JP2008133187A - Piezoelectric single crystal material and piezoelectric device using the same - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a piezoelectric single crystal material in which the temperature dependency of piezoelectric property in an orientation preferable as a device is improved, and to provide a piezoelectric device using the same. <P>SOLUTION: The piezoelectric single crystal material has a Ca<SB>3</SB>Ga<SB>2</SB>Ge<SB>4</SB>O<SB>14</SB>structure, comprises La, Sr, Ta, Ga and Si as main components, and is expressed by compositional formula: La<SB>3-x</SB>Sr<SB>x</SB>Ta<SB>y</SB>Ga<SB>6-y-z</SB>Si<SB>z</SB>O<SB>14</SB>wherein, x, y, and z exist within a compositional range obtained by successively connecting (x=0, y=0.35, z=0.3), (x=0.2, y=0.4, z=0.4), (x=0.8, y=0.5, z=0.8), (x=1.2, y=0.5, z=1.2), (x=1.0, y=0.3, z=1.4), (x=0.6, y=0.1, z=1.4), (x=0.2, y=0, z=1.2), (x=0, y=0.15, z=0.7), and (x=0, y=0.35, z=0.3). <P>COPYRIGHT: (C)2008,JPO&INPIT

Description

  The present invention relates to a novel piezoelectric single crystal material and a piezoelectric device using the piezoelectric single crystal material.

In recent years, a piezoelectric single crystal material belonging to the space group P321 and having a Ca ₃ Ga ₂ Ge ₄ O ₁₄ structure has a large piezoelectric characteristic compared to quartz and has a piezoelectric temperature characteristic close to that of quartz. .

  Therefore, there are many reports on the material constants related to piezoelectricity of this type of single crystal material, and there are also reports on the optimal substrate orientation and wave propagation direction of surface acoustic wave piezoelectric devices by computer simulation using the material constants. ing. As the optimum substrate orientation, an orientation in which the phase velocity variation with respect to the temperature change of the elastic wave satisfies the requirements of the piezoelectric device and has the largest electromechanical coupling coefficient is generally selected. For this reason, the optimum substrate orientation and propagation direction in the single crystal composition reported so far are different from those having the largest electromechanical coupling coefficient.

  Due to the recent trend of digitization, there has been a great demand for a substrate material having a higher electromechanical coupling coefficient, and research on new compositions has been conducted. However, research on these new compositions began with the search for “single crystallizable” compositions, and the temperature characteristics of the piezoelectric and material constants that were most notable for the device were not considered from the search stage.

Reported La ₃ Ga ₅ SiO ₁₄ (LGS), La ₃ Ta _0.5 Ga _5.5 O ₁₄ (LTG), and La ₃ Nb _0.5 Ga _5.5 for the optimal cut for surface acoustic wave piezoelectric elements. It is known that the piezoelectric single crystal material having a composition of O ₁₄ (LNG) is positive in the temperature coefficient primary term of the phase velocity of the elastic wave in the crystal orientation and propagation direction showing the maximum electromechanical coupling coefficient. .

The present applicant has found that a piezoelectric single crystal material having such a Ca ₃ Ga ₂ Ge ₄ O ₁₄ structure can increase the electromechanical coupling coefficient mainly by adjusting the A site, and Sr ₃ TaGa ₃ Si _2. A piezoelectric single crystal material of O ₁₄ (STGS) has been proposed (Patent Document 1). However, although this piezoelectric single crystal material STGS has a high electromechanical coupling coefficient, the temperature coefficient primary term of the phase velocity of the elastic wave is negative in the crystal orientation and propagation direction showing the maximum electromechanical coupling coefficient. I understood.

The applicant further has La _{3 -x} Sr _x Ta _{0.5 + 0.5 x} Ga _{5.5-0.5 x} , which has a Ca ₃ Ga ₂ Ge ₄ O ₁₄ structure and can increase the electromechanical coupling coefficient. A piezoelectric single crystal material having a composition of O ₁₄ (LSTG) (0 <x ≦ 0.15) is proposed (Patent Document 2). With respect to this piezoelectric single crystal material LSTG, when the temperature dependence of the resonance frequency of the X-plate Y-direction stretching vibration was measured, as shown in FIG. 1, La ₃ Ta _0.5 Ga _5.5 O ₁₄ before element substitution and In comparison, it was confirmed that the temperature coefficient first-order term approaches zero. From this result, it is presumed that the primary term of the phase velocity temperature coefficient of the elastic wave can be changed from negative to positive by changing the oxygen-coordinated cation site substitution element from Sr ²⁺ to La ³⁺ .

