JP7525876B2 - Frequency Filters - Google Patents

Description

The present invention relates to a frequency filter that filters the frequency of high-frequency power and is used in mobile phone base stations and the like. Here, "high-frequency power" and "high-frequency voltage" described below refer to the power and voltage of a high-frequency signal obtained by receiving electromagnetic waves and/or the high-frequency signal used when transmitting electromagnetic waves. The frequencies of such high-frequency power and high-frequency voltage are typically within the range of 50 MHz to 30 GHz.

In mobile phone services, many base stations are installed throughout a city to send and receive radio waves to and from mobile phone terminals. In both the terminal and the base station, a specific frequency is extracted from the many frequencies contained in the electromagnetic waves received by the antenna, and high-frequency power having that specific frequency is generated.

Conventionally, in terminals, electromagnetic waves with multiple frequencies received by an antenna are passed through a high-frequency power frequency filter to extract high-frequency power with a specific frequency. Examples of frequency filters for such high-frequency power include a surface acoustic wave (SAW) filter, which has a pair of interdigitated electrodes arranged on the surface of a piezoelectric thin film, and a bulk acoustic wave (BAW) filter, which has a pair of electrodes on both sides of a piezoelectric thin film (see, for example, Patent Document 1).

JP 2012-049758 A JP 2020-072284 A

SAW filters and BAW filters have low voltage resistance between electrodes and are unable to filter the frequencies of the high-power radio frequency power that needs to be processed at base stations. For this reason, cell phone base stations have traditionally used cavity resonators to filter frequencies. However, while SAW filters and BAW filters can be miniaturized to a few millimeters square, cavity resonators are several hundred millimeters square, making them difficult to miniaturize. In recent years, there has been a demand for installing small base stations in urban areas where electromagnetic waves from large base stations have difficulty reaching, such as places in the shadows of buildings. However, using cavity resonators in such small base stations results in a problem of the base station occupying a large space, which increases installation costs.

The problem that this invention aims to solve is to provide a frequency filter that uses a piezoelectric thin film and can be miniaturized, and that can filter the frequencies of the high-power radio frequency power handled by mobile phone base stations.

The frequency filter according to the present invention, which is made to solve the above problems, comprises:
a) a laminate in which a predetermined number of first layers made of a first piezoelectric material and having a polarization in one predetermined direction and second layers made of a second piezoelectric material and having a component of polarization perpendicular or parallel to the first layer that is 180° different from the component of polarization in the first layer, or a non-piezoelectric layer made of a non-piezoelectric insulating material, are alternately repeated one by one, and a thickness _d1 of the first layer and a thickness _d2 of the second layer are within the range of 0.5×( _v2 / _v1 ) _d1 ≦ _d2 ≦2.0×(v2/ _v1 )d1, where _v1 is the sound velocity in the _first layer and _v2 is the sound velocity in the _second layer;
b) a pair of electrodes disposed so as to sandwich the laminate in a lamination direction,
The predetermined number is set so that the frequency filter has a power resistance to 10 W of high-frequency power.

In the frequency filter according to the present invention, when high frequency power containing many frequencies is input between a pair of electrodes, high frequency power containing those frequencies, a high frequency voltage, is generated between the electrodes. As a result, each of the first layers provided on every other layer of the layers constituting the laminate vibrates with the same phase for each frequency at a specific number of frequencies determined by the thickness _d1 and the sound speed _v1 among the many frequencies due to the piezoelectric effect.

On the other hand, the second layer vibrates as follows when it is made of a piezoelectric layer and when it is made of a non-piezoelectric layer:

When the second layer is made of a piezoelectric layer, it vibrates at a specific number of frequencies determined by the thickness _d2 and the sound speed _v2 due to the piezoelectric effect. Here, since _d2 has a value of ( _v2 / _v1 ) _d1 or a value close to it (0.5 to 2.0 times), each second layer vibrates at the specific number of frequencies (i.e., the same frequency as the first layer) with the same phase for each frequency. Also, since the component of the polarization in a direction perpendicular or parallel to the first layer is oriented in a direction different by 180° from the component of the polarization in the first layer in the said direction, the phase of the vibration of the second layer is in the opposite phase to that of the first layer with respect to the vibration direction of the first layer (180° different). As a result, the vibrations of the first layer and the second layer are combined, and a resonance having a plurality of wave numbers that are half integers per layer and the specific plurality of frequencies corresponding to the plurality of wave numbers occurs in the entire laminate.

