JPWO2013065493A1 - Impact detection / recording device - Google Patents

Abstract

Provided is an impact detection / recording device that does not require a power source, and can detect and store a single impact and can hold the single impact. A piezoelectric element 2 that converts impact energy into electric energy and outputs the output; a ferroelectric body 4 connected to the piezoelectric element 2 and having one main surface and the other main surface facing the one main surface; And a ferroelectric memory 3 having a first electrode 5 formed on one main surface side of 4 and a second electrode 8 formed on the other main surface side of the ferroelectric. 3 can record at least one of the first and second states derived from the electronic polarization of the ferroelectric 4, and the capacitance-voltage characteristic of the ferroelectric memory 3 has hysteresis, In the hysteresis curve of capacitance-voltage characteristics, Vth> 0 or Vth <0, where the threshold voltage Vth is the voltage value having the largest capacitance difference between the curve at the time of voltage rise and the curve at the time of voltage drop. Is an impact detection and recording device.

Description

  The present invention relates to an impact detection / recording apparatus capable of detecting and recording an external impact, and more particularly to an impact detection / recording apparatus including a piezoelectric element and a ferroelectric memory.

  Conventionally, various sensors have been proposed for detecting and recording impacts in electronic devices and vehicles. Patent Document 1 below discloses a piezoelectric vibration energy sensor including a charge generating piezoelectric body that converts vibration energy into electric energy and a ferroelectric memory that repeats polarization inversion by the generated charge. When vibration such as impact is applied, the piezoelectric body generates electric charges. This charge reverses the polarization of the ferroelectric memory. When a large number of vibrations are repeatedly applied, the polarization inversion is repeated, and the residual polarization decreases due to fatigue. The number of applied vibrations can be determined from the degree of decrease in remanent polarization. Since the piezoelectric body and the ferroelectric memory are provided, the number of vibrations applied without a power source can be measured.

JP 2004-61347 A

  In the piezoelectric vibration energy sensor described in Patent Document 1, the number of applied vibrations can be measured with no power source. The number of vibrations is measured over a long period of time due to fatigue due to polarization reversal, that is, a decrease in residual polarization. Therefore, in Patent Document 1, even if a large impact is applied once, the single impact cannot be detected and recorded.

  Further, since remanent polarization is used, a large piezoelectric element that converts very large vibration energy into electrical energy is required. Therefore, downsizing is difficult.

  An object of the present invention is to provide an impact detection / recording device that does not require a power source, and can detect and record a single impact and hold it. .

  An impact detection / recording apparatus according to the present invention converts an impact energy into electrical energy and outputs the piezoelectric element, and is connected to the piezoelectric element and has one main surface and the other main surface facing the one main surface. A ferroelectric memory, a first electrode formed on one main surface side of the ferroelectric, and a second electrode formed on the other main surface side of the ferroelectric With. In the present invention, the ferroelectric memory can record at least one of the first and second states derived from the electronic polarization of the ferroelectric. The capacitance-voltage characteristic of the ferroelectric memory has hysteresis. In this hysteresis curve of capacitance-voltage characteristics, when the voltage value having the largest capacitance difference between the curve at the time of voltage rise and the curve at the time of voltage drop is defined as the threshold voltage Vth, Vth> 0 or Vth < 0.

  The fact that the first and second electrodes are respectively formed on the one main surface side and the other main surface side of the ferroelectric is not limited to that each electrode is formed directly on the ferroelectric. Instead, it may be via a semiconductor or a buffer layer as in the embodiment.

In a specific aspect of the impact detection / recording apparatus according to the present invention, an inversion voltage required to invert the ferroelectric memory from the first state to the second state is V ₀ , and the second state is the second state. the inversion voltage for inverting the first state when the _{V 1, V 0 <0 <} V 1 or _V 0>0> is _{V 1.} In this case, the polarization state is reversed and recorded with time, and there is no possibility that information is lost.

