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

Abstract

To provide a piezoelectric laminate and a piezoelectric element, in which a piezoelectric characteristic and a driving stability are improved.SOLUTION: A piezoelectric laminate and a piezoelectric element, comprise: a lower electrode layer; and a piezoelectric film having a perovskite oxide as a main component on a substrate in this order. The piezoelectric film includes an oxygen deficiency region in a region contacted to a lower electrode layer. In the case where a mean value of an oxygen amount in the region positioned at a center of three regions obtained by dividing the piezoelectric film into three to a thickness direction is a first mean oxygen amount, and a mean value of the oxygen amount in the oxygen deficiency region is a second means oxygen amount, a ratio R of the second means oxygen amount to the first mean oxygen amount is less than 0.97, and a thickness of the oxygen deficiency region is 120nm or more, and is 1 / 3 or less of a whole thickness of the piezoelectric film.SELECTED DRAWING: Figure 1

Description

[Technical Field]

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

[0002] Perovskite oxides such as lead zirconate titanate (Pb(Zr,Ti)O3, hereinafter referred to as PZT) are known as materials with excellent piezoelectric and ferroelectric properties. Piezoelectrics made of perovskite oxides are used in the piezoelectric films of piezoelectric elements. Piezoelectric elements include a lower electrode, a piezoelectric film, and an upper electrode on a substrate (see, for example, Patent Document 1). Piezoelectric elements are used in a variety of devices, including memories, inkjet heads (actuators), micromirror devices, angular velocity sensors, gyro sensors, ultrasonic elements (PMUT: Piezoelectric Micromachined Ultrasonic Transducers), and vibration-powered energy harvesting devices.

[0003] When applying piezoelectric elements to devices, high piezoelectric properties are desirable because the higher the piezoelectric properties, the better the device performance. Among piezoelectric materials, PZT has particularly excellent piezoelectric properties and is therefore widely used in a variety of devices. PZT is known to have a pyrochlore structure that does not exhibit piezoelectricity, in addition to the perovskite structure that exhibits piezoelectricity. In particular, when depositing a PZT film, the pyrochlore phase is likely to grow in the early stages of film formation, so in order to improve the piezoelectric properties, it is important to obtain a PZT film in which the pyrochlore phase is suppressed. The presence of the pyrochlore phase in a PZT film leads to reduced piezoelectric properties and reduced drive stability.

[0004] A well-known method for suppressing the growth of the pyrochlore phase during PZT film formation is to use a seed layer (see, for example, Patent Documents 2 and 3). By using SrRuO3 (strontium ruthenate), PbTiO3 (lead titanate), SrTiO3 (strontium titanate), or the like as a seed layer and forming a PZT film on the seed layer, it is possible to suppress the pyrochlore structure and form a PZT film with a good perovskite structure. SrRuO3, PbTiO3, SrTiO3, and the like are materials that are unlikely to form the pyrochlore structure and can stably form a perovskite structure. [Prior art documents] [Patent documents]

[0005] [Patent Document 1] International Publication No. 2015 / 045845 [Patent Document 2] Japanese Patent Application Publication No. 2019-052348 [Patent Document 3] Japanese Patent Application Laid-Open No. 2017-59751 Summary of the Invention [Problem to be solved by the invention]

[0006] The growth of the pyrochlore phase can be suppressed by forming a PZT film on the seed layer described above. However, providing a seed layer between the lower electrode layer and the PZT film, which is made of a material with a different composition from the lower electrode layer and the PZT film, increases the process load and manufacturing costs due to an increase in the number of steps and the number of materials.

[0007] The technology disclosed herein has been made in consideration of the above circumstances, and aims to provide a piezoelectric laminate, a piezoelectric element, and a method for manufacturing a piezoelectric laminate that suppresses the growth of the pyrochlore phase and improves the piezoelectric characteristics and drive stability without increasing the process load or manufacturing costs. [Means for solving the problem]

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

[0009] The piezoelectric laminate of the present disclosure is a piezoelectric laminate including a lower electrode layer and a piezoelectric film containing a perovskite oxide as a main component, in this order, on a substrate, the piezoelectric film has an oxygen deficiency region in a region in contact with the lower electrode layer, the average oxygen amount in a central region among three regions obtained by dividing the piezoelectric film into three equal regions in the thickness direction is defined as a first average oxygen amount, and the average oxygen amount in an oxygen deficiency region is defined as a second average oxygen amount, the ratio R of the second average oxygen amount to the first average oxygen amount is less than 0.97, The thickness of the oxygen deficiency region is 120 nm or more and is 1 / 3 or less of the total thickness of the piezoelectric film.

[0010] In the piezoelectric laminate of the present disclosure, the ratio R is preferably 0.91 or more and 0.95 or less.

[0011] In the piezoelectric laminate of the present disclosure, the thickness of the oxygen deficiency region is preferably 150 nm or more and ¼ or less of the total thickness of the piezoelectric film.

[0012] In the piezoelectric laminate of the present disclosure, the piezoelectric film is preferably a uniaxially oriented film oriented in the (100) direction.

[0013] In the piezoelectric laminate of the present disclosure, the polarization direction of the piezoelectric film is preferably a direction from the lower electrode layer side toward the film surface of the piezoelectric film.

[0014] In the piezoelectric laminate of the present disclosure, the perovskite oxide preferably contains Pb, Zr, Ti, and O.

[0015] In the piezoelectric laminate of the present disclosure, the perovskite oxide preferably contains one or more elements selected from V, Nb, Ta, Sb, Mo, and W at the B site.

[0016] In the piezoelectric laminate of the present disclosure, the lower electrode layer in contact with the piezoelectric film is preferably an Ir layer oriented in the (111) plane.

[0017] The piezoelectric element of the present disclosure includes the above-described piezoelectric laminate and an upper electrode layer provided on the piezoelectric film of the piezoelectric laminate.

