Piezoelectric film, piezoelectric element, and method for manufacturing piezoelectric film

Abstract

The present invention provides: a piezoelectric film which has achieved a good balance between high withstand voltage and long-term reliability; a piezoelectric element; and a method for producing a piezoelectric film.　A piezoelectric film which is mainly composed of a perovskite oxide, while having two local maximum values in a current-voltage profile that shows the relationship between the voltage, which is acquired in a case where the piezoelectric film is sandwiched between a pair of electrode layers and a sweeping voltage from -40 V to +40 V is applied thereto at a first change rate of 10 kV / cm·sec, and the current that flows when the voltage is applied thereto.

Description

[Technical Field]

[0001] The present disclosure relates to a piezoelectric film, a piezoelectric element, and a method for manufacturing a piezoelectric film. [Background technology]

[0002] Lead zirconate titanate (Pb(Zr,Ti)O3, hereafter referred to as PZT) is known as a material with excellent piezoelectric and ferroelectric properties. Taking advantage of its ferroelectric properties, PZT is used in nonvolatile memory, such as FeRAM (Ferroelectric Random Access Memory). Furthermore, in recent years, the integration of PZT with MEMS (Micro Electro-Mechanical Systems) technology has led to the practical application of MEMS piezoelectric elements with PZT films. PZT films are used as the piezoelectric film in piezoelectric elements that have a lower electrode, a piezoelectric film, and an upper electrode on a substrate. These piezoelectric elements have been applied to a variety of devices, including inkjet heads (actuators), micromirror devices, angular velocity sensors, gyro sensors, and vibration-driven power generation devices.

[0003] Current-electric field (IE) measurement is known as one way to evaluate the electrical properties of piezoelectric films. In IE measurements, a piezoelectric film is sandwiched between a pair of electrodes, and the voltage applied between the electrodes is changed over time, and the current flowing between the electrodes when the voltage is applied is measured. The current measured in this case indicates that charge is accumulated in the electrodes, i.e., the polarization of the piezoelectric film is changing, and by integrating this with respect to time, a polarization-electric field (PE) curve is obtained.

[0004] Japanese Patent Application Laid-Open Publication No. 2019-21701 discloses a ferroelectric material whose PE curve exhibits two asymmetric hysteresis curves (double hysteresis) on either side of an electric field of 0. Materials exhibiting double hysteresis polarization-electric field characteristics have a remanent polarization Pr of 0 or a value close to 0, and therefore can be expected to exhibit a larger displacement than materials exhibiting single hysteresis with a large remanent polarization Pr when the same electric potential is applied.

[0005] Japanese Patent Application Laid-Open Publication No. 2010-16011 discloses a piezoelectric element having a barium titanate-based piezoelectric film, in which the PE curve exhibits hysteresis where the coercive field at which polarization P becomes 0 has the same sign. The presence of hysteresis where the coercive field at which polarization P becomes 0 has the same sign means that the electric field at which polarization reversal occurs is sufficiently large, making it possible to apply a large electric field. This means that the dielectric breakdown voltage (hereinafter referred to as "voltage resistance") is large and the voltage resistance is excellent. Since the applied voltage and the amount of displacement are proportional, being able to apply a larger voltage will result in a larger amount of displacement, so it is desirable for the piezoelectric film to have a high voltage resistance. Summary of the Invention [Problem to be solved by the invention]

[0006] The piezoelectric films disclosed in JP 2019-21701 A and JP 2010-16011 A have improved pressure resistance compared to conventional films, but have not been evaluated in terms of reliability, which indicates how long they can be used. According to the inventor's investigations, the conventional piezoelectric films disclosed in JP 2019-21701 A and JP 2010-16011 A were not sufficiently reliable. When applying piezoelectric films to various devices, high reliability is required in addition to sufficiently high pressure resistance.

[0007] The technology disclosed herein has been made in consideration of the above circumstances, and aims to provide a piezoelectric film, a piezoelectric element, and a method for manufacturing a piezoelectric film that combine high withstand voltage and long-term reliability. [Means for solving the problem]

[0008] Specific means for solving the above problems include the following aspects.

[0009] The piezoelectric film of the present disclosure is a piezoelectric film mainly composed of a perovskite oxide, The sample is sandwiched between a pair of electrode layers and a voltage sweep from -40 V to +40 V is applied at a first rate of change of 10 kV / cm sec. In the current-voltage profile, which shows the relationship between the voltage obtained and the current that flows when the voltage is applied, there are two maximal values.

[0010] It is preferable that the piezoelectric film of the present disclosure has only one maximum value in a current-voltage profile showing the relationship between the voltage obtained when the piezoelectric film is sandwiched between a pair of electrode layers and a voltage is applied in a sweep from -40 V to +40 V at a second change rate of 50 kV / cm·sec and the current that flows when the voltage is applied.

