Piezoelectric element and method for manufacturing the same

Abstract

To provide a piezoelectric element that improves a durability time while controlling the crystal orientation of a piezoelectric material of the piezoelectric element, and a manufacturing method of the piezoelectric element.SOLUTION: A piezoelectric element 30 according to the present invention includes a Pt film 23 of a lower electrode provided on the plane of an Si substrate 20, a PZT film 25 provided on the Pt film 23, and a Pt film 26 of the upper electrode provided on the PZT film 25. The PZT film 25 includes a vertically oriented portion 25a having a c-axis in a direction perpendicular to the Pt film 23, and a tilted alignment portion 25b having a c-axis inclined with respect to the c-axis of the vertically oriented portion 25a. The c-axis distribution of the tilted alignment portion 25b is discrete with respect to the c-axis of the vertically oriented portion 25a.SELECTED DRAWING: Figure 7

Description

[Technical field]

[0001] The present invention relates to a piezoelectric element used in an optical deflector or the like, and a method for manufacturing the piezoelectric element. [Background technology]

[0002] In recent years, there has been a growing need for sensor elements and actuator elements that are made up of systems with microstructures such as MEMS (Micro Electro Mechanical Systems). For this reason, progress has been made in the development of a direct thin film deposition method for depositing a piezoelectric crystal film directly on a silicon wafer.

[0003] In particular, in piezoelectric actuators for MEMS that use a crystalline film of lead zirconate titanate (PZT) as the piezoelectric material, orientation control is essential to obtain high piezoelectric characteristics.

[0004] For example, the piezoelectric actuator of Patent Document 1 has a support and a piezoelectric body formed on the support, and is equipped with a plurality of piezoelectric cantilevers that undergo bending deformation by piezoelectric drive, and is equipped with a plurality of independent electrodes for applying a drive voltage to the piezoelectric bodies of the plurality of piezoelectric cantilevers. The ends of the plurality of piezoelectric cantilevers are mechanically linked so that the bending deformations of each are accumulated, and each piezoelectric cantilever is bent and deformed independently by application of the drive voltage.

[0005] In the piezoelectric actuator, the torque and displacement generated at the tip of the piezoelectric cantilever depend on the piezoelectric properties of the piezoelectric body and the size of the cantilever. In other words, in order to obtain high piezoelectric properties, it is preferable to control the crystal orientation of the crystalline film of the piezoelectric material. [Prior art documents] [Patent documents]

[0006] [Patent Document 1] JP 2008-35600 A Summary of the Invention [Problem to be solved by the invention]

[0007] Although it was possible to obtain high piezoelectric properties by controlling the crystal orientation of the PZT crystalline film, which is the piezoelectric material used in piezoelectric actuators (piezoelectric elements), there was a problem in that it was not possible to obtain sufficient durability.

[0008] The present invention has been made in consideration of the above circumstances, and aims to provide a piezoelectric element that improves the durability while controlling the crystal orientation of the piezoelectric material of the piezoelectric element, and a method for manufacturing the piezoelectric element. [Means for solving the problem]

[0009] The piezoelectric element of the first invention comprises a substrate having at least one flat surface, a first electrode film provided on the flat surface of the substrate, a tetragonal piezoelectric crystal film provided on the first electrode film, and a second electrode film provided on the surface of the piezoelectric crystal film facing the first electrode film, wherein the piezoelectric crystal film is a uniaxially oriented polycrystalline body consisting of columnar crystal grains whose tetragonal c-axes are oriented in a direction perpendicular to the first electrode film, and the polycrystalline body includes a vertically oriented portion having a c-axis in a direction perpendicular to the first electrode film, and an inclined orientation portion having a c-axis inclined with respect to the c-axis of the vertically oriented portion, wherein the c-axis of the vertically oriented portion and the c-axis of the inclined orientation portion each have a distribution, and the c-axis distribution of the inclined orientation portion is discrete with respect to the c-axis distribution of the vertically oriented portion.

[0010] The piezoelectric element of the present invention comprises a first electrode film, a tetragonal piezoelectric crystal film on the first electrode film, and a second electrode film on the piezoelectric crystal film. Here, "on" includes the case where there is no direct contact with the film on the lower surface side. A tetragonal crystal is a crystal in which the axial lengths a, b, and c of the unit lattice satisfy the relationship a=b≠c. Also, in the case of a tetragonal crystal, <100> The direction is the a-axis direction. <010> The direction is the b-axis direction. <001> The direction is the c-axis direction. <001> The planes perpendicular to the axis are the (001) plane and the c plane.

[0011] The piezoelectric crystal film (columnar grain boundaries) of the present invention includes a vertically oriented portion and an inclined orientation portion having a c-axis oriented at an angle from the c-axis of the vertically oriented portion, and when a voltage is applied (when the voltage is increased), it displaces without impairing its characteristics, and when the voltage application is stopped (when the voltage is decreased), it returns to its original state. Furthermore, the piezoelectric crystal film of the present invention including the inclined orientation portion has a higher withstand voltage than a piezoelectric crystal film consisting only of a vertically oriented portion. Furthermore, the c-axis distribution of the inclined orientation portion is discrete compared to the c-axis distribution of the vertically oriented portion. This allows the piezoelectric element to have high long-term reliability without deteriorating its performance as a piezoelectric element, and has a high withstand voltage.

[0012] In the piezoelectric element of the first aspect of the invention, it is preferable that the c-axis of the inclined orientation portion is inclined in all circumferential directions of 0° to 360° with the c-axis of the vertical orientation portion as a rotation axis.

[0013] The c-axis of the vertically oriented portion is perpendicular to the plane of the substrate, but the c-axis of the inclined orientation portion is inclined in the omnidirectional direction (0° to 360°) around the c-axis of the vertically oriented portion as the axis of rotation. Therefore, the c-axis of the inclined orientation portion extends at any angle in the radial direction with respect to the c-axis of the vertically oriented portion.

[0014] In the piezoelectric element of the first invention, the inclination angle φ of the c-axis of the inclined orientation portion with respect to the c-axis of the perpendicular orientation portion of the piezoelectric crystal film is preferably greater than 6° and less than 19°.

