Dielectric thin film substrate and method for manufacturing the same, optical waveguide element and optical modulation element

Abstract

The present invention provides a dielectric thin film substrate having a dielectric thin film made of lithium niobate, which can form an optical waveguide for an optical modulation element with low optical loss, and a method for manufacturing the same. [Solution] The dielectric thin film-attached substrate 1 comprises a single crystal substrate 2 oriented along the c axis in the in-plane direction and a dielectric thin film 3 formed in contact with the single crystal substrate 2, wherein the dielectric thin film 3 is a lithium niobate film whose c axis is aligned in one direction of the in-plane direction. Preferably, the angle between the c-axis direction of the single crystal substrate 2 and the c-axis direction of the lithium niobate film is 0.4° to 5°.

Description

Technical Field

[0001] The present invention relates to a substrate with a dielectric thin film, a method for manufacturing the substrate with a dielectric thin film, an optical waveguide element, and an optical modulation element.

Background Art

[0002] Conventionally, as an optical modulation element, there is one using a lithium niobate film epitaxially grown on a substrate. For example, Patent Document 1 describes an optical modulation element using a substrate with a dielectric thin film. Patent Document 1 describes a substrate with a dielectric thin film including a single crystal substrate and a dielectric thin film made of c-axis oriented lithium niobate epitaxially formed on the main surface of the single crystal substrate.

[0003] Also, Patent Document 2 describes a laminated structure including a single crystal substrate, a dielectric layer made of lithium niobate, and a buffer layer provided between the single crystal substrate and the dielectric layer. Patent Document 2 describes that the c-axis of the crystal constituting the dielectric layer of the laminated structure is substantially parallel to the main surface of the single crystal substrate, and the c-axis of the crystal constituting the buffer layer is substantially parallel to the main surface of the single crystal substrate.

Prior Art Documents

Patent Documents

[0004]

Patent Document 1

Patent Document 2

Summary of the Invention

Problems to be Solved by the Invention

[0005] Light propagating through the optical waveguide of an optical modulator exists in two modes: TE (Transverse Electric) mode and TM (Transverse Magnetic) mode. In TE mode, the polarization direction of the light is in the in-plane direction of the substrate of the optical modulator. In TM mode, the polarization direction of the light is perpendicular to the in-plane direction of the substrate of the optical modulator. The optical characteristics, such as the effective refractive index and modulation efficiency of the light incident on the optical waveguide of the optical modulator, differ between TE mode and TM mode.

[0006] In an optical modulation element having an optical waveguide made of a lithium niobate film, the electro-optic coefficient is largest when the polarization direction of the light incident on the optical waveguide coincides with the c-axis of the lithium niobate film. Therefore, for example, if the c-axis of the optical waveguide made of a lithium niobate film in an optical modulation element is parallel to the thickness direction of the substrate, when TE mode light emitted from a laser light source is incident on it, the polarization direction of the light and the direction of the c-axis of the lithium niobate film are different, so the voltage required for modulation becomes high.

[0007] One way to solve this problem is to convert the TE mode light emitted from the laser light source into TM mode light using an optical converter, and then inject it into the optical waveguide made of lithium niobate film in the optical modulation element. By using this method, if the c-axis of the optical waveguide made of lithium niobate film is parallel to the thickness direction of the substrate, the polarization direction of the light incident on the optical waveguide can be made to match the c-axis of the lithium niobate film. However, when using such a method, it is impossible to avoid the optical loss that occurs when converting TE mode light to TM mode light.

[0008] In contrast, when using a lithium niobate film with its c-axis parallel to the in-plane direction of the substrate as the optical waveguide for an optical modulation element, there is no need to convert the TE mode light emitted from the laser light source into TM mode light before it enters the optical waveguide. Therefore, unlike when using a lithium niobate film with its c-axis parallel to the thickness direction of the substrate, there is no optical loss associated with converting TE mode light to TM mode light. However, when a buffer layer is provided between the single crystal substrate and the dielectric layer, as in the laminated structure described in Patent Document 2, problems such as leakage of propagating light occur when the dielectric layer is used as an optical waveguide for an optical modulation element.

[0009] The present invention has been made in view of the above problems, and aims to provide a dielectric thin film substrate and a method for manufacturing the same, which have a dielectric thin film made of lithium niobate and can form an optical waveguide for an optical modulation element with a low driving voltage and low optical loss. Furthermore, the present invention aims to provide an optical waveguide element that can be used as an optical waveguide for an optical modulation element, comprising a dielectric thin film substrate having a dielectric thin film made of a lithium niobate film, and having a low driving voltage and low optical loss. Furthermore, the present invention aims to provide an optical modulation element with low driving voltage and low optical loss, which is equipped with a dielectric thin film substrate having a dielectric thin film made of lithium niobate. [Means for solving the problem]

[0010] A dielectric thin film substrate according to one aspect of the present invention comprises a single crystal substrate oriented along the c-axis in the in-plane direction and a dielectric thin film formed in contact with the single crystal substrate, wherein the dielectric thin film consists of a lithium niobate film whose c-axis is aligned in one direction in the in-plane direction. [Effects of the Invention]

[0011] The dielectric thin film substrate of the present invention comprises a single crystal substrate oriented along its c-axis in the in-plane direction and a dielectric thin film formed in contact with the single crystal substrate, wherein the dielectric thin film consists of a lithium niobate film whose c-axis is aligned in one direction in the in-plane direction. Therefore, for example, by forming an optical waveguide into which TE mode light is incident using the dielectric thin film of the dielectric thin film substrate of the present invention, it is possible to form an optical waveguide for an optical modulation element with a low driving voltage and low optical loss, in which the polarization direction of the light incident on the optical waveguide coincides with the c-axis of the lithium niobate film. Therefore, the dielectric thin film substrate of the present invention can be preferably used as a material for optical waveguide elements and optical modulation elements.

[0012] Furthermore, the optical waveguide element of the present invention is equipped with a dielectric thin film substrate of the present invention and has an optical waveguide made of a lithium niobate film whose c-axis is aligned in one direction in the in-plane direction. For this reason, the optical waveguide element of the present invention can be suitably used as an optical waveguide for an optical modulation element with a low driving voltage and low optical loss, for example, when TE mode light is incident on the optical waveguide, the polarization direction of the light incident on the optical waveguide coincides with the c-axis of the lithium niobate film.

