JP5721126B2 - Piezoelectric sensor - Google Patents

Description

  The present invention relates to a piezoelectric sensor.

  2. Description of the Related Art Conventionally, piezoelectric sensors such as an acceleration sensor, a pressure sensor, and an acoustic emission (hereinafter abbreviated as “AE” sensor) that use the piezoelectric effect of a piezoelectric body have been widely used. Among the piezoelectric sensors, the AE sensor is used for member deterioration diagnosis, process abnormality diagnosis, and the like. Since it can be used for such a wide range of applications, the piezoelectric sensor is regarded as an extremely useful sensor (for example, Patent Document 1).

  Usually, a piezoelectric sensor employs a structure in which electrodes are provided on a bulk piezoelectric body. In actual use of the piezoelectric sensor, the piezoelectric body is generally used by being fixed in a metal casing.

  In addition, piezoelectric sensors often need to handle weak voltage signals. Especially when detecting AE, the detection accuracy can be improved by using a filter so that signal components in unnecessary bands such as noise can be cut. It is increasing.

JP 7-135345 A (published May 23, 1995)

  However, the conventional piezoelectric sensor has a problem in that it is difficult to reduce the thickness of the detection unit, so that it cannot be used in a required place or a detection failure may occur due to noise caused by the environment. ing.

  More specifically, a conventional piezoelectric sensor has a structure that is usually housed in a metal casing. The metal casing is mainly used to protect internal piezoelectric elements from impacts and the like, and at the same time suppress noise from the outside environment. Therefore, the minimum thickness of the conventional piezoelectric sensor is 2.4 mm (PAC (Physical Acoustics Corporation) S9225). Therefore, when there is no parallel space of 2.4 mm or more in the thickness direction of the piezoelectric sensor, it is impossible to install this cylindrical piezoelectric sensor in the first place. In addition, in an environment where a high frequency power source in which strong electromagnetic waves act on the piezoelectric sensor is used, electromagnetic noise cannot be sufficiently removed even by using a metal casing and a filter. It may not be detected.

  Thus, in spite of the possibility of use in a wide range of applications, the use of piezoelectric sensors is actually limited by the installation environment.

  As an example of the problem of the installation environment, the inside of a high frequency plasma processing apparatus which is one of semiconductor manufacturing apparatuses can be cited. In the processing tank of the plasma processing apparatus, since a high frequency is used to generate plasma, a large electric field change occurs, and as a result, strong electromagnetic noise is generated. It is considered that various deterioration phenomena occur with the generation of AE in the plasma generation region or the components in the vicinity thereof, but usually there is only a gap of about 1 mm to 2 mm at most in the vicinity of the plasma generation region. . That is, the conventional piezoelectric sensor cannot be used due to the limitation due to the presence of the strong electromagnetic noise and the narrow installation space with only the gap.

  The present invention has been made in view of the above-described conventional problems, and an object of the present invention is an environment that could not be used in the past because it can be thinned and strong electromagnetic noise is generated by the action of high frequency. An object of the present invention is to provide a piezoelectric sensor that can be used.

  In order to solve the above problems, the piezoelectric sensor of the present invention has the same dielectric constant and capacitance as the first piezoelectric film, the first electrode, the insulator layer, the second electrode, and the piezoelectric film. A piezoelectric sensor in which a second piezoelectric film or a dielectric film is laminated, wherein the first piezoelectric film and the second piezoelectric film are arranged with the same polar face facing each other, and the insulating layer The area in the layer direction is the same as or smaller than the area in the layer direction of the first piezoelectric film, and is the same as or smaller than the area in the layer direction of the second piezoelectric film or dielectric film.

  Accordingly, the piezoelectric sensor can be installed with the installation pressure applied to the first piezoelectric film and the second piezoelectric film or the dielectric film being more effectively increased. As a result, it is possible to improve the detection performance related to the maximum amplitude of the detection signal detected by the first piezoelectric film and the second piezoelectric film or the dielectric film, and it is thin by having a laminated structure of each member. A piezoelectric sensor can be realized. In addition, since it is a differential structure in which the same polarity surfaces of both piezoelectric bodies including the first piezoelectric film and the second piezoelectric film or dielectric film are arranged to face each other, noise is reduced to a significant level. It is possible to detect a signal even in an environment using a high-frequency power source that is difficult to generate.

  In the piezoelectric sensor of the present invention, it is preferable that the insulator layer exhibits flexibility.

  Thereby, even when a strong mechanical load is applied in the vertical direction of the piezoelectric sensor, the insulator layer can receive the load, so that the occurrence of failure of the piezoelectric sensor due to breakage of the constituent members or the like can be reduced.

  In the piezoelectric sensor of the present invention, the insulator layer is preferably a polyimide layer.

  Polyimide exhibits excellent chemical resistance and heat resistance among polymer insulating materials, and is optimal for enduring use in a wide range of usage environments. In addition, since it has high electrical insulation and is excellent in the effect of attenuating ultrasonic waves, it is difficult to transmit ultrasonic waves propagated from one thickness direction to the other, which is effective for making the insulator layer thinner. is there.

  In the piezoelectric sensor of the present invention, the polyimide layer preferably has a thickness of 200 μm or more and 600 μm or less.

  Thereby, the thickness of the insulator layer can be kept in a thin range while setting the maximum amplitude of the detection signal detected by the piezoelectric sensor to a large value, which can contribute to the thinning of the piezoelectric sensor.

