JP4345473B2 - Manufacturing method of surface acoustic wave device - Google Patents

Description

The present invention relates to a method of manufacturing a surface acoustic wave element used in the band-pass filter or resonator or the like, and more particularly, to a method of manufacturing a surface acoustic wave element in which the domain structure of the piezoelectric substrate is improved.

  A surface acoustic wave is a wave that concentrates its energy and propagates near the surface of a medium. The surface acoustic wave element has a structure in which at least one interdigital transducer (IDT) is formed on a piezoelectric substrate. The IDT has a structure in which at least two comb-shaped electrodes are arranged so that the electrode fingers of the comb-shaped electrodes are interleaved with each other. By applying an electric signal between the comb-shaped electrodes, the surface wave is excited. Is done.

  A surface acoustic wave element can generally be reduced in size and is suitable for use in a high-frequency region, and is therefore widely used in bandpass filters, signal sources, signal processing devices, etc. for various AV equipment and communication equipment. .

  Conventionally, Rayleigh waves have been mainly used as surface acoustic waves, but recently, SH type surface waves such as BGS waves and Love waves have also been used.

As the piezoelectric substrate material of the surface acoustic wave element, a single crystal such as quartz, LiTaO _3, or LiNbO ₃ is mainly used. In addition, a PbTiO ₃ system (PT system) or a PbZrO ₃ -PbTiO ₃ system (PZT) is used. Piezoelectric ceramics mainly composed of solid solution are also used. In addition, a piezoelectric substrate formed by forming a piezoelectric film such as ZnO or PZT on an insulating substrate is also used.

  In order to obtain a desired characteristic in a surface acoustic wave device, an optimum design is necessary in designing an IDT, a surface acoustic wave vibration mode, and a piezoelectric substrate material, and various studies have been made so far.

  A Rayleigh wave, which is a general surface acoustic wave, propagates on each isotropic and anisotropic medium surface, and the surface wave is a P wave (Primary Wave, longitudinal wave) that is a displacement component in the traveling direction (x direction). ) And an SV wave (Vertical Shear Wave) which is a displacement component parallel to the depth direction (z direction). And 90% or more of the vibration energy is concentrated within one wavelength from the substrate surface, which shows an excellent energy concentration as a surface acoustic wave.

  However, since the P wave and the SV wave are complicatedly reflected at the boundary surface, mode conversion occurs at the time of reflection. Therefore, in order to use the surface acoustic wave element as a resonator, it is necessary to separately form a reflector, and the element size tends to be large. In particular, in the region where the frequency is low, the pitch between the electrodes of the IDT is increased, and thus the element size tends to be further increased.

  Further, as a defect that the vibration energy of the surface wave is concentrated on the surface, there is a problem that the characteristic is easily deteriorated in a reliability test or the like because it is easily influenced by the surface of the substrate.

  In contrast, an SH wave having a displacement component (y-direction component) in a direction perpendicular to the wave traveling direction (x-direction component) and the substrate depth direction (z-direction component) is an elastic surface that does not exist in an isotropic body. It is known that it is a wave and propagates on the surface of an anisotropic medium such as a piezoelectric medium or a layered medium.

  A BGS wave is one of SH waves, and propagates on various piezoelectric single crystals or piezoelectric ceramic substrates whose c-axis or polarization axis is parallel to the substrate surface.

  Unlike a Rayleigh wave, an SH wave such as a BGS wave has an advantage that a resonator can be formed without forming a reflector. This is because the SH wave is a wave having only a displacement component in the y direction parallel to the xy plane, and is completely reflected without causing mode conversion on the free end face of the high dielectric constant substrate. Therefore, the end surface of the piezoelectric substrate can be used as a reflecting surface, and the reflector can be omitted. Therefore, in the frequency region of several tens to several hundreds MHz where the element size tends to be large when Rayleigh waves are used, the size of the surface acoustic wave element can be effectively reduced by using the SH wave. it can.

  When the same piezoelectric substrate is used, the SH wave exhibits different characteristics from the Rayleigh wave in terms of electromechanical coupling coefficient (k), temperature characteristics, propagation speed, and the like. Therefore, application utilizing the characteristics is also expected.