[0018] A method for manufacturing a piezoelectric laminate according to the present disclosure is a method for manufacturing a piezoelectric laminate including a lower electrode layer and a piezoelectric film containing a perovskite oxide as a main component, in this order, on a substrate, the method comprising: The method includes a piezoelectric film formation step of sputtering a piezoelectric film on the lower electrode layer, in which the piezoelectric film is formed to a predetermined thickness at the initial stage of film formation using a first oxygen volume fraction, and then the remaining thickness is formed using a second oxygen volume fraction higher than the first oxygen volume fraction.

[0019] In the method for manufacturing a piezoelectric laminate according to the present disclosure, the predetermined thickness is preferably 25 nm or more. [Effects of the Invention]

[0020] According to the piezoelectric laminate, piezoelectric element, and method for manufacturing a piezoelectric laminate of the present disclosure, it is possible to improve the piezoelectric characteristics and driving stability without increasing the process load and manufacturing costs. [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] FIG. 4 is a schematic diagram showing the change in the amount of oxygen in the thickness direction of a piezoelectric film. [Figure 3] FIG. 2 is an enlarged schematic view of a piezoelectric film. [Figure 4] 1 is an XRD chart of Example 1. [Figure 5] 1 is an XRD chart of Comparative Example 1. [Figure 6] FIG. 1 is a diagram showing TOF-SIMS data of Example 1 and Comparative Example 1. 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] "Piezoelectric laminate and piezoelectric element" FIG. 1 is a cross-sectional schematic diagram showing the layer structure of a piezoelectric laminate 5 and a piezoelectric element 1 including the piezoelectric laminate 5 according to an embodiment. As shown in FIG. 1, the piezoelectric element 1 includes a piezoelectric laminate 5 and an upper electrode layer 18. The piezoelectric laminate 5 includes a substrate 10, a lower electrode layer 12, and a piezoelectric film 15 containing a perovskite oxide as a main component, which are laminated on the substrate 10. Here, "lower" and "upper" do not refer to upper and lower in the vertical direction; rather, the electrode disposed on the substrate 10 side across the piezoelectric film 15 is simply referred to as the lower electrode layer 12, and the electrode disposed on the opposite side of the piezoelectric film 15 from the substrate 10 is simply referred to as the upper electrode layer 18.

[0024] The piezoelectric film 15 is mainly composed of perovskite oxide. Here, "mainly composed" means that 80 mol % or more of the piezoelectric film 15 is made up of perovskite oxide. More preferably, 90 mol % or more of the piezoelectric film 15 is made up of perovskite oxide. Furthermore, it is more preferable that approximately 100 mol % of the piezoelectric film 15 is made up of perovskite oxide (however, unavoidable impurities are included).

[0025] The piezoelectric film 15 has an oxygen-deficient region 15b in a region in contact with the lower electrode layer 12. When the piezoelectric film 15 is divided into three equal regions in the thickness direction, and the average oxygen amount in a central region 15a (hereinafter referred to as central region 15a) is defined as a first average oxygen amount, and the average oxygen amount in the oxygen-deficient regions 15b is defined as a second average oxygen amount, the ratio R of the second average oxygen amount to the first average oxygen amount is less than 0.97.

[0026] Perovskite oxides are generally represented by ABO3. A represents an A-site element, B represents a B-site element, and O represents oxygen. The stoichiometric ratio of A:B:O is 1:1:3. The stoichiometric ratio of the A-site element to the B-site element is A / B=1, but A / B may deviate from 1 as long as the perovskite structure is maintained. In particular, when A, which will be described later, is mainly composed of Pb, it is preferable that A / B>1. Details of the A-site element and the B-site element will be described later.

[0027] The A-site elements, B-site elements, and their composition ratios are the same throughout the piezoelectric film 15. In contrast, the oxygen-deficient region 15b of the piezoelectric film 15 contains less oxygen than the central region 15a. The composition of the perovskite oxide in the oxygen-deficient region 15b is ABO b (b<3). This means that the amount of oxygen bonded in the perovskite oxide is less than the stoichiometric ratio, and oxygen vacancies exist. On the other hand, the perovskite oxide constituting the central region 15a has an ABO a where a=3 is the standard. a may deviate from 3 as long as the perovskite structure is maintained and the relationship with b, which will be described later, is satisfied. However, since the closer the ratio is to the stoichiometric ratio, the larger the piezoelectric constant, so a=3 is preferable. Here, b / a is equal to the ratio R of the second average oxygen amount to the first average oxygen amount. In other words, R=b / a.

[0028] As described above, in the piezoelectric element 1 of this embodiment, the ratio R is less than 0.97 (R=b / a<0.97). The ratio R is preferably 0.91 or more and 0.95 or less (0.91≦R=b / a≦0.95).

[0029] The thickness tb of the oxygen deficiency region 15b is 120 nm or more and 1 / 3 or less of the overall thickness t of the piezoelectric film 15. The thickness tb of the oxygen deficiency region 15b is preferably 150 nm or more and 1 / 4 or less of the overall thickness t of the piezoelectric film 15, and more preferably 250 nm or more. The overall thickness t of the piezoelectric film 15 is preferably about 0.4 μm to 5 μm, and more preferably 1 μm to 5 μm.

[0030] The oxygen signal intensity, which is proportional to the amount of oxygen in the piezoelectric film 15, can be measured by time-of-flight secondary ion mass spectrometry (TOF-SIMS). In the present disclosure, the ratio R of the second average oxygen amount to the first average oxygen amount is a value calculated based on data of the oxygen signal intensity measured by TOF-SIMS. The thickness of the oxygen deficiency region 15b is also determined based on data measured by TOF-SIMS.

