Piezoelectric film and frequency filter using the piezoelectric film

Abstract

When produced by the usual method of depositing a raw material on a substrate using a sputtering method or the like, the polarization in the thickness direction becomes N-polarization opposite to that of a conventional AlN piezoelectric film, and a conventional Al t Si x N piezoelectric film or Al 1-x Ge x A piezoelectric film having a larger electromechanical coupling coefficient k t in the thickness direction than that of an Al 【Solution means】The piezoelectric film 10 is formed by replacing 0.05% or more and less than 2.4% of the Al atoms in AlN with Si atoms or 0.05% or more and less than 3.5% of the Al atoms with Ge atoms, and further replacing 0.5% or more and 45% or less of the Al atoms with Sc. The piezoelectric film 10 is a thin film composed of Al 1-x-y Si x Sc y N (0.0005 ≦ x < 0.024, 0.005 ≦ y ≦ 0.5) or Al 1-x-y Ge x Sc y N (0.0005 ≦ x < 0.035, 0.005 ≦ y ≦ 0.45).

Description

[Technical Field]

[0001] The present invention relates to a piezoelectric film, which is a film made of a piezoelectric material, and a frequency filter using a piezoelectric film, which is used in communication equipment and the like. [Background technology]

[0002] Mobile devices such as smartphones, and base stations that transmit and receive electromagnetic waves to and from these devices, use frequency filters to extract currents of specific frequencies from electromagnetic waves containing various frequencies received by the antenna, which are converted into signal currents. One such frequency filter is the bulk acoustic wave (BAW) filter.

[0003] A BAW filter comprises a piezoelectric film and a pair of electrodes sandwiching the piezoelectric film in the thickness direction. When an AC voltage of a signal current containing various frequencies, converted from electromagnetic waves, is applied between these electrodes, the piezoelectric film sandwiched between them generates mechanical vibrations containing the same various frequencies as the AC voltage through piezoelectric conversion. Among these mechanical vibrations, resonance is formed within the piezoelectric film due to a mechanical vibration whose frequency (resonant frequency) is an integer multiple of half of the wavelength λ (nλ / 2: n is a natural number) that matches the thickness of the piezoelectric layer. This mechanical vibration with the resonant frequency is then converted into an AC voltage by the piezoelectric film. On the other hand, mechanical vibrations with frequencies other than the resonant frequency are attenuated within the BAW filter. Due to these actions, only those signal currents containing the resonant frequency that are input to the BAW filter pass through the filter. In mobile terminals and base stations, multiple BAW filters are used in combination to extract electromagnetic waves (signal currents) of a specific frequency band from electromagnetic waves with a wide frequency band.

[0004] BAW filters include single-layer BAW filters, which use only one piezoelectric film with polarization oriented in a specific direction, and higher-mode BAW filters, which have a laminated structure with two types of piezoelectric films with different polarization directions stacked alternately. In higher-mode BAW filters, the thickness of a single piezoelectric film is half the wavelength of the mechanical vibration corresponding to the frequency of the current being passed through. Because such piezoelectric films are stacked, the overall thickness of the laminated structure is greater than that of a single-layer BAW filter that passes through a current of the same frequency. This improves the voltage withstand capability, making higher-mode BAW filters particularly suitable for use as frequency filters for base stations where high voltage withstand capability is required. Furthermore, in frequency filters for mobile terminals, since the input impedance is a predetermined value (50Ω), it is necessary to set the capacitance C = εS / d (ε: dielectric constant, S: area, d: thickness) to a predetermined value accordingly. Therefore, as the thickness d increases, the area S is increased, which increases the volume of the laminated structure. As a result, higher-order mode BAW filters have a larger heat capacity and improved heat resistance compared to single-mode BAW filters, making them suitable for use as frequency filters in mobile devices.

