JP6736795B2 - Composite substrate for electro-optical element - Google Patents

Description

The technology disclosed herein relates to a composite substrate for an electro-optical element (eg, a light modulator) that utilizes the electro-optical effect.

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-optical effect. The electro-optical element is used, for example, in optical radio wave fusion communication, and is being developed in order to realize high-speed and large-capacity communication.

For example, Japanese Patent Laid-Open No. 2010-85789 discloses an optical modulator. This optical modulator includes an electro-optical crystal substrate having an electro-optical effect, and an optical waveguide is formed inside the substrate. 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-optic crystal substrate but also reduces dielectric loss, thereby facilitating high-speed operation of the optical modulator and driving at a low voltage. The composite substrate in which the electro-optic crystal substrate and the low dielectric constant substrate are combined can be suitably used not only for the optical modulator but also for various electro-optical elements.

In the conventional composite substrate, the electro-optic crystal substrate and the low dielectric constant substrate are bonded by an adhesive. With such a structure, the adhesive may deteriorate with time, resulting in peeling of the composite substrate, and further, the peeling may cause damage such as cracks. In order to avoid such a problem, it is possible to directly bond the electro-optic crystal substrate and the low dielectric constant substrate without using an adhesive. However, when the electro-optic crystal substrate and the low-dielectric constant substrate are directly bonded, an amorphous element composed of the electro-optic crystal substrate and the low-dielectric constant substrate is provided between the electro-optic crystal substrate and the low-dielectric constant substrate. A layer is formed. This amorphous layer has no crystallinity and optical properties are different from those of both substrates, and the interface between the electro-optic crystal substrate and the amorphous layer is not flat. Such an uneven interface may scatter (for example, irregular 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 same, which can avoid or reduce the problems described above.

This specification discloses a composite substrate for an electro-optical element. This composite substrate includes an electro-optic crystal substrate having an electro-optic effect and an electro-optic crystal substrate, and a low-dielectric constant substrate having a lower dielectric constant than the electro-optic crystal substrate, a low-dielectric constant substrate and an electro-optic substrate. It is located between the crystal substrate and the low-refractive index layer having a lower refractive index than the electro-optic crystal substrate and between the low-dielectric constant substrate and the low-refractive index layer. An amorphous layer composed of the constituent elements and the constituent elements of the low refractive index layer is provided.

The composite substrate described above can be manufactured by the following manufacturing method. This manufacturing method comprises a step of forming a low refractive index layer having a lower refractive index than that of the electro-optical crystal substrate on the main surface of the electro-optical crystal substrate having an electro-optical effect, and the low-refractive index layer being formed. And a step of directly bonding a low dielectric constant substrate having a dielectric constant lower than that of the electro-optic crystal substrate to the main surface of the electro-optic crystal substrate. The term “direct bond” as used herein means a bond in which atoms are diffused between two members to be bonded and a covalent bond is formed between the atoms.

According to the manufacturing method described above, a composite substrate in which the electro-optic crystal substrate and the low dielectric constant substrate are laminated can be manufactured without the need for an adhesive. In the manufactured composite substrate, an amorphous layer resulting from direct bonding is formed between the low dielectric constant substrate and the low refractive index layer. That is, the low refractive index layer is interposed between the amorphous layer and the electro-optical crystal substrate, and the amorphous layer does not contact the electro-optical crystal substrate. Therefore, the light transmitted through the electro-optic crystal substrate is not scattered or absorbed at the amorphous layer or the uneven interface between the amorphous layer and the electro-optic 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. it 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-section of the composite substrate 10 of an Example.

The figure which shows 1 process of the manufacturing method of the composite substrate 10 of an Example.

The figure which shows 1 process of the manufacturing method of the composite substrate 10 of an Example.

The figure which shows 1 process 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 for forming an electric field are added to the electro-optic crystal substrate 12, and an optical waveguide region 36 provided in the electro-optic crystal substrate 12 is added.

