JP2008085194A - Ferroelectric device - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a ferroelectric device which can suppress a power consumption without the need for continuing applying a voltage to keep a predetermined piezoelectric deformation, dielectric constant, and refractive index. <P>SOLUTION: A ferroelectric material is provided with a memory function of a piezoelectric deformation, a dielectric constant and a refractive index by the imprinting of an electric field. A piezoelectric device includes the ferroelectric material having the memory function for selectively keeping predetermined binary piezoelectric deformations without applying a voltage. A ferroelectric memory device includes the ferroelectric material having the memory function for selectively keeping predetermined binary dielectric constants without applying a voltage. An electrooptic device includes the ferroelectric material having the memory function for selectively keeping predetermined binary refractive indexes without applying a voltage. <P>COPYRIGHT: (C)2008,JPO&INPIT

Description

  The present invention relates to a ferroelectric device such as a piezoelectric device, a ferroelectric memory device, and an electro-optical device provided with a ferroelectric material.

  A ferroelectric is a material having spontaneous polarization that can be controlled by an external electric field. Among the ferroelectric characteristics, piezoelectric characteristics, dielectric characteristics, and optical characteristics show strong non-linearity with respect to an external electric field, but it is common knowledge that there is no memory function. That is, when the external electric field is returned to 0, the same piezoelectric strain, the same dielectric constant, and the same refractive index are obtained regardless of the direction of the spontaneous polarization value.

For example, a bending type piezoelectric actuator switch bends and deforms when a voltage is applied, and functions as a mechanical relay that turns the switch on or off. To maintain the on or off state, Must continue to be applied. Similarly, a memory that uses dielectric constant control by applying a voltage and an optical prism that uses a change in refractive index by applying a voltage must continue to apply a voltage in order to maintain its physical property values.
JP 2000-22091 A

  Therefore, in ferroelectric devices such as conventional piezoelectric devices, ferroelectric memory devices, and electro-optic devices that use a ferroelectric material, a voltage is applied to maintain a predetermined piezoelectric strain, dielectric constant, and refractive index. There was a problem that the power consumption would be increased.

  Therefore, in view of the above problems, the present invention provides a ferroelectric device capable of suppressing power consumption without having to continue to apply a voltage in order to maintain predetermined piezoelectric strain, dielectric constant, and refractive index. The purpose is to do.

  The ferroelectric device according to claim 1 of the present invention is a ferroelectric device provided with a memory function of piezoelectric distortion, dielectric constant, refractive index, piezoelectric constant, electromechanical coupling coefficient, Young's modulus, or pyroelectric constant by electric field imprinting. A dielectric material is provided.

  The ferroelectric device according to claim 2 of the present invention is the ferroelectric device according to claim 1, wherein the ferroelectric material is composed of lead zirconate titanate, lead titanate, barium titanate, lithium niobate, lead lanthanum zirconate titanate. It is either.

  According to a third aspect of the present invention, there is provided a piezoelectric device comprising a ferroelectric material having a memory function for selectively holding a predetermined binary piezoelectric strain without applying a voltage, and the memory function is achieved by electric field imprinting. It is what was given.

  According to a fourth aspect of the present invention, there is provided a dielectric constant detection type ferroelectric memory device comprising a ferroelectric material having a memory function of selectively holding a predetermined binary dielectric constant without applying a voltage. The memory function is provided by electric field imprinting.

  According to a fifth aspect of the present invention, there is provided an electro-optical device comprising a ferroelectric material having a memory function that selectively retains a predetermined binary refractive index without applying a voltage. It is given by.

  According to the ferroelectric device of the present invention, it is not necessary to continue to apply a voltage in order to maintain a predetermined piezoelectric strain, dielectric constant, and refractive index, and a ferroelectric device capable of suppressing power consumption is provided. can do. In addition, by adding a memory function to the piezoelectric characteristics, dielectric characteristics, and optical characteristics of the ferroelectric material, it is possible to provide an unprecedented ferroelectric device.

  The ferroelectric device of the present invention includes a ferroelectric material provided with a memory function of piezoelectric distortion, dielectric constant, refractive index, piezoelectric constant, electromechanical coupling coefficient, Young's modulus, or pyroelectric constant by electric field imprinting. It is a thing.

