JP4984943B2 - Piezoelectric sensor device and recording device - Google Patents

Description

  The present invention relates to a piezoelectric sensor device having a piezoelectric sensor having a structure in which a piezoelectric film is sandwiched between a pair of electrodes, and a recording apparatus incorporating the piezoelectric sensor device.

  The piezoelectric sensor generates a strain in the sensing unit due to the application of stress, and outputs an electrical signal corresponding to the magnitude of the strain. In addition, the piezoelectric sensor may be used as a vibration sensor or an impact sensor because the sensing unit is distorted by vibration or impact and outputs an electrical signal. In the piezoelectric sensor, since a signal corresponding to the magnitude of the distortion of the sensing unit is output, the sensitivity increases as the thickness of the normal sensing unit decreases. However, a general piezoelectric sensor is made of a bulk material, and it is extremely difficult to make the thickness of the sensing portion 10 μm or less.

  In recent years, information devices have been required to be miniaturized and highly functional, and it has been studied to incorporate a piezoelectric sensor (vibration sensor), a gyro sensor, or the like. In addition, in a recording apparatus such as an optical disk apparatus or a magnetic disk apparatus, it has been studied to incorporate a piezoelectric sensor in order to precisely control the flying height of the head.

  A film-like piezoelectric sensor having a structure in which a piezoelectric film formed using thin film technology is sandwiched between a pair of electrodes has been developed as an ultra-small and highly sensitive piezoelectric sensor suitable for such applications. In general, a piezoelectric body exhibits piezoelectricity by a polarization treatment that applies a voltage higher than the coercive electric field. However, in the case of a film-like piezoelectric sensor, spontaneous polarization may exist immediately after the piezoelectric body is formed. . However, in this case as well, the polarization amount increases and the sensitivity of the piezoelectric sensor increases when the polarization treatment is performed.

In addition, there exist some which were described in patent documents 1-4 as a prior art considered to be related to this invention. Patent Document 1 describes a piezoelectric sensor having a structure in which a piezoelectric oxide film having a perovskite structure containing PbTiO ₃ or BaTiO ₃ is sandwiched between a pair of platinum electrodes. Patent Document 2 describes manufacturing a piezoelectric sensor by processing a wafer by a photolithography method.

Patent Document 3 describes a magnetic disk device including an actuator that moves a magnetic head in a track direction using a piezoelectric thin film. Patent Document 4 describes an impact (piezoelectric) sensor having a structure in which an antiferroelectric film (piezoelectric / electrostrictive material film) is sandwiched between a pair of electrodes. In the piezoelectric sensor disclosed in Patent Document 4, after an antiferroelectric film is formed, an electric field higher than the coercive electric field is applied to cause layer transition, thereby obtaining a shape memory state (power storage state). As a result, the charge accumulated in the antiferroelectric film when an impact is applied is released, and an electrical output is obtained.
JP-A-9-181369 JP 2001-358375 A JP 2005-123421 A JP 7-83950 A

  In a piezoelectric sensor composed of a bulk material, the polarization characteristics hardly deteriorate with time. However, the film-shaped piezoelectric sensor formed by using the thin film technology has a problem that the polarization characteristic deteriorates with time due to the influence of the stress received from the substrate in accordance with the detection of pressure and the temperature, and the sensitivity is lowered. is there.

  An object of the present invention is to provide a piezoelectric sensor device capable of maintaining the sensitivity of a piezoelectric sensor having a structure in which a piezoelectric film is sandwiched between a pair of electrodes in a good state for a long period of time, and capable of performing a highly reliable measurement, and its It is to provide a recording apparatus incorporating a piezoelectric sensor device.

According to one aspect of the present invention, a piezoelectric sensor having a structure in which a piezoelectric film having a thickness of 1.5 μm or less is sandwiched between a pair of electrodes, a charge amplifier that converts an output of the piezoelectric sensor into a voltage, and the piezoelectric sensor A voltage that can apply an electric field higher than the coercive electric field of the piezoelectric film and a voltage that can drive the charge amplifier, and a voltage generated by the power supply is supplied to the charge amplifier, and is set in advance. And a switch circuit for switching the connection destination of the power source from the charge amplifier to the piezoelectric sensor every time .

