JP2013247216A - Piezoelectric element and ink jet head including the same - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a piezoelectric element reliability of which is enhanced by suppressing the peeling of a piezoelectric during continuous driving, even if the piezoelectric is deposited by sputtering, and to provide an ink jet head including the piezoelectric element.SOLUTION: A piezoelectric element 13 includes a piezoelectric 16a deposited on a diaphragm 15 by sputtering, a lower electrode 16c and an upper electrode 16b formed to sandwich the piezoelectric 16a in the thickness direction. A portion of the upper electrode 16b is formed on the piezoelectric 16a with an insulating film 18 interposed therebetween. The piezoelectric 16a is located directly under the insulating film 18, and has a first region R1 where the polarization direction during deposition by sputtering is maintained, and a second region R2 corresponding to the insulating film 18 non-forming region and having an inverted polarization direction.

Description

  The present invention relates to a piezoelectric element formed by sputtering on a diaphragm, a piezoelectric element having a lower electrode and an upper electrode, and an ink jet head provided with the piezoelectric element.

  Conventionally, piezoelectric bodies including lead zirconate titanate (PZT) have been used for actuators and sensors. A piezoelectric element that functions as an actuator includes a lower electrode, a piezoelectric body, an upper electrode, a diaphragm, and the like. When a voltage is applied to the lower electrode and the upper electrode sandwiching the piezoelectric body, the piezoelectric body expands and contracts, and thereby the vibration plate vibrates (displaces).

  In recent years, due to the need for miniaturization and higher density, piezoelectric elements have been required to be thinner and smaller. In particular, when a piezoelectric element is used as an actuator of an ink jet head, it is necessary to mount nozzles at high density in order to increase the definition of a printed image. Therefore, downsizing of the piezoelectric element is important. Conventionally, a bulk material has been used as the piezoelectric body. However, in order to reduce the thickness and size of the piezoelectric element, it is effective to reduce the thickness of the piezoelectric body. In this regard, for example, in Patent Document 1, a thin and small piezoelectric element is manufactured by forming a piezoelectric body into a thin film by sputtering or the like and patterning the piezoelectric body (piezoelectric thin film) by using a photolithography method.

  By the way, the piezoelectric body formed by the sputtering method has a characteristic that the polarization direction faces the direction from the lower electrode to the upper electrode. Therefore, when the upper electrode is an address electrode and the lower electrode is a common electrode, it is possible to operate the piezoelectric element by applying a negative voltage to the upper electrode. However, a drive circuit that applies a negative voltage is more expensive than a drive circuit that applies a positive voltage. In addition, when the lower electrode is used as an address electrode and a positive voltage is applied to the piezoelectric element to drive the piezoelectric element, a leakage current is generated between the lower electrodes corresponding to two adjacent piezoelectric bodies, and in the worst case, it is not intended. Electrical crosstalk occurs where ink is ejected from the pressure chamber.

  Therefore, in Patent Document 1, a piezoelectric element can be operated using an inexpensive drive circuit or power source by reversing the polarization direction of a piezoelectric film formed by sputtering and applying a positive voltage to the upper electrode. In addition, the occurrence of leakage current is reduced to improve electrical crosstalk.

JP 2009-218360 A (refer to claim 1, paragraphs [0006] to [0014], etc.)

  However, when the piezoelectric material is formed into a thin film by sputtering as in Patent Document 1, there is a possibility that the piezoelectric material may be peeled off when the piezoelectric element is continuously driven. There is a problem with sex.

  When the piezoelectric body is thinned, there are some which show characteristics different from the bulk, and one of them is a decrease in driving stability. When the piezoelectric body is formed as a thin film, it is very difficult to control the crystal of the piezoelectric body, and the piezoelectric body is greatly influenced by the base and grows. At this time, if the crystallinity of the piezoelectric body is good, the amount of displacement of the piezoelectric body is large and the variation in piezoelectric characteristics is small, so that good ink ejection characteristics can be obtained. On the other hand, when the crystallinity of the piezoelectric body is poor, the displacement amount of the piezoelectric body becomes small, and the variation in the piezoelectric characteristics increases, resulting in poor ink ejection characteristics. Therefore, when the piezoelectric body is formed as a thin film, a base film (a base film having a good compatibility with the piezoelectric body) for improving the crystallinity of the piezoelectric body is required. Examples of such a base film include a lower electrode made of platinum (Pt) or iridium (Ir).

  However, when the piezoelectric body is formed as a thin film, the adhesion of the piezoelectric body to the base film is poor, and therefore, when the piezoelectric element is continuously driven, the piezoelectric body is peeled from the base film. In Patent Document 1, no measures are taken for suppressing peeling of a piezoelectric body formed by sputtering.

  The present invention has been made to solve the above-described problems, and its purpose is to suppress the peeling of the piezoelectric body during continuous driving even when the piezoelectric body is formed by sputtering. Thus, a piezoelectric element capable of improving reliability and an ink jet head including the piezoelectric element are provided.

  A piezoelectric element according to one aspect of the present invention includes a piezoelectric body formed on a vibration plate by a sputtering method, and a lower electrode and an upper electrode formed so as to sandwich the piezoelectric body in a film thickness direction. And a part of the upper electrode is formed on the piezoelectric body through an insulating film, and the piezoelectric body is located immediately below the insulating film and is polarized during film formation by sputtering. A first region in which the direction is maintained, and a second region in which the polarization direction is reversed corresponding to a region where the insulating film is not formed.

  Since a part of the upper electrode is formed on the piezoelectric body through the insulating film, the polarization process is performed by applying a voltage higher than the coercive electric field between the upper electrode and the lower electrode. In the first region of the piezoelectric body located immediately below, only the second region can be reversed in polarity while maintaining the polarization direction during film formation by sputtering.

