JP2005322890A - Piezoelectric/electrostrictive film element - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a high-sensitivity sensor with regard to a piezoelectric/electrostrictive film element that is related to the piezoelectric/electrostrictive film element, and is used for especially actuators utilizing bending displacement, for example, microphones, sensors of fluid characteristics or sound pressure, very small weight, acceleration or the like, for example, viscosity sensors. <P>SOLUTION: The piezoelectric/electrostriction film element is formed by sequentially laminating a lower electrode and an auxiliary electrode, a piezoelectric/electrostriction film and an upper electrode on a substrate, comprising ceramics having a thin diaphragm section provided with a thick diaphragm on the circumference. In this case, the length of the upper electrode is 30% or higher and 70% or lower of that of the thin diaphragm. <P>COPYRIGHT: (C)2006,JPO&NCIPI

Description

  The present invention relates to a piezoelectric / electrostrictive membrane element, and in particular, an actuator that uses bending displacement, such as a microphone, and a sensor such as a fluid characteristic, sound pressure, minute weight, or acceleration, such as a piezoelectric / electric sensor used for a viscosity sensor. The present invention relates to a strain film type element.

  Piezoelectric / electrostrictive electrostrictive membrane elements have been conventionally used as actuators and various sensors. A piezoelectric / electrostrictive film type element used as a sensor is used for measuring characteristics such as density, concentration, and viscosity of a fluid as disclosed in Patent Document 1, for example. Such an element is used as a sensor by utilizing the fact that there is a correlation between the amplitude of the piezoelectric vibrator and the viscous resistance of the fluid in contact with the vibrator.

  Generally, the vibration form in the mechanical system such as the vibration of the vibrator can be replaced with an equivalent circuit in the electric system, and the vibrator is made to vibrate the piezoelectric / electrostrictive membrane vibrator in the fluid. Detecting changes in the electrical constant of the equivalent circuit of the piezoelectric body that constitutes the vibrator by receiving mechanical resistance based on the viscous resistance of the fluid, and measuring characteristics such as viscosity, density, and concentration of the fluid Is possible. Measurable fluids mean liquids and gases, and not only liquids consisting of a single component, such as water, alcohol, oil, but also soluble or insoluble media dissolved or mixed or suspended in these liquids. Includes liquids, slurries, and pastes.

  In addition, examples of the electrical constant to be detected include loss factor, phase, resistance, reactance, conductance, susceptance, inductance, capacitance, etc. Especially, one maximum or minimum change point near the resonance frequency of the equivalent circuit. The loss factor or phase that it has is preferably used. As a result, not only the viscosity of the fluid but also the density and concentration can be measured. For example, the sulfuric acid concentration in the sulfuric acid aqueous solution can be measured. In addition to the electrical constant, as an index for detecting the change in the vibration form, the change in the resonance frequency can be used if there is no particular problem from the viewpoint of measurement accuracy and durability.

Such a piezoelectric / electrostrictive film type element is laminated on a substrate 1 made of ceramics having a thin diaphragm portion 3 having a thick portion 2 as a peripheral portion as shown in FIG. The auxiliary electrode 8 is formed at a position independent of the lower electrode 4, and a part of the auxiliary electrode is formed so as to enter a part below the piezoelectric / electrostrictive film 5. With such a configuration, the upper electrode 6 can be continuously formed on the auxiliary electrode 8 and the surface of the piezoelectric / electrostrictive film 5 without being disconnected, and the connection reliability of the upper electrode 6 is improved. In FIG. 1, the fluid to be measured exists in the cavity 10 and is introduced through the through hole 9. Furthermore, by forming the auxiliary electrode 8 continuously from the thin diaphragm portion 3 to the thick portion 3, a stable device characteristic and a device applicable under any use conditions can be obtained.
JP-A-8-201265 JP 2002-261347 A

  In a sensor element that performs sensing by detecting such electrical characteristics based on vibration, for example, when detecting a change in resonance frequency due to a phase peak, as shown in FIG. A sharp phase peak is desirable as a sensor element with high resolution and sensitivity. However, in the past, a peak with sufficient sharpness could not be obtained, which was an obstacle when performing highly accurate detection.