JP-A-11-171696 Japanese Patent Laid-Open No. 2000-349587 H. Takeda et al. (Journal of ALLOYSAND C0MPOUNDS 290 (1999) 79-84)

However, in order to make the first order term of the phase velocity temperature coefficient of the elastic wave in the crystal orientation and propagation direction with a high electromechanical coupling coefficient zero, it is necessary to increase the Sr substitution amount from the composition shown in Patent Document 2. It is reported in Non-Patent Document 1 that a solid phase having a Ca ₃ Ga ₂ Ge ₄ O ₁₄ structure cannot be obtained from such a melt composition.

  Accordingly, an object of the present invention is to provide a piezoelectric single crystal material in which the temperature dependence of piezoelectric characteristics in a desired orientation as a device is improved, and a piezoelectric device using the piezoelectric single crystal material.

According to the present invention, it has a Ca ₃ Ga ₂ Ge ₄ O ₁₄ structure, the main components are composed of La, Sr, Ta, Ga and Si, and the composition formula La _3-x Sr _x Ta _y Ga _{6-y. -z} Si _z O ₁₄ , and x, y, and z in this composition formula are (x = 0, y = 0.35, z = 0.3), (x = 0.2, y = 0. 4, z = 0.4), (x = 0.8, y = 0.5, z = 0.8), (x = 1.2, y = 0.5, z = 1.2), ( x = 1.0, y = 0.3, z = 1.4), (x = 0.6, y = 0.1, z = 1.4), (x = 0.2, y = 0, z = 1.2), (x = 0, y = 0.15, z = 0.7), and (x = 0, y = 0.35, z = 0.3). A piezoelectric single crystal material in the range and a piezoelectric device using the piezoelectric single crystal material are provided.

The inventors of the present application examined crystallization of a five-component system in which SiO ₂ is added to a four-component system of La ₂ O ₃ , SrO, Ta ₂ O ₅ , and Ga ₂ O ₃ presented in Patent Document 2. The composition range in which the Sr substitution amount can be further increased was searched, and a composition range in which the Sr addition amount was coordinated up to 1.2 was obtained. Thereby, not only can the electromechanical coupling coefficient be increased, but also the phase velocity temperature coefficient of the elastic wave in the crystal orientation and propagation direction can be improved.

According to the present invention, for a five-component system of La ₂ O ₃ , SrO, Ta ₂ O ₅ , Ga ₂ O ₃ , SiO ₂ , La _{3 -x} Sr _x Ta _y Ga _{6-yz in} a predetermined composition range. By growing the Si _z O ₁₄ single crystal, it is possible to obtain a substrate material with improved temperature characteristics that is optimal for the mode of the target piezoelectric device. That is, not only can the electromechanical coupling coefficient be increased, but also the phase velocity temperature coefficient of the elastic wave in the crystal orientation and propagation direction can be improved.

Crystallization of a five-component system of La ₂ O ₃ , SrO, Ta ₂ O ₅ , Ga ₂ O ₃ , and SiO ₂ having a Ca ₃ Ga ₂ Ge ₄ O ₁₄ structure is examined.

The Ca ₃ Ga ₂ Ge ₄ O ₁₄ structure is composed of _four sites: an oxygen 8-coordinate A site, a 6-coordinate B site, and a 4-coordinate C and D site having different sizes. Of these, in the La ₂ O ₃ —SrO—Ta ₂ O ₅ —Ga ₂ O ₃ —SiO ₂ system, the A site (three) is La and Sr, the B site (one) is Ta and Ga, and the D site (two). ) Are partially substituted by Si and Ga, and Ga is substituted alone at the C site (three). That is, since elements other than Ga ₂ O ₃ replace only one of the A, B, and D sites, the average charge and ionic radius of each site can be adjusted independently. Moreover, since the Ca ₃ Ga ₂ Ge ₄ O ₁₄ structure is oxygen 14, the total charge of cations needs to be +28. Therefore, when the S-coordination number at the A site is x, the Ta-coordination number at the B site is y, and the Si coordination number at the D site is z, the composition formula is La _3-x Sr _x Ta _y Ga _{6-y −z} Si _z O ₁₄ , and from the charge formula, the following formula: −x + 2y + z = 1 (0 <x <3, 0 <y <1, 0 <z <2)
Must be satisfied.

