JP5093084B2 - Method for producing potassium niobate thin film - Google Patents

Description

  The present invention relates to a method for manufacturing a potassium niobate single crystal thin film, a surface acoustic wave device, a frequency filter, a frequency oscillator, an electronic circuit, and an electronic apparatus.

With the remarkable development of the communication field centering on mobile communications such as mobile phones, the demand for surface acoustic wave elements used in these fields is rapidly expanding. The direction of development of the surface acoustic wave device is in the direction of downsizing, higher efficiency, and higher frequency, similar to mobile phones, etc. For this purpose, a larger electromechanical coupling coefficient (hereinafter referred to as k ² ), larger Surface acoustic wave propagation velocity is required. For example, when used as a high frequency filter, a high k ² is desired to obtain a passband with a small loss and a wide bandwidth. In order to increase the resonance frequency, there is a limit to the design rules for forming the pitch of an inter-digital electrode (hereinafter referred to as IDT), and therefore, a material with a higher sound speed is desired. Furthermore, in order to obtain stabilization of characteristics in the operating temperature region, it is necessary that the center frequency temperature coefficient (TCF) is small.

Conventionally, a structure in which an IDT is formed on a single crystal of a piezoelectric material has been used for a surface acoustic wave device. Typical examples of the piezoelectric single crystal include quartz, lithium niobate (hereinafter, LiNbO ₃ ), lithium tantalate (hereinafter, LiTaO ₃ ), and the like. For example, LiNbO ₃ having a large k ² is used in the case of an RF filter that requires a wide band and a low loss in the pass band. On the other hand, in the case of an IF filter that requires stable temperature characteristics even in a narrow band, a crystal having a small TCF is used. Furthermore, LiTaO ₃ in which k ² and TCF are respectively between LiNbO ₃ and quartz plays an intermediate role. However, even the largest LiNbO ₃ of ^{k 2,} were k 2 of about 20%.

Recently, in a single crystal of potassium niobate (hereinafter referred to as KNbO ₃ ) (a = 0.5695 nm, b = 0.5721 nm, c = 0.3973 nm, hereinafter according to the present exponent display as an orthorhombic crystal), a large k ² A cut angle indicating a value was found. It was predicted by calculation that the 0 ° Y-cut X propagation (hereinafter, 0 ° Y-X) KNbO ₃ single crystal plate exhibits a very large value of k ² = 53% (see, for example, Non-Patent Document 1). ). Also, it was confirmed in experiments that the 0 ° Y-XKNbO ₃ single crystal plate exhibits a large value of k ² to 50%, and the filter using the rotated Y-XKNbO ₃ single crystal plate from 45 ° to 75 ° It has been reported that the oscillation frequency exhibits zero temperature characteristics near room temperature (see, for example, Non-Patent Document 2). These single crystal plates are used as surface acoustic wave substrates (see, for example, Patent Document 1).

In a surface acoustic wave device using a piezoelectric single crystal substrate, characteristics such as k ² , temperature coefficient, and sound velocity are values specific to the material and are determined by a cut angle and a propagation direction. Although the 0 ° Y-XKNbO ₃ single crystal plate is excellent in k ² , the zero temperature characteristics of the rotated Y-XKNbO ₃ single crystal plate from 45 ° to 75 ° do not show near room temperature. In addition, the propagation speed is slower than that of the same perovskite oxide, strontium titanate (hereinafter, SrTiO ₃ ) and calcium titanate (hereinafter, CaTiO ₃ ). Thus, the high sound speed, high k ² , and zero temperature characteristics cannot all be satisfied only by using the KNbO ₃ single crystal plate.

Therefore, it is expected that a piezoelectric thin film is deposited on some substrate and the film thickness is controlled to improve sound speed, k ² , and temperature characteristics. Examples include those in which a zinc oxide (ZnO) thin film is formed on a sapphire substrate (for example, see Non-Patent Document 3) and those in which a LiNbO ₃ thin film is formed (for example, see Non-Patent Document 4). Therefore, it is expected that KNbO ₃ is also thinned on the substrate to improve all the characteristics.

Here, the piezoelectric thin film is preferably oriented in the optimum direction in order to extract its k ² and temperature characteristics, and is a flat and dense epitaxial film in order to minimize loss due to leaky wave propagation. It is desirable. Here, the k ² to 50% Y-XKNbO ₃ thin film corresponds to pseudo cubic (100), and the k ² to 10% 90 ° Y-XKNbO ₃ thin film corresponds to pseudo cubic (110). . Therefore, for example, by using a SrTiO ₃ (100) or (110) single crystal substrate, a K ² to 50% Y-XKNbO ₃ thin film or a k ² to 10% 90 ° Y-XKNbO ₃ thin film can be obtained. it can.

When a KNbO ₃ thin film is formed by a general thin film formation method such as a conventional vapor phase method or a sol-gel method, the saturated vapor pressure of K is significantly higher than that of Nb, so that K evaporates as compared with Nb. Easily, the composition in the thin film after fabrication shifts to the Nb-excess side compared to the starting composition. In order to compensate for this composition shift, a target with excessive K in advance was used (for example, see Non-Patent Document 5).
However, as is clear from the binary phase diagram of K ₂ O—Nb ₂ O ₅ shown in FIG. 19 (see, for example, Non-Patent Document 6), the K excess composition side of KNbO ₃ has 3K ₂ O · Nb. ₂ O ₅ compound is present, coexist as KNbO ₃ and _{3K 2 O · Nb 2 O KNbO} 3 at eutectic temperature 845 ° C. below ₅ and _3K 2 O · _Nb 2 _{O 5} are both solid phase, Nb of KNbO ₃ There is a 2K ₂ O.3Nb ₂ O ₅ compound on the excess composition side, and KNbO ₃ and 2K ₂ O.3Nb ₂ O ₅ coexist as solid phases at a melting point of KNbO ₃ of 1039 ° C. or lower. Therefore, in the case of manufacturing by the vapor phase method, if the composition of K: Nb = 50: 50 is not strictly present when the starting material ablated by the laser reaches the substrate, the Nb excess will be exceeded even if it shifts to the K excess side. Even if it is shifted to the side, the produced thin film contains a different phase, and a single phase cannot be obtained.

On the other hand, in the case of a KNbO ₃ bulk single crystal, a large single crystal is obtained by pulling with a seed crystal from a liquid phase having a slightly K excess composition than K: Nb = 50: 50 by the Top Seed Solution Growth (TSSG) method or the like. (For example, refer nonpatent literature 7.). In this case, in the K ₂ O—Nb ₂ O ₅ binary phase diagram shown in FIG. 19, K ₂ O: Nb ₂ O ₅ = 50: 50 to K ₂ O: Nb ₂ O ₅ = 65: 35 or so. It is obtained by placing a starting material having an intermediate composition in the coexistence region of KNbO ₃ and a liquid phase existing at a eutectic temperature of 845 ° C. or higher between KNbO ₃ and 3K ₂ O · Nb ₂ O ₅ . That is, in FIG. 20, when the starting material having the composition C ₁ is cooled from the liquidus temperature T ₁ to the crystal growth temperature T ₂ , KNbO ₃ is precipitated from the liquid phase, and the liquid phase shows T ₂ as the liquidus. up composition C ₂ to a temperature shifted to K-rich side. Since the crystal growth rate at this time becomes faster as C ₁ -C ₂ is larger, KNbO ₃ and 3K ₂ O · Nb ₂ O ₅ having a composition close to KNbO ₃ but slightly K-excess than KNbO ₃ are used as much as possible. The eutectic temperature is cooled to around 845 ° C. However, the above behavior is in the atmosphere, and is also in the case where a KNbO ₃ bulk single crystal is grown from a large amount of liquid phase.

On the other hand, a method has been developed in which a crystal growth process in which a single crystal is precipitated from a liquid phase in the atmosphere by the TSSG method is applied to a thin film production process by a vapor phase method under reduced pressure. Tri-Phase-Epitaxy method is one such method, is deposited on the substrate holding the vapor temperature of the solid-liquid coexisting region, in which precipitating solid phase from the liquid phase, NdBa ₂ An application example has been reported in which a single crystal thin film is obtained by selectively etching only the residue of the liquid phase BaCuO ₂ .CuO after the growth of the single crystal thin film in the Cu ₃ O _x material (see, for example, Non-Patent Document 8).
Eletron. Lett., Vol.33 (1997) 193 Jpn. J. Appl. Phys., Vol. 37 (1998) 2929 Jpn. J. Appl. Phys., Vol. 32 (1993) 2337 Jpn. J. Appl. Phys., Vol. 32 (1993) L745 Appl. Phys. Lett. Vol.68 (1996) 1488 J. Am. Chem. Soc. Vol. 77 (1955) 2117 J. Crystal Growth Vol. 78 (1986) 431 Appl. Phys. Lett. Vol. 80 (2002) 61

However, only the selective residue above be directly applied to conventional Tri-Phase-Epitaxy method KNbO ₃ method for producing a single crystal thin film, KNbO ₃ liquid phase _3K after a single crystal thin film growth 2 _O · _Nb 2 O ₅ It was impossible to etch. Therefore, a liquid phase remains on the surface of the single crystal and a thin film having excellent surface morphology cannot be obtained.
The present invention has been made in view of the above circumstances, a method for producing a single-phase high-quality KNbO ₃ single crystal thin film having excellent surface morphology on various single crystal substrates, and the thin film obtained by this method. It is an object of the present invention to provide a surface acoustic wave device, a frequency filter, a frequency oscillator, an electronic circuit, and an electronic device that have a high k ^{2 and} are excellent in wideband, downsizing, and power saving.