  As described above, in the configuration in which the piezoelectric body formed by the sputtering method has the first region and the second region having different polarization directions, between the upper electrode and the lower electrode in order to drive the piezoelectric element. When a voltage is applied to (a potential difference is applied), one of the first region and the second region causes the piezoelectric body to expand in the film thickness direction (shrink in the direction perpendicular to the film thickness direction), and on the other hand Shrinks in the film thickness direction (extends in a direction perpendicular to the film thickness direction). Thereby, the peeling stress applied to the edge of the piezoelectric body due to the expansion or contraction of the piezoelectric body in one region can be reduced by the contraction or extension of the piezoelectric body in the other region. As a result, even when the piezoelectric element is continuously driven, it is possible to suppress the piezoelectric body formed by the sputtering method from being peeled off from the underlying lower electrode, and the reliability of the element can be improved.

  In the above configuration, the polarization direction of the first region is a direction from the lower electrode to the upper electrode, and the polarization direction of the second region is a direction from the upper electrode to the lower electrode. Also good.

  In this case, when a positive voltage is applied to the upper electrode, the second region of the piezoelectric body extends in the film thickness direction and the diaphragm vibrates. Therefore, when a positive voltage is applied to the upper electrode and the second region of the piezoelectric body is used for driving the element, the peeling of the piezoelectric body can be suppressed.

  In the above configuration, it is desirable that a positive polarity voltage is applied to the upper electrode. In this case, the piezoelectric element can be driven using a drive circuit that is less expensive than a drive circuit that applies a negative voltage.

  In the above configuration, the first region may be positioned so as to surround the second region. In this case, it is easy to form an insulating film so as to cover the edge of the piezoelectric body. Since the insulating film covers the edge of the piezoelectric body in this way, the peeling stress applied to the edge of the piezoelectric body is relieved by the insulating film, so that peeling of the piezoelectric body can be further suppressed.

  In the above configuration, when the thickness of the piezoelectric body is d μm, it is desirable that the second region is separated from the end face of the piezoelectric body by d / 2 μm or more. In this case, the peeling stress applied to the edge of the piezoelectric body due to the displacement of the second region can be reduced, and the peeling of the piezoelectric body can be suppressed.

  In the above configuration, the piezoelectric body is preferably composed mainly of lead zirconate titanate (PZT). In this case, a thin film piezoelectric body can be formed by sputtering using a material containing PZT as a main component, and a piezoelectric body having good piezoelectric characteristics can be formed.

  In the above configuration, the thickness of the piezoelectric body is preferably 2 to 10 μm. If the thickness of the piezoelectric body is too thin below the lower limit, a sufficient amount of displacement cannot be obtained, and if the thickness of the piezoelectric body is too thick above the upper limit, the film of the piezoelectric body tends to be damaged.

  The said structure WHEREIN: It is desirable for the said upper electrode to be formed inside the formation area of the said piezoelectric material. In this case, a short circuit between the upper electrode and the lower electrode can be avoided.

  The piezoelectric element having the above configuration may further include a substrate having an opening, and the diaphragm may be formed on the substrate so as to cover the opening.

  In this configuration, the piezoelectric element can be used as a pump (including an ink jet head) by filling the opening with a discharge medium such as gas or liquid. In addition, the piezoelectric element can be used as an ultrasonic sensor by vibrating the diaphragm with ultrasonic waves and detecting the electric field generated at that time.

  An ink jet head according to an aspect of the present invention includes a piezoelectric element having the above-described configuration, and ejects ink contained in an opening of the substrate as a pressure chamber by vibrating the diaphragm of the piezoelectric element. is there.

  According to the above-described piezoelectric element, even when a piezoelectric body is formed by sputtering, peeling of the piezoelectric body during continuous driving can be suppressed, and the reliability of the element can be improved. In addition, a highly reliable inkjet head can be realized.

  In the ink jet head, the pressure chamber forming region is inside the lower electrode forming region, and the piezoelectric material forming region is inside the pressure chamber forming region above the pressure chamber. The upper electrode forming region may be inside the body forming region.

  In such an ink jet head, the pressure chamber, the piezoelectric body, and the upper electrode are arranged at a high density, and an ink jet head capable of high-quality image printing and having stable printing characteristics can be realized.

  According to the above configuration, when the element is driven, the peeling stress applied to the edge of the piezoelectric body due to the expansion or contraction of the piezoelectric body in one of the first region and the second region is reduced in the other region. It can be reduced by contraction or expansion of the piezoelectric body. As a result, even when a piezoelectric element including a piezoelectric body formed by sputtering is continuously driven, the piezoelectric body can be prevented from peeling from the base, and reliability can be improved.

1 is a cross-sectional view illustrating a schematic configuration of an inkjet head according to an embodiment of the present invention. It is sectional drawing which expands and shows the P section of FIG. It is a top view of the piezoelectric element which the said inkjet head has. FIG. 4 is a plan view of the piezoelectric element shown with hatching on an insulating film in a state where a part of the piezoelectric element is not shown. FIG. 3 is a plan view of the piezoelectric element shown with hatching in the first region and the second region of the piezoelectric body in a state where a part of the piezoelectric element is not shown. FIG. 5 is a cross-sectional view illustrating a part of each manufacturing process in the method for manufacturing the inkjet head of Example 1. It is sectional drawing which shows a part of remaining of each manufacturing process in the said manufacturing method. It is sectional drawing which shows a part further remaining of each manufacturing process in the said manufacturing method. It is sectional drawing which shows a part further remaining of each manufacturing process in the said manufacturing method. It is sectional drawing which shows a part further remaining of each manufacturing process in the said manufacturing method.

  An embodiment of the present invention will be described below with reference to the drawings.

[1. Configuration of inkjet head]
FIG. 1 is a cross-sectional view illustrating a schematic configuration of an inkjet head 1 according to the present embodiment. The inkjet head 1 is formed by bonding a head substrate 10 and a wiring substrate 20 via an adhesive resin layer 30. A box-shaped manifold 40 that constitutes an ink chamber 41 in which ink is stored is provided on the upper surface of the wiring substrate 20 (the surface opposite to the head substrate 10) via an attachment plate 50.