When the second layer is a non-piezoelectric layer, the second layer does not vibrate due to the piezoelectric effect, but receives vibration from the first layer and vibrates at the same specific multiple frequencies as the first layer. When _d2 has a value of ( _v2 / _v1 ) _d1 or a value close to it (0.5 to 2.0 times), the vibration having the frequency of the first layer satisfies the resonance condition in the second layer. As a result, similar to the case where the second layer is a piezoelectric layer, resonance having the specific multiple frequencies corresponding to multiple wave numbers that are half integers per layer occurs in the entire laminate.

As described above, whether the laminate is made of piezoelectric layers or non-piezoelectric layers, when high-frequency power having multiple frequencies is input, the laminate resonates at specific frequencies among the multiple frequencies. As a result, only the high-frequency power having the specific frequencies is amplified, so the product according to the present invention functions as a frequency filter for high-frequency power.

In the present invention, the more the total number of first and second layers (the above-mentioned predetermined number) is, the thicker the insulator (piezoelectric or non-piezoelectric insulator) between the electrodes is, and therefore it is possible to filter frequencies of high-frequency power with higher power. In the field of mobile phones, the output of transmitted electromagnetic waves is a maximum of about 1 W at a terminal, whereas it is 10 to 100 W at a base station (see, for example, Patent Document 2). Therefore, in the frequency filter of the present invention, the above-mentioned predetermined number, i.e., the total number of first and second layers, is set so as to have a power resistance of 10 W (or more) according to the output of the electromagnetic waves. The more this number of layers is, the higher the power resistance is. In addition, since the frequency to be filtered is determined by the thickness of each layer, there is no effect even if the number of layers is increased.

Power durability is generally determined by taking into account deterioration during use and assuming that a certain amount of power is continuously input for a certain period of time. When the frequency filter of the present invention is used for a mobile phone base station, this specified period is at least three years.

When the second piezoelectric material is used as the material for the second layer, the second piezoelectric material may be the same material as the first piezoelectric material, or may be a different material. When the non-piezoelectric insulating material is used as the material for the second layer, the insulating material may be a layer made of an insulating material that is not a piezoelectric body, or may be a material that can generate piezoelectricity but does not generate piezoelectricity in the entire second layer because the polarization is randomly oriented within the second layer.

When the second layer is a non-piezoelectric layer, unlike when it is made of a piezoelectric layer, there is no need to adjust the relationship between the polarization directions of the first layer 211 and the second layer 212 during fabrication, making it easier to fabricate than when it is made of a piezoelectric layer.

The frequency filter according to the present invention preferably further comprises an acoustic Bragg reflector on one side of the pair of electrodes. The acoustic Bragg reflector is formed by alternately stacking two types of layers with different acoustic impedances. By determining the thickness of each layer of the acoustic Bragg reflector so as to generate Bragg reflection of sound waves having the frequency of the vibration generated within the stack, it is possible to prevent the vibration energy of the stack from leaking to the outside.

