JP4228827B2 - Piezoelectric ultrasonic sensor and method for adjusting resonance frequency thereof - Google Patents

Description

  The present invention relates to a piezoelectric ultrasonic sensor that detects a distance to a measurement object using a piezoelectric element.

  Conventionally, a piezoelectric ultrasonic sensor that receives reflected ultrasonic waves from a measurement object is known as a sensor that measures the position and shape of the measurement object. This sensor detects the distance to the measurement object by the time from the emission of the ultrasonic wave to the reception of the reflected wave by the ultrasonic wave receiving element using the piezoelectric element. Since ultrasonic waves have a slower propagation speed than light, distance measurement is easy. Further, as described in Patent Document 1, for example, ultrasonic receiving elements using piezoelectric elements are arranged in an array, and a delay process is performed on the output of each ultrasonic receiving element to electrically By performing scanning, it is possible to detect not only the distance to the measurement object but also the shape thereof.

In these conventional piezoelectric ultrasonic sensors, for example, a concave portion is formed on a silicon substrate, a diaphragm is provided, and a piezoelectric material thin film and electrodes formed on both surfaces of the piezoelectric material thin film are formed on one side of the diaphragm. An ultrasonic receiving element is provided. When the reflected wave from the measurement object is received, the diaphragm vibrates, and the piezoelectric material generates a voltage according to the vibration. Therefore, by measuring the voltage between the electrodes, it can be seen that the reflected wave from the measurement object has been received.

Conventional piezoelectric ultrasonic sensors exhibit various forms of vibration depending on the frequency of sound waves propagating in the medium. For example, the resonance frequency fr of the ultrasonic receiving device of a single layer die A Fulham is given by Equation 1.

As can be seen from Equation 1, the resonance frequency fr of the ultrasonic receiving element is affected by variations in the shape of the diaphragm and variations in the thickness of the thin film of the piezoelectric material. In particular, in a piezoelectric ultrasonic sensor in which a plurality of ultrasonic receiving elements are arranged in an array, it is very difficult to manufacture each ultrasonic receiving element so that the resonance frequency is the same. There is a variation in the resonance frequency of the ultrasonic receiving element. In addition, even in an ultrasonic sensor having a single ultrasonic receiving element, it is difficult to align the resonance frequency characteristics of each mass produced ultrasonic sensor. The variation in the resonance frequency appears as a difference in electrical conversion efficiency (sensitivity) with respect to the frequency when a sound wave having a specific frequency is input. For this reason, there has been a problem that the accuracy of checking the position of the object is lower than when it is assumed that the resonance frequencies of all the ultrasonic receiving elements are uniform. The same applies to the ultrasonic sensor disclosed in Patent Document 1.
JP 2002-156451 A

  The present invention has been made to solve the above-described problem. A plurality of ultrasonic receiving elements are arranged in an array, and each ultrasonic receiving element has substantially the same resonance frequency, and its An object of the present invention is to provide a resonance frequency adjusting method.

In order to achieve the above object, the invention of claim 1 is a piezoelectric ultrasonic sensor, wherein a substrate on which a plurality of diaphragms are formed, a thin film of piezoelectric material formed on one surface of the substrate, and the piezoelectric material Of the thin film of the metal thin film electrode formed on both sides of the portion corresponding to one diaphragm,
Corresponding to one diaphragm, ultrasonic receiving elements composed of a thin film of the piezoelectric material and two electrode regions facing each other are arranged in an array, and a DC voltage is applied to one ultrasonic receiving element. as or border the diaphragm, a difference in air pressure of a portion of both sides is provided, or by combining these, and wherein the changing the shape or the internal stress of the diaphragm at the ultrasonic wave receiving element.

According to a second aspect of the present invention, in the piezoelectric ultrasonic sensor according to the first aspect, the DC voltage is once increased to a voltage higher than a predetermined voltage and then applied so as to gradually decrease to a predetermined voltage. Features .

