JP7448901B2 - piezoelectric device - Google Patents

Description

The present invention relates to a piezoelectric vibrator that vibrates by thickness shear vibration.

There is an increasing demand for improved characteristics of piezoelectric devices. For example, in a high-precision temperature-compensated crystal oscillator (TCXO), the frequency-temperature characteristics of the crystal oscillator itself are measured, and this temperature characteristic is approximated by a high-order function, such as a 4th to 7th order function. There is a demand for making the temperature characteristics of the output from the TCXO as flat as possible by compensating the frequency according to an approximate formula. In order to satisfy such requirements, it is ideal that the approximation curve regarding the frequency-temperature characteristics of the crystal resonator itself has a correlation coefficient of 1. However, in reality, a so-called frequency dip occurs where the actual frequency deviates from the approximate curve at various temperatures.

As a technique for suppressing frequency dips, for example, there is a technique disclosed in Patent Document 1 filed by the applicant of this application. Specifically, for each of the first extraction electrode provided on the first main surface of the piezoelectric piece and the second extraction electrode provided on the second main surface, an unnecessary vibration suppression electrode is provided in an opposite region across the piezoelectric piece. This is a technology that has been developed.
According to this technique, the absolute value of the frequency dip can be suppressed compared to the case where this structure is not used.

Japanese Patent Application Publication No. 2018-137715

According to the technique disclosed in Patent Document 1, the absolute value of the frequency dip can be suppressed. However, it has not been possible to change the temperature at which the frequency dip exceeds the specifications and becomes a problem (hereinafter sometimes referred to as frequency dip occurrence temperature). If there were a technology that could change the temperature at which the frequency dip occurs, it would be useful as a design method for piezoelectric devices.
This application was filed in view of these points, and therefore, the purpose of this application is to provide a piezoelectric device having a novel structure that can control the temperature at which a frequency dip occurs.

In order to achieve this objective, according to the piezoelectric device of this application, the piezoelectric device includes a piezoelectric piece and excitation electrodes provided on the front and back sides of the piezoelectric piece.
A conductive film is provided on at least a partial region of the front and back sides of the piezoelectric piece, which is separated by a distance G from the edge of the excitation electrode on the front and back sides, the conductive film being electrically connected on the front and back sides,
The distance G is characterized in that it is a distance that adjusts the temperature at which the frequency dip exceeds the specification and becomes a problem.
Note that when conductive films are provided at multiple locations on the piezoelectric piece, the distance G referred to here may be the same or different for each conductive film. That is, depending on the temperature at which the frequency dip occurs, the distance may be the same or the distance may be different.

According to the piezoelectric device of this application, the frequency dip generation temperature can be controlled.

(A) and (B) are explanatory diagrams of the piezoelectric device 10 of the first embodiment. (A), (B), and (C) are explanatory diagrams of the results of trial production of the piezoelectric device 10 of the first embodiment. FIG. 3 is an explanatory diagram following FIG. 2 of the trial production results of the piezoelectric device 10 of the first embodiment. It is an explanatory view of piezoelectric device 50 of other embodiments. (A) is an explanatory diagram of a piezoelectric device 60 of still another embodiment, and (B) is an explanatory diagram of a piezoelectric device 70 of still another embodiment. It is an explanatory view of piezoelectric device 80 of still other embodiment.

Embodiments of each invention of this application will be described below with reference to the drawings. Note that the drawings used in the explanation are merely shown schematically to the extent that these inventions can be understood. Moreover, in each figure used for explanation, the same number is attached|subjected and shown about the same component, and the explanation may be abbreviate|omitted. Furthermore, the shapes, dimensions, materials, etc. described in the following embodiments are merely preferred examples within the scope of the present invention. Therefore, the present invention is not limited only to the following embodiments.

1. Piezoelectric device of first embodiment 1-1. Structure of Piezoelectric Device FIG. 1 is a diagram illustrating the structure of a piezoelectric device 10 of the first embodiment. In particular, FIG. 1(A) is a plan view of the piezoelectric device 10, and FIG. 1(B) is a cross-sectional view taken along line PP in FIG. 1(A). Note that in FIG. 1(A), illustration of the lid member 21 shown in FIG. 1(B)) is omitted.

This piezoelectric device 10 includes a piezoelectric piece 11, a first excitation electrode 13a, a first extraction electrode 13b, a second excitation electrode 13c, a second extraction electrode 13d, a conductive film 15, and a container 17. , a conductive adhesive 19, and a lid member 21. These constituent components will be explained in detail below.