[0031] Fig. 2 is a diagram showing an example of oxygen signal intensity measured by TOF-SIMS. The vertical axis represents oxygen signal intensity, which is proportional to the amount of oxygen, and the horizontal axis represents the position in the thickness direction of the piezoelectric film 15. In the example of Fig. 2, the oxygen signal intensity in the piezoelectric film 15 exhibits a substantially constant value from the upper electrode layer 18 side to the central region 15a, and gradually decreases from the central region 15a toward the lower electrode layer 12. As shown schematically in Fig. 2, the actually acquired signal value contains noise, so the signal value fluctuates slightly.

[0032] The following describes how to determine the ratio R of the first average oxygen content to the second average oxygen content. Two imaginary lines k1 (k1 = t / 3) and k2 (k2 = 2t / 3) are drawn to divide the piezoelectric film 15 into thirds in the thickness direction. The region between imaginary lines k1 and k2 is the central region 15a. First, the average value α of the signal at each thickness position in the central region 15a (hereinafter referred to as the central region average signal value α) is determined. In the region closer to the lower electrode layer 12 than the central region 15a, a region where the signal value drops from the central region average signal value α is visually determined. A linear fit is performed on the data in this region to draw an approximate line m. The intersection point of this approximate line m and the central region average signal value α is designated as c. The region from the interface with the lower electrode layer 12 to this intersection point c is considered to be the oxygen-deficient region 15b. In other words, the distance from the interface with the lower electrode layer 12 to the intersection point c is the thickness tb of the oxygen-deficient region 15b in the piezoelectric film 15. Then, the average value β of the signal values in this oxygen-deficient region 15b (hereinafter referred to as oxygen-deficient region average signal value β) is calculated.

[0033] As described above, the central region average signal value α and the oxygen deficiency region average signal value β are obtained from the TOF-SIMS data, and β / α is calculated. Since the oxygen signal intensity is proportional to the amount of oxygen, the ratio β / α of the oxygen deficiency region average signal value β to the central region average signal value α is equal to the ratio R of the average amount of oxygen in the oxygen deficiency region 15b (second average oxygen amount) to the average amount of oxygen in the central region 15a (first average oxygen amount) (R=β / α).

[0034] As described above, the thickness tb of the oxygen-deficient region 15b and the ratio R of the second average oxygen amount to the first average oxygen amount can be obtained from the TOF-SIMS data.

[0035] The A-site element A of the perovskite oxide constituting the piezoelectric film 15 is, for example, one or a combination of two or more of Pb (lead), Ba (barium), La (lanthanum), Sr, Bi (bismuth), Li (lithium), Na (sodium), Ca (calcium), Cd (cadmium), Mg (magnesium), and K (potassium).

[0036] The B-site element B is, for example, one or a combination of two or more of Ti (titanium), Zr (zirconium), V (vanadium), Nb (niobium), Ta (tantalum), Cr (chromium), Mo (molybdenum), W (tungsten), Mn (manganese), Fe (iron), Ru, Co (cobalt), Ir, Ni (nickel), Cu (copper), Zn (zinc), Ga (gallium), In, tin, antimony (Sb), and lanthanide elements.

[0037] The perovskite oxide contained in the piezoelectric film 15 is preferably lead zirconate titanate (PZT) containing Pb, Zr, Ti, and O. The PZT perovskite oxide may have an element other than Pb added to the A site and an element other than Zr and Ti added to the B site, as long as the perovskite structure can be maintained.

[0038] In particular, it is preferable that the perovskite-type oxide is a compound represented by the following general formula (1) containing an additive B1 at the B site of PZT. Pb{(Zr x Ti 1-x ) 1-y B1 y}O n (1) Here, B1 is preferably one or more elements selected from V (vanadium), Nb (niobium), Ta (tantalum), Sb (antimony), Mo (molybdenum), and W (tungsten). Most preferably, B1 is Nb. Here, 0 < x < 1 and 0 < y < 0.4. n is a as described above in the central region 15a and b as described above in the oxygen-deficient region 15b.

[0039] B1 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 B1 is these elements, a very high piezoelectric constant can be realized in combination with Pb of the A-site element.

[0040] Note that the piezoelectric film 15 preferably has a columnar structure film having a columnar structure including a large number of columnar crystals 17, as shown in the cross-sectional schematic view in FIG. 3. The large number of columnar crystals 17 preferably extend non-parallel to the surface of the substrate 10 (see FIG. 1) and are a uniaxially oriented film oriented in the (100) direction. By adopting an oriented structure, greater piezoelectricity can be obtained. Also, as shown in FIG. 3, the polarization direction P of the piezoelectric film 15 is preferably in the direction from the lower electrode layer 12 to the upper electrode layer 18. That is, in the piezoelectric laminate 5, the polarization direction P of the piezoelectric film 15 is preferably in the direction from the lower electrode layer 12 side to the film surface of the upper electrode layer 18 side.

[0041] In PZT-based perovskite oxides, when V, Nb, Ta, Sb, Mo, or W is added as B1 in the general formula (1), the polarization from the lower electrode side to the upper electrode side becomes stable. Therefore, immediately after sputtering (as deposited), the piezoelectric film 15 is formed with a polarization direction from the lower electrode layer to the upper electrode layer. Nb is particularly preferable as B1.

[0042] Furthermore, as shown in FIG. 3, the piezoelectric film 15 may contain a pyrochlore phase 16 at the interface with the lower electrode layer 12. As will be described in detail later, even when the pyrochlore phase 16 is contained, the pyrochlore phase 16 is sufficiently suppressed compared to conventional piezoelectric films. The pyrochlore phase 16 is preferably 20 nm or less. Note that the pyrochlore phase 16 is not uniformly formed on the surface of the lower electrode layer 12, but grows partially as shown in FIG. 3. A method for calculating the thickness of the pyrochlore phase 16 will be explained in the Examples. Note that when the pyrochlore phase 16 is 20 nm or less, almost no peak of the pyrochlore phase is observed in an XRD (X-ray diffraction) chart (see Examples).