9 shows a modified example of the composite substrate 10, in which a second low refractive index layer 20 is added between the low dielectric constant substrate 14 and the amorphous layer 18.

FIG. 9 is a diagram showing a method of manufacturing the composite substrate 10 of the modified example shown in FIG. 7.

7 shows a modification of the composite substrate 10, in which a ridge portion 13 is formed on the upper surface 12 a of the electro-optic crystal substrate 12.

A modified example of the composite substrate 10 is shown, and a first electrode 42 and a second electrode 44 are added as compared with the modified example shown in FIG. 9. 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 modified example of the composite substrate 10 is shown, and a conductive layer 22 is added along the lower surface 14b of the low dielectric constant substrate 14 as compared with the modified example 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. 11.

A modified example of the composite substrate 10 is shown, and a first electrode 52 and a second electrode 54 are added as compared with the modified example shown in FIG. 9. In this modification, the c-axis (c-axis) of the electro-optical crystal substrate 12 is perpendicular to the electro-optical crystal substrate 12.

13 shows a modified example of the composite substrate 10, in which a conductive layer 22 is added along the lower surface 14b of the low dielectric constant substrate 14 as compared with the modified example shown in FIG.

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

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

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

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

17 shows a modification of the composite substrate 10, in which an optical waveguide region 56 is added in the ridge portion 13 as compared with the modification shown in FIG.

7 shows a modified example of the composite substrate 10, in which the low refractive index layer 16 has a multilayer structure including a main layer 16a and a surface layer 16b.

The figure explaining 1 process (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. 23 is a diagram illustrating one step (direct bonding) of the method of manufacturing the composite substrate 10 shown in FIG. 22.

In one embodiment of the present technology, the low refractive index layer may have a lower dielectric constant than the electro-optic crystal substrate. With 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 an embodiment of the present technology, the composite substrate may further include a second low refractive index layer that is located between the low dielectric constant substrate and the amorphous layer and has a lower refractive index than the electro-optic crystal substrate. .. In this case, the amorphous layer may be composed of an element forming the low refractive index layer and an element forming the second low refractive index layer, instead of the element forming the low dielectric constant substrate. Such a composite substrate can be manufactured by directly bonding the electro-optic crystal substrate having the low refractive index layer formed thereon and the low dielectric constant substrate having the second low refractive layer formed thereon. At this time, if the materials of the low refractive index layer and the second low refractive index layer are the same or have similar structures, high quality direct bonding can be relatively easily performed.

In one embodiment of the present technology, a ridge portion may be formed on the surface of the electro-optic 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 in the electro-optical crystal substrate. However, since the optical waveguide region doped with impurities has a small step in the refractive index due to the doping of impurities and the light confinement effect is small, the near-field image (near field diameter) of light becomes relatively large. As a result, in the electro-optical element manufactured from the composite substrate, the electric field efficiency is reduced, so that the required driving voltage is increased. Therefore, the element size also becomes large. The ridge type optical waveguide is preferable from the viewpoint of lowering the driving voltage and downsizing of the electro-optical element.

In the above-described embodiments, the c-axis (that is, the crystal main axis) of the electro-optical crystal substrate may be parallel to the electro-optical crystal substrate. That is, the electro-optic crystal substrate may be an x-cut substrate. In this case, the composite substrate includes 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 principal axis) of the electro-optical crystal substrate may be perpendicular to the electro-optical crystal substrate. That is, the electro-optic 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 on a region of the surface of the electro-optic crystal substrate excluding the ridge portion. 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 where the composite substrate has a first electrode and a second electrode, at least one of between the first electrode and the electro-optic crystal substrate and between the second electrode and the electro-optic crystal substrate. A low refractive index film having a lower refractive index than the electro-optic crystal substrate may be formed. According to such a configuration, at the interface between the electro-optic crystal substrate and the first electrode (or the second electrode), it is possible to prevent light transmitted through the electro-optic crystal substrate from being absorbed or scattered by the electrode. You can

