Composite substrates for electro-optical components

Abstract

The present invention provides a composite substrate, the peeling of which is significantly suppressed, and when it is made into an electro-optical element, the propagation loss of light is small, and it can be driven at high speed and low voltage. In addition, it can realize a very thin electro-optical element that can maintain excellent reliability even in a harsh high-temperature environment. The composite substrate (100) for electro-optical element of an embodiment of the present invention comprises, in the following order: an electro-optical crystal substrate (10) having an electro-optical effect, a first high dielectric constant layer (21), a second high dielectric constant layer (22), and a supporting substrate (30). The first high dielectric constant layer (21) and the second high dielectric constant layer (22) are directly bonded, and an amorphous layer (40) is formed at the bonding interface between the first high dielectric constant layer (21) and the second high dielectric constant layer (22).

Description

Technical Field

[0001] The present invention relates to a composite substrate for an electro-optical element. Background Art

[0002] Various electro-optical elements are known. Electro-optical elements can convert electrical signals into optical signals by utilizing the electro-optic effect. Electro-optical elements are used, for example, in photoelectric wave fusion communications. In order to achieve high-speed and large-capacity communications, low power consumption (low driving voltage), and low space occupation, electro-optical elements are being developed. Therefore, electro-optical elements have begun to adopt a structure using, for example, a composite substrate. As a composite substrate for electro-optical elements, there is known a composite substrate in which an electro-optical crystal substrate having an electro-optical effect and a supporting substrate are directly bonded by means of a thin film layer (such as a high dielectric constant oxide film) to achieve integration. However, such a composite substrate has the following problems. When the electro-optical crystal substrate and the thin film layer are directly bonded, light propagation loss sometimes occurs. When the thin film layer and the supporting substrate are directly bonded, high-speed driving sometimes becomes difficult, and depending on the situation, the bonding itself is sometimes poor and a composite substrate cannot be obtained.

[0003] Prior art literature

[0004] Patent Literature

[0005] Patent Document 1: Japanese Patent No. 4174377 Summary of the Invention

[0006] The main purpose of the present invention is to provide a composite substrate whose peeling is significantly suppressed and, when made into an electro-optical element, has low light propagation loss and can be driven at high speed and low voltage. In addition, it is possible to realize a very thin electro-optical element that can maintain excellent reliability even in harsh high-temperature environments.

[0007] A composite substrate for an electro-optical element according to an embodiment of the present invention comprises, in this order: an electro-optical crystal substrate having an electro-optical effect, a first high dielectric constant layer, a second high dielectric constant layer, and a supporting substrate. The first high dielectric constant layer and the second high dielectric constant layer are directly bonded, and an amorphous layer is formed at the bonding interface between the first and second high dielectric constant layers.

[0008] In one embodiment, for the composite substrate for the electro-optical element, the first high dielectric constant layer is formed directly on the electro-optical crystal substrate, and the second high dielectric constant layer is formed directly on the supporting substrate. The supporting substrate contains silicon oxide as a main component, and the argon concentration in the supporting substrate is 1.0 atomic % or less. In one embodiment, for the composite substrate for the electro-optical element, the first high dielectric constant layer is formed directly on the electro-optical crystal substrate, a low dielectric constant layer is formed directly on the supporting substrate, and the second high dielectric constant layer is formed directly on the low dielectric constant layer. The low dielectric constant layer contains silicon oxide as a main component, and the argon concentration in the low dielectric constant layer is 1.0 atomic % or less. In these composite substrates for electro-optical elements, the argon concentrations in the first high dielectric constant layer and the second high dielectric constant layer may be 1.0 atomic % to 10 atomic %, respectively.

[0009] In one embodiment, in the electro-optical element composite substrate, the first high dielectric constant layer is formed directly on the electro-optical crystal substrate, the low dielectric constant layer is formed directly on the support substrate, and the second high dielectric constant layer is formed directly on the low dielectric constant layer. The low dielectric constant layer is composed of one selected from silicon oxide, aluminum oxide, magnesium fluoride, and calcium fluoride.

[0010] In one embodiment, in the electro-optical element composite substrate, the first high dielectric constant layer is formed directly on the electro-optical crystal substrate, and the second high dielectric constant layer is formed directly on the support substrate. The electro-optical crystal substrate has a thickness of 0.1 μm or greater and less than 1.0 μm, the first high dielectric constant layer has a thickness of 0.01 μm or greater, and the combined thickness of the first and second high dielectric constant layers is 0.10 μm or less. The electro-optical element composite substrate may further include a low dielectric constant layer formed directly on the support substrate, with the second high dielectric constant layer formed directly on the low dielectric constant layer.

[0011] In one embodiment, the support substrate is made of one selected from the group consisting of silicon, glass, sialon, mullite, aluminum nitride, silicon nitride, magnesium oxide, sapphire, quartz, crystal, gallium nitride, silicon carbide, and gallium oxide.

[0012] In one embodiment, the thickness of the electro-optical crystal substrate is 0.1 μm to 0.8 μm. In one embodiment, the thickness of the electro-optical crystal substrate is 0.2 μm to 0.6 μm.

[0013] In one embodiment, the electro-optical crystal substrate is composed of one selected from the group consisting of lithium niobate, lithium tantalate, potassium titanyl phosphate, potassium lithium niobate, potassium niobate, potassium tantalate niobate, and a solid solution of lithium niobate and lithium tantalate.

[0014] In one embodiment, the thickness of the first high dielectric constant layer is 0.01 μm to 0.08 μm, and the thickness of the second high dielectric constant layer is 0.001 μm to 0.04 μm.

[0015] In one embodiment, the first high dielectric constant layer and the second high dielectric constant layer are each made of one selected from the group consisting of tantalum oxide, niobium oxide, titanium oxide, aluminum oxide, hafnium oxide, and silicon.

[0016] In one embodiment, the low dielectric constant layer has a thickness exceeding 10 μm and not greater than 20 μm.

[0017] Effects of the Invention

[0018] According to an embodiment of the present invention, two high-dielectric constant layers are provided in a composite substrate for an electro-optical element. These two high-dielectric constant layers are directly bonded to integrate the electro-optical crystal substrate and the support substrate. This allows for a composite substrate in which delamination is significantly suppressed, light propagation loss is low when the electro-optical element is formed, and high-speed and low-voltage drive are possible. In one embodiment, the support substrate or the low-dielectric constant layer formed directly on the support substrate is composed primarily of silicon oxide, and the argon concentration in the support substrate or the low-dielectric constant layer is set to 1.0 atomic % or less. This allows for a very thin electro-optical element that maintains the excellent effects described above and excellent reliability even in harsh high-temperature environments. In one embodiment, the thickness of the first high-dielectric constant layer is set to a predetermined value or greater, and the combined thickness of the first and second high-dielectric constant layers is set to a predetermined value or less. This allows for maintaining the excellent effects described above and making the electro-optical crystal substrate very thin, resulting in an extremely thin electro-optical element. BRIEF DESCRIPTION OF THE DRAWINGS

[0019] Figure 1 This is a schematic perspective view of a composite substrate for an electro-optical element according to one embodiment of the present invention.

[0020] Figure 2 yes Figure 1 Schematic cross-sectional view of a composite substrate for an electro-optical element.

[0021] Figure 3 Schematic cross-sectional views of composite substrates for electro-optical devices according to Comparative Examples 1, 3, 5, 7, 9, and 11.

[0022] Figure 4 Schematic cross-sectional views of composite substrates for electro-optical devices according to Comparative Examples 2, 4, 6, 8, 10, and 12.

[0023] Figure 5This is a transmission electron microscope image showing the state of the bonding interface between the first high dielectric constant layer and the second high dielectric constant layer in the composite substrate for an electro-optical element of Example 7. DETAILED DESCRIPTION

[0024] Hereinafter, embodiments of the present invention will be described; however, the present invention is not limited to these embodiments.

[0025] A. Overall Structure of Composite Substrates for Electro-Optical Devices

[0026] Figure 1 This is a schematic perspective view of a composite substrate for an electro-optical element (hereinafter sometimes simply referred to as a composite substrate) according to one embodiment of the present invention; Figure 2 yes Figure 1 A schematic cross-sectional view of a composite substrate. Typically, Figure 1 As shown, the composite substrate according to the embodiment of the present invention can be manufactured in the form of a so-called wafer. The size of the composite substrate can be appropriately set according to the purpose. For example, the diameter of the wafer can be 4 inches (about 10 cm). Generally, multiple electro-optical elements can be manufactured from a single composite substrate. It should be noted that the composite substrate is not limited to the form of a wafer and can be manufactured and provided in various forms.

