above-mentioned measurement method based on SEM observation. The second method is a measurement method using X-rays. In this method, X-rays are irradiated to the second electrode 3, and the amount of transmitted X-rays is measured, thereby obtaining the attenuation caused by the second electrode 3. By performing an inverse operation on the obtained attenuation, the thickness t3 of the second electrode 3 can be measured. Regardless of which method is used, the measurement results are substantially the same.

[0087] The doping amount in the first electrode 2 can be measured by epitaxial resistivity measurement, air gap CV measurement, mercury CV measurement, surface charge distribution, secondary ion mass spectrometry, or spreading resistance measurement, etc. The measurement results are substantially the same regardless of the method.

[0088] When viewed in a cross section perpendicular to the direction in which the optical waveguide 1 extends, the width w2 of the first electrode 2 on the optical waveguide 1 side is greater than the width w1 of the optical waveguide 1. The width w2 of the first electrode 2 on the optical waveguide 1 side means the width of the surface of the first electrode 2 closest to the optical waveguide 1. In the present embodiment, the length of the surface of the first electrode 2 in contact with the optical waveguide 1 in the direction perpendicular to the stacking direction is the width w2.

[0089] [Effect]

[0090] In the optical modulator 10 according to the present embodiment, the first electrode 2 includes a semiconductor material, and the second electrode 3 includes a metal material. That is, of the first electrode 2 and the second electrode 3 that apply an electric field to the optical waveguide 1, the first electrode 2 is a semiconductor electrode. Compared with a metal electrode, a semiconductor electrode can suppress internal stress and can be formed thick. By forming the semiconductor electrode thick, power consumption can be suppressed, and a broadband and high-frequency signal can be applied to the semiconductor electrode. In addition, the semiconductor electrode can be formed while suppressing internal stress, so that the generation of cracks caused by internal stress can be suppressed.

[0091] The semiconductor material used for the first electrode 2 is, for example, a silicon semiconductor material in which impurities are doped in Si. The first electrode 2 may be a silicon single crystal substrate or a semiconductor silicon layer formed on a substrate. For example, when the first electrode 2 is a silicon single crystal substrate, the internal stress of the first electrode 2 can be reduced compared to the case where a semiconductor silicon layer is formed on a substrate. Therefore, the first electrode 2 can be formed thicker while further suppressing the internal stress of the first electrode 2.

[0092] In the optical modulator 10 according to the present embodiment, of the first electrode 2 and the second electrode 3 for applying an electric field to the optical waveguide 1, the second electrode 3 is a metal electrode. The metal material constituting the second electrode has a higher electrical conductivity than a semiconductor material. Therefore, compared with the case where both the first electrode and the second electrode are semiconductor electrodes, electrical loss can be suppressed.

[0093] The second electrode 3 as a metal electrode can be formed on the optical waveguide 1 by sputtering or evaporation without an adhesive layer. Therefore, light absorption by the adhesive layer can be prevented. In addition, the adhesive layer can be prevented from diffusing into the optical waveguide 1 and changing the refractive index of the optical waveguide 1.

[0094] In this embodiment, the first electrode 2 is stacked on the optical waveguide 1, and the second electrode 3 is stacked on the optical waveguide 1 on the opposite side of the first electrode 2. In this case, in the stacking direction, the optical waveguide 1 exists between the first electrode 2 and the second electrode 3. Therefore, the electric field generated by the first electrode 2 and the second electrode 3 can be efficiently applied to the optical waveguide 1.

[0095] In the present embodiment, the second electrode 3 includes a metal material, and the main component of the metal material is, for example, a noble metal. When the second electrode 3 is exposed to the atmosphere, impurities from the material in contact with the second electrode 3 or gas molecules flying out from other materials by degassing may diffuse into the second electrode. If such diffusion occurs in the second electrode 3, the impedance changes and deviates from the initial design value. If the impedance of the second electrode 3 deviates from the design value, the modulation characteristics of the optical modulator 10 are affected. In the case where the metal material constituting the second electrode 3 is mainly composed of a noble metal that is not easy to react chemically, the modulation characteristics of the optical modulator 10 are stable. Furthermore, if the main component of the metal material constituting the second electrode 3 is a noble metal, the resistance value of the second electrode 3 can be suppressed, and the power consumption can be suppressed.

[0096] In this embodiment, when viewed in a cross section perpendicular to the direction in which the optical waveguide 1 extends, the length (width w2) of the surface of the first electrode 2 on the optical waveguide 1 side in the direction perpendicular to the stacking direction is greater than the length (width w1) of the optical waveguide 1. In this case, an electric field can be applied to the entire region of the optical waveguide 1. In addition, when the optical waveguide 1 is formed on the first electrode 2, stress can be relieved.