[0013] Furthermore, the optical modulation element of the present invention is provided with a dielectric thin film substrate of the present invention, and has an optical waveguide made of a lithium niobate film whose c-axis is aligned in one direction in the in-plane direction, and a first electrode and a second electrode provided on the dielectric thin film made of lithium niobate film and spaced apart in the in-plane direction and facing each other, with the optical waveguide positioned between the first electrode and the second electrode. For this reason, when TE mode light is incident on the optical waveguide of the optical modulation element of the present invention, the polarization direction of the light incident on the optical waveguide coincides with the c-axis of the lithium niobate film, resulting in a low driving voltage and low optical loss.

[0014] Furthermore, in the optical waveguide element and optical modulation element of the present invention, the direction of the c-axis of the lithium niobate film forming the optical waveguide coincides with the polarization direction of light in TE mode. For this reason, in the optical waveguide element and optical modulation element of the present invention, it is not necessary to convert the polarization direction of the light by an optical conversion device before the light is incident on the optical waveguide, and no optical loss occurs due to the conversion of the polarization direction of the light.

[0015] The present invention relates to a method for manufacturing a dielectric thin film-coated substrate, which includes a dielectric thin film formation step in which a lithium niobate film, with its c-axis aligned in one direction in the in-plane direction, is grown on a single crystal substrate with its c-axis oriented in the in-plane direction by vapor phase growth. Therefore, the dielectric thin film-coated substrate of the present invention can be easily manufactured according to the present invention's method for manufacturing a dielectric thin film-coated substrate. [Brief explanation of the drawing]

[0016] [Figure 1] FIG. 1(a) is a schematic cross-sectional view showing a substrate with a dielectric thin film according to an embodiment of the present invention. FIG. 1(b) is a plan view showing a single crystal substrate forming the substrate with a dielectric thin film shown in FIG. 1(a). FIG. 1(c) is a plan view showing the dielectric thin film forming the substrate with a dielectric thin film shown in FIG. 1(a). [Figure 2] FIGS. 2(a) and 2(b) are diagrams for explaining the arrangement of Al atoms in a sapphire single crystal substrate. FIG. 2(a) is a schematic diagram showing the arrangement as viewed from the c-axis direction, and FIG. 2(b) is a schematic diagram showing the arrangement on the a-plane (1100). FIGS. 2(c) and 2(d) are diagrams for explaining the arrangement of Nb atoms and Li atoms in a lithium niobate film. FIG. 2(c) is a schematic diagram showing the arrangement as viewed from the c-axis direction, and FIG. 2(d) is a schematic diagram showing the arrangement on the a-plane (1100). [Figure 3] FIG. 3 is a plan view showing an example of an optical waveguide element 100 provided with the substrate 1 with a dielectric thin film shown in FIG. 1. [Figure 4] FIG. 4 is a cross-sectional view taken along line A-A' of the optical waveguide element 100 shown in FIG. 3. [Figure 5] FIG. 5 is a plan view showing an example of a Mach-Zehnder type optical modulator 200 provided with the substrate 1 with a dielectric thin film shown in FIG. 1. [Figure 6] FIG. 6 is a cross-sectional view taken along line B-B' of the optical modulator 200 shown in FIG. 5. [Figure 7] FIG. 7 is a profile of the X-ray diffraction intensity obtained by measuring (φ scan) the in-plane X-ray diffraction intensity of the lithium niobate film forming the dielectric thin film 3 of the substrate with a dielectric thin film in Example 1. [Figure 8] FIG. 8 is a drawing showing the measurement results of the poles of the X-ray diffraction of the lithium niobate film forming the dielectric thin film 3 of the substrate with a dielectric thin film in Example 1. [Figure 9]FIG. 9 is a profile of X-ray diffraction intensity obtained by measuring (φ scan) the in-plane X-ray diffraction intensity of a sapphire single crystal substrate forming the single crystal substrate 2 of the substrate with a dielectric thin film of Example 1 and a lithium niobate film forming the dielectric thin film 3. [Figure 10] FIG. 10(a) is a profile of X-ray diffraction intensity showing an enlarged part of FIG. 9, and is a profile in the vicinity of the angle of 85° on the φ axis. FIG. 10(b) is a profile of X-ray diffraction intensity showing an enlarged part of FIG. 9, and is a profile in the vicinity of the angle of 265° on the φ axis.

DETAILED DESCRIPTION OF THE INVENTION

[0017] The present inventors have solved the above problems and, when used as a material for an optical modulation element having an optical waveguide made of a lithium niobate film into which TE mode light emitted from a laser light source or the like is incident, have a dielectric thin film with a dielectric thin film capable of forming an optical modulation element with a low driving voltage and low optical loss. Focusing on the c-axis direction of the lithium niobate film, the inventors have intensively studied as follows.

[0018] That is, when the present inventors make TE mode light incident on an optical waveguide made of a lithium niobate film of an optical modulation element, in order to make the polarization direction of the light coincide with the direction of the c-axis of the lithium niobate film, as the optical waveguide, one having a c-axis aligned in one direction in the in-plane direction of the substrate It was thought that a lithium niobate film should be used.

[0019] However, in the conventional technology, it was not possible to grow a lithium niobate film having a c-axis aligned in one direction in the in-plane direction of the substrate on the single crystal substrate. As a method for solving this problem, it is conceivable to use a method of attaching a lithium niobate film having a c-axis aligned in one direction in the in-plane direction of the substrate on the substrate. However, the method of attaching a dielectric thin film on the substrate is not preferable because of poor productivity and high cost.

[0020] Therefore, the inventors have conducted extensive research on a method for growing a lithium niobate film in contact with a substrate, in which the c-axis is aligned in one direction within the plane of the substrate. As a result, we discovered that by growing a lithium niobate film on a single crystal substrate with its c-axis oriented in the in-plane direction using vapor phase growth, it is possible to form a lithium niobate film in which the c-axis is aligned in one direction in the in-plane direction of the substrate, and thus conceived the present invention.

[0021] The present invention includes the following embodiments. [1] A single crystal substrate oriented along the c axis in the in-plane direction, and a dielectric thin film formed in contact with the single crystal substrate, A substrate with a dielectric thin film, wherein the dielectric thin film is a lithium niobate film in which the c-axis is aligned in one direction of the in-plane direction.

[0022] [2] The dielectric thin film substrate according to [1], wherein the angle between the c-axis direction of the single crystal substrate and the c-axis direction of the lithium niobate film is 0.4° to 5°. [3] The dielectric thin film substrate according to [1], wherein the single crystal substrate is a sapphire single crystal substrate.