  Here, the ferroelectric device refers to all devices to which a ferroelectric material is applied, and widely includes piezoelectric devices, ferroelectric memory devices, electro-optical devices, and the like.

  Further, the ferroelectric material is not limited to a specific one, but for example, lead zirconate titanate (PZT), lead titanate, barium titanate, lithium niobate, lead lanthanum zirconate titanate. (PLZT).

  The piezoelectric device according to the present invention includes a ferroelectric material provided with a memory function of piezoelectric distortion by electric field imprinting. This memory function selectively holds a predetermined binary piezoelectric strain without applying a voltage.

  The dielectric constant detection type ferroelectric memory device of the present invention includes a ferroelectric material provided with a dielectric constant memory function by electric field imprinting. This memory function selectively holds a predetermined binary dielectric constant without applying a voltage.

  The electro-optic device of the present invention includes a ferroelectric material provided with a memory function of refractive index by electric field imprinting. This memory function selectively holds a predetermined binary refractive index without applying a voltage.

  The present invention proposes a mechanism capable of maintaining a piezoelectric strain, a dielectric constant, and a refractive index at predetermined values even when no voltage is applied in a ferroelectric device. Hereinafter, the principle will be described.

  As shown in the graph representing the relationship between the electric flux density (D) and the electric field (E) as a concept of the conventional ferroelectric substance on the left side of FIG. 1, the ferroelectric substance applies the electric field after applying a positive electric field. The electric flux density when it is zero takes a positive value P +, and the electric flux density when the electric field is zero after applying a negative electric field takes a negative value P−. Such a change in the electric flux density is caused by the reversal of the polarization direction. Thus, since it has a memory function of storing the direction of spontaneous polarization, the ferroelectric is applied to FeRAM which is a nonvolatile memory. The absolute value of the positive value P + and the negative value P- is the same.

  On the other hand, the graph showing the relationship between the piezoelectric strain (S), the dielectric constant (ε), and the refractive index (n) with respect to the electric field applied to the ferroelectric is symmetric, and even if the polarization direction of the ferroelectric is reversed. These physical property values are constant. These physical property values are the same when the input voltage is zero after applying a positive electric field and when the input voltage is zero after applying a negative electric field. Therefore, the ferroelectric does not have a memory function with respect to piezoelectric distortion, dielectric constant, and refractive index.

  Next, in the case where an offset electric field is applied in the dielectric application direction, that is, an electric field imprint is applied, the respective graphs are shifted in the horizontal direction as shown on the right side of FIG. Then, due to the reversal of the polarization direction, the electric flux density (D), the piezoelectric strain (S), the dielectric constant (ε), and the refractive index (n) show different values from those before the polarization reversal. Thus, the memory function is imparted to the ferroelectric by electric field imprinting. This memory function selectively retains predetermined binary piezoelectric strain, dielectric constant, and refractive index without applying an electric field. Ferroelectric materials having two stable piezoelectric strains, dielectric constants, and refractive indices due to such spontaneous polarization can be applied to various ferroelectric devices including various switches with reduced power consumption.

  Electric field imprinting is known as a phenomenon in which the polarization of a ferroelectric substance becomes wrinkled by repeated polarization inversion due to the application of an electric field, making it difficult to invert. It has been observed in experiments on the ferroelectric thin film used. This electric field imprinting changes the voltage (at the time of “0” writing and at the time of “1” writing) required at the time of bit writing using polarization inversion. In a ferroelectric memory, it is required to eliminate the electric field imprint in order to reverse the polarization symmetrically with respect to the electric field. Accordingly, research has been conducted on how to eliminate electric field imprinting, and attempts have been made to remove the substrate interface layer.

  Currently, the causes of electric field imprint are (1) a non-inverted ferroelectric layer near the drive electrode interface, (2) lattice mismatching between the ferroelectric thin film and the drive electrode, and (3) a ferroelectric substrate. The reason is the space charge layer due to lattice defects near the interface, but there are still many unclear points.

  In the present invention, this electric field imprint is actively used to impart a memory effect to the piezoelectric strain, dielectric constant, and refractive index of the ferroelectric material, and the ferroelectric material to which this memory effect is imparted has been used. The present invention provides various ferroelectric devices that are completely absent from the above.

