JP2002368576A - Surface acoustic wave element - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a surface acoustic wave(SAW) element having new configuration, with which temperature fluctuation characteristics similar to a thickness- shear vibrator can be obtained and a practical capacitance ratio can be reduced by basically using the vibration principle of an end face reflection type vibrator. SOLUTION: This element has a piezoelectric substrate and an cord electrode formed on any one of front and rear sides of the piezoelectric substrate, the side to propagate the SAW vibration, which is generated by the cord electrode, on the piezoelectric substrate is made mechanically free and further, the thickness of the piezoelectric substrate is reduced rather than a wavelength λof the SAW vibration.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an elastic wave device, and more particularly, to an elastic wave device that realizes high stability in a high frequency band.

[0002]

2. Description of the Related Art In recent years, as the operating frequency of electronic equipment has become higher, it has become necessary to increase the frequency and stability of elastic wave elements used in electronic equipment, such as oscillators for microcomputer clocks and oscillators. I have.

[0003] Conventional techniques for elastic wave devices generally use thickness-shear vibration, and special techniques use BGS (Bleustein-Gulyaev-Shimizu / Nakamur).
a) An end face reflection type vibrator using waves is used.

In the above-described thickness-sliding vibrator, electrodes are provided on both surfaces of a substrate, and a drive signal is applied between the electrodes, so that vibration waves are generated in the thickness direction of the substrate. Therefore, the thickness of the element substrate becomes thinner as the vibration wave becomes higher in frequency,
And thickness accuracy is important. For this reason, in the thickness-sliding vibrator, a high-precision element processing technique has been required.

On the other hand, an end face reflection type vibrator has a configuration in which a drive electrode formed on one surface of a substrate generates an elastic wave on the substrate surface. For this reason, since the vibration frequency does not depend on the thickness of the element substrate, the element is characterized in that the processing of the element is relatively easy. However, compared to a thickness-slip oscillator,
There is a problem that the frequency changes greatly depending on the temperature environment used.

Further, in the end face reflection type vibrator, a ratio (capacity ratio) of a capacitance existing in series with a resistance component and a capacitance existing in parallel with the resistance component in an equivalent circuit basically represented by a parallel resonance circuit is obtained. Large, which has generally limited their use.

[0007]

SUMMARY OF THE INVENTION Accordingly, an object of the present invention is to basically use the vibration principle of the above-mentioned end face reflection type vibrator, to obtain the same temperature fluctuation characteristics as the above-mentioned thickness-sliding vibrator, and to obtain a practical use. It is an object of the present invention to provide an acoustic wave device having a novel configuration capable of reducing the above capacitance ratio.

[0008]

In view of the above-mentioned object of the present invention, the present inventor has obtained preferable characteristics by a novel structural approach, and an elastic wave device according to the present invention has a piezoelectric substrate and a piezoelectric substrate. Having an interdigital electrode formed on at least one of the front and back surfaces of the piezoelectric substrate, and mechanically free the surface of the piezoelectric substrate of the acoustic wave vibration generated by the interdigital electrode that propagates, Further, the thickness of the piezoelectric substrate is smaller than the wavelength λ of the elastic wave vibration.

As a preferred embodiment of the acoustic wave device for achieving the above object of the present invention, the thickness of the piezoelectric substrate is
The wavelength is not more than 0.5λ with respect to the wavelength λ of the elastic wave vibration.

Further, as a preferred embodiment of the acoustic wave device which achieves the above object of the present invention, the same pattern of interdigital electrodes is formed on at least the front and back surfaces of the piezoelectric substrate, and the vibration on both the front and back surfaces of the piezoelectric substrate is achieved. Are characterized by coupling resonance.

In a preferred embodiment of the elastic wave device for achieving the object of the present invention, the piezoelectric substrate has fixed portions at both ends where elastic wave vibration of the piezoelectric substrate is sufficiently attenuated. The distance is, with respect to the wavelength λ of the elastic wave vibration,
It is not less than 5λ.