[0097] In the present embodiment, when viewed in a cross section perpendicular to the direction in which the optical waveguide 1 extends, the area of ​​the first electrode 2 is larger than the area of ​​the second electrode 3. Since the second electrode 3 is made of a metal material, the resistance value is sufficiently small even if the cross-sectional area is not increased. On the other hand, since the first electrode 2 is made of a semiconductor material having a lower electrical conductivity than the metal material, the resistance value can be reduced by making the cross-sectional area larger than the second electrode 3. In the present embodiment, the cross-sectional area of ​​the first electrode 2 is larger than the cross-sectional area of ​​the second electrode 3, so the resistance value of the first electrode 2 becomes small, and power consumption can be suppressed.

[0098] In this embodiment, the second electrode 3 as a metal electrode is used as a signal electrode, and the first electrode 2 as a semiconductor electrode is used as a ground electrode. The second electrode 3 includes a metal material with high conductivity and low attenuation of high-frequency signals. Therefore, if the second electrode 3 is set as a signal electrode, the driving voltage can be suppressed.

[0099] <Second embodiment>

[0100] Figure 2 1 is a cross-sectional view schematically showing a configuration of an optical modulator 10A according to the second embodiment. The optical modulator 10A is different from the optical modulator 10 according to the first embodiment in that the optical modulator 10A includes a low dielectric constant layer 4 .

[0101] In the first embodiment described above, the first electrode 2 is in contact with the optical waveguide 1. Even when the first electrode 2 is in contact with the optical waveguide 1, as long as the difference in refractive index between the first electrode 2 and the optical waveguide 1 is large, light can be confined in the optical waveguide 1 by adjusting the thickness of the optical waveguide 1 or the like. However, if the doping amount of impurities in the first electrode 2 is increased, the refractive index of the first electrode 2 decreases and approaches the refractive index of the optical waveguide 1. Therefore, light may leak from the optical waveguide 1 to the first electrode 2.

[0102] In the present embodiment, the first electrode 2 is arranged with a gap between it and the optical waveguide 1. That is, the first electrode 2 is separated from the optical waveguide 1 in the stacking direction. The first electrode 2 does not contact the optical waveguide 1. The size of the gap between the first electrode 2 and the optical waveguide 1 is, for example, not less than 0.750 μm and not more than 1.675 μm. In this specification, the size of the gap between the first electrode 2 and the optical waveguide 1 means the shortest distance from the first electrode 2 to the optical waveguide 1. In the case of the present embodiment, the distance from the first electrode 2 to the optical waveguide 1 in the stacking direction is the shortest distance from the first electrode 2 to the optical waveguide 1.

[0103] The low dielectric constant layer 4 has a refractive index smaller than that of the optical waveguide 1. The low dielectric constant layer 4 is provided in a gap between the first electrode 2 and the optical waveguide 1. In the optical modulator 10A, the low dielectric constant layer 4 is stacked on the first electrode 2, and the optical waveguide 1 is stacked on the low dielectric constant layer 4. That is, the optical waveguide 1 is indirectly stacked on the first electrode 2 via the low dielectric constant layer 4, and the first electrode 2 and the optical waveguide 1 are not in contact. The low dielectric constant layer 4 preferably covers the entire surface of the optical waveguide 1 that is opposite to the low dielectric constant layer 4. On the other hand, similarly to the first embodiment, the second electrode 3 is directly stacked on the optical waveguide 1 and in contact with the optical waveguide 1.

[0104] The main component of the low dielectric constant layer 4 is typically SiO 2 . As the main component of the low dielectric constant layer 4 , oxides such as Al 2 O 3 , LaAlO 3 , LaYO 3 , ZnO, HfO 2 , MgO, and Y 2 O 3 , or polymers such as BCB (benzocyclobutene) and PI (polyimide) may be used.

[0105] The low dielectric constant layer 4 is formed on the first electrode 2 by, for example, CVD, evaporation, sputtering, etc. Then, a material substrate having an electro-optical effect can be arranged on the low dielectric constant layer 4, and the material substrate and the low dielectric constant layer 4 can be bonded. Hereinafter, as described in the first embodiment, the optical waveguide 1 and the second electrode 3 can be formed on the low dielectric constant layer 4.

[0106] In the present embodiment, the first electrode 2 is not in contact with the optical waveguide 1. Furthermore, a low dielectric constant layer 4 having a refractive index lower than that of the optical waveguide 1 is provided in the gap between the first electrode 2 and the optical waveguide 1. Therefore, compared with the case where the first electrode 2 and the optical waveguide 1 are in contact, light passing through the optical waveguide 1 is less likely to leak to the first electrode 2 and is less likely to be absorbed by the first electrode 2. Therefore, light loss can be suppressed.

[0107] For example, if the semiconductor material used for the first electrode 2 is a silicon semiconductor material, a low dielectric constant layer 4 of SiO2 can be formed on the first electrode 2 of the silicon semiconductor material by a thermal oxidation method. In this case, the low dielectric constant layer 4 has good adhesion to the first electrode 2, and foreign matter is not easy to enter the interface between the first electrode 2 and the low dielectric constant layer 4. Therefore, at the interface between the first electrode 2 and the low dielectric constant layer 4, electrical loss can be suppressed. In addition, the reliability and life of the optical modulator 10A can be improved. This is because if foreign matter gathers at the interface between the first electrode 2 and the low dielectric constant layer 4, and the electric field is concentrated on the gathered foreign matter, the optical modulator 10A may be damaged.