[0005] Aluminum nitride (AlN) is one of the materials used for piezoelectric films in BAW filters. AlN has an electromechanical coupling coefficient k related to vibrations in the thickness direction of the piezoelectric film. t AlN is known as a material with high piezoelectric properties, meaning it has a high conversion efficiency between the voltage of an electrical signal applied in the thickness direction and the mechanical vibrations generated in the same direction. However, when a piezoelectric film made of AlN is fabricated using the usual method of depositing raw materials onto a substrate using sputtering, etc., the crystal is always formed such that aluminum (Al) atoms are located on the upper surface (opposite the substrate) and nitrogen (N) atoms are located on the lower surface (substrate side), and only upward polarization (called "Al polarization") can be formed. For this reason, conventionally, it has not been possible to fabricate a higher-mode BAW filter using only a piezoelectric film made of AlN, and only single-component BAW filters or higher-mode BAW filters made in combination with a piezoelectric film made of a material other than AlN that forms downward polarization have been fabricated.

[0006] Recently, 2.4 to 13% of the Al atoms in AlN were replaced with silicon (Si) atoms to form Al 1-x Si x N (x = 0.024 - 0.13). In the piezoelectric film (Al 1-x Si x N piezoelectric film), it was revealed that a downward polarization (N polarization) is formed, where N atoms are located on the upper most surface and Al atoms are located on the lower most surface (Non-Patent Document 1). Also, 3.5 to 11.9% of the Al atoms in AlN were replaced with germanium (Ge) atoms to form Al 1-x Ge x N (x = 0.035 - 0.119). In the piezoelectric film (Al 1-x Ge x N piezoelectric film), it was also revealed that N polarization is formed (Non-Patent Documents 2 and 3). Non-Patent Documents 1 to 3 further describe a high-order mode BAW filter in which piezoelectric films composed of an Al 1-x Si x N piezoelectric film or an Al 1-x Ge x N piezoelectric film and a piezoelectric film composed of AlN with Al polarization are alternately laminated.

Prior Art Documents

Patent Documents

[0007]

Patent Document 1

Non-Patent Documents

[0008]

Non-Patent Document 1

[0009] Al of N polarization as described in Non-Patent Document 1 1-x Si x N piezoelectric film and Al with N polarization as described in Non-Patent Documents 2 and 3. 1-x Ge x N piezoelectric films have a higher electromechanical coupling coefficient in the thickness direction than AlN piezoelectric films with Al polarization that do not contain Si or Ge. tThe coefficient is small, meaning the piezoelectric properties are low. Therefore, Al-polarized AlN piezoelectric film and N-polarized Al 1-x Si x N piezoelectric film or Al 1-x Ge x A higher-order mode BAW filter, which consists of alternating stacks of N piezoelectric films, cannot achieve a wide passband or low insertion loss.

[0010] The problem that this invention aims to solve is that when manufactured by a conventional method of depositing raw materials onto a substrate using a sputtering method, the polarization in the thickness direction becomes an N polarization opposite to that of conventional AlN piezoelectric films, and conventional AlN piezoelectric films having an N polarization. 1-x Si x N piezoelectric film and Al 1-x Ge x The electromechanical coupling coefficient in the thickness direction is greater than that of an N piezoelectric film. t The objective is to provide a piezoelectric film with a large capacitance, and a frequency filter using the piezoelectric film. [Means for solving the problem]

[0011] The piezoelectric film according to the present invention, which was made to solve the above problems, is an AlN in which 0.05% or more and less than 2.4% of the Al atoms are replaced with Si atoms, or 0.05% or more and less than 3.5% are replaced with Ge atoms, and furthermore, 0.5% or more and 45% of the Al atoms are replaced with Sc (scandium). 1-x-y Si x Sc y N(0.0005≦x<0.024, 0.005≦y≦0.45) or Al 1-x-y Ge x Sc y It is a thin film composed of N (0.0005 ≤ x < 0.035, 0.005 ≤ y ≤ 0.45).

[0012] In AlN that does not contain Si or Ge, it is known that the piezoelectric properties in the thickness direction can be improved by substituting 0.5 to 50% of the Al atoms with Sc atoms (for example, Patent Document 1. Note that in the same document, the electromechanical coupling coefficient k t Instead, the piezoelectric strain constant d in the thickness direction 33The piezoelectric properties are evaluated using [a specific method / tool]. However, it is generally considered difficult to replace one of the two types of atoms constituting a binary compound with two other types of atoms, and therefore it has also been considered difficult to replace some of the Al atoms in AlN with two types of atoms, Si (or Ge) and Sc. In fact, the inventors attempted to replace 3% of the Al atoms in AlN, which fall within the range where N polarization is realized in Non-Patent Document 1, with Si, and further to replace 5% of the Al atoms with Sc atoms, but were unable to produce a piezoelectric material having a crystal structure similar to AlN.

