JP2001165785A - Pressure sensor and its manufacturing method - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a mechanical-electrical conversion element for maximizing the features of split electrode structure. SOLUTION: In the pressure sensor and its manufacturing method, a first electrode (a), a piezoelectric body (c), and a second electrode (b) are laminated on a vibration plate that is made of insulation ceramic substrate, the first electrode (a) and the second electrode (b) are provided with each split pattern, and a piezoelectric body capacitor that is made of the first electrode (a), the piezoelectric body (c), and the second electrode (b) is connected in series according to the number of divisions of the electrode.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

[0001] 1. Field of the Invention [0002] The present invention relates to a pressure sensor which is a transducer for converting mechanical energy into electrical energy.

[0002]

2. Description of the Related Art An element for converting mechanical energy into an electric signal such as a pressure sensor, that is, a pressure sensor, has been commercialized for a long time as an application of piezoelectric ceramics. In recent years, elements using the piezoelectric effect of piezoelectric ceramics, that is, devices using electromechanical conversion, have been demanded for miniaturization and high sensitivity, as well as for receiving ultrasonic waves in water and as heart sound sensors in the field of medical equipment. There are also applications such as vibration detection elements of fluid flow meters that use fluid vibration, and their specifications range from very low frequency microphones to ultrasonic waves.
There is a need for high sensitivity in a wide frequency range. The principle of operation of these elements using piezoelectric ceramics is to apply stress to the diaphragm bonded to the piezoelectric element and output electric charges by the piezoelectric effect.However, the magnitude of the stress applied to the diaphragm given by classical mechanics The height is proportional to the cube of the diaphragm diameter and inversely proportional to the diaphragm thickness to the fourth power. Therefore,
In response to the demand for higher sensitivity, it is conceivable to increase the diameter of the element, but this is contrary to miniaturization, so other contrivances are required.

Here, the piezoelectric effect will be briefly described. Piezoelectric ceramics that have undergone polarization processing generate charges proportional to the stress. A piezoelectric constant is defined as an index indicating this efficiency, and its dimension is C (coulomb) / N
(Newton). Therefore, a material exhibiting a high piezoelectric constant may be developed as a high electromechanical transducer. However, when an output is obtained as an electric signal, a value obtained by dividing the amount of electric charge by the capacitance becomes an output voltage, and therefore, it is necessary to develop a material having a high piezoelectric constant and a low relative dielectric constant. However, a material having a high piezoelectric constant has a high relative permittivity, so that it is difficult to achieve both. Further, a material having a high piezoelectric constant also has a high temperature dependence of characteristics, and it is extremely difficult to exhibit characteristics with little fluctuation at −20 to 80 ° C. in a normal environment. It is also preferable to devise not only the characteristics of the material itself but also its structure. There is a so-called piezoelectric composite in which a void is intentionally added to piezoelectric ceramics. The concept of the piezoelectric composite was proposed in 1980, and is classified into nine types according to the matrix form. As an initial prototype, the 3-3 composite was manufactured by a manufacturing method using the structure of a natural plant, and the dielectric constant was successfully reduced without lowering the piezoelectric constant.
However, piezoelectric composites are not strongly employed in the present invention because of their poor reliability in terms of mechanical strength. As another method, by adopting a configuration in which individual piezoelectric capacitance elements divided according to the number of divisions are connected in series by employing divided electrodes, the apparent capacitance has been successfully reduced.

[0004] In connection with the present invention, for example, there is the following prior art. JP-A-7-225168, Japanese Patent Application No. 4-160
204, Japanese Patent Application No. 5-87997, Journal of the Acoustical Society of Japan 49, 9 (1993) 629, JJAP, vol. 24 (1
985) Supplement 24-2, p. 416

In Japanese Patent Application Laid-Open No. Hei 7-225168, an element structure is devised so as to use a divided electrode pressure sensor and reduce variations in individual sensor elements which are problematic when operation is amplified. . The aforementioned Japanese Patent Application No. 4-16
0204 and Japanese Patent Application No. 5-87997 propose an electro-mechanical conversion element in which an insulating ceramic is provided with a piezoelectric film on a substrate and an inverse piezoelectric effect of a piezoelectric body is used. . The development like the present invention is not listed. Also, JJAP, vo1.24 (1985) Su
pplement 24-2, p. No. 416 reports that a piezoelectric buzzer was formed on an insulating ceramic substrate.