  In the piezoelectric sensor of the present invention, the first piezoelectric film and the second piezoelectric film or the dielectric film are formed on the first substrate and the second substrate, both having conductivity, respectively. It is preferable that the outer edge portion of the first substrate and the outer edge portion of the second substrate are joined so that the conductivity is not impaired.

  As a result, even when external electromagnetic noise acts on the piezoelectric sensor, the outer shell of the sensor covered with the conductive material is kept at an equal potential regardless of the location. A noise signal having the same strength as that of the second piezoelectric film or dielectric film is detected. As a result, in the differential signal between the first piezoelectric film and the second piezoelectric film or dielectric film, the influence of the electromagnetic noise is remarkably reduced and is not ideally detected.

  In the piezoelectric sensor of the present invention, both the first substrate and the second substrate can bend, and the thickness of the first piezoelectric film and the second piezoelectric film or dielectric film can be increased. Are preferably 10 μm or less.

  Thereby, it becomes possible to join those outer edge parts in the state which bent the said 1st board | substrate and the 2nd board | substrate to the required extent, and can give a phase shape to a piezoelectric sensor.

  Further, in the piezoelectric sensor of the present invention, the lead wire connected to each of the first electrode and the second electrode, and the connection for transmitting the voltage signal obtained from the first electrode and the second electrode via the lead wire. It is preferable that at least a part of the cable including the conductor is covered with an insulator.

  Since at least a part of the cable is covered with an insulator, a piezoelectric sensor that is hardly affected by electromagnetic noise can be provided. In particular, when used in an environment where a strong high-frequency electric field acts, a large electromagnetic noise acts on the cable, which is very effective.

  As described above, the piezoelectric sensor of the present invention is a piezoelectric sensor in which the first piezoelectric film, the first electrode, the insulator layer, the second electrode, and the second piezoelectric film or the dielectric film are laminated. The second piezoelectric film or the dielectric film has the same dielectric constant and capacitance as the first piezoelectric film, and the first piezoelectric film and the second piezoelectric film have the same polarity. The areas in the layer direction of the insulator layer are the same as or smaller than the area in the layer direction of the first piezoelectric film, and the layer direction of the second piezoelectric film or dielectric film It is the same or smaller than the area.

  As a result, it is possible to improve the detection capability regarding the maximum amplitude of the detection signal detected by the first piezoelectric film and the second piezoelectric film or the dielectric film, and the piezoelectric sensor can be thinned by the laminated structure of each member. realizable. In addition, since the differential structure includes the first piezoelectric film and the second piezoelectric film or the dielectric film, it is difficult to reduce noise to a significant level in an environment using a high-frequency power source. Even if it exists, there exists an effect that the detection of a signal is possible.

It is sectional drawing which shows one Embodiment of the piezoelectric sensor in this invention. It is a side view which shows the piezoelectric sensor connected with the cable in this invention. It is a figure which shows the sensor part of the piezoelectric sensor shown in FIG. It is a block diagram which shows a uniaxial compression tester regarding a piezoelectric sensor. It is a graph which shows the relationship between the maximum amplitude of a detection waveform regarding the piezoelectric sensor of this invention, and a load load, and the relationship between the maximum amplitude at the time of 2.0 kN load, and an applied frequency. It is sectional drawing which shows the non-differential type piezoelectric sensor for a comparison. It is a graph which shows the signal detected with the piezoelectric sensor of this invention, and the non-differential type sensor for a comparison. It is a graph which shows the signal detected when processing a Ti wafer. 3 is a graph showing data of each sample based on Table 1. 3 is a graph showing data of each sample based on Table 1. It is a graph which shows the relationship between the polyimide thickness and the maximum amplitude value in each applied voltage frequency. 6 is a graph based on a test result of a comparative piezoelectric sensor of Comparative Example 1.

  An embodiment of the present invention will be described below with reference to FIGS. FIG. 1 is a cross-sectional view showing a basic structure of a piezoelectric sensor 50 according to the present embodiment. The piezoelectric sensor 50 includes a piezoelectric film (first piezoelectric film) 2, a lower electrode (first electrode) 3, and a polyimide layer (insulator layer) 4 on a substrate (first substrate) 1, and further includes an upper electrode. (Second electrode) 5, a piezoelectric film (second piezoelectric film) 6, a substrate (second substrate) 7, and the like. Hereinafter, each component will be described.

  The substrates 1 and 7 are base members for supporting the piezoelectric film 2 and the like disposed therebetween. The materials for the substrates 1 and 7 are not particularly limited, and known materials can be used. For example, various insulators such as silicon, sapphire, alumina, zirconia, magnesia, silicon carbide, glass, silicon nitride, Semiconductors can be used. Further, heat-resistant alloys such as so-called superalloys, stainless steel, other metal materials and alloys, intermetallic compounds, and conductive compounds such as titanium carbide and titanium nitride may be used.

  The piezoelectric films 2 and 6 generate a voltage by the piezoelectric effect. A known piezoelectric material may be used as the material of the piezoelectric film 2, and various oxides, nitrides, polymers, and the like can be used. In other words, single crystal such as lead zirconate titanate (PZT), crystal, lithium niobate, lithium tantalate, lithium tetraborate, potassium niobate, langasite crystal, orientation such as zinc oxide and aluminum nitride Crystal, polyvinylidene fluoride, and the like. In particular, from the viewpoint of thinning the piezoelectric sensor, a thinner piezoelectric body is desirable, and considering the ease of manufacture and the like, it is preferable to form the thin film piezoelectric body directly on the substrate 1. Excellent heat resistance, crystal stability and piezoelectricity, low dielectric constant and high dielectric breakdown resistance, so thin-film piezoelectric body made of aluminum nitride or its solid solution is preferable and can be used in a wider range of environments Become.