The present invention employs the following means in order to solve the above problems.
The method for producing a potassium niobate thin film of the present invention comprises a step of depositing a potassium niobate layer under conditions where the oxygen partial pressure is 1.33 × 10 ⁻¹ Pa to 13.3 Pa and the substrate temperature is 750 ° C. or less. And heating the substrate temperature to 850 ° C. or more, and forming the potassium niobate layer from K _x Nb _1-x O ₃ (0.5 ≦ x ≦ 0.65). K _x Nb _1-x O ₃ (0.5 ≦ x ≦) under the conditions of the process and the oxygen partial pressure of 1.33 × 10 ⁻¹ Pa to 13.3 Pa and the substrate temperature of 750 ° C. to 850 ° C. 0.65).
The method for producing a potassium niobate single crystal thin film is a method for producing a potassium niobate single crystal thin film by a vapor phase method, and is a eutectic of potassium niobate and 3K ₂ O · Nb ₂ O ₅ at a predetermined oxygen partial pressure. When the temperature and molar composition ratio at the point E are T _E and x _E (x is the molar ratio of potassium (K) and niobium (Nb) when expressed by K _x Nb _1-x O _y ), A raw material in a vapor phase is supplied to the substrate so that x immediately after being deposited on the substrate is in a range of 0.5 ≦ x ≦ x _E , and the oxygen partial pressure and the complete melting temperature at x are set. When T _m is set, the temperature T _s of the substrate is maintained in the range of T _E ≦ T _s ≦ T _m , and a precipitation step for depositing potassium niobate single crystal and a solid phase portion and a liquid phase portion coexist. evaporation step for evaporating the _K x _{Nb 1-x O y} from the liquid phase portion of the Characterized in that it comprises.
Further, in the present invention, it is preferable that the potassium niobate single phase single crystal thin film is continuously grown by repeating the evaporation step and the precipitation step.
According to this method, after depositing a potassium niobate single crystal from K _x Nb _1-x O _y in which a solid phase portion and a liquid phase portion deposited on a substrate coexist, a residual liquid having a composition deviation As a result, the single-layer potassium niobate single crystal with suppressed compositional deviation can be deposited. Thereby, a potassium niobate single crystal thin film having excellent surface morphology can be obtained, and by using such a potassium niobate single crystal thin film, a surface acoustic wave device excellent in k ² can be produced. it can.

In the present invention, it is preferable that the substrate has a crystal axis oriented on both the vertical and in-plane directions of the substrate surface, and the single crystal is epitaxially grown on the substrate.
According to this method, it is possible to obtain a potassium niobate single crystal thin film having a uniform orientation throughout the entire thin film using the substrate as a seed crystal. Accordingly, the surface acoustic wave device excellent in k ² can be obtained from this potassium niobate single crystal thin film. Can be produced.

The present invention uses a substrate having a thermal expansion coefficient larger than that of potassium niobate and having a perovskite structure pseudo-cubic unit cell with (100) orientation and in-plane orientation over the entire substrate surface. Is preferred.
Moreover, it is preferable to use a strontium titanate (100) single crystal substrate as the substrate.
According to this method, a potassium niobate single crystal can be deposited with orthorhombic (110) orientation on the substrate. Further, using a general-purpose perovskite oxide strontium titanate is single crystal substrate (100) single crystal substrate, it is possible to obtain a potassium niobate single crystal thin film having good surface morphology, furthermore, k ² is An excellent surface acoustic wave device of up to about 30% can be produced.

In the present invention, a substrate having a thermal expansion coefficient smaller than that of potassium niobate and having a perovskite structure pseudo-cubic unit cell with (100) orientation and in-plane orientation over the whole substrate surface is used. It is characterized by.
Further, it is preferable to use a substrate composed of a silicon single crystal substrate and a buffer layer epitaxially grown on the silicon single crystal substrate.
According to this method, the potassium niobate single crystal epitaxially grown on the substrate can be precipitated in an orthorhombic (001) orientation, and niobium obtained on a silicon single crystal substrate which is an inexpensive single crystal substrate. from potassium single crystal thin film, k ² can be manufactured up to about 50% and excellent surface acoustic wave device.

In the present invention, as the buffer layer, a first buffer layer made of NaCl type oxide and a second buffer layer made of simple perovskite type oxide epitaxially grown on the first buffer layer are produced. It is characterized by doing.
Further, as the buffer layer, a first buffer layer made of a fluorite oxide, a layered perovskite oxide epitaxially grown on the first buffer layer, and an epitaxial growth on the layered perovskite oxide A second buffer layer composed of a simple perovskite oxide is manufactured.
According to this method, since a suitable buffer layer is formed between the silicon single crystal and the potassium niobate single crystal, the potassium niobate single crystal is formed even on the silicon single crystal substrate which is an inexpensive single crystal substrate. the can be a film, it is possible to obtain an excellent surface acoustic wave device of close values from the potassium niobate single crystal thin film by theory about 50% of k ^2.

In the present invention, the substrate is composed of a substrate body made of any material of quartz, quartz, SiO ₂ coated silicon, and diamond coated silicon, and a buffer layer formed on the substrate body. The buffer layer used is a first buffer layer grown in-plane on the substrate regardless of the crystal orientation of the substrate surface, and a second buffer made of oxide epitaxially grown on the first buffer layer. The layer is manufactured by a vapor phase method involving ion beam irradiation.
According to this method, a high-quality potassium niobate single crystal thin film is formed on a substrate made of a material such as quartz, quartz, SiO ₂ -coated silicon, or diamond-coated silicon that is suitable for a surface acoustic wave device and inexpensive. it can be from the potassium niobate single crystal thin film, k ² can be obtained up to about 50% and excellent surface acoustic wave device.

The present invention is characterized in that the first buffer layer is made of a NaCl type oxide and the second buffer layer is made of a simple perovskite type oxide.
The first buffer layer is made of a fluorite oxide, and the second buffer layer is made of a layered perovskite oxide and a simple perovskite oxide epitaxially grown on the layered perovskite oxide. It is characterized by that.
According to this method, a high-quality potassium niobate single crystal thin film can be formed on a substrate of any material such as inexpensive quartz, quartz, SiO ₂ -coated silicon, diamond-coated silicon, etc. An excellent surface acoustic wave device having a value closer to the theoretical value of k ² of about 50% can be obtained from the potassium single crystal thin film.

The surface acoustic wave device of the present invention includes a potassium niobate single crystal thin film manufactured by the manufacturing method according to the present invention.
According to the surface acoustic wave device, since the potassium niobate single crystal thin film having a large k ² is provided, the surface acoustic wave device can be downsized.

The frequency filter of the present invention includes the surface acoustic wave device according to the present invention.
A frequency oscillator according to the present invention includes the surface acoustic wave device according to the present invention.
According to this frequency filter and frequency oscillator, it is possible to realize a small filter and a wide band of filter characteristics.

An electronic circuit according to the present invention includes the frequency oscillator according to the present invention.
This frequency oscillator has a wide band filter characteristic, is small in size, and can cope with power saving.
An electronic apparatus according to the present invention includes at least one of the frequency filter, the frequency oscillator, and the electronic circuit according to the present invention.
According to this electronic device, it is possible to improve the miniaturization, the broadband, and the power saving.

Next, a first embodiment of the present invention will be described with reference to FIGS.
As shown in FIG. 1D, the KNbO ₃ single crystal thin film 10 according to this embodiment is epitaxially grown on the substrate 11 and the orthorhombic (001) or orthorhombic (110) orientation on the substrate 11. And a KNbO ₃ single crystal layer 12.
The substrate 11 includes a SrTiO ₃ single crystal substrate 13 having crystal axes of (100) orientation in the direction perpendicular to the surface and (001) orientation in the plane, and K grown epitaxially on the SrTiO ₃ single crystal substrate 13, And an initial layer 14 made of Nb, Sr, Ti, or O.
The thermal expansion coefficient of the SrTiO ₃ single crystal substrate 13 is 11.1 × 10 ⁻⁶ (K ⁻¹ ), and the thermal expansion coefficient of the c-axis of the KNbO ₃ single crystal layer 12 is 14.1 × 10 ⁻⁶ (K Although it is smaller than ⁻¹ ), the thermal expansion coefficients of the a and b axes are larger than 5.0 × 10 ⁻⁶ (K ⁻¹ ) and 0.5 × 10 ⁻⁶ (K ⁻¹ ).
The initial layer 14 is an interface layer for transferring the structure from SrTiO ₃ to KNbO ₃ , and the structure and composition are not limited, but may be epitaxially grown on the SrTiO ₃ single crystal substrate 13.