[0011] In the piezoelectric film of the present disclosure, when the maximum value on the lower voltage side of the two maximum values ​​in the current-voltage profile acquired at the first rate of change is P1 and the maximum value on the higher voltage side is P2, 0.3≦P2 / P1 It is preferable that the following is satisfied.

[0012] In the piezoelectric film of the present disclosure, when the maximum value on the lower voltage side of the two maximum values ​​in the current-voltage profile acquired at the first rate of change is P1 and the maximum value on the higher voltage side is P2, P2 / P1≦1 It is preferable that the following is satisfied.

[0013] In the piezoelectric film of the present disclosure, the perovskite oxide preferably contains Pb, Zr, Ti, and O.

[0014] In the piezoelectric film of the present disclosure, the perovskite oxide is a compound represented by the following general formula (1): Pb{(Zr x Ti 1-x ) 1-y M y}O3(1) 0 <x<1、0<y<0.3 Preferably, M is one or more elements selected from V, Nb, Ta, Sb, Mo and W.

[0015] The piezoelectric film of the present disclosure is preferably an oriented film oriented in the (100) plane.

[0016] The piezoelectric film of the present disclosure has a piezoelectric constant d 31 It is preferable that the magnitude of the dielectric constant is 200 pm / V or more and the breakdown voltage is 50 V / μm or more.

[0017] The piezoelectric element of the present disclosure is a piezoelectric element comprising a substrate, a lower electrode, a piezoelectric film, and an upper electrode laminated in this order on one surface of the substrate, and the piezoelectric film is the piezoelectric film of the present disclosure.

[0018] In the method for manufacturing a piezoelectric film disclosed herein, when a piezoelectric film is formed on a deposition substrate by sputtering, deposition is performed in the early stages of deposition with the deposition substrate grounded or with a positive bias voltage applied to the deposition substrate, and then deposition is performed with a negative bias voltage applied to the deposition substrate.

[0019] In the method for manufacturing a piezoelectric film of the present disclosure, the negative bias voltage is preferably −20V to −100V. [Effects of the Invention]

[0020] According to the present disclosure, a piezoelectric film, a piezoelectric element, and a method for manufacturing a piezoelectric film that combine high withstand voltage and long-term reliability can be obtained. [Brief explanation of the drawings]

[0021] [Figure 1] FIG. 2 is a cross-sectional view showing a layer structure of a piezoelectric element according to an embodiment. [Figure 2] 1 is a schematic diagram of an IE profile for a piezoelectric film according to the present disclosure. [Figure 3] FIG. 2 is a diagram showing a schematic configuration of an evaluation sample. [Figure 4] FIG. 1 is a diagram showing an XRD chart. [Figure 5] FIG. 10 is a diagram showing the IE profile of Comparative Example 2. [Figure 6] FIG. 1 is a diagram showing an IE profile of Example 1. [Figure 7]FIG. 10 is a diagram showing an IE profile of Example 2. [Figure 8] FIG. 10 is a diagram showing an IE profile of Example 3. [Figure 9] FIG. 10 is a diagram showing an IE profile of Example 4. [Figure 10] FIG. 10 is a diagram showing the IE profile of Example 5. [Figure 11] FIG. 10 is a diagram showing the IE profile of Comparative Example 3. [Figure 12] FIG. 10 shows the IE profile of Example 9. DETAILED DESCRIPTION OF THE INVENTION

[0022] Hereinafter, embodiments of the present invention will be described with reference to the drawings. In the drawings, the thicknesses and ratios of each layer are appropriately modified for ease of viewing, and do not necessarily reflect the actual thicknesses and ratios.

[0023] 1 is a cross-sectional view showing the layer structure of a piezoelectric element 1 according to one embodiment. As shown in Fig. 1, the piezoelectric element 1 includes a substrate 11 on which a lower electrode layer 12, a piezoelectric film 15, and an upper electrode layer 18 are arranged in this order.

[0024] The piezoelectric film 15 is mainly composed of a perovskite oxide represented by the general formula ABO3. The piezoelectric film 15 has a profile (hereinafter referred to as a first profile) showing the relationship between voltage and current obtained when a voltage sweep from −40 V to +40 V is applied between the lower electrode layer 12 and the upper electrode layer 18 at a first change rate of 10 kV / cm·sec, the profile having two maximum values ​​P1 and P2 (see FIG. 2). Furthermore, the piezoelectric film 15 preferably has only one maximum value P in a profile (hereinafter referred to as a second profile) showing the relationship between voltage and current obtained when a voltage sweep from −40 V to +40 V is applied between the lower electrode layer 12 and the upper electrode layer 18 at a second change rate of 50 kV / cm·sec (see FIG. 2).