The head substrate 10 is configured by laminating a nozzle plate 11 made of a Si (silicon) substrate, an intermediate plate 12 made of a glass substrate, and a piezoelectric element 13 in this order. The piezoelectric element 13 includes a pressure chamber plate 14 formed by a support layer (Si substrate) of an SOI (Silicon on Insulator) substrate, and an active layer (Si substrate) and a BOX layer (SiO ₂ (silicon oxide)) of the SOI substrate. The diaphragm 15 and the actuator unit 16 are formed. Note that a thermal oxide film made of SiO ₂ may be formed on the surface of the active layer opposite to the BOX layer.

  The nozzle plate 11 is formed with a nozzle 11a serving as an ink ejection hole. The intermediate plate 12 is formed with a communication passage 12a through which the inside of the pressure chamber 14a formed in the pressure chamber plate 14 and the nozzle 11a of the nozzle plate 11 communicate with each other. A communication passage 12b is formed for communicating the through-hole 14b to be formed and the pressure chamber 14a.

  The pressure chamber plate 14 of the piezoelectric element 13 is formed with a pressure chamber 14 a that is formed as an opening of the substrate and accommodates ink as an ejection medium, and a through hole 14 b that forms an ink flow path from the ink chamber 41. Has been. A plurality of pressure chambers 14 a may be formed on the pressure chamber plate 14. In this case, the nozzle 11a and the actuator unit 16 (particularly, a piezoelectric body 16a and an upper electrode 16b described later) are provided corresponding to each of the plurality of pressure chambers 14a. The upper wall of the pressure chamber 14 a is constituted by the diaphragm 15, and the lower wall is constituted by the intermediate plate 12.

  The vibration plate 15 is provided on the pressure chamber plate 14 so as to cover the pressure chamber 14a from the wiring board 20 side, and discharges ink in the pressure chamber 14a to the outside through the nozzle 11a by vibration. The diaphragm 15 is formed with a communication passage 15 a that communicates the through hole 31 that penetrates the adhesive resin layer 30 and the through hole 14 b of the pressure chamber plate 14.

The actuator unit 16 includes a piezoelectric body 16a, and an upper electrode 16b and a lower electrode 16c formed so as to sandwich the piezoelectric body 16a in the film thickness direction (vertical direction). By applying a voltage between the upper electrode 16b and the lower electrode 16c, the piezoelectric body 16a is deformed (elongated or contracted), and thereby the diaphragm 15 vibrates. The piezoelectric body 16a is formed by sputtering using a material mainly composed of lead zirconate titanate (PZT; Pb (Zr, Ti) O ₃ ).

  The lower electrode 16 c is formed on the surface of the diaphragm 15. The formation region of the lower electrode 16c is larger than the formation region of the pressure chamber 14a. Note that an adhesive layer made of, for example, titanium (Ti) or titanium oxide (TiOx) may be provided below the lower electrode 16c.

  The piezoelectric body 16a and the upper electrode 16b are stacked on the lower electrode 16c so as to correspond to the pressure chamber 14a on a one-to-one basis. The piezoelectric body 16a is provided on the lower electrode 16c from above the pressure chamber 14a to above the side wall of the pressure chamber 14a (so as to straddle the boundary between the pressure chamber 14a and the side wall).

  The upper electrode 16b is formed on the inner side of the formation region of the piezoelectric body 16a and is formed on the piezoelectric body 16a from above the pressure chamber 14a to above the side wall of the pressure chamber 14a. On the upper electrode 16b outside the pressure chamber 14a, a first bump 17 made of, for example, a gold stud bump is formed to project toward the wiring board 20. The first bump 17 is electrically connected to the upper electrode 16b.

The wiring board 20 has an upper wiring 23 formed on the upper surface of a substrate body 21 made of a Si substrate via a wiring protective layer 22 made of SiO ₂ . The upper wiring 23 is electrically connected to an end portion of the wiring board 20 by an FPC (flexible printed circuit board) on which a driving circuit is mounted and an ACF (anisotropic conductive film). The upper wiring 23 is covered with a wiring protective layer 26 made of SiO ₂ .

A part of the upper wiring 23 faces the lower surface of the substrate body 21 through a through hole 21a formed in the substrate body 21, and the lower surface of the substrate body 21 is interposed with a wiring protective layer 22 made of SiO _2. The lower wiring 27 formed in this manner is electrically connected. A wiring protective layer 28 made of SiO ₂ is formed on the lower wiring 27, but a part of the lower wiring 27 is exposed through the opening 28 a of the wiring protective layer 28. A second bump 29 is formed on the lower wiring 27 exposed at the position of the opening 28 a so as to protrude toward the head substrate 10. The second bump 29 is formed of, for example, a solder bump and is electrically connected to the lower wiring 27. Further, the second bump 29 and the first bump 17 described above are electrically connected between the head substrate 10 and the wiring substrate 20.

  In addition, an ink supply path 21 b is formed in the wiring board 20. The ink supply path 21 b is opened on the upper surface of the wiring board 20, and the ink in the ink chamber 41 flows from the opening 26 a and is guided to the through hole 31.

  The adhesive resin layer 30 is an adhesive layer for adhering the head substrate 10 and the wiring substrate 20, and is composed of, for example, a thermosetting photosensitive adhesive resin sheet. By using such a photosensitive adhesive resin sheet, an unnecessary portion of the adhesive resin layer 30 can be easily removed by exposure and development processing, and a desired pattern layer can be easily formed.

  The adhesive resin layer 30 is attached in advance to the surface (lower surface) of the wiring protective layer 28 of the wiring substrate 20 in order to ensure a gap corresponding to the thickness of the adhesive resin layer 30 between the head substrate 10 and the wiring substrate 20. Thereafter, it is bonded to the head substrate 10.