According to a third aspect of the present invention, in the piezoelectric ultrasonic sensor according to the first or second aspect, an ultrasonic receiving element having a resonance frequency higher than a desired resonance frequency is formed, and compressive stress is applied to the ultrasonic receiving element. The desired resonance frequency is set .

A fourth aspect of the present invention, the piezoelectric ultrasonic sensor according to claim 1, the recess for forming the diaphragm of the substrate covered with a glass plate, be different values of external pressure the pressure in said recess It is characterized by .

The invention of claim 5 is formed on both surfaces of a substrate on which a plurality of diaphragms are formed, a thin film of piezoelectric material formed on one surface of the substrate, and a portion corresponding to one diaphragm of the thin film of piezoelectric material. A piezoelectric ultrasonic sensor comprising an electrode of a metal thin film and an ultrasonic receiving element corresponding to one diaphragm and arranged in the form of an array of the piezoelectric material thin film and two electrode regions facing each other A resonance frequency adjusting method according to claim 1, wherein a direct-current voltage is applied to an ultrasonic receiving element having a resonance frequency different from the reference resonance frequency based on a resonance frequency of any of the ultrasonic receiving elements, or the diaphragm By setting a difference in the air pressure on both sides of the boundary, or combining these, the resonance frequency of each ultrasonic wave receiving element is made substantially the same value It is characterized by that .

According to a sixth aspect of the present invention, in the method for adjusting the resonance frequency of the piezoelectric ultrasonic sensor according to the fifth aspect, the smallest value among the resonance frequencies of the respective ultrasonic receiving elements is used as a reference value, and a resonance frequency larger than the reference value is set. A compressive stress is applied to the ultrasonic receiving element .

The invention of claim 7 is a method for adjusting the resonance frequency of a piezoelectric ultrasonic sensor, a substrate on which a plurality of diaphragms are formed, a piezoelectric material thin film formed on one surface of the substrate, and the piezoelectric material thin film. Of the thin film formed on both surfaces of the portion corresponding to one diaphragm, and composed of the thin film of piezoelectric material and the two electrode regions facing each other. The receiving elements are arranged in an array, and a direct current voltage is applied to one ultrasonic receiving element, or the pressure on both sides of the diaphragm is set as a boundary, or by combining these, The resonance frequency of the ultrasonic receiving element is changed .

According to the first aspect of the present invention, an external force is applied when the resonance frequency of the ultrasonic receiving element is different from a desired value due to variations in the shape of the diaphragm or variations in the thickness of the thin film of the piezoelectric material. Since the apparent longitudinal elastic modulus is changed by changing the shape or internal stress of the diaphragm in the ultrasonic receiving element, the resonance frequency of the ultrasonic receiving element can be adjusted to a substantially desired value. As a result, in the piezoelectric ultrasonic sensor in which the ultrasonic receiving elements are arranged in an array, the resonance frequency of each ultrasonic receiving element can be made substantially the same value, and the accuracy of checking the position of the object can be improved. Can do. In addition, when a direct-current voltage is applied between electrodes facing each other constituting the ultrasonic receiving element to give a potential difference, the piezoelectric material causes distortion in the thin film of the piezoelectric material. This strain acts as an internal stress in the diaphragm in which four sides are constrained. Depending on the polarity of the DC voltage applied between the electrodes, it is possible to set the compression stress or the tensile stress. By appropriately controlling the value and polarity of the DC voltage to be applied, the resonance frequency of the ultrasonic receiving element can be adjusted to a desired value. Alternatively, when a pressure difference is provided on both sides of the diaphragm as a boundary, a load is applied to the entire diaphragm to change the shape or internal stress of the diaphragm in the ultrasonic receiving element. Therefore, even with this configuration, the apparent longitudinal elastic modulus of the diaphragm can be changed, and the resonance frequency of the ultrasonic receiving element can be adjusted to a substantially desired value.

According to the invention of claim 2 , with respect to the hysteresis characteristic of the piezoelectric effect, the voltage is made higher than the predetermined voltage once to reach the saturation region, and then the voltage is gradually lowered to the predetermined voltage. The same portion of the curve can be used to control the strain of the thin film of piezoelectric material and the relationship between the voltage value and the internal stress can be uniquely determined. As a result, it is possible to accurately adjust the resonance frequency of the ultrasonic receiving element to a desired value.