[0027] The composite substrate 100 shown in the example is provided in the following order: an electro-optical crystal substrate 10 having an electro-optic effect, a first high dielectric constant layer 21, a second high dielectric constant layer 22, and a supporting substrate 30. In an embodiment of the present invention, the first high dielectric constant layer 21 and the second high dielectric constant layer 22 are directly bonded. The electro-optical crystal substrate 10 and the supporting substrate 30 are integrated by direct bonding of the two high dielectric constant layers. For example, the first high dielectric constant layer 21 is formed on the surface of the electro-optical crystal substrate 10 by sputtering, and the second high dielectric constant layer 22 is formed on the surface of the supporting substrate 30 by sputtering, and the first high dielectric constant layer 21 and the second high dielectric constant layer 22 of each stack are directly bonded. It should be noted that, representatively, as shown in the example shown in the figure, an amorphous layer 40 is formed at the directly bonded bonding interface. In the example shown in the figure, a low dielectric constant layer 50 is formed on the side of the second high dielectric constant layer 22 of the supporting substrate 30. The low dielectric constant layer 50 is an optional layer provided according to the purpose and can be omitted. The composite substrate 100 may further have an optional layer not shown in the figure. The type, function, number, combination, configuration position, etc. of such layers can be appropriately set according to the purpose. In particular, the structure below the supporting substrate 30 or the low dielectric constant layer 50 (if present) (on the opposite side of the electro-optical crystal substrate) can be appropriately set according to the purpose. For example, a metal film can be provided below the supporting substrate 30 or the low dielectric constant layer 50 (if present). By providing such a metal film, when an electro-optical element is made of a composite substrate, unwanted ripples (noise, fluctuations) in the output signal of the electro-optical element can be suppressed to maintain normal operation. It should be noted that in this specification, "high dielectric constant layer" and "low dielectric constant layer" mean that the dielectric constants of the first high dielectric constant layer 21 and the second high dielectric constant layer 22 are relatively larger than the dielectric constant of the low dielectric constant layer 50. The absence of low-k dielectric constant layer 50 means that the dielectric constants of first high-k dielectric constant layer 21 and second high-k dielectric constant layer 22 are relatively greater than the dielectric constant of support substrate 30. In other words, the dielectric constants of first high-k dielectric constant layer 21, second high-k dielectric constant layer 22, and low-k dielectric constant layer 50 are not defined by specific values ​​of their respective dielectric constants. Furthermore, the relative magnitude of the dielectric constants of these layers relative to the dielectric constant of the electro-optical crystal substrate is not restricted.

[0028] By integrating the electro-optical crystal substrate 10 and the support substrate 30 through direct bonding, it is possible to effectively suppress peeling of the composite substrate. As a result, damage to the electro-optical crystal substrate (e.g., cracks) caused by such peeling can be effectively suppressed. Furthermore, by performing direct bonding without the use of an adhesive, the adverse effects caused by deterioration and deformation of the adhesive can be eliminated, thereby achieving higher reliability. Furthermore, there is no dielectric loss caused by the adhesive.

[0029] According to an embodiment of the present invention, by directly bonding the first high dielectric constant layer 21 and the second high dielectric constant layer 22, the following advantages can be obtained. When the electro-optical element is thinned (representatively, the thickness of the electro-optical crystal substrate is made to be less than 1 μm), it is preferably reinforced by compounding with a supporting substrate. In addition, it is known that in order to make such a composite substrate (electro-optical element) meet the speed matching conditions to achieve high-speed and low-voltage driving, it is effective to provide a high dielectric constant layer. Previously, development was only carried out for the electro-optical crystal substrate and the supporting substrate (low dielectric constant substrate). However, if the thickness of the electro-optical crystal substrate is less than 1 μm as described above, the microwave effective dielectric constant (refractive index) is sometimes too small. By providing a high dielectric constant layer between the electro-optical crystal substrate and the supporting substrate, the excessive reduction of the microwave effective dielectric constant (refractive index) can be suppressed, thereby enabling it to meet the speed matching conditions. Here, when the high dielectric constant layer is a single layer, in order to integrate the electro-optical crystallization substrate and the supporting substrate by direct bonding, direct bonding of the electro-optical crystallization substrate and the high dielectric constant layer or direct bonding of the high dielectric constant layer and the supporting substrate is required. The inventors of the present invention have newly discovered that the position of the amorphous layer that may be formed in the direct bonding of a single high dielectric constant layer has a greater impact on the characteristics of the electro-optical element (such as an optical modulator), and have completed the present invention. That is, if the electro-optical crystallization substrate and the high dielectric constant layer are directly bonded, the amorphous layer formed at the bonding interface develops toward the electro-optical crystallization substrate. As a result, light scattering and / or absorption occurs in the electro-optical crystallization substrate, and in addition, the electro-optical constant of the electro-optical crystallization substrate is insufficient. If the high dielectric constant layer and the supporting substrate are directly bonded, the material constituting the supporting substrate (substantially atoms) can diffuse and move toward the high dielectric constant layer via the amorphous layer formed at the bonding interface. As a result, sometimes the dielectric constant of the high dielectric constant layer decreases and / or the conductivity increases, resulting in an electric shielding effect. As a result, it is impossible to satisfy the speed matching condition, and sometimes high-speed and low-voltage driving becomes difficult. In addition, even if an attempt is made to directly bond the high dielectric constant layer and the supporting substrate, sometimes the bonding itself is poor and a composite substrate cannot be obtained. Relative to direct bonding with the help of a single high dielectric constant layer like this, according to an embodiment of the present invention, the electro-optical crystallization substrate and the supporting substrate are integrated by directly bonding two high dielectric constant layers, and an amorphous layer can be formed between the two high dielectric constant layers, and the amorphous layer can be separated from the electro-optical crystallization substrate and the supporting substrate. As a result, the effect of the composite substrate and the effect of the high dielectric constant layer can be well maintained, and the adverse effects of the amorphous layer can be prevented. As a result, a composite substrate can be achieved in which peeling is significantly suppressed, the propagation loss of light is small when an electro-optical element is made, and high-speed and low-voltage driving can be performed.

[0030] In this specification, “direct bonding” means: the components of the composite substrate ( Figure 1 and Figure 2 In the example of the first high dielectric constant layer 21 and the second high dielectric constant layer 22, the bonding is performed without the aid of an adhesive. The form of direct bonding can be appropriately set according to the composition of the layers or substrates to be bonded. For example, direct bonding can be achieved by the following procedure. In a high vacuum chamber (e.g., 1×10 -6 Pa or so), the bonding surfaces of the components to be bonded (layers or substrates) are irradiated with a neutralizing beam. Accordingly, each bonding surface is activated. Next, the activated bonding surfaces are brought into contact with each other in a vacuum atmosphere and bonded at room temperature. The load during this bonding can be, for example, 100N to 20000N. In one embodiment, when a neutralizing beam is used for surface activation, an inert gas is introduced into the chamber, and a high voltage is applied from a DC power supply to an electrode arranged in the chamber. If it is a structure like this, the electrons move due to the electric field generated between the electrode (positive electrode) and the chamber (negative electrode), generating a beam of atoms and ions of the inert gas. The ion beam in the beam reaching the grid is neutralized at the grid, so that a beam of neutral atoms is emitted from the high-speed atomic beam source. The atomic species constituting the beam are preferably inert gas elements (such as argon (Ar), nitrogen (N)). The voltage for activation by beam irradiation is, for example, 0.5kV to 2.0kV, and the current is, for example, 50mA to 200mA.

