JP7075448B2 - Composite substrate for electro-optic elements - Google Patents

Description

The techniques disclosed herein relate to composite substrates for electro-optic elements (eg, light modulators) that utilize electro-optic effects.

For example, an electro-optical element such as an optical modulator is known. The electro-optical element can convert an electric signal into an optical signal by utilizing the electro-optic effect. Electro-optic elements are used, for example, in optical radio wave fusion communication, and their development is underway in order to realize high-speed and large-capacity communication.

For example, Japanese Patent Laid-Open No. 2010-85789 discloses an optical modulator. This light modulator is provided with an electro-optical crystal substrate having an electro-optical effect, and an optical waveguide is formed therein. A reinforcing substrate having a lower dielectric constant than the electro-optical crystal substrate is bonded to the electro-optical crystal substrate. This reinforcing substrate not only mechanically reinforces the electro-optical crystal substrate, but also reduces the dielectric loss, thereby facilitating the high-speed operation of the optical modulator and the driving at a low voltage. A composite substrate in which such an electro-optical crystal substrate and a low dielectric constant substrate are combined can be suitably adopted not only for an optical modulator but also for various electro-optical elements.

In the conventional composite substrate, the electro-optical crystal substrate and the low dielectric constant substrate are bonded by an adhesive. With such a configuration, the adhesive may deteriorate over time, causing peeling of the composite substrate, and further, damage such as cracks may occur due to this peeling. In order to avoid such a problem, it is conceivable to directly bond the electro-optical crystal substrate and the low dielectric constant substrate without using an adhesive. However, when the electro-optical crystal substrate and the low-dielectric-constant substrate are directly bonded, the amorphous composition between the electro-optical crystal substrate and the low-dielectric-constant substrate is composed of the elements of the electro-optical crystal substrate and the elements of the low-dielectric-constant substrate. Layers are formed. This amorphous layer has no crystallinity, its optical properties are different from those of both substrates, and the interface between the electro-optical crystal substrate and the amorphous layer is not flat. Such an uneven interface may scatter (for example, diffuse reflection or leakage) or absorb light transmitted through the electro-optic crystal substrate.

Therefore, the present specification provides a composite substrate and a method for manufacturing the composite substrate, which can avoid or reduce the above-mentioned problems.

The present specification discloses a composite substrate for an electro-optic element. In this composite substrate, an electro-optical crystal substrate having an electro-optical effect and an electro-optical crystal substrate are laminated, and a low-dielectric-constant substrate having a lower refractive index than the electro-optical crystal substrate, a low-dielectric-constant substrate, and electro-optics are laminated. It is located between the crystal substrate and the low refractive index layer having a lower refractive index than the electro-optical crystal substrate, and is located between the low refractive index substrate and the low refractive index layer. It includes an amorphous layer composed of an element constituting the element and an element constituting the low refractive index layer.

The composite substrate described above can be manufactured by the following manufacturing method. In this manufacturing method, a step of forming a low refractive index layer having a lower refractive index than that of the electro-optical crystal substrate and a low refractive index layer are formed on the main surface of the electro-optical crystal substrate having an electro-optical effect. The main surface of the electro-optical crystal substrate is provided with a step of directly joining a low-refractive index substrate having a lower refractive index than that of the electro-optical crystal substrate. The term "direct bonding" as used herein means a bonding in which atoms are diffused between two members to be bonded and a covalent bond is formed between the atoms.

According to the above-mentioned manufacturing method, it is possible to manufacture a composite substrate in which an electro-optical crystal substrate and a low dielectric constant substrate are laminated without the need for an adhesive. In the manufactured composite substrate, an amorphous layer due to direct bonding is formed between the low dielectric constant substrate and the low refractive index layer. That is, a low refractive index layer is interposed between the amorphous layer and the electro-optical crystal substrate, and the amorphous layer does not come into contact with the electro-optical crystal substrate. Therefore, the light transmitted through the electro-optical crystal substrate is not scattered or absorbed at the amorphous layer or the uneven interface between the amorphous layer and the electro-optical crystal substrate. In addition, since the low refractive index layer in contact with the electro-optical crystal substrate has a lower refractive index than the electro-optical crystal substrate, it is possible to suppress leakage of light transmitted through the electro-optical crystal substrate like a clad in an optical fiber. can. By using this composite substrate, it is possible to manufacture an electro-optical element capable of high-speed operation and driving at a low voltage.

The perspective view which shows typically the composite substrate 10 of an Example.

The figure which shows typically the cross-sectional structure of the composite substrate 10 of an Example.

The figure which shows one step of the manufacturing method of the composite substrate 10 of an Example.

The figure which shows one step of the manufacturing method of the composite substrate 10 of an Example.

The figure which shows one step of the manufacturing method of the composite substrate 10 of an Example.

A modification of the composite substrate 10 is shown, in which electrodes 32 and 34 that form an electric field on the electro-optical crystal substrate 12 and an optical waveguide region 36 provided in the electro-optical crystal substrate 12 are added.

A modification of the composite substrate 10 is shown, in which a second low refractive index layer 20 is added between the low dielectric constant substrate 14 and the amorphous layer 18.

The figure which shows the manufacturing method of the composite board 10 of the modification shown in FIG.