[0013] Therefore, the inventors attempted to reduce the amount of Al atoms substituted with Si or Ge atoms to less than 2.4% (in the case of Si atoms) or less than 3.5% (in the case of Ge atoms), which is lower than the range described in Non-Patent Documents 1 to 3, and to substitute 0.5 to 45% of the Al atoms with Sc atoms. Here, the upper limit of the amount of Sc atom substitution was set to 45%, which is lower than the upper limit of 50% described in Patent Document 1, taking into consideration the substitution of a portion of the Al atoms with Si or Ge atoms. As a result, the inventors succeeded in fabricating a piezoelectric film made of a piezoelectric material having a crystal structure similar to AlN by depositing the raw material on a substrate using the sputtering method. Furthermore, the inventors found that the fabricated piezoelectric film has N polarization despite the reduced amount of Si and Ge atom substitution, and has a higher electromechanical coupling coefficient in the thickness direction than an AlN film made of AlN that does not contain Si, Ge, or Sc atoms. t We found that this value increases. As a result, the piezoelectric film according to the present invention was completed.

[0014] The piezoelectric film according to the present invention can be manufactured by depositing a raw material consisting of constituent atoms other than N (Al, Si or Ge, and Sc) on a substrate made of sapphire or the like, which has a hexagonal crystal structure similar to AlN and a lattice constant close to that of AlN, under a gas atmosphere containing nitrogen. A substrate made of a material having a face-centered cubic structure may be used instead of a hexagonal crystal. Alternatively, a substrate may be made by forming a thin metal layer of platinum or the like on the surface of a plate made of the same material as the substrate. This allows the metal layer to be used as one electrode of a BAW filter.

[0015] Also, Al that does not contain Si or Ge 1-Z Sc Z A film made of N (0≦z<1; for z=0, it is AlN) may be used as the substrate. Conversely, the piezoelectric film according to the present invention may be used as the substrate, and an Al film that does not contain Si or Ge may be placed on it. 1-Z Sc Z It is also possible to form a film made of N. The AlN film thus produced, which does not contain Si or Ge, has Al polarization. By combining these methods, a piezoelectric film according to the present invention can be made which contains Si or Ge and Sc and has N polarization, and which does not contain Si or Ge and has Al polarization. 1-Z Sc Z A piezoelectric film laminate can be obtained in which N films are repeatedly stacked alternately. This piezoelectric film laminate can be used in a higher-mode BAW filter.

[0016] In other words, the frequency filter according to the present invention is Al 1-Z Sc Z A first piezoelectric film consisting of N (0 ≤ z < 1), and AlN in which 0.05% or more but less than 2.4% of the Al atoms are replaced with Si atoms, or 0.05% or more but less than 3.5% are replaced with Ge atoms, and furthermore, 0.5% or more but less than 45% of the Al atoms are replaced with Sc. 1-x-y Si x Sc y N(0.0005≦x<0.024, 0.005≦y≦0.45) or Al 1-x-y Gex Sc y A piezoelectric film laminate is formed by alternately stacking a second piezoelectric film consisting of N (0.0005≦x<0.035, 0.005≦y≦0.45) and a third piezoelectric film. A pair of electrodes are provided so as to sandwich the piezoelectric film laminate in the stacking direction. This is a bulk elastic wave filter equipped with [specific features / features]. [Effects of the Invention]

[0017] According to the piezoelectric film of the present invention, by using a conventional method of depositing raw materials onto a substrate using a sputtering method, it is possible to form an N polarization, which is a polarization in the thickness direction opposite to the Al polarization in conventional AlN piezoelectric films, and a conventional AlN piezoelectric film having an N polarization. 1-x Si x N piezoelectric film and Al 1-x Ge x The electromechanical coupling coefficient in the thickness direction is greater than that of an N piezoelectric film. t It can be made larger.