  However, in the SH wave, (1) the surface concentration of the vibration energy of the surface wave is low, (2) the bulk wave is radiated inside during propagation, and (3) when reflected at the end face, There is also a problem that the propagation loss is essentially large due to the fact that the SH wave is easily affected by the end face. Therefore, it is necessary to reduce these propagation losses when applying SH waves. Conventionally, propagation loss is reduced by improving the processing accuracy of the element, improving the end face condition, and roughening the bottom surface of the substrate and part of the end face for the purpose of reducing bulk waves that cause spurious (Patent Document 1). It was done.

  However, even if the propagation loss is reduced as described above, in the SH type surface wave, when a piezoelectric substrate having a small electromechanical coupling coefficient k is used, the impedance Za at the antiresonance frequency and the impedance Zr at the resonance frequency are used. The ratio Za / Zr is small, and it is difficult to obtain narrow band filter characteristics.

In addition, when a substrate having a large electromechanical coupling coefficient k is used, the impedance ratio Za / Zr can be increased and a broadband filter can be formed. However, the weather resistance, particularly the resonance frequency when left in a humidity state, can be formed. There was a problem that the shift amount increased.

On the other hand, Patent Document 2 has a multilayer structure of two or more layers that differ in composition or electrical and mechanical characteristics in the thickness direction perpendicular to the electrode formation surface, or in relation to the composition or electrical and mechanical characteristics. By using a surface acoustic wave element characterized by having an inclined structure, the surface concentration on a piezoelectric substrate having a small electromechanical coupling coefficient k is controlled by mainly controlling the surface concentration of vibration energy of the surface acoustic wave. In order to reduce the loss due to the lowering of the degree and to make the piezoelectric substrate with a large electromechanical coupling coefficient k less susceptible to the influence of temperature, humidity, etc., suppressing excessive concentration of vibration energy on the substrate surface, etc. Za / Zr and moisture resistance are improved.
Japanese Patent Publication No. 8-21830 JP 2003-51734 A

  As described above, the characteristics of the surface acoustic wave can be improved by controlling the vibration state. However, when a control method such as that described in Patent Document 2 is used to control the vibration state represented by the surface concentration of the vibrational energy of the surface acoustic wave, the processing process becomes complicated. There was a problem that the increase in cost was also a concern.

An object of the present invention is to solve the drawbacks of the surface acoustic wave device using a conventional SH wave as described above, the reduction of propagation loss, improved weather resistance obtained easily and inexpensively that may be a surface acoustic wave element of reduced An object of the present invention is to provide a manufacturing method that makes it possible.

The present invention has been made to achieve the above object, and a method of manufacturing a surface acoustic wave device according to the present invention includes a step of preparing a piezoelectric substrate having first and second main surfaces, and the piezoelectric device. Applying a uniformly distributed electric field in a direction parallel to the first main surface of the conductive substrate and uniformly polarizing the electric field application direction is opposite to that during the polarization and in the thickness direction of the piezoelectric substrate. In contrast, the method includes polarization using an electric field wrapping phenomenon so that the electric field strength is distributed in a gradient, and forming an interdigital transducer on the first main surface of the piezoelectric substrate. Features. According to the onset bright, first, a piezoelectric substrate having a second major surface, and an interdigital transducer disposed on the first main surface side of the piezoelectric substrate, the minute of the piezoelectric substrate extreme is the thickness direction perpendicular to the first major surface, that have a slope varying domain structure, SH wave type surface acoustic wave device is provided.

  That is, the vibration state of the surface acoustic wave represented by the surface concentration degree of the vibration energy of the surface acoustic wave is greatly influenced by the electrical characteristics and mechanical characteristics of the piezoelectric substrate. In the present invention, the electrical characteristics and mechanical characteristics of the piezoelectric substrate are controlled by controlling the domain structure (degree of polarization) as described above.

  Conventionally, as an example of controlling the domain structure (polarization degree) of a surface acoustic wave device, a polarization processing method for performing domain orientation processing in an arbitrary direction, or an aging treatment after polarization processing to stabilize the domain structure There was a way to do. On the other hand, in the present invention, as described above, the domain structure is controlled so that the degree of polarization changes in an inclined manner in the thickness direction of the piezoelectric substrate, and as will be apparent from the examples described later. Therefore, the electrical characteristics and mechanical characteristics of the piezoelectric substrate can be controlled at low cost and easily without adding complicated additional steps to the conventional method of manufacturing the surface acoustic wave device.