[0031] In one embodiment, typically, the support substrate contains silicon oxide as a main component. In addition, the argon concentration in the support substrate is typically less than 1.0 atomic %, preferably less than 0.8 atomic %. The lower the argon concentration in the support substrate, the more ideal it is, and its lower limit can be, for example, 0.1 atomic %. By making the support substrate have such a structure, the following effects can be obtained. According to an embodiment of the present invention, as shown above, even if the electro-optical crystallization substrate is thinned to, for example, about 1 μm, the peeling of the bonding portion can be significantly suppressed. On the other hand, in recent years, it is hoped that the electro-optical crystallization substrate will be further thinned. In this regard, the inventors of the present invention have newly discovered that if the electro-optical crystallization substrate is thinned to less than 1.0 μm (for example, 0.6 μm), the electro-optical crystallization substrate sometimes peels off in a harsh high-temperature environment (for example, after a long-term heating reliability test). The inventors of the present invention have conducted intensive research on such peeling and found that by improving the film quality of the high dielectric constant layer and reducing the thickness of the high dielectric constant layer, the peeling of the electro-optical crystallization substrate in a harsh high-temperature environment can be significantly suppressed. For example, by controlling the argon concentration of the high dielectric constant layer to be typically less than 10 atomic % and the thickness of the high dielectric constant layer to be typically less than 0.2 μm, the peeling of the electro-optical crystallization substrate can be significantly suppressed even in a harsh high-temperature environment. In addition, the inventors of the present invention have conducted in-depth research on the elements for forming such a high dielectric constant layer with excellent film quality and relatively thin, and found that it is sufficient to control the state of the substrate or layer for forming the high dielectric constant layer. In addition, a supporting substrate composed of silicon oxide as the main component for forming the second high dielectric constant layer can form a second high dielectric constant layer with excellent film quality and relatively thin. That is, if the supporting substrate is constructed as described above, a composite substrate with a very thin electro-optical crystallization substrate (for example, a thickness of less than 1 μm) that can significantly suppress peeling even in a harsh high-temperature environment can be achieved. As a result, a very thin electro-optical element that can maintain excellent reliability even in a harsh high-temperature environment can be achieved. Such an effect solves the problem that was only recognized when the electro-optical crystallization substrate was further thinned, and it is an unexpected excellent effect. If the peeling of the electro-optical crystallization substrate in a harsh high-temperature environment as described above is suppressed, the other excellent effects brought about by the embodiment of the present invention can be maintained in a harsh high-temperature environment. Specifically, according to the embodiment of the present invention, even in a harsh high-temperature environment, the propagation loss of light can be maintained at a low level, and high-speed and low-voltage driving can be maintained. It should be noted that, for example, the argon concentration in the supporting substrate can be controlled within the above-mentioned desired range by forming it using a sol-gel method to purify the supporting substrate, irradiating the formed supporting substrate with soft X-rays, or a combination of these methods. In addition, in this specification, the first high dielectric constant layer and the second high dielectric constant layer are sometimes collectively referred to as "high dielectric constant layer".When it is necessary to distinguish the first high dielectric constant layer from the second high dielectric constant layer, “first” and “second” are clearly described.

[0032] In one embodiment, when a low dielectric constant layer is provided, the low dielectric constant layer may be configured to include silicon oxide as a main component, and the argon concentration in the low dielectric constant layer may be less than 1.0 atomic %. By configuring the low dielectric constant layer in this manner, the same effect as that obtained by controlling the argon concentration of the supporting substrate can be obtained. In addition, if it is configured in this manner, the options for configuring the supporting substrate can be increased. For example, the supporting substrate may be configured from a material other than silicon oxide. It should be noted that the argon concentration in the low dielectric constant layer may be controlled by adjusting the argon partial pressure during formation of the low dielectric constant layer (typically during sputtering).

[0033] As the thickness of the high dielectric constant layer, any appropriate thickness can be adopted. The thickness of the high dielectric constant layer can be, for example, 0.001 μm to 1.0 μm, or 0.001 μm to 0.1 μm, or 0.01 μm to 0.1 μm. If the thickness of the high dielectric constant layer is in such a range, it has the following advantages, namely, it is possible to suppress the excessive decrease in the microwave effective dielectric constant (refractive index) caused by the low dielectric constant layer or the supporting substrate, and at the same time, it is possible to reduce the increase in the microwave effective dielectric constant. In addition, if the thickness of the high dielectric constant layer is in such a range (especially if the thickness is less than 0.1 μm), even if the electro-optical crystal substrate is made very thin (for example, less than 1 μm), it is possible to significantly suppress the peeling of the electro-optical crystal substrate under a harsh high temperature environment. As a result, a very thin electro-optical element that can maintain excellent reliability even under a harsh high temperature environment can be achieved. The combined thickness of the first high dielectric constant layer and the second high dielectric constant layer can be, for example, 0.005 μm to 0.2 μm, 0.008 μm to 0.15 μm, 0.01 μm to 0.1 μm, or 0.03 μm to 0.08 μm. By setting the combined thickness of the first high dielectric constant layer and the second high dielectric constant layer within such a range, the effect of controlling the thickness of each high dielectric constant layer can be more pronounced.

[0034] In one embodiment, the thickness of the first high dielectric constant layer is typically 0.01 μm or more, preferably 0.02 μm or more, and more preferably 0.03 μm or more. The thickness of the first high dielectric constant layer can be, for example, 0.08 μm or less, and can be, for example, 0.07 μm or less. In addition, the total thickness of the first high dielectric constant layer and the second high dielectric constant layer is typically 0.10 μm or less, preferably 0.02 μm to 0.10 μm, more preferably 0.02 μm to 0.08 μm, and further preferably 0.03 μm to 0.07 μm. If it is a structure like this, it is possible to maintain the excellent effect of integrating the electro-optical crystal substrate and the supporting substrate by directly bonding the two high dielectric constant layers (representatively, significant suppression of peeling, and suppression of light propagation loss when making an electro-optical element, and high-speed and low-voltage drive), and the electro-optical crystal substrate can be made very thin. In other words, if it is a structure like this, even if the electro-optical crystallization substrate is thinned to, for example, less than 1 μm, for example, less than 0.8 μm, for example, less than 0.7 μm, for example, less than 0.6 μm, it is possible to maintain the above-mentioned excellent effect. If the thickness of the first high dielectric constant layer is too small, then during direct bonding (more specifically, during neutralization beam irradiation), argon atoms sometimes diffuse into the electro-optical crystallization substrate and / or the crystallinity of the electro-optical crystallization substrate deteriorates. As a result, it is sometimes impossible to achieve a good driving voltage and / or the propagation loss of light increases. If the total thickness is too large, it becomes difficult to meet the speed matching condition, and sometimes the modulation band is reduced. As the thickness of the second high dielectric constant layer, the thickness of the first high dielectric constant layer satisfies the above-mentioned desired range and the total thickness of the first high dielectric constant layer and the second high dielectric constant layer satisfies the above-mentioned desired range, and any appropriate thickness can be adopted. The thickness of the second high dielectric constant layer is preferably 0.001 μm to 0.04 μm, more preferably 0.005 μm to 0.035 μm, and even more preferably 0.01 μm to 0.03 μm. When the thickness of the second high dielectric constant layer is within this range, the function of the second high dielectric constant layer can be sufficiently ensured.

[0035] Hereinafter, the components (substrates or layers) of the composite substrate will be described in detail.

[0036] B. Electro-optical crystal substrate

[0037] The electro-optical crystal substrate 10 can be a layer (functional layer) having an electro-optical effect in the electro-optical element. For example, a part or all of the electro-optical crystal substrate 10 can be an optical waveguide that propagates light in the electro-optical element. The electro-optical crystal substrate 10 has an upper surface exposed to the outside and a lower surface located within the composite substrate. The electro-optical crystal substrate 10 is composed of crystals of a material having an electro-optical effect. Specifically, for the electro-optical crystal substrate 10, if an electric field is applied, the optical constant (such as the refractive index) can change. In one embodiment, the c-axis of the electro-optical crystal substrate 10 can be parallel to the electro-optical crystal substrate 10. That is, the electro-optical crystal substrate 10 can be an X-cut substrate or a Y-cut substrate. In another embodiment, the c-axis of the electro-optical crystal substrate 10 can be perpendicular to the electro-optical crystal substrate 10. That is, the electro-optical crystal substrate 10 can be a Z-cut substrate. The thickness of the electro-optical crystal substrate 10 can be set to any appropriate thickness according to the purpose. The thickness of the electro-optical crystal substrate 10 can be, for example, 0.1 μm to 10 μm. As described later, the composite substrate is reinforced with a supporting substrate, so the thickness of the electro-optical crystal substrate can be thinned. The thickness of the electro-optical crystal substrate is preferably 0.2 μm or more, more preferably 0.3 μm or more, and further preferably 0.45 μm or more. If the lower limit of the thickness of the electro-optical crystal substrate is in such a range, the propagation loss of light can be reduced in the electro-optical element. On the other hand, the thickness of the electro-optical crystal substrate is preferably 5.0 μm or less, more preferably 2.8 μm or less, further preferably 1.0 μm or less, further preferably less than 1.0 μm, particularly preferably 0.8 μm or less, and especially preferably 0.6 μm or less. If the upper limit of the thickness of the electro-optical crystal substrate is in such a range, the high-speed and low-voltage driving performance of the electro-optical element can be improved. In addition, if the thickness of the electro-optical crystal substrate is in such a range, the effect of using a high dielectric constant layer becomes significant. That is, the propagation loss of light can be suppressed and a higher-speed and lower-voltage drive can be achieved. Furthermore, according to an embodiment of the present invention, even when using such an extremely thin electro-optical crystal substrate, it is possible to suppress defects in a harsh high-temperature environment, thereby realizing an extremely thin electro-optical element that can maintain excellent reliability even in a harsh high-temperature environment.