[0023] [4] The dielectric thin film substrate according to [1], wherein the lithium niobate film is grown by vapor phase growth. [5] The dielectric thin film substrate according to [1], wherein the lithium niobate film is grown by sputtering.

[0024] [6] A substrate with a dielectric thin film as described in any of [1] to [5] is provided, An optical waveguide element having an optical waveguide made of the dielectric thin film. [7] A substrate with a dielectric thin film as described in any of [1] to [5] is provided, An optical waveguide made of the dielectric thin film, The device has a first electrode and a second electrode provided on the dielectric thin film and arranged opposite to each other with spaced apart in the in-plane direction, An optical modulation element in which the optical waveguide is positioned between the first electrode and the second electrode.

[0025] [8] A manufacturing method for producing a dielectric thin film substrate as described in any of [1] to [5], A method for manufacturing a substrate with a dielectric thin film, comprising a dielectric thin film deposition step of growing a lithium niobate film on a single crystal substrate in which the c-axis is oriented in the in-plane direction, by vapor phase growth, wherein the c-axis is aligned in one direction in the in-plane direction. [9] The method for manufacturing a dielectric thin film substrate according to [8], wherein the vapor phase growth method is a sputtering method.

[0026] The dielectric thin film substrate, the method for manufacturing the dielectric thin film substrate, the optical waveguide element, and the optical modulation element of this embodiment will be described in detail below with appropriate reference to the drawings. In the drawings used in the following description, characteristic parts may be enlarged for convenience in order to make the features of the present invention easier to understand. Therefore, the dimensional ratios of each component may differ from those of the actual components. The materials, dimensions, etc. exemplified in the following description are examples only, and the present invention is not limited to them, and can be implemented with appropriate modifications without changing the essence of the invention.

[0027] [Substrate with dielectric thin film] Figure 1(a) is a schematic cross-sectional view showing a dielectric thin film substrate according to one embodiment of the present invention. Figure 1(b) is a plan view showing a single crystal substrate on which the dielectric thin film substrate shown in Figure 1(a) is formed. Figure 1(c) is a plan view showing the dielectric thin film forming the dielectric thin film substrate shown in Figure 1(a). As shown in Figures 1(a) to 1(c), the dielectric thin film substrate 1 of this embodiment has a substantially circular single crystal substrate 2 in plan view and a dielectric thin film 3 formed in contact with the main surface 2a of the single crystal substrate 2.

[0028] (Single crystal substrate 2) The single-crystal substrate 2 on which the dielectric thin film substrate 1 of this embodiment is formed is c-axis oriented in the in-plane direction, and the crystal orientation of the main surface 2a is the a-plane. The arrow shown in Figure 1(b) indicates the c-axis direction of the single-crystal substrate 2. The single-crystal substrate 2 can be any single-crystal substrate c-axis oriented in the in-plane direction, and known single-crystal substrates can be used. Preferably, the single-crystal substrate 2 has a refractive index lower than lithium niobate, and for example, a sapphire single-crystal substrate or a silicon single-crystal substrate can be used.

[0029] In the dielectric thin film substrate 1 of this embodiment, it is particularly preferable to use a sapphire single crystal substrate as the single crystal substrate 2. The sapphire single crystal substrate has a lower refractive index than lithium niobate (LiNbO3). For this reason, for example, when the dielectric thin film 3 of the dielectric thin film substrate 1 is used as the optical waveguide layer of an optical waveguide element and / or optical modulation element, it can function as a cladding layer. Therefore, when the single crystal substrate 2 is a sapphire single crystal substrate, the dielectric thin film 3 can be suitably used as the optical waveguide layer of an optical waveguide element and / or optical modulation element without providing a separate layer between the single crystal substrate 2 and the dielectric thin film 3.

[0030] (Dielectric thin film 3) The dielectric thin film 3 forming the dielectric thin film substrate 1 in this embodiment is made of a lithium niobate film. The lithium niobate film has its c-axis aligned in one direction in the in-plane direction of the single crystal substrate 2. The arrow shown in Figure 1(c) indicates the c-axis direction of the lithium niobate film forming the dielectric thin film 3.

[0031] The composition of the lithium niobate film forming the dielectric thin film 3 is given by the general formula LixNbAyOz (wherein A is an element other than Li, Nb, or O; x is between 0.5 and 1.2; y is between 0 and 0.5; and z is between 1.5 and 4). In the formula, A represents an element other than Li, Nb, and O. Examples of elements represented by A include K, Na, Rb, Cs, Be, Mg, Ca, Sr, Ba, Ti, Zr, Hf, V, Cr, Mo, W, Fe, Co, Ni, Zn, Sc, Ce, and Ta. There may be only one element represented by A, or two or more elements. In the formula, x is between 0.5 and 1.2, preferably between 0.9 and 1.05. In the equation, y ranges from 0 to 0.5. In the formula, z is between 1.5 and 4, preferably between 2.5 and 3.5.

[0032] The lithium niobate film forming the dielectric thin film 3 is mainly composed of lithium niobate (LiNbO3). Because lithium niobate has a large electro-optic constant, it is suitable as a material for the optical waveguide layer of optical waveguide elements and optical modulation elements. It is desirable that the lithium niobate film forming the dielectric thin film 3 be a single phase consisting of the LiNbO3 phase.

[0033] The thickness of the dielectric thin film 3 can be, for example, 0.5 μm to 2 μm. A thickness of 0.5 μm or more is preferable because, when the dielectric thin film 3 of the dielectric thin film-attached substrate 1 is used as an optical waveguide layer for an optical modulation element, it can be applied to a wide range of light from visible light to infrared light. Furthermore, if the thickness of the dielectric thin film 3 is 2 μm or less, when the dielectric thin film 3 of the dielectric thin film-attached substrate 1 is processed into a ridge shape, the occurrence of cracks in the lithium niobate film forming the dielectric thin film 3 can be effectively suppressed.

[0034] The lithium niobate film forming the dielectric thin film 3 is preferably grown on a single crystal substrate 2 by vapor deposition. A lithium niobate film grown by vapor deposition in contact with a single crystal substrate 2 oriented along the c axis in the in-plane direction will have an angle of 0.4° to 5° between the c-axis direction of the single crystal substrate 2 (indicated by the arrow in Figure 1(b)) and the c-axis direction of the lithium niobate film (indicated by the arrow in Figure 1(c)).

