WO2022264426A1

WO2022264426A1 - Method for forming lithium niobate crystal thin film, and laminate including lithium niobate crystal thin film

Info

Publication number: WO2022264426A1
Application number: PCT/JP2021/023264
Authority: WO
Inventors: 方省赤澤
Original assignee: 日本電信電話株式会社
Priority date: 2021-06-18
Filing date: 2021-06-18
Publication date: 2022-12-22
Also published as: JPWO2022264426A1

Abstract

When applying LN crystals as an optical element, it is desirable to form a thin film of c-axis-oriented LN crystals on a substrate having high light transmittance, and it is preferable that the substrate also have high electrical conductivity in consideration of cases in which an electric field is applied to the LN crystals. Therefore, the present disclosure provides: a method for forming an LN crystal thin film to be applied as an optical element, the method involving forming c-axis-oriented LN crystals into a film on a substrate having high light transmittance and a large difference in refractive index from LN crystals; and a laminate thereof.

Description

Method for depositing lithium niobate crystal thin film and laminate containing lithium niobate crystal thin film

The present disclosure relates to a method for forming a c-axis oriented lithium niobate crystal thin film, and a laminate including the c-axis oriented lithium niobate crystal thin film.

　Integration of optical elements into a single optical circuit is a trend in the development of optical communication devices in recent years. In particular, silicon photonics compatible with complementary metal oxide semiconductor (CMOS) circuits provides an important platform for building systems that integrate the functions of electronic and optical devices.

A high-refractive-index-difference optical waveguide whose main constituent materials are silicon (Si) and silica (SiO ₂ ) can be curved or Y-branched with a sharp curvature on a micrometer scale. Therefore, there is an advantage that selective extraction of optical signals in a limited space and addition operation can be realized. However, since the function of an optical waveguide composed only of Si and _SiO2 remains passive, a new functional element must be inserted into the optical circuit in order to handle the optical signal more actively. is required. Candidate materials for the newly inserted functional element include ferroelectric materials having properties such as ferroelectricity, electro-optic effect, nonlinear optical effect, and photorefractive effect.

Lithium niobate (LiNbO ₃ : hereinafter referred to as LN), which is a composite of lithium oxide Li ₂ O and niobium oxide Nb ₂ O ₅ at a molar ratio of 1:1, is highly available as a single crystal and has a typical strength. Since it is a dielectric material, it is applied to electro-optic modulators, optical deflectors, wavelength converters, and the like. The crystal system of the LN crystal is a trigonal system, and its lattice constants are a=5.148 Å and c=13.86 Å. In order to maximize the characteristics of the LN crystal as an optical waveguide, it is preferably a single crystal, but it is technically difficult to insert an LN single crystal into an optical waveguide formed on a substrate. In the examples reported so far, after bonding an LN crystal wafer to a _SiO2 substrate, the LN crystal is polished to a micrometer-order thickness, and then processed into an optical waveguide shape by photolithography and etching. (see, for example, Non-Patent Document 1). However, this process is not suitable for production because it requires advanced technology for bonding and takes time for polishing.

On the other hand, a method for forming a thin film such as a sputtering method has the advantage that a polycrystalline LN crystal film whose orientation and film thickness are controlled can be formed anywhere on the substrate. There is a drawback that the characteristics as an optical element are inferior to the LN single crystal. However, if the crystallinity of LN polycrystals can be brought closer to that of single crystals by controlling the orientation or the like, there is a good possibility that they will be put to practical use even if some deterioration in characteristics is taken into account. From this point of view, many studies have been conducted on LN polycrystalline film formation.

In studies on such LN polycrystalline film formation, Si has been widely used as a substrate because of its low cost and high versatility. However, Si is not suitable as a substrate for forming an optical waveguide because of its high light absorption. On the other hand, SiO ₂ has a high light transmittance and a low refractive index of 1.457 at a wavelength of 633 nm, so it is a material that can serve as a clad layer for all the materials that constitute the core of the optical waveguide. In fact, the refractive index of SiO ₂ is much smaller than the ordinary refractive index n ₀ =2.28 and the extraordinary refractive index n _e =2.20 of the LN crystal. Therefore, even in an optical waveguide using an LN crystal as a core, SiO ₂ is an effective material as a cladding layer, and it can be said that the LN crystal formed on the SiO ₂ substrate is important for application as an optical element. However, in general, when forming an LN crystal film on a _SiO2 substrate, it is difficult to obtain a film with high crystallinity, and it is not always possible to obtain a thin film of LN crystal with c-axis orientation. is known (see, for example, Non-Patent Document 2).

Since the c-axis of the LN crystal coincides with the direction of spontaneous polarization, it is required to selectively orient the crystal orientation to the c-axis in order to maximize the function as an optical element. As a method for realizing this, there is a method of growing on a single crystal substrate lattice-matched to the LN crystal, and so far, many experimental results of forming an epitaxial film of an LN crystal on a sapphire C-plane substrate have been reported. (For example, see Non-Patent Document 3). There are also attempts to control the crystal orientation on the A-plane and R-plane of sapphire. In addition, an example of growing an epitaxial film of an LN crystal on a substrate of MgO (111), which is a single crystal of magnesium oxide (MgO), or MgO (100) has been reported (for example, Non-Patent Document 4 reference). However, MgO substrates, which are highly available, are generally limited to a size of about 10×10 mm, and are not suitable for practical use as optical elements.

