JP6819005B2 - Bulk elastic wave resonator - Google Patents

Description

The present invention relates to a temperature-compensated bulk elastic wave resonator.

In recent years, with the worldwide spread of smartphones and the ever-increasing demand for wireless communication services using microwaves, commonly known as wearables and IoT (Internet of Things), radio waves are a limited resource. In order to effectively use (microwave), it is required to selectively extract radio waves of a required frequency from microwaves overflowing in space. For example, at present, it is expanding to services that use not only the frequency band of 2.5 GHz or less but also the high frequency band of 3 GHz or more, and a high frequency filter is used to selectively extract radio waves in a predetermined frequency band. There is. High-frequency files of this type are required to have high performance, such as no temperature drift and steep skirt characteristics.

In addition, in order to support the frequency band used all over the world, more than 10 high frequency filters are installed in one smartphone, and because it is small and has excellent filter characteristics, it has high frequency. SAW (Surface Acoustic Wave) resonators are often used as filters.

On the other hand, SAW resonators have a limit in realizing a high frequency band of 3 GHz or more and a wide pass band, and bulk elastic wave (BAW) resonators are being used for filters that require high performance. .. In the future, it is expected that the demand for bulk elastic wave resonators will further increase, based on the prediction that the frequency band used will be crowded even in the high frequency band of 3 GHz or higher.

Currently, a piezoelectric film is sandwiched between an upper electrode film and a lower electrode film, and an air layer is provided directly above or below these electrodes, and the elastic wave is a bulk elastic wave in which the reflectance of the elastic wave on the surface of the upper electrode film or the lower electrode film is enhanced. A resonator is used.

By the way, in a bulk elastic wave resonator, the resonance frequency changes as the temperature changes due to the combination of the effect that the piezoelectric film material and the electrode film material thermally expand with temperature and the effect that the propagation speed of elastic waves changes with temperature. .. As a result, the pass band of the high-frequency filter composed of the bulk elastic wave resonator fluctuates depending on the temperature. Therefore, in consideration of such fluctuations, the frequency band to be used must be set narrow with a margin, and there is a problem that radio waves, which are a finite resource, cannot be effectively utilized. Further, if a strict specification that guarantees a predetermined pass band is set, there is a problem that the manufacturing yield is lowered.

Therefore, methods such as laminating a thin film material having a temperature dependence of the inverse code with the piezoelectric film or using an electrode film material having a temperature dependence of the inverse code with the piezoelectric film have been proposed for temperature compensation ( For example, Patent Documents 1 to 3).

FIG. 4 shows a cross-sectional view of a conventional temperature-compensated bulk elastic wave resonator. As shown in FIG. 4, a lower electrode film 2, a piezoelectric film 3, a temperature compensation film 4, and an upper electrode film 5 are laminated in this order on a silicon substrate 1 serving as a support substrate. Further, an opening 6 is formed in which a part of the laminated film is removed, and the structure communicates with the recess 7 formed under the lower electrode film 2.

Before forming the lower electrode film 2, the recess 7 is flattened by etching and removing a part of the silicon substrate 1 and filling it with a sacrificial layer (not shown), and then flattening the lower electrode film 2, the piezoelectric film 3, and temperature compensation. After forming the film 4 and the upper electrode film 5, an opening 6 is formed to expose a part of the sacrificial layer, and the sacrificial layer is etched and removed through the opening 6.

By forming the temperature compensation film 4 as shown in FIG. 4, it is possible to reduce the temperature dependence of the resonance frequency. The temperature compensation film 4 is not limited to the case where it is arranged between the piezoelectric film 3 and the upper electrode film 5 as shown in FIG. 4, but also on the upper electrode film 5 or between the lower electrode film 2 and the piezoelectric film 3. The arrangement can be changed, such as arranging between the piezoelectric film 3 and the upper electrode film 5, respectively.