[0035] When the angle between the c-axis direction of the lithium niobate film forming the dielectric thin film 3 and the c-axis direction of the single crystal substrate 2 is within the above range, the crystallinity of the c-axis aligned in one direction within the plane is good (the full width at half maximum of the rocking curve is small), which is preferable because it is possible to form an optical waveguide for an optical modulation element with a low driving voltage and low optical loss.

[0036] Here, the reason why the c-axis direction of a lithium niobate film grown by vapor phase growth in contact with a single crystal substrate 2 oriented c-axis in the in-plane direction is misaligned with the c-axis direction of the single crystal substrate 2 will be explained using a diagram, with the example of a lithium niobate film grown by vapor phase growth in contact with a sapphire single crystal substrate oriented c-axis in the in-plane direction.

[0037] Figures 2(a) and 2(b) illustrate the arrangement of Al atoms in a sapphire single crystal substrate. Figure 2(a) is a schematic diagram showing the arrangement as viewed from the c-axis direction, and Figure 2(b) is a schematic diagram showing the arrangement on the a-plane (1100). a1 in Figure 2(a) and c1 in Figure 2(b) are the lattice constants of the sapphire single crystal. As shown in Figures 2(a) and 2(b), in a sapphire single crystal substrate, Al atoms are arranged diagonally on the a-plane with respect to the c-axis direction (up and down direction in Figure 2(b)).

[0038] Furthermore, Figures 2(c) and 2(d) illustrate the arrangement of Nb and Li atoms in a lithium niobate film. Figure 2(c) is a schematic diagram showing the arrangement as viewed from the c-axis direction, and Figure 2(d) is a schematic diagram showing the arrangement on the a-plane (1100). a2 in Figure 2(c) and c2 in Figure 2(d) are the lattice constants of the lithium niobate crystal. As shown in Figures 2(c) and 2(d), in a lithium niobate film, Li and Nb atoms are arranged parallel to each other and obliquely with respect to the c-axis direction (up and down direction in Figure 2(d)) on the a-plane.

[0039] The tilt of the Al atom arrangement with respect to the c-axis in a sapphire single crystal substrate, and the tilt θ of the Nb and Li atom arrangements with respect to the c-axis in a lithium niobate film, can be calculated using the following equation (1).

[0040]

number

[0041] The lattice constant a1 of the sapphire single crystal substrate and the lattice constant a2 of the lithium niobate crystal are different values. Furthermore, the lattice constant c1 of the sapphire single crystal substrate and the lattice constant c2 of the lithium niobate crystal are also different values. Therefore, generally, c1 / a1 and c2 / a2 are different. Consequently, the tilt of the Al atom arrangement with respect to the c-axis in the sapphire single crystal substrate calculated by equation (1) does not match the tilt of the Nb and Li atom arrangements with respect to the c-axis in the lithium niobate film.

[0042] When a lithium niobate film (LiNbO3 film) is epitaxially grown on a sapphire single crystal substrate oriented along the c-axis in the in-plane direction using vapor phase growth, the Li and Nb atoms, which are the raw materials for the lithium niobate film, that reach the surface of the sapphire single crystal substrate are aligned according to the arrangement of Al atoms in the underlying sapphire single crystal substrate. However, as mentioned above, the inclination of the Al atom arrangement with respect to the c-axis of the sapphire single crystal substrate does not coincide with the inclination of the Nb and Li atoms arrangement with respect to the c-axis of the lithium niobate film. Therefore, when the Al atoms of the sapphire single crystal substrate and the Nb and Li atoms of the lithium niobate film are aligned, the c-axis direction of the lithium niobate film and the c-axis direction of the sapphire single crystal substrate are misaligned.

[0043] In a lithium niobate film grown by vapor deposition in contact with a single crystal substrate 2 oriented c-axis in the in-plane direction, the angle between the c-axis direction of the single crystal substrate 2 and the c-axis direction of the lithium niobate film is determined by the difference in the tilt between the arrangement of atoms in the single crystal substrate and the arrangement of Nb and Li atoms in the lithium niobate film, which is due to the difference in lattice constants a and c between the single crystal substrate 2 and the lithium niobate crystal, as well as the conditions of the vapor deposition method used to grow the lithium niobate film.

[0044] In this embodiment, a lithium niobate film grown by vapor deposition in contact with a single crystal substrate 2 oriented c-axis in the in-plane direction has a c-axis tilt of 0.4° to 5° with respect to the c-axis of the single crystal substrate 2. 0.4° occurs when the c2 / a2 of the vapor-deposited lithium niobate film matches the c2 / a2 of bulk lithium niobate. The c2 / a2 of the vapor-deposited lithium niobate film varies depending on the vapor deposition conditions. Generally, c2 / a2 is smaller than the bulk value. Therefore, the tilt |θ1-θ2| between the c-axis of the sapphire substrate and the c-axis of the lithium niobate becomes greater than 0.4°. On the other hand, if c2 / a2 becomes too small, the amount of Li contained in the lithium niobate film deviates significantly from the stoichiometric composition, or the lithium niobate film becomes difficult to epitaxially grow. At this lower limit of c2 / a2, the inclination between the c-axis of the sapphire substrate and the c-axis of lithium niobate becomes 5°.

[0045] The difference between the dielectric thin film-attached substrate 1 of this embodiment, which has a lithium niobate film manufactured in this manner, and a bonded substrate manufactured by, for example, bonding a separately manufactured lithium niobate film to a single crystal substrate 2 oriented along the c axis in the in-plane direction, such that the c axis direction is tilted by 0.4° to 5° with respect to the c axis direction of the single crystal substrate 2, cannot be specified by any wording that identifies the structure or properties of the materials.

[0046] Furthermore, there is no means to analyze and identify a lithium niobate film grown by vapor phase growth in contact with a single crystal substrate 2 oriented c-axis in the in-plane direction, based on measurements. Finding a means to analyze and identify the above-mentioned dielectric thin film-attached substrate 1 based on measurements would require a significant amount of trial and error. Therefore, it is technically impossible or impractical to analyze and identify the structure or properties of the above-mentioned dielectric thin film-attached substrate 1 based on measurements.

[0047] When the lithium niobate film forming the dielectric thin film 3 is grown by vapor deposition, examples of vapor deposition methods include vacuum deposition, sputtering, and chemical vapor deposition (CVD). Among these, it is preferable that the lithium niobate film is grown by sputtering, as this is the simplest method.