When this relational expression is used, a triangular diagram similar to a normal three-component system is obtained for the La ₂ O ₃ —SrO—Ta ₂ O ₅ —Ga ₂ O ₃ —SiO ₂ system with a well-balanced charge as shown in FIG. Can be created. However, in the same figure, the Ta coordination number y and the Si coordination number z are given numerical values in the opposite direction to normal.

  In the composition range thus obtained, a composition that can be single-crystallized is actually searched.

  As a method for searching for the composition, a μ-PD (micro pull-down) method is used. In this method, as shown in FIG. 3, raw material powder is inserted into a crucible 2 made of Pt or Pt—Rh alloy installed between two tubular furnaces 1 and connected to the crucible 2 (not shown). The crucible 2 is heated with the Joule heat by flowing a current from a direct current power source, and the raw material powder inside is melted to create the melt 3.

  Next, a rod-shaped seed crystal 4 is brought into contact with the melt 3 and a pulling shaft 6 is lowered in an atmosphere having an appropriate temperature gradient by an after heater 5 to grow a fiber-shaped single crystal 7.

  The composition that can be single-crystallized by this method can basically be single-crystallized by a single crystallizing method by melt solidification, that is, a commercially useful method such as Cz method, FZ method, Bridgman method. Therefore, this μ-PD method is useful as a composition search technique.

  About the obtained single crystal, observation with a stereomicroscope etc. and phase identification by powder X-ray diffraction are performed, and crystallinity is confirmed.

According to the above method, the solid phase of the Ca ₃ Ga ₂ Ge ₄ O ₁₄ structure is changed from the melt by changing x, y, and z of the composition of La _{3 -x} Sr _x Ta _y Ga _6-yz Si _z O _14. It was discriminated whether it was precipitated in the phase. The result is shown in FIG.

In the figure, ● represents the case where the fiber became a single phase of the Langasite phase from the start to the end of the growth, ○ represents the case where a different phase appeared in the later stage of the growth, and × represents the case where a different phase appeared from the initial phase of the growth. It is. In the case of ○, single crystallization is possible in the region where the solidification rate is small. _{Accordingly, La 3-x Sr x Ta} y Ga 6-y-z Si z O 14 composition range capable single crystal in composition, in FIG. 4, the area indicated by ○ and ●, i.e. 0 <x ≦ 1 0.2, 0 <y ≦ 0.5, 0 <z ≦ 1.4.

Of the compositions obtained as described above, La ₂ SrTa _0.5 Ga _4.5 SiO ₁₄ as Example 1 described later, and La _2.8 Sr _0.2 Ta _0.4 Ga _5. as Example 2 is described _. Crystal growth of a ₂ Si _0.4 O ₁₄ composition was performed, and the temperature change of the resonance frequency in the X-plate Y-direction elongation vibration mode was measured. The measurement results are shown in FIG. For comparison, La ₃ Ga ₅ SiO ₁₄ (LGS), La ₃ Ta _0.5 Ga _5.5 O ₁₄ (LTG), Sr ₃ TaGa ₃ Si ₂ O ₁₄ (STGS) and Japanese Patent Laid-Open No. 2000-349587 are shown in the same figure. The measurement result of La _2.925 Sr _0.075 Ta _0.5375 Ga _5.4625 O ₁₄ (LSTG) having the same composition as that disclosed in the publication is also shown.

  From the figure, in particular, in the composition of Example 1, there is a tendency that the resonance frequency decreases as the temperature increases due to an increase in the amount of Sr.