Additionally, if an electric field needs to be applied to the LN crystal to function as an optical element, the substrate preferably has not only high light transmission but also high electrical conductivity. On the other hand, since SiO ₂ and sapphire mentioned above have high insulating properties, when an LN crystal film is formed on these substrates, the electrodes must be arranged side by side on the LN crystal. An optical element having such a structure naturally has a limit to the magnitude of the electric field that can be applied, so it is difficult to apply it to an optical element that requires a high electric field, such as an optical deflector or an optical modulator. On the other hand, if an LN crystal is formed on a substrate having high conductivity, the substrate acts as a lower electrode, so forming an electrode on the LN crystal can ensure a high electric field. This is also advantageous in aligning the polarization directions of the ferroelectric domains by poling. As such an attempt, an epitaxial film of Pt (111), which is a single crystal of platinum (Pt), was formed on a Si (111) substrate, and a Pt (100) epitaxial film was formed on an MgO (100) substrate. Furthermore, an example of forming an LN crystal film thereon has been reported (for example, Non-Patent Document 4). However, if the LN film is formed directly on the metal film in this way, the light beam propagating in the LN crystal film and the metal film may interfere with each other, and the light intensity may be attenuated. It is necessary to increase the thickness. From this point of view, instead of the metal film, for example, it is possible to use a transparent conductive film such as ITO, but in this case, it is reported that the crystal orientation of the LN crystal becomes random orientation (for example, , Non-Patent Document 5).

Thus, in applying LN crystals as optical elements, it is desirable to form a thin film of c-axis oriented LN crystals on a substrate having high light transmittance. In particular, considering the case where an electric field is applied to the LN crystal, the substrate preferably has a higher electrical conductivity. However, at present, there is no report on a technique for forming a c-axis oriented LN crystal film on a substrate that satisfies such conditions, which is suitable for practical use in optical elements.

The present disclosure has been made in view of such problems, and an object of the present disclosure is to provide a thin film of LN crystal for application as an optical element, which has high light transmittance and large refraction of LN crystal. An object of the present invention is to provide a method for forming a c-axis oriented LN crystal film on a substrate having a difference in index and a laminate thereof.

In order to solve the above problems, the present disclosure provides a deposition method for depositing a c-axis oriented lithium niobate crystal thin film on a substrate using an electron cyclotron resonance (ECR) plasma sputtering method. After a step of preparing a substrate and depressurizing a film forming chamber of a film forming apparatus for carrying out the ECR plasma sputtering method, argon plasma is generated in a state in which no potential is applied to the lithium niobate target. A step of removing moisture adsorbed on the wall surface of the film forming chamber of the apparatus, generating ECR plasma while supplying argon and oxygen to the film forming chamber of the film forming apparatus, and further applying radio frequency (RF) power to the lithium niobate target. depositing particles of a lithium niobate target on said substrate by applying a to and causing a sputtering phenomenon on the surface of the lithium niobate target, wherein the substrate is ZnO/ _SiO2 , Z-cut quartz, or (111) oriented strontium titanate (STO) single crystal, wherein in the step of depositing particles of the lithium niobate target on the substrate, the c-axis oriented lithium niobate on the substrate. Provided is a method for forming a lithium niobate crystal thin film, characterized by forming a crystal thin film.

4 is a flow chart illustrating a method for depositing an LN crystal film on a _SiO2 substrate using ECR plasma sputtering according to the present disclosure; FIG. 3 shows XRD patterns of LN crystals deposited on SiO ₂ by ECR plasma sputtering at various deposition temperatures and oxygen flow rates. 4 is a flowchart illustrating a method for depositing ZnO as a buffer layer on SiO ₂ and depositing c-axis oriented LN crystals thereon according to an embodiment of the present disclosure; FIG. 4 is a diagram showing an XRD pattern of a laminate in which an LN crystal is deposited on ZnO/SiO ₂ with a ZnO deposition temperature of 500° C. and a film thickness of 70 nm, in an embodiment of the present disclosure. FIG. 4 is a diagram showing an XRD pattern of a laminate obtained by forming an LN crystal film on ZnO/SiO ₂ with a ZnO film forming temperature of 500° C. and a film thickness of 10 nm, in an embodiment of the present disclosure. FIG. 4 is a diagram showing an XRD pattern of a laminate obtained by forming an LN crystal film on ZnO/SiO ₂ with a ZnO film forming temperature of 400° C. and a film thickness of 70 nm, in an embodiment of the present disclosure. 4 is a flow chart illustrating a method for depositing c-axis oriented LN crystals on Z-cut quartz according to one embodiment of the present disclosure. FIG. 4 is a diagram showing an XRD pattern of a laminate in which an LN crystal is deposited on a Z-cut quartz substrate in an embodiment of the present disclosure; 4 is a flow chart illustrating a method for depositing c-axis oriented LN crystals on Z-cut quartz according to one embodiment of the present disclosure. FIG. 4 is a diagram showing an XRD pattern of a laminate in which an LN crystal is deposited on a Z-cut quartz substrate in an embodiment of the present disclosure; 4 is a flowchart illustrating a method for depositing a c-axis oriented LN crystalline film on STO(111) according to an embodiment of the present disclosure; FIG. 4 is a diagram showing an XRD pattern of a laminate in which an LN crystal is deposited on STO (111) in an embodiment of the present disclosure;

Embodiments of the present invention will be described below with reference to the drawings. In order to form a c-axis oriented LN crystal on a substrate having high light transmittance and a large difference in refractive index from the LN crystal, selection of the substrate is important. In one embodiment of the present disclosure, as a substrate, a two-layer body (hereinafter referred to as ZnO/SiO ₂ ) in which zinc oxide (ZnO) is deposited as a buffer layer on SiO ₂ using a sputtering method, Z-cut quartz, Three types of strontium titanate (SrTiO ₃ : hereinafter referred to as STO) single crystals were selected. The characteristics of each substrate material and the reasons for selection are described below.