[0018] Furthermore, according to the frequency filter of the present invention, the electromechanical coupling coefficient k of the second piezoelectric film is t Because this can be increased, the filtering characteristics can be improved. [Brief explanation of the drawing]

[0019] [Figure 1] A schematic diagram showing the piezoelectric film of this embodiment fabricated on a substrate. [Figure 2] A diagram showing the crystal structure of AlN-based materials when Al polarization (a) and N polarization (b) are formed. [Figure 3] This figure shows the configuration of the magnetron sputtering apparatus used to fabricate the piezoelectric film in this embodiment. [Figure 4] This graph shows the results of 2θ-ω scan measurements performed using an X-ray diffractometer for Si 0.3% sample (a) and Si 0.5% sample (b). [Figure 5] This graph shows the results of ω-scan measurements performed using an X-ray diffractometer for Si 0.3% and Si 0.5% samples. [Figure 6] This graph shows the results of φ-scan measurements performed using an X-ray diffractometer for Si 0.3% sample (a) and Si 0.5% sample (b). [Figure 7] This graph shows the results of measuring the anti-resonance frequency while varying the DC bias voltage in the thickness direction for Si0.3% sample (a) and Si0.5% sample (b). (a) also shows the measurement results for Al0.6Sc0.4N, a comparative example having Al polarization. [Figure 8] Graphs showing measured and theoretical values ​​of conversion loss obtained to determine the electromechanical coupling coefficient kt in the thickness direction for Si 0.3% sample (a) and Si 0.5% sample (b). [Figure 9] A schematic diagram showing the BAW filter of this embodiment using the piezoelectric film of this embodiment. [Modes for carrying out the invention]

[0020] Embodiments of the piezoelectric film and BAW filter according to the present invention will be described with reference to Figures 1 to 9.

[0021] (1) Al 1-x-y Si x Sc y N piezoelectric film, Al 1-x-y Ge x Sc y N piezoelectric film The piezoelectric film 10 of this embodiment (Figure 1) is made of Al 1-x-y Si x Sc y N(0.0005≦x<0.024, 0.005≦y≦0.45) or Al 1-x-y Ge x Sc y The film is made of a piezoelectric material having a composition represented by N(0.0005≦x<0.035, 0.005≦y≦0.45). Unless otherwise specified, "Al 1-x-y Si x Sc y "N" refers to a material that satisfies the requirements "0.0005≦x<0.024, 0.005≦y≦0.45", and "Al 1-x-y Ge x Sc y"N" refers to a material that satisfies the requirements "0.0005 ≤ x < 0.035, 0.005 ≤ y ≤ 0.45". This piezoelectric film 10 has a hexagonal crystal structure similar to AlN, and by fabricating it on a substrate 90 made of a material with a lattice constant close to that of AlN, an N polarization is formed in which the polarization P faces the substrate 90.

[0022] Here, we will explain Al polarization and N polarization using Figure 2. Al polarization is the polarization (also called electric polarization or dielectric polarization) that is normally present in a piezoelectric film made of AlN that does not contain Si, Ge, and Sc. As shown in Figure 2(a), it refers to polarization formed such that Al atoms (ions) are located on the outermost surface of the piezoelectric film (the surface furthest from the substrate 90), and N atoms (ions) are located on the surface facing the substrate 90. Since Al ions are positive ions and N ions are negative ions, and the direction of polarization is defined as moving from the negative charge side to the positive charge side, Al polarization is polarization that faces away from the substrate 90. On the other hand, as shown in Figure 2(b), N polarization is formed such that N ions are located on the outermost surface of the piezoelectric film 10, and Al ions or Si ions, Ge ions, or Sc ions substituted for Al ions are located on the surface facing the substrate 90, and polarization P faces the substrate 90 side. As described above, the piezoelectric film 10 of this embodiment is N-polarized and has the crystal structure shown in Figure 2(b).

[0023] In this embodiment, the substrate 90 is made of a single crystal of a material having a hexagonal or face-centered cubic (a type of cubic) crystal structure and a lattice constant close to that of AlN, and a metal layer 92 made of a metallic material is deposited on the surface of a single crystal plate 91 in which the (0001) plane is parallel to the surface (the c axis is perpendicular to the surface). The metal layer 92 does not need to be a single crystal, but in this embodiment it is formed by epitaxial growth on the single crystal plate 91.