[0043] The substrate 10 is not particularly limited, and examples thereof include substrates of silicon, glass, stainless steel, yttrium-stabilized zirconia, alumina, sapphire, silicon carbide, etc. The substrate 10 may be a laminated substrate such as a silicon substrate with a thermally oxidized film, in which an SiO2 oxide film is formed on the surface of a silicon substrate.

[0044] The lower electrode layer 12 is an electrode paired with the upper electrode layer 18 and used to apply 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) may also be used. It is particularly preferable that the lower electrode layer 12 has an Ir layer oriented in the (111) plane on the surface that contacts the piezoelectric film 15.

[0045] The thickness of the lower electrode layer 12 is not particularly limited, but is preferably about 50 nm to 300 nm, and more preferably 100 nm to 300 nm.

[0046] The upper electrode layer 18 is an electrode that pairs with the lower electrode layer 12 and applies a voltage to the piezoelectric film 15. There are no particular limitations on the main component of the upper electrode layer 18, and examples include electrode materials that are generally used in semiconductor processes, as well as conductive oxides such as ITO (Indium Tin Oxide), LaNiO3, and SRO (SrRuO3), and combinations of these.

[0047] There are no particular restrictions on the thickness of the upper electrode layer 18, but it is preferably about 50 nm to 300 nm, and more preferably 100 nm to 300 nm.

[0048] In the piezoelectric stack 5 and piezoelectric element 1 of this embodiment, the piezoelectric film 15 has an oxygen-deficient region 15b in a region in contact with the lower electrode layer 12. When forming the piezoelectric film 15, which is mainly composed of a perovskite oxide, on the lower electrode layer 12, the growth of the pyrochlore phase can be suppressed by first growing the perovskite oxide having oxygen vacancies. In particular, when the average oxygen content in the central region of the three equal regions obtained by dividing the piezoelectric film 15 into three thickness regions is defined as the first average oxygen content, and the average oxygen content in the oxygen-deficient regions is defined as the second average oxygen content, the ratio R of the second average oxygen content to the first average oxygen content is less than 0.97, and the thickness tb of the oxygen-deficient region 15b is greater than 100 nm, thereby ensuring the suppression of the growth of the pyrochlore phase. Since the pyrochlore phase can be sufficiently suppressed, the piezoelectric stack 5 and piezoelectric element 1 can be obtained, including a piezoelectric film 15 containing a good perovskite oxide. The piezoelectric film 15 has a suppressed pyrochlore phase, which allows for high piezoelectric properties and higher driving stability than conventional piezoelectric films. Furthermore, the oxygen-deficient region 15b is 1 / 3 or less of the overall thickness t of the piezoelectric film 15, which prevents degradation of the piezoelectric properties and driving stability. Here, driving stability means that stable driving is possible over a long period of time.

[0049] Furthermore, the piezoelectric stack 5 and piezoelectric element 1 of this embodiment use the oxygen-deficient region 15b as a seed layer, but the composition is the same throughout the piezoelectric film 15 except for the amount of oxygen, so increases in process load and manufacturing costs can be suppressed compared to conventional piezoelectric elements that include a seed layer made of a different material from the piezoelectric film. In other words, according to the technology disclosed herein, the piezoelectric stack 5 and piezoelectric element 1 can improve piezoelectric characteristics and drive stability without increasing process load and manufacturing costs.

[0050] The suppression effect of the pyrochlore phase is particularly high when the ratio R of the second average oxygen content in the oxygen deficiency region 15b to the first average oxygen content in the central region 15a is 0.91 or more and 0.95 or less. Furthermore, when the thickness of the oxygen deficiency region 15b is 150 nm or more and ¼ of the entire thickness of the piezoelectric film 15, it is possible to achieve both the suppression effect of the pyrochlore phase and the maintenance of a high piezoelectric constant.

[0051] The piezoelectric film 15 contains a perovskite oxide, and when it contains a perovskite oxide containing Pb, the pyrochlore phase is likely to form in the early stages of film formation. Therefore, when the piezoelectric film 15 contains a perovskite oxide containing Pb as its main component, the effect of suppressing the pyrochlore phase is particularly high. Among the perovskite oxides containing Pb, PZT-based perovskite oxides containing Pb, Zr, Ti, and O are particularly preferred because of their excellent piezoelectric properties. In particular, when the perovskite oxide is a compound represented by the above general formula (1) and B1 is one or more elements selected from V, Nb, Ta, Sb, Mo, and W, even better piezoelectric properties can be obtained.

[0052] Furthermore, as described above, by adding the B1 element to the B site, the polarization direction of the piezoelectric film 15 can be oriented from the lower electrode layer to the upper electrode layer immediately after film formation, thereby achieving high piezoelectricity without requiring a polarization treatment.

[0053] Ir is suitable for the lower electrode layer 12. Ir has good adhesion to the piezoelectric film 15, which can prevent peeling between the lower electrode layer 12 and the piezoelectric film 15. Ir has high conductivity and can reduce the resistance of the lower electrode layer 12. Furthermore, compared to other metals such as Au, Ir is less susceptible to elemental diffusion, even at high temperatures during piezoelectric film formation, and therefore has high drive stability. On the other hand, when Ir is deposited by sputtering or the like, it preferentially orients in the (111) plane. When a PZT-based perovskite oxide is deposited on this (111) plane, a pyrochlore phase is likely to form in the early stages of film formation. Therefore, when depositing the piezoelectric film 15 on a lower electrode layer 12 made of an Ir layer with a (111) plane orientation, providing an oxygen-deficient region 15b in the region in contact with the lower electrode layer 12 is particularly effective in suppressing the growth of the pyrochlore phase.

[0054] "Method of manufacturing piezoelectric laminate and piezoelectric element" The piezoelectric laminate and the piezoelectric element can be manufactured as follows. The piezoelectric stack 5 can be obtained by depositing the lower electrode layer 12 on the substrate 10, followed by depositing the piezoelectric film 15, and the piezoelectric element 1 can be obtained by depositing the upper electrode layer 18 on the piezoelectric film 15. Sputtering deposition is suitable for depositing the layers 12, 15, and 18.