  In another specific aspect of the impact detection / recording apparatus according to the present invention, an absolute value of the threshold voltage Vth of the ferroelectric memory is in a range of 0.5V to 3V. Within this range, the threshold voltage can be easily shifted from 0V to detect and record the impact, and the impact can be more reliably recorded by the charge generated by the piezoelectric element.

  In still another specific aspect of the impact detection / recording apparatus according to the present invention, the ferroelectric memory further includes a semiconductor layer in which the ferroelectric is stacked between a second electrode. In this case, a diode structure is formed by joining the ferroelectric and the semiconductor layer. Therefore, the threshold voltage Vth can be easily shifted from 0 by selecting the material constituting the diode structure.

  In still another specific aspect of the impact detection / recording apparatus according to the present invention, a buffer layer is provided between the semiconductor layer and the ferroelectric substance. In this case, the value of the threshold voltage Vth can be easily controlled by selecting the material and thickness of the buffer layer.

According to the impact detection / recording apparatus of the present invention, since the threshold voltage Vth in the ferroelectric memory is shifted from 0V, the inversion voltage V necessary for inversion from the first state to the second state. The absolute value of the inversion voltage V ₁ necessary for inversion from ₀ to the first state is different from 0. Therefore, when V ₀ > V ₁ or V ₁ > V ₀ , the initial state is set to the first state or the second state, and when an impact is applied, the state is switched to the second state or the first state. Thus, a single impact can be reliably recorded and held.

  Therefore, according to the present invention, it is possible to detect a single impact with no power source and to reliably record and hold the ferroelectric memory.

FIG. 1 is a front view showing a ferroelectric memory used in the impact detection / recording apparatus according to the first embodiment of the present invention. FIG. 2 is a circuit diagram of the impact detection / recording apparatus according to the first embodiment of the present invention. FIG. 3 is a diagram showing capacitance-voltage characteristics of the ferroelectric memory used in the first embodiment of the present invention. FIG. 4 is a diagram showing an X-ray diffraction profile of the ferroelectric frame manufactured in the first embodiment of the present invention. FIG. 5 is a diagram for explaining the memory operation of the ferroelectric memory used in the impact detection / recording apparatus according to the first embodiment of the present invention. FIG. 6 is a diagram for explaining the threshold voltage Vth and the first and second inversion voltages in the impact detection / recording apparatus according to the first embodiment of the present invention. FIG. 7 is a diagram showing the capacitance-voltage characteristics of the ferroelectric memory of the comparative example in which the threshold voltage V is located at 0V. FIG. 8 is a schematic diagram showing a circuit configuration for evaluating the impact detection / recording apparatus according to the first embodiment of the present invention. FIG. 9 is a schematic diagram for explaining an impact generation jig for measuring the descent of the impact detection / recording apparatus according to the first embodiment of the present invention. FIG. 10 is a diagram showing impact recording results in an experimental example of the impact detection / recording apparatus according to the first embodiment of the present invention. FIG. 11 is a diagram showing an impact recording result in an experimental example of the impact detection / recording apparatus of the first comparative example of the present invention. FIG. 12 is a diagram showing impact recording characteristics when a ferroelectric memory of sample number 2 as a comparative example is used. FIG. 13 is a diagram showing impact recording characteristics when a ferroelectric memory of Sample No. 2 as a comparative example is used.

  Hereinafter, the present invention will be clarified by describing specific embodiments of the present invention with reference to the drawings.

  FIG. 2 is a schematic configuration diagram of the impact detection / recording apparatus according to the first embodiment of the present invention. The impact detection / recording apparatus 1 includes a piezoelectric element 2 and a ferroelectric memory 3. The piezoelectric element 2 converts the applied impact energy into electrical energy and outputs it. Examples of such piezoelectric elements include various piezoelectric elements that have been used as well-known impact sensors. For example, a bimorph type piezoelectric element, a unimorph type piezoelectric element, etc. can be mentioned.