The KNbO ₃ single crystal layer 12 is formed by a vapor phase method involving ion beam irradiation. In this embodiment, a thin film is produced by a pulsed laser deposition (PLD) method. As shown in FIG. 2, a film forming apparatus 15 used at the time of film formation includes a process chamber 16 capable of reducing the pressure inside, a film forming base material 17 disposed facing the SrTiO ₃ single crystal substrate 13, A base material supporting part 18 on which the film forming base material 17 is placed and made self-revolving is provided, and a holding part 19 for holding the SrTiO ₃ single crystal substrate 13 is provided.
The film forming apparatus 15 includes a RHEED source 21 used when analyzing the thin film 20 by a reflection high energy electron diffraction (abbreviated as RHEED) method, and the RHEED source 21 to the SrTiO ₃ single crystal substrate 13. And an RHEED screen 22 for detecting a beam incident on and reflected by the thin film 20 deposited thereon.

In the PLD method, while a thin film is formed on a substrate, an oxygen atmosphere in which the internal space of the process chamber 16 is set to a very low pressure, for example, a pressure that is about one thousandth of atmospheric pressure, ArF or KrF excimer laser beam 23 is pulsedly irradiated to the rotating film forming base material 17, and components constituting the film forming base material 17 by this irradiation are plasma plume (plasma or molecular state) 24. As a film forming method in which the thin film 20 is deposited on the film formation surface by flying up to the SrTiO ₃ single crystal substrate 13.

Next, a method for manufacturing the KNbO ₃ single crystal thin film 10 according to the present embodiment will be described.
In this manufacturing method, when the temperature and molar composition ratio at the eutectic point E of KNbO ₃ and 3K ₂ O · Nb ₂ O ₅ at a predetermined oxygen partial pressure are T _E and x _E (x is K _x Nb The molar composition ratio of potassium (K) and niobium (Nb) expressed by _1-x O _y ), and the composition x in the liquid phase immediately after being deposited on the substrate 11 is 0.5 ≦ x ≦ the plasma plume 24 which is a raw material of the gas phase as the range of x _E is supplied to the substrate 11, also when the complete melting temperature at this oxygen partial pressure and the x and T _m, the temperature T _s of the substrate 11 An evaporation step for evaporating the residual liquid of K _x Nb _1-x O _y deposited on the substrate 11 from the plasma plume 24 while maintaining T _E ≦ T _s ≦ T _m , and K _x Nb _1-x from O _y KNbO ₃ single crystal and precipitation step of precipitating on the substrate 11 It is provided.
Hereinafter, a manufacturing method is demonstrated in order.

First, prior to the evaporation step, steps of depositing by supplying _K x _{Nb 1-x O y} on the SrTiO ₃ single crystal substrate 13.
First, the SrTiO ₃ single crystal substrate 13 is immersed in an organic solvent and degreased and cleaned using an ultrasonic cleaner. As the organic solvent, for example, a mixed solution in which ethyl alcohol and acetone are mixed at 1: 1 can be used, but the organic solvent is not limited thereto.

First, the initial layer 14 is formed on the SrTiO ₃ single crystal substrate 13 that has been degreased and cleaned.
After the SrTiO ₃ single crystal substrate 13 is loaded in the holding unit 19, the SrTiO ₃ single crystal substrate 13 is introduced into the process chamber 16, and the degree of vacuum is reduced to 1.33 × 10 ⁻⁶ Pa (1 × 10 ⁻⁸ Torr). Subsequently, oxygen gas is introduced so that the oxygen partial pressure is 1.33 Pa (1 × 10 ⁻² Torr), and the temperature is raised to 500 ° C. at 20 ° C./min using an infrared lamp (not shown). In the RHEED pattern from the SrTiO ₃ <010> direction at this time, a streak-like diffraction pattern is observed as shown in FIG.
In addition, conditions, such as a temperature increase rate, a substrate temperature, and a pressure, are not restricted to this.

After the pressure becomes constant, the molar ratio x between potassium (K) and niobium (Nb) as expressed by K _x Nb _1-x O _y is in the range of 0.5 ≦ x ≦ x _E. A K _0.6 Nb _0.4 O _y film-forming base material 17a of _0.6 is disposed so as to face the SrTiO ₃ single crystal substrate 13 and have a distance of 30 mm or more and 50 mm or less. The substrate temperature is 500 ° C. or higher and 850 ° C. or lower, and the oxygen partial pressure during deposition is 1.33 × 10 ⁻¹ Pa (1 × 10 ⁻³ Torr) or higher and 13.3 Pa (1 × 10 ⁻¹ Torr) or lower. Then, an excimer laser beam 23 having a laser energy density of ² J / cm ² or more and 3 J / cm ² or less and a laser frequency of 1 Hz or less is irradiated on the surface of the film forming base material 17a.

Here, the pulsed light of the KrF excimer laser beam 23 is incident under the conditions of a laser energy density of 2.5 J / cm ² , a laser frequency of 10 Hz, and a pulse length of 10 ns. Then, a plasma plume 24 made of K, Nb, and O is generated on the surface of the film forming base material 17a. The plasma plume 24 is applied to a SrTiO ₃ single crystal substrate 13 disposed at a position 40 mm away from the film forming base material 17a under conditions of a substrate temperature of 500 ° C. and an oxygen partial pressure of 1.33 Pa (1 × 10 ⁻² Torr). Irradiation is performed for 2 minutes to deposit 2 nm of a K _0.6 Nb _0.4 O _y layer 25 as shown in FIG.
The conditions are not limited to the above as long as the plasma plume 24 can sufficiently reach the SrTiO ₃ single crystal substrate 13 and can be epitaxially grown as the initial layer 14.

Since the eutectic point E of KNbO ₃ and 3K ₂ O.Nb ₂ O ₅ at the oxygen partial pressure is 750 ° C. to 800 ° C., the K _0.6 Nb _0.4 O _y layer 25 is epitaxially grown at this stage. Not done. In the RHEED pattern from the SrTiO ₃ <010> direction at this time, it is observed that the diffraction pattern disappears as shown in FIG. Therefore, the temperature is raised to 20 ° C./min using an infrared lamp (not shown) up to the complete melting temperature of 850 ° C. in the oxygen partial pressure and composition.
Then, a spot pattern as shown in FIG. 3C appears as an RHEED pattern at 750 ° C. to 800 ° C.
Above 800 ° C., K _0.6 Nb _0.4 O _y almost evaporates from the coexisting state of the solid-liquid phase of KNbO ₃ , and the remaining KNbO ₃ reacts with the SrTiO ₃ single crystal substrate 13. . Thus, as shown in FIG. 1B, the initial layer 14 made of any one of K, Nb, Sr, Ti, and O is epitaxially grown.

Next, the evaporation step and the precipitation step will be described.
The substrate temperature is 750 ° C. or more and 850 ° C. or less, and the oxygen partial pressure during deposition is 1.33 × 10 ⁻¹ Pa (1 × 10 ⁻³ Torr) or more and 13.3 Pa (1 × 10 ⁻¹ Torr) or less. , Excitation laser beam 23 having a laser energy density of ² J / cm ² or more and 3 J / cm ² or less and a laser frequency of 5 Hz or less is irradiated on the surface of the film forming base material 17a of K _0.6 Nb _0.4 O _y To do.

Here, the pulsed light of the KrF excimer laser beam 23 is incident under the conditions of a laser energy density of 2.5 J / cm ² , a laser frequency of 10 Hz, and a pulse length of 10 ns. Then, a plasma plume 24 made of K, Nb, and O is generated on the surface of the film forming base material 17a. The plasma plume 24 is disposed at a position 40 mm away from the film forming base material 17a, and the SrTiO ₃ single crystal substrate 13 is in a condition of a substrate temperature of 800 ° C. and an oxygen partial pressure of 1.33 Pa (1 × 10 ⁻² Torr). Is irradiated for 60 minutes to deposit a K _0.6 Nb _0.4 O _y layer 25a of 200 nm as shown in FIG. The solid phase portion 26 and the liquid phase portion 27 coexist in the K _0.6 Nb _0.4 O _y layer 25a immediately after deposition.