[0048] The fact that the lithium niobate film forming the dielectric thin film 3 has its c-axis aligned in one direction in the in-plane direction of the single crystal substrate 2 can be confirmed, for example, by measuring the intensity of in-plane X-ray diffraction (φ scan) using an X-ray diffractometer. Specifically, when the lithium niobate film has its c-axis aligned in one direction in the in-plane direction of the single crystal substrate, two diffraction peaks are observed in the X-ray diffraction intensity profile with respect to the φ axis, and the angular difference between the φ axes in which the two diffraction peaks are observed is 180°.

[0049] The fact that the lithium niobate film forming the dielectric thin film 3 is a single-crystal film in which the c-axis is aligned in one direction in the in-plane direction of the single-crystal substrate 2 can be confirmed, for example, by measuring the X-ray diffraction of the poles using the in-plane diffraction method with an X-ray diffractometer. Specifically, if the lithium niobate film is a single-crystal film, the number and position of the observed diffraction spots will match the reference diffraction data.

[0050] Furthermore, the angle between the c-axis direction of the single-crystal substrate 2 and the c-axis direction of the lithium niobate film can be calculated, for example, using an X-ray diffractometer to measure the in-plane X-ray diffraction intensity (φ scan) for both the single-crystal substrate 2 and the lithium niobate film. Specifically, it can be calculated by the angular difference between the maximum value of the diffraction peak in the X-ray diffraction intensity profile of the single-crystal substrate 2 with respect to the φ axis and the maximum value of the diffraction peak in the X-ray diffraction intensity profile of the lithium niobate film with respect to the φ axis, where the diffraction peaks were observed.

[0051] [Manufacturing method for substrates with dielectric thin films] Next, the manufacturing method of the dielectric thin film-attached substrate 1 according to this embodiment will be described with an example. When manufacturing the dielectric thin film-attached substrate 1 of this embodiment, for example, a dielectric thin film 3 made of a lithium niobate film is formed in contact with the main surface 2a of the single crystal substrate 2 using the method shown below (dielectric thin film deposition process).

[0052] (Dielectric thin film deposition process) In the dielectric thin film deposition process, a lithium niobate film is epitaxially grown on the main surface 2a of a single crystal substrate 2, whose c-axis is oriented in the in-plane direction, by vapor phase growth. Examples of vapor phase growth methods for growing the dielectric thin film 3 include vacuum deposition, sputtering, and chemical vapor deposition (CVD).

[0053] Among these methods, sputtering is preferred as the vapor phase growth method for growing the dielectric thin film 3. When using sputtering as the deposition method for dielectric thin film 3, a target having a composition in the range of Li / (Li+Nb) = 48 mass% to 51 mass% can be used.

[0054] The shape of the target used for depositing the dielectric thin film 3 is not particularly limited. The target is preferably circular, having a planar area at least twice that of the single-crystal substrate 2, in order to obtain a dielectric thin film 3 with a uniform thickness. Furthermore, the deposition of the dielectric thin film 3 is preferably carried out by arranging the circular target coaxially with the circular single-crystal substrate 2, in order to obtain a dielectric thin film 3 with a uniform thickness.

[0055] When using sputtering as the deposition method for the dielectric thin film 3, for example, the distance between the target and the single crystal substrate 2 can be set to 100 mm to 200 mm, a mixed gas of Ar and O2 can be used as the sputtering gas, the O2 ratio in the sputtering gas can be set to 35% to 60%, and the gas pressure can be set to 0.05 Pa to 0.15 Pa. After performing pre-sputtering for 100 to 300 seconds, the temperature of the single crystal substrate 2 can be set to 450°C to 700°C, and a power of 1500 W to 2000 W can be applied to deposit the film until the desired thickness is achieved.

[0056] This results in a dielectric thin film 3 consisting of a lithium niobate film whose c-axis is aligned in one direction in the in-plane direction of the single crystal substrate 2, and in which the angle between the c-axis direction of the single crystal substrate 2 and the c-axis direction of the lithium niobate film is 0.4° to 5°. It is preferable to deposit the dielectric thin film 3 in a so-called single step without changing the deposition conditions midway through the process. Through the above steps, the dielectric thin film-coated substrate 1 of this embodiment is obtained.

[0057] The dielectric thin film substrate 1 of this embodiment has a single crystal substrate 2 oriented along the c-axis in the in-plane direction and a dielectric thin film 3 formed on the single crystal substrate 2, wherein the dielectric thin film 3 consists of a lithium niobate film whose c-axis is aligned in one direction in the in-plane direction. Therefore, an optical waveguide element having an optical waveguide made of the dielectric thin film 3 of the dielectric thin film substrate 1 of this embodiment is a so-called x-cut optical waveguide element. For this reason, for example, by using the dielectric thin film substrate 1 of this embodiment to form an optical waveguide into which TE mode light is incident, it is possible to form an optical waveguide for an optical modulation element with a low driving voltage and low optical loss, in which the polarization direction of the light incident on the optical waveguide coincides with the c-axis of the lithium niobate film.

[0058] [Optical waveguide element] Figure 3 is a plan view showing an example of an optical waveguide element 100 equipped with the dielectric thin film substrate 1 shown in Figure 1. Figure 4 is a cross-sectional view of the optical waveguide element 100 shown in Figure 3 along line AA'. In the optical waveguide element 100 shown in Figures 3 and 4, the components of the dielectric thin film substrate 1 shown in Figure 1 are given the same reference numerals, and their descriptions are omitted.

[0059] The optical waveguide element 100 shown in Figures 3 and 4 has an optical waveguide consisting of a ridge portion 4 formed by processing the dielectric thin film 3 on the dielectric thin film substrate 1 shown in Figure 1 into a ridge shape (convex shape). The ridge portion 4 of the optical waveguide element 100 is the part through which TE mode light propagates. The optical waveguide element 100 shown in Figures 3 and 4 can be manufactured by processing the dielectric thin film 3 on the dielectric thin film substrate 1 shown in Figure 1 into a ridge shape (convex shape). Known methods such as etching can be used to process the dielectric thin film 3 into a ridge shape.