Here, regardless of the application such as a mobile phone base station, a frequency filter having a laminate in which a first layer which is a piezoelectric layer and a second layer which is a non-piezoelectric layer made of a non-piezoelectric insulating material are alternately laminated one by one has not been known so far. By using a non-piezoelectric layer as the second layer in this way, it is not necessary to control the second layer so that polarization occurs in a predetermined direction during fabrication, so that the laminate can be easily fabricated. In this case, even if the power resistance is lower than that required for a frequency filter for a mobile phone base station, the frequency filter can be used for other applications. In consideration of these points, another aspect of the frequency filter according to the present invention is as follows:
a) a laminate in which a predetermined number of piezoelectric layers each made of a first piezoelectric material and having polarization oriented in a predetermined direction and a second layer made of a non-piezoelectric insulating material are alternately repeated one by one, and a thickness _d1 of the first layer and a thickness _d2 of the second layer are laminated together in a range of 0.5×( _v2 / _v1 ) _d1 ≦ _d2 ≦2.0×(v2/ _v1 )d1, where _v1 is the sound velocity in the first _layer and _v2 is the sound velocity in the _second layer;
and b) a pair of electrodes disposed so as to sandwich the laminate in a lamination direction. Here, the predetermined direction can be any direction.

The present invention makes it possible to obtain a frequency filter using a piezoelectric thin film that can filter the frequencies of the high-power radio-frequency power handled by mobile phone base stations.

1 is a schematic diagram showing a first embodiment of a frequency filter for a mobile phone base station according to the present invention; 4A and 4B are diagrams showing the directions of polarization of a first layer and a second layer in the frequency filter of the first embodiment. FIG. 2 is a schematic diagram showing an example of a magnetron sputtering apparatus for manufacturing the frequency filter of the first embodiment. 5A to 5C are schematic diagrams illustrating a manufacturing process of the frequency filter according to the first embodiment. FIG. 4 is a schematic diagram showing a modification of the frequency filter of the first embodiment. 5 is a graph showing the results of experiment and calculation of conversion loss for the frequency filter (supported by a substrate made of quartz glass) according to the first embodiment. 11 is a graph showing the results of impedance obtained by experiment and calculation for a frequency filter according to a modified example of the first embodiment. 7 is a graph showing the results of experimental and calculated conversion loss for frequency filters ((a) to (c)) having different numbers of first and second layers from the example shown in FIG. 6 and a reference frequency filter ((d)). FIG. 4 is a schematic diagram showing a second embodiment of a frequency filter for a mobile phone base station according to the present invention;

Using Figures 1 to 9, we will explain an embodiment of a frequency filter for a mobile phone base station (hereinafter simply referred to as a "frequency filter") according to the present invention.

(1) Frequency Filter of the First Embodiment
(1-1) Configuration of the Frequency Filter of the First Embodiment Fig. 1 shows the configuration of a frequency filter 10 of the first embodiment. This frequency filter 10 has a laminate 11 in which a predetermined number of first layers 111 and second layers 112 are alternately stacked, and a first electrode 121 and a second electrode 122 provided to sandwich the laminate 11 in the stacking direction (the vertical direction in Fig. 1).

The first layer 111 is made of a piezoelectric material. In this embodiment, the second layer 112 is also made of a piezoelectric material. The piezoelectric material of the first layer 111 is called the first piezoelectric material, and the piezoelectric material of the second layer 112 is called the second piezoelectric material. The first piezoelectric material and the second piezoelectric material may be the same piezoelectric material or different piezoelectric materials. For the first piezoelectric material and the second piezoelectric material, various piezoelectric materials can be used, such as zinc oxide (ZnO), aluminum nitride (AlN), AlScN (Al _1-x Sc _x N, 0<x<1) in which part of Al (aluminum) in aluminum nitride is replaced with Sc (scandium), (Zn, Mg)O or (Zn, Ca)O in which zinc oxide is doped with Mg or Ca, lead titanate (PbTiO ₃ :PTO), and lead zirconate titanate (PbTi _1-x Zr _x O ₃ :PZT) in which part of Ti (titanium) in lead titanate is replaced with Zr (zirconium).

The first layer 111 and the second layer 112 each have a polarization oriented in the direction described below.

The direction of the polarization _P1 of the first layer 111 may be perpendicular to the first layer 111, parallel to the first layer 111, or even inclined to the first layer 111 (i.e., pointing in a direction that is neither parallel nor perpendicular).