[0015] By setting the inclination angle φ of the c-axis of the inclined orientation portion to be greater than 6° and less than 19°, the piezoelectric crystal film including the inclined orientation portion can have improved electric field resistance and durability, and can prevent deterioration of the piezoelectric characteristics.

[0016] In the piezoelectric element of the first invention, it is preferable that the intensity ratio Rp of the inclined orientation portion to the perpendicular orientation portion in an X-ray rocking curve is 0.1 to 1.

[0017] By setting the intensity ratio Rp within the range of 0.1 to 1, it is possible to enhance the effect of the inclined orientation portion, while preventing the orientation of the piezoelectric crystal film from becoming more disordered and thus preventing the piezoelectric properties from deteriorating.

[0018] In the piezoelectric element of the first invention, it is preferable that the piezoelectric crystal film is made of lead zirconate titanate (PZT) and the first electrode film is made of platinum (Pt).

[0019] In the present invention, lead zirconate titanate (PZT) and platinum (Pt), which are optimal materials for the piezoelectric crystal film and the first electrode film, respectively, are used. In particular, by creating a piezoelectric element using PZT, which has high piezoelectric properties, as the piezoelectric crystal film, when the piezoelectric element is applied to an optical scanner module, for example, it can be driven at high speed with low voltage and can obtain a large scanning angle.

[0020] In the piezoelectric element of the first invention, the film surface of the first electrode film is preferably a (111) surface.

[0021] If the film surface of the first electrode film is a (111) plane, then when a piezoelectric crystal film is formed on the upper surface of the first electrode film, the crystal orientation characteristics can be improved.

[0022] A method for manufacturing a piezoelectric element according to a second aspect of the present invention includes the steps of: forming an amorphous metal oxide film on a substrate having at least one flat surface; forming a first electrode film on the metal oxide film, the first electrode film being a conductive metal film having a (111) surface; forming an SRO film on the first electrode film; forming a piezoelectric crystal film on the SRO film, the piezoelectric crystal film being a uniaxially oriented tetragonal polycrystal having columnar crystal grains in a direction perpendicular to the first electrode film, the columnar crystal grains having a vertical orientation portion having a c-axis in a direction perpendicular to the first electrode film and an inclined orientation portion having a c-axis inclined at an inclination angle φ of the vertical orientation portion relative to the c-axis in a range of more than 6° and less than 19°; and forming a second electrode film on a surface of the piezoelectric crystal film facing the first electrode film.

[0023] The method for manufacturing a piezoelectric element of the present invention includes the steps of forming a metal oxide film on a substrate, forming a first electrode film, forming an SRO film, forming a piezoelectric crystal film, and forming a second electrode film. The piezoelectric crystal film includes a vertically oriented portion and an inclined orientation portion (wherein the inclination angle φ of the c-axis is greater than 6° and less than 19°) having a c-axis that is inclined from the c-axis of the vertically oriented portion. When a voltage is applied (when the voltage is increased), the inclined orientation portion is displaced without losing its characteristics, and when the voltage application is stopped (when the voltage is decreased), it returns to its original state. This piezoelectric crystal film has a higher withstand voltage than a piezoelectric crystal film consisting only of a vertically oriented portion, and therefore the high withstand voltage allows the piezoelectric element to have high long-term reliability without deteriorating its performance.

[0024] In the method for producing a piezoelectric element according to the second aspect of the present invention, it is preferable that the step of forming the piezoelectric crystal film uses an arc discharge reactive ion plating method.

[0025] By using the arc discharge reactive ion plating method, it is possible to efficiently form a high-quality piezoelectric crystal film with high adhesion. [Brief description of the drawings]

[0026] [Figure 1] 1 is a schematic diagram of an optical scanner module including a piezoelectric element of the present invention. [Diagram 2] FIG. 2 is a perspective view of a two-dimensional optical deflector included in the optical scanner module. [Figure 3A] A diagram explaining the operation of the meandering type piezoelectric actuator of a two-dimensional optical deflector (1). [Figure 3B] A diagram explaining the operation of the meandering type piezoelectric actuator of a two-dimensional optical deflector (2). [Figure 4] FIG. 2 is a diagram for explaining details of a two-dimensional optical deflector. [Diagram 5] 3 is a flowchart for manufacturing a piezoelectric element of the present invention. [Figure 6] FIG. 2 is a diagram illustrating a cross-sectional structure of a piezoelectric element according to the present invention. [Figure 7]FIG. 7 is a diagram for explaining an image of the crystal orientation of a PZT film of the piezoelectric element in FIG. 6. [Figure 8] FIG. 1 is a diagram illustrating the results of X-ray diffraction θ-2θ measurement of a PZT film. [Figure 9] FIG. 1 is a diagram illustrating the results of X-ray rocking curve measurement of a PZT film. [Figure 10] 1 is a graph showing the relationship between the electric field and the leakage current density of a PZT film. [Figure 11] This is a table comparing the measurement results of the electric field resistance, durability time, and piezoelectric constant of the PZT film with those of the comparative form. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0027] Hereinafter, the piezoelectric element of the present invention and the method for manufacturing the piezoelectric element will be described with reference to the drawings.

[0028] 1 is a schematic diagram of an optical scanner module 1 including a piezoelectric element of the present invention. The optical scanner module 1 is a device used in, for example, a picoprojector, a head-up display, a vehicle headlight, etc., and is mainly composed of a two-dimensional optical deflector 2, a laser light source 3, and a control device 5.

[0029] The two-dimensional optical deflector 2 is manufactured using semiconductor processes and MEMS technology, and emits light while scanning it by reflecting light incident from a certain direction with a rotating mirror (micromirror 9).

[0030] The movable frame 2a of the two-dimensional optical deflector 2 has a micromirror 9, semi-annular piezoelectric actuators 10a and 10b, torsion bars (elastic beams) 13a and 13b, etc. Laser light 4a emitted from the laser light source 3 is incident on the rotating micromirror 9 and reflected, and the scanned emitted light (laser light 4b) scans, for example, a projection surface of a picoprojector.

