Piezoelectric element and method for manufacturing same

Abstract

The present invention proposes: a piezoelectric element with which a compressive stress as a residual stress of a piezoelectric film can be increased; and a method for manufacturing the same. A piezoelectric element according to the present invention comprises a substrate 10, and a lower electrode layer 21, a piezoelectric film 20, and an upper electrode layer 22 which are sequentially stacked on the substrate 10. The piezoelectric film 20 is a metal oxide piezoelectric body (lead zirconate titanate (PZT)) having a perovskite structure, wherein the atomic ratio of each of metal ions (divalent Pb ions, Ti ions, and Zr ions) of the A-site and metal ions (tetravalent Pb ions, Ti ions, and Zr ions) of the B-site of the perovskite structure periodically changes in the film thickness direction (refer to Fig. 2).

Description

Piezoelectric Element and Method for Manufacturing the Same

[0001] The present invention relates to a piezoelectric element and a method for manufacturing the same.

[0002] The present invention relates to a manufacturing method for imparting residual compressive stress to a piezoelectric thin film in order to enhance the piezoelectric constant (an index of the displacement amount of a piezoelectric material with respect to an applied electric field) of the piezoelectric material. It is known that for a piezoelectric film having a perovskite structure typified by PZT, when the in-plane residual stress of the piezoelectric film at room temperature is compressive, the perovskite structure extends in the out-of-plane direction, that is, the spontaneous polarization increases by c-axis orientation, and the piezoelectric constant increases. Conversely, when the in-plane residual stress of the film is tensile, the perovskite structure shrinks in the out-of-plane direction, that is, the spontaneous polarization (residual polarization) becomes small by a-axis orientation, and the piezoelectric constant decreases.

[0003] In Patent Document 1 and Non-Patent Document 1, when PZT is formed into a film on a substrate (such as sapphire or a metal such as stainless steel) having a larger coefficient of thermal expansion (CTE) than that of the piezoelectric thin film, for example, PZT, at a high temperature and cooled to room temperature, the shrinkage amount of the PZT thin film becomes smaller than the shrinkage amount of the substrate, so it is described that PZT has compressive stress.

[0004] According to Non-Patent Document 1, when PZT is formed on each of a sapphire substrate having a CTE larger than that of PZT and a glass substrate having a CTE smaller than that of PZT, the PZT on the sapphire substrate that becomes compressive stress has a larger c-axis orientation ratio, and as a result, the spontaneous polarization having a positive correlation with the piezoelectric constant becomes larger.

[0005] A single-crystalline silicon substrate used in semiconductor manufacturing is optimally used as a substrate for MEMS applications because it is low-cost, easily available, has excellent mechanical properties, high heat resistance, and a three-dimensional processing process is established.

[0006] However, the CTE of a silicon single crystal is 2.6 [ppm / K], which is smaller than the CTE of PZT (7.0 [ppm / K]). Therefore, the amount of shrinkage of PZT during cooling after deposition at high temperature is greater than the amount of shrinkage of the silicon substrate, resulting in tensile stress on the PZT film and preventing the development of a large piezoelectric constant. In other words, the method of applying compressive stress to PZT by using a substrate with a larger CTE than the piezoelectric film, as described in Patent Document 1 and Non-Patent Document 1, cannot be applied to a silicon single crystal substrate.

[0007] As a means of applying compressive stress to a piezoelectric thin film formed on a silicon single crystal, a method has been proposed in which an intermediate layer (buffer layer) with a lattice constant smaller than that of the piezoelectric film is inserted between the silicon substrate and the piezoelectric film, and the piezoelectric film is epitaxially grown. In this method, the misfit is absorbed by elastic strain, and the piezoelectric film is constrained by the buffer layer and grows out of plane, i.e., it undergoes c-axis orientation and an increase in the piezoelectric constant (see, for example, Patent Document 2).

[0008] However, as the piezoelectric film becomes thicker and the distance from the buffer layer increases, the lattice strain due to misfit is relieved by the transition, and the compressive stress decreases. For this reason, the piezoelectric film thickness is limited to 2 to 100 nm, and it cannot be applied to piezoelectric films of micron size required for actuator applications such as MEMS mirrors.

[0009] As a means of forming thick compressive stress PZT on a silicon substrate, a PZT film has been proposed in which layers of 4 nm thick film are stacked using a multi-target sputtering apparatus, with the lead content of the A-site of the perovskite structure being constant, and the ratios of Ti and Zr in the B-site varying (see, for example, Non-Patent Document 2). Specifically, this means uses a 4 nm thick PZT film with a Zr atomic ratio (= Zr / (Zr+Ti)) of 0.65 as a reference, and repeatedly stacks 4 nm PZT films with varying Zr atomic ratios in the range of 0.3 to 0.9 to form a superlattice structure.

[0010] Since the lattice constant of PZT increases with the Zr ratio, if the lattice constant of PZT-X is greater than that of PZT-65, PZT-X will elongate (tensile stress), and if it is less, PZT-X will contract (compressive stress) (see Figure 16). Layers of PZT with a Zr composition lower than 58% will have compressive stress, and the piezoelectric constant will increase. The Ti and Zr compositions fluctuate periodically, while the lead and oxygen compositions remain constant.

[0011] International Publication No. WO2009 / 157189A1, Japanese Patent Publication No. 2007-099618

[0012] J. Euro. Ceram. Soc. 41 (2021) 6991 Appl. Phys. Lett. 122, 122902 (2023) J. Appl. Phys. 97,074101, (2005)

[0013] However, as can be seen from Figure 16, the total stress of the superlattice structure film, which is the sum of PZT-65 and PZT-X, is almost zero because the compressive and tensile stresses cancel each other out. Therefore, it is not possible to obtain a piezoelectric film with a large compressive stress as a whole.