[0108] The penetration depth of the evanescent light in the low dielectric constant layer 4 can be estimated based on the wavelength of the light (carrier) passing through the optical waveguide 1. When the first electrode 2 and the optical waveguide 1 are separated by more than the wavelength of the carrier, the evanescent light can be prevented from contacting the first electrode 2. Therefore, the size of the gap between the optical waveguide 1 and the first electrode 2, that is, the thickness (length in the stacking direction) of the low dielectric constant layer 4 is preferably greater than the wavelength of the light passing through the optical waveguide 1.

[0109] For example, if the size of the gap between the first electrode 2 and the optical waveguide 1 is 0.750 μm or more as in the present embodiment, the thickness of the low dielectric constant layer 4 becomes larger than the penetration depth of the evanescent light, and the light passing through the optical waveguide 1 is less likely to leak to the first electrode 2. As described above, the size of the gap between the first electrode 2 and the optical waveguide 1 may be 1.675 μm or less. If the size of the gap between the first electrode 2 and the optical waveguide 1 is 1.675 μm or less, the size of the electric field on the optical waveguide 1 can be ensured even if the voltage applied between the first electrode 2 and the second electrode 3 is not increased.

[0110] <Third embodiment>

[0111] Figure 3 1 is a cross-sectional view showing a schematic structure of an optical modulator 10B according to the third embodiment. The optical modulator 10B is different from the optical modulator 10 according to the first embodiment in that the optical modulator 10B includes a low dielectric constant layer 5 .

[0112] In the present embodiment, the second electrode 3 is arranged with a gap between it and the optical waveguide 1. That is, the second electrode 3 is separated from the optical waveguide 1 in the stacking direction. The second electrode 3 does not contact the optical waveguide 1. The size of the gap between the second electrode 3 and the optical waveguide 1 is, for example, not less than 0.750 μm and not more than 1.675 μm. In this specification, the size of the gap between the second electrode 3 and the optical waveguide 1 means the shortest distance from the second electrode 3 to the optical waveguide 1. In the case of the present embodiment, the distance from the second electrode 3 to the optical waveguide 1 in the stacking direction is the shortest distance from the second electrode 3 to the optical waveguide 1.

[0113] The low dielectric constant layer 5 has a refractive index smaller than that of the optical waveguide 1. The low dielectric constant layer 5 is provided in a gap between the second electrode 3 and the optical waveguide 1. In the optical modulator 10B, the low dielectric constant layer 5 is stacked on the optical waveguide 1, and the second electrode 3 is stacked on the low dielectric constant layer 5. That is, the optical waveguide 1 is indirectly stacked with respect to the second electrode 3 via the low dielectric constant layer 5, and the second electrode 3 is not in contact with the optical waveguide 1. The low dielectric constant layer 5 preferably covers the entire surface of the optical waveguide 1 that is opposite to the low dielectric constant layer 5. On the other hand, similarly to the first embodiment, the first electrode 2 is directly stacked on the optical waveguide 1 and in contact with the optical waveguide 1.

[0114] Examples of the main component of low dielectric constant layer 5 include the same main component as low dielectric constant layer 4 of the second embodiment. The main component of low dielectric constant layer 5 may be the same as or different from the main component of low dielectric constant layer 4 .

[0115] The low dielectric constant layer 5 is formed on the optical waveguide 1 by, for example, CVD, vapor deposition, sputtering, etc. It is sufficient to form the optical waveguide 1 on the first electrode 2 as described in the first embodiment before forming the low dielectric constant layer 5. Then, a metal layer is formed on the low dielectric constant layer 5 by sputtering, vapor deposition, etc., and the second electrode 3 is formed on the formed metal film as described in the first embodiment.

[0116] In the present embodiment, the second electrode 3 is not in contact with the optical waveguide 1. Furthermore, a low dielectric constant layer 5 having a refractive index lower than that of the optical waveguide 1 is provided in the gap between the second electrode 3 and the optical waveguide 1. Therefore, compared with the case where the second electrode 3 and the optical waveguide 1 are in contact, light passing through the optical waveguide 1 is less likely to leak to the second electrode 3 and is less likely to be absorbed by the second electrode 3. Therefore, light loss can be suppressed.

[0117] In this embodiment, the size of the gap between the second electrode 3 and the optical waveguide 1 is 0.750 μm or more and 1.675 μm or less. Therefore, similarly to the second embodiment, the size of the electric field on the optical waveguide 1 can be ensured while suppressing light loss.

[0118] <Fourth embodiment>

[0119] Figure 4 1 is a cross-sectional view showing a schematic structure of an optical modulator 10C according to the fourth embodiment. The optical modulator 10C is different from the optical modulator 10 according to the first embodiment in that it includes a low dielectric constant layer 4 and a low dielectric constant layer 5. From another point of view, the optical modulator 10C is an optical modulator that combines the structure of the second embodiment and the structure of the third embodiment.