[0031] The control device 5 transmits control signals to the movable frame 2a and the laser light source 3 through wiring (not shown). The semi-annular piezoelectric actuators 10a, 10b of the movable frame 2a are driven by these control signals, and the torsion bars 13a, 13b connected thereto are twisted, thereby rotating the micromirror 9. The control signals also control the on / off and brightness of the laser light 4a from the laser light source 3.

[0032] 2, in the two-dimensional optical deflector 2, a movable frame 2a is disposed in the center of an outer frame support 11. In addition, meandering type piezoelectric actuators 6a and 6b are disposed on both sides of the movable frame 2a, and the outer side of the movable frame 2a and the inner side of the outer frame support 11 are joined together.

[0033] The meandering type piezoelectric actuators 6a and 6b have a structure in which multiple cantilevers are arranged in parallel with their longitudinal directions adjacent to each other and are folded back at the top and bottom ends to be connected in series. As will be described in detail later, the meandering type piezoelectric actuators 6a and 6b are also driven by control signals from the control device 5, causing the movable frame 2a to rotate back and forth in the horizontal direction, i.e., around the X-axis in the figure.

[0034] As described above, by driving the semi-annular piezoelectric actuators 10a and 10b, the micromirror 9 rotates back and forth around the Y-axis in the figure, which coincides with the axis of the torsion bars 13a and 13b.

[0035] As a result, when the laser light 4a is reflected by the micromirror 9, the two-dimensional optical deflector 2 emits the light in front of the two-dimensional optical deflector 2, and can further perform scanning in two directions, the X-axis direction and the Y-axis direction.

[0036] The dotted areas of the meandering piezoelectric actuators 6a, 6b and the half-ring piezoelectric actuators 10a, 10b are the regions of the piezoelectric elements in which a piezoelectric crystal film is formed. As will be described in detail later, the piezoelectric crystal film is composed of a vertically oriented portion oriented along the orientation axis and an inclined orientation portion oriented at a predetermined inclination angle relative to the orientation axis. This allows the piezoelectric elements formed in the piezoelectric actuators to have high piezoelectric characteristics and long-term reliability.

[0037] Electrode pads 7a to 7e (hereinafter referred to as electrode pads 7) and electrode pads 8a to 8e (hereinafter referred to as electrode pads 8) are disposed below the outer frame support 11. The electrode pads 7 and 8 are electrically connected so as to apply a drive voltage to each electrode of the meandering type piezoelectric actuators 6a and 6b and the half-ring type piezoelectric actuators 10a and 10b.

[0038] It should be noted that the optical deflector can function without the meandering type piezoelectric actuators 6a and 6b. In this case, the movable frame 2a serves as an outer frame support, and the micromirror 9 rotates back and forth around the Y axis to form a one-dimensional optical deflector.

[0039] Next, the operation of the meandering type piezoelectric actuator 6a will be described with reference to FIGS. 3A and 3B.

[0040] As described above, the two-dimensional optical deflector 2 enables the micromirror 9 to rotate back and forth around the X-axis by operating the meandering type piezoelectric actuators 6a and 6b (hereinafter referred to as piezoelectric actuators 6a and 6b).

[0041] FIG. 3A is a cut-out view of the piezoelectric actuator 6a arranged on the left side when the two-dimensional optical deflector 2 in FIG. 2 is viewed from the front side (the side where the piezoelectric crystal film shown by the pointillism drawing part is visible when viewed from above. It may also be referred to as the "upward direction"). The piezoelectric actuator 6a is in the form of four piezoelectric cantilevers arranged side by side. Each piezoelectric cantilever is mainly composed of a piezoelectric crystal film of lead zirconate titanate (PZT: Pb(Zr,Ti)O3), which is a piezoelectric crystal, and electrode films sandwiching the film, and has a piezoelectric element (details will be described later). In the following, the piezoelectric cantilevers are referred to as 6a(1), 6a(2), 6a(3), and 6a(4) in order from the side farthest from the movable frame 2a.

[0042] For example, in a meandering type piezoelectric actuator 6a, a first voltage is applied to the odd-numbered piezoelectric cantilevers 6a(1) and 6a(3), and a second voltage in opposite phase to the first voltage is applied to the even-numbered piezoelectric cantilevers 6a(2) and 6a(4).

[0043] By doing this, as shown in FIG. 3B, the odd-numbered piezoelectric cantilevers 6a(1), 6a(3) can be bent upward (in FIG. 3B, convex), and the even-numbered piezoelectric cantilevers 6a(2), 6a(4) can be bent downward (in FIG. 3B, concave).

[0044] Although not shown, the piezoelectric actuators 6b are designated as piezoelectric cantilevers 6b(1), 6b(2), 6b(3), and 6b(4) in order from the side closest to the movable frame 2a. b (1), 6 b (3) is bent downward to deform the even-numbered piezoelectric cantilever 6 b (2), 6 b (4) can be bent and deformed upward.

[0045] This displaces the micromirror 9 so that the lower side (the torsion bar 13b side) of the micromirror 9 is pushed up and the upper side (the torsion bar 13a side) of the micromirror 9 is pushed down (the movable frame 2a moves in the U direction around the X-axis). Similarly, by applying reverse voltages to the piezoelectric cantilevers 6a(1)-(4) and 6b(1)-(4), the micromirror 9 can be displaced in the reverse direction. In this way, the micromirror 9 can be rotated around the X-axis.

[0046] Next, the movable frame 2a will be described in detail with reference to FIG.

[0047] 4 is a perspective view of the movable frame 2a as seen obliquely from the front. In the initial state, the micromirror 9 has a normal line extending from the center O to the front side facing straight forward.

[0048] The circular micromirror 9 is supported by torsion bars 13a and 13b in the Y-axis direction and is disposed at the center of the movable frame 2a. The reflective surface of the micromirror 9 is formed by a thin metal film of gold (Au), platinum (Pt), aluminum (Al), or the like, for example, by sputtering or electron beam deposition. The shape of the micromirror 9 is not limited to a circle, and may be an ellipse or other shape.