Further, as a preferred embodiment of the acoustic wave device that achieves the above object of the present invention, an electrode is formed on a side surface of the piezoelectric substrate, and the electrodes formed on the side surface use the electrodes formed on both sides of the piezoelectric substrate. The present invention is characterized in that the interdigitated electrodes formed in the above are electrically connected.

Further, as a preferred embodiment of the acoustic wave device which achieves the above object of the present invention, a solid electrode is formed on the entire surface of the piezoelectric substrate opposite to the surface on which the interdigital electrodes are formed. It is characterized by.

In a preferred embodiment of the acoustic wave device for achieving the object of the present invention, an end face of the piezoelectric substrate is a reflection surface of the elastic wave vibration.

Further, as a preferred embodiment of the acoustic wave device which achieves the object of the present invention, it is preferable that grating electrodes or grooves for reflectors are formed on both sides of the interdigital transducer formed on the piezoelectric substrate. Features.

Further, as a preferred embodiment of the acoustic wave device which achieves the above object of the present invention, one of the interdigital transducers formed on both the front and back surfaces of the piezoelectric substrate is electrically short-circuited or has a load impedance. It is characterized by having a terminated configuration.

Further, a preferred embodiment of the acoustic wave device which achieves the object of the present invention is characterized in that the piezoelectric substrate is
Using a LiTaO ₃ or LiNbO ₃ substrate, the crystal axis direction is cut out around an envelope obtained by rotating the Y axis around the Z axis by 36 ± 20 °, and the propagation direction of the elastic vibration is set to the X direction.

The features of the present invention will become more apparent from the embodiments described with reference to the following drawings.

[0019]

FIG. 1 is a perspective view of a piezoelectric element showing a first embodiment according to the present invention. Drive electrodes 2 and 3 are formed in the same pattern on both front and back surfaces of the piezoelectric substrate 1. Further, it has electrode pads 4 and 5 electrically and mechanically joined to the drive electrodes 2 and 3 in the longitudinal direction of the piezoelectric substrate 1, and through the electrode pads 4 and 5, one surface side of the piezoelectric substrate 1 (FIG. 1).
Then, input and output are performed with the drive electrodes 2 and 3 on the back side.

FIG. 2 shows an electrical equivalent circuit of the piezoelectric element. A series connection of a capacitance Ca, an inductance L and a resistance R, and a capacitance Cb connected in parallel with the series connection
Having. The capacitance ratio mentioned above indicates the ratio γ of the series capacitance Ca and the parallel capacitance Cb, and is expressed by γ = Cb / Ca. It is known to those skilled in the art that when the capacitance ratio γ is large, the use of the piezoelectric element is greatly restricted.

Next, an example of a trial production of a piezoelectric element having the configuration shown in FIG. 1 will be described below. The drive electrodes 2 and 3 formed on the back surface of FIG. 1 connected to the electrode pads 4 and 5 generate elastic wave vibration on the surface of the piezoelectric substrate 1.

When the wavelength of the elastic wave vibration is λ, the thickness (H) 6 of the piezoelectric substrate 1 of the prototype is 0.25λ, and the thickness of the part floating in the air by the electrode pads 4 and 5 is The length (LF) 7 was 15λ.

The driving electrode material is NiCr / Au (film thickness 50 nm / 4).
00 nm), the intersecting length 8 of the interdigital electrodes by the driving electrodes 2 and 3 is 5λ, and the number of pairs of electrodes is one.
The interdigital electrodes are arranged on the front and rear surfaces, and the driving electrodes, that is, the driving electrodes electrically connected to the electrode pads 4 and 5 are only on the rear surface side in FIG.

Further, the piezoelectric substrate 1 is cut out at an angle obtained by rotating the Y axis by 40 ° about the Z axis with respect to the crystal axis XYZ, and has a propagation direction of the vibration wave in the X direction. LiTaO ₃ substrate was used.

The vibrator prototyped under the above-described conditions is shown in FIG.
Are as shown in the following table.