By the way, depending on the material, the temperature compensating film 4 needs to be thickly laminated, and there is a problem that the characteristics of the resonator deteriorate, such as reducing the piezoelectric coupling coefficient and lowering the Q value. Therefore, there is a demand for a bulk elastic wave resonator that realizes temperature compensation without deteriorating the characteristics of the resonator.

Japanese Patent No. 5047594 Japanese Patent No. 4805836 Japanese Patent No. 4037825

The conventionally proposed temperature-compensated bulk elastic wave resonator uses a thin film material having a temperature dependence of the opposite sign to the piezoelectric film, or an electrode film material having a temperature dependence of the opposite sign to the piezoelectric film. This makes it possible to compensate for the temperature, but there is a problem that the characteristics of the resonator are deteriorated, such as reducing the piezoelectric coupling coefficient and lowering the Q value. An object of the present invention is to provide a temperature-compensated bulk elastic wave resonator that solves these problems.

In order to achieve the above object, the bulk elastic wave resonator according to claim 1 of the present application has a piezoelectric film, an upper electrode film and a lower electrode film sandwiching the piezoelectric film, and a temperature coefficient having a temperature coefficient opposite to that of the piezoelectric film. In a bulk elastic wave resonator in which a multilayer film including a compensation film is laminated, the thickness of the lower electrode film, the piezoelectric film, and the upper electrode film is an integer of 1/2 of the wavelength of the resonance frequency of the resonator. The temperature compensation film is twice as thick, and the thickness of the temperature compensation film is an integral multiple of 1/2 of the wavelength of the resonance frequency, and the temperature compensation film has a temperature coefficient opposite to that of the piezoelectric film. It is a multilayer film composed of a film having a temperature coefficient having the same code as that of the piezoelectric film .

The bulk elastic wave resonator according to claim 2 of the present application is a temperature compensating film in which a lower electrode film, a piezoelectric film and an upper electrode film are laminated on a substrate provided with a recess, and further has a temperature coefficient opposite to that of the piezoelectric film. In a bulk elastic wave resonator in which a multilayer film including and is laminated, the thickness of the lower electrode film, the piezoelectric film, and the upper electrode film is an integral multiple of 1/2 the wavelength of the resonance frequency of the resonator. Yes, the thickness of the temperature compensation film is an integral multiple of 1/2 the wavelength of the resonance frequency, and the temperature compensation film has a temperature coefficient opposite to that of the piezoelectric film and the piezoelectric film. It is characterized by being a multilayer film composed of a film having a temperature coefficient having the same code as the film .

The bulk elastic wave resonator according to claim 3 of the present application is the bulk elastic wave resonator according to any one of claims 1 or 2, wherein the temperature compensation film is on the upper electrode film, directly under the lower electrode film, or the upper portion. It is characterized in that it is laminated both on the electrode film and directly below the lower electrode film .

According to the present invention, the total thickness of the lower electrode film, the piezoelectric film and the upper electrode film is set to an integral multiple of 1/2 of the wavelength of the resonance frequency of the resonator, and the thickness of the temperature compensation layer. By making it an integral multiple of 1/2 the wavelength of the frequency, it is possible to obtain the same characteristics as a bulk elastic wave resonator having a structure without a temperature compensation film, and to realize temperature compensation.

Further, according to the present invention, temperature compensation can be performed even if the material and the laminated structure of the electrode and the piezoelectric film are changed, and a bulk elastic wave resonator having excellent characteristics can be realized.

It is sectional drawing of the bulk elastic wave resonator of 1st Example of this invention. It is sectional drawing of the bulk elastic wave resonator of the 2nd Example of this invention. It is sectional drawing of the bulk elastic wave resonator of the 3rd Example of this invention. It is sectional drawing of the conventional bulk elastic wave resonator.

In the bulk elastic wave resonator of the present invention, the total thickness of the lower electrode film, the piezoelectric film, and the upper electrode film is set to an integral multiple of 1/2 of the wavelength of the resonance frequency of the resonator, and the thickness of the temperature compensation layer is increased. By setting the value to an integral multiple of 1/2 of the wavelength of the resonance frequency of the resonator, temperature compensation and characteristic maintenance as a resonator are realized. Hereinafter, examples of the present invention will be described.