[0049] The torsion bars 13a and 13b are connected to the movable frame 2a beyond the joints between the micromirror 9 at one end and the semi-ring-shaped piezoelectric actuators 10a and 10b (hereinafter referred to as piezoelectric actuators 10a and 10b) at the other end. In this way, the torsion bars 13a and 13b are connected to the movable frame 2a, so that the reciprocating rotation around the Y axis is stabilized.

[0050] The piezoelectric actuators 10a and 10b are disposed in positions surrounding the micromirror 9 from the outside. The piezoelectric actuators 10a and 10b are coupled to torsion bars 13a and 13b on the Y axis, and coupled to fixed bars 14a and 14b, which are part of the outer frame support 11, on the X axis.

[0051] Although details will be described later, the piezoelectric actuators 10a and 10b also have a piezoelectric element structure in which a PZT piezoelectric crystal film is sandwiched between a lower electrode and an upper electrode by a semiconductor planar process. By applying a voltage to the piezoelectric crystal film via the lower electrode and the upper electrode, the piezoelectric actuators 10a and 10b are bent and deformed, twisting the torsion bars 13a and 13b.

[0052] In the piezoelectric actuators 10a and 10b, dividing grooves 18 are formed on straight lines inclined at 45° to the Y-axis, dividing the piezoelectric crystal film in the circumferential direction. In addition, since the torsion bars 13a and 13b extend on the Y-axis, the piezoelectric crystal film is also divided in the circumferential direction at these positions.

[0053] When manufacturing the movable frame 2a using MEMS technology, first, a piezoelectric crystal film for the piezoelectric actuators 10a, 10b is uniformly formed on the entire circumference including the torsion bars 13a, 13b. After that, the piezoelectric crystal film on the torsion bars 13a, 13b and the separating groove 18 is removed by etching.

[0054] Piezoelectric actuator 10a is divided into sections 16a to 16c, successively from the top, by two dividing grooves 18. On the other hand, piezoelectric actuator 10b is divided into sections 17a to 17c, successively from the top, by two dividing grooves 18.

[0055] This allows the application of drive voltages to the piezoelectric crystal films of the sections 16a-16c and 17a-17c individually. For example, by applying a predetermined voltage V1 to the sections 16a, 16c, and 17b, and applying a voltage V2 having an opposite phase to V1 to the sections 16b, 17a, and 17c, the micromirror 9 can be swung around the Y axis. Also, by separating the piezoelectric crystal films as described above, the same deflection angle can be obtained with about half the voltage required when the films are not separated, and power consumption can be reduced. In addition, the action and reaction of the torsion springs of the torsion bars 13a and 13b are also added, making it possible to reduce power consumption.

[0056] Next, the structure of the piezoelectric element formed in each piezoelectric actuator and the manufacturing method thereof will be described with reference to FIGS.

[0057] Fig. 5 is a flow chart for manufacturing the piezoelectric element. Fig. 6 shows a cross-sectional structure of a piezoelectric element 30 manufactured by the manufacturing method of Fig. 5, and Fig. 7 shows a crystal orientation image of the PZT film 25 of the piezoelectric element 30.

[0058] First, a plate-shaped Si core substrate 27 is prepared with a silicon oxide (SiO2) film 21 having a thickness of about 1 μm formed on the upper surface of a silicon (Si) substrate 20 having a thickness of 400 μm (see FIG. 6). Then, an amorphous titanium oxide (TiO2) film 22 made of a metal oxide having a thickness of 5 nm is formed as an adhesive layer on the Si core substrate 27 by a sputtering method at room temperature (for example, 20°C to 30°C) (STEP 01). The temperature at which the film is formed is the temperature of the Si core substrate 27 on which the film is to be formed.

[0059] Subsequently, a conductive Pt film 23 having a thickness of 200 nm is formed as a lower electrode (first electrode) on the upper surface of the TiO2 film 22 by a sputtering method at a temperature of 400° C. (STEP 02). In this step, the orientation of the Pt film 23 is controlled so that the full width at half maximum (FWHM) of the main peak of the X-ray rocking curve in the (111) plane reflection (diffraction) is 3°≦FWHM≦10°.

[0060] Subsequently, a 20 nm-thick SrRuO3 (SRO) film 24 having a perovskite structure is formed as a buffer layer on the upper surface of the Pt film 23 by sputtering (RF magnetron sputtering) at a temperature of 750° C. (STEP 03).

[0061] In the present invention, it is preferable to deposit the amorphous TiO2 film 22, the Pt film 23 having an X-ray rocking curve FWHM of 3° to 10° on the (111) plane, and the SRO film 24 as a buffer layer in the same apparatus. Alternatively, it is preferable to deposit the films in the above-mentioned steps without exposing them to the atmosphere by using an apparatus connected by a load lock chamber. This operation can improve the crystal orientation characteristics of the PZT film 25 to be deposited in the next step.

[0062] Next, a tetragonal PZT film 25 having a thickness of 4 to 5 μm is formed as a piezoelectric crystal film on the upper surface of the SRO film 24 at a temperature of 550° C. by an arc discharge reactive ion plating (ADRIP) method (STEP 04).

[0063] The arc discharge reactive ion plating method is a method of forming a film by evaporating or ionizing a target (PZT) using vacuum arc discharge. The characteristics of the arc discharge allow for the formation of a dense film with good adhesion, making it suitable for mass production and process stability.

[0064] Finally, a Pt film 26 having a thickness of 120 nm is formed as an upper electrode (second electrode) by sputtering on the upper surface of the PZT film 25 (STEP 05). This completes the piezoelectric element 30 used in the meandering type piezoelectric actuators 6a and 6b, the half-ring type piezoelectric actuators 10a and 10b, etc.