  In the conventional piezoelectric / electrostrictive membrane element shown in FIG. 1, as shown in FIG. 4, the actual vibration deformation shape deviates from the ideal vibration deformation shape. As a result, a sharp change in electrical characteristics could not be obtained.

  The present invention has been made to address this problem. That is, an object of the present invention is to provide a piezoelectric / electrostrictive membrane element that can match an ideal vibration deformation shape with an actual vibration deformation shape. Another object of the present invention is to provide a highly sensitive sensor element.

  That is, the piezoelectric / electrostrictive film type device according to the present invention sequentially forms a lower electrode and an auxiliary electrode, a piezoelectric / electrostrictive film, and an upper electrode on a substrate made of ceramics having a thin diaphragm portion having a thick portion at the peripheral portion. A laminated piezoelectric / electrostrictive film type element, wherein the length of the upper electrode is not less than 30% and not more than 70% of the length of the thin diaphragm portion. is there.

  Further, the piezoelectric / electrostrictive film type element according to the present invention has a lower electrode and an auxiliary electrode, a piezoelectric / electrostrictive film, and an upper electrode in order on a substrate made of ceramics having a thin diaphragm portion having a thick portion at a peripheral portion. A laminated piezoelectric / electrostrictive film element, wherein the width of the upper electrode is 70% or more of the width of the thin diaphragm portion 3.

  Here, the length of the upper electrode and the length of the thin diaphragm portion refer to the distance between both ends of each member in the direction parallel to the longitudinal direction of the cavity portion 10, and the width of the upper electrode and the thin wall portion The width of the diaphragm portion refers to the distance between both ends of each member in the direction parallel to the short direction of the cavity portion 10.

  The piezoelectric / electrostrictive membrane element provided by the present invention matches the ideal vibration deformation shape with the actual vibration deformation shape, so that a sharp change in the electrical characteristics can be obtained, thereby improving the resolution. And a sensor element with high sensitivity can be provided.

  BEST MODE FOR CARRYING OUT THE INVENTION Hereinafter, the best mode for carrying out the present invention will be described. It should be understood that design changes, improvements, and the like can be made as appropriate.

  As shown in FIG. 1, the piezoelectric / electrostrictive film type vibrator according to the present invention has a lower electrode 4 and a piezoelectric / electrostrictive film 5 on a ceramic substrate 1 composed of a thin diaphragm portion 3 and a thick portion 2. The upper electrode 6 is formed as an integral structure formed by sequentially laminating by a normal film forming method.

  One end of the lower electrode 4 on the auxiliary electrode 8 side is formed with a length up to a position not exceeding the thin diaphragm portion 3. The auxiliary electrode 8 is located on the same plane as the lower electrode 4, at a position independent of the lower electrode 4, so as to enter the lower side of the piezoelectric / electrostrictive film 5, and from the thick portion 1 on the side opposite to the lower electrode 4. It is continuously formed with a predetermined length up to the thin diaphragm portion 3. The piezoelectric / electrostrictive film 5 is formed so as to straddle the lower electrode 4 and the auxiliary electrode 8, and the upper electrode 6 is formed so as to be electrically connected to the auxiliary electrode 8 across the piezoelectric / electrostrictive film 5 and the auxiliary electrode 8.

  The length Lu of the upper electrode 6 is set to 30% to 70% of the length Ld of the diaphragm portion 3. Note that the connection portion of the upper electrode 6 for conducting with the auxiliary electrode 8 does not contribute to the substantial operation of the piezoelectric / electrostrictive film type element, and is not included in the length Lu here.

  If this length exceeds 70%, a sharp peak cannot be obtained as a result, and if it is shorter than 30%, the electric constant value itself becomes small, resulting in a decrease in sensitivity.