ZnO deposited by sputtering tends to be strongly c-axis oriented on many substrates, even if the temperature of the substrate during deposition is room temperature. A ZnO crystal has a wurtzite crystal structure belonging to the hexagonal crystal system, and its lattice constant is a=3.25 Å. If the C plane of the LN crystal is rotated 60° with respect to the buffer layer ZnO and grows, the length of the unit cell is 5.63 Å, and the lattice constant of the LN crystal, a = 5.148 Å, is approximately match. Therefore, although the growth of LN crystal film on ZnO is not epitaxial growth, it is expected to promote lattice-matched growth of LN crystal and to form a c-axis oriented LN crystal film.

Z-cut quartz is a type of quartz whose main component is _SiO2 , and is a single-crystal material preferentially oriented to Quartz (0001). And since the lattice constants are a = 4.91 Å and c = 5.41 Å, the Quartz (0001) plane of Z-cut quartz is the C plane of the LN crystal (lattice constant a = 5.148 Å) , the lattice constant difference is as small as about 5%. Therefore, it can be expected that the LN crystal film will grow epitaxially on the Quartz (0001) plane of Z-cut quartz due to the same lattice constant as the six-fold symmetrical crystal structure. In addition, since Z-cut quartz contains SiO ₂ as a component as described above, the refractive indices for light with a wavelength of 633 nm are n ₀ =1.544 and n _e =1.553, which are the same as SiO ₂ . Therefore, Z-cut quartz functions as a clad layer in an optical waveguide with an LN crystal core.

The STO crystal has a cubic crystal structure in which Sr occupies the A site and Ti occupies the B site, and its lattice constant is a = 3.905 Å. The STO (111) surface has a structure in which oxygen atoms are arranged in a hexagonal lattice, and the distance between oxygen atoms is 5.52 Å. This value corresponds to the lattice constant a=5.148 Å of the LN crystal, and the lattice constant difference is as small as about 7%. Therefore, the LN crystal is expected to grow epitaxially when deposited on the STO (111) surface. be.

Thus, in the present disclosure, an LN crystal is deposited by an electron cyclotron resonance (hereinafter referred to as ECR) plasma sputtering method on a substrate where the LN crystal is expected to be c-axis oriented. Prior to describing the method of forming a c-axis oriented LN crystal film on each of the three types of substrates described above, an LN crystal film was formed directly on a _SiO2 substrate, and the crystal orientation was determined by the film formation conditions. We will explain the contents of the evaluation to see if it changes.

FIG. 1 is a flowchart illustrating a method for depositing an LN crystal film on a _SiO2 substrate using ECR plasma sputtering according to the present disclosure. A method for forming an LN crystal on a _SiO2 substrate according to the present disclosure includes step 11 of creating a substrate having _SiO2 formed on the surface layer by thermally oxidizing a Si(111) single crystal; In a film forming apparatus for forming a film, after the film forming chamber is decompressed by a vacuum pump, argon plasma is generated without applying a potential to the LN target, and the moisture adsorbed on the wall surface in the film forming chamber is removed; By generating ECR plasma and applying high frequency (Radio Frequency: hereinafter referred to as RF) power to the LN target, a sputtering phenomenon is generated on the LN target surface, and the particles emitted from the LN target are deposited on the substrate. and step 13 of performing. Here, as an example, the film thickness of SiO ₂ in step 1 is set to 500 nm. Further, in step 12, the final vacuum degree of the film formation chamber was set to 9×10 ⁻⁵ Pa. Further, in step 13, the microwave power for generating the ECR plasma was set to 500W, and the RF power applied to the LN target was also set to 500W. Further, in step 13, the substrate was placed on a heater, and the substrate and the growing thin film were held at a desired temperature (hereinafter referred to as film forming temperature) for film formation. In addition, in order to maintain the degree of oxidation of the LN crystal to be deposited, oxygen was supplied in addition to argon, which is a sputtering gas, into the deposition chamber. The flow rate of argon was fixed at 8 sccm.

In such a method, the film formation temperature and oxygen flow rate were varied, and the accompanying changes in crystallinity of the LN crystal were evaluated. The film formation temperature was varied in the range of 400 to 460° C., and the oxygen flow rate was varied in the range of 0.5 to 3.0 sccm. Note that the film thickness of the LN crystal is 1 μm under any conditions. The crystallinity was evaluated by crystal structure analysis by X-ray diffraction (hereinafter referred to as XRD) method. The characteristic X-rays used in this method are CuKα rays, and the scan mode is θ/2θ.