[0120] In the present embodiment, both the first electrode 2 and the second electrode 3 are arranged with a gap between them and the optical waveguide 1. Furthermore, low dielectric constant layers 4 and 5 having a refractive index lower than that of the optical waveguide 1 are provided in the gap between the first electrode 2 and the optical waveguide 1 and in the gap between the second electrode 3 and the optical waveguide 1, respectively. As a result, it becomes difficult for light passing through the optical waveguide 1 to leak to each of the first electrode 2 and the second electrode 3. Therefore, in the optical modulator 10C, the loss of light can be further suppressed compared with the optical modulators 10A and 10B.

[0121] <Fifth embodiment>

[0122] Figure 5 1 is a cross-sectional view schematically showing a structure of an optical modulator 10D according to a fifth embodiment. The optical modulator 10D is different from the optical modulator 10C according to the fourth embodiment in the structure of a first electrode 2A.

[0123] Reference Figure 5 , the first electrode 2A includes a convex portion 2Aa. In the first electrode 2A, the convex portion 2Aa is provided on the surface located on the optical waveguide 1 side in the stacking direction, and protrudes toward the optical waveguide 1. In the example of the present embodiment, the first electrode 2A includes a convex portion 2Aa and a base portion 2Ab. The base portion 2Ab is arranged on the opposite side of the optical waveguide 1 in the stacking direction. The convex portion 2Aa protrudes toward the optical waveguide 1 from the surface 2Aba of the base portion 2Ab. The convex portion 2Aa and the base portion 2Ab may each have a substantially rectangular cross-section. In the optical modulator 10D, a low dielectric constant layer 4 is stacked on the convex portion 2Aa of the first electrode 2A, and the convex portion 2Aa is in contact with the low dielectric constant layer 4.

[0124] When viewed in a cross section perpendicular to the direction in which the optical waveguide 1 extends, the width w2Ab of the base 2Ab is greater than the width w2Aa of the convex portion 2Aa. The width w2Aa means the width of the surface 2Aaa of the convex portion 2Aa closest to the optical waveguide 1. In the present embodiment, the length of the surface 2Aaa of the convex portion 2Aa in the direction perpendicular to the stacking direction that is in contact with the low dielectric constant layer 4 is the width w2Aa. The width w2Aa of the convex portion 2Aa is greater than the width w1 of the optical waveguide 1.

[0125] In the optical modulator 10D of the present embodiment, the convex portion 2Aa can concentrate the electric field on the optical waveguide 1. Therefore, the voltage applied between the first electrode 2A and the second electrode 3 can be reduced, and the power consumption can be suppressed.

[0126] exist Figure 5 In the example of FIG. 1 , the first electrode 2A including the protrusion 2Aa and the base 2Ab is, for example, a silicon single crystal substrate. The protrusion 2Aa and the base 2Ab can be formed by performing photolithography and etching on a mother substrate of silicon single crystal.

[0127] exist Figure 6 , a modified example of the optical modulator 10D according to the fifth embodiment is shown. Figure 6 In the present invention, as a substrate for providing the first electrode 2A, an SOI substrate 20 is used. The SOI substrate 20 includes an oxide film 21 and active layers 22a and 22b arranged to sandwich the oxide film 21 from both surfaces thereof. In the SOI substrate 20, one active layer 22a becomes the first electrode 2A. If the active layer 22a of the SOI substrate 20 is used, patterning for forming the first electrode 2A can be easily performed.

[0128] The first electrode 2A may be applied to the optical modulator 10 according to the first embodiment. In this case, the convex portion 2Aa is in contact with the optical waveguide 1. The first electrode 2A may be applied to the optical modulator 10A according to the second embodiment. In this case, the convex portion 2Aa is in contact with the low dielectric constant layer 4 and is not in contact with the optical waveguide 1. The first electrode 2A may be applied to the optical modulator 10B according to the third embodiment. In this case, the convex portion 2Aa is in contact with the optical waveguide 1.

[0129] <Sixth embodiment>

[0130] Figure 7 2 is a cross-sectional view schematically showing a structure of an optical modulator 10E according to a sixth embodiment. The optical modulator 10E is different from the optical modulator 10D according to the fifth embodiment in the structure of a first electrode 2B.

[0131] Reference Figure 7 , the first electrode 2B includes a protrusion 2Ba and a base 2Bb. The protrusion 2Ba protrudes from the surface 2Bba of the base 2Bb toward the optical waveguide 1. When observed in a cross section perpendicular to the direction in which the optical waveguide 1 extends, the closer to the optical waveguide 1, the smaller the length (width) of the protrusion 2Ba in the direction perpendicular to the stacking direction becomes. The protrusion 2Ba can have a minimum width w2Ba in the surface 2Baa on the side of the optical waveguide 1. In the present embodiment, the surface 2Baa of the protrusion 2Ba is in contact with the low dielectric constant layer 4. The protrusion 2Ba may also have a cross section that is substantially trapezoidal in shape. In this case, the surface 2Baa of the protrusion 2Ba is equivalent to the upper base of the trapezoid.