[0014] Therefore, the present invention proposes a piezoelectric element and a method for manufacturing the same that can increase the compressive stress as residual stress in a piezoelectric film.

[0015] The piezoelectric element of the present invention comprises a substrate and a lower electrode layer, a piezoelectric film, and an upper electrode layer sequentially laminated on the substrate, wherein the piezoelectric film is a metal oxide piezoelectric material having a perovskite structure, and the atomic ratios of A-site metal ions and B-site metal ions in the perovskite structure change periodically and continuously in the direction of film thickness.

[0016] The present invention relates to a method for manufacturing a piezoelectric element, comprising the steps of forming a piezoelectric film by evaporating the metal constituting the metal ions of site A and the metal constituting the metal ions of site B from each of a plurality of evaporation sources located at the bottom of the vacuum chamber, while a wafer rotation holder, which holds a wafer constituting the substrate and is positioned at the top of the internal space of a vacuum chamber, is revolved around an orbital axis, and the wafer is rotated around a rotation axis, thereby depositing these metals onto the wafer. The method is characterized in that, for at least one evaporation source, the distance R1 between the orbital axis and the evaporation source is set to be greater than the distance R2 between the orbital axis and the rotation axis. For this reason, the distance and elevation angle between the wafer and each evaporation source fluctuate periodically due to the orbit. As a result, the composition of all metal ions deposited on the wafer changes periodically and continuously.

[0017] This is an explanatory diagram of the configuration of a piezoelectric element as one embodiment of the present invention. This is an explanatory diagram regarding the perovskite structure. This is an explanatory diagram of the configuration of an ADRIP apparatus. This is an explanatory diagram regarding the relative arrangement of the wafer and evaporation source in an ADRIP apparatus. This is an explanatory diagram regarding the relative arrangement of the wafer and evaporation source in an ADRIP apparatus. This is a diagram showing a cross-sectional TEM image of the piezoelectric film according to Example 1. This is a diagram showing the changes in the film thickness direction of the atomic ratio composition ratios of Pb (solid line), Zr (dotted line), and Ti (double-dotted line) of the piezoelectric film 20 constituting the piezoelectric element of Example 1, based on the TEM-EDX (energy-dispersive X-ray spectroscopy) spectrum. This is a diagram showing the changes in the film thickness direction of the variation in the composition ratio when the average composition ratio of Pb is set to 100 (solid line) from the EDX spectrum of the piezoelectric film 20 in Example 1, and the fitting of the variation amount by a sine wave (dotted line). This is a diagram showing the EDX spectrum of the piezoelectric film 20 in Example 2. This is a diagram showing the amplitude when the lead composition variation in the film thickness direction of the piezoelectric film 20 in Example 2 is approximated (fitted) by a sine wave. This figure shows the EDX spectrum of the piezoelectric film 20 in Comparative Example 1. This figure shows the amplitude when the lead composition variation in the film thickness direction of the piezoelectric film 20 in Comparative Example 1 is approximated (fitted) with a sine wave. This figure shows the EDX spectrum of the piezoelectric film 20 in Comparative Example 2. This figure shows the amplitude when the lead composition variation in the film thickness direction of the piezoelectric film 20 in Comparative Example 2 is approximated (fitted) with a sine wave. This figure shows the EDX spectrum of the piezoelectric film 20 in Comparative Example 3. This figure shows the amplitude due to the lead composition variation in the film thickness direction of the piezoelectric film 20 in Comparative Example 3. This figure explains a process to suppress the influence of noise in the cross-sectional lead composition analysis by the EDX method and to clarify the definition of the upper and lower limits of the lead composition variation. This figure shows the X-ray diffraction (XRD) θ-2θ spectrum of the piezoelectric film 20 in Example 1. This figure shows the X-ray diffraction (XRD) θ-2θ spectrum of the piezoelectric film 20 in Example 2. This figure shows the X-ray diffraction (XRD) of the piezoelectric film 20 in Comparative Example 2. This is an explanatory diagram regarding the stress of each PZT layer according to the Zr ratio in the prior art.

[0018] (Configuration of the piezoelectric element) The piezoelectric element shown in Figure 1, as one embodiment of the present invention, comprises a substrate 10, and a lower electrode layer 21, a piezoelectric film 20, and an upper electrode layer 22 stacked on the substrate 10 in order from bottom to top. An insulating layer 12 is formed between the substrate 10 and the lower electrode layer 21. An adhesion layer 211 is formed between the insulating layer 12 and the lower electrode layer 21. A conductive buffer layer 212 is formed between the lower electrode layer 21 and the piezoelectric film 20.

[0019] The substrate 10 is made of, for example, a single-crystal silicon substrate. Alternatively, the substrate 10 may be made of a silicon-on-insulator (SOI) wafer consisting of a single-crystal silicon active layer (e.g., 40 μm thick), an embedded silicon oxide layer (e.g., 1 μm thick), and a support layer (e.g., 350 μm thick). The insulating layer 12 is made of, for example, silicon oxide (SiO2). The insulating layer 12 may be made of silicon nitride (SiNx). The insulating layer 12 may be omitted. The lower electrode layer 21 and the upper electrode layer 22 are each made of, for example, Pt. The lower electrode layer 21 is made of Ir, IrO ２ The upper electrode layer 22 may be composed of Ir, Ru, Au, and Ti. The adhesion layer 211 may be, for example, TiO x It is composed of Ti, ZrO x , Ta, TaO x It may be composed of the following. The adhesion layer 211 may be omitted. The conductive buffer layer 212 is composed of, for example, SrRuO3. The conductive buffer layer 211 may be composed of LaNiO3. The conductive buffer layer 211 may be omitted.