[0026]

[Table 1] Therefore, the capacity ratio γ of the above prototype example is given by γ = Cb / Ca
= 12.5.

FIG. 3 is a diagram showing the relationship between the substrate thickness (H / λ) normalized by the wavelength λ and the capacitance ratio (γ) for the piezoelectric element having the structure shown in FIG. As can be easily understood from this figure, it can be understood that the capacitance ratio decreases as the piezoelectric substrate 1 becomes thinner. Conventional piezoelectric elements have a capacitance ratio of 70 to 80 as a general value. On the other hand, according to the principle of the present invention, by setting the substrate thickness (H / λ) to 1 or less, it is possible to obtain a capacitance ratio equal to or less than the capacitance ratio of the conventional piezoelectric element. Further, the effect of setting the thickness of the piezoelectric substrate 1 to 0.5λ or less is remarkable as an effect.

FIG. 4 shows the piezoelectric substrate thickness (8) normalized to the wavelength (λ) when the electrode intersecting length (8) is 5λ and the number of electrode pairs is one pair and three pairs in the piezoelectric element having the configuration shown in FIG. H
/ Lambda) and a diagram showing the temperature change rate 10- ⁶ (ppm) / ℃ relationship.

From this figure, it can be understood that the smaller the thickness (H / λ) of the piezoelectric substrate 1 and the smaller the number of electrode pairs, the higher the temperature stability (degree of temperature fluctuation). In FIG. 4, it is considered that the temperature stability converges to a constant value regardless of the number of electrode pairs as the number of electrode pairs increases. Therefore, FIG.
The broken line with 10 electrode pairs is an expected value.

The temperature stability of the conventional piezoelectric element is generally about 40 ppm / ° C., whereas the thickness (H / λ) of the piezoelectric substrate 1 is set to 1 or less to reduce the temperature fluctuation rate of the conventional piezoelectric element. it is possible to 40 × 10- ⁶ or less.

FIG. 5 is a diagram comparing the characteristics of the thickness-slip vibrator and the end face reflection vibrator and the characteristics of the present invention, with the temperature fluctuation rate set to a reference value of zero at a room temperature of 25 ° C. In the figure, the horizontal axis represents temperature, the vertical axis represents fluctuation rate ppm, the characteristic line A represents the temperature fluctuation characteristics of the thickness-sliding end face vibrator, and the characteristic lines B and C represent the end face reflection vibrator and the piezoelectric vibration of the present invention, respectively. This is the temperature fluctuation characteristic of the child.

Here, the temperature fluctuation rate generally shows a quadratic curve characteristic having the lowest value at the top. In FIG. 5, as described above, the thickness sliding oscillator has a stable temperature fluctuation characteristic (characteristic line A) and has a peak near normal temperature (25 ° C.). On the other hand, the temperature fluctuation (characteristic line B) is large near the normal temperature (25 ° C.) of the end face reflection oscillator, and the peak of the characteristic curve is much higher than the normal temperature (at least 75 ° C. or more in FIG. 5). Is understood). The present invention is similar to the end face reflection oscillator as a basic structure,
A temperature fluctuation characteristic (special use line C) similar to that of the thickness-slip oscillator is obtained.

Here, from the above description, in order to obtain the effects of the temperature stability and the reduction of the capacitance ratio according to the present invention, only the portion where the elastic vibration generated by the drive electrodes 2 and 3 propagates has a predetermined substrate. The thickness may be within the range of the thickness (H / λ).

Therefore, the structure of the embodiment shown in FIG. 6 is possible as a structure for facilitating handling. In FIG.
Only the end portions corresponding to the electrode pads 4 and 5 of the piezoelectric substrate 1 and the regions other than the portions 10 and 11 for fixing the piezoelectric elements, where the elastic vibration propagates, have a predetermined thickness (1λ) or less. At these ends, the elastic wave vibration is sufficiently attenuated.