  An example of such a ferroelectric device is a smart piezoelectric switch that keeps its position without power supply. By configuring a piezoelectric actuator with a memory holding function using a ferroelectric material imparted with a memory effect, it can be applied to an RF-MEMS switch or the like.

  In the conventional piezoelectric actuator, in order to maintain a certain piezoelectric displacement, it was necessary to continue to apply a voltage. However, in the piezoelectric actuator according to the present invention, it is possible to continue to maintain a certain shape even when the voltage is zero. . This drive can be driven with a pulse voltage. By using the electric charge stored in the external capacitor as a drive source, the drive can be performed with low power consumption. Further, a multi-value shape memory can be obtained by a structure in which a plurality of piezoelectric driving units are stacked.

  The piezoelectric actuator according to the present invention can be applied to any shape type such as a bimorph piezoelectric element and a laminated piezoelectric element. Also, regarding piezoelectric materials, not only lead zirconate titanate (PZT), but also polycrystals and single crystals such as lead titanate, barium titanate, lithium niobate, and lead lanthanum zirconate titanate (PLZT) It can be used for any material.

  In addition, the present invention can be applied to a recording device based on a new principle that holds a change in dielectric constant as recording data. That is, a memory based on a new principle is realized by detecting the current value when AC voltage is applied as data. Unlike the conventional charge recording type FeRAM, since the dielectric constant detection is performed, a very simple structure can be obtained, and as a result, it is possible to contribute to higher density of the device.

  Moreover, it can be applied to an optical smart switch based on a change in refractive index. At present, an optical switch in which a triangular electrode is arranged on the top of a ferroelectric and the light propagation direction is controlled by applying a voltage has been studied. In this case, the propagation direction of light changes with the magnitude of the voltage, but by using a ferroelectric material with a memory effect, the propagation of light in a certain direction can be achieved without applying a voltage. It becomes possible to maintain the direction.

  Furthermore, it can be applied to an optical storage element. If a photodetector is prepared in the light propagation direction of the above-mentioned optical smart switch, it becomes possible to detect the polarization direction of the ferroelectric, so that it can be applied as an optical storage element.

  In addition to the above, the ferroelectric device of the present invention can also be applied as a piezoelectric sensor used as a sensor for acceleration, gyro and the like. Here, piezoelectric sensors having different sensitivities can be obtained by storing the piezoelectric constants. This is the same as storing the electromechanical coupling coefficient, that is, the conversion efficiency between electric energy and mechanical energy. Further, since the Young's modulus of the piezoelectric constant is a function of the electromechanical coupling coefficient, the Young's modulus is also stored in memory.

  In addition, in the ferroelectric material, the polarization value changes with temperature change, and there is an effect of a pyroelectric effect in which electric charges are excited on the surface. Therefore, it is possible to configure temperature sensors having different sensitivities by storing the pyroelectric constants.

  Such a ferroelectric device with a memory function to which electric field imprinting is applied is expected to be applied to mobile devices and the like that are particularly required to be reduced in size and weight while suppressing power consumption.

  Next, a method for applying electric field imprint will be described. The method for applying electric field imprinting is not limited to the following method.

  As shown in the center of FIG. 1, a ferroelectric film 2 made of a ferroelectric material is formed on a conductive substrate 1 made of a conductive material, and an electrode 3 is placed on the ferroelectric film 2. An electric field imprint is applied to the ferroelectric film 2 by continuously applying an electric field of 1 kV / mm or more for 1 hour or more in an environment of 100 ° C. or more. Graphs representing respective relationships of electric flux density (D), piezoelectric strain (S), dielectric constant (ε), and refractive index (n) with respect to an electric field applied to the ferroelectric film 2 subjected to electric field imprinting are as follows: As shown on the right side of FIG.

  The degree of electric field imprinting can be controlled by temperature, electric field, and time. By controlling temperature, electric field, and time, electric field imprinting can be performed according to the purpose and application of the ferroelectric device. it can. The higher the temperature, the stronger the electric field, and the longer the time, the greater the degree of electric field imprinting, and the greater the deviation of the graph shown in FIG. Further, if the ferroelectric film 2 is thinned, a strong electric field can be applied even at the same voltage.