  The impact detection / recording apparatus 1 of this embodiment includes a ferroelectric memory 3 connected in parallel to a piezoelectric element 2. The ferroelectric memory 3 records and holds a single impact based on the electric energy applied from the piezoelectric element 2. The structure of the ferroelectric memory 3 will be described with reference to FIG.

FIG. 1 is a front view of a ferroelectric memory 3 used in this embodiment. The ferroelectric memory 3 has a ferroelectric thin film 4 as a ferroelectric layer. The ferroelectric material constituting the ferroelectric thin film 4 is not particularly limited. For example, SrBi ₂ Ta ₂ O ₉ , Pb (Zr, Ti) O ₃ , (Bi, La) ₄ Ti ₃ O ₁₂ , BiFeO _{3 and the} like can be mentioned. In the present embodiment, the ferroelectric thin film 4 is made of SrBi ₂ Ta ₂ O ₉ .

  A first electrode 5 is formed on one main surface of the ferroelectric thin film 4. The other main surface of the ferroelectric thin film 4 is laminated on the upper surface of the semiconductor substrate 6 with a buffer layer 7 interposed therebetween. The semiconductor substrate 6 is made of an appropriate semiconductor that can form a diode junction with the ferroelectric thin film 4. In the present embodiment, the semiconductor substrate 6 is made of Si.

The buffer layer 7 is provided in order to suppress deterioration due to mutual diffusion between the ferroelectric thin film 4 and the semiconductor substrate 6. The material constituting the buffer layer 7 is not particularly _{limited, HfO x, Al 2 O 3} , etc. _ZrO 2, CeO 2, LaAlO _3, or _La 2 _{O 3} and the like. In the present embodiment, the buffer layer 7 is made of HfO _x (where x is about 2). What is necessary is just to select suitably the material which comprises the buffer layer 7 according to the ferroelectric material which comprises the ferroelectric thin film 4, the semiconductor material which comprises the semiconductor substrate 6, etc. FIG. In addition, the threshold voltage described later can be controlled by selecting the material of the buffer layer 7.

  The threshold voltage Vth may be set to an appropriate value by stacking buffer layers made of a plurality of materials or using a solid solution of a plurality of materials.

  A second electrode 8 is formed on the lower surface of the semiconductor substrate 6, that is, on the other main surface side of the ferroelectric thin film 4. Therefore, the ferroelectric thin film 4 is sandwiched between the first electrode 5 and the second electrode 8.

  In the present embodiment, the first electrode 5 and the second electrode 8 are formed at positions facing each other with the ferroelectric thin film 4 therebetween, but one main surface and the other main surface of the ferroelectric thin film 4 If a voltage can be applied between them, it is not always necessary to form them at positions facing each other.

The first electrode 5 and the second electrode 8 can be formed of an appropriate metal material. In the present embodiment, the first electrode 5 and the second electrode 8 are made of Pt. However, in addition to Pt, an appropriate metal or alloy such as Au, Ag, Al, Pd, Ir, or Cu, or an oxide such as SrRuO ₃ , PtO _x (where x is an integer of 1 to 3), IrO ₂ , and PdO. Can be used.

  It is also possible to adjust a threshold voltage Vth described later by adjusting the materials of the first and second electrodes 5 and 8. In the present embodiment, the first electrode 5 and the second electrode 8 are disposed so as to face each other with the ferroelectric thin film 4, the buffer layer 7 and the semiconductor substrate 6 therebetween. The form of the two electrodes 5 and 8 facing each other via the ferroelectric thin film 4 is not limited to this.