[0024] The single crystal plate 91 is manufactured by cutting a single crystal so that the (0001) plane is parallel to the surface. Sapphire (Al2O3), which has a hexagonal crystal structure, can preferably be used as the material for the single crystal plate 91. When the material for the single crystal plate 91 is sapphire, platinum (Pt) can preferably be used as the material for the metal layer 92. Platinum is a material with a cubic face-centered cubic structure, but when deposited on the surface of a sapphire single crystal plate 91, the metal layer 92 is formed so that the (111) plane of platinum is parallel to the (0001) plane of the sapphire. This is because both the (0001) plane of sapphire and the (111) plane of platinum are made up of constituent atoms that are closely packed in a triangular lattice, and the interatomic distance between them is small. The piezoelectric film 10 is fabricated on the surface of the metal layer 92, but as described above, the (111) plane of platinum has a structure similar to the (0001) plane of sapphire, Al 1-x-y Si x Sc y N and Al 1-x-y Ge x Sc y N grows crystallically in the same way as when it is fabricated on the (0001) plane of sapphire. The metal layer 92 can be used as one of a pair of electrodes in the BAW filter described later.

[0025] Note that the material of the substrate 90 listed here is just an example, and other materials may be used. For example, for the material of the single crystal plate 91, instead of sapphire, silicon carbide (SiC), silicon (Si), magnesium oxide (MgO), strontium titanate (SrTiO3), gallium arsenide (GaAs), germanium (Ge), etc. can be used. Also, when alternately laminating the Al-polarized piezoelectric film made of AlN and the piezoelectric film 10 of the present embodiment as in the BAW filter described later, the Al-polarized piezoelectric film can be used as the substrate of the piezoelectric film 10 of the present embodiment. For the material of the metal layer 92, instead of platinum (Pt), aluminum (Al), titanium (Ti), copper (Cu), molybdenum (Mo), hafnium (Hf), tungsten (W), iridium (Ir), ruthenium (Ru), gold (Au), zirconium (Zr), etc. can be used. When the electrode in contact with the piezoelectric film 10 is not necessary, the single crystal plate 91 may be used alone as the substrate 90.

[0026] In this embodiment, Al 1-x-y Si x Sc y N or Al 1-x-y Ge x Sc y To fabricate the piezoelectric film 10 made of a single crystal of Al 1-x-y Si x Sc y N or Al 1-x-y Ge x Sc y A piezoelectric film made of a polycrystal of N is also included in the present invention. When fabricating such a piezoelectric film made of a polycrystal, a substrate made of a material having a hexagonal crystal structure such as sapphire and composed of a polycrystal with the c-axis oriented in a direction perpendicular to the substrate may be used.

[0027] An example of the manufacturing method of the piezoelectric film 10 of this embodiment will be described. In this example, the piezoelectric film 10 is manufactured using the known magnetron sputtering apparatus 80 shown in FIG. 3. The magnetron sputtering apparatus 80 includes a processing chamber 81, a magnetron electrode 82 placed in the processing chamber 81, a substrate holder 83 suspended from the ceiling of the processing chamber 81 and grounded so as to face the magnetron electrode 82, a high-frequency power source 84 that applies a high-frequency voltage between the magnetron electrode 82 and the substrate holder 83 (grounded), a gas inlet 85 that introduces a processing gas containing nitrogen gas into the processing chamber 81, and a gas outlet 86 that discharges gas from the processing chamber 81 by a vacuum pump not shown. The magnetron electrode 82 has a permanent magnet at its lower part and functions as a target holder that holds the target T, which is the material of the piezoelectric film 10, on its upper surface. The substrate holder 83 has a heater (not shown) built therein and can heat the substrate 90.