  In addition, by changing the synthesis conditions of the ferroelectric material without applying a treatment to apply a strong electric field at a high temperature as described above, a state similar to that in which electric field imprinting is performed is created and the degree of control is controlled. can do. For example, epitaxial growth of a ferroelectric material crystal causes electric field imprinting. (1) Non-inverted ferroelectric layer in the vicinity of the drive electrode interface, (2) Lattice error between the ferroelectric thin film and the drive electrode Matching, (3) a space charge layer due to lattice defects in the vicinity of the substrate interface of the ferroelectric, and the like can be generated, and a state similar to that in which electric field imprinting is performed can be obtained.

  Alternatively, in a ferroelectric material that has been polarized in one direction, a memory effect similar to that applied to electric field imprinting can be achieved by partially inverting the polarization by adjusting the driving electric field. can do.

  In the present invention, “electric field imprint” means to create a state similar to that in which electric field imprinting is performed by changing the synthesis conditions, and in a ferroelectric material polarized in one direction, the driving electric field is It includes creating a state similar to that in which electric field imprinting is performed by performing polarization reversal partially by appropriately adjusting.

  Hereinafter, the present invention will be described in more detail with specific examples, but the present invention is not limited to the following examples, and various modifications can be made.

  An example in which a ferroelectric material provided with a memory function by electric field imprinting is applied to a bending type piezoelectric actuator as a piezoelectric device will be described.

  As shown on the left side of FIG. 2, a conventional bending type piezoelectric actuator using a ferroelectric material bends due to a change in the piezoelectric distortion of the ferroelectric material when a voltage is applied. It returns to the value of and becomes the original linear shape. Thus, the conventional bending type piezoelectric actuator using a ferroelectric does not have a memory function.

  On the other hand, a bending type piezoelectric actuator using a ferroelectric material provided with a memory function by electric field imprinting stops application of voltage after applying a positive voltage as shown on the right side of FIG. However, the bent shape is maintained at a predetermined memory function stable point. Next, after applying a negative voltage, when the application of the voltage is stopped, the shape is held at another memory function stable point. As described above, the bending type piezoelectric actuator using the ferroelectric material provided with the memory function by the electric field imprint has a memory function.

  In the conventional piezoelectric actuator, driving is performed in an external electric field range where polarization is not reversed. This is a graph showing the relationship between the external electric field and the displacement due to piezoelectric strain, that is, when the so-called piezoelectric strain butterfly curve is symmetric with respect to the piezoelectric strain axis, the piezoelectric strain does not change even if the polarization is reversed. This is because the direction of polarization distortion with respect to the external electric field is reversed when reversed.

  Actually, a memory function was given to a commercially available bending type piezoelectric actuator (LPD3713X manufactured by Nippon Ceratech) by electric field imprinting.

  As shown in FIG. 3, the bending type piezoelectric actuator used has an overall thickness of 0.6 mm, and 0.2 mm thick lead zirconate titanate (PZT) 12 on top and bottom of a metal shim 11 having a thickness of 0.2 mm. Attached, 13.4 mm wide, 37 mm in total length, 28 mm PZT, and a bimorph structure that fixes the remaining 9 mm metal-only part. A driving electrode 13 is provided on the PZT, and the front and back surfaces are connected by a conductive wire 14. In this embodiment, the conductive wire 14 is cut and used as a unimorph that drives only one side of PZT. In addition, this actuator has been previously polarized, but by applying a relatively large voltage of 180 V at 5 Hz prior to electric field imprinting, the actuator was brought into an unpolarized initial state.

  The actuator was placed in an electric furnace (DKN302 manufactured by Yamato Scientific Co., Ltd.) that had been set to 150 ° C in advance, and connected to a high-voltage power supply (M-2601 manufactured by Mestech) outside the electric furnace, and a DC voltage of 700 V was applied. . After 2 hours, the power supply was disconnected and short-circuited, and then removed from the electric furnace, and the relationship between applied voltage and displacement due to piezoelectric strain, the so-called piezoelectric strain butterfly curve was measured. The result is shown in FIG.