A modification of the composite substrate 10 is shown, and the ridge portion 13 is formed on the upper surface 12a of the electro-optical crystal substrate 12.

A modification of the composite substrate 10 is shown, and the first electrode 42 and the second electrode 44 are added as compared with the modification shown in FIG. In this modification, the c-axis (c-axis) of the electro-optical crystal substrate 12 is parallel to the electro-optical crystal substrate 12.

A modification of the composite substrate 10 is shown, and the conductive layer 22 is added along the lower surface 14b of the low dielectric constant substrate 14 as compared with the modification shown in FIG.

A modified example of the composite substrate 10 is shown, and a support substrate 24 bonded to the conductive layer 22 is added as compared with the modified example shown in FIG.

A modification of the composite substrate 10 is shown, and the first electrode 52 and the second electrode 54 are added as compared with the modification shown in FIG. In this modification, the c-axis (c-axis) of the electro-optical crystal substrate 12 is perpendicular to the electro-optic crystal substrate 12.

A modification of the composite substrate 10 is shown, and the conductive layer 22 is added along the lower surface 14b of the low dielectric constant substrate 14 as compared with the modification shown in FIG.

A modified example of the composite substrate 10 is shown, and a support substrate 24 bonded to the conductive layer 22 is added as compared with the modified example shown in FIG.

A modified example of the composite substrate 10 is shown, and an optical waveguide region 56 is added in the ridge portion 13 as compared with the modified example shown in FIG.

A modified example of the composite substrate 10 is shown, and an optical waveguide region 56 is added in the ridge portion 13 as compared with the modified example shown in FIG.

A modified example of the composite substrate 10 is shown, and an optical waveguide region 56 is added in the ridge portion 13 as compared with the modified example shown in FIG.

A modified example of the composite substrate 10 is shown, and an optical waveguide region 56 is added in the ridge portion 13 as compared with the modified example shown in FIG.

A modification of the composite substrate 10 is shown, in which the low refractive index layer 16 has a multilayer structure including a main layer 16a and a surface layer 16b.

It is a figure explaining one step (direct joining) of the manufacturing method of the composite substrate 10 shown in FIG.

A modification of the composite substrate 10 is shown, in which the second low refractive index layer 20 has a multilayer structure including a main layer 20a and a surface layer 20b.

FIG. 22 is a diagram illustrating one step (direct bonding) of the manufacturing method of the composite substrate 10 shown in FIG. 22.

In one embodiment of the present invention, the low refractive index layer may have a lower dielectric constant than the electro-optical crystal substrate. According to such a configuration, the dielectric loss in the low refractive index layer can be reduced, and the high-speed operation of the electro-optical element and the driving at a low voltage can be further facilitated. Although not particularly limited, the low refractive index layer can be composed of at least one of silicon oxide, tantalum oxide, aluminum oxide, glass, magnesium fluoride and calcium fluoride.

In one embodiment of the present technique, the composite substrate is located between the low dielectric constant substrate and the amorphous layer, and may further include a second low refractive index layer having a lower refractive index than the electro-optical crystal substrate. .. In this case, the amorphous layer may be composed of an element constituting the low refractive index layer and an element constituting the second low refractive index layer instead of the element constituting the low dielectric constant substrate. Such a composite substrate can be manufactured by directly bonding a low-dielectric-constant substrate on which a second low-refractive index layer is formed to an electro-optical crystal substrate on which a low-refractive index layer is formed. At this time, if the materials of the low refractive index layer and the second low refractive index layer are the same or similar in structure, high quality direct bonding can be performed relatively easily.

In one embodiment of the present technique, a ridge portion may be formed on the surface of the electro-optical crystal substrate. If the ridge portion is formed in advance on the composite substrate, it is possible to easily manufacture an electro-optical element that requires a ridge-type optical waveguide. In addition to or instead of the ridge portion, an optical waveguide region doped with an impurity (for example, titanium or zinc) may be formed on the electro-optical crystal substrate. However, in the optical waveguide region where impurities are doped, the step in the refractive index due to the doping of impurities is small and the light confinement effect is small, so that the near-field image (near field diameter) of light is relatively large. As a result, in the electro-optical element manufactured from the composite substrate, the electric field efficiency is lowered, so that the required drive voltage is increased. Therefore, the element size also increases. The ridge type optical waveguide is preferable from the viewpoint of lowering the drive voltage and reducing the size of the electro-optical element.

In the above-described embodiment, the c-axis (that is, the crystal spindle) of the electro-optical crystal substrate may be parallel to the electro-optical crystal substrate. That is, the electro-optical crystal substrate may be an x-cut substrate. In this case, the composite substrate is provided with a first electrode provided on one side surface of the ridge portion and a second electrode provided on the other side surface of the ridge portion and facing the first electrode with the ridge portion interposed therebetween. May be further provided. These first and second electrodes can be used as electrodes for applying an electric signal (that is, an electric field) to the ridge type optical waveguide when manufacturing an electro-optical element from a composite substrate.

Alternatively, the c-axis (that is, the crystal spindle) of the electro-optical crystal substrate may be perpendicular to the electro-optical crystal substrate. That is, the electro-optical crystal substrate may be a z-cut substrate. In this case, the composite substrate further includes a first electrode provided on the top surface of the ridge portion and a second electrode provided in a range excluding the ridge portion on the surface of the electro-optic crystal substrate. You may prepare. These first and second electrodes can be used as electrodes for applying an electric signal (that is, an electric field) to the ridge type optical waveguide when manufacturing an electro-optical element from a composite substrate.