[0025] FIG. 2 shows a schematic example of a current-electric field profile (IE profile) illustrating the relationship between applied voltage and current for a piezoelectric film 15 of the present disclosure. Note that the horizontal axis represents the electric field, calculated by dividing voltage by film thickness. However, the current-electric field profile behaves in the same way as a current-voltage (IV) profile. That is, if there are two peaks in the IE profile, there will also be two peaks in the IV profile; if there is only one peak in the IE profile, there will also be only one peak in the IV profile. In FIG. 2, the solid line represents the first profile obtained when the applied voltage is changed at a first rate of 10 kV / cm·sec, and the dotted line represents the second profile obtained when the applied voltage is changed at a second rate of 50 kV / cm·sec. As shown in FIG. 2, the maximum value P in the second profile occurs at approximately the same voltage as the lower maximum value P1 of the two maximum values ​​P1 and P2 in the first profile.

[0026] The inventors have found that even for piezoelectric films primarily composed of perovskite oxides, for which no difference was observed in crystal structure evaluation by X-ray diffraction (XRD) or composition evaluation by X-ray fluorescence analysis (XRF), when a voltage sweep from −40 V to +40 V was applied at the first change rate, the first profile obtained sometimes had two maxima and sometimes only one maximum (see Examples below). Furthermore, they have found that for piezoelectric films primarily composed of perovskite oxides, when the first profile had two maxima, it was possible to achieve both high voltage resistance and high reliability compared to piezoelectric films with only one maximum (see Examples below). Note that a voltage sweep rate of 10 kV / cm·sec corresponds to a frequency much lower than the frequency typically used for IV measurements. Therefore, IV measurements at such a voltage sweep rate are not commonly performed.

[0027] As described above, the mechanism by which the piezoelectric film 15 having two maximum values ​​in the first profile can achieve both high withstand voltage and high reliability is not clear, but the present inventors speculate as follows.

[0028] When the IE profile is converted into a PE profile, the maximum values ​​of the IE profile appear as inflection points in the PE profile. It is known that the number of inflection points in the PE profile corresponds to the number of phase transitions (see Patent Document No. 2010-16011). That is, the reason why the first profile has two maximum values ​​P1 and P2 is thought to be because two phase transitions occur in the piezoelectric film due to the voltage sweep.

[0029] There are two possible modes in which two phase transitions occur in a piezoelectric film: a first mode in which one phase undergoes two phase transitions in the piezoelectric film, and a second mode in which the piezoelectric film has two phases with different response voltages, and the two phases with different response voltages undergo phase transitions sequentially.

[0030] In the conventional piezoelectric film, the first profile had only one maximum (see the solid line in FIG. 5 ), and the voltage at which this maximum occurred was equivalent to the voltage at which the maximum P1, which is located on the lower voltage side of the two maximums P1 and P2 in the first profile of the present piezoelectric film 15 (see the solid line in FIG. 6 ), occurred. That is, of the two maximums P1 and P2 in the first profile, the maximum P1 located on the lower voltage side is thought to represent a phase transition caused by a phase equivalent to that contained in the conventional piezoelectric film. On the other hand, the maximum P2 located on the higher voltage side is thought to be caused by a phase different from that contained in the conventional piezoelectric film. While it is unclear whether the two phase transitions occurred in the first or second manner described above, we presume that the inclusion of the phase that generates the maximum P2 allows the present piezoelectric film 15 to achieve high withstand voltage and high reliability. The inventors believe that the phase that generates the maximum P2 is more stable than the phase that generates the maximum P1, which is why the present piezoelectric film 15 achieves high withstand voltage and high reliability.

[0031] 2, the piezoelectric film 15 preferably has two maximum values ​​P1 and P2 in the first profile and only one maximum value in the second profile. In this case, the maximum value in the second profile occurs at approximately the same voltage as the maximum value P1 on the lower voltage side in the first profile.

[0032] 2, when the piezoelectric film 15 has two maximum values ​​P1 and P2 in the first profile and only one maximum value that occurs at approximately the same voltage as the maximum value P1 in the second profile, the component that contributes to the generation of the maximum value P2 is considered to be a phase that cannot respond to the first rate of change and does not undergo a phase transition at the first rate of change. In other words, of the two maximum values ​​P1 and P2 that appear in the first profile, the maximum value P1 that occurs at a relatively low voltage is considered to be associated with a phase transition due to a phase with a relatively fast response speed (small time constant), and the maximum value P2 that occurs at a relatively high voltage is considered to be associated with a phase transition due to a phase with a relatively slow response speed (large time constant).

[0033] Generally, in perovskite oxides, the longer it takes for polarization to change when a phase transition occurs due to an applied voltage (the slower the response speed), the more stable the crystal structure. That is, if the piezoelectric film 15 of the present disclosure has two maximum values ​​P1 and P2 in the first profile and only one maximum value in the second profile, it is presumed that the high breakdown voltage and high reliability are realized by including a component with a relatively slow response speed, i.e., a stable component, in addition to a component with a relatively fast response speed that contributes to driving.