[0006] The problems of the prior art will be described below. JJ
AP, vo 1.24 (1985) Supplement
24-2, p. In the method shown by reference numeral 416, a high-performance device cannot be realized unless the invention is devised. According to the split electrode method described in the above-mentioned Journal of the Acoustical Society of Japan and Japanese Patent Application Laid-Open No. 7-225168, a first electrode and a second electrode are arranged on both sides of a piezoelectric ceramic whose thickness is controlled, and then the diaphragm is used as a diaphragm. It is bonded to a glass filler reinforced epoxy plate via an adhesive. The characteristic required for the diaphragm is to use a material with high mechanical strength in order to efficiently generate stress applied to the piezoelectric body derived from bending deformation due to stress, and generally use 42 alloy of brass or Ni-based. Used. In order to fulfill the function of the split electrode, the diaphragm needs to have insulating properties. Therefore, brass and 42 alloy are not suitable, and a glass filler reinforced epoxy plate is used. However, such organic materials have insufficient strength, and cannot efficiently transmit stress to the piezoelectric body. Further, the diaphragm and the piezoelectric ceramics are joined via an acrylic anaerobic adhesive, but this is not preferable for the same reason. Specifically, due to variations in the thickness of the adhesive layer,
In addition to the sensor sensitivity fluctuating, the adhesive layer having high elastic compliance behaves as a stress absorber, causing deterioration of frequency characteristics. Furthermore, the accuracy of the superposition of the electrodes on both surfaces is lower than in the case where the electrodes are formed on the same plane, and it is difficult to arrange finely divided electrodes.

That is, the disadvantages of the conventional split electrode type pressure sensor are that an insulating substrate having insufficient mechanical strength is used, and the piezoelectric body and the insulating substrate are joined via an adhesive. In addition, a split electrode has to be formed on both sides of the piezoelectric layer, and as a result, it has not led to a sufficient performance enhancement. An object of the present invention is to solve the above-mentioned problems of the split electrode, and to provide a mechanical-electrical conversion element that maximizes the features of the split electrode structure, and to provide the conversion element.

[0008]

A feature of the present invention is that a first electrode, a piezoelectric body and a second electrode are laminated on a diaphragm made of an insulating ceramic substrate,
Have a divided pattern, and are arranged so as to take a configuration in which a piezoelectric capacitor composed of a first electrode, a piezoelectric body, and a second electrode is connected in series according to the number of divisions of the electrode. And a pressure sensor. In the split electrode type pressure sensor of the present invention, since the piezoelectric layer is directly formed on the ceramics diaphragm, the sensor sensitivity and the frequency band can be improved.

The split electrode type pressure sensor according to the present invention comprises an insulating ceramic as a diaphragm, a first electrode on the diaphragm,
It is preferable to use a so-called build-up method (FIG. 3) in which a piezoelectric body and a second electrode are stacked and formed. Since the split electrode type pressure sensor is manufactured by a build-up method, it is possible to effectively reduce the size of the element and increase the number of split electrodes. Since the first electrode and the piezoelectric body are formed on the diaphragm by screen printing and baking, the diaphragm is also limited to a substrate having high heat resistance. Specifically, the screen printing of the electrode layer and the piezoelectric layer and the subsequent sintering step are performed at a maximum of 1300 ° C., and therefore must have heat resistance higher than that.

Since the diaphragm has excellent resistance to the process of forming a piezoelectric film, it is possible to provide a diaphragm having high stability and reproducibility of piezoelectric characteristics and high reliability as a sensor element. Insulating ceramic substrates are preferred. As the insulating ceramic material, alumina, partially stabilized zirconia, and the like are preferable. In particular, silicon is included in conventional ceramic materials as SiO ₂ , but this silicon is not very preferable in forming lead-based piezoelectric ceramics. For example, SiO ₂ —Al
_{For 2} O ₃ ceramics, it must be at least 99% alumina. As the zirconia stabilizer, those partially stabilized with ceria, yttria, and magnesia are preferable, and in particular, yttria partially stabilized (4 mo).
1%) is preferred. These ceramic raw material powders are formed by a well-known sheet forming technique, for example, a doctor blade method.
After firing, it may be adjusted and manufactured to have a thickness of 10 to 100 μm. The green sheet produced by such a method may be cast on a disk at the stage of the sheet. Generally, these ceramic green sheets are 150
It is fired at a temperature of 0 ° C. or more.