[0055] In the process of forming the piezoelectric film 15, the oxygen volume fraction in the film formation gas in the film formation chamber is changed during sputtering. Specifically, the piezoelectric film 15 is formed to a predetermined thickness at the initial stage of film formation using a first oxygen volume fraction, and then the remaining thickness is formed using an increased second oxygen volume fraction. By making the first oxygen volume fraction smaller than the second oxygen volume fraction, an oxygen-deficient region 15b with a lower oxygen content than the central region 15a is formed in the region in contact with the lower electrode layer 12, and then a piezoelectric film 15 containing a perovskite-type oxide with an oxygen content of stoichiometric or near-stoichiometric ratio is grown.

[0056] For example, the oxygen volume fraction at which a perovskite oxide film with a stoichiometric ratio can be formed is determined in advance, and the oxygen volume fraction at which a perovskite oxide film with a stoichiometric ratio can be formed is set as the second oxygen volume fraction. The first oxygen volume fraction is set to an oxygen volume fraction that is smaller than the second oxygen volume fraction and that forms a perovskite oxide with a smaller oxygen bond amount than the stoichiometric ratio. The second oxygen volume fraction is, for example, approximately 8% or more and 15% or less, and the first oxygen volume fraction is, for example, approximately 2% or more and less than 8%.

[0057] The predetermined thickness formed at the first oxygen volume fraction is preferably 15 nm or more, more preferably 25 nm or more. For example, a film is formed at the first oxygen volume fraction to a thickness of 15 nm to 200 nm, preferably 25 nm to 150 nm, and more preferably 50 nm to 100 nm. The oxygen volume fraction is then changed from the first oxygen volume fraction to a second oxygen volume fraction, and the film is formed at the second oxygen volume fraction until the desired thickness is reached. By forming the film to a constant thickness at the first oxygen volume fraction in this manner, the growth of the pyrochlore phase in the early stages of film formation can be suppressed. By forming the piezoelectric film 15 at the first oxygen volume fraction to a thickness of 25 nm or more in the early stages of film formation, the effect of suppressing the pyrochlore phase can be more reliably achieved. Furthermore, when forming a film of a perovskite-type oxide containing Pb, the pyrochlore phase is likely to grow in the early stages of film formation, and therefore, the present manufacturing method is highly effective in suppressing the growth of the pyrochlore phase.

[0058] In the film formation process of the piezoelectric film 15, conditions other than the oxygen volume fraction are not changed. That is, film formation conditions other than the oxygen volume fraction, such as the target, the power input to the target, the degree of vacuum (film formation pressure) in the film formation chamber, and the substrate temperature, are kept the same until the entire piezoelectric film 15 is formed. This allows the piezoelectric film 15 to have the same composition in the thickness direction, except for the oxygen content.

[0059] In the piezoelectric film 15 obtained by depositing a film to a predetermined thickness with a first oxygen volume fraction and then depositing a film to a desired thickness t with a second oxygen volume fraction, the oxygen deficiency region 15b extends over a wider range than the thickness deposited with the first oxygen volume fraction (see Examples). Therefore, the thickness tb of the oxygen deficiency region 15b in the piezoelectric film 15 in the piezoelectric stack 5 and piezoelectric element 1 obtained by the above manufacturing method is different from the predetermined thickness deposited with the first oxygen volume fraction in the film deposition process.

[0060] The piezoelectric element 1 or piezoelectric laminate 5 of each of the above embodiments can be applied to ultrasonic devices, mirror devices, sensors, memories, and the like. [Example]

[0061] Specific examples and comparative examples of the piezoelectric element of the present disclosure will be described below. First, a method for fabricating the piezoelectric element of each example will be described. A radio frequency (RF) sputtering device was used to deposit each layer. Conditions other than the piezoelectric film were the same for each example. The fabrication method will be described with reference to the reference numerals of each layer of the piezoelectric element 1 shown in FIG. 1.

[0062] (Bottom electrode layer deposition) A 4-inch silicon substrate with a thermal oxide film was used as the substrate 10. The lower electrode layer 12 was formed on the substrate 10 by RF (radio-frequency) sputtering. Specifically, a 50 nm thick TiW layer was formed on the substrate 10 as the lower electrode layer 12, and then a 200 nm thick Ir layer was formed. In other words, the lower electrode layer 12 had a two-layer structure consisting of a TiW layer and an Ir layer. The sputtering conditions for each layer were as follows:

[0063] -TiW layer sputtering conditions- Target-substrate distance: 100 mm Target input power: 600W Ar gas pressure: 0.5 Pa Substrate temperature setting: 350℃

[0064] -Ir layer sputtering conditions- Target-substrate distance: 100 mm Target input power: 600W Ar gas pressure: 0.1 Pa Substrate temperature setting: 350℃

[0065] (Piezoelectric film deposition) The substrate 10 with the lower electrode layer 12 was placed in an RF sputtering device, and a 2 μm thick Nb-doped PZT film with a Nb doping amount of 12 at % in the B site was formed as the piezoelectric film 15. 1.3 Zr 0.435 Ti 0.445 Nb 0.12 An O3 target was used. The film was formed at a deposition rate of 100 nm / minute for a total of 20 minutes, resulting in a piezoelectric film 15 of about 2 μm. The sputtering conditions were as follows:

[0066] -Piezoelectric film sputtering conditions- Target-substrate distance: 60 mm Target input power: 500W Vacuum level: 0.3 Pa, Ar and O2 mixed atmosphere Substrate temperature setting: 700℃

[0067] The above conditions were common to the entire piezoelectric film, and the oxygen volume fraction conditions in the film formation chamber were varied between Examples and Comparative Examples. For each Example and Comparative Example, the oxygen volume fraction conditions during piezoelectric film formation were as follows: In Comparative Examples 1 and 2, the same oxygen volume fraction was used throughout the entire piezoelectric film, and in Examples 1 to 7, a first oxygen volume fraction was used in the initial stage of piezoelectric film formation, and then changed to a second oxygen volume fraction.