The piezoelectric piece 11 is capable of thickness-shear vibration, and is a variety of piezoelectric pieces including a crystal piece. Typically, it is an AT-cut crystal piece or a twice-rotation-cut crystal piece, such as an SC cut. In the case of this embodiment, the piezoelectric piece 11 is an AT-cut crystal piece having a square planar shape, specifically a rectangular shape. More specifically, the piezoelectric piece 11 is a so-called X-long crystal piece, in which the long side is the side along the X-axis direction of the crystal, and the short side is the side along the Z' axis of the crystal. However, a so-called Z' long crystal piece may also be used. Furthermore, a crystal piece that is not square in plan view, such as an elliptical or circular crystal piece, may be used, but a square piece is preferable.

Further, a first excitation electrode 13a is provided on one main surface of the piezoelectric piece 11, and a second excitation electrode 13c is provided on the other main surface of the piezoelectric piece 11. The first excitation electrode 13a and the second excitation electrode 13c are provided to face each other with the piezoelectric piece 11 in between. Further, the first extraction electrode 13b is extended from a part of the first excitation electrode 13a to one short side of the piezoelectric piece 11. Further, the second extraction electrode 13d is extended from a part of the second excitation electrode 13c to the one short side of the piezoelectric piece 11. These excitation electrodes and extraction electrodes can be made of any suitable metal film.

Further, the conductive film 15 is provided on at least a part of the front and back areas of the piezoelectric piece 11, which is separated by a distance G from the edge of each of the first excitation electrode 13a and the second excitation electrode 13c. . In this embodiment, the conductive film 15 is provided at a distance G away from both edges of each of the excitation electrodes 13a and 13c in the direction along the short side of the piezoelectric piece 11.
The conductive films 15 provided on the front and back sides are electrically connected to each other via the side surfaces of the piezoelectric piece 11. The conductive film 15 is typically a metal film, more typically a metal film made of the same material as the first and second excitation electrodes 13a and 13c.

The first excitation electrode 13a, the second excitation electrode 13c, and the conductive film 15 are preferably formed at the same time. Specifically, a plating frame having openings corresponding to the first excitation electrode 13a, the second excitation electrode 13c, and the conductive film 15 is used, and any suitable device such as a sputtering device or a vapor deposition device is used. It is preferable to attach a metal film for electrode formation to the piezoelectric piece 11 using a membrane device, and form the first excitation electrode 13a, the second excitation electrode 13c, and the conductive film 15 on the piezoelectric piece 11 at the same time. Alternatively, a metal film for forming the first excitation electrode 13a, the second excitation electrode 13c, and the conductive film 15 is attached to the piezoelectric wafer, and this metal film is formed using known film formation techniques and photolithography techniques. The first excitation electrode 13a, the second excitation electrode 13c, and the conductive film 15 may be simultaneously formed by patterning using the piezoelectric wafer, and then each piezoelectric piece may be separated from the piezoelectric wafer.

This is because when the first excitation electrode 13a, the second excitation electrode 13c, and the conductive film 15 are formed at the same time, the distance G can be formed more stably than when they are not formed. That is, although the details will be described later, the distance G that contributes to the adjustment of the temperature at which the frequency dip occurs can be controlled with high accuracy, and as a result, the adjustment of the temperature at which the frequency dip occurs can be performed with good control.

In this case, the container 17 includes a recess 17a, a connection pad 17b, and an external terminal 17c. For example, it is a known ceramic package.
The recess 17a has a shape and size that accommodates the piezoelectric piece 11. The connection pads 17b are provided at predetermined positions in the recess 11a of the container 11 so that the piezoelectric piece 11 can be held near both ends of one side of the piezoelectric piece 11. The external terminal 17c is provided on the outer bottom surface of the container 17. The connection pad 17b and the external terminal 17c are electrically connected by via wiring (not shown) provided in the container 17.
The piezoelectric piece 11 is electrically and mechanically connected to the connection pad 17b of the container 17 by the conductive adhesive 19 near both ends of one side thereof and at the positions of the ends of the first and second extraction electrodes 13b and 13d. The connection is fixed. That is, the piezoelectric piece 11 is fixed to the container 17 with a cantilever support structure. This container 17 is then sealed with a lid member 21.

1-2. Effects of the conductive film 15 Next, the effects of the conductive film 15 will be described with reference to experimental results.
In the piezoelectric device 10, thickness shear vibration is excited by an oscillation circuit (not shown) and first and second excitation electrodes 13a, 13c. In principle, this vibration is confined within the region of the excitation electrode of the piezoelectric piece 11 and continues. However, a portion of the vibration often reaches the edge of the piezoelectric piece 11, and in such cases, depending on the distance from the edge of the excitation electrode to the edge of the piezoelectric piece 11, unnecessary reflection of the vibration may occur. Unnecessary vibrations occur that are harmful to the main vibrations. For example, if the position of the excitation electrodes provided on the front and back sides of the piezoelectric piece 11 deviates from a predetermined position, and the distance from the edge of the excitation electrode to the edge of the piezoelectric piece changes from the predetermined distance, unnecessary vibrations may occur. occurs. For example, the distance from the edge of the excitation electrode to the edge of the piezoelectric piece may be caused by misalignment of the plating frame with respect to the piezoelectric piece during the formation of the excitation electrode, or by variations in external dimensions and shape due to variations in the processing of the piezoelectric piece itself. The distance may vary from a predetermined distance, and unnecessary vibrations occur.