Using such a substrate, a first electrode layer is formed by screen printing and firing. If the electrode material is formed by screen printing using a # 320 stainless steel mesh, a film having a film thickness of 5 μm and a resolution of 80 μm lines and spaces can be obtained, so that the size of the entire device (diaphragm diameter) is as small as 5 mm. That is, even if the image is divided into 12, there is no problem in terms of resolution. A commercially available platinum paste or a silver-palladium alloy (20/80) is used. The firing temperature at this time is 13 mm even for the platinum material having the highest temperature.
Fired at 00 ° C., it goes without saying that the ceramic diaphragm is within the allowable range of heat resistance.

Next, printing and firing of the piezoelectric material are performed. The overlay accuracy in printing is approximately ± 50 μm, and from this point, the build-up method is also excellent in forming a fine pattern.
Extensive equipment is required to achieve an accuracy of 50 μm or less). The diameter of the piezoelectric pattern is made smaller than the outer diameter of the first electrode pattern for the polarization process described later. As the piezoelectric material, lead zirconate titanate (PZT) ceramics has been used for a long time. In consideration of good aging characteristics after the polarization treatment, here, PZT (52/48) obtained by adding 0.3 mol% of Nb ₂ O ₅ and 0.1 mol% of manganese is used. Also, a previously reported ceramic composition may be used. The paste formulation for printing is kneaded with a general organic vehicle. The firing of these piezoelectric ceramics is approximately 125
It is carried out at 0 ° C. or lower.

Next, an electrode for polarization processing is once formed. This electrode is disposed only for the polarization treatment, and is thereafter removed, and finally the second divided electrode is disposed. It is desirable that the electrodes subjected to the polarization treatment can be easily removed in a later step, and are manufactured by a simple film forming method. Generally, it is sufficient to deposit about 50 nm by gold sputtering. After the polarization treatment, it can be removed only by simple polishing (lap finishing). In the polarization process, the entire first electrode pattern is applied to the common one electric field side, and the electrode on the opposing surface is applied as an opposing electrode by a DC electric field at an electric field intensity of 30 kV / cm at a temperature of 125 ° C. for 20 minutes. By this process, the piezoelectric ceramic elements of each section are polarized in the same direction. The electrode arranged for the polarization treatment is removed, and the second electrode is arranged at a low temperature at which depolarization of the piezoelectric ceramic does not occur. The Curie point of this PZT composition is 340 ° C.
Preferably, the electrode film is formed at a temperature of 00 ° C. or lower. For example, a layer made of only the organic vehicle may be formed by screen printing, and then formed by metal mask evaporation.

FIG. 1 shows a configuration in which piezoelectric capacitance elements are connected in series when the number of divided sections of an electrode is n. Further, the reduction of the capacitance of the element by the division method is expressed by the following equation (1).
Indicated by

[0015]

(Equation 1)

In the above formula, C _{1 to} C _n are the respective capacitances of the divided sections ₁ to _n in the overall view of the divided electrode system shown in FIG. 2, C ₀ is the capacitance before division, and n is the capacitance of the electrode. Indicates the number of sections respectively. Assuming that the number of electrode sections is n, n sections are connected in series, so that the combined capacitance is (1 / n ² ) C ₀ . However, since the generated charge of each section is 1 / n, the composite output voltage is the number of divisions n
Becomes a value directly proportional to. If n = 10 is adopted, the output voltage can be ten times as large.

[0017]

Embodiments of the present invention will be described below with reference to the drawings.