[0065] Through the above steps, as shown in FIG. 6, a piezoelectric element 30 is formed on the upper surface of a Si core substrate 27 consisting of a Si substrate 20 and a SiO2 film 21, in which a TiO2 film 22 as an adhesion layer, a Pt film 23 as a lower electrode (LE) on the upper surface of the TiO2 film 22, an SRO film 24 as a buffer layer on the upper surface of the Pt film 23, a PZT film 25 on the upper surface of the SRO film 24, and a Pt film 26 as an upper electrode (UE) on the upper surface of the SRO film 24.

[0066] The PZT film 25 of the piezoelectric element 30 thus formed is a polycrystalline film in which a plurality of columnar PZT crystal grains are bonded in a direction perpendicular to the surface of the Pt film 23 (SRO film 24) serving as the lower electrode. In other words, the piezoelectric element 30 has a structure in which the PZT film 25 consisting of a group of columnar crystal grains is disposed between the surface of the Pt film 23 serving as the lower electrode and the surface of the Pt film 26 serving as the upper electrode.

[0067] Next, the crystal orientation of the PZT film 25 will be described with reference to FIG.

[0068] The PZT crystal forming the PZT film 25 is a tetragonal crystal. <100> The direction is the a-axis, <010> The direction is the b axis, <001> When the direction is the c-axis, the a-axis and b-axis are perpendicular to each other, and the c-axis is perpendicular to the plane containing the a-axis and b-axis. In addition, the c-axis direction of the tetragonal crystal is long, and the crystal is polarized in the c-axis direction.

[0069] 7 is a diagram showing two vertically oriented portions 25a, 25a and two inclined orientation portions 25b, 25b' of PZT crystals constituting the PZT film 25. Each of the vertically oriented portions 25a, 25a and the inclined orientation portions 25b, 25b' corresponds to one unit of crystal grain of the polycrystalline film forming the PZT film 25. Also, each of the vertically oriented portions 25a, 25a and the inclined orientation portions 25b, 25b' may coexist in one unit of the polycrystalline body.

[0070] In the following description, when the two vertical alignment sections 25a, 25a are equivalent, they will be simply referred to as "vertical alignment section 25a", and when the two inclined alignment sections 25b, 25b' are equivalent, they will be simply referred to as "inclined alignment section 25b".

[0071] In the two vertically oriented portions 25a, the c-axis of the PZT crystal is oriented in a direction perpendicular to the plane of the Pt film 23 (SRO film 24). In the tilted orientation portion 25b, the c-axis is tilted at an inclination angle φ (for example, φ=10°) with respect to the c-axis of the vertically oriented portion 25a.

[0072] 7 is a schematic diagram of an inclined crystal lattice, but in the PZT film 25 in which columnar PZT crystal grains are bonded, only the internal orientation axes of the columnar PZT crystal grains are vertical or inclined. Although not shown, the c-axis of the inclined orientation portion 25b is inclined at an arbitrary angle (0° to 360° in the full circumferential direction) in the radial direction with respect to the c-axis of the vertical orientation portion 25a. The inclined orientation portion 25b is scattered (dispersed) in the PZT crystal film made of the vertical orientation portion 25a.

[0073] The a-axis (b-axis) of each PZT crystal contained in the PZT film 25 faces in various directions with the c-axis oriented perpendicular to the surface of the Pt film 23 (SRO film 24) as the rotation axis. That is, the PZT film 25 is a uniaxially oriented polycrystalline body.

[0074] Moreover, the c-axes of the vertically oriented portion 25a and the inclined oriented portion 25b each have a gradient distribution, such as a normal distribution, etc. Hereinafter, when the c-axis of the vertically oriented portion 25a and the c-axis of the inclined oriented portion 25b are mentioned, they are referred to as representative values ​​in the respective c-axis distributions (they may also be referred to as main c-axes).

[0075] When a voltage of a first polarity (second polarity) is applied to the lower electrode (Pt film 23) and upper electrode (Pt film 26) sandwiching the PZT film 25 made of such uniaxially oriented polycrystalline material, the c-axis expands (or contracts) and the plane perpendicular to the c-axis compresses (or expands) isotropically. This causes the piezoelectric actuators 6a, 6b, 10a, and 10b in which the piezoelectric element 30 is formed to operate as described above. In addition, since the PZT film 25 is compressed (or expanded) isotropically in the film surface direction, it can be used for any shape of film. Piezoelectric It can also be used as an actuator.

[0076] Although the PZT film 25 of this embodiment is made of PZT crystals including the vertically oriented portion 25a and the inclined oriented portion 25b, any piezoelectric film made of uniaxially oriented columnar polycrystals including the vertically oriented portion and the inclined oriented portion may be used, and is not limited to PZT crystals. Examples include barium titanate (BaTiO3), lead titanate (PbTiO3), (NaK)NbO3, etc.

[0077] In addition, the PZT film 25 consisting of uniaxially oriented columnar polycrystals including the vertical orientation portion 25a and the inclined orientation portion 25b in this embodiment can be formed by a step (STEP 01) of forming an amorphous TiO2 film 22 on a Si core substrate 27, a step (STEP 02) of forming a Pt film 23 (lower electrode) having an X-ray rocking curve FWHM of the (111) plane of 3° to 10°, and a step (STEP 03) of forming a buffer layer consisting of an SRO film 24, and a step (STEP 04) of forming a PZT crystal (piezoelectric film) on the laminated film formed in the step.

[0078] The manufacturing method of the above embodiment is merely an example, and is not limited to the manufacturing method of the present embodiment as long as a piezoelectric film consisting of uniaxially oriented columnar polycrystals including vertically oriented portions and inclined oriented portions is obtained.

[0079] Next, the structure and film-forming method of the piezoelectric element of the comparative embodiment will be described. The piezoelectric element of the comparative embodiment differs from the manufacturing method of the piezoelectric element 30 of the embodiment only in that the TiO2 film is annealed after the TiO2 film is formed (STEP 01), and that the Pt film is formed at a high temperature. The differences will be described below.

[0080] In the comparative PZT film, a TiO2 film is formed by sputtering at room temperature (for example, 20°C to 30°C), and then the TiO2 film is annealed at a temperature of 750°C in an annealing device to form a rutile crystal structure.