  In order to obtain a sharp peak, the shape of the upper electrode 6 that contributes to the length Lu of the upper electrode 6 in the length direction is preferably line symmetric. This is because it can be made to vibrate by emphasizing only the inherent vibration.

  The material of the ceramic substrate 1 is preferably a material having heat resistance, chemical stability, and insulation. This is because, as will be described later, when the lower electrode 4, the piezoelectric / electrostrictive film 5 and the upper electrode 6 are integrated, heat treatment may be performed, and the piezoelectric / electrostrictive film type element as the sensor element has liquid characteristics. This is because the liquid may be conductive or corrosive when sensing.

  Examples of ceramics that can be used from this viewpoint include stabilized zirconium oxide, aluminum oxide, magnesium oxide, mullite, aluminum nitride, silicon nitride, and glass. Among these, stabilized zirconium oxide can be preferably used because it can maintain high mechanical strength and has excellent toughness even when the thin diaphragm portion is formed thin.

  The thickness of the thin diaphragm portion 3 of the ceramic substrate 1 is generally set to 50 μm or less, preferably 30 μm or less, and more preferably 15 μm or less in order to prevent the drive amplitude from being reduced.

  Further, the surface shape of the thin diaphragm portion may be any shape such as a rectangle, a square, a triangle, an ellipse, or a perfect circle. In order to obtain an ideal deformed shape, a rectangular shape, an oval shape, or an elliptical shape with an aspect ratio of 1.5 or more is preferable.

  A lower electrode 4 and an auxiliary electrode 8 are formed on the surface of the ceramic substrate 1.

  At this time, the width of the lower electrode 4 may be larger than the width of the diaphragm portion 3, or may be smaller than the width of the piezoelectric / electrostrictive film 5.

  On the other hand, the auxiliary electrode 8 is continuously formed from the end of the ceramic substrate 1 opposite to the lower electrode 4 to a predetermined position on the thin diaphragm portion 3. The end portions on the thick portion of the lower electrode 4 and the auxiliary electrode 8 are connected to different terminal electrodes 11, respectively.

  The lower electrode 4 and the auxiliary electrode 8 may be made of different materials or the same material, and a conductive material having good bonding properties to both the ceramic substrate 1 and the piezoelectric / electrostrictive film 5 is used.

  Specifically, an electrode material mainly composed of platinum, palladium, rhodium, silver, or an alloy thereof is preferably used. In particular, when forming a piezoelectric / electrostrictive film, a heat treatment for sintering is performed. In this case, platinum and an alloy containing this as a main component are preferably used.

  Various known film forming techniques are used to form the lower electrode 4 and the auxiliary electrode 8. Specifically, thin film formation methods such as ion beam, sputtering, vacuum deposition, CVD, ion plating, plating, and thick film formation methods such as screen printing, spraying, and dipping are appropriately selected. A method and a screen printing method are preferably selected.

  In the case where a bonding layer for bonding the piezoelectric / electrostrictive film 5 and the thin diaphragm portion 3 is provided in the gap between the lower electrode 4 and the auxiliary electrode 8, prior to the formation of the piezoelectric / electrostrictive film 5, FIG. The bonding layer 7 may be formed at a position as shown.

  The formation of the bonding layer 7 makes the rigidity on the diaphragm portion 3 uniform, which is preferable for obtaining an ideal vibration deformation shape.

  As the bonding layer 7 made of an insulator, any material of an organic material or an inorganic material may be used as long as the adhesion and bonding properties between the piezoelectric / electrostrictive film 5 and the ceramic substrate 1 are high.

  Moreover, it is highly reliable that the thermal expansion coefficient of the material used as the bonding layer 7 has an intermediate value between the thermal expansion coefficient of the substrate material and the thermal expansion coefficient of the material used for the piezoelectric / electrostrictive film 5. Is more preferable. When the piezoelectric / electrostrictive film 5 is heat-treated for sintering, the material constituting the bonding layer 7 is a material used for the piezoelectric / electrostrictive film 5 to which a small amount of glass component is added, or a piezoelectric / A glass material having a softening point equal to or higher than the heat treatment temperature of the electrostrictive film 5 is preferably used because it has high adhesion and bonding properties to both the piezoelectric / electrostrictive film 5 and the ceramic substrate 1.