FIG. 2 shows XRD patterns of LN crystals deposited on SiO ₂ by ECR plasma sputtering at various deposition temperatures and oxygen flow rates. When the deposition temperature is 400 ° C. and the oxygen flow rate is 0.5 sccm, the XRD pattern of the deposited LN crystal shows that the peak intensity of LN (006) corresponding to the c-axis orientation is higher than the peak intensity of LN (006) corresponding to the c-axis orientation. The peak intensity of (122) is stronger. Therefore, it can be inferred that the LN crystal formed under these conditions has a weak c-axis orientation. On the other hand, when the film formation temperature was increased to 460° C. at the same oxygen flow rate, the peak intensity of LN(006) increased significantly. The peak intensity of LN (202), which is the second strongest in the same XRD pattern, is one order of magnitude smaller than the peak intensity of LN (006), so the LN crystal formed under this condition is a c-axis oriented LN crystal. I can say there is. Even when the film formation temperature is raised to 460° C., no peak corresponding to the Li-deficient LiNb ₃ O ₈ phase is detected. Furthermore, when the oxygen flow rate is increased to 1.5 sccm while the film formation temperature is kept at 460° C., the XRD pattern of the LN crystal shows that the LN (202) peak is increased while the LN (006) peak intensity is maintained. It can be seen that the c-axis orientation is becoming stronger. On the other hand, at the same deposition temperature of 460 °C, when the oxygen flow rate is increased to 3 sccm, the peak intensity of LN(006) decreases significantly, LN(012), LN(110), LN(202), LN(116) , LN (122), etc., were detected, and it was found that polycrystals with random orientation were formed. This means that the presence of oxygen in the interstitial sites of the growing LN crystal caused the LN crystal to split into multiple crystallites oriented in various orientations due to the oxygen excess at the thin film growth surface.

From the above, it is possible to form a c-axis oriented LN crystal film on a _SiO2 substrate by the method described above, and when the film formation temperature is 460 ° C., the oxygen flow rate is 0.5 to 1.5 sccm. It can be seen that c-axis oriented LN crystals can be obtained in the range of . As described above, when LN is usually deposited on SiO ₂ by sputtering, it is difficult to align the c-axis, but in the present disclosure, LN crystals deposited on amorphous SiO ₂ A film was formed in which the orientation of the film was aligned in the c-axis direction. This is attributed to minimizing the surface energy by terminating the LN film surface with the closest-packed C-plane. In the above-described film forming method according to the present disclosure, this condition is met when the oxygen flow rate is relatively low, ie, 0.5 to 1.5 sccm. However, when considering application as an optical element, an LN crystal with a stronger c-axis orientation is required. Also, as described above, SiO ₂ is unsuitable as a substrate when an electric field needs to be applied to the LN crystal. Therefore, the present disclosure proposes a method of forming an LN crystal having a stronger c-axis orientation using each of the three types of substrates described above. Embodiments using each substrate will be described below.

(First embodiment)
A first embodiment of the present disclosure will be described below with reference to the drawings. This embodiment is an example in which a ZnO/SiO ₂ substrate in which ZnO is deposited as a buffer layer on SiO ₂ is used as a substrate, and a c-axis oriented LN crystal film is deposited on the ZnO.

FIG. 3 is a flowchart illustrating a method for depositing ZnO as a buffer layer on SiO ₂ and depositing c-axis oriented LN crystals thereon according to one embodiment of the present disclosure. The method of forming an LN crystal according to the present embodiment includes step 31 of forming a substrate having SiO ₂ formed on the surface layer by thermally oxidizing a Si(111) single crystal, and forming a film of ZnO as a buffer layer. In the film apparatus, after the pressure in the film forming chamber is reduced by a vacuum pump, argon plasma is generated without applying a potential to the Zn target to desorb moisture adsorbed on the wall surface of the film forming chamber (step 32), and a ZnO film is formed. ECR plasma is generated while supplying oxygen in the deposition chamber of the deposition apparatus, and RF power is applied to the Zn target to generate a sputtering phenomenon on the Zn target surface and release from the oxidized Zn target surface. In step 33 of depositing the particles on SiO ₂ and in the film forming apparatus for forming the LN crystal film, after the film forming chamber is decompressed by a vacuum pump, plasma is generated without applying a potential to the LN target, A step 34 for removing moisture adsorbed on the wall surface of the film forming chamber, generating ECR plasma in the film forming chamber of the film forming apparatus for forming the LN film, and further applying RF power to the LN target, thereby removing the LN target surface generating a sputtering event at 35 to deposit particles emitted from the LN target onto the ZnO. In this embodiment, as an example, the size of Si (111) used in step 31 is 4 inches, and the film thickness of SiO ₂ formed is 140 nm. The ECR plasma sputtering method performed in step 33 was a reactive sputtering method using oxygen, film formation temperatures were 400° C. and 500° C., and ZnO film thicknesses were 10 nm and 70 nm. The conditions for forming the LN crystal in steps 34 and 35 were the same as in the case of forming the LN crystal on SiO ₂ described above. However, the film formation temperature and the oxygen flow rate were fixed under the conditions (460° C. and 1.5 sccm, respectively) under which the LN crystal with the strongest c-axis orientation could be formed when the LN crystal was formed on SiO ₂ . The film thickness of the formed LN crystal film is 1 μm.

In this way, the crystallinity of the LN crystal deposited on ZnO/SiO ₂ was evaluated using XRD. In this embodiment, in addition to the measurement in which the scan mode is θ/2θ, only the 2θ angle of the detector is scanned while the X-ray incident angle is 1.5° with respect to the thin film surface of the LN crystal. A Grazing Incident X-ray Diffraction (hereinafter referred to as GIXRD) pattern was also obtained. Further, a rocking curve was obtained for some of the detected peaks, and the orientation was evaluated in more detail from the shape and half-value width.

