Method for manufacturing piezoelectric film, method for manufacturing piezoelectric element, and method for manufacturing piezoelectric device

Abstract

This method for manufacturing a piezoelectric film includes a piezoelectric film formation step for forming a piezoelectric film, in which at least two types of crystal films are layered onto an amorphous film. The piezoelectric film formation step includes: a first crystal film formation step in which a piezoelectric material is sputtered onto the amorphous film in a gas atmosphere containing argon to form a first crystal film that includes the piezoelectric material as a principal constituent and that includes argon; and a second crystal film formation step in which the piezoelectric material is sputtered onto the first crystal film in a gas atmosphere containing krypton to form a second crystal film that includes the piezoelectric material as a principal constituent and includes krypton.

Description

[Technical Field]

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

[0002] Piezoelectric films have high piezoelectric properties, and therefore, piezoelectric elements including piezoelectric films are widely used in piezoelectric devices such as sensors, such as pressure sensors and acceleration sensors, high frequency filter devices, and piezoelectric actuators.

[0003] When a piezoelectric film is formed by crystal growth on a conductive film on a substrate, the crystals of the piezoelectric film are oriented in the c-axis direction, and the piezoelectric film has high crystal orientation, resulting in high piezoelectric properties. A piezoelectric element equipped with a piezoelectric film with such high piezoelectric properties can have excellent piezoelectric properties. However, the materials used to form the piezoelectric film tend to be limited to materials with a crystal system similar to that of the conductive film. Therefore, various methods have been proposed for producing piezoelectric films with high crystal orientation using an amorphous conductive film as an underlayer.

[0004] As a method for manufacturing a piezoelectric film on an amorphous conductive film, for example, a method for manufacturing a thin film piezoelectric resonator has been disclosed in which an amorphous metal film made of tantalum aluminum is formed on a first metal film on a substrate, a second metal film oriented in a direction perpendicular to the surface of the amorphous metal film is formed on the amorphous metal film, and a piezoelectric film oriented in a direction perpendicular to the surface of the second metal film is formed on the second metal film (see, for example, Patent Document 1). [Prior art documents] [Patent documents]

[0005] [Patent Document 1] Japanese Patent Application Publication No. 2006-191356 Summary of the Invention [Problem to be solved by the invention]

[0006] However, Patent Document 1 does not consider film stress. When the method for manufacturing a thin film piezoelectric resonator in Patent Document 1 is used, the piezoelectric film has high piezoelectric properties, but the film stress is correspondingly large, which increases the compressive stress of the piezoelectric film and makes the piezoelectric film more likely to bend. This poses a problem of the possibility of warping or cracking in the piezoelectric film. When warping or cracking occurs in the piezoelectric film, peeling easily occurs between the substrate and the piezoelectric film, which adversely affects the piezoelectric properties of the piezoelectric element when used in the piezoelectric element.

[0007] An object of one aspect of the present invention is to provide a method for manufacturing a piezoelectric film that can manufacture a piezoelectric film having low film stress while improving crystal orientation on an amorphous conductive film. [Means for solving the problem]

[0008] One aspect of the piezoelectric film according to the present invention includes a piezoelectric film formation step of forming a piezoelectric film in which at least two types of crystalline films are stacked on an amorphous film, and the piezoelectric film formation step includes a first crystalline film formation step of sputtering a piezoelectric material on the amorphous film in a gas atmosphere containing Ar to form a first crystalline film containing the piezoelectric material as a main component and also containing Ar, and a second crystalline film formation step of sputtering the piezoelectric material on the first crystalline film in a gas atmosphere containing Kr to form a second crystalline film containing the piezoelectric material as a main component and also containing Kr. [Effects of the Invention]

[0009] One aspect of the method for manufacturing a piezoelectric film according to the present invention makes it possible to manufacture a piezoelectric film having low film stress while improving crystal orientation on an amorphous conductive film. [Brief explanation of the drawings]

[0010] [Figure 1] FIG. 2 is a schematic cross-sectional view showing the configuration of a piezoelectric film. [Figure 2] FIG. 10 is an explanatory diagram for measuring the amount of warpage of a piezoelectric film. [Figure 3]1 is a flowchart illustrating a method for manufacturing a piezoelectric film according to an embodiment of the present invention. [Figure 4] FIG. 1 is a schematic cross-sectional view showing the configuration of a piezoelectric element including a piezoelectric film. [Figure 5] 3 is a flowchart illustrating a method for manufacturing a piezoelectric element according to an embodiment of the present invention. [Figure 6] FIG. 10 is a schematic cross-sectional view showing an example of another configuration of a piezoelectric element. [Figure 7] FIG. 10 is a schematic cross-sectional view showing an example of another configuration of a piezoelectric element. DETAILED DESCRIPTION OF THE INVENTION

[0011] Hereinafter, embodiments of the present invention will be described in detail. To facilitate understanding of the description, the same components in each drawing will be assigned the same reference numerals, and duplicate explanations will be omitted. The scale of each member in the drawings may differ from the actual scale. In this specification, unless otherwise specified, "to" indicating a range of values ​​means that the values ​​before and after it are included as the lower and upper limits.

[0012] A method for manufacturing a piezoelectric film according to this embodiment will be described. Before describing a method for manufacturing a piezoelectric film according to this embodiment, a piezoelectric film obtained by the method for manufacturing a piezoelectric film according to this embodiment will be described.

[0013] [Piezoelectric film] FIG. 1 is a schematic cross-sectional view showing the configuration of a piezoelectric film. As shown in FIG. 1, the piezoelectric film 1 includes a first crystalline film 11 and a second crystalline film 12 laminated in this order, and is provided on an amorphous film 2. The piezoelectric film 1 can be used in a piezoelectric element in a state where it is provided on the amorphous film 2. The piezoelectric film 1 is formed on the amorphous film 2 by the piezoelectric film manufacturing method according to this embodiment, and can have high crystal orientation and low film stress. Details of the crystal orientation and film stress will be described later. In this embodiment, the piezoelectric film 1 may have at least one of the first crystalline film 11 and the second crystalline film 12 formed of two or more layers.