Here, since the KrF excimer laser beam 23 is pulsed light, the plasma plume 24 is intermittently supplied to the SrTiO ₃ single crystal substrate 13.
During this pulse supply, the KNbO ₃ single crystal layer 12 is precipitated from the solid phase portion 26 and the remaining liquid phase portion 27 is evaporated as shown in FIG. To do.
Thus, the KNbO ₃ single crystal layer 12 is epitaxially grown by 200 nm.
When the obtained KNbO ₃ single crystal layer 12 is observed, the diffraction pattern shown in FIG. 3D and the smooth surface state shown in FIG. 4 can be confirmed.
From this result and the X-ray diffraction result shown in FIG. 5, when KNbO ₃ and SrTiO ₃ are indicated by orthorhombic and cubic indices, respectively, KNbO ₃ (110) / SrTiO ₃ (100) is perpendicular to the film surface. It can be confirmed that the KNbO ₃ single crystal thin film 10 having an orientation relationship of KNbO ₃ <001> // SrTiO ₃ <001> in the in-plane direction can be obtained.
Here, KNbO ₃ is orthorhombic (110) oriented because it receives a compressive stress from the SrTiO ₃ single crystal substrate 13 having a relatively large thermal expansion coefficient in the temperature lowering process, and a, b, c, This is because the c-axis having the largest coefficient of thermal expansion is oriented in the substrate plane.
If the plasma plume 24 can sufficiently reach the SrTiO ₃ single crystal substrate 13 and the evaporation rate balance between the deposition rate and the liquid phase part 27 can be taken so that the liquid phase part 27 does not remain, each condition is as described above. It is not limited.
In the above-described deposition step, the film forming base material 17a is K _0.6 Nb _0.4 O _y with x = 0.6, but within the range of 0.5 ≦ x ≦ x _E under the oxygen partial pressure. Then, a similar KNbO ₃ single crystal thin film can be obtained.

Next, the surface acoustic wave device 28 according to this embodiment will be described.
As shown in FIG. 6, the surface acoustic wave device 28 includes the above-described KNbO ₃ single crystal thin film 10.
Hereinafter, a method for manufacturing the surface acoustic wave element 28 will be described.
First, a pair of KNbO ₃ single crystal thin films 10 is formed on the KNbO ₃ single crystal thin film 10 under conditions of a substrate temperature of 45 ° C. and a degree of vacuum of 6.65 × 10 ⁻⁵ Pa (5 × 10 ⁻⁷ Torr) by vacuum deposition using metallic aluminum (Al). Al electrodes 29a and 29b are deposited.
The substrate temperature and the degree of vacuum are not limited to these.
Next, a continuous process of a patterning process by resist application, exposure, dry etching, and resist removal is performed on the Al electrodes 29a and 29b to form a pair of IDTs 30a and 30b.
In this way, the surface acoustic wave element 28 is manufactured.

In the surface acoustic wave device 28 obtained, IDT30a, speed of sound determined from the delay time _{V open} of the surface acoustic wave propagating between 30b was 4000 m / s. Further, IDT30a, ^{k 2} obtained between 30b from the difference between the delay time _{V short} of the surface acoustic wave when covered with a metal thin film was 25%.
When the KNbO ₃ single crystal thin film was not used, k ² obtained was about 10% even when the sound speed was 4000 m / s, and thus a sufficiently large k ² value could be obtained.
In addition, even if potassium sodium tantalate niobate is used instead of potassium niobate as the film forming base material 17a, K _1-x Na _x Nb _1-y Ta _y O ₃ (0 ≦ x ≦ 1, 0 ≦ y A solid solution thin film of ≦ 1) is similarly obtained.

According to this method for manufacturing a single crystal thin film of KNbO _3, a single phase with a suppressed compositional deviation is also provided on the SrTiO ₃ (100) single crystal substrate 13 which is a perovskite oxide single crystal substrate which is a general-purpose substrate. The KNbO ₃ single crystal can be precipitated with an orthorhombic (110) orientation, and the KNbO ₃ single crystal thin film 10 having excellent surface morphology can be obtained. In addition, the surface acoustic wave device 28 excellent in k ² can be manufactured from the KNbO ₃ single crystal thin film 10.

Next, a second embodiment according to the present invention will be described with reference to FIGS. In the following description, the same reference numerals are given to the components described in the above embodiment, and the description thereof is omitted.
The difference from the second embodiment is the first embodiment, KNbO ₃ single crystal thin film 10 of the first embodiment, KNbO ₃ on the substrate 11 made of SrTiO ₃ single crystal substrate 13 and the initial layer 14. Whereas the single crystal is manufactured by epitaxial growth, in the second embodiment, on the substrate 31 comprising the silicon (hereinafter referred to as Si) single crystal substrate 31a and the buffer layer 32 epitaxially grown thereon. The KNbO ₃ single crystal is epitaxially grown through the initial layer 14 to manufacture the KNbO ₃ single crystal thin film 33.

The Si single crystal substrate 31 a has a thermal expansion coefficient of 3.0 × 10 ⁻⁶ (K ⁻¹ ), and the thermal expansion coefficient of the b-axis of the KNbO ₃ single crystal layer 12 is 0.5 × 10 ⁻⁶ (K ^The coefficient of thermal expansion of the a and c axes is smaller than 5.0 × 10 ⁻⁶ (K ⁻¹ ) and 14.1 × 10 ⁻⁶ (K ⁻¹ ). The substrate surface is coated with a natural oxide film.
As shown in FIG. 7, the buffer layer 32 includes a first buffer layer 34 and a second buffer layer 35 epitaxially grown on the first buffer layer 34.
The first buffer layer 34 includes a first buffer layer 34a made of yttria-stabilized zirconia (hereinafter referred to as YSZ) and a first buffer layer 34b made of CeO ₂ epitaxially grown on the first buffer layer 34a. Yes.

The first buffer layer 34a and the first buffer layer 34b are made of a metal oxide. Examples of the metal oxide include a metal oxide having a NaCl structure or a fluorite structure. Among these, at least one of MgO, CaO, SrO, BaO, or a solid solution containing these, including a metal that thermodynamically bonds to oxygen more than Si, or YSZ, CeO ₂ , ZrO ₂ , or these It is preferable to use at least one of solid solutions containing Here, YSZ is epitaxially grown with cubic (100) orientation as the first buffer layer 34a, and CeO ₂ is epitaxially grown with cubic (100) orientation as the first buffer layer 34b.

The second buffer layer 35 is formed on a second buffer layer 35a obtained by epitaxially growing YBa ₂ Cu ₃ O _x that is a layered perovskite oxide in a tetragonal or orthorhombic (001) orientation, and the second buffer layer 35a. The second buffer layer 35b is formed by epitaxially growing SrTiO ₃ , which is a simple perovskite oxide, with a cubic (100) orientation.
The KNbO ₃ single crystal layer 12 is an orthorhombic crystal (110) or (001) epitaxially grown on the initial layer 14 made of any of K, Nb, Sr, Ti, and O epitaxially grown on the second buffer layer 35. ) Oriented KNbO ₃ single crystal.
When the first buffer layer 34 is made of a metal oxide having a NaCl structure such as MgO, the same effect can be obtained even if only SrTiO ₃ is epitaxially grown in a cubic (100) orientation as the second buffer layer 35. Can be obtained.

Next, a method for manufacturing the KNbO ₃ single crystal thin film 33 according to this embodiment will be described.
Also in the present embodiment, as in the first embodiment, an evaporation step of evaporating the residual liquid of K _x Nb _1-x O _y deposited on the substrate 31 from the plasma plume 24, and K _x Nb _1-x And a precipitation step of depositing a KNbO ₃ single crystal on the substrate 31 from O _y .
Hereinafter, a manufacturing method is demonstrated in order.

  The Si single crystal substrate 31a is immersed in an organic solvent and degreased and cleaned using an ultrasonic cleaner. As the organic solvent, for example, a mixed solution in which ethyl alcohol and acetone are mixed at 1: 1 can be used, but the organic solvent is not limited thereto. Further, since the natural oxide film is left, it is not necessary to perform a process of removing the natural oxide film such as RCA cleaning or hydrofluoric acid cleaning, which is a typical cleaning method for a normal Si single crystal substrate.

Next, a method for forming the buffer layer 32 by the PLD method using the film forming apparatus 15 shown in FIG. 2 will be described.
First, the first buffer layer 34a is formed on the Si single crystal substrate 31a.
The degreased and cleaned Si single crystal substrate 31a is loaded into the holding unit 19, and then introduced into the process chamber 16 to reduce the pressure to 1.33 × 10 ⁻⁶ Pa (1 × 10 ⁻⁸ Torr), and an infrared lamp (not shown) And heated to 700 ° C. at 10 ° C./min. Since the natural oxide film partially evaporates as SiO in the temperature range of 500 ° C. or higher, the degree of vacuum rises to 1.33 × 10 ⁻⁴ Pa (1 × 10 ⁻⁶ Torr). It becomes a constant value of .65 × 10 ⁻⁵ Pa (5 × 10 ⁻⁷ Torr) or less. It should be noted that the temperature rise rate, the substrate temperature, the pressure, and the like are not limited to these conditions as long as the new thermal oxide film is not formed on the surface of the Si single crystal substrate 31a.
At this time, the surface of the Si single crystal substrate 31a remains covered with a natural oxide film so that no diffraction pattern is observed in the RHEED pattern from the Si <011> direction shown in FIG. Yes.