FIG. 4 is an XRD pattern of a laminate in which an LN crystal is deposited on ZnO/SiO ₂ with a ZnO deposition temperature of 500° C. and a film thickness of 70 nm in an embodiment of the present disclosure. FIG. 4(a) is the XRD pattern with the scan mode θ-2θ, FIG. 4(b) is the GIXRD pattern, and FIG. 4(c) is the rocking curves for the peaks of LN(006) and ZnO(002), respectively. showing. As shown in FIG. 4A, the peak of ZnO (002) corresponding to the c-axis direction of ZnO, which is the buffer layer, is detected with a high intensity of 1×10 ⁵ counts. On the other hand, other peaks derived from ZnO were detected, such as ZnO (102), ZnO (110), ZnO (103), and ZnO (004). is smaller than that of , by more than two orders of magnitude. From this, it can be said that the 70 nm-thickness ZnO film formed as the buffer layer according to this embodiment is a film strongly c-axis oriented. In addition, in FIG. 4A, only LN (006) is detected as the peak for LN crystal, so the LN crystal formed reflects ZnO (002) and is strongly c-axis oriented. I can ask. On the other hand, peaks such as LiNb ₃ O ₈ (−602), LN(006), and LN(116) are also detected in the GIXRD pattern shown in FIG. 4(b). This indicates that crystallites having orientations different from the c-axis are also mixed. However, in the rocking curves of LN (006) and ZnO (002) shown in FIG. It is suggested that the formed LN crystal film has a certain degree of c-axis orientation.

FIG. 5 is an XRD pattern of a laminate in which an LN crystal is deposited on ZnO/SiO ₂ with a ZnO deposition temperature of 500° C. and a film thickness of 10 nm in an embodiment of the present disclosure. FIG. 5(a) shows an XRD pattern with a scan mode of θ-2θ, and FIG. 5(b) shows a GIXRD pattern. Compared to the case where the ^ZnO film thickness is 70 nm shown in FIG. , LN(104), LN(116), and LN(018), whose crystal orientation is not the c-axis, are detected with peak intensities comparable to those of LN(006). Furthermore, in the GIXRD pattern shown in FIG. 5(b), it can be seen that these diffraction peaks have peak intensities comparable to those of the θ/2θ scan. This suggests that the LN crystals deposited on ZnO/SiO ₂ with a ZnO film thickness of 10 nm were almost unaffected by the c-axis orientation of ZnO, and the crystals grew so as to have a random orientation. be done. It is known that, as a general tendency, when ZnO is deposited on _SiO2 by sputtering, the crystallinity is the worst in the initial stage, and the crystallinity at the top of the film improves as the film thickness increases. . Therefore, even in this embodiment, when the film thickness of ZnO is 10 nm, the film thickness is thin, so the c-axis orientation of ZnO is weak, and rather it acts to increase the surface roughness of the LN growth surface. It is thought that the LN crystal also became a polycrystalline structure with random orientation along with it.

FIG. 6 is an XRD pattern of a laminate in which an LN crystal is deposited on ZnO/SiO ₂ with a ZnO deposition temperature of 400° C. and a film thickness of 70 nm in an embodiment of the present disclosure. FIG. 6(a) is an XRD pattern with a scan mode of θ-2θ, FIG. 6(b) is a GIXRD pattern, and FIG. 6(c) is a rocking curve for LN (006) and ZnO (002) peaks, respectively. showing. As shown in FIG. 6A, the detected diffraction peaks for both ZnO and LN crystals are all peaks related to the c-axis direction, and both ZnO and LN crystals have higher , c-axis orientation becomes stronger. In addition, in the GIXRD pattern of FIG. 6 (b), only a weak LN (006) was detected as a peak derived from the LN crystal, which also indicates that the LN crystal is strongly c-axis oriented. is suggested. Furthermore, the rocking curves of ZnO (002) and LN (006) shown in FIG. It is considered to have sex. That is, it can be said that by setting the film formation temperature of ZnO to 400° C., an LN crystal with a stronger c-axis orientation was formed as compared to the case where the film formation temperature was 500° C. This is because the surface roughness of the deposited ZnO was reduced by lowering the deposition temperature of ZnO, and the film thickness was 70 nm, which was sufficient to have a strong c-axis orientation. Along with this, it is considered that the c-axis orientation of the LN crystal also became stronger.

Based on the above, an LN crystal film was formed using the ECR plasma sputtering method at a film formation temperature of 400 to 500 ° C., using a ZnO/SiO ₂ substrate in which ZnO having a film thickness of 70 nm was formed on SiO ₂ . By doing so, an LN crystal having a strong c-axis orientation can be formed. ZnO has a high light transmittance, so that light loss is small. Therefore, the LN/ZnO/SiO ₂ laminate deposited according to this embodiment can be applied to an optical waveguide having an LN crystal core.

Further, although the film thickness of ZnO is set to 70 nm in the present embodiment, the same effect can be obtained if the film thickness is 70 to 100 nm. If it is thicker than this, problems may arise in that the surface roughness of ZnO increases and the internal stress increases due to the difference in coefficient of linear expansion from that of SiO ₂ . Conversely, if the film thickness is less than this, the crystallinity of ZnO is low, and it is conceivable that the LN crystal formed thereon does not exhibit a strong c-axis orientation.

In addition, in this embodiment, the ZnO film formation method is a reactive sputtering method using oxygen, but a sputtering method using a ZnO target and argon also produces the same effect.