  The amount of polarization of the piezoelectric film constituting the piezoelectric sensor decreases with time, and the sensitivity of the piezoelectric sensor decreases accordingly. When a polarization process is performed in which an electric field higher than the coercive electric field is applied to the piezoelectric film of the piezoelectric sensor, the amount of polarization of the piezoelectric film is recovered and the sensitivity of the piezoelectric sensor is also recovered. In the present invention, the above polarization process is performed at regular intervals by an electronic circuit, so that the detection sensitivity of the piezoelectric sensor can be maintained in a good state for a long period of time, and highly reliable measurement is possible. .

  Hereinafter, embodiments of the present invention will be described with reference to the accompanying drawings.

(Piezoelectric sensor)
FIG. 1A is a perspective view showing a film-like piezoelectric sensor, and FIG. 1B is a cross-sectional view of a sensing portion of the piezoelectric sensor.

  The inventors of the present application created a unimorph type film-shaped piezoelectric sensor 10 as shown in FIGS. 1A and 1B in order to examine the change with time of the sensitivity of the film-shaped piezoelectric sensor. The piezoelectric sensor 10 is formed in a strip shape, and one end is fixed to the base material 11. The sensing portion 13 has a width W of 1.0 mm and a length L of 2.0 mm. As shown in FIG. 1B, the lower electrode 14 made of Pt (platinum) is formed on the Si (silicon) substrate 12. The piezoelectric film 15 made of PZT and the upper electrode 16 made of Pt are laminated. The thickness of the piezoelectric film 15 is 1.5 μm and is formed by a sol-gel method.

  The piezoelectric sensor 10 was subjected to polarization treatment by applying a voltage of 20 V, and the capacitance was measured using an LCR meter. As the amount of polarization of the piezoelectric film increases, the relative permittivity increases, so the capacitance of the piezoelectric sensor 10 also increases. Therefore, the amount of polarization of the piezoelectric film can be known by measuring the capacitance of the piezoelectric sensor 10.

  The inventors of the present application examined the change in the capacitance of the piezoelectric sensor 10 for 1300 hours after the polarization treatment. The result is shown in FIG. As shown in FIG. 2, the capacitance of the piezoelectric sensor 10 immediately after the polarization process was approximately 18 nF, but it was confirmed that the capacitance decreased with the passage of time. This means that the sensitivity of the film-like piezoelectric sensor 10 decreases with time.

  Table 1 below shows the result of measuring the sensitivity by applying vibration to the piezoelectric sensor 10 immediately after performing the polarization treatment using a vibrator.

この表１に示すように、分極処理直後の圧電センサ１０の感度は２．３ｐＣ／Ｇであった。一方、分極処理から１３００時間経過した後の圧電センサ１０に対し、同様に加振器により振動を加えて感度を測定した結果、圧電センサ１０の感度は２．０ｐＣ／Ｇであった。このことから、フィルム状圧電センサの感度は、時間の経過とともに低下することが確認された。

As shown in Table 1, the sensitivity of the piezoelectric sensor 10 immediately after the polarization treatment was 2.3 pC / G. On the other hand, as a result of measuring the sensitivity by applying vibration to the piezoelectric sensor 10 after 1300 hours from the polarization treatment in the same manner using a vibrator, the sensitivity of the piezoelectric sensor 10 was 2.0 pC / G. From this, it was confirmed that the sensitivity of the film-like piezoelectric sensor decreases with time.

  Next, the inventors of the present application examined changes in characteristics of the film-like piezoelectric sensor 10 due to temperature. That is, the capacitance was measured immediately after the polarization of the piezoelectric sensor 10, and after further heating at a temperature of 70 ° C. for about 5 minutes, the capacitance was measured by returning to room temperature. The results are shown in Table 2 below.

この表２に示すように、加熱前のフィルム状圧電センサの静電容量は１８ｎＦであったが、加熱後の静電容量は１５ｎＦであった。このことから、フィルム状圧電センサは、温度の変化によっても感度が低下することが確認された。

As shown in Table 2, the capacitance of the film-like piezoelectric sensor before heating was 18 nF, but the capacitance after heating was 15 nF. From this, it was confirmed that the sensitivity of the film-like piezoelectric sensor was lowered even when the temperature changed.