  However, in the impact detection / recording apparatus 1 of the present embodiment, it is necessary for the ferroelectric memory 3 to record and hold the impact based on the electrical energy applied from the piezoelectric element 2. Therefore, as described below, the threshold voltage Vth is shifted from 0V. That is, Vth> 0 or Vth <0, whereby a single impact can be recorded and held. Therefore, the ferroelectric thin film 4 is bonded to the semiconductor substrate 6 via the buffer layer 7, or the ferroelectric thin film 4 is directly stacked on the semiconductor substrate 6 to have the diode junction. It is desirable. Thereby, the threshold voltage Vth can be easily shifted from 0V.

  Next, the operation of the ferroelectric memory 3 will be described with reference to FIG. 3, FIG. 5 and FIG.

  FIG. 3 is a diagram showing the capacitance-voltage characteristics of the ferroelectric memory 3. As illustrated, the capacitance-voltage characteristic curve has hysteresis. That is, the curve at the time of voltage rise and the curve at the time of voltage drop are not coincident and are shifted.

  In the present embodiment, as shown in FIG. 6, the threshold voltage Vth is about 1.2V, and Vth> 0V. Here, the threshold voltage Vth refers to a voltage value at which the difference in capacitance is greatest between the curve at the time of voltage rise and the curve at the time of voltage drop in the hysteresis of the capacitance-voltage characteristic.

  Further, the applied voltage at the portion where the difference in applied voltage is the same with the same capacitance value between the curve at the time of voltage rise and the curve at the time of voltage drop is defined as a memory window. In FIG. 6, the size of this memory window is 0.6V.

As shown in FIGS. 3 and 6, in the capacitance-voltage characteristics of the ferroelectric memory 3, the capacitance increases to about 150 pF when the applied voltage is around −3 V to −4 V, and the applied voltage is +2 V. When it is about ˜4 V, it can be seen that the capacitance is as low as about 14 pF. Using this capacitance-voltage characteristic, the first state, that is, the “0” state, and the second state “1” can be distinguished. The “0” state is a case where the electronic polarization of the ferroelectric thin film 4 is directed to the first electrode side, and the “1” state is a state where the electronic polarization direction is directed to the semiconductor substrate 6 side. It corresponds. The direction does not change depending on the polarity of the semiconductor substrate to be used, but the polarity of the capacitance-voltage characteristic is reversed. In the ferroelectric memory 3, polarization inversion can be caused by applying a voltage. That is, when a voltage is applied, the first state is “0” or the second state, ie, “1”, or the electronic polarization of the ferroelectric is completely “0” or “1”. In other words, an intermediate state in which most of the states are “0” and “1” can be obtained. The voltage for switching to the first state, that is, the state of “0” is the inverted voltage V _0, and the voltage for switching to the “1” state is the inverted voltage V ₁ . In a ferroelectric memory, generally, when having the above-described hysteresis, V ₀ and V ₁ exist at positions symmetrical with respect to the threshold voltage Vth.

On the other hand, in the ferroelectric memory 3 of the present embodiment, the threshold voltage Vth is shifted to 1.2V instead of 0V. Therefore, _{V 0} is -1 V, _{V 1} is about + 4V. That is, the absolute values of the inversion voltage V ₀ and the inversion voltage V ₁ are different.

Therefore, in the ferroelectric memory 3 of the present embodiment, the reset voltage V ₀ and the inverted voltage V ₁ are reset so that the state of “1” having a high absolute value is set as the initial state. Even if it is left in the “1” state, it has a memory characteristic, so that state is maintained.

When a single impact is applied to the piezoelectric element 2 described above, energy is converted into electrical energy by the impact. Then, electric charges are generated in the piezoelectric element 2 and an output based on the electric charges is applied to the ferroelectric memory 3. When this non-rectified electrical energy is input to the ferroelectric memory 3 having a diode junction structure, that is, when the reverse voltage V ₀ or a voltage close to the reverse voltage V ₀ is applied, the ferroelectric memory 3 The state is shifted from the “1” state to the “0” state or an intermediate state between the two states. This change can be confirmed by detecting the source / drain current by adding the capacitance value of the diode or the source / drain electrodes.