The direction of the polarization _P2 of the second layer 112 is such that a component (parallel component) P2 _// of the polarization _P2 parallel to the first layer 111 and the second layer 112 is in a direction that differs by ₁₈₀ ° from the parallel component _P1 // of the polarization P1 of the first layer 111 (see FIG. 2 ), or a component (vertical component) _P2⊥ of the polarization _P2 perpendicular to the first layer 111 and the second layer 112 is in a direction that differs by 180° from the vertical component _P1⊥ of the polarization _P1 of the first layer 111. In addition, although an example in which the polarizations _P1 and _P2 are both inclined (neither parallel nor perpendicular) to the first layer 111 and the second layer 112 is shown in FIGS. 1 and ₂ , the polarizations _P1 and/or P2 may be parallel to the first layer 111 and the second layer 112, or the polarizations _P1 and/or _P2 may be perpendicular to the first layer 111 and the second layer 112.

The thickness _d2 of the second layer 112 is within the range of 0.5×( _v2 / _v1 ) to 2.0×( _v2 / _v1 ) _d1 times the thickness _d1 of the first layer 111. Here, _v1 is the sound velocity in the first layer 111, and _v2 is the sound velocity in the second layer 112. When the first layer 111 and the second layer 112 are made of the same piezoelectric material, _v1 = _v2 usually holds, so _d2 may be set to 0.5 to 2.0 times _d1 , and typically the first layer 111 and the second layer 112 may be set to the same thickness.

The first layer 111 and the second layer 112 are alternately stacked one layer at a time as described above, for a total of a predetermined number of layers. The more layers there are, the higher the power resistance. Therefore, in order to ensure the power resistance of the frequency filter 10 as a frequency filter used in mobile phone base stations, the number of layers of the first layer 111 and the second layer 112 is determined so that the frequency filter 10 has a power resistance with an upper limit in the range of 10 to 100 W. Whether such power resistance is satisfied can be confirmed by performing simulations or preliminary experiments.

The number of layers in the laminate 11 is preferably as large as possible to ensure sufficient power resistance of 10 W (or more), but if there are too many layers, manufacturing will be more time-consuming, so the number can be determined appropriately taking these factors into consideration.

The first electrode 121 and the second electrode 122 correspond to the pair of electrodes described above. There is no particular restriction on the material of the first electrode 121 and the second electrode 122 as long as it is conductive.

(1-2) Manufacturing Method of Frequency Filter of First Embodiment A manufacturing method of the frequency filter 10 of the first embodiment will be described with reference to Figs. 3 and 4. Fig. 3 shows a magnetron sputtering device 30 for manufacturing the frequency filter 10. The magnetron sputtering device 30 has a vacuum vessel 31, the inside of which is evacuated by a vacuum pump (not shown). A magnetron electrode 32 and a substrate holder 33 inclined with respect to the upper surface of the magnetron electrode 32 are provided inside the vacuum vessel 31. A plate-shaped target T, which is the material of each layer of the frequency filter 10, is placed on the upper surface of the magnetron electrode 32. A high-frequency power source 34 is connected to the magnetron electrode 32 via a matching box 341. The substrate holder 33 is made of a plate-shaped conductor and is grounded. A cooling mechanism 35 is provided at one end of the substrate holder 33 to cool the one end of the substrate holder 33 with cooling water.

When using a gas together with a target for the material of the first layer 111 and/or the second layer 112, a gas inlet (not shown) is provided in the vacuum vessel 31. For example, when producing the first layer 111 and/or the second layer 112 made of AlN or AlScN, an Al or Sc target is used, and nitrogen gas is introduced into the vacuum vessel 31 from the gas inlet during production.

When manufacturing the frequency filter 10, first, the substrate S is attached to the substrate holder 33, and a target TM made of metal is placed on the upper surface of the magnetron electrode 32. In this state, the target TM is sputtered to produce a first electrode 121 on the surface of the substrate S (Figure 4(a)).