According to the invention of claim 3 , in general, the internal stress is applied in the direction in which the compressive stress is applied to the property of the piezoelectric material which is weak against the tensile stress and easily damaged, so that the thin film of the piezoelectric material is prevented from being damaged. Thus, while improving the reliability of the ultrasonic sensor, the resonance frequency can be adjusted to a desired value at the same time.

According to the invention of claim 4 , for example, the concave portion for forming the diaphragm of the substrate is covered with a glass plate at a pressure different from the atmospheric pressure, and then the diaphragm is easily used under atmospheric pressure. As a boundary, a difference can be provided in the air pressure on both sides.

According to the fifth aspect of the present invention, in the piezoelectric ultrasonic sensor in which the ultrasonic receiving elements are arranged in an array, the resonance frequency different from the reference resonance frequency with reference to the resonance frequency of any of the ultrasonic receiving elements. By applying an external force to the ultrasonic receiving element having the above, the resonance frequency of each ultrasonic receiving element can be made substantially the same value, and the accuracy of confirming the position of the object by the ultrasonic sensor can be improved.

According to the invention of claim 6 , since the smallest value among the resonance frequencies of each ultrasonic receiving element is used as a reference value, compressive stress is applied to the ultrasonic receiving element having a resonance frequency higher than the reference value. It is possible to prevent damage to the thin film of the piezoelectric material and improve the reliability of the ultrasonic sensor.

According to the invention of claim 7, in the piezoelectric ultrasonic sensor ultrasonic receiving elements are arranged in an array, as possible out to align the resonance frequency of the ultrasonic receiving elements substantially equal.

  The best mode for carrying out the present invention will be described with reference to the drawings. FIG. 1 shows the configuration of the piezoelectric ultrasonic sensor according to the first embodiment of the present invention. The piezoelectric ultrasonic sensor 1 according to the first embodiment is an array type ultrasonic sensor in which, for example, a total of 16 4 × 4 ultrasonic receiving elements 2 are arranged in a matrix.

  Next, the configuration of one ultrasonic receiving element 2 is shown in FIG. As shown in FIG. 2, a concave portion 4 having a substantially rectangular (square) cross section is formed from one side of the silicon substrate 3 corresponding to the matrix arrangement, and a thin diaphragm 5 is formed at the bottom of the concave portion 4. Is formed. A substantially rectangular (square or rectangular) lower electrode 6 made of a metal thin film is formed on the flat surface side of the silicon substrate 3 corresponding to the diaphragm 5, and a piezoelectric material thin film 7 is formed on the lower electrode 6. Has been. Furthermore, a substantially rectangular (square or rectangular) upper electrode 8 is formed so as to face the lower electrode 6 through the thin film 7 of piezoelectric material. And wiring is connected to the lower electrode 6 and the upper electrode 8, and a signal is taken out between these.

  By the way, as described above, the resonance frequency fr of the ultrasonic receiving element represented by the equation 1 is affected by variations in the shape of the diaphragm 5 and variations in the thickness of the thin film 7 of the piezoelectric material. In the piezoelectric ultrasonic sensor 1 in which a plurality of ultrasonic receiving elements 2 are arranged in an array, it is considered that the resonance frequency of each ultrasonic receiving element 2 varies. In the first embodiment, the piezoelectric effect of the piezoelectric material is used, and a direct current voltage is applied in advance between the lower electrode 6 and the upper electrode 8 as an external force to give a potential difference, thereby causing distortion in the thin film 7 of the piezoelectric material. . The distortion of the piezoelectric material thin film 7 acts as internal stress in the diaphragm 5 in which four sides are constrained. Depending on the polarity of the DC voltage applied between the lower electrode 6 and the upper electrode 8, a compressive stress or a tensile stress can be obtained. As a result, the shape or internal stress of the diaphragm 5 changes, and the apparent longitudinal elastic modulus changes. Therefore, the resonance frequency of the ultrasonic receiving element 2 is desired by appropriately selecting the value and polarity of the DC voltage to be applied. Can be adjusted to the value. By performing such adjustment of the resonance frequency for each ultrasonic receiving element 2, the resonance frequencies of all the ultrasonic receiving elements 2 can be made substantially the same value, and the object of the object by the piezoelectric ultrasonic sensor 1 can be adjusted. Position confirmation accuracy can be improved.