Further, the piezoelectric / electrostrictive film 5 is made of (Bi _0.5 Na _0.5 ) TiO ₃ described later or a material containing this as a main component, or (1-x) (Bi _0.5 Na _0.5 ) TiO ₃ -xKNbO ₃ (x is mol (1-x) (Bi _0.5 Na _0.5 ) TiO ₃ —xKNbO ₃ (x is a molar fraction of 0 < The bonding layer 7 obtained by adding a small amount of a glass component to a material having x ≦ 0.5) as a main component has high adhesion between the piezoelectric / electrostrictive film 5 and the ceramic substrate 1, and the piezoelectric / electrostrictive film during heat treatment. 5 and the substrate 1 can be prevented from being adversely affected.

That is, by forming the bonding layer 7 with a material obtained by adding a small amount of a glass component to (1-x) (Bi _0.5 Na _0.5 ) TiO ₃ —xKNbO ₃ (x is 0 <x ≦ 0.5 in terms of mole fraction), Since it has the same components as the piezoelectric / electrostrictive film 5, the adhesiveness with the piezoelectric / electrostrictive film 5 is high, and there are few problems due to the diffusion of different elements that are likely to occur when glass is used alone, and KNbO ₃ is used. As a result, the substrate 1 is highly reactive and enables strong bonding. In addition, since the main component of the bonding layer is (1-x) (Bi _0.5 Na _0.5 ) TiO ₃ —xKNbO ₃ (x is 0.08 ≦ x ≦ 0.5 in terms of mole fraction), the bonding layer exhibits almost piezoelectric characteristics. Therefore, vibration, displacement, and stress are not generated with respect to the electric field generated in the lower electrode 4 and the auxiliary electrode 8 during use, so that stable element characteristics can be obtained.

  A normal thick film method is used to form these bonding layers 7, and in particular, a stamping method, a screen printing method, or an inkjet method is suitable when the size of the portion to be formed is about several tens to several hundreds of μm. Used for. Further, when the bonding layer 7 needs to be heat-treated, it may be heat-treated before the next piezoelectric / electrostrictive film 5 is formed, or may be simultaneously heat-treated after the piezoelectric / electrostrictive film 5 is formed.

The piezoelectric / electrostrictive film 5 is formed so as to straddle the lower electrode 4, the auxiliary electrode 8, and the coupling layer 7. The material of the piezoelectric / electrostrictive film may be any material that exhibits a piezoelectric / electrostrictive effect, such as lead zirconate, lead titanate, lead zirconate titanate (PZT), etc. Lead-based ceramic piezoelectric / electrostrictive materials, barium titanate and titavari-based ceramic ferroelectrics based on it, polymer piezoelectric materials represented by polyvinylidene fluoride (PVDF), or (Bi _0.5 Na _0.5 ) Bi-based ceramic piezoelectric bodies represented by TiO ₃ and Bi layered ceramics. Of course, it is needless to say that a mixture, a solid solution, or a material obtained by adding an additive to these, which has improved piezoelectric / electrostrictive characteristics, can be used.

  The PZT-based piezoelectric body is suitably used as a sensor material having high piezoelectric characteristics and capable of high sensitivity detection. Particularly in the present invention, the substrate is composed of a material mainly composed of at least one selected from lead titanate, lead zirconate, lead magnesium niobate, lead nickel niobate. It is more suitably used because it has low reactivity with the material, segregation of components during heat treatment hardly occurs, treatment for maintaining the composition can be performed well, and the intended composition and crystal structure are easily obtained. .