Next, a target T1, which is the material of the first layer 111, is placed on the upper surface of the magnetron electrode 32. In this state, the target T1 is sputtered while one end of the substrate holder 33 is cooled by the cooling mechanism 35, thereby forming a first layer 111 on the surface of the substrate S (FIG. 4(b)). At this time, since the substrate holder 33 is inclined with respect to the upper surface of the magnetron electrode 32 and one end of the substrate holder 33 is cooled by the cooling mechanism 35, a temperature gradient is generated within the surface of the substrate S, and therefore the polarization _P1 of the first layer 111 is inclined with respect to the first layer 111.

Next, the substrate S is temporarily removed from the substrate holder 33, rotated 180° around its normal axis, and then reattached to the substrate holder 33 (FIG. 4(c)). At the same time, a target T2, which is the material of the second layer 112, is placed on the upper surface of the magnetron electrode 32. In this state, the target T2 is sputtered while cooling one end of the substrate holder 33 with the cooling mechanism 35, to produce one layer of the second layer 112 on the surface of the first layer 111 (FIG. 4(d)). At this time, because the substrate S is rotated as described above, the substrate holder 33 is inclined as in the production of the first layer 111, and one end of the substrate holder 33 is cooled by the cooling mechanism 35, the polarization _P2 of the second layer 112 is oriented in a direction inclined with respect to the second layer 112, and the component of the polarization _P2 parallel to the second layer 112 is in the opposite direction to the polarization _P1 of the first layer 111.

Then, the next first layer 111 is fabricated on the second layer 112 by the same method as the first first layer 111, and the next second layer 112 is fabricated on the first layer 111 by the same method as the first second layer 112. This operation is repeated. In this way, a predetermined number of first layers 111 and second layers 112 are fabricated (Figure 4(e)).

Finally, a target TM made of metal is placed on the upper surface of the magnetron electrode 32, and the target TM is sputtered in this state to produce a second electrode 122 on the last of the first layer 111 and second layer 112 on the substrate S (FIG. 4(f)). Thereafter, the substrate S is removed as necessary (substrate S may be left in place), thereby completing the frequency filter 10.

(1-3) Operation of the Frequency Filter of the First Embodiment In the frequency filter 10 of the first embodiment, when high frequency power including a large number of frequencies is input between the first electrode 121 and the second electrode 122, a high frequency voltage including the large number of frequencies and having a magnitude corresponding to the magnitude of the high frequency power is generated between the electrodes. As a result, the high frequency voltage including the large number of frequencies is also applied in the thickness direction to each of the first layers 111 and each of the second layers 112 included in the laminate 11 (the sum of the voltages applied to each layer corresponds to the voltage between the first electrode 121 and the second electrode 122).

Then, each first layer 111 vibrates at a plurality of specific frequencies determined by the thickness _d1 and sound speed _v1 of the first layer 111 among the many frequencies, with the same phase for each frequency, due to the piezoelectric effect. At this time, since the polarization _P1 of each first layer 111 faces the same direction, each first layer 111 vibrates in the same direction. On the other hand, each second layer 112 vibrates at a plurality of specific frequencies determined by the thickness _d2 and sound speed _v2 of the second layer 112, due to the piezoelectric effect. The phase of vibration of the second layer 112 is opposite to that of the first layer 111, since the direction (second direction) of the parallel component _{P2 //} of the polarization P2 of the _second layer 112 is 180° different from the direction (first direction) of the parallel component P1 _// of the polarization P1 of the first layer 111. Furthermore, since _d2 is within the range of 0.5×( _v2 / _v1 ) to 2.0×( _v2 / _v1 ) _d1 times d1, the first layer 111 and _the second layer vibrate at the same specific multiple frequencies and enter into a resonant state. These specific multiple frequencies correspond to multiple wave numbers that are half integers per layer.

As a result of the formation of such a resonant state, only the high-frequency power having the specific multiple frequencies is amplified. Therefore, the device according to the first embodiment functions as a frequency filter.

The frequency filter 10 of the first embodiment has the number of layers of the first layer 111 and the second layer 112 determined so that it has power resistance for a specified power in the range of 10 to 100 W, and therefore can be used in mobile phone base stations with an output that corresponds to such power.