After the pressure becomes constant, the film forming base material 17a made of YSZ is disposed so as to face the Si single crystal substrate 31a and have a mutual distance of 30 mm or more and 50 mm or less. The substrate temperature is 650 ° C. or higher and 750 ° C. or lower, and the oxygen partial pressure during deposition is 1.33 × 10 ⁻³ Pa (1 × 10 ⁻⁵ Torr) or higher and 1.33 × 10 ⁻² Pa (1 × 10 ^−4). (Torr) or less, the surface of the film forming base material 17a is irradiated with an excimer laser beam 23 having a laser energy density of ² J / cm ² or more and 3 J / cm ² or less and a laser frequency of 5 Hz or more and 15 Hz or less.
At this time, each condition is not limited to the above range as long as Y and Zr plasma can selectively reach the substrate and can be epitaxially grown as YSZ while removing the natural oxide film on the substrate as SiO.
However, depending on conditions, even if the YSZ first buffer layer 34a is formed, oxygen may be supplied to the interface with the Si single crystal substrate 31a to form a new oxide film.
If ZrO ₂ forms a solid solution as a cubic crystal, La, Ce, Pr, Nd, Pm, Sm, Eu, Gd, Tb, Dy, Ho, Er, Any one element of Tm, Yb, Lu, Mg, Ca, Sr, and Ba may be added.

Here, the pulsed light of the KrF excimer laser beam 23 is incident under the conditions of a laser energy density of 2.5 J / cm ² , a laser frequency of 10 Hz, and a pulse length of 10 ns. Then, a plasma plume 24 made of Y, Zr, and O is generated on the surface of the film forming base material 17a. The plasma plume 24 is applied to a Si single crystal substrate 31a disposed at a position 40 mm away from the film forming base material 17a, at a substrate temperature of 700 ° C. and an oxygen partial pressure of 6.65 × 10 ⁻³ Pa (5 × 10 ⁻⁵ Torr). The YSZ first buffer layer 34a is epitaxially grown by 5 nm as shown in FIG. Epitaxial growth can be confirmed from the fact that a streak-like diffraction pattern appears in the RHEED pattern from the Si <011> direction shown in FIG.

Subsequently, the first buffer layer 34b is formed.
The base material support portion 18 is rotated so that the film formation base material 17b made of CeO ₂ is positioned to face the Si single crystal substrate 31a. The surface of the film forming base material 17b is irradiated with pulsed light of a KrF excimer laser beam 23 as described above. The irradiation conditions at this time are the same as in YSZ.
Here, the laser energy density is 2.5 J / cm ² , the laser frequency is 10 Hz, and the pulse length is 10 ns.
Then, a plasma plume 24 made of Ce and O is generated on the surface of the film forming base material 17b. The plasma plume 24 is applied to a Si single crystal substrate 31a disposed at a position 40 mm away from the film forming base material 17b, at a substrate temperature of 700 ° C. and an oxygen partial pressure of 6.65 × 10 ⁻³ Pa (5 × 10 ⁻⁵ Torr). ) For 10 minutes to epitaxially grow the CeO ₂ first buffer layer 34b by 10 nm as shown in FIG. Epitaxial growth can be confirmed from the fact that spot-like diffraction patterns appear in the RHEED pattern from the Si <011> direction shown in FIG.
Each condition is not limited to the above as long as it can be epitaxially grown as CeO ₂ . Further, if CeO ₂ forms a solid solution as cubic crystals, the same effect can be obtained even if Pr or Zr is added.

Next, the second buffer layer 35a is formed.
The base material support 18 is rotated so that the film forming base material 17c made of YBa ₂ Cu ₃ O _x is positioned to face the Si single crystal substrate 31a. The surface of the film forming base material 17c is irradiated with pulsed light of a KrF excimer laser beam 23 as described above. The irradiation conditions at this time are as follows: the substrate temperature is 550 ° C. or more and 650 ° C. or less, and the oxygen partial pressure during deposition is 1.33 × 10 ⁻¹ Pa (1 × 10 ⁻³ Torr) or more and 13.3 Pa (1 × 10 ^−1). Torr) is the same as in YSZ except that it is less than or equal to Torr).
Here, the laser energy density is 2.5 J / cm ² , the laser frequency is 10 Hz, and the pulse length is 10 ns.
Then, a plasma plume 24 made of Y, Ba, Cu, and O is generated on the surface of the film forming base material 17c. The plasma plume 24 is applied to a Si single crystal substrate 31a disposed at a position 40 mm away from the film forming base material 17c under the conditions of a substrate temperature of 600 ° C. and an oxygen partial pressure of 1.33 Pa (1 × 10 ⁻² Torr). Irradiating for 2 minutes, the YBa ₂ Cu ₃ O _x second buffer layer 35a is epitaxially grown by 2 nm as shown in FIG. Epitaxial growth can be confirmed from the fact that a streak-like diffraction pattern appears in the RHEED pattern from the Si <011> direction shown in FIG.

Each condition is not limited to the above as long as Y, Ba, Cu plasma can reach the substrate at a _constant ratio of 1: 2: 3 and can be epitaxially grown as YBa ₂ Cu ₃ O _x . Further, instead of YBa ₂ Cu ₃ O _x , M ₂ RuO ₄ (M represents any one element of Ca, Sr, Ba), RE ₂ NiO ₄ (RE is La, Ce, Pr, Nd, Pm, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu, Y.) and NiO solid solution, REBa ₂ Cu ₃ O _x (RE is La, Ce) , Pr, Nd, Pm, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, and Lu.), (Bi, RE) ₄ Ti ₃ O ₁₂ (RE = La, Ce, Pr, Nd, Pm, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu, Y represents any one element.
The same effect can be obtained by using ().

Then, the second buffer layer 35b is formed.
The base material support portion 18 is rotated so that the film formation base material 17d made of SrTiO ₃ faces the Si single crystal substrate 31a. The surface of the film forming base material 17d is irradiated with pulsed light of a KrF excimer laser beam 23. The irradiation conditions at this time are as follows: the substrate temperature is 550 ° C. or more and 650 ° C. or less, and the oxygen partial pressure during deposition is 1.33 × 10 ⁻¹ Pa (1 × 10 ⁻³ Torr) or more and 13.3 Pa (1 × 10 ^−1). Torr) is the same as in YSZ except that it is less than or equal to Torr).
Here, the laser energy density is 2.5 J / cm ² , the laser frequency is 10 Hz, and the pulse length is 10 ns.
Then, a plasma plume 24 made of Sr, Ti, and O is generated on the surface of the film forming base material 17d. The plasma plume 24 is placed on a Si single crystal substrate 31a disposed at a position 40 mm away from the film forming base material 17d under conditions of a substrate temperature of 600 ° C. and an oxygen partial pressure of 1.33 Pa (1 × 10 ⁻² Torr). Irradiation is performed for 1 minute, and as shown in FIG. 7, the SrTiO ₃ second buffer layer 35b is epitaxially grown by 100 nm.
Epitaxial growth can be confirmed from the fact that spot-like diffraction patterns appear in the RHEED pattern from the Si <011> direction shown in FIG.

Each condition is not limited to the above as long as Sr, Ti plasma can reach the Si single crystal substrate 31a at a constant ratio of 1: 1 and can be epitaxially grown as SrTiO ₃ . Further, instead of SrTiO ₃ , MTiO ₃ (M represents any one element of Ca and Ba), REAlO ₃ (RE is La, Ce, Pr, Nd, Pm, Sm, Eu, Gd, Tb. , Dy, Ho, Er, Tm, Yb, Lu, and Y.), MAlO ₃ (M represents any one element of Mg, Ca, Sr, and Ba), REGaO ₃ (RE represents any one element of La, Ce, Pr, Nd, Pm, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu, and Y). The same effect can be obtained.

The initial layer 14 is formed on the formed buffer layer 32 by the same method as in the first embodiment described above. However, in the first embodiment, the K _0.6 Nb _0.4 O _y layer is first deposited at 500 ° C. and then heated to 800 ° C. to obtain the initial layer 14. The same effect can be obtained even when deposited at a temperature of 0 ° C.
In the present embodiment, the pulse light of the KrF excimer laser beam 23 is incident on the film forming base material 17e under the conditions of a laser energy density of 2.5 J / cm ² , a laser frequency of 10 Hz, and a pulse length of 10 ns. And the plasma plume 24 which consists of K, Nb, and O used as the raw material of a gaseous-phase state is generated on the surface of the film-forming base material 17e. The plasma plume 24 is applied to a Si single crystal substrate 31a disposed at a position 40 mm away from the film forming base material 17e under the conditions of a substrate temperature of 800 ° C. and an oxygen partial pressure of 1.33 Pa (1 × 10 ⁻² Torr). Irradiating for 5 minutes, a K _0.6 Nb _0.4 O _y layer 25 of 5 nm is deposited as shown in FIG.
Each condition is not limited to the above as long as the plasma plume 24 can sufficiently reach the Si single crystal substrate 31a and can be epitaxially grown as the initial layer 14.