[0038] As the material constituting the electro-optical crystal substrate 10, any appropriate material may be used as long as the effects of the embodiments of the present invention are obtained. Representative examples of such materials include dielectrics (e.g., ceramics). Specific examples include lithium niobate (LiNbO3: LN), lithium tantalate (LiTaO3: LT), potassium titanyl phosphate (KTiOPO4: KTP), and potassium lithium niobate (K x Li (1 -x)NbO2: KLM), potassium niobate (KNbO3: KN), potassium tantalate niobate (KNb x Ta(1 -x)O3:KTN), solid solution of lithium niobate and lithium tantalate.

[0039] C. Support substrate

[0040] The support substrate 30 has an upper surface located inside the composite substrate and a lower surface exposed to the outside. The purpose of providing the support substrate 30 is to increase the strength of the composite substrate, thereby making the thickness of the electro-optical crystal substrate thinner. As the support substrate 30, any appropriate structure can be adopted. Specific examples of materials constituting the support substrate include: silicon (Si), glass, silicon aluminum oxynitride ceramics (Si3N4-Al2O3), mullite (3Al2O3·2SiO2, 2Al2O3·3SiO2), aluminum nitride (AlN), silicon nitride (Si3N4), magnesium oxide (MgO), sapphire, quartz, crystal, gallium nitride (GaN), silicon carbide (SiC), gallium oxide (Ga2O3). In one embodiment, the support substrate 30 contains silicon oxide as the main component as described above. That is, the support substrate can be composed of, for example, glass. It should be noted that the closer the linear expansion coefficient of the material constituting the support substrate 30 is to the linear expansion coefficient of the material constituting the electro-optical crystal substrate 10, the more ideal it is. This configuration can suppress thermal deformation (typically, warping) of the composite substrate. Preferably, the linear expansion coefficient of the material constituting the support substrate 30 is within a range of 50% to 150% of the linear expansion coefficient of the material constituting the electro-optical crystal substrate 10. From this perspective, the support substrate 30 can be made of the same material as the electro-optical crystal substrate 10. In particular, when using LN or LT, a substrate with suppressed pyroelectric properties can be used.

[0041] The thickness of the support substrate may be any appropriate thickness as long as it has the reinforcing effect of the composite substrate. The thickness of the support substrate is, for example, 100 μm to 1000 μm. If the thickness of the support substrate is too thin, the reinforcing effect and operability may sometimes become insufficient. If the thickness of the support substrate is too thick, the following problems may sometimes arise: (1) The thickness of the substrate increases, making it difficult to flow in the conventional process; (2) The obtained electro-optical element becomes thicker, and the package size becomes larger than before; (3) The heat dissipation of the support substrate becomes insufficient; (4) Ripple is easily generated in the low-frequency region.

[0042] As described above, a low dielectric constant layer 50 can be formed on the supporting substrate 30. By providing a low dielectric constant layer, regardless of the supporting substrate, the speed matching condition can be satisfied only with a low dielectric constant. In addition, there is no atomic movement toward both the second high dielectric constant layer 22 and the supporting substrate 30, which can increase the difference in dielectric constant at the interface (resulting in a difference in refractive index). As a result, it is possible to provide an electro-optical element with less deviation during manufacturing or less time-varying changes due to the environment, as designed, without increasing the thickness of the high dielectric constant layer. In addition, by providing a low dielectric constant layer, the options for the material of the supporting substrate can be increased. As a low dielectric constant layer, any suitable composition can be used as long as it has such an effect. Specific examples of the material constituting the low dielectric constant layer 50 include silicon oxide (SiO2), aluminum oxide (Al2O3), magnesium fluoride (MgF2), and calcium fluoride (CaF2). As described above, the low dielectric constant layer may be configured to contain silicon oxide as a main component, and the argon concentration in the low dielectric constant layer may be 1.0 atomic % or less.

[0043] As the thickness of the low dielectric constant layer, any appropriate thickness can be adopted. The thickness of the low dielectric constant layer can be, for example, 0.6μm to 20μm, or 5μm to 15μm, or more than 10μm and less than 20μm, or 12μm to 20μm, or 12μm to 15μm. If the thickness of the low dielectric constant layer is in such a range, it has the following advantages, that is, regardless of the supporting substrate, or the low dielectric constant layer is mainly used, it can meet the speed matching condition. When the low dielectric constant layer is thicker (for example, when the thickness exceeds 10μm), the effect brought about by controlling the argon concentration of the low dielectric constant layer can become significant. That is, by making the argon concentration of the low dielectric constant layer within the above-mentioned desired range, even if the low dielectric constant layer is thicker, the total amount of argon in the low dielectric constant layer can be prevented from becoming excessively large. As a result, even if the low dielectric constant layer is thick, the argon concentration of the high dielectric constant layer can be controlled to be below a predetermined value, thereby significantly suppressing delamination of the electro-optical crystal substrate even in a severe high-temperature environment.

[0044] D. High dielectric constant layer

[0045] The first high dielectric constant layer 21 and the second high dielectric constant layer 22 can be of the same composition (substantially the same constituent material and thickness), or can be of different compositions. Preferably, the first high dielectric constant layer 21 and the second high dielectric constant layer 22 can be composed of the same material. When the first high dielectric constant layer 21 and the second high dielectric constant layer 22 of different constituent materials are directly bonded to each other, the materials (substantially atoms) constituting the first high dielectric constant layer and the second high dielectric constant layer may diffuse and move with each other via the amorphous layer formed at the bonding interface. As a result, the portion near the amorphous layer of the first high dielectric constant layer and the second high dielectric constant layer may become a composition different from that of the other portions. As a result, an unexpected increase in conductivity and / or the generation of excessive stress may occur. By constituting the first high dielectric constant layer and the second high dielectric constant layer of the same material, such an undesirable situation can be prevented.

[0046] The high dielectric constant layer may be any suitable material that suppresses excessive reduction in the microwave effective dielectric constant (refractive index) to achieve high-speed and low-voltage driving. Specific examples of materials constituting the high dielectric constant layer include tantalum oxide (Ta2O5), niobium oxide (Nb2O5), titanium oxide (TiO2), aluminum oxide, hafnium oxide, and silicon (e.g., amorphous silicon).

[0047] The thickness of the first high dielectric constant layer, the thickness of the second high dielectric constant layer, and the total thickness of the first high dielectric constant layer and the second high dielectric constant layer are as described in the above section A.

[0048] The argon concentration in the high dielectric constant layer can be, for example, 1.0 atomic % to 10 atomic %, or 1.0 atomic % to 8.0 atomic %, or 1.0 atomic % to 6.0 atomic %, or 1.0 atomic % to 5.0 atomic %, or 2.0 atomic % to 10 atomic %, or 4.0 atomic % to 10 atomic %, or 5.0 atomic % to 10 atomic %. If the argon concentration in the high dielectric constant layer is in such a range, even if the electro-optical crystal substrate is very thin (for example, less than 1 μm), the peeling of the electro-optical crystal substrate in a harsh high temperature environment can be significantly suppressed. As a result, a very thin electro-optical element that can maintain excellent reliability even in a harsh high temperature environment can be realized. As described above, by configuring the supporting substrate or the low dielectric constant layer (if present) to contain silicon oxide as the main component and making the argon concentration in the supporting substrate or the low dielectric constant layer less than 1.0 atomic %, such an argon concentration in the high dielectric constant layer can be achieved.