(1-4) Modification of the Frequency Filter of the First Embodiment FIG. 5 shows a frequency filter 10A which is a modification of the first embodiment. This frequency filter 10A is provided with an acoustic Bragg reflector 13 on the surface of the first electrode 121 opposite to the laminate 11 in the frequency filter 10 of the first embodiment. The acoustic Bragg reflector 13 is formed by alternately laminating two types of layers 131 and 132 having different acoustic impedances. The thickness of each layer 131 and 132 is determined according to the material of each layer 131 and 132 so that Bragg reflection of sound waves having the frequency of the vibration of the laminate 11 occurs. By providing such an acoustic Bragg reflector 13, it is possible to suppress the energy of the vibration of the laminate 11 from leaking to the outside.

(1-5) Experimental and Calculation Results for the Frequency Filter of the First Embodiment Next, the results of the experiment and calculation performed on the frequency filter of the first embodiment will be described. In the experiment, the frequency filter 10 of the first embodiment supported by a substrate made of quartz glass and the frequency filter 10A of the modified example (i.e., the frequency filter 10 provided with the acoustic Bragg reflector 13) were fabricated. In the fabricated frequency filters 10 and 10A, the laminate 11 was made of 6 first layers 111 and 6 second layers 112, totaling 12 layers. The material of both the first layer 111 and the second layer 112 was Sc _0.43 Al _0.57 N (ScAlN, x=0.43). The thickness d ₁ of the first layer 111 and the thickness d ₂ of the second layer 112 were both fabricated with a target value of 5 μm, but since the measured value of the total thickness of the obtained laminate 11 was 52 μm, the actual d ₁ and d ₂ were about 4.3 (52/12) μm on average. In the calculation, the values of _d1 and _d2 were set to 5 μm. The target value of _d1 and _d2 in the experiment and the value used in the calculation, "5 μm," were set so that the frequency filter 10A functions in a frequency range centered on 400 MHz. The acoustic Bragg reflector 13 was fabricated so as to generate Bragg reflection of a sound wave with a frequency of 400 MHz.

The following experiment was performed on the fabricated frequency filter 10. A high-frequency electric field was applied between the first electrode 121 and the second electrode 122. This high-frequency electric field was converted into mechanical vibration by the inverse piezoelectric effect in the laminate 11. The mechanical vibration was reflected by the bottom surface of the quartz glass substrate and converted into a high-frequency voltage by the piezoelectric effect in the laminate 11. In this experiment, this high-frequency voltage was measured, and the insertion loss was calculated based on the obtained high-frequency voltage. Then, the mechanical loss occurring in the quartz glass substrate was subtracted from the obtained insertion loss to calculate the conversion loss, which is the efficiency of electromechanical conversion due to the piezoelectric effect. In addition, the same structure as in the experiment was constructed using an equivalent circuit model, and the theoretical conversion loss was calculated. The results of these experiments and calculations are shown in Figure 6. These experimental and calculation results indicate that the conversion loss is reduced at many frequencies, including around 400 MHz (0.4 GHz), meaning that the filter functions as a frequency filter.

Figure 7 shows the results of measuring the (electrical) impedance of the fabricated frequency filter 10A, along with the calculation results. Both the experimental and calculated values show an impedance of over 200 Ω at frequencies around 400 MHz, demonstrating that the frequency filter 10A has sufficient power resistance.

In this case, the frequency was set to a relatively small value of 400 MHz to simplify the experiment, but by making the thicknesses of the first layer 111 and the second layer 112 thinner and increasing the number of layers of the first layer 111 and the second layer 112 accordingly, it is possible to obtain a frequency filter that can be used in higher frequency bands (e.g., 3.5 GHz band or 4.5 GHz band) used in mobile phone base stations.