When the length of the region where the elastic vibration propagates, which is the length between the portions 10 and 11 for fixing the piezoelectric element in the structure of the embodiment shown in FIG. A relationship is obtained. FIG. 7 is a diagram showing the relationship between LF and the resonance resistance value in the structure of the embodiment in FIG. From this figure, it can be understood that when LF is 5λ or more, a preferable small value of the resonance resistance is obtained.

Next, in the basic structure of the present invention shown in FIG. 1, an extended modification of the drive electrodes 2 and 3 will be described.

FIG. 8 shows a first extended modification of the drive electrodes 2 and 3 in the structure shown in FIG. This is an embodiment configuration in which side electrodes 12, 13 for electrically connecting drive electrodes 2, 3 formed on both surfaces of the piezoelectric substrate 1 are formed on the side surfaces of the piezoelectric substrate 1.

By providing the side electrodes 12 and 13 as described above, elastic vibration occurs on the front and back surfaces of the piezoelectric substrate 1, and therefore, the driving efficiency of the vibration can be further increased.

FIG. 9 shows a second extended modification of the drive electrodes 2 and 3 in the structure shown in FIG. Instead of one of the drive electrodes 2, 3 formed on both surfaces of the piezoelectric substrate 1, that is, the drive electrodes 2, 3 on the side opposite to the side connected to the input / output electrode pads 4, 5, a metallized electrode 14 was used on the entire surface. This is an example configuration. With such a structure, it is possible to further enhance the electromechanical coupling between the input and output.

FIG. 10 shows a further extended modification of the drive electrodes 2 and 3 in the structure shown in FIG. Drive electrode 2.3
Are formed with grating electrodes 15 and 16 for reflectors on both sides of the reflector. Instead of reflection at the end face of the piezoelectric substrate 1,
The configuration is such that the grating electrodes 15 and 16 reflect elastic waves.

According to such a configuration, since the processing accuracy of the end face of the piezoelectric substrate in the end face reflection type vibrator is not affected, the manufacturing becomes easy.

FIG. 11 shows a modification of the embodiment of FIG. 10, in which the piezoelectric substrate 1 is replaced with the grating electrodes 15 and 16.
Grooves 17 and 1 for reflection are provided on both sides of the upper drive electrodes 2 and 3.
8 is formed by etching or the like.

Next, an example of using the drive electrodes 2 and 3 on the side not connected to the electrode pads 4 and 5 in the configuration of FIG. 1 will be described.

FIG. 12 shows the driving elements 2 and 2 of the piezoelectric element having the structure shown in FIG. 1 which are not connected to the electrode pads 4 and 5.
3 is electrically short-circuited or terminated with impedance 19. In this embodiment, substantially the same effect as the configuration in which one surface of the piezoelectric substrate 1 shown in FIG.

Although the embodiments of the present invention have been described using a piezoelectric vibrator as an example, the present invention is not limited to this. That is, a filter can be obtained using a piezoelectric element having improved temperature stability and capacitance ratio based on the principle of the present invention.

FIG. 13 is a view showing a cross section of the piezoelectric substrate 1 having the structure shown in FIG. The drive electrodes 2 and 3 on the front and back surfaces are electrically connected between the terminals 20-21 and 20'-21 '.
Therefore, a filter is configured by using the terminals 20-20 'and 21-21' as input / output electrodes, respectively. In such a configuration, the size of the filter element can be reduced.

In the configuration shown in FIG. 13, the generated elastic wave propagates in the thickness direction of the piezoelectric substrate 1 and is detected by the output electrode on the unit side. So, as a function,
It can be considered that it also has a function as a thickness slip oscillator.

FIG. 14 is a view showing a filter in which two sets of drive electrodes I and II are formed on one surface of the piezoelectric substrate 1 in any of the above embodiments. That is, the configuration is such that two sets of the drive electrodes 2 and 3 of the piezoelectric substrate 1 are continued. On the other surface of the piezoelectric substrate 1, it is possible to have the electrode configuration shown in FIGS. Thereby, two sets of drive electrodes I, I formed on one surface of the piezoelectric substrate 1 are formed.
The filter is electrically coupled through the electrode gap 30 between I, and a filter with high temperature stability is obtained.