In addition, in the present embodiment, ZnO may be reduced to such an extent that the crystal structure is not destroyed. By doing so, ZnO becomes a transparent conductive film, so that it becomes possible to have high conductivity while maintaining high light transmittance. This is because oxygen vacancies generated by reduction form donor levels that release two electrons. As a method for reducing ZnO, for example, a method of using a ZnO target in film formation by sputtering of ZnO and introducing hydrogen in addition to argon to the sputtering gas can be used.

Furthermore, it is possible to further increase the electrical conductivity by doping ZnO with aluminum (Al) or gallium (Ga) to form AZO or GZO. This is because the doped Al or Ga forms a substitutional solid solution in the Zn site of ZnO to form a donor level that emits one electron. As a method of doping ZnO with Al or Ga, for example, a method using AZO or GZO as a target in ZnO film formation by sputtering can be used.

In this way, if ZnO is given high conductivity, for example, by forming a ZnO film also on the upper part and forming a sandwich structure in which the LN crystal is sandwiched between ZnO transparent electrodes, it is possible to use the electro-optic effect for conduction. It can be applied to wave-type devices.

(Second embodiment)
A second embodiment of the present disclosure will be described below with reference to the drawings. This embodiment is an example in which a c-axis oriented Z-cut quartz is used as a substrate and a c-axis oriented LN crystal film is formed thereon.

FIG. 7 is a flowchart illustrating a method for depositing c-axis oriented LN crystals on Z-cut quartz according to one embodiment of the present disclosure. The method of forming a c-axis oriented LN crystal film on a Z-cut quartz according to the present embodiment comprises the following steps: step 71 of preparing a c-axis oriented Z-cut quartz substrate; After decompressing the film chamber with a vacuum pump, argon plasma is generated without applying potential to the LN target to remove moisture adsorbed on the wall surface of the film forming chamber 72, ECR plasma is generated, and the LN target is generating a sputtering phenomenon on the LN target surface by applying RF power to the LN target to deposit particles ejected from the LN target onto the Z-cut quartz substrate. The size of the Z-cut quartz substrate in this embodiment is 3 inches. In addition, the film formation method of the LN crystal is the ECR plasma sputtering method, and the film formation conditions are the conditions under which the LN crystal with the strongest c-axis orientation is obtained when the LN crystal film is formed on the SiO ₂ described above, that is, , the film formation temperature was 460° C., and the oxygen flow rate was 1.5 sccm. The film thickness of the LN crystal is 1 μm as in the first embodiment. The crystallinity of the LN crystal film formed by such a method was evaluated using XRD in the same manner as in the first embodiment.

FIG. 8 is an XRD pattern of a stack of LN crystals deposited on a Z-cut quartz substrate in one embodiment of the present disclosure. FIG. 8(a) shows an XRD pattern with a scan mode of θ-2θ, FIG. 8(b) shows a GIXRD pattern, and FIG. 8(c) shows a rocking curve for the peak of LN(006). Note that the notation of Q shown in the drawing is an abbreviation for Quartz. As shown in FIG. 8A, the diffraction peaks derived from Z-cut quartz are Q(001), Q(002), Q(003), and Q(004), and the peaks corresponding to the c-axis direction are mainly detected. From this, it was confirmed that the Z-cut quartz substrate used in this embodiment has a strong c-axis orientation. This is also proved by the fact that no peak derived from Z-cut quartz is detected in the GIXRD pattern of FIG. 8(b). On the other hand, the diffraction peak derived from the LN crystal is the strongest in LN (006) corresponding to the c-axis direction, and the peak intensity reaches 2 × 10 ⁴ counts, so this LN crystal is affected by the substrate. , c-axis oriented crystals. However, weak peaks of LN(202), LN(018), and LN(10 10 ), which do not correspond to the c-axis direction, are also detected, suggesting that the crystallinity is somewhat disturbed. Furthermore, even in the GIXRD pattern of FIG. No peaks are detected. Therefore, according to this embodiment, although the LN crystal film formed on the Z-cut quartz substrate is c-axis oriented, crystallites that do not correspond to the c-axis direction coexist in the film. It is thought that there are

By using such a method, it is possible to form a c-axis oriented LN crystal film on a Z-cut quartz substrate, although some crystallites do not correspond to the c-axis direction. As described above, since Z-cut quartz has a composition of SiO ₂ , its refractive index is n=1.54, which is the same as SiO ₂ . Therefore, when a structure in which an LN crystal is deposited directly on SiO ₂ is required, an optical waveguide can be provided that reflects the characteristics of the c-axis oriented LN crystal while maintaining the refractive index of SiO ₂ .

(Modification of Second Embodiment)
As described above, by using the film formation method according to the second embodiment, it is possible to form a c-axis oriented LN crystal film on a Z-cut quartz substrate. It is a state in which crystallites that do not correspond to coexist, and the crystallinity is not high. However, by adding a step of solid-phase crystallization to the film forming method according to the second embodiment, it is possible to improve the crystallinity of the LN crystal. The details are described below.