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

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

In the above-described embodiments, the composite substrate may further include a support substrate bonded to the conductive layer. With such a configuration, it is possible to reduce the thickness of the electro-optic crystal substrate and the low dielectric constant substrate while maintaining the mechanical strength of the composite substrate by the support substrate. In the electro-optical element manufactured from the composite substrate, as the thickness of the electro-optic crystal substrate and the low dielectric constant substrate becomes smaller, the frequency at which an electric signal resonates in the composite substrate (resonance frequency) can be shifted to the high frequency side. For example, if the total thickness of the electro-optic crystal substrate and the low dielectric constant substrate is 60 μm or less, the resonance frequency is 300 GHz or more. In this case, the electro-optical element can avoid the problem that a high-frequency (so-called millimeter wave band) electric signal of 100 GHz 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 present those skilled in the art with details for implementing the preferred embodiments of the invention and is not intended to limit the scope of the invention. Also, the additional features and inventions disclosed below can be used separately or in combination with other features and inventions to provide further improved composite substrates, and methods of using and making them. ..

In addition, the combination of features and steps disclosed in the following detailed description is not essential in carrying out the present invention in the broadest sense, and is only for explaining a representative specific example of the present invention. It is described. Furthermore, the various features of the exemplary embodiments above and below, as well as those of the independent and dependent claims, are set forth herein in providing additional and useful embodiments of the invention. It is not necessary to combine them according to the specific examples shown or in the order in which they are listed.

All the features described in the present specification and/or the claims are, apart from the configuration of the features described in the embodiments and/or the claims, individually as a limitation to the disclosure as originally filed and the specific matters claimed, And are intended to be disclosed independently of each other. Furthermore, all numerical ranges and statements regarding groups or groups are intended to disclose the original disclosure as well as any intermediate constructions as a limitation to the particular matter claimed.

With reference to the drawings, a composite substrate 10 of an embodiment and a manufacturing method thereof will be described. The composite substrate 10 of the present embodiment can be applied to various electro-optical elements such as an optical modulator. As shown in FIG. 1, the composite substrate 10 of the present embodiment is manufactured in the form of a so-called wafer and provided to the manufacturer of the electro-optical element. As an example, composite substrate 10 has a diameter of 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 wafer form, and may be manufactured and provided in various forms.

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

The material constituting the electro-optic crystal substrate 12 is not particularly limited, but lithium niobate (LiNbO ₃ :LN), lithium tantalate (LiTaO ₃ :LT), potassium titanate phosphate (KTiOPO ₄ : KTP), niobate. potassium-lithium _{(K x Li (1-x} ) NbO 2: KLN), potassium niobate (KNbO _3: KN), potassium tantalate niobate _{(KNb x Ta (1-x} ) O 3: KTN), niobium It may be either a solid solution of lithium oxide and lithium tantalate. The electro-optic crystal substrate 12 may have an electro-optic effect of changing 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. On the upper surface 14a side of the low dielectric constant substrate 14, the electro-optic crystal substrate 12 described above is laminated. The low dielectric constant substrate 14 is made of a material having a lower dielectric constant than the electro-optic crystal substrate 12. The dielectric constant (relative dielectric constant) of the low dielectric constant substrate 14 is not particularly limited, but is preferably 5 or less. As an example, the material forming the low dielectric constant substrate 14 may be, for example, quartz or 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-optic crystal substrate 12. The low dielectric constant substrate 14 adjacent to the electro-optic crystal substrate 12 can reduce dielectric loss when an electric field is applied to the electro-optic crystal substrate 12. Therefore, by using the composite substrate 10, it is possible to manufacture an electro-optical element that can be driven at high speed and at 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 refractive index lower than that of the electro-optic 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. To be done. The material forming the low refractive index layer 16 is not particularly limited, but 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 μm or more and 1 μm or less. As an example, the low-refractive index layer 16 in the present embodiment suppresses the dielectric loss and reduces the effective dielectric constant of the electric signal, and thus the dielectric loss (electric loss) of the electro-optic crystal substrate 12 is increased so that the light modulation can be performed efficiently. tan δ) or a material having 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 forming the low dielectric constant substrate 14 and an element forming 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 described later, the composite substrate 10 can be manufactured by directly bonding the low dielectric constant substrate 14 to the electro-optic crystal substrate 12 on which the low refractive index layer 16 is formed. The amorphous layer 18 is a layer generated in 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 that contacts the low refractive index layer 16) and the lower surface 18b of the amorphous layer 18 (that is, the interface that contacts the low dielectric constant substrate 14) are not necessarily flat.