  The thickness of the piezoelectric films 2 and 6 is within a range in which the piezoelectric films 2 and 6 can play a role as a detector, and is preferably thinner to make the piezoelectric sensor 50 thinner, and is 150 μm or less. be able to. A more preferable range is 1 μm or more and 10 μm or less at which a good thin film piezoelectric material can be easily formed, and a more preferable range is that a piezoelectric film formation failure is unlikely to occur, Even if there is a subsequent damage, it is 3 μm or more and 6 μm or less, which hardly impairs the function as a sensor.

  Polyimide double-sided tapes 2a and 6a are disposed on the outer periphery of the piezoelectric films 2 and 6, thereby causing a short circuit between the substrate 1 and the lower electrode 3 or the inner conductor 3c, and the substrate 7 and the upper electrode 5 or the inner portion. A short circuit with the conductor 5c can be prevented.

  The piezoelectric sensor 50 is provided with piezoelectric films 2 and 6 and has a differential structure. Of the two piezoelectric films, the piezoelectric film 2 is disposed on the detection surface side, and the piezoelectric film 6 is disposed for reference to obtain a differential signal. For this reason, the piezoelectric films 2 and 6 have the same relative dielectric constant and capacitance, and the same polar surfaces are arranged to face each other. Further, instead of the piezoelectric film 6 for reference, a dielectric having the same relative permittivity and capacitance as the piezoelectric film 2 may be used. For example, the piezoelectric film 2 may be an aluminum nitride thin film piezoelectric body having piezoelectricity and orientation, and the dielectric may be an aluminum nitride thin film having no orientation and no piezoelectricity. . The lower electrode 3 transmits the voltage generated in the piezoelectric film 2 to the internal conductor (conductive wire) 3c, and the upper electrode 5 transmits the voltage generated in the piezoelectric film 6 to the internal conductor (conductive wire) 5c. Thus, the lower electrode 3 is connected to the inner conductor 3c, and the upper electrode 5 is connected to the inner conductor 5c.

  The lower electrode 3 and the upper electrode 5 have the same configuration, for example, copper foil. The lower electrode 3 and the upper electrode 5 include polyimide double-sided tapes 3 a and 5 a that play a role of bonding to the polyimide layer 4. Further, a polyimide film 3 b is provided between the lower electrode 3 and the piezoelectric film 2, and a polyimide film 5 b is provided between the upper electrode 5 and the piezoelectric film 6. This prevents the piezoelectric body from being damaged due to an accident and short circuit between the lower electrode 3 and the substrate 1 or the upper electrode 5 and the substrate 7 existing above and below the piezoelectric body, thereby preventing the sensor from becoming unusable. ing. In the piezoelectric sensor 50, copper foil is used for the lower electrode 3 and the upper electrode 5, but instead of copper foil, platinum, aluminum, nickel, iron, gold, silver, copper, palladium, chromium, and alloys thereof Any of these may be used.

  Both the surfaces of the inner conductors 3c and 5c are covered with primary outer conductors 3d and 5d through an inner insulating layer, and are further protected with primary insulating covering layers 3f and 5f, respectively. Further, all of these are covered by the secondary outer conductor 8a and the secondary insulating coating layer 8b on the surface thereof. Thus, since the inner conductors 3c and 5c are doubly shielded by the outer conductor, the structure can reduce the influence of noise received by the measurement environment.

  In the piezoelectric sensor 50, a polyimide layer 4, which is an insulator layer, is disposed between the lower electrode 3 and the upper electrode 5 via polyimide double-sided tapes 3 a and 5 a. The polyimide layer 4 may be a laminate of a plurality of polyimide films, or may be constituted by a single polyimide film or sheet. When laminating, each polyimide film can be bonded with a silicone-based adhesive or the like, and for example, a polyimide double-sided tape may be used. It is preferable to form a thick polyimide layer by laminating a polyimide film using such an adhesive because a flexible polyimide layer can be formed rather than using a single polyimide sheet having the same thickness.

  In the piezoelectric sensor 50, the thickness of the polyimide layer 4 is set to 440 μm. The thickness can be appropriately changed, and a preferable range is 200 μm or more and 600 μm or less. A lower limit value of 200 μm or more is preferable in that the maximum amplitude value of the detection signal detected by the piezoelectric sensor 50 increases. On the other hand, the maximum amplitude value of the detection signal further increases as the thickness of the polyimide layer 4 increases, but hardly increases when the thickness exceeds 600 μm. Therefore, it is possible to contribute to the thinning of the piezoelectric sensor 50 by setting the preferable upper limit value to 600 μm.

  The shape of the polyimide layer 4 in the layer direction is a shape that fits into both regions when overlapped with regions in the film surface formed by the piezoelectric films 2 and 6. That is, regarding the area of the polyimide layer 4 in the layer direction, the area of the piezoelectric film 2 is the same as or smaller than the area of the piezoelectric film 2 and the piezoelectric film 6 (a dielectric film is used instead of the piezoelectric film 6). Is the same as or smaller than the area of the dielectric film) in the layer direction. Thus, when the piezoelectric sensor 50 is configured, an installation pressure is applied to the piezoelectric films 2 and 6 in accordance with the area of the polyimide layer in the layer direction, and all of the installation load is applied to the piezoelectric film (or the dielectric film). The installation pressure can be effectively increased. As a result, the detection capability regarding the maximum amplitude of the detection signal detected by the piezoelectric films 2 and 6 can be improved.