[0014] In this specification, the thickness direction (vertical direction) of the piezoelectric film 1 is defined as the Z-axis direction, and the lateral direction (horizontal direction) perpendicular to the thickness direction is defined as the X-axis direction. The second crystal film 12 side of the Z-axis direction is defined as the +Z-axis direction, and the first crystal film 11 side is defined as the -Z-axis direction. In the following explanation, for convenience of explanation, the +Z-axis direction will be referred to as up or upward, and the -Z-axis direction will be referred to as down or downward, but this does not represent a universal up-down relationship.

[0015] 1, the first crystalline film 11 is provided on the amorphous film 2. The first crystalline film 11 contains a piezoelectric material having a wurtzite crystal structure (wurtzite crystal material) as a main component, and contains Ar as an additive element.

[0016] The term "main component" means that the content of the wurtzite crystal material is 95 atm % or more, preferably 98 atm % or more, and more preferably 99 atm % or more.

[0017] The wurtzite crystal structure of the first crystalline film 11 is represented by the general formula AB (A is an electropositive element and B is an electronegative element). The wurtzite crystal material has a hexagonal unit cell and a polarization vector parallel to the c-axis.

[0018] The wurtzite crystal material preferably exhibits piezoelectric properties above a certain level and can be crystallized in a low-temperature process at 200°C or less. The wurtzite crystal material is represented by the general formula AB, and contains a positive element A selected from the group consisting of Zn, Al, Ga, Cd, and Si. Examples of wurtzite crystal materials that can be used include zinc oxide (ZnO), zinc sulfide (ZnS), zinc selenide (ZnSe), zinc telluride (ZnTe), aluminum nitride (AlN), gallium nitride (GaN), cadmium selenide (CdSe), cadmium telluride (CdTe), and silicon carbide (SiC). Among these, ZnO is preferred as the wurtzite crystal material because it is relatively easy to achieve good c-axis orientation even in a low-temperature process. These materials may be used alone or in combination. When two or more wurtzite crystal materials are used in combination, one or more of these components may be contained as the main component, and other components may be contained as optional components.

[0019] The wurtzite crystal material preferably contains ZnO, and more preferably consists essentially of ZnO. "Substantially" means that the wurtzite crystal material may contain, in addition to ZnO, inevitable impurities that may be inevitably contained during the manufacturing process.

[0020] When two or more wurtzite crystal materials are used in combination, the respective piezoelectric films may be laminated.

[0021] In addition to the above-mentioned ZnO, ZnS, ZnSe, and ZnTe, the wurtzite crystal material may contain alkaline earth metals such as Mg, Ca, and Sr, or metals such as vanadium (V), titanium (Ti), zirconium (Zr), silica (Si), and lithium (Li) in a predetermined range of proportion. These components may be contained in the elemental state or in the oxide state. For example, if the wurtzite crystal material contains Mg in addition to ZnO, the Mg can be contained as MgO. These components can distort the crystal lattice of ZnO by entering the Zn site of ZnO, thereby improving the piezoelectric properties.

[0022] As described above, the first crystalline film 11 contains Ar as an additive element, and may contain at least one of Kr, Xe, and Rn in an amount smaller than that of Ar.

[0023] The content of Ar in the first crystalline film 11 is not particularly limited, and may be within a range that allows the first crystalline film 11 to have a wurtzite crystal structure.

[0024] There are no particular limitations on the method for measuring the Ar content in the first crystalline film 11 as long as it is a measurable method. The Ar content in the first crystalline film 11 may be measured, for example, by Rutherford backscattering spectroscopy (RBS) using a Pelletron 3SDH (manufactured by NEC Corporation) as a measuring device, or by secondary ion mass spectrometry using dynamic SIMS (D-SIMS) or the like.

[0025] The thickness of the first crystalline film 11 is preferably 1 nm to 20 nm, more preferably 3 nm to 15 nm, and even more preferably 5 nm to 10 nm. If the thickness of the first crystalline film 11 is 1 nm to 20 nm, the second crystalline film 12 can be formed to have high crystal orientation even if the non-crystalline film 2 is amorphous.

[0026] In this specification, the thickness of the first crystalline film 11 refers to the length in the direction perpendicular to the main surface of the first crystalline film 11. The thickness of the first crystalline film 11 may be, for example, the thickness measured at an arbitrary location on the cross section of the first crystalline film 11, or may be the average value of measurements measured at several arbitrary locations. Hereinafter, the definition of thickness is the same for other members.

[0027] The second crystalline film 12 contains a wurtzite crystalline material as a main component and Kr as an additive element. The wurtzite crystalline material of the second crystalline film 12 contained in the first crystalline film 11 is similar to the wurtzite crystalline material, so details are omitted.

[0028] As described above, the second crystalline film 12 contains Kr as an additive element, and may contain at least one of Ar, Xe, and Rn in an amount smaller than that of Kr.

[0029] The content of Kr in the second crystalline film 12 is not particularly limited, and may be within a range that can enhance the c-axis orientation of the second crystalline film 12 and suppress an increase in film stress.

[0030] The Kr content in the second crystalline film 12 can be measured by RBS or the like, similarly to the Ar content.

[0031] The thickness of the second crystalline film 12 is not particularly limited, and when applied to a piezoelectric element, the second crystalline film 12 should have sufficient piezoelectric properties, i.e., polarization properties proportional to pressure, and should also reduce the occurrence of cracks in the first crystalline film 11, suppress leak paths between electrodes, and stably exhibit piezoelectric properties. The thickness of the second crystalline film 12 may be, for example, 5 μm or less.

[0032] The film density of the second crystalline film 12 is not particularly limited and can be designed as appropriate, as long as it is within a range that can enhance the crystalline orientation of the second crystalline film 12 and suppress an increase in film stress. Note that, like the first crystalline film 11, the method for measuring the film density is not particularly limited and, for example, XRR or the like can be used.