[0060] The optical waveguide element 100 shown in Figures 3 and 4 has an optical waveguide made of a dielectric thin film 3 on a dielectric thin film substrate 1 shown in Figure 1. Therefore, the optical waveguide of the optical waveguide element 100 of this embodiment is made of a lithium niobate film whose c-axis is aligned in one direction in the in-plane direction of the single crystal substrate 2, and is an x-cut optical waveguide element in which light propagates in TE mode. For this reason, the optical waveguide element 100 of this embodiment can be suitably used as an optical waveguide for an optical modulation element with low optical loss, for example, when TE mode light is incident on the optical waveguide, the polarization direction of the light incident on the optical waveguide coincides with the c-axis of the lithium niobate film.

[0061] Furthermore, in the optical waveguide element 100 of this embodiment, the direction of the c-axis of the lithium niobate film forming the optical waveguide coincides with the polarization direction of TE mode light. Therefore, the optical waveguide of the optical waveguide element 100 of this embodiment can be coupled to the output port of the laser light source, allowing TE mode light emitted from the laser light source to be directly incident. Thus, in the optical waveguide element 100 of this embodiment, unlike, for example, a z-cut optical waveguide element made of a lithium niobate film with the c-axis aligned in the thickness direction, there is no need to convert TE mode light to TM mode light using an optical conversion device before injecting light into the optical waveguide, and no optical loss occurs due to the conversion of the polarization direction of the light.

[0062] [Optical Modulator] Figure 5 is a plan view showing an example of a Mach-Zehnder type optical modulation element 200 equipped with the dielectric thin film substrate 1 shown in Figure 1. Figure 6 is a cross-sectional view of the optical modulation element 200 shown in Figure 5 along the BB' line. In the optical modulation element 200 shown in Figures 5 and 6, the components of the dielectric thin film substrate 1 shown in Figure 1 are given the same reference numerals, and their descriptions are omitted.

[0063] The optical modulation element 200 shown in Figures 5 and 6 is a device that modulates light propagating within the optical waveguide 10 by applying a voltage to a Mach-Zehnder interferometer formed by the optical waveguide 10. As shown in Figure 5, the optical waveguide 10 has a first optical waveguide 10a and a second optical waveguide 10b branched from a single input optical waveguide, and an output optical waveguide 10c formed by combining the first optical waveguide 10a and the second optical waveguide 10b.

[0064] The optical modulation element 200 shown in Figures 5 and 6 has a ridge portion 4 formed by processing the dielectric thin film 3 on the dielectric thin film substrate 1 shown in Figure 1 into a ridge shape (convex shape). In the optical modulation element 200, an optical waveguide 10 is formed by the ridge portion 4. As shown in Figures 5 and 6, two strip-shaped second electrodes 8a and 8b and one strip-shaped first electrode 7 are arranged approximately parallel to each other on the slab portion made of the dielectric thin film 3. The slab portion made of the dielectric thin film 3 is formed by thinning a portion of the upper surface of the dielectric thin film 3 in the dielectric thin film substrate 1 shown in Figure 1 by an etching method or the like.

[0065] As shown in Figures 5 and 6, the second electrode 8a and the first electrode 7 are positioned opposite each other with a space between them in the in-plane direction of the single crystal substrate 2, and the first optical waveguide 10a is positioned between the second electrode 8a and the first electrode 7. Also, the second electrode 8b and the first electrode 7 are positioned opposite each other with a space between them in the in-plane direction of the single crystal substrate 2, and the second optical waveguide 10b is positioned between the second electrode 8b and the first electrode 7. In this embodiment, the distances between the second electrode 8a and the first electrode 7 and the first optical waveguide 10a, and the distances between the second electrode 8b and the first electrode 7 and the second optical waveguide 10b are all approximately the same.

[0066] The first electrode 7 and the second electrodes 8a and 8b can be made of known conductive films, such as single-layer conductive films like Au films, Cu films, Al films, or ITO (indium tin oxide) films, or multilayer films of Ti films and Au films. The first electrode 7 and the second electrodes 8a and 8b may be made of the same material, or they may be made of different materials.

[0067] In this embodiment, the first electrode 7 functions as a signal electrode. The second electrodes 8a and 8b are reference electrodes set to a reference potential. The first electrode 7 and the second electrode 8a apply a predetermined voltage in the in-plane direction to the dielectric thin film 3 that changes the refractive index of the first optical waveguide 10a. The first electrode 7 and the second electrode 8b apply a predetermined voltage in the in-plane direction to the dielectric thin film 3 that changes the refractive index of the second optical waveguide 10b.

[0068] Preferably, the first optical waveguide 10a, positioned between the first electrode 7 and the second electrode 8a, and the second optical waveguide 10b, positioned between the first electrode 7 and the second electrode 8b, extend in a direction substantially perpendicular to the c-axis direction of the dielectric thin film 3 (up and down direction in Figure 5, left and right direction in Figure 6). In this case, as shown in Figures 5 and 6, by aligning the extending directions of the first electrode 7 and the second electrodes 8a and 8b in a direction substantially perpendicular to the c-axis direction of the dielectric thin film 3, the amount of phase change of light propagating through the first optical waveguide 10a and the second optical waveguide 10b with respect to the applied voltage is maximized. Therefore, the operating voltage of the Mach-Zehnder interferometer of the optical modulation element 200 can be lowered.

[0069] As shown in Figures 5 and 6, a buffer layer 5 is formed on the ridge portion 4 that forms the optical waveguide 10. As shown in Figure 6, the buffer layer 5 is formed to cover the upper and side surfaces of the ridge portion 4 that form the second optical waveguide 10b and the first optical waveguide 10a. The buffer layer 5 is also embedded between the first electrode 7 and the second electrodes 8a and 8b, with the upper surfaces of the first electrode 7 and the second electrodes 8a and 8b exposed. As the buffer layer 5, for example, one made of an SiO2 film or one made of a thin film in which an oxide of a metal element is added to SiO2 can be used.

[0070] [Manufacturing method for optical modulation elements] The optical modulation element 200 shown in Figures 5 and 6 can be manufactured, for example, by the manufacturing method described below. First, the dielectric thin film 3 on the dielectric thin film substrate 1 shown in Figure 1 is processed into a ridge shape (convex shape) using a known method such as etching, thereby forming an optical waveguide 10 consisting of the ridge portion 4, and also forming a slab portion consisting of the dielectric thin film 3.