Example 1 FIGS. 2 (a) to 2 (d) show electrode patterns of the divided electrode system manufactured. Diameter of diaphragm ceramic is 10m
m, thickness 30 μm, outer diameter of split electrode 8 mm, thickness 4
μm, the diameter of the piezoelectric ceramic layer was 7 mm, the thickness was 30 μm, and the outer diameter of the second divided electrode was 6.2 mm. This distribution is an optimized distribution in which the stress distribution is not canceled when the diaphragm (d) is fixed at the periphery. Further, assuming miniaturization, a diaphragm (d) having an outer diameter of 5 mm was manufactured according to the same distribution rule. As the number of divisions increases, the overall capacitance decreases. However, when considering the actual electrode lead, the area given to the electrode lead becomes 1 mm ² in the division number 12 when the maximum diameter is φ5 mm, and the further division is performed. Indicates that there are restrictions on the mounting aspect. In the above-mentioned element having a diameter of 10 mm, 2S manufactured by SANYO
The sensor sensitivity was measured using K596E (FET).
The sample without a split electrode has a voltage of 220 mV / mmH ₂ O, whereas the sample with 12 splits has a voltage of 2.6 V / mmH ₂ O.
Reached. This is because the area of the insulating portion between the divided electrodes was not taken into consideration, and when this area was converted, the sensitivity was increased by a factor of 12 which is almost equal to the number of divisions.

A split electrode type pressure sensor using a conventional glass filler reinforced epoxy diaphragm and this ceramic diaphragm sensor were compared. The conventional one is a pressure sensor manufactured by Primo Corporation. Fit to commercially available shape (φ1
0 mm), and were devised so that there was no difference in resonance characteristics due to the shape. This is because the resonance frequency depends on the diaphragm diameter. In addition, since the piezoelectric materials used are different, the evaluation of the output (V / mmH ₂ O) with respect to the unit pressure is not appropriate. Here, an AC pressure fluctuation (sinusoidal shape) was given, and the frequency characteristics were evaluated. 0.5Hz to 600Hz for glass filler reinforced epoxy diaphragm
The output was constant up to and had a resonance point at about 1.5 kHz. On the other hand, the sensor of the present invention has a frequency of 0.5 Hz to 1.2 kHz.
The output signal strength is constant up to 1.5 kHz
Has a resonance point, leading to a significant improvement in bandwidth. That is, a split electrode type pressure sensor was obtained using an insulating ceramic substrate. The sensor sensitivity matched the theoretical calculation, and a sensitivity 12 times higher than that of a normal sensor not using a split electrode was obtained. In addition, since it does not require bonding with a glass filler reinforced epoxy diaphragm or adhesive, the frequency band has been improved.

Example 2 A partially stabilized zirconia ceramics calcined powder manufactured by Hayashi Chemical Co., Ltd. is used to form a green sheet having a thickness of 30 μm after firing by a doctor blade method. The formulation was such that a solvent of ethyl cellosolve and toluene was added to a polyvinyl butyral resin, and dibutyl phthalate was used as a plasticizer.
Electrode 1 (a) uses platinum paste made by Tanaka Kikinzoku Co., Ltd.
Formed on a screen printing plate of # 325 stainless mesh, the minimum pattern obtained at this time was 50 μm.
After the squeegee, drying and firing at 1300 ° C. for 2 hours were performed. Next, the composition of the PZT piezoelectric ceramic is as described above.
To this calcined powder, ethyl cellulose as a binder, α-terpineol, a cellosolve-based solvent, and a surfactant are added, and the mixture is kneaded with a three-roll mill to produce a desired ceramic paste.

Screen printing is performed using stainless mesh (#
200) to obtain a thick-film ceramic sintered body by pattern transfer, drying, degreasing, and firing. At this time, at a high temperature during firing, the lead component in the ceramics is volatilized. Therefore, the firing is performed in a lead atmosphere using a crucible having good airtightness of magnesia or a setter. Firing conditions are 120
Performed at about 0 ° C. for 2 hours. 100 μmL of patterned piezoelectric film formed by optimizing printing conditions
Finished to & S dimensions. An electrode for polarization processing is formed to a thickness of 50 nm by gold sputtering, and the electric field strength is 25 kV /
cm, a processing temperature of 125 ° C., and a processing time of 20 minutes, the gold electrode was removed by polishing. Finally, an electrode 2 (b) patterned by a metal mask deposition was formed, and gold having good solder joint of the electrode lead was used. In this way, a pressure sensor having a split electrode system could be formed on the insulating ceramic substrate (d) without using an adhesive layer.
Although it depends on the required sensitivity, the minimum sensor element size by this method can be made up to φ3 mm. That is, in the pressure sensor of the present invention which is manufactured by the build-up method, the formation accuracy is dramatically improved, and the size of the element is reduced. As a result, external dimensions φ3
mm 12-electrode pressure sensor was developed.