[0033] The amorphous film 2 has a piezoelectric film 1 provided on its upper principal surface (top surface). Any material can be used for the amorphous film 2. Examples of such materials include oxide conductive films such as ITO (indium tin oxide), IZO (indium zinc oxide), IZTO (indium zinc tin oxide), and IGZO (indium gallium zinc oxide), metal films, and laminated films in which an insulating amorphous film is provided on a crystalline transparent conductive film. Note that the amorphous film 2 is not necessarily formed only from amorphous components, and may also include locally crystalline portions.

[0034] The amorphous film 2 can suppress unevenness and grain boundaries at the interface between the amorphous film 2 and the first crystalline film 11. Therefore, the amorphous film 2 can suppress unevenness on the surface of the amorphous film 2 and the generation of grain boundaries that can cause leak paths. In addition, the upper first crystalline film 11 can grow with good crystal orientation without being affected by the crystal orientation of the amorphous film 2.

[0035] As described above, the piezoelectric film 1 is obtained by the method for manufacturing a piezoelectric film according to this embodiment, and can have high crystal orientation and low film stress.

[0036] The crystalline orientation of the piezoelectric film 1 is indicated by the full width at half maximum (FWHM) obtained when the surface of the piezoelectric film 1 is measured by the X-ray rocking curve (XRC) method. That is, the crystalline orientation of the piezoelectric film 1 is expressed by the FWHM of the peak waveform of the rocking curve obtained when the diffraction from the (0002) plane of the crystal of the piezoelectric material contained as the main component of the piezoelectric film 1 is measured by the XRC method. When the piezoelectric material contained in the piezoelectric film 1 has a wurtzite crystal structure such as ZnO, the FWHM indicates the degree of parallelism of the crystals constituting the piezoelectric material in the c-axis direction. Therefore, the FWHM of the peak waveform of the rocking curve obtained by the XRC method can be used as an indicator of the c-axis orientation of the piezoelectric film 1. Therefore, the smaller the FWHM of the rocking curve, the better the crystalline orientation of the piezoelectric film 1 in the c-axis direction can be evaluated.

[0037] Furthermore, the crystal orientation of the piezoelectric film 1 can be evaluated using the XRC method, measuring the diffraction from the (0002) plane of the crystals of the piezoelectric material contained in the piezoelectric film 1, and also using the peak intensity in addition to the FWHM of the rocking curve. That is, the crystal orientation of the piezoelectric film 1 can also be evaluated using the value obtained by dividing the integrated value of the peak intensity by the FWHM as an evaluation value. In this case, the stronger the peak intensity of the rocking curve and the smaller the FWHM, the better the c-axis orientation of the piezoelectric material can be evaluated. Therefore, the greater the evaluation value obtained by dividing the integrated value of the peak intensity by the FWHM, the better the crystal orientation of the piezoelectric film 1 can be evaluated.

[0038] The method for evaluating the film stress of the piezoelectric film 1 is not particularly limited as long as it can evaluate the film stress of the piezoelectric film 1, and various measurement methods can be used for evaluation. The film stress of the piezoelectric film 1 can be evaluated, for example, from the amount of warpage.

[0039] The amount of warpage of the piezoelectric film 1 can be determined by calculating the average height between the position where the piezoelectric film 1 contacts the substrate and each corner of the piezoelectric film 1 when the piezoelectric film 1 is placed on a substrate with the convex side facing downward. For example, as shown in FIG. 2, if the piezoelectric film 1 is shaped like a square (3 cm × 3 cm) with sides of 3 cm in plan view, the average height between the contact surface of the piezoelectric film 1 with the substrate and the four corners of the piezoelectric film 1 is defined as the amount of warpage of the piezoelectric film 1. If the amount of warpage is less than a predetermined value (e.g., 10 mm), the amount of warpage of the piezoelectric film 1 can be evaluated as good. Note that the amorphous film 2 is smooth and hardly warps. Therefore, even if the amount of warpage is measured when the piezoelectric film 1 is placed on the amorphous film 2, the measured amount of warpage can be evaluated as the amount of warpage of the piezoelectric film 1.

[0040] The crystal orientation and film stress of the piezoelectric film 1 are determined by the relative relationship between the thicknesses of the first crystalline film 11 and the second crystalline film 12. As described above, the second crystalline film 12 is formed to be thicker than the first crystalline film 11. Therefore, depending on the ratio of the thickness of the second crystalline film 12 to the thickness of the piezoelectric film 1, the crystal orientation and film stress of the piezoelectric film 1 can be evaluated as being substantially the crystal orientation and film stress of the second crystalline film 12.

[0041] [Method of manufacturing piezoelectric film] A method for manufacturing a piezoelectric film according to this embodiment will now be described. Fig. 3 is a flowchart showing a method for manufacturing a piezoelectric film according to this embodiment. As shown in Fig. 3, the method for manufacturing a piezoelectric film according to this embodiment includes an amorphous film preparation step (step S11), a first crystalline film formation step (step S12), and a second crystalline film formation step (step S13). Each step will be described below.

[0042] First, an amorphous film 2 is prepared. The method for forming the amorphous film 2 is not particularly limited, and may be either a dry process or a wet process. If a dry process is used as the method for forming the amorphous film 2, a thin amorphous film 2 can be easily formed. Examples of dry processes include sputtering and vapor deposition, and examples of wet processes include plating. By using sputtering as the method for forming the amorphous film 2, a thin amorphous film 2 with high density can be easily obtained. For this reason, sputtering is preferred as the method for forming the amorphous film 2. Alternatively, a commercially available product may be used as the amorphous film 2.

[0043] A first crystalline film 11 is formed above the amorphous film 2 by sputtering using a target containing a wurtzite-type crystalline material in a gas atmosphere containing Ar (first crystalline film forming step: step S12).

[0044] As the sputtering method, for example, DC (direct current) or RF (radio frequency) magnetron sputtering method can be used.