FIG. 1 is a cross-sectional view of a bulk elastic wave resonator according to a first embodiment of the present invention. The bulk elastic wave resonator shown in FIG. 1 has a different structure of the temperature compensation film 4 from the bulk elastic wave resonator of the conventional example described above.

For example, when aluminum nitride (AlN) is used as the piezoelectric film and molybdenum (Mo) is used as the electrode film, the temperature coefficients of the materials are about -25 ppm / ° C. and about -60 ppm / ° C., respectively, and the temperature compensation film 4 is provided. The thin film bulk elastic wave resonator having no conventional structure has a negative temperature coefficient of about -40 ppm / ° C., and the resonance frequency tends to decrease as the temperature rises.

Therefore, in the present invention, the lower electrode film 2, the piezoelectric film 3, and the upper electrode film 5 are laminated and formed, and the temperature compensation film 4 is further laminated and formed on the upper electrode film 5.

In a bulk elastic wave resonator having such a structure, a predetermined resonance frequency is determined by the thickness, acoustic impedance, layer density, etc. of each layer of the lower electrode film 2, the piezoelectric film 3, the upper electrode film 5, and the temperature compensation film 4. have.

Here, in this embodiment, the thickness of the laminated film of the lower electrode film, the piezoelectric film, and the upper electrode film is set to an integral multiple of 1/2 of the wavelength of the resonance frequency of the resonator, and the thickness of the temperature compensation film is also the resonator. It is an integral multiple of 1/2 of the wavelength of the resonance frequency of. However, when the piezoelectric film has a single piezoelectric crystal orientation direction, the total thickness of the lower electrode film, the piezoelectric film, and the upper electrode film is an odd multiple of 1/2 of the wavelength of the resonance frequency of the resonator. And. When composed of two layers of piezoelectric films having crystal orientation directions that are approximately equal in thickness and exhibit piezoelectricity in opposite directions, the total thickness of the lower electrode film, the two layers of piezoelectric film, and the upper electrode film is resonated. It should be twice the wavelength of the resonance frequency of the vessel. Further, in the case of a multilayer structure in which piezoelectric films having crystal orientation directions opposite to each other are alternately laminated with even layers, the total thickness is set to half the number of layers of the wavelength of the resonance frequency of the resonator. ..

For simplicity, the piezoelectric film has a single piezoelectric crystal orientation direction, the total thickness is 1/2 of the resonance frequency wavelength, and the temperature compensation film thickness is also 1/2 of the resonance frequency. The effect of the present invention will be described in the case of first-order resonance. Since the same concept applies to the case of high-order resonance that is an integral multiple of 2 or more, the description thereof will be omitted.

In this case, the lower surface of the lower electrode film, the upper surface of the upper electrode film (corresponding to the lower surface of the temperature compensation film), and the upper surface of the temperature compensation film form nodes of strain or stress distribution in bulk elastic wave vibration, and are equivalent to one wavelength in total. The elastic wave is excited and becomes a resonance state. The strain distribution in the laminated structure of the lower electrode film, the piezoelectric film, and the upper electrode film matches the strain distribution of the bulk elastic wave resonator composed of the lower electrode film without the temperature compensation film, the piezoelectric film, and the upper electrode film. Therefore, the piezoelectric coupling coefficients that determine the strain distribution in the film are kept substantially the same, and there is no deterioration in characteristics. On the other hand, as shown in the conventional example of FIG. 4, it is composed of a laminated structure of a lower electrode film, a piezoelectric film, a temperature compensation film and an upper electrode film, and the lower surface of the lower electrode film and the upper surface of the upper electrode film are nodes of strain distribution. In the case of the resonator, the strain distributions of the lower electrode film, the piezoelectric film, and the upper electrode film differ depending on the presence or absence of the temperature compensation film, and therefore the piezoelectric coupling coefficient does not deteriorate, as compared with the present invention. It is a very different place.