Next, as in the first embodiment, the process proceeds to an evaporation step and a precipitation step.
In this embodiment, the pulsed light of the KrF excimer laser beam 23 is incident under the conditions of a laser energy density of 2.5 J / cm ² , a laser frequency of 10 Hz, and a pulse length of 10 ns. Then, a plasma plume 24 made of K, Nb, and O is generated on the surface of the film forming base material 17e. The plasma plume 24 is disposed at a position 40 mm away from the film forming base material 17e, and is applied to the Si single crystal substrate 31a under the conditions of a substrate temperature of 800 ° C. and an oxygen partial pressure of 1.33 Pa (1 × 10 ⁻² Torr). Irradiating for 360 minutes, a K _0.6 Nb _0.4 O _y layer 25a is deposited to 200 nm as shown in FIG.
The K _0.6 Nb _0.4 O _y layer 25a immediately after deposition is solidified with the initial layer 14 as the nucleus of crystal growth during the pulse supply of the KrF excimer laser beam 23 as in the first embodiment. The KNbO ₃ single crystal layer 12 is deposited from the phase portion 26 and the remaining liquid phase portion 27 is evaporated.
Thus, the KNbO ₃ single crystal layer 12 is epitaxially grown by 200 nm.
Epitaxial growth can be confirmed from the fact that spot-like diffraction patterns appear in the RHEED pattern from the Si <011> direction shown in FIG.

Thus, in combination with the X-ray diffraction results shown in FIG. 9, KNbO ₃ , SrTiO ₃ , YBa ₂ Cu ₃ O _x , CeO ₂ , YSZ, and Si are orthorhombic, cubic, tetragonal, cubic, When the cubic crystal and cubic crystal are indicated by an index, KNbO ₃ (001) / SrTiO ₃ (100) / YBa ₂ Cu ₃ O _x (001) / CeO ₂ (100) / YSZ (100) is perpendicular to the film surface. / Si (100), KNbO ₃ <110> // SrTiO ₃ <010> // YBa ₂ Cu ₃ O _x <100> // CeO ₂ <011> // YSZ <011> // Si < It can be confirmed that a KNbO ₃ single crystal thin film 33 having an orientation relationship of 011> is obtained.
Here, KNbO ₃ has an orthorhombic (001) orientation because it receives a tensile stress from the Si single crystal substrate 31a having a relatively small thermal expansion coefficient in the temperature lowering process, and a, b, c, among the three axes. This is because the b-axis having the smallest coefficient of thermal expansion is oriented in the substrate plane.
If the plasma plume 24 can sufficiently reach the Si single crystal substrate 31a and the evaporation rate balance between the deposition rate and the liquid phase portion 27 can be taken so that the liquid phase portion 27 does not remain, each condition is limited to the above. Is not something
In the above-described deposition step, the film forming base material 17e is set to K _0.6 Nb _0.4 O _y with x = 0.6, but within the range of 0.5 ≦ x ≦ x _E under the oxygen partial pressure. Then, a similar KNbO ₃ single crystal thin film can be obtained.

A pair of IDTs 30a and 30b are formed on the above-described KNbO ₃ single crystal thin film 33 by the same method as in the first embodiment, and the surface acoustic wave device 36 according to this embodiment shown in FIG. 10 is manufactured.
In the surface acoustic wave device 36 obtained, IDT30a, speed of sound determined from the delay time _{V open} of the surface acoustic wave propagating between 30b was 5000 m / s. In addition, k ² obtained from the difference from the delay time V _short of the surface acoustic wave when the IDTs 30a and 30b are covered with a metal thin film is 35% because the KNbO ₃ single crystal thin film 33 is (001) oriented. The value was higher than that of the surface acoustic wave device 28 obtained in the first embodiment.
In addition, even if potassium sodium tantalate niobate is used instead of potassium niobate as the film forming base material 17a, K _1-x Na _x Nb _1-y Ta _y O ₃ (0 ≦ x ≦ 1, 0 ≦ y A solid solution thin film of ≦ 1) is similarly obtained.

According to this method of manufacturing a KNbO ₃ single crystal thin film, by forming the buffer layer 32 on the Si single crystal substrate 31a, the KNbO ₃ single crystal is deposited using the inexpensive Si single crystal substrate 31a. _Three single crystal thin films 33 can be obtained.
Further, since the KNbO ₃ single crystal is precipitated in the orthorhombic (001) orientation, the surface acoustic wave device 36 having an excellent k ² of about 50% at maximum can be produced from the KNbO ₃ single crystal thin film 33.

Next, a third embodiment according to the present invention will be described with reference to FIGS. In the following description, the same reference numerals are given to the components described in the above embodiment, and the description thereof is omitted.
The third embodiment differs from the second embodiment in that in the second embodiment, the buffer layer 32 is formed on the Si single crystal substrate 31a and the KNbO ₃ single crystal is epitaxially grown. On the other hand, in the third embodiment, the buffer layer 32 is formed on the quartz substrate (substrate body) 40 and the KNbO ₃ single crystal is epitaxially grown.

As shown in FIG. 11, the KNbO ₃ single crystal thin film 41 according to this embodiment includes a substrate 42 and a KNbO ₃ single crystal layer 12 epitaxially grown thereon.
The substrate 42 includes a quartz substrate 40 and a buffer layer 32 formed on the quartz substrate 40.
The constituent material of the quartz substrate 40 may be any material other than quartz, such as quartz, SiO ₂ coated silicon, and diamond coated silicon, and is a ceramic such as a polycrystalline YSZ substrate or an amorphous material such as a glass substrate. May be. Further, the perovskite oxide may have a crystal structure that cannot be epitaxially grown. Here, it is a general-purpose crystal that is important as a substrate for a surface acoustic wave device. The quartz substrate 40 has a thermal expansion coefficient of 0.5 × 10 ⁻⁶ (K ⁻¹ ), and the thermal expansion coefficient of the b-axis of the KNbO ₃ single crystal layer 12 is 0.5 × 10 ⁻⁶ (K ⁻¹ ). The coefficient of thermal expansion of the a and c axes is smaller than 5.0 × 10 ⁻⁶ (K ⁻¹ ) and 14.1 × 10 ⁻⁶ (K ⁻¹ ).

The buffer layer 32 includes a first buffer layer 34 and a second buffer layer 35 made of SrTiO ₃ which is a simple perovskite oxide epitaxially grown in a cubic (100) orientation on the first buffer layer 34. ing.
The first buffer layer 34 is made of a metal oxide. Examples of the metal oxide include a metal oxide having a NaCl structure or a fluorite structure. Among these, at least one of MgO, CaO, SrO, BaO, or a solid solution containing these, including a metal that thermodynamically bonds to oxygen more than Si, or YSZ, CeO ₂ , ZrO ₂ , or these It is preferable to use at least one of solid solutions containing Furthermore, the in-plane orientation direction may be independent of the crystal orientation of the substrate surface.

The first buffer layer 34 of the present embodiment is a NaCl-type oxide, and is composed of MgO that is grown in an in-plane orientation with cubic crystals (100).
When a fluorite type oxide such as YSZ or YSZ / CeO ₂ is used for the first buffer layer 34, a structure having the following structure is epitaxially grown and used.
That is, as the second buffer layer 35, a metal oxide having a layered perovskite structure such as YBa ₂ Cu ₃ O _x is epitaxially grown in a tetragonal or orthorhombic (001) orientation, and further, SrTiO ₃ is cubic thereon. The structure is epitaxially grown with (100) orientation.
The KNbO ₃ single crystal layer 12 is composed of an orthorhombic (110) or (001) orientation of the KNbO ₃ single crystal.

Next, the manufacturing method of the above-described KNbO ₃ single crystal thin film 41 will be described below step by step.
Also in the present embodiment, as in the above-described embodiment, an evaporation process for evaporating the residual liquid of K _x Nb _1-x O _y deposited on the substrate 42 from the plasma plume 24, and K _x Nb _1-x O and a precipitation step of depositing a KNbO ₃ single crystal on the substrate 42 from _y .
Hereinafter, a manufacturing method is demonstrated in order.
First, the quartz substrate 40 is immersed in an organic solvent and degreased and cleaned using an ultrasonic cleaner. As the organic solvent, for example, a mixed solution in which ethyl alcohol and acetone are mixed at 1: 1 can be used, but the organic solvent is not limited thereto.

Subsequently, the buffer layer 32 is formed on the quartz substrate 40 by the film forming apparatus 15 shown in FIG.
First, the first buffer layer 32 is formed on the quartz substrate 40.
The degreased and washed quartz substrate 40 is loaded into the holding unit 19 and then introduced into the process chamber 16, and 1.33 × 10 ⁻² Pa (1 × 10 ⁻⁴ Torr) at a partial pressure ratio of argon: oxygen = 100: 1. The mixed gas is introduced so that the pressure becomes.
The pressure condition is not limited to this.

After the pressure becomes constant, the film forming base material 17a made of Mg or MgO is disposed so as to face the crystal substrate 40 and have a distance of 30 mm or more and 50 mm or less. The deposition base material 17a is formed under conditions where the pressure during deposition is 1.33 × 10 ⁻³ Pa (1 × 10 ⁻⁵ Torr) or more and 1.33 × 10 ⁻² Pa (1 × 10 ⁻⁴ Torr) or less. An excimer laser having a laser energy density of ² J / cm ² or more and 3 J / cm ² or less and a laser frequency of 5 Hz or more and 15 Hz or less is irradiated on the surface.
Note that each condition is not limited to this as long as in-plane orientation growth is possible as MgO.