[0045] The amorphous film 2 is placed on a film formation plate that serves as an anode in a film formation chamber of a sputtering device. The film formation plate may be, for example, rotatable. By placing the amorphous film 2 on the film formation plate, the first crystalline film 11 can be formed on the amorphous film 2 in a batchwise manner.

[0046] Alternatively, the amorphous film 2 may be wound around a drum roll, which is a film-forming roll, instead of a film-forming plate, as an anode. By placing a drum roll in the film-forming chamber, it becomes possible to continuously form the first crystalline film 11 on the amorphous film 2 while transporting the amorphous film 2 in a roll-to-roll manner.

[0047] A target containing a wurtzite crystalline material is used as the cathode.

[0048] The target containing the wurtzite crystal material may be a single or multiple targets containing the wurtzite crystal material contained as a main component in the first crystalline film 11. The single or multiple targets are arranged to face the film formation plate with a gap therebetween.

[0049] When multiple targets are used as cathodes, each target contains a different type of material that constitutes the wurtzite crystalline material contained as the main component of the first crystalline film 11. When multiple targets are used, for example, a target containing Zn, a target containing Si or Sn, and a target containing Al or Mg can be used. Furthermore, each target may be a metal oxide target containing oxygen. It is preferable to arrange the multiple targets in the film formation chamber at intervals. During sputtering, the power applied to each target is adjusted according to the type of wurtzite crystalline material contained in the first crystalline film 11, thereby adjusting the atomic ratio between the materials that constitute the first crystalline film 11.

[0050] When a single target is used as the cathode, the single target contains the wurtzite crystalline material contained in the first crystalline film 11. When a single target is used, an alloy target can be used in which the atomic ratio of the wurtzite crystalline material contained in the first crystalline film 11 is adjusted. For example, an alloy target containing Zn, Si or Sn, and Al or Mg can be used. The alloy target may be a metal oxide target containing a wurtzite crystalline material and oxygen.

[0051] Ar gas and oxygen gas are supplied as sputtering gases into a film formation chamber of a sputtering apparatus to create a gas atmosphere containing Ar and oxygen in the film formation chamber. A mixed gas containing Ar gas and oxygen gas may be supplied into the film formation chamber, or Ar gas and oxygen gas may be supplied separately.

[0052] The degree of vacuum in the deposition chamber was 1.0×10 -4 Pa or less is sufficient, 1.0 × 10 -6Pa~1.0×10 -4 It is preferable to adjust the pressure to 1.0×10 Pa. -5 Pa ~ 6.0 × 10 -5 It is more preferable that the vacuum level in the film formation chamber is within the above-mentioned preferable range. When the vacuum level in the film formation chamber is within the above-mentioned preferable range, the first crystalline film 11 is easily formed so as to contain a wurtzite crystalline material.

[0053] The pressure of the gas atmosphere in the film formation chamber during sputtering is not particularly limited, as long as it is within a range in which Ar is likely to be incorporated into the crystal lattice of the piezoelectric material that constitutes the wurtzite crystal material and which allows the formation of the first crystalline film 11. The pressure of the gas atmosphere may be, for example, 2.0 Pa or less.

[0054] When Ar gas and oxygen gas are supplied to the deposition chamber to form a gas atmosphere containing Ar gas and oxygen gas, the ratio of the flow rate of oxygen to the total flow rate of Ar and oxygen in this gas atmosphere can be appropriately selected depending on the type of gas, the oxygen content in the wurtzite crystal material, etc., and is preferably 0.1% to 20.0%, and more preferably 1.0% to 10.0%. If the oxygen flow rate ratio is within the above-mentioned preferred range, the wurtzite crystal material is likely to be formed in the first crystal film 11.

[0055] Next, the second crystalline film 12 is formed on the upper surface of the first crystalline film 11 by sputtering using a target containing a wurtzite-type crystalline material in a gas atmosphere containing Kr (second crystalline film forming step: step S13).

[0056] The second crystal film formation process (step S13) is similar to the first crystal film formation process (step S12) except that Kr gas is used instead of Ar gas as the sputtering gas and the pressure of the gas atmosphere is different, so only the differences will be explained.

[0057] In the second crystalline film formation process (step S13), Kr gas and oxygen gas are used as sputtering gases. Kr gas and oxygen gas are supplied into the film formation chamber of the sputtering device, and a gas atmosphere containing Kr gas and oxygen gas is created in the film formation chamber. A mixed gas containing Kr gas and oxygen gas may be supplied into the film formation chamber, or Kr gas and oxygen gas may be supplied separately.

[0058] The pressure of the gas atmosphere in the film formation chamber during sputtering is not particularly limited, as long as it is within a range in which Kr is easily incorporated into the crystal lattice of the piezoelectric material that constitutes the wurtzite crystal material and the second crystalline film 12 can be formed. The pressure of the gas atmosphere may be, for example, 5 Pa or less.

[0059] As a result, the piezoelectric film 1 is obtained.

[0060] After the second crystalline film 12 is formed, the entire piezoelectric film 1 may be heat-treated at a temperature (for example, 130° C.) lower than the melting point or glass transition point of the amorphous film 2. This heat treatment crystallizes the amorphous film 2, thereby reducing its resistance.

[0061] As described above, the method for manufacturing a piezoelectric film according to this embodiment includes a first crystalline film forming step (step S12) and a second crystalline film forming step (step S13). In the first crystalline film forming step (step S12), the first crystalline film 11 containing a wurtzite crystalline material as a main component and containing Ar is formed on the amorphous film 2, and in the second crystalline film forming step (step S13), the second crystalline film 12 containing a wurtzite crystalline material as a main component and containing Kr is formed on the first crystalline film 11.