In an embodiment in which the composite substrate has a first electrode and a second electrode, at least one between the first electrode and the electro-optical crystal substrate and between the second electrode and the electro-optical crystal substrate. , A low refractive index film having a lower refractive index than that of the electro-optical crystal substrate may be formed. According to such a configuration, at the interface between the electro-optical crystal substrate and the first electrode (or the second electrode), the light transmitted through the electro-optical crystal substrate is suppressed from being absorbed or scattered by the electrode. Can be done.

In the embodiment in which the electro-optical crystal substrate has a ridge portion, an optical waveguide region containing impurities may be formed along the longitudinal direction of the ridge portion. According to such a configuration, a desired optical waveguide can be easily formed by changing the region where impurities are doped without changing the ridge portion.

In one embodiment of the present technique, the composite substrate may further include a conductive layer located on the opposite side of the electro-optical crystal substrate with respect to the low dielectric constant substrate. Such a conductive layer can suppress the leakage of an electric field from a low dielectric constant substrate in an electro-optic element manufactured from a composite substrate.

In the above-described embodiment, the composite substrate may further include a support substrate bonded to the conductive layer. According to such a configuration, the thickness of the electro-optical crystal substrate and the low dielectric constant substrate can be reduced while maintaining the mechanical strength of the composite substrate by the support substrate. In the electro-optical element manufactured from the composite substrate, the frequency (resonance frequency) at which the electric signal resonates in the composite substrate can be shifted to the high frequency side as the thickness of the electro-optical crystal substrate and the low dielectric constant substrate becomes smaller. For example, if the total thickness of the electro-optical crystal substrate and the low dielectric constant substrate is 60 micrometers or less, the resonance frequency is 300 gigahertz or more. In this case, the electro-optical element can avoid the problem that a high frequency (so-called millimeter wave band) electric signal such as 100 gigahertz is attenuated by substrate resonance.

Hereinafter, typical and non-limiting specific examples of the present invention will be described in detail with reference to the drawings. This detailed description is merely intended to provide those skilled in the art with details for carrying out preferred embodiments of the invention and is not intended to limit the scope of the invention. In addition, the additional features and inventions disclosed below may be used separately or together with other features and inventions to provide further improved composite substrates and methods of their use and manufacture. ..

Further, the combination of features and processes disclosed in the following detailed description is not essential for carrying out the present invention in the broadest sense, and is particularly for explaining a representative specific example of the present invention. It is to be described. In addition, the various features of the above and below representative examples, as well as the various features of those described in the independent and dependent claims, are described herein in providing additional and useful embodiments of the invention. It does not have to be combined according to the specific examples given or in the order listed.

All features described herein and / or claims are, separately, as a limitation to the disclosure at the time of filing and the specific matters claimed, apart from the composition of the features described in the examples and / or claims. And it is intended to be disclosed independently of each other. In addition, all numerical ranges and statements relating to groups or groups are made with the intent of disclosing their intermediate composition as a limitation to the disclosure at the time of filing and the specific matter claimed.

The composite substrate 10 of the embodiment and the manufacturing method thereof will be described with reference to the drawings. The composite substrate 10 of this embodiment can be used for various electro-optical elements such as optical modulators. As shown in FIG. 1, the composite substrate 10 of this embodiment is manufactured in the form of a so-called wafer and is provided to a manufacturer of an electro-optical element. As an example, the diameter of the composite substrate 10 is approximately 10 cm (4 inches). Usually, a plurality of electro-optical elements are manufactured from one composite substrate 10. The composite substrate 10 is not limited to the form of the wafer, and may be manufactured and provided in various forms.

As shown in FIGS. 1 and 2, the composite substrate 10 includes an electro-optical crystal substrate 12. The electro-optic crystal substrate 12 has an upper surface 12a exposed to the outside and a lower surface 12b located inside the composite substrate 10. A part or all of the electro-optical crystal substrate 12 is an optical waveguide that transmits light in an electro-optical element manufactured from the composite substrate 10. The electro-optical crystal substrate 12 is composed of crystals of a material having an electro-optical effect. Specifically, when an electric field is applied to the electro-optical crystal substrate 12, the refractive index of the electro-optical crystal substrate 12 changes. In particular, when an electric field is applied along the c-axis of the electro-optical crystal substrate 12, the refractive index of the electro-optical crystal substrate 12 changes significantly. Here, the c-axis of the electro-optical crystal substrate 12 may be parallel to the electro-optic crystal substrate 12. That is, the electro-optical crystal substrate 12 may be, for example, an x-cut or y-cut substrate. Alternatively, the c-axis of the electro-optical crystal substrate 12 may be perpendicular to the electro-optic crystal substrate 12. That is, the electro-optical crystal substrate 12 may be, for example, a z-cut substrate. The thickness T2 of the electro-optical crystal substrate 12 is not particularly limited, but may be, for example, 1 micrometer or more and 50 micrometer or less.