In the present invention, the lower surface of the lower electrode film, which is the lowermost surface of the resonator, and the upper surface of the upper electrode film, which is the uppermost surface, are always nodes of strain distribution regardless of temperature. The sign of the temperature characteristic of the temperature compensation film is opposite to that of the temperature characteristic of the piezoelectric film and the electrode film. Therefore, when the wavelengths of the piezoelectric film and the electrode film become shorter due to temperature fluctuations, the wavelengths of the temperature compensation film become longer, and by canceling each other, the resonance frequency is kept almost constant regardless of the temperature, or the resonance frequency fluctuates due to the temperature. It will be relaxed.

The bulk elastic wave excited by the piezoelectric film seeps into the temperature compensation film, and elastic loss occurs due to the viscosity of the temperature compensation film. Therefore, the Q value decreases somewhat, but the piezoelectric film as in the conventional example. Compared with the structure in which the temperature compensation film is sandwiched between the electrode film and the electrode film, the reduction is slight.

As described above, since the upper surface and the lower surface of the temperature compensation film may be the nodes of the strain distribution, the thickness of the temperature compensation film is not only 1/2 of the wavelength of the resonance frequency but also an integral multiple of it. There may be.

The temperature compensation film 4 may be a silicon dioxide film, an impurity-doped silicon dioxide oxide film, a silicon oxide film having a different oxygen composition, or a silicon oxide nitride film (SiON).

These films are composed of a lower electrode film 2, a piezoelectric film 3, and an upper electrode film 5 when the temperature compensation film 4 has a film thickness that is an integral multiple of 1/2 the wavelength of the resonance frequency of the bulk elastic wave resonator. It is sufficient to select a film having characteristics that compensate for the temperature dependence of the portion having a negative temperature coefficient that is an integral multiple of 1/2 of the wavelength of the resonance frequency.

Next, a second embodiment will be described. In the first embodiment, the case where the temperature compensating film 4 is laminated on the upper electrode film 5 has been described, but the present invention is not limited to this. Specifically, as shown in FIG. 2, it may be arranged directly under the lower electrode film 2.

Also in this embodiment, the thickness of the laminated film of the lower electrode film, the piezoelectric film, and the upper electrode film is set to an integral multiple of 1/2 of the resonance frequency of the resonator, and the thickness of the temperature compensation film is also the resonance frequency of the resonator. It is an integral multiple of 1/2 of the wavelength of. With this configuration, the structure described in the first embodiment is turned upside down, and the operating principle and its effect are the same as those in the first embodiment.

Next, a third embodiment will be described. In the first or second embodiment described above, the case where the temperature compensation film 4 is composed of a single film has been described, but the present invention is not limited thereto. Specifically, as shown in FIG. 3, a metal film 8 having a negative temperature coefficient is laminated on a temperature compensating film 4 having a positive temperature coefficient, and a multilayer film of the temperature compensating film 4 and the metal film 8 is formed. It may be configured to function as a temperature compensation film. Molybdenum is an example of the metal film 8 having a negative temperature coefficient. Instead of the metal film, a dielectric film having a negative temperature coefficient or a semiconductor film such as silicon may be used.

In this embodiment, the thickness of the laminated film of the lower electrode film, the piezoelectric film, and the upper electrode film is set to an integral multiple of 1/2 of the wavelength of the resonance frequency of the resonator, and a multilayer composed of a temperature compensation film, a metal film, or the like. The total thickness of the membrane is also an integral multiple of 1/2 the wavelength of the resonance frequency of the resonator. When the temperature compensation effect becomes an issue with a single temperature compensation film and the resonator as a whole exhibits the opposite temperature dependence, this embodiment in which a plurality of films having negative and positive temperature coefficients are laminated. The configuration of is valid. Under the constraint of the present invention that the thickness of the entire temperature compensation film must be an integral multiple of 1/2 the wavelength of the resonance frequency of the resonator, the temperature of the resonance frequency can be adjusted by adjusting each film thickness of the laminated multilayer film. This is because the fluctuation can be completely suppressed.