[0062] By forming the first crystalline film 11 on the amorphous film 2, even if the amorphous film 2 is amorphous, the orientation of the second crystalline film 12 formed on the first crystalline film 11 can be made to depend on the orientation of the crystals of the wurtzite crystalline material contained in the first crystalline film 11. Because the first crystalline film 11 and the second crystalline film 12 contain the same wurtzite crystalline material as the piezoelectric material, when the second crystalline film 12 is formed on the first crystalline film 11, the crystals of the wurtzite crystalline material contained in the second crystalline film 12 can be grown in alignment with the orientation of the crystals contained in the first crystalline film 11. Therefore, the first crystalline film 11 functions as a so-called underlayer for the second crystalline film 12, and when the second crystalline film 12 is formed on the first crystalline film 11, the crystalline orientation of the wurtzite crystalline material contained in the second crystalline film 12 can be improved.

[0063] Furthermore, when an inert gas is used as the sputtering gas, the first crystalline film 11 is formed by sputtering in a gas atmosphere containing Ar. Because Ar has a smaller atomic weight than other inert gases such as Kr, the number of recoil ions, such as Ar ions and oxygen ions, that strike the amorphous film 2 due to recoil from the target is likely to increase. Therefore, during the formation of the first crystalline film 11, the surface of the amorphous film 2 is bombarded by recoil Ar particles, which are high-energy particles, while the first crystalline film 11 is formed on the amorphous film 2. As a result, the first crystalline film 11 tends to have high crystallinity and film stress. On the other hand, the second crystalline film 12 is formed by sputtering in a gas atmosphere containing Kr. Because Kr has a larger atomic weight than Ar, the number of recoil ions, such as Kr ions and oxygen ions, that strike the first crystalline film 11 due to recoil from the target is less than the number of attacks on the amorphous film 2 by Ar ions and oxygen ions during the formation of the first crystalline film 11. Therefore, the surface of the first crystalline film 11 tends to be less likely to be bombarded by high-energy particles. Therefore, when the second crystalline film 12 is formed, it tends to be formed while suppressing the film stress of the amorphous film 2. Therefore, by including Kr in the second crystalline film 12, the second crystalline film 12 can have an improved orientation while reducing the film stress occurring in the second crystalline film 12. The piezoelectric characteristics of the piezoelectric film 1 depend on the characteristics of the first crystalline film 11 and the second crystalline film 12. Therefore, by using the method for manufacturing the piezoelectric film 1, it is possible to manufacture a piezoelectric film 1 on the amorphous film 2 that has an improved crystal orientation and low film stress.

[0064] Generally, piezoelectric films can improve their piezoelectric properties by orienting their crystalline polarization axis in the vertical axis direction of the substrate and achieving high crystalline orientation. When a piezoelectric film is a crystalline film, a highly oriented piezoelectric film can be obtained by epitaxially growing it on a single-crystal substrate corresponding to the orientation plane. Therefore, it is relatively difficult to grow a highly oriented crystalline film on a crystalline underlayer with low lattice matching. Therefore, in order to obtain a highly oriented crystalline film as a piezoelectric film, the selection of materials for the crystalline underlayer that serves as the underlayer is limited. Even if an amorphous film is used as the crystalline underlayer, it is difficult to grow a highly oriented crystalline film by epitaxially growing a crystalline film on an amorphous film.

[0065] According to the method for manufacturing a piezoelectric film of this embodiment, a first crystalline film 11 is formed on an amorphous film 2, and then a second crystalline film 12 is formed on the first crystalline film 11. This allows the second crystalline film 12 to be formed with high orientation while suppressing an increase in film stress. The higher the crystalline orientation of the piezoelectric film 1, the higher the energy conversion efficiency from electrical energy to mechanical energy, and therefore the greater the displacement of the piezoelectric film 1 in the thickness direction. Furthermore, the more the film stress of the piezoelectric film 1 is suppressed, the more it is possible to suppress the occurrence of cracks or breaks in at least one of the first crystalline film 11 and the second crystalline film 12 that constitute the piezoelectric film 1. Therefore, according to the method for manufacturing a piezoelectric film of this embodiment, a piezoelectric film 1 that exhibits high piezoelectric efficiency in the thickness direction and has excellent durability can be obtained.

[0066] In the piezoelectric film manufacturing method according to this embodiment, the piezoelectric material used in the first crystalline film forming step (step S12) and the second crystalline film forming step (step S13) can contain Zn. This allows the first crystalline film 11 and the second crystalline film 12 to contain ZnO as a wurtzite crystalline material. ZnO has a wurtzite crystalline structure and exhibits higher crystal orientation in the c-axis direction than piezoelectric materials with other crystalline structures. This ensures that the first crystalline film 11 and the second crystalline film 12 have high crystal orientation. Furthermore, since ZnO has a high correlation between crystal orientation and piezoelectric properties, it is easy to improve energy conversion efficiency. Therefore, according to the piezoelectric film manufacturing method according to this embodiment, the crystal orientation of the resulting piezoelectric film 1 can be improved with increased certainty. When the piezoelectric film 1 is applied to a piezoelectric element, the piezoelectric properties of the piezoelectric element can be improved.

[0067] In the method for manufacturing a piezoelectric film according to this embodiment, sputtering can be performed using a target containing a piezoelectric material in at least one of the first crystalline film formation step (step S12) and the second crystalline film formation step (step S13). This allows the type of piezoelectric material contained in the first crystalline film 11 and the second crystalline film 12 to be easily changed depending on the type of target used. Therefore, according to the method for manufacturing a piezoelectric film according to this embodiment, when forming at least one of the first crystalline film 11 and the second crystalline film 12, the type of piezoelectric material contained in at least one of the first crystalline film 11 and the second crystalline film 12 can be easily changed depending on the application.

[0068] Since the piezoelectric film 1 has the above-mentioned properties, it can be suitably used as a piezoelectric layer of a piezoelectric element.

[0069] [Piezoelectric element] A piezoelectric element including a piezoelectric obtained by the method for manufacturing a piezoelectric film according to this embodiment will be described. The piezoelectric element includes a first electrode, a piezoelectric layer, and a second electrode on a substrate, and the piezoelectric film 1 according to this embodiment shown in FIG. 1 is used for the piezoelectric layer.