  One end (upper end) of the through hole 31 that penetrates the adhesive resin layer 30 communicates with the ink supply path 21 b of the wiring board 20 described above. On the other hand, the other end (lower end) of the through hole 31 communicates with a communication path 15 a formed in the vibration plate 15 of the head substrate 10. Accordingly, the ink in the ink chamber 41 passes through the opening 26a, the ink supply path 21b, the through hole 31, the communication path 15a, the through hole 14b, and the communication path 12b in this order, as indicated by a broken line A1 in FIG. 14a.

  In the inkjet head 1 described above, the first bump 17 on the head substrate 10 side and the second bump 29 on the wiring substrate 20 side are electrically connected, so that the drive voltage from the drive circuit of the wiring substrate 20 is (Drive signal) is supplied to the upper electrode 16b of the actuator section 16 through the upper wiring 23, the lower wiring 27, the second bump 29, and the first bump 17 in this order. The flow of electricity at this time is indicated by a broken line B in FIG. Thus, when a drive voltage is supplied to the upper electrode 16b with respect to the lower electrode 16c of the actuator section 16, the piezoelectric body 16a is deformed (elongated or contracted) by the piezoelectric effect, and the diaphragm 15 vibrates. As a result, pressure for ejection is applied to the ink in the pressure chamber 14a, and the ink in the pressure chamber 14a is ejected as fine droplets from the nozzle 11a through the communication path 12a (broken line A2 in FIG. 1). reference).

[2. Details of piezoelectric element)
Next, details of the piezoelectric element 13 will be described. FIG. 2 is an enlarged cross-sectional view showing a portion P in FIG. In the present embodiment, a part of the upper electrode 16 b in the actuator unit 16 is formed on the piezoelectric body 16 a via the insulating film 18. The insulating film 18 is formed on the piezoelectric body 16a and the lower electrode 16c so as to cover the edge (end) 16e of the piezoelectric body 16a. The piezoelectric body 16a corresponds to the first region R1 located immediately below the insulating film 18 and the non-formation region of the insulating film 18 (which is not covered with the insulating film 18 and is in direct contact with the upper electrode 16b). 2 regions R2. The first region R1 is a region in which the polarization direction during film formation by sputtering is maintained, and the second region R2 is a region in which the polarization direction during film formation by sputtering is reversed.

  Here, in the film formation of the piezoelectric body 16a by the sputtering method, the polarization direction of the piezoelectric body 16a is usually the direction from the lower electrode 16c to the upper electrode 16b. In the present embodiment, only the second region R2 of the piezoelectric body 16a is inverted in polarization direction, so that the polarization direction of the first region R1 is the polarization direction at the time of sputtering film formation, from the lower electrode 16c to the upper electrode 16b. The polarization direction of the second region R2 is the direction from the upper electrode 16b to the lower electrode 16c.

  In this way, the polarization direction of the first region R1 and the second region R2 of the piezoelectric body 16a can be made different because a part of the upper electrode 16b is placed on the piezoelectric body 16a via the insulating film 18. By forming. That is, when the polarization process is performed by applying a voltage higher than the coercive electric field (threshold of the electric field for polarization inversion) between the upper electrode 16b and the lower electrode 16c, in the second region R2 of the piezoelectric body 16a, the above-described anti-electric field Although the polarization inversion occurs due to the electric field, since the insulating film 18 is interposed between the first region R1 and the upper electrode 16b, the electric field applied to the piezoelectric body 16a becomes equal to or lower than the coercive electric field, and the polarization inversion does not occur. . Therefore, by providing the insulating film 18, it is possible to perform a polarization process in which only the second region R2 undergoes polarization inversion, whereby the first region R1 and the second region having different polarization directions in the piezoelectric body 16a. Region R2 can coexist.

  3 is a plan view of the piezoelectric element 13, and FIG. 4 is a plan view of FIG. 3, in which the upper electrode 16b and the first bump 17 are not shown, and the insulating film 18 is hatched for convenience. FIG. 5 shows the first region R1 and the second region of the piezoelectric body 16a with the upper electrode 16b and the first bump 17 omitted in the plan view of FIG. The region R2 is hatched for convenience. Also from these drawings, it can be seen that the insulating film 18 is formed so as to cover the edge 16e of the piezoelectric body 16a.

  The insulating film 18 has an opening 18a inside the formation region of the pressure chamber 14a, and has an opening 18b outside the formation region of the pressure chamber 14a. The upper electrode 16b is in direct contact with the second region R2 of the piezoelectric body 16a inside the opening 18a. The first bump 17 is provided on the upper electrode 16b that contacts the piezoelectric body 16a inside the opening 18b.

  Further, as shown in FIG. 5, the first region R1 of the piezoelectric body 16a is positioned so as to surround the second region R2 (along a plane perpendicular to the film thickness direction). That is, the first region R1 is located outside the second region R2 of the piezoelectric body 16a, and the edge of the first region R1 is the edge 16e of the piezoelectric body 16a.

[3. Inkjet head manufacturing method)
Hereinafter, a method for manufacturing the inkjet head 1 including the piezoelectric element 13 having the above-described configuration will be described as Examples 1 and 2. For comparison with Examples 1 and 2, Comparative Example 1 will also be described.

Example 1
6-10 is sectional drawing which shows each manufacturing process in the manufacturing method of the inkjet head 1 of Example 1. FIG. In addition, the cross section in these drawings shall each respond | correspond to the AA 'line arrow cross section of FIG. Moreover, the numerical values such as the thickness of each layer shown below are merely examples, and are not limited to the numerical values described.

First, as shown in FIG. 6A, an SOI substrate (active layer (thickness 5 μm) / BOX layer (thickness 0) having a thermal oxide film 19a (thickness 0.1 μm) made of SiO ₂ formed on both sides, as shown in FIG. .2 μm) / support layer (thickness 600 μm)). At this time, the pressure chamber plate 14 is constituted by the support layer of the SOI substrate, and the diaphragm 15 is constituted by the BOX layer 15b and the active layer 15c of the SOI substrate.