[0068] "Example 1" The film was formed at an oxygen volume fraction of 5% for only the first minute of film formation (thickness 100 nm), and then at an oxygen volume fraction of 10% for the remaining 19 minutes.

[0069] "Example 2" The film was formed with an oxygen volume fraction of 6% for only the first minute of film formation (thickness 100 nm), and then for the remaining 19 minutes, the oxygen volume fraction was 10%.

[0070] "Example 3" The film was formed at an oxygen volume fraction of 7.5% for only the first minute of film formation (thickness 100 nm), and at an oxygen volume fraction of 10% for the remaining 19 minutes.

[0071] Example 4 The film was deposited at an oxygen volume fraction of 2% for only the first minute of deposition, that is, to a thickness of 100 nm, and then at an oxygen volume fraction of 10% for the remaining 19 minutes.

[0072] "Example 5" The film was deposited at an oxygen volume fraction of 5% for only the first 2 minutes of deposition, that is, to a thickness of 200 nm, and then at an oxygen volume fraction of 10% for the remaining 18 minutes.

[0073] "Example 6" The film was deposited with an oxygen volume fraction of 5% for only the first 30 seconds (0.5 minutes), ie, to a thickness of 50 nm, and then with an oxygen volume fraction of 10% for the remaining 19 minutes and 30 seconds. "Example 7" The film was deposited with an oxygen volume fraction of 5% for only the first 15 seconds (approximately 0.25 minutes) of deposition, ie, to a thickness of 25 nm, and then with an oxygen volume fraction of 10% for the remaining 19 minutes and 45 seconds.

[0074] "Example 8" The film was deposited with an oxygen volume fraction of 5% for only the first 10 seconds (approximately 0.17 minutes) of deposition, that is, to a thickness of 17 nm, and then with an oxygen volume fraction of 10% for the remaining 19 minutes and 50 seconds.

[0075] "Comparative Example 1" The film was formed for 20 minutes (thickness: 2 μm) with an oxygen volume fraction of 10%.

[0076] "Comparative Example 2" The film was formed for 20 minutes (thickness: 2 μm) with an oxygen volume fraction of 5%.

[0077] Table 1 shows the conditions that were varied in forming the piezoelectric film in each example. [Table 1]

[0078] (Top electrode layer formation) Next, the substrate 10 after the formation of the piezoelectric film 15 was placed in the film formation chamber of an RF sputtering device, and an ITO (Indium Tin Oxide) target was used to form an ITO layer with a thickness of 200 nm as the upper electrode layer 18. Before forming the upper electrode layer 18, a lift-off pattern for an evaluation sample was created on the piezoelectric film 15, and the upper electrode layer 18 was formed on the lift-off pattern. The film formation conditions for the upper electrode layer 18 were as follows:

[0079] -Upper electrode layer sputtering conditions- Target-substrate distance: 100 mm Target input power: 200W Degree of vacuum: 0.3 Pa, Ar and O2 mixed gas (O2 volume fraction 5%) Substrate temperature setting: RT (room temperature)

[0080] (Formation of electrode pattern for evaluation) After the upper electrode layer 18 was formed, the upper electrode layer 18 was lifted off along a lift-off pattern by a lift-off method, and the upper electrode layer 18 was patterned.

[0081] By the above steps, a piezoelectric laminate substrate of each example was fabricated, which had the lower electrode layer 12, the piezoelectric film 15 and the patterned upper electrode layer 18 on the substrate 10.

[0082] (Preparing the evaluation sample) -Evaluation sample 1- A cantilever was fabricated as evaluation sample 1 by cutting out a rectangular piece of 2 mm×25 mm from the piezoelectric laminate substrate.

[0083] -Evaluation sample 2- A 25 mm×25 mm portion having the upper electrode layer 18 patterned in a circle with a diameter of 400 μm at the center of the surface of the piezoelectric film 15 was cut out from the piezoelectric laminate substrate, and used as evaluation sample 2.

[0084] <Evaluation of piezoelectric properties> The piezoelectric constant d 31 The larger the absolute value of the piezoelectric constant d31, the higher the piezoelectric properties, and the smaller the absolute value, the lower the piezoelectric properties. Piezoelectric constant d 31 The measurement was carried out using the above-mentioned evaluation sample 1, with a sinusoidal voltage of -10 V±10 V applied, according to the method described in I. Kanno et al. Sensor and Actuator A 107 (2003) 68. The results are shown in Table 2.

[0085] <Evaluation of driving stability> A time-dependent dielectric breakdown (TDDB) test was conducted to evaluate drive stability. Using evaluation sample 2, the lower electrode layer 12 was grounded in a 120°C environment, a voltage of -40 V was applied to the upper electrode layer 18, and the time (hr) from the start of voltage application until breakdown occurred was measured. The measurement results are shown in Table 2. Dielectric breakdown was defined as the flow of a current of 1 mA or more when a voltage was applied to a 400 μm circular upper electrode layer. The TDDB test was conducted for 1000 hours, and samples that did not experience breakdown up to 1000 hours were marked with 1000 in Table 2.

[0086] <Crystallinity evaluation> The crystallinity of the piezoelectric film of each example of the piezoelectric laminate was evaluated by XRD analysis using RINT-ULTIMA III manufactured by RIGAKU.

[0087] (Evaluation of peak intensity derived from pyrochlore phase) Figure 4 shows the XRD chart for Example 1, and Figure 5 shows the XRD chart for Comparative Example 1. The crystallinity evaluation was carried out using the piezoelectric laminate before the formation of the upper electrode layer. The intensity of the heterophase pyrochlore (222) was determined from the XRD chart obtained for each example. The region where the pyrochlore (222) phase was detected was around 29°, and the peak intensity derived from the pyrochlore (222) phase was determined by removing background noise from the obtained XRD diffraction intensity (counts). Also, from the XRD chart, py(222) / {pr(100)+pr(110)+pr(111)}×100% was calculated as the pyrochlore ratio.