  Preferably, the area of the polyimide layer 4 in the layer direction is 80% or less with respect to the area of the piezoelectric films 2 and 6 in the layer direction (upper limit value). As a result, even if there is a slight misalignment during the production of the piezoelectric sensor, it can be arranged without deviating from the region of the piezoelectric film 2, 6, so that a large pressure can be preferably applied to the piezoelectric film 2, 6. . Further, the lower limit is not particularly limited, but if it is too small, it becomes difficult to produce the piezoelectric sensor 50, and the region where the piezoelectric film is subjected to pressure is narrowed. Since the capacity is also reduced, the signal-to-noise ratio (S / N) is consequently reduced, which is not preferable. Therefore, it is better to adjust the area in the layer direction of the insulator layer so that the capacitance of the piezoelectric film calculated from the area in the layer direction of the insulator is sufficiently larger than the stray capacitance due to the cable or the like, For example, it may be 500 pF or more.

As an example, when an aluminum nitride thin film piezoelectric body is used, assuming that the relative dielectric constant is 8.8, when the thickness of the piezoelectric film is 3 μm, it is larger than 20 mm ² (about the area of a circle having a diameter of 5 mm). It is preferable to do so.

The piezoelectric film is preferably made of an aluminum nitride thin film because the area of the polyimide layer 4 in the layer direction is preferably 80% or less with respect to the area of the piezoelectric films 2 and 6 in the layer direction. In the case of a film, the area in the layer direction of the piezoelectric film is preferably 25 mm ² or more.

  The piezoelectric sensor 50 has a structure including the polyimide layer 4 from the viewpoint of easy molding and flexibility. Since the polyimide layer 4 has flexibility, failure of the piezoelectric sensor 50 due to breakage or the like can be reduced. However, the piezoelectric sensor according to the present embodiment can also be configured by using an insulator such as a sapphire, quartz, silicon nitride, alumina, or silica ceramic material instead of the polyimide layer 4. .

The piezoelectric sensor 50 has the above-described structure, and can realize a thin-layer structure without using a strong metal casing. Specifically, when the thickness of the central polyimide layer 4 is 440 μm, the substrates 1 and 7 are set to 100 μm, the piezoelectric films 2 and 6 are set to 3 μm, and the lower electrode 3 and the upper electrode 5 are set to 150 μm. The thickness of these layers is 946 μm (0.946 mm), which is about 1 mm at most including the other layers. It can be seen that this is much thinner than the minimum thickness of the conventional piezoelectric sensor of about 2.4 mm .

  In addition, sensors using ordinary electronic signals are affected by electromagnetic waves. In the case of an AE sensor that uses a piezoelectric body as a detector and uses a weak piezoelectric signal, it is possible to detect a finer AE signal by using a filter for cutting noise in an unnecessary frequency band. Yes.

  On the other hand, the piezoelectric sensor 50 according to the present embodiment is of a differential type provided with two piezoelectric films 2 and 6 with the same polarity surfaces facing each other. Further, since the polyimide layer 4 is disposed between the piezoelectric films 2 and 6, the AE signal detected by the piezoelectric film 2 on the detection side is significantly attenuated by the polyimide layer 4. For this reason, the AE signal detected by the piezoelectric film 6 is very small or hardly detected.

  In particular, a conductive material is selected as the substrates 1 and 7, and the outer peripheral portions of the piezoelectric films 2 and 6 are formed while leaving the outer edges, and the outer edges of the substrates 1 and 7 are joined so that the conductivity is not impaired. For example, the shield effect can be exhibited as in the case of using a metal casing, and electrical noise can be suppressed.

  Furthermore, if a material that can be bent, such as a thin plate of metal or alloy, is selected as the substrates 1 and 7 and the piezoelectric films 2 and 6 of sufficient thickness are formed, all the members of the piezoelectric sensor are flexible. Will have sex. Therefore, a piezoelectric sensor having a curvature can be obtained by using a specific tool or the like to form an integrated piezoelectric sensor. In other words, even if the sensor installation surface is a curved surface, it can be a piezoelectric sensor with a shape that matches the curvature of the sensor, so that it is effective while applying sufficient installation pressure to the detection surface even when installing on a curved surface. Can be installed. That is, it is possible to provide a thin piezoelectric sensor that is strong against electromagnetic noise and can be installed on a curved surface.

  As described above, since the piezoelectric sensor 50 has a differential configuration, a weak AE signal can be detected with high sensitivity from the differential signal of the AE signal detected by the piezoelectric films 2 and 6. . According to the piezoelectric sensor 50, it is possible to detect an AE signal even in an environment using a high-frequency power source in which it is difficult to remove noise.

  FIG. 2 is a side view showing the piezoelectric sensor 50 connected to the cable 60. The piezoelectric sensor 50 is connected to the end of the cable 60 and has a shape that can be installed at a narrow measurement location. As a preferred form, the surface of the cable 60 is covered with an insulator 61, and the insulator at the end is fixed so as not to move easily. Thereby, even in an environment such as high-frequency plasma, the piezoelectric sensor 50 that is hardly affected by electromagnetic waves can be provided, and AE can be measured. Further, a connection terminal 62 is provided at the other end of the cable 60 so that it can be connected to a preamplifier.