  As described above, the capacitance of the piezoelectric sensor 10 immediately after the polarization process was 18 nF, but the capacitance of the piezoelectric sensor 10 was measured after 40 hours from the polarization process, and was 16 nF. After the polarization process was performed again on the piezoelectric sensor 10, the capacitance returned to 18 nF. As described above, when the polarization process is performed on the piezoelectric sensor whose sensitivity is lowered, the capacitance is restored to the original value, so that the sensitivity is considered to be recovered.

  As described above, in a film-like piezoelectric sensor using a piezoelectric film, the sensitivity can be recovered by performing the polarization process even if the sensitivity is lowered. On the other hand, if the polarization process is performed at regular time intervals, it is possible to avoid a decrease in sensitivity of the piezoelectric sensor and to always detect stress with a good sensitivity. Since stress cannot be detected while the polarization process is being performed, there is a possibility that the detection of stress may be hindered if the time interval for performing the polarization process is short. However, from the results of experiments by the inventors of the present application, it has been found that the capacitance of the film-like piezoelectric sensor has hardly changed at the time when one hour has elapsed since the polarization treatment. Therefore, the time interval for performing the polarization process may be 1 hour or more.

  Next, the inventors of the present application considered the voltage required for the polarization treatment of the film-like piezoelectric sensor. The film-like piezoelectric sensor outputs an electric charge corresponding to the applied stress. For this reason, the output of the piezoelectric sensor is input to the charge amplifier, converted into a voltage by the charge amplifier, and output to the detection circuit. If the drive voltage of the charge amplifier is different from the voltage applied to the piezoelectric sensor during the polarization process, two types of power supplies are required, which causes an increase in cost. In order to make the drive power supply of the charge amplifier and the power supply used for the polarization processing of the piezoelectric sensor common, assuming that the power supply voltage is Vcc, the thickness of the piezoelectric film is t, and the coercive electric field of the piezoelectric film is Ec, t ≦ It is necessary to satisfy the relationship of Vcc / Ec.

  As a result of measuring the PE (polarization-applied voltage) hysteresis characteristics of the film-like piezoelectric sensor 10 used in the above-described experiment, it was found that the coercive electric field (Ec) of the piezoelectric film was 33 kV / cm. Assuming that the drive voltage (Vcc) of the charge amplifier is 5 V, from the above inequality, by setting the thickness (t) of the piezoelectric film to 1.5 μm, the power supply for the charge amplifier and the power supply used for the polarization processing of the piezoelectric sensor It became clear that can be shared. The piezoelectric film having this thickness can be formed relatively easily by a sol-gel method or other methods.

(Piezoelectric sensor device)
FIG. 3 is a circuit diagram showing the piezoelectric sensor device according to the embodiment of the present invention.

  As shown in FIG. 1, the piezoelectric sensor 10 is a film-like piezoelectric sensor having a structure in which a piezoelectric film (PZT) is sandwiched between a pair of electrodes. One terminal (electrode) of the piezoelectric sensor 10 is connected to the input terminal of the charge amplifier 22 and one terminal of the switch 23 a of the switch circuit 23. The other terminal of the piezoelectric sensor 10 is connected to the negative electrode side of the power supply 24 through a ground wiring.

  The charge amplifier 22 includes an operational amplifier 27 and a capacitor 28 connected between the input terminal and the output terminal of the operational amplifier 27. The output terminal of the charge amplifier 22 (the output terminal of the operational amplifier 27) is connected to the output terminal 25. One power supply terminal of the charge amplifier 22 is connected to one terminal of the switch 23b of the switch circuit 23, and the other power supply terminal is connected to the negative electrode side of the power supply 24 through a ground line.

  The power supply 24 has a negative electrode side connected to the ground wiring and a positive electrode side connected to the other terminals of the switches 23 a and 23 b of the switch circuit 23. These switches 23 a and 23 b are on / off controlled by a timer 26. However, the switch 23a and the switch 23b are driven exclusively. When the switch 23a is on, the switch 23b is off, and when the switch 23a is off, the switch 23b is on.