FIG. 15 shows a structure in which two sets of drive electrodes I and II are arranged in parallel on one surface of the piezoelectric substrate 1 in any of the above embodiments. On the other surface of the piezoelectric substrate 1, it is possible to have the electrode configuration shown in FIGS. As a result, the filter is acoustically coupled through the space gap 31 between the two sets of drive electrodes I and II formed in one surface of the piezoelectric substrate 1, and a filter with high temperature stability is obtained.

Here, cutting out of the piezoelectric substrate 1 applied to the present invention will be described. FIG. 16 shows the definition of the crystal rotation angle.
Is shown in Generally, a LiTaO ₃ substrate or a LiTaO ₃ substrate is used as a piezoelectric substrate material. Crystal axis Z of piezoelectric substrate 1
, And the propagation direction of the vibration is the X direction.

FIGS. 17 and 18 show a LiTaO ₃ substrate, and FIGS.
The sound velocity with respect to the rotation angle on the LiNbO ₃ substrate is set to 0 (FIG. 17,
FIG. 19) and the relationship between the electromechanical coupling coefficients (FIGS. 18 and 20). The LiTaO ₃ substrate has a rotation angle of 33 ° (Fig. 1
8), when the LiNbO ₃ substrate has a rotation angle of 36 ° (FIG. 20), the coupling coefficient becomes maximum. The rotation angle may be in the range of 36 ° ± 20 ° in order to obtain the effect of the present invention.

Therefore, in applying the present invention, it is advantageous to use a piezoelectric substrate cut out at a crystal axis rotation angle at which the electromechanical coupling coefficient is maximized.

[0053]

As described above with reference to the drawings, according to the present invention, by reducing the thickness of the elastic wave vibrator to be less than the wavelength λ of the vibration propagating through the piezoelectric substrate, the elastic wave vibrator having a high temperature stability and a small capacitance ratio can be obtained. Provision is possible. Further, downsizing of the low frequency band and stable production of the elastic vibrator in the high frequency band can be performed, which greatly contributes to cost reduction.

[Brief description of the drawings]

FIG. 1 is a perspective view of a piezoelectric element showing a first embodiment of an elastic wave element according to the present invention.

FIG. 2 is a diagram showing an electrical equivalent circuit of the piezoelectric element having the configuration shown in FIG.

3 is a diagram showing a relationship between a substrate thickness (H / λ) normalized by a wavelength λ and a capacitance ratio (γ) for the piezoelectric element having the configuration of FIG. 1;

FIG. 4 is a diagram showing temperature characteristics of the piezoelectric element having the configuration of FIG. 1;

5 is a diagram comparing the temperature characteristics of the piezoelectric element having the configuration of FIG. 1 with those of the piezoelectric element having the conventional configuration.

FIG. 6 is a diagram showing a configuration of another embodiment of the present invention.

FIG. 7 is a diagram showing the relationship between the length of floating the piezoelectric substrate and the resonance resistance in the configuration of FIG. 6;

FIG. 8 is a view showing the configuration of another embodiment of the drive electrode of the present invention.

FIG. 9 is a view showing the configuration of still another embodiment relating to the drive electrode of the present invention.

FIG. 10 is a further extended modification of the drive electrodes 2 and 3 with respect to the structure shown in FIG.

FIG. 11 is a diagram showing a modification of the embodiment in FIG. 10;

12 is a configuration in which the drive electrodes 2 and 3 on the side not connected to the electrode pads 4 and 5 are electrically short-circuited or terminated with a load impedance 19 in the piezoelectric element having the configuration of FIG.

13 is a diagram illustrating a cross section of the piezoelectric substrate 1 having the configuration of FIG. 1 and illustrating a filter configuration.

14 is a diagram showing a filter in which two sets of drive electrodes I and II are formed on one surface of the piezoelectric substrate 1 shown in FIG.