[0070] Fig. 4 is a schematic cross-sectional view showing the configuration of a piezoelectric element. As shown in Fig. 4, the piezoelectric element 20A includes a substrate 21, a first electrode 22, a piezoelectric layer 23, and a second electrode 24, which are laminated in this order from the substrate 21 side. The piezoelectric layer 23 includes a first crystalline film 231 and a second crystalline film 232, which are laminated in this order. Since the piezoelectric layer 23 is formed of the piezoelectric film 1 according to this embodiment shown in Fig. 1, a description of the piezoelectric layer 23 will be omitted. Note that the piezoelectric element 20A may not include the second electrode 24 depending on the application, etc.

[0071] The base material 21 is a substrate on which the first electrode 22 is disposed. Any material can be used for the base material 21, and a plastic base material, a silicon (Si) substrate, a metal plate, a glass base material, or the like can be used.

[0072] When a plastic substrate is used, it is preferable to use a flexible material that can impart flexibility to the piezoelectric element including the piezoelectric layer 23 .

[0073] Examples of materials that can be used to form the plastic substrate include polyethylene terephthalate (PET), polyethylene naphthalate (PEN), polycarbonate (PC), acrylic resin, cycloolefin polymer, and polyimide (PI). Among these materials, PET, PEN, PC, acrylic resin, and cycloolefin polymer are transparent materials and are suitable for use in piezoelectric elements having a piezoelectric layer 23 when the electrodes used are transparent. Furthermore, when optical transparency is not required for piezoelectric elements having a piezoelectric layer 23, such as in healthcare products such as pulse monitors and heart rate monitors, or in-vehicle pressure detection sheets, the plastic substrate may be formed from the above-mentioned materials or a translucent or opaque plastic material.

[0074] The thickness of the substrate 21 is not particularly limited, and can be any thickness appropriate depending on the application of the piezoelectric element 20A, the material of the substrate 21, etc. For example, when the substrate 21 is a plastic substrate, the thickness of the substrate 21 may be 1 μm to 250 μm. The method for measuring the thickness of the substrate 21 is not particularly limited, and any measuring method can be used.

[0075] The first electrode 22 is provided on the upper main surface (top surface) of the substrate 21. The first electrode 22 can be made of the same material as the amorphous film 2 used in the piezoelectric film 1 according to this embodiment shown in FIG. 1. If the substrate 21 is a conductive material such as a metal plate, the substrate 21 can also function as an electrode, and therefore the first electrode 22 does not need to be provided. In this case, a metal plate may be used as the substrate 21, and a laminate having the amorphous film 2 provided thereon may be used.

[0076] The first electrode 22 may be formed as a thin film on a part of or the entire surface of the base material 21, or a plurality of first electrodes 22 may be provided in parallel stripes.

[0077] The thickness of the first electrode 22 can be designed appropriately, and is preferably 3 nm to 100 nm, and more preferably 10 nm to 50 nm, for example. If the thickness of the first electrode 22 is within the above-mentioned preferred range, the function as an electrode can be exhibited and the piezoelectric element 20A can be made thinner.

[0078] The second electrode 24 can be provided on the second crystalline film 12. The second electrode 24 can be made of any conductive material, and the same material as the first electrode 22 can be used.

[0079] Similar to the first electrode 22, the second electrode 24 may be formed as a thin film on a part of or the entire surface of the second crystalline film 12, or a plurality of second electrodes 24 may be provided in parallel stripes.

[0080] [Manufacturing method of piezoelectric element] A method for manufacturing a piezoelectric element according to this embodiment will now be described. Fig. 5 is a flowchart showing a method for manufacturing a piezoelectric element according to this embodiment. As shown in Fig. 5, the method for manufacturing a piezoelectric element according to this embodiment includes a first electrode forming step (step S21), a first crystal film forming step (step S22), a second crystal film forming step (step S23), and a second electrode forming step (step S24). Each step will now be described.

[0081] First, the first electrode 22 is formed on the upper surface of the base material 21 (first electrode forming step: step S21).

[0082] The method for forming the first electrode 22 is not particularly limited, and may be either a dry process or a wet process. If a dry process is used as the method for forming the first electrode 22, a thin first electrode 22 can be easily formed. Examples of dry processes include sputtering and vapor deposition, and examples of wet processes include plating. If sputtering is used as the method for forming the first electrode 22, a thin first electrode 22 can be easily formed. Therefore, sputtering is preferred as the method for forming the first electrode 22.

[0083] The first electrode 22 may be formed on the entire upper surface of the substrate 21. Alternatively, the first electrode 22 may be formed into an arbitrary shape by processing the first electrode 22 into a pattern having a predetermined shape by etching or the like. For example, the first electrode 22 may be patterned into stripes and a plurality of first electrodes 22 may be arranged in the form of stripes.

[0084] Next, a wurtzite crystal material is sputtered above the first electrode 22 in a gas atmosphere containing Ar and oxygen using a target containing the wurtzite crystal material (first crystal film forming step: step S22). The first crystal film forming step (step S22) is similar to the first crystal film forming step (step S12) in the piezoelectric film manufacturing method according to the present embodiment described above, and therefore details thereof will be omitted.

[0085] Next, a wurtzite crystal material is sputtered onto the upper surface of the first crystal film 11 in a gas atmosphere containing Kr and oxygen using a target containing the wurtzite crystal material by sputtering to form the second crystal film 12 (second crystal film forming step: step S13). The second crystal film forming step (step S23) is similar to the second crystal film forming step (step S13) in the piezoelectric film manufacturing method according to the present embodiment described above, and therefore details thereof will be omitted.

[0086] Next, a second electrode 24 having a predetermined shape is formed on the upper surface of the second crystalline film 12 (second electrode forming step: step S24).

[0087] The second electrode 24 can be formed using the same method as that for the first electrode 22 .