[0132] In the optical modulator 10E of the present embodiment, in the cross section of the convex portion 2Ba, the closer to the optical waveguide 1, the smaller the width of the convex portion 2Ba becomes. In this case, the side surface 2Bab of the convex portion 2Ba can be made relatively smoothly continuous with respect to the surface 2Bba of the base 2Bb. More specifically, the side surface 2Bab of the convex portion 2Ba can be smoothly connected to the surface 2Bba of the base 2Bb at an obtuse angle, that is, a shape close to a curve. Thus, compared with the case where the side surface 2Bab of the convex portion 2Ba is connected to the surface 2Bba of the base 2Bb at a right angle, it is possible to make it less likely that electrical loss will occur at the boundary between the convex portion 2Ba and the base 2Bb.

[0133] In the present embodiment, the side surface 2Bab of the convex portion 2Ba is inclined at a fixed inclination relative to the surface 2Baa. However, the inclination of the side surface 2Bab relative to the surface 2Baa may be varied.

[0134] The first electrode 2B may be applied to each of the optical modulators 10 , 10A, and 10B according to the first to third embodiments.

[0135] <Seventh embodiment>

[0136] Figure 8 10F is a cross-sectional view schematically showing a configuration of an optical modulator 10F according to a seventh embodiment. The optical modulator 10F is different from the optical modulator 10D according to the fifth embodiment in that the optical modulator 10F includes a metal thin layer 6 .

[0137] Reference Figure 8 The metal thin layer 6 is provided on the surface of the first electrode 2A on the optical waveguide 1 side. Specifically, the metal thin layer 6 is provided on the surface 2Aaa on the optical waveguide 1 side of the convex portion 2Aa. The thickness t6 of the metal thin layer 6 is thinner than the thickness t2 of the first electrode 2A. The cross-sectional area of ​​the metal thin layer 6 is smaller than the cross-sectional area of ​​the first electrode 2A. In addition, the thickness t6 of the metal thin layer 6 is thinner than the thickness t3 of the second electrode 3.

[0138] The metal thin layer 6 has a large conductivity and a small attenuation of high-frequency signals. If the metal thin layer 6 is disposed on the surface 2Aaa of the optical waveguide 1 side of the convex portion 2Aa, the driving voltage can be suppressed. Furthermore, in the convex portion 2Aa of the first electrode 2A, the high-frequency signal propagates more through the surface layer due to the skin effect, so it is preferred that the conductivity near the surface layer is greater. In this regard, if the metal thin layer 6 is disposed on the surface of the optical waveguide 1 side of the first electrode 2A, the resistance value can be reduced and the attenuation of the signal can be suppressed. In this case, the convex portion 2Aa can be used for low-frequency signals.

[0139] When the first electrode 2A includes a silicon semiconductor material, as described above, the thickness t2 of the first electrode 2A is, for example, not less than 0.43 μm and not more than 500 μm. The thickness t6 of the metal thin layer 6 is, for example, not less than 10% and not more than 50% of the thickness t2 of the first electrode 2A. The cross-sectional area of ​​the metal thin layer 6 is, for example, not less than 10% and not more than 50% of the cross-sectional area of ​​the first electrode 2A. The cross-sectional area of ​​the metal thin layer 6 may also be 0.043 μm. 2 Above and 2500μm 2 The metal thin layer 6 can be formed using, for example, the same metal material as that of the second electrode 3 .

[0140] The metal thin layer 6 may be applied to each of the optical modulators 10, 10A, 10B, and 10C according to the first to fourth embodiments. In this case, the metal thin layer 6 is provided on the surface of the first electrode 2 having no convex portion. The metal thin layer 6 may be applied to the optical modulator 10E according to the sixth embodiment. In this case, the metal thin layer 6 is provided on the surface 2Baa of the convex portion 2Ba of the first electrode 2B.

[0141] <Eighth embodiment>

[0142] Fig. 9 1 is a cross-sectional view schematically showing a structure of an optical modulator 10G according to an eighth embodiment. The optical modulator 10G is different from the optical modulator 10C according to the fourth embodiment in the structure of an optical waveguide 1C and the arrangement of a first electrode 2C and a second electrode 3C.

[0143] Reference Fig. 9 The optical modulator 10G includes an optical waveguide 1C, a first electrode 2C, a second electrode 3C, and a low dielectric constant layer 4C. The optical waveguide 1C includes a substrate portion 1Ca and a protrusion 1Cb. The protrusion 1Cb protrudes from the surface of the substrate portion 1Ca. The protrusion 1Cb substantially functions as an optical waveguide. The low dielectric constant layer 4C is stacked on the optical waveguide 1C. More specifically, the low dielectric constant layer 4C is stacked on the substrate portion 1Ca and the protrusion 1Cb.

[0144] The first electrode 2C and the second electrode 3C are stacked on the low dielectric constant layer 4C. The first electrode 2C and the second electrode 3C are arranged in parallel with a gap between them. Specifically, in the cross-sectional view of the optical modulator 10G, the first electrode 2C and the second electrode 3C are arranged in a direction substantially perpendicular to the stacking direction of the optical waveguide 1C and the low dielectric constant layer 4C. In the direction substantially perpendicular to the stacking direction, the first electrode 2C is arranged on one side of the protrusion 1Cb, and the second electrode 3C is arranged on the other side of the protrusion 1Cb. The first electrode 2C and the second electrode 3C can form a potential difference between each other to apply an electric field to the protrusion 1Cb of the optical waveguide 1C.