In general, an amorphous layer resulting from direct bonding is composed of elements constituting the materials located above and below it, has no crystallinity, and different elements may be taken in from the outside. different. In addition, 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 the present embodiment, the low refractive index layer 16 having an unaffected thickness is interposed between the amorphous layer 18 and the electro-optic crystal substrate 12, and the amorphous layer 18 is the electro-optic crystal substrate. I don't touch 12. Therefore, the light transmitted through the electro-optic 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 the electro-optical crystal substrate 12, the leakage of light propagating through the electro-optical crystal substrate 12, like a clad in an optical fiber, is prevented. It is possible to suppress and propagate the optical waveguide.

As described above, in the composite substrate 10 of the present embodiment, the low dielectric constant substrate 14 is provided adjacent to the electro-optic crystal substrate 12, so that when the electric field is applied to the electro-optic crystal substrate 12, the dielectric The loss can be reduced. Further, since the electro-optic 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 that reliability can be secured. .. Since the amorphous layer 18 resulting from the direct junction is separated from the electro-optic crystal substrate 12 by the low refractive index layer 16, the light transmitted through the electro-optic crystal substrate 12 can propagate to the output side without loss. it can.

Next, a method of manufacturing the composite substrate 10 will be described with reference to FIGS. 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 (c-axis is parallel to the substrate), and when a domain-inverted portion is formed, the c-axis forms an angle within 10° with the horizontal plane of the substrate. May be an offset substrate. Alternatively, it may be a z-cut substrate (the c-axis is perpendicular to the substrate). Next, as shown in FIG. 4, the low refractive index layer 16 is formed on the lower surface 12b of the electro-optic crystal substrate 12. The low refractive index layer 16 can be formed by vapor deposition (physical vapor deposition or chemical vapor deposition), although not particularly limited thereto. The lower surface 12b of the electro-optic crystal substrate 12 is one main surface of the electro-optic crystal substrate 12. Next, as shown in FIG. 5, the 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-optic 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.

Regarding the above-mentioned direct bonding, the specific procedure and processing conditions are not particularly limited. It can be appropriately determined according to each material of the layer or the substrate to be bonded to each other. As an example, in the manufacturing method of the present embodiment, first, in the high vacuum chamber (for example, about 1×10 ⁻⁶ Pascal), each bonding surface is irradiated with a neutralizing beam. As a result, each joint surface is activated. Then, the activated bonding surfaces are brought into contact with each other in a vacuum atmosphere to bond them at room temperature. The load at the time of joining can be set to 100 to 20000 Newtons, for example. 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 supply to the electrodes arranged in the chamber. As a result, electrons are moved by the electric field generated between the electrode (positive electrode) and the chamber (negative electrode), and a beam of atoms and ions is generated by the inert gas. Among the beams that have reached the grid, the ion beam is neutralized by the grid, so that a beam of neutral atoms is emitted from the fast atom beam source. The atomic species forming the beam are preferably an inert gas element (eg, argon (Ar), nitrogen (N), etc.). The voltage upon activation by beam irradiation can be 0.5 to 2.0 kilovolts and the current can be 50 to 200 milliamps.