  The voltage applied to measure the piezoelectric strain butterfly curve was applied to the bimorph after the output from the function generator (NF1946) was amplified by a high-speed amplifier (NF4010). Displacement due to piezoelectric strain was measured with a laser displacement meter (LC2400 manufactured by Keyence Corporation). The relationship between the applied voltage and the displacement due to the piezoelectric strain was obtained using an oscilloscope, where the applied voltage was Ch1 and the displacement due to the piezoelectric strain was Ch2.

  From the results shown in FIG. 4, it was found that the piezoelectric strain butterfly curve was shifted in the positive direction of the voltage axis. In this graph, the positive direction of the voltage axis, that is, the horizontal axis, takes the direction of the electric field from the central metal shim 11 as the lower electrode toward the upper electrode 13. Therefore, the horizontal axis represents the inversion of the voltage applied to the upper electrode 13. In the electric field imprint process, 700 V is applied to the upper electrode 13 and the polarization direction is stable downward. Therefore, the electric field imprint is generated so that the polarization in the negative direction becomes stable. As a result, the piezoelectric strain butterfly curve is expected to shift to the right, which coincides with the result shown in FIG.

  As a result of applying the same 700 V DC voltage at 150 ° C. for another 2 hours, the piezoelectric strain butterfly curve was further shifted to the right as shown in FIG. Further, when a DC voltage of 700 V in the reverse direction was applied for 2 hours at a temperature of 150 ° C. using a new actuator, the piezoelectric strain butterfly curve shifted to the left in the reverse direction as shown in FIG.

  As described above, in this example, it was confirmed that the magnitude and direction of the electric field imprint can be controlled by the direction of the electric field and the time for applying the voltage.

  Next, the shape memory memory characteristics were evaluated using an actuator to which a DC voltage of 700 V was applied for a total of 4 hours in the environment of 150 ° C. Displacement due to piezoelectric strain was measured by applying a positive pulse and a negative pulse alternately with a pulse width of 0.1 seconds. Note that the drive voltage was set to 0 after application of these pulses for inversion of polarization. From the results shown in FIG. 7, it was found that when the voltage was set to 0 after polarization reversal, the displacement of about 0.12 mm was maintained compared to before the polarization reversal. That is, it has been confirmed by electric field imprinting that it has two piezoelectric strain stable points along the polarization direction and functions as a shape memory piezoelectric actuator.

  In this embodiment, the PZT unimorph has been described as an example. However, the present invention is not limited to this structure, and can be applied to piezoelectric actuators of all structures. Since the piezoelectric displacement is maintained even at a voltage of 0, it can contribute to suppressing power consumption by applying to a small switch such as an RF-MEMS switch or a micro-channel valve. Further, the driving voltage can be reduced by pulse driving, and further, the invention can be applied to a multi-value shape memory type actuator by devising the structure of the actuator.

  An example in which the electric field imprint is performed according to the synthesis condition of the ferroelectric material will be described.

(1) Lead titanate thin film (Synthesis condition 1)
An epitaxial thin film of lead titanate (PTO) was formed on a SrRuO ₃ substrate by a hydrothermal synthesis method. The reaction temperature was 150 ° C., the reaction time was 24 hours. As raw materials, 2.26 g of lead nitrate (Pb (NO ₃ ) ₂ ), 0.393 g of titanium oxide (TiO ₂ ), and 15 ml of 10N KOH aqueous solution were used. The obtained PTO thin film had a thickness of 100 nm, and a platinum electrode was formed on the thin film by vapor deposition.

  FIG. 8 and FIG. 9 show the hysteresis curves representing the relationship between polarization and dielectric constant with respect to the applied voltage of the obtained PTO thin film, respectively.

  The PTO thin film of this example showed a hysteresis curve shifted in the left direction without being subjected to a treatment for applying a strong electric field at a high temperature.

(2) Lead titanate thin film (Synthesis condition 2)
An epitaxial thin film of lead titanate (PTO) was formed on an epitaxial SrRuO ₃ thin film on a SrTiO ₃ (STO) (100) single crystal by a hydrothermal synthesis method. The reaction temperature was 150 ° C., the reaction time was 24 hours, and 1.0 g of lead nitrate, 0.2 g of titanium oxide, and 15 ml of 10N aqueous KOH solution were used as raw materials. The obtained PTO thin film had a thickness of 430 nm, and a platinum electrode was formed on the thin film by vapor deposition.