In such a case, in the present invention, since the conductive film 15 is provided at a distance G from the edges of the first and second excitation electrodes 13a and 13c, this conductive film 15 has the effect of suppressing unnecessary vibrations. shows. That is, by changing the distance G between the excitation electrodes 13a, 13c and the conductive film 15, the temperature at which the frequency dip occurs can be adjusted.

In order to deepen the understanding of the effect of the conductive film 15 described above, the above effect will be further explained using the results of a trial production of the piezoelectric device 10.
The piezoelectric device 10 described using FIG. 1 was prototyped with the dimensions of each part as described below.
As shown in FIG. 1(A), the X dimension of the piezoelectric piece 11, Xs=4 mm, the Z' dimension of the piezoelectric piece 11, Zs=1.85 mm, and the X dimension of the first and second excitation electrodes 13a, 13c, Xe= A piezoelectric device having a diameter of 1.4 mm, a Z' dimension Ze of the first and second excitation electrodes 13a and 13c of 0.96 mm, and an oscillation frequency of 38.88 MHz was fabricated. The excitation electrodes 13a and 13c were eccentrically centered by a distance a, specifically, 0.35 mm, toward the tip end of the piezoelectric piece 11 with respect to the piezoelectric piece 11. Note that the distance G between the excitation electrodes 13a, 13c and the conductive film 15 was set to three levels: G=0.13 mm, G=0.21 mm, and G=0.26 mm. Note that the number of samples of piezoelectric devices of three levels was 10 each.

Next, the frequency-temperature characteristics of all three types of piezoelectric devices described above were measured in the range of -40°C to 105°C in steps of 1°C. Furthermore, for the measured temperature characteristics of each piezoelectric device, an approximate expression using a quartic function was determined by the least squares method. Furthermore, for each piezoelectric device, find the difference Δf between the frequency in the above approximate formula and the actual measurement frequency for each measurement temperature, and divide this Δf by the oscillation frequency F, the value Δf/F (hereinafter, this is the frequency Dip (frequency Dip, unit: ppm) was determined.
FIGS. 2A, 2B, and 2C are diagrams in which the horizontal axis represents temperature, the vertical axis represents frequency dip, and the frequency dips of the three prototypes described above are plotted. However, in order to prevent the diagram from becoming complicated, the diagram does not show the plots of all prototypes, but plots the frequency dips of several prototypes for each level.

In addition, the first temperature at which the frequency dip exceeds 0.4 ppm in absolute value is defined as the frequency dip occurrence temperature, and the frequency dip occurrence temperature of each prototype is calculated from the frequency dip data of the 30 prototypes measured. Extracted. The extraction results are shown in Table 1 below. Note that the criterion value of 0.4 ppm is just an example.

Further, FIG. 3 shows the relationship between the frequency dip occurrence temperature and the distance G for the 30 prototypes, with the horizontal axis representing the distance G and the vertical axis representing the temperature.
From FIG. 2, FIG. 3, and Table 1, it can be seen that there is a correlation between the distance G and the temperature at which the frequency dip occurs. Specifically, the average value of the frequency dip occurrence temperature of 10 prototypes with distance G = 0.13 mm is 82.6 ° C, and the average value of the frequency dip occurrence temperature of 10 prototypes with distance G = 0.21 mm is The average frequency dip generation temperature of 10 prototypes at 49.2°C and distance G = 0.26mm is 30.3°C. It can be seen that as the distance G becomes smaller, the temperature at which the frequency dip occurs changes to the higher temperature side. This shows that the temperature at which the frequency dip occurs can be adjusted by changing the distance G between the excitation electrodes 13a, 13c and the conductive film 15.

2. Other Embodiments In the first embodiment, the conductive film 15 was provided on the side along the Z'-axis direction of the crystal with respect to the excitation electrodes 13a and 13c. Therefore, the above case is particularly effective in suppressing unnecessary modes caused by reflection of waves propagated in the Z'-axis direction of the crystal. However, in piezoelectric devices, unnecessary modes also occur due to waves propagating along the X-axis direction of the crystal. Therefore, the area where the conductive film 15 is provided can be changed in various ways depending on the unnecessary mode. Some embodiments will be shown below.