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-optic crystal substrate 12 on the upper surface 12 a of the electro-optic crystal substrate 12. The material forming the electrodes 32 and 34 may be a conductor and may be a metal such as gold. Between the electrodes 32 and 34 and the electro-optic crystal substrate 12, titanium (Ti), chromium (Cr), nickel (Ni), platinum (Pt) is provided in order to prevent peeling and migration of the electrodes 32 and 34. Other layers may intervene. The numbers, positions, and shapes 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 for each electro-optical element. If 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.

Additionally or alternatively, the optical waveguide region 36 may be provided in the electro-optic crystal substrate 12 by doping impurities. In the electro-optic 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 a 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-optic crystal substrate 12. The second low refractive index layer 20 may be made of a material having a lower dielectric constant than the electro-optic crystal substrate 12 in order to suppress dielectric loss. The second low refractive index layer 20 can be made 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 of manufacturing the composite substrate 10 shown in FIG. 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-optic crystal substrate 12 on which the low refractive index layer 16 is formed. It can be manufactured by directly bonding 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 low refractive index layer 16 and the second low refractive index layer 20 are made of the same material or have similar structures, high quality direct bonding can be relatively easily performed.

As shown in FIG. 9, a ridge portion 13 may be formed on the upper surface 12 a of the electro-optic crystal substrate 12. The ridge portion 13 is a protruding portion that extends in an elongated shape along the upper surface 12a. The ridge portion 13 constitutes a ridge-type optical waveguide in the electro-optical element in which the composite substrate 10 is manufactured. If 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 μm or more and 10 μm 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-optic crystal substrate 12. The number, position, and shape of the ridge portions 13 are not particularly limited. As an example, when the composite substrate 10 is used for manufacturing a Mach-Zehnder type electro-optic modulator, it is preferable that at least two ridge portions 13 extending in parallel be formed.

As shown in FIG. 10, the composite substrate 10 having the ridge portion 13 may be further provided with a first electrode 42 and a second electrode 44. Here, when the c-axis of the electro-optic crystal substrate 12 is parallel to the electro-optic crystal substrate 12, the first electrode 42 may be provided on the one side surface 13 a of the ridge portion 13. .. The second electrode 44 is preferably provided on the other side surface 13b of the ridge portion 13 and faces the first electrode 42 with the ridge portion 13 interposed therebetween. With such a configuration, the first electrode 42 and the second electrode 44 can apply an electric field parallel to the c-axis to the ridge portion 13 that serves as an optical waveguide in the electro-optical element. The material forming 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 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. May be. Such a low-refractive index film functions as a clad layer and can suppress 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 14 b 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 forming the conductive layer 22 may be a conductive material, 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 is in contact with the low dielectric constant substrate 14, and titanium (Ti), chromium (Cr), nickel (Ni), platinum (Pt), or the like is used to prevent peeling or migration of the conductive layer 22. It 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. With such a configuration, it is possible to reduce the thickness of the electro-optic crystal substrate 12 and the low dielectric constant substrate 14 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 thicknesses of the electro-optical crystal substrate 12 and the low dielectric constant substrate 14, the higher the resonance frequency for an electric signal. For example, if the total thickness of the electro-optic crystal substrate 12 and the low dielectric constant substrate 14 is 60 μm or less, the resonance frequency is 300 GHz or more. In this case, the electro-optical element can process a high-frequency (so-called millimeter wave band) electric signal of 100 GHz without any problem.

