JP6626661B2 - Piezoelectric vibrator - Google Patents

Description

  The present invention relates to a structure of an excitation electrode of a piezoelectric vibrator, and more particularly to an improvement in aging performance of a tuning-fork type piezoelectric vibrator that requires high frequency accuracy.

  2. Description of the Related Art Piezoelectric vibrators represented by quartz vibrators have been widely used as frequency references for electronic devices requiring high frequency accuracy, such as watches, because of their good frequency accuracy. Among them, particularly when used in a device that requires a high frequency accuracy, the effect of a change over time (hereinafter, referred to as aging) of the oscillation frequency becomes a problem, and improvement is desired.

  Hereinafter, a general tuning fork type crystal resonator will be described as an example of the piezoelectric resonator. First, a conventional tuning-fork type crystal resonator will be described with reference to FIGS.

  6A and 6B are diagrams showing a configuration of a piezoelectric vibrator which is a conventional tuning-fork type crystal vibrator. FIG. 6A is a front view of a main surface side of the piezoelectric vibrator, and FIG. It is the side view seen from B direction of 6 (a). The piezoelectric vibrator 1 includes a base 2 and a plurality of vibrating legs 3 extending from the base 2, and an excitation electrode 4 such as a metal thin film is formed on a main surface and side surfaces of the vibrating leg 3. I have. Here, a portion where the vibrating leg 3 and the base 2 are in contact is referred to as a root 5. Further, the piezoelectric vibrator 1 has a terminal electrode formed on the base 2, a lead wiring for connecting the terminal electrode and the excitation electrode 4, and a frequency adjustment film. Illustration is omitted. The tuning fork shape is cut out from a quartz wafer by a technique such as photoetching or machining, and is formed.

  FIG. 7 is a diagram for explaining the excitation of the vibrating leg 3, and is a cross-sectional view taken along the line AA of FIG. Here, the width direction of the vibrating leg 3 will be described as X, the extending direction will be Y, and the thickness direction of the vibrating leg 3 (base 2) will be Z. As shown in FIG. 7, the excitation electrode 4 is formed so that the main surface and the side surface have different polarities, and an electric field is applied between the different poles. In FIG. 7, this electric field is indicated by an arrow 11, and the direction of the arrow indicates an electric field from a high voltage to a low voltage. The X-axis component (indicated as an arrow 12) in the electric field described above is from the center of the cross section of the vibrating leg 3 toward both sides in the left vibrating leg 3 in FIG. 7, and from both sides in the right vibrating leg 3 in FIG. It extends toward the center of the cross section. When an electric field in the X-axis direction is applied to the crystal, the crystal is distorted by the piezoelectric effect, and this becomes a driving force for the vibration of the piezoelectric vibrator 1.

  FIG. 8 shows a state of bending vibration of the piezoelectric vibrator 1. The distortion near the root 5 is the largest, and becomes smaller toward the tip of the vibrating leg 3. When the piezoelectric vibrator 1 is mounted and sealed in a package on the end side of the base 2 and connected to an oscillation circuit, it oscillates at a frequency near the resonance frequency and is used as a reference signal of the device.

  Usually, the excitation electrode 4 is formed uniformly from the vicinity of the root 5. This is because, at the time of resonance, the distortion near the root 5 is largest, and if the excitation electrode 4 is present there, oscillation can be performed with high electro-mechanical conversion efficiency. That is, the CI value (crystal impedance) as the resonance resistance is low.

  The ordinary piezoelectric vibrator 1 in which the excitation electrode 4 is formed up to the root 5 has the advantage of a low CI value and has a general configuration, but can be said to be in a disadvantageous state with respect to aging performance.

What determines the oscillation frequency of the vibrator is mainly the resonance frequency of the piezoelectric vibrator 1. A possible cause of the aging change is that the film stress of the excitation electrode film attached on the quartz crystal changes with time. An excitation electrode film is formed on the crystal by sputtering or the like in order to drive the vibrator.Film stress generated at the interface between the crystal and the excitation electrode film during the formation of the excitation electrode film or during a heat process is It will be released over time. The film stress is also an element that affects the resonance frequency. When the film stress is released, the resonance frequency changes. When the resonance frequency changes, the oscillation frequency of the vibrator also changes. Thus, an aging change occurs. Regardless of whether it is for excitation or not, the film stuck on the crystal similarly causes aging change. Changes in the film stress near the root 5 where the distortion is large tend to particularly affect the resonance frequency.