  In addition, in the ferroelectric memory 3 of the present embodiment, even if the second and subsequent impacts are applied, the first impact recording is reliably maintained. That is, as described above, the threshold voltage Vth is shifted from 0 to 1.2 V in the hysteresis of the capacitance-voltage characteristics. Therefore, the stability of the “0” state and the “1” state are different. More specifically, a voltage having a small absolute value may be applied to make the state “0”, and a voltage larger than “0” is needed to make the state “1”. . In other words, a larger voltage is required to return to the “1” state.

  On the other hand, the voltage that can be extracted by the piezoelectric effect by the piezoelectric element is usually about 3 V or less in the piezoelectric element used for this type of impact sensor. Therefore, even after a large impact is applied once as described above after being reset to the “1” state, the electric energy from the piezoelectric element 2 is reset even if the state is reset to the “0” state or an intermediate state between the two states. Then, a large voltage that cannot be reset to the “1” state is required. Therefore, even if a large impact is recorded in the ferroelectric memory 3 and then a plurality of impacts are applied to the piezoelectric element 2 a plurality of times, the recording due to the one impact described above is not erased.

  Therefore, according to this embodiment, it is possible to detect a single impact with no power source, and to record and hold the impact. Further, even if a large number of impacts are applied later, it is not reset, and the ferroelectric memory 3 is not reset without applying a reset voltage from the outside. Therefore, according to the present embodiment, it is possible to record and hold that a large impact that causes a failure is applied to, for example, an electronic device in which the impact detection / recording apparatus of the present embodiment is mounted. As a result, it is possible to reliably detect and record the occurrence of a large impact caused by use or an accident that would normally not be considered.

  Note that a general ferroelectric memory uses a “0” or “1” state, that is, a binary value, whereas in this embodiment, a “0” state and a “1” state Intermediate states can also be used.

In addition, it was shown that the inversion voltage V ₀ required for the state of “0” is −1V, and the magnitude of the inversion voltage V ₁ for the state of “1” is about + 4V. Since the inverted voltage V ₀ and the inverted voltage V ₁ appear symmetrically with the threshold voltage Vth as described above, the values can be read from the capacitance-voltage polarity in FIG. It is not always necessary to set -1V and 4V.

In the present embodiment, Vth is set to a positive voltage of about 1.2 V, but Vth may be a negative voltage, that is, Vth <0. Even in that case, since the absolute values of the inversion voltage V ₀ and the inversion voltage V ₁ are different, it is possible to detect and record that a large impact has been applied once as in the above embodiment.

  As described above, the characteristic of the impact detection / recording apparatus of this embodiment is that the threshold voltage Vth of the ferroelectric memory 3 is shifted from 0V. As described above, the threshold voltage Vth can be shifted from 0 V by selecting the type of each material constituting the ferroelectric memory 3 and the thickness of each material layer. This will be explained in more detail by giving specific examples below.

In the following experimental example, a ferroelectric memory 3 was manufactured using a ferroelectric thin film 4 made of SBT (SrBi ₂ Ta ₂ O ₉ ), which is known as a representative ferroelectric material. Strontium carbonate (SrCO ₃ ), tantalum oxide (Ta ₂ O ₅ ), and bismuth oxide (Bi ₂ O ₃ ) were weighed so as to have a composition of SrBi _2.2 Ta ₂ O ₉ . These weighed materials were wet-ground using PSZ balls and mixed. Note that Bi ₂ O ₃ was added excessively because Bi ₂ O ₃ might vaporize during film formation.

  The mixed powder placed as described above was dried and calcined in the atmosphere at a temperature of 900 ° C. for 2 hours. The calcined material was pulverized using PSZ balls to obtain a calcined powder. The calcined powder was mixed with water and polyvinyl alcohol and dried to obtain a press raw material.