FIG. 4 is a plan view showing another embodiment of the piezoelectric device 50. In the case of the piezoelectric device 50 of this embodiment, the conductive film 15a for frequency dip generation temperature adjustment is provided in a region on the tip side of the piezoelectric piece 11 and a distance G away from the excitation electrodes 13a and 13c. be. However, as already mentioned, the distance G is a value that is set depending on where the frequency dip generation temperature is adjusted, and is not necessarily the value in the first embodiment (as described below). (same in piezoelectric devices of other embodiments).

FIG. 5(A) is a plan view showing a piezoelectric device 60 in yet another practical form. In the case of the piezoelectric device 60 of this embodiment, the conductive film 15a for frequency dip generation temperature adjustment is placed on the tip side of the piezoelectric piece 11 in a region separated by a distance G from the excitation electrodes 13a and 13c. 4. Similarly, a conductive film 15b for frequency dip generation temperature adjustment is provided in a region on the side of the piezoelectric piece 11 supported by the conductive adhesive and separated by a distance G from the excitation electrodes 13a and 13c. It is provided in

FIG. 5(B) is a plan view showing a piezoelectric device 70 in yet another practical form. In the case of the piezoelectric device 70 of this embodiment, the conductive films 15, 15a, and 15b are provided with a structure that is a combination of the structure shown in FIG. 1 and the structure shown in FIG. 5(A). This is effective in suppressing unnecessary modes in the Z' direction and the X direction of the piezoelectric piece. As already mentioned, the distance G between the conductive films 15, 15a, and 15b may be the same or different. Further, for example, the distance G may be different between the left and right conductive films 15 (the same applies in the first embodiment).

Each of the embodiments described above is an example in which the present invention is applied to a piezoelectric device having a cantilevered structure. However, as shown in FIG. 6, the present invention can also be applied to a piezoelectric device having a so-called double-sided structure in which a piezoelectric piece is held at two opposing ends. In that case, a conductive film 15c is provided on each of the two sides of the piezoelectric piece 11 that are not supported by the conductive adhesive 19 at a distance G from the excitation electrode 13a (13c).

Furthermore, in each of the figures used to explain each embodiment described above, the length of the conductive film 15 is the same as the length of the excitation electrode, but the length of the conductive film 15 may vary depending on the design. It may be shorter than the length of the electrode (such as the conductive film 15b in FIG. 4), or it may be longer. However, considering the purpose, the length of the conductive film 15 is preferably the same as or slightly shorter than the length of the excitation electrode. Further, the conductive film 15 is preferably arranged so as to face at least the central part of the side of the excitation electrode. Since the vibration intensity is strongest near the center of the excitation electrode, it can be considered that vibration leakage is stronger near the center of the side of the excitation electrode than in other areas.Therefore, the conductive film 15 is placed opposite this area. This is because it is considered a good idea to do so.

Further, in each of the above-described embodiments, the container 17 has a recess 17a that accommodates the piezoelectric plate 11, but the cap has a flat base on which the piezoelectric plate 11 is placed, and a recess that contains the piezoelectric plate 11. Of course, the present invention can also be applied to a piezoelectric device in which a container is configured with a lid member having a shape.

10: piezoelectric device of the first embodiment,
13a: first excitation electrode, 13b: first extraction electrode,
13c: second excitation electrode, 13d: second extraction electrode,
15, 15a, 15b, 15c: Conductive film (film for frequency dip generation temperature adjustment)
17: Container, 17a recess,
17b: Connection pad, 17c: External terminal,
19: Conductive adhesive, 21: Lid member,
50, 60, 70, 80: Piezoelectric devices of other embodiments

Claims (3)

An AT-cut crystal piece with a rectangular shape in plan view, with the long side along the X axis of the crystal and the short side along the Z' axis of the crystal, and an excitation device with a rectangular shape in plan view provided on the front and back of the crystal piece. A piezoelectric device comprising an electrode for
The AT-cut crystal piece has a long side of 4 mm, a short side of 1.85 mm, a dimension along the long side of the excitation electrode of 1.4 mm, and a dimension of the excitation electrode along the short side. 0.96mm, oscillation frequency is 38.88MHz,
Disposed in the front and back regions of the crystal piece , along the pair of opposing long sides and spaced apart from the edges of the front and back excitation electrodes by a distance G of 0.13 to 0.21 mm. A piezoelectric device comprising a conductive film . 2. The piezoelectric device according to claim 1, wherein the conductive films provided on the front and back sides are electrically connected via side surfaces of the crystal piece. 3. The piezoelectric device according to claim 1 , wherein the conductive film is a metal film made of the same material as the excitation electrode.

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