  FIG. 3 is a top view showing the piezoelectric sensor 50 of FIG. The piezoelectric sensor 50 is integrated by joining the outer edges of the metal substrates 1 and 7 to enhance the shielding effect, and is joined so as to have a curvature of R150. Therefore, the detection surface (lower surface) is R150. It has an optimal shape for installation on the concave surface. Since the curvature of the detection surface can be variously set by using an appropriate jig at the time of joining, it can be changed as appropriate. For example, by using the substrates 1 and 7 having a thickness of 20 μm or more and 500 μm or less, a curved surface having a curvature of R10 to a flat surface can be obtained.

  As described above, when the piezoelectric sensor 50 is installed at the installation location, since the shape according to the installation location is given, it is easy to integrate the piezoelectric sensor 50 and the installation location.

  The installation pressure in the polyimide layer 4 will be described in detail below. The inventors of the present invention have found that it is necessary to increase the installation pressure of the piezoelectric film with respect to the electrode in order to improve the detection performance of the piezoelectric sensor by intensive studies on the piezoelectric sensor. In the conventional piezoelectric sensor, since a piezoelectric body, an electrode, and the like are formed on the inner surface of the casing, the installation pressure is not received by the piezoelectric body or the like formed inside the casing. In contrast, in the piezoelectric sensor 50 according to the present embodiment, the load applied at the time of installation is directly transmitted to the piezoelectric body. For this reason, it is considered that load dependency may appear in the detection characteristics. Hereinafter, the result of the test performed using the jig 72 which can apply equal installation pressure according to the curvature about the piezoelectric sensor 50 which has a curved surface shape is shown.

  First, an apparatus for evaluating the characteristics of the piezoelectric sensor 50 will be described. FIG. 4 is a configuration diagram showing a uniaxial compression tester 70 used for evaluating the piezoelectric sensor. The uniaxial compression tester 70 is a tester for causing the piezoelectric sensor 50 to detect a pseudo AE under a predetermined load (installation pressure) condition. As shown in the figure, the uniaxial compression tester 70 includes push rods 71 and 71a, a jig 72, an excitation sensor 73, a function generator 74, an oscilloscope 75, and a preamplifier 76.

  The push rods 71 and 71a sandwich and fix the jig 72 and the excitation sensor 73. The jig 72 is made of an Al alloy and is a pair of jigs having a concave surface and a convex surface having a curvature of R150. The piezoelectric sensor 50 is sandwiched between the jigs 72 while maintaining a curved surface provided in advance. In this way, the piezoelectric sensor 50 having a situation can be installed in an installation space sandwiched between such situations. As the excitation sensor 73, a so-called broadband general-purpose AE sensor having a flat frequency characteristic over a region from 100 kHz to 1 MHz is used. Hereinafter, the operation of the uniaxial compression tester 70 when a compression load is applied will be described.

  During the compression test, first, a static load is applied to the jig 72 and the excitation sensor 73 between the push rods 71 and 71a. When a predetermined voltage signal is applied to the excitation sensor 73 while keeping the static load constant, an elastic wave is generated in the jig 72 because the excitation sensor 73 is driven, and this propagates to the piezoelectric sensor 50 as a pseudo AE. . The elastic wave detected by the piezoelectric sensor 50 is converted into a voltage signal, amplified by the preamplifier 76, and then taken into the oscilloscope 75.

  Next, test results using the uniaxial compression tester 70 will be described. FIG. 5 is a graph showing the result of a simple evaluation test of the piezoelectric sensor 50 performed by applying one wavelength of a sine wave signal having an amplitude of 20 V to the excitation sensor 73. FIG. 5A shows the relationship between the maximum amplitude of the detected waveform at the applied voltage frequency of 300 kHz and the applied load, and FIG. 5B shows the maximum amplitude of the detected waveform at the applied load of 2.0 kN and the applied frequency. Shows the relationship. Note that both graphs in FIGS. 5A and 5B are the results of tests performed with the gain of the preamplifier 76 increased by 100 times.

As shown in FIG. 5A, when a load of 1.5 kN or more is applied to the jig 72, the maximum amplitude of the differential output signal of the piezoelectric sensor 50 is kept substantially constant and stable. Furthermore, it is stable with almost no change at 2.0 kN or more. Thus, if the load which the jig 72 exerts on the board | substrates 1 and 7 is 1.5 kN or more, since a favorable maximum amplitude can be obtained, it is very preferable. In the piezoelectric sensor 50, since the area of the insulating layer in the layer direction is 50 mm ² , it can be said that it is very preferable to apply a pressure of 30 MPa or more to the insulating layer.

  FIG. 5B shows the relationship between the output voltage and the applied frequency when the load applied to the jig 72 (to the piezoelectric sensor 50) is fixed at 2.0 kN. Since the broadband AE sensor having a flat output characteristic from 100 kHz to 1 MHz is used as the excitation sensor 73, the piezoelectric sensor 50 may be considered to have a peak of the output characteristic in the vicinity of about 200 to 300 kHz.