  When the switch 23a is off and the switch 23b is on, the stress detection mode is set. In this stress detection mode, a voltage corresponding to the stress applied to the piezoelectric sensor 10 is output from the output terminal 25.

  On the other hand, when the switch 23a is on and the switch 23b is off, the polarization processing mode is set. In this polarization processing mode, a voltage is applied from the power supply 24 to the piezoelectric sensor 10 via the switch 23a, and the piezoelectric film is subjected to polarization processing. At this time, no drive voltage is supplied to the charge amplifier 22, so no signal is output from the output terminal 25.

  In the present embodiment, after the stress detection mode continues for one hour, the polarization processing mode is set for the time required for the polarization processing, and then the operation returns to the stress detection mode again. Usually, the time required for the polarization treatment is about 1 second at the longest.

(Magnetic recording device)
FIG. 4 is a plan view showing a magnetic recording apparatus (magnetic disk apparatus) according to an embodiment of the present invention.

  The magnetic recording device 30 holds a disk-shaped magnetic recording medium (magnetic disk) 31, a spindle motor (not shown) that rotates the magnetic recording medium 31, a magnetic head 32, and the magnetic head 32 in its housing. Suspension 33, actuator 34 for controlling the suspension 33 in the radial direction of the magnetic recording medium 31, and driving and controlling the spindle motor and actuator 34, and writing and reading data to and from the magnetic recording medium 31 via the magnetic head 32. And an electronic circuit 35. The magnetic head 32 includes a writing element (inductive head) that converts an electric signal into a magnetic signal and a reading element (magnetoresistance effect element) that converts a magnetic signal into an electric signal.

  When the magnetic recording medium 31 is rotated at a high speed by the spindle motor, the magnetic head 32 slightly floats from the magnetic recording medium 31 due to the air flow generated by the rotation of the magnetic recording medium 31. In this state, the actuator 34 moves the magnetic head 32 in the radial direction of the magnetic recording medium 31, and data is written to or read from the magnetic recording medium 31. A piezoelectric sensor is attached to the suspension 33 or the HGA (head gimbal assembly), as will be described later, and changes in the flying height of the magnetic head 32 are detected.

  5A and 5B are perspective views showing the suspension 33 and the piezoelectric sensor 10 and the magnetic head 32 attached to the suspension 33. FIG. FIG. 5A detects the flying height fluctuation of the magnetic head 32 using the piezoelectric lateral effect, and FIG. 5B uses the piezoelectric vertical effect to detect the flying height fluctuation of the magnetic head 32. It is to detect.

  In the example of FIG. 5A, the suspension 33 bends when the flying height of the magnetic head 32 fluctuates, and at the same time, the piezoelectric sensor 10 also bends. This electric charge is transmitted to the charge amplifier via the wiring 33a formed on the surface of the suspension 33. Here, it is assumed that the charge amplifier and the switch circuit (see FIG. 3) are incorporated in the electronic circuit 35. The wiring 33a and the suspension 33 are electrically insulated by an insulating layer such as polyimide interposed between them. Note that a terminal on the ground side of the piezoelectric sensor 10 may be connected to the suspension 33, and the suspension 33 may be used as a ground wiring.

  In the example of FIG. 5B, since stress is applied to the piezoelectric sensor 10 when the flying height of the magnetic head 32 varies, a charge corresponding to the stress is generated in the piezoelectric sensor 10. This charge is transmitted to the charge amplifier via wiring (not shown) formed on the surface of the suspension 33, as in the example shown in FIG.

  The piezoelectric sensor 10 can also be used as a retract sensor for retracting the magnetic head 32 when an impact is applied, or as a vibration sensor for detecting the vertical and horizontal vibrations of the magnetic head 32.

(Operation of the piezoelectric sensor device)
FIG. 6 is a flowchart showing the operation of the piezoelectric sensor device incorporated in the magnetic recording apparatus described above. The operation of the piezoelectric sensor device will be described with reference to FIG. 6, FIG. 3, and FIG.