FIG. 9 is a flowchart illustrating a method for depositing c-axis oriented LN crystals on Z-cut quartz according to one embodiment of the present disclosure. In the method shown in FIG. 9, after the method shown in FIG. 7, a step 74 of performing heat treatment for the purpose of solid-phase crystallization is added to the laminated body in which the LN crystal is formed on the Z-cut quartz substrate. include. Here, the film formation temperature during the film formation in step 73 was room temperature, the heat treatment temperature for solid phase crystallization was 600° C., and the holding time was 1 hour. Crystallinity was evaluated by XRD in the same manner as in the first embodiment for the laminated body in which the LN crystal was formed on the Z-cut quartz substrate formed by such a method.

FIG. 10 is an XRD pattern of a stack of LN crystals deposited on a Z-cut quartz substrate in one embodiment of the present disclosure. FIG. 10(a) shows an XRD pattern with a scan mode of θ-2θ, FIG. 10(b) shows a GIXRD pattern, and FIG. 10(c) shows a rocking curve for the peak of LN(006). As shown in FIG. 10( a ), the XRD pattern of the LN crystal film formed by the method including the step 74 as described above is similar to FIG. 8( a ), and the diffraction peak derived from the LN crystal is , and LN(006) are predominant, but diffraction peaks of LN(018) and LN(10 10 ) are also slightly detected. Also in the GIXRD pattern of FIG. 10(b), a diffraction peak not corresponding to the c-axis direction is detected as in FIG. 8(b). That is, the LN crystal deposited by the present embodiment has crystallites that do not correspond to the c-axis direction, as in the LN crystal film deposited by the method that does not include the step 74 described in the second embodiment. It can be said that it is in a state of However, the peak intensity of LN(006) is 5×10 ⁴ counts, which is more than double that of FIG. 8(a). In addition, the rocking curve of FIG. 10(c) also has a narrower half width than that shown in FIG. 8(c), indicating Bragg reflection centered at ω=19.5°. . Therefore, by setting the film formation temperature to room temperature and further applying the heat treatment in step 74, an LN crystal having a stronger c-axis orientation and improved crystallinity can be formed on the Z-cut quartz substrate. In FIG. 10(a), the diffraction peak of LiNb ₃ O ₈ (−602) derived from LiNb ₃ O ₈ is detected, so this LN crystal film contains LiNb ₃ O ₈ . it is conceivable that. This is considered to be due to the fact that Li in the LN crystal was evaporated at a certain rate in the form of Li ₂ O by the heat treatment (see, for example, Non-Patent Document 6).

In this example, the film formation temperature of the LN crystal was set to room temperature. Effective. In addition, although the temperature of the heat treatment after film formation is set to 600° C. in this embodiment, the same effect can be obtained as long as the temperature is in the range of 500 to 650° C. At a temperature below this, the crystallization of the LN crystal film is insufficient, and conversely, if the heat treatment is performed at a temperature above this, re-evaporation of Li in the LN crystal occurs.

(Third embodiment)
A second embodiment of the present disclosure will be described below with reference to the drawings. This embodiment is an example in which an STO (111) single crystal is used as a substrate and a c-axis oriented LN crystal film is formed thereon.

FIG. 11 is a flowchart illustrating a method for depositing a c-axis oriented LN crystalline film on STO (111) according to one embodiment of the present disclosure. The method of forming a c-axis oriented LN crystal film on STO (111) according to the present embodiment includes step 111 of preparing a single crystal substrate of STO (111), and a film forming apparatus for forming an LN crystal. After depressurizing the film formation chamber with a vacuum pump, argon plasma is generated without applying potential to the LN target, and step 112 of removing moisture adsorbed on the wall surface in the film formation chamber, ECR plasma is generated, and a step 113 of applying RF power to the LN target to generate a sputtering phenomenon on the LN target surface, depositing particles emitted from the LN target onto the STO (111). In this embodiment, the film formation temperature was set to 350° C. in the LN crystal film formation shown in step 113 . Other film formation conditions such as the oxygen flow rate were the same as the film formation conditions for the LN crystal in the first and second embodiments. Also, the film thickness of the LN crystal was set to be the same at 1 μm. The crystallinity of the LN crystal deposited on the STO (111) was evaluated by XRD in the same manner as in the first and second embodiments.

FIG. 12 is an XRD pattern of a stack of LN crystals deposited on STO(111) in an embodiment of the present disclosure. FIG. 12(a) shows an XRD pattern with a scan mode of θ-2θ, FIG. 12(b) shows a GIXRD pattern, and FIG. 12(c) shows a rocking curve for the peak of LiNb ₃ O ₈ (−602). ing. As shown in FIG. 12(a), only LN (006) was detected as a diffraction peak derived from the LN crystal, indicating that the LN crystal is c-axis oriented. Diffraction peaks of LiNb ₃ O ₈ (−301) and LiNb ₃ O ₈ (−602) derived from LiNb ₃ O ₈ are also detected. As described above, the conditions for forming the LN crystal film in this embodiment are not significantly different from those in the first and second embodiments. Therefore, the formation of _LiNb3O8 , which is limited only to the case of STO ₍ 111), can be attributed not to the deposition process, but rather to the chemical interaction between the deposited LN crystal film and STO ( For example, Li atoms diffuse into STO). Since the LN (006) plane and the LiNb ₃ O ₈ (−602) plane are in an epitaxial relationship (see, for example, Non-Patent Document 6), it is natural for both to coexist. That is, it can be said that the film formed according to this embodiment is a mixed film in which LiNb ₃ O ₈ and LN coexist (hereinafter referred to as LiNb ₃ O ₈ /LN). Further, in the GIXRD pattern of FIG. 12(b), no diffraction peak corresponding to the c-axis direction is detected, and in the rocking curve shown in FIG. was observed, it was confirmed that this LiNb ₃ O ₈ /LN was epitaxially grown.