[0020] The piezoelectric film 20 is composed of a metal oxide piezoelectric material having a perovskite-type crystal structure as shown in Figure 2. Large ionic ions, such as divalent ions, are located at the corner positions (A-site) of the unit cell. Small ionic ions, such as tetravalent ions, are located near the center of the unit cell (B-site). Oxygen ions are located at the face-center positions of the unit cell. In the perovskite structure, polarization occurs and ferroelectricity is exhibited because the ionic positions at B-site are slightly offset from the lattice center.

[0021] The atomic ratios of Pb, Zr, and Ti, which are the metal ions of A-site and B-site in the perovskite structure constituting PZT, change periodically in the direction of the film thickness of the piezoelectric film 20. When the piezoelectric film 20 is composed of lead zirconate titanate (PZT) having a perovskite structure, the atomic ratios of lead ions, zircon ions, and titanium ions change periodically in the direction of the film thickness.

[0022] In a typical PZT, the A-site mainly contains stable divalent lead ions, while the B-site mainly contains stable tetravalent zircon or titanium ions. As described above, the ratio of lead in the piezoelectric film 20 changes periodically in the direction of film thickness, so layers with an excess of lead (lead-rich layers) and layers with a deficiency of lead (lead-deficient layers) periodically appear relative to the capacity of the A-site. Because divalent lead ions have an extremely large ionic radius, excess lead ions cannot penetrate the gaps between the atoms constituting the perovskite structure and do not become interstitial atoms. On the other hand, when lead ions leave the A-site in the lead-deficient layer, creating lattice vacancies, the lattice dimensions shrink, but no change in the lattice constant can be confirmed from X-ray diffraction. Therefore, it can be explained that in the lead-rich layer, the excess lead becomes tetravalent ions and is contained in the B-site, while in the lead-deficient layer, titanium or zircon becomes divalent ions and is contained within the A-site. In fact, Non-Patent Document 3 reports that when PZT thin films are formed by sputtering, divalent titanium or zircon ions are incorporated into a portion of A-site, and tetravalent lead ions are incorporated into a portion of B-site.

[0023] However, hereafter, "metal ions in A-site" refers to the metal ions of the main metals that occupy A-site with the correct proportions of each atom. In PZT, the metal ions in A-site are Pb ions. Similarly, "metal ions in B-site" refers to the metal ions of the main metals that occupy B-site with the correct proportions of each atom. In PZT, the metal ions in B-site are Zr and Ti ions.

[0024] The period of change in the atomic ratios of Pb, Zr, and Ti is preferably within the range of 1 to 20 nm. If the period is less than 1 nm, a superlattice will not form in the piezoelectric film 20 due to the interdiffusion of metal ions. If the period exceeds 20 nm, the quality of the piezoelectric film 20 will deteriorate due to lead (Pb) deficiencies.

[0025] It is preferable that the periodically changing atomic ratio of Pb metal ions periodically changes in the film thickness direction within a range of 98% or less and 102% or more, based on the average atomic ratio of Pb metal ions in the piezoelectric film 20. If the range of variation in Pb metal ions is narrower than this range, compressive stress cannot be obtained, and instead tensile stress is obtained, making it impossible to improve the piezoelectric constant. On the other hand, if the periodically changing atomic ratio of Pb metal ions is set to a lower limit of 92% or less and an upper limit of 108% or more, a non-piezoelectric crystalline phase other than the perovskite structure is generated due to lead deficiency or lead excess, and the piezoelectric constant decreases. In addition, the long-term reliability of the device is lost due to Pb ion deficiency. To summarize the appropriate range for the periodic lead composition, the lower limit is 98% or less and the upper limit is 102% or more, and more preferably the lower limit is 92% to 96% and the upper limit is 104% to 108%.

[0026] (Manufacturing method) First, in the thermal oxidation process, the single-crystal silicon substrate constituting the substrate 10 is thermally oxidized to form an insulating layer 12 made of silicon oxide (for example, with a thickness of 1 μm).

[0027] Next, in the sputtering process, a magnetron sputtering apparatus is used to create an adhesion layer 211 (for example, TiO xA base layer (10 nm thick), a lower electrode layer 21 (e.g., Pt, 150 nm thick), and a conductive buffer layer 212 (e.g., SrRuO3, 20 nm thick) are formed in sequence.

[0028] Next, the piezoelectric film 20 is formed by arc discharge reactive ion plating (ADRIP process), in which the raw material metal is heated and evaporated in a high-density oxygen plasma generated in a vacuum chamber by a plasma gun, and a ferroelectric oxide is formed by the reaction of each metal vapor with oxygen in the vacuum chamber or on the substrate. The arc discharge reactive ion plating apparatus (ADRIP apparatus) shown in Figure 3 is used for the formation of the piezoelectric film 20 (see Japanese Patent Publication No. 6757544). As shown in Figure 3, a Pb evaporation source 402-1, a Zr evaporation source 402-2, and a Ti evaporation source 402-3 are provided in the lower part of the internal space of the vacuum chamber 401 for independently evaporating Pb, Zr, and Ti, respectively. Above the Pb evaporation source 402-1, the Zr evaporation source 402-2, and the Ti evaporation source 402-3, vapor amount sensors 402-1S, 402-2S, and 402-3S are provided, respectively. A wafer rotation holder 403 with a heater is provided at the top of the internal space of the vacuum chamber 401 for placing the wafer 403a.