The material constituting the electro-optical crystal substrate 12 is not particularly limited, but is limited to lithium niobate (LiNbO ₃ : LN), lithium tantalate (LiTaO ₃ : LT), potassium niobate (KTIOPO ₄ : KTP), and niobate. Potassium niobate (K _x Li _(1-x) NbO ₂ : KLN), potassium niobate (KNbO ₃ : KN), tantalate / potassium niobate (KNb _x Ta _(1-x) O ₃ : KTN), niobate It may be either a solid solution of lithium acid and lithium tantalate. The electro-optical crystal substrate 12 may have an electro-optical effect that changes other optical constants in addition to or instead of the refractive index.

The composite substrate 10 further includes a low dielectric constant substrate 14. The low dielectric constant substrate 14 has an upper surface 14a located inside the composite substrate 10 and a lower surface 14b exposed to the outside. The above-mentioned electro-optical crystal substrate 12 is laminated on the upper surface 14a side of the low dielectric constant substrate 14. The low dielectric constant substrate 14 is made of a material having a lower dielectric constant than the electro-optical crystal substrate 12. The dielectric constant (relative permittivity) of the low dielectric constant substrate 14 is not particularly limited, but is preferably 5 or less. As an example, examples of the material constituting the low dielectric constant substrate 14 include quartz and glass. The thickness T4 of the low dielectric constant substrate 14 is also not particularly limited, but may be larger than the thickness T2 of the electro-optical crystal substrate 12. The low dielectric constant substrate 14 adjacent to the electro-optical crystal substrate 12 can reduce the dielectric loss when an electric field is applied to the electro-optical crystal substrate 12. Therefore, by using the composite substrate 10, it is possible to manufacture an electro-optical element capable of high-speed operation and driving at a low voltage.

The composite substrate 10 further includes a low refractive index layer 16. The low refractive index layer 16 is provided along the lower surface 12b of the electro-optical crystal substrate 12 and is located between the low-dielectric-constant substrate 14 and the electro-optical crystal substrate 12. The low refractive index layer 16 has a lower refractive index than the electro-optical crystal substrate 12. As a result, the light transmitted through the electro-optical crystal substrate 12 is easily totally reflected on the lower surface 12b of the electro-optical crystal substrate 12 (that is, the interface in contact with the low refractive index layer 16), and leakage from the electro-optical crystal substrate 12 is suppressed. Will be done. The material constituting the low refractive index layer 16 is not particularly limited, and may be, for example, one or more of silicon oxide, tantalum oxide, aluminum oxide, glass, magnesium fluoride and calcium fluoride. The thickness T6 of the low refractive index layer 16 is also not particularly limited, but may be, for example, 0.3 micrometer or more and 1 micrometer or less. Although it is an example, the low refractive index layer 16 in this embodiment has a dielectric loss (more than the electro-optical crystal substrate 12) so as to suppress the dielectric loss, reduce the effective dielectric constant of the electric signal, and efficiently perform optical modulation. It is composed of tan δ) or a material with a low dielectric constant.

The composite substrate 10 further includes an amorphous layer 18. The amorphous layer 18 is located between the low dielectric constant substrate 14 and the low refractive index layer 16. The amorphous layer 18 has an amorphous structure and is composed of an element constituting the low dielectric constant substrate 14 and an element constituting the low refractive index layer 16. The thickness T8 of the amorphous layer 18 is not particularly limited, but may be 0.5 nanometers or more and 100 nanometers or less. As will be described later, the composite substrate 10 can be manufactured by directly bonding the low dielectric constant substrate 14 to the electro-optical crystal substrate 12 on which the low refractive index layer 16 is formed. The amorphous layer 18 is a layer generated by this direct bonding, and is formed by diffusing the atoms of the low refractive index layer 16 and the low dielectric constant substrate 14, respectively. Therefore, the upper surface 18a of the amorphous layer 18 (that is, the interface in contact with the low refractive index layer 16) and the lower surface 18b of the amorphous layer 18 (that is, the interface in contact with the low dielectric constant substrate 14) are not necessarily flat.

In general, an amorphous layer caused by direct bonding is composed of elements constituting the materials located above and below it, and is not crystalline, and different elements may be taken in from the outside, so that the material has optical properties located above and below. different. Further, the interface of the amorphous layer is not flat, and absorption or scattering may occur optically. Therefore, if the amorphous layer 18 is in direct contact with the electro-optical crystal substrate 12, the light transmitted through the electro-optical crystal substrate 12 is attenuated by the amorphous layer 18. On the other hand, in the composite substrate 10 of this embodiment, a low refractive index layer 16 having a thickness that is not affected is interposed between the amorphous layer 18 and the electro-optical crystal substrate 12, and the amorphous layer 18 is an electro-optical crystal substrate. Not in contact with 12. Therefore, the light transmitted through the electro-optical crystal substrate 12 is not scattered by the amorphous layer 18 or the upper surface 18a. In addition, since the low refractive index layer 16 in contact with the electro-optical crystal substrate 12 has a lower refractive index than that of the electro-optical crystal substrate 12, light leakage transmitted through the electro-optical crystal substrate 12 is prevented like a cladding in an optical fiber. It can be suppressed and propagated through optical waveguides.