[0088] The second electrode 24 may be formed on the entire surface of the second crystalline film 12, or may be formed in any suitable shape. For example, when the first electrode 22 is patterned in stripes, the second electrode 24 may be formed so that multiple stripes extend in a direction perpendicular to the direction in which the stripes of the first electrode 22 extend in a plan view.

[0089] As a result, the piezoelectric element 20A is obtained.

[0090] After the second electrode 24 is formed, the entire piezoelectric element 20A may be heat-treated at a temperature (for example, 130°C) lower than the melting point or glass transition point of the base material 21. This heat treatment can crystallize the first electrode 22 and the second electrode 24, thereby reducing their resistance. The heat treatment is not essential, and may not be performed after the formation of the piezoelectric element 20A in cases such as when the base material 21 is made of a material that is not heat-resistant.

[0091] In this way, the piezoelectric element 20A includes the piezoelectric layer 23 between the first electrode 22 and the second electrode 24, and the piezoelectric layer 23 exhibits excellent piezoelectric properties and can reduce film stress, thereby stably exhibiting high piezoelectric efficiency in the thickness direction of the piezoelectric layer 23. Therefore, the piezoelectric element 20A can maintain its excellent piezoelectric properties for a long period of time.

[0092] The piezoelectric element 20A has excellent piezoelectric properties and can therefore be suitably used in piezoelectric devices, such as devices that utilize the piezoelectric effect, such as force sensors for touch panels, pressure sensors, acceleration sensors, and acoustic emission (AE) sensors, as well as speakers, transducers, high-frequency filter devices, piezoelectric actuators, and optical scanners that utilize the inverse piezoelectric effect.

[0093] (Other aspects) In this embodiment, the piezoelectric element 20A is not limited to the above configuration, and may have another configuration as long as it has a first electrode 22 and a piezoelectric layer 23 on a substrate 21 and the piezoelectric layer 23 can exhibit excellent piezoelectric properties in the thickness direction. An example of another configuration of the piezoelectric element 20A is shown below.

[0094] As shown in FIG. 6, the piezoelectric element 20B does not necessarily have to include the second electrode 24.

[0095] As shown in FIG. 7, the piezoelectric element 20C may include an adhesive layer 26 between the piezoelectric layer 23 and the second electrode 24, and a substrate 27 on the upper surface of the second electrode 24.

[0096] The adhesive layer 26 suppresses leak paths caused by cracks or pinholes that occur in the piezoelectric layer 23. If metal grain boundaries or protrusions are present at the interface between the first electrode 22 and the piezoelectric layer 23 or the interface between the piezoelectric layer 23 and the second electrode 24, when a crack or the like occurs in any of the first electrode 22, the piezoelectric layer 23, or the second electrode 24, the crack or the like will form a leak path between the first electrode 22 and the second electrode 24, resulting in loss of polarization. By providing the adhesive layer 26 between the piezoelectric layer 23 and the second electrode 24, the piezoelectric element 20C suppresses the formation of leak paths and maintains good piezoelectric properties of the piezoelectric layer 23.

[0097] The base material 27 can be made of the same material as the base material 21 .

[0098] An example of a method for manufacturing the piezoelectric element 20C will be described. For example, a first laminate is formed by laminating a first electrode 22 and a piezoelectric layer 23 in this order on a substrate 21. Meanwhile, a second laminate is formed by forming a second electrode 24 on a substrate 27. Thereafter, the piezoelectric layer 23 and the second electrode 24 are bonded together via an adhesive layer 26 so that the piezoelectric layer 23 of the first laminate faces the second electrode 24 of the second laminate. In this way, the piezoelectric element 20C is manufactured.

[0099] The piezoelectric element 20C has a piezoelectric strain constant d 33 Since the value is large and the leak path between the electrodes can be suppressed, it is possible to have better piezoelectric properties. [Example]

[0100] Hereinafter, the embodiment will be described in more detail with reference to examples and comparative examples, but the embodiment is not limited to these examples and comparative examples.

[0101] <Manufacturing of piezoelectric elements> [Example 1] (Production of the first electrode) On a substrate (PET, thickness: 50 μm), a DC sputtering method (discharge power: 400 W, gas pressure: 0.4 Pa, ultimate vacuum: 6.0 × 10) was used in an atmosphere of a mixed gas of Ar gas and O2 gas (Ar gas: O2 gas = 99:1). -5 Using a SiO 2 thin film (In content: 10.4 wt %), an IZO film (amorphous film (first electrode)) was formed to a thickness of 100 nm.

[0102] (Fabrication of piezoelectric film) Next, a piezoelectric film consisting of a first crystalline film and a second crystalline film was formed on the obtained IZO film.

[0103] (Preparation of the first crystalline film) Next, on the obtained IZO film, a DC sputtering method (discharge power: 500 W, gas pressure: 0.2 Pa, ultimate vacuum: 6.0 × 10) was used in an atmosphere of a mixed gas of Ar gas and O2 gas (Ar gas: O2 gas = 87:13). -5 A Mg-doped ZnO thin film with a hexagonal wurtzite structure and a mass ratio of ZnO and MgO of 88 wt%:12 wt% was formed as the first crystalline film using a ZnO / MgO ion beam sputtering system (PA). The thickness of the first crystalline film was 10 nm.

[0104] (Preparation of the second crystalline film) Next, on the first crystal film, a mixture of Kr gas and O2 gas (Kr gas: O2 gas = 100:10) was deposited by DC sputtering (discharge power: 500 W, gas pressure: 0.7 Pa, ultimate vacuum: 6.0 × 10 -5 A Mg-doped ZnO thin film with a hexagonal wurtzite structure, containing ZnO and MgO in a mass ratio of 88 wt%:12 wt%, was deposited as the second crystalline film using a ZnO / MgO ion exchanger (PA). The thickness of the second crystalline film was 190 nm.

[0105] In this way, a piezoelectric element was fabricated, which included a first electrode and a piezoelectric film (first crystalline film and second crystalline film) laminated in this order on the substrate.