[0145] The optical modulator 10G according to this embodiment also achieves the same effects as the optical modulator 10C according to the fourth embodiment. The metal thin layer 6 according to the seventh embodiment can also be applied to the optical modulator 10G according to this embodiment.

[0146] <Ninth embodiment>

[0147] Fig.10 2 is a cross-sectional view schematically showing a structure of an optical modulator 10H according to a ninth embodiment. The optical modulator 10H is different from the optical modulator 10 according to the first embodiment in the structure of a first electrode 2D.

[0148] Reference Fig.10 The first electrode 2D has a surface layer 2Da on the optical waveguide 1 side and a remaining portion 2Db. In the example of the present embodiment, the surface layer 2Da is arranged adjacent to the optical waveguide 1 in the first electrode 2D. The surface layer 2Da is, for example, a portion within a range of 10% of the length (thickness) of the first electrode 2D in the stacking direction of the first electrode 2D relative to the optical waveguide 1 from the surface of the first electrode 2D on the optical waveguide 1 side. The remaining portion 2Db means a portion of the first electrode 2D other than the surface layer 2Da. In the first electrode 2D, the concentration of impurities doped in the semiconductor material is higher in the surface layer 2Da than in the remaining portion 2Db. That is, the first electrode 2D has different concentrations of impurities, i.e., doping amounts, in the surface layer 2Da and the remaining portion 2Db. For example, the concentration of impurities in the surface layer 2Da is higher than that in the remaining portion 2Db by more than 10%. Such a concentration distribution of impurities in the first electrode 2D can be formed by a thermal diffusion method or an ion implantation method, etc.

[0149] In the first electrode 2D, the concentration of impurities may change sharply at the boundary between the surface layer 2Da and the remaining part 2Db, or may gradually become thinner as it moves away from the surface layer 2Da in the stacking direction. The concentration of impurities in the first electrode 2D can be measured by epitaxial resistivity measurement, air gap CV measurement, mercury CV measurement, surface charge distribution, secondary ion mass spectrometry or extended resistance measurement. Regardless of which method is used, the measurement results are substantially the same. The difference between the concentration of impurities in the surface layer 2Da and the concentration of impurities in the remaining part 2Db can be confirmed by any of the above-mentioned measurement methods. Specifically, by implementing the above-mentioned measurement method, a distribution diagram of the impurity concentration of the first electrode 2D in the depth direction from the surface of the optical waveguide 1 side is obtained. Based on the obtained distribution diagram of the impurity concentration, the integral average of the impurity concentration of the surface layer 2Da and the integral average of the impurity concentration of the remaining part 2Db are calculated as the concentration of impurities in the surface layer 2Da and the concentration of impurities in the remaining part 2Db, respectively. That is, the impurity concentration of the surface layer 2Da is averaged by the integral of the impurity concentration in the range from the surface of the first electrode 2D on the optical waveguide 1 side to 10% of the depth (thickness) of the first electrode 2D, and the impurity concentration of the remaining range is averaged by the integral of the impurity concentration in the remaining portion 2Db. The obtained impurity concentration of the surface layer 2Da is higher than the obtained impurity concentration of the remaining portion 2Db by, for example, 10% or more.

[0150] In the optical modulator 10H according to the present embodiment, the surface layer 2Da on the optical waveguide 1 side of the first electrode 2D is doped with impurities at a higher concentration than the remaining portion 2Db of the first electrode 2. In this case, in the first electrode 2D, a region with high conductivity can be locally present near the optical waveguide 1, and attenuation of high-frequency signals can be suppressed by the skin effect.

[0151] The first electrode 2D may be applied to each of the optical modulators 10A, 10B, 10C, 10D, 10E, and 10F according to the second to seventh embodiments.

[0152] <Tenth embodiment>

[0153] Fig.11 10 is a cross-sectional view schematically showing the structure of an optical modulator 10I according to the tenth embodiment. The optical modulator 10I is different from the optical modulator 10G according to the eighth embodiment in the structure of the first electrode 2E.

[0154] Reference Fig.11The first electrode 2E has a surface layer 2Ea on the side of the raised portion 1Cb of the optical waveguide 1C and a remaining portion 2Eb. The surface layer 2Ea on the side of the raised portion 1Cb is a surface layer of the first electrode 2E where the electric field applied to the raised portion 1Cb together with the second electrode 3C passes. In the example of the present embodiment, the surface layer 2Ea is a surface layer of the first electrode 2E located on the side of the raised portion 1Cb that substantially functions as an optical waveguide in a direction (width direction) perpendicular to the stacking direction of the first electrode 2E with respect to the optical waveguide 1C. The surface layer 2Ea is, for example, a portion within a range from a surface located on the side of the raised portion 1Cb in the width direction of the first electrode 2E to 10% of the length of the first electrode 2E in the width direction. The remaining portion 2Eb means a portion of the first electrode 2E other than the surface layer 2Ea. Similar to the first electrode 2D of the ninth embodiment described above, in the first electrode 2E, the concentration of impurities doped in the semiconductor material is higher in the surface layer 2Ea than in the remaining portion 2Eb. Therefore, even the optical modulator 10I according to this embodiment achieves the same effect as the optical modulator 10H according to the ninth embodiment.