[0034] If we assume that one phase undergoes two phase transitions in the first profile, i.e., a first phase transition from the first phase to the second phase and a second phase transition from the second phase to the third phase, then the second profile does not have a second phase transition. This means that the second phase is a stable phase. Since the piezoelectric film 15 contains a stable second phase that does not undergo a phase transition when measured at the second rate of change, it can be assumed that high withstand voltage and high reliability are achieved.

[0035] On the other hand, if we assume that the piezoelectric film 15 includes two phases with different response voltages, the maximum value P2, which is on the higher voltage side of the two maximum values ​​in the first profile, can also be considered to be a phase that has a slow response speed and does not respond at the second change rate because it is stable.

[0036] Of the two maximum values ​​in the current-voltage profile measured at the first rate of change, the maximum value on the lower voltage side is P1, and the maximum value on the higher voltage side is P2. 0.3≦P2 / P1 It is preferable that the following is satisfied.

[0037] The peak of the maximum value B is thought to be caused by stable components in the piezoelectric film 15, and reliability can be improved by making the current peak ratio of the stable components 30% or more of the other components.

[0038] Also, P2 / P1≦1 It is preferable that the following is satisfied.

[0039] As mentioned above, when applied to a device, the component that generates strain in the piezoelectric film and causes displacement in the element is thought to be the component that mainly contributes to the maximum value P1 (the component with a relatively fast response speed). By satisfying P2 / P1≦1, the component that mainly contributes to displacement can account for more than half of the current peak ratio, thereby achieving higher piezoelectricity.

[0040] The piezoelectric film 15 of the present disclosure is primarily composed of perovskite oxide. Here, "primarily composed of perovskite oxide" means that 80 mol % or more of the piezoelectric film is made of perovskite oxide. Preferably, the piezoelectric film 15 is made of perovskite oxide (however, it inevitably contains impurities).

[0041] The perovskite oxide is preferably lead zirconate titanate (PZT), which contains Pb (lead), Zr (zirconium), Ti (titanium) and O (oxygen).

[0042] In particular, it is preferable that the perovskite-type oxide is a compound represented by the following general formula (1) containing an additive M in the B site of PZT. Pb{(Zr x Ti 1-x ) 1-y M y}O3(1) Here, M is preferably one or more elements selected from V (vanadium), Nb (niobium), Ta (tantalum), Sb (antimony), Mo (molybdenum), and W (tungsten). Here, 0 < x < 1 and 0 < y < 0.3. In the general formula (1), Pb:{(Zr x Ti 1-x ) 1-y M y}:O is based on 1:1:3, but may deviate within the range where a perovskite structure can be obtained.

[0043] M may be a single element such as only V or only Nb, or a combination of two or more elements such as a mixture of V and Nb, or a mixture of V, Nb, and Ta. When M is these elements, a very high piezoelectric constant can be realized in combination with the Pb of the A-site element.

[0044] The piezoelectric film 15 is preferably an (100)-plane-oriented film. Here, "the piezoelectric film 15 is an (100)-plane-oriented film" means that the piezoelectric film 15 is a film in which the (100) plane is preferentially oriented substantially parallel to the film surface in a state where no voltage is applied. Here, "preferential orientation" means that the orientation degree of the (100) plane is 60% or more. The orientation degree is calculated from an XRD chart obtained by XRD (X-Ray Diffraction) measurement. The orientation degree = Σ(100) peak / Σ((100) peak + (110) peak + (111) peak)). In the (100)-plane-oriented piezoelectric film 15, a high piezoelectric constant d 31 can be obtained.

[0045] Piezoelectric constant d 31It is preferable that the magnitude of the piezoelectric constant is 200 pm / V or more and the withstand voltage is 50 V / µm or more. The method for measuring the piezoelectric constant and the withstand voltage will be described in the Examples section below.

[0046] The thickness of the piezoelectric film 15 is usually 200 nm or more, for example, 0.2 μm to 5 μm, and preferably 1 μm or more.

[0047] The piezoelectric element 1 is configured so that an electric field is applied to the piezoelectric film 15 in the film thickness direction by the lower electrode layer 12 and the upper electrode layer 18. Each layer of the piezoelectric element 1 other than the piezoelectric film will be described below.

[0048] The substrate 11 is not particularly limited and examples thereof include substrates of silicon, glass, stainless steel, yttrium-stabilized zirconia, alumina, sapphire, silicon carbide, etc. The substrate 11 may also be a laminated substrate such as an SOI (Silicon on Insulator) substrate in which an SiO2 oxide film is formed on the surface of a silicon substrate.