Figure 8 shows the results of experimental and calculated conversion loss when the total number of layers, the first layer 111 and the second layer 112, is (a) 8 layers, (b) 4 layers, and (c) 2 layers. The thickness of each layer is the same as that of the 12-layer case shown in Figures 6 and 7 (target value for the experiment). For reference, similar experiments and calculations were also performed for the case where there is only one first layer 111 and no second layer 112 (not included in the present invention) (Figure 8(a)). The fewer the total number of layers, the fewer the positions in the graph where the conversion efficiency is minimal, but in all cases, the minimum value exists at a position where the horizontal axis is around 400 MHz, and it can be seen that the device operates at a frequency of around 400 MHz. However, reducing the number of layers reduces the power durability. Therefore, it is advisable to determine the total number of layers, including the first layer 111 and the second layer 112, taking into consideration power durability and cost (which increases as the number of layers increases).

(2) Frequency Filter of Second Embodiment Fig. 9 shows the configuration of a frequency filter 20 of the second embodiment. This frequency filter 20 has a laminate 21 in which a predetermined number of first layers 211 and second layers 212 are alternately repeated, and a first electrode 221 and a second electrode 222 provided to sandwich the laminate 21 in the lamination direction. Of these components, the first electrode 221 and the second electrode 222 are similar to the first electrode 121 and the second electrode 122 in the frequency filter 10 of the first embodiment, respectively.

In the second embodiment, a non-piezoelectric layer made of a non-piezoelectric insulating material is used for the second layer 212. Typical examples of such non-piezoelectric insulating materials include dielectrics that do not have piezoelectricity, such as _SiO2 . Alternatively, the second layer 212 may be made of a material that can produce piezoelectricity but does not produce piezoelectricity as a whole because the polarization is randomly oriented within the second layer 212.

The first layer 211 is made of a piezoelectric material. The polarization direction of the first layer 211 is arbitrary as long as each first layer 211 is in the same direction. Therefore, the polarization direction of the first layer 211 may be perpendicular to the first layer 211, parallel to the first layer 211, or even inclined to the first layer 211 (neither perpendicular nor parallel).

In the example shown in FIG. 9, the layer closest to the first electrode 221 in the laminate 21 is the first layer (piezoelectric layer) 211, but this layer may be the second layer (non-piezoelectric layer) 212 (so that the first layer 211 and the second layer 212 in the example shown in FIG. 9 are interchanged for the entire laminate 21).

The thickness _d2 of the second layer 212 is _set within the range of 0.5×( _v2 /v1) to 2.0×( _v2 / _v1 ) _d1 times the thickness _d1 of the first layer 211, as in the first embodiment.

The number of layers in the first layer 111 and the second layer 112 is set to have an upper limit of power resistance within the range of 10 to 100 W to ensure the power resistance required for a frequency filter used in a mobile phone base station, as in the first embodiment.

The frequency filter 20 of the second embodiment can be basically manufactured in the same manner as the frequency filter 10 of the first embodiment. However, when the direction of the polarization _P1 of the first layer 211 is perpendicular to the first layer 211 as shown in Fig. 9, it is not necessary to tilt the substrate with respect to the upper surface of the magnetron electrode or to form a temperature gradient within the surface of the substrate, and it is sufficient to arrange the substrate parallel to the upper surface of the magnetron electrode and then manufacture the first layer 211 and the second layer 212 by sputtering.

In the frequency filter 20 of the second embodiment, when high-frequency power including a large number of frequencies is input between the first electrode 221 and the second electrode 222, a high-frequency voltage including the frequencies and having a magnitude corresponding to the magnitude of the high-frequency power is generated between the electrodes. As a result, each first layer 211 vibrates at a plurality of specific frequencies among the plurality of frequencies, determined by the thickness d ₁ of the first layer 211 and the sound speed v ₁ , with the same phase for each frequency, due to the piezoelectric effect. On the other hand, although no vibration due to the piezoelectric effect occurs in the second layer 212, the second layer 212 vibrates at the same plurality of specific frequencies as the first layer 211 upon receiving vibration from the first layer 211. Here, the thickness d ₂ of the second layer 212 is the same as the thickness d ₁ of the first layer 211 or is within a range of 0.5×(v ₂ /v ₁ ) times to 2.0×(v ₂ /v ₁ ) times d ₁ times close to d ₁ , so that the vibration having the frequency of the first layer 211 satisfies the resonance condition in the second layer 212. As a result, resonance having the specific multiple frequencies corresponding to multiple wave numbers that are half integers per layer occurs throughout the laminate 21. This allows the product according to the second embodiment to function as a frequency filter for the frequencies.