  The press raw material was formed into a disk having a diameter of 20 mm and a thickness of 5 mm by a pressing device. The formed disk-shaped molded body was heated in the atmosphere at 400 ° C. to remove the binder. Thereafter, it was fired in the air at 1150 ° C. for 4 hours in a sealed cage to obtain a sintered compact target.

A p-type Si semiconductor substrate was prepared as the semiconductor substrate 6. On this semiconductor substrate 6, HfO _x was deposited by sputtering to form a buffer layer 7. The buffer layer 7 was formed under the conditions of Rf power = 100 W, substrate temperature = 400 ° C., gas flow rate: 25 sccm (Al: O ₂ = 24: 1), and pressure 0.25 Pa. Thereafter, the temperature was raised to 800 ° C. in a vacuum of 1 × 10 ⁻⁴ Pa, and then annealed for 10 minutes and cooled to 400 ° C. Annealing was carried out at 400 ° C. for 30 minutes in an oxygen atmosphere of 0.25 Pa to form the buffer layer 7.

  On the buffer layer 7 of the semiconductor substrate 6 on which the buffer layer 7 was formed, the ferroelectric thin film 4 was formed by the pulse laser deposition method using the sintered body target. A KrF excimer laser was used as the laser. The ferroelectric thin film 4 was formed at a substrate temperature of 750 ° C., an oxygen partial pressure of 33 Pa, and a laser energy of 120 mA.

  The powder obtained by pulverizing the formed ferroelectric thin film 4 was evaluated by X-ray diffraction measurement. The results are shown in FIG.

As is apparent from FIG. 4, it can be seen that the SrBi ₂ Ta ₂ O ₉ film is formed from 2θ of X-ray diffraction.

  After forming the ferroelectric thin film 4 by changing the film formation time, the first electrode and the second electrode made of Pt were formed to a thickness of 20 nm.

  Ferroelectric memory elements 11 to 14 shown in Table 1 below were obtained by setting the film thickness of the ferroelectric thin film 4 to 130, 250, 400, or 600 mm.

  As a representative example, the capacitance-voltage characteristics of the ferroelectric memory element 12 were measured using an LCR meter (manufactured by Hewlett-Packard Company, product number: HP4284). In the measurement, a probe using a W probe was used to apply a voltage between the first electrode and the second electrode in steps of 0.1 V from −3 V to +3 V to −3 V with a frequency of 1 MHz and an amplitude of 50 mV. . The capacitance in this case was measured, and the voltage dependency of the capacitance was evaluated. The measurement was performed with the second electrode connected to the ground potential.

  The capacitance-voltage characteristics obtained as described above are the characteristics shown in FIG. 3, FIG. 5 and FIG.

  As described above, hysteresis appears, and the capacitance is rapidly decreased by applying a positive voltage.

  For comparison, FIG. 7 shows a capacitance-voltage curve of a ferroelectric memory in which the threshold voltage Vth is not shifted. In FIG. 7, hysteresis appears, but the threshold voltage Vth is 0V.

  As described above, the impact detection / recording apparatus according to the present embodiment uses the hysteresis on the CV characteristic in the ferroelectric memory 3. Therefore, the ferroelectric memory in which the hysteresis does not appear cannot be used in the present invention. For example, when the voltage is swept in a very small voltage range such as -1V → 1V → -1V during measurement, no hysteresis appears. This is because the electronic polarization of the ferroelectric does not reverse. Therefore, in order to detect and record an impact as described above using the ferroelectric memory 3, it is necessary to apply a voltage higher than the coercive electric field to the ferroelectric thin film so that the hysteresis appears.

In the normal ferroelectric memory, the threshold voltage Vth is located at 0V. On the other hand, since the ferroelectric memory used in this embodiment has the diode junction and the MFIS structure, all applied voltages are not applied to the ferroelectric thin film 4. Therefore, when the hysteresis appears and the memory window becomes the largest, the voltage at which the memory window becomes 0.3 V or more is defined as the inverted voltages V ₀ and V ₁ .