[0049] The lower electrode layer 12 is an electrode for applying a voltage to the piezoelectric film 15. The main component of the lower electrode layer 12 is not particularly limited, and examples thereof include metals or metal oxides such as Au (gold), Pt (platinum), Ir (iridium), Ru (ruthenium), Ti, Mo, Ta, Al (aluminum), Cu (copper), and Ag (silver), as well as combinations thereof. ITO (indium tin oxide), LaNiO3, and SRO (SrRuO3) may also be used. Various adhesion layers and seed layers may be included between the piezoelectric film 15 and the lower electrode layer 12 and between the lower electrode layer 12 and the substrate 11.

[0050] The upper electrode layer 18 is paired with the lower electrode layer 12 and serves as an electrode for applying a voltage to the piezoelectric film 15. The main component of the upper electrode layer 18 is not particularly limited, and examples thereof include the materials exemplified for the lower electrode layer 12, as well as electrode materials commonly used in semiconductor processes, such as chromium (Cr), and combinations thereof. However, it is preferable to use an oxide conductor in the region in contact with the piezoelectric film 15. Specific examples include ITO, iridium oxide, and SRO, as well as LaNiO3 or doped ZnO. By providing an oxide conductor in the region of the upper electrode layer 18 in contact with the piezoelectric film 15, oxygen elements are less likely to escape from the piezoelectric film 15 compared to when a metal is in direct contact with the piezoelectric film 15, which effectively suppresses degradation of piezoelectricity.

[0051] Here, "lower" and "upper" do not mean upper and lower in the vertical direction; rather, the electrode arranged on the substrate side across the piezoelectric film is referred to as the lower electrode, and the electrode arranged on the opposite side of the piezoelectric film from the substrate is referred to as the upper electrode.

[0052] There are no particular limitations on the thickness of the lower electrode layer 12 and the upper electrode layer 18, but it is preferably about 50 nm to 300 nm, and more preferably 100 nm to 300 nm.

[0053] A method for manufacturing the piezoelectric film 15 will now be described.

[0054] A lower electrode layer 12 is formed on a substrate 11, which is used as a film-forming substrate 10. A piezoelectric film 15 is formed on the film-forming substrate 10 by sputtering.

[0055] In the initial stage of deposition of the piezoelectric film 15, deposition is performed with the deposition substrate 10 grounded or with a positive bias voltage applied to the deposition substrate 10. Thereafter, the bias voltage is switched to a negative bias voltage, and deposition is continued until a desired film thickness is reached.

[0056] The positive bias voltage applied during initial film formation is preferably 0 V (ground) to 60 V, more preferably 20 V to 40 V. The negative bias voltage that can be switched midway is preferably −20 V to −100 V, more preferably −40 V to −80 V.

[0057] The bias voltage applied to the deposition substrate 10 can be switched from ground or positive to negative bias voltage any time after the initial nucleation of the perovskite structure of the piezoelectric film 15 is completed, but preferably after a film of several tens of nanometers or more has been deposited. On the other hand, in order to obtain a film thickness by applying high plasma energy as a negative bias voltage to the deposition substrate 10, it is preferable that the film thickness deposited with a ground or positive bias voltage be several hundred nanometers or less. The switching timing is when the film thickness deposited with a ground or positive bias voltage reaches 30 nm to 300 nm.

[0058] Thus, according to the method for manufacturing a piezoelectric film of this embodiment, when the piezoelectric film 15 is formed by sputtering, the film formation substrate 10 is grounded or a positive bias voltage is applied to the film formation substrate 10 during initial film formation, and then the bias voltage applied to the film formation substrate 10 is switched to negative, and film formation is performed with a negative bias voltage applied to the film formation substrate 10. By using this film formation method, it is possible to obtain the piezoelectric film of the above embodiment, which exhibits two maxima in the first profile.

[0059] When forming a perovskite oxide film by sputtering, it is known that a piezoelectric film formed with a negative bias voltage applied to the deposition substrate exhibits lower piezoelectric performance than a piezoelectric film formed with a positive bias voltage. This is thought to be due to so-called plasma damage, in which defects are generated by the collision of plasma ions with the deposition surface. To prevent crystal defects caused by plasma damage, conventionally, piezoelectric films have been formed with a positive bias voltage applied. Furthermore, the bias voltage has not been changed during the piezoelectric film formation process. In the piezoelectric film manufacturing method of this embodiment, during initial film formation, i.e., during the generation of perovskite oxide crystal nuclei, the deposition substrate 10 is grounded or a positive bias voltage is applied to the deposition substrate 10, thereby minimizing plasma damage and enabling the formation of highly crystalline crystal nuclei. After the formation of highly crystalline crystal nuclei, the bias voltage applied to the deposition substrate 10 is switched to a negative voltage, and the piezoelectric film is deposited to the desired thickness. Because highly crystalline nuclei are formed during the initial film formation, a highly crystalline perovskite structure can be obtained even when film formation is performed under conditions in which a negative bias voltage is applied to the film formation substrate 10, which may cause plasma damage. Furthermore, it is believed that forming a piezoelectric film while a negative bias voltage is applied to the film formation substrate 10 allows a stable phase to be formed in the film. [Example]

[0060] Examples and comparative examples of the present disclosure will be described below. Fig. 3 shows a schematic diagram of an evaluation sample 2 for the piezoelectric film of the example.