The material forming the support substrate 24 is not particularly limited. For example, the support substrate 24 is silicon (Si), glass, sialon _{(Si 3 N 4 -Al 2 O} 3), mullite _{(3Al 2 O 3 · 2SiO 2} , 2Al 2 O 3 · SiO 2), aluminum nitride (AlN ), silicon nitride (Si ₃ N ₄ ), magnesium oxide (MgO), sapphire, quartz, quartz, gallium nitride (GaN), silicon carbide (SiC), gallium oxide (Ga ₂ O ₃ ) substrate May be However, in order to suppress thermal deformation (particularly warpage) of the composite substrate 10, the linear expansion coefficient of the material forming the support substrate 24 is preferably close to the linear expansion coefficient of the material forming the electro-optic crystal substrate 12. Although not particularly limited, the linear expansion coefficient of the material forming the support substrate 24 is preferably within ±50% of the linear expansion coefficient of the material forming the electro-optic crystal substrate 12. As an example, the material forming the support substrate 24 may be the same as the material forming the electro-optic 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 of manufacturing the composite substrate 10 shown in FIG. 12, the conductive layer 22 and the supporting substrate 24 may be joined by direct joining. 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 resulting from direct bonding is formed between the conductive layer 22 and the supporting substrate 24. This amorphous layer is composed of the element that constitutes the conductive layer 22 (mainly platinum) and the element that constitutes the support substrate 24.

As shown in FIG. 13, the c-axis of the electro-optic 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 12 a of the electro-optic crystal substrate 12. Further, the first electrode 52 and the second electrode 54 may be provided on the upper surface 12 a of the electro-optic crystal substrate 12. However, it is preferable that the first electrode 52 be provided on the top surface 13c of the ridge portion 13, and the second electrode 54 be in a range of the upper surface 12a of the electro-optic crystal substrate 12 excluding the portion of the ridge portion 13. It should be provided. With such a configuration, the first electrode 52 and the second electrode 54 can apply an electric field in parallel to the c-axis to the ridge portion 13 that serves as 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 14 b 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 similarly to the composite substrate 10 shown in FIG. Thereby, the thickness of the electro-optic 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-optic crystal substrate 12 and the low dielectric constant substrate 14, the resonance frequency for an electric signal can be made sufficiently higher than a high frequency band such as a millimeter wave band. Also in the method of manufacturing the composite substrate 10 shown in FIG. 15, the conductive layer 22 and the support substrate 24 can be joined by direct joining. 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 to 19, in various composite substrates 10 disclosed in the present specification, the optical waveguide region 56 containing impurities is provided 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 same as the composite substrate 10 shown in FIG. 9 except that an optical waveguide region 56 is added in the ridge portion 13. The composite substrate 10 shown in FIG. 17 is the same as the composite substrate 10 shown in FIG. 13, except that an optical waveguide region 56 is added in the ridge portion 13. The composite substrate 10 shown in FIG. 18 is the same as the composite substrate 10 shown in FIG. 14, except that an optical waveguide region 56 is added in the ridge portion 13. The composite substrate 10 shown in FIG. 19 is the same as the composite substrate 10 shown in FIG. 15 with the optical waveguide region 56 added inside the ridge portion 13. Although not shown, also in the other composite substrate 10 shown in FIGS. 10 to 12, similarly, an optical waveguide region containing an impurity is formed inside the ridge portion 13 along the longitudinal direction of the ridge portion 13. May be.

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 forming the main body layer 16a may be one or more of silicon oxide, tantalum oxide, aluminum oxide, glass, magnesium fluoride, and calcium fluoride. On the other hand, for the material forming the bonding layer 16b, for example, direct bonding such as tantalum oxide (Ta ₂ O ₅ ), niobium oxide (Nb ₂ O ₅ ), aluminum oxide (Al ₂ O ₃ ), or titanium oxide (TiO ₂ ). It is good that the material is suitable for. With such a configuration, as shown in FIG. 21, the low refractive index layer 16 having the bonding layer 16b can be directly bonded to the low dielectric constant substrate 14 in a good condition. The low refractive index layer 16 having a multi-layer structure can be adopted in all the embodiments disclosed in this 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. 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 forming the bonding layer 20b may be a material suitable for direct bonding, such as tantalum oxide, niobium oxide, aluminum oxide, or titanium oxide. With such a configuration, as shown in FIG. 23, the low refractive index layer 16 on the electro-optic crystal substrate 12 side can be directly bonded well to the second low refractive index layer 16 having the bonding layer 20b. it can. 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 this specification.