  Therefore, since the electrical characteristics and mechanical characteristics of the piezoelectric substrate can be adjusted by controlling the domain structure (degree of polarization), a surface acoustic wave element having desired characteristics can be provided with high accuracy and at low cost.

  Aging refers to intentionally accelerating changes over time in various characteristics by raising the temperature of a piezoelectric body or the like and holding it for a certain period of time, and is also referred to as “withering”.

  In a specific aspect of the method for manufacturing a surface acoustic wave device according to the present invention, a piezoelectric ceramic is used as the piezoelectric substrate.

  As the SH wave, various SH waves such as a BGS wave, a love wave, and a leaky elastic wave can be used.

  Since the surface acoustic wave device according to the present invention has a domain structure in which the degree of polarization of the piezoelectric substrate changes in the thickness direction perpendicular to the first main surface on which the interdigital transducer is formed, The electrical characteristics and mechanical characteristics of the conductive substrate can be controlled in a tilted manner. Therefore, the control of the vibration mode represented by the surface concentration degree of the vibrational energy of the surface acoustic wave can be easily and highly accurately achieved by controlling the polarization structure. This is because the polarization structure can be controlled relatively easily and with high accuracy by externally applied energy such as an electric field.

  Moreover, since it is only necessary to control the polarization structure, a complicated process is not required in the production, and the mechanical characteristics and electrical characteristics can be improved without increasing the cost. A surface acoustic wave element having characteristics can be provided at low cost.

  In the present invention, when a piezoelectric ceramic is used as the piezoelectric substrate, an SH type surface acoustic wave element having desired mechanical characteristics and electrical characteristics can be easily configured according to the present invention.

  The surface acoustic wave device according to the present invention uses SH waves, but preferably BGS waves are used. In the case of a surface acoustic wave element using BGS waves, the surface concentration of surface wave energy is low. Therefore, by controlling the polarization structure of the piezoelectric substrate in accordance with the present invention, the impedance ratio Za / Zr is increased and narrow band is obtained. The filter characteristics can be easily realized and the weather resistance can be improved.

  In the method for manufacturing a surface acoustic wave device according to the present invention, an electric field having a uniform distribution is applied in a direction parallel to the first main surface of the piezoelectric substrate to uniformly polarize the electric field. The piezoelectric substrate is polarized by performing polarization using the wraparound phenomenon of the electric field so that the application direction is opposite and the electric field strength is distributed in a gradient with respect to the thickness direction of the piezoelectric substrate.

  Therefore, since the polarization treatment is performed so as to have a domain structure that changes in the thickness direction as described above, similarly to the surface acoustic wave element of the present invention, the electrical characteristics and mechanical properties of the piezoelectric substrate Therefore, the surface acoustic wave element having desired characteristics can be easily obtained. Further, since the polarization can be easily performed by applying an electric field or the like, the manufacturing process is not complicated.

  Hereinafter, the present invention will be described in more detail by describing specific examples of the present invention.

  First, with reference to FIG.1 and FIG.2, the specific embodiment of the surface acoustic wave element of this invention is described.

A surface acoustic wave element 1 shown in FIG. 1 has a rectangular plate-shaped piezoelectric substrate 2. The piezoelectric substrate 2 is made of PbTiO ₃ or PbZrO ₃ —PbTiO ₃ based piezoelectric ceramics. An interdigital transducer 3 is formed on the upper surface 2 a as the first main surface of the piezoelectric substrate 2. The interdigital transducer 3 has a pair of comb-shaped electrodes 3a and 3b. The electrode fingers of the comb-shaped electrodes 3a and 3b are disposed so as to be inserted into each other. In the surface acoustic wave element 1, the piezoelectric substrate 2 is polarized in the direction indicated by the arrow P. That is, it is polarized in a direction parallel to the electrode fingers of the comb-shaped electrodes 3a and 3b, and the surface wave propagation direction A is a direction orthogonal to the electrode fingers.