Furthermore, the frequency filter 20 has the first layer 211 and the second layer 212 stacked alternately one layer at a time as described above, totaling a predetermined number of layers, thereby ensuring the power resistance required for a frequency filter used in a mobile phone base station.

Unlike the frequency filter 10 of the first embodiment, the frequency filter 20 of the second embodiment does not require adjustment of the relationship between the polarization directions of the first layer 211 and the second layer 212, and can therefore be manufactured more easily without using a special manufacturing method such as that shown in FIG. 4. On the other hand, in the frequency filter 10 of the first embodiment, since not only the first layer 111 but also the second layer 112 are piezoelectric, the second layer 112 also becomes a source of vibration, and therefore it is possible to obtain a stronger output signal than the frequency filter 20 of the second embodiment.

The frequency filter 20 of the second embodiment may be used for purposes other than a mobile phone base station. In that case, depending on the purpose (for example, for a mobile phone terminal), power resistance to 10 W of high-frequency power is not required.

(3) Others Although the frequency filter 10 of the first embodiment, the frequency filter 10A of the modified example thereof, and the frequency filter 20 of the second embodiment have been described above, the present invention is not limited to these three examples, and various modifications are possible. For example, the frequency filter 20 of the second embodiment can be provided with an acoustic Bragg reflector, similar to the frequency filter 10A of the modified example of the first embodiment.

10, 10A, 20...frequency filter 11, 21...laminated body 111, 211...first layer 112, 212...second layer 121, 221...first electrode 122, 222...second electrode 13...acoustic Bragg reflector 131, 132...layer constituting acoustic Bragg reflector 30...magnetron sputtering apparatus 31...vacuum vessel 32...magnetron electrode 33...substrate holder 34...high frequency power supply 341...matching box 35...cooling mechanism

Claims (4)

a) a laminate in which a predetermined number of first layers made of a first piezoelectric material and having a polarization in one predetermined direction and second layers made of a second piezoelectric material and having a component of polarization perpendicular or parallel to the first layer that is 180° different from the component of polarization in the first layer, or a non-piezoelectric layer made of a non-piezoelectric insulating material, are alternately repeated one by one, and a thickness _d1 of the first layer and a thickness _d2 of the second layer are within the range of 0.5×( _v2 / _v1 ) _d1 ≦ _d2 ≦2.0×(v2/ _v1 )d1, where _v1 is the sound velocity in the _first layer and _v2 is the sound velocity in the _second layer;
b) a pair of electrodes disposed so as to sandwich the laminate in a lamination direction,
The predetermined number is set so that the frequency filter has a power resistance to 10 W of high-frequency power. The frequency filter of claim 1, characterized in that the second layer is made of the non-piezoelectric layer. The frequency filter according to claim 1 or 2, further comprising an acoustic Bragg reflector on one side of the pair of electrodes. a) a laminate in which a predetermined number of piezoelectric layers each made of a first piezoelectric material and having polarization oriented in a predetermined direction and a second layer made of a non-piezoelectric insulating material are alternately repeated one by one, and a thickness _d1 of the first layer and a thickness _d2 of the second layer are laminated together in a range of 0.5×( _v2 / _v1 ) _d1 ≦ _d2 ≦2.0×(v2/ _v1 )d1, where _v1 is the sound velocity in the first _layer and _v2 is the sound velocity in the _second layer;
b) A frequency filter comprising a pair of electrodes arranged to sandwich the laminate in the lamination direction.

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