  As shown in FIGS. 10, 11, and 12, the hysteresis curves representing the relationship of displacement, polarization, and dielectric constant with respect to the applied voltage of the obtained PTO thin film are further to the left than in the case of the above synthesis condition (1). It was something that shifted.

(3) Lead zirconate titanate thin film An epitaxial thin film of lead zirconate titanate (PZT) was formed on an epitaxial SrRuO ₃ thin film on a SrTiO ₃ (STO) (100) single crystal by a hydrothermal synthesis method. The reaction temperature was 150 ° C, the reaction time was 9 hours, and 2.64 g of lead nitrate, 0.74 g of zirconium oxychloride octahydrate (ZrOCl ₄ 8H ₂ O), 0.2 g of titanium oxide, and 15 ml of a 10N aqueous KOH solution were used. . The obtained PZT thin film had a thickness of 500 nm, and a platinum electrode was formed on the thin film by vapor deposition.

  The hysteresis curve representing the polarization relationship with respect to the applied voltage of the obtained PZT thin film was shifted to the right as shown in FIG.

  As described above, even if a process for applying a strong electric field at a high temperature is not performed, by changing the synthesis condition of the ferroelectric material, a state similar to that in which the electric field imprint is performed or the degree thereof is controlled. It was confirmed that

It is explanatory drawing which shows the characteristic of the ferroelectric material to which the memory function of the piezoelectric distortion, the dielectric constant, and the refractive index was provided by the electric field imprint. It is explanatory drawing which shows operation | movement of the piezoelectric actuator of this invention provided with the ferroelectric material to which the memory function of the piezoelectric distortion was provided by the electric field imprint. It is the upper side figure and side view which show the bending type piezoelectric actuator used in Example 1. FIG. 3 is a graph showing a piezoelectric strain butterfly curve for 2 hours of electric field imprint processing in Example 1. FIG. 3 is a graph showing a piezoelectric strain butterfly curve for 4 hours of electric field imprint processing in Example 1. FIG. It is a graph which shows the piezoelectric distortion butterfly curve at the time of performing the electric field imprint process in Example 1 in the reverse direction. 3 is a graph showing driving of the piezoelectric actuator of Example 1. It is a graph which shows the voltage-polarization hysteresis curve of the lead titanate thin film of Example 2 (synthesis condition 1). It is a graph which shows the voltage-dielectric-constant hysteresis curve of the lead titanate thin film of Example 2 (synthesis condition 1). It is a graph which shows the voltage-displacement hysteresis curve of the lead titanate thin film (synthesis condition 2) of Example 2. It is a graph which shows the voltage-polarization hysteresis curve of the lead titanate thin film of Example 2 (synthesis condition 2). It is a graph which shows the voltage-dielectric-constant hysteresis curve of the lead titanate thin film (synthesis condition 2) of Example 2. 3 is a graph showing a voltage-polarization hysteresis curve of a lead zirconate titanate thin film of Example 2.

Claims (5)

A ferroelectric material comprising a ferroelectric material provided with a memory function of piezoelectric distortion, dielectric constant, refractive index, piezoelectric constant, electromechanical coupling coefficient, Young's modulus, or pyroelectric constant by electric field imprinting device. 2. The ferroelectric device according to claim 1, wherein the ferroelectric material is any one of lead zirconate titanate, lead titanate, barium titanate, lithium niobate, and lead lanthanum zirconate titanate. A piezoelectric material comprising a ferroelectric material having a memory function for selectively holding a predetermined binary piezoelectric strain without applying a voltage, the memory function being provided by electric field imprinting device. A dielectric material comprising a ferroelectric material having a memory function that selectively retains a predetermined binary dielectric constant without applying a voltage, the memory function being provided by electric field imprinting Rate detection type ferroelectric memory device. A ferroelectric material having a memory function for selectively holding a predetermined binary refractive index without applying a voltage, the memory function being provided by electric field imprinting Optical device.

Images