  As a method of improving the performance by changing such a normal excitation electrode structure, for example, a crystal resonator disclosed in Patent Literature 1 or Patent Literature 2 is known.

  Patent Literature 1 reports that the aging performance and the Q value have been improved by arranging the excitation electrodes except for the maximum distortion portion of the crystal resonator. If the excitation electrode film is not originally formed, there is no change in the film stress that affects the oscillation frequency.

  Patent Literature 2 proposes that an excitation electrode and a separate piezoelectric film are formed on a base material of quartz, and out-of-plane vibrations are generated using the piezoelectricity of the piezoelectric film provided above instead of the piezoelectricity of quartz. This is a vibrator to be driven, which has a different structure from a normal crystal vibrator, but it is said that the vibration is stabilized by shortening the length of the piezoelectric film.

JP-A-56-52922 (page 2, FIG. 5) JP 2012-15886 A (page 5, FIG. 1)

  However, in the technique described in Patent Document 1, although the electrode film in the maximum strain portion is removed, although the aging change is smaller than that in the case where the excitation electrode is formed up to the root, the specific dimensions excluding the electrode film are not described. Only the dimensions including the base are described, and the dimensions of the vibrating leg without the electrodes are not mentioned. As shown in FIG. 5 of Patent Document 1 (not shown here), simply removing the maximum distortion portion did not provide the high aging performance required for devices requiring high accuracy such as watches.

  Further, in Patent Document 2, a lower limit dimension of a portion where a piezoelectric film used for driving is not formed is specified, but an upper limit dimension is not specified. In this case, the driving portion becomes very small. However, if the driving portion is too small, the CI value, which is the resonance resistance, becomes high, and there is a problem that the power consumption of the oscillation circuit increases. Further, when the vibrator is used for an application requiring high frequency accuracy, a thin film of an electrode is usually formed on a highly stable crystalline material such as quartz. When the dimensions recommended in Patent Literature 2 are used for a material such as quartz having a small electromechanical coupling constant among piezoelectric materials, the CI value and the power consumption are particularly increased, and it is difficult to actually use the material.

  An object of the present invention is to provide a piezoelectric vibrator in which the aging characteristics of the frequency are improved and the increase in the CI value is suppressed.

  The vibrator of the present invention employs the following configuration to achieve the above object.

A base, a vibrating leg extending from the base, an excitation electrode formed on the vibrating leg, a terminal electrode formed on the base, and a wiring for connecting the terminal electrode and the excitation electrode; A piezoelectric vibrator that bends and vibrates, wherein the lead-out wiring is thinner than the excitation electrode at least in a portion formed on the vibration leg, and the excitation electrode includes a main surface and a side surface of the vibration leg. When the total length in the extending direction of the vibrating leg is L, and the distance from the base of the vibrating leg to the end of the excitation electrode on the base side is M, M / L is 0.3 state, and are 0.5 or less, the lead wiring connected to the excitation electrode formed on a side surface of the vibrating leg is a region of the side surface of the vibrating leg, of the vibration legs The excitation electrode and the base formed on the side surface; Characterized in that formed in the region between.

  With this configuration, since the M / L is 0.3 or more, the aging performance is sufficiently improved even for high-precision applications. Further, since the M / L is 0.5 or less, the CI value does not deteriorate and the power consumption does not increase too much. Therefore, it is possible to obtain a high-precision vibrator having both the characteristics of high aging performance and good power consumption.

It is the front view and side view of the tuning fork type piezoelectric vibrator of this example. It is a graph of CI value with respect to M / L. It is a graph of the aging rate with respect to M / L. It is a graph of the aging rate with respect to the CI value when M / L is changed. 5 is a graph plotting the gradient of FIG. 4 against M / L. It is the front view and side view of the conventional tuning fork type piezoelectric vibrator. FIG. 7 is a sectional view taken along line AA of the piezoelectric vibrator in FIG. 6. It is a front view showing the bending vibration of the conventional tuning fork type piezoelectric vibrator.