  As described above, the shift amount of the threshold voltage Vth of the ferroelectric memory 3 can be adjusted by selecting a material. Regarding the change in the threshold voltage in such a ferroelectric memory, for example, (Jpn. -Semiconductor Structure for One Transistor-Type Ferroelectric Memory by Rapid Thermal Annealing). That is, it has been conventionally known that the threshold voltage Vth changes in the MFIS structure using the ferroelectric thin film by changing the heat treatment condition. Therefore, the conditions for shifting the threshold voltage Vth in the ferroelectric memory 3 can be known in accordance with a conventionally known technique. However, the technique that actively uses such a shift of the threshold voltage Vth is not described in the above-mentioned document.

  Next, the effect of the above embodiment was evaluated by applying an actual impact. That is, as shown in FIG. 8, the LCR meter 22 and the piezoelectric element 2 were connected to the ferroelectric memory 3 via the switch 21. Further, an impact generating jig 31 shown in FIG. 9 was prepared. In the impact generating jig 31, a post 33 is erected on an iron plate 32. A guide 34 that can slide up and down along the post 33 is provided. The piezoelectric element 2 was fixed to the guide 34, and the guide 34 was dropped onto the iron plate 32 from a position having a height of 25 cm, 80 cm, or 120 cm. The impact due to the drop was detected and recorded by the circuit of FIG. 8 including the piezoelectric element 2 and the ferroelectric memory 3.

  First, the ferroelectric memory 3 was connected to the LCR meter 22 and a voltage was applied so as to be in a reset state. The capacitance at that time was read. Next, the LCR meter 22 was disconnected from the ferroelectric memory 3 by the switch 21 and connected to the piezoelectric element 2. The piezoelectric element 2 was fixed to the guide 34 in a state where the piezoelectric element 2 was connected to the ferroelectric memory 3 by wiring or the like, and the impact due to the dropping as described above was applied. Then, electric energy generated by the impact was written in the ferroelectric memory 3. Thereafter, the switch 21 was switched, and the ferroelectric memory 3 was reconnected to the LCR meter 22 to read the capacitance. Thereby, it was evaluated whether or not the impact could be recorded.

  The piezoelectric element 2 includes a first piezoelectric element having an acceleration detection sensitivity of 0.35 pC / G and a capacitance of 740 pF, and a second piezoelectric element having an acceleration detection sensitivity of 0.84 pC / G and a capacitance of 770 pF. And prepared. The greater the acceleration detection sensitivity, the greater the charge generated by the same magnitude of impact. The results are shown in Table 2 below.

  Sample numbers 1 and 2 in Table 2 correspond to comparative examples, and no impact could be recorded even when dropped from any height. In contrast, Sample Nos. 3 to 10 were able to detect and record an impact when falling from at least one of the height positions.

  For example, sample No. 7 could not record an impact when dropped from a height of 25 cm. Similarly, in sample No. 9, the impact could not be recorded when dropped from a height of 25 cm and 80 cm. However, in the case of sample numbers 7 and 9, the impact can be recorded when dropped from a height of 80 cm or 120 cm. Therefore, it can be seen that one impact can be detected and recorded according to the present invention by adjusting the threshold voltage and the reverse voltage according to the magnitude of the impact to be recorded.

  10 and 11 show the results when the sample number 5 is dropped from 80 cm. At a stage where a voltage of 3 to 4 V is applied for 5 seconds and reset processing is performed, the ferroelectric memory has a capacitance of 117 pF, and this state corresponds to a state of “1”. Next, it was dropped from a height of 80 cm according to the drop test at the portion indicated by arrow A1 in FIG. As a result, it increased to 138 pF by the LCR meter 22 as shown in the figure. Note that the electrostatic capacity did not change even when the impact was applied again after recording the impact. On the other hand, as shown in FIG. 11, after applying a voltage of −1 V for 5 seconds and measuring the capacitance of the ferroelectric memory, it was 138.8 pF. This is a state of “0”. The connection is switched again by the switch 21, and the piezoelectric element 2 is dropped from the height of 80 cm at the timing of the arrows B1 and B2, and then the switch 21 is switched, and the capacitance of the ferroelectric memory is changed by the LCR meter 22. I read it. As a result, as shown in FIG. 11, the electrostatic capacitance hardly changed to 139.3 pF, and recording could not be performed.