[0029] Furthermore, upstream of the vacuum chamber 401, there is a pressure gradient type plasma gun 404 for introducing inert gas (for example, 10 sccm of Ar gas and 100 sccm of He gas) to maintain the arc discharge, and an O2 gas inlet 405 for introducing oxygen (O2) gas, which is the oxygen raw material for the PZT layer. On the other hand, downstream of the vacuum chamber 401, there is an exhaust port 406 connected to a vacuum pump (not shown).

[0030] In the arc discharge plasma 407 shown in Figure 3, O2 gas (e.g., 200 sccm) is excited to generate oxygen radicals, and the Pb evaporation source 402-1, Zr evaporation source 402-2, and Ti evaporation source 402-3 are simultaneously evaporated and reacted with the oxygen radicals. At this time, the temperature of the wafer 403a is controlled to, for example, 600°C. Next, when the Pb evaporation amount, Zr evaporation amount, and Ti evaporation amount, measured by the vapor amount sensors 402-1S, 402-2S, and 402-3S respectively, stabilize, the substrate shutter (not shown) is switched from a closed state to an open state, and the deposition of the perovskite phase PZT film is started. After a predetermined time, the substrate shutter is switched from an open state to a closed state, and then the evaporation amount is turned off. In this way, a piezoelectric film 20 (e.g., 5 μm thick) composed of PZT is formed.

[0031] The relative arrangement of the wafer 403a and each of the evaporation sources 402-1 to 3 is adjusted such that the ratio (R1-R2) / TS of the deviation (R1-R2) between the horizontal distance R1 between the orbital axis R1 of the wafer rotating holder 403 and the rotation axis R2 of the wafer 403a, to the vertical distance TS between the evaporation plane and the wafer 403a, satisfies the condition defined by the relation 0 < (R1-R2) / TS ≤ 0.35. Preferably, the ratio (R1-R2) / TS satisfies the condition defined by the relation 0.15 < (R1-R2) / TS ≤ 0.25.

[0032] Finally, in the sputtering process, the upper electrode layer 22 (for example, 150 nm thick) is formed using a magnetron sputtering apparatus. As a result, the piezoelectric element shown in Figure 1 is fabricated.

[0033] (Example 1) As shown in Figure 4, the wafer rotating holder 403 can simultaneously hold, for example, four 6-inch silicon wafers 403a. To ensure uniformity of the film thickness, each wafer 403a rotates on its own axis of rotation around its center, and the wafer rotating holder 403 revolves around its own axis. The distance R2 between the revolving axis of the wafer rotating holder 403 and the rotation axis of the wafer 403a is set to 150 mm (see Figure 4). The distance TS between the wafer rotating holder 403 and the virtual plane (evaporation plane) in contact with the respective evaporation surfaces of the Pb evaporation source 402-1, Zr evaporation source 402-2, and Ti evaporation source 402-3 is set to 600 mm (see Figure 5). The horizontal distance R1 between the centers of the Pb evaporation sources 402-1, Zr evaporation source 402-2, and Ti evaporation source 402-3, which are arranged at equal intervals of 120° in the circumferential direction, and the orbital axis of the wafer rotation holder 403 was set to 180 mm (see Figures 4 and 5).

[0034] When the deposition rate of the piezoelectric film 20, composed of PZT, is 32 nm / min, the orbital speed of the wafer rotating holder 403 was controlled to 10 rpm so that the superlattice period becomes 3.2 nm. As a result, each wafer 403a passes over the Pb evaporation source 402-1, the Zr evaporation source 402-2, and the Ti evaporation source 402-3 in sequence every 6 seconds.

[0035] As a result, the piezoelectric element of Example 1 was fabricated. It was confirmed that the atomic ratios of Pb, Zr, and Ti in the piezoelectric film 20 constituting the piezoelectric element of Example 1 changed with a period of approximately 3.2 nm in the film thickness direction. Figure 6 shows a cross-sectional image of the piezoelectric film 20 taken with a transmission electron microscope (TEM). From Figure 6, the existence of a so-called superlattice pattern due to the periodic change in composition can be seen.

[0036] Figure 7A shows the changes in the film thickness direction of the atomic ratio composition ratios of Pb (solid line), Zr (dotted line), and Ti (double-dotted line) of the piezoelectric film 20 constituting the piezoelectric element of Example 1, as determined by TEM-EDX (energy-dispersive X-ray spectroscopy). Here, the composition ratio is the value obtained by dividing the atomic ratio of each component by the sum of the atomic ratios of Zr and Ti, and is obtained by normalizing with the atomic ratios of Pb, Zr, Ti, and oxygen of the PZT film, which are determined by X-ray fluorescence analysis (XRF) with correct absolute values ​​of composition. From the EDX spectrum shown in Figure 7A, it can be confirmed that the atomic ratios (composition ratios) of Zr and Ti change periodically in the film thickness direction in a complementary manner, and at the same time, the atomic ratio of Pb also changes periodically. The periods of Ti and Zr change with a difference of approximately 1 / 2 period. The peaks and troughs of the periods of Ti and Pb roughly coincide.