FIG. 15 shows a configuration in which two sets of drive electrodes I and II are arranged in parallel on one surface of a piezoelectric substrate 1 shown in FIG.

FIG. 16 is a diagram showing a definition of a crystal rotation angle.

FIG. 17 is a diagram illustrating a relationship between a rotation angle and a sound speed in a LiTaO ₃ substrate.

FIG. 18 is a diagram illustrating a relationship between an electromechanical coupling coefficient and a rotation angle in a LiTaO ₃ substrate.

FIG. 19 is a diagram showing a relationship between a rotation angle and a sound speed in a LiNbO ₃ substrate.

FIG. 20 is a diagram showing a relationship between a rotation angle and an electromechanical coupling coefficient in a LiNbO ₃ substrate.

[Explanation of symbols]

 Reference Signs List 1 piezoelectric substrate 2, 3 drive electrode 4, 5 pad electrode 6 element thickness (H) 7 distance between pad electrodes (LF) 8 drive electrode cross length 9 element thickness 10, 11 side electrode 12 metallized electrode 13, 14 grating electrode 15 , 16 (15 ', 16') Electrical line 17, 18 Electrical line 19 Groove 20, 21 Gap

 ──────────────────────────────────────────────────続 き Continuing from the front page (72) Inventor Masaaki Ono 460 Oyama Oyama, Suzaka City, Nagano F-term in Fujitsu Media Devices Limited (Reference) 5J097 AA21 AA29 BB11 EE09 FF03 FF08 GG03 GG04 KK05 5J108 AA03 AA07 BB01 BB03 CC04 CC12 DD02 FF02

Claims (10)

[Claims] 1. A piezoelectric substrate comprising: a piezoelectric substrate; and interdigital transducers formed on at least one of the front and back surfaces of the piezoelectric substrate, and the acoustic wave generated by the interdigital transducer propagates through the piezoelectric substrate. An elastic wave device, wherein a surface is mechanically free, and a thickness of the piezoelectric substrate is smaller than a wavelength λ of the elastic wave vibration. 2. The elastic wave device according to claim 1, wherein the thickness of the piezoelectric substrate is 0.5λ or less with respect to the wavelength λ of the elastic wave vibration. 3. The acoustic wave device according to claim 1, wherein IDTs of the same pattern are formed on at least both front and back surfaces of the piezoelectric substrate, and vibrations on both front and back surfaces of the piezoelectric substrate are coupled and resonated. 4. The piezoelectric substrate according to claim 1, further comprising fixed portions at both end portions of the piezoelectric substrate at which elastic wave vibration is sufficiently attenuated, wherein a distance between the fixed portions is equal to a wavelength λ of the elastic wave vibration. An acoustic wave device having a wavelength of 5λ or more. 5. The piezoelectric substrate according to claim 3, wherein electrodes are formed on side surfaces of the piezoelectric substrate, and the electrodes formed on the side surfaces electrically connect the interdigital electrodes formed on both front and back surfaces of the piezoelectric substrate. An elastic wave device characterized by the above-mentioned. 6. The acoustic wave device according to claim 1, wherein a solid electrode is formed on the entire surface of the piezoelectric substrate opposite to the surface on which the interdigital electrodes are formed. 7. The elastic wave device according to claim 1, wherein an end face of the piezoelectric substrate is a reflection surface of the elastic wave vibration. 8. The acoustic wave device according to claim 1, wherein grating electrodes or grooves for reflectors are formed on both sides of the interdigital transducer formed on the piezoelectric substrate. 9. The acoustic wave device according to claim 3, wherein one of the interdigital transducers formed on both front and back surfaces of the piezoelectric substrate is electrically short-circuited or terminated with a load impedance. 10. The method of claim 1, as the piezoelectric substrate, LiTaO ₃ or, using a LiNbO ₃ substrate,
An elastic wave device, wherein the crystal axis direction is cut out around an envelope obtained by rotating the Y axis around the Z axis by 36 ± 20 °, and the propagation direction of the elastic vibration is set to the X direction.