  Then, titanium (Ti) is formed to a thickness of 20 nm on the surface of the thermal oxide film 19a on the active layer 15c side by sputtering, and thereafter, Ti is oxidized in an oxygen atmosphere to form an adhesion made of titanium oxide (TiOx). Layer 19b is formed. The adhesion layer 19b is a film for improving adhesion between the thermal oxide film 19a serving as the base and the lower electrode 16c. If the adhesion layer 19b is too thin, the adhesion is deteriorated, and if it is too thick, the crystallinity of platinum (Pt) constituting the lower electrode 16c is deteriorated. Therefore, the thickness of the adhesion layer 19b is preferably about 5 nm to 40 nm. Note that the adhesion layer 19b may remain Ti, and even in this case, the adhesion between the thermal oxide film 19a and the lower electrode 16c can be improved.

Subsequently, as shown in FIG. 6B, a lower electrode 16c (thickness: 100 nm) made of Pt is formed by sputtering. Pt has both a role as an electrode for applying a voltage to the piezoelectric body 16a and a role as an orientation control film for improving the crystallinity of PZT constituting the piezoelectric body 16a. If the lower electrode 16c is made thicker, the resistance becomes lower, which is preferable. However, if the lower electrode 16c is made too thick, the smoothness of the surface becomes worse and the crystallinity of PZT formed thereon becomes worse. Pt is preferably oriented in the <111> direction with respect to the substrate surface. At this time, PZT is easily oriented in the <100> direction, and good crystallinity is obtained. As other materials for the lower electrode 16c, iridium (Ir), iridium oxide (IrOx), strontium ruthenate (SRO; SrRuO ₃ ), or the like can be used.

  Next, as shown in FIG. 6C, PZT constituting the piezoelectric body 16a is formed on the lower electrode 16c at a temperature of 600 ° C. by sputtering. The thinner the piezoelectric body 16a is, the more difficult it is to peel off. However, if the thickness is less than 2 μm and becomes too thin, the force for displacing the diaphragm 15 is reduced, so that a certain thickness is required. On the other hand, if the thickness of the piezoelectric body 16a exceeds 10 μm and becomes too thick, the piezoelectric body 16a itself tends to be damaged, and cracks are likely to occur. Therefore, the thickness of the piezoelectric body 16a is preferably 2 to 10 μm. In this case, the displacement of the diaphragm 15 can be increased, and the durability of the piezoelectric body 16a can be improved. In this embodiment, the thickness of the piezoelectric body 16a is 5 μm. In addition, it is preferable that PZT as the piezoelectric body 16a adopts a perovskite structure and has a <100> orientation from the viewpoint of obtaining good piezoelectric characteristics.

  Furthermore, as a method of stably forming a PZT composition and crystal orientation with good reproducibility, a buffer layer made of an insulating oxide having a perovskite structure such as lead lanthanum titanate (PLT) is previously formed on the lower electrode. There is a method of forming PZT on the buffer layer formed on 16c. It is effective to apply such a method to this embodiment.

  The piezoelectric body 16a only needs to contain PZT as a main component, and an additive such as lanthanum (La) may be included in PZT. The phrase “PZT is a main component” means that the proportion of PZT in the piezoelectric body 16a is 80% or more. Thus, since the piezoelectric body 16a has PZT as a main component, the thin film piezoelectric body 16a can be formed by sputtering. PZT is very effective as a material exhibiting good piezoelectric characteristics. Further, since the formed piezoelectric body 16a can be processed by a photolithography method described later, a thin and fine pattern can be obtained.

  Next, as shown in FIG. 7A, the pressure chamber plate 14 is polished to a thickness of 200 μm by chemical mechanical polishing (CMP).

  In this embodiment, the design thickness of the pressure chamber plate 14 (support layer of the SOI substrate) is 200 μm. At this time, if PZT is formed on an SOI substrate whose support layer originally has a thickness of 200 μm, the substrate is warped due to the temperature at the time of film formation, and the in-plane uniformity is reduced. Therefore, in order to form a PZT film with a small warpage of the substrate, the PZT film is formed on the SOI substrate having the support layer having a thickness of 600 μm as described above, and the support layer is polished after the film formation to obtain a desired thickness. It has been adjusted.

  Next, as shown in FIG. 7B, a resist is applied on the piezoelectric body 16a, and exposure and development are performed to form a resist pattern 61 for patterning the piezoelectric body 16a by etching. Then, using the resist pattern 61 as a mask, ICP (Inductively Coupled Plasma) dry etching using chlorine-based or fluorine-based gas is used to form a piezoelectric body 16a having a diameter of 200 μm as shown in FIG. 7C. A PZT pattern having a circular portion is formed. The etching at this time may be wet etching using a hydrofluoric acid solution.

  Subsequently, as shown in FIG. 8A, silicon nitride (SiNx) constituting the insulating film 18 is formed on the lower electrode 16c by plasma CVD (Chemical Vapor Deposition) so as to cover the patterned piezoelectric body 16a. To form a thickness of 50 nm. At this time, by appropriately setting the thickness of SiNx, the electric field density applied to the first region R1 and the second region R2 of the piezoelectric body 16a during the polarization process described later is made different, The polarization directions in the regions can be made different from each other. Further, when the piezoelectric element 13 is driven, the piezoelectric body 16a in the first region R1 can be operated in a direction to reduce the peeling stress of the piezoelectric body 16a.

  Regarding the film thickness setting of SiNx as the insulating film 18, the film quality is preferably about 20 nm to 1000 nm. If the insulating film 18 is too thin, sufficient uniformity cannot be obtained, and if it is too thick, the film is easily damaged. Further, the preferred film thickness range depends greatly on the design conditions and process conditions such as the crystallinity, thickness, dielectric constant, coercive electric field characteristics of the PZT film, the size and driving conditions of the diaphragm 15, and so on depending on the application. It is desirable to set by performing an optimization test together.