[0037] Figure 7B shows the variation in the amount of variation in composition ratio in the film thickness direction (solid line) and the sinusoidal fitting of the amount of variation (dotted line) when the average composition ratio of Pb is set to 100, based on the EDX spectrum of the piezoelectric film 20 in Example 1. From Figure 7B, it was found that the amplitude of the amount of variation approximated by a sinusoidal wave, i.e., the variation range of Pb (composition ratio 1.002), is ±3.5%. Similarly, when the variation ranges of Zr (composition ratio 0.624) and Ti (composition ratio 0.376) are approximated by sinusoidal waves (not shown), they are ±11.0% and ±15.1%, respectively. The reason why the variation range of Pb is smaller than that of Zr and Ti is that some of the Pb that reaches the substrate revolts from the substrate.

[0038] The following describes how to estimate the composition variation range (sine wave amplitude A (%)). When the average composition ratio that varies sinusoidally with respect to the film thickness depth X is set to 100 (%), the composition ratio Y is expressed by the following equation (1), where T is the superlattice period and Φ is the phase: Y = A sin(2πX / T + Φ) + 100 ... (1)

[0039] Figure 12 illustrates a process to suppress the influence of noise in cross-sectional lead composition analysis using the EDX method and to clarify the definition of the upper and lower limits of lead composition variation. As shown in Figure 12, first, drift correction is performed on the cross-sectional EDX spectral data ((i) raw data) using Python or similar software so that the average of the composition variation range becomes 100% at predetermined periods (one or more periods) ((ii) drift correction). Then, when the drift-corrected composition variation data is subjected to a Fast Fourier Transform (FFT), the superlattice period T and phase Φ are obtained. In addition, peaks corresponding to the period can be obtained ((iii) FFT). These values ​​are substituted into equation (1) above, and the amplitude, i.e., the composition variation range (%), is calculated using the nonlinear least squares method based on the sine wave model ((iv) sine fitting: sine fitting is performed with the period range of the peak). By doing so, the amplitude of the sine wave is determined and defined as the variation range. This makes it possible to suppress the influence of noise in cross-sectional lead composition analysis using the EDX method. If the EDX spectral data has a slope, appropriate baseline correction should be performed using methods such as the least squares method. To obtain sufficient measurement accuracy, the length of the EDX spectral data in the film thickness depth direction should be at least five times the superlattice period.

[0040] (Piezoelectric Properties) Figure 13 shows the X-ray diffraction (XRD) θ-2θ spectrum of the piezoelectric film 20 in Example 1. When the ratio of peaks is calculated with the sum of the intensities of the PZT(001), (110), and (111) diffraction peaks as the denominator, a perovskite structure was obtained in which 98% or more were oriented in the PZT(001) direction, as shown in Figure 13. It is preferable that the ratio of the peak orientation in the PZT(001) direction, with the sum of the intensities of the PZT(001), (110), and (111) diffraction peaks as the denominator, be 90% or more, and more preferably 98% or more. Also, in Figure 13, PZT(110) does not appear, and the PZT(111) peak is less than 5% of the PZT(001) peak. It is preferable that both PZT(110) and PZT(111) are less than 5% of the PZT(001) peak. Note that PZT(001) and PZT(100) are equivalent, but are shown as PZT(001) in Figure 13. Furthermore, it was shown that there are no other phases such as pyrochlore phases that do not have piezoelectric properties. The presence of satellite peaks originating from the superlattice structure was confirmed on both sides of the PZT(001) and PZT(002) peaks. The full width at half maximum of the (004) peak measured by XRD rocking curve was 0.69°, which is less than 1°, confirming that the piezoelectric film 20 has a high-quality crystal structure with aligned orientations.

[0041] In a known example of a PZT superlattice structure in which two types of films having different compositions / structures reported conventionally are alternately laminated, the amount of Pb at the A-site is constant, and Zr and Ti at the B-site have a stepped periodic concentration gradient in the film thickness direction. On the other hand, the PZT superlattice structure of Example 1 has a feature in which all the composition ratios of Pb, Zr, and Ti constituting the PZT film vary continuously in a periodic and sinusoidal approximation. When the phases of the periodic variations of both are determined from the composition variation data of Zr and Ti subjected to FFT processing, it is 180°, and the atomic ratios of both change complementarily in the film thickness direction. Basically, only divalent metal ions are accommodated in the A-site of the perovskite structure. For this reason, in the piezoelectric film 20, in the Pb-deficient layer at the A-site, Zr and / or Ti are accommodated as divalent ions, and at the B-site, Pb is accommodated as tetravalent ions. Thereby, the distortion of the perovskite crystal structure due to the excess or deficiency of lead is corrected.

[0042] Generally, when a PZT film is formed on a silicon substrate at a high temperature and cooled to room temperature, residual stress becomes tensile stress because PZT shrinks more than the substrate due to the difference in the thermal expansion coefficients of silicon and PZT, and it is known that the piezoelectric constant decreases due to the tensile stress. The piezoelectric film 20 (PZT superlattice film) in Example 1 was confirmed to have a compressive stress of residual stress of -15 [MPa] due to the periodic expansion of the PZT film due to the periodic variation of Pb. After forming upper and lower electrodes by a semiconductor process from the substrate on which the PZT film was formed, a strip-shaped cantilever element having a length of 30 mm and a width of 5 mm was cut out, and when one end was fixed and current was applied to both electrodes, the displacement amount of the other end was measured with a laser Doppler meter. As a result, it was confirmed that the piezoelectric constant d31 when an electric field of 10 [V / μm] was applied was -190 [pm / V].