  First, in step S11, the timer 26 starts measuring time. While the timer 26 counts time, the electronic circuit 35 drives the spindle motor, the actuator 34 and the magnetic recording head 32 to write data to the magnetic recording medium 31 and read data from the magnetic recording medium 31. . Further, the switch 23 a is turned off and the switch 23 b is turned on, so that the drive voltage is supplied to the charge amplifier 22. A voltage corresponding to the stress applied to the piezoelectric sensor 10 is output from the charge amplifier 22.

  Thereafter, when a certain time (here 1 hour) elapses, the timer 26 times up in step S12. As a result, the process proceeds to step S13, and an interrupt signal is generated.

  By this interrupt signal, the process proceeds to step S14, and the drive of the magnetic head 32 is interrupted. In step S15, the magnetic head 32 is retracted (unloaded) from the magnetic recording medium 31 to a predetermined position.

  Next, the process proceeds to step S16, where the switch 23a is turned on and the switch 23b is turned off. Thereby, it transfers to step S17 and the piezoelectric sensor 10 is connected to the power supply 24 via the switch 23a. Then, an electric field higher than the coercive electric field is applied to the piezoelectric film of the piezoelectric sensor 10 to perform polarization processing.

  When the polarization process is completed, the process proceeds to step S18, where the switch 23a is turned off and the switch 23b is turned on. Thereafter, the process proceeds to step S19, where the magnetic head 32 is loaded onto the magnetic recording medium 31, and the head drive is resumed in step S20. When the drive of the magnetic head 32 is resumed in this way, the process returns to step S11, the timer 26 is reset, and time measurement is started. Thereafter, the above steps are repeated, and the polarization process is performed on the piezoelectric film of the piezoelectric sensor 10 at regular time intervals.

  In the present embodiment, since the polarization process is automatically performed on the piezoelectric film of the piezoelectric sensor 10 at regular intervals, the sensitivity of the piezoelectric sensor 10 can be maintained in a good state for a long period of time. Therefore, there is an effect that the reliability of the detection result by the piezoelectric sensor 10 is remarkably improved.

  Further, according to the present embodiment, since the power source used for the polarization process of the piezoelectric sensor 21 and the drive power source of the charge amplifier 22 are made common, the power circuit is simplified and an increase in cost is avoided.

  In the above-described embodiment, the case where the piezoelectric film constituting the piezoelectric sensor is a PZT film has been described. However, the present invention is also applied to a piezoelectric sensor having a piezoelectric film made of a material exhibiting a piezoelectric effect other than PZT. be able to. In the above-described embodiment, the case where the present invention is applied to a magnetic recording apparatus has been described. However, the present invention can also be applied to an optical disc apparatus.

  Hereinafter, various aspects of the present invention will be collectively described as supplementary notes.

(Appendix 1) a piezoelectric sensor having a structure in which a piezoelectric film is sandwiched between a pair of electrodes;
And an electronic circuit that applies an electric field equal to or greater than a coercive electric field to the piezoelectric film of the piezoelectric sensor at regular intervals.

  (Appendix 2) The electronic circuit includes a power source, a switch circuit disposed between the power source and the piezoelectric sensor, and a timer that controls the switch circuit according to a set time. The piezoelectric sensor device according to appendix 1.

  (Supplementary Note 3) The electronic circuit further includes a charge amplifier that converts the output of the piezoelectric sensor into a voltage, and the drive voltage of the charge amplifier is supplied from the power supply via the switch circuit. The piezoelectric sensor device according to attachment 2.

  (Supplementary note 4) Supplementary note 3 wherein t ≦ Vcc / Ec is satisfied, where Vcc is the voltage of the power source, Ec is the coercive electric field of the piezoelectric film, and t is the thickness of the piezoelectric film. The piezoelectric sensor device according to 1.

  (Additional remark 5) The said timer controls the said switch circuit when applying the electric field more than the coercive electric field of the said piezoelectric material film, and stops supply of the drive voltage to the said charge amplifier, It is characterized by the above-mentioned. The piezoelectric sensor device according to 1.