  The present invention is characterized in that the aging characteristics of the frequency in the piezoelectric vibrator are improved, and the excitation electrode film is formed in a range where the rise of the CI value can be suppressed. Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings.

  As the material of the piezoelectric vibrator, quartz crystal, lithium niobate, lithium tantalate, sapphire, and the like having piezoelectric characteristics are known. In this embodiment, a tuning fork type bending using a typical crystal as the piezoelectric vibrator is used. Description will be given as a vibrator. The present invention is not limited to the quartz crystal vibrator, but can be applied to other forms by changing the type of the piezoelectric material and the polarity of the electrode each time. Further, the present invention can be applied to a piezoelectric vibrator of a type in which an elastic material such as silicon is used as a substrate and a piezoelectric film is formed on the elastic material and driven.

  FIG. 1 is a diagram showing a configuration of a piezoelectric vibrator according to an embodiment of the present invention. FIG. 1A is a front view of a main surface side of the piezoelectric vibrator, and FIG. It is the side view seen from C direction of a).

  The piezoelectric vibrator 6 includes a base 7 and a pair of parallel vibrating legs 8 extending from the base 7, and excitation electrodes 9 are formed on the main surface and side surfaces of the vibrating legs 8. Here, a portion where the vibrating leg 8 and the base 7 are in contact with each other is defined as a root 10, and an end of the excitation electrode 9 on the base 7 side is defined as an excitation electrode end 13. Further, the piezoelectric vibrator 6 has a terminal electrode formed on the base 7, a lead wiring for connecting the terminal electrode and the excitation electrode 9, and a frequency adjustment film. Illustration is omitted.

  The shape of the piezoelectric vibrator 6 is processed by photoetching or machining from a plate material cut out at a predetermined angle with respect to the crystal axis of quartz. Since the shape processing method of the piezoelectric vibrator 6 can be formed by using a known technique, the description is omitted.

The excitation electrode 9 is disposed at a position separated from the base 10 of the vibrating leg 8 by a predetermined distance, and is formed of a conductive thin film such as a metal. As a method for forming the conductive thin film, a known method such as sputtering or vapor deposition can be used, and detailed description is omitted.
The position of the vibrating leg 8 separated from the base 10 by a predetermined distance, that is, the position where the excitation electrode end 13 in the vibrating leg 8 is formed, is determined by the total length of the vibrating leg 8 in the extending direction from the base 7 as L Assuming that the distance from the base 10 of the vibrating leg 8 to the excitation electrode end 13 is M, the M / L is in the range of 0.3 to 0.5. In other words, the excitation electrode 9 is not formed in a range where the M / L is 0.3 or more and 0.5 or less from the root 10.
Note that the above-described wiring for applying a voltage to the excitation electrode 9 may be present in a range where the excitation electrode 9 is not formed, if necessary. When the lead wiring is formed, as one example, the shape of the excitation electrode becomes thinner at the distance M.
The principle of excitation by applying a voltage to each excitation electrode 9 is the same as that shown in the sectional view of FIG.

  The piezoelectric vibrator thus manufactured is mounted and sealed in a package, and used as a piezoelectric device by being connected to an oscillation circuit.

  Next, the grounds that the position M / L of the excitation electrode end 13 formed on the vibrating leg 8 is 0.3 or more and 0.5 or less will be described with reference to FIGS.

  2 to 5 are diagrams showing the relationship between the aging performance and the CI value when the dimension of the distance M is changed. FIG. 2 is a diagram showing the CI value with respect to the ratio (M / L) of the distance M to the total length L. 3 is a graph of the aging ratio with respect to M / L. FIG. 4 is a graph in which the CI value at a predetermined M / L is plotted on the horizontal axis and the aging ratio is plotted on the vertical axis, and FIG. 5 is a graph in which the gradient of FIG. . Here, the aging ratio is expressed as the rate of aging change when M = 0, that is, when the aging change in the shape of FIG. 6 of the conventional example is 100%, and the smaller the value is, the smaller the change with time is. It can be said that this is a good oscillator.

As shown in FIG. 2, the CI value increases exponentially as M / L increases. This is because the larger the distance M, the smaller the area of the excitation electrode 9 becomes, and the lower the efficiency of excitation becomes.
FIG. 2 shows that when M / L exceeds 0.5, the CI value sharply increases. If the CI value rises, the excitation efficiency deteriorates and the power consumption for oscillation increases, which is not suitable for use. Further, a sharp rise in the CI value indicates that the manufacturing variation of the CI value increases when some dimensional error occurs, and is not suitable for manufacturing when the M / L exceeds 0.5. .