[0061] First, a method for manufacturing the evaluation sample 2 having the piezoelectric film of the example and the comparative example will be described.

[0062] (Bottom electrode layer deposition) A Si wafer substrate was used as the substrate 21. A lower electrode layer 22 was formed on the substrate 21. Specifically, as the lower electrode layer 22, a 20 nm thick Ti layer and a 150 nm thick Ir layer were laminated on the substrate 21 in this order.

[0063] (Piezoelectric film deposition) Next, a piezoelectric film 25 was formed on the lower electrode layer 22 by sputtering. Specifically, the RF (radio-frequency) sputtering device described in JP 2009-57599 A was used as the sputtering device. The sputtering device includes a vacuum chamber, a sputtering electrode that holds a target material and generates plasma, and a substrate holder that is positioned opposite the sputtering electrode and holds a deposition substrate. The substrate holder is connected to an impedance adjustment circuit. Adjusting this impedance connection circuit allows the bias voltage of the deposition substrate held by the substrate holder to be changed. Using this sputtering device, a piezoelectric film 25 was formed on a deposition substrate 20 that includes a substrate 21 and a lower electrode layer 22 on it. The sputtering conditions were as follows: Target size: 8 inches Distance between target and deposition substrate: 100 mm Deposition substrate temperature: 550℃ Atmosphere: Ar and O2 mixture (O2 volume fraction 2.5%) Film formation pressure: 0.5 Pa Power input to target: 3kW

[0064] At the beginning of the deposition of the piezoelectric film 25, a first bias voltage was applied to the deposition substrate 20, and then the bias voltage was changed to a second bias voltage to deposit the film. The first bias voltage and the second bias voltage for each example were as shown in Table 1 below. In each example, the first bias voltage was a positive bias voltage (specifically, +40 V), and the second bias voltage was a negative bias voltage (specifically, -20 V to -100 V). In Comparative Examples 1 to 3, the bias voltage applied to the deposition substrate 20 was not changed. The first bias voltage was switched to the second bias voltage when the thickness of the piezoelectric film 25 reached 100 nm.

[0065] In Comparative Examples 1 and 2 and Examples 1 to 8, a 2 μm Nb-doped PZT film was formed as the piezoelectric film 25. In Comparative Example 3 and Example 9, a 2 μm PZT film was formed as the piezoelectric film 25. In Comparative Examples 1 and 2 and Examples 1 to 8, Pb 1.3 Zr 0.43 Ti 0.44 Nb 0.13 For Comparative Example 3 and Example 9, a Pb target was used. 1.3 Zr 0.52 Ti 0.48 An O3 target was used.

[0066] (Top electrode layer formation) An upper electrode layer 28 was formed by sputtering on the surface of the piezoelectric film 25. The upper electrode layer 28 was made of ITO and had a thickness of 100 nm. As described above, a laminate including the piezoelectric film of each example or comparative example was prepared. Using this laminate, an evaluation sample was prepared according to the procedure described below, and the piezoelectric film of each example and comparative example was evaluated.

[0067] <Crystallinity evaluation> XRD measurements were performed to confirm the crystallinity of the Nb-PZT piezoelectric films of Examples 1 to 8 and Comparative Examples 1 and 2. As an example, FIG. 4 shows an XRD chart obtained for Example 2. In the XRD chart, the peak at 22° is the (100) plane of the perovskite structure, and the peak at 44.5° is the (200) plane of the perovskite structure. This piezoelectric film has crystallinity oriented in the (100) plane. The XRD charts obtained for all Examples and Comparative Examples other than Example 2 were substantially similar to the XRD chart shown in FIG. 4. That is, in the 2θ range of 20° to 50°, the only peaks of the perovskite structure were (100) and (200), and no significant differences were observed in the magnitude or half-width of each peak.

[0068] <Composition evaluation> The compositions of the piezoelectric films, which are Nb-PZT, of Examples 1 to 8 and Comparative Examples 1 and 2 were evaluated by XRF measurement. As a result, the composition of the piezoelectric films was found to be Pb a{(Zr x Ti 1-x ) 1-y Nb y}O z In this case, a=1.12, x=0.52, and y=0.12 for Examples 1 to 8 and Comparative Examples 1 and 2. In other words, no significant difference was observed in the composition of the piezoelectric films of Examples 1 to 8 and Comparative Examples 1 and 2.