  As described above, in the combination of the ferroelectric memory of sample number 5 and the piezoelectric element, if the state is reset to “1”, the impact can be recorded and held by the electric energy from the piezoelectric element. I understand. Further, it is possible to detect and record a single impact as described above. In addition, since the sensitivity is very high, not only can one impact be recorded, but a small piezoelectric element of 3 × 3 × 0.5 mm type can be used as a sensor. Therefore, it can be seen that an impact detection / recording apparatus that can be mounted on a small portable terminal or the like can be configured.

  For comparison, FIGS. 12 and 13 show the results when a ferroelectric memory of sample number 2 was used and dropped from the piezoelectric element 80 cm in the same manner as described above.

  FIG. 12 shows the results when the ferroelectric memory of sample number 2 is used and an impact is applied at the timing of the arrow X2 after switching to the state “1” by the arrow X1. FIG. 13 shows the result when an impact is applied at the timing of the arrow Y2 after switching to the state of “0” at the timing of the arrow Y1. In either case, the impact could not be detected and recorded. This is because, in Sample No. 2, a large voltage of about ± 2 V is required in any case to switch the ferroelectric memory 3 to the “0” or “1” state. Therefore, with the output of a small piezoelectric element that can be mounted on a cellular phone or the like, the electronic polarization cannot be reversed in the ferroelectric memory of Comparative Example 2. Thus, it can be seen that a large piezoelectric element must be used.

DESCRIPTION OF SYMBOLS 1 ... Impact detection and recording apparatus 2 ... Piezoelectric element 3 ... Ferroelectric memory 4 ... Ferroelectric thin film 5 ... 1st electrode 6 ... Semiconductor substrate 7 ... Buffer layer 8 ... 2nd electrode 21 ... Switch 22 ... LCR meter 31 ... Impact generating device 32 ... Iron plate 33 ... Post 34 ... Guide

Claims (5)

A piezoelectric element that converts impact energy into electrical energy and outputs it;
A ferroelectric material connected to the piezoelectric element and having one main surface and the other main surface facing the one main surface; a first electrode formed on one main surface side of the ferroelectric; and A ferroelectric memory having a second electrode formed on the other main surface side of the ferroelectric,
The ferroelectric memory can record at least one of the first and second states derived from the electronic polarization of the ferroelectric, and the capacitance-voltage characteristic of the ferroelectric memory has hysteresis. In the hysteresis curve of capacitance-voltage characteristics, Vth> 0 or Vth, where the threshold voltage Vth is the voltage value having the largest capacitance difference between the curve at the time of voltage rise and the curve at the time of voltage drop. <0, impact detection / recording device. When an inversion voltage necessary to invert the ferroelectric memory from the first state to the second state is V ₀ and an inversion voltage for inversion from the second state to the first state is V ₁ 2. The impact detection / recording apparatus according to claim ₁ , wherein V ₀ <0 <V ₁ or V ₀ >0> V ₁ is satisfied.   3. The impact detection / recording apparatus according to claim 1, wherein an absolute value of the threshold voltage Vth of the ferroelectric memory is in a range of 0.5V to 3V.   The impact detection / recording apparatus according to claim 1, wherein the ferroelectric memory further includes a semiconductor layer stacked between the ferroelectric and the second electrode.   The impact detection / recording apparatus according to claim 4, wherein a buffer layer is provided between the semiconductor layer and the ferroelectric substance.

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