[0081] Furthermore, a Pt film with a thickness of 200 nm is formed as a lower electrode on the upper surface of the TiO2 film by sputtering at a temperature of about 800°C. The FWHM of the X-ray rocking curve of the (111) plane of the Pt film thus formed is less than 3°. As a result, the PZT film formed under the same conditions as the embodiment after the formation of the Pt film is composed of only vertically oriented parts.

[0082] Next, various measurement data of the piezoelectric element 30 (PZT film 25) of this embodiment and a piezoelectric element of a comparative embodiment will be described with reference to FIGS.

[0083] 8 shows the results of measuring the PZT film 25 using the X-ray diffraction θ-2θ measurement method. In this measurement, a piezoelectric element without an upper electrode (Pt film 26) is used because the upper electrode (Pt film 26) blocks X-rays. It is also possible to use a piezoelectric element in which the Pt film 26 is partially etched after forming the Pt film 26 as the upper electrode. Note that the crystal orientation characteristics of the PZT film 25 do not change even if an upper electrode is not provided.

[0084] The θ-2θ measurement method of X-ray diffraction (XRD) is a technique for analyzing crystals using the Bragg reflection of X-rays based on the crystal plane spacing. Specifically, X-rays are incident at an angle θ based on the horizontal crystal plane of the sample, and among the X-rays reflected (diffracted) from the sample, reflected X-rays at an angle 2θ to the incident angle θ of the incident X-rays are detected, and the reflected X-ray intensity is examined. Note that when the horizontal plane of the sample is used as the reference, the incident angle of the X-rays is ω, so it is sometimes written as ω-2θ.

[0085] 8, the horizontal axis is the angle 2θ [deg] of the reflected X-ray, and the vertical axis is the reflected X-ray intensity ([cps: counts per sec]). The scan angle is 20° to 50°, and Cu-Kα1 is used as the incident X-ray.

[0086] As shown in the figure, the reflected X-ray intensity curve of the θ-2θ measurement of the PZT film 25 of this embodiment detects only the reflection peaks of the (001) plane (plane perpendicular to the c-axis) of the PZT crystal at 21.82° and the (002) plane (plane perpendicular to the c-axis) of the PZT crystal at 44.44°. Also, a reflection peak of the (111) plane of the Pt film 23, which is the lower electrode, is detected at 39.70°. Thus, it can be seen that the PZT crystal of the PZT film 25 is only c-axis oriented with respect to the (111) plane of the Pt film 23.

[0087] In addition, in the PZT film of the comparative embodiment, only peaks of the (001) plane reflection, the (002) plane reflection, and the (111) plane reflection of the Pt film were observed, as in the embodiment (not shown). From this, it was confirmed that the PZT film of the comparative embodiment also had a c-axis orientation on the (111) plane of the Pt film via the SRO film.

[0088] Next, the results of the X-ray rocking curve measurement of the PZT film 25 will be described with reference to FIG.

[0089] X-ray rocking curve measurement is a technique in which the X-ray incidence angle θ and the detector reflection angle 2θ are fixed so that reflection from a specific crystal plane is obtained, and the sample stage (PZT film) is scanned (ω scan) from the incidence direction to the reflection direction, and the orientation distribution of the crystal plane (c-plane), in other words, the distribution of the crystal orientation axis perpendicular to the crystal plane, is examined from the obtained X-ray reflection intensity curve (rocking curve).

[0090] 9 shows rocking curves of the (001) plane of the PZT film 25 of the embodiment and the PZT film of the comparative embodiment. Here, the horizontal axis is the relative angle Δω [deg] with the angle ω at which the reflected X-ray intensity is maximum as the reference (0°), and the vertical axis is the reflected X-ray intensity Intensity [cps] (linear scale with 2000 cps increments). The solid line is the rocking curve of the PZT film 25 of the embodiment, and the dashed line is the rocking curve of the PZT film of the comparative embodiment. The magnification is adjusted so that the reflected X-ray intensity of the comparative embodiment is the same as the maximum value of the embodiment.

[0091] The rocking curve (solid line) of the PZT film 25 of the embodiment is composed of a main peak having a peak (main peak Pm) at Δω=0° and a sub-peak having a peak (sub-peak Ps) different from the main peak at two positions of Δω=±10°. The FWHM of the main peak and the two sub-peaks are 5.0° and 8.36°, respectively. The intensity ratio Rp (IntPs / IntPm) of the reflected X-ray of the sub-peak Ps to the main peak Pm was 0.25 (2811 / 11314) for the Rp of the sub-peak Ps at Δω=10, and 0.30 (3358 / 11314) for the Rp of the sub-peak Ps at Δω=-10.

[0092] The PZT film 25 provides substantially the same rocking curve at any position and even when the PZT film 25 is rotated. That is, on the film surface of the PZT film 25, the vertical orientation portion 25a and the inclined orientation portion 25b are evenly formed.

[0093] In addition, when the peak of the main peak (main peak Pm) of the rocking curve is set at Δω=0°, the position of the sub-peak (sub-peak Ps) Δωps corresponds to the tilt angle φ of the principal c-axis of the inclined alignment portion 25b with respect to the principal c-axis of the perpendicular alignment portion 25a. That is, in this embodiment, |Δωps|=φ (here, 10°).

[0094] From the above, the vertically oriented portion 25a is a crystal group having a c-axis tilt distribution with a FWHM of 5.0°, and the tilted orientation portion 25b is a crystal group having a c-axis tilt distribution with a FWHM of 8.36°. Both crystal groups are discrete crystal groups with the c-axis orientations corresponding to the main peak Pm and the sub-peak Ps, respectively, as the distribution centers. The c-axis of the tilted orientation portion 25b is tilted in the entire circumferential direction from 0° to 360° with respect to the c-axis of the vertically oriented portion 25a. The abundance ratio of the tilted orientation portion 25b to the vertically oriented portion 25a is set to an abundance ratio that does not impair the inverse piezoelectric effect (here, Rp=0.25 and 0.30).