Here, pulsed light of a KrF excimer laser beam 23 is incident on the Mg film forming base material 17a under the conditions of a laser energy density of 2.5 J / cm ² , a laser frequency of 10 Hz, and a pulse length of 10 ns. Then, an Mg plasma plume 24 is generated on the surface of the film forming base material 17a. The plasma plume 24 is irradiated on the quartz crystal substrate 40 disposed at a position 40 mm away from the film forming base material 17a for 10 minutes under the condition of pressure 1.33 × 10 ⁻² Pa (1 × 10 ⁻⁴ Torr). As shown in FIG. 11, the MgO first buffer layer 34 is epitaxially grown by 10 nm.

At this time, an argon ion beam is irradiated onto the substrate from a direction that is 45 degrees with the normal direction on the surface of the quartz substrate 40. Here, the ion beam source source is preferably a Kauffmann ion source, and the acceleration voltage of the ion beam is preferably about 200 eV and the current is preferably about 10 mA.
The substrate temperature is not particularly controlled by a heater or the like, but the substrate temperature rises to 50 to 70 ° C. due to the impact of the argon ion beam.

After depositing the MgO first buffer layer 34, the SrTiO ₃ second buffer layer 35 is epitaxially grown to 100 nm by the same method as in the second embodiment to obtain the substrate 42.
On the formed buffer layer 32, the initial layer 14 is deposited by 5 nm by the same method as in the second embodiment described above.
Similarly, the process proceeds to an evaporation step and a precipitation step, and a K _0.6 Nb _0.4 O _y layer 25a is deposited on the initial layer 14 to a thickness of 200 nm.
The K _0.6 Nb _0.4 O _y layer 25a immediately after deposition is solidified with the initial layer 14 as the nucleus of crystal growth during the pulse supply of the KrF excimer laser beam 23 as in the first embodiment. The KNbO ₃ single crystal layer 12 is deposited from the phase portion 26 and the remaining liquid phase portion 27 is evaporated.
Thus, the KNbO ₃ single crystal layer 12 is epitaxially grown by 200 nm.

Thus, when the index displayed KNbO ₃ and SrTiO ₃ with orthorhombic and cubic, _{respectively, KNbO 3 (001) / SrTiO} 3 (100) / MgO (100) to the film surface in the vertical direction, KNbO in-plane direction ₃ A KNbO ₃ single crystal thin film 41 having an orientation relationship of <110> // SrTiO ₃ <010> // MgO <010> is formed.
Here, KNbO ₃ is in the orthorhombic (001) orientation because it receives tensile stress from the quartz substrate 40 having a relatively low thermal expansion coefficient in the temperature lowering process, and is the first of the three axes a, b, c. This is because the b-axis having a small coefficient of thermal expansion is oriented in the substrate plane.
Each condition is limited to the above as long as the plasma plume 24 can reach the quartz substrate 40 sufficiently and the evaporation rate balance between the deposition rate and the liquid phase portion 27 can be obtained so that the liquid phase portion 27 does not remain. is not.
In the above-described deposition step, the film forming base material 17a is K _0.6 Nb _0.4 O _y with x = 0.6, but within the range of 0.5 ≦ x ≦ x _E under the oxygen partial pressure. Then, a similar KNbO ₃ single crystal thin film can be obtained.

A pair of IDTs 30a and 30b is formed on the above-described KNbO ₃ single crystal thin film 41 by the same method as in the second embodiment, and the surface acoustic wave device 43 according to this embodiment shown in FIG. 12 is manufactured.
In the surface acoustic wave device 43 obtained, IDT30a, speed of sound determined from the delay time _{V open} of the surface acoustic wave propagating between 30b was 3000 m / s. Further, k ² obtained from the difference from the delay time V _short of the surface acoustic wave when the IDTs 30a and 30b are covered with a metal thin film is 35% because the KNbO ₃ single crystal thin film 41 has the (001) orientation. Also in this embodiment, the value was higher than that of the surface acoustic wave device 28 obtained in the first embodiment.
In addition, even if potassium sodium tantalate niobate is used instead of potassium niobate as the film forming base material 17a, K _1-x Na _x Nb _1-y Ta _y O ₃ (0 ≦ x ≦ 1, 0 ≦ y A solid solution thin film of ≦ 1) is similarly obtained.

According to this method of manufacturing a single crystal thin film of KNbO ₃ , the buffer layer 32 is formed on a substrate made of any material such as quartz, quartz, SiO ₂ coated silicon, diamond coated silicon, which is suitable for a surface acoustic wave device and inexpensive. By forming it, a KNbO ₃ single crystal can be deposited to obtain a KNbO ₃ single crystal thin film. In addition, since the KNbO ₃ single crystal thin film is deposited in the orthorhombic (001) orientation, the surface acoustic wave device 43 having an excellent k ² of about 50% at maximum can be produced from the KNbO ₃ single crystal thin film 41. .

Next, the frequency filter provided with the surface acoustic wave device according to the present invention will be described.
A frequency filter 60 shown in FIG. 13 absorbs a surface acoustic wave element 61 made of any one of the KNbO ₃ single crystal thin films 10, 33, and 41 and a surface acoustic wave propagating on the surface of the surface acoustic wave element 61. Sound absorbing portions 62a and 62b.
A pair of IDTs 63 a and 63 b are formed on the upper surface of the surface acoustic wave element 61. The IDT electrodes 63a and 63b are made of Al or an Al alloy and have a thickness set to about 1/100 of the IDT pitch.
A high frequency signal source 64 is connected to the IDT electrode 63a, and a signal line 65 having terminals 65a and 65b at the ends is connected to the IDT 63b.
The sound absorbing parts 62a and 62b are formed so as to sandwich the IDTs 63a and 63b.

In the frequency filter 60, when a high frequency signal is output from the high frequency signal source 64, it is applied to the IDT 63 a and a surface acoustic wave is generated on the upper surface of the surface acoustic wave element 61. The surface acoustic wave propagates on the upper surface of the surface acoustic wave element 61 at a speed of about 4000 m / s. Of the surface acoustic waves, the surface acoustic waves propagated from the IDT 63a to the sound absorbing portion 62a are absorbed by the sound absorbing portion 62a. However, among the surface acoustic waves propagated to the IDT 63b side, a surface acoustic wave having a specific frequency determined according to the wiring pitch of the IDT 63b or a frequency in a specific band is converted into an electric signal.
Most of the rest passes through the IDT 63b and is absorbed by the sound absorbing part 62b.

  According to the frequency filter 60, only the surface acoustic wave having a specific frequency or a frequency in a specific band among the electric signals supplied to the IDT 63a can be obtained with high efficiency (filtering).

A frequency oscillator 70 shown in FIG. 14 includes a surface acoustic wave element 71 made of any one of the KNbO ₃ single crystal thin films 10, 33, and 41.
On the upper surface of the surface acoustic wave element 71, an IDT 72 and a pair of IDTs 73 a and 73 b are formed with the IDT 72 interposed therebetween. These IDTs 72, 73a, 73b are made of Al or an Al alloy, and the thickness is set to about 1/100 of the IDT pitch.
The IDT 72 further includes a pair of comb-like electrodes 72a and 72b. A high frequency signal source 74 is connected to one comb-like electrode 72a, and a signal line 76 having terminals 75a and 75b is connected to the other comb-like electrode 72b.

In this frequency oscillator 70, when a high frequency signal is output from the high frequency signal source 74, this frequency signal is applied to the comb-like electrode 72a, and the surface acoustic wave propagating to the IDT 73a side on the upper surface of the surface acoustic wave element 71, and the IDT 73b. A surface acoustic wave propagating to the side is generated. This surface acoustic wave has a velocity of about 4000 m / s.
Of these surface acoustic waves, surface acoustic waves having a specific frequency component are reflected by the IDTs 73a and 73b, respectively, and a standing wave is generated between the IDTs 73a and 73b. Among the standing waves, specific frequency components resonate and the amplitude increases.
A part of the surface acoustic wave having this frequency component or a frequency component in a specific band is extracted from the comb-like electrode 72b and has a frequency (or a certain band) according to the resonance frequency of the IDTs 73a and 73b. Frequency) electrical signal is taken out from the terminals 75a and 75b.

FIG. 15 shows an example in which the frequency oscillator 70 is applied to a VCSO (Voltage Controlled SAW Oscillator) 80. The VCSO 80 is mounted inside a casing 81 made of metal (aluminum or stainless steel). In the VCSO 80, an IC (Integrated Circuit) 83 and a frequency oscillator 84 are mounted on a SAW substrate 82. The IC 83 controls the frequency applied to the frequency oscillator 84 in accordance with a voltage value input from an external circuit (not shown).
IDTs 86a, 86b, and 86c are formed on the surface acoustic wave element 85 included in the frequency oscillator 84. On the SAW substrate 82, a wiring 87 for electrically connecting the IC 83 and the frequency oscillator 84 is patterned. The IC 83 and the wiring 87 are connected by wire wires 88a and 88b such as gold wires and are also electrically connected.