  (Appendix 6) The piezoelectric sensor device according to appendix 1, wherein the piezoelectric film is made of PZT and has a thickness of 1.5 μm or less.

  (Supplementary note 7) The piezoelectric sensor device according to supplementary note 1, wherein a time interval for performing a process of applying an electric field higher than the coercive electric field to the piezoelectric film is 1 hour or more.

(Appendix 8) a disc-shaped recording medium;
A head for writing or reading data to or from the recording medium;
A suspension for holding the head;
An actuator for driving the suspension;
A piezoelectric sensor configured by a piezoelectric film and a pair of electrodes arranged with the piezoelectric film interposed therebetween and attached to the suspension;
And an electronic circuit that applies an electric field equal to or higher than a coercive electric field to the piezoelectric film of the piezoelectric sensor at regular intervals.

  (Supplementary note 9) The supplementary note 8, wherein the electronic circuit controls the actuator to retract the head from the recording medium when an electric field higher than a coercive electric field is applied to the piezoelectric film. Recording device.

  (Supplementary note 10) The recording apparatus according to supplementary note 8, wherein the piezoelectric sensor detects a variation in a flying height of the head.

FIG. 1A is a perspective view showing a film-like piezoelectric sensor, and FIG. 1B is a cross-sectional view of a sensing portion of the piezoelectric sensor. FIG. 2 is a diagram showing the results of examining the change in capacitance of the piezoelectric sensor over 1300 hours after the polarization treatment. FIG. 3 is a circuit diagram showing the piezoelectric sensor device according to the embodiment of the present invention. FIG. 4 is a plan view showing the magnetic recording apparatus according to the embodiment of the present invention. FIGS. 5A and 5B are perspective views showing the piezoelectric sensor and the magnetic head attached to the suspension. FIG. 6 is a flowchart showing the operation of the piezoelectric sensor device incorporated in the magnetic recording device.

Explanation of symbols

10: Piezoelectric sensor,
11 ... base material,
12 ... Si substrate,
13 ... Sensing part,
14 ... lower electrode,
15 ... piezoelectric film,
16 ... upper electrode,
22 ... Charge amplifier,
23 ... Switch circuit,
23a, 23b ... switches,
24 ... Power supply,
25 ... Output terminal,
26: Timer,
27. Operational amplifier,
28: Capacitor,
30 ... Magnetic recording device,
31 ... Magnetic recording medium,
32 ... Magnetic head,
33 ... suspension,
34 ... Actuator,
35: Electronic circuit.

Claims (3)

A piezoelectric sensor having a structure in which a piezoelectric film having a thickness of 1.5 μm or less is sandwiched between a pair of electrodes;
A charge amplifier that converts the output of the piezoelectric sensor into a voltage;
A voltage source capable of applying an electric field higher than the coercive electric field of the piezoelectric film to the piezoelectric sensor and generating a voltage capable of driving the charge amplifier;
A piezoelectric sensor device comprising: a switch circuit that supplies a voltage generated by the power source to the charge amplifier and switches a connection destination of the power source from the charge amplifier to the piezoelectric sensor every preset time . 2. The condition according to claim 1 , wherein t ≦ Vcc / Ec is satisfied, where Vcc is the power supply voltage, Ec is the coercive electric field of the piezoelectric film, and t is the thickness of the piezoelectric film. Piezoelectric sensor device. A disk-shaped recording medium;
A head for writing or reading data to or from the recording medium;
A suspension for holding the head;
An actuator for driving the suspension;
A piezoelectric sensor comprising a piezoelectric film having a thickness of 1.5 μm or less and a pair of electrodes disposed with the piezoelectric film interposed therebetween, and attached to the suspension;
A charge amplifier that converts the output of the piezoelectric sensor into a voltage;
A voltage source capable of applying an electric field higher than the coercive electric field of the piezoelectric film to the piezoelectric sensor and generating a voltage capable of driving the charge amplifier;
A recording apparatus comprising: a switch circuit that supplies a voltage generated by the power source to the charge amplifier and switches a connection destination of the power source from the charge amplifier to the piezoelectric sensor every preset time .

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