  In this embodiment, the insulating film 18 is made of SiNx, but various other insulating films such as an inorganic insulating film and an organic insulating film can be used. In particular, a material that is excellent in film formation uniformity and that can be finely processed is suitable as the insulating film 18.

  Next, as shown in FIG. 8B, a resist is applied on the insulating film 18, exposed and developed, and a resist pattern 62 for patterning the insulating film 18 by etching is formed. Then, using the resist pattern 62 as a mask, by ICP dry etching, as shown in FIG. 8C, the insulating film 18 covers the edge 16e of the piezoelectric body 16a and has an opening 18a having a diameter of 186 μm. Form. The opening diameter of the opening 18a corresponds to the size of the second region R2 (see FIG. 2) of the piezoelectric body 16a. That is, the second region R2 is positioned corresponding to the non-formation region (opening 18a) of the insulating film 18.

  Subsequently, as shown in FIG. 9A, the upper electrode 16b is formed so as to cover the insulating film 18 and the piezoelectric body 16a exposed to the opening 18a. More specifically, first. Ti is formed by sputtering to form a 20 nm thick Ti layer, and then gold (Au) is formed by sputtering to form a 300 nm thick Au layer. An upper electrode 16b made of an Au layer is formed. The Ti layer of the upper electrode 16b functions as an adhesion layer for closely bonding the Au layer and the piezoelectric body 16a (PZT), and the Au layer functions as an electrode. The material constituting the upper electrode 16b is not limited to the above-described Ti or Au, but other than that, Pt, chromium (Cr), aluminum (Al), copper (Cu), silver (Ag) , Ir, IrOx, SRO, and the like can be used.

  Next, as shown in FIG. 9B, a resist is applied on the upper electrode 16b, exposed and developed, and a resist pattern 63 for leaving the upper electrode 16b on the movable portion of the piezoelectric body 16a by etching. Form. Then, the pattern of the upper electrode 16b having a circular portion with a diameter of 196 μm is formed by dry etching using the resist pattern 63 as a mask, as shown in FIG. 9C.

  Subsequently, as shown in FIG. 10A, a resist is applied to the back surface of the pressure chamber plate 14, that is, the surface opposite to the formation side of the piezoelectric body 16a, and exposure and development are performed. Form. This resist pattern 64 corresponds to the pattern of the movable part of the diaphragm (pressure chamber 14a). In this embodiment, the resist pattern 64 is a hole pattern having an opening 64a having a diameter of 230 μm.

  Then, as shown in FIG. 10B, using the resist pattern 64 as a mask, the pressure chamber plate 14 was deep drilled by the Bosch process of the ICP apparatus to form a pressure chamber 14a having a diameter of 230 μm. In the piezoelectric element 13 having such a configuration, the formation region S2 of the pressure chamber 14a is formed inside the formation region S1 of the lower electrode 16c, and the formation region S3 of the piezoelectric body 16a is formed inside the formation region S2 of the pressure chamber 14a. There is a formation region S4 of the upper electrode 16b inside the formation region S3 of the piezoelectric body 16a.

  Thereafter, as shown in FIG. 1, the pressure chamber plate 14 and the intermediate plate 12 are bonded together by anodic bonding, and the intermediate plate 12 and the nozzle plate 11 are bonded together by anodic bonding, whereby the head substrate 10 is completed.

  In this example, after the head substrate 10 was completed as described above (after anodic bonding), the piezoelectric body 16a was subjected to polarization treatment. Specifically, the polarization process was performed by applying a voltage such that the potential of the upper electrode 16b of the piezoelectric element 13 was +50 V and the potential of the lower electrode 16c was GND (ground potential). As a result, as shown in FIG. 2, in the first region R1 of the piezoelectric body 16a covered with the insulating film 18, the polarization direction at the time of sputtering film formation is maintained as it is (the polarization direction changes from the lower electrode 16c to the upper electrode). In the second region R2 of the piezoelectric body 16a that is not covered with the insulating film 18, the polarization direction during sputtering film formation is reversed (the polarization direction is the direction from the upper electrode 16b to the lower electrode 16c). .

  In this embodiment, the polarization process is performed after the head substrate 10 is completed. The polarization process is performed before the completion of the head substrate 10, that is, before the anodic bonding. This is because there is a possibility that the polarization state is changed by high temperature and the piezoelectric characteristics are deteriorated. For example, if the head substrate 10 can be completed without using anodic bonding, such as bonding each plate with an adhesive, the polarization treatment may be performed before the head substrate 10 is completed.

  Finally, the polarization-treated head substrate 10 and the wiring substrate 20 are bonded together via the adhesive resin layer 30, and the manifold 40 constituting the ink chamber 41 is attached to the upper surface of the wiring substrate 20 via the mounting plate 50. Thus, the inkjet head 1 is completed (see FIG. 1).

(Example 2)
In Example 2, the diameter (opening diameter) of the opening 18a of the insulating film 18 is adjusted so that the width of the first region R1 of the piezoelectric body 16a is half the thickness (5 μm) of the piezoelectric body 16a. Except for 5 μm, it is the same as Example 1. That is, in Example 2, when the thickness of the piezoelectric body 16a is d μm, the first area R1 is set so that the second area R2 is separated from the end face 16s (see FIG. 2) of the piezoelectric body 16a by d / 2 μm. The width of was set.

(Comparative Example 1)
Comparative Example 1 is the same as Example 1 except that the piezoelectric body 16a is subjected to the same polarization treatment without forming the insulating film 18. Since the polarization process is performed without forming the insulating film 18, the polarization direction of the piezoelectric body 16a is opposite to the direction of sputtering deposition over the entire area of the piezoelectric body 16a (the direction from the upper electrode 16b to the lower electrode 16c). It is. In the inkjet head of Comparative Example 1, the diameter of the upper electrode 16a was set to 186 μm, which is the same as the diameter of the opening 18a of the insulating film 18 of Example 1.