[0043] (Example 2) The horizontal distance R1 between the centers of each of the Pb evaporation source 402-1, the Zr evaporation source 402-2, and the Ti evaporation source 402-3 arranged at equal intervals of 120° in the circumferential direction and the revolution axis of the wafer rotation holder 403 was set to 210 mm (see FIGS. 4 and 5). Other than this, the piezoelectric element of Example 2 was manufactured by forming the piezoelectric film 20 in the ADRIP process under the same conditions as in Example 1.

[0044] FIG. 14 is a diagram showing the X-ray diffraction (XRD) θ-2θ spectrum of the piezoelectric film 20 in Example 2. As shown in FIG. 14, there are no heterogeneous phases such as a PZT pyrochlore phase having no piezoelectricity. When the ratio of the peaks is determined with the sum of the intensities of the PZT (001), (110), and (111) diffraction peaks as the denominator, it is shown that the perovskite structure is oriented at 98% or more in the (001) direction. It is preferable that the ratio of the peak orientation in the PZT (001) direction is 90% or more, and more preferably 98% or more, with the sum of the intensities of the PZT (001), (110), and (111) diffraction peaks as the denominator. Also, in FIG. 14, PZT (110) does not appear, and the peak of PZT (111) is 5% or less of the peak of PZT (001). It is preferable that both PZT (110) and PZT (111) are 5% or less of the peak of PZT (001). Note that PZT (001) and PZT (100) are equivalent, but in FIG. 14, it is described as PZT (001). Satellite peaks derived from the superlattice structure were observed on both sides of the PZT (001) and PZT (002) peaks. The full width at half maximum of the (004) peak by XRD rocking curve measurement is 0.76°, and since it is 1° or less, it was confirmed that the piezoelectric film 20 has a high-quality crystal structure with aligned orientations. Also, the piezoelectric film 20 is a single-oriented polycrystalline film.

[0045] FIG. 8A is a diagram showing the EDX spectrum of the piezoelectric film 20 in Example 2. As shown in FIG. 8A, it was confirmed that the composition ratios of all of Pb, Zr, and Ti vary periodically and continuously. Similar to Example 1, generally, the periods of Ti and Zr are shifted by about a half period.

[0046] Figure 8B shows the amplitude when the lead composition variation in the film thickness direction of the piezoelectric film 20 in Example 2 is approximated (fitted) with a sine wave. When the drift is corrected from the EDX spectrum shown in Figure 8A, it was found that the variation range of Pb (composition ratio 1.017) approximated with a sine wave shown in Figure 8B is ±5.2%. Similarly, when the variation ranges of Zr (composition ratio 0.637) and Ti (composition ratio 0.363) are approximated with sine waves (not shown), they are ±12.2% and ±17.9%, respectively. By moving the Pb evaporation source 402-1, Zr evaporation source 402-2, and Ti evaporation source 402-3 further horizontally from the wafer 403a than in Example 1, the periodic composition variation of the PZT superlattice became larger than in Example 1, and the residual stress of the PZT superlattice film became a compressive stress of -32 [MPa]. As a result, measurements using a cantilever fabricated in the same manner as in Example 1 confirmed that the piezoelectric constant d31 when an electric field of 10 [V / μm] is applied is -220 [pm / V].

[0047] The Pb evaporation source 402-1, Zr evaporation source 402-2, and Ti evaporation source 402-3 do not need to be arranged at equal intervals of 120° in the circumferential direction, nor do their distances R1 need to be the same; it is sufficient that the R1 of at least one evaporation source is set to be greater than the distance R2.

[0048] (Comparative Example 1) The horizontal distance R1 between the centers of the Pb evaporation sources 402-1, Zr evaporation source 402-2, and Ti evaporation source 402-3, which are arranged at equal intervals of 120° in the circumferential direction, and the orbital axis of the wafer rotation holder 403 was set to 120 mm (see Figures 4 and 5). Except for this, the piezoelectric element of Comparative Example 1 was manufactured by forming the piezoelectric film 20 in the ADRIP process under the same conditions as in Example 1.

[0049] Figure 9A shows the EDX spectrum of the piezoelectric film 20 in Comparative Example 1. Figure 9B shows the amplitude when the lead composition variation in the film thickness direction of the piezoelectric film 20 in Comparative Example 1 is approximated (fitted) with a sine wave. The variation range of Pb (composition ratio 1.013) approximated with a sine wave shown in Figure 9B was ±1.3%. When the variation ranges of Zr (composition ratio 0.623) and Ti (composition ratio 0.381) are similarly approximated with sine waves (not shown), it was confirmed that the atomic ratio changes periodically in the film thickness direction with variation ranges of ±8.5% and ±10.8%, respectively. The XRDθ-2θ spectrum of the piezoelectric film 20 in Comparative Example 1 shows that it has a perovskite structure with more than 98% oriented in the (001) direction, and the full width at half maximum of the (004) peak measured by XRD rocking curve was 0.62°, confirming that the homogeneity of the crystal structure is sufficiently high.

[0050] However, by moving the Pb evaporation source 402-1, Zr evaporation source 402-2, and Ti evaporation source 402-3 closer to the wafer 403a in the horizontal direction than in Example 1, the periodic compositional fluctuations of the PZT superlattice became smaller than in Example 1, and the residual stress of the PZT superlattice film became a tensile stress of +24 [MPa]. As a result of the piezoelectric film 20 having tensile stress, it was confirmed that the piezoelectric constant d31 when an electric field of 10 [V / μm] was applied was -165 [pm / V].