  Next, as shown in FIG. 3, the aging ratio decreases as M / L increases, indicating that the aging characteristics are improved. This is because, as the distance M increases, the area of the excitation electrode 9 decreases, and as a result, the change in the film stress generated between the vibration leg 8 and the excitation electrode 9 decreases. That is, it can be said that the CI value and the aging ratio have a trade-off relationship.

  FIG. 4 is a graph of the aging ratio with respect to the CI value at each M / L. Point D on the graph is M / L = 0.2, point E is M / L = 0.3, point F is M / L = 0.4, point G is M / L = 0.5, and point H is It is a value at the time of M / L0.6. As shown in FIG. 4, it can be seen that up to a certain point, the CI value does not increase (deteriorate) even though the aging characteristic is improved. In a region where aging can be improved without showing a significant increase in the CI value, the aging characteristics should be improved by increasing the distance M as much as possible.

FIG. 5 shows an approximate curve of the graph of FIG. 4 and plots the gradient against the corresponding M / L. As shown in FIG. 5, the absolute value of the gradient converges small when M / L is around 0.3. Therefore, in an area where M / L is 0.3 or more, aging is a good area even in consideration of a balance between CI values. In addition, when the M / L is less than 0.3, the manufacturing variation of the aging ratio increases when some dimensional error occurs, and when the M / L is less than 0.3, the reliability is increased. Not suitable for lacking. Aging can be reduced to 20% or less of the current state, and the performance is sufficient for device applications aiming at high accuracy.

  Here, application of the piezoelectric vibrator of this embodiment to an oscillation device will be considered. As an example, in an oscillator circuit of a portable device represented by a timepiece, a current consumption is required to be 100 nA or less in consideration of a battery size and a battery life which are usually required. In this case, the required CI value of the vibrator is usually about 100 kΩ. Further, for example, the power consumption of the oscillation circuit can be reduced by changing the process of the CMOS transistor and lowering the threshold voltage of oscillation, but in this case, the allowable value of the CI value is about 750 kΩ or less.

As described above, in order to use an oscillation device requiring a high frequency accuracy, such as a timepiece, the excitation electrode end 13 on the base 7 side of the excitation electrode 9 formed on the vibration leg 8 is required. The position must be formed so that the M / L is in the range of 0.3 or more and 0.5 or less.
The piezoelectric vibrator thus manufactured can obtain high aging accuracy and can be used as a vibrator in which the CI value is suppressed from increasing.

  In the piezoelectric vibrator of the present invention, the aging performance is sufficiently improved, but the CI value is not excessively increased. Therefore, it is a piezoelectric vibrator exhibiting practically sufficient aging performance and low power consumption for applications such as timepieces that require high accuracy in frequency characteristics.

1, 6 Piezoelectric vibrator 2, 7 Base 3, 8 Vibration leg 4, 9 Excitation electrode 5, 10 Root 11 Electric field 12 X direction electric field 13 Excitation electrode end M Distance from root to excitation electrode end L L Vibration leg full length

Claims (2)

A base, a vibrating leg extending from the base, an excitation electrode formed on the vibrating leg, a terminal electrode formed on the base, and a wiring for connecting the terminal electrode and the excitation electrode; And a piezoelectric vibrator that bends and vibrates,
The routing wiring is thinner than the excitation electrode at least in a portion formed on the vibration leg,
The excitation electrode is formed on both the main surface and the side surface of the vibrating leg,
When the total length in the extending direction of the vibrating leg is L, and the distance from the base of the vibrating leg to the end of the excitation electrode on the base side is M, M / L is 0.3 or more and 0.5 or more. Ri der below,
The routing wiring connected to the excitation electrode formed on the side surface of the vibrating leg is an area of the side surface of the vibration leg, and the excitation electrode formed on the side surface of the vibration leg and the base A piezoelectric vibrator formed in the area between the piezoelectric vibrator and the piezoelectric vibrator.   The piezoelectric vibrator according to claim 1, wherein the M / L is 0.4 or more and 0.5 or less.

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