[0069] <Measurement of piezoelectric constant> The piezoelectric constant of the piezoelectric element in each example was measured by the following method. The laminate produced as described above was cut into a strip of 2 mm × 25 mm to produce a cantilever, and a sine wave voltage of −10 V ± 10 V, i.e., a bias voltage of −10 V and an amplitude sine wave voltage of 10 V, was applied to measure the piezoelectric constant d 31 The measurement results and evaluation are shown in Table 1. Piezoelectric constant d 31 The evaluation criteria were as follows: A:200pm / V or more B: 150 pm / V or more, less than 200 pm / V C: Less than 150 pm / V

[0070] <Measurement of withstand voltage> The breakdown voltage of the piezoelectric element was measured by the following method. The laminate produced as described above was cut into a 25 mm × 25 mm square, and the upper electrode layer 28 was patterned into a 400 μm diameter circle by lift-off (see Figure 3). The lower electrode layer 22 was grounded, and the upper electrode layer 28 was set to a negative potential. The voltage was increased at a rate of 1 V / sec, and the voltage at which a current of 1 mA or more flowed was considered to be the breakdown voltage. A total of 10 measurements were performed, and the average value (absolute value) was defined as the breakdown voltage. The measurement results and evaluation are shown in Table 1. The evaluation criteria for pressure resistance were as follows: A:75V / μm or more B: 50 μm or more, less than 75 V / μm C: Less than 50V / μm

[0071] <I-V Measurement> For the I-V measurement, the same evaluation samples as those used for the breakdown voltage measurement were used. The lower electrode layer 22 was grounded, and a voltage was applied by sweeping from -40 V to 40 V to the upper electrode layer 28 at a first change rate of 10 kV / μm·sec, and the current value was measured; similarly, the lower electrode layer 22 was grounded, and a voltage was applied by sweeping from -40 V to 40 V to the upper electrode layer 28 at a second change rate of 50 kV / μm·sec, and the current value was measured. Strictly speaking, due to the convenience of the measuring device, the applied voltage was changed stepwise every 1 V, and by controlling the holding time at each step, the voltage rise time was controlled and adjusted to the desired voltage change rate.

[0072] The I-E profiles obtained by the above measurement are shown in FIGS. 5 to 12. FIGS. 5 to 12 show the I-E profiles of Comparative Example 2, Examples 1 to 8, Comparative Example 3, and Example 9, respectively. The solid line is the profile measured at the first change rate (10 kV / μm·sec), and the broken line is the profile measured at the second change rate (50 kV / μm·sec). For Examples 1 to 5 and 9 shown in FIGS. 6 to 10 and FIG. 12, there were two maximum values P1 and P2 (two peaks) in the first profile measured at the first change rate, and there was one maximum value in the second profile measured at the second change rate. The same was true for Examples 6 to 8 not shown. On the other hand, for Comparative Examples 2 and 3 shown in FIGS. 5 and 11, there was only one maximum value in both the first profile measured at the first change rate and the second profile measured at the second change rate. For Comparative Example 1, there was also only one maximum value in any of the profiles measured at the first change rate and the second change rate, similar to Comparative Example 2. Here, the maximum value refers to the point indicating the maximum value (see FIG. 2) in a state where the influence of the S / N of the measurement has been removed from the profile. For the two maximum values, the ratio P2 / P1 of the maximum value P1 on the lower voltage side to the maximum value P2 on the higher voltage side was obtained as the peak intensity ratio, respectively. The peak intensity ratio P2 / P1 for each example is shown in Table 1. <​​​The reliability test used evaluation samples similar to those used in the breakdown voltage measurement. In an environment of 120°C, the lower electrode layer 22 was grounded, and a voltage of -40 V was applied to the upper electrode layer 28, and the time (hr) from the start of voltage application until breakdown occurred was measured. The measurement results and evaluation are shown in Table 1. The reliability test was performed for 1000 hours, and those that did not experience breakdown over 1000 hours were marked with "1000<" in Table 1, since the time until breakdown occurred exceeded 1000 hours. The evaluation criteria for the reliability test were as follows: A: More than 500hr B: 200 hours or more, less than 500 hours C: Less than 200 hours

[0074] [Table 1]

[0075] The overall evaluation results from the viewpoints of piezoelectric constant, withstand voltage, and reliability are shown in Table 1. Taking practicality into consideration, the lowest evaluation result from the individual evaluations of piezoelectric constant, withstand voltage, and reliability was used as the overall evaluation.

[0076] Among Comparative Examples 1 to 3, in which the bias voltage was not changed during film formation, Comparative Examples 2 and 3, in which a positive bias voltage was applied, received evaluations of A and B for the piezoelectric constant and withstand voltage, indicating practically acceptable results. However, the reliability test rated C, indicating insufficient long-term reliability. On the other hand, Examples 1 to 9 of the present disclosure received evaluations of B or higher in all reliability tests, demonstrating long-term reliability. Furthermore, Examples 1 to 9 received evaluations of B or higher, indicating practically acceptable results, and can be considered piezoelectric films that combine high withstand voltage and reliability. Table 1 clearly shows that piezoelectric films with two peaks in the IE profile obtained by measurement at the first rate of change have higher long-term reliability than piezoelectric films with only one peak in the IE profile obtained by measurement at the first rate of change. In particular, Examples 2 to 5, which were Nb-PZT piezoelectric films and had a peak intensity ratio P2 / P1 of 0.3 or greater and 1.2 or less, demonstrated particularly high reliability, exceeding 1,000 hours.