[0071] Next, second electrodes 8a, 8b and first electrode 7 are formed in contact with the upper surface of the slab portion made of dielectric thin film 3 using known methods such as sputtering, vacuum deposition, and plating. Subsequently, a buffer layer 5 is formed to cover the upper and side surfaces of the ridge portion 4 and to fill the space between the first electrode 7 and the second electrodes 8a and 8b using known methods such as sputtering, vacuum deposition, pulsed laser ablation (PLD), and chemical vapor deposition (CVD). Through the above process, the optical modulation element 200 shown in Figures 5 and 6 is obtained.

[0072] The optical modulation element 200 of this embodiment includes a dielectric thin film substrate 1 as shown in Figure 1. Therefore, the optical waveguide 10 of the optical modulation element 200 of this embodiment is made of a lithium niobate film whose c-axis is aligned in one direction in the in-plane direction of the single crystal substrate 2. Consequently, when TE mode light is incident on the optical waveguide of the optical modulation element 200 of this embodiment, the polarization direction of the light incident on the optical waveguide coincides with the c-axis of the lithium niobate film, resulting in a low driving voltage and low optical loss.

[0073] Furthermore, since the optical modulation element 200 of this embodiment has an optical waveguide 10 made of a lithium niobate film with its c-axis aligned in one direction in the in-plane direction of the single crystal substrate 2, the first optical waveguide 10a and the second optical waveguide 10b can be arranged between the first electrode 7 and the second electrode 8a, and between the first electrode 7 and the second electrode 8b, respectively, which are spaced apart in the in-plane direction and facing each other on the dielectric thin film 3 made of lithium niobate film. For this reason, the optical modulation element 200 of this embodiment has a different design compared to, for example, the case in which the optical waveguide is made of a lithium niobate film with its c-axis in the thickness direction.

[0074] Furthermore, in the optical modulation element 200 of this embodiment, the direction of the c-axis of the lithium niobate film forming the optical waveguide coincides with the polarization direction of the TE mode light. For this reason, for example, it is not necessary to convert the TE mode light to TM mode light using an optical conversion device before injecting light into the optical waveguide, thus eliminating optical loss associated with the conversion of the polarization direction of the light, and also eliminating the need to secure space for installing an optical conversion device. Therefore, the optical modulation element 200 of this embodiment can be suitably used, for example, as an optical communication device.

[0075] Although preferred embodiments of the present invention have been described above, the present invention is not limited to the embodiments described above. It goes without saying that the present invention can be modified in various ways without departing from the spirit of the invention, and these modifications are also included within the scope of the present invention. [Examples]

[0076] "Example 1" The dielectric thin film substrate 1 shown in Figures 1(a) to 1(c) was manufactured using the method described below. As single crystal substrate 2, a 4-inch sapphire single crystal substrate was prepared in which the c-axis was oriented in the in-plane direction (the main surface 2a was the a-plane).

[0077] (Dielectric thin film deposition process) In the dielectric thin film deposition process, a dielectric thin film 3 was deposited on the main surface 2a of the single crystal substrate 2 by epitaxial growth using the sputtering method. The target used was a circular object with a diameter of 8 inches and a composition of Li / (Li+Nb) = 50 atomic percent.

[0078] The dielectric thin film 3 was deposited by positioning the target coaxially with the single crystal substrate 2 such that the distance between the target and the main surface 2a of the single crystal substrate 2 was 180 mm. Furthermore, for the deposition of the dielectric thin film 3, a mixed gas of Ar and O2 was used as the sputtering gas, with the O2 ratio in the sputtering gas set to 40% and the gas pressure to 0.09 Pa. After pre-sputtering for 180 seconds, the temperature of the single crystal substrate 2 was set to 600°C, and the film was deposited by applying a power of 1800 W. The dielectric thin film 3 was deposited in a so-called single-step manner without changing the deposition conditions midway through the process. Through the above steps, the dielectric thin film-coated substrate 1 of Example 1 was obtained.

[0079] The dielectric thin film-attached substrate 1 of Example 1 obtained in this manner was evaluated (Evaluation 1) to (Evaluation 3) using a fully automated multi-purpose X-ray diffractometer (SmartLab (rotating cathode type) manufactured by Rigaku Corporation) by the following methods, specifically the sapphire single crystal substrate on which the single crystal substrate 2 is formed and the lithium niobate film on which the dielectric thin film 3 is formed.

[0080] A CuKα light source (wavelength = 1.54186 Å) was used for X-ray irradiation. The output of the light source was set to 45 kV × 200 mA. Furthermore, for the lithium niobate (LiNbO3) single crystal, JCPDS (ASTM) card number 20-0631 was referred to as the reference diffraction data, and for the sapphire single crystal, JCPDS (ASTM) card number 10-0173 was referred to.

[0081] (Rating 1) The lithium niobate film provided on the dielectric thin film substrate 1 of Example 1 was subjected to in-plane diffraction, and the intensity of X-ray diffraction with respect to the φ axis was measured (scanned). The X-ray diffraction intensity was measured with the monochromator detector fixed at 2θχ = 38.94° so that the diffraction plane (006) perpendicular to the c-axis direction of LiNbO3 could be observed. The X-ray diffraction intensity was measured by placing the dielectric thin film substrate 1 on a horizontal rotating stage and varying the stage rotation angle φ from 0° to 360° while irradiating it with X-rays. The results are shown in Figure 7.

[0082] Figure 7 shows the X-ray diffraction intensity profile obtained by measuring (φ scan) the in-plane X-ray diffraction intensity of the lithium niobate film forming the dielectric thin film 3 on the dielectric thin film substrate of Example 1. As shown in Figure 7, only two diffraction peaks originating from the diffraction plane (006) were observed in the X-ray diffraction intensity profile. Also, as shown in Figure 7, the angular difference of the φ axis where the two diffraction peaks were observed was 180°. From these results, it was confirmed that the c-axis of the LiNbO3 provided on the dielectric thin film substrate 1 of Example 1 is aligned in one direction in the in-plane direction of the single crystal substrate 2.

[0083] (Rating 2) X-ray diffraction at the poles of the lithium niobate film provided on the dielectric thin film substrate 1 of Example 1 was measured using in-plane diffraction. The monochromator detector was set to an angle of 2θχ = 38.94°, and the slit configuration was adjusted to allow observation of diffracted light with a lattice spacing d in the range of 2.1 to 2.6 Å. The dielectric thin film substrate 1 was then placed on a horizontal rotating stage, and the diffracted X-rays at the poles were measured while irradiating with X-rays. The results are shown in Figure 8.