[0155] exist Fig.12 , a modified example of the optical modulator 10I according to the tenth embodiment is shown. Fig.12 The surface layer 2Ea may be a surface layer of the first electrode 2E located on the optical waveguide 1C side in the stacking direction of the first electrode 2E with respect to the optical waveguide 1. In this case, the surface layer 2Ea is, for example, a portion ranging from the surface located on the optical waveguide 1C side in the stacking direction to 10% of the thickness of the first electrode 2E. Even with this structure, the same effects as those of the optical modulator 10H according to the ninth embodiment can be achieved.

[0156] As mentioned above, although the embodiment related to this disclosure was described, this disclosure is not limited to the said embodiment, Unless it deviates from the summary, various changes can be made.

[0157] The structure of the optical modulator involved in the present disclosure is summarized below.

[0158] <1>

[0159] An optical modulator comprising:

[0160] Optical waveguides, comprising materials having an electro-optic effect;

[0161] A first electrode comprising a semiconductor material; and

[0162] The second electrode includes a metal material and is arranged to form a potential difference with the first electrode to apply an electric field to the optical waveguide.

[0163] <2>

[0164] The optical modulator according to <1>, wherein:

[0165] It also includes: a low dielectric constant layer having a refractive index smaller than that of the optical waveguide,

[0166] Among the first electrode and the second electrode, at least the first electrode is arranged with a gap between it and the optical waveguide.

[0167] The low dielectric constant layer is disposed in the gap.

[0168] <3>

[0169] The optical modulator according to <1>, wherein:

[0170] It also includes: a low dielectric constant layer having a refractive index smaller than that of the optical waveguide,

[0171] The first electrode and the second electrode are respectively arranged with a gap between them and the optical waveguide.

[0172] The low dielectric constant layer is disposed in the gap.

[0173] <4>

[0174] The optical modulator according to <2> or <3>, wherein:

[0175] The size of the gap is greater than or equal to 0.750 μm and less than or equal to 1.675 μm.

[0176] <5>

[0177] The optical modulator according to any one of <2> to <4>, wherein:

[0178] The semiconductor material is a silicon semiconductor material in which impurities are doped into silicon.

[0179] The main component of the low dielectric constant layer is SiO2.

[0180] <6>

[0181] The optical modulator according to any one of <1> to <5>, wherein:

[0182] The first electrode is stacked on the optical waveguide,

[0183] The second electrode is stacked on the optical waveguide on the opposite side of the first electrode.

[0184] <7>

[0185] The optical modulator according to <6>, wherein:

[0186] The first electrode includes a convex portion provided on a surface located on the optical waveguide side in a stacking direction of the first electrode, the optical waveguide, and the second electrode, and protruding toward the optical waveguide.

[0187] <8>

[0188] The optical modulator according to <7>, wherein:

[0189] When viewed in a cross section perpendicular to the direction in which the optical waveguide extends, the length of the convex portion in the direction perpendicular to the stacking direction decreases as it is closer to the optical waveguide.

[0190] <9>

[0191] The optical modulator according to any one of <6> to <8>, wherein:

[0192] When viewed in a cross section perpendicular to the direction in which the optical waveguide extends, a length of a surface of the first electrode on the optical waveguide side in a direction perpendicular to a stacking direction of the first electrode, the optical waveguide, and the second electrode is greater than a length of the optical waveguide.

[0193] <10>

[0194] The optical modulator according to any one of <1> to <9>, wherein:

[0195] The semiconductor material is a silicon semiconductor material obtained by doping impurities in silicon.

[0196] <11>

[0197] The optical modulator according to <10>, wherein:

[0198] The concentration of the impurity in the first electrode is 1.0×10 17 cm -3 Above and 1.0×10 22 cm -3 the following.

[0199] <12>

[0200] The optical modulator according to any one of <1> to <11>, wherein:

[0201] The refractive index of the first electrode is less than 3.

[0202] <13>

[0203] The optical modulator according to any one of <10> to <12>, wherein:

[0204] The first electrode is a silicon single crystal substrate.

[0205] <14>

[0206] The optical modulator according to any one of <10> to <12>, wherein:

[0207] The first electrode is an active layer of the SOI substrate.

[0208] <15>

[0209] The optical modulator according to any one of <1> to <14>, wherein:

[0210] The main component of the metal material is a precious metal.

[0211] <16>

[0212] The optical modulator according to any one of <1> to <15>, wherein:

[0213] When viewed in a cross section perpendicular to the direction in which the optical waveguide extends, the area of ​​the first electrode is larger than the area of ​​the second electrode.