[0095] On the other hand, the rocking curve (dashed line) of the PZT film of the comparative embodiment has only a single-peak reflection peak based on the (001) plane at Δω=0°. The FWHM of the reflection peak was 3.1°. In other words, the PZT crystals of the PZT film of the comparative embodiment are composed only of vertically oriented portions, and the vertically oriented portions are a crystal group having a narrower c-axis tilt distribution than the embodiment. In other words, the PZT film of the comparative embodiment is a PZT film with excellent orderliness in the crystal orientation of the PZT crystals.

[0096] FIG. 10 is a graph showing the relationship between the electric field and the leakage current density of the PZT film 25. As shown in FIG.

[0097] In FIG. 10, the horizontal axis represents the electric field [V / μm] applied to the PZT film 25 when a voltage is applied to the lower electrode (Pt film 23) and upper electrode (Pt film 26) of the piezoelectric element 30, and the vertical axis represents the leakage current density [nA / cm 2 The relationship between the electric field and the leakage current density of the piezoelectric element 30 of the embodiment is shown by a solid line, and the relationship between the electric field and the leakage current density of the piezoelectric element of the comparative embodiment is shown by a dashed line.

[0098] In this measurement, after poling (polarization processing) the PZT film of the piezoelectric element, the voltage increasing side (upper curve) is measured, followed by the decreasing side (lower curve). The voltage is applied in the following pattern: 0 [V] → Measurement voltage 1 → 0 [V] → Measurement voltage 2 → .... In addition, the applied voltage is avoided in the unstable region, and is applied in the region where the electric field is 2.0 [V / μm] or more.

[0099] In the piezoelectric element 30 of the embodiment, when the voltage is increased (upper curve of the solid line), the current in the first direction (positive value) is about 30 [nA / cm 2 ]. Next, when the voltage is reduced (the curve below the solid line), the leakage current flowing in the first direction (positive value) decreases when the electric field is 10.0 to 8.0 [V / μm], the leakage current becomes zero when the electric field is 8.0 [V / μm], and the leakage current flowing in the second direction (negative value) increases when the electric field is 8.0 to 2.0 [V / μm]. In other words, the piezoelectric element 30 causes the polarity of the leakage current to reverse when the voltage is reduced, except in the unstable region. The reason why the polarity of the leakage current does not reverse immediately when the voltage is reduced is thought to be due to hysteresis of the piezoelectric crystal.

[0100] On the other hand, in the comparative piezoelectric element, when the voltage was increased (upper curve of the dashed line), the current in the first direction (positive value) was about 70 nA / cm 2 Next, when the voltage is lowered (the curve below the dashed line), the leakage current flowing in the first direction (positive value) at an electric field of 10.0 to 2.0 [V / μm] is approximately 70 to 0 [nA / cm 2 ].

[0101] As described above, in the piezoelectric element 30 of the embodiment, when a voltage is applied to the electrodes (Pt film 23, Pt film 26), the leakage current is large when the voltage is increased and is small when the voltage is decreased. Furthermore, the direction of the leakage current is reversed and increases from an early stage of the voltage decrease. On the other hand, in the piezoelectric element of the comparative embodiment, the leakage current is large when the voltage is increased and only becomes small when the voltage is decreased.

[0102] Such a characteristic in which the direction of leakage current flow is reversed and increases when the voltage is reduced is believed to be due to an improvement in the leakage current resistance characteristic during crystal deformation of the PZT film 25 when the inverse piezoelectric effect (deformation of the piezoelectric element) is applied to the piezoelectric element 30 of the embodiment.

[0103] Finally, FIG. 11 shows the electric field resistance that is the operating limit electric field of the piezoelectric element 30 of the embodiment and the piezoelectric element of the comparative embodiment, as well as the endurance time and the piezoelectric constant d in a durability test simulating the operating conditions of the meandering type piezoelectric actuators 6a and 6b and the half-ring type piezoelectric actuators 10a and 10b used in the two-dimensional optical deflector 2. 31 A table comparing the measurement results is shown below.

[0104] The electric field resistance, durability, and piezoelectric constant d31 of the piezoelectric element 30 of the embodiment are 21.8 [V / μm], 4000 hours or more, and 152 [pm / V], respectively. On the other hand, the electric field resistance, durability, and piezoelectric constant d31 of the piezoelectric element of the comparative embodiment are 14.0 [V / μm], 1000 hours or less, and 152 [pm / V], respectively. comparison Piezoelectric elements of different shapes Child and Furthermore, even in the case of the piezoelectric element 30 of the embodiment in which the inclined orientation portion 25b is provided, the piezoelectric constant d31 is improved in comparison.

[0105] The piezoelectric element 30 of this embodiment has improved electric field resistance and endurance time due to the structure described below.

[0106] First, the PZT crystals forming the PZT film 25 of this embodiment are columnar uniaxially oriented polycrystals with the tetragonal c-axis oriented in a direction perpendicular to the film surface of the Pt film 23 (SRO film 24) (the directions of the a-axis and b-axis are arbitrary). Second, the columnar uniaxially oriented polycrystals are provided with vertically oriented portions 25a and inclined oriented portions 25b. Also, the inclined oriented portions 25b are dispersed on the film surface of the PZT film 25. Third, an inclination distribution is provided for the c-axis of the vertically oriented portion 25a and the c-axis of the inclined oriented portion 25b (there is an axis distribution for both the main peak and the sub-peak).

[0107] With the above-mentioned structure, the film distortion that occurs when an electric field is applied to the PZT film 25 to deform the PZT crystals can be dispersed and alleviated throughout the PZT film 25, making it possible to reduce the electric field resistance. Special In particular, by providing the c-axis of the inclined orientation portion 25b spaced apart (discrete) by an angle φ from the c-axis of the vertical orientation portion 25a, the leakage current resistance characteristic is improved, and the durability can be extended.