  In the surface acoustic wave element 1, a BGS wave is excited by applying an alternating voltage between the comb electrodes 3a and 3b, and the BGS wave propagates along the propagation direction A and is reflected by the end faces 2b and 2c. That is, the facing end faces 2b and 2c are parallel to each other, and constitute a reflecting end face of an end face reflection type surface wave resonator using a BGS wave.

  The surface acoustic wave device 1 according to the present embodiment is characterized by the domain structure (polarization degree) of the piezoelectric substrate 2. This will be described with reference to FIG.

  FIG. 2 is a schematic cross-sectional view for explaining a method of polarizing the piezoelectric substrate 2, and shows a state seen from the end face 2c side of FIG.

  When the piezoelectric substrate 2 is polarized, the polarization is first performed uniformly. That is, as shown in FIG. 2A, polarization electrodes 6a, 6b, 7a, and 7b are formed, and a voltage of the same level is applied to the polarization electrode 6a and the polarization electrode 7a, so that the polarization electrode 6b and the polarization electrode 6b are polarized. A voltage having a certain potential difference from them is applied to the electrode 7b (polarization method A).

  Subsequently, as shown in FIG. 2 (b), a voltage is applied only to the polarization electrodes 6a and 6b in a direction opposite to the polarization method A (polarization method B), so as to be inclined with respect to the thickness direction of the substrate. A domain structure with an increased degree of polarization can be obtained. Further, as shown in FIG. 2 (c), a domain in which the degree of polarization is inclined with respect to the thickness direction of the substrate by applying a voltage to only the polarization electrodes 7a and 7b in the opposite direction to the polarization method A. A structure can also be obtained (polarization method C).

  In this manner, by applying "tilt type reverse polarization after uniform polarization once", distortion during polarization can be reduced. Conventional examples (Japanese Patent No. 2508334, Japanese Patent Laid-Open No. 5-160463) Warping and cracking during polarization, as occurred in This is because the 180 ° domain and the non-180 ° domain are uniformly polarized with respect to the thickness direction of the substrate in the first uniform polarization, and then only the 180 ° domain that is easily oriented by the gradient polarization of the reverse electric field is inverted. This is probably because the degree of polarization can be controlled more stably without generating.

  In the surface acoustic wave device 1 of the present embodiment, as described above, the piezoelectric substrate 2 has a domain structure in which the degree of polarization is changed in an inclined manner in the thickness direction orthogonal to the upper surface 2a. Compared with the case where a piezoelectric substrate is used, the electrical characteristics and mechanical characteristics of the piezoelectric substrate can be controlled by controlling the polarization structure. This point will be described later based on a specific embodiment.

  In the polarization structure obtained by the polarization method B shown in FIG. 2B, the degree of polarization is changed so that the degree of polarization is lower on the upper surface 2a than on the lower surface 2f side. Conversely, as shown in (c), the domain structure may be formed such that the degree of polarization decreases from the upper surface 2a side to the lower surface 2f side.

In the surface acoustic wave element 1 of the above embodiment, the piezoelectric substrate 2 is made of piezoelectric ceramics as described above, but may be a single crystal such as LiTaO ₃ or LiNbO ₃ .

  In addition, the present invention can be applied not only to BGS waves but also to surface acoustic wave elements using other SH waves such as Love waves.

  Next, specific examples will be described.

  Two types of piezoelectric substrates of 50 mm × 50 mm × thickness 0.7 mm made of porcelain compositions α and β shown in Table 1 below were prepared. The porcelain composition α gives a piezoelectric ceramic having a smaller electromechanical coupling coefficient k than the porcelain composition β.

  Next, samples (1) to (4) were prepared by the substrate composition and polarization method shown in Table 2 below. Detailed production procedures of the samples (1) to (4) are as follows.

  [Sample (1)]... A piezoelectric substrate made of the porcelain composition α is polarized on the upper surface of the piezoelectric substrate 2 and polarized electrodes 6a and 6b on the lower surface as shown in the polarization method A of FIG. Electrodes 7a and 7b are formed, and an electric field of 2.5 to 4 kV / mm is applied for 10 minutes to 60 minutes between the polarizing electrodes 6a and 6b and between the polarizing electrodes 7a and 7b in oil at 80 ° C. to 200 ° C. Then, the piezoelectric substrate 2 was uniformly fully polarized. Next, as shown in the polarization method B in FIG. 2B, the piezoelectric substrate is immersed in oil at 60 ° C. to 100 ° C. using only the polarization electrodes 6a and 6b formed on the upper surface of the piezoelectric substrate. Then, an electric field of 0.5 to 3.0 kV / mm was applied in a pulse manner in the direction opposite to that shown in FIG.