10: Composite substrate 12: Electro-optic crystal substrate 13: Ridge portion 13a, 13b: Side surface of the ridge portion 13c: Top surface of the ridge portion 14: Low dielectric constant substrate 16: Low refractive index layer 18: Amorphous layer 20: Second low Refractive index layer 22: Conductive layer 24: Support substrates 32, 34, 42, 44, 52, 54: Electrodes 36, 56: Optical waveguide region

Claims (12)

A composite substrate for an electro-optical element, comprising:
An electro-optic crystal substrate having an electro-optic effect,
While the electro-optic crystal substrate is laminated, a low dielectric constant substrate having a lower dielectric constant than the electro-optic crystal substrate,
Located between the low dielectric constant substrate and the electro-optic crystal substrate, a low refractive index layer having a lower refractive index than the electro-optic crystal substrate,
Located between the low-dielectric constant substrate and the low-refractive index layer, an amorphous layer composed of an element forming the low-dielectric constant substrate and an element forming the low-refractive-index layer,
A conductive layer located on the opposite side of the electro-optic crystal substrate to the low dielectric constant substrate,
Bei to give a,
The conductive layer has a layer of gold, silver, copper, aluminum, platinum, or an alloy containing at least two of them, and titanium, chromium, nickel is used as an underlying layer in contact with the low dielectric constant substrate. , Further comprising a layer of platinum,
Composite board. The composite substrate according to claim 1, wherein the low refractive index layer has a lower dielectric constant than the electro-optic crystal substrate. The composite substrate according to claim 2, wherein the low refractive index layer is composed of at least one of silicon oxide, tantalum oxide, aluminum oxide, glass, magnesium fluoride and calcium fluoride. Further comprising a second low-refractive index layer located between the low-dielectric constant substrate and the amorphous layer and having a lower refractive index than the electro-optic crystal substrate,
The amorphous layer is composed of an element forming the low refractive index layer and an element forming the second low refractive index layer, instead of the element forming the low dielectric constant substrate. The composite substrate according to any one of 3 above. The composite substrate according to claim 4, wherein the material forming the second low refractive index layer is the same as the material forming the low refractive index layer. The composite substrate according to claim 1, wherein a ridge portion is formed on the surface of the electro-optic crystal substrate. The c-axis of the electro-optic crystal substrate is parallel to the electro-optic crystal substrate,
It further includes 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. The composite substrate according to claim 6. The c-axis of the electro-optic crystal substrate is perpendicular to the electro-optic crystal substrate,
The first electrode provided on the top surface of the ridge portion, and the second electrode provided on a region of the surface of the electro-optic crystal substrate excluding the portion of the ridge portion. 7. The composite substrate according to item 6. The composite substrate according to claim 6, wherein an optical waveguide region containing an impurity is formed in the ridge portion along a longitudinal direction of the ridge portion. The composite substrate according to claim 1, further comprising a support substrate bonded to the conductive layer. The composite substrate according to claim 10, further comprising an amorphous layer located between the conductive layer and the supporting substrate, the amorphous layer being composed of an element forming the conductive layer and an element forming the supporting substrate. .. A method of manufacturing a composite substrate for an electro-optical element, comprising:
On the main surface of the electro-optic crystal substrate having an electro-optic effect, a step of forming a low refractive index layer having a lower refractive index than the electro-optic crystal substrate,
A step of directly bonding a low dielectric constant substrate having a lower dielectric constant than the electro-optic crystal substrate to the main surface of the electro-optic crystal substrate on which the low refractive index layer is formed,
A step of forming a conductive layer on the side opposite to the electro-optic crystal substrate with respect to the low dielectric constant substrate;
Equipped with
The conductive layer has a layer of gold, silver, copper, aluminum, platinum, or an alloy containing at least two of them, and titanium, chromium, nickel is used as an underlying layer in contact with the low dielectric constant substrate. , Further comprising a layer of platinum,
Production method.

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