[0049] E. Amorphous layer

[0050] Amorphous layer 40 is a layer formed at the bonding interface by direct bonding between first high dielectric constant layer 21 and second high dielectric constant layer 22. As its name suggests, amorphous layer 40 has an amorphous structure and is composed of the elements that constitute first high dielectric constant layer 21 and the elements that constitute second high dielectric constant layer 22. Typically, the amorphous layer may further contain atomic species (typically argon or nitrogen) that form a neutral atomic beam for direct bonding. The content of such atomic species in the amorphous layer may be, for example, 1.5 atomic % to 2.5 atomic %.

[0051] The thickness of the amorphous layer may be, for example, 0.1 nm to 100 nm, or 2 nm to 15 nm.

[0052] The amorphous layer 40 is formed by diffusing atoms of the constituent materials of the first high dielectric constant layer 21 and the second high dielectric constant layer 22 in direct bonding of these layers. Therefore, the upper surface (interface with the first high dielectric constant layer 21) and the lower surface (interface with the second high dielectric constant layer 22) of the amorphous layer are not necessarily flat. The arithmetic mean roughness of the upper surface and the lower surface of the amorphous layer can be, for example, 0.1nm to 10nm. In addition, sometimes the upper and lower parts of the amorphous layer have different compositions due to such a formation process. In the case where an amorphous layer like this is formed at the interface between the electro-optical substrate or the supporting substrate and the high dielectric constant layer, as described above, the amorphous layer itself has an adverse effect on the electro-optical crystalline substrate, or sometimes the constituent material of the supporting substrate diffuses through the amorphous layer and has an adverse effect on the high dielectric constant layer. According to the embodiment of the present invention, by directly bonding the first high dielectric constant layer 21 and the second high dielectric constant layer 22 , an amorphous layer can be formed isolated from both the electro-optical crystal substrate and the support substrate, thereby preventing such inconvenience.

[0053] Example

[0054] Hereinafter, the present invention will be specifically described with reference to Examples, but the present invention is not limited to these Examples.

[0055] <Example 1>

[0056] As an electro-optical crystal substrate, a 4-inch diameter X-cut lithium niobate substrate was prepared, and as a supporting substrate, a 4-inch diameter silicon substrate (500 μm thick) was prepared. First, tantalum oxide was sputtered on the electro-optical crystal substrate to form a first high dielectric constant layer with a thickness of 0.03 μm. Next, silicon oxide was sputtered on the supporting substrate to form a low dielectric constant layer with a thickness of 10.0 μm. The obtained low dielectric constant layer was slightly CMP-polished to reduce the arithmetic average roughness Ra of the surface of the low dielectric constant layer. Next, the surface of the low dielectric constant layer was cleaned, and tantalum oxide was sputtered on the cleaned surface to form a second high dielectric constant layer with a thickness of 0.03 μm. Here, an atomic force microscope was used to measure the □10 μm arithmetic average roughness of the interface between the second high dielectric constant layer and the low dielectric constant layer, and the arithmetic average roughness of the interface between the low dielectric constant layer and the supporting substrate. The results showed that both were 0.2 nm per □10 μm. Next, after cleaning the surfaces of the first and second high dielectric constant layers, the first and second high dielectric constant layers are directly bonded to each other, thereby integrating the electro-optical crystal substrate and the supporting substrate. Direct bonding is performed as follows. The electro-optical crystal substrate and the supporting substrate are placed in a vacuum chamber. -6 Pa and below 10 -5 Pa in a vacuum, irradiate the bonding surface of the electro-optical crystallization substrate and the supporting substrate (the surface of the first high dielectric constant layer and the second high dielectric constant layer) with a high-speed Ar neutral atom beam (acceleration voltage 1kV, Ar flow rate 60sccm) for 70 seconds. After irradiation, place the electro-optical crystallization substrate and the supporting substrate for 10 minutes to cool, then make the bonding surface of the electro-optical crystallization substrate and the supporting substrate (the beam irradiation surface of the first high dielectric constant layer and the second high dielectric constant layer) contact, pressurize with 4.90kN for 2 minutes, and bond the electro-optical crystallization substrate and the supporting substrate. After bonding, grind until the thickness of the electro-optical crystallization substrate is 0.5μm, and obtain Figure 2 In the obtained composite substrate for an electro-optical element, no defects such as peeling were observed at the bonding interface.

[0057] Using the composite substrate obtained above, an optical waveguide (ridge waveguide) and electrodes were formed to create an optical modulator. With the inter-electrode gap set to 3 μm and the electrode length L set to 1 cm, the product of the half-wavelength voltage Vπ and the electrode length L (Vπ·L) was 1.0 Vcm. The propagation loss of the optical waveguide was 0.5 dB. Furthermore, the modulation band of the optical modulator was measured using a lightwave component analyzer and found to be 50 GHz. No ripple was detected in the modulation characteristics below this frequency.

[0058] <Example 2>

[0059] A quartz glass substrate (thickness 500 μm) was used as the supporting substrate, and a low dielectric constant layer was not formed on the supporting substrate. Figure 2 A similar composite substrate for an electro-optical element (however, there was no low dielectric constant layer between the second high dielectric constant layer and the supporting substrate) was obtained, and no defects such as peeling were observed at the bonding interface.

[0060] An optical modulator was fabricated using the resulting composite substrate. The product of the half-wavelength voltage (Vπ) and the electrode length (L), Vπ·L, was 1.0 Vcm. The propagation loss of the optical waveguide was 0.5 dB. Furthermore, the modulation band was 50 GHz, and no ripple was detected in the modulation characteristics below this frequency.

[0061] <Comparative Example 1>

[0062] The same method as in Example 1 was used to obtain the electro-optical crystal substrate 10 except that the first high dielectric constant layer was not formed on the electro-optical crystal substrate (ie, the electro-optical crystal substrate 10 and the second high dielectric constant layer 22 were directly bonded). Figure 3 In the obtained composite substrate for an electro-optical element, no defects such as peeling were observed at the bonding interface.

[0063] An optical modulator was then fabricated using the resulting composite substrate. The product of the half-wavelength voltage (Vπ) and the electrode length (L), Vπ·L, was 1.2 Vcm. The propagation loss of the optical waveguide was 1.0 dB. Furthermore, the modulation band was 50 GHz, and no ripple was detected in the modulation characteristics below this frequency.

[0064] The increase in half-wavelength voltage is believed to be due to the growth of the amorphous layer 40 formed at the bonding interface into the electro-optical crystal substrate 10, which reduces the electro-optical effect of the lithium niobate crystal in this region. The refractive index change caused by the applied voltage in the distributed optical field in this region decreases, reducing the phase shift of light propagating through the optical waveguide. Consequently, it is speculated that the half-wavelength voltage of the optical modulator increases.

[0065] The increased light propagation loss is believed to be due to absorption and / or scattering by the amorphous layer 40 formed at the bonding interface. It is speculated that the amorphous layer, a mixed layer of lithium niobate and tantalum oxide, absorbs light due to compositional variations within the amorphous layer and / or internal stress during its formation, causing light to be scattered at the interface between the amorphous layer and the electro-optical crystal substrate.

[0066] Comparative Example 2

[0067] The same method as in Example 1 was used to obtain a quartz crystal. The quartz crystal was obtained except that the second high dielectric constant layer was not formed on the supporting substrate (ie, the first high dielectric constant layer 21 and the low dielectric constant layer 50 were directly bonded to each other). Figure 4 Composite substrates for electro-optical devices. The resulting composite substrates for electro-optical devices exhibited delamination defects at the bonding interface. Delamination occurred in approximately 30% of the total composite substrate area. Furthermore, attempts to fabricate another composite substrate (wafer) in the same manner as described above also resulted in a complete failure to bond.

[0068] Furthermore, an optical modulator was fabricated using the composite substrate described above with approximately 30% delamination. The product of the half-wavelength voltage (Vπ) and the electrode length (L), Vπ·L, was 1.0 Vcm. The propagation loss of the optical waveguide was 0.5 dB. Furthermore, the modulation band was 40 GHz, and no ripple was detected in the modulation characteristics below this frequency.