[0088] The strength from each side was determined as follows. The average number of counts at 2θ of 25° to 28° was taken as background noise N. The intensity of py(222) was determined by dividing N by the maximum number of counts in the 2θ range of 28° to 30°. The intensity of pr(100) was determined by dividing N by the maximum number of counts in the 2θ range of 21° to 23°. The intensity of pr(110) was determined by dividing N by the maximum number of counts in the 2θ range of 30° to 32°. The intensity of pr(111) was determined as the value obtained by dividing N from the maximum number of counts in the 2θ range of 37.5° to 39.5°.

[0089] In Figures 4 and 5, the peak values of the perovskite phase (100) are comparable. However, differences are observed in the pyrochlore phase (222). As shown in Figure 5, in Comparative Example 1, there is a clear pyrochlore phase (222) peak near 29°. In Example 1 shown in Figure 4, the pyrochlore phase (222) peak value is lower than in Comparative Example 1, and is barely observed. As shown in Figures 4 and 5, a PZT film uniaxially oriented in (001) was obtained.

[0090] <Pyrochlore phase thickness evaluation> For the examples and comparative examples, TEM (Transmission Electron Microscope) images were taken, and the thickness of the pyrochlore phase was determined from the TEM images. In the piezoelectric film, since the contrast in the TEM image is different between the pyrochlore phase and the perovskite phase, the region of the pyrochlore phase can be specified and the thickness can be calculated. In addition, it was observed that columnar crystals of perovskite-type oxides were formed in the portions of the piezoelectric film other than the pyrochlore phase. The thickness of the pyrochlore phase was calculated as the average thickness because the pyrochlore phase is not uniformly formed on the surface of the lower electrode layer.

[0091] Specifically, using the contrast adjustment function of the image processing software, the original image is binarized with a predetermined threshold value, and the pyrochlore phase is extracted using the edge extraction function of the image processing software. In this case, the threshold value is set so that only those that can remove noise as much as possible and can clearly be distinguished as the pyrochlore phase are extracted. When the contour of the pyrochlore-type oxide layer is unclear in the binarized image, an outline is drawn empirically while looking at the binarized image, and the inside is filled. The area of the extracted pyrochlore phase is calculated from the number of pixels of the image processing software and divided by the field width of the TEM image to obtain the average layer thickness. Here, Photoshop (registered trademark) was used as the image processing software. The thickness of the pyrochlore phase obtained as described above is shown in Table 2.

[0092] <Measurement of the amount of oxygen in PZT: TOF-SIMS> TOF-SIMS is a technique for irradiating a solid sample with an ion beam (primary ion), mass-separating the ions (secondary ions) emitted from the surface using the difference in their flight times, and measuring the amount of a specific element. In this example, the analysis was performed using the following apparatus. Measuring apparatus: TOF_SIMS5 manufactured by ION-TOF Primary ion source: Bi Sputtering ion source: Cs

[0093] A sample of 1 cm × 1 cm was cut out from each example of the piezoelectric laminate substrate to prepare a sample for TOF-SIMS measurement, and the piezoelectric film 15 was peeled off at the interface with the lower electrode layer 12. Measurement was performed by irradiating an ion beam onto a 50 μm square area from the peeled surface of the piezoelectric film 15 from which the lower electrode layer 12 had been peeled off.

[0094] Table 2 shows the results of determining the oxygen amount in the central region of the piezoelectric film, the oxygen amount in the oxygen-deficient region, and the thickness of the oxygen-deficient region from the measurement data obtained by TOF-SIMS.

[0095] As an example, FIG. 6 shows the oxygen content measurement data for the piezoelectric films of Example 1 and Comparative Example 1. As shown in FIG. 6, first, imaginary lines k1 and k2 were drawn to divide the piezoelectric film into three equal parts in the thickness direction to define the central region. The average oxygen signal intensity α of the central region (central region average signal value α) was then calculated. The central region average values α were equivalent in Example 1 and Comparative Example 1. Next, in the region closer to the lower electrode layer than the central region, a region where the signal value decreased from the central region average signal value α was visually determined. Linear fitting was performed on the data for this region to draw an approximate line. The thickness of the oxygen deficiency region was calculated from the interface on the lower electrode side (here, the peeled surface) to the intersection of the approximate line and the central region average signal value α. The average oxygen signal intensity β in the oxygen deficiency region was then calculated. In FIG. 6, the oxygen deficiency region for Example 1 is shown as Oxygen Deficiency Region Substance 1, and the oxygen deficiency region for Comparative Example 1 is shown as Oxygen Deficiency Region Ratio 1. Also, β Actual 1 is the average oxygen signal intensity for Example 1, and β Ratio 1 is the average oxygen signal intensity for Comparative Example 1.

[0096] Table 3 shows the average signal value α of the central region, the average signal value β of the oxygen deficiency region, the ratio R (= β / α) of the average oxygen content of the oxygen deficiency region (second average oxygen content) to the average oxygen content of the central region (first average oxygen content), and the thickness tb of the oxygen deficiency region for each example.

[0097] The evaluation results are shown in Tables 2 and 3. [Table 2] [Table 3]

[0098] Examples 1 to 8, in which films were formed at the first oxygen volume fraction to a thickness of 17 nm or more and the oxygen deficiency region was 120 nm or more in thickness, had higher piezoelectric constants and higher driving stability than Comparative Examples 1 and 2. Among Examples 1 to 8, Examples 1 to 7, in which films were formed at the first oxygen volume fraction to a thickness of 25 nm or more and the oxygen deficiency region was 150 nm or more in thickness, had a higher effect of suppressing the pyrochlore phase than Example 8, and good results were obtained in both the piezoelectric constant and driving stability.