[4. Durability test)
Next, durability tests of the inkjet heads (piezoelectric elements) of Examples 1 and 2 and Comparative Example 1 were performed. In this durability test, the upper electrode 16b is a signal line, the lower electrode 16c is GND, the drive waveform of the voltage applied to the upper electrode 16a is a trapezoidal wave of 50 kHz, 0 to 25 V, and the rise time of the trapezoidal wave is 1 μs. The piezoelectric element was continuously driven with a fall time of 1 μs.

  As a result, in the inkjet head of Comparative Example 1, PZT peeling occurred when the continuous driving time exceeded 300 hours. In contrast, the inkjet heads of Examples 1 and 2 were continuously driven up to 1000 hours, but PZT peeling did not occur.

  Table 1 shows parameters of the inkjet heads (piezoelectric elements) in Examples 1 and 2 and Comparative Example 1, the maximum amplitude of the diaphragm, and the peeling stress (simulation value) of the piezoelectric body. The maximum amplitude of the diaphragm and the peeling stress of the piezoelectric body are shown as relative values when the value of Comparative Example 1 is 100.

  In Examples 1 and 2 and Comparative Example 1, the maximum amplitude of the diaphragm was approximately 600 nm in measured value (absolute value), and there was almost no difference. In addition, the peeling stress applied to the edge of the piezoelectric body is 24.3% lower than that in Comparative Example 1 in Example 1, and it can be seen that the peeling stress is considerably reduced as compared with Comparative Example 1. . Moreover, also about Example 2, it turns out that peeling stress is reduced about 10% rather than the comparative example 1. FIG. Thus, in Examples 1 and 2, it is considered that PZT peeling is less likely to occur as described above as a result of the reduction in peeling stress compared to Comparative Example 1.

  Further, from Table 1, in Examples 1 and 2, it is considered that there is almost no decrease in the maximum amplitude of the diaphragm. Therefore, according to the configurations of the first and second embodiments, the peeling stress can be reduced and the peeling of PZT can be suppressed without substantially reducing the maximum amplitude of the diaphragm (corresponding to the piezoelectric characteristics of the piezoelectric body). It can be said.

  As described above, in this embodiment, a part of the upper electrode 16b is formed on the piezoelectric body 16a with the insulating film 18 interposed therebetween. Therefore, the piezoelectric element positioned directly below the insulating film 18 during the polarization process. In the first region R1 of the body 16a, only the second region R2 can be inverted in polarization while maintaining the polarization direction during film formation by sputtering. When a positive voltage is applied to the upper electrode 16b to drive the piezoelectric element 13, the piezoelectric body 16a expands in the film thickness direction (contracts in a direction perpendicular to the film thickness direction) in the second region R2, and the first region The piezoelectric body 16a contracts in the film thickness direction in one region R1 (extends in a direction perpendicular to the film thickness direction). Thereby, the peeling stress applied to the edge 16e of the piezoelectric body 16a due to the displacement of the piezoelectric body 16a in the second region R2 can be reduced by the reverse displacement of the piezoelectric body 16a in the first region R1. As a result, even when the piezoelectric element 13 is continuously driven, the piezoelectric body 16a formed by sputtering can be prevented from peeling from the lower electrode 16c, and the reliability can be improved.

  In the present embodiment, the polarization direction of the first region R1 of the piezoelectric body 16a is the direction during sputtering film formation, that is, the direction from the lower electrode 16c to the upper electrode 16b, and the polarization of the second region R2 The direction is a direction from the upper electrode 16b toward the lower electrode 16c. In this way, by setting the polarization direction of each region, a positive voltage is applied to the upper electrode 16b, the second region R2 of the piezoelectric body 16a is extended in the film thickness direction, and the diaphragm 15 is vibrated. The element 13 can be driven. Therefore, when a positive voltage is applied to the upper electrode 16b and the second region R2 of the piezoelectric body 16a is used for driving the element, peeling of the piezoelectric body 16a can be suppressed.

  In addition, since the drive circuit that applies a positive voltage is less expensive than the drive circuit that applies a negative voltage, such a low-cost drive circuit can be obtained by applying a positive polarity voltage to the upper electrode 16b. By using this, the piezoelectric element 13 can be driven.

  If the cost is not considered, the piezoelectric element 13 can be driven by applying a negative voltage to the upper electrode 16b. In this case, in the setting of the polarization direction shown in FIG. 2, the second region R2 of the piezoelectric body 16a contracts in the film thickness direction (extends in the direction perpendicular to the film thickness direction), and the first region R1 becomes the film. Elongates in the thickness direction (shrinks in a direction perpendicular to the film thickness direction). Accordingly, even in this case, the peeling stress applied to the edge 16e of the piezoelectric body 16a due to the displacement of the piezoelectric body 16a in the second region R2 is reduced by the reverse displacement of the piezoelectric body 16a in the first region R1. It is possible to suppress the peeling of the piezoelectric body 16a during continuous driving.

  As shown in FIG. 5, the first region R1 of the piezoelectric body 16a is positioned so as to surround the second region R2, and the first region R1 is located outside the second region R2. Therefore, the insulating film 18 can be easily formed so as to cover the edge 16e of the piezoelectric body 16a. By forming the insulating film 18 in this way, the peeling stress applied to the edge 16e of the piezoelectric body 16a can be relieved by the insulating film 18, so that the peeling of the piezoelectric body 16a can be further suppressed.

  Note that the patterning shape of the insulating film 18 may be changed so that the second region R2 is positioned so as to surround the first region R1 of the piezoelectric body 16a. In this case, the displacement of the first region R1 of the piezoelectric body 16a can be used for driving the element. However, in this case, since the first region R1 is located inside the second region R2, the insulating film 18 cannot be formed so as to cover the edge 16e of the piezoelectric body 16a, but the first region Since the direction of displacement of the piezoelectric body 16a is opposite between R1 and the second region R2, the peeling stress of the piezoelectric body 16a can be reduced, and the peeling of the piezoelectric body 16a can be suppressed.