[0051] (Comparative Example 2) The horizontal distance R1 between the centers of the Pb evaporation source 402-1, Zr evaporation source 402-2, and Ti evaporation source 402-3, which are arranged at equal intervals of 120° in the circumferential direction, and the orbital axis of the wafer rotation holder 403 was set to 300 mm (see Figures 4 and 5). Except for this, the piezoelectric element of Comparative Example 2 was fabricated by forming the piezoelectric film 20 in the ADRIP process under the same conditions as in Example 1.

[0052] Figure 15 shows the X-ray diffraction (XRD) of the piezoelectric film 20 in Comparative Example 2. As shown in Figure 15, the XRD θ-2θ spectrum of Comparative Example 2 also shows a diffraction peak in the PZT(110) direction, indicating a polycrystalline structure with microcrystals of various directions, and a pyrochlore crystal phase without ferroelectricity also appears near 2θ = 34.5°. On the other hand, in Figures 13 and 14, which show the X-ray analysis (XRD) of Examples 1 and 2, no pyrochlore phase is observed near PZT(110) or 2θ = 34.5°. The ratio of the PZT(001) peak, with the sum of the intensities of the PZT(001), (110), and (111) diffraction peaks as the denominator, was 62%, suggesting disorder in the crystal structure. Furthermore, the full width at half maximum of the (004) peak measured by XRD rocking curve was 1.61°, confirming disorder in the crystal structure.

[0053] Figure 10A shows the EDX spectrum of the piezoelectric film 20 in Comparative Example 2. Figure 10B shows the amplitude when the lead composition variation in the film thickness direction of the piezoelectric film 20 in Comparative Example 2 is approximated (fitted) with a sine wave. The variation range of Pb (composition ratio 1.015) approximated with a sine wave shown in Figure 10B was ±8.8%. When the variation ranges of Zr (composition ratio 0.613) and Ti (composition ratio 0.388) are similarly approximated with sine waves (not shown), it was confirmed that the composition ratio changes periodically in the film thickness direction with variation ranges of ±15.5% and ±22.1%, respectively. On the other hand, it was confirmed that the variation range was large, suggesting disorder in the crystal structure. This is thought to be due to the low lead composition region formed because the amount of divalent Zr or Ti ions that can be stored in A-site is insufficient due to the relatively large variation period of lead.

[0054] The residual stress of the piezoelectric film 20 was a compressive stress of -22 [MPa], but due to the disorder of the piezoelectric film 20, it was confirmed that the piezoelectric constant d31 when an electric field of 10 [V / μm] was applied was -150 [pm / V]. (Comparative Example 3) One wafer 403a was placed at the center of the wafer rotating holder, and the orbital axis and rotational axis were aligned. The piezoelectric element of Comparative Example 3 was manufactured by forming the piezoelectric film 20 in the ADRIP process under the same conditions as in Example 1.

[0055] In Comparative Example 3, the full width at half maximum of the (004) peak measured by XRD rocking curve of the piezoelectric film 20 was 0.66°, confirming that high-quality crystal growth with aligned orientations was observed.

[0056] Figure 11A shows the EDX spectrum of the piezoelectric film 20 in Comparative Example 3. Figure 11B shows the amplitude due to lead composition variation in the film thickness direction of the piezoelectric film 20 in Comparative Example 3. According to Figures 11A and B, since the distance between the deposition source and the substrate does not change even when the substrate rotates, it was confirmed that there is no periodic compositional change in the film thickness direction for all of Pb (composition ratio 1.001), Zr (composition ratio 0.608), and Ti (composition ratio 0.392). Furthermore, it was confirmed that although the θ-2θ spectrum of the piezoelectric film 20 in Comparative Example 3 shows a perovskite structure with more than 98% oriented in the (001) direction, satellite peaks originating from a superlattice structure do not appear. In Comparative Example 3, since the wafer 403a does not substantially revolve, it is presumed that there is no bias in the cross-sectional direction of the constituent elements and therefore it does not form a superlattice.

[0057] The XRDθ-2θ spectrum of the piezoelectric film 20 in Comparative Example 3 was confirmed to indicate a perovskite structure in which more than 98% of the film is oriented in the (001) direction. The residual stress of the piezoelectric film 20 in Comparative Example 5 was a tensile stress of +88 [MPa], and the piezoelectric constant d31 when an electric field of 10 [V / μm] was applied was confirmed to be -125 [pm / V].

[0058] Table 1 summarizes the measurement results for the ADRIP process of the examples and comparative examples, including the orbital speed of the wafer rotating holder 403, the superlattice period of the piezoelectric film 20, the arrangement relationship between the evaporation sources 401-1 to 3 and the wafer 403a ((R1-R2) / TS), the Pb fluctuation range, Zr fluctuation range, Ti fluctuation range, (004) full width at half maximum in XRD, residual stress, and piezoelectric constant d31.

[0059] From the results of Comparative Examples 1 and 2 in Table 1, it can be seen that a Pb variation range greater than ±1.3% and less than ±8.8% is preferable. Therefore, the appropriate range for the composition is 92% to 98% at the lower limit and 102% to 108% at the upper limit. Furthermore, referring to Example 1, it is even more preferable that the appropriate range for the composition is 92% to 96% at the lower limit and 104% to 108% at the upper limit.