  As is well known, after applying a voltage of a predetermined polarity to a piezoelectric material to generate distortion, applying a reverse polarity voltage to generate reverse distortion causes the hysteresis characteristics shown in FIG. It does not return to the original state. Therefore, when the piezoelectric ultrasonic sensor 1 is used, the voltage is always made higher than a predetermined voltage (a voltage at which a predetermined resonance frequency is obtained) to reach the saturation region, and then the voltage is gradually increased to a predetermined value. It is preferable to reduce the voltage. In this way, the distortion of the piezoelectric material thin film 7 can be controlled using the same part of the hysteresis characteristic curve, and the value of the voltage applied between the lower electrode 6 and the upper electrode 8 is generated in the diaphragm 5. The relationship with the internal stress to be determined can be uniquely determined. As a result, the resonance frequency of the ultrasonic receiving element 2 can be accurately adjusted to a desired value. Such control is not necessarily performed in all cases, and the detection accuracy required for the piezoelectric ultrasonic sensor 1 (accuracy of checking the position of the object) and the variation in the resonance frequency of the ultrasonic receiving element 2 are not necessarily required. This may be done only when necessary, depending on the range of.

  Further, it is generally known that a piezoelectric material is strong against compressive stress but weak against tensile stress and easily damaged. With regard to the individual ultrasonic receiving elements 2, the ultrasonic receiving element 2 having a resonance frequency higher than the desired resonance frequency is formed, and compressive stress is applied to the ultrasonic receiving element so that the desired resonance frequency is obtained. Adjust it. In this way, there is almost no possibility of tensile stress acting on the thin film 7 of the piezoelectric material, and the resonance frequency of each ultrasonic receiving element 2 can be adjusted while preventing the thin film 7 of the piezoelectric material from being damaged. In addition, regarding the entirety of the plurality of ultrasonic elements 2 arranged in an array, an ultrasonic wave having a resonance frequency higher than the reference value, with the smallest value among the resonance frequencies of each ultrasonic receiving element 2 being a reference value. Since compressive stress is applied to the receiving element 2, damage to the specific ultrasonic receiving element 2 can be prevented and the reliability of the entire ultrasonic sensor 1 can be improved, and an object formed by the piezoelectric ultrasonic sensor 1 can be improved. The accuracy of confirming the position of can be improved.

Here, in the piezoelectric ultrasonic sensor 1 in which a plurality of ultrasonic receiving elements 2 are arranged in an array, the resonance frequency needs to be adjusted when the resonance frequencies of the ultrasonic receiving units 2 are not aligned. A brief explanation will be given. As shown in FIG. 4, when the distance between the two ultrasonic receiving elements 2a and 2b is L, and the azimuth angle of the reflected wave incident on each ultrasonic receiving element 2X and 2Y is θ, one ultrasonic receiving element 2b The time for the reflected wave to reach is delayed by the distance D (D = L · sin θ) than the time for the reflected wave to reach the other ultrasonic receiving element 2a. Further, the sensitivity of each of the ultrasonic receiving elements 2a and 2b is slightly different due to variations in the resonance frequency. Assuming that the received waveform by the ultrasonic receiving element 2a is f _a (t) and the received waveform by the ultrasonic receiving element 2b is f _b (t), the received waveforms f _a (t) and f _b (t) are respectively shown in FIG. The waveform is as shown in 5 (a), and is expressed by the following Equation 2 and Equation 3.