[0214] <17>

[0215] The optical modulator according to any one of <1> to <16>, wherein:

[0216] The second electrode is a signal electrode, and the first electrode is a ground electrode.

[0217] <18>

[0218] The optical modulator according to any one of <1> to <17>, wherein:

[0219] The invention further comprises a metal thin layer provided on a surface of the first electrode on the optical waveguide side and having a thickness thinner than that of the first electrode.

[0220] <19>

[0221] The optical modulator according to any one of <1> to <18>, wherein:

[0222] The surface layer on the optical waveguide side of the first electrode is doped with impurities at a higher concentration than other portions of the first electrode.

[0223] Description of Reference Numerals

[0224] 10, 10A, 10B, 10C, 10D, 10E, 10F, 10G, 10I: optical modulator;

[0225] 1. 1C: optical waveguide;

[0226] 1Ca: substrate part;

[0227] 1Cb: ridge;

[0228] 2, 2A, 2B, 2C, 2D, 2E: first electrode;

[0229] 2Aa, 2Ba: convex portion;

[0230] 2Ab, 2Bb: base;

[0231] 2Da, 2Ea: surface layer;

[0232] 2Db, 2Eb: remaining part;

[0233] 20: SOI substrate;

[0234] 21: Oxide film;

[0235] 22a, 22b: active layer;

[0236] 3. 3C: second electrode;

[0237] 4, 5, 4C: low dielectric constant layer;

[0238] 6: Thin metal layer.

Claims

1. An optical modulator comprising: Optical waveguides, comprising materials having an electro-optic effect; a first electrode comprising a semiconductor material; and The second electrode includes a metal material and is arranged to form a potential difference with the first electrode to apply an electric field to the optical waveguide.

2. The optical modulator according to claim 1, wherein It also includes: a low dielectric constant layer having a refractive index smaller than that of the optical waveguide, Among the first electrode and the second electrode, at least the first electrode is arranged with a gap between it and the optical waveguide. The low dielectric constant layer is disposed in the gap.

3. The optical modulator according to claim 1, wherein: It also includes: a low dielectric constant layer having a refractive index smaller than that of the optical waveguide, The first electrode and the second electrode are respectively arranged with a gap between them and the optical waveguide. The low dielectric constant layer is disposed in the gap.

4. The optical modulator according to claim 2 or 3, wherein: The size of the gap is greater than or equal to 0.750 μm and less than or equal to 1.675 μm.

5. The optical modulator according to any one of claims 2 to 4, wherein: The semiconductor material is a silicon semiconductor material in which impurities are doped into silicon. The main component of the low dielectric constant layer is SiO2.

6. The optical modulator according to any one of claims 1 to 5, wherein: The first electrode is stacked on the optical waveguide, The second electrode is stacked on the optical waveguide on the opposite side of the first electrode.

7. The optical modulator according to claim 6, wherein: The first electrode includes a convex portion provided on a surface located on the optical waveguide side in a stacking direction of the first electrode, the optical waveguide, and the second electrode, and protruding toward the optical waveguide.

8. The optical modulator according to claim 7, wherein: When viewed in a cross section perpendicular to the direction in which the optical waveguide extends, the length of the convex portion in the direction perpendicular to the stacking direction decreases as it is closer to the optical waveguide.

9. The optical modulator according to any one of claims 6 to 8, wherein: When viewed in a cross section perpendicular to the direction in which the optical waveguide extends, a length of a surface of the first electrode on the optical waveguide side in a direction perpendicular to a stacking direction of the first electrode, the optical waveguide, and the second electrode is greater than a length of the optical waveguide.

10. The optical modulator according to any one of claims 1 to 9, wherein: The semiconductor material is a silicon semiconductor material obtained by doping impurities in silicon.

11. The optical modulator according to claim 10, wherein: The concentration of the impurity in the first electrode is 1.0×10 17 cm -3 Above and 1.0×10 22 cm -3 the following.

12. The optical modulator according to any one of claims 1 to 11, wherein: The refractive index of the first electrode is less than 3.

13. The optical modulator according to any one of claims 10 to 12, wherein: The first electrode is a silicon single crystal substrate.

14. The optical modulator according to any one of claims 10 to 12, wherein: The first electrode is an active layer of the SOI substrate.

15. The optical modulator according to any one of claims 1 to 14, wherein: The main component of the metal material is a precious metal.

16. The optical modulator according to any one of claims 1 to 15, wherein: When viewed in a cross section perpendicular to the direction in which the optical waveguide extends, the area of ​​the first electrode is larger than the area of ​​the second electrode.

17. The optical modulator according to any one of claims 1 to 16, wherein: The second electrode is a signal electrode, and the first electrode is a ground electrode.

18. The optical modulator according to any one of claims 1 to 17, wherein: The invention further comprises a metal thin layer provided on a surface of the first electrode on the optical waveguide side and having a thickness thinner than that of the first electrode.

19. The optical modulator according to any one of claims 1 to 18, wherein: The surface layer on the optical waveguide side of the first electrode is doped with impurities at a higher concentration than other portions of the first electrode.

Images