Moreover, the temperature change of the resonance frequency in the Y plate X direction thickness deviation vibration mode was measured. The measurement results are shown in FIG. In the same figure, the comparison with the existing langasite crystal is also made as in the case of FIG. Since the second order coefficient with respect to the temperature of the resonance frequency is larger than that in FIG. 5, the characteristics are parabolic. The La _2.8 Sr _0.2 Ta _0.4 Ga _5.2 Si _0.4 O ₁₄ of Example 2 obtained in this study has a maximum value of about 50 ° C. and near room temperature, and its resonance The maximum change in frequency from −20 ° C. to 80 ° C. decreased to 330 ppm. Thus, it was confirmed that the resonance frequency temperature characteristic of the fundamental vibration mode can be greatly changed by growing a single crystal having a composition in which the amount of Sr is adjusted. This means that the temperature characteristics of the material constant can be greatly changed by adjusting the composition parameters x, y, and z. Actually, the thickness deviation mode is used as the monolithic filter, but the La _2.8 Sr _0.2 Ta _0.4 Ga _5.2 Si _0.4 O ₁₄ composition is Y-cut as shown in FIG. Good temperature characteristics can be obtained. Furthermore, a better temperature characteristic can be obtained by increasing the Sr amount so that the maximum value of the resonance frequency with respect to the temperature is adjusted to about 30 ° C. Furthermore, when the wafer cut required for the piezoelectric device is parallel to the crystal plane, the orientation accuracy can be easily adjusted to 0.1 ° or less, so that the process can be simplified and device variations can be reduced. Further, by adjusting the composition of the surface acoustic wave device using the piezoelectric single crystal material, it is possible to realize a high band by improving the temperature characteristics in the cut having the largest coupling coefficient.

  FIG. 7 shows an example of a single crystal manufacturing apparatus used in this example.

  As shown in the figure, the crucible 10 is installed at the center of the heat insulating material 11, and a refractory housing 12 is disposed on the upper portion of the crucible 10. An opening 12a is provided at the center of the top wall of the refractory housing 12, and a lifting shaft 14 having a seed crystal 13 attached to the lower end extends vertically from a power source (not shown) and penetrates the opening 12a. Yes. Around the heat insulating material 11 and the refractory housing 12, a refractory cylinder 15 having an opening 15a through which the crystal pulling shaft 14 passes is arranged on the top wall. A high-frequency induction coil 16 is wound outside the refractory housing 12, and the crucible 10 is induction-heated by flowing a high-frequency current to maintain the crystal material melt 17 at a predetermined temperature.

In Example 1, a high frequency oscillator having a frequency of 70 kHz was used. In the manufacturing apparatus shown in FIG. 7, about 350 g of La ₂ SrTa _0.5 Ga _4.5 SiO ₁₄ was inserted into an Ir crucible 10 having a diameter of 50 mm, a height of 50 mm, and a thickness of 1.5 mm. The growth was performed in an atmosphere in which ₁ vol% O ₂ was mixed in N _2, and a La ₂ SrTa _0.5 Ga _4.5 SiO ₁₄ single crystal with [001] orientation was used as the seed crystal 13 at a speed of 0.5 mm / h. I raised it. As a result, a transparent La ₂ SrTa _0.5 Ga _4.5 SiO ₁₄ single crystal 18 having a diameter of 20 mmφ and a length of 110 mm as shown in the photograph of FIG. 8 was obtained. Note that x, y, and z in the composition formula in this example are x = 1.0, y = 0.5, and z = 1.0.

  An X-plate thickness Y elongation vibration resonator and a Y-plate thickness X shear vibration resonator were prepared from the obtained crystal, and the resonance frequency was determined in the range of -20 ° C to + 80 ° C. The changes were 1815 ppm and 732 ppm, respectively.

A high frequency oscillator having a frequency of 70 kHz was used. In the manufacturing apparatus shown in FIG. 7, an Ir crucible 10 having a diameter of 50 mm, a height of 50 mm, and a thickness of 1.5 mm is placed on a La _2.8 Sr _0.2 Ta _0.4 Ga _5.2 Si _0. Approximately 370 g of ₄ O ₁₄ was inserted. The growth was performed in an atmosphere in which 1 vol% O ₂ was mixed in N ₂ , using a La ₃ Ta _0.5 Ga _5.5 O ₁₄ single crystal with [001] orientation as the seed crystal 13 and a speed of 0.5 mm / h. It was raised at. As a result, a transparent La _2.8 Sr _0.2 Ta _0.4 Ga _5.2 Si _0.4 O ₁₄ single crystal 18 having a diameter of 20 mmφ and a length of 100 mm as shown in the photograph of FIG. 9 was obtained. . Note that x, y, and z in the composition formula in this example are x = 0.2, y = 0.4, and z = 0.4.