  For comparison, a difference from the piezoelectric sensor 50 will be described by using a non-differential piezoelectric sensor 150 different from the piezoelectric sensor 50 according to the present invention. FIG. 6 is a cross-sectional view showing the structure of a comparative non-differential piezoelectric sensor 150. The non-differential piezoelectric sensor 150 has a structure including a piezoelectric film 102 and an electrode 103 between substrates 101 and 107. Further, the electrode 103 is provided with a polyimide tape 103a and a polyimide film 103b. The electrode 103 is connected to the inner conductor 103c, and the inner conductor 103c is covered with a primary outer conductor 103d.

  Similar to the piezoelectric sensor 50, the non-differential piezoelectric sensor 150 is provided with a cable and a connection terminal, and the cable length and the like are all set to the same conditions. The non-differential piezoelectric sensor 150 is attached to the uniaxial compression tester 70 instead of the piezoelectric sensor 50, and the relationship between the maximum amplitude of the detected waveform and the load load at the applied voltage frequency of 300 kHz under the same conditions as the piezoelectric sensor 50 is shown. It was measured. As a result, the non-differential piezoelectric sensor 150 did not reach the saturation point as shown in FIG. 5A even when a load of 3 kN or more was applied. Therefore, according to the piezoelectric sensor 50, the piezoelectric films 2 and 6 can be sufficiently pressed against the lower electrode 3 and the upper electrode 5 with a smaller load, respectively, and the insertion of the polyimide layer 4 is extremely difficult. It turns out that it is effective.

  Furthermore, in order to compare the piezoelectric sensor 50 and the non-differential type piezoelectric sensor 150, FIG. 7 shows the result of a comparison test performed under the same conditions with a common configuration such as a cable length. FIG. 7 is a graph showing the relationship between the maximum amplitude of the detected waveform and the applied frequency when the load is 2.0 kN.

  When the graph showing the result of the piezoelectric sensor 50 according to the present invention shown in FIG. 7 (upper graph) and the graph showing the result of the non-differential piezoelectric sensor 150 (lower graph) are compared, the dependence on the applied frequency is Although similar, it can be seen that the piezoelectric sensor 50 is approximately 200 mV above the voltage level detected by the non-differential piezoelectric sensor 150. In other words, it can be seen that the piezoelectric sensor 50 has the polyimide layer 4 so that it can exert a higher pressure on the piezoelectric films 2 and 6 even at the same installation pressure, and the detection performance is improved.

  Finally, the superiority of the piezoelectric sensor 50 according to the present embodiment being a differential type will be described. When the non-differential piezoelectric sensor 150 was used for a detection test for abnormal discharge generated during plasma etching, it was found that a very large spike signal was generated. An example of the waveform detected during the test is shown in FIGS.

  Through detailed investigations by the present inventors, it has been found that the spike-like signal in FIG. 8 is an electromagnetic signal generated by abnormal discharge (hereinafter referred to as “abnormal discharge signal”). However, it is difficult to sufficiently cut the spike signal shown in FIG. As a result, the gain of the preamplifier must be suppressed to 40 dB or less. In normal AE measurement, it is necessary to amplify the sensor signal by about 60 to 80 dB in the entire measurement system, so that the spike-like signal detected here is an extremely large obstacle for detecting the target AE. I understand.

  Therefore, FIG. 8B shows the result of using the piezoelectric sensor 50 according to the present embodiment as a measure for lowering the level of the spike signal. FIG. 8B is a graph showing a response result of the piezoelectric sensor 50 to the abnormal discharge signal.

  Since the spike signal is an electromagnetic signal, if it jumps into each of the two signal lines (inner conductors) in the differential cable or the piezoelectric body connected to them at the same level, two In principle, the spike-like signal can be canceled by taking the difference between the voltage signals obtained via the signal lines.

  Furthermore, when the signal shown in FIG. 8B is detected by the piezoelectric sensor 50, the gain is set to 30 dB larger than that of the non-differential piezoelectric sensor 150 shown in FIG. The amplitude of the spike signal is suppressed to below the same level. Also, the attenuation is extremely fast. Therefore, even if the AE signal arrives from the installation surface side before and after the spike signal, the installation surface side piezoelectric body can detect the AE signal, but the other upper surface side piezoelectric body can detect the polyimide spacer. The AE signal can hardly be detected due to the attenuation effect by the (insulator layer). As a result, by extracting the difference signal between them, the necessary AE signal can be reliably detected while lowering the level of the spike-like signal, so that the AE signal can be detected.

  Further, in the environment affected by the high-frequency electric field as in the case of generating high-frequency plasma, the vicinity of the cable 60 of the piezoelectric sensor 50 is easily affected by noise. On the other hand, as shown in FIG. 1, in the piezoelectric sensor 50, the inner conductors 3c and 5c are doubly shielded by the primary outer conductors 3d and 5d and the secondary outer conductor 8a, respectively. Further, the cable 60 of the piezoelectric sensor 50 is covered with an insulator 61. As a result, a strong noise countermeasure can be taken at the cable portion, which can contribute to a reduction in the influence of spike signals.

  Although this invention is demonstrated more concretely based on an Example and FIGS. 9-12, this invention is not limited to this. Those skilled in the art can make various changes, modifications, and alterations without departing from the scope of the present invention.

[Differential signal generation]
Based on the detection signals detected on the receiving surface and the upper surface of the piezoelectric sensor, a differential signal was obtained by the following method. Specifically, differential signals were obtained using the following two methods as appropriate.