As described above, in the composite substrate 10 of this embodiment, since the low dielectric constant substrate 14 is provided adjacent to the electro-optical crystal substrate 12, when an electric field is applied to the electro-optical crystal substrate 12, it is dielectric. The loss can be reduced. Further, since the electro-optical crystal substrate 12 and the low dielectric constant substrate 14 are directly bonded without using an adhesive, there is no dielectric loss due to the adhesive, and there is no alteration or deformation, so reliability can be ensured. .. Since the amorphous layer 18 due to the direct bonding is separated from the electro-optical crystal substrate 12 by the low refractive index layer 16, the light transmitted through the electro-optical crystal substrate 12 can be propagated to the output side without loss. can.

Next, a method for manufacturing the composite substrate 10 will be described with reference to FIGS. 3 to 5. First, as shown in FIG. 3, the electro-optic crystal substrate 12 is prepared. The electro-optic crystal substrate 12 may be an x-cut or y-cut substrate (the c-axis is parallel to the substrate), and when a polarization inversion portion is formed, the c-axis is at an angle within 10 ° with the horizontal plane of the substrate. It may be an offset substrate forming the above. Alternatively, it may be a z-cut substrate (c-axis is perpendicular to the substrate). Next, as shown in FIG. 4, a low refractive index layer 16 is formed on the lower surface 12b of the electro-optical crystal substrate 12. The film formation of the low refractive index layer 16 is not particularly limited, but can be performed by thin film deposition (physical vapor deposition or chemical vapor deposition). The lower surface 12b of the electro-optical crystal substrate 12 is one main surface of the electro-optic crystal substrate 12. Next, as shown in FIG. 5, a low dielectric constant substrate 14 is prepared, and the low dielectric constant substrate 14 is directly bonded to the lower surface 12b of the electro-optical crystal substrate 12 on which the low refractive index layer 16 is formed. At this time, the above-mentioned amorphous layer 18 is formed between the low dielectric constant substrate 14 and the low refractive index layer 16. As a result, the composite substrate 10 shown in FIGS. 1 and 2 is manufactured.

The specific procedure and processing conditions for the above-mentioned direct joining are not particularly limited. It can be appropriately determined depending on each material of the layer or the substrate to be bonded to each other. Although it is an example, in the manufacturing method of this embodiment, first, each joint surface is irradiated with a neutralized beam in a high vacuum chamber (for example, about 1 × ^10-6 pascals). As a result, each joint surface is activated. Then, in a vacuum atmosphere, the activated joining surfaces are brought into contact with each other and joined at room temperature. The load at the time of this joining can be, for example, 100 to 20000 Newton. In this manufacturing method, when the surface is activated by the neutralizing beam, an inert gas is introduced into the chamber, and a high voltage is applied from a DC power source to the electrodes arranged in the chamber. As a result, the electric field generated between the electrode (positive electrode) and the chamber (negative electrode) causes electrons to move, and a beam of atoms and ions due to the inert gas is generated. Of the beams that reach the grid, the ion beam is neutralized by the grid, so that the beam of neutral atoms is emitted from the high-speed atomic beam source. The atomic species constituting the beam is preferably an inert gas element (for example, argon (Ar), nitrogen (N), etc.). The voltage when activated by beam irradiation can be 0.5 to 2.0 kilovolts, and the current can be 50 to 200 mA.

As shown in FIG. 6, the composite substrate 10 may be provided with electrodes 32 and 34 for forming an electric field on the electro-optical crystal substrate 12 on the upper surface 12a of the electro-optic crystal substrate 12. The material constituting the electrodes 32 and 34 may be a conductor, and may be a metal such as gold. Titanium (Ti), chromium (Cr), nickel (Ni), platinum (Pt) are used between the electrodes 32 and 34 and the electro-optical crystal substrate 12 in order to prevent peeling and migration of the electrodes 32 and 34. Etc. may intervene. The number, position, and shape of the electrodes 32 and 34 are not particularly limited. For example, the number of electrodes 32 and 34 can be appropriately determined according to the number of electro-optical elements manufactured from the composite substrate 10 and the number of electrodes 32 and 34 required by each electro-optical element. When the electrodes 32 and 34 are provided in advance on the composite substrate 10, the manufacturer of the electro-optical element can easily manufacture the electro-optical element from the composite substrate 10.

In addition, or instead, the optical waveguide region 36 may be provided in the electro-optical crystal substrate 12 by doping impurities. In the electro-optical crystal substrate 12, the refractive index can be selectively (that is, locally) increased by doping with a specific impurity such as titanium or zinc, whereby the optical waveguide region 36 can be formed. .. The number, position, and shape of the optical waveguide regions 36 are also not particularly limited. For example, the number of optical waveguide regions 36 can be appropriately determined according to the number of electro-optical elements manufactured from the composite substrate 10 and the number of optical waveguide regions 36 required by each electro-optical element. When the optical waveguide region 36 is provided in advance on the composite substrate 10, the manufacturer of the electro-optical element can easily manufacture the electro-optical element from the composite substrate 10.

As shown in FIG. 7, the composite substrate 10 may be further provided with the second low refractive index layer 20. The second low refractive index layer 20 is located between the low dielectric constant substrate 14 and the amorphous layer 18, and is made of a material having a lower refractive index than the electro-optical crystal substrate 12. Further, the second low refractive index layer 20 may be made of a material having a dielectric constant lower than that of the electro-optical crystal substrate 12 in order to suppress the dielectric loss. The second low refractive index layer 20 can be composed of, for example, at least one of silicon oxide, tantalum oxide, aluminum oxide, glass, magnesium fluoride and calcium fluoride. The second low refractive index layer 20 may be made of the same material as the low refractive index layer 16 or may be made of a different material.