[0077] In all of the piezoelectric films of Examples 1 to 9, a positive bias voltage was applied to the film-forming substrate 10 in the initial stage of film formation, and then the bias voltage was switched to a negative voltage. The piezoelectric films of Comparative Examples 1 and 2 and Examples 1 to 8 were formed under the same film-forming conditions except for the bias voltage. XRD and XRF analysis revealed no difference between the piezoelectric films of Comparative Examples 1 and 2 and Examples 1 to 8. That is, the crystal structures and composition ratios observed by XRD and XRF were indistinguishable. However, as described above, a clear difference was observed between the piezoelectric films of Examples and Comparative Examples in the IE profiles measured at the first rate of change, in that the piezoelectric films had one or two maxima (peaks). Furthermore, as shown in Table 1, it is clear that a piezoelectric film having two maxima in the IE profiles measured at the first rate of change can achieve both high breakdown voltage and high reliability. In particular, piezoelectric films with higher reliability were obtained by adjusting the second bias voltage between −40 V and −100 V, as in Examples 2 to 5. Furthermore, piezoelectric films with good piezoelectric properties, withstand voltage, and reliability were obtained when the first bias voltage was in the range of 0 to 60 V, as in Examples 3 and 6 to 8.

[0078] The disclosure of Japanese Patent Application No. 2021-051660, filed on March 25, 2021, is incorporated herein by reference in its entirety. All publications, patent applications, and technical standards mentioned in this specification are herein incorporated by reference to the same extent as if each individual publication, patent application, or technical standard was specifically and individually indicated to be incorporated by reference.

Claims

1. A piezoelectric film mainly composed of a perovskite oxide, a current-voltage profile showing the relationship between the voltage obtained when a voltage sweep from −40 V to +40 V is applied at a first change rate of 10 kV / cm sec while sandwiched between a pair of electrode layers and the current that flows when the voltage is applied has two maximum values; a piezoelectric film sandwiched between the pair of electrode layers and having a voltage sweep applied from −40 V to +40 V at a second change rate of 50 kV / cm sec; and a current-voltage profile showing the relationship between the voltage and the current that flows when the voltage is applied, the current-voltage profile showing the relationship between the voltage and the current that flows when the voltage is applied, wherein the piezoelectric film has only one maximum value.

2. When the maximum value on the lower voltage side of the two maximum values ​​in the current-voltage profile acquired at the first rate of change is P1 and the maximum value on the higher voltage side is P2, 0.3≦P2 / P1 The piezoelectric film according to claim 1 , which satisfies the following:

3. When the maximum value on the lower voltage side of the two maximum values ​​in the current-voltage profile acquired at the first rate of change is P1 and the maximum value on the higher voltage side is P2, P2 / P1≦1 The piezoelectric film according to claim 1 or 2, which satisfies the following:

4. The piezoelectric film according to claim 1 , wherein the perovskite oxide contains Pb, Zr, Ti, and O.

5. The perovskite oxide is a compound represented by the following general formula (1): Pb{(Zr ｘ Ti １－ｘ ) １－ｙ M ｙ }O ３ (1) 0<x<1, 0<y<0.3, 5. The piezoelectric film according to claim 4, wherein M is one or more elements selected from the group consisting of V, Nb, Ta, Sb, Mo, and W.

6. The piezoelectric film according to claim 1 , which is an oriented film oriented in the (100) plane.

7. Piezoelectric constant d ３１ The piezoelectric film according to claim 1 , wherein the magnitude of the dielectric constant is 200 pm / V or more and the withstand voltage is 50 V / μm or more.

8. A substrate; a piezoelectric element having a lower electrode, a piezoelectric film, and an upper electrode laminated in this order on one surface of the substrate, A piezoelectric element, wherein the piezoelectric film is the piezoelectric film according to claim 1 .

9. A method for manufacturing a piezoelectric film according to any one of claims 1 to 7, comprising the steps of: When the piezoelectric film is formed on a film formation substrate by sputtering, A method for manufacturing a piezoelectric film, wherein, in the early stage of film formation, film formation is performed with the film formation substrate grounded or with a positive bias voltage applied to the film formation substrate, and then film formation is performed with a negative bias voltage applied to the film formation substrate.

10. 10. The method for manufacturing a piezoelectric film according to claim 9, wherein the negative bias voltage is −20V to −100V.

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