(1) Method 1
One digital oscilloscope (Yokogawa Electric Corporation DL1640L) with 4CH and two low noise preamplifiers for AE (NF circuit design block 9913) having a fixed amplification factor of 40 dB are used. Each signal amplified by connecting each output terminal of the differential sensor to a preamplifier is measured with an oscilloscope, and a differential signal is generated by using an arithmetic function of the oscilloscope. Therefore, the three waveforms of the waveform from both output terminals and the differential signal can be observed simultaneously.

(2) Method 2
In order to obtain a differential signal from one output terminal, a differential amplifier (NF circuit design block 5307) was used. When this is used, signals from the two output terminals of the differential sensor can be amplified to an appropriate level and obtained as a differential signal at one output terminal. If the signal from the differential amplifier is input to the oscilloscope, the differential signal waveform can be observed. Since the gain of this differential amplifier is 1000 times at the maximum, a discriminator (NF circuit design block AE9922) is sandwiched between the amplifier and the oscilloscope when further amplifying (FIG. 8 (b) ).

  Method 1 was used when it was desired to check individual signals simultaneously (as shown in FIGS. 9, 10, and 11), and method 2 was used otherwise.

  First, a piezoelectric sensor similar to the piezoelectric sensor 50 of FIG. 1 was produced. As each member, an IN600 substrate (Niraco Co., Ltd.) having a thickness of 100 μm, a width of 17 mm, and a length of 30 mm was used as the substrates 1 and 7.

  The piezoelectric films 2 and 6 are aluminum nitride thin film piezoelectric bodies having a thickness of 3 μm, a width of 7 mm, and a length of 7 mm, and were formed at predetermined positions on the substrates 1 and 7 using a high-frequency magnetron sputtering apparatus. Further, it was confirmed that the piezoelectric films 2 and 6 had the same film thickness and relative dielectric constant, and the surfaces after the film formation had the same polarity surface. Since the film formation areas of these piezoelectric bodies are the same, the capacitance is naturally the same.

  Polyimide double-sided tapes 2a and 6a were prepared by using a double-sided Kapton tape (P-223, manufactured by Permacel) with a thickness of 114 μm, a width of 13 mm, and a length of 13 mm, and a central area of 7 mm in width and 7 mm in length. The position was adjusted so that the center of the hollowed portion and the center of the piezoelectric film were aligned, and the film was pasted on the substrates 1 and 7.

  The double-sided polyimide tapes 3a and 5a have a thickness of 114 μm, a width of 11 mm, and a length of 11 mm. A double-sided Kapton tape (Permacel P-223), and the polyimide films 3b and 5b have a thickness of 12.5 μm and a width. Kapton 50H (Toray DuPont Co., Ltd.) prepared to 9 mm and 9 mm in length was used.

  Each of the lower electrode 3 and the upper electrode 5 uses a copper foil (Niraco Co., Ltd.) having a thickness of 40 μm, a width of 5 mm, and a length of 5 mm, respectively, a polyimide double-sided tape 3a and a polyimide film 3b, and a polyimide double-sided tape 5a and a polyimide film, respectively. 5b is sandwiched between the conductive members other than the inner conductors 3c and 5c which are in contact with each other so as not to be short-circuited.

  Further, the polyimide layer 4 has five different thicknesses, and four types of piezoelectric sensors 50 were produced. That is, (a) 50 μm (one 50 μm thick polyimide film), (b) 125 μm (one 125 μm thick polyimide film), (c) 250 μm (two 125 μm thick polyimide film), (d) 667 μm (125 μm) (Thick polyimide film 3 sheets + 146 μm thick polyimide double-sided tape 2 sheets), (e) 938 μm (125 μm thick polyimide film 4 sheets + 146 μm thick polyimide double-faced tape 3 sheets). The polyimide film used was Toray DuPont 200H or 500H, and the polyimide double-sided tape used was 760H manufactured by Teraoka Seisakusho.

  Each member such as the substrate, the piezoelectric film, and the polyimide film was sandwiched at an installation pressure of 750 N using a pair of jigs having parallel surfaces without giving a curvature. Moreover, the total thickness from the board | substrate 1 of the five types of obtained samples 1 to the board | substrate 7 was about 0.7 mm-1.7 mm.

  Further, as the cable 60 including the inner conductors 3c, 5c, etc., a cable with a shield coating is manufactured and used based on a Teflon (registered trademark) coaxial cable (RG196A / U). As the connection terminal 62, Sakaguchi Electric Heat Co., Ltd. RG6910 and HUBER + SUHNER SMA-50-1-4 / 111 were used, respectively.

  Five types of piezoelectric sensors manufactured from the above members are installed in the uniaxial compression tester 70 shown in FIG. 4, and the excitation sensor 73 is supplied to the excitation sensor 73 for one wavelength of the amplitude of the sine wave signal by the method described in the embodiment. And a simple evaluation test was performed on each sample. The test results obtained are shown in Table 1.

  In Table 1, the uppermost row shows the thickness of the polyimide film layer. The second row shows a signal source. In the piezoelectric sensor, the piezoelectric film 2 corresponds to the receiving surface, and the piezoelectric film 6 corresponds to the upper surface. Here, the difference {(1)-(2)} between the voltage on the receiving surface and the voltage on the upper surface is defined as differential. In the leftmost column, the applied frequency (kHz) is described, and the maximum amplitude (V) corresponding to the applied frequency is shown for each reception surface, top surface, and differential.