Example 3 The flexural deformation of a peripherally fixed disk is proportional to the cube of the diameter,
Is inversely proportional to the fourth power of. Element size was φ5mm
At this time, the thickness of the diaphragm (d) was based on a displacement of 30 μm.
Case (the amount of deformation is proportional to the generated stress),
The displacement amount decreases to 1/120. About this
The diaphragm (d) has a diameter of 20 mm, and
Even if the thickness is m, the reduction amount is reduced to half, and
Depends on the size of the blade, but generally stable with a doctor blade
The thickness of 20 μm to 100 μm that can be built
It can be said that it is preferable as the thickness of the moving plate (d). Diaphragm
Prepare various ceramics as Lamix material,
Printed PZT was made. A particular problem is that the electrode 1
Because (a) is divided, diaphragm ceramic
There is a part where PZT comes into direct contact with
During formation, the reaction between the two proceeds, and it does not show sufficient piezoelectricity.
It is thought that it is not. After testing various ceramics
In particular, alumina and yttria partially stabilized
It was Luconia ceramics. Showa Denko Aluminum
In the silica composite material, the silica content is 10%, 1
%, 0.4%, 0%
As a result, when 1% or more of silica is contained, the piezoelectricity deteriorates.
Was observed. From this result, the silica content was 0.4%
The following alumina ceramics are preferred. Also partially stable
Zirconia is ZrO which constitutes PZT ceramics₂
After firing, PZT / ZrO ₂Strong at the interface
A good bond was obtained. Magnesia ceramics from ceramics knowledge
Has low reactivity with lead oxide and the PZT characteristics after firing
Although excellent, the adhesion strength at the interface is insufficient,
Not suitable for use. That is, the ceramic of the diaphragm (d)
Extraction of suitable materials for the housing material
Highly qualitative, reproducible and highly reliable sensor elements
Came to offer.

[0023]

According to the present invention, since a piezoelectric body is directly formed on a ceramic diaphragm, a pressure sensor capable of improving sensor sensitivity and a frequency band is provided. Claim 2 The insulating ceramic substrate is used for a piezoelectric film forming process.
Because of its excellent resistance, a pressure sensor having high stability and reproducibility of piezoelectric characteristics and high reliability as a sensor element is provided. [Claim 3] Since a build-up method is used as a manufacturing method of the split electrode type pressure sensor, the element can be downsized and the split electrodes can be made high.

[Brief description of the drawings]

FIG. 1 is a diagram illustrating a configuration in which piezoelectric capacitance elements are connected in series when the number of divided sections of an electrode is n.

FIG. 2 is a view showing each divided piezoelectric gas capacitive element when the number of divided sections of an electrode is four. (A) is a diagram showing the number of each section of the electrode. (B) is a diagram showing the number of each divided section of the electrode. (C) A diagram showing the number of each section of the electrode. (D) is a diagram showing the number of each divided section of the electrode.

FIG. 3 is a schematic sectional view of a pressure sensor.

FIG. 4 is a diagram showing a first electrode pattern, a second electrode pattern, and the number of divisions.

[Explanation of symbols]

 a electrode 1 b electrode 2 c piezoelectric body d diaphragm

Claims (3)

[Claims] 1. A first electrode, a piezoelectric body and a second electrode are laminated on a vibration plate made of an insulating ceramic substrate, and the first and second electrodes each have a divided pattern. A pressure sensor, wherein a piezoelectric capacitor composed of a first electrode, a piezoelectric body, and a second electrode is arranged in series according to the number of divisions of the electrode. 2. The split electrode type pressure sensor according to claim 1, wherein a main composition of the insulating ceramic substrate is made of alumina or zirconia. 3. The method according to claim 1, wherein the first electrode and the piezoelectric body are formed on the vibrating substrate by screen printing and baking, and thereafter, the second electrode is formed by an arbitrary method. Or a method for manufacturing a pressure sensor.