  [Sample (2)]... A piezoelectric substrate made of the porcelain composition β is polarized on the upper surface of the piezoelectric substrate 2 and polarized electrodes 6a and 6b on the lower surface as shown in the polarization method A of FIG. Electrodes 7a and 7b are formed, and an electric field of 2.5 to 4 kV / mm is applied for 10 to 60 minutes in oil at 80 to 200 ° C. between the polarizing electrodes 6 a and 6 b and between the polarizing electrodes 7 a and 7 b. Applied, the piezoelectric substrate 2 was uniformly fully polarized. Next, as shown in the polarization method C in FIG. 2 (c), the piezoelectric substrate is immersed in oil at 60 ° C. to 100 ° C. using only the polarization electrodes 7a and 7b formed on the lower surface of the piezoelectric substrate. Then, an electric field of 0.5 to 3.0 kV / mm was applied in a pulse manner in the direction opposite to that shown in FIG.

  [Sample (3)] A piezoelectric substrate made of the porcelain composition α is polarized on the upper surface of the piezoelectric substrate 2 and the polarizing electrodes 6a and 6b on the lower surface as shown in the polarization method A of FIG. Electrodes 7a and 7b are formed, and an electric field of 2 to 3.5 kV / mm is applied for 10 minutes to 60 minutes in oil at 80 ° C. to 150 ° C. between the polarizing electrodes 6a and 6b and between the polarizing electrodes 7a and 7b. Applied and polarized (conventional method).

  [Sample (4)]... A piezoelectric substrate made of the porcelain composition β is polarized on the upper surface of the piezoelectric substrate 2 and the polarization electrodes 6a and 6b on the lower surface as shown in the polarization method A of FIG. Electrodes 7a and 7b are formed, and an electric field of 2 to 3.5 kV / mm is applied for 10 minutes to 60 minutes in oil at 80 ° C. to 150 ° C. between the polarizing electrodes 6a and 6b and between the polarizing electrodes 7a and 7b. Applied and polarized (conventional method).

  The polarizing electrodes of the piezoelectric substrates of the samples (1) to (4) produced as described above were removed with an etching solution and ultrasonically cleaned. Thereafter, aging was performed at a temperature of 150 ° C. to 200 ° C. for 10 minutes to 60 minutes to obtain each aged piezoelectric substrate.

  In addition, although the stability of a domain structure is improved by aging, aging is not necessarily an essential process in this invention.

  Next, Al was vapor-deposited on the upper surface of the piezoelectric substrate obtained as described above, and was patterned by photolithography to form one interdigital transducer 3. Furthermore, the piezoelectric substrate was cut into dimensions of about 1 mm × about 2 to 3 mm × thickness 0.7 m, and four types of piezoelectric substrates were obtained.

As described above, four types of surface acoustic wave elements of Samples (1) to (4) were prepared, and the impedance analyzer k _BGS wave electromechanical coupling coefficient k _BGS and each of 40, 80, 120 and 160 MHz The impedance ratio Za / Zr _BGS between the antiresonance point and the resonance point for each resonance frequency was measured. Further, a moisture resistance test was performed in which each surface acoustic wave element was left in an environment of a temperature of 60 ° C. and a relative humidity of 95% for 1000 hours, and the resonance frequency shift amount ΔFr was measured. The results are shown in FIGS.

In addition to the above evaluation, as shown in FIG. 7, the vibration energy of the BGS wave at each resonance frequency was simulated by the finite element method for each k _BGS value.

As apparent from FIGS. 3 and 4, the surface acoustic wave device of the sample (1) has an impedance ratio Za / Zr in a low frequency range of 40 to 80 MHz, even though k _BGS is very low, about 20%. Is 45 dB or more, which is a value capable of forming a filter. Therefore, it can be seen that the impedance ratio Za / Zr can be greatly improved as compared with the sample (3) produced by the conventional method.