  From the results of Examples 1 and 2, it can be said that when the thickness of the piezoelectric body 16a is d μm, it is desirable that the second region R2 be separated from the end face 16s of the piezoelectric body 16a by d / 2 μm or more. When the distance between the second region R2 and the end face 16s is shorter than d / 2 μm, when the second region R2 is displaced, the peeling stress applied to the edge 16e of the piezoelectric body 16a increases, and the first Since the width of the region R1 is reduced, the effect of reducing the peeling stress is reduced by the first region R1, and as a result, the piezoelectric body 16a is considered to be easily peeled off. Therefore, by separating the second region R2 from the end surface 16s by d / 2 μm or more, the peeling stress applied to the edge 16e of the piezoelectric body 16a can be reduced, and the peeling of the piezoelectric body 16a can be suppressed.

  Even when the insulating film 18 is formed so as to cover the edge 16e of the piezoelectric body 16a, the insulating film 18 hardly adheres to the end face 16s of the piezoelectric body 16a with a desired thickness, and the insulating film 18 is covered at the edge portion. Defects are likely to occur. For this reason, if the upper electrode 16b is formed larger than the formation region S3 of the piezoelectric body 16a, the upper electrode 16b and the lower electrode 16c may be short-circuited near the edge of the piezoelectric body 16a.

  In this respect, in this embodiment, as shown in FIG. 10B, the upper electrode 16b is formed inside the formation region S3 of the piezoelectric body 16a and does not straddle the edge 16e of the piezoelectric body 16a. Short circuit between the upper electrode 16b and the lower electrode 16c can be avoided.

  The inkjet head 1 of the present embodiment includes the above-described piezoelectric element 13 having a thin film piezoelectric body 16a, and ejects ink stored in the pressure chamber 14a by vibrating the vibration plate 15 of the piezoelectric element 13. Therefore, it is possible to realize a thin and highly reliable inkjet head 1 with excellent durability.

  Further, in the inkjet head 1, as shown in FIG. 10B, there is a formation region S2 of the pressure chamber 14a inside the formation region S1 of the lower electrode 16c, and the formation of the pressure chamber 14a is above the pressure chamber 14a. The formation area S3 of the piezoelectric body 16a is inside the area S2, and the formation area S4 of the upper electrode 16b is inside the formation area S3 of the piezoelectric body 16a. Therefore, the pressure chamber 14a, the piezoelectric body 16a, and the upper electrode 16b Thus, the inkjet head 1 can be realized with high quality image printing and stable printing characteristics.

  Although the case where the piezoelectric element 13 is applied to the inkjet head 1 has been described above, the piezoelectric element 13 of the present embodiment can also be applied to an ultrasonic sensor. That is, in the configuration in which the diaphragm 15 is provided on the substrate (Si substrate, pressure chamber plate 14) so as to cover the opening (pressure chamber 14a), the opening is filled with a discharge medium such as gas or liquid (for example, ink). By doing so, the piezoelectric element 13 can be used as a pump (for example, the inkjet head 1). In addition, the vibration plate 15 can be vibrated by sound waves or ultrasonic waves, and an electric field corresponding to the displacement of the piezoelectric body 16a at that time can be detected via the upper electrode 16b and the lower electrode 16c. The element 13 can be used as an ultrasonic sensor.

  The piezoelectric element of the present invention can be used for, for example, an inkjet head or an ultrasonic sensor.

1 Inkjet head 13 Piezoelectric element 14 Pressure chamber plate (substrate)
14a Pressure chamber (opening)
DESCRIPTION OF SYMBOLS 15 Diaphragm 16a Piezoelectric body 16b Upper electrode 16c Lower electrode 16e Edge 16s End surface 18 Insulating film R1 1st area | region R2 2nd area | region

Claims (12)

A piezoelectric element having a piezoelectric body formed on a vibration plate by a sputtering method, and a lower electrode and an upper electrode formed so as to sandwich the piezoelectric body in a film thickness direction,
A part of the upper electrode is formed on the piezoelectric body via an insulating film,
The piezoelectric body is located immediately below the insulating film, corresponds to a first region where the polarization direction during film formation by sputtering is maintained, and a region where the insulating film is not formed, and the polarization direction is reversed. And a second region. The polarization direction of the first region is a direction from the lower electrode toward the upper electrode;
2. The piezoelectric element according to claim 1, wherein the polarization direction of the second region is a direction from the upper electrode toward the lower electrode.   The piezoelectric element according to claim 1, wherein a positive polarity voltage is applied to the upper electrode.   The piezoelectric element according to claim 1, wherein the first region is positioned so as to surround the second region.   The piezoelectric element according to claim 4, wherein the insulating film is formed so as to cover an edge of the piezoelectric body. When the thickness of the piezoelectric body is d μm,
6. The piezoelectric element according to claim 4, wherein the second region is separated from the end face of the piezoelectric body by d / 2 μm or more.   The piezoelectric element according to claim 1, wherein the piezoelectric body contains lead zirconate titanate as a main component.   The piezoelectric element according to claim 1, wherein the piezoelectric body has a thickness of 2 to 10 μm.   The piezoelectric element according to claim 1, wherein the upper electrode is formed inside a region where the piezoelectric body is formed. A substrate having an opening;
The piezoelectric element according to claim 1, wherein the diaphragm is formed on the substrate so as to cover the opening.   An ink jet head comprising the piezoelectric element according to claim 10, wherein ink accommodated in an opening of the substrate as a pressure chamber is ejected by vibrating the diaphragm of the piezoelectric element. There is a formation region of the pressure chamber inside the formation region of the lower electrode,
Above the pressure chamber,
There is a formation region of the piezoelectric body inside the formation region of the pressure chamber,
The inkjet head according to claim 11, wherein the upper electrode formation region is inside the piezoelectric material formation region.

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