FIG. 8 shows a method for manufacturing the composite substrate 10 shown in FIG. 7. As shown in FIG. 8, in the composite substrate 10 having the second low refractive index layer 20, the second low refractive index layer 20 is formed on the lower surface 12b of the electro-optical crystal substrate 12 on which the low refractive index layer 16 is formed. It can be manufactured by directly joining the filmed low-dielectric-constant substrate 14. According to this manufacturing method, the amorphous layer 18 shown in FIG. 7 is formed between the low refractive index layer 16 and the second low refractive index layer 20. Here, if the materials of the low refractive index layer 16 and the second low refractive index layer 20 are the same or the structures are similar, high quality direct bonding can be performed relatively easily.

As shown in FIG. 9, a ridge portion 13 may be formed on the upper surface 12a of the electro-optical crystal substrate 12. The ridge portion 13 is a protruding portion extending elongated along the upper surface 12a. The ridge portion 13 constitutes a ridge type optical waveguide in the electro-optic element in which the composite substrate 10 is manufactured. When the ridge portion 13 is formed in advance on the composite substrate 10, it is possible to easily manufacture an electro-optical element that requires a ridge-type optical waveguide. The width W of the ridge portion 13 is not particularly limited, but may be 1 micrometer or more and 10 micrometer or less. The height T1 of the ridge portion 13 is also not particularly limited, but may be 10% or more and 95% or less of the thickness T2 of the electro-optical crystal substrate 12. The number, position, and shape of the ridge portions 13 are also not particularly limited. As an example, when the composite substrate 10 is used in the manufacture of a Mach-Zehnder type electro-optic modulator, it is preferable that two ridge portions 13 having at least a part extending in parallel are formed.

As shown in FIG. 10, the composite substrate 10 having the ridge portion 13 may be further provided with the first electrode 42 and the second electrode 44. Here, when the c-axis (c-axis) of the electro-optical crystal substrate 12 is parallel to the electro-optical crystal substrate 12, the first electrode 42 may be provided on one side surface 13a of the ridge portion 13. .. The second electrode 44 may be provided on the other side surface 13b of the ridge portion 13 and may face the first electrode 42 with the ridge portion 13 interposed therebetween. According to such a configuration, the first electrode 42 and the second electrode 44 can apply an electric field in parallel with the c-axis to the ridge portion 13 which is an optical waveguide in the electro-optical element. The material constituting the first electrode 42 and the second electrode 44 may be a conductor, and may be a metal material such as gold. Further, a low refractive index film having a lower refractive index than that of the electro-optical crystal substrate 12 is provided between the first electrode 42 and the electro-optical crystal substrate 12 and between the second electrode 44 and the electro-optical crystal substrate 12. You may. Such a low refractive index film functions as a clad layer and can suppress the loss of light transmitted through the ridge portion 13.

As shown in FIG. 11, the composite substrate 10 may be provided with a conductive layer 22 along the lower surface 14b of the low dielectric constant substrate 14. Such a conductive layer 22 can be used as a ground electrode in an electro-optical element manufactured from the composite substrate 10. In this case, the conductive layer 22 can suppress the electric field applied by the first electrode 42 and the second electrode 44 from leaking from the lower surface 14b of the low dielectric constant substrate 14. The material constituting the conductive layer 22 may be a conductor, for example, gold (Au), silver (Ag), copper (Cu), aluminum (Al), platinum (Pt), or at least two of them. It may have a layer of alloy containing one. The conductive layer 22 may have a single-layer structure or a multi-layer structure. The conductive layer 22 is a base layer that comes into contact with the low dielectric constant substrate 14, and in order to prevent peeling and migration of the conductive layer 22, titanium (Ti), chromium (Cr), nickel (Ni), platinum (Pt), etc. are used. May have a layer of. The conductive layer 22 shown in FIG. 11 can be similarly provided on the other composite substrate 10 described above.

In addition, as shown in FIG. 12, the composite substrate 10 may further include a support substrate 24 bonded to the conductive layer 22. According to such a configuration, the thickness of the electro-optical crystal substrate 12 and the low dielectric constant substrate 14 can be reduced while maintaining the mechanical strength of the composite substrate 10 by the support substrate 24. In the electro-optical element manufactured from the composite substrate 10, the smaller the thickness of the electro-optical crystal substrate 12 and the low dielectric constant substrate 14, the higher the resonance frequency with respect to the electric signal. For example, if the total thickness of the electro-optical crystal substrate 12 and the low dielectric constant substrate 14 is 60 micrometers or less, the resonance frequency is 300 gigahertz or more. In this case, the electro-optical element can process a high frequency (so-called millimeter wave band) electric signal such as 100 gigahertz without any problem.