  An X-plate thickness Y elongation vibration resonator and a Y-plate thickness X shear vibration resonator were prepared from the obtained crystal, and the resonance frequency was determined in the range of -20 ° C to + 80 ° C. The changes were 732 ppm and 332 ppm, respectively.

A high frequency oscillator having a frequency of 70 kHz was used. In the manufacturing apparatus shown in FIG. 7, a La _2.4 Sr _0.6 Ta _0.4 Ga _4.8 Si _0.8 is added to a Pt crucible 10 having a diameter of 50 mm, a height of 50 mm, and a thickness of 1.5 mm _. Approximately 360 g of ₈ O ₁₄ was inserted. The growth is carried out in an atmosphere in which 1 vol% O ₂ is mixed in N ₂ , and the seed crystal 13 is a La _2.4 Sr _0.6 Ta _0.4 Ga _4.8 Si _0.8 O ₁₄ single crystal with [001] orientation. Was pulled up at a speed of 0.3 mm / h. As a result, a transparent La _2.4 Sr _0.3 Ta _0.4 Ga _4.8 Si _0.8 O ₁₄ single crystal 18 having a diameter of 20 mmφ and a length of 68 mm as shown in the photograph of FIG. 10 was obtained. . Note that x, y, and z in the composition formula in this example are x = 0.6, y = 0.4, and z = 0.8.

  The above-described embodiments and examples are all illustrative and do not limit the present invention, and the present invention can be implemented in various other modifications and changes. Therefore, the scope of the present invention is defined only by the claims and their equivalents.

It is a figure which shows the change with respect to temperature of the resonant frequency in the X plate Y direction elongation vibration of the conventional material. It is a triangle figure of the composition in the present invention. It is a figure for demonstrating the fiber crystal growth by micro-PD method. It is a figure which shows the composition range in which single crystallization obtained in this invention is possible. It is a figure showing the temperature change of the resonant frequency of the elongation vibration mode of the single crystal obtained in the Example of this invention. It is a figure showing the temperature change of the resonant frequency of the thickness shear mode of the single crystal obtained in the Example of this invention. It is a figure for demonstrating the single crystal manufacturing apparatus used in the Example of this invention. ₂ is a photograph showing an example of growing a La ₂ SrTa _0.5 Ga _4.5 SiO ₁₄ single crystal in Example 1. FIG. Is a photograph showing the development example of _{La 2.8 Sr 0.2 Ta 0.4 Ga 5.2} Si 0.4 O 14 single crystal in Example 2. 4 is a photograph showing an example of growing a La _2.4 Sr _0.6 Ta _0.4 Ga _4.8 Si _0.8 O ₁₄ single crystal in Example 3.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 Tubular furnace 2, 10 Crucible 3, 17 Melt 4, 13 Seed crystal 5 After heater 6 Pulling shaft 7, 18 Single crystal 11 Heat insulating material 12 Refractory housing 14 Lifting shaft 15 Refractory cylinder 16 High frequency induction coil

Claims (5)

Ca ₃ Ga ₂ Ge ₄ O ₁₄ has a structure made of a main component is La, Sr, Ta, Ga and Si, the composition formula _{La 3-x Sr x Ta y} Ga 6-y-z Si z O 14 X, y, z in the composition formula are (x = 0, y = 0.35, z = 0.3), (x = 0.2, y = 0.4, z = 0. 4), (x = 0.8, y = 0.5, z = 0.8), (x = 1.2, y = 0.5, z = 1.2), (x = 1.0, y = 0.3, z = 1.4), (x = 0.6, y = 0.1, z = 1.4), (x = 0.2, y = 0, z = 1.2) , (X = 0, y = 0.15, z = 0.7), and (x = 0, y = 0.35, z = 0.3) are in the composition range obtained by sequentially connecting. Piezoelectric single crystal material.   2. The piezoelectric single crystal material according to claim 1, wherein x, y, and z in the composition formula are x = 0.2, y = 0.4, and z = 0.4.   2. The piezoelectric single crystal material according to claim 1, wherein x, y, and z in the composition formula are x = 1, y = 0.5, and z = 1.   2. The piezoelectric single crystal material according to claim 1, wherein x, y, and z in the composition formula are x = 0.6, y = 0.4, and z = 0.8.   A piezoelectric device using the piezoelectric single crystal material according to any one of claims 1 to 4.

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