[0106] In addition, samples similar to the first and second crystalline films prepared in the process of fabricating the piezoelectric element were prepared. The Ar or Kr content contained in the first and second crystalline film samples, as well as the crystal orientation and warpage of the piezoelectric film, were measured. The measurement results are shown in Table 1.

[0107] (Ar or Kr content) The Ar or Kr content in the prepared samples was measured using Rutherford backscattering spectroscopy (RBS) with a Pelletron 3SDH (NEC Corporation) under the following measurement conditions and evaluation criteria. When measuring the Ar or Kr content in the first and second crystalline films, a very small amount of Kr was sometimes measured in addition to Ar in the first crystalline film, and a very small amount of Ar was sometimes measured in addition to Kr in the second crystalline film. These very small amounts of Ar and Kr were determined to be trace amounts of Ar or Kr that had migrated from one film to the other through the interface between the first and second crystalline films after the second crystalline film was formed, and were treated as not being Ar or Kr contained in the first and second crystalline films. ((Measurement conditions)) Incident ions: 4He ++ Incident energy: 2300 keV ·Incidence angle: 0deg ·Scattering angle: 140deg Specimen current: 10nA Beam diameter: 2mmφ In-plane rotation: None ·Irradiation amount: 80μC

[0108] (crystal orientation) The surface of the prepared sample was measured by XRC to measure the diffraction from the (0002) plane of the crystal of the main component contained in the sample. The full width at half maximum (FWHM) of the peak waveform of the rocking curve obtained was calculated and used as the crystal orientation of the piezoelectric layer. The measurement results are shown in Table 1.

[0109] (Warpage amount) The amount of warping of the prepared samples was measured. The prepared samples were piezoelectric elements, and the amount of warping of the piezoelectric element corresponded to the amount of warping of the piezoelectric film, so the amount of warping of the piezoelectric element was evaluated as the amount of warping of the piezoelectric film.

[0110] The prepared sample was formed into a square in plan view and placed on a 3 cm square PET film. With the sample placed face down, the average height of the surface where the placed surface was in contact with the PET film and each of the four corners was calculated to determine the amount of warping of the piezoelectric film. A warping amount of 4.0 mm or less was evaluated as good. The measurement results are shown in Table 1.

[0111] [Comparative Example 1] The same procedure as in Example 1 was carried out except that the first crystal film was not formed and the thickness of the second piezoelectric layer was set to 200 nm.

[0112] Table 1 shows the measurement results of the characteristics of the piezoelectric film in each of the examples and comparative examples.

[0113] [Table 1]

[0114] From Table 1, it can be seen that in Example 1, the half-width of the piezoelectric film was 3.7°, which was below 4.0°, the point at which piezoelectricity begins to saturate, and therefore the crystal orientation was good. Furthermore, the amount of warping of the piezoelectric film was 3.60 mm, which means that the residual stress in the piezoelectric layer was also low and warping was suppressed to a low level. On the other hand, in Comparative Example 1, the half-width of the piezoelectric element was 4.4°, which means that the crystal orientation was low. Furthermore, the amount of warping of the piezoelectric layer was 5.27 mm, which means that the residual stress in the piezoelectric layer was also low and warping was not suppressed.

[0115] Therefore, it was confirmed that the piezoelectric film of Example 1 had small crystal orientation and suppressed warpage, maintained high piezoelectric properties, and satisfied the conditions for use. On the other hand, it was confirmed that the piezoelectric film of Comparative Example 1 had high crystal orientation and warpage, did not satisfy the conditions for use, and may have practical problems.

[0116] Therefore, unlike the piezoelectric element of Comparative Example 1, the piezoelectric element of Example 1 has a piezoelectric film consisting of a first crystalline film and a second crystalline film formed above the first electrode containing amorphous material, which can prevent breakage or cracks from occurring in the piezoelectric film.Therefore, it can be said that a piezoelectric element that can be equipped with a piezoelectric film can exhibit excellent piezoelectric properties.

[0117] Although the embodiments have been described above, they are presented as examples and the present invention is not limited to the above embodiments. The above embodiments can be implemented in various other forms, and various combinations, omissions, substitutions, modifications, etc. can be made without departing from the spirit of the invention. These embodiments and their modifications are included within the scope and spirit of the invention, and are also included in the scope of the invention and its equivalents as set forth in the claims.

[0118] This application claims priority based on Patent Application No. 2021-056824, filed with the Japan Patent Office on March 30, 2021, and the entire contents of Patent Application No. 2021-056824 are incorporated herein by reference. [Explanation of symbols]

[0119] 1 Piezoelectric film 2. Amorphous film 11, 231 First crystal film 12, 232 Second crystal film 20A, 20B, 20C Piezoelectric elements 21 Base material 22 First electrode 23 Piezoelectric layer 24 Second electrode

Claims

1. a piezoelectric film forming step of forming a piezoelectric film in which at least two types of crystalline films are laminated on an amorphous film; The piezoelectric film forming step includes: a first crystalline film forming step of sputtering a piezoelectric material on the amorphous film in a gas atmosphere containing Ar to form a first crystalline film containing the piezoelectric material as a main component and also containing Ar; a second crystal film forming step of sputtering the piezoelectric material on the first crystal film in a gas atmosphere containing Kr to form a second crystal film containing the piezoelectric material as a main component and also containing Kr; A method for manufacturing a piezoelectric film comprising:

2. The method for manufacturing a piezoelectric film according to claim 1 , wherein the piezoelectric material contains Zn.

3. 3. The method for producing a piezoelectric film according to claim 1, wherein at least one of the first crystalline film forming step and the second crystalline film forming step is performed by sputtering using a target containing the piezoelectric material.

4. a first electrode forming step of forming the amorphous film as a first electrode on at least one main surface of a substrate; A method for producing a piezoelectric film according to any one of claims 1 to 3; A method for manufacturing a piezoelectric element, comprising:

5. A method for manufacturing a piezoelectric device, comprising the method for manufacturing a piezoelectric element according to claim 4.

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