The material constituting the support substrate 24 is not particularly limited. For example, the support substrate 24 includes silicon (Si), glass, Sialon (Si ₃ N ₄ -Al ₂ O ₃ ), Murite ( ₃ Al _{2 O 3.2SiO 2 ,} 2Al ₂ O ₃ SiO ₂ ), and aluminum nitride (AlN). ), Silicon nitride (Si ₃ N ₄ ), magnesium oxide (MgO), sapphire, quartz, crystal, gallium nitride (GaN), silicon carbide (SiC), gallium oxide (Ga ₂ O ₃ ). May be. However, in order to suppress thermal deformation (particularly warpage) of the composite substrate 10, the coefficient of linear expansion of the material constituting the support substrate 24 should be closer to the coefficient of linear expansion of the material constituting the electro-optical crystal substrate 12. Although not particularly limited, the coefficient of linear expansion of the material constituting the support substrate 24 is preferably within ± 50% of the coefficient of linear expansion of the material constituting the electro-optical crystal substrate 12. As an example, the material constituting the support substrate 24 may be the same as the material constituting the electro-optical crystal substrate 12. The support substrate 24 shown in FIG. 12 can be similarly provided on the other composite substrate 10 described above together with the conductive layer 22.

In the method for manufacturing the composite substrate 10 shown in FIG. 12, the conductive layer 22 and the support substrate 24 may be bonded by direct bonding. In this case, the surface layer of the conductive layer 22 may be made of platinum. Platinum is a suitable material for direct bonding. Therefore, when the surface layer of the conductive layer 22 is made of platinum, the electro-optic crystal substrate 12 on which the conductive layer 22 is formed can be satisfactorily directly bonded to the support substrate 24. In this case, in the manufactured composite substrate 10, an amorphous layer due to direct bonding is formed between the conductive layer 22 and the support substrate 24. This amorphous layer is composed of an element (mainly platinum) constituting the conductive layer 22 and an element constituting the support substrate 24.

As shown in FIG. 13, the c-axis (c-axis) of the electro-optical crystal substrate 12 may be perpendicular to the electro-optic crystal substrate 12. Even in this case, the ridge portion 13 may be formed on the upper surface 12a of the electro-optical crystal substrate 12. Further, the first electrode 52 and the second electrode 54 may be provided on the upper surface 12a of the electro-optical crystal substrate 12. However, the first electrode 52 is preferably provided on the top surface 13c of the ridge portion 13, and the second electrode 54 is in the range excluding the ridge portion 13 of the top surface 12a of the electro-optical crystal substrate 12. It should be provided. According to such a configuration, the first electrode 52 and the second electrode 54 can apply an electric field in parallel with the c-axis to the ridge portion 13 which is an optical waveguide in the electro-optical element.

As shown in FIG. 14, the composite substrate 10 shown in FIG. 13 may also be provided with the conductive layer 22 along the lower surface 14b of the low dielectric constant substrate 14. As described above, the conductive layer 22 can be used as a ground electrode in the electro-optical element manufactured from the composite substrate 10. Further, as shown in FIG. 15, a support substrate 24 may be bonded to the conductive layer 22 in the same manner as the composite substrate 10 shown in FIG. As a result, the thickness of the electro-optical crystal substrate 12 and the low dielectric constant substrate 14 can be reduced while maintaining the mechanical strength of the composite substrate 10. As described above, by reducing the thickness of the electro-optical crystal substrate 12 and the low dielectric constant substrate 14, the resonance frequency with respect to the electric signal can be made sufficiently higher than the high frequency band such as the millimeter wave band. Also in the method for manufacturing the composite substrate 10 shown in FIG. 15, the conductive layer 22 and the support substrate 24 can be bonded by direct bonding. In this case, as described above, when the surface layer of the conductive layer 22 is made of platinum, the direct bonding between the conductive layer 22 and the support substrate 24 can be satisfactorily performed.

As illustrated in FIGS. 16-19, in the various composite substrates 10 disclosed herein, the optical waveguide region 56 containing impurities is located inside the ridge portion 13 along the longitudinal direction of the ridge portion 13. It may be formed. The composite substrate 10 shown in FIG. 16 is the composite substrate 10 shown in FIG. 9 in which the optical waveguide region 56 is added in the ridge portion 13. The composite substrate 10 shown in FIG. 17 is the composite substrate 10 shown in FIG. 13 in which the optical waveguide region 56 is added in the ridge portion 13. The composite substrate 10 shown in FIG. 18 is the composite substrate 10 shown in FIG. 14 in which the optical waveguide region 56 is added in the ridge portion 13. The composite substrate 10 shown in FIG. 19 is the composite substrate 10 shown in FIG. 15 in which the optical waveguide region 56 is added in the ridge portion 13. Although not shown, similarly in the other composite substrates 10 shown in FIGS. 10 to 12, an optical waveguide region containing impurities is formed inside the ridge portion 13 along the longitudinal direction of the ridge portion 13. You may.

As shown in FIG. 20, the low refractive index layer 16 of the composite substrate 10 may have a multilayer structure including a main layer 16a and a bonding layer 16b. The bonding layer 16b is a layer located between the main layer 16a and the amorphous layer 18 and in contact with the amorphous layer 18. As described above, the material constituting the main layer 16a may be one or more of silicon oxide, tantalum oxide, aluminum oxide, glass, magnesium fluoride and calcium fluoride. On the other hand, regarding the material constituting the bonding layer 16b, for example, tantalum oxide (Ta ₂ O ₅ ), niobium oxide (Nb ₂ O ₅ ), aluminum oxide (Al ₂ O ₃ ), titanium oxide (TIO ₂ ) and the like are directly bonded. It is good that the material is suitable for. According to such a configuration, as shown in FIG. 21, the low refractive index layer 16 having the bonding layer 16b can be satisfactorily directly bonded to the low dielectric constant substrate 14. The low refractive index layer 16 having a multilayer structure can be adopted in all the embodiments disclosed in the present specification.