[0069] The reason for the reduction in the modulation bandwidth is believed to be a decrease in the dielectric constant of the first high-k dielectric layer 21. It is speculated that during direct bonding, an amorphous layer 40 forms at the interface between the first high-k dielectric layer 21 and the low-k dielectric layer 50. This decreases the dielectric constant of a portion of the first high-k dielectric layer due to diffusion of silicon oxide, making it impossible to suppress the decrease in the effective dielectric constant and effective refractive index of the electrical signal. This leads to a deviation from the velocity matching condition, resulting in a decrease in the modulation bandwidth.

[0070] <Examples 3 to 16 and Comparative Examples 3 to 12>

[0071] Composite substrates for electro-optical devices were fabricated with the configuration shown in Table 1 and observed for defects such as delamination at the bonding interface. Furthermore, light modulators were fabricated from the resulting composite substrates and evaluated in the same manner as in Example 1. The results are shown in Table 1.

[0072] Table 1

[0073]

[0074] As shown in Table 1, according to the embodiments of the present invention, by directly bonding two high-dielectric-constant layers (a first high-dielectric-constant layer and a second high-dielectric-constant layer) to integrate the electro-optical crystal substrate and the support substrate, a composite substrate for an electro-optical element with suppressed delamination can be obtained. Furthermore, it can be seen that the composite substrate for an electro-optical element according to the embodiments of the present invention can realize an electro-optical element (e.g., an optical modulator) that has low light propagation loss and is capable of high-speed, low-voltage operation.

[0075] Furthermore, regarding the composite substrate for electro-optical element of Example 7, the bonding interface between the first high dielectric constant layer and the second high dielectric constant layer was observed using a transmission electron microscope (TEM). The TEM image (magnification: 2 million times) is shown in FIG. Figure 5 .Depend on Figure 5It was confirmed that an amorphous layer was formed at the bonding interface between the first high dielectric constant layer and the second high dielectric constant layer. In addition, EDX (energy dispersive X-ray analysis) was used to investigate the composition near the interface between the first high dielectric constant layer and the amorphous layer, the amorphous layer, and the interface between the amorphous layer and the second high dielectric constant layer. The results are shown in Table 2. As can be seen from Table 2, argon, which constitutes a neutral atomic beam for direct bonding, is contained in the amorphous layer and its vicinity. Oxygen detected near the amorphous layer is caused by moisture adsorbed by the fixture of the film forming device or oxidation after film formation. Oxygen can be intentionally doped from the perspective of optical properties, electrical properties, or bonding strength as needed.

[0076] Table 2

[0077]

[0078] <Example 17>

[0079] As an electro-optical crystal substrate, an X-cut lithium niobate substrate with a diameter of 4 inches was prepared, and as a supporting substrate, a glass substrate with a diameter of 4 inches (thickness 500μm) was prepared. The supporting substrate (glass substrate) was formed by a sol-gel method to achieve high purity. The argon ion concentration of the glass substrate was measured by energy dispersive X-ray analysis, and the result was 1 atomic%. A composite substrate was made using the above substrate. Specifically as follows. First, tantalum oxide was sputtered on the electro-optical crystal substrate and the glass substrate, respectively, to form a first high dielectric constant layer and a second high dielectric constant layer with a thickness of 0.03μm, respectively. The argon concentration of the formed first high dielectric constant layer and the second high dielectric constant layer was measured by energy dispersive X-ray analysis, and the result was 1 atomic%. Here, an atomic force microscope was used to measure the □10μm arithmetic average roughness of the interface between the first high dielectric constant layer and the electro-optical crystal substrate, and the arithmetic average roughness of the interface between the second high dielectric constant layer and the supporting substrate. The results were both 0.2nm in □10μm. Furthermore, the arithmetic mean roughness of the surfaces of the first high dielectric constant layer and the second high dielectric constant layer was measured, and the results were both 0.2nm in terms of □10μm. Next, after cleaning the surfaces of the first high dielectric constant layer and the second high dielectric constant layer, the first high dielectric constant layer and the second high dielectric constant layer were directly bonded to each other, thereby integrating the electro-optical crystal substrate and the supporting substrate. Direct bonding was performed as follows. The electro-optical crystal substrate and the supporting substrate were placed in a vacuum chamber and - 6 Pa and less than 10 -5Pa in a vacuum, irradiate the bonding surface of the electro-optical crystallization substrate and the supporting substrate (the surface of the first high dielectric constant layer and the second high dielectric constant layer) with a high-speed Ar neutral atom beam (acceleration voltage 1kV, Ar flow rate 60sccm) for 70 seconds. After irradiation, the electro-optical crystallization substrate and the supporting substrate are left to cool for 10 minutes, and then the bonding surface of the electro-optical crystallization substrate and the supporting substrate (the beam irradiation surface of the first high dielectric constant layer and the second high dielectric constant layer) are brought into contact, and pressurized at 4.90kN for 2 minutes to bond the electro-optical crystallization substrate and the supporting substrate. After bonding, the electro-optical crystallization substrate is ground to a thickness of 0.6μm to obtain a structure having an electro-optical crystallization substrate / first high dielectric constant layer / amorphous layer / second high dielectric constant layer / support substrate (i.e., from Figure 2 The composite substrate for an electro-optical element was obtained by removing the low dielectric constant layer from the structure of FIG. No defects such as peeling were observed at the bonding interface of the obtained composite substrate for an electro-optical element.

[0080] Using the composite substrate obtained above, an optical waveguide (ridge waveguide) and electrodes were formed to create an optical modulator. With the gap between the electrodes set to 3 μm and the electrode length L set to 1 cm, the product of the half-wavelength voltage Vπ and the electrode length L (Vπ·L) was 1.0 Vcm. The propagation loss of the optical waveguide was 0.5 dB. Furthermore, the modulation band of the optical modulator was measured using a lightwave component analyzer and found to be 50 GHz. No ripple was detected in the modulation characteristics below this frequency.

[0081] Furthermore, the optical modulator was subjected to a reliability test (a high-temperature test at 80°C for 500 hours) and evaluated in the same manner as above. The results showed no change in the measured values ​​of the half-wavelength voltage Vπ, the propagation loss of the optical waveguide, or the modulation bandwidth. No delamination of the electro-optical crystal substrate was observed during the visual inspection. Thus, the optical modulator of this example demonstrated exceptional reliability in a harsh, high-temperature environment. These results are summarized in Table 3.

[0082] <Comparative Example 17a>

[0083] As a supporting substrate, an argon-containing glass substrate was used. The argon ion concentration of the glass substrate was measured by energy dispersive X-ray analysis, and the result was 2 atomic %. Using this supporting substrate, a composite substrate for an electro-optical element was obtained in the same manner as in Example 17 except for this. Here, the argon concentration of the first high dielectric constant layer and the second high dielectric constant layer was measured by energy dispersive X-ray analysis, and the result was 11 atomic %. In the composite substrate for an electro-optical element obtained, no defects such as peeling were observed at the bonding interface. Furthermore, an optical modulator was made from the obtained composite substrate in the same manner as in Example 17. The product Vπ·L of the half-wavelength voltage Vπ and the electrode length L of the obtained optical modulator was 1.0 Vcm, and the propagation loss of the optical waveguide was 0.5 dB. In addition, the modulation band of the optical modulator was 50 GHz, and no ripple was detected for the modulation characteristics below this frequency. Furthermore, the optical modulator was subjected to the same reliability test as in Example 17. As a result, the electro-optical crystal substrate peeled off, and the characteristics could not be evaluated. These results are summarized in Table 3. In addition, for easy comparison, the numbers of the comparative examples and reference examples shown in Table 3 correspond to the numbers of the examples.

[0084] <Examples 18 to 21, Comparative Examples 17b to 21b, Reference Examples 17a to 21b>

[0085] Composite substrates for electro-optical devices were fabricated using the configurations shown in Table 3 and observed for defects such as delamination at the bonding interface. Furthermore, optical modulators were fabricated from the resulting composite substrates and evaluated in the same manner as in Example 1. Furthermore, the optical modulators were subjected to the same reliability tests as in Example 17. The results are shown in Table 3.

[0086] Table 3

[0087]

[0088] Table 3 shows that, according to the embodiments of the present invention, by forming the support substrate primarily with silicon oxide and setting the argon concentration in the support substrate to 1.0 atomic % or less, a very thin electro-optical element capable of maintaining excellent reliability even in harsh, high-temperature environments can be achieved. Furthermore, the reference examples show that this effect is unique to the case where the electro-optical crystal substrate is thinned to less than 1 μm.