This VCSO 80 is used as, for example, a VCO (Voltage Controlled Oscillator) 91 of a PLL (Phase Locked Loop) circuit 90 shown in FIG. The PLL circuit 90 further includes an input terminal 92, a phase comparator 93, a reduction filter 94, and an amplifier 95.
The phase comparator 93 compares the phase (or frequency) of the signal input from the input terminal 92 and outputs an error voltage signal whose value is set according to the difference. The reduction filter 94 passes only the low frequency component at the position of the error voltage signal output from the phase comparator 93, and the amplifier 95 amplifies the signal output from the reduction filter 94. The VCO 91 is an oscillation circuit whose oscillation frequency continuously changes within a certain range according to an input voltage value.

The PLL circuit 90 operates so that the difference from the phase (or frequency) input from the input terminal 92 decreases. That is, when the frequency of the signal output from the VCO 91 is synchronized with the frequency of the signal input from the input terminal 92, thereafter, it matches the signal input from the input terminal 92 except for a certain phase difference, and the input signal changes. Output a signal that follows
According to the frequency oscillator 70, since any one of the KNbO ₃ single crystal thin films 10, 33, and 41 is provided, it is small and inexpensive, and obtains a high-performance filter characteristic that can cope with a wideband signal. Can do.

Next, the electronic circuit 100 having the frequency filter 60 and the frequency oscillator 70 and having the electrical configuration shown in FIG. 17 will be described. The electronic circuit 100 is provided in, for example, a mobile phone (electronic device) 101 shown in FIG. The cellular phone 101 includes a liquid crystal display unit 102 and operation buttons 103.
The electronic circuit 100 includes a transmitter 104, a transmission signal processing circuit 105, a transmission mixer 106, a transmission filter 107, a transmission power amplifier 108, a transmission / reception duplexer 109, an antenna 110, a low noise amplifier 111, a reception filter 112, and a reception mixer 113. A reception signal processing circuit 114, a receiver 115, a frequency synthesizer 116, a control circuit 117, and an input / display circuit 118.

The transmitter 104 is a microphone or the like that converts a sound wave signal such as voice into a radio wave signal, and the transmission signal processing circuit 105 performs D / A conversion processing and modulation on the electric signal output from the transmitter 104. Processing such as processing is performed. The transmission mixer 106 mixes the signal output from the transmission signal processing circuit 105 using the signal output from the frequency synthesizer 116. The transmission filter 107 is the frequency filter 60 shown in FIG. 13 and passes only a signal having a required frequency among the intermediate frequencies (IF), and cuts a signal having an unnecessary frequency. The passed signal is converted into an RF signal by a conversion circuit (not shown). The transmission power amplifier 108 amplifies the power of the RF signal and outputs it to the transmission / reception duplexer 109.
The transmitter / receiver demultiplexer 109 transmits the amplified RF signal from the antenna 110 in the form of a radio wave. Further, the received signal received from the antenna 110 is demultiplexed and output to the low noise amplifier 111.

The low noise amplifier 111 amplifies the input signal and outputs it to a conversion circuit (not shown), and the conversion circuit converts this signal into IF. The reception filter 112 is the frequency filter 60 shown in FIG. 13 and passes only a signal having a required frequency in the IF and cuts an unnecessary signal.
The reception mixer 113 mixes the signal output from the reception filter 112 using the signal output from the frequency synthesizer 116.
The reception signal processing circuit 114 performs processing such as D / A conversion processing and demodulation processing on the electrical signal output from the reception mixer 113.
The receiver 115 includes a small speaker that converts a radio wave signal into a sound wave signal such as a voice.

The frequency synthesizer 116 includes the PLL circuit 90 shown in FIG. 16 described above, divides a signal output from the PLL circuit 90, generates a signal to be supplied to the transmission mixer 106 and the reception mixer 113, and further divides a part of the signal. Then, a signal to be supplied to the reception mixer 113 is generated. These signals are individually set in the transmission filter 107 and the reception filter 112.
The input / display circuit 118 is used to display the state of the device to the user of the cellular phone 101 and to input a user instruction. The input / display circuit 118 is provided on the liquid crystal display unit 102 and the operation buttons 103 shown in FIG. Equivalent to.
The control circuit 117 controls the overall operation of the mobile phone 101 by controlling the transmission signal processing circuit 105, the reception signal processing circuit 114, the frequency synthesizer 116, and the input / display circuit 118.
According to the electronic circuit 100 and the mobile phone 101, since any one of the KNbO ₃ single crystal thin films 10, 33, and 41 is provided, it is possible to improve the size reduction, the broadband, and the power saving.

The technical scope of the present invention is not limited to the above embodiment, and various modifications can be made without departing from the spirit of the present invention.
For example, in the above-described embodiment, the mobile phone 101 is used as the electronic device, and the electronic circuit 100 provided in the mobile phone 101 is used as the electronic circuit. However, the present invention is not limited to the mobile phone, and various mobile communication devices can be used. The present invention can be applied to a device and an electronic device provided in the device.

  Furthermore, the present invention can be applied not only to mobile communication devices but also to communication devices used in a stationary state such as tuners that receive BS (Broadcast Satellite) and CS (Commercial Satellite) broadcasts, and electronic circuits provided therein. . In addition to communication devices that use radio waves propagating in the air as communication carriers, electronic devices such as HUBs that use high-frequency signals propagating in coaxial cables, or optical signals propagating in optical cables, and the electronics provided therein It can also be applied to circuits.

It is a cross-sectional view of a KNbO ₃ thin film in the first embodiment. It is a perspective view which shows the film-forming apparatus in 1st Embodiment. It is a RHEED pattern photograph observed in a 1st embodiment. The scanning electron micrograph of the thin film surface obtained in 1st Embodiment. The X-ray diffraction pattern result of the thin film surface obtained in 1st Embodiment. It is a figure which shows the cross section of the surface acoustic wave element in 1st Embodiment. It is a diagram illustrating a KNbO ₃ thin film of the cross section of the second embodiment. It is a RHEED pattern photograph observed in a 2nd embodiment. The X-ray diffraction pattern result of the thin film surface obtained in 2nd Embodiment. It is a figure which shows the cross section of the surface acoustic wave element in 2nd Embodiment. It is a figure which shows the cross section of the KNbO3 thin film in _3rd Embodiment. It is sectional drawing of the surface acoustic wave element in 3rd Embodiment. It is a perspective view of the frequency filter in the embodiment of the present invention. It is a perspective view which shows the frequency oscillator in embodiment of this invention. It is a perspective view which shows the frequency oscillator in embodiment of this invention. It is a block diagram which shows the PLL circuit of embodiment of this invention. It is a block diagram which shows the electronic circuit in embodiment of this invention. It is a perspective view which shows the mobile telephone in embodiment of this invention. A two-component phase diagram of the _{K 2 O-Nb 2 O 5} . It is a state figure showing the relation between the composition ratio of KNbO ₃ and temperature.

Explanation of symbols

  10, 33, 41 Potassium niobate single crystal thin film, 11, 31, 42 substrate, 12 Potassium niobate single crystal layer (potassium niobate single crystal), 13 Strontium titanate single crystal substrate (substrate), 24 Plasma plume (raw material) ), 26 Solid phase part, 27 Liquid phase part, 28, 36, 43, 61, 71, 85 Surface acoustic wave element, 31a Silicon single crystal substrate (substrate), 32 Buffer layer, 34 First buffer layer, 35 Second Buffer layer, 40 Crystal substrate (substrate body), 60 Frequency filter, 70, 84 Frequency oscillator, 100 Electronic circuit, 101 Mobile phone (electronic device)

Claims (5)

Depositing a potassium niobate layer under conditions where the oxygen partial pressure is 1.33 × 10 ⁻¹ Pa to 13.3 Pa and the substrate temperature is 750 ° C. or less;
Heating the substrate temperature to 850 ° C. or higher, and forming the potassium niobate layer from K _x Nb _1-x O ₃ (0.5 ≦ x ≦ 0.65). When,
Under the conditions of an oxygen partial pressure of 1.33 × 10 ⁻¹ Pa to 13.3 Pa and a substrate temperature of 750 ° C. to 850 ° C., K _x Nb _1-x O ₃ (0.5 ≦ x ≦ 0.65 A deposition step of depositing
A method for producing a potassium niobate thin film comprising: Wherein in the deposition step, the manufacturing method of the potassium niobate film according to claim 1, wherein the vapor phase method for depositing a _{K x Nb 1-x O 3} . The K _x Nb _1-x O ₃ (0.5 ≦ x ≦ 0.65) K _x Nb _1-x O ₃ is deposited by making a pulse light incident on a film forming base material in the deposition step. Or the manufacturing method of the potassium niobate thin film of 2 . The method for manufacturing a potassium niobate thin film according to any one of claims 1 to 3, wherein the substrate is made of strontium titanate. The method for producing a potassium niobate thin film according to any one of claims 1 to 3, wherein the substrate is made of silicon.

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