  The maximum amplitude of the detection signal on the upper surface of each sample is reduced to about one tenth of the maximum amplitude of the detection signal on the reception surface, and it can be seen that the influence of the piezoelectric body on the upper surface can be ignored. It was also confirmed that the detection waveform on the receiving surface and the waveform by the differential signal corresponded well. The graph based on the data of each sample based on Table 1 is shown to Fig.9 (a)-(c) and Fig.10 (a), (b).

  Table 2 shows particularly 200 kHz to 400 kHz data having a particularly high output value in Table 1. When the data of 200 kHz to 400 kHz were examined, the following tendency was found. This will be specifically described below with reference to FIG. FIG. 11A is a graph showing the relationship between the polyimide thickness and the maximum amplitude value when the applied voltage frequency is 200 kHz. Similarly, FIG. 11B is a graph showing the relationship between the polyimide thickness and the maximum amplitude when the applied voltage frequency is 300 kHz, and FIG. 11C is the applied voltage frequency of 400 kHz.

  As shown in FIGS. 11A to 11C, when the thickness of the polyimide layer 4 is 200 μm or more, the maximum amplitude of the detection signal increases. On the other hand, the maximum amplitude hardly increases even when the polyimide layer 4 exceeds 600 μm. For this reason, it is understood that the thickness of the polyimide layer 4 is preferably 200 μm or more and 600 μm or less from the viewpoint of keeping the piezoelectric sensor thin while obtaining a large maximum amplitude.

[Comparative Example 1]
A comparative piezoelectric sensor was produced in the same manner as in Example 1 except that the polyimide layer 4 was not inserted. Using the produced comparative piezoelectric sensor, a piezoelectric sensor was installed in the uniaxial compression tester 70 in the same manner as in Example 1, and a simple evaluation test was performed under the same conditions as in Example 1. A graph based on the obtained test results is shown in FIG.

  As shown in FIG. 12, since the polyimide layer 4 is not laminated in the comparative piezoelectric sensor, the maximum amplitude of the detection signal on the upper surface of the piezoelectric body is one half or more of the maximum amplitude of the detection signal on the reception surface.

  For this reason, compared with the piezoelectric sensor of Example 1, it was not possible to confirm a good correspondence between the waveform of the differential signal and the detected waveform on the receiving surface. From this comparison result, it is clear that the piezoelectric sensor of Example 1 has an advantage.

  The present invention is not limited to the above-described embodiments, and various modifications can be made within the scope of the claims, and the technical means disclosed in different embodiments can be appropriately combined. Such embodiments are also included in the technical scope of the present invention.

  The piezoelectric sensor according to the present invention can be used as an acceleration sensor, a pressure sensor, and an acoustic emission sensor even in an environment where the installation location is particularly narrow and an electromagnetic noise is often generated such that a high frequency power supply is used.

1 substrate (first substrate)
2 Piezoelectric film (first piezoelectric film)
3 Lower electrode (first electrode)
3c Inner conductor (conductor)
4 Polyimide layer (insulator layer)
5 Upper electrode (second electrode)
5c Inner conductor (conductor)
6 Piezoelectric film (second piezoelectric film)
7 Substrate (second substrate)
8a Secondary outer conductor 8b Secondary insulation coating layer 50 Piezoelectric sensor 60 Cable 61 Insulator 62 Connection terminal 70 Uniaxial compression tester 71 / 71a Push rod 72 Jig 73 Excitation sensor 74 Function generator 75 Oscilloscope 76 Preamplifier

Claims (5)

A piezoelectric sensor in which a first piezoelectric film, a first polyimide film, a first electrode, an insulator layer, a second electrode, a second polyimide film, and a second piezoelectric film or a dielectric film are laminated in this order. ,
The second piezoelectric film or dielectric film has the same relative dielectric constant and capacitance as the first piezoelectric film,
The first piezoelectric film and the second piezoelectric film are disposed with the same polar face facing each other,
The first piezoelectric film is disposed on the detection surface side of the piezoelectric sensor,
The area of the layer direction of the insulating layer is smaller than the area of the layer direction of the first piezoelectric film, and, rather smaller than the area of the layer direction of the second piezoelectric film or a dielectric film,
The insulator layer is flexible;
The insulator layer is a polyimide layer formed to increase a maximum amplitude value of a detection signal detected by the first electrode;
A thickness of the polyimide layer is 200 μm or more and 600 μm or less . The first piezoelectric film and the second piezoelectric film or dielectric film are respectively formed on a first substrate and a second substrate having conductivity,
2. The piezoelectric sensor according to claim 1 , wherein the outer edge portion of the first substrate and the outer edge portion of the second substrate are joined so as not to impair the conductivity. Both the first substrate and the second substrate can bend,
3. The piezoelectric sensor according to claim 2 , wherein the first piezoelectric film and the second piezoelectric film or the dielectric film have a thickness of 10 μm or less. A conducting wire connected to each of the first electrode and the second electrode;
A connection terminal for transmitting a voltage signal obtained from the first electrode and the second electrode through the conducting wire, and
The piezoelectric sensor according to claim 2 or 3 , wherein at least a part of the cable including the conducting wire is covered with an insulator. The area in the layer direction of the insulator layer is 80% or less with respect to the area in the layer direction of the first piezoelectric film, and the area of the area in the layer direction of the second piezoelectric film or dielectric film respect, the piezoelectric sensor according to any one of claims 1 to 4, characterized in that 80% or less.

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