[0028] When manufacturing the piezoelectric film 10, first, the target T is placed on the upper surface of the magnetron electrode 82, and the substrate 90 is attached to the substrate holder 83 with its surface facing downward (the target T side). The target T contains Al of the piezoelectric film 10 to be manufactured. 1-x-y Si x Sc y N or Al 1-x-y Ge x Sc yThe N composition used contains Al, Si or Ge, and Sc, which are elements other than N (nitrogen), in the same proportions as the nitrogen. After setting up the target T and substrate 90 in this way, the air in the processing chamber 81 is discharged from the gas outlet 86 by a vacuum pump, and then the processing gas is introduced into the processing chamber 81 from the gas inlet 85. The processing gas may be nitrogen gas alone, or it may be a mixture of nitrogen gas and an inert gas such as argon gas. Then, a high-frequency voltage is applied between the magnetron electrode 82 and the substrate holder 83 by the high-frequency power supply 84. As a result, a plasma is formed in which the molecules of the processing gas are ionized, and the ionized particles move due to the electric and magnetic fields formed near the target T and collide with the target T. As a result, a part of the target T is knocked away and sputtered particles are generated, and these sputtered particles are incident on the surface of the substrate 90. The sputtered particles containing Al, Si or Ge, and Sc that are incident on the surface of the substrate 90, and the nitrogen plasma in which nitrogen atoms are plasma-generated in the processing chamber 81, cause Al 1-x-y Si x Sc y N or Al 1-x-y Ge x Sc y A piezoelectric film 10 having the composition of N is formed.

[0029] Next, we will describe an example of a piezoelectric film 10 that has actually been fabricated. In this example, Al 0.837 Si 0.003 Sc 0.16 N(x=0.003, y=0.16) and Al 0.875 Si 0.005 Sc 0.11 Samples of piezoelectric film 10 were prepared for two different compositions, N(x=0.005, y=0.11). Hereafter, the former sample will be referred to as the "Si0.3% sample" and the latter as the "Si0.5% sample," based on their Si content.

[0030] The fabrication conditions are as follows. Except where conditions differ for each sample, the Si 0.3% and Si 0.5% samples were fabricated under the same conditions. The substrate 90 was prepared by epitaxially growing a platinum metal layer 92 to a thickness of 40 nm on the surface of the sapphire single crystal plate 91 described above. The processing gas was a mixture of nitrogen and argon gases in a partial pressure ratio of 1:4, introduced into the processing chamber 81 at a pressure of 0.5 Pa. The distance between the surface of the target T and the surface of the substrate 90 was 20 mm. The power supplied from the high-frequency power supply 84 was 100 W. During film deposition, the substrate 90 was heated to 110°C for the Si 0.3% sample and to 300°C for the Si 0.5% sample using a heater and thermocouple built into the substrate holder 83. Thus, the higher the Si content, the higher the heating temperature of the substrate 90 should be. The film deposition time was 35 minutes for the Si 0.3% sample and 30 minutes for the Si 0.5% sample. The thickness of the fabricated piezoelectric film 10 was 2.1 μm for the Si 0.3% sample and 1.5 μm for the Si 0.5% sample.

[0031] For each prepared sample, a 2θ-ω scan measurement was performed using an X-ray diffractometer. The results are shown in Figure 4. In both the Si0.3% and Si0.5% samples, a peak was observed around 2θ=36°, which is approximately the same position as the X-ray diffraction peak at the (0002) plane of AlN. This indicates that this peak is Al 1-x-y Si x Sc y The X-ray diffraction peak at the (0002) plane of N, where Al 1-x-y Si x Sc y This means that N has a crystal structure similar to AlN. The peaks around 2θ = 40° and 42° are due to the platinum metal layer 92 and the sapphire single crystal plate 91 of the substrate 90, respectively.

[0032] Also, 1-x-y Si x Sc yAn ω-scan measurement was performed by fixing 2θ at the location where an X-ray diffraction peak was obtained on the (0002) plane of N, and then scanning only ω (by fixing the position of the X-ray detector in the X-ray diffractometer and rotating only the sample stage in the direction of the elevation angle of the piezoelectric film 10). The results are shown in Figure 5. A peak was observed around ω=18° for both the Si0.3% sample and the Si0.5% sample. The presence of a peak in the ω scan indicates that the sample is a single crystal or an oriented polycrystalline material. In other words, this embodiment yields a single crystal or an oriented polycrystalline material in which the (0002) plane is parallel to the surface of the substrate 90, or in other words, the c-axis is perpendicular to the surface of the substrate 90. The full width at half maximum (FWHM) of the peak was 2.6° for the Si0.3% sample and 2.4° for the Si0.5% sample, both indicating that a highly oriented single crystal or oriented polycrystalline material was obtained.