[0060]

[0061] Although the examples primarily describe PZT, any composition of piezoelectric perovskite structure is acceptable. For example, it can be applied to bismuth sodium titanate, potassium sodium niobate, barium strontium titanate, etc. Doped PZT, such as niobium, lanthanum, or manganese, can also be used. Furthermore, composite oxides composed of two or more perovskite structures, such as magnesium niobate / lead titanate (PMN-PT), are also acceptable. At least one of the metal ions in the A-site and B-site constituting the perovskite structure must change periodically and continuously. It is presumed that the periodic expansion of the film due to the periodic fluctuations in the A-site and B-site metal ion compositions causes residual stress to become compressive stress. In the case of PZT, the A-site metal ion is Pb, and the B-site metal ions are Zr and Ti. All atoms may undergo periodic and continuous changes. Even when applying multi-source sputtering or pulsed laser deposition (PLD) as the film deposition method, a superlattice structure similar to that of the present invention can be formed as long as the substrate is in orbit and the arrangement of the multiple targets of the deposition method is outside the substrate's orbital position. If a lower electrode layer or a conductive buffer layer having a single-crystal structure is used as the substrate, the perovskite structure can undergo epitaxial growth to form a single-crystal film having a superlattice structure. The crystal structure constituting the superlattice structure may be any of tetragonal, rhombohedral, orthorhombic, or monoclinic.

Claims

1. A piezoelectric element comprising a substrate and a lower electrode layer, a piezoelectric film, and an upper electrode layer sequentially laminated on the substrate, wherein the piezoelectric film is a metal oxide piezoelectric material having a perovskite structure, and the atomic ratios of A-site metal ions and B-site metal ions of the perovskite structure change periodically and continuously in the direction of film thickness.

2. A piezoelectric element comprising a substrate and a lower electrode layer, a piezoelectric film, and an upper electrode layer sequentially laminated on the substrate, wherein the piezoelectric film is lead zirconate titanate (PZT) having a perovskite structure, and the atomic ratio of at least one of titanium and zircon and lead changes periodically and continuously in the film thickness direction.

3. A piezoelectric element according to claim 2, wherein the atomic ratio of titanium and zircon changes periodically and continuously in the film thickness direction.

4. A piezoelectric element according to claim 2, wherein in a lead-rich layer where the atomic ratio of lead in the piezoelectric film is relatively high, the lead ions are contained in B-site as tetravalent ions, and in a lead-deficient layer where the atomic ratio of lead in the piezoelectric film is relatively low, the titanium ions and zircon ions are contained in A-site as divalent ions.

5. A piezoelectric element according to claim 2, wherein the period of change in the atomic ratio of at least one of lead, titanium, and zircon is included in the range of 1 to 20 nm.

6. A piezoelectric element according to claim 2, wherein the periodically changing atomic ratio of lead is periodically changed in the film thickness direction within a range of 98% or less and 92% or more, and 102% or more and 108% or less, based on the atomic ratio of lead in the piezoelectric film.

7. A piezoelectric element according to claim 6, wherein the periodically changing atomic ratio of lead is periodically changed in the film thickness direction within a range of 96% or less and 92% or more, and 104% or more and 108% or less, based on the atomic ratio of lead in the piezoelectric film.

8. A piezoelectric element according to claim 7, characterized in that the XRDθ-2θ spectrum of the piezoelectric film has diffraction peak intensities of 5% or less in the PZT, (110) and (111) directions compared to the diffraction peak intensity in the PZT(001) direction.

9. A piezoelectric element according to claim 7, characterized in that the XRDθ-2θ spectrum of the piezoelectric film has a diffraction peak intensity in the PZT(001) direction of 90% or more of the sum of the intensities of the PZT(001), (110), and (111) diffraction peaks.

10. A piezoelectric element according to claim 1 or 2, wherein the substrate is a silicon single crystal substrate.

11. A piezoelectric element according to claim 1 or 2, wherein an insulating layer is formed between the substrate and the lower electrode layer.

12. A piezoelectric element according to claim 1 or 2, wherein an adhesion layer made of a metal oxide is formed between the substrate and the lower electrode layer.

13. A piezoelectric element according to claim 1 or 2, wherein a buffer layer is formed between the lower electrode layer and the piezoelectric film, and the piezoelectric film is made of polycrystalline PZT with a preferred orientation in the (001) direction.

14. A method for manufacturing a piezoelectric element according to claim 1 or 2, comprising the step of forming the piezoelectric film by evaporating the metal constituting the metal ions of A-site and the metal constituting the metal ions of B-site from each of a plurality of evaporation sources located at the bottom of the internal space of the vacuum chamber, while a wafer rotation holder, which is disposed at the top of the vacuum chamber and holds a wafer constituting the substrate, is revolved around an orbital axis, and the wafer is rotated around a rotation axis, and thereby depositing these metals onto the wafer, wherein for at least one evaporation source, the distance R1 between the orbital axis and the evaporation source is set to be greater than the distance R2 between the orbital axis and the rotation axis.

15. A method for manufacturing a piezoelectric element according to claim 14, wherein for at least one evaporation source, the ratio (R1-R2) / TS of the deviation (R1-R2) between the distance R1 between the orbital axis and the evaporation source and the distance R2 between the orbital axis and the rotation axis, to the vertical distance TS between the evaporation plane, which is a virtual horizontal plane including the evaporation surfaces of the plurality of evaporation sources, and the wafer, satisfies the condition defined by the relation 0 < (R1-R2) / TS ≤ 0.

35.

16. A method for manufacturing a piezoelectric element according to claim 15, wherein the ratio (R1-R2) / TS satisfies the condition defined by the relation 0.15 < (R1-R2) / TS ≤ 0.25.

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