[0099] In both Comparative Examples 1 and 2, the piezoelectric film was formed with the same oxygen volume fraction throughout the entire film. The piezoelectric film of Comparative Example 1 was formed with an oxygen volume fraction throughout the entire film that resulted in a perovskite phase with a stoichiometric oxygen bond amount, while the piezoelectric film of Comparative Example 2 was formed with an oxygen volume fraction throughout the entire film that resulted in a perovskite phase with a lower oxygen bond amount than the stoichiometric oxygen bond amount. Although the oxygen volume fraction was not changed in Comparative Examples 1 and 2, an oxygen-deficient region having oxygen defects was formed at the interface with the lower electrode layer in both cases. However, compared to Examples 1 to 8, in which the film was formed with a first oxygen volume fraction at the beginning of film formation and then the oxygen volume fraction was increased to a second oxygen volume fraction, the difference between the oxygen content in the central region and the oxygen-deficient region was smaller, and the thickness of the oxygen-deficient region was also thinner. When the entire piezoelectric film was formed with an oxygen volume fraction of 5%, as in Comparative Example 2, an effect equivalent to the suppression effect of the pyrochlore phase in the Examples was obtained. From these results, it can be concluded that growing a perovskite oxide with an oxygen bond amount less than the stoichiometric oxygen bond amount in the region in contact with the lower electrode layer is effective in suppressing the formation of the pyrochlore phase. On the other hand, when the film was formed with an oxygen volume fraction of 5% in Comparative Example 2, the piezoelectric constant was significantly reduced to half compared to when the film was formed with an oxygen volume fraction of 10% in Comparative Example 1. From these results, it is clear that perovskite oxides containing oxygen vacancies have a lower piezoelectric constant and lower driving stability than perovskite oxides with fewer oxygen vacancies or a stoichiometric ratio.

[0100] In Examples 1 to 8, each had an oxygen-deficient region in contact with the lower electrode layer, and the ratio R of the first average oxygen content (the average oxygen content in the central region) to the second average oxygen content (the average oxygen content in the oxygen-deficient region) was less than 0.97. The thickness of the oxygen-deficient region was 120 nm or more and less than 1 / 3 of the total thickness of the piezoelectric film. In Examples 1 to 8, the growth of the pyrochlore phase was suppressed without providing a seed layer with a different composition from the piezoelectric film. In other words, the pyrochlore phase was suppressed without increasing the process load or manufacturing cost. By making the oxygen-deficient region 1 / 3 or less of the total thickness of the piezoelectric film, it was possible to suppress the decrease in the piezoelectric constant and driving stability, and also to achieve an improvement in the piezoelectric constant and driving stability by suppressing the pyrochlore phase. [Explanation of symbols]

[0101] 1 Piezoelectric element 5 Piezoelectric laminate 10 Substrate 12 Lower electrode layer 15 Piezoelectric film 15a central area 15b Oxygen deficiency region 16 Pyrochlore phase 17 Columnar crystals 18 Upper electrode layer t Piezoelectric film thickness tb Thickness of oxygen deficiency region

Claims

1. A piezoelectric laminate including a substrate, a lower electrode layer, and a piezoelectric film mainly composed of a perovskite oxide, in this order, the piezoelectric film has an oxygen deficiency region in a region in contact with the lower electrode layer, the piezoelectric film is divided into three equal regions in the thickness direction, and an average value of the oxygen amount in a central region of the three regions is defined as a first average oxygen amount, and an average value of the oxygen amount in the oxygen deficiency region is defined as a second average oxygen amount, the ratio R of the second average oxygen amount to the first average oxygen amount is less than 0.97, The thickness of the oxygen deficiency region is 120 nm or more and is 1 / 3 or less of the total thickness of the piezoelectric film. The perovskite oxide comprises Pb, Zr, Ti, and O.

2. The piezoelectric stack according to claim 1 , wherein the ratio R is equal to or greater than 0.91 and equal to or less than 0.

95.

3. 3. The piezoelectric stack according to claim 1, wherein the thickness of the oxygen deficiency region is 150 nm or more and ¼ or less of the entire thickness of the piezoelectric film.

4. 4. The piezoelectric stack according to claim 1, wherein the piezoelectric film is a uniaxially oriented film oriented in the (100) direction.

5. 5. The piezoelectric stack according to claim 4, wherein the polarization direction of the piezoelectric film is a direction from the lower electrode layer side toward the film surface of the piezoelectric film.

6. 2. The piezoelectric stack according to claim 1, wherein the perovskite oxide contains one or more elements selected from the group consisting of V, Nb, Ta, Sb, Mo, and W at the B site.

7. 7. The piezoelectric laminate according to claim 1, wherein the lower electrode layer in contact with the piezoelectric film is an Ir layer oriented in the (111) plane.

8. The piezoelectric laminate according to any one of claims 1 to 7; an upper electrode layer provided on the piezoelectric film of the piezoelectric laminate.

9. A method for manufacturing a piezoelectric laminate having a lower electrode layer and a piezoelectric film containing a perovskite oxide as a main component, in this order, on a substrate, the method comprising: the piezoelectric film is made of a perovskite oxide containing Pb, Zr, Ti, and O, and has an oxygen-deficient region in a region in contact with the lower electrode layer, the oxygen-deficient region having a thickness of 120 nm or more and being one-third or less of the total thickness of the piezoelectric film; a piezoelectric film deposition step of sputtering the piezoelectric film on the lower electrode layer, wherein in the piezoelectric film deposition step, the film is deposited to a predetermined thickness at an initial stage of film deposition with a first oxygen volume fraction, and subsequently deposited to the remaining thickness with a second oxygen volume fraction higher than the first oxygen volume fraction; where: the predetermined thickness is 25 nm or more and 150 nm or less, the first oxygen volume fraction is 2% or more and less than 8%, The second oxygen volume fraction is 8% or more and 15% or less. A method for manufacturing a piezoelectric laminate.

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