As shown in FIG. 22, when the composite substrate 10 further includes the second low refractive index layer 20, the second low refractive index layer 20 also has a multilayer structure including the main layer 20a and the bonding layer 20b. May be good. The bonding layer 20b is a layer located between the main layer 20a and the amorphous layer 18 and in contact with the amorphous layer 18. As described above, the material constituting the bonding layer 20b may be a material suitable for direct bonding, such as tantalum oxide, niobium oxide, aluminum oxide, and titanium oxide. According to such a configuration, as shown in FIG. 23, the low refractive index layer 16 on the electro-optic crystal substrate 12 side can be satisfactorily directly bonded to the second low refractive index layer 16 having the bonding layer 20b. can. In the examples shown in FIGS. 22 and 23, the low refractive index layer 16 on the electro-optic crystal substrate 12 side may have a single-layer structure or may not have the bonding layer 16b. The low refractive index layer 20 having a multilayer structure can be adopted in all the embodiments disclosed in the present specification.

10: Composite substrate 12: Electro-optical crystal substrate 13: Ridge portion 13a, 13b: Ridge portion side surface 13c: Ridge portion top surface 14: Low refractive index substrate 16: Low refractive index layer 18: Amorphic layer 20: Second low Refractive index layer 22: Conductive layer 24: Support substrate 32, 34, 42, 44, 52, 54: Electrode 36, 56: Optical waveguide region

Claims (10)

A composite substrate for electro-optical elements
With an electro-optical crystal substrate having an electro-optic effect,
The electro-optical crystal substrate is laminated, and a low dielectric constant substrate having a dielectric constant lower than that of the electro-optical crystal substrate is used.
A low refractive index layer located between the low refractive index substrate and the electro-optical crystal substrate and having a lower refractive index than the electro-optical crystal substrate.
An amorphous layer located between the low dielectric constant substrate and the low refractive index layer,
A second low refractive index layer, which is located between the low dielectric constant substrate and the amorphous layer and has a lower refractive index than the electro-optical crystal substrate,
Equipped with
Each of the low refractive index layer and the second low refractive index layer has a main body layer and a bonding layer made of a material different from the main body layer.
The amorphous layer is composed of an element constituting the bonding layer of the low refractive index layer and an element constituting the bonding layer of the second low refractive index layer.
Composite board. The composite substrate according to claim 1, wherein the low refractive index layer has a dielectric constant lower than that of the electro-optical crystal substrate. The composite substrate according to claim 2, wherein the main layer of the low refractive index layer is composed of at least one of silicon oxide, tantalum oxide, aluminum oxide, glass, magnesium fluoride and calcium fluoride. The composite substrate according to any one of claims 1 to 3, wherein the low dielectric constant substrate has a relative permittivity of 5 or less. The composite substrate according to any one of claims 1 to 4, wherein the material constituting the main layer of the second low refractive index layer is the same as the material constituting the main layer of the low refractive index layer. The composite substrate according to any one of claims 1 to 5, wherein a ridge portion is formed on the surface of the electro-optical crystal substrate. The c-axis of the electro-optical crystal substrate is parallel to the electro-optic crystal substrate, and is parallel to the electro-optic crystal substrate.
A first electrode provided on one side surface of the ridge portion and a second electrode provided on the other side surface of the ridge portion and facing the first electrode with the ridge portion interposed therebetween are further provided. , The composite substrate according to claim 6. The c-axis of the electro-optical crystal substrate is perpendicular to the electro-optic crystal substrate, and is perpendicular to the electro-optic crystal substrate.
The claim further comprises a first electrode provided on the top surface of the ridge portion, and a second electrode provided in a range excluding the ridge portion on the surface of the electro-optical crystal substrate. 6. The composite substrate according to 6. The composite substrate according to any one of claims 6 to 8, wherein an optical waveguide region containing impurities is formed in the ridge portion along the longitudinal direction of the ridge portion. A method for manufacturing a composite substrate for an electro-optical element.
A step of forming a low refractive index layer having a refractive index lower than that of the electro-optical crystal substrate on the main surface of an electro-optical crystal substrate having an electro-optic effect.
A step of forming a second low-refractive index layer having a refractive index lower than that of the electro-optical crystal substrate on the main surface of the low-dielectric-constant substrate having a lower refractive index than the electro-optical crystal substrate.
A step of directly joining the main surface of the low refractive index substrate on which the second low refractive index layer is formed to the main surface of the electro-optical crystal substrate on which the low refractive index layer is formed.
Equipped with
Each of the low refractive index layer and the second low refractive index layer has a main body layer and a bonding layer made of a material different from the main body layer.
In the direct bonding step, the bonding layer of the low refractive index layer and the bonding layer of the second low refractive index layer are bonded by direct bonding.
Production method.

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