From the above, it can be said that an epitaxial film of LiNb ₃ O ₈ /LN with c-axis orientation can be formed according to the present embodiment. It has been confirmed that the XRD pattern of the deposited LiNb ₃ O ₈ /LN crystal film exhibits a diffraction pattern similar to that of FIG. . That is, in the method of forming the LiNb ₃ O ₈ /LN crystal film according to the present embodiment, the same effect can be obtained if the film forming temperature is between 350 and 550°C.

It should be noted that by doping STO used as a substrate in this embodiment with niobium (Nb) to such an extent that the crystal structure does not collapse, it is possible to make the STO a transparent conductive film, thereby increasing the electrical conductivity. . With such a configuration, it can be applied to a waveguide type device utilizing the electro-optic effect.

In a waveguide type optical element using an LN crystal as a core, it is expected to be put into practical use as an element with higher signal light transmission efficiency than before, and as an element using the electro-optic effect.

Claims

A deposition method for depositing a crystal thin film of c-axis oriented lithium niobate on a substrate using an electron cyclotron resonance (ECR) plasma sputtering method, comprising:
providing the substrate;
After reducing the pressure in the film forming chamber of the film forming apparatus for carrying out the ECR plasma sputtering method, argon plasma is generated in a state in which no potential is applied to the lithium niobate target, so that the wall surface of the film forming chamber of the film forming apparatus is A step of desorbing the moisture adsorbed on the
ECR plasma is generated while supplying argon and oxygen to the film forming chamber of the film forming apparatus, and high frequency (RF) power is applied to the lithium niobate target to cause a sputtering phenomenon on the surface of the lithium niobate target. depositing particles of the lithium niobate target onto the substrate by generating;
with
said substrate is either ZnO/ SiO2 , Z-cut quartz, or (111) oriented strontium titanate (STO) single crystal, and depositing particles of said lithium niobate target onto said substrate. 2. A method of depositing a lithium niobate crystal thin film according to claim 1, wherein the lithium niobate crystal thin film is deposited on the substrate in a c-axis orientation.
wherein the substrate is ZnO/ SiO2 ;
The step of preparing the substrate comprises:
a step of thermally oxidizing a (111)-oriented Si single crystal to form the substrate in which SiO 2 is formed on the surface layer of the Si;
After reducing the pressure in the film forming chamber of the film forming apparatus for forming a ZnO film, argon plasma is generated in a state in which no potential is applied to the Zn target or the ZnO target. a step of removing moisture adsorbed on the wall surface in the deposition chamber;
ECR plasma is generated in the deposition chamber of the deposition apparatus for depositing the ZnO, RF power is applied to the Zn target or the ZnO target, and a sputtering phenomenon occurs on the surface of the Zn target or the ZnO target. depositing the ZnO particles on the SiO2 heated to 400-500° C. by generating
with
depositing the ZnO particles on the SiO2 heated to 400 to 500° C., a crystalline film of the ZnO having a c-axis orientation and a thickness of 70 to 100 nm on the SiO2 ; 2. The method of forming a lithium niobate crystal thin film according to claim 1, wherein the film is formed by
The substrate is the Z-cut quartz,
2. The lithium niobate according to claim 1, wherein the step of depositing the particles of the lithium niobate target on the substrate is performed while the temperature of the Z-cut quartz is maintained at 460.degree. A method for forming a crystal thin film.
The substrate is the Z-cut quartz,
The step of depositing particles of the lithium niobate target on the substrate is performed while the temperature of the Z-cut quartz is maintained within a range of room temperature to 300° C.,
2. The method of claim 1, wherein the film forming method further comprises a step of performing a heat treatment of holding the substrate on which the lithium niobate crystal thin film is formed at a temperature in the range of 500° C. to 650° C. for 1 hour. 2. The method for forming a lithium niobate crystal thin film according to 1 above.
the substrate is a single crystal of the STO;
The step of depositing the particles of the lithium niobate target on the substrate is performed while the temperature of the STO single crystal is maintained within a range of 350°C to 550°C. Item 2. A method for forming a lithium niobate crystal thin film according to item 1.
a substrate;
a c-axis oriented lithium niobate crystal thin film deposited on the substrate;
A laminate containing a lithium niobate crystal thin film comprising
A laminate, wherein the substrate is either c-axis oriented ZnO/SiO 2 , Z-cut quartz, or (111) oriented strontium titanate (STO) single crystal.
the substrate is the ZnO/ SiO2 ;
7. The laminate according to claim 6, wherein said ZnO is a transparent conductive film.
the substrate is the ZnO/ SiO2 ;
8. Laminate according to claim 6 or 7, characterized in that said ZnO is Al-doped AZO or Ga-doped GZO.
the substrate is the Z-cut quartz;
7. The c-axis oriented lithium niobate crystal thin film formed on the substrate is a mixed film in which the c - axis oriented LiNb3O8 and LiNbO3 coexist. The laminate according to .
the substrate is a single crystal of the STO;
7. The c-axis oriented lithium niobate crystal thin film formed on the substrate is a mixed film in which the c - axis oriented LiNb3O8 and LiNbO3 coexist. The laminate according to .
the substrate is a single crystal of the STO;
11. The laminate according to claim 6, wherein the STO single crystal is a Nb-doped transparent conductive film.