[0089] <Example 22>

[0090] As an electro-optical crystal substrate, a 4-inch diameter X-cut lithium niobate substrate was prepared, and as a supporting substrate, a 4-inch diameter silicon substrate (500 μm thick) was prepared. First, tantalum oxide was sputtered on the electro-optical crystal substrate to form a first high dielectric constant layer with a thickness of 0.03 μm. Next, silicon oxide was sputtered on the supporting substrate to form a low dielectric constant layer with a thickness of 12.0 μm. The obtained low dielectric constant layer was slightly CMP-polished to reduce the arithmetic mean roughness Ra of the surface of the low dielectric constant layer. Next, the surface of the low dielectric constant layer was cleaned, and tantalum oxide was sputtered on the cleaned surface to form a second high dielectric constant layer with a thickness of 0.03 μm. Here, the argon ion concentration of the low dielectric constant layer was measured by energy dispersive X-ray analysis, and the result was 1 atomic %. It should be noted that the argon concentration in the low dielectric constant layer was controlled by changing the argon partial pressure during sputtering. In addition, the argon concentration of the first high dielectric constant layer and the second high dielectric constant layer was measured respectively by energy dispersive X-ray analysis, and the result was 1 atomic %. Furthermore, an atomic force microscope was used to measure the arithmetic average roughness of the interface between the first high dielectric constant layer and the electro-optical crystal substrate at 10 μm, and the arithmetic average roughness of the interface between the second high dielectric constant layer and the supporting substrate. The results showed that both were 0.2 nm in terms of 10 μm. Furthermore, the arithmetic average roughness of the surfaces of the first high dielectric constant layer and the second high dielectric constant layer was measured. The results showed that both were 0.2 nm in terms of 10 μm. Next, after cleaning the surfaces of the first high dielectric constant layer and the second high dielectric constant layer, the first high dielectric constant layer and the second high dielectric constant layer were directly bonded to each other, thereby integrating the electro-optical crystal substrate and the supporting substrate. Direct bonding is performed as follows. The electro-optical crystal substrate and the supporting substrate are placed in a vacuum chamber at 10 -6 Pa and less than 10 -5 Pa in a vacuum, irradiate the bonding surface of the electro-optical crystallization substrate and the supporting substrate (the surface of the first high dielectric constant layer and the second high dielectric constant layer) with a high-speed Ar neutral atom beam (acceleration voltage 1kV, Ar flow rate 60sccm) for 70 seconds. After irradiation, place the electro-optical crystallization substrate and the supporting substrate for 10 minutes to cool, then make the bonding surface of the electro-optical crystallization substrate and the supporting substrate (the beam irradiation surface of the first high dielectric constant layer and the second high dielectric constant layer) contact, pressurize with 4.90kN for 2 minutes to bond the electro-optical crystallization substrate and the supporting substrate. After bonding, grind until the thickness of the electro-optical crystallization substrate is 0.6μm to obtain a Figure 2 The composite substrate for an electro-optical element having the structure shown in FIG. In the obtained composite substrate for an electro-optical element, no defects such as peeling were observed at the bonding interface.

[0091] Using the composite substrate obtained above, an optical waveguide (ridge waveguide) and electrodes were formed to create an optical modulator. With the gap between the electrodes set to 3 μm and the electrode length L set to 1 cm, the product of the half-wavelength voltage Vπ and the electrode length L (Vπ·L) was 1.0 Vcm. The propagation loss of the optical waveguide was 0.5 dB. Furthermore, the modulation band of the optical modulator was measured using a lightwave component analyzer and found to be 50 GHz. No ripple was detected in the modulation characteristics below this frequency.

[0092] Furthermore, the optical modulator was subjected to a reliability test (80°C, 500 hours of high-temperature maintenance) and the same evaluation as above. The results showed no change in the measured values ​​of the half-wavelength voltage Vπ, the optical waveguide propagation loss, and the modulation bandwidth. No delamination of the electro-optical crystal substrate was observed during the visual inspection. Thus, the optical modulator of this example demonstrated extremely excellent reliability in a harsh high-temperature environment. These results are summarized in Table 4.

[0093] <Comparative Example 22>

[0094] As a low dielectric constant layer, a silicon oxide layer (thickness 12.0 μm) with an argon concentration of 2 atomic % was formed, and a composite substrate for an electro-optical element was obtained in the same manner as in Example 22, except that this was formed. The argon concentration in the low dielectric constant layer was controlled by changing the argon partial pressure during sputtering. Here, the argon concentration of the first high dielectric constant layer and the second high dielectric constant layer was measured by energy dispersive X-ray analysis, and the result was 11 atomic %. In the composite substrate for an electro-optical element obtained, no defects such as peeling were observed at the bonding interface. Furthermore, an optical modulator was prepared from the obtained composite substrate in the same manner as in Example 22. The product Vπ·L of the half-wavelength voltage Vπ and the electrode length L of the obtained optical modulator was 1.0 Vcm, and the propagation loss of the optical waveguide was 0.5 dB. In addition, the modulation band of the optical modulator was 50 GHz, and no ripple was detected for the modulation characteristics below this frequency. Furthermore, the optical modulator was subjected to the same reliability test as in Example 22. As a result, the electro-optical crystal substrate peeled off, and the characteristics could not be evaluated. These results are summarized in Table 4. In addition, for easy comparison, the numbers of the comparative examples and reference examples shown in Table 4 correspond to the numbers of the examples.

[0095] <Examples 23 to 26, Comparative Examples 23 to 26, Reference Examples 22 to 26>

[0096] Composite substrates for electro-optical devices were fabricated using the configurations shown in Table 4 and observed for defects such as delamination at the bonding interface. Furthermore, optical modulators were fabricated from the resulting composite substrates and evaluated in the same manner as in Example 1. Furthermore, the optical modulators were subjected to the same reliability tests as in Example 22. The results are shown in Table 4.

[0097] Table 4

[0098]

[0099] Table 4 shows that, according to the embodiments of the present invention, by forming the low-k dielectric layer primarily from silicon oxide and setting the argon concentration in the low-k dielectric layer to 1.0 atomic % or less, a very thin electro-optical element can be realized that maintains excellent reliability even in harsh, high-temperature environments. Furthermore, the reference examples show that this effect is unique to the case of thinning the electro-optical crystal substrate to less than 1 μm.

[0100] <Example 27>

[0101] As an electro-optical crystal substrate, a 4-inch diameter X-cut lithium niobate substrate was prepared, and as a supporting substrate, a 4-inch diameter silicon substrate (500 μm thick) was prepared. First, tantalum oxide was sputtered on the electro-optical crystal substrate to form a first high dielectric constant layer with a thickness of 0.01 μm. Next, silicon oxide was sputtered on the supporting substrate to form a low dielectric constant layer with a thickness of 12.0 μm. The obtained low dielectric constant layer was slightly CMP-polished to reduce the arithmetic average roughness Ra of the surface of the low dielectric constant layer. Next, the surface of the low dielectric constant layer was cleaned, and tantalum oxide was sputtered on the cleaned surface to form a second high dielectric constant layer with a thickness of 0.03 μm. Here, an atomic force microscope was used to measure the □10μm arithmetic average roughness of the interface between the second high dielectric constant layer and the low dielectric constant layer, and the arithmetic average roughness of the interface between the low dielectric constant layer and the supporting substrate. The results showed that both were 0.2 nm in terms of □10μm. Next, after cleaning the surfaces of the first and second high dielectric constant layers, the first and second high dielectric constant layers are directly bonded to each other, thereby integrating the electro-optical crystal substrate and the supporting substrate. Direct bonding is performed as follows. The electro-optical crystal substrate and the supporting substrate are placed in a vacuum chamber. -6 Pa and less than 10 -5 Pa in a vacuum, irradiate the bonding surface of the electro-optical crystallization substrate and the supporting substrate (the surface of the first high dielectric constant layer and the second high dielectric constant layer) with a high-speed Ar neutral atom beam (acceleration voltage 1kV, Ar flow rate 60sccm) for 70 seconds. After irradiation, place the electro-optical crystallization substrate and the supporting substrate for 10 minutes to cool, then make the bonding surface of the electro-optical crystallization substrate and the supporting substrate (the beam irradiation surface of the first high dielectric constant layer and the second high dielectric constant layer) contact, pressurize with 4.90kN for 2 minutes to bond the electro-optical crystallization substrate and the supporting substrate. After bonding, grind until the thickness of the electro-optical crystallization substrate is 0.6μm to obtain a Figure 2 In the obtained composite substrate for an electro-optical element, no defects such as peeling were observed at the bonding interface.