[0033] Furthermore, a φ scan (in which the piezoelectric film 10 is rotated in the azimuthal direction while 2θ and ω are fixed) was performed. The results are shown in Figure 6. In both the Si0.3% and Si0.5% samples, six peaks appear at equal intervals while the azimuthal angle φ is rotated 360°. This indicates that the obtained samples are single crystals.

[0034] Next, an experiment was conducted to determine the polarity of the polarization. In this experiment, a DC bias voltage was applied in the thickness direction of the piezoelectric film 10, and the anti-resonance frequency f was determined using a network analyzer while varying the DC bias voltage. p The following was measured. Generally, in two piezoelectric films with different polarization directions in the thickness direction, changing the DC bias voltage in the thickness direction results in an anti-resonant frequency f p They shift in opposite directions. That is, one has an anti-resonant frequency f. p The frequency increases, while the anti-resonant frequency f increases. p Therefore, this measurement was performed on both the Si 0.3% sample and the Si 0.5% sample, and for comparison, Al, which is known to have Al polarization, was also measured. 0.6 Sc 0.4 Similar measurements were performed for N (which does not contain Si). The measurement results are shown in Figure 7. The anti-resonant frequency f increases with increasing DC bias voltage.p In the comparative example, Al 0.6 Sc 0.4 While the N-polarization shifts in an increasing direction, both the Si0.3% and Si0.5% samples in this embodiment shift in a decreasing direction. From these results, it was confirmed that both the Si0.3% and Si0.5% samples have N-polarization that is the opposite of that of the comparative example.

[0035] Next, for each sample prepared, the electromechanical coupling coefficient k in the thickness direction is determined. t Measurements were performed to determine the conversion loss for each frequency using a network analyzer, and Mason's equivalent circuit model was used to best match the obtained measurements. t The following was determined. Figure 8 shows graphs overlaid with the measured conversion loss and the theoretical conversion loss calculated using Mason's equivalent circuit model for both the Si0.3% and Si0.5% samples. The measured and theoretical conversion losses for both samples show good agreement. t The value is the square of k. t 2 It is obtained as follows, and in the Si 0.3% sample, k t 2 =4.6%, in the Si 0.5% sample, k t 2 = 5.3%. On the other hand, according to Non-Patent Document 1, Al has N polarization. 1-x Si x k of N(x=0.024-0.13, no Sc) piezoelectric film t 2 The maximum is 3.2%, and also, AlN(Al) has Al polarization as described in the same document. 1-x Si x At N, x=0) the k of the piezoelectric film t 2 The percentage is 4.2%.

[0036] Thus, according to this embodiment, and the conventional Al having N polarization as described in Non-Patent Document 1, 1-x Si x The electromechanical coupling coefficient in the thickness direction is greater than that of an N piezoelectric film. tIt was confirmed that a piezoelectric film 10 with a large N polarization was obtained. Furthermore, the piezoelectric film 10 of this embodiment is Al 1-x Si x Not only N piezoelectric films, but also AlN piezoelectric films with Al polarization that do not contain Si are k t It has a large size and excellent piezoelectric properties.

[0037] (2) Al 1-x-y Si x Sc y BAW filter using N piezoelectric film Al of this embodiment 1-x-y Si x Sc y N piezoelectric films can be suitably used in higher-mode BAW filters. Hereinafter, such a higher-mode BAW filter will be described as a BAW filter according to one embodiment of the present invention.