The operating principle and its effect of this embodiment are the same as those of the first and second embodiments.

Although the examples of the present invention have been described above, it goes without saying that the present invention is not limited to the above examples. Specifically, the piezoelectric film is not limited to aluminum nitride, and scandium aluminum nitride (Al _1-x Sc _x N), zinc oxide (ZnO), and lead zirconate titanate (PZT) can also be used. It is possible. Further, the upper electrode film or the lower electrode film can be formed of a metal thin film such as platinum (Pt), titanium (Ti), iridium (Ir), and ruthenium (Ru) instead of molybdenum (Mo). Similarly, the metal film used as the temperature compensation film may be formed of a metal thin film such as platinum (Pt), titanium (Ti), iridium (Ir), ruthenium (Ru) instead of molybdenum (Mo). it can.

Further, the temperature compensation film having a multilayer structure shown in FIG. 3 may be laminated and arranged directly under the lower electrode film 2 as described in the second embodiment. In this case, even if the structure is such that the metal film 8 is laminated on the silicon substrate 1, the temperature compensation film 4 is laminated on the metal film 8, and the lower electrode film 4 is laminated on the temperature compensation film 4, the metal film 8 and the temperature are also laminated. The stacking order of the compensation film 4 may be reversed, and the lower electrode film 4 may be laminated on the metal film 8. Further, the temperature compensation film is not limited to the two-layer structure, and may have a multi-layer structure of three or more layers.

Furthermore, there is no problem even if the temperature compensation film is arranged both on the upper electrode and directly under the lower electrode. In this case, by setting each temperature compensation film to an integral multiple of 1/2 the wavelength of the resonance frequency of the resonator, the same effect as in the case described in the above embodiment can be obtained.

1: Silicon substrate 2: Lower electrode film 3: Piezoelectric film 4: Temperature compensation film 5: Upper electrode film, 6: Opening, 7: Recess, 8: Metal film

Claims (3)

In a bulk elastic wave resonator in which a piezoelectric film, an upper electrode film and a lower electrode film sandwiching the piezoelectric film, and a multilayer film including the piezoelectric film and a temperature compensation film having a temperature coefficient of the opposite sign are laminated.
The thickness of the lower electrode film, the piezoelectric film, and the upper electrode film is an integral multiple of 1/2 the wavelength of the resonance frequency of the resonator, and the thickness of the temperature compensation film is the resonance frequency. and it is an integer multiple of half of the wavelength,
A bulk elastic wave resonator, wherein the temperature compensation film is a multilayer film composed of a film having a temperature coefficient having a temperature coefficient opposite to that of the piezoelectric film and a film having a temperature coefficient having the same code as that of the piezoelectric film . Bulk elastic wave resonance in which a lower electrode film, a piezoelectric film, and an upper electrode film are laminated on a substrate provided with a recess, and a multilayer film including the piezoelectric film and a temperature compensation film having a temperature coefficient of the opposite sign is laminated. In the vessel
The thickness of the lower electrode film, the piezoelectric film, and the upper electrode film is an integral multiple of 1/2 the wavelength of the resonance frequency of the resonator, and the thickness of the temperature compensation film is the resonance frequency. and it is an integer multiple of half of the wavelength,
A bulk elastic wave resonator, wherein the temperature compensation film is a multilayer film composed of a film having a temperature coefficient having a temperature coefficient opposite to that of the piezoelectric film and a film having a temperature coefficient having the same code as that of the piezoelectric film . In the bulk elastic wave resonator according to any one of claims 1 or 2, the temperature compensating film is laminated on the upper electrode film, directly under the lower electrode film, or both on the upper electrode film and directly under the lower electrode film. A bulk elastic wave resonator characterized by being

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