[0102] Using the composite substrate obtained above, an optical waveguide (ridge waveguide) and electrodes were formed to create an optical modulator. With the gap between the electrodes set to 3 μm and the electrode length L set to 1 cm, the product of the half-wavelength voltage Vπ and the electrode length L (Vπ·L) was 1.0 Vcm. The propagation loss of the optical waveguide was 0.5 dB. Furthermore, the modulation band of the optical modulator was measured using a lightwave component analyzer and found to be 50 GHz. No ripple was detected in the modulation characteristics below this frequency.

[0103] <Example 28>

[0104] A composite substrate for an electro-optical element was produced in the same manner as in Example 27, except that the thickness of the first high-dielectric-constant layer was changed to 0.05 μm. No defects such as delamination were observed at the bonding interface of the resulting composite substrate for an electro-optical element. Furthermore, an optical modulator was produced from the resulting composite substrate and evaluated in the same manner as in Example 1. The results are shown in Table 5.

[0105] <Example 29>

[0106] A composite substrate for an electro-optical element was produced in the same manner as in Example 27, except that the thickness of the first high-dielectric-constant layer was changed to 0.07 μm. No defects such as delamination were observed at the bonding interface of the resulting composite substrate for an electro-optical element. Furthermore, an optical modulator was produced from the resulting composite substrate and evaluated in the same manner as in Example 1. The results are shown in Table 5.

[0107] <Comparative Examples 27 to 29 and Reference Examples 27 to 28>

[0108] A composite substrate for an electro-optical element was produced using the configuration shown in Table 5. No defects such as delamination were observed at the bonding interface of the resulting composite substrate for an electro-optical element. Furthermore, an optical modulator was produced using the resulting composite substrate and evaluated in the same manner as in Example 1. The results are shown in Table 5.

[0109] <Example 30>

[0110] A composite substrate for an electro-optical element was produced in the same manner as in Example 27, except that a glass substrate (500 μm thick) was used as the supporting substrate and that the second high dielectric constant layer was formed directly on the supporting substrate instead of forming the low dielectric constant layer. No defects such as delamination were observed at the bonding interface of the resulting composite substrate for an electro-optical element. Furthermore, an optical modulator was produced using the resulting composite substrate and evaluated in the same manner as in Example 1. The results are shown in Table 5.

[0111] <Examples 31 and 32, Comparative Examples 30 and 32, and Reference Examples 29 and 30>

[0112] A composite substrate for an electro-optical element was produced using the configuration shown in Table 5. No defects such as delamination were observed at the bonding interface of the resulting composite substrate for an electro-optical element. Furthermore, an optical modulator was produced using the resulting composite substrate and evaluated in the same manner as in Example 1. The results are shown in Table 5.

[0113] Table 5

[0114]

[0115] As can be seen from Table 5: According to an embodiment of the present invention, even if the electro-optical crystal substrate is thinned to less than 1 μm (for example, 0.6 μm), the excellent effects of integrating the electro-optical crystal substrate and the supporting substrate by directly bonding the two high dielectric constant layers can be maintained (representatively, significant suppression of peeling, suppression of light propagation loss when making electro-optical elements, and realization of high-speed and low-voltage driving).

[0116] Furthermore, regarding the structures corresponding to Examples 27 to 32, Comparative Examples 27 to 32, and Reference Examples 27 to 30, it was confirmed that the same results were obtained even if the materials forming the first high dielectric constant layer and the second high dielectric constant layer were changed to Al2O3, Nb2O5, or amorphous silicon, respectively.

[0117] Industrial applicability

[0118] The composite substrate according to the embodiment of the present invention can be preferably used in an electro-optical element (eg, an optical modulator).

[0119] Explanation of symbols

[0120] 10 Electro-optical crystal substrate

[0121] 21. First high dielectric constant layer

[0122] 22 Second high dielectric constant layer

[0123] 30 Support substrate

[0124] 40 Amorphous layer

[0125] 50 Low dielectric constant layer

[0126] 100 Composite substrate for electro-optical components

Claims

1. A composite substrate for an electro-optical element, wherein: The invention comprises, in the following order: an electro-optical crystal substrate having an electro-optical effect, a first high dielectric constant layer, a second high dielectric constant layer, and a supporting substrate. The first high dielectric constant layer and the second high dielectric constant layer are directly bonded, and an amorphous layer is formed at the bonding interface between the first high dielectric constant layer and the second high dielectric constant layer. The first high dielectric constant layer is directly formed on the electro-optical crystal substrate, the low dielectric constant layer is directly formed on the support substrate, and the second high dielectric constant layer is directly formed on the low dielectric constant layer. The low dielectric constant layer contains silicon oxide as a main component and further contains argon, and the argon concentration in the low dielectric constant layer is 1.0 atomic % or less. The dielectric constants of the first high dielectric constant layer and the second high dielectric constant layer are greater than the dielectric constant of the low dielectric constant layer.

2. The composite substrate for an electro-optical element according to claim 1, wherein The argon concentrations in the first high dielectric constant layer and the second high dielectric constant layer are respectively 1.0 atomic % to 10 atomic %.

3. A composite substrate for an electro-optical element, wherein: The invention comprises, in the following order: an electro-optical crystal substrate having an electro-optical effect, a first high dielectric constant layer, a second high dielectric constant layer, and a supporting substrate. The first high dielectric constant layer and the second high dielectric constant layer are directly bonded, and an amorphous layer is formed at the bonding interface between the first high dielectric constant layer and the second high dielectric constant layer. The first high dielectric constant layer is directly formed on the electro-optical crystal substrate, and the second high dielectric constant layer is directly formed on the supporting substrate. The supporting substrate contains silicon oxide as a main component, The thickness of the electro-optical crystal substrate is greater than or equal to 0.1 μm and less than 1.0 μm. The thickness of the first high dielectric constant layer is greater than 0.01 μm. The total thickness of the first high dielectric constant layer and the second high dielectric constant layer is 0.10 μm or less. The dielectric constants of the first high dielectric constant layer and the second high dielectric constant layer are greater than the dielectric constant of the support substrate.

4. The composite substrate for an electro-optical element according to claim 3, wherein: The composite substrate for an electro-optical element further includes a low dielectric constant layer formed directly on the support substrate, and the second high dielectric constant layer is formed directly on the low dielectric constant layer.

5. The composite substrate for an electro-optical element according to claim 1, wherein The support substrate is composed of one selected from the group consisting of silicon, glass, sialon, mullite, aluminum nitride, silicon nitride, magnesium oxide, sapphire, quartz, crystal, gallium nitride, silicon carbide, and gallium oxide.

6. The composite substrate for an electro-optical element according to any one of claims 1 to 5, wherein: The thickness of the electro-optical crystal substrate is 0.1 μm to 0.8 μm.

7. The composite substrate for an electro-optical element according to claim 6, wherein: The thickness of the electro-optical crystal substrate is 0.2 μm to 0.6 μm.

8. The composite substrate for an electro-optical element according to any one of claims 1 to 5, wherein: The electro-optical crystal substrate is composed of one selected from the group consisting of lithium niobate, lithium tantalate, potassium titanyl phosphate, potassium lithium niobate, potassium niobate, potassium tantalate niobate, and a solid solution of lithium niobate and lithium tantalate.

9. The composite substrate for an electro-optical element according to any one of claims 1 to 5, wherein: The thickness of the first high dielectric constant layer is 0.01 μm to 0.08 μm, and the thickness of the second high dielectric constant layer is 0.001 μm to 0.04 μm.

10. The composite substrate for an electro-optical element according to any one of claims 1 to 5, wherein The first high dielectric constant layer and the second high dielectric constant layer are each made of one selected from the group consisting of tantalum oxide, niobium oxide, titanium oxide, aluminum oxide, hafnium oxide, and silicon.

11. The composite substrate for an electro-optical element according to claim 1 or 4, wherein: The low dielectric constant layer has a thickness exceeding 10 μm and not greater than 20 μm.

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