Further, when platinum or an alloy containing platinum as a main component is used for the lower electrode 4 and the auxiliary electrode 8, the bondability with these is higher, the characteristic variation of the element is reduced, and high reliability is obtained. Therefore, (Bi _0.5 Na _0.5 ) TiO ₃ or a material containing this as a main component is preferably used. Among these, in particular, (1-x) (Bi _0.5 Na _0.5 ) TiO ₃ —xKNbO ₃ (x is a molar fraction, 0 ≦ x ≦ 0.06) or a material mainly composed thereof has relatively high piezoelectric characteristics. Since it has, it is used more suitably.

  Such a piezoelectric / electrostrictive material is formed as the piezoelectric / electrostrictive film 5 by various known film forming methods in the same manner as the lower electrode 4 and the auxiliary electrode 8. Among these, screen printing is preferably used from the viewpoint of low cost.

The piezoelectric / electrostrictive film 5 thus formed is heat-treated as necessary, and integrated with the lower electrode 4, the auxiliary electrode 8 and the coupling layer 7. In order to suppress the variation in the characteristics of the element and to increase the reliability, when it is necessary to further strengthen the bonding property between the piezoelectric / electrostrictive film 5, the lower electrode 4, the auxiliary electrode 8, and the coupling layer 7, (Bi _0.5 Na _0.5 ) TiO ₃ or a material based on this, in particular, (1-x) (Bi _0.5 Na _0.5 ) TiO ₃ —xKNbO ₃ (x is the molar fraction 0 ≦ x ≦ 0.06) or the main It is preferable to heat-treat at a temperature of 900 ° C. to 1400 ° C., preferably 1000 ° C. to 1300 ° C., using the material as a component. The same applies when using PZT-based materials. At this time, it is preferable to perform heat treatment while controlling the atmosphere together with the evaporation source of the piezoelectric / electrostrictive material so that the piezoelectric / electrostrictive film 5 does not become unstable at a high temperature.

  Further, the upper electrode 6 is continuously formed on the piezoelectric / electrostrictive film 5 thus formed so as to extend from the piezoelectric / electrostrictive film 5 to the auxiliary electrode 8.

  As the material of the upper electrode 6, a conductive material having high bondability with the piezoelectric / electrostrictive film 5 is used, and the upper electrode 6 is formed by the same film forming method as the lower electrode 4 and the auxiliary electrode 8.

  Further, the upper electrode 6 is heat-treated as necessary after the film is formed, and is joined to the piezoelectric / electrostrictive film 5 and the auxiliary electrode 8 to form an integral structure. It is the same as that of the lower electrode 4 that such a heat treatment is not always necessary.

  In order to obtain an ideal deformed shape, it is desirable that the rigidity is uniform on the diaphragm portion 3. For this purpose, the lower electrode 4, the auxiliary electrode 8, the coupling layer 7, the piezoelectric / electrostrictive film 5, the upper portion The electrode 6 is preferably integrated with the diaphragm portion 3 by heat treatment, rather than being bonded using an adhesive.

  The length Lu of the upper electrode 6 is set to 30% to 70% of the length Ld of the diaphragm portion 3. The width of the upper electrode 6 is set to 70% or more with respect to the width of the diaphragm portion 3 in order to obtain an ideal deformed shape. In addition, when comparing the width | variety of the upper electrode 6 and the diaphragm part 3, it shall compare in the longest part among the lengths of each width | variety.

  In order to obtain a sharp peak, the shape of the upper electrode 6 in the width direction is desirably line symmetric. This is because it can be made to vibrate by emphasizing only the inherent vibration.

  Further, it is desirable that the center of the upper electrode 6 and the diaphragm portion 3 coincide with each other, but the deviation from the center is 5% or less with respect to the length of the diaphragm portion 3 in the length direction of the diaphragm portion 3, and the diaphragm in the width direction. If it is 10% or less with respect to the width of the portion 3, an ideal deformed shape can be obtained, and there is no particular problem.