[0108] In order to improve the electric field resistance and durability, it is preferable that the inclination angle φ of the c-axis of the inclined orientation section 25b is greater than 6° and less than 19°. If the inclination angle φ of the c-axis of the inclined orientation section 25b is set to 6° or less, the inclination angle φ will be assimilated with the angular distribution of the c-axis of the vertical orientation section 25a, resulting in a decrease in the electric field resistance and durability. In addition, if the inclination angle φ of the c-axis of the inclined orientation section 25b is set to 19° or more, <111> This is because the c-axis appears and the piezoelectric properties of the PZT film deteriorate. In addition, taking into consideration the angular distribution of the c-axis of the inclined orientation portion 25b, the inclination angle φ is preferably 7° or more and 13° or less.

[0109] The ratio of the inclined orientation portion 25b of the PZT film 25 is such that the intensity ratio Rp of the reflected X-rays is 0.1 to 1 in If Rp is less than 0.1, the effect of the inclined orientation portion 25b cannot be obtained. If Rp is more than 1, the c-axis orientation of the PZT film 25 becomes too disturbed, and the piezoelectric constant d 31 butA suitable Rp is 0.15 or more at which the improvement in electric field resistance characteristic due to the inclined orientation portion 25b saturates, or 0.45 or less at which the improved durability life value does not decrease.

[0110] As described above, the PZT crystals forming the PZT film 25 of the piezoelectric element 30 of the embodiment include the vertically oriented portion 25a having a c-axis in a direction perpendicular to the plane of the Pt film 23 (SRO film 24) and the inclined oriented portion 25b having a c-axis inclined (tilted) by an angle φ with respect to the c-axis, thereby providing high withstand voltage, excellent durability, and unimpaired d 31 This allows the piezoelectric element 30 of the present invention to provide a highly reliable two-dimensional optical deflector.

[0111] Furthermore, by improving the withstand voltage and durability of the piezoelectric element 30 of the embodiment, it is possible to apply a high voltage (electric field) to the piezoelectric element 30, for example, to increase the movable range of the actuator.

[0112] The above describes an embodiment for carrying out the present invention, but the present invention is not limited to the above embodiment, and can be modified as appropriate without departing from the gist of the present invention.

[0113] For example, the thickness of the thin film constituting the piezoelectric element 30 and the method of film formation in Fig. 6 are merely examples and can be changed. The piezoelectric crystal film may be made of a material having piezoelectric properties other than PZT. In addition, the TiO2 film 22 as an adhesion layer and the SRO film 24 as a buffer layer are not essential components of the piezoelectric element of the present invention and may be omitted. [Explanation of symbols]

[0114] Reference Signs List 1...optical scanner module, 2...two-dimensional optical deflector, 3...laser light source, 5...control device, 6a, 6b...meander type piezoelectric actuator, 9...micromirror, 10a, 10b...half-ring type piezoelectric actuator, 20...Si substrate, 21...SiO2 film, 22...TiO2 film, 23...Pt film (lower electrode), 24...SRO film, 25...PZT film, 25a...vertical alignment portion, 25b, 25b'...inclined alignment portion, 26...Pt film (upper electrode), 27...Si core substrate, 30...piezoelectric element.

Claims

1. a substrate having at least one planar surface; a first electrode film provided on the flat surface of the substrate; a tetragonal piezoelectric crystal film provided on the first electrode film; a second electrode film provided on a surface of the piezoelectric crystal film facing the first electrode film, the piezoelectric crystal film is a uniaxially oriented polycrystalline body made of columnar crystal grains in which the c-axis of the tetragonal crystal is oriented in a direction perpendicular to the first electrode film, The polycrystalline body has a vertically oriented portion having a c-axis in a direction perpendicular to the first electrode film; an inclined orientation portion having a c-axis inclined with respect to the c-axis of the vertical orientation portion, Each of the c-axes of the vertically oriented portion and the c-axes of the inclined oriented portion has a distribution, and the c-axis distribution of the inclined oriented portion is discrete with respect to the c-axis distribution of the vertically oriented portion, The inclination angle φ of the c-axis of the inclined orientation portion is greater than 6° and less than 19°, A piezoelectric element characterized in that an intensity ratio Rp of the inclined orientation portion to the perpendicular orientation portion in an X-ray rocking curve is 0.1 to 1.

2. 2. The piezoelectric element according to claim 1, wherein the c-axis of the inclined orientation portion is inclined in all directions from 0° to 360° with the c-axis of the vertical orientation portion as the axis of rotation.

3. 3. The piezoelectric element according to claim 1, wherein the inclination angle φ of the c-axis of the inclined orientation portion with respect to the c-axis of the vertical orientation portion of the piezoelectric crystal film is greater than 6° and less than 19°.

4. 4. The piezoelectric element according to claim 1, wherein an intensity ratio Rp of the inclined orientation portion to the perpendicular orientation portion in an X-ray rocking curve is 0.1 to 1.

5. 5. The piezoelectric element according to claim 1, wherein the piezoelectric crystal film is made of lead zirconate titanate, and the first electrode film is made of platinum.

6. 5. The piezoelectric element according to claim 4, wherein the film surface of the first electrode film is a (111) surface.

7. depositing an amorphous metal oxide film on at least one planar substrate; forming a first electrode film made of a conductive metal film having a (111) surface on the metal oxide; forming an SRO film on the first electrode film; a polycrystalline body having a uniaxial orientation of a tetragonal crystal, the polycrystalline body having columnar crystal grains on the SRO film in a direction perpendicular to the first electrode film, forming a piezoelectric crystal film on the columnar crystal grains, the piezoelectric crystal film having a vertically oriented portion having a c-axis in a direction perpendicular to the first electrode film and an inclined portion having a c-axis inclined at an inclination angle φ of more than 6° and less than 19° with respect to the c-axis of the vertically oriented portion; forming a second electrode film on a surface of the piezoelectric crystal film that faces the first electrode film; A method for producing a piezoelectric element, wherein an intensity ratio Rp of the inclined orientation portion to the perpendicular orientation portion in an X-ray rocking curve is 0.1 to 1.

8. 8. The method for producing a piezoelectric element according to claim 7, wherein an arc discharge reactive ion plating method is used in the step of forming the piezoelectric crystal film.

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