  This is presumably because, in the sample (1), the domain structure in which the polarization degree is changed in a gradient is realized, so that the degree of surface concentration of the BGS wave is increased, thereby reducing the propagation loss.

Further, the improvement of the impedance ratio Za / Zr is mainly achieved in the 40 to 80 MHz band when the electromechanical coupling coefficient k _BGS is 20% as shown in FIG. It is considered that the surface concentration at (40 to 80 MHz) is particularly bad and the propagation loss is reduced by improving the surface concentration.

  Furthermore, as can be seen from FIG. 7, the sample (1) has a high frequency characteristic that is substantially equal to or higher than that of the sample (3) manufactured by the conventional method. The sample (1) tends to be somewhat deteriorated in moisture resistance, but the level of moisture resistance required for the reliability of the narrow-band bandpass filter is ensured.

As apparent from FIGS. 5 and 6, in the sample (2), the electromechanical coupling coefficient k _BGS is as large as about 50%, and the impedance ratio Za / Zr is about 45 dB that can form a broadband filter up to about 100 MHz. Value. In the sample (2), the surface concentration of excessive surface waves is reduced. Therefore, in the sample (2), the impedance ratio Za / Zr is somewhat lower in the 40 to 80 MHz band than the sample (4) produced by the conventional method, but the impedance ratio is not at a level that causes practical problems. I understand that.

  Furthermore, it can be seen that the sample (2) has a smaller resonance frequency shift after the moisture resistance test than the sample (4) produced by the conventional method, and therefore the moisture resistance is improved.

  This is because the sample (2) has a domain structure in which the degree of polarization changes in an inclined manner, so that excessive concentration of surface waves on the substrate surface that is easily affected by temperature, humidity, etc. is alleviated. This is probably because the weather resistance is enhanced.

1 is a perspective view showing a surface acoustic wave element according to an embodiment of the present invention. (A) is a typical end view for explaining the process of performing the conventional polarization process in the manufacturing method of the surface acoustic wave element of one embodiment of the present invention, and (b) and (c) are the present invention. Each typical end view for demonstrating the process of performing a polarization process in the manufacturing method of the surface acoustic wave element of one Embodiment. The figure which shows the relationship between the resonant frequency and impedance ratio of each surface acoustic wave element using the piezoelectric substrate polarization-processed on various polarization conditions. The figure which shows the relationship between the leaving time in the humidity of the surface acoustic wave element using the piezoelectric substrate polarization-processed on various polarization conditions, and the amount of resonance frequency shifts. The figure which shows the relationship between the resonant frequency and impedance ratio of each surface acoustic wave element using the piezoelectric substrate polarization-processed on various polarization conditions. The figure which shows the relationship between the leaving time in the humidity of the surface acoustic wave element using the piezoelectric substrate polarization-processed on various polarization conditions, and the amount of resonance frequency shifts. The figure which shows the relationship between the penetration frequency which shows the resonant frequency of the surface acoustic wave element comprised using the piezoelectric substrate which has various electromechanical coupling coefficients, and vibration energy distribution.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 ... Surface acoustic wave element 2 ... Piezoelectric substrate 2a ... Upper surface 2b, 2c ... End surface 2f ... Lower surface 3 ... Interdigital transducer 6a, 6b, 7a, 7b ... Electrode for polarization

Claims (3)

Preparing a piezoelectric substrate having first and second main surfaces;
Applying a uniformly distributed electric field in a direction parallel to the first main surface of the piezoelectric substrate to perform polarization uniformly;
A step of performing polarization using an electric field wraparound phenomenon such that the electric field application direction is opposite to that during polarization and the electric field strength is distributed in a gradient with respect to the thickness direction of the piezoelectric substrate;
Forming an interdigital transducer on the first main surface of the piezoelectric substrate. A method of manufacturing an SH wave type surface acoustic wave element. As the piezoelectric substrate, a piezoelectric ceramics, a manufacturing method of the SH wave type surface acoustic wave device according to claim 1. The manufacturing method of the SH wave type | mold surface acoustic wave element of Claim 1 or 2 with which a BGS wave is excited.

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