[0038] As shown in Figure 9, the BAW filter 20 of this embodiment has Al having the above-mentioned N polarization on the surface of the metal layer 92 of the substrate 90. 1-x-y Si x Sc y A piezoelectric film 10 made of N is formed, and on the surface of the piezoelectric film 10 (opposite side from the metal layer 92 side), Al having Al polarization is formed. 1-Z Sc Z A first piezoelectric film 211 consisting of N (0 ≤ z < 1) is formed. In the BAW filter 20, Al has N polarization. 1-x-y Si x Sc y The piezoelectric film 10 made of N is called the second piezoelectric film 212, and the metal layer 92 is called the first electrode 221. Furthermore, the piezoelectric film laminate 21 is formed by repeatedly stacking the first piezoelectric film 211 and the second piezoelectric film 212 alternately. In addition, a second electrode 222 made of gold is provided on the surface of the piezoelectric film laminate 21 (the side opposite to the metal layer 92). Therefore, the first electrode 221 and the second electrode 222 are provided so as to sandwich the piezoelectric film laminate 21.

[0039] According to the BAW filter 20 of this embodiment, the electromechanical coupling coefficient k is lower than that of a conventional piezoelectric film having N polarization. t Large Al 1-x-y Six Sc y By using the N-based piezoelectric film 10 as the second piezoelectric film 212, the filtering characteristics can be improved.

[0040] In Figure 9, four layers each of the first piezoelectric film 211 and the second piezoelectric film 212 are provided, but the number of these layers is not limited to this example and can be arbitrary. The materials of the first electrode 221 and the second electrode 222 are not limited to the above example (platinum for the former and gold for the latter), and the first electrode 221 can use the materials of the various metal layers 92 described in the description of the piezoelectric film 10 of this embodiment, and the second electrode 222 can use any conductive material (it may be a metal or a non-metallic material). The material of the single crystal plate 91 can also be the various materials described in the description of the piezoelectric film 10 of this embodiment, and instead of a single crystal, a substrate made of polycrystalline material with the c axis oriented perpendicular to the substrate may be used.

[0041] In the above example, the second piezoelectric film 212 was provided on the surface of the substrate 90, and then the first piezoelectric film 211 and the second piezoelectric film 212 were alternately laminated. However, the first piezoelectric film 211 may be provided on the surface of the substrate 90, and then the second piezoelectric film 212 and the first piezoelectric film 211 may be alternately laminated.

[0042] Although embodiments and several modifications of the piezoelectric film and BAW filter according to the present invention have been described above, the present invention is not limited thereto, and various modifications are possible. [Explanation of Symbols]

[0043] 10… Piezoelectric film 20...BAW filter 21… Piezoelectric film laminate 211...First piezoelectric film 212...Second piezoelectric film 221...1st electrode 222…Second electrode 80…Magnetron sputtering apparatus 81… Processing Room 82…Magnetron electrodes 83... Circuit board holder 84…High frequency power supply 85...Gas inlet 86...Gas outlet 90... Circuit board 91... Single crystal plate 92...Metal layer

Claims

1. In AlN, 0.05% or more but less than 2.4% of the Al atoms are substituted with Si atoms, or 0.05% or more but less than 3.5% are substituted with Ge atoms, and furthermore, 0.5% or more but less than 45% of the Al atoms are substituted with Sc. 1-x-y Si x Sc y N (0.0005 ≤ x < 0.024, 0.005 ≤ y ≤ 0.45) or Al 1-x-y Ge x Sc y A piezoelectric film is a thin film composed of N (0.0005 ≤ x < 0.035, 0.005 ≤ y ≤ 0.45).

2. Al 1-Z Sc Z A first piezoelectric film composed of AlN (0 ≤ z < 1), where 0.05% or more but less than 2.4% of the Al atoms in AlN are substituted by Si atoms or 0.05% or more but less than 3.5% are substituted by Ge atoms, and further 0.5% or more but 45% or less of the Al atoms are substituted by Sc (scandium), respectively 1-x-y Si x Sc y AlN (0.0005 ≤ x < 0.024, 0.005 ≤ y ≤ 0.45) or Al 1-x-y Ge x Sc y A piezoelectric film laminate in which a second piezoelectric film composed of AlN (0.0005 ≤ x < 0.035, 0.005 ≤ y ≤ 0.45) is alternately laminated A pair of electrodes are provided so as to sandwich the piezoelectric film laminate in the stacking direction. A bulk acoustic wave filter, which is a BAW filter equipped with [a specific feature].

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