  In addition, it is preferable that the ratio of the area which the part effective for operation | movement of the upper electrode 6 occupies with respect to the area of the diaphragm part 3 is 15% or more and 40% or less. This is because if this ratio is 15% or more, vibration necessary for sensing can be obtained. Moreover, if it is 40% or less, rigidity advantageous for vibration can be obtained.

  FIG. 5 shows a piezoelectric / electrostrictive film element for a sensor as one embodiment of the present invention. The actual vibration deformation shape when the shape of the upper electrode is a rectangle and Lu / Ld is 0.42 and 0.57 (Example 1) is shown in FIG.

  As is apparent from FIG. 5, the shape is close to the ideal vibration deformation shape, and the phase peak with respect to the frequency of these elements is as shown in FIG. 2, and a sharp peak was obtained.

  Further, as shown in FIG. 7, when the shape of the upper electrode is diamond and Lu / Ld is 0.57 (Example 2), the actual vibration deformation shape is ideal as shown in FIG. As shown in FIG. 2, the phase peak with respect to the frequency of these elements became a sharp peak.

  Further, when the upper electrode is circular as shown in FIG. 9 and Lu / Ld is 0.42 (Example 3), the actual vibration deformation shape is compared with the conventional example as shown in FIG. Thus, it became close to an ideal vibration deformation shape, and the phase peak with respect to the frequency of these elements was as shown in FIG. 2, and a sharp peak was obtained.

  When the lower electrode 4, the bonding layer 7, the piezoelectric / electrostrictive film 5, and the upper electrode 6 are bonded by heat treatment, they may be heat-treated each time they are formed. You may heat-process. Needless to say, the heat treatment temperature is appropriately selected in order to suppress the deterioration due to good bondability and diffusion of constituent elements during the heat treatment.

  Further, in FIG. 1, the through hole 9 is formed in the cavity 10, but the structure below the cavity 10 where the element contacts the fluid may be any structure such as a simple cavity structure without a lid, Not limited.

  Further, the end of the piezoelectric / electrostrictive film 5 in the length direction may be disposed so as not to exceed the thin diaphragm portion 3 so that the piezoelectric / electrostrictive film 5 does not straddle the thick portion 2.

It is explanatory drawing which shows embodiment of the conventional piezoelectric / electrostrictive film type | mold element. It is explanatory drawing which shows a peak shape desirable as a piezoelectric / electrostrictive film type | mold element. It is explanatory drawing which shows the peak shape in the conventional piezoelectric / electrostrictive film type | mold element for use. It is explanatory drawing which shows the vibration deformation shape in the past. It is explanatory drawing which shows embodiment of the piezoelectric / electrostrictive film type | mold element which concerns on this invention. It is explanatory drawing which shows the vibration deformation shape in FIG. It is explanatory drawing which shows other embodiment of the piezoelectric / electrostrictive film type | mold element which concerns on this invention. It is explanatory drawing which shows the vibration deformation | transformation shape in FIG. It is explanatory drawing which shows other embodiment of the piezoelectric / electrostrictive film type | mold element which concerns on this invention. It is explanatory drawing which shows the vibration deformation shape in FIG.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 ... Board | substrate, 2 ... Thick part, 3 ... Diaphragm part, 4 ... Lower electrode, 5 ... Piezoelectric / electrostrictive film, 6 ... Upper electrode, 7 ... Coupling layer, 8 ... Auxiliary electrode, 9 ... Through-hole, 10 ... Cavity part, 11 ... terminal electrode.

Claims (2)

A piezoelectric / electrostrictive film type element in which a lower electrode and an auxiliary electrode, a piezoelectric / electrostrictive film, and an upper electrode are sequentially laminated on a substrate made of ceramics having a thin-walled diaphragm portion with a thick-walled portion at the periphery,
The piezoelectric / electrostrictive film type device, wherein the length of the upper electrode is 30% or more and 70% or less of the length of the thin diaphragm portion.   2. The piezoelectric / electrostrictive film element according to claim 1, wherein the width of the upper electrode is 70% or more of the width of the thin diaphragm portion.

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