When such received waveforms f _a (t) and f _b (t) are synthesized, the result is as shown in Expression 4. When attention is paid to the sine component of Expression 4, waveforms similar to those of Expression 2 and Expression 3 are represented. However, since a cos component is applied to this, a ghost (beat) due to an amplitude fluctuation component of frequency 1 / T = (ω _a −ω _b ) / 4π occurs as shown in FIG. As a result, the accuracy of confirming the position of the object by the piezoelectric ultrasonic sensor 1 is lowered. Here, when the resonance frequencies of the two ultrasonic receiving elements 2a and 2b are made uniform, ω _a ≈ω _b , the cos component can be made substantially constant, and generation of a ghost due to an amplitude variation component can be reduced or prevented. Can do. As a result, the accuracy of confirming the position of the object by the piezoelectric ultrasonic sensor 1 can be improved. Therefore, it is necessary to make the resonance frequencies of the respective ultrasonic receivers 2 substantially the same value.

  Next, a second embodiment of the present invention will be described. In the first embodiment, a direct-current voltage is applied as an external force to the thin film 7 of the piezoelectric material to cause distortion in the thin film 7 of the piezoelectric element, thereby changing the apparent longitudinal elastic modulus of the diaphragm 5. In the second embodiment, an atmospheric pressure difference is provided on both sides of the diaphragm 5 so that a load is applied to the entire thin portion of the diaphragm 5 to change the apparent longitudinal elastic modulus.

  FIG. 6 shows the configuration of one ultrasonic receiving element 2 of the piezoelectric ultrasonic sensor 1 according to the second embodiment. As can be seen from FIG. 6, the opening of the recess 4 formed in the silicon substrate 3 is closed with a glass plate 9, and the pressure P1 inside the recess 4 is set to a value different from the atmospheric pressure P0 (for example, P0> P1). ing. With such a configuration, a load due to a pressure difference is applied to the entire diaphragm 5.

  As a method of manufacturing such an ultrasonic sensor 1, for example, after forming the lower electrode 6, the thin film 7 of piezoelectric material and the upper electrode 8 on the silicon substrate 3, the pressure in the manufacturing apparatus is set to P1, and the state Then, the glass plate 9 is fixed to the side of the silicon substrate 3 where the concave portion 4 is formed by adhesion or the like. If it does so, the gas of the atmospheric | air pressure P1 will be sealed in the inside of the recessed part 4. FIG. Then, by using the piezoelectric ultrasonic sensor 1 under the atmospheric pressure P0, the apparent longitudinal elastic modulus of the diaphragm 5 is changed, and the resonance frequency of the ultrasonic receiving element 2 can be set to a desired value.

  For the individual ultrasonic receiving elements 2 arranged in an array, it is not necessarily advantageous in terms of man-hours and costs to make the resonance frequency the same value by the method according to the second embodiment. . Therefore, a plurality of ultrasonic receiving elements 2 having a resonance frequency higher than a desired resonance frequency are formed using the method according to the second embodiment, and each ultrasonic receiving element 2 is in accordance with the first embodiment. A compression stress may be applied using a technique to obtain a desired resonance frequency.

  Further, the diaphragm 5 may have a rectangular shape so that the resonance frequencies in different vibration modes are close to each other and the Q value of the resonance point is lowered. In Equation 1, a is a rectangular short side and b is a long side. The resonance frequency values of the vibration mode of m = 1 and n = 2 (mode indicated by X in FIG. 7) and the vibration mode of m = 2 and n = 1 (vibration mode indicated by Y in FIG. 7) approach each other. It becomes possible to detect ultrasonic waves corresponding to the vibration mode (broadband).

  In each of the above embodiments, the array type ultrasonic sensor in which the ultrasonic receiving elements 2 are arranged in a matrix is illustrated, but the present invention is not limited to this, and the ultrasonic receiving element is 1 Needless to say, the present invention can also be applied to a piezoelectric ultrasonic sensor having only one. In that case, the resonance frequency characteristics of each ultrasonic sensor to be mass-produced can be made uniform.