[0084] Figure 8 shows the measurement results of the X-ray diffraction poles of the lithium niobate film forming the dielectric thin film 3 on the dielectric thin film substrate of Example 1. As shown in Figure 8, 11 diffraction spots were observed. The positions of the 11 observed diffraction spots corresponded to the spots (006), (00-6), (110), (113), (11-3), (202), (20-2), (022), (02-2), (2-10), and (-120) in the reference diffraction data for the LiNbO3 single crystal when the measured diffraction plane is the a-plane. This confirmed that the dielectric thin film 3 on the dielectric thin film substrate 1 of Example 1 is a LiNbO3 single crystal film.

[0085] (Rating 3) For the sapphire single crystal substrate of the dielectric thin film substrate 1 of Example 1, the intensity of X-ray diffraction with respect to the φ axis was measured (scanned) by in-plane diffraction. The X-ray diffraction intensity was measured with the monochromator detector fixed at an angle of 2θχ = 41.68° so that the diffraction plane (006) perpendicular to the c-axis direction of the sapphire single crystal could be observed. Furthermore, the X-ray diffraction intensity was measured without moving the relative position of the dielectric thin film substrate 1 from the state in which the X-ray diffraction intensity of the lithium niobate film with respect to the φ axis was measured in (Evaluation 1). This allowed confirmation of the relative position of LiNbO3 with respect to the c-axis direction. Then, similar to (Evaluation 1), the X-ray diffraction intensity was measured while irradiating with X-rays and changing the stage rotation angle φ in the range of 0° to 360°. The results are shown in Figures 9, 10(a), and 10(b).

[0086] Figure 9 shows the X-ray diffraction intensity profile obtained by measuring (φ scan) the in-plane X-ray diffraction intensity of the sapphire single crystal substrate forming the single crystal substrate 2 of the dielectric thin film substrate in Example 1. Figure 9 shows the X-ray diffraction intensity profile of the sapphire single crystal substrate, as well as the X-ray diffraction intensity profile of the lithium niobate film forming the dielectric thin film 3 shown in Figure 7, which was measured in (Evaluation 1). Figure 10(a) is an enlarged portion of the X-ray diffraction intensity profile of Figure 9, showing the profile around a φ-axis angle of 85°. Figure 10(b) is an enlarged portion of the X-ray diffraction intensity profile of Figure 9, showing the profile around a φ-axis angle of 265°. In Figures 9, 10(a), and 10(b), the dotted lines show the results for the sapphire single crystal substrate, and the solid lines show the results for the lithium niobate film.

[0087] As shown in Figure 9, in the X-ray diffraction intensity profile, only two diffraction peaks originating from the diffraction plane (006) were observed. Also, as shown in Figure 9, the angular difference of the φ axis where the two diffraction peaks were observed was 180°. From these results, it was confirmed that the sapphire single crystal substrate on which the dielectric thin film substrate 1 of Example 1 is formed has its c axis aligned in one direction in the in-plane direction of the single crystal substrate 2. Therefore, it was confirmed that the crystal orientation of the main surface 2a is the a-plane.

[0088] Furthermore, as shown in Figure 9, the profile of the sapphire single crystal substrate, indicated by the dotted line, does not match the peak position angle of the lithium niobate film, indicated by the solid line. This confirms that the c-axis direction of the sapphire single crystal substrate and the c-axis direction of the lithium niobate film are misaligned.

[0089] Furthermore, as shown in Figures 10(a) and 10(b), the two diffraction peaks (maximum values) in the X-ray diffraction intensity profile of the sapphire single crystal substrate with respect to the φ axis were 86.5° and 266.5°. Also, as shown in Figures 10(a) and 10(b), the two diffraction peaks (maximum values) in the X-ray diffraction intensity profile of the lithium niobate film with respect to the φ axis were 84.2° and 264.2°. The angular difference in the φ axis where the diffraction peaks of the sapphire single crystal substrate and the lithium niobate film were observed (86.5 - 84.2 = 266.5 - 264.2 = 2.3) was 2.3°. This confirmed that the angle between the c-axis direction of the single crystal substrate 2 and the c-axis direction of the lithium niobate film in the dielectric thin film substrate 1 of Example 1 is 2.3°. [Explanation of Symbols]

[0090] 1 Dielectric thin film coated substrate, 2 Single crystal substrate, 2a Main surface, 3 Dielectric thin film, 4 Ridge portion, 5 Buffer layer, 7 First electrode, 8a, 8b Second electrode, 10 Optical waveguide, 10a First optical waveguide, 10b Second optical waveguide, 10c Output optical waveguide, 100 Optical waveguide element, 200 Optical modulation element.

Claims

1. The system comprises a single crystal substrate oriented along the c-axis in the in-plane direction, and a dielectric thin film formed in contact with the single crystal substrate. A dielectric thin film substrate comprising a lithium niobate film in which the c-axis is aligned in one direction of the in-plane direction.

2. The dielectric thin film substrate according to claim 1, wherein the angle between the c-axis direction of the single crystal substrate and the c-axis direction of the lithium niobate film is 0.4° to 5°.

3. The dielectric thin film substrate according to claim 1, wherein the single crystal substrate is a sapphire single crystal substrate.

4. The dielectric thin film substrate according to claim 1, wherein the lithium niobate film is grown by vapor phase growth.

5. The dielectric thin film substrate according to claim 1, wherein the lithium niobate film is grown by sputtering.

6. A substrate with a dielectric thin film according to any one of claims 1 to 5 is provided, An optical waveguide element having an optical waveguide made of the dielectric thin film.

7. A substrate with a dielectric thin film according to any one of claims 1 to 5 is provided, An optical waveguide made of the dielectric thin film, The device has a first electrode and a second electrode provided on the dielectric thin film and arranged opposite to each other with spaced apart in the in-plane direction, An optical modulation element in which the optical waveguide is positioned between the first electrode and the second electrode.

8. A manufacturing method for producing a dielectric thin film substrate according to any one of claims 1 to 5, A method for manufacturing a substrate with a dielectric thin film, comprising a dielectric thin film deposition step of growing a lithium niobate film on a single crystal substrate in which the c-axis is oriented in the in-plane direction, by vapor phase growth, wherein the c-axis is aligned in one direction in the in-plane direction.

9. The method for manufacturing a dielectric thin film substrate according to claim 8, wherein the vapor phase growth method is a sputtering method.

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