The front view which shows the structure of the piezoelectric ultrasonic sensor which concerns on the 1st Embodiment of this invention Side surface sectional drawing which shows the structure of one ultrasonic receiving element of the piezoelectric ultrasonic sensor in 1st Embodiment Graph showing hysteresis characteristics of piezoelectric material The figure explaining the time delay of the reflected wave which injects into two ultrasonic receiving elements (A) is a waveform diagram showing a received waveform by a single ultrasonic receiving element, (b) is a waveform diagram obtained by combining received waveforms by two ultrasonic receiving elements. Side surface sectional drawing which shows the structure of one ultrasonic receiving element of the piezoelectric type ultrasonic sensor in the 2nd Embodiment of this invention Graph showing the relationship between vibration mode and resonance frequency when the diaphragm shape is rectangular

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 Piezoelectric ultrasonic sensor 2 Ultrasonic receiving element 3 Silicon substrate 4 Recessed part 5 Diaphragm 6 Lower electrode 7 Thin film of piezoelectric material 8 Upper electrode 9 Glass plate

Claims (7)

A substrate having a plurality of diaphragms formed thereon; a piezoelectric material thin film formed on one surface of the substrate; and a metal thin film electrode formed on both surfaces of a portion of the piezoelectric material corresponding to one diaphragm. Prepared,
Corresponding to one diaphragm, ultrasonic receiving elements composed of a thin film of the piezoelectric material and two electrode regions facing each other are arranged in an array,
Applying a DC voltage to one ultrasonic receiving element or providing a difference in air pressure on both sides of the diaphragm as a boundary, or combining them, the shape of the diaphragm in the ultrasonic receiving element or A piezoelectric ultrasonic sensor characterized by changing an internal stress. 2. The piezoelectric ultrasonic sensor according to claim 1 , wherein the direct current voltage is once increased to a voltage higher than a predetermined voltage and then gradually decreased to a predetermined voltage. Than the desired resonant frequency to form a high ultrasonic wave receiving element resonance frequencies, the addition of compressive stress in the ultrasonic receiving device, to claim 1 or claim 2 characterized in that said desired resonant frequency The piezoelectric ultrasonic sensor described. Piezoelectric ultrasonic sensor according to claim 1, wherein the recesses for forming the diaphragm of the substrate covered with a glass plate, characterized by different values of external pressure the pressure in the recess. A substrate having a plurality of diaphragms formed thereon; a piezoelectric material thin film formed on one surface of the substrate; and a metal thin film electrode formed on both surfaces of a portion of the piezoelectric material corresponding to one diaphragm. A method of adjusting the resonance frequency of a piezoelectric ultrasonic sensor corresponding to one diaphragm, wherein the ultrasonic receiving elements are arranged in an array in the form of a thin film of the piezoelectric material and two electrode regions facing each other. And
With reference to the resonance frequency of any of the ultrasonic receiving elements, a direct current voltage is applied to the ultrasonic receiving element having a resonance frequency different from the reference resonance frequency , or the portions on both sides of the diaphragm are used as a boundary. A method for adjusting the resonance frequency of a piezoelectric ultrasonic sensor, characterized in that the resonance frequency of each ultrasonic wave receiving element is made substantially the same value by providing a difference in atmospheric pressure or combining them . A reference value the smallest value of the resonance frequencies of the ultrasonic receiving device, according to claim 5, characterized in that to the ultrasonic receiving elements having a larger resonance frequency than the reference value, applying a compressive stress Method for adjusting resonance frequency of piezoelectric type ultrasonic sensor. A substrate having a plurality of diaphragms formed thereon; a piezoelectric material thin film formed on one surface of the substrate; and a metal thin film electrode formed on both surfaces of a portion of the piezoelectric material corresponding to one diaphragm. An ultrasonic receiving element that corresponds to one diaphragm and that is composed of a thin film of the piezoelectric material and two electrode regions facing each other is arranged in an array, and a DC voltage is applied to one ultrasonic receiving element. The piezoelectric ultrasonic sensor is characterized in that the resonance frequency of the ultrasonic receiving element is changed by applying a difference between the pressures on both sides of the diaphragm or by combining them with the diaphragm as a boundary . Resonance frequency adjustment method.

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